Polyurethane Elastomers

Second Edition

Polyurethane Elastomers

Second Edition

C. HEPBURN

Formerly

Senior Lecturer in Rubber Technology (1969-1991), Institute of Polymer Technology and Materials Engineering,

Loughborough University of Technology, Loughborough, Leicestershire LEll 3TU, UK

Presently

Professor in Polymer Engineering, Department of Mechanical and Industrial Engineering,

University of Ulster, Shore Road, Newtownabbey, Belfast BT3700B, UK

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WITH 156 TABLES AND 127 ILLUSTRATIONS

I'; 1992 ELSEVIER SCIENCE PUBLISHERS LTD

British Library Cataloguing in Publication Data

Hepburn, C. (Claude) Polyurethane elastomers.-2nd ed. I. Title 678.72

ISBN 1-85166-589-7

Library of Congress Cataloging-in-Publication Data

Hepburn, C. (Claude) Polyurethane elastomers / by C. Hepburn.--2nd ed.

p. cm. Includes bibliographical references and index

ISBN 1-85166-589-7 1. Polyurethanes. 2. Elastomers. J. Title

TP1180.P8H46 1991 668.4'239--dc20 91-734

CIP

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To Evelyn for her everlasting love and dedication to our family

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Cardinal Newman

Preface to the Second Edition

Since the first edition was published in 1982 the expansion and diversifica­tion of the polyurethane industry has continued unabated as these materials have steadily penetrated into most applications due to their unique combination of strength, high elongat'ion at break and enormous range of hardnesses. This market expansion has attracted new chemical suppliers with novel polyol, diisocyanate and chain extender systems which have resulted in new generations of polyurethane elastomers. Of particular importance is the growing use of aliphatic diisocyanates which enable colour stable and transparent polyurethanes to be produced in both highly flexible and very rigid forms.

C. HEPBURN

vii

Preface to the First Edition

Since their discovery in the 1930s the polyurethane macromolecules have fascinated engineers and scientists by their unique combination of unusual properties and ability to be processed, shaped and formed by almost all known manufacturing techniques. Until the mid-1970s polyurethanes were regarded as speciality materials due to their relatively high costs and hence only relatively modest tonnages were used by industry. As we increasingly progress into the 1980s this situation is changing rapidly to one of enormous expansion in the use of polyurethanes which is primarily concerned with applications that require urethane elastomers. One particular process has been responsible for this growth, namely reaction injection moulding of liquid polyurethane elastomers; the growth in use of thermoplastic poly­urethane elastomers, despite their high price, has also been an important market expansion factor.

In the early days of polyurethane exploitation the technology was usually practised by urethane specialists and avoided, if possible, by others or only applied in its simplest of forms. Today it is necessary for many development and production personnel to be able to 'practise' poly­urethane elastomer technology with a good understanding of the chemical, physical and engineering design factors that must be combined for success­ful product manufacture and application. Information on polyurethane elastomers is widely available in the scientific press but scattered and diffuse through its dissemination in a multitude of journals and specialist books. The aim of this monogram has been to distil into a single volume the essentials of this often complex polyurethane technology in a form that is usable by all technical and semi-technical people who wish to become their own polyurethane elastomer experts. In particular the chapter on reaction injection moulding has been designed for use by both non-chemists and chemists.

IX

x PREFACE TO THE FIRST EDITION

The original work on this topic, published in 1969 by P. Wright and A. P. C. Cumming as Solid Polyurethane Elastomers, was widely used by the urethane industry and acknowledgement is made of this pioneering work which has been so useful in guiding the compilation of this present book. A bibliography is supplied for further reading and current awareness; the latter is best attempted by scanning the specialist Urethane Abstracts.

The author has been primarily concerned with an account of industrially important and hence established materials and processes; therefore the many alternative polyurethane systems which must presently be classified as primarily of research interest are not included in the present text. The author would be pleased to receive suggestions for the incorporation of material in any future work.

C. HEPBURN

Acknowledgements

The author gratefully acknowledges the use of material from Chapter 8c in the book Block Copolymers, edited by D. C. Allport and W. H. Janes, Applied Science Publishers, in the compilation of Chapter 3. He also wishes to acknowledge the help he obtained in calling upon material from the book Solid Polyurethane Elastomers written by P. Wright and A. P. C. Cumming, which has been used as a basis for Chapters 4,5 and 7 and for the reproduction of Tables 9.1, 13.4, 13.6, 13.8, 13.9-13.11.

xi

Contents

Preface to the Second Edition

Preface to the First Edition

Acknowledgements .

l. Chemistry and Basic Intermediates. Introduction .

V]J

IX

xi

Basic Chemistry 2 Basic Structure of a Polyurethane Elastomer. 4 Synthesis of Basic Urethane Building Blocks. 7

Isocyanates . 7 Storage Life and Reactions of Isocyanates with Themselves-Dimers

(Uretidinediones) 15 Uretidiones 19 Polyols. 19 Polyesters 21 ~~~~ M Chain Extenders and Crosslinking Agents 27 Catalysts 27

2. Polyurethane Elastomer Chemistry. 29 Liquid Processing Routes 29 Millable Elastomer Synthesis. 30 Thermoplastic Linear Urethane Elastomers 34 Concept of Equivalent Weights 36 Concept of the Complete Urethane Chemical System 38

Polyol Quantization by Chemical Analysis. 40 Other Isocyanate Analytical Terms 42 Other Useful Terms 43 Example Calculation for a Typical Polyurethane Elastomer Synthesis 45

Blends 46 Calculation of Isocyanate Proportions for a Water Blow Foam System. 48 Effect of Flexible to Rigid Segment on Urethane Properties 50

Xlll

XIV CONTENTS

3. Property-Structure Relationships in Polyurethanes 51 Introduction . 51 Flexible Segments 52 Polyethers 55 Polycaprolactones 56 Rigid Segments 57 Aliphatic Versus Aromatic Diisocyanates 59 Aliphatic Diisocyanates and Transparency 59 Effect of Diisocyanate Structure on the Relative Thermal Stability of

Polyurethane Elastomers 61 Chain Extension of Unsymmetrical Diisocyanates . 65 Q~~ . ~ Chain Extension of Symmetrical Diisocyanates with Diols 73 Synthesis. 73 Even Molar Ratio Effects 76

Influence of the Diol Structure on Thermal Stability of the PU Elastomer: Molar Ratios 1:2:1 76

Loss Tangent Values and Chain Extender Composition. 77 Uneven Molar Ratio Effects with Respect to the Diisocyanate 80 Comparison of the Thermal Stability Effects of a Diamine with Diols

(as Chain Extenders) in Polyurethane Elastomers (Molar Ratio 1: 3 :2) 82 Differential Scanning Calorimetry (DSC) as a Means of Predicting

Thermal Stability (Molar Ratio 1 :2:1) 84 DSC Studies and the Molar Ratio 1:2.6:1 85 DSC as a Means of Predicting the Relative Thermal Stability of a Diamine

(polacure) with Diol Chain Extenders 87 Contribution of Crosslinks to Properties 87 Crosslinking and Thermal Stability in Polyurethane Elastomers 89 Synthesis Technique 90 In-Situ Isocyanate and Isocyanurate Crosslinking . 92 PU Elastomer Compositions . 92 Influence of% Free NCO on the Properties of the PU Elastomer at Room

Temperature . 97 Strength Properties at Elevated Temperature. 99 Dynamic Mechanical Thermal Analysis (DMT A) 99 Thermal Equilibrium Properties 101 Low Temperature Stiffening and Glass Transition Temperature (Tg) 101 Hazards . 102 Thermal Stability and Crosslink Structure in PUs. 103 Structure in Polyurethane Elastomers 103 Continued Crystal Growth 106 Plasticization . 106

4. Reaction Rates, Catalysis and Surfactants 107 Reaction Rates 107

Diisocyanates 107 Polyols. 110 Temperature 112

CONTENTS XV

Catalysis. 113 Acid Catalysis and Prepolymer Stability (Shelf Life) 113 Shelf Life Stability of Prepolymers 113 Base Catalysis 115 Additional Comments on RIM Catalysis 117 Tertiary Amines . 117 Organometallic and Other Metal Compounds 118 Catalyst Combinations 120

Surfactants (Surface Active Agents) 120 Ionic Surfactants. 120 Non-ionic Surfactants . 120

5. Liquid Polyurethane Elastomer Systems. 122 Stable Prepolymer Systems 124

Simple Hand Casting Procedure. 125 Chain Extenders . 127 Proces~ing of TD I Prepolymers 129 Processing Variables . 131 Type of Curative. 132 Reaction Temperature 134 Other Representative Polymer Systems 135

Unstable Prepolymer System . 152 Water Crosslinked Millable Vulkollan for Compression Moulding 158 Cellular Polyurethane Elastomers . 158

Vulkollan System 158 Cellular Urethanes from Stable Prepolymers 159 Solvents 160

Compression Moulding of Cast Elastomers . 161 Spray Application of Urethane Elastomers 163 Note on Water Content of Polypols of Urethane Elastomers. 164 Alternative Chain Extenders to MOCA . 165

3,3' -Dimethyl-4,4' -diaminodicyclohexylmethane . 166 Diols as Chain Extenders . 167

Synthesis and Properties of Polyurethane Elastomers Based on Aromatic Versus Aliphatic Diisocyanates 169

CHDI Prepolymer Synthesis and Chain Extension. 169 PPDI Prepolymer Synthesis and Chain Extension. 171

6. Reaction Injection Moulding (Liquid Injection Moulding) . 174 The Process 178 RIM Formulation . 179

Water. 181 Physical Properties of RIM Urethane Systems 182 RIM Machine Design 182 Energy Requirements of RIM. 186 Mechanism of Impingement Mixing 187 Mould Design. 187

Transition Zone . 189

XVI CONTENTS

Moulding 191 Ribs and Radius. 191 Runners 192 Mould Sealing 193

Processing Economics 194 Reinforced Reaction Injection Moulding Machinery 194

Dispersion of Fibre Reinforcement 198

7. Millable Polyurethane Elastomers 199 Sulphur Vulcanized Grades 200 Millable Polyether Polyurethane Elastomers 201 Plasticization . 206 Filler Reinforcement 207 Isocyanate-Cured Urethane Rubber 207 Peroxide-Cured Polyurethane Elastomers 217

Cure Temperature 218 Effect of Fillers . 220

Combined Peroxide and Diisocyanate Cures 220

8. Polyurethane Adhesives 221 Reasons for Adhesive Properties of Polyurethane and Diisocyanate-Based

Adhesives. 223 Isocyanate Reactivity . 224 Self Polymerization 224 Surface Wetting Properties. 225 Polarity 225 Graded Physical Properties. 225 Reaction with Unreactive Surfaces 226

Reasons for the Apparent Universal Applications of Polyurethanes as Adhesives 226

Methods of Using Isocyanates as Adhesives 227 As an Isocyanate Primer. . 227 By In-Situ Polyurethane Polymerization 229 As Polyurethane Elastomer Without or With Added Polyisocyanates 231

Thermoplastic Polyurethane Elastomer Types 233 Elastomer Gum Types . 234 Blocked Di- or Polyisocyanate Adhesives 235 Aqueous Dispersions of Polyurethanes . 239 Film and Tape Polyurethane Adhesives. 241 Powdered Polyurethane . 241 Stabilization of Adhesives 242

9. Thermoplastic (Linear Polyurethane Elastomers) 244 Partially Crosslinked Thermoplastic Polyurethanes 245 Synthesis of Thermoplastic Polyurethane 245

Preparation. 247 Notes on Large-Scale Production Synthesis Procedures 249

Processing of Thermoplastic Polyurethanes . 249

Extrusion Injection Moulding Calendering Hot-Melt Calendering. Film Lamination

Solution Applications Reactive Coatings .

100%-SoJids Reactive Systems Commercial1y Available TPU s

CONTENTS

Comment About the Set Properties of TPUs . Microporous Thermoplastic Polyurethanes Blends Processing of Transparent Thermoplastic Polyurethane Elastomer

Injection Moulding. Thermoplastic Polyurethane Elastomers as Hydraulic Seal Materials CHDI-Based Polyurethanes .

Polyester TPUs . Mixed Aliphatic Diols as Chain Extenders (CHDM and BOO) . Polyether TPUs .

Thermal Stability . Polycaprolactone/CHDI Polyurethanes Polycaprolactone/PPDI Polyurethanes Polyether/PPDI Polyurethanes Polyether/CHDI Polyurethanes . Energy Absorption (tan b) . Post-Cure Prediction Responses.

Conclusions

XVII

250 251 253 254 255 256 256 257 257 260 260 263

by 264 264 267 268 277 277 278 278 278 278 279 279 279 279

10. Water Dispersions of Polyurethane Elastomers 281 Self-Emulsifying Latex 281 Emulsified Latex 282 Polyurethane Ionomers 284 Special Features of Water-Dispersed Polyurethane Elastomers 286

Anionic Dispersions 287 Cationic Dispersions . 288

Practical Example for the Preparation of a Polyurethane Dispersion (Melt-Dispersion Technique) . 290

II. Analysis and Characterization of Polyurethane Elastomers 292 Diisocyanates . 292

Measurement of NCO Content . 293 Total Chlorine: Principle of the ASTM Method 01638 295 Acidity 295 Hydrolyzable Chlorine 296 Isomer Ratio 297 Activity of Diisocyanate 297 Diisocyanates in the Atmosphere 298

Polyol Analysis 303

XVlll CONTENTS

Hydroxyl Number 303 Primary and Secondary Hydroxyl Groups . 305 Water Content 306 Acid Number 307 Measurement of pH (Apparent) 307 Hydrolytic Stability 308 Unsaturation 308 Peroxide Content 310 General Analytical Parameters for Polyols . 310

Infrared Spectroscopy Techniques . 311 Determination of Isomer Ratio of TDI by Infrared Spectroscopy 318 Determination ofUnreacted NCO Groups in a Polyurethane Elastomer 320 An Additional Rapid Infrared Method for the Quantitative Analysis of

NCO Present in a Polyurethane Elastomer 321 Chromatography Techniques. 323

Gas-Liquid Chromatography 323 Thin Layer Liquid Chromatography 325

Thermal Analysis . 328 Identification of the Components in a Polyurethane 329

Polyesters 329 Polyethers . 329

Differentiation of Polymers Present in a Polyurethane 330 General Analysis of Miscellaneous Elements in a Polyurethane Elastomer 333

Colour Reactions for Polyurethane Identification 334 References 336

12. Special Types of Polyurethane Elastomers 337 Hydroxy-Terminated Polybutadienes 337

Process Oils and Plasticizers 339 Low-Temperature Resistance 339 Transparent Polyurethane Elastomers 339 Amine-Terminated Polyols . 350 Fluorinated Polyolls . 351 UV Light Curable PU Systems 352

13. Properties and Applications 355 Tensile Properties 357

In Tension . 357 In Compression 358

Load Deflection Properties 359 Comparison of Physical Properties of Polyurethane Elastomers with

Common Rubbers and Plastics 372 Energy Absorption Properties 372 Wear Resistance. 373 Friction Properties 375 Environmental and Ageing Properties 376 Light Resistance. 383 High Energy Radiation Effects 383 Electrical Properties 387

CONTENTS xix

Applications . 390 Current Types of PU Elastomer 391

Solid Tyres . 392 Pneumatic Polyurethane Tyres 392 Microcellular Shoe Solings . 395 Blends of TPU with other Thermoplastics 397 Rollers 397 Automotive Body Panels and Bumpers 398 Rain-Erosion Protection of Surfaces . 398 Cold-Curing Elastomers 398 Low-Speed Tyre-Filling Compositions 398

Water Vapour Permeable PU Elastomers 399 Polyurethane Elastomers in Medicine 400 Millable Polyurethane Elastomers . 403

Fillers for Cold-Cast Polyurethane 404 Plasticizers for Polyurethane Elastomers 405

Manufacturing Factors 405 Moulding 405 Shrinkage . 405 Mould Lubricant 405 Mould Cleaning 406 Machining . 406

14. Health Hazards and Precautions 407 Toluene Diisocyanate (TDI) 409 Diphenylmethane Diisocyanate (MDI) 409 1,5-Naphthalene Diisocyanate (NDI) . 411 Toxicity Data for the Analogues Aromatic and Aliphatic Diisocyanates 411

General Toxicity Problems 413 Handling Isocyanates and Manufacture of Polyurethane Products 413 Skin Irritation 414

Protective Clothing 416 First Aid 416

Specific Hazards Associated with the Manufacturing Processes 417 Urethane Rubbers 417 Manufacture of Moulded Articles 417 Surface Coatings. 418 Printing Inks 419 Adhesives 420

Isocyanate Monitoring and Detection 420 Analytical Monitoring Techniques 421 Alternative Method 423 Determination of TDI in Solvent Atmosphere 424

Diamines 425 Polyols and Glycols 425 Waste Disposal 426

Relerences and Bihliography 427

Index. 429