special purpose elastomers.pdf

3Special-Purpose Elastomers

Sudhin DattaExxonMobil Chemical Co., Baytown, Texas, U.S.A.

I. INTRODUCTION TO SPECIALTY ELASTOMERS

The worldwide production capacity for all elastomers is around 17,500kilotons per year (kt/yr) and is split between natural rubber and syntheticelastomers (1). Historically, the split has ranged from 40% to 50% naturalrubber, though the level of natural rubber being used has been decreasingdue to the emergence of new solution-polymerized elastomers and special-purpose polymers. Specialty elastomers make up a small but important por-tion of the arena of synthetic elastomers and consist of elastomers that havean annual estimated consumption of about 1000 kt/yr or less. These elasto-mers occupy critical application areas principally because they have one ormore unique attributes. This distinction in the attributes of the specialtyelastomers compared to general-purpose rubbers (GPR) and the structuralfeatures responsible for this distinction are illustrated in Table 1. Morton (2)and Dick (3) compiled excellent reviews of special-purpose elastomers. Thepurpose of this chapter is therefore to provide a brief overview of this im-portant class of polymers and, as appropriate, provide references for furtherreading.

It is important to realize that specialty elastomers have a variety ofcomponents and structures that have been selected to provide key perform-ance attributes. These compositions have been recognized by the ISO 1629nomenclature procedures (Table 2) and have been commonly used todistinguish them. For example, the word ‘‘rubber’’ is inserted after the nameof the monomers from which the rubber was prepared and is applicable to

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Copyright © 2004 by Taylor & Francis

Table 1 Specialty Elastomers—Estimated Volumes in 2001 and Key Attributes

Elastomera Key attribute Structural feature

Estimated

volume (kt/yr)

EPM, EPDM Environmental resistance Saturated backbone 1200Plastomer Environmental resistance;

compatibility with iPPSaturated backbone;higher a-olefin

copolymer

1000

CR Resistance tohydrocarbon solvents

Chlorination ofisoprene rubber

310

CSM Strong resistance to

solvents

Ionic interactions and

chlorinated backbone

280

EAM Resistance to hydrocarbonsolvents; heat resistance

Ethylene backbone withfunctionality

75

HNBR Environmental resistance;oxidation resistance

Saturated backbone;nitrile groups bycopolymerization

45

ACM Oxidation resistance Saturated backbone 8Silicone (Q) Wide temperature service Stable silicon–oxygen

bonds, low Tg for

polymers

42

Polyether (O) Excellent low-temperatureproperties

Ether linkage 11

Fluoroelastomers Chemical resistance Chemically unreactive

carbon–fluorine bonds

8

a EPM = ethylene-propylene copolymer; EPDM = ethylene-propylene-diene terpolymer; CR =

chloroprene rubber; CSM = chlorosulfonated polyethylene; EAM = ethylene-vinyl acetate copolymer;

HNBR = hydrogenated acrylonitrile-butadiene rubber; ACM = acrylic rubber; Q = polysiloxane

rubber; O = oxygenated rubber.

Table 2 Generic Nomenclature of Synthetic Elastomers

M Saturated polymethylene chain

N Nitrogen on the polymer chainO Carbon and oxygen in the polymer chainR Unsaturated carbon chain

Q Silicon and oxygen in the polymer chainT Carbon, oxygen, and sulfur in the polymer chain

(polysulfide elastomers)U Carbon, nitrogen, and oxygen in polymer

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elastomers that contain high levels of unsaturation. Polybutadiene is thusdenoted as BR and styrene-butadiene copolymer as SBR. Elastomers thathave substituted carboxylic acid groups (UCOOH) on the polymer chainbegin with an X. For example, carboxylated chloroprene rubber would bedenoted XCR. Rubbers containing a halogen begin with the elementemployed. Bromobutyl rubber is therefore denoted as BIIR. Chlorobutylrubber is described as CIIR (4). Table 3 presents the nomenclature for over 20commercial elastomers (5).

Table 3 International Abbreviations for Elastomers

ACM Copolymer of ethyl acrylate or other acrylate plus a lowlevel of other unsaturated monomer for vulcanization

AU Polyester polyurethane

BR Butadiene rubberBIIR Bromo-isobutene-isoprene (bromobutyl) rubberCFM Polychlorotrifluoroethylene

CIIR Chloro-isobutene-isoprene (chlorobutyl) rubberCM ChloropolyethyleneCR Chloroprene rubberCSM Chlorosulfonyl polyethylene

EAM Ethylene-vinyl acetate copolymerEPDM Terpolymer of ethylene, propylene, and a diene with

unsaturation to facilitate vulcanization

EPM Ethylene-propylene copolymerEU Polyether polyurethaneHNBR Hydrogenated acrylonitrile-butadiene rubber

IIR Isobutene-isoprene rubber (butyl rubber)IM IsobuteneIR Polyisoprene (synthetic)

MQ Silicone rubber with methyl groups on the polymer chain(e.g., dimethyl polysiloxane)

NBR Acrylonitrile-butadiene rubberE-SBR Emulsion styrene-butadiene rubber

S-SBR Solution styrene-butadiene rubberXNBR Carboxylated acrylonitrile-butadiene rubberXSBR Carboxylated styrene-butadiene rubber

YAU Thermoplastic polyester polyurethaneYEU Thermoplastic polyether polyurethaneYSBR Block copolymer of styrene and butadiene

YSIR Block copolymer of styrene and isoprene

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II. THE NEED FOR SPECIALTY SYNTHETIC ELASTOMERS

A wide variety of specialty synthetic elastomers have been developed toovercome some of the performance deficiencies of natural rubber (NR) andthe larger volume general-purpose rubbers (GPRs) such as styrene-butadienerubber (SBR) and butadiene rubber (BR). Some of these deficiencies are

1. Poor resistance to light, oxygen, and ozone weathering2. Relatively poor heat resistance3. Poor resistance to organic fluids

A more recent innovation is the replacement of larger volume GPRs byspecialty synthetic elastomers to exactly match the performance of theelastomer to the intended use. This is mostly due to the ease of manipulationof the structure of the specialty elastomers in low-volume applications,because corresponding changes for GPR and NR on a small scale iseconomically unattractive.

Specialty elastomers are valued for particular properties or combina-tions of properties typically unavailable in the large-volume elastomers. Thispremiumon property enhancement affords the polymer designer considerablelatitude in the selection of monomer(s) and polymer chain architecture as wellas the process to be used for the synthesis. Thus specialty elastomers typicallyconsist of more than one monomer in an effort to deliver a combination ofproperties not available from a single monomer. A notable example of thistrend is the development of EPMelastomers, which contain both ethylene andpropylene as the principal monomers. In these copolymers the absence ofpropylene would lead to crystalline polyethylene whereas the absence ofethylene would lead to thermally unstable polypropylene.

There are six distinct and important tools employed in matching thestructure of the synthetic specialty elastomer to its intended use:

1. The composition of the elastomer2. The microstructure and orientation of the monomers3. The use of a combination of monomers4. Segregation of the different monomers into portions of a single

chain (block copolymers)5. The architecture of the elastomer as defined by distribution of

composition and molecular weight6. The use of long-chain branching to improve the fabrication and

processing of elastomers

The use of the composition of the synthetic specialty elastomer as a toolin matching the properties of the polymer is shown in Fig. 1. Two of the mostimportant properties of elastomers are the ability to withstand weathering

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due to oxygen, ozone, and light (as shown by the continuous-use temperature)and the ability to withstand organic fluids. Figure 1 shows graphically thetemperature resistance of a variety of synthetic elastomers. Elastomers with asaturated backbone are more resistant to weathering than those with unsat-urated backbones. Thus EPM is significantly better than BR or NR. Theultimate weathering is for elastomers where the CUC backbone is replacedwith the SiUOUSi backbone of the Q elastomers. Figure 1 also shows theordering of these elastomers with respect to resistance to organic solvents.Elastomers with strong polarity are more resistant to polar organic fluids andoils than those that are composed entirely of hydrocarbons. Thus nitrile(NBR), acrylic rubber (ACM), and fluorinated rubber (FKM) are moresolvent-resistant than SBR, EPDM, or butyl rubber (IIR). Chlorinatedpolymers, such as polychloroprene (CR), which are intermediate in polarity,are intermediate between these extremes.

The microstructure and the orientation of the monomer units in thesynthetic elastomer can be altered by the choice of reaction conditions andcatalysts.Natural rubber (NR) exists as a single, naturally derived isomer. It isimpossible to manipulate the properties of NR by changing the stereochem-istry. cis-1,4-Isoprene rubber (IR) is the synthetic analog of NR, whereas itsisomeric form trans-1,4 IR is a tough, semicrystalline polymer. IR polymerscontaining intermediate amounts of these isomers display intermediate

Figure 1 Graphical correlation of solvent and temperature resistance of elas-

tomers. For abbreviations see text.

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properties. A similar example is the difference between isotactic and atacticpolypropylene. These isomers differ in the stereochemical orientation of themethyl group on the propylene monomer. The former is a thermoplastic,whereas the latter can be used as an elastomer. In general, the ability of apolymer to exhibit elastomeric properties depends on the flexibility of rotationaround its backbone. Rigid stereochemical isomers often crystallize inordered crystals, which prevents these polymers from exhibiting elastomericproperties.

A number of synthetic specialty elastomers contain two or more mono-mers. EPM,which contains ethylene and propylene, and SBR, which containsstyrene and butadiene, are examples. The mixture of monomers is used eitherto lower the (Tg) (for SBR) or the crystallinity (for EPM). In addition, thepresence of the two monomers allows a tailoring of the properties of the syn-thetic elastomer that cannot be attempted with homopolymers such as NR.Normally the two monomers are randomly mixed in the copolymer. Theresulting copolymer, which contains a mixture of the two monomers, has acompositional microstructure determined by the reactivity ratio. The com-positional microstructure is the number ofmonomer units in runs of one, two,three, and more. The reactivity ratio (which could be defined as the relativetendency of the monomers to cross-react) for a pair of monomers A and B isindicated by rAB and is the product (RAARBA/RABRBB), where RIJ is thekinetic rate constant for the insertion of the Jmonomer into a growing poly-merization chain terminated by the Imonomer. The reactivity ratio is largelydetermined by the structure of the polymerization catalyst and the monomersas well as by the physical conditions of the polymerization such as temper-ature and concentrations. Changes in the reactivity ratio rarely lead to runlengths of any monomer greater than 7–10 units in length for an equimolarcopolymer. A more direct way to control the polymerization sequence or therun length of a particular monomer leading to single-monomer run lengths ofseveral to several hundred is the practice of sequential addition of eithermonomers or prepolymers made from a single monomer. This geometricalisomer of the copolymer, which allows the segregation of monomers intolarge macroscopic sections of the polymer chain, is designated a blockpolymer (6).

These sections of the same monomeric composition may be small por-tions of the chain several monomers in length as in thermoplastic polyure-thane elastomers (TPU-E) or sections that are several hundred monomerunits long as in SBS. These segregated copolymers show unexpected elasto-meric properties such as extreme elongation (f1000%) and excellent tensilestrength even when unvulcanized. An example of this is the difference in themechanical properties of SBS, a block copolymer, and SBR, the corre-sponding random copolymer, which is composed of the same monomer units

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in essentially the same ratio. This segregation in structure is possible becauseof the corresponding segregated polymerization process during which onemonomer is added in preference to the other. These architectural details of theelastomers are important in fulfilling the needs of particular applications andcannot be achieved for the larger volume rubbers, for which neither theselection of the monomer nor the polymerization condition can be altered.

The use of copolymers in specialty elastomers often leads to additionalstructural features such as intermolecular compositional differences byinducing differences in the comonomer distribution within the copolymer.These compositional differences typically lead to improvements in the me-chanical properties of the vulcanized copolymers beyond those that are acces-sible for the average composition in the absence of compositional differences.

Copolymerization of two or more monomers can lead to severalgeometric isomers of the same polymer. The practical specialty polymersare variations of three limiting cases:

1. Alternating Polymerization. The two types of monomer unitsalternate in the polymer chain until the monomer in the minor con-centration is exhausted.

2. Statistical Polymerization. The two monomers enter into a poly-mer chain in a statistically random manner, with their averageconcentration in the chain corresponding to their feed ratio.

3. Block Polymerization. Complete polymerization of one mono-mer occurs prior to polymerization of the second monomer.

III. VULCANIZATION OF SPECIALTY ELASTOMERS

Elastomers are vulcanized after compounding with fillers, plasticizers, cura-tives, accelerators, and minor amounts of antioxidants. The vulcanizationreaction is conducted by heating the formed or extruded rubber part to atemperature such that the cross-linking reaction is initiated. Most syntheticelastomers have specific curing sites, formed by the introduction of a specialmonomer, where the cross-linking occurs. These special monomers includeisoprene for isobutylene-isoprene rubber (IIR), halogenation of IIR to formCIIR or BIIR, diene for EPDM, and alkenoic acids for ethylene acrylaterubber (EAM). Because the number of cross-linking sites is limited comparedto GPR, the conditions and chemistry for cross-linking are typically moresevere for synthetic elastomers. The reaction conditions are maintained untilthe cross-link density is significant before the temperature is lowered. Presscure, transfer molding, steam cure, hot air cure, and injection molding areacceptable methods of curing.

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IV. SYNTHESIS AND MANUFACTURING PROCESS

All of the commonly known polymerization processes are used for thesynthesis of specialty elastomers. These include both the chain growth processand condensation polymerization. The chain growth process can be propa-gated by 1) free radicals, 2) ionic (cationic or anionic) polymerization, and/or3) coordination or metal complex polymerization.

In free radical polymerization, heat, light, or electrochemical reactionsproduce free radical generators (initiators) from molecules added to themonomer for this purpose. The best chemical initiators are formed by thedecomposition of peroxides, hydroperoxides, or azo compounds. The mostfrequently used electrochemical initiators are redox systems, where thereaction of reducing and oxidizing agents forms free radicals. After initiationof the polymerization, the propagation step occurs, during which the polymerchain grows through stepwise addition of monomer molecules until the chainis terminated. The growing polymer chain can react with other molecules(chain transfer agents) that are present to start new chains. The polymerradical becomes deactivated, and a chain transfer radical forms that, in turn,starts a new monomer radical and thus a new polymer chain. Alcohols, alkylhalides, mercaptans, and xanthogen disulfide are examples of practical chainmodifiers.

Anions or cations, as opposed to free radicals, are used as chaininitiators in ionic polymerization. In ionic polymerization there is a transferof charge from the initiator ion to the monomer. Therefore, positively ornegatively charged initiator ions result in cationic or anionic polymerization,respectively. Brønsted or Lewis acids can initiate a cationic polymerization.For anionic polymerizations, initiators such as alkyllithium compounds areused. The activation energies for the initiation and polymerization reactionsare often small, and ionic polymerizations are often conducted at low tem-peratures. The extent of formation of the dissociated initiator, and thus itsefficiency, is dictated by the dielectric constant of the solvent. Chain transferor termination is the principal feature of ionic polymerization, which limitsthe molecular weight of the elastomer. However, at low polymerizationtemperatures for cationic polymerization or low temperature ambient con-ditions for anionic polymerization, these chain transfer or terminationprocesses do not occur. Thus, very narrow molecular weight distributionscan be obtained.

A particular case of coordination polymerization is commonly knownas anionic polymerization. This is used mostly for the polymerization ofconjugated dienes and styrene. These polymerizations are initiated by acarbon-centered organic anion of an alkali metal. Polymerization proceedsby stepwise addition of the diene to the anion, which always leaves the

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growing terminus as an anion coordinated to the alkali metal cation. Homo-polymers and copolymers of styrene and 1,3-dienes are made by this pro-cedure. A recent innovation is living anionic polymerization. Under carefullyselected polymerization conditions where the initiation of polymerizationis instantaneous and all termination pathways are suppressed, the polymer-ization leads to intermolecular uniform polymers similar in both composi-tion andmolecular weight (MWD=1).However,modifying the compositionof the monomers during polymerization can change the intramolecular com-position. These techniques lead to formation of styrene-butadiene blockpolymers.

In coordination polymerization reactions, the initiator exists as a metal-centered complex attached to the polymer chain and the polymerizationprogresses by insertion of new monomer molecules into this complex. Themost common initiators for these insertions are the Ziegler–Natta catalysts.These are reaction products of aluminum alkyl halides with transition metalsalts of group IV–VIII elements. Because a monomer unit enters the polymerchain in a sterically controlled coordination, sterically and geometricallyregular polymers are obtained with coordination catalysts. Thus in thepolymerization of butadiene, pure cis-1,4, trans-1,4, or trans-1,2 microstruc-tures can be made.

In contrast to chain growth polymerization, in which each chain growsfrom a single insertion point where fresh monomer is introduced and thenincorporated into the chain, in step growth polymerization individual mono-mer molecules are incorporated into small prepolymer units that are dormantfor a while and then assemble into high molecular weight polymers. Thusinsertion in step growth processes may occur at hundreds of points for anysingle polymer chain. Several intermediate molecular weight polymers oftenexists simultaneously, and the average molecular weight increases continu-ously during polymerization. Polysulfide rubber (TM), polyurethane rubber(AU), and polysiloxane rubber (Q) are examples of polymers obtained bycondensation polymerization. In all of these polymerizations, multifunctionalcompounds react to form elastomers. If bifunctional compounds are usedexclusively, linear molecules result. The presence of at least one trifunctionalcomponent per macromolecule yields branched or cross-linked elastomers.

There are several processes for the production of specialty elastomers:

1. Emulsion2. Bulk3. Solution4. Suspension

Emulsion polymerization is primarily used in free radical chain poly-merization. At least four components are required here—water-insoluble

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monomers, water, emulsifiers, and water-soluble initiators—in addition tochain modifiers and polymerization stoppers. The emulsifier causes themonomer to be emulsified into small droplets, which aggregate into micelles.Initiator radicals react with the monomer in the micelles. The micelles growlarger by absorbing new monomer from the surroundings to form latex.Emulsion polymerization leads to very high conversion of monomer becausethe termination reactions are avoided. In comparison with homogeneouspolymerization processes, in emulsion polymerization, 1) water is efficient atdissipating the heat of polymerization; 2) viscosity is independent of themolecular weight of the polymer, leading to elastomers with very highmolecular weight; and 3) polymerization can be conducted in simple reactionvessels. Although originally developed for the use of peroxide initiators(reaction temperature >100jC) to form ‘‘hot polymers,’’ the use of redoxinitiators at low polymerization temperatures (near room temperature) hasled to more uniform and thus more valuable ‘‘cold polymers.’’

In bulk polymerization the polymerization takes place with the pureliquid monomer as reactionmedium. The heat of polymerization is dissipatedthrough external or evaporative cooling. The first synthetic BR elastomerswere made by bulk polymerization.

In solution polymerization the monomer is dissolved in an organicsolvent. Heat of polymerization is removed by refrigerating the solvent. Afterinitiation of the polymerization reaction, the viscosity of the reactionmediumrises with the degree of polymerization, and this limits the extent of chaingrowth.

Suspension polymerization is carried out using a liquid monomer as thesolvent. However, in contrast to bulk polymerization, the resulting polymer isinsoluble in the monomer and precipitates as a suspension in the reactionmedium. In suspension polymerization the viscosity of the reaction mediumincreases only slightly and thus very high molecular weight elastomers can bemade. For instance, suspension polymerization processes are of importancefor the production of ultrahigh molecular weight EPDM for which ethylene,the diene monomer, and polymerization aids are dissolved in the liquefiedpropylene and then polymerized.

The recent increase in the amounts of synthetic elastomers made insolution reflects some inherent advantages. In general, compared to othersynthetic processes solution polymerization leads to

1. Greater control of both intermolecular and intramolecular com-position

2. Attenuation of competing intermolecular reactions, e.g., branch-ing

3. Easier removal of catalyst residues

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4. Easier diffusion of monomers for elastomers with high meltingpoints (Tm) such as those that are semicrystalline, high glass tran-sition temperatures (Tg), or very high bulk viscosities

5. Easier removal of the heat of polymerization

Although directed polymerization is the synthesis method of choice fora majority of specialty elastomers, a significant number of these elastomersare made by the postpolymerization chemical modification of existingpolymers. The value of these polymers is sufficient to justify the additionalcost of a second chemical transformation step in their synthesis. Thechlorination of polyethylene and the bromination of IIR are notable exam-ples. These chemical modification reactions are conducted almost exclusivelyin solution. Solution processes are favored over the cheaper slurry oremulsion polymerization processes because they afford better control overthe extent and speed of the chemical modification reactions when all of thereactant is in solution.

V. SOLVENT-RESISTANT SPECIALTY ELASTOMERS

Solvents encompass a broad class of apolar organic fluids. This includesparaffins, aromatics, cycloalkanes, olefins, chlorinated hydrocarbons, andfatty acids and their esters but not small polar molecules such as methanol.Vulcanizates of hydrocarbon elastomers easily swell in these solvents and losemost of their tensile strength, elongation, and resistance to set or abrasion.Thus specialty elastomers that resist hydrocarbon solvents are an importantcommercial development. The class of solvent-resistant elastomers incorpo-rates large amounts of strongly polarizing groups such as esters, nitriles, andhalogens to raise the solubility parameter of the elastomer such that it is nolonger miscible with the solvents.

The polarity of acrylonitrile makes NBR resistant to common hydro-carbon solvents (Fig. 2). Increasing the acrylonitrile concentration of NBRimproves its resistance to swelling in oil (Fig. 3). However the trade-off isdeterioration in crack resistance, elasticity, and low-temperature properties.Figure 4 illustrates the effect of acrylonitrile on the brittle temperature of theelastomer (7). However, NBR is an unsaturated elastomer and is easilydegraded by weathering. Most of these shortcomings can be alleviated byhydrogenating the double bonds to form hydrogenated NBR (HNBR).

Table 4 displays a generic compound formulation suitable to serve as aninnertube of a high-pressure hydraulic hose. Nitrile polymers are typicallycompounded with low-structure, large particle size carbon blacks such as

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N660 or N762. In addition, a plasticizer is added along with stearic acid andzinc oxide for the cure system.

Acrylic elastomers (ACM) are copolymers of various alkyl acrylatesdiffering in the nature of their alkyl group. They contain the strongly polarester group. Acrylate elastomers are saturated. Chloroprene rubber (CR) isprepared by emulsion polymerization of chloroprene. It resists hydrocarbonsolvents and shows improved flame resistance due to the incorporatedchlorine. In addition, it is also more resistant to oxidation than NR owingto the electron-withdrawing effect of the chlorine groups, which makes theunsaturation less subject to ambient oxidation. Saturated chlorinated syn-thetic elastomers such as chlorinated polyethylene (CPE) are made byreacting polyethylene with chlorine. Chlorosulfonated polyethylene (CSM)is made similarly by reacting polyethylene with both chlorine and sulfurdioxide.

A. NBR Derivatives

The most important specialty solvent-resistant elastomer is HNBR made bythe almost complete catalytic hydrogenation of the unsaturation derived frombutadiene moieties. Nippon Zeon makes HNBR almost exclusively. The

Figure 2 NBR nitrile (acrylonitrile-butadiene copolymer).

Figure 3 NBR acryonitrile content and oil swell (ASTM #3).

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small amount of residual unsaturation is used for vulcanization. This resultsin a product with much improved resistance to oxidation and weathering butwith little or no sacrifice of solvent resistance or elastomer performance. As inNBR, HNBR is also available with different ACN contents. TheHNBR has ahigher glass transition temperature though it has significantly enhancedsolvent resistance. Applications for HNBR take advantage of the excellent

Figure 4 NBR acrylonitrile content and brittle point.

Table 4 Generic NBR/SBR Hydraulic Hose Tube

Formula (phr)

NBR (40% acrylonitrile) 80E-SBR (1500) 20N660 110

Stearic acid 1Zinc oxide 5Dioctyl phthalate 16

Sulfur 2TBBS 1TMTD 0.1

Typical property targets

Tensile strength (MPa) 17.00Ultimate elongation (%) 170.00Volume swell (%) 25

(ASTM #3 oil, aged 3 days at 70jC)

Source: Ref. 6.

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resistance to organic fluids. However, the increased high-temperature per-formance of the HNBRmakes it far superior to NBR in critical applications.

Chemically modified NBR containing pendant carboxylic acids is madeby copolymerizing methacrylic or acrylic acid with butadiene and acryloni-trile (XNBR). These polymers, which contain 2–6 wt% acidmonomer, can bevulcanized with polyvalentmetal ions, which results in a large improvement inabrasion resistance. This acid functionality leads to excellent polymer–fillerinteraction in compounds. XNBR polymers require care when mixing, andtemperatures typically should not exceed 135jC. The compounds tend to behard, display good abrasion resistance, and have high tensile and tear strengthcompared to conventional nitrile compounds.

Nitrile polymers are often blended with other elastomers to 1) improveozone resistance, 2) lower temperature properties, 3) improve aging resist-ance, and 4) improve abrasion resistance. Table 5 is a simple illustration onhow blends with other elastomers will impact nitrile compound properties.

B. Polychloroprene

Polychloroprene (CR) was the first commercially developed synthetic elasto-mer. The current worldwide capacity of CR is 310 kt. DuPont Dow has 49%of the capacity with facilities in the United States, United Kingdom, andJapan. There are five other producers of CR. Commercial CR polymerizationis typically conducted in aqueous emulsion (Fig. 5). Historically it wasproduced first by dimerization of acetylene, though in more recent years thechloroprene monomer has been produced from the halogenation of butadi-ene. Molecular weight is controlled with chain transfer agents. Mercaptanchain transfer agents are used, but other materials such as xanthogendisulfides have also been employed. The latter provide reactive end groups.

Table 5 Blending of Other Polymers with NBR

Polymer blend Effect Comment

SBR Reduces cost Noncompatible due to differencesin solubility parameters

BR Improves low-temperatureproperties

Improves flex resistance

EPDM Improves ozone resistance 70/30 NBR/EPDM blends

optimumCR Improves flex properties Best at low NBR AN contentResins Increase hardness Improve abrasion

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Molecular weight is also controlled by copolymerization with sulfur. Thiscopolymer contains polysulfide units that are then cleaved chemically to lowerthe molecular weight. The predominant microstructure of CR is the head-to-tail trans-1,4-CR, although other structural units are also present. The highconcentration of this repeat unit is responsible for the crystallinity of CR andfor the ability of the material to crystallize under strain. Other insertionpathways such as 3,4 insertion in minor amounts lead to easily vulcanizedsites on the polymer. The presence of these structural irregularities does leadto lower crystallinity and tensile strength. The total amount of insertionmodes other than trans-1,4 increases with polymerization temperature from5%at�40jC to about 30% at 100jC. CRmade with high trans-1,4 content istough and is used for adhesives. CR made at higher temperature, with morechain irregularities, tends to crystallizemuchmore slowly and is more suitablefor elastomer uses (Fig. 5).

Polychloroprene polymers are often differentiated by the use of anappended letter. These designations do not reflect the composition of thepolymer but are derived from the manufacturing and process conditions.Ultimately, the process conditions affect the microstructure—i.e., branching,molecular weight distribution, and isomer distribution—and thus the com-pounding and ultimate physical properties of the polymer. The G family ofDuPont polymers, containing residual thiuram disulfide, can be cured withmetallic oxides. The W, T, and xanthate-modified families require the add-ition of an organic accelerator, often in combination with a cure retarder, forpractical cures. G, W, and T are trade designations.

Polychloroprene rubber has been successful because it combines envi-ronmental resistance and toughness, especially in dynamic applications

Figure 5 Chloroprene production from dimerization of acetylene and polymer-ization. (From Ref. 7.)

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involving heat buildup and resistance to flex cracking. The high strength ofCR vulcanizates is a result of the tendency of CR to crystallize under stress.The rate of crystallization and the melting point of CR both increase withincreasing trans-1,4 microstructure. The resistance of CR to aging is higherthan that of other diene elastomers because of the presence of an electroneg-ative chlorine atom on the repeat unit double bond. Polyester plasticizers aremore effective than hydrocarbon oils in reducing the Tg of vulcanizates.However, the plasticizers are also effective in promoting crystallization. Formany applications, where cost and processibility are the objective, naphthenicand aromatic oils are used. CR has a well-balanced combination of propertiesincluding processibility, strength, flex and tear resistance, flame resistance,and adhesion, together with sufficient heat, weather, and ozone resistance formost applications. Typical applications involve power transmission andtiming belts, automotive boots (rubber piece around the constant-velocityjoint in the front wheels of cars), airsprings, and truck engine mounts. A largeamount of CR is used in adhesives. CR tends to have high uncured strengthdue to strain-induced polymer crystallization. CR latex and solvent adhesiveshave been used in foil laminating adhesives, facing adhesives, and construc-tion mastics. CR latex can be used in a variety of special applications to makebinders, coatings, dipped goods, elasticizers, and foam where either dry orsolvent-based processes would be impractical. The water-based systems arenow preferred because they have high solids content and minimal solventemissions but are unaffected by polymer rheology and have extremely smallparticle size.

Over the years, almost every vinyl and diene monomer has been testedwith CR in free radical polymerization. Copolymers usually contain only alimited amount of random comonomer. Methacrylic acid, for example,promotes adhesion to substrates and increases cohesive strength. A largenumber of graft polymers of CR have been described, but the only ones ofcommercial significance are those made with acrylates and methacrylate.These are particularly useful for adhesion to elasticized poly(vinyl chloride)(PVC). The graft polymers may be made by either solution or emulsionpolymerization. For a good review of the compounding of polychloropreneelastomers, further reference should be made to Colbert (8) and Dunn andVara (9) Table 6 displays a simplified hydraulic hose cover compoundformulation that might serve as a starting point in a new compounddevelopment study.

C. Acrylic Elastomers

Acrylic (ACM) elastomers are a limited volume specialty elastomer with aworldwide capacity of about 8 ktons. This volume is included in the

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worldwide capacity for the more common ethylene-vinyl acetate copolymer(EAM). ACM manufacture is distributed among the United States, westernEurope, and Japan. Its production is intimately associated with automotiveproduction. The major producers for ACM include Enichem Elastomers andZeon Chemical, with approximately equal market shares. Ricon Resinsproduces a small amount of lowmolecular weight ACMpolymer for adhesiveapplications.

Acrylic elastomers are produced by free radical polymerization usingprincipally aqueous suspension and emulsion polymerization. ACM elasto-mers consist of a majority (f97–99 wt%) of ethyl acrylate, butyl acrylate, and2-methoxy ethyl acrylate monomers. In addition, a small amount of cure sitemonomers is added to facilitate vulcanization. The acrylic esters constitutingthe majority of the polymer chain determine the physical, chemical, andmechanical properties of the polymer and its vulcanizates. Cure site mono-mers have an acrylate double bond for polymerization into the ACM and areactive group for the vulcanization process. Two of the most importantclasses of cure site monomers are reactive chlorine-containing monomers andepoxy/carboxyl-containing monomers.

Acrylic elastomer has both a saturated backbone, which is responsiblefor the heat and oxidation resistance, and ester side groups, which contributeto the marked polarity. The cure sites present in the ACM also affect theexpected properties. Reactive chlorine cure sites generally give excellent heataging as shown by retention of elongation. The alkali metal carboxylate-

Table 6 Generic Hydraulic HoseCover Compound

Formula (phr)Polychloroprene 100N762 100Aromatic oil 20

MgO 4Antiozonant 2Wax 3

DPG 1TMTD 1Sulfur 0.75

ZnO 5Typical target propertiesTensile strength (MPa) 14

Ultimate elongation (%) 300

Source: Ref. 9.

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sulfur cure system has been widely used for this type of curemonomer since itsintroduction in the early 1960s. Epoxy/carboxyl cure monomers give goodcompression set and good resistance to hydrolysis. New vulcanizationsystems based on quaternary ammonium salts have been introduced tovulcanize epoxy/carboxyl cure sites. These have been found to be effectivealso in chlorine-containing ACM.

Its resistance to aging at high temperatures and insensitivity to organicfluids makes ACM useful in automotive underhood parts. These include lipand shaft seals, O-rings, oil pan and cover valve gaskets, and hoses.

D. Chlorosulfonated Polyethylene

The worldwide production capacity of chlorosulfonated polyethylene (CSM)(Fig. 6). elastomers is about 55 kt. DuPont Dow is the principal manufac-turer, with over 90% of the market share and with plants in the United Statesand Northern Ireland. Toyo Soda in Japan provides the balance of theproduct.

Chlorosulfonated polyethylene is notmade from componentmonomersbut from the chemical modification of polyethylene. The chlorination andchlorosulfonation of polyethylene are carried out simultaneously or sequen-tially using carbon tetrachloride as the reaction solvent. The elastomericcharacter of CSMarises from the inherent flexibility of the polyethylene chaindue to reduced crystallinity. Substitution of chlorine in the polymer chainprovides sufficient molecular irregularity to reduce crystallinity. The sulfonylchloride groups provide cross-linking sites for non-peroxide vulcanization.CSMproperties can vary fromelastomeric to plastic depending on the amountof chlorine and sulfonyl chloride substitution.

The presence of chlorine and sulfonyl chloride in the chain requiresweak bases in CSM compounds to react with acidic by-products. Acceptablebases include magnesia, litharge, organically bound lead oxide, calciumhydroxide, synthetic hydrotalcite, and epoxy resins. The presence of sulfonylchloride groups contributes to adhesion and mechanical reinforcement byreaction with fillers. In addition, the presence of chlorine imparts significantflame and light resistance to CSM compounds. However, all of these pro-perties depend on the extent and distribution of the chlorination and chlo-

Figure 6 Simplified chemical structure of CSM.

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rosulfonation. CSM polymers can be easily functionalized by replacement ofthe halogen either on the backbone or on the sulfonyl group. Acid, ester, andamide derivatives of the sulfonyl chloride groups have been prepared.

The sulfonyl chloride group is the cure site for CSM and determines therate and state of cure. Carbon black fillers give the best reinforcement as wellas significant resistance to photochemical degradation. Ester plasticizersprovide the best combination of low-temperature flexibility, heat resistance,and mechanical properties.

Chlorosulfonated polyethylene elastomers are used in specialty end-useapplications such as coating, adhesives, roofing membranes, pond andreservoir liners, electrical wiring, insulation, automotive and industrial hoses,tubing and belts, and molded goods. A typical starting formulation forcompound development is compiled in Table 7 for reference purposes.

E. Ethylene-Acrylic Elastomers

The worldwide capacity for acrylate rubbers is about 75 kt. E.I. DuPont is thesole producer of ethylene-acrylic (EAM) polymers. EAM polymers are madeby the free radical polymerization of ethylene, methyl acrylate, and analkenoic acid. A small amount (1–5mol%) of an alkenoic acid is incorporatedto provide sites for cross-linking with diamines. Copolymers of ethylene andmethyl acrylate have recently been commercialized.

Ethylene-acrylic elastomer is an amorphous polymer due to randomplacement of the acrylate comonomer along the ethylene backbone. Thepolymer is saturated, making it highly resistant to aging and weathering evenin the absence of antioxidants. In addition, the methyl acrylate-to-ethyleneratio determines both the low-temperature properties and the resistance toorganic fluids. EAM elastomers have aging, heat, and fluid resistance in

Table 7 Model Formula for a CSM-Based Compound Development Study

Material Loading (phr) Purpose

CSM 100 PolymerMgO 4 Acid acceptorPentaerythritol 3 Activator for acid acceptor

Clay 80 Filler (carbon black preferred)Oil 25 PlasticizerWaxes 3 Processing aidSulfur 1 Vulcanizing agent

TMTD, TMTS 2 Accelerator

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addition to acceptable elastomeric properties. In particular, they show largelytemperature-stable vibrational damping properties and the ability to formflame-resistant elastomeric compounds with combustion products that havelow toxicity and corrosiveness. Figure 7 illustrates some of themonomers thatcan potentially be used to produce such polymers.

VI. TEMPERATURE-RESISTANT ELASTOMERS

Temperature-resistant elastomers can be classified into two groups: 1) thosethat are suitable for automotive underhood applications where only transienttemperatures are greater than 100jC and 2) those that are suitable for muchhigher temperature use such as for gasket sealant for engines. Polyolefinelastomers with saturated backbones are the material of choice for the firstkind of application and have gradually supplanted other unsaturated elas-tomers in spite of the difficulty in achieving physical targets with theseelastomers. In the truly temperature-resistant elastomers of the second kind,the hydrocarbon structure has been replaced with more refractory buildingblocks. The most common of these are silicone rubbers (Q). These are basedon chains of SiUOUSi rather than CUC units and owe their temperatureresistance to their unique structure. Silicone rubbers are almost exclusivelypolydimethylsiloxanes. The other heat-resistant elastomer, fluorocarbonelastomer (FKM), is derived from perfluorinated versions of common poly-olefins such as polypropylene and polyethylene. Its thermal resistance arisesfrom the nonreactivity of the carbon–fluorine bond. Elastomers with a CUCskeleton that has a saturated backbone chains but retains a small amount ofpendant unsaturation (for vulcanization) resist environmental aging betterthan diene elastomers. Degradation by oxygen, ozone, and light is slow on thesaturated backbone and causes only infrequent chain cleavage.

Figure 7 Monomers for acrylic-based elastomers.

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A. Ethylene-Propylene Copolymers and Ethylene-Propylene-Diene Terpolymers

The worldwide production of ethylene-propylene copolymers (EPM) andethylene-propylene-diene terpolymers (EPDM) is approximately 1150 kt. Inthe United States the producers are Uniroyal (11% of worldwide capacity),Exxon Chemical (11%), DuPont/Dow Elastomers (12%), and Copolymer/DSM (9%). Western European producers are Exxon Chemical (10%), DSM(7%), and Degussa-Huls (5%). In Japan the producers are Mitsui Petro-chemical (8%), Japan Synthetic Rubber (7%), DSM Idemitsu (6%), andSumitomo Chemical (5%). There are additional smaller capacities in Brazil(2%),Korea (2%), and India (2%). In addition to these solution plants, Bayer(7%) and EniChem (10%) have suspension plants in the United States andEurope, respectively. Additional U.S. capacity (12% of worldwide capacity)has been announced by Dow (formerly Union Carbide) using a novel gas-phase process.

Even though EPM and EPDM elastomers have been available for morethan 30 years, the technology for these products, both their production andtheir application, is still under development. Themost widely used process is asolution polymerization in which the polymer is produced in a hydrocarbonsolvent. EPM and EPDM rubbers are produced in continuous processes.Slurry polymerization is conducted in liquid propylene. Special reactordesigns withmultiple feeding locations to achieve special molecular structureshave been developed. Gas-phase polymerization of EPDM is possible as anextension of the ubiquitous gas-phase processes for polyethylene and poly-propylene.

As manufactured today, EPM and EPDM are polymers based on theearly work of Natta and coworkers. Generically an EPM is 60 mol% ethyleneand 40 mol% propylene. Analogous EPDM polymer contains in addition1.5 mol% nonconjugated diene such as ethylidene norbornene or dicyclo-pentadiene (Fig. 8). The comonomers are statistically distributed along themolecular chain. EPM is a saturated synthetic elastomer because it does notcontain any unsaturation. It is inherently resistant to degradation by heat,light, oxygen, and, in particular, ozone. EPDM, which contains pendantunsaturation, is only slightly less stable to aging than EPM. The properties ofEPMcopolymers are dependent on the relative content of ethylene units in thecopolymer chain and the variation in the comonomer composition of differentchains. EPM and EPDM polymers with greater than 60 mol% ethylene areincreasingly crystalline and tough.

Ethylene-propylene copolymer can be vulcanized with peroxides. Thesmall amount of a third diene monomer in EPDM permits conventionalvulcanization with sulfur and other vulcanization systems such as resins at the

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pendant sites of unsaturation. EPM and EPDM elastomers have to becompounded with reinforcing fillers, e.g., carbon black, if enhanced mechan-ical properties are required. Paraffinic oils are widely used as plasticizers. InEPM and EPDM compounds, mechanical properties depend on the compo-sition of the elastomers and the types and amounts of fillers. In general, theelastic properties are better than those of many other synthetic rubbers butnot as good as those of NR or SBR. The resistance of EPM and EPDM toheat and aging is much better than that of SBR and NR. EPM and EPDMvulcanizates have excellent resistance to inorganic or highly polar fluids suchas dilute acids, alkalise, and alcohol. However, the resistance to aliphatic,aromatic, or chlorinated hydrocarbons is very poor. The electrical insulatingand dielectric properties of the pure EPM and EPDM are very good, but incompounds they are also strongly dependent on the choice of compoundingingredients.

Among the synthetic elastomers, EPM and EPDM have the fastestgrowing markets, owing to their excellent ozone resistance in comparison tothe diene elastomers. This growth still comes from replacement of thesecommodity rubbers by virtue of their better ozone and thermal resistance.Another facet of the growth is that EPDM rubber can be extended with fillersand plasticizers to an extremely high level in comparison with the otherelastomers and still maintain excellent processibility and properties in end-usearticles. The main uses of EPM or EPDM are in 1) automotive applicationssuch as profiles, hoses, and seals (41% of worldwide consumption); 2)building and construction as profiles, roofing membranes, and seals (21%);

Figure 8 EPDM termonomers.

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3) cable insulation and jacketing (6%); and 4) molded appliance parts. Animportant application for EPDM is in blends with general-purpose rubbers.Considerable amounts of EPM and EPDM are also used in blends withthermoplastics (28%), and substantial amounts of EPM are used as additivesto lubrication oils (12%) because of its excellent heat and shear stability underthe operating conditions of automobile engines. Table 8 illustrates the rangeof EPDMpolymers available commercially from ExxonMobil (10). Polymersvary in termonomer content,Mooney viscosity, which is essentially a functionof molecular weight; and ethylene content. Two model compound formula-tions are displayed in Table 9, which can be used as a reference for building anew compound formulation.

B. Silicone Rubber

The majority of silicone rubber (Q) elastomers (>60%) are made by GeneralElectric. Laur and Dow Corning have about 10% of the remaining capacity,while smaller manufacturers (Wacker, Bayer, and Rhone-Poulenc) have thebalance.

Q rubbers are made commercially either by the multistep hydrolysis ofdimethyldichlorosilane or by the ring-opening polymerization of the cyclicoligomer octamethylcyclosiloxane. In the hydrolysis procedure the chlorine

Table 8 ExxonMobil Chemical Vistalon EPDMGrades

Vistalon grade Wt%

407–878Copolymer of Mooney range 18–51

Ethylene range 44–781703–3708Terpolymer ENB range 0.9–3.8 (1–4)

Mooney range 25–52Ethylene range 55–70

2504–7800Terpolymer ENB range 4.2–6.0 (4–6)

Mooney range 20–82Ethylene range 55–79

6505–9500

Terpolymer ENB range 8–11Mooney range 51–83Ethylene range 53–69

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atoms are hydrolyzed and replaced with oxygen atoms bonding a pair ofsilicon atoms. Water is frequently replaced with methanol, which leads to theformation of methyl chloride rather than the more corrosive hydrochloricacid. In the ring-opening polymerization, strong acid or strong base catalystsare used to produce high molecular weight Q. The ring-opening polymeriza-tion process can be conducted in an aqueous emulsion procedure usingdodecylbenzenesulfonic acid as the catalyst.

Most Q polymers have the repeat unit empirical formula ((CH3)2SiO)nand are referred to as polydimethylsiloxanes. The elastomer consists ofalternating silicon and oxygen atoms with two methyl groups on each silicon.A significant departure from most other elastomers is the absence of carbonfrom the backbone. Three reaction types are predominantly employed for theformation of vulcanized Q: peroxide-induced free radical vulcanization,hydrosilylation addition cure, and condensation cure. Silicones have alsobeen cross-linked using radiation to produce free radicals or to inducephotoinitiated reactions.

Q elastomers do not crystallize even under strain and have very poorphysical properties. Unfilled silicone rubber has a tensile strength of only0.345 MPa. Q vulcanizates are reinforced with f25% finely divided fumedsilica. This reinforcing filler increases tensile strength, tear properties, andabrasion resistance. The SiUOUSi bonds in Q have a much lower energy ofactivation for rotation than CUC or CUO bonds. This makes Q elastomersflexible and rubbery even at very low temperatures, and this elastomericproperty is little affected by temperature changes. This feature combined withtheir refractory characteristics makes these elastomers useful over a widetemperature range.

Table 9 Model Formulas for Initiation of New Compound Development Work

Roofing cover or sheeting Radiator hose

EPDM 100 EPDM 100

N330 carbon black 120 N660 carbon black 100Clay or talc 30 Calcium carbonate 30Paraffinic oil 90 Paraffinic oil 100

Zinc 5 Zinc 3Oxide OxideStearic acid 2 Stearic acid 1MBTS 2.50 DTDM 2

TMTD 0.50 ZDBDC 2TETD 0.50 TMTD 2Sulfur 0.80 Sulfur 0.75

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Q elastomers are used for electrical insulation, medical devices, seals,surface-treated fillers, elastic textile coatings, and foams. Liquid injectionmolding is used for electrical connectors, O-ring seals, valves, electricalcomponents, health care products, and sporting equipment such as gogglesand scuba diving masks.

C. Fluorocarbon Elastomers

The principal producers of fluorocarbon (FKM) polymers are 3M,Ausimont,and E.I. DuPont Dow in the United States. In the Far East, production ismaintained by Asahi Glass and Daikin. The annual worldwide FKM usage isabout 8 kt. About 40% of this is in the United States, 30% in Europe, and20% in Japan.

High pressure, free radical, and aqueous emulsion polymerization aretypically used to prepare these elastomers. The initiators are organic orinorganic peroxides, e.g., ammonium persulfate. The emulsifying agent isusually a fluorinated acid soap. FKM elastomers are the perfluoro derivativesof the common polyolefins. The monomers are the perfluorinated derivativesof ethylene and propylene. Copolymers of different olefins are available; theyare noncrystalline polymers that are elastomeric when cross-linked. Thevulcanized FKM polymers are dimensionally stable and chemically inert inhostile environments and in a variety of organic fluids such as oils andsolvents. This chemical resistance spans a wide temperature range. Inaddition, vulcanized FKM polymers show extraordinary self-lubricatingproperties due to their low surface energy. FKMelastomers can be vulcanizedwith one of three distinct procedures, with diamine, bisphenol-onium, andperoxide curing agents. The bisphenol-onium cure system is the most widelyused. FKM elastomers are resistant to heat, chemicals, and solvents.

The major use of FKM polymers is in the automotive industry for suchitems as engine, gasket, and fuel system components (hoses andO-rings). Thisapplication is fueled by increased demands from higher use temperatures,alcohol-containing fuels, and aggressive lubricants. Other major segmentsinclude petroleum, petrochemical, and industrial pollution control andindustrial hydraulic and pneumatic applications.

D. Phosphazenes

The worldwide capacity for phosphazenes (FZ) is less than 0.1 kt. Albemarle(Ethyl Corporation) is the sole supplier. Phosphazene polymers are made in atwo-step process. First, the trimer hexachlorocyclotriphosphazene is poly-merized in bulk to polydichlorophosphazene, a chloropolymer. The chlo-ropolymer is then dissolved and reprecipitated to remove unreacted starting

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material. Nucleophilic substitution with alkyl or aryloxide substitution of thehalide in solution provides the elastomer.

Polyphosphazenes have a backbone of alternating nitrogen and phos-phorus atoms with two substituents on each phosphorus atom. The backboneis isoelectronic with that of silicones; these polymer backbones share thecharacteristics of thermal stability and high flexibility.

Two elastomers with unique property profiles have been commercial-ized. One has fluoroalkoxy substituents that provide resistance to manyfluids, especially to hydrocarbons. FZ elastomer is a translucent pale browngum with a glass transition temperature of �68jC to �72jC. The gum canbe cross-linked by using peroxides such as dicumyl peroxide and a,aV-bis(t-butylperoxy)diisopropylbenzene. FZ elastomers have excellent resistanceto hydrocarbons and inorganic acids, as is expected for a fluorinatedelastomer. They are strongly affected by polar solvents but are more resistantto amines than most other fluorinated elastomers. This material also has abroad use temperature range and useful dynamic properties. The otherelastomer (aryloxyphosphazene) has phenoxy and p-ethylphenoxy substitu-ents. It has flame retardant properties without containing halogens. It may becured using either peroxides or sulfur.

Varying the polymer can produce coatings, fluids, elastomers, andthermoplastic materials. These variations include changes in the molecularweight and the substituents on the phosphorus. These materials have beensuggested for use in biomedical devices, including implants and drug carriers.However, initial applications have been largely in military and aerospaceareas.

E. Polyethers

Nippon Zeon, with plants in the United States and Japan, has a nearmonopoly on polyether rubbers. These include epichlorohydrin homopoly-mer rubber (CO), epichlorohydrin-ethylene oxide copolymer (ECO), epichlo-rohydrin (ECH)-ethylene oxide terpolymers (ETER), propylene oxidehomopolymer rubber (PO), and propylene oxide-allyl glycidyl ether copoly-mers (GPO). There are no production facilities in western Europe for theseelastomers. Worldwide production is about 11 kts/yr, with Nippon Zeonhaving about 90% of the market share and Daiho the balance.

Polymerization is conducted either in solution or in slurry at 40–130jCin toluene, benzene, heptane, or diethyl ether. Trialkylaluminum–water andtrialkylaluminum–water–acetylacetone catalysts are used. Chain propaga-tion is by a cationic charge transfer mechanism. The polyethers include agroup of minor elastomers made by ring-opening polymerization of epoxidesand include CO, ECO, ETR, PO, and GPO. CO and ECO are linear and

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amorphous. Because it is asymetrical, ECH monomer can polymerize in thehead-to-head, tail-to-tail, or head-to-tail fashion. The commercial polymer is97–99% head-to-tail and atactic. The commercial products are essentiallyamorphous.

Polyethers are remarkable because of an exceptional combination ofproperties. The epichlorohydrin homopolymer has low gas permeability andis better than IIR. It is resistant to ozone and has low hysteresis. The polymeris flame retardant owing to its high chlorine content. Its resilience is poor atroom temperature but improves upon heating. ECO is less flame retardantowing to its lower chlorine content. It has some impermeability to gases buthas better low-temperature flexibility. It also exhibits good heat resistance.Polyethers are resistant to apolar organic fluids such as oils and aliphatic/aromatic solvents.

The polyethers are important in automotive applications such as fuel,air, and vacuum hoses; vibration mounts; and adhesives. Other uses includedrive and conveyor belts, hoses, tubing, and diaphragms; pump partsincluding inner coatings, seals, and gaskets; printing rolls and blankets; fabriccoatings for protective clothing; pond liners; and membranes in roofingmaterial. In the automotive areas they are used as constant-velocity boots,dust and fuel hose covers, mounting isolators, and hose and wire covers.

F. Ring-Opened Polymers

Ring-opening metathesis polymerization of cyclopentene is carried out ineither chlorinated or aromatic solvents to give trans-polypentenamer (TPA).This polymer is not commercially available. The same polymerization tech-nique is used to convert cyclooctene to trans-cyclooctenemer (TOR).Degussa-Huls is the sole producer of TOR. Their capacity is estimated at13.5 kt.

VII. SUMMARY

Specialty elastomers have structures and compositions designed to havespecific attributes. The principal characteristic of these polymers, in compar-ison to general-purpose rubbers, is that they have either resistance to solventsor resistance to elevated temperatures or, preferably, both. In comparison tothe high-volume rubbers, the quantities of these specialty polymers are small.However, as the mission profiles for products using special-purpose elasto-mers become more demanding, the need for such materials can be expectedto increase.

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REFERENCES

1. International Institute of Synthetic Rubber Producers, Inc. Worldwide Rubber

Statistics. Houston, TX: Int Inst Synthet Rubber Producers, 2003.2. Morton M. Rubber Technology. 3rd ed. New York: Van Nostrand Reinhold,

1987.3. Dick J. Rubber Technology: Compounding and Testing for Performances.

Berlin: Hanser, 2001.4. International Standards Organization. Rubbers and Latices—Nomenclature.

ISO 1629. ISO, New York, NY: ANSI, 1995.

5. International Institute of Synthetic Rubber Producers, Inc. The SyntheticRubber Manual. 14th ed. Houston, TX: Int Inst Synth Rubber Producers, 1999.

6. Stevens MP. Polymer Chemistry: An Introduction. New York: Oxford Uni-

versity Press, 1999.7. Barbin WW, Rodgers MB. The science of rubber compounding. In: Mark JE,

Erman B, Eiri FR, eds. Science and Technology of Rubber. Orlando, FL:Academic Press, 1994.

8. Colbert GP. Solvent resistant elastomers—neoprene, hypalon, and chlorinatedpolyethylene. In: Barawal KC, Stevens HL, eds. Basic Elastomer Technology.Washington, DC: Rubber Division, Am Chem Soc, 2001.

9. Dunn JR, VaraRG.Oil resistant elastomers for hose applications. Rubber ChemTechnol 1983; 56:557–574.

10. ExxonMobil, 2003. www.exxonmobilchemical.com/

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