analysis of rubber and rubber-like polymers fourth edition m.j.r. loadman kluwer academic copy

Analysis of Rubber and

Rubber-like Polymers

Fourth Edition

M.J.R. Loadman

KLUWER ACADEMIC PUBLISHERSDORDRECHT / BOSTON / LONDON

Library of Congress Cataloging-in-Publication Data

ISBN O 412 81970 8

Published by Kluwer Academic Publishers,P.O. Box 17, 3300 AA Dordrecht, The Netherlands.

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Preface

The first edition of this book (1958) described an analytical situationwhich had existed for a number of years for maintaining quality controlon vulcanizates of natural rubber although the situation had recentlybeen disturbed by the introduction of a range of synthetic rubberswhich required identification and quantitative estimation.

For the former purpose 'wet' chemistry, based on various imperfectlyunderstood organic reactions, was pressed into service. Alongside thiswas the first introduction of instrumental analysis, using the infraredspectra of either the polymers or, more usually, their pyrolytic productsto 'fingerprint7 the material. The identification of a range of organicaccelerators, antioxidants and their derivatives which had been intro-duced during the 1920s and 30s was, in the first edition, dealt with by acombination of column chromatography and infrared spectroscopy orby paper chromatography.

Quantitative procedures were, however, still classical in the traditionof gravimetric or volumetric assays with an initially weighed sampleyielding, after chemical manipulation, a carefully precipitated, driedand weighed end product, or a solution of known composition whoseweight or titre, as a percentage of the initial sample, quantified thefunction being determined.

The second edition of this work (1968) consolidated the newer techni-ques which had been introduced in the first without adding to themalthough, in other applications of analytical chemistry, instrumentalanalysis had already brought about a transformation in laboratorypractice.

In 1983 the third edition was published and gave full credit tomodern instrumentation in all spheres of the analysis of rubber andrubber-like polymers, describing techniques and illustrating applicationswhere equipment still at the 'research stage' could add to the strengthof the analysts' armoury of the future. Nevertheless, the financial stric-

tures confronting modern 'instrumental7 laboratories were appreciatedso, within each area of analysis, there was a variety of techniquespresented, from the I)UITI test', costing essentially nothing, to thoseusing instrumentation costing many tens of thousands of pounds.

In this, the fourth edition, the structure of the previous edition hasbeen maintained and expanded in that each chapter provides acomplete package of information on a particular topic as viewed by anenquirer or analyst rather than discussing the range of uses of a parti-cular instrument or technique. After covering a range of topics, thebook continues by showing how specific primary analytical data can beintercorrelated and how this can then be expressed in the technologicallanguage of compound or product 'formulation7. Finally, the validity ofany conclusions drawn from the analytical data is discussed in terms ofits statistical significance so that a reasoned interpretation may be madeof the final information package.

The impact of 'health and safety' oriented legislation has taken its tollof many of the older chemical methods of analysis. Not only are thechemicals used now considered potentially hazardous, but it is alsoimportant to note that many of the older methods present in the litera-ture of the last century have not been fully validated against thethousands of new substances which may, today, be found in a commer-cial rubber product and which may interfere with a colorimetric or spottest which would have been perfectly satisfactory in earlier times. Manynew or extended instrumental techniques have, however, replaced thosewhich have been eliminated, and, at the same time, the opportunity hasbeen taken to invite my colleagues in the Materials CharacterizationGroup of the Tun Abdul Razak Research Centre to comment on,rewrite, or expand any areas which they believed to be deficient.Because these experts operate under areas of instrumental expertise andthe book is structured under topics of interest to the rubber analyst ortechnologist, individual contributions are scattered throughout the textand I can only claim to have attempted to produce a coherent whole!

To my staff, in alphabetical order, I give my thanks: Bob Crafts(elemental analysis and statistics), Paul Cudby (microscopical techni-ques), Jim Gleeson (GC and TLC), Colin Hull (NMR, thermal methodsand carbon black), Kevin Jackson (spectroscopic and thermal methods),Chris Lewan (LC and GPC), and Sue Stephens (GC and TLC). To othersof my staff whose contributions were indirect in that they freed thoselisted above to make their direct contributions I also offer my thanks.

Acknowledgement is also due to the Board of the Tun Abdul RazakResearch Centre (TARRC) for permission to undertake this project andfor the facilities made available to my staff and me.

MJRL (1998)

Acknowledgements

In a book of this nature it is inevitable that a wide range of publicationsbe consulted to afford as balanced a picture as possible of the currentposition in the analysis of rubber and rubber-like polymers. From thesepublications many tables and figures have been culled to illustraterelevant points throughout the text and it is with much gratitude that Iand the publisher thank the copyright holders for permission to usetheir data. The very number of these necessitates only the briefest ofcomments but this brevity in no way reduces the sincerity of our appre-ciation to:

The British Standards Institution for Figure 6.3, taken from BS 7164:Part 24: 1966 and the American Society for Testing and Materials,together with A. Krishen (1974) for Figures 7.10, 7.11 and 7.12. Fullcopies of these documents may be obtained from 389 Chiswick HighRoad, London W4 4AL and 100 Barr Harbour Drive, West Consho-hocken, PA19428, USA respectively. The National Institute of Standardsand Technology, Technology Administration, US Department ofCommerce, for permission to reprint Table 11.7.

John Wiley & Sons, Inc. with Evans, Higgins, Lee and Watson (1960) /.Appl. Polym. ScL for Figure 5.2; with Gelling, Loadman and Sidek (1979)/. Polym. ScL Polym. Chem. Edn. for Figures 7.16, 7.17 and 7.18; with Kimand Mendelkern (1972) /. Polym. ScL Part A2 for Figure 7.20; with Leeand Singleton (1979) /. Appl. Polym. ScL for Figure 7.21 and withBillmeyer (1971) Textbook of Polymer Science, 2nd Edn for Figure 8.2

The Managing Editor of Rubber Chemistry and Technology with Swarinand Wims (1974) for Figures 12.2, 12.3 and 12.4 as well as Tables 12.6and 12.7; Sircar and Lamond (1978) for Figure 12.5; Brazier and Nickel(1975) for Table 12.5 and Pautrat et al (1976) for Figure 11.4.

The Editor of The European Journal for the following tables whichappeared in Rubber /.: White (1967) for Table 4.2 and Lamond andGillingham (1970) for Tables 11.5 and 11.6.

The American Chemical Society with Carman (1973) Macromolecules forFigure 8.14 and Krishen (1972) Anal. Chem. for Figure 7.9.

The Editor of Materials World with Davies and Kam (1967) /. IRI forTable 11.3, Ney and Heath (1968) /. IRI for Figures 7.7 and 7.8,McSweeney (1970) /. IRI for Figure 3.4, Davey et al. (1978) Plast. andRubb. Mat. and Applic. for Figure 6.4 and Charsley and Dunn (1981)Plast. and Rubber Process Applic. for Figure 11.5.

MCM Publishing for allowing Figures 11.2 and 11.3 to be taken fromMaurer (197Oa), Rubber Age, Elsevier Science-NL, Sara Burgerhartstraat25, 1055 KV Amsterdam for allowing Figure 7.23 to be reprinted fromThermochim. Acta (1980), 39, 593 (Goh) and Addison Wesley LongmanLtd for permission to reproduce Davies and Goldsmith's table of '%ageof Student's t distribution' from 'Statistical Methods in Research andProduction (0-852-45087-X) as Figure 14.1.

Figure 10.2 was supplied by the Parr Instrument Company and ispublished with its permission, Figures 6.5 and 6.6 are published withthe permission of Dionex (UK) Ltd whilst Figure 7.13 was provided by,and is published with the permission of, the Perkin Elmer Corporation.

Finally I thank the Director of the Rubber Research Institute of Malaysiafor permission to use the data shown in Tables 6.2 (Davey (1989) /. NatRubber Res.) and 14.2 and the Board of the Tun Abdul Razak ResearchCentre (TARRC), through the Director of Research, for permission torefer to unpublished work carried out within the Research Centre overmany years and for Figure 7.2 taken from the house publication, NRTechnol. (G.M.C. Higgins and M.J.R. Loadman, 1970). Work carried outunder the earlier name of the Research Centre - the Malaysian RubberProducers' Research Association (MRPRA) - is credited to that name.

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Contents

Preface ............................................................................ xii

Acknowledgements ......................................................... xiv

1. Introduction ............................................................. 1

The Nature of Materials ....................................................... 1 The Historical Perspective .................................................. 4 Scope of the Book ............................................................... 14 The Analytical Problem ....................................................... 16 Compositional Categories ................................................... 19 References .......................................................................... 22

2. Sampling and Sample Preparation ........................ 25

Analysis of Average Composition ....................................... 25 Homogenization of Sample ................................................. 27 Analysis of Localized Composition ..................................... 28 Size of Test Portion ............................................................. 29 Sample Preparation ............................................................ 29

3. Extraction ................................................................ 31

Preliminary Remarks ........................................................... 31 Nature of the Extraction Process ........................................ 32 Standard Apparatus for Determination of Extract

Level ............................................................................. 37 Choice of Solvent ................................................................ 38 Time of Extraction ............................................................... 40 Rapid Extraction .................................................................. 41 Microwave Extraction .......................................................... 42

vi Contents

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Micro Scale Extraction ........................................................ 43 Multiple Extractions ............................................................. 44 Specific Extractions ............................................................. 45 Supercritical Fluid Extraction ............................................... 46 Latex .................................................................................... 47 Thermal Extraction .............................................................. 48 Adsorption/Extraction .......................................................... 49 References .......................................................................... 52

4. Analysis of Extracts ............................................... 54

Identifications with no Separation ....................................... 55 Identification with Separation .............................................. 65 Identification after Separation ............................................. 77 References .......................................................................... 79

5. Solution Methods .................................................... 81

Theoretical Considerations ................................................. 82 Practical Considerations ..................................................... 88 References .......................................................................... 93

6. Quantitative Elemental Analysis ........................... 95

Carbon and Hydrogen ......................................................... 95 Nitrogen ............................................................................... 96 Oxygen ................................................................................ 98 Chlorine and Bromine ......................................................... 100 Fluorine ................................................................................ 103 Silicon .................................................................................. 105 Phosphorus ......................................................................... 106 Sulphur ................................................................................ 109 Ion Chromatography (IC) .................................................... 123 References .......................................................................... 126

Contents vii


7. Instrumental Polymer Analysis ............................. 129

Introduction .......................................................................... 129 Infrared Spectroscopy (IR) .................................................. 129 Nuclear Magnetic Resonance Spectroscopy (NMR) .......... 143 Pyrolysis-Gas Chromatography (PGC) .............................. 148 Derivative Thermogravimetry (DTG) ................................... 154 Differential Scanning Calorimetry (DSC) ............................ 163 Scanning Electron Microscopy (SEM) ................................ 168 References .......................................................................... 171

8. Polymer Characterization ...................................... 174

Molar Mass .......................................................................... 174 Microstructure ...................................................................... 193 Metathesis ........................................................................... 201 Latex Particle Sizing ............................................................ 202 References .......................................................................... 205

9. Blend Morphological Analysis ............................... 208

Light Microscopy (LM) ......................................................... 208 Scanning Electron Microscopy (SEM) ................................ 209 Transmission Electron Microscopy (TEM) .......................... 211 SEM Based Scanning Transmission Electron

Microscopy (S(T)EM) ................................................... 212 TEM Based Scanning Transmission Electron

Microscopy (STEM) ..................................................... 214 Microtomy and Associated Techniques .............................. 215 Freeze Fracture ................................................................... 226 Chemical Staining ............................................................... 226 Chemical Etching ................................................................ 229 Case Study .......................................................................... 231

viii Contents


Swollen Vulcanized Elastomer Network Observation ........ 238 References .......................................................................... 242

10. Inorganic Fillers and Trace Metal Analysis .......... 243

Ashing .................................................................................. 243 Bulk Filler Analysis .............................................................. 251 Trace Metals ........................................................................ 252 Analysis of Prepared Solutions ........................................... 252 Total Sample Elemental Analysis ....................................... 256 References .......................................................................... 263

11. Carbon Black ........................................................... 265

Obtaining Free Carbon Black from the Rubber Matrix ....... 265 Types of Carbon Black ........................................................ 267 Analysis of Carbon Black Particles and Aggregates .......... 270 Analysis of Carbon Black Type ........................................... 270 Surface Area Measurements .............................................. 274 Black Type by Thermogravimetry ....................................... 279 Carbon Black Dispersion in Vulcanizates ........................... 284 Other Techniques Used to Examine Carbon Black ............ 285 References .......................................................................... 287

12. Formulation Derivation and Calculation ............... 290

Polymer Content .................................................................. 290 Formulation Derivation ........................................................ 303 Formulation Calculation ...................................................... 309 References .......................................................................... 310

13. Blooms and Visually Similar Phenomena ............ 312

True Blooms ........................................................................ 312 Modified Blooms .................................................................. 314

Contents ix


Pseudo Blooms ................................................................... 314 Surface Contamination ........................................................ 315 Hazing of Transparent Rubbers .......................................... 315 Staining/Discoloration .......................................................... 315 Pre-Analytical Check-List .................................................... 317 Analytical Methods .............................................................. 319 Removal of Bloom Prior to Analysis ................................... 320 References .......................................................................... 323

14. Validity of Results ................................................... 325

Introduction .......................................................................... 325 Meaningful Information from Imprecise Data ...................... 328 Traceability .......................................................................... 341 Validation of Analytical Methods ......................................... 343 References .......................................................................... 346

Appendices .................................................................... 347

Appendix A Table of Official National and International Standards ............................................... 347

Appendix B Elastomers: Nomenclature, Description and Properties ............................................................. 352

Appendix C Intercorrelation of Analytical Techniques ....... 359

Author Index .................................................................. 369

Index ............................................................................... 361

Introduction I

THE NATURE OF MATERIALS

Most readers will have their own idea of the materials they wouldexpect to find in a book such as this but it is not easy to define them ina way which makes it immediately apparent whether or not any one is,or is not, included. The difficulty has been in part avoided by giving anappendix of the materials which have been considered in drawing upthe methods and schemes in this book but, even so, some attempt mustbe made at definition, or at least at description.

Natural rubber derived from the tree Hevea brasiliensis is the proto-type of a wide range of materials which have a high extensibilitycombined with an ability to recover from extension. It is usual to referto these materials as highly elastic, and to group their properties ashigh elasticity.

These properties have been found to be dependent on a certain typeof molecular structure, and rubber-like materials have physical proper-ties similar to natural rubber because they, too, have the same molecularpattern in their structure. The essential features of this structure are theability of certain atoms to unite forming long, very flexible, chainscoupled with the existence of a range of molecular attractions betweenthe chains which modifies the degree of flexibility. If the chains areperfectly uniform and symmetrical, the molecular attraction betweenthem will reduce flexibility and even lead to crystal formation. If theyare completely irregular then the material will possess little strengthand will break at a comparatively low extension. For dimensional stabi-lity over long periods of time it is further necessary that the molecularchains should be linked together by occasional crosslinks to form athree-dimensional network. All the materials in the appendix conformin structure to the first two of the above requirements but those whichalso conform to the last condition, that of possessing a crosslinkedthree-dimensional structure, are those referred to as rubbers whereas

the remainder are rubber-like. In general one may say that the rubber-like materials are the flexible plastics or thermoplastics. Rubber andrubber-like materials are therefore materials composed of long-chainmolecules, which show high elasticity and it is this property which ledto the generic term elastomer being coined by Fisher in 1939.

The term polymer has not yet been introduced into this discussion; ifwe consider the long molecular chain of plastic sulphur, the sulphuratoms themselves form the simple unit from which it is built, but mostother long-chain molecules are formed by the repetition of a rathermore complex unit consisting of several atoms, which will constitute thebackbone, to which other atoms or atomic groupings are appended.When this unit is repeated to build up a long molecular chain, the unitis defined as the monomer and the polymer can be represented as(monomer)n. In this instance, the polymer should more correctly bereferred to as a homopolymer.

Details of methods whereby polymers can be built from monomersare outside the scope of this book but it should be realized at this earlystage that many important synthetic rubbers are copolymers derivedfrom two or more monomers being mixed before polymerization. Inthese situations, the polymer chain does not necessarily contain aregular and uniform alternating sequence of the monomer units nor arethey necessarily distributed randomly along the length of the chain.Indeed the extent to which monomers exist in 'blocks', and the lengthof the blocks, plays an important part in determining the properties ofthat particular copolymer.

The use of only one monomer naturally leads to a greater regularityin the polymer chain but even a relatively simple monomer such as 1,3-butadiene can give rise to polymeric irregularity due to combination inthe 1,2 instead of the 1,4 position and due to the possibility of thespatial configuration around the central double bond of each unit of thepolymerized material being either trans or cis. (It will be appreciatedthat polymerization of butadiene involves a loss of unsaturation sinceCH2=CH-CH=CH2 becomes, in the polymer, (-CH2-CH=CH-CH2—)n with the monomer units connecting end to end during the 1,4-polymerization process). This method of polymerization is the additionmethod and the resulting polymers are addition polymers.

An alternative approach to the formation of a long molecular chain isthe condensation of two or more types of molecular units (monomers)each possessing two, and no more than two, functional groups accom-panied by the elimination of some simple molecule such as hydrogenchloride or water. It is by this method that the alkyd resins used inpaint technology are made from dibasic acids and glycols with theelimination of water. There is, however, a limit to the size of themolecule that can be made in this way and this limit is below the size

where useful rubber-like properties are developed. In order to achievesufficient polymerization a further stage is added whereby the mediumlength molecules obtained from the condensation process are subse-quently linked together by other di-functional compounds such as thehighly reactive diisocyanates. This process is the basis of some impor-tant synthetic rubber-like materials such as highly elastic lacquers andfoams. Unlike addition polymers, polycondensates must, from theirchemical nature, be completely regular although not all the mediumlength molecular chains will be the same length.

So far, only the formation of chain molecules has been discussed butrubbers possess a crosslinked three-dimensional structure which resultsfrom chemical 'joins' or 'links' from one chain to another. These cross-links must be sufficiently frequent to prevent chains from sliding overeach other but not frequent enough to destroy their essential flexibility;in practice about 1% of monomer units take part in the crosslinkingprocess. Convenient crosslinking agents are chemically dissimilar fromthe chains they link and, as is well known, sulphur is the mostcommon. The term curing was used by Charles Goodyear in the USAto describe the process of heating natural rubber with sulphur to 'cure'it of its propensity to turn brittle on cooling and sticky when hot whilst,in the UK, the term vulcanization was preferred. Today, whilst curingand vulcanization are used synonymously for the sulphur crosslinkingof elastomers, the term curing is also applied to all other forms of cross-linking where three-dimensional networks are built up from polymerchains without the use of sulphur.

Given a suitable choice of solvent and temperature, most highpolymers (that is, polymers with very long polymer chain lengths) willdissolve, but ease of solution decreases with increasing molecular sizeand crosslinking effectively gives a molecule whose molecular size isthe same as its physical size. At this extreme, dissolution of the polymerin solvent cannot occur but mixing of solvent molecules with thenetwork of the polymer is possible and this is the mechanism of solventswelling. The swollen polymer still retains the crosslinked structure ofits unswollen state but is rendered softer, more flexible and weakerbecause the strength and rigidity of the network structure are no longerenhanced by the attraction of the polymer chains for each other. Thecontrolled mixing of a polymer with a suitable solvent can be used toadvantage in modifying the properties of some rubber-like materialsand this is the basis of the conversion of rigid polyvinylchloride to aflexible plastic. Pure polyvinylchloride is a hard, horn-like materialwhich can be mixed with a solvent such as dibutylphthalate to give thefamiliar plastic material which is sufficiently soft and flexible forgarments to be made from it. In this application the solvent is referredto as a plasticizer and it functions by separating the molecular chains

Type

Rigid polyvinylplastics

Polyurethanes

Polyether esters

Styrene block/starcopolymers

Thermoplasticelastomers

Soft component

Plasticizer

Polyether glycol orPolyester glycol (A)

Polyether glycol (A)

Polyolefin (A)

'Soft' elastomer i.e.EP rubber or NR

Hard component

Polyvinyl chloride orEthyiene-vinyl

acetate copolymer

Methylene (diphenylisocyanate) (B)

1,4-Benzenedicarboxylic acid

(terephthalic acid)(B)

Polystyrene (B)

Polypropylene orPolyethylene

Structure

Swollen polymer

(AB)2

(AB)2

ABA or ABAA

physical blend

Table 1.1 Classification of thermoplastic materials

one from another. Popular technical usage historically restricted theterm plasticizer to solvents used with synthetic materials such as polyvi-nylchloride and nitrile rubbers and used the terms softener or extender,depending upon the level present, for materials performing a similarfunction in natural and general-purpose rubbers. More recently thisdistinction has become blurred.

Following from this discussion, rubber-like materials, or thermoplas-tics, can conveniently be divided into five types as illustrated in Table1.1.

THE HISTORICAL PERSPECTIVE

It would be inappropriate in a book of this nature to consider the histor-ical perspective of these materials in any depth but, having identifiedthe categories into which the various materials fall, a knowledge oftheir historical development should be both of interest to, and advanta-geous for, any analyst working in this area.

NATURAL RUBBER

The history of natural rubber over the last three thousand years is afascinating story and in many areas it is confused in detail where, eventoday, the truth is sometimes obscure. It also involved many dozens offamous scientists who, space decrees, must forfeit mention here.

The oldest rubber known was reputed to have been found in 1924, inGermany, fossilized in lignite deposits some 60 million years old(Schidrowitz and Dawson, 1952), and this could be the same materialdescribed by Auleytner (1953) which was again found in Germany anddated to the Eocene period, some 30 million years ago. An attempt bythe editor to trace this material in 1994 at its last known location Qagiel-Ionian University, Cracow) met with failure. There seems to be only oneother reference to natural rubber appearing in the 'old world' and thisis to the Ethiopians making play-balls and other rubber objects whichthen spread to ancient Egypt. Herodotus attributed their origins to theLydians. With these exceptions, the early history of rubber is solely astory of the 'new world', centred round the equatorial regions of SouthAmerica and Mexico.

The earliest records which refer to natural rubber in the Americas areAztec picture writings dating from the 6th Century AD which showthat rubber was used as a material for paying tributes and was alsoassociated with devil-worship. In the Mayan city of Chichen Itzaexcavations have uncovered many sacrificial items (including humanremains), rubber figurines and torches with rubber cores which wereburnt to generate thick black smoke, possibly to suggest rain clouds -homeopathic witchcraft!

There is no doubt that sport was a fundamental part of the pre-Columbian Americas and it seems that one game which spread throughthe whole region was an early version of handball or basketball. Thesame game was played as far south as Paraguay and north into what isnow Arizona. In 1993 Stuart described a rubber ball found in El Manatiwhich was over three thousand years old.

Although the ball game appears to have covered a vast area, thesame is not true for the artefacts manufactured by the natives of theAmazon or Peruvian regions, possibly because these had more practicaland/or religious significance. One example was the use of rubber forthe manufacture of shoes. The Amazonian native was concerned withprotecting his feet and did this by a straight over-dipping process, withhis feet as the mould, to produce a perfectly fitting pair of galoshes.

The earliest western references to rubber inevitably involve Christo-pher Columbus but the honour for the first certain reference to rubberin print belongs to Pietro Martire d'Anghiera (1530) who talked of'gummi optima', and described how it was obtained as a white juicefrom certain trees which dried to a transparent material whose proper-ties were improved by fumigation.

For a few years the literature flowed. Captain Gonzalo Fernandez deOviedo y Valdes (1535) gave a detailed description of the ball gamesplayed in the Greater Antilles whilst Antonio de Herrera Tordesillas(1601) described how Cortez had watched such a game at the court of

Montezuma. In 1615 Torquemada documented the first applicationsother than play-balls. He described how his soldiers were taught bynatives to waterproof their clothing by dipping it in the milky juicefrom the rubber tree and he also described the making of footwear,bottles and a variety of hollow goods by the process of dipping overclay formers then breaking out the latter. However, neither the reportsnor the rubber products which came out of the Americas stimulatedmore than a passing interest in Europe. The latter were just regarded ascuriosities and there was no appreciation of the commercial landslidewhich was to come.

From 1615 to 1736 rubber-related literature was minimal but from thelatter date the start of the western rubber industry can be dated. Thiswas due to the activities of two Frenchmen, Charles de Ia Condamineand Francois Fresneau. La Condamine was born at the turn of the17th/18th Century and was a soldier, social climber, dilettante, andpoet but he was also a friend of Voltaire and had interests in chemistry,astronomy and botany. When the Paris Academy of Science organisedtwo expeditions to determine the exact shape of the Earth, he seemed areasonable choice to lead one which was sent to Ecuador (or Esmer-aldis).

Soon after his arrival in Quito, in 1736, he sent a package of rubber tothe Academy with a long memoir describing many aspects of its originsand production. These included the words 'Heve' as the name of thetree from which the milk or 'latex' flowed and the name given to thematerial by the Maninas Indians: 'cahuchu' or 'caoutchouc'. He laterdescribed the smoking procedure by which the natives made the rubberstable and the wide range of goods which were produced, including thefollowing. 'They [the natives on the banks of the Amazon] make bottlesof it in the shape of a pear, to the neck of which they attach a flutedpiece of wood. By pressing them, the liquid they contain is made toflow out through the flutes and, by this means, they become realsyringes/ From this the Portuguese called the tree 'pao de Xiringa'(syringe wood) and the rubber tappers or harvesters 'Seringueiros'.

The present name for the tree which is universally accepted as produ-cing the best rubber is 'Hevea braziliensis' and this is the source of allmodern plantation rubber. It was not, however, the tree whichproduced much of the rubber spoken of in pre-Columbian times. Thetree which Ia Condamine called 'Heve' was 'Castilloa elastica', but he didnot realise that the one he described a decade later, the 'pao de Xiringa'or Seringa tree, was different. In 1775, Fusee-Aublet identified andnamed 'Hevea guyahensis' as the rubber-producing tree of the Guianasand it was left to Willdenhow in 1811 (Dean, 1987), Director of theBerlin Botanical Gardens, to classify the Seringa as 'Hevea braziliensis'.Meanwhile, Persoon (1807) had proposed the name 'Siphonia elastica'

and the matter was only laid to rest by Muller (1865-6) who suppressedthe classification 'Siphonia' in favour of 'Hevea braziliensis' some 50 yearslater.

'Latex', the word used by Ia Condamine to describe the juice of thetree, was derived from the Spanish word for milk and remains in use tothis day. The name 'rubber' was coined in 1770 by the scientist JosephPriestley when he found some in a shop of artists' materials, being soldto erase pencil marks by rubbing them out. The full name of 'indiar-ubber', intended to reflect the perceived source of the material, soonbecame shortened.

La Condamine's word 'caoutchouc' is generally taken to be based onthe Indian 'caa ochu' - 'the tree that weeps' - but in view of the earlyreligious significance of rubber it is interesting to note that in adictionary of Kechuan language of the ancient Incas, Holguin (1608)translated 'cauchu' as 'he who casts the evil eye' whilst other writershave also noted the connection between the word and things magical. Ithas also been related to a native word for blood, and this couldcomplete the circle to the weeping (bleeding) tree. Regardless of whichis correct (and both could be), these are the likely origins of the currentGerman and French words, 'kautschuk' and 'caoutchouc'.

One final observation about the confusion of words: the reader ofolder books about rubber production in Amazonia will see rubberdescribed as 'fine Para' or the like. This is named after the port of Para,close to the mouth of the Amazon river. However, the whole state, asubstantial part of Brazil, is also called Para whilst the town is alsoknown as Belem.

Before returning to France, Ia Condamine met Fresneau who was atrained engineer and amateur botanist. Fresneau became infected withIa Condamine's enthusiasm for rubber and was the first Europeanperson to consider it as a potential industrial material. When Ia Conda-mine returned to France, Fresneau remained in Guiana, detailing allaspects of rubber production, treatment and usage and forwarding hisreports to his friend for publication. In 1751 Ia Condamine presented apaper by Fresneau to the French Academy (eventually published in1755) which described many of the latter's findings and this can trulybe called the first scientific paper on rubber. Fresneau deserves onefurther mention. After he returned to France in 1749 he continued todevote his life to research into rubber and, according to his biographerand descendant, the Comte de Chasseloup Laubat (1942), he eventuallyconcluded that turpentine was the ideal solvent from which to preparerubber solutions which could be used to emulate latex in the manufac-ture of articles in Europe, the latex itself being too unstable to ship toEurope. This enabled rubber to become an industrial raw material andjustified Fresneau's title as 'the father of the rubber industry'.

For the next fifty years after the work of Fresneau, progress was slowbut then, between 1820 and 1839, there was a resurgence of interest as,in the UK, Hancock invented his machine to convert lumps of solidrubber into a useable homogenous gum, a process he called 'pickling7

to confuse his competitors, Macintosh developed his three-layer water-proof fabric and, in North America, Chaffee invented his rubber milland calender, the designs of which are basically the same as those inuse today. Chaffee also founded the Roxburgh Rubber Co., the firstAmerican rubber company.

In 1839 Goodyear discovered that heating a mix of rubber, white leadand sulphur resulted in a highly elastic material which was rubber'cured' of its problems. It no longer went brittle in the cold and soft inthe heat - nor did it seem to putrefy so easily. Thus the process ofheating rubber with sulphur became known as the curing process. Inthe UK, Hancock acquired of some of Goodyear's cured rubber and,identifying sulphur as the 'magic' ingredient, developed a curingprocess which he patented ten weeks before Goodyear. The namevulcanization was coined by a friend of Hancock's - a Mr Brockedon -and fell into popular use.

In 1857 Thomas Hancock published his classic guide to the UKrubber industry and his illustrations give some idea of the breadth ofuses to which rubber was being put. Not many are missing from a listof today since they include airproof products, hoses and tyres, nautical,domestic and travel equipment as well as a range of seals, washers andmedical devices.

As early as 1791 the idea of transplanting the South Americanrubber tree to more convenient (and politically more acceptable)locations was proposed by James Anderson but it was not until the187Os that Sir Joseph Dalton Hooker brought the concept to fulfilment.Around 1870 Sir Clements Markham was feeling very pleased withhis new knighthood, bestowed on him by Queen Victoria for having're-located' the cinchona (quinine) tree to India, and was looking fornew ideas. The idea of repeating the process with the Hevea treeseemed appealing and, through his contacts with the Cabinet, theConsul in Para was asked to obtain some Hevea seeds. In 1873 thefirst (2000) seeds came to England but only 12 germinated and thesedied either at Kew or in India. Hooker then suggested that a planterhe knew be commissioned to collect some seeds. Thus HenryWickham arrived on the scene.

The story of how Henry Wickham brought his famous Hevea seedsout of South America to Kew Gardens and thence to Ceylon (Sri Lanka)and on to Malay(si)a has been told many times - mostly by Wickhamhimself with more and more added refinements until his death in 1928but even the earlier versions seem to owe more to poetic licence than

fact if one judges by his wife's diaries and other contemporaneousreports (Wolf and Wolf, 1936; Dean, 1987).

Interestingly, whilst Brazil continues to revile the name of Wickhamfor carrying out what was later to be called 'an exploit hardly defensiblein international law', the country glorifies the names of FranciscoInocentcio de Souza Coutinho, who smuggled seeds of many spicesfrom Cayenne to Para in 1797, and Francisco de MeIo Palheta who hadbeen able to charm the wife of the French Governor into providing himwith, amongst other forbidden fruit, seeds of that flavour of delight -coffee - in 1727.

There is, however, no doubt that some 70 000 seeds arrived fromBrazil as a result of Wickham's exploits and that just 2397 germinated.In August 1876, 1919 of these were sent to Ceylon and 90% survivedthe journey to Colombo, arriving in September of that year. It was thendiscovered that no-one had arranged for the freight charges to be paidand only after furious correspondence the matter was finally settled.History does not relate how many survived but by 1880 there wereonly 320 of the original stock remaining in the plantation at Henerat-goda. It does relate that 100 were also sent to Singapore, again with noarrangement for freight charges to be paid, and that these all died.

The importance of tracing these seeds and seedlings lies in the factthat in 1876 Markham also arranged for Robert Cross to travel to Brazilto 'back up' Wickham by shipping further Hevea stock to England.These were shipped mainly as seedlings and in the spring of 1887 itwas recorded that only 26 had survived. By the end of 1877, Kew haddistributed over 3000 seedlings, much more than their primary stock, sothere must have been considerable propagation from cuttings and,within this set, a further 100 were sent to Ceylon - of which 22 wereforwarded to Singapore. The planters noted that these were quitedifferent from other Heveas they had seen and this led Henry Ridley,the Director of the Singapore Botanical Gardens and the man who,more than any other, could claim to have got the Malaysian rubberindustry off (or into) the ground, to suggest that these were 'cross'plants and that 'it was from these 22 plants ... that three quarters of thecultivated plants of Hevea braziliensis have sprung'. The questionremains: who should be called 'the father of the plantation rubberindustry'?

It must be appreciated that the story of natural rubber is not onlythat of Hevea braziliensis although, in the industrialized world, mostother sources were of only passing importance. By far the most impor-tant in the closing years of the nineteenth century and first decade ofthe twentieth was the Congo vine.

Before Stanley's epic three year journey from Zanzibar to the mouthof the Congo in 1877, the centre of Africa was a blank on any map.

However, the fact that he had made the journey, and the stories he hadto tell, opened up the possibility of commercial exploitation of theCongo basin. Stanley first tried to interest the British but they had otherthings on their minds so he turned to Leopold II, King of the Belgians,who was quick to realise the potential profits to be made. Amongst theraw materials available for exploitation was rubber from the Congovine and when it was explained to the natives that the Europeanswanted it and would pay for it, they could hardly believe their luck.However, it was not long before the proverb 'lootoji loo Ie iwa' (rubber isdeath) came into being. In 1887, 30 tons came down the Congo but by1908 the total had reached 50 000 tons. Without the wild rubber ofAmerica and Africa the world of 1914 would have been a very differentplace. By 1914 the world's output of plantation rubber had equalledthat of wild rubber and by 1918, plantation rubber was the only naturalway forward. The story of wild rubber was essentially over.

THE SYNTHETIC RUBBERS

The first phase in the search for a synthetic rubber was the fundamentalscientific research in which natural rubber was broken down so that itsstructure could be determined, followed by the recombination of themonomer unit, or other low molecular weight materials with a similarchemical structure which could be obtained from commercially sensiblesources, to give an elastomeric material.

C.G. Williams decomposed rubber by pyrolysis as early as 1860 andidentified 'spirit7, 'oil' and 'tar' - the 'spirit' or volatile substance henamed isoprene and correctly gave its elemental composition as C5H8.In 1879 Bouchardat reported the recombination of isoprene to a rubberymaterial as did Wallach (1887) in Germany and Tilden (1892) in the UK,the last having correctly proposed the structure of isoprene as 2-methyl-1,3-butadiene ten years earlier (1882) and having written (1884) of thepossible industrial significance of polymerizing isoprene if it could beobtained from a more convenient source. Tilden used turpentine as thesource of his isoprene and there is an interesting footnote to history inthat a small container labelled 'Sir William Tilden's Rubber' recentlycame to light at Birmingham University. This was analysed usingproton and 13C NMR spectroscopy at the laboratories of the Tun AbdulRazak Research Centre by C. D. Hull (1995) and unambiguously identi-fied as poly-(2,3-dimethylbutadiene), not polyisoprene. This is difficultto reconcile with the information which Tilden gave in his presentationto the Birmingham Philosophical Society on May 18th 1892 but it maybe that a number of experiments were set up and that this one,assumed to be with isoprene, actually contained 2,3-dimethy!butadiene.The extra interest here is that, although initial polymerization studies

were carried out using isoprene as 'feedstock', Kondakoff polymerized2,3-dimethylbutadiene in 1900 to produce 'methyl rubber'. This becamethe first commercial rubber when it was produced by Hoffmann andCoutelle, working for Bayer, in 1909. The timing was propitious withthe Great War on the horizon. Hoffmann deserves more than a passingmention because he was also involved in the invention of new accelera-tors and antidegradents which were essential to impart reasonableperformance to the methyl rubber he was manufacturing.

In 1912 the first synthetic car tyres were made of this elastomer forProfessor Duisberg and these were followed with a set for KaiserWilhelm II. One, at least, of these is still in existence and was displayedat an exhibition 'Rubber, The Fascinating Material' which touredEurope during 1995-6. When the author tapped one of these tyres itwas as hard as rock!

Germany was obtaining natural rubber from America before thatcountry entered the Great War but, from 1916, its problems becameacute and production of 'methyl rubber' was recommenced with some2.5 thousand tons being manufactured by the war's end.

Russia was also active during this period with Lebedev polymerising1,4-butadiene in 1910 and Ostromislensky taking out a patent on thesynthesis of PVC and PVBr in 1912. In 1913 Ostromislensky published abook detailing a vast range of procedures for synthesizing differentfeedstocks. However, neither the Russian nor the American syntheticrubber industries were under the same pressures as Germany and, withthe price of natural rubber low, there was little incentive for other thanacademic research. At this point mention should be made of the UKfirm, Strange and Graham Ltd of London, which patented (Mathewsand Strange, 1910) the use of sodium as the first chemical polymeriza-tion catalyst. It should not be imagined that the procedures used topolymerize the various dienes were similar to those in use today; therewere many routes to polymerization affording nominally the samematerials with, generally, very poor and unpredictable properties. Theywere also time consuming, reaction times being measured in weeks oreven months!

The situation changed drastically in 1922 when the StephensonReduction Plan, which cut production from the British controlledplantations to force up the price of the natural material, was introduced.Over the next three years there was a tenfold price rise followed by acatastrophic fall as producers outside the control of Britain flooded themarket. It was this uncertainty which was a major catalyst for the nextphase in the development of the synthetics.

One of the first of these materials was far removed from the work ofthe preceding years in that it was prepared, by accident, by J.C. Patrickin the early 20s (although not patented until 1932) and was an ethylene

polysulphide - the first of the 'Thiokols' which are still in use assealants today. Working independently in Switzerland, Baer (1926)produced a similar material on which IG Farbenindustrie based itsPerdurens. In the States the thiokol rubbers were referred to as GR-Prubber.

IG (which now included Bayer) resumed its research in 1925 andcame on stream with Buna (polybutadiene rubber, BR) as well as twocopolymers synthesized by mixing two different monomers togetherbefore the polymerization stage - Buna S (styrene butadiene copolymer,SBR or GR-S in America) and Buna-N (butadiene acrylonitrilecopolymer, NBR or GR-A). These had reached laboratory production by1930 but then there was a further hiatus as the bottom dropped out ofthe natural rubber market yet again. In 1933, when Hitler came topower, work restarted with a vengeance. One important feature of thesenew polymerizations was that they took place in an aqueous emulsionand were very much more efficient than the earlier gas phase reactions.Unfortunately, there seems to be no record as to whether the emulsionprocess was serendipity or was intended to mimic the biosynthesis ofnatural rubber.

Given the political situation in Europe during this period it is,perhaps, ironic that IG and the Standard Oil Co. of New Jersey formeda joint study group with carefully designated areas of co-operation andprivacy. At that time IG was making acetylene, its primary feedstockfor elastomer synthesis, from calcium carbide (its private field) but inabout 1930 it changed to natural gas and Standard was then entitled toan interest. Thus Standard held the US patents to all the Buna rubbers,a crucial factor in the development of the American synthetic rubberindustry as the Second World War developed.

A further valuable material to come out of the IG/Standard agree-ment was butyl rubber. Originally synthesized by IG as polyisobutyleneit had no olefinic groups remaining after polymerization and thereforecould not be vulcanized. Standard added a little butadiene andproduced a vulcanizable product with a low level of residual unsatura-tion. At that time all of America's rubber development programme wasprivately funded and, when the Second World War started, indigenousAmerican synthetic rubber production was in its infancy. In 1941 it wasproducing less than 1% of the country's consumption and of this some227 metric tons was SBR. The first government-owned plant came onstream in mid 1942 and by 1945 the year's production exceeded 830 000metric tons. Thus is the incentive of war and the availability of blankcheques!

There was one other major elastomer which made its appearanceduring this period and that was polychloroprene. This originated in theacademic work of Father Julius Nieuwland (1922) into the dimerization

of acetylene to form vinyl acetylene and when Du Pont de Nemoursbecame aware of this work its significance was appreciated. Theaddition of hydrogen chloride across the acetylenic bond wouldproduce 2-chloro-l,3-butadiene, a substance analogous to isopreneexcept that the side chain methyl group had been replaced by achlorine atom. This was prepared by Carothers et al. (1931) and calledchloroprene. It polymerized to give polychloroprene although this isoften called by Du Font's trade name, initially Duprene and laterNeoprene. Although being last in this 'between-the-wars' part of thehistory of the synthetics, it was the first real commercial syntheticrubber. Over the same period Russia also synthesized a polychloroprene- Sovprene.

The American contribution to synthetic rubber production during thewar paid for a vast amount of fundamental research as well as produc-tion technology but, when the war finished in 1945, the cycle of cheapnatural rubber leading to diminished research completed another circle.However, in the early 1950s Ziegler and Natta revolutionized theindustry with their new catalysts which enabled high cis 1,4-polybuta-diene to be synthesized whilst novel organo-metallic catalysts also ledto the synthesis of epichlorohydrin and propylene oxide. The thirdphase of production techniques had arrived.

All of the elastomers mentioned so far have been either homopoly-mers, that is one monomer polymerized, or random copolymers but,when some structure is fed into this randomness, quite different proper-ties can be obtained and this is the principle behind many of today'sthermoplastic elastomers. In these materials there are soft 'rubbery7

regions to provide extensibility coupled with 'glassy' regions whichserve as physical network junctions at their operating temperatures butbecome thermoplastic and thus mouldable (or remouldable) when theyare heated (see Table 1.1). Their nomenclature gives an indication oftheir structure, thus polyisoprene, 'tipped' at both ends withpolystyrene, is designated SIS. These have been available now for some25 years and are taking an ever-increasing share of the elastomermarket, recent figures suggesting about 20% of the non-tyre market.Other materials with similar properties are alloys of plastics and elasto-mers such as natural rubber and polypropylene. As with the syntheticrubbers, the range of these materials is vast and they have a number ofbooks devoted solely to them. The interested reader is referred to, interalia, publications by Legge, Holden and Schroeder (1987) or De andBhowmick (1990).

Although quite different from the classic concept of a vulcanized orcrosslinked elastomer their requirements for analysis are similar tothose of conventional vulcanizates and their particular differences willbe highlighted throughout the analytical test procedures where relevant.

Of the many elastomers not covered by this historical introductionthere is one class which must be mentioned as it is unique in containingno carbon - it is thus not even an organic material. This is the class ofsilicone rubbers which were introduced in 1944 (Hyde, 1944).

SCOPE OF THE BOOK

There are four comprehensive sources of analytical methods for rubberwritten in English: the publications of the International Organization forStandardization (ISO), British Standards Institution (BS), ComiteEuropeen de Normalisation (CEN) and the American Society for Testingand Materials (ASTM). Each provides standard methods for performinga range of analyses, the details given being precise and comprehensive,covering everything from the design of suitable apparatus and thequality of reagents to the manipulative details for each step. For manyestimations alternative methods are given. As BS Standards are revisedthey are generally double referenced with both BS and ISO referencesand it should also be noted that where a BS and CEN Standard co-exist,the former must be withdrawn if there is conflict between the two. It isnot proposed that this book should supersede the published works ofthe standardizing bodies but rather that it should supplement them andfor reference a list of current ISO, BS, ASTM and DIN (DeutschesInstitut Fur Normung. e.V.) documents relevant to the analysis ofrubber and rubber-like materials is given in Appendix A.

There are two ways in which supplementation is needed and shouldbe useful. In the first place, the standard methods often give no indica-tion of when they should be used or why one method is preferable toanother. In the second place, there is no attempt to incorporate thediscrete methods into an analytical scheme designed for this or thatpurpose. When an analyst is asked to investigate a faulty product, or toadvise on suitable procedure for factory control, he or she needs aconspectus of available methods together with information illustratingtheir use, range and limitations. In short there is a need for a criticalassessment of analytical practice in the field of the material in question,and it is for such a person that the present work is intended.

In the following chapters an attempt is made to assess criticallythe tools and practice of analysis applied in the field of rubber andrubber-like materials. Although the major concern of this book is theidentification and estimation of the components of the complexmaterial of a manufactured product, this includes, of necessity,certain aspects of raw rubber analysis. Published standard methodsare not in general repeated here and only where a method is not ina British or International Standards publication are procedural detailsfully set out.

At the time of publication of the first edition of this book, booksdevoted solely or even principally to the analysis of rubber and rubber-like materials had been rare although most textbooks on rubberchemistry and, more recently, on high polymers, devote some space tothe topic. The first textbook of analysis was either that of Ditmar or thatof Pontio, since both were published in 1909. Die Analyse Des Kautschuksder Guttapercha Balata und ihrer Zusatze is the title of Ditmar's work. Itcontains much discussion on the theory of the constitution of rubber,the preparation of chemical derivatives such as brominated rubber, andthe alleged structures of these. Pontio's Analyse du Caoutchouc et de IaGutta-Percha is altogether lighter but nevertheless contains the essentialprocesses for the examination of rubber from various botanical sourcesas well as alternative methods for analysing vulcanized rubber. The firstwork in English seems to have been that of Caspari (1914) which, inspite of its title, India-Rubber Laboratory Practice, was concerned almostexclusively with analysis. Of course, analysis had been dealt with exten-sively in Weber's much earlier book The Chemistry of Rubber (1902), andthe popularity of this work may have accounted for the lack of a bookspecifically on the subject. Tuttle followed in 1922 with the firstAmerican book, The Analysis of Rubber, and after this there was a gapuntil the Ministry of Supply published, at first for limited circulationonly, its Users' Memorandum No U.9, Identification and Estimation ofNatural and Synthetic Rubbers, in 1944, and a revised edition in 1946.This was actually a pamphlet rather than a book, and the first realtextbook of analysis dealing with synthetic as well as natural rubberwas that of H. E. Frey, Methoden zur Chemischen Analyse von Gummi-mischungen published by Springer in 1953. Roff in 1956 dealt extensivelywith analytical matters in his reference book Fibres, Plastics and Rubberswhich has the advantage of giving the salient features in a concise formand setting them out in relation to other high polymers covering agreater range of properties than are dealt with here.

The journals Analytical Chemistry and Rubber Chemistry and Technologyhave published critical reviews in the field, such as Analysis, Compositionand Structure of Rubber and Rubber Products (Tyler, 1967). Full textbooksof methods and critical discussions have also been published in theUnited States by Try on and Horowitz (1963), Tyler and Try on (1963)and the very extensive study in three volumes edited by Kline (1959,1962). In England, the publication by Haslam and Willis (1965) entitledIdentification and Analysis of Plastics, now in its second edition withSquirrell as co-author (1972), includes many data on rubbers as well asplastics.

The two atlases, Infra Red Analysis of Polymers, Resins and AdditivesVolumes I and II by Hummel and Scholl (1969, 1973), revised in threevolumes. Atlas of Polymer and Plastics Analysis (Hummel, 1981a, b;

Scholl, 1981), provide many thousands of reference spectra as well asmuch practical analytical advice.

The publication of new books in this field has been limited; forinstance the Handbook of Analysis of Synthetic Polymers and Plastics, byUrbanski et al. appeared in 1977 but it is a reprint of a Polish publica-tion of 1972. More recently the trend has been towards producing booksof conference papers, which lack specificity and tend to be a mixture ofliterature surveys, promotional literature and speculative research, orvolumes such as Applied Polymer Analysis and Characterization, VoI II,1991, edited by Mitchell, which describe a wide range of technicaladvances but leave one searching for their applicability in the 'real',rather than 'research', world.

Although the first edition of this work could claim priority in its fieldthis was not the case for the second, third, or this, the fourth edition.However, the justification for the second and third editions still holdstrue; no other work seems to deal with the problems of the generalanalyst or technologist, nor do other books discuss the significance ofeach individual analysis in the total concept of the vulcanizate formula-tion, the relevance of state of cure or of blooming, or the analysis ofdegraded materials to provide data on the reason for, or mechanism of,degradation.

The opportunity has been taken to continue to expand details ofmodern instrumental techniques but it remains a fact that many rubberindustry laboratories and factories will not have these facilities and thussome pre-instrumental methods are still covered providing as wide arange as possible for each type of analysis. The increasing pressures of'Health and Safety' legislation, however, inevitably mean that a numberof useful experiments have had to be deleted.

The practising technologist, or rubber-chemist, who provides ananalytical service will soon find that as well as analysing vulcanizates,he or she will be asked to study thermoplastics, compounds prior tovulcanization, raw rubbers and possibly latex. At each stage throughoutthis edition variations in experimental technique which will broaden thescope of the analytical procedure are described and discussed.

THE ANALYTICAL PROBLEMA rubber vulcanizate, or rubber-like product, can be considered toconsist of five major classes of materials:

1. polymers2. plasticizers/oils3. solid fillers4. ancillary chemicals and their residues5. adventitious materials

and herein lies the paradox since this is the breakdown which is oftenrequired but it is the very breakdown which the analyst cannot directlyobtain. Polymers contain extractable materials which appear in (3)whilst carbon black can contain up to 1% sulphur as well as 5% othercomponents. Polymers may be used as dust-free carriers for curativeswhilst inorganic powders can be used to carry organic curatives such asperoxides. Some inorganics, such as whiting, decompose during thermalanalysis and none of these is classically 'pure'. Oils and plasticizers maybe metered accurately into a mix but suffer a degree of loss due toleakage in the mixer whilst many protective additives are complexmixtures, components of which may react differently during cure andageing. A formulation analysis therefore usually consists of a set ofanalytical data followed by inspired interpretation. The more informa-tion there is, the more closely will the derived formulation reflect thetrue composition.

The qualitative and quantitative separation and identification of any,or all, of these chemicals can be a complex and time-consuming processand it is thus important to consider the purpose for which an analysis isrequired, what degree of accuracy is needed, and which of a variety ofavailable methods, if any, will enable it to be achieved. It is worthremembering that Parkinson's Law applies as much to the analyticallaboratory as elsewhere, and here it may be stated: 'Whenever newequipment, techniques or automation are introduced, demand willincrease to fully occupy the equipment available'. Nothing is as costeffective as a sceptical approach to the question of the need for a parti-cular analysis.

Analysis of rubbers or rubber-like materials in a commercial consul-tancy tend to fall in one or more of the following categories:1. complete analysis of a competitor's product;2. partial analysis, e.g. fillers only, or nature and percentage of polymer,

under similar circumstances;3. reasonably complete analysis of representative samples purchased to

a defined specification;4. specific analysis, e.g. type and level of antioxidant (problems often

linked to environmental or toxicological concerns);5. analysis as a means of checking product behaviour, e.g. pH of

aqueous extract of a gasket intended for use in contact with metal;6. analysis of deteriorated or faulty products to determine, if possible,

the cause;7. analysis to detect factory errors.

The reasons for desiring to know the exact make-up of a competingproduct may or may not be regarded as an ethical problem but this hasbut little bearing on the analytical problem. One point which should

always be borne in mind is that the cost of identifying and quantifyingevery component in a product is exceedingly high in both work hoursand range of equipment required. A selective approach to the depth ofanalysis coupled with an input from an experienced rubber technologistwill generally provide the most cost-effective route to a formulationequivalent to or better than the one being investigated.

Sometimes it is an interest in the cost of materials which promptsanalysis, and then differentiation between various antioxidants or stabi-lizers would probably be unnecessary since the cost difference, if onewere substituted for another, would be insignificant relative to smallbulk ingredient changes. In general, analysis for costing purposes onlyrequires the identification and estimation of the polymer and bulkfillers.

The routine examination of a certain percentage of products,purchased to a specification which lays down their composition, israrely carried out nowadays in areas of general rubber goods orengineering products although it is common in areas of medical orpharmaceutical products. Many organizations regard a specification aslaying down the performance required, leaving the manufacturer toachieve this in his or her own way. This largely abolishes the need forextensive analysis and substitutes the easier and cheaper methods ofphysical testing, before and after accelerated ageing if this is required.This can, however, cause problems for the analyst if he or she is askedby a user to comment on the reasons for a particular product's failureto meet its required performance specification. With no knowledge ofthe polymer, fillers or other chemicals present, a complete analysis willbe necessary in order to see whether or not it would be expected tomeet the specification, even if correctly mixed and cured, before consid-ering possible errors in manufacture. It might also cause the organiza-tion problems if it is multi-sourcing components and nominallyidentical products in one application have different compositions.

On the other hand, certain contracts contain a clause requiring disclo-sure of the materials of manufacture and some of the reasons for thisare not sufficiently appreciated. Where the rubber or rubber-likematerial is in contact with complex materials such as explosives, livingtissue, food or medical supplies, the manufacturer cannot be expectedto foresee all possible effects of the chemicals incorporated into thefinished article which he or she supplies. Even the user may not havesufficient knowledge of which ingredients are, or are not, acceptable.Disclosure by the manufacturer allows consideration of the materials bythird parties with a wider field of knowledge but disclosure without thepossibility that subsequent departures from the disclosed formula willbe detected offers no safeguard. Where health or safety is at stake,analyses may be necessary on every batch but disclosure considerably

lightens the analyst's task, since the analysis can be designed aroundthe known formula with the omission of many steps that would beessential were the product unknown. Care must be taken, however, thatin designing one particular analytical sequence it does not become sospecific that it excludes the observation of extra-specification materials,i.e. whilst designing an analytical protocol to make sure that one parti-cular antioxidant is present, the protocol must be broad enough toconfirm that others are absent.

Further reasons for the analysis of rubber and rubber-like materialsare those of examining deteriorated or faulty products and here itshould be remembered that there is often a long chain between customcompounder, component manufacturer, trade component user (one ormore) and final retail product purchaser. At any stage of the manufac-turing or assembling processes the rubber component may be rejectedor the product may be returned after use or misuse. In any event it willbe necessary to carry out an investigation to a greater or lesser extentand, almost inevitably, in the early stages of such an investigation thefaulty article will be examined by analysis. It may be, for example, thata bloom has developed in the warehouse; the analyst is consulted onthe nature of the bloom and once this is unambiguously known theproblem is usually more than half solved.

The improvement in physical testing of materials has led manufac-turers increasingly to depend upon physical properties as a criterion ofcorrect manufacture, but advances in instrumental techniques overrecent years should make each factory manager consider whether anyparticular one could be of use to him or her in his or her search forquality. Typically, a vulcanized product can be analysed non-destruc-tively for sulphur content in under two minutes whilst an 'oil',polymer, black, inorganic filler analysis can be obtained on a few milli-grams of sample in under ten minutes. Perhaps more importantly still,batches of uncured compound can be checked and adjusted if necessarybefore cure, thus preventing wastage and reducing product variability.Even if absolute identifications are not carried out, compositionalprofiles and accepted deviations can be defined and mixtures 'flagged'if they fall outside permitted ranges.

COMPOSITIONAL CATEGORIES

POLYMERS

The elastomeric phase of a rubber product is just one of the categorieswhich has been defined and even this expands beyond just polymeridentification when one realizes that several polymers could have beenblended together to optimize a particular property, or to cheapen a

compound without damaging its properties sufficiently to put it out ofspecification. Historically there is no one reference which introduces theconcept of rubber blends to the manufacturing industry but it probablyoccurred within days of the first synthetic elastomers being prepared.The precise ways in which the various polymers intermix if they areblends, or their structure if they are copolymers (block or random), canalso critically affect the final performance of the product. One must alsoconsider the level of polymer in the product, the blend ratio if morethan one polymer is present, whether the polymers are vulcanized ornot - thermoplastic or thermoset - and the morphology of the system.

PLASTICIZERS AND OILS

The level of complexity of these materials is close to that of thepolymers with a wide range of materials being documented, each ofwhich may be uniquely selected to impart specific properties to a parti-cular elastomeric product and, as with elastomers, blends are often usedto improve further or refine product properties. These materials havedifferent solubility properties in different solvents and the wrong choicecan lead to incorrect raw data from which erroneous conclusions will bedrawn.

SOLID FILLERS

In addition to blending and plasticizing the polymers, it is frequentlydesirable to incorporate powders into the materials to increase theirbulk, alter their density, reduce their resilience, cheapen their cost, or tomodify some special property. This practice certainly extends back tothe beginning of the nineteenth century and probably back to theAztecs. The powders are incorporated before crosslinking and aredispersed in the polymer, which provides the continuous matrix.

The bulk filler may consist of a single material or may be a mixtureof several components and an error in determining the total fillerloading can arise from the nature of the fillers themselves. Thus, precipi-tated calcium carbonate may contain up to 5% of stearic acid and, sincecalcium stearate is soluble, the material remaining will differ from thatoriginally added to the polymer by amounts up to 5.5%. As a furthercomplication, 'rubber grade' stearic acid is only some 40% stearic, 57%palmitic and 2% myristic acid so an appropriate analysis must be usedor the limitations of the chosen one realised. Some clays used in rubberand PVC compounds contain added organic materials, 2-3% of whichmay be extractable, leading to analytical figures which differ from thosethe compounder would claim.

In describing the analysis of fillers, a distinction is made between

carbon black and inorganic fillers as the identification of the formerrequires completely different techniques and there are different factorswhich are important.

ANCILLARY CHEMICALS AND THEIR RESIDUES

In the case of rubber vulcanizates, the formulation complexity does notend with the major 1^uIk' components because the final crosslinkingstage is rarely carried out by sulphur alone. Accelerators are added toboth speed up and 'fine tune' the chemistry of the rubber-sulphurreaction, zinc oxide to 'activate' the accelerator, and some fatty acid,usually 'stearic' acid, to assist in the activation. These materials firstappeared at the dawn of the synthetic era with the dithiocarbamatesbeing invented by Bruni (1919), mercaptobenzthiazole (MBT) by Bruniand Romani (1921), diphenylguanidine (DPG) by Weiss (1922) andmercaptobenzthiazole disulphide (MBTS) by Sebrell and Boord (1923).These earliest materials are still the materials of choice in many applica-tions today.

It will be appreciated that both rubber vulcanizates and rubber-likematerials, natural or synthetic, are organic in nature, and age in thepresence of air. This ageing is partially counteracted or deferred (butnever prevented) by small amounts of stabilizing agents which may bepresent in natural materials or added during the manufacture ofsynthetic materials. Even so, more of these materials are usually addedwhen mixing the polymer with the other ingredients. Rubbers withsome degree of unsaturation are stabilized with antioxidants or antiozo-nants whilst, with PVC, a metal oxide may be added to protect againstloss of hydrogen chloride.

For over a century wax has been appreciated as an inert coatingwhich will prevent oxygen coming in contact with a substrate of rubber(Schidrowitz and Dawson, 1952) but, even today, this may be wiped offa product as being unsightly, thus negating its whole purpose. Amine-based antioxidants were first used at the turn of the century but it tookuntil the 1950s for the non-staining phenolic antioxidants to make theirpresence felt. Most of today's protective materials are developments ofthese two categories and the developments continue as ever greaterservice demands are placed on modern elastomers.

ADVENTITIOUS 3VtATERIALS

This category would normally include dirt contamination, present ineither the polymer or compounding ingredients, together with proteinand other insoluble non-rubbers from natural rubber, or catalystresidues from synthetic polymers. The analysis of any of these could be

significant in terms of both problem solving and polymer identification.Many of these adventitious materials are the subject of environmentalor health and safety-related controls, examples being nitrosaminesderived from dithiocarbamate curatives or the trace metals covered byregulations such as EN71.3.

It should also be appreciated that adventitious materials can be gener-ated during the manufacturing process, thus thiurams will form dithio-carbamates and these, in turn, will lead to N-nitrosamines, the levels ofwhich are restricted in a range of products.

A product made from rubber, or a rubber-like material, can thus beconsidered to be a mechanical mixture of polymer(s), plasticizer(s) orextending oil(s) and inert powders (or fillers as they are usually called)together with a number of other ingredients which may be regarded asbeing dissolved or suspended in the polymer. The temperature at whichthe mixing and vulcanization steps are carried out, coupled with thepresence of a range of reactive species, cause changes in the composi-tion of these 'other ingredients' so that they frequently no longer existin the form in which they were added to the vulcanizate and it willtherefore be necessary to identify the products derived from them todeduce their original presence.

No discussion of rubber analysis is complete without intelligentanticipation of the errors expected, and their significance in the interpre-tation of the results. In some areas of chemical analysis it is quitepossible, and reasonable, to quote percentages to two places ofdecimals, with equivalent implied precision for those componentspresent at much lower levels. In the field of rubber analysis these levelsof accuracy are neither sought nor, usually, attained and typically onewould expect an accuracy of no better than 1-2% of the measuredvalue.

The meaning of the term 'accuracy' is discussed at length in the finalchapter of this book but it should be borne in mind at this stage thatthere is little to be gained by analysing components to a much greateraccuracy than that with which they were added to the mix, whilstarguably the accuracy need only be sufficient to indicate technologicallysignificant variations from the norm. It should also be remembered thatvirtually none of the materials used in the rubber industry could beconsidered 'pure' as one would normally define the term and thus,however accurate the analysis itself is, it will not enable a moreaccurate estimation of the actual added material to be made.

REFERENCES

d'Anghiera, P.M. (1530) De Orbe Nouo, Compluti (now Alcala) folio xxxv.Auleytner, J. (1953) Bulletin de I'Academie Polonaise des Sciences, 1, 5, 189.

Baer, J. (1926) BP 279,406.Bouchardat, G. (1879) Compte rend. 89, 1117.Bruni, G. (1919) DRP 380774.Bruni, G. and Romani, E. (1921) Ind. Rubb. J. 62, 18.Carothers, W.H., Williams, L, Collins, A.M. and Kirby, J.E . (1931) /. Amer.

Chem. Soc. 53, 4203.Caspari, W. A. (1914) India-Rubber Laboratory Practice, Macmillan, London,de Chasseloup Laubart, F. (1942) Francois Fresneau Pere de Caoutchouc, Paris.Ia Condamine, C. M. (1755) Sur une Resine elastique nouvellement decouverte par

M. Fresneau, in Histoire et Memoires de I'Academic pour I'annee 1751, 319 Paris.De, S.K. and Bhowmick, A.K. (eds) (1990) Thermoplastic Elastomers from Rubber-

Plastic Blends, Ellis Horwood, London.Dean, W. (1987) Brazil and the Struggle for Rubber, Cambridge University Press,

Cambridge.Ditmar, R. (1909) Die Analyse des Kautschuks der Guttapercha Balata und ihrer

Zusatze, Hartleben, Vienna and Leipzig.Fisher, H.L. (1939) Ind. Eng. Chem. 31, 941.Frey, H.E. (1953) Methoden zur Chemischen Analyse von Gummimischungen,

Springer Verlag, Berlin.Fusee-Aublet, J.B.C. (1755) Histoire des Plantes de Ia Guiane Frangaise, London and

Paris, 2, 871.Hancock, T. (1857) The Origin and Progress of the CAOUTCHOUC or India-rubber

Manufacture in England, London.Haslam, J. and Willis, H.A. (1965) Identification and Analysis of Plastics, Iliffe,

London.Haslam, J., Willis, H.A. and Squirrell, D.C.M. (1972) Identification and Analysis of

Plastics, 2nd edn., Iliffe, London,de Herrera Tordesillas, A. (1601) Historia General de los Hechos de los Castillanos,

1, 231, Madrid.Hoffman, F. and Coutelle, C. (1909) GP 260690.Holguin, D.G. (1608) Vocabulario de Ia Lengua General de todo el Peru llamada

Lengua Quichua, o del Inca, Ciudad de los Reyes (Lima).HuU, C.D. (1995) TARRC internal report reference D576.Hummel, D.O. (198Ia) Atlas of Polymer and Plastics Analysis, Volume I, Polymers,

Structures and Spectra, Carl Hanser Verlag, Munich.Hummel, D.O. (198Ib) Atlas of Polymer and Plastics Analysis, Volume II, Plastics,

Fibres, Rubbers, Resins, Carl Hanser Verlag, Munich.Hummel, D.O. and Scholl. F.K. (1969) Infra Red Analysis of Polymers, Resins and

Additives, an Atlas: Volume I, Plastics, Elastomers, Fibres and Resins, CarlHanser Verlag, Munich.

Hummel, D.O. and Scholl, F.K. (1973) Infra Red Analysis of Polymers, Resins andAdditives, an Atlas: Volume II, Additives and Processing Aids, Carl HanserVerlag, Munich.

Hyde, J.F. (1944) BP 561136/561226.Kline, G.M. (1959) Analytical Chemistry of Polymers I, Interscience, New York.

Idem. II and III (1962).Kondakoff, I, (1900) /. Prakt. Chem. 62, 172.Lebedev, S.V. (1910) /. Russ. Phys. Chem. Soc. 42, 949.Legge, N.R., Holden, G. and Schroeder, H.E. (eds) (1987) Thermoplastic Elasto-

mers. A Comprehensive Review, Hanser Publishers, Munich.

Mathews, F.E. and Strange, E.H. (1910) EP 24790.Ministry of Supply (1944) Identification and Estimation of Natural and Synthetic

Rubbers, Users' Memorandum U.9, London, and (1946) Users' MemorandumU.9A, London.

Mitchell, J. (ed.) (1991) Applied Polymer Analysis and Characterization, Vol. 2,Hanser, Munich.

Miiller (1865-6) Linnoea, Vol. xxxiv.Nieuwland, J.A. (1922) Science 56, 486.Ostromislensky, I. (1912) GP 264123.Ostromislensky, I. (1913) Caoutchouc and its Analogues, Moscow,de Oviedo y Valdes, G.F. (1535) Historia natural y general de las Indias, Seville.Patrick, J.C. (1932) USP 1,890,191.Persoon, C.H. (1807) Synopsis Planarium sive Encheiridicum, Paris, 2, 588.Pontio, M. (1909) Analyse du Caoutchouc et de Ia Gutta-Percha, Gauthier-Villars,

Paris.Roff, WJ. (1956) Fibres, Plastics and Rubbers, Butterworth, London.Schidrowitz, P. and Dawson, T.R. (eds) (1952) History of the Rubber Industry,

Heffer and Sons, Cambridge.Scholl, F.K. (1981) Atlas of Polymer and Plastics Analysis, Volume III, Additives and

Processing Aids, Carl Hanser Verlag, Munich.Sebrell, L.B. and Boord, C.E. (1923) Am. Soc. 45, 2390.Stuart, G.E. (1993) National Geographic, November 1993, 101.Tilden, W.A. (1882) Chem. News 46, 120.Tilden, W.A. (1884) /. Chem. Soc. 47, 411.Tilden, W.A. (1892) Chem. News 65, 265.Torquemada, J. (1615) Monarchia Indiana 2, 664, Seville.Try on, M. and Horowitz, E. (1963) Methods for the analysis of rubber and related

products, in Handbook of Analytical Chemistry, Meites, L. (ed.), McGraw-Hill,New York.

Turtle, J.B. (1922) The Analysis of Rubber, Chemical Catalog Co., New York.Tyler, W.P (1967) Rubber Chem. Technol 40, 238.Tyler, W.P. and Tryon, M. (1963) in Standard Methods of Chemical Analysis, 6th

edn, Welcher, FJ. (ed.), 2B, 43, Van Nostrand, Princeton.Urbanski, J., Czerwinski, N., Janicka, K., Majewska, F. and Zowall, H. (1977)

Handbook of Analysis of Synthetic Polymers and Plastics, Halsted Press, NewYork.

Wallach, O. (1887) Annalen 238, 88.Weber, C.O. (1902) The Chemistry of Rubber, Griffin, London.Weiss, M.L. (1922) USP 1411231.Wolf, H. and Wolf, R. (1936) Rubber, Covici Friede, New York.

Sampling and sample r\

preparation tL

It is essential that the material actually analysed either be representativeof the material available, or be that most appropriate for solving theparticular problem presented.

International and British Standards distinguish between the sample(that which one is given to analyse) and the test portion (that whichone separates from the sample and uses entire for a given investigation).Using this differentiation, our discussion centres upon the choice of atest portion and its subsequent treatment so that the maximum relevantinformation is obtained.

Procedures for the sampling of both natural and synthetic latices arefully detailed in ISO 123-1985 whilst for raw rubber ISO 1795-1992should be consulted. It is not intended to cover these here as they areextremely detailed and specific. Our more immediate concern is toindicate the problems confronting an analyst when examining acompounded, thermoplastic or thermoset elastomer.

In mixing and processing rubber and rubber-like polymers, powdersare added and form a disperse phase in a matrix or continuous phaseof polymer. The degree of dispersion may vary considerably both overshort distances and long distances. The analyst is usually given asample on which an analysis is needed and before taking the testportion required for a given determination he or she ensures that thetest portion is appropriate for that particular analysis.

ANALYSIS OF AVERAGE COMPOSITION

In those cases where one or more aspects of the overall formulation isor are to be determined, it is necessary that the test portion is takenfrom a large enough volume to ensure that inhomogeneities arisingeither from mixing or from peculiarities of the particular manufacturingprocess can be averaged out by homogenization prior to the relevant

analysis. The following examples, though not exhaustive, are illustrativeof the general principles to be observed.

THIN CALENDERED SHEETS OR PROOFINGS

Economic considerations will usually prevent sampling from the centreof the length of a roll but the extreme ends should be avoided andsamples taken near to both ends rather than at one end only. Thesamples should be from the entire width, preferably cut diagonally.Whether separate test portions are cut from a sample from each end or,alternatively, whether the samples from the two ends are blended andhomogenized before taking the test portion, will depend on the natureof and reason for the analysis.

DIPPED GOODSMany rubber products such as catheters, condoms and gloves are madeby a dipping process and, particularly in the last instance, care shouldbe taken in the choice of a piece for analysis. There is no doubt thatcure residues and protective agents can, on occasion, become concen-trated in the fingertips of gloves and, unless there is a specific reasonfor a different course of action, samples should be taken from the centreof the palm or the equivalent region on the back of the glove. Alldipped products should be sampled with the possibility of dippingvariability in mind.

SMALL MOULDED ARTICLES

The quantity of material required for a particular set of analyses willlargely determine the number of mouldings required. A sufficientnumber of mouldings to allow all necessary analyses to be carried outshould be homogenized together before taking the test portion(s).

LARGE MANUFACTURED ARTICLES

In this case the nature of, and reason for, the analysis will influenceprofoundly the procedure to be followed. Composite articles such astyres must be sectioned and dismantled, the various components beingseparated and handled separately. Homogenization of the separatedcomponents will usually but not necessarily be carried out.

RUBBERIZED FABRICS

Thick rubberized fabrics may sometimes be separated by cutting with a

razor blade, but in cases where this is not feasible it is often possible toseparate rubber from fabric after swelling the rubber with vapours of asuitable solvent such as chloroform or dichloromethane. The rubber isfreed of solvent by evaporation in air or vacuum at room temperature,and then homogenized.

If rubber cannot be separated from the fabric, then the material mustbe analysed as a whole, after cutting into small pieces (ISO, to pass a2mm sieve; ASTM, 1.5mm square).

HOMOGENIZATION OF SAMPLE

Two methods are available for rubbers and rubber-like polymers: (i) thesample may be finely divided by cutting or grinding and the cuttingswell mixed before taking the test portion, or (ii) the mixing may becarried out by passing through the tightly closed nip of a roll mill, thecutting of the test portion being delayed until after sheeting the sample.In some cases a piece of the homogenized sheet can itself form the testportion without the necessity for finely dividing by cutting. In all casesit is essential to ensure that any extraneous foreign matter is excludedfrom the sample prior to homogenization.

International, British and American standardizing bodies prefercomminution of the material by passing through the cold, tightly closedrolls of a two-roll rubber-mill. If this machinery is available it isundoubtedly the best way to prepare the material for most analyticalprocedures and anyone regularly analysing rubber-like samples wouldbe well advised to install one. The rolls need not be machine driven asperfectly satisfactory results can be obtained with a long handle oneach roll and human effort to turn them. Failing this, a rotating raspmay be used but is not favoured. Rasping causes a considerable localtemperature rise which can lead to chemical reactions such as'maturing' processes, and reaction of any residual sulphur, whilstoxidation occurs with consequent increase in extractable material. Also,it is unsuitable for unvulcanized rubber and the rubber-like plastics.The obvious alternative, grinding or buffing, is not acceptable since thepowder obtained will be oxidized and contaminated with material fromthe grinding wheel. Cutting with scissors or knife (razor blade) is labor-ious but is essential if a mill is not available. The InternationalStandard ISO 4661 Part II, 1987, allows cutting and specifies thatmaterial 'shall be comminuted to pass a 1.7mm aperture sieve'. TheASTM Standard on rubber products, ASTM D 297-1993, also allowscutting but requires the sample to be rubbed or passed through a 14-mesh sieve (this sieve has an opening of 1.4mm). Both specificationsrequire the sheeting, if this is the method of preparation used, to be to0.5 mm or less in thickness.

ANALYSIS OF LOCALIZED COMPOSITION

There are many occasions when homogenization of a sample destroysthe very features which are important in a particular investigation. Thesituations where homogenization is inappropriate are too varied for acomprehensive discussion to be presented; nevertheless, the followingexamples are illustrative and highlight the need for closely defining theanalytical problem and designing both the sampling and the analysisprocedures appropriately.

VULCANIZATION STATE OF THICK ARTICLES

An example where homogenization might not be desirable is the deter-mination of the 'free' sulphur content of a truck tyre. Assuming thatthis is required because inadequate vulcanization is suspected, it wouldbe reasonable to take the test portion only from the inner face of thetread rubber rather than from a homogenized cross-section of the tread.Similarly with large blocks of rubber for mounting engines or for bridgebearings, where the state of cure may well vary with the distance fromthe surface, free sulphur determinations carried out on test portionstaken separately from the centre and outer parts of the block would bemore informative than those carried out on a test portion taken from ahomogenized cross-section of the block.

ANALYSIS OF BLOOMS (SEE CHAPTER 13)

Where blooms form on the surface of a rubber mix or rubber article, itis clearly inappropriate to homogenize the bulk material prior to identi-fication of the bloom.

BOND FAILURE PROBLEMS

Bonds between rubber and metal are sensitive to the state of cure of therubber. It is, however, the state of cure in the immediate vicinity of themetal which is important, and so the test portion must be taken fromthis area rather than from a homogenized cross-section.

INHOMOGENEITY AND POOR DISPERSION

These can cause a variety of problems, such as variable physical proper-ties, article-to-article variation, unevenness of colour etc. Such problemscan be investigated by the reverse of the normal procedure. By cuttingdown on the size of the test portion, and with no homogenizationstage, an idea may be obtained of the degree of inhomogeneity existing,

provided that the analytical technique employed is sufficiently sensitiveto cope with the small sample size.

PHASE MORPHOLOGY WITHIN A BLEND (SEE CHAPTER 9)

In this area, an awareness of the artefacts which are inevitably intro-duced during the manufacturing process is a prerequisite to selecting anappropriate volume to sample. For example, injection moulded testplaques are subject to high levels of flow orientation but these are at aminimum level at a point roughly a quarter of the way up the plaquedirectly opposite the tab, so blocks for sectioning should be removedfrom this region if the bulk morphology is the major concern but fromother well defined areas if it is the orientation, or flow effect, which isbeing studied. Likewise, in many commercial products, edge, surfaceand bulk orientation effects are likely to be present and may make theselection of a genuinely artefact-free volume difficult. In a series ofsimilar samples, once a sampling point has been established, it shouldbe adhered to for the whole series. Finally, to judge how representativea thin section is of the bulk it is often more appropriate to check theentire length of one section, where the regions will be separated fromanother by lmm or more, than to check one section against the nextwhere the separation between the two will only be 150 nm or so.

SIZE OF TEST PORTIONThe size of the test portion must be chosen with several factors borne inmind.1. It must be sufficiently large to allow the carrying out of all the analy-

tical techniques which might be required.2. It must be sufficiently large to give adequate sensitivity for each

technique being employed.3. It must be sufficiently large to average out any irrelevant inhomo-

geneities.4. It must be within reasonable limits such that handling during subse-

quent analysis is not an insuperable problem.5. It must be sufficiently small that reagent volumes and apparatus are

not unpractically large.6. It must be sufficiently small that relevant inhomogeneities of adventi-

tious contaminants are not swamped out.

SAMPLE PREPARATIONAlthough International and other standards organizations define thematerial actually being analysed as the test portion, the general analyst,

and indeed the scientist, in the English-speaking world, uses the wordsample to refer to the material he is actually analysing. Indeed this isimplicit both in the heading of this section, and in the heading of thecorresponding parts of International Standards, which also use theword sample.

In conformity with common convention, therefore, the word sampleis used from this point onwards throughout the book to refer to thatpiece of material which the analyst is actually using. The sample maybe one section of the test portion, taken to carry out one of a series ofinterrelated analyses, or it may be a discrete micro-portion on whichone specific analysis will be carried out, and which will not necessarilybe representative either of the bulk material or of another micro-sample.

Having selected a test portion which is most appropriate for a givenanalytical problem, the analyst must then decide on the most appro-priate procedure for preparation of the sample to be analysed.

The exception to this general nomenclature is in terms of microsco-pical analysis in which specimen is often used to denote that portion ofthe sample that has been prepared or is undergoing preparation (oftenby a lengthy procedure) for examination.

In general, sample preparation techniques other than the initialhomogenization procedure are specific to the analytical technique beingemployed. An exception to this is solvent extraction, partial or exhaus-tive, which is considered in some detail in the next chapter.

Other preparation techniques range in complexity from cutting intothin strips with scissors, through hot pressing, microtoming, or ashing,to sophisticated total or selective degradative procedures used primarilyfor infrared or nuclear magnetic resonance spectroscopic investigation.These preparation techniques are considered during the discussions ofthe particular analytical technique, in subsequent chapters.

Extraction O

PRELIMINARY REMARKSAlthough the concept of extraction is thoroughly understood by mostanalysts, its applications to the analysis of rubbers and rubber-likematerials are diverse and complicated. As a first step it is advisable todifferentiate between extraction, solution and dissolution. Extraction ishere defined as the procedure for removing organic additives fromthe polymer/black/inorganic components without simultaneously re-moving significant amounts of the polymeric phase, whilst solution anddissolution involve the removal of polymer from the remaining compo-nents. It must, however, be borne in mind that most polymers, even inthe uncompounded state, contain non-rubbers which will be extractedby these techniques, and in any quantitative extraction due correctionmust be made for them. In general those extracted organic materials areof low molar mass, but they may include polymeric plasticizers, facticeand mineral rubber, more realistically considered as plasticizers thanpolymers.

Extractions do not necessarily require solvents: useful informationmay be provided by a thermal extraction whilst extraction using asolvent may be carried out in the cold, or heated, for periods of timeranging from seconds to days, and be either quantitative, qualitative, orselective depending upon the exact nature of the experiment.

It would be realistic to say that in the vast majority of cases thepurpose of an 'extraction' is to use an appropriate solvent to provideessentially complete separation of the extractable materials from thebulk matrix so that each can be examined without interference from theother; for this reason the classic theory of extraction merits detailedconsideration. The choice of an 'appropriate solvent' is a potential diffi-culty. Until a completely extracted sample is available identification ofan unknown polymer may not be possible but, paradoxically, until thepolymer has been identified, one does not know the correct solvent to

use for extraction. This difficulty is more apparent than real since theanalyst will usually obtain some information from the appearance ofthe sample, its use, colour or smell. If a wrong solvent is mistakenlyused for extraction, the fault will be detected and remedial action taken.All the solvents commonly employed for hot extractions and certainlythose recommended in these pages, will usually extract all extractablematerial; where they fail is in extracting some polymer as well as thenon-polymeric material. In such a case, the polymer is available foridentification but the extract will be too great and may mislead theanalyst as to its composition and nature. As soon as the polymer isidentified, however, the analyst will realize his or her mistake and willtake steps to correct it if information on its amount or composition isrequired.

Natural rubber, being the oldest of the class of materials we areconsidering, serves as the prototype for extraction procedures. Henri-ques (1892) introduced extraction with alcoholic 'potash7 to removefactice and an abstract by Weber in 1894 records the extraction of'asphaltum' by cold nitrobenzene. Henriques also used carbon disul-phide in 1894 to extract vulcanized rubber, and Holde, about thesame time, used ether-alcohol mixtures. Acetone seems to have beenused at the turn of the century and Weber (1902) argues in favour ofits use, while Caspari (1914) records it as the standard extractant.Acetone is very suitable for the extraction of natural rubber but is notthe best solvent for use with all synthetic rubbers. It is not suitable forextracting unvulcanized synthetic rubbers or thermoplastic materialssuch as polyvinylchloride; these remarks are amplified later in thischapter.

NATURE OF THE EXTRACTION PROCESS

The extraction of soluble substances from a rubber by a solventutilizing a continuous extraction process as described later in thischapter is a diffusion controlled process. In the case of a vulcanizedrubber, the substance on which most extractions are likely to becarried out, the rubber acts as a semi-permeable membrane. Sometime after the start of the extraction, the rubber is swollen to itsmaximum extent by imbibition of the extracting liquid which forms arelatively concentrated solution, inside the rubber, of the substances tobe extracted. The solvent outside the rubber is continually renewed sothe concentration outside the rubber is virtually zero and diffusion ofthe soluble substances follows the direction of the concentrationgradient. The rubber acts as a semi-permeable membrane by reason ofits crosslinked nature, giving a mesh the size of which limitsabsolutely the size of the molecules which can diffuse out. Since the

process is taking place in a (relatively) non-ionizing solvent withlargely neutral molecules and the mesh itself is non-ionizing, many ofthe considerations normally important with semi-permeablemembranes can be ignored. Instead of a single mesh size there willbe, of course, a size-range depending on the distribution of the cross-links forming the structure, and thermal agitation will lead to varia-tion in the space through which a molecule could move. However, theoccurrence of the maximum space given by the fully extended chainsforming the sides of any mesh will have a finite probability and there-fore only molecules corresponding to this fully stretched mesh size, orsmaller, will be extractable. For a soft vulcanized natural rubber thismesh will have sides of about 20 nm so that molecules whosesmallest dimension, considered as a radius, exceeds 10 nm will beinextractable. The more the rubber is swollen by solvent, the morerapidly will extraction occur but the absolute limit to the size ofmolecules which can be extracted will not be affected. When themolecular size is near the limit of extraction, the effect of swelling onthe rate of extraction can make all the difference between extraction ina few hours and extraction necessitating weeks. This may beillustrated by reference to bitumen. The higher molar mass portion ofthis can be extracted from vulcanized natural rubber in a few hours ifchloroform is used but even after extraction for many hours withacetone, a constant weight will not be achieved.

The fundamental law in the study of extraction processes as examplesof diffusion phenomena is Pick's first law (Eq. 3.1). If dmg of thesubstance diffuses in time At across an area A under a concentrationgradient dc/dx, then

m=-DA.dc/dx.dt (3.1)where D is the diffusion coefficient, which is defined by Pick's law. Theelimination of m from Pick's first law gives the general differentialequation of diffusion sometimes known as Pick's second law (Eq. 3.2):

dc/dt = Dd2c/dx2 (3.2)The various solutions to this which can be obtained after the impositionof certain boundary conditions are discussed by Barrer (1941). The casewith which we are concerned may be visualized as diffusion from athin membrane of, say, 0.5mm thickness, the concentration of solublematerial in the membrane being given by the ratio of the extractablesolids to the solvent imbibed, and the concentration falling discontinu-ously to zero at the interface between membrane and liquid. This is, ofcourse, an idealized approximation, but as the solvent surrounding thesample is continuously agitated by the arrival of freshly condensedsolvent and is also completely drained at frequent intervals, it suffices

as a model. For this model, the amount of extractable material (w) leftin the membrane at any time t is given by:

m = 0.0405C0.£ l/(2n + l)2 exp[-(3.95 x 103)(2n + l)2 Dt] (3.3)n=0

In this equation C0 is the initial concentration of extractable material inthe volume of the membrane, and provided consistent units are usedfor C0 and ra (grams or moles) the diffusion coefficient (D) is given incm2 s-1.

The magnitude of D is a function of molecular size and some idea ofits variation can be obtained from Figure 3.1, which gives D plottedagainst log (molar mass) for an aqueous system and is derived fromdata given by Alexander and Johnson (1949). It will be observed thatthe diffusion coefficient for oxygen in water is about 2 x ICT5 (at 18 0C)whereas values given in Table 3.1 for nitrogen, which has about thesame molecular size as oxygen, show that for normal vulcanizedrubbers it is of the order of 2 x 1(T7.

Any increase in the value of D due to the higher temperature and tothe presence of the solvent needs to be set against its decrease withincreasing molar mass, but with the smaller molecules the presence ofsolvent would bring D up to the same order as exhibited by moleculesof similar size in water. It is clear from Figure 3.1 that, in the absenceof the restraining influence of a semi-permeable membrane, the diffu-sion coefficient decreases relatively slowly with increasing molar massbut once the diffusing molecule approaches the size of the membranemesh, it will cause D to drop rapidly to zero. It seems probable that,for the resins and plasticizers normally extracted from rubbers, D isabout HT6 - 1(T7.

We are now able to discuss the question of completeness of extrac-tion. If we return to Eq. 3.3 it will be seen that it converges very rapidlyindeed and a reasonable approximation can be obtained by expandingfor two terms only and rearranging to give Eq. 3.4.

m/0.0405C0 = exp( - 3.95. 1O3Df) +1/9 exp( - 35.55.103 Dt) (3.4)

Table 3.1 Diffusion coefficients for nitrogen in polymers

Polymer Temperature, 0C D, Cm2S-1

Vulcanized polychloroprene 27.1 1.9 x 10~7

Butadiene copolymers:Acrylonitrile 17 0.66 x 1(T7

Methacrylate 20 3.4 x 10~7

Styrene 20 2.4 x 10~7

Logio (molar weight)Figure 3.1 Coefficient of diffusion of water as a function of molecular size.

Initially, when f = 0, ra/0.0405C0 = 1.111, whereas, if the summationhad been carried to infinity, this expression would be equal to 1.234since, by definition and for the thickness of membrane taken,ra/C0/ = 0.05. Figure 3.2 shows a plot on double logarithmic scale ofra/0.0405C0 obtained from the right-hand side of Eq. 3.4 against log tfor a time scale of 10-105 seconds (about 28 hours) and for two valuesof D. Whilst it is true that an infinite time is required to complete theextractions, the amount unextracted after even a short time can bebelow that detectable by the analytical operation involved. WhenD = 10~7, the unextracted material is reduced to 1% of its initial value in3 hours and to 0.1% in 5 hours after which the amount remainingbecomes too small to have any analytical meaning.

The diffusion equation with the awkward summation of exponentialsin its integrated form has been avoided by experimentalists indiscussing the effect of extraction time and other variables. In addition,it is rare in analytical practice to be extracting from a polymer a singlemolecular species of definite molar mass. In the past, extraction wasalways from raw or vulcanized natural rubber where the mixture ofextractable substances certainly defied any attempt to ascribe a definite,even if average, value of D, because the range of molar masses of thenon-rubber constituents increases smoothly from that of quebrachitol

Oxygen

Nitric acid

Sodium chlorideOxalic acid

Diffu

sion

coeff

icien

t, D

x IO5

Logio (time in seconds)

Figure 3.2 Influence of diffusion coefficient on extraction time.

and those of the fatty acids to molar masses of tens of thousands.However, the shapes of the curves in Figure 3.2 indicate that definiteextract levels should be obtained if the polymer is a true membrane, i.e.if it is either crosslinked or, if free from crosslinks, free of polymermaterial of low molar mass that might be soluble in the extractingsolvent. Failure to extract to constant weight is most likely to be achemical phenomenon due to slow oxidative scission giving a constantsupply of material of low molar mass derived from the polymer.

The simple application of the diffusion equation assumes the concen-tration to fall to zero at the surface of the rubber. This is not true sinceeach of the pieces of equipment illustrated in Figure 3.3 has a finite timebetween siphoning and, during each cycle, there is a build-up in theconcentration of extracted materials in the solution surrounding therubber sample. It will also be appreciated that the rubber is alwayswetted by the extracting liquid and a layer of this remains even aftersiphoning has removed the bulk of it. The effect of this will be todecrease the concentration gradient thus depressing the value of thediffusion coefficient.

The diffusion theory expounded above gives a reasonable physicalpicture of the extraction process and, when applied quantitatively,gives values for the parameters of the equation used which are of theright order. The corollary, that extraction can never be complete, is notof analytical significance for the amount remaining unextracted when Dis of the order of 1(T6 can be reduced below the limits of analyticalsensitivity within reasonable periods of extraction. This is no longer so

Log,

o[0. 0

405C

oJ

when high molar mass polymers are to be extracted from another cross-linked polymer. Polymethylmethacrylate of high molar mass (intrinsicviscosity 6.6) was extracted with acetone from a natural rubber vulcani-zate only to the extent of 14.6% of the amount there, after 27 days(Cooper and Smith, 1962). Their data do not allow calculation of thediffusion coefficient but an approximate treatment suggests that it issmaller than 10~10.

STANDARD APPARATUS FOR DETERMINATION OF EXTRACTLEVEL

The apparatus used for extraction should preferably be of all-glassconstruction and two forms recommended in the InternationalStandards Organization document ISO 1407:1992 are illustrated inFigure 3.3(a) and (b) which, in addition, allows a form of extractionapparatus in which a coiled metal condenser is inserted into the neck ofa conical flask, which is closed by a metal disc through which thecondenser tube passes, and is integral with it. This form of apparatus,usually known as the Underwriters, is illustrated in ASTMD 297-93 andis shown here as Figure 3.3(c). The objections to it are that when severalare connected in series the tubing tends to prevent the closing platesitting squarely on the flask; flask irregularities have a similar effect,both resulting in a loss of solvent, a loss aggravated by the fact that the

Figure 3.3 Four basic types of extraction apparatus suitable for the extraction ofrubber and rubber-like polymers.

condenser is necessarily tightly coiled and its effective surface areasmall. In addition, a condenser cut from block tin is expensive and itscheaper equivalent, dipped or plated brass, has been known to containpin-holes in the plating, flaws identified as the cause of polymerizationof the extractant (acetone).

Figure 3.3(d) illustrates the routine extraction apparatus used in theauthor's laboratory. A 150cm3 round-bottomed flask is used since thisis preferred for convenient heating on a bank of heating mantles, andenables the solvent to be removed under reduced pressure at lowtemperature. The Soxhlet cup, although having a capacity of only 8 cm3,will hold the weight of sample usually extracted (3 g) and has consider-able advantages in terms of the length of time required for extractionfor reasons discussed later.

Raw rubbers, unvulcanized compounded rubbers and some thermo-plastic materials often become tacky during extraction and tend tocoalesce, thus invalidating the quantitative extraction data. This can beovercome with sheeted samples by placing them between lens tissue ornylon filter cloth prior to extraction whilst, for cut-up samples BS 1673:Part 11-1954 (now withdrawn) suggests the use of silver sand to dilutethe polymer. In all cases the anti-coalescing materials should beextracted before use.

A valuable technique with thermoplastics and compounded rubbersis to prepare a thin film using a hot laboratory press. Temperatures upto 18O0C may be required for some thermoplasts but it is a simplematter to press for a few seconds and obtain a sheet 0.2-0.5 mm thick.Similarly a compound can be lightly cured by holding it at approxi-mately 15O0C for 1-2 minutes and the sheet will then have a muchreduced tendency to flow during the extraction although the extract willthen contain cure residues, together with the original curatives. If theedges of the sheet are discarded there is no evidence for degradation ofthe polymer in this time scale.

CHOICE OF SOLVENTFor the extraction of natural rubber, whether vulcanized or raw, acetoneis usually specified as it fulfils most of the criteria for a good extractant.These are that the polymer should be swollen slightly by the solventbut should not be soluble in it; it is convenient that it should boil at atemperature well below that of any extracted material so that it can beremoved easily from the extract without loss of any extracted liquid orheat damage to any solids, and, in addition, the solvent should be inertto any possible ingredient of the extract and not objectionable by virtueof excessive toxicity, inflammability or odour (although it must beremembered that all solvents are toxic to some extent). It is advanta-

Table 3.2 Solvents for the extraction of rubbers and rubber-like polymers

Elastomers Raw/compounded Vulcanized

NR acetone acetonemethanol methanol

2-propanol butanonesynthetic polyolefins: acetone acetone(i.e. BR/SBR/IR etc) 2-propanol butanoneUR acetone acetone

butanone butanoneCR and NBR light petroleum (60-80) light petroleum (60-80)

methanol methanol2-propanol 2-propanol

PVC diethyl ethermethanol -

Thermoplastic block methanolcopolymers (ie SIS)NR/PE/PP types methanolEPR/PE/PP types methanol

geous if the solvent used is cheap, as the reuse of recovered solventcarries with it a certain element of risk and should be avoided.

Table 3.2 indicates solvents which have been found generally accep-table in the extraction of common elastomers, but attention is alsodrawn to Table 3.3 which gives a much broader picture of the resistance

Table 3.3 Resistance of rubbers to various liquids

Rubber Aliphatic Aromatic Halogenated Ketones Alcohols Animal Water& veg.

oils

Natural P P P G G P-G Ecis- P l P P P G G G - P ES B R P P P G F P - G G - ENR P P P G - E E E G - Ecis BR P P P G F-G P-G EE P R etc. P P - F P G P P EC R G F P P F G F - GN B R E F P P E E F - GAU/EU E F F P - F G G GO T E E G G G E FMQ etc. P - G P - F F F - G G F FFPM/CFM E E G P E E EC S M F F P P G G GAcrylates E E P P P E E

Ratings: E = excellent, G = good, F = fair, P = poor.For polymer types see Appendix B.

of a range of rubbers to various solvent types, and may help in theselection of an unusual solvent for a specific application. Acetone usedfor extraction should be free from its polymers and from water and thismeans that for all accurate work, if it is taken other than from a newlyopened container which has been stored in the dark, it should be redis-tilled before use. Methanol is also a useful solvent for the qualitativeextraction of natural rubber, raw or vulcanized, for subsequent chroma-tographic examination of the extract. Whilst additives are generallyextracted quantitatively, the extract appears cleaner than when obtainedwith acetone.

Extraction of polar synthetic rubbers may be with ether or withlight petroleum of specified boiling range. If the latter is preferred itshould be noted that the Soxhlet-type extraction apparatus sometimesgives trouble due to the lower boiling components of the solventcreating vapour locks in the siphon tube. The remedy is to use aSoxhlet with an external syphon (see Figure 3.3(d)). Ether, being ofconstant boiling point, is free from this trouble provided that excessiveheat is not applied to the flask, preventing the condensate fromrunning back.

Methanol has been found to be an acceptable solvent for selectivelyextracting plasticizers from block copolymers such as SBS or SIS, andthose based on a polyolefin/polypropylene blend, whilst it also affordsa good separation of the polymeric plasticizers used with acrylonitrilerubbers and chloroprenes (Williamson, 1957). Robertson and Rowley(1960) recommend the carbon tetrachloride-methanol azeotrope for theremoval of polymeric plasticizers from PVC but this is unlikely to findfavour today because of the toxicity of carbon tetrachloride. Because ofthe vast range of solvents suggested by different authors it is imperativethat any quantitative results, or specifications, define fully the solventsystem used and the extraction process.

TIME OF EXTRACTIONRubber extractions with acetone or the other solvents listed are usuallycarried out overnight, with ASTMD297-93 and ISO 1407:1992 tending toagree on some 300 cycles through the extraction cup, although ISOallows as few as 160. If the sample has been properly comminuted thelonger times required by these standards are probably unnecessaryalthough, when new apparatus is used, the extraction rate should bechecked, as also should its behaviour with any unusual solvents beforeleaving extractions overnight. A simple rubber compound containingonly a minimum of added materials is usually quantitatively extractedin periods less than eight hours but this gives the necessary margin ofsafety to allow for the presence of unusual substances.

RAPID EXTRACTION

The apparatus already illustrated in Figure 3.3(d), using the low-volumeSoxhlet extractor, enables relatively rapid extractions to be carried out.The cycle time is of the order of one minute (with 3g rubber) and thusafter three hours the minimum number of ISO passes has beenexceeded and, in the author's experience, essentially quantitative extrac-tion of general purpose vulcanizates or compounds etc. has occurred.The particular advantage of reducing the extraction time from eight tothree hours is that an extraction with subsequent examination of theextract and residue can be carried out within one working day.

An even more rapid method has been proposed by Kress (1956), andis referenced in ISO 1407. The procedure is as follows:

Sheet out Ig of the vulcanizate with the mill nip set as tightly aspossible. Make several passes with a single unfolded sheet until thesample is as coherent as possible. Where this is not possible place themilled crumbs into a filter paper envelope and for uncured stocksandwich between filter paper. Cut, with scissors, a test portion ofbetween 80 and HOmg and drop the weighed test piece into theboiling solvent and continue rapid boiling for 30 minutes. (Kress uses20cm of solvent in a 250cm conical flask with a condenser butstates that a beaker of solvent on a hot-plate and covered with awatch glass is equally satisfactory.) After extraction, press the testpiece between 'folds of absorbent paper towels' to remove excesssolvent and dry at 105-11O0C for 10 minutes.Kress advocates the use of a mixture of methyl ethyl ketone and

ethanol, 75:25 by volume, and weighing the test portion before andafter in order to determine the extract quantitatively. This mixed solventis chosen empirically to give results in line with the standard ASTMacetone extraction, MEK itself giving too high a figure. However, weare not here concerned with Kress's suggestion that his method shouldreplace the existing quantitative procedure but only that it provides aconvenient means of obtaining quickly an extracted sample for qualita-tive examination and for this purpose the use of MEK without ethanolis probably preferable.

A completely different philosophy was adopted by Higgins (1978)who used a high-speed macerator which generated ultrasonic pulses toextract quantitatively 2-5 g samples of raw rubbers in a matter ofminutes. Samples of SMR 5, 20, 50 and 5-LV rubbers, together with anexperimental set of oil-extended NRs (OENR) containing up to 25% oilwere examined by the rapid method and ASTM D297. The resultsillustrated that the method allows a rapid determination of the extractlevel with extraction times of less than 5 minutes.

Subsequent unpublished work has shown that this technique isequally valid for the analysis of compounded or vulcanized rubber andthus has a particular significance in that it allows the cold extraction ofunvulcanized samples permitting the identification of the addedcuratives themselves, rather than their decomposition products, particu-larly important for the identification of thiazole and thiuram accelera-tors. It is, of course, quite possible to use a 'cold Soxhlet' extractor, inwhich the hot solvent vapours by-pass the sample holder and only aftercondensing do they flow over the material being extracted, but this isextremely time consuming and cumbersome whilst the extractedsubstances are still contained within the boiling extractant for theperiod of the experiment.

MICROWAVE EXTRACTION

Accelerated extraction using microwave heating is a relatively recentdevelopment, the first commercial ovens only appearing some 10-15years ago. The main attraction of this type of apparatus is its ability toachieve extraction temperatures above the normal boiling point of theextraction solvent, without using high pressure containment vessels.

Microwave heating achieves super-heated conditions because thecontainer is not heated directly by the microwaves. The microwaves areusually at a frequency corresponding to rotational energy bands of -C-H or -O-H, that is around 4000 cm"1 or 2450 MHz. Although the vesselsused to contain the samples are often made from PTFE, or similar inertpolymers, any residual -C-H linkages present in non fully fluorinatedpolymers are rigidly fixed and incapable or absorbing the microwaveradiation therefore the solvent is heated from the inside outwards,rather than from the outside in, as is the more usual situation. Thismode of heating also promotes superheating since it reduces the poten-tial for nucleation of bubbles by asperities on the surface of thecontainer.

When boiling does eventually occur, the temperature of the solventdoes not return to its 'normal' boiling temperature, but it remains at anelevated one which is characteristic of the particular solvent being used.

This higher temperature enables extraction to be carried out in signifi-cantly shorter times and the extraction times for microwave extractionare of the same order as those for micro scale extraction. It should,however, be noted that these elevated temperatures will tend toincrease the possibility of further cure-related chemical reactions or thedecomposition of labile additives during the period of the extraction.

This technique has been used extensively in extracting additives fromplastics (Freitag and John, 1989; Neilson, 1991) but less so for elasto-mers. The additives for which extraction was demonstrated by these

authors were all either actual rubber processing chemicals, or closelyrelated substances but the difficulty of using the data from these papersas a guide to the suitability of microwave extraction for rubber productsrelates directly to the difficulty of producing very small particles fromrubbery materials without inducing chemical changes to title analytes, adifficulty which is also relevant to supercritical fluid extraction asmentioned below. Since the extraction process is diffusion-limited, thelarger particles inevitably require longer extraction times which mayreduce the cost effectiveness of the extraction process.

MICRO SCALE EXTRACTIONThe first published account describing applications and a procedure forcarrying out extractions of rubber on the micro scale is that of Wyatt(1941). An illustration of his apparatus was included in the secondedition of this book (Wake, 1969) but it appears obsolete today since thepractising analyst would tend to use standard 'micro glassware' notavailable in 1941. The size of the test portion was some 20 mg and bystrict definition, therefore, the method should be considered a semi-micro one as the sample is over 10 mg in weight. Nevertheless theextract will normally be between 1 and 5mg thus the use of the word'micro' could be acceptable. Wyatt's results indicate the rapidity ofextraction using this apparatus, 2 hours being sufficient for an extractionwith acetone as compared with 7 hours for the conventional modernmacro method. The shorter time equates well with the time requiredusing the small extraction apparatus illustrated in Figure 3.3(d) when 3hours is generally adequate for a 3g sample. Wyatt also gives data forchloroform and alcoholic 'potash' extractions, and again illustrates thatthe more rapidly obtained results have recoveries comparable withthose of the slower macro method.

The test portion size of 20 mg is also a point which requires consid-eration. If a piece of material of adequate weight is available (ISO1407 requires 3-5 g) then there is little point in carrying out a microextraction unless there is a specific reason as discussed in Chapter 2.In this case the problem of obtaining a representative sample does notarise as one is specifically looking for differences on the micro scale.Should a micro extraction be required of a 5g sample such that it isrepresentative of the whole, then it should be appreciated that the20 mg constitutes only some 1/250 part of this, and it is advisable tocarry out a two-part homogenization by taking a 0.1-0.2 g samplefrom the initially homogenized 5g and carrying out a further homoge-nization before taking the 20 mg samples. The statistical rationalebehind this argument and the validity of micro sampling techniquesare given in Chapter 14.

MULTIPLE EXTRACTIONS

In rubber technology some use is made of factice and mineral rubberwith both natural and general-purpose synthetic rubbers whilst poly-esters are increasingly used as plasticizers for PVC and oil-resistingsynthetic rubbers. Factice, also known as rubber substitute, exists intwo broad classes distinguished by colour and known as 'light sub/and 'dark sub/. Actually, the colour difference signifies a chemicaldifference of some importance to the analyst. Brown, or dark sub., isformed by reacting together a mixture of vegetable oils and sulphurand is a polymeric material of moderate molar mass (about 7000)(Stamberger and Knight, 1928), whereas white, or light sub., is formedfrom the same oils by reaction with sulphur chloride (S2Cl2) and themolecule, probably of similar molar mass, contains chlorine as well assulphur. Factice is not extractable from vulcanized rubber by acetonealthough its presence usually leads to a slight increase in the acetoneextract due to the presence of small quantities of free oils; neither doesa simple change of solvent, to chloroform for example, enable this to bedone, so recourse has to be made to a degradative extraction withalcoholic potash after the rubber has undergone an initial extractionwith acetone.

Mineral rubber is the 'trade name7 given to asphaltic hydrocarbonsused as 'extenders' or cheap filling material or processing aids forrubbers when high-grade mechanical properties are not required.Asphalt, whether derived from native asphalt or obtained as a distilleryresidue from some petroleum sources, consists of a mixture of oils,resins, and asphaltenes the latter two being of medium high molarmass, and it is part of these which resist extraction with acetone. Ifasphalt is suspected, the procedure to adopt is to follow the acetoneextraction with one of chloroform until a colourless liquid is obtainedfrom the extraction cup since no other materials in common commercialuse in rubber vulcanizates are insoluble in acetone but soluble in chloro-form. However, it does not suffice to quote the chloroform extract soobtained as the mineral rubber content, since part of the mineral rubberwill have been extracted by acetone and this part will depend on thesource of the mineral rubber and probably also on the temperature ofvulcanization of the compound. In matching a specification for acompound to the analytical figures this must be allowed for.

Rather similar to the factice problem in natural rubber is that ofpolymeric plasticizers in the oil-resistant synthetic rubbers. Typicalmaterials are polypropylene adipate and polypropylene sebacate. Themolar mass is not very high and, as with condensation polymers gener-ally, the molar mass distribution is rather broad. This causes difficultyin finding a solvent which will extract ordinary monomeric plasticizers

without extracting any of the polymeric plasticizer and, in fact, thiscannot be done. Ether, which is normally used, definitely takes outsome of the polymeric plasticizer. Difficulty is also experienced at theother end of the molar mass distribution in finding a solvent which willcompletely remove the high molar mass material without removing thebase polymer. Acetone will successfully remove all the polymeric plasti-cizer but will also remove some of the rubber, particularly that of thebutadiene-acrylonitrile type.

As already mentioned, Williamson (1957) has shown that methanolcan be used successfully in certain cases for the analysis of polymericplasticizers.

SPECIFIC EXTRACTIONSMultiple extractions are, by definition, specific since the first must leavesomething behind for the subsequent ones. There is, however, no needfor subsequent extractions to be carried out if the first one extracts thesubstance to be analysed either qualitatively, reproducibly or quantita-tively depending on the purpose of the exercise and this is the conceptbehind a specific extraction.

There are two fundamental reasons for carrying out such an analysis.The first is to extract an analyte cleanly whilst leaving behind possiblecontaminants or interfering substances whilst the second is to extract anarticle in a way which mimics a process it might experience during itsservice life so that any substances being leached or extracted can beinvestigated.

An example of the former is the aqueous acid extraction and subse-quent estimation of hydroxylamine from raw or vulcanized naturalrubber (Davey and Loadman, 1988) whilst the latter is often related tohealth and safety considerations where a water-based extractionmedium may be used to emulate a physiological activity. Examples ofthis are the procedure of Blosczyk (1992) for extracting MBT and ZMBTfrom rubber products with water, although Edwards (1994) claimedthat the insolubility of ZMBT was such that it was not extracted unlessthe extractant had been acidified prior to the extraction being carriedout, and the analysis for volatile N-nitrosamines in baby feeding bottleteats and soothers by the German (BGA) method of 1984 (soon to bereplaced by CEN Standard in response to the EU directive 93/11/EEC).It should be noted that the American (FDA) procedure for nitrosamineanalysis uses dichloromethane as extractant on the grounds that this isexhaustive and gives a maximum level of nitrosamines which, poten-tially, could be bio-available.

Recently there has been a growing interest in the levels of bio-avail-able nitrosamines in a wide range of products such as the BGA

Table 3.4 Chemical compositions of artificial saliva and artificial sweat

Artificial saliva Artificial sweat

sodium bicarbonate 4.2 g potassium chloride 0.3 gsodium chloride 0.5 g sodium chloride 4.5 gpotassium carbonate 0.2 g sodium sulphate 0.3 gdistilled water 100Om3 ammonium chloride 0.4g

!active acid (90%) 3.Ogurea 0.2 gdistilled water 100Om3

Recommendation 21 'special category' products, condoms, gloves andcatheters. Only in some cases are analytical procedures described andthis must cause concern for the analyst who is confronted with aproduct for which no documented or recognized procedure exists. Forinstance, when analysing latex gloves or condoms, considerationshould be given to using artificial perspiration instead of artificialsaliva. As Table 3.4 illustrates, these are appreciably different chemi-cally although whether the difference is significant in terms ofextracting bio-available nitrosamines and nitrosatable amines appearsnot to have been documented.

Some representative Standards are DIN 53160, which measures theresistance to saliva and perspiration of coloured toys, and the EuropeanStandard, EN71-3, which is concerned with the bio-available toxicelements in children's toys. Here the extractant is hydrochloric acid ofpH 1.0-1.5, being intended to simulate the potential for dissolution ofingested materials in the stomach.

Regulations concerning rubber in contact with food are typified bythe FDA Code of Federal Regulations, title 21 which defines, inter alia,extraction limits with water for aqueous based foods and n-hexane forfatty ones whilst the BGA Recommendation 21 identifies four othercategories as well as the 'special category' and sets limits for eachcategory when extracted with water, 10% aqueous ethanol and 3%aqueous acetic acid.

SUPERCRITICAL FLUID EXTRACTIONSupercritical fluid extraction can be considered an extension of both thespecific and sequential extraction procedures described above in thatselective extraction may be achieved by varying the temperature,density (pressure), flow and time of the extracting liquefied gas. Byprogressively altering one of the parameters, typically the pressure andhence density of the extractant, a controlled extraction can be achievedwhich mirrors the use of solvent gradients in HPLC analysis and,

indeed, SFE is often directly coupled to HPLC to exploit this 'pre-column' selectivity (King, 1989).

Nevertheless two major disadvantages remain in applying thistechnique to the analysis of rubbers or rubber-like materials. Firstly, it isexperimentally complex and the validation of any result is a time-consuming exercise for which there is no short cut whilst secondly, andperhaps of more pragmatic importance, it is necessary for a solid testportion to be prepared as a finely divided powder, with individualparticles in the size range 10-50 |im. Whilst this is possible if one uses amacerator followed by cryogenic grinding and sieving it may well bedifficult to justify the time and cost involved in the operation. It mustalso be remembered that the minor extractable components of therubber might have undergone at the least a quantitative change duringthe preparation of the powder. The reader who wishes to consider thisalternative approach to 'solvent' extraction is referred to excellentreviews by Gere and Derrico (1994).

LATEX

ISO documentation provides three standards concerned with theconversion of the liquid latex to solid rubber. For our purposes two ofthem - ISO 124-1992 and ISO 498-1992 - can be considered the same inthat they both afford total solids in the form of a thin sheet of dryrubber. The difference is that whereas ISO 124 is solely concerned withobtaining a total solids value, and dries a layer of latex at 7O0C (16 h) or10O0C (2h), ISO 498 is specifically written to prepare a smooth thin filmof rubber and thus dries at 35 0C to constant weight, a procedure takingseveral days. The third document (ISO 126-1995) describes the determi-nation of the dry rubber content by coagulation with acetic acid, separa-tion from the latex liquids, and subsequent drying at 70 0C. It is obviousthat this sample will be quite different from the other two in that mostof the non-rubbers will have been separated from the rubber itself, andit should not therefore be used for further general analysis if this couldbe significant.

The dried latex may be extracted by any appropriate solvent in thesame way as a raw rubber or vulcanizate, but its physical state can beused to advantage in preparing a suitable sample for extraction. Chin etal. (1975) used a rotary evaporator to spread a thin film of NR latex(LA-SPP) over the sides of a 150cm3 round-bottomed flask which wasthen simultaneously coagulated and extracted by the addition of amethanol-acetic acid mixture. After a few minutes of rotating thesolution in contact with the latex film, effectively quantitative extractionof pentachlorophenol was achieved in the form of a solution amenableto direct analysis.

A similar procedure was adopted by Edwards (1981) who prepared alatex (LATZ) film as described above and used pure methanol to extracttetramethyl thiuram disulphide (TMTD) by rotating the flask containingthe solvent and latex film for 1 hour before analysis of the extract byliquid chromatography. It should be commented that no TMTD(<lppm) was found in a number of samples of latex examinedalthough quantitative recovery was obtained from freshly 'doped'latices.

THERMAL EXTRACTION

At the beginning of this chapter it was noted that the definition of'extraction7 did not mention solvents and there are times when a'solvent-free' extraction, relying on the volatility of one or more compo-nents of the product, may be used to advantage.

McSweeney (1970) demonstrated a thermal extraction methodcoupled with subsequent TLC separation for the identification ofcompounding ingredients in rubbers. The apparatus, which is particu-larly suitable for the examination of small amounts of rubbers (Figure3.4), consists of a small furnace surrounding a tube through whichnitrogen is passed. As the temperature of the oven is slowly raised to250 0C, the nitrogen sweeps volatilized material directly on to the startline of a TLC plate. By progressively moving the TLC plate while theoven temperature is raised, thermally fractionated material is deposited

thermocouple & probe

PTFEsleeve

variableresistance

ticplate

plateholder

on railssample

along the start line so that, after development of the plate, informationconcerning the relative volatilities of the compounds can be obtained.

Samples of a few milligrammes of a vulcanizate are quite adequatefor the qualitative identification of curatives, cure residues, protectiveagents and extender oils by this procedure, although the techniquemakes no pretence to quantitative accuracy.

A very simple thermal extraction apparatus may be constructed froma tin which has had a hole drilled in the lid into which a septum hasbeen fitted. The product, in any appropriate form, is placed in the tinwhich is then heated in an air oven set at a selected temperature.Samples of the atmosphere inside the tin may be taken at intervals andanalysed by gas chromatography or, on a somewhat larger scale, by gasphase infra red spectroscopy. For an additional several tens ofthousands of pounds it is possible to purchase a 'headspace analyser7

which can be directly interfaced to a gas chromatograph although forone's money one does get sophisticated temperature control coupledwith full auto-sampling facilities. The applications for this type of equip-ment usefully cover all those substances which may be too volatile tohold in the liquid phase of a conventional solvent extraction and couldrange from volatile organic contaminants to residual acrylonitrilemonomer in nitrile rubbers, the latter being the subject of ASTMD4322.

Certain modern programmable temperature vapourisers (PTVs) areable to operate as thermal desorption units since it is a simple matter toprogramme into the analysis both the maximum temperature to bereached and the thermal ramp required to attain it. When such instru-ments are coupled to a gas chromatograph with a mass selectivedetector substantial amounts of information are available very easilyfrom samples of vulcanizate weighing a few milligrams. Figures 3.5 and3.6 illustrate the total ion chromatograms obtained from the sequentialthermal desorption and pyrolysis of a sliver (approximately 5mg) of anatural rubber vulcanizate. The thermal desorption total ion chromato-gram (260 0C) shows antioxidant 2246 and wax, the hydrocarbon profileallowing a reasonable characterization of the latter, whilst the pyrogramshows two major peaks identified as isoprene and dipentene from theirmass spectra.

ADSORPTION/EXTRACTION

Any atmosphere, be it that from that of a rubber factory or an environ-mental chamber similar to, but rather more sophisticated than, the tincan mentioned earlier, can be considered to be a 'headspace7 waiting foranalysis. In the rubber and plastics industry, compounding rubber andplastic products (and curing the former) can result in the evolution offumes and vapours potentially hazardous to health and it is therefore

Time (min)Figure 3.5 Total ion chromatogram of thermally desorbed species (26O0C).

necessary, and a legal requirement, for these to be monitored to reducethe risks of diseases arising from occupational exposure to potentiallyharmful substances.

Perhaps the most simple form of monitoring involves the use ofdedicated absorption tubes containing chemicals which react with the

Abundance

Time (min)

Figure 3.6 Total ion chromatogram of pyrolysis products (60O0C) from a naturalrubber vulcanizate.

Abundance

lsopre

neDip

enten

e

particular substance being monitored to give a directly readable colourchange. This is quick and relatively inexpensive but requires a differenttube for each substance or group of substances being monitored.

Of more general use is a procedure whereby the vapours areadsorbed on an inert material and then subsequently extracted ordesorbed for identification and quantification by an appropriate analy-tical technique. The initial entrapment stage can be carried out bypassive adsorption on to a disc badge or tube containing the adsorbingsubstance for a specific period of time or, alternatively, the atmospherecan be drawn through a collector tube using a small portable pumpwhere the volume of air sampled in the time of the experiment isrecorded.

The entrapped substances can be released from the adsorbingmaterial by either solvent extraction for subsequent analysis or bythermal desorption directly into the measuring equipment usingdedicated and automated equipment. In general the thermal desorptionis preferred since this eliminates potential losses at the solvent extrac-tion stage. It has the added bonus of 'refreshing' the sample tubes sothat they are ready for further use.

It is worth noting that the sampler and adsorbing material arebecoming ever more sophisticated as the use of the technique grows.For instance, a nitrosamine-specific atmospheric monitor currently onthe market consists of a basic unit which can be fitted to mostmonitoring pumps, is designed with male/female Luer fittings so that itcan 'nest' with a back-up monitor to check for saturation of the primarymonitor, be sealed with plugs for shipment between sampling andanalysis, and also be easily flushed with solvent, directly from a Luersyringe, for analysis. The adsorbing material consists of an initial 'trap'to hold any amines in the atmosphere followed by a nitrosamine 'trap'which contains a nitrosation inhibitor to further maintain the stability ofthe entrapped nitrosamines.

REFERENCESAlexander, A.E. and Johnson, P. (1949) Colloid Science, Clarendon Press, Oxford.Barrer, R.M. (1941) Diffusion In and Through Solids, Cambridge University Press,

London.Blosczyk, G. (1992) Deutsche Lebensmittel-Rundschau, 88,12, 392.Caspari, W.A. (1914) India-Rubber Laboratory Practice, Macmillan, London.Chin, H.C., Singh, M.M. and Higgins, G.M.C. (1975) Internal Rubber Conf.,

Kuala Lumpur.Cooper, W. and Smith, R.K. (1962) /. Appl. Polym. ScL, 6, 64.Davey, J.E. and Loadman, M.J.R. (1988) /. Nat Rubb. Res. 3 (1), 1.Edwards, A.D. (1981) Unpublished work at MRPRA.Edwards, A.D. (1994) Unpublished work at MRPRA.

Freitag, W. and John, O. (1989) Die Angewandte Makromolekulare Chemie, 175,181-185, (Nr. 2952).

Gere, D.R. and Derrico, E.M. (1994) LC-GC Int., 7, 6, 325 and 7, 7, 370.Henriques, R. (1892) /. Soc. Chem. Ind. 11, 477.Higgins, G.M.C. (1978) NR Technol. 9, 68.King, J.W. (1989) /. Chromatog. Sd. 27, 355.Kress, K.E. (1956) Rubb. World 134, 709.Lindley, P.B. (1966) Rubber. 'Kempe's Engineers Year Book', 1, 1341.McSweeney, G.P. (1970) /. Inst. Rubb. Ind. 4, 245.Neilson, R.C. (1991) J. Liq. Chrom. 14 (3), 503-519.Robertson, M.W. and Rowley, R.M. (1960) British Plastics 33, 26.Stamberger, P. and Knight, B.C.J.G. (1928) /. Chem. Soc. 2791.Wake, W.C. (1969) The Analysis of Rubber and Rubber-like Polymers, Maclaren,

London.Weber, C.O. (1894) /. Soc. Chem. Ind. 13, 987.Weber, C.O. (1902) The Chemistry of India-Rubber, Griffen, London.Williamson, A.G. (1957) RABRM Laboratories, unpublished work.Wyatt, G.H. (1941) Analyst 66, 362.

Analysis of extracts T"

Extracts of compounded rubbers may contain materials added ascuratives, protective agents, processing aids, property modifiers (e.g.resins) and plasticizers, separately or in combination. In addition, therewill be chemicals added during manufacture of synthetic rubbers (e.g.catalyst residues, antioxidants and surfactants) or naturally occurringchemicals retained during processing of natural rubber. Vulcanizedrubber will also contain degradation products arising from the curativesand may also contain materials originating during the actual service lifeof the product such as butterfat in milking machine tubes or detergentsin washing machine gaskets.

Any elemental sulphur present in the compound or vulcanizate willalso be found in the extract but consideration of the methods availablefor quantifying this will be deferred until Chapter 6 when the wholerange of sulphur analyses will be considered.

In view of the diverse range of chemicals which may be present in arubber extract, and the multiplicity of possibilities within each class ofcompound, it is likely that a separation stage will be needed either toprecede any determination or to be an integral part of the procedure. Incertain instances this may have been achieved, at least in part, by theuse of a selective extraction procedure as described in the previouschapter.

There are occasions when chemical spot tests can be applied withoutcarrying out any separation and some of these are described brieflylater in this chapter. Nevertheless, their use is of ever decreasing impor-tance for three fundamental reasons:1. Health and safety requirements for risk assessments and documenta-

tion of all laboratory processes using chemicals has decreased theavailability of many of the 'spot test' reagents.

2. The possibility of interferences from other materials is always presentand this is of ever-growing importance as many of the spot tests still

documented were developed half a century or more ago, beforemany chemicals currently in use were available, so they could not beincluded in the initial validations of the tests.

3. The availability of instrumental analytical techniques has greatlyincreased in the last 20-30 years and techniques such as high perfor-mance liquid chromatography (HPLC), gas chromatography (GC),with a range of different detectors, infra red/Raman spectroscopy(IR/R) ultra violet spectroscopy (UV) and nuclear magneticresonance spectroscopy (NMR) are often accessible to provide veryspecific analytical information.

In this chapter, we initially consider various selective analyses andspot tests for curatives, protective agents and other additives, and alsodiscuss the much more specific identifications possible with moderninstrumental techniques.

IDENTIFICATIONS WITH NO SEPARATIONThe use of more or less specific chemical reagents to treat unchromato-graphed extracts still has some applicability, particularly when knownsystems are being checked, possibly for quality control purposes. Thusit is frequently possible to check whether a para phenylene diamineantioxidant is present, although it is not usually possible to distinguishbetween particular compounds within this general class. A few of themore useful chemical tests are presented below, but more extensivecompilations are available in the literature (Hummel and Scholl, 1981;Haslam et al, 1972; Crompton, 1971).

PLASTICIZERS

Dependent upon the type of polymer, two completely distinct types ofplasticizer may be used. For hydrocarbon polymers (NR, IR, SBR, BR.etc.). various grades of mineral (hydrocarbon) oil are used. Polyvinylchloride, polyurethane rubbers, and to a lesser extent nitrile rubber, onthe other hand, are usually plasticized with oxygenated compounds,such as esters. Although the phthalates are arguably the mostcommonly used plasticizers, flame-retardant phosphate esters and otheraliphatic esters such as adipates and sebacates, useful for goodflexibility at low temperatures, are of growing importance. Haslam et al.(1951) provide some useful spot tests for the presence of phthalates,phenolic and cresylic and phosphate ester plasticizers.

Except where mey are added primarily as processing aids, cis is \¥i£case for low levels of hydrocarbon oils in black-filled hydrocarbonrubbers, plasticizers are usually present in quantities far in excess ofother additives. Under these circumstances the general type of plasti-

cizer may be identified by evaporation of the solvent from the totalsolvent extract, and comparison of the infra red spectrum of the residuewith those of commercial plasticizers. For extensive examples ofpublished spectra, see Hummel and Scholl (1981) and Haslam et al(1972). A much more specific identification may be achieved using gaschromatography with a mass sensitive detector as described later in thechapter.

Tlte use of plasticizer mixtures is a continuing aspect of polyvinylchloride technology and is becoming more common in nitrile orpolychloroprene rubber vulcanizates as manufacturers attempt to 'finetune' their formulations. It may therefore be a useful preliminary step toinvestigate whether the extracted material is a single substance or amixture. Haslam and Willis (1965) recommend a preliminary examina-tion by dissolving the extract in carbon tetrachloride and collectingfractions from a silica gel/Celite column as it is eluted successively withcarbon tetrachloride mixed with 1.5, 2.0, 3.0 and 4.0% isopropyl ether.After removal of the eluting solvent, these authors suggest density,refractive index, UV fluorescence, or boiling point under reducedpressure measurements to see whether a mixture is present. Doolittle(1954) and Kline (1967) both provide a range of physical properties forsome of the commoner plasticizers, the review of the latter being themore exhaustive. In another approach, Collins (1955) has stated thatmixtures of phthalate esters can be separated by simple distillation on asemi-microscale, preferably under reduced pressure. After either ofthese separations, one would, today, use IR, NMR or GC-MS to subse-quently identify the isolated materials.

FACTICE

Although factice is, strictly speaking, not a plasticizer, being added tomixes for a whole range of reasons including the improvement ofdimensional stability and rigidity of unsupported articles in hot aircures, the improvement of extrusion and calendering, and the improve-ment of surface finish and electrical insulation, it does nevertheless actas a softener, improving filler dispersion and decreasing the energyrequirements of mixing. It may therefore be appropriately discussed atthis point. Dark factice is prepared by heating vegetable oil with 10-25% of its weight of sulphur at 130-150 0C. White factice, on the otherhand, is produced by reacting vegetable oil at 30-5O0C with sulphurmonochloride. If, therefore, analysis of an unknown rubber productindicates a higher level of bound sulphur than might reasonably beexpected for its crosslink density, then the presence of factice is onepossible explanation.

The detection and measurement of factice content depends upon the

fact that the origin of the factice is a vegetable oil, which consists ofglycerides (glyceryl esters of fatty acids). After the rubber has beenthoroughly extracted it is treated with alcoholic potassium hydroxide tohydrolyse the glycerides and the liberated fatty acids are then estimatedeither gravimetrically or by esterification followed by gas chromato-graphy. The gravimetric procedure is detailed in the German StandardDIN 53 588. A variant of it, in which the rubber is extracted first withacetone and then with chloroform instead of n-hexane, is given inASTM D 297-1993 Method No. 21.

TESTS FOR SOME SPECIFIC ADDITIVES

In addition to the above tests on unseparated (or unchromatographed)extracts, it is also possible to check the presence of, and determinequantitatively, a variety of other additives which are put into rawrubber or raw latex.

A hydroxylamine salt (sulphate or chloride) is added to some gradesof raw natural rubber and raw natural rubber latex in order to conferconstant viscosity properties on the rubber by preventing the occurrenceof storage hardening reactions. Sodium pentachlorophenate (SPP) usedto be added to Malaysian NR latex to prevent bacterial degradation ofthe latex. Its use in that country has now been proscribed but it maystill be used by latex producers in other countries. Boric acid is used fora similar purpose (British Patent 825280-1960) and methods for thedirect quantitative determination of all three in rubber extracts are avail-able.

Determination of Hydroxylamine (Berg and Becker, 1940)

Five grammes of thinly sheeted raw rubber is extracted by refluxingfor 16 h in 45cm3 of 0.8 M sulphuric acid. The acid extract is quantita-tively transferred to a 50cm3 volumetric flask, and made up to themark with 0.8 M sulphuric acid. A mixture of 1 cm3 extract, 1 cm3 offresh 8-hydroxyquinoline solution (1% w/v in ethanol) and 3cm3 ofIM sodium carbonate solution is placed in a test tube of 30cm3

capacity, and oxygen or air bubbled through for 5 minutes. The tubeis stoppered and warmed to 40 0C for 20 min in a water-bath. Thesolution is diluted to 10 cm3 with water in a volumetric flask, and thegreen colour measured in a colorimeter or spectrophotometer set to700 nm.

Zinc ions interfere with the above method which cannot thereforebe used without modification for compounds or vulcanizates. Interfer-ence by zinc ions can be prevented by addition of 3cm3 of a 10%solution of DCTA (l^-diaminocyclohexane-N^N^.'N'-tetraacetic acid)

in IM sodium carbonate solution, as complexing agent. This modifica-tion of the above method is due to A.M. Petric of the MRPRA Labora-tories.

Determination of boric acid

Boric acid, in aqueous extracts of rubber or rubber products, issimply determined by neutralization of the extract to phenolphthalein,addition of mannitol, and titration with 0.02 M sodium hydroxidesolution.

Total boron in rubbers can be determined using a modification of themethod due to Hayes and Metcalf (1962).

A finely chopped rubber sample (0.1 g) is covered with sodium carbo-nate (0.1-0.2 g) in a closed platinum crucible and ignited until fusionis complete (approximately 3 minutes). The cooled residue isdissolved in lcm 50% acetic acid in water, transferred to a 10cm3

graduated flask, and made up to the mark with glacial acetic acid.Aliquots (lcm3) of this solution are placed in polythene bottles and3cm3 of curcumin-acetic acid reagent (0.12% w/w in glacial aceticacid) and 3cm3 of 1:1 acetic acid/sulphuric acid added. After 20minutes at 2O0C, 50cm3 absolute ethanol is added, and the volumeadjusted to 100cm3 in a 100cm3 volumetric flask, using absoluteethanol. The optical density is measured at 555 nm.

Determination of sodium pentachlorophenate

Sodium pentachlorophenate can be determined rapidly and accuratelyin Hevea latex concentrate by reaction with 4-aminophenazone to give ablue complex which is measured at 600 nm. The method is described indetail by Chin et al. (1975). The presence of sodium salicylate, added tofreeze-thaw stabilized natural rubber latex concentrate, does not inter-fere, but ammonia in high ammonia grades of latex must first bereduced in concentration to 0.2% (w/w) by the addition of formalin orboric acid solution.

ANALYSIS FOR ANTIOXIDANTS

Although antioxidants have been determined directly on the rubberitself, for example by Luongo (1965) and Miller and Willis (1959), themethod is applicable only to uncured mixes not containing carbonblack, and then only to materials of known composition, and will notbe considered further. Many techniques are available for the detectionof antioxidants after separation from the polymer by extraction (e.g.

Wheeler, 1968). Some are mentioned below in the section on ultra-violet absorption spectroscopy. Others, highly specific in nature, willbe discussed in the sections on gas, thin layer and liquid chromato-graphy. For information on the more classical procedures, the readeris referred to the more specialist literature of Hilton, 1958; Wheeler,1968; Crompton, 1971; Haslam et al, 1972; and Hummel and Scholl,1981.

ANALYSIS FOR ACCELERATORS

Accelerators are active vulcanization ingredients, many of which arechemically altered during the vulcanization process. It is thereforeimportant to recognize that whilst most accelerators can be recoveredunchanged from raw mixes, only the 'vulcanization residues7 can berecovered from cured products. Sulphenamide accelerators give amineand mercaptobenzothiazole (MBT) or its zinc salt (ZMBT), mercaptoben-zothiazyl disulphide (MBTS) gives MBT or ZMBT, and thiurams givethe corresponding dithiocarbamates. Guanidines give, in part, thearomatic amine, but can also be detected unchanged.

In addition, one should note that chemical changes may also takeplace during extraction of raw mixes since hot extraction can bringabout the same reactions that these chemicals undergo during cure.Cold extraction, whilst possibly not being quantitative, is to be preferredfor the identification of accelerators in a compounded but unvulcanizedmix.

Extraction with acetone is known to cause decomposition of thiuramdisulphides to give dithiocarbamates (Wake, 1969). This can be avoidedby using solvents such as 2-propanol, methanol or dichloromethane andit is therefore essential to use one of these if a distinction between theaccelerators is required. An alternative procedure is to use a cold extrac-tion technique although, if minute traces of thiuram remaining aftercure are being sought, acetone should still be avoided.

Few chemical tests are used today, emphasis instead being placed onthe various chromatographic techniques which are discussed later inthis chapter. The reader interested in these older techniques is referredto the work of Hofmann and Ostromov (1968, 1969), Brock and Louth(1955) and Crompton (1971).

NATURAL RUBBER EXTRACTS

Rubber obtained from Hevea brasiliensis exudes from the tapping cutas latex containing about 40% total solids. The serum forming thecontinuous phase is a complex solution and the interface betweenrubber and serum is stabilized by a mixture of surface active materials

in which proteins predominate. For use as latex, this 40% latex isconcentrated by one of several possible processes and stabilizers areadded; for use in the form of sheet or crumb rubber the latex is coagu-lated and, after suitable treatment, dried. Rubber goods prepared fromlatex are thus generally associated with a larger proportion of the solidsinitially in the serum or at the interface than are sheet or bale rubbers.However, normal commercial rubber still contains detectable quantitiesof the various impurities which betray its natural origin. The mostconvenient of these from an analytical point of view are protein and P-sitosterol, although it should be mentioned that one form of purifiednatural rubber is enzymatically deproteinized and only contains some10% of the normal protein content whilst skim rubber contains verysubstantially more. It is not intended here to distinguish between thevarious grades of natural rubber nor to consider the various additionsto latex, but to concentrate on that regular question: 'Is the polyisoprenenatural or synthetic?' and to extend this to cover both deproteinizedand skim rubbers. It is mentioned that these methods are non-instru-mental, and that other methods will be covered under the variousinstrumental techniques described in subsequent chapters.

P-sitosterolFrom an analytical perspective one major difference between proteinand p-sitosterol is that the former is not extracted by a solvent such asacetone or methanol, whilst the latter is. Its separation by thin layerchromatography and subsequent visualization by a specific sprayreagent will be considered later in this chapter (Davies, 1967) and thepractical aspects will not be repeated, but two points merit comment.The first is the question of reliability of observation; the experiences ofMcSweeney (1970) over many years have been that the analyst hasnever failed to find p-sitosterol in samples, raw or vulcanized, whichcontain more than about 10% by weight of natural rubber, providedthat the sample has not been subjected to extraction with an organicsolvent before being received for analysis - if any other component isfound to be present by TLC examination, p-sitosterol will be observed iforiginally present. These analyses include samples cured at up to 200 0Cand an eighty-year-old sewer pipe seal which must have suffered every-thing other than organic solvent extraction!

A few samples of reclaim have been examined and here too, P-sitos-terol has been found although, if it is compounded with new naturalrubber, this may be of limited value. The second point is the quantifica-tion of the P-sitosterol content, to enable the natural rubber level to bedetermined. There is intrinsically no difficulty in quantifying theanalysis, but unpublished studies at TARRC, using a silylation/gas

chromatographic technique, showed that there was a substantial varia-tion in (3-sitosterol contents for different samples of rubber and thus theanalysis is not particularly useful. It is probable that a visual estimationof the natural rubber content, based on the size of the TLC spot, andexperience, gives an equally valid figure.

Protein

A method was developed by Loadman and McSweeney (1970) in whichthe protein in acetone-extracted samples containing natural rubber washydrolysed and the resulting amino acids separated by paper chromato-graphy.

A sample of the extracted sample (10 g), lightly milled or cut intoslivers, is placed in a thick-walled glass tube (10 cm x 1 cm i.d.), 6 Mhydrochloric acid (3cm3) added, the tube sealed and heated to 10O0Cfor 16 hours after which it is cooled, opened and the solutionexamined directly by paper chromatography using n-butanol-acetone-water (60:20:20) as eluent, and ninhydrin as the visualizingspray. A characteristic series of spots clearly indicates the amino acidsfrom the natural rubber protein.

Interestingly a similar hydrolysis process, followed by derivatizationand quantitative analysis by HPLC has been developed as a means ofquantifying water extractable protein in sensitive medical devices suchas examination and surgeons' gloves (Heese, Koch and Lacher, 1996).There has been a growing interest in the water extractable proteincontent of products such as these over the last decade due to agrowth in observed protein allergenic reactions. These, in turn, wereprobably due to the increased use of exam gloves in the medical andrelated professions as a result of the AIDS crisis, coupled with themarketing of inferior quality products manufactured by inexperiencedcompanies which sprang up to meet the perceived demand. Thecurrent method of choice for determining the water extractable proteincontent of these products is described as the 'Modified LowryMethod' although this description is generic since methods beingdeveloped by ISO and CEN differ in detail from those put forward bythe Rubber Research Institute of Malaysia (RRIM) and ASTM. At thistime in the West, testing is normally carried out to ASTM D5712 orthe draft prEN 455-3.

Deproteinized natural rubber

Both low-protein and enzymatically deproteinized natural rubber stillcontain (3-sitosterol and thus, in the unextracted state, cause no difficulty

in analysis. If the sample has been previously extracted with an organicsolvent, then the 'cleanliness' of the sample, together with a nitrogencontent of typically below 0.05%, precludes a non-instrumentaltechnique but several instrumental methods remain valid.

Skim rubberSkim rubber has a much higher protein (hence nitrogen) content thanthe normally available grades of rubber. The analysis of many hundredsof samples of various grades of Standard Malaysian Rubber has shownnitrogen contents to be independent of grade, in the range 0.2-0.5%,whilst the International Standard ISO 2000-1989 specifies a maximum of0.6%. There is a much greater variability in the nitrogen content of skimrubbers but the lower limit appears to be about 1.5%, with an upperlimit of 3.5%. There should not therefore be a problem in identifyingskim rubber in the raw state, and as it is generally used in the form ofsheets the problem of dilution with 'normal' rubber does not oftenoccur.

The Rubber Research Institute of Malaysia (RRIM) (1954) published amethod to distinguish between the more usual grades of natural rubberand skim rubber or, indeed, to detect adulteration of the former withthe latter. The method relies on the difference in specific gravitiesbetween the two, and the fact that normal grades of raw natural rubberhave specific gravities below 0.93 (Davey, 1973). A solution of ethanol(AR) 40.8% w/w in distilled water at 25 0C has exactly this density andis used for the flotation test. In this solution, skim, or natural rubberadulterated with more than about 5% skim, will sink whilst normalgrades will float. The sample is pre-wetted by dipping it in a solution ofdetergent:water:ethanol (5: 45: 55 v/v) and rinsed with some of the testsolution prior to immersion in a fresh portion of the test solution whichis used for the actual test.

Particular care should be taken to ensure that the samples contain noentrapped air bubbles and it is advised that at least half a dozen smallsamples of the order of 1-2 mm3 be examined in order to minimize thispossibility.

ANALYSIS FOR OTHER EXTRACTANTS

Detailed descriptions of the chemical analysis for other extractants suchas processing aids, blowing agents, peptisers or their residues arebeyond the scope of this book and the interested reader is referred tothe review by Hummel and Scholl (1981). Today, these are generallybest carried out using one of the instrumental techniques describedbelow. With the vast variety of these materials on the market, it is up to

the ingenuity of the analyst to find the one most appropriate to theprevailing circumstances.

ULTRAVIOLET SPECTROSCOPY (UV)

Although ultraviolet spectroscopy is generally associated today withversatile detectors for liquid chromatography, it provides a highly sensi-tive means of measuring the concentrations of many substancesincluding most antioxidants, many vulcanization accelerators andsulphur in extracts of rubber vulcanizates without prior separation(Fikhlengol'ts et al., 1966). Ultraviolet absorption bands are, however,usually very broad, and often relatively featureless so the technique isof limited diagnostic value, particularly in situations where two or moresimilar materials may be present simultaneously.

There are nevertheless occasions when useful information may beobtained by combining ultraviolet spectroscopy with treatment of theextract with a selective chemical reagent. If, for example, the ultravioletspectrum of an extract (in a solvent which is adequately transparent) isrecorded both before and after treatment with a selective chemicalreagent, the disappearance of one band or the appearance of a band ata new position is relatively easy to detect, even in the presence ofseverely overlapping bands. Indeed by measuring the ultravioletspectrum of the treated extract relative to that of the untreated extract,by placing one solution in the sample beam and the other in the refer-ence beam (difference spectroscopy) ultraviolet absorption bands whichare unaffected by the chemical treatment can be cancelled out leavingonly the differences to be observed.

Some of the more useful treatments for commonly observedcompounding ingredients are shown in Table 4.1, the information beingcompiled from papers by Lloyd (1962) and Wexler (1963).

The relatively low specificity of the ultraviolet method can be put touse as a method for obtaining a crude estimate of the total antioxidantpresent. Indeed Blois (1958) demonstrated that antioxidants of a widevariety of types discharge the intense violet colour from the stable freeradical diphenylpicrylhydrazyl (DPPH), replacing it by a yellow colour.Measurement of the optical density at 517nm, at which wavelengthonly the purple free radical absorbs, gives an estimate of the totalantioxidant present. Glavind (1963) used this method to determine totalantioxidant in biological systems (mammalian blood and liver).

Although the method appears to be relevant to polymers, it does notyet seem to have been used for this purpose. The method would havethe advantage that naturally occurring antioxidants in natural rubber(such as the sitosterols) would be determined, although thiol-containingproteins also present might interfere.

Table 4.1 Significant UV spectral data for some compounding ingredients

Compounding UV max Chemical Reaction UV maxingredient reagent product

Mercaptobenzthiazyl 282 SnCI2/HCI MBT/amine 327sulphenamideSulphur 261 Sodium H2S

borohydridePhenolic a/o's 280-290 Potassium Phenolate 300-310

hydroxide anionDithiocarbamates 273 Copper Copper 435

sulphate complexMercaptobenzthiazole 327 Copper Copper insol.(MBT) sulphate complexTetramethylthiuram 282 Copper Copper 434disulphide (TMTD) sulphate complex

INFRARED SPECTROSCOPY (IR)

Infrared examination of complete extracts tends to be less useful thanultraviolet spectroscopy because of the extreme complexity of thespectra of organic compounds. Nevertheless, it can be useful, particu-larly if considerable quantities of plasticizer or extender oils are present.Once again, Hummel and Scholl (1981) have provided a large compila-tion of reference spectra.

Cooper et. al (1971) described how IR spectroscopy could be used toidentify the type of hydrocarbon oil used as an extender by reference tothe relative intensities of bands at 13.9, 12.3 and 6.26 jim whilst, a fewyears earlier, White (1967) had already shown how the intensities of thesame bands could be compared with those of n-heptane, phenanthreneand toluene to obtain a semi-quantitative assessment of the paraffin/naphthenic/aromatic contents (Table 4.2).

ATOMIC ABSORPTION SPECTROSCOPY (AAS) / INDUCTIVELY COUPLEDPLASMA-ATOMIC EMISSION SPECTROSCOPY (ICP-AES)

AAS and ICP-AES are the methods of choice for the identification andquantitation of many elements, be they at the trace level or present as acomponent of a bulk filler in rubber product. The techniques will bediscussed in detail in Chapter 10 but, as already mentioned in Chapter3, there is sometimes a requirement for the determination of specificelements in the extract of a raw elastomer or a product manufacturedfrom that elastomer. The requirements are often health-related, such as

EN71, and can involve a variety of different solvents in selective extrac-tions, but, once given the extract, these techniques provide the requiredanalytical data extremely quickly and with both a high specificity andsensitivity.

IDENTIFICATION WITH SEPARATION

The complexity of modern mixes, coupled with the large number ofaccelerators, antioxidants and other additives which are commerciallyavailable as well as the ever-increasing need to carry out analyses inever shorter times, has made it inevitable that separative techniqueshave become of prime importance in the detailed analysis of rubberextracts.

Chromatography is the general term covering a wide range of separa-tive techniques all of which have the same fundamental principles ofoperation, and all of which can be used, under differing circumstances,for the separation (and often identification) of the chemical constituentsof rubber extracts.

All chromatographic techniques involve a stationary phase and amobile phase which passes through, around, or over it, dependingupon its nature. If a mixture of components is placed at one end of thesystem, each individual component will be carried with the mobilephase, at a rate dependent upon its relative affinity for the two phases.Different chemicals will thereby become separated from each other.

The different chromatographic techniques which will be consideredhere are column and paper chromatography, thin layer chromatography(TLC), high performance liquid chromatography (HPLC) with thespecific sub-category of ion chromatography (IC)), and gas chromato-graphy (GC). Gel permeation chromatography (GPC) is considered indetail in Chapter 9 and is based on the sieving action of a porousstationary phase to separate materials which, although chemicallysimilar, have different molecular shapes or molar masses. It will not beconsidered further here.

COLUMN/PAPER CHROMATOGRAPHY

The earliest comprehensive separative scheme in the field of rubberchemical analysis was that of Bellamy et al. (1947). This depended onchromatography on an alumina column of the acetone extract dissolvedin benzene, and elution of components from the column. The variouseluted fractions were then re-chromatographed after the addition ofcobalt oleate. Later workers favoured treating the acetone extract toeffect a preliminary separation by chemical means into acidic andalkaline components prior to separation of the components by chroma-

tography either on a column or on paper. An example of the latter isthat of Zijp (1956) whose procedure was given in some detail in thesecond edition of this book (Wake, 1969) and will not be repeated here.Unfortunately the rate of movement of the solvent through the papertends to be very slow by the ascending method, and overnight runsused to be the norm. Downward development is faster, but the resol-ving power of the method becomes inferior. Other means of speedingup the process, such as circular paper chromatography, are available,but for the chromatography of organic materials, thin layer chromato-graphy has now displaced column and paper chromatography almostentirely.

THIN LAYER CHROMATOGRAPHY (TLC)

The ease of use of this technique coupled with its speed of operationand low cost make it particularly suitable for the rubber laboratory,both for identification of unknown formulations and for quality controlpurposes. The technique is closely related to paper chromatography,having a stationary phase, which may be cellulose, silica or alumina forexample, applied as a thin layer on a rigid support which is commonlyglass or plastic. It does not, however, suffer from the disadvantages ofpaper chromatography since it is quick, with a run taking perhaps halfan hour, and it has a much higher resolving power than has paperchromatography. Indeed high performance grades of surface coatings,with much smaller particle sizes and a narrower distribution of particlesizes (TLC particle size being typically in the 5-20 (im range, whilst thatof HPTLC is in the 4-8|im range), give plates which will separatematerials adequately within the space of 5cm or so, still furtherspeeding up the process.

Pre-prepared plates can be purchased readily although, if a largenumber of thin layer chromatographic analyses are to be carried out,this will prove more costly than the alternative of coating one's own.For this purpose commercial spreaders are available, although platescan be coated using spacers and a glass rod.

The operation of the chromatographic technique has been describedby Stahl (1965), Zweig and Sherma (1972), and many other authors; itsapplication to rubber antidegradant analysis is given in some detail inASTM D 3156 and in ISO 4645.2-1984. In essence, the procedure is toplace a small spot of a dichloromethane solution of the rubber extracton to the coating on the plate, about 2 cm from the bottom, and then tostand the plate in a glass tank, with a lid, containing the desired solventsystem in the bottom to a depth of about 1 cm. The solvent travels upthe layer by capillary attraction. When the solvent front has reached therequired height, the plate is removed from the tank and air dried prior

to visualization of the separated spots with a reactive chemical spray, orrepeated development of the plate either in the same or differentsolvent. The mobility is measured in terms of RF which is given by theequation:

distance travelled by the compoundRF =

distance travelled by the solvent front

Thin layer chromatography is usually regarded as being semi-quantita-tive; the logarithm of the spot area being proportional to the quantity ofmaterial in the spot and, with practice, the visual comparison of a spotsize with those of a series of spots containing standard quantities of theauthentic material will suffice to provide a realistic estimate of quantityof material present. More accurate quantitative results can, in theory, beobtained by the use of a scanning densitometer but, in practice, extractsfrom rubbers tend to be so complex that it is rarely possible to measureone component without serious interference from some others.

During the development of the chromatogram it is necessary topreserve an environment for the TLC plate which is fully saturated withthe solvent by using a glass tank lined with absorbent paper which dipsinto the solvent at the bottom of the tank. If this is not carried out, thenunder certain conditions of temperature and humidity, plates will takean abnormally long time to develop, and apparent compound mobilitieswill be much higher than expected.

Some analysts favour the use of a short column chromatographicpretreatment of the extract to remove any oil present prior to TLCexamination. However, this procedure is not to be recommended aspolar materials might be retained on the column while elementalsulphur may well be eluted rapidly with the oil. An alternative methodfor separating oils from the bulk of the rubber chemicals and their cureresidues is to develop the plate initially in light petroleum, prior toelution with normal chromatographic solvents. The light petroleumcarries the oil near to the solvent front and out of the way of othercomponents. The plate is then developed in the required solvent. Thisprocedure prevents the oil from interfering with the separation of othercomponents in the extract, whilst ensuring that all materials are alwayson the plate and have the potential to be developed. The initial run inpetroleum has the additional advantage of separating free sulphur at anRF of about 0.6. If the TLC plate has a fluorescent layer, then inspectionof the dried plate under ultraviolet light renders any free sulphurpresent visible as a dark spot on a fluorescent background, as describedby McSweeney (1971) and Davies and Thuraisingham (1968). Thepresence of free sulphur may be confirmed by spraying with aqueoussilver nitrate, which gives a brown-coloured spot. Alternatively, after

examination under ultraviolet light, the plate may be developed in thenormal chromatographic solvent. As will be shown in Chapter 6, thepresence of free sulphur in a cured product may well indicate that theproduct is undercured.

Since Yuasa and Kamiya (1964) published their paper on the use ofTLC for the identification of additives in acetone extracts of rubber, averitable flood of papers on the subject has appeared, and paperchromatographic techniques have been almost entirely displaced.Comprehensive reviews of the TLC of rubber chemicals have beengiven by Kreiner and Warner (1969), Crompton (1971), Wheeler (1968),Kreiner (1971), Hummel and Scholl (1981) and Hofmann and Ostromov(1968, 1969). Hofmann and Ostromov (1968) have also used thin layerchromatography extensively in their compilation of methods for theinvestigation of rubbers in relation to German health regulationsalthough the tendency now is to replace these with GC or HPLC.

A useful review for the rubber-technologist is that of McSweeney(1970) who gives the practical details needed regarding technique andspray reagents useful for the analysis of rubber extracts. In a laterpaper, Higgins and McSweeney (1974) gave a method whereby amineresidues, produced by decomposition of sulphenamide acceleratorsduring cure, can be identified and hence the nature of the originalsulphenamide deduced.

The point has already been made that rubber extracts tend to containrather a large number of chemicals and, in the author's laboratory, ithas been found profitable to standardize on a fluorescent grade of silicagel G as stationary phase and on a single solvent system following thelight petroleum pre-run (light petroleum/diethyl ether, 60:40 v/v), sothat the positions of all the various spots gradually become familiar tothe analyst. When solvent mixtures are used they should be replacedfrequently in order to prevent significant changes in the mixture due todifferential evaporation of the solvents leading to variable retentiontimes. In cases where particular problems arise, such as two compo-nents being inadequately resolved, or polar materials being presentwhich do not move off the start line (e.g. DPG), then an alternativesolvent system can be chosen and the separation repeated. Thus theguanidines separate if, after running the plate in the above solvent, theplate is re-run in a mixture of acetone and ammonia (98:2 v/v). Theycan be visualized by spraying with sodium hypochlorite solution.

The most generally useful spray reagents are 2,6-dibromo-para-benzo-quinone-4-chlorimine and the corresponding dichloro-compound. Bothare popularly known as Gibbs' reagent and are widely reactive, givingcolours for different chemical types which range from a greyish whiteto yellow for certain phenolic antioxidants, through brown for dithiocar-bamates, orange for MBT, to greens and purples for amine antioxidants

and a wide range of other nitrogen-containing materials. Dithizonespray reagent is particularly useful for visualizing zinc compounds,with which it gives a rich pink colour. An aqueous solution of coppersulphate is particularly sensitive towards dithiocarbamates, whilstanisaldehyde and sulphuric acid (5% each) in ethanol is a useful generalspray which nevertheless shows some specificity.

Davies (1967) described how natural rubber may be distinguishedfrom synthetic polyisoprene by the detection of p-sitosterol, a naturallyoccurring and very persistent compound which is able to survive mostnormal rubber treatments including vulcanization. McSweeney (1970)improved the procedure by using the more selective spray reagent,cupric acetate (3% in 8% phosphoric acid). On heating with sterols, thisgives a red colour which, on further heating, turns first to blue and thento dark grey. The background does not darken, spot colours are stable,and very few other compounds react with the reagent. The copperacetate spray can be applied even after the plates have been sprayedwith Gibbs' reagent.

Higgins and McSweeney (1974) showed how the amines producedfrom sulphenamides during vulcanization can be reacted with analkaline solution of 7-nitrobenzo-2,l,3-oxadiazol (NBD) in methyl ethylketone solution. The NBD derivatives thus formed are extracted bytoluene, transferred to chloroform and chromatographed on a fluores-cent grade of silica gel in toluene/ethyl acetate (4:1, v/v). Comparisonof the TLC spot positions with the positions of spots due to the NBDderivatives of authentic amines enables the amine portion of thesulphenamide accelerator to be identified.

Thin layer chromatographic procedures have been developed for theidentification of a wide range of rubber additives, and the interestedreader is referred to the specialist literature, as detailed in the reviewslisted above. Only two others will be mentioned briefly here, the firstbeing the determination of castor oil in crumb grades of natural rubber.This method, originally developed by Davies and Tunnicliffe (1967),was improved by McSweeney (1972) and the method has been adoptedby the International Organization for Standardization as ISO 6225/1-1984 (whilst ISO 6225/2-1990 offers a GC method). The main improve-ment introduced in ISO 6225/1 was the use of the anisaldehyde sprayreagent. This reagent is much more selective than the phosphomolybdicacid spray used in the original method and avoids interference byoverlapping compounds which are usually present. The anisaldehydespray reagent gives mauve spots which turn green on heating. If, afterheating, the plate is resprayed with anisaldehyde there is an immediateincrease in the green coloration

The second is the use of thin layer chromatography for the determina-tion of the type of hydrocarbon extending oil. Extending oils used in

Table 4.2 Composition of hydrocarbon oils (Courtesy, European Rubber Journal)

Composition %Paraffinic oils Naphthenic oils Aromatic oils

Paraffinic carbon 50-68 40-60 34-44Naphthenic carbon 26-40 20-60 10-34Aromatic carbon 2-7 21-29 30-50Specific gravity 0.86-0.90 0.90-0.92 0.96-1.03

the rubber industry contain oils of paraffinic, naphthenic and aromatictypes. Those oils, known loosely as paraffinic, naphthenic and aromaticoils, do in fact consist of mixtures of different proportions of the threemolecular types as shown in Table 4.2.

Thin layer chromatographic analysis of oil type has been investigatedby Killer and Amos (1966), Gilchrist et al (1972), Peurifoy et al (1970)and Hellmann (1974). An alternative column chromatographic proce-dure (ASTM D2007) was listed by the American Society for Testing andMaterials in 1990 but is absent in 1996.

McSweeney (1970), in the author's laboratory, has used two proce-dures for the analysis of oil type. The first, and simpler of the two,relies upon straightforward comparison of the thin layer chromatogramof a rubber extract with those of authentic samples of the three types ofoil. A fluorescent grade of silica gel is used as the stationary phase,whilst the mobile phase is light petroleum of boiling range 40-6O0C.Two methods of visualization of the partly separated oils are possible.The plate is allowed to dry, and then examined in ultraviolet light. Inlight of wavelength 254 run, naphthenic and aromatic oils appear asultraviolet absorbent spots (i.e. dark spots on a fluorescent background),with the naphthenic oils having higher mobility. In light of wavelength366 nm, aromatic components show a blue fluorescence. In additionMcSweeney (1972) found that the plate may be sprayed with a freshlyprepared solution of anisaldehyde and concentrated sulphuric acid (5%each) in ethanol. On gently heating with a hot air blower, naphthenicoils give a predominantly pink colour whilst aromatic oils give a bluecolour. Aliphatic oils are not observed. Additional resolution of thevarious types of oil can be obtained by developing the chromatogramtwice.

A refinement of the above method, also due to McSweeney (1972),uses two-dimensional development of the thin layer chromatogram. Theunknown sample is spotted on to a square TLC plate about 4cmdiagonally from one corner, and the chromatogram run in one directionin light petroleum as above. The plate is then allowed to dry, turned

through 90 °, and then redeveloped in a mixture of light petroleum/diethyl ether (60/40). The resultant pattern of colours obtained afterspraying as above is used to determine the type of oil.

In both the above methods, paraffinic components (which do notreact to the sprays) can be seen as fleeting spots of high mobility as thesolvents are drying, and as a colourless area defined by the crescentshaped naphthenic oil spot after spraying. It is also frequently possibleto detect the paraffinic oil as a non-wetted area if the plate is sprayedwith a fine mist of water.

The anisaldehyde spray used for oils is also useful for detection ofoxidative degradation in NR. If an extract of a sample of oxidizedrubber is run under the 'standard' conditions mentioned above andthen sprayed with anisaldehyde a pink streak will slowly develop fromthe origin to an RF of around 0.6. This streak is not produced by theoxidised rubber itself, but by the oxidation products of non-rubbersfound in NR.

As mentioned earlier, thin layer chromatograms can be extremelycomplex, particularly when one remembers that many commercialantioxidants (and sometimes also accelerators) consist of a number ofdifferent components. In addition there are numerous occasions whentwo materials have identical or nearly identical mobilities and eachmasks the characteristic colour reaction of the other. On some occasionsthe problem can be overcome by the use of specific sprays which reactto only one of the overlapping components. On other occasions moresophisticated 'tricks' must be used. Some examples of these follow(McSweeney, 1981).

Agerite White

The antioxidant Agerite White has the same mobility and similar sprayreaction to Gibbs' reagent as has mercaptobenzothiazole (MBT). Thematerials can readily be separated by making use of the acidic nature ofthe MBT. After the initial development in light petroleum, the plate isexposed to ammonia vapour in a covered tank. The plate is thenimmediately placed in the second development tank, containing thelight petroleum/diethyl ether solvent. The MBT, being present now asthe highly polar ammonium salt, remains on or near the start line of theTLC plate, whereas the Agerite White travels with its normal mobility.

N-2-propyl-N'-phenyl-para-phenylenediamine

The converse procedure may be carried out in the case of the overlap-ping (N-2-propyl-N'-phenyl-/w#-phenylenediamine) and MBT. Para-phenylene diamines, being strongly basic, react readily to form hydro-

chloride salts when the TLC plate is held in hydrogen chloride vapour.The amine will then have very low mobility, and the MBT will travel atits normal speed.

Cyclohexyl Benzothiazyl Sulphenamide (CBS)When an unvulcanized compound has been cold extracted, CBS willfrequently be present in the extract. This has a high mobility, but isvery difficult to detect on spraying with Gibbs' reagent. Brief exposureof the TLC plate to hydrogen sulphide gas reduces the compound toMBT which then gives the characteristic orange colour of MBT. Othersulphenamide accelerators will act in a similar way, but with differingretention factors.

ThioureaThiourea sometimes occurs in mixes and vulcanizates, but it is difficultto detect since it has only a weak reaction to spray reagents. If thedeveloped plate is first held in ammonia vapour and then immediatelysprayed with Gibbs' reagent, thiourea gives a very strong purple spot.

Wingstay L / Lowinox CPL / 22CP46 and TocopherolThese antioxidants, which are nominally chemically equivalent, are diffi-cult to separate from Tocopherol, a sterol found in NR. A usefultechnique to achieve separation is the use of trough TLC. The plate isfirst run in light petrol as usual and dried. A special developing tank (atrough tank) is used for the second run in which there is a barrieraround 2cm high along the bottom of the tank separating the frontfrom the back. Light petrol is added to the rear trough, absorbent paperis then used to line the back of the tank, dipping into the petrol. Afterallowing sufficient time for equilibration (around half an hour) the plateis put into the front trough and left for 15 minutes, after which diethylether is carefully added to the front trough by means of a long funnel.This technique offers the advantage of increased resolution as thinbands are formed rather than spots, but is much more time consumingthan using a solvent mixture as the solvents are discarded after one use.

HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)

In the early 1970s the introduction of micro particulate stationaryphases, coupled with high pressure pulse-free pumps under micropro-cessor control, led to the transformation of classical liquid columnchromatography to the modern HPLC. Reviews of its application to the

rubber industry have been given by Sidwell (1980) and Sullivan et al.(1976) whilst Gross and Strauss used it in the study of plasticizers(1977), stabilizers (1976), and antidegradants and accelerators (1979).

In the last decade there has been a further substantial improvementin the separative ability of HPLC, in the main due to the maturation ofinstrument technology. There is now widely available a sophisticatedselection of pumps, detectors and computer software, as well asadvanced column packings, which can be purchased as one completepackage or separately as modular units.

The increased environmental awareness of chemicals in rubber andpolymer products, coupled with the ever-growing health and safetyregulations, has led to an increase in demand for quality assessmentand control in a wide range of sensitive products such as gloves,condoms, soothers, medical components, mattresses etc. HPLC isinvaluable in the rubber and polymer laboratory for identification andquantification of the major rubber chemicals. The technique is relativelyquick, provided an appropriate solution of the analyte can be prepared,and detection limits are routinely at the ppm level.

HPLC fundamentally can be divided into normal and reverse phaseoperation. Normal phase HPLC utilizes a polar stationary phasecoupled with a non-polar mobile phase so that separation of the variouscomponents of the extract then depends on the adsorption of thecomponents on to the stationary phase (polar-polar interaction) and therelative ease with which they can be returned to solution by the eluent.Reverse phase HPLC (RPLC) uses a non-polar stationary phase and apolar mobile phase, thus separation is dependent on the polarity of themobile phase (nonspecific hydrophobic interactions).

As many organic substances show low solubility in water but highsolubility in many water-miscible organic solvents, the popularity ofRPLC exploded in the late 1970s, and today represents the most widelyused mode of HPLC.

Since separation can be manipulated by altering the polarity of themobile phase, it provides a very flexible procedure which can beadjusted to meet specific requirements of the analysis and this has ledto RPLC becoming the most popular type of liquid chromatography inthe field of rubber chemical analysis. Programmable solvent delivery,varying the relative concentrations of a number of solvents with time,gives excellent results over short run times while re-equilibration timesbetween runs do not cause significant delays. The use of solvent gradi-ents involves the initial use of a weakly eluting (polar) solvent changingprogressively to a strongly eluting (non-polar) solvent over the courseof the separation. This produces good resolution of the poorly retainedsolutes at the beginning of the separation, while also eluting the morestrongly retained solutes within a reasonable time. The type of solvent

system can be used to separate a wide range of compounds whilstclosely eluting peaks under one set of conditions can often be resolvedmore completely by decreasing the rate of change of eluent ratio overthe specific time period. It is possible to have linear or exponentialprofiles for solvent gradients on some pump models to provide evengreater flexibility.

Columns for HPLC are available with a range of packing materialsspecific to the type of analysis required. The choice of column willdepend upon the compounds to be separated and the solvent systemused; the most commonly used columns in RPLC of rubber chemicalsbeing octadecyl-silane, (ODS-2 and ODS-B). A recent review of columndevelopment is given by Majors (1994). Guard columns are useful asthey can decrease column costs by prolonging the lifetime of thecolumn by protecting it against mobile phase and sample contaminants.Isocratic HPLC pumps are still available but gradient models are moreversatile and it would almost certainly be a false economy not to optfor the increased flexibility of the latter if new equipment were beingpurchased.

There is a wide choice of high purity solvents available which areessential for this application. These are designated 'HPLC Grade' bymanufacturers but each batch should be checked as they can varyconsiderably whilst remaining 'in specification'. When used in combina-tion to give solvent gradients, methanol, acetonitrile, THF and watergive sufficient retention and selectivity for most reverse phase separa-tions.

Computer controlled autosamplers have added yet another dimensionto the flexibility of HPLC (as, indeed, they have done to many instru-mental techniques). These allow many samples to be analysed with theminimum of attention, enabling standards to be run at frequent inter-vals and multiple injections from the same vial whilst some systemsallow a variety of gradient programs to be built into one set of runs.They also offer a significant benefit in quantitative analyses since repro-ducibility of the sample injection volume is improved relative tomanual injections.

The range of available detectors continues to grow. UV/visible lightabsorbance detectors can be fixed or variable wavelength, or rapidscanning over a defined spectral range. Photodiode-array detectors alsoprovide instant spectra at time slices throughout the chromatogram butthese tend to be less sensitive than the modern scanning spectrophot-ometer. Refractive index detectors are often used but they are not verysensitive and are totally non-selective whilst programmed solventmixing may lead to baseline disruption.

Developments in LC detector technology has been discussed byFielden (1992) whilst many LC method optimization schemes have been

introduced and are available as commercial packages. A review of theseis given by Dolan and Snyder (1990).

ION CHROMATOGRAPHY (IC)

Ion chromatography is a particular type of liquid chromatographywhich will be discussed in detail in Chapter 6, where it is applied toaspects of elemental analysis. Nevertheless it merits mention herebecause of its ability to separate, identify and quantify both cations andions present in aqueous solutions such as latex sera or aqueous extractsof rubber products. Two particular areas of significance are those ofionic surfactants and organic acids (Crafts et al, 1990).

GAS CHROMATOGRAPHY (GC)

Gas chromatography is a widely used technique in the polymer analy-tical laboratory and it both complements and supplements HPLC inthat it is useful for the identification and quantification of the morevolatile components of polymer systems, such as antioxidants, antiozo-nants, accelerators, plasticizers, residual monomer, fatty acids and nitro-samines. Less volatile compounds, such as the higher fatty acids andsome accelerators, can be examined after using one of the many avail-able derivatization techniques to convert them to a more volatile form.

Its versatility arises largely from the multitude of detector typeswhich can be used with the gas chromatograph, although there is also ahuge choice of columns available, as well as a number of differenttechniques for introducing the sample into the chromatography column.

The basis of the technique is that a mobile phase (the carrier gas)containing the volatilized sample passes over a stationary phase whichwill adsorb components differentially leading to partition of the consti-tuents based upon boiling point and, dependent upon the nature of thestationary phase, polarity. There are a very large number of stationaryphases available, some designed for very specific separations whilstothers have a wide range of applications.

Traditionally packed columns were used, in which the stationaryphase was used to coat a support such as brick dust or diatomite whichwas then packed into a large bore (2-4 mm internal diameter) columnmade from glass or stainless steel. Coating of the support and packingthe column could be performed in the laboratory to produce thecolumn required for a particular analysis. Packed columns have beenlargely superseded in modern GC systems by capillary columns,usually made from fused silica and having an internal diameter of 0.2-0.75 mm. The most commonly used form of capillary column is the wallcoated open tubular (WCOT) column in which, as the name suggests,

the stationary phase is coated directly onto the inside of the column.Advantages of capillary columns over packed include shorter analysistimes, and greatly enhanced sensitivity and resolution. To be weighedagainst these advantages is the high cost of capillary columns comparedwith their packed counterparts.

There are three carrier gases in general use; hydrogen, helium andnitrogen. Nitrogen will give the best separation, but will require longerrun time than the other two, consequently hydrogen and helium arebecoming the mobile phases of choice in modern laboratories. Theobvious safety implications of using hydrogen should be considered.The use of clean carrier gas is of paramount importance to preventcolumn degradation and noise, hence the use of traps in the carrier gasline to remove moisture, oxygen and hydrocarbons is essential.

The sample is normally introduced into the GC via syringe injection(manually or by use of an auto sampler) of a solution into a heatedinjection port, although for analysis of gas phase samples heatedheadspace samplers and purge and trap systems are available in whichthe needle is permanently in the injection port. This last technique is ofparticular importance in environmental monitoring where varioustrapping devices are available which can then be thermally 'extracted'directly into the GC. The ultimate extension of this is high temperaturethermolytic fragmentation of large molecules within the GC injectionarea to provide low molar mass volatile materials whose analysisenables a reconstruction of the parent compound. This is the principleof polymer identification by pyrolysis-GC as detailed in Chapter 7.

As mentioned previously there are a number of detector types avail-able, a few of which will be described briefly:

FLAME IONIZATION DETECTOR (FID)

This is the commonest and probably the easiest to use and maintain ofGC detectors, consisting of a needle jet at the end of the columnthrough which the carrier gas exits whereupon combustion takes placein a hydrogen/air mix. This combustion produces a stream of ions thatare detected by polarized electrodes in the top of the detector. The FIDis very sensitive toward organic compounds, with very little responsetoward those containing no carbon.

MASS SELECTIVE DETECTOR (MSD), ALSO KNOWN AS THE MASSSPECTROMETER

Until recent years MSDs were prohibitively expensive for smallerlaboratories, but very powerful benchtop systems are now available atreasonable cost. In the electron ionization MSD the sample undergoes

electron bombardment fragmenting into a series of charged specieswhich are separated according to the mass/charge ratio. Whenfragmented under constant conditions compounds will have a character-istic fragmentation pattern which can be compared to an on-line libraryof mass spectra to enable identification of unknowns. The relativeexpense of the system compared to the FID is justified by this ability toidentify unknown compounds as well as to quantify them. Other varia-tions of the MSD are available which fragment the sample usingchemical ionization or which separate fragments according to their timeof flight (TOF).

NITROGEN PHOSPHORUS DETECTOR (NPD)

Also known as the Thermionic Specific Detector or the Alkali FlameIonization Detector. This detector is similar to an FID in that combus-tion of the sample takes place in a hydrogen/oxygen flame, but there isa 'bead' of an alkali metal salt suspended in the flame across which anelectrical current is passed. As its name implies, it is one of a range ofvery selective detectors, being highly specific to compounds containingnitrogen and phosphorus. As such it has application in the quantitationof residual acrylonitrile monomer in nitrile rubber vulcanizates. Theprocedure is documented in ASTM D4322.

THERMAL ENERGY ANALYSER (TEA)

This is another detector of considerable importance to the rubberindustry since it exhibits an extremely high specificity to N-nitrosocompounds and is able to detect and quantify all the nitrosamineswhich are currently the subject of regulation within the rubber industryat levels at least an order of magnitude below their regulated levels. Itoperates by pyrolytically breaking the N-NO bond to form nitrosylradicals which are then oxidized to electronically excited nitrogendioxide which, in turn, decays rapidly back to its ground state with theemission of a characteristic radiation measured by a photomultiplier.

IDENTIFICATION AFTER SEPARATION

The three spectroscopic techniques, infrared, nuclear magnetic resonanceand Raman are all used extensively for the identification of organicsubstances and thus have an obvious place in any book where this typeof analysis is required. However, once the substance has been isolatedby an appropriate chromatographic technique, its identification becomesa matter of using the instrumental technique and this is beyond theterms of this book.

Isolation, or trapping, of the eluted components also tends to beinstrument-related, thus components may be condensed in a cold trapafter elution through a GC column although it may take a number ofruns to trap enough material if an analytical, as opposed to a semi-preparative column, has to be used to achieve the necessary separation.If HPLC is the separative technique of choice the cold trap can bedispensed with but dedication is still required to collect the fractionsunless the chromatograph is fitted with an automatic fraction collector.

Infrared spectroscopy has been used extensively for the identificationof small amounts of material separated from rubber extracts by thinlayer chromatography and various methods have been used for trans-ferring the separated material to be identified from the TLC plate to thespectrometer and there is no reason why these procedures should notbe used to obtain samples for analysis by Raman or nuclear magneticresonance spectroscopy as these techniques have become more avail-able. An obvious procedure is to scrape the spot (and adsorbent) fromthe TLC plate, or to suck it off with a mini glass vacuum cleaner, andthen wash the material from the adsorbent using a polar solvent, forexample in a small glass tube. Examples of a variety of designs ofapparatus for this purpose are given by Crompton (1971). The methoddoes, however, suffer from the disadvantage that it is difficult to avoidcontamination of the sample by minute traces of adsorbent. These caninterfere with the infrared examination but would have little influenceon either the Raman or nuclear magnetic resonance spectra.

This potential problem with the infrared spectrum can, however, beavoided by 'chromatography' of the separated substance on a (commer-cially available) porous wedge of potassium bromide powder (Rice,1967; Garner and Packer, 1968). Material scraped from the TLC plate isplaced into a glass tube with a flat base. The wedge of potassiumbromide is placed on the sample and is prevented from touching thesides of the glass tube by means of a glass or metal ring. The tube isclosed with a plastic cap in which a 3mm diameter hole has beendrilled centrally. About 0.5cm3 of a suitable solvent, e.g. methanol,carbon tetrachloride or toluene, is added to the tube. The solvent risesup the wedge, carrying the sample with it. Solvent evaporates from thetop of the wedge so that the sample becomes concentrated at this point.This process may take 1-5 hours depending on the volatility of thesolvent and on the room temperature. After drying, the tip of the potas-sium bromide wedge is removed, crushed, and compressed into a thindisc which is examined in the infrared spectrometer. The spectrumobtained can then be compared with those of known compounds, anenormous compilation of which is given by Hummel and Scholl (1981).Volume 2, Part a/II, published in 1984 also includes some Ramanspectra. Perhaps the most comprehensive set of NMR reference spectra

is the Aldrich Library of 13C and 1H NMR spectra which is available asa three volume set or on CD-ROM but, unlike the Hummel and Schollcompilation, is not specifically orientated towards polymer-relatedsubstances.

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land, Ohio.

Solution methods O

In the early part of Chapter 3 the terms 'extraction', 'solution7 and'dissolution' were used and 'extraction' was defined as the practice ofremoving organic additives from the bulk sample without simulta-neously removing significant amounts of the polymer itself. It is nownecessary to define and distinguish between the other two terms.

Solution is the procedure whereby one or more polymers is or areremoved from the extracted bulk sample by the addition of a suitablesolvent, or solvent mixture, in such a way that the polymer(s) may berecovered unchanged from the solution.

Dissolution allows a combination of degradation and solvent treat-ments so that the polymeric phase undergoes chemical reactions whichincrease its solubility but do not enable it to be recovered unchangedfrom the resulting solution.

Dissolution is dealt with in a degree of detail in the various chapterswherein its application is relevant, typically in Chapter 7 where a disso-lution procedure involving oxidation of the rubber in a solution ofboiling orfto-dichlorobenzene (Barnes et al., 1944; Clark and Scott, 1970)is described together with the 'dry' oxidation procedures of LiGotti(1972) and Carlson et al (1970). These represent one end of the dissolu-tion spectrum, with the polymer possibly being only partially solubi-lized, and suffering relatively little degradation, whilst at the other endis the completely destructive dissolution procedure as described byKolthoff and Gutmacher (1950) where the object is not to recover thepolymer but to free the carbon black and inorganic fillers of allpolymeric materials (Chapter 11). In addition to these methods it isworth noting that Barnard (1956) describes a scheme for the completeremoval of natural rubber from grafts with polymethylmethacrylate orpolystyrene by ozonolysis of the natural rubber. This scheme should beapplicable to any system containing a mixture of polyunsaturated andsaturated polymers and is particularly interesting because, havingshown that there was degradation of the remaining polymer after allthe natural rubber had been destroyed, he added di-n-butyl sulphide to

the solution prior to ozonolysis. This, having a reactivity to ozone inter-mediate between that of natural rubber and the other polymers,provided a 'buffer time zone' so that he could be sure of completeozonolysis of the former, without measurable degradation of the latter.

Here, however, we are concerned with the preparation of solutionsand their significance to the analyst beyond their obvious purpose ofproviding a solution for subsequent analysis by one or more of therelevant techniques.

THEORETICAL CONSIDERATIONSThe parameters governing the solubility of a specific polymer in asolvent are numerous and complex and it would be extremely difficult,if at all possible, to predict the behaviour of every polymer with everysolvent.

There are, however, several generalizations which may be made. Forinstance, a distinction exists between the dissolving of polymers of highmolar mass which are partly crystalline and those which have nocrystallinity, i.e. are totally amorphous. In the case of the former, thecrystalline regions have first to be melted and thus usually it is neces-sary to heat the solvent to a temperature near the melting point of thepolymer to effect a rapid solution.

In the case where a specific interaction occurs between the polymerand solvent, solution will be obtained without undue difficulty. Atypical example is the solution of Nylon 6.6 in cold formic acid due tohydrogen bonding between the two.

If we consider an amorphous polymer and its interaction with aliquid, solution will occur if the free energy of mixing (AG) is negative:

AG = AH-TAS (5.1)

As the entropy of mixing (AS) is usually large and positive for theorganic solvents we are concerned with, the sign of ZlG depends uponthe size and sign of AH.

If there is a strong interaction between the polymer and the solvent,AH will be negative and thus solution will occur. If, however, there areonly dispersive forces involved in the solution we can use the expres-sion of Hildebrand and Scott (1949) to determine the heat of mixing:

AH=$s$p(*s-*p)2 (5.2)

where (f)s and <pp equal the volume fractions of polymer and solventrespectively, <^s and <^ equal the solubility parameters of polymer andsolvent respectively and () is defined by

^ = (AE/V)1/2 (5.3)

where ZlE is the heat of vaporization, and AE/V (i.e. AE per unitvolume) is the cohesive energy density (CED).

In this situation ZJH must be positive and therefore should be as smallas possible so that ZlG can have as large a negative value as possible(Eq. (5.1)) and this will result if the solubility parameters of the solventand polymer are equal.

We are now in a position, if we know the solubility parameters of arange of solvents and polymers, to see which are similar and thusprobably (or possibly) will allow solution to occur, and which arewidely different, making solution unlikely. Note that Zl H is related tothe square of the difference between solubility parameters, thus thedirection of the difference is inconsequential and there is no advantagein choosing a solvent with a solubility parameter value which is lowerthan that of the polymer.

Whilst there is intrinsically no difficulty in obtaining a CED value,and hence the solubility parameter, for any solvent, it cannot bemeasured directly for polymers as they do not vaporize. Severalindirect methods for obtaining the data have been proposed by Gee(1943), Bristow and Watson (1958), and Small (1953) who used methodsbased on swelling, viscosity and the summation of individual values forsmall molecular groups, respectively. If we move from an ideal torealistic situation it is necessary to make some allowance for hydrogenbonding as in many polymer-solvent systems this will have a significanteffect on solubility. Burrell (1955) divided solvents into three classes:

1. poor hydrogen bonding capability: aliphatic/aromatic hydrocar-bons, chlorinated hydrocarbons, nitro-paraffins;

2. moderate hydrogen bonding capability: esters, ketones;3. strong hydrogen bonding capability: alcohols, acids, amines.

He then used these, together with solubility parameters, to selectsolvents for particular polymers. Crowley et al. (1966, 1967) haveproposed a three parameter concept of solubility, based on solubilityparameter, hydrogen bonding and dipole forces. Table 5.1 lists a rangeof common solvents for polymers together with numerical values forthese parameters. Hydrogen bonding values of less than 4 can beconsidered poor, 4-10 moderate, and greater than 10 strong (Mellan,1968). Table 5.2 provides solubility parameter data for a range ofpolymers from a variety of sources, including Mellan (1968), Small(1953), Bristow and Watson (1958) and Brandrup and Immergut (1989).The spread includes results obtained by different authors using severalmethods and allows for differences due to the different hydrogenbonding properties of the solvents used.

Table 5.3 shows the solubility of a range of polymers and plasticsusing a scale of 1 to 6 defined in a footnote to the table.

Table 5.1(a) Polymer solvent (alphabetical order)

Substance Solubility Hydrogen Dipoleparameter bonding moment

Acetone 9.9 9.7 2.9Acetonitrile 11.9 6.3 3.9Acetylacetone 10.8 8.4 3.1n-Butanol 11.4 18.7 1.7Carbon disulphide 10.0 O OCarbon tetrachloride 8.6 O OChlorobenzene 9.5 1.5 1.6Chloroform 9.3 1.5 1.2Cyclohexane 8.2 O OCyclohexanol 11.4 18.7 1.7Cyclohexanone 9.9 11.7 2.7n-Decane 6.6 O ODiacetone alcohol 9.2 13 2.5o-Dichlorobenzene 10.0 O ON, N-Diethyl acetamide 9.9 12.3 2.0Diethyl ether 7.4 13.0 1.2N,N-Diethyl formamide 10.6 11.7 2.0N,N-Dimethyl formamide 12.1 11.7 2.0Dimethyl sulphoxide 12.9 7.7 4.0Dioxan 10.0 9.7 ODipropyl sulphone 11.3 7.7 4.5Ethyl acetate 9.1 8.4 1.8Ethyl alcohol 12.7 18.7 1.7Ethyl benzene 8.8 1.5 0.6Ethylene glycol 14.6 20.6 2.3n-Hexane 7.3 O OMethyl alcohol 14.5 18.7 1.7Methylene chloride 9.7 1.5 1.5Methyl ethyl ketone 9.3 7.7 2.7Nitrobenzene 10.0 2.8 4.3Nitromethane 12.7 2.5 3.4Piperidine 8.7 24.2 2.22-Propyl acetate 8.4 8.6 1.9Pyridine 10.7 18.1 2.2Styrene 9.3 1.5 OTetrahydrofuran 9.1 8.6 1.6Toluene 8.9 4.5 0.4Trichloroethane (1,1,2-) 9.6 1.5 1.2Water 23.4 39.0 1.8Xylene (commercial 8.8 4.5 0.4mixture)

Table 5.1 (b) Polymer solvent (in order of increasing 5)

Polymer 5 Polymer 8

n-Decane 6.6 N,N-Diethyl acetamide 9.9n-Hexane 7.3 Acetone 9.9Diethyl ether 7.4 Dioxan 10.0Cyclohexane 8.2 o-Dichlorobenzene 10.02-Propyl acetate 8.4 Carbon disulphide 10.0Carbon tetrachloride 8.6 Nitrobenzene 10.0Piperidine 8.7 N,N-Diethyl formamide 10.6Ethyl benzene 8.8 Pyridine 10.7Xylene (commercial mixture) 8.8 Acetylacetone 10.8Toluene 8.9 Dipropyl sulphone 11.3Ethyl acetate 9.1 n-Butanol 11.4Tetrahydrofuran 9.1 Cyclohexanol 11.4Diacetone alcohol 9.2 Acetonitrile 11.9Chloroform 9.3 N.N-Dimethyl formamide 12.1Methyl ethyl ketone 9.3 Ethyl alcohol 12.7Styrene 9.3 Nitromethane 12.7Chlorobenzene 9.5 Dimethyl sulphoxide 12.9Trichloroethane (1,1,2-) 9.6 Methyl alcohol 14.5Methylene chloride 9.7 Ethylene glycol 14.6Cyclohexanone 9.7 Water 23.4

Table 5.2 Solubility parameters of polymers

Polymer Range of Elastomer Range ofquoted quotedpolymer polymervalues values

Polyethylene 7.7-8.4 NR/PI 7.9-8.5Polypropylene 9.4 SBR (4-40% S) 8.1-8.6Polystyrene 8.5-10.6 BR 8.0-8.6Polyacrylonitrile 12.3-12.8 UR 7.5-8.0Polyvinylacetate 8.5-9.5 CR 8.1-9.4Polyvinylchloride 8.5-11.0 NBR (18-30% ACN) 8.7-9.3Polymethylmethacrylate 8.9-12.7 NBR (40% ACN) 10.4-10.5Styrene acrylonitrile 10.6-11.2 Silicone(s) 7.0-11.0Ethylene vinylacetate 7.8-10.6 Chlorinated rubber 9.4

Thiokol(s) 9.0-10.0Polyurethane(s) 9.8-10.3EPDM(s) 7.5-8.6

Table 5.3 Solubility of polymers in various solvents

Solvent A B C D E M N O P Q

Acetone 6 1 6 6 6 5 1 3 1 3Acetonitrile 6 5 6 6 6 6 6 4 1 6n-Butanol 6 6 6 1 5 6 6 6 5 6Carbon disulphide 1 4 1 1 1 6 5 1 4 1Carbon tetrachloride 1 5 1 1 1 6 5 1 1 1Chlorobenzene 1 1 1 1 1 4 1 1 1 1Chloroform 1 1 1 1 1 5 1 1 1 1Cyclohexane 1 6 1 1 1 6 6 1 4 6Cyclohexanol 5 5 5 1 5 6 6 6 1 6Cyclohexanone 1 1 1 1 1 1 1 1 1 1Diacetone alcohol 6 1 6 6 6 6 5 5 1 3o-Dichlorobenzene 1 1 1 1 1 4 1 1 1 1Diethyl ether 1 6 1 1 1 6 6 3 4 6Dimethyl formamide 6 1 5 5 6 4 1 1 1 1Dimethyl sulphoxide 6 5 6 6 6 2 1 6 1 1Dioxan 1 1 4 5 1 3 1 1 1 1Ethyl acetate 3 1 5 5 5 6 1 1 1 1Ethyl alcohol 9 6 % 6 6 6 6 6 6 6 6 1 6Ethyl benzene 1 1 1 1 1 6 5 1 1 1Ethylene glycol 6 4 6 6 5 6 6 6 6 6Hexane 3 6 1 1 1 6 5 3 6 6Methyl alcohol 6 5 6 6 6 6 6 6 1 6Methyl ethyl ketone 4 1 4 1 4 3 1 1 1 1Methylene chloride 1 1 1 1 1 2 1 1 1 1Nitrobenzene 1 1 1 6 1 2 1 1 1 1Nitromethane 6 1 6 6 6 6 6 5 1 6Styrene 1 1 1 1 1 5 2 1 1 1Tetrahydrofuran 1 1 1 1 1 1 1 1 1 1Toluene 1 1 1 1 1 5 2 1 1 1Trichlorethane 1 1 1 1 1 6 5 1 1 1Pyridine 1 1 5 4 1 2 1 1 1 1Xylene 1 1 1 1 1 6 4 1 4 1

1 = soluble, 2 = virtually soluble, 3 = strongly swollen, 4 = swollen 5 = marginally swollen,and 6 = no visible effect.Polymer Code:Elastomers PlasticsA = SBR M= PVCB = NBR N = PMMAC = NR/IR O = PSD = UR P = PV acetateE = BR Q= Chlorinated PP

THETA TEMPERATURE

It will be apparent from inspection of Tables 5.1 and 5.2 that manysolvents should dissolve a particular polymer and indeed may well doso but it is eventually necessary to choose a particular one. Leavingaside specific applications, which will be mentioned later, there arecertain theoretical considerations which will influence the choice.

If one considers a polymer dissolved in any solvent, the polymerchain will be extended, to a degree depending upon the polymer-solvent interaction, so that the mean end-to-end chain length (/) isgreater than would be predicted for a random coil configuration (d).The ratio l/d is known as the expansion factor, a.

As the solution cools the configuration approaches that of a randomcoil (oc -> 1) and at a specific temperature for each polymer-solventsystem one reaches the theta temperature at which a is unity. This isalso the critical miscibility temperature for a polymer of infinitely highmolar mass and may be determined by measuring the critical miscibilitytemperature for a series of polymer fractions, and extrapolating toinfinite molar mass.

It will be apparent that if the theta temperature of a particularpolymer solvent system is near room temperature, the polymer will benear to precipitation and thus the solvent will be a bad one. A goodsolvent will have its theta temperature substantially below zero.

Mixtures of two solvents may be used to adjust the theta temperatureof a particular system and indeed this is the principle of precipitation. Ifthe theta temperature of a solvent is adjusted until it is close to roomtemperature, the solvent for that particular polymer of that particularmolar mass is called the theta solvent.

Flory (1942) and Huggins (1942) introduced the Flory-Huggins inter-action constant (x) in order to define numerically the concept of'goodness of solvent' as described above. The critical value of x is 0.5and for a given polymer-solvent system / must be below 0.5 for solubi-lity to occur.

The practical significance of the constant may be appreciated byconsidering the determination of molar mass by membrane osmometry(Chapter 8).

The working equation is given:

h/c = K/Mn + bc (5.4)where

b = (0.5-x) (5.5)For a theta solvent (when x approaches 0.5)

h/c = K/Mn (5.6)

where K is the product of the gas constant (R) and the absolutetemperature (T). Thus no calibration plot is required and the molarmass (M) may be calculated simply by measuring the osmotic pressure(h) for one solution of concentration (c). At the other extreme, the'better' the solvent the smaller will be %, the steeper the slope of the h/cvs. c graph and the greater the error of measuring the intercept. Oneshould therefore choose a solvent which has its theta temperature nearto the operating temperature of the osmometer so that the graph isrelatively flat. Gee (1940, 1944) used a mixed solvent system ofbenzene/methanol to approach these conditions in the analysis ofnatural rubber, but the use of mixed solvents is potentially dangerousbecause of differences in diffusion rates and relative volatilities.

GUIDELINES TO SOLUBILITY

From the mass of published empirical data on solubility, together witha consideration of the scientific reasons put forward in this chapter,Billmeyer (1962) listed some generalities which still have validity:

1. Similarity of chemical and structural makeup of solvent and polymerfavours solubility.

2. Solubility is inversely related to molar mass.3. Solubility is inversely related to melting-point.4. The presence of polar groups in a polymer will reduce solubility in

non-polar solvents (increase in polymer-polymer interaction).5. Solubility of copolymers is a function of the relative amounts of each

monomer.

These were later expanded by Hanson (1967) to include systemscontaining two or more polymers:

1. Polymers must be individually soluble in a solvent for them to bemutually soluble in it.

2. Individual solubility does not guarantee mutual solubility, particu-larly if:(a) both are of high molar mass;(b) the solvent is a poor one for one of the polymers;(c) the polymers do not have similar and overlapping d ranges.

3. As the concentration increases, one polymer in a mutually solublesystem can become insoluble.

PRACTICAL CONSIDERATIONSPerhaps the most important practical consideration for an analyst is thatof quickly deciding which solvent is likely to dissolve a particularpolymer and therefore Table 5.3 is included, but the choice of which

particular solvent to use must obviously be governed by many factors,not least being the reason for obtaining the solution. Thus for manyelastomers the preparation of a cast film for infrared spectroscopicstudy is carried out from a chloroform solution but this would be nouse if the ultraviolet spectrum of the solution were required. The lattercould possibly be obtained from a solution in tetrahydrofuran but,conversely, this is a poor solvent from which to cast a film for infraredexamination as it is too volatile and gives bubbles in the polymer film.

PREPARATION OF A SOLUTION

Certain points are worth bearing in mind when the appropriate solventhas at last been chosen. Polymer solutions tend to be unstable, with thepolymer often being prone to oxidation. This is not only true of unsatu-rated polymers such as natural rubber (Bateman, 1954) but also ofsaturated polymers whose viscosity has long been known to be timedependent, to a different extent, in different solvents (Mead and Fuoss,1946; Morrison et a/., 1946). The practice of adding stabilizers iswidespread but this must obviously depend upon the purpose ofobtaining the solution, and a balance must be struck between the timetaken for solution to become complete and the time scale over whichthe polymer molar mass may be considered to be stable.

Most polymers dissolve slowly and, whilst this can be hastened byheating or agitation, care should be taken since, as long ago as 1956,Grassie remarked that there are many references to the effect that 'overenthusiastic' shaking may result in the depolymerization of thepolymer. If the sample of polymer is a powder, and this is added to awarm solvent, it may 'lump up', resulting in a very long time tocomplete solution; it is better to cool the solvent to a little above itsfreezing-point and then add the powder with gentle agitation so that itis completely wetted, before raising the temperature to speed up thesolution processes.

REMOVAL OF SOLVENT

In many applications it is required to remove a solvent after preparingthe solution. Two typical examples would be the preparation of a castfilm for infrared spectroscopic analysis and the quantitative measure-ment of the concentration of a solution after filtration to remove carbonblack, filler or gel. In either case it is imperative that the solvent beremoved completely, and this is by no means simple. Haslam andWillis (1965) illustrated an extremely simple apparatus (Figure 5.1)which has its uses if a vacuum hotplate is not available. If the solventhas a high vapour pressure when frozen, then the technique of freeze

Figure 5.1 Apparatus for drying washed polymers.

drying may be used to advantage. The solution is rapidly frozen andthe solvent removed under high vacuum, without allowing melting tooccur. This leaves the polymer in a very fine expanded form, the idealphysical state from which to remove final traces of the solvent.

Evans ei al. (1960) examined in detail the solvent-retaining propertiesof cyclized natural rubber with a wide range of halogenated solvents,benzene and carbon disulphide, and showed how substantial amountsremain after 'drying'. He also illustrated the effect of altering the levelof cyclization on the retention of carbon tetrachloride under fixed condi-tions. Both sets of data are illustrated, in Table 5.4 and Figure 5.2. Otherparticularly noteworthy examples of polymer-solvent pairs which aredifficult to separate are polyvinyl chloride-tetrahydrofuran andpolystyrene-toluene/benzene. Note: it should be remembered thatbenzene is a carcinogen and should never be used if an alternative,such as toluene, is available.

to vacuum

PERCENT UNSATURATION

Figure 5.2 Dependence of solvent retention by films of cyclized rubber on theirunsaturation. (Courtesy J. Appl. Polym. Sc/.).

As the solvents become higher, boiling exacerbates the situation, andone way round this is to wash the nominally dried samples with a lowboiling liquid which is miscible with the solvent, but which will notdissolve the polymer.

If thermogravimetric analysis (TGA) is available it is well worthwhilecarrying out routine checks on all polymers which are supposed to be'dry'. Any residual solvent present will be volatized at a relatively lowtemperature in an atmosphere of nitrogen (< 200 0C). Many publishedinfrared spectra show the polymer to be contaminated with solvent and

MO

LES

C

CU

RE

TAIN

ED

PER

IO

O

C5H

8UN

ITS

Table 5.4 Solvent retention of films of cyclized natural rubber

Molecules solvent retained per 100 moles ofC5H5 units in polymer

Solvent 250C 10O0C

CCI4 13.42 8.55CHCI3 9.77 1.64CH2CI2 0.1CH3CHCI2 7.76 2.89CH2CICH2CI 9.75 5.54CHCI2CHCI2 13.1CHCI = CHCI 4.31 4.1CCI2 = CCI2 3.8 6.17CBr4 2.05CHBr3 11.7 8.3CH2Br2 10.5 7.3C6H6 2.3CS2 1.84

After 24 hours at room temperature and atmospheric pressure, solvent retention for CCI4,19.75; after 3 days at 2O0C. and 1O-4 mm Hg, 17.75; after 5 hours at 1550C and 1Cr4ImTiHg,4.56.The films were cast from various solvents at atmospheric pressure and room temperatureand pumped for 3 hours at 250C and 10O0C, at 0.5 mm Hg. In all cases the films were lessthan 0.2 mm thick.

it is often advisable to cast the films concurrently from differentsolvents so that one is not misled by an unassigned band. It should alsobe noted that solvent bands do not always occur in identical positionsin different matrices, thus the 14.85 Jim band of benzene, as it appearsin cyclohexane solution, shifts to 14.75 jim when present as a residualpeak in polystyrene.

SELECTIVE SOLUTION

There are many occasions when one may wish only to dissolve onepolymer from a material which contains several such materials. One isto obtain detailed microstructural information in the absence of poten-tial interference from the other polymers present whilst another is in theexamination of laminates when a knowledge of the total polymercomposition from, say, transmission infrared spectroscopy may enableone to dissolve sequentially the polymer layers and thus obtain thelaminate composition in full detail.

The technique is also useful in the examination of thermoplastics ofthe rubber-plastic mixture type when successive extractions with arange of solvents such as methyl alcohol-acetone-ethylene dichloride-

tetrahydrofuran followed by infrared spectroscopic analysis of thepolymer soluble in each successive solvent will provide data on theinterrelationships between the various monomer species known to bepresent.

A more sophisticated procedure for multiple extractions has beendescribed by Ceresa (1962) who analysed block and graft copolymersystems such as polyvinyl acetateipolyethylene and natural rubberipoly-methylmethacrylate using solvent pairs consisting of a solvent and non-solvent for each polymer. Successive extractions with progressivelydifferent mole fractions of the two solvents afforded separation into thetwo homopolymers and block/graft copolymers of proportionatelydifferent compositions.

However, one should always be aware that selective solution cangive misleading results in certain circumstances, the most common oneprobably being where one of the polymers exists as a discrete phase ina matrix of the other, rather than the two being co-continuous.Attempts to dissolve the discrete phase from the insoluble matrix willoften be frustrated due to the exceedingly slow diffusion of solvent andthe resulting polymer solution through the sample (Chapter 3). Thediffusion rate can be increased by using a solvent mixture such thatbulk swelling occurs, or by using the converse technique of dissolutionfollowed by selective precipitation.

SELECTIVE PRECIPITATION

This undoubtedly affords the purest materials from any complex blendor mixture of copolymer and polymer and consists of preparing asolution of the total polymeric phase prior to selectively precipitatingthe individual homopolymers by the addition of the appropriate non-solvent. A dual precipitation will remove both homopolymers, leavingthe graft (if present), with its intermediate solution characteristics, insolution.

This process can be extended until it becomes fractional precipita-tion, and it is then used to separate different molar mass fractions of aparticular homopolymer, or graft/block copolymers of different, andprogressively graded, proportions of each polymer.

REFERENCESBarnard, D. (1956) /. Polym. ScL 22, 213.Barnes, R.B., Williams, V.Z., Davis. A.R. and Giesecke, P. (1944) Ind. Eng. Chem.

Anal. 16, 9.Bateman, L, (1954) Quart. Rev. Chem. Soc. 8, 147.Billmeyer, F.W. Jr (1962) Polymer Science, Interscience, New York.Brandrup. J. and Immergut, E.H. (1989) Polymer Handbook, 3rd edn, Wiley, New

York.

Bristow, G. and Watson, W.F. (1958) Trans. Faraday Soc. 54, 1731, 1742.Burrell, H. (1955) Off. Dig. 27 No. 369, 726.Carlson, D.W., Ransaw, H.C. and Altenau, A.G. (1970) Analyt. Chem. 42, 1248.Ceresa, RJ. (1962) Block and Graft Copolymers, Butterworths, London.Clark, J. and Scott, R. (1970) Rubber Chem. Technol 43, 1332.Crowley, J.D., league, G.S. Jr and Lowe, J.W. Jr (1966) /. Paint Technol. 38, (496),

269.Crowley, J.D., league, G.S. Jr and Lowe, J.W. Jr (1967) J. Paint Technol. 39, (504),

19.Evans, MB., Higgins, G.M.C., Lee, D.F. and Watson, W.F. (1960) J. Appl. Polym.

ScL 4, 367.Flory, PJ. (1942) /. Chem. Phys. 10, 51.Gee, G. (1940) Trans. Faraday Soc. 36, 1141.Gee, G. (1943) Trans. IRl, 18, 266.Gee, G. (1944) Trans. Faraday Soc. 40, 462.Grassie, N. (1956) Chemistry of High Polymer Degradation Processes, Butterworths,

London.Hanson, C.M. (1967) J. Paint Technol. 39, (505), 104.Haslam, J. and Willis, H.A. (1965) Identification and Analysis of Plastics, Iliffe,

London.Hildebrand, J. and Scott, R. (1949) The Solubility of Non-Electrolytes, 3rd edn,

Reinhold, New York.Huggins, M.L. (1942) Ann. N. Y. Acad. ScL 43, 1.Kolthoff, LM. and Gutmacher, R.G. (1950) Analyt. Chem. 22, 1002.LiGotti, I. (1972) Paper presented at 20th meeting, ISO TC45-WGI, Cologne.Mead, DJ. and Fuoss, R.M. (1946) /. Am. Chem. Soc. 64, 277.Mellan, I. (1968) Compatibility and Solubility, Noyes Development Corp., London.Morrison, J.A., Homes, J.M. and Mclntosh, R. (1946) Canad. J. Res. 24B, 179.Small, P.A. (1953) /. Appl. Chem. 3, 71.

Quantitative elemental r*

analysis D

At a time when ever more sophisticated instrumental equipment isavailable for the analysis of elastomers and their products, it should beremembered that one of the oldest of the modern analytical techniques- quantitative elemental analysis - still has a considerable role to playin the analysis of an extracted polymer, compound or vulcanizate sinceit can provide valuable information on many topics of interest to therubber technologist. These can range from the quantitative analysis ofblends, the homogeneity of mixes and the nature of various degradativeprocesses, to the state of cure of rubber products.

One aspect of using elemental analysis to determine polymer contentand composition should not be overlooked. The measured element will,inevitably, be less than 100% of the total polymer content and couldwell be only a very small percentage. The calculation of the amount ofpolymer present from the elemental data will therefore require a scalingfactor which will be the relative error of the elemental determinationtogether with the batch-to-batch variability of the element duringmanufacture of the polymer.

CARBON AND HYDROGEN

Although it is possible to use complete elemental analysis to bothidentify and quantify the proportions of elastomers present in a vulcani-zate, the increasing sophistication of modern formulations makes inter-pretation of the results of any carbon and hydrogen analysesparticularly difficult although it should be noted that there is onecurrent standard (ISO 4655) which provides for the use of carbon andhydrogen determinations to estimate the amount of styrene in a rawstyrene butadiene copolymer.

Numerous commercial instruments are available for carrying outcarbon, hydrogen (and nitrogen) analyses. In general these rely onburning the sample in oxygen and driving the volatile products through

a tube packed with a solid oxidant which ensures 100% conversion tocarbon dioxide and water. Combustion products from interferingelements such as sulphur and halogen are removed with appropriatereagents packed sequentially in the tube whilst oxygen is removed andthe oxides of nitrogen reduced to nitrogen itself in a heated reductiontube filled with copper powder. The evolved gas is then analysed gravi-metrically or instrumentally for carbon dioxide, water and nitrogen sothat the original proportions of carbon, hydrogen and nitrogen may becalculated. It is generally possible to automate this equipment so thatthe throughput can be doubled or trebled by operating the instrumentthroughout the night.

Childs and Henner (1970) have compared some instrumental andclassical methods of analysis for these elements and for the analyst whodoes not wish to purchase the commercial equipment, complete detailsfor the construction of a manual gravimetric unit are given by Ma andRittner (1979).

NITROGEN

There are, in essence, only two methods which need be considered forthe analysis of nitrogen in rubber. The first is the Dumas method, inwhich the rubber is destroyed by oxidation and the oxides of nitrogenreduced to nitrogen using copper powder, whilst the second, due toKjeldahl (1883), relies on the action of concentrated sulphuric acid toform ammonium hydrogen sulphate which is then treated with alkali toliberate ammonia. A third method, proposed by ter Meulen (1924),appears never to have become popular. The Dumas method is that usedby automatic commercial carbon, hydrogen and nitrogen analysers andwas considered briefly above. A comparison of the two major methodshas been made by Dunke (1967) who used them to measure thenitrogen content of nitrile-butadiene rubbers; he preferred the Kjeldahlmethod due to the smaller standard deviation of its results.

Dunke's method, in its current micro form, was described by Ma andZuazaga in 1942 and has changed little since. The procedure is fullydescribed in ISO 1656-1996 and in the Rubber Research Institute ofMalaysia (RRIM) Test Methods, SMR Bulletin No 7 (1973). Broadlyspeaking, samples will fall into three categories: natural rubber (forprotein content), nitrile-butadiene rubbers (for acrylonitrile content), andthe remainder which will include polyurethanes and other nitrogen-containing polymers. The amount of sample taken should be adjusted,where possible, to give approximately lmg of ammonia and togenerate this, a typical sample requirement for a normal grade ofnatural rubber would be 200 mg. When the method is applied to nitro-genous polymers, such as nitrile-butadiene rubbers, then the sample

size will need to be adjusted accordingly, remembering that any adjust-ment must allow for the non-NBR substances present in a commercialvulcanizate. This information will, of course be required if the elementaldata is to be interpreted correctly.

One point should be highlighted: the normal catalyst system which isadded to the digestion solution consists of selenium, copper sulphateand potassium sulphate, in the weight ratio 1:4:30. However, ifurethane rubbers, or those containing heterocyclic aromatic compoundswith nitrogen in the ring (cf. vinyl pyridine), are similarly treated, a lowresult will be obtained unless the catalyst is changed to mercuric oxide:potassium sulphate (30:6). In this case sodium thiosulphate should beadded at the distillation stage to decompose the mercury:ammoniacomplex.

The nitrogen content of natural rubber is related to the protein leveland, although the nitrogen content of proteins varies, a conversionfactor for nitrogen to protein of 6.25 is generally considered an accep-table value. The protein content of natural rubber varies dependingupon its source and to the methods used in its processing. Representa-tive raw rubbers would be expected to have a nitrogen content in therange 0.3-0.6% but rubbers prepared from concentrated latex willusually have one of two nitrogen levels, the normal latex grades havinggenerally lower levels than the 'dry' rubbers, with values around 0.2%,whilst 'skim7 rubber, with its higher protein content, will have appreci-ably higher values, often in the range 1.5-2.5%.

Further types of natural rubber are either currently available or indevelopment in which the protein has been reduced by one of severalprocesses prior to coagulation of the latex. This can show nitrogenlevels as low as 0.04% but a more usual value for commercially avail-able material would be about 0.06-0.1%. No doubt this will be an areawhere market-driven forces arising from natural rubber latex proteinallergy will lead to a further lowering of the protein content in thecoming years.

The acrylonitrile level of a nitrile-butadiene rubber varies between15% and 50% with the level reflecting the degree of oil resistancepossessed by the copolymer. Also available commercially are isoprene-acrylonitrile copolymers and polyvinylchloride/nitrile-butadiene blends.Nitrogen levels in the raw polymer are thus in the 4-13% range and a20 mg sample of the raw material will generate the appropriate level ofammonia.

The use to which the measured nitrogen value is put will dependupon the other data available and illustrates the interrelationshipsbetween techniques which should be used, wherever possible, toconfirm both quantitative and qualitative data. The polymer type (sayNBR) will be known from pyrolysis-infrared, NMR spectroscopy or

pyrolysis-gas chromatography whilst the temperature of the glasstransition (Tg) will, in the absence of any interfering polymers whichmight be present in a blend, indicate the acrylonitrile content of thenitrile-butadiene rubber. Thermogravimetric analysis (TGA) will notdirectly give the polymer loading since the 'pyrolysate' weight loss willrequire a correction for the carbonaceous residue left by the nitrilerubber (and the level of this correction will depend on the acrylonitrileloading - see Chapter 7) but the nitrogen level can be used to obtainthe acrylonitrile level. The observed 'pyrolytic' and 'combustible7 weightlosses observed during TGA can then be corrected to give 'polymer'and 'black' loadings.

The presence of a copolymer can make this type of analysis moredifficult, particularly if a chlorinated polymer is present as this may alsoleave a carbon residue when pyrolysed. A C NMR spectroscopicmethod for resolving this issue is described in Chapter 7 but, in thecontext of elemental analysis, a combination of carbon, hydrogen,chlorine and nitrogen determinations will enable the polymers to becompletely characterized.

It is emphasized that these analyses should always be carried out onextracted samples in order to remove added nitrogenous materials suchas amine antioxidants and cure residues. A thiazole cure inevitablyresults in some accelerator fragments becoming chemically attached tothe polymer chain, but these contribute negligible additional nitrogen.The use of a reversion-resistant urethane cure, however (Baker et al.,1970), can increase the measured nitrogen content by between 0.2% and0.6% of the polymer weight and this, again, emphasizes the need for a'total analytical overview' rather than one narrow analysis.

OXYGENIn spite of the importance of oxygen to the organic chemist, it wasprobably the last element for which an accurate quantitative procedurewas developed. Historically it has been calculated by difference, a riskybusiness as this not only entails a summation of all the errors of themeasured element concentrations, but also risks a larger error due to thepresence of a major element not having been included in the analysis.

The first satisfactory procedure was developed by Schultze (1939) andmodified to the micro scale by Zimmermann (1939). In 1947 Aluise andco-workers used a method developed by Unterzaucher (1940) to analysea range of synthetic rubbers and in 1948 Chambers published a detailedappraisal of the same method in the analysis of natural rubber. Theprinciple is simple and consists of pyrolysing the sample in a stream ofpure nitrogen after which the gaseous products are passed over carbonheated to UOO0C or more. All the oxygen is converted to carbon

monoxide which is then oxidized to carbon dioxide by iodine pentoxide.The amount of iodine pentoxide consumed is estimated iodometricallyusing a sodium thiosulphate titration. Chambers claimed that interfer-ence from halogens and sulphur could be removed by the incorporationof a soda-asbestos-filled scavenging tube prior to the oxidation step. Asystem similar in principle is recommended by the Association ofOfficial Analytical Chemists (AOAC) (1984) but this uses a tube filledwith copper powder to remove interfering materials, copper oxide toconvert the carbon monoxide to dioxide, and a gravimetric finish.

In view of the fact that automatic carbon, hydrogen and nitrogenanalysers determine carbon content by measuring carbon dioxide, it isnot surprising that several have been modified to determine the carbondioxide obtained from the oxygen in a sample. The alterations entailchanging the reaction tubes and temperatures, but using the samedetector system. It seems probable that at levels of oxygen greater than0.2% there is little to choose between the three basic techniques:iodometric, gravimetric and instrumental.

A problem arises if the analysis is carried out on a fluoropolymer, ashydrogen fluoride reacts with silica, from the tubing of the instrument,to give water and silicon tetrafluoride, the latter of which will enhancethe observed nitrogen detector signal and thus give an inflated valuefor the nitrogen content. A number of modifications to the basictechnique have been developed to circumvent this: Ehrenberger et al.(1963) used a nickel combustion tube and Cruikshank and Rush (1962)a platinum one, whilst Olson and Kulver (1970) preferred to use thecompletely different technique of isotope dilution analysis, firstdescribed by Grosse et al. as early as 1946.

An excellent review, which has dated little, for quantitativelymeasuring the oxygen content of organic materials is presented byDavies (1969) whilst Ma and Rittner (1979) also provide useful data andtechniques but without specific reference to polymers.

Oxygen analyses in the polymer field, be they on raw rubbers orvulcanizates, tend to fit into one of two categories, those involvingrelatively high levels of oxygen, when the oxygen is an element withinthe polymer repeat unit, and those when the oxygen content is low,being due to oxygen-containing impurities (e.g. protein in NR) or oxida-tion. One example of the former is the determination of the level ofmethyl methacrylate in methacrylate-grafted natural rubber, supplied asMG49 (49% w/w polymethylmethacrylate) in a blend with naturalrubber, whilst examples of the latter are self evident.

If studies of oxidative degradation are being made it is essential thata suitable control sample be analysed concurrently. This is not alwayseasy to find by inspection and it is advisable to take a number ofsamples progressively deeper into the bulk of the material being

analysed so that the l3ase' level may be determined with a degree ofreliability. Smith et al. (1972) claimed to have used a commercialelemental analyser in the oxygen mode to measure the oxygen contentsof petroleum products at levels below 1% whilst Loadman and Oliver(1996) took core samples from a large engineering bearing andmeasured the progressive decrease in oxygen level in a series of 2mmslivers taken progressively in from the outer face of the core. Triplicateresults from each sliver were consistent in showing the measuredoxygen content to have a scatter less than 0.1% in the range 3% to the'bulk' level of about 1.5%.

Chambers (1948) illustrated the changes in the oxygen content ofnatural rubber brought about by drying exhaustively, solvent extractingand milling for various periods of time. He reported that no specialprecautions were observed in selecting the samples and would thussupport the general contention that rubber is normally homogeneous,with regard to oxygen content, at the l-5mg level unless there arespecific reasons for it not being so.

CHLORINE AND BROMINE

There is a considerable range of polymers which contain chlorine, andalthough specific chemical tests have been developed for certain types(Wake, Tidd and Loadman, 1983), they suffer from two major disadvan-tages. The first is that many use potentially dangerous chemicals, asviewed from a modern 'health and safety' perspective, whilst thesecond is that with the ever-increasing number and types of chemicalsbeing used by the rubber industry, the possibility of interferencesbecomes ever greater.

The quantitative estimation of chlorine is thus a fundamental part ofany analysis involving a chlorinated polymer such as polychloroprene,polyvinyl chloride or chlorosulphonated polyethylene and has evenmore significance when these materials are blended with polyolefinrubbers. Thermogravimetric analysis of all chlorinated polymers canproduce a carbonaceous residue and, in the presence of a polyolefin, thethermally liberated hydrogen chloride will attack the olefinic doublebonds, distorting both the TGA data and any IR spectrum obtainedfrom the pyrolysate.

Bromine is normally only found in bromobutyl rubber and its levelwill be low, about 2% of the polymer loading. The methods describedfor estimating chlorine are equally applicable to bromine.

INSTRUMENTAL METHODS OF ANALYSIS

X-ray fluorescence analysis is an ideal, although relatively expensive,technique for providing quick quantitative elemental data on a range of

elements present in a rubber compound or vulcanizate. An analysis canbe carried out, typically for sulphur, zinc or halogen, within the spaceof 1-2 minutes and this is ideal for repetitive analyses such as qualitycontrol. The one potential difficulty which can arise in practice is thequality and cleanliness of the actual surface since any contamination orirregularities can result in considerable scatter of results. Calibrationstandards must be prepared and the sample preparation must mirrorthe procedure used to obtain the analytical surface.

A scanning electron microscope (SEM) fitted with an energy disper-sive X-ray spectrometer as described in Chapter 10 can also providevery quick data which can only be considered semi-quantitative.Wavelength dispersive detectors are also available and these can beconsidered quantitative although they are difficult and timeconsuming to operate. Obviously, the cost of both these spectro-meters, coupled to an SEM, precludes them from consideration assimple analytical tools but, if they are available for other purposeswithin a laboratory complex, their suitability for a particular analysisshould be considered.

CONVERSION OF 'ORGANIC CHLORINE TO CHLORIDE

Furnace tube combustion for the determination of sulphur and halogensis still used, but is being increasingly superseded by the oxygen flaskcombustion technique, largely because of the much greater speed of thelatter. The reader interested in pursuing the furnace tube details isreferred to earlier editions of this text or the work of Bobanski andSucharda (1936), Phillips (1949) and Stern and Hinson (1953).

The most popular procedure today for determination of chlorine andsulphur in organic compounds is via oxygen flask combustion. Thisprocedure relies on the simple ignition of the sample in a flask filledwith oxygen; chlorine and other halogens are quantitatively convertedto the corresponding halide which can then be estimated by any conve-nient method.

The apparatus is illustrated in Fig. 6.1 and shows a conical flaskfitted with a stopper through which platinum electrodes pass, one ofwhich terminates in a loop into which a platinum gauze cup fits.Flasks of various dimensions and shapes have been proposed (Hempel,1892; Mikl and Pech, 1953; Schoniger, 1955), but a typical capacity,based on a polymer containing about 2mg of chlorine, would be 500-1000cm3 whilst the absorbing solution would consist of 1-5 cm3 of0.05 M potassium hydroxide solution, 0.2cm3 of 30% hydrogenperoxide and 10cm3 of distilled water. The flask is thoroughly flushedwith oxygen after which the sample, wrapped in tissue paper or lenstissue, is placed in the platinum cup and the absorbing solution added

Figure 6.1 Oxygen combustion flask.

prior to combustion by the application of a high voltage across theelectrodes. After the combustion the flask is shaken vigorously andallowed to stand for an hour before titration. The solution should beinspected to confirm that a 'clean burn' has occurred and if there areany black particles to be seen the analysis must be aborted. Olderpublished papers differ as to the reliability of this method for halogenanalysis, Haslam and Willis (1965) regarding it as semi-quantitative(although interestingly Haslam et al (1972) claim it to be a veryaccurate method for determining the polyvinyl chloride content of ablend of this with polytetrafluoro-1-ethylene) and the French having aNational Standard on the technique. Childs et al. (1963) found thatsome bromine was produced during the oxygen flask combustion ofbromo compounds and he used hydrazine sulphate to reduce thebromine to bromide prior to its estimation.

Experience in the author's laboratory would indicate that, provided amaximum 20 mg of halogenated polymer is burnt in a single combus-

tion, incomplete combustion is quite rare. Equally, there has been noevidence for the presence of bromine or any halogen oxy-acid beingformed during the combustion and the procedure is now a standardmethod within both ISO and ASTM. The addition of hydrogen peroxideis not strictly necessary for the analysis of halogens in polymers.However, its use is still recommended for the combustion of vulcani-zates as it ensures that any sulphur present in the sample is quantita-tively converted to sulphate thus preventing interference betweenchloride and sulphite if ion chromatography is used for the finalchlorine estimation.

DETERMINATION OF CHLORIDE AND BROMIDE

Titrimetric and gravimetric methods are still useful for relatively highlevels of halogen in polymers. Indeed with great care even quite lowlevels have been routinely measured. The trend nowadays, however, istowards more instrumental techniques, including chlorine, bromine andfluorine selective electrodes as well as completely general instrumentssuch as ion chromatography. Although ion selective electrodes can givevery reliable results from solutions where the constituents are known,they can give rise to problems where the solutions are less well charac-terized since all ion selective electrodes suffer from interferences wherenon-target species react with the electrodes.

Oxygen flask combustion, as described above for the determination ofchlorine alone, is able to produce solutions of any of the halogens eitheralone or in combination. The estimation of the halogens in the solutionfrom oxygen flask combustion is commonly carried out by ion chroma-tography which has the particular advantage of having a large dynamicrange (it can quantify both high and low concentrations without succes-sive dilutions) whilst it is also able to measure low levels of onehalogen in the presence of a large excess of another. Titrimetric proce-dures are unable to deal with much greater than a 10:1 excess of onehalogen over another, and ion specific electrodes are not particularlyspecific for individual halogens in a mixed solution.

The uses of ion chromatography, together with its principles of opera-tion, are discussed in more detail at the end of this chapter.

FLUORINE

The determination of fluorine in polymeric materials is of constantinterest. There are two possible approaches, the burning of thecompound in oxygen and its destruction by oxidative fusion. Thecombustion method is discussed by Freier and co-workers (1955) whoworked mainly with highly fluorinated liquids but who also report a

successful combustion with polytetrafluoroethylene. The combustionwas carried out in a fused quartz combustion tube containing platinumcontacts and quartz chippings. The latter participate in the reactionwhich may be represented as

(C2F4)+ SiO2+ O2 -> SiF4+ 2CO2

The silicon tetrafluoride is absorbed in water wherein it is hydrolyzed,producing hydrofluoric acid which can be titrated directly usingphenolphthalein as indicator.

Haslam and co-authors (1972) distinguish between fluorinatedpolymers with less than 20% by weight fluorine and those with more.In the case of the former, combustion by the oxygen flask methodfollowing the works of Willard and Horton (1950) and Gel'man andKiparenko (1965) has been shown to give excellent results.

The mixtures to be ignited were prepared from the samples (10-20 mg), together with polyethylene foam (25 mg), wrapped in filterpaper (25 mg) impregnated with potassium nitrate. After combustionand absorption in 5cm3 of water the fluoride was titrated againstthorium nitrate solution. Light and Mannion (1969), however, claimedthat the oxygen flask gives good results with polytetrafluoroethylene(contrary to the experience of Haslam). They used a polycarbonatecombustion flask and dodecyl alcohol as a combustion aid.

Fluoride can also be determined by oxygen flask combustion using aquartz flask followed by quantification with a fluoride electrode (Oliver,1996). Details of the procedure, which needs to be very carefullycontrolled if precise results are to be obtained, are not, unfortunately,published.

The fusion method is typified by the use of the Parr bomb (seeChapter 10). A sample of about 0.2 g is mixed with 10 g of potassium asan accelerator and placed in the bomb. The charge of sodium peroxideappropriate to the bomb size is then added followed by immediatesealing and firing. The fluorine is recovered as a soluble fluoride andcan either be determined gravimetrically as calcium fluoride, or titrime-trically, using eerie nitrate. Ma and Gwirtsman (1957) describe a micromethod using the Parr micro-bomb whilst Schroder and Waurick (1960)chose fusion with metallic sodium.

Haslam and Whettem (1952) preferred to use an electrically heatedbomb, again charged with sodium peroxide but with starch as accel-erator, to carry out the combustion, and a titration using alizarin red asindicator to determine the resultant fluoride loading. It is probable thatthis, and all titrimetric methods, could be improved by the use of afluoride-specific ion selective electrode, as described by Light andMannion (1969).

SILICONThe occurrence of this element in a rubber, or rubber-like material,could be due to its presence either in the elastomeric phase, as a siliconerubber or oil, or in the filler as a silicate. The distinction between thesilicone (either rubber or oil) and the silicate is easily made by pyrolysis,followed by infrared spectroscopic examination of the pyrolysate(Chapter 7). The analysis of inorganic ashes is covered in Chapter 10 sohere we are concerned with the organosilicones.

The major problem confronting the analyst is that of destroying theorganosilicone material without losing volatile silicon-containingfragments, or producing the extremely stable silicon carbide. On boththese counts dry ashing is to be avoided although the oxygen flaskmethod has been used by the Schwarzkopfs (1969) who added concen-trated sulphuric acid to the flask to dehydrate silicic acid and weighedthe silica thus obtained.

Smith (1960) described a wet digestion procedure, using a mixture offuming nitric and sulphuric acids, with a direct weighing of the residueas silica. However, as rubber products typically contain insoluble non-silicaceous fillers, such as titanium dioxide, a subsequent treatment withhydrofluoric acid and measurement of the weight loss is the preferredprocedure. The problem of removing non-silicaceous fillers was alsoconsidered by Shcherbacheva (1957) who used carbonate fusion, after asulphuric acid digestion, to solubilize all but silicic acid.

It is worth noting here that titanium dioxide is often added to asilicate-filled rubber product, or one made of silicone rubber, as a'brightener' and it should be recognised that simple hydrofluoric acidtreatment of mixed silica and titanium dioxide does not necessarily givethe correct quantitative result for the silica content. The use of hydro-fluoric acid to remove silica requires also the addition of sulphuric acidto promote loss of silicon as silicon tetrafluoride by suppressing theformation of oxyacids. Whilst titanium dioxide does not convert to thesulphate on treatment with sulphuric acid in the absence of hydrofluoricacid, in its presence the conversion does occur via the intermediatetetrafluoride and, in consequence, there will be a weight increase due toconversion of titanium dioxide to titanium sulphate.

The problem can also be compounded by the presence of calcinedsilicates in which the metals are equally reluctant to produce sulphatesuntil first released from the silicate matrix by hydrofluoric acid. Theseweight gains can make assessing the level of silicone or silicate inrubber products extremely difficult and may necessitate a full elementalanalysis so that corrections for all the weight gains can be made, withthe inevitable accumulation of errors, before the silica content is calcu-lated.

Fusion in metal bombs has been advocated by Smith (1960) as thebest way of destroying the organic part of the molecule; he preferredsodium as did Debal (1972) although Wetters and Smith (1969)advocated potassium hydroxide. The silicon in the fusion mixture maythen be determined gravimetrically by complexing with molybdic acidand treating with 8-hydroxyquinoline (oxine) to give the three compo-nent complex which is filtered and ignited at 50O0C to afford SiO2.12MoO3.

The silicon released into solution from the fusion mixture may beestimated titrimetrically by adding a known excess of oxine to the silico-molybdate complex and, after dilution to a known volume, and filtra-tion, titration of an aliquot against standard bromide-bromate solution.Both techniques are described by McHard et al. (1948) who point outthat an empirical standardization of the bromide-bromate solutionshould be made, using a pure organosilicone, to compensate for a slightdeviation from the theoretical factor.

A further titrimetric method has been described by Bartusek (1973)whilst Debal (1972) reports a colorimetric method based on the silico-molybdate complex.

Silicone rubber and silicone oils can be conveniently converted tosilica by heating in a pressure vessel at 100-1150C with concen-trated nitric acid. The organic part of the molecule is destroyed andas the system is fully enclosed during the reaction there is no possi-bility of loss of volatile silicon compounds. The silica produced canthen be determined either gravimetrically or colorimetrically. Theacidic solution can also be used to determine the levels of cationssuch as magnesium, calcium, aluminium and potassium and hencethe particular silicate used in the formulation can be completelycategorized.

PHOSPHORUSPhosphorus occurs in natural rubber latex as free ort/iophosphate, sugarphosphates and phospholipids. It is also added to the latex, as diammo-nium phosphate, to precipitate magnesium phosphate from certainlatices which have a high magnesium content, prior to centrifugation,and thus improve the stability of that latex. Unfortunately, if too muchphosphate is added the stability will decrease again; thus in any situa-tion where the stability of a natural rubber latex is suspect, thephosphorus content should be determined.

In order to be certain of analysing for the total phosphorus content adried film should first be prepared from the latex, care being taken toadhere to the sampling procedures discussed in Chapter 2, and detailedin ISO 1231.

The non-instrumental analysis of a 'dry7 rubber sample forphosphorus content can be divided into two parts; first, the removal ofthe polymer and the production of a solution of phosphate ions, andsecond the development of a colour which can be measured spectropho-tometrically and the intensity of which is proportional to the concentra-tion of phosphorus.

The first stage may use any of the three procedures described above -Kjeldahl acid digestion, oxygen flask combustion, or fusion in a sealedbomb. In all cases it is necessary to boil the derived aqueous solutionfor a few minutes prior to the development of the colour to ensure thatthe phosphorus is present in the final solution as orthophosphate. If theoxygen flask combustion method is to be used, it should be remem-bered that phosphorus is one of the elements which tends to combinewith platinum. There is a potential for the loss of phosphorus ontoplatinum sample holders and one way of avoiding this is to replace thebasket with a quartz spiral to carry the sample, as described by Corner(1959).

The fusion procedure (using sodium peroxide) for the determinationof phosphorus in organic compounds has been described by Fennell etal (1957) and Christopher et al. (1964) who found no problems with thetechnique although they did comment that the sodium peroxide theyused had a significant phosphorus content.

The author's experience is that the acid digestion procedure is theone of choice and this is described in detail below. Two colorimetricmethods are available: one, generating a yellow colour, due to avanadiphosphomolybdate complex, is preferred for milligram quanti-ties of phosphorus whilst the other, which produces a blue or blue/green colour (molybdenum blue), is more sensitive and used fordeterminations in the microgram range. There are marked differencesbetween the two methods: the yellow colour is quite stable andrelatively insensitive to slight variation in experimental procedurewhilst the molybdenum blue colour is extremely sensitive to smallvariations in pH, concentration, temperature, light, etc., and indeedspectrophotometric examination of the colours produced underdifferent conditions has shown maxima between 650 and 900 nm(Ma and Rittner, 1979).

It is thus essential that a standard, reproducible procedure is used todevelop the colour, and that calibration standards, prepared from stocksolutions of (say) potassium dihydrogen phosphate are run concurrentlyunder these conditions. Unfortunately the rubber analyst is normallyconcerned with the measurement of relatively low levels of phosphorusin rubber and will tend to use the molybdenum blue method. For thisreason the following procedure, used in the author's laboratory, is givenin detail.

PHOSPHORUS DETERMINATION IN RUBBER (COURTESY OF TARRC)

A weight of sample (0.1-0.5 g) such that the phosphorus content isless than 100 jig is placed in a micro Kjeldahl flask (10cm3 capacity),2 cm3 of concentrated sulphuric acid added, and the solution warmeduntil charring just begins. Portions of concentrated nitric acid(0.2cm3) are added, the solution being heated after each additionuntil reaction has ceased and then cooled, until a total of 10cm3 hasbeen used. A final addition of 0.25 cm3 is made after which heating iscontinued until there is no further reaction. After cooling, distilledwater (10cm3) is added carefully and the solution heated gently toboiling. It is boiled until acid fumes are observed at the mouth of theflask. This is repeated after the addition of a further 5 cm3 of distilledwater. The solution should now be colourless; if not 60% perchloricacid (0.5cm3) is added and the solution gently heated further, takingparticular care and using a safety screen. The final clear solution isdiluted to 50cm3 with distilled water in a volumetric flask.

A suitable aliquot (initially 5cm3) is neutralized to Congo Redpaper with concentrated ammonia solution and transferred to a50 cm3 flask, distilled water being used to dilute the solution to about25 cm3. Ammonium molybdate solution (see end of method) (5 cm3) isadded, the solution shaken and the reducing solution (see end ofmethod) (5cm3) added. The flask is then placed in a boiling waterbath for 30 minutes, ensuring that the solution is below the waterline. It is then cooled to ambient temperature and the final dilution to50 cm3 made in the volumetric flask.

The absorbance of this solution is then measured at 700 nm againsta 'blank' solution prepared by taking all the reagents (with the excep-tion of the rubber) through the complete procedure. The phosphoruscontent is calculated by reading from a calibration graph, preparedby measurement of the absorbencies of a series of standard phosphatesolutions which have had their colours developed concurrently.• Ammonium molybdate solution: a solution of 1Og ammonium

molybdate in 100 cm3 distilled water is poured slowly into a cooledsolution of 300cm3 50% aqueous sulphuric acid. This solution isstored in the dark.

• Reducing solution: sodium metabisulphite (4Og), sodium sulphite(1 g) and Metol (0.2 g) are dissolved in 100 cm3 distilled water. Thissolution has a shelf life of no more than one week and ideallyshould be freshly prepared for each analysis. It is emphasized thatmany other procedures are published for developing the molyb-denum blue colour and these are equally valid but could well givean absorbance maximum at a wavelength different from the700 nm found for this one.

A very convenient and substantially less time consuming procedureuses pressure bomb digestion of the rubber sample (0.2-0.25 g) withconcentrated nitric acid (2cm3) overnight at 10O0C. The resultingsolution is diluted with deionised water to a convenient volume and thephosphorus measured by inductively coupled plasma-atomic emissionspectroscopy (ICP-AES). This has the dynamic range capability (andlinearity) to measure phosphorus concentrations from Img/kg to atleast lOOOmg/kg. As with other techniques which are capable ofmeasuring several components within a single analytical run, the use ofICP-AES is particularly advantageous when other elements need to bedetermined for other purposes. Fuller details of ICP-AES are discussedin Chapter 10.

SULPHURThe reasons for carrying out sulphur analyses are diverse, as indeed arethe types of sulphur which the analyst may need to quantify. Estima-tions may need to be carried out for the determination of the polymerpresent, as with thioplast rubbers or chlorosulphonated poly-ethylene,but the commonest use is as part of the analysis of a sulphur vulcanizedrubber.

Much work has been carried out over the years by various authors,including Bateman et al (1963), Moore (1964), Craig (1957), Campbelland Wise (1964), and Scheele (1961), whilst they were studying themechanism of vulcanization and, as a result, not only is the analysis ofa rubber for its total sulphur content required, but so too is the analysisfor free (elemental) sulphur, and for sulphide sulphur. The level of freesulphur in a vulcanizate gives an indication of undercure, while thelevel of sulphide sulphur can give an indication of overcure. It may alsobe advantageous to determine the level of sulphur which is intrinsic tothe added carbon black and which, for the purposes of vulcanizationchemistry, can be considered inert since it can be recovered, unused,after vulcanization.

DETERMINATION OF TOTAL (OR COMBINED) SULPHUR

The determination of total and combined sulphur is carried out by thesame procedure, the only difference being that combined sulphur is thetotal sulphur remaining after solvent extraction.

Methods of determining the total sulphur content of an organicmaterial have a long history of change and development, during whichnumerous methods have been proposed and adopted only to pass outof fashion after a few years. Johnson and Messenger (1933) give a verycomplete historical review starting at the original work of Henriques

(1892) and Weber (1894) whilst methods for determining the totalsulphur content of rubbers have been reviewed by Auler (196Ia).

The method adopted must depend on what is needed from theanalysis. Some products will contain sulphides or sulphates of zinc,barium or calcium, and it must be decided in advance whether theanalytical method to be chosen is to include any of these.

Methods available include combustion, both furnace tube and oxygenflask, oxidative fusion and X-ray fluorescence. Wet oxidation, popularfor many years, has now been phased out of both British and Interna-tional Standards.

The major advantage of the furnace tube method is its ability todetermine some insoluble inorganic sulphates, such as barytes, by usingcombustion aids. This is not possible with oxygen flask methods but,conversely, the oxygen flask, especially when combined with ionchromatography, is immune to the problem of zinc in the reactionsolution which may block the indicator used for titration. Crafts andDavy (1989) have shown that rubber samples containing as little as0.5% chlorine will give titrimetric results consistently low by about 10%for sulphur, solely due to the volatility of zinc chloride and the effectthat zinc has on the titration indicator. The use of ion chromatographycompletely avoids this problem whilst allowing the chlorine level to beestimated simultaneously.

It is also possible to determine sulphur via the pressure digestionprocedure with nitric acid, and this will allow phosphorus and otherelements to be determined simultaneously. Thus the choice betweenoxygen flask and bomb digestion may well depend on the additionalelements which need to determined.

Furnace tube combustion methodCombustion of the test portion in a stream of oxygen or air followed byabsorption of the products is an elegant method in the tradition ofclassical organic analysis. This method was used by Eaton and Day asearly as 1917 but, even then, they emphasized the need to limit itsapplication to certain classes of compound. The difficulty lay in therange of decomposition temperatures covered by the various substanceswhich could be present. The temperature of combustion needs to besufficiently high to decompose all the organic material present whilst,ideally, not being high enough to cause decomposition or volatilizationof any inorganic sulphur compounds.

The principal causes of trouble are the presence of lithopone, which isa mixture of barium sulphate and zinc sulphide, of barium sulphate asbarytes and of calcium sulphate. The formation of sulphides andsulphates from zinc oxide, which is virtually always present, and

calcium carbonate or magnesium carbonate which may be present is apossibility if too low a furnace temperature is chosen and, since zincsulphate decomposes at 85O0C, a minimum furnace temperature ofabout 950 0C is recommended.

A thorough study and testing of a modified combustion method wascarried out many years ago in various laboratories of the DunlopRubber Co. Ltd, and the method finally published from the DunlopResearch Centre has probably been more thoroughly tried out underroutine conditions than other analytical method. The method is a semi-micro one derived from the Grate procedure. Since it was first described(BS 903-1950) it has been modified (Bauminger, 1955, 1956a, b), writtenup as an International Standard (ISO 6528.3-1988) and, as describedbelow, can be used to determine total or unextracted sulphur in fullycompounded natural and synthetic rubbers including those containingchlorine and/or nitrogen. It is relatively fast, with results being avail-able within an hour and the operating procedure is simple.

Simple combustion in oxygen requires a temperature of 1350 0C if allthe sulphur likely to be present in rubber compounds is to be included,but the use of a mixture of vanadium pentoxide and zinc oxide(Bauminger, 1955) enables quantitative recovery of all forms of sulphurby combustion at 1000 0C. The combustion tube must be of translucentor transparent silica and the furnace should be capable of beingcontrolled to +20 0C.

The following procedure is set out in detail in ISO 6528.3-1988, detailsof the apparatus required being shown in Fig. 6.2:

The rubber sample (10-50 mg) is mixed with 1 g of catalyst consistingof 0.8 g dry vanadium pentoxide and 0.2 g zinc oxide, and burned ina stream of oxygen at a temperature of 1000 0C. Combustion productsare absorbed by hydrogen peroxide solution, 3 cm3 of 30% hydrogenperoxide being mixed with 30 cm3 water, 15 cm3 of this being addedto the main absorption vessel and 5 cm3 to the other. If nitrogen andhalogen are known to be absent (the trivial nitrogen content ofnatural rubber can be ignored), then the sulphuric acid can be deter-mined by titration with standard 0.02 M sodium hydroxide solutionusing a methyl red/methylene blue indicator. In all other cases abarium perchlorate titration is used. Small amounts of zinc chloridemay distil over from rubbers containing chlorine, and will interferewith the barium perchlorate titration. This interference may beremoved by passing the combined absorbing solutions slowlythrough a short ion exchange column. Sufficient propan-2-ol is addedto the absorbent solution to bring it to 70-90% alcohol by volume. Afew drops of Thorin indicator solution is added, followed by suffi-cient methylene blue solution to change the colour from orange to

Figure 6.2 Apparatus for the determination of total sulphur by the furnace tube method.

Flow meterPurifying train

Silica combustion tube

Magnetic block 'Combustion furnace

Silica rod Combustion boat

Absorbing vesselsNeedle valve

Calcium chloride tube

yellow. The solution is then titrated to a permanent pink colour using0.01 M barium perchlorate in a mixture of 80/20 (v/v) propan-2-oland water.

Oxygen flask combustion methodThe oxygen flask combustion technique was described in detail abovein relation to its use for the determination of chlorine and bromine. Theapparatus has the advantage of being less costly than that for thefurnace tube combustion method. A single determination takes longerthan does an analysis in the furnace tube but if multiple analyses arerequired then the oxygen flask method is much more rapid since abatchwise procedure can be used, a facility not available with thefurnace tube method.

Another feature of the oxygen flask technique is that, by definition,combustion takes place in an atmosphere of oxygen, but it is also in theabsence of catalyst. This means that any stable sulphur containing fillerssuch as lithopone or barytes will not be decomposed, and their sulphurcontents will not be included in the determination. Any zinc sulphateproduced, either directly by reaction with the sulphuric acid from theburning of the sulphur with zinc oxide, or indirectly by oxidation ofzinc sulphide, will be determined since zinc sulphate is very watersoluble and will easily dissolve during the sample work-up. Calciumsulphate is not completely insoluble and at the amounts likely to bepresent, bearing in mind that the total sample is likely to be approxi-mately 30 mg, will normally be fully dissolved in the 10ml of absorbent.This may of course be an advantage in that the rubber analyst isprimarily interested in vulcanization-derived sulphur. Similar non-detection by the combustion tube method of sulphur present ininorganic compounds may be achieved by omission of the catalystduring the combustion stage.

The procedure to be followed is spelled out in detail in ISO 6528.1-1992 (BS 7164 Sect. 23.1-1993): 20-40 mg of the finely milled test portionis wrapped in paper and placed in the combustion flask which is thenfilled with oxygen and sealed. The sample and paper are ignited, andany sulphuric acid produced during the combustion is absorbed inhydrogen peroxide solution contained in the bottom of the combustionflask.

If determination is to be by titrimetry, the method used for the deter-mination of the sulphuric acid formed is precisely the same as thatdescribed above for the furnace tube combustion method. Interferingzinc ions must in this case be removed, whether or not halogen ispresent, by passage of the absorbent solution through a short ionexchange column prior to carrying out the titration.

Table 6.1 Total sulphur determinations

Sample Absorbing solution Theoretical %S Found S%

Masterbatch H2O2 2.44 2.48, 2.49Masterbatch H2O2 2.20 2.23,2.21,2.28+ 10 pts ZnOMasterbatch+ H2O2 1.22 0.54,0.63,0.07100 pts CaCO3

Masterbatch + H2O2/HCI 1.22 1.19, 1.18100 pts CaCO3

For carrying out titrimetric determinations when calcium carbonate isknown to be present, Davey (1979) found that if one uses a mixture of0.25cm3 concentrated hydrochloric acid, 2cm3 water and lcm3 6%hydrogen peroxide as the absorbing solution, any calcium sulphatedissolves and the calcium carbonate is decomposed. After passagedown an ion exchange column the sulphuric acid which remains can betitrated in the usual way. The results shown in Table 6.1 were obtainedusing this titrimetric procedure.

X-ray fluorescence method

X-ray fluorescence analysis, as mentioned earlier in the chapter, is acostly, but very powerful, technique which is capable of measuring theconcentration of a wide range of elements in a given sample. In thistechnique (Jenkins, 1974) the sample is bombarded with X-rays from anX-ray source, and this causes the sample to emit X-ray radiation of alower energy (i.e fluorescence). The energies and intensities of theemitted X-rays are measured, and used to provide a rapid and accurateidentification and estimation of the elements in the sample.

Such instruments are not commonly available to many rubberanalysts. However, there are on the market small X-ray fluorescenceanalysers which have radioactive elements to excite the fluorescence ofthe elements in the sample. In place of the expensive dispersive analy-sers used for large installations, these analysers use a series of filters toisolate the X-ray fluorescence of the element of interest, and can providea direct readout of the percentage sulphur content of a sample in underone minute.

Samples which are thermoplastic, including unvulcanized compound,are best prepared as discs of 2-5 cm diameter by hot pressing against(for example) cellophane, which can be peeled off prior to the analysis.It must be remembered that this technique is relatively surface depen-

dent so that surface contamination must be kept to an absoluteminimum. Surface texture is also important. This does not causeproblems when samples can be hot pressed; however, for vulcanizatesthis is not possible, and a standardized method of surface treatment isessential. Davey and Loadman (1977) found that a suitable method ofsample preparation for cured products was to cut a plug of rubber witha cork borer. The plug end is then cut as smoothly as possible with arazor blade and smoothed with fine emery cloth. The ground surface iswiped clean with cotton wool dampened with methanol. Provided thatthe standards with which the unknown is compared are prepared in thesame way very accurate results are obtained with a 50 second measure-ment time; a series of ten different analyses gave a sulphur content of1.263% with a standard deviation of 0.013%.

Choice of a suitable radioactive element source, and use of the appro-priate filters, enables the instrument to be applied to the analysis ofother elements, such as zinc or halogen. Although relatively expensive,the speed and ease of operation, without even needing to weigh thesample, makes this an ideal instrument for quality control monitoring,particularly of compounded rubber mixes. However, it should be notedthat there is a need for careful matrix matching between the samplesand standards and this is a severe limitation in the application of thetechnique to samples of unknown composition.

INTRINSIC (INACTIVE) SULPHUR IN CARBON BLACK

The sulphur content of carbon black can vary from a few parts permillion to a percent or more according to the method of manufactureand the feedstock used (Studebaker, 1957) and this can severely influ-ence any calculation carried out to reconstruct the original formulation.Davey (1989) showed how a range of carbon blacks recovered fromdifferent types of vulcanizates by controlled pyrolysis below 600 0C,with subsequent acid leaching to remove soluble inorganic fillers,retained their original sulphur levels. The work was carried out usingconventional and efficient NR formulations but there is no reason tosuppose that the data are not equally valid for other polymers. Theresults are also of interest in indicating that the intrinsic sulphur presentin carbon black does not play any part in the curing of the vulcanizateand can truly be considered inactive. The determinations were carriedout using the furnace tube combustion procedure described above, andthe data are tabulated in Table 6.2.

DETERMINATION OF FREE (ELEMENTAL) SULPHUR

Most rubber mixes contain elemental sulphur and a sulphur-containingaccelerator. During vulcanization the sulphur gradually becomes

Table 6.2 Sulphur determinations on recovered carbon blacks*

Black No. intrinsic Measured sulphur % on recovered blackssulphur %(raw black) Pyrolysed Conventional Efficient

raw black cure cure

1 0.90 0.88 0.88 0.882 1.13 1.08 1.04 1.093 1.57 1.55 1.57 1.624 0.61 0.57 0.72 0.655 0.75 0.74 0.80 0.88

*mean of duplicate values (Courtesy Rubber Research Institute of Malaysia)

combined with the rubber, as too does some of the accelerator (Moore,1964). Determination of the level of free sulphur in a raw mix is impor-tant for checking whether compounding has been carried out correctly.Of equal, if not more, importance is the determination of free sulphur ina cured product, since any appreciable level found must indicate thatthe product is undercured. Such a finding would explain deficiencies inphysical properties in the product. In addition, vulcanizates maycontain higher levels of free sulphur than the rubber can dissolve, theresult being an unsightly or even harmful bloom of sulphur.Methods for the determination of free sulphur in rubber have beenreviewed by Auler (196Ib), and several methods were published in BS903 Part B7. The nitric acid and bromine methods of the latter are nowlittle used, being replaced by the sulphite method (Mackay and Avons,1940) whilst the copper spiral method (Hardmann and Barbehenn, 1935)is retained as ISO7269-1995 (BS7164 Sect. 24-1996). This remains the bestof the methods, being subject to fewer interferences although, in theabsence of sulphur donor accelerators, the sulphite method is ofcomparable accuracy.

Copper spiral method

Full details of this procedure are given in ISO7269-1995 (BS7164 Sect.24-1996).

A thinly sheeted or finely chopped sample of rubber (0.5 g) is acetoneextracted in a Soxhlet extractor, with a coiled piece of clean coppergauze in the flask containing the boiling acetone. Sulphur extractedfrom the rubber reacts with the copper to form a black layer ofcopper sulphide. The acetone in the extraction flask is filtered off, andthe copper spiral(s) washed with hot acetone. The extraction flask,

now containing the copper spiral(s), filter funnel and filter, isassembled into the apparatus shown in Figure 6.3. 50cm3 of dilutehydrochloric acid is added slowly to the extraction flask through thefunnel, and the flask allowed to stand at room temperature for 5minutes. The solution is brought slowly to the boil, and boiled for30-40 minutes. Any hydrogen sulphide formed is swept by a streamof nitrogen into the absorption flask which contains bufferedcadmium acetate solution as also do the gas washing bottles. Thehydrogen sulphide is trapped as a quantitative precipitate ofcadmium sulphide. Excess standard iodine solution (0.025 M) isadded, and the wash bottle swirled gently until all the cadmiumsulphide has been dissolved. Residual iodine is then back-titratedwith standard (0.05 M) sodium thiosulphate solution.

Interferences with this method have been shown by Davey (1981) to berelatively few (Table 6.3) but since the method depends upon an extrac-tion stage, low results will be obtained if raw mixes have been preparedusing insoluble sulphur unless prolonged (up to 72 hours) extractionperiods are used. After cure, however, any sulphur remaining will beextractable. For uncured mixes, care should be taken to minimizeheating, since this can cause curing to take place during the initialstages of the extraction. Later in the extraction, accelerators will havebeen removed and heating is unlikely to be harmful.

Sulphite methodApproximately 2g of finely divided or thinly sheeted sample isplaced in a conical flask and 100cm3 of 0.05 M sodium sulphitesolution, together with 3-5 cm3 of liquid paraffin, to minimizefrothing, added. The mouth of the flask is covered with a watchglass, and the contents of the flask are boiled gently for 4 hours.During this period, sulphur reacts with the sodium sulphite to formsodium thiosulphate. After cooling, 5g activated charcoal is addedand the flask allowed to stand for 30 minutes during which timeaccelerator residues are adsorbed on the charcoal. Insoluble residuesare removed by filtration, and to the filtrate is added 10cm3 offormaldehyde solution (400 g/L) to complex with the excess sodiumsulphite. After standing for 5 minutes, 5cm3 glacial acetic acid isadded, and the sodium thiosulphate formed is reacted with excess0.025 M iodine solution. Excess iodine is back-titrated with 0.05 Msodium thiosulphate solution using starch as indicator.

Alternatively the thiosulphate can be determined directly in the reactionsolution, without any pretreatment, by using ion chromatography. This

3mm boreFigure 6.3 Apparatus for determination of free sulphur by the copper spiralmethod (Courtesy BSI).

A!5OmI

capacity

B 34 Joint

B14 Joint

Nitrogen

2mm bore

B 24Joint

capacity

B IO Joint (or spherical joint ifrequired)

B 14 Joint

B 14 Joint

Table 6.3 Effect of various commercial accelerators on the copper spiral and sulphite methods of analysis

Apparent% free S(sulphitewithout

charcoal)

30.333.123.823.78.9

20.567.426.517.647.0

5.215.913.1

Apparent% free S(sulphitemethod)

26.420.021.020.0

3.538.616.633.321.4

5.17.60.7

12.411.2

Apparent% free S

(copper Spiralmethod)

22.520.010.36.14.61.40.9

<0.1<0.1<0.1<0.1<0.1<0.1<0.1

Total S%found

58.245.820.634.939.445.824.842.040.231.932.024.836.936.8

Total S%(Theory)

57.150.027.133.842.253.325.842.241.933.231.424.238.438.5

\bbreviation

DPTHDPTDDTDM

MBS/MORZIX

TMTD

TMTMZDMCZMBTETUCBSMBT

MBTS

Chemical name /

Dipentamethylene thiuram hexasulphideDipentamethylene thiuram tetrasulphide4,4'-Dithiodimorpholine2-(morpholino dithio)benzthiazyl sulphenamideZinc isopropyl xanthateTetra methyl thiuram disulphideThiocarbamyl sulphenamideTetra methyl thiuram monosulphideZinc dimethyl dithiocarbamateZinc 2-mercaptobenzothiazoleEthyl thioureaN-cyc/ohexyl-2-benzothiazyl sulphenamide2-Mercaptobenzothiazole2,2'-Dibenzothiazyl disulphide

eliminates the quite lengthy work-up and results in the sulphite methodtaking roughly the same time as the copper spiral method to complete.

As can be seen from work due to Davey (1981) presented in Table6.3, many accelerators give significant interference in this method, withsubstantial proportions of those accelerators acting as though they werefree sulphur. Fortunately accelerators tend to be used at low percentagelevels, but the level of interference is often so large that the analysis ofunvulcanized mixes should not be undertaken by the bisulphitemethod. In addition to the interferences shown by accelerators, Davey(1981) has shown that other classes of compounds can also interferewith the sulphite method. Thus the antioxidant Santoflex 13 reacts asthough 11% of it were free sulphur, and the peptizer Renacit VII reactsas though 32% of it were free sulphur. Neither compound interfereswith the copper spiral method.

Although the sulphite method is less robust than the copper spiralmethod because of its susceptibility to interferences, it can, with ionchromatographic thiosulphate determination, measure free sulphur atthe ppm level. The copper spiral method is limited to levels of freesulphur above 0.03%.

Other methodsPoulton and Tarrant (1951) describe a procedure to determine thesulphur in an extract by polarography. This method is much less proneto interference by accelerators, but has never become widely used. Atthe time it was developed polarographs were cumbersome and not easyto use. Present-day commercial polarographs are much easier to useand the technique is worthy of reconsideration.

High performance liquid chromatography (Chapter 4) is a verypowerful tool for the analysis of rubber extracts. Sulphur is one suchcompounding ingredient which can be so analysed, with high sensi-tivity and with a high degree of specificity. Reverse phase chromato-graphy using an ODS2 column, gradient elution with acetonitrileiwaterand UV detection at 270 nm will provide excellent specificity and adetection limit measured in ppm.

DETERMINATION OF SULPHIDE SULPHUR

In unvulcanized but compounded rubbers the only source of sulphidesulphur will normally be metallic sulphides present in inorganic fillerssuch as lithopone. In vulcanizates, however, the situation is different.As vulcanization proceeds most of the sulphur which was initiallyadded as elemental sulphur becomes bound to the rubber network butsome reacts with the zinc oxide present to generate zinc sulphide. At

optimum cure the level of sulphur combined with the rubber reaches amaximum, decreasing on further heating and crosslink degradationreactions occur. The zinc sulphide level, however, continues to increaseduring these crosslink degradation reactions as well as during the initialvulcanization stage (Bateman et al., 1963) . Subsequent to this work, asurvey of published data has shown (Tidd, 1975) that for a substantialnumber of cure systems the ratio zinc sulphide/network combinedsulphur ranges between 0.17 and 0.21 at optimum cure. On substantialovercure, this figure can increase to as high as 0.9. At higher curetemperatures degradation reactions are, relatively, more important, andthe overall levels of zinc sulphide are higher.

On the other hand, certain cure systems, particularly those containingzinc dithiocarbamates, habitually give very low figures of around 0.02-0.03. Clearly, either the actual mechanism of vulcanization changesaccording to the accelerator being used, or zinc sulphide once formed inthe normal manner is consumed by further reaction with the vulcaniza-tion additives or their reaction products. Evidence that at least the lattertakes place was obtained by Morrison (1982) who has found that afterheating zinc sulphide with MBTS (mercapto-benzothiazyl disulphide)only low levels of zinc sulphide are detectable in the product.

The method therefore has limitations for the study of unknownsamples. However, under quality control conditions, where thecompound used is known, the technique comes into its own. If the curetemperature is held constant, then increases in the sulphide sulphurlevel indicate an increasing time of cure. Similarly if the cure time isknown, as it is in many situations, an increase in sulphide level mustreflect an increase in cure temperature.

The original method for the determination of sulphide sulphur in avulcanizate was published in BS 903 Part BlO. Davey et al (1978),however, significantly improved the procedure in a number of respects,and their method now appears in ISO 8054-1996 (BS7164 Sect. 25 -1996).

The finely chopped or thinly sheeted rubber is extracted with acetone,and the extracted rubber is dried thoroughly and treated with amixture of concentrated hydrochloric acid lcm3, water lcm3, andglacial acetic acid 5cm3, either in the apparatus described above forthe determination of free sulphur by the copper spiral method, orpreferably in the improved apparatus produced by Davey et al. (1978)(Figure 6.4).

All ground glass joints are lubricated with glycerol, and the gaswashing bottles contain buffered cadmium acetate. The glacial aceticacid/hydrochloric acid mixture (50cm3) is introduced via the funnel

Figure 6.4 Apparatus for sulphide sulphur determination. (Courtesy Plast. andRubb. Mat. and Applic.)

into the flask containing the rubber. The contents of the flask are heatedgradually to boiling, and boiling is continued for one hour. During theheating, hydrogen sulphide liberated from the sulphide sulphur isswept into the gas absorption flasks containing buffered cadmiumacetate solution by a slow stream of nitrogen. The precipitatedcadmium sulphide is determined, as detailed under free sulphur, byreaction with iodine and back-titration of the excess iodine with sodiumthiosulphate solution.

funnel

The main features highlighted by Davey et al. (1978) are:

1. The use of hydrochloric/glacial acetic acid. The organic nature of thissolvent gives more rapid penetration of the rubber by the acid, andhence a shorter reaction time.

2. Routine acetone extraction is not necessary in most cases, and evenintroduces possible errors since any acetone remaining in the samplewill distil into the cadmium acetate solution and react with iodinewhen that is added.

3. Milling of the sample should be kept to an absolute minimum sincethis treatment allows oxidation to occur and zinc sulphide to becomeprogressively depleted, although the total sulphur level remainsconstant.

4. Milling is more effective than comminution for permitting rapidingress of the acid mixture, and allows the reaction to be complete inabout 1 hour, whereas comminuted samples require up to 5 hours'refluxing.

ION CHROMATOGRAPHY (IC)Ion chromatography is a specific form of liquid chromatography and isused for the separation and quantification of both cationic and anionicspecies at levels between mg/L and ng/L. The procedure is illustratedschematically in Figure 6.5, and a representative chromatogram isshown in Figure 6.6. The technique is included in this chapter becauseof its general relevance to the quantification of chloride, bromide,fluoride, phosphate, sulphate etc. in aqueous solutions although itsrelevance to the examination of aqueous extracts of rubber, latex serumand similar applications should not be overlooked. One specific applica-tion is in the analysis of latex serum, by direct injection on to thecolumn, for both phosphate and sugar phosphates as described byCrafts (1982). After chromatographic separation using a separatorcolumn, the various ions are modified to highly conductive acids oralkalis in the suppressor column before passing to a detector. As withother forms of chromatography, identification and quantification isbased on retention time and peak area respectively.

It will be appreciated that this technique has numerous applicationsand it has the potential to affect substantial time savings in manylaboratories, as well as replacing many chemical analytical procedureswhich may well entail health and safety risk assessments and theirconcomitant controlled operating procedures. The American Society forTesting and Materials (ASTM) , ISO/BSI and the FDA have all nowpublished standardised methods for trace and high level measurementsusing this technique.

WasteFigure 6.5 Schematic representation of common ion chromatography. (Repro-duced with permission of the Dionex Corporation).

There are several modes of separation available which enable almostall ionizable molecules to be determined. High performance ion chroma-tography (HPIC) separates inorganic and low molecular divalentorganic anions, an ion interaction chromatography also known asmobile phase ion chromatography (MPIC) allows the separation of highmolecular weight anions (or cations) such as anionic surfactants, andion exclusion chromatography (ICE) is used to separate monovalent

ConductivityDetector

Suppressor

Separator

Eluent(NaOH)

Sample In

Resin

Resin

Time

Sign

al

ResinResin

ResinResin

MinutesFigure 6.6 Separation of common anions. (Reproduced with permission of theDionex Corporation).

Inje

ct

organic anions. The above categorization is only intended to indicatethe general type of separation for which the technique was developed.In practice most anions can be separated under all of the separationmodes, provided the eluents and detectors are altered appropriately.This flexibility enables almost any combination of anions to beseparated efficiently, especially when gradient elution facilities are avail-able to enhance the separations.

The above comments are directed primarily at anion analysis;however, columns are available to allow metallic elements and aminesto be separated with equal facility.

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347.Association of Official Analytical Chemists (AOAC) (1984) Official Methods of

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Physics ofRubberlike Substances, Bateman, L., ed., Maclaren, London.Bauminger, B.B. (1955) Kaut. u. Gummi Kunstst. 8, WT31.Bauminger, B.B. (1956a) Analyst 81, 12.Bauminger, B.B. (1956b) Trans. IRI 32, 21X.Bobanski, B. and Sucharda, E. (1936) Semi-micro Methods for the Elementary

Analysis of Organic Compounds, A. Gallonkamp & Co., London.Campbell, R.H. and Wise, R.W. (1964) Rubber Chem. Technol 37, 635, 650.Chambers, W.T. (1948) Paper presented at the Rubber Technology Conference, June,

London.Childs, C.E. and Henner, E.B. (1970) Microchem. J. 15, 590.Childs, C.E., Cheng, J., Meyers, E.E., Laframboise, E. and Balodis, R.B. (1963)

Microchem. J. 7, 266.Christopher, A.J., Fennell, T.R.F.W. and Webb, J.R. (1964) Talanta 11, 1323.Corner, M. (1959) Analyst 84, 41.Crafts, RC. (1982) Unpublished work at MRPRA.Crafts, RC. and Davey, J.E. (1989) Unpublished work at MRPRA.Craig, D. (1957) Rubber Chem. Technol. 30, 1291.Cruikshank, S.S. and Rush, C.A. (1962) Microchem. ]., Symp. Ser. 2, 467.Davey, J.E. (1979) Unpublished work at MRPRA.Davey, J.E. (1981) Unpublished work at MRPRA.Davey, J.E. (1989) /. Nat Rubber Res. 4, 4, 284.Davey, J.E. and Loadman, M.J.R. (1977) Unpublished work at MRPRA.Davey, J.E., Edwards, A.D. and Higgins, G.M.C. (1978) Plastics and Rubb. Mater.

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Instrumental polymer -y

analysis /

INTRODUCTION

Whilst there is no doubt that many chemical and physical tests areuseful in the analysis of an unknown vulcanizate, and it would befoolhardy to ignore some of the simpler ones such as the 'burn' test, theanalyst with instrumental facilities will find that these rapidly tend tosupersede the classical chemical methods in the routine analysis ofpolymeric materials.

The purpose of the instrumental examinations described in thischapter is to identify the particular polymer, or blend of polymers, in asample and, in the case of a blend, to quantify the component ratio.Although numerous techniques have been covered in the literature overthe past 50 years or so, the most widely used today are infrared spectro-scopy (IR), gas chromatography (GC) and nuclear magnetic resonancespectroscopy (NMR). However, there have been increasing contributionsfrom thermal techniques such as derivative thermogravimetry (DTG)and differential scanning calorimetry (DSC) whilst advances in Ramanspectroscopy are enabling this too to provide valuable information. Inaddition, the electron microscope (both scanning, transmitting andscanning-transmitting) has added substantially to an understanding ofpolymer blend morphology. It should be emphasized that this chapteris not concerned with the microstructure of the polymer, a subjectwhich will be discussed in Chapter 8, whilst Chapter 9 is dedicated tomicroscopical techniques and blend morphological studies.

INFRARED SPECTROSCOPY (IR)

The infrared spectroscopic examination of a rubber, or rubber-likematerial, can be separated into three distinct parts:1. sample preparation and presentation

2. running the spectrum3. interpretation of the spectral data.

However, before considering any of these it is pertinent to examinewhat options the analyst has in terms of the different techniquescovered by the general term 'infrared spectroscopy'.

If we firstly consider the main methods of obtaining spectra, that ofshining the infrared light through a test portion is self descriptive butrequires one to have a sufficiently thin film of material. The secondalternative, of reflecting light off the surface, together with a complexarray of methods by which this may be achieved, has become widelyused in recent years. This concept was initially proposed by Fahrenfort(1961), and is illustrated diagrammatically in Figure 7.1. This system iscorrectly described as multiple internal reflectance spectroscopy (MIR)whilst the optional name, attenuated total reflectance (ATR), whichtends to be more generally used, should really be confined to a crystalgiving only one internal reflection. The technique is based on thephenomenon of the total internal reflection of light at the interfacebetween media of differing refractive indices. In the system illustrated,the rhombohedral crystal is made of a material of high refractiveindex and when a substance is pressed hard against it a spectrum ofthat substance is obtained in which the absorption bands are closelyrelated in position to those of a conventional transmission spectrum.The major difference is that in reflectance spectroscopy, penetration ofthe infrared light is wavelength dependent, being greater at higherwavelengths (lower wave numbers), and this gives a relative increasein spectral intensity as one progresses through the spectrum from2.5 jim (4000Cm'1) to 25|im (400cm'1).

A particular advantage of MIR is in examining thin films of elasto-meric materials or packaging, as described initially by Leukroth (1970),when the presence of laminates or surface treatments can be deduced

sample

crystal

sample

Figure 7.1 Diagrammatic representation of MIR.

by comparing reflectance spectra with transmission spectra or bycomparing MIR spectra of laminate surfaces after treatment withdifferent solvents (Beauchaine and Rosenthal, 1987; Andersen andMuggli, 1981; Andersen, 1984).

A second form of reflection IR spectroscopy is known as specularreflection in which IR radiation reflecting off the front surface of thesample is collected. Since specular reflectance is often measured close tonormal incidence the reflected energy is small (<10% of source)although in regions of strong absorption it is much greater. The datarecorded are usually very different from those obtained from conven-tional transmission spectra as the bands are distorted due to interactionwith a component from the refractive index dispersion. This has, untilrecently, meant that the spectra obtained cannot be readily identified butthe advent of computer collection techniques, together with the avail-ability of Kramers-Kronig transform programs, now enables specularreflectance spectra to be converted into transmission-like spectra whichare then more readily compared with published databases.

More recently, specialized methods of specular reflection spectroscopyhave been used to study black coloured or filled vulcanizates.Claybourn et al. (1991) have shown that black polyethylene films can bestudied and this has been extended to carbon black filled (up to 50 phr)vulcanizates. The spectra produced by this method remain relativelypoor in quality and the best method of identification is via the deriva-tive spectrum which produces sharp peaks at characteristicwavelengths. However, these are of little use in terms of componentquantitation. There are further disadvantages to the use of specularreflectance in that bands (known as Restrablen bands) which are due tostrong reflected radiation at a specific energy are generated by thetechnique. These bands are unique to specular reflectance experimentsand must be identified within each spectrum before any interpretationof the data can take place.

Whilst the improved design of the MIR apparatus has contributedtowards improving the quality of reflectance spectra, the advent ofFourier Transform (FT) infrared instrumentation has been the majorfactor in advancing the technique. This has been in part due to theability of the modern computerized instruments to accumulate multiplespectra, and thus increase the signal to noise ratio, but also because ofthe greater fundamental light throughput of the FT experiment, lightfrom all frequencies being collected simultaneously rather than by aspectral scan. One particular development resulting from these improve-ments has been the ability to attach microscopes to FT-IR or FT-Ramanspectrometers and so record spectra from a very closely defined surfaceof a sample which has been deemed of interest by microscopical investi-gation.

A further area of IR spectroscopy to be developed in the 1980s wasphotoacoustic spectroscopy (PAS) FT-IR. This technique involvesplacing the sample to be examined in an acoustic cell and focusinginfrared radiation incident to the sample in the cell. The absorption ofradiation produces heating and other vibrational changes in the samplewhich can be detected as a noise (acoustic) signal. This signal is thenmathematically transformed into an IR spectrum. The theory of PASwas presented first by Rosencwaig (1980) and further described andexpanded by Griffiths and de Haseth (1986) and McClelland (1987). Thetechnique would appear, at first glance, to be a perfect technique for thestudy of rubbers due to the lack of any sample preparation. Unfortu-nately there are several drawbacks which limit its application. First, andpossibly the most significant, is the absorption of the IR radiation andthe photoacoustic signal by carbon black. Interestingly this material wasoriginally used as the standard for a background scan, to be subtractedfrom all subsequent spectra to produce the true spectrum, but wassuperseded (Carter et al., 1989) by a rubber filled with 30-50 phr carbonblack which they declared to be the ideal background material. It isthus obvious that examination of carbon black-filled rubbers will poseconsiderable problems!

Second, the technique is not quantitative. The thermal diffusionlength, optical opacity, effective thermal thickness and physical thick-ness of the sample all affect the signal response and many of these arefrequency dependent. The response throughout the whole samplecannot be considered constant and therefore cannot be reliably correctedsince the parameters will vary with each piece prepared for analysis.

Two further problems are operational, the time required to obtain aspectrum and the need for an expert operator. The sampling procedurefor the PAS experiment is relatively simple but the actual obtaining ofspectral data is far from straightforward and requires extensive experi-ence of the technique. If sufficient care is not taken then spuriousartefacts can easily be introduced into the spectrum (Rockley et al.,1984).

In spite of these problems, PAS has a role to play in infrared investi-gative studies but it would be difficult to justify it in cost effectiveterms as a routine analytical tool.

SAMPLE PREPARATION AND PRESENTATION

Two main methods by which the infrared spectrum of any sample maybe obtained have been discussed: by shining light through it, or bybouncing or reflecting light off its surface. In both cases the spectro-meter compares the energy spectrum of the infrared source before andafter absorption by the sample and generates a spectrum characteristic

of the sample showing the wavelengths at which it absorbs energy.Carbon black interferes by absorbing substantial amounts of energy inthe full spectral range whilst inorganic fillers superimpose their spectraon that of the polymer or polymer blend. Ideally, and if possible, thepolymer should be separated from the fillers before its spectrum isobtained although Corish (1960) obtained recognizable spectra frommicrotomed sections of filled vulcanizates and this work has beenexpanded by Bruck (1988). Excepting this, the sample may be offered tothe spectrometer as a film of the polymer itself, as a partially degradedbut still polymeric film, or as a liquid film prepared from the severelydegraded polymer. The last of these generally produces a spectrumwhich is quite different from that of the parent polymer but is, never-theless, characteristic of it and thus perfectly adequate for identificationwhen compared against reference spectra.

In all cases of sample preparation, the material under examinationshould have been extracted prior to spectroscopic analysis to removecomponents which could complicate spectral interpretation. This isparticularly important with plasticized materials such as PVC when theonly substance observed in the spectrum of a plasticized product couldwell be the plasticizer.

Selection of the appropriate infrared technique will depend upon thetype of sample. Thermoplastic materials can best be examined as filmsobtained by hot pressing at 150-18O0C for a few seconds beforemounting directly in the instrument although the film should be lessthan 100 Jim thick and this can be difficult to achieve. Latex can beexamined as a thin film cast directly on to a plate made from a non-water-soluble but infrared-transparent material such as silver chloride.The few drops of latex can be dried at 100 0C in a minute or two withthe transition from milky white to a clear golden colour indicating thecomplete removal of water. An alternative method of preparing a thinfilm of an elastomer is to cast one from solution. This can be doneeither on a rock salt plate from a suitable solvent or onto a sheet ofglass or mercury, from which it can be lifted on a frame for examinationalthough today, health and safety considerations would mitigate againstthe latter support medium. It is obvious that any chosen solvent mustmeet certain criteria:

• It must dissolve the sample entirely.• It must be inert towards the sample.• It must be completely volatile.• It should leave a smooth film (i.e. must not evaporate too quickly at

whatever temperature is chosen).

Typical solvents are chloroform, dichloromethane, toluene and tetra-hydrofuran and the analyst should become acquainted with the

absorption bands of the solvent of choice to ensure that the cast filmsare solvent free and that the spectrum is not misinterpreted. This isespecially important for toluene which has a tendency to be difficult toremove in the final stages of film preparation. It should be noted that,as a general principle, complete solution is an essential prerequisite fora reliable analysis, the significance and potential problems of selectivesolution and dissolution having been discussed in earlier chapters.

Vulcanizates are not amenable to direct solution techniques but cansometimes be solubilized if a degradation step is carried out first. Inone of the earliest papers on the examination of rubbers by infraredspectroscopy, Barnes et al. (1944) separated the polymer by dissolutionin a high boiling solvent (orf/zo-dichlorobenzene) and removed thecarbon black and other fillers by filtration. A similar procedure wasdescribed by Dinsmore and Smith (1948) in an extremely detailed papertabulating solvents of preference for a range of gum and vulcanizedpolymers. Clark and Scott (1970) improved the rate and extent of disso-lution by the addition of Pepton 22 (2,2'-dibenzamidodiphenyl disul-phide) to the o-dichlorobenzene before refluxing for 7 hours prior to acomplex work-up procedure. The method is, however, time consumingand Dinsmore estimated six work hours per sample on a routine basis!Furthermore, the solvents used for the degradation process are quiteaggressive and are subject to more controls than they were a generationago. Mineral fillers are removed by centrifugation but if carbon black ispresent a filter aid may be added and the solution filtered. In eithercase the clear solution is evaporated to low volume under nitrogen anda thin film of the elastomer cast from this solution. The spectrum isthen recorded and generally has the advantage over pyrolytic techni-ques in that the structural features of the elastomer are not destroyedand the spectrum will be closely similar to that of the unvulcanizedelastomer, albeit showing signs of oxidative degradation. The maindisadvantage is that one must wait for complete dissolution of thesample to be certain that all components of a blend are solubilized but,in some instances, the prolonged times which this entails can result inthe complete oxidative degradation, and hence loss, of some of them. Inaddition, newer polymers and polymer blends are becoming increas-ingly resistant to oxidative decomposition and this procedure cannot beguaranteed to work in every case.

An alternative to 'wet' degradation is 'dry' degradation as describedby LiGotti (1972) and Carlson et al (1970). In their procedures thesample is heated in air prior to dissolution according to the followingscheme:

About 2g of milled, extracted and dried vulcanizate are placed in atest-tube which is plugged with cotton wool and placed in an oven at

20O0C for 10 minutes. The sample is then transferred to a beakercontaining 50 cm3 of trichloroethylene and this is heated on a boilingwater bath for 30 minutes. The solution is then filtered, the clearsolution evaporated to low volume under nitrogen, and a film castfor infrared examination.

As the heating time is much shorter than in the wet method, there isless oxidation and the spectrum is even nearer that of the unvulcanizedpolymer. However, complete dissolution is rarely achieved so thisprocedure is perhaps better reserved for obtaining microstructural dataon polymeric systems where the blend composition has been deter-mined by another method. It has, for instance, proved helpful in deter-mining the level of 1-2 vinyl groups in 'high vinyl7 BR and SBRpolymers which were first thermally degraded and then examined asthin films by IR. This process can also be used in the preparation ofsamples for examination by NMR spectroscopy as described later in thischapter.

The most certain way to be sure that a polymer, or blend ofpolymers, has been completely removed from the filler matrix is tofragment the polymer into volatile species of low molar mass which canbe collected and characterized. This procedure, usually carried outeither under vacuum or in an inert gas such as nitrogen, is pyrolysisand was originally proposed as a routine method for the identificationof vulcanized elastomers by Harms (1953) who heated vulcanizates in ahorizontal test-tube in a Bunsen flame and collected the pyrolysatewhich condensed in the mouth of the tube for spectroscopic analysis.Although crude, this is still a relatively effective qualitative method inthe hands of an expert. However, in the same year Kruse and Wallace(1953) recommended an aluminium block, heated to 440-4650C, withholes drilled for thermometer and test-tube, as a more stable heatsource. Cleverley and Herrmann (1960) went a step further and used atemperature gradient up to 20O0C to extract materials of low molarmass thermally before increasing the temperature to 40O0C to pyrolysethe polymer. Gross (1975) has reviewed degradation methods of disso-lution and pyrolysis and published the spectra of many polymersobtained by both methods whilst BS 4181-1990 (equivalent to ISO 4650-1984) describes all three methods of sample preparation and illustratesboth pyrolysate and cast film spectra for a range of common elastomers.

Even with complete pyrolysis there are still quantitative variations inthe infrared spectra of the pyrolysates and there appears to be generalagreement between authors that this variability in spectra of the samepolymer or, more particularly, of polymer blends, is due to variation inthe temperature of pyrolysis. Lerner and Gilbert (1964) note that thepyrolysis of an NR-SBR blend gives different ratios of the ll.Ojim

(910cm l) and 11.25|im (888cm l) bands (due respectively to pyrolysisproducts of polybutadiene (ex SBR) and polyisoprene) for pyrolysis inthe 400-500 0C range from those obtained by pyrolysis at 850-950 0C.

In order to obtain quantitative data it is essential that a pyrolysisapparatus capable of reproducible operation is constructed in which theatmosphere, temperature gradient of heating and final temperature areclosely controlled. If it is not intended to trap all of the pyrolysate, astandardized trapping system should be devised. ISO 4650 uses thetest-tube/Bunsen method but also offers the option of an electricallyheated furnace with the pyrolysate condensed in an open tube.MacKillop (1968) described an electrically heated furnace, with pyrolysisat up to 390 0C ± 10 0C occurring in a vacuum and collection of all thecondensate, whilst Higgins and Loadman (1970, 1971) used a systemwhereby the furnace is maintained at a constant temperature (5350C)and the sample (approximately 100 mg) is always inserted to the sameposition so that, after pyrolysis for 15 minutes the temperature isconstant at 515 0C ± 5 0C. The apparatus is illustrated in Figure 7.2. Thetemperature of 5150C was found to be optimum for this particularsystem as below this there was incomplete pyrolysis of the SBR whilstabove 55O0C a reduction in the observed aromatics, presumed to bedue to loss of styrene in the volatile, uncondensed fraction, wasobserved.

The variation in spectra of pyrolysates obtained at different tempera-tures has been used by Dawson and Sewell (1975) to differentiatebetween natural and synthetic polyisoprene. Black filled vulcanizates of

B14 jointPTFEsleeve

thermocouple & probe

pyrolysate

furnace

Figure 7.2 Pyrolysis apparatus (Higgins and Loadman; 1970; 1971). (CourtesyTARRC).

the two elastomers give quite different spectra when pyrolysed at350 0C, and in the case of blends this effect can be quantified. Analogousdifferences can be found in thermogravimetric analyses when these arecarried out at a slow heating rate. This will be considered in more detaillater in this chapter.

A more comprehensive analysis of the total pyrolysate can beobtained in either a combined pyrolysis and gas cell, where one side-wall of the gas cell is an MIR crystal the temperature of which can beindependently controlled, or with the attachment of a FT-IR spectro-meter to the output of a thermogravimetric analyser to record IRspectra at selected points during thermogravimetric analysis. Witheither of these attachments it is possible to examine the gas phaseproducts and liquid condensate of the same pyrolysate after one experi-ment (Truett, 1977).

RUNNING THE SPECTRUM

Most modern infrared spectrometers are Fourier Transform instruments.These require an initial background scan to be run which is thensubtracted from subsequent spectra to produce the sample spectrum,the sample compartment generally being flushed with dry air ornitrogen to remove water vapour and carbon dioxide bands from thespectrum. A typical specification from one manufacturer to test for anadequate atmosphere within the spectrometer is to examine abackground spectrum for the ratio of the water band at 1657.9cm"1 tothe baseline at 1651.9cm"1:

(Il657.9 — Il651.9) / Il657.9

A ratio of less than 0.1 should be achieved with a suitable dry air ornitrogen supply (dew point below -30 0C) for any modern FT-IR instru-ment. A very useful guide to the theory and everyday use of FT-IRinstrumentation can be found in a book by Griffiths and de Haseth(1986).

For older grating (or even prism) instruments there is no need to runa background spectrum as the energy from the infrared source is splitinto two which follow closely similar paths through the 'samplechamber' but with only one beam actually passing through or beingreflected off the sample. The difference in energies between the beams isthen recorded as the spectral range is scanned. It is, however, still goodpractice to use an enclosed sample compartment to prevent localizedchanges in the atmosphere from affecting the spectrum. It is also impor-tant to realise that both atmospheric contamination and solvents insolution spectra can completely absorb all the energy from the IR radia-tion thus there will be no energy for the sample to absorb and no

difference between the sample and reference beams. In an FT instru-ment this will be obvious but in a grating (ratio recording) instrumentthere will only be a flat baseline and this has led inexperienced opera-tors to misinterpret spectral data. Problems may also be experienced bybubbles materializing in the liquid film or the cell leaking whilst thespectrum is being run. This is indicated by a gradual fall-off in peakintensity as one progresses through the spectrum and, if undetected,can render quantitative data totally unreliable.

Instrumental precautions which should be observed are to make surethat the scan speed is sufficiently slow for very sharp peaks not to bedistorted and to ensure that the gain (amplification) is set within accep-table limits. It is, of course, always good practice for both grating andFT spectrometers to record a known standard spectrum and to compareit with previous ones to ensure that no instrumental anomalies orartefacts have developed within the system before carrying out theanalysis of an unknown substance.

INTERPRETATION OF SPECTRAL DATA

Spectra will either be of a polymer (or polymer blend) itself, or of itspyrolysate. In both cases matching to reference spectra gives thequickest and most reliable means of identification of both blends andsingle polymer systems. The advent of computer-controlled IR systemsenables spectral matching to be achieved by an automated process andmany commercial libraries of IR spectra are available. However anymatch found by such a process should still be checked by an experi-enced operator as many search processes will involve only the matchingof certain pre-set parameters within each spectrum.

It is unfortunately contradictory that there will often be small differ-ences between nominally identical pyrolysis spectra of the samepolymer, whilst the analyst will be on the look-out for slight differencesfrom 'authentic' spectra as an indication of the presence of a minorcomponent. This can be partly overcome by building up an 'in-house'reference spectrum library with the spectra obtained under standardconditions but otherwise it remains a problem which only experiencecan resolve. Although a 'fresh' analyst will feel overwhelmed by thevast number of published spectra it will soon be realized that most ofthe samples analysed fall into a relatively small group, and onlyoccasionally will a full search have to be carried out.

Blends of polymers cause additional problems as, unless they havebeen observed previously, only an experienced operator will recognizethe presence of two or more polymers in one spectrum. Blends alsocause problems for search programs since, obviously, they contain thepeaks associated with all the polymeric ingredients. Some programs

allow reference spectra to be combined and their relative contributionsaltered by an iterative process to provide a match which providesquantitative blend data as well as compositional data but these shouldbe used with extreme caution as small differences between the spectraof nominally identical reference pyrolysates can lead to very different'interpretations7 by the software. The computer must always beconsidered only an aid to common sense and experience, not theirreplacement! It is worth noting here that demands for ever extendedoperating lives of rubber products in harsh working environmentscontinue to lead to the development of new polymers and polymerblends and any reference library of 'base7 materials will need continualupdating.

Some materials will break down under pyrolysis to liberate aggres-sive chemicals, such as hydrogen chloride from polychloroprenes,which will then react with the double bonds of a polyolefin whichcould be present in the blend. In some cases this can completelyobscure the presence of the second polymer and its presence will onlybe indicated by, say, quantitative chlorine analysis or a non-destructivetechnique such as swollen state NMR spectroscopy as described later inthis chapter.

One of the best sources of IR spectra for rubbers, plastics and manyof the chemicals used in the rubber industry are the Atlases ofHummel and Scholl (1984). Large numbers of polymer spectra are alsofound in Haslam et al. (1972) whilst, as already mentioned, Gross(1975), BS 4181-1990 and ISO 4650-1984 illustrate both film andpyrolysate spectra. Cleverley (1979) has published the spectra of awide range of packaging films. IR spectral libraries are available frommany instrument manufacturers and chemical suppliers; however, asmentioned earlier, the creation of an in-house database of spectra isalways preferable where practical and has the additional benefit thatall pyrolysis spectra recorded will be produced under the same condi-tions.

There are inevitably problems with the identification of polymers atlevels of less than 10% by pyrolytic techniques although some, such asSBR, will be visible at 5% or below. Chloroprene sometimes decom-poses completely leaving no pyrolysate (as described above), whilsthalobutyl will be indistinguishable from the butyl rubber itself. EPR orEPDM could well be missed at levels of up to 30% in NR and poly-butadiene may be confused with chlorosulphonated polyethylene. PVCand chlorinated polyethylene are effectively indistinguishable bypyrolysis techniques. For these reasons it is emphasized that it isalways advisable to use more than one analytical technique if there isany doubt about the identity of the polymer(s). It should always beremembered there could be supporting evidence from other analyses

which are being carried out for different reasons thus EPR will notcontain cure ingredients whereas vulcanized EPDM will and the shapeof the first derivative weight loss plot during the polymer decomposi-tion stage of a thermogravimetric analysis can also provide evidenceas to the probability of there being more than one polymer present inthe sample.

Quantitative analysis of polymer blends by pyrolysis-infrared spectro-scopy has generated many hundreds of publications. It is completelybeyond the scope of this book to consider these in detail but Table 7.1lists a number of references which are relevant.

Table 7.1 Quantitative polymer blend analysis

PolymerSystem

SBR-BR

BR-NR/IR

NR-SBR-BR

EPDM

(Termonomer)

ABR

NBR-ABR

NR-IRcis/trans NR/IR

Authors

Clark, J. K. and Scott, R.A.Higgins G. M. C. and LoadmanMJ. R.Mills, W. and Jordan MJ.MacKillop, D.A.Higgins, G. M. C and Loadman,MJ.R.MacKillop, D.A.Binder, J. L.Higgins, G. M. C. and Loadman,MJ.R.Mills, W. and Jordan MJ.MacKillop, D.A.Jasper, B. T.Takeuchi, T., Tsuge, S., andSugimura, Y.Brame, E.G. Jr, Barry, J. E. andToy, FJ. JrGardner, IJ., Cozewith, C. andVerstrate, S.Altenau, A.G., Headley, L.M.,James, C. O. and Ramsaw, H. C.Seism, AJ.

Ruzicka, B. and Krotki, E.

Dawson, B. and Sewell, P.R.Cunneen, J.I, Higgins, G. M. C.and Watson, W.F.

References

J, Ap pi. Polym. Sd. 14,1 (1970)NR Technol. 10, 1 (1970)

J. IRI 4, 60 (1970)Analyt. Chem 40, 607 (1968)NR Technol. 10, 1 (1970)

Analyt. Chem. 40, 607 (1968)/App/. Spectroscopy 23,1 (1969)NR Technol. 10, 1 (1970)

J. IRI 4, 60 (1970)Analyt. Chem. 40, 607 (1968)J. IRI 3, 72 (1969)Analyt. Chem. 41, 184 (1969)

Analyt. Chem. 44, 2022 (1972)

Rubber Chem. Technol. 44 (4),1015 (1971)Analyt. Chem. 42, 1280 (1970)

Analyt. Chem. Acta 42, 177(1968)Chem. Anal. (Warsaw) 116,1207 (1971)Rubber lnd. 9(5), 180 (1975)J. Polym. Sd. 15,1 (1959)

RAMAN SPECTROSCOPY

The initial theory of Raman spectroscopy was proposed by Smekal(1923) with the first practical demonstration of the effect being achievedby Raman and Krishnan (1928) and, at almost the same time, byMandelstam and Landsburg (1928). However, for reasons which willbecome obvious, it was of little practical significance in the field ofpolymer analysis until the last decade.

Raman spectroscopy is based on the inelastic scattering of light fallingincident on to a material. A small fraction (it can be as low as ICT15) ofthe incident light is scattered inelastically at a different frequency fromthat of the incident light and the shift in this inelastically scatteredradiation from the incident or exciting radiation is recorded as theRaman spectrum. The light can be scattered to a frequency eithergreater or less than the incident light; the latter are known as Stokeslines whilst the former are known as anti-Stokes lines. Whilst these twolines are often portrayed as being of equal intensity, the ratio of theStokes to anti-Stokes radiation is usually about 10:1 and thus the Stokeslines are usually measured in the Raman experiment. A Ramanspectrum is similar in presentation to an IR spectrum but, whereas theIR spectrum is very sensitive to polar groups such as carbonyls and lesssensitive to non-polar groups such as carbon-carbon double bonds, theRaman spectrum is the opposite, being more sensitive to the non-polarspecies. It will thus be apparent that recording both the Raman and IRspectra of a sample provides its complete vibrational spectrum.

As Raman spectroscopy is a reflectance technique it can be used tostudy polymeric species without the need for any sample preparationbeyond extraction to simplify the spectra obtained. The ease ofsampling together with the sensitivity of the technique to carbonunsaturation and thus configurational information would seem to makeRaman spectroscopy an ideal tool in the study of rubbers and otherpolymers. However, this technique has not enjoyed the samewidespread application to the analysis of polymers that IR spectroscopyhas because of two important factors; the time required to obtain aRaman spectrum and the phenomenon of fluorescence. The latter inparticular tended to limit the technique in the polymer field to thestudy of purified materials (Kurosaki, 1988) or in specialist researchtechniques. In an attempt to alleviate these problems Chase (1987),Hallmark (1987) and Hendra and Mould (1988) used the Fourier Trans-form technique combined with near infrared lasers to produce a Ramanspectrometer suitable for routine use which also has the advantage ofreducing the incidence of fluorescence. This form of Raman spectro-scopy is known as near infrared Fourier Transform (NIR FT) Ramanspectroscopy.

The use of a near infrared laser as an energy source reduces theproblem of fluorescence since the electronic transitions from the groundstate which produce this effect are rare in this domain. Furthermore, thelow energy of a near infrared source reduces both the tendency of thesample to absorb the incident radiation and the possibility of photo-degradation. The use of Fourier Transform collection techniques,coupled with the addition of multiple scans to improve the signal tonoise ratio and a simple slot-in sampling procedure analogous to thatused in IR spectroscopy, but without the requirement of light transmis-sion through the sample, allow for further improvements in spectralquality and uniformity. Instrumentation and applications of NIR FT-Raman spectroscopy have been documented in a book by Hendra,Jones and Warnes (1991).

As Raman spectroscopy is a reflective technique there is no need tosolubilize or otherwise change the sample in order to satisfy the condi-tions for analysis. Vulcanized samples or products made from mouldedthermosets or thermoplastics can be examined as easily as rawmaterials, whilst latex can be examined directly without the need forpreparing a dried film or degrading the polymer. Problems can beexperienced with very dark coloured materials in that the laser excita-tion beam may be absorbed, leading to sample heating, whilst theproblem of fluorescence may still occur with some brightly colouredmaterials or some oxidatively degraded polymers, even with a nearinfrared source, and this will prevent a Raman spectrum beingobtained.

The low sensitivity to polar groups of the Raman experiment hasimportant experimental advantages. Filled polymers may be examinedwithout excessive interference from the inorganic component, thussilica-filled vulcanizates can be studied with only a nominal effect beingobserved from the silica, in marked contrast to the equivalent IRspectrum where there would be little visible beyond the Si-O resonances(Hendra and Jackson, 1994). This has added significance in that the onlycriterion for a sample container is that it is transparent in the visibleregion of the spectrum. The fact that glass fits this criterion leads togreatly simplified sampling for liquids and suspensions such as latex.

As with IR spectroscopy, the presence of carbon black in a samplecauses problems. If it is present at a level much above 5 phr a combina-tion of sample heating and absorbance of the Raman signal by thecarbon black prevents any spectral data from being obtained. Due tothe fundamental nature of the problem it is very unlikely that it will beresolved experimentally and thus, in black-filled samples, the Ramanexperiment suffers from the same limitations as conventional IR spectro-scopy. Nevertheless, the potential for NIR-FT Raman spectroscopy inthe study of polymers is extensive and should expand to rival that of IR

spectroscopy. It is particularly relevant where identification is requiredwithin a very short timescale with the absolute minimum of samplepreparation and it would thus be ideal for quality assurance.

Qualitative Raman spectroscopy is analogous to IR spectroscopy, inthat material identification is generally by comparison of the spectrumwith those of known standards, although published databases arenecessarily much smaller in size, but in areas where quantitation isrequired Raman spectroscopy has the advantage that there is a linearrelationship between the characteristic peak height (or area) and theamount of material present, rather than the logarithmic relationshipfound in IR spectroscopy. Hendra et al. (1992) have demonstrated thequantitation of BR/SBR blends, nitrile levels in NBRs and the identifica-tion of a range of elastomeric materials using Raman spectroscopywhilst Frankland et al. (1991), in a detailed study of butadienes andbutadiene co-polymers, recommend Raman spectroscopy as the bestmethod for determining isomer ratio. An updating review of the signifi-cance of Raman spectroscopy to polymers by Gerrard and Maddams(1986) covers isomerization and orientation. A useful atlas of FT-Ramanpolymer spectra has also been published by Agbenyega and Hendra(1993).

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY (NMR)

NMR spectroscopy is a highly sophisticated analytical tool requiringhigher levels of training and experience than many of the other techni-ques covered in this book in order to prevent the experimental processand subsequent data manipulation from affecting the final results. Here,therefore, is emphasized the range of information which the techniquemay provide so that the rubber analyst may work with the NMRspecialist to maximize the usefulness of this technique.

The two nuclei of most interest to the rubber analyst are carbon andhydrogen. It is possible to examine a wide range of other nuclei but thisrequires even more specialized instruments which tend to be confinedto the research environment. In the case of hydrogen it is the 1H isotopewhich affords a spectrum (1H-NMR) and for carbon the 13C isotope(13C-NMR). The principle of NMR spectroscopy is the same for bothnuclei. The nuclei are spinning particles which generate their ownmagnetic fields thus, when they are placed in a strong externalmagnetic field, they can align (precess) either parallel (in accordancewith) or anti-parallel (in opposition to) to the applied field. Thisproduces two energy levels and, upon absorption of radio frequencyenergy, transitions from the lower to the upper levels (states) can beinduced. The frequency at which this occurs is dependent upon themagnetic field experienced by each nucleus which, in turn is determined

by the surrounding bonding electrons which themselves generate 'mini-magnetic7 fields. The net field experienced by any particular nucleuswill therefore depend on its chemical environment and these smalldifferences will provide a 'spectrum' of absorbed radio-frequency. Themodification of the applied field is called shielding. Frequencies ofabsorption are quoted relative to a standard chemical (typically tetra-methylsilane, TMS), which is more highly shielded, for both 13C and for1H, than most compounds. The difference between the radio frequencyabsorption of a particular nucleus and that of TMS is known as thechemical shift and is calculated in hertz (Hz); however, if the frequencyshift were to be expressed in these units it would vary with thefrequency of the surrounding magnetic field. An alternative notationhas therefore been adopted which expresses the shift as a difference inHz from the TMS frequency divided by the field strength of the instru-ment in MHz. The values then obtained, as parts per million, will beindependent of the frequency of the NMR spectrometer but will still becharacteristic of the magnetic field of the individual nuclei. Thus a90 Hz shift observed with a 90 MHz instrument corresponds to 1 ppm asdoes a 400 Hz shift with a 400 MHZ machine. For spectra plotted on thesame ppm scale, any particular band position will be constant but theband width, being a certain number of hertz, will appear to decrease asthe magnetic field strength increases, making it easier to resolve peakswhich are close together.

The range of a proton spectrum is typically 0-10 ppm (although someprotons are shifted by up to 15 ppm or more) and for a 13C spectrum,200 ppm. By convention the signal due to TMS (zero ppm) is placed atthe right-hand side of the spectrum. 1H-NMR spectra, as usuallyobtained from solutions of polymers, are not sharp bands, but a seriesof moderately broad symmetrical peaks, whilst the 13C spectra appearas very sharp (first order) bands. It is also of crucial importance to notethat the area under each peak in a 1H-NMR spectrum is proportional tothe number of that type of hydrogen nucleus present but for 13C-NMRspectra, as normally obtained, this is not strictly true although specia-lized instrumental procedures can be used to provide reasonably quanti-tative peak areas. For further study of the fundamental principles of 1H-and 13C-NMR spectroscopy the reader is referred to books by Stothers(1972), Abraham and Loftus (1978), Fukushima and Roeder (1981) andSlicter (1990).

Conventional 'high resolution NMR' spectra are obtained fromsolutions in which the solvent is either proton-free (CC^) or deuterated(CDCl3, C6D6 etc.). Modern NMR spectrometers use the deuteriumsignal from these solvents to 'lock' the spectrum in position so if thesolvent does not contain deuterium, a suitable deuterated referencematerial must be added. Numerous publications exist on the determina-

ppm ppmFigure 7.3 1H-NMR spectrum of an EPR. Figure 7.4 13C-NMR spectrum of a

high ethylene EPR.

tion of structure and microstructure by these techniques such as that byTonelli (1989) but since the latter is the subject of Chapter 9 only appli-cations related to the former, polymer identification and blend analysis,are considered here.

A representative pair of spectra is shown in Figures 7.3 and 7.4. Fromthe 1H-NMR spectrum we can see bands typical of an ethylenepropy-lene copolymer. Integration (measurement of the relative peak areas)will give the percentage of protons from the propylene methyl groupwhich will enable the mole ratio, and thence weight % polypropylene tobe calculated. The 13C-NMR spectrum shows many more differentcarbons than the five one would naively expect, and this is due tovarious microstructural features such as sequence distribution etc. asconsidered in the next chapter.

Very early articles by Bovey et al. (1959) and Bovey and Tiers (1960)illustrated the use of 1H-NMR for monomer ratio determinations instyrene copolymers and since that date numerous authors have usedthe technique to examine quantitatively a range of copolymers ofstyrene with methyl acrylate, methyl methacrylate, propylene,butadiene, isoprene, ethylene, a-methyl styrene, para-methyl a-methylstyrene and 2-ethyl hexyl acrylate.

Polyurethanes have also been the subject of several publications onthe use of NMR spectroscopy in structural analysis and the reader isparticularly referred to those of Yeager and Becker (1977) who use 1H-NMR, and Delides et al. (1981) who use 13C-NMR.

It is obvious from these few examples that, provided a solution of thesample can be obtained, an NMR spectrum can provide vast amountsof data, particularly since the rules for predicting the chemical shifts of

the various nuclei are the same for polymers as for low molar massmaterials and are extremely well documented and understood.

The fact that conventional NMR spectroscopy requires a solutionmust be considered a severe limitation in the study of commercialelastomeric materials which are most commonly presented in the formof cured materials. Although all the methods of solution/dissolutionpreviously discussed can be used, i.e. Werstler (1980) who describes amodification of the or^o-dichlorobenzene dissolution procedure andtabulates 13C-NMR chemical shift data for a large number of polymers,NMR spectroscopy is rarely applied to the analysis of these degraded ordepolymerized species.

An alternative procedure to conventional solution NMR has beendeveloped for examining of solid material and this is called 'solid state'NMR spectroscopy. However, it requires a very much more compli-cated, and therefore expensive, 'solid state7 instrument. Nevertheless,spectra have been widely reported ranging from an early review byCarman (1979) to more detailed studies by Fyfe (1983), Komoroski(1986) and Kinsey (1990).

This can be an extremely powerful analytical tool but it should beremembered that solid state NMR can, with some ease, producespurious peaks and other anomalies as a result of the experimentalprocedures required to obtain a spectrum. The interpretation of thesespectra, which in no instance should be regarded as quantitative, is bestleft to those people with a background in both NMR and polymersciences.

In 1989, Loadman and Tinker showed how continuous wave (CW)1H-NMR spectra could be obtained from slivers of swollen rubbervulcanizates using a conventional 'solution7 spectrometer with nomodifications, and how the information thus obtained could be used todetermine the crosslink densities of individual elastomeric componentsof vulcanized blends. The basis for the work was the observation thatthe signals in NMR spectra of polymers are considerably broader thanthose of simple molecules and that the signal width increases progres-sively as does the crosslink density due to the related reduction in chainmobility. In 1992, Brown, Loadman and Tinker expanded the techniqueto 13C-NMR spectroscopy, using a Fourier Transform (FT) instrument.A higher field strength instrument (30OMHz) was used resulting inbetter resolution of the various polymer resonances. The excellentquality of the spectra can be seen in Figure 7.5. A review of this workwas presented by Tinker (1995).

Recently Hull and Jackson (1997) illustrated how the technique of'swollen state7 NMR spectroscopy could be used to advantage in theanalysis of polymer blends which are not amenable to the more usualanalytical approaches, perhaps the most impressive example being a

Chemical Shift (ppm)

Figure 7.5 Swollen state 1H-NMR spectra of NR, cross link density = 41 mol/m3

(upper) and 114 mol/m3 (lower).

blend of epoxidized NR and PVC (Figure 7.6). Whilst the problemswhich arise from spectral broadening and reduced signal intensityrelated with increasing crosslink density can make the quantitation of

ENR assignments:

chemical shift (ppm) from TMS

Figure 7.6 Swollen state 13C-NMR spectrum of an ENR-50 : PVC blend.

polymer blends open to error, copolymer analysis such as the acryloni-trile level of a nitrile rubber or the epoxide level of epoxidized NR(ENR) can be reliably achieved even in the presence of anotherelastomer.

PYROLYSIS-GAS CHROMATOGRAPHY (PGC)When analysing a sample by pyrolysis-infrared spectroscopy the mostimportant problems encountered are in selecting appropriate pyrolysisapparatus and conditions to provide informative and reproducible data.Pyrolysis-gas chromatography not only has these problems but, inaddition, the chromatographic conditions have to be optimized toprovide worthwhile data on as broad a range of polymers as possible.It seems probable that this wide range of variables, leading to greatdifficulty in correlating interlaboratory data, has provided one of thereasons why pyrolysis-gas chromatography has not received the level ofacceptance accorded to the infrared technique. Nevertheless, manypapers have been published since the earliest one of Davison et al.(1954) which itself was published almost a century after Williams (1862)pyrolytically decomposed natural rubber and identified isoprene anddipentene amongst the pyrolysis products without access to chromato-graphic separative techniques.

In principle it is still possible to carry out the pyrolysis stage in asealed system remote from the gas chromatograph and then inject thepyrolysate into the column in a way analogous to the infrared methodalthough, in practice, this procedure is rarely used for routine analysesalthough it has some applicability in areas of research. For instance,Dawson and Sewell (1975) and Gelling et al. (1979) used low tempera-ture pyrolysis at 35O0C followed by gas chromatographic analysis todistinguish between natural and synthetic polyisoprenes. A comparisonof the ratios of the yields of l-methyl-4 (l-methylethenyl)cyclohexaneand l-methyl-4 (1-methylethyl) benzene for both raw and black filledvulcanizates also enabled Gelling to distinguish between lithium alkyl-and Ziegler Natta-catalysed synthetic polyisoprenes.

The more usual approach is to pyrolyse the sample, raw or vulca-nized, at the head of the GC column and to chromatograph all thevolatiles, producing a trace on the recorder known as a pyrogram. Inorder that good, and reproducible, resolution of the eluted componentsis obtained there are certain features which must be considered.

• The temperature of pyrolysis should be adequate to give a highconcentration of volatile components.

• The temperature rise should be extremely rapid to reduce the gaschromatographic injection time to a minimum.

• The temperature rise profile should be completely reproducible.

• Secondary reactions, which complicate the chromatogram, should bekept to a minimum.

• The material of the pyrolyser unit should be inert with respect to asmany polymers as possible.

• The sample size must be small, again to keep the 'injection time' low,and the pyrolysis temperature profile reproducible through thesample.

The two most commonly used forms of pyrolyser are the Curie-point(inductively heated) and the resistively (conductively) heated pyrolyser.The Curie-point pyrolysis units (first reported by Giacabbo and Simonin 1964) rely on the fact that ferromagnetic materials heat up rapidlywhen exposed to a radio frequency field. At a certain temperature thematerial will become paramagnetic and maintain the temperatureknown as its Curie-point. Different temperatures can be achieved byusing different metals; the Curie-point of nickel is 360 0C, iron is 770 0C,and other temperatures can be achieved by the use of alloys. Thesample is usually held in a coil or clamp of the ferromagnetic material.

Resistively heated pyrolysers usually take the form of a platinum coilinto which the sample is inserted (alternatively a platinum ribbon maybe used which is coated with the sample). Heating is achieved byapplying an electric current through the coil. This type of unit achievesa very fast rise time, or thermal ramp, and, unlike Curie-point pyroly-sers, the final temperature is infinitely variable, being only currentdependent. There were early concerns about catalytic reactions occur-ring on the metal surface and of carbon deposition causing ageing ofthe filament but both of these concerns were overcome by placing thesample in an inert quartz tube inside the coil. Whilst this will increasethe thermal rise time, it also provides some degree of thermal'buffering' against local hot-spots and provides a consistent ramp.

More recently, injection ports have become available which, as wellas being pressure programmable, are temperature programmable over avery wide range (-5O0C to 60O0C) and have rapid ramp times (vanLieshout et al. 1996). These can be retrofitted to existing gas chroma to-graphs and enable a single sample to be first heated to a moderatetemperature (e.g. 260 0C) in order to drive off volatile components suchas plasticizers and antioxidants and then pyrolysed at a much highertemperature in a subsequent run. We have come full circle since thework of Cleverly and Herrmann (1960).

In both inductive and conductive heaters the heat is applied as apulse. Another option is the static mode furnace reactor which consistsof a continuously heated furnace into which the sample is dropped orpushed. The two main drawbacks of this type of unit are that it usuallyrequires large samples and the furnace provides a large head space.

This inevitably leads to variations in the thermal ramp experiencedthroughout the sample so the degradation pattern of the volatileproducts will vary, leading to irreproducible results. The large headspace also reduces chromatographic resolution. Some interesting results,indicating the way in which yields of primary products fall withincreasing sample size, are given by Ney and Heath (1968) for NR, SBRand BR (Figure 7.7). The results are justified on the grounds that the

a butadiene( BR )

b vinylcyclohexene

c styrene

d butadiene ( SBR )

e vinylcyciohexene

f lsoprene ( NR }

g dipentene

Figure 7.7 Pyrolysis products as a function ot sample size, pyrolysis temperature54O0C. (Courtesy J. IRI.)

Sample size (mg)

lower temperature gradients experienced by larger samples will allowextended times for secondary reactions to occur.

Of equal significance are plots of relative yields of isoprene from thepyrolysis of NR at various temperatures, illustrated by both Ney andHeath (1968) and Krishen (1972) in Figures 7.8 and 7.9 respectively. Itwill be noted that the yield found by Ney and Heath peaks at about6250C (lmg) whilst that of Krishen is at 70O0C for a similar sampleweight (0.5-0.8 mg). One must therefore conclude that the furnacetemperatures were not truly representative of the sample temperatures.

In spite of the apparent development of the pyrolyser from arelatively crude furnace tube to the Curie-point and conductivelyheated systems, there still appears little agreement in the literature as to

a 1mg sample

b 5mg sample

c 9mg sample

Pyrolysis temp. 0CFigure 7.8 Yield of isoprene vs. temperature. (Courtesy J.IRI.)

% y

ield

Pyrolysis temp. 0CFigure 7.9 Yield of isoprene vs. temperature. (Courtesy Analyt. Chem.)

the best device to use. An interlaboratory check (Gough and Jones,1975) concluded: "Provided that conditions of pyrolysis and chromato-graphy are specified, it is possible to achieve readily identifiablepyrograms from the same polymer in different matrices, and from onelaboratory to another". The details specified were that the pyrolysershould be Curie-point (70O0C or 77O0C) with a 5-10 second pulse time.Little more recent comparative data is available although the currentstandard on pyrolysis-gas chromatography (ISO 5475, 1978) allows theuse of the furnace/silica tube, platinum coil, or Curie-point pyrolyserbut makes the point most strongly that fingerprint comparisons, theretention times of particularly indicative peaks and quantitative peak

pe

ak

a

rea /

mic

rog

ram

o

f s

am

ple

area comparison must be made against standards obtained on identicalequipment, and preferably at the same time.

The mathematical interpretation of the pyrogram data offers, like allother aspects of pyrolysis-gas chromatography, a degree of flexibility!Krishen (1974) recommends a weighed sample and an absolute calcula-tion based on the areas of specific peaks per unit sample weight andtwo graphs are reproduced here to illustrate the scatter of the results(Figures 7.10 and 7.11). These are compared with the method of calcula-tion more normally used for estimating natural rubber contents - thecalculation of area % isoprene and diprene in the total pyrolysate whichis illustrated in Figure 7.12. Cole et #/.(1966) preferred to use peakheight ratios and provide a large amount of experimental data on arange of polymer systems.

It would seem that quantification of good chromatographic data is nota major problem. It is up to the analyst to make sure that the data areas good as possible by:

1. obtaining a quick and reproducible heating to a specific temperature;2. using a constant sample weight;3. choosing the best columns for either general or specific analyses;4. optimizing the GC conditions (isothermal/programmed) for good

resolution in a reasonable time;5. running standards of a similar composition to the sample immedi-

ately before and after it.

Figure 7.10 Absolute isoprene peakareas for a range of NR blends.(Courtesy ASTM.)

Figure 7.11 Absolute diprene peakareas for a range of NR blends.(Courtesy ASTM.)

% Natural rubber % Natural rubber

Dipr

ene

peak

are

a

lsop

rene

pea

k ar

ea

Figure 7.12 Area % of diprene and isoprene for two series of NR vulcanizates.(Courtesy ASTM.)

Three old, but nevertheless still valid, publications illustrate largenumbers of pyrograms, that of Cole et al. (1966) giving quantitative dataas well. The polymers covered are tabulated (Table 7.2) to provide somebase data for an analyst considering entering this field. As alreadymentioned, the readily available ISO 4650 illustrates both film andpyrolysate spectra for eight common elastomers.

DERIVATIVE THERMOGRAVIMETRY (DTG)

Although thermogravimetry is the oldest of the thermoanalytical techni-ques, it was only in 1966 that the first derivative thermogravimetriccurves were published by Smith (1966a, b). The advent of instrumenta-tion enabling a continuous record of the first derivative of the weight

Natural rubber %

diprene

isoprene

Are

a %

Table 7.2 References to pyrogram collections

Cole et al. (1966) Cianetti and Pecci (1969) Alekseeva (1980)

NR in NR/NBR NR/IR NR/IRHR in IIR/CR SBR SBRNBR in NBR/CR NBR BRCSM in CSM/CR BR methylstyrene-BRACN in NBR UR UREPM & EPDM CR NBR

EPM CRCSM EU

Silicones AUFluorosilicones ABR

CIIRBIIR

loss plot against time (or temperature) to be recorded has since enabledthermogravimetric analysis to make a substantial contribution to thefield of polymer analysis, not least because the information it providescan be considered 'free' in the context of conventional thermogravi-metry. Thermogravimetry in relation to formulation analysis will beconsidered in Chapter 12 but here we are only concerned with thederivative mode, DTG, for the information it can provide on polymertypes and blend complexity.

The principle of operation is illustrated in Figure 7.13. The sample,weighing a few milligrams, is suspended from a microbalance in asmall furnace, the whole being enclosed in a glass tube to enable theatmosphere to be controlled. In older instruments a recorder provides asimultaneous reading of temperature and rate of weight change as thetemperature programme proceeds whilst in more modern instrumentsthe chart recorder, temperature programmer and balance controlmodules are replaced with a PC which allows data to be digitallystored and thus manipulated after the experiment has finished (Yuen etol., 1980). A typical curve, as described by Loadman and McSweeney(1975), is illustrated in Figure 7.14.

Most elastomers, when heated in an inert atmosphere, undergothermal degradation in the temperature range 330-53O0C. Table 7.3shows the significance of the technique in that the temperature ofmaximum rate of decomposition, Tmax, varies with the thermal stabilityof the polymer whilst, provided that the decomposition affords solelyvolatile products, the area under each curve gives a true and absoluteindication of the polymer content. No calibration curve is thereforerequired for quantitative blend analysis if good curve deconvolutioncan be achieved. Most polymers undergo pyrolysis quantitatively with

Figure 7.13 Principle of operation of a thermogravimetric analyser. (CourtesyPerkin Elmer Corporation.)

the notable exceptions of the chloroprenes, acrylonitrile copolymers andother halogen containing polymers. These will be discussed in moredetail in Chapter 12.

HEATING RATE AND CALIBRATION

It is important to appreciate the effect of altering the heating rate. Asheat must pass from the furnace to the sample, it is inevitable that in aheating cycle the latter will always be at a lower temperature than theformer. The temperature difference is known as thermal lag and

Quartz

Purge in

Sample in pan

Microfurnace

Thermocouple

Purge out

Balance

Sensor

Figure 7.14 First derivative thermogram and temperature vs. time plots.

obviously increases with the heating rate. The extent of the effect for aparticular instrument (Stanton Redcroft TG750) is shown in Figure 7.15.Bearing this in mind, all quoted Tmax values should, in principle, be

Table 7.3 7max for various raw elastomers (minor weight loss peaks not reported)

Brazier & Nickel Loadman & Tidd Hull & Jackson(1975) (1976) (1996)

Instrument Dupont Stanton Redcroft Perkin-ElmerModel 951 TG750 TGA 7Atmosphere N2 50 cm3 min~1 N2 30 cm3 min~1 N2 30 cm3 min~1

Heating Rate 1O0C min~1 1O0C min~1 2O0C min'1Polymer Tmax (0C) Tmax (0C) Tmax (0C)

NR 373 370 370IR 373 370 —BR (various) 460 458 456-460SBR (23.5%) 445-449 447 443-449UR 386 382 381EPDM 460 460 458CR (various) 375-78, 454-5 367, 449 364, 378, 452CSM (various) 335-40,465-479 318,464 —ACN (various) 370-405 400 381-392

n i t r o g e n oxygen

weightloss

rate ofweight loss

temperature

BLACK

EPDM

O 1Heating rate C min

Figure 7.15 Variation in observed 7max for NR with heating rate.

corrected to zero heating rate but the procedure is very time consumingand for practical purposes, where all polymers have similar thermalconductivities and the samples are generally of a very similar size, itcan be dispensed with. This is particularly true if one is using a moderncomputer controlled instrument when calibration of the instrument canmore easily allow for the thermal lag.

Temperature calibration of thermogravimetric analysers has beendiscussed by Stewart (1969) and Norem et al.(l969; 1970). Calibrationprocedures fall into three main types, each being based on the measure-ment of some known thermal properties:1. examination of a standard material which has a known and well

defined mass loss temperature;2. the use of a material with known and reproducible thermal transi-

tion;3. The use of reference materials with magnetic properties which are

removed at well defined temperatures (Curie-points).Whilst the first approach is appealing in that the calibrant weight loss isdirectly related to the TGA experiment, it does have the drawback ofbeing dependent on the nature of the environment in which the sampleis positioned, particularly the nature of the environmental gas, itsbuoyancy and flow rate. It is possible to calibrate in a range of environ-ments but, realistically, since the environment changes throughout the

experiment, it may not be possible to use different calibration criteriafor these different environments.

The second method requires a standard with a suitable thermaltransition. Potassium nitrate, potassium chromate and tin were allsuccessfully used by Stewart (1969) for calibration but to detect thetransition it is necessary to have an additional thermocouple in contactwith the standard, a feature which may require changes to the normaloperating procedure and will thus not be totally valid when applied toa 'conventional' experiment.

The last method is probably the most common and uses Curie-points(the temperature at which a material loses its magnetic properties).Methods vary from manufacturer to manufacturer but are based on themethod used by Norem et al. (1969, 1970) which examines materialssuch as tin, iron and some alloys in a magnetic environment, a smallmagnet being positioned to modify the apparent weight of the sample.Once the Curie-point is reached the loss of ferromagnetic charactercauses a sudden change in weight which can then be related to theindicated temperature. Once an instrument has been temperaturecalibrated under set conditions (heating rate, gas flow rate and samplepositioning) these conditions should not be changed without recalibra-tion taking place.

It should be noted here that at the time of writing no national trace-able standards for Curie-point determinations exist and this can causedifficulties for laboratories seeking accredited status for tests involvingthis equipment.

POLYMER IDENTIFICATION

Whilst the use of Tmax for polymer identification is perfectly valid, ithas obvious limitations in that polymers with similar thermal stabilitieswill decompose at similar temperatures. Table 7.3 lists Tmax values fromthree sources in which different equipment, nitrogen flow rates andheating rates were used. The first two examples use a heating rate of10 0C a minute whilst the last uses 20 0C a minute but with theapparatus calibrated at the appropriate heating rate to correct for anythermal lag which might effect the data. The results show very goodagreement and suggest that, although one would not rely solely on aDTG Tmax to identify an unknown polymer, the range of possiblematerials can be reduced to a very few, whilst the origins of the sample,together with any other additional data, could afford a positive identifi-cation. In many cases, the distinction between a blend and singlepolymer being present will be made.

Further information may also be derived from the shape of thederivative trace. Brazier and Nickel (1975), Sircar (1977) and Gelling et

Temperature (0C)Figure 7.16 DTG curves of unvulcanized NR and Natsyn 2200 rubber: NR( ); Natsyn ( ); NR + 50phr carbon black ( ); Natsyn + 50phrcarbon black ( ). (Courtesy J. Polym. ScL)

al. (1979) discussed and illustrated how natural and synthetic cis-polyi-soprene may be distinguished by their different DTG curves. The rawpolymers show essentially identical curves, the addition of 50phrcarbon black produces some indication of a high temperature secondpeak (Figure 7.16), but vulcanization (2.5 phr S, 0.6 phr CBS) results inquite obvious differences (Figure 7.17). Sircar postulated that this wasdue to cyclization of the polyisoprene, possibly encouraged in the caseof the synthetics by the polymerization catalyst residues. Strongsupporting evidence for this was supplied by Gelling et al. whoillustrated the DTG curve of cyclized NR and also that of purifiedNatsyn 2200 in which the catalyst residues had been removed by micro-filtration and centrifugation (Figure 7.18). In this last instance the curvewas indistinguishable from that of NR.

POLYMER BLEND QUALIFICATION

Provided that the components present in a blend give smooth DTGcurves, reasonably resolved from each other and with little carbonac-eous residue, there is no difficulty in measuring relative areas under

Temperature (0C)

Figure 7.17 Comparison of the DTG curves of black-filled vulcanized NR( ); Natsyn 2200 ( ); and Cariflex IR 305 or 309 ( ). (Courtesy J.Polym. Sd.)

curves, or peak heights, and thus obtaining an adequate estimate of theblend composition. Loadman (1976) used a relative peak height methodto analyse blends of natural rubber and polypropylene with a reason-able degree of absolute accuracy (±1%). This method can again beimproved by computerization where the weight trace can be substitutedby the derivative trace and the areas derived therein. In addition, peakdeconvolution routines can be applied to quantify partially resolvedpeaks although these should always be treated with care as they requiresome input regarding the number of component curves which are to bedeconvoluted. In instances where one component is too dilute to give aseparate Tmax, measurements of the height of the trace at fixed displace-ments from the observed Tmax can be taken and, with calibration, levelsin the 0-5% range for polypropylene have been measured reproducibly.

Maurer (1973, 1974) studied NR-EPDM and NR-SBR-EPDM blendswhilst Brazier and Nickel (1975) analysed quantitatively blends of NR-BR and NR-SBR-EPDM, which show substantial peak overlap, bydetermining 'response factors', i.e. the peak height per unit of massdegraded for each component elastomer at Tmax in blends of various

Temperature (0C)Figure 7.18 DTG of NR ( ); Natsyn 2200 ( ); and purified Natsyn2200 ( ) black filled vulcanizates compared with cyclized NR (- - - • -).(Courtesy J. Polym. Sd.)

compositions. As will be obvious from the section on calibrationearlier in this Chapter, these factors will only apply for a given set ofexperimental conditions. It is, however, particularly advantageous thatEPDM and polypropylene are so much more thermally stable than thepolyolefin rubbers (Figure 7.14) since their levels may easily be deter-mined in blends with NR, one of the most difficult combinations toquantify by the more usual pyrolysis-IR or pyrolysis-GC techniques.

Chlorinated polymers are easily identified by their characteristic rapidloss of weight as hydrogen chloride is evolved at a specific temperature.This occurs well before the decomposition of the polymer back-boneand can be an important factor in formulation analysis as discussed inChapter 12. However, when these polymers are blended with polyole-fins secondary reactions occur in which the hydrogen chloride reactswith the olefinic double bond and then decomposes again as thetemperature increases further. Depending on the ratio of the two typesof material there can be considerable distortion of the derivative weight

loss curve which will certainly render it of little use quantitatively and,in the case of low chloropolymer-high polyolefin blends, could disguisethe presence of the chloropolymer completely.

This behaviour can be considered an extreme case of a Tmax valuebeing modified by the presence of a second polymer but it is notunique. Sircar and Lamond (1975a) illustrated this effect in a study ofperoxide cured blends of NR-BR over a blend ratio of 80:20 to 20:80.Both elastomers showed a fall in Tmax as their loadings decreased. Inthe case of NR this was from 36O0C at 80% to 3470C at 20% whilst forBR it was 465 0C at 80%, falling to 450 0C at 20%. Factors such as thistend to relegate DTG to the area of providing supporting evidence forthe identity of a polymer or blend of polymers rather than as a stand-alone technique.

DTG has been used in conjunction with TGA by Jackson (1995) toquantify NR-EPDM-carbon black masterbatches and the gel fractionsremaining after dissolution of the soluble portion and removal of thefree carbon black. He was able to show that there was a strong prefer-ence for the carbon black to be associated with the NR phase.

DIFFERENTIAL SCANNING CALORIMETRY (DSC)

Differential scanning calorimetry (DSC) and differential thermal analysis(DTA) provide similar information about a material in that they bothrespond to enthalpy (heat content) changes reflecting a chemical orphysical change within it. DTA is, however, more difficult to quantita-tively relate to enthalpy changes as the observed measurement is adifference in temperature between sample and reference whilst, in thecase of DSC, it is the difference in energy input required to maintain thetwo at identical temperatures. In the latter case calibration against themelting endotherm of a suitable standard allows absolute values ofthermal properties to be obtained. Two types of DSC systems are avail-able, described as power compensated and heatflux, but each givesessentially identical information.

In order to illustrate the application of these techniques to polymeranalysis, only the more commonly used DSC will be considered. Fig7.19 illustrates the events which may be observed in the DSC examina-tion of a sample and the various regions may be analysed to provideuseful information. The first stage in any analysis is the preparation ofthe test portion and this immediately highlights the difficulty inobtaining reproducible data since, as with TGA/DTG, heat transferthrough the container to the sample must be both uniform and reprodu-cible. Although methods such as melting the polymer in the samplecontainer (Dannis, 1963) have been advocated, it is essential that anysuch method must be fully reversible and this is not always the case,

Figure 7.19 Idealized DSC curve for a polymeric material in an air atmosphere.

neither has it any applicability to vulcanized elastomers! The authorrecommends that a sliver some 1-1.5 mm thick, stamped to a disc aboutlmm smaller than the diameter of the sample pan, will provide anideal sample for encapsulation in that the disc fits into the aluminiumsample container, is flat for good heat transfer from the sample heaterand leaves sufficient room for the capsule to seal together. A referencecapsule containing an equivalent weight of aluminium lids or aluminaaffords a more linear baseline than an empty reference cell.

GLASS TRANSITION TEMPERATURES

Glass transition temperatures (Tg's) provide useful non-destructive(except for the requirements of sampling) analytical data on polymers,and the Tg's of many elastomers covered in this book may be found inAppendix B. The presence of one broad 'average' Tg or two discreteTg's for polymer blends affords information on the compatibility of thetwo phases; NR-SBR vulcanizates show two distinct transitions at-72 0C and -60 0C respectively whilst SBR-BR samples show a gradualprogression from that of SBR (-6O0C) to that of the BR used (e.g.-1120C for 55NF). The relative displacements of the two Tg's or theoverall profile of the single Tg enable quantitative blend analysis to becarried out (Loadman, 1986). It is reported by Landi (1972), Chandlerand Collins (1969) and Jorgensen et al (1973) that uncured nitrile-butadiene or nitrile-isoprene copolymers exhibit two Tg's when theacrylonitrile content is less than 35-36% of the polymer content, whilsta similar phenomenon enables SBS block copolymers to be distin-guished from randomly polymerized SBR. Fielding-Russell (1972)

crystallization

oxidation

melting

glass transition

Endo

ther

mic

Exot

herm

icHea

t flo

w r

ate

comments that the polystyrene Tg position varies with different poly-butadiene homopolymers due to their plasticization of the polystyreneto different extents. Ikeda et al. (1969) have evaluated these observationsfor the quantification of SBS block copolymers and obtained goodresults. More recently Hull (1997) has devised a method based on Tgmeasurements for determining the NR content of fibre boards in thepresence of other polymers.

CRYSTALLIZATION AND MELTING

Although the enthalpy of crystalline melting has been used extensivelyin the analysis of plastics, it has only rarely been used to study elasto-mers. Kim and Mandelkern (1972) investigated NR latex and purifiedNR and observed two melting regions as illustrated in Figure 7.20 andEdwards (1975) observed two morphological forms by electron micro-scopy which he correlated with these events. Loadman and Davey(1978) studied a range of NR crepes by both density gradient tubeand DSC in order to assess the usefulness of the latter in measuringlow levels of crystallinity in sole crepes and found it to be as accurateas the former method and much easier to carry out. One problemthey highlighted was the difficulty of preparing the test pieces sincethe energy required to cut the crystallized material could result inlocalized melting at the cut. It is important to realize that the meltingpoint of NR is related to the temperature at which crystallization hasoccurred, the former being some 30 0C higher than the latter. This has

Temperature (0K)

Figure 7.20 DSC fusion curve of purified NR. Sample crystallized at -250C for 6hour heating rate 50C min~1. (Courtesy J. Polym. Sd.)

Mix time (Minutes)Figure 7.21 Fusion enthalpy of BR as a function of blending time. Formulations:20 BR, 80 SBR, 20 carbon black. Mixing schemes: A - free mixing, B - blackpremixed in BR, C - black premixed in SBR. (Courtesy J. Appl. Polym. Sd.)

considerable technological significance since NR will crystallize, albeitslowly, at 1O0C and will thus have to be heated to 4O0C or more toremove the crystallization whilst a sample which has crystallized onlow temperature storage could well melt as soon as it is returned toambient temperature.

Also of technological significance is the observation by Sircar andLamond (1973b) that BR showed a loss of crystallinity when mixedwith a second elastomer (NR, IR, EPDM, CR-IIR, NBR or CR), both inthe presence and absence of carbon black. Lee and Singleton (1979)observed essentially the same behaviour in BR-SBR blends for whichthe enthalpy of BR fusion was determined as a function of mixing timeby three different procedures. Figure 7.21 illustrates the dependence ofthe BR fusion enthalpy on mixing time for free mixing. The magnitudeof the changes in the enthalpy are consistent with the absence of thetransfer of carbon black between the phases.

The problems of sample placement and reproducibility have alreadybeen mentioned but dynamic processes such as crystallization can bemarkedly affected both by the treatment which the sample receives inthe DSC capsule itself and by its thermal history prior to sampling. This

MIXING SCHEME CAH m

joule/m

g

Temperature 0CFigure 7.22 DSC curves of BR crystallization; effect of thermal history, (a) -untreated, (b) - heated and quenched, (c) - heated and slowly cooled.

is clearly illustrated in Figure 7.22 which shows curves for the heatingof BR after various cooling procedures. Rapid cooling (b) preventscrystallization from being completed before the Tg is passed, at whichpoint no more crystallization can occur. On heating through the Tg,crystallization continues until a temperature is reached at which meltingcommences. This crystallization on heating can be completely elimi-nated (c) if the sample is cooled sufficiently slowly for crystallization tobe complete before the Tg is reached.

HIGH TEMPERATURE EVENTS

In a series of papers covering a vast range of polymers, Sircar andLamond (1972-75) described how, above vulcanization temperatures,exothermic and endothermic events corresponding to high-tempera-ture reactions in the vulcanizate could be measured. Several elasto-mers undergo reactions which can be related to the specificchemical structure of the chain. For example, cyclization events inBR, SBR and NBR elastomers were detected quantitatively in theDSC and the enthalpy associated with the process was used byboth Sircar and Lamond (1973a) and Sircar and Voet (1970) foranalytical purposes.

In an inert atmosphere, the overall thermal degradation patternresults in a characteristic DSC profile which depends upon the type ofthe elastomer present. The profile is often complex, but its generalshape has been used in the fingerprinting of elastomer samples, and the

exothe

rmic

endo

therm

icHe

at flow

rate

Figure 7.23 DSC curves of various polyisoprenes heated in oxygen, at 20 ml min 1

flow, scan speed 160C min"1. (1) - NR (0.74 mg), (2) - Natsyn 2200 (0.62 mg), (3)- gutta percha (0.68 mg), (4) - 'trans PIP' 100 (0.68 mg). (Courtesy Thermochim.Ada.)

subsequent identification of elastomer mixtures. Oxidative degradationusually results in an even more complex DSC profile but, again, cangive a characteristic fingerprint for a particular system. This may alsobe used for elastomer identification but is not generally a favouredoption as the reaction or reaction products can cause damage to theheating modules. Figure 7.23 illustrates a series of results by Goh (1980)showing how oxidative degradation can distinguish between naturaland synthetic polyisoprenes.

The two techniques, DTG and DSC, which have been discussed inthis chapter represent the most common, and generally most informa-tive, of a whole range of techniques which broadly can be categorizedas 'thermoanalytical'. Table 7.4 gives some indication of the breadth ofavailable techniques but, because of space considerations and theirspecificity of application, the interested reader will have to pursue thesethrough more specialist publications such as the Journal of ThermalAnalysis and Thermochimica Ada.

SCANNING ELECTRON MICROSCOPY (SEM)

There are many ways of Visually' examining the surface or cross-section of a piece of rubber, with instruments ranging from a simplelens (x 10), through a light microscope (x 10 - x 400), scanning electronmicroscope (x 20 - x 300 000), to a transmission electron microscope

Temperature 0K

exothe

rmic

endo

therm

ic

Heat

flow ra

te

Table 7.4 Classification of thermoanalytical techniques

Measured property

Mass

Temperature

EnthalpyDimensionsMechanical characteristicsAcoustic characteristics

Optical characteristicsElectrical characteristicsMagnetic characteristics

Derived technique(s)

Thermogravimetrylsobaric mass-change determinationEvolved gas detectionEvolved gas analysisEmanation thermal analysisThermoparticulate analysisHeating curve1

Diff. thermal analysisDiff. scanning calorimetryThermodilatometryThermomechanical analysisThermosonimetryThermoacoustimetryThermoptometryThermoelectrometryThermomagnetometry

Acceptedabbreviation

TG

EGDEGA

DTADSC

TMA

1 'Reverse heating', or cooling leads to a 'cooling curve'.*As defined in For Better Thermal Analysis, 2nd edn (1980) ICTA, Rome.

(x 1000 - x 1000 000 or more). Similarly the different elements presentmay be identified and, within limits, quantified by such techniques asX-ray fluorescence (XRF) and electron spectroscopy for chemicalanalysis (ESCA). Whilst all of these have their place in the analysis ofboth raw polymers and commercial elastomeric products, some indeedproviding information not obtainable by any other method, thescanning electron microscope, with an integral X-ray analyser, offers aunique combination of advantages which merits its inclusion in thischapter, as well as in Chapters 10 and 13 dealing with fillers andblooms respectively. It should also be noted that Chapter 9 is dedicatedto the use of a range of microscopical techniques in the field of blendmorphological analysis.

The first true SEM was built by Von Ardenne (1938), but it was notuntil the mid 1960s that instruments became commercially available andthe technique could be considered to have arrived. Most current appli-cations are biomedical, biological or metallurgical and there remainslittle published on the application of the SEM to rubber analysis. In thecontext of this chapter two areas merit comment.

VISUAL ASPECTS OF POLYMER ANALYSIS

Latex particle size and size distribution are of critical importance in themanufacture of foam rubber goods, as they both reflect on the viscosityof the latex solution. Procedures to treat latex particles prior to micro-scopical examination are covered in Chapter 8 so here we shall just notethat the technique enables us to differentiate between natural andsynthetic latices by the particle size and size distribution. Naturalrubber latex shows a range of sizes (0.4-4 |im diameter) whilst thesynthetics are much smaller with few particles above 0.2 |im. As theseare too small to produce a good foam they are generally agglomeratedusing the process of Talalay (1963), but it is still a simple matter todistinguish the agglomerates from individual latex particles.

ELEMENTAL ANALYSIS

One aspect of elemental analysis which is uniquely suited to the SEMwith X-ray analyser is the examination of halogen-containing materialsto see whether the halogen, easily identified as chlorine or bromine, ispresent in the bulk of the rubber or as a surface skin. This can beachieved in two ways. Since the depth of penetration of the electronbeam depends upon its accelerating voltage, typically 5-50 keV, asurface-halogenated sample will appear relatively richer in halogen,compared with any 'bulk' elements, as the voltage is decreased, and thebeam penetrates less into the rubber behind the surface film. Alterna-tively, the sample may be sectioned and the concentration of halogenmeasured across the section, a uniform halogen concentrationthroughout the sample obviously indicating a blend whilst a zeroconcentration in the bulk of the rubber and a high concentration at thesurface indicates surface treatment. So-called 'edge effects' cansometimes interfere with the analysis of very thin surface films and oneway of avoiding these is to press together gently two samples of thetest piece with the two suspect faces in contact. A halogen-rich surfacelayer will then appear as a symmetrical peak when the halogen concen-tration is scanned across the width of the two test pieces. An analogousprocedure may be used with latex-dipped products to analyse for thepresence of laminates of different polymers. If the polymers are halogenfree it is often possible to observe filler differences which will distin-guish laminates from blends. This method of X-ray mapping theconcentration of a specific element, and matching the map to the visualdisplay, has also been used to identify crumbed scrap rubber in anarticle, and to estimate its level in the total product. Such a techniqueenables one to make sense of bulk polymer or filler analyses, whichotherwise may seem surprisingly complex.

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Alekseeva, K.V. (1980) /. Anal and Appl Pyrol 2, 19.Andersen, M.E. (1984) Microbeam Anal. 19, 115.Andersen, M.E. and Muggli, R.Z. (1981) Analyt. Chem. 53, 1772.Von Ardenne, M. (1938) Z. Physik 109, 553.Barnes, R.B., Williams, V.Z., Davis, A.R. and Giesecke, P. (1944) lnd. Eng. Chem.

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744.Briick, D. (1988) Proc. 133rd Meeting of ACS. (Rubber Division).Carlson, D.W., Ransaw, H.C. and Altenau, A.G. (1970) Analyt. Chem. 42,1278.Carman, CJ. (1979) Am. Chem. Soc. Symp. Ser. 103, 97.Carter, R.O., Paputa Peck, M.C., Samus, M.A. and Killgoar, P.C. Jr (1989) Appl

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Technol 39, 259.Corish, PJ. (1960) /. Appl. Polym. ScL 4, 86.Dannis, M.L. (1963) /. Appl. Polym. ScL 7, 231.Davison, W.H.T., Slaney, S. and Wragg, A.L. (1954) Chem. lnd. 1356.Dawson, B. and Sewell, P.R. (1975) /. IRI 9, 180.Delides, C., Pethrick, R.A., Cunliffe, A.V. and Klein, P.G. (1981) Polymer 22,

1205.Dinsmore. H.L. and Smith. D.C. (1948) Analyt. Chem. 20, 11.Edwards, B.C. (1975) /. Polym. ScL Polym. Phys. Edn. 13, 1387.Fahrenfort, J. (1961) Spectrochim. Ada 17, 698.Fielding-Russell, G.S. (1972) Rubber Chem. Technol. 45, 252.Foxton, A.A., Hillman, D.E. and Mears, P.R. (1969) /. IRI 3, 179.Frankland, J.A., Edwards, H.G.M., Johnson, A.F., Lewis, LR. and Poshyachinda,

S. (1991) Spectrochim. Acta 47A, 1511.Fukushima, E. and Roeder, S.B.W. (1981) Experimental Pulse NMR; A Nuts and

Bolts Approach, Addison-Wesley Publishing Co., London.Fyfe, C.A. (1983) Solid State NMR For Chemists, CFC Press, Guelph, Canada.

Gelling, LR., Loadman, M.J.R. and Sidek, B.D. (1979) /. Polym. ScL Polym. Chem.Edn. 17, 1383.

Gerrard, D.L. and Maddams, W.F. (1986) Appl. Spectrosc. Rev. 22, 251.Giacabbo, H. and Simon, W. (1964) Phann. Ada HeIv. 39, 162.Goh, S.H. (1980) Thermochim. Ada 39, 353.Gough, T.A. and Jones, C.E.R. (1975) Chromatographia 8, 12.Griffiths, P.R. and de Haseth, J.A. (1986) Fourier Transform Infra-red Spectroscopy,

J.Wiley & Sons, New York.Gross, D. (1975) Rubber Chem. Technol. 48, 289.Hallmark, V.M. (1987) Spectroscopy 2, 40.Harms, D.S. (1953) Analyt. Chem. 25, 1140.Haslam, J., Willis, H.A. and Squirrell, D.C.M. (1972) Identification and Analysis of

Plastics, 2nd edn, IHffe, London.Hendra, PJ. and Jackson. K.D.O. (1994) Spedrochim. Ada 5OA, 1987.Hendra, PJ. and Mould, H. (1988) Int. Laboratory 18, 34.Hendra, PJ., Jones, C. and Warnes, G. (1991) Fourier Transform Raman Spectro-

scopy; Instrumentation and Chemical Applications, Ellis Horwood, London.Hendra, PJ., Jones, CJ., Wallen, PJ., Ellis, G., Kip, BJ., van Duin, M., Jackson,

K.D.O. and Loadman, M.J.R. (1992) Kautsch. Gummi Kunstst. 45, 910.Higgins, G.M.C. and Loadman, M.J.R. (1970) NR Technol. 10, 1.Higgins, G.M.C. and Loadman, M.J.R. (1971) Ind. Comma 15, 50.Hull, C.D. (1997) Confidential Report, TARRC.Hull, C.D. and Jackson, K.D.O. (1996) Unpublished work at TARRC.Hull, C.D. and Jackson, K.D.O. (1997) Paper presented at IRC 97, Kuala

Lumpur, Malaysia.Hull, C.D., Jackson, K.D.O. and Loadman, M.J.R. (1996) J. Nat. Rubb. Res. 9(1),

23.Hummel, D.O. and Scholl, F.K. (1984) Infrared Analysis of Polymers, Resins and

Additives. An Atlas, VoIs 1-3, Carl Hanser Verlag, Munich.Ikeda, R.M., Wallach, M.L. and Angelo, RJ. (1969) Block Polymers, S.L. Aggarwal

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46, 1087.Kim, H.G. and Mandelkern, L. (1972) /. Polym. Sd. Part A.2 10, 1125.Kinsey, R.A. (1990) Rubber Chem. Technol 63, 407.Komoroski, R.A. (ed.) (1986) High Resolution NMR Spectroscopy of Synthetic

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Chromatogr. 19, 193.LiGotti, I. (1972) Paper Presented at 20th Meeting, ISO TC45-WCI, Cologne.Loadman, M.J.R. (1976) Unpublished work at MRPRA.

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Anal. 221.Norem, S.D., O'Neill, MJ. and Gray, A.P. (1970) Thermochim. Acta, 1, 29.Raman, C.V. and Krishnan, K.S. (1928) Nature 121, 501.Rockley, M.G., Ratcliffe, A.E., Davis, D.M. and Woodard, M.K. (1984) Appl

Spectrosc. 38, 553.Rosencwaig, A. (1980) Photoacoustics and Photoacoustic Spectroscopy, John Wiley

and Sons, New York.Sircar, A.K. (1977) Rubber Chem. Technol. 50, 71.Sircar, A.K. and Lamond, T.G. (1972) Rubber Chem. Technol. 45, 329.Sircar, A.K. and Lamond, T.G. (1973a) /. Appl. Polym. Sd. 17, 2569.Sircar, A.K. and Lamond, T.G. (1973b) Rubber Chem. Technol. 46, 178.Sircar, A.K. and Lamond, T.G. (1973c) Thermochim. Acta 7, 287.Sircar, A.K. and Lamond, T.G. (1975a) Rubber Chem. Technol. 48, 301.Sircar, A.K. and Lamond, T.G. (1975b) Rubber Chem. Technol. 48, 631.Sircar, A.K. and Lamond, T.G. (1975c) Rubber Chem. Technol. 48, 640.Sircar, A.K. and Lamond, T.G. (1975d) Rubber Chem. Technol 48, 653.Sircar, A.K. and Voet, A. (1970) Rubber Chem. Technol. 43, 1327.Slicter, C.P. (1990) Principles of Magnetic Resonance, 3rd edn, Springer-Verlag,

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Polymer Q

characterization O

Two features of polymer characterization are of special interest to therubber analyst, the first being the molecular weight, or molar mass, ofthe polymer and the second the microstructure of the polymer chain.

MOLAR MASS

It will be appreciated that a polymer does not consist of a large numberof molecules of identical molar masses, and thus there is no such valueas the molar mass of a polymer. Instead there are most ^likely to bequoted two values called the number-average molar mass (Mn) and theweight-average molar mass (Mw).

A third value, designated Mz, may occasionally be met. Described asthe Z-average molar mass it has no trivial name and is the third term ofthe power series from which the other two molar masses are derived:

f>Mz £ «,-Mi2 f>,-Mi3

/=1 Z=I Z=I

Mn = ; Mw = ; Mz = .X X X

^n1 I>/M/ 2>;M/2

Z=I Z=I Z=I

where HI is the number of molecules of molar mass Mi. The Z-averagemolar mass is especially sensitive to high molar mass components inthe polymer.

The difference between number- and weight-average molar masses isimportant. The weight-average value is always greater than thenumber-average one except in an ideal system of uniform molar mass(mono-disperse system) when the two are equal. This leads to the ratioMw/Mn being used_to describe the dispersity, or spread, of molarmasses in a sample. Mw is more sensitive to high molar mass compo-nents whilst the converse is true for Mn. It is a simple matter to illus-trate the difference by considering two polymers, one in which all themolecules have a molar mass of 100 000, and the other in which all the

molecules are of exactly 10000 molar mass. In the former case Mn =Mw = 100000 whilst in the latter, Mn = Mw = 10000.

If equal weights of each are mixed, the resulting measured valueswould be

Mn = 18 200 Mo; = 55 000

but if equal numbers of molecules of each are taken,

Mn = 55 000 Mw = 92 000In subsequent sections dealing with methods of measurement, the typeof molar mass so measured will be indicated.

There are many methods for determining some form of averagemolar mass of a polymer but relatively few for studying the distributionof that molar mass. Both these properties are important in helping tounderstand variations in processing such as mastication (Baijol, 1972),cold flow characteristics (Purdon and Mati, 1966) and adhesive proper-ties (Koldunovich et al. 1968). A number of the more relevant andsuccessful ones will be discussed in some detail.

END GROUP ANALYSIS (Mn)

With certain linear polymers it is possible to estimate the number ofend groups by chemical analysis and so derive the number-averagemolar mass. The total number of chain ends is twice the number ofpolymer molecules, but if each polymer molecule contains one group ofa particular type at the end of its chain then the number of that endgroup equates with the number of molecules. It is obviously of funda-mental importance that the particular group which is the subject of theanalysis is confined only to the end of each chain and also, withbranched polymers, that each branch is not terminated with the groupof interest. (Obviously this situation would provide data on 'branching'if each branch was so tipped, and a 'true' value of the molar mass wasobtained by a different method.) Given these criteria it will be apparentthat there are innumerable methods of analysis and it must be up to theanalyst to find the most suitable one for any particular end group.

Techniques to be considered include chemical methods, used by Ogget al. as early as 1945 to estimate terminal hydroxyl groups, infraredspectroscopy, first used by Pfarm et al. a year later, and also nuclearmagnetic resonance, pyrolysis-gas chromatography and mass spectro-metry. It may also be advantageous to react the functional group chemi-cally prior to analysis. Heacock (1963) estimated carboxyl groups in thepresence of carbonyl groups by reacting the former with sulphur tetra-fluoride to obtain the thionyl halides which were then quantitativelymeasured by infrared spectroscopy, whilst Edwards and Loadman

(1976) determined the molar mass of hydroxyl tipped polystyrenes byreaction with hexamethyldisilazane and trimethylchlorsilane in pyridinesolution followed by measurement of the trimethylsilyl end grouprelative to a quantitatively added standard using NMR spectroscopy.The replacement of one proton by nine greatly enhances the sensitivityand usefulness of the technique and Mn values up to 80000 weremeasured.

MEASUREMENT OF COLLIGATIVE PROPERTY (Mn)

A colligative property is one which depends primarily upon thenumber of molecules in the system and not upon their nature. Perhapsone of the earliest observed was the relationship between the depressionof freezing point and the concentration of the freezing solution(cryoscopy) enshrined as Blagden's Law some two hundred years ago.Others include the elevation of boiling point (ebulliometry), the reduc-tion of osmotic pressure (membrane osmometry, MO) and the reductionof vapour pressure (vapour pressure osmometry, VPO).

Under ideal conditions a general equation defines the calculation ofMn:

X/c = K/Mn+ be (8.1)

where X is the colligative property, c is the concentration of thesolution, and K and b are 'constants' which differ for each techniqueand for each piece of equipment.

In practice a series of solutions of varying concentrations is preparedand the colligative property measured for each. A graph is then plotted,as illustrated in Figure 8.1, of X/c against c, the number-average molarmass being the reciprocal of the intercept of the plot, after extrapolationto infinite dilution (C = O), with the abscissa. This method requires aknowledge of the constant K which is normally obtained by calibrationagainst standards of known molar masses.

In rare instances the solutions do not behave ideally and the plotobtained shows a distinct curvature due to the concentration depen-dence of the colligative property becoming significant. In these cases aplot of ^/(X/c) vs. c usually provides an acceptably straight line forreliable extrapolation to zero concentration.

Cryoscopy and ebulliometry

Cryoscopy and ebulliometry can be considered together briefly as theydo not feature largely in polymer analysis although a number of publi-cations, Newitt and Kokle (1966) (cryoscopy) and Ezrin (1968) (ebullio-metry), have appeared. These indicate that the techniques are valid up

Figure 8.1 Diagrammatic illustration of the determination of Mn from colligativeproperty measurements.

to molar masses of some 30000, although there are problems withsupercooling and frothing respectively. Commercial instrumentationdoes not, however, appear to be available, probably due to the muchgreater ease of operation of the vapour pressure osmometer.

Vapour pressure osmometry

The technique of indirectly measuring the lowering of vapour pressureof a solvent due to the presence of a dissolved material was firstproposed by Pasternack et al. (1962). It is based on measurement of thetemperature difference between droplets of pure solvent and of polymersolution maintained in an isothermal atmosphere saturated with thesolvent vapour. The temperature difference results from the differentrates of solvent evaporation from and condensation on to the twodroplets. As with the two previous methods, the colligative property, atemperature difference, is measured electronically and the value of K isobtained by calibration against appropriate standards of known molarmasses.

In the 1970s and 80s there appeared a number of papers whichquestioned the validity of an absolute constant in the determination byVPO of a molar mass and these illustrated how the 'constant7 K varieswith molar mass; see Bersted (1973), Brzezinski et al. (1973), Morris(1977), and Marx-Figini and Figini (1980). Edwards (1977) tabulated Kvalues for a range of molar mass standards as shown in Table 8.1.However, examination of the literature quoted shows that all thesestudies were carried out using a Hewlett Packard 302B instrument.

Table 8.1 Relationship between Mn and K (solvent,toluene at 4O0C) (Hewlett Packard 302B)

Substance Mn K

8-Hydroxyquinoline 145 10670Hexachlorobenzene 285 11600Polystyrene standard 970 12300Polystyrene standard 2300 13300Polystyrene standard 8500 14000Polystyrene standard 17500 14950

When Edwards (1981) repeated this work with a Corona-Wescan 232Avapour pressure osmometer he found that K was indeed a constant, anobservation also made by Burge (1979). It would seem, therefore, thateach analyst must calibrate his or her particular instrument for the fullmolar mass range over which it is likely to be used, since subtle designand manufacturing differences are obviously important.

Membrane osmometry

Membrane osmometry is the last of the techniques to be consideredwhich relies upon a colligative property of a polymer. The success ofthis technique depends upon a membrane, between the polymersolution and solvent, being permeable to the latter but completelyimpermeable to the polymer molecules in solution. In practice thismeans that whereas VPO finds application in the 500-50 000 Mn range,MO is restricted to polymers having no components with molar massesless than 15000; for unfractionated polymers this means an effectiveminimum Mn of 50 000.

The principle of operation is simple. A solution of the sample isplaced above the membrane below which is the pure solvent connectedto a reservoir by a flexible tube. The height of the solvent reservoir isautomatically adjusted to keep a bubble, introduced into a capillarytube below the membrane, in line with an optical detector, and thusequalize the rate of migration of solvent molecules from both sides ofthe membrane. The colligative property is then directly measured as h(the height difference between the membrane and solvent meniscus). Inpractice the displacement is converted to a voltage and plotted on achart recorder or monitor to allow the observation of slow drifts due tothe diffusion of any relatively low molar mass components which mightbe present. Extrapolation to zero time provides a realistic value for Mn

although Elias (1968) reviewed theoretical treatments of this problemand showed that even such extrapolation fails to provide a 'true' figure.

Unlike the previously described techniques, membrane osmometryrequires no calibration as the constant K = RT where R is the gasconstant and T the absolute temperature.

VISCOMETRY (Mn OR Mw)

The use of a viscometer to measure the viscosity (T/) of a solution of apolymer, followed by the calculation of intrinsic viscosity ([TJ]) andhence its molar mass, has been a standard procedure for many years,with one of the earliest reports being that of Staudinger and Heuer(1930). Many analysts will have used a viscometer since schooldays andthus the apparatus requires little description, although ISO 3105 may beconsulted for the design of various accepted glass capillary viscometers,the type generally used in measuring polymer solution viscosity. Theprinciple of operation is extremely simple in that the time (t) taken for asolution of known concentration to flow between two marks on a capil-lary tube is compared with the time taken by the solvent (t0), and theratio is a measure of the viscosity of that solution. Full practical detailswere described in the British Standard BS 5858-1980 which is nowwithdrawn. Successive dilutions afford a range of concentrations (c) andtimes (t) from which the intrinsic viscosity [rj] may be calculated.

The viscosity of the solution (solvent) = rj (rj0) from which r]sp (thespecific viscosity) may be obtained:

n - no t-t0 (8.2)risp = - —no to

and hence [^], using either the Huggins equation (1942)

-£--[,!+KH M% <8'3)

or the Schultz-Blaschke equation (1964),

^-= W + KH M2 rjsp <8-4>Several authors, including Rudkin and Wagner (1975), have described'one point' methods for determining intrinsic viscosity but Tidd (1976)found these to have a degree of concentration dependence whenapplied to natural rubber and does not advocate their use unlesssample limitation so demands. Khan and Bhargava (1980) published anew mathematical approach to the one point method for polystyreneand styrene-acrylonitrile copolymers.

Layec-Raphalen and coauthors (1979) raised the problem of theassociation of macromolecules in dilute solution, together with the effectthat this had on viscosity measurement, and showed how associationconstants could be calculated from viscosity measurements.

Conversion of the intrinsic viscosity ([?/]) to a molar mass dependsupon the Mark-Houwink-Sakurada expression:

M=KMa (8.5)

where K and a are empirical constants. Depending upon the source ofthese constants, M can be either number-average (Mn) or weight-average (Mw). Calculations based on molar mass methods such asmembrane osmometry (MO), vapour pressure osmometry (VPO) or endgroup estimation will give Mn, whilst those based on light scattering(LS) or sedimentation measurements will give Mw. Many hundreds ofK and oc values are tabulated in the Polymer Handbook, edited byBrandrup and Immergut (1975) (cf. 3rd edn 1989) together withsolvents, temperatures and techniques used.

Theoretical arguments have been put forward to suggest that eq. (8.5)is not valid over a wide range of molar masses when M refers to Mn orMw (Kurata and Stockmeyer, 1963) but it does seem in practice thatvalid results for many polymer-solvent systems are obtained if K and ahave been determined with reference samples spanning the range ofinterest. Dondos (1977) used single and dual solvent systems to study arange of polystyrene samples and showed that, whereas a classical plotof log [rj] vs. log Mw deviates from linearity at values of Mw < 150 000,a linear relationship exists when l/[rj] is plotted vs. 1/M'/2. Thus areliable calibration plot may usefully be constructed for any polymersubject to regular analysis.

LIGHT SCATTERING (Mw?)

The use of equipment to measure the light scattering behaviour ofsolutions of polymers and the calculation therefrom of weight-averagemolar masses appears never to have reached the popularity of thetechniques described earlier for the determination of number-averagemolar mass. It has also tended to have been eclipsed by gel permeationchromatography (GPC). This situation has recently changed with theintroduction of low angle and multi-angle laser light scattering, andevaporative light scattering.

Much of this earlier neglect was due to the calculations required,together with the lack of reproducibility of results obtained by differentworkers and the difficulties experienced in the analysis of standardsamples due to problems of sample preparation. Nevertheless thetechnique can be precise and has been used successfully on samples

with molar masses between 10000 and 10000000. Early examples oflight scattering instruments were scanning devices incorporating onephotomultiplier detector, a mercury arc lamp with filters, and a centralstage upon which the sample could be positioned to make measure-ments at specific angles. Soon, a laser replaced the mercury arc lampand low angle laser light scattering (LALLS) was developed.

In 1984, Wyatt directed the development of the first commerciallyviable simultaneous multi-angle instruments, multi-angle laser lightscattering (or MALLS) which determine directly the molar mass andsize of molecules in solution. Coupled to a GPC or thermal field flowfractionation system these obviate the need for column calibration, refer-ence standards and pump speed dependence. The increased sensitivityprovided by the laser light source allows relatively dilute solutions andvery low cell volumes (0.1 jul) to be used so that solution clarification ismuch easier (McConnell, 1978). Multi-angle light scattering instrumentsalso have the advantage of determining branching ratios directly. Anexcellent review of this topic is provided by Wyatt (1992).

An evaporative light-scattering detector (ELS) is of use for the analysisof polymers and polymer additives. With this detector the solvent isevaporated from the eluent as it passes down a drift tube, leaving thesolute particles to scatter the light from the light source. This scatteredlight is collected by a photomultiplier tube and amplified to give ananalogue signal which generates the chromatogram. The theory of thisdetector is discussed by Moury and Oppenheimer (1984). Although theELS detector has a useful part to play in polymer analysis it is, unlikeUV, refractive index (RI) and laser light scattering, destructive.

The problems of sample preparation for all the light scattering techni-ques can be divided into two: choice of solvent, and clarification of theresulting polymer solution. The former affects the sensitivity and, hence,the accuracy of measurement, with the two major requirements beingthat the refractive index of the solvent is as different as practicable fromthat of the polymer, and that the solvent itself is low-scattering. It is thelatter, however, which gives the biggest source of variability in results.A general procedure for clarifying solutions of polymers is that ofsequential filtration and centrifugation, typically as described byJennings (1966). Unfortunately when one reads comments such as thoseof Doty and Bunce (1952), who claim that purification by centrifugationbecomes more difficult as the concentration of the solution increases,and Witnauer et al (1955) who reach exactly the opposite conclusion,one realizes that a careful study of each polymer system is required toestablish the most viable working conditions. It is also advisable tostudy an unknown polymer in a selection of solvents to reduce thepossibility of artefacts such as polymer-polymer or polymer-solventinteractions.

The basic equation which relates the molar mass to the extent of lightscattering is the Debye equation derived by Debye (1944, 1947):

Y c l 2Ec <*aK — = =— + —— + ... (8.6)R9 Mw RT

where B is the second osmotic virial coefficient.This assumes that the scattering particles are small, compared with

the wavelength of the light. If this is invalid, allowance must be madefor dissymmetry of scattering throughout the molecule by introducingthe particle scattering factor P(O):

K-g- = - 1 +-g*-+... (8.7)R0 MwP(O) RT

where K is a calculated constant, requiring a knowledge of, amongstother things, the refractive index of the solution and its variation withpolymer concentration (Brice and Halwer, 1951), c is the concentration,R0 the excess scattering intensity of the solution over the solvent.

Calculation of RQ from the experimentally obtained data became thesubject of a number of papers and is dependent upon the solventsystem and cell arrangement used (Leblanc, 1962; Kratohvil, 1966;Miyake et 0/., 1970). These data are then treated according to themethod of Zimm (1948) to afford the most accurate graphical methodfor the derivation of Mw. As the scattering angle approaches zero,P(0)~l, the reciprocal of the particle scattering function, can beexpressed as:

lim P(fl)-1 = 1 + - ( s2) sin2 0/2 (8.8)0 ^ 0 3X

to give:

^J-(i + <s2>sin20/2U^ (8.9)R0 Mw \ 3A2 / RT

from which it will be seen that a plot of Kc/Re against sin2 9/2 + kc willhave a common intercept of 1/M at zero concentration and zero angle.A typical Zimm plot is illustrated in Figure 8.2 for Kc/Re againstsin20/2 + kc (where k is a convenient arbitrary constant, in this case100). Measurements are made at different angles (0i-0g) for a range ofsolution concentrations (C^-C4) and these are plotted as shown (O).Extrapolation of both concentration and angle plots to a common inter-cept (at 0 = 0,_c = 0) (•) with the abscissa gives the weight-averagemolar mass (Mw) as the reciprocal of the intercept, i.e. 1030 000 in theillustrated example.

There is a belief that the angular dissymmetry of the scattered lightwill afford information on the molecular shape of a polymer and whilst

Figure 8.2 Zimm plot showing the light scattering from a sample of polystyrene inbutanone (Billmeyer, 1971). (Courtesy John Wiley & Sons.)

this is a reasonable theoretical deduction, the practical position appearsto be that this may only be so under exceptional circumstances (Benoit,1968). Carpenter (1966) has concluded that a similar situation prevailswith regard to dispersity measurements using this technique. It shouldalso be borne in mind that severe restrictions apply to the interpretationof data obtained from a copolymer solution, although Benoit andBushuk (1958) developed a theory to encompass copolymers which hasbeen evaluated by a number of workers such as Benoit and Leng (1961),Prud'homme and Bywater (1971), Shimura et al (1964), Jordan (1968),Spatorico (1974) and AIi (1978). Their results, however, show varyingdegrees of success.

GEL PERMEATION CHROMATOGRAPHY (Mn, Mw, Mz)

Gel permeation chromatography (GPC) (sometimes referred to as sizeexclusion chromatography or SEC) is a particular form of liquid

chromatography in which a solution of a polymer is pumped through aseries of columns, each packed with a gel of specific pore size, underconstant flow conditions, to a suitable detector positioned at the end ofthe final column. The range of pore sizes is such that it compares withthe dimensions of the polymer molecules; thus the largest molecules,which can penetrate few pores, take the shortest route through thecolumn and are eluted first whilst the smaller ones, which can enterproportionately more pores, take progressively longer to reach thedetector. Rubbers, having broad molar mass distributions, arecommonly analysed using a series of columns with packings of differentpore sizes connected sequentially, the packing with the largest pore sizebeing first. However, 'mixed bed' columns are now commercially avail-able and these are packed with a mixture of materials with a range ofpore sizes.

There is no doubt that, since the early applications of GPC to thecharacterization of molar mass distributions of polydisperse systems byVaughan (1960), Brewer (1961) and Moore (1964), the technique hasbecome the one chosen for the routine study of a vast range ofpolymers (although the calculations carried out to obtain accurate molarmass values rely on calibration data for each system obtained by themethods discussed earlier).

Although conceptually simple, the reproducibility of experimentalconditions can generate severe practical problems if care is not takenand the right equipment is not chosen. Undoubtedly the biggest poten-tial problem is the pumping system, as the flow of solution through thecolumns must proceed at an absolutely reproducible and constant rate.Not only does this require a relatively sophisticated pump andassociated controls, but also care must be taken in sample preparationusing filtration or centrifugation to remove any gel or other insolubleswhich might progressively block the columns.

Problems with dead space, injection systems and column channellingalso occur and, whilst any chromatographer will be aware of these, it isimportant to note that in the field of GPC they become much moresignificant than in most other chromatographic systems as one is rarelyconcerned just with separating peaks, but more with the absolute reten-tion time (volume) and detailed peak shape.

A wide variety of detectors is available, one of the earliest being thedifferential refractometer, described by Moore (1964) and still in usewhere the more modern detectors lack sensitivity or selectivity in theanalysis of certain polymer-solvent systems. Calculations generallyassume that the refractive index of a polymer is independent of itsmolar mass and, whilst this is certainly true when the molar massexceeds a few thousand, Barrall et al. (1968) have shown how a smallcorrection is required for low molar mass polymers. It must also be

realized that differential refractometry is not satisfactory for monitoringthe fractionation of a mixture of chemically different polymers unlessresponse factors are known for each component.

Many other detection systems have been used, including viscometry,thermal conductivity, LALLS as already described by McConnell (1978),MALLS, and ELS. Spectroscopic detectors, either ultraviolet or infrared,are today used extensively and, if an instrument with a variablewavelength facility is used, there is considerable flexibility in thebreadth and selectivity of the data which may be acquired.

Potentially, both of these techniques have sensitivities many timesgreater than that of the differential refractometer but the sensitivityfor two polymers may vary by orders of magnitude depending upontheir different structures and the specificity of the monitoringwavelength. Ross and Castro (1968) used infrared (2940cm"1) toobtain data on polyethylene with perchlorethylene as solvent whilstTerry and Rodriguez (1968) monitored methyl methacrylate (1731 cm"1)and styrene (698cm"1). Birley et al. (1978) have discussed theoreticalaspects of quantitative infrared Spectroscopic detection. Runyon et al.(1969) used an ultraviolet spectrophotometer (260 run) to detect styreneand, sequentially, a differential refractometer to give 'total detection7

for styrene-butadiene copolymers. Spectroscopic types of detectors areparticularly useful as the flow can be stopped at any time and a fullspectrum obtained. Cooper et al. (1969) have shown that there isessentially no loss of resolution with flows interrupted for severalhours.

GPC can usefully be applied to distinguish between residual (free)and rubber-bound chemical modifiers and to quantify the latter.Edwards (1992) showed how chemical modifiers which act as sulphurdonors and silica-coupling agents can be detected by their UV absor-bance when bound to rubber. By measuring the molar mass distributionof the modified rubber at a wavelength where the rubber itself does notcontribute to the absorbance, and comparing this with the molar massprofile of the unmodified rubber, calculations can be made to determinethe percentage modification.

In recent years there has been a tendency to use shorter, narrow borecolumns packed with smaller diameter particles in order to reduce theanalysis times of GPC runs. This was originally described as high-pressure GPC (HPGPC) but the wording was soon altered to high-performance GPC. Early workers such as Gudzinowicz and Alden(1971) reduced analysis times appreciably but, because they did nothave packing materials of sufficiently small particle diameter, theirresolution suffered. This problem has now been overcome with theavailability of particles less than 10 m in diameter, compared with theearlier diameters of 50|im and much has been made of the fact that

molar mass distribution measurements can now be carried out in lessthan ten minutes (Kato et al, 1974; Kirkland, 1976; Unger and Kern,1976). Edwards (1981), however, was concerned at differences found inthe analysis of natural rubber using both a 'normal' GPC system withcolumn packing particles some SOjim in diameter and an HPGPCsystem where the particles were about 10|im diameter. Whereas theformer gave a typical bimodal distribution for the natural rubber(Subramanium, 1972), the latter showed a steady decrease in apparentMn and a reduction in the relative concentration of those componentsof higher molar masses with increasing flow rate. The area under eachGPC trace was normalized against that for a polystyrene standard ofbroad molar mass distribution (MwDl from the NPL).

The results, reported for both natural rubber and a synthetic polyiso-prene, are given in Table 8.2 and show that the passage of a dilutesolution of polyisoprene of higher molar mass through tightly packedmicroparticulate columns, at high flow rates, produces shear forcessufficient to cause severe degradation of the largest molecules. It isimperative, therefore, that molar mass determinations are carried out atthe slowest flow rates consistent with practical considerations andseveral runs may be necessary to determine valid conditions.

Similar observations have been made for polystyrene (Kirkland, 1976)and polyisobutene (Huber and Lederer, 1980) whilst a recent studycarried out by Polymer Laboratories, UK (1996) to determine the mostsuitable column set to use for the analysis of natural rubber showed themixed bed A (20 Jim) column series to be preferred over the mixed bedB (10 jam) series since the former gave adequate resolution whilst

Table 8.2 Molar masses obtained at different flow rates using 10(im particles inHPGPC

Flow rate Back pres. Mn Mw Mn/Mwcm3 min~1 p.s.i.

Car if lex IR 3050.3 150 593000 1942000 3.280.7 500 377000 1078000 2.882.0 1200 302000 797000 2.644.0 1850 237000 611000 2.58

NR(RRIM 501)0.3 150 152000 828000 5.450.7 500 145000 664000 4.572.0 1200 124000 433000 3.504.0 1850 112000 344000 3.07

minimizing back-pressure build-up which can lead to shear degrada-tion.

Having discussed the problems of obtaining a reliable representativechromatogram it is now necessary to consider how molar mass data canbe obtained from the chromatogram.

A primary calibration is usually carried out with polystyrenes of aslow a dispersity as can be obtained. Commercial packages are availablewith polystyrene calibrants pre-prepared on two spatulas, allowingfrequent and consistent calibration with the minimum of effort.

The _^most_representative^_ molar mass can be described as (MnMw)1/2

so, as Mn/Mw_-+ 1, then Mn = Mw = Mrep. This is a reasonable approxi-mation if Mn /Mw < 1.1 and a plot of elution volume at peak maximum(V) against logMrep gives the primary calibration. From this, anychromatogram can be expressed in terms of 'molar mass polystyreneequivalent' which, although useful in certain empirical applications,does not provide absolute molar mass data for systems other thanlinear polystyrene.

If samples of a broader molarjnass distribution are used to constructa calibration curve, and the Mn and Mw values are available, therelationship between molar mass and elution volume, assuming thecalibration curve to be linear over the molar mass range of a givenfraction, becomes

lnM = AV + D (8.10)

where M is the molar mass of the fraction eluted at volume V. Anumber of methods, such as those by Hamielec et al. (1969) and Roy(1976), have been used to evaluate the constants A and D but in essencethey are iterative methods based_on the assumption that only A signifi-cantly influences the calculated Mw/Mn ratio. An initial value of A isobtained, by assuming a log-normal molar mass distribution, when itcan be shown that

A = 4[ln Mw/Mn]l/2/W (8.11)

where W is the peak width. After an initial calculation, the process isrepeated with adjustments to A until calculated and experimentalvalues of Mw/Mn agree. The availability of computer- or micropro-cessor-controlled GPC equipment has made this a much less tediouscalculation than it used to be.

In 1967 Benoit et al. proposed the Universal Calibration Mark-Houwink Method and showed that for all polymers a single calibrationcurve exists for the relationship between the hydrodynamic volumelog [^]M and the elution volume V in a given solvent system. Inpractice there may be slight deviations from the universal curve at itsextreme ends, for reasons of molecular shape, as described and

discussed by Ambler and Mclntyre (1975). At any given elution volumefor polymer 1 and polymer 2 the relationship given in Eq. (8.12) holds:

[^]M1=[T12]M2 (8.12)

where DfI] = K1Mi06 as given in Eq. (8.5). From Eq. (8.12) is derived thebasic universal calibration equation:

1OgM1 = (1 + QC1)-1 1OgK2XK1 + (1 + oc2)/(l + on) logM2 (8.13)

The Mark-Houwink constants for the two polymers must of course beobtained at the same temperature and in the same solvent. Eq. (8.13)can be rewritten as:

[rji]/[rj2]= Mn2/Mn1= Mw2/Mw1 = V (8.14)

where B is designated the Benoit factor and is a constant for the parti-cular polymer being considered.

If a primary calibration has been obtained as described earlier (and asis shown in Figure 8.3 for monodisperse samples of polystyrene), it isonly necessary to have one monodisperse standard sample _o_f anotherpolymer to construct its calibration curve. A knowledge of Mn or Mwtogether with the measurement of its elution volume under identicalconditions to those used to obtain the polystyrene calibration curveenables the point X to be found, and B calculated after obtaining the

Elution Volume

Figure 8.3 Representative GPC plot of log M against elution volume.

polystyrene equivalent molar mass P at the same elution volume. It isthen a simple matter to construct the full calibration curve for the newpolymer.

Using this calibration curve it is now possible to convert a graphicalprintout of detector response h(V) at elution volume V into a normal-ized molar mass plot, showing w(M), the fractional weight of a polymerhaving molar mass M, plotted against M, via Eq. (8.15):

•*•>-**">-15555 <fU5)

Figure 8.4 shows the experimental data, in the form of a graphicalprintout, together with the appropriate calibration graph and, superim-posed (and calculated from the latter), a graph of dV/d(log M) againstlog M. It is apparent that at any volume V one can read off values forh(V), dV/d(log M) and log M (hence M), and calculate w(M).

Implicit in Eq. (8.15) is the condition that the detector response isdirectly related to the weight concentration of the polymer. If this is notvalid then a correction must be introduced. It will also be noted thatEq. (8.15) contains the reciprocal of the slope of the calibration plot, animportant point in converting the V axis to an M axis. This can beignored if the primary calibration is linear over the whole range ofmolar masses being studied, but must be included if curvature ispresent - as it almost inevitably is.

Inevitably, the situation is never as clearcut as it seems. As alreadymentioned, Ambler and Mclntyre (1975) described how there are devia-tions from the universal calibration with extremes of spatial configura-tions of the polymer chains whilst Narasimhan et al. (1981) illustrated aconcentration effect, and a 'second polymer' effect, on the elutionvolumes of polystyrenes and polybutadienes.

For a more detailed consideration of many of the problems ofconverting primary GPC data into absolute molar mass values thereader is referred to papers by Letot et al. (1980), Chaplin and Ching(1980), Dawkins (1977), Gilding et al (1981) and Busnel (1982).

No mention has been made so far of peak broadening. The GPCcolumn does not have an infinite resolving power, and thus theobserved peak is always broader than its true molar mass distributionwarrants. Tung (1966) has shown how this may be corrected for,_but asit is generally accepted that the effect is insignificant when Mw /Mn >2,the usual situation for the rubber analyst, it is not discussed further.

THERMAL FIELD FLOW FRACTIONATION

As has already been noted, an essential prerequisite of GPC (orSEC) is that one has a true solution which is free from gel which

Elution Volume

Figure 8.4 (b) Calibration graph and calculated relationships dWd(log M) againstelution volume.

Elution Volume

Figure 8.4 (a) Experimental data recorder response against elution time.

Calibration graph

ConstructeddWd log M graph

dW

dlo

gM

could block the separating column. Even microgel, which might beable to pass through the column without blocking it, will rupture,or shear, providing a distorted pattern of molar mass distribution.The examination of a clear true solution of a polymer will thereforeprovide only part of the picture if the material has a measurablegel or microgel content which has been removed prior to analysisby GPC.

Recently, Fulton et al. (1996) have begun to investigate the applic-ability of thermal field flow fractionation (ThFFF) to the characterizationof high molar mass polymers. When used together with a multi-anglelaser light scattering (MALLS) detector the technique has allowedabsolute molar mass and size distributions to be obtained without theneed for calibration standards.

The technique can be considered hyphenated as the fractionationstage followed by the light scattering detection stage are complemen-tary in obtaining the final data. Separation of the sample intofractions of graded molar mass is achieved by applying a tempera-ture gradient (104°C/cm) across a thin channel, typically 125 |imthick, and injecting the polymer solution at one end. The tempera-ture gradient drives polymer molecules towards the colder wallwhilst mass diffusion results in the molecules of smaller molar masspreferentially migrating from the colder region to the warmer one.Since the velocity of the eluting solvent is higher in the highertemperature regions, separation is achieved with the lower molarmass materials being eluted earlier. The multi-element detector isderived from the classical technique for determining absolute molarmass and size. The particular detector used has 18 photodetectorsspaced round a special flow cell in a geometry which permits simul-taneous measurements to be made over a wide range of angles. Theobserved scattering for a particular 'time slice' is then extrapolatedto 'zero angle' and this value is used to determine the absolutemolar mass of that time slice. Data can also be used to obtain themolecular size distribution and the size/molar mass data can thenbe further used to investigate parameters such as molecular confor-mation and chain branching.

The technique obviously has very considerable potential for providinga total 'package' of information which will be useful not only for theore-tical purposes but also on the factory floor where small batch to batchmolecular variations within a polymer can have significant implicationsto the compounder.

The techniques described so far in this chapter have a general applic-ability to the determination of the molar masses of polymers but anumber of others have been used for specific applications, three ofwhich are considered briefly.

DIFFERENTIAL SCANNING CALORIMETRY / DIFFERENTIAL THERMALANALYSIS (Mn)

These techniques are discussed in detail in Chapter 7 but, in the contextof molar mass measurements, we are particularly concerned with themeasurement of glass transition temperatures (Tg).

Fox and Flory (1950, 1954) were the first to observe that, for ahomologous series of fractionated polystyrenes, there is a linear relation-ship between the glass transition temperature and the inverse of thenumber-average molar mass:

T*=T*ro-i (8-16)

where Tg is the glass transition temperature for the particular polymermolar mass, and Tg oo the glass transition temperature for the samepolymer of infinite molar mass. It will be appreciated that as Mnincreases, the difference between Tg and Tg oo will become too small foraccurate measurement and thus the method is only useful for materialsof relatively low molar masses, typically up to 5000 for polyisoprene(Kow et al, 1982), 50000 for polystyrene (Loadman and Tinker, 1980)and 70000 for poly aery lonitrile (Keavney and Eberlin, 1960). However,given that reference compounds covering the molar mass range ofinterest are available, this probably constitutes one of the simplestmethods for rapidly checking the molar mass of a homopolymer ofrelatively low molar mass.

ULTRACENTRIFUGATION (Mw)

This is probably the most intricate of existing methods for determiningthe absolute molar mass of a high polymer but, although it shows agood deal of success in dealing with relatively compact high molarmass materials such as proteins, problems such as entanglements andextended chain effects render the results much less useful for thepolymer analyst.

SEDIMENTATION EFFECTS (Mw OR Mz)

Many sedimentation experiments have been carried out under differentexperimental conditions, but they may be generally summarized asbeing time consuming and of restricted use whilst the results arecomplicated to evaluate. The interested reader is referred to representa-tive publications on the determination of molar mass distribution byScholte (1970), and on the use of a mixed solvent system to generate adensity gradient, and hence separate polystyrenes of different tacticities,by Morawetz (1965).

MICROSTRUCTURE

Polymer microstructure is most simply defined as the arrangement ofthe various monomer units which constitute that polymer within thepolymer chain. A study of the type and distribution of the monomericspecies will give information on isomer specificity, stereospecificity andchain branching whilst, in the case of copolymers, data on the distribu-tion of the chemically different monomers will indicate the randomnessor 'blockiness' of the copolymerization. It will be apparent that thesefeatures are of crucial concern to both the manufacturer and user ofsynthetic polymers, as relatively small structural changes within thechain can have appreciable effects on the physical properties of theproduct manufactured therefrom.

If the monomer units are all identical and contain no asymmetriccentres, then the structure is completely defined without ambiguity.This would be the situation with an ideal polyethylene (CH2)n. Amonomer such as isoprene, however, could polymerize to give fourisomerically different monomer units as illustrated in Figure 8.5. It istherefore necessary not only to identify and quantify each of the fourpossible structures but also to obtain data, if possible, on the monomersequence distribution throughout the chain.

On many occasions the repeat units within the polymer chain will

frans1,4-

Figure 8.5 Isoprene: isomeric options on polymerization.

isotactic syndiotactic

Figure 8.6 Isotactic and syndiotactic polymerization.

contain asymmetric centres and then we must consider tacticity, whichdescribes the relative configurations of these asymmetric centres. Such amonomer is illustrated diagrammatically in Figure 8.6. It maypolymerize with the configuration of the asymmetric centres identical(isotactic), regularly alternating (syndiotactic) or random (atactic).

An alternative nomenclature, which allows one to define in absolutedetail larger monomer sequences, considers an existing chain and thenadds the next monomer in either a meso configuration (m) relative tothe last monomer of the chain (i.e. in the same sense) or in racemicconfiguration (r) (i.e. the opposite sense). Thus m represents a pair ofmonomers in a like configuration, building to a triad (mm) analogous tothe isotactic structure illustrated, whilst rr is analogous to the syndio-tactic triad illustrated. The chains can then continue to grow in a specifi-cally defined way (Figure 8.7). Although this is an area of extremecomplexity, much of which is beyond the terms of reference of thisbook, the rubber-analyst should have some idea of the types of datawhich may be obtained.

MONOMER TYPEExactly the same techniques may be used to study the types ofmonomer present as are described, in Chapter 7, on the instrumental

monomer

Figure 8.7 Monomer chains.

analysis of polymers. Perhaps the most significant difference is that,whereas in the earlier applications we were generally concerned withidentifying the major species, much of the microstructural analysis isconcerned with the presence of low levels of specific isomeric struc-tures.

As long ago as 1946, Field et al. used infrared spectroscopy toexamine various polyisoprenes qualitatively and some years laterRichardson and Sacher (1953) quantified the analysis in terms of all fourpossible isomers illustrated earlier. It is interesting to note that whereasone of the modern techniques for analysing a polymer blend byinfrared spectroscopy is computerized curve matching against referencespectra of the pure compounds, Richardson and Sacher confirmed theircalculations by manually computing the summed spectra of specificregions from their reference data, and comparing these with the experi-mentally determined spectra. Cunneen and co-workers (1959) also usedinfrared spectroscopy to determine the cis and trans contents of isomer-ized natural rubber.

Raman spectroscopy may be used in a similar way to that of IRspectroscopy but with the added advantage that the band intensities aredirectly proportional to the concentration of the species present. Arecent review of various methods for determining the microstructure ofpolybutadienes by Edwards et al. (1991) recommended Raman spectro-scopy as having distinct advantages over both IR and NMR techniques.

Pyrolysis-gas chromatography is described in detail in Chapter 7 and,as with IR, it will be apparent what sort of data are available, particu-larly if the monomer source of specific peaks can be identified. Tsuge etal. in 1980 and Tsuge (1981) described a combined pyrolysis-hydrogena-tion glass capillary gas chromatograph and illustrated how this wasused to study the microstructure of polyethylene, polypropylene andcopolymers of the two.

The use of DSC for the determination of polymer molar masses hasbeen noted but it may also be used in specific applications to providemicrostructural data. For instance Kow et al. (1982) point out that the Tgof synthetic polyisoprenes is linearly related to the 1,4 content, whilstinspection of the data given in Appendix B will show many occasionswhen a relationship between Tg and a microstructural feature exists. Itis worth noting, however, that changes in the cis:trans ratio, both forisoprene and butadiene, do not measurably affect the Tg's.

Of all the techniques suitable for the determination of microstructure,NMR is undoubtedly the best and many thousands of papers have beenpublished on virtually every aspect of polymer analysis. One exampleof proton (1H) NMR and one of carbon (13C) NMR will suffice to illus-trate the point. Figures 8.8 and 8.9 illustrate the 1H-NMR spectra ofnatural and a synthetic polyisoprene. The trans methyl group and 3,4

Figure 8.8 1H-NMR spectrum of NR.

trans

Figure 8.9 1H-NMR spectrum of 3,4/high trans Pl.

Table 8.3 Low trans contents of isomerizedNR measured by 1H-NMR spectroscopy

Sample % trans contentby NMR by IR

a 2.9 2.4b 3.9 3.6c 4.6 4.2d 5.5 5.2

units are clearly observed in the latter and can be estimated by peakarea integration, as originally reported by Golub et al (1962). Loadman(1978) used a reference solution of pure natural rubber, with appro-priate sensitivity and resolution adjustments, to define exactly the cismethyl peak within the cis-tmns envelope, together with a modificationof the infrared method of Cunneen et al. (1959), to measure the transcontent of isomerized natural rubbers. The results in Table 8.3 showthat good agreement can be obtained even at low levels of isomeriza-tion.

The set of spectra illustrated in Figures 8.10-8.12 shows 13C-NMRspectra of the alkyl regions of natural rubber, isomerized natural rubberand a synthetic polyisoprene.

As with 1H-NMR spectra the signals due to the trans and 3,4 speciesare clearly visible but with greatly improved resolution and, by usingspectral accumulation techniques, very low levels of these may bemeasured, the limit of detection currently being of the order of 0.005-0.01 moles per 100 isoprene units.

The application of solvent swollen NMR spectroscopy (Chapter 7) tomicrostructure determination means that in many cases informationwhich could previously only be derived from elastomers in solution cannow be gained from crosslinked or otherwise insoluble materials (Hullet al., 1994; Hull and Jackson 1997). The spectra obtained by thistechnique are rarely well resolved and require an extensive knowledgeof the relative band positions of the polymers being investigated.

MONOMER DISTRIBUTION

Of those techniques described for the identification of the monomericspecies, all can contribute something to the study of monomer sequencebut most are of little general significance when compared with 13C-NMR spectroscopy.

Infrared, or pyrolysis infrared, spectroscopy can be used to distin-guish between block and random copolymers; for instance the spectrum

transcis

Figure 8.12 13C-NMR line spectrum ofsynthetic (high cis) Pl.

Figure 8.11 13C-NMR line spectrum ofisomerized NR.

Figure 8.10 13C-NMR line spectrum of NR.

of a pyrolysate of an SBS block copolymer is the summed spectra of thepyrolysates of polystyrene and polybutadiene and is quite differentfrom that of a random SBR copolymer pyrolysate, but beyond this thereis little detailed information available.

Raman spectroscopy can also be used in the study of tacticity, crystal-linity and even orientation. Broad Raman bands of low intensity areassociated with materials of low stereochemical purity (e.g. syndiotacticpolypropylene) whereas sharp intense bands are characteristic of a moreordered material like isotactic polypropylene (Fraser et aL, 1973).Crystallization will strongly influence the intensity of the Ramanspectrum with intensity increasing with crystallinity. This can beobserved in samples of SBR where, as the styrene content increases tothe point where the styrene blocks can form a crystal structure, thestyrene band at 1000cm"1 suddenly increases in intensity. The methodof determining crystallinity by Raman spectroscopy has been coveredby Gerrard and Maddams (1986), updating earlier work by Strobl andHagedorn (1978). Rubbers can be induced to crystallize under pressureor strain and Wang et al. (1989) showed the crystallization of syntheticpolyisoprene under pressure whilst strain crystallization of NR andpolychloroprene has also been demonstrated (Jones and Hendra, 1990and Wallen, 1991).

Tsuge (1981) extended the application of pyrolysis hydrogenation gaschromatography to a study of the high molar mass fragments from thepyrolysis of polyethylene and several polypropylenes of varying tacti-city. An examination of those products containing greater than twelvecarbons (PP tetramer) showed that information on stereoregularitycould be obtained. He does, however, emphasize the points made in thediscussion on pyrolysis-gas chromatography of the need for a standar-dized PGC system to produce universally significant and correctabledata.

Differential scanning calorimetry provides the data already discussedwhich, in the appropriate situation, can afford information relating tothe 'blockiness' of a copolymer, and also provide an indication of thelength of the blocks. Roovers and Toporowski (1974) and Kow et al.(1982) have studied the Tg's of star-shaped polystyrenes and polyiso-prenes respectively. The polystyrene work established a relationshipbetween Tg and the number of chain ends in polystyrene stars withmolar masses < 90000 but the latter authors found no variation withpolyisoprenes of molar masses > 10000. The probable explanation ofthis is that the polystyrenes had molar masses in the non-asymptoticregion of their molar mass vs. Tg plot whereas the polyisoprenes wereall in the asymptotic region.

Perhaps one of the classic illustrations of the use of 1H-NMR spectro-scopy in the analysis of microstructural differences is that of

Figure 8.13 1H-NMR spectrum of polymethylmethacrylate.

polymethylmethacrylate (PMMA), shown in Figure 8.13 and describedby Bovey and Tiers (1960). Three distinct signals are observed due tothe central methyls of the isotactic, heterotactic and syndiotactic 'triads'(or blocks of three monomers).

At about the same time, Bovey et al (1959) and Bovey and Tiers(1960) reported the analysis of many styrene copolymers and showedthat, as well as obtaining the styrenerbutadiene ratio, there was infor-mation available on the size of the styrene blocks. Below about eightconsecutive styrene units the aromatic peak appears as a singlet, butabove eight a doublet is seen. Random polymerization requires 80%styrene before the average block size reaches eight, hence with moststyrene-containing copolymers (if the styrene loading can simulta-neously be obtained from the spectrum) this method gives a rapidqualitative criterion to distinguish between the random and blockcopolymers.

Of all the examples of the application of 13C-NMR spectroscopy tothe analysis of microstructure only one relatively simple one isillustrated here to indicate the amount of information available. This isfor free radical initiated polyvinylchloride and uses the data of Carmanet al (1971) and Carman (1973).

The 13C-NMR spectrum is illustrated in Figure 8.14. The spectrumshows five bands assignable to methylene carbons and a further seven(on expanding the spectrum) originating from methine carbons. Thislarge number of signals is due to differences in chemical shifts of thecarbons brought about by the variation in configuration of the neigh-

CH3 Resonance CH3 triad resonances- SyndiotacticHeterotacticIsotactic

P-Methyleneresonances

PPM from TMS

ppm from TMSFigure 8.14 NMR noise decoupled spectrum of free radical initiated PVC intrichlorobenzene at 12O0C, 25.2 MHz (Carman, 1973). (Courtesy Macromolecules.)

bouring groups. The assignments made by Carman are shown on thespectrum with the rr and mr bands actually splitting into two.

The particular advantages of 13C-NMR spectroscopy are that one isable to make extremely precise calculations of a particular chemicalshift using established incremental data for carbon atoms several timesremoved from the one being studied and intrinsic instrumental resolu-tion allows observation and quantification of these small differences.The interested reader can obtain a great deal of information frompublications by Chen (1968) and Randall (1977, 1979) which, althoughdated, provide a great deal of background data and discuss in detailmany specific polymers. The only deficiency in these earlier papers isthe relatively poor resolution resulting from the comparatively lowfield strengths of the available instrumentation. Werstler (1980)discusses the application of 13C-NMR to the analysis of cured filledelastomers.

METATHESIS

In 1967 Calderon et al. described a process which they called olefinmetathesis whereby vinylic olefins, when treated with a catalyst oftungsten hexachloride, ethyl alcohol and ethyl aluminium dichloride,were transformed according to the scheme:

2 R1CH=CHR2+ 2 R3CH=CHR4 -»

R1CH=CHR3 + R2CH=CHR4 + R1CH=CHR4 + R2CH=CHR3

Since that date a number of papers have been published which refer tothe use of this technique in polymer structure elucidation. ThusMichailov and Harwood (1970) treated BR and SBR with 2-butene toobtain 2,6-octadiene from (l,4-)-(l/4-) sequences within the BR, and both5-phenyl-2,8-octadiene and 4-phenyl cyclohexene from the (1,4-)-(styrene)-(l,4-) sequences. The products were identified by gas chroma-tography. A similar procedure was carried out by Stelzer et al (1977)who used 4-octene as the metathesis olefin whilst Kumar and Hummel(1982) used a catalyst of tungsten hexachloride and tetramethyl tin with4-octene to study the extent to which a crosslinked 1,4-polybutadienecould be solubilized with varying reaction times. Analysis was by GPCand they concluded that the soluble fraction (particularly the highermolar mass components) passed through a maximum value. Hummel etal (1982) described how peroxide-cured 1,4-polybutadiene, filled with awide range of materials, may be broken down with 1-octene and acatalyst of tungsten hexachloride so that the fillers may be quantita-tively removed after filtration.

LATEX PARTICLE SIZING

The application of specific analytical techniques to latex or latexproducts is not emphasized throughout this book since most aredirectly suitable or may be simply modified in terms of sample prepara-tion. There is, however, one characteristic of latex which does not applyto dry rubbers and that is the size of the latex particles. There areseveral techniques by which this parameter can be determined but eachof these has different strengths and weaknesses; ultimately thetechnique selected will be determined by the type of results wanted andthe equipment available.

SAMPLE PRETREATMENT FOR TRANSMISSION ELECTRON MICROSCOPY

It is, however, necessary to do some substantial preparative work priorto the microscopical examination since elastomer latices are, by defini-tion, film forming and any preparation that does not take this intoaccount will be wholly unsuccessful in producing valid results. Case-hardening is the technique used to retain the individuality of the latexparticles and an assessment of the different techniques available for thisis therefore required.

The most popular and most often used are bromination described byBrown (1947) and Schoon and Van Der Bie (1955), and osmiumtetroxide fixation described by Gomez and Hamzah (1989) and Kato(1966). Other techniques have been mentioned but only the first twomerit further description here.

Bromination

This technique seeks to case-harden the latex whilst in suspension byexposure to bromine and the referenced texts give different proceduresfor accomplishing this. A further procedure, described by Cobbold(1988), involves diluting the suspension with distilled water and addingbromine water to the suspension until a yellow endpoint has beenreached. A few drops of the suspension are transferred to a nebulizerand blown on to a TEM examination grid coated with a formvarsupport film. This is then micrographed with reference to a standard togive accurate measurements. The support film is made by dissolvingformvar in chloroform to form a 0.5% w/v solution and casting a filmon to a clean glass slide. Once the chloroform has evaporated the film isscored and floated off on to water from which it can be transferred toTEM examination grids.

There is one notable drawback to this technique which is that bromi-nation may cause a swelling of the particles of the order of 10%.Although this will not cause a variation in a size distribution, allowancesshould be made in terms of absolute measurements.

Treatment with osmium tetroxide

As with bromination, it is necessary to treat the latex whilst it is insuspension. Matters are made slightly more complicated by the hazardsinvolved with handling osmium tetroxide and suspensions that maystill contain unreacted osmium tetroxide. This is virtually a prerequisiteof the technique since there is no easily observable endpoint fromwhich to judge how much fixation is required to case-harden the parti-cles. Despite this, Gomez and Hamzah (1989) persevered and devised amethod that involved mixing a drop of latex with a drop of fixative asit was removed from the tree. Unfortunately they referred to the treat-ment being carried out for a 'suitable duration7 without giving anindication of how this was to be determined. The micrographs includedin their paper indicate that their technique was successful, but not everylaboratory has the advantage of being able to fix their latex as it isremoved from the tree!

Perhaps of greater practical use is the technique devised by Kato(1966) in which a drop from a suitably diluted latex suspension isplaced on a TEM grid coated with a support film. The grid is thenplaced in a tightly sealed glass jar containing an aqueous suspension ofosmium tetroxide for thirty minutes (although osmium tetroxidecrystals would probably be just as satisfactory). Provided that thesuspension does not dry too quickly the technique seems to be apractical alternative to bromination.

TRANSMISSION ELECTRON MICROSCOPY

The major advantage of the technique is that it is possible to observethe structure of the particles at the same time as they are being sized.The major disadvantage is that it is difficult to measure a great enoughnumber of particles for the data to be statistically significant. Indeed,very early in the development of techniques for this type of study itwas suggested by Cobbold and Gilmour (1971) that at least 10000 parti-cles should be counted. Clearly this requires some form of automatedcomputer analysis to be viable. Numerous systems are available andwith the advent of digital cameras and on-line processing this can becarried out using a transmission electron microscope (Chapter 9)without the need for the taking of any micrographs. Nevertheless, ablind faith in computer generated numbers can sometimes be misplacedand there will always be a place for micrographs and visual examina-tion. An experienced microscopist should be capable of screening aspecimen for representative areas, the measurement of which will giveat least a first order approximation (or better) of the particle size distri-bution.

Several points need to be made regarding the electron microscopy oflatex particles. Perhaps the most important is that it is essential that acalibration sample is included at the same magnification and time asthe particles being examined. Furthermore, once the work has beencommenced the magnification settings should not be changed because areturn to the precise magnification previously used cannot be guaran-teed.

As far as possible, exposure to the beam should be limited since evenfixed latex is not stable in the beam for long periods. For this reason itis recommended that a reasonably small second condenser lens apertureis used. If a cold stage is available, its stabilizing properties will makeits use well worth while.

PARTICLE SIZING BY PHOTON CORRELATION SPECTROSCOPY (PCS)

This technique, also known as dynamic light scattering (DLS) andquasi-elastic light scattering (QELS), is a very different way of deter-mining particle sizes and involves neither direct observation of theparticles nor lengthy fixative methods. Described by Pendle andSwinyard (1990), the technique depends on the measurement of fluctua-tions in the intensity of scattered light produced by the particles inBrownian motion by a rapid response photomultiplier coupled to acomputer. No calibration or preparation is required and the measure-ments can be carried out in about an hour.

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Polymer 30, 524.Werstler, D.D. (1980) Rubber. Chem. Technol. 53, 1191.Witnauer, L.P., Senti, F.R. and Stern, M.D. (1955) /. Polym. ScL 16, 1.Wyatt, PJ. (1992) Anal. Chim. Ada. 272, 1.Zimm, B.H. (1948) J. Chem. Phys. 16,1093.

Blend morphological Q

analysis \3

For the rubber technologist, the routine assessment of polymer blendmorphology should be considered essential in developing newmaterials. For many years rubbers have been blended together so thattheir individual properties can be combined to produce a materialwhich has a particular combination of properties not available in theindividual materials. It is only by the use of some form of microscopicaltool that one can literally observe the changes made to the phasemorphology of a blend by variations in cure times, curatives, fillers etc.This type of information is now being used to fine tune blends in orderto produce exactly the type of final material that is required.

The aim of this chapter is to discuss the microscopical techniqueswhich are available for morphological observation. The five techniqueswhich are described produce different but complementary informationand it is often the case that more than one technique will be required tobuild up a complete picture of the system under investigation. Since itis essential to decide what information actually is required beforeembarking on any form of microscopical analysis, the different techni-ques, with the information they provide, are described first, followed byan in-depth discussion of the various preparative methods required forthem. Towards the end of the chapter a worked example illustrates howsome of the techniques described can be applied.

LIGHT MICROSCOPY (LM)

Light microscopy should normally be the starting point in the microsco-pical examination of any new sample. Transmitted images are obtainedfrom thin sections taken from the bulk of the sample, effectively givinga cross-sectional image. However, there tends to be very little contrastbetween the phases of most elastomer blends using common light andconsequently little information is available unless one of the morespecialized LM techniques is applied.

Probably the example which supplies the most easily interpretablemicrographs is phase contrast LM. In this technique, differences inrefractive indices between the two (or more) phases are exploited toproduce contrast as discussed by Haynes (1984). For a number ofmaterials this technique can produce useful micrographs which giveconsiderable information about blend morphology, including floworientation, phase sizes and variations thereof, and degree of co-conti-nuity.

LM is, however, limited in resolution terms (resolution being definedas the minimum distance between two object features at which they canstill be seen as two features) to 0.25 j^m as expressed by Sawyer andGrubb (1987):

, A where d = resolutionk/VA A = wavelength of light used

k = a constantNA = Numerical Aperture

In practical terms, this means that no additional useful information canbe extracted from a light microscope beyond a magnification of aboutx 1000 and this will be a limiting factor in obtaining useful informationin some laboratories.

SCANNING ELECTRON MICROSCOPY (SEM)

SEM provides a relatively fast technique for observing blendmorphology at higher resolutions than are available with visible light.The technique is based on the observation of a surface and is thereforenot quite so suitable for observing the internal phase structure of indivi-dual phases.

An image is formed in the SEM by the scanning of a focused electronprobe across a specimen surface, under vacuum, while synchronized tothe raster scan on a TV-type display. Where the electron beam strikesthe surface, a number of different types of interaction take place,including the emission of electrons of differing energies. To form animage, one or more of the electron signals is selected and, as the regionunder inspection is scanned, the strength of the signal modulates thedisplay signal.

Although numerous interactions take place, this discussion will belimited to only two: secondary electrons and backscattered electrons.Brundle et al. (1992) describe secondary electrons as low energyelectrons which have been inelastically scattered by atomic electrons inthe sample, and backscattered electrons as high energy electrons whichhave been elastically scattered by atomic nuclei. Image contrast is gener-ated in different ways from these two types of signal. Changes in the

surface topography cause changes in the secondary electron signal andchanges in elemental composition cause changes to the backscatteredelectron signal. In other words a greater slope produces a strongersecondary electron signal and a higher atomic number produces astronger backscattered electron signal. An ideal specimen would there-fore be similar to that shown in Figure 9.1 in which an object made of aheavy metal protrudes significantly from the surface of a carbonsubstrate. The heavy metal produces a strong backscattered electronsignal by virtue of its high atomic number whilst the backscatteredsignal from the carbon substrate is comparatively weak. Thetopography of the protrusion gives rise to a strong secondary electronsignal. Unfortunately, in the world of polymer and elastomer blends,this idealized situation does not occur; the reality is more akin to thatdepicted in Figure 9.2.

Elastomers in general are comprised of very similar elements, all ofwhich are low in atomic number, hence producing little contrast from abackscattered electron signal. Furthermore, blends are often examinedas sections, so there is very little variation in topography and hencevery little contrast due to the secondary electron signal. Consequently, ifexamined directly, most sections of elastomers appear relatively feature-less by SEM unless some form of differential relaxation takes placebetween the phases after sectioning. Techniques therefore have to beintroduced to artificially increase the contrast in the SEM and two suchtechniques have predominated: chemical staining and chemical etching.These are both described later in this chapter with examples of stainsand etching materials in common use.

Figure 9.1 SEM idealized sample.

Carbon substrate

1 micron

Lead protrusion

The two phases have similar elemental composition.The section has little surface topography.Result: very little contrast. The sample appears featureless by SEM.Figure 9.2 SEM elastomer blends.

TRANSMISSION ELECTRON MICROSCOPY (TEM)

TEM provides very high resolution (and correspondingly sharp) imagesof the internal structure of samples prepared in the form of ultra-thinsections. Most instruments should be capable of a resolution of at least0.34 nm with a suitable test specimen, thereby giving more visual infor-mation about overall blend morphology and the structure within theindividual phases than the other techniques discussed. It should benoted that Agar et al. (1974) pointed out that TEMs with singlecondenser lens illumination are not suitable for polymer and elastomerstudies because of the lack of control and subsequent damage impartedto the specimen. It should also be noted that beam damage to thespecimen is minimized and resolution is maximized by examining thespecimen at 100 or 125 kV where this is within the capabilities of theinstrument.

Images are formed in a TEM by focusing an electron beam with aseries of condenser lenses on to an ultra-thin specimen, typically 150 nmthick or less. As electrons travel through the specimen, a proportioninteract with atomic nuclei or electrons from the specimen and aredeflected. The paths of these deflected electrons may take them outsidethe objective aperture, depending on the size of the aperture, and hencethe regions that deflect electrons appear dark to a degree dependent onscattering ability. This is the mechanism by which image contrast iscreated. The remainder of the signal is focused and enlarged by a seriesof electro-magnetic lenses on to either a phosphorescent viewing screen

Continuous phase (B)-C1 H, N and O

Discrete phase (A)-C, H and O

or a camera. The image is therefore formed from variations in themass/thickness of the section whilst the level of contrast is dependenton a number of factors and is maximized by the choice of a suitableobjective aperture. However, as with SEM, there are usually insufficientelemental differences between the elastomers of a blend to producemuch scatter so contrast is limited and it is usually necessary to induceadditional contrast by differentially staining the specimen. This isdescribed in greater detail later in the chapter.

There are some important limitations with TEM and perhaps themost significant is that it can be an extremely time consumingtechnique. It requires ultra-thin sections, usually less than 150 nm thick,and preparation of sections of this type can take from a few hours toseveral days dependent on the nature of the sample. In addition, lowmagnification images are often subject to considerable distortion onconventional TEMs and, unfortunately, images below x 4000 magnifica-tion usually provide the information of most interest in this field.Furthermore, there is no suitable instant film for direct electron imagingso negatives have to be developed and printed separately. Ultimately,although TEM is vital for high resolution work, it is not always practicalfor the routine assessment of phase morphology of large numbers ofsamples.

Two other transmitted electron imaging techniques are available andthese are described below.

SEM BASED SCANNING TRANSMISSION ELECTRONMICROSCOPY (S(T)EM)

Two types of S(T)EM exist, and within this context they could bedescribed as the 'missing link7 between TEM and LM. The first is themore conventional (and expensive) and generally requires a modifica-tion to the microscope to include a secondary electron detector under-neath a special specimen stage which permits observation intransmission. The second, proposed by Ansell and Stevenson (1993) andsubsequently described by Cudby and Gilbey (1995), is a much cheaperoption and involves the use of a simple mount which fixes to thenormal SEM stage and produces a redirected secondary electron signalat approximately 90° to the transmitted signal from an ultra-thinsample. This can be collected by a standard secondary electron detectorin the SEM without the addition of other detectors or modification tothe instrument. The first system is generally a factory fitted optionwhilst the second can be easily constructed for use on almost any SEMalthough the dimensions may vary depending on the design of theinstrument. The mount described here was originally sized for theHitachi S-2700 SEM.

Figure 9.3 The S(T)EM mount in cross-section.

The theory behind the technique is somewhat of a hybrid betweenSEM and TEM imaging and can be best described by reference toFigure 9.3. The S(T)EM mount consists of several copper components.The top is a hollow tube incorporating a locking cylinder in which theprepared specimen, placed on a standard TEM examination grid, isheld. This is positioned over a polished and gold coated angled platewhich, when placed on the stage in the SEM specimen chamber, isangled towards the secondary electron detector. The focused electronbeam is scanned across the area of interest of the section in the sameway as for a normal surface examination and when the electrons strikethe sample a portion of the signal is forward scattered as opposed tobackscattered. After transmission, these electrons strike the angled plateproducing a secondary electron signal which is directed at and detectedby the secondary electron detector. This produces an image, the charac-teristics of which are determined by the mass and thickness of thesection. As with TEM, the similarities in average atomic numberbetween the phases require that the material be differentially chemicallystained in order to produce useable levels of contrast in the final image.

"Transmitted"secondaryelectrons

CutawaySecondary electrondetector

This technique has several advantages, the first of which is cost. It isnot unusual for a TEM to be prohibitively expensive for a laboratory topurchase and maintain. SEMs are generally cheaper and more widelyavailable and this technique provides a very cost-effective method fortransmitted electron imaging. It provides higher resolution than with aconventional light microscope and image clarity approaches that of aTEM. The added versatility imparted to the SEM with its ease of use atlower magnifications (approximately x 1000-2000) and its digitalelectronics mean that the hybrid instrument is capable of imagingsections that would be difficult to view in the TEM, and since, unlikethe TEM, it can use instant film, results can be obtained more quickly.

TEM BASED SCANNING TRANSMISSION ELECTRONMICROSCOPY (STEM)This technique is probably the most common type of scanning transmis-sion tool and is usually an addition to a conventional TEM. Highresolution dedicated STEMs are available but are described by Sawyerand Grubb (1987) as being extremely expensive and generally unsui-table for polymer and elastomer work because of the ultra-highvacuums at which they operate. Nevertheless, a few points are worthnoting.

Images are formed in a similar manner to an SEM by scanning afocused beam across a specimen. The image is formed from theresulting rastered transmission signal which is displayed on a TV typedisplay. The technique has a number of advantages over conventionalTEM:

• The digital signal produced can be manipulated to obtain visibleimages with a very low beam current. This is extremely useful withunvulcanized rubber and other beam sensitive materials.

• The technique can also be used to undertake other analytical techni-ques such as energy dispersive microanalysis (EDX) and varioustypes of electron energy loss spectroscopies (EELS, PEELS etc.).Collectively this has become known as analytical electron micro-scopy. Elemental distributions can be resolved to between 5 and 50run.

• Different digital signals can be mixed.• Irradiation of the specimen is limited to the area being examined.

This is extremely useful with beam sensitive materials.• Digital images can easily be downloaded to a computer.

However, STEM instruments do not have the resolution of conventionalTEMs and they do not provide good diffraction images. For thesefunctions conventional TEM should be used.

MICROTOMY AND ASSOCIATED TECHNIQUES

After deciding on the type and level of information required, and whichmicroscopical technique is appropriate or available, the material to beexamined must be suitably prepared. This is often far more timeconsuming than the actual microscopy and may require a significantcommitment in terms of time to develop the relevant skills. The proce-dure of embedding the rubber before sectioning is not included sincethis can alter the observed phase morphology (Smith and Andries,1974) and, with modern cryo-preparative techniques, is redundant.

Generally speaking, if in doubt or if a new material is beingexamined it is better to start with LM to try and obtain an overview ofthe morphology. One should not be tempted to begin with STEM orTEM since this will limit the area that can be observed.

MICROTOMY USING A BASE-SLEDGE MICROTOME

Walter (1980) gives a general description of the base-sledge microtomeand this technique can only be considered for obtaining semi-thinsections for light microscopy. The chief difficulty is in achieving a lowenough temperature to be able to cut sections since the Tg of naturalrubber is - 72 0C and it is normally necessary to section at temperaturesbelow this. The base-sledge microtome has no thermally isolated cryo-chamber so maintaining it in this temperature range is difficult in anopen laboratory. If this is the only option, a base-sledge microtomewhich has provision for cooling the sample with liquid nitrogen mustbe used and temperatures should be reduced to as low a value as isrealistically possible, around -80 0C with some instruments. The qualityof the sections is usually dependent on the skill of the operator andsome laboratories have personnel who regularly produce sections forthe Cabot test (Chapter 11) of commendable quality with very basicequipment. The same is therefore possible with phase morphologicalanalysis.

Base-sledge microtomes generally use either glass knives or hardenedsteel blades. Glass knives are discussed in some detail below and arenormally sharper but less hard-wearing than hardened steel blades.Hardened steel blades need regular sharpening or 'stropping' and thisis a specialist job which requires training to obtain the correct knifeprofile. For sectioning hard frozen elastomers the knife profile shouldbe wedge or plane shaped.

ULTRAMICROTOMY USING A CRYO-ULTRAMICROTOME

The difference between microtomy and ultramicrotomy is in the thick-ness of the section obtained. Microtomy is used to obtain thick and

semi-thin sections for LM and incident SEM whilst ultramicrotomy is toobtain ultra-thin sections for TEM and STEM. The complicating factorwith samples made of rubber is that it is necessary to cut them belowtheir glass transition temperature, hence the usual terms: cryo-microtomy and cryo-ultramicrotomy.

The latter is a difficult subject covered in numerous texts which tendto deal with it from a biological standpoint (Sorvall, 1965; Reid, 1975;Sawyer and Grubb, 1987). This requires significant alteration when it isto be applied to elastomers and the following adds some detail relevantto this application.

High resolution phase morphological analysis of elastomer blends isnormally carried out using one or more of the electron microscopicaltechniques described earlier although, where phases are of sufficientsize, observations can, of course, be made by light microscopy. Thefollowing discussion centres on electron microscopy but all the techni-ques can be applied to sectioning on an ultramicrotome for light micro-scopy with the sometimes useful proviso that the operator need not bequite so diligent in producing ultra-thin sections. Sections for any formof transmitted electron microscopy must, however, be extremely thinand can only be obtained by ultramicrotomy. As already mentioned,this usually requires the operator to section at temperatures well belowthe lowest Tg of the system and this can only be done reproduciblywith specialized equipment which can maintain cryogenic temperaturesstable to +/-0.10C. Companies such as RMC and Leica both manufac-ture this type of equipment.

At this point it is pertinent to discuss how thin a usable ultra-thinsection should be, since time can be wasted trying to cut sections whichare thinner than necessary. In terms of obtaining an image it is unlikelythat an operator with a 10OkV TEM will be able to obtain meaningfulimages from sections that are thicker than 200 nm in a simple gumblend. However, it is not normally necessary to reject any sectiongreater than 50 nm thick as being of no use. Certainly the thinner thesection is, the higher the resolution is of the image that can be obtainedfrom that section, but very few examinations in this context require 1-5nm resolution! A more important criterion is that sections of differentsamples in the same group are cut to similar thicknesses so thatmeaningful comparisons can be made, even to the extent of not usingan extremely thin section from one material if others in the same seriescannot be sectioned so successfully. It is therefore necessary to applysome common sense and an awareness of economics to the subject.Experience suggests as a general rule of thumb that the thinner thesection, the longer it will take to obtain the conditions required toproduce it and consequently the more it will cost in real terms. Cosslett(1951; 1956), showed that the maximum resolution obtainable was

approximately one tenth of the thickness of the section. Some moderninstruments have capabilities for correcting chromatic blurring arisingfrom sections that are thicker than optimum, but Cosslett's figure stillserves as a useful guide. In other words the section could be up to tentimes as thick as the resolution required, provided instrumental limita-tions are taken into account. Therefore, if the operator plans to use TEMfor imaging at reasonably low magnification and requires a resolutionof only 20 nm then, in theory, the sections used could be up to 200 nmthick, although at this thickness he or she is at the limit of what isusable even with a 100 kV instrument. Had the operator spent extratime trying to obtain sections that were 50 nm thick then it could beargued that the time could have been used more effectively. Commonsense also dictates that if the morphological or structural features ofinterest are likely to be much smaller than the anticipated section thick-ness then there is a danger of overlapping features and confusion ofdetail. However, this may not necessarily be a problem provided thatthe operator is willing to spend some time interpreting the image. Forexample, the TEM micrograph of the NR/EPDM blend considered as aworked example towards the end of this chapter is thick enough toreveal a great deal about the structure of the EPDM phase within theNR matrix although there is some confusion of detail regarding themicrostructure within the phases. It is also worth noting that heavilyfilled materials always require thinner sections because of the increasedelectron scatter produced by the filler.

Several aspects of ultramicrotomy warrant detailed descriptions andthese are included below:

the knifesectioning temperaturesize and shape of the block faceknife angle, clearance angle and sectioning speedsectioning using a trough liquidsectioning without a trough liquidsectioning on to ice.

The knifeFor ultramicrotomy there are currently only two choices, a glass knifeor a diamond knife. The advantage of a diamond knife is that it isintrinsically sharper than a glass knife and remains so for far longer.However, diamond knives are extremely expensive to buy andmaintain, with sharpening costs about half the value of the originalknife. They must be used with great care and only for ultra-thinsectioning, never for trimming the block since the knife edge is extre-mely fragile and easily damaged. It is also important to note that

diamond knives are very prone to damage from the various particulatefillers used in rubber and rubber-like materials. Regular resharpening isextremely expensive and, although sections obtained with diamondknives are usually thinner and less prone to artefacts than thoseobtained with glass knives, there is little doubt that, in this field, glassknives are the more economic option.

In contrast to diamond, glass knives are cheap to make and are dispo-sable after use. They are usually produced in-house by a dedicatedknife making device, such as those made by RMC and Leica, whichreproducibly score and break glass into knives. Texts rarely mentionthat making a glass knife can be a time-consuming process and that thebest knives are made by a slow break usually taking in excess of fifteenminutes. This is achieved by carefully adjusting the pressure applied tothe scored glass rhomboid and should result in the stress mark, whichcan be seen starting at the far left-hand end of the edge and curvingrapidly away from the edge, being very faint. With practice it ispossible to break knives in which the mark is almost invisible. A goodslow break can often be recognized in the way in which the two halvesof the knife remain stuck together when removed from the knife maker.

There are essentially two regions on the knife edge. The right-handtwo thirds of the knife is generally less suitable for ultramicrotomy dueto increasing roughness. This part of the knife is best used for trimmingthe block face prior to sectioning. The left-hand third of the edge is farmore regular and seems to become sharper towards the left. There is atendency for the edge to become rough where the stress mark joins itbut once again a slow break reduces this problem. Knives can, ofcourse, be broken more quickly for LM where ultra-thin sections are notrequired.

Sectioning temperatureTo consider sectioning temperature effectively, it is first necessary toconsider the nature of the 'cut' itself. A simple attempt at cutting avulcanizate at room temperature with a razor blade illustrates the diffi-culties involved with cutting this type of material and, in order toobtain ultra-thin sections of high quality, it is necessary to change thecutting behaviour of the elastomer by reducing its temperature to belowits glass transition temperature (Tg). However, using natural rubberwith a glass transition temperature of -720C as an example, it isunlikely to be sufficient simply to reduce the temperature to -8O0C.This is because, when a material is sectioned below its Tg, energy isliberated in the form of heat at the tip of the knife which can raise thelocalized temperature substantially, possibly above the Tg if the generaltemperature is not low enough. The effect of this would be that the

knife would have a tendency to stick into the sample causing a numberof artefacts including tearing. It has been suggested by Reid (1975) andCobbold (1988) that a localised temperature rise of 5O0C is possiblealthough 20-30 0C seems more likely since natural rubber can besectioned quite successfully at -100 0C. The rise in temperature will beinfluenced by other sectioning conditions including the sectioning speedsince the faster the cut, the more energy is being put into the system.

Size and shape of the block face

The size and shape of the block face can dictate whether a sample willbe sectioned successfully or not, and it is generally found that smallerblock faces are easier to section than larger ones. The best results areusually obtained from block faces approximately 0.5 mm wide with thelongest face oriented vertically. For LM preparation, sections can belarger. In terms of the shape of the block face, the literature is againbiased towards biological specimens and some ideas can be distinctlyunhelpful since most biological materials are embedded in a hardpolymer or resin prior to sectioning. These can be sectioned at roomtemperature into a water filled trough to produce long ribbons offloating sections. The block face shape that is often suggested tomaximize the chance of ribbon forming is illustrated in Figure 9.4.However, when sectioning at cryogenic temperatures ribbon forming isnot possible and neither is the trapezoid shape desirable since its broadleading edge will quickly blunt the edge of the knife when sectioningelastomers. Two other shapes are proposed in Figure 9.5; the first ofthese consists of a triangular shaped block with the sharp edge pointedtowards the knife. This is a useful shape but has as its greatest pitfallthe problem of the section not detaching from the block at the broadend when a cutting cycle is complete. A simple modification can be

Figure 9.4 Biologist's preferred block face shape.

Direction ofsectioning

Insert of block faceand ribbon of

floating sections inknife trough

Blockface

Figure 9.5 Preferred polymer and elastomer block face shapes.

made to the shape by cutting away the top corners, thereby makingdetachment more easy. As with many aspects of sectioning, this is asmall change but it can make a big difference to the ease with whichsections can be prepared. It can be difficult to trim the block face tosuch a precise shape and this is best accomplished by trimming theblock face to a triangle using a razor blade prior to mounting it in themicrotome. It is then cooled to the required temperature at which it canbe trimmed using a small pre-chilled micro-scalpel.

Knife angle, clearance angle and sectioning speedProvided that the correct temperature has been established, it is oftenthe interplay between these three sets of conditions which determinewhether sectioning will be successful or not. The distinction betweenknife angle and clearance angle should first be drawn and these areillustrated in Figure 9.6.

When working with glass knives a compromise must be drawnbetween sharpness and longevity. Shallower angled knifes, e.g. withangles of 35°, tend to be sharper than knives made with large knifeangles, e.g. 60°, but are short-lived, especially with filled materials. Thechoice of the knife angle depends largely on the type of specimen underconsideration and on the preferred conditions of the particular operatorand the way in which he or she sections. If the sample is very hardthen using a larger angled knife may be necessary but this will be at thecost of sharpness and consequently the sections may not be of such adesirable thickness. For most elastomer applications it has been foundthat a 45° knife seems to provide the optimum conditions.

The clearance angle is determined by the hardness of the specimen at

Pentagonal block face (orhexagonal if bottomcorner removed) for

elastomers

Triangular block face forpolymers

Figure 9.6 Knife angle and clearance angle.

the temperature used to section it. The harder the block is, the largerthe clearance angle will need to be in order for sections to be cut. Thisangle can be varied between 0° and 10° on most ultramicrotomes.However, it should also be noted that the steeper the angle, the moreprone the sections will be to compression. There is also a tendency forthe knife to scrape across the surface of hard block faces if the clearanceangle is too shallow and if this happens it will cause a rapid blunting ofthe knife edge which will be further degraded when the knife finallycuts because the section will be far thicker than originally intended. It istherefore up to the operator to judge the specimen and set the clearanceangle accordingly. At the end of the day there is no substitute forexperience.

Sectioning speed is dictated by both the sample and the conditionsselected by the operator. No clear guidelines can be set although it isoften better to start at a slower speed and increase if possible. Withpractice it becomes more easy to section manually since the operatormaintains more control than if automated motor drive is selected.However, some sections require manipulation with a single-hair brushas they are being sectioned and under these circumstances, automaticcontrol is essential.

Knife

Clearanceangle

Specimenblock

Knife angle

Sectioning using a trough liquidThis is another area where many of the skills and practices developedfor biological specimens are largely redundant with elastomers. Thepractice of sectioning into water at room temperature cannot be accom-plished with elastomers, unless they have been embedded, because ofthe cryogenic temperatures involved. There are some trough liquidsavailable that do not freeze at these temperatures, although sections donot float on these in the same way that they do on water at roomtemperature. An effective example is n-propanol which can be useddown to about -12O0C. Good results can be obtained from having asmall amount of n-propanol in the trough which can be swept carefullyup the knife to the edge using a single-hair brush. Care must be takennot to add too much since its relatively high viscosity leads to its easilybeing dragged over the edge of the knife and on to the sample, atwhich point it usually becomes necessary to stop and clean the block.Careful sectioning will lead to sections sliding down the n-propanolinto the reservoir. This can be aided by gently sweeping sections downusing a single-hair brush. When sufficient sections have been taken,more n-propanol is added to the trough to raise the level and sectionsare retrieved by scooping them up with a slightly bent TEM grid heldin a pair of cross-over tweezers. These sections are then quickly butgently laid onto the surface of a water-filled Petri dish. The interactionbetween the n-propanol and the water usually leads to the sectionsbeing floated off on to the water and flattening out due to surfacetension. The required sections can then be chosen and removed on afresh TEM examination grid for TEM and STEM or removed using awire loop before placing on a slide for LM.

Sectioning without a trough liquidIn numerous cases it is found that sectioning with a trough liquid is notappropriate. Examples include materials which show some kind ofinteraction with water or n-propanol, such as epoxidized naturalrubber, and many unvulcanized materials in which distortions in thesections, artefacts and consequently misleading information have beenobserved. In these cases, or if one is simply unsure about the nature ofthe sample, then sectioning without a trough liquid is the technique ofchoice. Sections are collected on the knife itself and are then carefullypositioned on a TEM grid by holding the grid against the knife (awayfrom the knife edge) and moving the sections with a single-hair brush.The main disadvantage with sectioning dry is that a buildup of staticelectricity in the chamber can lead to sections being difficult to handleand prone either to sticking to the sample block face or the knife, or

flying around the cryo-chamber when attempts are made to move them.Anti-static guns are available but they are expensive and careless usecan damage the sensitive electronics of a cryo-unit. Once again the bestroute to good results is persistence, even after seeing one's best sectionflying off into the nether regions of the microtome! (There is, of course,no flattening action caused by the surface tension of the water in thisinstance.)

Sectioning on to iceThis technique, recently developed by Cudby (1995), is ideal forsectioning materials that have a tendency to curl (e.g. NR/BR blends)although it can only be implemented if the cryo-ultramicrotome beingused has an inbuilt defrost unit which automatically raises the tempera-ture to just above ambient.

The apparatus is prepared by placing a knife with a trough filledwith distilled water (with a slight negative meniscus) in the knifeholder. The apparatus is cooled in the usual way, freezing the water inthe knife trough. Sections are cut and arranged on the ice surface. Aftersufficient sections have been taken, the apparatus is warmed until theice melts. As it turns from ice to slush the resulting surface tensionflattens out the sections. Prior to melting, some manipulation of thesections is possible so that they are touching without overlappingalthough care must be taken above the Tg not to distort them. Once allthe ice has melted, the sections can be retrieved on a TEM grid. This isa time-consuming technique since the apparatus has to be repeatedlywarmed and cooled between samples. It does, however, give goodresults with difficult specimens.

COMMON PROBLEMS WITH SECTIONING

No description of sectioning would be complete without a discussion ofsome of the common problems encountered and some of the solutions.

CurlingCurling during sectioning of vulcanized material usually suggests thatthe section is too thick. This is solved 'simply' by cutting thinnersections! A more difficult type of curling occurs when seemingly flatsections of appropriate thickness curl as they are brought up fromcryogenic temperatures to room temperature. In many cases the mostlikely cause of this is inbuilt stresses in the material from moulding.While the material is a coherent whole it retains its shape but in somecases, when a section is removed from the bulk cryogenically and

allowed to warm to room temperature, the relaxation of stresses in thedifferent phases causes the section to curl. This seems to be a particu-larly common problem in blends containing elastomers with verydifferent Tg's such as NR and BR. As the section is rapidly warmed, BRwill return to being elastomeric near -UO0C (dependent on the type ofBR) whereas NR will remain a glass until its temperature is raised closeto -70 0C. The change in behaviour of the BR phase while the NR phaseis still a glass often leads to the section curling.

There are two techniques for overcoming this problem. The first is theice-sectioning method described above. The second, developed byCudby (1989), which is a lot less controllable, is to use a triangularblock face as this will provide a section with a tendency to curl from allthree edges resulting in the curl effectively bracing itself and thusleaving a flat region at the centre of the section. There is some depen-dence on the section being the same thickness throughout but practice,perseverance (and luck) can bring success. If bake-out facilities are avail-able on the cryo-unit then the first technique is usually more predict-able.

Knife marksThese are an inevitable consequence of sectioning an elastomer whichcontains particulate matter such as zinc oxide, silica, carbon black etc.The action of the knife on a hard filler particle as it traverses thespecimen can damage the knife edge at that point. From then on, thedamage to the knife edge will be translated on to any section and blockface as it passes over that point on the knife and it will appear as along line in the direction of sectioning. The more filler present, the morequickly the knife will be damaged. If a diamond knife is used then thedamage may not occur as quickly but, once damaged, the line will betransmitted to any sections from subsequent operations with that partof the knife until it is resharpened.

Knife marks do, however, have an important use. They can reveal thedirection of sectioning which may be important when trying to decidewhether the shape of a structure has been influenced by compression(see below). Where the knife marking is severe or publication isrequired of a marked image, a computer imaging macro can be used toimprove the visual appearance of an image which has been digitallycollected (Gilbey, 1996).

CompressionWhen a section is removed from a block, it often appears to be shorterthan the vertical face of the block from which it was removed. This is

known as compression. In more severe cases wrinkles appear at rightangles to the sectioning direction. Usually the effect can be reduced bychanging the clearance angle and/or the sectioning speed. Bad compres-sion generally results from too high a sectioning speed and/or too steepa clearance angle. Temperature may also be a function and lowering thetemperature can sometimes improve section quality.

Chatter

Chatter occurs when a high frequency vibration is set up between thespecimen block and the knife which leads to regular variations in thethickness of the section. This is observed as parallel lines at right anglesto the sectioning direction. As with compression its cause is usually acombination of wrong clearance angle, sectioning speed and tempera-ture.

Inconsistent sections

Once again in biological circles it is expected that the correctsectioning conditions will lead to a ribbon of ultra-thin sectionsfloating on the water in the trough. It has often been expressed bythose such as Reid (1975) that once the conditions are correctly set,sections will be cut serially, i.e. on every cutting stroke. This is anunrealistic expectation with elastomers. For most technologicalmaterials it is likely that the operator will be unable to stay withany region of the knife for a prolonged period of time before itbecomes blunt. The time taken for this to occur depends largely onthe material and on the conditions that the operator is attempting touse but it is quite conceivable that the knife will become sufficientlyblunt after only five or ten cutting cycles for the operator to need tomove to the next part of the edge and with the usable knife edgebeing quite short it is therefore necessary to obtain the correct condi-tions quickly. With practice and experience a good operator canassess which conditions should be set up initially from the type ofmaterial. In general terms, the harder the block the steeper the clear-ance angle that will be required. Sectioning speed is more difficult toassess and seems to depend on too many factors (including operatorpreference) to be able to give complete guidelines. It is felt that it isbetter to start at slow speeds and shallow clearance angles becausethese will do less damage to the knife edge if they are incorrect.Having block faces that are not too long will maximize the life spanof a portion of the knife since it will be cutting less material on eachcycle. Effectively this means that less time is wasted on an unsuitableset of conditions.

FREEZE FRACTURELaboratories which do not have access to expensive ultramicrotomyequipment but nevertheless operate an SEM can still examine surfaces ifthey are freshly exposed by freeze fracture before being treated byetching and/or staining. Freeze fracturing is a simple technique whichinvolves cooling the sample and a sharp instrument to equilibrium inliquid nitrogen, a process that can take up to thirty minutes dependingon the sample size. The operator, wearing protective goggles andgloves, then places the sharp instrument on the sample and delivers asingle sharp blow with a mallet. The sample will break and the exposedsurfaces can then be examined directly in the SEM or treated in someway to enhance whatever intrinsic differences may be present beforeexamination.

CHEMICAL STAININGChemical staining is a routine procedure to enhance the discriminationbetween different polymers in a blend. The elemental similarities ofmany elastomers dictate that little differential contrast will existbetween them and hence phase morphology is difficult to observe.However, although most elastomers are elementally similar, they oftenhave significant chemical differences and it is then possible preferen-tially to react one of the elastomeric phases with a chemical containinga heavy atom to produce elemental contrast. This is referred to asdifferential chemical staining. There are numerous stains available, butfor the purpose of obtaining differential contrast in elastomers there areonly three which merit attention.

OSMIUM TETROXIDE

This is probably the most useful stain for morphological examination. Itreacts with unsaturated carbon-carbon bonds as shown in Figure 9.7and a knowledge of the different polymers in a blend usually makes itpossible to predict which will be stained to the greater degree.Commentators such as Sawyer and Grubb (1987) and Kato (1967) differas to which is the most efficient way to stain material with somesuggesting that the shaped elastomer block from which sections are tobe taken should be heavily stained for some time prior to ultrami-crotomy whilst others suggest that the material should be sectioned firstand then stained. Experience at TARRC (Cudby, 1991) would suggestthat the latter is the more reproducible and, generally, the less timeconsuming method.

Specialized equipment exists for carrying out this staining but aneffective and cheap piece of apparatus can be made from a sintered

Figure 9.7 Staining with osmium tetroxide.

glass crucible and a glass weighing jar. Crystals of osmium tetroxideare placed in the weighing jar, the sintered glass crucible placed on topof them and examination grids with sections on them are then placed inthe crucible. The lid of the jar is replaced and the sections are thenexposed to the vapour without them being in direct contact with thecrystals. Although it is advisable that each material be tested to deter-mine the length of time needed to produce the optimum stain contrast,one or two hours is usually sufficient with most blends. It must beremembered that osmium tetroxide is extremely toxic and safety regula-tions for its use must be observed scrupulously.

The effect of staining depends on whether the material is beingexamined in transmission or via incident illumination. These arediscussed below but generally speaking, the heavier the stain, the lowerthe number of electrons that will be transmitted resulting in stainedregions appearing dark in the TEM but, when viewed from above, i.e.by SEM, the effect is reversed and heavily stained regions give rise tobrighter regions due to an increased level of backscattered electrons.

RUTHENIUM TETROXIDE

This stain, although less toxic than osmium tetroxide, is a far morepowerful oxidizing agent and is consequently more difficult to handle.

It is a relatively recent addition to microscopy stains and althoughdocumented by several authors such as Sawyer and Grubb (1987), Vitaliand Montani (1980) and Trent et al (1983) the extent of its use has notyet been fully determined. It is a useful material where additionalstaining is required since it will also stain aromatic rings and numerousunsaturated systems as well as providing additional stability andcontrast in an electron beam. The disadvantage is its expense, avail-ability, and ease of use.

In practical terms a similar staining procedure to the one describedfor osmium tetroxide should be followed except that sections must beplaced on gold grids, not merely gilded ones. Any other type will berapidly oxidized and rendered useless. Exposure to the stain should bein the order of minutes for most sections of elastomers and care shouldbe taken to avoid artefacts caused by over-exposure leading to thedeposition of ruthenium oxide.

As a consequence of being an extremely strong oxidizing agent,ruthenium tetroxide is very difficult to store. It is usually purchased asa yellow aqueous solution in a sealed ampoule. Once opened it has ashelf life measured in days unless it is kept in scrupulously clean glass-ware under nitrogen in a fridge.

URANYL ACETATE

This has been in use for many years as a simple staining mediumfor improving contrast in biological materials and has been describedby several authors such as Lewis and Knight (1977) and Kay (1965).In the polymer field it has a particular applicability in the examina-tion of natural rubber latex films where it offers a useful means ofdelineating latex particle boundaries by virtue of its ability to stainproteinaceous material. The technique is very simple although localregulations for the handling and disposal of radioactive materialsmust be followed. Uranyl acetate is an alpha particle emitter andgreat care must be taken not to ingest it. The texts referenced aboveprovide several different staining techniques but a simple alternativeprocedure is to place sections that have been taken from a latex filmand mounted on TEM examination grids, as described above, in a70:30 ethanoliwater saturated solution of uranyl acetate for twohours. Again, care should be taken to avoid contact between speci-mens and undissolved crystals and this can simply be done byplacing a watch glass in the solution and putting the grids on thewatch glass. After staining, the grids should be carefully butthoroughly washed in a solution of 70:30 ethanol:water to removeany remaining crystals.

Figure 9.8 Differential chemical etching.

CHEMICAL ETCHING

As an alternative to staining, differential chemical etching can be usedto reveal important three-dimensional information about a blend whichcannot be obtained by other means. Its use has been described by manyauthors including Sawyer and Grubb (1987) and Cudby (1989). In thistechnique a chemical that will preferentially chemically etch one of thephases is introduced on to the surface for a predetermined time. Thesurface can then be imaged directly by SEM or it can be replicated by asingle or two stage replication procedure and the replica can beexamined in the TEM. Particular care needs to be taken in interpretingthese results, as illustrated in Figure 9.8 which diagrammatically illus-trates an etched surface viewed at right angles to the viewing directionused in the SEM. From this diagram it is clear that overhangs of the un-etched phase can give misleading information about the sizes and

Direction of view

Unetched

phase

Location of

etched phase

Figure 9.9 SEM micrograph of NR/ENR blend etched with phosphotungstic acid.

shapes of the etched phase simply by obscuring the field of view of theobserver from the space under such an overhang. This can also causeapparent discrepancies in the observed blend ratio.

Some success has been achieved in observing NR/ENR blends, asshown in Figure 9.9. This specimen has been treated with phospho-tungstic acid which has etched away the NR phase, leaving the ENR25phase intact. The final appearance gives a better idea of the threedimensionality than simple staining would, with the ENR25 phasetaking on the appearance of a sponge.

For the sake of completion, plasma and ion etching should bementioned but these techniques are extremely prone to the productionof artefacts, difficult to control and hence not recommended for elasto-mers.

CASE STUDYThe aim of this section is to show how four different techniques can beapplied to a specimen in order to obtain phase morphological informa-tion. The specimen in question is an NR/EPDM blend at a 60:40 blendratio which was prepared for examination by phase contrast LM,incident SEM, S(T)EM and TEM. However, because the amount of infor-mation obtained from phase contrast light microscopy is limited for thisblend, an additional example is included to show its capabilities with asuitable specimen.

Figure 9.10 therefore, is a phase contrast light micrograph of an NR/NBR blend (25:75), in which the blend ratios can be used to determinethat the lighter, continuous phase is the NR phase and the darker,discrete phase is the NBR. It also shows that the zinc oxide particles arepreferentially located in the NBR phase. This is phase contrast LM at itsmost useful when important information can be obtained by an experi-enced operator in little more than one or two hours work.

Figure 9.11 is also a phase contrast light micrograph and is the first ofthe NR/EPDM micrographs. It is quite clear that the amount of infor-mation available in this instance is limited. Little can be inferred aboutthe phase morphology or any other aspect of the blend except that thephase size is small with a cross-sectional diameter in the order of0.5 (im. The original micrograph was taken on high resolution film atx 500 magnification and printed at x 1000 thus approaching the limita-tions of the technique and still not providing much of the requiredinformation. With current blending technology, phase sizes are tendingto become ever smaller so the use of conventional LM to obtainmeaningful information is becoming increasingly rare and so examplessuch as Figure 9.11 are not in the minority.

Figure 9.12 is the same NR/EPDM except that this time it has been

Figure 9.10 Phase contrast light micrograph of section from NR/NBR blend.

Figure 9.11 Phase contrast light micrograph of section from NR/EPDM blend.

Figure 9.12 SEM micrograph of section from NR/EPDM blend.

examined at higher magnification by SEM. The specimen wasprepared by sectioning after which it was stained in osmium tetroxidevapour for one hour and then earthed with carbon paste to the SEMstub on which it was mounted. This was coated with gold/palladiumand examined in the SEM at 10 kV accelerating voltage. The low accel-erating voltage was used to avoid deep penetration of the beam sincethis would have resulted in the visualization of material significantlybelow the surface of the section which would give a confused image.The effect of the stain has been to render the NR phase lighter since itgives a stronger backscattered electron signal when scanned. Thedark, discrete EPDM phase can be observed to contain a stained NRmicro-phase.

Figure 9.13 is a S(T)EM micrograph of a section of the samematerial at the same magnification as the SEM image, thus permit-ting direct comparison. The microscope was operating with an accel-erating voltage of 3OkV to maximize transmission of the signal. Anyless would have resulted in excessive specimen heating. The effectof imaging in transmission has been to reverse the contrast, withthe stained NR phase now obstructing the passing of some of thesignal and thus appearing darker. The NR micro-phase in theEPDM is confirmed and in addition, an EPDM micro-phase in theNR can be observed. There is also a stronger suggestion of co-conti-nuity than can be observed by SEM incident examination. Thistechnique therefore offers additional information and would be thebetter alternative when low levels of filler are present since thesecan be better observed in transmission. The disadvantage of thetechnique is that the section must be ultra-thin for transmittedimaging, and this inevitably takes longer to prepare. Incidentimaging requires less attention to the thickness of the section sincethe depth to which one observes structure will be limited by theaccelerating voltage used. It should also be noted that this techniqueis not suitable for highly filled materials and these must beobserved using TEM.

The final image in this series, Figure 9.14, is the same NR/EPDMexamined using a transmission electron microscope operating at100 kV at about the same magnification as Figures 9.12 and 9.13. Theimage is demonstrably sharper than the S(T)EM image and, whilst theS(T)EM image was taken at near to the maximum magnification ofthat technique, the same magnification on the TEM is at its low end.Consequently, the specimen could easily be examined at higher magni-fication to obtain more information about the internal phase structureof either phase if necessary. For the examination here, Figure 9.14serves to show similar information as that seen in Figure 9.13 but athigher resolution.

Figure 9.13 S(T)EM micrograph of section from NR/EPDM blend.

Figure 9.14 TEM micrograph of section from NR/EPDM blend.

SWOLLEN VULCANIZED ELASTOMER NETWORKOBSERVATION

This technique, initially reported some time ago by Shiibashi (1987), hasundergone considerable development recently by Cook et al. (1992). It isrelatively simple and involves converting the elastomer or blend beinganalysed into a semi-interpenetrating polymer network by swelling thevulcanized material to equilibrium in styrene. The latter is thenpolymerized, effectively locking' the elastomer into its swollen state.The swollen sample block is sectioned and the resulting specimens arestained, usually with osmium tetroxide, to delineate the swollen rubbernetwork.

From the micrographs obtained it is possible to relate the swollennetwork mesh size to the physical crosslink density of the material. Itis therefore possible to use this technique to measure localized cross-link densities and to observe blends in which curative migration hasgiven rise to an unequal distribution of cross links within the twophases. An example of this is illustrated in Figure 9.15, an NR/NBRblend in which the NR phase is of a much lower physical crosslinkdensity and hence possesses a larger network mesh size. Direct obser-vation of the variation in absolute and relative crosslink densities ofthe individual polymers of various polymer blends with different curesystems thus offers a direct route to evaluating the character of thecure system and correlating it with the physical properties of thevulcanizate.

Perhaps more importantly, the technique has also been shown tohighlight weak spots within a material. As the material undergoesswelling, and subsequent phase separation on polymerization, theaction of the styrene is to swell any volumes in relation to theircrosslink density. In a region such as that surrounding an inertfiller particle, there is little or no interaction with the network andhence that region can swell much further because it is lessconstrained. This is often observed as large voids appearing withina network.

An extremely important application of this principle is in thefield of direct observation of the level of interfacial adhesion withina blend. A material that was known to have poor mechanicalproperties was swollen in styrene, as described above. The resultingmicrograph (Figure 9.16) showed large voids at the phase interfacesthus confirming them as failure sites. A similar material wastreated with a compatibilizer, a third elastomer which possessedgood interfacial properties with both phases, and Figure 9.17clearly shows it as a dark band of 'glue' holding the main twophases together.

Figure 9.15 TEM micrograph of section of swollen network from NR/NBR showing differential network mesh sizes.

Figure 9.16 TEM micrograph of section of swollen network from NR/NBR showing phase interfacial failure.

Figure 9.17 TEM micrograph showing the action of a compatibilizer in preventing interfacial bond failure.

REFERENCESAgar, A.W., Alderson, R.H. and Chescoe, D. (1974) Principles and Practice of

Electron Microscope Operation, North-Holland, Oxford.Ansell, P. and Stevenson, I. (1993) Private communication.Brundle, C.R., Evans, C.A. Jr and Wilson, S. (1992) Encyclopaedia of Materials

Characterization, Butterworth-Heinemann, London.Cobbold, AJ. (1988) Private communication.Cook, S., Cudby, P.E.F. and Tinker, AJ. (1992) Paper presented to Rubber Div.

Am. Chem. Soc. Meeting, Nashville.Cosslett, V.E. (1951) Practical Electron Microscopy, Butterworths, London.Cosslett, V.E. (1956) Brit. J. ofApp. Phys. 7, 10.Cudby, P.E.F. (1989) Unpublished work at MRPRA.Cudby, P.E.F. (1991) Unpublished work at MRPRA.Cudby, P.E.F. (1995) Unpublished work at MRPRA.Cudby, P.E.F. and Gilbey B.A. (1995) Rubber Chem. TechnoL 68, 342.Gilbey, B.A. (1996) Unpublished work at MRPRA.Haynes R. (1984) Optical Microscopy of Materials, International Textbook Co.,

London.Kato, K. (1967) Polym. Eng. ScL 7, 38.Kay, D.H. (1965) Techniques for Electron Microscopy, 2nd edn, Blackwell, Oxford.Lewis, P.R. and Knight D.P. (1977) Staining Methods for Sectioned Material, North-

Holland, Oxford.Reid, N. (1975) Ultramicrotomy, North-Holland, Oxford.Sawyer, L.C. and Grubb D.T. (1987) Polymer Microscopy, Chapman & Hall,

London.Shiibashi, T. (1987) Int. Polym. ScL and Tech. 14(12), 33.Smith, R.W. and Andries J.C. (1974) Rubber Chem. Technol. 47, 64.Sorvall (1965) Thin Sectioning and Associated Techniques for Electron Microscopy,

Norwalk.Trent, J.S., Scheinbeim J.I. and Couchman P.R. (1983) Macromolecules 16, 589.Vitali, R. and Montani E. (1980) Polymer 21, 1220.Walter, F. (1980) The Microtome, 2nd edn, Leitz.

Inorganic fillers and *j ^N

trace metal analysis I U

ASHING

One of the simplest of analytical procedures would seem to be that ofthe combustion of a test portion and the weighing of the residue. In theliteral sense this is of course true but in many cases merely weighingthe residue yields insufficient information, and the ash is the startingmaterial for further analyses, as in the determination of trace metals. Incarrying out an ash determination on a compounded polymer the aimis to pyrolyse the polymer, distilling off liquid and vapour, preferablywithout combustion, and, only when this is complete, to continue withoxidative heating.

The most important sources of gross error are carbonization, which isoften unavoidable, especially with polymers containing aromatic groupsor halogen, and frothing which leads to the physical loss of polymer.Carbonization leads to the formation of flakes of carbon which are oftenvery difficult to bum off and it is important these are not lost in aircurrents. Some raw polymers liquefy at an early stage when heatedover a Bunsen burner, and continued heating from underneath cancause boiling liquid to froth over the rim of the crucible. Styrene-butadiene copolymers (SBR) are particularly liable to froth, but theproblem can be avoided by wrapping the test portion in ashless filterpaper before placing it in the crucible (Milliken, 1952) whilst carefulcontrol over the rate of heating will also help to avoid the problem. It isalso preferable to avoid setting the polymer alight since this mayindicate an excessive heating rate with material being lost in thevapours.

For halogenated polymers 'acid ashing', using sulphuric acid to elimi-nate the halogen prior to pyrolysis of the polymer, avoids the loss ofvolatile compounds such as zinc chloride by converting them to theinvolatile sulphates. Where more volatile compounds such as arsenic ormercury need to be determined a contained system, either with wetashing or a IDOnTIb' digestion will be required.

TEMPERATURE OF ASHING

Owing to the decomposition of certain common compounding ingredi-ents, simple ashing gives reproducible values only if both time andtemperature of ashing are strictly controlled. ISO 247-1990 offers twoprocedures, the first, dry ashing, being unsuitable for compounded orvulcanized rubbers containing halogens whilst the other, acid ashing, isnot recommended for raw rubbers. The methods do not, in general,give the same result and thus the procedure used must be specified.

CHANGES IN MINERAL CONSTITUENTS DURING ASHING

The filler most likely to give distorted or unreliable results is whiting(calcium carbonate) since, as is well known, this decomposes on heatingwith the loss of carbon dioxide. Extrapolation from very early publisheddata (Johnston, 1910) shows the figures in Table 10.1 for the dissociationpressure of calcium carbonate.

In a closed furnace, with static air, carbon dioxide would tend toremain in the crucible with the ash and this would hold equilibrium ata level which would reduce the continuous decomposition which islikely in the open air, as when ashing is carried out over a Bunsenburner. Decomposition can be ignored below 520 0C, at which tempera-ture the dissociation pressure of calcium carbonate is about balanced bythe partial pressure of carbon dioxide in the atmosphere, but not above520 0C, where decomposition becomes possible. The temperature for ashdetermination in ISO 247-1990 of 550 0C + 25 0C seems therefore a littlehigh whilst the allowed option of 950 0C ± 25 0C will completely decom-pose the whiting to calcium oxide.

The importance of keeping the temperature low is reinforced whensilicates are present, as zinc oxide forms an insoluble silicate whenheated with clay above 70O0C. Poulton (1958) claims that this effect isnot found with silica but Gorsuch (1970) illustrates how, in the presenceof inorganic chlorides, reaction can occur with silica crucibles.

As already mentioned, the ISO specification for ashing includes a

Table 10.1 Dissociation pressureof calcium carbonate

0C mm Hg

300 0.000016400 0.0030500 0.14600 2.25

Table 10.2 Ashing of neoprene rubber containing zinc oxide

Composition of mix Ash%% Ash% Muffle Ash%

ZnO Neoprene Bunsen Furnace Calculated

O 100 0.5 0.5 —5 95 2.9 0.4 5.35

20 80 9.2 5.5 19.85100 O 97.1 91.5 —

warning about the presence of chlorine-containing polymers, and Sternand Hinson (1953) published a note demonstrating the magnitude ofthe effect with zinc oxide and polychloroprene. Some of their results areshown in Table 10.2. These authors presented evidence, in the samenote, that calcined magnesia could be quantitatively recovered fromneoprene by ashing. The question of ashing polychloroprene isdiscussed later, in the section on acid ashing.

Presumably one should also avoid ashing chlorosulphonatedpolyethylene except by the sulphated ash procedure. For a ratherdifferent reason care must be exercised in ashing heavily loaded 'hardrubbers' or ebonites. Smith et al. (1959) pointed out that the very highsulphur content can and does react with mineral fillers during ashing.In ebonite containing calcium carbonate both sulphide and sulphate canbe found in the residue depending on the conditions and time ofheating.

LOSS OF TRACE ELEMENTS DURING ASHING

Apart from the loss of major constituents of the ash there is always thepossibility that trace elements present may be lost by one route oranother. The element around which interest usually centres is copper;this can be used to illustrate several important points. As has alreadybeen discussed, loss by volatilization is minimized by pyrolysing thepolymer as gently as possible and above all by not letting the testportion catch fire. Historically, the most common cause of poorrecovery was the use of porcelain crucibles, since copper and othermetals could be absorbed by the hot glaze of the crucible. The use ofthe more modern silica crucibles will generally avoid this problem, buteven these become etched after a few determinations and thereafter careshould be taken with stained crucibles being immediately discardedsince absorption is occurring. An alternative is the use of inert materialto line the crucible. Magnesium oxide is used for this purpose and hasthe additional merit of assisting the ashing by helping to introduce

oxygen from the air held in the bulk of the powder whilst alsoabsorbing liquid on its surface. The use of platinum crucibles is afurther option but Zeitlin et al. (1958) reported that if a Bunsen burner isused to carry out the ashing, unburnt gas in the flame can reducecopper salts to metallic copper which then alloys with the platinum.

Another mechanism by which trace metals apparently can be lost isby reaction with other constituents of the material being analysed. Forexample, a reliable estimate of the copper content of a clay-containingrubber product will require that the silicate be destroyed by, forinstance, reaction with hydrofluoric acid. The copper may well bepartially extracted with a hydrochloric or nitric acid digestion of theash, but certainty of total extraction will require dissolution of the clay.

THERMOGRAVIMETRY

A specific application of the dry ashing technique, which completelysolves the problem found with whiting, is thermogravimetry, thederivative mode of which has been discussed in Chapter 7, and thequantitative aspects of which will be considered in depth in Chapter 12.It must, however, receive mention here because, having removed thepolymer by pyrolysis in an inert atmosphere over a temperature rangeof 250-550 0C, heating can be continued up to 800 0C and, should anywhiting be present, carbon dioxide will be evolved quantitatively,enabling the whiting content to be calculated. Because the heating iscarried out with an inert gas flow over the sample, the tendency ofcalcium carbonate to react with sulphur compounds is reduced to negli-gible proportions.

In the same instrument the atmosphere can be changed to air at anyspecific temperature so that carbonaceous residues can be removed bycombustion and a residual ash obtained which, depending upon theconditions chosen, will be similar to one or other of those producedusing the two 'dry7 methods of ISO 247-1990.

ACID ASHING

A method of overcoming some of the difficulties of dry ashing whilstavoiding all the complications of wet ashing was suggested by van derBie (1947). This has been called 'acid ashing' because it does not quitefit into the categories of either dry or wet ashing.

A modified procedure has now been accepted in ISO 247-1990 anddiffers from van der Bie's original suggestion in that it uses sulphuricacid instead of nitric acid and is recommended for halogenated rubbersalthough it is equally applicable whether the halogen is present as partof the polymer, has been applied to the polymer as a surface treatment

or has been incorporated as a filler or additive. However, whilst the useof nitric acid should, in principle, lead to the same total ash as directheating, the sulphated ash can give a different figure due to the thermalstability of sulphates produced from fillers during the ashing step.

The final heating of a sulphated ash from polychloroprene is criticalas both zinc and magnesium are normally present and it is necessary toensure the conversion of sulphates to oxides. This can be most easilyachieved by using the higher muffle furnace temperature of 95O0C.However, as shown by Gorsuch (1959), at higher temperatures some ofthe zinc oxide is fused into the silica crucible and, although the ash iscorrectly determined, the zinc cannot be readily re-extracted if quantita-tive estimation is required. In the presence of calcium carbonate there isno alternative but to weigh the ash as the sulphate. In most cases it issufficient to specify the precise method used to produce the ash,thereby facilitating comparison by other laboratories, rather than tryingto produce by sulphated or wet ashing the same result as would beobtained by dry ashing.

WET ASHING

Undoubtedly chromic acid systems are the most efficient for obtainingthe complete conversion of carbon compounds to carbon dioxide andwater (Houghton, 1945), but they are rendered unusable in trace metaldeterminations by the certainty of introducing, with the reagents, justthose elements which are most commonly determined. Nitric andsulphuric acids are not, of themselves, completely effective and it isusual to follow their use by supplementary oxidizing agents such ashydrogen peroxide or perchloric acid although the latter reagent, in theopinion of the author, is best avoided as there have been manyinstances of explosions when the last traces of organics are beingremoved by it.

It is important to recall that, relative to the metals to be determined,large quantities of reagent are added and evaporated, and concentrationof their impurities therefore occurs. For example, the nitric acid usedshould be at least of a grade specified for food analysis or a microanaly-tical reagent or, preferably, Aristar or doubly-distilled acid. (Reagentswith heavy metals such as lead at levels below 1 ppm may still lead toa considerable reagent 'blank'.) Gorsuch (1970) recommends a sophisti-cated wet digestion apparatus as illustrated in Figure 10.1, which has anumber of advantages in flexibility of operation depending upon theposition of the three-way tap (A).

This allows refluxing (position a), distillation (position b) and removalof some or all of the distillate (position c). Fractions of distillate can bediscarded, reserved for further treatment, or examined at leisure in the

Figure 10.1 Apparatus for controlled decomposition of organic material.

course of an investigation. In some cases it is convenient to use a two-necked flask instead of the single-necked flask shown, and to insert athermometer into the flask via the second neck. By this means, thelengths of the distillation stages can be controlled by the temperatureattained by the reaction solution; this can be of particular value withmixtures containing perchloric acid owing to the ease with which a run-away exothermic reaction can develop.

There is one essential difference between wet and dry ashing: thesample size for the latter is 5g but can be increased to 2Og withoutinconvenience and the ash may be dissolved into as small a volume ascan conveniently be handled. With wet ashing, 2 g is about the practicallimit for sample size and the volume of reagents added dictate that 25-50ml final volumes are usually used in order to keep the final solutionat acceptable acid concentrations. This means that poorer detectionlimits and reduced precision will be achieved. It may also be noted that

whereas a dry ashing can be performed by an electric furnace, wetashing is performed by an analyst, i.e. it must be watched and it cantake a long time!

It should be apparent that it is impossible to state categorically thatone method of ashing is superior to another. In general it can be saidthat for many determinations the advantages and disadvantages can beweighed against each other on the grounds of convenience, but forcertain applications there are overriding reasons for choosing a specificone. If there is a possibility of volatilizing the element being analysedthen wet oxidation is virtually essential, but if extremely low levels ofelements are being measured, and reagent purity could become aproblem, dry or acid ashing would be preferred. As observed above,low levels of volatile elements are not amenable to either of thesemethods of analysis.

It is therefore best to consider each requested analysis separately,using one of the ISO procedures to obtain a standard ash value butusing thermogravimetric analysis to obtain a rapid, perfectly valid, ashcontent together with a whiting loading, if any is present, and toprovide a suitable sample for subsequent 'bulk7 analysis. However,separate specific ashing procedures may be required for the quantitativeelemental analysis of particular elements.

DIGESTION IN PRESSURE VESSELS

Strictly speaking, wet ashing is not an ashing technique at all, as thereis no dry solid produced which can be weighed to give an ashcontent. The intention of wet ashing is purely to produce a solutionwhich can then be used to estimate the content of, usually, metals bysome quantitative technique. Unfortunately the sensitivity of thesemeasurement techniques to excessively high acid levels requires thatthe wet ashed solution be diluted to maintain acidity levels withinacceptable limits. An alternative procedure, used extensively in theauthor's laboratory is to oxidize the rubber with nitric acid in apressurised 130mb' and then to treat the resulting solution as thoughit were a wet ash solution. For this reason the procedure is consideredhere although it also falls within the 'total sampling' categorydiscussed later in the chapter.

The major advantage of the bomb digestion procedure is that theamount of acid required to achieve complete digestion is very muchless than that required for a wet digestion at atmospheric pressure. Atypical wet digestion of a 1 g sample of rubber would require 15 ml ofconcentrated nitric acid and 5ml of concentrated sulphuric acid with,perhaps, 20ml of lOOvol hydrogen peroxide to complete the oxidation.A bomb digestion of 0.25 g of rubber needs just 2ml of concentrated

nitric acid, and no other oxidant. It is also worth noting that whereasthe wet digestion requires up to 4 hours close supervision, the bomb issimply placed in an oven at 100 0C and left overnight. Given the greatlyreduced volumes of reagents used, it becomes much more cost effectiveto use the purest grades of reagent available thereby minimizing thereagent blank. The digested solution is usually diluted to 5ml forgreatest sensitivity, giving a dilution factor of 20, a figure comparable tothat used during wet ashing.

The Parr bomb, named after the company which developed it, wasthe forerunner for a range of sealed digestion vessels and it is illustratedin Figure 10.2. Modern high pressure reaction vessels are now availablewith a range of wall thicknesses to withstand different operatingpressures and with an inert polytetrafluoroethylene (PTFE) lining tobroaden their scope. More applications of these bombs to the analysis ofrubber can be found in Chapter 6. Bernas (1968) and Uhrberg (1982)described the design of such a bomb for analysing volatile metals inbiological samples and, although little has been published concerningthe use of these devices for the preparation of samples for rubberanalysis, their advantages of speed of digestion, low sample/digestionmedium requirements and total sample containment should be selfevident.

Figure 10.2 The Parr bomb. (Courtesy Parr Instrument Co.)

Before choosing a particular bomb, it is advisable to carry outdetailed calculations of the pressure which could be generated duringuse. This involves converting the polymer to its equivalent of carbondioxide and water (and other oxides as appropriate) and the nitric acidinto nitrogen oxides and water. A maximum temperature must then beselected, based on an assessment of the likely heat evolved during theinitial exothermic reaction between the polymer and nitric acid. In workcarried out at the author's laboratory it was estimated that 150 0C is alikely maximum temperature and this is used in the calculations. Theactual, rather than nominal, capacity of the bomb needs to be ascer-tained. The charge of both polymer and acid can be varied to ensurethat the maximum pressure generated is likely to be no more than 60-70% of the pressure limit of the vessel.

A typical analysis at the author's laboratory involves approximately200 mg of rubber and 2 ml of concentrated nitric acid. The bomb (25 mlcapacity) is placed in an oven at 10O0C overnight and the digestedsample worked up in the morning. Digestion is normally complete intwo to five hours, depending on the polymer, but overnight is conve-nient and allows absolute confidence that the digestion has proceededto completion.

BULK FILLER ANALYSIS

Historically the identification of the bulk fillers in a rubber vulcanizatehas been made by infrared spectroscopic examination of the dry ash,preferably that obtained at approximately 50O0C as either a paste (ormull) prepared by grinding with liquid paraffin, or as finely dispersedparticles in a matrix of potassium bromide pressed under high pressureto form a thin disc. Comprehensive sets of reference spectra have beenpublished (Scholl, 1981; Corish, 1961) and from these it is a simplematter to identify the inorganic component as a clay, silica itself,whiting, barium sulphate etc. The main disadvantages of this simpletechnique are that it does not show zinc oxide, which has no infraredspectrum and it is not always easy to identify the components of mixedfiller systems since the observed absorptions are relatively broad andfew, unlike the complex patterns characteristic of organic molecules.Jackson (1997) has recently described how Raman spectroscopy can beused to advantage in this area since many fillers give relatively sharpbands in their Raman spectra and he has published comparative Ramanand infrared spectra of a number of common fillers. Other than inexceptional cases it would not be possible to quantify the analysis offiller mixtures by these techniques, but the author's experience is thatsuch analysis is rarely required. If necessary, classical methods ofinorganic analysis, which are beyond the scope of this book, or the

all-embracing techniques of atomic spectroscopy, are equally able toanalyse trace metals and bulk fillers.

TRACE METALSA few years ago the only trace metallic components which the rubberanalyst routinely was required to comment on were copper, iron and,possibly, manganese - all prodegradants - and occasionally lead.Pressures of 'health and safety7 have led to a massive expansion of thislist, and no doubt it will continue to grow. The Australian Standard AS1647, the British BS5665 (EN71) and the UK Toy safety regulations of1989, for instance, all list antimony, arsenic, barium, cadmium, lead,mercury and chromium as controlled elements in certain products. TheISO Standards for the colorimetric determination of copper (ISO 1654-1971), iron (ISO 1657-1986) and manganese (ISO 1655-1975) in rawrubber, and for manganese in compounded rubber (ISO 1397-1975),have now been superseded by the series ISO6101 parts 1-5 (1986-90),which cover the determination of lead, zinc, manganese, copper andiron in both raw rubber and rubber products by atomic absorptionspectrophotometry. There is also an ISO standard (ISO 2454-1982) forthe determination of zinc by EDTA titration.

In view of the detailed descriptions given, and the policy of notgenerally describing ISO standards in detail, these are not consideredfurther, the interested reader being referred to the standards themselves.Little has been published about the levels of these elements in rawrubbers but Crafts (1992) published a comprehensive survey of thelevels of some seventeen elements found in raw Malaysian naturalrubber including distribution by both grades and types of producers.

ANALYSIS OF PREPARED SOLUTIONS

ATOMIC SPECTROSCOPY

The common techniques under this general heading which are currentlybeing used are atomic absorption spectrophotometry (AAS), inductivelycoupled plasma-atomic emission spectrophotometry (ICP-AES), induc-tively coupled plasma mass spectrometry (ICP-MS), flame photometry,arc spectrometry and dc plasma spectrometry.

All of these techniques are capable of determining both filler andtrace levels of metals in rubber products, although ICP-MS is lesssuited to high level measurements. Flame photometry is only suitablefor a restricted range of elements and, unfortunately, many of themore important ones, from a rubber point of view, are outside thatrange. Arc spectrometers and dc plasma share many characteristics in

common with ICP and will not be discussed separately. The mostcommonly used techniques of AAS and ICP are considered below inmore detail.

Atomic absorption spectrophotometry (AAS)Atomic absorption spectrophotometry is one of the two currentmethods of choice for the routine estimation of large numbers ofdifferent elements at concentrations ranging from many per cent to lessthan 1 ppm. The principle is extremely simple and involves the genera-tion of a cloud of atoms of the elements being studied in a flame. Theproportion of the atoms in the ground state is dependent on thetemperature of the flame and may be calculated from the Maxwell-Boltzmann equation. Electrons can be raised to higher energy levels bythe absorption of photons.

Metallic and semi-metallic elements contain valence electrons whichare excited by photons of specific wavelengths in the range 190-800 nm.For each element the difference in energies between any two specificlevels is virtually identical for all its atoms although a little spread isintroduced due to atomic collisions and the so-called Doppler broad-ening, due to random translational motion, which become greater as thetemperature rises. If radiation of the precise wavelength, correspondingto the energy difference between two electronic energy levels within anatom, is passed through the flame and its reduction in intensitymeasured, this will lead directly to the quantitation of that particularelement.

The ideal photon source for each element is its own specific emissionline as found in the emission spectrum generated in a hollow cathodelamp or electrodeless discharge lamp for that particular element. Inpractice the relationship between concentration and absorption is onlylinear over a small concentration range but, as samples are bracketed bystandards during a routine run, this causes few problems and it shouldbe remembered that successive dilutions can always be used tooptimize the concentration range. The particular advantages of thistechnique on a cost basis are clearly obvious as each additional elementmay be analysed by the purchase of one extra lamp, and most instru-ments have multi-lamp turrets so that one lamp can be changed whilstanother is in operation.

The cloud of atoms is produced directly in the flame by aspirating asolution of the sample into the flame. The choice of gases for the flame,flame geometry, temperature and other operating parameters can easilybe optimized for each element, so one is only left with making sure thatthe solution being examined is truly representative of the originalsample.

The analysis of compounds and vulcanizates containing mixed fillerscan present particular difficulties. Specifically, mixtures containingcalcium salts, silicates and titanium dioxide need to be approachedcarefully if all of the components are to be quantified. The major inter-ferences which have to be considered are that calcium salts produceinsoluble calcium sulphate with the sulphuric acid added duringremoval of silica by hydrofluoric acid, and titanium dioxide, whilstbeing unaffected by sulphuric acid alone, is converted to titaniumfluoride by hydrofluoric acid and thence to titanium sulphate by thesulphuric acid. This sulphate is not converted back to the oxide byheating at 550 0C, although it would probably be so converted at 900 0C.

A general scheme, which should cover most filler/polymer combina-tions is as follows:

1. Carry out a sulpha ted ash at 55O0C in a platinum crucible. (Thisremoves halogens which may be present in halogenated polymers.)

2. Add HF at a rate of approximately 5ml/g of ash and approxi-mately 0.25ml of sulphuric acid. Evaporate on a water-bath untilthe HF has been removed and then gently heat the crucible over abunsen burner until all the white fumes of sulphuric acid havebeen expelled. If necessary, due to large amounts of silica beingpresent, repeat the treatment. When all the sulphuric acid has beenexpelled, dry in the muffle furnace at 55O0C. (This removes silicaquantitatively as silicon tetrafluoride.) [Note: all handling of hydro-fluoric acid must be carried out with due regard for health andsafety. All operations must be carried out in a fume cupboard withappropriate personal safety equipment.]

3. Add a few ml of hydrofluoric acid and a few drops of sulphuricacid, evaporate to remove all the HF. Add HCl (3 M) and dilute tovolume with the same acid. Titanium sulphate will remain solublein this acid and therefore 3 M HCl should be used for all subse-quent dilutions.

This scheme is reasonably general but cannot cover all possibilities. Ifit is required to measure calcium, zinc and silica in a vulcanizate with achlorinated polymer, then the sulphated ash will be necessary to avoidloss of zinc during ashing but this will lead to difficulty in dissolvingthe calcium sulphate. There is no realistic alternative to measuring thecalcium and zinc in different test portions with suitably modifiedashing procedures. Thus it should be clear that the above should onlybe used as a guide, and the precise procedure to be followed needs tobe evaluated in the light of known potential interferences in each indivi-dual sample.

In the early stages of the discussion on atomic absorption spectropho-tometry the point was made that various proportions of atoms of

different elements were in their ground states (the exact figure beingdependent on the flame temperature). Some, therefore, are in excitedstates and most modern instruments can make use of this by operatingin the emission mode, directly measuring the strength of the emissionsignals as the excited species return to their ground states. In practicethe flame temperature is usually increased a little (typically from about2800 0C to 3300 0C) to increase the population of the excited states butthe sensitivity is still relatively low compared with atomic absorptionspectrophotometry. The low sensitivity of emission measurements,when using an AAS instrument for that measurement, is due primarilyto limitations of the optics and electronics of the AAS and the relativelylow temperature of the emission cell, not to any inherent insensitivity ofemission techniques.

Plasma spectroscopy

For general use almost all plasma instruments use inductively coupledplasmas (ICP), dc plasma instruments being used for specialist applica-tions. The ICP plasma maintains a temperature in the region of 8000-10 000 0C, in which range most atoms are in excited states and elementswill exhibit a number of ionized states. Each species generates asubstantial number of emission lines, the number running intothousands for transition elements, and this provides a large choice forinterference-free analysis. When compared with AAS the detectionlimits and quantitative precision are at least as good, and often muchbetter, whilst the concentrationrresponse relationship stays linear over arange covering several orders of magnitude. The multiplicity of linescan, however, be a problem since interference with primary emissionlines is common and less intense lines have to be used for the analysis.This inevitably leads to reductions in sensitivity and detection limitsbelow those ideally attainable.

Modern instruments have reduced the significance of this problemby moving to gratings of high resolution, 5-10 pm (picometres) istoday the norm, compared with 50-100 pm twenty years ago. Thisimproved resolution has reduced the problems with spectral interfer-ences to such an extent that unknown samples can be analysed withreasonable confidence. It remains sensible, however, to carry outquantitative determinations on two different lines, consistent datagiving reassurance that there is no interfering species under one ofthem. The particular advantage of ICP over flame AAS is its ability tocarry out multi-element analyses, using either a sequential searchprogramme, when up to 16 elements can be identified and quantifiedin two minutes or so, or a multichannel analyser, in which a numberof dedicated data processors analyse up to twenty pre-identified

elements simultaneously. The latter has much smaller solution require-ments, as does ICP generally since it uses sample aspiration rates ofapproximately Iml/min compared to 4-5ml/min for AAS. Couplethis with auto-sampling capabilities, full microprocessor control tooptimize the performance of the instrument, and built-in programs forbackground correction, spectral overlap correction etc., and thestrength of the technique is obvious.

The fact remains that aspiration of the sample into the plasma stillrequires the sample to be in solution, so the problems discussed earlierremain. ICP coupled with mass spectroscopy can achieve the sensitivityobtainable by total sample analysis using atomic absorption spectropho-tometry coupled with electrothermal atomization but at a significantadditional cost. For these reasons it is unlikely that either plasmatechnique will ever replace atomic absorption spectrophotometrycompletely.

For a more detailed discussion of the treatment of specific elementslikely to be met by the analyst in the elastomer field the reader isdirected to the references suggested in Table 10.3 as well as the range ofstandards documented in Appendix A.

TOTAL SAMPLE ELEMENTAL ANALYSISIn view of the problems associated with the preparation of a trulyrepresentative ash and its subsequent dissolution, it is obviously ofinterest to consider whether one can dispense with these steps anddirectly analyse the material with no pre-treatment, other than thecutting of a suitably sized test portion.

DESTRUCTIVE ELEMENTAL ANALYSIS

The techniques which can be used to give a rapid elemental analysis ofa total sample of rubber with the minimum of pre-treatment mayconveniently be divided into two groups: those which result in thedestruction of the sample and those which enable it to be recoveredessentially undamaged for subsequent further analyses. The two groupswill be considered in this order and it will be apparent that the two arecomplementary rather than in competition; the destructive methodsgiving an indication of the average composition of the elements in asample whereas the non-destructive tests generally provide a measureof the surface levels or, at best, a measure to a limited penetrationdepth. Obviously, sectioning will convert a "bulk' sample into a'surface' for this application and this often provides useful referencedata against which to interpret the true surface data.

Table 10.3 Selected references for the quantitative recovery of inorganics fromorganic matrices

Element

Alkali metalsAluminiumAntimonyArsenic

Cadmium

ChromiumCopper

IronLead

MagnesiumManganeseMercury

NickelSelenium

Zinc

Problems/points covered

excessive heatretention in SiO2

wet oxidationoxygen flask-alloyvarious digestionswet oxidationacetic acid extractionwet ashingacid ashingwet vs dry ashingalloy with platinumlow temp, oxidation withexcited oxygenloss with chlorideswet vs sulphated vsoxygen flaskloss with H2SO4/Ca saltsloss at 740 0Cwet vs dry ashingwet ashing H2S(VKMnO4wet ashingwet vs dry ashingwet oxidationloss with H2SO4/HN03perchloric acidlosses on dryinglosses with chloridelosses with clayslosses with chloroprene

Authors

Joyet (1951)Sandell (1944)Gorsuch (1962)Corner (1959)Banks et a/. (1948)Gorsuch (1959)Moldrai and Petrescu (1965)Down and Gorsuch (1967)Middleton and Stucking (1954)Gorsuch (1960)Zeitlin et a/. (1958)Gleit and Holland (1962)

Gorsuch (1960)Belcher et at. (1958)Gorsuch (1959)

Davidson (1962)Heckman (1967)Gage (1961)Anal. Meth. Comm. (1965)Gorsuch (1970)Klein (1941)Gorsuch (1959)Kelleher and Johnson (1961)Stanton and McDonald (1965)Gorsuch (1970)Poulton (1958)Stern and Hinson (1953)

Oxygen flask combustion

An alternative to dry, sulphated or wet ashing is to use the oxygenflask method as described in Chapter 6. This has been used successfullyin the analysis of many elements but there are dangers with lead(Belcher et al., 1958) and arsenic (Corner, 1959) as each may form analloy with the platinum foil container. This can be avoided by using asilica spiral coil instead of the platinum foil container.

Bomb digestionA further option is to use a digestion technique, and this is preferablycarried out in a sealed vessel, or 'bomb'. The section on wet ashing/digestion earlier in this chapter provides further information on thistechnique.

Total sample atomic absorption spectrophotometry (electrothermalatomization, ETA)The problems of ensuring that a solution prepared for atomic absorp-tion spectrophotometric analysis contains the elements to be measuredin the same relationship as that in which they exist in the originalsample can be overcome by using an electrothermal atomizer attachedto an atomic absorption spectrophotometer.

A solid sample weighing about 1 mg - accurately weighed - is heatedunder reproducibly programmed conditions to remove the polymericphase, ashed, and then atomized very rapidly. The concentration of theelement under investigation can then be determined from the integratedpeak area (not the steady state reading as obtained with an aspiratedsolution) relative to either solids of known composition or solutionstandards dropped into the atomizer and given the same pre-treatment.The atomizer usually consists of a small graphite tube or furnace, or atantalum cup, which is subjected to a programmed heating cycleoptimized to the system under investigation. There are some problemswith elements such as titanium which form stable carbides or nitridesbut these can be overcome using, for example, coated graphite tubes oran argon purge gas. Other problems relate to the precise reproducibilityof positioning of the sample in the furnace but the procedure, firstproposed by L'vov (1961), offers appreciable advantages in total sampleanalysis with a less obvious one being that one can 'build up' theconcentration of a trace component by carrying out a succession ofpyrolyses and ashings on several test portions before the final atomiza-tion step.

A major disadvantage of this procedure is the influence that the exactcondition of the graphite surface can have on the response of an analytesince a fresh graphite tube and one which has undergone 10 or 20atomizations can have very different responses. This introduces therequirement for repeated standardization for accuracy of analysis.Several manufacturers have now produced instruments which automatethe sample introduction for liquid samples but this facility is not avail-able for solid samples. The points made previously, and in Chapter 14,about the need to be certain that a 1 mg test portion is representative ofthe whole material should always be borne in mind.

NON-DESTRUCTIVE ELEMENTAL ANALYSIS

The second group of techniques to be considered which enableselemental analytical data to be obtained from a sample of rubber, be itraw, compounded or vulcanized, consists of bombarding the samplewith one form of energy and monitoring specific induced effects charac-teristic of each atomic species present. It will be appreciated that thisaffords elemental data but does not indicate the structural environmentof that element thus, unlike infrared spectroscopic analysis, there will beno distinction between silicon in silica, an inorganic silicate or siliconerubber. This must be deduced from other data.

One example of such a technique has already been discussed inChapter 6 where a radioisotope was used to provide X-ray fluorescenceidentification and quantification of certain elements. There arenumerous others as reference to Analytical Chemistry Reviews: SurfaceCharacterization shows, but two merit discussion here as they illustrate acomplementary pair of effects. In one, the bombardment of a samplewith X-rays liberates electrons whilst in the other, bombardment withelectrons releases characteristic X-rays.

Electron spectroscopyThe basis of electron spectroscopy is the measurement of the kineticenergy of electrons emitted from a sample in a vacuum followingionization by a monochromatic X-ray source, the latter usually beinggenerated by the bombardment of a pure metal such as aluminiumwith a beam of electrons. This technique can be modified in a numberof ways to produce subtly different data, but the ones which have themost significance in this application are X-ray photoelectron spectro-scopy (XPS) also known as electron spectroscopy for chemical analysis(ESCA) and Auger electron spectroscopy (AES).

A knowledge of the kinetic energy of the ejected core electrontogether with the irradiating energy enables the binding energy of thecore electron to be calculated, and hence its identity to be established.These instruments are, however, very expensive and in consequencetheir use tends to be limited to areas of research such as catalysis,surface structures and electronic properties rather than the routineidentification of inorganic constituents of an elastomeric product.

The converse technique would be the bombardment of a sample withan electron beam and monitoring the X-rays generated thereby. Thismerits rather more detailed discussion because it is the principle of X-ray analysis in the electron microscope (as opposed to the more usualVisual' detection mode which was discussed earlier (Chapter 9) withparticular reference to thermoplastics) and it will feature again in

Chapter 13 on the analysis of blooms and other surface effects. Themany areas which may be investigated using this combination of ascanning electron microscope (SEM) and an X-ray analyser make thevery powerful, if expensive, technique worthy of consideration. Even ifthe analyst is not able to justify purchase of such equipment, contractservices exist, and it is thus important that its capability is appreciated.

Wavelength-dispersive X-ray analysis

There are two distinct and quite different types of X-ray detector whichmay be fitted to an SEM - those which operate in the wavelength-dispersive mode and those which use energy dispersion to produce an'element spectrum'. The former is achieved by diffraction with a crystalspectrometer, the principles of operation of which are illustrated inFigure 10.3. The X-rays leave the sample in all directions but someimpinge on the specially shaped crystal which can be fitted so that theangle of incidence (6) of the X-rays may be altered. Bragg's law states:

™.L (10.1)nwhere A represents the wavelength of the diffracted X-ray.

The range of wavelengths which one crystal can cover is limited so itis usual to have a set, each with different lattice spacings, fitted to the

crystal

electron beam

specific X rays

lattice spacing (d)

detector

sample

X rays ofall wavelengths

Figure 10.3 Operation of a crystal spectrometer.

instrument so that for a given range of O a much larger range of X-rayscan be covered. As the wavelength of the X-ray is dependent upon theatomic number of the element, identification of an element merelyrequires measurement of the angle 6 and reference to a table of charac-teristic angles for each element.

The spectrometer can scan through the region of the characteristicwavelength and record an energy peak, the area of which is propor-tional to the concentration of that element (within the limitations of thematrix effects discussed later).

Energy-dispersive X-ray analysis

The energy-dispersive system is not mechanical nor does it dependupon the separation of different X-rays; it depends entirely on electro-nics, the key being the detector which is a lithium drifted silicon crystal,Si(Li). This has the property that electron-hole pairs are produced whenX-rays fall upon it and these are collected as a current pulse. Thenumber of electron-hole pairs generated by a specific X-ray is depen-dent upon the energy of that X-ray (i.e. the particular element) and apulse-height analyser is used to assign each pulse to a particularchannel of a multichannel analyser. Each channel is thus dedicated toX-rays of a specific energy and hence to a particular element. As thereis no scanning and there is random impingement of all the X-rays onthe detector there is effectively a simultaneous accumulation of the fullelemental spectrum although, in practice, special ultra-thin or window-less detectors are required to extend the useful element range belowsodium to low atomic mass elements such as beryllium.

The spectrum can be displayed on a visual display unit withinseconds of beginning the analysis and a typical hard copy printout,obtained after 100 seconds' analysis, is illustrated in Figure 10.4.

K alpha lines

Figure 10.4 X-ray dispersive spectrum of a rubber vulcanizate.

The major differences, therefore, between the two techniques are thatthe energy-dispersive system gives a rapid 'total' spectrum whereas thewavelength-dispersive system provides very high resolution peaks forindividual elements, but would be extremely tedious, if not impractic-able, to use to obtain a full elemental analysis in spectral form since thiswrould require mechanical scanning and several crystal changes. Thelatter is, however, very much more sensitive, and allows detection at100 ppm, whilst the former is realistically limited to about 0.1%(1000 ppm).

QUANTITATION OF ENERGY-DISPERSIVE X-RAY ANALYTICAL DATA

Unfortunately the quantitative side of energy-dispersive X-ray analysisis by no means as clear cut as the qualitative side. Problems can bedivided into two areas which may be called intrinsic and specific. Inthe intrinsic area one must consider background counts in the areabeing monitored, dead time (when the pulse processor is handling onepulse and rejecting others until the first is dealt with) and interferencefrom other elements. In the specific area there is absorption of theelectron beam by species other than the one of interest, secondary fluor-escence (where the characteristic radiation of an element is additionallyexcited by X-rays of a higher energy than the critical energy of theelement being monitored) and a diminution of the X-rays beingmonitored due to their absorption by other elements of the matrix asthey pass through the bulk sample on their way to the detector. Theseare collectively called matrix effects and various methods have beenadopted to correct for them (Lucas-Tooth and Price, 1961; Lucas-Toothand Pyne, 1964). Most instrument manufacturers supply 'correctionprograms', such as the popular ZAF correction, for the computers usedwith their analysers but these tend to refer to smooth, mirror-likesurfaces of materials of generally high atomic mass, quite unlike thoseexperienced in the analysis of rubber where we have a fundamentallylow atomic mass matrix containing well dispersed particulate fillers aswell as some elements in organic molecules which are dissolved in therubber.

Progress has been made in corrections for dealing with roughsurfaces (Brundle, Evans and Wilson, 1992) but problems persist withthe quantitation of fillers in elastomers. It would be fair to add that ifaccurately known controls are available which bracket, in all elements,the 'unknown' composition, reasonable results can be attained. It seemsbetter, therefore, at the present time, to regard the energy-dispersive X-ray spectrum of a bulk sample as an extremely rapid indicator of theelements present (with a lower element 'cut off depending on thematerial used to make the detector window) together with their approx-

imate concentrations, which then can be estimated more accurately byother techniques.

Again it is emphasized that these are surface analyses and thus, whilstfull use can be made of this, together with the magnifying power of themicroscope, to identify individual particles, or areas of inhomogeneity,such investigations should always be coupled with the examination of afreshly cut section through the sample to obtain an authentic 'bulk'spectrum against which the surface data can be assessed.

REFERENCESAnalytical Methods Committee (1965) Analyst 90, 515.Banks, C.K., Sultzberger, J.A., Mourina, FA. and Hamilton, C.S. (1948) /. Am.

Pharm. Assoc. 37, 13.Belcher, R., Macdonald, A.M.S. and West, T.S. (1958) Talanta \, 408.Bernas, B. (1968) Analyt. Chem. 40, 1682.van der Bie, GJ. (1947) India-Rubber J. 113, 499, 502, 541.Brundle, CR., Evans, CA. Jr and Wilson, S. (1992) Encyclopaedia of Materials

Characterization, Butterworth-Heinemann, London.Corish, PJ. (1961) /. Appl Polym. ScL 5(13), 53.Corner, M. (1959) Analyst 84, 41.Crafts, RC. (1992) /. Nat. Rubb. Res., 7(4), 240Davidson, J. (1962) Analyst 77, 263.Down, J.L. and Gorsuch, T.T. (1967) Analyst 92, 398.Gage, JC. (1961) BnY. /. Ind. Med. 18, 287.Gleit, C.E. and Holland, W.D. (1962) Analyt. Chem. 34, 1454.Gorsuch, T.T. (1959) Analyst 84, 135.Gorsuch, T.T. (I960) PhD thesis, London.Gorsuch, T.T. (1962) Analyst 87, 112.Gorsuch, T.T. (1970) The Destruction of Organic Matter, Pergamon Press, Oxford.Heckman, M. (1967) /. Assoc. Offtc. Anal. Chem. 50, 45.Houghton, AA. (1945) Analyst 70, 118.Jackson, K.D.O. (1997) /. NaL Rubb. Res. 12, 102.Johnston, J. (1910) /. Amer. Chem. Soc. 32, 938.Joyet, C. (1951) Nucleonics 9b, 42.Kelleher, WJ., and Johnson, MJ. (1961) AnalyL Chem. 33, 1429.Klein, A.K. (1941) /. Assoc. Offic. Anal. Chem. 24, 363.Lucas-Tooth, HJ. and Price, BJ. (1961) Metallurgia 64, 149.Lucas-Tooth, HJ. and Pyne, C. (1964) Advances in X-ray Analysis 7, 523.L'vov. B.V. (1961) Spectrochim. Acta 17, 761.Middleton, G. and Stucking, R.E. (1954) Analyst 79, 13.Milliken, L.T. (1952) Rubber Age (N. Y.) 71, 64.Moldrai, T. and Petrescu, G. (1965) Industrie Uscara 12, 522.Poulton, FCJ. (1958) Unpublished work at the Dunlop Rubber Co.Sandell. E.B. (1944) Colorimetric Determination of Trace Metals, Interscience, New

York.Scholl, F.K. (1981) Atlas of Polymer and Plastics Analysis Volume III (Additives and

Processing Aids), Carl Hanser Verlag, Munich.

Smith, M., Stickland, F.G. and Tarbin, F.G. (1959) Trans. IRI 35, 210.Stanton, R.E. and McDonald, AJ. (1965) Analyst 90, 497.Stern, HJ. and Hinson, D. (1953) India-Rubber /. 125, 1010.Uhrberg, R. (1982) Analyt. Chem. 54, 1906.Zeitlin, H., Fredyma, M.M. and Iheda, G. (1958) Analyt. Chem. 30, 1284.

Carbon black I I

In the early part of the twentieth century, just prior to the First World(or Great) War, it was discovered that carbon black could be added torubber in quite considerable quantities as a 'filler7 and that it wasunique in the way in which it reinforced or improved the properties ofthe final product rather than just cheapening it by extending its bulk tothe detriment of its physical performance.

Frank and Marckwald (1923) prepared identical products using bothGerman (lampblack) and American (oilblack) carbon blacks and foundthat their physical properties were significantly different with theformer giving a more elastic product and the latter a much toughermaterial. It was realized that carbon blacks produced by different routesand from different starting materials could be used to impart a widerange of different physical properties to a rubber product and today thematerial is available in many grades, the correct choice of which is ofcrucial importance to the performance of the finished product.Parameters which are important include the amount added to therubber, its particle size, its available surface area and its dispersionwithin the mix.

It is therefore important that the analyst is able to determine all ofthese parameters and in this chapter we are primarily concerned withthe classification of the carbon black and its distribution in the rubbermatrix. The amount of carbon black can be determined by thermogravi-metric analysis and this is discussed in detail in Chapter 12.

OBTAINING FREE CARBON BLACK FROM THE RUBBERMATRIX

Over a period of many years the carbon black content of a vulcanizedrubber product was estimated by a method which depended ondestruction of the rubber by nitric acid followed by separation of the

black by filtration and its subsequent drying and weighing. Finally, theloss in weight on ignition of the dried residue provided the carbonblack loading. This method was first proposed by Jones and Porritt(1914) and is described in one of the earliest books on the analysis ofrubber, that by Tuttle (1922).

There are today two fundamentally different methods which are incommon use to recover carbon black from a compound or vulcanizate.These will be considered separately below, as will their applicability tothe range of elastomeric materials now available which are likely tocontain this material.

DESTRUCTION AND FILTRATION METHODS

It is instructive to follow the steps whereby the original methods for thedestruction of the rubber by nitric acid have led to the modern recom-mended standard procedures. Scott and Wilmott (1941) were the first torealize the need for a modified method when dealing with syntheticrubbers. Their procedure consisted of swelling the finely divided rubberin hot nitrobenzene, then adding 25% (v/v) nitric acid and heating on ahot plate. Neoprene disintegrates and dissolves in the nitrobenzene in afew minutes. The whole is then heated on a steam-bath for about onehour after which xylene is added and the mixture filtered hot. Afterfiltration the carbon is washed with hot xylene and then with acetonebefore drying and igniting. The problem of disintegrating the moreresistant synthetic rubbers was pursued by Louth (1948) who found itnecessary to introduce an ether extraction step to deal with the organicmaterial derived from the decomposed rubber.

The method proposed by Galloway and Wake (1946) for estimatingthe polymer in a compounded and vulcanized butyl rubber leads to thesimultaneous estimation of carbon black, the level of which can beobtained by drying and igniting the residue on a sintered crucible.

The use of methods based on dissolution for estimating carbon blackhas not found universal favour, removal of the black from the solutionof the polymer by filtration being tedious since most solvents dispersethe black instead of aggregating it and because any polymer istenaciously absorbed on to the black and is difficult to remove comple-tely. Obviously, the more degraded the rubber is, the less viscous thesolution and the more easily is the black washed free of it. This leadsnaturally to the use of oxidation catalysts which are particularly usefulfor rubbers based on butadiene which show a tendency to crosslinkingrather than chain-scission when subjected to oxidative attack by oxygenalone. Kolthoff and Gutmacher (1950) were the first to use tertbutylhydroperoxide in the presence of osmium tetroxide to hasten the disso-lution of a range of rubbers in boiling paradichlorobenzene.

The modification of Kolthoff and Gutmacher's method adopted for BS903-1964 dropped the use of osmium tetroxide and used only terbutylhydroperoxide as the oxidation catalyst.

Louth (1948) reported the determination of carbon black in butyl,natural, butadiene-styrene and polychloroprene rubbers with anestimated standard deviation independent of the polymer of about0.15%. Kolthoff and Gutmacher gave results which show a standarddeviation of only 0.09% on a carbon black content of 30% although thisrepresents accuracy unlikely to be achieved in the ordinary routinelaboratory.

REMOVAL OF POLYMER BY DISTILLATION

Another approach to the isolation of carbon black from a black-filledelastomer is by pyrolysis, whereby the rubber (and other organicspecies present) are thermally fragmented by heating the sample inan inert atmosphere and then distilled off as volatile fragments. In1949 Bauminger and Poulton described a method for determiningpolymer loading by controlled pyrolysis in an inert atmosphere andpointed out that the residual carbon is a function of the proportionof material other than hydrocarbon in the polymer. This approachcan be over-simplistic as it does not take into account any inorganicsubstances which may be present but, nevertheless, it may be consid-ered the introduction of thermogravimetry (TG) to the rubber labora-tory. Its subsequent development, through instrumentation, to afforda quantitative estimate of carbon black loading is discussed in detailin Chapter 12.

For the purposes of this chapter we will accept that TG can givequantitative data on carbon black loadings and consider what furtherinformation may be obtained concerning the carbon black in a vulcani-zate. However, it should be noted here that not only must due allow-ance be made for any inorganics in the sample, but also that thepyrolysis of some polymers leads to additional carbonaceous residueand that this must be allowed for both quantitatively and in terms of itsinterference with any method used to obtain further classification dataon the black.

TYPES OF CARBON BLACK

Carbon blacks used in the rubber industry were initially of relativelyfew types and were classified according to the properties of the rubbercompound and/or vulcanizate derived therefrom. As the technologicalbase for the manufacture of carbon blacks developed it became obviousthat a more detailed categorization was required and some indication of

the complexity is given by Gerspacher et al. (1995) who pointed out thatthere are three basic processes used in carbon black production, thechannel, thermal and furnace processes with the most significant, the oilfurnace process, accounting for over 95% of the world's current produc-tion and providing more than 20 different grades. Two further processesexist, the lampblack process, developed by the Chinese to manufactureink and lacquer and the acetylene process which gives a black whichfinds a small use in the manufacture of electrically conducting rubberproducts.

Any attempt at categorization requires an understanding of thephysical and physiochemical properties of the various grades of carbonblack so let us begin by considering exactly what carbon black is.

Carbon black is an extremely pure form of carbon which consists ofextremely small particles which, ideally, approximate to spheres inshape but are rarely seen individually as they fuse together in chains orclusters, referred to as aggregates. These in turn tend to cluster togetherin agglomerates which are believed to break up on mixing with rubber.Aggregates, on the other hand, may occasionally fracture but in essencerepresent the units of carbon found within a vulcanizate. The type ofaggregate indicates the structure of the black which may be consideredto reflect the ratio of the surface area exposed to the rubber moleculesto that hidden from the rubber inside pores or channels too small forthe rubber molecules to penetrate; the higher the structure, the greaterthe number of particles per aggregate. The parameters which mayrequire defining are thus:

• basic sphere size• structure (aggregate size and shape)• absolute surface area• rubber-available surface area

Carbon blacks are currently identified in a variety of ways such asgroup number, name or symbol, and ASTM designation (ASTMD1765). The interrelationships between these, together with other datawhich will be referred to throughout this section, are given in Table11.1.

Two general points are worth noting: first, as well as the designation'N' (normal) one may meet 'S' (slow) which serves to distinguish theslow curing channel blacks (or modified furnace blacks) from thenormal furnace blacks and second, whilst the first of the ASTMnumbers is identical with the old group number, the last two arearbitrarily assigned and therefore have no scientific significance. Thevalues listed are target values or are ranges taken from the literatureproduced by a range of manufacturers.

Table 11.1 Analytical data on carbon blacks

CTABtypicalvalue(m2/g)

1261281111048382874142763633

9

DBPmeasured

range(ml/100 g)

110-119113

108-120124

70-8095-110

120-130100-113115-125

18085-9565-8533-36

TypicalIodine No

(nng/g)

1451601211088282904243

1003627

BET Nitrogensurface area

measured(range m2/g)

125-160143

115-130103

75-10570-9080-10035-5235-52

8026-4017-335-10

Permittedrange (nm)

11-1911-1920-2520-2526-3026-3026-3040-4840-4840-4849-6061-100

201-500

Group No.

1122333555679

Symbol

SAFSAFISAF

HAF-LSHAFHAF-HSFEF-LSFEFFEF-HSGPFSRF-HMMT

ASTM Type by namedesign

N 110 Super abrasion furnaceN 115 Super abrasion furnaceN 220 Intermediate SAFN 299N 326 High abrasion furnace LSN 330 High abrasion furnaceN 347 High abrasion furnace HSN 539 Fast extruding furnace LSN 550 Fast extruding furnaceN 582 Fast extruding furnace HSN 660 General purpose furnaceN 772 Semi-reinforcing furnace HMN 990 Medium thermal

ANALYSIS OF CARBON BLACK PARTICLES ANDAGGREGATES

It was as early as 1920 that Weigand estimated the size of a carbonblack particle by light microscopy but since the advent of the electronmicroscope just prior to 1940 the latter has been universally used forthis purpose. Hess and Herd (1993) provide a recent review, with manyreferences, of available techniques ranging from the earliest light micro-scopical studies through X-ray diffraction, transmission electron micro-scopy, scanning electron microscopy to some of the atomic force andscanning tunnelling microscopical techniques. The last two are of parti-cular interest to those wishing to examine the surface microstructure ofcarbon black as discussed by Niedermeier, Stierstorfer et al (1994),Niedermeier, Raab et al. (1994), and Raab et al (1997).

In any discussion on 'particle sizing' it is necessary to be clearwhether one is referring to particles (the basic sphere size) or aggre-gates, although the visualization of either requires the same techniqueand uses a transmission electron microscope (TEM). Hess et al. (1969)described a technique whereby carbon black aggregates from thevirgin black could be dispersed for examination. The techniqueconsists of adding a few milligrams of carbon black to a smallamount of chloroform which is then treated with low power ultra-sonics. One drop of the resulting suspension is pipetted on to a TEMexamination grid that has been coated with a carbon support film.This can then be examined directly in the TEM to give a micrographsuch as is illustrated in Figure 11.1. Various shape and sizing opera-tions can then be carried out either manually or automaticallydepending on the level of equipment available. The same authorsalso examined vulcanizates, after sampling by ultramicrotomy, andwere thus able to obtain data on aggregate breakdown duringcompounding.

ANALYSIS OF CARBON BLACK TYPE

SPECTROPHOTOMETRIC METHODS

In earlier times, when the available types of carbon black were fewer,identification of the type could quite simply be made on the blackrecovered by nitric acid disintegration of the rubber by means of itstinting strength (Dawson et al., 1947; Scott and Wilmott, 1947). Valuesare still listed for the Group 1 to Group 3 blacks in ASTM Dl 765 andexperimental details are supplied in ASTM D3265. The use of tintingstrength might still be worthwhile in the laboratory where more elabo-rate apparatus is not available and where, perhaps, a distinction is

Figure 11.1 Representative TEM micrograph of virgin carbon black.

required between a limited number of known blacks of which authenticsamples are available.

The tinting strength is obtained by grinding O.Ig of the carbon blackwith linseed oil with the addition of zinc oxide in small quantities togive a standard shade of grey. The tinting strength is then expressed asthe ratio of the weight of zinc oxide to that of black and comparison ismade with known samples. A valuable ancillary test to this is a glosstest reported by Dawson et al. (1947) and ascribed by them to D. F.Twiss. In this a small quantity of the black is rubbed out on a filterpaper with a metal spatula and the colour and gloss compared with arange of blacks. These are both simple tests but yield quite surprisinglyconstant results, and their empirical nature and the need for standardsshould not prejudice the analyst against them for they are valuable,quite rapid, and inexpensive.

Kress and Stevens-Mees (1970) put the gloss test on a quantitativebasis when, with the assumption that increasing particle size wouldgive an increase in reflectance, they developed a method for measuringthe reflectance of a black filled vulcanizate, without prior isolation ofthe black or destruction of the polymer, at 540 nm using a spectrophot-ometer with a reflectance attachment and a gloss black tile as referencematerial. The results (Table 11.2) show that a reasonable relationshipexists between the percentage reflectance and particle size as judged byan electron microscope, although there is a marked polymer depen-dency on absolute values.

Other workers have also used spectrophotometers to classify carbonblacks but in most cases suspensions have been prepared in a liquidmedium prior to examination. Fiorenza (1956) prepared a suspension ofcarbon black in a benzene solution of natural rubber prior to measuring

Table 11.2 Relationship between particle size and percentage reflectance for arange of carbon blacks in sets of vulcanizates identical except for polymer type

Type of Particle % Reflectance in matrix of:black size (nm)ASTM BR NR OESBR UR

N 110 17 20 24 37 29N 220 20 26 30 34 38N 330 27 22 34 39 42S 300 28 28 28 35 38N 550 33 40 48 50 54N 660 55 43 48 50 54N 761 75 55 58 66 62N 880 180 76 79 77 81N 990 470 82 87 91 99

the absorbance at two wavelengths - 430 nm and 750 run - and calcu-lating a colour index (Ic):

Ic = log IQ/I (430 nm)/log I0/! (750nm) (11.1)

This procedure was also used for uncured compounds and vulcanizatesbut in the case of the latter the rubber was initially destroyed usingconcentrated nitric acid. Later, thermal degradation, as described byKolthoff and Gutmacher (1950), was used to render the vulcanizatesoluble. It was also noted that for free carbon black a suspension couldbe prepared in aqueous gum Arabic, rather than the rubber solutionpreviously used.

The method for rubber containing carbon black was subsequentlyinvestigated by Davies and Kam (1967) who, finding it time consumingand prone to error due to incomplete separation of the black from therubber, resorted to pyrolysis using a simple furnace ascribed toChambers (1958) followed by an acid wash before dispersing the blackin an aqueous gum acacia solution for measurement of the colour indexat wavelengths of 425 nm and 675 nm. The slightly differentwavelengths used from those of Fiorenza result in marginally differentvalues for the colour index. Davies and Kam compared the colour indexvalues of specific blacks in their virgin states, and in compounded andvulcanized formulations (Table 11.3) and also showed that, at least foran HAF black, the values were unchanged when obtained from vul-canizates based on NR, SBR, BR and a's-BR. A standard deviation of+/-0.Ol is claimed.

Table 11.3 Colour index (/c) values of various blacks (Courtesy J.IRI)

Type of carbon Colour Index (/c)black Free state Recovered

1 2

Lampblack 0.95 0.95Fine thermal 0.96 0.98 0.97Semi-reinforcing 1.03 1.01 1.01Fine ext. furnace 1.17 1.16 1.16High modulus 1.21 1.22 1.22High abrasion 1.35 1.36Superconductive 1.38 1.40Intermediate 1.44 1.43Low modulus 1.48 1.47Medium process 1.55 1.57 1.56

1. Uncured rubber sample. 2. Cured rubber samples.

Davey (1970) investigated the effect that silica had on these valuesand reported that over the full range of particle sizes a weight of silicaequal to that of the black could be tolerated with no alteration of theobserved colour index.

Aqueous dispersions of carbon black have also been studied by Ng(1978) who carried out turbidity measurements in the ultraviolet regionof the spectrum (200-300 nm) to predict not only the arithmetic meanparticle size, but also the size distribution. He used a procedure forremoving the rubber from a vulcanizate which consisted of ozonolysisof a comminuted vulcanizate suspended in chloroform at -20 0C.

SURFACE AREA MEASUREMENTSA second group of analytical procedures for estimating the type orgrade of carbon black in a vulcanizate relies on the direct measurementof the surface area of the black. Many authors, however, includingMicek et al (1968) and Kolthoff and Gutmacher (1952), have shown thatthe reinforcing ability of a black is a function of the 'available7 or'external7 surface area of the carbon black, rather than the total surfacearea, which includes the pores in the carbon black aggregates, whichare not accessible to the large rubber molecule. Thus the total surfacearea gives an indication of the particle size and the external surface areaan indication of the structure.

TOTAL SURFACE AREA

Methods for the determination of total surface area of carbon blacksinclude the 'BET7, the 'iodine adsorption7 and the 'CTAB7 methods.

BET method (nitrogen adsorption)In 1938 Brunauer et al. published work on the surface adsorption ofnitrogen and this has been the standard (BET) method of total surfacearea measurement ever since. A wide range of instrumentation is nowavailable to do this and, although the original apparatus is described byBarr and Anhorn (1949), its fragility and intricacy have led to the devel-opment of more rugged and simpler equipment. The reader is referredto the work of McFearin (1962), Kremens et al (1965) and Atkins (1964).An ISO standard (ISO 4652) exists which uses a commercially availableinstrument, the Ni-Count-1, which, in turn, is based on a techniquedeveloped by the Phillips Petroleum Company (Krecji and Roland,1965) whilst the American Society for Testing and Materials describesunder ASTM D 3037-1978 four accepted procedures, one of which isthat using the Ni-Count-1.

Iodine adsorptionThis provides a method for estimating total surface area of carbon blackswhich has the advantage that no equipment other than that found in anormal chemical laboratory is required. It shows a good correlation withthe nitrogen surface area results, as indicated in Table 11.1, although ifthe black is highly acidic the iodine volume can be low. The procedure isgiven in ASTM D1510 (ISO 1304) but is described in full because of itsusefulness and ease of operation (Schubert et al., 1969).

The weight of carbon to be used is dependent upon the iodine adsorp-tion number. For iodine adsorption numbers up to 130, use l.OOOg ofcarbon; from 130 to 500 use 0.500Og; over 500 use 0.250Og. It may benecessary to run the test with l.OOOg of carbon and repeat the testwith a lower weight of sample after a value is obtained. Weigh thedried carbon into a 100 cm3 screw cap bottle or a stoppered test-tubewhose length is approximately 200mm and ID approximately300mm. Pipette into the bottle or tube 50cm3 of 0.0236 M iodinesolution (commercially available) containing 6.000 g iodine and 57.0 gpotassium iodide per litre. Cap or tightly stopper the containerimmediately. Shake the iodine/carbon black mixture vigorously for 5minutes on a laboratory shaker. The carbon must be intimately mixedwith the iodine and, if it is difficult to wet, add a few drops ofethanol, correcting for any change in iodine normality. Centrifugeimmediately until the carbon settles. Decant the iodine solution into a100 cm3 beaker and immediately pipette 20 cm3 of the solution into a250cm3 Erlenmeyer flask. Titrate with 0.0394 M sodium thiosulphatesolution containing 9.7810 g of sodium thiosulphate pentahydrate and5cm3 of 1-pentanol per litre. Treat a reagent blank in the samemanner. If a centrifuge is not available, filter into a 250cm3 Erlen-meyer flask through a long-stem funnel plugged with fine glass wool.To avoid volatilization of any iodine pass the stem of the funnelthrough a bored cork which fits the neck of the flask and cover thefunnel with a watchglass. Pipette and titrate as above. Calculate theiodine adsorption number, I, in mg of iodine/g of carbon, as follows:

I=(B-S)/B (50/W) (M) (253.82) (11.2)

where B = volume of thiosulphate used in the titration of the blank incm3, S = volume of thiosulphate used in the titration of the sample incm3, W = weight of carbon in g and M = molarity of the iodinesolution.

Reinforcing grades of black have iodine adsorption numbers typicallyin the 70-160 mg/g range whilst semi-reinforcing ones are in the 30-45 mg/g range.

Table 11.4 Comparison of BET surface areas of recovered blacks from differentvulcanizates

BET Surface area: m2 /g

Grade Original black ex NR ex SBR ex SBR/BR ex UR

N220 120 — 113 — —N234 120 119 — — 129N326 76 — — 71 —N339 94 96 100 — 108N347 86 87 88 — 92N357 74 79 82 — 83N539 42 — 47 46 —N550 39 45 45 — 45N660 30 — — 34 —N765 23 — — — 21

Both of these techniques were developed for virgin carbon blacks andtherefore, if they are to be used on vulcanizates, we must know theirvalidity for recovered blacks. Brown et al. (1979) provide a mass of dataon the BET nitrogen surface area of blacks recovered by pyrolysis ofblack filled elastomers (NR, SBR, alone and blended with BR, and UR)followed by grinding at high speed in an analytical mill (Janke &Kunkel type AIOS) before analysis. The original data illustrate blackloadings between 20 and 75pphr and show little effect with loading.Table 11.4 is for loadings of 40-50 pphr.

A similar table (Table 11.5) obtained by Lamond and Gillingham(1970) confirms the variations from the original total surface areas andsuggests that, given a knowledge of the polymer type, recovery bypyrolysis is a valid technique for obtaining the carbon black, the particle

Table 11.5 BET surface areas of blacks recovered from vulcanizates (CourtesyEuropean Rubber J.)

BET Surface area: m2 / g

Grade Original black ex NR ex SBR ex SBR/BR

N110 144.6 132.5 127.5 123.9N220 124.2 124.3 112.9 110.0N285 103.6 104.0 95.1 81.8N330 82.9 86.4 78.2 74.5N550 39.7 45.2 45.1 —N785 30.5 38.1 33.5 —N990 7.4 12.2 10.8 —

size of which can then be related to that of the originally addedmaterial.

Lamond and Gillingham (1970) and Lamond and Price (1970) alloweda much larger molecule than those considered above to be adsorbed onto the surface of the carbon black. This provides arguably the bestmeasure of true surface area as it is less influenced by chemical differ-ences between blacks and is less sensitive to porosity.

Procedure for the determination of the Aerosol OT absorption value:50cm3 of Aerosol OT solution (4g/l) is shaken for 30min with 1 g ofthe carbon black sample and the carbon black is then separated fromthe OT solution by centrifuging at 24000rev/min. 10cm3 aliquots ofthe OT solution are titrated (before and after treatment with carbonblack) with cetyltrimethylammonium bromide (CTAB) (concentration1.125g/l) using a modification of the method proposed by Barr et al.(1948). On addition of CTAB to the OT mixture, the chloroformemulsifies in the aqueous layer and the chloroform-OT-CTAB mixturetakes on a milky appearance. On continued addition of the CTAB, de-emulsification occurs and the chloroform layer (containing someemulsified water) separates out. At this point, bubbles are observedin the chloroform layer (probably caused by emulsified water) andthe titration is continued until these bubbles disappear instanta-neously after allowing the agitated mixture to stand.

The amount of OT adsorbed is given by the following expression:

(decrease in CTAB titre). C . 6.322/W (11.3)

where C = the concentration of CTAB (g/1) and W = the sample weight(g). Data on virgin and recovered carbon blacks are given in Table 11.6.

ASTM (D3765) defines a similar procedure - the CTAB surface area

Table 11.6 The OT no. of recovered carbon blacks from a range of vulcanizates(Courtesy European Rubber J.)

Black recovered from:

Grade Original black NR SBR SBR/BR

N110 103.9 98.2 99.1 100.1N220 95.9 96.1 92.0 91.0N285 77.8 81.7 78.3 81.7N347 70.7 61.7 66.7 73.6N330 65.0 71.5 62.0 68.9N550 33.5 29.4 32.1 —N785 26.3 24.1 26.2 —

,N980 6.3 10.0 7.8 —

test method - in which the original mixing is between the black andCTAB with subsequent titration with Aerosol OT. Reinforcing grades ofblack have CTAB values typically in the 80-140 m2/g range whilstsemi-reinforcing are in the 30-45 m2/g range.

Magee (1995) has recently reviewed both the BET (NSA) and CTABprocedures and suggests that the 'statistical thickness surface area'(STSA) is a valuable parameter which can be measured simultaneouslywith the NSA and offers a number of (other) advantages over theCTAB method in the determination of the 'available' surface area of acarbon black.

EXTERNAL SURFACE AREA

DBP test

The classical procedure for measuring the external surface area of acarbon black, and thus assessing its structure, is the oil absorptionmethod of Sweitzer and Goodrich (1944) whereby alkali-refined linseedoil is mixed with Ig of carbon black until just sufficient is added toallow the mix to cohere as a single mass. This can be carried out oneither raw or recovered black with the latter again having been groundprior to testing. It will be appreciated that, although this is a valid andquick test in the hands of an experienced operator, it is highly subjectiveand could benefit from automation. This has now been carried out, andthe automated method is covered in detail in various InternationalStandards such as ISO 4656/1 as well as ASTM D2414. In this proce-dure dibutylphthalate (DBP) is automatically added from a burette to atest portion which is kept in motion by rotating paddles. As the DBP isadded the powder changes to a semi-liquid mass with an increase intorque. At the limit of adsorbed DBP the torque peaks and this triggersa closing of the automatic burette. The absorption number is expressedas ml DBP per 100 g of black with low structure values being typically60-80 ml/100 g moving through intermediate to high stucture wherevalues are in excess of 120 ml/100 g. An extension of the DBP procedurecrushes the black before analysis to break up some of the weaker aggre-gates and this is described under ASTM D3493.

All of the methods here described are regularly used and thus achoice must depend upon the facilities of the laboratory and the depthof information required. In general terms it may be concluded thatprovided the vulcanizate is properly treated to isolate the carbonblack, that is, solvent extracted, followed by pyrolysis under nitrogenor vacuum, a subsequent acid wash, water wash and drying at lowtemperature (approximately 1050C) followed by a light grinding tobreak up the agglomerates without damaging the aggregates, the

recovered black may be analysed in the same way as a virgin blackand, if due consideration is given to its history and to the polymericbase from which it was recovered, valid and useful information willbe obtained.

BLACK TYPE BY THERMOGRAVIMETRY

A new dimension was added to the analysis of carbon black type witha publication by Maurer (197Oa) in which he claimed that thermogravi-metry (TG) could be used, in certain systems, to identify differentcarbon blacks. The initial basis for this statement is shown in Figure11.2 which illustrates a plot of residual sample weight against tempera-ture for a pair of vulcanizates identical apart from the type of blackused.

After pyrolysis in nitrogen to remove oil and polymer the samplewas cooled to 2750C, the atmosphere changed to air, and the samplereheated at 150C per minute. The different oxidative stabilities of thetwo blacks, MPC and SRF, are illustrated and this represents a potentialmethod for distinguishing between them. The procedure was improvedto allow for the difficulty in determining the exact temperature at whichweight loss of the carbon by oxidation commenced, by measuring Ti5and T50, the temperatures at which 15% and 50% of the carbon blackweight were lost. It is well known that a piece of vulcanizate retains itsshape after pyrolysis and thus the residue must be a porous matrixthrough which air can permeate freely. There should therefore be adegree of correlation between the BET surface area and the oxidizabilityof the black. Indeed this was found to be the case shown in Figures 11.3and 11.4 for Ti5 (Maurer, 197Oa; Pautrat et al, 1976). However, the twographs are by no means superimposable and thus standards will benecessary for any system under consideration. Maurer (197Ob) showed afurther complication when he illustrated the effect of different curesystems on the onset temperature of oxidation of the carbon blacks.This is shown in Table 11.7 for a butyl vulcanizate.

However, the literature contains other references which suggest thatthe situation regarding the use of TG for black type identification isfar from straightforward. Spacsek et al. (1977) were not able toconfirm the relationship between Ti5 and surface area whilst Schwartzand Brazier (1978) reported a number of blacks which fell appreciablyoff the correlation line of a T2Q vs. surface area plot. These latterauthors also provide data which suggest that nominally identicalblacks manufactured at different locations have different oxidationcharacteristics.

Attempts to clarify these situations have been numerous and varia-tions such as isothermal oxidation tried (Maurer, 1974), but the current

Figure 11.2 Detection of carbon black differences in standard formulation.(Courtesy Rubber Age.)

Table 11.7 Effect of cure system and black type on decomposition of butyl rubbervulcanizates

Cure Average values Polymer Black decomp.system decomp. (0C) (0C)

% % % temp. final onset finalpolymer black ash 50% loss temp. temp. temp.

A 65.0 31.0 3.0 420 456 497 55365.2 30.8 4.0 418 455 529 58264.9 31.4 3.8 416 452 550 59664.8 31.8 3.3 412 454 551 605

B 65.0 32.0 3.3 418 453 513 55866.0 30.3 3.7 417 455 543 58865.1 31.3 3.6 415 451 567 60865.0 32.0 3.0 411 450 581 623

C 65.3 31.8 3.0 421 457 528 56165.8 31.3 2.8 421 464 601 61264.8 32.5 2.8 420 455 597 61764.0 33.0 3.3 416 452 595 628

D 63.3 30.0 6.7 417 464 427 46163.0 30.0 7.0 413 448 468 47962.8 30.4 6.9 414 453 464 49762.5 31.0 6.8 407 442 461 529

Reprinted courtesy of the National Institute of Standards and Technology, TechnologyAdministration, U.S. Department of Commerce. Not copyrightable in the United States.

carbon black / ash

(nitrogen)

polymer / oil

Wei

ght

% r

emai

ning

Figure 11.4 Virgin carbon black combustion vs. surface area. (Courtesy RubberChem. Technol.)

Figure 11.3 Carbon black in UR: vulcanizate combustion vs. surface area.(Courtesy Rubber Age.)

Su

rfac

e ar

ea (

m2/g

)S

urf

ace

are

a (m

2/g

)

position seems to have changed little since it was summarized byCharsley and Dunn (1981) who studied the following:

Cure systems (pphr:)

• A - Altax (1.0), Tellurac (1.5), sulphur (1.0), ZnO (5.0), stearic acid(2.0)

• B - Sulfasan R (2.0), Tuads (2.0), ZnO (5.0), stearic acid (2.0)• C - SP1055 (12.0), ZnO (5.0), stearic acid (2.0)• D - Altax (4.0), GMF (1.5), red lead (5.0), ZnO (5.0), stearic acid

(2.0)

The maximum temperature reached during pyrolysis: up to about60O0C there is little change in the T15 but above this there is arelatively linear increase of about 1O0C in T15 for each 5O0C rise infinal temperature of pyrolysis.

Isothermal hold during pyrolysis: again 60O0C appears significant;there is no observable increase in T15 if the hold is up to 30 minutesat 540 0C but there is a gradual increase if the hold is at 655 0C. Thisamounts to about 10C per 2 minutes' hold.

Effect of air-flow rate over the sample: there is a regular decrease inT15 as the air-flow increases; a T15 value of 5970C at a flow rate of5Cm3ITUn"1 falls to 5630C at a SOcn^min'1 flow rate.

Having optimized and standardized all these conditions, the authorsproduced the results shown graphically in Figure 11.5 where the errorbars represent the range for six nominally identical analyses of eachsample. They also comment that these values are for a set of resultsdetermined on one day and that repeat analyses at a later date gavethe same degree of spread but with the curve displaced by a fewdegrees.

Effect of cure: although the full range of cure conditions as used byMaurer (197Oa) (Table 11.7) was not examined, it was shown thatappreciably different T15 values were obtained for sulphur andperoxide cured vulcanizates of EPDM and SBR.

From these data Charsley and Dunn conclude:

"The experimental variables which are found to affect significantlythe measured T15 value for compounded carbon blacks are: (a) themaximum temperature achieved during the pyrolysis step, and (b)the flow rate of air and the heating rate used during the oxidationstep.

There is a definite correlation between the T15 value and the surfacearea of a carbon black, both in its free form and when compoundedin a rubber.

Figure 11.5 T15 values for carbon blacks in NR rubber formulations. (CourtesyRubber Chem. TechnoL)

The T15 value is dependent on the cure method of the rubber andhas also recently been reported to depend on the manufacturingsource of the carbon black. This technique, therefore, cannot berecommended as suitable for the identification of a carbon black typein an unknown formulation. It can be used, however, as a routinequality control check on batch rubbers/'

These comments, together with the observation that the techniqueappears valid for checking the consistency of a specific grade of virginblack from a particular source, still describe the current position in theapplication of TG to black identification.

(Data and conclusions of Charsley and Dunn reprinted with permis-sion of Rubber Chem. TechnoL)

surface area

CARBON BLACK DISPERSION IN VULCANIZATES

The dispersion of carbon black has a strong influence on a number ofvulcanizate physical properties and, over the years, numerous techni-ques for the assessment of dispersion have been developed althoughthese vary in the depth of information they generate.

For many industrial products it may be necessary only to judge thequality of dispersion from low resolution light microscopical techniques.Alternatively, some research materials may require high resolutiontransmission electron microscopy in order to observe the preferentiallocation of the carbon black. The following is a brief review of some ofthe techniques available, beginning with light microscopical techniques.It should be noted that low resolution light microscopical imaging willoften gain by being supported by electron microscopy which can afforda clearer impression of the dispersion.

THE CABOT DISPERSION TEST

This technique, described by Medalia and Walker (1970), requires thecutting of semi-thin sections using a base-sledge microtome equippedwith a liquid nitrogen stage and either a sharp steel blade or a freshlycleaved glass knife. The knife is wetted with xylene and sections ofapproximately 2|im thick are cut from the frozen mounted sampleblock. These sections are removed using a brush and deposited in adish containing xylene from which the best sections can be removedand mounted on glass slides using a suitable mountant. This type ofsectioning is described in considerable detail in Chapter 9, although it isworth mentioning here that the production of useful thin sections takesa good deal of practice and patience. A grading of the dispersion isobtained by comparing five fields from the specimen, viewed in trans-mission using a light microscope fitted with a Cabot graticule, with aCabot Dispersion Classification Chart.

THE CUT-SURFACE AND TORN-SURFACE METHODS

These methods provide an alternative to the Cabot test and offer somemeasure of automation in the form of such equipment as the OptigradeDispergrader. Technology is now reaching the point where imageanalysis tools should make full automation possible, but for thepurposes of this book, the manual technique is more relevant. Incontrast to the Cabot test, surfaces are viewed using incident illumina-tion thus negating the rather time consuming sectioning technique. Thetorn-surface method is described in detail by Sweitzer ei al. (1958) andshould be regarded as a technique for judging the level of agglomera-

tion of carbon black. Stumpe and Railsback (1964) developed thetechnique into a more reproducible method in which surfaces are cutusing a razor blade prior to examination. Surfaces are examined usingoblique illumination and compared with micrographs of standardsranging from 1, a very poor dispersion, to 10, an excellent dispersion.

TRANSMISSION ELECTRON MICROSCOPY

Transmission electron microscopy can be thought of as an extension oftransmitted light microscopy in terms of examining black dispersionalthough, as Morrell (1977) rightly points out, since such a small area isviewed using electron microscopy, it is necessary for the observer to bethorough in his or her examination since it is far too easy to missimportant information by working at too high a magnification. Thetechnique is very useful for observing variations in carbon black phasedistribution in blends as discussed by Herd and Bomo (1995).

Kruse (1973) offers a detailed description of rubber microscopy as awhole and deals with the electron microscopy of black filled vulcani-zates in detail whilst for a more detailed discussion of agglomeratestructure, Wolff and Wang (1993) describe the association of carbonblack aggregates into agglomerates with a chain-like structure or clusterreferred to as secondary structure or filler structure.

The techniques required for specimen preparation by cryo-ultrami-crotomy are described in detail in Chapter 9. However, it should benoted that even moderate loadings of 30-40 phr will require evengreater care in sectioning. Section thicknesses should be less than100 nm (as opposed to less than 200 nm for phase morphological obser-vation) to permit high resolution imaging.

OTHER TECHNIQUES USED TO EXAMINE CARBON BLACK

The rubber analyst is mainly concerned with the determination of thecarbon black type and distribution of that black in a rubber product butthe following methods merit recognition for use in trying to extend ourunderstanding of carbon black and its interactions with rubbers.

INVERSE GAS CHROMATOGRAPHY (IGC)

Wang, Wolff and Donnet (1991) have used IGC to study the surfaces ofdifferent grades of carbon black, observing the thermodynamicparameters and surface energies as they relate to the total surface areas.This was followed up in later study (Wang and Wolff, 1992) of a seriesof carbon blacks including graphitized and non-graphitized carbonblacks. They concluded that high energy sites play a dominant role in

elastomer reinforcement and that the smaller particle size blacks possessa greater number of high energy centres.

NEUTRON SCATTERING

Small angle neutron scattering (SANS) has the ability to probe thecarbon particle structure. A significant study has been carried out byHjelm et al (1994) using a method of contrast variation to probe theinternal structure of aggregates.

RAMAN SPECTROSCOPY AND X-RAY SCATTERING

Gruber, Zerda and Gerspacher (1993; 1994) followed up some earlierdevelopmental work on the use of Raman spectroscopy in the examina-tion of graphites and coals and extended their work to the applicabilityof this method to the characterization of blacks in relation to microstruc-ture based upon relative peak intensities. They provided a comprehen-sive study of a range of carbon blacks using Raman scattering andfound it complementary to X-ray diffraction. As well as providingquantitative data, the technique is useful in providing qualitative differ-ences between grades of black with varying microcrystalline dimensionsand graphitic ordering. They found that all carbon blacks obtainedusing the furnace process possess a similar crystalline size of around25A.

A development of X-ray methods using wide angle X-ray scatteringby Gerspacher and Lasinger (1988) has been useful in determining inter-planar spacing and stacking height. This has shown differences betweenblacks of the same grade which exhibit differences in rubber reinforce-ment potential.

SURFACE COMPOSITION ANALYSIS

It has long been known that carbon black has chemical sites on thesurface which contain components other than carbon. The presence ofsuch reactive sites, which have the potential to interact with the rubber,is an area which merits consideration in the field of rubber-black inter-action. As an example of this Ayala et al. (1990) used a variety of techni-ques including IGC, Gas Chromatography-mass spectrometry (GCMS),secondary ion mass spectrometry (SIMS) and X-ray photoelectricspectrometry (XPS) in their examination of the surface composition ofcarbon blacks with the aim of measuring carbon black surface interac-tions with SBR and UR. They found a complex hydrogen functionalitywhich was preserved even after heating at 90O0C and concluded thatthis was a primary factor relating to carbon black surface activity.

MODELS OF CARBON BLACK USING FRACTAL DIMENSIONS

It has been suggested that conventional Euclidean geometry is inade-quate to measure the complexity of carbon morphology. Irregularobjects, defined as fractals, can be measured in terms of non-integerdimensions and it is suggested that carbon black is a typical fractalobject. Hess and Herd (1993) and Le Mehaute et al (1993) used severalmethods to try and predict the fractal dimensions of carbon black. Herdet al. (1991) show there is a general correlation of the mass fractaldimension with measurements by DBF methods such that mass fractalincreases with decreasing DBP values.

Further work on fractal analysis has been carried out and related tomodified BET gas adsorption methods (Zerda et al., 1992), and X-rayand neutron scattering (Gerspacher and O'Farrel, 1991; Reich et al.,1990). Li et al. (1996) used fractal analysis in a comparison of simulatedblack aggregates to actual commercial carbon blacks in a modellingstudy and fractal dimensions used by Kluppel and Heinrich (1995) toexplore black aggregates in rubber and the implications for carbonblack reinforcement of rubber and subsequent properties.

To the reader wishing to pursue the complexities of carbon blackanalysis beyond the range of this book, it is suggested that the paper byGerspacher et al. (1995) entitled 'Furnace Carbon Black Characterization:Continuing Saga' is an ideal place to start although it should be notedthat the authors claim that this topic has been the subject of over 50 000published articles in the second half of the century!

REFERENCESAtkins, J.H. (1964) Analyt. Chem. 36, 579.Ayala, J.A., Hess W.M., Dotsan, A.O. and Joyce, G.A. (1990) Rubber Chem.

Technol 63, 747.Barr, T., Oliver, J. and Stubbings, W.V. (1948) /. Soc. Chem. Ind. 67, 45.Barr, W. and Anhorn, V. (1949) Scientific and Industrial Glassblowing and Labora-

tory Techniques, Instrument Publishing Co., Pittsburgh.Bauminger, B.B. and Poulton, F.C.J. (1949) Analyst 74, 351.Brown, W.A., Schleifer, D.E. and Patel, A.C. (1979) Paper to Rubber Div. Am.

Chem. Soc. Meeting, Atlanta.Brunauer, S., Emmett, R.H. and Teller, E. (1938) /. Am. Chem. Soc. 60, 309.Chambers, W.T. (1958) Unpublished work at MRPRA.Charsley, E.L. and Dunn, J.G. (1981) Plast. and Rubber Process. Appln. 1, 3. (See

also (1982) Rubber Chem. Technol. 55, 382.)Davey, J.E. (1970) Unpublished work at MRPRA.Davies, J.R. and Kam, F.W. (1967) /. IRI1, 231.Dawson, T.R., Porritt, B.D. and Scott, J.R. (1947) /. Rubber Res. 16, 199.Fiorenza, A. (1956) Rubber Age 80, 69.Frank, F. and Marckwald, E. (1923) Gummi-Zeitung 36, 1459.

Galloway, P.O. and Wake, W.C. (1946) Analyst 71, 505.Gerspacher, M. and Lasinger, C. (1988) Paper to Rubber Div. Am. Chem. Soc.

Meeting, Dallas.Gerspacher, M. and O'Farrell, C.P. (1991) Elastomerics 123, 4, 35.Gerspacher, M., OTarrell, C.P., Nikiel, L. and Yang, H.H. (1995) Paper to

Rubber Div. Am. Chem. Soc. Meeting, Cleveland.Gruber, T.C., Zerda, T.C. and Gerspacher, M. (1993) Carbon 31, 1209.Gruber, T.C., Zerda, T.C. and Gerspacher, M. (1994) Carbon 32, 1377.Herd, C.R. and Bomo, F. (1995) Kaut. u Gummi Kunstst. 48(9), 588.Herd, C.R., McDonald, G.C. and Hess, W.M. (1991) Paper to Rubber Div. Am.

Chem. Soc. Meeting, Toronto. (See also Rubber Chem. Technol 65, 107.)Hess, W.M. and Herd, C.R. (1993) in Carbon Black, Science and Technology,

Donnet, J.-B., Bansal, R.C. and Wang, M.-J. (eds) Marcel Dekker Inc., NewYork.

Hess, W.M., Ban, L.L. and McDonald, G. (1969) Paper to Rubber Div. Am.Chem. Soc. Meeting, Los Angeles.

Hjelm, R.P., Wampler, W.A., Seeger, P.A. and Gerspacher, M. (1994) /. Mat. Res.9, 3210.

Jones, H.W. and Porritt, B.D. (1914) Rubber Ind. London, 199.Kluppel, M. and Heinrich, G. (1995) Rubber Chem. Technol. 68, 623.Kolthoff, LM. and Gutmacher, R.G. (1950) Analyt. Chem. 22, 1002.Kolthoff, LM. and Gutmacher, R.G. (1952) /. Phys. Chem. 56, 740.Krecji, J.C. and Roland, C.H. (1965) Paper to Rubber Div. Am. Chem. Soc.

Meeting, Cleveland.Kremens, J., Lagarius, J.S. and Deitz, V.R. (1965) Paper to Pittsburgh Conf. on

Analyt. Chem. and Appl. Spectrosc.Kress, K.E. and Stevens-Mees, F. (1970) Rubber Age 102, 49.Kruse, J. (1973) Rubb. Chem. & Technol 46(3), 653.Lamond, T.G. and Gillingham, C.R. (1970) Rubber J. 152, 65.Lamond, T.G. and Price, C.R. (1970) Rubber J. 152, 49.Li, Q., Monas-Zloczower, I. and Feke, D. (1996) Rubber Chem. Technol 69, 8.Louth, G.D. (1948) Analyt. Chem. 20, 717.Magee, R.W. (1995) Rubber Chem. Technol 68, 590.Maurer, JJ. (197Oa) Rubber Age 102, 47.Maurer, JJ. (197Ob) NBS Spec. Publ (US) 338, 165.Maurer, JJ. (1974) /. Macromol. Sd. Chem. 178, 73.McFearin, T.C. (1962) Rubber Age 91, 611.Medalia, A.I. and Walker, D.F. (1970) Evaluating Dispersion of Carbon Black in

Rubber, Technical Report RG-124 Revision 2, Cabot Corporation, CarbonBlack Division, Boston, Mass.

Le Mehaute, A., Gerspacker, M. and Tricot, C. (1993) in Carbon Black, Science andTechnology, Donnet, J.-B., Bansal, R.C. and Wang, M.-J. (eds), Marcel DekkerInc., New York.

Micek, E., Lyon, F. and Hess, W.M. (1968) Rubber Chem. Technol 41, 1271.Morrell, S.H. (1977) Progr. Rubb. Technol. 40, 105.Niedermeier, W., Raab, H., Stierstorfer, J., Kreitmeier, S. and Goritz, D. (1994)

Kaul u Gummi, Kunstst. 47, 799.Niedermeier, W., Stierstorfer, J., Kreitmeier, S., Metz, O. and Goritz, D. (1994)

Rubber Chem. & Technol 67, 148.Ng, T.S. (1978) Prog. Coll and Polym. Sd. 65, 271.

Pautrat, R., Metivier, B. and Marteau, J. (1976) Rubber Chem. Technol. 49, 1060.Raab, H., Frohlich, J. and Goritz, D. (1997) Proceedings of International Rubber

Conference, 171, Kuala Lumpur.Reich, M.H., Russo, S.P., Snook, J.K. and Wagenfold, H.K. (1990) J.Colloid. Inter-

face ScL 135, 252.Schubert, B., Ford, E.P. and Lyon, F. (1969) Encyclopedia of Industrial Chemical

Analysis 8, 179, Wiley, New York.Schwartz, RV. and Brazier, D.W. (1978) Thermochim. Ada 26, 349.Scott, J.R. and Wilmott, W.H. (1941) India-Rubber }. 101, 177.Scott, J.R. and Wilmott, W.H. (1947) /. Rubber Res. 16, 204.Spacsek, K., Somolo, A. and Soos, I. (1977) /. Thermal Anal 11, 211.Stumpe, N.A. and Railsback, H.E. (1964) Rubber World, 151(3), 41.Sweitzer, C.W. and Goodrich, W.C. (1944) Rubber Age 55, 469.Sweitzer, C.W., Hess, W.M. and Callan, J.E. (1958) Rubber World 138(6), 869.Tuttle, J.B. (1922) Analysis of Rubber, Chem. Catalog Co., New York.Wang, M.-J. and Wolff, S. (1992) Rubber Chem. Technol. 65, 890.Wang, M.-J., Wolff, S. and Donnet, J.-B. (1991) Rubber Chem. Technol. 64, 714.Weigand, W.B. (1920) Canad. J. Chem. 4,160.Wolff, S. and Wang, M.-J. (1993) in Carbon Black, Science and Technology, Donnet,

J.-B., Bansal, R.C. and Wang, M.-J. (eds), Marcel Dekker Inc., New York.Zerda, T.W., Yang, H.H. and Gerspacher, M. (1992) Rubber Chem. Technol 65,

130.

Formulation derivation u r\

and calculation I lL

In the preceding chapters of this book, methods and instrumentationhave been discussed whereby specific qualitative and quantitativeanalyses may be carried out. The major omission to date is the overalldetermination of polymer content. The purpose of this chapter is to fillthis gap and to illustrate how the primary analytical data - that is, theresults actually obtained - may be manipulated and correlated toprovide the best estimate of the formulation actually used in themanufacture of the article.

POLYMER CONTENTVarious methods for determining specific polymer loadings will havebecome apparent to the reader of the earlier chapters - typically theestimation of chloroprene rubber content based on chlorine analysis(although even this is not as straightforward as it seems since, beforeone can carry out the calculation, one has to know the chlorine contentof that material sold as polychloroprene) - but what we require initiallyis a basic analytical scheme which subsequently can be developed asmore information on the sample becomes available.

The possibility of removing the organic part of a compound, be itrubber or plastic, by a simple heating process which leaves theinorganic fillers behind is very attractive. If the material is firstextracted with a suitable solvent, thus removing plasticizers and cureresidues, quantitative combustion of the remainder should provide abasic separation between polymer and inorganic residues, providedthat carbon black is not present. If it is, then pyrolysis, rather thancombustion, of the remainder after extraction should provide a figurefor the total polymer content and also a value for the combined carbonblack and inorganic fillers' contents. It will also provide material in theform of a pyrolysate which can be used, as described in Chapter 7, for

identifying the polymer. If complete pyrolysis were always possible ananalytical scheme could be built around the process without furtherado. Unfortunately this is not the case but nevertheless the procedure issufficiently valid in so many cases that it does provide our 'starting-point'.

When a long-chain molecule is heated it can behave in more than oneway. The possibilities are:1. fracture of the chain at points randomly disposed along it leading to

a steady fall in molar mass;2. fracture of the chain at certain weak points along it; this also will

lead to a drop in molar mass but the molar mass distribution willdiffer from that obtained by random fracture;

3. depolymerization by 'peeling off monomeric units; this should,ideally, cause the molar mass of the residue to fall much more slowlythan the other two processes;

4. polymerization of material formed by depolymerization; this canbecome evident as crosslinking or chain branching and may lead to ahard gel being formed which remains stable at a given temperature,a rise in degradation temperature being required before it too breaksdown;

5. decomposition of the molecule, either whilst it still has a high molarmass or else by decomposition of the monomer formed by depoly-merization; this will become apparent by the evolution of, forexample, hydrogen chloride or water, or by carbonization.

All degradation processes involve free radicals and will thus bemodified or suppressed by suitably active molecules if they remain inthe system at a sufficiently high temperature. A number of papers havebeen written, and will be referred to later in this chapter, which showthat the carbon black loadings, determined by pyrolysis of unextractedvulcanizates, are generally higher than would be expected from theformulations. This could indicate a modification of the decompositionroute by the extractable ingredients present.

Some of the earliest work on the quantitative pyrolysis of polymerswas carried out by Bauminger and Poulton (1949). This was incidentalto their main purpose of studying the carbon black (Chapter 11) butthey published some data on the residual carbon contents of a numberof polymers. These data are tabulated in Table 12.1. Some years later,Brazier (1980) and Sircar and Lamond (1978) published additional datawhich are reproduced in Table 12.2.

It is interesting to note the difference in values obtained forNeoprene by Bauminger and the later authors. The major differenceappears to be in the heating rate, as the earlier pair of workers insertedthe samples directly into a hot furnace whilst the later ones increased

Table 12.1 Residual carbon from pyrolysis of rawpolymers at 60O0C

Polymer Residualcarbon %

Crepe rubber 0.27Smoked sheet rubber 0.23Butadiene-styrene rubber 0.23Butyl rubber 0.10Butadiene-acrylonitrile rubber (I) 1.5Butadiene-acrylonitrile rubber (II) 1.9Butadiene-acrylonitrile rubber (III) 3.7Ethylene disulphide rubber 2.9PVA 2.7PVC 5.9Neoprene 12.3/15.4

the temperature of the samples slowly from ambient to 550 0C. Work inthe author's laboratory using heating rates of 3O0C per minute(Loadman, 1975) has also given a value of about 20% for Neoprene W,together with 3% for Viton B and 5% for Hypalon 30.

Table 12.2 Carbonaceous residues (55O0C) for various raw polymers heated innitrogen

Polymer type - % Carbonaceous Polymer type - % Carbonaceousname residue (55O0C) name residue (55O0C)

All polymers FKM - Viton A 4.0containing only FKM - Viton B 3.0C/H <1% FKM- Viton C10 7.0

FKM - Viton E60 3.7Silicones <1%

NBR-CIIR/BIIR <1% ACN content:

18.5 2.1CR-Neoprene W 21.0% 25.0 2.7CR - Neoprene GT 22.0% 28.8 2.9CR - Neoprene AJ 23.0% 32.2 5.2

34.1 5.5CSM - Hypalon 20 2.0 38.5 6.1CSM - Hypalon 40 3.5 47.0 11.6CSM - Hypalon 45 2.0 47.5 12.5

94.8 44.0PU/AU - urethanes 1-5 (higher values

indicate morearomatics)

It can be seen that the vast majority of elastomers leave a carbonac-eous residue of less than 1%, whilst those leaving 1-5% can be reason-ably corrected for. It is only with the higher values that there can beproblems which require more than a simple mathematical correction.

In 1958 Chambers described a very simple furnace system to pyrolysequantitatively hydrocarbon polymers (Figure 12.1) and noted thatinorganic fillers could have significant weight losses which weredifferent at 55O0C and 80O0C. Chalk (whiting or calcium carbonate)merited particular attention, losing less than 1% of its weight at thelower temperature but 42% at the higher. Subsequently Higgins andLoadman (197O7 1971) replaced the Bunsen burner with a controlledtemperature tube furnace, put a 'U' tube receiver on the outlet from thefurnace to collect the pyrolysate for IR examination, and found thatmodern white spot nitrogen, or the boil-off from a bulk liquid nitrogenstorage system, was so low in oxygen that the heated copper gauze wasno longer required. This apparatus has already been illustrated inChapter 7.

There does not, at the moment, exist an ISO standard for the quanti-tative determination of polymer content by a pyrolytic method as suchalthough ISO 1408 uses the furnace tube to determine black loadingquantitatively on selected polymers whilst ASTM D 297 allows thepolymer to be determined as (100% - ash% - black% - extract%), withthe first two being determined by pyroly sis/combustion.

The current availability of thermogravimetric analysers, coupled withan increased awareness of the amounts of information available from acontinuous plot of weight (and/or the first derivative of weight loss)against temperature with a reproducible heating cycle in a controlledatmosphere, have led to the 'manual tube furnace' method being essen-tially discarded for weight loss determinations although it is still has aplace when a 'high tech7 TGA system breaks down. The pyrolysis tuberemains an essential part of any pyrolysis-infrared spectroscopic proce-dure for the analysis of polymer type. Details concerning the designand operation of TGAs feature in Chapter 7 whilst here we areconcerned with the quality of data the instruments provide and theinterpretation of that data.

Regardless of the choice of instrumentation, the first point to beconsidered is whether the sample should be extracted prior to thermo-gravimetric analysis or not. To a large extent this depends upon theinformation required. If a full thin layer chromatographic examinationis to be carried out in addition to thermogravimetric analysis, there isno advantage in not using the extracted sample. If, on the other hand,the study is being carried out on a quality control basis, then thematching of integral and derivative curves with those of a standardmaterial will give the necessary data without extraction. An excellent

Figure 12.1 Pyrolysis apparatus.

N 20 mesh stainlessgauze roll

IO cm rollreduced Cugauze atdull red

Table 12.3 Rapid TG analysis for quality control of masterbatch synthetic rubbers

Sample1 A B C D

% Carbon 25.0 30.0 34.5 35.0expected:

% Carbondetermined:1 24.6 30.0 34.8 3472 25.2 30.2 34.8 3503 25.2 30.2 34.5 3514 24.8 30.2 34.5 3435 24.9 30.0 34.3 3496 25.2 30.8 34.8 34.9

Mean 24.98 30.23 34.62 34.90Std dev. 0.25 0.29 0.21 0.141 W - oil-extended SBR + HAF, laboratory prepared. 1B' - oil-extended BR + HAF,laboratory prepared. 1C' - BR + HAF, laboratory prepared. 1D' - oil-extended BR + HAF,commercial production.

review was published in 1969 by Maurer which is perfectly valid today.In it he considers the many options open to the analyst when carryingout an investigation on a vulcanizate using thermogravimetric analysis.He discusses the merits of solvent and of thermal extraction, togetherwith the effect of sample size and heating rate, on the reproducibility ofthe data obtained.

Most published data refer to the analysis of unextracted vulcanizatesor masterbatches, and it is of relevance to note that whereas Harris(1977) determined the carbon black content of a range of BR and SBRmasterbatches, with and without oil extension, with a high degree ofprecision as shown in Table 12.3, Pautrat et al. (1975) found black levelsconsistently higher than calculated for vulcanizates of several polymerswith a range of carbon blacks, as illustrated in Table 12.4.

Jaroszynska et al. (1977) studied vulcanizates of the same four elasto-mers, including oil extended BR and SBR, and found black loadingshigher than compounded (with the exception of one SBR sample).Brazier and Nickel (1975) list a series of production compounds (Table12.5) and again find consistently high black loadings.

More recently, Jackson (1996) has illustrated the long term stability ofa particular thermogravimetric analyser by monitoring the data gener-ated during the monthly analysis of a standard black-filled NR/SBRvulcanizate. The results, obtained over a year during which time theinstrument was regularly stripped for cleaning and had its quartz

Table 12.4 Determination of carbon blacks in various elastomers

Black Elastomer Added Black1 Found Black1 Difference1

HAF EPDM 30.4 31.3 +0.9NR 28.2 30.0 +1.8NR 31.5 33.7 +2.2SBR 30.0 31.0 +1.0

SRF EPDM 41.7 43.4 +1.7NR 33.2 34.4 +1.2NR 31.5 33.0 +1.5

MT EPDM 48.5 51.6 +3.1UR 34.0 35.5 +1.2NR 31.5 33.5 +2.0

1 These values are absolute weights (mg) of carbon black in the samples taken. (Courtesy,Rev. Gen. Caoutch. PlasL).

Table 12.5 Typical TG/DTG analysis of production compounds (Courtesy RubberChem. Technol.)

Batch No. % % % % % %NR BR1 Oil2 Black3 Ash4 Sulphur

nominal 33 22 8.8 30.3 1.65 0.821 31.3 21.4 8.6 31.7 2.1 0.822 32.8 23.1 8.0 31.1 2.3 0.803 31.7 21.0 8.9 32.2 2.2 0.714 31.6 21.4 8.8 31.6 2.4 0.795 32.9 21.9 8.9 31.8 2.2 0.956 31.1 21.9 8.5 32.5 2.4 0.897 30.7 21.4 8.1 31.9 2.5 0.818 30.8 22.4 8.9 32.3 2.1 0.869 31.4 22.5 8.5 32.2 2.2 0.8210 31.6 22.4 8.7 32.2 2.3 0.8811 32.4 22.7 9.4 32.1 2.1 0.8112 31.6 22.0 8.9 32.2 2.2 0.8413 33.2 22.4 9.3 31.9 2.2 0.8114 32.2 22.5 8.5 31.8 2.4 0.9015 33.2 23.3 8.6 32.5 2.5 0.7616 31.0 22.3 8.6 31.7 2.2 0.8217 32.0 23.4 8.3 32.5 2.1 0.7418 31.9 22.6 8.4 32.5 2.1 0.7119 33.2 22.2 8.4 32.0 2.3 0.7020 32.0 23.3 8.5 32.1 2.4 0.851 + 0.5%; 2Wt% loss at 30O0C; 3Wt% loss at 55O0C in nitrogen (-ash%); 4Residual weightat 5750C in oxygen.

jacket, thermocouple and furnace replaced with re-calibration after eachevent, show for the total polymer, carbon black and ash values respec-tively, standard deviations (± ranges) of 0.25 (± 0.4), 0.39 (±0.5) and0.37 (±0.6).

On the basis of these data there seems little doubt that, for polymerswhich pyrolyse completely and contain carbon black and/or oil, a priorextraction in the case of a vulcanizate enables a true elastomer contentto be obtained by TGA whereas the similar treatment of an unextractedvulcanizate gives a small amount (0.5-2%) of carbonaceous residue,possibly due to the interactions discussed at the beginning of thechapter. The problems of quantifying the polymer content of anunextracted vulcanizate or masterbatch in the presence of oils, orindeed other volatiles, have been considered in detail by Swarin andWims (1974).

Figure 12.2 shows the thermal curve for the pyrolysis and combustionof a sample based on an EPDM formulation whilst Figure 12.3 showsthe three possible methods of obtaining the 'intercept point'. 'A' relieson the superimposition of the pure 'standard' polymer curve on that ofthe sample to identify the boundary between the weight loss of theplasticizer and that of the polymer, 'B' uses the intercept of the twomeasurable tangents, whilst 'C' measures the weight loss at theminimum of the derivative plot.

The quantitative results shown in Table 12.6 suggest that the refer-ence method is the most accurate, but the derivative method requiresno reference material, often difficult or time-consuming to obtain in thecase of a polymer blend, and has a perfectly acceptable accuracy fornormal usage.

A further problem which must be addressed is that of polymerswhich leave a carbonaceous residue on pyrolysis. Of these the chloro-prenes (CR) and nitrile-butadiene copolymers (NBR) are by far the mostimportant of the elastomers which are found in black filled products,although urethane rubbers should also be borne in mind. Polyvinylchloride (PVC), either plasticized or with a copolymer, is probably themost common material of this type in the area of light colouredhalogen-containing products.

The usual procedure for the analysis of halogenated polymers (CRand PVC) is to extract quantitatively and then determine the combinedpyrolysate and combustible levels of the extracted polymers by thermo-gravimetry. A chlorine content of the extracted material is also obtainedand then calculation of the polymer level based on the experimentallyfound halogen contents for the pure polymers (approximately 35% CR;56% PVC) gives the required data. If the polymeric phase has beenfound to consist of more than one polymer, or in the case of PVC, acopolymer, a complete elemental analysis of the acetone extracted

Figure 12.2 Thermogravimetric curves of EPDM rubber formulation showing weight losses for extender oil, polymer and carbon black.(Courtesy Rubber Chem.Technol.)

Temperature, °C

Carbon Black

Polymer

Temperature, 0CFigure 12.3 Methods for the determination of oil and polymer in EPDM rubber bythermogravimetry. 1A', reference overlay method; 1B', extrapolation method; 1C',derivative method. (Courtesy Rubber Chem. Technol.)

Weigh

t perc

ent re

mainin

gWe

ight p

ercen

t rema

ining

Weigh

t perc

ent re

mainin

g

Table 12.6 Comparison of methods for the determination of oil and polymercontents of EPDM formulations (two samples, A and B) (Courtesy Rubber Chem.Technol.)

Composition %1

Expected Reference Extrapolation Derivativepolymer method methodmethod

Oil Polymer Oil Polymer Oil Polymer Oil Polymer

A 21.6 47.5 21.6 47.0 20.7 47.9 22.1 46.5B 27.3 33.5 27.3 33.0 25.1 35.2 27.5 32.8

1 Each value is the average of three determinations.

sample to give absolute values for carbon, hydrogen, oxygen andchlorine levels, when compared to the total pyrolysate and combustiblefigure, usually enables the polymer system to be fully quantified, asdescribed in Chapter 6.

Nitrile-butadiene copolymers and polyurethanes pose more of aproblem due to the variability of their nitrogen contents. Swarin andWims (1974), using an unextracted NBR vulcanizate, illustrated (Figure12.4) how there is an initial pyrolysis loss of plasticizer and cureresidues etc., followed by most of the elastomer. When the weight lossstabilizes (550 0C) the heating is stopped, the furnace cooled to 300 0C,the inert gas (N2) flow changed to air, and the heating recommencedat 1O0C per minute. There is an initial gain in weight as oxidation ofthe residual elastomer occurs followed by a weight loss in the 400-50O0C region shown to be due to its subsequent volatilization. This isfollowed by the 'true carbon' weight loss. The quantitative data givenin Table 12.7 would suggest that the method is valid but, unfortu-nately, Swarin and Wims do not define the particular grade of NBRwhich they used.

Both Sircar and Lamond (1978) and Pautrat et al (1975) describeproblems in differentiating between residual carbon from some nitrile-butadiene rubbers and several types of carbon black. Sircar andLamond (1978) therefore preferred to use glass transition measurements(Tg) and the relationship illustrated in Figure 12.5 to quantify theacrylonitrile loading of the nitrile-butadiene copolymer, and then toobtain a value for the carbonaceous residue of that copolymer from thedata in Table 12.2. Obviously any sample being subjected to a study todetermine its glass transition temperature should be extracted prior tothat study so that any plasticizing effect of an extending oil is removed.This method is equally valid for nitrile-butadiene/polyolefin blends as a

Figure 12.4 Thermogravimetric curves of NBR rubber formulation showing weight losses for plasticizers, rubber and carbon black.(Courtesy, Rubber Chem. Techno!.)

Temperature, 0C

Carbon Black

Polymer

Plasticizers

Table 12.7 TG analysis of prepared NBR formulations1 (Courtesy Rubber Chem.Technol.)

A B C

Plasticizer % Expected 15.6 15.6 11.3Determined 16.0 16.0 11.3std dev. 0.1 0.1 0.2

Polymer % Expected 57.5 57.5 56.8Determined 57.7 57.6 56.0std dev. 0.3 0.2 0.2

Carbon Expected 23.3 23.3 28.3Black % Determined 23.3 22.2 27.8

std dev. 0.3 0.1 0.3

Ash % Expected 3.6 3.6 3.6Determined 4.0 4.2 4.9std dev. 0.1 0.2 0.1

1Four determinations on each sample.

separate Tg is still observed, enabling the particular NBR to be identi-fied and the total polymer loading calculated as above.

Recently, Hull et al (1994) have shown how swollen state 13C NMRspectroscopy can be used to determine the acrylonitrile loading of arange of commercial nitrile-butadiene rubber vulcanizates with a highdegree of precision whilst Hendra et al. (1992) showed that FT Ramanspectroscopy could be used to quantify the components of several

Percent acrylonitrile in NBRFigure 12.5 Variation of glass transition temperature of NBR with nitrite content.(Courtesy, Rubber Chem. Technol.)

copolymers or polymer blends, including nitrile-butadiene rubbers.These techniques are discussed in more detail in Chapter 7 but itshould be remembered here that the Raman technique is restricted tosystems which do not contain carbon black.

The problem of carbonaceous residues is less easy to resolve forpolyurethanes and there would appear to be little option other than theidentification of the elastomeric constituents as described in Chapter 6(Dawson et al., 1970) or an estimate of the carbonaceous residue afteridentifying the type of polyurethane by pyrolysis-infrared spectroscopyor another appropriate technique. In almost every case this latter mustbe the quicker method and will probably be of sufficient accuracy formost purposes.

COPOLYMERS

Somewhere between physical blends and random copolymers lie theblock copolymers, their exact position depending upon their degree of'blockiness'. It will be apparent that if the thermal stabilities of the twomonomers, as homopolymers, are sufficiently far apart it will bepossible to distinguish between the alternatives of block and randomcopolymerization whilst more detailed interpretation of the data mightprovide information on the extent of the blockiness itself. Studies in thisfield have been reported by Chiu (1966), Baer (1964) and Hendra et al.(1992).

LATEX

Standards do exist for the quantitative determination of natural rubber,or butadiene homo- or copolymer loadings in latex (ISO 126: ISO 2028)but these methods tend to be time consuming and Loadman (1975) hasused thermogravimetric analysis to obtain the data in a few minutes.Perhaps more significantly, Loadman and Tidd (1976) showed how thiscould be used to obtain the polymer and whiting loadings of a mixedlatex carpet backing compound, with a precision quite adequate forquality control purposes, thus enabling a manufacturer to check in tenminutes that settling had not occurred whilst the compounded latexwas waiting to be used.

FORMULATION DERIVATION

By the time that we are interested in deriving the formulation of anelastomeric sample, be it a vulcanizate or thermoplastic elastomer, wewill have accumulated an appreciable amount of primary analyticaldata which will require further manipulation before it can give arealistic estimate of the formulation.

It is doubtful whether any analyst would claim to be able to describea complete formulation with an absolute quantitative certainty and,even if it could be done, it would be a non cost-effective exercise. If aformulation match is the aim, it is the role of the analyst to assist thetechnologist by obtaining cost effective primary analytical data and thenderiving a good approximation to the formulation so that there is avalid base from which to start, or, if quality control is the reason for theanalyses, to provide sufficiently reliable and comprehensive data toenable a formulation to be checked for both correctness and consistency.

The discussion on consistency of compounding is better suited toinclusion in Chapter 14, whilst formulation derivation will be consid-ered here in the form of the reconstruction of some vulcanizate formula-tions.

Relatively little has been published on the quantitative aspects offormulation derivation although Loadman and McSweeney (1975)showed how a micro-sample of 10 mg of vulcanizate could be sub-divided and analysed to provide the maximum amount of data (Table12.8), and how the results therefrom are converted to parts perhundred rubber (pphr) and compared with the mixed formulation(Table 12.9).

They do not, however, provide details of the calculations used, butpapers by Putman et al (1979) and Leyden and Rabb (1979) do, theformer using a scheme analogous to thermogravimetry on an extractedsample whilst the latter quantified the polymer loading by chemicalmethods. Putman and colleagues, however, made many assumptions,

Table 12.8 Breakdown of 10 mg vulcanizate sample for formulation analysis

mgO

% Total sulphur% Total zinc

2% Sulphide sulphur

3Deproteinized NR or not ? % Acetone extract:

4Carbon black type Natural or synthetic polyisoprene

6 Antioxidant% Unextractable sulphur Antiozonant

8 Cure residues% Polymer and type(s) Free su|phur0/0 Black Oil (extenders)% Inorganic filler and type(s) Zjnc% Zinc oxide

10

Table 12.9 Analytical results obtained from a 5mg sample

Experimental results Results as parts per hundred rubber

Extracted sample % as found as mixed

Polymer(type) 55.2 (79NR:21 Polymer 79NR: 8ONR:BR) 12BR 2OBR

Black (type) 40.1 (HAF) Black 71 73Ash 4.7 Ash 8.3 6.7(Ash not ZnO) 1.1 Ash not ZnO 2.0 O

Total sampleZnO 2.9Sulphur 1.46 Sulphur ca. 2Acetone extract 19.0

Acetone extractB-sitosterol ex NRNonox ZAMBT and ca. 1 Nonox ZA 2.5 2.7Cyclohexylamineex CBS highOilZinc stearate aromatic Oil ca. 30 ca. 33

Zinc stearate 3.6

particularly on sulphur loadings which they did not determine. Sincemuch importance is placed on sulphur analysis in this book with regardnot only to conventional vs. efficient cure systems but also to the stateof cure of a vulcanizate, examples are considered from the data ofLeyden and Rabb but results have also been calculated taking correctionfactors used in the author's laboratory to illustrate the extent of theadjustments which they introduce.

Each piece of the primary analytical data will be considered in turnand show the type of corrections which need making in order toconvert it to a technologically meaningful value.

EXTRACT

Extraction may be carried out with a solvent, or thermally using athermogravimetric analyser or some other apparatus involving a heatedinert atmosphere. For most analytical schemes the thermal methodsafford values essentially identical with the solvent ones and, whilst avariety of solvents may be used, acetone extraction is taken as thestandard procedure for an unknown system. However, as discussed inChapter 3, it may be necessary to repeat the extraction with a more

appropriate solvent as information on the polymer type(s) becomesavailable.

The acetone soluble content of different polymers is of course avariable, but the same is also true of nominally identical polymers, due,for instance, to variations in the soap contents of synthetic rubbers, andto inter-clonal differences in the case of natural rubber. The biggestrange of values is found with styrene-butadiene copolymers, which cancontain up to 8% extractables when emulsion polymerized, but onlyabout 2% when solution polymerized. In general, values of 1-4% coverthe vast majority of rubbers and rubber-like materials. Putman takes2.5% for a natural rubber/polybutadiene (50/50) blend and 1% for anethylene-propylene terpolymer whilst Leyden takes 3.7% for a natural/styrene-butadiene/butadiene (50/25/25) vulcanizate, zero for nitrile(Hycar 1042) and 8% for a styrene-butadiene copolymer formulation.This last figure indicates that he is allowing 2.3% for the NR/BR in thefirst example. Leyden also makes a deduction of 1.5% from hismeasured acetone extract value due to 'miscellaneous materials' andthis is not restored elsewhere in the calculation. It could indicate thelevel of self-polymerization found for the acetone under his extractionprocedure and, in this context, it is worth noting that determination ofthe acetone extract level by weighing the dried extract tends to give amarginally higher value than does measuring the weight loss of theelastomeric sample.

POLYMER

As has been indicated already, for most practical purposes the polymer,black and ash loadings of an extracted sample are determined bythermogravimetric analysis directly, or by calculating the percentagepolymer as (100% - black% - ash%) (Putman et al, 1979). In these casesthe only corrections required to the observed polymer loadings are forany carbonaceous residues which may have been generated, and theaddition of the raw polymer extract levels which will then requirededucting from the extract levels themselves.

If a chemical or structure-specific method for determining thepolymer content is used, the situation is quite different because afurther correction must now be introduced to allow for the unextractednon-rubbers. Thus in a quantitative determination of NR via, forexample, the Kuhn-Roth method it is the rubber hydrocarbon which isdetermined and this probably accounts for only some 95% of theextracted sample. In the case of the quantification of styrene-butadienerubber by the determination of styrene content, other corrections mustbecome insignificant relative to the assumption of the styrene content inthe copolymer.

BLACK

Black loadings, as determined by thermogravimetric analysis with asubsequent correction for any carbonaceous residue from the polymer,represent the best approximation to the true levels present, although it isagain emphasized that care must be taken if whiting is suspected ofbeing present. For this reason a thermogravimetric analyser used with anitrogen atmosphere up to 80O0C, prior to the introduction of air oroxygen, is again recommended over the more usually suggested proce-dure of stopping the pyrolysis around 600 0C, cooling to, say, 300 0C andthen running up to 80O0C in an atmosphere of oxygen or air. With agraphical presentation not only can the carbon dioxide be measured bythe earlier procedure, but one can gain additional or confirmatory datafrom a study of the weight-loss and derivative curves. Modern blackshave ash and volatile levels well below 1% at manufacture although,being hygroscopic, they can pick up moisture on standing prior tocompounding. This becomes such a variable that it is quite unrealistic toattempt any further correction at the analytical stage; indeed Wake(1969), in discussing a series of interlaboratory trials prior to the intro-duction of BS 903-1964, expressed surprise at the level of agreementbetween the added carbon black loadings and the values found by themodified solution method of Kolthoff and Gutmacher (1950) adapted forBS 903-1964. He suggested that oxidation of the black, giving anincrease in weight, could counteract any loss of volatiles. It is a fact ofthe analyst's life that all through these manipulations many minorcorrections could be made, both adding to and subtracting from eachpiece of primary data. However, for practical purposes those not consid-ered here can be safely ignored and normally assumed to 'cancel out'.

ASH

Ash contents generally appear to require little correction beyond theinevitable one if whiting is present. This can be made using either thecarbon dioxide weight loss or the measured calcium content althoughthe latter is suspect as calcium oxide or other calcium-containingcompounds such as 'Caloxol' could have been added. Problemsassociated with ash determination by thermogravimetric analysis arecovered in Chapter 10 in the discussion on dry ashing and should beborne in mind, particularly if a chlorinated elastomer or high sulphurvulcanizate is being analysed.

SULPHUR

The intricacies of sulphur analysis have been discussed in detail inChapter 6. Here we are solely concerned with the corrections to and

implications of the total sulphur value. In the absence of a sulphur-containing polymer, or inorganic fillers such as barium sulphide,barium sulphate or calcium sulphate, the biggest correction that will berequired is for the sulphur content of the black, whilst it should beremembered there could be a further contribution from any extendingoils or plasticizers present in the sample. Leyden assumes that 0.5% ofthe sum of the acetone extract level (after the corrections describedearlier) and the carbon black loading is sulphur whilst in the author'slaboratory it is more usual to ignore the oil initially because of thepossible variability of its sulphur content and allow 0.8% of the blackloading as sulphur (this does not require a correction of the blackcontent as this 'intrinsic' sulphur is not extracted nor does it appear tobe removed by pyrolysis under conditions used to volatilize thepolymer). Should the corrected sulphur value still appear too high forthe application, or when compared with the combined and free sulphurvalues, the type and level of oil or plasticizer can be considered and afurther correction made if appropriate.

It must be remembered that the corrected sulphur content is not dueonly to the added elemental sulphur but that it includes the sulphurpresent in the accelerator. Table 12.10 gives values for the sulphurcontents of a number of accelerators and it will be appreciated howimportant this correction is, particularly if the cure system is a semi-efficient or efficient one.

With a corrected sulphur content, which can be described as the 'curesulphur', its value can then be used, together with a knowledge of thecure system obtained from an examination of the extract (Chapter 4), tocalculate the probable ratio of the sulphur loading to that of the accel-erator. Thus a 'cure sulphur' content of 2.7pphr can reasonably beassumed to be derived from an elemental sulphur addition of 2.5pphrwith the remaining 0.2pphr being from about 0.5pphr accelerator. Atthe other extreme a 'cure sulphur' of lpphr could be due to O.Spphrelemental sulphur and 2 pphr accelerator. On this basis an efficient (EV)and conventional cure system may easily be distinguished. A word ofwarning should be added: if a high 'cure sulphur' loading is obtainedthe presence of factice should be considered a possibility.

Table 12.10 Sulphur percentage in some common accelerators (to nearest percent)

CBS 24% TMTM 46%MBT 38% TMTD 53%MBTS 40% TETD 43%MOR 25% TBTD 25%

See also Table 6.3 (p. 119).

FORMULATION CALCULATION

The two following tables, 12.11 and 12.12, illustrate data from twovulcanizates which they analysed and for which they subsequentlycalculated the formulations (middle column). The right-hand column ineach case was calculated by the author from the published primarydata using the corrections discussed for the polyolefins in the case ofSample A, and adding a further one for the carbonaceous residue of thenitrile-butadiene rubber in Sample B. In each case the author took thesulphur analysis a stage further in interpretation, the left-hand figure inthe author's column relating to sulphur analysis represents the 'curesulphur' whilst the right-hand figures, in parentheses, represent thederived 'best estimate' of the levels of ingredients added.

These results illustrate the conclusions reached from many hundredsof such calculations: that most corrections do, indeed, cancel out andthat these mathematical manipulations provide an extremely goodindication of the formulation to which the sample was compounded.They certainly provide the rubber technologist with a base from whichto develop a practical formulation which will have properties matchingthe analysed one.

It is interesting to note that Leyden and Rabb give the measured'total sulphur level7 in Sample B as 0.95%. This must indicate either ananalytical or compounding error as just from 0.75 pphr S (S = 0.75),2.0pphr TMTD (S = 1.06) and 2.0 pphr MBTS (S = 0.8) there should be2.61 (or 1.42%) sulphur. If one includes a low figure from the black

Table 12.11 Calculations of the formulation of a conventionally cured vulcanizate

Sample A As compounded Derived Leyden Derived fromand Rabb published data

(1979) (author)3

NR 50 50 50SBR 25 25 25BR 25 25 25Black 50 49 50Oil 20 24 22ZnO 3.0 3 3Stearic acid 3.0 2 26PPD 2.25 2 2Sulphur 2.7b ] (2.2-2.4)

2.6Vulkacit DZ 1.14 2 J (1.6-0.8)a Author's corrections to the published data: (i) 2.5% of polymer extracted by acetone; (ii)0.8% of black loading is S.bNot corrected for sulphur in the accelerator.

Table 12.12 Calculations of the formulation of an efficiently cured vulcanizate

Sample B As Leyden and Author3

compounded Rabb (1979)

Hycar1042 100 100 100Black 60 66 60DOP 10 13 10Z n O 5 5 5Stearic acid 1.0 1 1Struktol WB 212 2.0Aminox 2.0Sulphur 0.75 1.5b } (0.5-0.8)TMTD 2.0 >1.25 \MBTS 2.0 J J (2.0-1.5)a Author's corrections: (i) 2.5% of polymer extracted by acetone; (ii) 0.8% of black is S; (iii)Hycar 1042 is 33% nitrile, so 5.3% carbonaceous residue is black.bNo correction for accelerators.

(0.5%) this gives 1.6%. The author's interpretation would suggest thatone of the accelerators could have been omitted (if the analytical resultis correct) and recourse should be made to a thin layer chromatographyinvestigation of the extract to resolve this anomaly.

REFERENCESBaer, M. (1964) /. Polym. ScL A2, 417.Bauminger, B.B. and Poulton, F.C.J. (1949) Analyst 74, 351.Brazier, D.W. (1980) Rubber Chem. Technol 53, 437.Brazier, D.W. and Nickel, G.H. (1975) Rubber Chem. Technol. 48, 661.Chambers, W.T. (1958) Unpublished work at MRPRA.Chiu, J. (1966) Applied Polymer Symposia 2, 25.Dawson, B., Hopkins, S. and Sewell, P.R. (1970) /. Appl. Polym. ScL 14, 35.Harris, J. (1977) Off. Plast. Caout. 24, 254. (See also Brazier (198O).)Hendra, P.J., Jones, C.H., Wallen, P.J., Ellis, G., Kip, B.J., van Duin, M., Jackson,

K.D.O.J. and Loadman, M.J.R. (1992) Kaut. u. Gummi 45, 910.Higgins, G.M.C. and Loadman, M.J.R. (1970) NR Technol. 10, 1.Higgins, G.M.C. and Loadman, M.J.R. (1971) Ind. Comma. 15, 50.Hull, C.D., Jackson, K.D.O.J. and Loadman, M.J.R. (1994) /. Nat. Rubb. Res. 9(1),

23.Jackson, K.D.O. (1996) Unpublished Work at TARRC.Jaroszynska, D., Kleps, T. and Tulak, D. (1977) Int. Polym. ScL Technol. 4, T20.Kolthoff, LM. and Gutmacher, R.G. (1950) Analyt. Chem. 22, 1002.Leyden, JJ. and Rabb, J.M. (1979) Paper presented to the Rubber Div. Am.

Chem. Soc. Meeting, Cleveland.Loadman, M.J.R. (1975) Unpublished work at MRPRA.Loadman, M.J.R. and McSweeney, G.P. (1975) Rev. Cen. Caoutch. Plast. 52, 805.

Loadman, M.J.R. and Tidd, B.K. (1976) Paper presented at 5th ConferenceEuropeenne des Plastiques et des Caoutchoucs, Paris.

Maurer, JJ. (1969) Rubber Chem. Technol. 42, 110.Pautrat, R., Metavier, B. and Marteau, J. (1975) Rev. Gen. Caoutch. Plast. 52, 273.Putman, J.B., Samples, C.R. and Knowles, T.M. (1979) Paper presented to the

Rubber Div. Am. Chem. Soc. Meeting, Cleveland.Sircar, A.K. and Lamond, T.G. (1978) Rubber Chem. Technol. 51, 647.Swarin, SJ. and Wims, A.M. (1974) Rubber Chem. Technol 47, 1193.Wake, W.C. (1969) The Analysis of Rubber and Rubber-like Polymers, 2nd edn,

Maclaren, London.

Blooms and visually i Q

similar phenomena I \J

There is no doubt that every rubber-technologist and analyst knowswhat is meant by the word bloom, occasionally called 'frosting', but itis quite apparent from investigations carried out at TARRC over anumber of years that these words include a wide variety of verydifferent effects. These can be divided conveniently into:

• true blooms• modified blooms• pseudo blooms• surface contamination

It is also worth including here a consideration of stains and discolora-tions, as these are sometimes confused with blooms. Even when this isnot the case, there are good reasons for treating them together. They areall visually offensive effects, and the appearance of a surface deposit ora colour change is quite sufficient to cause rejection of the productduring manufacture, storage or service. The consequences of this arejust as commercially harmful as mechanical damage or a morecatastrophic physical failure.

TRUE BLOOMS

The mechanism of the blooming of crystalline materials is simple inbroad theoretical outline (Nah and Thomas, 1980); the substancewhich blooms must have a limited but appreciable solubility in therubber and be present in excess of this solubility. This excess willexist as discrete particles throughout the mass of the rubber eitherbecause it has never dissolved or because, having dissolved at thetemperature of vulcanization, it has crystallized out on cooling. Thesediscrete particles can easily be seen in sections cut from pure gum

rubber and examined by transmitted light under the microscope.Wax is particularly notable as its crystals are anisotropic and crossedpolaroids enable them to be identified easily. In thus crystallizing itmust be assumed that local strain is set up in the rubber displacedby the formation of the crystal. This strain results in pressure on thecrystal, the solubility of which is increased thereby. At the freesurface crystals of the material can form without distortion of therubber and the solubility will be unaffected. The free energy ofcrystallization will therefore be less at the surface than in the bulk ofthe rubber; the solubility of the substance will also be slightly less.There will therefore be a concentration gradient of dissolved materialwhich will cause diffusion from the inside towards the surface andthis will persist until all the material crystallized in the bulk hasdissolved under the influence of pressure and diffused outwards. Themagnitude of the increased solubility due to pressure will, of course,be minute as also will the concentration gradient within the rubber,but large forces are not necessary to account for the observedphenomena.

Free sulphur is probably the most common substance to give a truebloom, and in a vulcanized product such a bloom is due to undercure.This, in itself, could result from a number of factors and the relationshipbetween the time of cure, temperature of cure and suitability of formu-lation should be considered first of all.

Zinc dithiocarbamates are also known to give blooms and of thethree common ones, the dimethyl-, diethyl- and dibutyl-dithiocarba-mates, it is the middle one which shows the most rapid and, over aperiod of time, the densest bloom. The order of solubility isZDMC < ZDEC < ZDBC and it is therefore concluded that the solublefraction of ZDMC is relatively low, resulting in a low rate of migration,whilst ZDBC is sufficiently soluble for the solubility limit not normallyto be exceeded and thus for there to be no bloom. It is unfortunate thatZDBC gives a slower rate of cure than the methyl or ethyl homologuesand therefore it is not always practicable to use it.

Of the other commonly used accelerators, mercaptobenzothiazole andzinc mercaptobenzimidazole have also been observed to bloom.

Many instances have been recorded of protective waxes being thecause of complaints concerning blooms. This appears to suggest alack of knowledge of the function of wax added to protect againstozone as the presence of a surface layer (bloom) of wax is the objectof its addition, and the reason protection is afforded to the rubber. Itmust be noted, however, that the extent of a wax bloom is not onlya function of its loading, but also of its melting point, and these twoparameters can be played off against each other for different applica-tions.

MODIFIED BLOOMS

Certain chemicals present within the matrix of a rubber vulcanizatereact, either deliberately or not, with constituents of the environmentand this results in a significantly different mechanism of blooming.Typical examples are the paraphenylenediamine (PPD) antiozonants,which protect the rubber by reacting with ozone to form an insolubleprotective skin on the surface. This results in a deficiency of PPD in thelayer of rubber nearest the atmosphere and there is a migration of thePPD from the bulk rubber to eliminate the concentration gradient. Asmore PPD reaches the surface layer it reacts with ozone and the processof migration continues until the 'skin' of oxidized PPD prevents furtherozone penetration, and enables a constant concentration to be estab-lished throughout the bulk of the rubber.

Paraphenylenediamines may also bloom by the 'true bloom'mechanism and it is therefore important that they should only be addedat levels up to their solubility. Although this is usually the case when aformulation is originally devised, subsequent modifications without afull realization of their significance have been known to take formula-tions 'over the limit'.

Zinc salts of carboxylic acids (in particular zinc stearate) constitutefurther examples of both true and modified blooms. Zinc stearate has aknown solubility in a's-polyisoprene of about 0.3% and thus theaddition of lpphr stearic acid and 2-5pphr zinc oxide should inevi-tably produce a bloom. However, it is also known that the solubility ofzinc stearate is greatly increased when it complexes with amines and,since these are usually present as accelerator decomposition products,or in natural rubber as supplied, the problem is less acute than it wouldappear at first glance. However, in moist atmospheres, a bloom of zincstearate reacts with water vapour to produce 'basic zinc stearate' whichforms on the surface as a solid layer, visually indistinguishable from abloom, and this is completely insoluble in the rubber. A true zincstearate bloom can be dissolved back into the rubber by heating, butthis is not the case with the basic salt.

PSEUDO BLOOMS

On a surprisingly large number of occasions it has been found that thematt effect on an initially smooth shiny surface has not been due to theblooming of a particular compound, or to deposition of a contaminant,but to the degradation of the rubber surface itself. The pitted surfacewhich develops on oxidative degradation results in sufficient lightscattering to give the impression of a bloom as is illustrated later in thischapter (Figure 13.1(d) page 321). This is particularly significant in view

of the data of Moakes (1950) who noted 'blooms' of calcium and zinccarbonate. There is no doubt that because of their complete insolubilitythese inorganic materials cannot migrate and therefore cannot bloom.This phenomenon is regularly observed in lightly coloured articles andis due to the even more extensive degradation of rubber surroundingthe filler particles which results in the exposure of these particles in a'crater' of rubber.

SURFACE CONTAMINATION

It is always difficult to decide by visual inspection whether a surfacedeposit is a bloom or contamination and it then rests with the analystto identify the surface material to such an extent that this can beresolved. One of the most obvious causes of surface contamination issilicone oil, used as a mould release agent. Not only does it impart anoily film to the surface but it also gives a base to which dirt anddusting powders may adhere.

The washing of rubber products also gives rise to contamination ifrinsing is inadequate and both inorganic salts and organic materialshave found their way on to the surface of rubber articles by this route.Inorganic fillers, used as dusting agents, tend to be present in the air ofmost factories and can adhere to freshly moulded rubber surfaces,giving the appearance of a bloom.

HAZING OF TRANSPARENT RUBBERS

Haze is defined as a cloudy appearance within the bulk of a transparentarticle and from a visual inspection it is often difficult to distinguishbetween it and a bloom. Blooms and surface breakdown have alreadybeen discussed so we must now consider opacity within the bulk of therubber itself. This will result from the presence of insoluble particles,micelles or droplets (in the case of liquids) having a different refractiveindex from rubber and so being able to cause light scattering. One of thecommonest causes of this is the use of zinc oxide either of the wronggrade or in excessive amounts and this problem can be eliminated bythe use of special fine-particle grades at levels not exceeding 1 pphr.

On the other hand, calcium oxide can cause this effect even at thelow levels required for desiccant purposes whilst the mal-dispersion ofotherwise suitable compounding ingredients is a further threat to trans-parency.

STAINING/DISCOLORATION

Although these terms tend to be used interchangeably it is probablybetter to consider discoloration as applying to the rubber article itself,

and staining as describing the effect produced on a material in contactwith the compounded or cured rubber. In the vast majority of casesthese effects are brought about by free sulphur or dithiocarbamates incontact with copper, as both copper sulphide and copper dithiocarba-mate are very dark coloured, and give a visible stain even at the parts-per-million level. Trace metals such as iron and copper in the rubberitself or in fillers such as clays or calcium carbonate (whiting) can alsogive rise to discoloration, as too can the use of zinc oxide with an over-high level of lead. If high levels of these metals are found, the problemthen is in identifying the source of their excessive levels and one areawhich should also be considered in coloured articles is the colorant ordye which was used. Perhaps one of the hardest problems is in definingat what level these elements become effective discolorants. There is nodoubt that the form in which the trace metals exist substantially affectsthe amount which can be tolerated in a vulcanizate but, broadlyspeaking, if one has discoloration problems and the copper levelexceeds 20ppm, or the iron ISOppm, then this is probably the rootcause of the effect.

The effects of staining antioxidants particularly paraphenylene-diamines are, of course, well known and a test for their presence wouldbe the first thing to carry out if a purple, blue or brown stain or disco-loration is observed.

Certain phenolic antioxidants are known to give a pink colour torubber products although this is generally faint and thus only noticedwith light-coloured or transparent formulations. Less well known is thephenomenon of pinking in latex or goods produced therefrom. This is alight-induced weak coloration which is reported by Sin Siew Weng(1982) only to occur when zinc diethyl dithiocarbamate has been added,and which may be removed by washing with dilute potassium hydr-oxide solution.

Less appreciated is the fact that some 'non-staining' antioxidants can,in fact, discolour light rubber products and stain materials which mightbe in contact with them. The mechanism employed by the phenolics forpreventing oxidation (or, more correctly, slowing its rate) does notrequire that they migrate to the surface of the rubber, indeed, it specifi-cally requires that they remain intimately dispersed within the bulk ofthe article. Nevertheless, conventional diffusion theory predicts that iftwo materials are in contact with one another and one contains asubstance not present in the second, that substance will attempt tomigrate from one to the other and this can result in the yellowing offabrics in contact with rubber which contains these antioxidants.

In order resolve the detailed chemistry of the yellowing, or coloration,of some phenolic antioxidants, Gleeson and Loadman (1996) investi-gated a large group of these 'non-staining' materials after oxidation in a

simulated urban environment of air and nitrogen oxides (NOx7S), initi-

ally using thin layer chromatography to identify those which producedyellow components, followed by preparative thin layer chromatographyand gas chromatography-mass spectroscopy to obtain the structures ofthe coloured compounds. They identified four distinct reactions whichgenerated coloured substances:• para-oxidation• orf/zo-nitration• para-nitration• oxidation of the phenol groupComparison of the structures of those antioxidants which discolouredwith those which did not showed that oxidation or nitration of the para-position only occurred when there was no mefa-substitution and here itis important to remember that the yellow derivatives are not only thenitrated phenols, which obviously require the presence of NOx's, butalso the quinones which do not, these being simple oxidation products.Of the antioxidants which did not discolour, all were either para-substi-tuted or, if they had no para- group, they were raefa-substituted, withthe substituent groups hindering the introduction of the relatively largenitro group into the para- or 4-position or preventing the oxidation tothe quinone.

PRE-ANALYTICAL CHECK-LISTBefore discussing in detail the methods available for the analysis ofblooms and the other effects described, it is worthwhile workingthrough a series of questions, the answers to which could assist infinding one's way through this maze: I am indebted to my colleague,Mr P.M. Lewis, for permission to quote his question-and-answerscheme.

Ql Has the bloom increased during storage? If not, a dustingpowder may be responsible.

Q2 Does the bloom disappear on heating? If not, basic zincstearate, an insoluble dusting powder such as talc or surfacedegradation may be responsible.

Q3 Is light or exposure in the open required for the bloom toform? If yes, surface degradation or certain antidegradents maybe responsible.

Q4 Can the bloom be removed by a solvent wipe? If not, try othersolvents (a complexing agent such as acetyl acetone or lacticacid will remove basic zinc stearate). If these also fail,embedded dusting agent or insolubles may be responsible.

Q5 Is the surface of the rubber pitted or roughened after asolvent wipe? If yes, surface degradation may be responsible.

If the answers to Ql, Q2 and Q4 are positive and those to Q3 and Q5negative there is probably a true bloom and the presence of an excessof a compounding ingredient or residue or a protective agent shouldbe suspected.

If the answers to Q4 and Q5 are negative, the haze may be due notto a bloom but to an opaque material within the rubber.

Interpretation of the solvent wipe data (Q4) should be treated withcaution since, in the case of oxidative surface damage, some solventswill swell the rubber so that the light scattering is reduced and thesurface seems 'cleaner'.

If the answers to Ql and Q2 are also negative the cause may be aninsoluble filler such as zinc oxide. Check whether the level of zincoxide exceeds 1 pphr.

If the answer to Q2 is positive, a vulcanization residue or absorbedwater may be responsible. If opaque specks are observed on holdingthe rubber up to the light, suspect the presence of zinc dimethyl-dithiocarbamate (or TMTD, TMTM). If water is responsible, theproblem should not recur after leaching and drying.

COLOUR CHANGES

Ql Does the discoloration/stain appear to be light-induced? Checkby comparing with a sample kept in the dark or with a surfacehidden from light. If 'yes7, an antidegradent may be responsible.A greyish or brownish discoloration may be indicative of anamine antioxidant, a pinkish discoloration may be indicative ofcertain phenolic antioxidants.

Q2 Does the discoloration/stain appear to be heat-induced? If 'yes'and there are no signs of ageing (e.g. stickiness, embrittlement,etc.) an amine antioxidant may be responsible.

Q3 Is the discoloration uniform or patchy? If patchy, externalcontamination is likely, although an additive in the rubbermay still be involved. This may also indicate non-uniformwashing or heating of a dipped product during manufac-ture.

Q4 Does the discoloration/stain appear after laundering, contactwith metal parts, or only when the rubber is in contact withfibres and textiles? If 'yes' to the first part, copper or ironcontamination may be responsible; see whether there is a brownstain. Check whether dithiocarbamates are present in the formu-lation. Identify the antioxidant.

Q5 Is the discoloration/stain accompanied by poor ageing? If 'yes'this is further evidence that copper or iron may be responsibleor that there may be an oxidized phenolic antioxidant present.

ANALYTICAL METHODS

It must first be decided whether to examine the surface as it stands orto attempt to remove any bloom present and carry out a subsequentanalysis on the separated bloom. Some idea of the best approach toadopt will have been gleaned from the tests above, so let us considerfirst the examination of the surface without attempting to removeanything. To do this there are three basic techniques in addition to theuse of a lens or microscope: spot tests, multiple internal reflectanceinfrared spectroscopy (MIR) and scanning electron microscopy with anX-ray analyser (Chapter 7).

SPOT TESTS

Spot tests appear to be restricted to the detection of free sulphur; twoextremely sensitive and specific tests are as follows.

Behaviour with carbon disulphide

If one small drop of carbon disulphide is spotted on to the bloom thedrop spreads out and then dries off, leaving a clean dull circular areasurrounded by a line of recrystallized sulphur. The yellow crystallineappearance of this ring is quite characteristic and is not obtained withaccelerators and antioxidants (Figure 13.1(b), page 321).

Kirchhof's piperidine test

The surface of a white or brightly coloured rubber is spotted with piper-idine. In the presence of free sulphur a yellow or deep orange-redcoloration occurs presumably due to the formation of polysulphidepiperidine compounds (Kirchhof, 1925).

MULTIPLE INTERNAL REFLECTANCE

Multiple internal reflectance techniques have been described in Chapter7 and it will be apparent that if one obtains a spectrum of the surface2-10 jim of a sample with a bloom on the surface, much of thespectrum will be due to that bloom. Although this is true in principle,the sample requirements of a flat piece some 5cm by 2cm in area,together with the virtual absence of carbon black, which absorbs all

infrared radiation, reduces its usefulness appreciably, and eliminatesmany samples immediately. Nevertheless blooms have been identifiedusing this technique with Petit and Carter (1964) identifying zincstearate on the surface of a soling material. Excess dusting agents orfiller pseudo blooms have similarly been identified although care mustbe taken when examining a white filled vulcanizate by this technique asthere will inevitably be the spectrum of the filler superimposed on thatof the elastomer even in the absence of bloom.

If the presence of a bloom is suspected on a black filled or otherproduct but no useful spectrum is obtainable, it is frequently worth-while to remove the product from the MIR plate and inspect the surfaceof the plate to see whether any transfer of bloom has occurred. Aspectroscopic examination of this coated plate could then provide aspectrum free from the rubber background and thus make it moreeasily recognizable. This is particularly true of silicone oils, zinc stearateand other 'sticky' materials, including degraded rubber which mayitself have been thought to be a bloom.

SCANNING ELECTRON MICROSCOPY

The use of the scanning electron microscope has been discussed indetail previously (Chapter 7) and from this the application to bloomanalysis will be self-evident. Figure 13.1(a)-(d), p. 321) (Loadman andBrown, 1982) clearly illustrate the type of information which is availablevery quickly by this technique. Figure 13.1(a) shows characteristicsulphur crystals bloomed to the surface of a badly cured product, X-rayelemental analysis of an individual crystal showing only sulphurpresent, whilst Figure 13.2(b) again shows a sulphur bloom, now after adrop of carbon disulphide has been placed on the bloom and allowedto evaporate. Figure 13.1(c), on the other hand, shows a sample whichwas suspected of having a bloom but which was found to have asurface fungal growth. Figure 13.1(d) illustrates an example of stress-induced oxidative degradation of an injection moulded sample whichhad been allowed to cure partly prior to moulding. The matt, ratherthan shiny, surface finish led both the manufacturer and customerindependently to suspect that blooming had occurred.

REMOVAL OF BLOOM PRIOR TO ANALYSIS

When it is necessary to remove the bloom from the surface of therubber for subsequent examination the first decision to be made iswhether it is better to use a dry or a wet method and, if wet, whichsolvent to use. A 'dry wipe' has certain advantages in that one isunlikely to extract chemicals close to, but under, the rubber surface.

Figure 13.1 (a) Bloom of sulphur crystals on an incompletely cured rubber vulcanizate; (b) bloom of sulphur crystals with one drop ofCS2 spotted onto the bloom; the 'atol' is uniquely characteristic of free sulphur; (c) 'bloom': actually a fungal growth; (d) 'bloom':actually loss of surface gloss due to stress-relaxing oxidation.

As early as 1950 Galloway and Foxton described a procedure foridentifying a bloom of free sulphur:

To detect sulphur in a bloom on an article, fold in four a piece offilter paper (preferably the slow-absorbing, alkali-resistant type) andrub one of the fold edges over the surface; if the bloom is light, firsttreat the surface with a drop of carbon disulphide, and then rub thepaper round the outline left when the solvent evaporates. Unfold thepaper and add 1 drop (about 0.05cm3) of strong (20-30%) aqueoussodium hydroxide solution, followed by 1 drop of pyridine. A blue-green colour in the pyridine, rapidly succeeded by an orange orbrown stain on the paper at the fold, indicates the presence ofsulphur.

This procedure is quite general for other surface blooms in that thespraying of the filter paper with the more specific of the thin layerchromatographic visualizing sprays described in Chapter 4 will oftenidentify the bloom components. The paper can alternatively be extractedwith a few drops of a suitable solvent, and the extract examined byTLC, LC, IR, NMR, GC-MS etc. in the same way as a complete rubberextract.

On many occasions it is possible to scrape the bloomed surface with anew clean razor blade and thus remove the bloom. This is again thenavailable for examination by the previously described techniques,including IR with the substance as a 'smear' or as a potassium bromidedisc, DSC or melting point apparatus, the last being particularly usefulin the absence of a GC-MS for identifying paraffin waxes.

A third technique is to remove any solid surface material by pressingan adhesive tape against it and then either examining the tape bymicroscopy or extracting it and analysing the extract by any of thestandard techniques. Obviously it is essential to treat a 'control' sampleof the tape in an analogous analytical fashion but these tapes tend to berelatively 'clean' when compared with the rubber from which thebloom has been removed and almost invariably provide more discrimi-nation than examining the material in situ. This technique is particularlyuseful when examining degraded rubber as the transfer of fragments ofthe rubber itself, or fillers from the degraded surface, makes them easyto identify.

If none of the 'dry' extraction procedures proves conclusive thensolvent-based techniques must be used. Amos (1967) described in detaila procedure by which he removed blooms from the surface of ethylenepropylene rubbers with cotton wool swabs moistened with chloroformand then extracted the swabs to obtain solutions of the removed chemi-cals for thin layer chromatographic analysis. The particular problemwith wet swabbing is in being certain that there is no significant

penetration of the solvent into the rubber surface, with the consequentleaching of components which do not form any part of the bloom.Preferred, although still not ideal, solvents would thus be those such asacetone or methanol which, whilst being good solvents for most of thechemicals known to bloom, do not swell the polymers appreciably. Areduction in temperature of the rubber and solvent will help to reducediffusion and so afford a less contaminated bloom; a temperature reduc-tion from +2O0C to -7O0C could reduce the effective penetration one-hundredfold. One particular application of this technique has beenreported by Edwards et al. (1976) who used a mixture of acetyl acetoneand 2-propanol (10:90) to complex and thus render soluble the normallyinsoluble basic zinc stearate/palmitate bloom by soaking for up to 93hours at -260C. Identification and quantification of the zinc and fattyacids were by atomic absorption spectrometry and gas chromatographyrespectively.

It is recommended that in the analysis of a bloom involving wetswabbing or soaking, a clean control area should be identically treated.This area should ideally be obtained by cutting away the surface andcertainly not by washing an area clean! Only those components foundto be present in the bloom at a level significantly higher than in the'control area7 should be considered to be components of the bloom.

The analysis of stains and discolorations is often extremely difficultbecause of the very small quantities of materials involved. Staininggenerally results from the diffusion of a chemical in the rubber productto its surface and then its migration into the material in contact with therubber. The chemical may be coloured in its own right, in which case itwill be removed by extraction or swabbing and identified by comparingan extract of the swab with those of the rubber and the material, or itmay be colourless but react with another chemical in the material or inthe environment to produce the staining colour. If the derived productsare soluble in a particular solvent, at least this can be observed and,particularly for the derivatives of the phenolic antioxidants mentionedearlier, GC-MS must be the analytical method of choice.

In some cases it will not be possible to remove the stain with thenormal range of solvents and it will be impossible to identify absolutelythe cause of the problem but, nevertheless, with sufficient analyses ofgood and bad regions, slight but constant differences may becomeapparent which make constructive comments possible.

REFERENCESAmos, R. (1967) /. Chromatog. 31, 263.Edwards, A.D., McSweeney, G.P., Roberts, A.D. and Tidd, B.K. (1976) Unpub-

lished work at MRPRA.

Galloway, P.D. and Foxton, R.N. (1950) /. Rubber. Res. 19, 74.Gleeson, J.G. and Loadman, MJ.R (1996) Paper presented to the Polym. Chem.

Div. Am. Chem. Soc. Meeting, Florida.Kirchhof, F. (1925) Gummi Ztg. 39, 849; (1955) Chem. Ztg. 79, 434.Loadman, MJ.R. and Brown, J. (1982) Unpublished work at MRPRA.Moakes, R.C.W. (1950) RAPRA Bull 4, 9 (circulated to members only).Nah, S.H. and Thomas, A.G. (1980) /. Polym. Sd. Polym. Phys. Edn. 18, 511.Petit, D. and Carter, A.R. (1964) Adhesion of Translucent Rubbers: Application of

Infrared Spectrometry to the Problem, British Boot, Shoe and Allied TradesResearch Association, Kettering.

Sin Siew Weng (1982) Private communication.

Validity of results I T"

INTRODUCTION

Throughout the previous chapters we have been concerned with theacquisition of data, much of it numerical, and its subsequent manipula-tion to provide information in whatever form it may be required.During these manipulations the assumption has been made that theanalytical result is exact. However, as is only too obvious to any analystwho carries out even duplicate analyses, any replication of an analyticalmeasurement will, in general, give a set of results differing by measur-able amounts. It is therefore essential that the results from any measure-ment procedure can be assessed critically to determine the extent ofvariation which may be expected from a particular procedure andthereby provide a basis for estimating the likely variability, or uncer-tainty, associated with a specific set of analytical results.

It is essential for the analyst to understand what limits must beplaced on the primary data so that any values passed to a third partymay be properly explained to prevent misinterpretation or misrepresen-tation and, in order to do this, it is necessary to examine the ways bywhich, and reasons for which, a result can deviate from the 'true value'(whatever that is), and to define some terms rather more rigorouslythan usual.

The four terms most commonly used to describe these deviationsfrom the 'truth' are precision, accuracy, bias and error so let us start byconsidering what these mean.

PRECISION, ACCURACY, BIAS AND ERROR

The term precision describes the spread of results obtained from anapparently identical set of analyses, the lower the precision the greaterthe spread or scatter of those data points. What must be appreciated isthat this says nothing about how near these data points are to the 'true'answer, only how near they are to each other.

If we eliminate the scatter between these results, by methodsdiscussed later, we shall have a single point which has a certain value,the closeness of which to the 'true' value is defined as its trueness. If weonly have one point to begin with the term accuracy replaces trueness.

Although, ideally, an analyst would wish to generate a value whichis exact, in practice, as discussed above, the result is a value which isone of a range of possible values. It is not uncommon for analysescarried out under different conditions to consistently achieve resultswhich are precise within each set of conditions, but which maintain aconsistent difference from the 'true' value for each set of conditionsused. This difference is termed the bias and, in the above description,each set of conditions will show a different bias.

There are two factors which can account for the discrepancy betweenthe 'true' value and measured value and these are described as randomerror and systematic error. Random error is so called because it isgenerated randomly and cannot be eliminated, however carefully theanalysis is carried out, whilst the systematic error is a reflection of theparticular analyst/laboratory/method/etc., thus whenever the sameparticular set of conditions is used, then the same systematic error willoccur.

An important characteristic of systematic errors is that they are estab-lished for a closely defined set of conditions. If one considers thesystematic error which could occur due to two different analystschoosing slightly different shades of an indicator to signify an end-pointwhen they were both using a titrant which had had its concentrationdetermined by a third party, this error would remain constant as longas each analyst continued to be consistent in the choice of end-point.However, if the number of analysts carrying out the titration wereincreased to, say, twenty and each of these had their own specific end-point shade, then the magnitude of the error associated with the shadeselection could be estimated as it would have been converted to arandom error. This is a general rule with systematic errors. Providedthat a sufficient number of independent estimates of a parameter exist,then a systematic error can be converted into a random error and themagnitude of the error more easily determined. The major caveats arethat there are sufficient numbers of independent estimates (it is usuallyconsidered that 10 is a minimum with 15-20 being preferred) and thatthe values are genuinely independent. There is a third considerationwhich can sometimes be overlooked, the systematic error has to be dueto a continuous variable, not a discrete variable. That is, although theerror due to, say, taking the end-point of screened methyl red at thefirst tinge of red on grey, compared to using the development of a fullred is the maximum error, there are an infinite number of intermediatecolours which could be chosen and the particular colour selected will

depend on a huge range of factors including the colour vision of theanalyst. This would therefore be a suitable subject for simple conversionof the systematic error into a random error. However, if the error weredue to a burette being read at the top of the meniscus rather than thebottom, then, since almost all analysts would read the meniscus ateither the top or the bottom and virtually none at intermediatepositions, the systematic error could not be transformed into a randomerror in the same way. The advantage of being able to transformsystematic errors into random errors is that a better estimate of the'true' value of the analyte is possible.

Collaborative interlaboratory test programmes are often designedround these principles with the collective data being used in statisticalinterpretations as described later in the chapter.

Precision is defined (ISO5725) as The closeness of agreement betweenindependent test results obtained under stipulated conditions' but it isimportant to appreciate that there are two levels of precision, onerelating to repeatability (r), where the minimum of variables arechanged, for instance when a series of replicate analyses are carried outby one individual in one laboratory over a short period of time, andreproducibility (R), where as many variable factors are changed aspossible, perhaps where the data are obtained on identical test portionsby different analysts at different times working in different laboratories.Not surprisingly, the spread of data obtained during reproducibilitystudies is significantly larger than that obtained during repeatabilitystudies.

The very term 'identical', as used earlier, immediately raisesproblems. If the material is not perfectly homogeneous, each testportion taken for analysis may be considered to possess a differentvalue for the quality being determined thus, however precise themethod employed and however careful the analyst is to exclude othersources of variation, different test portions will show different values.Even if we consider the method and care used to be such that the testportion value is determined with absolute precision, a number of suchdeterminations still enable us to make only an estimation of the meanvalue for the bulk material, although that estimate will become moreand more precise as the number of test portions analysed increases. Theprocedure adopted for taking the samples can significantly influencethis process and this is why most specifications lay down rules togovern the mode of sampling. Unfortunately, in the real world ofrubber product analysis, the sampling, or test portion selection, hasoften taken place before the analyst receives the piece from which isweighed, or otherwise measured, the test portion required for a parti-cular analysis. The significance of sampling therefore has to be consid-ered if a statistical method of evaluating the results is to be used.

MEANINGFUL INFORMATION FROM IMPRECISE DATA

All measurements are an approximation and thus imprecise. Whethermeasuring the length of a line with a rule or the volume of titrant usingthe graduations of the burette, the actual value quoted is always anestimate. Even changing to instrumental measurement does not avoidthe problem. At best it simply automates the estimation, probablyleading to increased precision but, at the same time, it may well lead toan increased error as in the case of HPLC peaks where the baselineestimation of a small peak situated on the tail of a larger peak can bedifficult to assess instrumentally. The choice for the analyst is sometimesbetween high precision coupled with a possibly significant error byinstrumental data handling or a lower precision but a better control ofgross errors by intuitive (or experienced) manual interpretation of thedata.

Having accepted that a series of measurements will inevitably lead toa set of results which, whilst being similar, are nevertheless notidentical, these can be plotted as a frequency distribution, that is thenumber of values found within a range is plotted against the mid-pointvalue of that range. A curve drawn through the points is very oftenfound to conform to a shape which is known as a normal distribution,(Figure 14.1) although the smoothness of the curve will depend on thenumber of data points available. In some cases this curve is slewed orskew, but an initial mathematical manipulation of the values beingplotted can often improve it.

Within the data set there are three values which can be used to definethe most probable analytically correct value. These are the mean, themedian and the mode. The mean is also known as the average and issimply the sum of all the values divided by the number of values. Themedian is the middle value when the data are ordered, that is when thedata are arranged in order from smallest to largest (or vice versa) themedian is the middle item defined as the (n+l)/2th data point for oddnumbers of data and n/2th item for even numbers of data. The modeis that value which occurs most frequently. For normally distributeddata with a large number of data points the three characteristic valuesshould be identical and, indeed, are generally very similar. However, aswill be seen later, they vary in their ability to cope with data which arecontaminated with errors.

The mean is widely used as the characteristic value, or statistic,associated with a set of data simply because it is easy to calculate and itenables a simple assessment of the optimum value of the measurementto be made without recourse to the other values. However, by itself, itdoes not fully describe the variation between the values which gotogether to give the mean - it does not indicate the precision of the

Figure 14.1 Normal distribution curve.

measurement. For this purpose it is common to use the standard devia-tion (o) to provide a measure of the width of the distribution curve asillustrated in Figure 14.1. The standard deviation is calculated as shownin Eq. 14.1 or Eq. 14.2, the latter being the easier expression to calculate.

/ _(x-^_ (M1)

V ^ nor

* = \/I>V^^ (14-2)

In practice, since 100% sampling is not carried out, one does not havea value for the true population mean and an estimate of this must bemade from the few analyses actually carried out. Under these conditionsa more realistic estimate of the sample standard deviation (s) isobtained by the equation:

-^W <i4-3)

mean

Fre

qu

en

cy

of

rea

din

gs

It will be realized that once n becomes reasonably large s approaches ain value and, also, that the mean will increase in precision as thenumber of results from which it is derived increases. This increase inprecision is quantitatively measured by the standard error of the mean;if s is the estimated standard deviation, the standard error of the mean(SE) is given by s/^/n.

We can, therefore, express analytical results as the mean of n of obser-vations plus or minus their standard error, i.e. the 'most representative'value together with an indication of the spread of results from whichthis value was calculated.

The significance of these numbers, or statistics, is that they reduce agreat deal of data to two simple numbers which have the ability both todescribe the behaviour of the results already produced and also topredict the likely range of values which will be produced in the futurefrom further analyses carried out under the same conditions. Thesepredictions can then be used as the basis for assessing whether theanalytical procedure continues to function properly or to determinewhether a product is being produced within a specification. Obviouslythe latter relies on the former having been shown to be true by, say, theanalysis of standard reference materials.

Other terms which may be met in this area of statistical investigationare confidence limits and uncertainty.

CONFIDENCE LIMITS

The confidence limits associated with a measurement are limits whichidentify the range of values within which the analytically 'true' value isasserted to lie with a specified probability.

Confidence limits are an assertion about the particular measurementresult which has been achieved. They do not, unless the results arecombined with others from different measurement series, allow anyconclusion to be drawn about the range of results which might beattained in a future measurement exercise.

UNCERTAINTY

Uncertainty is, in general, applicable to a method. Therefore it is ofgeneral applicability and can be used to define the likely range ofvalues within which future values will fall. Thus uncertainty estimatescan be used to derive values for the number of replicates required toensure that measurement precision is adequate for the purpose of themeasurements.

In many respects confidence limits have the same function as uncer-tainty bounds. The difference is that uncertainty bounds are established

to demonstrate the capability of methods, whereas confidence limits arestrictly tied to a particular measurement series.

COEFFICIENT OF VARIATION

It is sometimes of more interest to examine the relative variability ofsets of results which have different mean values. In these instances theconcept of coefficient of variation (V), defined mathematically below, isused.

V=1-^ (14.4)X

SIGNIFICANCE TESTS

Double-sided significance tests

In the normal distribution, as illustrated in Figure 14.1, it is apparentthat the frequencies of readings fall off the further one gets from themean. It can be shown that 95% of all readings occur in the area within±1.96(7 boundaries whilst 99% fall inside the area when the boundariesbecome ±2.6(7. The area in which there is a certain probability that theresults will fall is the mathematical representation of the confidencelimits of that result; thus a statement saying that the 95% confidencelimits are ±1.96(7 means the same as saying that there is a 95%probability that the result lies within the measured values ±1.96(7.Confidence limits of 95% (i.e. 1 in 20 being outside the range indicated)are those generally quoted but obviously any value required can becalculated. This can also be used to calculate the confidence limits of apopulation mean (x). At the 95% level it can be said that the true meanwill lie in the range

mean = x ± 1.96 -^- (14.5)Vn

If n becomes sufficiently large then (1.96a/^/n) approaches zero and xbecomes equal to the true population mean.

This relationship is particularly important as it enables us to calculatehow many replicate analyses (n) are required to obtain a given precisionif the population standard deviation (cr) or the coefficient of variation(V) is known.

Suppose an analysis has a coefficient of variation of 0.3% whilst ananswer is required with a 95% probability of its being within 0.2% ofthe true value. Then

1.96x0.3 ^ ,.A,.— = 0.2 (14.6)A/ft

and nine determinations will be required.

Another question which often confronts the analyst is that of decidingwhether the difference between duplicate results is within allowablelimits, or whether they are significantly different, in which case theinterpretation is that either an error has been made or the samples areinhomogeneous. Given that we are still using 95% confidence limits,two results may be considered sufficiently close for the difference to beacceptable if they are within 2.77G where G is already known from alarge number of determinations. This value is known as the least signif-icant difference and has a further importance in that it can beexpanded to show whether different means of two sets of analyses aresignificantly different or not. The least significant difference betweenmeans then becomes

1.96aVlM + l/n2 (14'7)

(It can be seen that if the two results are from single analyses, theequation reduces to (1.96^/2) G = 2.77(7.)

Single-sided significant testsAll the tests described so far are double sided, that is the variationboth sides of the mean is of equal significance. In certain cases,however, one may only be concerned that a minimum specified valueis being exceeded, that a certain contaminant is not exceeding a givenvalue, or that the uptake of a chemical during service is below speci-fied limits.

If we consider the normal distribution curve, it is apparent that thesituation we are describing is that shown in Figure 14.2 which illustratesa single-sided function. In the double-sided situation (Table 14.1) wecan determine that there is a 90% probability of all values lying within± 1.64o- of the mean. As the normal distribution curve is symmetricalthere is 45% probability of a value lying between (x - 1.640-) and x anda 45% probability of a value lying between x and (x+ 1.64cr). Now in asingle-sided function 50% of the values will lie on one side of the meanwhilst the probability level for values on the other side is the same asthat for each side of the double-sided function; thus for a single-sidedfunction the 95% confidence limit for a maximum value is representedby 50%+ 45% which is the mean value (x) + 1.64(7 and for 99% confi-dence limits, 235(j.

Exactly the same arguments can be applied to the confidence limits ofa population mean (Eq. 14.5); by replacing 1.96 with 1.64 one can say thatit is 95% certain that a population mean is greater than (x - 1.64cr/^/n)or conversely that it is 95% certain that the population mean is less than(x + 1.64<rA/n).

Figure 14.2 Single-sided normal Gaussian distribution function. f(U) = shadedarea = probability (u < U).

Student's t test

Most of the statistical interpretations covered so far require that onehas sufficient data to have obtained a meaningful value for thestandard deviation (G) and thus its squared term (cr2) - the variance.In many practical instances this is not the case so further considerationmust be given to the information which is available. Eq. (14.3)indicated the means whereby an estimate of the sample standarddeviation (s) is obtained when only a limited number of readings isavailable.

It will be apparent that the lower the number of readings used tocalculate s the greater must be the limits between which a particularresult will be found for a given probability value. It thereforebecomes invalid to use the values determined for use with a(known as U, the standard normal deviate values) for calculationwith s, and these must be replaced by Student's i, the values ofwhich are dependent upon the number of readings taken and maybe obtained from Table 14.1. It should be noted that the values of iare arranged according to the symbol y which represents the numberof degrees of freedom. This indicates the number of independentcomparisons which can be made between the individual values in a

Table 14.1 Table of percentage points of Student's t distribution (Davies andGoldsmith, 1972)

Degree For use in double-sided For use in single-sidedy percentage point percentage point

1% 5% 10% 1% 5% 10%

1 63.66 12.71 6.31 31.82 6.31 3.082 9.92 4.3 2.92 6.97 2.92 1.893 5.84 3.18 2.35 4.54 2.35 1.644 4.60 2.78 2.13 3.75 2.13 1.535 4.03 2.57 2.02 3.37 2.02 1.486 3.71 2.45 1.94 3.14 1.94 1.447 3.50 2.36 1.89 3.00 1.89 1.428 3.36 2.31 1.86 2.90 1.86 1.409 3.25 2.26 1.83 2.82 1.83 1.3810 3.17 2.23 1.81 2.76 1.81 1.3711 3.11 2.20 1.80 2.72 1.80 1.3612 3.05 2.18 1.78 2.68 1.78 1.3613 3.01 2.16 1.77 2.65 1.77 1.3514 2.98 2.15 1.76 2.62 1.76 1.3515 2.95 2.13 1.75 2.60 1.75 1.3416 2.92 2.12 1.75 2.58 1.75 1.3417 2.90 2.11 1.74 2.57 1.74 1.3318 2.88 2.10 1.73 2.55 1.73 1.3319 2.86 2.09 1.73 2.54 1.73 1.3320 2.85 2.09 1.72 2.53 1.72 1.3325 2,79 2.06 1.71 2.49 1.71 1.3230 2.75 2.04 1.70 2.46 1.70 1.3140 2.70 2.02 1.68 2.42 1.68 1.3060 2.66 2.00 1.67 2.39 1.67 1.30120 2.62 1.98 1.66 2.36 1.66 1.29

Reprinted by permission of Addison Wesley Longman Ltd.

set of analytical data. If, as is usually the case, the mean (x) hasbeen calculated from a set of n observations, there are n - I degreesof freedom (y). Note that the nth comparison is not independent asa knowledge of x and n - \ values must define the nth valueunambiguously. If, on the other hand, the mean is known indepen-dently and only the standard deviation needs to be estimated fromthe results of n samples, then there are n individual comparisons,hence n degrees of freedom.

The calculation of degrees of freedom can become complicated incertain complex statistical analyses and for further information thereader is referred to the work of Davies and Goldsmith (1972).

It has already been pointed out that as the number of readings usedto obtain s increases, s tends towards a. Student's t table illustrates that

as the number of readings increases, so t tends towards U. We can thuscompare the 'ideal' and 'practical' equations:

1 Q6/T(a) Confidence limit for a mean Ideal x ± —

(95% probability) V"_ ts

Practical x± —Vn

(b) Least significant difference T . , „ /1 1between: Ideal L96f fV^ +

(i) two means (different n values) /5% probability Practical ts x /— + —

V m n2

(ii) two means (same n values) ry95% probability Ideal 1.96(7 / —

f~2Practical ts + / —V n

and so on.Note that for the first example (a) we will obtain t from the 95%

column at a level where y = n — 1 whilst for the others (b)(i),V = ( H 1 - I ) + (H2 - 1) and (b)(ii), y = (n-l) + (n- 1), i.e. (2n-2).

ANALYSIS OF VARIANCE

Analysis of variance is a procedure for apportioning the variability in aset of results between various possible sources of error. We havealready identified sources such as homogenization and sampling, aswell as those relating to the differences between repeatability and repro-ducibility, and if one considers the processes of compounding andvulcanizing rubber products, it is obvious that the list can be extendedmuch further! An appropriate analysis of variance will allow therelative contribution from each of these factors to be estimated but it isimportant to note that in order to achieve the required separation of thevarious contributions it is necessary that the whole experiment bedesigned with that aim in mind. In general it is not satisfactory simplyto gather together results from a few sets of analyses and then attempt

to derive meaningful estimates of the errors associated with individualerror sources.

It should be realized that this technique can be used to decide, onrational grounds, what size of sample should be homogenized beforethe test portion is taken if there is extant information on the samplingerror as well as the experimental error. It can be shown that if a portionof the sample is to be homogenized before the test portion is taken,then, under normal conditions, that portion should be not less thanseven times the final test portion. The significance of this can be appliedto, say, the case of micro-scale acetone extraction, where the determina-tion requires a sample of 20 mg. From the argument above it would besafe to take the test portion from a sample of O.Ig except that thechoosing of the 0.1 g piece itself should normally be a sampling opera-tion made after homogenization. Thus double homogenization is aprecaution which is always advisable when milligram quantities formthe test piece and it will ensure adequate safety when the sample erroris unknown.

Whilst the statistical arguments offer considerable insights into thelogic behind what may be intuitive feelings, their application tosubstances such as commercial rubber vulcanizates must be treatedwith great care and with due consideration not only of the sample andtest piece but also of the information required. A re-reading of Chapter3 could well be appropriate at this time.

OUTLIERS

Reference to Figure 14.5 sample 2, day 1 later in this chapter, showssome analytical results which are appreciably different from the mainset. These are referred to as outliers and the most extreme is marked(I). Occasions will arise when one must decide whether such valuesshould be included in, or rejected from, the bulk of the data. Inprinciple a reading should never be rejected unless there is a validreason for so doing. Possible reasons can be summarized:• operator error• instrument malfunction• damaged sample• statistical reasons

The first two are self-evident, although sometimes difficult to prove.Rejection on the grounds of a damaged sample is critically dependentupon the reason for the test. If a sheet of rubber is being used tomeasure gas permeability, the presence of a pinhole flaw will give avalue which will be rejected but, on the other hand, if the gas perme-ability is being used to check the quality of the rubber sheet or fabric,

the occasional pinhole will be extremely important. Each sample mustbe judged on its merits.

There are statistical methods for deciding which outlying valuesshould be rejected and again it should be pointed out that these assumea normal distribution. If the data do not, approximately at least, fit thispattern, the statistical treatment will not be strictly valid and the conclu-sions should not be interpreted too rigidly. It is also the case that, forstatistical tests for outliers, low significance limits are used such as 1%,with levels greater than 5% being avoided. Tests exist to detect one ormore outliers on one or both sides of the normal distribution curve, buthere we will initially describe two tests to check the most commonsituation of there being an outlier at one side of the distribution. Bothtests are simplified by tabulating the results in order of increasing value,with XI being the smallest, X2 the next and so on to Xn, the largest.

T test

The first test, the T test, measures the distance the extreme reading(either XI if a low outlier is suspected, or Xn if a high one is suspected)lies from the mean value of the readings (x). This value is then dividedby the sample standard deviation (s) and the quotient (T) is noted onthe graph of T vs. log n. This is illustrated (Figure 14.3) for twoprobabilities, indicating 1 in 20 (5%) and 1 in 100 (1%) boundaries foracceptability. If we do not specify on which side the outlier is beingtested for, the probability of its being on either side is twice that of itsbeing on a specified one. The 1% probability boundary would thenbecome the 2% (1 chance in 50).

Dixon's test

The second test, Dixon's test, compares the interval between thesuspected outlier and one of its neighbours, with the spread of theresults between a large number of readings, the exact intervalsmeasured depending upon the number of readings taken. These arelisted under criterion 1 to criterion 4 and again the resulting value iscompared with the boundary lines shown in Figure 14.4 to see whetherit falls in the acceptable value or possible outlier region.

Criterion 1: 3-7 readings (n = 3-7)

If the suspected outlier is on the low side, the interval between it (XI)and its neighbour (x2) is (x2 - XI). The interval between a large numberof readings (all for this low number of readings, 3-7) is (xn - XI) thus:

Dixon's significance level (ds) = (x2 - Xi)/(xn - XI)

log nFigure 14.3 7 test for outliers (using data of Grubbs and Beck, 1972).

Figure 14.4 Dixon's test for outliers (using data of Dixon, 1953).

Acceptable value

Doubtful value (outlier)

Criterion number

Acceptable value

Doubtful value

(outlier)

Signifi

cance

level

Similarly if the outlier is suspected of being on the high side of thedistribution:

(ds) = (Xn -Xn^1)/(Xn -X1)

Criterion 2: n = 8-10

For low outlier: (ds) = (x2 - x\)l(xn-\ - *i)For high outlier: (ds) = (xn - xn_i)/(xn - X2)

Criterion 3: n = 11-13

For low outlier: (ds) = (x3 - Xi)/(xn^ - XI)

For high outlier: (ds) = (xn - xn_2)/(xn ~ X2)

Criterion 4: n > 14

For low outlier: (ds) = (x3 - Xi)/(xn,2 - XI)

For high outlier: (ds) = (xn - xn_2)/(xn - X3)

Inevitably, with borderline cases, the two tests sometimes show differ-ences of opinion. As a general rule the Dixon's test is the quicker andcan be used to screen many results rapidly but Ferguson (1961) showedthe T test to be the better one to use should a single outlier besuspected. In the case of two outliers, one on each side, one can againuse the T test on the reading which is further from the mean and if thisproves likely to be an outlier reject the value and recalculate x, s andthe T value for the other possible outlier. Alternatively one may preferto use the test of Teitjen and Moore (1972). If it is suspected that tworeadings on the same side are outliers it is probably simplest to ignorethe outer one and use the T test on the remaining suspect reading. Ifthis proves to be a probable outlier it is apparent that the rejected onemust also be. One can also refer to the more sophisticated method ofGrubbs (1969).

The two tests are illustrated on the low reading shown in Figure 14.5sample 2, day 1 (marked j).

n = 72 X = OAIl s = 0.037

Using the T test: (x-x)/s = T(0.411 - 0.30)70.037 = 3.0 (just acceptable at 5%)

Using the Dixon's test: ds = (x3 - Xi)/(xn-2 - XI)

(0.33 - 0.30)7(0.46 - 0.30) = 0.1875

The graph (Figure 14.4) does not continue to 72 readings but thevalue of ds is obviously very low which suggests acceptability. Onemay therefore conclude that it is reasonable to expect a value as lowas 0.3 in this set so the sample is probably from the same set as theothers.

GRAPHICAL DATA PRESENTATION

In the early part of the chapter it was noted that a set of analyticalresults would be expected to have a normal distribution about amean and that mean would represent the analytical answer, althoughit might include various error statements. If the distribution of thedata points does not approximate to normal then we are in theclassical position of statistical analysis - rubbish in can only lead torubbish out.

A simple way of checking is to use a graphical interpretation of thedata as shown in Figure 14.5 which shows the reported data derivedfrom an interlaboratory check on the ash contents of natural rubbersamples; 24 laboratories, two samples of rubber, each divided into twotest portions, one test portion of each rubber being analysed in triplicateon one day, and the procedure repeated on the next.

Simple statistical interpretation shows that the pairs of means are ingood qualitative agreement although, as inspection of Table 14.2indicates, sample 2 shows a higher mean than sample 1. The values forsample 2, however, are grouped around the 0.4% level whereas thosefor sample 1 show rather more spread, with a number of values in the0.2-0.3% region. It is also noticeable that the analytical variationappears relatively constant, with the low values holding for all sixanalyses of a particular sample. If a bar graph as illustrated in Figure14.5 had been plotted it would have immediately become clear thatsample 1 is not going to give a normal distribution; indeed it is devel-oping a clear bimodal distribution, one mean of which looks similar tothat of sample 2 whilst the other is appreciably lower. The mostobvious conclusion from these data is that two quite different samplesare involved. One accounts for all 24 test portions of sample 2 and 13 ofthe 24 in the case of sample 1, whilst the other accounts for theremaining 11 test portions of sample 1. These data can still be analysedstatistically rather than discarding the total experiment, but in adifferent way from the obvious one which assumes a typical distribu-tion pattern. One thus identifies six sets of results: 1.1 low, 1.1 high, 1.2low, 1.2 high, 2.1 and 2.2. Whilst the numbers of samples in the four '1sets' are relatively small, one can obtain means, standard deviations,standard errors etc. as a check on the laboratory and replication preci-sions.

Measured ash content (%)

Figure 14.5 Bar graph of interlaboratory ash determinations.

TRACEABILITY

Statistical interpretation of analytical data is usually carried out on theprimary data, obtained as a titration volume or as an integrated peakarea from a chromatogram. However, a result which is simply a titra-tion volume, a peak area or an absorbance reading, is of little value.The significance of the primary data is determined by its conversion toa concentration which is usually performed via a calibration function.

sample 1 dayl

sample! day2

sample2 dayl

sample2 day2

Frequency of

measurement

Table 14.2 lnterlaboratory ash analysis (raw NR)

Sample 1 Sample 1 Sample 2 Sample 2

L a b 1 2 3 1 2 3 1 2 3 1 2 3

1 .30 .29 .31 .28 .30 .30 .40 .39 .43 .42 .40 .402 .34 .39 .30 .44 .41 .42 .31 .35 .30 .41 .41 .433 .29 .28 .29 .26 .26 .26 .46 .48 .46 .42 .43 .434 .42 .42 .43 .44 .42 .43 .42 .43 .42 .40 .41 .405 .39 .41 .39 .41 .45 .43 .37 .37 .36 .42 .41 .416 .45 .47 .45 .45 .41 .43 .47 .47 .47 .42 .45 .427 .40 .42 .42 .44 .43 .46 .38 .36 .35 .35 .36 .368 .28 .29 .28 .31 .29 .31 .40 .42 .40 .39 .37 .399 .31 .29 .30 .30 .28 .33 .41 .41 .39 .39 .39 .3910 .43 .41 .41 .40 .42 .41 .40 .42 .42 .42 .42 .4111 .41 .40 .42 .45 .42 .44 .44 .44 .45 .45 .44 .4112 .32 .31 .32 .31 .31 .31 .41 .40 .41 .40 .40 .4013 .38 .44 .47 .40 .42 .43 .42 .45 .46 .42 .46 .4114 .39 .38 .40 .42 .45 .44 .38 .39 .39 .44 .42 .4315 .38 .39 .39 .37 .39 .37 .42 .41 .42 .43 .42 .4216 .25 .24 .27 .28 .27 .25 .39 .33 .37 .35 .39 .3717 .46 .46 .46 .42 .42 .42 .44 .44 .44 .40 .40 .4018 .30 .28 .29 .29 .29 .29 .43 .44 .41 .42 .42 .4119 .31 .28 .28 .26 .28 .30 .43 .46 .43 .36 .39 .4620 .27 .25 .27 .28 .26 .27 .41 .39 .40 .40 .42 .4021 .29 .28 .30 .30 .29 .32 .40 .43 .43 .41 .41 .4522 .42 .39 .41 .39 .44 .44 .45 .38 .38 .46 .40 .3723 .43 .40 .41 .43 .45 .44 .39 .40 .38 .42 .41 .4424 .30 .30 .29 .30 .33 .29 .43 .45 .45 .45 .41 .40

x 0.358 0.362 0.411 0.410

Data reproduced with permission of the Rubber Research Institute of Malaysia.

The calibration is the first step in traceability, the term used to describethe sequence of connections which ensure that each calibration can bereferred reliably to an agreed authentic reference material or measure.

Traceability is most easily understood in terms of general metrology.The International Standard Kilogram, located in Paris, is used tostandardize reference kilogram weights by National measurementauthorities such as, in the UK, the National Physical Laboratory, andthese are used to calibrate weights used by certified testing andcalibration laboratories. These second tier reference weights are thenused to produce weights which are used within laboratories on aroutine basis or the analytical laboratory may choose to furthercalibrate its 'working7 weights against its certified weights so that thelatter can be maintained in pristine condition. Thus the workingweights can be seamlessly traced back through the individual refer-

ence weights used to calibrate them and thence back to the originalNational Standard Weight.

With chemical standards the situation is less clear-cut since there isno single pure reference material against which all others can becalibrated. It thus falls on the analyst to either purchase referencechemicals with some certificate of analysis by an accredited test houseor to go to considerable lengths him- or herself to prove that thesubstance has characteristics appropriate for the particular applicationrequired of it.

In the field of rubber chemical and product analysis there is an oftenunappreciated problem associated with quantitation vs. pure referencematerials and that is that many of the additives are 'commercial grade'chemicals and thus an estimate of the amount added will be correctchemically but incorrect in terms of the weight of the chemical addedduring compounding. This topic was mentioned in Chapter 4 butdeserves mention again here.

VALIDATION OF ANALYTICAL METHODS

Statistical interpretation of a mass of analytical data may be the idealway of obtaining a 'true' analytical result but it is rarely practical ongrounds of economy of staff time and equipment usage. The analyst ismore often than not only able to carry out a single or duplicate analysisusing a regular method which has a degree of familiarity or, less often,to take a published method and use it to obtain a result in which he orshe can express a degree of confidence. Statistical methods of dealingwith the uncertainty in the former category of these measurements hasbeen mentioned earlier in the chapter whilst, in the latter, the analystcan only rely on the published levels of precision and his or her abilityas a competent analyst.

Whilst this may be adequate for 'in-house' investigative analyses,there are a growing number of areas where it is not acceptable andwhere the analyst has to validate the method, often with reference to aparticular substrate in which the analyte resides. Perhaps the mostimportant area is that of submission of analytical data to regulatoryauthorities as part of a drug acceptance programme.

Method validation is inevitably time consuming and involves anumber of fundamental steps.

VALIDATION STEPS

Instrument and other apparatus details

Every component of the analytical equipment used must bedocumented and adequately checked and serviced. If any piece is

changed then it should be proved by experiment that the change has noeffect on the data.

ChemicalsThese should all be certified and of the highest (or, in the case of'rubber grade' materials, most appropriate) quality. In-house purifica-tions must be verified by techniques which confirm both the composi-tion of the material and its purity.

Instrument check and internal robustnessA protocol should be documented which will permit testing of the'instrumental analytical package' to an extent whereby any deviationfrom the expected performance which will introduce a bias into thecollected data will be discernible. This protocol should be used to checkthe total system before each set of analyses is carried out.

Sample treatment and analytical protocolThese, again should be documented and limitations identified. If, forany reason, the defined treatments and protocol cannot be adhered to,when compared with a documented method, it must be shown that thedeviations do not introduce any error or bias into the result.

Calculation of resultsThe procedure used should be fully documented and explained. Anyextension of the calculation to draw conclusions about the meaningful-ness of the data should be fully explained.

Validation experimentsValidation experiments, using standard reference (and, where available,traceable) materials, must be carried out before the method can be used.The various areas which need to be addressed are:

• Has the technique the required accuracy or trueness?• Can a smooth correlation be established between the concentration of

the analyte and the data value generated by the analytical instrumen-tation? Of particular interest in this area are so-called 'matrix effects'which can be defined most simply as 'does the behaviour ofstandard solutions remain the same in the presence of the analytesolution or are there substances present which can disguise or distortthe apparent level or identity of the analyte?'

• Has the method the required precision? There are normally threeaspects to this part of a validation and these concern the analyticalrepeatability as well as sample to sample repeatability (i.e. they illus-trate instrumental, intra-sample and inter-sample variability). Infor-mation on the first two is fundamental to any conclusions to whichthe last leads one regarding sample or batch inhomogeneity.

• Can the sampling procedures be shown to be appropriate for theexperiment?

• What are the detection limits? This is probably one of the most diffi-cult areas to satisfactorily quantify in spite of the statisticalapproaches which are available and merits rather more detailedconsideration.

LIMITS OF DETECTION (LOD) AND QUANTITATION (LOQ)

These can be estimated in several ways, with both the FDA and USP(United States Pharmacopoeia) offering various approaches. Two arebased on 'visual inspection' whilst, for an instrumental technique suchas chromatography, peak height measurement relative to the standarddeviation of the base line noise can be used.

It is axiomatic that the limit of detection (LOD) is that value abovewhich there is a defined probability that the observed value is greaterthan zero. A realistic value for the LOD is often quoted as three timesthe noise standard deviation (SD) and a number of commercial softwarepackages allow the operator to select a 'window' wherein the baselineappears smooth and to calculate this value, often as well as other usefulfactors, signal max. and signal min. Whilst this argument is perfectlyjustifiable in terms of the statistics discussed earlier and the probabilityof a value (the data point) exceeding some mean value, it seems not totake into account the rate of collection of data points. In most chromato-graphic analyses a peak, however small, will consist of an appreciablenumber of data points and, whilst it may require a single one to bethree times the noise SD before its presence can be confirmed, thepresence of a noisy peak on a noisy baseline is usually obvious atperhaps half that value.

The limit of quantitation is sometimes confused with the limit ofdetection but, when the difference is understood, it is generally taken asthree times the LOD, i.e. at a signalrnoise ratio of 9. It is also defined(USP) as the level at which a value can be calculated with reasonableaccuracy so we now move from objective to subjective decisions andthus out of statistics to a pragmatic estimate based on the purpose ofthe analysis and the requirements of the enquirer. If a peak can bequantified at three times the detection limit, this implies that theaccuracy of measurements is 3 ± 1 unit of area whilst precisely at the

detection limit, the value becomes 1 + 1. At the detection limit, theprecision is therefore ± 100%, falling to ± 33% at the LOQ valuedefined above.

Both the FDA and USP allow regression analysis to be used in asses-sing detection limits and the results obtained in the author's laboratoryusing this procedure would support a somewhat lower value than the'3 x SD' and '9 x SD' defined above.

For single or duplicate analyses, visual assessment must be theprimary criterion for assessing 'non-detectability' and a procedurewhich seems often acceptable is to identify a very small 'blip' in thechromatogram, to integrate it as though it were the component inquestion and then calculate its 'concentration'. Depending on one'sassessment of the ease of measuring the 'blip' one can then define arealistic detection limit.

Although this discussion has concentrated on chromatographic data,the application can be extended to most analyses where there is aprintout of primary analytical data.

Finally, it should never be forgotten that any precision statementmust correlate with the data quoted for limits of quantitation.

REFERENCESDavies, O.L. and Goldsmith, P.L. (1972) Statistical Methods in Research and

Production, 4th edn, Oliver and Boyd, Edinburgh.Dixon, WJ. (1953) Biometrics 9, 74.Ferguson, T.S. (1961) Rev. Inst. Int. de Stat. 3, 29.Grubbs, F.E. (1969) Technometrics 11, 1.Grubbs, F.E. and Beck, G. (1972) Technometrics 14, 847.Teitjen, J.E. and Moore, R.H. (1972) Technometrics 14, 583.

Appendix A

Table of Official National and

International Standards

Many countries have standard test procedures and methods of analysiswhich are accepted as having been validated and are therefore consid-ered reliable for certification purposes or in cases of dispute or arbitra-tion. The following table lists the reference numbers of many of thosepublished by the International Standards Organization (ISO), the BritishStandards Institute (BSI), The American Society for Testing andMaterials (ASTM) and the German Deutsches Institut fur Normung e.V.(DIN) which are relevant to the rubber industry. CEN Standards do notfeature in the following table since they are generally product-orientatedwhilst most of the standards listed are composition-orientated. It wouldnot be practicable to list all the product-related standards which relateto the rubber industry although a few appear in the text where theyhave a particular relevance.

Very few new British and German Standards are now beingproduced since the greater part of the effort is now being directedtowards The European (CEN) Standards. In the UK these will then beadopted as the British Standard, since it is obligatory upon EUStandards Institutions to use the ENs when they are produced and,indeed, they supersede the equivalent national one if one exists. Britishadopted European Standards are labelled BS/EN... and if CEN hasadopted the ISO standard the designation will become BS/EN/ISO...British adopted ISO standards not adopted by CEN will become BS/ISO... German standards will be similarly dual numbered.

Gradually, therefore, a new numbering system will come into use

and it will be essential that the full identifying characters and numbersare quoted since, for instance, BS1434 relates to copper specification inelectrical goods whilst BS/ISO1434 relates to bale coating on a bale ofnatural rubber.

ISO BS ASTM DIN

GeneralRubber vocabulary 1382 3558 D1566 53501Rubber, latex nomenclature 1629 BS/ISO1629 D1418Test sieves, specn and use 565,2194 1629 Ell 4188Glass viscometers 3105/4 188 D746Rubber, preparation of

test pieces 4661/1.2 1673,5738 D3183 535025858, 6315,5923, 7164.

LatexSampling 123 6057/2,3397 D1417,D1076 53562Total solids 124 6057/3.2 D1417,D1076 53563NR, alkalinity 125 6057/3.3 D1076 53565NR, dry rubber content 126 6057/3.4 D1076 53564NR, KOH value 127 6057/3.5 D1076 53566NR, density 705 6057/3.7 D1076 53597NR, coagulum 706 6057/3.8 D1076 53594NR, concentrate dry

film preparation 498 6057/3.24NR, concentrate determination of:

volatile fatty acid no. 506 6057/3.6pH 976 6057/3.9 D1076,D1417 53606Viscosity 1652 6057/3.11 D1076,D1417Copper (photometric) 8053 7164/28.2 D1278 53569Copper (AAS) 6101/3 7164/28.1 D4004Manganese (photometric) 7780 7164/26.2 D1278Manganese (AAS) 6101/4 7164/26.1 D4004Iron (photometric) 1657 7164/27.2 D1278 53620Iron (AAS) 6101/5 7164/27.1 D4004NR, boric acid 1802 6057/3.12 D1076 53605NR, centrifuged/creamed-

ammonia preserved specn 2004 6057/1.1 D1076NR, sludge 2005 6057/3.13 D1076 53592SBR, volatile unsaturates 2008 6057/3.15 D1417 53675BR, mono/copolymer -

dry polymer preparation 2028 6057/3.16 D1417SBR, volatiles 2058 3397 D1417 53526Synthetic, codification 2438 6057/1.3 53549SBR, bound styrene 3136 3397 D1417

ISO BS ASTM DIN

NBR, residual acn. monomer 3899 6057/3.22 D1417NBR, bound acrylonitrile 3900 6057/3.21SBR, reinforced, bound styrene 4655 6057/3.19 01416,01417

Raw rubbersAsh 247 7164/5 D1416JD1278 53568Volatile matter 248 7164/6.1 D1416,D1278NR, dirt 249 7164/8 D1278 53527Viscosity (Mooney) 289 903/A58 D3346 53523Solvent extraction 1407 7164/3 D1416,D1278 53553NR, bale coating 1434 BS/ISO1434 D2449Bale wrapping 7948Copper (photometric) 8053 7164/28.2 D1278 53569Copper (AAS) 7164/28.1Manganese (photometric) 7780 7164/26.2 D1278 53589Manganese (AAS) 6101/4 7164/26.1 D4004NR, nitrogen 1656 7164/21 D3533Iron (1,10-phenanthroline) 1657 7164/27.2 D1278 53620Iron (AAS) 6101/5 7164/27.1 D4004Bale sampling and sample

preparation 795 6315 D1485 53525NR, guide to specification 2000NR, specification D2227SBR, organic acid/soap 7781 7164/9 D1416SBR, copolymers:

bound styrene 2433 4656/9 D1416NR, colour index 4660 7596 D3157Single polymer (PGC) 7270 D1417Isoprene content 5945 7164/7.1 D1278SBR, block styrene content 6235 D3314Detection of factice D297 53588Oil, extender and processing

by chromatography D2008

Compounded and vulcanized rubbersAsh 247 7164/5 D297 53568Zinc (EDTA) 2454 5923/2 D297 53581Zinc (AAS) 6101/1 7164/29.1 D4004Manganese (photometric) 7780 7164/26.2 D1278Manganese (AAS) 6101/4 7164/26.1 D4004 53589Iron (AAS) 6101/5 7164/27.1 D4004Iron (photometric) 1657 7164/27.2 D1278Copper (AAS) 6101/3 7164/28.1 53569Copper (photometric) 8053 7164/28.2 D1278Lead 6101/2 7164/30.1 D4004 53599Solvent extract 1407 7164/3 D297 53553

ISO BS ASTM DIN

Carbon black (pyrolytic) 1408 7164/14 D297 53585Vulcanized rubber storage 3574Density 2781 903/A1 D297Total sulphur content 6528/1 7164/23.1 D297 53561Free sulphur content 7269 7164/24Inorganic sulphide sulphur 8054 7164/25Bromine and/or chlorine

determination 7725 7164/22.2 D297 3566Test for staining with

organic materials 3865 903/A33 53540Test piece (dimensions) 4648 903/A38Prepn of test pieces

for chemical analysis 4461/1 903/A36 D3183Isoprene content 5945 7164/7.1 53621Antidegradant by TLC 4645 6630 D3156 53622Antidegradant by HPLC 11089 7164/31.1Determination of accelerators by

gas and thin layerchromatography 11389 7164/32.1

Carbon blackReference grades D4678Bulk/bin delivery sampling 1124 5293/1 D1900 53602Ash 1125 5293/7 D1506 53586Loss on heating 1126 5293/5 D1509 53552Total sulphur 1138 5293 D1619 53584Iodine number 1304 5293/10 D1510 53582Package shipments, sampling 1126 5293/1 D1799Sieve residue, determination 1437 5293/6 D1514Sieve residue, specification 1867 Dl 765Volatiles 1868 D1765Toluene extract, light

transmission 3858 5293 D1618Nitrogen surface area 4652 5293/11 D3037Prepn sample for surface

area determination 6894 5293/19Surfactant surface area 6810 5293/12DBP no. (plasticorder) 4656/2 5293/18 D9414 53601DBP 4656/1 5293/17 D3493Tinting strength 5435 5293/13 D3265,3493Solvent extractables 6209 5293/16 53553

Also relevant are:Classification for CB TR12245 5923/21 D1765Visual inspection for dispersion D2663Agglomerate counts D2663

ISO BS ASTM DIN

Standard test/formulations for evaluation of:NR 1658 5738 D3184 53670UR 2302 4470 D3188 53670SBR 2322 6995 D3185 53670CR 2475 5375 D3190 53670BR 2476 5047 D3189 53670EPDM 4097 6063 D3568SBR (black/oil MB) 4659 5563 D3186Standard test formulations

for carbon black in NR D3192Standard test formulations

for carbon black in SBR 3257 5293/20 D3191

Statistical standards (not solely applicable to rubbers)Precision of test methods 5725 BS/ISO5725*Statistics vocabulary 3534 BS/ISO3534*Sampling procedures and tables for

inspection by attributes 2859 6001/0-3*Statistical interpretation of test results, estimation

of mean, confidence limits 2602 2846/2*Statistical interpretation of data techniques and tests

relating to means and variants 2854 2846/4*Statistical interpretation of data, determination

of a statistical tolerence level 3207 2846/3*Statistical intepretation of data, comparison of two

means for paired observations 3301 2846/6*Statistical interpretation of data, power of tests

relating to mean and variances 3494 2846/5**Much of the content of these standards is covered in ASTM D4483 which is purelyfor the rubber and carbon black industries.

Relevant ISO standards in course of preparationExtractable proteins in NR

medical gloves ISO/CD12243 (+ prEN455/3 and ASTM 5712 (published))Sulfenamide type accelerators, method of test ISO/11235p-phenylenediamine antidegradants, test methods ISO/11236Carbon black - iodine no., potentiometric method ISO/DAM/1304Determination of composition in selected polymers ISO/CD9924Determination of microstructure by IR in BR ISO/CD12965Determination of unsaturation in HNBR by IR ISO/CD14558Determination of sulphur by automated methods ISO/CDl5671Determination of nitrogen content by automated method ISO/CDl5672Determination of monomers and other organic compounds

in raw rubbers by an automated thermal desorption no number yettechnique allocated

Appendix B

Elastomers: nomenclature,

description and properties

The classification system used in the rubber industry is based on thatdescribed in ISO 1629-1976. The last letter of the identification codedefines the basic group to which the polymer belongs whilst theearlier ones provide more specific information and in many casesdefine the polymer absolutely. It is unrealistic to attempt a compre-hensive catalogue of all elastomers in an appendix of this nature. It ishoped that the most common ones, and those which could becomecommon in the next decade, are included. Tg's and density values aregiven where available but it should be noted that, except in the caseof a stereospecific homopolymer, variations in structure and blendcomposition will have an effect on these values whilst there is afurther dependence on the molar mass and method of measurementused. The figures quoted are all DTA/DSC values and the samplesare of such a molar mass that variation in this would have an insig-nificant effect on the Tg.

'M' GROUP: RUBBERS HAVING A SATURATED -C-C- MAINCHAIN

IM: Polyisobutylene (e.g. VISTANEX), a soft inert plastic; lowmolecular weight material used as a plasticizer andadhesive.

d = 0.91 Tg -70^-730C

EPM: Copolymer of ethylene and propylene; the rubber-like materialshave a wt/wt composition between 70-30 and 30-70.d = 0.87 Tg50/50 -6O0C

EPDM: A terpolymer of ethylene, propylene and a di- or polyenegiving pendent olefin groups as crosslinking sites (e.g.NORDEL). An ozone- and oxidation-resistant rubber.

d = 0.85 Tg (NORDEL 1470/1660) -66 °CSM: Chlorosulphonated polyethylene (e.g. HYPALON), containing

both C-Cl and C-SO2Cl groups. Cl content 20-45%; S content0.5-2.5%. Optimum properties 30% Cl, 1.5% S; ozone-resistantrubber also used in varnishes.d = 1.27 Tg (PiYPALON 30) +90C

FPM: Fluoro/fluoroalkyl groups on C-C backbone (e.g. VITON,FLUOREL - copolymers of hexafluoropropylene and vinylidenefluoride)(e.g. TECHNOFLON copolymer of vinylidene fluoride and1-hydropentafluoropropylene.d = 1.85 Tg (VITON B) -18 0C

CFM: As above, but containing Cl as well as F; vinylidene fluoride(VF): chlorotrifluoroethylene (CTFE) copolymer (e.g.VOLTALEF, KEL F).d = 1.85 Tg VF -45 0C, CTFE +52 0CAll the fluoropolymers are thermally stable and relatively inert.Various copolymers show a linear relationship between CTFEwt % and Tg.

'O' GROUP: RUBBERS HAVING CARBON AND OXYGEN IN THEMAIN CHAINCO: Poly(epichlorohydrin) (HERCLOR H) - the parent material

from which came:ECO: Copolymer of epichlorohydrin and ethylene oxide (HERCLOR

C)d = 1.27 Tg -470C

GPO: Copolymer of propylene oxide and allyl glycidyl ether (PAREL)d = 1.01 Tg (PAREL 58) -73 0C

All these materials have good heat resistance and excellent lowtemperature properties.

'Q' GROUP: SILICONE RUBBERS

MQ: Polydimethylsiloxane; depending on the molar mass this can bean oil, wax or rubber.

Tg -127 0C typically

MPQ: As MQ with the addition of phenylmethylsiloxane.Tg -86 0C typically

MPVQ: As above but with vinyl groups.

MFQ: As MQ but fluorinated.

These are all relatively stable thermally and because of theircold cure characteristics may be used as electrical insulants,seals, moulds, etc.

'R' GROUP: RUBBERS HAVING AN UNSATURATED CARBONBACKBONE

ABR: Refers to copolymers of butadiene and methyl methacrylate(e.g. BUTAKON ML) used to impregnate paper

Tg -57 0C at 25% PMMA

but also includes the terpolymer with acrylonitrile (primer,before adhesive layer applied) and tetrapolymer with styrene(used as a synthetic rubber).

BR: Poly(butadiene) - available as high cis (98%+), high trans (98% +)and anywhere in between. Can also have vinyl groups present atany level.

d = 0.91-0.93 Tg -107 0C (100% cis and 100%trans) -> -15 0C (100% vinyl)

A linear relationship exists between these ranges dependingupon the vinyl content, not affected by cis/trans ratio. General-purpose rubbers usually 90%+ cis or about 45% cis 45% trans10% vinyl. High vinyls have some specialist uses.

CR: Poly(p-chlorobutadiene) (e.g. CHLOROPRENE, NEOPRENE).Two main types, 'G7, amber in colour with large molar massrange centred at about 100000; 'W, white, molar mass ofnarrower range and centred about 200 000. Used as an adhesive

or where oil or ozone resistance required; gaskets, subaquasuits, etc.

d = 1.23 Tg -450C

UR: Copolymer of isobutylene and isoprene (BUTYL). Only a smallamount of diene added (approximately 5%) to give crosslink-able sites. Has a low gas permeability, hence uses in inflatableproducts, and as general-purpose rubber.

d = 0.92 Tg -670C -> -750C(varies with isoprene loading)

CIIR: Chlorinated UR ) with 2-3% w/w halogen to decrease gas> permeability and improve self-adhesion on

BIIR: Brominated UR J building

(e.g. HYCAR 2202, BUTYL HT 1066, 1088). Uses as for UR.d = 0.95 Tg (HT 1066) -7O0C

IR: Synthetic ds-poly(isoprene) (e.g. CARIFLEX, NATSYN. SKI3)cis level 90-99%, remainder trans and vinyl. General-purposerubber.

d = 0.91 Tg -68^-70°

NBR: Copolymer; acrylonitrile and butadiene (e.g. KRYNAC,NITRILE) available with a wide range of ACN loadings to alterhardness; oil-resistant applications.

d = 0.95 -> 1.05 Tg linear dependency upon ACN loading;65 0C at 15% -» O 0C at 50% for therandom copolymer. Also available isterpolymer (see ABR) and tetrapolymerwith styrene.

NR: C/s-poly(isoprene) natural rubber, essentially 100% cis, trans/vinyl <0.1%. Contains about 95% polyisoprene. Various gradesavailable RSS, SMR, SIR, SLR, NIG with number identifyinggrade - 5, 10, 20. Also modified NR - PA, SP, OENR, ENR,DPNR. NR was the original general purpose (GP) rubber.

d = 0.92 Tg -720C

SBR: Random copolymer of styrene and butadiene. Styrene levelvaries from 10% to 80% but the general purpose level is 23.5%.Many types available and the exact type identified by anumeric code. General purpose rubber. Vast amounts used intyres.

d = 0.93 Tg variable - 60 0C at 23.5% styrene-380C at 36% styrene

Also available as ter/tetra polymer systems (see ABR andNBR).

T" GROUP: RUBBERS HAVING CARBON (OXYGEN) ANDSULPHUR IN THE MAIN CHAINOT: Polymer of Ws-chloroalkylether (or formal), with sulphur. Most

common one uses Ws-2-chloroethylformal; CH2(OCH2CH2Cl)2(with a little 1,2,3-trichloropropane for crosslinking) THIOKOLST.

d = 13 Tg -590C

EOT: As above, but copolymerized with ethylene dichloride. All ofthese smell strongly of sulphur and are used for oil and solventseals. The liquid polymers cold cure and find a wide acceptanceas sealants in the building trade. Popular ones include:Poly (ethylene disulphide) d = lA Tg -270C

Poly (butyl ether disulphide) d = 1.1 Tg -760C

'U' GROUP: POLYMER CHAIN CONTAINS CARBON, OXYGENAND NITROGENAU: Polyesterurethanes 1 0 ^1 , . 1 ^ 1 1 1 - ,J I See Chapter I, Table 1.1,-,T T r> i ,1 4-u I f°r structural details.EU: Polyether urethanes J

A wide range of materials used as oil-resistant materials, inoxidation-resisting applications and as lightweight shoe soling.d= 1.2 Tg AU often around -30 0C

Tg EU often around - 50 0CBut both values variable

Although not elastomers, certain polymeric materials merit inclusionhere because of their application as rubber-like materials:PVC: Poly(vinylchloride); hard brittle material (d = lA) often copoly-

merized with vinylidine chloride, vinyl acetate, styrene, ABR,ethylene vinyl acetate etc. for a wide range of applications.When plasticized, usually with esters such as phthalates, itbecomes quite 'rubbery', used in conveyer belts, paints,

varnishes, floor coverings, erasers (rubbers), flexible tubing,Wellington boots and many cheap 'rubber' goods. Thermo-plastic.

PE: Polyethylene; a wide range of types available - HDPE (highdensity PE) and LDPE (low density PE). Numerous applications• medical implants to polythene bags, blended with elastomerssuch as EPDM to produce thermoplastic elastomers. Typedistinguished by m.p. - LDPE < 110 0C, HDPE up to 1360C.Reclaimed material gives a combination thermal curve.

PP: Polypropylene; similar applications to PE but higher melting(165 0C). Also used to make thermoplastic elastomers.

PS: Polystyrene; only occasionally met as a reinforcing plasticwithin a continuous elastomeric phase (e.g. shoe soling) but canbe considered to be present in some thermoplastic elastomerssuch as the block copolymers:• SIS styrene-isoprene-styrene• SBS styrene-butadiene-styreneSpectral and some thermal data show the styrene as'polystyrene' rather than randomly dispersed styrene as in SBR.

TPR: Thermoplastic rubber; for full classification, see Chapter 1, Table1.1.

Chlorinated rubber: Refers specifically to chlorinated naturalrubber, used for paints and adhesives. The theoretical level for(C5H8Cl2)n is 51% but commercial chlorinated rubber contains65% Cl which is ascribed to the structure

C CH3 Cl ClI I I I

(C-C-C-C)nI

Cl

which requires 68.3% Cl.Rubber hydrochloride: Again refers specifically to hydrochlori-nated natural rubber - usually with about 90% of the doublebonds hydrochlorinated (30% Cl). Plasticized material producedas film (e.g. PLIOFILM) was used for packaging.M. G. rubber: natural rubber to which methyl methacrylate hasbeen grafted, commercial materials generally contain 30% or49% w/w methacrylate.

GuttaPercha: I Polyisoprene, with 100% of the units trans', pure\ material not unlike PVC in feel and, when

Balata: J plasticized, can have similar uses.

Chicle: A naturally occurring mixture of cis and trans polyiso-prene (25:75), with resins, used in chewing-gum.

Guayule: Natural ds-polyisoprene isolated from the shrubParthenium argentatum by solvent extraction. Uses and proper-ties as for NR, but smell reminiscent of gin. Efforts to developcommercial exploitation have not been particularly successful.

For comprehensive details of manufacturers, trade names, detailedtechnical data, and application of the whole range of elastomers,together with similar details for chemicals used in the rubber industry,the interested reader is referred to the two annual publications: RubberRed Book, Communication Channels Inc., 6285 Barfield Road, Atlanta,GA. 30328, USA; and The Blue Book, Bill Communications Inc., 633Third Avenue, New York, USA.

Appendix C

lntercorrelation of analytical

techniques

In spite of the many requests to the rubber analyst to 'analyse thismaterial' it is axiomatic that there is no one analytical technique whichwill provide all the answers to satisfy the real interest behind such asimple request. Indeed it is often the case that the enquirer him- or her-self does not know what information is required since the analysis isoften to find out why a product or compound has not performed in thepredicted way. The effect may be obvious but the causes could bemany.

In Figure C.I an attempt has been made to show how many of theanalytical procedures discussed in earlier chapters 'interlock' and datafrom one leads naturally to another. There are five basic routes one maytake initially, depending on one's understanding of the analyticalproblem and rubber technology but, from then on, the analyst has todesign the most cost effective route to the answer, feeding back eachpiece of information until the picture becomes clear.

The figure is not intended to be a comprehensive flow chart but,hopefully, it will act as a stimulus to the analyst and can be shown tothe non-analytical enquirer to show what the simple request posed atthe beginning of this appendix entails!

Figure C.1 Interrelationship between analytical procedures relevant to the rubber analyst.

VisualexaminationEye -, Lens

"MicroscopeTransmissionelectron mic

Scanningelectron mic

+/•X-rayScanning

transmissionelectron mic

Thermal methodsof analysisDifferentialscanning

calorimetry

! PyrolysisThermo-

gravimetricanalysis

Structural data byspectroscopy

InfraredRaman

Ultraviolet

Nuclearmagneticresonance

Identification withno separation

Vapour pressure osmometer, Membrane osometer, ViscometerAtomic absorption spectrophotometer, Atomic emission spectrophotometer

SAMPLE

Purity byseparation

chromatography

Thin layerGas liquid

Liquid

Gel permeation

ion

Estimate ofcomplexity

Specificequipment

residue

extract

Isolation ofproducts

Specific analyses

Carbon %Hydrogen %Nitrogen %Oxygen %

Solvent extract %

Different typesof Sulphur %

Other elements %

Volatiles%Dirt%

Colour %Fatty acids %

pH

Ash%Carbon black type

Author index

Abraham, RJ. 144Agar, A.W. 211Agbenyega, J. K. 143Alden, K. 185Alderson, R. H. 211Alexander, A. E. 34AU, S. 183Altenau, A. G. 81, 134Aluise, V. A. 98Ambler, M. R. 188, 189Amos, R. 70, 322Andersen, M.E. 131Andries J. C. 215Angelo, R. J. 165Anhorn, V. 274Ansell, P. 212Von Ardenne, M. 169Askill, L N. 189Atkins, J. H . 274Auler, H. 110, 116Auleytner, J. 22Avons, C. H. J. 116Ayala, J.A. 286

Baer, M. 303Baer, J. 23Baijol, M. D. 175Baker, C. S. L. 98Balke. S. T. 187Balodis, R.B. 102Ban, L. L. 270Banks, C. K. 263Barbehenn, H. E. 116

Barnard, D. 81, 98Barnes, R. B. 81, 134Barnes, D. E. 185Barr, T. 277Barr, W. 274Barrall, E. M. II 184Barrer, R. M. 33Bartusek, P. 106Bateman, L. 89, 109, 121Bauminger, B. B. 267, 291Beauchaine, J. P. 131Becker, E. 57Becker, J. W. 145Becker, W. W. 98Belcher, R. 257Bellamy, L. J. 65Benoit, H. 183, 187Berg, R. 57Bernas, B. 250Bersted, B. H. 177Bhacca, N. S. 197Bhargava. C. S. 179Bhlowmick, A.K. 13van der Bie, G. J. 202, 246Billmeyer, F. W. Jr. 88Birley, A. W. 185Blois, M. S. 63Blosczyk, G. 45Blyumina, S. B. 175Bobanski, B. 101Bomo, F. 285Boord, C. E. 21Bouchardat, G. 10

Bovey, F. A. 145, 200Brady, P. 177Brandrup, J. 83, 180Brazier, D. W. 159, 161, 279, 291, 295Brewer, P. I. 184Brice, B. A. 182Bristow, G. 83Brock, M. J. 59Brown, J. 320Brown, P. S. 146Brown, W. A. 276Brown, W. E. 202Brtick, D. 133Brunauer, S. 287Brundle, C. R. 209, 262Bruni, G. 21Bruzzone, A. R. 185Brzezinski, J. 177Bunce, B. H. 181Burge, D. E. 178Burns, C. M. 189Bushuk, W. 183Busnel, J. P. 189Burrell, H. 83Bywater, S. 183

Calderon, N. 201Callan, J. E. 284Campbell, R. H. 73, 109Cantow, M. J. R. 184Carlson, D. W. 81, 134Carman, C. J. 146, 200Carothers, W. H. 13Carpenter, D. K. 183Carter, A. R. 320Carter, R. O. 132Caspar!, W. A. 15, 32Castro, M. E. 185Ceresa, R. J. 93Chalmers, J. 131Chambers, W. T. 98, 100, 273, 293Chandler, L. A. 164Chaplin, R. P. 189Charsley, E. L. 282Chase, B. 141Chen, H. Y. 201Cheng, J. 102Chescoe, D. 211

Childs, C. E. 96, 102Chin, H. C. 47, 58Ching, W. 189Chiu, J. 303Christopher, A. J. 107Clark, H. C. 106Clark, J. 81Clark. J. K. 134Claybourn, M. 131Cleverley, B. 135, 139, 149Cobbold, A. J. 203, 204, 219Cole, H. M. 153, 154Collins, A. M. 13Collins, E. A. 164Collins, J. H. 56Colombel P. 131Cook, S. 238Cooper, A. R. 185Cooper, W. 37, 79Corish, P. J. 133, 251Corner, M. 107, 257Cosslett, V. E. 216Couchman P. R. 228Coutelle, C. 11Crafts, R. C. 75, 110, 123, 252Craig, D. 109Cramers, C. A. 149Crompton, T. R. 55, 59, 68, 78Crowley, J. D. 83Cruikshank, S. S. 99Cudby, M. E. A. 199Cudby, P. E. F. 212, 223, 226, 229, 238Cui, Q. 199Cunliffe, A. V. 145Cunneen, J. 1. 195, 197Czerwinski, N. 16

Dannis, M. L. 163Davey, J. E. 45, 62, 75, 110, 114, 115,

117, 120, 121, 123, 165, 274Davidson, J. 263Davies, D. H. 99Davies, J. R. 60, 67, 69, 273Davies, O. L. 334Davis, A. R. 81, 134Davis, D. M. 132Davison, W. H. T. 148Dawkins, J. V. 185, 189

Dawson, B. 136, 146, 303Dawson, T. R. 270, 272Day, F. W. F. 110De, S.K. 13Dean, W. 69Debal, E. 106Debye, P. 182Deitz, V. R. 274Delides, C. 145Derrico, E. M. 47Dinsmore. H. L. 134Ditmar, R. 15Dixon, W. J. 337Dolan, J. W. 75Dondos, A. 180Donnet, J-B. 285Doolittle, A. K. 56Dotsan, A. O. 286Doty, P. 181Down, J. L. 263van Duin, M. 143, 302, 303Dunke, M. 96Dunn, J. G. 282

Eaton, B. J. 110Eberlin, E. C. 192Edwards, A. D. 45, 48, 121, 123, 175,

177, 178, 185, 186, 323Edwards. B. C. 165Edwards, H. G. M. 143, 195Ehrenberger, F. 99Ehrmantraut, H. C. 177Elias, H. G. 179Ellis, G. 143, 302, 303Emmett, R. H. 287Epshtein, V. G. 175Evans, C. A. Jr. 209, 262Evans, M. B. 90Ezrin, M. 176

Feke, D. 287Fennell, T. R. F. W. 107Ferguson, T. S. 339Fetters, L. J. 192, 195, 199Field, J. B. 195Fielden, P. R. 74Fielding-Russell. G. S. 164Figini, R. V. 177

Fikhlengol'ts, V. S. 63Filipovich, G. 145, 200Fiorenza, A. 272Fisher, H.L. 2Flory, P. J. 87, 192Ford, E. P. 275Fox, T. G. 192Foxton. A. A. 171Foxton, R. N. 322Frank, F. 265Frankland, J. A. 143, 195Fraser, G.V. 199Fredyma, M. M. 246Freier, H. E. 103Freitag, W. 42Frey, H. E. 15Frohlich, J. 270Fukushima, E. 144Fulton, W.S. 191Fuoss, R. M. 89Fuqua. S. A. 197Fusee-Aublet, J. B. C.Fyfe, C. A. 146

Gage, J. C. 263Gall, M. J. 199Galloway, P. D. 266, 322Garner, H. R. 78Gee, G. 83, 88Gehman, S. D. 195Gelling, I. R. 148, 159Gel'man, N. E. 104Gerbach, S. 99Gere, D. R. 47Gerrard, D. L. 143, 199Gerspacher, M. 286, 287Giacabbo, H. 149Giesecke, P. 81, 134Gilbert, R. C. 135Gilbey B. A. 212, 224Gilchrist, C. A. 70Gilding, D. K. 189Gillingham, C. R. 276, 277Gilmour, R. E. 204Glavind, J. 63Gleeson, J. G. 316Gleit, C. E. 263Glewala, H. 177

Goh, S. H. 168Goldsmith, P. L. 334Goldstein, J. H. 200Golub, M. A. 197Gomez, J. B. 202, 203Goodrich, W. C. 278Goritz, D. 270Gorsuch, T. T. 244, 247Gough, T. A. 152Grassie, N. 89Gray, A. P. 158, 159Griffiths, P. R. 132, 137Gross, D. 73, 135, 139Grosse, A. V. 99Groyer. S. 202Grubb D. T. 209, 214, 216, 226, 228,

229, 242Grubbs, F. E. 339Gruber, T.C. 286Grubisic, Z. 187Gudzinowicz, D. J. 185Gutmacher, R. G. 81, 266, 273, 274, 307Gwirtsman, J. 104

Hagedorn, W. 199Hall, R. T. 98Hallmark, V.M. 141Halwer, M. 182Hamielec, A. E. 187Hamilton, C. S. 263Hamzah S. 202, 203Hanson, C. M. 88Hardmann, A. F. 116Harms, D. S. 135Harris, J. 296Harwood, H. J. 202de Haseth, J. A. 132, 137Hashimoto, T. 186Haslam, J. 15, 55, 56, 59, 89, 102, 104,

139Hayes, M. R. 58Haynes R. 209Heacock, J. F. 175Heath, A. B. 150, 151Heckman, M. 263Heese, A. 61Heinrich, G. 287Hellmann, H. 70

Hempel, W. 101Hendra, P. J. 141, 142, 143, 199, 302,

303Henner, E. B. 96Henriques, R. 32, 109Herd, C.R. 270, 285, 287Herrmann, R. 135, 149Hess, W. M. 270, 274, 284, 286, 287Heuer, W. 179Higgins, G. M. C. 41, 47, 58, 68, 69,

90, 121, 123, 136, 195, 197, 297Hildebrand. J. 82Hillman, D. E. 171Hilton, C. L. 59Hindin, S. G. 99Hinson, D. 101, 245Hjelm, R. P. 286Hoffman, F. 11Hofmann, W. 59, 68Holden, G. 13Holland, W. D. 263Homes, J. M. 89Hopkins, S. 303Horowitz, E. 15Horton, C. A. 104Houghton, A. A. 247Huang, R. Y. M. 189Huber, C. 186Huggins, M. L. 87, 179Hull, C. D. 10, 146, 165, 197, 302Hummel, D. O. 15, 55, 56, 59, 62, 64,

68, 78, 139Hummel, K. 202Hyde, J. F. 14

Iheda, G. 246Ikeda, R. M. 165Immergut, E. H. 83, 180

Jackson, K. D. O. J. 142, 143, 146, 163,197, 251, 295, 302, 303

Janicka, K. 16Janssen, H-G. 149Jaroszynska, D. 295Jenkins, R. 114Jennings, B. R. 181John, O. 42Johnson, A. F. 143, 195

Johnson, J. F. 184, 185Johnson, M. J. 263Johnson, P. 34Johnson, R. N. 109Johnston, J. 244Jones, C. E. R. 152Jones, C. H. 199, 302, 303Jones, C. J. 143Jones, H. W. 266Jordan, E. F. 183Jorgensen, A. H. 164Joyce, G. A. 286Joyet, C. 263

Kam, F. W. 273Kambe, H. 183Kamiya, K. 68Kato, K. 202, 203, 226Kato, Y. 186Kay, D. H. 228Keavney, J. J. 192Kelleher, W. J. 263Kern, R. 186Khan, H. U. 179Kido, S. 186Killer, F. C. A. 70Killgoar, P. C. Jr 132Kim, H. G. 165King, J. W. 47Kinsey, R. A 146Kip, BJ. 143, 302Kiparenko, L. M. 104Kirby, J. E. 13Kirchhof, F. 319Kirkland, J. J. 186Kirshenbaum, A. D. 99Kjeldahl, J. 96Klein, A. K. 263Klein, P. G. 145Kleps, T. 295Kline, G. M. 15, 56Kluppel, M. 287Knight, B. C. J. G. 44Knight, D.P. 228Knowles,T. M. 304, 306Koch, H. W. 61Kokle, V. 176Koldunovich, E. B. 175

Kolthoff, I. M. 81, 266, 273, 274, 307Komas-Colka, A. 177Komoroski, R. A. 146Kondakoff, I. 11Kow, C. 192, 195, 199Kratohvil, J. P. 182Krecji, J. C. 274Kreiner, J. G. 68Kreitmeier, S. 270Kremens, J. 274Kress, K. E. 41, 272Krishen, A. 151, 153Krishnan, K. S. 141Kruse, J. 285Kruse, P. F. 135Kuhls, G. H. 73Kulver, S. 99Kumar, V. G. 202Kurata, M. 180Kurosaki, K. 141Kyriacos, D. 185

Lacher, U. 61Laframboise, E. 102Lagarius, J. S. 274Lamond, T. G. 163, 166, 167, 276, 277,

291, 300Landi, V. R. 164Lasinger, C. 286Lawrie, J. H. 65Layec-Raphalen, M. N. 180Le Clair, B. P. 187Leblanc, A. 182Lechner, H. 202Lederer, K. H. 186Lee, B. 166Lee, D. F. 90Legge, N. R. 13Leng, M. 183Lerner, M. 135Lesec, J. 189Letot, L. 189Lewis, I. R. 143, 195Lewis, P. R. 228Leyden, J. J. 304, 309Li, Q. 287van Lieshout, M. H. P. M. 149Light, T. S. 104

LiGotti, I. 81, 134Lin, D. 199Liu, Z. 199Lloyd, D. G. 63Loadman, M. J. R. 45, 61, 100, 115,

136, 143, 146, 148, 155, 159, 161,164, 165, 175, 192, 197, 292, 293,302, 303, 304, 316, 320

Loftus, P. 144Louth, G. D. 59, 266, 267Lowe, J. W. Jr. 83Lucas-Tooth. H. J. 262Luongo, J. P. 58L'Vov, B. V. 258L'Vov, Yu. A. 63Lynes, A. 70Lyon, F. 274, 275

Ma, T. S. 96, 99, 104, 107McClelland, J. F. 132McConnell, M. L. 181, 185Macdonald, A. M. S. 257McDonald, A. J. 264McDonald, G. 270McDonald, G. C. 287McFearin, T. C. 274McHard, J. A. 106Mclntosh, R. 89Mclntyre, M. 188, 189Mackay, J. G. 116MacKillop, D. A. 136McSweeney, G. P. 48, 61, 67, 68, 69,

70, 71, 75, 155, 304, 323Maddams, W. F. 143, 199Magee, R. W. 278Majewska, F. 16Majors, R. E. 74Mandelkern, L. 165Mandelstam, L. 141Mann, W. 99Mannion, R. F. 104Marckwald, E. 265Mark, H. 175Marteau, J. 279, 295, 300Marx-Figini, M. 177Mathews, F.E. 11Mati, R. D. 175Maurer, J. J. 161, 279, 282, 295

Mead, D. J. 89Mears, P. R. 171Medalia, A.I. 284Le Mehaute, A. 287Mellan, I. 83Messenger, T. H. 109Metavier, B. 279, 295, 300Metcalf, J. 58Metz, O. 270ter Meulen, H. 96Meyers, E. E. 100, 102Micek, E. 274Michailov, L. 202Middleton. G. 263Mikl, O. 101Miller, R. G. T. 58Milliken, L.T. 243Mita, A. 183Miyake , Y. 182Moakes, R. C. W. 315Moldrai. T. 263Monas-Zloczower, I. 287Montani E. 228Moore, C. G. 109, 116, 121Moore, J. C. 184Moore, R. H. 339Morawetz, H. 192Morrell, S.H. 285Morris, C. E. M. 177Morrison, J. A. 89Morrison, N. J. 121Morton, M. 192, 195, 199Mould, H. 141Mourey, T. H. 181Mourina, F. A. 263Muggli, R. Z. 131

Nagaya, T. 195Nah, S. H. 312Narasimhan, V. 189Nataka, M. 182Neilson, R. C. 42Newitt, E. J. 176Ney, E. A. 150, 151Ng, T. S. 274Nickel, G. H. 159, 161, 295Niedermeier, W. 270Nieuwland, J. A. 12

Nippoldt, B. W. 103Norem, S. D. 158,159

O'Neal, M. J. 70Ogg, C. L. 175Ol'shanskaya, L' 'A. 175Oliver, B. J. 100, 104Oliver, J. 277Olson, P. B. 99, 103Oppenheimer, L. E. 18Ostromov, H. 59, 68O'Farrell, C. P. 287O'Neill, M. J. 158, 159

Packer, H. 78Paputa Peck, M. C. 132Pasternack, R. A. 177Patel, A. C. 276Patrick, J. C. 11Pautrat, R. 279, 295, 300Pech, J. 101Pendle, T. D. 204Persoon, C. H. 6Pethrick, R. A. 145Petit, D. 320Petrescu. G. 263Petterson, D. L. 153, 154Peurifoy, P. V. 70Pfann, H. F. 175Phillips, W. M. 101Pierce. S. L. 187Pontio, M. 15Porritt, B. D. 266, 270Porter, M. 98, 109, 121Porter, W. L. 175Poshyachinda, S. 143, 195Poulton, F. C. J. 64, 120, 244, 267, 291Press, E. W. S. 65Price, B. J. 262Price, C. R. 277Priel, Z. 180Prud'homme, J. 183Purdon, J. R. 175Putman, J. B. 304, 306Pyne, C. 262

Quivoron, C. 189

Raab, H. 270Rabb, J. M. 304, 309Railsback, H. E. 285Raman, C. V. 141Randall, J. C. 201Ransaw, H. C. 81, 134Ratcliffe, A. E. 132Reed, A. M. 189Reich, M.H. 287Reid, N. 216, 219, 225Rempp, P. 187Rice, D. D. 78Richardson, W. S. 195Rittner, R. C. 96, 107Roberts, A. D. 323Roberts, M. W. 107Robertson, M. W. 40Rockley, M. G. 132Rodriguez, F. 185Roeder, S. B. W. 144Roff, W. J. 15Roland, C. H. 274Romani, E. 21Roovers, J. E. L. 199Rosencwaig, A. 132Rosenthal, R. J. 131Ross, J. A. 185Rowley, R. M. 40Roy, B. R. 187Rudd, J. F. 185Rudkin, A. 179Runyon, J. R. 185Rush, C. A. 99Russo, S. P. 287

Sacher, A. 195Samples, C. R. 304, 306Samus, M. A. 132Sandell. E. B. 263Sang, J. 199Saville, B. 109, 121Sawyer, L. C. 209, 214, 216, 226, 228,

229, 242Scheele, W. 109Scheinbeim J. I. 228Schidrowitz, P. 21Schleifer, D. E. 276Scholl, F. K. 15, 16, 55, 56, 59,62, 64,

68, 78, 139, 251Scholte, Th. G. 192Schoniger, W. 101Schoon, Th. G. F. 202Schroder, E. 104Schroeder, H. E. 13Schubert, B. 275Schultze, M. 98Schwartz, N. V. 279Schwarzkopf, F. 105Schwarzkopf, O. 105Scott, J. R. 266, 270, 272Scott, K. W. 201Scott, R. 81, 82Scott, R. A. 134Sebrell, L. B. 21Seeger, P. A. 286Senti, F. R. 181Servais, P. C. 106Sewell, P. R. 64, 136, 148, 303Shaner, W. C. Jr. 100Shcherbacheva, M. A. 105Sherma, J. 66Shiibashi, T. 238Shimura, Y. 183Sidek, B. D. 148, 159Sidwell, J. A. 73Silberberg, A. 180Simon, W. 149Sin Siew Weng 316Singh, M. M. 47, 58Singleton, C. 166Sircar, A. K. 159, 163, 166, 167, 291,

300Slaney, S. 146Slicter, C. P. 144Sljaka, V. A. 153, 154Small, P. A. 83Smekal, A. 141Smith, A. J. 100Smith, D. A. 154Smith. D. C. 134Smith, D. S. 153, 154Smith, J. C. B. 105, 106Smith, M. 245Smith, R. C. 106Smith, R. K. 37Smith, R. W. 215

Snook, J.K. 287Snyder, L.R. 75Somolo, A. 279Soos, I. 279Soppet, W. W. 55Sorvall 216Spacsek, K. 279Spatorico, A. L. 183Squirrell, D. C. M. 15, 55, 56, 59, 102,

104, 139Staat, F. C. 98Stahl, E. 66Stamberger, P. 44Stanton. R. E. 264Staudinger, H. 179Steel, G. 70Stelzer, F. 202Stephens, I. S. 75Stern, H. J. 101, 245Stern, M. D. 181Stevens-Mees, F. 272Stevenson, I. 212Stewart, L. N. 158, 159Stickland. F. G. 245Stierstorfer, J. 270Stockmeyer, W. H. 180Stothers, J. B. 144Strange, E. H. 11Strauss, K. 73Strobl, G. R. 199Stubbings, W. V. 277Stucking, R. E. 263Studebaker, M. L. 115Stumpe, N. A. 285Subramanium, A. 186Sucharda, E. 101Sugimura, Y. 195Sullivan, A. B. 23Sultzberger, J. A. 263Swarin, S. J. 297, 300Sweitzer, C. W. 278, 284Swinyard, P. E. 204

Talalay, L. 170Tarbin, F. G. 245Tarpley, A. R. 200Tarrant, L. 120Teague, G. S. Jr. 83

Teitjen, J. E. 339Teller, E. 287Terry, S. L. 185Thomas, A. G. 312Thorpe, W. M. H. 191Thummer, R. 202Thuraisingham, S. T. 67Tidd, B. K. 100, 121, 179, 303, 323Tiers, G. V. D. 145, 200Tilden, W. A. 10Tinker, A. J. 146, 192, 238Tonelli, A. E. 145Toporowski, P. M. 199Trent, J. S. 228Tricot, C. 287Truett, W. L. 137Try on, M. 15Tsuge, S. 195, 199Tulak, D. 295Tung, L. H. 185, 189Tunnicliffe, M. E. 69Tuttle, J. B. 15, 266Tyler, W. P. 15

Uhrberg, R. 250Unger, K. 186Unterzaucher, J. 98Urbanski, J. 16

Vaughan, M. F. 184Vitali, R. 228Voet, A. 167

Wagenfold, H. K. 287Wagner, A. R. 179Wake, W. C. 43, 59, 66, 100, 266Walker, D.F. 284Wallace, W. B. 135Wallach, M. L. 165Wallach, O. 10Wallen, P. J. 143, 199, 302, 303Walter F. 215Wampler, W. A. 286Wang, J. 199Wang, M- J. 285Warner, W. C. 68Warnes, G. 142Watson, D. S. 199

Watson, W. F. 83, 90, 195, 197Waurick, U.104Webb, J. R. 107Weber, C. O. 15, 32, 110Weiblen, D. G. 103Weigand, W. B. 270Weiss, M. L. 21Werstler, D. D. 146, 201West, T. S. 257Wetters, J. H. 106Wexler, A. S. 63Wheeler, D. A. 59, 68Whettem, S. M. A. 104White, D. W. 64White, R. J. 191Whitham, B. T. 70Willard, H. H. 104Williams, C. H. G. 10, 148Williams, L 13Williams, V. Z. 81, 134, 175Williamson, A. G. 40, 45Willis, H. A. 15, 55, 56, 58, 59, 89, 102,

104, 139, 199Willits, C. O. 175Wilmott, W. H. 266, 270Wilson, S. 209, 262Wims, A. M. 297, 300Wise, R. W. 109Witnauer, L. P. 181Wolf, H. 9Wolf, R. 9Wolff, C. 180Wolff, S. 265Woodard, M. K. 132Woodford, D. E. 195Woods, L. A. 70Wragg, A. L. 146Wyatt, G. H. 43Wyatt, P. J. 181

Yamamoto, M. 186Yamasaki, H. 182Yang, H. H. 287Yang, M. 199Yeager, F. W. 145Yuasa, T. 68

Zeitlin, H. 246

Zerda, T. C. 286Zerda, T. W. 287Zhoa, Y. 199Zijp, J. W. H. 66Zimm, B. H. 182

Zimmermann, W. 98Zolotareva, R. V. 63Zowall, H. 16Zuazaga, G. 96Zweig, G. 66

361 This page has been reformatted by Knovel to provide easier navigation.

Index

Index terms Links

A accelerators, identification 59

accuracy 326

acid ashing procedure 243 246

addition polymers 2

additives, identification 57

adsorption monitoring 50

adventitious materials 21

Agerite White, identification 71

alkali flame ionization detector see nitrogen phosphorus detector

analysis techniques and classification categories 16 interrelationships 359

analysis of variance procedure 335

antioxidants cause of staining 316 determination 58

ash content 307

ashing procedure 243 loss of trace elements 245 mineral constituents changes 244 temperature control 244 thermogravimetry 246

362 Index terms Links


atomic absorption spectroscopy (AAS) 64 253 electrothermal 258

atomic spectroscopy techniques 252

attenuated total reflectance (ATR) see multiple internal reflectance spectroscopy (MIR)

Auger electron spectroscopy (AES) 259

average value 328

B Benoit factor 188

BET method (nitrogen adsorption), for surface area measurement 274

Blagden’s law 176

block copolymers 303

block face size and shape, for ultramicrotomy 219

blooms analytical methods 28 319 modified 314 multiple internal reflectance infrared spectroscopy 319 pre-analytical checklist 317 pseudo 314 removal prior to analysis 320 scanning electron microscopy 320 spot tests 319 true 312 see also surface contamination

bomb digestion technique 249 258

bond failure problems 28

boric acid, determination 58

bromination technique, latex sample preparation 203

bromine, quantitative analysis 100



bulk filler analysis 251

Buna rubbers 12

butyl rubber 12

C Cabot dispersion test 284

calendered sheets, sampling 26

carbon, quantitative analysis 95

carbon black 265 analysis by thermogravimetry 279 analysis of particles and aggregates 270 analysis of type 270 categorization 267 colour index 273 dispersion in vulcanizates 284 from rubber matrix 265 loadings 307 models using fractal dimensions 287 other examination techniques 285 surface area measurements 274 surface composition analysis 286

carbon disulphide, spot test 319

carbonization errors 243

CEN Standards see European (CEN) Standards

checklist, pre-analytical 317

chemical etching 229 231

chemical shift (in NMR) 144

chemical staining procedure, differential 226

chlorine, quantitative analysis 100

coefficient of variation 331



cohesive energy density (CED) 83

colligative property measurement 176

column chromatography 65

compositional categories 16 19

confidence limits 330

contamination, surface 315

copolymer 2

22CP46 (antioxidant), identification 72

cryoscopy 176

crystallization process 165

CTAB surface area test method 277

Curie-point determinations 159 pyrolysers 149

curing process 3 8 effect of conditions 282

cut-surface/torn-surface methods, for carbon black dispersion 284

cyclohexyl benzothiazyl sulphenamide (CBS), identification 72

D data

graphical presentation 340 use of imprecise 328

DBP test method, for surface area measurement 278

Debye equation 182

degradation, at high temperatures 167

derivative thermogravimetry (DTG) 154 calibration 158 heating rate 156 polymer blend quantification 160



derivative thermogravimetry (DTG) (Continued) polymer identification 159

destructive elemental analysis 256

differential scanning calorimetry (DSC) 163 crystallization and melting 165 glass transition temperatures 164 high temperature events 167 for molar mass determination 192 195 for monomer distribution determination 199

differential thermal analysis, for molar mass determination 192

diffusion theory 35

digestion vessel see Parr bomb

dipped goods, sampling 26

discoloration 315 colour changes 318

dissolution procedure, definition 81

Dixon’s test (for outliers) 337

Dumas method (nitrogen determination) 96

Dunke’s method (nitrogen determination) 96

dynamic light scattering technique (DLS) see photon correlation spectroscopy

E ebulliometry 176

elastomers classification system 352 definition 2 thermoplastic 13

electron spectroscopy techniques 259 for chemical analysis (ESCA) 259



electrothermal atomization (ETA) 258

elemental analysis, with SEM 170

end group analysis 175

errors random 326 systematic 326

European (CEN) Standards 347

evaporative light scattering detector (ELS) 181

extender 4

extraction process adsorption 49 basis 32 definition 31 for formulation derivation 305 latex 47 microscale 43 microwave method 42 multiple 44 rapid method 41 solvent selection 38 specific extractions 45 standard apparatus 37 supercritical fluid 46 thermal 48 timing 40

F factice 44

identification 56

Fick’s laws (diffusion process) 33

fillers 20



flame ionization detector (FID) 76

Flory-Huggins interaction constant 87

fluorine, quantitative analysis 103

formulation calculation 309 derivation 303

Fourier Transform infrared instrumentation 131 137

fractal analysis, carbon black 287

fractional precipitation 93

freeze fracturing techniques 226

frosting see blooms

G gas chromatography (GC) 75

gel permeation chromatography (GPC) 183

glass transition temperatures 164 192

H hazing 315

high performance liquid chromatography (HPLC) 72 autosamplers 74 detector developments 74 reverse phase (RPLC) 73

high-performance GPC (HPGPC) 185

high-pressure GPC see high-performance GPC (HPGPC)

homogenization, sample 27

homopolymer 2

Huggins equation (viscosity) 179

hydrogen, quantitative analysis 95

hydroxylamine, determination 57



I identification methods

after separation 77 with no separation 55 with separation 65

inductively coupled plasma spectroscopy (ICP) 255

inductively coupled plasma-atomic emission spectroscopy (ICP-AES) 64

infrared (IR) spectroscopy 64 78 data interpretation 138 for monomer analysis 195 running the spectrum 137 sample preparation 132 types available 129

inhomogeneity, sampling for 28

instrumental examination techniques 129

international standards 347

inverse gas chromatography (IGC) 285

iodine adsorption method, for surface area measurement 275

ion chromatography (IC) 75 123

K Kirchof’s piperidine test 319

knife selection, for ultramicrotomy 217 220

Kuhn-Roth method (polymer determination) 306

L latex

derivation 7 extraction process 47



latex (Continued) particle sizing techniques 202 sample pretreatments for transmission electron microscopy 202 standards 303

least significant difference 332

light microscopy (LM) 208 231

light scattering behaviour, for molar mass determination 180

limit of detection (LOD) 345

limit of quantitation (LOQ) 345

localized analysis 28

low angle laser light scattering (LALLS) 181 185

Lowinox CPL, identification 72

M manufactured articles, sampling 26

Mark-Houwink-Sakurada expression (viscosity) 180 188

mass selective detector (MSD) 76

mass spectrometer see mass selective detector

mean value 328

measurements, use of imprecise 328

median value 328

membrane osmometry 178

metathesis process 201

methyl rubber 11

microtomy 215 using base-sledge microtome 215

microwave extraction 42

mineral rubber 44

mode 328



molar mass determination 174 by colligative property measurement 176 by end group analysis 175 by gel permeation chromatography 183 by light scattering behaviour 180 by thermal field flow fractionation 189 by viscometry 179

monomer definition 2 distribution 197 type determination 194

morphological analysis techniques 208 case study 231 chemical etching 229 231 chemical staining 226 freeze fracture 226 microtomy 215 sectioning problems 223 swollen vulcanized elastomer network 238

moulded articles, sampling 26

multi-angle laser light scattering (MALLS) 188 191

multiple internal reflectance infrared spectroscopy (MIR) 130 blooms 319

N N-2-propyl-N’-phenyl-para-phenylenediamine, identification 71

natural rubber analysis 59 deproteinized 61 history 4

neutron scattering technique 286



nitrogen, quantitative analysis 96

nitrogen phosphorus detector (NPD) 77

non-destructive elemental analysis 259

normal distribution curve 328

nuclear magnetic resonance spectroscopy (NMR) 77 143 for polymer analysis 195 197

199 solid state 146 swollen state 146 197

O olefin metathesis 201

osmium tetroxide (stain) 226 latex sample preparation 203

osmometry techniques, for molar mass determination 177

outlying values (outliers) 336

oxygen, quantitative analysis 98

oxygen flask combustion technique 257

P paper chromatography 65

paraphenylenediamine (PPD), cause of blooms 314

Parr bomb (digestion vessel) 250

particle sizing (of carbon black) 270

phase morphology, within blend 29

phosphorus, quantitative analysis 106

photoacoustic spectroscopy (PAS) 132

photon correlation spectroscopy (PCS) 204

plasticizers definition 3



plasticizers (Continued) identification 20 55

polychloroprene 12

polycondensates 3

polymer microstructure 193 monomer distribution 197 monomer sequence distribution 193 monomer type 194

polymeric plasticizers 44

polymers analysis visual aspects 170 blend quantification by DTG 160 categories 19 content determination 290 definition 2 identification by DTG 159 loadings 306

precision 325 327

proofings, sampling 26

protective materials 21

protein, determination 61

pyrolysis procedure 135 290 apparatus 149 293 problems 297

pyrolysis-gas chromatography (PGC) 148 for monomer analysis 195 199

Q quasi-elastic light scattering (QELS) see photon correlation

spectroscopy (PCS)



R Raman spectroscopy

for carbon black study 286 for identification of organic substances 77 for monomer analysis 195 199 near infrared Fourier Transform (NIR FT) 141 principles 141

random errors 326

repeatability 327

reproducibility 327

resistively heated pyrolysers 149

Restrablen bands 131

rubber, name derivation 7

rubber substitute see factice

rubberized fabrics, sampling 26

ruthenium tetroxide (stain) 227

S sample

definition 25 30 homogenization 27 preparation 29

scanning electron microscopy (SEM) 168 for blooms 320 for morphological analysis 209 235

Schultze-Blashke equation (viscosity) 179

sectioning techniques chatter problems 225 compression problems 224 curling problems 223



sectioning techniques (Continued) inconsistent sections 225 knife marks 224 on to ice 223 using trough liquid 222 without trough liquid 222

sedimentation experiments, for determination of molar mass 192

selective precipitation 93

selective solution 92

SEM-based scanning transmission electron microscopy (S(T)EM) 212 235

shielding (in NMR) 144

significance tests double-sided 331 single-sided 332

silicon, quantitative analysis 105

silicone rubbers 14

E-sitosterol, determination 60

size exclusion chromatography (SEC) see gel permeation chromatography (GPC)

skim rubber, analysis 62

small angle neutron scattering (SANS) technique 286

sodium pentachlorophenate, determination 58

softener 4

solubility parameters guidelines 88 practical considerations 88 theoretical considerations 82

solution preparation 89 procedure definition 81



solution (Continued) selective 92

solvent removal 89

solvent selection, extraction process 38

specific extraction process 45

specimen, definition 30

spectrophotometric analysis, of carbon black 270

specular reflection spectroscopy 131

stabilizing agents 21

staining effects 315

standard deviation 329

standard error 330

standard normal deviate values 333

standard test procedures, validated 347

Standards, International 347

Student’s t test 333

sulphur cause of blooms 313 copper spiral determination method 116 determination in carbon black 115 determination of free (elemental) 115 furnace tube combustion determination method 113 loadings 307 oxygen flask combustion determination method 113 quantitative analysis 109 sulphide determination 120 sulphite determination 117 120 X-ray fluorescence determination method 114

supercritical fluid extraction process 46

surface area measurements, carbon black 274



surface contamination 315

swollen vulcanized elastomer network observation 238

synthetic rubbers, development 10

systematic errors 326

T T test (for outliers) 337

TEM-based scanning transmission electron microscopy (STEM) 214

temperature selection, for ultramicrotomy 218

test portion definition 25 30 selection 24 size 29

test results 325 deviations 325

thermal energy analyzer (TEA) 77

thermal field flow fractionation (ThFFF) 189

thermionic specific detector see nitrogen phosphorus detector (NPD)

thermogravimetry (TG) for carbon black identification 279 for carbon black isolation 267 derivative see derivative thermogravimetry dry ashing 246

thermoplastics 2

theta temperature 87

thin layer chromatography (TLC) 66 additives identification 69 extending oil determination 69

thiokol rubbers 12

thiourea, identification 72



Tocopherol, identification 72

total sample elemental analysis destructive 256 non-destructive 259

trace metals 252 loss in ashing 245

traceability, to reference material 341

transmission electron microscopy (TEM) for carbon black dispersion 285 latex sample pretreatments 202 for morphological analysis 211 for particle sizing 204 scanning (STEM) 214

trueness (value) 326

U ultracentrifugation, for molar mass determination 192

ultramicrotomy, using cryo-ultramicrotome 215

ultraviolet spectroscopy (UV) 63

uncertainty bands 330

uranyl acetate (stain) 228

V validation

of analytical methods 343 standard reference experiments 344

vapour pressure osmometry 177

viscometry, for molar mass determination 179

vulcanization process definition 3 8



vulcanization process (Continued) degradation procedures 134 in thick articles 28

W waxes, protective 313

wet ashing procedure 247

Wingstay L, identification 72

X X-ray photoelectron spectroscopy (XPS) 259

energy-dispersive 261 262 wavelength-dispersive 260

X-ray scattering, for carbon black study 286

Z zinc dithiocarbamates, cause of blooms 313

zinc salts, cause of blooms 314

analysis of rubber and rubber-like polymers fourth edition m.j.r. loadman kluwer academic copy

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analysis of rubber andrubber

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