REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS A CONCISE GUIDE TO INDUSTRIAL POLYMERS
Johannes Karl Fink Montanuniver...
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REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS A CONCISE GUIDE TO INDUSTRIAL POLYMERS
Johannes Karl Fink Montanuniversität Leoben Leoben, Austria
Copyright © 2005 by William Andrew, Inc. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the Publisher. Cover art © 2005 by Brent Beckley / William Andrew, Inc. ISBN: 0-8155-1515-4 (William Andrew, Inc.) Library or Congress Catalog Card Number: 2005007686 Library of Congress Cataloging-in-Publication Data Fink, Johannes Karl. Reactive polymers : fundamentals and applications : a concise guide to industrial polymers / by Johannes Karl Fink. p. cm. -- (PDL handbook series) Includes bibliographical references and index. ISBN 0-8155-1515-4 (acid-free paper) 1. Gums and resins, Synthetic. 2. Gums and resins--Industrial applications. I. Title. II. Series. TP1185.R46F56 2005 668'.374--dc22 2005007686 Printed in the United States of America This book is printed on acid-free paper. 10 9 8 7 6 5 4 3 2 1 Published by: William Andrew Publishing 13 Eaton Avenue Norwich, NY 13815 1-800-932-7045 www.williamandrew.com NOTICE To the best of our knowledge the information in this publication is accurate; however the Publisher does not assume any responsibility or liability for the accuracy or completeness of, or consequences arising from, such information. This book is intended for informational purposes only. Mention of trade names or commercial products does not constitute endorsement or recommendation for their use by the Publisher. Final determination of the suitability of any information or product for any use, and the manner of that use, is the sole responsibility of the user. Anyone intending to rely upon any recommendation of materials or procedures mentioned in this publication should be independently satisfied as to such suitability, and must meet all applicable safety and health standards.
PDL EDITOR’S PREFACE The publication of Reactive Polymers Fundamentals and Applications: A Concise Guide to Industrial Polymers in the PDL (Plastics Design Library) series gives me a special pleasure. The author, Dr. Johannes Karl Fink, has brought together an impressive array of information about the reactions polymers undergo, often resulting in cross-linking. The masterful interweaving of the applications of these reactions with the discussion of chemistry has rendered this book easy to read for a variety of scientists and engineers. The reader learns about the incredible penetration of reactive plastics into every day life whether in a dentist’s office or an office copier. On the more cutting edge, the book includes information about carborane copolymers, which far exceed the temperature resistance of any existing polymers. Launched in 1990, the Plastics Design Library (PDL) has established itself within the materials engineering community as the "go to" source for information on plastics, elastomers, and adhesives. PDL is a unique series of reference and data books essential to the daily work of practicing engineers and scientists in applied industries. The library encompasses the areas of non-metallic materials with a special emphasis on plastics, elastomers, coatings, and adhesives. Important current and future materials, processing technologies, and applications are emphasized. Future titles are planned in the newer areas of commercial activity such as nanocomposites, bio-based polymers, recycling and mathematical modeling. The breadth of the coverage ranges from the nature and selection of materials to design and fabrication to specification and performance. The uniqueness of PDL is in its balance of practical and theoretical aspects with a clear emphasis of the practical over the theoretical. The encyclopedic format of the books lends itself to easy reading and referral while the theoretical sections provide the curious reader with an opportunity for more in-depth learning. Ample references throughout each book serve as both bibliography and additional learning. Our hope is that this book will meet the needs of people who work with the reactions of polymers for any reason. Sina Ebnesajjad 2005 i
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Sina Ebnesajjad, Editor Plastics Design Library Dr. Sina Ebnesajjad is a senior technical consultant at the DuPont Company, where he has been in a variety of technical assignments since 1982. Sina is the author of three handbooks on the science, technology and applications of Fluoroplastics, published by William Andrew, Inc. He is the Series Editor for the Fluorocarbon Handbook Series that includes six handbooks on fluoroplastics, fluoroelastomers, fluorinated coatings and fluoroionomers. He has been the Editor of Plastics Design Library since September 2004.
PREFACE Most of the synthetic polymers are produced in chemical plants and delivered to a plastics manufacturer who does the formulating, blending, extruding or molding in order to fabricate articles. The processes required for the final product are purely physical that occur essentially without any chemical reaction of the polymer. Since most of the polymers are immiscible, there is not much room to modify the polymer properties during the plastics manufacturing. The properties of the final product are often modified by the actions of additives. A minor number of polymers, usually called resins, are delivered as precursors by the chemical industry to the manufacturer. Here, the manufacturer gets to the final article by a chemical reaction. There also exists an in-between state where polymers can be modified by reactive extrusion and grafting. The modification of polymers is advantageous if comparatively small changes of certain properties are needed that cannot be achieved in chemical plants. Since many different precursors of the final resin can be combined, the variability of, and thus the ability to modify, the final properties are much more pronounced in comparison to the rest of polymers. This is the topic with which the present book deals, namely, chemical reactions that take place during the final stage of part fabrication from plastics. The text does not deal with the chemical reactions needed to produce resin precursors and the synthesis of polymers. However, chemical topics relevant to the part manufacturer are elaborated here. These range from the manufacture of glass-fiber-reinforced articles such as boats made by the amateur and in a small scale dockyard to what takes place when a dentist is filling teeth. Industrial processes for the plastics batch fabrication are described, in addition to their end uses. The text describes the basic principles of reactive resins as well as the most recent developments. Paints, coatings and adhesives that are constituted from resins are not dealt with here, even when the curing mechanisms are similar. The past art is discussed by reference to monographs, whereas the recent developments are documented by references in the scientific literature and the patent literature after 2000. In some topics, e.g., urea-formaldehyde resins, the present research activity is low. In other areas, such as resins used for nanocomposites, there are many recent papers. Even those resins, for which the research activity is rather dormant at the moiii
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Reactive Polymers Fundamentals and Applications
ment, find widespread use and well established applications. They are not covered here because they are presented in general reviews cited at the beginning of the respective chapters. Newer applications of these resins are discussed in detail. The text originates from a lecture manuscript developed by the author that has been expanded into a monograph. The original literature presented here covers the period until July 2004. The text is at a level that a chemist with a general eduction in polymer chemistry should understand. Further, the text is addressed to the advanced student of plastics engineering and the practicing engineer.
HOW TO USE THIS BOOK Utmost care has been taken to present reliable data. Because of the vast variety of material presented here, however, it cannot be complete in all relevant aspects, and it is recommended that the reader study the original literature for complete information. Therefore, the author cannot assume responsibility for the completeness and validity of, nor for the consequences of, the use of the material presented here. Every attempt was made to identify trademarked products in this volume; however, there were some that the author was unable to locate, and we apologize for any inadvertent omission.
Index There are three indices: an index of acronyms, an index of chemicals, and a general index. Unfortunately the acronyms presented in the literature are not always consistent. This means that in a few cases the same acronym stands for different terms. Further, in the literature the acronyms are sometimes expanded in a different way, in particular for chemical names. The author has not unified the system of chemical names, even when the same compound appears with different names, because otherwise back tracing in the original literature would be difficult. I apologize here for this somewhat unsatisfactory situation. In the index of chemicals, compounds that occur extensively, e.g., “styrene”, are not included at every occurrence, but rather when they appear in an important context.
Preface
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ACKNOWLEDGEMENTS I am indebted to our local library, Dr. Lieselotte Jontes, Dr. Johann Delanoy, Franz Jurek, Friedrich Scheer, and Christian Slamenik for support in literature acquisition. I express my gratitude to all the scientists who have carefully published their results concerning the topics dealt with here. The book could not have been compiled otherwise. I would like to thank Dr. Sina Ebnesajjad, Editor of Plastics Design Library (PDL) for his review and comments on the manuscript. The Editorial Staff of William Andrew, Inc. have been most supportive of this project, especially Ms. Millicent Treloar, Senior Acquisitions Editor, who has been tireless in her efforts. Thank you Ms. Joan Bally for copy editing the manuscript. J. K. F.
Plastics Design Library
PDL Editor: Sina Ebnesajjad Founding Editor: William A. Woishnis Fluoroelastomers Applications in Chemical Processing Industries ISBN: 0-8155-1502-2 Khaladkar, P. R., Ebnesajjad, S., Pub. Date: 2005, 592 Pages The Effect of Sterilization Methods on Plastics and Elastomers, 2nd Ed. ISBN: 0-8155-1505-7 Massey, L. K., Pub. Date: 2005, 408 Pages Extrusion: The Definitive Processing Guide and Handbook ISBN: 0-8155-1473-5 Giles, H. F., Jr., Wagner,J. R., Jr., Mount, E. M., III, Pub. Date: 2005, 572 Pages Film Properties of Plastics and Elastomers, 2nd Ed. ISBN 1-884207-94-4 Massey, L. K., Pub. Date: 2004, 250 Pages Handbook of Molded Part Shrinkage and Warpage ISBN 1-884207-72-3 Fischer, J., Pub. Date: 2003, 244 Pages Fluoroplastics, Volume 2: Melt-Processible Fluoroplastics ISBN 1-884207-96-0 Ebnesajjad, S., Pub. Date: 2002, 448 Pages Permeability Properties of Plastics and Elastomers, 2nd Ed. ISBN 1-884207-97-9 Massey, L. K., Pub. Date: 2002, 550 Pages Rotational Molding Technology ISBN 1-884207-85-5 Crawford, R. J., and Throne, J. L. , Pub. Date: 2002, 450 Pages Specialized Molding Techniques & Application, Design, Materials and Processing ISBN 1-884207-91-X Heim, H. P., and Potente, H., Pub. Date: 2002, 350 Pages
Chemical Resistance CD-ROM (3rd Ed.) ISBN 1-884207-90-1 Plastics Design Library Staff, Pub. Date: 2001, CD-rom Plastics Failure Analysis and Prevention ISBN 1-884207-92-8 Moalli, J., Pub. Date: 2001, 400 Pages Fluoroplastics, Volume 1: Non-Melt Processible Fluoroplastics ISBN 1-884207-84-7 Ebnesajjad, S., Pub. Date: 2000, 365 Pages Coloring Technology for Plastics ISBN 1-884207-78-2 Harris, R. M., Pub. Date: 1999, 333 Pages Conductive Polymers and Plastics in Industrial Applications ISBN 1-884207-77-4 Rupprecht, L. M., Pub. Date: 1999, 302 Pages Imaging and Image Analysis Applications for Plastics ISBN 1-884207-81-2 Pourdeyhimi, B., Pub. Date: 1999, 398 Pages Metallocene Technology in Commercial Applications ISBN 1-884207-76-6 Benedikt, G. M., Pub. Date: 1999, 325 Pages Weathering of Plastics ISBN 1-884207-75-8 Wypych, G., Pub. Date: 1999, 325 Pages Dynamic Mechanical Analysis for Plastics Engineering ISBN 1-884207-64-2 Sepe, M.. Pub. Date: 1998, 230 Pages Medical Plastics: Degradation Resistance and Failure Analysis ISBN 1-884207-60-X Portnoy, R. C., Pub. Date: 1998, 215 Pages
Metallocene Catalyzed Polymers ISBN 1-884207-59-6 Benedikt, G. M., and Goodall, B. L., Pub. Date: 1998, 400 Pages Polypropylene: The Definitive User’s Guide and Databook ISBN 1-884207-58-8 Maier, C., and Calafut, T., Pub. Date: 1998, 425 Pages Handbook of Plastics Joining ISBN 1-884207-17-0 Plastics Design Library Staff, Pub. Date: 1997, 600 Pages Fatigue and Tribological Properties of Plastics and Elastomers ISBN 1-884207-15-4 Plastics Design Library Staff, Pub. Date: 1995, 595 Pages Chemical Resistance, Volume 1 ISBN 1-884207-12-X Plastics Design Library Staff, Pub. Date: 1994, 1100 Pages Chemical Resistance, Volume 2 ISBN 1-884207-13-8 Plastics Design Library Staff, Pub. Date: 1994, 977 Pages The Effect of UV Light and Weather on Plastics and Elastomers ISBN 1-884207-11-1 Plastics Design Library Staff, Pub. Date: 1994, 481 Pages The Effect of Creep and Other Time Related Factors on Plastics and Elastomers ISBN 1-884207-03-0 Plastics Design Library Staff, Pub. Date: 1991, 528 Pages The Effect of Temperature and Other Factors on Plastics ISBN 1-884207-06-5 Plastics Design Library Staff, Pub. Date: 1991, 420 Pages
Contents
1
Unsaturated Polyester Resins 1.1 History . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Monomers . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Monomers for the Unsaturated Polyester (UP) 1.2.2 Vinyl Monomers . . . . . . . . . . . . . . . . 1.2.3 Specialities . . . . . . . . . . . . . . . . . . . 1.2.4 Synthesis . . . . . . . . . . . . . . . . . . . . 1.2.5 Manufacture . . . . . . . . . . . . . . . . . . 1.3 Special Additives . . . . . . . . . . . . . . . . . . . . 1.3.1 Inhibitors . . . . . . . . . . . . . . . . . . . . 1.3.2 Thickeners . . . . . . . . . . . . . . . . . . . 1.3.3 Emission Suppressants . . . . . . . . . . . . . 1.3.4 Fillers . . . . . . . . . . . . . . . . . . . . . . 1.3.5 Reinforcing Materials . . . . . . . . . . . . . 1.3.6 Mold Release Agents . . . . . . . . . . . . . . 1.3.7 Low-profile Additives . . . . . . . . . . . . . 1.3.8 Interpenetrating Polymer Networks . . . . . . 1.3.9 Polyurethane Hybrid Networks . . . . . . . . 1.3.10 Flame Retardants . . . . . . . . . . . . . . . . 1.3.11 Production Data . . . . . . . . . . . . . . . . 1.4 Curing . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 Initiator Systems . . . . . . . . . . . . . . . . 1.4.2 Promoters . . . . . . . . . . . . . . . . . . . . 1.4.3 Initiator Promoter Systems . . . . . . . . . . . 1.4.4 Polymerization . . . . . . . . . . . . . . . . . 1.5 Properties . . . . . . . . . . . . . . . . . . . . . . . . vii
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1 1 2 2 8 10 13 15 17 17 18 18 19 23 25 26 28 30 31 34 34 35 37 40 40 44
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Reactive Polymers Fundamentals and Applications 1.5.1 Structure Properties Relationships . . . . . . . 1.5.2 Hydrolytic Stability . . . . . . . . . . . . . . 1.5.3 Recycling . . . . . . . . . . . . . . . . . . . . 1.6 Applications and Uses . . . . . . . . . . . . . . . . . 1.6.1 Decorative Specimens . . . . . . . . . . . . . 1.6.2 Polyester Concrete . . . . . . . . . . . . . . . 1.6.3 Reinforced Materials . . . . . . . . . . . . . . 1.6.4 Coatings . . . . . . . . . . . . . . . . . . . . 1.7 Special Formulations . . . . . . . . . . . . . . . . . . 1.7.1 Electrically Conductive Resins . . . . . . . . . 1.7.2 Fluoro Copolymers . . . . . . . . . . . . . . . 1.7.3 Toner Compositions . . . . . . . . . . . . . . 1.7.4 Pour Point Depressants . . . . . . . . . . . . . 1.7.5 Biodegradable Polyesters . . . . . . . . . . . 1.7.6 Bone Cement . . . . . . . . . . . . . . . . . . 1.7.7 Compatibilizers . . . . . . . . . . . . . . . . . 1.7.8 Reactive Melt Modification of Poly(propylene) References . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
Polyurethanes 2.1 History . . . . . . . . . . . . 2.2 Monomers . . . . . . . . . . . 2.2.1 Diisocyanates . . . . . 2.2.2 Polyols . . . . . . . . 2.2.3 Other Polyols . . . . . 2.2.4 Polyamines . . . . . . 2.2.5 Chain Extenders . . . 2.2.6 Catalysts . . . . . . . 2.2.7 Blowing . . . . . . . 2.3 Special Additives . . . . . . . 2.3.1 Fillers . . . . . . . . . 2.3.2 Reinforcing Materials 2.3.3 Flame Retardants . . . 2.3.4 Production Data . . . 2.4 Curing . . . . . . . . . . . . . 2.4.1 Recycling . . . . . . . 2.5 Properties . . . . . . . . . . . 2.5.1 Mechanical Properties
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Contents 2.5.2 Thermal Properties . . . . 2.5.3 Weathering Resistance . . 2.6 Applications and Uses . . . . . . 2.6.1 Casting . . . . . . . . . . 2.7 Special Formulations . . . . . . . 2.7.1 Interpenetrating Networks 2.7.2 Grafting with Isocyanates 2.7.3 Medical Applications . . . 2.7.4 Waterborne Polyurethanes 2.7.5 Ceramic Foams . . . . . . 2.7.6 Adhesion Modification . . 2.7.7 Electrolytes . . . . . . . . References . . . . . . . . . . . . . . . .
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3 Epoxy Resins 3.1 History . . . . . . . . . . . . . . . . . . . . 3.2 Monomers . . . . . . . . . . . . . . . . . . . 3.2.1 Epoxides . . . . . . . . . . . . . . . 3.2.2 Phenols . . . . . . . . . . . . . . . . 3.2.3 Specialities . . . . . . . . . . . . . . 3.2.4 Manufacture . . . . . . . . . . . . . 3.3 Special Additives . . . . . . . . . . . . . . . 3.3.1 Toughening Agents . . . . . . . . . . 3.3.2 Antiplasticizers . . . . . . . . . . . . 3.3.3 Lubricants . . . . . . . . . . . . . . 3.3.4 Adhesion Improvers . . . . . . . . . 3.3.5 Conductivity Modifiers . . . . . . . . 3.3.6 Reinforcing Materials . . . . . . . . 3.3.7 Interpenetrating Polymer Networks . 3.3.8 Organic and Inorganic Hybrids . . . 3.3.9 Flame Retardants . . . . . . . . . . . 3.3.10 Production Data . . . . . . . . . . . 3.4 Curing . . . . . . . . . . . . . . . . . . . . . 3.4.1 Initiator Systems . . . . . . . . . . . 3.4.2 Compounds with Activated Hydrogen 3.4.3 Coordination Catalysts . . . . . . . . 3.4.4 Ionic Curing . . . . . . . . . . . . . 3.4.5 Photoinitiators . . . . . . . . . . . .
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4
Reactive Polymers Fundamentals and Applications 3.4.6 Derivatives of Michler’s Ketone . . . . . . . . . 3.4.7 Epoxy Systems with Vinyl Groups . . . . . . . . 3.4.8 Curing Kinetics . . . . . . . . . . . . . . . . . . 3.4.9 Thermal Curing . . . . . . . . . . . . . . . . . 3.4.10 Microwave Curing . . . . . . . . . . . . . . . . 3.5 Properties . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Hybrid Polymers and Mixed Polymers . . . . . 3.5.2 Recycling . . . . . . . . . . . . . . . . . . . . . 3.6 Applications and Uses . . . . . . . . . . . . . . . . . . 3.6.1 Coatings . . . . . . . . . . . . . . . . . . . . . 3.6.2 Foams . . . . . . . . . . . . . . . . . . . . . . 3.6.3 Adhesives . . . . . . . . . . . . . . . . . . . . . 3.6.4 Molding Techniques . . . . . . . . . . . . . . . 3.6.5 Stabilizers for Polyvinyl Chloride . . . . . . . . 3.7 Special Formulations . . . . . . . . . . . . . . . . . . . 3.7.1 Development of Formulations . . . . . . . . . . 3.7.2 Restoration Materials . . . . . . . . . . . . . . . 3.7.3 Biodegradable Epoxy-polyester Resins . . . . . 3.7.4 Swellable Epoxies . . . . . . . . . . . . . . . . 3.7.5 Reactive Membrane Materials . . . . . . . . . . 3.7.6 Controlled-release Formulations for Agriculture 3.7.7 Electronic Packaging Application . . . . . . . . 3.7.8 Solid Polymer Electrolytes . . . . . . . . . . . . 3.7.9 Optical Resins . . . . . . . . . . . . . . . . . . 3.7.10 Reactive Solvents . . . . . . . . . . . . . . . . . 3.7.11 Encapsulated Systems . . . . . . . . . . . . . . 3.7.12 Functionalized Polymers . . . . . . . . . . . . . 3.7.13 Epoxy Resins as Compatibilizers . . . . . . . . 3.7.14 Surface Metallization . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .
189 193 193 197 197 198 199 200 203 203 203 203 203 204 204 204 205 205 205 206 206 206 207 207 210 211 212 212 215 215
Phenol/formaldehyde Resins 4.1 History . . . . . . . . . . . . . . . 4.2 Monomers . . . . . . . . . . . . . . 4.2.1 Phenol . . . . . . . . . . . 4.2.2 o-Cresol . . . . . . . . . . 4.2.3 Formaldehyde . . . . . . . 4.2.4 Multihydroxymethylketones
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Contents
4.3
4.4
4.5
4.6
4.7
4.2.5 Production Data of Important Monomers 4.2.6 Basic Resin Types . . . . . . . . . . . . 4.2.7 Specialities . . . . . . . . . . . . . . . . 4.2.8 Synthesis . . . . . . . . . . . . . . . . . 4.2.9 Catalysts . . . . . . . . . . . . . . . . . 4.2.10 Manufacture . . . . . . . . . . . . . . . Special Additives . . . . . . . . . . . . . . . . . 4.3.1 Low Emission Types . . . . . . . . . . . 4.3.2 Boric Acid-modified Types . . . . . . . 4.3.3 Fillers . . . . . . . . . . . . . . . . . . . 4.3.4 Flame Retardants . . . . . . . . . . . . . Curing . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Model Studies . . . . . . . . . . . . . . 4.4.2 Experimental Design . . . . . . . . . . . 4.4.3 Water Content . . . . . . . . . . . . . . 4.4.4 Influence of Pressure . . . . . . . . . . . 4.4.5 Wood . . . . . . . . . . . . . . . . . . . 4.4.6 Novolak Curing Agents . . . . . . . . . 4.4.7 Resol Resin Hardeners . . . . . . . . . . 4.4.8 Ester-type Accelerators . . . . . . . . . . 4.4.9 Ashless Resol Resins . . . . . . . . . . . 4.4.10 Recycling . . . . . . . . . . . . . . . . . Applications and Uses . . . . . . . . . . . . . . 4.5.1 Binders for Glass Fibers . . . . . . . . . 4.5.2 Molding . . . . . . . . . . . . . . . . . 4.5.3 Novolak Photoresists . . . . . . . . . . . 4.5.4 High Temperature Adhesives . . . . . . 4.5.5 Urethane-modified Types . . . . . . . . 4.5.6 Carbon Products . . . . . . . . . . . . . Special Formulations . . . . . . . . . . . . . . . 4.6.1 Chemical Resistant Types . . . . . . . . 4.6.2 Ion Exchange Resins . . . . . . . . . . . 4.6.3 Brakes . . . . . . . . . . . . . . . . . . 4.6.4 Waterborne Types . . . . . . . . . . . . 4.6.5 High Viscosity Novolak . . . . . . . . . 4.6.6 Foams . . . . . . . . . . . . . . . . . . 4.6.7 Visbreaking of Petroleum . . . . . . . . Testing Methods . . . . . . . . . . . . . . . . . .
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Reactive Polymers Fundamentals and Applications 4.7.1 4.7.2 4.7.3 4.7.4 4.7.5 4.7.6 4.7.7 References .
5
Water Tolerance . . . . . Salt Tolerance . . . . . . Free Phenol Content . . . Free Formaldehyde . . . . pH . . . . . . . . . . . . Solids Content . . . . . . o-Cresol Contact Allergy . . . . . . . . . . . . . . . .
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274 274 275 275 275 275 275 275
Urea/formaldehyde Resins 5.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Synthesis of Resin . . . . . . . . . . . . . . . . . . . . . 5.2.1 Formaldehyde . . . . . . . . . . . . . . . . . . 5.2.2 Urea . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Ammonia . . . . . . . . . . . . . . . . . . . . . 5.2.4 Diketones . . . . . . . . . . . . . . . . . . . . . 5.2.5 Specialities . . . . . . . . . . . . . . . . . . . . 5.2.6 Polymerization . . . . . . . . . . . . . . . . . . 5.2.7 Manufacture . . . . . . . . . . . . . . . . . . . 5.3 Special Additives . . . . . . . . . . . . . . . . . . . . . 5.3.1 Modifiers . . . . . . . . . . . . . . . . . . . . . 5.3.2 Flame Retardants . . . . . . . . . . . . . . . . . 5.3.3 Production Data of Important Monomers . . . . 5.4 Curing . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Measurement of Curing . . . . . . . . . . . . . . . . . . 5.6 Properties . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.1 Formaldehyde Release . . . . . . . . . . . . . . 5.6.2 Storage . . . . . . . . . . . . . . . . . . . . . . 5.7 Applications and Uses . . . . . . . . . . . . . . . . . . 5.7.1 Glue Resins . . . . . . . . . . . . . . . . . . . . 5.7.2 Binders . . . . . . . . . . . . . . . . . . . . . . 5.7.3 Foundry Sands . . . . . . . . . . . . . . . . . . 5.8 Special Formulations . . . . . . . . . . . . . . . . . . . 5.8.1 Ready-use Powders . . . . . . . . . . . . . . . . 5.8.2 Cyclic Urea Prepolymer in PF Laminating Resins 5.8.3 Liquid Fertilizer . . . . . . . . . . . . . . . . . 5.8.4 Soil Amendment . . . . . . . . . . . . . . . . . 5.8.5 Microencapsulation . . . . . . . . . . . . . . .
283 283 283 283 284 284 284 284 286 290 290 290 291 291 291 292 293 293 293 294 294 294 294 294 294 295 295 295 296
Contents
xiii
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Melamine Resins 6.1 History . . . . . . . . . . . . . . . . . 6.2 Monomers . . . . . . . . . . . . . . . . 6.2.1 Melamine . . . . . . . . . . . . 6.2.2 Other Modifiers . . . . . . . . 6.2.3 Synthesis . . . . . . . . . . . . 6.2.4 Manufacture . . . . . . . . . . 6.3 Properties . . . . . . . . . . . . . . . . 6.4 Applications and Uses . . . . . . . . . 6.4.1 Wood Impregnation . . . . . . 6.5 Special Formulations . . . . . . . . . . 6.5.1 Resins with Increased Elasticity 6.5.2 Microspheres . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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7 Furan Resins 7.1 History . . . . . . . . . . . . . . . . . . . . . . 7.2 Monomers . . . . . . . . . . . . . . . . . . . . . 7.2.1 Furfural . . . . . . . . . . . . . . . . . . 7.2.2 Furfuryl Alcohol . . . . . . . . . . . . . 7.2.3 Specialities . . . . . . . . . . . . . . . . 7.2.4 Synthesis . . . . . . . . . . . . . . . . . 7.3 Special Additives . . . . . . . . . . . . . . . . . 7.3.1 Reinforcing Materials . . . . . . . . . . 7.3.2 Production Data of Important Monomers 7.4 Curing . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Acidic Curing . . . . . . . . . . . . . . 7.4.2 Oxidative Curing . . . . . . . . . . . . . 7.4.3 Ultrasonic Curing . . . . . . . . . . . . 7.5 Properties . . . . . . . . . . . . . . . . . . . . . 7.5.1 Recycling . . . . . . . . . . . . . . . . . 7.6 Applications and Uses . . . . . . . . . . . . . . 7.6.1 Carbons . . . . . . . . . . . . . . . . . . 7.6.2 Chromatography Support . . . . . . . . 7.6.3 Composite Carbon Fiber Materials . . . 7.6.4 Foundry Binders . . . . . . . . . . . . . 7.6.5 Glass Fiber Binders . . . . . . . . . . .
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299 299 299 299 300 300 302 302 303 303 304 304 304 304
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307 307 307 308 309 309 309 310 310 311 311 312 312 312 313 313 313 313 314 315 316 316
xiv
Reactive Polymers Fundamentals and Applications 7.6.6 7.6.7 7.6.8 References .
8
Oil Field Applications . . . . . . . Plant Growth Substrates . . . . . . Photosensitive Polymer Electrolytes . . . . . . . . . . . . . . . . . . . .
Silicones 8.1 History . . . . . . . . . . . . . . . . 8.2 Monomers . . . . . . . . . . . . . . . 8.2.1 Chlorosilanes . . . . . . . . . 8.2.2 Silsesquioxanes . . . . . . . . 8.2.3 Hydrogen Silsesquioxanes . . 8.2.4 Alkoxy Siloxanes . . . . . . . 8.2.5 Epoxy-modified Siloxanes . . 8.2.6 Silaferrocenophanes . . . . . 8.2.7 Synthesis . . . . . . . . . . . 8.2.8 Manufacture . . . . . . . . . 8.3 Modified Types . . . . . . . . . . . . 8.3.1 Chemical Modifications . . . 8.3.2 Fillers . . . . . . . . . . . . . 8.3.3 Reinforcing Materials . . . . 8.4 Curing . . . . . . . . . . . . . . . . . 8.4.1 Curing by Condensation . . . 8.5 Crosslinking . . . . . . . . . . . . . . 8.5.1 Condensation Crosslinking . . 8.5.2 Peroxides . . . . . . . . . . . 8.5.3 Hydrosilylation Crosslinking . 8.6 Properties . . . . . . . . . . . . . . . 8.6.1 Silicone Rubber . . . . . . . 8.6.2 Thermal Properties . . . . . . 8.6.3 Electrical Properties . . . . . 8.6.4 Surface Tension Properties . . 8.6.5 Antioxidants . . . . . . . . . 8.6.6 Gas Permeability . . . . . . . 8.6.7 Weathering . . . . . . . . . . 8.7 Applications and Uses . . . . . . . . 8.7.1 Antifoaming Agents . . . . . 8.7.2 Release Agents . . . . . . . . 8.7.3 Sealing and Jointing Materials
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317 317 317 319
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321 321 322 322 322 322 324 325 325 327 330 331 331 331 332 332 332 334 334 335 335 336 336 336 338 338 338 338 340 340 340 340 341
Contents 8.7.4 Electrical Industry . . . . 8.7.5 Medical Applications . . . 8.8 Special Formulations . . . . . . . 8.8.1 Polyimide Resins . . . . . 8.8.2 Thermal Transfer Ribbons 8.8.3 Self-Assembly Systems . 8.8.4 Plasma Grafting . . . . . 8.8.5 Antifouling Compositions References . . . . . . . . . . . . . . . .
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9 Acrylic Resins 9.1 History . . . . . . . . . . . . . . . . . . . . . . 9.2 Monomers . . . . . . . . . . . . . . . . . . . . . 9.2.1 Specialities . . . . . . . . . . . . . . . . 9.2.2 Synthesis . . . . . . . . . . . . . . . . . 9.2.3 Manufacture . . . . . . . . . . . . . . . 9.3 Special Additives . . . . . . . . . . . . . . . . . 9.3.1 Ultraviolet Absorbers . . . . . . . . . . 9.3.2 Flame Retardants . . . . . . . . . . . . . 9.3.3 Production Data of Important Monomers 9.4 Curing . . . . . . . . . . . . . . . . . . . . . . . 9.4.1 Initiator Systems . . . . . . . . . . . . . 9.4.2 Promoters . . . . . . . . . . . . . . . . . 9.5 Properties . . . . . . . . . . . . . . . . . . . . . 9.5.1 Electrical Properties . . . . . . . . . . . 9.5.2 Hydrolytic and Photochemical Stability . 9.5.3 Recycling . . . . . . . . . . . . . . . . . 9.6 Applications and Uses . . . . . . . . . . . . . . 9.6.1 Acrylic Premixes . . . . . . . . . . . . . 9.6.2 Protective Coatings in Electronic Devices 9.6.3 High-performance Biocomposite . . . . 9.6.4 Solid Polymer Electrolytes . . . . . . . . 9.7 Special Formulations . . . . . . . . . . . . . . . 9.7.1 Silane and Siloxane Acrylate Resins . . . 9.7.2 Marble Conservation . . . . . . . . . . . 9.7.3 Tackifier Resins . . . . . . . . . . . . . 9.7.4 Drug Release Membranes . . . . . . . . 9.7.5 Support Materials for Catalysts . . . . .
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341 341 342 342 342 343 343 344 345
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349 349 350 350 352 353 356 356 356 358 358 358 360 360 360 361 361 361 361 362 363 363 365 365 365 366 366 367
xvi
Reactive Polymers Fundamentals and Applications 9.7.6 9.7.7 9.7.8 9.7.9 References .
Electron Microscopy . Stereolithography . . Laminated Films . . . Ink-jet Printing Media . . . . . . . . . . . . .
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367 367 367 368 369
10 Cyanate Ester Resins 10.1 History . . . . . . . . . . . . . . . . . . . . . . . 10.2 Monomers . . . . . . . . . . . . . . . . . . . . . . 10.2.1 Specialities . . . . . . . . . . . . . . . . . 10.2.2 Synthesis . . . . . . . . . . . . . . . . . . 10.3 Special Additives . . . . . . . . . . . . . . . . . . 10.3.1 Fillers . . . . . . . . . . . . . . . . . . . . 10.3.2 Flame Retardants . . . . . . . . . . . . . . 10.4 Curing . . . . . . . . . . . . . . . . . . . . . . . . 10.4.1 Thermal Curing . . . . . . . . . . . . . . 10.4.2 Curing with Epoxy Groups . . . . . . . . . 10.4.3 Curing with Unsaturated Compounds . . . 10.4.4 Initiator Systems . . . . . . . . . . . . . . 10.5 Properties . . . . . . . . . . . . . . . . . . . . . . 10.5.1 Modelling . . . . . . . . . . . . . . . . . 10.6 Applications and Uses . . . . . . . . . . . . . . . 10.6.1 Composites . . . . . . . . . . . . . . . . . 10.6.2 Electronic Industry . . . . . . . . . . . . . 10.6.3 Spacecraft . . . . . . . . . . . . . . . . . 10.7 Special Formulations . . . . . . . . . . . . . . . . 10.7.1 PT Resins . . . . . . . . . . . . . . . . . . 10.7.2 Blends with Epoxies . . . . . . . . . . . . 10.7.3 Bismaleimide Triazine Resins . . . . . . . 10.7.4 Siloxane Crosslinked Resins . . . . . . . . 10.7.5 Alloys with Thermoplastics . . . . . . . . 10.7.6 Coupling Agents for Cyanate Ester Resins References . . . . . . . . . . . . . . . . . . . . . . . . .
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373 373 373 373 375 377 377 378 379 379 381 382 384 385 385 385 385 385 386 386 386 386 387 388 389 391 391
11 Bismaleimide Resins 11.1 Monomers . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.1 4,4′ -Bis(maleimido)diphenylmethane . . . . . . 11.1.2 2,2′ -Diallyl bisphenol A . . . . . . . . . . . . . 11.1.3 Poly(ethylene glycol) End-capped with Maleimide
397 397 397 397 400
Contents 11.1.4 Bismaleimide Bisimides . . . . . . . . . . 11.1.5 Maleimide Epoxy Monomers . . . . . . . 11.1.6 Phosphorous-containing Monomers . . . . 11.1.7 Multiring Monomers with Pendant Chains 11.1.8 Reactions of Maleimides . . . . . . . . . . 11.1.9 Specialities . . . . . . . . . . . . . . . . . 11.2 Special Additives . . . . . . . . . . . . . . . . . . 11.2.1 Tougheners and Modifiers . . . . . . . . . 11.2.2 Fillers . . . . . . . . . . . . . . . . . . . . 11.2.3 Titanium dioxide . . . . . . . . . . . . . . 11.2.4 Reinforcing Materials . . . . . . . . . . . 11.2.5 Flame Retardants . . . . . . . . . . . . . . 11.3 Curing . . . . . . . . . . . . . . . . . . . . . . . . 11.3.1 Monitoring Curing Reactions . . . . . . . 11.3.2 Polymerization . . . . . . . . . . . . . . . 11.3.3 Interpenetrating Networks . . . . . . . . . 11.4 Properties . . . . . . . . . . . . . . . . . . . . . . 11.4.1 Thermal Properties . . . . . . . . . . . . . 11.4.2 Water Sorption . . . . . . . . . . . . . . . 11.4.3 Recycling . . . . . . . . . . . . . . . . . . 11.5 Applications and Uses . . . . . . . . . . . . . . . 11.5.1 Biochemical Reagents . . . . . . . . . . . 11.6 Special Formulations . . . . . . . . . . . . . . . . 11.6.1 Adhesives . . . . . . . . . . . . . . . . . . 11.6.2 Phosphazene-triazine Polymers . . . . . . 11.6.3 Phosphazene-triazine Polymers . . . . . . 11.6.4 Porous Networks . . . . . . . . . . . . . . 11.6.5 Nonlinear Optical Systems . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . 12 Terpene Resins 12.1 History . . . . . . . . . . . . . . . . . . . . . . 12.2 Monomers . . . . . . . . . . . . . . . . . . . . . 12.2.1 Resin . . . . . . . . . . . . . . . . . . . 12.2.2 Turpentine . . . . . . . . . . . . . . . . 12.2.3 Rosin . . . . . . . . . . . . . . . . . . . 12.2.4 Production Data of Important Monomers 12.3 Curing . . . . . . . . . . . . . . . . . . . . . . .
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xviii
Reactive Polymers Fundamentals and Applications
12.3.1 Homopolymers . . . . . . . . . . . . . . . . . . 12.3.2 Copolymers . . . . . . . . . . . . . . . . . . . . 12.3.3 Terpene Phenolic Resins . . . . . . . . . . . . . 12.3.4 Terpene Maleimide Resins . . . . . . . . . . . . 12.4 Properties . . . . . . . . . . . . . . . . . . . . . . . . . 12.4.1 Solubility . . . . . . . . . . . . . . . . . . . . . 12.4.2 Adhesive Properties . . . . . . . . . . . . . . . 12.4.3 Characterization . . . . . . . . . . . . . . . . . 12.4.4 Recycling . . . . . . . . . . . . . . . . . . . . . 12.5 Applications and Uses . . . . . . . . . . . . . . . . . . 12.5.1 Sealants . . . . . . . . . . . . . . . . . . . . . . 12.5.2 Pressure-sensitive Adhesives . . . . . . . . . . . 12.5.3 Polyacrylate Hot-melt Pressure-sensitive Adhesives 12.5.4 Hot-Melt Adhesives . . . . . . . . . . . . . . . 12.5.5 Coatings . . . . . . . . . . . . . . . . . . . . . 12.5.6 Sizing Agents . . . . . . . . . . . . . . . . . . . 12.5.7 Toner Compositions . . . . . . . . . . . . . . . 12.5.8 Chewing Gums . . . . . . . . . . . . . . . . . . 12.6 Special Formulations . . . . . . . . . . . . . . . . . . . 12.6.1 Toughener for Novolaks . . . . . . . . . . . . . 12.6.2 Fluoro Copolymers . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Cyanoacrylates 13.1 Monomers . . . . . . . . . . . . . . . . . . . . 13.1.1 Synthesis . . . . . . . . . . . . . . . . 13.1.2 Crosslinkers . . . . . . . . . . . . . . 13.1.3 Commercial Products . . . . . . . . . 13.2 Special Additives . . . . . . . . . . . . . . . . 13.2.1 Plasticizers . . . . . . . . . . . . . . . 13.2.2 Accelerators . . . . . . . . . . . . . . 13.2.3 Thickeners . . . . . . . . . . . . . . . 13.2.4 Stabilizers . . . . . . . . . . . . . . . 13.2.5 Primers . . . . . . . . . . . . . . . . . 13.2.6 Diazabicyclo and Triazabicyclo Primers 13.2.7 Polyamine Dendrimers . . . . . . . . . 13.3 Curing . . . . . . . . . . . . . . . . . . . . . . 13.3.1 Photo Curing . . . . . . . . . . . . . .
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451 452 452 454 455 455 456 458 459 459 460 460 461 462 463 463 465 465 466 466 467 467 471 471 471 472 473 475 475 477 480 480 482 483 484 485 485
Contents 13.4 Properties . . . . . . . . . . . 13.5 Applications and Uses . . . . 13.5.1 Manicure Composition 13.5.2 Tissue Adhesives . . . References . . . . . . . . . . . . . .
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486 486 486 487 489
14 Benzocyclobutene Resins 14.1 Modified Polymers . . . . . . . . . . . . . . 14.1.1 Thermotropic Copolymers . . . . . . 14.1.2 BCB-modified Aromatic Polyamides 14.1.3 BCB End-capped Polyimides . . . . 14.1.4 Flame Resistant Formulations . . . . 14.2 Crosslinkers . . . . . . . . . . . . . . . . . . 14.2.1 Modified Poly(ethylene terephthalate) 14.3 Applications and Uses . . . . . . . . . . . . 14.3.1 Applications in Microelectronics . . 14.3.2 Optical Applications . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . .
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493 498 498 498 498 501 501 501 501 502 504 504
15 Reactive Extrusion 15.1 Extruder . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.1 Heat of Polymerization . . . . . . . . . . . . . . 15.1.2 Ceiling Temperature . . . . . . . . . . . . . . . 15.1.3 Strategy of Reactive Extrusion . . . . . . . . . . 15.2 Compositions of Industrial Polymers . . . . . . . . . . . 15.2.1 Poly(styrene) . . . . . . . . . . . . . . . . . . . 15.2.2 Poly(tetramethylene ether) and Poly(caprolactam) 15.2.3 Polyamide 12 . . . . . . . . . . . . . . . . . . . 15.2.4 Poly(butyl methacrylate) . . . . . . . . . . . . . 15.2.5 Poly(carbonate) . . . . . . . . . . . . . . . . . . 15.3 Biodegradable Compositions . . . . . . . . . . . . . . . 15.3.1 Poly(lactide)s . . . . . . . . . . . . . . . . . . . 15.3.2 Biodegradable Fibers . . . . . . . . . . . . . . . 15.3.3 Poly(ε-caprolactone) . . . . . . . . . . . . . . . 15.3.4 Cationically Modified Starch . . . . . . . . . . . 15.3.5 Blends of Starch and Poly(acrylamide) . . . . . 15.3.6 Blends of Protein and Polyester . . . . . . . . . 15.4 Chain Extenders . . . . . . . . . . . . . . . . . . . . . . 15.4.1 Recycling of Poly(ethylene-terephthalate) . . . .
507 508 510 510 511 512 513 513 513 514 514 517 519 521 521 523 523 523 524 524
xx
Reactive Polymers Fundamentals and Applications 15.4.2 Modified Poly(ethylene terephthalate) 15.4.3 Poly(butylene terephthalate) . . . . . 15.5 Related Applications . . . . . . . . . . . . . 15.5.1 Transesterification . . . . . . . . . . 15.5.2 Hydrolysis and Alcoholysis . . . . . 15.5.3 Flame Retardant Master Batch . . . . References . . . . . . . . . . . . . . . . . . . . . .
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16 Compatibilization 16.1 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . 16.2 Basic Terms . . . . . . . . . . . . . . . . . . . . . . . . 16.2.1 Thermodynamic Compatibility . . . . . . . . . 16.2.2 Thermodynamic Models . . . . . . . . . . . . . 16.2.3 Particle Size . . . . . . . . . . . . . . . . . . . 16.2.4 Interfacial Slip . . . . . . . . . . . . . . . . . . 16.2.5 Interpolymer Radical Coupling . . . . . . . . . 16.2.6 Technological Compatibility . . . . . . . . . . . 16.3 Compatibilization by Additives . . . . . . . . . . . . . . 16.3.1 Poly(ethylene) Blended with Inorganic Fillers . . 16.3.2 Filler Materials without Chemical Compatibilizers 16.3.3 Modified Inorganic Fillers . . . . . . . . . . . . 16.3.4 Clay Nanocomposites . . . . . . . . . . . . . . 16.3.5 Thermoplastic Elastomers . . . . . . . . . . . . 16.3.6 Polyamide 6,6 and Poly(butylene terephthalate) . 16.3.7 Poly(ethylene)/Wood Flour Composites . . . . . 16.3.8 Recycled Polyolefins . . . . . . . . . . . . . . . 16.3.9 Block Copolymers . . . . . . . . . . . . . . . . 16.3.10 Impact Modification of Waste PET . . . . . . . 16.3.11 Starch . . . . . . . . . . . . . . . . . . . . . . . 16.3.12 Blends of Cellulose and Chitosan . . . . . . . . 16.4 Reactive Compatibilization . . . . . . . . . . . . . . . . 16.4.1 Coupling Agents for Compatibilization . . . . . 16.4.2 Vector Fluids . . . . . . . . . . . . . . . . . . . 16.4.3 Poly(ethylene) and Polyamide 6 . . . . . . . . . 16.4.4 PEO and PBT . . . . . . . . . . . . . . . . . . 16.4.5 Poly(ethylene-octene) and Polyamide 6 . . . . . 16.4.6 Ethylene Acrylic Acid Copolymers and Polyamide 6 . . . . . . . . . . . . . . . . . . . . . . .
524 525 525 525 526 526 526 531 532 532 532 533 533 534 534 534 538 538 538 539 540 540 541 541 542 542 544 544 545 545 548 549 549 551 552 552
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16.4.7 16.4.8 16.4.9 16.4.10 16.4.11 16.4.12
Sisal Fibers . . . . . . . . . . . . . . . . . . . . Thermotropic Liquid Crystalline Polyesters . . . Ionomers and Ionomeric Compatibilizers . . . . Poly(styrene) . . . . . . . . . . . . . . . . . . . Polyolefins/Poly(ethylene oxide) . . . . . . . . . Poly(phenylene sulfide)/Liquid Crystalline Polymers . . . . . . . . . . . . . . . . . . . . . . . 16.4.13 LDPE/Thermoplastic Starch . . . . . . . . . . . 16.4.14 PE and EVA . . . . . . . . . . . . . . . . . . . 16.4.15 SBR and EVA . . . . . . . . . . . . . . . . . . 16.4.16 NBR and EPDM . . . . . . . . . . . . . . . . . 16.4.17 NBR and PA6 . . . . . . . . . . . . . . . . . . 16.4.18 Poly(carbonate)–Poly(vinylidene fluoride) Blends 16.4.19 Bisphenol A-poly(carbonate) and ABS Copolymers 16.4.20 Kevlar™ . . . . . . . . . . . . . . . . . . . . . 16.4.21 Polyamides . . . . . . . . . . . . . . . . . . . . 16.4.22 Polyethers . . . . . . . . . . . . . . . . . . . . 16.4.23 Polyurethane and Poly(ethylene terephthalate) . 16.5 Functionalization of End Groups . . . . . . . . . . . . . 16.5.1 Mechanisms . . . . . . . . . . . . . . . . . . . 16.5.2 Amino-terminated Nitrile Rubber . . . . . . . . 16.5.3 Functionalization of Olefinic End Groups of Poly(propylene) . . . . . . . . . . . . . . . . . . . . 16.5.4 Muconic Acid Grafted Polyolefin Compatibilizers 16.5.5 Polyfunctional Polymers and Modified Polyolefin References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Rheology Control 17.1 Melt Flow Rate . . . . . . . . . . . 17.2 Rheology Control Techniques . . . 17.2.1 Pelletizing . . . . . . . . . 17.3 Peroxides for Rheology Control . . 17.3.1 Hydroperoxides . . . . . . 17.3.2 Peroxides . . . . . . . . . . 17.3.3 Diacyl Peroxides . . . . . . 17.3.4 Ketone Peroxides . . . . . . 17.3.5 Masterbatches of Peroxides 17.3.6 Peresters . . . . . . . . . .
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552 553 554 556 558 559 559 559 560 560 561 561 561 563 563 564 566 566 566 569 569 573 573 574 587 587 587 589 590 590 591 593 593 595 595
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17.3.7 Properties of Peroxides . . . . . . . . . . . . . . 17.3.8 Azo Compounds . . . . . . . . . . . . . . . . . 17.4 Scavengers . . . . . . . . . . . . . . . . . . . . . . . . 17.4.1 Stable Nitroxyl Radicals . . . . . . . . . . . . . 17.5 Mechanism of Degradation . . . . . . . . . . . . . . . . 17.6 Ultra High Melt Flow Poly(propylene) . . . . . . . . . . 17.7 Irregular Flow Improvement . . . . . . . . . . . . . . . 17.8 Heterophasic Copolymers . . . . . . . . . . . . . . . . . 17.9 Poly(propylene) . . . . . . . . . . . . . . . . . . . . . . 17.9.1 Long Chain Branched Poly(propylene) . . . . . 17.9.2 Effect of MFR on Temperature and Residence Time References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Grafting 18.1 The Techniques in Grafting . . . . . . . . . . . . . . . . 18.1.1 Parameters that Influence Grafting . . . . . . . . 18.1.2 Free Radical Induced Grafting . . . . . . . . . . 18.1.3 Grafting Using Stable Radicals . . . . . . . . . 18.2 Polyolefins . . . . . . . . . . . . . . . . . . . . . . . . 18.2.1 Monomers for Grafting onto Polyolefins . . . . 18.2.2 Mechanism of Melt Grafting . . . . . . . . . . . 18.2.3 Side Reactions . . . . . . . . . . . . . . . . . . 18.2.4 Viscosity . . . . . . . . . . . . . . . . . . . . . 18.2.5 Ceiling Temperature . . . . . . . . . . . . . . . 18.2.6 Effect of Initiator Solubility . . . . . . . . . . . 18.2.7 Distribution of the Grafted Groups . . . . . . . . 18.2.8 Effect of Stabilizers on Grafting . . . . . . . . . 18.2.9 Radical Grafting of Polyolefins with Diethyl Maleate . . . . . . . . . . . . . . . . . . . . . . 18.2.10 Inhibitors for the Homopolymerization of Maleic anhydride . . . . . . . . . . . . . . . . . . . . . 18.2.11 Inhibitors for Crosslinking . . . . . . . . . . . . 18.2.12 Special Initiators . . . . . . . . . . . . . . . . . 18.2.13 Maleic anhydride . . . . . . . . . . . . . . . . . 18.2.14 Polyolefins Grafted with Itaconic Acid Derivatives 18.2.15 Imidized Maleic Groups . . . . . . . . . . . . . 18.2.16 Oxazoline-modified Polyolefins . . . . . . . . . 18.2.17 Modification of Polyolefins with Vinylsilanes . .
595 600 601 601 601 605 605 606 608 608 608 609 611 611 611 614 614 616 616 617 618 618 620 620 622 622 622 623 623 624 629 629 630 630 631
Contents
xxiii
18.2.18 Ethyl Diazoacetate-modified Polyolefins . . . . 18.2.19 Grafting Antioxidants . . . . . . . . . . . . . . 18.2.20 Comonomer Assisted Free Radical Grafting . . . 18.2.21 Radiation Induced Grafting in Solution . . . . . 18.2.22 Characterization of Polyolefin Graft Copolymers 18.2.23 PVC/LDPE Melt Blends . . . . . . . . . . . . . 18.3 Other Polymers . . . . . . . . . . . . . . . . . . . . . . 18.3.1 Poly(styrene) Functionalized with Maleic anhydride . . . . . . . . . . . . . . . . . . . . . . . 18.3.2 Multifunctional Monomers for PP/PS Blends . . 18.3.3 Poly(ethylene-co-methyl acrylate) . . . . . . . . 18.3.4 n-Butyl methacrylate Grafted onto PVC . . . . . 18.3.5 Starch Esterification . . . . . . . . . . . . . . . 18.3.6 Starch Grafted Acrylics . . . . . . . . . . . . . 18.3.7 Thermoplastic Phenol/Formaldehyde Polymers . 18.3.8 Polyesters and Polyurethanes . . . . . . . . . . 18.3.9 Polyacrylic Hot-melt Pressure-sensitive Adhesive 18.4 Terminal Functionalization . . . . . . . . . . . . . . . . 18.4.1 Ene Reaction with Poly(propylene) . . . . . . . 18.4.2 Styrene-butadiene Rubber . . . . . . . . . . . . 18.4.3 Diels-Alder Reaction . . . . . . . . . . . . . . . 18.5 Grafting onto Surfaces . . . . . . . . . . . . . . . . . . 18.5.1 Grafting onto Poly(ethylene) . . . . . . . . . . . 18.5.2 Grafting onto Poly(tetrafluoroethylene) . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Acrylic Dental Fillers 19.1 History . . . . . . . . . . . . . . . . . 19.2 Polymeric Composite Filling Materials . 19.3 Monomers . . . . . . . . . . . . . . . . 19.3.1 Acrylics and Methacrylics . . . 19.3.2 Cyclic Monomers . . . . . . . 19.3.3 Epoxy Monomers . . . . . . . 19.3.4 Highly Loaded Composite . . . 19.4 Radical Polymerization . . . . . . . . . 19.4.1 Chemical Curing Systems . . . 19.4.2 Photo Curing . . . . . . . . . . 19.4.3 Curing Techniques . . . . . . .
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631 632 633 636 636 637 637 637 637 638 638 639 639 640 640 642 642 642 643 643 644 644 647 649 657 658 658 659 659 665 666 668 668 668 673 676
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Reactive Polymers Fundamentals and Applications
19.4.4 Dual Initiator Systems . . . . . . 19.5 Inhibitors . . . . . . . . . . . . . . . . . 19.6 Additives . . . . . . . . . . . . . . . . . 19.6.1 Fillers and Reinforcing Materials 19.6.2 Pigments . . . . . . . . . . . . . 19.6.3 Photostabilizers . . . . . . . . . . 19.6.4 Caries Inhibiting Agents . . . . . 19.6.5 Coloring or Tint Agents . . . . . 19.6.6 Adhesion Promoter . . . . . . . . 19.6.7 Thermochromic Dye . . . . . . . 19.7 Properties . . . . . . . . . . . . . . . . . 19.7.1 Water Sorption . . . . . . . . . . 19.7.2 Cytotoxicity . . . . . . . . . . . 19.8 Applications . . . . . . . . . . . . . . . . 19.8.1 Filling Techniques . . . . . . . . 19.8.2 Primer Emulsions . . . . . . . . References . . . . . . . . . . . . . . . . . . . .
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676 677 677 677 681 681 682 682 682 686 686 686 687 687 687 688 688
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693 694 695 696 696 697 697 698 700 701 701
Index Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . General . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
703 703 724 757
20 Toners 20.1 Toner Components . . . . . . . . . . . . . . . . 20.2 Toner Resins . . . . . . . . . . . . . . . . . . . 20.3 Manufacture of Toner Resins . . . . . . . . . . . 20.3.1 Suspension Polymerization . . . . . . . 20.3.2 Terephthalic Ester Resins . . . . . . . . 20.3.3 Unsaturated Ester Resins . . . . . . . . . 20.3.4 Toner Resins with Low Fix Temperature 20.3.5 Toners for Textile Printing . . . . . . . . 20.4 Characterization of Toners . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . .
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1 Unsaturated Polyester Resins Unsaturated polyester resins consist of two polymers, i.e., a short chain polyester containing polymerizable double bonds and a vinyl monomer. The curing reaction consists of a copolymerization of the vinyl monomer with the double bonds of the polyester. In the course of curing, a three-dimensional network is formed. Unsaturated polyester resins belong to the group of so-called thermosets. There are several monographs and reviews on unsaturated polyesters and unsaturated polyester resins.1–7 We will differentiate between unsaturated polyesters and unsaturated polyester resins. Unsaturated polyesters are the polyesters as they emerge from the condensation vessel. They are rarely sold as such, because they are brittle at room temperature and difficult to handle. Instead, whenever a polyester is freshly synthesized in a plant, it is mixed with the vinyl monomer in the molten state. Thus materials that are viscous at room temperature, with a styrene content of ca. 60% are sold. Such a mixture of an unsaturated polyester with the vinyl polymer is referred to here as an unsaturated polyester resin.
1.1 HISTORY It was realized long ago that some natural oils as well as alkyde resins can be dried by certain additives and used as coatings. This drying results from a polymerization of the unsaturated moieties in the ester molecules. Next it was discovered that the addition of styrene would accelerate the drying. 1
2
Reactive Polymers Fundamentals and Applications
CH3 HO CH2
CH2 OH
HO CH
Ethylene glycol
CH2 OH
Propylene glycol CH3
HO CH2
C CH2 OH CH3
Neopentyl glycol CH2OH H3C
CH2 C CH2OH CH2OH
Trimethylol propane
CH2 CH CH2 OH OH OH Glycerol
Figure 1.1: Diols and Triols Used for Unsaturated Polyester Resins
The invention of unsaturated polyester resins is ascribed to Carleton Ellis (1876–1941). The first patents with regard to polyester resins emerged in the 1930’s.8–10 Commercial production started in 1941 already reinforced with glass fibers for radar domes, also referred to as radomes.
1.2
MONOMERS
According to the composition of an unsaturated polyester resin, the monomers can be grouped in two main classes, i.e., components for the polyester and components for the vinyl monomer.
1.2.1 Monomers for the Unsaturated Polyester (UP) Monomers used for unsaturated polyesters are shown in Table 1.1 and in Figures 1.1 and 1.2. Unsaturated diols are only rarely used.
Unsaturated Polyester Resins
Table 1.1: Monomers for Unsaturated Polyesters Saturated alcohols
Remarks
1,2-Propylene glycol Ethylene glycol
Most common glycol Less compatible with styrene than Propylene glycol Good drying properties Good hydrolysis resistance Trifunctional alcohol, for branched polyesters. Danger of crosslinking during condensation Flame retardant Trifunctional alcohol, cheaper than glycerol Weather resistant for coatings11, 12
Diethylene glycol Neopentyl glycol Glycerol Tetrabromobisphenol A (TBBPA) Trimethylolpropane Trimethylolpropane mono allyl ether (TMPAE) Undecanol Saturated acids and anhydrides Phthalic anhydride Isophthalic acid Terephthalic acid HET acid Tetrabromophthalic anhydride Adipic acid Sebacic acid o-Carboxy phthalanilic acid Unsaturated acids and anhydrides Maleic anhydride Fumaric acid Itaconic acid
Used as chain stopper Remarks Most common anhydride Good hydrolysis resistance Superior hydrolysis resistance Flame retardant systems. In fact, even when addressed as HET acid, the HET anhydride is used Flame retardant systems Soft resins Soft resins 13
Remarks Most common Copolymerizes better with styrene than maleic anhydride
3
4
Reactive Polymers Fundamentals and Applications
COOH
O O
COOH
O Phthalic anhydride
Isophthalic acid
Cl COOH
Cl Cl
Cl
O Cl COOH Terephthalic acid
H
O HET-anhydride
O O
H
O
Cl
O
Maleic anhydride
COOH CH CH HOOC Fumaric acid
Figure 1.2: Acids and Anhydrides Used for Unsaturated Polyester Resins
Unsaturated Polyester Resins 1.2.1.1
5
Alcohol Components
The most common alcohol components are 1,2-propylene glycol and ethylene glycol. Ether containing alcohols exhibit better air drying properties and are used in topcoats. Polyesters based on unsaturated diols can be prepared by the transesterification of diethyl adipate with unsaturated diols, e.g., cis-2-butene-1,4-diol, and 2-butyne-1,4-diol. The transesterification method is a suitable procedure for the preparation of unsaturated polyesters in comparison to the direct polycondensation.14 cis-2-Butene-1,4-diol, the most available aliphatic unsaturated diol, has been used to produce some valuable polymers such as graftable unsaturated segmented polyurethanes and crosslinkable polyesters for medical purposes. 1.2.1.2
Acid and Anhydride Components
A general purpose industrial unsaturated polyester is made from 1,2-propylene glycol, phthalic anhydride, and maleic anhydride. The most commonly used vinyl monomer is styrene. Maleic anhydride without phthalic anhydride would yield a polyester with a high density of double bonds along the polyester chain. This would result in a high crosslinking density of the cured product, thus in a brittle product. Therefore, the unsaturated acid component is always diluted with an acid with non-polymerizable double bonds. Note that aromatic double bonds also will not polymerize with vinyl components. The double bond in HET acid will not polymerize. Fumaric acid copolymerizes well with styrene, but fumaric acid is more costly than maleic anhydride. Therefore, maleic anhydride is the preferred unsaturated acid component. Another aspect is that during the condensation of fumaric acid, 2 mol of water must be removed from the reaction mixtures, whereas in the case of maleic anhydride only 1 mol of water must be removed. Anhydrides are preferred over the corresponding acids because of the higher reactivity. Isophthalic acid and terephthalic acid cannot form an anhydride. These compounds do not condense as fast as phthalic anhydride. On the other hand, the polyesters from isophthalic acid and terephthalic acid are more stable than those made from phthalic anhydride. That is why these polyesters with neopentyl glycol are used in aggressive environments and also as gel coats and top coats. A gel coat is the first layer of a multi layer material; the top coat is the layer on the opposite side. For instance, if a polyester boat is built, the gel coat is first painted into the model. Then
6
Reactive Polymers Fundamentals and Applications
a series of glass fiber reinforced laminates are applied, and finally the top coat is painted. Isomerization. During the synthesis of the polyester, maleic anhydride partly isomerizes to fumaric acid. The isomerization follows a secondorder kinetics because of the catalysis by maleic acid. The activation energy of the isomerization is ca. 63.2 kJ/mol.15 2-Methyl-1,3-propanediol offers significant process advantages to resin producers because it is an easily handled liquid, it has a high boiling point, and it has two primary hydroxyl groups for rapid condensations. Polyester resins produced from 2-methyl-1,3-propanediol using conventional condensation polymerization, however, have relatively low fumarate contents (60 to 70%), and simply increasing the reaction temperature to promote isomerization causes color problems. The two-step process helps increase the degree of isomerization for such systems. First, the aromatic dicarboxylic acid is allowed to react with 2-methyl-1,3-propanediol at a temperature of up to 225°C to produce an ester diol intermediate. In the second step, the intermediate reacts with maleic anhydride and with 1,2-propylene glycol. The resulting unsaturated polyester resin has a fumarate content greater than about 85%.16 The high fumarate content helps the resins to cure quickly and thoroughly with vinyl monomers, giving the resulting thermosets excellent water resistance. 1.2.1.3
Amine Modifiers
The adducts of ethylene oxide or propylene oxide with N,N ′ -diphenylethane-1,2-diamine or N,N-dimethyl-p-phenylene diamine and ethylene oxide with N,N ′ -diphenylhexane-1,6-diamine can be used as modifiers. When used in amounts up to 2%, the amines substantially reduce the gelation time of these modified unsaturated polyesters. However, as the reactivity of the resins increases, their stability decreases.17, 18 1.2.1.4
Dicyclopentadiene
Dicyclopentadiene is used in a wide variety of applications, including elastomers, flame retardants, pesticides, and resins for adhesives, coatings and rubber tackifiers. Approximately 30% of the production is used for unsaturated polyester resins because of its valuable properties.19
Unsaturated Polyester Resins
O
7
O OH
OH H
+ OH
O
O
O
Figure 1.3: Ene Reaction between Maleic acid and Dicyclopentadiene
O
O O
O
O
O
+
O
O
Figure 1.4: Retro Diels-Alder Reaction of Dicyclopentadiene and Diels-Alder Reaction between Maleic acid units and Cyclopentadiene
Dicyclopentadiene polyester resins are synthesized from dicyclopentadiene, maleic anhydride, and a glycol. The reaction is performed in the presence of water to generate maleic acid from the maleic anhydride to form dicyclopentadiene maleate. The ene reaction is shown in Figure 1.3. The maleate is esterified with the glycol to form the unsaturated polyester resin.20, 21 The ene adduct serves to form end-capped polyesters. At higher temperatures dicyclopentadiene undergoes a retro Diels-Alder reaction and can add to the unsaturations of fumaric acid and maleic acid (as pointed out in Figure 1.4), to form nadic acid units When the dicyclopentadiene-modified unsaturated polyester is used for a molding material, the polyester is usually mixed with a radically polymerizable monomer and a polymerization initiator. This allows the viscosity or curing time of the molding material to be suitable for the molding operation.
8
Reactive Polymers Fundamentals and Applications Table 1.2: Vinyl Monomers for Unsaturated Polyester Resins Monomer
Remarks
Styrene p-Vinyltoluene α-Methylstyrene Methyl acrylate Methyl methacrylate Diallyl phthalate Triallyl cyanurate
Most common, but carcinogenic Not really a substitute for styrene Slower curing reaction to avoid thermal stress Good optical properties
Dicyclopentadiene-modified unsaturated polyesters yield molded articles with excellent performance. The function of dicyclopentadiene is to impart air drying characteristics, low-profile properties, high heat distortion, excellent weathering performance, and increased filler dispersibility in the resulting polymer.22
1.2.2 Vinyl Monomers The vinyl monomer serves as solvent for the polyester and reduces its viscosity. Further, it is the agent of copolymerization in the course of curing. Vinyl monomers for unsaturated polyester resins are shown in Table 1.2 and in Figure 1.5. 1.2.2.1
Styrenes
Styrene is the most widely used vinyl monomer for unsaturated polyesters. However, styrene has a carcinogenic potential: therefore,replacing styrene by some other vinyl monomer has been discussed for years. With larger amounts of styrene the rigidity of the material can be increased. α-Methylstyrene forms less reactive radicals, and thus slows down the curing reaction. Therefore, α-methylstyrene is suitable for decreasing the peak temperature during curing. Polar vinyl monomers, such as vinylpyridine, improve the adhesion of the polyester to glass fibers, which is useful in preventing delamination. 1.2.2.2
Acrylates and Methacrylates
Acrylates improve outdoor stability. Methyl methacrylate, in particular, enhances the optical properties. The refractive index can be varied with
Unsaturated Polyester Resins
CH CH2 CH CH2
CH3 Styrene
p-Vinyltoluene
CH3 C CH2 CH2 CH2 CH2 C
α-Methylstyrene
O O CH3
Methyl methacrylate
O C O CH2 CH CH2
C O CH2 CH CH2 O Diallyl phthalate
Figure 1.5: Vinyl Monomers for Unsaturated Polyester Resins
9
10
Reactive Polymers Fundamentals and Applications
CH2CH2 O CH2CH2 O CH2CH2 O
O CH
CH
CH2
CH2
Triethylene glycol divinyl ether
Figure 1.6: Vinyl ethers
mixtures of styrene and methyl methacrylate close to that of glass, so that fairly transparent materials could be produced. 1.2.2.3
Vinyl Ethers
Various vinyl and divinyl ethers have been used as substitutes for styrene. Divinyl ethers with unsaturated polyesters are used preferably in radiation curable compositions and coatings. However, special formulations containing no styrene but triethylene glycol divinyl ether (c.f. Figure 1.6) are available that can be used for gel coats.23 Propenyl ethers are generally easier to prepare than their corresponding vinyl ethers. The propenyl ethers are simply prepared by isomerization of the corresponding allyl ethers. Due to the steric effect of the methyl groups in the propenyl ether molecules, they are expected to be much less reactive than their vinyl ether analogs.24 Examples for propenyl ethers are ethoxylated hexanediol dipropenyl ether and, 1,1,1-trimethylolpropane dipropenyl ether. 1.2.2.4
Other Vinyl Monomers
Triallyl cyanurate enhances the thermal stability of the final products. Since the compound is trifunctional, it enhances the crosslinking density.
1.2.3 Specialities 1.2.3.1
Monomers for Waterborne Unsaturated Polyesters
Waterborne unsaturated polyesters are used for wood coatings. They have UV-sensitive initiator systems. The basic constituents are selected from
Unsaturated Polyester Resins
11
ethylene glycol, 1,2-propylene glycol, diethylene glycol, and tetrahydrophthalic anhydride, terephthalic acid, and trimellitic anhydride.25 The vinyl monomer is trimethylolpropane diallyl ether. The UV-sensitive compound is 2-hydroxy-2-methylphenylpropane-1-one. When diluted with water, the resins exhibit a proper viscosity in the range of 2,500 cps. The cured products show good tensile properties and weatherability. Another method used to make unsaturated polyesters water soluble is to introduce polar hydrophilic groups such as carboxylic and sulfonic groups into the resin molecule, which ensures a good dispersibility in water. An example of such a compound is sodium 5-sulfonatoisophthalic acid. Instead of styrene, glycerol monoethers of allyl alcohol and unsaturated fatty alcohols are used as vinyl monomer.26 Unsaturated polyester resins diluted in water are used for particleboards and fiberboards. They are modified with acrylonitrile and also used as mixtures with urea/formaldehyde (UF) resins. A mixture of a UP resin and a UF resin allows the production of boards which have considerably higher mechanical properties than those bonded exclusively with UF resins.27 1.2.3.2
Low Emission Modifiers
Several methods have been proposed for reducing volatile organic compounds (VOCs) emissions: • Adding skin forming materials, • Replacement of the volatile monomer with a less volatile monomer, • Reduction in the amount of the monomer in the compositions, and • Increasing the vinyl monomers by attaching them onto the polyester chain. Low Volatile Monomers. Styrene can be partly substituted for by low volatile monomers, such as bivalent metal salts of acrylic acid or methacrylic acid. Examples include zinc diacrylate, zinc dimethacrylate, calcium diacrylate, and calcium dimethacrylate.28 The metal salt monomer is typically a solid, and therefore has much lower vapor pressure than, e.g., styrene. The acrylate functionality copolymerizes readily with styrene. Due to the bivalent metal ions, the acrylates
12
Reactive Polymers Fundamentals and Applications
act as crosslinkers of the ionomer type. Therefore, an additional crosslinking occurs in comparison to pure styrene. Acrylate-modified Unsaturated Polyesters. Acrylate-modified unsaturated polyesters may be used for low-viscosity resins and resins with low emission of volatile monomers. In commercially available unsaturated polyester resin applications, up to 50% of styrene or other vinyl monomers are used. During curing some of the organic monomer is usually lost to the atmosphere, which causes occupational safety hazards and an environmental problem. Tailoring the polyester by synthesizing branched structures and incorporating additional vinyl unsaturations has been proposed. The diol alcohols used for condensation may be partly replaced by glycidyl compounds in order to obtain low molecular weight methacrylate or acrylatemodified or terminated polyesters.29 Suitable glycidyl compounds include glycidyl methacrylate and glycidyl acrylate. Not more than 60 mol-% of the alcohols can be replaced by glycidyl compounds.23, 30 These polyesters have low viscosities because of the branched structures. In addition to the maleic or fumaric units, they bear additional unsaturations resulting from the pending reactive acrylate or methacrylate moieties. For this reason these types need less vinyl monomer (styrene) to increase the crosslinking density of the cured product. The increased unsaturation results in a higher reactivity, which in turn leads to an increase in heat distortion temperature and better corrosion resistance, good pigmentability and excellent mechanical and physical properties.31 Such resins are therefore suitable as basic resins in gel coats. 1.2.3.3
Epoxide-based Unsaturated Polyesters
Epoxide-based unsaturated polyesters are obtained from the reaction of half esters of maleic anhydride of fumaric acid with epoxy groups from epoxide resins. For example, n-hexanol reacts easily with maleic anhydride to form acidic hexyl maleate. This half ester is then used for the addition reaction with the epoxy resin.32 Allyl alcohol in the unsaturated resins enhances their properties. The glass-transition temperatures of the epoxy fumarate resins exceed 100°C. The glass-transition temperatures epoxy maleates are higher than 70°C. The resins have good chemical resistance.33
Unsaturated Polyester Resins
HO
13
OH C O
O C O
NH C
Figure 1.7: o-Carboxy phthalanilic acid13
1.2.3.4
Isocyanates
Isocyanates, such as toluene diisocyanate can be added to a formulated resin, such as polyester plus vinyl monomer. The gelation times increase with the concentration of toluene diisocyanate. Also the viscosity increases strongly. Resins with only 3% of toluene diisocyanate are thixotropic.34 An increase in the viscosity is highly undesirable.
1.2.3.5
o-Carboxy phthalanilic acid
A new acid monomer, o-carboxy phthalanilic acid, c.f., Figure 1.7, has been synthesized from o-aminobenzoic acid with phthalic anhydride. This monomer was condensed with different acids and glycols to prepare unsaturated polyesters. These polyesters were admixed with styrene and cured. The final materials were extensively characterized.13, 35 It was found that the styrene/poly(1,2-propylene-maleate-o-carboxy phthalanilate) polyester resin has the highest compressive strength value and the best chemical resistance and physical properties among the materials under investigation.
1.2.4 Synthesis The synthesis of unsaturated polyesters occurs either by a bulk condensation or by azeotropic condensation. General purpose polyesters can be condensed by bulk condensation, whereas more sensitive components need the azeotropic condensation technique, which can be performed at lower temperatures. The synthesis in the laboratory scale does not differ significantly from the commercial procedure.
14
Reactive Polymers Fundamentals and Applications
1.2.4.1
Kinetics of Polyesterification
The kinetics of polyesterification have been modelled. In the models, the asymmetry of 1,2-propylene glycol was taken into account, because it bears a primary and secondary hydroxyl group. The reactivities of these hydroxyl groups differ by a factor of 2.6. The relative reactivity of maleic and phthalic anhydride towards 1,2-propylene glycol, after the ring opening of both anhydrides is complete, increases from ca. 1.7 to 2.3 when the temperature is increased from 160 to 220°C.36 The rate constants and Arrhenius parameters are estimated by fitting the calculated conversion of the acid with time to the experimental data over the entire range of conversion. For the copolyesterification reactions involving two acids, a cross-catalysis model is used.37 The agreement between model predictions and experimental data has been proved to be satisfactory. For example, the energy of activation for the condensation reaction of 2-methyl-1,3-propanediol (MPD) with maleic anhydride was obtained to 65 kJ/mol, and with phthalic anhydride 82 kJ/mol was obtained.
1.2.4.2
Sequence Distribution of Double Bonds
The polycondensate formed by the melt condensation process of maleic anhydride, phthalic anhydride, and 1,2-propylene glycol in the absence of a transesterification catalyst has a non-random structure with a tendency towards blockiness. On the other hand, the distribution of unsaturated units in the unsaturated polyester influences the curing kinetics with the styrene monomer. Segments containing double bonds close together appear to lower the reactivity of the resin due to steric hindrance. This is suggested by the fact that the rate of cure and the final degree of conversion increase as the average sequence length of the maleic units decreases. Due to the influence of the sequence length distribution on the reactivity, the reactivity of unsaturated polyester resins may be tailored by sophisticated condensation methods. Methods to calculate the distributions have been worked out.38, 39 Monte Carlo methods can be used to investigate the effects of the various rate constants and stoichiometry of the reactants. Also, structural asymmetry of the diol component and the influence of the dynamics of the ring opening of the anhydride is considered.
Unsaturated Polyester Resins
15
O
O CH3
OH
O + CH CH2 OH OH O
O CH CH2 OH O
CH3
Figure 1.8: Reaction of Maleic anhydride with 1,2-Propanediol
1.2.5 Manufacture Unsaturated polyesters are still produced in batch. Continuous processes have been invented, but are not widespread. Most common is a cylindrical batch reactor equipped with stirrer, condenser, and a jacket heater. Thus the synthesis in laboratory and in industry is very similar. The typical size of such reactors is between 2 and 10 m3 . We now illustrate a typical synthesis of an unsaturated polyester. The reactor is filled at the room temperature with the glycol, in slight excess to compensate the losses during the condensation. Losses occur because of the volatility of the glycol, but also due to side reactions. The glycol may eliminate water at elevated temperatures. Then maleic anhydride and phthalic anhydride are charged to the reactor. Typical for a general purpose unsaturated polyester resin is a ratio of 1 mol maleic anhydride, 1 mol phthalic anhydride, and 1.1 mol 1,2-propylene glycol. Further, other components, such as adhesion promoters, can be added. The reactor is sparged with nitrogen and slowly heated. At ca. 90°C the anhydrides react with the glycol in an exothermic reaction. This is the initial step of the polyreaction, shown in Figure 1.8. At the end of the exothermic reaction a condensation catalyst may be added. Catalysts such as lead dioxide, p-toluenesulfonic acid, and zinc acetate40 affect the final color of the polyester and the kinetics of curing. Temperature is raised carefully up to 200°C, so that the temperature of the distillate never exceeds ca. 102 to 105°C. Otherwise the glycol distills out. The reaction continues under nitrogen or carbon dioxide atmosphere. The sparging is helpful for removing the water. Traces of oxygen could cause coloration. The coloration emerges due to multiple conjugated double bonds. Maleic anhydride is helpful in preventing coloration, because the series of conjugated double bonds are interrupted by a DielsAlder reaction. In the case of sensitive components, e.g., diethylene glycol,
16
Reactive Polymers Fundamentals and Applications
even small amounts of oxygen can cause gelling during the condensation reaction. There are certain variations of water removal. Simply sparging with inert gas is referred to as the melt condensation technique. In the case of thermal sensitive polyesters, the water may be removed by the azeotrope technique. Toluene or xylene is added to the reaction mixture. Both compounds form an azeotrope with water. During reflux, water separates from the aromatic solvent and can be collected. In the final stage, the aromatic solvent must be removed either by enhanced sparging or under vacuum. The azeotrope technique is in general preferred, because condensation proceeds faster than in the case of melt condensation. Vacuum also can be used to remove the water, although this technique is used only rarely for unsaturated polyesters because of the risk of removal of the glycol. At early stages, the progress of the condensation reaction can be controlled via the amount of water removed. In the final stage, this method is not sufficiently accurate and the progress is monitored via the acid number. Samples are withdrawn from the reactor and are titrated with alcoholic potassium hydroxide (KOH) solution. The acid number is expressed in milligrams KOH per gram of resin. Even though other methods for the determination of the molecular weight are common in other fields, the control of the acid number is the quickest method to follow the reaction. The kinetics of self-catalyzed polyesterification reactions follows a third-order kinetic law. Acid catalyzed esterification reactions follow a second-order kinetics. In the final stage of the reaction, the reciprocal of the acid number is linear with time. General purpose unsaturated polyester resins are condensed down to an acid number of around 50 mg KOH/g resin. This corresponds to a molecular weight of approximately 1000 Dalton. After this acid number has been reached, some additives are added, in particular polymerization inhibitors, e.g., hydroquinone, and the polyester is cooled down, to initiate the mixing with styrene. The polyester should be cooled down to the lowest possible temperature. In any case the temperature of the polyester should be below the boiling point of the vinyl monomer. There are two limiting issues: 1. If the polyester is too hot, after mixing with the vinyl monomer a preliminary curing may take place. In the worst case the resin may gel.
Unsaturated Polyester Resins
17
Table 1.3: Inhibitors and Retarders for Unsaturated Polyester Resins Inhibitor
Retarder
Hydroquinone 1,4-Naphthoquinone p-Benzoquinone Chloranil Catechol Picric acid
2,4-Pentanedione
2. If the polyester is too cold, its viscosity becomes too high, which jeopardizes the mixing process. Mixing can occur in several ways: either the polyester is poured into styrene under vigorous stirring, or under continuous mixing, or the styrene is poured into the polyester. The last method is preferred in the laboratory. After mixing, the polyester resin is then cooled down to room temperature as quickly as possible. Finally some special additives are added, such as promoter for preaccelerated resin composition. An unsaturated polyester resin is not miscible in all ratios with styrene. If an excess of styrene is added, a two-phase system will emerge. The resins have a slightly yellow color, mainly due to the inhibitor. The final product is filtered, if necessary, and poured into vats or cans.
1.3 SPECIAL ADDITIVES 1.3.1 Inhibitors There is a difference between inhibitors and retarders. Inhibitors stop the polymerization completely, whereas retarders slow down the polymerization rate. Inhibitors influence the polymerization characteristics. They act in two ways: 1. Increasing the storage time 2. Decreasing the exothermic peak during curing Common inhibitors are listed in Table 1.3. Inhibitors are used to increase the storage time, and also to increase the pot life time. Sometimes a combination of two or more inhibitors is used, since some types of inhibitors act more specifically on the storage time and others influence the pot life time.
18
Reactive Polymers Fundamentals and Applications
The storage time of an unsaturated polyester resin increases with the amount of inhibitor. Storage at high temperatures decreases the possible shelf life. On the other hand, high doses of inhibitor detrimentally influence the curing of the resin. Higher amounts of radical initiators are required in the presence of high doses of inhibitors. The exothermic peak during curing is reduced. This influences the degree of monomer conversion. A high degree of conversion is needed to have optimal properties.
1.3.2 Thickeners 1.3.2.1
Multivalent Salts
For sheet molding compounds and bulk molding compounds, the resins are thickened. This can be achieved particularly with MgO, at a concentration of about 5%. It is believed that it first interacts with the carboxylic acid group on chains. Then a complex is formed with the salt formed and the carboxylic acid groups of other chains, leading to an increase in viscosity. The maximum hardness is achieved at 2% MgO with an increase from 190 MPa to 340 MPa for the specimen cured at room temperature. High temperature curing decreases hardness. 1.3.2.2
Thixotropic Additives
For gel coat applications, fumed silica, precipitated silica or an inorganic clay can be used. Hectorite and other clays can be modified by alkyl quaternary ammonium salts such as di(hydrogenated tallow) ammonium chloride. These organoclays are used in thixotropic unsaturated polyester resin systems.41
1.3.3 Emission Suppressants If a polyester is exposed to open air during curing, the vinyl monomer can easily evaporate. This leads to a change in the composition and thus to a change in the glass transition temperature of the final product.42 Still more undesirable is the emission of potentially toxic compounds. There are several approaches to achieving products with low emission rates. The earliest approach has been the use of a suppressant which reduces the loss of volatile organic compounds. The suppressants are often
Unsaturated Polyester Resins
19
waxes. The wax-based products are of a limited comparability with the polyester resin. The wax-based suppressants separate from the system during polymerization or curing, forming a surface layer which serves as a barrier to volatile emissions. For example, a paraffin wax having a melting point of about 60°C significantly improves the styrene emission results. Waxes with a different melting point from this temperature will not perform adequately at the low concentrations necessary to maintain good bonding and physical properties while inhibiting the styrene emissions.43 The waxy surface layer must be removed before the next layer can be applied, because waxes are likely to cause a reduction in the interlaminar adhesion bond strength of laminating layers. Suppressants selected from polyethers, polyether block copolymers, alkoxylated alcohols, alkoxylated fatty acids or polysiloxanes show a suppression of the emission as well and better bonding properties.44–46 Unsaturated polyesters that contain α,β-unsaturated dicarboxylic acid residues and allyl ether or polyalkylene glycol residues (so-called gloss polyesters) require no paraffin for curing the surface of a coating, because the ether groups initiate an autoxidative drying process.47
1.3.4 Fillers Examples for fillers include calcium carbonate powder, clay, alumina powder, silica sand powder, talc, barium sulfate, silica powder, glass powder, glass beads, mica, aluminum hydroxide, cellulose yarn, silica sand, river sand, white marble, marble scrap, and crushed stone. In the case of glass powder, aluminum hydroxide, and barium sulfate the translucency is imparted on curing.48 Common fillers are listed in Table 1.4. Fillers reduce the cost and change certain mechanical properties of the cured materials. 1.3.4.1
Inorganic Fillers
Bentonite. Ca-bentonite is used in the formulation of unsaturated polyester-based composite materials. Increasing the filler content, at a constant styrene/polyester ratio, improves the properties of composites. Maximum values of compressive strength, hardness, and thermal conductivity of composites are observed at about 22.7% of styrene, whereas the water absorption capacity was a minimum at a styrene content of 32.8%.49
20
Reactive Polymers Fundamentals and Applications Table 1.4: Fillers for Unsaturated Polyester Resins Filler
Reference
Bentonite Calcium carbonate Clay Glass beads Flyash Woodflour Rubber particles Nanocomposites
49 50 51 52 53 54 55–57
Montmorillonite. Sodium montmorillonite and organically modified montmorillonite (MMT) were tested as reinforcing agents. Montmorillonite increases the glass transition temperatures. At 3-5% modified montmorillonite content, the tensile modulus, tensile strength, flexural modulus and flexural strength values showed a maximum, whereas the impact strength exhibited a minimum. Adding only 3% of organically modified montmorillonite improved the flexural modulus of an unsaturated polyester by 35%. The tensile modulus of unsaturated polyester was also improved by 17% at 5% of montmorillonite.51 Instead of styrene, 2-hydroxypropyl acrylate (HPA) as a reactive diluent has been examined in preparing an unsaturated polyester/montmorillonite nanocomposite.58 The functionalization of MMT can be achieved with polymerizable cationic surfactants, e.g., with vinylbenzyldodecyldimethyl ammonium chloride (VDAC) or vinylbenzyloctadecyldimethyl ammonium chloride (VOAC). Polymerizable organophilic clays have been prepared by exchanging the sodium ions of MMT with these polymerizable cationic surfactants.59 With an unsaturated polyester, nanocomposites consisting of UP and clay were prepared. The dispersion of organoclays in UP caused gel formation. In the UP/VDAC/MMT system, intercalated nanocomposites were found, while in the UP/VOAC/MMT system partially exfoliated nanocomposites were observed. When the content of organophilic montmorillonite is between 25% and 5%, the mechanical properties, such as the tensile strength, the impact strength, the heat resistance, and the swelling resistance of the hybrid are enhanced. The properties are better than those of composites prepared with pristine or non-polymerizable quaternary ammonium-modified montmorillonite.60
Unsaturated Polyester Resins
21
Flyash. Flyash is an inexpensive material that can reduce the overall cost of the composite if used as filler for unsaturated polyester resin. A flyash-filled resin was found to have a higher flexural modulus than those of a calcium carbonate-filled polyester resin and an unfilled resin. Flyash was found to have poor chemical resistances but good saltwater, alkali, weathering, and freeze-thaw resistances.52 An enhancement of the tensile strength, flexural strength, and impact strength is observed when the flyash is surface-treated with silane coupling agents.61
1.3.4.2
Wood flour
Plant-based fillers like sawdust, wood flour and others are utilized because of their low density, and their relatively good mechanical properties and reactive surface. The main disadvantage is the hygroscopicity62 and the difficulties in achieving acceptable dispersion in a polymeric matrix. Surface modification of these materials can help reduce these problems. Wood flour can be chemically modified with maleic anhydride to improve the dispersion properties and adhesion to the matrix resin. This treatment decreases the hygroscopicity, but excessive esterification has to be avoided, because it leads to the deterioration of the wood flour, adversely affecting its mechanical properties.53 The incorporation of wood flour into the resin increases the compression modulus and the yield stress but decreases the ultimate deformation and toughness in all cases. Thermogravimetric analysis of wood flour indicates changes in the wood structure occur as a consequence of chemical modifications. Alkaline treatment reduces the thermal stability of the wood flour and produces a large char yield. In composites a thermal interaction between fillers and matrix is observed. Thermal degradation of the composites begins at higher temperature than the neat wood flours.63
1.3.4.3
Rubber
Rubber particles toughen the materials.54, 64 They act also as low-profile additives. A low-profile additive, in general, diminishes shrinking in the course of curing.
22
Reactive Polymers Fundamentals and Applications
Toughening. Rubbers with functional groups have been tested in blends of unsaturated polyesters with respect to improving the mechanical properties. In particular, functional rubbers such as hydroxy terminated poly(butadiene), epoxidized natural rubber, hydroxy-terminated natural rubber, and maleated nitrile rubber were tested. The performance of a maleic anhydride-grafted-nitrile rubber is superior to all other rubbers studied. The improvement in toughness, impact resistance, and tensile strength is achieved without jeopardizing other properties.65 Rubber as Low-profile Additive. A low-profile additive consisting of a styrene-butadiene rubber solution is prepared by heating styrene with hydroquinone up to 50°C. Into this liquid a styrene-butadiene rubber is dissolved to obtain a resin solution having a solid content of 35%. This solution is taken as a low-profile additive.66 1.3.4.4
Nanocomposites
Only a few nanocomposite materials are commercially available and these materials are very expensive. In order to make a successful nanocomposite, it is very important to be able to disperse the filler material thoroughly throughout the matrix to maximize the interaction between the intermixed phases. Titanium dioxide. Titanium dioxide nanoparticles with 36 nm average diameter have been investigated. The nanoparticles have to be dispersed by direct ultrasonification.55 The presence of the nanoparticles has a significant effect on the quasi-static fracture toughness. Even at small volume fractions an increase in toughness is observed. The changes in quasi-static material properties in tension and compression with increasing volume fraction of the nanoparticles are small due to the weak interfacial bonding between the matrix and the filler. The dynamic fracture toughness is higher than quasi-static fracture toughness. Quite similar experimental results have been presented by another group.67 Titanium dioxide nanoparticles can also be bound by chemical reaction to the polyester itself.68 Aluminum Oxide. It was observed that the addition of untreated, Al2 O3 particles does not result in an enhanced fracture toughness. Instead, the fracture toughness decreases.56, 57 However, adding an appropriate amount
Unsaturated Polyester Resins
23
Table 1.5: Reinforcing Materials for Unsaturated Polyesters Fiber
Reference
Glass fibers Jute Sisal Hemp Wollastonite Barium titanate
69 70 71 70 72
of (3-methacryloxypropyl)trimethoxysilane to the liquid polyester resin during particle dispersion process leads to a significant enhancement of the fracture toughness due to the crack trapping mechanism being promoted by strong particle-matrix adhesion. For example, the addition of 4.5% volume fraction of treated Al2 O3 particles results in a nearly 100% increase in the fracture toughness of the unsaturated polyester.
1.3.5 Reinforcing Materials Suitable reinforcing materials are shown in Table 1.5. The application of reinforcement fibers is strongly governed by the relation of the price of matrix resin and fiber. Therefore, expensive fibers, such as carbon fiber, are usually used with epoxide resins, not with unsaturated polyester resins. If the fiber is expensive and has superior properties, then the matrix resin should have superior properties. 1.3.5.1
Glass Fibers
The most common “fillers” are reinforcing materials, like glass fibers. Because of the unavoidable shrinking during curing, interfacial stresses between resin and glass fiber arise that lower the adhesion forces. To enhance the adhesion, glass fibers are surface-modified. Silane coupling agents such as (3-methacryloxypropyl)trimethoxysilane and (3-aminopropyl)triethoxysilane are preferably used. In the case of (3-methacryloxypropyl)trimethoxysilane the pendent double bonds may take part in the curing reaction; thus chemical linkages between resin and glass surface are established.
24
Reactive Polymers Fundamentals and Applications
The surface free energy and the mechanical interfacial properties especially showed the maximum value for 0.4% silane coupling agent.73, 74 In an E-glass/vinylester composite it was observed that the fibers significantly inhibit the final conversion.75 1.3.5.2
Wollastonite
A suitable coupling agent for wollastonite is (3-methacryloxypropyl)trimethoxysilane. In such a treated wollastonite-unsaturated polyester composite, the tensile and flexural strength increase initially with the wollastonite content and then decrease. The flexural strength reaches an optimum value at 30% wollastonite content, whereas the tensile strength reaches an optimum point at 50% wollastonite content.70 1.3.5.3
Carbon Fibers
Reports on carbon fiber reinforced polyester are rare.76 Carbon fibers have mainly been used in aerospace with epoxide resins or high temperature thermoplastics, whereas polyesters have found application in large-volume and low-cost applications with primarily glass fibers as reinforcement. The combination of carbon fibers and polyester matrix is becoming more attractive as the cost of carbon fibers decreases. In comparison to epoxide resins, unsaturated polyester exhibits a relatively low viscosity. This property makes them well suited for the manufacture of large structures.77 The interfacial shear strength with untreated carbon fibers increases with increasing degree of unsaturation of the polyester. The unsaturation is adjusted by the amount of maleic anhydride in the feed. This is explained by a contribution of chemical bonding of the double bonds in the polymer to the functional groups of the carbon fiber surface.77 1.3.5.4
Natural Fibers
Agrowastes and biomass materials, e.g., sawdust, wood fibers, sisal, bagasse, etc. are slowly penetrating the reinforced plastics market, presently dominated by glass fibers and other mineral reinforcements. These fillers have very good mechanical properties and low density, and are loaded into polymeric resin matrices to make useful structural composite materials.62
Unsaturated Polyester Resins
25
Jute. Jute as reinforcing fiber is particularly significant from an economic point of view. On a weight and cost basis, bleached jute fibers are claimed to have better reinforcement properties than other fibers.69 Sisal. Sisal fiber is a vegetable fiber having specific strength and stiffness that compare well with those of glass fiber. Most synthetic resins are, however, more expensive than the sisal fiber, making these composites less attractive for low-technology applications. Therefore, for sisal fibers naturally occurring resol-type resins, cashew nut shell liquid is an attractive alternative.78 For unsaturated polyester composites the surface treatment of sisal fibers is done with neopentyl(diallyl)oxy tri(dioctyl)pyrophosphatotitanate as the coupling agent.70, 79 In a sisal/wollastonite reinforcing system for unsaturated polyester resins, the tensile strength and the flexural strength drop with increasing sisal content. Sisal composites with unsaturated polyesters can be formulated to be flame retarded using decabromodiphenyloxide and antimony trioxide to reach a satisfactory high state of flame retardancy.80
1.3.6 Mold Release Agents Mold release agents are needed for the molding processes, i.e., for the manufacture of bulk molding compounds and sheet molding compounds. There are two classes of mold release agents: 1. External mold release agents, 2. Internal mold release agents. External mold release agents are applied directly to the mold. This procedure increases the manufacturing time and must be repeated every one to five parts. In addition, the mold release agent builds up on the mold, so the mold must be cleaned periodically with a solvent or washing agent. This is costly and time consuming. Internal mold release agents are added directly into the molding compound. Since they do not have to be continuously reapplied to the mold, internal mold release agents increase productivity and reduce cost. There are mostly internal mold release agents, e.g., metal soaps, amine carboxylates, amides, etc. Zinc stearate acts by exuding to the surface
26
Reactive Polymers Fundamentals and Applications
of the molding compound, thereby contacting the mold and providing lubrication at the mold surface to permit release. Liquid mold release agents are liquid zinc salts and phosphate esters and higher fatty acid amines.81 The amine salts are obtained simply by neutralizing the acids with appropriate amines.
1.3.7 Low-profile Additives Low-profile additives (LPA) reduce the shrinking of the cured products. Shrinking causes internal voids and reduced surface quality. Thermoplastic resins are added to reduce shrinking, e.g., poly(vinyl acetate). This additive absorbs some styrene in the early stages of curing. When the temperature is increased in the course of curing, the styrene eventually evaporates and consequently a counter pressure is formed which counterbalances the shrinking. The successful performance of low-profile additives depends essentially on the phase separation phenomena in the course of curing, c.f. Section 1.4.4.2. The effects of poly(vinyl acetate), poly(vinyl chloride-co-vinyl acetate), and poly(vinyl chloride-co-vinyl acetate-co-maleic anhydride), have been studied.82, 83 The curing rate decreases with an increase of the molecular weight of the low-profile additive which causes the chain entanglement effect. The plasticizing effect is reduced with an increase in the molecular weight of the low-profile additive.84 Low-profile additives with higher molecular weight and lower content of additive seem to work better under low-temperature curing conditions.85 Polymers from the acrylic group have been tested as low-profile additives. In particular, binary copolymers from methyl methacrylate and nbutyl acrylate, and ternary copolymers from methyl methacrylate, n-butyl acrylate, and maleic anhydride have been studied.86, 87 The volume fraction of microvoids generated during the curing process is governed by the stiffness of the UP resin, the compatibility of the uncured ST/UP/LPA systems, and the glass-transition temperature of the low-profile additive. A good volume shrinkage control can be achieved by raising the curing temperature slowly to allow sufficient time for phase separation, and going to a high final temperature to allow the formation of microvoids.88 Dilatometric studies in the course of curing of a low-profile resin
Unsaturated Polyester Resins
27
containing poly(vinyl acetate)89 have revealed that there are two transition points in both volume and morphological changes in the course of curing. The thermoplastics start to be effective on shrinkage control at the first transition point when the low-profile additive-rich phase and the unsaturated polyester resin-rich phase become co-continuous. At the second transition point when the fusion among the particulate structures is severe, the shrinkage control effect vanishes. The relative rate of polymerization in the two phases plays an important role in shrinkage control. Instead of poly(vinyl acetate) a copolymer with acrylic acid or itaconic acid should have better properties as a low-profile additive. This is based on the assumption that the presence of acid groups on the copolymer chain changes the selectivity of the cobalt promoter, and therefore, the relative reaction rate in the thermoplastic-rich and the unsaturated polyester resin-rich phases during polymerization. Itaconic acid is about twice as acidic as acrylic acid and more reactive than maleic acid or fumaric acid. The two carboxyl groups allow the introduction of larger amounts of acidity into the copolymer even at rather low comonomer concentrations in comparison to acrylic acid. The monoester of 2-hydroxyethyl acrylate and tetrachlorophthalic anhydride also has been proposed as a comonomer. The acidity of tetrachlorophthalic anhydride is much stronger than that of itaconic acid because of the four chloro substituents in its structure. Samples with an acid-modified low-profile additive showed an earlier volume expansion during curing, as a result of faster reaction in the low-profile additive-rich phase.90 The relative reaction rate in the two phases can be controlled in addition to the selectivity control by the low-profile additive in a reverse manner, i.e., by the addition of secondary vinylic comonomers and special promoters. Secondary monomers, such as divinylbenzene and trimethylolpropane trimethacrylate, were added to the formulation. 2,4-Pentandione was chosen as co-promoter.91 In fact, the combination of trimethylolpropane trimethacrylate and 2,4-pentandione increased the reaction rate in the low-profile rich-phase. Methyl methacrylate was tested as a secondary monomer.92 At a low ratio of methyl methacrylate to styrene, the amount of residual styrene decreases and the volume shrinkage of the resin system remains unchanged. However, at a high ratio of methyl methacrylate to styrene, the amount of residual styrene can be substantially reduced. This advantageous behav-
28
Reactive Polymers Fundamentals and Applications
ior occurs because of the monomer reactivity ratios. However, the study of shrinkage shows that methyl methacrylate has a negative effect on the shrinkage control. Styrene has a polymerization shrinkage of 15% and methyl methacrylate has a shrinkage of 20%. Therefore, the addition of methyl methacrylate contributes to a larger volume shrinkage. The performance of a low-profile additive becomes less effective when the molar ratio of methyl methacrylate to styrene exceeds 0.1. A dual initiator system, i.e., methylethylketone peroxide/tert-butylperoxybenzoate, was used in combination with cobalt octoate as a promoter. tert-Butylperoxybenzoate cannot be considered a low temperature initiator because the reaction temperature needs to reach almost 90°C to ensure the proper progress of the reaction. On the other hand, tertbutylperoxybenzoate is more active compared to methylethylketone peroxide at high temperatures, because the latter completely decomposes. tert-Butylperoxybenzoate is therefore a good initiator to finish the reaction. Volume shrinkage measurements of the resin system initiated with dual initiators revealed that a good performance of the low-profile additive was achieved at low temperatures (e.g., 35°C) and high temperatures (100°C) but not at intermediate temperatures.93 It was found that in bulk molding compounds calcium stearate, which is primarily used as an internal mold release agent, is active as a low-profile additive.94 Even when added in small quantities, some internal mold release agents may provoke the formation of a polyester-rich phase in the form of spherical globules ca. 60 µ m.
1.3.8 Interpenetrating Polymer Networks An interpenetrating polymer network is a mixture of two or more polymers that are not necessarily independently crosslinked. If another polymer that is capable of crosslinking separately is added to an unsaturated polyester resin, the physical properties can be enhanced dramatically. Other special types of such systems are also addressed as hybrid systems. 1.3.8.1
Polyurethanes
For example, besides the unsaturated polyester resin, compounds that simultaneously form a crosslinkable polyurethane are added, such as poly-
Unsaturated Polyester Resins
29
glycols and diisocyanates.95 The rate of reaction of one component might be expected to be reduced due to the dilutional effects by the other components.96 However, during free radical polymerization, the reaction may become diffusion controlled and a Trommsdorff effect emerges. The Trommsdorff effect consists of a self-acceleration of the overall rate of polymerization. When the polymerizing bulk becomes more viscous as the concentration of polymer increases, the mutual deactivation of the growing radicals is hindered, whereas the other elementary reaction rates, such as initiation and propagation, remain constant. For an unsaturated polyester resin – polyurethane system, the rate of the curing process increased substantially in comparison to the pure homopolymers. Collateral reactions between the polyurethane isocyanate groups and the terminal unsaturated polyester carboxyl groups were suggested that may lead to the formation of amines, c.f. Eq. 1.1. R−N = C = O + R′ COOH → R−NHCO−O−CO−R′ → R−NHCO − R′ + CO2
(1.1)
These amines may act as promoters of the curing process. Moisture, which does not influence the curing reaction of the unsaturated polyester resin, would also lead to the formation of amines by the reaction of water with the isocyanate groups.97 A tricomponent interpenetrating network system consisting of castor oil-based polyurethane components, acrylonitrile, and an unsaturated polyester resin (the main component) was synthesized in order to toughen the unsaturated polyester resin. By incorporating the urethane and acrylonitrile structures, the tensile strength of the matrix (unsaturated polyester resin) decreased and flexural and impact strengths were increased.98 1.3.8.2
Epoxides
Mixtures of unsaturated polyester resin systems and epoxy resins also form interpenetrating polymer networks. Since a single glass transition temperature for each interpenetrating polymer network is observed, it is suggested that both materials are compatible. On the other hand, an interlock between the two growing networks was suggested, because in the course of curing, a retarded viscosity increase was observed.99 A network interlock
30
Reactive Polymers Fundamentals and Applications
is indicated by a lower total exothermic reaction during simultaneous polymerization in comparison to the reaction of the homopolymers.100 In bismaleimide-modified unsaturated polyester–epoxy resins, the reaction between unsaturated polyester and epoxy resin could be confirmed by IR spectral studies.101 The incorporation of bismaleimide into epoxy resin improved both mechanical strength and thermal behavior of the epoxy resin. 1.3.8.3
Vinylester Resins
Unsaturated polyesters modified with up to 30% of vinylester oligomer are tougheners for the unsaturated polyester matrix. The introduction of vinylester oligomer and bismaleimide into an unsaturated polyester resin improves thermomechanical properties.102 1.3.8.4
Phenolic Resins
An interpenetrating network consisting of an unsaturated polyester resin and a resol-type of phenolic resin improves heat resistance but also helps to suppress the smoke, toxic gas, and heat release during combustion in comparison to a pure unsaturated polyester resin.103 1.3.8.5
Organic-inorganic Hybrids
Organic-inorganic polymer hybrid materials can be prepared using an unsaturated polyester and silica gel. First an unsaturated polyester is prepared. To this polyester the silica gel precursor is added, i.e., tetramethoxysilane, methyltrimethoxysilane, or phenyltrimethoxysilane. Gelling of the alkoxysilanes was achieved at 60°C using HCl catalyst in the presence of the unsaturated polyester resin. It was confirmed by nuclear magnetic resonance spectroscopy that the polyester did hydrolyze during the acid treatment. Finally, the interpenetrating network was formed by photopolymerization of the unsaturated polyester resin.104 It is assumed that between the phenyltrimethoxysilane and the aromatic groups in the unsaturated polyester resin π-interactions arise.
1.3.9 Polyurethane Hybrid Networks The mechanical properties of the unsaturated polyester resin can be greatly improved by incorporating a polyurethane linkage into the polymer net-
Unsaturated Polyester Resins
31
work. The mechanical properties also can be altered by the techniques used in segmented polyurethanes. The basic concept is to use soft segments and hard segments. The polyester is prepared with an excess of diol and dilute with styrene as usual. Additional diols as chain extenders are blended into the resin solution. 4,4′ -Diphenylmethane diisocyanate dissolved in styrene is added to form the hybrid linkages. Suitable peroxides are added to initiate the radical curing. The curing starts with the reaction between the isocyanates and the hydroxyl groups, thus forming the polyurethane linkage. Then the crosslinking reaction takes place.105 The mechanical properties of the hybrid networks were generally improved by the incorporation of a chain extender at room temperature. Hexanediol increased the flexibility of the polymer chains, resulting in a higher deformation and impact resistance of the hybrid networks. Hybrid networks with ethylene glycol as the chain extender are stiffer.
1.3.10 Flame Retardants Flame retardant compositions can be achieved by flame retardant additives, by flame retardant polyester components, and by flame retardant vinyl monomers. Halogenated compounds are still common, but there is a trend towards substituting these compounds with halogen free compositions. In halogenated systems, bromine atoms mostly are responsible for the activity of the retardant. On the other hand, a disposal problem arises when a pyrolytic recycling method is intended at the end of the service times of such articles. Flame retardants are summarized in Table 1.6. In general, bromine compounds are more effective than chlorine compounds. Suitable additives are chlorinated alkanes, brominated bisphenols and diphenyls. Antimony trioxide is synergistic to halogenated flame retardants. It acts also as a smoke suppressant in various systems.106 1.3.10.1
Flame Retardant Additives
Decabromodiphenyloxide. Decabromodiphenyloxide with 2% of antimony trioxide increases the oxygen index values linearly with the bromine content. Some improvement of the mechanical properties can be achieved by adding acrylonitrile to the polyester.107 Decabromodiphenyloxide with
32
Reactive Polymers Fundamentals and Applications Table 1.6: Flame Retardants for Unsaturated Polyester Resins Flame Retardant Aluminum hydroxide Melamine diphosphate Melamine cyanurate Ammonium polyphosphate Antimony trioxide Zinc hydroxystannate 2-Methyl-2,5-dioxo-1-oxa-2-phospholane Decabromodiphenyloxide HET acid 2,6,2′ ,6′ -Tetrabromobisphenol A (TBBPA) Tetrachlorophthalic anhydride Tetrabromophthalic anhydride
Remarks Reference
108 109
Synergist Reactive Reactive Reactive Reactive Reactive
110–112 113 114 115
antimony trioxide increases the activation energy of the decomposition of the unsaturated polyester.114 Aluminum Hydroxide. Fillers, such as aluminum hydroxide, yield crystallization water at higher temperatures, thus achieving a certain flame retardancy. At high degrees of filling in the range of 150 to 200 parts of aluminum hydroxide per 100 parts of unsaturated polyester resin, it is possible to achieve self-extinguishing and a low smoke density. A disadvantage of such systems is that the entire material has a high density. The density can be reduced, if hollow filler is used for reinforcement.116 Lower amounts of aluminum hydroxide are sufficient, if red phosphorus and melamine or melamine cyanurate is admixed.108 Magnesium hydroxide acts in a similar way to aluminum hydroxide. Ammonium polyphosphate. Ammonium polyphosphate is a halogenfree flame retardant for unsaturated polyester resin composites.109 Commonly used are ammonium polyphosphates having the general formula (NH4 )n+2 Pn O3n+1 . A significant reduction of the flame spread index is achieved by a combination of a polyhydroxy compound, a polyphosphate, melamine, cyanuric acid, melamine salts, e.g., melamine cyanurate, and a polyacrylate monomer.117 The fire retardant polyacrylate component should be distinguished
Unsaturated Polyester Resins
33
O O
H3C P CH2 O C
CH2
O P CH2
CH2
C O
CH3
O
Figure 1.9: Ring opening of 2-Methyl-2,5-dioxo-1-oxa-2-phospholane113
from the unsaturated monomers that may be included as crosslinkers in the resin systems. It cannot be ruled out that the polyacrylate may become involved in the crosslinking reactions of such systems. However, it has been observed that the fire retardant effect of the polyacrylates is also effective in those resin systems that do not involve curing by way of unsaturated groups. Preferred polyacrylates are those having backbones of a type that is known to contribute to char formation, for example, those having alkylene or oxyalkylene backbones.118 Reactive Phosphor Compound. Oxaphospholanes are heterocyclic compounds. Certain derivatives are reactive to alcohols and can be incorporated in a polyester backbone. Due to their phosphor content they also act as flame retardants, with the advantage that they are chemically bound to the backbone.113 The ring opening reaction of 2-methyl-2,5-dioxo-1-oxa2-phospholane is shown in Figure 1.9. As a side effect, phosphoric compounds increase the adhesion of the final products, without toughening too much. Expandable Graphite. The flammability of crosslinked unsaturated polyester resins is reduced by the addition of expandable graphite even at levels as low as 7 phr. Expandable graphite is particularly useful when used in combination with ammonium polyphosphate or with a halogenated flame retardant.119 1.3.10.2
Flame Retardant Polyester Components
The flame retardant can be also built in the polymer backbone. Examples are HET acid, tetrachlorophthalic anhydride, and tetrabromophthalic anhydride. The mechanical properties decrease with increasing halogen content
34
Reactive Polymers Fundamentals and Applications
in the backbone.120 HET acid is used for fireproof applications, e.g., for panels in subways, etc. 1.3.10.3
Flame Retardant Vinyl Monomers
Dibromostyrene is a suitable brominated vinyl monomer.121 However, it is not commonly used. Dibromoneopentylglycol and diallyl ether of bromobutenediol have been used as curing agents for unsaturated polyester resins in paint coatings. Both monomers act effectively against inhibition by oxygen. The bromine content decreases the flammability of the final products. The monomers can be obtained in a direct allylation by the use of allyl bromide. The resins can be photocured in a system consisting of mono- or diazide and hydroxyalkylphenone.122 Flame-retardant polyester resin polymers wherein the ability of the polyester resin to transmit light is not significantly affected can be formulated using, instead of antimony trioxide, organic antimony compounds together with halogen flame retardants. Antimony ethylene glycoxide (i.e., ethylene glycol antimonite) can be incorporated in the polyester backbone. Antimony tri-alloxide and antimony methacrylate are vinyl monomers.123 The antimony alkoxides can be prepared by dissolving antimony trichloride in a slight excess of the corresponding alcohol in an inert solvent, e.g., carbon tetrachloride or toluene and sparging with anhydrous ammonia. The antimony acylates are prepared by mixing the stoichiometric amounts of saturated or unsaturated acid and antimony alkoxide. In addition to good light transmission, polyester resins may contain a smaller proportion of combined antimony than those produced using antimony trioxide and still retain their self-extinguishing properties. Moreover, a smaller proportion of chlorine, than for formulations using antimony trioxide, is sufficient to retain the self-extinguishing properties.
1.3.11 Production Data Global production data of the most important monomers used for unsaturated polyester resins are shown in Table 1.7.
1.4
CURING
Curing is achieved in general with a radical initiator and a promoter. A promoter assists the decomposition of the initiator delivering radicals, even
Unsaturated Polyester Resins
35
Table 1.7: Global Production/Consumption Data of Important Monomers and Polymers124 Monomer
Mill. Metric tons
Methyl methacrylate Styrene Phthalic anhydride Isophthalic acid Dimethyl terephthalate (DMT) and terephthalic acid (TPA) Adipic Acid Bisphenol A Maleic anhydride 1,4-Butanediol Dicyclopentadiene Unsaturated Polyester Resins
Year
Reference
2 21 3.2 0.270
2002 2001 2000 2002
125
75 2 2 1.3 1 0.290 1.6
2004 2001 1999 2001 2003 2002 2001
129
126 127 128
130 131 132 133 134 135
at low temperatures at which the initiator alone does not decompose at a sufficient rate. Promoters are also addressed as accelerators.
1.4.1 Initiator Systems Even when a wide variety of initiators are available, common peroxides are used for cold curing and hot curing. Coatings of unsaturated polyester resins are cured with light sensitive materials. Peroxide initiators include ketone peroxides, hydroperoxides, diacyl peroxides, dialkyl peroxides, alkyl peresters, and percarbonates. Azo compounds, such as 2,2′ -azobis(isobutyronitrile) and 2,2′ -azobis(2-methylbutyronitrile) are also suitable. These curing agents can be used alone, or two or more can be used in combination. Some peroxide initiators are shown in Table 1.8.
1.4.1.1
In-Situ Generated Peroxides
Allyl alcohol propoxylate can generate a peroxide in-situ in the presence of metal salt promoter. This peroxide cures the unsaturated polyester resin. The curing proceeds with a very low exothermic reaction and low product shrinkage.136
36
Reactive Polymers Fundamentals and Applications Table 1.8: Peroxide Initiators Peroxide Type
Example
Ketone Peroxides
Methylethylketone peroxide Acetylacetone peroxide Cumene hydroperoxide Dibenzoyl peroxide Dicumyl peroxide tert-Butylcumyl peroxide tert-Butylperoxy-2-ethylhexanoate tert-Butylperoxybenzoate tert-Amylperoxybenzoate tert-Hexylperoxybenzoate bis(4-tert-Butylcyclohexyl)peroxydicarbonate
Hydroperoxides Diacyl peroxides Dialkyl peroxides Alkyl peresters
Percarbonates
1.4.1.2
Functional Peroxides
Peroxides can be functionalized. Functional peroxides based on pyromellitic dianhydride, poly(ethylene glycol)s and tert-butyl hydroperoxide contain two types of functional groups: 1. Carboxylic groups that can participate in ionic reactions, 2. Peroxide groups that can initiate certain radical reactions. The oligoesters are able to form three-dimensional networks when heated to 130 °C.137 1.4.1.3
Photoinitiators
Photoinitiators are mostly used for coating applications. Some common photoinitiators are listed in Table 1.9. A common problem is yellowing during curing. α-Aminoacetophenones and thioxanthone derivatives impart yellowness. Such derivatives are used in thin layers. Although suitable initiators for clear systems have become available only in the last few years, photoinitiators for pigmented systems have been developed for some time. Problems with regard to the absorbtion of ultraviolet light, needed for curing, arise when the coating is pigmented or when it is UV stabilized for outdoor applications. Ultraviolet stabilizers consist of ultraviolet absorbers or hindered amine light stabilizers. The curing performance depends on the pigment absorption and particle size.
Unsaturated Polyester Resins
37
Table 1.9: Common Photoinitiators Photoinitiator
Reference
Benzoin methyl ether 2,2-Dimethoxy-2-phenylacetophenone 2-Hydroxy-2-methylphenylpropane-1-one α-Hydroxy-acetophenone Bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide 2-Hydroxy-2-methyl-1-phenyl-propan-1-one 2,4,6-Trimethylbenzoyldiphenylphosphine oxide Bis(2,6-dichlorobenzoyl)-(4-propylphenyl)phosphine oxide
138
139 140
The adsorption of bisacylphosphine oxides is in the near UV-visible range, and thus at much lower energy than other common photoinitiators. Those photoinitiators therefore allow the curing of thick pigmented layers. Acylphosphine oxides were originally used in dental applications. Acylphosphine oxides and bisacylphosphine oxides are prone to solvolysis attack; that is why the carbon phosphor bond is shielded by bulky groups. Earlier investigations on acylphosphine oxides, in particular 2,4,6-trimethylbenzoyldiphenylphosphine oxide, did not show any advantage over 2,2-dimethoxy-2-phenylacetophenone. It was even concluded that acylphosphonates cannot be considered useful photoinitiators.141, 142 A mixture of bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide and 2-hydroxy-2-methyl-1-phenyl-propan-1-one is suitable for curing thick pigmented furniture coatings.140 The structures of these compounds are shown in Figure 1.10. Further, a combination of a bis-acylphosphineoxide and an α-hydroxy-acetophenone photoinitiator overcomes the limitations imposed by filtering of UV radiation by the pigments and provides a balanced cure.139 The chloro compounds, e.g., bis(2,6-dichlorobenzoyl)-(4-propylphenyl)phosphine oxide are less satisfactory, c.f. Figure1.11.
1.4.2 Promoters There is a difference between hydroperoxides such as methylethylketone peroxide and peroxides, such as dibenzoyl peroxide. Redox promoters, e.g., cobalt naphthenate, can stimulate the decomposition of hydroperoxides catalytically, whereas they cannot stimulate the decomposition of di-
38
Reactive Polymers Fundamentals and Applications
OCH3 O
CH3O O
C P C OCH3
H3C
CH3O
CH3
CH2
C CH2
CH CH3
CH3 Bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine O CH3 C C OH CH3 2-Hydroxy-2-methyl-1-phenyl-propan-1-one
Figure 1.10: Bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine and 2-Hydroxy-2-methyl-1-phenyl-propan-1-one
peroxides. Therefore, for hydroperoxides only catalytic amounts of metal salts are necessary, whereas the salts do not act readily on diperoxides. The mechanism of catalytic action of metal salts is shown in Eq. 1.2. ROOH + Co2+ → RO · +OH− + Co3+ ROOH + Co3+ → ROO · +H+ + Co2+
(1.2)
The cobalt ion is either oxidized or reduced by the peroxides depending on its value. If too much promoter is added, then the exotherm can be very high. Since the thermal conductivity of polymers is small, the heat of reaction cannot be transported out of the resin. The material would overheat and gas bubbles would form. Promoters can be metal soaps, e.g., cobalt octoate or manganese octoate, or further metal chelates such as cobalt acetylacetonate and vanadium acetylacetonate. These promoters are redox promoters and amine compounds such as N,N-dimethylaniline. These accelerators can be used alone, or two or more kinds of them can be used in combination. Examples of promoters are shown in Table 1.10. The auxiliary accelerator is used for enhancing the potency of the accelerator and includes, for
Unsaturated Polyester Resins
Cl
39
Cl O
O
C P C Cl
H3C
CH2
Cl
CH2
Bis(2,6-dichlorobenzoyl)-(4-propylphenyl)phosphine oxide CH3 O O H3C
C P CH3
2,4,6-Trimethylbenzoyl-diphenylphosphine oxide
Figure 1.11: 2,4,6-Trimethylbenzoyldiphenylphosphine oxide and Bis(2,6-dichlorobenzoyl)-(4-propylphenyl)phosphine oxide
Table 1.10: Promoters Promoter Type
Example
Metal soaps
Cobalt octoate Manganese octoate Cobalt acetylacetonate Vanadium acetylacetonate N,N-Dimethylaniline
Metal chelates Amine compounds
40
Reactive Polymers Fundamentals and Applications Table 1.11: Initiator Promoter Systems Initiator
Promoter
Methylethylketone peroxide Dibenzoyl peroxide Di-tert-butyl peroxide tert-Butylperoxybenzoate
Cobalt naphthenate N,N-Dimethylaniline
Temperature °C 20 60 130 130
example, acetylacetone, ethyl acetoacetate and anilide acetoacetate. These auxiliary accelerators can be used alone, or two or more of them can be combined.
1.4.3 Initiator Promoter Systems Some common initiator promoter systems are shown in Table 1.11. Methylpropylketone peroxide offers some advantage over methylethylketone peroxide, as the curing times are shorter.143 Diperoxyketal initiators are used for high-temperature molding processes. Dichloroacetic acid is a suitable promoter. It does not negatively influence the pot life and the cure cycle.144
1.4.4 Polymerization The initiators together with the accelerator initiate a crosslinking copolymerization. The monomer reactivity ratios for the system styrene/fumarate indicate an alternating system, i.e., a styrene radical reacts with a fumarate unit, and vice versa. On the other hand, the system styrene/maleate will tend to form blocks. Therefore, the fumarate system yields final products with better properties. Fortunately the maleate unit isomerizes during the condensation reaction. If a nonazeotropic composition is used, then the ratio of styrene to polymerizable double bonds in the polyester varies in the course of curing. Such systems show a decrease in network density in the course of conversion.145 1.4.4.1
Kinetics of Curing
The kinetics of curing can be conveniently investigated by differential scanning calorimetry and infrared spectroscopy. Both methods have been com-
Unsaturated Polyester Resins
41
pared.146 The overall conversion measured by differential scanning calorimetry is in-between the styrene consumption and the consumption of the pending double bonds in the polyester obtained by infrared spectroscopy. The curing of laminates containing 50 to 70% glass fiber mat can be monitored by Raman spectroscopy.147 Also, white and lightly colored gel coats can easily be monitored by Raman spectroscopy, but fluorescent problems are encountered with heavily colored pigments. Using differential scanning calorimetry, both isothermal runs and temperature programmed runs can be used. Usually a complete conversion is not achieved during ordinary curing. There are two portions of reaction enthalpy that can be investigated under laboratory conditions, 1. The enthalpy characterizing the styrene homopolymerization and copolymerization during curing, 2. A residual enthalpy that can be determined by heating up to near the degradation point of the resin. At isothermal curing experiments, it was found that the sum of enthalpy of polymerization and residual enthalpy depends on the curing (isothermal) temperature.148 An unsaturated polyester resin initiated with a curing system of methylethylketone peroxide and a cobalt salt as promoter was studied by dynamic scans from −100°C to 250°C at heating rates from 2°C/min to 25°C/min. The amount of heat generated by a curing reaction decreases with increasing heating rate. The energy of activation of the overall reaction is around 90 kJ/mol. The traces can be fitted by either an empiric model or a model based on the theory of free-radical polymerization.149 The rate of curing depends on the amount of initiator added to the mixture. A universal isoconversional relationship of the type t = d − b ln[I]0 +
Ea RT
(1.3)
was established that expresses the dependency of the curing time t on the temperature, T, and the initial concentration of the initiator [I]0 and the energy of activation Ea .150 Gel Point. At a certain conversion the reacting mixture rather suddenly changes its appearance: it gels. The gel point is an important parameter for
42
Reactive Polymers Fundamentals and Applications
the pot life time. The gel point can be determined most simply by stirring from time to time, although there are other more sophisticated methods available. A well-known phenomenon in radical polymerization is the acceleration at moderate conversion which is addressed as the Trommsdorff effect. This effect can also be observed in crosslinking polymerization. The increase in rate causes a temperature rise in the bulk material. It was found that the gel time corresponds closely to the initial rise of the temperature.151 The same is true when inhibitors are added or when the curing system is changed. For example, the addition of a tert-butyl catechol inhibitor increases the gel time in a linear fashion and the exothermic reaction is similarly delayed. An increase in the concentrations of initiator (either methylethylketone peroxide or acetyl acetone peroxide) or cobalt octoate promoter decreases the gel time. The gel point has been extrapolated by thermal mechanical analysis, as the point at which the shrinkage rate drops to zero and the dimensions of the material show no appreciable change.152 The curing characteristics can also be measured by the change of ultrasonic properties in the course of curing.153 The sound velocity is constant until the gel point is reached. Afterwards the sound velocity increases to a plateau. Reaching the plateau indicates the end of the chemical reaction. The attenuation reaches a maximum which is attributed to the vitrification. The transition into the vitreous state causes a strong change of the acoustic properties. The glass transition temperature increases continuously with conversion. When the glass transition temperature reaches the polymerization temperature, then vitrification occurs. Vitrification strongly hinders the mobility of the reactive groups. For this reason, the polymerization reaction slows down or stops before complete conversion is reached. The increase of the longitudinal sound velocity with curing time can be associated with the increase of longitudinal modulus L′ , while the irreversible viscous losses are responsible for the increase of sound attenuation. Kinetic Model. To describe the curing behavior of sheet molding compounds, a kinetic model based on radical polymerization mechanisms was developed.154 In the model, three radical reaction steps are involved:
Unsaturated Polyester Resins
Initiation : I0 → 2R· Propagation : R ·n +M → R·n+1 Inhibition : R· → Products
43
(1.4)
Here I0 is the (initial) initiator concentration, R·n a growing radical with chain length n, and M a monomer unit. R· refers to the total concentration of growing radicals. The kinetic constants were experimentally obtained by differential scanning calorimetry (DSC) measurements in model unsaturated polyester resins. Another kinetic model has been presented that is based on the irreversible thermodynamic fluctuation theory. Because the glass transition temperature is related to molecular relaxation processes, the chemical kinetics also can be explained in terms of fluctuation theory.155 The physical or mechanical properties of polymers during curing can be expressed by Eq. 1.5 P(∞) − P(t) (1.5) = exp −(t/τ)β . P(∞) − P(0) P(t), P(∞), and P(0) is some property at times t,∞, and 0, β is a constant, and τ is the curing relaxation time, τ ∝ exp(H/RT ), where H is the activation energy of the curing reaction. If the property P is addressed as the monomer concentration, then the left hand term in Eq. 1.5 is the fraction of unreacted monomer 1 − α. Thus the conversion α is a function of the curing relaxation time, reaction time, and the reaction temperature.155, 156
1.4.4.2
Phase Separation
A phase separation may occur in the course of curing, when styrene is in excess. In this case a crosslinked phase and a poly(styrene) rich phase appear. In the case of unsaturated polyester systems, the phase separation occurs mainly by chemical changes of the system, in contrast to the more common thermally induced phase separation. The phase separation is therefore addressed as a chemically induced phase separation. Thermodynamic models have been established to understand this phenomenon.157 The final morphology of the resin is primarily determined by the phase separation process and the gelation resulting from the polymerization.158 The cured polymer of a single phase resin shows a flake-like structure, while spherical particles form in the two-phase system.159
44
Reactive Polymers Fundamentals and Applications
The phase behavior can be observed by measuring the glass transition temperatures where shoulders are observed in the presence of a twophase system. The shoulders become more evident utilizing dynamic mechanical analysis by plotting log tan δ vs. temperature.160 Phase separation is an important feature in low-profile resin systems. Here the system separates in a thermoplastic-rich phase and in an unsaturated polyester-rich phase. This two-phase structure provides a weak interface where microcracking can initiate and microvoids can form to compensate the shrinkage.90 In such systems an optical microscope equipped with a heating chamber is employed to observe the phase separation process during curing. At the same time, conversion is monitored by infrared spectroscopy. The results show that the copolymerization routes locate between the azeotropic and the alternating copolymerization line, and shift gradually toward the azeotropic line.
1.5
PROPERTIES
1.5.1 Structure Properties Relationships The properties can be widely influenced by the choice of the components, since there is a wide variety of compounds. Some aspects are briefly indicated in Table 1.1. Aliphatic chains, both in the acid moiety and in the diol moiety, will result in comparatively soft materials. Therefore, 1,2-butanediol and diethylene glycol and adipic acid will make the resin softer than phthalic anhydride. The rigidity decreases in the following order: 1,2-propanediol, 2,3-butanediol, 1,4-butanediol, dipropylene glycol, diethylene glycol. For acids the rigidity decreases in the order orthophthalic acid, isophthalic acid, succinic acid, adipic acid, glutaric acid, isosebacic acid, and pimelic acid.1 More rigid materials do not absorb water as much as flexible materials. Therefore, because there is less water available, the rigid materials show better resistance to hydrolysis. Bisphenol A and neopentyl glycol-containing resins shield the access of small molecules to the ester group and therefore they exhibit a better chemical resistance. The crosslink density grows with the amount of maleic anhydride feed. The rigidity can be controlled with the content of maleic anhydride in the polyester. The glass transition temperature also increases with increasing crosslinking density.
Unsaturated Polyester Resins
45
The resistance against hydrolysis increases, as the ester linkages are more stable. Bulky alcohol molecules, like neopentyl glycol, cyclohexanediol, or hydrogenated bisphenol A, are used for hydrolytic resistant materials. The alcohols are used in combination with isophthalic acid and terephthalic acid.
1.5.2 Hydrolytic Stability The ester group is a weak link with regards to hydrolysis. Hydrolysis occurs in aqueous media and is enhanced at elevated temperatures and in particular in alkaline media. The long-term behavior of glass fiber-reinforced plastic pipes was tested in an aqueous environment at 20°C. The strength of the wet pipes after a 1000 hour loading reduced to about 60% of the dry strength in short-term loading.161
1.5.3 Recycling 1.5.3.1
Poly(ethylene terephthalate) Waste Products
Oligomers obtained from depolymerization of poly(ethylene terephthalate) waste products can be reused. The glycolysis products can be used for the synthesis of polyester polyols for rigid polyurethane foams and also for the synthesis of unsaturated maleic or fumaric polyester resins. Bis(2-hydroxyethyl)terephthalate (BHET) is the main product from the glycolysis of poly(ethylene terephthalate). A mixture of maleic anhydride and sebacic acid is added and a condensation is performed.162 The glycolysis reaction is conducted by heating poly(ethylene terephthalate) and the glycol in a nitrogen atmosphere at a temperature preferably within a range from 200°C to 260°C to obtain a terephthalate oligomer48 containing two to three terephthalate units. Zinc acetate is a suitable transesterification catalyst.163 Unsaturated polyesters based on the glycolyzed poly(ethylene terephthalate)with propylene glycol or diethylene glycol and mixtures of both glycols show a broad bimodal molecular weight distribution. Larger molecular weight oligomers were obtained with increasing diethylene glycol contents in the glycol mixtures. The tensile modulus decreased and the toughness of cured products increased with increasing diethylene glycol content.164
46
Reactive Polymers Fundamentals and Applications
A study of the glycolysis of waste bottles made from poly(ethyleneterephthalate) and back condensation with maleic anhydride indicated that the type of glycol used in glycolysis had a significant effect on the characteristics of the uncured and cured resins.165 On the other hand, it was also found that no separation of the type of bottles was needed before glycolysis, since the resins prepared from either water bottles, soft drink bottles, or a mixture of both bottles showed all the same characteristics. The properties of materials recycled in this way have been presented in detail.166 Similarly, residues from the manufacture of dimethyl terephthalate has been tried as feedstock for the aromatic acid component and condensed with maleic anhydride.167 The complete process of how to come from a poly(ethylene terephthalate) to a suitable unsaturated polyester resin composition is described in detail in the literature.168 The glycolysis products can be directly incorporated in an unsaturated polyester resin composition. However, toluene diisocyanate as an intermediating agent must be added. The isocyanate accelerates the curing significantly.169 It is proposed that at the beginning of the curing, the isocyanate reacts with the oligo glycols to form chain-extended products. The glycolysis product acts as a modifier that improves the mechanical properties of the resulting composites. The procedure allows an effective utilization of the waste products. It is reasonable to use only partly glycolyzed products, when the molecular mass of the degradation products is still higher. 1.5.3.2
Cured Unsaturated Polyester Resin Waste
Cured unsaturated polyester resin waste can be decomposed with a decomposition component such as a dicarboxylic acid or a diamine to obtain resin raw material. The unsaturated polyester resin is re-synthesized with this resin raw material.170 It is also possible to synthesize polyurethane resin by reacting the glycolic raw material with a diisocyanate compound.171
1.6
APPLICATIONS AND USES
The properties can be adjusted in a wide range, since a wide variety of basic materials can be used. Consequently, unsaturated polyesters have a very wide area of application. They can be used either as pure resin or with
Unsaturated Polyester Resins
47
fillers, or reinforced, respectively. One of the early uses of unsaturated polyesters was to produce cast items such as knife and umbrella handles, encapsulation of decorative articles, and electronic assemblies.
1.6.1 Decorative Specimens Pure resins can be used for embedding of decorative specimens. Together with a photosensitive curing formulation, furniture coatings are on the market. The most important casting application is the manufacture of buttons.
1.6.2 Polyester Concrete Polymer concrete is usually composed of silica sand and a binder consisting of a thermoset resin, such as unsaturated polyester. Polyester concrete is more resistant to chemicals than conventional concrete. An unsaturated polyester concrete is developed by adding the methyl methacrylate monomer to the resin to improve the early-age strength and the workability of the UP polymer concrete.172 The study revealed that the workability is remarkably improved as the methyl methacrylate content is increased. The ratio of filler to binder is an important parameter for the workability.
1.6.3 Reinforced Materials Bulk and sheet molding compounds are used in a wide variety of areas such as transportation, electrical applications, building, and construction. Reinforced unsaturated polyester resins are used for the manufacture of articles for sanitary furniture, panels, pipes, boats, etc. There are several techniques for manufacturing the final products, i.e., • • • • • •
Hand lay-up process Fiber spray-up process Cold press molding, hot press molding Sheet molding, bulk molding Wet-mat molding Pultrusion
In the hand lay-up process, parts in an open, glass reinforced, mold are produced. First the mold surface is treated with release wax and then
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Reactive Polymers Fundamentals and Applications
coated with a special polyester resin, the so-called gel coat. Then glass fibers are placed into the mold and impregnated with the formulated resin which cures after a short time. This procedure is repeated several times, until the desired thickness is reached. Finally a top coat is placed. In this way, for example, glass fiber reinforced boats can be fabricated. The fiber spray-up process is an improvement of the hand lay-up process. A spray system is used to apply both chopped glass strands and the polyester resin. The spray system places simultaneously resin, catalyst, and glass strands by means of air pressure. The fiber spray-up process is much faster than hand lay-up process and can be automated. In cold press molding and hot press molding, a preimpregnated fiber is placed in presses and cured there. In sheet molding and bulk molding, the resin is mixed with the reinforcing material, either in bulk form or as mats or sheets. To the resin a thickener is added. The articles are formed in presses. The situation is similar in wet-mat molding. In pultrusion, the reinforcement fiber is wheeled off a spool, dipped into a resin mixture, and pulled through a heated die to cure the compound.
1.6.4 Coatings Unsaturated polyester resins are used for a wide variety of coatings. The formulations are usually thixotropic. Curing is mostly achieved by UVsensitive initiators. 1.6.4.1
Powder Coatings
Thermosetting powder coatings have gained considerable popularity over liquid coatings for a number of reasons. Powder coatings are virtually free of harmful fugitive organic solvents normally present in liquid coatings. They give off little, if any, volatiles to the environment when cured. This eliminates solvent emission problems and exposure risk of workers employed in the coating operations. Powder coatings also improve working hygiene, since they are in dry solid form with no messy liquids associated with them to adhere to the clothes of the workers and the coating equipment. Furthermore, they are easily swept up in the event of a spill without requiring special cleaning and spill containment supplies.
Unsaturated Polyester Resins
49
Table 1.12: Special Applications of Polyester Resins Application
Reference
Polyester concrete Bone cement Coatings Road paints Electronic and microwave industries Electrically conductive resins Toner material Compatibilizers Pour point depressants Reactive melt modifier
173 174 175 121 176 177 178 179 180
Another advantage is that they are 100% recyclable. Over sprayed powders are normally recycled during the coating operation and recombined with the original powder feed. This leads to very high coating efficiencies and minimal waste generation. However, in spite of the many advantages, powder coatings traditionally have not been suitable for heat sensitive substrates, such as wood and plastic articles, due to the high temperatures demanded to fuse and cure the powders. Unsaturated polyester powder coatings are available that undergo rapid polymerization at low temperatures, making them particularly attractive for coating of heat sensitive substrates. Low temperature curable unsaturated polyester powder coatings contain polyols with active hydrogens. Allylic, benzylic, cyclohexyl, and tertiary alkyl hydrogen atoms are readily abstracted during free radical induced curing to form the corresponding stable allylic, benzylic, cyclohexyl, and tertiary alkyl free radicals, all of which promote curing at the surface of the coating film in an open air atmosphere. A suitable polyol is 1,4-cyclohexanedimethanol.181
1.7 SPECIAL FORMULATIONS Unsaturated polyester resins have a broad field of application. Unsaturated polyester resins for special purposes are summarized in Table 1.12.
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Reactive Polymers Fundamentals and Applications
1.7.1 Electrically Conductive Resins Electrically conductive resins can be formulated by the addition of carbon black particles. The particles have a strong tendency to agglomerate in a low-viscosity resin. The agglomeration process generates electrically conductive paths already in the uncured state. The fully cured resins containing carbon black above percolation concentration have a constant, temperature-independent conductivity, over a wide temperature range.176
1.7.2 Poly(ε-caprolactone)-perfluoropolyether Copolymers Basically, fluorinated materials are attractive modifying agents because of their unique properties such as chemical inertness, solvent and high temperature resistance, barrier properties, low friction coefficient and low surface tension. These properties can be imparted to other polymeric materials by blending or copolymerization. This type of modification has been usually achieved by the use of fluorine-containing comonomers of low molecular weight which usually lead to homogeneous UP resin and therefore have to be added in significant amounts to achieve an appreciable performance improvement. Furthermore, the high cost of fluorinated monomers leads to very expensive polymeric materials. Unsaturated polyester resins can be modified by hydroxy-terminated telechelic∗ perfluoropolyethers as comonomers during the synthesis of the polyester.182 A disadvantage of this approach is the reactivity of these materials. A fraction of perfluoropolyether does not react. Another method of introducing fluorine into the unsaturated polyester resins is simply blending fluorinated materials. A problem arises, however, because fluorinated polymers are usually immiscible with nonfluorinated polymers. They segregate in a separate phase with poor adhesion to the non-fluorinated matrix, leading to poor mechanical properties. However, separate block or graft copolymers containing fluorinated segments can be prepared that are compatible with the unsaturated polyester resin. Poly(ε-caprolactone)-perfluoropolyether block copolymers are prepared by ring opening polymerization of ε-caprolactone with fluorinated ´ claw of a crab, an oligomer or polymer with well defined τε´ λoς: end and χηλη: ˜ end groups, often star branched, whereas τηλη means far, therefore better telochelic ∗ from
Unsaturated Polyester Resins
51
hydroxy ethers of the formula 1.6. Titanium tetrabutoxide is used as catalyst.183 H−(OC2 H4 )n −OCH2 CF2 O−(C2 F4 O)p × (1.6) (CF2 O)q −CF2 CH2 O−(C2 H4 O)n −H This polymer can be added to an ordinary unsaturated polyester resin and cured with conventional initiator systems. Applications of fluorine-modified unsaturated polyester resins include thermosetting resins for gel coating with excellent resistance to corrosion, water and atmospheric agents, formulations for resins and foams, etc.
1.7.3 Toner Compositions Toner resins, and consequently toners are propoxylated bisphenol A fumarate resins that are crosslinked in a reactive extrusion process in the presence of the liquid 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane as initiator.177 1,1-Di(tert-butylperoxy)-3,3,5-trimethylcyclohexane has advantages in comparison to the conventionally used dibenzoyl peroxide. Dibenzoyl peroxide generates benzoic acid as a by-product, which is undesirable. Benzoic acid is difficult to remove from the crosslinked resin in that it condenses in a vacuum system, rapidly clogging the system and requiring frequent apparatus shutdowns for cleaning. As a result of the difficulty in the removal of the benzoic acid by-product, the crosslinked toner resin contains a significant amount of acids. Such acidity has been found to negatively affect the charging, the humidity sensitivity of the charging, and the background density properties of the toners. Crosslinked resins are used in making toner. The resins can be subsequently melt blended or otherwise mixed with a colorant, charge carrier additives, surfactants, emulsifiers, pigment dispersants, flow additives, etc. The resultant product can then be pulverized to form toner particles. UV curable resins for incorporation in toner particles are powders based on unsaturated polyesters and polyurethaneacrylates with bis-ethoxylated 2,2-bis(4-hydroxyphenyl)propane or bis-propoxylated 2,2-bis(4-hydroxyphenyl)propane.184 The toner particles can be prepared by melt kneading the toner ingredients, i.e., toner resin composition, charge control agent, pigment, etc.
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Reactive Polymers Fundamentals and Applications
After the melt kneading the mixture is cooled and the solidified mass is pulverized.
1.7.4 Pour Point Depressants Copolymers of dialkyl fumarates and dialkyl maleates with vinyl acetate and vinylpyrrolidone are effective as flow improvers and pour point depressants, respectively. Among a series of similar polymers, copolymers based on didodecyl fumarate vinyl acetate are the most effective pour point depressants.179 These polymers are suitable additives for improving the flow properties and viscosity index of lubricating oils.
1.7.5 Biodegradable Polyesters Aliphatic polyesters are almost the only promising structural materials for biodegradable plastics. In fact, aliphatic unsaturated polyesters, succinic fumaric units, and 1,4-butanediol are biodegradable as such. However, the condensation of aliphatic polyesters derived from diacids and diols failed to obtain high-molecular weight polyesters. Effective transesterification catalysts, high vacuum technique and chain extenders enable the synthesis of high-molecular weight polyesters with improved mechanical properties.185
1.7.6 Bone Cement An unsaturated polyester, made from propylene glycol and fumaric acid, is suitable as resorbable bone cement. Depending on the molecular weight, poly(propylene) fumarate is a viscous liquid. A filler of calcium gluconate/hydroxyapatite is used. An injectable form of a resorbable bone cement can be crosslinked in-situ. The material cures to a hard cement degradable by hydrolysis.186 Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide has been found useful as photoinitiator for poly(propylene) fumarate, for the treatment of large bone defects.187 Citric acid and sodium bicarbonate as the foaming agent develop porosity in the material by generating carbon dioxide during the effervescence reaction.174
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1.7.7 Compatibilizers An unsaturated polyester is a suitable compatibilizer for styrene-butadiene and acrylonitrile-butadiene (NBR) rubber blends. By the addition of 10 parts unsaturated polyester per hundred parts of rubber, the degree of compatibility was greatly enhanced. The rheological and mechanical properties of the blends were also improved.178
1.7.8 Reactive Melt Modification of Poly(propylene) Melt blending of poly(propylene) with a low molecular weight unsaturated polyester in the presence of peroxide in a batch mixer and a twin-screw extruder improves the morphology. Under these conditions competitive degradation and crosslinking reactions take place. These reactions result in a significant change in the viscosity ratio. Rheological studies show that depending on the process conditions some reacted PP/UP blends have a pronounced suspension behavior due to the presence of the dispersed polyester gel particles in a low molecular weight poly(propylene) matrix. Infrared studies of the blends suggest the presence of block or graft structures that promote the compatibility in the treated blends. Such blends are suitable as compatibilizers for blends of poly(propylene) with high molecular weight thermoplastic polyester blends.180
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Reactive Polymers Fundamentals and Applications of cured unsaturated polyester resins. I. miscibility, curing behavior, and glass-transition temperatures. J. Appl. Polym. Sci., 91(5):3369–3387, March 2004. J. P. Dong, J. G. Huang, F. H. Lee, J. W. Roan, and Y. J. Huang. Effects of poly(methyl methacrylate)-based low-profile additives on the properties of cured unsaturated polyester resins. II. volume shrinkage characteristics and internal pigmentability. J. Appl. Polym. Sci., 91(5):3388–3397, March 2004. X. Cao and L. J. Lee. Control of shrinkage and final conversion of vinyl ester resins cured in low-temperature molding processes. J. Appl. Polym. Sci., 90(6):1486–1496, November 2003. W. Li and L. J. Lee. Low temperature cure of unsaturated polyester resins with thermoplastic additives I. dilatometry and morphology study. Polymer, 41(2):685–696, January 2000. W. Li, L. J. Lee, and K. H. Hsu. Low temperature cure of unsaturated polyester resins with thermoplastic additives III. modification of polyvinyl acetate for better shrinkage control. Polymer, 41(2):711–717, January 2000. X. Cao and L. J. Lee. Effect of co-promoter and secondary monomer on shrinkage control of unsaturated polyester (UP)/styrene (St)/low-profile additive (LPA) systems cured at low temperatures. J. Appl. Polym. Sci., 82(3): 738–749, October 2001. X. Cao and L. J. Lee. Control of volume shrinkage and residual styrene of unsaturated polyester resins cured at low temperatures. II. effect of comonomer. Polymer, 44(5):1507–1516, March 2003. X. Cao and L. J. Lee. Control of shrinkage and residual styrene of unsaturated polyester resins cured at low temperatures: I. effect of curing agents. Polymer, 44(6):1893–1902, March 2003. C. Mavon, A. Chambaudet, and F. Jaffiol. Phase separation in styrenated polyester resin containing a PVAc low profile additive and metallic stearates. Macromol. Symp., 166:179–187, March 2001. M. Ruffier, G. Merle, and N. Vincent. Fractal characterization of the fissures occurring during polymerization of low-profile unsaturated polyester resins. Polym. Bull., 30:111–118, 1993. Y. S. Yang and L. J. Lee. Polymerization of polyurethane-polyester interpenetrating polymer network (IPN). Macromolecules, 20:1490–1495, 1987. X. Ramis, A. Cadenato, J. M. Morancho, and J. M. Salla. Polyurethaneunsaturated polyester interpenetrating polymer networks: thermal and dynamic mechanical thermal behaviour. Polymer, 42(23):9469–9479, October 2001. S. Guhanathan, R. Hariharan, and M. Sarojadevi. Studies on castor oilbased polyurethane/polyacrylonitrile interpenetrating polymer network for toughening of unsaturated polyester resin. J. Appl. Polym. Sci., 92(2): 817–829, April 2004.
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140. W. Rutsch, K. Dietliker, D. Leppard, M. Köhler, L. Misev, U. Kolczak, and G. Rist. Recent developments in photoinitiators. Prog. Org. Coat., 27(1–4): 227–239, January 1996. 141. J. E. Baxter, R. S. Davidson, and H. J. Hageman. Use of acylphosphine oxides and acylphosphonates as photoinitiators. Polymer, 29(9):1569–1574, September 1988. 142. J. E. Baxter, R. S. Davidson, and H. J. Hageman. Acylphosphine oxides as photoinitiators for acrylate and unsaturated polyester resins. Eur. Polym. J., 24(5):419–424, 1988. 143. E. A. Syed, B. Vries, and F. W. K. Koers. Methyl propyl ketone peroxide formulations and their use in processes to cure unsaturated polyester resins. US Patent 6 642 287, assigned to Akzo Nobel N.V. (Arnhem, NL), November 4 2003. 144. C. S. Sheppard. Peroxy compounds. In H. F. Mark, N. Bikales, C. G. Overberger, and G. Menges, editors, Encyclopedia of Polymer Science and Engineering, volume 11, page 1. Wiley Interscience, New York, 2nd edition, 1988. 145. L. M. Marroyo, X. Ramis, and J. M. Salla. Behavior of nonazeotropic compositions of a styrene-unsaturated polyester resin analyzed through FTIR spectroscopy and dynamic mechanical thermal analysis. J. Appl. Polym. Sci., 89(13):3618–3625, September 2003. 146. K. de la Caba, P. Guerrero, I. Mondragon, and J. M. Kenny. Comparative study by DSC and FTIR techniques of an unsaturated polyester resin cured at different temperatures. Polym. Int., 45(4):333–338, April 1998. 147. M. Skrifvars, P. Niemela, R. Koskinen, and O. Hormi. Process cure monitoring of unsaturated polyester resins, vinyl ester resins, and gel coats by Raman spectroscopy. J. Appl. Polym. Sci., 93(3):1285–1292, August 2004. 148. N. Delahaye, S. Marais, J. M. Saiter, and M. Metayer. Characterization of unsaturated polyester resin cured with styrene. J. Appl. Polym. Sci., 67(4): 695–703, January 1998. 149. J. L. Martin, A. Cadenato, and J. M. Salla. Comparative studies on the non-isothermal DSC curing kinetics of an unsaturated polyester resin using free radicals and empirical models. Thermochim. Acta, 306(1-2):115–126, November 1997. 150. X. Ramis and J. M. Salla. Effect of the initiator content and temperature on the curing of an unsaturated polyester resin. J. Polym. Sci., Part. B: Polym. Phys., 37(8):751–768, April 1999. 151. W. D. Cook, M. Lau, M. Mehrabi, K. Dean, and M. Zipper. Control of gel time and exotherm behaviour during cure of unsaturated polyester resins. Polym. Int., 50(1):129–134, January 2001. 152. X. Ramis, A. Cadenato, J. M. Morancho, and J. M. Salla. Curing of a thermosetting powder coating by means of DMTA, TMA and DSC. Polymer, 44(7):2067–2079, March 2003.
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153. F. Lionetto, R. Rizzo, V. A. M. Luprano, and A. Maffezzoli. Phase transformations during the cure of unsaturated polyester resins. Mater. Sci. Eng., A, 370(1-2):284–287, April 2004. 154. V. Massardier-Nageotte, F. Cara, A. Maazouz, and G. Seytre. Prediction of the curing behavior for unsaturated polyester-styrene systems used for monitoring sheet moulding compounds (SMC) process. Composites Science and Technology, 64(12):1855–1862, September 2004. 155. H. S.-Y. Hsich. Kinetic model of cure reaction and filler effect. J. Appl. Polym. Sci., 27(9):3265–3277, September 1982. 156. P. Li, X. P. Yang, Y. H. Yu, and D. S. Yu. Cure kinetics, microheterogeneity, and mechanical properties of the high-temperature cure of vinyl ester resins. J. Appl. Polym. Sci., 92(2):1124–1133, April 2004. 157. R. Mezzenga, B. Pettersson, and J. A. E. Manson. Thermodynamic evolution of unsaturated polyester-styrene-hyperbranched polymers. Polym. Bull., 46(5):419–426, June 2001. 158. W. Li and L. J. Lee. Low temperature cure of unsaturated polyester resins with thermoplastic additives II. structure formation and shrinkage control mechanism. Polymer, 41(2):697–710, January 2000. 159. C. P. Hsu and L. J. Lee. Structure formation during the copolymerization of styrene and unsaturated polyester resin. Polymer, 32(12):2263–2271, 1991. 160. E. M. S. Sanchez, C. A. C. Zavaglia, and M. I. Felisberti. Unsaturated polyester resins: influence of the styrene concentration on the miscibility and mechanical properties. Polymer, 41(2):765–769, January 2000. 161. M. Farshad and A. Necola. Effect of aqueous environment on the long-term behavior of glass fiber-reinforced plastic pipes. Polymer Testing, 23(2): 163–167, April 2004. 162. M. E. Tawfik. Preparation and characterization of water-extended polyester based on recycled poly(ethylene terephthalate). J. Appl. Polym. Sci., 89(13): 3693–3699, September 2003. 163. S. H. Mansour and N. E. Ikladious. Depolymerization of poly(ethylene terephthalate) waste using 1,4-butanediol and triethylene glycol. J. Elastomer Plast., 35(2):133–148, April 2003. 164. D. J. Suh, O. O. Park, and K. H. Yoon. The properties of unsaturated polyester based on the glycolyzed poly(ethylene terephthalate) with various glycol compositions. Polymer, 41(2):461–466, January 2000. 165. V. Pimpan, R. Sirisook, and S. Chuayjuljit. Synthesis of unsaturated polyester resin from postconsumer PET bottles: Effect of type of glycol on characteristics of unsaturated polyester resin. J. Appl. Polym. Sci., 88(3):788–792, April 2003. 166. A. Viksne, M. Kalnins, L. Rence, and R. Berzina. Unsaturated polyester resins based on PET waste products from glycolysis by ethylene, propylene, and diethylene glycols and their mixtures. Arab. J. Sci. Eng., 27(1C):33–42, June 2002.
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167. B. Gawdzik, T. Matynia, and E. Zarebska. Synthesis of unsaturated polyester resins based on dimethyl terephthalate process residue. Przem. Chem., 81(8):525–527, August 2002. 168. T. Yasumura and C. Yoshioka. Process for producing unsaturated polyester and unsaturated polyester resin composition. US Patent 6 353 036, assigned to Dainippon Ink and Chemicals, Inc. (Tokyo, JP), March 17 2002. 169. P. Radenkov, M. Radenkov, G. Grancharov, and K. Troev. Direct usage of products of poly(ethylene terephthalate) glycolysis for manufacturing of glass fibre reinforced plastics. Eur. Polym. J., 39(6):1223–1228, June 2003. 170. S. Kubota, O. Ito, and H. Miyamoto. Method of recycling cured unsaturated polyester resin waste. US Patent 5 776 989, assigned to Wakayama Prefecture (Wakayama, JP); Miyaso Chemical Co. (Wakayama, JP), July 7 1998. 171. S. Kubota, O. Ito, and H. Miyamoto. Method of recycling unsaturated polyester resin waste and recycling apparatus. US Patent 5 620 665, assigned to Miyaso Chemical Co. (Wakayama, JP), April 15 1997. 172. K. S. Yeon, N. J. Jin, Y. H. Kwon, and K. W. Ryu. Workability and strength properties of MMA-modified UP polymer concrete. J. Polym. Eng., 23(5): 385–398, September–October 2003. 173. R. E. Hefner, Jr. Polymer modified unsaturated polyester for polyesteramide resin polymer concrete. US Patent 4 777 208, assigned to The Dow Chemical Company (Midland, MI), October 11 1988. 174. K. U. Lewandrowski, D. D. Hile, B. M. J. Thompson, D. L. Wise, W. W. Tomford, and D. J. Trantolo. Quantitative measures of osteoinductivity of a porous poly(propylene fumarate) bone graft extender. Tissue Eng., 9(1): 85–93, February 2003. 175. E. Blot and C. Stock. Road paint compositions containing an unsaturated polyester resin. US Patent 5 907 003, assigned to Lafarge Materiaux De Specialites (FR), May 25 1999. 176. M. Narkis, I. Rafail, G. Victor, A. Tzur, R. Tchoudakov, and A. Siegmann. Electrically conductive thermosetting resins containing low concentrations of carbon black. J. Appl. Polym. Sci., 76(7):1165–1170, May 2000. 177. J. L. Leonardo, Y. Lipovetskaya, S.-R. Nilmarie, H. Chang, D. Li, K. B. Sheth, D. A. Harrington, J. J. Ianni, P. L. Jacobs, J. S. Kittelberger, L. J. Kurtic, Jr., R. E. Lutz, D. J. O’ Keefe, and E. F. Young. Cross-linked polyester toners and process of making such toners. US Patent 6 359 105, assigned to Xerox Corp, March 19 2002. 178. S. H. Mansour, S. Y. Tawfik, and M. H. Youssef. Unsaturated polyester as compatibilizer for styrene-butadiene (SBR)/acrylonitrile-butadiene (NBR) rubber blends. J. Appl. Polym. Sci., 83(11):2314–2321, March 2002. 179. A. A. A. Abdel-Azim and R. M. Abdel-Aziem. Polymeric additives for improving the flow properties and viscosity index of lubricating oils. J. Polym. Res.-Taiwan, 8(2):111–118, June 2001.
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180. C. Wan, S. H. Patel, and M. Xanthos. Reactive melt modification of polypropylene with a crosslinkable polyester. Polym. Eng. Sci., 43(6): 1276–1288, June 2003. 181. J. Muthiah, J. J. Kozlowski, N. B. Shah, P. H. Radcliffe, and E. G. Nicholl. Unsaturated polyester powder coatings with improved surface cure. US Patent 6 048 949, assigned to Morton International, Inc. (Chicago, IL), April 11 2000. 182. F. Pilati, M. Toselli, M. Messori, U. Credali, C. Tonelli, and C. Berti. Unsaturated polyester resins modified with perfluoropolyethers. J. Appl. Polym. Sci., 67(10):1679–1691, March 1998. 183. M. Messori, M. Toselli, F. Pilati, and C. Tonelli. Unsaturated polyester resins modified with poly(ε-caprolactone)-perfluoropolyethers block copolymers. Polymer, 42(25):9877–9885, December 2001. 184. S. Tavernier, S. De Meutter, and D. van Wunsel. Radiation curable toner particles. EP Patent 0 821 281, assigned to Agfa Gevaert NV, January 28 1998. 185. M. S. Nikolic, D. Poleti, and J. Djonlagic. Synthesis and characterization of biodegradable poly(butylene succinate-co-butylene fumarate)s. Eur. Polym. J., 39(11):2183–2192, November 2003. 186. K. U. Lewandrowski, J. D. Gresser, D. L. Wise, R. L. White, and D. J. Trantolo. Osteoconductivity of an injectable and bioresorbable poly(propylene glycol-co-fumaric acid) bone cement. Biomaterials, 21(3):293–298, February 2000. 187. J. P. Fisher, D. Dean, and A. G. Mikos. Photocrosslinking characteristics and mechanical properties of diethyl fumarate/poly(propylene fumarate) biomaterials. Biomaterials, 23(22):4333–4343, November 2002.
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2 Polyurethanes Polyurethanes consist basically of two components, an isocyanate component and a diol component. The diol component can be a polyether endcapped diol or a polyester end-capped diol. The urethane structure may be identified as the esters of carbamic acid or ester amides of a carbonic acid. The urethane formation is achieved by the addition of a tertiary amine and an organometallic compound. There are many monographs on the topic,1–11 the most recent of W. Dias Vilar12 and Klempner.13 Polyurethanes also find use in medical applications.14, 15 They are used to a large extent as adhesives16 and as coatings.
2.1 HISTORY Polyurethane was first described by Bayer∗ in 1937.17 The first polyurea was composed from hexane-1,6-diamine and hexane-1,6-diisocyanate. Two diisocyanates used at that time, diphenylmethane-4,4′ -diisocyanate and naphthalene-1,5-diisocyanate, are still key products in polyurethane chemistry. Besides O. Bayer, H. Rinke, A. Hoechtlen, P. Hoppe and E. Meinbrenner contributed significantly to the development of polyurethanes. In 1940, toluene diisocyanate was introduced. From the beginning polyurethanes were utilized as foams, coatings, and cast elastomers. ∗ Otto
Bayer, born in Frankfurt/Main 1902, died 1982
69
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Reactive Polymers Fundamentals and Applications
Cl R
NH2 +
C O
R
N C O
Cl
Figure 2.1: Synthesis of Isocyanates
2.2
MONOMERS
Monomers for the synthesis of polyurethanes consist of two types, i.e., diisocyanates and polyols.
2.2.1 Diisocyanates The basic synthesis of isocyanates is shown in Figure 2.1. The synthesis starts with an amine, aliphatic or aromatic and phosgene. The isocyanate is formed by the elimination of two molecules of HCl. Phosgene Route. The synthesis route via phosgene was invented in 1884 by Hentschel, although isocyanates had been discovered in 1848 by Wurtz. The synthesis runs via two basic steps, i.e. 1. Formation of the carbamic chloride, 2. Elimination of hydrochloric acid. The industrial synthesis has to minimize the various side reactions that may occur, as shown in Figure 2.2. Phosgene-free Route. There is also a phosgene-free synthesis route, because of the hazards of handling phosgene. The route is shown in Figure 2.3. The synthesis starts with nitrobenzene; from that the ethyl urethane is directly formed with carbon monoxide and ethanol. The urethane is dimerized by a carbonylation reaction. Finally, by heating the urethane is decomposed into the isocyanate and the alcohol. Typical diisocyanates are shown in Table 2.1. Aromatic diisocyanates are shown in Figure 2.4. The highly volatile isocyanates are very toxic. During curing there is also an emission of the unreacted isocyanate. The emission also depends on the reactivity of the particular isocyanate, as
Polyurethanes
R
R
NH2 + HCl
NH3
71
Cl
R R
N C O + H N R H
H
N
H
N
C O R R
R
NH2
Cl +
R
N
H
N
C O
C O
Cl
NH2
H
R
Figure 2.2: Side Reactions in Isocyanate Synthesis: Salt Formation with HCl generated, Formation of Urea from Amine and Isocyanate, Formation of Urea from Amine and Phosgene
NO2
CO + CH3CH2OH
NH C O CH2 CH3 O
CHO
CH3
CH3 CH2
CH2 O
O C HN
CH2
NH C O
O
OCN
CH2
NCO
Figure 2.3: Phosgene-free Synthesis of Diisocyanates
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Reactive Polymers Fundamentals and Applications
CH3 NCO
CH3 NCO
OCN NCO Toluene 2,4-diisocyanate
OCN
Toluene 2,6-diisocyanate
CH2
NCO
4,4'-Diphenyl methane diisocyanate NCO CH2
NCO
2,4'-Diphenyl methane diisocyanate NCO
OCN Naphthalene 1,5-diisocyanate
Figure 2.4: Aromatic Diisocyanates
Polyurethanes
73
Table 2.1: Isocyanates for Polyurethanes Isocyanate
Remarks
Hexamethylene diisocyanate Isophorone diisocyanate Dicyclohexylmethane-4,4′ -diisocyanate 4,4′ -Diisocyanato dicyclo hexylmethane 2,4-Toluene diisocyanate
Color-free Color-free
2,6-Toluene diisocyanate 1,5-Naphthalene diisocyanate 4,4′ -Methylene diphenyl diisocyanate 4,4-Methylene biscyclohexyl diisocyanate (HMDI) 1,2-Bis(isocyanate)ethoxyethane (TEGDI) Macromonomers Lysine-diisocyanate
A mixture of 65% 2,4 isomer and 35% 2,6 isomer is most common Lower volatile then TDI Extremely soft18 See Ref.19 Biodegradable formulations20
detected in a mixture of 2,4′ -methylene diphenyl diisocyanate (2,4′ -MDI) and 4,4′ -methylene diphenyl diisocyanate (4,4′ -MDI). Because of the high reactivity with moisture, the analysis requires special techniques; less than 5 n g/m3 can be detected.21 2.2.1.1
Toluene diisocyanate
In technical applications, toluene diisocyanate (TDI) is used either as pure 2,4-isomer or as a blend of the 2,4- and 2,6-isomers. Two blend qualities are available, TDI-80/20 and TDI-65/35, which means 80% 2,4-isomer with 20% 2,6-isomer, and 65% 2,4-isomer with 35% 2,6-isomer, respectively. The two isocyanate groups have unequal reactivity; the isocyanate group at the p-position is more reactive. Toluene diisocyanate is synthesized from toluene via dinitrotoluene, reduction of the nitro group with hydrogen (c.f. Figure 2.5) and phosgenation as shown in Figure 2.1. The nitration of toluene is achieved in a two-step procedure. In the first step a mixture of the ortho, para, and meta isomers (63% o-isomer, 33% p-isomer, 4% m-isomer) is obtained. The isomers can be separated by distillation. When p-nitrotoluene is used in the second nitration step, a
74
Reactive Polymers Fundamentals and Applications
CH3
CH3 O2N
HNO3/H2SO4
NO2
H2 O CH3 H2N
CH3
C NH2
Cl
Cl
OCN
NCO
Figure 2.5: First Steps of the Synthesis of Toluene diisocyanate
100% 2,4-dinitrotoluene is obtained. The nitration of o-nitrotoluene finally yields the TDI-65/35 quality. If the blend obtained from the first step is directly reacted, the TDI-80/20 quality will be obtained. 2.2.1.2
Diphenylmethane diisocyanate
Diphenylmethane diisocyanate (MDI) has a lower vapor pressure and is therefore less toxic than TDI. The synthesis of MDI starts with the condensation of aniline with formaldehyde as shown in Figure 2.6 for the ortho adducts. In fact, 2,2′ - and 2,4′ - and 4,4′ -isomers are formed, the yield of the dimer of 4,4′ -diphenylmethane diamine being in an amount of ca. 50%. The isocyanates are obtained then in the usual way by phosgenation. The crude mixture can be directly used. However, the mixture can be separated or otherwise modified in order to obtain products with more convenient properties. 4,4′ -MDI has a melting point around 38°C. It forms insoluble dimers when stored above the melting point. Further, it tends to crystalize. A mixture of 2,4′ -MDI and 4,4′ -MDI shows a lowering of the melting point with a minimum of 15°C at 50% p-isomer. 2.2.1.3
Aliphatic Diisocyanates
A disadvantage of aromatic diisocyanates is that they become yellow to dark brown when they are cured. This limits the fields of applications.
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75
NH2 + CH2
H 2N
NH2
O
CH2
NH2 CH2
NH2
NH2 CH2
CH2
Figure 2.6: Condensation of Aniline with Formaldehyde
Aliphatic diisocyanates are colorless, but have other disadvantages. In particular, the mechanical properties of the final products, such as such as elongation, tensile strength and flexibility, are inferior. However, aliphatic isocyanates find important applications in coating formulations. Aliphatic diisocyanates include 1,6-hexane diisocyanate (HDI), isophorone diisocyanate (IPDI), dicyclohexylmethane-4,4′ -diisocyanate, i.e., hydrogenated MDI, c.f. Figure 2.7. In general, aliphatic are less reactive than aromatic isocyanates. Due to steric hinderance, the affinity of m-tetramethylxylene diisocyanate to water is so small that it can be dispersed in water without reacting. 2.2.1.4
Modified Diisocyanates
The isocyanates can be modified in several ways, i.e. by dimerization, oligomerization with diols, or capping the isocyanate group. Dimerization. Diisocyanates can be dimerized, by splitting off carbon dioxide, to the respective carbodiimides. The carbodiimide can react further with an excess of isocyanate to a uretonimine, c.f. Figure 2.8. Such
76
Reactive Polymers Fundamentals and Applications
OCN CH2 CH2 CH2 CH2 CH2 CH2 CH2
NCO
Hexamethylene diisocyanate NCO CH2 NCO
H3C C CH3 CH3
H3C CH3
NCO
Isophorone diisocyanate
C H3C
NCO
m-Tetramethylxylene diisocyanate
Figure 2.7: Aliphatic Diisocyanates: 1,6-Hexane diisocyanate, Isophorone diisocyanate, m-Tetramethylxylene diisocyanate
compounds have now three isocyanate groups in the molecule, i.e., they have a functionality of three. The properties of MDI can be varied in wide ranges, and consequently can be used for different applications. The crude MDI is used for rigid foams. Pure 4,4′ -MDI is used, among other applications, for shoe soles and also for thermoplastic polyurethanes. Biuret Reaction. Water hydrolyzes the isocyanate group very quickly. Therefore it is essential to store the isocyanate material moisture-free. On the other hand, the action of water can be purposefully used to modify isocyanates. A biuret is formed by the reaction of a substituted urea with isocyanate, as shown in Figure 2.9. The substituted urea itself can be obtained by the reaction of water with isocyanate. An amine is formed in the course of hydrolysis that condenses immediately with water to the substituted urea. The substituted urea is the reagent for the biuret reaction as explained above. Prepolymers. If a glycol or a glycol ether is reacted with an excess of a diisocyanate, then a prepolymer is formed. In this reaction one diol couples two molecules of diisocyanate, as schematically shown in Figure 2.10. Also, branched alcohols, like 1,1,1-trimethylolpropane, can be used.
Polyurethanes
NCO
NCO
NCO NCO
CH2
CH2
CH2 CH2
NCO NCO
N
N
C
C N
N
N C O
CH2
CH2
CH2
NCO
NCO
NCO
Figure 2.8: Formation of Uretonimine
R N C O H N C O R N R’
H R N C O N C O R
N
Figure 2.9: Biuret Formation of Isocyanates
R’
77
78
Reactive Polymers Fundamentals and Applications
CH2
OCN
NCO
+ HO CH2
CH2
O CH2 CH2
OH
H O OCN
CH2
N C O CH2 CH2 O CH2 CH2 O
OCN
CH2
N C H O
Figure 2.10: Formation of Prepolymers
Polyurethanes
79
In this case ideally a trifunctional isocyanate is formed. When the stoichiometric ratio of isocyanate groups to alcohol groups is more then two, appreciable amounts of unreacted diisocyanate is left behind, which causes an increased toxicity. If the diisocyanate is sufficiently volatile, the unreacted residual diisocyanate can be removed by distillation under vacuum. Such mixtures are liquids at room temperature. Because of larger structure the prepolymers are less volatile and therefore less toxic. Toluene diisocyanate and isophorone diisocyanate possess two isocyanate groups with different reactivities. When forming the prepolymer, the more reactive group is reacted. The less reactive group is left unreacted. The properties of the final product can be adjusted by the selection of the components and the amounts making the prepolymer. For example, prepolymers based on poly(ethylene oxide) or poly(propylene oxide) will be used for hydrophilic gels, whereas hydrophobic polyols will result in hydrophobic polyurethanes. For hydrophobic polyurethanes, polyols with very nonpolar backbones, e.g., hydroxyl functional poly(butadiene), can be used to introduce the hydrophobicity.22 By choosing the stoichiometric ratio of NCO to OH groups, the content of free isocyanate groups can be adjusted from 2% to 20%. Viscosity is an important parameter for the processability of the raw materials. The viscosity increases with molecular weight and decreases with the content of unreacted isocyanate. The viscosity also increases with increasing allophanate formed, because this is a crosslinking reaction. The allophanate formation is favored at temperatures above 60 to 80°C and catalyzed by alkaline residues in polyether polyols, if any is present. Therefore, to increase the storage time of the prepolymer, acid stabilizers such as benzoyl chloride, acetyl chloride, or p-toluenesulfonic acid can be added. End-capped Diisocyanates. The reaction of the isocyanate group with alcohols to form the urethane functionality is thermoreversible. At elevated temperatures the urethane decomposes into the isocyanate. This reaction is utilized at the phosgene-free route of synthesis of isocyanates. On the other hand, the reversibility can be used in the preparation of end-capped, or blocked diisocyanates. The isocyanate group is allowed to react with compounds containing acidic hydrogen atoms. In this way the isocyanate group is masked and not accessible for other reactants. At elevated temperatures the retro reaction takes place, the isocyanate group is set free, and in presence of amines the
80
Reactive Polymers Fundamentals and Applications
urethane can be formed. A necessary condition for the concept to work properly is that the unblocking reaction takes places at lower temperatures than the thermal decomposition of the urethane group. The temperatures for the retro reaction of unblocking are between 90 and 160°C. Aromatic isocyanates are less stable than aliphatic isocyanates. The temperature of unblocking decreases in the following order for the types of blocking agents: alcohols > lactams > ketoximes > active methylene groups containing compounds. Suitable blocking agents are phenol, ethyl acetoacetate, ε-caprolactam, methylethylketoxime, diethyl malonate, and 3,5-dimethylpyrazole. N,N ′ -Carbonylbiscaprolactam (CBC), c.f. Figure 2.11, offers an isocyanate-free route to new families of thermosets and reactive resins with caprolactam-blocked isocyanates. CBC reacts with primary amines into blocked isocyanates at 100 to 150°C. The reaction is also suitable for highly functional amine dendrimers and polymers. With polyols, a ring-opening of the caprolactam occurs. Catalysts include zirconium alcoholates, magnesium bromide or dibutyltin dilaurate (DBTDL). N-carbamoyl caprolactam end groups are formed by a nucleophilic attack of the hydroxy group at one of the CBC caprolactam rings and subsequent ring opening. Thus, the corresponding blocked ester-functional isocyanates are formed. The CBC derivatives are attractive crosslinking agents and interfacial coupling agents for adhesives and coatings. Further, due to the non-toxic CBC-intermediates and polyesterurethanes, they are also suitable for medical applications.23, 24 When the ring opening reaction is done with poly(propylene oxide)-based triols, then crosslinked polyurethanes are obtained.25 Thus, 1,2-bis-[2(2-hydroxy-5-methylphenyl)-5-benzotriazolyl]-ethane (BHMBE) reacts with the phenolic hydroxyl groups and is thus a reactive UV-absorber.26 The synthesis starts from 4,4′ -diaminodibenzyl in several steps. The structure is shown in Figure 2.12. Isocyanurate. The formation of an isocyanurate is in fact a trimerization of an isocyanate (Figure 2.13). Trimers from toluene diisocyanate and hexamethylene diisocyanate are available. Such isocyanate isocyanurate structures are trifunctional, i.e., they have three isocyanate groups pending. They can be modified to become more hydrophilic, if one isocyanate group is allowed to be coupled with a polyglycol, e.g., poly(ethylene oxide) or poly(propylene oxide).
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81
O R X C
+
N
N H
O O
O N C
N
+ RXH
O O
H
R X
N C
N
O
O
O
Figure 2.11: Reaction of N,N ′ -Carbonylbiscaprolactam with a Nucleophile RXH. Top: ring elimination with formation of caprolactam. Bottom: ring opening reaction.23
OH
HO N N N
H 3C
N CH2 CH2
N N CH3
Figure 2.12: 1,2-Bis-[2(2-hydroxy-5-methylphenyl)-5-benzotriazolyl]-ethane26
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Reactive Polymers Fundamentals and Applications
O R 3 R NCO O
N
N N
R O
R
Figure 2.13: Trimerization: Formation of an Isocyanurate Structure
Macromonomers. A macromonomer is a polymer that contains reactive groups, here isocyanate groups. A macromonomer from 2-(dimethylamino)ethyl methacrylate that bears a 1-(isopropenylphenyl)-1,1-dimethylmethyl isocyanate group has been synthesized. However, 2-(dimethylamino)ethyl methacrylate (DMAEMA) reacts with 2-mercaptoethanol preferably in an addition reaction that acts as chain transfer agent in radical telomerization. In this way, an adduct of the methacrylate and the mercapto compound is formed. The structure of the adduct and the product of functionalization are shown in Figure 2.14. The oligomers can be then functionalized with 1-(isopropenylphenyl)-1,1-dimethylmethyl isocyanate (TMI), resulting in macromonomers.19 α,α′ -Dihydroxyl-poly(butyl acrylate) prepared by atom transfer radical polymerization (ATRP) has been used as a macromonomer with two hydroxyl groups at one end. This macromonomer was used for chain extension of diphenyl-methane-4,4-diisocyanate to obtain comb-like oligo isocyanates, as shown in Figure 2.15. These materials have potential interest as pressure-sensitive adhesives (PSA).27 In a completely different way rodlike macromonomers were obtained. In a first step, the N=C bond n-hexyl isocyanate was polymerized by titanium catalysts in a living polymerization. The living chain end was deactivated by methacryloyl chloride to result in a methacrylic-terminated poly(n-hexyl isocyanate.28 Block copolymers from n-hexyl isocyanate and isoprene have been obtained by a living polymerization technique.29 The living anionic polymerization proceeds very fast and therefore low temperatures −98°C, are required to control the selectivity. 3,5-Bis(4-aminophenoxy)benzoic acid, c.f. Figure 2.16, is a monomer from the type AB2 . It can be polycondensed to form dendritic polymers. These polymers contain pendant amino groups that can be crosslinked with diisocyanates.30
Polyurethanes
O
O
S H CH2
CH C O
CH2
CH3
CH2
CH2
CH2 C O
CH2
S
CH3
CH2
CH2
N H3C CH3
OH
CH2 CH2 N H3C CH3
CH2 OH
O
CH3 H2CH
83
C CH3
O
S CH2
CH2 C O
CH2
CH3
CH2
C NH C O CH2
CH2
CH3
N H3C CH3
Figure 2.14: Adduct from 2-(dimethylamino)ethyl methacrylate and 1-(isopropenylphenyl)-1,1-dimethylmethyl isocyanate19
CH3 O
CH2
O CH2
N C O CH2 C CH2 O C N H
CH2
O CH2
O
CH2
CH2
C O
O C H3C C CH2 CH CH2 CH3
NCO
H
CH
C O O CH2
CH2
CH2
CH3
Figure 2.15: Comb-like Oligo Isocyanates27
CH3
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Reactive Polymers Fundamentals and Applications
H 2N
O O C OH
H 2N
O
Figure 2.16: 3,5-Bis(4-aminophenoxy)benzoic acid
2.2.1.5
Enzymatic Synthesis of Polyurethanes
Polyurethanes have been synthesized using the enzyme Candida antarctica lipase B. The use of enzymatic methods offers the possibility to reverse the conventional process by creating the urethane first and then using a low temperature enzymatic polyester synthesis to build the polymer. A novel series of biscarbamate esters and polyesters also could be obtained.31 2.2.1.6
Synthesis of Urethanes via Carbonate Esters
The synthesis of urethanes avoiding handling of isocyanates is also possible by the reaction of amines or diamines with ethylene carbonate. The scheme is shown in Figure 2.17. Urethane dimethacrylates suitable for dental fillers have been synthesized in this way. For example, ethylene carbonate in two-fold excess was reacted with 1,6-hexane diamine to obtain a urdiol. This was reacted with methacrylic anhydride.32
2.2.2 Polyols Polyols are the second basic component beside diisocyanates. There are two types of polyols, 1. Polyether polyols, 2. Polyester polyols. 2.2.2.1
Polyether Polyols
Most widely used are polyether polyols. Monomers commonly used for polyether polyols are listed in Table 2.2.
Polyurethanes
H 2N O O O
NH2
CH2
H 2C
CH2
H 2C
O O O
O HO(CH2)2O C HN
C O(CH ) OH 2 2 NH
O
Figure 2.17: Reaction of Ethylene Carbonate with 1,6-Hexane diamine
Table 2.2: Monomers for Polyether Polyols Monomer Propylene oxide Ethylene oxide Butylene oxide Tetrahydrofuran
Remarks As copolymer with propylene oxide In fibers and elastomers
85
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Reactive Polymers Fundamentals and Applications
R
R
O
OH + B
O + CH2 CH2 CH3
R
O
R
O CH2
CH2 O CH3
Figure 2.18: Initial Steps of the Formation of Polyether polyols
Anionic Ring Opening. Polyols with a molecular weight between 1,000 and 6,000 Dalton and a functionality between 1.8 and 3.0 are used in flexible foams and elastomers. Polyols with a molecular weight below 1,000 Dalton and high functionalities result in high crosslinked rigid chains and are used in rigid foams and high performance coatings. The polymerization is initiated with an alcohol and a strong base. The base is usually potassium hydroxide that forms initially the monomeric alcoholate. The alcoholate anion is subjected to a series of ring opening reactions of the epoxide or the cyclic ether. The basic mechanism is sketched in Figure 2.18. In the case of nonsymmetric epoxides the alcoholate anion attacks the less hindered carbon atom of the epoxide, as shown in Figure 2.18. Therefore, polyols composed exclusively from propylene oxide bear secondary hydroxyl groups as end groups. Secondary hydroxyl groups are less reactive than primary hydroxyl groups. To get polyols with the more reactive primary hydroxyl groups, the polymerization is started with propylene oxide, and in the final stage ethylene oxide is added. Ethylene oxide improves the water solubility of the polyol. Due to the mechanism of polymerization without termination in preparing polyether polyols, the molecular weight distribution of the polyols exhibits a Poisson distribution. This is narrower than the distribution of polyester polyols. Instead of alcohols, amines can also be used. Typical initiator alcohols are propylene glycol, glycerol, trimethylolpropane, triethanolamine, pentaerythritol, sorbitol, or sucrose. Sucrose results in highly branched polyols suitable for rigid foams, whereas the alcohols with a lower functionality are used for flexible materials. Amines include ethylene diamine, toluene diamine, 4′ ,4′ -diphenyl-
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87
methane diamine, and diethylenetriamine. The resulting polyols exhibit a higher basicity than the polyols with an alcohol as initiator and are therefore more reactive with isocyanates. A side reaction of the base in polymerization is the isomerization reaction. For example, propylene oxide isomerizes to allyl alcohol. As a consequence, vinyl-terminated monofunctional polyols are formed. Such monofunctional polyols are addressed as monols. Such compounds have negative influence on the mechanical properties of the final products. The formation of monols can be suppressed by using special catalysts, e.g., zinc hexacyanocobaltate. This type of catalyst is referred to as double metal cyanide catalyst. Grafted Polyols. Copolymer polyols are obtained by grafting styrene or acrylonitrile to poly(propylene oxide). The radicals attack the tertiary hydrogen sites (−CH2 CHtert (−CH3 )−O) in the poly(propylene oxide) as a transfer reaction to the poly(propylene oxide). Originally pure acrylonitrile was used for grafting, but the so formed copolymer polyols cause discoloration problems in slabstock flexible foams. For this reason styrene/acrylonitrile copolymer polyols were developed. Vinyl Functionalized Polyols. Another method is to functionalize the polyols with a vinyl moiety. This is achieved by reaction of the polyols with maleic anhydride, or methacryloyl chloride. Of course the functionality of the polyols must be greater than two with respect to the hydroxyl group, because hydroxyl groups are lost. If to the vinyl functionalized polyol a polymerizing vinyl monomer mixture is added, the pendent vinyl group polyols take part in the polymerization reaction. With respect to the vinyl polymer a comb-like structure is formed, the teeth of the “comb” being the polyol moieties. The styrene is hydrophobic, and at higher conversion the backbone of the comb may collapse to yield a spherical structure. The polyol chains are at the surface of the sphere. Polyurea-modified Polyols. Urea urethane polyols and polyurea-modified polyols are another type of polyols. They are synthesized in a twostage reaction. 1. In the first stage a diamine or an amino alcohol is allowed to react with an excess of diisocyanate. The amine groups react with
88
Reactive Polymers Fundamentals and Applications the isocyanate group to form urea groups, whereas the hydroxy groups react with the isocyanate group to form urethane groups. The excess of isocyanate causes the formation of an isocyanate end-capped prepolymer. In the case of a diamine, isocyanates are formed that contain exclusively urea groups in their backbone. In the case of amino alcohols isocyanates containing urea and urethane in the backbone are formed. Suitable diamines are hydrazine, ethylene diamine, etc. 2. In the second stage a diol or a polyol in molar excess with respect to the unreacted isocyanate groups is added. The pending isocyanate groups react with the hydroxy groups to form chain-extended polymeric polyols. The reaction of diamines with isocyanates proceeds fast in comparison to the reaction of polyols with isocyanates.
Autocatalytic Polyols. The alkylamine group can be introduced in a polyol chain by using N-alkylaziridine or N,N-dialkyl glycidylamine as a comonomer with ethylene oxide or propylene oxide. Since the amine groups in the chain catalyze the reaction of the hydroxyl groups with the isocyanate, this type of polyol is called autocatalytic.33 Autocatalytic polyols require less capping with primary hydroxyls, that is, less ethylene oxide capping to obtain the same performance in flexible molded foam than conventional polyols when used under the same conditions. Moreover, low emission polyurethane polymers can be made with autocatalytic polyols. 2.2.2.2
Polyester Polyols
Typical monomer combinations for polyester polyols are shown in Table 2.3. Polyesters from Acid and Alcohols. The polyesters are produced by preheating the diol to ca. 90°C and adding the acid into it. The reaction temperature is raised gently up to 200°C to completion. Inert gas or vacuum is used to remove the water. The condensation is an equilibrium reaction, and a Schulz-Flory distribution of the molecular weight is obtained. The condensation is catalyzed by acids, bases, and transition metal compounds. However, catalysts should be used with care, because they
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89
Table 2.3: Monomers for Polyester Polyols Acid Alcohol Components
Uses
Adipic acid, diethylene glycol, 1,1,1-trimethylolpropane Adipic acid, phthalic acid, 1,2-propylene glycol, glycerol Adipic acid, phthalic acid, oleic acid, 1,1,1-trimethylolpropane Adipic acid, ethylene glycol, diethylene glycol Adipic acid, ethylene glycol, 1,4-butanediol ε-caprolactone, various diols
Flexible foam
Castor oil, glycerol, trimethylolpropane
Semi-rigid foam Rigid foam Shoe soles Elastomers Ring opening condsation Transesterification
could have undesirable effects on the subsequent curing reaction. Condensation catalysts based on tin and other transition metals added only in the ppm range did not show negative effects on the later procedures and properties. The hydroxyl numbers increase from flexible foams to rigid foams from 60 mgKOH/g up to 400 mgKOH/g. Acids for soft foams are aliphatic acids, such as adipic acid, whereas phthalic anhydride increases the rigidity. Terephthalic acid or isophthalic acid are used in high performance hard coatings and adhesives. Such foams are improved to be flame resistant. Foams based on aromatic polyester polyols show charring upon exposure to flame. Polyesters based on terephthalic acid are manufactured by transesterification of dimethyl terephthalate. Also poly(ethylene terephthalate) waste materials, such as polyester fibers or soft drink bottles, can be recycled by glycolysis to obtain suitable polyols. Triols, such as glycerol and 1,1,1-trimethylolpropane, will result in branched polyesters. Alcohols for flexible foams are ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, etc. Longer chains result in a greater hydrolytic stability, simply because there are fewer ester groups in the structure. Polyesters from a single acid component and a single alcohol component are crystalline. The crystallinity can be reduced by using mixtures of diols or mixtures of different polyesters.
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Reactive Polymers Fundamentals and Applications
Mixed polyesters from waste acids of the production of nylon contain adipic acid, glutaric acid, and succinic acid. The acids can be also hydrogenated to obtain the respective diols that can be used in the condensation. The ester group in polyester polyols is sensitive to hydrolysis attack. The hydrolysis stability can be improved with additives that react with carboxylic and alcoholic groups, which are formed during the hydrolysis. These additives include oxazolines, epoxy compounds, and carbodiimide structures. In particular, polyester polyols can be stabilized by the addition of 1 to 2% of hindered aromatic carbodiimides. These compounds are scavengers for the acid generated by ester hydrolysis. The acid would catalyze further hydrolysis. Polyester polyols can contain 10 to 20% of vinyl polymers. The vinyl polymers improve the hydrolysis stability, hardness and the form stability. ε-Caprolactone based polyesters. Another synthesis route for aliphatic polyester polyols is the ring opening polymerization of ε-caprolactone with various glycols. These include diethylene glycol, 1,4 butanediol, neopentyl glycol, or 1,6-hexanediol. Branched products are obtained by adding 1,1,1-trimethylolpropane or glycerol to a bifunctional alcohol. Higher branched polyesters utilize pentaerythritol. The poly(ε-caprolactone)-containing polyesters exhibit a greater hydrolysis resistance and lower viscosity than comparable polyadipate glycols.
2.2.3 Other Polyols 2.2.3.1
Hydrocarbon Polyols
Hydrocarbon polyols can be obtained by living anionic polymerization of butadiene initiated by sodium naphthalene, which is the common route to polymerize butadiene. However, the living chains are finally terminated by adding ethylene oxide or propylene oxide. By adding water a poly(butadiene) with primary and secondary hydroxyl groups is obtained. Hydroxy-terminated poly(butadiene) is also accessible by free-radical polymerization of butadiene, initiated by hydrogen peroxide. The major advantage of hydrocarbon polyols is the high chemical resistance. The low glass transition temperature keeps its elastomeric properties down to
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91
extremely low temperatures. The double bonds in the chain or pendent double bonds open the possibility of further reactions, like vulcanization and other chemical reactions. The functionality of these diols is two, therefore they can be used for thermoplastic polyurethanes. 2.2.3.2
Polythioether Polyols
Polythioether polyols include products obtained by condensing thiodiglycol either alone or with other glycols, alkylene oxides, dicarboxylic acids, formaldehyde, amino-alcohols, or aminocarboxylic acids. 2.2.3.3
Polyacetal Polyols
Polyacetal polyols are prepared by reacting glycols such as diethylene glycol, triethylene glycol, or hexanediol with formaldehyde. Suitable polyacetals may also be prepared by polymerizing cyclic acetals. 2.2.3.4
Acrylic Polyols
Acrylic polyols are obtained by copolymerization of acrylic monomers, such as ethyl acrylate, n-butyl acrylate, acrylic acid, methyl methacrylate, or styrene with minor amounts of 2-hydroxyethyl acrylate or 4-hydroxybutyl acrylate. Styrene, if added, makes the acrylic polyol more hydrophobic. Acrylic polyols are used in two-component coating systems. They exhibit good chemical resistance and weatherability. 2.2.3.5
Liquefied Wood
Liquefied wood can be obtained by the liquefaction of benzylated wood wastes using dibasic esters as solvent with hydrochloric acid as catalyst. The reaction is completed at 80°C after 3 hours. Liquefied wood acts as a diol component for, e.g., TDI, IPDI, and HDI. Polyurethane resins from liquefied wood have a higher thermal stability than the traditional polyurethane resins.34
2.2.4 Polyamines The amine functionality reacts with the isocyanate group to a urea moiety. In this way an amine group corresponds to a hydroxy group that reacts with the isocyanate group to a urethane moiety.
92
Reactive Polymers Fundamentals and Applications
Hydroxyl end groups in polyether polyols can be converted into amine end groups by reductive amination. This type of compound is called an amine-terminated polyether, or simply polyetheramine. Polyetheramines are suitable for soft segments of polyurea resins.
2.2.5 Chain Extenders Chain extenders, curing agents, and crosslinkers are low molecular compounds for improving properties of the final products. Examples are shown in Table 2.4. Chain extenders are difunctional compounds. Glycols are used in polyurethanes. Diamines or hydroxylamines are used in polyureas and mixed polyurethane ureas. Low-molecular weight polyamines react with the isocyanate group very fast, and can be used in reactive injection molding, where short cycles are essential. 2,2′ -Pyromellitdiimidodisuccinic anhydride (DA) can act as a chain extender for isocyanates in the presence of polyols. In a first stage, the polyol is allowed to react with the isocyanate compound to get isocyanateterminated oligomers. In the second stage, the 2,2′ -pyromellitdiimidodisuccinic anhydride reacts with the oligomer, splitting off carbon dioxide to result in a poly(urethane-imide-imide). This class of polyurethane has a higher thermal stability than conventional polyurethanes.35 Chain extenders with the triazene structure are photosensitive compounds.36 They are used together with another extender as a coextender. Because the resulting triazene polyurethanes become crosslinked by exposure to UV irradiation, they have a potential use as negative-resist polymers.
2.2.6 Catalysts Catalysts are necessary to obtain the desired end products. The final properties depend strongly on the content of urethane, urea, allophanate, biuret, and isocyanurate bonds. Therefore, catalysts govern the final properties of the materials. The nature of the catalysts also greatly influences the reaction time and the properties of the final product. The catalysts can be classified into three main categories: 1. Catalysts for blowing, 2. Catalysts for gelling, and 3. Catalysts for crosslinking.
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93
Table 2.4: Chain Extenders Compound Ethylene glycol Diethylene glycol Propylene glycol Dipropylene glycol 1,4 Butanediol 2-Methyl-1,3-propylene diol N,N ′ -bis(2-hydroxypropylaniline) Water 1,4-Di(2-hydroxyethyl)hydroquinone Diethanolamine Triethanolamine 1,1,1-Trimethylolpropane Glycerol Dimethylol butanoic acid (DMBA) Hydrazine Ethylene diamine (EDA) 1,4-Cyclohexane diamine Isophorone diamine 4,4′ -bis(sec-Butylamine)dicyclohexylmethane 4,4′ -bis(sec-Butylamine)diphenylmethane Diethyltoluene diamine 4,4′ -Methylene bis(2-chloroaniline) 4-Chloro-3,5-diamino-benzoic acid isobutylester 3,5-Dimethylthio-toluene diamine Trimethylene glycol-di-p-aminobenzoate 4,4′ -Methylene bis(3-chloro-2,6-diethylaniline) 1-(α-Naphthyl)-3,3-di(2-hydroxyethyl)triazene-1 (NT-D) 1-Phenyl-3,3-di(2-hydroxyethyl)triazene-1 (PT-D)
Remarks
Waterborne chain extender37
Both isomers Both isomers Photosensitive36 Photosensitive36
94
Reactive Polymers Fundamentals and Applications Table 2.5: Catalysts Classified According to the Reaction
Reaction
Catalyst Type
Trimerization
Strong bases, quaternary ammonium salts, phosphines Phosphorous compounds Alkaline metal hydroxides Tertiary amines, organometals, metal salts Tertiary amines Metal salts Tin and zinc salts
Dimerization Polymerization Addition to alcohols Reaction with water Addition to urethane Addition to amines
From the chemical view, catalysts for producing polyurethanes can be divided into two general types: tertiary amines and organo-tin compounds. Organometallic tin catalysts predominantly favor the gelling reaction, while amine catalysts exhibit a more varied range of blow/gel balance. A lot of catalysts have been described and reviewed.6, 38 The choice of the catalyst depends on which reaction and which structure is to be favored. Table 2.5 lists types of catalysts that are suitable for the individual reactions. It is important to tune the kinetics of the individual reactions properly. For example, if the blowing reactions take place significantly before the sufficient progress of gelling (crosslinking), the viscosity of the reacting material is low, causing the carbon dioxide to escape, and will not yield a foam. On the other hand, if the gelling (or crosslinking reaction) occurs too fast, the blowing gas cannot expand the material. Thus, it is necessary to balance the individual reactions. This balance can be readily controlled by the nature and quantity of the catalyst used.
2.2.7 Blowing Chemical blowing is effected by the reaction of isocyanate and water. The rate of blowing increases with the catalyst and water content.39 As an intermediate, carbamic acid is formed. The carbamic acid is not stable; it decomposes into an amine and carbon dioxide. Carbon dioxide expands the polyurethane into a foam. There are also physical blowing agents available. In this case the foam is generated by the evaporation of the blowing agent supported by
Polyurethanes
95
external heating but also by the temperature rise due to the formation of the polyurethane from the diisocyanate and the polyol. Suitable reagents for physical blowing were previously fluorocarbons and chlorofluorocarbons. The latter class of substance has been removed because of its ozone depletion potential. Pentane is a substitute for chlorofluorocarbons. The release of the physical blowing agents occurs in three ways when a foamed material is recycled or shredded:40 1. The instantaneous release from cells split or damaged by the shredding, 2. The short-term release from cells adjacent to the cut surface , and 3. The long-term release by normal diffusion processes. Formic acid has been proposed as a chemical blowing agent.41, 42 Formic acid can behave either as an acid or an aldehyde. In contrast to water that yields exclusively carbon dioxide, formic acid upon contact with an isocyanate group reacts to initially liberate carbon monoxide and further decomposes to form an amine with a release of carbon dioxide, according to the following reaction: 2 Φ−NCO + HCOOH → CO + CO2 + Φ−NH−CO−NH−Φ
(2.1)
Aside from its zero ozone depletion potential, a further advantage of using formic acid is that 2 mol of gas are released for every mole of formic acid present, whereas a water-isocyanate reaction results in the release of only 1 mol of gas per mol of water. In both the water-isocyanate and the formic acid-isocyanate reactions, the isocyanate is consumed and one must add a proportionate excess of isocyanate to compensate for the loss. However, since formic acid is a more efficient blowing agent than water, the number of moles of formic acid necessary to produce the same number of moles of gas as a water-isocyanate reaction is greatly reduced, thereby reducing the amount of excess isocyanate and leading to a substantial economic advantage.43 It is believed that liberation of carbon monoxide and subsequently carbon dioxide in the reaction Eq. 2.1 proceeds at a slower rate than the release of carbon dioxide in a water-isocyanate reaction for two reasons: 1. The anhydride is more stable than the carbamic acid formed in a water-isocyanate reaction and, therefore, requires more thermal energy to decompose, and
96
Reactive Polymers Fundamentals and Applications 2. The reaction is a two-step reaction rather than the one-step reaction present in a water-isocyanate reaction.
The exothermic reaction in a polyol composition containing formic acid proceeds in a more controlled manner than in an all water blown reaction. Formic acid in combination with hydrochlorofluorocarbons improves the mechanical and thermal properties. It exhibits a delayed action and thus a prolonged gel time. Rigid foams produced with formic acid possess excellent dimensional stability at low densities.43 However, the generation of carbon monoxide during the curing and corrosion problems are evident drawbacks. 2.2.7.1
Gelling and Crosslinking
Gelling reactions are discussed as curing reactions that do not blow, but yield linear urethanes. These reactions are similar to crosslinking reactions, from the chemical view. The technical term “curing” is not common in polyurethanes, except for unsaturated polyester technology, epoxies, etc., because the resulting final products are often not hard, e.g., flexible foams. The basic reactions in the course of polyurethane formation are shown in Figure 2.19. These include the reaction of isocyanate with a polyol to yield a polyurethane, the formation of urea from an isocyanate and an amine, and the blowing reaction. Other reactions are the formation of a biuret, c.f. Figure 2.9 and the trimerization, c.f. Figure 2.13. The action of a catalyst can be studied conveniently with model compounds. Suitable experimental techniques are liquid chromatography,infrared spectroscopy, and nuclear magnetic resonance spectroscopy. Infrared spectroscopy conveniently monitors the disappearance of the isocyanate group. Raman spectroscopy is advantageous in two ways. Since the Raman effect is a scattering process, samples of any shape or size can be examined. Moreover, Raman spectroscopy measurements can be conducted remotely using inexpensive, communications grade, fused-silica optical fibers.44 Nuclear magnetic resonance spectroscopy suffers from the disadvantage that the spectroscopic shifts of the urethane, urea, allophanate, and biuret linkages are very similar. Rheological techniques are also suitable for monitoring the progress of curing.45–47 The dynamic viscosity has been measured as a function of
Polyurethanes
R N C O
R N C O
H O R’
H O R’
R N C O
R N C O H N
H N R’
R’
R N C O
R N C O
H O H
H O H
R N
H H
R N C O H N C O R O R’
97
CO2
H R N C O N C O R
O R’
Figure 2.19: Basic Reactions in Polyurethane Formation: Reaction of Isocyanate with a Polyol; Formation of Urea from Isocyanate and Amine; Chemical Blowing with Water; Allophanate formation
98
Reactive Polymers Fundamentals and Applications
time and found to be independent of the shear rate.47 A simple technique of this kind is to drop metal ball bearings consecutively into a growing foam. The position of the ball bearings in the final foam reflects the viscosity profile. The simultaneous measurement of the height of the foam gives information of the degree of expansion. The gel times can be used to evaluate the activity of catalysts. In particular, it was found that the activity of catalysts, among them organometallic catalysts, decreases in the order Bi > Pb > Sn > triethylamine > . . ..46 The rheological properties determined by dynamic mechanical techniques can be sensitive to the rate of mechanical deformation. The rate of expansion or possibly the rate of foam rise can be used characterizing the activity of certain catalysts. A combined measurement of the expansion and the weight loss permits characterizing the mass of CO2 trapped within a foam, the mass of CO2 lost, and the total mass of CO2 generated during curing. There are three major classes of catalysts: tertiary amines, organic salts, and organometallics. Often the chemical nature of the catalysts is not disclosed in the patent literature. However, a compilation of chemical structures of commercially available catalysts useful in the manufacture of flexible foams is available.48 Nevertheless, it is often difficult to establish structure-property relationships because of the unavailability of information. 2.2.7.2
Tertiary Amine Catalysts
Commercially used amines are summarized in Table 2.6 and shown in Figure 2.20. Amine catalysts are often delivered as a solution in dipropylene glycol. This makes the dosage of small quantities easier. Tertiary amines are used most commonly to catalyze the urethane formation. They catalyze both gelling and blowing reactions but not the formation of isocyanurate. Tertiary amines are often formulated with organotin compounds. As the basicity increases, the crosslinking is favored. A known problem is volatility that causes odor. Further, the migration of amine catalysts can cause a discoloration when the final polyurethane is used with poly(vinyl chloride) (PVC). This problem emerges in the automotive industry and is addressed as “vinyl staining”. The discoloration of poly(vinyl chloride) bound to polyurethane has
Polyurethanes
99
Table 2.6: Tertiary Amine Catalysts Amine
Remarks
1,4-Diazabicyclo[2.2.2]octane (DABCO) Bis(2-dimethylaminoethyl)ether (BDMAEE)
Widely employed High-resiliency foams, heavy blowing catalyst Polyester slabstock foam Polyester slabstock foam High vapor pressure, improves skin formation in molded foam Highly volatile cure catalyst Low odor Uretdiones
N-Ethylmorpholine N-Methylmorpholine N′ ,N′ -dimethylpiperazine Triethylamine N,N-dimethylethylamine Substituted pyridines 2-Azabicyclo[2.2.1]heptane N-(3-Dimethylaminopropyl)-2-ethylhexanoic acid amide N,N,N ′ ,N ′ ,N ′′ -pentamethyldiethylene triamine N,N-dimethylcyclohexylamine N,N-dimethylbenzylamine N,N-Dimethylethanolamine 3-Hydroxy-1-azabicyclo[2.2.2]octane 2-(2-N,N-Diethylaminoethoxy)ethanol 5-Dimethylamino-3-methyl-1-pentanol 1-(2-hydroxypropyl)imidazole 1-(3′ -Aminopropyl)imidazole 1-(3′ -(Imidazolinyl)propyl)urea Bis(3-(N,N-dimethylamino)propyl)amine, chain-extended with polyol and polyisocyanate
49
Heavy blowing catalyst Odorous liquid Polyester flexible foams Polyether flexible foams Reactive catalyst Reactive catalyst Reactive catalyst, low odor50 Reactive catalyst Reactive catalyst51 51
52
100
Reactive Polymers Fundamentals and Applications
O CH2 N CH2 CH2
CH2 CH2 CH2
N
N
CH2
1,4-Diazabicyclo[2.2.2]octane
CH3
N-Ethylmorpholine CH3
H3C N CH2
CH2
O CH2
CH2
N CH3
H3C Bis(2-dimethylaminoethyl)ether H3C CH2
N
N CH2 CH3
2-Methyl-2-azabicyclo[2.2.1]heptane
CH2
OH
H3C N,N-Dimethylethanolamine
Figure 2.20: Tertiary Amine Catalysts: 1,4-Diazabicyclo[2.2.2]octane, N-Ethylmorpholine, Bis(2-dimethylaminoethyl)ether, 2-Azabicyclo[2.2.1]heptane, N,NDimethylethanolamine
Polyurethanes
101
been attributed to the catalyzed dehydrochlorination of the PVC by the residual amine catalyst.53 Amine-free catalyst systems based on carboxylates are helpful to avoid this phenomenon.54, 55 The activity of amines increases with increasing basicity. However, the activity is negatively influenced by steric hindrance. The urethane formed by the reaction catalyzes further formation of urethane. Amines of the general structure RR′ N(CH2 )n OR′′ are effective blowing catalysts at n = 2, but good gelling catalysts at n = 3. Triethylene diamine is a synonym for 1,4-diazabicyclo[2.2.2]octane, which is both an excellent gelling and blowing catalyst. It is the most used tertiary amine in the production of polyurethanes. The unusual high activity of 1,4-diazabicyclo[2.2.2]octane emerges from a lack of steric hindrance in spite of its moderate basicity. Its complex with boric acid exhibits a reduced odor. Bis(2-dimethylaminoethyl)ether is used to produce high-resiliency foam, because it promotes the reaction of the isocyanate with water. It is often used together with triethylene diamine. N-ethylmorpholine and Nmethylmorpholine have lower activity and are therefore used in the production of polyester slabstock foam, where only catalysts with lower activity are needed. N-Methylmorpholine, N-ethylmorpholine and triethylamine belong to the group of skin cure catalysts. These are tertiary amines with high vapor pressure. They volatilize from the developing foam to the foam mold surface, thus promoting an additional reactivity there. Substituted hexahydro-s-triazines, like 1,3,5-tris(3-dimethylaminopropyl)-s-hexahydrotriazine and hexamethylenetetramine56 and alkylated imidazoles, like 1-methylimidazole or 1,2-dimethylimidazole57–60 (Figure 2.21) are also used in both high resiliency and rigid foams. An amidine contains a chemical structure as presented in Eq. 2.2.
C
N N
(2.2)
Certain bicyclic amidines (Fig. 2.22) exhibit a high gelling activity coupled with low volatility. However, these materials are sensitive to heat, light, and oxygen. 1,8-Diazobicyclo[5.4.0]undec-7-ene or 1,5-diazobicyclo[4.3.0]non-5-ene in combination with primary amines can catalyze the reaction of phenol blocked isocyanates.61 The bicyclic catalyst is capa-
102
Reactive Polymers Fundamentals and Applications
H3C H3C
CH3 N H2C H2C H2C
N
N
CH2
CH2
CH2
N CH3
N
CH3
CH2
CH2
CH2
N
CH3
1,3,5-Tris(3-dimethylaminopropyl)-s-hexahydrotriazine N N H2C
N
CH2
H2C
N CH2 N CH2
Hexamethylenetetramine
N CH3 1-Methylimidazole
Figure 2.21: 1,3,5-tris(3-dimethylaminopropyl)-s-hexahydrotriazine, hexamethylenetetramine, 1-methylimidazole
ble of unblocking phenol blocked isocyanate groups, and can effect curing within an hour at ambient temperature. Among the amidines the bicyclic amidines have greater activity than the monocyclic amidines.62 Alkylamino amides, i.e. secondary amides with a pendent tertiary amine with the basic structure [(CH3 )2 N(CH2 )3 ]2 NCOR are odorless and have a high resistance to hydrolysis.63 For example, formaldehyde can be condensed with N,N-bis(3-dimethylamino-n-propyl)amine. Ammonia is evolved to yield N,N-bis[3-(dimethylamino)propyl]formamide.
N N 1,8-Diazobicyclo [5.4.0] undecene-7
N N 1,5-Diazobicyclo [4.3.0] non-5-ene
Figure 2.22: 1,8-Diazobicyclo[5.4.0]undec-7-ene, 1,5-Diazobicyclo[4.3.0]non5-ene
Polyurethanes
103
These types of compounds are strong gelling catalysts. Combination of the latter compound with a weak blowing catalyst, such as methoxyethylmorpholine has been described.64 Formamide-type catalysis can be used to replace the highly volatile dimethylpiperazine. The use of N,N-Bis[3-(dimethylamino)propyl]formamide as the sole catalyst produces a tight foam. Blends with methoxyethylmorpholine or optionally with 2,2′ -oxybis(N,N-dimethylethanamine) are strong blowing catalysts. They improve flow, skin cure, and de-mold times in flexible molded polyether foams.64 Still less volatile catalysts can be prepared using bifunctional oxalic esters instead of formic acid derivatives.65 This class is addressed as alkylamino oxamides. An aqueous catalyst mixture is obtained to form the salts by, e.g., salicylic acid. Alternative catalysts have cyclic structures, e.g., bis[N-(3-imidazolidinylpropyl)]oxamide, or bis[N-(3-morpholinopropyl)]oxamide. Headspace gas chromatography was applied to measure the fugitivity. The oxalic acid amide adducts were not volatile under the conditions of analysis. To combine good in-mold flowability and fast curing, delayed-action catalysts were developed. Reduced reactivity in reactive injection molding is sometimes desirable so that large molds could be filled completely before cure. The activity of an amine catalyst can be delayed by adding acids, such as formic acid, 2-ethylhexanoic acid, or amino acids.66 The amine salt is less active then the free amine. As the curing proceeds the temperature rises. At elevated temperatures the amine salt dissociates to the free amine and acid. Zwitterionic salts from triethylene diamine and tetra-n-butylammonium chloroacetate also delay the reaction. The effect of controlled catalysis may be realized in improved reactivity profiles, for instance, delayed initiation or accelerated cure.67, 68 A disadvantage in the usage of amine salts is the possibility of corrosion, a negative influence on the long-term properties of the final product. Half esters of diethylene glycol with maleic anhydride or phthalic anhydride can be used to neutralize, or block amines, such as bis(2-dimethylaminoethyl)ether (BDMAEE). Such types of blocked amines are noncorrosive, delayed-action catalysts for flexible foams.69 The reaction can be performed in one stroke, allowing phthalic anhydride to react with BDMAEE in diethylene glycol. Acid-blocked amine catalysts have an unpleasant odor associated
104
Reactive Polymers Fundamentals and Applications
with their use, especially when the polyurethane mixtures are cured in an oven at temperatures above 120°C. This unpleasant odor also remains in the final product, making these catalysts unsuitable for some applications.70 The incorporation of active hydrogens, such as primary and secondary hydroxyl groups and amino groups, into the catalyst structure is suitable to reduce odors and emissions. 2.2.7.3
Mechanisms of Tertiary Amine Catalysts
Two basic mechanisms for tertiary amine-catalyzed formation of urethane are under discussion. The first mechanism deals with the formation of an isocyanate-amine complex followed by reaction with an alcohol. This mechanism suggests that the nucleophilicity of the amine is the dominant factor. The second mechanism postulates an amine-alcohol complex that reacts with the isocyanate. According to this mechanism, the amine basicity is the dominant factor. The mechanism based on an isocyanate-amine complex seems to be more generally accepted. It is suggested that Lewis bases are activating the alcohols.71 2.2.7.4
Reactive Catalysts
If the catalysts are modified with a group that reacts with isocyanates, then the catalysts can be incorporated into the polyurethane material. For example, triethanolamine has three hydroxy functions and is at the same time a tertiary amine. Other compounds include an adduct of glycidyl diethylamine with 2(di-methylamino)ethanol.72, 73 A hydroxy functional tertiary amine can be produced by a Michael type reaction followed by reductive amination of the cyano group, as exemplified with 1-(3-dimethylaminopropoxy)-2-butanol in Figure 2.23. Since the butanol can attack the acrylonitrile either with the primary hydroxyl group or with the secondary hydroxyl group, in fact an isomeric mixture will be obtained.74 In the same way an adduct with 1-methylpiperazine can be obtained. Reactive catalysts typically show a high activity in the initial stage of polymerization and then a reduced activity when they are included in the growing polymer.
Polyurethanes
H2C
CH C N
H2C CH C N O H
O H CH2
CH CH2
CH2
CH3
CH3 H2C CH CH2 N CH3
O H CH CH2
CH CH2
CH3
OH
OH
CH2
105
CH3
N CH3
+ H2 - NH3
CH3
OH 1-(3-Dimethylaminopropoxy)-2-butanol
Figure 2.23: Synthesis of a Hydroxy Functional Tertiary Amine: 1-(3-Dimethylaminopropoxy)-2-butanol
2-Dimethylaminoethyl urea or N,N ′ -Bis(3-dimethylaminopropyl) urea contains the ureido group which enables the catalysts to react into the polyurethane matrix. These reactive catalysts can be used as gelling catalysts or blowing catalysts with complementary blowing or gelling cocatalysts, respectively, which may or may not contain reactive functional groups to produce polyurethane foam materials. The reactive catalysts produce polyurethane foams which have no amine emissions.75 Examples for reactive catalysts include 3-quinuclidinol (3-hydroxy1-azabicyclo[2.2.2]octane),76, 77 propoxylated 3-quinuclidinol, 3-hydroxymethyl quinuclidine,78 and 2-(2-N,N-diethylaminoethoxy)ethanol. Propoxylated 3-quinuclidinol is a liquid, which is soluble in dipropylene glycol, whereas 3-quinuclidinol is a high melting solid. 3-Methyl-3-hydroxymethyl quinuclidine may be prepared by reacting ethylpyridine with formaldehyde to afford 2-methyl-2-(4-pyridyl)-1,3-propanediol which is hydrogenated to 2-methyl-2-(4-piperidyl)-1,3-propanediol which in turn is cyclized to the quinuclidine product.78 2-(2-N,N-diethylaminoethoxy)ethanol is superior with regard to vinyl staining. Combinations of a nonreactive catalyst and a reactive catalyst, e.g., N,N-bis(3-dimethylaminopropyl)formamide and dimethylaminopropylurea, have been proposed for foams for interior components of automo-
106
Reactive Polymers Fundamentals and Applications
biles.79 Such low-volatility catalysts do not emit vapors over time or under the effects of heat which would otherwise cause nuisance fogging of windshields, and also reduce the chemical content of the air inside vehicles to which a driver and passengers are otherwise exposed. 2.2.7.5
Anionic Catalysts
Anionic catalysts favor the isocyanurate formation. Isocyanurate units are built by trimerizing an isocyanate. The isocyanurate group improves properties such as thermal resistance, flame retardancy, and chemical resistance. In quaternary ammonium carboxylates, alkali metal carboxylates and substituted phenols such as 2,4,6-tris(dimethylaminomethyl)phenol, the active species is the anion. This is different from amine salt catalysts where the active species is the free amine. Examples for quaternary ammonium carboxylates are benzylammonium carboxylate,80 tetramethylammonium pivalate, and methyldioctyldecylammonium pivalate (C8 H17 )2 (C10 H21 )(CH3 )N+− O2 CC(CH3 ).81 Tetraalkylammonium fluorides and cesium fluoride are extremely selective catalysts for the formation of isocyanurate.82 The trimerization of diisocyanates produces not only the trimer, i.e., monoisocyanurate, but also higher oligomers. The viscosity of the demonomerized polyisocyanate increases as the oligomer content increases. The deactivation of the catalyst is necessary in order to terminate the trimerization and to ensure the storage stability of the polyisocyanate. The degree of trimerization can be controlled by the addition of a catalyst inhibitor. After adding the catalyst inhibitor, the trimerization stops.83 Suitable catalyst inhibitors are compounds which enter into chemical reactions with quaternary ammonium fluorides. Examples include calcium chloride or alkyl chlorosilanes such as ethyl chlorosilane, or substances which adsorptively bind quaternary ammonium fluorides, such as silica gel. Further organic acids or acid chlorides deactivate the catalysts. Potassium octoate and tertiary phosphines are other catalysts useful for the dimerization and trimerization of isocyanates. Carboxylic acids favor the formation of urea bond compounds.84, 85 Potassium acetate is a general purpose catalyst. 2.2.7.6
Organometallic Catalysts
Commonly used organometallic catalysts are shown in Table 2.7.
It is
Polyurethanes
107
Table 2.7: Organometallic Catalysts Compound
Remarks
Dibutyltin dilaurate (DBTDL) Stannous octoate Dibutyltin diacetate Dibutyltin dimercaptide Lead naphthenate Lead octoate Dibutyltin bis(4-hydroxyphenylacetate) Dibutyltin bis(2,3-dihydroxypropylmercaptide) Ferric acetylacetonate
Standard Compound Polyether-based slabstock foams
Hydrolytically stable Elastomers
believed that the catalytic action occurs by a ternary complex of the isocyanate, hydroxyl, and the organometallic compound. A Lewis acid-isocyanate complex is formed followed by complexation with the alcohol.71 For gelling reactions, organometallic catalysts are more selective than tertiary amines. Some organotin compounds lose their activity in the presence of water or at high temperatures. As in the case of amine catalysts, the activity decreases in sterically hindered compounds. Also, solvent effects are observed. The solvent effect is relevant for solvent-based coating formulations. Dialkyltin dimercaptides, such as dibutyltin dilauryl mercaptide, exhibit good storage times when admixed with other catalyst components.86 Dibutyltin dilaurate catalyzes the formation of urethane suppressing the formation of allophanates and isocyanurates.87 With high resiliency foams (HR), where more reactive polyols are generally employed, very few tin catalysts can be used because the foam cell walls are less prone to rupture than with conventional foams, and this can result in shrinkage problems.56 Bis(2-acyloxyalkyl)diorganotins exhibit only a small activity at room temperature. However they decompose at elevated temperatures into diorganotin dicarboxylates, which are the active species and olefins. For this reason they are also referred to as latent catalysts. This effect can be used to tailor catalysts. One advantage of the latent catalysts of the formula like Figure 2.24 is, therefore, to be able to mix the starting materials with the latent catalyst without catalysis of the reaction taking place and to initiate the catalysis of the reaction by heating the mixture to the decomposition temperature of the latent catalyst. 2-Acetoxyethyl-dibutyltin
108
Reactive Polymers Fundamentals and Applications
CH3
CH3 C O Bu Bu
O
Sn
H
Cl
CH2
Bu = CH3
C O
hν ∆
CH
CH2
Bu
Bu
O
Sn CH2
CH2
Cl CH2
CH2
Figure 2.24: Synthesis of 2-Acetoxyethyl-dibutyltin chloride from Chlorodibutyltin hydride and Vinyl acetate
chloride is prepared from chlorodibutyltin hydride and vinyl acetate, c.f. Figure 2.24, and it is decomposed by heat at 90°C within one hour.88, 89 Another latent tin catalyst consists of the adduct of a tin carboxylate or other tin compound with a sulfonylisocyanate, such as dibutyltin dilaurate or dibutyltin methoxide and tosyl isocyanate.90 Tin alkoxides or tin hydroxides have a far higher catalytic activity than the tin carboxylates. These additional compounds are extremely sensitive to hydrolysis, alcoholysis and are decomposed by the presence of water. Moisture can be supplied by the substrate, the atmosphere or by compounds containing reactive groups toward isocyanate, in particular hydroxyl groups, with release of the catalysts. Before hydrolytic or alcoholytic decomposition of the addition compounds takes place, these compounds are completely inert towards isocyanate groups. They give rise to no side reactions which would impair the storage stability of organic polyisocyanates. Combinations of organotin catalysts and hydrogen chloride extend the pot-life time in coating compositions without changing the cure time.91 Bismuth neodecanoate and combinations of bismuth and zirconium carboxylic acid salts also exhibit longer pot-life times combined with rapid curing.92 However, catalysts based on bismuth are water sensitive and deactivate in the presence of moisture. Polymeric metal catalysts are less prone to migrate. They can be synthesized by reacting a diorganotin dichloride or dibutyltin oxide with a hydroxymercaptan, such as 3-mercapto-1,2-propanediol with water removal. A viscous polymeric material is obtained.93 Dibutyltin bis(4-hydroxyphenylacetate) and dibutyltin bis(2,3-dihydroxypropylmercaptide) are hydrolytically particularly stable. The hy-
Polyurethanes
109
droxy functionality allows an incorporation in the polyurethane chain.94 A low odor and migration resistant organotin catalyst consists of the reaction products of dibutyltin oxide and aromatic aminocarboxylic acids, e.g., 3,5-diaminobenzoic acid to result in tin-di-n-butyl-di-3,5-amino benzoate.95
2.3 SPECIAL ADDITIVES Chemical formulations of polyurethane foams are based on the following ingredients: 1. 2. 3. 4. 5. 6. 7. 8.
Polyol, Isocyanate, Catalysts, Water, Blowing agent, Surfactant, Pigment, Additives.
2.3.1 Fillers 2.3.1.1
Rectorite Nanocomposites
Rectorite (REC) is a clay mineral with a 1:1 regular interstratification of a dioctahedral mica and a dioctahedral smectite. Rectorite has been used to yield intercalated or exfoliated thermoplastic polyurethane rubber nanocomposites by melt processing intercalation. X-ray diffraction and transmission electron microscopy clarified that the composites with lower amounts of clay are intercalation or part exfoliation nanocomposites. The mechanical properties of the composites are substantially enhanced.96 2.3.1.2
Zeolite
Zeolite has been used for modifying the structure of polyurethane membranes and to improve their properties. Membranes with zeolite content between 10 and 70%, have been prepared. The preparation method induces an anisotropy in the membranes. The membranes have therefore an
110
Reactive Polymers Fundamentals and Applications
asymmetric structure consisting of the top skin, i.e., the active layer, the substructure, and the bottom skin.97 2.3.1.3
Iron Particles
The sound absorption characteristic within a certain frequency bandwidth of a flexible polyurethane foam can be changed, when 2 to 5 µ m carbonyl iron particles are incorporated, when constant intensity magnetic fields are applied.98
2.3.2 Reinforcing Materials 2.3.2.1
Nanosilica Particles
Polyurethane ionomers in an aqueous emulsion were reinforced with hydrophobic nanosilica to give composites. The aqueous emulsion was stable and the particle size increased as the content of hydrophobic nanosilica was increased. The reinforcing effects of nanosilica on the mechanical properties were examined in various tests. The composites showed an enhanced thermal and water resistance.99 Nanosized SiO2 particles can be prepared via the sol-gel process. In a sol-gel process, the inorganic mineral is formed and deposited in-situ in the organic polymer matrix, for example, aqueous emulsions of cationic polyurethane ionomers, mixed with tetraethoxysilane, hydrolyze by the action of acid. In this way, silica nanocomposites, based on poly(ε-caprolactone glycol) as soft segment, and isophorone diisocyanate as hard segment, and 3-dimethylamino-1,2-propanediol as chain extender were prepared.100 Mechanical properties are improved by the incorporation of the particles. The particles do not essentially affect the low temperature-resistant properties, but improve the heat-resistance of the resin.101 The dispersion of the particles can be enhanced by a surface modification with (3-aminopropyl)triethoxysilane.102 Polyurethane/filler composites also can be prepared by mixing the polyol with a solution of the silica in methylethylketone, then stripping the methylethylketone. This solution is then reacted with a diisocyanate, and then chain-extended with 1,4-butanediol. Atomic force microscopy revealed that the filler particles were evenly distributed in the hard and soft phases.103
Polyurethanes
111
Table 2.8: Flame Retardants for Polyurethanes
2.3.2.2
Compound
Reference
Expandable graphite Triethyl phosphate Ammonium polyphosphate Melamine cyanurate Poly(epichlorohydrin) (PECH) 3-Chloro-1,2-propanediol, reactive
105 106 107 107 108 109
Layered Silicate Nanocomposites
High performance nanocomposites that consist of a polyurethane elastomer (PUE) and an organically modified layered silicate have been described.104 The polyurethane is based on poly(propylene glycol), 4,4′ -methylene bis(cyclohexyl isocyanate) and 1,4-butanediol. The tensile strength and strain at break for these PUE nanocomposites increases more than 150%. An isocyanate index of 1.10 results in the best improvement in stress and elongation at break. Polyurethane/organophilic montmorillonite (PU/OMT) nanocomposites have an enhanced tensile strength and improved thermal properties, in comparison to unmodified polyurethane.110 An amphiphilic urethane precursor with hydrophilic poly(ethylene oxide) (PEO) was used to prepare nanocomposites containing Na+ -montmorillonite.111
2.3.2.3
Nanoclays
Waterborne polyurethane/poly(methyl methacrylate) hybrid materials were reinforced with exfoliated organoclay. The size of the particles in the emulsion increased when the contents of PMMA or organoclay was increased. X-ray measurements showed an effective exfoliation of the silicate layer in the polymer matrix.112
2.3.3 Flame Retardants Flame retardants, recently described, are summarized in Table 2.8.
112 2.3.3.1
Reactive Polymers Fundamentals and Applications Poly(epichlorohydrin)
Poly(epichlorohydrin) (PECH) was phosphorylated by the reaction the P-H bond of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) with the pendent chloromethyl groups of PECH. A phosphorus-containing PECH with hydroxyl terminal groups is thus obtained.108 From this compound a phosphorous-containing polyurethane is obtained by the reaction with 2,4-toluene diisocyanate. The polymers are useful as multifunctional modifiers for epoxy resins and for improving the toughness and flame retardancy. 2.3.3.2
Expandable Graphite
The protective shield in a polyurethane expandable graphite (EG) system consists of expanded worms of graphite embedded in the tarry degraded matrix of polyurethane.105 The expansion of EG is due to a redox process between H2 SO4 , intercalated between graphite layers, and the graphite itself that originates the blowing gases according to the reaction: C + 2 H2 SO4 → CO2 + 2 H2 O + 2 SO2
(2.3)
Expandable graphite can be used in poly(isocyanurate) polyurethane foams in order to improve fire behavior of such foams. In order to obtain a completely halogen-free material, water blown foams must be prepared thus avoiding the use of hydrochlorofluorocarbons or hydrofluorocarbons. The limiting oxygen index of the material without expandable graphite is at 24% and reaches 30.5% in presence of 25% of expandable graphite.113 Triethyl phosphate shows a synergistic effect with expandable graphite.106 Further expandable graphite or triethyl phosphate do not worsen the mechanical properties. Ammonium polyphosphate, melamine cyanurate, and expandable graphite were tested in a comparative study. Expandable graphite showed the best results.107 2.3.3.3
Charring Agents
In the case of ammonium polyphosphate, the blowing effect is less important105 than in expandable graphite. Ammonium polyphosphate, melamine cyanurate and expandable graphite are compounds that form char layers that provide a thermal isolation.
Polyurethanes
113
Table 2.9: Global Production/Consumption Data of Important Monomers and Polymers115 Monomer
Mill. Metric tons
Phosgene Toluene diisocyanate p,p′ -Methylene diphenyl diisocyanate (MDI) Ethyleneamines Phthalic anhydride Maleic anhydride 1,4-Butanediol Polyurethane foams (flexible and semi-rigid) Polyurethane foams (rigid) Polyurethane elastomers Urethane surface coatings
5 1.3 2.4 0.248 3.2 1.3 1 2.3 1.6 0.581 1.5
Year
Reference
2002 2000 2000 2002 2000 2001 2003 2001 2001 2001 1999
116 117 117 118 119 120 121 122 122 123 124
However, the action takes place in different ways. Ammonium polyphosphate leads to the formation of a char layer through a series of processes consisting of initial peroxide formation, decomposition to alcohols and aldehydes, formation of alkyl-phosphate esters, dehydration and subsequent char formation.114 Thermogravimetric studies showed that the addition of ammonium polyphosphate accelerates the decomposition of the matrix but leads to an increase in the amount of high-temperature residue, under an oxidative or inert atmosphere. This stabilized residue acts as a protective thermal barrier during the intumescence fire retardancy process. The resulting char consists of an aromatic carbonaceous structure which condenses and oxidizes at high temperature. In the presence of ammonium polyphosphate, a reaction between the additive and the polymer occurs, which leads to the formation of a phosphocarbonaceous polyaromatic structure.125 Melamine cyanurate acts in an endothermic decomposition and gives off ammonia. Still nitrogen-containing polymers form then a char layer.107
2.3.4 Production Data Global Production Data of the most important monomers used for unsaturated polyurethane resins are shown in Table 2.9.
114
2.4
Reactive Polymers Fundamentals and Applications
CURING
The isocyanurate formation and isocyanate degree of conversion can be measured simultaneously by means of FT-IR spectroscopy.126 The curing behavior of polyurethanes based on modified methylene diphenyl diisocyanate and poly(propylene oxide) polyols has been studied using isothermal Fourier-transform infrared (FTIR) spectroscopy , differential scanning calorimetry (DSC) and adiabatic exothermic experiments. Increasing the concentration of the catalyst, i.e., dibutyltin dilaurate (DBTDL) or decreasing the molecular weight of the polyol raises the rate of reaction and shifts the DSC exothermic peak temperature to lower temperatures. However, the heat of reaction remains constant. A marked increase in reaction rate is observed when an ethylene oxide end-capped polyol is used instead of a standard propylene oxide end-capped polyol. The conversion of isocyanate for several concentrations of dibutyltin dilaurate (DBTDL) fits a second-order kinetics. The activation energy of curing is independent of the molecular weight of the hydroxy compound.127 However, the activation energy depends on the extent of conversion.47 With isocyanate reactive hot-melt adhesives an autocatalytic effect was observed. The autocatalysis is not dependent on the structure of diols but on the isocyanates.128
2.4.1 Recycling 2.4.1.1
Solvolysis
In recycling, catalysts can effect a reduction of the time required to recycle polyurethanes via hydrolysis and glycolysis. The products of polyurethane recycling are a complex mixture of alcohols and amines. Useful catalysts for recycling include titanium tetrabutoxide, potassium acetate, sodium hydroxide or lithium hydroxide. uncatalyzed polyurethane recycling is also possible. The recovery and purification of the polyol-containing liquid products can be achieved by the distillation of the glycolysis products. The amount of recoverable products by distillation reaches a maximum of 45%, when a process temperature of 245 to 260°C is applied.129
Polyurethanes 2.4.1.2
115
Ultrasonic Reactor
High resiliency polyurethane foam has been recycled by the application of high-power ultrasound in a continuous ultrasonic reactor. The foam has been decrosslinked at various screw speeds and various ultrasound amplitudes, then blended at different ratios with the virgin polyurethane rubber and then cured. In comparison to the ground recycled samples, the blends of the decrosslinked samples are easier to mix and exhibit enhanced properties.130 2.4.1.3
Polyacetal-modified Polyurethanes
Polyacetals are thermally stable but undergo a degradation by treatment with aqueous acid even at room temperature. Therefore, polyacetals are candidates for degradable polymers for chemical recycling. Polyurethane elastomers with degradable polyacetal soft segments have been synthesized.131 The polyurethanes were synthesized from polyacetal glycol and 4,4-diphenylmethane diisocyanate. 1,4-Butanediol was used as a chain extender. For comparison, samples containing a polyether glycol instead of the polyacetal glycol were prepared. Acid treatment indicated that the degradation took place. 2.4.1.4
Production Wastes
Waste residue from the production of toluene diisocyanate was used as a modifier in making improved waterproofing bitumen. The degree of improvement of the softening point could be correlated with the blend morphology.132
2.5 PROPERTIES 2.5.1 Mechanical Properties Copolymers of propylene oxide and ethylene oxide are used for softer foams in comparison with polyols obtained exclusively from propylene oxide. In comparison with polyether polyurethanes, polyester polyurethanes are more resistant to oil, grease, solvents, and oxidation. They exhibit better mechanical properties. On the other hand, polyester polyurethanes are less chemically stable and are also sensitive to microbiological attack.
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Reactive Polymers Fundamentals and Applications
2.5.2 Thermal Properties Additives, in particular nanocomposites, have a positive effect on the thermal properties. On heating up to degradation, the urethane structure undergoes a retro reaction into isocyanates. Therefore, highly poisonous products can be formed. The isocyanates yield depends greatly on the specific combustion conditions selected, such as temperature, ventilation, and fuel load. The mechanism of thermal degradation has been sketched.133 Polyurethane undergoes a depolycondensation. Volatile diisocyanate and isocyanate-terminated fragments are formed.134 In laboratory combustion experiments, isocyanates could be detected in the gaseous effluent. They were analyzed using impinger flasks containing 1-(2-methoxyphenyl)piperazine (MOPIP) as derivatizing reagent. The derivatives were analyzed by high performance liquid chromatography and tandem mass spectrometry. Isocyanic acid, aliphatic isocyanates, alkenyl isocyanates, and other derivatives were found.135 Heavy metals influence the thermal degradation. Manganese, cobalt, and iron ions favor the polyurethane degradation. Chromium and copper ions reduce the initial thermal stability of the polyurethane and have a catalytic effect on the second stage of its decomposition, but enhance the thermal stability of its intermediate decomposition products. By the modification of polyurethanes with these transition metal ions, changes in the decomposition mechanism of the polyurethane are induced.136
2.5.3 Weathering Resistance In aliphatic polyurethane-acrylate (PUA) resins, usually used for coatings, the urethane linkage is the most sensitive bond type with respect to photodegradation. The materials exhibit good weathering properties.137
2.6
APPLICATIONS AND USES
2.6.1 Casting Cold casting and hot casting systems are available. A polyurethane/poly(styrene-co-divinylbenzene) system can be cured at room temperature, in a one-step process.138
Polyurethanes
117
Table 2.10: Interpenetrating Polymer Networks Polyurethane
Further Component
Castor oil-based polyurethane Polyurethane– poly(ethylene oxide) Polyurethane Polyurethane Polyurethane ionomer Polyurethane Polyurethane Polyurethane Polyurethane Polyurethane Polyurethane Polyurethane Polyurethane Polyurethane
Poly(acrylonitrile), unsaturated polyester resin Poly(acrylonitrile)
Reference
Vinylester resin Poly(styrene) Poly(vinyl chloride) Poly(acrylate) latex Poly(methacrylate) Poly(butyl methacrylate) Poly(acrylamide) Nitrokonjac glucomannan Epoxy resin Poly(vinylpyrrolidone) Poly(benzoxazine) Poly(allyl diglycol carbonate)
139 140 141 142 143 144, 145 146–148 149 150 151 152, 153 154 155 156
2.7 SPECIAL FORMULATIONS 2.7.1 Interpenetrating Networks Several types of interpenetrating networks with polyurethanes have been prepared and characterized. These types are summarized in Table 2.10. In a tricomponent interpenetrating polymer network composed of castor oil, toluene diisocyanate, acrylonitrile, ethylene glycol diacrylate, and an unsaturated polyester resin, it was found that the tensile strength of the unsaturated polyester (UP) matrix was decreased and flexural and impact strengths were increased upon incorporating polyurethane/polyacrylonitrile (PU/PAN) networks.139 Poly(methyl methacrylate-co-2-methacryloyloxyethyl isocyanate) can be crosslinked with various diols that result in polyurethane structures. The crosslinking kinetics of diols, such as ethylene glycol (EG), 1,6-hexanediol, and 1,10-decanediol (DD) has been investigated, and second-order kinetics was observed. The rate constants decreased from EG to DD.146 The addition of nanosized silicon dioxide can improve compatibility, damping and phase structure of interpenetrating networks.152
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Reactive Polymers Fundamentals and Applications
CH2OH
CH2OH
O
O O
OH NH2 H 3C
CH2
NH
n
C O
N C O
n
NH H 3C
CH3 O C N
O
OH
CH2 CH3
CH3 O C N
CH3
Figure 2.25: Reaction of Chitosan with Isophorone
2.7.2 Grafting with Isocyanates 2.7.2.1
Chitosan
Chitosan is a linear polysaccharide obtained from the N-deacetylation of chitin. The amino group in chitosan can be reacted with an isocyanate, as shown in Figure 2.25, exemplified with isophorone diisocyanate. If in addition a polyol is present, then the second isocyanate group in isophorone can react with the polyol and longer pendent polyurethane chains can be formed.157
2.7.3 Medical Applications 2.7.3.1
Siloxane-based Polyurethanes
Polyurethane elastomers are used for medical implants. Deficiencies of conventional polyurethanes include deterioration of mechanical properties and degradation by hydrolysis reactions. Polyurethanes with improved long-term biostability are based on polyethers, hydrocarbons, poly(carbonate)s, and siloxane macrodiols. These components are intended to replace the conventional polyesters and polyethers. Siloxane-based polyurethanes show excellent biostability.158
Polyurethanes 2.7.3.2
119
Blood Compatibility
Polyurethanes are widely used as blood-contacting biomaterials because they exhibit good biocompatibility and further due to their mechanical properties. However, the blood compatibility is not adequate for certain applications. Modification of the surface is an effective way to improve the blood compatibility. Sulfonic and carboxyl groups can effectively improve the blood compatibility of polyurethane. Films of polyurethane containing acrylic acid were exposed to a sulfur dioxide plasma to graft sulfonic acid group on its surfaces. During the preparation of the films by dissolution, acrylic acid polymerizes to some extent.159 Carboxybetaine has been grafted onto polyurethane. A three-step procedure was used. First, the film surfaces were treated with hexamethylene diisocyanate in presence of DBTDL. Then, N,N-dimethylethylethanolamine (DMEA) or 4-dimethylamino-1-butanol (DMBA), respectively, was allowed to react in toluene with the pendent isocyanate groups. Finally, carboxybetaines were formed in the surface by ring opening involving the tertiary amine of DMEA or DMBA and β-propiolactone (PL).160 Similarly, sulfobetaines can be formed on the surface by the reaction of 1,3-propanesulfone (PS) instead of PL.161, 162 A polyurethane containing a phosphorylcholine structure has improved blood compatibility. The phosphorylcholine moiety consists of (6-hydroxy)hexyl-2-(trimethylaminonio)ethyl phosphate (HTEP). A segmented polyurethane (SPU) containing the phosphorylcholine structure was synthesized from diphenylmethane diisocyanate, soft segment polytetramethylene glycol (PTMG), and HTEP, with 1,4-butanediol (BD) as a chain extender.163 The phosphorylcholine structure on the surface of the SPU was proven by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and water contact angle measurements. 2.7.3.3
Degradable Polyurethanes
Longitudinal lesions in the meniscus are frequent orthopedic problems of the knee. The repair by simple techniques is limited to the vascular part of the meniscus. For the repair of the avascular part of the meniscus, a scaffold consisting of polyurethane foam has been developed. The scaffold is intended to assist the body in the formation of new meniscus cell tissue.
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Reactive Polymers Fundamentals and Applications
A segmented polyurethane with poly(ε-caprolactone) as the soft segment and 1,4-butanediisocyanate and 1,4-butanediol as uniform hard segments was chosen.164 The material has a micro phase separated morphology and excellent mechanical properties. Foams were prepared for a porous scaffold. The scaffold was tested by implantation in the knees of beagles. It was found that meniscus-like tissue had been formed in the scaffold. Another biodegradable, sponge-like polyurethane scaffold consists of lysine-diisocyanate (LDI) and glycerol. Ascorbic acid (AA) was copolymerized with LDI-glycerol.20 The cytocompatibility of polyurethane porous scaffolds is improved by photo grafting of methacrylic acid or poly(2-hydroxyethyl acrylate) onto the surface.165, 166 Polyurethanes can be degraded by esterase. This may contribute to the failure of medical implants. A strong dependence on the enzyme concentration for polyurethanes with different hard segment chemistry was established.167 2.7.3.4
Prevention of Polyurethane Heart Valve Cusp Calcification
The calcification of polyurethane prosthetic heart valve leaflets is highly undesirable. Polyurethane valves modified with covalently linked bisphosphonate groups are resistant to calcification, but the highly polar bisphosphonate groups on the polyurethane surface attract sodium counter ion, therefore, water absorption is increased. However, attaching diethylamino groups to the bisphosphonate-modified polyurethane will reduce water absorption.168
2.7.4 Waterborne Polyurethanes Waterborne polyurethanes are used mainly for coatings, but also for composites and nanocomposites. They are covered briefly, with special attention to their chemistry. Water dispersable paints can be produced from polyester polyol, isophorone diisocyanate and hydrophilic monomers such as dimethylol propionic acid (DMPA) and tartaric acid (TA).169 Phosphorus-containing flame retardant water-dispersed polyurethane coatings were also synthesized by incorporating a phosphorus compound into the polyurethane main chain.170
Polyurethanes
121
Table 2.11: Composites Made From Waterborne Polyurethane Materials Second Compound
Reference
Starch Carboxymethyl konjac glucomannan (CMKGM) Casein Carboxymethyl chitin Soy flour
174 175 176 177, 178 179
Bis(4-aminophenyl)phenylphosphine oxide (BAPPO) was obtained from bis(4-nitrophenyl)phenylphosphine oxide by the reduction of the nitro groups.171 The stability of waterborne dispersions can be improved by using a continuous process of preparation.172 Acetone addition has a large effect on the particle diameter.173 Waterborne anionomeric polyurethane-ureas can be made from dimethylol terminated perfluoropolyethers, isophorone diisocyanate, dimethylol propionic acid, and ethylene diamine. The materials are obtained as stable aqueous dispersions. Surface properties and chemical resistance were estimated by the measurement of contact angles and spot tests with different solvents. The surface hydrophobicity was not affected by the composition. Water-sorption behavior is however sensitive to the content of carboxyl groups in the polymer.180 Another type of waterborne polyurethane-urea anionomers consists of isophorone diisocyanate, poly(tetramethylene ether) glycol, dimethylol butanoic acid (DMBA), and hydrazine monohydrate (HD). Ethylene diamine (EDA), 1,4-butane diamine (BDA) are chain extenders. The pendent carboxylic groups are neutralized by ammonia/copper hydroxyde or triethylamine (TEA).181 Table 2.11 summarizes composites made from waterborne polyurethane materials. Composite materials were prepared by blending carboxymethyl konjac glucomannan (CMKGM) and a waterborne polyurethane (WPU). A blend sheet with 80% CMKGM exhibited good miscibility and higher tensile strength (89.1 MPa) than that of both of the individual materials, i.e. waterborne polyurethane sheets (3.2 MPa) and CMKGM (56.4 MPa) sheets. With an increase of CMKGM content, the tensile strength, Young’s modulus, and thermal stability increased significantly, attributed
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Reactive Polymers Fundamentals and Applications
to intermolecular hydrogen-bonding between CMKGM and WPU.175 Waterborne polyurethane and casein have been prepared by blending at 90°C for 30 min, and then crosslinking with ethanedial. Water resistance of the materials proved to be quite good.176
2.7.5 Ceramic Foams Organic polymers can be used in the manufacture of ceramic components. The organic polymers are admixed with the inorganic ceramic components, either to ceramic powder or to an inorganic monomer, as processing aids. Such a mixture can be processed in injection molding machines or by other techniques. The organic polymer supports the process of shaping a green part. Subsequently it is volatilized by pyrolysis or oxidation during heating. Ceramic foams can be produced with polyurethane and ceramic powder mixtures.182
2.7.6 Adhesion Modification In order to increase the compatibility between polyamide 6 and thermoplastic polyurethane, the polyurethane was reactively modified.183
2.7.7 Electrolytes Polymer electrolytes are used as solid electrolyte materials in rechargeable lithium batteries and electrochromic devices. Solid polymer electrolytes (SPE) have been introduced since the discovery of poly(ethylene oxide)electrolytes.184–186 In polyethers, the dissociation of alkali-metal salts occurs by the formation of transient crosslinks between the ether oxygen groups in the host polymer and alkali-metal cations. The anion is usually not solvated. The main deficiency of polyether-type electrolytes is the high degree of crystallization of the polyether. Thermoplastic polyether polyurethanes (TPU), doped with various alkali metal salts, have also been studied as polymer electrolytes. TPU exhibits good mechanical properties, a tough crystallinity of the polyether segments is reduced. Polyurethanes can be modified with chelate groups in order to enhance the electrical properties. ((3-(4-(1-(4-(3-(Bis-carboxymethylamino) 2-hydroxy-propoxy) phenyl)-1-methyl-ethyl) phenoxy) 2-hydroxypropyl)
Polyurethanes
123
CH3 CH2
O
HO CH CH2 N
C CH3
O CH2 CH OH CH2
H 2C
CH2
N H2C CH2
O C
C O
O C C O
HO
OH
HO OH
Figure 2.26: ((3-(4-(1-(4-(3-(Bis-carboxymethylamino) 2-hydroxy-propoxy) phenyl)-1-methyl-ethyl) phenoxy) 2-hydroxypropyl) carboxy methylamino) acetic acid
carboxy methylamino) acetic acid (EPIDA), c.f. Figure 2.26, is such a chelate. The molecule bears hydroxyl functions, which are basically reactive with isocyanate groups. Therefore, it can be built into a polyurethane chain.187 These electrolytes, due to the chelating groups, exhibit a significant interaction of the Li+ ions. A change in polymer morphology is also observed. An increase in the glass transition temperature of the soft segment occurs. Porous polymers, based on polyurethane/polyacrylate, can be prepared by emulsion polymerization. During the production, no organic solvent is used. The synthesis proceeds in four steps, listed here.188 1. A prepolymer is prepared from 2,4-toluene diisocyanate and poly(propylene glycol). 2,4-toluene diisocyanate is in a two-fold excess. 2. 2-Hydroxyethyl methacrylate (HEMA) is added to the prepolymer. The hydroxyl groups react with the residual isocyanate groups. 3. Again poly(ethylene glycol) is added in order to react with the remaining isocyanate groups. A macromonomer with pendant double bonds is obtained. 4. The macromonomer is emulsified and polymerized by the addition of 2,2′ -azobis(isobutyronitrile). The ionic conductivity is about 10−3 S cm−1 at room temperature. This conductivity is useful for many practical electrochemical applications. A light-emitting electrochemical cell (LEC) is composed of a blend
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Reactive Polymers Fundamentals and Applications
of semiconducting polymer and polymer electrolyte mixture. An electrochemical cell was built from poly(p-phenylene vinylene) (PPV), as light-emitting material and lithium ion conducting waterborne polyurethane ionomer as solid electrolyte.189 The polyurethane was prepared from a poly(ethylene glycol), α,α′ -dimethylol propionic acid and isophorone diisocyanate.
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174. X. D. Cao, L. N. Zhang, J. Huang, G. Yang, and Y. X. Wang. Structureproperties relationship of starch/waterborne polyurethane composites. J. Appl. Polym. Sci., 90(12):3325–3332, December 2003. 175. G. Yang, Q. Huang, L. Zhang, J. Zhou, and S. Gao. Miscibility and properties of blend materials from waterborne polyurethane and carboxymethyl konjac glucomannan. J. Appl. Polym. Sci., 92(1):77–83, April 2004. 176. N. G. Wang, L. Zhang, Y. S. Lu, and Y. M. Du. Properties of crosslinked casein/waterborne polyurethane composites. J. Appl. Polym. Sci., 91(1): 332–338, January 2004. 177. M. Zeng, L. N. Zhang, N. G. Wang, and Z. C. Zhu. Miscibility and properties of blend membrane of waterborne polyurethane and carboxymethylchitin. J. Appl. Polym. Sci., 90(5):1233–1241, October 2003. 178. M. Zeng, L. Zhang, and Y. Zhou. Effects of solid substrate on structure and properties of casting waterborne polyurethane/carboxymethylchitin films. Polymer, 45(10):3535–3545, May 2004. 179. Y. Chen, L. N. Zhang, and L. B. Du. Structure and properties of composites compression-molded from polyurethane prepolymer and various soy products. Ind. Eng. Chem. Res., 42(26):6786–6794, December 2003. 180. S. Turri, M. Levi, and T. Trombetta. Waterborne anionomeric polyurethane-ureas from functionalized fluorovolvethers. J. Appl. Polym. Sci., 93(1): 136–144, July 2004. 181. Y. S. Kwak, S. W. Park, and H. D. Kim. Preparation and properties of waterborne polyurethane-urea anionomers - influences of the type of neutralizing agent and chain extender. Colloid Polym. Sci., 281(10):957–963, October 2003. 182. T. Takahashi, H. Munstedt, M. Modesti, and P. Colombo. Oxidation resistant ceramic foam from a silicone preceramic polymer/polyurethane blend. J. Eur. Ceram. Soc., 21(16):2821–2828, December 2001. 183. J. Nagel, M. B. Brauer, B. Hupfer, D. Lehmann, and K. Lunkwitz. Adhesion modification of thermoplastic polyurethane and chemical influences on the adhesion in composites with pa 6. Kautsch. Gummi Kunstst., 57(5): 240–247, May 2004. 184. B. Scrosati, A. Magistris, C. M. Mari, and G. Mariotto, editors. Fast Ion Transport in Solids : [Proceedings of the NATO Advanced Research Workshop on Fast Ion Transport in Solids, Belgirate, Italy, September 20 - 26, 1992]. NATO ASI series : Series E, Applied sciences. Kluwer Academic Publishers, Dordrecht, 1993. 185. W. A. V. Schalkwijk and B. Scrosati, editors. Advances in Lithium-Ion Batteries. Kluwer Academic Publishers, Dordrecht, 2002. 186. B. Scrosati, editor. Application of Electroactive Polymers. Chapman and Hall, London, 1993. 187. S.-M. Lee, C.-Y. Chen, C.-C. Wang, and Y.-H. Huang. The effect of EPIDA units on the conductivity of poly(ethylene glycol)-4,4′ -diphenylmethane
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diisocyanate-EPIDA polyurethane electrolytes. Electrochim. Acta, 48(6): 669–677, February 2003. 188. X. Huang, T. Ren, and X. Tang. Porous polyurethane/acrylate polymer electrolytes prepared by emulsion polymerization. Mater. Lett., 57(26-27): 4182–4186, September 2003. 189. H.-L. Wang, A. Gopalan, and T.-C. Wen. A novel lithium single ion based polyurethane electrolyte for light-emitting electrochemical cell. Mater. Chem. Phys., 82(3):793–800, December 2003.
3 Epoxy Resins Epoxy resins are formed from an oligomer containing at least two epoxide groups and a curing agent, usually either an amine compound or a diacid compound. A great variety of such resins is on the market. There are many monographs on epoxy resins available.1, 2
3.1 HISTORY N. Prileschajew discovered in 1909 that olefins can react with peroxybenzoic acid to epoxides.3 Schlack claimed in 1939 a polymeric material based on amines and multi functional epoxides.4 Castan∗ , in the course of searching for dental materials claimed the preparation of bisphenol A diglycidyl ether (DGEBA).5, 6 A similar material, but higher in molecular weight, was invented by S. O. Greenlee.7 Epoxy resins came on the market around 1947. The first major intended application was as coating material.
3.2 MONOMERS 3.2.1 Epoxides Epichlorohydrin is the monomer used for the synthesis of glycidyl ethers and glycidyl esters. Epichlorohydrin (1-chloro-2,3-epoxypropane) is synthesized from propene via allyl chloride. A number of epoxides are shown ∗ Pierre
Castan, born in Bern 1899, died in Geneva 1985
139
140
Reactive Polymers Fundamentals and Applications
O CH2
O C
O
O CH3 O
H3C O
O
O
O O
O
Figure 3.1: Cycloaliphatic Epoxides
in Table 3.1. Reactive diluents, i.e. monofunctional epoxide compounds are shown in Table 3.2. The curing of cycloaliphatic epoxides proceeds easily with anhydrides, but is too slow with amines. Synthetic procedures for including styrenic, cinnamoyl, or maleimide functionalities, into cycloaliphatic epoxy compounds, have been described.8 3.2.1.1
Epoxide Equivalent Weight
The equivalent weight of the epoxide used is an important parameter for the amount of curing agent needed. The common method to determine the equivalent weight is the titration procedure with HBr in glacial acetic acid. However, a method for the determination of the epoxide equivalent weight in liquid epoxy resins using proton nuclear magnetic resonance (1 H-NMR) spectroscopy has been described.9
3.2.2 Phenols Bisphenol A is the most important ingredient in standard epoxy resins. It is prepared by the condensation of acetone with phenol. The latter two compounds can be prepared in the Hock process by the oxidation of cumene. Phenolic products are shown among others in Table 3.3 and Figure 3.2. The hydroxyl and amino functions are epoxidized with epichlorohydrin.
Epoxy Resins
141
Table 3.1: Epoxides Epoxide
Remark/Reference
Epichlorohydrin
Used for the formation of glycidyl ethers and esters
Butadiene diepoxide 1,4-butanediol diglycidyl ether (1,4-BDE) Glycerol diglycidyl ether 1,3-Didodecyloxy-2-glycidylglycerol Poly(butadiene) epoxides Vinylcyclohexene epoxide Styrene oxide ( = ethenylphenyloxirane) Glycidyl methacrylate (GMA) Epoxidized linseed oil Epoxy methyl soyate Epoxy allyl soyate Vernonia oil Triglycidyl isocyanurate Triglycidyloxy phenyl silane 2-(6-Oxid-6H-dibenz[c,e][1,2]oxaphoshorin-6-yl)-1,4-benzenediol 3,4-Epoxycyclohexyl-methyl3,4-epoxycyclohexane carboxylate 2,3,8,9-Di(tetramethylene)1,5,7,11-tetraoxaspiro[5.5]undecane Bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate Epoxidized cyclololefins Fluoro-epoxides Biphenyl-based epoxies Terephthaloylbis(4-oxybenzoic) acid DGEBA adduct Bis[3-(2,3-epoxypropyl thio)phenyl]sulfone 4,4′ -Dihydroxychalcone-epoxy oligomer
10
Amphiphilic polymers, for potential use as emulsifiers and solubilizing agents11 Flexible Both with vinyl and epoxy function Both with vinyl and epoxy function12 Both with vinyl and epoxy function 13 14 14
Naturally epoxidized, E-12,13-epoxyoctadeca-E-9-enoic acid esters15–17 Flame retardant18 Flame retardant19 Coatings Dental applications20 Dental applications20 Multifunctional, c.f. Figure 3.1 21
Liquid crystalline, c.f. Figure 3.3 Liquid crystalline22 Optical applications23 Optical applications24
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Reactive Polymers Fundamentals and Applications
Table 3.2: Reactive Diluents Reactive Diluent Phenyl glycidyl ether (PGE) Styrene oxide Allyl glycidyl ether Tetraethyl orthosilicate caprolactone diol adducts 2-Hydroxy-4(2,3-epoxypropoxy)benzophenone exo-3,6-Epoxy-1,2,3,6-tetrahydrophthalimidocaproic acid exo-3,6-Epoxy-1,2,3,6-tetrahydrophthalic anhydride
Remark/Reference
Cationic curable coatings25 Reactive photostabilizer for wood26 Polymers show anticarcinogenic activity27 Polymers show anticarcinogenic activity28, 29
Table 3.3: Compounds for Glycidyl Functionalization for Epoxide Resins Compound a
Remark/Reference
Bisphenol A Standard resins Bisphenol F Phenol novolak Naphthyl or limonene-modified Bis- Improved mechanical properties, rephenol A formaldehyde novolak duced water absorption30 Cresol novolak Tetrakis(4-hydroxyphenyl)ethane Increases crosslinking density p-Aminophenolb Higher reactivity at amine curing 31 Aminopropoxylate 4,4′ -Diaminodiphenylmethane b Hexahydrophthalic acid c 32 1,3-Bis(3-aminopropyl)tetramethyldisiloxane Tetrabromobisphenol A For flame retardant formulations Bishydantoin Isocyanurate Powder coatings Cresol Reactive diluent 1,4-Butanediol Reactive diluent a : Compounds are epoxidized at the hydroxyl function with epichlorohydrin b : Compounds epoxidized at the amino function with epichlorohydrin c : Compounds epoxidized at the carboxyl function with epichlorohydrin
Epoxy Resins
HO
CH2
OH
Bisphenol-F
HO
HO
NH2
p-Aminophenol
OH
CH3 C
HO
OH
CH3 CH CH
HO
Bisphenol-A
OH
H2N
CH2
NH2
Tetrakis(4-hydroxyphenyl)ethane
4,4’-Diaminodiphenylmethane
OH
OH
OH CH2
CH2
CH2
Novolac
Figure 3.2: Compounds for Epoxide Resins
143
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Reactive Polymers Fundamentals and Applications
3.2.3 Specialities 3.2.3.1
Hyperbranched Polymers
Hyperbranched polymers are highly branched macromolecules that are prepared through a single-step polymerization process.33 Many polymers of this type are also known as dendrimers, because their structure resembles the branches of a tree. Also, star-like and comb-like polymers belong to the class of hyperbranched polymers. However, hyperbranched polymers are built up from dendritic, linear, and terminal units. They can be synthesized via three routes: 1. Step-growth polycondensation of ABx monomers, 2. Self-condensing vinyl polymerization of AB∗ monomers, 3. Multibranching ring-opening polymerization of latent ABx monomers. The methods of synthesis available allow a wide variety of different polymer types. Further special properties can be imparted by suitable end capping reactions. This type of polymer has unique properties that are characteristic for dendritic macromolecules, such as low viscosity, good solubility, and a high functionality. Dendrimers are used in medical fields, as carriers of organic compounds. Hyperbranched polymers are easier to synthesize in large quantities and are used as tougheners, plasticizers, antiplasticizers and curing agents.34, 35 Hyperbranched polymers (HBP) with hydroxyl terminal groups can initiate curing by a proton donor-acceptor complex. In curing a tetrafunctional epoxy resin, the activation energy is lower than in an epoxy system with linear polymers.36 Hyperbranched polymers strongly enhance the curing rate due to the catalytic effect of hydroxy groups.37 The gel time increases with increasing functionality from diglycidyl ether of bisphenol A (DGEBA) to tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM).38 A hydroxyl-functionalized HBP reduced the gel time of the blends because of the accelerating effect of -OH groups to the epoxy curing reaction. Star-like epoxy polymers can be rooted from polyhydroxy fullerene with a cycloaliphatic epoxy monomer.39 Around 8 to 10 epoxy units can be attached to the fullerene core. The addition of small amounts of hyperbranched polymer to an epoxy system enhances dramatically its toughness. The critical strain energy
Epoxy Resins
145
release rate DGEBF resin can be increased by a factor of 6 by the addition of only 5% of hyperbranched polymer.40 At higher concentrations, a phase separation is indicated by two glass transition temperatures.41 In composite materials, resins modified by hyperbranched polymers allow higher volume fractions of fibers for producing void-free laminates in comparison to unmodified resins.42 3.2.3.2
Liquid Crystalline Epoxide Resins
Initially a few technical terms concerning liquid crystals are recalled. There are textbooks on liquid crystals, e.g., that of Collings and Hird.43 Liquid Crystal. Liquid crystals were discovered by the Austrian chemist and botanist Friedrich Reinitzer, who found that cholesterol benzoate did not melt into a clear liquid, but remained turbid. On further heating the turbid liquid turned suddenly clear. This transition point is now called the clearing point. For this reason, in addition to the common states of aggregation, the liquid crystalline state was established. The term liquid crystal goes back to the German physicist Otto Lehmann. Liquid crystals are formed mostly by rod-like molecules. They are sometimes addressed as mesomorphic phases. Materials that can form such phases are called mesogens. An ordinary fluid is called isotropic, i.e., its properties are independent of direction. A liquid crystal is orientated, or likewise an anisotropic liquid. This means that the molecules are oriented preferably in a certain direction. Such an anisotropic fluid is a nematic liquid crystal. A liquid crystal more similar to a solid is a smectic phase. Here the molecules are arranged in layers, but within the layers the molecules have no fixed positions. Polymers. Liquid crystalline polymers exhibit a number of improved properties in comparison with traditional plastics, in particular increased elastic moduli at high temperatures, reduced coefficients of thermal expansion, increased decomposition onset temperatures, and reduced solvent absorption. Suitable epoxide monomers are based on biphenyl moieties.44 Monomers for liquid crystalline epoxide resins are shown in Figure 3.3. It is believed that micro-Brownian motion in the polymer chain is increasingly suppressed as the mesogen concentration increases. This effect causes an
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Reactive Polymers Fundamentals and Applications
O CH2 C
CH2
O
O CH2
H
O CH2
H H3C
O CH2 C
C
CH2
O
CH3 O CH2
H
C
O CH2
H H3C
O O CH2 C (CH2)n CH2 H
CH3
O CH2 (CH2)n C H
O CH2
Figure 3.3: Monomers for Liquid Crystalline Epoxide Resins
increase in the thermal decomposition onset temperatures, a decrease of the coefficient of thermal expansion, and a decrease in water absorption. When the diglycidyl ether of bisphenol A is cured with sulfanilamide, a crosslinked network with liquid crystalline properties is obtained.45 Sulfanilamide has two different amine functions of unequal reactivity. This causes the formation of a smectic phase when it is used as a curing agent. Polarized optical microscopy indicates that the epoxy monomer does not show a liquid cristalline (LC) phase. Also a mixture of sulfanilamide and diglycidyl ether of bisphenol A does not show LC properties. An isotropic liquid is formed above the melting point. However, when the reaction between epoxy and amine proceeds, an LC texture is developed, which is locked in the crosslinked network by the nematic arrangement.
3.2.4 Manufacture 3.2.4.1
Epoxides
Epoxides can be manufactured by the epoxidation reaction, in particular 1. By direct oxidation, 2. Via peroxyacids, 3. In-situ epoxidation,
Epoxy Resins
147
4. By hypochlorite reaction, and 5. By reaction with fluoro complexes. Direct Oxidation. Olefins can epoxidized by oxidizing them in the vapor phase in the presence of a silver catalyst. The catalyst is activated by adding small amounts of dichloroethane to the reaction mixture. The direct oxidation with oxygen is less important for the synthesis of epoxies used for epoxy resins, in favor of peroxyacids. Certain Schiff bases that are attached on polymers allow the direct oxidation of olefins. A polymer bound Schiff base ligand is prepared from poly(styrene) bound salicylaldehyde and glutamic acid. With complexes of these catalysts, cyclohexene, 1-octene, 1-decene, 1-dodecene and 1-tetradecene can be oxidized by molecular oxygen.46 Peroxyacids. Also, organic peroxides can serve as an oxygen source. Unsaturated fatty acids and their esters are epoxidized with peroxyacetic acid. Originally peroxybenzoic acid was used, which is highly selective. However, this reagent is comparatively expensive. Several other peroxyacids have been investigated; they are in general less efficient. The reaction of olefins with peroxyacids is a single-step reaction. Hydrogen peroxide itself is a rather poor epoxidation oxidant, however, it is used to generate the peroxyacids that are much more active. The peroxyacids are prepared by reacting hydrogen peroxide with the corresponding acid. The reaction is an equilibrium reaction. Highly concentrated peroxyacids can be obtained by adding anhydrides, or removing the water by azeotropic distillation. Another route to prepare peroxyacids starts from the anhydride and sodium peroxide, in presence of an acid as catalyst. There should not be even traces of heavy metals present that cause a loss in activity of the hydrogen peroxide. For technical synthesis, peroxyacetic acid is used most frequently, because it has a high equivalent weight, a high efficiency for epoxidation, and a sufficient stability. In-Situ Epoxidation. The peroxyacids can be regenerated during the epoxidation reaction with hydrogen peroxide. In this way all the hazards in preparation and handling of the peroxyacids as such are avoided. The reaction is heterogeneous and the peroxyacid has to be regenerated under conditions that would result in ring opening of the epoxide. Therefore, only
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Reactive Polymers Fundamentals and Applications
fast epoxidation reactions can be conducted utilizing the in-situ technique. For this reason, the most reactive peroxyacids are also selected. These are in particular the 3-nitroperoxybenzoic acid and 4-nitroperoxybenzoic acid. Less reactive olefins must still be epoxidized with the peroxyacids formed in a previous step. The ring opening of the epoxide with the acid formed from the peroxyacid can be minimized, allowing the phases utmost separation. This means there should be only small agitation. On the other hand, with certain solvent combinations the epoxide and the acid are mutually insoluble. Hypochlorite. Partially fluorinated epoxides can be prepared by the oxidation of the corresponding olefins by NaOCl or NaOBr with phase transfer catalysts, e.g., methyltricaprylylammonium chloride.47 For example, hexafluoroisobutene reacts with the solution of sodium hypochlorite in water at 0 to 10°C giving the corresponding epoxide in a yield of 65 to 70%. Fluoro Complex. By reacting diluted fluorine with aqueous acetonitrile, a complex HOF × CH3 CN is formed. This complex is a very efficient oxygen transfer agent. It was shown to be useful to obtain various types of epoxides that are otherwise difficult to synthesize. The products can be obtained in a single-step reaction with high yield.48 3.2.4.2
Glycidyl Ethers
In the simplest case a glycidyl ether for an epoxy resin is prepared by the reaction of bisphenol A (and epichlorohydrin), as pointed out in Figure 3.4. In the first step DGEBA is formed, however, the condensation can proceed further. The reaction proceeds in two steps. First the epoxide ring is opened and then the ring is formed again, as shown in Figure 3.5. Hydrogen chloride is evolved during the condensation and captured with caustic soda. The ring opening occurs such that the primary carbon atom is attacked and thus a 1,2-chlorohydrin (ΦCH2 CH(OH)CH2 Cl) is formed, as shown in Figure 3.5. However in a side reaction the secondary carbon atom is also attacked and thus a 1,3-chlorohydrin (HOCH2 CH(Φ)CH2 Cl) is formed. If the degree of dehydrochloration is not complete, then 1,2-chlorohydrin end groups also may be present.
Epoxy Resins
O CH2
CH3 CH CH2
Cl + HO
OH
C CH3
CH3
O CH CH2 O
CH2
C
O
CH3
CH2 CH OH
O CH2
CH CH2 O
CH3
CH2
C
O
CH3
Figure 3.4: Synthesis of an Epoxide Oligomer
O OH + CH2
CH CH2
Cl
OH O
CH2
CH CH2
Cl
-HCl O O
CH2
CH CH2
Figure 3.5: Formation of the Glycidyl Ether
n
149
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Reactive Polymers Fundamentals and Applications
Concerning the nomenclature, the situation is confusing. There are many synonyms for the glycidyl ethers. The Chemical Abstracts name for diglycidyl ether of bisphenol A (DGEBA) is 2,2′ -[(1-Methylethylidene)bis(4,1-phenyleneoxymethylene)]bis(oxirane), and there are some 12 other synonyms of chemical names in use, besides the trade names. We focus back to the main reaction. The newly formed epoxide groups from the second step of the reaction may again undergo a reaction with the phenolic group, and in the case of a bifunctional phenol, such as bisphenol A, the molecule grows. The degree of oligomerization (n − 1 in Figure 3.4) can vary from 1 to approximately 25. The oligomer is liquid at room temperature when n is smaller than one and becomes solid when n is larger than two. The degree of polymerization that can be achieved depends on the ratio of bisphenol A to epichlorohydrin. If epichlorohydrin is in excess, then the diglycidyl ether will be the main product. Impurities such as water can substantially decrease the degree of polymerization by side reactions. Water reacts with epichlorohydrin to form a glycol.
3.2.4.3
Fluorinated Epoxides
The incorporation of fluorine enhances the chemical and the thermal stability, the weathering resistance. Further the surface tension is lowered and thus the hydrophobicity is enhanced. Fluorinated epoxy monomers have been synthesized from fluorinated diols, such as 2,2,3,3,4,4,5,5-octafluoro-hexane-1,6-diol or 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluoro-decane-1,10-diol by etherification with allyl chloride and subsequent oxidation of the allyl group.21 In UV curing, the monomers showed a higher reactivity than hexanediol diglycidyl ether. The adduct of 2-chlorobenzotrifluoride and glycerol diglycidyl ether (DGEBTF) has been co-reacted with DGEBA using 4,4′ -diaminodiphenylmethane as hardener.49 The introduction of the trifluoromethyl group into the chain of the epoxy resin results in an improvement of the dielectric and mechanical properties. Further the glass transition temperature is lowered. The glass transition temperature of a pure DGEBA resin is 193°C whereas the glass transition temperature of the DGEBTF resin is 105 °C. This indicates that the introduction of fluorine enhances the mobility of the network.
Epoxy Resins
151
Table 3.4: Toughening Agents for Epoxy Resins Compound Class
Reference
Poly(ethylene) phthalates Poly(ethylene phthalate-co-ethylene terephthalate) Hyperbranched aliphatic polyester Hyperbranched block copolyethers Epoxidized soyabean oil Copolymers of 2-ethylhexyl acrylate and acrylic acid Methacrylic microgels Terpolymers of N-phenylmaleimide, styrene and p-hydroxystyrene Triblock copolymer poly(styrene-b-ethylene-co-buteneb-styrene) Poly(benzimidazole) Poly(phenylene oxide Silicon-modified polyurethane oligomers Poly(dimethylsiloxane) polymers Epoxy-aminopropyltriethoxysilane Poly(ether ether ketones) Polyetherimides Carboxylated polymers Phenolic hydroxy-terminated polysulfones Liquid rubbers Liquid rubbers carboxyl-terminated with poly(2-ethylhexyl acrylate) Poly(vinyl acetate) Rubbery epoxy based particles Glass beads
50 51 52, 53 54 14, 55, 56 57 58 59 60 61 62 63 64 65 66–68 69 70, 71 72 73–75 76 77 78, 79
3.3 SPECIAL ADDITIVES 3.3.1 Toughening Agents Highly crosslinked epoxy resins are brittle. For various applications they need to be toughened. Toughening agents are summarized in Table 3.4. Extensive literature on toughening of polymers is available.80–83 The toughening mechanisms of elastomer-modified epoxy systems are different from flexibilized epoxy systems. • Flexibilized epoxy systems reduce mechanical damage through lowering modulus or plasticization; this allows stress to be relieved
152
Reactive Polymers Fundamentals and Applications through distortion of the material.84 • Elastomer-toughened epoxy systems in general maintain a large percentage of the modulus and temperature resistance of the unmodified resin system. Stress is absorbed by cavitation of the elastomer particles and shear banding in the cavitated zone. Elastomer-toughened epoxy systems can tolerate a certain degree of damage by preventing growth of a crack. In this way the damaged region remains local.85
When using thermoplastic-modified thermosets, compromises between toughness and thermal stability associated with the rubber toughening of thermosets can be avoided. Another advantage of using the reaction induced phase separation procedure is that by the adequate selection of cure cycles and initial formulations, a variety of morphologies can be generated. However, the fracture toughness is significantly improved with a nonreactive thermoplastic, only, when bicontinuous or inverted phase structures are formed. On the other hand, when the phase separation produces thermoplastic-rich particles that are dispersed in a continuous thermosetrich matrix, little or no improvement of the fracture properties is obtained. This is mainly due to the poor adhesion between the phases.60 Basically, functionalized thermoplastics are capable of forming a chemical linkage between the phases. This interphase bonding could improve the adhesion properties. However, the reactivity of the modifier can also complicate the behavior and the control of the phase separation process. 3.3.1.1
Polyvinylic Compounds
Many polyvinylic compounds increase the flexibility and are used as toughening agents. Poly(styrene). Blends of poly(styrene) (PS) with an epoxy monomer (DGEBA) and a tertiary amine, benzyldimethylamine (BDMA), are initially miscible at 120°C. However, at very low conversions a phase separation occurs. Here, at the cloud point, a sharp decrease of the light transmittance is observed. There is a significant difference between the refractive indices of poly(styrene) and the DGEBA/BDMA solution. The refractive index of the epoxy network increases in the course of polymerization. Due
Epoxy Resins
153
to the continuous increase of the refractive index of the epoxy phase during curing, finally the refractive indices of both phases match, so that the final materials at complete conversion appear transparent.86 Copolymers of Styrene and Acrylonitrile. In an epoxy system containing tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and a 4,4′ -diaminodiphenylsulfone (DDS) hardener, blends with poly(styrene-co-acrylonitrile) (SAN) up to 40 phr show complete miscibility over the entire range.87 The glass transition temperature and the curing characteristics can be modelled with various theories.88 In several systems autocatalytic curing kinetics is observed.89–93 Copolymers of Phenylmaleimide, Benzyl methacrylate, and Styrene. The vinylic compounds can be polymerized in-situ during the curing of the epoxy system.94 A suitable monomer system consists of three monomers: phenylmaleimide, benzyl methacrylate, and styrene. An advantage is that by the admixing of the monomers the viscosity of the uncured resins drops significantly. Graft Polymers of Ethylene/vinyl acetate to Methyl methacrylate. A graft polymer synthesized by grafting ethylene/vinyl acetate (EVA) onto poly(methyl methacrylate) thus resulting in a poly(ethylene-co-vinyl acetate)graft-poly(methyl methacrylate) exhibits a special performance. The EVA moieties are initially immiscible in the uncured epoxide formulation. The PMMA moieties are initially miscible, however they separate during curing. Therefore, EVA-g-PMMA as modifier yields stable dispersions of EVA blocks, favored by the initial solubility of PMMA blocks. So the PMMA acts initially as a compatibilizer for the epoxy moieties.95 Blends of Poly(methyl methacrylate) and Poly(ethylene oxide). Blends of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) form a single phase in the melt. In solid mixtures of these polymers, phase separation is often observed. In blends of an epoxy resin with PMMA, PEO acts as a compatibilizer. The morphology of the resulting polymer mixture may be changed dramatically by only small amounts of PEO. The stiffness is controlled by the corresponding matrix of the ternary mixture, but both strength and fracture toughness are a function of the resulting morphology.96
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Reactive Polymers Fundamentals and Applications
Poly(benzimidazole). The incorporation of poly(benzimidazole) into a difunctional epoxy resin matrix enhances both the glass transition temperature of the matrix and its toughness.61 Multilayer Particles. Multilayer particles of PMMA can be manufactured formed by emulsion polymerization. They consist of alternate glassy and rubbery layers. The outer layer bears glycidyl groups to allow a chemical bonding of the particles onto the cured resin. This type of toughening particles is more effective than acrylic toughening particles or a liquid carboxyl-terminated butadiene-acrylonitrile rubber.97 3.3.1.2
Polycondensates
Aromatic polyesters that are prepared from aromatic dicarboxylic acids and 1,2-ethanediol are improving the toughness of bisphenol A diglycidyl ether epoxy resins. In particular, phthalic anhydride, isophthalic acid, terephthalic acid and 2,6-naphthalene dicarboxylic acid, and mixtures of these compounds are used. The aromatic polyesters are soluble in the epoxy resin without solvents and are effective modifiers for toughening the epoxy resins.50 The inclusion of 20% poly(ethylene phthalate) increases the fracture toughness of a cured resin by 130% with no loss of mechanical and thermal properties.51 Instead of 1,2-ethanediol, 1,4-cyclohexanedimethanol can be used to obtain poly(1,4-cyclohexylenedimethylene phthalate).98 Other flexibility enhancers are polyamide, polyetherimides,66, 67 carboxylated polymers,69 phenolic hydroxy-terminated polysulfones,70 and fatty diamines. Polyetherimide. In blends of an epoxy system of diglycidyl ether of bisphenol A and nadic methyl anhydride, a phase separation occurs by the addition of polyetherimide in the course of curing. The phase separation is not observed without polyetherimide. By increasing the amount of polyetherimide in the blends, the final conversion is decreased. This indicates that polyetherimide hinders the cure reaction between the epoxy and the curing agent.99 Homogeneous structures are formed at low polyetherimide concentration (5 phr).100 Poly(ether ether ketone). Poly(ether ether ketone) (PEEK) is a tough, semi-crystalline high performance thermoplastic polymer with good ther-
Epoxy Resins
155
O O
O
C
C O C O
H3C
O
PEEK-C CH3 C
CH3
O
O
C
PEEK-T
Figure 3.6: Poly(ether ether ketone)s
mal and mechanical properties. Because of its semi-crystalline nature, it is difficult to blend this material with epoxy resins. Phenolphthalein poly(ether ether ketone) (PEEK-C) is miscible with TGDDM. Several methods, including dynamic mechanical analysis, Fourier-transform infrared spectroscopy, and scanning electron microscopy indicate that the cured blends are homogeneous. With increasing PEEK-C content, the tensile properties of the blends decrease slightly. The fracture toughness factor also decreases. This happens presumably due to the reduced crosslink density of the epoxy network. Inspection of the fracture surfaces of fracture toughness test specimens by scanning electron microscopy shows the brittle nature of the fracture for the pure epoxy resins and its blends with PEEK-C.101 A lower curing temperature favored the homogeneous morphology in amine cured DGEBA+PEEK-C blends.102 In general, the processing of blends with PEEK should be easier, by using PEEK with terminal functional groups and bulky pendant groups. However, poly(ether ether ketone) based on tertiary butyl hydroquinone (PEEK-T) showed a decreasing rate of reaction with increasing PEEK-T content. The rate of reaction also decreased with the isothermal curing temperature. This can be explained by the phase separation. As the curing reaction proceeds, the thermoplastic component undergoes a phase sepa-
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Reactive Polymers Fundamentals and Applications
ration. The separated thermoplastic could retard the curing reaction. The dispersed particle size increases with the lowering of curing temperature and with an increase in the thermoplastic material added.65 Poly(ether ether ketone)s are shown in Figure 3.6. Chain-extended Ureas. The synthesis of chain-extended ureas runs via a two-stage process. In the first stage, a prepolymer with isocyanate end groups is synthesized by the reaction of poly(propylene) glycol and toluene diisocyanate. In the second step, the prepolymer is end-capped with dimethylamine or imidazole, to result in an amine-terminated chain-extended urea (ATU) or an imidazole-terminated chain-extended urea, respectively, with flexible spacers.103 This type of toughening agent accelerates the curing of the epoxide groups significantly because of the amino functions in the molecule. 3.3.1.3
Liquid rubbers
The addition of elastomers to epoxy adhesives can improve peel strength, fracture resistance, adhesion to oily surfaces and ductility. Liquid rubbers, like carboxyl, amine, or epoxy-terminated butadiene/acrylonitrile rubbers, are used as toughening agents.72, 104 Liquid rubber modifiers are initially miscible with the epoxy resin. However, in the course of curing a phase separation takes place. Carboxy-terminated butadiene/acrylonitrile copolymers (CTBN) are particularly suitable because of their miscibility in many epoxy resins. The carboxyl group can react easily with an epoxy group. If a CTBN is not prereacted with an epoxy resin, the carboxylic acid groups can react during curing. Solid acrylonitrile-butadiene rubbers (NBR), in particular with high content of acrylonitrile are also suitable tougheners.105 A high content of acrylonitrile in the rubber imparts better compatibility between NBR and the epoxy resin. 3.3.1.4
Silicone Elastomers
CTBN and amine-terminated butadiene-acrylonitrile elastomers (ATBN) lose the desired mechanical properties in the high temperature region and in the low temperature region. Silicone rubbers are superior in this aspect. However, silicone rubbers are completely immiscible with epoxy resins
Epoxy Resins
157
and cannot be used for this reason. The addition of a silicone grafted poly(methyl methacrylate) is effective to stabilize the interface of the silicone rubber and the epoxy resin and helps to disperse the silicone rubber in the epoxide matrix in this way. The molecular weight of the silicone segment strongly affects the effectiveness of the compatibilizer. With increasing particle diameter of the silicone the fracture toughness decreases and drops eventually below the unmodified resin.106 For a carboxyl-terminated dimethyl siloxane oligomer used as a rubber modifier, aramid/silicone block copolymers were used as compatibilizers.107 The aramid-type blocks have phenolic groups on the aromatic rings. These groups can react with the epoxy resin to cause the compatibilization. 3.3.1.5
Rubbery Epoxy Compounds
Instead of liquid rubber, rubbery epoxy based particles obtained from an aliphatic epoxy resin can be blended with another epoxy resin to act as toughening agents themselves.77 One of the limitations of epoxy-CTBN adducts is their high viscosity; however, there are also low-viscosity types available. 3.3.1.6
Phase Separation
During curing of polymer resin blends, a phase separation occurs. The phase separation can be characterized by 1. 2. 3. 4. 5.
Small angle X-ray scattering, Light transmission, Light scattering, Transmission electron microscopy, and Atomic force microscopy.
The viscosity at the cloud point can have a strong effect on the final morphology and mechanical properties of the resin. The phase separation mechanisms are dependent on the initial modifier concentration and on the ratio of the phase separation rate to the curing rate. The curing temperature has a strong effect on the extent of phase separation. Annealing allows the phase separation process to proceed further.67 The extent of phase separation depends on the cure cycle, as shown in blends of a standard epoxy resin and poly(methyl methacrylate). The
158
Reactive Polymers Fundamentals and Applications
extent of phase separation can be diminished or suppressed by longer precuring times at lower temperatures, before the main curing is started.108 In addition, the phase separation can be controlled by the choice of the curing agents. In the case of poly(methyl methacrylate) as modifier, in an epoxy system, based on DGEBA some hardeners effect a phase separation before gelation and others do not. For example, 4,4′ -diaminodiphenylsulfone (DDS) and 4,4′ -methylenedianiline (MDA) result in a phase separation, but for 4,4′ -methylene bis(3-chloro-2,6-diethylaniline) (MCDEA) no phase separation is observed.109 3.3.1.7
Preformed Particles
Preformed particles do not require phase separation and remain in that shape in which they were added to the neat resin or composite. Therefore, these particles may be synthesized prior to the resin formulation and then added to the thermosetting resin or formed in-situ, i.e., during the formulation of the resin, before the resin is cured.110 Prereacted urethane microspheres can be formed by dynamic vulcanization method in liquid diglycidyl ether of bisphenol A. The prereacted particles are then added to an uncured epoxy resin system and cured. The mechanical and adhesion properties do not depend on any curing condition of epoxy resin because the particles are stable, in contrast to a process where a phase separation occurs during curing.111 3.3.1.8
Inorganic Particles
In contrary to rubber, the toughening of inorganic particles is rather modest. However, the toughening by inorganic particles has an advantage insofar as it can also improve the modulus. Rubber toughens such that the increase in toughness is accompanied at the expense of a decrease in the modulus. The toughening of inorganic particles is explained by the crack front bowing mechanism.112–114 A crack front increases its length by changing its shape when it interacts with two or more inhomogeneities in a brittle material. The inorganic particles inside the polymer matrix can resist a crack propagation. When a crack propagates in a rigid particle filled composite, the rigid particles try to resist. Because of this resistance, the primary crack front has to change its direction between the rigid particles (bowing), thus forming
Epoxy Resins
159
a secondary crack front. The bowed secondary crack front now has more elastic energy stored than the straight unbowed crack front. A crack front starts to bow out between particles, when it meets the particles. Microcracking with debonding has been proposed as one of the toughening mechanisms of glass bead-filled epoxies. Three types of micro-mechanical deformations can be distinguished:78 1. Step formation 2. Debonding of glass beads and diffuse matrix shear yielding 3. Micro-shear banding Among the micro-mechanical deformations, micro-shear banding is considered the major toughening mechanism for glass bead-filled epoxies. Step formation and combined debonding and diffuse matrix yielding are secondary toughening mechanisms.79
3.3.2 Antiplasticizers Antiplasticizers are additives for increasing the strength and modulus of the respective material. They act via strong interactions with the epoxide matrix. Epoxides with antiplasticizers characteristically115 1. Have a sufficiently high value of the glass transition temperature as needed for the applications, 2. Exhibit a higher modulus and higher toughness around room temperature, 3. Exhibit a lower water uptake at equilibrium. Antiplasticizers for epoxide resins are shown in Table 3.7. The addition of the reaction product of 4-hydroxyacetanilide and 1,2-epoxy-3-phenoxypropane (EPPHAA) to an epoxide resin increases the tensile strength and the shear modulus of the cured system.116 The mechanism of antiplasticization can be formulated in terms of hindrance of the short-scale cooperative motions in the glassy state as a dynamic coupling between the epoxy polymer and the antiplasticizer molecule.117 In systems where the antiplasticizers have a poor affinity to the resin, a phase separation during curing occurs. The mobility of the constituting groups can be characterized by nuclear magnetic resonance techniques.118
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Reactive Polymers Fundamentals and Applications
OH
O
O CH2 CH CH2 O
NH C CH3
EPPHAA OH O CH2 CH CH2 O
CH3
AM CH3 O O CH2 CH CH2 O
CH3
AO
Figure 3.7: Antiplasticizers for Epoxide Resins
3.3.3 Lubricants In automotive, aviation and the related industries, there is a tendency to use metallic materials with polymeric materials. For many parts in such applications, good tribological properties are required.119 Fluorinated polymers are known as low friction materials. This property arises due to their low surface energies. Fluorinated poly(aryl ether ketone) (12F-PEK) can be added to epoxy resins to improve the tribological properties. At low concentrations of 12F-PEK, homogeneous systems are obtained after curing. Above 10% 12F-PEK, a phase separation is observed. At still higher concentrations, an inversion of the morphology is observed. With fluoropolymer concentrations of 10% 12F-PEK, a friction reduction of 30% can be obtained.120
3.3.4 Adhesion Improvers Epoxy polyurethane hybrid resins are used in high strength adhesives. Elastomer-modified resins are used for adhesive formulations that cure under water.
Epoxy Resins
161
Table 3.5: Reinforcing Materials for Epoxides Material
Remark/Reference
Glass fibers Hollow glass fibers Carbon fibers Carbon nanotubes Graphite Aluminum Boron Aluminum borate whiskers Paper Poly(ethylene) fibers Polyaramid Fabric Cotton Flax
121–123 124 125–127 128–131 132–138 139, 140 141
Low density and extremely high strength 142
3.3.5 Conductivity Modifiers To modify the thermal and electrical properties, thermally and electrically conductive materials are added.
3.3.6 Reinforcing Materials 3.3.6.1
Composites and Laminates
Composites and laminates are made by reinforcing the polymers with continuous fibers. About 1/4 of the epoxy resins are reinforced materials. Reinforcing materials are shown in Table 3.5. Traditional composite structures are usually made of glass, carbon, or aramid fibers. The advances in the development of natural fibers in genetic engineering and in composite science offer significant opportunities for improved materials from renewable resources with enhanced support for sustainable applications. Biodegradable composites from biofibers and biodegradable polymers will serve to solve environmental problems.143 Often the surface of the fiber is chemically modified to increase the adhesion properties to the resin matrix. For example, glass fibers are coated with a silane coupling agent. The interfacial bonding between carbon fiber and epoxy resin can be improved by modification with poly(pyrrole). Poly(pyrrole) (PPy) can be deposited on carbon fibers via the oxidation-polymerization of pyrrole (Py) with ferric ions.144
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Reactive Polymers Fundamentals and Applications
Laminates are used for insulations. Impregnated sheets of woven glass, paper, and polyaramid fabric or cotton are laminated in large presses. These sheets are used for printed circuit boards in the electronics industry. 3.3.6.2
Nanocomposites
Polymer nanocomposites, in particular polymer-layered silicate nanocomposites, are a radical alternative to macroscopically filled polymers. The preparation of epoxy resin-based nanocomposites was first described by Messersmith and Giannelis.145 Extensive work on epoxy based nanocomposites has been done and is reviewed among other polymers in the literature.146, 147 Organoclays. Organoclays are used as precursors for nanocomposites in many polymer systems. Usually montmorillonite is used for organoclays. Montmorillonite belongs to the 2:1 layered silicates. Its crystal structure consists of layers of two silica and a layer of either aluminum hydroxide or magnesium hydroxide. Water and other polar molecules can enter between the unit layers because of the comparatively weak forces between the layers. Substitution of the ions originally in the layers by such ions with different charges generates charged interlayers. The stacked array of clay sheets separated by a regular spacing is addressed as gallery. For true nanocomposites, the clay nanolayers must be uniformly dispersed in the polymer matrix, to avoid larger aggregations. Small aggregations are still addressed as nanocomposites, as intercalated nanocomposites, ordered exfoliated nanocomposites, and disordered exfoliated nanocomposites.148 Originally, intercalation was the insertion of an extra day into a calendar year. Exfoliation refers to the peeling of rocky materials into sheets due to weathering. Clay nanolayers in elastomeric epoxy matrices dramatically improve both the toughness and the tensile properties.145, 149 The dimensional stability, the thermal stability and the chemical resistance can also be improved with clay nanolayers.150 Exfoliated clays are formed when the clay layers are well separated from one another and individually dispersed in the continuous polymer matrix. Since exfoliated nanocomposites exhibit a higher phase homogeneity than the intercalated clays, exfoliated clays are more effective in improving the properties of the nanocomposites.
Epoxy Resins
163
Successful nanocomposite synthesis depends not only on the cure kinetics of the epoxy system but also on the rate of diffusion of the curing agent into the galleries, because it affects the intragallery cure kinetics. The nature of the curing agent influences these two phenomena substantially and therefore the resulting structure of the nanocomposite. The curing temperature controls the balance between the extragallery reaction rate of the epoxy system and the diffusion rate of the curing agent into the galleries.151 It was found that the activity energy decreases with the addition of organic montmorillonite.152 Hexahydrophthalic anhydride (HHPA) is usually used for hot curing of epoxy resins. With an alkoxysilane, it also acts as a condensation agent.153 Hot curing of montmorillonite-layered silicates has been described with methyltetrahydrophthalic anhydride.154 An exfoliated epoxy-clay nanocomposite structure can be synthesized by loading the clay gallery with hydrophobic onium ions and then allowing diffusion in the epoxide and a curing agent. The degree of exfoliation increases with decreasing curing agent.155 Clays exert catalytic effects on the curing of epoxy resins.156 An organically modified montmorillonite, prepared by a cation exchange reaction between the sodium cation in montmorillonite and dimethyl benzyl hydrogenated tallow ammonium chloride is suitable for high degrees of filling for epoxy resins.157 Nanocomposites exhibit a significant increase in thermal stability in comparison to the original epoxy resin.158 Quaternary ammonium ions both catalyze the epoxy curing reactions and plasticize the epoxy material. This causes a large reduction in glass transition temperature and lowers the storage modulus. Plasticization is small for aromatic epoxy resins, but large for aliphatic resins. Therefore, aromatic epoxy-clay systems may result in a complete exfoliation of the clay galleries, whereas mixtures of aliphatic and aromatic epoxy may produce intercalated systems.159 Poly(oxypropylene)amine intercalated montmorillonite is highly organophilic and compatible with epoxy materials.160 Star branched functionalized poly(propylene oxide-block-ethylene oxide) was used with an organophilic modified synthetic fluorohectorite as compatibilizer for nanocomposites. The polarity of the polyol could be tailored by the type of functionalization. A mixture of two epoxy resins, tetraglycidyl 4,4′ -diaminodiphenylmethane and bisphenol A diglycidyl ether, cured with 4,4′ -diaminodiphenylsulfone, was used as matrix material.161 The hybrid nanocomposites were composed of intercalated clay particles
164
Reactive Polymers Fundamentals and Applications Table 3.6: Interpenetrating Polymer Networks
Epoxide
Further Component
Diglycidyl ether of bisphenol A Aliphatic epoxide resin
Unsaturated polyesters
162
Vinylester resin (Bisphenol A glycidylmethacrylate adduct in styrene with layered silicate nanoparticles) Bisphenol A diacrylate
10
Cyanate ester Silica Hexakis(methoxymethyl)melamine 2,2′ -Diallyl bisphenol A (DBA) Polyaniline
164
Diglycidyl ether of bisphenol A Epoxide bismaleimide resin Epoxide-amine network Diglycidyl ether of bisphenol A Novolak epoxy resin Epoxy resin
Reference
163
165 166 167 168
as well as separated PPO spheres in the epoxy matrix. Phenolic alkylimidazolineamides were also used to exchange the interlayer sodium cations of the layered silicates.169 Electric capacitors based on epoxy clay nanocomposites can be integrated into electronic devices.170
3.3.7 Interpenetrating Polymer Networks Interpenetrating polymer networks are ideally compositions of two or more chemically distinct polymer networks held together exclusively by their permanent mutual entanglements.171 In practice, interactions of both networks beyond entanglement may occur, for instance, intercrosslinking. In a simultaneous interpenetrating polymer network, the two network components are polymerized concomitantly. In a sequential interpenetrating polymer network, the first network is formed and then swollen with a second crosslinking system, which is subsequently polymerized. Interpenetrating polymer networks are known to remarkably suppress creep phenomena in polymers. The motion of the segments in interpenetrating polymer networks is diminished by the entanglement between the networks. Interpenetrating polymer networks including epoxide resins as one of the components are summarized in Table 3.6.
Epoxy Resins 3.3.7.1
165
Curing Kinetics
If a thermosetting system is cured at a temperature below its maximally attainable glass transition temperature, vitrification occurs during cure. The vitrification slows down the reaction. The reaction may freeze before reaching full conversion. In contrast, in an interpenetrating network, if one component (I) reacts more slowly than the other component (II), the former component (I) may act as a plasticizer of the polymeric component (II). This allows a faster reaction of the second component (II) and a more thorough cure without vitrification.172 In the simultaneous curing of a vinylester resin (VER) and an epoxy resin a reduction in reaction rate due to the dilution of each reacting system by the other resin components is observed. The radical polymerization of an acrylate monomer is hardly affected by the oxygen inhibition effect, while the cationic polymerization of an epoxy monomer is enhanced by the atmosphere humidity.173 The decomposition of peroxides is known to be accelerated by amines. In fact, if for the radical curing of the vinylester component peroxides are used instead of azo compounds, a strong redox interaction between the peroxide and the amine used for curing the epoxide component is observed. In such systems the peroxide decomposes too quickly to develop its full power for curing the vinylester system. Further, there is an interaction between the vinyl groups of the vinylester system and the amine via a Michael addition. The curing performance of the epoxide resin is less affected by the radical initiator.174 3.3.7.2
Unsaturated Polyesters
In mixtures of epoxy based on diglycidyl ether of bisphenol A and unsaturated polyesters, the curing monitored with differential scanning calorimetry indicated a higher rate constant than the pure epoxide resin. It is believed that the hydroxyl end group of the unsaturated polyester in the blend provides a favorable catalytic environment for the epoxide curing.162 The interpretation of the viscosity development suggests that an interlock between the two growing networks exists that causes a retarded increase of the viscosity.175 The introduction of unsaturated polyester into epoxy resin improves toughness but reduces the glass transition temperature.176
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Reactive Polymers Fundamentals and Applications
Functional Peroxides. Peroxy ester oligomers can be obtained by condensation of anhydrides with poly(ethylene glycol)s and tert-butyl hydroperoxide. Suitable anhydrides are pyromellitic dianhydride and the tetrachloroanhydride of pyromellitic anhydride. The resulting esters contain carboxylic groups and peroxy groups. These compounds can be used as curing agents for unsaturated polyesters as such and for hybrid resins consisting of an epoxy resin and an unsaturated polyester resin.177 3.3.7.3
Acrylics
For interpenetrating polymer networks consisting of diglycidyl ether of bisphenol A (DGEBA) and bisphenol A diacrylate as radically polymerizable component, 4,4′ -methylenedianiline and dibenzoyl peroxide are suitable curing agents. The curing can be achieved between 65°C and 80°C. The kinetics of curing of the epoxide takes place as a combination of an uncatalyzed bimolecular reaction and a catalyzed termolecular reaction. The kinetics of curing of the acrylate runs according to a first-order reaction.163 In the mixture, the rate constants are lower than in the separate systems. Also the activation energies in the mixtures are higher. It is believed that chain entanglements between the two networks cause a steric hindrance for the curing process. The vitrification restrains the chain mobility that is reflected as a decrease of the rate constants. The incorporation of the methacryloyl moiety in an epoxide resin improves the weathering stability and the photostability of the system.178, 179 3.3.7.4
Urethane-modified Bismaleimide
Urethane-modified bismaleimide (UBMI) can be introduced and partially grafted to the epoxy oligomers by polyurethane grafting agents. Afterwards, a simultaneous bulk polymerization technique can be used to prepare interpenetrating networks.180 The tensile strength increases to a maximum value with increasing UBMI content, then decreases with further increasing UBMI content. If the polyurethane grafting agent contains poly(oxypropylene) polyols the interpenetrating network shows a two-phase system, whereas in the case of poly(butylene adipate) a single phase system is observed. The better compatibility of poly(butylene adipate) base networks results in a higher impact strength. An intercrosslinked network of bismaleimide-modified polyurethane-epoxy systems was prepared from the bismaleimide having ester link-
Epoxy Resins
167
ages, polyurethane-modified epoxy, and cured in the presence of 4,4′ -diaminodiphenylmethane. Infrared spectral analysis was used to confirm the grafting of polyurethane into the epoxy skeleton. The prepared matrices were characterized by mechanical, thermal, and morphological studies. The changes of the properties depend on the relative amounts of the moieties used. The incorporation of polyurethane into the epoxy skeleton increases the mechanical strength and decreases the glass transition temperature, thermal stability, and heat distortion temperature. On the other hand, the incorporation of bismaleimide with ester linkages into a polyurethane-modified epoxy system increases the thermal stability, tensile and flexural properties, and decreases the impact strength, glass transition temperature, and heat distortion temperature.181 3.3.7.5
Electrically Conductive Networks
Electrically conductive polymers could find use in rechargeable batteries, conducting paints, conducting glues, electromagnetic shielding, antistatic formulations, sensors, electronic devices, light-emitting diodes, coatings, and others. Low concentrations of polyaniline can make the polymer electrically conductive when a co-continuous microstructure could be achieved. For the preparation of conductive polyaniline epoxy resin composites, a doped polyaniline is blended with the epoxy resin. Plasticizers are added to assist in the dispersion of the conductive polymer. The curing agent must be selected in order to avoid dedoping.168 The grafting onto the nitrogen of polyaniline was achieved by the ring-opening graft copolymerization of 1,2-epoxy-3-phenoxypropane. By the degree of grafting, the solubility, the optical and the electrochemical properties of the grafted polyaniline can be tailored.182
3.3.8 Organic and Inorganic Hybrids An organic-inorganic hybrid interpenetrating network has been synthesized from an epoxide-amine system and tetraethoxysilane (TEOS). The kinetics of the formation of the silica structure in the organic matrix, and its final structure and morphology, depend on the method of preparation of the interpenetrating network. In the sol gel process, hydrolysis and polymerization of TEOS are performed at room temperature in isopropyl alcohol. The hybrid network can be prepared by two procedures.
168
Reactive Polymers Fundamentals and Applications
In the one-step procedure, all reaction components are mixed simultaneously. In the two-step procedure, TEOS is hydrolyzed in the first step, then mixed with the organic epoxy components and polymerized under the formation of silica and epoxide networks. Large compact silica aggregates, with 100 to 300 nm diameter, are formed by the one-stage process of polymerization. In the two-stage process the partial hydrolysis of TEOS effects an acceleration of the gelation. This results in somewhat smaller silica structures. The most homogeneous hybrid morphology with the smallest silica domains of size 10 to 20 nm can be achieved in a sequential preparation of the interpenetrating network.165, 183 An increase in modulus by two orders of magnitude was achieved at a silica content below 10%.184 Phenolic novolak/silica and cresol novolak epoxy/silica hybrids can be prepared in a similar manner with TEOS.185
3.3.9 Flame Retardants Flame retardancy can be imparted by suitable monomers and curing agents. Flame retardants can be grouped into halogen-containing compounds, the most important being tetrabromobisphenol A, halogen free systems containing aluminum trihydrate with red phosphorus, and phosphate esters.186 Flame retardants that are used in epoxide resins are shown in Table 3.7. Triglycidyloxy phenyl silane cured with 4,4′ -diaminodiphenylmethane and others gives highly flame retardant polymers.18 Heating in air indicates that a silicon-containing carbon residue formed is superior in preventing oxidative burning. 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is synthesized by a multi step reaction from o-phenylphenol and phosphorus trichloride. From this compound, an adduct with p-benzoquinone, 2-(6-oxid6H-dibenz[c,e][1,2]oxa-phosphorin-6-yl)-1,4-benzenediol (ODOPB), can be obtained. ODOPB can be used as a reactive flame-retardant in o-cresol formaldehyde novolak epoxy resins for electronic applications.19, 187 A related compound, 2-(6-Oxid-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)methanol (ODOPM) can be used as flame-retardant hardener for o-cresol-formaldehyde novolak epoxy (CNE) resin in electronic applications.188 Some phosphorous-containing flame retardants are shown in Figure 3.9.
Epoxy Resins
169
Table 3.7: Flame Retardants for Epoxide Resins Compound
Remark/Reference
Tetrabromobisphenol A-based epoxies Triglycidyloxy phenyl silane 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) 10-(2,5-Dihydroxyphenyl)-9,10-dihydro-9-oxa10-phosphaphenanthrene-10-oxide (DHPDOPO) Bis(m-aminophenyl)methylphosphine oxide (BAMPO) Bismaleimide(3,3′ -bis(maleimidophenyl)) phenylphosphine oxide (BMPPPO) Bis(3-glycidyloxy)phenylphosphine oxide Bis(4-aminophenoxy)phenylphosphine oxide (BAPP) Tris(2-hydroxyphenyl)phosphine oxides Bis(3-diethylphosphono-4-hydroxyphenyl)sulfide (DPHS) Benzoguanamine-modified phenol biphenylene components Melamine phosphate 2,4,6-Tris(2,4,6-tribromophenoxy)-1,3,5-triazine 2,2′ -[(1-Methylethylidene)bis[(2,6-dibromo-4,1-phenylene) oxy]]bis[4,6-bis[(2,4,6-tribromophenyl)oxy]]-1,3,5-triazine Carbon black a c.f. Figure 3.8
18 19, 189 190 191 192 193 194 195, 196 197 198 199 200 a 200 a 201
170
Reactive Polymers Fundamentals and Applications
Br
Br
Br
O Br
N N
Br
O Br
N O
Br
Br
Br Br
Br
Br
O Br
N N
Br
O Br
N O
Br
Br
H3C C CH3
Br
Br O N
Br O Br
Br
N N
Br O Br
Br
Figure 3.8: Top: 2,4,6-Tris(2,4,6-tribromophenoxy)-1,3,5-triazine, Bottom: 2,2′ -[(1-Methylethylidene)bis[(2,6-dibromo-4,1-phenylene)oxy]] bis[4,6-bis[(2,4,6-tribromophenyl)oxy]-1,3,5-triazine200
Epoxy Resins
O P O CH2 OH ODOPM
171
O P O HO
OH
ODOPB
Figure 3.9: 2-(6-Oxid-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)methanol (ODOPM), 2-(6-Oxid-6H-dibenz[c,e][1,2]oxa-phosphorin-6-yl)-1,4-benzenediol (ODOPB)
Other phosphorus-containing epoxy resins can be obtained from the addition reaction of DOPO and the glycidyl ether of cresol-formaldehyde novolak.202, 203 The cured products are highly flame resistant. In the presence of a phosphorous-containing hardener, bis(m-aminophenyl)methylphosphine oxide (BAMPO), the volatilization of the cured resin is reduced and aromatization is accelerated. This results in a larger yield of stable char. This behavior is attributed to the flame retardant action of BAMPO. However, at high content of BAMPO this effect is overwhelmed by flame quenching due to the volatilization of the phosphoruscontaining moieties from BAMPO.191 Further, bismaleimide(3,3′ -bis(maleimidophenyl))phenylphosphine oxide (BMPPPO), is a phosphorus-containing compound that is soluble in organic compounds. Interpenetrating networks can be prepared by simultaneously curing an epoxy/diaminodiphenylmethane system and BMPPPO. The cured resin system exhibits a glass transition temperature around 212°C, thermal stability at temperatures beyond 350°C, and excellent flame retardancy with a limiting oxygen index (LOI) of 40%.192 Phosphorous-containing diamines have been prepared that act as curing agents for epoxy resins.204 The compounds and their synthesis are shown in Figure 3.10. When cured with phosphorus-containing curing agents, the epoxy resins show extremely high LOI values of up to 49. Amine-based curing agents destabilize a brominated epoxy resin by a mechanism of the nucleophilic substitution of bromine. As a result, a brominated epoxy resin releases products of pyrolysis about 100°C lower than a nonbrominated epoxy resin.205
172
Reactive Polymers Fundamentals and Applications
O
O H 2N
C
P
NH2
+ H O P
O
H2SO4/HNO3
O2N
O
NO2
P
SnCl2/HCl/EtOH
H 2N O P H 2N
O
C O P
NH2
O
NH2
P
O
2-DOPO-A
BAPPPO
Figure 3.10: 2-DOPO-A, Bis(4-aminophenyl)phenylphosphine oxide (BAPPO)204, 206
Epoxy Resins
173
Table 3.8: Global Production/Consumption Data of Important Monomers and Polymers207 Monomer
Mill. Metric tons
Ethylene oxide Ethyleneamines Epichlorohydrin Epoxy Resins
14.7 0.248 0.640 0.65
Year
Reference
2002 2002 1999 1999
208 209 210 211
3.3.10 Production Data Global production data for the most important monomers used for unsaturated epoxy resins are shown in Table 3.8.
3.4 CURING 3.4.1 Initiator Systems The epoxide group reacts with several substance classes. Only a few of the possible reactions are used for curing in practice. Curing agents of epoxy resins can be subdivided into three classes: 1. Compounds with active hydrogens, 2. Ionic initiators, and 3. Hydroxyl coupling agents. The most commonly used curing reaction is based on the polyaddition reaction, thereby opening the epoxide ring. The glycidyl group can be cured by amines and other nitrogen-containing compounds such as polyamides. Many of the amines effect curing at room temperature. This type of curing is called a cold curing. The reactivity of an epoxy compound with an amine depends on the structure of the compounds. The relative reaction rates of the secondary amine to the primary amine can be explained in terms of substitution effects.212 Anhydrides are active only at elevated temperatures. This type of curing is addressed as hot curing.
174
Reactive Polymers Fundamentals and Applications
O R
NH2 + CH2
OH CH
R NH
CH2
OH
OH R NH
CH2
CH
R
O CH2
N
CH2
CH
CH2
CH
CH
OH
O R
CH
OH + CH2
OH CH
R
O
CH2
CH
Figure 3.11: Reaction of the Glycidyl Group with an Amine and with a Hydroxy Group
3.4.2 Compounds with Activated Hydrogen 3.4.2.1
Amines
Both primary and secondary amines can be used. From a chemical point of view, the active hydrogen attached to the nitrogen group effects an addition reaction, as the epoxide group is opened. The curing of the diglycidyl oligomer with a diamine occurs in three stages: 1. Linear coupling of the oligomer, 2. Formation of a branched structure, and 3. Crosslinking. The basic reaction between the glycidyl groups with a primary amine is shown in Figure 3.11. The first reaction in Figure 3.11 is the addition reaction of primary amine hydrogen with an epoxy group. The product of this reaction is a secondary amine. The secondary amine may react with another epoxy group to form a tertiary amine, as shown in the second reaction, Figure 3.11. Usually the secondary amine is less reactive than the primary amine. The ratio of the kinetic constants is approximately 1/2. Both reactions are autocatalyzed by OH groups formed during the process. The third reaction shown is the etherification reaction between epoxy functions and hydroxyl groups. In most systems, this reaction can
Epoxy Resins
175
Table 3.9: Amines Suitable for Curing Compound
Remarks
Ethylene diamine Diethylenetriamine Triethylenetetramine Hexamethylene diamine
Fast curing, low viscosity Fast curing, low viscosity Fast curing, low viscosity Slower curing, needs elevated temperature, flexible materials Needs elevated temperature, good adhesive
Diethylaminopropylamine Isophorone diamine 1,2-Diaminocyclohexane Bis-p-aminocyclohexylmethane Bisaminomethylcyclohexane Menthane diamine N-aminoethyl piperazine Diaminodiethyl toluene m-Phenylene diamine 4,4′ -Diaminodiphenylmethane 3,3′ ,5,5′ -Tetraethyl-4,4′ -diamino diphenylmethane 4,4′ -Diamino-3,3′ -dimethyl dicyclohexylmethane (DCM) 1,5-Naphthalene diamine
Needs elevated temperature, good potlife Fast curing Mixture of 2,6-diamino-3,5-diethyl toluene and 2,4-diamino-3,5-diethyl toluene Chemical resistant materials Chemical resistant materials Flame retardant191 Cycloaliphatic diamine213, 214
be neglected. However, it has been shown that this reaction takes place using 4,4′ -methylene bis(3-chloro-2,6-diethylaniline) (MCDEA) as curing catalyst. On the other hand, with 4,4′ -diaminodiphenylsulfone (DDS) and 4,4′ -methylenedianiline (MDA) as catalysts the etherification was not observed.109, 215 Typical nitrogen compounds used for cold curing are shown in Tables 3.9, and 3.10, and in Figures 3.12 and 3.13. There are many possibilities for formulating a curing system from primary and secondary amines, and also with tertiary amines. Tertiary amines catalyze the reaction. Other catalysts are complexes of boron trifluoride complexes, quaternary ammonium salts, thiocyano compounds, etc. Retarders are certain ketones and diacetone alcohol.
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Reactive Polymers Fundamentals and Applications
Table 3.10: Polymeric Amines and Hetero Functional Amines Compound Poly(propylene oxide)diamine Trimercaptothioethylamine Polymercaptopolyamines 2,4-Diamino-4′ -methylazobenzene (DMAB) 4,4′ -Dithiodianiline Dicyandiamide 4,4′ -Diaminodiphenylsulfone bis(m-aminophenyl)methylphosphine oxide (BAMPO) 4,4′ -Methylene bis[3-chloro2,6-diethylaniline] Olefin oxide polyamine adducts Glycidyl ether polyamine adducts Diamide of dimerized linoleic acid and ethlyene diamine Ketimines 2,5-Bis(aminomethyl)bicyclo[2.2.1] heptane di(methylisopropyl ketimine) Substituted imidazolines, e.g., 2-ethyl-4-methylimidazole, 1-methylimidazole Sulfanilamide Polysilazane-modified polyamines
Remarks Optical applications23, 216 In combination with customary amine hardeners217 Optical applications218 Reversible crosslinking219 Common for adhesives Chemical resistant materials 191 67
Fast curing, low toxicity Fast curing For adhesives Low viscosity, long potlife, latent hardening catalysts Norbornane diketimine220 Wide range in stoichiometry 45, 221–223
Thermal resistant224
Epoxy Resins
H2N
CH2
CH2
NH CH2
CH2
177
NH2
Diethylenetriamine H2N
CH2
CH2
CH2
CH2
CH2
CH2
NH2
Hexamethylenediamine CH3
CH2
CH3
CH2
N CH2
CH2
CH2
NH2
Diethylaminopropylamine CH3
CH3
H2N C CH3
H2N
CH2 CH2
N H
NH2
Menthanediamine
N-Aminoethyl piperazine
Figure 3.12: Aliphatic Nitrogen Compounds for Curing: Diethylenetriamine, Hexamethylene diamine, Diethylaminopropylamine, Menthane diamine, N-aminoethyl piperazine
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Reactive Polymers Fundamentals and Applications
H2N
NH2
NH2 H2N
m-Phenyenediamine
1,5-Napthalene diamine O
H2N
S
NH2
O 4,4’-Diaminodiphenylsulfone
H2N
CH2
NH2
4,4’-Diaminodiphenylmethane
Figure 3.13: Aromatic Nitrogen Compounds for Curing: m-Phenylene diamine, 1,5-Naphthalene diamine, 4,4′ -Diaminodiphenylsulfone, 4,4′ -Diaminodiphenylmethane
Epoxy Resins
179
Certain cyclic amines, such as 1,2-bis(aminomethyl)cyclobutane and isomers of diaminotricyclododecane increase the pot life time. Polyamines and dicyanamide are preferably used for adhesive formulations. Phenolic hydroxyl groups exert autocatalysis at low conversions with respect to the ring opening of the epoxide group, thereby adding the amine groups. In the later stage of curing the amine groups are largely consumed and the phenolic hydroxyl groups start to react with the residual epoxide groups.225 A suitable accelerator for adhesive formulations is 2,4,6-tris(dimethylaminomethyl)phenol. Most low molecular amines are toxic and also sensitive to the carbon dioxide in air. Therefore, the various adducts of the amines have been developed to mitigate this drawback. 3.4.2.2
Ketimines
Ketimines form the active amine structure by addition of water; thus they act as delayed-action catalysts. 3.4.2.3
Amino Amides
Amide-based compounds are used to achieve special properties and desired curing characteristics, such as lower toxicity, less sensitive final properties to the stoichiometry, lower peak temperatures for large castings. The active group in curing is not directly the amide group, but the attached primary and secondary amino groups present in the molecule. The amide group is helpful for achieving the other benefits, mentioned above. Examples for amino amides are adducts of polyamines with fumaric acid or maleic acid, or fatty acids. Similar to amines, in amine amides the reaction can be accelerated with boron trifluoride complexes, Mannich bases, etc. 3.4.2.4
Metal salts
Zirconium tetrachloride catalyzes effectively the nucleophilic opening of epoxide rings by amines. This has been used for the efficient synthesis of β-amino alcohols.226 Zinc bromide and zinc perchlorate are also active in this manner.227 However, it seems that this catalyst is not used for the curing of epoxy resins.
180
Reactive Polymers Fundamentals and Applications Table 3.11: Anhydrides for Hot Curing
Anhydride
Remark/Reference
Dodecenyl succinic anhydride Hexahydrophthalic anhydride 3-Methyl-1,2,3,6-tetrahydrophthalic anhydride (MeTHPA) Hexahydro-4-methylphthalic anhydride Tetrahydrophthalic anhydride Methyltetrahydrophthalic anhydride Phthalic anhydride Methyl nadic anhydride HET anhydride Pyromellitic dianhydride (PMDA) 5-(2,5-Dioxotetrahydrofuryl)3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DMCDA) Glutaric anhydride Styrene-maleic anhydride copolymers
Liquid
3.4.2.5
N,N-dimethylbenzylamine as accelerator228
Liquid 229
Biodegradable Formulations230 Low molecular weight copolymers
Phenols
Bisphenol A is a main ingredient for the manufacture of glycidyl ethers. Polyfunctional phenols can be used to cure epoxy resins. This method did not find large commercial use, except in the development of highly chemically resistant coatings. The curing reaction is completely similar to the curing reaction of amines. Phenoplasts. Polyfunctional phenols can be applied as phenol/formaldehyde condensates of the novolak-type. In this field a wide variety has been examined, including phenolic adducts of chloromethylated diphenyl oxide, tetrabrominated bisphenol, and phenol adducts of poly(butadiene) 3.4.2.6
Anhydride Compounds
Typical anhydride compounds used for hot curing are shown in Table 3.11 and in Figure 3.14. Most anhydrides need elevated temperatures to be active. The anhydride group is not active in the absence of acidic or basic
Epoxy Resins
O
O C12H23
O
O
O
O Dodecenylsuccinic anhydride
Phthalic anhydride
O
O
O
O
O
O Tetrahydrophthalic anhydride
H3C CH O O O Methylnadic anhydride
Hexahydrophthalic anhydride
O O O
O O O
Pyromellithic anhydride
Figure 3.14: Anhydrides for Hot Curing
181
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Reactive Polymers Fundamentals and Applications
catalysts; instead the anhydride group must be converted into the carboxyl group. This can be achieved by hydrolysis by natural occurring moisture, or by alcoholysis. The reaction of an anhydride is accelerated by a tertiary amine or by complexes of metal salts, such as ferric acetylacetonate.231 The reaction of the anhydride group, as well as the acid group with the epoxide group, results in an ester linkage, with all the advantages and disadvantages of the ester link. Anhydrides are in some cases preferred over amines because they are less irritating to the skin, have longer pot life times, and low peak temperatures. Aromatic and cycloaliphatic anhydrides find wide applications for molding and casting techniques. 3.4.2.7
Polybasic Acids
The carboxyl group is capable of opening the epoxide group. Theoretically, the optimum stoichiometry is one acid group by one epoxide group. In practice an excess of acid is used. 3.4.2.8
Polybasic Esters
To obtain tough materials, the epoxides can be cured by the insertion reaction into ester groups. The curing agent is formed in-situ by the radical polymerization of N-phenylmaleimide and p-acetoxystyrene.232 2,5-Dimethyl-2,5-bis(benzoylperoxy)hexane is suitable, because its decomposition temperature of 110°C is close to the desired cure temperature of 100°C. The two monomers copolymerize satisfactorily in the absence of the epoxy compound. The advantage of using the in-situ technique of polymerization is that the initial composition has low viscosity. The insertion mechanism is shown in Figure 3.15. Compared to epoxy systems cured with a phenol resin, the copolymer of N-phenylmaleimide and p-acetoxystyrene shows a significantly higher glass transition temperature.
3.4.3 Coordination Catalysts Coordination catalysts consist of metal alkoxides, such as aluminum isopropyloxide, metal chelates, and oxides. Coordinative polymerization results in high molecular weight and stereospecific species.
Epoxy Resins
183
O O
O
H3C C O + CH2
H 3C C O CH CH2
O CH2
CH CH2 O
N
O
CH CH2
CH CH2 O
N
O
Figure 3.15: Insertion of the Epoxide into a Pendent Ester Group
3.4.4 Ionic Curing 3.4.4.1
Anionic Polymerization
The anionic polymerization of epoxides can be initiated by metal hydroxides, and secondary and tertiary amines. The rate of curing is low in comparison to other curing methods. Therefore, anionic polymerization has not found wide industrial application. Moreover, the mechanical properties of the final materials are not satisfactory. 3.4.4.2
Cationic Polymerization
Cationic polymerization can lead to a crosslinking process if diepoxides are taken as monomers. Thus, a wide variety of compounds can be used catalytically as cationic curing initiators for epoxy resins that act at a high rate. Moreover, their low initial viscosities and fast curing make them good candidates for rapid reactive processing. Cationic polymerization is initiated by Lewis acids. A lot of metal halogenides have been shown to be active, such as AlCl3 , SnCl4 , TiCl4 , SbCl5 or BF3 , but the most commonly used compound is boron trifluoride. In practice, boron trifluoride is difficult to handle and the reaction runs too fast. Therefore, the compound is used in complexed form, e.g., as an ether complex or an amine complex. The strength of the ether and amine complexes can be related to the base strength of the ether and amine,
184
Reactive Polymers Fundamentals and Applications Table 3.12: Latent Catalysts Compound
Reference
N-Benzylpyrazinium hexafluoroantimonate N-Benzylquinoxalinium hexafluoroantimonate Benzyl tetrahydrothiophenium hexafluoroantimonate o,o-Di-tert-butyl-1-piperidinylphosphonamidate o-tert-Butyl-di-1-piperidinylphosphonamidate o,o-di-tert-Butyl phenylphosphonate o,o-Dicyclohexyl phenylphosphonate
233 233 234 235 235 236 236
respectively. Since the reactivity of a complex depends on the dissociation constant, some predictions on the activity of the complex can be made. Water or alcohols cause chain transfer reactions. The alcohol attacks the positively charged end of the growing polymer chain and forms an ether linkage or a hydroxyl group, respectively. The released proton can initiate the growth of another polymer chain. Diols and triols yield polymers with pendent hydroxyl groups. Therefore, diepoxides or higher functional epoxides are polymerized in the presence of diols or triols, etc.; branched and crosslinked products may appear. In the cationic UV curing of an aliphatic epoxy compound it was observed that the polymerization rate decreased strongly after a conversion level of less than 10%. This effect was not caused by the glass transition temperature. However, the addition of 1,6-hexanediol (HD) raised the conversion at room temperature.237 There are photolatent and thermolatent catalyst systems. A great variety of those catalysts have been reviewed.238 Besides the direct thermolysis of the initiator, also indirect methods are viable. Table 3.12 provides a list of latent catalysts. Spiroorthocarbonate. The cationic curing reaction of a bisphenol A-type epoxy resin in the presence of a spiroorthocarbonate can be performed with borontrifluoride dietherate. The spiroorthocarbonate undergoes a double ring opening reaction.239 The conversion of the epoxy groups increases as the content of the spiroorthocarbonate increases. 3,9-Di(p-methoxybenzyl)-1,5,7,11-tetra-oxaspiro[5.5]undecane, c.f. Figure 3.16 as spiroorthocarbonate, can be synthesized by the reaction of 2-methoxybenzyl-1,3-propanediol with dibutyltin oxide. Differential scanning calorimetry shows two peaks that are attributed
Epoxy Resins
185
O O H3C O
O CH3 O O
3,9-Di(p-methoxy-benzyl)-1,5,7,11-tetra-oxaspiro(5,5)undecane O O O
O
O O
3,23-Dioxatrispiro[tricyclo[3.2.1.0<2,4>]octane-6,5’1,3-dioxane-2’2"-1,3-dioxane-5",7’"-tricyclo[3.2.1.0<2,4>octane]
Figure 3.16: 3,9-Di(p-methoxybenzyl)-1,5,7,11-tetra-oxaspiro[5.5]undecane and 3,23-dioxatrispirotricyclo[3.2.1.0[2.4]]octane-6,5′ -1,3-dioxane-2′ 2′′ 1,3-dioxane-5′′ ,7′′′ -tricyclo[3.2.1.0[2.4]octane]
to the polymerization of the epoxy group, and to the copolymerization of the spiroorthocarbonate with epoxy groups or homopolymerization, respectively. Copolymers containing a spiroorthocarbonate are capable of yielding a hard, non-shrinking matrix resin. Examples of these copolymers include a 2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane spiroorthocarbonate, and 3,23-dioxatrispirotricyclo[3.2.1.0[2.4]]octane6,5′ - 1,3-dioxane-2′ 2′′ -1,3-dioxane-5′′ ,7′′′ -tricyclo[3.2.1.0[2.4]octane] and cis,cis-, cis,trans-, and trans,trans-configurational isomers of 2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane. These spiroorthocarbonates were determined to undergo an expansion of 3.5% during homopolymerization and demonstrated acceptable cytotoxicity and genotoxicity properties. These properties make them promising components of composite resin matrix materials.20 Trifluoromethanesulfonic acid salts. Triflic acid, i.e., trifluoromethanesulfonic acid, CF3 SO3 H is a known strong acid. Lanthanide triflates are Lewis acids and they maintain their catalyst activity even in aqueous solution. The strong electronegativity of the trifluoromethanesulfonate anion enhances the Lewis acid character of the initiator. Therefore, lanthanide triflates are excellent catalysts in the ring opening of the epoxy compounds.240
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Reactive Polymers Fundamentals and Applications
Phosphonic Acid Esters. Phenylphosphonic esters decompose into phenylphosphonic acid and the corresponding olefins at 150 to 170°C. In the presence of ZnCl2 they can initiate a cationic polymerization of glycidyl phenyl ether (GPE) to molecular weights up to 2000 to 7000 Dalton.236 Examples are o,o-di-1-phenylethyl phenylphosphonate, o,o-di-tertbutyl phenylphosphonate, and o,o-dicyclohexyl phenylphosphonate. These compounds can be synthesized from phenylphosphonic dichloride and the corresponding alcohols. Phosphonamidates. Phosphonamidates are thermally latent initiators, suitable for the polymerization of epoxides.235 These compounds, such as o,o-di-tert-butyl-1-piperidinylphosphonamidate and further o-tert-butyl-di-1-piperidinylphosphonamidate can be synthesized from phosphorus oxychloride and piperidine in the presence of triethylamine, followed by the reaction with tert-butyl alcohol in the presence of sodium hydride. No polymerization of epoxide resins occurs below 110°C, whereas the curing proceeds rapidly above 110°C. At room temperature a mixture of epoxide and phosphonamidate is stable for months.
3.4.5 Photoinitiato
