Reactive polymers fundamentals and applications: a concise guide to industrial polymers

REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS A CONCISE GUIDE TO INDUSTRIAL POLYMERS Johannes Karl Fink Montanuniver...

Author: Johannes Karl Fink

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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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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

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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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400 402 402 405 405 412 417 417 421 421 422 422 423 423 423 429 431 431 431 433 433 433 433 433 435 435 435 435 438

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447 447 447 448 450 450 450 451

xviii


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

xxii


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

xxiv


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


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


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


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


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


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


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


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


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


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


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.


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


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.


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


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


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


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


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


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


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


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


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


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


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-


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


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-


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


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


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


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.


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


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


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-


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


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:


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


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.


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


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


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

48


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.


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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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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


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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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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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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

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70


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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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

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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


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


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


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


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

82


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

84


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

86


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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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.

90


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

Polyurethanes

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


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


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


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


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


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


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


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


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


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


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.

116


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


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

118


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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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

122


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

124


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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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


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

142


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

144



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

146


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

148


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

150


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

154


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-

156


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


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

160


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.


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

162


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

166


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


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


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


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


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


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.

176


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

178


H2N

NH2

NH2 H2N

m-Phenyenediamine

1,5-Napthalene diamine O

H2N

S

NH2

O 4,4’-Diaminodiphenylsulfone

H2N

CH2

NH2


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

182


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]octane-6,5’1,3-dioxane-2’2"-1,3-dioxane-5",7’"-tricyclo[3.2.1.0octane]

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

186


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 Photoinitiators Photoinitiation is one of the most efficient methods for achieving very fast polymerization. Often the reaction can be completed within less than one second.241 Curing with ultraviolet light has been developed for the coating area, printing inks and adhesives. The mechanism of photo curing consists mostly of a cationic photopolymerization of epoxides. The kinetics of the photoinduced reactions can be monitored by differential photocalorimetry.242 The major drawback of differential photocalorimetry is the rather long response time in comparison to the curing rate. The well-known use of radical generating photoinitiators in vinylcontaining systems is not applicable in pure epoxy systems. There is an exception when the epoxide resin is mixed with a vinyl monomer that bears the hydroxyl functionality or the amide functionality. The radical generating photoinitiator reacts then with the vinyl monomer.243 Common photoinitiators for epoxy systems are shown in Table 3.13. In the photoinduced curing of epoxides, the propagating polymer cations cannot deactivate one another, but require a deactivation by another species present in the polymerization mixture. Therefore, after the light

Epoxy Resins

187

Table 3.13: Photoinitiators for Epoxides Compound Aryl diazonium tetrafluoroborates 4,4-Bis(N,N-dimethyl-N-(2-ethoxycarbonyl-1-propenyl)ammonium hexafluoro antimonate)benzophenone Calixarene derivatives 9-Fluorenyl tetramethylene sulfonium hexafluoroantimonate Cyclopentadiene-Fe-arene hexafluorophosphate

Reference 244 245 246 247

is switched off, a pronounced postpolymerization reaction can be monitored.248 The conversion in the dark may contribute up to 80% of the total curing process. The overall polymerization quantum yield reaches ca. 200 mol per photon. It has been shown that polyglycols, i.e., polyols from 1,2-diols, slow down the cationic polymerization, whereas polyols made from 1,4-diols do not show this effect.234 Also the addition of small amounts of crown ethers (12-crown-4 ether) retards the polymerization. This behavior is attributed to complexes that are formed only with glycol-like structures that reduce the effective concentration of cations available to initialize the polymerization. 3.4.5.1

Aryl Diazonium Tetrafluoroborates

The azo group in aryl diazonium tetrafluoroborates decomposes on ultraviolet radiation into the aromatic compound, nitrogen and boron trifluoride. The latter compound initiates a cationic polymerization of the epoxide resin. The evolution of nitrogen limits the applications to thin films. 3.4.5.2

Aryl Salts

Other efficient photoinitiators are based on the photolysis of diaryliodonium and triarylsulfonium salts, that when decomposed liberate strong Brønsted bases. These bases initiate the cationic polymerization. It has been shown that diaryliodonium hexafluoroantimonate initializes photochemically the cationic copolymerization of 3,4-epoxycyclohexylmethyl-3′ ,4′ -epoxycyclohexane carboxylate and triethylene glycol methylvinyl ether.249 Epoxy-functionalized silicones can be synthesized by

188


O

S Thioxanthone

Anthracene

Figure 3.17: Thioxanthone, Anthracene

rhodium-catalyzed, chemoselective hydrosilation of vinyl ethers with siloxanes or silane.250 Epoxidized soyabean oil accelerates the crosslinking reaction of aromatic diepoxides in the presence of a triarylsulfonium photoinitiator.251 The photoinitiated copolymerization leads within seconds to a fully cured insoluble material showing increased hardness, flexibility, and scratch resistance. In interpenetrating networks, constructed by vinyl polymers and epoxides by photo curing, a mixture of a radically decomposing photoinitiator and a cationic photoinitiator is used. Examples are a mixture of a hydroxyphenylketone and a diaryliodonium hexafluorophosphate salt. During the UV curing of a mixture of acrylate and epoxide monomers, the epoxides react slower than acrylates.173 The low efficiency of the initiation process is caused by the low ultraviolet absorbance of cationic photoinitiators. However, photosensitizers can improve the performance. Combinations of photo curing and thermal curing in interpenetrating networks of a vinyl polymer and an epoxide are possible. Such a combination of crosslinkable resins allows the partial or complete cure of each component independent of the other.252 3.4.5.3

Photosensitizers

Photosensitizers can be used to improve characteristics of photo curing for pigmented materials. These photosensitizers exhibit significant UV absorption in the near UV and transfer the absorbed energy to a cationic photoinitiator.253 Examples for photosensitizers are anthracene and thioxanthone derivatives, such as 2,4-diethylthioxanthone, isopropylthioxanthone, c.f. Figure 3.17. Photoinitiators are iodonium salts that exhibit a compara-

Epoxy Resins

189

CH3

CH2

H3C

CH2

CH3

OH OH

HO

CH2

CH2 OH

HO OH

H3C

CH2

CH2

CH3

CH3

Figure 3.18: p-Methylcalix[6]arene

tively low triplet state energy. 3.4.5.4

Calixarenes

Calixarenes are by-products in the phenol/formaldehyde condensation to prepare bakelite. They found attention for their application as surfactants, chemoreceptors, electrochemical and optical sensors, solid-phase extraction phases, and stationary phases for chromatography.254 The hydroxyl groups in calixarenes (c.f. Figure 3.18) can be protected with tert-butoxycarbonyl groups, trimethylsilyl groups, and cyclohexenyl groups, respectively. In this way the hydroxyl group does not react with an epoxide group. The phenol groups can be restored if a compound is present that generates acids photolytically.245

3.4.6 Derivatives of Michler’s Ketone 4,4-Bis(N,N-dimethyl-N-(2-ethoxycarbonyl-1-propenyl)ammonium hexafluoro antimonate)benzophenone (MKEA) is synthesized from 4,4′ -bis(dimethylamino)benzophenone (Michler’s ketone) and ethyl α-(bromomethyl)acrylate, c.f. Figure 3.19. MKEA initiates cationic photopolymerization of cyclic ethers, like cyclohexene oxide (CHO) via a conventional addition

190


O

H 3C N

CH3

C

N

+ Br

CH2

CH3

H 3C

C CH2 C O O CH2 CH3

H2C C CH2 O C O

CH3

O

CH3

N+

C

N+ CH2

C CH2

CH3

C O

CH3 SBF6

-

SBF6

-

O

CH2

CH2

CH3

CH3 MKEA

Figure 3.19: Synthesis of 4,4-Bis(N,N-dimethyl-N-(2-ethoxycarbonyl-1-propenyl)ammonium hexafluoro antimonate)benzophenone (MKEA)

fragmentation mechanism. MKEA belongs to the group of addition-fragmentation catalysts. The mechanism of initiation of MKEA is shown in Figure 3.20. This initiator does not require supplementary free radical sources. It is suggested that radicals stemming from the photoinduced hydrogen abstraction participate in addition fragmentation reactions to yield reactive species capable of initiating cationic polymerization.244 Monomers with strong electron donors such as N-vinyl carbazole, isobutyl vinyl ether, and n-butyl vinyl ether undergo explosive polymerization upon illumination of light. In the case of cyclohexene oxide there is an induction period, owing to the trace impurities present, but afterwards, the polymerization proceeds readily. 3.4.6.1

Photoinitiator Systems

Visible light photoinitiator systems include an iodonium salt, a visible light sensitizer, and an electron donor compound.20

Epoxy Resins

R* + H2C C CH2 O C

191

CH3 N+ CH3

O CH2 CH3

R*

H2C C* CH2 O C

CH3 N+ CH3

O CH2 CH3

R*

H2C C* CH2 O C

CH3 + N*+ CH3

O CH2 CH3

Figure 3.20: Mechanism of Initiation of 4,4-Bis(N,N-dimethyl-N-(2-ethoxycarbonyl-1-propenyl)ammonium hexafluoro antimonate)benzophenone (MKEA)

192


Examples of useful aromatic iodonium complex salt photoinitiators include diaryliodonium hexafluorophosphates and diaryliodonium hexafluoroantimonates, such as (4-(2-hydroxytetradecyloxyphenyl))phenyliodoniumhexafluoroantimonate, (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate (OPIA), and (4-(1-methylethyl)phenyl)(4-methylphenyl)iodonium tetrakis pentafluorophenylborate. These salts are more thermally stable, promote faster reaction, and are more soluble in inert organic solvents than are other aromatic iodonium salts of complex ions. Diphenyl iodonium hexafluoroantimonate has a photoinduced potential greater than N,N-dimethylaniline. The second component in the photoinitiator system is the photosensitizer. Desirably, the photoinitiator should be sensitized to the visible spectrum to allow the polymerization to be initiated at room temperature using visible light. The sensitizer should be soluble in the photopolymerizable composition, free of functionalities that would substantially interfere with the cationic curing process, and capable of light absorption within the range of wavelengths between about 300 and about 1000 nanometers. Suitable sensitizers include compounds in the following categories: • • • •

α-Diketones Ketocoumarins Aminoarylketones p-Substituted aminostyrylketones

For applications requiring deep cure (e.g., cure of highly filled composites), it is preferred to employ sensitizers having an extinction coefficient below about 1000 l mol−1 cm−1 at the desired wavelength of irradiation for photopolymerization, or alternatively, the initiator should exhibit a decrease in absorptivity upon light exposure. Many of the α-diketones exhibit this property, and are particularly preferred for dental applications. A suitable photosensitizer is camphorquinone. The third component of the initiator system is an electron donor compound. The electron donor compound should be soluble in the polymerizable composition. Further, suitable compatibility and interplay with the photoinitiator and the sensitizer and other properties, like shelf stability, should be fulfilled. The donor is typically an alkyl aromatic polyether or an alkyl, aryl amino compound wherein the aryl group is optionally substituted by one or more electron withdrawing groups. Examples of suitable electron withdrawing groups include carboxylic acid, carboxylic acid ester,

Epoxy Resins

193

ketone, aldehyde, sulfonic acid, sulfonate, and nitrile groups. In practice, the following compounds find application: 4,4′ -Bis(diethylamino)benzophenone, 4-Dimethylaminobenzoic acid (4-DMABA), Ethyl-4-dimethylamino benzoate (EDMAB), 3-Dimethylaminobenzoic acid (3-DMABA), 4-Dimethylaminobenzoin (DMAB), 4-Dimethylaminobenzaldehyde (DMABAL), 1,2,4-Trimethoxybenzene (TMB), and N-Phenylglycine (NPG).

3.4.7 Epoxy Systems with Vinyl Groups Besides pure epoxy systems, mixed systems such as epoxy acrylates are in use. These systems can be cured with radical photoinitiators. Examples for such initiators are 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butan-1-one (BDMB), 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one (TPMK), 2,2-dimethoxy-1,2-diphenylethan-1-one (BDK), and hydroxy-2-methyl-1-phenyl-propanone.255

3.4.8 Curing Kinetics There are various methods to investigate the kinetics of curing, including 1. 2. 3. 4. 5. 6. 7. 3.4.8.1

Viscometry, Differential scanning calorimetry, Modulated differential scanning calorimetry, Dielectric analysis, Dynamic mechanical analysis, In-situ Fourier transform infrared spectroscopy, and Fluorescence response. Viscometry

In the course of curing, the crosslinking density and the viscosity as well as the modulus of the resin system increase. The viscoelastic properties can be measured in a torsional motion.256

194 3.4.8.2

Reactive Polymers Fundamentals and Applications Differential Scanning Calorimetry

Differential scanning calorimetry is the only direct reaction rate method which operates in two modes: constant temperature or linear programmed mode. Several methods to evaluate the data obtained by differential scanning calorimetry are available.257 The isoconversional method258 is frequently used to calculate the energies of activation and evaluating the dependence of the effective activation energy on the extent of conversion.259 Relations are available between the degree of conversion, the time dependence of the conversion, and the direct measurable parameters, i.e., viscometry, differential scanning calorimetry and dynamic mechanical analysis. The equation is always second-order although the coefficients to this equation are different for the individual methods. The DSC technique becomes insensitive at conversions shortly after the gel point.260 However, changes in the heat capacity can be indicators of the onset and the finishing of the vitrification.214 Differential scanning calorimetry allows statements concerning the reaction mechanism of curing. The ring opening reaction between phenyl glycidyl ether and aniline was investigated by DSC. The reaction resembles the diepoxy-diamine cure mechanism. However, it was detected that besides that from the epoxy ring opening reaction, another exothermic process at the last stages of the reaction takes place. It was concluded that the reaction of epoxy ring opening by aniline occurs by two concurrent pathways,261, 262 an uncatalyzed one and an autocatalyzed one. 3.4.8.3

Temperature Modulated Differential Scanning Calorimetry

In temperature modulated differential scanning calorimetry (TMDSC), the sample is subjected to a sinusoidal temperature change. The instruments are called differential AC-calorimeters. This particular method can measure the storage heat capacity and the loss heat capacity, i.e., the reversible part of heat that can be withdrawn again by cooling, and a part of heat consumed by chemical reaction. A complex heat capacity with a real part (storage heat capacity) and an imaginary part (loss heat capacity) can be defined.263 The treatment is similar to other complex modules in mechanics. During the curing, the glass transition temperature rises steadily. The reaction induced vitrification takes place when the glass transition temperature rises above the curing temperature. This transition can be fol-

Epoxy Resins

195

lowed simultaneously with the reaction rate in TMDSC.264, 265 Modulated differential scanning calorimetry allows detecting of reaction induced phase separations. The apparent heat capacity changes, as phase separation occurs. The cloud point can be determined with optical microscopy, and there is a correspondence between the optical method and the calorimetry method.266, 267 In an amine curing system, a complex formed from the primary amine and the epoxide was postulated that initiates the curing reaction. The reactions of the primary amine and the secondary amine with an epoxy-hydroxyl complex are comparatively slow and thus rate determining during the whole curing process.264, 268 In an epoxy-anhydride system some complications have been elucidated.269 Temperature modulated DSC can be used with advantage during isothermal curing of semi-interpenetrating polymer networks.270 3.4.8.4

Dielectric Analysis

Dielectric analysis271 is based on the measurement of the dielectric permittivity ε′ and the dielectric loss factor ε′′ in the course of curing. The complex dielectric constant ε∗ may be expressed by ε∗ = ε′ − iε′′

(3.1)

The permittivity is proportional to the capacitance and depends on the orientation polarization. The orientation polarization results from the change in the dipole moment due to the chemical reaction and also from the change of the concentration of dipoles due to the volume contraction during the curing reaction. The loss factor corresponds to the energy loss. Both dielectric and mechanical measurements are suitable techniques for monitoring the curing process. Also, phase-separation processes can be monitored by dielectric analysis, because dielectric measurements are sensitive to interfacial charge polarization. Dipolar relaxation indicates the vitrification through the α-relaxation process in both phases.272 Further, dielectric sensor measurements have the advantage that they can be made in the laboratory as well as in-situ in the fabrication tool in a production line.273 A relation between the dielectric response and other methods measuring the gel point has been established in epoxy systems.214 Dielectric analysis, in combination with other experimental techniques, can be used to establish a time-temperature-transition (TTT) diagram. The curing must be measured in a series of experiments at differ-

196


ent temperatures. In such a diagram gelation, vitrification, full cures, and phase-separation are marked.274 A technique involving simultaneous dielectric and near infrared measurements has been used for monitoring the curing of blends of a diglycidyl ether bisphenol A epoxy resin with a 4,4′ -diaminodiphenylmethane hardener and various amounts of poly(methyl methacrylate) as modifier.275 3.4.8.5

In-Situ Fourier Transform Infrared Spectroscopy

During the curing reaction, the appearance or disappearance of various characteristic infrared bands can be monitored. This method yields more information than a single parameter, e.g., as obtained from a DSC measurement. However, there is more work needed to calibrate the system properly than in a DSC experiment. Multivariate analysis, in particular alternating least squares (ALS), allows calculation of the concentration profiles and the spectra of all species involved in the reaction of curing epoxy resins.276 During curing, the intensity of the epoxy group, at 789 to 746 cm−1 decreases.277 For example, based on such experiments, in the curing of a dicyanate ester (1,1-bis(4-cyanatophenyl)ethane) with a bisphenol A epoxide, the formation of an oxazoline structure has been proposed.278 3.4.8.6

Fluorescence Response

Fluorescence is a very sensitive and non-destructive technique to monitor the curing. The fluorescence response from chemical labels and probes enables the changes to be followed in the surroundings of the chemical label. In the curing process, the viscosity may change about six orders of magnitude. A change in the viscosity of the medium leads to a decrease in the non-radiative decay rate and consequently a change in the fluorescence quantum yield. The reaction medium acts as a thermal bath for the excited fluorescent molecule. When the monomers become fixed in forming a crosslinked polymer, a reduction of translational, rotational, and vibrational degrees of freedom in the bath takes place. Therefore, a reduction in the number of non-radiative deactivation pathways and an increase in fluorescence intensity occurs. 1-Pyrenesulfonyl chloride (PSC) was used as a chemical label for silica epoxy interfaces, the surface coated with (3-aminopropyl)triethoxysilane, because it reacts easily with amine groups, yielding sulphonamide

Epoxy Resins

H 3C

197

CH3 N

NH2

C O

OH

9-Anthroic acid

CH2

O S O CH2

NH

DNS-EDA

Figure 3.21: 9-Anthroic acid, 5-dimethylaminonaphthalene-1-(2-aminoethyl)sulfonamide (DNS-EDA)

derivatives.279 Also 9-anthroic acid, its ester derivatives and 5-dimethylaminonaphthalene-1-(2-aminoethyl)sulfonamide (DNS-EDA), c.f. Figure 3.21 are common fluorescence dyes.280, 281

3.4.9 Thermal Curing By investigating the curing of a commercial epoxy prepolymer with imidazole curing agents, it has been verified that the cure schedule influences the properties of the end product. The highest thermal stability of the polymers can be achieved by isothermal cure schedules. Samples cured by a temperature program showed lower glass transition temperatures. In a series of temperature programmed curing experiments, a lower heating rate resulted in higher transition temperatures and superior thermal stability. The initial and postcure schedules are thus of critical importance for the final properties of the polymer.282

3.4.10 Microwave Curing Due to increasing application in the aerospace and microelectronics industries the demand for accelerated curing has emerged. In particular, for the microelectronics industry, the curing of thermoset systems has become a bottleneck of the whole production process. Besides photo curing, curing with γ-rays and electron beams is an alternative. Microwave curing of materials has the potential to deliver several major advantages over conventional thermal processing. One of these is

198


a decrease in the time necessary for manufacture since another potential advantage is that the power is directed to the sample. The microwave energy is absorbed throughout the body of the material rather than relying on thermal conduction and convection. Therefore, the energy consumed is less than thermal curing. Experiments with the diglycidyl ether of bisphenol A and three types of curing agents, i.e., 4,4′ -Diaminodiphenylsulfone, 4,4′ -Diaminodiphenylmethane, and m-phenylene diamine with various energies of microwave energy showed that in comparison to thermal curing microwave curing is faster. The glass transition temperatures are somewhat lower in the case of the products cured with microwave technology in comparison to those cured by thermal methods.283 However, the curing performance is strongly dependent on the curing agent used.284 The interfacial shear strengths in those composites cured with microwave techniques are comparable with being thermally postcured.285

3.5

PROPERTIES

Mechanical properties of epoxy resins can be correlated and traced back to the constituting monomers. The mechanical properties of epoxy resins depend on the flexibility of the segments and on the crosslinking density. Epoxy resins shrink in the course of curing less than vinyl resins. It is important to distinguish between the shrinkage that occurs before gelling and after gelling. Only a shrinkage that occurs after gelling results in residual stress in the final product. Epoxy resins can exhibit several thermal transition regions, depending on the chemical nature of the monomers. These transitions influence the curing. If a glass transition occurs during curing at the temperature applied, the individual reactive parts of the pendant molecules can no longer move sufficiently and the curing reaction freezes at this conversion. However, raising the temperature effects further curing. Cycloaliphatic epoxy resins have a low viscosity. The cured resins exhibit a high glass transition temperature. On the other hand, they exhibit low break elongation and toughness because of their high crosslinking density. Epoxy resins show good electrical properties. Of course, the electrical properties are affected by the moisture content. On the other hand, the resins can be made electrically conductive, by metal particles such as

Epoxy Resins

199

Table 3.14: Epoxies Based on Hybrid Polymers Compounds

Remark/Reference

Siloxane polymer with pendant epoxide rings Epoxy polyurethane hybrid resins Maleimide-epoxy resins

286–288 289

silver and copper. Epoxy resins adhere by forming strong bonds with the majority of surfaces, therefore, an important application is in adhesives. Epoxy resins have excellent resistance to acids, bases, organic and inorganic solvents, salts, and other chemicals.

3.5.1 Hybrid Polymers and Mixed Polymers Hybrid polymers and mixed polymers are summarized in Table 3.14. These include silicone-epoxy hybrid polymers, urethane-epoxy hybrid polymers, and maleimide-epoxy polymers. 3.5.1.1

Epoxy-Siloxane Copolymers

A siloxane polymer with pendant epoxide rings on the side chain of the polysiloxane polymer backbone, when blended with diglycidyl bisphenol A ether and cured, increases the mobility of the crosslinked network and the thermal stability. Graft siloxane polymer with pendant epoxide rings can be synthesized by the hydrosilylation of poly(methylhydrosiloxane) with allyl glycidyl ether.286 Aminopropyl-terminated poly(dimethylsiloxane) blended in an epoxy resin shows an outstanding oil and water repellency in coatings.290 The peel strength of a pressure-sensitive adhesive affixed to the modified epoxy resin also decreases. Polyether/poly(dimethylsiloxane)/polyether triblock copolymers added in amounts of 5 ca. phr, efficiently reduce the static friction coefficient of the cured blends upon steel.291 Silsesquioxanes are organosilicon compounds with the general formula [RSiO3/2 ]n , c.f. Figure 8.1, at page 323. Silsesquioxane (SSO) solutions were reacted with diglycidyl either of bisphenol A with 4-dimethylaminopyridine as initiator, to result in SSO-modified epoxy networks. The modification with SSO increased the elastic modulus in the glassy state. This is explained by an increase in the cohesive energy density.292

200 3.5.1.2

Reactive Polymers Fundamentals and Applications Maleimide-Epoxy Resins

Maleimide-epoxy resins are based on N-(p-carboxyphenyl)maleimide and allyl glycidyl ether.289 The resin can be cured thermally and is suitable as one component resin.

3.5.2 Recycling 3.5.2.1

Solvolysis

The recycling of wastes of epoxy resins is very difficult, because of the inherent infusibility and insolubility of the materials. Often the composite materials contain reinforcing fibers, metals, and fillers.293 Efficient destruction of the organic material in composites can be achieved by thermolysis processes or by incineration processes. These methods yield considerable amounts of non-combustible residues or decomposition products that are not attractive for further use. Valuable recycled materials can be obtained by solvolysis methods. Here, the depolymerization products and reinforcing fibers can be retrieved. By glycolysis with diethylene glycol, the ester linkages of a bis epoxide (diglycidyl ether of bisphenol A) that is cured with a di anhydride, are cleaved. The transesterification is catalyzed by titanium n-butoxide. In the case of 70% glass fiber/epoxide-anhydride composites, the glass fibers can be recovered. The liquid depolymerization products may be converted to polyols, components for unsaturated polyester resins, etc. The glycolysis of amine cured epoxide resins shows no volatile nitrogen compounds. The most favored path of degradation is the decomposition of the ether linkage of bisphenol A to yield oligomers with phenol groups.294 The separation of the phenolic compounds from the glycolysis products can be achieved by liquid-liquid extraction. The glycolysis products can be basically used as a polyol in production of polyurethane. However, the hydroxyl value is much too high for polyurethane production. It has been suggested to use the solvolysis products from epoxide resins in combination with other solvolysis products, e.g., solvolysis products from wastes from PET for semi interpenetrating networks based on PET hydrolyzate and epoxies.295

Epoxy Resins 3.5.2.2

201

Reworkable Epoxies for Electronic Packaging Application

Epoxy resins show excellent longevity and resistance to ageing. This is due to the formation of an insoluble and infusible crosslinked network during the cure cycle. This property is sometimes seen as a drawback from the repairability standpoint. During the manufacture of expensive electronic parts, such as multi chip modules, several chips are mounted onto one high density board. If one chip is damaged, then the whole board will become useless. The same is true if some special electronic parts in a board need to be modified because of progress in technology. Therefore, the availability of a reworkable material, that is, one that undergoes controlled network breakdown, expands the potential routes to repairing, replacing, or removing assembled structures and devices. Implementing reworkable materials early could expand recycling concerns that could be faced in the near future. An effective solution is to use thermally reworkable epoxide resins for underfilling.296, 297 In such systems, the cured epoxy network can be degraded by locally heating to a suitable temperature, and the faulty chip could be replaced. Commercial cycloaliphatic epoxides degrade at about 300°C. Epoxides with secondary or tertiary ester bonds (as shown in Figure 3.22) have been demonstrated to decompose at temperatures between 200°C and 300°C.216, 298 The epoxides are cycloaliphatic compounds and can be basically derived by the esterification of cyclohexenoic acid with α-terpineol with subsequent epoxidation. Diepoxides with carbamate and carbonate groups299 also degrade in this temperature range. In comparison to chemical degradation methods, heat to degrade the network can be localized more easily in the rework process, thereby allowing for more precise control over the region of the board that will be reworked. Instead of branched ester structures, ether structures, c.f. Figure 3.22, bottom, are also suitable candidates for thermolabile linkages in epoxides.231 Thio links can be used to form a reversible network.219 Further diepoxides connected via acetal links can be used for the introduction of reversible chemical links.300 This type of network can be degraded in acidic solvents.

202


O O

O

O CH3

O O

O

CH3

O O

O

O

CH3 CH3

O O

O

O

CH3

O CH3

O

O

CH3 CH CH2

O

O

1,2-Bis (2,3-epoxycyclohexyloxy) propane O

O

CH3 CH3 C

CH

O

O

CH3 2-Methyl-2,4-bis (2,3-epoxycyclohexyloxy) pentane

Figure 3.22: Epoxides with Thermally Cleavable Groups for Controlled Network Breakdown: Top Esters, Bottom Ethers. 1,2-Bis(2,3-epoxycyclohexyloxy)propane, 2-Methyl-2,4-bis(2,3-epoxycyclohexyloxy)pentane

Epoxy Resins

203

3.6 APPLICATIONS AND USES 3.6.1 Coatings The largest applications of epoxy resins are in coatings. Epoxy resin coatings have excellent mechanical strength and adhesion to many kinds of surfaces. They are corrosion resistant and resistant to many chemicals. Coatings find applications in various paints, white ware, and automotive and naval sectors, for heavy corrosion protection of all kinds. Epoxy coating formulations are available both as liquid and solid resins. Epoxy acrylic hybrid systems are used as coatings for household applications, e.g., indoor and outdoor furniture and metal products. Waterborne coatings are dispersions of special formulations of the resins with suitable surfactants. These materials can be applied by electrodeposition techniques. Powders can be applied as coatings by fluidizedbed techniques.

3.6.2 Foams Epoxy resins can be fabricated to make foams. Foamable compositions have been described from a novolak resin, an epoxy resin, and a blowing agent. Water can act as a blowing agent, especially when higher density foams are required. Novolak resins are typically suspended in an aqueous solution, that is the blowing agent.301 Encapsulated calcium carbonate or anhydrous sodium bicarbonate are suitable blowing agents.302 Phosphoric acid is used to catalyze the polymerization of resin and it also reacts with the carbonate core to generate a blowing gas to form voids.

3.6.3 Adhesives Approximately 5% of total epoxy resin production is used in adhesive applications. Epoxide resin adhesives are formulated two-component systems that cure at room temperature, and as hot curing systems in the form of films and tapes. Among others, acrylates are suitable modifiers for epoxy adhesives.

3.6.4 Molding Techniques Epoxy resins are used in all known reactive molding techniques. Non-reinforced articles can be molded with aluminum molds. This is used for

204


electrical coil covering, etc. In electronic industries various embedment techniques, i.e., casting and potting, and impregnation are important applications. Laminated sheets are used for the fabrication of printed circuit boards in the electronics industry. Pultrusion and lamination are common techniques. Laminated articles are also used in building constructions for concrete molds, honeycomb cores, reinforced pipes, etc. Epoxy resins are superior to polyesters where adhesion and underwater strength are important.

3.6.5 Stabilizers for Polyvinyl Chloride Epoxy resins with monofunctional epoxy groups in the prepolymer are effective in stabilizing polyvinyl chloride against dehydrochlorination during processing and use, in comparison to tribasic lead sulphate. Lead-based stabilizers for polyvinyl chloride are mostly banned and only allowed for a few applications. For example, the replacement of lead-based stabilizers by epoxy stabilizers will improve the environmental toxicity of lead in water flowing through PVC pipes.303

3.7

SPECIAL FORMULATIONS

3.7.1 Development of Formulations In practice, epoxy resins are composed of a wide variety of individual components. To obtain a composition with the desired properties, a great deal of know-how is required. A solid knowledge of the structure-property relationships can serve as a valuable tool for the art of formulation.304 On the other hand, there are methods that are helpful in the development of formulations. In particular, statistical methods can save time. An overview of such methods is given in the standard book of Box and Hunter.305 Instead, most studies on epoxy formulation are done by the “one variable at a time” method. This means that only one parameter of interest is changed while the other remaining parameters are kept constant. This strategy provides admittedly valuable information, however, it does not allow a good insight into mutual interactions of formulation parameters. The usefulness of statistical methods in the field of formulation of epoxy adhesives has been demonstrated in the literature.306

Epoxy Resins

205

3.7.2 Restoration Materials A variety of epoxy resins are used for the consolidation of stone monuments in an outdoor environment. For these applications good weathering resistance and sufficient penetration depth is mandatory. A suitable epoxy monomer for restoration materials is 3-glycidoxypropyltrimethoxysilane (GLYMO) and amine curing agent is (3-aminopropyl)triethoxysilane (ATS). This monomeric composition penetrates deep enough to exceed the maximum moisture zone and creeps beyond the damaged layers. The alkoxysilane groups are hydrolytically unstable and generate silanol groups which may crosslink with one another, and also form bonds to the hydroxyl groups present in the stone, thus anchoring the organic polymer onto the lithic matrix.307, 308 The curing kinetics of hybrid materials prepared from diglycidyl ether of bisphenol A and GLYMO has been investigated using poly(oxypropylene)diamine as a hardener.309 The total conversion of epoxy groups was found to decrease with increasing content of GLYMO. The experimental data scattered, which was attributed to an uncontrolled initial hydrolysis of GLYMO caused by the varying air humidity during the sample preparation.

3.7.3 Biodegradable Epoxy-polyester Resins Biodegradable epoxy-polyester resins consist of polyesters with pendent epoxidized allyl groups.230 These polyesters are synthesized from succinic anhydride and allyl glycidyl ether and butyl glycidyl ether with benzyltrimethylammonium chloride as catalyst. The butyl glycidyl ether acts as a diluent for the allyl functionalities, in order to reduce the amount of pendant allyl groups in the chain. The epoxidation of the polyesters is achieved by m-chloroperbenzoic acid. The epoxy-polyester resins can be cured with glutaric anhydride.

3.7.4 Swellable Epoxies Hydrophilic polymers find applications in medicine and agriculture, owing to their biocompatibility.310 Crosslinked structures, prepared from sucrose and 1,4-butanediol diglycidyl ether (1,4-BDE) are hydrogels with water regain values of 30%.311

206


The crosslinking is achieved with triethylamine or sodium hydroxide. Triethylamine gives rise to end-capped diethylamine groups. By this reaction the ethyl group is left behind as ethyl ether in the sucrose. The ring-opening polymerization of epoxy end-terminated poly(ethylene oxide) (PEO) can serve to synthesize crosslinked materials with an exceptional swelling behavior.312 These gels have attracted interest for use as drug delivery platforms.

3.7.5 Reactive Membrane Materials Reactive membrane materials can be prepared from 2-hydroxyethyl methacrylate and glycidyl methacrylate by radical photopolymerization. Enzymes, such as cholesterol oxidase, can be directly immobilized on the membrane by the reaction of the amino groups of the enzyme and the epoxide groups of the membrane. The immobilization improves the pH stability of the enzyme as well as its thermal stability. The immobilized enzyme activity remains quite stable.313 Poly(2-hydroxyethyl methacrylate) membranes can be also activated by direct treatment with epichlorohydrin. On such materials poly(L-lysine) could be immobilized.314 Such a membrane with immobilized poly(Llysine) can be utilized as an adsorbent for DNA adsorption experiments.

3.7.6 Controlled-release Formulations for Agriculture In order to introduce pendant dichlorobenzaldehyde functionalities as acetals, the epoxide functionalities in linear and crosslinked poly(glycidyl methacrylate) are hydrolyzed to diol groups. In the second step the pendant diol groups in the polymers are acetalized by dichlorobenzaldehyde.315 Dichlorobenzaldehyde is a bioactive agent that is slowly released under various conditions.

3.7.7 Electronic Packaging Application In flip-chip manufacturing, filled polymers serve as underfilling. Underfilling is the plastic material which is inserted in the gap between integrated circuit and the substrate. The gap is approximately 50 to 75 µ m wide. The underfilling is used to couple the chip and the substrate mechanically. It decreases the residual stress in the solder joints caused by thermal expansion.

Epoxy Resins

207

The materials used for underfilling should have good wetting characteristics, significant adhesion, high conductivity, and should not form voids. The prevention of void formation is essential for thermal conductivity. Low viscosity of the monomeric resin is essential to achieve void-free underfillings. A resin with a lower viscosity allows the addition of a greater amount of filler. The viscosity of a benzoxazine resin can be reduced by the incorporation of a low-viscosity epoxy resin. The benzoxazine resin imparts a low water uptake, a high char yield, and mechanical strength. The epoxy resin reduces the viscosity of the mixture and results in higher crosslinking density and improved thermal stability of the material. A melt viscosity of about 0.3 Pa s at 100°C can be achieved.316

3.7.8 Solid Polymer Electrolytes The interest in solid polymer electrolytes arises from the possibility of applications of polymer ionic conductors in energy storage systems, electrochromic windows, and fuel cells or sensors operating from subambient to moderate temperatures.317 Hosts for solid polymer electrolytes are poly(ethylene oxide) (PEO), segmented polyurethanes with poly(ethylene oxide)/ poly(dimethylsiloxane)318 and with poly(ethylene oxide)/perfluoropolyether319 blocks, respectively, as well as crosslinked epoxy-siloxane polymer complexes.320, 321 The copolymers are immersed in a liquid electrolyte (1 M LiClO4 in propylene carbonate) to form gel-type electrolytes. Solvent-free solid polymer electrolytes are based on polyether epoxy crosslinked with poly(propylene oxide) polyamines.322 The crosslinked polyether networks are doped with LiClO4 . The network is prepared by mixing epoxy monomer, the curing agent dissolved in acetone and LiClO4 . To obtain films the mixture is poured on plates and cured at elevated temperatures. The electric conductivity of the polymer electrolyte is dependent on interactions between ions and the host polymer.

3.7.9 Optical Resins 3.7.9.1

Lenses

In comparison to glasses, plastics have low density, i.e., comparative low weight, are fragmentation-resistant and can be easily dyed. Therefore, optical materials made from organic polymers are attractive for optical ele-

208


ments such as lenses of eyeglasses and cameras. However, the refractive index of the standard resins is relatively small. Therefore, there is a need to use materials with high refractive index and low chromatic aberration. The introduction of sulfur into the monomers raises the refractive index. Sulfur-containing resins have a high refractive index, low dispersion, and a good heat stability.23, 216 Components for epoxy resin with high refractive index are obtained from bis(3-mercaptophenyl)sulfone (BEPTPhS) and epichlorohydrin. A sulfur-containing curing agent is trimercaptotriethylamine (TMTEA) which can be obtained from triethanolamine. Besides sulfurcontaining epoxies, with tailor-made polyphosphazenes, refractive indices ranging from 1.60 to 1.75 can be achieved.323 3.7.9.2

Liquid Crystal Displays

In liquid crystal displays (LCDs), control of the alignment of the liquid crystal (LC) molecules is one of the most important issues with respect to the quality of LCDs. The rubbing method does not satisfy the recent demands for alignment quality. The photoalignment method reduces contaminations that lower the contrast ratio and electrostatic buildup that can cause failure of thin film transistors.324 Nematic liquid crystalline materials can be aligned homogeneously on a photoreactive polymer film when exposed to linearly polarized light. Thermal stability and photostability of the alignment layer is a crucial parameter and the alignment layer must be transparent in the visible region for a display device. Certain photocrosslinkable polymer systems meet these demands. Derivatives of cinnamic ester and cinnamic acid are suitable candidates for phototransformations. In particular, the anisotropic [2+2] cycloaddition of the cinnamate moiety can induce an irreversible alignment of a low molecular weight liquid crystal. Polymers with the chalcone group in the side chain react in a similar way. A chalcone-epoxy compound can be synthesized from 4,4′ -Dihydroxychalcone and epichlorohydrin in the same way as with bisphenol A. In this photoreactive epoxy oligomer, the photosensitive unsaturated carbonyl moieties are located in the main chain. For the polymerization of the epoxy groups, triarylsulfonium hexafluoroantimonate (TSFA) is a suitable photoinitiator. The photodimerization of the chalcone precedes the photopolymerization of the epoxy groups. Under continuous irradiation, the anisotropic

Epoxy Resins

209

O HO

C CH CH

OH

1,3-Bis-(4-hydroxy-phenyl)-propenone

Figure 3.23: 4,4′ -Dihydroxychalcone

photocrosslinked chain molecules can be frozen by the photopolymerization of the epoxy groups at both ends of the compound. Without a photoinitiator, the end groups of the oligomer are not fixed. Therefore, there are two kinds of photochemical reactions that enhance the photostability of the induced optical anisotropy.24 3.7.9.3

Holography

Materials for high-resolution holograms, which can be used on holographic optical elements such as heads-up display, consist of a bisphenol-type epoxy resin and a radically polymerizable aliphatic monomer. A diaryliodonium salt and 3-ketocoumarin (KCD) are used as a complex initiator. The formation of the image is based on the radical polymerization of the monomer initiated by a holographic exposure, followed by the cationic polymerization of the epoxy resin by UV-exposure after post-exposure baking.325 3.7.9.4

Nonlinear Optical Polymers

Second-order nonlinear optical (NLO) polymeric materials are of interest because of their potential applications in integrated optical devices, such as waveguide electro-optic modulators, switches, and optical frequency doubling devices. The interest in these polymeric materials is mainly due to their large optical nonlinearities, low dielectric constants, and ease of production. For practical use, the poled polymers must possess large secondorder optical nonlinearities which should be sufficiently stable at ambient temperature for a long period of time. A high crosslinking density and stiffness makes interpenetrating networks attractive for such applications. The possibility of introduction of chromophores that impart the nonlinear optical properties is essential.

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An example for an NLO active interpenetrating polymer network is an epoxy prepolymer and a phenoxy-silicon polymer. 4,4′ -Nitrophenylazoaniline (Disperse Orange 3) functionalized with crosslinkable acryloyl groups is incorporated into the epoxy prepolymer. The epoxy prepolymer forms a network through acryloyl groups which are reactive at high temperatures without the aid of any catalyst or initiator. The phenoxy-silicon polymer is based on an alkoxysilane dye made of (3-glycidoxypropyl)trimethoxysilane and Disperse Orange 3, and 1,1,1-tris(4-hydroxyphenyl)ethane, as a multifunctional phenol. The two networks are formed simultaneously and separately at 200°C.326 Interpenetrating polymer networks based on crosslinked polyurethane/epoxy based polymer can be obtained by simultaneously crosslinking phenol-capped isocyanates with 2-hydroxypropyl acrylate and curing epoxy prepolymers. To each of these components phenylazo-benzothiazole chromophore groups are linked. The crosslinked polyurethane and the epoxy based polymer show glass transition temperatures of 140 and 178°C, respectively, whereas the interpenetrating network shows two Tg ’s at 142 and 170°C. Thin, transparent poled films of the crosslinked polymers can be prepared by spin-coating, followed by thermal curing and corona poling at 160°C. The polymers exhibit a long-term stability of the dipole alignment at 120°C.327

3.7.10 Reactive Solvents Polymers can be processed more easily by using solvents. The disadvantage the necessary removal of the solvent. This might be tedious and a time consuming step. Also, environmental hazards may arise. Reactive solvents are those that polymerize after the molding process. In this case, no removal is necessary. Accordingly, intractable polymers can be processed by the utilization of reactive solvents. The polymers are dissolved in a liquid curable resin. Then the homogeneous solution is transferred into a mold. The curing of the reactive solvent takes place in the mold. In the course of curing, molecular weight of the resin increases. A phase separation and phase inversion are likely to take place. The dissolved polymer should become the continuous matrix, and the reactive solvent is dispersed as particles in the matrix. So the final properties of the system are dominated by the properties of the thermoplastic phase. The main advantage of this procedure is a lower processing temper-

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ature because of decrease with viscosity. There is no need to remove the solvent which is bounded to the polymer. 3.7.10.1

Poly(butylene terephthalate)

Although poly(butylene terephthalate) can be relatively easily processed, a further improvement of the processing is required when a difficult flow length or mold geometry has to be mastered.328 3.7.10.2

Poly(phenylene ether)

Poly(2,6-dimethyl-1,4-phenylene ether) (PPE) can be dissolved at elevated temperatures in an epoxy resin and the solution can be easily transferred into a mold or into a fabric.329 During the curing of epoxy resin, a phase separation and a phase inversion occurs. The originally dissolved PPE then becomes the continuous phase. The dispersed epoxy particles become an integral part of the system and act as fillers or as toughening agents, depending on the type of epoxy resin. An important parameter for the final physical and mechanical properties is the size of the dispersed particles. The size of the dispersed phase is governed by the competition between the coalescence of dispersed droplets, and the vitrification or gelation rate, respectively, induced by the curing process. For the coalescence, the viscosity of the system plays an important role which is dependent on the curing temperature. The viscosity can be further controlled by adding another thermoplastic material such as poly(styrene). Blends of poly(phenylene ether) and an epoxy resin cured with dicyandiamide materials show a two-phase morphology. To improve the uniformity and miscibility, triallyl isocyanurate (TAIC) can be used as an insitu compatibilizer.330 Also the fracture toughness of the modified systems is improved by adding TAIC.

3.7.11 Encapsulated Systems Photopolymerizable liquid encapsulants (PLE) for microelectronic devices may offer important advantages over traditional transfer molding compounds. A PLE is comprised of an epoxy novolak-based vinylester resin, fused silica filler, a photoinitiator, a silane coupling agent, and optionally of a thermal initiator.331

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3.7.12 Functionalized Polymers The epoxy group can be used to functionalize various polymers, to achieve certain desired properties. 3.7.12.1

Tougheners

Vinylester-urethane hybrid resins (VEUH) can be toughened by functionalized polymers.332 Suitable basic materials for toughening are nitrile rubber, hyperbranched polyesters, and core/shell rubber particles. These materials can be functionalized with vinyl groups, carboxyl groups, and epoxy groups. Toughness is improved in VEUH when the functional groups of the modifiers react with the secondary hydroxyl groups of a bismethacryloxy vinylester resin and with the isocyanate groups of the polyisocyanate compound. Functionalized epoxy and vinyl hyperbranched polymers are less efficient as toughness modifiers in comparison to functionalized liquid nitrile rubber. They show no adverse effect on the mechanical properties.

3.7.13 Epoxy Resins as Compatibilizers Most polymers are not miscible with one another. This lack of miscibility results in poor properties of polymeric blends. However the properties can be improved by adding compatibilizers. Due to the inherent reactivity of the epoxy group, an interfacial chemical bonding can be achieved which results in small particle sizes of the blend. This enhances the properties of the blends. Some compatibilizers based on epoxy compounds are shown in Table 3.15. 3.7.13.1

Polyamide Blends

Blends of polyamide 6 and epoxidized ethylene propylene diene (e-EPDM) can improve the toughness of polyamide 6. The particle size of e-EPDM is much smaller than that of unepoxidized EPDM (u-EPDM) rubber in a polyamide 6 matrix. It is believed that the epoxy group in e-EPDM reacts with the polyamide 6 to form a graft copolymer. Thus an interfacial compatibilization takes place.333 Styrene/glycidyl methacrylate (SG) copolymers are miscible with syndiotactic poly(styrene) (s-PS). In blends of polyamide 6 (PA6) with syndiotactic poly(styrene), the epoxide units in the SG phase are capable

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Table 3.15: Compatibilizers Based on Epoxy Compounds for Various Polymers Polymer 1

Polymer 2

Compatibilizer

PA6 PA6

PS ABS

PA6

PP

PBT PBT

PPE SAN

Styrene/glycidyl methacrylate copolymers Glycidyl methacrylate/ methyl methacrylate copolymers (GMA/MMA) Poly(ethylene) functionalized with maleic anhydride Low molecular weight epoxy compounds Terpolymers of methyl methacrylate, glycidyl methacrylate (GMA), and ethyl acrylate

PA6 PS PBT ABS PP SPE SAN

Polyamide 6 Poly(styrene) Poly(butylene terephthalate) Acrylonitrile butadiene styrene (ABS) copolymers Poly(propylene) Poly(phenylene ether) Styrene/acrylonitrile copolymers

of reacting with the PA6 end groups. Copolymers of styrene/glycidyl methacrylate are effective in reducing the s-PS domain size and improving the interfacial adhesion. The best compatibilization is found with a content of 5% glycidyl methacrylate (GMA) in the SG copolymer. Both the strength and modulus of the blend are improved by the addition of the SG copolymers. However, a loss in toughness is observed at loadings of copolymer. The addition of SG copolymer to the blend has little influence on the crystallization behavior of the polyamide component. The crystallinity of s-PS is reduced.334 Blends of nylon 6 with acrylonitrile/butadiene/styrene (ABS) copolymers and with styrene/acrylonitrile copolymers (SAN) can be prepared using glycidyl methacrylate/methyl methacrylate (GMA/MMA) copolymers as compatibilizing agents.335 Known compatibilizers for blends of low density poly(ethylene) (LDPE) and polyamide 6 (PA6) are ethylene/acrylic acid copolymers (EAA), maleic anhydride functionalized polyethylenes, and an ethylene/glycidylmethacrylate copolymer (EGMA). The effectiveness of EGMA as a reactive compatibilizer is comparable to that of the EAA copolymers. However the effectiveness is lower than that of poly(ethylene) functionalized with maleic anhydride. A possible reason is the reaction of the pendent

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epoxy groups with the amide groups that attach the polyamide molecules together and hinder the dispersion in this way.336 In blends of poly(propylene) and polyamide 6, poly(ethylene) functionalized with maleic anhydride showed better compatibilization than glycidyl methacrylate. The compatibilizing effect of the PP-MA for the PP/Ny6 blends was more effective than poly(propylene) functionalized with glycidyl methacrylate.337 Glycidyl methacrylate copolymers are miscible with SAN. The epoxide unit can react with the polyamide end groups. The compatibilizer can form graft copolymers at the polyamide/SAN interface during melt processing. Incorporation of the compatibilizer does not significantly improve the impact properties of nylon 6/ABS blends. The direct mixing of polyamide and poly(propylene) leads to incompatible blends with poor properties. Poly(propylene) functionalized with glycidyl methacrylate can be used as a compatibilizer in the blends of PP and nylon 6.338 3.7.13.2

Poly(butylene terephthalate)

Poly(butylene terephthalate) (PBT) and poly(phenylene ether) (PPE) can be compatibilized by low molecular weight epoxy compounds.339 Terpolymers of methyl methacrylate, glycidyl methacrylate (GMA), and ethyl acrylate are effective reactive compatibilizers for blends of (Poly(butylene terephthalate) (PBT) with styrene/acrylonitrile copolymers (SAN) or ABS materials.340 During melt processing, the carboxyl end groups of PBT react with epoxide groups of GMA to form a graft copolymer. In blends of poly(butylene terephthalate) with an ethene/ethyl acrylate copolymer (E/EA), which show the general features of uncompatibilized polymer blends, such as a lack of interfacial adhesion and a relatively coarse unstabilized morphology, no evidence of transesterification reaction was found. In contrast, blends containing both virgin and modified E/MA/GMA terpolymers show a complex behavior. Two competitive reactions take place during the melt blending: 1. Compatibilization due to interfacial reactions between PBT chain ends and terpolymer epoxide groups, resulting in the formation of E/MA/GMA/PBT graft copolymer, and 2. Rapid crosslinking of the rubber phase due to the simultaneous presence of hydroxyl and epoxide groups on E/MA/GMA chains.

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The competition reactions between compatibilization and crosslinking are dependent on the type of the terpolymer, since the modified E/MA/GMA contains hydroxyl groups before mixing. The in-situ compatibilization reaction of the pendent epoxy groups with PBT causes the formation of E/MA/GMA hydroxyl groups.341 The concentration of carboxyl groups at the PBT chain ends influences the rate of compatibilization but not the final morphology. The lower the concentration, the slower the morphology development. Ternary blends of PBT/(E-MA-GMA/E-MA) exhibit a very fine morphology. Here the development of the morphology is mildly influenced by the crosslinking rate of the rubber phase caused by the shear rate in the mixing chamber.342

3.7.14 Surface Metallization Established methods for the metallization of a polymer surface are343 1. Electroless plating, 2. Vacuum deposition or metal spraying, and 3. Coating using a metallic paint. A more recent method has been described that utilizes the reduction of metal ions incorporated directly in the polymer. It has been shown that cobalt or nickel ions integrated in an epoxy network could be reduced to the pure metal by dipping the film in an aqueous NaBH4 solution.229

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4 Phenol/formaldehyde Resins Phenolic resins are known as the oldest thermosetting polymers. They still have many industrial applications in sectors such as automotive, computing, aerospace, and building. Reviews concerning phenolic resins are given, for example, by Gardziella and by Burkhart.1–3 Phenolic resins are thermosetting resins produced by the condensation of a phenol with an aldehyde wherein water is produced as a byproduct. Typically, the phenol is phenol itself and the aldehyde is formaldehyde, but substituted phenols and higher aldehydes have been used to produce phenolic resins with specific properties such as reactivity and flexibility. The variety of phenolic resins available is quite large as the ratio of phenol to aldehyde, the reaction temperature, and the catalyst selected can be varied.4 Phenolic resins fall into two broad classes: 1. Novolak resins, 2. Resol resins. Resol resins are single stage resins and novolak resins are two-stage resins. Resol resins are typically produced with a phenol, a molar excess of formaldehyde, and an alkaline catalyst. The reaction is controlled to create a non-crosslinked resin that is cured by heat without additional catalysts to form a three-dimensional crosslinked insoluble, infusible polymer. In contrast, novolak resins are typically produced with formaldehyde, at molar excess of phenol, and an acid catalyst. 241

242

Reactive Polymers Fundamentals and Applications Table 4.1: Types of Novolak Resins Novolak Resin Type High ortho novolak General-purpose novolak High para novolak

Ratio ortho:para 75:25 45:55 38:62

Suitable acid catalysts include the strong mineral acids such as sulfuric acid, phosphoric acid, and hydrochloric acid as well as organic acid catalysts such as oxalic acid, p-toluenesulfonic acid, and inorganic salts such as zinc acetate or zinc borate. The reaction produces a thermoplastic polymer that can be melted but will not crosslink upon the application of heat alone. The resulting novolak thermoplastic resin can be crosslinked by the addition of a novolak curing agent. There are various types of novolak resins with different ortho-to-para ratios of the methylene linkages: high ortho novolak resins (HON), general-purpose novolak resins (GPN), and high para novolak resins (HPN). The characterization is listed in Table 4.1. Resol resins require no additional curing agents. They can be cured by heat reactive. However, they have a low shelf life. Curing for resols and hexamine-cured novolaks proceeds at 150 to 200°C.

4.1

HISTORY

As early as in 1872, Baeyer∗ reported about reactions of phenols and aldehydes that give resinous substances. In 1899, Arthur Smith patented phenol/formaldehyde (PF) resins to replace ebonite as electrical insulation. In 1899, Arthur Smith filed patent application for a method for substituting ebonite, wood, etc.5 In 1907 Baekeland † mixed phenol and formaldehyde and obtained phenol/formaldehyde resins. In 1907 he filed the first of 117 patents on phenol/formaldehyde resin systems.6 Before he was engaged in phenolic resins, Baekeland worked on the development of copying papers. Such a product became famous under the name Velox. Formica was first produced by Herbert Faber and Daniel O’Conor as an electrical insulator in 1910. Formica is a composite that ∗ Adolf † Leo

von Baeyer, born in Berlin 1835, died in Starnberg 1917 Hendrick Baekeland, born in Gent 1863, died 1944

Phenol/formaldehyde Resins

243

Table 4.2: Phenolic Monomers Phenol

Remark/Reference

Phenol Bisphenol A Bisphenol F Bisphenol B Resorcinol Cresols m-Cresol p-Cresol 2-Cyclohexyl-5-methylphenol Xylenols m-Aminophenol m-Methoxyphenol β-Naphthol Cardanol Cardol

Most common 2,2-Bis(4-hydroxyphenyl)propane Bis(4-hydroxyphenyl)methane 2,2-Bis(4-hydroxyphenyl)butane Methylphenols Photoresists7 Photoresists7 Photoresists7 8 8

Table 4.3: Aldehyde-type Components Aldehyde

Remark/Reference

Formaldehyde Paraformaldehyde Butyraldehyde

Most common

Glyoxal Multihydroxymethyl ketones

9

Hot-melt adhesives and as binders for non-wovens10 Improved optical properties11 12, 13

consists of layers of paper impregnated with phenolic and melamine resins. In 1952 the first long-playing records and singles were manufactured from polyvinyl chloride which replaced shellacs and phenolics previously used.

4.2 MONOMERS Derivatives of phenol that are suitable for use for phenol/formaldehyde resins are listed in Table 4.2. They include bisphenol A, bisphenol B, resorcinol, cresols, and xylenols. Derivatives of formaldehyde that are suitable for use for phenol/formaldehyde resins are listed in Table 4.3. They include paraformaldehyde, acetaldehyde, propionaldehyde, butyraldehyde and glyoxal, trioxane, furfural, and furfurol.

244


CH3

CH3 + CH

CH CH3

CH2

CH3

H+ -H2O

CH3 C O OH

O C

CH3

CH3

OH CH3

C

CH3

O

Figure 4.1: Synthesis of Phenol and Acetone

4.2.1 Phenol The peroxidation of cumene is the preferred route to phenol, accounting for over 90% of world production. The process which is also referred to as the Hock™ Process or Cumox™ Process, consists of 1. Liquid-phase oxidation of cumene to cumene hydroperoxide (CHP), and 2. Decomposition of the concentrated CHP to phenol and acetone. The synthesis is shown in Figure 4.1. The main use of phenol is as a feedstock for phenolic resins, bisphenol A, and caprolactam. It is also used in the manufacture of many products including insulation materials, adhesives, lacquers, paint, rubber, ink, dyes, illuminating gases, perfumes, soaps, and toys.

4.2.2 o-Cresol o-Cresol is used mostly as an intermediate for the production of pesticides, epoxy resins, dyes, and pharmaceuticals, but also as a component of disinfectants and cleaning agents. o-Cresol is readily biodegradable and has a


245

Table 4.4: Uses of Formaldehyde Chemical Phenol/formaldehyde resins urea/formaldehyde resins Wood adhesives Foundry materials Polyacetal resins 1,4-Butanediol Methylene bis(4-phenyl isocyanate) Pentaerythritol Controlled-release fertilizers Melamine/formaldehyde resins Paraformaldehyde Chelating agents Herbicides Trimethylolpropane Pyridine chemicals Neopentyl glycol Nitroparaffin derivatives Textile chemicals Trimethylolethane

low bioaccumulation or geoaccumulation potential. Approximately 60% of o-cresol is obtained from coal-tar and crude oil by using classical techniques such as distillation, stripping, and liquid-liquid extraction. The remaining 40% is obtained synthetically by the alkylation of phenol with methanol.

4.2.3 Formaldehyde Formaldehyde is a basic industrial chemical. It is used for the production of a variety of chemicals, as shown in Table 4.4. Formaldehyde is a colorless, highly flammable gas that is sold commercially as 30 to 50% aqueous solutions. Formaldehyde is used predominantly in the synthesis of resins, with urea/formaldehyde resins, phenolic-formaldehyde resins, pentaerythritol, and other resins. About 6% of uses are related to fertilizer production. Formaldehyde find application in a variety of industries, including the medical, detergent, cosmetic, food, rubber, fertilizer, metal, wood, leather, petroleum, and agricultural industries.14

246


O H3C C CH2

CH2

(O CH2)n OH CH2O

O H3C C CH2

CH2

(O CH2)n OH -1

Figure 4.2: Multihydroxymethylketones


Mill. Metric tons

Formaldehyde Benzene Bisphenol A Phenol Phenolic resins Resorcinol

24 44 2 6.4 2.9 0.046

Year

Reference

2003 2003 1999 2001 2001 2002

16 17 18 19 20 21

4.2.4 Multihydroxymethylketones Multihydroxymethylketones are the reaction products of ketones with a large excess of formaldehyde.13 They are used as reactive solvents for melamine and other applications, but also can act as a source of formaldehyde, because they will decompose back, as shown in Figure 4.2. A mixture of phenol in a multihydroxymethylketone produces a special type of a modified phenol/formaldehyde resin.

4.2.5 Production Data of Important Monomers Production data of raw materials for phenolic resins are shown in Table 4.5. Only a minor part of the formaldehyde produced is consumed for making phenol resins. Bisphenol A is also used in other resin systems, mainly for epoxide resins.


247

4.2.6 Basic Resin Types 4.2.6.1

Novolak Resins

A novolak resin is a precondensate consisting of at least one phenol, or a phenol derivative, and at least one aldehyde. Novolak resins are used, for example, in rubber preparations which serve the production of belts, tubes and tires. These resins can reinforce the rubber preparations by contributing hardness and high moduls with low deformation after curing. The reinforcement is explained by the formation of a three-dimensional network within the rubber upon curing. 4.2.6.2

Resol Resins

Phenolic resol resins are typically made by condensation polymerization of phenol and formaldehyde in the presence of a catalyst at temperatures between 40°C and 100°C. An alkaline catalyst is essential. If an acid catalyst would be used, an uncontrolled curing during the preparation of the prepolymer would occur. On the other hand, in principle, curing of the resol prepolymer could be achieved by acidifying. Due to the low yield of the phenol and formaldehyde condensation under the normal reaction conditions, a typical resol resin contains a high percentage of free monomers, i.e., phenol and formaldehyde. These free monomers are volatile and highly toxic. Reducing the level of free monomers in such resins, thus reducing their emissions into the environment during application processes, has been one of the most heavily researched areas by both phenolic resin producers and resin users for many years.22 Resol refers to phenolic resins that contain useful reactivity, as opposed to the cured resins. At this stage, the product is fully soluble in one or more common solvents, such as alcohols and ketones, and is fusible at less than 150°C.


Modification with Lignin

Lignin (poly(phenylpropane) units) from waste black can be used for a partial substitution of the phenol in a phenol/formaldehyde resin. The amount

248


C4H9

S OH H9C4

OH

H2C

HO

C4H9

OH CH2

S

C4H9

Figure 4.3: 2,14-Dithiacalix[4]arene

of replaced phenol with lignin in the resin can be increased by hydrolysis of the lignin with hydrochloric acid.23 The modification of PF resins with cornstarch and lignin promotes the condensation reactions. Increased molar masses and a high yield of methylene bridges are found.24

4.2.7.2

Hydrogen peroxide modifier for particleboards

The addition of H2 O2 to a phenolic resin results in greater reactivity of the phenolic resin and increases the mechanical properties of particleboards. No significant influence of H2 O2 on the water resistance of the particleboards has been observed.25

4.2.7.3

Calixarenes

Calixarenes are cyclic phenol/formaldehyde oligomers. They have unusual and interesting properties. 2,14-Dithiacalix[4]arene, c.f. Figure 4.3, can be prepared by acid-catalyzed cyclocondensation of 2,2′ -thiobis[4-tert-butylphenol] with formaldehyde.26


O

OH

O

249

O

-

OH

O

-

O HCHO

CH2

OH

O

CH2OH

H

-

O

O

O

HCHO

H CH2

OH

CH2OH

Figure 4.4: Reaction Mechanism for the Addition of Formaldehyde on Phenol in Basic Medium27

4.2.8 Synthesis 4.2.8.1

Mechanism

The basic mechanism of the addition of formaldehyde is shown in Figure 4.4. The catalyst can be a hydroxide anion and a metal cation. The hydroxide anion contributes to the formation of phenates by abstracting the alcoholic proton. The rate constants correlate with the radius of the metal cation, as shown in Table 4.6. The metal hydroxide catalysts can be classified into two families according to the valency of the cation: KOH, NaOH, and LiOH; and Ba(OH)2 and Mg(OH)2 . 4.2.8.2

Kinetic Models

The kinetics of the polymerization of resol has been modelled taking into account the phenol and formaldehyde equilibria. The kinetic parameters

250

Reactive Polymers Fundamentals and Applications Table 4.6: Rate Constants, and Ionic Radius27 Cation K+ Na+ Li+ Ba2+ Ca2+ Mg2+

k[l mol−1 h−1 ]

Ionic radius [Å]

0.106 0.119 0.153 0.164 0.226 0.413

3 4 6 5 6 8

have been obtained by adjusting the experimental data. The influence of the type and amount of catalyst, the initial pH, the initial molar ratio of formaldehyde to phenol and the condensation temperature on the kinetic rate constants can be described.28 4.2.8.3

Preparation

A resol type phenol/formaldehyde resin may be prepared by reacting a molar excess of formaldehyde with phenol under alkaline reaction conditions. Formaldehyde is used in an amount between about 0.5 and 4.5 mol per mol of phenol, with the preferred ranges dependent on the application. The free formaldehyde is typically between 0.1% and 15%. The free phenol content is typically between 0.1% and 20%. Reaction Conditions. Alkaline reaction conditions are established by adding an alkaline catalyst to an aqueous solution of the phenol and formaldehyde reactants. During the initial reaction of the phenol and formaldehyde, only that amount of alkaline catalyst necessary to produce a resin need be added to the reaction nature. Typically, an amount of 0.005 to 0.01 mol of alkaline catalyst per mol of phenol is used. Sodium hydroxide is the most popular catalyst. Polycondensation of phenol and formaldehyde is typically carried out at a temperature in the range from about 30°C to about 110°C, over a reaction time of about 1 hour to about 20 hours, using a formaldehyde to phenol mole ratio in the range from about 1 to about 6.22 Formaldehyde to Phenol Ratio. A typical phenolic resin to be used as a binder for fiberglass is made at a formaldehyde/phenol mole ratio as high


251

as 6, to virtually eliminate free phenol in the resin. The high formaldehyde/phenol ratio required to achieve the very low free phenol concentration results in free formaldehyde concentrations as high as 20%. The high percentage of free formaldehyde in the resin must be scavenged by the addition of a large amount of urea or any other formaldehyde scavengers.22 4.2.8.4

Structure

A part of a structure of a novolak resin and a resol resin is shown in Figure 4.5. A resol prepolymer differs from a novolak resin in that it contains not only methylene bridges but also reactive methylol groups and dimethylene ether bridges. 13 C-NMR spectroscopy has proven to be the most successful and informative analytical tool to analyze resol resins. Using chromium(III)acetylacetonate as a relaxation agent, quantitative 13 C-NMR spectra can be obtained.29

4.2.9 Catalysts The common catalysts for the phenol/formaldehyde resol synthesis are shown in Table 4.7. The catalyst type influences the rate of reaction of phenol and formaldehyde and the final properties of the resins.27 The substitution of phenol with formaldehyde in the ortho-position versus para-position increases in the following sequence of hydroxide catalyst metals: K < Na < Li < Ba < Sr < Ca < Mg.30 Among the tetraalkylammonium hydroxides, it is advantageous to use tetramethyl- or tetraethylammonium hydroxides as catalysts rather than tetrapropyl- or tetrabutylammonium hydroxides, because the resins prepared with the last two catalysts have a limited miscibility of the resins obtained with water.30 4.2.9.1

Inorganic Catalysts

Phenolic resins are widely used as binders in the fiberglass industry. Most resins for the fiberglass industry are catalyzed with inorganic catalysts because of their low cost and non-volatility. When an inorganic base-catalyzed phenolic resin is mixed with urea solution, a so-called premix or prereact, certain components of the phenolic resin, such as tetradimers, crystallize out, causing the blockage of lines,

252


OH

OH

OH

CH2

CH2

CH2

OH CH2 OH CH2 Novolak OH

OH

OH

CH2

CH2

CH2 OH CH2

CH2 O CH2 OH CH2OH Resol

Figure 4.5: Structure of Novolak and Resol Resins


253

Table 4.7: Common Catalysts for the Phenol/formaldehyde Resol Synthesis Catalyst

Reference

Sodium hydroxide, potassium hydroxide, lithium hydroxide Magnesium hydroxide, calcium hydroxide, barium hydroxide Sodium carbonate Calcium oxide, magnesium oxides Tertiary amines, triethylamine 2-Dimethylamino-2-methyl-1-propanol, 2-(dimethylamino)-2-(hydroxymethyl)-1,3-propanediol Tri(p-chloro phenyl)phosphine, triphenylphosphine Tetraalkylammonium hydroxide

31, 32

32 32 30

interrupting normal operations, and the loss of resin. The crystallized material is difficult to dissolve and hinders uniform application of the resin to the glass fiber. The tetradimer tends to crystallize in premix solutions of inorganic base-catalyzed resins and urea. Precaution must be taken with the inorganic base-catalyzed resins to avoid tetradimer crystal growth. For example, problems can be minimized by regular cleaning of the storage tanks and lines, and by shortening the time between the preparation and use of the premix solution.22 4.2.9.2

Organic Catalysts

Phenolic resins catalyzed with an organic catalyst are especially useful for applications where high moisture resistance and higher mechanical strength are required. When a phenolic resin such as PF resin catalyzed with an organic catalyst is mixed with an amino resin such as urea/formaldehyde (UF) resin, the resultant PF/UF or PF/U is expected to be much more storage stable and to have much less tetradimer precipitation or crystallization. The organic catalyst, unlike an inorganic base, will increase the solubility of the phenolic resin in the PF/UF solution. A PF/UF mixture or premix is often used as a binder in the fiberglass industry.22 The activation energy of curing of UF resins is generally higher than that of PF resins, but the curing rates of UF resins are faster than those

254


of PF resins.33 Tertiary amino alcohols have been found to be very effective catalysts for the polycondensation of phenol and formaldehyde, and yet they are essentially non-volatile so that attendant amine emissions are negligible. Because the tertiary amino alcohols are organic catalysts, they produce resins which are essentially ashless, and thus are particularly useful in the manufacture of resins suitable for use in many industries. The resulting phenol/formaldehyde resol resin is characterized by the high moisture resistance and high mechanical strength of resins produced with the use of organic rather than inorganic catalysts. These organic catalysts also produce phenol/formaldehyde resol resins having superior tetradimer storage stability, when mixed with a formaldehyde scavenger such as urea.22 The tertiary amino alcohol catalyst remains in the resulting reaction product, and at least a portion of the catalyst becomes chemically bound to the polymeric matrix in the resol resin. The presence of the hydroxyl functionality on the amino alcohol molecule acts as a plasticizer and increases the flow of the hot resin melt, thereby increasing the resin efficiency and yielding a stronger bond of the resin with materials which are integrated with the resin, such as fiberglass. The chemical bonding of the catalyst to the polymeric matrix also further inhibits catalyst emissions in the finished product.22

4.2.10 Manufacture 4.2.10.1

Exothermic Hazards

The reaction of phenol with formaldehyde is highly exothermic. Therefore, there is a hazard situation owing to the high released heat in case of improper operation in industrial scale. From kinetic data, the conditions of a thermal explosion has been modelled.34

4.2.10.2

Distillation

When a distillation step is required, the distilled resin can be solvated in an alcohol, such as methanol, isopropanol, or ethyl alcohol. This is typical for paper saturating resins.35 These resins are usually neutralized to a pH of 6.5 to 7.5 with acid to give lighter color cure.


255

4.3 SPECIAL ADDITIVES 4.3.1 Low Emission Types It is often desirable to scavenge the free formaldehyde prior to application. This is done for several reasons: 1. To reduce the extent of human exposure during manufacture, 2. To reduce the free formaldehyde emissions during the forming and curing of the insulation product, 3. To reduce the free formaldehyde prior to the addition of an acid catalyst, 4. To reduce the cost of the binder, and 5. To improve the anti-punk properties of the resin.

4.3.1.1

Release of Phenol

A problem is the release of volatile organic components, such as phenol, into the atmosphere during curing. Typical levels of free phenol in a phenol/formaldehyde resin are in the range of 5 to 15%. One method of reducing the free phenol level in the base phenol/formaldehyde resin is to increase the amount of formaldehyde (relative to the phenol) in the resin as manufactured. Unfortunately this usually results in a more brittle resin that when cured is unacceptable for manufacturing postforming laminates.35

4.3.1.2

Urea Scavenger

Often urea cannot be added to the phenolic resin by the manufacturer, because the mixture of phenolic resin and urea, i.e., the premix, is not sufficiently stable to permit its storage for two to three weeks without tetradimer precipitation.22 If urea is added as scavenger to the phenolic resin in a premix system, it lasts many days before the resin has to be used. During this time, virtually all the free formaldehyde in the resin reacts with the added urea. The free formaldehyde content in the premix can then be as low as 0.1%. The use of such a ready-for-sale premix system reduces the emission of free monomer.22

256 4.3.1.3

Reactive Polymers Fundamentals and Applications Scavengers for Formaldehyde

The most common scavengers for formaldehyde are chemical species containing a primary or secondary amine functionality. Examples include urea, ammonia, melamine, and dicyandiamide. The most common, and the most economical, amine species is urea.35 The addition of formaldehyde scavengers to a phenol/formaldehyde resin requires a finite period of time to achieve equilibrium with the free formaldehyde in the resin. The process of reaching this equilibrium is referred to as prereaction, and the time to reach the equilibrium is referred to as the prereact time. Prereact times vary with temperature and amine species. When urea is used, the prereaction times range from 4 to 16 hours, depending on temperature. The mole ratio of formaldehyde to formaldehyde scavenger is important, and the conditions must be optimized to achieve the best performance of the binder resin. With urea, the mole ratio of formaldehyde to urea is optimally maintained between 0.8 and 1.2. At a lower level, the opacity increases significantly along with the ammonia emissions. At higher levels the formaldehyde emissions increase and the risk of precipitation of dimethylolurea is greatly increased. That is why in traditional binders using urea as the formaldehyde scavenger, the extension level is dictated by the amount of free formaldehyde in the base resin. However, disadvantages arise when the resins are prereacted with urea prior to forming the binder. Because free formaldehyde stabilizes the tetradimer in the resin, prereacting with urea will reduce the amount of free formaldehyde in the resin, hence reducing the shelf life of the formulation. In addition, prereacting with urea takes time, requires prereact tanks and binder tanks, and urea needs to be stored in heated storage tanks. 4.3.1.4

Over-condensing

The reduction of residual free formaldehyde can be achieved by over-condensing the resin.36 An over-condensed resol denotes a resin in which a relatively high proportion of large oligomers is formed at the end of the condensation stage. It has a high average molecular weight, higher than 500 Dalton. Such a resol is obtained by increasing the reaction time or the reaction temperature to ensure a virtually quantitative conversion of the initial phenol while going beyond the monocondensation to monomethylolphenols. It contains a very low proportion of free phenol and volatile


257

phenolic compounds capable of polluting the atmosphere at the site of use. 4.3.1.5

Aqueous Dispersions of Phenol/formaldehyde Resins

Aqueous dispersions of phenol/formaldehyde resol resins are frequently used in the manufacture of mineral fiber insulation materials, such as insulating glass fiber batts for walls, in roofs and ceilings, and insulating coverings for pipes. Typically, after the glass fiber has been formed, the still hot fiber is sprayed with aqueous binder dispersion in a forming chamber or hood, with the fibers being collected on a conveyer belt in the form of a wool-like mass associated with the binder. In some cases, a glass fiber web is sprayed with the aqueous dispersion. Both resol and urea-modified resol resins have been employed for this purpose. The urea contributes to the punking resistance of the binder (i.e., resistance to exothermic decomposition at elevated temperatures), and reduces volatile compounds when the resin is cured at elevated temperatures.37 To improve the performance of the binder for glass fibers, a lubricant composition, such as a mineral oil emulsion, and a material promoting the adhesion of the resol resin to the glass fibers, such as a suitable silane, can be added. An example of an adhesion-improving silane is (3-aminopropyl)triethoxysilane.37

4.3.2 Boric Acid-modified Types Boric acid-modified phenolic resins (BPFR) show excellent performance, such as thermal stability, mechanical strength, electric properties and further shielding of neutron radiation.38 Because bisphenol F has a methylene group, it shows a higher freedom of rotation in contrast to bisphenol A-based materials. The reaction of bisphenol F, formaldehyde and then with boric acid is shown in Figure 4.6. At elevated temperatures the boric acid forms a six-membered ring structure containing a boron oxygen coordination bond.39 The curing reaction of BPFR follows an autocatalytic kinetics mechanism.40 The cured structure of BPFR formed from the paraformaldehyde method is different from BPFR formed from the formalin method. The structure in this curing BPFR does not contain ether bonds and carbonyl groups. The thermal stability of this BPFR is better than BPFR formed from formalin.41

258


HO

CH2

OH

CH2O CH2

HO CH2 HO

CH2

OH

OH

H3BO3

HO

CH2

OH

HO CH2

CH2

O

HO CH2

CH2

O

B OH

HO

H2C

CH2

CH2 O O CH2 B O O H

OH

CH2

Figure 4.6: Reaction of Bisphenol F with Formaldehyde and Boric Acid


259

CH2

R

O CH2

R

Figure 4.7: Formation of Xanthens42

The weight loss for a common bisphenol F resin is more than 99% at 580°C, while the boron-modified resin shows only 45% at 700°C. The situation is completely similar for a bisphenol A resin.38

4.3.3 Fillers Many fillers are known for PF resins, such as glass fibers, ceramic material, and organic fiber materials. 4.3.3.1

Jute Reinforcement

Jute textile can be recycled into composites using 12 to 30% of phenol/formaldehyde (PF) resin. The dimensional stability of the produced composites can be improved by acetylating or by steam treatment of the jute textile. Steaming the jute textile is superior to acetylation in improving the dimensional stability. A steamed jute textile exhibits much less irreversible and reversible swelling than acetylated or untreated jute textile.43

4.3.4 Flame Retardants 4.3.4.1

Pyrolysis products

The main products of pyrolysis of both novolaks and resols are phenol, 2-methylphenol, 4-methylphenol, 2,4,6-trimethylphenol, and xanthens.42 Xanthens arise because of cyclization reactions, as shown in Figure 4.7.

260


OH

OH H3C OH H3C

CH2

O

H2C

CH3

OH CH2

CH3

CH3 OH

CH3

H3C

OH H2C

CH3

CH3

CH3

Figure 4.8: Self-condensation of 2-Hydroxymethyl-4,6-dimethylphenol

4.3.4.2

Brominated Phenol/formaldehyde Resin

A brominated phenol/formaldehyde resin with ca. 10% bromine has been shown to be a good plywood adhesive that shows a high shear strength, good flame retardancy, and good resistance to both hot and cold water.44

4.4

CURING

4.4.1 Model Studies The mechanism of curing has been investigated using model compounds. 2-Hydroxymethyl-4,6-dimethylphenol condenses at 120°C into bis(2-hydroxy-3,5-dimethylbenzyl)ether and bis(2-hydroxy-3,5-dimethyl-benzyl)methylene as shown in Figure 4.8. The ether is formed much faster than the methylene compound. Phenol does not act as an acid catalyst for ether hydrolysis. Previous results suggest that during curing at temperatures above 150°C, quinone methides have been proposed as key intermediates. However, at temperatures below 150°C, quinone methides have not been considered as important, which has been contradicted.45 Figure 4.9 illustrates the formation of quinone methides. Quinone methides can be formed by the intramolecular dehydration of 2-hydroxymethyl-4,6-dimethylphenol. Further, they can be formed by a retro Diels-Alder reaction of a trimer. With phenol and 2-methylphenol, a quinone methide attacks exclusively


OH H3C

OH CH2

O -H2O

H3C

CH3

O H3C

261

CH2

CH3

OH

OH CH2

CH3

H3C

OH CH2

CH3

Figure 4.9: Formation and Reaction of Quinone Methides

the free ortho site of the phenol. Therefore, a high ortho bridged resin should be formed under conditions that favor the formation of an ortho quinone methide. This would require a resin which contains predominately ortho hydroxymethyl substituents, and condensation at high temperature, preferably in solvents which encourage the dehydration of the ortho hydroxymethyl functionality.45 Phenoxy bridges are shown to be formed by ether exchange between phenolic OH and a bridging ether. Evaluation of nonisothermal DSC curing data by isoconversional analysis revealed that the activation energy changes with conversion in the course of curing. The data were interpreted to show that the curing process of phenol/formaldhyde resins undergoes a change in the reaction mechanism from a kinetic to a diffusion regime.46

4.4.2 Experimental Design Factorial experiments have been conducted to find the effect of the monomer feed on the structure of resol resins.47, 48 The amount of ortho and para methylol phenols increases with the F/P ratio. An increased condensation viscosity also increases the weight-average molecular weight. Among the parameters investigated, the viscosity has the strongest effect on the molecular weight. Several other useful relations could be established by the statistical approach.

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4.4.3 Water Content The amount of water in a powder resol resin was shown to play an important part in the curing kinetics. In the initial curing stages, water acts as a diluent and retards the curing. At the higher conversions, water acts as a plasticizer and contributes to enhancing the final conversion.49

4.4.4 Influence of Pressure Curing under high-pressure conditions reveals competition between the oxidation and polymerization reactions. This results in fewer methylene bridges and more free ortho-positions. Thus, a consequent lower degree of polymerization is reached.50

4.4.5 Wood The activation energy of the curing reaction of a PF resin generally increases when PF resin is mixed with wood. This is caused mainly by the decrease of the pH resulting from the presence of wood.51 Further, wood decreases the curing enthalpy. This effect can be interpreted in that the final conversions are lowered.

4.4.6 Novolak Curing Agents Several curing agents for novolak resins are known in the art, including formaldehyde, paraformaldehyde, and hexamethylenetetramine. The most common curing agent is hexamethylenetetramine, which reacts upon heating to yield ammonia and cured resin. These curing agents complete the crosslinking reaction to convert a thermoplastic novolak resin to an insoluble infusible state.4 4.4.6.1

Hexamethylenetetramine

When hexamethylenetetramine is used, ammonia evolves during curing of the novolak resin. In addition, novolak curing agents like hexamethylenetetramine typically require curing temperatures as high as 150°C. The cure temperatures can be lowered by the addition of acids, but this often introduces other problems such as die staining, die sticking, and sublimation of organic acids into the atmosphere.

Phenol/formaldehyde Resins 4.4.6.2

263

Triazine-type Hardeners

Hardeners of the triazine-type are alkoxylated melamine/formaldehyde resins or alkoxylated benzoguanamine-formaldehyde resins. These hardeners have a water solubility of less than 15% by weight and contain from 1 to 2.5 melamine or benzoguanamine rings per molecule. About 7% to 15% of triazine hardener is used. The triazine hardeners are prepared from melamine or benzoguanamine and formaldehyde with at least 4 mol formaldehyde per mol melamine or benzoguanamine to produce melamine/formaldehyde resins or benzoguanamine-formaldehyde resins, e.g., hexakis(methylol)melamine in the case of a melamine/formaldehyde resin. These formaldehyde resins are subsequently alkoxylated with, e.g., butoxymethyl groups.52 Melamine resins typically require either an acid catalyst or elevated temperatures to cure a novolak resin. Melamine resin curing agents also tend to cure novolak resins more slowly than hexamethylenetetramine. They produce a lesser extent of cure, and frequently produce formaldehyde in a side reaction.4 4.4.6.3

Substituted Melamines

The most common methylene donor for crosslinking novolak resins is hexamethylenetetramine (HMTA), but it has the following drawbacks: • It raises problems of health and safety. • When novolak resins are used with HMTA in the presence of rubbers intended to adhere to metal reinforcements, this bond may take deteriorate. HMTA as hardener can be replaced by another methylene donor, hexa(methoxymethyl)melamine (H3M).8 This can be used in conjunction with a urea, amide, or imide, such as propionamide. This compound liberates methanol instead of ammonia in the course of curing.8

4.4.7 Resol Resin Hardeners Resol resins have also been used as a curing agent for novolak resins.53 A comparatively large amount of resol is required to achieve a reasonable crosslink density.

264


A disadvantage is that resol resins have a limited shelf life, caused by self-condensation wherein the phenolic nuclei are bridged by methylene groups. Resol resins may contain significant levels of free phenol and formaldehyde that may present environmental concerns. Conventional resol resins typically contain 4 to 6% free phenol and may contain approximately 1% free formaldehyde.4 4.4.7.1

Benzoxazine Curing Agent

An alternative curing agent for a novolak resin is a benzoxazine polymer.54 Benzoxazine may be an intermediate product in the reaction of hexamethylenetetramine (HMTA) and phenol or substituted phenols. Benzoxazine is the Mannich product of a phenolic compound, an aldehyde and a primary amine. A benzoxazine polymer composition may be manufactured by combining an alcoholate of an amino triazine such as melamine, guanamine, benzene guanamine, an aldehyde and a resol, and allowing these to react under conditions favorable to benzoxazine formation.

4.4.8 Ester-type Accelerators Certain esters can accelerate the curing of PF resins, for example, ethyl formate, propylene carbonate, γ-butyrolactone, and triacetin.55 A mechanism for the action of these accelerators has been proposed. The first step consists of transesterification with the methylol group of the resin. Then, the ester group is attacked by another aromatic compound in the ortho or para position, or it is converted to a reactive quinone methide intermediate, which reacts by the quinone methide mechanism.

4.4.9 Ashless Resol Resins Ashless resins are prepared from organic ingredients; no inorganic catalyst should be used. The catalysts commonly used in phenolic resin production are sodium hydroxide and triethylamine (TEA). TEA is very volatile and toxic. Its emission into the atmosphere is regulated by government agencies. Ashless and low-ash phenolic resol resins having no amine odor can be prepared by reacting phenol and formaldehyde in the presence of a low volatile and strongly basic tertiary amino alcohol, such as 2-dimethylamino-2-methyl-1-propanol (DMTA) or 2-(dimethylamino)-2-(hydroxymeth-


265

Table 4.8: Basicity of Amines and Aminoalcohols Amine

pHa

Triethylamine 2-Dimethylamino-2-methyl-1-propanol 2-(Dimethylamino)-2-(hydroxymethyl)-1,3-propanediol (80%) a 0.010 N aqueous solution

10.8 10.7 10.6

yl)-1,3-propanediol (DMAMP).22 These tertiary amino alcohols have a boiling point above 250°C. A comparison of the pH of 0.010 N solutions of these amino alcohols along with that of triethylamine is shown in Table 4.8, indicating that DMTA and DMAMP (80%) are as basic as TEA.22

4.4.10 Recycling 4.4.10.1

Porous Fiberboard from Waste Newspapers

Flame retardant and waterproof porous fiberboards can be manufactured from waste newspapers by using a foaming agent and a reinforcing phenol/formaldehyde resin.56 A water-soluble phenol/formaldehyde resin of the resol type is used in amounts of 11% to obtain best quality product. To increase the porosity, a foaming agent is admixed.

4.4.10.2

Sewage Treatment Process

Modified wastes from phenol/formaldehyde resins and expanded poly(styrene) can be used in sewage treatment processes.57 Amino derivatives of novolak wastes and sulfonated derivatives of novolak and expanded polystyrene wastes are synthesized. These compounds are basically anionic polyelectrolytes, and they exhibit good flocculation properties in purification processes of a sewage waters of coal-mines, or steel plants. The purification is effected in that the impurities in the waste water interact with the polymeric material thus forming insoluble particles. The main mechanisms of destabilization of the polymeric electrolyte are bridging of the individual molecules by the impurity mosaic flocculation, and charge neutralization. In mosaic flocculation, the polyelectrolyte adsorbs locally onto the impurity, so that opposite charged regions may be formed.58

266

Reactive Polymers Fundamentals and Applications Table 4.9: Uses for Phenol/formaldehyde Resins35

4.5

Use

Remarks

Insulation Plywood and engineered lumber Oriented strand board High pressure laminating resins Paper saturating resins Open or closed cell foams Abrasive binders Friction binders Coated foundry sand binders

Coating fibers Wafer board resins Use for flat surfaces Oil filter, overlay, paint roller tubes Floral foam supports


Phenol/formaldehyde resins are used to make a variety of products including consolidated wood products such as plywood, engineered lumber, hard board, fiberboard, and oriented strand board. Other products include fiberglass insulation, laminates, abrasive coatings, friction binders, foams, foundry binders, and petroleum recovery binders. They are also used as paper saturating resins for oil filters, overlay, paint roller tubes, etc.35 Uses of phenol/formaldehyde resins are summarized in Table 4.9. Phenol/formaldehyde foam resins are used to make open or closed cell foams when cured. Such foams are primarily used to make floral foam supports for retaining flower stems in water. The foam is able to soak up water many times its original weight to provide water for the flowers. These foams are primarily open cell (with openings in cell walls). Other uses for phenol/formaldehyde foams are dense foams used for models (similar to balsa wood), to hold jewelry, and to make molds for foot prosthetics. Closed cell foams find use in barrier and insulation applications. Further uses of phenol/formaldehyde resins include abrasive binders, friction binders, and phenol/formaldehyde coated foundry sand binders.

4.5.1 Binders for Glass Fibers Glass fibers are generally mass produced in two types: 1. Bulk or blown fiber for insulation and allied applications, 2. Continuous-filament, or reinforcing fibers.


267

In either form, the raw fiberglass is abrasive and fragile. Damage to the individual glass fibers can occur as a result of the self-abrasive motion of one fiber passing over or interacting with another fiber. The resulting surface defects cause reduction in the overall mechanical strength of the fiberglass. Consequently, binders have been developed to prevent these problems. A typical binder may prevent the destructive effects of self-abrasion without inhibiting the overall flexibility of the finished glass fiber product. Good resistance and resilience to extreme conditions of elevated humidity and temperature are beneficial in view of the wide variety of applications of glass fiber/binder compositions. The amount of binder present in a fiberglass product is dependent on several factors including the product shape, the type of service required, compressive strength requirements, and anticipated environmental variables such as temperature.59 4.5.1.1

Phenolic Binders

Traditionally, the performance parameters required for insulation fibers have been satisfied only with phenol/formaldehyde resins. Therefore, glass fiber binders have been almost exclusively based on phenol/formaldehyde resins. These systems typically include aminoplast resins such as melamine and urea, silicone compounds, soluble or emulsified oils, wetting agents, and extenders or stabilizers. Typically phenolic binders contain large amounts of low molecular weight species including phenol, formaldehyde, and volatile phenol/formaldehyde adducts such as 2-methylolphenol and 4-methylolphenol. During the curing process, these volatile low molecular weight components are released into the atmosphere in substantial volumes as volatile organic compounds (VOCs). Since the process of manufacturing fiberglass typically involves spraying large volumes of phenol/formaldehyde binders into high volume air streams, and then curing the product in convection ovens that involve high volumes of air, fiberglass manufacturers have an urgent need to reduce their VOC emissions, particularly with regard to formaldehyde.59 Reducing the free formaldehyde content of typical phenol/formaldehyde binders affects the final product quality, because an excess of formaldehyde is essential for curing and bonding in such systems. Attempts

268


to convert free formaldehyde into less obnoxious and dangerous chemicals have involved the addition of ammonia or urea. Such additions were intended to convert free formaldehyde into hexamethylenetetramine or a mixture of mono and dimethylol ureas. Unfortunately, urea, hexamethylenetetramine, and mono and dimethylol ureas can all contribute to the production of trimethylamine, which gives the cured phenolic binder and finished product an undesirable fishy odor. In addition, nitrogen-containing compounds can decompose to yield ammonia and other potentially harmful volatile compounds. Phenol/formaldehyde resins require careful handling procedures. Since the cooked resin must be refrigerated until use, refrigerated trucks and holding tanks are required. Even with refrigeration, the storage time of a phenolic resin is typically 15 days.

4.5.2 Molding In the plastics molding field, phenolic resins have been a preferred choice as molding material for precision parts that must function in hostile environments. Phenolic resins form crosslinked structures with excellent dimensional, chemical, and thermal stability at elevated temperature.

4.5.3 Novolak Photoresists A positive photoresist composition can comprise a 1,2-naphthoquinonediazide compound and a novolak resin. The composition is sensitive to ultraviolet rays.7, 60 The photosensitizer is mainly a naphthoquinonediazidesulfonic acid ester. The content of the photosensitizer is 20 to 60 parts per 100 parts by weight of the substituted phenol novolak resin. Suitable phenols for this special application are mixtures of 2-cyclohexyl-5-methylphenol, mcresol, and p-cresol. The phenols are condensed with formaldehyde. Suitable solvents include methylisobutylketone or 2-heptanone.7 The unexposed photoresist is not soluble in alkaline medium. The insolubility is attributed due to an azo coupling of the sensitizer with the novolak polymer. Exposure to UV-light converts the o-diazonaphthoquinone into an indene carboxylic acid, c.f. Figure 4.10. The carboxylic groups enhance the solubility of the lacquer, so that the material becomes soluble in an alkaline medium.


269

O

O +

C

-

N N

hν SO3H

SO3H H2O

O C OH

SO3H

Figure 4.10: Conversion of o-Diazonaphthoquinone by Radiation

4.5.4 High Temperature Adhesives Resol melamine dispersions in which melamine is solubilized are used as high temperature adhesives, e.g., for glass fibers. The resin has low formaldehyde content and a high alkali ratio. The uncured resins compositions show improved water solubility.61

4.5.5 Urethane-modified Types Cured phenol/formaldehyde resins show considerable fragility and a low impact resistance. The hydroxymethyl groups present enable a chemical modification with urethane oligomers with isocyanate end groups. The addition of polyurethane improves the elasticity of the compositions and introduces coupling sites that increase the adhesion properties.62 In order to decrease the reactivity of the PF resin with oligomer isocyanate groups and to expand the shelf life of such compositions, the methylol groups can be etherified with butanol. The etherification with butanol proceeds in the presence of phosphoric acid.

270


4.5.6 Carbon Products Phenol/formaldehyde polymers are increasingly used as precursors for the production of carbon replacing ceramic and pitch-based materials in refractory applications. 4.5.6.1

Mechanism of Carbonization

The pore sizes of carbons obtained from phenol/formaldehyde resins depend strongly on the ratio of formaldehyde to phenol (FP) in the initial formulation. Higher formaldehyde-to-phenol ratios result in higher surface areas when measured with nitrogen, but similar surface areas measured with carbon dioxide.63 This leads to the conclusion that the microporous structure of the carbon powder is extremely narrow. A comparison of a resin with a molar ratio of phenol to formaldehyde (F/P) F/P=1.2 and F/P=1.8 showed that the differences between the cured resins persisted after heating to 400°C, when methylene bridge degradation becomes significant. However, no substantial differences are observable after heating to 500°C. 4.5.6.2

Carbon Membranes

Carbon membranes have gained great interest because of their thermal and chemical stability. Carbon membranes are classified according to pore size as microfiltration membranes. The mean pore diameter ranges from 0.02 to 10 µ m, usually from 0.1 to 1 µ m. Ultrafiltration membranes have a mean pore diameter from 1 to 100 nm: gas separation membranes have a mean pore diameter of less than 1 nm. Activated carbons can be prepared by controlled pyrolysis of either natural products, such as coconut shell at 800°C, coal at 400 to 600°C, wood at 300 to 500°C, or polymeric materials, such as phenol/formaldehyde resins at 900°C, poly(furfuryl alcohol) at 600°C, or polyimide at 550 to 800°C.64 Carbon molecular sieve membranes for the separation of hydrogen - nitrogen and hydrogen - methane mixtures have been prepared from a novolak phenol/formaldehyde resin. The liquid resin is used to form a film on a porous substrate by dip-coating. A carbon molecular sieve membrane is then obtained by carbonization of the film. The pore structure of the carbon membranes can be closely controlled by adjusting the degree of curing of the raw material. The final pore


271

diameter increases with the amount of hexamethylenetetramine used for curing.65, 66 Porous carbon membranes can also be formed on a macroporous clay support. The process consists of carbonizing a solvent-containing non-interpenetrating crosslinked resol-type phenol/formaldehyde (PF) resin film on a macroporous clay substrate. The porous structure of the membrane seems to result from the evaporation of solvent at the film-making stage, along with in-situ crosslinking and carbonization.64 4.5.6.3

Porous Carbon Beads

Porous carbon beads can be prepared by carbonization at 1000°C of phenol/formaldehyde beads under nitrogen or carbon dioxide atmosphere, followed by oxidation with boiling nitric acid.67 The carbonization atmospheres have a remarkable influence on the porosity development and structural changes of the resulting carbon spheres. In comparison with a N2 atmosphere, a CO2 atmosphere yields more surface pits, a higher surface area, and a higher micropore volume of the carbon spheres. 4.5.6.4

Carbon Urea Impregnation

Carbons from phenol/formaldehyde resins, which are glass-like, contain a high proportion of closed pores that are not accessible to gas molecules. Opening of these closed pores contributes to an increase in the porosity. Urea, which decomposes at 130 to 400°C, can be employed as an additive to the resin precursor. The escape of the degradation gases produced by the impregnated urea during resin carbonization promotes the formation of micropores in the resulting char. Carbons of ca. 2000 m2 /g can be obtained at 70% burn-off by 10% urea impregnation in the resin, while at a similar burn-off level, carbons obtained from pure resin can have a surface area around 1400 m2 /g.68 The burn-off level is the weight loss in % of the maximum weight loss. 4.5.6.5

Nitrogen-containing Carbon Catalysts

Activated carbons, because of their high accessible surface area, are used as supports for certain catalysts. Often, the presence on the surface of heteroelements, such as oxygen, nitrogen, and sulfur, stabilizes loaded metallic catalysts. Surface oxygen or surface nitrogen can effectively catalyze the

272


reduction of NO with NH3 at low temperatures, compared to metal oxide catalysts. Carbon catalyst supports containing nitrogen can be prepared using implantation of nitrogen by treatment with NH3 or HCN of a carbon that has been previously oxidized. Another method consists of the use of nitrogen-containing polymers. For example, active carbons containing up to 4.5% nitrogen can be prepared by carbonization in argon and steam activation of a vinylpyridine resin.69 Activated carbon catalysts can be produced from the phenol/formaldehyde resins that are prepared using ammonia. After synthesis, m-phenylene diamine (MPDA) dissolved in alcohol is added to the resin as an additional nitrogen source. Up to 30% of m-phenylene diamine is required by the process. The resin can be carbonized using a gasification method. This method consists of carbonization of the resin in N2 by heating at 10°C/min from room temperature to 800°C, followed by gasifying the resulting carbon in oxygen at 400°C.70 The main difference between pyrolysis and gasification as used here, is that pyrolysis is conducted in inert atmosphere, but gasification is a combined thermal treatment in inert gas, ie., N2 and an oxidizing gas, i.e. O2 . Ultimate analysis shows that up to 10% of nitrogen can be incorporated in the carbon char.

4.6


4.6.1 Chemical Resistant Types Alkaline resistance can be improved by etherification of the phenolic hydroxyl group. Not more than one third of the phenolic hydroxyl group should be etherified, otherwise the reactivity would decrease too much.

4.6.2 Ion Exchange Resins Commercially available ion exchange resins are produced from polymers such as phenol/formaldehyde, styrene-divinylbenzene, acrylonitrile, acrylates, and polyamines.71 These polymers can be modified by halomethylation, sulfonation, phosphorylation, carboxylation, etc. This additional reaction enables the production of an ion exchange resin with specific reactive sites, thereby exhibiting greater selectively towards particular metal ions or other anions


273

or cations. In conventional practice, the ion exchange resins are produced in bead or granular form, the bead size generally varying from 40 µ m to greater than 1 mm in diameter. A particular advantage of phenolic resins is that they are chemically resistent.

4.6.3 Brakes Phenolic-bonded composites for Industrial brake applications often contain partially dehydrated vermiculite particles to generate friction. Dehydration and rehydration processes of vermiculite should take place. The maximum detected temperature on the friction surface of certain investigated composite samples after friction test was 900°C.72

4.6.4 Waterborne Types Generally waterborne laminating resins are similar to the solvent-borne types except they lack an organic solvent and have usually lower molecular weight than their solvent-borne counterparts. Because they have lower molecular weight, they typically have a higher level of free phenol.

4.6.5 High Viscosity Novolak High molecular weight, thermoplastic phenol/formaldehyde is a suitable compatibilizer for poly(propylene)-phenol/formaldehyde resins. The materials can be blended by reactive extrusion. A phenol/formaldehyde resin with high molecular weight is required in reactive extrusion to obtain a favorable viscosity ratio. A mixture of phenol and cresols, tert-butylphenols, and resorcinol is used as phenol component. The resins are highly linear with a molecular weight in the range of 10 to 30 k Dalton.73

4.6.6 Foams In order to foam the resin, surfactants or wetting agents are mixed into the resin to create bubbles. Then a low boiling liquid such as CFC, HCFC, pentane or hexane is added to the mixture. A strong acid is added to the resin to initiate curing of the phenol/formaldehyde resin. This reaction generates heat causing the low boiling liquid to vaporize within the bubbles in the resin. Consequently a foam is created from this mixture. Typically,

274


within 10 minutes the foam rises to its maximum height and hardens when fully cured.

4.6.7 Visbreaking of Petroleum Mild thermal cracking (visbreaking) of the gas oil fraction boiling above 350°C can be achieved in the presence of 0.5% of a polymeric phenol/formaldehyde sulfonate (PFS) used as a promoter. The addition of PFS as a promoter accelerates the free radical chain reaction.23

4.7

TESTING METHODS

Several test methods are commonly used to characterize a phenolic resin.22 Subsequently,these methods are described.

4.7.1 Water Tolerance Distilled water at 25°C is gradually added to 10 g resin until the resin solution turns hazy. The water tolerance of a resin is an indication of the miscibility of the resin with water. It is an important parameter for resin used in fiberglass binders since the phenolic resin is normally diluted with water to a concentration as low as 2%. Maintaining a clear solution without phase separation at such dilution is essential for trouble-free processing and for high quality film properties. Typically a water tolerance of 25 times is required. The higher the water tolerance of the resin is, the lower is the molecular weight of the resin. A low molecular weight resin has more polar end groups than a more condensed resin.

4.7.2 Salt Tolerance For the salt tolerance test, a 10% sodium chloride solution is added to the phenolic resin solution gradually until the resin solution turns hazy. This is another method to measure the ability of the resin to mix with water and remain clear without precipitation, similar to water tolerance except that it is more challenging to the resin.


275

4.7.3 Free Phenol Content The free phenol content is measured by gas chromatography. It is the amount of phenol in the resin at the end of synthesis. A lower number is preferred for increased resin efficiency and lower emissions.

4.7.4 Free Formaldehyde The free formaldehyde content is measured commonly by the hydroxylamine titration method. This is the amount of formaldehyde left unreacted with phenol in the resin at the end of synthesis. A lower number is preferred for higher resin use efficiency and lower emissions.

4.7.5 pH The pH measures the basicity of the resin. A certain basic pH should be preferably maintained for the resin to be free of precipitation and to have a high water tolerance.

4.7.6 Solids Content The solids content measures the concentration of the phenolic resin which is not evaporable at the test temperature for the duration of the test. The phenolic resin placed in an aluminum dish and is kept in a 150°C oven for 2 hours.

4.7.7 o-Cresol Contact Allergy The presence of o-cresol was established as a contact sensitizer in a phenol/formaldehyde resin.74

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4. P. A. Waitkus and T. N. Morrison. Polymer composition for curing novolac resins. US Patent 6 569 918, assigned to Plastics Engineering Company (Sheboygan, WI), May 27 2003. 5. A. Smith. Verfahren zur Herstellung eines Ersatzmaterials für Ebonit, Holz u. dgl. DE Patent 112 685, October 10 1899. 6. L. H. Baekeland. Method of making insoluble products of phenol and formaldehyde. US Patent 942 699, assigned to Baekeland, December 7 1909. 7. K. Hashimoto, H. Osaki, and Y. Uetani. Positive resist composition comprising a novolac resin made from a cycloalkyl substituted phenol. US Patent 5 792 586, assigned to Sumitomo Chemical Company, Limited (Osaka, JP), August 11 1998. 8. B. M. Chauvin and O. Durel. Process and composition for the use of substituted melamines as hardeners of novolac resins. US Patent 5 763 558, assigned to Compagnie Generale Des Etablissements Michelin-Michelin & Cie (Clermont-Ferrand Cedex, FR), June 9 1998. 9. P. J. Gelling, J. E. B. Hunt, and J. D. Marshman. Continuous production of phenol-formaldehyde resin and laminates produced therefrom. US Patent 4 413 113, assigned to Formica Limited (North Shields, GB2), November 1 1983. 10. H. Petersen, H.-J. Krause, K. Fischer, A. Segnitz, and H. Zaunbrecher. Cocondensates based on phenol-butyraldehyde resins, their preparation and their use. US Patent 4 276 209, assigned to BASF Aktiengesellschaft (DE), June 30 1981. 11. A. H. Gerber. Phenol-novolacs with improved optical properties. US Patent 6 316 583, assigned to Borden Chemical, Inc. (Columbus, OH), November 13 2001. 12. R. Lubczak. Novel phenol-formaldehyde resins, 1 - novolaks. Macromol. Mater. Eng., 287(9):619–626, October 2002. 13. R. Lubczak. Novel phenol-formaldehyde resins, 2 - resols prepared using reactive solvents. Macromol. Mater. Eng., 288(1):66–70, January 2003. 14. R. G. Liteplo, R. Beauchamp, M. E. Meek, and R. Chénier. Formaldehyde. Concise International Chemical Assessment Document 40, World Health Organization, Geneva, 2002. 15. R. Gubler, editor. Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, 1950–to present. (Internet: http://ceh.sric.sri.com/). 16. S. Bizzari. Report “Formaldehyde”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, January 2004. (Internet: http://ceh.sric.sri.com/). 17. E. Gartner. Report “Benzene”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, September 2003. (Internet: http://ceh.sric.sri.com/).


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18. E. Greiner, T. Kaelin, and G. Toki. Report “Bisphenol A”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, February 2001. (Internet: http://ceh.sric.sri.com/). 19. S. Bizzari. Report “Phenol”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, February 2002. (Internet: http://ceh.sric.sri.com/). 20. E. Greiner. Report “Phenolic Resins”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, April 2002. (Internet: http://ceh.sric.sri.com/). 21. F. Hajduk and A. Kishi. Report “Resorcinol”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, September 2001. (Internet: http://ceh.sric.sri.com/). 22. G. Wu. Low-volatile and strongly basic tertiary amino alcohols as catalyst for the manufacture of improved phenolic resins. US Patent 5 623 032, assigned to Angus Chemical Company (Buffalo Grove, IL), April 22 1997. 23. H. I. Nabih, A. M. A. Omar, and R. M. Habib. Visbreaking of a heavy gas oil fraction promoted by phenol formaldehyde sulphonate polymer. Afinidad, 60(505):289–294, May–June 2003. 24. M. Turunen, L. Alvila, T. T. Pakkanen, and J. Rainio. Modification of phenol-formaldehyde resol resins by lignin, starch, and urea. J. Appl. Polym. Sci., 88(2):582–588, April 2003. 25. R. Czarnecki and J. Lecka. H2 O2 as a modifier of phenol-formaldehyde resin used in the production of particleboards. J. Appl. Polym. Sci., 88(14): 3084–3092, June 2003. 26. N. Kon, N. Iki, Y. Yamane, S. Shirasaki, and S. Miyano. Facile synthesis of thiacalix[n]arenes (n=4, 6, and 8) consisting of p-tert-butylphenol and methylene/sulfide alternating linkage and metal-binding property of the n=4 homologue. Tetrahedron Lett., 45(1):207–211, January 2004. 27. M.-F. Grenier-Loustalot, S. Larroque, D. Grande, P. Grenier, and D. Bedel. Phenolic resins: 2. influence of catalyst type on reaction mechanisms and kinetics. Polymer, 37(8):1363–1369, April 1996. 28. C. C. Riccardi, G. A. Aierbe, J. M. Echeverria, and I. Mondragon. Modelling of phenolic resin polymerisation. Polymer, 43(5):1631–1639, March 2002. 29. R. Rego, P. J. Adriaensens, R. A. Carleer, and J. M. Gelan. Fully quantitative 13 C NMR characterization of resol phenol-formaldehyde prepolymer resins. Polymer, 45(1):33–38, January 2004. 30. B. Kaledkowski and J. Hetper. Synthesis of phenol-formaldehyde resole resins in the presence of tetraalkylammonium hydroxides as catalysts. Polymer, 41(5):1679–1684, March 2000. 31. A. Knop and L. A. Pilato. Phenolic resins. Springer Verlag, Berlin, 1985. 32. J. Li. High catalyst phenolic resin binder system. US Patent 6 307 009, assigned to Owens Corning Fiberglas Technology, Inc. (Summit, IL), October 23 2001.

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33. G. B. He and B. Riedl. Phenol-urea-formaldehyde cocondensed resol resins: Their synthesis, curing kinetics, and network properties. J. Polym. Sci., Part. B: Polym. Phys., 41(16):1929–1938, August 2003. 34. K.-M. Luo, S.-H. Lin, J.-G. Chang, K.-T. Lu, C.-T. Chang, and K.-H. Hu. The critical runaway condition and stability criterion in the phenol-formaldehyde reaction. Journal of Loss Prevention in the Process Industries, 13(2):91–108, March 2000. 35. F. C. Dupre, M. E. Foucht, W. P. Freese, K. D. Gabrielson, B. D. Gapud, W. H. Ingram, T. M. McVay, R. A. Rediger, K. A. Shoemake, K. K. Tutin, and J. T. Wright. Cyclic urea-formaldehyde prepolymer for use in phenolformaldehyde and melamine-formaldehyde resin-based binders. US Patent 6 379 814, assigned to Georgia-Pacific Resins, Inc. (Atlanta, GA), April 30 2002. 36. B. Lericque, S. Tetart, C. Labbe, and P. Espiard. Phenolic resin for glue sizing composition, preparation method and glue sizing composition containing same. US Patent 6 342 271, assigned to Isover Saint-Gobain (Courbevoie, FR), January 29 2002. 37. V. Malhotra, W. Walliser, S. G. Watson, P. C. Herault, D. Tessari, P. Espiard, S. Tetart, and B. Malhieuxe. Low emission formaldehyde resin and binder for mineral fiber insulation. US Patent 6 646 094, assigned to CertainTeed Corporation (Valley Forge, PA); Borden Chemical, Inc. (Columbus, OH), November 11 2003. 38. J. Gao, Y. Liu, and F. Wang. Structure and properties of boron-containing bisphenol-a formaldehyde resin. Eur. Polym. J., 37(1):207–210, January 2001. 39. J. Gao, L. Xia, and Y. Liu. Structure of a boron-containing bisphenol-f formaldehyde resin and kinetics of its thermal degradation. Polym. Degrad. Stabil., 83(1):71–77, January 2004. 40. Y. F. Liu and J. G. Gao. Curing kinetics of boron-containing phenol-formaldehyde resin formed from paraformaldehyde. Int. J. Chem. Kinet., 34(11): 638–644, November 2002. 41. Y. Liu, J. Gao, and R. Zhang. Thermal properties and stability of boron-containing phenol-formaldehyde resin formed from paraformaldehyde. Polym. Degrad. Stabil., 77(3):495–501, 2002. 42. M. Sobera and J. Hetper. Pyrolysis-gas chromatography-mass spectrometry of cured phenolic resins. J. Chromatogr. A, 993(1-2):131–135, April 2003. 43. M. L. Hassan. Recycling of jute textile in phenol formaldehyde-jute composites. J. Appl. Polym. Sci., 90(13):3588–3593, December 2003. 44. A. Petsom, S. Roengsumran, S. Hanphichanchai, and P. Sangvanich. Brominated phenol-formaldehyde resin as an adhesive for plywood. J. Appl. Polym. Sci., 89(7):1918–1924, August 2003. 45. K. Lenghaus, G. G. Qiao, and D. H. Solomon. Model studies of the curing of resole phenol-formaldehyde resins. Part 1: The behaviour of ortho quinone methide in a curing resin. Polymer, 41(6):1973–1979, March 2000.


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46. G. He, B. Riedl, and A. Aït-Kadi. Model-free kinetics: Curing behavior of phenol formaldehyde resins by differential scanning calorimetry. J. Appl. Polym. Sci., 87(3):433–440, January 2003. 47. T. Holopainen, L. Alvila, P. Savolainen, and T. T. Pakkanen. Effect of f/p and oh/p molar ratios and condensation viscosity on the structure of phenolformaldehyde resol resins for overlays - a statistical study. J. Appl. Polym. Sci., 91(5):2942–2948, March 2004. 48. H. Holopainen, L. Alvila, T. T. Pakkanen, and J. Rainio. Determination of the formaldehyde-to-phenol molar ratios of resol resins by infrared spectroscopy and multivariate analvsis. J. Appl. Polym. Sci., 89(13):3582–3586, September 2003. 49. G. B. He, B. Riedl, and A. Ait-Kadi. Curing process of powdered phenolformaldehyde resol resins and the role of water in the curing systems. J. Appl. Polym. Sci., 89(5):1371–1378, August 2003. 50. N. Gabilondo, M. D. Martin, I. Mondragon, and J. M. Echeverria. Polymerization of formaldehyde and phenol at different pressures. High Perform. Polym., 14(4):415–423, December 2002. 51. G. B. He and B. Riedl. Curing kinetics of phenol formaldehyde resin and wood-resin interactions in the presence of wood substrates. Wood Sci. Technol., 38(1):69–81, April 2004. 52. A. H. Gerber. Curatives for phenolic novolacs. US Patent 5 648 404, assigned to Borden Inc. (Columbus, OH), July 15 1997. 53. K. Jellinek, A. Gardziella, K.-H. Schwieger, P. Adolphs, and J. Suren. Nonwoven textiles. US Patent 4 745 024, assigned to Rutgerswerke Aktiengesellschaft (DE), May 17 1988. 54. C. K. Johnson and J. P. Chen. Benzoxazine polymer composition. US Patent 5 910 521, assigned to Borden Chemical, Inc. (Columbus, OH), June 8 1999. 55. A. H. Conner, L. F. Lorenz, and K. C. Hirth. Accelerated cure of phenolformaldehyde resins: Studies with model compounds. J. Appl. Polym. Sci., 86(13):3256–3263, December 2002. 56. C. P. Chang and S. C. Hung. Manufacture of flame retardant foaming board from waste papers reinforced with phenol-formaldehyde resin. Bioresour. Technol., 86(2):201–202, January 2003. 57. W. M. Bajdur and W. W. Sulkowski. Application of modified wastes from phenol-formaldehyde resin and expanded polystyrene in sewage treatment processes. Macromol. Symp., 202:325–337, September 2003. 58. G. Tchobanoglous, F. L. Burton, and H. D. Stensel, editors. Wastewater Engineering : Treatment and Reuse. McGraw-Hill series in civil and environmental engineering. McGraw-Hill, Boston, 4th edition, 2003. 59. T. J. Taylor, W. H. Kielmeyer, C. M. Golino, and C. A. Rude. Emulsified furan resin based glass fiber binding compositions, process of binding glass fibers, and glass fiber compositions. US Patent 6 077 883, assigned to

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60.

61. 62.

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65.

66.

67.

68.

69.

70.

71. 72.

73.

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74. M. Bruze and E. Zimerson. Contact allergy to o-cresol—a sensitizer in phenol-formaldehyde resin. American Journal of Contact Dermatitis, 13(4): 198–200, December 2002.

5 Urea/formaldehyde Resins Urea/formaldehyde glue resins are the most important type of urea/formaldehyde-resins. Monographs on the chemistry of urea/formaldehyde resins include those by Dunky, Meyer and Pizzi, Dijk.1–4 The industrial production of urea/formaldehyde glue resins for the wood-working industry started in 1931. Environmental concerns demanded a change of the formulation of urea/formaldehyde-resins to decrease the molar ratio of formaldehyde to urea to avoid formaldehyde emissions.

5.1 HISTORY The reaction of urea with formaldehyde was first noted in 1884, with commercial interest in the polymers commencing at about 1918 with a patent issued to Hanns John (1891–1942).5–7 However, in 1896 Carl Goldschmidt described precipitates formed when aqueous solutions of urea and formaldehyde were reacted under acidic conditions.8 It is believed that the primary precipitate formed by Goldschmidt and empirically identified as C5 H10 O3 N4 was, in fact, a cyclically structured condensation product.9

5.2 SYNTHESIS OF RESIN 5.2.1 Formaldehyde Formaldehyde is available in many forms. Paraform (solid, polymerized formaldehyde) and formalin solutions (aqueous solutions of formaldehyde, 283

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sometimes with methanol, in 37%, 44%, or 50% formaldehyde concentrations) are commonly used forms. Formaldehyde is also available as a gas. Typically, formalin solutions are the preferred source of formaldehyde.

5.2.2 Urea Solid urea, such as prill, and urea solutions, typically aqueous solutions, are commonly available. Further, urea may be combined with another moiety, typically formaldehyde, often in an aqueous solution.

5.2.3 Ammonia Ammonia is used to reduce the free formaldehyde content. Ammonia is available in various gaseous and liquid forms, particularly including aqueous solutions at various concentrations. Any of the commercially-available aqueous ammonia-containing solutions are the preferred form. Such solutions typically contain between 10 to 35% ammonia. A solution having 35% ammonia can be used, providing stability and control problems can be overcome. An aqueous solution containing about 28% ammonia is particularly preferred. Ammonia or late additions of urea are commonly used to reduce free formaldehyde levels in urea/formaldehyde polymer systems. Ammonia reduces the cured polymers’ resistance to hydrolysis. The addition of urea tends to produce a polymer that releases smoke during the cure cycle.10

5.2.4 Diketones Small additions of acetylacetone and ammonia to urea/formaldehyde resin can bind the free formaldehyde.11 The addition causes the formation of 2,6-dimethyl-3,5-diacetyl-1,4-dihydropyridine (3,5-diacetyl-1,4-dihydrolutidine) by a Hantz reaction, as shown in Figure 5.1.


Cationic Urea Formaldehyde Resins

A water soluble cationic resin is prepared by initially reacting urea and formaldehyde at a formaldehyde to urea mole ratio of 2 to 3 together with triethanolamine in a urea to triethanolamine mole ratio of 2 to 3. The resin formed is made cationic by acidifying it to a pH of 1.5, with a strong

Urea/formaldehyde Resins

CH3

CH3 C O CH2 C O CH3

285

H 3C CH2O, NH4OH

C

O

H N H 3C

C

O CH3

Figure 5.1: Binding of Formaldehyde by Acetylacetone

inorganic acid such as hydrochloric, sulfuric, or nitric acid, followed by prompt neutralization to a pH of 6 to 7. A pH above 7 is discouraged as this retards the cure of the resin. Cationic urea formaldehyde resins with polymers containing vinylamine units improve the properties of paper with respect to dry strength and wet strength.12 Suitable polymers containing polymerized vinylamine units can be prepared by hydrolysis of homopolymers and copolymers containing polymerized N-vinylamide units. Examples for such polymers are a homopolymer of N-vinylformamide and a copolymer of methacrylic acid and N-vinylformamide. The amide group is often only partly hydrolyzed, say to an extent of 25%. The cationic urea/formaldehyde resins are infinitely dilutable with water. Aqueous solutions of cationic urea/formaldehyde resins typically have a solid content between 25 and 45%. The aqueous resin solutions or the solid products obtained therefrom are used as additives for increasing the dry and wet strength of paper in papermaking. The resins, in the form of aqueous solutions, are added to the paper stock prior to sheet formation.

Water-soluble Resins. Water-soluble cationic urea/formaldehyde resins are obtained by condensing urea and formaldehyde in the presence of polyamines. The reactants are first precondensed in alkaline pH range, then condensed in acidic pH range until gel formation begins. They are subjected to post-condensation, for example, with formaldehyde, and are subsequently neutralized.12

286 5.2.5.2

Reactive Polymers Fundamentals and Applications Melamine-modified Resins

Urea is the standard nitrogen-containing component in urea/formaldehyde resins. Resins with improved properties can be obtained by substitution of the urea with melamine. Sulfitation of the methylol groups can improve the resin properties. Still another approach is the co-condensation with amines and the introduction of urea-terminated amines. Melamine improves the resistance against attack by humidity and water, especially at elevated temperatures. Melamine contents up to 25% are used. 1,1,2,2-Tetramethoxyethane (TME) is a high boiling point diacetal (165°C). It can be synthesized from glyoxal. Such acetals improve the performance of melamine/urea/formaldehyde resins. The acetal as cosolvent increases the solubility of both the unreacted melamine and the oligomers in water. Thus a more effective reaction can be achieved.13 The improvement of mechanical properties by the addition of acetals such as methylal and ethylal occurs because of the by increased effectiveness and participation of the melamine to the crosslinking reactions.14 Iminoamino methylene base intermediates are obtained by the decomposition of hexamethylenetetramine in the presence of strong anions − such as SO2− 4 and HSO4 . These compounds improve the weathering resistance of hardened melamine/urea/formaldehyde resins.15

5.2.6 Polymerization The synthesis of a urea/formaldehyde (UF) resin proceeds via the methylolation of urea and condensation of the methylol groups. The reaction can be conducted in an aqueous medium because of the good solubility of both urea and formaldehyde. The basic reactions are shown in Figure 5.2. The methylolation of urea is done in alkaline or slightly acidic solution in a two-fold excess of formaldehyde. Following methylolation, further condensation into methylene urea oligomers occurs, with a degree of oligomerization of 4 to 8. Because of the functionality of the nitrogen, branched products can be formed. Ether bridges also may be formed. These ether bridges can be rearranged into methylene bridges, expelling formaldehyde. Dimethylol urea is not a stable compound. In the presence of another formaldehyde reactive compound, dimethylol urea will donate its two formaldehyde groups to the more stable phenol, ammonia, melamine, etc. This leaves raw urea in the resin which reduces the durability significantly.10


O

O + H 2C O

C H 2N

C

NH2

H 2N

NH CH2

OH

O C H 2N

NH CH2

OH

O

O

C H 2N

C NH CH2

O CH2 N

O

O

C H 2N

NH2

C NH CH2 N

NH2

Figure 5.2: Basic Reactions of Urea and Formaldehyde

287

288

Reactive Polymers Fundamentals and Applications Table 5.1: Feed for a Urea Formaldehyde Resin16 Reactant

mol

Formalin solution, 50% CH2 O 14.5 Ethylene diamine 0.3 Urea (first charge) 12.1 NH4 OH, 28% 6.1 UFC 85: water a 14.4 CH2 O 34.5 Urea 7.2 Urea (second charge) 3.5 Alum (KAlSO4 × 12H2 O) 50% 0.2 NaOH 25% 0.02 Latent catalyst 0.02 Water 1.6 a : 25% urea, 60% formaldehyde and 15% water

5.2.6.1

UF three-step preparation

The resin is prepared by reacting urea and formaldehyde in a three-step process. 1. Urea and formaldehyde are reacted in the presence of ammonia, at an excess formaldehyde of 1.2 to 1.8. A cyclic triazone/triazine polymer is formed at 85 to 95°C within 2 hours. 2. A thermosetting polymer is formed from the cyclic polymer. To the reaction mixture containing triazole/triazine polymer, an additional portion of formaldehyde is added, preferably with additional urea, to yield a higher cumulative F/U mole ratio of 2 to 2.7. The pH is adjusted to 6.0 to 6.4. 3. If the resin is not used immediately, a third neutralization step should be employed, preferably with sodium hydroxide. Use of ammonia or late additions of urea are common techniques for reducing free formaldehyde content of urea/formaldehyde polymer systems. 5.2.6.2

Synthesis Procedure

An example of a synthesis procedure is given here. The reactants that are used to prepare a urea/formaldehyde resin are listed in Table 5.1. The re-


289

sin is prepared by charging the 50% formalin, ethylene diamine, and urea into a reactor and heating the mixture to 45°C to dissolve the urea. Then NH4 OH is added which causes the mixture to have an exothermic reaction reaching a temperature of 83°C. The reaction mixture is heated further to 95°C and maintained at that temperature for 90 minutes. A cyclic polymer is formed in this initial phase of the chemical reaction. The triazone concentration can be over 50% of the total polymer mix at this stage of the synthesis, depending on the molar ratios of the ingredients. The pH of the mixture is maintained between 8.7 and 9.3 by adding 25% NaOH as needed ( a total of 0.4 mol). The reaction mixture is then cooled to 85°C. UFC 85 (25% urea, 60% formaldehyde, and 15% water) and a second charge of urea are added to the reaction mixture. The temperature is thereafter maintained at 85°C for 10 minutes. The pH is adjusted from about 6.2 to 6.4 by adding a total of 0.2 mol of alum (KAlSO4 × 12H2 O) in increments over a course of 25 minutes. The reaction mixture was cooled to 80°C, and after 15 minutes, further cooled to 75°C. After 7 minutes, the reaction mixture is cooled to 55°C, 26.9 g 25% NaOH is added, and then the mixture is further cooled to 35°C. A latent catalyst was added and the reaction mixture is cooled to 25°C. The pH is finally adjusted to 7.6 to 8.2 with 25% NaOH. The free formaldehyde content of the resin is 0.59%. After 24 hours the free formaldehyde content drops to 0.15%. The viscosity of the resin is 573 cP.

5.2.6.3

Cyclic Melamine/urea/formaldehyde Prepolymer

Cyclic urea prepolymers may be used as modifiers of thermosetting phenol/formaldehyde and melamine/formaldehyde-based resins for a variety of end uses. These prepolymers are urea/formaldehyde polymers containing at least 20% triazone and substituted triazone compounds. The use of cyclic urea prepolymer in such resin binders provides properties superior to those obtained from using the resin alone, in many applications. The resins are modified with the cyclic urea prepolymer, either by reacting into the base resin system, blending with the completed base resin system, or blending into a binder preparation. Suitable primary amines can be used in the formulation, such as methylamine, ethylamine, and propylamine, ethanolamine, cyclopentylamine, ethylene diamine, hexamethylene diamine, and linear polyamines. A methylolated cyclic urea prepolymer is typically prepared by re-

290


acting urea, ammonia, and formaldehyde and then reacting with 2 mol of formaldehyde to produce a methylolated cyclic urea prepolymer having 50% solids. 13 C-NMR indicates that 42.1% of the urea is contained in the triazone ring structure, 28.5% of the urea was di/tri-substituted, 24.5% of the urea is mono-substituted, and 4.9% of the urea is free. This cyclic urea prepolymer is then reacted into a standard phenol/formaldehyde resin during the cook cycle of the phenol/formaldehyde resin. In many systems, a cyclic prepolymer is either cooked into the resin or added to a resin.10

5.2.7 Manufacture The production of UF resins is usually achieved in three stages.3 1. Methylolation: urea reacts with aqueous formaldehyde under alkaline conditions at temperatures up to 100°C. 2. Condensation: the condensation of methylols in slightly acidic medium yields oligomers with different molar mass and various functionalities. The condensation is then stopped by adding alkaline substances. 3. Post treatment: evaporation of excess water and formaldehyde, or addition of secondary urea to decrease the ratio of formaldehyde to urea. The multistage process is useful to fulfill the requirements of retaining the reactivity and the strength of the cured resin under the condition of minimal emission of formaldehyde during service.

5.3

SPECIAL ADDITIVES

5.3.1 Modifiers A requirement is that the additives does not increase the viscosity of the suspension significantly, but would improve the toughness and the moisture resistance of UF resin. Thermoplastic acrylic copolymers with different degrees of hydrophilicity were added to a UF resin. The copolymers consist of two or three monomers selected from methyl methacrylate, acrylamide, acrylic acid, 1-vinyl-2-pyrrolidinone, ethyl acrylate, and vinyl acetate. The SEM micrographs of cured thermoplastic-modified UF showed a phase-separated thermoplastic structure in a continuous UF phase when


291


Mill. Metric tons

Urea Formaldehyde Amino resins

110 24 8.4

Year

Reference

2002 2003 2002

18 19 20

UF was modified with a self-dispersed and surfactant-stabilized polymer type. However, when the UF was modified with a water-soluble polymer, a single phase was detected.21, 22 A water soluble, styrene-maleic anhydride copolymer can be used a modifier for binder resins. Glass fiber mats made with the modified binder composition exhibit an enhanced wet tensile strength, wet mat strength, tear strength, and dry tensile strength. Because of this strength improvement, the mat processing speeds through the cure oven can be significantly increased without risking breakage of the continuous mat.23

5.3.2 Flame Retardants As flame retardants, ammonium hydrogenphosphate ((NH4 )2 HPO4 ) and sodium tetraborate (Na2 B4 O7 ) were tested together with mineral fillers such as vermiculite, phlogopite, clay, etc. The increased flame resistance results from the evolution of noncombustible gases.24

5.3.3 Production Data of Important Monomers Production data of important raw materials are shown in Table 5.2. Urea is mostly used as a fertilizer. Only a small fraction is used for urea/formaldehyde resins. Formaldehyde is used not exclusively for urea/formaldehyde resins. Other major uses are phenol/formaldehyde resins, polyacetal resins, pentaerythritol 1,4-butanediol and hexamethylenetetramine. Amino resins include melamine/formaldehyde resins and melamine/urea/formaldehyde resins, besides urea/formaldehyde resins.

5.4 CURING During curing, an insoluble, infusible, three-dimensional network is constructed. Curing is initiated by lowering the pH. This is achieved by the ad-

292


N H 2C O + (NH4)2SO4

N

N N + H2SO4

Figure 5.3: Reaction of Ammonium Sulfate with Formaldehyde

dition of acids, such as phosphoric acid or maleic acid. Acidic salts, e.g., aluminum sulfate, or urea phosphate can be added. Further anhydrides, such as maleic anhydride, decompose in aqueous medium into acids. Ammonium sulfate reacts with formaldehyde to form hexamethylenetetramine and sulfuric acid, as shown in Figure 5.3. Ammonium chloride is now avoided for acidification in favor of ammonium sulfate. Residual ammonium chloride forms hydrochloric acid during the combustion of wood-based panels. It is suspected that the chlorine promotes the formation of chlorodioxins. Usually 2 to 3% of ammonium salt-based on the solid content of resin are sufficient as catalyst. Excess catalyst causes over-curing. Brittle resins are then formed, with less water resistance. Formaldehyde is the primary reactive component in urea/formaldehyde resins. A higher reactivity and a higher crosslinking density of the final network formation can be achieved by a higher formaldehyde-to-urea ratio. On the other hand, free formaldehyde is undesirable for toxicological reasons. Resins with very low formaldehyde content exhibit several drawbacks of the final product. These can be minimized, however by a special condensation process, the use of special accelerators, and by the modification of the formulation with melamine.

5.5

MEASUREMENT OF CURING

Curing can be monitored by thermal methods, as well as utilizing spectroscopic methods. The curing reaction in an ammonium chloride catalyzed system starts at around 100°C, whereas in an uncatalyzed system the curing reaction starts between 120 to 180°C.25 In comparison to PF resins, the activation energy of curing of UF resins is generally higher. Nevertheless,


293

the curing rates of UF resins are faster.26 The pH values in UF formulations have a significant influence on the rate constants, but they affect the activation energy of curing marginally. The curing reaction in the presence of wood has been measured using 15 N-distortionless enhancement by polarization transfer nuclear magnetic resonance spectroscopy (DEPT NMR).27 A DEPT pulse sequence was employed to follow the curing of urea/formaldehyde resin.

5.6 PROPERTIES 5.6.1 Formaldehyde Release Typically, when urea/formaldehyde resins are cured, they release formaldehyde into the environment. Formaldehyde can also be released from the cured resin, particularly when the cured resin is exposed to acidic environments. Such formaldehyde release is undesirable, particularly in enclosed environments. Formaldehyde is malodorous and is considered hazardous to human and animal health. Various techniques have been used to reduce formaldehyde emission from urea/formaldehyde resins. Use of formaldehyde scavengers and methods for resin formulation, including addition of urea as a reactant late in the resin formation reaction, are techniques used to reduce formaldehyde emission. However, the use of formaldehyde scavengers often is undesirable, not only because of the additional cost, but also because it affects the properties, of the resin. For example, using ammonia as a formaldehyde scavenger reduces the resistance of the cured resin to hydrolysis. Later addition of urea to reduce free formaldehyde concentration in the resin generally yields a resin that must be cured at a relatively low rate to avoid smoking. The stability of the resin can also be adversely affected by such treatments.16 Instead of urea, triethanolamine can be added to a mixture of urea and formaldehyde.

5.6.2 Storage During storage, urea/formaldehyde resins undergo reactions that result in structural changes. Methylene groups adjacent to secondary amino groups are formed by the main reaction. This reaction proceeds between the free terminal hydroxymethyl and amino groups.28

294

5.7



5.7.1 Glue Resins The main application of urea/formaldehyde resins is in the adhesive industry. Urea/formaldehyde glues are used in pressed wood products such as particle board, and plywood as laminating resins. Because of the potential for formaldehyde release, UF resins have been modified for indoor applications. Formaldehyde is a potent primary irritant.

5.7.2 Binders Typical binders used to bind glass fiber mats include urea/formaldehyde resins, phenolic resins, melamine resins, bone glue, polyvinyl alcohols, and latices. These binder materials are impregnated directly into the fibrous mat and set or cured by heating to obtain the desired integrity in glass fibers. The most widely used glass mat binder is urea/formaldehyde, because it is relatively inexpensive.16

5.7.3 Foundry Sands In the manufacturing of low nitrogen-containing foundry sands, the hexamine crosslinker is replaced partly with another crosslinking agent that does not contain nitrogen. Nitrogen, when present in coated foundry sand can give rise to nitrogen defects during steel casting. It is preferable to have as low of nitrogen content as possible. Usually this other crosslinking agent is a thermosetting resol phenol/formaldehyde resin. During the manufacturing of low nitrogen-containing sands, a novolak resin is added, followed by the resol resin and then the hexamine.10

5.8


5.8.1 Ready-use Powders For small scale applications, e.g., as adhesive, ready-use powders of urea/formaldehyde resins are dissolved in water. The formulation contains fillers, extenders, hardeners, scavengers, and other additives which have to be mixed with water only.


295

5.8.2 Cyclic Urea Prepolymer in PF Laminating Resins Phenol/formaldehyde resins used to manufacture high pressure laminates are typically produced by reacting phenol and formaldehyde by means of an alkaline catalyst such as sodium hydroxide.10 Typical mole ratios of formaldehyde to phenol range from 1.2 to 1.9 mol of formaldehyde per mol of phenol. Catalyst levels range from 0.5 to 3%. The materials are reacted to a suitable endpoint, cooled under vacuum, and usually distilled to remove the water present from the formaldehyde solution as well as the water of condensation from the polymerization reaction. They may be used in this state or an organic solvent such as methanol can be added to reduce the solids concentration and viscosity of the mixture. A cyclic urea prepolymer in phenol-formaldehyde resins acts as a plasticizer for the resin. This makes the laminate more post-formable and tougher. Products produced with such resins resist chipping and breakage during machining steps. Diluting the phenol-formaldehyde resin with cyclic urea prepolymer reduces the free phenol and other volatile phenolic moiety levels of the phenol-formaldehyde resin which reduces air pollution. Because of the plasticizing effect achieved with the cyclic urea prepolymer, higher F/P mole ratio PF resins (traditionally more brittle) can be used which further reduces the free phenol and volatile phenolic moiety levels.

5.8.3 Liquid Fertilizer Urea/formaldehyde-based liquid fertilizers can provide nitrogen to the soil. In addition to nitrogen, phosphorous and potassium are considered major nutrients essential for plant growth. Long-term stability of high nitrogen liquid urea/formaldehyde fertilizers can be achieved by forming either a high percentage (more than 30%) of cyclic triazone structures or by condensing the urea/formaldehyde resin into small urea/formaldehyde polymer chains.29

5.8.4 Soil Amendment Urea/formaldehyde resin foams are used as a soil amendment for agricultural applications. The amendment by UF foams does not influence the pH

296


and causes insignificant alterations to the physical properties of the soil by slightly increasing total porosity, water availability, and the porosity, and by reducing the bulk density.30

5.8.5 Microencapsulation Self-healing polymers and composites with microencapsulated healing agents offer a possibility for long-lived polymeric materials. Healing agents have been microencapsulated using urea/formaldehyde resins. The microcapsules must have sufficient strength to remain intact during polymer processing. However, the microcapsules should break when the polymer is damaged.31

REFERENCES 1. A. Pizzi. Urea-formaldehyde adhesives. In A. Pizzi, editor, Advanced Wood Adhesives Technology. Marcel Dekker, New York, 1994. 2. B. Meyer. Urea-Formaldehyde Resins. Addison-Wesley, London, 1979. 3. M. Dunky. Urea-formaldehyde glue resins. In J. C. Salamone, editor, The Polymeric Materials Encyclopaedia: Synthesis, Properties and Applications, pages 1598–1599. CRC Press, Boca Raton, FL, 1999. 4. H. van Dijk. The chemistry and application of amino crosslinking agents or aminoplasts. In P. Thomas, editor, Waterborne & Solvent Based Surface Coating Resins and Their Applications, volume 5,2. Wiley, Chichester, 1999. 5. H. John. Verfahren zur Herstellung von Kondensationsprodukten aus Formaldehyd und Harnstoff bzw. Thioharnstoff oder anderen Harnstoffderivaten. AT Patent 78 251, October 9 1919. 6. H. John. Process for the manufacture of condensation products of formaldehyde and carbamide or carbamide derivatives. GB Patent 151 016, January 16 1922. 7. H. John. Manufacture of aldehyde condensation product capable of technical utilization. US Patent 1 355 834, assigned to Hans John (Prague), October 19 1920. 8. C. Goldschmidt. Über die Einwirkung von Formaldehyd auf Harnstoff. Ber. dtsch. Chem. Ges., 29:2438–2439, 1896. 9. J. E. Carlson. Cross-linked urea-formaldehyde polymer matrix compositions containing cyclic intermediate structures. US Patent 4 429 075, assigned to Chem-Nuclear Systems, Inc. (Columbia, SC), January 31 1984. 10. F. C. Dupre, M. E. Foucht, W. P. Freese, K. D. Gabrielson, B. D. Gapud, W. H. Ingram, T. M. McVay, R. A. Rediger, K. A. Shoemake, K. K. Tutin, and J. T. Wright. Cyclic urea-formaldehyde prepolymer for use in phenolformaldehyde and melamine-formaldehyde resin-based binders. US Patent


11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

297

6 379 814, assigned to Georgia-Pacific Resins, Inc. (Atlanta, GA), April 30 2002. V. Z. Maslosh, V. V. Kotova, and O. V. Maslosh. Effect of acetylacetone on the residual content of formaldehyde in urea-formaldehyde resin. Russ. J. Appl. Chem., 75(8):1369–1370, August 2002. K. Flory, A. Stange, M. Kroener, and N. Sendhoff. Cationic urea/formaldehyde resins, their preparation and their use in the paper industry. US Patent 5 478 656, assigned to BASF Aktiengesellschaft (Ludwigshafen, DE), December 26 1995. M. Zanetti, A. Pizzi, and P. Faucher. Low-volatility acetals to upgrade the performance of melamine-urea-formaldehyde wood adhesive resins. J. Appl. Polym. Sci., 92(1):672–675, April 2004. M. Zanetti, A. Pizzi, M. Beaujean, H. Pasch, K. Rode, and P. Dalet. Acetalsinduced strength increase of melamine-urea-formaldehyde (MUF) polycondensation adhesives. II. solubility and colloidal state disruption. J. Appl. Polym. Sci., 86(8):1855–1862, November 2002. C. Kamoun, A. Pizzi, and M. Zanetti. Upgrading melamine-urea-formaldehyde polycondensation resins with buffering additives. I. the effect of hexamine sulfate and its limits. J. Appl. Polym. Sci., 90(1):203–214, October 2003. L. R. Graves. Urea-formaldehyde resin composition and method of preparation thereof. US Patent 5 674 971, assigned to Georgia-Pacific Resins, Inc. (Atlanta, GA), October 7 1997. R. Gubler, editor. Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, 1950–to present. (Internet: http://ceh.sric.sri.com/). D. H. Lauriente. Report “Urea”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, July 2004. (Internet: http://ceh.sric.sri.com/). S. Bizzari. Report “Formaldehyde”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, January 2004. (Internet: http://ceh.sric.sri.com/). E. Greiner. Report “Amino Resins”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, February 2004. (Internet: http://ceh.sric.sri.com/). P. Rachtanapun and P. Heiden. Thermoplastic polymers as modifiers for urea-formaldehyde (UF) wood adhesives. I. procedures for the preparation and characterization of thermoplastic-modified UF suspensions. J. Appl. Polym. Sci., 87(6):890–897, February 2003. P. Rachtanapun and P. Heiden. Thermoplastic polymers as modifiers for urea-formaldehyde (UF) wood adhesives. II. procedures for the preparation and characterization of thermoplastic-modified UF wood composites. J. Appl. Polym. Sci., 87(6):898–907, February 2003.

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23. S.-G. Chang, L. R. Graves, C. R. Hunter, and S. L. Wertz. Modified ureaformaldehyde binder for making fiber mats. US Patent 6 084 021, assigned to Georgia-Pacific Resins, Inc. (Atlanta, GA), July 4 2000. 24. A. N. Egorov, Y. I. Sukhorukov, G. V. Plotnikova, and A. K. Khaliullin. Fireproofing coatings based on urea resins for metallic structures. Russ. J. Appl. Chem., 75(1):152–155, January 2002. 25. J. Lisperguer and C. Droguett. Curing characterization of urea formaldehyde resins by differential scanning calorimetry (DSC). Bol. Soc. Chilena Quim., 47(1):33–38, March 2002. 26. G. B. He and B. Riedl. Phenol-urea-formaldehyde cocondensed resol resins: Their synthesis, curing kinetics, and network properties. J. Polym. Sci., Part. B: Polym. Phys., 41(16):1929–1938, August 2003. 27. B. D. Gill, M. Manley-Harris, and R. A. Thomson. Use of natural abundance 15 N DEPT NMR to investigate curing of urea-formaldehyde resin in the presence of wood fibers. Magn. Reson. Chem., 41(8):622–625, August 2003. 28. P. Christjanson, K. Siimer, T. Pehk, and I. Lasn. Structural changes in urea-formaldehyde resins during storage. Holz als Roh- und Werkst., 60(6): 379–384, December 2002. 29. K. D. Gabrielson. Controlled release urea-formaldehyde liquid fertilizer resins. US Patent 6 632 262, assigned to Georgia-Pacific Resins, Inc. (Atlanta, GA), October 14 2003. 30. P. A. Nektarios, A.-E. Nikolopoulou, and I. Chronopoulos. Sod establishment and turfgrass growth as affected by urea-formaldehyde resin foam soil amendment. Scientia Horticulturae, 100(1-4):203–213, March 2004. 31. E. N. Brown, M. R. Kessler, N. R. Sottos, and S. R. White. In situ poly(ureaformaldehyde) microencapsulation of dicyclopentadiene. J. Microencapsul., 20(6):719–730, November–December 2003.

6 Melamine Resins Melamine resins rely on 1,3,5-triazine-2,4,6-triamine and formaldehyde. They are similar to urea formaldehyde polymers.

6.1 HISTORY The industrial use of melamine resin started in the late 1930s when the Swiss company CIBA began the industrial production of melamine from dicyandiamide.1, 2 Earlier, the use of this resin was limited because of its high price. Now melamine can be produced cheaper from urea, so the economical situation is improved.

6.2 MONOMERS 6.2.1 Melamine Melamine may be partially or totally replaced with other suitable aminecontaining compounds. Alternatives to melamine include urea, thiourea, dicyandiamide, 2,5,8-triamino-1,3,4,6,7,9,9b-heptaazaphenalene (melem), (N-4,6-diamino-1,3,5-triazin-2-yl)-1,3,5-triazine-2,4,6-triamine (melam), melon, ammeline, ammelide, substituted melamines, and guanamines.3 The melamine homologues melam, melem, and melon have higher thermal stability than pure melamine. These compounds are also used as flame retardants. 299

300


Substituted melamines include alkyl melamines and aryl melamines. Representative examples of some alkyl-substituted melamines include methylmelamine, dimethylmelamine, trimethylmelamine, ethylmelamine, and 1-methyl-3-propyl-5-butylmelamine. Typical examples of an aryl-substituted melamine are phenylmelamine or diphenylmelamine. Melamine and related compounds are shown in Figure 6.1. Foams and fibers exhibit increased elasticity, when some of the melamine is replaced by a substituted melamine, e.g., N-mono-, N,N ′ -bis- and N,N ′ ,N ′′ -tris(5-hydroxy-3-oxapentyl)melamine.4 However, based on considerations of cost and availability, standard melamine is generally preferred.

6.2.2 Other Modifiers Suitable resin modifiers are ethylene diamine, melamine, ethylene ureas, and primary, secondary, and tertiary amines. Dicyandiamide can be also incorporated into the resin. The concentrations of these modifiers in the reaction mixture may vary typically from 0.05 to 5.00%. All these modifiers promote hydrolysis resistance, polymer flexibility, and lower formaldehyde emissions.5

6.2.3 Synthesis Similar to urea, melamine reacts with formaldehyde in weakly alkalineaqueous media to form methylol compounds. Melamine is hexafunctional, so up to hexamethylol monomers can be formed. Hexamethylol melamine is shown in Figure 6.2. The further condensation proceeds under neutral and acidic conditions, thereby forming methylene or dimethylene ether bonds. A pure melamine resin gels within a few days at room temperature. Because of this undesired property, melamine resins are blended with urea resins. 6.2.3.1

Etherified Resins

Etherified resins are prepared by the reaction of melamine with formaldehyde under the conditions of pH around 6 and reflux temperature in the presence of a large amount of butanol. Xylene cycles out the water formed by the condensation reaction by azeotropic distillation and accelerates the etherification in this way.

Melamine Resins

301

H NH2

N

H 2N N

N

H 2N

N

N N

NH2

Melam

NH2

NH2

N H 2N

N

N N

N

N

N NH2

N

Melem

H 2N

H 2N

N N

N

N NH

N

n Melon

NH2

N N

N NH2

NH2

Melamine

N

N

N

NH2

N

N

HH H N

N C N

H C N

H Benzoguanamine

Dicyandiamide

Figure 6.1: Melamine, Melam, Melem, Melon, Benzoguanamine, Dicyandiamide

302


NH2

N

H 2N N

+ H 2C O

N NH2

CH2OH

HOH2C N HOH2C

N

N N

CH2OH

N N

HOH2C

CH2OH

Figure 6.2: Hexamethylol melamine from Melamine and Formaldehyde

6.2.4 Manufacture Melamine is mixed with neutralized formaldehyde solution. The excess of formaldehyde is about threefold. The mixture is heated to 75 to 85°C. When the solution becomes cloudy, water is admixed. Then fillers can be admixed for molding resins. The mixture is dried at 70 to 80°C. while the condensation reaction still proceeds. In the co-condensation of melamine and urea, due to the difference in reactivity of melamine and urea, the condensation of melamine moiety is quicker than the urea moiety.

6.3

PROPERTIES

Phenol/formaldehyde resins and melamine/formaldehyde resins are standard resins used for many products. The choice of resin depends on the desired properties. Phenol/formaldehyde resins are strong and durable and relatively inexpensive, but are generally colored resins. Melamine resins are water clear but are more expensive. They are generally used for products where the color or pattern of the substrate is retained with a transparent

Melamine Resins

303

melamine protective coating or binder. The emission of formaldehyde in melamine/urea/formaldehyde resins is decreased as the melamine content is increased.6 This is explained due to the stronger bonding between triazine carbons of melamine than those of urea carbons. Sulfonated melamine/formaldehyde resins exhibit good solubility in water.7

6.4 APPLICATIONS AND USES Melamine based resins are widely used as adhesives for wood, as resins for decorative laminates, varnish, and moldings, and for improving the properties of paper and cellulosic textiles. In comparison to urea formaldehyde resins, a melamine-based resin has higher resistance against heat and moisture. Etherified melamine resins are often used in combination with alkyd resins for production of decorative laminates. Modification of textiles by melamine is used to impart crease resistance and shrinkage. The wet strength of paper is greatly improved by the use of melamine resins as wet-end additives. Acoustic ceiling tiles are backcoated with melamine resins in order to make them more rigid and humidity-resistant when installed in suspended ceilings. Melamine resins are also used for the preparation of decorative or overlay paper laminates. This application is due to their excellent color, hardness, and solvent, water, and chemical resistance, heat resistance, and humidity resistance. Molded articles, such as dinnerware, are prepared with a combination of melamine/formaldehyde resins and urea/formaldehyde resins. The resins are combined because the melamine/formaldehyde resin is too expensive by itself. The such articles made from these resins are generally not very water-resistant or dimensionally stable.5

6.4.1 Wood Impregnation Melamine/formaldehyde (MF) belongs to the hardest and stiffest isotropic polymeric materials used for decorative laminates, molding compounds, adhesives, coatings, and other products. Due the high hardness and stiffness, and low flammability, MF resins can be used to improve the properties of solid wood. An MF resin can penetrate the amorphous region of

304


wood. It has been established that significant portions of a suitable MF resin penetrate the secondary cell wall layers and middle lamella of softwoods.8

6.5


6.5.1 Resins with Increased Elasticity In foams and fibers, with increased elasticity, the melamine is partly replaced by a hydroxy alkyl substituted melamine. To prepare these resins, melamine and substituted melamine are polycondensed together with formaldehyde. The feed may also contain small amounts of customary additives, such as disulfite, formate, citrate, phosphate, polyphosphate, urea, dicyandiamide, or cyanamide.4 Moldings are produced by curing the resins in a conventional manner by adding small amounts of acids, preferably formic acid. Foams can be produced by foaming an aqueous solution or dispersion containing the melamine/formaldehyde precondensate, an emulsifier, a blowing agent and a curing agent.

6.5.2 Microspheres Monodisperse melamine/formaldehyde microspheres have been prepared via a dispersed polycondensation technique. The nucleation and growth of the particles were achieved within short periods. A continuous coagulation occurred even in the presence of surfactants.9 Microcapsules are interesting because of the controlled-release properties of the respective encapsulated substances. A fragrant oil could be microencapsulated by an in-situ polymerization.10 The particle sizes ranged from 12 to 15 µ m. The efficiency of encapsulation of the fragrant oil reached up to 67-81%. Microcapsules were prepared in a capillary flow microreactor and also in a batch experiment. The microcapsules obtained from the microreactor showed smaller particle diameters and a narrower particle size distribution than those obtained in a batch experiment.11

REFERENCES 1. M. Higuchi. Melamine resins (overview). In J. C. Salamone, editor, The Polymeric Materials Encyclopaedia: Synthesis, Properties and Applications, pages 837–838. CRC Press, Boca Raton, FL, 1999.

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2. Gesellschaft für Chemische Industrie in Basel. Verfahren zur Herstellung von 2.4.6-Triamino-1.3.5-triazin (Melamin). CH Patent 189 406, assigned to Ciba AG, February 28 1937. 3. G. M. Crews, S. Ji, C. U. Pittman, Jr., and R. Ran. Ammeline-melamineformaldehyde resins (amfr) and method of preparation. US Patent 5 254 665, assigned to Melamine Chemicals, Inc. (Donaldsonville, LA), October 19 1993. 4. J. Weiser, W. Reuther, G. Turznik, W. Fath, H. Berbner, and O. Graalmann. Melamine resin moldings having increased elasticity. US Patent 5 162 487, assigned to BASF Aktiengesellschaft (Ludwigshafen, DE), November 10 1992. 5. F. C. Dupre, M. E. Foucht, W. P. Freese, K. D. Gabrielson, B. D. Gapud, W. H. Ingram, T. M. McVay, R. A. Rediger, K. A. Shoemake, K. K. Tutin, and J. T. Wright. Cyclic urea-formaldehyde prepolymer for use in phenolformaldehyde and melamine-formaldehyde resin-based binders. US Patent 6 379 814, assigned to Georgia-Pacific Resins, Inc. (Atlanta, GA), April 30 2002. 6. S. Tohmura, A. Inoue, and S. H. Sahari. Influence of the melamine content in melamine-urea-formaldehyde resins on formaldehyde emission and cured resin structure. J. Wood Sci., 47(6):451–457, 2001. 7. L. H. Su, S. R. Qiao, J. Xiao, X. Tang, G. D. Zhao, and S. W. Fu. Synthesis and properties of high-performance and good water-soluble melamine-formaldehyde resin. J. Appl. Polym. Sci., 81(13):3268–3271, September 2001. 8. W. Gindl, F. Zargar-Yaghubi, and R. Wimmer. Impregnation of softwood cell walls with melamine-formaldehyde resin. Bioresour. Technol., 87(3): 325–330, May 2003. 9. I. W. Cheong, J. S. Shin, J. H. Kim, and S. J. Lee. Preparation of monodisperse melamine-formaldehyde microspheres via dispersed polycondensation. Macromol. Res., 12(2):225–232, April 2004. 10. H. Y. Lee, S. J. Lee, I. W. Cheong, and J. H. Kim. Microencapsulation of fragrant oil via in situ polymerization: effects of ph and melamine-formaldehyde molar ratio. J. Microencapsul., 19(5):559–569, September–October 2002. 11. T. Sawada, M. Korenori, K. Ito, Y. Kuwahara, H. Shosenji, Y. Taketomi, and S. Park. Preparation of melamine resin micro/nanocapsules by using a microreactor and telomeric surfactants. Macromol. Mater. Eng., 288(12): 920–924, December 2003.

7 Furan Resins Furan resins are condensation products of furfuryl alcohol (FA). The resins are derived from vegetable cellulose, a renewable resource.1 Furans as constituents of polymers have been reviewed.2

7.1 HISTORY In Latin, furfur means bran. Furfural was first isolated in 1832 (or 1821) by Döbereiner∗ , as a by-product of the synthesis of formic acid. In 1840 the ability of furfural to form resins was discovered by Stenhous.3 The industrial production of furfural started in 1922, and one year later the first furan-based resins emerged. Early patents on furan resins include that of Claessen4 and one for synthetic resins, (actually mixed phenol furan resins) suitable for use in molding gramophone records.5

7.2 MONOMERS Monomers suitable for furan resins are listed in Table 7.1. One of the chief advantages in furan resins stems from the fact that they are derived from vegetable cellulose. Suitable sources of vegetable cellulose are corn cobs, sugar cane bagasse, oat hulls, paper mill by-products, biomass refinery ∗ Johann Wolfgang Döbereiner, born in Hof an der Saale 1780, in Germany, died in Jena

1849

307

308

Reactive Polymers Fundamentals and Applications Table 7.1: Monomers for Furan Resins6 Furan Compound

Remarks or Reference

Furan Furfural Furfuryl alcohol 5-Hydroxymethylfurfural (HMF) 5-Methylfurfural 2-Furfurylmethacrylate Bis-2,5-hydroxymethylfuran 2,5-Furandicarboxylic acid

OH

Glass fiber binder

OH

OH

O

OH OH

7

OH

HO

OH OH

OH

O

C

H

O

Figure 7.1: Mechanism of the Formation of Furfural

eluents, cottonseed hulls, rice hulls, and food stuffs such as saccharides and starch.6 Pentoses hydrolyze to furfural and hexoses give 5-hydroxymethylfurfural on acid digestion.8

7.2.1 Furfural Furfuraldehyde is a by-product from the sugar cane bagasse which produces resins with an excellent chemical stability and low swelling. 2-Furan formaldehyde or furfural is made from agricultural materials by means of hydrolysis. The mechanism of formation of furfural is shown in Figure 7.1. It is a light yellow to amber colored transparent liquid. Its color gradually deepens to brown during storage. It tastes like apricot kernel. It is mainly

Furan Resins

309

used in lubricant refinement, furfuryl alcohol production, and pharmaceutical production. Furfural is the chief reagent used to produce materials such as furfuryl alcohol, 5-hydroxymethylfurfural (HMF), bis(hydroxymethyl)furan (BHMF), and 2,5-dicarboxyaldehyde-furan. The furan-containing monomers in turn can undergo reactions to produce various furan-containing monomers with a wide variety of substituents as shown in Table 7.1.

7.2.2 Furfuryl Alcohol Furfuryl alcohol is made from furfural by reduction with hydrogen. It is a colorless transparent liquid and becomes brown, light yellow, or deep red, when exposed in the air. It can be mixed with water and many organic solvents such as alcohol, ether, acetone, etc., but not in hydrocarbon products.


Furan-based Polyimides

Polyimides based on poly(2-furanmethanol-formaldehyde) can be prepared by a Diels-Alder reaction (DA) of the respective furan resin with bismaleimides.9 The Diels-Alder reaction proceeds in tetrahydrofuran (THF) or in bulk. The tetrahydrophthalimide intermediates aromatize in the presence of acetic anhydride. Polyimides based on the furan resin exhibit good thermal stability.

7.2.4 Synthesis Furan-based monomers can polymerize through two well-known mechanisms. The first involves chain or polyaddition polymerization, which is initiated by free radical, cationic or anionic promoters. Polymerization produces macromolecules with furan rings pendant on the main chain. The second method is a polycondensation, also referred to as polymerization condensation. Polymers and copolymers resulting from acid catalyzed condensation reactions result in macromolecules with furan rings in the main chain.6 As a general rule, the furan resins formed by polycondensation reactions have stiffer chains and higher glass transition temperatures. These reactions may involve self-condensation of the furan monomers described

310


O

CH2

O

O

OH

H+

CH2+

O

CH2+

CH2

O

O

CH2

CH2

OH

OH

Figure 7.2: Acid Catalyzed Self-condensation of Furfuryl Alcohol

above, as well as condensation reactions of such monomers with aminoplast resins, organic anhydrides, and aldehydes such as formaldehyde, ketones, urea, phenol, and other suitable reagents. Most common, furan resins are produced by acid catalyzed condensation reactions. The condensation results in linear oligomers, the furan rings being linked with methylene and methylene-ether bridges, c.f. Figure 7.2. The synthesis of furan resins proceeds in a pH range of 3 to 5, at a temperature range of 80 to 100°C. The condensation is stopped, when a desired viscosity value is reached, by neutralizing the liquid resin. Furfuryl alcohol can also be condensed with formaldehyde to obtain furan-formaldehyde resins. The content of free formaldehyde can be lowered by the addition of urea at the late stages of synthesis.

7.3

SPECIAL ADDITIVES

7.3.1 Reinforcing Materials Aramid fibers were used as reinforcing material for a phenol resin and a furan resin. A comparative study of the mechanical performance of the materials showed that the furan resin is more suitable as a matrix than the phenol resin.10

Furan Resins

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Table 7.2: Global Production Data of Furan Resins Related Components11 Monomer Mill. Metric tons Formaldehyde 24 Furfural 0.225

Year 2003 1999

Reference 12 13

7.3.2 Production Data of Important Monomers The most important industrial furan resins are based on 2-furfuryl alcohol. The largest producers of furfural are China and the Dominican Republic. Production data are shown in Table 7.2. Around 30% of the furfural consumption is used to produce furfuryl alcohol, which is mainly consumed by the production of furan resins.

7.4 CURING Materials known to be suitable for curing furan resins include inorganic and organic acids. Examples of suitable organic and inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, tartaric acid, and maleic acid. Friedel-Crafts catalysts include aluminum trichloride, zinc chloride, aluminum bromide, and boron fluoride. Resins with improved fire resistance are cured with a mixture of trimethylborate, boric anhydride, and p-toluenesulfonic acid.14 Salts of both inorganic and organic acids may also be used. Ammonium sulfate is preferred. Ammonium sulfate is a latent catalyst which may become active at approximately 110 to 150°C. Suitable organic salts are the urea salt of toluenesulfonic acid, the polyammonium salts of polycarboxylic acids such as the diammonium salts of citric acid, and the ammonium salts of maleic acid. Cyclic anhydrides such as maleic anhydride are also suitable for use as catalysts. It is believed that polyester co-polymers are formed between the anhydride and the free hydroxylated species present in the resin. Maleic acid promotes the polymerization reaction. Furthermore, it is believed that maleic acid may preferentially reduce the emission of bis(hydroxymethyl)furan monomer during the curing process. A significant reduction of volatile organic compounds (VOCs), will use a catalyst system comprised of maleic acid and ammonium sulfate.6

312


CH O

CH2

O

O

CH2

O CH

CH2

Figure 7.3: Crosslinked Methylene Bridges

7.4.1 Acidic Curing The resin can be crosslinked by an acidic catalyst. The reaction is not sensitive to air. The main route of curing is an additional condensation reaction at the free α-hydrogen of furan rings. These positions are connected by methylene bridges.

7.4.2 Oxidative Curing The oxidative crosslinking of furfuryl alcohol (FA) polycondensates proceeds at temperatures of 100 to 200°C. Structures with tertiary carbon atoms, as shown in Figure 7.3, could be identified.

7.4.3 Ultrasonic Curing Ultrasonic treatment, i.e., sonication during the curing process of a furan resin, showed changes of the curing performance. p-Toluenesulfonic acid was added as curing catalyst in the proportion of 0.3%. Fine carbons were also incorporated. Using an ultrasonic homogenizer in the presence of carbonaceous fine particles showed an increased curing rate of the furan resin. This, in turn, increased the polymerization degree with an increase in ultrasound intensity. The increase of curing rate was also observed by small additions of carbonaceous fine particles. In this case, the curing accelerated with an increase in the specific surface area of the additives.15 The increase of curing rate is believed to result from cavitation. The curing reaction proceeds slowly in the absence of cavitation and simple stirring fails to produce such a marked increase in the rate of reaction. The curing is accelerated by heat, oxygen, and the addition of phenol and urea.

Furan Resins

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7.5 PROPERTIES 7.5.1 Recycling Research has been conducted to introduce pendent furan groups into polymers such as poly(styrene) via copolymerization with a suitable comonomer. The pendent furan moieties can be crosslinked with a bismaleimide to achieve polymers with better performance. In order to recycle these crosslinked materials, heating experiments with an excess of 2-methylfuran were performed in order to induce the retro Diels-Alder reaction and break-up the network. The reaction proceeded in this manner and the original copolymers could be recovered from the treatment. Therefore, the introduction of furan units is a potential path of recycling crosslinked polymers by thermal treatment with a diene in excess.16 The Diels-Alder reaction between styrene-furfuryl methacrylate copolymer samples and bismaleimide can be monitored the ultraviolet absorbance of the maleimide group at 320 nm or by 13 C-NMR spectroscopy.7

7.6 APPLICATIONS AND USES Furan resins are used mainly in the foundry industry, as sand binders for casting molds and cores. Furan resins are often used in combination with other resins. Furan resins are highly corrosion resistant. Therefore, they have found use in mortars and in cements. Improved mechanical properties are implemented by reinforcing with glass fibers.

7.6.1 Carbons Porous Carbon. Furan resins form a porous carbon by pyrolysis at 450°C. Glass-like Carbon. Glass-like carbon is identified as an excellent carbon artifact due to its characteristics such as hardness and shape stability. The microstructure of glass-like carbon consists of a non-graphitic alignment of hexagonal sheets. It has unique properties such as great hardness compared with other carbon materials and impermeability for gases.17 Glass-like carbon is of interest in the battery and semiconductor industries. Glass-like carbon is prepared by heat-treatment on thermosetting resins in inert atmosphere.

314


Figure 7.4: SEM photographs of glass-like carbon derived from furan resin. (a) 300, (b) 600 °C.18 (Reprinted with permission from Elsevier)

During the heat-treatment of a furan resin, weight loss is very rapid up to 500°C, then continues gradually up to 1000°C, and then the weight stays almost constant above 1000°C. SEM photographs of heat treated glass-like carbon reveal a large increase of micro-grain size in the range of 60 to 105 nm when treated at 2000°C. Up to 2500°C, the grain size decreases to 27 to 40 nm due to graphitization.18 There is a structural correlation between the micro-texture of the furan resin and the glass-like carbon formed from the particular resin. The pore structure in glass-like carbon can be characterized by small-angle X-ray scattering (SAXS) technique. The scattering intensities grow gradually with increasing heat-treatment temperature up to 1600 to 1800°C, and then the intensities increase abruptly at a temperature higher than 1800°C. The dependence of the structural change of a glass-like carbon from a furan resin is almost the same as that of a phenolic resin. However, it was found that the carbon prepared at 1200°C from furan resin shows the largest interlayer spacing in the carbon matrix and at the same time the smallest value of the gyration radius for the pores.17

7.6.2 Chromatography Support Conventionally used packing materials for liquid chromatography are a chemically bonded type of packing material based on silica gel and a pack-

Furan Resins

315

ing material based on synthetic resin. The silica gel-based packing material has relatively strong in mechanical properties and in its swelling or shrinking characteristics against various organic solvents. Therefore, it has a high resolving power and is superior in exchangeability of eluent for analysis. However, the silica gel-based packing material has problems in that the silica gel dissolves under acidic or alkaline conditions and the solubility of the silica gel in an aqueous solution increases when warmed, resulting in durability problems. The packing material of synthetic resin, on the other hand, is known to be high in acid- and alkali-resistivity, and has a good chemical durability as a packing material. However, since the mechanical strength of the particles is small, it has been difficult to convert them into finer particles. Raw materials which are highly chemically stable and exhibit high mechanical strength are graphitized carbon black. A packing material for liquid chromatography is produced by mixing carbon black, a synthetic resin which can be graphitized, and pitches. Suitable synthetic resins are phenolic resins, furan resins, furfural resins, divinylbenzene resins, or urea resins.19 The pitches can be petroleum pitches, coal-tar pitches, and liquefied coal oil. The mixture is granulated and heated up to 3000°C in an inert atmosphere.

7.6.3 Composite Carbon Fiber Materials Impregnation of carbon fibers and subsequent pyrolysis at 1000°C improves strength of carbon fibers.20 A yarn is passed through a bath containing a carbonizable resin precursor, such as a partially polymerized furfuryl alcohol. It is advantageous to add a latent catalyst along with the precursor. Suitable catalysts are a complex of boron trifluoride and ethylamine or maleic anhydride. The use of a latent catalyst allows the application of a low-viscosity solution to the fiber with subsequent polymerization at the elevated temperatures. If the precursor were to polymerize significantly prior to application, the treating bath would be so viscous that would allow only a coating to be formed. For high performance composite carbon fiber reinforced carbonaceous material which is compositely reinforced with carbon fibers, prepregs of woven fabrics of carbon fibers are impregnated with a resin such as phenol resin, furan resin, epoxy resin, urea resin, etc. They then are lamin-

316


ated as a matrix and molded under heat and pressure, and after carbonization, they are further graphitized by heating to a temperature of 3000°C.21

7.6.4 Foundry Binders Furans are somewhat more expensive than other binders, but the possibility of sand reclamation is advantageous. One of the most commercially successful no-bake binders is the phenolic-urethane no-bake binder. This binder provides molds and cores with excellent strengths that are produced in a highly productive manner. Furan-based binders have less VOC, free phenol level, low formaldehyde, and produce less odor and smoke during core making and castings. However, the curing performance of furan binders is much slower than the curing of phenolic urethane no-bake binders. Furan binders can be modified to increase their reactivity, for instance by formulating with urea/formaldehyde resins, phenol/formaldehyde resins, novolak resins, phenolic resol resins, and resorcinol. Nevertheless, these modified furan binders do not provide the cure speed needed in foundries that require high productivity. Therefore, an activator, which promotes the polymerization of furfuryl alcohol, is added. Resorcinol pitch is used for this purpose.22 Further components in such a formulation are polyester polyols or polyether polyols, and a silane, such as (3-aminopropyl)triethoxysilane. The curing process of urea-modified furan resins in sands has been investigated by infrared spectroscopy.23

7.6.5 Glass Fiber Binders An alternative to phenol/formaldehyde-based fiberglass binders is furanbased binders. Furan binders provide many of the advantages of phenolic binders while resulting in substantially reduced VOC emissions. Water as a significant component can be used. Formaldehyde is not a significant curing or decomposition by-product, and the furan resins form very rigid thermosets. Emulsified furan resins can be used. Emulsified furan-based glass fiber binding compositions are advantageous since they allow the use of furan resins that have high molecular weights or the addition of other materials which would give rise to the formation of two-phase systems.6 A suitable surfactant to be added to the furan binder compositions is sodium

Furan Resins

317

dodecyl benzene sulfonate. It may be added in an amount from 0.05 to 1.0%.

7.6.6 Oil Field Applications Wells in sandy, oil-bearing formations are frequently difficult to operate because the sand in the formation is poorly consolidated and tends to flow into the well with the oil. Sand production is a serious problem because the sand causes erosion and premature wearing out of the pumping equipment. It is a nuisance to remove from the oil at a later point in the operation. Furan resin formulations can be used for in-situ chemical sand consolidation.24

7.6.7 Plant Growth Substrates Conventional mineral wool plant growth substrates are based on a coherent matrix of mineral wool of which the fibers are mutually connected by a cured binder. There is a need to reduce the phytotoxicity of the chemicals used. The phytotoxicity may result from the phenolic binder materials. If a phenolic resin is used as binder, a wetting agent must be added in order to impart the hydrophobic mineral wool matrix with hydrophilic properties. However, the use of a furan resin allows the abandonment of the use of a wetting agent. A disadvantage of the use of a furan resin is its comparatively high price. Therefore, the traditional phenol/formaldehyde resin substituted only partly by a furan resin, is sufficient to maintain or to achieve the desired properties.25, 26

7.6.8 Photosensitive Polymer Electrolytes Both conjugated furan chromophores and polyethers can be grafted onto chitosan to result in a photosensitive polymer electrolyte. The furan chromophore consists of conjugated furan chromophores of 5-[2-(5-Methyl furylene vinylene)]furancarboxyaldehyde,27 c.f. Figure 7.5. The graft polymer can be photocrosslinked. The photochemical reaction consists of a π2 +π2 cycloaddition reaction of the vinylene double bonds of the furan moiety so that two pendent vinylene groups form a four membered ring. The crosslinking reaction is shown in Figure 7.6

318


O O H 3C

C

O

H

Figure 7.5: 5-[2-(5-Methyl furylene vinylene)]furancarboxyaldehyde

OH

OH O

O

O

O NH

NH CH2

CH3

O

O

CH2 O

CH3

O

O

O

O

O

CH3

CH2

CH2

NH

CH3

NH

O

O OH

O

O OH

Figure 7.6: Photo Crosslinking of the Furylene Vinylene Units Grafted on Chitosan

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319

REFERENCES 1. Z. László-Hedvig and M. Szesztay. Furan resins (2-furfuryl alcohol based). In J. C. Salamone, editor, The Polymeric Materials Encyclopaedia: Synthesis, Properties and Applications, pages 548–549. CRC Press, Boca Raton, FL, 1999. 2. A. Gandini and M. N. Belgacem. Furans in polymer chemistry. Prog. Polym. Sci., 22(6):1203–1379, 1997. 3. I. F. C. B.V. Historical overview and industrial development. (Internet: http://www.furan.com). 4. C. Claessen. Process for the treatment of wood or other substances containing cellulose for the purpose of obtaining cellulose and artificial resin, asphalt, lac and the like. GB Patent 160 482, March 17 1921. 5. J. S. Stokes. Improvements in and relating to synthetic resin composition. GB Patent 243 470, December 3 1925. 6. T. J. Taylor, W. H. Kielmeyer, C. M. Golino, and C. A. Rude. Emulsified furan resin based glass fiber binding compositions, process of binding glass fibers, and glass fiber compositions. US Patent 6 077 883, assigned to Johns Manville International, Inc. (Denver, CO); QO Chemicals, Inc. (West Lafayette, IN), June 20 2000. 7. E. Goiti, F. Heatley, M. B. Huglin, and J. M. Rego. Kinetic aspects of the Diels-Alder reaction between poly(styrene-co-furfuryl methacrylate) and bismaleimide. Eur. Polym. J., 40(7):1451–1460, July 2004. 8. C. Moreau, M. N. Belgacem, and A. Gandini. Recent catalytic advances in the chemistry of substituted furans from carbohydrates and in the ensuing polymers. Top. Catal., 27(1-4):11–30, February 2004. 9. K. S. Patel, K. R. Desai, K. H. Chikhalia, and H. S. Patel. Polyimides based on poly(2-furanmethanol-formaldehyde). Adv. Polym. Technol., 23(1):76–80, 2004. 10. K. Kawamura and S. Ozawa. Characterization of fiber reinforced plastics made from aramid and phenol resin or furan resin. Kobunshi Ronbunshu, 59(1):51–56, 2002. 11. R. Gubler, editor. Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, 1950–to present. (Internet: http://ceh.sric.sri.com/). 12. S. Bizzari. Report “Formaldehyde”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, January 2004. (Internet: http://ceh.sric.sri.com/). 13. J. Levy and Y. Sakuma. Report “Furfural”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, July 2001. (Internet: http://ceh.sric.sri.com/).

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14. N. Meyer and M. Cousin. Furan resins of improved fire resistance. US Patent 4 355 145, assigned to Societe Chimique des Charbonnages SA (Paris La Defense, FR), October 19 1982. 15. K. Hoshi, T. Akatsu, Y. Tanabe, and E. Yasuda. Curing properties of furfuryl alcohol condensate with carbonaceous fine particles under ultrasonication. Ultrason. Sonochem., 8(2):89–92, April 2001. 16. C. Gousse, A. Gandini, and P. Hodge. Application of the Diels-Alder reaction to polymers bearing furan moieties. 2. Diels-Alder and retro-Diels-Alder reactions involving furan rings in some styrene copolymers. Macromolecules, 31(2):314–321, January 1998. 17. K. Fukuyama, T. Nishizawa, and K. Nishikawa. Investigation of the pore structure in glass-like carbon prepared from furan resin. Carbon, 39(13): 2017–2021, November 2001. 18. Y. Korai, K. Sakamoto, I. Mochida, and O. Hirai. Structural correlation between micro-texture of furan resin and its derived glass-like carbon. Carbon, 42(1):221–223, 2004. 19. H. Ichikawa, A. Yokoyama, T. Kawai, H. Moriyama, K. Komiya, and Y. Kato. Packing material for liquid chromatography and method of manufacturing thereof. US Patent 5 270 280, assigned to Nippon Carbon Co., Ltd. (Tokyo, JP); Tosoh Corporation (Yamaguchi, JP), December 14 1993. 20. M. Katz. Improvement of carbon fiber strength. EP Patent 0 251 596, assigned to Du Pont, January 7 1988. 21. T. Kawakubo and E. Oota. Process for preparation of carbon fiber composite reinforced carbonaceous material. US Patent 5 096 519, assigned to Mitsubishi Pencil Co., Ltd. (JP), March 17 1992. 22. K. K. Chang. Furan no-bake foundry binders and their use. US Patent 6 593 397, assigned to Ashland Inc. (Dublin, OH), July 15 2003. 23. M. Bilska and M. Holtzer. Application of fourier transform infrared spectroscopy (FTIR) to investigation of moulding sands with furan resins hardening process. Arch. Metall., 48(2):233–242, 2003. 24. P. Shu. Water compatible chemical in situ and sand consolidation with furan resin. US Patent 5 522 460, assigned to Mobil Oil Corporation (Fairfax, VA), June 4 1996. 25. E. L. Hansen and J. F. De Groot. Process for the manufacture of a mineral wool plant growth substrate. US Patent 6 562 267, assigned to Rockwool International A/S (Hedenhusene, DK), March 13 2003. 26. J. F. De Groot and T. B. Husemoen. Hydrophilic plant growth substrate, comprising a furan resin. US Patent 6 032 413, assigned to Rockwool International A/S (DK), March 7 2000. 27. A. Gandini, S. Hariri, and J.-F. Le Nest. Furan-polyether-modified chitosans as photosensitive polymer electrolytes. Polymer, 44(25):7565–7572, December 2003.

8 Silicones In contrast to most organic polymers, in silicones the backbone is made of silicon and oxygen. Silicon is together with carbon in the fourth group of the periodic system, therefore a similar behavior of these elements can be expected.

8.1 HISTORY Kipping∗ started with the synthesis of organic silicon compounds by treating SiCl4 with magnesium-based organometallic compounds. These compounds are now called Grignard reagents, invented by Victor Grignard in 1900. Hyde† , at Corning, developed a flexible, high temperature binder for glass fibers and synthesized the first silicone polymer. The potential applications in other fields, such as electric industries soon became apparent. Eugene George Rochow‡ at General Electric developed synthesis of silicones that is now used.1, 2 His first patent dates at 1941.3, 4 In 1949,the silly putty was invented by James Wright when mixing silicone oil with boric acid. Silly putty acts like both a rubber and a putty. ∗ Frederic

Stanley Kipping, born in Upper Broughton (UK) 1863, died in 1949 Franklin Hyde, born in Solvay, New York 1903, died in 1999 ‡ Eugene George Rochow, born in Newark, New Jersey 1909, died in 2002 † James

321

322

8.2


MONOMERS

8.2.1 Chlorosilanes The synthesis of silanes and siloxanes starts from chlorosilanes such as dimethyldichlorosilane. Other products are derived from this compound that also serve as monomers. Thus, in silicone chemistry, the term monomer is not as clearly defined as in other fields of polymer chemistry.

8.2.2 Silsesquioxanes Silsesquioxane resins are used in industrial applications in the automotive, aerospace, naval, and other manufacturing industries. Silsequioxane resins exhibit excellent heat and fire resistant properties that are desirable for such applications. These properties make the silsesquioxane resins attractive for use in fiber-reinforced composites for electrical laminates, and structural use in automotive components, aircraft, and naval vessels. There is a need for rigid silsesquioxane resins that has increased flexural strength, flexural strain, fracture toughness, and fracture energy, without significant loss of modulus or loss of thermal stability. In addition, rigid silsesquioxane resins have low dielectric constants and are useful as interlayer dielectric materials. Rigid silsesquioxane resins are also useful as abrasion resistant coatings. These applications require that the silsesquioxane resins exhibit high strength and toughness.5 The formation of silsesquioxanes is shown in Figure 8.1. Silsesquioxanes are organosilicon compounds with the formula [RSiO3/2 ]n . [R7 Si7 O9 (OH)3 ], as shown in Figure 8.1, can be synthesized in one step via the hydrolytic condensation of RSiCl3 or RSi(OMe)3 . A single Si-O-Si linkage in a fully condensed R8 Si8 O12 framework can be cleaved selectively by strong acids (e.g., HBF4 /BF3 or triflic acid.6

8.2.3 Hydrogen Silsesquioxanes Hydrogen-silsesquioxane resins are useful precursor substances for silicacontaining ceramic coatings. Hydrogen silsesquioxane resins are ladder or cage polymers.7 The general structure is shown in Figure 8.2. When trichlorosilane is subjected to hydrolytic condensation caused by direct contact with water, the reaction occurs abruptly, and gels are formed. Accordingly, various methods for manufacturing hydrogen-silsesquioxane resins

Silicones

323

R OH

Si O Cl

R

H 2O

O

OH R OH O Si Si R

O

Si

R Si Cl

Si

Cl

O

O

O

R

O Si

Si O

R

R

Figure 8.1: Formation of Silsesquioxanes: [R7 Si7 O9 (OH)3 ]

H

HO

HO

Si O Si

H O

O

H

H

H O

Si O Si O O Si O Si

O

H

H

H

Si O Si O O Si O Si

OH

OH

H

H n H

O Si

H Si

O O H

O Si

Si

Si H

O

O

O

O

H

Si O

H

O O

H

Si

Si O

H

Figure 8.2: Hydrogen silsesquioxane resins.7 Top: Ladder Form, Bottom: Cage Form

324


that do not form gels have been proposed. The hydrogen-silsesquioxane resin can be manufactured in an aromatic hydrocarbon solution of trichlorosilane. The hydrolytic condensation is then performed as a two-phase reaction with concentrated sulfuric acid. Concentrated sulfuric acid and aromatic hydrocarbon react to produce an arylsulfonic acid hydrate, and the water in this hydrate contributes to the hydrolytic condensation of trichlorosilane. Therefore, the hydrogensilsesquioxane resin produced by this hydrolytic condensation is obtained from the organic phase. When water is added to the concentrated sulfuric acid phase in order to recover and reuse the arylsulfonic acid, precipitation occurs, thus rendering the arylsulfonic acid unsuitable for reuse. For this reason, large quantities of organic solvent and sulfuric acid are lost using this method. A method for complete reuse of the solvent, the sulfuric acid and surfactants, essentially without loss of these compounds, has been described. The method utilizes a two-phase system consisting of an aqueous phase: 1. An aqueous solution consisting of sulfuric acid and an organic sulfonic acid, e.g., p-toluenesulfonic acid monohydrate, and 2. The organic phase consisting of a diluted solution of organic sulfonic acid in a halogenated hydrocarbon solvent. The trichlorosilane must be soluble in this solvent, and the solvent should not react with sulfuric acid. Examples are isopropyl chloride, chlorobenzene, and others. This method results in hydrogen-silsesquioxane resins at a high yield. The loss of the organic solvent used in the organic phase is small, and the precipitation of benzenesulfonic acid, etc., in the aqueous phase due to supersaturation can also be eliminated. The organic solvent, the sulfuric acid and the organic sulfonic acid used in the aqueous phase can be effectively reused.8

8.2.4 Alkoxy Siloxanes Examples of alkoxy siloxanes are listed in Table 8.1. Trifunctional siloxane units and tetrafunctional siloxane units are used to improve the physical properties of curable epoxy resins. Branched silicone resins with trifunctional siloxane units are highly heat-resistant and have an excellent capacity for film-formation, which is why they are used as electrical insulating

Silicones

325

Table 8.1: Epoxy-containing Siloxanes9 Siloxane Methyltrimethoxysilane Methyltriethoxysilane Ethyltrimethoxysilane Ethyltriethoxysilane Vinyltrimethoxysilane Phenyltrimethoxysilane 3,3,3-Trifluoropropyltrimethoxysilane Dimethyldimethoxysilane Methylphenyldimethoxysilane Methylvinyldimethoxysilane Diphenyldimethoxysilane Dimethyldiethoxysilane Methylphenyldiethoxysilane Tetramethoxysilane Tetraethoxysilane (TEOS) Tetrapropoxysilane Dimethoxydiethoxysilane

materials, and heat-resistant paints and coatings.9

8.2.5 Epoxy-modified Siloxanes Siloxanes with pendent epoxy groups are listed in Table 8.2. Epoxy-containing silicone resins are prepared either by the co-hydrolysis and condensation of epoxy-containing trialkoxysilane and diorganodialkoxysilane or by the base-catalyzed equilibration polymerization of cyclic diorganosiloxane and epoxy-containing trialkoxysilane.9 Epoxy-containing silicone resins have broad molecular weight distributions and do not exhibit a softening point or a distinct glass transition temperature.

8.2.6 Silaferrocenophanes Silaferrocenophanes are of considerable interest because they may serve as precursors to unusual ceramic materials. Polymers can be made by ring opening polymerization as shown in Figure 8.3. Other ferrocenophanes bridged by heteroatoms such as germanium and phosphorus have been synthesized. In the presence of methylphenylchlorosilane or diphenylchlorosilane, i.e., silanes with pendent hydrogen, telechelic polymers can

326


Table 8.2: Epoxy-containing Siloxanes9 Siloxane 3-Glycidoxypropyl(methyl)dimethoxysilane 3-Glycidoxypropyl(methyl)diethoxysilane 3-Glycidoxypropyl(methyl)dibutoxysilane 2-(3,4-Epoxycyclohexyl)ethyl(methyl)dimethoxysilane 2-(3,4-Epoxycyclohexyl)ethyl(phenyl)diethoxysilane 2,3-Epoxypropyl(methyl)dimethoxysilane 2,3-Epoxypropyl(phenyl)dimethoxysilane 3-Glycidoxypropyltrimethoxysilane (GLYMO) 3-Glycidoxypropyltriethoxysilane 3-Glycidoxypropyltributoxysilane 2-(3,4-Epoxycyclohexyl)ethyltrimethoxysilane 2-(3,4-Epoxycyclohexyl)ethyltriethoxysilane 2,3-Epoxypropyltrimethoxysilane 2,3-Epoxypropyltriethoxysilane

Fe

CH3 Si

Fe

CH3 Si CH3

CH3

Figure 8.3: Ring Opening Polymerization of Silaferrocenophanes

Silicones

327

Table 8.3: Products Obtained by the Rochow Synthesis10 Silane Methyldichlorosilane Methyltrichlorosilane Dimethyldichlorosilane Trimethylchlorosilane

Yields [%]

Boiling Points [°C]

0.5 8–18 80–85 2–4

41 66 70 57

be produced with the hydrogen bearing silanes as end group11, 12 Apart from silaferrocenophanes, ferrocenophanes with conjugated double bonds instead of silicon are of interest because of their electrical properties.13

8.2.7 Synthesis 8.2.7.1

Direct Synthesis

Silicones are synthesized via methylchlorosilanes by the Müller-Rochow process. The reaction is carried out at temperatures of 250 to 300°C and 2 to 5 bars. A copper catalyst used with antimony, cadmium, aluminum, zinc, and tin is effective for improving the activity. However, lead would act as an inhibitor. A finely homogenized mixture of silicon and copper is introduced into a fluidized bed reactor. The reactor is fluidized by gaseous methylchloride. The reactants are separated from the solid components and on cooling a crude liquid silane mixture is obtained. Silicon conversions of 90 to 98% and methylchloride conversions of 30 to 90% can be achieved. The reaction is strongly exothermic and requires a precise control. Dimethyldichlorosilane is the main product. Other major products obtained are shown in Table 8.3. The selectivity for producing dimethyldichlorosilane is highly sensitive to trace amounts of other metals present. The selectivity for dimethyldichlorosilane is reduced if the Cu, Zn, or Sn concentrations exceed the generally used concentrations or if the reaction temperature exceeds 300°C. A silver promoter increases the selectivity to dimethyldichlorosilane.14, 15 The crude silane mixture is then separated in distillation columns. A high separating capacity is needed, because the boiling points of CH3 SiCl3 and (CH3 )2 SiCl2 differ by only 4°C. A high purity is required, because even a small amount of CH3 SiCl3 leads to branched and eventually gelled products.

328 8.2.7.2

Reactive Polymers Fundamentals and Applications Hydrosilylation

The hydrosilylation reaction consists of the addition of hydrogen-containing silanes to products with double or triple bonds. This reaction is suitable for introducing organo functions into silicone compounds. Therefore, hydrosilylation is extensively used to synthesize organofunctional silicones with pendant vinyl groups, amino groups, etc.16 In a further step, chlorine atoms, hydrogen atoms, and alkoxy groups can undergo a nucleophilic substitution. The hydrosilylation reaction requires often high temperatures.

Vinyl Groups. The hydrosilylation of aromatic compounds containing vinyl unsaturation can lead to radical polymerization of the monovinylaromatic compounds, especially at elevated temperature. The use of radical polymerization inhibitors, such as phenols or quinones, is often necessary, however, most of these inhibitors are not sufficiently active at elevated temperatures and require the presence of oxygen to improve their activity. However, special conditions and precautions make the use of a radical polymerization inhibitor unnecessary. Styrene and α-methylstyrene can be hydrosilylated with heptamethyltrisiloxane with a Karstedt platinum catalyst at 90°C.17 When 4-vinyl-1-cyclohexene is reacted with a hydrogenchlorosilane, both the vinylic double bond and the double bond in the cyclohexene ring react. Thereby an organic silicon compound of the formula given in Figure 8.4 is obtained in which the hydrogenchlorosilane is added to each of the two double bonds in 4-vinyl-1-cyclohexene. The cyclohexane ring within the molecule imparts a high hardness and scratch resistance and is useful as a coupling agent to be added to paints for use in automobiles, buildings and adhesives. The compound is also useful as an intermediate to an alkoxysilane coupling agent.18

8.2.7.3

Grignard Synthesis

The Grignard synthesis is suitable to introduce organic groups to silicon. The Grignard synthesis is used on a laboratory scale. An example for a Grignard synthesis is shown in Figure 8.5. With water, methylphenyldichlorosilane condenses to a linear polymer.

Silicones

CH3

CH3 + H Si Cl

H Si Cl + Cl

Cl

CH3 CH3

Si Cl

Si

Cl

Cl

Cl Cl

+

CH3

CH3

Si

Si Cl

Cl

Cl

Figure 8.4: Hydrosilylation of 4-Vinyl-1-cyclohexene

Cl

Cl Mg

Br + Cl

Si Cl

Si

Cl

Cl

Cl CH3

CH2

Mg

Br + Cl

Si Cl

Cl CH3

CH2

Si

+ MgClBr

Cl

Figure 8.5: Grignard Synthesis

Cl + MgClBr

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330


8.2.7.4

Condensation

Hydrolysis of chlorosilanes results in silanols. These silanols are not stable and undergo a polycondensation. Intramolecular and intermolecular condensation takes place. Intermolecular condensation yields linear siloxanes, and intramolecular condensation yields cyclic products. When trichlorosilanes undergo hydrolysis, highly crosslinked silicone resins are obtained. The reaction can be catalyzed by acids. An equilibrium between the linear siloxanes and cyclic siloxanes can be established. If the catalyst is deactivated, the condensation stops and the cyclic products that consist mostly of a tetramer can be removed by distillation. On the other hand, cyclic siloxanes can be transformed to polymers in the presence of alkali. If the catalyst is not deactivated then cyclic siloxane forms until the equilibrium is established. In equilibrium ca. 20% of cyclic products are present, which is relevant to the recycling of polysiloxanes. Chain Stoppers. To obtain stable or functional terminal groups, chain stoppers are added. The reaction proceeds under continuous cleavage and recombination of siloxane bonds. The reaction is catalyzed by acids. Bodying. Bodying is a technology that consists of the base-catalyzed depletion of the silanol groups in a silicone resin prepared by the hydrolysis and condensation of organoalkoxysilane. In this process the molecular weight of the silicone resin is simply increased, while control of the molecular weight, softening point, and glass transition temperature is not possible.9 Crosslinking. The degree of crosslinking depends on the presence of either tetrachlorosilane SiCl4 for the production of very rigid resins, or (CH3 )2 SiCl2 for softer grades.

8.2.8 Manufacture Commercially produced silicone resins comprise: • • • •

Non-meltable solids Soluble reactive resins Silsesquioxanes High reactive alkoxysiloxanes with molecular weight.

Silicones

331

8.3 MODIFIED TYPES 8.3.1 Chemical Modifications Reactive alkoxysiloxanes can undergo a reaction with functional organic resins. The modification of methylpolysiloxanes is achieved by substituting the methyl groups with other organic groups, e.g., lower alkyl chains or functional groups like vinyl groups, or by copolymerization with organic polymers, e.g., poly(ethylene oxide) or poly(propylene oxide). 8.3.1.1

Amine Functions

Aminofunctional silicones impart extreme softness. Such materials are appreciated in textiles because of the improved wear comfort. In textile dyeing uniformity of color fixation is achieved by efficient control of foaming in the dyeing bath. 8.3.1.2

Functionalized Silanes

Reactive silanes or siloxanes can also include functionalities such as: vinyl, hydride, allyl, or other unsaturated groups. For surface coating, hexamethyldisiloxane and tetramethyldivinyldisiloxane are used.5 Mixtures of siloxanes with trimethyl silyl groups and dimethylvinyl silyl groups are also common. 8.3.1.3

Crosslinking Agents

Crosslinking agents include alkoxysilanes such as methyltrimethoxysilane, dimethyldimethoxysilane, etc., or oxime silanes, for example, methyltris(methylethylketoxime)silane.19 Crosslinking accelerators include amines, tin compounds such as dibutyltin diacetate, or dibutyltin dilaurate.19

8.3.2 Fillers The silicone network does not exhibit much mechanical strength. Mechanical strength is imparted by the interaction of a filler with the polymer. Fumed silica shows the strongest reinforcing effect. Other fillers include quartz flour, iron oxide, and carbon black.

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8.3.3 Reinforcing Materials Fiber reinforced, silicone matrix resin composites find many applications in structural components. Fiber reinforcement often takes the form of woven glass fiber mats. Woven carbon fiber mats offer a higher modulus reinforcing media, but they are more expensive than glass fibers. Other fiber compositions such as aramid, nylon, polyester, and quartz fibers may be used for reinforcement. Other fibrous forms, such as non-woven mats and layers of loose fibers, may also be used in silicone-based composite applications.20 Fiber reinforced, silicone matrix resin composites in multilayer laminated form are strong and fire resistant. They find applications in interiors of airplanes and ships. They are also used in electrical applications, such as wiring boards and printed circuit boards, requiring flexural strength and low weight. Suitable resin types are typically highly branched and crosslinked polymer molecules, when cured. To facilitate the impregnation process, silicone precursor formulations may be diluted with toluene. The toluene is then evaporated from the composite.

8.4

CURING

8.4.1 Curing by Condensation Curing by condensation releases alcohol, amines, acetic acid, or other volatile reaction products. The polymerization reaction does proceed in the absence of water. This fact is utilized in one component systems that form polymers by means of atmospheric humidity. To avoid premature curing, the components are packed in compartments that are free of moisture and tight to permeation of moisture. Methoxysilanes can condense with chlorosilanes releasing methylchloride,21 as shown in Figure 8.6. The reaction is catalyzed by ferric chloride. 8.4.1.1

Platinum Complexes for Hydrosylilation

Additional crosslinking occurs by reaction of compounds with pendant vinyl groups. Certain platinum complexes catalyze the hydrosylilation re-

Silicones

CH3

CH3

Si O CH3 + Cl

Si O

CH3

CH3

CH3

333

CH3

Si O Si O CH3

CH3

CH3 Cl

Figure 8.6: Condensation of Methoxysilanes with Chlorosilanes

action. Suitable platinum catalysts are chloroplatinic acid, dichlorobis(triphenylphosphine)platinum(II), platinum chloride, platinum oxide, and also complexes of platinum compounds. For example, a Karstedt catalyst is a complex of chloroplatinic acid with 1,3-divinyl-1,1,3,3-tetramethyldisiloxane and 1,3-diethenyl-1,1,3,3-tetramethyldisiloxane.5 Synergistic catalyst systems are mixtures of the compounds H2 PtCl6 and RuCl3 × nH2 O.22 The hydrosilylation reaction proceeds at room temperature. However, using inhibitors the temperature can be increased. 8.4.1.2

Hydrosilylation Inhibitors

Hydrosilylation inhibitors fall into two general classes.23 One class is composed of materials that effectively inhibit hydrosilylation over a wide range of temperatures and can be volatilized out of the organosilicon composition to allow hydrosilylation to proceed. Examples of this class are pyridine, acrylonitrile, 2-ethenylisopropanol, and perchloroethylene. The other class of inhibitors is materials that are non-volatile. The inhibitory effect of these materials is overcome by heating, whereupon hydrosilylation takes place. Examples of this latter class are the reaction product of a siloxane having silicon-bonded hydrogen atoms, a platinum catalyst, and an acetylenic alcohol, organic phosphines and phosphites, benzotriazole, organic sulfoxides, metallic salts, aminofunctional siloxanes, ethylenically unsaturated isocyanurates olefinic siloxanes, dialkyl carboxylic esters, and unsaturated amides. Examples of inhibitors are shown in Table 8.4. In polyethers, oxidation impurities inhibit the hydrosilylation of the polyethers, however, the exact identities of these inhibitors are unknown. They are believed to include acetal hydroperoxides, allyl hydroperoxides and free radicals localized at the tertiary carbon atoms in the hydrophobic

334

Reactive Polymers Fundamentals and Applications Table 8.4: Inhibitors for Platinum Catalysts Inhibitor Methylbutynol Ethynyl cyclohexanol Diphenylphosphine 3-Methyl-1-dodecyn-3-ol 3,7,11-Trimethyl-1-dodecyn-3-ol

Remarks Most preferred5 Release coatings24

segments (e.g., propylene oxide) of unsaturated polyethers. Oxidation impurities are most likely to occur in polyethers which have been stored for a long period with no or insufficient quantities of antioxidant. However, they may also be present in freshly prepared polyethers which may have gotten too hot in the presence of air or oxygen. Polyethers can be stabilized with mixtures of ascorbic acid and sodium ascorbate and allyl polyethers.25 8.4.1.3

Salts

A commercially available curing catalyst material comprises zinc octoate and choline octoate.20 8.4.1.4

Polymethylsilazanes

Polymethylsilazanes are synthesized by the reaction of ammonia with dimethyldichlorosilane and methyltrichlorosilane. They are effective room temperature curing agents for silicone resins. However, ammonia is released in the course of curing.26

8.5

CROSSLINKING

Crosslinking can be achieved by different reactions at high temperatures for HTV-rubber and at room temperature for RTV-rubber. The liquid RTVsilicone rubber can crosslink both by condensation and by addition mechanisms.

8.5.1 Condensation Crosslinking Condensation crosslinking occurs between α,ω-dihydroxypoly(dimethylsiloxane)s and silicates in the presence of inorganic compounds. The cross-

Silicones

335

linking density depends on the functionality and concentration of the crosslinking agent and the nature of the catalyst.

8.5.2 Peroxides Crosslinking at higher temperatures in the range 100 to 160°C is achieved by the addition of peroxides. Suitable formulations contain a small amount of vinyl groups.

8.5.3 Hydrosilylation Crosslinking The hydrosilylation reaction is suitable for final crosslinking or curing reactions. 8.5.3.1

Thermoplastic Elastomers

Hydrosilylation crosslinking can be used to prepare a thermoplastic elastomer. A thermoplastic elastomer is a polymer or polymeric blend that can be processed and recycled in the same way as a conventional thermoplastic material. However, it has some properties and functional performance similar to those of vulcanized rubber at the service temperature. Elastomeric rubber blends are used in the production of high performance thermoplastic elastomers, particularly for the replacement of thermoset rubbers in various applications. High performance thermoplastic elastomers, in which a highly vulcanized rubbery polymer is intimately dispersed in a thermoplastic matrix, are generally known as thermoplastic vulcanizates. Hydrosilylation crosslinking of a rubber acts via the unsaturated groups present from norbornene and diene components. Even at low concentrations of hydrosilylation agent and catalyst, a rubber can be fully crosslinked in a dynamic vulcanization process and provide a thermoplastic elastomer product with excellent physical properties and oil resistance. Suitable hydrosilylation agents are methylhydrogen polysiloxanes, methylhydrogen dimethyl-siloxane copolymers, bis(dimethylsilyl)alkanes, and bis(dimethylsilyl)benzene.27 Platinum catalyst concentrations of 0.1 to 4 ppm are sufficient. The preparation is done by mixing the rubber and silicone hydride at 180°C. Then a solution of the platinum catalyst is added. The rubber is dynamically vulcanized by mixing until maximum torque is reached.

336

8.6


PROPERTIES

8.6.1 Silicone Rubber Silicone rubber consists essentially of silicone polymers and fillers. Silicone rubber formulations with molecular weights of more than 100 k Dalton and can flow, in contrast to other polymers. 8.6.1.1

HTV-silicone rubber

Silicone polymers for solid silicone rubber (HTV-silicone rubber) have molecular weights of 500 k Dalton to 1000 k Dalton, yet exhibit a pasty consistency. 8.6.1.2

RTV-silicone rubber

Pourable silicone rubber (RTV-silicone rubber) has a liquid consistency and molecular weights in the range of 10 k Dalton to 20 k Dalton.

8.6.2 Thermal Properties The service temperatures of silicones cover a wide range, from −60 to +250°C. Silicone rubber retains its elasticity to temperatures down to −60°C. The glass transition temperature is 120°C. At temperatures greater than 150 °C silicone rubbers are superior to other elastomers with respect to their thermal properties.28 Silicone rubber exhibits a flash point of 750°C and an excellent flame retardancy. However, on combustion, it releases toxic or corrosive gases. 8.6.2.1

Boron Siloxane Copolymers

Polymers containing boron within the polymer are high temperature oxidatively stable materials. It has been known that the addition of a carborane within a siloxane polymer significantly increases the thermal stability of such siloxane polymers.29 Hybrids of organic and inorganic components, made from 1,7-Bis(chlorotetramethyldisiloxy)-m-carborane, 1,3-dichlorotetramethyldisiloxane and 1,4-dilithio-1,3-butadiyne are shown in Figure 8.7. Oxidative crosslinking in air is found for poly(m-carborane-di-methylsiloxane) around 420°C.21 Such polymers can be converted into ceramics

Silicones

H3C Si CH3 CH3 Cl

CH3

CH3

CH3

O

Si O Si C C Si O Si Cl CH3 CH3 Z CH3 CH3

H3C Si CH3 C C

Z=B10H10

C C H3C Si CH3

Li

C C C C

O

Li

H3C Si CH3 C CH3 Cl

C

CH3

H3C Si CH3

Si O Si Cl CH3

Z

O

CH3

H3C Si CH3 B B

B

B

B

C

C

B

B B

B

B

Figure 8.7: Polymers from 1,7-Bis(chlorotetramethyldisiloxy)-m-carborane, 1,3-dichlorotetramethyldisiloxane and 1,4-dilithio-1,3-butadiyne

337

338


by pyrolysis. Carbon fibers coated with poly(carborane-siloxane-acetylene) can be protected against oxidation at elevated temperatures.30 8.6.2.2

Microcellular Ceramics

Microcellular foams were produced by means of poly(methyl methacrylate) as a sacrificial templating agent. Poly(methyl methacrylate) microbeads, were mixed in with a methylsilicone resin powder. The samples were heated up to 300°C and after one hour pyrolyzed at 1200°C. A silicon oxycarbide (SiOC) ceramic microcellular foam was obtained.31

8.6.3 Electrical Properties Silicone rubbers and resins are very efficient in insulating. The dielectric strength, the resistivity, and the dielectric constant do not change significantly with temperature.

8.6.4 Surface Tension Properties Unmodified silicones exhibit hydrophobic properties. When spread out as films they impart water repellency to the carrier material. The surface tension is only around 30 m N/m. Silazanes significantly improve water-repellent properties of silicone resins.19 Examples of hexaorganodisilazanes include hexamethyldisilazane, 1,3-dihexyltetramethyldisilazane, 1,3-di-tert-butyltetramethyldisilazane, 1,3-di-n-butyltetramethyldisilazane, and 1,3-diphenyltetramethyldisilazane.

8.6.5 Antioxidants Iron-containing polysilazanes exhibit an antioxidation effect on silicone oil and rubber.32 This type of polymer was synthesized by the polycondensation of silazane lithium salts with iron trichloride. The synthesis is shown in Figure 8.8. The gelling time of a silicone oil increased from 3 to 1000 hours at 300°C in air by an addition of 5% of polysilazane.

8.6.6 Gas Permeability Silicones have an extraordinarily high gas permeability. They find use in medical applications, e.g., as contact lenses, so that the oxygen in air can

Silicones

H H3C H3C

Si

N

N

Li

CH3 Si N

H3C BuLi

CH3

Si

H3C

Si H H H3C CH3

N

N

CH3 Si N

CH3

Si H H H3C CH3 BuLi

Li H3C Si

N

Li

CH3

H3C

Si

BuLi

CH3 N N Si H Li H3C CH3

H3C

H3C

Si

H3C

H3C

Si

CH3

Li

H3C

Li

Li

Si CH3

H3C Si

N

CH3

H3C H3C

H3C

CH3

FeCl3

CH3

CH3 Si

CH3 N N Si Li Li H3C CH3

H3C

Si

N

Si

N

N

Si

CH3

H3C

Si

N Li

Si

CH3

CH3

CH3 Fe H3C CH3 Si Li Li Si N N Si CH3 H3C Si CH3 H3C

Figure 8.8: Synthesis of Iron-containing Polysilazanes

339

340


contact the cornea of the eye. Another medical application is the use as permeable membrane in heart-lung machines.

8.6.7 Weathering Silicone rubber is highly resistant to ozone and radiation. Therefore, it exhibits good weathering properties.

8.7


Silicone products are used for a wide variety of applications, including building and construction material, medical applications, sealing, impregnation, putty, surface treatments, and painting applications. Silicone rubbers can be fabricated into tubing, hose, gaskets, and seals. Silicone oils are oligomeric chains of poly(dimethylsiloxane). The fluids are thermally stable and chemically resistant. They can serve as excellent lubricants.

8.7.1 Antifoaming Agents The low surface tension enables silicones to be used as antifoaming agents, foam stabilizers, and free-flowing agents, e.g., in paints. 8.7.1.1

Antifoaming Agents

Suspension polymerization of PVC, silicone antifoaming agents are an important constituent. Also, foaming in spinning baths of man-made fibers can be controlled by silicone antifoaming agents.

8.7.2 Release Agents 8.7.2.1

Mold Release Agents

Silicones are used as mold release agents in the rubber and plastics industries. Molds made from silicone rubber itself are common. Silicone resins are typically applied to surfaces by dissolving the silicone resin in volatile solvents. Evaporation of the solvent leaves behind the silicone resin in the desired location, e.g., on the surface of the mold for release, or in the cavities and interstices of the port as a sealant. Then,

Silicones

341

with the application of heat or chemicals, the resin is cured in-situ, forming a hard, polymeric network. Waterborne silicone release agents are common. An advantage of using water as a carrier is that the presence of water can prevent or delay silanol condensation of the resin. A catalyst may be added and stored in a water-based composition without inducing immediate curing. Hence, the use of water as a carrier improves the shelf life of the composition. The most significant difficulty associated with using water as a carrier is that silicone resins are relatively immiscible in water. Water-based silicone resin compositions can be formulated using conventional surfactants. Large amounts of surfactant, however, are usually required, and the dispersion formed may not be very stable. The dispersion can be stabilized with a hydrophobically modified polycarboxylic acid.33 8.7.2.2

Paper Release Agents

Crosslinkable silicone polymers are used as silicone release papers. These have a wide range of applications for labels and coatings.

8.7.3 Sealing and Jointing Materials Silicone seals have found widespread uses in cars, gaskets, household engines, and medical devices. Silicone jointing materials are used for expansion joints on building facades, connecting aluminum or plastic, and in the sanitary field, e.g., for bathroom tiles. Various silicone rubber grades have been developed with different curing systems.

8.7.4 Electrical Industry The high insulating power of silicones is appreciated in the electrical industry. Applications are in cables, electrical motors, seals, and heating elements. Silicone rubber rollers are used in photocopying devices, and facsimile devices.

8.7.5 Medical Applications Silicones are mostly inert to living organisms. They are considered nontoxic materials and can be used in pharmaceutical and medical applications.

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8.8



8.8.1 Polyimide Resins Polyimide resins are commonly used as materials for printed circuit boards and heat-resistant adhesive tapes because of their high heat resistance and superior electrical insulation properties. Common basic materials are 3,3′ ,4,4′ -diphenylsulfone tetracarboxylic dianhydride (DSDA) 4,4′ -hexafluoropropylidenebisphthalic dianhydride (6FDA), 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) and 3,3′ -dihydroxy4,4′ -diaminobiphenyl (HAB). They are used as resin varnish to form surface protective films and interlayer insulating films of electronic parts and semiconductor materials. Commonly a solution is prepared by dissolving a polyimide precursor such as polyamic acid. The solution is coated on a substrate, followed by removal of the solvent. Then high-temperature treatment effects the dehydration cyclization and the product obtained is used as polyimide resin. To improve the solubility of a polyimide resin in solvents, improve its adhesive force to substrates and impart flexibility, a siloxane group can be introduced into the polyimide skeleton. Such siloxane materials are diaminosiloxanes, i.e., straight chain silicones having amino groups at both terminals. Therefore, types with a small content of cyclic siloxane oligomers have been developed.34

8.8.2 Thermal Transfer Ribbons Thermal transfer printing is advantageous because relatively low noise levels are attained during printing. Thermal transfer printing is widely used in special applications such as printing of machine readable bar codes and magnetic alpha-numeric characters. Most thermal transfer ribbons employ poly(ethylene terephthalate) (PET) polyester as a substrate. The functional layer which transfers ink, also referred to as the thermal transfer layer, is deposited on one side of the substrate, and a protective backcoat is deposited on the other side of the poly(ethylene terephthalate) substrate. Untreated poly(ethylene terephthalate) will not pass under a thermal print head without problems. The side of the poly(ethylene terephthalate) substrate which comes in contact with the print head, i.e., the side opposite the thermal transfer layer, must be protected during the printing process.

Silicones

343

Failure to do so will result in the sticking of poly(ethylene terephthalate) to the heating elements during the heating cycle. Poly(ethylene terephthalate) is also an abrasive material which will cause unacceptable wear on the print head. Therefore, conventional thermal transfer ribbons which employ a poly(ethylene terephthalate) substrate have treated backsides. The substrate is coated on the reverse side to form a barrier between the poly(ethylene terephthalate) and the print head. The backcoats are usually comprised of silicone polymers. The most common backcoats are silicone oils and UV cured silicones. The silicone oils can be delivered directly to the PET substrate or via an organic solvent. However, for environmental reasons solvent-free coatings are used. A water-soluble silicone block copolymer consists of silicone resin blocks and water-soluble poly(ethylene oxide) blocks or poly(propylene oxide) blocks.35

8.8.3 Self-Assembly Systems The adhesion between a substrate and a polymer can be improved by using molecular self-assembling polymers. Typical self-assembly systems include silanes on hydroxylated substrates, such as glass surfaces or silicon wafers. The mechanical stability of a self-assembled polymer film can be increased by incorporating sticker groups in the polymer chain to introduce additional interactions between the sticker groups and the substrate solid surface. This is why silane functionalized poly(styrene) and poly(methyl methacrylate) were polymerized in the presence of a silane coupling agent, mercaptopropyltrimethoxysilane (MPS), which is also an effective chain transfer agent in the radical polymerization.36

8.8.4 Plasma Grafting Cornstarch granules could be surface functionalized in a high frequency plasma with ethylene diamine. In the second step the material was grafted with dichlorodimethylsilane. A poly(dimethylsiloxane) graft-copolymer layer on the modified surface was detected.37 The starch graft-copolymer might find use as a reinforcing component in silicone-rubber materials.

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8.8.5 Antifouling Compositions Aquatic animal and plant organisms such as barnacles, oysters, ascidians, polyzoans, serupulas, sea lettuces and green layers adhere and grow on the surface of marine structures, resulting in damage. For example, the aquatic organisms can adhere to the bottom of a ship, increasing the frictional resistance between the ship body and water. The increased resistance results in higher fuel costs. Some industrial plants use sea water for cooling. Fouling of intake pipes by aquatic animals and plants can hinder the induction of cooling water resulting in a drop in cooling effectiveness. A wide range of marine structures such as undersea construction, piers, buoys, harbor facilities, fishing nets, ships, marine tanks, water conduit raceway tubes of power plants and coastal industrial plants are affected. 8.8.5.1

Biopolymers

Marine organisms are initially attracted to and subsequently attach to a surface by chemical and physical means. Biopolymers such as polypeptides and polysaccharides comprise the outermost layer of marine organisms, and in some cases the marine organism exudes a glue, which is typically comprised of similar material, by which it attaches to a substrate. Biopolymers are very polar, and initial physical attachment to a substrate easily occurs when the substrate contains polar groups to which these biopolymers can form hydrogen-bonds. Further chemical attachment can take place by reactions between the biopolymers and a substrate. A hydrophobic surface is one which contains very little to no polar groups; thus, a hydrophobic surface expresses very few toeholds for marine organisms to adhere. The only type of attraction would be Van der Waals forces, which are very weak. 8.8.5.2

Toxicants

Various antifouling compositions have been developed to prevent the adherence of the aquatic organisms. Toxicants containing copper, tin, arsenic and mercury, and others been proposed. Further, strychnine, atropine, creosote, and phenol have been proposed. However, even the effective compositions have disadvantages. These compositions prevent fouling by a toxic mechanism. Effectiveness of the

Silicones

345

compositions requires that a lethal concentration of poison be maintained in the water immediately adjacent to the surface of the marine structure. Eventually, the poison is completely leached into the water and the composition is exhausted and must be replaced. Further, the poisons are toxic to humans and aquatic life and can be a major source of pollution in busy harbors and in waterways. 8.8.5.3

Fouling Release Coatings

Fouling release coatings, i.e., coatings which do not allow organisms to adhere to the marine body surface, have been proposed as alternatives to the toxicity-based antifouling agents. Particularly suitable are curable fluorinated silicone resins formed by replacing some but not all of SiH groups in an end-capped fluoroalkyl group-containing polyalkylhydrosiloxane. Otherwise, a fluorinated silicon resin can be blended with a non-fluorinated organopolysiloxane resin prior to crosslinking. Examples of suitable unsaturated fluoroalkyls include nonafluorohexene, 1H-1H-2H-perfluoroheptene, 1H-1H-2H-perfluorooctene, 1H-1H2H-perfluorononene and 3,3,4,4,5,5,5-heptafluoro-1-pentene. Fluorosilicons are prepared by reacting a polyalkylhydrosiloxane and an unsaturated fluoroalkyl compound. A suitable catalyst is an organic transition metal salt, such as zinc octoate. The fluorinated silicon resin is then crosslinked either by the pendant groups of the silicon resin itself, or with added compounds. Such added components can be methyltriethoxysilane or octyl triethoxysilane or a tetraethoxysilane or a fluoroalkyltriethoxysilane. The coating consists of more than one layer, an anticorrosive layer, the adhesion promoting layer, and the release layer. Adhesion promoting layers include a moisture curable grafted copolymer that further includes poly(dialkylsiloxane) and n-butyl acrylate, styrene, vinyl chloride and vinylidene chloride that is grafted onto the siloxane backbone. An aminofunctionalized polysiloxane is active as adhesion promoter. The release layer consists of the fluorinated polysiloxane.38

REFERENCES 1. E. G. Rochow. Silicone and silicones : About Stone-Age Tools, Antique Pottery, Modern Ceramics, Computers, Space Materials, and How They All Got

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Reactive Polymers Fundamentals and Applications that Way. Springer-Verlag, Berlin, 1987. H. A. Liebhafsky, S. Small Liebhafsky, and G. Wise. Silicones Under The Monogram: A Story of Industrial Research. Wiley-Interscience, New York, 1978. E. G. Rochow. Methyl silicones and related products. US Patent 2 258 218, assigned to General Electric, October 7 1941. E. G. Rochow and W. I. Patnode. Preparation of organosilicon halides. US Patent 2 380 996, assigned to General Electric, August 7 1945. Z. Li, F. J. McGarry, D. E. Katsoulis, J. R. Keryk, D. F. Bergstrom, K. S. Kwan, and B. Zhu. Hydrosilyation cured silicone resin containing colloidal silica and a process for producing the same. US Patent 6 646 039, assigned to Dow Corning Corporation (Midland, MI), November 11 2003. F. J. Feher, R. Terroba, and J. W. Ziller. Base-catalyzed cleavage and homologation of polyhedral oligosilsesquioxanes. Chem. Commun., pages 2153–2154, 1999. R. C. Camilletti, M. J. Loboda, and K. W. Michael. Semiconductor chips suitable for known good die testing. US Patent 5 693 565, assigned to Dow Corning Corporation (Midland, MI), December 2 1997. L. E. Carpenter, II and T. Michinio. Method for manufacturing hydrogensilsesquioxane resin. US Patent 6 353 074, assigned to Dow Corning Corporation (Midland, MI), March 5 2002. Y. Morita, J. Nakanishi, K. Tanaka, and T. Saruyama. Epoxy group-containing silicone resin and compositions based thereon. US Patent 5 952 439, assigned to Dow Corning Toray Silicone Co., Ltd. (Tokyo, JP), September 14 1999. A. Tomanek. Silicones, synthesis, properties, and applications. In J. C. Salamone, editor, The Polymeric Materials Encyclopaedia: Synthesis, Properties and Applications, pages 1462–1463. CRC Press, Boca Raton, FL, 1999. A. Bartole-Scott, R. Resendes, and I. Manners. Transition metal-catalyzed ring-opening polymerization of silicon-bridged [1]ferrocenophanes in the presence of functional silanes: Molecular weight control and synthesis of telechelic poly (ferrocenyl silanes). Macromol. Chem. Phys., 204(10): 1259–1268, July 2003. M. J. MacLachlan, J. Zheng, K. Thieme, A. J. Lough, I. Manners, C. Mordas, R. LeSuer, W. E. Geiger, L. M. Liable-Sands, and A. L. Rheingold. Synthesis, characterization, and ring-opening polymerization of a novel [1]silaferrocenophane with two ferrocenyl substituents at silicon. Polyhedron, 19(3): 275–289, February 2000. R. W. Heo, J.-S. Park, J. T. Goodson, G. C. Claudio, M. Takenaga, T. A. Albright, and T. Randall Lee. Romp of t-butyl-substituted ferrocenophanes affords soluble conjugated polymers that contain ferrocene moieties in the backbone. Tetrahedron, 60(34):7225–7235, August 2004.

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14. K. M. Lewis, R. A. Cameron, J. M. Larnerd, and B. Kanner. Direct synthesis process for organohalohydrosilanes. US Patent 4 973 725, assigned to Union Carbide Chemicals and Plastics Company Inc. (Danbury, CT), November 27 1990. 15. L. N. Lewis, R. E. Colborn, and J. M. Bablin. Method for improving selectivity for dialkyldichlorosilane. US Patent 6 407 276, assigned to General Electric Company (Schenectady, NY), June 18 2002. 16. B. Marciniec, editor. Comprehensive Handbook on Hydrosilylation. Pergamon Press, Oxford, New York, 1st edition, 1992. 17. P. Branlard, G. Mignani, P. Olier, and C. Willemin. Method for preparing silicones with arylalkyl function(s) by hydrosilylation. US Patent 6 437 163, assigned to Rhodia Chimie (Courbevoie Cedex, FR), August 20 2002. 18. Y. Tonomura, T. Kubota, and M. Endo. Hydrosilylation of 4-vinyl-1-cyclohexene. US Patent 6 245 925, assigned to Shin-Etsu Chemical Co., Ltd. (Tokyo, JP), June 12 2001. 19. S. Akamatsu and M. Sasaki. Curable silicone resin composition. US Patent 6 255 373, assigned to Dow Corning Toray Silicone Company, Ltd. (Tokyo, JP), July 3 2001. 20. F. J. McGarry, B. Zhu, and D. E. Katsoulis. Silicone resin based composites interleaved for improved toughness. US Patent 6 660 395, assigned to Dow Corning Corporation (Midland, MI), December 9 2003. 21. M. Patel and A. C. Swain. Thermal stability of poly(m-carborane-siloxane) elastomers. Polym. Degrad. Stabil., 83(3):539–545, March 2004. 22. K. D. Klein, W. Knott, and D. Windbiel. Synergistic catalyst system and process for carrying out hydrosilylation reactions. US Patent 6 307 082, assigned to Goldschmidt AG (Essen, DE), October 23 2001. 23. J. R. Keryk, P. Y. K. Lo, and L. E. Thayer. Silicone release coatings containing higher alkenyl functional siloxanes. US Patent 4 609 574, assigned to Dow Corning Corporation (Midland, MI), September 2 1986. 24. J.-M. Frances, R. S. Dordick, and A. Soldat. Silicone compositions comprising long chain α-acetylenic alcohol hydrosilylation inhibitors. US Patent 5 629 387, assigned to Rhone-Poulenc Chimie (Courbevoie Cedex, FR), May 13 1997. 25. K. M. Lewis and R. A. Cameron. Treatment of polyethers prior to hydrosilylation. US Patent 5 986 122, assigned to Witco Corporation (Greenwich, CT), November 16 1999. 26. Y. M. Zhang, Y. Huang, X. L. Liu, and Y. Z. Yu. Studies on the silicone resins cured with polymethylsilazanes at ambient temperature. J. Appl. Polym. Sci., 89(6):1702–1707, August 2003. 27. R. E. Medsker, D. R. Hazelton, G. W. Gilbertson, S. Abdou-Sabet, K.-S. Shen, R. L. Hazelton, and Ravishanka. Hydrosilylation crosslinking of thermoplastic elastomer. US Patent 6 251 998, assigned to Advanced Elastomer Systems, L.P. ; Exxon Chemical Patents, Inc., June 26 2001.

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28. P. R. Dvornic and R. W. Lenz. High Temperature Siloxane Elastomers. Hüthig & Wepf, Basel, New York, 1990. 29. T. M. Keller and D. Y. Son. High temperature ceramics derived from linear carborane-(siloxane or silane)-acetylene copolymers. US Patent 6 265 336, assigned to The United States of America as represented by the Secretary of the Navy (Washington, DC), July 24 2001. 30. T. M. Keller. Oxidative protection of carbon fibers with poly(carborane-siloxane-acetylene). Carbon, 40(3):225–229, March 2002. 31. P. Colombo, E. Bernardo, and L. Biasetto. Novel microcellular ceramics from a silicone resin. J. Am. Ceram. Soc., 87(1):152–154, January 2004. 32. Y. M. Li, Z. M. Zheng, C. H. Xu, C. Y. Ren, Z. J. Zhang, and Z. M. Xie. Synthesis of iron-containing polysilazane and its antioxidation effect on silicone oil and rubber. J. Appl. Polym. Sci., 90(1):306–309, October 2003. 33. J. Wang. Water-based silicone resin compositions. US Patent 5 804 624, September 8 1998. 34. M. Sugo and H. Kato. Polyimide silicone resin, process for its production, and polyimide silicone resin composition. US Patent 6 538 093, assigned to Shin-Etsu Chemical Co., Ltd. (Tokyo, JP), March 25 2003. 35. J. D. Roth. Water soluble silicone resin backcoat for thermal transfer ribbons. US Patent 6 245 416, assigned to NCR Corporation (Dayton, OH), June 12 2001. 36. F. Zhou, W. M. Liu, T. Xu, S. J. Liu, M. Chen, and J. X. Liu. Preparation of silane-terminated polystyrene and polymethylmethacrylate self-assembled films on silicon wafer. J. Appl. Polym. Sci., 92(3):1695–1701, May 2004. 37. Y. H. C. Ma, S. Manolache, M. Sarmadi, and F. S. Denes. Synthesis of starch copolymers by silicon tetrachloride plasma-induced graft polymerization. Starch-Stärke, 56(2):47–57, February 2004. 38. A. E. Mera and K. J. Wynne. Fluorinated silicone resin fouling release composition. US Patent 6 265 515, assigned to The United States of America as represented by the Secretary of the Navy (Washington, DC), July 24 2001.

9 Acrylic Resins Acrylic resins are polymers of acrylic or methacrylic esters. They are sometimes modified with monomers such as acrylonitrile and styrene. The most common acrylates are methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate. Methacrylates include methyl methacrylate, ethyl methacrylate, butyl methacrylate, and higher alcohol esters. The resins are used either as molding powders or casting syrups. Acrylic resins are often used as hybrid resins in combination with urethanes, epoxides and silicones. Since coatings are not the primary goal of this topic, coating applications will be dealt with only marginally, even when acrylic resins contribute highly to this topic. Acrylic resins are also widely used for dental applications. We treat this topic because of its importance in a special chapter. Here we focus on non-dental applications of acrylic resins. An overview on acrylic and methacrylic ester polymers is given in the literature.1, 2

9.1 HISTORY Acrylic acid was obtained through the air oxidation of acrolein by Redtenbacher in 1843. Methacrylic acid was first prepared in 1865. Otto Röhm observed the polymerization of acrylics. The production of acrylates started in 1927 by Röhm&Haas. In 1936 poly(methyl methacrylate) was prepared by a casting process. 349

350


O CH2

O

CH C O CH3

CH2

CH C O CH2 CH3

Methyl acrylate

Ethyl acrylate

O CH2

CH C O CH2

CH

CH2

CH2

CH2

CH2

CH3

CH2 CH3 2-Ethylhexyl acrylate O CH2

CH C O O

CH2

CH2

CH C O CH2 O CH2

CH

C CH2

CH3

CH2

C O

Trimethylolpropane triacrylate

Figure 9.1: Acrylate-based Monomers

9.2

MONOMERS

A large variety of monomers is known, because of the possibility of esterifying the acrylic acid and methacrylic acid with various alcohols. The most common monomers are shown in Table 9.1. Some acrylate-based monomers are shown in Figure 9.1.


Cyclohexyl methacrylates

Polymers containing cyclohexyl methacrylate and related compounds such as 4-methylcyclohexylmethyl methacrylate exhibit high weather resistance. This is due to its low hygroscopic functional group. It is used for coating materials.

Acrylic Resins

351

Table 9.1: Monomers for Acrylic Resins Monomer Acrylic Monomers Acrylic acid Methyl acrylate Ethyl acrylate n-Butyl acrylate 2-Ethylhexyl acrylate Trimethylolpropane triacrylate (TMPTA) Aziridine derivatives Methacrylic Monomers Methyl methacrylate Ethyl methacrylate 2-Hydroxyethyl methacrylate (HEMA) n-Butyl methacrylate Ethylene glycol dimethacrylate Poly(ethylene glycol)dimethacrylate 3-Methacryloxypropyl-trimethoxysilane (MPTS) Cyclohexyl methacrylate 4-Methylcyclohexylmethyl methacrylate Methacryloyl isocyanate 2-Methacryloyloxyethyl isocyanate Methyl N-methacryloylcarbamate Phenyl N-methacryloylcarbamate 2-Ethylhexyl N-methacryloylcarbamate 2-Isocyanatoethyl methacrylate (IEM) a Crosslinker b Standard c Flexible d Improved weatherability

Remarks

Reference

a

3

a

4

b

c a 5 d

6 7 7 7 8 8 9 10

352


9.2.1.2

Methacryloyl Isocyanate and Derivatives

Alkenoylcarbamates can be readily polymerized by themselves or with any other vinyl compounds. The carbamates formed from alcohols with a small number of carbon atoms are available in a stable solid form under the atmospheric condition and can be dissolved easily in various solvents. The acylurethane structure contributes to an enhancement of cohesion. Therefore, copolymers containing alkenoylcarbamate units show various advantageous properties such as high elasticity and good adhesion. The introduction of an epoxy or aziridino group introduces further reactive moieties. The modification to a blocked isocyanate structure provides the alkenoylcarbamate compounds with the latent reactivity of an isocyanate group, which will be actualized from the blocked isocyanate structure under heating. Methyl N-methacryloylcarbamate, phenyl N-methacryloylcarbamate, benzyl N-methacryloylcarbamate, and a series of other methacryloyl carbamates can be synthesized from methacryloyl isocyanate by adding the appropriate alcohols to methacryloyl isocyanate.8 The synthesis is shown in Figure 9.2. The reaction is conducted at low temperatures. Also an exchange of the alcohol group in the carbamate is possible. For example, ethyl N-methacryloylcarbamate can be reacted with 2-ethylhexyl alcohol in the presence of a radical polymerization inhibitor such as hydroquinone at 120°C. The ethoxy moiety is then replaced by the 2-ethylhexyloxy moiety to result in 2-ethylhexyl N-methacryloylcarbamate. This product is a viscous liquid.9

9.2.2 Synthesis 9.2.2.1

Monomers

Acrylic acid is synthesized by the oxidation of propene via acrolein. Methyl methacrylate is synthesized from acetone via the acetone cyanhydrin (ACH). The reactions are shown in Figure 9.3. The conventional process for the synthesis of methyl methacrylate runs via the acetone cyanhydrin. Other technical processes include • The ACH-based process (Röhm&Haas, Mitsubishi Gas Chemical), • The i-butylene oxidation process (Lucky, Japan Methacrylic), • The tert-butanol oxidation process (Kyodo, Mitsubishi Rayon),

Acrylic Resins

CH3 CH2

CH3

CH3OH

C C N C O

CH2

353

H

C C N C O

O

O

O CH3

N-Methacryloylcarbamate

CH3 CH2

C C N C O O

OH

CH3 CH2

H

C C N C O O

O

Phenyl N-methacryloylcarbamate

Figure 9.2: Synthesis of Methyl N-methacryloylcarbamate and Phenyl N-methacryloylcarbamate8

• The propyne carbonylation (Shell, ICI), and • The hydrocarbonylation of ethene.11 9.2.2.2

Esterification

The reaction of methacrylic acid with an alcohol results in the respective ester. Also, an olefin can be added to the acid in the presence of anhydrous catalysts. Ethylene oxide reacts to form the hydroxy alkyl esters. Diazomethane reacts to the methyl esters. The reactions are shown in Figure 9.4.

9.2.3 Manufacture Various structural elements, such as rods, sheets, tubes, and molding powders are produced by bulk polymerization. The most common method for the production of sheets is the batch cell method. The polymerization process releases a lot of heat and has to be carried out slowly, in order to avoid an adverse effect on the optical properties. If the polymerization in bulk quantities proceeds too fast, even the boiling point can be crossed and

354


CH2

O2

CH CH3

O CH2

CH C H

O2

O CH2

CH C

CH2

H

O

CH3 C O

CH3 HCN

CH C OH

HO C C N

CH3

CH3

H2SO4 CH2 C C NH2 CH3 H2SO2 CH3

OH

CH3 CH2

C C OCH3 O

Figure 9.3: Synthesis of Acrylic acid and Methyl methacrylate

Acrylic Resins

CH3 CH2

CH3

C C OH + HO

R

CH2

O

C C OR O

CH3 CH2

C C OH + CH2

CH

R

O CH3 C C O CH2 CH2 R

CH2

O O

CH3 CH2

C C OH + H2C

CH2

O CH3 CH2

C C O CH2

CH2

OH

O

CH3 CH2

C C OH O

CH3 + CH2N2

CH2

C C OCH3 O

Figure 9.4: Esterification Reactions of Methacrylic Acid

355

356

Reactive Polymers Fundamentals and Applications Table 9.2: Ultraviolet Absorbers for Acrylic Resins12 Compound Benzotriazole Ultraviolet Absorbers 2-(5-Methyl-2-hydroxyphenyl)benzotriazole 2-[2-Hydroxy-3,5-bis(α, α-dimethylbenzyl)phenyl]-2H-benzotriazole 2-(3,5-Di-tert-butyl-2-hydroxyphenyl)benzotriazole 2-(3-tert-Butyl-5-methyl-2-hydroxyphenyl)-5-chlorobenzothiazole 2-(3,5-Di-tert-Butyl-2-hydroxyphenyl)-5-chlorobenzothiazole 2-(3,5-Di-tert-amyl-2-hydroxyphenyl)benzotriazole 2-(2′ -Hydroxy-5′ -tert-octylphenyl)benzotriazole 2-Hydroxybenzophenone Ultraviolet Absorbers 2-Hydroxy-4-methoxybenzophenone 2-Hydroxy-4-octoxybenzophenone 2,4-Dihydroxybenzophenone 2-Hydroxy-4-methoxy-4′ -chlorobenzophenone 2,2′ -Dihydroxy-4-methoxybenzophenone 2,2′ -Dihydroxy-4,4′ -dimethoxybenzophenone Salicylic Acid Phenyl Ester Ultraviolet Absorbers p-tert-Butylphenyl salicylate p-Octylphenyl salicylate

thus bubbles are formed. Inhomogeneous temperature distribution during polymerization my cause streaks in the material.

9.3

SPECIAL ADDITIVES

9.3.1 Ultraviolet Absorbers Examples of ultraviolet absorbers are shown in Table 9.2.

9.3.2 Flame Retardants Flame resistance can be imparted by incorporating certain organic phosphoric acid esters to acrylic resins. Some flame retardants are shown in Table 9.3 and in Figure 9.5. However, these organic phosphoric acid esters usually have a plasticizing effect. They are likely not only to substantially lower the heat distortion temperature of the acrylic resin products, but also to lower its

Acrylic Resins

357

Table 9.3: Flame Retardants13, 14 Compound

Remarks

13

Phosphoric acid esters Chlorinated polyphosphates Halogenated polyphosphonate Alkyl acid phosphate Tetrabromobisphenol A 2,2-Bis(4-hydroxy-3,5-dibromophenyl)propane Tricresyl phosphate Tris(2-chloroethyl)phosphate Antimony trioxide Zirconium hydroxide Barium metaborate Tin oxide

O CH2

O

Reference 13 13

Synergist

13 14 14 14 14

Inorganic Inorganic Inorganic Inorganic

14 14 14 14

O

O P O CH P O CH P O CH2 CH3 O

CH2

CH2

Cl

CH2

CH2

CH2

Cl

CH2

CH2

Cl

Cl

CH3 O

CH2 Cl

Figure 9.5: Chlorinated Polyphosponate (Phosgard C-22 R™, Monsanto)

358


Table 9.4: Global Production Data of Important Monomers and Polymers15 Monomer

Mill. Metric tons

Methyl methacrylate Acrylic surface coatings Acrylic resins and plastics

2 0.92 0.85

Year

Reference

2002 2003 2000

16 17 18

mechanical strength. Further, the water absorptivity of the resin products tends to increase by the incorporation of such flame retardants, and when used outdoors, the resin products are likely to undergo deformation or crazing upon absorption of water. For a copolymer of methyl methacrylate, α-methylstyrene, styrene, maleic anhydride, and methacrylic acid, a synergism has been observed. When two types of flame retardants, i.e., a halogen-containing condensed phosphoric acid ester or a halogenated polyphosphonate and an alkyl acid phosphonate, are combined, superior flame resistance and physical properties will be imparted by the synergistic effect of the components. Here, it is possible to reduce the amount of the main flame retardant to a level of about 20% even when the flame resistance should meet the standard of V-0 of the UL Standards.19 Therefore, it is possible to avoid the deterioration of the physical properties, particularly the deterioration of the heat resistance, which is a serious problem when a great amount of the flame retardant is added.13

9.3.3 Production Data of Important Monomers Global production data of the most important monomers used for acrylic ester resins are shown in Table 9.4.

9.4

CURING

The polymerization of acrylic resins occurs essentially by a radical mechanism.

9.4.1 Initiator Systems Traditional radical polymerization initiators may be used for the casting polymerization. Common catalysts are shown in Table 9.5. Polymerization

Acrylic Resins

Table 9.5: Polymerization Initiators for Casting Initiator Azobis-type

Remarks Catalysts13

2,2′ -Azobis(isobutyronitrile) 2,2′ -Azobis(2,4-dimethylvaleronitrile)

preferred preferred

Diacylperoxide-type Catalysts13 Lauroyl peroxide Dibenzoyl peroxide Bis(3,5,5-trimethylhexanoyl)peroxide

preferred

Perester-type Catalysts20 tert-Amylperoxy-2-ethylhexanoate tert-Butylperoxy-2-ethylhexanoate Percarbonate-type Catalysts13 bis(4-tert-Butylcyclohexyl)peroxydicarbonate UV Curing Catalysts3 2,2-Dimethoxy-2-phenylacetophenone (DMPA) benzophenone (BP) and methyldiethanolamine (MDEA) Acyl-phosphine oxide

preferred

21

359

360


initiators particularly suitable for the continuous sheet-forming process are those having a decomposition temperature, at a half-life of 10 hours, in the range of 40°C to 80°C.

9.4.2 Promoters Special initiator systems for cold curing were developed. An effective initiator promoter system consists of a zinc 2-ethylhexanoate solution, a cobalt 2-ethylhexanoate, and as peroxide source tert-butylperoxybenzoate.22

9.5

PROPERTIES

Acrylic resins are appreciated for their exceptional clarity and optical properties. Acrylics show a slow burning behavior and can be formulated as self-extinguishing. Acrylic resins are excellent in transparency, translucency, surface gloss, and weather resistance and further have a high surface hardness and a superior design adaptability. Therefore, they find a wide variety of applications in interior materials for vehicles, exterior materials for household electrical appliances and building materials (exterior) for example, regardless of whether they are outdoor or indoor applications. However, acrylic resins generally exhibit poor flexibility and low impact resistance and, therefore, pose a problem in that they are prone to fracture when given an extraneous load or impact.

9.5.1 Electrical Properties Acrylic resins are easily electrically charged by friction because of their high surface resistivity. Thus they are deteriorated in appearance by adhesion of rubbish or dust, or they bring about an undesirable situation by electrostatic electrification in parts of electronic equipment. Antistatic properties to the acrylic resin can be imparted by23 • Kneading a surfactant with the acrylic resin, or applying a surfactant on the surface of the acrylic resin, • Kneading a vinyl copolymer having a poly(oxyethylene) chain and a sulfonate, carboxylate or quaternary ammonium salt structure, with an acrylate resin,

Acrylic Resins

361

• Kneading a polyether ester amide with a methyl methacrylatebutadiene-styrene copolymer, • Adding a functional polyamide elastomer, • Adding a polyamide-imide elastomer having a low content of hard segments.

9.5.2 Hydrolytic and Photochemical Stability Methacrylate-based polymers have a better hydrolytic stability than the corresponding acrylate polymers. They are much more stable than vinyl acetate polymers. Acrylic and methacrylic resins are not very sensitive to ultraviolet radiation. However, ultraviolet absorbers improve stability. Adding ultraviolet absorbers, e.g., to arcylic windows, also protects the interior from UV radiation.

9.5.3 Recycling Poly(methyl methacrylate) depolymerizes nearly qualitatively (ca. 96%) on pyrolysis into the monomer. This property is attractive for thermal recycling of unmixed poly(methyl methacrylate) wastes. The situation in the case of acrylates is different.

9.6 APPLICATIONS AND USES Acrylic resins have been widely used as materials for various parts of electronics products, household appliances, office automation appliances, etc. because of their excellent transparency and stiffness.23 They are used in the sanitary sector, as surrogate for ordinary glass.

9.6.1 Acrylic Premixes An acrylic resin composition can be used as raw material for an acrylic premix for producing an acrylic artificial marble. Acrylic artificial marbles are obtained by blending an acrylic resin with inorganic fillers such as aluminium hydroxide. They have an excellent appearance, soft feeling and weatherability and are widely used for kitchen counters, lavatory dressing tables, waterproof pans, etc.

362


Artificial marbles are generally produced by filling a slurry mold. This mold is obtained by dispersing inorganic fillers in an acrylic syrup. The filled slurry must be cured at relatively low temperature. The acrylic syrup has a comparatively low boiling temperature. Consequently a long time is required for molding which causes low productivity. To overcome these drawbacks, the acrylic syrup can be blended with a crosslinked resin powder having a specific degree of swelling. On the other hand, an acrylic premix for an artificial marble, with excellent low shrinking properties has been prepared by blending the acrylic syrup with a thermoplastic acrylic resin powder, which is poorly soluble in the syrup. The acrylic syrup consists essentially of methyl methacrylate or a (meth)acrylic monomer mixture and poly(methyl methacrylate) or an acrylic copolymer. To impart strength, solvent resistance, and dimension stability to a molded article, instead of pure methyl methacrylate monomer, a polyfunctional (meth)acrylic monomer may be added. It is preferable to replace the methyl methacrylate monomer by neopentyl glycol dimethacrylate up to 50%, since the molded article then has a remarkably excellent surface gloss.24 The acrylic syrup may be obtained by dissolving the component acrylic polymer in the monomer, or a syrup can be obtained by partial polymerization of the component in the monomer. To the premix curing agents, such as dibenzoyl peroxide, lauroyl peroxide, tert-butyl hydroperoxide, cyclohexanone peroxide, methylethylketone peroxide, tert-butylperoxyoctoate, tert-butylperoxybenzoate, dicumyl peroxide, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,2′ -azobis(isobutyronitrile) are added. The filler then is added and heat and pressure curing takes place for 10 minutes under conditions of a mold temperature of 130°C and a pressure of 100 kg/cm2 to obtain an acrylic artificial marble with a thickness of 10 mm.

9.6.2 Protective Coatings in Electronic Devices In semiconductor devices including a ferroelectric film or a dielectric film with a high dielectric constant, a surface coating has been proposed. This coating is made of an acrylic resin which prevents the degradation of the polarization properties of the ferroelectric or a film with a high dielectric constant, respectively, on the semiconductor device.25

Acrylic Resins

363

9.6.3 High-performance Biocomposite An environmentally friendly acrylic resin has been examined with respect to a high-performance biocomposite. Parameters such as the onset of curing reaction, the degree of cure at certain temperatures, and the swelling equilibrium data were analyzed. The crosslinking density after curing the resin at 180°C for 10 min indicates the completion of curing to a major extent under those conditions.26

9.6.4 Solid Polymer Electrolytes In conventional batteries with electrolytic solutions, the possible leakage of the electrolyte solution or elution of the electrode substance outside the battery presents a problem in long-term reliability. Batteries and electric double-layer capacitors using a solid polymer electrolyte are free of these problems. Also, these can be easily reduced in thickness. For installing a solid polymer electrolyte into a battery or electric double-layer capacitor, a method of using an electrolyte and a trifunctional methacrylic compound as the main components for the solid polymer electrolyte has been developed. The monomer for the solid polymer electrolyte is prepared from 2-isocyanatoethyl methacrylate and a branched oligo glycol. Such a glycol can be prepared by the etherification of glycerol, ethylene glycol, and propylene glycol. The isocyanate group adds with the pendent hydroxyl groups resulting in a trifunctional methacrylate ester.10 The synthesis is shown in Figure 9.6. Also, a mixture of poly(ethylene glycol)dimethacrylate and methoxypoly(ethylene glycol) mono methacrylate can serve as a monomer for solid polyelectrolyte polymers.27 It is desirable for a three-dimensional network structure to be formed. The polymerization is initiated by conventional peroxides or azo compounds. The whole formulation for a battery consists of: 1. 2. 3. 4. 5. 6.

Methacrylic monomer, Polymerization initiator, Polymerization retarder, Electrolyte salt, Organic solvent, and Inorganic particles.

The process for manufacturing a complete battery is described in detail elsewhere.10

364


CH3 CH2

CH C O CH2 CH3

CH2

NCO HOR CH2

CH2

NCO HOR CH

CH2

NCO HOR CH2

O

CH C O CH2 CH3

CH2

CH2

O

CH C O CH2 O

H O

CH3 CH2

CH C O CH2 CH3

CH2

O

CH C O CH2 CH3

CH2

CH2

H O CH2

O

CH C O CH2

N C OR CH2

N C OR CH H O

CH2

N C OR CH2

O

R = (CH2CH2O)n(CH2CHO)m CH3

Figure 9.6: Synthesis of a Methacrylate Monomer for a Solid Electrolyte10

Acrylic Resins

365

9.7 SPECIAL FORMULATIONS 9.7.1 Silane and Siloxane Acrylate Resins Weather resistant resin coatings can be prepared by an addition polymerization reaction of n-butyl acrylate, methyl methacrylate, n-butyl methacrylate, and 3-methacryloxypropyl-trimethoxysilane (MPTS). The weather resistant silicone/acrylic resin coatings are then blended with TiO2 . The viscosity of the resin decreases with increasing content of MPTS, whereas the thermal stability at high temperature increases.5, 28, 29 Coatings with 30% MPTS have especially good weather resistant properties. UV-curable formulations for UV-transparent optical fiber coatings have been developed. Poly(dimethylsiloxane-acrylate) resins showed the best performance with respect to the monomer reactivity and the UV-transparency of the polymer coating. An acyl-phosphine oxide proved to be the best suited because of its high reactivity, fast photolysis, and lack of absorbing of the by-products of photo curing at the wavelength of operation.21

9.7.2 Marble Conservation Marble and stone used as building materials are susceptible to environmental damage. Acrylic resins can be used as carriers of suitable pigments for the protection of the surface of a monument. Copolymers of ethyl methacrylate and methyl acrylate have been extensively applied as a protective agent for stone building materials since the 1950s. The photodegradation of acrylic resins containing titanium dioxide pigments has been studied under UV irradiation. Two kinds of TiO2 , anatase and a mixture of anatase and rutile, were used in different concentrations. The changes caused by the irradiation treatment were monitored by Fourier transform infrared spectroscopy, gel permeation chromatography, and solubility measurements. The presence of anatase pigment significantly improved the photostability.30 Films of acrylic resins of varying compositions were applied both on a dolomitic white marble support and on potassium bromide disks and exposed to UV light. The main degradation pathway under ultraviolet irradiation is the chain scission. The rates of photodegradation may be related to the type of ester group and to the presence of the α-methyl group in the main chain.31

366


Blends of acrylic resins and fluoroelastomer are also suitable materials in stone protection. Films of a copolymer of ethyl methacrylate and methyl acrylate blended with a copolymer of vinylidene fluoride and hexafluoropropene were investigated by means of FT-IR spectroscopy and FT-IR microspectroscopy before and after UV and thermal treatments. A high content of fluoroelastomer increases the stability of these blends. A solution of a blend described above in tetrahydrofuran was successfully applied to a marble surface of the Saint Maria Cathedral in Lucca, Toscana.32 9.7.2.1

Degradation by Lipase

Layers of an aged acrylic resin, a fifteenth-century tempera painting on panel and a nineteenth-century oil painting on canvas have been removed by the action of lipases. Lipases are hydrolytic enzymes that act on glycerol ester bonds. These enzymes are a less aggressive alternative to highly polar organic solvents or alkaline mixtures.33

9.7.3 Tackifier Resins Acrylic resins are suitable as tackifier resins in pressure-sensitive adhesive applications. They can be prepared by free-radical polymerization. The acrylic tackifier is then blended with a natural rubber base in various ratios. Investigation of the mechanical properties showed that blends with a good pressure-sensitive adhesive performance have a higher loss of tangent-δ at higher frequencies.34

9.7.4 Drug Release Membranes The feasibility of a transdermal delivery system (TDS) for 17-β-estradiol was investigated by in vitro release studies. Unilaminate adhesive devices capable of releasing 17-β-estradiol in a controlled fashion over a period up to 216 hours have been developed using acrylic resins. The release of drug from the adhesive devices seems to obey a zero-order kinetics. Acetyltributyl citrate (ATBC), triethyl citrate (TEC), propylene glycol (PG) and myristic acid (MA) are plasticizers that can modify the release patterns of the drug. The study demonstrated that the acrylic resins are suitable polymers for the preparation of 17-β-estradiol TDS adhesive devices.35

Acrylic Resins

367

9.7.5 Support Materials for Catalysts Palladium-tin catalysts can be deposited on acrylic resins bearing carboxylic functional groups. The resins act as support materials for the catalysts. The catalysts are suitable for the selective hydrogenation of 100 ppm aqueous nitrate solutions. The materials exhibit different Sn contents and show different reduction temperatures. A high COOH content in the support is important in the control of the selectivity of the catalysts limiting ammonia formation.36 Copper ion catalysts can be immobilized on acrylic resins (rather than acrylonitrile resins) with aminoguanidyl groups. They are prepared by modification of the nitrile groups in an acrylonitrile, vinyl acetate, and divinylbenzene terpolymer using aminoguanidine carbonate. The catalysts act on the oxidation of hydroquinone to p-benzoquinone with hydrogen peroxide as oxidant. The catalytic activity and selectivity in the Cu(II)-resin system increases in comparison to the reaction without a catalyst and the reaction with native Cu(II) ions.37

9.7.6 Electron Microscopy Acrylic resins can be used for low-temperature embedding of samples in electron microscopy.38

9.7.7 Stereolithography Three-dimensional objects can be built without the use of molds by stereolithography. The objects are obtained layer by layer by polymerizing a low-viscosity liquid resin under a laser beam. The kinetic behavior of the resin is essential for a complete curing which occurs in the small zone exposed to laser irradiation. The isothermal kinetic behavior of a commercial acrylic resin for stereolithography has been analyzed by differential photocalorimetric analysis. A kinetic model accounting for the effect of autoacceleration, the vitrification, and light intensity has been set up.39

9.7.8 Laminated Films Laminated films of acrylic resins are constituted of a soft layer formed of an acrylic resin with rubber particles incorporated and a hard layer also formed of an acrylic resin.

368


By incorporating rubber particles into an acrylic resin, the flexibility is improved while all other positive properties, such as transparency and surface gloss, are maintained. To have a hard outer surface two films, one of them filled with rubber particles and one being unfilled, are combined. The laminated film is excellent in surface hardness, flexibility and ability to prevent whitening from occurring during molding or forming and hence is suitable for use as a surface material for moldings, such as interior materials for vehicles, exterior materials for household electrical appliances and building materials (exterior), which are obtained by a molding or forming process requiring bending or stretching.12

9.7.9 Ink-jet Printing Media An ink-jet printing system is one wherein ink droplets are jetted onto a surface of a printing medium and attached thereon. Therefore, the surface of the printing medium needs to rapidly absorb the jetted ink droplets. The printing media for use in the ink-jet printing system are not limited to paper but include various materials such as transparent resin films for overhead films and metals. Some of these printing media have no hydrophilic surfaces. Therefore, in order to clearly print information, an ink-receiving layer needs to be provided on the surface of a substrate constituting the printing medium. The ink-jet printing media are required to have the following characteristics: • Permeation of ink inside the ink-receiving layer must be rapidly made, color running should not take place, and clear color having high chromaticity can be reproduced. • In the multi-color printing using a combination of ink components, each ink component must be rapidly absorbed even if ink dots are superposed on the same surface of the printing medium, and in the high-speed printing, the printed surface must be free from staining, and the ink absorption rate and the ink absorption quantity both must be satisfactory. • The printing medium must have water resistance, and even if the printed image is contacted by water, running or bleeding of ink of the image must not take place. • Even if the ink-jet printing media are stored in the superposed state, they must be free from blocking.

Acrylic Resins

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• Even if the printed matter is stored for a long period of time, color fading should not take place. Proposals for forming an ink-receiving layer containing hydrophilic resins include40 • • • • •

Starch and other water-soluble cellulose derivatives, Polyvinyl alcohol, modified polyvinyl alcohol, Polyvinyl pyrrolidone, Polyvinyl acetal, and Hydrophilic acrylic copolymer having a crosslinked structure on the substrate surface.

A hydrophilic acrylic copolymer having a crosslinked structure consists of acrylamide, methacrylamide and other amides, acrylic acid and acrylic esters, such as glycidyl acrylate and the corresponding methacrylate derivatives, respectively. Examples of crosslinking agents include divinylbenzene, ethylene glycol acrylate, and triethylene glycol diacrylate. A suitable polymerization initiator is 2,2′ -azobis(2-amidinopropane)hydrochloride or dibenzoyl peroxide in toluene. The polymerization is carried out in aqueous isopropyl alcohol and poly(oxyethylene) nonylphenyl ether.40 The copolymer was isolated and a dispersion was prepared that was applied on wood-free paper and poly(ethylene terephthalate).

REFERENCES 1. J. W. Nemec and W. Bauer. Acrylic and methacrylic ester polymers. In J. I. Kroschwitz, editor, Encyclopedia of Polymer Science and Engineering, volume 1, pages 211–234. John Wiley & Sons, Inc., New York, 2nd edition, 1985. 2. H. W. Coover, Jr. and J. M. McIntire. Acrylic and methacrylic ester polymers. In J. I. Kroschwitz, editor, Encyclopedia of Polymer Science and Engineering, volume 1, pages 234–305. John Wiley & Sons, Inc., New York, 2nd edition, 1985. 3. S. Yin, A. Merlin, A. Pizzi, X. Deglise, B. George, and M. Sylla. Structureproperty relationship and influences of phenolic compounds on the mechanical and thermomechanical properties of UV-cured acrylic resin networks. J. Appl. Polym. Sci., 92(6):3499–3507, June 2004. 4. F. Xie, Z. H. Liu, and D. Q. Wei. Curing kinetics and properties of acrylic resin cured with aziridine crosslinker. Chin. J. Polym. Sci., 20(1):65–70, January 2002.

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5. H. S. Park, D. J. Chung, H. S. Hahm, S. K. Kim, W. B. Im, and S. J. Kim. Preparation and physical properties of weather resistant silicone/acrylic resin coatings. J. Chem. Eng. Jpn., 37(2):158–165, February 2004. 6. H. Nomura, Y. Kimata, H. Kanai, Y. Yokota, and M. Yoshida. Weatherability of hals copolymerized acrylic resin precoated steel sheet. Tetsu To Hagane-J. Iron Steel Inst. Jpn., 89(1):128–134, January 2003. 7. K. Nakamura, Y. Yokota, K. Takahashi, and M. Yoshida. Cyclohexylalkyl (meth) acrylate ester-based resin composition. US Patent 6 686 413, assigned to Nippon Shokubai Co., Ltd. (Osaka, JP), February 3 2004. 8. S. Urano, R. Mizuguchi, N. Tsuboniwa, K. Aoki, Y. Suzuki, and T. Itoh. Carbamate physical property-improving reagent. US Patent 4 935 413, assigned to Nippon Paint Co., Ltd. (Osaka, JP), June 19 1990. 9. E. Yamanaka, N. Tsuboniwa, T. Morimoto, M. Furukawa, and S. Urano. Production of unsaturated carbamic acid derivative. US Patent 5 606 096, assigned to Nippon Paint Co., Ltd. (Osaka, JP), February 25 1997. 10. M. Takeuchi and S. Naijo. Thermopolymerizable composition for battery use. US Patent 6 562 513, assigned to Showa Denki Kabushiki Kaisha (Tokyo, JP), May 13 2003. 11. B. W. L. Jang, M. R. Gogate, J. J. Spivey, J. R. Zoeller, R. D. Colberg, and G. N. Choi. Synthesis of methyl methacrylate from coal-derived syngas. US Department of Energy Reports, Fischer Tropsch Archive 94065/20, Research Triangle Institute, Research Triangle Park, NC, 1999. 12. K. Koyama and Y. Tadokoro. Acrylic resin laminated film and laminated molding using the same. US Patent 6 692 821, assigned to Sumitomo Chemical Company, Limited (Osaka, JP), February 17 2004. 13. S. Tayama and N. Kusakawa. Flame resistant acrylic resin composition and process for its production. US Patent 4 533 689, assigned to Mitsubishi Rayon Company, Limited (Tokyo, JP), August 6 1985. 14. F. Sawaragi and H. Sonezaki. Abrasion-resistant coating composition for acrylic resin molded article. US Patent 6 177 138, assigned to Nippon ARC Co., Ltd. (Chiba, JP), January 23 2001. 15. R. Gubler, editor. Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, 1950–to present. (Internet: http://ceh.sric.sri.com/). 16. S. Bizzari. Report “Methyl Methacrylate”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, August 2003. (Internet: http://ceh.sric.sri.com/). 17. E. Linak and A. Kishi. Report “Acrylic Surface Coatings”. In Chemical Economics Handbook (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA, April 2004. (Internet: http://ceh.sric.sri.com/). 18. J. Lacson, A. Leder, and M. Yoneyama. Report “Acrylic Resins and Plastics”. In Chemical Economics Handbook (CEH). SRI Consulting, a Di-

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20.

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22.

23.

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26. 27.

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29.

30.

31.

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vision of Access Intelligence, Menlo Park, CA, April 2001. (Internet: http://ceh.sric.sri.com/). Underwriter Laboratories. UL 94: Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. Underwriters Laboratories, Inc., 333 Pfingsten Road, Northbrook, IL, 1.6th edition, 2000. C. D. Diakoumakos, Q. Xu, F. N. Jones, J. Baghdachi, and L. M. Wu. Synthesis of acrylic resins for high-solids coatings by solution and separation polymerization. J. Coat. Technol., 72(908):61–70, September 2000. F. Masson, C. Decker, S. Andre, and X. Andrieu. UV-curable formulations for UV-transparent optical fiber coatings I. acrylic resins. Prog. Org. Coat., 49(1):1–12, January 2004. S.-I. Nonaka, R. B. Frings, C. Jaroszewski, and G. F. Grahe. Initiator composition for polymerizing unsaturated monomers. US Patent 6 087 458, assigned to Dainippon Ink and Chemicals, Inc. (Tokyo, JP), July 11 2000. K. Kawakami, Y. Ishibashi, and T. Suzuki. Acrylic resin composition. US Patent 5 574 101, assigned to Asahi Kasei Kogyo Kabushiki Kaisha (Osaka, JP), November 12 1996. Y. Ikegami, S. Koyanagi, Y. Kishimoto, and Y. Nakahara. Acrylic resin composition, acrylic premix, process for producing acrylic artificial marble and thickening agent. US Patent 6 323 259, assigned to Mitsubishi Rayon Co., Ltd. (Tokyo, JP), November 27 2001. K. Umeda and K. Matsunaga. Semiconductor device including acrylic resin layer. US Patent 6 730 948, assigned to Matsushita Electric Industrial Co., Ltd. (Osaka, JP), May 4 2004. T. Behzad and M. Sain. Cure study of an acrylic resin to develop natural fiber composites. J. Appl. Polym. Sci., 92(2):757–762, April 2004. T. Sato, K. Hata, and T. Maruo. Polymer battery and method of manufacture. US Patent 6 696 204, assigned to Nisshinbo Industries, Inc. (Tokyo, JP), February 24 2004. H. S. Park, I. M. Yang, J. P. Wu, M. S. Kim, H. S. Hahm, S. K. Kim, and H. W. Rhee. Synthesis of silicone-acrylic resins and their applications to superweatherable coatings. J. Appl. Polym. Sci., 81(7):1614–1623, August 2001. H. S. Park, S. R. Kim, H. J. Park, Y. C. Kwak, H. S. Hahm, and S. K. Kim. Preparation and characterization of weather resistant silicone/acrylic resin coatings. J. Coat. Technol., 75(936):55–64, January 2003. P. Spathis, E. Karagiannidou, and A. E. Magoula. Influence of titanium dioxide pigments on the photodegradation of paraloid acrylic resin. Stud. Conserv., 48(1):57–64, 2003. M. J. Melo, S. Bracci, M. Camaiti, O. Chiantore, and F. Piacenti. Photodegradation of acrylic resins used in the conservation of stone. Polym. Degrad. Stabil., 66(1):23–30, October 1999.

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32. E. Benedetti, A. D’ Alessio, M. F. Zini, E. Bramanti, N. Tirelli, P. Vergamini, and G. Moggi. Characterization of acrylic resins and fluoroelastomer blends as potential materials in stone protection. Polym. Int., 49(8):888–892, August 2000. 33. R. Bellucci, P. Cremonesi, and G. Pignagnoli. A preliminary note on the use of enzymes in conservation: The removal of aged acrylic resin coatings with lipase. Stud. Conserv., 44(4):278–281, 1999. 34. Y. C. Leong, L. M. S. Lee, and S. N. Gan. The viscoelastic properties of natural rubber pressure-sensitive adhesive using acrylic resin as a tackifier. J. Appl. Polym. Sci., 88(8):2118–2123, May 2003. 35. M. Rafiee-Tehrani, N. Safaii-Nikui, H. Peteriet, and T. Beckert. Acrylic resins as rate-controlling membranes in novel formulation of a nine-day 17 βestradiol transdermal delivery system: In vitro and release modifier effect evaluation. Drug Dev. Ind. Pharm., 27(5):431–437, 2001. 36. A. Roveda, A. Benedetti, F. Pinna, and G. Strukul. Palladium-tin catalysts on acrylic resins for the selective hydrogenation of nitrate. Inorg. Chim. Acta, 349:203–208, June 2003. 37. I. Owsik and B. Kolarz. The oxidation of hydroquinone to p-benzoquinone catalysed by cu(II) ions immobilized on acrylic resins with aminoguanidyl groups Part 1. J. Mol. Catal. A-Chem., 178(1-2):63–71, January 2002. 38. P. Gounon. Low-temperature embedding in acrylic resins. In Electron Microscopy Methods and Protocols, volume 117 of Methods In Molecular Biology, pages 111–124. Humana Press, Inc., Totowa, 1999. 39. A. Maffezzoli and R. Terzi. Effect of irradiation intensity on the isothermal photopolymerization kinetics of acrylic resins for stereolithography. Thermochim. Acta, 321(1-2):111–121, November 1998. 40. M. Sato and M. Yamagishi. Hydrophilic acrylic copolymers, hydrophilic acrylic resin particles and ink-jet recording media. US Patent 6 063 488, assigned to Soken Chemical & Engineering Co., Ltd. (JP), May 16 2000.

10 Cyanate Ester Resins Cyanate ester resins are a comparatively new generation of thermosetting resins. They are characterized by the cyanate group as a reactive group. Most materials of this class are aromatic. Cyanate esters exhibit attractive physical, electrical, thermal, and processing properties. Blends with epoxy and bismaleimide are common. Major applications are in microelectronics, aerospace, and related areas.1–3

10.1 HISTORY Cyanate chemistry was discovered in 1964. Cyanate ester resins have been commercialized since the late 1970s.

10.2 MONOMERS Cyanate esters bear basically two cyanate groups (-OCN) attached to an aromatic ring. Also aryl cyanate esters with additional allyl groups are known, e.g., 1-allyl-2-cyanatobenzene. Allyl-modified types act as reactive diluents in combination with bismaleimide resins.

10.2.1 Specialities Modifications in the thermal and mechanical properties are achieved by blending cyanate esters with epoxies. This is used in particular for adjust373

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ing the tack, drape flow, and other rheological properties. Functionalized thermoplastic oligomers can be also used for modification. 10.2.1.1

Monofunctional Cyanates

Monofunctional cyanates such as dinonylphenol cyanate can be used to modify a fluorinated bisphenol A dicyanate monomer. The monofunctional cyanate reduces the crosslink density in the cured network. Long network chains between the branching points can be prepared by polymeric endcapped dicyanates. 10.2.1.2

Alkenyl-modified Resins

Cyanate ester monomers, functionalized with alkenyl groups, raise the glass transition temperature when used in conjunction with bismaleimide resins.4 Alkenyl groups are addressed as reactive modifiers because they have the ability to react with both homopolymers (cyanate ester and bismaleimide) to form networks. 10.2.1.3

Low-dielectric Cyanates

Fluoroaliphatic Cyanates. Fluoroaliphatic cyanates can be prepared from a fluoro methylol precursor, such as HOCH2 (CF2 )6 CH2 OH, and cyanogen bromide. A solution of cyanogen bromide is reacted with a fluoromethylol precursor with triethylamine as catalyst at −20°C. The product is recovered by dilution with a water-immiscible organic solvent, extraction with water, separation, drying, and concentration of the organic phase.5 Resins from such materials have a very low permittivity to electric fields, as needed for improving the performance in microelectronics. The length of the fluoromethylene chain correlates with decreasing dielectric constant, decreasing moisture absorption, and increasing thermal stability. In general, fluoroaliphatic cyanate resins have dielectric constants in the range of 2.3 to 2.6, tan δ loss lower than 0.02, and a low moisture absorption. o-Methylated Cyanates. The ortho-methylation of a bis(4-cyanatocumyl)benzene cyanate ester showed a further decrease of the dielectric constant. However, other physical properties are also affected. The glass transition

Cyanate Ester Resins

375

OH + Cl C N

- HCl

O C N

H 2O

O O C NH2

Figure 10.1: Formation of Cyanates, Hydrolysis with Water to a Carbamide (not desired)

temperature decreases by 40°C and the coefficient of thermal expansion increases; thermal stability is reduced.6

10.2.2 Synthesis Cyanates are formed by the reaction of phenols with cyanogen halides. The reaction is shown in Figure 10.1. A tertiary amine is catalytically active. The reaction is sensitive to traces of water. Water hydrolyzes aryl cyanates into carbamates. On the other hand, if the condensation is conducted at temperatures near 0°C, the water seems not to affect the reaction.7 Low boiling esters can be purified by distillation. The impurities can be reduced by proper selection of the solvent used in crystallization and washing steps after the synthesis of the cyanate ester. Polymeric cyanate esters can be purified by repeated precipitation processes. The synthesis of various bisphenol dicyanate monomers has been reported. Several cyanate esters are commercially available. Monomers for cyanate ester resins are listed in Table 10.1 and shown in Figure 10.2. Most of the monomers are based on bisphenols. Other cyanates are obtained by

376


CH3 NCO

C

OCN

CH3 2,2-Bis(4-cyanatophenyl)propane CH3 NCO

C

OCN

H 1,1-Bis(4-cyanatophenyl)ethane CH3

H3C H NCO

C

OCN

H CH3

H3C

Bis(3,5-dimethyl-4-cyanatophenyl)methane NCO

S

OCN

Bis(4-cyanatophenyl)thioether CH3 NCO

C CH3 CH3

C CH3

CH3 NCO

C CH3

1,3-Bis(4-cyanatophenyl-1-(methylethylidene))benzene

Figure 10.2: Bisphenol-based Cyanate Esters


377

Table 10.1: Monomers Compound

Remark/Reference

2,2-Bis(4-cyanatophenyl)propane 1,1-Bis(4-cyanatophenyl)ethane Bis(4-cyanatophenyl)methane Bis(3,5-dimethyl-4-cyanatophenyl)methane 1,3-Bis(4-cyanatophenyl-1-(methylethylidene))benzene Bis(4-cyanatophenyl)thioether Bis(4-cyanatophenyl)ether 1,3-Bis(4-cyanatophenyl-1-(1-methylethylidene))benzene 1,1-Dibromo-2,2-bis(4-cyanatophenyl)ethylene 1,1-Dichloro-2,2-bis(4-cyanatophenyl)ethylene 2,2-Bis(4-Cyanatophenyl)1,1,1,3,3,3-hexafluoropropane 4,4-Dicyanatobiphenyl Resorcinol dicyanate 2,7-dihydroxynaphthalene dicyanate 1,1-bis(3-methyl-4-cyanatophenyl)cyclohexane a Flame Retardant

AroCy B-10 AroCy L-10

7

XU 366 8 a

8 a

9 10

reactions of novolak and cyan halides.11 The carbon atom in the cyanato group is highly electrophilic. It is therefore prone to a nucleophilic reagent attack.

10.3 SPECIAL ADDITIVES 10.3.1 Fillers 10.3.1.1

Silica

In semiconductor encapsulation, a large amount of inorganic filler, typically 65% is used. As is the case of epoxy based encapsulants, and also in cyanate ester composites, a silica filler increases the conductivity, Young’s modulus, and dielectric constant. The filler decreases the thermal expansion. A high degree of interfacial adhesion between the untreated silica filler and the cyanate ester matrix is obtained.12 10.3.1.2

Silicate Nanocomposites

Nanocomposites improve the properties of cyanate resins.13 The addition of silicate nanocomposites increases the onset of the thermal decompo-

378


sition. The glass transition temperature Tg increases from 354°C for the neat resin, to 387°C for a 2.5% loading with nanocomposites. The fracture toughness and the flexural modulus increase by 30% with a loading of 5%.

10.3.2 Flame Retardants In general, cyanate ester resins exhibit a better flame retardancy than epoxy resins. In electronic applications, laminates are generally required to possess a wide range of favorable properties, including high mechanical strength, good thermal stability, good chemical resistance, low heat distortion, a high resistance to aging, good electric insulation properties, consistent dimensional stability over a wide temperature range, good adhesion to glass and copper, a high surface resistivity, a low dielectric constant and loss factor, ease of drillability, low water absorption, and a high corrosion resistance. Additionally or even equally important is the limited flammability. Epoxy resins alone or in combinations with cyanate esters or other additives, which are widely used in the electronic industry for printed circuit board (PCB) laminate applications, meet these requirements only because they contain approximately 30 to 40% brominated aromatic epoxy components. This is 17% to 32% as elemental bromine. Antimony and halogen compounds have been added to resins in order to impart flame retardancy. The problem with these brominated compounds is that, although they have excellent flame-retardant properties, they also have some undesirable properties. The chemical decomposition of aromatic bromine compounds releases free bromine radicals and hydrogen bromide, which are highly corrosive. Additionally, when highly brominated aromatics decompose in the presence of oxygen, they may form the highly toxic brominated dibenzofurans as some past studies have shown. Consequently, interest in displacing the use of brominated aromatic epoxies emerged. Fillers with an extinguishing flame effect, such as antimony trioxide, aluminum oxide hydrates, aluminum carbonates, magnesium hydroxides, borates, and phosphates, have been proposed for the replacement of brominated aromatics. However, all these fillers have the disadvantage that they often seriously impair the mechanical, chemical, and electrical properties of the laminates. In the case of antimony trioxide, it is listed as a carcinogen.


379

The flame-retardant effect of red phosphorus has also been investigated in some cases combined with finely divided silicon dioxide or aluminum oxide hydrate. Such compositions when used in electronic applications may lead to corrosion due to the formation of phosphoric acid in the presence of moisture. In addition, organic phosphorus compounds, such as phosphoric acid esters, phosphonic acid esters and phosphines, were proposed as flame-retardant additives. These alternatives have not been promising due to plasticization effects that they impart to the base resin. Other useful phosphorus-containing compounds include propanephosphonic anhydride, and ethylmethylphosphinic anhydride.14 These flame retardants are allowed to react with the epoxide component. Monomers with the phenylphosphine oxide structure exhibit good thermooxidative properties and increased yields of char when heated.

10.4 CURING Essentially no volatile by-products are formed in the course of curing. Many cyanate esters do not shrink during cure.

10.4.1 Thermal Curing Cyanate ester resins are polymerized by a cyclotrimerization of the cyanato functions. The cyclotrimerization produces aryloxytriazine rings which serve as the crosslink sites in the final thermoset matrix. The cyclotrimerization is shown in Figure 10.3. Very high temperatures, beyond 300°C, are usually required for the crosslinking by cyclotrimerisation of cyanate ester groups in uncatalyzed systems.15 However, suitable catalysts are available and usually catalysts are added. Then, the triazine rings are formed around 180°C.16 In fact, the mechanism of cyclotrimerization is much more complicated.17 Analysis of the initial products of curing by gel permeation chromatography indicates that the dimer is a straight chain with a primary amino group. The triazine ring in the trimers seems to exert a strong catalytic effect on the remaining cyanate groups so that the reactivity from the stage of trimers is significantly increased. The reactivities of the higher intermediates decrease up to the heptamer. The monomer consumption in the initial stage of curing follows a second-order rate kinetics.18 In the case of a novolak-type cyanate ester monomer, an autocatalytic behavior

380


O

N

C N

C

O C N O

N

O N

O N

O

Figure 10.3: Cyclotrimerization of Cyanate Esters


381

Table 10.2: Curing Agents Compound

Reference

Zinc octoate Bis(1-methyl-imidazole)zinc(II)dicyanate Bis(1-methyl-imidazole)zinc(II)dioctoate Bis(1-methyl-imidazole)zinc(II)diacetyl-acetonate Aluminium(III)acetylacetonate and dodecylphenol Cobalt(II)acetylacetonate and nonylphenol Dibutyltin dilaurate 2,2′ -Diallyl bisphenol A (DBA)

19

20 21 22 15

was observed.23 The same is true for epoxy blends.24 Various techniques for monitoring the curing of a bisphenol A dicyanate ester resin have been screened, including UV, fluorescence, phosphorescence, and IR techniques. During curing, a very strong luminescence emission has been found. The fluorescence emission intensity around 420 nm first increases followed by a decrease with a small red shift as the cure reaction proceeds. The aromatically substituted cyanurates formed during curing exhibit an inner filter effect and are thus responsible for the observed emission and its trend in intensity.25 As catalysts, Lewis acids and carboxylic salts of transition metal are suitable. For example, zinc naphthenate and nonylphenol cure the ester at 149°C. Further catalysts are given in Table 10.2. Besides trimerization, the formation of dimers and higher oligomers was observed in small amounts. Aryl cyanates are converted cleanly at 25°C to 1,3,5-triazines by catalytic amounts of titanium tetrachloride in dichloromethane. A mechanism has been proposed involving a rate-limiting nucleophilic attack of the cyanate nitrogen on the cyanato carbon of a cyanate-titanium tetrachloride complex. The subsequent steps are fast.26

10.4.2 Curing with Epoxy Groups Curing by the reaction with epoxy groups is also possible. The reaction is shown in Figure 10.4. The reaction is proposed to run via an intermediate trimer of the cyanate ester. The epoxy component acts as a toughener for cyanate ester resins. The chemical structure of the cyanate monomer can affect the cur-

382


+

O C N

CH2

CH

O

N O O C

Figure 10.4: Reaction of Cyanate Esters with an Epoxide to Produce an Oxazoline Structure

ing reactions and thermal properties of the final product. In a comparative study, 2,2′ -bis(4-cyanatophenyl)propane was blended and cured with epoxy based bisphenol or tetramethyl bisphenol. The oxazolidinone ring structure is dominant when curing the bisphenol epoxy system, whereas a cyanurate ring is predominant in the curing reaction of a tetramethyl bisphenol epoxy system. This is attributed to the bulky methyl groups. The crosslinked cyanurate structure has a higher thermal stability than the linear oxazolidinone structure.27 In epoxy/dicyanate blends containing an amine curing agent, the cure rate increases with increasing dicyanate content. The reaction mechanism is autocatalytic and is second-order.24

10.4.3 Curing with Unsaturated Compounds Figure 10.5 shows the reaction of cyanate esters with unsaturated compounds, exemplified with a maleimide and an acetylenic compound. Phenolic hydroxy groups have a catalytic effect on the cyclotrimerisation of cyanate esters. 2,2′ -Diallyl bisphenol A (DBA), with two phenolic hydroxy groups, has been used as a catalyst for the crosslinking of a cyanate ester (CE). The double bonds on DBA can readily copolymerize with bismaleimide to form an interpenetrating polymer network (IPN).15 This type of resin system is addressed as self-catalytic. The crosslinks are formed by different reactions. By such a combination, cyanate esters can be cured at a lower temperature while largely maintaining their superior dielectric properties.


R N

O O

O N

N O

R N

O

O C N

O

N C O

R C C R

O N O

N R

R

Figure 10.5: Reaction of Cyanate Esters with Unsaturated Compounds

383

384


However, these resins exhibit somewhat lower mechanical properties. The flexural strength, flexural strain at break, and impact strength of the BMI/DBA-CE IPN cured resin systems are relatively lower than that calculated by the rule of mixtures, i.e., BMI/DBA and CE.28

10.4.4 Initiator Systems 10.4.4.1

Encapsulated Initiators

To combine properties such as long pot life, short cure time, and high glass transition temperature, small particles of effective hardeners were encapsulated to make them insoluble and nonreactive when mixed with the resin at room temperature. By this technique, pot lifes of more than 3 months could be reached, whereas the same cyanate ester gels and becomes solid within 30 min at the room temperature, if a neat hardener is used instead of the capsules. However, on heating, the capsules open and the curing reaction starts immediately. Low-temperature systems with cure times less than 5 min at 80°C can reach glass temperatures of about 140°C. A glass transition temperature of 220°C after only 10 sec curing time can be achieved with certain formulations. Such systems are also addressed as snap cure resin systems. They can be easily mixed with a lot of common additives such as minerals, tougheners, metallic powders, and others to cover a wide range of performance characteristics.29 10.4.4.2

Photoinitiators

Cyanate esters can be rendered photosensitive by mixing with a cationic photoinitiator. Photosensitive compositions containing cyanate esters can be used as permanently retained etch masks, solder masks, plating masks, dielectric films, and protective coatings. The cured products can withstand temperatures up to 360°C. The materials, depending upon the type of photoinitiator selected, can be used as both positive and negative resists. Suitable photoinitiators are arylacyldialkyl and hydroxyaryldialkyl sulfonium salts. When a negative working photoresist is desired, the photoinitiator employed is one which will generate a Lewis acid upon exposure to actinic light. Examples of such photoinitiators are iron arenes. Furthermore, photosensitizers can be added. Suitable photosensitizers include perylene(peri-dinaphthalene), anthracene derivatives (e.g., 9-methylanthrac-


385

ene), dyes (e.g., acridine orange, acridine yellow, benzoflavin), and titanium dioxide.30 Modified resins, containing epoxy acrylate, a cyanate ester compound, and an anhydride are used.11

10.5 PROPERTIES Cyanate monomers have a low toxicity with an LD50 of 3 g/kg.31 Cyanate ester resins are superior to epoxy resins, phenolic resins, and bismaleimide resins. They combine the advantages of epoxies, the fire resistance of phenolics, and the high-temperature performance of polyimides. The crosslinked networks of cyanacrylate resins can exhibit a Tg higher than 300°C. They are thermally stable up to 475°C. A systematic study on the effect of silica fillers in an AroCy B cyanate ester polymer in the range from 15% to 70% on the thermal, mechanical, and conductivity properties has been presented.12

10.5.1 Modelling The prediction of the physical and mechanical properties of new potential poly(cyanurate)s prior to synthesis is an important issue for future technological application. Several important properties have been predicted by molecular dynamics programs. For example, the glass transition temperature (Tg ) can be simulated by monitoring changes in cell volume while keeping the number of the atoms, the pressure, and the total energy constant.31 Also, the curing behavior under process conditions has been modelled.32

10.6 APPLICATIONS AND USES 10.6.1 Composites Dicyanates of bisphenol derivatives are currently used in composites with established reinforcements such as carbon fiber, glass fiber, silica cloth, and pitch-based graphite fibers.

10.6.2 Electronic Industry Cyanate ester laminates are primarily used in the electronic industry for printed circuit boards. These laminates show a low dielectric constant, loss factor, and superior peel strength with respect to copper.

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10.6.3 Spacecraft High-temperature composite solar array substrate panels for spacecraft applications to orbit the planet Mercury are made from pitch-fiber composite material containing cyanate ester resins. The thermal, mass and stiffness requirements suggested the panels should be fabricated from a high conductivity and stiffness pitch-fiber composite material capable of withstanding short-term temperatures as high as 270°C.33

10.7 SPECIAL FORMULATIONS 10.7.1 PT Resins To obviate certain disadvantages attendant to phenolic resins, a modified multifunctional phenolic cyanate/phenolic triazine copolymer (PT) has been developed. This resin type has greater oxidative, mechanical, and thermal stability than conventional phenolic resins. Further, it did not produce volatile by-products during crosslinking. In addition, the PT resins possess better elongation properties and higher glass transition temperatures than the conventional phenolic resins.34

10.7.2 Blends with Epoxies Blending of cyanate esters with epoxy resins is one of the most important modifications. Most commercial cyanate ester prepregs are, in fact, made from cyanate ester/epoxy blends. During curing, a complicated reaction occurs in the blends. The cyanate resin can act here as a latent catalyst for the epoxy resin.35 In blends, the following mechanisms of curing have been postulated: 1. 2. 3. 4. 5.

Polycyclotrimerization, Formation of oxazoline, Insertion of epoxy groups into cyanurate, Formation of tetrahydrooxazolooxazole, and Ring cleavage and reformation of oxazoline.

In dicyanate-novolak epoxy resin blends, most of the oxazolidinone is formed by isomerization of oxazoline rather than by insertion of epoxy into isocyanurate.36 The moisture uptake of certain dicyanate-epoxy novolak blends is substantially lower than that of the homopolycyanate.37


387

Formulations made from bisphenol A cyanate ester and diglycidyl ether of bisphenol A epoxy resin, or o-cresol formaldehyde novolak epoxy resin, have enhanced processing characteristics.38 Resins of high crosslink density and high glass transition temperature appeared to exhibit a larger reduction in glass transition temperature upon plasticization by moisture compared to those with lower crosslink density.39 Epoxy backbones with hard-soft segments were tailored to improve the toughness. Epoxide and cyanate ester resins with isophthalic and terephthalic groups in the backbone, i.e., 1,3-[di(4-glycidyloxy diphenyl-2,2′ -propane)]isophthalate (DGDPI) and 1,4-[di(4-cyanato diphenyl-2,2′ -propane)]terephthalate (DCDPT) exhibit higher Tg compared to a standard epoxy system. The increase in the Tg is attributed to the cyanate ester and rigid aromatic backbones present.40, 41

10.7.3 Bismaleimide Triazine Resins Bismaleimide triazine resins (BT) are used for high density circuit boards because of their good thermal stability. Bismaleimide triazine resins consist of bismaleimide, a cyanate ester, and epoxy compounds.42 A BT resin can be cured with peroxide initiators, such as dicumyl peroxide or dibenzoyl peroxide, or metal salt catalysts. Cuprous oxide at a prepreg surface layer attracts more cyanate ester resins but less bismaleimide resin from the prepreg to its surface than the cupric oxide. A copper surface affects the curing extent of the BT resin in contact and the cupric oxide has a more pronounced effect than the cuprous oxide. This surface effect can extend at least two microns deep into the BT prepreg from the contacted interface.43 The thermal degradation of BT resins results mainly from the epoxy constituent. However, in the presence of copper oxides, the degradation in the BT occurs not only in the epoxy resin but also in the cyanate ester component. The incorporation of cyanate ester into an epoxy resin improves the flexural and impact strength. The incorporation of bismaleimide (BMI) increases the stress strain properties with a reduction in impact strength. The moisture resistance increases with both increasing cyanate ester and BMI content.44, 45 However, glass transition temperature and heat deflection temperature decrease with increasing cyanate ester content. The incorporation of

388


bismaleimide into an epoxy resin enhances the thermal properties according to its percentage of content.46 The addition of bismaleimide to a cyanate ester results in an increase in fracture toughness. Dynamic mechanical analysis suggests two glass transition temperatures. This indicates that the material has a two-phase morphology and can be addressed as an interpenetrating network.47 For interpenetrating polymer networks based on BMI resin and cyanate ester resins, a BMI resin modified with 2,2′ -diallyl bisphenol A was utilized. Thermal curing with a cyanate ester resin results in an interpenetrating network. The flexural strength, flexural strain at break, and impact strength of such a cured resin is lower than that calculated by a linear contribution. Single damping peaks are detected for the cured resin systems, which suggests a substantial degree of interpenetration between two networks.28 At the gel point, the storage modulus G′ and loss modulus G′′ of the IPN follow a power law with the oscillation frequency.48 BMI/ DBA-CE IPN resin systems combine a low dielectric constant and loss, high-temperature resistance, and good processability.49 2,2-bis(4-cyanatophenyl)propane, and a 2,2-bis[4-(4-maleimido phenoxy)phenyl]propane, have a similar backbone structure. The monomer blend shows a eutectic point at equimolar composition with a melting point of 15°C. When cured together in a bismaleimide-triazine network, polymers of varying compositions can be obtained. The simultaneous curing of the blend can be transformed to a sequential curing by catalyzing the dicyanate curing process using dibutyltin dilaurate. The cured blends undergo a two-stage decomposition, corresponding to the poly(cyanurate) and poly(bismaleimide).50

10.7.4 Siloxane Crosslinked Resins Cyanate ester monomers linked to dimethylsiloxane are cured in the same way as cyanate esters by a cyclotrimerization of the cyanate group to a cyanurate structure. The cured resins are homogeneous rubbery castings with Tg ranging from 15 to −43°C. The dielectric constants show a strong dependence on frequency. The tan δ increases with the chain length of the siloxane but exhibits only a small frequency dependence.51


389

Table 10.3: Thermoplastic Modifiers for Cyanate Ester Resins Thermoplastic Modifier

Reference

Poly(ether imide) Maleimide-styrene terpolymers Polyarylates Polysulfones Polyoxypropylene glycol

52 53 54 55 56

10.7.5 Alloys with Thermoplastics Cyanate esters can be alloyed with thermoplastics. This improves the fracture toughness and the moisture resistance. Thermoplastic modifiers are summarized in Table 10.3 10.7.5.1

Poly(ether imide)

Bisphenol A dicyanate blends with a poly(ether imide) exhibit a phase separation during curing. The poly(ether imide) phase separates at the early stages of curing, before gelation, but this phase separation does not affect the kinetics of the cyclotrimerization.57 The phase structure changes from a separated phase, via a co-continuous phase, to phase inversion with the increase of the content of poly(ether imide). The co-continuous phase morphology is attributed to a spinodal decomposition. The admixture of poly(ether imide) increases the tensile strength and elongation at break. Time resolved light scattering indicates that the evolution of the phase separation is governed by a viscoelastic relaxation process.52, 58 10.7.5.2

Maleimide-Styrene Terpolymers

A terpolymer composed of N-Phenylmaleimide, N-(p-hydroxy)phenylmaleimide and styrene has pendent reactive p-hydroxyphenyl groups. This polymer was used to improve the toughness of cyanate ester resins. Copolymers composed of N-phenylmaleimide and styrene are also effective. An increase in the fracture toughness up to 135% could be achieved, with a slight loss of flexural strength but retention of flexural modulus and glass transition temperature.53

390


The styrene-hydroxyphenyl maleimide copolymer does not impair the mechanical properties of cyanate ester resins, in contrast to other modifiers.22 10.7.5.3

Polyarylates

Polyarylates prepared from bisphenol A and phthaloyl chloride and the diacid dichlorides are soluble in a cyanate ester resin and can be used to improve the brittleness of the resin. The polyarylates include poly(2,2-di(4-phenylene)propane phthalate) (PPA), poly(2,2-di(4-phenylene)propane phthalate-co-2,2-di(4-phenylene)propane isophthalate) (IPPA), and poly(2,2-di(4-phenylene)propane phthalate-co-2,2-di(4-phenylene)propane terephthalate) (TPPA). The most effective modification of the cyanate ester resin can be attained under the condition of a co-continuous phase structure of the modified resin.54, 59 10.7.5.4

Polysulfones

The miscibility of bisphenol A dicyanate and polysulfone decreases with an increase in molecular weight of the polysulfone. Concomitantly, the viscosity of the blend increases. During curing, the phase separation mechanism depends on the viscosity of the blends. At the onset point of phase separation, the viscosity determines the morphology of the blends. Increasing viscosity suppresses the nucleation and growth. Therefore, the viscosity of the blends at the onset point of phase separation is the critical parameter that determines the morphology of the blends.55 10.7.5.5

Reactive Blending

Inhomogeneous modified poly(cyanurate)s have been created by reactive blending of a bisphenol A dicyanate ester and polyoxypropylene glycol (PPG). A finely divided morphology with highly interpenetrated phases, i.e., a poly(cyanurate) rich phase, a mixed phase, and a polyoxypropylene glycol rich phase is formed. The glass transition temperature of the modified network matrix at increasing PPG content is lowered. This is attributed due to the incorporation of PPG in the network, the decrease of the final conversion of the cyanate, and the increase of the free polyoxypropylene glycol which acts as plasticizer.56


391

10.7.6 Coupling Agents for Cyanate Ester Resins Cyanate ester resins have utility in a variety of composite, adhesive, and coating applications, where adhesion between the cyanate ester resin and a surface is of critical importance. A coupling agent to enhance the adhesion is 3-glycidoxypropyltrimethoxysilane. 3-(2-cyanatophenyl)propyltrimethoxysilane or 3-(4-cyanatophenyl)propyltrimethoxysilane can be synthesized from 2-allylphenol or 4-allylphenol, respectively and trimethoxysilane. A Karstedt catalyst is used for the hydrosylilation.60 The cyanate ester formulations including the coupling agent can be coated or mixed with a substrate to provide coated composites or filled molded articles.

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11 Bismaleimide Resins Bismaleimide resin systems are noted for their high-strength, high-temperature performance, particularly as matrix resins in fiber-reinforced prepregs and composites. They are bridging the gap between the relatively low temperature-resistant epoxy systems and the very high temperature-resistant polyimides. Unfortunately, bismaleimides are somewhat brittle, and thus subject to impact induced damage.

11.1 MONOMERS Monomers for bismaleimide resins are summarized in Table 11.1 and are shown in Figure 11.1.

11.1.1 4,4′ -Bis(maleimido)diphenylmethane The most important monomer is 4,4′ -bis(maleimido)diphenylmethane (BMI). BMI has a melting temperature of 155 to 156°C and it polymerizes radically above the melting point. Networks resulting from BMI are very brittle.

11.1.2 2,2′ -Diallyl bisphenol A BMI can be used together with 2,2′ -Diallyl bisphenol A (DBA). DBA copolymerizes with BMI. The reaction is an ene reaction that leads to a chain 397

398


Table 11.1: Monomers for Bismaleimide Resins Compound 4,4′ -Bis(maleimido)diphenylmethane

Reference (BMIM)a

Bisphenol A bismaleimide (BMIP)b 2,2-Bis[4-(4-maleimido phenoxy)phenyl]propane (BMIP)b 2,2′ -Diallyl bisphenol A (DBA) 1,3-Bis(maleimidomethyl)cyclohexane

1

Multi Ring Maleimides N,N-4,4-Diphenylmethanebismaleimide N,N-4,4-Diphenyl ether bismaleimide (BMIE) N,N-4,4-dibenzylbismaleimide Bis(4-maleimidophenyl)sulfone (BMIS) 1,6-Hexane bismaleimide

2

Divalent metal bismaleimides 4-(N-maleimidophenyl)glycidyl ether (MPGE) 4,4′ -Bismaleimidophenylphosphonate

5

3 3 3 4

6 7

8 Bismaleimide bisimides 9 Imides with pendant naphthalene 10, 11 Ester-containing bismaleimides 12 Cardo ester bismaleimides 12 Poly(aminoaspartimide)s a also BMI, BMDPM and BDM, however BMI is used in general for bismaleimides b BMIP is not uniquely used in the literature. BMIP stands for either Bisphenol A bismaleimide or 2,2-Bis[4-(4-maleimido phenoxy)phenyl]propane

Bismaleimide Resins

O

399

O N

CH2

N

O

O BMIM

O

O N

O

N

O

O BMIE

O

O O N

S

N

O O

O BMIS

O

O CH3 N

O

O

C

N

CH3 O

O BMIP O

O O N

S O

2+ -

O- M

O O S

N

O

O

O Divalent metal bismaleimide

Figure 11.1: Bis(4-maleimidophenyl)methane (BMIM), Bis(4-maleimidophenyl)ether (BMIE), Bis(4-maleimidophenyl)sulfone (BMIS), 2,2-Bis[4-(4-maleimido phenoxy)phenyl]propane (BMIP), Divalent metal bismaleimide

400


extension reaction. Subsequently a Diels-Alder reaction follows. Such copolymers exhibit less brittleness, because the crosslinking density is less than that of pure BMI resins. Mixtures of 2,2′ -Diallyl bisphenol A ether and 1,4-diallyl phenyl ether also have been used. These compounds are reactive diluents for BMI because they reduce the apparent viscosity of the BMI.13 N,N ′ -Diallyl p-phenyl diamine (DPD) is a reactive diluent in this sense. Reducing the viscosity is important for the preparation of the advanced composites by techniques such as resin transfer molding (RTM).14 For example, instead using diallyl bisphenol A, a novolak resin can be obtained from diallyl bisphenol A and formaldehyde using p-toluenesulfonic acid as catalyst. The resin is then reactively blended with bisphenol A bismaleimide (BMIP) and cured through an Alder-ene reaction at high temperatures. The materials are useful as adhesives.15 The lap shear strength properties are not significantly affected by the structure of the particular BMI used. It has been demonstrated that using 2,2′ -Diallyl bisphenol A gives products with better adhesion at elevated temperature.16

11.1.3 Poly(ethylene glycol) End-capped with Maleimide The addition of maleimido end-capped poly(ethylene glycol) (PEG) to a bismaleimide resin (4,4′ -bis(maleimido)diphenylmethane) (BDM) enhances the processability of the BDM resin significantly. The processing temperatures of the BDM resin increase from approximately 20 to 80°C. However, the modified resins show a decreased thermal stability of the blended BDM resin, and the coefficient of thermal expansion increases. The curing behavior and the thermal and mechanical properties are independent of the molecular weight of the PEG segment.17

11.1.4 Bismaleimide Bisimides The monomers for bisimide resins are prepared by reacting N,N ′ -(4-aminophenyl)-p-benzoquinone diimine (QA) with maleic anhydride or 5-norbornene-2,3-dicarboxylic anhydride (also called nadic anhydride) in glacial acetic acid as shown in Figure 11.2. The cured resins exhibit a char residue at 800°C in nitrogen atmosphere greater than 55%. Chain-extended types with flexible ether linkages, i.e., 1,3-bis(4-maleimido phenoxy)benzene or 1,4-bis(4-maleimido phenoxy)benzene, show a lower thermal stability than the neat resins.8

Bismaleimide Resins

O

401

O N

N

N

N

O

O O O O

H2N

N

N

NH2

O O O O

O N

O

N

N

N O

Figure 11.2: Bismaleimide Adducts of N,N ′ -(4-Aminophenyl)-p-benzoquinone diimine with Maleic anhydride and Nadic anhydride8

402


11.1.5 Maleimide Epoxy Monomers The use of 4-(N-maleimidophenyl)glycidyl ether (MPGE) is a convenient approach for synthesizing BMIs with epoxy linkage backbones.6 MPGE is synthesized from N-(4-hydroxyphenyl)maleimide and epichlorohydrin by using benzyltrimethylammonium chloride as a catalyst.18 In a similar manner, maleimide-modified epoxy compounds can be prepared from N-(4-hydroxyphenyl)maleimide (HPM) with the diglycidyl ether of bisphenol A.19 The reaction scheme is shown in Figure 11.3. Triphenylphosphine and methylethylketone were utilized as catalyst and solvent, respectively. The resulting compounds bear both the oxirane ring and the maleimide group. Curing can be achieved by amine curing agents, such as 4,4′ -diaminodiphenylmethane (DDM) and dicyandiamide (DICY). The incorporation of maleimide groups into epoxy resins provides a cyclic imide structure and high crosslinking density. The cured resins show high char yields and high LOI values up to 30. Further, specific chemical groups can be introduced into the BMI bridging linkages, such as silicon groups and phosphorus groups. The dimerization is shown in Figure 11.4. The cured resin with silicone exhibits a limiting oxygen index of greater than 50.

11.1.6 Phosphorous-containing Monomers A phosphorus-containing bismaleimide (BMI) monomer, bis(3-maleimidophenyl)phenylphosphine oxide (BMIPO), can be accessed by the imidization of bis(3-aminophenyl)phenylphosphine oxide. This bismaleimide exhibits a good solubility in common organic solvents and a wide processing window.20, 21 It is an excellent flame retardant with a high glass transition temperature, high onset decomposition temperature, and high limiting oxygen index. Copolymers with BMIPO, BMI, and epoxy based 4,4′ -methylenedianiline (DDM) are homogeneous products without phase separation.22 Epoxy resins can be modified by 3,3′ -bis(maleimidophenyl)phenylphosphine oxide. The cured resins have good thermal properties.23 Further, phenyl-(4,4′ -bismaleimidophenyl)phosphonate and ethyl-(4,4′ -bismaleimidophenyl)phosphonate were tested as flame retardants in epoxy systems. The flame retardancy of phosphonate-containing epoxy systems was improved significantly with BMI.24 An increase of the BMI com-

Bismaleimide Resins

O

O O + H 2N

N

OH O

O

O

OH

CH2

O

CH

CH2 CH

CH2

CH2

O

O

H3C C CH3

H3C C CH3

O O

N

+

O

CH2 O

O

CH2

O

CH CH2

CH OH OH

N

O CH2

O

Figure 11.3: Synthesis of Epoxy-modified Maleimide Monomers19

403

404


O N

O CH2

O

O CH CH2

MPGE HO R OH

O OH N

O CH2

CH CH2

O

O R O

O

CH2 CH CH2

O

N

OH O R= one of the following groups CH3 C CH3

O P O

Si

Figure 11.4: Dimerization of 4-(N-maleimidophenyl)glycidyl ether (MPGE) with Functional Diols

Bismaleimide Resins

405

Table 11.2: Reactions of Maleimides Reaction Type Radical polymerization Diels-Alder reaction with a pentamathylcyclopentadiene derivative Diels-Alder reaction with furans

Reference 25 26

pounds also increased the storage modulus and glass transition temperature but reduced the mechanical strength of the epoxy blends. More bulky phosphorous-containing bismaleimides have been obtained by the reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene10-oxide (DOPO) and 4,4′ -bis(maleimido)diphenylmethane.27, 28 The glass transition temperatures of the cured resins decrease with phosphorus content. The limiting oxygen index (LOI) is improved by the incorporation of DOPO.

11.1.7 Multiring Monomers with Pendant Chains The synthesis of multiring monomers with long pendant chains is shown in Figure 11.5. The synthesis runs via a two-fold Friedel-Crafts reaction, followed by a reduction of the dinitro compounds. The diamines are then reacted with maleic anhydride into bismaleimides. The properties of the crosslinked poly(benzylimide) are not strongly affected by the presence of the long alkyl chains. Therefore, linear thermoplastic polyimides with good thermal stability can be obtained.2

11.1.8 Reactions of Maleimides We summarize some of the reactions of maleimides, all of them suitable for obtaining polymers. The reactions are given in Table 11.2.

11.1.8.1

Radical Polymerization

The double bond in the maleic group undergoes an ordinary radical polymerization.

406


CH3

CH3

H 3C

+

Cl

CH2

NO2

R R=C16H33;C8H17;C6H13

CH3 O2N

H 2C

CH2

NO2

CH3

H 3C R

CH3 H 2N

H 2C

CH2

H 3C

CH3

NH2

R O O O O

O CH3 N

O

H 2C

CH2

H 3C

CH3

N O

R

Figure 11.5: Multiring Monomers with Flexible Side Chains2

Bismaleimide Resins 11.1.8.2

407

Michael Addition

The Michael addition is an addition of resonance-stabilized carbanions to activated double bonds. The Michael addition is thermodynamically controlled. It was first described in 1887.29 α, ω-Polyaminoglycols. Amino-terminated oligomers based on propylene glycol, ethylene glycol, and dimethylsiloxane, have been chain-extended via Michael additions with bismaleimides. The polymers have a degree of polymerization up to 15. The polymers are either linear or crosslinked, depending on the starting materials and the conditions of preparation.30, 31 Maleimide-urethanes. The reaction of 4-maleimidophenyl isocyanate and oligoether diols or oligoester diols results in bismaleimide-containing urethane groups. The bismaleimides can be chain-extended by means of a Michael reaction into linear polymers.32 The reaction scheme is shown in Figure 11.6. Chain extenders are 4,4′ -diaminodiphenylmethane and 4,4′ -oxydianiline. Elastic films are obtained that show good mechanical properties and a better thermal stability than the traditional polyurethane elastomers. 11.1.8.3

Diels-Alder Reaction

Chain Extension. Bismaleimide oligomers can be synthesized by chain extension reaction utilizing a Diels-Alder reaction as shown in Figure 11.7. In the first step, a bisfuranylmethylcarbamate is formed from toluene diisocyanate (TDI), or hexamethylene diisocyanate with two mol of furfuryl alcohol. The furan (via its double bonds) then reacts with a bismaleimide, such as 4,4′ -bis(maleimido)diphenylmethane using a DielsAlder reaction.33 These bismaleimide oligomers can be used as a toughness modification agent for other BMI resins. Finally, the ether link in the original furan moiety is eliminated by acetic anhydride, and replaced by an aromatic group. Furan-containing Adducts. Furan-terminated compounds react with BMI at 70°C to an oxygen-containing cycloadduct. The simple adducts are obtained from the monofunctional dienophiles. Crosslinked products are obtained from the coupling of furanic polymers with the bisdienophiles.26

408


O

O N

NCO + HO

OH + OCN

N

O

O

O

O

O

C O N

O

O C

NH

HN

N

O

O H 2N

O

NH2

4,4’-Oxydianiline H 2N

CH2

NH2


Figure 11.6: Dimaleimide urethanes and Michael Reaction with aromatic Diamines

Bismaleimide Resins

OH OCN R NCO +HO CH2

CH2

O

O O

CH2

O C

O

O C O CH2

O

HN R NH O N O O

O O

CH2

O C

O C O CH2

O

HN R NH O

O

O

N

N O CH3 O CH2

O C

O

C O C CH3

O C O CH2

HN R NH O

O N

O

O N

Figure 11.7: Chain Extension Reaction

409

410


On heating the polymerized materials in various solvents with high boiling points, no soluble products were obtained. This indicates the absence of a retro Diels-Alder reaction. It was concluded that aromatization of the imino heterocycles arising from the cycloaddition took place, resulting in irreversible crosslinks. For example, 1,1′ -(1-methylethylidene)bis(4-(1-(2-furanylmethoxy)-2-propanolyloxy))benzene reacts with several bismaleimides, such as N,N ′ -hexamethylenebismaleimide and N,N ′ -p-phenylenedimaleimide. In a subsequent polymerization in the presence of acetic anhydride the aromatization of the tetrahydrophthalimide intermediates occurs.34 Networks from the linear copolymer Poly(styrene-co-furfuryl methacrylate) can be prepared by Diels-Alder reaction at 25°C by adding bismaleimide.35 In such a crosslinked copolymer, an endothermic peak without a glass transition is observed. On reheating the sample, a glass transition is found. This is attributed to the formation of a linear copolymer produced by the retro Diels-Alder reaction in the course of the first heat treatment.36 Bisdienes. Phenylated poly(dihydrophthalimide)s have been synthesized from 3,3′ -(oxydi-p-phenylene)bis(2,4,5-triphenylcyclopentadienone), 3,3′ -(p-phenylene)bis(2,4,5-triphenylcyclopentadienone), N,N ′ -o-phenylenedimaleimide, N,N ′ -m-phenylenedimaleimide, and N,N ′ -p-phenylenedimaleimide.37 Ketonic adducts are formed as intermediates, but the carbon monoxide evolution proceeds spontaneously. Difunctional cyclohexadienes with dihydrophthalimide as central units can act as bisdienes in Diels-Alder polymerization polyadditions with bis(4-(1,2,4-triazoline-3,5-dione-4-yl)phenyl)methane as the difunctional dienophile. The introduction of phenyl side groups increases the solubility.38 Pyrones. Pyrones also behave as diene and react with bismaleimides, thus forming a bis-cycloadduct.39 Diabietylketone. Another bisdiene is the dehydrodecarboxylation product of abietic acid, also addressed as diabietylketone.40 The dehydrodecarboxylation reaction is shown in Figure 11.8. A Diels-Alder polymerization of diabietylketone with 4,4′ -diphenylmethanedimaleimide (bismaleimide) is possible. The resulting polymer is expected to be a poly(ketoimide) with hydrophenanthrene moieties in the backbone. However,

Bismaleimide Resins

411

C O

COOH

Figure 11.8: Dimerization of Abietic Acid by Dehydrodecarboxylation

it was found that the repeating units are bismaleimide and diabietylketone units not in a molar ratio of 1:1, but in a ratio of 5:1 to 6:1. This observation was explained by the difference between the rates of the two concomitant reactions, i.e., the homopolymerization of bismaleimide and the Diels-Alder polymerization. On the other hand, a polymer of the two monomer units in a ratio of 1:1 can be obtained by the dehydrodecarboxylation of the diacid resulting from the Diels-Alder reaction between abietic acid and 4,4′ -diphenylmethanedimaleimide and also by the polycondensation of the ketone of maleated abietic acid with 4,4′ -diaminodiphenylmethane. The polymers are stable in air up to 360°C. Photochemical Generation of Dienes. Certain dienes, such as o-quinodimethanes can be generated by photochemical reactions.41 When the photochemical generation occurs in the presence of bismaleimides, the dienes may react immediately with the bismaleimide in a Diels-Alder reaction, thus forming a polymer. Naphthols. Several 2-naphthols undergo a Diels-Alder addition reaction with maleimides. This reaction can be utilized in curing bismaleimides. For example, 7-allyloxy-2-naphthol satisfactorily cures bismaleimides.42

412


Urethane-imides. Poly(ester-urethane-imides) can be prepared by the Diels-Alder polyaddition of 1,6-hexamethylene-bis(2-furanylmethylcarbamate) with various bismaleimides that contain ester groups in the backbone.43 Triol Extenders. Poly(bismaleimide-ether) polymers with functional pendant groups can be obtained from a Michael polyaddition of flexible bismaleimides, such as N,N-4,4-diphenylmethanebismaleimide, N,N4,4-diphenyl ether bismaleimide and N,N-4,4-dibenzylbismaleimide to trifunctional monomers, such as glycerol and phenolphthalein. Additionally, the hydroxyl functional poly(bismaleimide-ether) can be modified with cinnamoyl moieties.3 Hyperbranched Polyamides. Monomers that contain the diphenylquinoxaline group are 2,3-bis(4-aminophenyl)quinoxaline-6-carboxylic acid (BAQ) and 2,3-bis(4-(4-aminophenoxy)phenyl)quinoxaline-6-carboxylic acid (BAPQ), c.f. Figure 11.9.These compounds form hyperbranched aromatic polyamides on polycondensation. Although the monomers are structurally similar, the properties of both monomers and the respective hyperbranched polymers are different. BAQ reacts normally with BMI in a Michael addition fashion, followed by homopolymerization of the excess BMI. However, BAPQ seems to initiate a free radical polymerization of BMI at room temperature. This unexpected property of BAPQ suggests it can be used as a prototype for the development of low-temperature, thermally curable thermosetting resin systems for high-temperature applications.44


1,3-Bis(maleimidomethyl)cyclohexane

Imides are often substantially insoluble in ordinary organic solvents and are soluble only in high boiling aprotic polar solvents, such as N-methyl2-pyrrolidone, N,N-dimethylacetamide, etc. This is a drawback when impregated varnishes are prepared by dissolving the imides in these solvents. High temperature is required for removing the solvents and the solvents are liable to remain in the prepregs formed from the varnishes, causing foaming in the laminates and considerably lowering the quality of flexible printed circuits (FPC).

Bismaleimide Resins

NH2

COOH N N

NH2 2,3-Bis(4-aminophenyl)quinoxaline-6-carboxylic acid

O

COOH

NH2

N N

O

NH2

2,3-Bis[4-(4-aminophenoxy)phenylquinoxaline-6-carboxylic acid

Figure 11.9: Monomers for Hyperbranched Oligoamides44

413

414


1,3-Bis(maleimidomethyl)cyclohexane is a bismaleimide compound that is readily soluble in a variety of ordinary low boiling point organic solvents.1 For instance, it is soluble in acetone, methylethylketone, tetrahydrofuran, chloroform, and N,N-dimethylformamide. Despite its aliphatic structure, the monomer can provide good heat-resistant bismaleimide resins by thermal polymerization. 11.1.9.2

Siliconized Bismaleimides

Siliconized epoxy-1,3-bis(maleimido)benzene has been synthesized from siloxanes.45, 46 In the first step, epoxy based on the diglycidyl ether of bisphenol A, and 4,4′ -diaminodiphenylmethane (DDM) was extended with (3-aminopropyl)triethoxysilane. The pendent ethoxysilane groups were further reacted with a hydroxy-terminated poly(dimethylsiloxane) (HTPDMS) with dibutyltin dilaurate as catalyst. The scheme is shown in Figure 11.10. 1,3-Bis(maleimido)benzene is prepared from m-phenylene diamine and maleic anhydride. Finally, the 1,3-bis(maleimido)benzene is dissolved in the siliconized epoxy system at 125°C. To this mixture, a stoichiometric amount of DDM is added homogenized. This mixture is cured at 120°C for 1 hour and postcured at 205°C. The curing is a comparatively complex process. It is proposed that the curing is due to the following reactions:46 1. Oxirane ring opening reaction with active amine hydrogens, 2. Autocatalytic reaction of the oxirane ring with pendent hydroxyl groups of epoxy resin, 3. Addition reaction of the amine groups of DDM with double bonds of BMI (Michael addition), and 4. Homopolymerization reaction of BMI. Bismaleimides with silicone linkages can also be prepared via the Diels-Alder reaction of bismaleimide-containing silicone and bisfuran containing silicone. The bismaleimides are soluble in low boiling point solvents, and the cured resins are stable up to 350 to 385°C.47 Still another reaction path to prepare bismaleimides with silicone groups is the reaction of N-(4-hydroxyphenyl)maleimide with dichlorodimethylsilane. In a second step, the adduct is reacted with a polysiloxane that is terminated with hydroxyl groups.48

Bismaleimide Resins

CH2

CH CH2 + H H + CH2 CH N O O CH2

CH2

CH2 CH2 CH3

CH2

O Si O CH2

CH3

O OH +

Si

CH2

OH

CH3 + + OH

Si

Si

CH2

CH CH2 OH

N

CH2

CH2

CH

CH2

OH

CH2 CH2 Si

O Si O Si O Si

Figure 11.10: Formation of a Silane-modified Epoxy Resin

415

416


11.1.9.3

Maleimide Phenolic Resins

Phenolic novolak resins with pendant maleimide groups are accessible by the polymerization of a mixture of phenol and N-(4-hydroxyphenyl)maleimide (HPM) with formaldehyde in the presence of an acid catalyst.49 HPM is less reactive than phenol toward formaldehyde. In fact, N-phenylmaleimide is also reactive towards phenol and formaldehyde. Curing is done by both possible typical reaction mechanisms for these groups. Around 150 to 170°C, there is a condensation reaction of the methylol groups formed in minor quantities on the phenyl ring of HPM. The curing at around 275°C is associated with the addition polymerization reaction of the maleimide groups. Polymerization studies of non-hydroxy-functional N-phenyl maleimides indicate that the phenyl groups of these molecules are activated toward an electrophilic substitution reaction by the protonated methylol intermediates formed by the acid-catalyzed reaction of phenol and formaldehyde.

Allyl-functional Novolak. The maleimide-functional phenolic resin can be reactively blended with an allyl-functional novolak. This system undergoes a multistep curing process over a temperature range of 110 to 270°C. The presence of maleimide reduces the isothermal gel time of the blend. Increasing the allylphenol content decreases the crosslinking in the cured matrix, leading to an enhanced toughness and to improved mechanical properties of the resultant composites. Increasing the maleimide content results in an enhanced thermal stability.50

Epoxy-functional Novolak. Epoxy-novolak (EPN) resins have been ′ cured together with a 1,1 -(methylene di-4,1-phenylene)bismaleimide. A suitably blended EPN and BMI with 30% bismaleimide shows higher Tg than the neat resin. With an increase of bismaleimide, the thermal stability is increased. A single exothermic reaction is observed on curing. The morphology of the cured samples indicates the formation of a homogeneous network in the blends.51, 52

Bismaleimide Resins

417

Table 11.3: Modifiers Compound

Reference

2,2′ -Diallyl

bisphenol A (DBA) Reactive rubbers Polysulfone Polyetherimide Poly(hydantoin) 4,4′ -Bis(o-propenylphenoxy)benzophenone N-Phenylmaleimide-styrene copolymer Acetylene-terminated polymers 2,4-Di(2-allylphenoxy)-6-(2-naphthyloxy)-1,3,5-triazine (DAPNPT) 2,4-Di(2-allylphenoxy)-6-N,N-dimethylamino-1,3,5-triazine 2,6-Di(4-aminophenoxy)benzonitrile (DAPB) Poly(propylene phthalate)

53

54 55 56 57

11.2 SPECIAL ADDITIVES 11.2.1 Tougheners and Modifiers The toughness of bismaleimide resins is a major problem that is limiting the field of application. The toughness can be improved by adding reactive components that reduce the crosslinking density. Modifiers are summarized in Table 11.3.

11.2.1.1

Reactive Rubbers

Blending with reactive liquid rubbers such as carboxyl-terminated butadiene acrylonitrile rubbers increases the toughness.

11.2.1.2

Polyetherimide

Polyetherimide (PEI) is highly effective as a toughness improver for a bismaleimide resin. Increasing the modifier content increases the miscibility of the two phases. At a content of 20% PEI, the morphological structure of the modified resin changes from a dispersed system to a particle cocontinuous structure and eventually with still more PEI to a phase inverted system.53 Polyetherimide is also used in bismaleimide resins composed of 4,4′ -bis(maleimido)diphenylmethane and 2,2′ -Diallyl bisphenol A.58, 59

418


11.2.1.3

Polyesterimide

Polyesterimides can be used to improve the toughness of bismaleimide resins, composed of 4,4′ -bis(maleimido)diphenylmethane (BDM) and 2,2′ -Diallyl bisphenol A (DBA). The fracture energy of the cured samples increases with the increase of polyesterimide content in the modified bismaleimide system.60

11.2.1.4

Polysiloxanes

The addition of alkenylphenols such as 2-allylphenol, 2-propenylphenol, and 2,2′ -diallyl bisphenol A increases the toughness of bismaleimide resin systems, but the degree of toughness obtained is less than that ultimately desirable. Polysiloxanes that are capped with alkenylphenols are compatible with bismaleimide resins and can be used in appreciable amounts to toughen such resins. The toughened systems maintain a high degree of thermal stability. A 2-allylphenoxy-terminated diphenyldimethylpolysiloxane can be prepared from an epoxy-terminated siloxane, 2-allylphenol and triphenylphosphine as catalyst.61

11.2.1.5

Poly(ether ketone ketone)

Poly(ether ether ketones) (PEEK) to improve the brittleness of the bismaleimide resin include poly(phthaloyl diphenyl ether) (PPDE), poly(phthaloyl diphenyl ether-co-isophthaloyl diphenyl ether) (PPIDE), and phthaloyl diphenyl ether-co-terephthaloyl diphenyl ether (PPTDE). The bismaleimide resin is a mixture of 4,4′ -bis(maleimido)diphenylmethane and 2,2′ -Diallyl bisphenol A. It was shown that PPIDE with 50 mol-% isophthaloyl unit is more effective as a modifier for the bismaleimide resin than the other poly(ether ketone ketone)s. The most effective modification for the cured resins could be achieved with a co-continuous phase or a phase-inverted structure of the modified resins.62 Similarly, in a three-component bismaleimide resin composed of 4,4′ -bis(maleimido)diphenylmethane (BDM), 2,2′ -diallyl bisphenol A (DBA), and o,o′ -dimethallyl bisphenol A, PPIDE and PPTDE are more effective as modifiers than PPDE.63

Bismaleimide Resins 11.2.1.6

419

Triazines

2,4-di(2-allylphenoxy)-6-(2-naphthyloxy)-1,3,5-triazine (DAPNPT) can be prepared by the reaction of cyanuric chloride with 2-allylphenol followed by a treatment with 2-naphthol.54 The procedure is shown in Figure 11.11. Copolymers of DAPNPT with 4,4′ -bis(maleimido)diphenylmethane (BMDPM) show improved mechanical properties compared to pure BMDPM. The copolymer shows up to 10 times higher impact strength and 3 times higher shear strength. However, the impact strength and the shear strength dramatically decrease when the molar ratio of DAPNPT/BMDPM in the copolymer exceeds 1:2. Completely analogous, as shown in Figure 11.11, 2,4-di(2-allylphenoxy)-6-N,N-dimethylamino-1,3,5-triazine can be prepared by the reaction of 2-allylphenol with cyanuric chloride and then by dimethylamine.55 This monomer is a modifier for bismaleimide resins. It effectively improves the mechanical properties of the resin without greatly decreasing heat resistance of the resin.

11.2.1.7

Others

Polyamide-imide (PAI), poly(phenylene sulfide) cannot be used in BMI allyl systems. These compounds have poor miscibilities with allyl compounds.

11.2.1.8

Boric Esters

Boron can be incorporated into allylic compounds by esterification of allylphenol and boric acid. Such compounds are suitable as comonomers in the polymerization of bismaleimide resins. The cured resins show an excellent thermal stability. No weight loss was observed when the copolymer was heated up to 465°C in nitrogen atmosphere. The char yields at 800°C in nitrogen are more than 50%.64 Allyl boron compounds improve the ablative properties of bismaleimide resins.65

420


Cl OH

+

N

Cl

HO

N

+

Cl

N

Cl N O

N O

N

HO

O N O

N N

O

Figure 11.11: Preparation of 2,4-Di(2-allylphenoxy)-6-(2-naphthyloxy)-1,3,5-triazine

Bismaleimide Resins

421

11.2.2 Fillers 11.2.2.1

Aluminum nitride Ceramic Powders

To prevent the failure of integrated circuites (IC) during processing and operation, materials with a low dielectric constant, and a silicon compatible coefficient of thermal expansion (CTE) ca. 4.0×10−6 K−1 are needed. A low dielectric constant reduces the delay time of signal transmission. Further, a high glass transition temperature and a high conductivity is substantial, especially in high powered ICs. Silica has a high thermal conductivity, but it has a high dielectric constant of around 40. Aluminum nitride66 has a melting point of 2230°C and is highly chemically inert. It is used in refractory materials, also in conjunction with silica nitride and boron nitride. Aluminum nitride (AlN) ceramic is superior to silica, since it not only has a high thermal conductivity of up to 320 W/K and a compatible CTE with silicon, but it also has a relatively low dielectric constant (ca. 8.9). AlN ceramic powders, used as fillers in a modified bismaleimide resin, change the curing performance. The addition of AlN increases the activation energy of curing of the BMI. Also, the glass transition temperature is raised slightly.67 11.2.2.2

Silsesquioxane Nanofillers

Silsesquioxane nanofillers in a bismaleimide modified novolak resin exhibit improvements in the glass transition temperature and the heat resistance of the material. The modulus at high temperatures is also improved. The particle size of the dispersed phase was about 100 nm, and particle aggregates were observed.68

11.2.3 Titanium dioxide Ternary hybrids of bismaleimide-polyetherimide-titanium dioxide were synthesized by sol-gel reaction. A 10% solution of BMI prepolymer in N-methyl-2-pyrrolidone was mixed with 30 phr of polyetherimide. Dibutoxybis(acetylacetonato)titanium(IV) was obtained from tetrabutyltitanate and acetylacetone. This compound was added, and after stirring again tetrabutyltitanate and acid were added. After drying, the resulting film was thermally cured. The titanium dioxide particles were dispersed uniformly in both the PEI-rich phase and the BMI-rich phase,

422

Reactive Polymers Fundamentals and Applications Table 11.4: Flame Retardant Bismaleimides Compound Bis(3-maleimidophenyl)phenylphosphine oxide (BMIPO) 3,3′ -Bis(maleimidophenyl)phenylphosphine oxide Phenyl-(4,4′ -bismaleimidophenyl)phosphonate Ethyl-(4,4′ -bismaleimidophenyl)phosphonate 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)

Reference 20–22 23 24 24 27, 28

having a mean diameter of around 50 nm.69 Increasing titanium dioxide content improves the mechanical properties. However, the thermal decomposition temperatures of the hybrids decrease from 374°C of the unfilled resin to 294°C of a resin with a titanium dioxide content of 20 phr. It is believed that titanium dioxide exerts a catalytic effect in this aspect.


Silica Coatings

The usual way to reinforce is to add the reinforcing material to polymeric materials. For medical applications, ceramic coatings have been applied to a bismaleimide. Non-reinforced BMI specimens are coated with a thin, protective layer of a dense silicate ceramic material. Testing of the Vickers hardness on the coated and uncoated BMI specimens indicates that the coatings adhere well to the substrate.70

11.2.5 Flame Retardants Bismaleimide resins are flame retardant, because they are comprised of aromatic groups and nitrogen. Therefore, for many applications, flame retardancy is not a major problem. Phosphorous-containing monomers have been described as flame retardants. They are used not only for bismaleimides, but also for epoxy systems. Flame retardants are shown in Table 11.4.

Bismaleimide Resins

423

11.3 CURING 11.3.1 Monitoring Curing Reactions 11.3.1.1

DSC

Experimental data for a kinetic model of a modified bismaleimide resin were obtained by isothermal DSC. A curing mechanism involving multiple reactions was established. The reaction is dominated by different mechanisms at different stages of curing. At the beginning of curing, an autocatalytic reaction was observed.71 A reaction model was set up, and the activation energy and the frequency factor were calculated.72 11.3.1.2

Dielectric Method

A dielectric sensor for the cure monitoring of high temperature composites has been developed. The on-line cure monitoring of a bismaleimide resin was performed using a Wheatstone bridge type circuit and a high temperature dielectric sensor.73 11.3.1.3

Infrared Spectroscopy

An in-situ technique for studying the polymerization kinetics has been developed. Fourier self-deconvolution of the spectra was used to enhance the peak separations and the calculation of the peak areas needed for quantitative monitoring of the curing process. During curing of 1,1′ -(methylene di-4,1-phenylene)bismaleimide (MDP-BMI) with 4,4′ -diaminodiphenylmethane (DDM), a substantial difference in the reactivity between primary and secondary amine was observed.74

11.3.2 Polymerization 11.3.2.1

Gel Point

When a monomer containing two double bonds is incorporated into a radically growing chain, it is first incorporated with one double bond only. The polymer chain then will bear pendant double bonds, but initially no crosslinks. There are special cases where, after incorporation of the first double bond, the second, now pendant double bond will be consumed by the same growing radical. This behavior is termed backbiting, or if it occurs more randomly, intramolecular cyclization.

424


The pendant double bonds may react in a further stage of the polymerization with another growing chain. Accordingly a complete polymer chain becomes part of another growing chain by the reaction of a single pendant double bond. The molecular weight of the polymer grows rapidly until a certain stage of conversion is reached and a gel is formed. The formation of networks during the copolymerization of styrene with various maleimide compounds was investigated.75 In particular, pmaleimidobenzoic anhydride, or mixtures of p-maleimidobenzoic anhydride, methyl-p-maleimidobenzoate, and styrene were studied. In resin systems containing bismaleimides, during radical polymerization, the concentrations of pendent double bonds in copolymers, calculated from the consumption of monomers and copolymer composition, follow the general trend typical for vinyl-divinyl copolymerization. At the end of polymerization, a substantial fraction of pendent maleimide bonds remains in the system. The conversions at the gel point are much higher than for ring-free copolymerization due to cyclization and the steric hindrance of the pendent double bonds. 11.3.2.2

Thermal Polymerization

In stoichiometric formulations of 1,1′ -(methylene di-4,1-phenylene)bismaleimide, modified with 2,2′ -Diallyl bisphenol A, during the thermal curing, copolymerization and homopolymerization do not overlap with each other.76 The reactions progress sequentially and homopolymerization occurs only when the copolymerization is completed. This conclusion is based on the Tg –conversion relationship that was modelled by the DiBenedetto equation.77 The DiBenedetto equation, Eq. 11.1 is based on the corresponding states. Tg,α=0 = 1 +C1 α +C1 α2 Tg Tg α C1 C2

(11.1)

Glass transition temperature Conversion Constant, characteristic for the system Constant, characteristic for the mobility of the repeating units

In a modified diallyl bisphenol A/diaminodiphenylsulfone/bismaleimide resin, the different temperature regimes were characterized by IR spectroscopy. The major crosslinking occurs below 150°C. At 190°C the maleimide moiety is converted into succinimide.78

Bismaleimide Resins

425

Cure Reaction Pathways. In a homopolymerized bismaleimide resin system, the maleimide ring addition is the only observable reaction with conventional methods. When the maleimide is cured in the presence of an amine, the Michael addition of the amine to the maleimide ring can be observed. In solution, using special reagents and conditions, a ring-opening aminolysis reaction has been observed. Such a reaction has been postulated as a curing mechanism for bismaleimides. It has been verified that such an aminolysis reaction, accompanied by ring opening, occurs to a significant extent during the cure of a neat BMI resin. This partial structure can remain in the network even after a hightemperature postcure treatment. The existence of the amide product has been demonstrated in bismaleimide resin formulations selectively labelled with 13 C atoms and 15 N atoms.79 Cure Kinetics and Mechanism. Maleimide reacts with allylphenols in an ene reaction via an intermediate Wagner-Jauregg reaction, followed by a Diels-Alder reaction.80, 81 The Wagner-Jauregg reaction is essentially a Diels-Alder addition of BMI to the ene adduct of BMI and the allylphenol. The reaction shows a strong dependency on the electron density of the BMI. The Diels-Alder reaction is facilitated by an increased electrophilicity of the dienophile. However, a reverse trend is observed for the Wagner-Jauregg reaction. Therefore, it was concluded that this reaction could follow a mechanism different from the conventional Diels-Alder reaction, although the final product looks the same as in the Diels-Alder reaction.82 In a mixture of 4,4′ -bis(maleimido)diphenylmethane and 2,2′ -diallyl-bisphenol A (BMDM/DABPA) and other models, it was established that the cure mechanism consists of a combination of step-wise and chain polymerization and polycondensation reactions:83 1. Step-wise ene addition reaction of allyl group to maleimide, (shown in Figure 11.12). 2. Chain polymerization of the maleimide and the propenyl groups generated by first reaction. The chain polymerization is the main crosslinking reaction. The mechanism of the reaction involving monofunctional model compounds differs from the curing of the actual system because of steric hindrances in 2,2′ -diallyl bisphenol A, which retard reversible Diels-Alder reactions, and

426


OH

O

OH +

O

N

N

O

O

O OH

N

O

N

O

O

O N

O

N

O

O

OH

O

N

N

O O

O

O

N

O

Figure 11.12: Ene Reaction of Allylphenol and Maleimide, Followed by Wagner-Jauregg Reaction and a Diels-Alder Reaction

Bismaleimide Resins

427

different reactivity of maleimide groups.84 Another mechanism of crosslinking is the dehydration reaction of phenol groups. The dehydration of phenolic groups necessarily involves the 1:1 adduct of maleimide and allyl function as a reactant.85 The homopolymerization of maleimide groups proceeds autocatalytically under the action of free radicals generated by thermal decomposition of maleimide propenyl groups donor-acceptor pairs. The steric hindrance in 2,2′ -diallyl-bisphenol A prevents the reversible Diels-Alder reaction. The methylated analog of 2,2′ -diallyl bisphenol A shows a higher reactivity in thermal free-radical polymerization.86 The curing kinetics of bismaleimide modified with diallyl bisphenol A has been modelled by an autocatalytic and nth -order model.87 Microwave Curing. A comparative study between thermal and microwave curing of bismaleimide resin was done. The degree of cure was determined with differential scanning calorimetry. No difference in the chemical reactions taking place during the microwave cure and the thermal cure was detected. Samples that were cured with a conventional oven showed slightly higher glass transition temperatures than the microwavecured samples at higher conversions.88 11.3.2.3

Photo Curing

N-alkylmaleimides homopolymerize in the absence of a photoinitiator when exposed to UV light in solvents bearing a labile hydrogen.89 Since the maleimide is a chromophore, it is considered a photoinitiator together with a co-initiator. A co-initiator may be methyldiethanolamine, trimethylolpropane trismercaptopropionate, or poly(ethylene glycol). Maleimide/vinyl ether systems belong to electron donor/electron acceptor monomers. Maleimide acts as an electron acceptor and the vinyl ether acts as an electron donor. With stoichiometric maleimide-vinyl ether mixtures, the reaction proceeds within seconds upon UV exposure.90 The initiation reaction is shown in Figure 11.13. The initiator radicals are formed by hydrogen abstraction from the excited maleimide molecules. Highly crosslinked polymer networks can be obtained. The molecular structure of bismaleimides is quite rigid because of the presence of aromatic rings. The presence of the aromatic rings, as well

428


O H C C H

O H

C N R

hν

C

CH O CH2

H R’

N R

H C

O CH2

C*

CH2

O

CH O CH*

R’

Figure 11.13: Photoinitiation in Donor Acceptor Systems91

as the resultant high crosslinked density during thermal curing, give the cured product its high heat-resistance, resulting in a high Tg and a high mechanical strength. For the radiation curing of bismaleimides, comonomers such as ′ 2,2 -diallyl bisphenol A and 4-hydroxybutylvinyl ether have been tested. Unlike N-alkylmaleimide and N-phenylmaleimide, BMI does not react with vinyl ether without a photoinitiator. Triphenylphosphine oxide is a suitable photoinitiator. 2,2′ -Diallyl bisphenol A, which is a good property modifier for BMI in thermal curing formulation, does not polymerize with either BMI or 4-hydroxybutylvinyl ether, even in the presence of a photoinitiator. However, 2,2′ -diallyl bisphenol A is a co-initiator and speeds up the reaction of a ternary system.91 11.3.2.4

Anionic Initiators

Several maleimides can be polymerized by nanometer sized Na+ /TiO2 initiators. The temperature for the polymerization initiated by nanometer sized Na+ /TiO2 is lower than that for the radical polymerization. An anionic mechanism resulting from the catalysis by Na+ /TiO2 as the counter ion is proposed.92, 93 11.3.2.5

Diels-Alder Polymerization

A monomer suitable for Diels-Alder polymerization is shown in Figure 11.14. The reaction between α, α′ -dibromo-m-xylene (DBMX) and sodium 1,2,3,4,5-pentamethylcyclopenta-1,3-dienide gives the respective pentamathylcyclopentadiene derivative,25 as depicted in Figure 11.14.

Bismaleimide Resins

CH3 H3C

Br

Na

CH2

CH2

CH3

Br H3C

CH3

CH3

CH3

H3C

CH3

CH3

+

Na

H3C CH3 CH 2 H3C

CH3 CH3

429

+

CH2 CH3 CH3 CH3

H3C CH3

Figure 11.14: Synthesis of Bis-1,3-methyl-1,2,3,4,5-pentamethylcyclopenta2,4-diene benzene

The Diels-Alder polymerization is shown in Figure 11.15. The reaction must be performed in dimethylformamide at 140 to 150°C because of the low solubility of the BMI.

11.3.3 Interpenetrating Networks 11.3.3.1

Polyurethane Bismaleimide

In polyurethane/poly(urethane-modified bismaleimide-bismaleimide) interpenetrating polymer networks (PU/P(UBMI-BMI) IPNs) interpenetration occurs at the hard segment domains of PU, which leads to an enhancement of the phase separation of PU. The dispersing tendency of the dispersed phase increases.94 Poly(butylene adipate)-based polyurethane-crosslinked epoxy (BMI/ PU-EP IPN) and bismaleimide from interpenetrating networks are prepared by using the simultaneous bulk polymerization technique. It was demonstrated that the bismaleimide was dissolved primarily in the polyurethane domains of the epoxy matrix to form a compatible system, thereby increasing the mechanical strength of the BMI/PU-EP IPNs.95, 96 An epoxy based on poly(propylene oxide) has a better grafting effect

430


H3C CH3 CH 2

CH2 CH3 CH3

CH3

H3C CH3

CH3

CH3 CH 2

N R N

CH3

H3C

O

O

O

O

CH2 CH3 O

O N

O

O

N

Figure 11.15: Diels-Alder Polymerization

due to higher compatibility between the BMI than poly(butylene adipate) epoxies.97 The incorporation of chain-extended BMI into polyurethane-modified epoxy systems increases the thermal stability, and tensile and flexural properties, but decreases the impact strength and the glass transition temperature.98 11.3.3.2

Unsaturated Polyester Bismaleimide

A bismaleimide resin monomer can be readily dissolved in the uncured polyester matrix up to a concentration of about 20%.99 Spectroscopic investigation during curing indicates that the crosslinking process is strongly affected by the presence of the bismaleimide in the system. The maleimide groups react preferentially with the styrene. The styrene radical reacts with both the unsaturated polyester and the maleimide moieties so that a crosslinked structure can emerge. When the maleimide groups are fully consumed, the curing proceeds as in a neat resin. The bismaleimide effects an increase of the crosslinking density of the final product. Further, the bismaleimide increases the overall stiffness of the network.

Bismaleimide Resins

431

11.4 PROPERTIES In comparison to epoxy resins, BMI resins exhibit a higher tensile strength and modulus, excellent chemical and corrosion resistance, better dimensional stability, and good performances at elevated temperature.

11.4.1 Thermal Properties Among two high temperature adhesives, based on epoxy and bismaleimide, the bismaleimide-based adhesive shows a better high temperature performance and is more resistant to thermal aging than an epoxy based resin.100 There are relationships between structure and thermal properties of polymers.101

11.4.2 Water Sorption In general, a disadvantage of thermoset resins is their tendency to absorb significant amounts of water when exposed to humid environments. The absorbed moisture has detrimental effects on material performance. The temperature dependence of moisture content in equilibrium is controversial. It has been reported that the equilibrium moisture content is independent of temperature,102 but also that it is dependent on the temperature.103, 104 From the viewpoint of thermodynamics, the temperature dependence of the solubility is governed by the enthalpy of dissolution d ln cs = −∆Hs /R. d1/T

(11.2)

During hydrothermal cycling experiments, the molecular network structure of BMI appears to change.105, 106 It was concluded that in the course of water absorption at elevated temperatures, a chemical degradation can occur. This is part of an aging mechanism. IR spectra obtained by the reflection technique during water absorption show that the band at 1600 cm−1 increases.107 This band is attributed to the N − H stretching of an amine and also of an amide. The hydrolysis reaction is shown in Figure 11.16. It is assumed that the hydrolysis is similar to the reverse reaction of formation of a bismaleimide.107 When a BMI resin was stored in water at temperatures of up to 70°C for a period of 18 months, blistering and severe microcracking occurred, leading to severe weakening of the materials.103

432


O

O

OH

H2O N

N O

O H

H2O

O OH H OH N H O

Figure 11.16: Hydrolysis of a Cured Bismaleimide Resin

The presence of about 10 to 15% of alkenyl-substituted cyanate in dicyanate and bismaleimide blends leads to a marked reduction in moisture absorption in comparison with an unmodified bismaleimide/cyanate blend containing a comparable amount of bismaleimide. The modified samples display thermal stabilities that are indistinguishable from cured resins that have not undergone immersion.108 The moisture transport can be correlated to the glass transition temperature and the network properties. The network structure can be systematically varied by the initial monomer composition and the conditions of curing.109

11.4.2.1

Multivariate Analysis

An analysis of samples subjected to accelerated ageing tests shows that simple near infrared spectroscopic measurements on virgin materials can predict results otherwise obtained from dynamic mechanical thermal analysis, and can provide correlations with thermogravimetric analysis. Therefore, a rapid screening method for multivariate analysis has been proposed, in conjunction with a combinatorial approach for the development of advanced composites.110

Bismaleimide Resins

433

11.4.3 Recycling Various styrene copolymers containing comonomers with a pendant furan ring were subjected to Diels-Alder reactions with a monomaleimide or a bismaleimide. When the materials are heated in the presence of excess of 2-methylfuran, the retro Diels-Alder reaction is induced. The process is rather a trans Diels-Alder reaction. The maleimides are released with the furanic additive. Concomitantly the original copolymers can be recovered. The reaction is of interest because of the possibility of recycling crosslinked polymers by a simple thermal treatment.111

11.5 APPLICATIONS AND USES 11.5.1 Biochemical Reagents Bismaleimides are used as reagents in biochemical investigations.112 Bismaleimide is used as a crosslinking reagent for the synthesis of bifunctional antibodies. The use of a solid-phase reactor in the preparation of the bifunctional antibodies eliminates many time-consuming separation steps between fragmentation and conjugation steps.113

11.6 SPECIAL FORMULATIONS 11.6.1 Adhesives For high-temperature usage, i.e., above 200°C, either bismaleimides or polyimides are suitable. These are supplied as films, with or without a carrier. Epoxies are not generally used at temperatures beyond 150°C, although there are some modified epoxies that can be used up to 200°C.114 11.6.1.1

Void Control

Polyimides have a higher service temperature than bismaleimides. However, bismaleimides offer some advantages as they do not generate volatiles during cure. When volatiles are created during curing a high void content in the adhesive can develop. There are several methods to control the voids. These include114 • Vacuum release technique. The joint to be bonded is placed in an oven under vacuum. The temperature is increased in order to

434

Reactive Polymers Fundamentals and Applications reduce the viscosity of the adhesive. When the vacuum is released, the voids collapse to a negligible volume. • Another method uses an autoclave where hydrostatic pressure can be applied. The hydrostatic pressure compresses the gas in a void and reduces its volume.

11.6.1.2

Thermally Reversible Adhesives

A formulation of thermally reversible adhesives consists of a diepoxy compound and aliphatic diamines. The diepoxy compound is formed by the Diels-Alder reaction between epoxy-containing furans and a bismaleimide. The epoxy resin is cured with aliphatic diamines.115 At temperatures above 90°C the retro Diels-Alder reaction occurs, which leads to a significant loss in the shear modulus. The loss of the shear modulus is reversible with temperature. Therefore, the formulation can act as a thermally reversible adhesive. The adhesive bonds are easily broken at elevated temperature where the modulus is low. 11.6.1.3

Adhesion Improvement

In order to improve the adhesion of Kevlar™∗ fibers to a 2,2-bis[4-(4-maleimido phenoxy)phenyl]propane (BMPP) resin, the surface of the fibers can be chlorosulfonated. The fibers are immersed in a solution of chlorosulfonic acid in dichloromethane at −10°C. After the chlorosulfonation, the surface concentration of carbon decreases. In the subsequent reaction with ethylene diamine, allylamine, the O/N ratio again decreases. On the other hand, the O/N ratio was increased by hydrolysis treatment. The interfacial shear strength (IFSS) is determined by pull-out experiments of the fiber from the matrix calculated by the relationship τ= τ F d L

F . dL

(11.3)

Interfacial shear strength Pull-out force Diameter of the fiber Embedded length of the resin

The interfacial shear strength (IFSS) between Kevlar fibers and the BMPP resin increases slightly due to the chemical treatment.116, 117 In ∗ Kevlar

is a trademark of DuPont company

Bismaleimide Resins

435

graphite/bismaleimide composites, the treatment with ammonia has been shown to be promising for the improvement of adhesion.118

11.6.2 Phosphazene-triazine Polymers Polyquinoline/bismaleimide blends are miscible thermosetting polymers. Thermogravimetry shows a 5% weight loss between 450 and 535°C for thin films at 5 to 60% of bismaleimide loading. The glass transition temperatures are between 275 and 360°C.119

11.6.3 Phosphazene-triazine Polymers Phosphazene-triazine polymers can be obtained by curing a ternary blend of tris(2-allylphenoxy)triphenoxy cyclotriphosphazene (TAP), tris(2-allylphenoxy)-s-triazine (TAT) and bis(4-maleimidophenyl)methane (BMM). The maleimide component increases the thermal stability. The tensile strength decreases and the modulus increases with increasing maleimidecontent. Tensile properties improve for an allyl/maleimide ratio of two.120

11.6.4 Porous Networks Network structures have been prepared by in-situ polymerization of a mixture of N-phenylmaleimide and 1,1′ -(methylene di-4,1-phenylene)bismaleimide in 80% poly(vinylidene difluoride-co-hexafluoropropylene) (PVDH). The maleimide monomers are forming thermoreversible gels with PVDH. After polymerization, porous networks are obtained by removing the PVDH by solvent extraction. The poly(maleimide) networks are stable up to 380°C in an inert atmosphere. It is suggested that these networks may be used for thermally stable membranes.121

11.6.5 Nonlinear Optical Systems Thermally stable second-order nonlinear optical polymeric materials based on bismaleimide contain chromophores with excellent thermal stability, such as the N-maleimide of Disperse Orange 3. The synthesis of the monomer is shown in Figure 11.17. A full interpenetrating polymer network can be formed by the simultaneous reaction of bismaleimide and a solgel process of the alkoxysilane dyes. The dynamic thermal and temporal

436


O O + H 2N

N N

NO2

O

O N

N N

NO2

O

Figure 11.17: Synthesis of the Maleimide of Disperse Orange 3

stabilities of the interpenetrating network are much better than those of comparable non-interpenetrating networks.122 Azo chromophores with allyl groups at one or two ends of the molecules can be thermally cured with bis(maleimidodiphenyl)methane to give crosslinked and chromophore-modified bismaleimide resins. The resins show no appreciable decomposition up to 300°C. By incorporating a chromophore into the network of a BMI resin, an improvement of the thermal stability of the materials is achieved.123 Examples of azo chromophore allyl compounds include (4-(N,Ndiallyl)-4′ -nitrophenyl)azoaniline, allyl-4-[(4-N-allyl-N-ethyl)aminophenylazo]-α-cyanocinnamate, and allyl-4-[(4-N,N-diallyl)aminophenylazo]α-cyanocinnamate, c.f. Figure 11.18.

Bismaleimide Resins

CH2

CH2

CH

CH

H2C

N

CH2

CH2 CH3 CH H2C CH2 N

CH2

CH2

CH

CH

H2C

N

437

CH2

N

N

N

N

N

N

NO2

CH

CH

C CN

C CN

C O

C O

O

O

CH2

CH2

CH

CH

CH2

CH2

Figure 11.18: Azo Chromophore Allyl Compounds: (4-(N,N-Diallyl)-4′ -nitrophenyl)azoaniline, allyl-4-[(4-N-Allyl-N-ethyl)aminophenylazo]-α-cyanocinnamate, allyl-4-[(4-N,N-Diallyl)amino-phenylazo]-α-cyanocinnamate

438


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54.

55.

56.

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61.

62.

63.

64.

65. 66.

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12 Terpene Resins Terpenes are widespread in nature, mainly in plants as constituents of essential oils. Some terpenes are pure hydrocarbons, but there are also terpenes with hydroxyl functions and carbonyl functions. Terpenes provide plants and flowers with fragrance.

12.1 HISTORY Polyterpene resins were discovered in 1789 when turpentine was treated with sulfuric acid to produce a crude resin. Turpentine is a semi-fluid resin obtained from pines. Turpentine is used as a thinner, antiseptic, drug, pesticide, insecticide, and raw material for the chemical industry. Rouxeville observed that a great number of hydrocarbons may be changed in their composition so that an artificial product that is formed is distinguished by a new composition. This occurs by oxidation or polymerization. He patented a process of polymerization by treatment with sulfuric acid.1, 2

12.2 MONOMERS Terpenes and related monomers are relatively non-toxic liquids that may be obtained from natural renewable nonpetroleum sources. The structure of terpenes can be essentially derived from isoprene. This fact is known as the isoprene rule. The terpene unit consists of two isoprene units. There447

448


CH3 CH2

CH3 CH3

CH3

CH3

CH3

α-Pinene

β-Pinene

Limonene

Figure 12.1: Terpenes

fore, terpenes have the general molecular formulas (C5 H8 )2n . According to the number of isoprene units, terpenes are classified into monoterpenes (2 isoprene units), sesquiterpenes (3 isoprene units), diterpenes (4 isoprene units), triterpenes (6 isoprene units), and tetraterpenes (8 isoprene units). Polyterpene resins are low molecular weight hydrocarbon polymers prepared by cationic polymerization or copolymerization of monoterpenes such as α-pinene and β-pinene or limonene. These types of terpenes are bicyclic terpene hydrocarbons. The structures of some terpenes are shown in Figure 12.1. α-Pinene and β-pinene and limonene are liquid at room temperature. Naturally occurring terpene compounds are shown in Table 12.1. Natural rubber, or poly(isoprene), is a polyterpene which consists of up of 1000 to 5000 isoprene units.

12.2.1 Resin Crude resin is obtained by tapping living pine trees. It is a thick, sticky, but still fluid material. Due to occluded moisture, the material is milky-gray in color. The resin contains a certain amount of forest debris, such as pine needles, insects, etc. The separation of resin into its component parts, namely rosin and turpentine, involves two basic operations: cleaning and distillation. The approximate composition of crude resin, as it is received at the plant for processing, is 70% rosin, 15% turpentine, and 15% debris and water. In the first stage of refinement, the resin is diluted with turpentine and heated. During purification by filtering of the hot diluted resin, all extraneous materials, both solid and soluble are removed. Filtration is usu-

Terpene Resins

Table 12.1: Naturally Occurring Terpene Compounds Monoterpenes α-Pinene β-Pinene Limonene Camphene Myrcene Terpinene Terpinolene Phellandrene Chrysanthemol Allo-ocimene Carene Dipentene

In oil of turpentine In various vegetable oils In various vegetable oils In eucalyptus oil 3

Sesquiterpenes Longifolene Carophyllene

From Himalaya pine

Diterpenes Retinol Abietic acid Pimaric acid

Rare, but its provitamin β-carotine is widespread In rosin A mixture of rosin acids. Components of pine rosin

Triterpenes Betulin Lupeol Squalene Polyterpenes Natural rubber Guttapercha

In birch bark In lupin seeds In shark liver, and natural oils

449

450


Table 12.2: Global Production Data (2000) of Terpenes and Derivatives4 Terpene Rosin Rosin gum Turpentine Turpentine gum

Mill. Metric tons 1.20 0.72 0.33 0.10

ally followed by a washing step. The purified resin then undergoes a steam distillation.

12.2.2 Turpentine Turpentine is a clear, flammable liquid, with a pungent odor and a bitter taste. It is immiscible with water and has a boiling point above 150°C. Turpentine is a mixture of organic compounds, mainly terpenes, and its composition can vary considerably according to the species of pine from which it is derived. Fractional distillation of turpentine allows the isolation of α-pinene and β-pinene.

12.2.3 Rosin Rosin is the major product obtained from pine resin. It remains behind as the non-volatile residue after distillation of the turpentine and is a brittle, transparent, glassy solid. It is insoluble in water but soluble in many organic solvents.4 It consists mainly of a mixture of abietic acids and pimaric acids. Most rosin is used in a chemically modified form rather than in the raw state in which it is obtained.

12.2.4 Production Data of Important Monomers Global production data of the most important monomers used for unsaturated polyester resins are shown in Table 12.2. Major producers are The People’s Republic of China, Indonesia, Russia, Brazil, and Portugal.

Terpene Resins

451

CH3

CH3 +

CH3 CH3

H

H

H3C C+ CH3

Figure 12.2: Cationic Initiation of the Polymerization of Terpenes

12.3 CURING 12.3.1 Homopolymers Polymers are produced by the cationic polymerization of α-pinene, βpinene, or limonene. The initiation reaction is shown in Figure 12.2. The initial stage is the same for α-pinene and β-pinene. However, the subsequent propagation differs. β-Pinene and limonene contain vinyl double bonds and vinylidene double bonds that facilitate polymerization. α-Pinene does not contain such double bonds. This makes the chain propagation more difficult for αpinene. β-Pinene and limonene resins are manufactured by the reaction in an aromatic solvent such as xylene or toluene by a Lewis acid, such as anhydrous aluminum chloride as catalyst. Ethylaluminum dichloride is also a suitable catalyst.5 For α-pinene a co-catalyst is needed to reach a degree of polymerization higher than dimer. Alkyl silicon halides and antimony chloride are suitable co-catalysts.6–8 The polymers are different in structure and molecular weight, which has a direct effect on the areas of application. The cationic polymerization is performed at 30 to 60°C, with 1 to 3% AlCl3 in a solvent. The reaction is strongly exothermic. The reaction is then quenched with water, dilute alkali, or acid. The organic phase is washed with water to remove hydrochloric acid and catalyst residues. Then the solvent and lower molecular weight dimer oils are stripped until a material with the desired softening point is obtained.

452


12.3.2 Copolymers Copolymers of α-pinene, β-pinene, limonene, styrene, piperylene, cyclopentadiene, and vinyl toluene can be prepared. The monomers are copolymerized with Lewis acid catalysts. The copolymerization of terpenes with other monomers such as styrene extends their Hildebrand solubility parameter. Such copolymers are compatible with poly(butadiene) rubbers. This copolymer is used in the manufacture of disposable diapers.

12.3.3 Terpene Phenolic Resins Terpene phenol resins are used in a variety of applications including adhesive and ink formulations and in the manufacture of engineering thermoplastics. Commercial terpene phenol resins are typically produced by reacting a terpene with a phenol in a suitable solvent in the presence of a catalyst. After the reaction is substantially complete, the catalyst is deactivated with water or clay, and the resin is isolated from the reaction mass product by distillation to remove the solvent and by-products. In particular, a phenol-terpene-cyclic polyolefin resin can be synthesized by reacting a phenol, a terpene or a low molecular weight propylene polymer and a cyclic polyolefin in the presence of a Friedel-Crafts catalyst in an aromatic, naphthenic, or paraffinic hydrocarbon solvent. The monomer compounds can first be blended together, after which the catalyst is added in small amounts with stirring. This method is particularly suitable if relatively small amounts of phenol compound have to be incorporated. The phenolic copolymer may also be prepared according to a reverse cationic polymerization in a solvent. “Reverse” means that an activated complex is first formed between the catalyst and the phenol compounds and after that the remaining monomer units are added. This method makes it possible to incorporate higher proportions of phenol compounds. Both methods can be applied with or without a solvent. By using a solvent, the reaction can proceed at lower temperatures.9 12.3.3.1

Carene Resins

Carenes show a poor reactivity. Therefore, an improved procedure has been identified. For preparation of carene-phenol resins a phenol with a carene is reacted two steps. The first step comprises reacting the entire

Terpene Resins

CH3

CH3

H

H H 3C

H CH3

(+)-2-Carene

H 3C

453

CH3

H H CH3

(+)-3-Carene

H 3C

H CH3

(+)-4-Carene

Figure 12.3: Carenes

amount of phenol with about one-half the amount of carene in the presence of a catalyst. Then the rest of the carene is reacted with the condensation product obtained in the first step in the presence of the catalyst. The resulting phenol-carene resin is then reacted with a reactive terpene to give an improved resin.10 Terpene-phenol-aldehyde resins based on α-pinene, phenol, and additional formaldehyde have a high softening point, greater than 140°C.11 Manufacturing of terpene-phenol resins with low softening points, i.e., softening points in the range from about 80°C to about 110°C, is very difficult. The traditional methods for producing such resins use one of two approaches. In the first approach, diluents such as mineral oil or polyolefin oligomers are added to resins having higher softening points. This approach usually results in reduced adhesive or ink formulation performance or excessive amounts of volatiles in the resulting resin. The second approach is to synthesize the low softening point resin directly. Generally, the synthetic methods in current use produce base resins that cannot be finished to softening points below 110 °C without leaving substantial amounts of process solvents or phenol in the resin. Again, this results in decreased adhesive or ink formulation performance.12 A terpene phenol-based resin with a low softening point comprises the reaction of a phenol dissolved in an organic solvent with terpene and an acyclic monounsaturated olefin, such as a mixture of 1-diisobutylene and 2-diisobutylene in the presence of a Lewis acid catalyst, such as boron trifluoride.13 Boron trifluoride is used as an acetic complex.10 Cyclic polyolefins can produce products with unacceptable amounts

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of low molecular weight fractions. The low molecular weight fractions tend to volatilize or cause smoking during the preparation and use of hot melt adhesives. Therefore, the reaction product is washed and distilled to remove the solvent and any unreacted phenol. Further, the product may be sparged with an inert gas at a temperature up to 260°C to remove any remaining low molecular weight terpene phenol alkylates and terpene-terpene dimers. A high yield of relatively low softening point terpene phenol resin is produced. It has softening point in the range from about 70°to about 110°C. Terpene phenol resins with low melting points are not suitable for use in printing ink applications. For use in printing inks, the amount of vinyl aromatic units should be less than 5%. The small fraction of resin results in good solubility of the resin (in the ink) and an effective drying of the ink. In these types of applications, dicyclopentadiene may be used.9 Terpene phenolic resins are suitable for more polar adhesive resins. They are made by the addition of a terpene to phenol.

12.3.4 Terpene Maleimide Resins Resinous terpene maleimides are useful as tackifiers for elastomers. They are prepared by reacting equimolar amounts of non-conjugated monocyclic terpenes and maleic anhydride at temperatures between 140°C and 200°C. Iodine in amounts of 0.05% to 0.15% is used as catalyst. In the first step, maleic adducts with the terpene are formed, including both mono adducts and a minor amount of di-adducts. This adduct mixture is reacted with stoichiometric amounts of an aliphatic primary diamine, such as ethylene diamine. A terpene maleimide resin having an average molecular weight between about 500 Dalton and about 600 Dalton is recovered.14 A suitable terpene type is the terpene fraction containing about 90% terpinolene with the remainder being monocyclic terpene hydrocarbons and terpene alcohols. A certain procedure14 yields a product, which is a resinous terpene maleimide, with properties as summarized in Table 12.3. The terpene imide resins are soluble in aromatic hydrocarbons, chlorinated hydrocarbons, esters, ketones, ethers, and alcohols, but insoluble in aliphatic hydrocarbons. Due to their molecular weight and compatibility (as shown by cloud point of 160°C), the resins find utility as tackifiers in chemically

Terpene Resins

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Table 12.3: Properties of a Resinous Terpene Maleimide Property

Unit

Softening point Acid number Number-average molecular weight Cloud point

88 1 533 159

[°C] [mg KOH/g] [Dalton] [°C]

polar formulations. The terpene imides prepared are compounded with vinyl acetate-ethylene copolymers and a paraffin wax.

12.4 PROPERTIES 12.4.1 Solubility Low molecular weight terpene resins have an excellent solubility in elastomers. This makes them useful for adhesives. 12.4.1.1

Hildebrand Solubility Parameters

The Hildebrand solubility parameters δ can be predicted on the basis of the solubility of polymers in solvents with known Hildebrand solubility parameters. The Hildebrand solubility is defined as the square root of the cohesive energy density, which is a characteristic for the intermolecular interactions in a pure liquid or solid. The solubility parameter is related to the heat of mixing Hm in Eq. 12.1. ∆Hm = nsVs Φ p (δs − δ p )2 Hm ns Vs Φp δs δp

(12.1)

Heat of mixing Moles of solvent Molar volume of solvent Volume fraction of polymer Solubility parameter of solvent Solubility parameter of polymer

∆Gm decreases as ∆Gm decreases. Therefore, if the two Hildebrand solubility parameters approach one another, the heat of mixing approaches a minimum. The theory of solubility parameters was developed by Scatchard in 1931 and further refined by Hildebrand.15

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Terpene resins will be effective as solid solvents for an elastomer when their Hildebrand solubility parameters are close to the Hildebrand solubility parameters of the respective polymer. For example, from Table 12.4 it can be seen that pure polyterpene resins are suitable tackifiers for poly(ethylene), natural rubber, and poly(butadiene) polymers. Further, terpene phenol resins are suitable tackifiers for poly(vinyl acetate), poly(methyl methacrylate), and poly(ethylene terephthalate).

12.4.2 Adhesive Properties 12.4.2.1

Tackifiers

Low molecular weight polyterpene resins are addressed as tackifiers. They act as solid solvents for adhesive backbone polymers and can modify the ability of an adhesive formulation in wetting a surface. The resins impart a tack, as they modify certain adhesion characteristics. The adhesive properties are expressed in terms of shear adhesion, peel adhesion, and quick stick. Quick stick is the resistance to separation of the adhesive from substrate, bonded without pressure. The addition of the rosin ester to ethylene/vinyl acetate (EVA) copolymers produces a compatible mixture, whereas for a terpene resin a less compatible mixture is obtained. The increase in the vinyl acetate amount in the EVA decreases the crystallinity of EVA. Both the storage and the loss moduli decrease, but the peel strength and the immediate adhesion increase. The immediate adhesion of the EVA/tackifier blends is affected by both the compatibility and the rheological properties of the blends. An increase in the VA content increases the flexibility of the adhesives and thus a decrease in peel strength is obtained.16 The tackifying properties are used not only for adhesive purposes. For example, the cover material of a golf ball can consist of an ionomer resin with up to 50% of a tackifier such as terpene resins, or rosin ester resins.17 12.4.2.2

Cotackifiers

Polyterpene resins are compatible with paraffins. Therefore, they are also compatible with petroleum hydrocarbon resins that are used as cotackifiers. Other cotackifiers include rosins and coumarone-indene resins.

Terpene Resins

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Table 12.4: Hildebrand Solubility Parameters δ of Solvents and Polymers18, 19 Solvent n-Pentane n-Hexane Diethyl ether 1,1,1-Trichloroethane Turpentine Cyclohexane Xylene Ethyl acetate Benzene Methylethylketone Acetone Pyridine Ethanol Dimethyl sulfoxide n-Butanol Methanol Water Polymer Poly(ethylene) Poly(butadiene) Poly(styrene) Poly(vinyl acetate) Poly(methyl methacrylate) Poly(ethylene terephthalate) Rosin esters β-Pinene polyterpene Terpene phenol polymers a MPa1/2 ∼ 2.05× µ m =

δ M Pa1/2 a 14.4 14.9 15.4 15.8 16.6 16.8 18.2 18.2 18.7 19.3 19.7 21.7 26.2 26.4 28.7 36.2 48.0 δ M Pa1/2 16–17 16–17 17–20 19 19–26 19-22 18 16 19–21

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Reactive Polymers Fundamentals and Applications Table 12.5: Important Specifications of Terpene Resins Parameter Solubility Cloud point Softening point Toluene insolubles Color Viscosity at compounding temperatures Thermal stability Compatibility

Remarks

Ring and ball method Residues from catalyst Gardner scale

12.4.3 Characterization Some important specifications of terpene resins are shown in Table 12.5. 12.4.3.1

Rheological and Aging Characteristics

Polyterpene resins can modify the rheological and aging characteristics of adhesive backbone polymers. In this way the polymers themselves become usable as adhesives. It is possible to correlate rheological characteristics to the tack, shear, and peel. 12.4.3.2

Cloud Point

Higher molecular weight resins will have a higher cloud point in polymerpolymer combinations. 12.4.3.3

Softening Point

The softening point is related to the glass transition temperature and to the melt viscosity. Polyterpene resins have cyclic and polycyclic structures in the backbone. These provide high softening points at low molecular weights and low viscosities, which is very useful. Terpene resins are available in softening points from 25°C to 135°C. Most commercial resins have softening points of 85°C to 115°C. 12.4.3.4

Color

The color of a resin is monitored in a 50% heptane solution and is given in the Gardner scale.

Terpene Resins 12.4.3.5

459

Toluene Insolubles

The toluene insolubles is a measure of the amount of inorganic material in the resin. The toluene insolubles consist mainly of catalyst residues.

12.4.4 Recycling 12.4.4.1

Pyrolysis of Poly(isoprene)

Controlled thermal depolymerization at 300 to 380°C of cis-1,4-poly(isoprene) produces a liquid poly(isoprene) having a considerably lower molecular weight in comparison to the starting cis-1,4-polymer. The product is enriched in trans-1,4-units and 3,4-units together with vinylidene units. FT-IR spectroscopy shows that the end groups of liquid poly(isoprene) consist of diene, triene and tetraene moieties. Aldehyde groups conjugated with dienes and trienes are also analyzed. Pyrolysis opens a possible low cost source of raw materials, since the pyrolysis of natural and synthetic cis-1,4-poly(isoprene) produces mainly dipentene (DL-limonene) with small amounts of isoprene and other products. A 3,4-poly(isoprene)-rich polymer produces mainly dipentene. The residual of pyrolysis consists of 3,4-poly(isoprene). The crude dipentene obtained from cis-1,4-poly(isoprene) pyrolysis can be converted into a terpene resin usable in adhesive formulations and as thermoplastic rubber tackifier.20 12.4.4.2

Biodegradation

Films for packaging based on isotactic poly(propylene)-modified with natural terpene resins are biodegradable. It was found that a certain microbial community was able to erode the blend films but not the plain isotactic poly(propylene) film.21

12.5 APPLICATIONS AND USES Only three terpenes, i.e., α-pinene, β-pinene, and limonene have found commercial application in the manufacture of polyterpene resins. Polyterpene resins are used as pressure-sensitive adhesives, hot-melt adhesives, and sealants. Some polyterpene resins are used in chewing gum.

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12.5.1 Sealants 12.5.1.1

Moisture Barrier Films

Oriented poly(propylene) (OPP) is known for its inherent moisture barrier properties. However, certain applications require even greater resistance to water vapor transmission to increase the shelf life of the packed material. The incorporation of terpene polymers at low levels in high crystallinity poly(propylene) provides a product film having significantly improved moisture barrier properties. The addition of a terpene polymer increases the extent of amorphous orientation in the stretching of the poly(propylene), thereby restricting the diffusion of water molecules.3 Terpene polymers can also be added to poly(propylene) film materials to improve the heat seal properties.

12.5.2 Pressure-sensitive Adhesives Pressure-sensitive adhesives are used for adhesive tapes and labels. The substrates include paper, polyvinyl chloride, polyester, poly(propylene), or cellophane.22 Pressure-sensitive adhesives (PSA) can be prepared via hot melts, solvents, and waterborne systems. In solvent-containing systems, formulations with a high content of solid material are possible, requiring a minimum solvent recovery. Mixtures of polyterpene resins with different molecular weights can be used to establish the desired adhesive properties. 12.5.2.1

Hot-melt Extrusion

The production of PSA via hot-melt extrusion methods is the preferred route. The corresponding rubber is a styrene isoprene rubber or a styrene butadiene rubber. These block copolymers are elastomers, but become thermoplastic upon heating. 12.5.2.2

Waterborne Systems

For waterborne applied systems, the structural polymer and the tackifying resin must be supplied as dispersion. No solvent recovery system is necessary. For waterborne systems, the manufacture via rosin esters is easier than the emulsification of polyterpene resins. For carboxylated styrene

Terpene Resins

461

butadiene rubber, pure polyterpenes are suitable. For neoprene and acrylic rubbers, terpene phenol polymers are used. 12.5.2.3

Styrene-isoprene Block copolymers

Poly(styrene-b-isoprene-b-styrene) (SIS)/tackifier resin blends show a lower critical solution temperature phase transition at around 150°C and an upper critical solution temperature phase transition at around 200°C. The properties of the pressure-sensitive adhesive in SIS/tackifier resin blends change with the annealing temperature.23

12.5.3 Polyacrylate Hot-melt Pressure-sensitive Adhesives For industrial pressure-sensitive adhesive (PSA) tape applications it is very common to use polyacrylate PSAs. Polyacrylates possess a variety of advantages over other elastomers. They are highly stable toward UV light, oxygen, and ozone. Synthetic and natural rubber adhesives normally contain double bonds, which make these adhesives unstable to environmental effects. Further advantages of polyacrylates include their transparency and their serviceability within a relatively wide temperature range. Polyacrylate PSAs are generally prepared in solution by free-radical polymerization. A variety of polymerization methods are suitable for preparing low molecular mass PSAs. Chain transfer agents, such as alcohols or thiols, can be used. These chain transfer agents reduce the molecular weight and broaden the molecular weight distribution. Another controlled polymerization method is that of atom transfer radical polymerization (ATRP). The initiators include monofunctional or difunctional secondary or tertiary halides and, for abstracting the halides, complexes of certain metals. However, the metal catalysts have the side effect of adversely influencing the aging of the PSAs (gelling, transesterification). Unfortunately, the majority of metal catalysts are toxic, discolor the adhesive, and can be removed from the polymer only by means of complicated precipitations. Another method is to use a nitroxide-controlled polymerization process. It is possible to realize high conversions in combination with high molecular weight and low polydispersity. In order to increase the cohesion, the polymer is crosslinked. Curing takes place thermally, by UV crosslinking, or by electron beam curing. The polyacrylates are normally applied to the corresponding backing material from solution using a coating bar, and then dried. However,

462


it is difficult to produce PSA tapes with a high adhesive application rate, without bubbles. One solution to overcome this disadvantage is the hotmelt process. In this process, the PSA is applied to the backing material from the melt. Polyacrylate hot-melt pressure-sensitive adhesives with copolymerized photoinitiators and with a narrow molecular weight distribution can be processed very effectively in a melt process and can be crosslinked very efficiently by UV crosslinking.24 For the use of the polymers as pressure-sensitive adhesives, they are optimized by blending with tackifying resins. Tackifying resins include pinene resins, indene resins, and rosins, and derivatives.

12.5.4 Hot-Melt Adhesives Hot-melt adhesives are commonly used in bookbindery, for the manufacture of packaging, for coatings, diapers, other sanitary products, and tapes. Due to their properties, polyterpene resins are ideal materials for the formulation of hot melt systems. They are compatible with many structural polymers and they exhibit a high softening point together with a low melt viscosity. Therefore, upon melting, a low viscous mixture with the structural polymer is formed. Therefore, polyterpene resins are considered as the best modifiers to improve the tack and adhesion of elastomeric systems. Limonene polymers have a good hot tack performance. Most commonly used hot-melt mixtures are EVA (or block copolymers), resin tackifier, and wax blends. Most commercially available hot melt adhesives require temperatures of above 180°C to ensure complete melting of all the components and also to achieve a satisfactory application viscosity. Adhesive formulations that can be applied at temperatures below 120°C are prepared using low molecular weight components or a high wax content, however, the application viscosity and adhesive properties suffer. Softer or more amorphous components may be added in order to improve adhesion. However, these components reduce the effective heat resistance. Modified rosins and modified terpenes, having a molecular weightto-softening point ratio of less than about 10, when used as tackifier alone or in combination, in a hot melt provide adhesives that can be applied at low temperature and exhibit high heat resistance and good cold resistance.

Terpene Resins

463

A modified rosin or terpene is a phenolic-modified resin.25 The molecular weight-to-softening point ratio is the molecular weight of the modified rosin or modified terpene in Dalton divided by its softening point in °C.

12.5.5 Coatings Thermoplastic resin compositions consisting of a poly(phenylene ether) (PPE) and polyamide (PA) resin show outstanding properties that make them well suited for inline coating. They also find extensive use in external automobile components. They are used with paints such as acrylic urethane paint, acrylic amino paint, and polyester polyol paint, but these paints do not show sufficient adhesion to the PPE/PA resin composition, and when they are used for direct coatings, peeling of the coating film may occur. For this reason, the main method used is to first apply a coat of primer to the molded product, followed by a finish coat. However, with the tendency towards cutting costs, attitudes towards coatings tend to shift toward the primerless approach, and there is a changeover toward thinner coating films. When a specified terpene phenol resin is added to a PPE/PA resin composition, the coating film adhesion of the composition can be considerably improved.26 The terpene phenol resin must have a hydroxyl value of 50 or greater. If the hydroxyl value is too low, the adhesion will be poor. For example, a suitable terpene phenol resin is a copolymer of limonene and phenol or copolymer of α-pinene and phenol. It has an average molecular weight of 700 Dalton, a softening point of 120 to 150°C, and a hydroxyl value of 50 to 130 mg KOH/g.

12.5.6 Sizing Agents Sizing agents are used by the paper industry to give paper and paperboard some degree of resistance to wetting and penetration by aqueous liquids. There are two basic categories of sizing agents: acid and alkaline. Acid sizing agents are intended for use in acid papermaking systems, traditionally less than pH 5. Analogously, alkaline sizing agents are intended for use in alkaline papermaking systems, typically at a pH greater than 6.5. Most acid sizing agents are based on rosin. The development of sizing with a rosin-based size is dependent upon its reaction with papermaker’s alum, Al2 (SO4 )3 × 14 − 18H2 O. Since aluminum species that ex-

464


ist predominantly at a low pH (< pH 5) are required for the appropriate interactions needed to effect sizing, rosin and alum have been used primarily in acid papermaking systems. It has been shown that, by proper selection of addition points in the papermaking system. By using cationic dispersed rosin sizes, rosin-based sizes can be used in papermaking systems up to about pH 7, thus extending the range of acid sizes. However, due to the limitations imposed by alum chemistry, the efficiency of rosin-based sizes decreases above about pH 5.5.8 12.5.6.1

Alkaline Sizing

Sizing agents developed for papermaking systems above pH 6.5 are generally based on alkylketene dimer (AKD) or alkenyl succinic anhydride (ASA). Alkylketene dimer. Sizes based on alkylketene dimer (AKD) form covalent bonds with cellulose to give proper orientation and anchoring of the hydrophobic alkyl chains. This covalent bond formation makes AKD sizing very efficient and resistant to strong penetrants. However, AKD sizes have some limitations: small changes in the amount of size added can lead to large differences in sizing (steep sizing response curve), and there is a slow rate of sizing development (cure). Alkenyl succinic anhydride. The other major alkaline sizing agent is based on ASA. As with AKD, the development of sizing with ASA sizes is also dependent on the formation of covalent bonds with cellulose to give proper orientation and anchoring. ASA is more reactive than AKD, resulting in a greater sizing effect. However, the reaction rate with water is also greater, producing a hydrolyzate that is an inefficient sizing agent in alkaline systems. It also contributes to the formation of deposits on the papermaking machine. To minimize the formation of hydrolyzate, ASA is typically emulsified at the mill immediately before addition to the papermaking system. Cationic resins. Cationic resins have been used in the papermaking process.8 Sizing of paper can be done with an aqueous emulsion of a partially saponified terpene resin, 1 to 5% alum, and optionally a partially saponified

Terpene Resins

465

rosin. Sizing in the absence of alum is achieved with an aqueous dispersion of fortified rosin, a hydrocarbon resin and a vinyl imidazoline polymer as a retention aid. Another sizing composition for paper comprises an aqueous dispersion of partially neutralized rosin, a terpene polymer, and 1 to 5% aluminum sulfate. The addition of a cationic polyamine resin is used to anchor the rosin to the paper pulp. The cationic polyamine resin is of the type polyalkyleneamine-epihalohydrin resin.8 High levels of size or cationic resin cause no significant reduction in the papers coefficient of friction.

12.5.7 Toner Compositions Toner compositions containing copolymers based on styrene and myrcene can be synthesized by an anionic polymerization process. The reaction needs dried reagents. A resin was obtained with a Tg of 60°C, with Mn of 36 k Dalton and Mw of 64 k Dalton. Charge additives are quaternary ammonium bisulfates and distearyl methyl hydrogen ammonium bisulfate.27

12.5.8 Chewing Gums In general, chewing gums and bubble gums utilize as their gum base a combination of natural or synthetic elastomers. Preferably, polymers of limonene or other dipentenes with rosin-glycerol esters are used in the formulation of chewing gums. The gum base that is selected provides the chewing gum with its masticatory properties. A chewing gum base is normally admixed with sugars or synthetic sweeteners, perfumes, flavors, plasticizers, and fillers. Then it is milled and formed into sticks, sheets, or pellets. Cottonseed oil is sometimes also added to give the gum softness. Styrene butadiene rubber (SBR) is a synthetic elastomer that is widely used as a gum base in chewing gums. However, SBR is not widely used in manufacturing soft chew gums because it lacks the desired physical properties. Poly(isobutylene) is widely used in manufacturing soft chew gums even though it is much more expensive than SBR. In any case, chewing gum compositions are typically comprised of a water soluble bulk portion, a water insoluble chewing gum base portion and typically water insoluble flavoring agents. The water soluble portion dissipates with a portion of the flavoring agent over a period of time during chewing. The gum base portion is retained in the mouth throughout the chewing process.

466


The gum base includes a number of ingredients that are subject to deterioration through oxidation during storage. The insoluble gum base is generally comprised of elastomers, elastomer plasticizers, waxes, fats, oils, softeners, emulsifiers, fillers, texturizers and miscellaneous ingredients, such as antioxidants, preservatives, colorants, and whiteners. The compounds containing carbon-carbon double bonds, such as fats, oils, unsaturated elastomers, and elastomer plasticizers are susceptible to oxidation. The gum base commonly contains 15 to 35% by weight of the chewing gum. In chewing gum base natural or artificial antioxidants are utilized to stabilize the rubbery polymer. For instance, β-carotenes, acidulants (e.g., Vitamin C), propyl gallate, BHA, and BHT are commonly used to stabilize the rubber used in manufacturing chewing gum. Such antioxidants are included in the chewing gum base as a stabilizer to inhibit oxidation. Antioxidants are widely used in food products susceptible to degeneration, in one form or another, due to oxidation. Commercial applications include use in processed meat and poultry, salad dressings, seasonings, snacks, nuts, soup bases, edible fats and oils, natural foods, pet foods, and packaging. In addition to foods, antioxidants have been used to prevent oxidation in various cosmetic and toiletry products and in pharmaceutical preparations. The primary purpose in each of these applications is to prevent deterioration of desirable product characteristics by inhibiting oxidation.

12.6 SPECIAL FORMULATIONS 12.6.1 Toughener for Novolaks There is considerable technical interest in hydrophobically substituted, but nevertheless crosslinkable and grindable novolaks, since they have considerably better compatibility with hydrophobic substrates. In addition, it is desirable to control the crosslinking rate of novolak/crosslinking agent mixtures at a given temperature. It is furthermore desirable to reduce the high brittleness of the crosslinking products of novolaks. These difficulties can be overcome by means of modified novolaks which contain, as modifying components, terpenes and unsaturated carboxylic acids or derivatives of these compounds. Phenols that have at least one free ring hydrogen atom in the orthoor para-position can be substituted to the phenolic hydroxyl group with

Terpene Resins

467

terpenes in the presence of Lewis or protonic acids. This gives low-molecular-weight synthetic resins which have a relatively high melting point, but cannot be crosslinked. However, if a significant number of phenolic hydroxyl groups is still present after the reaction, terpene modified resins can be subjected to crosslinking reactions.28

12.6.2 Fluoro Copolymers A more uniform copolymer with a narrower molecular weight distribution for improved flex life can be obtained by the copolymerization of tetrafluoroethylene (TFE) and perfluoro (alkyl vinyl ether) (PAVE) in the presence of a terpene in an aqueous polymerization medium. This produces a melt-fabricable TFE/PAVE copolymer (PFA) having a uniformly distributed PAVE. The small amount of terpene added to the polymerization system does not decrease the rate of polymerization, but is present in an amount that is effective for improving the uniformity of the resin by narrowing the molecular weight distribution, i.e., in the ppm range.29

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mer, editors, Microbiology of Composting, pages 265–272. Springer-Verlag, Berlin, 2002. D. Satas, editor. Handbook of Pressure Sensitive Adhesive Technology. Satas & Associates, Warwick, RI, 3rd edition, 1999. S. Akiyama, Y. Kobori, A. Sugisaki, T. Koyama, and I. Akiba. Phase behavior and pressure sensitive adhesive properties in blends of poly(styreneb-isoprene-b-styrene) with tackifier resin. Polymer, 41(11):4021–4027, May 2000. M. Husemann and S. Zollner. UV-crosslinkable acrylic hotmelt PSAs with narrow molecular weight distribution. US Patent 6 720 399, assigned to tesa AG (Hamburg, DE), April 13 2004. D. L. Haner, B. Carillo, and J. Mehaffy. Hot melt adhesive composition. US Patent 6 593 407, assigned to National Starch and Chemical Investment Holding Corporation (New Castle, DE), July 15 2003. T. Ohtomo, K. Myojo, and H. Kubo. Compositions of polyphenylene ether and polyamide resins containing terpene phenol resins. US Patent 5 554 693, assigned to General Electric Company (Pittsfiled, MA), September 10 1996. M. K. Georges, N. A. Listigovers, S. V. Drappel, M. V. McDougall, and G. R. Allison. Toner compositions with styrene terpene resins. US Patent 5 364 723, assigned to Xerox Corporation (Stamford, CT), November 15 1994. W. Hesse, E. Leicht, and R. Sattelmeyer. Modified novolak terpene products. US Patent 5 096 996, assigned to Hoechst Aktiengesellschaft (DE), March 17 1992. T. Iwasaki and M. Kino. Process for manufacture of a copolymer of tetrafluoroethylene and perfluoro (alkyl vinyl ether). US Patent 6 586 546, assigned to DuPont-Mitsui Fluorochemicals Co. Ltd. (Tokyo, JP), July 1 2003.

13 Cyanoacrylates Cyanoacrylates were commercially introduced in 1950 by Tennesee Eastman Company. Cyanoacrylate adhesives are monomeric adhesives. They are generally quick-setting materials which cure to clear, hard glassy resins, useful as sealants, coatings, and particularly adhesives for bonding together a variety of substrates.1 Polymers of alkyl 2-cyanoacrylates are also known as superglues.

13.1 MONOMERS 13.1.1 Synthesis In 1895 Auwers and Thorpe2 attempted to synthesize diethyl-2,2-dicyanoglutarate (Figure 13.1) by base-catalyzed condensation of aqueous formaldehyde and ethyl cyanoacetate. They isolated a mixture of oily oligomers and an amorphous polymer of higher molecular weight.

CN

CN CH2

H C CH2

C C O R

R O C

O

O

CN C H C O R O

Figure 13.1: 2,4-Dicyanoglutaric Acid Ester

471

472


CN CH2 O

+

CN CH2

H C H

C O R

C O R O

C

-H2O

O

Figure 13.2: Synthesis of Cyanoacrylates: Knoevenagel Reaction

In fact ethyl 2-cyanoacrylate monomer was synthesized as an intermediate, which underwent an immediate polymerization reaction. The condensation of formaldehyde with cyanoacetate is still the most important method for the commercial production of the monomers, c.f. Figure 13.2. The reaction mechanism takes place as a base-catalyzed Knoevenagel condensation of cyanoacetate and formaldehyde to give an intermediate 2-substituted methylol derivative. A. E. Ardis3 at B.F. Goodrich (in 1947), found that the polymer-oligomer mixture obtained in the formaldehyde-cyanoacetate condensation reaction could be thermally depolymerized with acid catalysts. However, the monomer prepared by utilizing these methods was unstable and the yields were low. Later4 it was realized that the water is responsible for polymerization. Instead of aqueous formaldehyde, paraformaldehyde was used with an organic solvent to remove the water by azeotropic distillation. The stability of the monomer can be enhanced by the redistillation of the crude monomer in the presence of small quantities of acidic stabilizers, e.g., sulfur dioxide. Several other methods for cyanoacrylate monomer production have been described, including the pyrolysis of 3-alkoxy-2-cyanopropionates,5 transesterification of ethyl 2-cyanoacrylate,6 and displacement of cyanoacrylate monomer from its anthracene Diels-Alder adduct by treatment with maleic anhydride. This last method is used for the synthesis of monomers that are not accessible or may be difficult to prepare by the retropolymerization route, for example, difunctional cyanoacrylates,7 thiocyanoacrylates,8 and perfluorinated monomers.

13.1.2 Crosslinkers To improve the cohesive strength, difunctional monomeric crosslinking agents may be added to the monomer compositions. These include alkyl

Cyanoacrylates

473

Table 13.1: Commercially Available Cyanoacrylates Compound

Remarks

Methyl cyanoacrylate

Strongest bonding to metals, good stability against solvents General purpose > 100°C service temperature Flexible, medical applications9 Medical applications9 Medical applications9, 10 Weak odor

Ethyl cyanoacrylate Allyl cyanoacrylate n-Butyl cyanoacrylate Isobutyl cyanoacrylate 2-Octyl cyanoacrylate 2-Methoxyethyl cyanoacrylate 2-Ethoxyethyl cyanoacrylate Weak odor 2-Methoxy-1-methylethyl Weak odor cyanoacrylate

CN CH2

C

CN CH2

C

C O R

C O R

O

O

n

Figure 13.3: Basic Structure of Cyanoacrylate Monomers and Polymers

bis(2-cyanoacrylates), triallyl isocyanurates, alkylene diacrylates, alkylene dimethacrylates, 1,1,1-trimethylolpropane triacrylate, and alkyl bis(2-cyanoacrylates).11

13.1.3 Commercial Products Commercial products consist mainly of monofunctional monomers. Commonly encountered monomers are shown in Table 13.1. The monomers are usually low-viscosity liquids with excellent wetting properties. The basic structure of cyanoacrylate monomers and polymers is shown in Figure 13.3. The syntheses of the monomers and the raw materials are shown in Figures 13.4 and 13.5. Because of the high electronegativity of the nitrile group and the carboxylate groups, they undergo rapid anionic polymerization on contact with basic catalysts. The anionic polymerization is facilitated by the possibility of resonance structures as shown in Figure 13.6. The polymers formed in this way exhibit high molecular weights, usually more than 106 Dalton.

474


CN H C H

+

CH2 O

+

C O R

N

O

H

-H2O CN N CH2 C H C O R O

CN CH2

C C O R O

Figure 13.4: Synthesis of Cyanoacrylates: Mannich Reaction

Cl

Cl C C

H

CH3 Cl

O H2O

Cl

C O H

CH2

Cl2

C O H O

Figure 13.5: Synthesis of Chloro acetic acid

Cyanoacrylates

CN Y

-

+

CH2

C

Y C O R

CH2

C-

CN

C O R

O

O

N

N

C Y

CH2

C

C

Y C O R

O-

475

CH2

CC O R O

Figure 13.6: Resonance Structures of the Growing Anions

13.2 SPECIAL ADDITIVES 13.2.1 Plasticizers Adhesives based on cyanoacrylate esters are effective bonding agents for a wide variety of materials, but do not give a permanent bond in joints involving glass. A strong bond to glass is obtained initially but generally the joint fails after a period of weeks or months at room temperature conditions. The extremely rapid curing rate on glass caused by the basic nature of the surface is responsible for high stresses that are generated in the bond line immediately adjacent to the glass, at a molecular level. These stresses make the polymer in the bond line uniquely susceptible to chemical or physical degradation.12 Cyanoacrylate adhesive bonds also tend to be relatively brittle; therefore, the adhesive compositions are often plasticized.13 Typical plasticisers include various alkyl esters and diesters and alkyl and aromatic phosphates and phosphonates, diallyl phthalates and aryl and diaryl ethers. Plasticisers are summarized in Table 13.2 For glass bonding, dibutyl phthalate is a suitable plasticizer in nbutyl cyanoacrylate.12 The glass bonds were tested for durability by subjecting them to a sequence of washing cycles in a domestic dishwasher. The results shown in Table 13.3. suggest that the bond strength decreases with increasing proportions of plasticizer. Levels greater than about 40% result in bonds of reduced strength. The concentration of plasticizer needed

476


Table 13.2: Plasticizers11 Compound Dioctyl phthalate Dimethyl sebacate Triethyl phosphate Tri(2-ethylhexyl)phosphate Tri(p-cresyl)phosphate Glyceryl triacetate Glyceryl tributyrate Diethyl sebacate Dioctyl adipate Isopropyl myristate Butyl stearate Lauric acid Dibutyl phthalate Trioctyl trimellitate Dioctyl glutarate

Table 13.3: Durability of Bonds to Glass with Various Amounts of Plasticizer12 Dibutyl phthalate [%]

Bond Strength N/mm2

0 10 20 25 30 40 50 60 70 a No. of Dishwasher Cycles

2.70 3.20 3.20 1.94 2.46 1.86 0.80 0.32 0.08

Durabilitya 5 3 5 5 50 90 90 20 10

Cyanoacrylates

477

for good durability is about 30% to 50%.

13.2.2 Accelerators The esters of 2-cyanoacrylic acid are also commonly called quick-set adhesives, since they generally harden after a few seconds when used or the joined parts exhibit at least a certain degree of initial strength. However, in the case of some substrates, especially acidic substrates such as wood or paper, the polymerization reaction may be very greatly delayed. Acidic materials exhibit a pronounced tendency to draw the adhesive, which is often highly liquid, out of the joint gap by capillary action before hardening has taken place in the gap. Even in cases in which, for reasons of geometry, the adhesive must be applied in a relatively thick layer in the joint gap or in cases where relatively large amounts of adhesive are applied and relatively large drops of adhesive protrude from between the parts to be joined, rapid hardening throughout may rarely be achieved.14 Therefore, attempts have been made to accelerate the polymerization for such applications by means of certain additives. The methods used may roughly be divided into three categories: • Addition of accelerators directly to the adhesive formulation. This is possible to only a very limited extent, however, since substances having a basic or nucleophilic action, which would normally bring about a pronounced acceleration of the polymerization of the cyanoacrylate adhesive, are generally used at the expense of the storage stability of such compositions. • The second common method is the addition of the accelerators shortly before application of the adhesive in virtually a two-component system. However, such method has the disadvantage that the working life is limited after the activator has been mixed in. In addition, with the small amounts of activator that are required, the necessary accuracy of metering and homogeneity of mixing are difficult to achieve. • A third process is the use of activators in the form of a dilute solution. The solution is either sprayed onto the parts before they are bonded onto the places where the adhesive is still liquid after the substrates have been joined. The solvents used for such dilute solutions of activators are generally low-boiling organic solvents.

478


R1 (CH2

CH2

O)n

Si

O

R2 n = 4..10

Figure 13.7: Silacrown ethers15

Cure accelerators include crown ethers, calixarene compounds, silacrown compounds, and amines.

13.2.2.1

Silacrown Compounds

Silacrown compounds as additives give substantially reduced fixture and cure times on wood and other deactivating surfaces such as leather, ceramic, plastics and metals with chromate treated or acidic oxide surfaces. Silacrown accelerators have significantly lower reported acute toxicity than the crown ether compounds. The lower observed toxicity of silacrowns in comparison to crown-ethers may be related to the hydrolytic instability of the Si-O-C linkage. Thus, while the silacrown ring is stable in the cyanoacrylate composition, it will open up in biological environments, reducing both acute and chronic risk.16 Silacrowns are prepared by transesterification of alkoxysilanes with poly(ethylene glycol)s, i.e., they are reaction products of silanes but are not themselves silanes. Silacrown compounds are commercially available and are reportedly readily synthesized in good yield.15–18 Silacrown ethers are shown in Figure 13.7.

13.2.2.2

Calixarenes

Cyanoacrylate adhesive compositions that employ calixarene compounds as additives give substantially reduced fixture and cure times on wood and other deactivating surfaces such as leather, ceramic, plastics and metals with chromate-treated or ceramic oxide surfaces.19–21

Cyanoacrylates

N

479

N S S

N HOOC

N S S

COOH

N

N S S N

N

Figure 13.8: 2,2′ -Dipyridyl disulfide, 6,6′ -Dithiodinicotinic acid and Bis(4-tertbutyl-1-isopropyl-2-imidazolyl)disulfide

13.2.2.3

Amines

Solutions of lower fatty amines, aromatic amines, and dimethylamine are used that are sprayed on the surface before the cyanoacrylate is applied, or at the same time. Examples are N,N-dimethylbenzylamine, N-methylmorpholine, and N,N-diethyltoluidine. N,N-Dimethyl-p-toluidine, when subsequently applied to the joined parts, causes even relatively large amounts of adhesive to harden within seconds. The poly(cyanoacrylate) so formed is completely free of turbidity. Disadvantages are the very high volatility of the substance, which does not permit long waiting times between the application of the accelerator solution to the substrates to be bonded and the subsequent bonding process. The compound is also toxic.14 13.2.2.4

Disulfides

Examples of disulfides are dibenzodiazyl disulfide, 6,6′ -dithiodinicotinic acid, 2,2′ -dipyridyl disulfide, or bis(4-tert-butyl-1-isopropyl-2-imidazolyl)disulfide,14 c.f. Figure 13.8.The disulfides have a good accelerating action, but they nevertheless permit a long waiting time between application of the activator and application of the adhesive. In addition, they avoid spontaneous, merely superficial hardening.

480

Reactive Polymers Fundamentals and Applications Table 13.4: Thickeners Compound

Reference

Fumed silica Poly(cyanoacrylate) Poly(lactic acid) Poly(glycolic acid) Lactic-glycolic acid copolymers Poly(ε-caprolactone) Poly(3-hydroxybutyric acid) Polyorthoesters Polyacrylates Polymethacrylates

16 11 11 11 11 11 11 11 11 11

13.2.3 Thickeners Thickeners are added to increase the viscosity of 2-cyanoacrylate adhesive compositions. The 2-cyanoacrylate monomer generally has a low viscosity of several centipoise, and, therefore, the adhesive penetrates into porous materials such as wood and leather or adherents with a rough surface. Thus, good adhesion bond strengths are difficult to achieve. Thickeners are summarized in Table 13.4. Various polymers can be used as thickeners, and examples include poly(methyl methacrylate), methacrylate-type copolymers, acrylic rubbers, cellulose derivatives, poly(vinyl acetate), and poly(2-cyanoacrylate). A suitable amount of thickener is generally about 20% by weight or less based on the total weight of the adhesive composition. Fumed silica for use as thickener is treated with poly(dialkylsiloxane) or trialkoxyalkylsilanes.16 The purpose of the silane which is retained on the surface of the silica is to maintain the fumed silica in a dispersion within the composition.

13.2.4 Stabilizers Stabilizers have to be added both for the production and for storage. The stabilizer systems are added so that no polymerization would occur during transportation and storage in sealed drums, even at elevated temperatures and after long periods. After application polymerization occurs immediately. Accordingly, besides radical polymerization inhibitors, inhibitors against anionic poly-

Cyanoacrylates

481

Table 13.5: Stabilizers Compound

Reference

Sulfur dioxide 6-Hydroxy-5-[(4-sulfophenyl)azo]-2-naphthalenesulfonic acid Lactone Boron trifluoride Hydroquinone Catechol Pyrogallol p-Benzoquinone 2-Hydroxybenzoquinone p-Methoxyphenol tert-Butyl catechol Organic acid Butylated hydroxy anisole Butylated hydroxy toluene tert-Butyl hydroquinone Alkyl sulfate Alkyl sulfite 3-Sulfolene Alkylsulfone Alkyl sulfoxide Mercaptan Alkyl sulfide Dioxathiolanes

11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 22

merization are generally added to cyanoacrylate adhesives. Stabilizers are summarized in Table 13.5. A typical stabilizer to prevent radical polymerization is hydroquinone. Boron trifluoride prevents anionic polymerization.

13.2.4.1

Acidic Cation Exchanger

It has been proposed to add a strongly acidic cation exchanger as inhibitor. Cation exchangers are based on crosslinked poly(styrene)-containing sulfonic acid groups. The disadvantage of this approach is that the ion exchanger added can easily impede the outflow of the adhesive and that, as a solid, it does not act throughout the entire volume of the adhesive.

482


13.2.4.2

Acid Groups on Container Walls

It has been proposed to modify the surface of storage containers for cyanoacrylate adhesives in such a way that they contain acid groups.23 Although this proposal can be successfully implemented, it is afflicted by the problem that the inhibition occurs in the vicinity of the container wall. 13.2.4.3

Sulfur Compounds

Sulfur Dioxide. Another method of stabilizing cyanoacrylate adhesives is to add sulfur dioxide as an inhibitor. Although this measure has been successfully applied in practice, it is important to bear in mind that sulfur dioxide is a gaseous substance and that uniform addition is difficult so that quality variations can occur. In addition, sulfur dioxide can escape from the adhesive containers by diffusion during storage. Dioxathiolanes. Cyclic organic sulfates, sulfites, sulfoxides, sulfinates, for example, 2-oxo-1,3,2-dioxathiolanes, act in raising the ceiling temperature and hence to improve the thermal stability of the adhesives.22 4,5-Dimethyl-2-oxo-1,3,2-dioxathiolane is a liquid with a boiling point of 185°C. This is an inhibitor for the anionic polymerization and should be effective throughout the entire volume of the adhesive. It could be added more uniformly and more easily than gases. In addition, the discoloration of the adhesive during storage is prevented.24

13.2.5 Primers It is well known in the adhesive field that there are plastic substrates made from certain types of plastic materials which are extremely difficult to bond. Such difficult-to-bond materials include low surface energy plastics such as poly(ethylene) and poly(propylene) and highly crystalline materials such as polyacetals and poly(butylene terephthalate). As a consequence of the difficulty in bonding substrates made from these plastics materials with adhesives, various surface treatments have been employed where such materials require bonding. Examples of such surface treatments include corona discharge exposure of the substrate surface, acid etching, plasma treatment, etc. However, these methods are clearly not applicable to the bonding of plastic substrates in the domestic or household areas. Alternatively, various primer compositions have been developed which are de-

Cyanoacrylates

483

Table 13.6: Primers Compound

Reference

n-Octylamine 1,5-Diazabicyclo[4.3.0]non-5-ene 1,8-Diazabicyclo[5.4.0]undec-7-ene 1,5,7-Triazabicyclo[4.4.0]dec-5-ene Tetra-n-butyl ammonium fluoride Tributylphosphine N,N,N ′ ,N ′ -Tetramethylethylene diamine N,N,N ′ ,N ′ -Tetraethylethylene diamine N,N,N ′ ,N ′ -Tetramethyl-1,3-butane diamine N,N-Dimethyl-N′ ,N ′ -di(2-hydroxypropyl)-1,3-propane diamine N-2-Aminoethyl-3-aminopropyl-tris(2-ethylhexoxy)silane Imidazole derivatives 2-Phenyl-2-imidazoline Organometallic compounds Manganese(III)acetylacetonate

25 26, 27 26, 27 26, 27 28, 29 30 31 32 31 31 31 33 33 34 34

H N

N N

N

N

N N

Figure 13.9: 1,5-Diazabicyclo[4.3.0]non-5-ene, 1,8-Diazabicyclo[5.4.0]undec-7-ene and 1,5,7-Triazabicyclo[4.4.0]dec-5-ene

signed to be applied to the plastic substrate to be bonded prior to application of the adhesive.31 Primers contain mostly aminic structures. Some primers are listed in Table 13.6.

13.2.6 Diazabicyclo and Triazabicyclo Primers 1,5-Diazabicyclo[4.3.0]non-5-ene, 1,8-Diazabicyclo[5.4.0]undec-7-ene, and 1,5,7-Triazabicyclo[4.4.0]dec-5-ene are shown in Figure 13.9. It is well known that solutions of amines and other organic and inorganic bases will accelerate the curing of cyanoacrylate adhesives. Diazabicyclo and triazabicyclo compounds also confer adhesion to nonpolar substrates.26, 27 This primer acts in a two-component adhesive system comprising 2-cyanoacrylate adhesive and the azabicyclo primer. In poly(propylene)

484


NH2 CH2 H

CH2 N CH2 CH2

NH2

+

O C

(CH2)10 CH3

CH2 CH2 NH2

NH C

(CH2)10 CH3

CH2 CH2 N CH2 CH2 NH C

(CH2)10 CH3

CH2 CH2 NH C

(CH2)10 CH3

Figure 13.10: Condensation of Tris(2-aminoethyl)amine and Dodecyl aldehyde35

the application of 1,8-Diazabicyclo[5.4.0]undec-7-ene, the tensile shear bond strength raises to up to 74 kg/cm2 in comparison to 7 kg/cm2 without primer.

13.2.7 Polyamine Dendrimers Compounds with a variety of highly branched architectures are known, including cascade, dendrimer, hyperbranched, and comb-like architectures. The term multi-amine compounds refers to compounds with such branched architectures in which branching occurs via tertiary amine groups. For example, polyamine dendrimers are prepared by the condensation of tris(2-aminoethyl)amine (TAA) and dodecyl aldehyde followed by the reduction with tetra-n-butylammonium cyanoborohydride.35 The reaction is shown in Figure 13.10. The contact between the adhesive and the multi-amine compound may be accomplished by mixing immediately prior to bonding. Ordinarily, however, using the multi-amine compound in

Cyanoacrylates

485

a primer composition will provide the most practical and convenient application to the substrate and will give effective bonding improvement on polyolefin substrates.

13.3 CURING Cyanoacrylates can be polymerized both by radical and by anionic mechanisms. The polymerization of cyanoacrylates has been monitored by Raman spectroscopy.36 Cyanoacrylates polymerize comparatively slowly with free-radical initiators. However, in the presence of catalytic amounts of anionic bases and in the presence of covalent bases such as amines and phosphines, they polymerize extremely rapidly. The exceptionally fast rate of anionic polymerization of cyanoacrylates in the presence of a base, including water, made this class of monomers unique among all acrylic and vinyl monomers. Consequently, the anionic polymerization is initiated by traces of moisture which are to be found on almost all surfaces. Accordingly, cyanoacrylate adhesives set very quickly when introduced between two surfaces stored under ambient conditions. Of the alkyl cyanoacrylate family of monomers, the methyl- and ethyl-esters are used extensively in industrial and consumer-type adhesives. Consequently, most of the published work on the polymerization of cyanoacrylates focuses on anionic polymerization.

13.3.1 Photo Curing Although the predominant mechanism by which cyanoacrylate monomers undergo polymerization is anionic, free-radical polymerization is also known to occur. Radical polymerization of cyanoacrylate can be achieved in the presence of a radical forming component and a photosensitizer. The radical generating component can be dibenzoyl peroxide and the photoinitiator component is 2,4,6-triphenylpyrylium tetrafluoroborate (TPT).37 The chemical structures of these compounds are shown in Figure 13.11. Some metallocene salts are capable of generating both a cationic species and a free radical species upon exposure to radiation. Ferrocene and DAROCUR™ 1173 (2-hydroxy-2-methyl-1-phenyl1-propane) are photo catalysts suitable for cyanoacrylates.38 Radiation times of 5 to 15 seconds are sufficient.

486


C O O C O

O O BF4-

Figure 13.11: Dibenzoyl peroxide and 2,4,6-Triphenylpyrylium tetrafluoroborate

13.4 PROPERTIES The particular advantage of cyanoacrylate adhesives in terms of adhesives technology lies precisely in the high reactivity coupled with the high bond strengths of the final materials, especially to polar substrates. Due to high molar mass, good wetting properties, and polarity, poly(cyanoacrylate)s exhibit excellent adhesive properties. In addition, they have been found useful as polymeric binding agents in controlled drug delivery systems. They are also useful for dry etching processes.

13.5 APPLICATIONS AND USES Of the alkyl cyanoacrylate family of monomers, the methyl- and ethyl-esters are used extensively in industrial and consumer-type adhesives.

13.5.1 Manicure Composition Cyanoacrylate compositions are used as manicure compositions in treating chapped nails. When nails are manicured, it is generally observed that the moisture content in the nails becomes out of balance or lipids are eluted out from the nails. As a result, nail chapping proceeds under the manicure coating. Therefore, the nail chapping can be prevented by adding to manicure compositions a substance capable of keeping nails in good health or improving the nail health. Cyanoacrylates are hardened so quickly that the hardening reaction thereof is associated with heat generation. Therefore, when cyanoacrylates are applied to nails, there arises heat irritation. Avocado oil and jojoba

Cyanoacrylates

487

oil can be added as plasticizer. In addition, these oils can suppress the heat generation upon hardening without deteriorating the quick hardening properties of cyanoacrylates or impairing its storage stability. Furthermore, these natural oils may prevent nails from keratinization.39

13.5.2 Tissue Adhesives The isobutyl, n-butyl, and n-octyl cyanoacrylate esters are used clinically as blocking agents, sealants, and tissue adhesives due to their much lower toxicity as compared with their more reactive methyl and ethyl counterparts. Cyanoacrylate ester compositions can be sterilized using visible light irradiation at room temperature conditions.9 There has been a great deal of interest in using tissue adhesives in many surgical procedures in place of sutures and staples for a variety of reasons, including40 1. Ease of application and reduced clinician time, 2. Location of repairable site as in contoured locations, 3. Biomechanical properties as in weak organs, such as liver and pancreas, and 4. Minimized hypertrophy and scar formation as in plastic surgery. However, there have been a number of concerns associated with the alkyl cyanoacrylates. These include 1. Their low viscosity and associated difficulties in precise delivery at the application site in non-medical and medical applications, 2. Poor shear strength of the adhesive joint, particularly in aqueous environments in both medical and non-medical applications, 3. High modulus or stiffness of cured polymers at soft tissue application sites and associated mechanical incompatibility, which can lead to adhesive joint failure and irritation of the surrounding tissue, 4. Excessive heat generation upon application of monomers to living tissue due to exceptionally fast rate of curing resulting in necrosis, and 5. Site infection, among other pathological complications, associated with prolonged residence of the non-absorbable tissue adhesives.

488


Problems with sterilization may arise. For example, poly(2-octyl cyanoacrylate), degrades when exposed to a 160°C dry heat sterilization cycle or 20 to 30 kGy (2 to 3 MRad) of electron beam radiation.10 13.5.2.1

Bioabsorbable Polymers

Bioabsorbable polymers have been classified into three groups:40, 41 • Soluble, • Solubilizable, and • Depolymerizable. The most common materials used in bioabsorbable implants in orthopaedic surgery42 are polyglycolic acid (PGA), polylactic acid (PLA), and polydioxanone (PDS). Soluble polymers are water-soluble and have hydrogen-bonding polar groups, the solubility being determined by the type and frequency of the polar groups. Solubilizable polymers are usually insoluble salts, such as calcium or magnesium salts of carboxylic or sulfonic acid-functional materials which can dissolve by cation exchange with monovalent metal salts. Depolymerizable systems have chains that dissociate to simple organic compounds in vivo under the influence of enzymes or chemical catalysis. Ester of Triethylene glycol. Bioabsorbable tissue adhesives41, 43 are based on a methoxypropyl cyanoacrylate as the precursor of an absorbable tissue adhesive polymer and a polymeric, highly absorbable, liquid comprising an oxalate ester of triethylene glycol as a modifier to modulate the viscosity of the overall composition, lower the heat of polymerization, and increase the compliance and absorption rate of the cured adhesive joint. Copolymers of caprolactone, D,L-lactide, and glycolide also are considered as bioabsorbable.40, 44 Cyanoacrylate-capped Heterochain Polymers. Although the admixture of a polymeric modifier has been shown to be effective in addressing most of the medical and non-medical drawbacks of cyanoacrylate-based adhesives represented by methoxypropyl cyanoacrylate, there remain technical drawbacks in these systems, such as mutual immiscibility of two or more polymers.

Cyanoacrylates

489

Cyanoacrylate-capped heterochain polymers having two or more cyanoacrylate ester groups per chain have certain advantages. The heterochain polymer used for capping can be one or more absorbable polymers of the following types: polyester, polyester-carbonate, polyether-carbonate, and polyether-ester. The capped polymer can also be derived from a polyalkylene glycol such as poly(ethylene glycol), or a block copolymer of poly(ethylene glycol) and poly(propylene) glycol. The capping of the heterochain polymer can be achieved using an alkyl cyanoacrylate, or an alkoxyalkyl cyanoacrylate such as ethyl cyanoacrylate or methoxypropyl cyanoacrylate, respectively, in the presence of phosphorus-based acids or precursors. In fact the capping takes place as transesterification reaction. In the simplest case, a predried poly(ethylene glycol) is mixed with ethyl cyanoacrylate in the presence of pyrophosphoric acid under a dry nitrogen atmosphere. The reaction is allowed to proceed by heating for 5 hours at 85°C.40

REFERENCES 1. H. V. Coover, D. W. Dreifus, and J. T. O. Conner. Cyanoacrylate adhesives. In I. Skeist, editor, Handbook of Adhesives, chapter 27, pages 463–477. Van Nostrand Reinhold, New York, 3rd edition, 1990. 2. K. F. von Auwers and J. F. Thorpe. Liebigs Ann. Chem., 285:322, 1895. 3. A. E. Ardis. US Patent 2 467 927, assigned to B. F. Goodrich, New York (NY), April 19 1949. 4. F. B. Joyner and G. F. Hawkins. Method of making α-cyano-acrylates. US Patent 2 721 858, assigned to Eastman Kodak, Rochester, New York, October 25 1955. 5. A. E. Ardis. Preparation of monomeric alkyl-α-cyano-acrylates. US Patent 2 467 926, assigned to B. F. Goodrich, New York (NY), April 19 1949. 6. A. Vojtkov, K. A. Mager, Y. V. Kokhanov, A. M. Polyakova, and Y. B. Vojtekunas. Method of preparing cyanacrylic acid esters. SU Patent 726 086, assigned to Inst. Elementoorganicheskikh So. (SU), April 5 1980. 7. C. J. Buck. Modified cyanoacrylate monomers and methods for preparation. US Patent 4 012 402, assigned to Johnson and Johnson, (New Brunswick, NJ), March 15 1977. 8. S. Harris. The preparation of thiocyanoacrylates. J. Polym. Sci., Part. A: Polym. Chem., 19:2655–2656, 1981. 9. I. N. Askill, S. C. Karnik, and R. L. Norton. Methods for sterilizing cyanoacrylate compositions. US Patent 6 579 916, assigned to MedLogic Global Corporation (Devon, GB), June 17 2003.

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10. T. Hickey, U. A. Stewart, J. Jonn, and J. S. Bobo. Sterilized cyanoacrylate solutions containing thickeners. US Patent 6 743 858, assigned to Closure Medical Corporation (Raleigh, NC), June 1 2004. 11. J. C. Leung and J. G. Clark. Biocompatible monomer and polymer compositions. US Patent 5 328 687, assigned to Tri-Point Medical L.P. (Raleigh, NC), July 12 1994. 12. P. F. McDonnell, R. J. Lambert, E. P. Scott, G. M. Wren, and M. McGuinness. Cyanoacrylate adhesive compositions for bonding glass. US Patent 6 607 632, assigned to Loctite (R&D) Limited (Dublin, IE), August 19 2003. 13. L. Corp. Debondable cyanoacrylate adhesive composition. GB Patent 1 529 105, assigned to Loctite Corp, October 18 1978. 14. H. Misiak and I. Scheffler. Activator for cyanoacrylate adhesives. US Patent 6 547 917, assigned to Henkel Kommanditgesellschaft auf Aktien (Duesseldorf, DE), April 15 2003. 15. B. C. Arkles. Silacrown ethers, method of making same, and use as phasetransfer catalysts. US Patent 4 362 884, assigned to Petrarch Systems, Inc. (Levittown, PA), December 7 1982. 16. J.-C. Liu. Instant adhesive composition and bonding method employing same. US Patent 4 906 317, assigned to Loctite Corporation (Newington, CT), March 6 1990. 17. I. Haiduc. Silicone grease: A serendipitous reagent for the synthesis of exotic molecular and supramolecular compounds. Organometallics, 23(1):3–8, 2004. 18. G. Oddon and M. W. Hosseini. Silacrown ethers: Synthesis of macrocyclic diphenylpolyethyleneglycol mono- and disilanes. Tetrahedron Lett., 34(46): 7413–7416, November 1993. 19. J. M. Rooney, D. P. Melody, J. Woods, S. J. Harris, and M. A. McKervey. Instant adhesive composition utilizing calixarene accelerators. EP Patent 0 151 527, assigned to Loctite Ireland Ltd, August 14 1985. 20. S. J. Harris. Calixarene derivatives and use as accelerators in adhesive compositions. US Patent 4 866 198, assigned to Loctite Corporation (Newington, CT), September 12 1989. 21. S. J. Harris, M. A. McKervey, D. P. Melody, J. Woods, and J. M. Rooney. Instant adhesive composition utilizing calixarene accelerators. US Patent 4 636 539, assigned to Loctite (Ireland) Limited (Dublin, IE), January 13 1987. 22. S. Attarwala and P. T. Klemarczyk. Cyanoacrylate adhesives with improved cured thermal properties. US Patent 5 328 944, assigned to Loctite Corporation (Hartford, CT), July 12 1994. 23. R. Lier, R. Vogel, and H.-J. Heine. Stabilising cyanoacrylate ester(s) - by adding acid, e.g. p-toluenesulphonic or citric acid etc., to moulded plastics used in prodn., storage or use of ester(s). DE Patent 4 109 105, assigned to Henkel KGAA, September 24 1992.

Cyanoacrylates

491

24. H.-R. Misiak and D. Behn. Stabilized cyanoacrylate adhesives. US Patent 6 642 337, assigned to Henkel Kommanditgesellschaft auf Aktien (Duesseldorf, DE), November 4 2003. 25. P. F. McDonnell. Primer composition and use thereof in bonding non-polar substrates. EP Patent 0 295 013, assigned to Loctite Ireland Ltd, December 14 1988. 26. P. F. McDonnell and B. J. Kneafsey. Diazabicyclo and triazabicyclo primer compositions and use thereof in bonding non-polar substrates. EP Patent 0 295 930, assigned to Loctite Ireland Ltd, December 21 1988. 27. P. F. McDonnell and B. J. Kneafsey. Diazabicyclo and triazabicyclo primer compositions and use thereof in bonding non-polar substrates. US Patent 4 869 772, assigned to Loctite (Ireland) Ltd. (Tallaght, IE), September 26 1989. 28. J. C. Liu. Primer for bonding low surface energy plastics with cyanoacrylate adhesives. EP Patent 0 333 448, assigned to Loctite Corp, September 20 1989. 29. J. C. Liu. Primer for bonding low surface energy plastics with cyanoacrylate adhesives and bonding method employing same. US Patent 5 079 098, assigned to Loctite Corporation (Hartford, CT), January 7 1992. 30. S. Fukushige et al. Primer for cyanoacrylate adhesive. JP Patent 2 120 378, assigned to Koatsu Gas Kogyo Co Ltd, May 8 1990. 31. R. Grieves and K. G. M. Pratley. Adhesive primer. US Patent 5 837 092, assigned to Pratley Investments (Proprietary) Limited (ZA), November 17 1998. 32. P. F. McDonnell, G. M. Wren, and E. K. Welch, II. Consumer polyolefin primer. US Patent 5 314 562, assigned to Loctite Corporation (Hartford, CT), March 24 1994. 33. H. C. Nicolaisen and A. Rehling. Primer for cyanoacrylate adhesives and use thereof in a bonding method. US Patent 5 133 823, assigned to Henkel Kommanditgesellschaft auf Aktien (Duesseldorf, DE), July 28 1992. 34. A. Hiraiwa, K. Ito, and K. Kimura. Primer composition. US Patent 5 292 364, assigned to Toagosei Chemica Industry Co., Ltd. (Tokyo, JP), March 8 1994. 35. J. G. Woods and J. M. J. Frechet. Multi-amine compound primers for bonding of polyolefins with cyanoacrylate adhesives. US Patent 6 673 192, assigned to Loctite Corporation (Hartford, CT), January 6 2004. 36. E. Urlaub, J. Popp, V. E. Roman, W. Kiefer, M. Lankers, and G. Rossling. Raman spectroscopic monitoring of the polymerization of cyanacrylate. Chem. Phys. Lett., 298(1-3):177–182, December 1998. 37. H. R. Misiak. Radiation-curable, cyanoacrylate-containing compositions. US Patent 6 734 221, assigned to Loctite (R&D) Limited (Dublin, IE), May 11 2004. 38. S. Wojciak and S. Attarwala. Radiation-curable, cyanoacrylate-containing compositions. US Patent 6 726 795, April 27 2004.

492


39. K. Kishita and N. Ohsawa. Manicure composition for nail. US Patent 6 703 003, assigned to Three Bond Co., Ltd. (Tokyo, JP); Three Bond International Inc. (West Chester, OH), March 9 2004. 40. S. W. Shalaby. Cyanoacrylate-capped heterochain polymers and tissue adhesives and sealants therefrom. US Patent 6 699 940, assigned to Poly Med, Inc. (Anderson, SC), March 2 2004. 41. S. W. Shalaby. Biabsorbable polymers. In J. Swarbrick and J. C. Boylan, editors, Absorption of Drugs to Bioavailability of Drugs and Bioequivalence, volume 1 of Encyclopedia of Pharmaceutical Technology, pages 465–476. Marcel Dekker, Inc., New York and Basel, 1988. 42. P. B. Maurus and C. C. Kaeding. Bioabsorbable implant material review. Operative Techniques in Sports Medicine, 12(3):158–160, July 2004. 43. C. L. Linden and S. W. Shalaby. Absorbable tissue adhesives. US Patent 5 350 798, assigned to The United States of America as represented by the Secretary of the Army (Washington, DC), September 27 1994. 44. S. W. Shalaby. Polyester/cyanoacrylate tissue adhesive formulations. US Patent 6 299 631, assigned to Poly-Med, Inc. (Pendleton, SC), October 9 2001.

14 Benzocyclobutene Resins Benzocyclobutene (BCB) or bicyclo[4.2.0]octa-1,3,5-triene is also called cardene, cyclobutabenzene and cyclobutarene. Benzocyclobutene was first synthesized by Finkelstein in 1909 by the 1,4-elimination of bromine from α, α, α′ , α′ -tetrabromo-o-xylene,1 as shown in Figure 14.1. Finkelstein’s thesis was rejected for publication and was accidentally discovered more than 40 years later. 1,5-Hexadiyne trimerizes to give 1,2-bis(benzocyclobutenyl)ethane (BCBE). Various other methods of synthesis of benzocyclobutene derivatives have been reported.2 Suitable monomers are summarized in Table 14.1. The four-membered ring in benzocyclobutene (BCB) imparts a ring strain. Therefore, this class of molecules is especially reactive. BenzocyTable 14.1: Benzocyclobutene Derivatives Compound

Reference

Benzocyclobutene (BCB) Benzocyclobutene-maleimide 1,2-Dihydrocyclobutabenzene-3,6-dicarboxylic acid 2,6-Bis-4-benzocyclobutene benzo[1,2-d:5,4-d′ ]bisoxazole Isophthaloyl bis-4-benzocyclobutene 1-Methoxypoly(oxyethylene)benzocyclobutene 1-Benzocyclobutenyl vinyl ether 2,6-Bis(4-benzocyclobutenyloxy)benzonitrile 4-Trimethylsiloxybenzocyclobutene

493

3 4 5 5

494


Br

CHBr2

NaJ

(a) CHBr2

NaJ, H2

Br

(b)

O +

(c)

O O

(d)

+

O O O

+

Figure 14.1: a) Synthesis of Benzocyclobutene, (b) Isomerization, (c) DielsAlder Reaction with Maleic anhydride, (d) Cyclotrimerization of 1,5-Hexadiyne

Benzocyclobutene Resins

495

∆

Figure 14.2: Thermal Polymerization of o-Xylylene

clobutene derivatives serve as important building blocks for natural product syntheses and for polymers and advanced materials. In the presence of dienophiles the o-xylylene unit undergoes a Diels-Alder reaction. However, in the absence of a dienophile the o-xylylene unit polymerizes as shown in Figure 14.2. The BCB four-membered ring opens thermally around 200°C to produce o-quinodimethane (QDM), also known as o-xylylene.2, 6 o-Xylylene readily undergoes Diels-Alder reactions with available dienophiles or, in the absence of a dienophile, it reacts to give a dimer, 1,2,5,6-dibenzocyclooctadiene. The dimerization reaction is thermodynamically preferred over a Diels-Alder reaction. However, the Diels-Alder reaction is kinetically favored. Benzocyclobutene-maleimide monomers (c.f. Figure 14.3) polymerize to yield exceptionally tough resins with high glass transition temperatures. Upon heating at above 200°C, the benzocyclobutene ring opens to form o-xylylene, which then undergoes a cycloaddition or dimerization reaction. The cyclobutene structure in 1,2-Dihydrocyclobutabenzene3,6-dicarboxylic acid can act as a crosslinking site when incorporated in a polymer. Poly(benzo[1,2-d4,5-d′ ]bisthiazole-2,6-diyl)-1,4-phenylene (PBT) is a rod-like monomer. A thermal crosslinking occurs, when benzocyclobutene substructures are imbedded in PBT.7 The mechanism is shown in Figure 14.4. 1-Methoxypoly(oxyethylene)benzocyclobutene has been prepared by reacting 1-benzocyclobutenyl-1-hydroxyethyl ether with the mesylate of methoxypoly(oxyethylene). The Diels-Alder reactions of 1-methoxypoly(oxyethylene)benzocyclobutene with maleic anhydride and N-phenylmaleimide runs to 100% conversion.3 1-Benzocyclobutenyl vinyl ether has been prepared by the elimination of hydrogen bromide from 1-benzocyclobutenyl-1-bromoethyl ether. This compound was obtained from 1-bromobenzocyclobutene and ethylene glycol. 1-Benzocyclobutenyl vinyl ether can be polymerized by a cationic mechanism.4

496


O

COOH

C O N COOH

O (1)

(2)

O

O

N

N (3)

O

O

C

C

(4)

Figure 14.3: Benzocyclobutene-maleimide monomer (1), 1,2-Dihydrocyclobutabenzene-3,6-dicarboxylic acid (2) 2,6-Bis-4-benzocyclobutene benzo[1,2-d:5,4-d′ ]bisoxazole (3), Isophthaloyl bis-4-benzocyclobutene (4)


ClOC

497

COCl H 2N

SH

HS

NH2

+

ClOC

COCl

Polyphosphoric acid N

S

S

N

∆ N

S

S

N

Crosslinking

Figure 14.4: Crosslinking of Poly(benzo[1,2-d4,5-d′ ]bisthiazole-2,6-diyl)1,4-phenylene Modified with Benzocyclobutene Structures

498


The microwave curing of benzocyclobutene has been described.8 Microwave curing may speed up the manufacture of parts used in microelectronics.

14.1 MODIFIED POLYMERS 14.1.1 Thermotropic Copolymers Thermotropic polymers are polymers that are forming liquid crystalline phases in the melt. Thermotropic copolymers composed of hydroxybenzoic acid (HBA), hydroxynaphthoic acid (HNA), and systematically varying amounts of hydroquinone (HQ) and crosslinkable terephthalic acid have been described.9 Also, the chain extension is possible if a BCB functionality is on one or both ends of a polymer chain.2

14.1.2 BCB-modified Aromatic Polyamides The compressive strength of high modulus fibers such as Kevlar™ can be improved by the use of a latently crosslinkable monomer, such as 1,2-dihydrocyclobutabenzene-3,6-dicarboxylic acid. BCB modified aromatic polyamides as shown in Figure 14.5 can be synthesized by the condensation of terephthaloyl dichloride and 3,5-diaminophenyl-4-benzocyclobutenylketone.10 The polymers can be crosslinked by the application of heat. These polymers exhibited broad cure exotherms with onset temperatures in the range of 238°C, reaching maximum at 275°C. The polymers are useful as crosslinkable, thermoplastic matrix materials for rigid rod-like composites. Such composites include composites with poly(p-phenylene benzobisthiazole) (PBZT), as well as with other benzobisazole polymers.10

14.1.3 BCB End-capped Polyimides Benzocyclobutene-terminated polyimide oligomers are useful for high performance adhesive applications. They are more processable than conventional polyimide systems, yet form polymers that are thermally stable at temperatures above 200°C. For example, imide oligomers can be prepared from 4,4′ -[1,3-phenylene(1-methyl ethylidene)]bisaniline (Bis-M), and 4-amino-benzocyclobutene as chain stopper and 4,4′ -oxydiphthalic anhydride (ODPA) to arrive at a structure as shown in Figure 14.6. Benzo-


H 2N

NH2

+

O C

O

Cl

O

O

C

C Cl

O

C HN

NH C

O C

Figure 14.5: BCB-modified Aromatic Polyamides

499

500


H 2N

CH3

CH3

C

C

CH3

CH3 Bis-M

O

NH2

+

O O

O

O

+

H 2N O

O ODPA

4-amino-BCB

O

O O Ar

N

N O

O

Figure 14.6: BCB end-capped Polyimides Prepared from 4,4′ -[1,3-Phenylene(1-methyl ethylidene)]bisaniline (Bis-M), 4-Amino-benzocyclobutene, and 4,4′ -Oxydiphthalic anhydride (ODPA)11


501

cyclobutene-terminated polyimides can be cured to form polymers that exhibit high adhesive strength.11 The adhesives have been shown to withstand exposure to hot wet environment. For example, lap shear samples immersed in boiling water for three days and tested at room temperature are found to retain over 80% of their strength.

14.1.4 Flame Resistant Formulations Benzocyclobutene polymers are known as thermosetting polymers having high thermal stability, but they are flammable. Both the flammability and brittleness of BCB resins can be reduced by adding a brominated acrylate, such as pentabromobenzyl acrylate (PBA) monomer to the BCB resins and causing them to react to form a resulting flame retardant thermoset material. The PBA reduces the brittleness of the cured material by reducing the crosslinking density. PBA is advantageous because it reacts with the BCB to create a homogeneous system.12

14.2 CROSSLINKERS 14.2.1 Modified Poly(ethylene terephthalate) Thermally crosslinkable polyester copolymers can be synthesized by the incorporation of a benzocyclobutene-containing terephthalic acid derivative into poly(ethylene terephthalate) (PET). The cyclobutene moiety on the chain allows the reactive crosslinking at temperatures at ca. 350°C. No catalyst is needed and no volatile products are formed. Crosslinking occurs above the melting temperature of 250°C but below the degradation temperature of 400°C. Therefore, the material can be melt processed. The degradation temperature and the melting temperature decrease slightly with increased cyclobutene content. The recrystallization and glass transition temperature are insensitive to the cyclobutene content. The limiting oxygen index (LOI) increases with cyclobutene content.13

14.3 APPLICATIONS AND USES Polymer films from BCB formulations exhibit many desirable properties for microelectronic applications.14 In particular, they have a low dielectric constant and dissipation factor, low moisture absorption, rapid curing and

502


E-BCB

DVB-BCB CH3 Si CH3

CH3 O Si CH3

DVS-BCB

Figure 14.7: 1,2-Bis(4-benzocyclobutenyl)ethylene (E-BCB), Bis(benzocyclobutenyl)-m-divinylbenzene (DVB-BCB), and Bis(benzocyclobutenyl)divinyltetramethylsiloxane (DVS-BCB)

low temperature cure without generating by-products, minimum shrinkage in curing process, and no Cu migration issues.15 Due to these properties, applications have been found in bumping/ wafer level packaging, optical wave guides,16, 17 and flat panel display.

14.3.1 Applications in Microelectronics Several BCB-containing polymers have been investigated for their use in coating applications. Suitable monomers are analogues to trans-stilbene, e.g., 1,2-bis(4-benzocyclobutenyl)ethylene (E-BCB), bis(benzocyclobutenyl)-m-divinylbenzene (DVB-BCB), and bis(benzocyclobutenyl)divinyltetramethylsiloxane (DVS-BCB). The structures of the monomers are shown in Figure 14.7.


503

+

CH3 Si CH3 H3C Si CH3 O H3C Si CH3

H 3C

Si

CH3

Figure 14.8: Basic Curing Reaction and the Structure of the Polymer of DVSBCB2

14.3.1.1

Siloxane-modified Benzocyclobutene

For microelectronics applications, the polymer from DVS-BCB is commonly used, because it results in a polymer with a high glass transition temperature of greater than 350°C, a low dielectric constant, a low dissipation factor, low water absorption, and good adhesive properties.2 The basic curing reaction and the structure of the polymer are shown in Figure 14.8. With a siloxane bisbenzocyclobutene, high quality spin-on gate dielectric layers as thin as 50 nm have been fabricated over the semiconductor layer for polymer field-effect transistors by a solution process.18 It is desirable to get materials with low refractive index, and thus low dielectric constant. This can be achieved when the curing reaction is stopped before vitrification is reached.19 The treatment with ultraviolet light in the presence of ozone modifies the chemical properties of BCB, as the polymeric structure of BCB is degraded and becomes soluble in acetone. This behavior may be useful for BCB reworking after polymerization.20

504


O

+

Br

O

OH OH

Figure 14.9: Synthesis of BCB-acrylic acid21

14.3.1.2

Acid Functional Benzocyclobutenes

Acid functional polymers based on benzocyclobutene display excellent qualities of toughness, adhesion, dielectric constant, and low stress.21 The synthesis of BCB-acrylic acid is shown in Figure 14.9.

14.3.2 Optical Applications Diffractive gratings made from benzocyclobutene can withstand temperatures up to 300°C with only small optical and topographical changes after 45 min., whereas conventional photoresist gratings change drastically within a few minutes under these conditions.22 The fabrication of Bragg reflector mirrors for GaInAsP/InP lasers has been described.23 The process involves multiple sequential steps of CH4 /H2 reactive ion etching (RIE) and O2 plasma etching.

REFERENCES 1. G. Mehta and S. Kotha. Recent chemistry of benzocyclobutenes. Tetrahedron, 57(4):625–659, January 2001. 2. M. F. Farona. Benzocyclobutenes in polymer chemistry. Prog. Polym. Sci., 21(3):505–555, 1996. 3. R. S. Herati. Synthesis of 1-methoxypoly(oxyethylene)benzocyclobutene and its Diels-Alder reactions. J. Polym. Sci. Pol. Chem., 42(8):1934–1938, April 2004. 4. K. Chino, T. Takata, and T. Endo. Synthesis of a poly(vinyl ether) containing a benzocyclobutene moiety and its reaction with dienophiles. J. Polym. Sci. Pol. Chem., 37(1):59–67, January 1999. 5. L. S. Tan, N. Venkatasubramanian, P. T. Mather, M. D. Houtz, and C. L. Benner. Synthesis and thermal properties of thermosetting bis-benzocyclobutene-terminated arylene ether monomers. J. Polym. Sci. Pol. Chem., 36(14): 2637–2651, October 1998.


505

6. R. A. Kirchhoff and K. J. Bruza. Benzocyclobutenes in polymer synthesis. Prog. Polym. Sci., 18(1):85–185, 1993. 7. Y.-H. So. Rigid-rod polymers with enhanced lateral interactions. Prog. Polym. Sci., 25(1):137–157, February 2000. 8. R. V. Tanikella, S. A. B. Allen, and P. A. Kohl. Variable-frequency microwave curing of benzocyclobutene. J. Appl. Polym. Sci., 83(14):3055–3067, April 2002. 9. P. T. Mather, K. P. Chaffee, A. Romo-Uribe, G. E. Spilman, T. Jiang, and D. C. Martin. Thermally crosslinkable thermotropic copolyesters: synthesis, characterization, and processing. Polymer, 38(24):6009–6022, November 1997. 10. L.-S. Tan and N. Venkatasubramaian. Aromatic polyamides containing keto-benzocyclobutene pendants. US Patent 5 514 769, assigned to The United States of America as represented by the Secretary of the Air (Washington, DC), May 7 1996. 11. E. S. Moyer and D. J. D. Moyer. Benzocyclobutene-terminated polymides. US Patent 5 464 925, assigned to The Dow Chemical Company (Midland, MI), November 7 1995. 12. M. W. Wagaman and T. F. McCarthy. Flame retardant benzocyclobutene resin with reduced brittleness. US Patent 6 342 572, assigned to Honeywell International Inc. (Morris Township, NJ), January 29 2002. 13. E. Pingel, L. J. Markoski, G. E. Spilman, B. J. Foran, T. Jiang, and D. C. Martin. Thermally crosslinkable thermoplastic PET-co-XTA copolyesters. Polymer, 40(1):53–64, January 1999. 14. Y. H. So, P. Garrou, J. H. Im, and D. M. Scheck. Benzocyclobutene-based polymers for microelectronics. Chem. Innov., 31(12):40–47, December 2001. 15. K. Ohba. Overview of photo-definable benzocyclobutene polymer. J. Photopolym. Sci. Technol., 15(2):177–182, 2002. 16. C. W. Hsu, H. L. Chen, W. C. Chao, and W. S. Wang. Characterization of benzocyclobutene optical waveguides fabricated by electron-beam direct writing. Microw. Opt. Technol. Lett., 42(3):208–210, August 2004. 17. W. S. Sul, S. D. Kim, S. D. Lee, T. S. Kang, D. An, Y. H. Chun, I. S. Hwang, J. K. Rhee, and K. H. Ryu. Low-characteristic-impedance transmission line of a benzocyclobutene-based 3-dimensional structure at millimeter-wave frequencies. J. Korean Phys. Soc., 43(6):1076–1080, December 2003. 18. L. L. Chua, P. K. H. Ho, H. Sirringhaus, and R. H. Friend. High-stability ultrathin spin-on benzocyclobutene gate dielectric for polymer field-effect transistors. Appl. Phys. Lett., 84(17):3400–3402, April 2004. 19. K. C. Chan, M. Teo, and Z. W. Zhong. Characterization of low-k benzocyclobutene dielectric thin film. Microelectron. Int., 20(3):11–22, 2003. 20. B. Viallet, E. Daran, and L. Malaquin. Effects of ultraviolet/ozone treatment on benzocyclobutene films. J. Vac. Sci. Technol., A, 21(3):766–771, May–June 2003.

506


21. Y. H. So, R. A. DeVries, M. G. Dibbs, R. L. McGee, E. O. Shaffer, II, M. J. Radler, and R. DeCaire. Acid functional polymers based on benzocyclobutene. US Patent 6 361 926, assigned to The Dow Chemical Company (Midland, MI), March 26 2002. 22. A. Straat and F. Nikolajeff. Study of benzocyclobutene as an optical material at elevated temperatures. Appl. Optics, 40(29):5147–5152, October 2001. 23. M. M. Raj, J. Wiedmann, S. Toyoshima, Y. Saka, K. Ebihara, and S. Arai. High-reflectivity semiconductor/benzocyclobutene Bragg reflector mirrors for GaInAsP/InP lasers. Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap., 40(4A):2269–2277, April 2001.

15 Reactive Extrusion Reactive extrusion is an attractive route for polymer processing in order to carry out various reactions including polymerization, grafting, branching, and functionalization. There are monographs on reactive extrusion.1, 2 In this chapter, we deal mainly with the formation of polymers by reactive extrusion, i.e., reactive extrusion polymerization. Aspects of reactive extrusion are covered in other chapters: this includes grafting, compatibilization, and controlled rheology. Reactive extrusion polymerization involves polymerizing a liquid or solid monomer or a prepolymer during the residence time in the extruder to form a high molecular weight melt. Low-cost production and processing methods for biodegradable plastics are of great importance, since they enhance the commercial viability and cost-competitiveness of these materials. Reactive extrusion is an attractive route for the polymerization of cyclic ester monomers, without solvents, to produce high molecular weight biodegradable plastics. Extruders can be used for bulk polymerization of monomers, like methyl methacrylate, styrene, lactam, and lactide. From a mechanistic peerspective, nearly all kinds of polymerization have been performed in an extruder. These include radical polymerization, ionic polymerization, metathesis polymerization,3 and ring opening polymerization. The techniques of characterization and experimental setup for reactive extrusion can be found in the literature.4, 5 The technique is also attractive for melt spinning.6, 7 The economics of using an extruder as a bulk polymerization reactor are favorable when high throughputs and control of molecular weight 507

508


H ∆T

V R

∆T H

Figure 15.1: Balance of an Extruder

are realized. The limitation arises due to the residence time required to complete the polymerization, which ideally should be less than 5 minutes. There are significant kinetic, heat transfer, and diffusion-related issues in a bulk polymerization process that make it difficult to develop and design processing methods that result in high molecular weight polymer at high throughputs with a high conversion of the monomer. However, extruders are ideal process vehicles for this purpose as they can be tailored to give various flow patterns, residence time distributions and shear effects, each of which affect the polymerization and polymer quality.

15.1 EXTRUDER In this section the reactive extruder depicted in Figure 15.1 is modelled mathematically. There is an input of monomer on the left side with a volume rate of V˙ . The average residence time tr is then tr =

V , V˙

(15.1)

˙ when the volume of the extruder is V . The reaction rate in the extruder is R. Let us assume for simplicity sake that the rate of reaction is not dependent on the conversion. The conversion C, as a fraction is then ˙ C = tr R.

(15.2)

˙ To obain full conversion, i.e., C = 1, the residence time should be tr > 1/R. The rate of reaction heat generation is calculated by means of Eq. 15.3. ˙ 0V H˙ = RH

(15.3)

Reactive Extrusion H˙ H0 R˙ V

509

Rate of heat released in the whole extruder Heat released for full conversion in the unit volume Rate of reaction Volume of extruder

The heat released in the extruder must be conducted through the walls. Here we neglect that some of the heat is transported away with the melt. We also neglect that additional heat is generated by friction forces through kneading. The heat that can be transported though the walls of the extruder is given by the heat flow equation Eq. 15.4. H˙ = kA∆T.

(15.4)

Here k is the overall heat transfer coefficient ([J s−1 K−1 m−2 ] (different from the conductivity coefficient). The area A relates to the volume of the extruder with a geometry factor g. V = gA

(15.5)

In the case of a cylinder, V = r2 πh = g2rπh. Let us assume that the heat is transferred through the envelope of the cylinder. Combining Eq. 15.3 and Eq. 15.4, yields Eq. 15.6 V˙ H0 g . (15.6) ∆T = V k We have previously implied the condition of full conversion. Eq. 15.6 states that a temperature gradient will be created by the reaction in the extruder. We are restricted by the temperature difference by the cooling facilities. For example, the outer temperature is usually not set below the room temperature for economic reasons. On the other hand, the temperature inside the extruder cannot get too high. Otherwise the material will pyrolyze. The temperature gradient is limited. Now the temperature difference will be affected by the throughput. The throughput will be pushed to a maximum for economic reasons. The heat of reaction for a given process cannot be changed. However, if there are alternative processes found that achieve a material with identical properties, the process with a low heat of conversion should be selected. The geometry factor can be influenced by the design of the extruder. Clearly, a smaller diameter is advantageous. This will lead to a design of a longer machine, if a large volume is desired. The length of a machine is restricted by the mechanical properties of a screw. The situation is simpler in a chemical plant. There are bent loop reactors in which the material can freely flow.

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The model described is a very simple, because the temperature gradient in the melt, the residence time distribution, and many other parameters have not been taken into account. It does provide a basic insight into important parameters of the device. More sophisticated models are available in the literature. The reactive extrusion process in a single-screw extruder has been assessed by power law fluids undergoing isothermal homogeneous and heterogeneous reactions. The reaction was reported to be first-order. The equation of conservation of component species was transformed into an eigenvalue problem. Analytical solutions were developed for the concentration distribution in the extruder. Expressions for the conversion of the reactant and Sherwood number were given.8 Sometimes severe fluctuations in product quality have been observed. These fluctuations can be caused by thermal, hydrodynamic, or chemical instabilities.9 Some of these instabilities are dependent on the scale of the equipment. The experimental design is thus important when a reactive extrusion process is developed in the laboratory for scale up to larger machines.

15.1.1 Heat of Polymerization The performance of a reactive extrusion polymerization depends on the heat of polymerization itself. Table 15.1 summarizes heat and entropy of polymerization for selected compounds. A review of the data in Table 15.1 reveals that vinyl polymers, such as propene, styrene, and acrylics have a high enthalpy of polymerization. Polymers that are formed by ring opening polymerization have a relatively lower enthalpy of polymerization. From the point of view of heat transfer it is desirable to use monomers that have a lower polymerization heat, because the heat must be removed through the walls of the reactor which has a limited surface area. Only a low polymerization heat guarantees a high throughput.

15.1.2 Ceiling Temperature On the other hand, low polymerization heat implies low thermal stability from the view point of thermodynamics. The free enthalpy of polymerization is given by Eq. 15.7.

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Table 15.1: Heats and Entropies of Polymerization10 Compound

Statea

−∆H [kJ/mol]

−∆S [J/mol/K]

Temperature [°C]

84 67 77 56 70 59 24 19 -5 17 27

116

25 75 75 130 25 75 100 25 25 25 127

Propene lc Acrylic acid lc Acrylonitrile lc Methyl methacrylate lc Styrene lc Maleic anhydride ls 1,3-Dioxolan lc Tetrahydrofuran lc γ-Butyrolacton lc Caprolacton lc D,L-Lactide lc a lc: from liquid to crystalline ls: from liquid to solid

∆G = ∆H − T ∆S. ∆G ∆H ∆S

109 117 149 — 76 16 30 4 13

(15.7)

Free enthalpy of polymerization Enthalpy of polymerization Entropy of polymerization

If ∆G turns negative, then the polymer is no longer stable with respect to the monomer. Assuming an equilibrium is established, then the ceiling temperature Tc can be calculated by equating Eq. 15.7 to zero. Tc =

∆H T ∆S

(15.8)

The ceiling temperature yields reasonable results for vinyl monomers, but in the case of polymers formed by ring opening polymerization, unreasonable values are obtained.

15.1.3 Strategy of Reactive Extrusion As pointed out above, it is desirable to use materials with a low polymerization heat in reactive extrusion. Only then can a high throughput be obtained. On the other hand, it is possible to use a mixture of a polymer and monomer. The latter is then polymerized in the extruder. This concept can

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Reactive Polymers Fundamentals and Applications Table 15.2: Polymers Obtained by Reactive Extrusion Polymer

Reference

Radical Polymerization Poly(styrene) Poly(butyl methacrylate)

11 12

Ring opening Polymerization Poly(lactide)

13

Anionic Polymerization Poly(styrene) Styrene-butadiene copolymer Polyamide 12

14 15

Metathesis Polymerization Poly(octenylene)

3

reduce the amount of heat to be transferred. In compatibilization, a modified polymer is used, with only one chemically reactive group in the chain. In this case the heat of polymerization with respect to volume is reduced drastically. On the other hand, in injection molding of small articles with a high surface-to-volume ratio, the viscous melt often must be driven though small channels, before the melt is placed in the form. In the case of small articles only small amounts of material are needed. Therefore, the cost of the material used is less influential in the choice of the process. The cycle time can be reduced, if the form filling can be reduced. This can be done by selecting a material that is less viscous. In the case of small articles, the heat of polymerization is reduced. Therefore, reactive extrusion is possibly attractive, in the manufacture of small articles.

15.2 COMPOSITIONS OF INDUSTRIAL POLYMERS Before going into detail, we summarize the polymers that have been obtained by reactive extrusion according to the mechanism of reaction in Table 15.2.

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15.2.1 Poly(styrene) Styrene was polymerized in a twin-screw extruder. The polymerization reaction mainly occurred in the zone between 400 and 1000 mm along the screw axis in the extruder, corresponding to the residence time of the reactants ranging from 1 to 4 min in the extruder. Based on dimensionless analysis, a model of the residence time was established. A kinetic model of the polymerization was set up under the assumption that the screw extruder can be treated as an ideal plug flow reactor.11 A styrene-butadiene multiblock copolymer was synthesized by anionic polymerization in a twin-screw extruder. The polymerized materials exhibit a nanometer size styrene and butadiene multiblock structure. Further, they show an ultrahigh elongation at break, which differs considerably from conventional polymers made by traditional solution polymerizing methods.14 Poly(styrene) could be modified by reactive extrusion with trimethylolpropane triacrylate (TMPTA) and dicumyl peroxide (DCP).16 The TMPTA increased the molecular weight of PS by a coupling reaction. The coupling was enhanced in the presence of DCP at a high ratio of TMPTA to DCP.

15.2.2 Poly(tetramethylene ether) and Poly(caprolactam) A polyetheramide, composed of poly(tetramethylene ether) (PTMEG) as soft segment and poly(caprolactam) as the hard segment, is synthesized in a one-step, solvent-free process. No volatile by-product is formed during the process. An isocyanate-terminated telechelic PTMEG was premixed with caprolactam, and this mixture was allowed to react in the twin-screw extruder to form the polyetheramide triblock copolymer.17

15.2.3 Polyamide 12 Polyamide 12 was prepared in a reactive extrusion process by the anionic polymerization of lauryllactam.15 Sodium hydride was used as initiator and N,N ′ -ethylene-bisstearamide (EBS) was used as activator. The reaction was complete to 99.5% in less than 2 min at 270°C and could be performed in an internal mixer and a twin-screw extruder with co-rotating intermeshing screws. Rubber-toughened polyamide 12 blends were obtained when poly(ethylene-co-butyl acrylate) was dissolved in lauryllactam. 1,3-Phen-

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ylene-bis(2-oxazoline) is a suitable chain extender.18 It reacts with the terminal carboxyl groups of the polyamide. During the extrusion process, the residence time distribution (RTD) has been measured by the addition of ultraviolet and ultrasonic detectable tracers.19, 20

15.2.4 Poly(butyl methacrylate) Telomers of butyl methacrylate were obtained by reactive extrusion with 1-octadecanethiol as chain transfer agent. The transfer constant to 1-octadecanethiol was measured. It was shown that the use of relatively high ratio of chain transfer agent to monomer had no perceptible effect on the kinetics of telomerization.12

15.2.5 Poly(carbonate) Poly(carbonate)s, such as bisphenol A poly(carbonate), are typically prepared either by interfacial or melt polymerization methods. The reaction of a bisphenol such as bisphenol A with phosgene in the presence of water, a solvent such as methylene chloride, an acid acceptor such as sodium hydroxide, and a phase transfer catalyst such as triethylamine is typical of the interfacial methodology. The interfacial method for making poly(carbonate) has several inherent disadvantages. The process requires phosgene which is highly poisonous. Further, the process requires large amounts of organic solvent. The reaction of bisphenol A with diphenyl carbonate at high temperature in the presence of sodium hydroxide as a catalyst is typical for the melt polymerization method. The melt method, although obviating the need for phosgene or a solvent, such as methylene chloride, requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement of carbonate units along the growing polymer chains. Fries rearrangement gives rise to undesired and uncontrolled polymer branching which may negatively impact the polymer’s flow properties and performance. The melt method further requires the use of complex processing equipment capable of operation at high temperature and low pressure, and capable of efficient agitation of the highly viscous polymer melt during the relatively long reaction times required to achieve high molecular weight. On the other hand, poly(carbonate) can be formed under relatively mild conditions by reacting a bisphenol A with a diaryl carbonate formed

Reactive Extrusion

O C R

HO

O

515

C R O

Figure 15.2: Fries Rearrangement

by the reaction of phosgene with methyl salicylate. Early procedures used relatively high levels of transesterification catalysts such as lithium stearate in order to achieve the desired high molecular weight poly(carbonate). 15.2.5.1

Linear Poly(carbonate)

Poly(carbonate) is prepared by introducing an ester substituted diaryl carbonate, such as bis(methyl salicyl)carbonate, a bisphenol A, and a transesterification catalyst, e.g., tetrabutylphosphonium acetate (TBPA) into an extruder.21 Within the extruder, a molten mixture is formed in which the reaction between carbonate groups and hydroxyl groups occurs, giving rise to a poly(carbonate) product and an ester-substituted phenol by-product. The extruder may be equipped with vacuum vents which serve to remove the ester-substituted phenol by-product and thus drive the polymerization reaction toward completion. The molecular weight of the poly(carbonate) may be controlled by controlling, among other factors, the feed rate of the reactants, the type of extruder, the extruder screw design and configuration, the residence time in the extruder, the reaction temperature, the number of vents present in the extruder, and the (vacuum) pressure. The poly(carbonate) reaches a weight-average molecular weight of greater than 20,000 Dalton. In a special experimental design, the extruder included 14 segmented barrels, each barrel having a ratio of length to diameter of about 4, and six vent ports for the removal of the by-product methyl salicylate. Two vents were configured for the operation at atmospheric pressure and four vents were configured for operation under vacuum. The methyl salicylate formed as the polymerization reaction took place was collected by means of two condensers. The poly(carbonate)s have extremely low levels of Fries rearrangement products and possess a high level of endcapping. Contrary to this is a bisphenol A poly(carbonate) prepared by a melt reaction method in which the Fries reaction occurs. The Fries rearrangement is shown in Figure 15.2.

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15.2.5.2

Branched Poly(carbonate)s

Branched poly(carbonate) resins differ from most thermoplastic polymers used for molding in their melt rheology behavior. Most thermoplastic polymers exhibit non-Newtonian flow characteristics over essentially all melt processing conditions. However, in contrast to most thermoplastic polymers, certain branched poly(carbonate)s prepared from dihydric phenols exhibit Newtonian flow at normal processing temperatures and shear rates below 300 s−1 . Copolyester-carbonate resins are prepared analogous to the preparation of poly(carbonate), but a difunctional carboxylic acid is added. Usually the carboxylic acid is aromatic and used as halide, i.e., isophthaloyl dichloride and terephthaloyl dichloride. Aliphatic diacid components yield soft segment co-poly(carbonate)s. Poly(carbonate) and copolyester-carbonate resins can be branched by reaction with tetraphenolic compounds during synthesis. On the other hand, a poly(carbonate) resin possessing a certain degree of branching and molecular weight can be produced via reactive extrusion. This is achieved by melt extruding a linear poly(carbonate) resin with a specific branching agent and an appropriate catalyst system.22 The resulting molecular weight increases with branching, but can also decrease if conditions are chosen that favor degradation. Branching agents useful to branch linear poly(carbonate)s are polyacrylates and polymethacrylates, in particular pentaerythritol triacrylate (PETA). Organic peroxides include 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DHBP) and 2,5-dimethyl-2,5-di(tert-butylperoxy)-3-hexyne (DYBP). Upon melt extrusion, branching and crosslinking occurs in the poly(carbonate) resin melt. The material is compounded on a melt extruder, a co-rotating twin-screw extruder under reduced pressure of 0.5 atmospheres, at a temperature profile of 200 to 300°C. The assumed mechanism of branching consists of thermal decomposition of a radical initiator which attacks the methyl groups of the BPA units in order to create poly(carbonate) macroradicals. The macroradicals can be recombined by a radical branching agent (compound containing at least two double bonds) to generate a branched structure. The key to the process will be the lifetime of the radicals and the sensitivity of the poly(carbonate) backbone versus radicals. A copolyester-poly(carbonate)containing long chain aliphatic diacid moieties, such as dodecyl diacid,

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Table 15.3: Biodegradable Compositions Compounds

Reference

Poly(lactide)s Poly(ε-caprolactone) Poly(ε-caprolactone)-grafted starch Poly(propylene) wood flour composites Poly(ε-caprolactone), wood flour or lignin Starch and poly(acrylamide) Protein and polyester Poly(styrene)-grafted starch

23, 23 24, 25 26 27 28 29 30

is more sensitive to radical attack. Branched poly(carbonate) resins produced by reactive extrusion are useful blow-moldable resins exhibiting an enhanced melt strength and melt elasticity. The branched poly(carbonate) products are useful in applications such as22 • Profile extrusion: of wire and cable insulation, extruded bars, pipes, fiber optic buffer tubes, and sheets; • Blowmolding: of containers and cans, gas tanks, automotive exterior applications such as bumpers, aerodams, spoilers and ground effects packages; and • Thermoforming: of automotive exterior applications and food packaging.

15.3 BIODEGRADABLE COMPOSITIONS Many biodegradable compositions have been synthesized and investigated. These are summarized in Table 15.3. Poly(β-hydroxybutyrate-co-valerate), poly(butylene succinate), poly(ethylene succinate) and poly(ε-caprolactone) are biodegradable polymers which are thermally processable. Poly(β-hydroxybutyrate-co-β-hydroxyvalerate) (PHBV) can be made by both the fermentation process of carbohydrate and an organic acid by a microorganism, e.g., Alcaligenes Eutrophus, and by the use of transgenic plants. Polyalkylene succinate (PAS) is produced by the reaction between aliphatic dicarboxylic acids and ethylene glycol or butylene glycol. Poly(εcaprolactone) (PCL) is produced by the ring-opening polymerization of εcaprolactone.

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By grafting polar monomers onto poly(β-hydroxybutyrate-co-valerate), poly(butylene succinate), or poly(ε-caprolactone), the resulting modified polymer is more compatible with polar polymers and other polar substrates. Useful polar monomers, oligomers, or polymers include ethylenically unsaturated monomers containing a polar functional group, such as 2-hydroxyethyl methacrylate (HEMA) and poly(ethylene glycol)methacrylate (PEG-MA). The grafted biodegradable polymer may contain 1.5 to 20% of grafted polar monomers. Other reactive ingredients which may be added to the compositions include free-radical initiators, such as Lupersol™ 101. The amount of free-radical initiator ranges from 0.1 to 1.5%. A low dosage of free-radical initiator cannot initiate the grafting reaction. On the other hand, if the amount of free-radical initiator is too high, it will create undesirable crosslinking of the polymer composition. Crosslinked polymers are undesirable, because they cannot be processed into films, fibers or other products. The grafting reaction can be performed by a reactive-extrusion process.31 A particularly useful reaction device is a co-rotating twin-screw extruder having one or more ports. Such an extruder allows multiple feeding and venting ports and provides high intensity distributive and dispersive mixing. The grafting may be achieved in several ways. 1. All of the ingredients, including a biodegradable polymer, a freeradical initiator, and the polar monomer are added simultaneously to a melt mixing device or an extruder. 2. The biodegradable polymer may be fed to a feeding section of a twin-screw extruder and subsequently melted, and a mixture of a free-radical initiator and the polar monomer is injected into the biodegradable polymer melt under pressure. The resulting melt mixture is then allowed to react. 3. The biodegradable polymer is fed to the feeding section of a twinscrew extruder, then the free-radical initiator and the polar monomer are fed separately into the twin-screw extruder at different points along the length of the extruder. The heated extrusion is performed under high shear and intensive dispersive and distributive mixing resulting in a grafted biodegradable polymer of high uniformity. The modified polymer compositions have a greater compatibility

Reactive Extrusion

519

with water-soluble polymers, such as polyvinyl alcohol and poly(ethylene oxide), than the unmodified biodegradable polymers. The compatibility of modified polymer compositions with a polar material can be controlled by the selection of the monomer, the level of grafting and the blending process conditions. Tailoring the compatibility of blends with modified polymer compositions leads to better processability and improved physical properties of the resulting blend. The compositions are biodegradable so that the articles made from them could be degraded in aeration tanks by aerobic degradation, and by anaerobic degradation in wastewater treatment plants. PHBV allows only a low cooling rate, such that commercial use of this material is impractical. On the other hand, polylactic acid (PLA) is brittle. However, a blend of PLA and PHBV allows to the PHBV cool at an acceptable rate and also makes PLA more flexible such that these materials can be used.

15.3.1 Poly(lactide)s It is generally known that lactide polymers are unstable. The concept of instability has both advantages and disadvantages. The advantage is the biodegradation or other forms of degradation that occur when lactide polymers or articles manufactured from lactide polymers are discarded or composted after completing their useful life. A negative aspect of such instability is the degradation of lactide polymers during processing at elevated temperatures as, for example, during melt processing by end user. Thus, the same properties that make lactide polymers desirable as replacements for non-degradable petrochemical polymers also create undesirable effects during production of lactide polymer resins and processing of these resins. In general, poly(lactide) is a relatively brittle polymer with low impact resistance. Articles made of poly(lactide) may be brittle and prone to shatter under use conditions. For example, if poly(lactide) is made into articles such as razor holders, shampoo bottles, and plastic caps, these articles may be prone to undesirable shatter in use.23 However, compositions with modified physical properties can avoid these drawbacks.

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H3C

O

O O

O O

HO CH C O CH3

CH3

n

Figure 15.3: Ring Opening Polymerization of a Lactide

15.3.1.1

Ring Opening Polymerization of Lactide

The ring opening polymerization of a lactide using an equimolar complex of 2-ethylhexanoic acid tin(II) salt Sn(Oct)2 and triphenylphosphine PΦ3 as catalyst exhibits a high reactivity to polymerization that is too high to allow a continuous single-step reactive extrusion process for bulk polymerization. The catalyst also delays the occurrence of undesirable backbiting reactions. The ring opening polymerization is shown in Figure 15.3. A sophisticated screw design is required to ensure further enhancement of the polymerization reaction by using mixing elements and by the introduction of shear into the melt. It is possible to design a single stage process using reactive extrusion to polymerize the lactide into a poly(lactide) that can be fabricated by most any known polymer processing techniques.13 Possible uses of such polymers include food packaging for meat and soft drinks, films for agro-industry, and non-wovens in hygienic products.32 15.3.1.2

Functionalized Poly(lactide)s

A functionalized poly(lactide) is a polymer which has been modified to contain groups capable of bonding to an elastomer or which have a preferential solubility in the elastomer. Only a portion of the poly(lactide) needs to be functionalized in order to gain the benefit of improved impact strength, however, uniform distribution of the functionalization throughout the poly(lactide)-based polymer is preferred. The functionalized poly(lactide) can be created during the lactide polymerization process, for example, by copolymerizing a compound containing both an epoxide ring and an unsaturated bond. The functionalized poly(lactide) polymer, containing unsaturated bonds, can be blended and linked via free radical reactions to an elastomer which contains unsaturated bonds. The functionalized polymer can also be prepared subsequent to polymerization reaction, for ex-

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521

ample, by grafting a reactive group, such as maleic anhydride (MA), to the poly(lactide)-based polymer using peroxides.23 Typically, the resulting polymer compositions have an impact resistance of at least 0.7 ft − lb/in (120 kgs−2 ). and an impact resistance of at least about 1 ft − lb/in (180 kgs−2 ). A poly(lactide) can be also functionalized by radical grafting of maleic anhydride onto it.33, 34 A concentration of 2% MA in the presence of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane suffices to reach up to 0.7% MA grafted onto the poly(lactide). Increasing the initiator concentration results in an increase in the grafting of MA, but also in a decrease in the molecular weight of the polymer. Without initiator, extensive degradation was observed.

15.3.2 Biodegradable Fibers One of the most promising biodegradable polymers is poly(lactide) (PLA), in particular, from the viewpoint of environmental protection. PLA is of great interest due to its mechanical property profile, its thermoplastic processability, and its biodegradability. Further advantages of PLA compared to other biodegradable polymers are its renewable origin and low price. PLA is synthesized by the polycondensation of lactic acid or by the ring-opening polymerization of the lactide. In both cases, lactic acid is the starting monomer. Lactic acid is commercially produced by means of bacterial fermentation. Fibers from PLA can obtained in a high-speed melt spinning and spin drawing process.35 A copolymer of L-lactide and 8% meso-lactide is used that can be obtained by reactive extrusion polymerization.

15.3.3 Poly(ε-caprolactone) Bulk polymerization of ε-caprolactone in an extruder in the presence of starch to give a compatibilized blend of poly(ε-caprolactone), starch and grafted starch-g-poly(ε-caprolactone) is described in the literature. A suitable catalyst is aluminum isopropoxide. Aluminum isopropoxide can be generated in-situ by using tri ethyl aluminum or diisobutyl aluminum hydride. The lactone should contain less than 100 ppm water and should have an acid value less than 0.5 mg KOH/g. The presence of water and free acid in the reactant mixture is especially significant in the synthesis of high

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molecular weight poly(ε-caprolactone) polymer by reactive extrusion polymerization since it has a deleterious effect on the kinetics and ultimately leads to lower conversion of monomer to polymer. The impurities interact with the polymerization catalyst or the propagating species and lower the overall rate of polymerization. In cases where the monomer contains greater than 100 ppm water, the desired water content may be achieved by drying it using molecular sieves or calcium hydride (chemical method). The ring-opening polymerization of ε-caprolactone in the presence of starch leads to a poly(ε-caprolactone)-grafted starch. The reactant mixture is extruded at a temperature of 80 to 240°C with residence times up to 12 minutes.24

15.3.3.1

Blends with Starch

Films produced from poly(ε-caprolactone) and its copolymers which have low melting points, are tacky, as extruded, and noisy to the touch and have a low melt strength over 130°C. Due to the low crystallization rate of such polymers, the crystallization process proceeds for a long time after the production of the finished articles, followed by an undesirable change of properties with time. However, the blending of pre-blended starch with other polymers, such lactone polymers, improves their processability without impairing the mechanical properties and biodegradability properties.36 The improvement is particularly effective with polymers having low melting point temperatures from 40°C to 100°C. The pre-blends are obtainable by blending a starch-based component and a synthetic thermoplastic component, such as an ethylene-vinyl alcohol copolymer in the presence of a plasticizer. Suitable plasticizers are glycerol, sorbitol, and sorbitol monoethoxylate. Urea as additive can destroy hydrogen bonds of the starch. The addition of urea is advantageous for the production of blends for film-blowing. By means of extrusion, thermoplastic blends are obtained wherein the starch-based component and the synthetic thermoplastic component form an interpenetrating structure. In a first step, starch and an ethylene-vinyl alcohol copolymer (1:1) with minor amounts of plasticizer, and other additives such as urea are melt blended in a twin-screw extruder. This extrudate is pelletized. In a second step the extrudate from the first step is blended with poly(ε-caprolactone).

Reactive Extrusion 15.3.3.2

523

Blends with Wood Flour and Lignin

Poly(ε-caprolactone) was compounded in twin-screw extruder together with wood flour and lignin.27 Maleic anhydride-grafted poly(ε-caprolactone) (PCL-g-MA) was used as a compatibilizer. The grafting of maleic anhydride onto PCL was achieved with 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. Low contents of grafted maleic anhydride and PCL-g-MA were required to improve both mechanical properties and interfacial adhesion. The addition of lignin retarded the biodegradation.

15.3.4 Cationically Modified Starch Cationic wheat starch has been prepared by reactive extrusion in a twinscrew extruder. The modifiers are 2,3-epoxypropyltrimethylammonium chloride and 3-chloro 2-hydroxypropyltrimethylammonium in aqueous sodium hydroxide (NaOH).37 A high reaction efficiency can be reached if a low degree of substitution is adjusted.

15.3.5 Blends of Starch and Poly(acrylamide) Starch-poly(acrylamide) copolymers have been prepared by reactive extrusion with ammonium persulfate as initiator. The extrusion temperature had no significant impact on acrylamide conversion.28

15.3.6 Blends of Protein and Polyester Blends of soy protein and biodegradable polyester could be prepared with glycerol as compatibilizer.29 Miscibility was only achieved when the soy protein was processed with glycerol applying high shear at elevated temperatures in an extruder. There, a partial denaturation of the soy protein occurred. Extruder screws with large kneading blocks were preferred. Thermoplastic blends were obtained with high elongation and high tensile strength. When the concentration of protein was increased, a lower degree of crystallinity and a lower melting point was obtained. It is possible to use a soy protein concentrate instead of a more expensive soy protein with higher purity.

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15.4 CHAIN EXTENDERS 15.4.1 Recycling of Poly(ethylene-terephthalate) Chain extenders are low molecular weight compounds that can be used to increase the molecular weight of polymers. Pyromellitic dianhydride (PMDA) is a suitable chain extender to increase the molecular weight of poly(ethylene terephthalate) (PET) industrial scraps with low intrinsic viscosity. Industrial scraps coming from PET processing plants are in many cases uncontaminated. However, their viscosity is lowered by the first extrusion. PMDA has a melting point (283°C) close to that of PET and it reacts within a few minutes under the processing conditions of PET. PMDA is a tetrafunctional compound, therefore, branching can occur. The PET end groups consist of carboxyl and hydroxyl groups. The chain extension occurs by a polyaddition between the hydroxyl groups and the pyromellitic dianhydride. The crucial parameters of the process are the concentration of the chain extender, the residence time of the polymer in the extruder, and the working temperatures. Dry blends of PET chips and PMDA powder were prepared with different amounts of PMDA (0.25, 0.50, 0.75, and 1.00% by weight). These were vacuum dried for 12 h at 110°C and extruded at 280°C. The average residence time is approximately 150 s. An amount of PMDA from 0.50 to 0.75% is sufficient to result in an increase of Mw , a broadening of Mw /Mn , and branching phenomena. The recycled polymer from PET scraps is then suitable for film blowing and blow molding processes.38

15.4.2 Modified Poly(ethylene terephthalate) Multifunctional epoxy based modifiers, such as a tetraglycidyl diaminodiphenylmethane (TGDDM) resin, can be used to increase the melt strength of PET. The progress conversion with time can be measured by the change of torque in an internal mixer. With a stoichiometric concentration of TGDDM, the molecular weight distribution of modified PET shows an eight-fold increase of the z-average molecular weight (Mz ) and the presence of branched molecules of very large mass.39 Further, a tetrafunctional epoxy based additive can be used to extrude PET in order to produce PET foams. The molecular structure ana-

Reactive Extrusion

525

lysis and shear and elongation rheological characterization indicates that branched PET is obtained for small amounts, up to 0.4% of a tetrafunctional epoxy additive. Gel permeation chromatography (GPC) studies suggest that a randomly branched structure is obtained, the structure being independent of the modifier concentration.40 An increase in the degree of branching and the Mw and the broadening of the molecular weight distribution causes an increase in the Newtonian viscosity, the relaxation time, flow activation energy, and the transient extensional viscosity. On the other hand, the shear thinning onset and the Hencky strain at the fiber break decrease markedly.

15.4.3 Poly(butylene terephthalate) The chain extension reaction in poly(butylene terephthalate) (PBT) can be achieved by a diglycidyl tetrahydrophthalate with high-reactivity.41 The chain extender reacts with the hydroxyl and carboxyl end groups of PBT very fast and also at a comparatively high temperature. The chain extension reaction is complete within 2 to 3 min at temperatures above 250°C. The chain-extended PBT is thermally more stable than the original polymer. In order to obtain PBT resins with a high molecular weight, the reactive extrusion process is simpler and cheaper than the post-polycondensation method.

15.5 RELATED APPLICATIONS 15.5.1 Transesterification The transesterification is a different concept from polymerization. Transesterification of mixtures of polyesters and oligoesters allows synthesizing new types of polymers. Block copolyesters have been synthesized from poly(neopentyl isophthalate) and poly(ethylene terephthalate).42 The esterification of poly(neopentyl isophthalate) is somehow resistant to transesterification. Therefore, blocks instead of alternating polyesters will be obtained. Poly(neopentyl isophthalate) is expected to exhibit high barrier properties. Therefore, such materials are of interest in the field of beverage containers. Similarly, block co-polyesters of PET and poly(ε-caprolactone) have been synthesized by reactive extrusion. In the presence of stannous octoate, the ring-opening polymerization of ε-caprolactone can be initiated due

526


to the hydroxyl end groups of molten PET to form poly(ε-caprolactone) blocks.43 A block copolymer with a minimal degree of transesterification can be obtained under conditions of a fast distributive mixing of the ε-caprolactone into the high viscous PET.

15.5.2 Hydrolysis and Alcoholysis The continuous hydrolytic depolymerization of a poly(ethylene terephthalate) was carried out in a twin-screw extruder. The hydrolysis was achieved by the injection of saturated steam at high pressure. Low molecular weight products were obtained even at low residence times in the extruder. Therefore, high depolymerization rates should occur under the conditions selected.44 α, ω-Diols have been obtained by the alcoholysis of PET through reactive extrusion. The alcoholysis of PET with diols in the presence of dibutyltin oxide was carried out in a twin-screw extruder with residence times of ca. 1 min. Scissions of PET chains are taking place and oligoester α, ω-diols are formed with a number-average of around 1 k Dalton.45 The study revealed that oligoesters synthesized by reactive extrusion are quite similar to oligoesters synthesized by batch processes which last many hours.

15.5.3 Flame Retardant Master Batch A master batch of an intumescent flame retardant was prepared by reactive extrusion of melamine phosphate and pentaerythritol with a poly(propylene) carrier in a twin-screw extruder.46

REFERENCES 1. M. Xanthos, editor. Reactive Extrusion, Principles and Practice. Polymer Processing Institute Series. Hanser, München, 1992. 2. S. Al-Malaika, editor. Reactive Modifiers for Polymers. Blackie Academic & Professional, London, New York, 1997. 3. E. Haberstroh, W. Michaeli, and P. Schwarz. Synthesis of polyoctenylene by means of reactive extrusion. Kautsch. Gummi Kunstst., 52(3):184–187, March 1999. 4. W. Michaeli, H. Höcker, U. Berghaus, and W. Frings. Reactive extrusion of styrene polymers. J. Appl. Polym. Sci., 48:871–886, 1993.

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5. H. Kye and J. L. White. Continuous polymerization of caprolactam in a modular intermeshing corotating twin screw extruder integrated with continuous melt spinning of polyamide 6 fiber: Influence of screw design and process conditions. J. Appl. Polym. Sci., 52:1249–1262, 1994. 6. R. Beyreuther, B. Tandler, M. Hoffmann, and R. Vogel. Reactive extrusion and melt spinning - a new technological route to special fibres. J. Mater. Sci., 36(13):3103–3111, July 2001. 7. M. Hoffmann, B. Tandler, R. Beyreuther, and R. Vogel. Melt spinning on twin screw extruder. Part II: Polyolefin elastomer fibers by means of reactive extrusion. Kautsch. Gummi Kunstst., 53(11):632–637, November 2000. 8. S. Roy and A. Lawal. Isothermal pseudo-2d analysis of reactive extrusion in single-screw extruders. J. Reinf. Plast. Compos., 23(7):685–706, 2004. 9. L. P. B. M. Janssen. On the stability of reactive extrusion. Polym. Eng. Sci., 38(12):2010–2019, December 1998. 10. W. K. Busfield. Heats and entropies of polymerization, ceiling temperatures, equilibrium monomer concentration; and polymerization of heterocyclic compounds. In J. Brandrup and E. H. Immergut, editors, Polymer Handbook, chapter II, pages II/295–II/334. J. Wiley & Sons, New York, 3rd edition, 1989. 11. S. S. Gao, Z. Ying, Z. Anna, and H. N. Xia. Polystyrene prepared by reactive extrusion: kinetics and effect of processing parameters. Polym. Adv. Technol., 15(4):185–191, April 2004. 12. J. D. Chen, Y. Chalamet, and M. Taha. Telomerization of butyl methacrylate and 1-octadecanethiol by reactive extrusion. Macromol. Mater. Eng., 288(4): 357–364, April 2003. 13. S. Jacobsen, H. G. Fritz, P. Degee, P. Dubois, and R. Jerome. Single-step reactive extrusion of PLLA in a corotating twin-screw extruder promoted by 2-ethylhexanoic acid tin(II) salt and triphenylphosphine. Polymer, 41(9): 3395–3403, April 2000. 14. S. S. Gao, Z. Ying, Z. Anna, and H. N. Xia. Study on nanometer-size styrenebutadiene multiblock copolymer synthesized by reactive extrusion. J. Appl. Polym. Sci., 91(4):2265–2270, February 2004. 15. A. Wollny, H. Nitz, H. Faulhammer, N. Hoogen, and R. Mülhaupt. In situ formation and compounding of polyamide 12 by reactive extrusion. J. Appl. Polym. Sci., 90(2):344–351, October 2003. 16. B. K. Kim, K. H. Shon, and H. M. Jeong. Modification of polystyrene by reactive extrusion with peroxide and trimethylolpropane triacrylate. J. Appl. Polym. Sci., 92(3):1672–1679, May 2004. 17. B. H. Lee and J. L. White. Formation of a polyetheramide triblock copolymer by reactive extrusion; process and properties. Polym. Eng. Sci., 42(8): 1710–1723, August 2002. 18. Y. Chalamet, M. Taha, F. Berzin, and B. Vergnes. Carboxyl terminated polyamide 12 chain extension by reactive extrusion using a dioxazoline coupling

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31. J. H. Wang and D. M. Schertz. Reactive extrusion process for making modifiied biodegradable compositions. US Patent 6 579 934, assigned to Kimberly-Clark Worldwide, Inc. (Neenah, WI), July 17 2003. 32. S. Jacobsen, H. G. Fritz, P. Degee, P. Dubois, and R. Jerome. Continuous reactive extrusion polymerisation of l-lactide - an engineering view. Macromol. Symp., 153:261–273, March 2000. 33. D. Carlson, L. Nie, R. Narayan, and P. Dubois. Maleation of polylactide (PLA) by reactive extrusion. J. Appl. Polym. Sci., 72(4):477–485, April 1999. 34. D. Carlson, P. Dubois, L. Nie, and R. Narayan. Free radical branching of polylactide by reactive extrusion. Polym. Eng. Sci., 38(2):311–321, February 1998. 35. G. Schmack, D. Jehnichen, R. Vogel, B. Tandler, R. Beyreuther, S. Jacobsen, and H. G. Fritz. Biodegradable fibres spun from poly(lactide) generated by reactive extrusion. J. Biotechnol., 86(2):151–160, March 2001. 36. C. Bastioli, V. Bellotti, A. Montino, G. D. Tredici, R. Lombi, and R. Ponti. Biodegradable polymeric compositions based on starch and thermoplastic polymers. US Patent 5 412 005, assigned to Novamont S.p.A. (Milan, IT), May 2 1995. 37. A. Tara, F. Berzin, L. Tighzert, and B. Vergnes. Preparation of cationic wheat starch by twin-screw reactive extrusion. J. Appl. Polym. Sci., 93(1):201–208, July 2004. 38. L. Incarnato, P. Scarfato, L. Di Maio, and D. Acierno. Structure and rheology of recycled PET modified by reactive extrusion. Polymer, 41(18):6825–6831, August 2000. 39. S. Japon, L. Boogh, Y. Leterrier, and J. A. E. Manson. Reactive processing of poly(ethylene terephthalate) modified with multifunctional epoxy-based additives. Polymer, 41(15):5809–5818, July 2000. 40. S. Japon, A. Luciani, Q. T. Nguyen, Y. Leterrier, and J. A. E. Manson. Molecular characterization and rheological properties of modified poly(ethylene terephthalate) obtained by reactive extrusion. Polym. Eng. Sci., 41(8): 1299–1309, August 2001. 41. B. H. Guo and C. M. Chan. Chain extension of poly(butylene terephthalate) by reactive extrusion. J. Appl. Polym. Sci., 71(11):1827–1834, March 1999. 42. G. Moad, A. Groth, M. S. O’ Shea, J. Rosalie, R. D. Tozer, and G. Peeters. Controlled synthesis of block polyesters by reactive extrusion. Macromol. Symp., 202:37–45, September 2003. 43. W. M. Tang, N. S. Murthy, F. Mares, M. E. McDonnell, and S. A. Curran. Poly(ethylene terephthalate)-poly(caprolactone) block copolymer. I. synthesis, reactive extrusion, and fiber morphology. J. Appl. Polym. Sci., 74(7): 1858–1867, November 1999. 44. T. Yalcinyuva, M. R. Kamal, R. A. Lai-Fook, and S. Ozgumus. Hydrolytic depolymerization of polyethylene terephthalate by reactive extrusion. Int. Polym. Process., 15(2):137–146, June 2000.

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45. M. Dannoux, P. Cassagnau, and A. Michel. Synthesis of oligoester α,ω-diols by alcoholysis of PET through the reactive extrusion process. Can. J. Chem. Eng., 80(6):1075–1082, December 2002. 46. Q. Wang, Y. H. Chen, Y. Liu, H. Yin, N. Aelmans, and R. Kierkeis. Performance of an intumescent-flame-retardant master batch synthesized by twinscrew reactive extrusion: effect of the polypropylene carrier resin. Polym. Int., 53(4):439–448, April 2004.

16 Compatibilization There are innumerable binary, ternary, and multiple alloys known in metallurgy. Metals easily form mutual alloys. The situation is completely different in polymer chemistry, as is pointed out in the literature.1 Most of the organic polymers are not mutually miscible. Methods have been reported for compatibilization of immiscible blends of polymers by reactive mixing, in which functionalized versions of the polymeric components react in-situ to form a block copolymer compatibilizer. The fundamental requirements for a compatibilizer as additive and in reactive processing include the following:2, 3 • The interfacial tension must be optimized. • There must be sufficient mixing to achieve the desired fineness of morphological texture. • Some of the polymer molecules must contain chemical functional groups which can react to form primary bonds during the mixing/mastication process. The functional groups must be of sufficient reactivity for reactions to occur across melt phase boundaries. • The reactions must occur rapidly enough to be completed during processing in the extruder or mixer within a reasonable time. • The bonds formed as a result of reactive blending must be stable enough to survive subsequent processing. • The adhesion between the phases in the solid state should be enhanced. • The compatibilization reactions should be fast and irreversible. 531

532


Many new products can be manufactured by melt blending of polymers to achieve improved properties that are not available in any single polymeric material, e.g., toughness, chemical resistance, ease of fabrication, etc. The use of blends and alloys of immiscible polymers has increased because it is generally less expensive to develop a new blend composition than to develop new polymers based upon new monomers to meet the need for specialized polymers.

16.1 EQUIPMENT Compatibilization reactions are normally performed in an extruder. Also blenders have been used for discontinuous work. Ultrasonic assisted extrusion in the molten state has been described in extruders.4, 5 Ultrasonic horns placed at the exit of the extruder that are vibrating in the direction perpendicular to the flow direction introduce longitudinal ultrasonic waves into the polymer melt. The mechanical performance of polymer blends subjected was significantly enhanced by ultrasonic treatment in comparison to the performance of blends not subjected to ultrasonic treatment with a similar phase morphology. Ultrasonically assisted melt mixers also have been described.6 Poly(propylene)/poly(styrene)/clay nanocomposites and poly(methyl methacrylate)/clay nanocomposites were prepared by in-situ polymerization and ultrasonic assisted melt mixing.

16.2 BASIC TERMS 16.2.1 Thermodynamic Compatibility Compatibilizing methods and agents are required for such blends, since most polymers are mutually immiscible and have poor interfacial adhesion. Compatibilizers generally are believed to act at interfaces to improve interfacial interactions between immiscible polymeric species. It is recognized that miscibility between polymers is determined by a balance of enthalpic and entropic contributions to the free energy of mixing. While for small molecules the energy is high enough to ensure miscibility, for polymers the entropy is almost zero, causing enthalpy to be decisive in determining miscibility. The change in free energy of mixing (∆G) is written

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533

as ∆G = ∆H − T ∆S

(16.1)

where H is enthalpy, S is entropy, and T is temperature. For spontaneous mixing, ∆G must be negative, and so ∆H − T ∆S < 0.

(16.2)

This implies that exothermic mixtures (∆H < 0) will mix spontaneously, whereas for endothermic mixtures miscibility will only occur at high temperatures.2

16.2.2 Thermodynamic Models Flory’s solution theory was modified by Hamada.7 This modified theory was used to predict the miscibility of blends of poly(ethylene oxide) with poly(methyl methacrylate) (PEO-a-PMMA) and with poly(vinyl acetate) (PEO-PVAc).8 The interaction parameters of a PEO-a-PMMA blend with the weight ratio of PEO/aPMMA = 50/50 at the temperature range of 393 to 433 K and PEO-PVAc blends with different compositions and temperatures were calculated from the parameters of the equation of state. The interaction parameters of the PEO-a-PMMA blend turned out to be negative. The interaction parameters and excess volumes of PEO-PVAc blends are negative and increase with enhancing the content of PEO and the temperature. The miscibility of blends of poly(methyl methacrylate) (PMMA) and poly(ethylene oxide) (PEO) oligomers was studied by temperaturemodulated DSC. The miscibility domain is larger for the atactic and syndiotactic PMMA than for the isotactic isomer. The Flory-Huggins interaction parameter χ1,2 decreases with the increase of the PEO molecular weight as well as with the syndiotacticity of the PMMA and is lower for the PEO with alkyl modified chain ends.9

16.2.3 Particle Size Wu10 proposed Eq. 16.3 to predict the particle size for polyamide and polyester blends containing 15% ethylene/propylene rubber as a dispersed phase. 4γηr±0.84 d¯ = Gηm

(16.3)

534


d¯ G γ ηr = ηd /ηm ηm ηd

Average diameter of the droplet Shear rate Interfacial tension Viscosity ratio Viscosity of the matrix Viscosity of the dispersed phase

The positive exponent is for ηr > 1; the negative exponent is for ηr < 1.

16.2.4 Interfacial Slip The viscosity of uncompatibilized polymer blends often exhibits a negative deviation from a log-additivity rule at shear rates relevant to processing. This deviation is attributed to the interfacial slip. The interfacial slip arises from the loss of entanglements at the interface. The effect of reactive compatibilization on the interfacial slip has been studied in blends from ethylene/propylene rubber and polyamide 6. It has been demonstrated that the interfacial slip can be important in uncompatibilized systems whereas it is suppressed in compatibilized blends.11

16.2.5 Interpolymer Radical Coupling By using a solid-state shear pulverization technique in order to blend polymers, an interpolymer radical coupling reaction has been discovered.12, 13 This reaction leads to the formation of block copolymers. Pulverization leads to an intimate mixing, creating a large interfacial area between blend components, and to chain scission reactions, resulting in polymer radicals. These radicals can cause interpolymer coupling in the solid state. The interpolymer reaction was proved in a PMMA/PS blend using a pyrenelabelled PS. The blends were characterized by gel permeation chromatography with a fluorescence detector. The change in elution times was noted and taken as proof that the interpolymer radical coupling happened at mixing.

16.2.6 Technological Compatibility However, thermodynamic compatibility need not be attained. Technological compatibility, where the blend has useful properties, normally is sufficient. Mechanical or chemical techniques can be used to attain technological compatibility. Technological compatibility of immiscible polymers can be produced by

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• The addition of a compatibilizer before or during the mixing/blending process, • Adjustment of viscosity ratios to favor rapid formation of the desired phase morphology during mixing, • In-situ formation of a compatibilizer during the mixing/blending process, and • Introduction of crosslinks in one of the phases. The objective of technological compatibilization is to produce compositions which exhibit good ultimate properties, e.g., strength, elongation, fatigue life, etc. Compatibilized polymer blends exhibit at least some of the following differences from uncompatibilized polymer blends: reduced morphological dimensions (smaller domain sizes, thus smaller potential flaws); improved bonding or adhesion between phases; and reduced tendencies to form highly shaped domains during flow in processing, molding, etc. One approach to technological compatibilization is the addition of a compatibilizer before or during the mixing or blending process, respectively. Such compatibilizers are frequently a block copolymer. To be efficient, the compatibilizing block copolymer must possess segments with chemical structures or solubility parameters, which are similar to or the same as those of the polymers being blended. A sufficient amount of the compatibilizing polymer must be located at the interface of the polymer phases. Usually in this type of compatibilization, one block, A, is chemically similar to one of the polymer components of the blend and the other block, B, is chemically similar to the other blend component. This method is usually extremely successful in stabilizing the polymer blend. Nevertheless, this method is rarely used in commercial applications because of the high expense usually involved in synthesizing the block copolymer. Another method of promoting the presence of a compatibilizing block copolymer at the interfacial region is to use reactive mixing techniques, whereby the compatibilizing copolymer forms at the interface. In such cases, polymer molecules of one phase contain functional groups which chemically interact with molecules of a polymer in an adjacent phase, so that a compatibilizer could form in the interfacial regions where it is needed. In other words, reactive compatibilization involves the formation of a block or graft copolymer via a coupling reaction between the reactive functional groups of two additives.2 In Table 16.1 and Table 16.2, compatibilizers for various polymer blends are listed.

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Table 16.1: Compounds for Compatibilization of Polyolefines Blends

Compatibilizer

LDPE/PA6

Diethylsuccinate,14 glycidyl methacrylate,15 ethylene and acrylic acid copolymer,16 poly(ethylene) grafted with maleic anhydride Ethylene-propylene copolymer-g-methacryloyl carbamate17 Poly(ethylene-co-glycidyl methacrylate)18 Styrene/ethylene-butylene/styrene block copolymer19 Maleic anhydride20 Poly(ethylene-co-glycidyl methacrylate)21, 22 Styrene-butadiene triblock copolymer,23, 24 styrene-ethylene/propylene diblock copolymer (SEP)25 ε-Caprolactam and maleic anhydride grafted poly(propylene),26 isocyanate-modified PP,27, 28 poly(styrene-b-(ethylene-co-butylene)-b-styrene) grafted with maleic anhydride29 Grafted ethylene/propylene rubber (EPM-g-MA)30 Triallyl isocyanurate

LDPE/PET LDPE/Starch HDPE/HIPS HDPE/PA12 HDPE/PET PP/PS PP/PA6

EPDM/PTT Polyolefins

Abbreviations EPDM Ethylene propylene diene rubber HDPE High density poly(ethylene) HIPS High impact poly(styrene) LCP Liquid crystalline polymers LDPE Low density poly(ethylene) PA6 Polyamide 6 PA12 Polyamide 12 PE Poly(ethylene) PET Poly(ethylene terephthalate) PP Poly(propylene) PS Poly(styrene) PTT Poly(trimethylene terephthalate)

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Table 16.2: Compounds for Compatibilization Blends

Compatibilizer

PA6/PPO

Styrene/maleic anhydride copolymer,31, 32 poly(ethylene 1-octene),33 Styrene/glycidyl methacrylate (SG) copolymers,34 EPDM grafted with maleic anhydride,35 styrene-ethylene-butadiene-styrene block copolymer grafted with maleic anhydride35 Bisphenol A-type epoxy resins36 Styrene-maleic anhydride copolymer37 Maleic anhydride Maleic anhydride38 Poly(ethylene-co-glycidyl methacrylate)39–41 Copolymer of methyl methacrylate and acrylic acid42 Functionalized ethylene copolymers with n-butyl acrylate, maleic anhydride, epoxy, and acrylic acid43 Maleated and acrylic acid grafted polyethylenes44 Glycidyl methacrylate grafted rubber45 Multifunctional epoxies36, 46 poly(methyl methacrylate-co-maleic anhydride) copolymers47 Acrylic modified polyolefin-type ionomer48 Poly(styrene)-b-poly(ethylene-co-butylene) poly(styrene) triblock copolymer49 Interphase modifiers50 Surfactants51

PA6/PS

PA66/PBT PA66/PS PBT/EVA PBT/EVA/PA6 PBT/PEO PC/PVDF PE/Fillers PE/Wood flour PET PET/LCP ABS/PA6 PET/PA6 PS PS/EPR SBR/XNBR

Abbreviations ABS Acrylonitrile-butadiene-styrene EPR Ethylene/propylene rubber EVA Ethylene/vinyl acetate copolymer LCP Liquid crystalline polymers PBT Poly(butylene terephthalate) PEO Poly(ethylene-octene) PET Poly(ethylene terephthalate) PMMA Poly(methyl methacrylate) PPO Poly(2,6-dimethyl-1,4phenylene oxide) XNBR Carboxylated nitrile rubber blend

Abbreviations PA6 Polyamide 6 PA12 Polyamide 12 PA66 Polyamide 6,6 PC Poly(carbonate) PE Poly(ethylene) PP Poly(propylene) PS Poly(styrene) PVDF Poly(vinylidene fluoride) SBR Styrene butadiene rubber

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16.3 COMPATIBILIZATION BY ADDITIVES The next topic is compatibilization by additives and reactive compatibilization. This classification is somewhat arbitrary. In fact, additives react chemically with some ingredients of a blend to increase the compatibility of their constituents. Section 16.4 discusses reactive compatibilization issues where the compatibilizer is essentially formed during the blending.

16.3.1 Poly(ethylene) Blended with Inorganic Fillers Aluminum hydroxide and magnesium hydroxide blends with poly(ethylene) composites can be compatibilized with functionalized polyethylenes. Suitable compatibilizers are hydroxyl or carboxylic acid functionalized ethylene copolymers prepared with metallocene catalysts. Compatibilizer precursors are n-butyl acrylate, maleic anhydride, epoxy, and acrylic acid.43 These polymeric compatibilizers improve adhesion. The improved adhesion is reflected in the mechanical properties, such as stiffness and toughness. The flame retardancy imparted by the inorganic hydroxides does not deteriorate upon addition of compatibilizers.

16.3.2 Filler Materials without Chemical Compatibilizers Filler materials modify the melt elasticity and viscosity of a dispersed phase of a lower viscosity material sufficiently to result in a reduction of the efficiency of dispersed phase particle collisions in a higher viscosity matrix. Since the efficiency of such collisions is inversely related to the modulus of the dispersed phase, for instance, more elastic, higher modulus dispersed phase particles have a lower probability of coalescing upon collision. This reduces the coalescence of the dispersed phase under conditions of high shear experienced, for example, during injection molding. Selective precompounding or extrusion of polymer components with fillers can change the viscosity and/or elasticity of the dispersed phase polymer prior to forming the blend, so that a customized viscosity/elasticity ratio of the blend components may be achieved, obviating the need for added compatibilizers.52 Matrix phase polymers include a wide range of polymers. The volume of the matrix phase polymer is greater than about 65%. There is a wide range of suitable materials that may be used as filler material, such as carbon black, hydrated amorphous silica, fumed silica, fumed titanium

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539

dioxide, fumed aluminum oxide, diatomaceous earth, talc, and calcium carbonate. In general, blends may be formed by dispersing the filler material within the dispersed phase polymer to form a modified dispersed phase polymer, then dispersing the modified material within the matrix phase polymer. The first step is a melt blending of the filler with the dispersed phase polymer. The goal of the first step is to ensure that the filler is completely wetted by the polymer, and to take advantage of the strong interaction of the dispersed phase polymer and the filler surface. If the interactions are not sufficiently favorable, it may be desirable to pretreat the filler to ensure strong interactions between the filler and the dispersed phase so that the filler stays confined in the dispersed phase. The modified dispersed phase polymer is then mixed with the matrix phase polymer. Typically, this mixing occurs at a temperature which is less than the melting point of the matrix phase polymeric component. In the subsequent mixing step the components are well mixed in a melt mixing process. However, due to the strong interaction between the filler and the dispersed phase polymer, the filler stays substantially confined in the dispersed phase.

16.3.3 Modified Inorganic Fillers 16.3.3.1

Magnesium Hydroxide

For high molecular weight medium density poly(ethylene) (MDPE) compounds, the magnesium hydroxide filler can be surface-treated by fatty acid coatings. The surface treatment modifies the yield stress and modulus. Maxima are observed close to the monolayer coverage of the acid modifier. Acid-group terminated poly(ethylene) (ATPE) coatings produce the highest yield stress, as a result of physical interaction with the matrix polymer. The thermomechanical history during processing also modifies physical properties of MDPE/Mg(OH)(2) composites to some extent. Anisotropic effects include molecular orientation and filler particle alignment induced by shear stress during the injection molding process. The use of organo-acid coatings reduces polymer-particle surface interaction and thermodynamic work of adhesion, leading to an improved dispersion and enhanced mechanical properties.

540


16.3.3.2

Fumed Silica

The compatibility of PP/PS blends can be improved by the addition of nano-silica particles. Fumed SiO2 particles with a size of 10 to 30 nm are treated with octamethylcyclotetraoxysilane.53 Possible explanations for the compatibilization effect caused by the nano-silica particles have been pointed out: 1. The enhanced compatibility is caused by the adsorption of both PP and PS molecules onto the surface of the silica. 2. By the introduction of the particles, the viscosity changes. This causes a retardation of the coalescence of the dispersed PS particles. It seems that the compatibilization in PP/PS blends by fumed silica particles is controlled by the kinetics rather than by thermodynamics.

16.3.4 Clay Nanocomposites Maleic anhydride grafted poly(ethylene) (maleated poly(ethylene))/ clay nanocomposites can be prepared by simple melt compounding. The exfoliation and intercalation behaviors depend on the hydrophilicity of poly(ethylene) grafted with maleic anhydride and the chain length of organic modifier in the clay. When the number of methylene groups in the alkylamine acting as organic modifier is more than 16, an exfoliated nanocomposite is formed. Unmodified LLDPE shows only an intercalation, which does not depend on the initial spacing between clay layers.54 In a similar study another group used octadecylamine-modified montmorillonite clay.55

16.3.5 Thermoplastic Elastomers Thermoplastic elastomers with 50% poly(ethylene terephthalate) (PET), 30% compatibilizer, i.e., glycidyl methacrylate grafted rubber or glycidyl methacrylate-containing copolymer and 20% other rubbers, can be produced by melt blending with and without dicumyl peroxide initiated curing. The compatibility of the blend with PET is strongly improved when a nitrile rubber (NBR) with a high content of acrylonitrile and an ethylene/glycidylmethacrylate copolymer (EGMA) or an ethylene/propylene rubber grafted with GMA (EPR-g-GMA), are used as rubber and compatibilizer respectively.45

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541

The reactive compatibilization of poly(butylene terephthalate) (PBT) with an epoxide-containing rubber is influenced by the concentration of the reactive groups. The interfacial reaction is slow and not controlled by diffusion. The kinetics of the interfacial grafting only depends on the concentration of the reactive functions in the vicinity of the interface. The final particle size correlates to the amount of copolymer formed in-situ at the blend interface.56 Reactive compatibilization of a poly(butylene terephthalate) (PBT) and ethylene/vinyl acetate copolymer (EVA) can be achieved by maleic anhydride (MA). The graft copolymerization of EVA with MA is done, using dicumyl peroxide (DCP) as an initiator, by melt-free radical grafting. PBT is then blended with the EVA-g-MA obtained in the first step. The impact strength of PBT/EVA-g-MA (80/20) blend showed about threefold increase in comparison with a comparable PBT/EVA blend without compatibilizer.57

16.3.6 Polyamide 6,6 and Poly(butylene terephthalate) A bisphenol A-type solid epoxy resin is a low cost and efficient compatibilizer for immiscible and incompatible blends of polyamide 6,6 (PA66) and poly(butylene terephthalate) (PBT).36 The epoxy resin is able to form in-situ a PBT-co-Epoxy-co-PA66 mixed copolymer at the interface. This mixed copolymer with both segments structurally identical to both base polymers will anchor along the interface, and functions as an effective compatibilizer for the PA66/PBT blends.

16.3.7 Poly(ethylene)/Wood Flour Composites Functionalized polyolefins such as maleated and acrylic acid grafted polyethylenes, maleated poly(propylene) (PP-g-MA), and styrene-ethylene/butylene-styrene triblock copolymer (SEBS-g-MA) have been tested to reduce the interfacial tension between a poly(ethylene) matrix and the wood filler. Among these compounds, maleated linear low density poly(ethylene) shows a maximum tensile and impact strength of the composites. This is effected by the improved compatibility with the HDPE matrix.44

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16.3.8 Recycled Polyolefins Post-consumer high density poly(ethylene) (HDPE) has been investigated for use in recycling in large-scale injection moldings. Blends with recycled high density poly(ethylene) (re-HDPE) and low density poly(ethylene) (LDPE) or linear low density poly(ethylene) (LLDPE) are considered to improve the mechanical properties.58 The mechanical and rheological data show that LDPE is a better modifier for re-HDPE than LLDPE. The mechanical properties of re-HDPE/LLDPE blends are lower than the additive properties, i.e. they exhibit a negative synergism. This demonstrates the lack of compatibility between the blend components in the solid state. The mechanical properties of blends of recycled HDPE and LDPE are equal to or higher than calculated from linear additivity.

16.3.9 Block Copolymers Poly(styrene) (s-PS)/ethylene/propylene rubber (EPR) blends have been compatibilized with triblock copolymers such as poly(styrene)-b-poly(ethylene-co-butylene) poly(styrene) (SEBS).49 The size of the dispersed EPR phase in s-PS/EPR/SEBS blends decreases and the particle size distribution becomes narrower with increasing amounts of SEBS in the blends. Low molecular weight SEBS is more effective in increasing the impact strength of s-PS/EPR blend than a high molecular weight SEBS. This correlates with the fact that s-PS/EPR blends compatibilized by the low molecular weight SEBS have good adhesion between the s-PS matrix and dispersed EPR particles, whereas the s-PS/EPR blends compatibilized by the high molecular weight SEBS exhibit poor adhesion between phases. It is suggested that the blocks in the low molecular weight SEBS penetrate into the corresponding phase more easily than the blocks in the high molecular weight SEBS. The functionalization of a styrene-b-(ethylene-co-1-butene)-b-styrene triblock copolymer (SEBS) and styrene-co-butadiene (SBR) random copolymer takes place in the melt with diethyl maleate (DEM) and dicumyl peroxide (DCP) as initiator. Under these conditions, the functionalization proceeds with a large preference at the aliphatic carbons of the polyolefin block.59 The situation is similar, when maleic anhydride is used.60 Triblock copolymers of poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) or poly(styrene-b-(ethylene-co-butylene)) (SEB) can be used

Compatibilization

543

to compatibilize high density poly(ethylene) and syndiotactic poly(styrene) blends. The phase size of the dispersed s-PS particles is significantly reduced by the addition of all these copolymers, and the interfacial adhesion between the two phases is dramatically enhanced. The mechanical performance of the modified blends is dependent not only on the interfacial activity of the copolymers but also on the mechanical properties of the copolymers, in particular at a high copolymer concentration. The addition of compatibilizers to HDPE/s-PS blends results in a significant reduction in crystallinity of both HDPE and s-PS. The Vicat temperature of the blends indicates an improved heat resistance of the HDPE by addition of incorporation of 20% s-PS.61 The lack of adequate characterization techniques has been a hindrance to the effective exploitation and study of co-continuous morphologies in polymer blends. Blends of high density poly(ethylene) and poly(styrene) blends have been compatibilized with a triblock copolymer interfacial modifier. The influence of the triblock copolymer interfacial modifier, hydrogenated styrene-ethylene-butadiene-styrene, has been investigated by the measurement of the surface area and the pore dimensions of the blends after solvent extraction of one of the phases. The Brunauer Emmett Teller (BET) nitrogen adsorption technique and mercury porosimetry, respectively, have been used. Mercury porosimetry can lead to erroneous information, while the BET method appears to be both rapid and consistent with SEM observation. The specific surface area of the compatibilized co-continuous blend system is five-fold higher than that of its non-compatibilized counterpart, while the pore diameter of the extracted compatibilized blend is reduced five-fold. Using the BET technique, it is possible to generate an emulsification curve in the continuous region, demonstrating the efficiency of the interfacial modifier.62 The Brunauer Emmett Teller equation63 can be used to determine the surface area of solids. It takes into account a multiple layer absorption, in contrast to the earlier derived Langmuir adsorption isotherm. The volume of gas absorbed at the surface V is related to the pressure p applied by Eq. 16.4. 1 C−1 p p = + (16.4) V (p − p0 ) VmC VmC p0 Vm p0 C

Volume of a monomolecular layer Saturation pressure Constant related to activation energy of absorption and desorption

544


16.3.10 Impact Modification of Waste PET The impact properties of waste poly(ethylene terephthalate) can be improved by melt blending with a polyolefinic elastomer, in a co-rotating twin-screw extruder. The compositions have an elastomer content up to 10%. Poly(ethylene-co-acrylic acid) is a suitable compatibilizer for this system.64 The incorporation of polyolefinic elastomer improves the impact properties of PET significantly.

16.3.11 Starch Starch is a carbohydrate that is produced by many plants to store chemical energy. It is a polymer built from glucose. The main components are amylose which is a linear polymer and amylopectin which is branched.The ratio of amylose to amylopectin varies with origin. Most common are potato starch, maize starch, soybean starch, and rice starch. Sago is manufactured from the tree trunks of an Indian pomp tree. In grain form it is exported to Europe and America where it is used in the food industries.65 Cassava is also known as manioc, manihot, yucca, mandioca, sweet potato tree, and tapioca. It originates from tropical regions of America and is now cultivated in Africa and East Asia. It has a particular high starch content. The starch is collected from its tuberous roots. Tapioca starch is an important carbohydrate in tropical countries. When gelatinized it results in a highly cohesive paste. 16.3.11.1

Sago Starch

Sago starch is a possible filler for polyolefins. Sago starch can be chemically modified through esterification using 2-dodecen-1-yl succinic anhydride and propionic anhydride in solvents, such as N,N-dimethylformamide, triethylamine, or toluene. Evidence of anhydride modification was indicated by a weight gain of the material and was further confirmed by infrared spectroscopy. Starch modified with 2-dodecen-1-yl succinic anhydride and propionic anhydride can be used for the preparation of composites.66 In unmodified blends of starch and LLDPE, the tensile modulus and water absorption increase with increasing starch content. However, the tensile strength and the elongation at break show a decrease with increasing starch content. Modified starch shows improved

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545

mechanical properties and water absorption properties in comparison to unmodified starch. 16.3.11.2

Cassava and Tapioca Starch

Cassava starch can be chemically modified by radiation induced grafting with acrylic acid to obtain a cassava starch graft poly(acrylic acid). This product is further modified by esterification and etherification with poly(ethylene glycol) and propylene oxide, respectively. The chemical modifications of cassava starch cause it to become partially hydrophobic. It can be used for blending with LDPE.67 A functionalized epoxy resin, poly(ethylene-co-glycidyl methacrylate), can be used as a compatibilizer for blends of low density poly(ethylene) (LDPE) and tapioca starch. The mechanical properties are significantly improved by the addition of the epoxy compatibilizer, approaching values close to those of virgin LDPE. Scanning electron micrographs of the compatibilized blends show a ductile failure structure, which obviously contributes to the enhanced mechanical properties.18

16.3.12 Blends of Cellulose and Chitosan Blends of the naturally occurring polysaccharides, cellulose and chitosan, can be obtained in the solid phase by the combined action of high pressure and shear deformation. A diepoxide can act as a crosslinking agent, even when cellulose reacts with chitosan without the compatibilizer. The crosslinking agent reacts predominantly at the amino groups of chitosan, forming a three-dimensional network. The cellulose macromolecules are located within and partially bound with this network by the crosslinks. The formation of the network results in the insolubility of cellulose-chitosan compositions in acidic and alkaline aqueous media.68

16.4 REACTIVE COMPATIBILIZATION It is a common practice to blend existing polymers to obtain new materials, instead of searching for new monomers, which is often more costly and time-consuming. Addition of block copolymers or the use of functionalized homopolymers which can react to form copolymers in-situ is an effective method

546


CH3 C CH2 CH3 C H3C

NCO

TMI

Figure 16.1: 3-Isopropenyl-α,α-dimethylbenzene isocyanate (TMI)

for compatibilization of two immiscible phases in a polymer blend and prevention of coalescence. The role of compatibilization is to stabilize the morphology and modify the interfacial properties of the blend. This is achieved by adding or creating in-situ, during the blending process, a third component, often called an interfacial agent, emulsifier or compatibilizer.69 Reactive compatibilization allows generating the compatibilizer insitu at the interfaces directly during blending. The presence of a copolymer also accelerates the melting of polymer blends.70, 71 In the case of poly(propylene)/polyamide 6 (PA6), a graft copolymer can be formed easily during blending, if a fraction of the poly(propylene) chains are functionalized with a functional vinyl monomer such as maleic anhydride. The anhydride then reacts with the terminal amine groups of polyamide 6. Since the compatibilizer is formed in-situ at the interfaces, placing it at the interfaces is straightforward. In general, the functional groups must be stable enough under the process conditions, to withstand high temperature and exposure to air and humidity. Poly(propylene) can also be functionalized with 3-isopropenyl-α,α-dimethylbenzene isocyanate,69, 72 c.f. Figure 16.1. 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane (DHBP) is a suitable free-radical initiator for the functionalization of poly(propylene) with 3-isopropenyl-α,α-dimethylbenzene isocyanate (TMI). The free radical grafting is performed at 200°C. In the second step, an in-situ polymerization of ε-caprolactam in the presence of poly(propylene) and an in-situ compatibilization of the functional poly(propylene) with the PA6 occurs. Activator and catalyst for the

Compatibilization

547

polymerization of ε-caprolactam are sodium chloride and ε-caprolactam blocked hexamethylene diisocyanate. The compatibilizing efficiency is very high compared with that of the classical compatibilization method starting with a premade PP, PA6 and a maleic anhydride modified PP. In the classical compatibilization method, a copolymer of PP and PA6 is formed by an interfacial reaction between maleic anhydride functionalized PP and the terminal amine group of PA6. Thus, the amount of copolymer formation depends very much on the interfacial volume available in the system. This is usually very small for immiscible polymer pairs. On the other hand, when one polymer component is formed in-situ, the amount of the copolymer formation is no longer limited by the interfacial.69 Another type of reactive compatibilizer is 4,4′ -diphenylmethane carbodiimide (OCDI), 4,4′ -diphenylmethane bismaleimide (BMI), and 2,2′ -(1,4-phenylene)bisoxazoline (BOX). OCDI and BOX are chain extenders and react with the carbonyl groups of PA6. On the other hand, BMI has a lower reactivity. Grafting of BMI to PP chains improves the compatibility in a PA6/PP blend and increases PP adhesion to glass fiber.73 The functionalization of a propylene moiety with a bismaleimide is shown in Figure 16.2. Among acrylic acid (AA), bismaleimide (BMI) and maleic anhydride as compatibilizing agent for an isotactic poly(propylene) (IPP)/polyamide 6 blend, the effectiveness increases in the following order: IPP-AA < IPP-BMI 1.6 >1,8 AMPC Allyl-N-(4-methyl-phenyl)carbamate HCPA 4-Hydroxybutyl-2-chloro-2-phenylacetate NHCPA N-(2-Hydroxyethyl)-2-chloro-2-phenylacetamide BIAE 2-Bromoisobutyric acid ethylester a with respect to end groups Example 1: Initiator + end capping with allyl alcohol Examples 2 and 3: OH-functional initiator + end capping with the phenylurethane derivative of allyl alcohol Examples 4 and 6: Double end capping with the phenylurethane derivative of allyl alcohol Example 5: Double end capping with allyl alcohol.

Compatibilization

571

• Olefinically unsaturated dicarboxylic acids, e.g., fumaric acid, maleic acid, and itaconic acid and the corresponding di-tert-butyl esters, e.g., mono or di-tert-butyl fumarate and mono or di-tertbutyl maleate, • Olefinically unsaturated dicarboxylic anhydrides, e.g., maleic anhydride, sulfo- or sulfonyl-containing olefinically unsaturated monomers, e.g., p-styrenesulfonic acid, 2-methacrylamide2-methylpropenesulfonic acid or 2-sulfonyl(meth)acrylate, • Oxazolinyl-containing olefinically unsaturated monomers, e.g., vinyloxazolines and vinyloxazoline derivatives, and • Epoxy-containing olefinically unsaturated monomers, e.g., glycidyl (meth)acrylate or allyl glycidyl ether. The most common monomer for preparing functionalized poly(propylene) is maleic anhydride. Maleated poly(propylene) is commercially available. 16.5.3.2

Amine Functions in Poly(propylene)

A polyether monoamine containing ethylene oxide (EO) units and propylene oxide (PO) units is useful as a reactant with maleated poly(propylene) to form a reaction product that can be blended with poly(propylene).146 Generally, the polyether amines are made by aminating a polyol, such as a polyether polyol with ammonia in the presence of a catalyst such as the nickel-containing catalyst Ni/Cu/Cr. The mixing of the maleated poly(propylene) and polyetheramine may be carried out in a customary mixing apparatus including batch mixers, continuous mixers, kneaders, and extruders. For most applications, the preferred customary mixing apparatus is an extruder in which the polyetheramine is grafted onto the maleated poly(propylene). The residence time varies from about 25 to 300 seconds. The preferred temperature range is from about 190 to 260°C. Blends of poly(propylene), maleated poly(propylene), and Jeffamine™ M-2070 produced in an extruder exhibit the characteristics as shown in Table 16.4. Maleated poly(propylene) and polyether amine show improved paintability, improved impact resistance, and excellent mold flowability over blends of poly(propylene) and maleated poly(propylene).

572


Table 16.4: Properties of Poly(propylene) Blends with Poly(propylene) Modified Polyether Amines146 Example % MAL-PP % M2070 FM,[kpsi] StY, [psi] TE,% TSt [psi] NI [ft lb/in] UnI [ft lb/in] %MAL-PP % M2070 FM StY TE TSt NI UnI

16.5.3.3

1

2

3

4

5

6

20 20 20 30 30 30 0 2 4 0 2 4 284 255 226 289 256 201 8660 7980 7030 8750 7830 6170 8 16 10 4 13 5 4990 4770 4280 5000 4630 3720 0.161 0.220 0.386 0.123 0.139 0.220 12 14 10 10 14 5 % Maleated poly(propylene) % Jeffamine™ 2070 Rest filled with poly(propylene) to 100% Flexural modulus Stress at yield Tensile elongation Tensile strength Notched Izod impact Unnotched Izod impact

Amidoamine Functions in Poly(propylene)

Amidoamines can be obtained by reacting caprolactam, laurolactam or another cyclic lactam with a polyetheramine. The molar ratio of lactam to amine may vary in wide ranges. Water may be used to control the speed of the reaction and the molecular weight of the amidoamine product. The polyetheramines used to make the amidoamines are prepared from ethylene oxide and propylene oxide. Any combination of ethylene oxide and propylene oxide will work, however, the ratio of ethylene oxide to propylene oxide may be tailored to control the water absorption. The amount of ethylene oxide should be greater than about 90%. Maleated poly(propylene) is used for reaction of the amidoamines.147 The reaction takes place in an extruder in which the amidoamine reacts with the maleated poly(propylene) to form a reaction product at about 240°C to about 260°C. Blends of poly(propylene), maleated poly(propylene), and amidoamine can be produced in a single-screw extruder.

Compatibilization

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16.5.4 Muconic Acid Grafted Polyolefin Compatibilizers Muconic acid is also known as 2,4-hexadienedioic acid. cis,cis-muconic acid and cis,trans-muconic acid are commercially available. Due to its double bonds and diacid functionality, muconic acid can undergo a wide variety of reactions. Many muconic acid derivatives are known, including lactones, sulfones, polyamides, polyesters, thioesters, addition polymers, and other compounds. Such compounds have a wide variety of uses, including use as surfactants, flame retardants, UV light stabilizers, thermoset plastics, thermoplastics, and coatings. Muconic acid units grafted onto a polyolefin backbone are compatibilizers. The muconic acid group itself may have special advantages in the reactive compatibilization of certain polymers due to its particular chemical properties compared to other functional groups.148 To manufacture the compatibilizer, the polyolefin is melt extruded with muconic acid at a temperature in the range of about 180°C to 220°C. A suitable initiator is Lupersol™ 130, an organic peroxide free-radical initiator containing 2,4-dimethyl-2,5-di(tert-butylperoxy)-3-hexyne. When the polyolefin and muconic acid are mixed and free radical addition is initiated a hydrogen atom on a polyolefin carbon atom (either on the main chain or on a side group) is replaced by a muconic acid side group; the muconic acid loses one of its double bonds as one of its carbon atoms bonds to the polyolefin carbon atom in place of the lost hydrogen and the muconic acid side group picks up another hydrogen atom. Only little polymer degradation during muconic acid grafting compared to the known degradation produced by grafting acrylic acid and other units onto polyolefins is observed. Muconic acid graft copolymers exhibit a greater intrinsic viscosity retention than acrylic acid graft copolymers. Muconic acid graft copolymers are also far more ductile.

16.5.5 Polyfunctional Polymers and Modified Polyolefin By the reaction of a polyfunctional polymer with a modified polyolefin, crosslinked products may be formed. A copolymer of vinyloxazoline or 2-isopropenyl-2-oxazoline (IPO) and styrene produced by Dow, containing ca. 1% oxazoline, has been used for the reaction with carboxylic acid functional polyolefins.149, 150 A copolymer of styrene and 2-isopropenyl2-oxazoline (SIPO) and a copolymer of ethylene and acrylic acid have been melt blended at 280°C in an extruder.151

574


C

N CH 2 O CH2

+

O

O HO C

O

C N CH2

CH2

O C

H

Figure 16.4: Reaction of Oxazoline and Carboxylic Acid152

Other suitable polymers, being reactive with the oxazoline group, include those which contain amine, carboxylic acid, hydroxyl, epoxy, mercaptan, and anhydride in the polymer chain or as end groups. Examples are SIPO and a high density poly(ethylene)/maleic anhydride graft copolymer (HDPE/MA), a styrene/acrylonitrile/IPO terpolymer (SANIPO), and a propylene/acrylic acid copolymer (PAA) with /6acrylic acid, 75% SIPO, and 25% of a carboxylated polyester resin, sold as Vitel™ VPE6434, SIPO and a vinylidene chloride/methacrylic acid copolymer with 1% methacrylic acid.153 The reaction of the oxazoline group with a carboxylic acid group is shown in Figure 16.4.

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110. J. H. Wang and D. M. Schertz. Reactive extrusion method of making inverse phase blends of poly(ethylene oxide) and polyolefin. US Patent 6 225 406, assigned to Kimberly-Clark Worldwide, Inc. (Neenah, WI), May 1 2001. 111. T. G. Gopakumar, S. Ponrathnam, A. Lele, C. R. Rajan, and A. Fradet. In situ compatibilisation of poly(phenylene sulphide)/wholly aromatic thermotropic liquid crystalline polymer blends by reactive extrusion: morphology, thermal and mechanical properties. Polymer, 40(2):357–364, 1999. 112. F. J. Rodriguez-Gonzalez, B. A. Ramsay, and B. D. Favis. High performance LDPE/thermoplastic starch blends: a sustainable alternative to pure polyethylene. Polymer, 44(5):1517–1526, March 2003. 113. T. Moriyama, H. Takida, and T. Uemura. Saponified ethylene-vinyl acetate copolymer composition and the use thereof. US Patent 5 310 788, assigned to Nippon Gohsei Kagaku Kogyo Kabushiki Kaisha (Osaka, JP), May 10 1994. 114. B. G. Soares, F. F. Alves, M. G. Oliveira, A. C. F. Moreira, F. G. Garcia, and M. S. Lopes. The compatibilization of SBR/EVA by mercapto-modified EVA. Eur. Polym. J., 37(8):1577–1585, August 2001. 115. A. S. Sirqueira and B. G. Soares. The effect of mercapto- and thioacetatemodified EPDM on the curing parameters and mechanical properties of natural rubber/EPDM blends. Eur. Polym. J., 39(12):2283–2290, December 2003. 116. M. G. Oliveira, A. C. O. Gomes, M. S. M. Almeida, and B. G. Soares. Reactive compatibilization of NBR/EPDM blends by the combination of mercapto and oxazoline groups. Macromol. Chem. Phys., 205(4):465–475, March 2004. 117. M. G. Oliveira and B. G. Soares. Compatibilization of nitrile-butadiene rubber/ethylene-propylene-diene monomer blends by mercapto-modified ethylene-vinyl acetate copolymers. J. Appl. Polym. Sci., 91(3):1404–1412, February 2004. 118. B. G. Soares, A. S. Sirqueira, M. G. Oliveira, and M. S. M. Almeida. The reactive compatibilization of EPDM-based elastomer blends. Kautsch. Gummi Kunstst., 55(9):454–459, September 2002. 119. B. G. Soares, A. S. Sirqueira, M. G. Oliveira, and M. S. M. Almeida. Compatibilization of elastomer-based blends. Macromol. Symp., 189:45–58, November 2002. 120. B. G. Soares. Reactive compatibilization of nitrile rubber/EPDM blends. Kautsch. Gummi Kunstst., 56(7-8):396–400, July–August 2003. 121. R. Scaffalo, F. P. La Mantia, R. Bertani, and A. Sassi. Compatibilization of PA6/rubber blends by using an oxazoline functionalized rubber. Macromol. Symp., 202:67–76, September 2003. 122. N. Moussaif, C. Pagnoulle, J. Riga, and R. Jerôme. XPS analysis of the PC/PVDF interface modified by PMMA. location of the PMMA at the interface. Polymer, 41(9):3391–3394, April 2000.

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123. G. S. Wildes, T. Harada, H. Keskkula, D. R. Paul, V. Janarthanan, and A. R. Padwa. Synthesis and characterization of an amine-functional SAN for the compatibilization of PC/ABS blends. Polymer, 40(11):3069–3082, May 1999. 124. S. Saikrasun, T. Amornsakchai, C. Sirisinha, W. Meesiri, and S. BualekLimcharoen. Kevlar reinforcement of polyolefin-based thermoplastic elastomer. Polymer, 40(23):6437–6442, November 1999. 125. B. De Roover, J. Devaux, and R. Legras. PAmXD,6/PP-g-MA blends. I. compatibilization. J. Polym. Sci., Part A-1: Polym. Chem., 35:901–915, 1997. 126. B. De Roover, J. Devaux, and R. Legras. PAmXD,6/PP-g-MA blends. II. rheology and phase inversion location. J. Polym. Sci., Part A-1: Polym. Chem., 35:917–925, 1997. 127. B. De Roover, J. Devaux, and R. Legras. PAmXD,6/PP-g-MA blends. III. microstructure, blend melt viscosity, and copolymer concentration relationship. J. Polym. Sci., Part A-1: Polym. Chem., 35:1313–1327, 1997. 128. T. X. Xie and G. S. Yang. Effects of maleated styrene-(ethylene-co-butene)styrene on compatibilization and properties of nylon-12,12/nylon6 blends. J. Appl. Polym. Sci., 93(3):1446–1453, August 2004. 129. O. Okada, H. Keskkula, and D. R. Paul. Nylon 6 as a modifier for maleated ethylene-propylene elastomers. Polymer, 40(10):2699–2709, May 1999. 130. S. G. Cottis and K. M. Natarajan. In situ compatibilization of PPE/polyethylene copolymer blends. US Patent 5 286 793, assigned to Istituto Guido Donegani (Milan, IT); Enichem America, Inc. (Monmouth Junction, NJ), February 15 1994. 131. H. A. M. van Aert, G. J. M. van Steenpaal, L. Nelissen, P. J. Lemstra, J. Liska, and C. Bailly. Reactive compatibilization of blends of poly(2,6-dimethyl-1,4-phenylene ether) and poly(butylene terephthalate). Polymer, 42(7):2803–2813, March 2001. 132. K. Abe, S.-I. Yamauchi, and A. Ohkubo. Polyphenylene ether resin composition. US Patent 4 460 743, assigned to Mitsubishi Petrochemical Co., Ltd. (Tokyo, JP), July 17 1984. 133. S. Togo, A. Amagai, Y. Kondo, and T. Yamada. Solvent-resistant polyphenylene ether resin composition. US Patent 4 914 153, assigned to Mitsubishi Gas Chemical Company, Inc. (Tokyo, JP), April 3 1990. 134. C. K. Samios, K. G. Gravalos, and N. K. Kalfoglou. In situ compatibilization of polyurethane with poly(ethylene terephthalate). Eur. Polym. J., 36(5):937–947, May 2000. 135. G. H. Hu and M. Lambla. Chemical reactions between immiscible polymers in the melt: Transesterification of poly(ethylene-co-methylacrylate) with mono-hydroxylated polystyrenes. J. Polym. Sci., Part A-1: Polym. Chem., 33(1):97–107, January 1995.

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136. G.-H. Hu and M. Lambla. Catalysis and reactivity of the transesterification of ethylene and alkyl acrylate copolymers in solution and in the melt. Polymer, 35(14):3082–3090, July 1994. 137. E. Rizzardo and G. Moad. Living radical polymerization. In J. C. Salamone, editor, The Polymeric Materials Encyclopaedia: Synthesis, Properties and Applications, pages 795–797. CRC Press, Boca Raton, FL, 1999. 138. B. J. C., D. Derouet, F. Epaillard, J. C. Soutif, G. Legeay, and K. Dušek. Hydroxyl-terminated polymers obtained by free-radical polymerization - synthesis, characterization, and applications. Adv. Polym. Sci., 81:167–223, 1986. 139. B. Ameduri, B. Boutevin, and P. Gramain. Synthesis of block copolymers by radical polymerization and telomerization. Adv. Polym. Sci., 127:87–142, 1997. 140. M. Melchiors, D. Margotte, H. Höcker, H. Keul, and A. Neumann. Process for the production of telechelic polymers, telechelic polymers produced in this manner and use thereof. US Patent 6 455 645, assigned to Bayer Aktiengesellschaft (Leverkusen, DE), September 24 2002. 141. J. Wootthikanokkhan and B. Tongrubbai. Compatibilization efficacy of poly(isoprene-butyl acrylate) block copolymers in natural/acrylic rubber blends. J. Appl. Polym. Sci., 88(4):921–927, April 2003. 142. M. F. Diaz, S. E. Barbosa, and N. J. Capiati. Polypropylene/polystyrene blends: In situ compatibilization by Friedel-Crafts alkylation reaction. J. Polym. Sci., Part. B: Polym. Phys., 42(3):452–462, February 2004. 143. Y.-J. Sun, R. J. G. Willemse, T. M. Liu, and W. E. Baker. In situ compatibilization of polyolefin and polystyrene using Friedel-Crafts alkylation through reactive extrusion. Polymer, 39(11):2201–2208, 1998. 144. Y. Gao, H. L. Huang, Z. H. Yao, D. Shi, Z. Ke, and J. H. Yin. Morphology, structure, and properties of in situ compatibilized linear low-density polyethylene/polystyrene and linear low-density polyethylene/high-impact polystyrene blends. J. Polym. Sci., Part. B: Polym. Phys., 41(15): 1837–1849, August 2003. 145. R. Mülhaupt, T. Duschek, and B. Rieger. Functional polypropylene blend compatibilizers. Macromol. Symp., 48-49:317–332, 1991. 146. R. K. Evans, R. J. G. Dominguez, and C. R. J. Polyether monoamine with 36-44 EO units and 1-6 PO units. US Patent 6 465 606, assigned to Huntsman Petrochemical Corporation (Austin, TX), October 15 2002. 147. R. J. Clark. Amidoamine modification of polypropylene. US Patent 5 668 217, assigned to Huntsman Petrochemical Corporation (Austin, TX), September 16 1997. 148. P. N. Chen, Sr., M. M. Glick, M. M. Jaffe, and A. Forschirm. Muconic acid grafted polyolefin compatibilizers. US Patent 5 173 541, assigned to Hoechst Celanese Corp. (Somerville, DE), December 22 1997.

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17 Rheology Control 17.1 MELT FLOW RATE The technical terms melt flow index (MFI), melt flow rate (MFR), and melt flow number (MFN) are used synonymously. Throughout the text we prefer to use melt flow rate. The melt flow rate is the measure of a polymer’s ability to flow under certain conditions. It measures a melt flow rate, which is the amount of polymer that flows over a period of time under specified conditions. Typical melt flow units of measurement are dg/min. Melt flow provides an indication of the resin’s processability, such as in extrusion or molding, where it is necessary to soften or melt the polymer.1

17.2 RHEOLOGY CONTROL TECHNIQUES High melt flow rate poly(propylene) can be produced directly in a polymerization reactor, but its production is often limited by the solubility of hydrogen in the reaction. Hydrogen is the most effective chain transfer agent for propylene polymerization reactions, whether the reaction takes place in solution or in the bulk monomer.2 Another method for producing high melt flow rate poly(propylene) is to degrade low melt flow rate poly(propylene) using controlled rheology. Controlled rheology treatments are often employed as alternative techniques for producing high melt flow rate poly(propylene) because these treatments do not depend on hydrogen solubility. 587

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Controlled rheology treatments can also be used to increase production efficiency by converting the low melt flow rate polymers into high melt flow rate polymers without changing the reactors operating conditions. Thus, many manufacturers prefer controlled rheology treatments to produce high melt flow rate polymers. Poly(α-olefins), particularly poly(propylene), may have its weightaverage molecular weight decreased substantially, or its melt flow rate substantially increased, by controlled degradation of the polymer. This may be accomplished by 1. Reaction of the polymer with free radicals or free radical-producing agents such as peroxides, 2. Heat treatment, and 3. Subjecting the polymer to high shear,3 or combinations of these methods. The effect attained is that the polymer molecule scission occurs, resulting in an overall lowered molecular weight or elevated MFR. Early techniques have been developed to degrade and to narrow the molecular weight using high shear gradients at temperatures between the melting point and the temperature at which purely thermal degradation of the polyolefin occurs.3 The degradation of the polyolefin can be achieved by a metal salt catalyst.4 A crystalline polyolefin is mixed with a metal salt of a carboxylic acid, and the resultant mixture is heated in an atmosphere which is substantially free of oxygen to a temperature of 275 to 450°C. Also an organic anhydride catalyst is suitable for degradation of polyolefins at 200 to 400°C.5 The controlled oxidative degradation of propylene polymers has been further proposed by injecting oxygen or an oxygen-containing gas and an organic or inorganic peroxide. Next the melt is subjected to a high shear. An essentially odor-free propylene polymer can be recovered with a melt flow rate higher than that of the feed polymer.6 The addition and reaction of a peroxide with polymer is well known in the industry and is known generally as vis-breaking or peroxide degradation.7 Polymer resins produced with a low melt flow may need to be further modified after their initial polymerization to improve their processability. This is typically done through controlled rheology (CR) techniques

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wherein the molecular weight of the polymer is lowered, usually by the addition of peroxide, to improve its flowability. This secondary processing, however, adds additional processing steps and increases the cost of manufacturing. Controlled rheology processing may also degrade the polymer and leave peroxide residue so that its use may be limited in certain applications.

17.2.1 Pelletizing While vis-breaking is useful to the finishing of the polymer, it creates a need for an extra process step and adds expense to the process in equipment and process requirements. Provision of vis-breaking or controlled rheology (CR) process initiators prior to or during pelletization is a complex operation. Typical pelletizing equipment operates at shear rates or temperatures sufficient to trigger molecular degradation. However, molecular degradation should not take place during pelletizing. Instead, it is desirable that vis-breaking is the last process step prior to the final polymer transformation into its desired product. If vis-breaking already occurs in a preprocess, the material would result in a low-viscosity, sticky mass rather than discreet easily to handle pellets. This problem can be solved if the preparation of the pellets starts with a lower initial molecular weight polymer, or higher melt flow rate polymer. Such a material will generate less frictional heat in compounding into pellets. This means that the viscous dissipation of the heat will cause less peroxide activation and allow pelletizing at generally lower temperatures or at longer exposure periods. Using lower weight-average molecular weight Mw , or higher melt flow rate, the starting material requires less of the vis-breaking agent, such as peroxide, to reach the desired very high or ultra-high melt flow rates. A further benefit is the formation of less of the undesirable by-products of peroxide degradation. Poly(propylene) with 30 to 33 g/min MFR and with xylene solubles of about 2 to 6% material is compounded with peroxides and 0.025% calcium stearate. To obtain a polymer with MFR in the range of 120 to 150 g/min, about 1,800 to 2,000 ppm of peroxide will be added. The inclusion of other additives can be accomplished in a continuous blender in a separate step. The dry-blended material plus additive

590


is further compounded and pelletized under time and conditions where the peroxide would not react to decompose the polymer significantly. By keeping the residence time short and the temperature low to prevent significant molecular scission or degradation, the polymer can be neatly and cleanly pelletized. Thus, a useful, easily handled, and convenient pelletized product is created.7

17.3 PEROXIDES FOR RHEOLOGY CONTROL The technique for controlling the rheology of homopolymers and copolymers of poly(propylene) consists of peroxide degradation of these polymers. It is used to develop fluid products in an efficient way without having a detrimental effect in terms of production flow rates by reducing the number of basic polymerization powders. It is also possible to melt a propylene homopolymer or copolymer powder and to incorporate in it a peroxide before the extrusion followed by granulation. Peroxide radicals can cause chain scission resulting in shorter polymer chains, which increases the melt flow rate of the polymer. Such modification also causes a decrease in the flexural modulus versus non-degraded polymer of similar final melt flow rate. The drawback of this process is the fact that these products have mechanical properties, strength and shock resistance that are weaker than those of a product that is obtained directly after polymerization, extrusion and granulation, or a powder that has been extruded and pelletized for a second time. Actually, a poly(propylene) resin that is degraded by a peroxide may contain peroxide radicals, thus runs the risk of modifying the viscosity of the resin when it is processed at elevated temperatures. During this transformation, the peroxide again degrades the resin to reduce its viscosity. Now, during storage, the peroxide has the tendency to migrate and therefore to leave the resin. Thus, during the storage period, the resin may have a different behavior and show a viscosity that is different during or after processing, depending on whether there is a little or a lot of peroxide.8

17.3.1 Hydroperoxides Hydroperoxides for rheology control are shown in Table 17.1.

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Table 17.1: Hydroperoxides for Controlled Rheology8 Hydroperoxide

Remarks

tert-Butyl hydroperoxide tert-Amyl hydroperoxide Pinane hydroperoxide Cumene hydroperoxide 2,5-Dimethyl-2,5-di(hydroperoxy)hexane Diisopropylbenzene mono hydroperoxide

Most common Common

17.3.2 Peroxides Peroxides for rheology control are shown in Table 17.2. 17.3.2.1

4-(tert-Amylperoxy)-4-methyl-2-pentanol

4-(tert-Amylperoxy)-4-methyl-2-pentanol has over the years found utility as a reactant or a reaction catalyst which made use of its hydroxy functionality for various purposes.9 17.3.2.2

DHBP

Over time, because of its safety in handling and decomposition temperature, one specific peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DHBP) also known as LUPEROX™ 101, Trigonox™ 101, and Interox™, has become the industry standard for poly(propylene) modification.9 DHBP is a liquid, so the dosage and admixture are more comfortably compared to solid peroxides. Its half-life time is 5.9 s at 200°C. Typical decomposition side products of DHBP are considered to be acceptable for using it as an additive for food packages. 17.3.2.3

Di-tert-butyl peroxide

Di-tert-butyl peroxide (DTBP) has a particularly simple structure and from the commercial point of view is the most advantageous of these peroxides. However, it has high volatility and its use is therefore restricted. DTBP is only added in low concentrations, in the form of a master batch with a solid carrier. In addition, its ignition point is between 48 and 55°C, even under nitrogen. Safety issues in its use are therefore problematic. DTBP can be fed as a liquid via metering pumps to the extruder.

592


Table 17.2: Peroxides for Controlled Rheology8 Peroxide Dibenzoyl peroxide p-Chlorobenzoyl peroxide Lauroyl peroxide (Dodecanoyl peroxide) Decanoyl peroxide 3,5,5-Trimethylhexanoyl peroxide Acetyl peroxide 2,5-Dimethyl-2,5-di(benzoylperoxy)hexane 2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane 2,2-Di(tert-butylperoxy)butane 2,2-Di(tert-amyl)peroxypropane 4-(tert-Amylperoxy)-4-methyl-2-pentanol 1,1-Di(tert-butylperoxy)cyclohexane 1,1-Di(tert-butylperoxy)-3,3,5-trimethylcyclohexane 1,1-Di(tert-amylperoxy)cyclohexane 2,2-Bis(4,4-di-tert-butylperoxycyclohexyl)propane 2,5-Dimethyl-2,5-di(tert-butylperoxy)-3-hexyne Di-tert-butyl peroxide Di-tert-amyl peroxide 1,4-Di(tert-butylperoxyisopropyl)benzene 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane 1,1,4,4,7,7-Hexamethylcyclo-4,7-diperoxynonane 3,3,6,6,9,9-Hexamethylcyclo-1,2,4,5-tetraoxanonane 3,6,6,9,9-Pentamethyl-3-n-propyl-1,2,4,5-tetraoxacyclononane 3,6,6,9,9-Pentamethyl-3-(ethyl acetate)1,2,4,5-tetraoxacyclononane 3-Phenyl-3-tert-butylperoxyphthalide

Remarks

10 9

Lupersol™ 130

Trigonox™ 101 10

USP-138™

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593

However, when the peroxide is added as a liquid to the extruder, disadvantages are often encountered in relation to polymer properties, in particular the film properties of degraded propylene polymers. There is the danger of explosion within the extruder. Gaseous DTBP is capable of exploding even in an inert gas atmosphere. If a gas explosion of this type extends to involve liquid peroxide, wherever it is present it can damage the extruder.11 Other compounds of this type which are more expensive but easier to handle are frequently used in industrial applications.

17.3.3 Diacyl Peroxides Diacyl peroxides and hydroperoxides often exhibit an induced decomposition. Acyl peroxides decompose into acyloxy radicals. These radicals undergo β-scission very fast to give the corresponding alkyl radical or aryl radical and eject carbon dioxide. Therefore, the acyloxy group is not observed in the decomposition products.

17.3.4 Ketone Peroxides Methylethylketone peroxide and methylisobutylketone peroxide are known to be mixtures of several different ketone peroxide compounds, among which the noncyclic ketone peroxides predominate. However, these ketone peroxides do contain some small quantities of cyclic ketone peroxides which result from side reactions during the preparation of the methylethyl and methylisobutylketone peroxides. For example, in commercially available methylethylketone peroxides about 1 to 4% of the total active oxygen content is attributable to cyclic ketone peroxides. 17.3.4.1

Cyclic Ketone Peroxides

The cyclic ketone peroxides are exceptionally well suited for use in the modification of polymers. In general, the cyclic ketone peroxide trimers are less volatile and more reactive than the corresponding dimers. Cyclic peroxides can be made by reacting a ketone with hydrogen peroxide. Suitable ketones for use in the synthesis of the cyclic peroxides include methylethylketone, methylisobutylketone, diethylketone, and methylisopropylketone. Therefore, examples for cyclic peroxides are

594


CH3

CH3

CH3

H3C C O O C CH2 CH3

CH2

CH3

CH3

CH3

C O O C CH3 CH3

2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane H5C2 C

O

O

H3C O

CH3 C C2H5 O

O H3C

C

O C2H5

3,6,9-Triethyl-3,6,9-trimethyl-1,4,7-triperoxonane

Figure 17.1: 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane and 3,6,9-Triethyl-3,6,9-trimethyl-1,4,7-triperoxonane

cyclic methylethylketone peroxide, cyclic methylisobutylketone peroxide, cyclic diethylketone peroxide, and cyclic methylisopropylketone peroxide. Cyclic ketone peroxides are composed of at least two ketone peroxide entities which may be the same or different. Thus, cyclic ketone peroxides may exist in the form of dimers, trimers, etc. When cyclic ketone peroxides are prepared, a mixture usually is formed which predominantly exists of the dimeric and trimeric forms. The ratio between the various forms mainly depends on the reaction conditions during the preparation. The peroxides can be prepared, transported, stored, and applied as such or in the form of powders, granules, pellets, pastilles, flakes, slabs, pastes, and solutions. These formulations may optionally be phlegmatized, as necessary, depending on the particular peroxide and its concentration in the formulation . Other examples for cyclic ketone peroxides, are,12 e.g., 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, c.f. Figure 17.1. Cyclic ketone peroxides give a much higher degree of poly(propylene) degradation than their non-cyclic ketone peroxide counterparts. The degradation of polyolefins with cyclic ketone peroxides results in less yellowing than comparable processes employing their non-cyclic ketone peroxides. The principal advantage of these prod-

Rheology Control

595

ucts is that they do not produce tert-butanol as a decomposition by-product.

17.3.5 Masterbatches of Peroxides Masterbatching can be used to facilitate the process mixing the peroxide with the polyolefin. Masterbatching refers to a process of adding a small amount of poly(propylene), which has an organic peroxide and/or other additives within it, to a larger amount of poly(propylene) and subsequently blending and extruding in order to achieve the desired poly(propylene) characteristics. A problem in masterbatching is the melt blending of large amounts of peroxide into poly(propylene). This is difficult because the peroxide tends to decompose during the melt blending step. While some of the peroxide can survive the melt blending step, at least some degrades the poly(propylene). Another problem with mixing solid poly(propylene) pellets, flakes, or powder with a liquid organic peroxide is that the poly(propylene) does not usually form a homogeneous, free-flowing phase with the liquid organic peroxide. Usually, an absorbent, such as silica, is added to the poly(propylene) in order to facilitate the addition of organic peroxide in the masterbatching process. However, an absorbent which is added to the poly(propylene) can interfere with the processing of the poly(propylene) material. Therefore, poly(propylene) which could absorb liquid organic peroxide without the necessity of using other absorbents, such as silica, is of great economic and scientific value. A free-flowing material typically contains 80 to 90% of poly(propylene) and 10 to 20% of liquid organic peroxide. The organic peroxide used can be any liquid organic peroxide, for example: 2,5-dimethyl-2,5-di(tertbutylperoxy)-3-hexyne, dicumylperoxide or 2,5-dimethyl-2,5-di-tert-butylperoxyhexane.13

17.3.6 Peresters Peresters for controlled rheology are shown in Table 17.3. Peresters decompose into acyloxy and alkoxy radicals.

17.3.7 Properties of Peroxides Peroxides used in industrial applications are shown in Table 17.4.

596


Table 17.3: Peresters for Controlled Rheology8 Peresters tert-Butylperoxybenzoate tert-Butylperoxyacetate tert-Butylperoxy-3,5,5-trimethylhexanoate O,O-tert-Butyl-O-isopropyl monoperoxy carbonate O,O-tert-Butyl-O-(2-ethylhexyl)monoperoxy carbonate O,O-tert-Amyl-O-(2-ethylhexyl)monoperoxy carbonate tert-Butylperoxyisobutyrate tert-Butylperoxy-2-ethylhexanoate tert-Amylperoxy-2-ethylhexanoate tert-Butylperoxypivalate tert-Amylperoxypivalate tert-Butylperoxyneodecanoate tert-Butylperoxyisononanoate 2,5-Dimethylhexene-2,5-diperoxyisononanoate tert-Amylperoxyneodecanoate α-Cumylperoxyneodecanoate 3-Hydroxy-1,1-dimethylbutylperoxyneodecanoate tert-Butylperoxymaleate Ethyl-3,3-di(tert-butylperoxy)butyrate Ethyl-3,3-di(tert-amylperoxy)butyrate n-Butyl-4,4-di(tert-butylperoxy)valerate Di(2-ethylhexyl)peroxydicarbonate Dicyclohexylperoxydicarbonate

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597

Table 17.4: Industrial Used Peroxides for Controlled Rheology and Crosslinking14 Peroxide

Remarks

2,5-Dimethyl-2,5-di(tert-butylperoxy)3-hexyne 2,5-di(tert-Butylperoxy)hexyne Di(2-tert-butylperoxyisopropyl)benzene Dicumyl hydroperoxide tert-Butyl hydroperoxide 1,1-Di(tert-butylperoxy)-3,3,5-trimethylcyclohexane α,α′ -Bis(tert-butylperoxy)diisopropyl benzene

Lupersol™ 130, Mackine™ 201 DYBP™ Perkadox™ 14-40 Perkadox™ BC-FF

Peroxide for Other Uses Dibenzoyl peroxide MEK peroxide Dicumyl peroxide tert-Butylperoxybenzoate Lauroyl peroxide (Dodecanoyl peroxide)

Remarks Curing Curing Vulcanizing agent Esperox ™ 10 Laurox ™ W-25 Polymerization initiator

1,3-Di(2-tert-butylperoxyisopropyl)benzene Methylisobutylketone peroxide Methylethylketone peroxide

Varox™ 231-XL Varox™ VC-R vulcanization or crosslinking of elastomers

Trigonox ™ HM curing of unsaturated polyester resins Curing of unsaturated polyester resins

598


CH3

CH3

C O O C CH3

CH3

CH3

CH3

C O* CH3

*O C CH3

O 2

C

*CH3

CH3

Figure 17.2: Decomposition of Dicumylperoxide15

Some peroxides suffer from excessively long half-lifes. However, the half-life of the initiator should be shorter than the residence time of the resin in the extruder. A long half-life is undesirable because it will lead to product quality problems due to residual peroxide in resin, or lower productivity or higher resin color depending on the amount of undecomposed peroxide in the resin. These include longer residence times or higher temperatures in the extruder.9 Often the use of safety diluent is required. Diluents are undesirable in at least some poly(propylene) grades because they may produce smoking or dripping in an end user’s extruder. It has been reported that diluents are also undesirable for fiber or film grades where, for example, they may adversely affect the feel of the surface. 17.3.7.1

Mechanism of Decomposition

Peroxides decompose in a rather complicated way in a multistep reaction. The mechanism of decomposition of dicumylperoxide is shown in Figure 17.2. The mechanism of decomposition of dicumylperoxide is shown in Figure 17.3.

Rheology Control

CH3 CH3

CH3

CH3 CH3

CH3

O 4 *CH3 2 H3C C CH3

CH3

C O O C CH3

C O O C CH3

599

CH3

O

O C

C

CH3

CH3

Figure 17.3: Decomposition of 1,4-Di(tert-butylperoxyisopropyl)benzene15

17.3.7.2

Kinetics of Decomposition

Most studies on the decomposition of peroxides have been done in dilute solutions at low temperatures at which only small concentrations of radicals occur at low pressures. The conditions under which grafting occurs in the extruder are different in these aspects: • High temperatures, • High pressures, • High viscous environment. For these reasons there is not much knowledge concerning the radical reaction in the extruder. However there are some qualitative statements. High temperatures decrease the selectivity of radical reactions. High pressures reduce the tendency of chain scission. In high viscous media, diffusion controlled reactions are significantly slower than in low viscous solutions. From kinetic constants it may be concluded that tert-alkoxy radicals favor the abstraction of hydrogen atoms rather than the addition on vinyl groups. This tendency is enhanced at higher temperatures.16, 17 17.3.7.3

Half-life of Peroxides

The half-life of peroxides listed is shown in Table 17.5. If the residence time is in the range of five half-life times, then the decomposition of the peroxide will reach more than 97%. If the half-life time is very short in

600

Reactive Polymers Fundamentals and Applications Table 17.5: Half-life of Peroxides10

Peroxide

°C a,b

°C a,c

LUPEROX 101 2,2-Di(tert-amylperoxy)propane Di-tert-amyl peroxide MEK cyclic trimer 4-(tert-amylperoxy)-4-methyl-2-pentanol a One hour half-life time at the temperature specified b in dodecane c in poly(propylene)

140 128 143 – 141

145 – – 158 –

Table 17.6: Flash Points of Peroxides10 Peroxide

Flash point [°C a ]

LUPEROX 101 (92% assay) LUPEROX 101 (95% assay) Di-tert-amyl peroxide 4-(tert-Amylperoxy)-4-methyl-2-pentanol a Performed with ASTM D3278 b Depending on preparation

49 78 25a >60

comparison to the residence time, then the peroxide decomposes to a great extent in the initial stage of the process. This results in high concentrations of radicals, and secondary radicals in the polymer backbone, which may result in an enhanced crosslinking. The majority of today’s production processes require that the peroxide be mixed with solid poly(propylene) in a blender. Under such conditions, it is crucial that the peroxide has a high flash point for safety. Flash points can be determined using the small scale closed cup method (ASTM D3278). Flash points of commercially available peroxides are shown in Table 17.6

17.3.8 Azo Compounds Azo compounds are shown in Table 17.7. Azo compounds are advantageous over peroxides in that they show no or much less induced decomposition. However, the most common azonitriles, e.g., AIBN decompose too quickly at the required temperatures. In addition, the cyanoalkyl radicals

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601

Table 17.7: Azo Compounds for Controlled Rheology8 Azo Compounds 2,2′ -Azobis(2-acetoxy)propane 2,2′ -Azobis(isobutyronitrile) (AIBN) 2,2′ -Azobis(2,4-dimethylvaleronitrile) 2,2′ -Azobis(cyclohexanenitrile) 2,2′ -Azobis(2-methylbutyronitrile) 2,2′ -Azobis(2,4-dimethyl-4-methoxyvaleronitrile)

are comparatively unreactive to abstract hydrogen from a polyolefin.

17.4 SCAVENGERS 17.4.1 Stable Nitroxyl Radicals Incorporation of stable radicals that are always present after extrusion provides a better thermal stability to the products that are obtained, improves the UV resistance of the latter and reduces their tendency to depolymerize. In the case where a peroxide is also incorporated into the resin, the latter has a more stable viscosity over time because of comprising a reservoir of heat-reacting counter-radicals. However, the resin contains a reservoir of stable free radicals that have the tendency to neutralize the peroxide as soon as the latter is breaks down, thus reducing its degradation effects, regardless of whether its concentration is high or low. The storage time thus no longer has as much effect on the viscosity of the transformed resin.8 Stable nitroxyl radicals are shown in Table 17.8 and in Figure 17.4. The properties of stable nitroxyl radicals are described in the literature.18

17.5 MECHANISM OF DEGRADATION The degradation of poly(propylene) with peroxides is believed to occur via a series of free radical reactions involving steps such as initiation, scission, transfer and termination. The mechanism of degradation of a poly(propylene) by a peroxide is shown in Figure 17.5. First, the peroxide decomposes by a homolytic scission into two radicals. The tertiary carbon atom would yield most stable radicals and therefore is preferably attacked. The peroxide is deactivated by hydrogen transfer. In the next step, a scission of

602


Table 17.8: Stable Nitroxyl Radicals for Controlled Rheology8 Nitroxyl Radicals 2,2,5,5-Tetramethyl-1-pyrrolidinyloxy a 3-Carboxy-2,2,5,5-tetramethyl-pyrrolidinyloxy b 2,2,6,6-Tetramethyl-1-piperidinyloxyc 4-Hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy d 4-Methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy e 4-Oxo-2,2,6,6-tetramethyl-1-piperidinyloxy f Bis(1-oxyl-2,2,6,6-tetramethylpiperidine4-yl)sebacate g 2,2,6,6-Tetramethyl-4-hydroxypiperidine-1-oxyl monophosphonate N-tert-Butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide h N-tert-Butyl-1-dibenzylphosphono-2,2-dimethylpropyl nitroxide N-tert-Butyl-1-di(2,2,2-trifluoroethyl)phosphono-2,2-dimethylpropyl nitroxide N-tert-Butyl[(1-diethylphosphono)-2-methyl-propyl]nitroxide N-(1-Methylethyl)-1-cyclohexyl-1-(diethylphosphono)nitroxide N-(1-Phenylbenzyl)-((1-diethylphosphono)-1-methyl ethyl)nitroxide N-Phenyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide N-Phenyl-1-diethylphosphono-1-methyl ethyl nitroxide N-(1-Phenyl-2-methylpropyl)-1-diethylphosphono-1-methyl ethyl nitroxide N-tert-Butyl-1-phenyl-2-methylpropyl nitroxide N-tert-Butyl-1-(2-naphthyl)-2-methylpropyl nitroxide a PROXYL™ b 3-carboxy PROXYL™ c TEMPO™ d 4-hydroxy-TEMPO e 4-methoxy-TEMPO f 4-oxo-TEMPO g CXA 5415™ h DEPN

Rheology Control

H3C H3C

CH3 N -

O

H3C

CH3

H3C

CH3

PROXYLTM

603

N -

O

CH3

TEMPOTM

H3C

CH3 H CH3 H3C C C CH3 P N CH3 H2C O O O OCH3 CH2 C

CH3 DEPNTM CH3

H3C H3C O N H3C

O C O

(CH2)3

C O O

CH3 N O CH3 CH3

H3C CXA 5415TM

Figure 17.4: Stable Nitroxyl Radicals: 2,2,5,5-Tetramethyl-1-pyrrolidinyloxy (PROXYL™), 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide (DEPN), Bis(1-oxyl-2,2,6,6-tetramethylpiperidine4-yl)sebacate (CXA 5415™)

604


R O*

R O O R

*O R

R O* H CH2

H

C

CH2

H

C

CH3

CH2

CH3

C CH3

R O H H CH2

H

C

CH2

C*

CH3

H CH2

CH2

C*

C CH3

C*

*C CH2

CH3

H C

CH2

H CH2

CH3

CH2

CH3

CH3

H C

CH3

C

H

C

CH3

CH2

CH2

CH3

H

H CH

CH3

C

H C CH2 CH3

CH3

Figure 17.5: Mechanism of Degradation of a Poly(propylene) Chain

Rheology Control

605

the main chain takes place. The radicals migrate to find another radical. Finally two radicals terminate by a disproportionation. A termination by recombination would be unfavorable in this case. In addition to the peroxide induced degradation, other models include the thermal decomposition of peroxides.

17.6 ULTRA HIGH MELT FLOW POLY(PROPYLENE) Ultra high melt flow (UHMF) poly(propylene) generally has a melt flow of greater than about 30 g/min. The production of UHMF polymers can be achieved during their initial polymerization, without the need for secondary processing. This usually involves the addition of hydrogen during the polymerization reaction. Increasing the hydrogen concentration in the polymerization reactor, however, can result in the production of excessive xylene solubles, which is often undesirable. Equipment or process limitations may also limit the amount of hydrogen that can be used during the polymerization reaction.1

17.7 IRREGULAR FLOW IMPROVEMENT Molded parts made from typical controlled rheology-treated poly(propylene) tend to have inferior appearance and surface characteristics, and are often marred by flow marks such as tiger marks. Controlled rheology poly(propylene) has a narrow molecular weight distribution which results from the selective loss of longer molecular chains due to the action of the organic peroxides. This narrow molecular weight distribution does not permit good surface molding of the molded article due to the irregular flow of the molten polymer in the mold. This irregular flow will lead to the surface flaws. Therefore, the use of controlled rheology poly(propylene) in injection molding has been limited to applications that do not require good surface characteristics. The addition of a high molecular weight component to the controlled rheology materials will improve the irregular flow in the mold. It is believed that this improvement occurs because of the broadened molecular weight distribution. However, the addition of the high molecular weight component sacrifices the high MFR properties gained in controlled rheology treatment.

606


Thus, there is a need in the art for controlled rheology propylene polymers which have a high MFR and good surface characteristics when in injection molding. Poly(tetrafluoroethylene) (with a molecular weight above 1,000,000 Dalton or even above about 5,000,000 Dalton) can be dispersed by mechanically blending with propylene polymers in the same extruder which is used for a simultaneous or subsequent controlled rheology treatment. Poly(tetrafluoroethylene) preferably can be dispersed simultaneously with the controlled rheology treatment in the extruder. A surface-modified poly(tetrafluoroethylene) is particularly useful. This is an acrylic modified poly(tetrafluoroethylene), commercially available as Metablen™.2

17.8 HETEROPHASIC COPOLYMERS Poly(propylene) heterophasic copolymers are typically made up of three components. These include a poly(propylene) homopolymer, a rubbery ethylene propylene copolymer, and a crystalline ethylene-rich ethylene propylene copolymer. The typical heterophasic morphology of these polymers consists of the rubbery ethylene propylene copolymer being dispersed as generally spherical domains within the semi-crystalline poly(propylene) homopolymer matrix. Poly(propylene) copolymers can be modified to improve their impact strength. This can be done through the use of elastomeric modifiers or with peroxides. When using elastomeric modifiers, the elastomeric modifiers are melt blended with the poly(propylene) copolymer, with the increased elastomer content typically contributing to a higher impact strength. Examples of elastomeric modifiers include ethylene/propylene rubber (EPR) and ethylene propylene diene monomer (EPDM) rubber. In poly(propylene) heterophasic copolymers modified with peroxides during the controlled rheology process, performance improvements can be achieved by adjusting the conditions under which the controlled rheology is carried out. By slowing deactivation of the peroxide, impact copolymers with higher impact strength and lower stiffness values can be attained, while achieving the desired final melt flow characteristics. A slower decomposition of the peroxide during controlled rheology polymer modification also slows down the vis-breaking reactions. This allows the polymer fluff to remain at a higher viscosity for longer periods of time during the extrusion. It is believed that by maintaining the polymer

Rheology Control

607

viscosity at higher levels during extrusion, the rubber phase of the poly(propylene) copolymer is more uniformly dispersed, which in turn results in higher impact strength for the same polymer modified with peroxide having shorter decomposition times. Linear peroxides having at least two peroxide groups, such as 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, are particularly suitable for delayed decomposition. Other suitable peroxides are the cyclic ketone peroxides, such as those disclosed in the literature,12 e.g., 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane, c.f. Figure 17.1. Improvements in impact strength for poly(propylene) heterophasic copolymers have been observed by slowing the decomposition or increasing the half-life of the peroxide during degradation. This is accomplished through a reduction in extrusion temperatures. Alternatively, a peroxide with a longer half-life than would otherwise be selected may also be employed if required by the extrusion conditions. Normal extrusion temperatures for most controlled rheology of heterophasic copolymers are usually from about 230°C to about 290°C., but may be hotter depending upon the product being processed. By significantly reducing these temperatures, improvements in impact strength can be achieved. To achieve slower decomposition, the poly(propylene) heterophasic copolymer is extruded at temperatures sufficient to maintain the material in a molten state, but reduced from those used in conventional controlled rheology processes. Thus, extrusion temperatures may range anywhere from the minimum temperature to maintain the copolymer in a molten state up to about 215 °C. When such temperatures are employed, at least some amount of the peroxide will usually remain unconsumed within the extruded copolymer. In heterophasic poly(propylene) (PP), both the degradation and the functionalization mainly occur in the ethylene rich phase. A preferential attack of the free radicals at single tertiary hydrogens between ethylene units, or at the ends of a PP block adjacent to one or multiple ethylene units, results in a selective functionalization of the ethylene rich copolymers, regardless of the solubility parameter or decomposition rate of the peroxides. These tertiary hydrogen atoms are not sterically protected by adjacent methyl groups, and are therefore more accessible to the generated free radicals and the bulky maleic anhydride.19

608


17.9 POLY(PROPYLENE) Commercial poly(propylene) resins commonly are polymerized by conventional Ziegler-Natta catalyst systems and have a high molecular weight and a broad molecular weight distribution (MWD). The chemical structure of poly(propylene) is generally influenced by the kind of polymerization system used during its production. Because the MWD largely determines the rheological properties of poly(propylene) melts, this parameter must be controlled to improve the material response during processing and to achieve the diversity in polymer grades suitable for the different applications of poly(propylene). Establishing of a broad molecular weight distribution of the poly(propylene) in conventional reactors is difficult because it requires the addition of chain terminators and transfer agents. These operations decrease output of the reactor and are often uneconomical. The most important characteristic of peroxides is that the half-life time at 130°C must be higher than 1 hour and smaller than 10 hours. Examples of peroxides industrially accepted for this degradation reaction are given in Table 17.4.

17.9.1 Long Chain Branched Poly(propylene) Poly(propylene) with long chain branches can be obtained by reactive extrusion of a poly(propylene) in the presence of a peroxide, a polyfunctional acrylate monomer and thiuram disulfide as co-reactant. The thiuram disulfide gives two dithiocarbamate radicals by thermal decomposition. These radicals react with the PP radicals in a reversible reaction. Therefore, a decrease in the instantaneous concentration of free radicals is achieved, which favors the branching reaction. In this way the β-scission is reduced.20

17.9.2 Effect of MFR on Temperature and Residence Time The initiator decomposition rate and residence time distribution in the extruder increase with increasing temperature. The change of the screw speed affects mixing and residence time distribution. So the MFR should increase with increasing residence time. However, if the process is performed at a sufficient long residence time to allow all degradation reactions to complete, a further increase in residence time will not change the MFR.

Rheology Control

609

Is has been demonstrated with dicumyl peroxide (DCP) as radical generator, the melt flow rate increases with increasing amount of peroxide. Generally, the crystalline fraction of samples increases with increasing peroxide concentration.14

REFERENCES 1. K. P. Blackmon, L. P. Barthel-Rosa, S. A. Malbari, D. J. Rauscher, and M. M. Daumerie. Production of ultra high melt flow polypropylene resins. US Patent 6 657 025, assigned to Fina Technology, Inc. (Houston, TX), December 2 2003. 2. M. Fujii and S. Kim. Polypropylene materials with high melt flow rate and good molding characteristics and methods of making. US Patent 6 599 985, assigned to Sunoco Inc. (R&M) (Philadelphia, PA), July 29 2003. 3. G. Schmidtthomee, C. Alt, R. Herbeck, H. Moeller, and H. G. Trieschmann. Narrowing the molecular weight distribution of polyolefins. GB Patent 1 042 178, assigned to BASF AG, September 14 1966. 4. J. J. Baron, Jr. and J. P. Rakus. Thermal degradation of polyolefins in the presence of a metal salt carboxylic acid catalyst. US Patent 3 332 926, assigned to Allied Chem, July 25 1967. 5. R. L. McConnell and D. A. Weemes. Method for making polyolefin waxes by thermal degradation of higher molecular weight polyolefins in the presence of organic acids and anhydrides. US Patent 3 519 609, assigned to Eastman Kodak Co, July 7 1970. 6. E. G. Castagna, A. Schrage, and M. Repiscak. Process for controlled degradation of propylene polymers. US Patent 3 940 379, assigned to Dart Industries, Inc. (Los Angeles, CA), February 24 1976. 7. M. W. Musgrave. Pelletized polyolefin having ultra-high melt flow and its articles of manufacture. US Patent 6 423 800, assigned to Fina Technology, Inc. (Houston, TX), July 23 2002. 8. D. Bertin and P. Robert. Method for the production of a controlled rheological polypropylene resin. US Patent 6 620 892, assigned to Atofina (Puteaux, FR), September 16 2003. 9. L. Kasehagen, R. Kazmierczak, R. Cordova, and T. Myers. Safe, efficient, low t-butanol forming organic peroxide for polypropylene modification. US Patent 6 599 990, assigned to Atofina Chemicals, Inc. (Philadelphia, PA), July 29 2003. 10. R. J. Ehrig and R. C. Weil. Controlled-rheology polypropylene. US Patent 4 707 524, assigned to Aristech Chemical Corporation (Pittsburgh, PA), November 17 1987.

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11. K. Huber, J. Schwind, K. Lehr, H. Elser, H. Klassen, and K.-H. Kagerbauer. Peroxidic treatment of olefin polymers. US Patent 6 313 228, assigned to Basell Polyolefine GmbH (Ludwigshafen, DE), November 6 2001. 12. J. Meijer, A. H. Hogt, G. Bekendam, and L. A. Stigter. Modification of (co) polymers with cyclic ketone peroxides. US Patent 5 932 660, assigned to Akzo Nobel NV (Arnhem, NL), August 3 1999. 13. J. D. Adams, R. H. Dorn, M. J. King, J. L. Kulasa, N. J. Motto, L. J. Ostanek, and D. Petticord. High organic peroxide content polypropylene. US Patent 5 198 506, assigned to Phillips Petroleum Company (Bartlesville, OK), March 30 1993. 14. H. Azizi and I. Ghasemi. Reactive extrusion of polypropylene: production of controlled-rheology polypropylene (CRPP) by peroxide-promoted degradation. Polymer Testing, 23(2):137–143, April 2004. 15. T. Bremner and A. Rudin. Peroxide modification of linear low density polyethylene: A comparison of dialkyl peroxides. J. Appl. Polym. Sci., 49: 785–798, 1993. 16. G. Moad. The synthesis of polyolefin graft copolymers by reactive extrusion. Prog. Polym. Sci., 24(1):81–142, April 1999. 17. G. Moad. Corrigendum to “the synthesis of polyolefin graft copolymers by reactive extrusion”[progress in polymer science 1999;24:81-142]. Prog. Polym. Sci., 24(10):1527–1528, December 1999. 18. L. B. Volodarsky, V. A. Reznikov, and V. I. Ovcharenko. Synthetic Chemistry of Stable Nitroxides. CRC Press, Boca Raton, FL, 1994. 19. T. Kamfjord and A. Stori. Selective functionalization of the ethylene rich phase of a heterophasic polypropylene. Polymer, 42(7):2767–2775, March 2001. 20. D. Graebling. Synthesis of branched polypropylene by a reactive extrusion process. Macromolecules, 35(12):4602–4610, June 2002.

18 Grafting Pros and cons of grafting copolymers by reactive extrusion in comparison to other methods are:1, 2 + − − + − + +

essentially no solvents, intimate mixing of reactants compulsory, the high reaction temperatures needed, fast preparation, side reactions, e.g., degradation, crosslinking or discoloration, simple product isolation, extrusion is a continuous process.

Grafting takes place mostly by a radical reaction mechanism3 and is also called free radical grafting. However, there are other techniques for introducing functional groups into polymers, e.g., according to the Alder-ene reaction.4

18.1 THE TECHNIQUES IN GRAFTING 18.1.1 Parameters that Influence Grafting 18.1.1.1

Mixing

Efficient mixing of the individual components is of critical importance for the success of a graft process. The mixing efficiency is dependent on the screw geometry, the melt temperature, the pressure, the rheological prop611

612


erties of the polymer, and the solubilities of the monomer and the initiator, respectively, in the polyolefin. 18.1.1.2

Grafting Efficiency

In order to obtain a high grafting efficiency together with an effective suppression of the side reactions, it is necessary to transform the macroradicals on the backbone as far as possible into graft sites. In general, within reasonable limits, higher reaction temperatures, higher initiator levels, and lower throughput rates result in higher grafting efficiency. Peroxide Concentration. The grafting efficiency of maleic anhydride on low density poly(ethylene) (LDPE) increases as the concentration of the peroxide increases. Further, the grafting efficiency depends on the means of reactive processing. In a comparative study with varying experimental setup, the lowest efficiency was found for extrusion using a typical shaping extrusion head, a higher efficiency was found with a static mixer and the highest efficiency was found with a dynamic mixer. The dynamic mixer is a cavity transfer mixer that provides shear rates of the moving melt of about 100 s−1 . Propene Content. In a series of polyolefins with different ethene/propene, the efficiency of grafting of maleic anhydride (MA) both in the melt and in solution was studied. The maleic anhydride graft content is low for polyolefins with high propene content, increases as the propene content decreases, and reaches a plateau at propene levels below 50%. Branching and crosslinking occurs for polyolefins with low propene content, while degradation is the main side reaction for polyolefins with high propene content.5 Mechanochemistry. Shear stresses in the dynamic mixer cause a formation of radicals even in the absence of any peroxide. Therefore, grafting of maleic anhydride on LDPE even without the action of peroxide initiator is observed. The dynamic mixer helps to obtain a high grafting efficiency on LDPE using a small concentration of peroxide initiator. Under these conditions, grafting is not accompanied by a crosslinking reaction of the poly(ethylene) chains.6

Grafting 18.1.1.3

613

Screw Geometry

Reactive extruders usually have a modular construction. This allows flexible arrangements of the screw elements and barrel sections as needed. 18.1.1.4

Processing Temperature

The processing temperature is of critical importance. Too high processing temperatures will cause degradation reaction, and the initiator may decompose too quickly to be effective. 18.1.1.5

Processing Pressure

In contrast to temperature, a high processing pressure can improve the solubility of the monomer to be grafted and the solubility of the initiator in the polymer. 18.1.1.6

Residence Time

The residence time is governed by the overall throughput which can be adjusted by the screw speed, the screw design, and the geometry of the extruder. 18.1.1.7

Removal of By-Products

The unreacted monomers and decomposition products from the initiator, etc., are removed by the application of vacuum to the melt. 18.1.1.8

Consistency

Experiments of grafting maleic anhydride onto poly(propylene) by melt extrusion with dicumyl peroxide, where the poly(propylene) was fed either as powder or in granular form, showed that consistency plays a role on the degree of grafting.7 The grafting efficiency of powdered poly(propylene) was higher than that obtained for the granular form of poly(propylene). It is believed that the grafting of powder is more successful because a better initial mixing and less diffusional resistance during the grafting is provided.

614


Table 18.1: Reaction Temperatures for Coupling of Stable Radicals8 Polymer

Abbreviation

Low density poly(ethylene) High density poly(ethylene) Poly(propylene) Poly(styrene) Styrene-block copolymers Ethylene-propylene-diene modified Ethylene/propylene rubber

LDPE HDPE PP PS SB(S) EPDM EPR

Temperature [°C] 170–260 180–270 180–280 190–280 180–260 180–260 180–260

18.1.2 Free Radical Induced Grafting The most commonly used grafting method is free radical induced grafting. However, the efficiency of grafting cannot simply be increased by increasing only the concentration of the radical initiator. More important for the grafting efficiency are proper mixing and a sophisticated choice of proper comonomers. Grafting without radical initiator is also possible. In this case, the macroradicals are formed by a shear induced chain scission. Of course, this process is accompanied by degradation or crosslinking reactions.

18.1.3 Grafting Using Stable Radicals The technique of grafting using stable radicals involves two steps.8 1. A stable nitroxyl radical is grafted onto a polymer, which involves the heating of a polymer and a stable nitroxyl radical. 2. The grafted polymer of the first step is then heated in the presence of a vinyl monomer or oligomer to a temperature at which cleavage of the nitroxyl-polymer bond occurs and polymerization of the vinyl monomer is initiated at the polymer radical. The temperature applied in the first reaction step depends on the polymer and is for example, 50°C to 150°C above the glass transition temperature (Tg ) for amorphous polymers and 20°C to 180°C above the melting temperature (Tm ) for semi-crystalline polymers. Typical temperatures are summarized in Table 18.1. Stable nitroxyl radicals are collected in Table 18.2. The first step of the process is performed conveniently in an extruder or a kneading appa-

Grafting

615

Table 18.2: Stable Nitroxyl Radicals Compound Benzoic acid 2,2,6,6-tetramethyl-piperidin-1-oxyl-4-yl ester 4-Hydroxy-2,2,6,6-tetramethyl-piperidin-1-oxyl 4-Propoxy-2,2,6,6-tetramethyl-piperidin-1-oxyl Decanedioic acid bis(2,2,6,6-tetramethyl-piperidin-1-oxyl-4-yl)

ratus. In the extruder, a reduced pressure of less than 200 mbar is applied during extrusion. Volatile by-products may be removed thereby. Typical reaction times are from 2 min to 20 min. For the monomer grafting reactions, unsaturated monomers are selected from styrene, dodecyl acrylate, and other compounds. The second reaction step may be performed immediately after the first step, however it is also possible to store the intermediate polymeric radical initiator at room temperature for some time. Because the graft polymerization is a living polymerization, it can be started and stopped practically at will. The intermediate polymeric radical initiator is stable at room temperature and no loss of activity occurs up to several months. The reaction step may also be performed in a mixer or extruder. However, it is also possible to dissolve or disperse the polymer and to add the monomer to the solution. If the second reaction step is performed in a melt, a reaction time of 2 to 20 min is adequate. The grafted polymers are useful in many applications such as compatibilizers in polymer blends or alloys, adhesion promoters between two different substrates, surface modification agents, nucleating agents, coupling agents between filler and polymer matrix, or dispersing agents. The process is particularly useful for the preparation of grafted block copolymers. Grafted block copolymers of poly(styrene) and polyacrylate are useful as adhesives or as compatibilizers for polymer blends or as polymer toughening agents. Poly(methyl methacrylate-co-acrylate) diblock graft copolymers or poly(methyl acrylate-co-acrylate-co-methacrylate) triblock graft copolymers are useful as dispersing agents for coating systems, as coating additives or as resin components in coatings. Graft block copolymers of styrene, (meth)acrylates, or acrylonitrile are useful for plastics, elastomers, and adhesives.

616

Reactive Polymers Fundamentals and Applications Table 18.3: Monomers for grafting onto Polyolefins1 Vinyl Monomer

Remarks/References

Maleic anhydride Maleate esters Styrene Maleimide derivatives Methacrylate esters Acetoacetoxy methyl methacrylate Glycidyl methacrylate Acrylate esters Ricinoloxazoline maleate Vinylsilanes

Most common 9

Auxiliary monomer10 11 12 11 13, 14 15 16 17

18.2 POLYOLEFINS The synthesis of polyolefin graft copolymers by reactive extrusion has been reviewed by Moad.1, 2 The methods of modification can be classified as 1. Free radical induced grafting of unsaturated monomers onto polyolefins, 2. End-functional polyolefins by the ‘ene’ reaction, 3. Hydrosilylation, 4. Carbene insertion, and 5. Transformation of pending functional groups on polyolefins, e.g., by transesterification, alcoholysis.

18.2.1 Monomers for Grafting onto Polyolefins Monomers for grafting onto polyolefins are listed in Table 18.3. 18.2.1.1

Macromonomers

Polymeric or oligomeric vinyl compounds are addressed as macromonomers in the field of reactive extrusion. Examples for macromonomers are higher molecular acrylate esters, methacrylate esters, and maleimides. Macromonomers are less likely to undergo homopolymerization than low molecular vinyl compounds. This property arises due to steric effects. Thus they may not form longer pendent chains on the grafting sites consisting of homopolymers. A disadvantage of macromonomers is their low

Grafting

617

CH3 HC CH

C

O

C CH

CH3 O C C O

O C C O

OH OH

OH OH CH3 C O C6H12

CH C CH2

CH3

O C C O OH OH

Figure 18.1: Structure of Low Molecular Weight Fraction of Extrudates of Poly(propylene) with Maleic anhydride and Dicumyl peroxide

volatility. For this reason, an unreacted or excess compound may not easily be removed by vacuum treatment in the extrusion device.

18.2.2 Mechanism of Melt Grafting Functionalized poly(propylene) (PP) has been used extensively for compatibilization of immiscible poly(propylene)/polyamide and poly(propylene)/polyester blends. Also, the interfacial adhesion of PP with glass and carbon fibers can be improved. Further, functionalized PP is a processing aid for degradable plastics.18, 19 It is generally accepted that chain scission occurs during the peroxide initiated functionalization of PP.20 Maleic anhydride (MA) is appended to a tertiary carbon atom along the PP backbone as a single ring or as a short pendant chain due to the homopolymerization of MA.21 On the other hand, according to the ceiling temperature, there is no possibility for the homopolymerization of MA under the melt grafting process conditions at 190°C.22 Chemical analysis of the low molecular weight fraction of extrudates of poly(propylene) with maleic anhydride and dicumyl peroxide by mass spectrometry indicated the products shown in Figure 18.1. No MA oligomers or MA homopolymers are found in the low molecular weight fraction. The MA radicals always contain double bonds after termination. Peroxide residues are attached to MA molecules. A reduction of the molecular weight occurs when the degree of grafting increases. From the

618


inspection of the chemical structure of the low molecular weight residue, it can be concluded that the maleic anhydride is attached as a single moiety on the tertiary carbon atoms of the poly(propylene) backbone. From these experimental findings a mechanism of grafting has been proposed23 that is given in Figure 18.2. Furthermore, the grafting of maleic anhydride onto poly(propylene) has been studied by a Monte Carlo simulation method.24 The results presented in this study are in agreement with the experiments. The grafting efficiency of methyl methacrylate is similar to that of maleic anhydride.12

18.2.3 Side Reactions Side reactions accompany the grafting reaction of polyolefins. These include1, 2 1. 2. 3. 4. 5.

Radical induced crosslinking of the polyolefin substrate, Radical induced chain scission of the polyolefin substrate, Shear induced degradation of the polyolefin substrate, Homopolymerization of the monomer, and Side reactions which lead to a coloration of the product.

The extent of the side reactions depends on the type of polyolefin. Some poly(ethylene) types are sensitive to branching and crosslinking. This is due to the recombination of the macroradicals.25 Poly(propylene) and linear low density poly(ethylene) copolymers undergo degradation rather than crosslinking, although crosslinking may occur. Degradation is often favored to synthesize controlled rheology types.

18.2.4 Viscosity The formation of products with higher molecular weight is indicated by an increase of the apparent viscosity. On the other hand, by the introduction of polar groups during grafting, an increase of the viscosity is observed because of physical crosslinks of the individual molecules. Maleic anhydride has been grafted onto poly(propylene) in the presence of supercritical carbon dioxide. Supercritical carbon dioxide was used in order to reduce the viscosity of the poly(propylene) melt phase. A reduced viscosity should promote a better mixing of the reactants. The characterization of the products showed that the use of supercritical carbon

Grafting

O

O

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

O

O

CH3

CH3

CH3

619

O

O

CH3

CH3

Figure 18.2: Mechanism of Grafting of Maleic anhydride onto Poly(propylene)23 (abbreviated)

620


Table 18.4: Ceiling Temperatures for Important Monomers in Reactive Extrusion Grafting1, 26 Monomer

Ceiling Temperature [°C]

Maleic anhydride Styrene Methacrylate esters Acrylate esters

400 ∼200 >400

dioxide in fact resulted in improved grafting when high levels of maleic anhydride were used. No evidence of an improvement in the homogeneity of the product was observed. However, melt flow rate showed a reduction in the degradation of poly(propylene) during the grafting reaction when low levels of maleic anhydride were used.27

18.2.5 Ceiling Temperature The ceiling temperature is an important parameter for the ability of polymerization itself. We are dealing here with homopolymerization. The concept of the ceiling temperature is not restricted to a polymerization mechanism, because it deals with the thermodynamic equilibrium. Ceiling temperatures for important monomers in reactive extrusion grafting onto polyolefins are given in Table 18.4. The ceiling temperatures given in Table 18.4 could be important for the grafting of maleic anhydride and maleic esters.28, 29 The ceiling temperatures depend on the pressure and on the concentration of the monomer. They are usually calculated from the heats and the entropies of polymerization that are usually given at one atmosphere. In fact, the homopolymerization of maleic anhydride was observed at a higher temperature than 150°C, even when the ceiling temperature would not predict a polymerization reaction.

18.2.6 Effect of Initiator Solubility Experiments of grafting of itaconic acid (IA) onto a low density poly(ethylene) (LDPE) with various initiators in the course of the reactive extrusion revealed that the solubility of the peroxide initiator in the molten polymer is the most important parameter in the IA grafting onto LDPE. The kinetics of decomposition is an important parameter for the efficiency of grafting.

Grafting

621

Table 18.5: Solubility Parameters of Peroxides30 δ [Jcm−3 ]1/2

Peroxide

Dicumyl peroxide 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane Di-tert-butyl peroxide 2,2-Di(tert-butylperoxy)-5,5,6-trimethyl bicyclo[2.2.1]heptane 2,5-Dimethyl-2,5-di(tert-butylperoxy)-3-hexyne a calculated for 25°C

a

17.4 11.3 15.3 16.1 19.1

The solubility parameters of various peroxides are collected in Table 18.5. The solubility parameters δ in Table 18.5 are calculated from group contributions31 according to Eq. 18.1. s ∑i ∆Ei δ= (18.1) Na ∑i ∆Vi ∆Ei ∆Vi Na

Contribution of every atom and type of the intermolecular interaction in the molar cohesion energy van der Waals volume of a group constituting the molecule Avogadro number

The temperature dependence of δ can be expressed by Eq. 18.2: log δ(T ) = log δ(298K) − αk(T − 298)

(18.2)

k is a coefficient. For the polyolefin k = 1 and for the peroxides and the monomer k = 1.25. α is the linear thermal expansion coefficient. The cohesion energy density δ calculated from Eq. 18.1 correlates well with the values obtained from the heat of vaporization of the respective substances. Substances are thermodynamically miscible in the absence of strong specific interactions between them, if their solubility parameters differ by less than 2 (J cm−3 )1/2 . The solubility parameters of IA and LDPE are 24.6 (J cm−3 )1/2 and 16.1 (J cm−3 )1/2 , respectively. Therefore, IA and LDPE form a heterogeneous system in the melt. On the other hand, it is expected that some of the peroxides listed in Table 18.5 would dissolve in LDPE. It is assumed that radicals formed during peroxide decomposition interact first with LDPE macromolecules, then the formed macroradicals initiate the grafting reactions with IA. Peroxides, which are easily dissolved in LDPE, are most efficient in initiating the grafting reactions.30 It was found that neutralizing agents introduced into the initial reaction mixture increase the yield of LDPE-g-IA, when the carboxyl groups

622


were neutralized partially or totally. As neutralizing agents, zinc oxides and hydroxides as well as magnesium oxides and hydroxides can be used.32

18.2.7 Distribution of the Grafted Groups There is a lot of research presented in the literature, and there is still a controversy concerning the mechanism and the distribution and the structure of the grafted portions on the backbone. This is reviewed in detail by Moad.1

18.2.8 Effect of Stabilizers on Grafting The grafting of maleic anhydride onto poly(ethylene) is fully inhibited by adding a phenolic stabilizer to the reactive blend.33 In a system consisting of itaconic acid, linear low density poly(ethylene) and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane with Irganox™ 1010 (Ciba Geigy, Switzerland), i.e. the ester of 3,5-di-tert-butyl-4-hydroxyphenyl-propanoic acid and pentaerythritol, the grafting efficiency decreases slightly. However, at concentrations of the stabilizer greater than 0.3% some improvement in the grafting efficiency occurs and the melt viscosity is much lower.30 The efficiency of stabilizers on the grafting and on the crosslinking also depends on their solubility in the polymer and the monomer. For example, 1,4-dihydroxybenzene has an increased affinity toward the monomer and both reduces the yield of grafting and inhibits crosslinking.34

18.2.9 Radical Grafting of Polyolefins with Diethyl Maleate The use of maleate esters such as diethyl maleate or dibutyl maleate has been suggested because of their lower volatility and lower toxicity in comparison to maleic anhydride. However, maleate esters are less reactive towards free radical addition than maleic anhydride. Grafting polyolefins with diethyl maleate can be carried out in solution. However, the use of extruders as reactors has several economic advantages. The extruder screw is advantageously configured with different mixing elements after an additional feed zone downstream from the initial feed port for peroxide and diethyl maleate. Further there are no mixing elements beyond the vent port. Turbine mixing elements are used for the

Grafting

623

improved blending of the low-viscosity initiator and the diethyl maleate into the high-viscosity poly(ethylene). A vacuum vent port is used to eliminate the unreacted monomer. In the extruder, dicumyl peroxide (DCP), is used as initiator.35 The kinetics of the free radical grafting of diethyl maleate (DEM) onto linear poly(ethylene) initiated by dicumyl peroxide has been studied by differential scanning calorimetry (DSC). The activation energy Ea and the order of the reaction n depend on the conditions and vary with the feed composition. The values of Ea and n increase with increasing DCP/DEM ratio because of secondary reactions, such as chain extension and degradation. The data can be described by a mathematical model which can be used to select feed composition and process parameters to obtain the desired products.36

18.2.10 Inhibitors for the Homopolymerization of Maleic anhydride In a series of papers, Gaylord showed that various additives are effective in reducing both the amount of crosslinking and chain scission.37, 38 These additives include amides, such as N,N-dimethylacetamide, N,N-dimethylformamide, caprolactam, stearamide, sulfoxides such as dimethyl sulfoxide, and phosphites, such as hexamethylphosphoramide, triethyl phosphite. The action has been attributed to the electron donating properties of these compounds. It was shown that these compounds also act as inhibitors of the homopolymerization of maleic anhydride, thus reducing its grafting efficiency. However, it seems that these compounds are not effective in general at least, there was some controversy.1

18.2.11 Inhibitors for Crosslinking p-Benzoquinone, triphenyl phosphite and tetrachloromethane were found to be good inhibitors for the crosslinking reaction of LDPE.39 In the melt grafting of maleic anhydride onto an elastomeric ethylene-octene copolymer, N,N-dimethylformamide was used as an inhibitor to reduce the crosslinking reaction. Further N,N-dimethylformamide is a solvent for peroxide initiator. The melt grafting was carried out in a twinscrew extruder, in the presence of dicumyl peroxide as an initiator. However, increasing the initiator concentration increased the degree of grafting, and at the same time, increased the extent of crosslinking.40

624

Reactive Polymers Fundamentals and Applications Table 18.6: Functionalized Peroxides41

Compound 1,1-Dimethyl-3-hydroxybutyl-6-(hydroxy)peroxyhexanoate 1,1-Dimethyl-3-hydroxybutyl-2-(carboxy)perbenzoate 1,1-Dimethyl-3-hydroxybutyl-2-(carboxy)peroxycyclohexanecarboxylate 1,1-Dimethyl-3-hydroxypropyl-3-(carboxy)peroxypropanoate 1,1-Dimethyl-3-hydroxybutyl-3-(carboxy)-5-norbornene-2-ylperoxycarboxylate

18.2.12 Special Initiators 18.2.12.1

Bisperoxy Compounds

The decomposition of the two peroxy groups in bisperoxy compounds is not concerted. The two peroxy groups decompose independently to yield a variety of alkoxy and alkyl radicals. 18.2.12.2

Functionalized Peroxides

To optimize the chemical compatibility or solubility of the peroxides in a wide variety of polymeric systems, the organic character of these peroxides may be tailored by introducing suitable groups. Functionalized peroxides may be used as crosslinking, grafting and curing agents, initiators for polymerization reactions and as monomers for condensation polymerizations to form peroxy-containing polymers, which in turn can be used to prepare block and graft copolymers. Some functionalized peroxides are shown in Figure 18.3 and collected in Table 18.6. The half-life times of the peroxides at 180°C are ca. 0.27 min for LUPEROX™ PMA and LUPEROX™ TA-PMA and 0.31 min for Luperco™ 212-P75 and Lupersol™ 512. The peroxides are assumed to result in acrylic carboxyl groups and propionic carboxyl groups on the tertiary carbon atoms of poly(propylene) on recombination with the tertiary radicals formed previously. The highest acidity on the polymer backbone is obtained with LUPEROX™ PMA. With respect to the functional radicals, the peroxides which yield radicals that bear double bonds have a higher grafting efficiency. It is assumed that the alkenyl radicals have a higher reactivity with respect to alkyl radicals. Further, the increased grafting efficiency may arise since macroradicals can add across the double bond of the alkenyl groups.42

Grafting

CH3

O

625

O

H3C C O O C CH CH C OH

CH3 LuperoxTM PMA CH3

O

O

H3C C O O C CH2 CH2 C OH

CH3 LupercoTM 212-P75 CH3 CH3

O

O

CH2 C O O C CH CH C OH

CH3 LuperoxTM TA-PMA CH3 CH3

O

O

CH2 C O O C CH2 CH2 C CH3

OH

LupersolTM 512

Figure 18.3: Functionalized Peroxides, manufactured by Elf Atochem North America

626


O

CH3

O + OH CH

O O CH3 CH2

C O O H + O CH3 O

O HO CH3

O CH

O

O

CH3 HO

CH2

C O O CH3 O

Figure 18.4: Reaction of 1,1-Dimethyl-3-hydroxybutyl hydroperoxide with Maleic anhydride

Preparation of Functionalized Peroxides. There are several routes to preparing functionalized peroxides. 1,1-Dimethyl-3-hydroxybutyl hydroperoxide reacts with two units of glutaric anhydride or maleic anhydride in ring opening of the anhydride43 as shown in Figure 18.4. Similarly, 1,1-dimethyl-3-hydroxybutyl-2-(carboxy)perbenzoate can be prepared from phthalic anhydride by adding 1,1-dimethyl-3-hydroxybutyl hydroperoxide in equimolar quantities. Peroxyketals. The chemical modification of molten poly(ethylene) by thermolysis of peroxyketals involves the decomposition of three cyclic or acyclic peroxyketals. An ester function by coupling of an alkyl radical bearing such a function, arising from the peroxyketals, a polymer radical, generated from the poly(ethylene), were identified as grafting products.44 18.2.12.3

Induced Decomposition of Peroxides

Peroxides show an induced decomposition with amino-functional monomers such as diethylaminoethyl acrylate (DEAEMA) and diethylamino-

Grafting

Br H3C H C C H CH3

2,3-Dimethyl-2,3-diphenylbutane

Br

Br

Br

Br

Br Br

627

Br

Hexabromocyclododecane

Figure 18.5: Dicumyl and Hexabromocyclododecane

ethyl acrylate (TBAEMA). Instead of a peroxide an azo compound can be used as an radical initiator. 18.2.12.4

Grafting to Poly(ethylene) with Bicumene

Bicumene, i.e., dicumyl or 2,3-dimethyl-2,3-diphenylbutane, can serve as a radical initiator as an alternative to a peroxide. Compounds of the bicumene-type also serve as synergists for flame retardants polyolefin by using them in combination with a known flame-retardant for polyolefin such as hexabromocyclododecane (c.f. Figure 18.5) and 2,3-tris(dibromopropylene)phosphate. When a peroxide is employed as the reaction initiator, the peroxide serves as a graft polymerization initiator, but at the same time a portion of the peroxide induces a crosslinking reaction and a decomposition reaction of the polyolefin. Because of the crosslinking reaction or the decomposition reaction, the inherent physical properties of polyolefin deteriorate and the resulting modified product is unable to maintain the properties of the polyolefin. In addition, when the peroxide decomposes as the reaction proceeds, the decomposition products (e.g., butanol or other decomposition products) stain the modified product. For example, the modified product yields odor originating from the decomposition product, or turns to yellow because of the action of the decomposition product. The graft polymerization reaction starts more moderately and proceeds more selectively, in comparison to a conventional reaction using peroxide. Also, the crosslinking reaction or decomposition reaction of poly-

628


olefin is less, and the resulting modified polyolefin has the excellent physical properties of the unmodified polyolefin. For instance, when a linear low density poly(ethylene) is employed, the modified product thereof has a high mechanical strength at low temperatures.45 The bicumene-initiated modification of high density poly(ethylene) at 290°C provides no benefits in terms of selectivity when compared to a standard peroxide-based process operating at 180°C. However, the selectivity of linear low density poly(ethylene) modification is influenced by chain scission, which counteracted the molecular weight effects of macroradical combination.46 As compared with the case of using peroxide, the variation of melt index caused by the modification is smaller, and the modified product obtained shows a melt index only slightly different from that of the polyolefin employed as a starting material. Maleic anhydride is generally employed in the amount of 10−3 to −5 10 mol/g of polyolefin. When the amount of maleic anhydride exceeds 10−3 mol/g, the graft efficiency of maleic acid sometimes decreases, and unreacted maleic anhydride remains in a large amount. This results in an unfavorable effect on the physical properties of the resulting modified product. When the amount of maleic anhydride is less than 10−6 mol/g, the modification with the maleic anhydride is unsatisfactory, and accordingly the resulting modified product does not have sufficiently improved adhesive properties.45 The graft polymerization reaction is performed by heating a mixture of the polyolefin, maleic anhydride and the initiator under kneading.

18.2.12.5

Ultrasonic Initiation

The grafting of maleic anhydride onto high density poly(ethylene) also can be performed through ultrasonic initiation. Obviously, the ultrasonic waves can decrease the molecular weight of the grafted product and increase the amount of grafted maleic anhydride. In comparison to the initiation with peroxide, ultrasonic initiation can prevent the crosslinking reaction by adjusting the ultrasonic intensity. The mechanical properties of the improved HDPE glass fiber composite produced by ultrasonic initiatives are higher than in those produced by peroxide initiatives.47

Grafting

629

Table 18.7: Use of Maleic anhydride-grafted Linear low density poly(ethylene) as Compatibilizer System

Reference

Poly(propylene)/organoclay nanocomposites Low density poly(ethylene)/ethylenevinyl alcohol Poly(propylene)/Poly(styrene) Low density poly(ethylene)/rice starch

48 49 50 51

18.2.13 Maleic anhydride Maleic anhydride is most frequently used for grafting and functionalization of polyolefins. Many of the features are described in the general sections, e.g., Section 18.2.2. Systematic and quantitative studies of the graft copolymerization in batch and continuous mixers and kinetic data for poly(propylene) and maleic anhydride are available.52 In the melt grafting of maleic anhydride onto low density poly(ethylene)/poly(propylene) blends, in the presence of dicumyl peroxide (DCP), the blend had lower viscosity in comparison to exclusively pure poly(ethylene) under comparable conditions. However, the grafting degree of the MA grafted LDPE/PP (90/10) blend was almost the same as or a little higher than that of the MA grafted LDPE.53 Maleic anhydride can be grafted onto poly(propylene) using benzophenone (BP) as the photoinitiator.54 In comparison to thermally initiated grafting with peroxide initiators, photoinitiated grafting has a higher grafting efficiency. Maleic anhydride-grafted linear low density poly(ethylene) (LDPE-g-MA) is widely used as compatibilizer for various applications, as shown in Table 18.7.

18.2.14 Polyolefins Grafted with Itaconic Acid Derivatives 18.2.14.1

Poly(ethylene) Polyamide 6 Blends

Two-phase blends of polyamide 6 (PA6) and low density poly(ethylene) (LDPE) have been prepared. Here in the course of reactive extrusion, an in-situ grafting of itaconic acid (IA) on the LDPE takes place. The performance of blending was tested with neutralization and without neutralization of the acid groups of itaconic acid.55 The maximum increase with regard to the mechanical properties was achieved when magnesium hydroxide was used as a neutralizing agent.

630 18.2.14.2

Reactive Polymers Fundamentals and Applications Poly(propylene)

Functionalized poly(propylene) (PP) by radical melt grafting with monomethyl itaconate or dimethyl itaconate is a compatibilizer in PP/poly(ethylene terephthalate) (PET) blends. Blends with compositions 15/85 and 30/70 by weight of PP and PET, prepared in a single-screw extruder, revealed a very fine and uniform dispersion of the PP phase compared to the respective non-compatibilized blends. An improved adhesion between the two phases is shown. Dimethyl itaconate as compatibilizer derived agent exhibits only a small activity to increase the impact resistance of PET in PP/PET blend. However, monomethyl itaconate is active in this respect. This finding is attributed to the hydrophilic nature of monomethyl itaconate. The tensile strength of PET in non-compatibilized blends gradually decreases with increasing content of PP. Blends containing functionalized PP exhibit, in general, higher values.56

18.2.15 Imidized Maleic Groups The chemical modification of swollen HDPE particles in near critical propane seems to be much more effective in avoiding crosslinking than the conventional modification in the melt phase. High density poly(ethylene) grafted with 0.17% maleic anhydride (PE-g-MA) can be additionally modified with 1,4-diaminobutane (DAB). After formation of amic acid groups, the excess of diaminobutane is extracted with a near critical propane-ethanol mixture. Finally, the obtained PE-g-MA-DAB is imidized to the corresponding imide (PE-g-MI) in the melt. The obtained PE-g-MI shows no increased gel content with respect to the initial PE-g-MA. It appears that PE-g-MI samples react with the anhydride groups of a styrene/maleic anhydride copolymer (SMA) during melt blending of SMA with PE-g-MI, while the PE-g-MA do not react.57

18.2.16 Oxazoline-modified Polyolefins The free radical induced grafting of 2-isopropenyl-2-oxazoline (IPO) onto PP has been reported.58

Grafting

631

18.2.17 Modification of Polyolefins with Vinylsilanes Vinylsilanes, e.g., vinyltrimethoxysilane (VTMS) do not readily homopolymerize. The modification of polyolefins with vinylsilanes, such as vinyltrimethylsilane, vinyltriethylsilane, or 3-(trimethoxysilyl)propyl methacrylate aims to the preparation of a moisture curable crosslinked polyolefins. For example, the silane grafting of a metallocene ethylene-octene copolymer is carried out in a twin-screw extruder, in the presence of vinyltrimethoxysilane and dicumyl peroxide.17 These materials are used in the manufacture of electrical cables. 18.2.17.1

Vinyltriethoxysilane

Bicumene initiates the grafting of vinyltriethoxysilane (VTEOS) to poly(ethylene) efficiently over an uncommonly large range of operating temperatures. The analysis of kinetics of bicumene decomposition suggests that the initiation occurs via an autoxidation mechanism that is facilitated by the interaction of cumyl radicals with oxygen.46 The analysis of poly(ethylene-g-vinyltrimethoxysilane) by differential scanning calorimetry-successive self-nucleation and annealing (DSC-SSA) indicated that the distribution of pendant alkoxysilane grafts amongst polymer chains is not uniform. Fractionation and characterization of a graft-modified model compound, tetradecane-g-VTMS, showed that the composition distributions were influenced strongly by intramolecular hydrogen atom abstraction. It yields multiple grafts per chain as single pendant units and oligomeric grafts. The chain transfer to the methoxy substituent of VTMS grafts contributes significantly to the product distribution.59 The selectivity for the ratio of grafting to crosslinking shows a considerable scope for optimization through variation of monomer and peroxide loadings in the case of VTEOS as modifier, in contrast to maleic anhydride.60

18.2.18 Ethyl Diazoacetate-modified Polyolefins Ethyl diazoacetate and chloroethyl diazoacetate is inserted by a carbene insertion mechanism at 210°C. No radical initiator is needed, however the grafting efficiency is small.61, 62

632


H 3C H 3C

C

CH3

CH2

HO H 3C

O CH CH2

C H 3C

CH3 DBBA O

CH2 CH C O

O

CH2

CH2 CH C O CH2 C CH2 CH3 CH2 CH C O CH2 O TRIS

Figure 18.6: 3,5-Di-tert-butyl-4-hydroxybenzyl acrylate (DBBA) and 1,1,1-Trimethylolpropane triacrylate (TRIS)

18.2.19 Grafting Antioxidants Routes for grafting antioxidants onto polyolefins with high grafting yields have been reported. The antioxidant 3,5-di-tert-butyl-4-hydroxybenzyl acrylate (DBBA) reacts with the trifunctional coagent 1,1,1-trimethylolpropane triacrylate (TRIS), c.f. Figure 18.6, in the presence of a small concentration of a free-radical initiator in a poly(propylene) melt during processing. The major reaction is a homopolymerization of the antioxidant in the absence of TRIS. This results in low grafting levels. However, in the presence of TRIS, more than 90% grafting efficiency of DBBA on the polymer is monitored, 6% of DBBA is used. The mechanism of the grafting reaction could be established with decalin, used as a hydrocarbon model compound.15 The decalin adds with the hydrogen atom on the bridge to the double bond of DBBA. Quinoneimines containing an N-p-hydroxyphenyl and an N-p-aminophenyl substituent have a high antioxidant efficiency when added to isoprene rubber (IR), styrene butadiene rubber (SBR), ethylene/propylene rubber (EPR), and ethylene propylene diene monomer (EPDM) rubbers,

Grafting

H 3C C

H 3C

CH3

R O + H 2N

O H 3C

C

H 3C

633

R’

CH3

H 3C H 3C

C

CH3

N

O H 3C H 3C

R

C

R’

CH3

Figure 18.7: Synthesis of Quinoneimines

because they add to the allylic −CH of the polymer giving active adducts. The synthesis of the quinoneimines is shown in Figure 18.7. The retention of the protective activity after extraction of the material indicates the grafting of these compounds during the thermal or mechanical processing of the rubbers.63

18.2.20 Comonomer Assisted Free Radical Grafting The idea of using styrene as a comonomer originated from a detailed analysis of the mechanism of free radical grafting. To obtain high graft efficiency, together with a reduced degradation of polymer, it is essential that the macroradicals in the backbone react with the grafting monomers before they undergo chain scission of the backbone. If the primary monomer is not sufficiently reactive towards the macroradicals, it is helpful to add another monomer that reacts with the macroradicals faster than primary monomer. A further requirement is that the resulting pendent free radicals of the secondary monomer copolymerize readily with the primary monomer. It was shown that the addition of styrene can improve the graft efficiency of monomers such as hydroxyethyl methacrylate (HEMA) and

634


C

N CH 2 O CH2

+

O

O HO C

O

C N CH2

CH2

O C

H

Figure 18.8: Ring Opening of a Pendant Oxazoline Group

methyl methacrylate (MMA), glycidyl methacrylate (GMA), but not vinyl acetate (VAc) and ricinoloxazoline maleate (OXA). This is due to the fact that styrene copolymerizes readily with HEMA, MMA, and GMA, but not with VAc and OXA. The ring opening of a pendant oxazoline group is shown in Figure 18.8. Ricinoloxazoline maleate is a bifunctional compatibilizer agent. It can be grafted with the vinyl function of the maleate unit onto a poly(propylene) site by usual radical grafting, thus becoming oxazoline groups attached to the poly(propylene) chain. The oxazoline group can be reacted with the carboxyl groups of poly(butylene terephthalate).16 18.2.20.1

Styrene-assisted Grafting

Maleic anhydride. The low reactivity of MA with respect to free-radical polymerization is inherently due to its structural symmetry and the deficiency of the electron density around the double bond. It is clear that the addition of a monomer capable of donating electrons, i.e., an electron-rich comonomer, would activate an electron deficient monomer like MA by changing the electron density of the π-bond. The addition of styrene to a melt grafting system as a comonomer of maleic anhydride can significantly enhance the graft degree onto poly(propylene). The maximum graft degree is obtained when the molar ratio of maleic anhydride to styrene is approximately 1:1. Styrene improves the grafting reactivity of maleic anhydride and also reacts with maleic anhydride to form a styrene/maleic anhydride co-

Grafting

635

polymer (SMA) before grafting onto the poly(propylene) backbone. When the concentration of maleic anhydride is higher than that of styrene, some maleic anhydride monomer reacts with styrene to form SMA, but others can directly graft onto macroradicals on the poly(propylene) chain. When the amount of styrene added is higher than that of maleic anhydride, a part of the styrene monomer may preferentially react with the macroradicals to form macroradicals with styryl ends, while others copolymerize with maleic anhydride to yield SMA.10 On the other hand, styrene is ineffective as comonomer for maleate esters grafting onto PP.64 This arises from the low affinity of the styryl radical towards the maleate ester species. This could be predicted from the critical inspection of the monomer reactivity ratios of styrene and maleic esters. Glycidyl Methacrylate. The reactivity of glycidyl methacrylate (GMA) in free radical grafting onto poly(propylene) (PP) is low. However, adding styrene as a comonomer for glycidyl methacrylate increases both the rate and grafting efficiency. Further the degradation of PP is reduced. It is believed that when styrene is added to such a grafting system, styrene reacts first with PP macroradicals to form pendent styryl radicals. These styryl radicals are the starting point for a copolymerization with GMA to form a grafted PP.13 Poly(propylene) functionalized with glycidyl methacrylate has been used for the compatibilization of poly(propylene) and poly(butylene terephthalate) blends.65 Similar studies have been done for the grafting of glycidyl methacrylate onto linear low density poly(ethylene) (LLDPE).14 18.2.20.2

Increasing the Grafting Efficiency with Comonomers

The mechanisms that result in higher grafting yields by the addition of comonomers can be attributed to1 • Longer chain grafts, • More grafting sites, • Use of polyvinyl monomers. Longer chain grafts appear to be the favored alternating copolymerization of electron donor-electron acceptor forming monomer pairs. Examples are styrene, and maleic anhydride. More grafting sites emerge by a more efficient addition of the macroradicals on the backbone by the addition of

636


Table 18.8: Experimental Techniques for the Characterization of Modified Polyolefins Method

Remarks

Titration FTIR spectroscopy NMR spectroscopy

Maleic anhydride, glycidyl units Most widely used method Chemical shifts are very sensitive to the chemical environment Poor sensitivity

13 C

NMR spectroscopy

a comonomer. Polyfunctional monomers effect presumptive branching or crosslinking sites when once grafted onto the backbone. In this way a star shaped or comb shaped grafting center may emerge. An example for this concept is the use of a triacrylate monomer as comonomer.66

18.2.21 Radiation Induced Grafting in Solution A suitable solvent for the radiation induced graft copolymerization of styrene and maleic anhydride (Sty/MA) binary monomers onto high density poly(ethylene) (HDPE) is acetone. Untreated and treated grafted HDPE membranes have potential applications in dialysis.67 The hydrophilicity of the membrane, the degree of grafting and the molecular weight and chemical structure of the metabolites, such as urea, creatinine, uric acid, glucose, and phosphate salts, have a great influence on the transport properties of the membrane. The permeability increases with the degree of grafting. Basic metabolites show higher permeation rates through the modified membrane as acidic metabolites, in particular phosphate salts. The permeabilities of high molecular weight compounds are low.

18.2.22 Characterization of Polyolefin Graft Copolymers The characterization of the grafted functionality in modified polyolefins is difficult because the small number of modified units are overwhelmed by the normal polyolefin repeat units. The content of modified units is typically only about one to five modified units per molecule in a polymer of typical molecular weight of 20 to 40 k Dalton.1, 2 Some experimental techniques to characterize modified polyolefins are summarized in Table 18.8.

Grafting

637

Table 18.9: Polymers Used for Grafting Polymer

Grafting Agent

Reference

Poly(styrene) Poly(vinyl chloride) Poly(alkylene terephthalate) Starch Starch

Maleic anhydride n-Butyl methacrylate Nadic anhydride Vinyl acetate Methyl acrylate

68 69 70 71 72

18.2.23 PVC/LDPE Melt Blends In blends of a low density poly(ethylene) (LDPE) with polyvinyl chloride (PVC) during melt blending, chemical reactions take place.73 This is indicated by changes in the molecular weight, Mn and Mw number-average molecular weight, the polyene and the carbonyl indices, color changes, and the changes of the glass transition and decomposition temperatures. By mixing of LDPE to PVC and melt blending, short-chain LDPE grafted PVC (s-LDPE-g-PVC) copolymers are formed. On the other hand, the dehydrochlorination reaction of PVC was suppressed.

18.3 OTHER POLYMERS Table 18.9 summarizes polymer types other than polyolefins that have been used for grafting other units.

18.3.1 Poly(styrene) Functionalized with Maleic anhydride Maleic anhydride (MA) can be grafted to poly(styrene) (PS) by reactive extrusion in the presence of a free-radical initiator, namely 1,3-Bis(tertbutylperoxyisopropyl)benzene. Its half-life is about 2.5 min at 180°C. The introduction of the maleic anhydride units in PS proved to be very effective for controlling the morphology of blends of PA6 with modified PS. The rheological properties of the blends indicate the formation of long branching between the amine end groups of PA6 and the maleic anhydride unit of maleic anhydride grafted poly(styrene) (MPS) during melt mixing.68

18.3.2 Multifunctional Monomers for PP/PS Blends Polyolefins, do not have reactive functionalities. There are two commonly used approaches for compatibilization in reactive extrusion.74

638

Reactive Polymers Fundamentals and Applications 1. In the two-step process, polymers are functionalized selectively in the first step, and then blended in an extruder in the second step. The grafting reaction should occur between the functionalized groups during blending, and graft co-polymers are formed in-situ. 2. In the one-step process, low molecular weight compounds are added into the melted blends to initiate grafting and coupling reactions at the phase interface to form graft or block copolymers during the extrusion process.

Peroxides cause serious chain scission of the PP backbone, which affects the properties of the alloys. Multifunctional monomers, such as glycol trilinoleate (GTL), trimethylolpropane triacrylate (TMPTA), diethylene glycol diacrylate (DEGDA) or tripropylene glycol diacrylate (TPGDA) in combination with dicumyl peroxide (DCP) can suppress the PP degradation efficiently, and promote the grafting reaction to some extent at the same time. GTL is prepared by the esterification of glycerol with linoleic acid.74

18.3.3 Poly(ethylene-co-methyl acrylate) Maleic anhydride can be melt-grafted onto poly(ethylene-co-methyl acrylate). The grafting is enhanced with a comonomer, i.e., divinylbenzene or vinyl-4-tert-butylbenzoate. A suitable radical initiator is 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane (LUPERSOL™ 231). The processing temperature of the internal batch mixer is at 140°C. It was observed that styrene and vinyl-4-tert-butylbenzoate can significantly increase the amount of anhydride grafted. The styrene comonomer system is most efficient.75 The use of 1-dodecene in this system showed primarily a plasticizer effect.

18.3.4 n-Butyl methacrylate Grafted onto Poly(vinyl chloride) Melt grafting of n-butyl methacrylate onto poly(vinyl chloride) was achieved by a melt mixing process with a free-radical initiator.69 A maximum of 14% graft was obtained. The graft copolymer showed significant improvement in processability and both thermal and mechanical properties.

Grafting

639

18.3.5 Starch Esterification Starch esters with low degrees of substitution are prepared in aqueous media by batch methods.76 Extrusion is not used widely for modification of starch, however, it has great potential. Extruders have been used to manufacture carboxymethylated and cationic potato starch, starch phosphates, anionic starch, and oxidized starches.77–80 Starch esters can be synthesized by extruding 70% amylose starch with fatty anhydrides and sodium hydroxide as catalyst in a single-screw extruder. The sodium hydroxide neutralizes the organic acids formed in the course of the reaction. Acetic anhydride, propionic anhydride, heptanoic anhydride, and palmitic anhydride have been used.81 The degrees of substitution of esterified starch can be determined by hydrolyzing substituted groups with NaOH and then titrating back with acid. The degree of substitution coincides with the expected value from the monomer feeds. Some molecular weight reduction of the amylopectin fraction was detected in the esterified products from cornstarch with a 70% amylose content. Lower molecular weights and higher levels of anhydride resulted in the greatest reduction in starch molecular weight. The acid esters decrease the hydrophilic character of the starch. The introduction of heptanoic anhydride and palmitic anhydride result in a higher water absorption index. This is explained by the disruption of the crystalline structure of the starch. By disrupting the crystalline structure of the starch, the opportunity for hydrogen-bonding between starch and water is increased. Clearly, the heptanoic and palmitic acid residues provide a more significant steric hindrance for the formation of starch crystals than the smaller acetic and propionic acid residues. Another approach for the acetylation of starch is the use of vinyl acetate and sodium hydroxide.71 The acetylation reaction is accompanied by the hydrolysis of vinyl acetate and a consecutive hydrolysis reaction of the acetylated starch. The degree of substitution could be varied from 0.05 to 0.2.

18.3.6 Starch Grafted Acrylics Starch graft poly(methyl acrylate) (S-g-PMA) could be prepared from an aqueous cornstarch slurry and methyl acrylate by the initiation with ceric ions. At the end of the reaction, an additional small amount of ceric ion solution was added. After this addition no unreacted methyl acrylate mono-

640


mer remained.72 The grafted starch is intended for the use as loose-fill foam. This type of loose-fill foam has a better moisture and water resistance than other starch-based materials. Graft copolymers of starch and poly(acrylamide) could be prepared by reactive extrusion with ammonium persulfate as initiator.82

18.3.7 Thermoplastic Phenol/Formaldehyde Polymers Phenol/formaldehyde resins with high viscosity are needed in reactive extrusion with poly(propylene) to establish a favorable viscosity ratio. Most commercially available phenol/formaldehyde resins have a molar mass of 0.5 to 1 k Dalton. Only thermoplastic phenol/formaldehyde polymers of the novolak type meet the requirement of avoiding crosslinking in the extruder. High molecular weight novolak-type resins can be obtained by adjusting the ratio of formaldehyde to the phenol near unity.83

18.3.8 Polyesters and Polyurethanes A number of techniques for polymerizing radical polymerizable monomers with polyester resins and polyurethane resins to obtain graft or block reaction products have been published. The graft or block reaction products have been studied to improve, for example, the impact resistance of molding compounds by using them as a compatibilizer, the adhesiveness of paints and adhesives to substrates, the curing property of the paints and adhesives, and the dispersibility of pigments.84 The modification of high molecular weight polyesters introduces polymerizable unsaturated double bonds into the main chain or into the molecular terminal groups. The double bonds can be polymerized with radical polymerizable monomers by graft or block polymerization. Similarly, graft or block modifications for polyurethane can be achieved. When a high molecular weight polyester or polyurethane is grafted for the modification, crosslinking between the polyester molecules or the polyurethane molecules is more likely. 18.3.8.1

Polyesters

In the case of polyesters, the sum of polymerizable unsaturated double bonds is desirably up to 20 mol-% of the total acid components and diol

Grafting

641

components. When the sum exceeds 20 mol-%, various properties of the base resin itself are largely reduced. 18.3.8.2

Polyester Polyurethanes

The polyester polyurethanes should contain up to 30 polymerizable unsaturated double bonds in one molecule. 18.3.8.3

Radical Polymerizable Monomers

Radical polymerizable monomers are a mixture of an electron accepting monomer and an electron donor monomer. This combination allows controlling the gelation, even if the resin has a very large amount of unsaturated bonds. Electron donor monomers are styrene, α-methyl styrene, tertbutyl styrene, and N-vinyl pyrrolidone.85 Electron accepting monomers are fumaric acid, monoesters, and diesters of fumaric acid. Basically gelation can be avoided by a dilution of the polymeric vinyl groups by monomeric vinyl groups that are more prone to copolymerize. 18.3.8.4

Grafting Reaction

This technique is a graft polymerization of the polymerizable unsaturated double bond existing in the base resin, i.e., the main chain with the radical polymerizable monomers. The graft polymerization reaction is performed by reacting the base resin, which is dissolved in an organic solvent, with a mixture of the radical polymerizable monomers and a radical initiator. Suitable radical initiators are organic peroxides and organic azo compounds. The organic peroxides include dibenzoyl peroxide and tert-butylperoxypivalate and the organic azo compounds include 2,2′ -azobis(isobutyronitrile) and 2,2′ -azobis(2,4-dimethylvaleronitrile). A chain transfer agent such as octyl mercaptane, dodecyl mercaptane, 2-mercaptoethanol, and α-methyl styrene dimer may be used to control the grafted chain length. The solvents that can be utilized include methylethylketone, methylisobutylketone, cyclohexanone, toluene, xylene, ethyl acetate, and butyl acetate. The solvent itself should neither decompose the radical initiator by induced decomposition nor create a combination with the initiator which

642


O +

O

H ene

O O H

O enophil

O

Figure 18.9: Basic Mechanism of the Ene Reaction

causes a danger of explosion that has been reported between specific organic peroxides and specific ketones. Furthermore, it is important that the solvent has a suitably lower chain transfer constant as a reaction solvent for the radical polymerization.84

18.3.9 Polyacrylic Hot-melt Pressure-sensitive Adhesive A polyacrylic hot-melt pressure-sensitive adhesive is prepared as follows. A copolymer consisting of acrylic acid, tert-butyl acrylamide, maleic anhydride, 2-ethylhexyl acrylate, n-butyl acrylate is manufactured in acetone/isopropanol solution, with 2,2′ -azobis(2-ethylpropionitrile) as initiator in a batch reactor. This polymer contains anhydride groups that are useful for coupling. The polymer is then degassed from the solvent in an extruder. In the next step, the acrylic hot-melt is compounded with 2-hydroxypropyl acrylate. Pendent acrylate groups are formed in this way. This offers the advantage of very gentle crosslinking methods, since crosslinking can be carried out directly by way of the installed acrylate groups. The hot-melt exhibits viscoelastic behavior at room temperature.86

18.4 TERMINAL FUNCTIONALIZATION 18.4.1 Ene Reaction with Poly(propylene) Polyolefins prepared with Ziegler-Natta processes or metallocene catalysts may carry olefinic end groups. Olefinic end groups are also introduced by melt degradation. A poly(propylene) functionalized at the end groups with anhydride can be obtained via the Alder-ene reaction from a low molecular weight

Grafting

643

amorphous poly(propylene) by reactive extrusion. The Alder-ene reaction is a pericyclic reaction with a 6-center intermediate. It involves the reaction of an ene and a enophil. The ene moiety in the Alder-ene reaction is a double bond with an allylic hydrogen. The basic mechanism is shown in Figure 18.9. The ene reaction is reversible.87 However, the reverse reaction seems to be not a simple retro-ene process. The rate of the Alder-ene reaction depends on the acidity and basicity of ene and enophile, respectively. Lewis acids, like SnCl4 , TiCl4 , and AlCl3 develop fumes of hydrochloric acid during reaction. However, a less reactive Lewis acid, SnCl2 ·2 H2 O, can also catalyze the reaction without the drawback of developing HCl. The reaction is complete at 230°C within 5 min in the presence of a stable radical, such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), which acts as a free radical scavenger. This prevents the maleic anhydride from being grafted onto the backbone of the poly(propylene).4, 88 The maleation of polypropylene by reactive extrusion via the Alderene reaction produces a terminal functionality of the polymer without significant chain scission.

18.4.2 Styrene-butadiene Rubber The end capping of living anions of poly(styrene-butadiene) can be done with polymeric terminator molecules. A polar functional terminator is a block copolymer of poly(ethylene glycol) and poly(dimethylsiloxane) (PEG-PDMS) containing a chlorosilyl moiety at one chain end. This polymer is synthesized by two-step hydrosilylation reaction.89 The PEG-PDMS end groups behave as polar functional groups, showing an increase of the glass transition temperature and storage modulus in a composite of endcapped SBR with silica particles.

18.4.3 Diels-Alder Reaction A benzocyclobutene (BCB) capped polymer can be used to react in a Diels-Alder reaction with another polymer bearing a dienophile.90 4-(3-iodopropyl)benzocyclobutene was used to terminate an anionic polymerization of styrene to give a poly(styrene) end-capped with BCB. A copolymer of 1-hexene and 7-methyl-1,6-octadiene was prepared by Ziegler-Natta polymerization, with the pendant double bonds intended as the grafting sites. The reaction is illustrated in Figure 18.10.

644


CH

CH

CH2

CH2

CH2

CH2

CH2

CH2

CH

+

H

C

C H 3C

CH3

C

H3C CH3

Figure 18.10: Grafting of an Isomerized Benzocyclobutene Unit to a Polyolefin Dienophile90

18.5 GRAFTING ONTO SURFACES 18.5.1 Grafting onto Poly(ethylene) 18.5.1.1

Sulfonic Acid Groups

In order to introduce sulfonic acid groups on poly(ethylene), poly(ethylene) samples are irradiated with UV light in a gas atmosphere containing SO2 and air to achieve a photosulfonation of the surface. The surface modification is carried out under atmospheric pressure and is considered to be an inexpensive alternative to plasma modification techniques. The hydrophilicity of the PE surface increases considerably compared to unreacted PE. The depth of photomodification reached several µ. Because of the large depth of modification, the process may also be useful for the modification of membranes. In combination with projection lithography the process could be suitable for the manufacture of gratings in thin polymer films, as required for holographic recordings and distributed feedback lasers.91 18.5.1.2

Sulfate Groups

Sulfate groups at the surface of poly(ethylene) are introduced by immobilizing a precoated layer of either sodium 10-undecenyl sulfate (SUS) or sodium dodecyl sulfate (SDS) on the polymeric surface by means of an argon plasma treatment. SUS is synthesized by sulfating 10-undecene1-ol with the pyridine-SO3 complex. The presence of sulfate groups at the polymeric surfaces was confirmed by X-ray photoelectron spectroscopy

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(XPS). The presence of an unsaturated bond in the alkyl chain of the surfactant improved the efficiency of the immobilization process. About 25% of the initial amount of sulfate groups in the precoated layer was retained at the PE surface for SUS, but only 6% for SDS.92 18.5.1.3

Photochemical Bromination

The gas phase bromination of poly(ethylene), poly(propylene) and poly(styrene) film surfaces by a free-radical photochemical mechanism occurs with high regioselectivity. The surface bromination is accompanied by a simultaneous dehydrobromination. This results in the formation of long sequences of conjugated double bonds. Thus, the brominated polyolefin surface contains bromide moieties in different chemical environments.93 In contrast, the gas phase free radical photochemical chlorination of polyolefin films proceeds in a rather random way and is also accompanied by simultaneous dehydrochlorination. 18.5.1.4

Poly(thiophene)

Poly(thiophene) (PT) can be grafted on a PE film using three reaction steps. 1. PE films are brominated in the gas phase, yielding PE-Br. 2. A substitution reaction of PE-Br with 2-thiophene thiolate anion gives the thiophene-functionalized PE. 3. PT is grafted on the PE surface using chemical oxidative polymerization to give PE-PT. The polymerization is performed in a suspension solution of anhydrous FeCl3 in CHCl3 , yielding a reddish PE-PT film after dedoping with ethanol. Infrared spectroscopy reveals that the PT is grafted on PE in the 2,5-position. SEM imaging shows islands of PT on the PE film. The thickness of the islands is in the range of 120 to 145 nm. The conductivity of these thin films is in the range of 10−6 S cm−1 , which is a significant increase from the value of 10−14 S cm−1 measured for an ungrafted PE film.94 18.5.1.5

Acrylics

Ion beam-modified poly(ethylene) was exposed to the solutions of acrylic acid, acrylonitrile, and bromine.95 The chemical and structural changes

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were examined using spectroscopic techniques, electronparamagnetic resonance, and Rutherford back-scattering techniques. Acrylic acid, acrylonitrile, and bromine react with radicals and conjugated double bonds created by the ion irradiation in the poly(ethylene). The reactions in the ion beam-modified surface layer may lead to the creation of a grafted surface layer with a thickness of up to 150 nm. Surface photo grafting of high density poly(ethylene) (HDPE) powder can be achieved with a pretreated HDPE surface by benzophenone (BP). Onto such a surface, acrylic acid can be graft copolymerized by photo grafting in the vapor phase.96 The most suitable reaction temperature is 90°C. The grafting degree can reach a comparable high value of 10%. 18.5.1.6

Siloxane

The dyeing properties on high-strength and high modulus poly(ethylene) fibers are improved by building up a layer of a polysiloxane network. The grafting of siloxane onto poly(ethylene) proceeds first via a treatment with peroxide. Hydrogen peroxide in o-xylene emulsion is emulsified by sonication. The emulsion is effective for introduction of hydroxide groups onto the poly(ethylene) fiber surface. The treatment does not influence the tensile strength of the fiber. A polysiloxane network can be built up on the fiber surface by treating the surface with a (3-aminopropyl)triethoxysilane (APS) solution.97 In fact, this method can be used to dye a poly(ethylene) fiber surface. 18.5.1.7

Silicone

The surface graft copolymerization of hydrogen silicone fluid onto a low density poly(ethylene) (LDPE) film through corona discharge shows an improved hydrophobicity of the grafted LDPE films. However, the mechanical properties decrease slightly. Thus there is evidence that HSF can be graft copolymerized onto an LDPE film surface through corona discharge.98 18.5.1.8

Surface Crosslinking

Ultra-high molecular weight poly(ethylene) (UHMWPE) can be crosslinked at the surface by irradiation with electron beams.99 Attenuated total

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reflectance Fourier transform infrared spectroscopy (ATR-FTIR) infrared techniques suggest that the irradiation in air atmosphere introduced hydroperoxide groups into the polymer without formation of any other oxygencontaining groups. The generated hydroperoxides could be decomposed further by subsequent heat treatment of the irradiated polymer, resulting in crosslinking of UHMWPE chains in the region of the material near the surface. As a result of this surface modification, the surface hardness of UHMWPE substantially increases.

18.5.2 Grafting onto Poly(tetrafluoroethylene) 18.5.2.1

Diazonium Salts

Functionalization of poly(tetrafluoroethylene) (PTFE) surfaces can be achieved by diazonium salts. Reduced PTFE can be grafted by nitro and bromophenyldiazonium tetrafluoroborate salts in a manner similar to that used for carbon, except that no application of a reductive potential during grafting is required. The grafting is evidenced by cyclic voltametry, Xray fluorescence or time of flight single ion monitoring mass spectroscopy (TOF-SIMS).100–102 18.5.2.2

Epoxide-containing Monomers

A pretreated PTFE film with argon plasma can be further modified by a graft copolymerization with hydrophilic and epoxide-containing monomers. The grafting is initiated by UV light. Functional monomers for grafting include acrylic acid (AA), sodium salt of p-styrenesulfonic acid, N,N-dimethylacrylamide (DMAA), and glycidyl methacrylate (GMA). A stratified surface microstructure with a significantly higher ratio of substrate to grafted chains in the top surface layer than in the subsurface layer is always obtained. The grafted PTFE films show a number of new issues. These include:103 • Covalent immobilization of an enzyme, such as trypsin, for AA graft copolymerized surface, • Change transfer included coating of an electroactive polymer, such as polyaniline, for AA and styrenesulfonic acid graft copolymerized surfaces,

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Reactive Polymers Fundamentals and Applications • Adhesive-free adhesion between two PTFE surfaces, for AA, styrenesulfonic acid and DMAA graft copolymerized surfaces, • Improved adhesive bonding via interfacial crosslinking of the grafted chains, for GMA graft copolymerized surfaces.

18.5.2.3

2-Hydroxyethyl acrylate

Surface modifications of Ar plasma pretreated poly(tetrafluoroethylene) (PTFE) film via graft copolymerization improve the adhesion of copper. The PTFE film surface is initially modified by graft copolymerization with a monomer, such as 2-hydroxyethyl acrylate and acrylamide. These monomers contain the functional groups for epoxide groups. The modified PTFE surface is subsequently again exposed to an Ar plasma and subjected to UV induced graft copolymerization with glycidyl methacrylate.104 18.5.2.4

Glycidyl Methacrylate

The surface modification of a PTFE film is done by the deposition of glycidyl methacrylate (GMA) in the presence of H2 plasma activation of the PTFE substrates. The H2 plasma treatment results in an effective defluorination and hydrogenation of the PTFE surface. This enhances the adhesion of Cu vapor onto the PTFE surface. In addition, a plasma polymerization with glycidyl methacrylate is performed. High adhesion strength for the Cu on such a surface is obtained only in the presence of H2 plasma activation of the PTFE substrates prior to the plasma polymerization and deposition of GMA. In the absence of H2 plasma pre-activation, the deposited pp-GMA layer on the PTFE surface can be readily removed by acetone extraction. The enhancement of the adhesion of the Cu on the surface is attributed to the covalent bonding of the pp-GMA layer with the PTFE surface, the preservation of the epoxide functional groups in the pp-GMA layer, and the strong interaction of evaporated Cu atoms with the epoxide and carboxyl groups of the GMA chains.105 18.5.2.5

Oxygen and Ammonia Plasmas

PTFE can be treated in oxygen or ammonia plasmas in order to introduce oxygen-containing or nitrogen-containing groups, respectively. These

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groups increase the surface free energy and allow the adsorption of polyelectrolytes via electrostatic interactions.106 The effects of such a modification can be evaluated by means of contact angle measurements.

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96. J. X. Lei, J. Gao, R. Zhou, B. S. Zhang, and J. Wang. Photografting of acrylic acid on high density polyethylene powder in vapour phase. Polym. Int., 49(11):1492–1495, November 2000. 97. H. Fujimatsu, M. Imaizumi, N. Shibutani, H. Usami, and T. Iijima. Modification of high-strength and high-modulus polyethylene fiber surfaces for the purpose of dyeing. Polym. J., 33(7):509–513, 2001. 98. M. Li, J. X. Lei, J. Gao, and Z. J. Su. Surface graft copolymerization of hydrogen silicone fluid onto low density polyethylene film through corona discharge and the properties of grafted film. Polym.-Plast. Technol. Eng., 42(2):207–215, 2003. 99. O. N. Tretinnikov, S. Ogata, and Y. Ikada. Surface crosslinking of polyethylene by electron beam irradiation in air. Polymer, 39(24):6115–6120, November 1998. 100. C. Combellas, F. Kanoufi, D. Mazouzi, A. Thiebault, P. Bertrand, and N. Medard. Surface modification of halogenated polymers. 4. functionalisation of poly(tetrafluoroethylene) surfaces by diazonium salts. Polymer, 44(1):19–24, January 2003. 101. C. Combellas, F. Kanoufi, D. Mazouzi, and A. Thiebault. Surface modification of halogenated polymers: 5. localized electroless deposition of metals on poly(tetrafluoroethylene) surfaces. J. Electroanal. Chem., 556:43–52, September 2003. 102. C. Combellas, A. Fuchs, F. Kanoufi, D. Mazouzi, and S. Nunige. Surface modification of halogenated polymers. 6. graft copolymerization of poly(tetrafluoroethylene) surfaces by polyacrylic acid. Polymer, 45(14): 4669–4675, June 2004. 103. E. T. Kang, K. G. Neoh, K. L. Tan, B. C. Senn, P. J. Pigram, and J. Liesegang. Surface modification and functionalization of polytetrafluoroethylene films via graft copolymerization. Polym. Adv. Technol., 8(11): 683–692, November 1997. 104. S. Y. Wu, E. T. Kang, K. G. Neoh, and K. L. Tan. Surface modification of poly(tetrafluoroethylene) films by double graft copolymerization for adhesion improvement with evaporated copper. Polymer, 40(25):6955–6964, December 1999. 105. X. P. Zou, E. T. Kang, K. G. Neoh, C. Q. Cui, and T. B. Lim. Surface modification of poly(tetrafluoroethylene) films by plasma polymerization of glycidyl methacrylate for adhesion enhancement with evaporated copper. Polymer, 42(15):6409–6418, July 2001. 106. U. Lappan, H. M. Buchhammer, and K. Lunkwitz. Surface modification of poly(tetrafluoroethylene) by plasma pretreatment and adsorption of polyelectrolytes. Polymer, 40(14):4087–4091, June 1999.

19 Acrylic Dental Fillers Polymers in dental applications are used as restorative materials, cements, adhesives, cavity liners, and as protective sealants for pits and fissures. The use of composite resins is recommended for amalgam replacement. Polymers are further used as denture base materials, denture relines, crown and bridge resins, dental impressions, and duplicating materials. There are monographs on the topic.1–5 Polymeric materials used in dental applications must meet certain physical, chemical, biological, and aesthetic requirements. These requirements include • • • • • • •

Adequate strength Resilience, Abrasion resistance, Dimensional stability, Color stability, Resistance to body fluids, Tissue tolerance, low allergenicity, toxicity, mutagenicity, carcinogenic responses.

Further, the materials should be easy to use and should not be expensive. 657

658


19.1 HISTORY The first polymeric materials used in dental applications were guttapercha, celluloid, phenol/formaldehyde, and acrylic resins. Polymers such as acrylics, poly(styrene)s, poly(carbonate)s, and polysulfones can be injection-molded to yield dentures with outstanding toughness, high fatigue strength, and low water absorption. Most common are acrylic-based resins. However, other classes also gain importance, such as spiro orthocarbonates, cycloaliphatic epoxy compounds, cyclic ketene acetals and 2-vinylcyclopropanes,5, 6 because of the demand for low shrinkage materials. Often these monomers are in combination with acrylic-based resins. We will mention these classes briefly here. Acrylic resins have been used in the construction of denture bases since 1930.4 Multifunctional acrylates and methacrylates can be polymerized to crosslinked polymers to be used as restorative materials. A polymerization involving cold curing is carried out with redox initiators at ambient temperature. Bisphenol A diglycidyl ether dimethacrylate with a ceramic filler opened a new area in the state of the art. A silane coupling agent between ceramics and organic polymer increases the adhesion strength. The principle of photopolymerization for dental resins was introduced around 1975. Also, a polyurethane resin, based on polyurethane dimethacrylate and similar monomers was developed7 that can be cured with visible light.

19.2 POLYMERIC COMPOSITE FILLING MATERIALS An overview of the ingredients in a dental composite is given in Table 19.1. Dental polymeric composite filling materials consist of di- and trifunctional monomer systems that undergo crosslinking in the course of polymerization. Reinforcing fillers are silanized quartz, glass, and ceramics. The polymerization must be initialized effectively under oral conditions. Various additives may increase the chemical stability of the cured materials. Dental sealants mostly are not filled with reinforcing fillers. Dental composites may be used as two-component formulations or as a one-component formulation.

Acrylic Dental Fillers

659

Table 19.1: Constituents in a Dental Composite Compound Type Organic resin Initiator systems Polymerization inhibitors Fillers Pigments Coloring or tint agents Caries inhibiting agents Fluoride release agents UV-absorbers Stabilizers Surfactants Thickening agents

19.3 MONOMERS Common monomers are shown in Table 19.2. Some acrylics and methacrylics are shown in Figure 19.1.

19.3.1 Acrylics and Methacrylics Most common thermosets are methacrylate based, for example, 2,2-bis[p(2-hydroxy-3-methacryloxypropoxy)phenyl]propane, i.e., the bisphenol A adduct of glycidylmethacrylate (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA), c.f. Figure 19.1. 19.3.1.1

Urethane-modified Acrylics

The reaction product of 1,6-hexamethylenediisocyanate and ethylene glycoldimethacrylate or other glycol esters is also a suitable monomer. Other urethane dimethacrylates are 1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane (UDMA). Further polyurethane dimethacrylate (PUDMA) is commonly used as a principal polymer in dental restoratives of this type. Urethane derivatives of Bis-GMA exhibit lower viscosities and are more hydrophobic than Bis-GMA. In general, the viscosities of these monomers decrease with increasing chain length of the alkyl urethane substituent. Since Bis-GMA, PUDMA, and others are still highly viscous at

660


Table 19.2: Monomers for Dental Polymers Vinyl Monomer

Reference

2,2-Bis[4-(3-methacryloxy-2-hydroxypropoxy)phenyl]propane (Bis-GMA) Bisphenol A dimethacrylate (Bis-A-Dima) Ethoxylated Bis-A-Dima Triethylene glycol dimethacrylate (TEGDMA) Ethoxylated bisphenol A dimethacrylate (EBPDMA) Hydroxyethyl methacrylate (HEMA) Hydroxyethyl methacrylate maleic anhydride adduct (HEMAN) 1,1,1-Trimethylolpropane trimethacrylate (TMPTMA) Tetrahydrofurfuryl cyclohexene dimethacrylate (TCDM) Hexafunctional methacrylate ester (HME) 1,6-Hexanediol dimethacrylate (HDDMA) 2-Isocyanatoethyl methacrylate (IEM) Di-2-methacryloxyethyl-2,2,4-trimethylhexamethylene dicarbamate (UDMA) Polyurethane dimethacrylate (PUDMA) esters Tetrahydrofurfuryl methacrylate (THFMA) Glycidyl methacrylate (GMA) Methacryloyl-β-alanine (MBA) Methacryloyl glutamic acid Acryloyl-β-alanine Acryloyl glutamic acid Poly(carbonate)dimethacrylate (PCDMA) Cyclic Monomers α-Methylene-γ-butyrolactone Epoxy Monomers Cycloaliphatic diepoxide Epoxylated vinyl ether

8

9 10

11 11 11 11 12

Reference 13

Reference 14 14


OH

CH3

CH CH2

C

CH2

CH3

661

OH CH2

CH CH2

O

O

C O

O C

C CH3

H3C

CH2

C CH2

Bis-GMA O CH3 O C C CH2 CH2

O CH2 CH2

CH2

O CH2 CH2 O C C CH2 O CH3 TEGDMA CH3

CH2

C C O CH2

CH2

OH

O HEMA CH3 CH2

C C O CH2

O

O THFMA CH3 CH2

C C O CH2

CH2

N C O

O IEM CH3 CH2

C C O CH2

CH2 O

O GMA

Figure 19.1: 2,2-Bis[p-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (Bis-GMA), Triethylene glycol dimethacrylate (TEGDMA), Hydroxyethyl methacrylate (HEMA), Tetrahydrofurfuryl methacrylate (THFMA), 2-Isocyanatoethyl methacrylate (IEM), Glycidyl methacrylate (GMA)

662

Reactive Polymers Fundamentals and Applications Table 19.3: Example for a Resin Matrix9 Monomers

%

Ethoxylated bisphenol A dimethacrylate Polyurethane dimethacrylate ester 1,6-Hexanediol dimethacrylate

30 50 20

Initiator System

phr

Camphorquinone Ethyl-4-dimethylamino benzoate 2,4,6-Trimethylbenzoyldiphenylphosphine oxide

0.1 0.3 0.2

room temperature, they are generally diluted with an acrylate or methacrylate monomer having a lower viscosity, such as trimethylolpropyl trimethacrylate, 1,6-hexanediol dimethacrylate, or 1,3-butanediol dimethacrylate. Other dimethacrylate monomers, such as ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, poly(ethylene glycol)dimethacrylate (PEGDMA) and tetraethylene glycol dimethacrylate, are also in general use as diluents.7 The photopolymerization of these monomers shows high degrees of conversion of the vinyl groups in comparison to Bis-GMA. Polymers with lower polymerization shrinkages at equivalent degrees of vinyl conversion than Bis-GMA are obtained. The refractive indices of the urethane derivatives were similar to Bis-GMA. However the flexural strengths of the polymers are lower than that of the Bis-GMA homopolymer. The flexural strengths decrease with increasing chain length of the alkyl urethane substituent.15 An example for a resin matrix is shown in Table 19.3. 19.3.1.2

Isocyanatomethacrylates

The potential utility of isocyanatomethacrylates in dental adhesives arises from the possibility of dual modes of reaction, i.e., free-radical polymerization via the methacrylate double bonds, and the reaction via the NCO group with active hydrogens in a suitable compound to be admixed.10 19.3.1.3

Nematic Acrylics

Zero polymerization shrinkage is one of the most necessary features of a dental restorative so that accumulated stresses do not debond the dentinrestorative interface or fracture the tooth or restorative which can result in


663

marginal leakage and microbial attack. This feature is also important in bone repair and in accurate reproduction of photolithographic imprints and optical elements. Attempts have been made to reduce polymerization shrinkage by utilizing nematic liquid crystal monomers. The expected low polymerization shrinkage for such compounds originates from the high packing efficiency that already exists in the nematic state, thus minimizing the entropy reduction that occurs during polymerization. Liquid crystal monomers or prepolymers have another advantage in that the viscosity is lower than that of an isotropic material of the same molecular weight.16 An example for a liquid crystalline methacrylate is shown in Figure 19.2, i.e. 4,4′ -Bis(2-hydroxy-3-methacryloylpropoxy)biphenyl esterified with 4′ -cyano-4-biphenyloxyvaleric acid. The liquid crystalline di(meth)acrylate is synthesized by the reaction of 2,3-epoxypropoxy methacrylate with 4,4′ -dihydroxybiphenyl to form a methacrylate-terminated macromonomer having hydroxyl groups. The macromonomer hydroxyl groups are then esterified with 4′ -cyano-4-biphenyloxyvaleric acid. The (meth)acrylate polymerizes quantitatively and with very low volume shrinkage of less than 2.5%.17 19.3.1.4

Amino Acid Derivatives of Acrylics

It is known that unreacted 2-hydroxyethyl methacrylate (HEMA) in current resin-modified glass ionomer cements (RMGICs) shows potential cytotoxicity to pulp and surrounding tissues.18 Amino acid acrylate and methacrylate derivatives were found to be suitable in light curable glass-ionomer cements (LCGIC). Methacryloyl and acryloyl derivatives of the amino acids can be synthesized via the Schotten-Baumann reaction. Among several derivatives, methacryloyl-β-alanine (MBA) has a particularly low solution viscosity and a high compressive strength. The LCGIC system based on amino acid derivatives is free from HEMA. This system may eliminate a potential cytotoxicity in LCGICs caused by leached HEMA. Optimal MBA-modified cements are higher in certain mechanical properties in comparison to conventional cements.11 19.3.1.5

Phosphoric Esters

Phosphoric acid esters with pendant acrylate or methacrylate functions serve as adhesion promoters to fillers such as surface active glasses. Ex-

664


CH2 CH CH3 C O O CH2 HC H2C O O C O

CH2 H3C CH O C O CH2 O CH2 CH O O C

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

O

O

C

C

N

N

Figure 19.2: Nematic Monomer, 4,4′ -Bis(2-hydroxy-3-methacryloylpropoxy)biphenyl esterified with 4′ -cyano-4-biphenyloxyvaleric acid17, 19


665

amples are 2-(methacryloyloxy)ethyl phosphate, bis[2-(methacryloyloxy)ethyl]phosphate7 or pentaerythritol trimethacrylate monophosphate. They added up to 5% with respect to the organic curable composition. 19.3.1.6

Hydrophobic-Modified Acrylics

Hydrophobic composites stronger than methacrylate prepolymers, are the corresponding analogues where most of the hydrogens are replaced by fluorine.

19.3.2 Cyclic Monomers Cyclic monomers generally exhibit less shrinkage in the course of polymerization as the polymerization process occurs by a ring opening reaction, in contrast to vinyl monomer that is basically the ring opening of a two membered ring, i.e., the double bond. α-Methylene-γ-butyrolactone (MBL) is an expanding monomer and does not cause shrinkage of the material during polymerization. It can be described as the cyclic analog of methyl methacrylate, and it exhibits greater reactivity in free-radical polymerization than conventional methacrylate monomers.13 19.3.2.1

Spiroorthocarbonates

Spiroorthocarbonates (SOC), spiroorthoesters (SOE) and bicyclic orthoesters are attractive because they show a very low shrinkage or even expansion during polymerization.20, 21 Spiroorthocarbonates with polymerizable double bonds have been investigated; some of them are shown in Figure 19.3. A few are bearing methacrylic substructures.21 Spiroorthocarbonates with seven membered rings show a high tendency of ring opening when they undergo a radical polymerization. This is favorable for low shrinkage. Spiroorthocarbonate compounds that include epoxy groups as a substituent have been described.22 The synthesis of 7,26-Dioxatrispirobicyclo[4.1.0]heptane-4,5′-1,3-dioxane-2′ ,2′′ -1,3-dioxane-5′′ ,4′′ -bicyclo[4.1.0]heptane (DCHE) is shown in Figure 19.4. It has been concluded that although the polymerization shrinkage has been one of the main shortcomings of resin-based composites, the ring-opening polymerization of cyclic monomers has not been successfully achieved for commercial dental filling materials.5

666


O

O

O

O

O

O

O

O

O O

O

O

O

O

O O

O O

O O

O O

Figure 19.3: Spiroorthocarbonates with Polymerizable Double Bonds21

19.3.3 Epoxy Monomers Acrylate based compositions have the disadvantage of shrinking strongly at hardening. Epoxy compounds containing compositions are known; they can undergo cationic polymerization with low shrinkage. In this case, it is necessary to use a high energy light source for such a polymerization, e.g., a mercury vapor lamp, which cannot be used in medical practices because of the danger of combustion. Certain compositions are not completely cured and do not fulfill the requirements of adhesiveness and abrasiveness. To achieve a complete hardening, it is necessary to apply a thermic aftertreatment, which is not practicable in the mouth of a patient.14 However, a composition obtained by the combination of a cyclic diepoxide, tetrahydrofuran, diphenyliodoniumhexafluorantimonate and camphorquinone by means of accelerators, e.g., 4-dimethylaminobenzaldehyde, 4-dimethylaminophenethanol, dihydroxyethyl-p-toluidine, ethyl4-dimethylamino benzoate can be cured at wavelengths of 400 to 1000 nm. These materials can be used in dental applications.23 N,N-bis-hydroxyalkyl-p-aminobenzoic acid alkyl esters have an excellent efficiency as accelerators of the light induced hardening of a composition based on epoxy compounds.14


Bu OH + O Sn Bu OH

O Bu Sn O Bu

Cl

667

HO +

S C

HO

Cl

O +

S O

O O O O

O O O

O

O O

Figure 19.4: Synthesis of 7,26-Dioxatrispirobicyclo[4.1.0]heptane-4,5′ -1,3-dioxane-2′ ,2′′ -1,3-dioxane-5′′ ,4′′ -bicyclo[4.1.0]heptane (DCHE)22

668

Reactive Polymers Fundamentals and Applications Table 19.4: Chemical Curing Systems24, 25 Catalyst

Promoter

Dibenzoyl peroxide

Tertiary aromatic amines 4-(N,N-Dimethylamino)phenethyl alcohol26 Cobalt salt Thioureas Ascorbic acid Cu2+ Cl-compound Acid Oxygen

Organic peroxides Hydroperoxides Peroxides Barbituric acid Aryl borate Tri-n-butylborane

19.3.4 Highly Loaded Composite In general, a highly loaded composite looks very dry and is hard to handle. Suitable monofunctional monomers may be used to act as a diluent to control or reduce the viscosity of the resin as well as to provide fewer polymerization sites, both of which assist in formulating the composition. The addition of a viscosity controlling monofunctional monomer makes the composition and composites easier to handle.

19.4 RADICAL POLYMERIZATION Initiators for methacrylics generally fall within one of three categories: 1. Cold curing chemical systems that initiate polymerization upon admixing two or more compounds, 2. light initiated initiator systems, 3. heat initiated initiator systems.

19.4.1 Chemical Curing Systems Cold curing chemical systems include traditional free-radical polymerization initiators normally used with polymerizable ethylenically unsaturated materials and resins. A variety of catalysts for chemical polymerization have been proposed. The types of chemical curing systems are summarized in Table 19.4.

Acrylic Dental Fillers 19.4.1.1

669

Peroxide Amine Systems

For example, organic peroxide initiators and amine accelerations may be used. The initiators are admixed with the monomers shortly before application to the tooth or dental appliance.25 The kinetics of curing of several dimethacrylate monomers initiated by a dibenzoyl peroxide amine system has been studied using differential scanning calorimetry.26 A mathematical model was developed to describe the rate of polymerization. Here tertiary amines are aromatic tertiary amines, for example, ethyl-4-dimethylamino benzoate (EDMAB), 2-[4-(dimethylamino)phenyl] ethanol, N,N-dimethyl-p-toluidine (DMPT), bis(hydroxyethyl)-p-toluidine, and triethanolamine. Such accelerators are generally present in the range from about 0.5 to about 4.0% of the resin composition.9 The combination of the organic peroxide and the tertiary amine involves problems such as tinting the cured product due to oxidation of the amine compound and discoloration, and impairing the polymerization due to oxygen and acidic components. An acidic component would produce a quaternary salt which does not exhibit reducing ability upon reacting with the tertiary amine. The problem of tinting or discoloration causes the color tone to differ from that of a natural tooth when the catalyst is used for a dental restorative as represented by a composite resin, and deteriorates the aesthetic value. Impairing the polymerization means that the catalyst cannot be used for the dental adhesive that uses an acid group-containing polymerizable monomer as an essential component.24 19.4.1.2

Hydroperoxides Thiourea Systems

Other redox initiators are hydroperoxides with thioureas, and peroxides with ascorbic acid. A two-part system may be built up as follows: One part contains an initiator. The second part comprises filler and the co-initiator. The two parts are spatuled (mixed) to form a cement prior to placement on tooth. 19.4.1.3

Barbituric Acid-based Initiators

Catalyst systems based on barbituric acid are most generally used in the field of dental materials because of relatively low harmful effect on the body and ready availability. 1-benzyl-5-phenylbarbituric acid can be used

670


Bu

Bu

O

O2

B

Bu

Bu

O

Bu B Bu *O

Bu

Bu

Bu B

B

Bu

Bu O Bu

O

Bu

Bu

B Bu

Bu

B

O* Bu

Bu

Bu O

Bu B

Bu = CH3

CH2

CH2

CH2

Bu*

Bu

Figure 19.5: Radical Generating Mechanisms of Alkylboranes27

in combination with peroxide and with a heavy metal accelerator in a second component.28 However, a barbituric acid-type catalyst can cause problems, such as the difficulty in controlling the curing time and poor preservation stability.24, 29 19.4.1.4

Borane and Borate-based Initiators

Triethylborane initiates a very fast polymerization of methyl methacrylate. Since some radical inhibitors are active in the inhibition, the type of polymerization was identified as a radical polymerization. In a series of experiments, high molecular weight polymers were produced in the presence of p-benzoquinone. Consequently a speculative mechanism was postulated that an adduct of quinone and borane should be responsible for the initiation.30 The basic radical generating mechanism of borane systems is shown in Figure 19.5. Trialkylborane or the partial oxide thereof is an excellent promoter for chemical polymerization and is very active, but it is chemically unstable. Therefore, this catalyst must be packaged separately from other components, must be picked up in suitable amounts just before


671

it is used and must be mixed with other monomer components, requiring cumbersome operation, which is a drawback. Aryl borate as catalyst is easy to handle, does not cause the cured product to be tinted or discolored, and exhibits excellent preservation stability without, however, exhibiting sufficient activity for polymerization. The activity for polymerization is greatly enhanced when an aryl borate compound and an acidic compound are used in combination with a particular oxidizing agent. Suitable peroxides are methylethylketone peroxide, cumene hydroperoxide or tert-hexyl hydroperoxide. A mixture of 2-methacryloyloxyethyldihydrogen phosphate and bis(2-methacryloyloxyethyl)hydrogen phosphate is used as acidic compound. Optional metal compounds are ferric acetylacetonate and copper(II)acetylacetonate. The catalyst is easy to handle, exhibits high activity for polymerization even in the presence of oxygen or an acidic compound, and imparts a suitable degree of surplus operation time. Catalysts that contain a metal compound for promoting the decomposition of the organic peroxide exhibit a particularly high polymerizing efficiency. If the polymerizable monomer is an acidic compound, e.g., 11-methacryloyloxy-1,1-undecanedicarboxylic acid (MAC-10), then there is no need to add any other acidic compound, and no acidic compound elutes out from the obtained cured product when it is used. The mechanism of initiation of polymerization is proposed as follows: The aryl borate compound is decomposed due to the acid compound. Thereby an aryl borane compound is formed which is then oxidized with oxygen present in the atmosphere to form polymerizable radicals. It is further oxidized with an organic peroxide to form more radicals in the composition containing less oxygen. Thereby it serves as a highly active catalyst for chemical polymerization. The metal compound promotes the decomposition of the organic peroxide. Oxidation of the aryl borane compound with the organic peroxide is promoted lending the catalyst itself for use as a more active catalyst for chemical polymerization. The polymerization proceeds at ambient temperature even in a dark place to give an excellently cured product.24 Several techniques have been reported for increasing the work-life, e.g., slowing the cure rate of the polymerizable system by reducing the amount of initiators, adding inhibitors, or adding comonomers to decelerate the cure rate of the free radical composition.31 Examples for work-life extenders are allylsuccinic anhydride, 2-octen-1-ylsuccinic anhydride, iso-

672

Reactive Polymers Fundamentals and Applications Table 19.5: Two Component Formulation32 Base paste Pyrogenic silicic acid (particle size < 0.05 µ) Glass powder, silanized (average particle size 10 µ) 2,2-Bis(4-(oligo(ethoxy))phenyl)propanedimethacrylate N,N-Dimethyl-p-toluidine Hydroquinone monomethyl ether Tributylborane Catalyst paste Glass powder, silanized (average particle size 10 µ) 2,2-Bis(4-(oligo(ethoxy))phenyl)propane diacetate Dibenzoyl peroxide

% 5.00 32.50 61.48 0.50 0.02 0.50 % 32.00 65.50 2.50

butenylsuccinic anhydride, and itaconic anhydride. 19.4.1.5

Hybrid Initiator Systems

A hybrid initiator system acts on a mixture of epoxide monomers and acrylic group-containing monomers. The epoxide monomers are cured by a cationic reaction mechanism. The acrylic group-containing monomers are cured by a radical mechanism. Two initiator systems are needed to ensure polymerization.32 The first initiator system is comprised of boranes and hydrazones and releases species that initiate a curing reaction upon contact with oxygen. The second initiator system is comprised of iodonium compounds capable of radical fission. An oxygen-sensitive compound is used that, when brought into contact with oxygen, can form radicals that in turn can release acid from a saline initiator by means of another reaction sequence. The acid so formed can initiate a polymerization reaction, in particular a cationic polymerization reaction. This saline initiator is an iodonium compound, which, when activated by means of free radicals, can decompose into acids. Thus, there are two initiator systems which react with each other to control the course of the polymerization reaction. A two component material for temporary crowns and bridges, which is mixed in a ratio of 10/1 (base/catalyst) and cured without any smear layer, is shown in Table 19.5. One special advantage of the preparations and processing techniques lies in the fact that the


673

Table 19.6: Photoinitiators for acrylics Compound

Reference

Benzil Camphorquinone Benzoin methyl ether Isopropoxybenzoin Benzoin phenyl ether Benzoin isobutyl ether Eosine Titanocene

8, 33

33 33, 34

range of the activation time and the processing time is determined by the composition of the dental materials. The processor can influence, within specific limits, the requisite processing time by means of the intensity with which the materials are brought into contact with oxygen or air.

19.4.2 Photo Curing Light or photo curing or photosensitive polymerization initiation and curing systems are activated to harden and cure the composition by irradiation with visible or UV light. Visible light of a wavelength of about 400 to 500 nm initiates rapid and efficient curing within a few minutes. Preferably, the photoinitiator systems should be sensitive to light in a range of wavelength that is not harmful to the patient who is undergoing a dental procedure.25 Photoinitiators for acrylics are summarized in Table 19.6. Initiation by photo curing is achieved with α-diketone light-sensitive initiator compounds such as benzophenone or a derivative, or a 1,2-diketone such as benzil or camphorquinone (CQ) and derivatives. Certain tertiary aromatic amines act as accelerator compounds. Some compounds that may be suitable are ultraviolet light-sensitive initiators, like 1,2-diketones, benzophenones, substituted benzophenones, benzoin methyl ether, isopropoxybenzoin, benzoin phenyl ether, and benzoin isobutyl ether, as shown in Figure 19.6. An example for an effective photoinitiator system is camphorquinone (CQ) and ethyl-4-dimethylamino benzoate (EDMAB) or cyanoethylmethylaniline.8 The crosslinking density of the final product is dependent on the way the radiation energy is offered to the curing system.35

674


H 3C

CH3 CH3

O O

O

C C

O Camphorquinone

Benzil O

H 3C N

C O CH2

CH3

H 3C Ethyl 4-(dimethylamino)benzoate

F N Ti

F F N

F Titanocene

Figure 19.6: Photoinitiators


675

The photopolymerization or a Bis-GMA/TEGDMA resin was examined with light sources with extremely different intensities, i.e., 200 and 1800 m W/cm2 ). In general, the polymers irradiated using the high light intensity source showed a greater conversion. However, an increased light intensity also increased the maximum temperature reached during polymerization. Therefore, the greater conversion results form both a photopolymerization and a thermal polymerization. Extreme differences in the initiation rate do not significantly alter the mechanical properties of the polymer matrix as long as the conversions are similar.36 The kinetics of the photopolymerization of dental composites has been monitored in-situ by a modified Fourier Transform infrared spectrometer with attenuated total reflection. The experimental setup could reveal the kinetic stages of the photopolymerization of dental composites. The spectroscopic results correlate with the Vickers micro-hardness.37 Under comparable conditions, UDMA resins are significantly more reactive than Bis-GMA and EBADMA resins.38 In urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA)-based dental resins, differential scanning calorimetry (DSC) showed that the light-cured specimens contain residual living groups entrapped by the fast reaction, which lead to further reaction during postcure heat treatment. After an additional heating to 175°C above the exothermic peak, most of the residual groups in the light-cured specimen were found to have reacted. A single decrease in modulus and a single peak in the tanδ curve, was observed and no exotherm in the DSC curve.39 19.4.2.1

Tertiary Amine Reductants

In visible light curable compositions, the tertiary amines are most profitably acrylate derivatives such as dimethylaminoethyl methacrylate and, particularly, diethylaminoethyl methacrylate (DEAEMA) in amounts ranging from about 0.05 to about 0.5% of the resin composition.9 Other suitable tertiary amine reductants are tributylamine, tripropylamine, N-methyldiethanolamine, N-propyldiethanolamine, N-ethyldiisopropanolamine, triethanolamine and triisopropanolamine. One of the preferred tertiary amine reductants is ethyl-4-dimethylamino benzoate (EDMAB).25

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19.4.3 Curing Techniques Various techniques of curing are in use. Soft-start cure of a resin composite may give rise to a reduced contraction because of a possible flow before gelling. Soft-start cure may be achieved by a pulse-delay cure technique, where the polymerization is initiated by a short flash of light followed by a waiting time of several minutes before the final curing is performed.40, 41 Another soft-start technique is characterized by step-curing. Here a reduced intensity of curing light is used during the first part of the polymerization period. Next the intensity is increased.42 The use of a soft-start polymerization mode, from low to high, offers some modest advantages in curing effects, especially the delay in the original shrinkage-strain. In general, a higher conversion is accompanied by a higher shrinkage. Some reductions in the problems of shrinkage may be achieved by an acceptable reduction in degree of conversion. The pulse-cure method may give rise to a different structure of the polymer although the degree of conversion and the hardness in the final state are not affected by the curing method.43, 44 An initially slow cure may favor the formation of a relatively linear polymer. A slow start of the polymerization could be associated with relatively few centers of polymer growth, resulting in a more linear polymer structure with relatively few crosslinks. The differences in structure can be examined by studies of the Wallace hardness of cured swollen samples. The Wallace hardness measures the depth of penetration of a Vickers diamond under a predetermined load. The Wallace hardness is in fact a measure of softness, in that the higher the Wallace hardness, the softer is the material. A lower degree of conversion is associated with softer polymers after storage in ethanol. Polymers with the same degree of conversion respond differently to the softening action by ethanol swelling, dependent on the history of light exposure.45 In a series of experiments using microwave curing of acrylic resins with different cure cycles, it was shown that the porosity of the cured resin was not affected by the manner of curing.46

19.4.4 Dual Initiator Systems Initiator systems employing two or more initiators, i.e., light and self-curing or light and heat initiated systems, can also be formulated. Such multi-initiator systems may have utility in that they may in-


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clude a rapid cure initiator, and light or heat cure to impart significant polymerization in the dental office or dental laboratory. A light cure system in combination with a longer time self-cure initiator continues to cause further polymerization after the patient leaves the office and further secures the restorative to the tooth structure.25 Such dual cure light/heat systems, as well as their respective single initiator systems, are also desirable in that they may be formulated and packaged in one container or syringe, thereby avoiding the need for mixing by the dental professional before application. For example, such one-component systems exhibit good shelf life of more than a year when stored away from light at room temperature. If self-curing compositions are desired, the self-curing initiator may be packaged in one of two containers separately from the polymerizable components of the composition, with the contents of both containers being admixed shortly before use in the dental office.

19.5 INHIBITORS Polymerization inhibitors are mainly substituted phenols, for example, hydroquinone monomethyl ether (MEHQ) or 2,6-di-tert-butyl-4-methylphenol (BHT)

19.6 ADDITIVES 19.6.1 Fillers and Reinforcing Materials Fillers are summarized in Table 19.7. Filler particles can be silanized aluminum oxide, zirconium oxide, silicon oxide, barium glass, strontium glass, and silicate glasses. Spherical particles in the compositions improve the handling characteristics, such as bulk and consistency, and improve the filler packing for better restoration placement in cavity preparations by minimizing the flow and the slump of the composition. In most composites, fillers have a higher refractive index than the resin. Typical levels of filler are from about 50 to 80%. If a more finely particulated filler is used, the amounts of filler may be decreased due to the relative increase in surface area which attends the smaller sizes of particles. Particle size distributions may range from 0.02 to 50 µ. Both the chemical structure of the polymer matrix and the type of

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Reactive Polymers Fundamentals and Applications Table 19.7: Fillers for Dental Composites Material

Reference

Calcium hydroxy apatite Silanized aluminum oxide Zirconium oxide Siliconedioxide Barium glass Strontium glass Strontium fluoroaluminosilicate cement Silicate glass Functionalized metal oxide nanoparticles Silsesquioxane Polyamide 6 nanofibers

17

9 47

filler system can have significant effects on the strength and water sorption of dental composites.48 19.6.1.1

Sub-micron Size Fillers

Sub-micron size fillers are preferred to minimize surface wear and plucking of filler components from the restorative surface, as well as imparting a surface which may be easily polished by the dental professional. Filler particles have an average size of about 0.04 to 0.08 µ m.25 19.6.1.2

Glass Fibers

Glass fibers increase filler packing and improve filler self-orientation for high filler loadings.25 19.6.1.3

Functionalized Metal Oxide Nanoparticles

There have been efforts to generate functionalized metal oxide nanoparticles to make highly uniform composite materials. For example, aluminum tri-sec-butylate is dissolved in toluene, reacted with one allyl acetoacetate,49 c.f. Figure 19.7. The dispersed, individual metal oxide particles can be prepared by partially replacing the organic radical by a functional one and then by hydrolyzing to oxide with water. A silane functionalized polymer is also hydrolyzed with water to form a network crosslinked by the resultant silica particles. Elimination of


CH3 CH3 CH O

CH2

CH3

O

+

CH2

CH O Al O CH

CH3

CH3

CH3

C CH2 C O CH2 CH CH2 CH2 O CH3

CH3 CH3 CH O

CH2

CH2

CH O Al O CH2

CH3

CH3

CH

CH2

CH

CH2

CH

CH2

H 2O

O Al O CH2 O O Al O CH2

Figure 19.7: Functionalized Metal Oxide

679

680


the composite shrinkage induced by removal of volatile reaction products is attempted by utilizing ring strained alkenoxysilanes and polymerizable solvents where all reaction by-products contribute to the SiO2 network or the resultant interpenetrating, matrix, organic polymer. The expected packing disruption induced by the strained ring opening of the alkenoxysilane is a strategy for compensating for the shrinkage induced by conversion of double bonds to single bonds.16 19.6.1.4

Polyamide 6 Nanofiber

Electrospun Polyamide 6 nanofibers, as non-woven fabrics, were impregnated with the dental methacrylate of 2,2-bis[4-(3-methacryloxy-2-hydroxypropoxy)phenyl]propane/triethylene glycol dimethacrylate (Bis-GMA/TEGDMA) in order to prepare restorative composite resins.47 The polyamide 6 nanofibers used are much softer than inorganic fillers with a regular cylindrical shape with diameters ranging from 100 to 600 nm. Flexural strength (FS), elastic modulus (EY), and work of fracture (WOF) of the nanofiber reinforced composite resins were significantly increased by the admixture of relatively small amounts of Polyamide 6 nanofibers. 19.6.1.5

Silsesquioxane

A polyhedral oligomeric silsesquioxane (POSS) filler has several advantages. POSS-filled resins typically exhibit lower mass densities and greater stiffness, and are capable of withstanding higher temperatures, as well as higher levels of ionizing radiation. In addition, POSS-filled resins are capable of wetting fibers to desirably high degrees. The use of POSS with dental resin materials, particularly the acrylate or methacrylate resins, minimizes polymerization shrinkage and increases material toughness. The nanoscale dimensionality of the POSS fillers also allows for better aesthetic properties, including easier polishability and improved transparency.9 Functionalized POSS, also known as POSS monomers are particularly preferred. Herein one or more of the covalently bound organic groups are reactive with at least one component of the resin composition. In some cases, it is possible to have all of the covalently bound organic groups be reactive. POSS monomers may be prepared, for example, by corner-capping an incompletely condensed POSS, containing trisilanol groups with a substituted trichlorosilane.


681

Through variation of the substituted group on the silane, a variety of functional groups can be placed off the corner of the POSS framework, including halide, alcohol, amine, isocyanate, acid, acid chloride, silanols, silane, acrylate, methacrylate, olefin, and epoxide. Preferred functional groups are acrylate and methacrylate groups since they are involved in the polymerization reaction. 19.6.1.6

Calcium Phosphates

Amorphous calcium phosphates (ACP) are fillers for mineral releasing dental composites. When the ACP is stabilized by pyrophosphate (P2 O4− 7 ) ions, pyrophosphate retards the conversion of ACP to apatite. ACPs have a relatively high aqueous solubility and can release Ca2+ and PO4− ions. However, pyrophosphate stabilized ACP-filled composites have relatively poor mechanical properties, because ACP does not act as reinforcing filler such as commonly used silanized glass fillers. ACP can be hybridized with tetraethoxysilane or zirconyl chloride (ZrOCl2 ) and surface-treated with 3-methacryloxypropoxytrimethoxysilane (MPTMS) or zirconyl dimethacrylate (ZrDMA). In fact, a silica- or zirconia-hybridized ACP moderately improves the biaxial flexural strength of Bis-GMA/TEGDMA/HEMA/ZrDMA-based composites while maintaining their high anti-demineralizing and remineralizing potential. Thus adequate levels of calcium and phosphate ions are released.50

19.6.2 Pigments For aesthetic demands, pigments are added to provide the desired colors of the fillings, i.e., the colors of the neighboring teeth. Pigments are inorganic compounds of different kinds and blendings. There have been attempts to standardize color shades to facilitate clinical use and combining products. Many producers have adapted their color system to the Vita R shade system (Vita Zahnfabrik Company, Germany) and deliver a full range of shades in one-dose pre-filled tips.

19.6.3 Photostabilizers In order to protect the materials against photo degradation, photostabilizers are added. Photo degradation of the cured resin causes changes in color and the mechanical properties. Most common photostabilizers are salicylates,

682


in particular the phenyl esters of benzoic acid, ortho-hydroxybenzophenones, ortho-hydroxybenzotriazoles and substituted cinnamic esters. A few photostabilizers are shown in Figure 19.8.

19.6.4 Caries Inhibiting Agents Caries is the damage of bone tissue (not only dental) caused by infection. Caries dentium is the tooth decay where the dental enamel, i.e., dentin is damaged by bacteria residing in the mouth. Increased dental plaque supports the formation of acid metabolic products of the bacteria that act in the decalcination of the dentin. Moreover, sucrose acts in a similar way, as it forms dextranes that stick as plaques and decompose into lactic acid and pyruvic acid. Caries inhibiting agents are slow releasing fluoride agents to help inhibit caries from forming in the adjacent tooth structure.

19.6.5 Coloring or Tint Agents Coloring or tint agents may be included in small amounts of about 1% or less of the total composition. Such fillers can also be selected to be radio opaque. For example, appropriate amounts of radio opaque barium, strontium, or zirconium glass may be used as all or as part of the filler portion.

19.6.6 Adhesion Promoter The compositions may include an adhesion promoter. This may be a phosphorus-containing adhesion promoter, free from halogen atoms. Both polymerizable and non-polymerizable phosphorus derivatives are available. However polymerizable phosphorus materials having ethylenic unsaturation are advantageous. Examples of saturated and unsaturated phosphorus acid esters are shown in Table 19.8. The phosphoric ester of Bis-GMA can be obtained by the treatment of Bis-GMA with phosphorous oxychloride, as shown in Figure 19.10.


O

OH

O

683

OH

OC8H17

OH 2,4-Dihydroxybenzophenone

2-Hydroxy-4-octoxybenzophenone

NC O O O O NC

CN

C

O

O O O CN

1,3-bis-[2’-cyano-3’,3-diphenylacryloyl)oxy]-2,2-bis{[2-cyano-3’,3’-diphenylacryloyl)oxy]methyl}propane

Figure 19.8: 2,4-Dihydroxybenzophenone (Uvinul 3000 ™, BASF) 2-Hydroxy-4-octoxybenzophenone (Uvinul 3008 ™) 1,3-Bis[2′ -cyano-3′ ,3-diphenylacryloyloxy]-2,2-bis-[[2-cyano-3′ ,3′ -diphenylacryloyloxy]methyl]propane (Uvinul 3030™)

684


Table 19.8: Adhesion Promoters Compound

Reference

Pentaerythritol triacrylate monophosphate Pentaerythritol trimethacrylate monophosphate Dipentaerythritol pentaacrylate monophosphate Dipentaerythritol pentamethacrylate monophosphate Hydroxyethyl methacrylate monophosphate Methacryloyloxyethane-1,1-diphosphonic acid Methacrylate-terminated phosphoric acid ester 4-Methacryloxyethyl trimellitate 2,2′ -Bis(α-methacryloxy-β-hydroxypropoxyphenyl)propane diphosphonate Bis-GMA diphosphonate Bis-GMA diphosphate Dibutyl phosphite Di-2-ethylhexyl phosphite Di-2-ethylhexyl phosphate Glyceryl-2-phosphate Glycerophosphate dimethacrylate Glycerophosphoric acid Methacryloxyethyl phosphate Glyceryl dimethacrylate phosphate

17 17 17 17 17 51 7 52

53

CH3 H2C C C O O CH3 O H2C C

CH2

OH

C O CH2 C CH2 O P O CH3 CH2 H2C C

OH

O

O

Figure 19.9: Pentaerythritol trimethacrylate monophosphate


CH2

CH2

C CH3

C CH3

C O

C O

O

O

CH2

CH2

CH OH

CH O P O

CH2

CH2

O

Cl Cl

OH OH

O

P O , H2O Cl

H3C C CH3

H3C C CH3

O

O

CH2

CH2

CH OH

CH O P O

CH2

CH2

O

O

C O

C O

C CH3

C CH3

CH2

CH2

Figure 19.10: Phosphoric ester of Bis-GMA51

OH OH

685

686


19.6.7 Thermochromic Dye For aesthetic reasons, tooth-colored restoration materials are increasingly being used in restorative dentistry. These materials have the disadvantage that they can be visually distinguished from the natural tooth substance only with difficulty, with the result that the removal of excess material and also the reworking and matching of fillings is made difficult. The consequence is that, frequently, healthy tooth substance is unnecessarily removed or on the other hand, excess dental material is overlooked which can then, as a retention niche, encourage the formation of plaque and lead to periodontal problems. Also, when tooth-colored fillings are removed, because of the poor visibility of the transition area between filling and tooth substance, often either too much healthy tooth substance is removed or remains of the filling are overlooked. Similar problems result when using tooth-colored fixing materials for the cementing of tooth-colored restorations. A dental material can be formulated, where the color can be temporarily changed in a simple way such that the material can be visually distinguished from the natural tooth substance, but assumes its original color after a period sufficient for the working of the dental material. This object is achieved by adding a thermochromic dye to the formulation. Thermochromic dyes are preferred that are colorless at a temperature of approx. 37°C and which change color upon heating or preferably cooling, i.e., assume a color that can clearly be distinguished from the natural tooth substance. At a temperature of 37°C, the color of the dental material is thus determined by its intrinsic color. Thermochromic dyes are based on an acid-responsive component and an acidic component. Other thermochromic dyes are liquid crystalline cholesterol derivatives.54

19.7 PROPERTIES Dental materials are standardized by several documents.55, 56

19.7.1 Water Sorption According to the ISO standard for dental restorative resins, a suitable resin for its use as dental material must show a water sorption lower than


687

50 µg/mm3 and a solubility lower than 5 µg/mm3 .57 In resins and composites based on an ethoxylated bisphenol A glycol dimethacrylate (BisEMA) and a poly(carbonate)dimethacrylate (PCDMA), the water sorption and desorption was examined in both equilibrium and dynamic conditions in adjacent sorption-desorption cycles. The equilibrium water uptake from all resins was very small. However, it increased as the amount of PCDMA in the resin was increased.58 A maximum volume increase of 2% due to swelling was observed. The sorption of water of polymer filling materials affects the dimensional stability, the mechanical properties, and the bonding strength to the tooth. The maximum water sorption and the diffusion coefficient of water are important in determining the time-dependent mechanical properties and time-dependent hydroscopic expansion of resins for clinical use.59

19.7.2 Cytotoxicity Acrylic resins have been shown to be cytotoxic as a result of the substances that leach from the resin. The primary eluate is residual monomer.60 However, in organic leachables analyzed by gas chromatography-mass spectrometry, nearly the whole volatile compounds in the formulation as well as degradation products could be traced back. Among components detected were monomers, comonomers, initiators, stabilizers, decomposition products, and contaminants. In a study, 32 substances were identified and 17 were confirmed with reference substances.61 In order to minimize the cytotoxicity, utmost conversion must be achieved and monomers with minimal cytotoxicity should be selected. Common compounds in dental resin compositions have been tested with respect to estrogenic activity. Most compounds tested in the study do not show estrogenic activity, but some show activity.62

19.8 APPLICATIONS 19.8.1 Filling Techniques Compositions are applied to the tooth, preferably by syringe in incremental layers of about 0.5 to about 2 mm and cured for about 20 to 40 seconds, depending on the shade of the composition. Darker compositions have longer curing times. Additional layers follow until the cavity is completely filled to the cavosurface margin. Any excess material is removed immediately

688


from the surface and the restoration is finished and polished by conventional techniques such as diamonds, discs and polishing pastes. Such finishing also removes any oxygen-inhibited uncured or partially cured layer on the surface of the restoration, which if left in place, might cause staining of the surface over time.25

19.8.2 Primer Emulsions A self-etching dental adhesive primer composition is used as an adhesive or adhesion promoter to affix dental filling materials or bone cements to tooth material. Such a composition essentially comprises an emulsion of water immiscible polymerizable monomers, oligomers, and adhesion promoters in water. By using an emulsion of the polymerizable substances in water, the need for volatile organic solvents is avoided and biocompatibility is enhanced. Further, the composition comprises initiators, accelerators, and inhibitors and surfactants and colloidal silica particles to aid in the formulation of an emulsion and to help keep this stable. For example, a polymerizable surfactant may consist of the reaction product of isophorone diisocyanate with poly(ethylene glycol) monomethyl ether, cured with dibutyltin dilaurate. To this product glycerol dimethacrylate is added together with a radical polymerization inhibitor, which grafts to the polyurethane polymer.63 The product is a white, soft sticky solid at room temperature, partially soluble in water to give a light foam on shaking. The material has a melting range of about 35 to 37°C. The polymeric and polymerizable surfactant can be emulsified in water and phosphoric acid which provides the etching.

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35. M. S. Soh and A. U. J. Yap. Influence of curing modes on crosslink density in polymer structures. Journal of Dentistry, 32(4):321–326, May 2004. 36. L. G. Lovell, S. M. Newman, M. M. Donaldson, and C. N. Bowman. The effect of light intensity on double bond conversion and flexural strength of a model, unfilled dental resin. Dent. Mater., 19(6):458–465, September 2003. 37. R. L. Oréfice, J. A. C. Discacciati, A. D. Neves, H. S. Mansur, and W. C. Jansen. In situ evaluation of the polymerization kinetics and corresponding evolution of the mechanical properties of dental composites. Polymer Testing, 22(1):77–81, February 2003. 38. S. H. Dickens, J. W. Stansbury, K. M. Choi, and C. J. E. Floyd. Photopolymerization kinetics of methacrylate dental resins. Macromolecules, 36(16): 6043–6053, August 2003. 39. J. K. Lee, J. Y. Kim, and B. S. Lim. Dynamic mechanical properties of a visible light curable urethane dimethacrylate based dental resin. Polym. J., 35(11):890–895, 2003. 40. J. Kanca, III. and B. I. Suh. Pulse activation: reducing resin-based composite contraction stresses at the enamel cavosurface margins. American Journal of Dentistry, 12(3):107–112, June 1999. 41. A. Sahafi, A. Peutzfeldt, and E. Asmussen. Effect of pulse-delay curing on in vitro wall-to-wall contraction of composite in dentin cavity preparations. American Journal of Dentistry, 14(5):295–296, October 2001. 42. N. Silikas, G. Eliades, and D. C. Watts. Light intensity effects on resincomposite degree of conversion and shrinkage strain. Dent. Mater., 16(4): 292–296, July 2000. 43. E. Asmussen and A. Peutzfeldt. Influence of pulse-delay curing on softening of polymer structures. Journal of Dental Research, 80(6):1570–1573, June 2001. 44. E. Asmussen and A. Peutzfeldt. Influence of selected components on crosslink density in polymer structures. Eur. J. Oral Sci., 109(4):282–285, August 2001. 45. E. Asmussen and A. Peutzfeldt. Two-step curing: influence on conversion and softening of a dental polymer. Dent. Mater., 19(6):466–470, September 2003. 46. M. A. Compagnoni, D. B. Barbosa, R. F. de Souza, and A. C. Pero. The effect of polymerization cycles on porosity of microwave-processed denture base resin. J. Prosthet. Dent., 91(3):281–285, March 2004. 47. H. Fong. Electrospun nylon 6 nanofiber reinforced Bis-GMA/TEGDMA dental restorative composite resins. Polymer, 45(7):2427–2432, March 2004. 48. D. Skrtic and J. M. Antonucci. Effect of bifunctional comonomers on mechanical strength and water sorption of amorphous calcium phosphate- and silanized glass-filled Bis-GMA-based composites. Biomaterials, 24(17): 2881–2888, August 2003.

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49. R. Nass and H. Schmidt. Process for fixing inorganic species in an organic matrix. US Patent 5 064 877, assigned to Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung e.V. (DE), November 12 1991. 50. D. Skrtic, J. M. Antonucci, E. D. Eanes, and N. Eidelman. Dental composites based on hybrid and surface-modified amorphous calcium phosphates. Biomaterials, 25(7-8):1141–1150, March–April 2004. 51. I. Omura, J. Yamauchi, Y. Nagase, and F. Uemura. Adhesive compositions. US Patent 4 499 251, assigned to Kuraray Co., Ltd. (Kurashiki, JP), February 12 1985. 52. E. Masuhara, N. Nakabayashi, and M. Takeyama. Curable composition. US Patent 4 148 988, assigned to Mitsui Petrochemical Industries Ltd. (Tokyo, JP), April 10 1979. 53. L. J. Pranitis, Jr. and D. Ng. Single dose dental adhesive delivery system and method and adhesive therefor. US Patent 5 860 806, assigned to The Kerr Corporation (Orange, CA), January 19 1999. 54. A. Burgath, P. Burtscher, U. Salz, and V. Rheinberger. Thermochromic dental material. US Patent 6 670 436, assigned to Ivoclar Vivadent AG (LI), December 30 2003. 55. Dentistry – polymer-based filling, restorative and luting materials. ISO Standard 4049, International Organization for Standardization, Geneva, Switzerland, 2000. 56. Dentistry – polymer-based die materials. ISO Standard 14233, International Organization for Standardization, Geneva, Switzerland, 2003. 57. I. Sideridou, V. Tserki, and G. Papanastasiou. Study of water sorption, solubility and modulus of elasticity of light-cured dimethacrylate-based dental resins. Biomaterials, 24(4):655–665, February 2003. 58. I. Sideridou, D. S. Achilias, C. Spyroudi, and M. Karabela. Water sorption characteristics of light-cured dental resins and composites based on bis-EMA/PCDMA. Biomaterials, 25(2):367–376, January 2004. 59. K. Asaoka and S. Hirano. Diffusion coefficient of water through dental composite resin. Biomaterials, 24(6):975–979, March 2003. 60. J. H. Jorge, E. T. Giampaolo, A. L. Machado, and C. E. Vergani. Cytotoxicity of denture base acrylic resins: A literature review. J. Prosthet. Dent., 90(2): 190–193, August 2003. 61. V. B. Michelsen, H. Lygre, R. Skalevik, A. B. Tveit, and E. Solheim. Identification of organic eluates from four polymer-based dental filling materials. Eur. J. Oral Sci., 111(3):263–271, June 2003. 62. Y. Nomura, H. Ishibashi, M. Miyahara, R. Shinohara, F. Shiraishi, and K. Arizono. Effects of dental resin metabolites on estrogenic activity in vitro. J. Mater. Sci. -Mater. Med., 14(4):307–310, April 2003. 63. G. B. Blackwell. Self etching adhesive primer composition and polymerizable surfactants. US Patent 6 387 982, assigned to Dentsply DeTrey G.m.b.H. (DE), May 14 2002.

20 Toners Toners for developing electrical or magnetic latent images are used in various processes for forming and printing images. One such image forming process is electrophotography, which uses a photosensitive member generally formed of a photo conductive material, and wherein an electrical latent image is formed on the photosensitive member by various means. The electrical latent image is developed using a toner. The toner image thus developed is transferred to a printing material, such as paper, and then fixed thereto by heating or pressure, or by using solvent vapor thus obtaining a copy of the image.1 The following types of developers are conventionally used in dry development devices for electrophotography: 1. One-component-type magnetic developers comprising a toner containing magnetic powder. 2. One-component-type non magnetic developers comprising a toner containing no magnetic powder. 3. Two-component-type non magnetic developers comprising a toner containing no magnetic powder and a magnetic carrier, which is mixed with the toner in a fixed proportion. 4. Two-component-type magnetic developers comprising a toner containing magnetic powder and a magnetic carrier, which is mixed with the toner in a fixed proportion. Various development methods using such toners have been proposed and put into practical use. The toners used in these development methods 693

694


are generally manufactured by a pulverizing method in which a coloring agent, like a dye or pigment, is mixed with, and uniformly dispersed in, a thermoplastic resin serving as the binder. The mixed substance thus obtained is then finely pulverized and classified to provide a desired particle size distribution. Toners typically contain a principal resin or toner resin, colorant, and various functional additives such as release agents and charge control additives. Most toner compositions employ release agents such as waxes and/or silicone polymers. Poly(dimethylsiloxane) resins or oils exhibit excellent external release agent characteristics, i.e., when applied to fuser rolls, due to their extremely low surface energy. The property is highly desirable in contact-fusing electrophotography, because it is important to be able to release the toner from the hot-oiled fuser roll and thus prevent hot offset. Several different low molecular weight organic materials have been used in the toner industry to eliminate this hot offset phenomenon. Low molecular weight polyolefin waxes are by far the most common type of internal release agent. Each type of release agent has its own advantages and disadvantages. For example, polyolefins tend to crystallize to a significant extent. The crystallinity is between 70% and 90%. When these molecules crystallize, they segregate from the toner resin into a separate phase and form large wax domains which cause numerous print quality defects, as well as a wax imbalance between the toner fines and the average size toner particles. Poor homogeneity of these additives in the toner particles tends to cause a number of problems. The most important issues are low toner powder flow and variations in triboelectric charge distribution, which can lead to print quality defects.

20.1 TONER COMPONENTS Toners contain a primary or binder resin, known as a toner resin, such as a thermoplastic resin, a colorant such as a dye or pigment, and a charge control agent, releasing agents, and other additives. Several polymers are usable as the thermoplastic binder resin, including poly(styrene)s, styrene-acrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, and urethanes.2, 3 Examples of the colorant include: dyes and pigments such as carbon black, iron black, graphite, nigrosine, metallic complex of mono-

Toners

695

azo dye, ultramarine, copper phthalocyanine, methylene blue, chrome yellow, quinoline yellow, hanza yellow, benzene yellow, and various types of quinacridone pigments. When the colorant is contained in a non magnetic toner, the amount should be approximately 1% to 30%, in magnetic toners, ca. 60%. The colorant can be coated by a UV stabilizer.4 Toner compositions can contain charge enhancing additives, for example, 0.1% to 10% cetyl pyridinium chloride, distearyl dimethyl ammonium methyl sulfate, metal salicylates, etc. Other charge control agents include the sulfonated styrene-acrylate ester copolymers, calixarenes. Polymeric charge control agents can be compatibilized with the toner resin in the same manner as other polymeric components of the toner compositions. Toner compositions may also include colloidal silica, metal salts and metal salts of fatty acids such as zinc stearate as surface additives.

20.2 TONER RESINS The heterogeneity of the toner particles in the composition is believed to be the root cause of numerous problems throughout the serviceable life of toner in a printing device. The print quality black-on-white defect involving unwanted toner black spots on the printed product has been a recurring and sometimes serious problem in certain commercial printers. The black spots are highly visible in the background region of the print and are nonrepeating in nature. It is believed that the toner particle compositional uniformity is a major factor in the existence of such defects. Heterogeneity of the toner particles’composition is believed to be the root cause of observed selectivity throughout the life of the cartridge. The lack of heterogeneity in toners can be eliminated by the addition of two functional additives reactive with each other to form a stable reaction product. The copolymer reaction product apparently acts as a compatibilizer to improve the dispersion of various polymeric components, such as a release agent with the backbone structure of the toner resin. Toners contain a primary or binder resin, known as a toner resin, such as a thermoplastic resin, a colorant such as a dye or pigment, a charge control agent, releasing agents, and other additives. These components will be separately described. Any suitable binder resin can be used as the toner resin, including polyesters, epoxy resins, various polymers containing styrene, and acrylic

696

Reactive Polymers Fundamentals and Applications Table 20.1: Toner Composition1 Component Resin binder Carbon black Metal salicylate Poly(ethylene) wax Other additives Siloxane polymer Styrene-maleic anhydride copolymer Amino-siloxane polymer

% 90.0 5.0 2.5 1.0 1.0 0.25 0.5 0.25

acid derivatives. These may be used either singly or as mixtures. Polyesters are not preferred, because it is difficult to place reactive functional groups on a polyester resin backbone and because polyester resins are more reactive than styrene polymers, increasing the tendency to obtain random copolymers rather than block or graft copolymers. The polymer used for toner preparations can be a random styrene/acrylic copolymer, crosslinked with divinylbenzene. The two functional materials added to the toner formulation, which already contains a poly(dialkylsiloxane) oil, are a styrene/maleic anhydride copolymer and a diamine-terminated poly(dimethylsiloxane) polymer. The reaction takes place between the amino end groups and the anhydride side groups. During the extrusion and melt mixing of the toner materials, these functional groups react to form a fairly stable amic acid bond and thus, a polysiloxane/toner resin compatibilizer. An example for a toner composition with a styrenemaleic anhydride copolymer and an amino-siloxane polymer as compatibilizing agents is shown in Table 20.1. Reactive extrusion is accomplished in a continuous twin-screw extruder maintained within a temperature range of 135 to 210°C and at an appropriate torque. The molten extrudate is subsequently cooled by passage through a chilled roller assembly and the resulting ribbons are crushed.1

20.3 MANUFACTURE OF TONER RESINS 20.3.1 Suspension Polymerization A suspension polymerization method for the preparation of a toner has been described.5, 6 The organic phase, containing styrene monomer, n-

Toners

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butyl acrylate, divinylbenzene, 2,2′ -azobis(isobutyronitrile), paraffin and Phthalocyanine Blue T, is dispersed into the aqueous phase. The aqueous phase contains polyvinyl alcohol and sodium dodecylsulfate as suspension dispersants. Infrared studies suggest that the interactions between Phthalocyanine Blue T and the resin are mainly caused by physical forces.

20.3.2 Terephthalic Ester Resins The toner particles are prepared by an emulsion aggregation process.7 The toner resin is a sulfonated polyester made from dimethyl terephthalate, sodium sulfoisophthalate, 1,2-propanediol, and 2.5 mol-% diethylene glycol with dibutyltin oxide as catalyst. Subsequent to synthesis of the toner particles and addition of pigment, poly(pyrrole) is applied to the toner particle surfaces by an oxidative polymerization process. Using oxidants such as ferric chloride and tris(p-toluenesulfonato)iron(III) for the oxidative polymerization of the pyrrole monomer tends to result in formation of toner particles that become positively charged when subjected to triboelectric or inductive charging processes. Accordingly, toner particles can be obtained with the desired charge polarity without the need to change the toner resin composition, and can be achieved independently of any dopant used with the poly(pyrrole). The poly(pyrrole) in or on the toner particles generally imparts a high degree of color to the toner particle. These toners are usually preferred, where black images are desired. Similarly to pyrrole monomers, 3,4-ethylenedioxythiophene can be polymerized on the toner resin.8

20.3.3 Unsaturated Ester Resins Examples of linear unsaturated polyesters are low molecular weight condensation polymers formed by saturated and unsaturated diacids and diols. The resulting unsaturated polyesters are crosslinkable in two ways: 1. Due to double bonds along the polyester chain, and 2. Due to the functional groups such as carboxyl, hydroxy, and others, amenable to acid-base reactions. Suitable diacids and dianhydrides include succinic acid, isophthalic acid, terephthalic acid, phthalic anhydride, and tetrahydrophthalic anhydride. Unsaturated diacids or anhydrides, are fumaric acid, itaconic acid, and maleic anhydride. Suitable diols include propylene glycol, ethylene glycol, diethylene glycol, and propoxylated bisphenol A.

698


A particularly preferred polyester is poly(propoxylated bisphenol A fumarate). A propoxylated bisphenol A fumarate unsaturated polymer undergoes a crosslinking reaction with a chemical crosslinking initiator, such as 1,1-di-(tert-butylperoxy)cyclohexane. The crosslinking between chains will produce a large, high molecular weight molecule, ultimately forming a gel. The toners and toner resins may be prepared by a reactive melt mixing process wherein reactive resins are partially crosslinked. For example, low melt toner resins and toners may be fabricated by a reactive melt mixing process comprising the following steps.4 1. Melting reactive base resin, thereby forming a polymer melt, in a melt mixing device. 2. Initiating crosslinking of the polymer melt with certain liquid chemical crosslinking initiator and increased reaction temperature. 3. Retaining the polymer melt in the melt mixing device for a sufficient residence time that partial crosslinking of the base resin may be achieved. 4. Providing sufficiently high shear during the crosslinking reaction to keep the gel particles formed during crosslinking small in size and well distributed in the polymer melt. 5. Optionally devolatilizing the polymer melt to remove any effluent volatiles. The high temperature reactive melt mixing process allows for very fast crosslinking which enables the production of substantially only microgel particles, and the high shear of the process prevents undue growth of the microgels and enables the microgel particles to be uniformly distributed in the resin.

20.3.4 Toner Resins with Low Fix Temperature Toners made from vinyl-type binder resins, such as styrene-acrylic resins, may cause a problem which is addressed as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image is contacted, for a period of time, with a polyvinyl chloride surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl binder covers, and the fixed image adheres to the PVC surface. Crosslinked thermoplastic binder resins can be used as toners which

Toners

699

possess a low fix temperature and a high offset temperature, and which show a substantially minimized vinyl offset. The resin composition consists of a linear reactive base resin, an initiator, and a polyester with an amine functionality.2 The linear unsaturated polyester base resin is prepared from unsaturated diacids, e.g., maleic acid or fumaric acid and diols, like propylene glycol or propoxylated bisphenol A. Particularly suitable is poly(propoxylated bisphenol A fumarate). An amine-containing polyester is prepared from propoxylated 4,4′ -isopropylidene bisphenol A, N-phenyldiethanolamine, and fumaric acid. A peroxide, such as tert-butyl hydroperoxide, is used as radical initiator. In general, peroxides that thermally decompose at higher temperatures are preferred so that the amine promoted decomposition is favored at the polymer melt processing temperatures. To disperse small amounts of the peroxide thoroughly in the resin, a 0.6% master batch in poly[4,4′ -isopropylidenebisphenyl bispropanol bisether/fumaric acid] is formed. The peroxide/polyester mixture can be extruded at 120°C without decomposition of the peroxide under these conditions. In larger scale reactions the initiator can be added to the extruder by direct injection. To this blend in a next step, the amine containing polyester is then blended in an extruder. The amine polyesters are added in amounts from 1% to about 10%. The polymers are crosslinked in the molten state under high shear conditions, producing substantially uniformly dispersed microgels of high crosslinking density, preferably using certain chemical initiators as crosslinking agents in an extruder. The amine of the polyester reacts with the initiator to form free radicals. The tert-butoxy radical reacts with a vinyl bond in the polymer backbone which subsequently forms a crosslink between polymer chains when it, in turn, reacts with another vinyl bond in the polymer backbone. The crosslinked resin produced in this way is a clean and non-toxic polymer mixture comprising crosslinked gel particles and a noncrosslinked portion. 20.3.4.1

Fixing Performance of the Toner

The fixing performance of a toner can be characterized as a function of the temperature.2 The lowest temperature at which the toner adheres to the support medium is referred to as the cold offset temperature (COT). The maximum temperature at which the toner does not adhere to the fuser roll

700


is referred to as the hot offset temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images resulting, for example, in blurred images. This undesirable phenomenon is known as offsetting. Between the COT and HOT of the toner is the minimum fix temperature (MFT), which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is referred to as the fusing latitude.

20.3.5 Toners for Textile Printing The imaging of textiles and other materials using thermal transfer of sublimable dyes has been commercially practiced for more than 50 years. With the introduction of laser printers for use with personal computers, attempts were made with only limited success to incorporate thermal transfer sublimable dyes into toners to be used in these printers. The printers were intended to image in only one color, particularly black. However, when a toner was properly formulated for this application and a sublimable dye was incorporated into the toner, images could be formed which could then be thermally transferred by the application of sufficient heat to vaporize the dye. By this method, a single color image could be formed. Since many of these laser printers used replaceable cartridges to carry the toner to form the image in this electrophotographic process, several of these special thermal transfer toners could be installed in several cartridges, including toners containing the process color dyes for cyan, magenta, and yellow color imaging. Using a color separation program on a personal computer connected to such a laser printer, a skilled operator could effectively create a color separation of a full color image and print each separation by installing in turn the appropriate cartridge containing the indicated color: cyan, magenta, or yellow. By this method, an image containing the appropriate cyan, yellow and magenta thermal transfer dyes can be constructed stepwise.9 The requirements for toner materials for good textile performance, i.e., low initial modulus, and flexibility differ from the requirements for production of toner powders by grinding, i.e. brittleness.10, 11 Toner com-

Toners

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positions for use in textile printing are available.12

20.4 CHARACTERIZATION OF TONERS The powder flow test is a direct determination of the amount of energy necessary to pull apart aggregates of cohesive particles in a specified time. The powder flow of a toner is an important aspect to consider in designing a toner because of the required performance in the electrophotographic process.1 The powder flow test allows for the evaluation of the flowability of the toner by measuring the amount of toner passing through a sieve during a preset time relative to the initial loading of toner on the sieve. The sieve is supported on a cantilever and is vibrated at a frequency of 60 Hz. The intensity (amplitude) of the vibration is controlled using a voltage adjustment. Generally, a free flowing material will tend to flow steadily and consistently. Conversely, a non free-flowing material will tend to flow as agglomerated particles. The cohesiveness of the toner is also an important characteristic. The cohesiveness affects powder flow, the lower cohesion values being associated with higher powder flows. To measure the cohesiveness, a measured amount of toner is placed on a screen. Three screens of reducing size are placed in series so that the powder goes through increasingly smaller screens.

REFERENCES 1. B. P. Livengood, B. W. Baird, and G. P. Marshall. Reactive compatibilization of polymeric components such as siloxane polymers with toner resins. US Patent 6 544 710, assigned to Lexmark International, Inc. (Lexington, KY), April 8 2003. 2. P. G. Odell, S. V. Drappel, and M. S. Hawkins. Reactive melt mixing processes. US Patent 6 114 076, assigned to Xerox Corporation (Stamford, CT), September 5 2000. 3. K. A. Moffat, M. N. V. McDougall, R. Carlini, D. A. Hays, J. T. LeStrange, and P. J. Gerroir. Toner compositions comprising vinyl resin and poly (3,4-ethylenedioxythiophene). US Patent 6 689 527, assigned to Xerox Corporation (Stamford, CT), February 10 2004. 4. S. M. Silence, E. J. Gutman, and T. R. Hoffend. Toner compositions. US Patent 6 680 153, assigned to Xerox Corporation (Stamford, CT), January 20 2004.

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5. Y. F. Duan and Q. Zhang. Preparation of suspension polymerized color toners and correlation between ingredients and rheological behavior. J. Imaging Sci. Technol., 48(1):6–9, January–February 2004. 6. S. Kiatkamjornwong and P. Pomsanam. Synthesis and characterization of styrenic-based polymerized toner and its composite for electrophotographic printing. J. Appl. Polym. Sci., 89(1):238–248, July 2003. 7. J. R. Combes, K. A. Moffat, and M. N. V. McDougall. Toner compositions comprising polyester resin and polypyrrole. US Patent 6 743 559, assigned to Xerox Corporation (Stamford, CT), June 1 2004. 8. K. A. Moffat, R. Carlini, M. N. V. McDougall, D. A. Hays, and J. T. LeStrange. Toner compositions comprising polyester resin and poly (3,4-ethylenedioxythiophene). US Patent 6 730 450, assigned to Xerox Corporation (Stamford, CT), May 4 2004. 9. R. J. Thompson. Color toner containing sublimation dyes for use in electrophotographic imaging devices. US Patent 6 270 933, assigned to International Communication Materials, Inc. (Connellsville, PA), August 7 2001. 10. W. W. Carr, D. S. Sarma, L. Cook, S. Shi, L. Wang, and P. H. Pfromm. Xerographic printing of textiles: Polymeric toners and their performance. J. Appl. Polym. Sci., 78(14):2425–2434, December 2000. 11. W. W. Carr, F. L. Cook, H. Yan, and P. H. Pfromm. Application of dimer acid-based polyamide for xerographic toners for textiles printing. J. Appl. Polym. Sci., 81(10):2399–2407, September 2001. 12. A. Verhecken and P. Sterckx. Toner composition for use in textile printing. US Patent 6 007 955, assigned to Agfa-Gevaert, N.V. (Mortsel, BE), December 28 1999.

Index ACRONYMS AA Acrylic acid, 547, 647 Ascorbic acid, 120 ABS Acrylonitrile butadiene styrene, 213 ACH Acetone cyanhydrin, 352 ACP Amorphous calcium phosphates, 681 AEP 1-(2-Aminoethyl)piperazine, 563 AIBN 2,2′ -Azobis(isobutyronitrile), 601 AKD Alkylketene dimer, 464 AlN Aluminum nitride, 421 ALS Alternating least squares, 196 AMPC Allyl-N-(4-methyl-phenyl)carbamate, 570 APS (3-Aminopropyl)triethoxysilane, 646 ASA Alkenyl succinic anhydride, 464 ATBC Acetyltributyl citrate, 366

704


ATBN Amine-terminated butadiene-acrylonitrile elastomers, 156 ATPE Acid-group terminated poly(ethylene), 539 ATR-FTIR Attenuated total reflectance Fourier transform infrared spectroscopy, 119, 647 ATRP Atom transfer radical polymerization, 82, 461, 568 ATS (3-Aminopropyl)triethoxysilane, 205 ATU Amine-terminated chain-extended urea, 156 BAMPO Bis(m-aminophenyl)methylphosphine oxide, 169, 171, 176 BAPP 2,2-Bis[4-(4-aminophenoxy)phenyl]propane, 342 Bis(4-aminophenoxy)phenylphosphine oxide, 169 BAPPO Bis(4-aminophenyl)phenylphosphine oxide, 121, 172 BAPQ 2,3-Bis(4-(4-aminophenoxy)phenyl)quinoxaline-6-carboxylic acid, 412 BAQ 2,3-Bis(4-aminophenyl)quinoxaline-6-carboxylic acid, 412 BCB Benzocyclobutene, 493 BCBE 1,2-Bis(benzocyclobutenyl)ethane, 493 BD 1,4-Butanediol, 119 BDA 1,4-Butane diamine, 121 1,2-BDE 1,4-Butanediol diglycidyl ether, 141, 205 BDK 2,2-Dimethoxy-1,2-diphenylethan-1-one, 193 BDM 4,4′ -Bis(maleimido)diphenylmethane, 418 BDMA Benzyldimethylamine, 152 BDMAEE Bis(2-dimethylaminoethyl)ether, 99, 103

Index BDMB 2-Benzyl-2-dimethylamino-1-(4-morpholinophenyl)butan-1-one, 193 BenzOXA Benzoxazole, 557 BEPTPhS Bis(3-mercaptophenyl)sulfone, 208 BET Brunauer Emmett Teller, 543 BHET Bis(2-hydroxyethyl)terephthalate, 45 BHMBE 1,2-Bis-[2(2-hydroxy-5-methylphenyl)-5-benzotriazolyl]-ethane, 80 BHMF Bis(hydroxymethyl)furan, 309 BHT 2,6-Di-tert-butyl-4-methylphenol, 677 BIAE 2-Bromoisobutyric acid ethylester, 570 Bis-A-Dima Bisphenol A dimethacrylate, 660 Bis-GMA 2,2-Bis[4-(3-methacryloxy-2-hydroxypropoxy)phenyl]propane, 660 Bis-M 4,4′ -[1,3-Phenylene(1-methyl ethylidene)]bisaniline, 498 BMDPM 4,4′ -Bis(maleimido)diphenylmethane, 419 BMI 4,4′ -Bis(maleimido)diphenylmethane, 397 4,4′ -Diphenylmethane bismaleimide, 547 Bismaleimide, 387 BMIE N,N-4,4-Diphenyl ether bismaleimide, 398 Bis(4-maleimidophenyl)ether, 399 BMIM 4,4′ -Bis(maleimido)diphenylmethane, 398 Bis(4-maleimidophenyl)methane, 399 BMIP 2,2-Bis[4-(4-maleimido phenoxy)phenyl]propane, 398, 399 Bisphenol A bismaleimide, 398, 400 BMIPO Bis(3-maleimidophenyl)phenylphosphine oxide, 402, 422 BMIS Bis(4-maleimidophenyl)sulfone, 398, 399

705

706


BMPP 2,2-Bis[4-(4-maleimido phenoxy)phenyl]propane, 434 BMPPPO Bismaleimide(3,3′ -bis(maleimidophenyl))phenylphosphine oxide, 169, 171 BOX 2,2′ -(1,4-Phenylene)bisoxazoline, 547 BP Benzophenone, 359, 629, 646 BPFR Boric acid-modified phenolic resins, 257 BPO Dibenzoyl peroxide, 568 BT Bismaleimide triazine resins, 387 CBC N,N ′ -Carbonylbiscaprolactam, 80 CE Cyanate ester, 382 CHO Cyclohexene oxide, 189 CHP Cumene hydroperoxide, 244 CMKGM Carboxymethyl konjac glucomannan, 121 COT Cold offset temperature, 699 CQ Camphorquinone, 673 CR Controlled rheology, 588 CTBN Carboxy-terminated butadiene/acrylonitrile copolymers, 156 CTE Coefficient of thermal expansion, 421 CXA Bis(1-oxyl-2,2,6,6-tetramethylpiperidine4-yl)sebacate, 602 DA 2,2′ -Pyromellitdiimidodisuccinic anhydride, 92 Diels-Alder reaction, 309 DAB 1,4-Diaminobutane, 630 DABCO 1,4-Diazabicyclo[2.2.2]octane, 99

Index DAPB 2,6-Di(4-aminophenoxy)benzonitrile, 417 DAPNPT 2,4-Di(2-allylphenoxy)-6-(2-naphthyloxy)-1,3,5-triazine, 417, 419 DBA 2,2′ -Diallyl bisphenol A, 164, 381, 382, 397, 398, 417, 418 DBBA 3,5-Di-tert-butyl-4-hydroxybenzyl acrylate, 632 DBMX α, α′ -Dibromo-m-xylene, 428 DBTDL Dibutyltin dilaurate, 80, 107, 114, 567 DBTO Dibutyltin oxide, 567 DCDPT 1,4-[Di(4-cyanato diphenyl-2,2′ -propane)]terephthalate, 387 DCHE 7,26-Dioxatrispirobicyclo[4.1.0]heptane-4,5′ -1,3-dioxane-2′ ,2′′ 1,3-dioxane-5′′ ,4′′ -bicyclo[4.1.0]heptane, 665, 667 DCM 4,4′ -Diamino-3,3′ -dimethyldicyclohexylmethane, 175 DCP Dicumyl peroxide, 541, 609, 623, 629 DD 1,10-Decanediol, 117 DDM 4,4′ -Diaminodiphenylmethane, 402, 414, 423 4,4′ -Methylenedianiline, 402 DDS 4,4′ -Diaminodiphenylsulfone, 153, 158, 175 DEAEMA Diethylaminoethyl acrylate, 626 Diethylaminoethyl methacrylate, 675 DEGDA Diethylene glycol diacrylate, 638 DEM Diethyl maleate, 542, 623 DEPN N-tert-Butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide, 603 DEPT NMR Distortionless enhancement by polarization transfer nuclear magnetic resonance spectroscopy, 293

707

708


DGDPI 1,3-[Di(4-glycidyloxy diphenyl-2,2′ -propane)]isophthalate, 387 DGEBA Bisphenol A diglycidyl ether, 139 Diglycidyl ether of bisphenol A , 150 DGEBTF Adduct of 2-chlorobenzotrifluoride and glycerol diglycidyl ether, 150 DHBP 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane, 516, 546, 591 DHPDOPO 10-(2,5-Dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 169 DICY Dicyandiamide, 402 DMAA N,N-Dimethylacrylamide, 647 DMAB 2,4-Diamino-4′ -methylazobenzene, 176 4-Dimethylaminobenzoin, 193 3-DMABA 3-Dimethylaminobenzoic acid, 193 4-DMABA 4-Dimethylaminobenzoic acid, 193 DMABAL 4-Dimethylaminobenzaldehyde, 193 DMAEMA 2-(Dimethylamino)ethyl methacrylate, 82 DMAMP 2-(Dimethylamino)-2-(hydroxymethyl)-1,3-propanediol, 265 DMBA 4-Dimethylamino-1-butanol, 119 Dimethylol butanoic acid, 93, 121 DMCDA 5-(2,5-Dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, 180 DMEA N,N-Dimethylethylethanolamine, 119 DMPA 2,2-Dimethoxy-2-phenylacetophenone, 359 Dimethylol propionic acid, 120 DMPT N,N-Dimethyl-p-toluidine, 669

Index

709

DMT Dimethyl terephthalate, 35 DMTA 2-Dimethylamino-2-methyl-1-propanol, 264 DNS-EDA 5-Dimethylaminonaphthalene-1-(2-aminoethyl)sulfonamide, 197 DOPO 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 112, 168, 169, 405, 422 DPD p-Phenyl diamine, 400 DPHS Bis(3-diethylphosphono-4-hydroxyphenyl)sulfide, 169 DSC Differential scanning calorimetry, 43, 114, 623 DSC-SSA Differential scanning calorimetry-successive self-nucleation and annealing, 631 DSDA 3,3′ ,4,4′ -Diphenylsulfone tetracarboxylic dianhydride, 342 DTBP Di-tert-butyl peroxide, 591 DVB-BCB Bis(benzocyclobutenyl)-m-divinylbenzene, 502 DVS-BCB Bis(benzocyclobutenyl)divinyltetramethylsiloxane, 502 DYBP 2,5-Dimethyl-2,5-di(tert-butylperoxy)-3-hexyne, 516 E-BCB 1,2-Bis(4-benzocyclobutenyl)ethylene, 502 e-EPDM Epoxidized ethylene propylene diene, 212 E/EA Ethene/ethyl acrylate copolymer, 214 EAA Ethylene-co-acrylic acid, 555 Ethylene/acrylic acid copolymers, 213, 549 EBPDMA Ethoxylated bisphenol A dimethacrylate, 660 EBS N,N ′ -Ethylene-bisstearamide, 513 EDA Ethylene diamine, 93, 121

710


EDMAB Ethyl-4-dimethylamino benzoate, 193, 669, 673, 675 EG Ethylene glycol, 117 Expandable graphite, 112 EGMA Ethylene/glycidylmethacrylate copolymer, 213, 540 Poly(ethylene-co-glycidyl methacrylate), 552 EHMOXA Ethyl hydroxymethyl oxazoline, 557 EO Ethylene oxide, 571 EPDM Ethylene propylene diene monomer, 606, 632 EPIDA ((3-(4-(1-(4-(3-(Bis-carboxymethylamino) 2-hydroxy-propoxy) phenyl)-1-methyl-ethyl) phenoxy) 2-hydroxypropyl) carboxy methylamino) acetic acid, 123 EPN Epoxy-novolak, 416 EPPHAA 1,2-Epoxy-3-phenoxypropane, 159 EPR Ethylene/propylene rubber, 557, 606, 632 EPR-g-GMA Ethylene/propylene rubber grafted with GMA, 540 EPR-g-MA Ethylene/propylene rubber grafted with maleic anhydride, 564 EVA Ethylene/vinyl acetate, 153, 456 EVALSH Mercapto-modified EVA, 560 EY Elastic modulus, 680 12F-PEK Fluorinated poly(aryl ether ketone), 160 FA Furfuryl alcohol, 307, 312 6FDA Hexafluoropropylidenebisphthalic dianhydride, 342 FP Ratio of formaldehyde to phenol, 270

Index FPC Flexible printed circuits, 412 FS Flexural strength, 680 FTIR Fourier-transform infrared (spectroscopy), 114 GLYMO 3-Glycidoxypropyltrimethoxysilane, 205, 326 GMA Glycidyl methacrylate, 141, 213, 635, 648, 660, 661 GPC Gel permeation chromatography, 525 GPE Glycidyl phenyl ether, 186 GPN General-purpose novolak resins, 242 GTL Glycol trilinoleate, 638 H3M Hexa(methoxymethyl)melamine, 263 HAB 3,3′ -Dihydroxy-4,4′ -diaminobiphenyl, 342 HAP Hexa(allylamino)cyclotriphosphonitrile, 548 HBA Hydroxybenzoic acid, 498 HBP Hyperbranched polymers, 144 HCPA 4-Hydroxybutyl-2-chloro-2-phenylacetate, 570 HD 1,6-Hexanediol, 184 Hydrazine monohydrate, 121 HDDMA 1,6-Hexanediol dimethacrylate, 660 HDI 1,6-Hexane diisocyanate, 75 HDPE High density poly(ethylene), 542 HEMA 2-Hydroxyethyl methacrylate, 123, 351, 518 HEMAN Hydroxyethyl methacrylate maleic anhydride adduct, 660

711

712


HHPA Hexahydrophthalic anhydride, 163 HIPS High impact poly(styrene), 536 HMDI 4,4-Methylene biscyclohexyl diisocyanate, 73 HME Hexafunctional methacrylate ester, 660 HMF 5-Hydroxymethylfurfural, 308, 309 HMTA Hexamethylenetetramine, 263, 264 HNA Hydroxynaphthoic acid, 498 HON High ortho novolak resins, 242 HOT Hot offset temperature, 700 HPA 2-Hydroxypropyl acrylate, 20 HPM N-(4-Hydroxyphenyl)maleimide, 402, 416 HPN High para novolak resins, 242 HQ Hydroquinone, 498 HR Resiliency foams, 107 HTEP (6-Hydroxy)hexyl-2-(trimethylaminonio)ethyl phosphate, 119 HTPDMS Hydroxy-terminated poly(dimethylsiloxane), 414 IA Itaconic acid, 620, 629 IC Integrated circuites, 421 IEM 2-Isocyanatoethyl methacrylate, 351, 660 IFSS Interfacial shear strength, 434 IPDI Isophorone diisocyanate, 75

Index

713

IPN Interpenetrating polymer network, 382 IPO 2-Isopropenyl-2-oxazoline, 573, 630 IPP Isotactic poly(propylene), 547 IPPA Poly(2,2-di(4-phenylene)propane phthalate-co-2,2-di(4-phenylene)propane isophthalate), 390 IR Isoprene rubber, 632 KCD 3-Ketocoumarin, 209 Kevlar Poly(p-phenylene terephthalamide), 563 LC Liquid cristalline, 146 Liquid crystal, 208 LCDs Liquid crystal displays, 208 LCGIC Light curable glass-ionomer cements, 663 LCP Liquid crystalline polymers, 549 LDI Lysine-diisocyanate, 120 LDPE Low density poly(ethylene), 213, 542, 612, 620 LEC Light-emitting electrochemical cell, 123 LLDPE Linear low density poly(ethylene), 542, 635 LOI Limiting oxygen index, 405, 501 LPA Low-profile additives, 26 MA Maleic anhydride, 521, 541, 612, 637 Myristic acid, 366 MA-g-PP Maleic anhydride-grafted-poly(propylene), 563 MAC-10 11-Methacryloyloxy-1,1-undecanedicarboxylic acid, 671

714


MBA Methacryloyl-β-alanine, 660, 663 MBL α-Methylene-γ-butyrolactone, 665 MCDEA 4,4′ -Methylene bis(3-chloro-2,6-diethylaniline), 158 MDA 4,4′ -Methylenedianiline, 158, 175 MDEA Methyldiethanolamine, 359 MDI p,p′ -Methylene diphenyl diisocyanate, 113 Diphenylmethane diisocyanate, 74 MDP-BMI 1,1′ -(Methylene di-4,1-phenylene)bismaleimide, 423 MDPE Medium density poly(ethylene), 539 MEHQ Hydroquinone monomethyl ether, 677 MeTHPA 3-Methyl-1,2,3,6-tetrahydrophthalic anhydride, 180 MF Melamine/formaldehyde, 303 MFI Melt flow index, 587 MFN Melt flow number, 587 MFR Melt flow rate, 587 MFT Minimum fix temperature, 700 MKEA 4,4-Bis(N,N-dimethyl-N-(2-ethoxycarbonyl-1-propenyl)ammonium hexafluoro antimonate)benzophenone, 189–191 MMA Methyl methacrylate, 634 MMT Montmorillonite, 20 MOPIP 1-(2-Methoxyphenyl)piperazine, 116 MPD 2-Methyl-1,3-propanediol, 14

Index

715

MPDA m-Phenylene diamine, 272 MPGE 4-(N-Maleimidophenyl)glycidyl ether, 398, 402, 404 MPS Maleic anhydride grafted poly(styrene), 637 Mercaptopropyltrimethoxysilane, 343 MPTMS 3-Methacryloxypropoxytrimethoxysilane, 681 MPTS 3-Methacryloxypropyl-trimethoxysilane, 351, 365 MWD Molecular weight distribution, 608 NBR Acrylonitrile-butadiene, 53 Nitrile rubber, 540 NHCPA N-(2-Hydroxyethyl)-2-chloro-2-phenylacetamide, 570 NLO Nonlinear optical, 209 1 H-NMR Proton nuclear magnetic resonance spectroscopy, 140 NPG N-Phenylglycine, 193 NT-D 1-(α-Naphthyl)-3,3-di(2-hydroxyethyl)-triazene-1, 93 OCDI 4,4′ -Diphenylmethane carbodiimide, 547 ODOPB 2-(6-Oxid-6H-dibenz[c,e][1,2]oxa-phosphorin-6-yl)-1,4-benzenediol, 168 ODOPM 2-(6-Oxid-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)methanol, 168 ODPA 4,4′ -Oxydiphthalic anhydride, 498 OMT Organophilic montmorillonite, 111 OPIA (4-Octyloxyphenyl)phenyliodonium hexafluoroantimonate, 192 OPP Oriented poly(propylene), 460 OXA Ricinoloxazoline maleate, 634

716


PA Polyamide, 463 PA6 Poly(ε-caprolactam), 550 Polyamide 6, 213, 629 PAI Polyamide-imide, 419 PAVE Perfluoro (alkyl vinyl ether), 467 PBA Pentabromobenzyl acrylate, 501 PBO 2,2′ -(1,3-Phenylene)bis(2-oxazoline), 564 PBT Poly(benzo[1,2-d4,5-d′ ]bisthiazole-2,6-diyl)-1,4-phenylene, 495 Poly(butylene terephthalate), 214, 525, 541 PBZT Poly(p-phenylene benzobisthiazole), 498 PCB Printed circuit board, 378 PCDMA Poly(carbonate)dimethacrylate, 660 PCL Poly(ε-caprolactone), 517 PDClPO Poly(2,6-dichloro-1,4-phenylene oxide), 555 PDS Polydioxanone, 488 PE-g-MA Poly(ethylene) grafted with maleic anhydride, 549 PECH Poly(epichlorohydrin), 111, 112 PEEK Poly(ether ether ketone), 154 PEEK Poly(ether ether ketone), 418 PEEK-C Phenolphthalein poly(ether ether ketone), 155 PEEK-T Poly(ether ether ketone) based on tertiary butyl hydroquinone, 155 PEG Poly(ethylene glycol), 400

Index

717

PEG-MA Poly(ethylene glycol)methacrylate, 518 PEG-PDMS Block copolymer of poly(ethylene glycol) and poly(dimethylsiloxane), 643 PEGDMA Poly(ethylene glycol)dimethacrylate, 662 PEI Polyetherimide, 417 PEN Poly(ethylene 2,6-naphthalate), 555 PEO Poly(ethylene oxide), 153, 206, 533 Poly(ethylene-octene) copolymer, 551 PET Poly(ethylene terephthalate), 342, 540 PETA Pentaerythritol triacrylate, 516 PF Phenol/formaldehyde (resin), 242 PFS Phenol/formaldehyde sulfonate, 274 PG Propylene glycol, 366 PGA Polyglycolic acid, 488 PGE Phenyl glycidyl ether, 142 PHB Poly(p-hydroxybenzoate), 555 PHBV Poly(β-hydroxybutyrate-co-β-hydroxyvalerate), 517 PL β-Propiolactone, 119 PLA Poly(lactide), 521 Polylactic acid, 488, 519 PLE Photopolymerizable liquid encapsulants, 211 PMDA Pyromellitic dianhydride, 180, 524 PMMA Poly(methyl methacrylate), 153, 533

718


PO Propylene oxide, 571 POSS Polyhedral oligomeric silsesquioxane, 680 PP Poly(propylene), 607, 617 PP-g-MA Maleated poly(propylene), 541 PPA Poly(2,2-di(4-phenylene)propane phthalate), 390 PPDE Poly(phthaloyl diphenyl ether), 418 PPE Poly(2,6-dimethyl-1,4-phenylene ether), 565 Poly(phenylene ether), 214, 463 Poly(phenylene ether)s, 564 PPG Polyoxypropylene glycol, 390 PPIDE Poly(phthaloyl diphenyl ether-co-isophthaloyl diphenyl ether), 418 PPO Poly(2,6-dimethyl-1,4-phenylene oxide), 549, 555 PPOH 3-Phenyl-1-propanol, 567 PPS Poly(phenylene sulfide), 559 PPTDE Phthaloyl diphenyl ether-co-terephthaloyl diphenyl ether, 418 PPV Poly(p-phenylene vinylene), 124 PPy Poly(pyrrole), 161 PROXYL 2,2,5,5-Tetramethyl-1-pyrrolidinyloxy, 602 PS 1,3-Propanesulfone, 119 Poly(styrene), 152, 557 PSA Pressure-sensitive adhesives, 82, 460 PSC 1-Pyrenesulfonyl chloride, 196 PT Phenolic cyanate/phenolic triazine copolymer, 386

Index Poly(thiophene), 645 PT-D 1-Phenyl-3,3-di(2-hydroxyethyl)-triazene-1, 93 PTFE Poly(tetrafluoroethylene), 647 PTMEG Poly(tetramethylene ether), 513 PTMG Polytetramethylene glycol, 119 PTT Poly(trimethylene terephthalate), 536 PU Polyurethane, 69 PU/PAN Polyurethane/polyacrylonitrile, 117 PUA Polyurethane-acrylate, 116 PUDMA Polyurethane dimethacrylate, 659, 660 PUE Polyurethane elastomer, 111 PVC Poly(vinyl chloride), 98 PVDF Poly(vinylidene fluoride), 561 PVDH Poly(vinylidene difluoride-co-hexafluoropropylene), 435 QA N,N ′ -(4-Aminophenyl)-p-benzoquinone diimine, 400 QDM o-Quinodimethane, 495 re-HDPE Recycled high density poly(ethylene), 542 REC Rectorite, 109 RIE Reactive ion etching, 504 RMGICs Resin-modified glass ionomer cements, 663 RTD Residence time distribution, 514 RTM Resin transfer molding, 400

719

720


S-g-PMA Starch graft poly(methyl acrylate), 639 s-PS Syndiotactic poly(styrene), 212 SAN Poly(styrene-co-acrylonitrile), 153 Styrene/acrylonitrile copolymers, 214 SAXS Small-angle X-ray scattering, 314 SBR Styrene butadiene rubber, 465, 632 SDS Sodium dodecyl sulfate, 644 SEB Poly(styrene-b-(ethylene-co-butylene)), 542 SEBS Styrene-b-(ethylene-co-1-butene)-b-styrene triblock copolymer, 542 SEBS-g-MA Styrene-ethylene/butylene-styrene triblock copolymer, 541 SEM Scanning electron microscopy, 551 SG Styrene/glycidyl methacrylate, 212 SiOC Silicon oxycarbide, 338 SIPO 2-Isopropenyl-2-oxazoline, 573 SIS Poly(styrene-b-isoprene-b-styrene), 461 SMA Styrene/maleic anhydride copolymer, 549, 630, 635 SOC Spiroorthocarbonates, 665 SOE Spiroorthoesters, 665 SPE Solid polymer electrolytes, 122 SSO Silsesquioxane, 199 SUS 10-Undecenyl sulfate, 644 TA Tartaric acid, 120

Index TAA Tris(2-aminoethyl)amine, 484 TAIC Triallyl isocyanurate, 211 TAP Tris(2-allylphenoxy)triphenoxy cyclotriphosphazene, 435 TAT Tris(2-allylphenoxy)-s-triazine, 435 TBAEMA Diethylaminoethyl acrylate, 627 TBBPA 2,6,2′ ,6′ -Tetrabromobisphenol A, 32 Tetrabromobisphenol A, 3 TBPA Tetrabutylphosphonium acetate, 515 TCDM Tetrahydrofurfuryl cyclohexene dimethacrylate, 660 TDI Toluene diisocyanate, 73, 407 TDS Transdermal delivery system, 366 TEA Triethylamine, 121, 264 TEC Triethyl citrate, 366 TEGDI 1,2-Bis(isocyanate)ethoxyethane, 73 TEGDMA Triethylene glycol dimethacrylate, 659, 660, 675 TEMPO 2,2,6,6-Tetramethyl-1-piperidinyloxy, 603, 643 Tetramethyl-1-piperidinyloxy, 568 TEMPO 2,2,6,6-Tetramethyl-1-piperidinyloxy, 602 TEOS Tetraethoxysilane, 167, 325 TFE Tetrafluoroethylene, 467 TGDDM Tetraglycidyl diaminodiphenylmethane, 524 Tetraglycidyl-4,4′ -diaminodiphenylmethane, 144, 153 THF Tetrahydrofuran, 309

721

722


THFMA Tetrahydrofurfuryl methacrylate, 660 TLCP Thermotropic liquid crystalline polymer, 553 TMB 1,2,4-Trimethoxybenzene, 193 TMDSC Modulated differential scanning calorimetry, 194 TME 1,1,2,2-Tetramethoxyethane, 286 TMI 1-(Isopropenylphenyl)-1,1-dimethylmethyl isocyanate, 82 3-Isopropenyl-α,α-dimethylbenzene isocyanate, 546 TMPAE Trimethylolpropane mono allyl ether, 3 TMPTA Trimethylolpropane triacrylate, 351, 513, 638 TMPTMA 1,1,1-Trimethylolpropane trimethacrylate, 660 TMTEA Trimercaptotriethylamine, 208 TOF-SIMS Time of flight single ion monitoring mass spectroscopy, 647 TPA Terephthalic acid, 35 TPGDA Tripropylene glycol diacrylate, 638 TPMK 2-Methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, 193 TPP Triphenyl phosphite, 565 TPPA Poly(2,2-di(4-phenylene)propane phthalate-co-2,2-di(4-phenylene)propane terephthalate), 390 TPS Thermoplastic starch, 559 TPT 2,4,6-Triphenylpyrylium tetrafluoroborate, 485 TPU Thermoplastic polyether polyurethanes, 122 TRIS 1,1,1-Trimethylolpropane triacrylate, 632

Index

723

TSFA Triarylsulfonium hexafluoroantimonate, 208 TTT Time-temperature-transition, 195 UBMI Urethane-modified bismaleimide, 166 UDMA 1,6-Bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane, 659 Di-2-methacryloxyethyl-2,2,4-trimethylhexamethylenedicarbamate, 660 Urethane dimethacrylate, 675 UF Urea/formaldehyde, 11, 253, 286 UHMF Ultra high melt flow, 605 UHMWPE Ultra-high molecular weight poly(ethylene), 646 UP Unsaturated polyester, 117 VAc Vinyl acetate, 634 VDAC Vinylbenzyldodecyldimethyl ammonium chloride, 20 VEUH Vinylester-urethane hybrid resins, 212 VOAC Vinylbenzyloctadecyldimethyl ammonium chloride, 20 VOCs Volatile organic compounds, 11, 267, 311 VTEOS Vinyltriethoxysilane, 631 VTMS Vinyltrimethoxysilane, 631 WOF Work of fracture, 680 WPU Waterborne polyurethane, 121 XPS X-ray photoelectron spectroscopy, 119, 645 ZrDMA Zirconyl dimethacrylate, 681

724


CHEMICALS Abietic acid, 410, 411, 449 Acetaldehyde, 243 Acetic anhydride, 309, 407, 639 Acetoacetoxy methyl methacrylate, 616 Acetone cyanhydrin, 352 Acetonitrile, 148 2-Acetoxyethyl-dibutyltin, 108 2-Acetoxyethyl-dibutyltin chloride, 108 p-Acetoxystyrene, 182 Acetylacetone peroxide, 36 Acetyl chloride, 79 Acetyl peroxide, 592 Acetyltributyl citrate, 366 Acrolein, 349, 352 Acrylamide, 290, 369, 523, 648 Acrylic acid, 11, 91, 119, 151, 290, 350, 351, 547, 551, 560, 645–647 Acrylonitrile, 87, 104, 645 Acryloyl-β-alanine, 660 Acryloyl glutamic acid, 660 Adipic Acid, 35 Adipic acid, 3, 44, 89, 90 Allo-ocimene, 449 Allyl acetoacetate, 678 Allyl alcohol, 12 Allyl alcohol propoxylate, 35 Allyl-4-[(4-N-allyl-N-ethyl)aminophenylazo]-α-cyanocinnamate, 436 Allylamine, 434 Allyl bromide, 34 Allyl chloride, 139, 150 1-Allyl-2-cyanatobenzene, 373 Allyl cyanoacrylate, 473 Allyl-4-[(4-N,N-diallyl)aminophenylazo]-α-cyanocinnamate, 436 Allyl glycidyl ether, 142, 199, 200, 205, 571 7-Allyloxy-2-naphthol, 411 2-Allylphenol, 391, 418, 419 4-Allylphenol, 391 Allylsuccinic anhydride, 671 Aluminium(III)acetylacetonate, 381 Aluminum bromide, 311 Aluminum isopropoxide, 521 Aluminum isopropyloxide, 182

Index Aluminum nitride, 421 Aluminum tri-sec-butylate, 678 Aluminum trichloride, 311 4-Amino-benzocyclobutene, 498 o-Aminobenzoic acid, 13 N-2-Aminoethyl-3-aminopropyl-tris(2-ethylhexoxy)silane, 483 N-Aminoethyl piperazine, 175, 177 1-(2-Aminoethyl)piperazine, 563 1,3-Aminoethylpropanediol, 557 m-Aminophenol, 243 o-Aminophenol, 557 p-Aminophenol, 142 N,N ′ -(4-Aminophenyl)-p-benzoquinone diimine, 400 1-(3′ -Aminopropyl)imidazole, 99 (3-Aminopropyl)triethoxysilane, 23, 110, 205, 316, 414, 646 Ammonium polyphosphate, 32, 111, 112 Ammonium sulfate, 311 O,O-tert-Amyl-O-(2-ethylhexyl)monoperoxy carbonate, 596 tert-Amyl hydroperoxide, 591 Amylopectin, 639 Amylose starch, 639 tert-Amylperoxybenzoate, 36 tert-Amylperoxy-2-ethylhexanoate, 596 4-(tert-Amylperoxy)-4-methyl-2-pentanol, 591, 592 tert-Amylperoxyneodecanoate, 596 tert-Amylperoxypivalate, 596 Anatase, 365 Aniline, 74, 75 9-Anthroic acid, 197 Antimony trioxide, 25, 32, 34, 357, 378 Ascorbic acid, 120, 334, 668, 669 Atropine, 344 2-Azabicyclo[2.2.1]heptane, 99, 100 2,2′ -Azobis(2-acetoxy)propane, 557, 601 2,2′ -Azobis(2-amidinopropane)hydrochloride, 369 2,2′ -Azobis(cyclohexanenitrile), 601 2,2′ -Azobis(2,4-dimethyl-4-methoxyvaleronitrile), 601 2,2′ -Azobis(2,4-dimethylvaleronitrile), 359, 601, 641 2,2′ -Azobis(2-ethylpropionitrile), 642 2,2′ -Azobis(isobutyronitrile), 35, 359, 601, 641, 697 2,2′ -Azobis(2-methylbutyronitrile), 35, 601 Barbituric acid, 668–670

725

726


Barium metaborate, 357 Barium sulfate, 19 Barium titanate, 23 Benzenesulfonic acid, 324 Benzil, 673 Benzocyclobutene, 493 1-Benzocyclobutenyl-1-bromoethyl ether, 495 1-Benzocyclobutenyl-1-hydroxyethyl ether, 495 1-Benzocyclobutenyl vinyl ether, 493, 495 Benzoguanamine, 301 Benzoic acid, 51 Benzoic acid 2,2,6,6-tetramethyl-piperidin-1-oxyl-4-yl ester, 615 Benzoin isobutyl ether, 673 Benzoin methyl ether, 37, 673 Benzoin phenyl ether, 673 Benzophenone, 359, 629, 646, 673 p-Benzoquinone, 17, 168, 367, 481, 623, 670 Benzoxazole, 557 Benzoyl chloride, 79 Benzyldimethylamine, 152 2-Benzyl-2-dimethylamino-1-(4-morpholinophenyl)butan-1-one, 193 Benzyl methacrylate, 153 1-Benzyl-5-phenylbarbituric acid, 669 N-Benzylpyrazinium hexafluoroantimonate, 184 N-Benzylquinoxalinium hexafluoroantimonate, 184 Benzyl tetrahydrothiophenium hexafluoroantimonate, 184 Benzyltrimethylammonium chloride, 205, 402 Betulin, 449 Bicumene, 627 Bicyclo[4.2.0]octa-1,3,5-triene, 493 Bis-p-aminocyclohexylmethane, 175 2,5-Bis(aminomethyl)bicyclo[2.2.1]heptane di(methylisopropylketimine), 176 1,2-Bis(aminomethyl)cyclobutane, 179 Bisaminomethylcyclohexane, 175 3,5-Bis(4-aminophenoxy)benzoic acid, 82 Bis(4-aminophenoxy)phenylphosphine oxide, 169 2,2-Bis[4-(4-aminophenoxy)phenyl]propane, 342 2,3-Bis(4-(4-aminophenoxy)phenyl)quinoxaline-6-carboxylic acid, 412 Bis(m-aminophenyl)methylphosphine oxide, 169, 171, 176 Bis(3-aminophenyl)phenylphosphine oxide, 402 Bis(4-aminophenyl)phenylphosphine oxide, 121, 172 2,3-Bis(4-aminophenyl)quinoxaline-6-carboxylic acid, 412 1,3-Bis(3-aminopropyl)tetramethyldisiloxane, 142

Index

727

2,6-Bis-4-benzocyclobutene benzo[1,2-d:5,4-d′ ]bisoxazole, 493, 496 Bis(benzocyclobutenyl)-m-divinylbenzene, 502 Bis(benzocyclobutenyl)divinyltetramethylsiloxane, 502 1,2-Bis(benzocyclobutenyl)ethane, 493 1,2-Bis(4-benzocyclobutenyl)ethylene, 502 2,6-Bis(4-benzocyclobutenyloxy)benzonitrile, 493 4,4′ -Bis(sec-Butylamine)dicyclohexylmethane, 93 4,4′ -Bis(sec-Butylamine)diphenylmethane, 93 Bis(4-tert-Butylcyclohexyl)peroxydicarbonate, 36, 359 Bis(4-tert-butyl-1-isopropyl-2-imidazolyl)disulfide, 479 α,α′ -Bis(tert-butylperoxy)diisopropyl benzene, 597 1,1-Bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 51, 362 ((3-(4-(1-(4-(3-(Bis-carboxymethylamino) 2-hydroxy-propoxy) phenyl)-1-methyl-ethyl) phenoxy) 2-hydroxypropyl) carboxy methylamino) acetic acid, 123 1,7-Bis(chlorotetramethyldisiloxy)-m-carborane, 336, 337 Bis(4-cyanatocumyl)benzene cyanate, 374 1,1-Bis(4-cyanatophenyl)ethane, 196, 377 Bis(4-cyanatophenyl)ether, 377 2,2-Bis(4-Cyanatophenyl)1,1,1,3,3,3-hexafluoropropane, 377 Bis(4-cyanatophenyl)methane, 377 1,3-Bis(4-cyanatophenyl-1-(1-methylethylidene))benzene, 377 1,3-Bis(4-cyanatophenyl-1-(methylethylidene))benzene, 377 2,2′ -Bis(4-cyanatophenyl)propane, 382 2,2-Bis(4-cyanatophenyl)propane, 377, 388 Bis(4-cyanatophenyl)thioether, 377 1,3-Bis[2′ -cyano-3′ ,3-diphenylacryloyloxy]2,2-bis-[[2-cyano-3′ ,3′ -diphenylacryloyloxy]methyl]propane, 683 2,2-Bis(4,4-di-tert-butylperoxycyclohexyl)propane, 592 Bis(2,6-dichlorobenzoyl)-(4-propylphenyl)phosphine oxide, 37, 39 4,4′ -Bis(diethylamino)benzophenone, 193 Bis(3-diethylphosphono-4-hydroxyphenyl)sulfide, 169 Bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, 37 4,4′ -Bis(dimethylamino)benzophenone, 189 Bis(2-dimethylaminoethyl)ether, 99, 103 N,N-Bis(3-dimethylamino-n-propyl)amine, 102 Bis(3-(N,N-dimethylamino)propyl)amine, 99 N,N-Bis(3-dimethylaminopropyl)formamide, 105 N,N-Bis[3-(dimethylamino)propyl]formamide, 103 N,N ′ -Bis(3-dimethylaminopropyl)urea, 105 Bis(3,5-dimethyl-4-cyanatophenyl)methane, 377 4,4-Bis(N,N-dimethyl-N-(2-ethoxycarbonyl-1-propenyl)ammonium hexafluoro antimonate)benzophenone, 187, 189–191

728


Bis(dimethylsilyl)benzene, 335 1,2-Bis(2,3-epoxycyclohexyloxy)propane, 202 Bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate, 141 Bis[3-(2,3-epoxypropyl thio)phenyl]sulfone, 141 Bis(3-glycidyloxy)phenylphosphine oxide, 169 Bishydantoin, 142 2,2-Bis(4-hydroxy-3,5-dibromophenyl)propane, 357 Bis(2-hydroxy-3,5-dimethylbenzyl)ether, 260 Bis(2-hydroxy-3,5-dimethyl-benzyl)methylene, 260 Bis(2-hydroxyethyl)terephthalate, 45 Bis(hydroxyethyl)-p-toluidine, 669 2,2-Bis[p-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane, 661 4,4′ -Bis(2-hydroxy-3-methacryloylpropoxy)biphenyl, 664 Bis(hydroxymethyl)furan, 309 1,2-Bis-[2(2-hydroxy-5-methylphenyl)-5-benzotriazolyl]-ethane, 80 2,2-Bis(4-hydroxyphenyl)butane, 243 Bis(4-hydroxyphenyl)methane, 243 2,2-Bis(4-hydroxyphenyl)propane, 243 N,N ′ -Bis(2-hydroxypropylaniline), 93 Bis[N-(3-imidazolidinylpropyl)]oxamide, 103 1,2-Bis(isocyanate)ethoxyethane, 73 Bismaleimide(3,3′ -bis(maleimidophenyl))phenylphosphine oxide, 169, 171 1,3-Bis(maleimido)benzene, 414 4,4′ -Bis(maleimido)diphenylmethane, 397, 398, 405, 407, 418, 419, 425 1,3-Bis(maleimidomethyl)cyclohexane, 398, 414 1,3-Bis(4-maleimido phenoxy)benzene, 400 1,4-Bis(4-maleimido phenoxy)benzene, 400 2,2-Bis[4-(4-maleimido phenoxy)phenyl]propane, 388, 398, 399, 434 Bis(4-maleimidophenyl)ether, 399 Bis(4-maleimidophenyl)methane, 399 3,3′ -Bis(maleimidophenyl)phenylphosphine oxide, 402, 422 Bis(3-maleimidophenyl)phenylphosphine oxide, 402, 422 4,4′ -Bismaleimidophenylphosphonate, 398 Bis(4-maleimidophenyl)sulfone, 398, 399 Bis(3-mercaptophenyl)sulfone, 208 1,6-Bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane, 659 2,2-Bis[4-(3-methacryloxy-2-hydroxypropoxy)phenyl]propane, 660 2,2′ -Bis(α-methacryloxy-β-hydroxypropoxyphenyl)propane diphosphonate, 684 Bis(2-methacryloyloxyethyl)hydrogen phosphate, 671 Bis[2-(methacryloyloxy)ethyl]phosphate, 665 1,1-Bis(3-methyl-4-cyanatophenyl)cyclohexane, 377 Bis(1-methyl-imidazole)zinc(II)diacetyl-acetonate, 381 Bis(1-methyl-imidazole)zinc(II)dicyanate, 381

Index Bis(1-methyl-imidazole)zinc(II)dioctoate, 381 Bis-1,3-methyl-1,2,3,4,5-pentamethylcyclopenta-2,4-diene benzene, 429 Bis(methyl salicyl)carbonate, 515 Bis[N-(3-morpholinopropyl)]oxamide, 103 Bismuth neodecanoate, 108 Bis(4-nitrophenyl)phenylphosphine oxide, 121 2,2-Bis(4-(oligo(ethoxy))phenyl)propane diacetate, 672 2,2-Bis(4-(oligo(ethoxy))phenyl)propanedimethacrylate, 672 Bis(1-oxyl-2,2,6,6-tetramethylpiperidine4-yl)sebacate, 602, 603 Bisphenol A, 35, 142, 148, 150, 180, 243, 246, 514 Bisphenol A bismaleimide, 398, 400 Bisphenol A dicyanate, 381, 389, 390 Bisphenol A diglycidyl ether, 139 Bisphenol A diglycidyl ether dimethacrylate, 658 Bisphenol A dimethacrylate, 660 Bisphenol B, 243 Bisphenol F, 142, 243 4,4′ -Bis(o-propenylphenoxy)benzophenone, 417 Bis(4-(1,2,4-triazoline-3,5-dione-4-yl)phenyl)methane, 410 Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, 52 Bis(3,5,5-trimethylhexanoyl)peroxide, 359 Boric acid, 101, 257, 321, 419 Boric anhydride, 311 Boron fluoride, 311 Boron trifluoride, 183, 315, 453, 481 1-Bromobenzocyclobutene, 495 Butadiene diepoxide, 141 1,4-Butane diamine, 121 1,4-Butanediisocyanate, 120 1,2-Butanediol, 44 1,4-Butanediol, 35, 44, 52, 110, 113, 119, 142, 245, 291 2,3-Butanediol, 44 1,4-Butanediol diglycidyl ether, 141, 205 1,3-Butanediol dimethacrylate, 662 cis-2-Butene-1,4-diol, 5 tert-Butyl acrylamide, 642 n-Butyl acrylate, 26, 91, 345, 349, 351, 365, 538, 642, 697 tert-Butyl acrylate, 569 tert-Butyl alcohol, 186 tert-Butyl catechol, 42, 481 tert-Butylcumyl peroxide, 36 n-Butyl cyanoacrylate, 473, 475 N-tert-Butyl-1-dibenzylphosphono-2,2-dimethylpropyl nitroxide, 602

729

730


n-Butyl-4,4-di(tert-butylperoxy)valerate, 596 N-tert-Butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide, 602, 603 N-tert-Butyl[(1-diethylphosphono)-2-methyl-propyl]nitroxide, 602 o-tert-Butyl-di-1-piperidinylphosphonamidate, 184, 186 N-tert-Butyl-1-di(2,2,2-trifluoroethyl)phosphono-2,2-dimethylpropyl nitroxide, 602 Butylene oxide, 85 O,O-tert-Butyl-O-(2-ethylhexyl)monoperoxy carbonate, 596 tert-Butyl hydroperoxide, 36, 362, 591, 597, 699 tert-Butyl hydroquinone, 481 O,O-tert-Butyl-O-isopropyl monoperoxy carbonate, 596 n-Butyl methacrylate, 351, 365, 637, 638 tert-Butyl methacrylate, 569 2-(3-tert-Butyl-5-methyl-2-hydroxyphenyl)-5-chlorobenzothiazole, 356 N-tert-Butyl-1-(2-naphthyl)-2-methylpropyl nitroxide, 602 tert-Butylperoxyacetate, 596 tert-Butylperoxybenzoate, 28, 36, 40, 360, 362, 596, 597 tert-Butylperoxy-2-ethylhexanoate, 36, 596 tert-Butylperoxyisobutyrate, 596 tert-Butylperoxyisononanoate, 596 tert-Butylperoxymaleate, 596 tert-Butylperoxyneodecanoate, 596 tert-Butylperoxypivalate, 596, 641 tert-Butylperoxy-3,5,5-trimethylhexanoate, 596 N-tert-Butyl-1-phenyl-2-methylpropyl nitroxide, 602 p-tert-Butylphenyl salicylate, 356 Butyl stearate, 476 n-Butyl vinyl ether, 190 2-Butyne-1,4-diol, 5 Butyraldehyde, 243 γ-Butyrolactone, 264 Calcium hydroxy apatite, 678 Calcium stearate, 28, 589 Calixarene, 187 Camphene, 449 Camphorquinone, 662, 673 ε-Caprolactam, 80, 536, 546 N,N ′ -Carbonylbiscaprolactam, 80 Carboxymethyl chitin, 121 Carboxymethyl konjac glucomannan, 121 N-(p-Carboxyphenyl)maleimide, 200 o-Carboxy phthalanilic acid, 3, 13 3-Carboxy-2,2,5,5-tetramethyl-pyrrolidinyloxy, 602

Index Cardanol, 243 Cardene, 493 Cardol, 243 Carene, 449, 452 Carophyllene, 449 β-Carotine, 449 Casein, 121, 122 Cassava starch, 545 Catechol, 17, 481 Celluloid, 658 Cellulose, 545 Cetyl pyridinium chloride, 695 Chitosan, 118, 545 Chloranil, 17 Chloro acetic acid, 474 Chlorobenzene, 324 p-Chlorobenzoyl peroxide, 592 4-Chloro-3,5-diamino-benzoic acid isobutylester, 93 Chlorodibutyltin hydride, 108 1-Chloro-2,3-epoxypropane, 139 Chloroethyl diazoacetate, 631 m-Chloroperbenzoic acid, 205 Chloroplatinic acid, 333 3-Chloro-1,2-propanediol, 111 Chlorosulfonic acid, 434 Choline octoate, 334 Chrysanthemol, 449 Cinnamic acid, 208 Citric acid, 52, 311 Cobalt acetylacetonate, 38 Cobalt 2-ethylhexanoate, 360 Cobalt(II)acetylacetonate, 381 Cobalt octoate, 28, 38 Copper phthalocyanine, 695 Cornstarch, 248, 343, 639 Creatinine, 636 Creosote, 344 m-Cresol, 243, 268 o-Cresol, 168, 244, 275, 387 p-Cresol, 243, 268 Cumene hydroperoxide, 36, 244, 591, 671 α-Cumylperoxyneodecanoate, 596 Cupric oxide, 387

731

732


Cuprous oxide, 387 3-(2-Cyanatophenyl)propyltrimethoxysilane, 391 3-(4-Cyanatophenyl)propyltrimethoxysilane, 391 2-Cyanoacrylate, 480, 483 2-Cyanoacrylic acid, 477 4′ -Cyano-4-biphenyloxyvaleric acid, 663, 664 Cyanoethylmethylaniline, 673 Cyanogen bromide, 374 Cyanuric chloride, 419 Cyclobutabenzene, 493 Cyclobutarene, 493 1,4-Cyclohexane diamine, 93 1,4-Cyclohexanedimethanol, 49, 154 Cyclohexanone peroxide, 362 Cyclohexene, 147 Cyclohexene oxide, 189 Cyclohexenoic acid, 201 Cyclohexyl methacrylate, 350, 351 2-Cyclohexyl-5-methylphenol, 243, 268 Cyclopentylamine, 289 Decabromodiphenyloxide, 31, 32 Decalin, 632 Decanedioic acid bis(2,2,6,6-tetramethyl-piperidin-1-oxyl-4-yl), 615 1,10-Decanediol, 117 Decanoyl peroxide, 592 1-Decene, 147 Diabietylketone, 410 3,5-Diacetyl-1,4-dihydrolutidine, 284 2,2′ -Diallyl bisphenol A, 164, 381, 382, 397, 398, 400, 417, 418, 424, 425, 427, 428 (4-(N,N-Diallyl)-4′ -nitrophenyl)azoaniline, 436, 437 2,4-Di(2-allylphenoxy)-6-N,N-dimethylamino-1,3,5-triazine, 417, 419 2,4-Di(2-allylphenoxy)-6-(2-naphthyloxy)-1,3,5-triazine, 417, 419, 420 1,4-Diallyl phenyl ether, 400 Diallyl phthalate, 8 3,5-Diaminobenzoic acid, 109 1,4-Diaminobutane, 630 1,2-Diaminocyclohexane, 175 4,4′ -Diaminodibenzyl, 80 2,4-Diamino-3,5-diethyl toluene, 175 2,6-Diamino-3,5-diethyl toluene, 175 Diaminodiethyl toluene, 175 4,4′ -Diamino-3,3′ -dimethyldicyclohexylmethane, 175

Index

733

4,4′ -Diaminodiphenylmethane, 142, 163, 167, 175, 178, 196, 198, 402, 407, 414, 423 4,4′ -Diaminodiphenylsulfone, 153, 158, 163, 175, 176, 178, 198 2,4-Diamino-4′ -methylazobenzene, 176 2,6-Di(4-aminophenoxy)benzonitrile, 417 3,5-Diaminophenyl-4-benzocyclobutenylketone, 498 (N-4,6-Diamino-1,3,5-triazin-2-yl)-1,3,5-triazine-2,4,6-triamine, 299 Diaminotricyclododecane, 179 2-(3,5-Di-tert-amyl-2-hydroxyphenyl)benzotriazole, 356 Di-tert-amyl peroxide, 592 1,1-Di(tert-amylperoxy)cyclohexane, 592 2,2-Di(tert-amyl)peroxypropane, 592 1,5-Diazabicyclo[4.3.0]non-5-ene, 483 1,4-Diazabicyclo[2.2.2]octane, 99–101 1,8-Diazabicyclo[5.4.0]undec-7-ene, 483, 484 1,5-Diazobicyclo[4.3.0]non-5-ene, 102 1,5-Diazobicyclo[4.3.0]non-5-ene, 101 1,8-Diazobicyclo[5.4.0]undec-7-ene, 101, 102 o-Diazonaphthoquinone, 268 1,2,5,6-Dibenzocyclooctadiene, 495 Dibenzodiazyl disulfide, 479 Dibenzoyl peroxide, 36, 40, 359, 486, 568, 592, 597, 668 N,N-4,4-Dibenzylbismaleimide, 398, 412 1,1-Dibromo-2,2-bis(4-cyanatophenyl)ethylene, 377 Dibromoneopentylglycol, 34 Dibromostyrene, 34 α, α′ -Dibromo-m-xylene, 428 Dibutoxybis(acetylacetonato)titanium(IV), 421 Di-tert-butyl fumarate, 571 3,5-Di-tert-butyl-4-hydroxybenzyl acrylate, 632 2-(3,5-Di-tert-butyl-2-hydroxyphenyl)benzotriazole, 356 2-(3,5-Di-tert-Butyl-2-hydroxyphenyl)-5-chlorobenzothiazole, 356 3,5-Di-tert-butyl-4-hydroxyphenyl-propanoic acid, 622 2,6-Di-tert-butyl-4-methylphenol, 677 Di-tert-butyl peroxide, 40, 591, 592 2,2-Di(tert-butylperoxy)butane, 592 1,1-Di(tert-butylperoxy)cyclohexane, 592 1,1-Di-(tert-butylperoxy)cyclohexane, 698 α, α′ -Di(tert-butylperoxy)diisopropylbenzene, 557 1,3-Di(2-tert-butylperoxyisopropyl)benzene, 597 1,4-Di(tert-butylperoxyisopropyl)benzene, 592, 599 Di(2-tert-butylperoxyisopropyl)benzene, 597 1,1-Di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 51, 592, 638

734


o,o-Di-tert-Butyl phenylphosphonate, 184 Dibutyl phosphite, 684 Dibutyl phthalate, 475, 476 o,o-Di-tert-butyl-1-piperidinylphosphonamidate, 184, 186 1,3-Di-n-butyltetramethyldisilazane, 338 1,3-Di-tert-butyltetramethyldisilazane, 338 Dibutyltin bis(2,3-dihydroxypropylmercaptide), 107, 108 Dibutyltin bis(4-hydroxyphenylacetate), 107, 108 Dibutyltin diacetate, 107, 331 Dibutyltin dilaurate, 80, 107, 108, 114, 381, 414, 567, 688 Dibutyltin dilauryl mercaptide, 107 Dibutyltin dimercaptide, 107 Dibutyltin oxide, 184, 567, 697 2,5-Dicarboxyaldehyde-furan, 309 Dichloroacetic acid, 40 Dichlorobenzaldehyde, 206 1,1-Dichloro-2,2-bis(4-cyanatophenyl)ethylene, 377 Dichlorobis(triphenylphosphine)platinum(II), 333 Dichlorodimethylsilane, 343, 414 Dichloroethane, 147 Dichloromethane, 381, 434 1,3-Dichlorotetramethyldisiloxane, 336, 337 Dicumyl, 627 Dicumyl hydroperoxide, 597 Dicumyl peroxide, 36, 362, 541, 597, 609, 623, 629 4,4-Dicyanatobiphenyl, 377 1,4-[Di(4-cyanato diphenyl-2,2′ -propane)]terephthalate, 387 Dicyandiamide, 176, 299, 300, 402 Dicyclohexylmethane-4,4′ -diisocyanate, 73, 75 Dicyclohexylperoxydicarbonate, 596 o,o-Dicyclohexyl phenylphosphonate, 184 Dicyclopentadiene, 6, 7, 35, 454 Didodecyl fumarate, 52 1,3-Didodecyloxy-2-glycidyl-glycerol, 141 1,3-Diethenyl-1,1,3,3-tetramethyldisiloxane, 333 2-(2-N,N-Diethylaminoethoxy)ethanol, 99, 105 Diethylaminoethyl acrylate, 626, 627 Diethylaminoethyl methacrylate, 675 Diethylaminopropylamine, 175, 177 Diethyl-2,2-dicyanoglutarate, 471 Diethylene glycol, 3, 11, 15, 44, 89, 93, 103, 200, 697 Diethylene glycol diacrylate, 638 Diethylenetriamine, 87, 175, 177

Index

735

Di(2-ethylhexyl)peroxydicarbonate, 596 Di-2-ethylhexyl phosphate, 684 Di-2-ethylhexyl phosphite, 684 Diethylketone, 593 Diethyl maleate, 542, 623 Diethyl malonate, 80 Diethyl sebacate, 476 Diethylsuccinate, 536, 550 2,4-Diethylthioxanthone, 188 Diethyltoluene diamine, 93 N,N-Diethyltoluidine, 479 1,3-[Di(4-glycidyloxy diphenyl-2,2′ -propane)]isophthalate, 387 Diglycidyl tetrahydrophthalate, 525 1,3-Dihexyltetramethyldisilazane, 338 1,2-Dihydrocyclobutabenzene-3,6-dicarboxylic acid, 493, 495, 496, 498 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 112, 168, 169, 405, 422 Dihydrophthalimide, 410 1,4-Dihydroxybenzene, 622 2,4-Dihydroxybenzophenone, 356, 683 4,4′ -Dihydroxybiphenyl, 663 4,4′ -Dihydroxychalcone, 208, 209 3,3′ -Dihydroxy-4,4′ -diaminobiphenyl, 342 2,2′ -Dihydroxy-4,4′ -dimethoxybenzophenone, 356 1,4-Di(2-hydroxyethyl)hydroquinone, 93 Dihydroxyethyl-p-toluidine, 666 α,α′ -Dihydroxyl-poly(butyl acrylate), 82 2,2′ -Dihydroxy-4-methoxybenzophenone, 356 2,7-Dihydroxynaphthalene dicyanate, 377 10-(2,5-Dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 169 Diisobutyl aluminum hydride, 521 1-Diisobutylene, 453 2-Diisobutylene, 453 4,4′ -Diisocyanato dicyclo hexylmethane, 73 Diisopropylbenzene mono hydroperoxide, 591 1,4-Dilithio-1,3-butadiyne, 336 Di-2-methacryloxyethyl-2,2,4-trimethylhexamethylenedicarbamate, 660 o,o′ -Dimethallyl bisphenol A, 418 3,9-Di(p-methoxybenzyl)-1,5,7,11-tetra-oxaspiro[5.5]undecane, 184, 185 Dimethoxydiethoxysilane, 325 2,2-Dimethoxy-1,2-diphenylethan-1-one, 193 2,2-Dimethoxy-2-phenylacetophenone, 37, 359 N,N-Dimethylacetamide, 412, 623

736


N,N-Dimethylacrylamide, 647 Dimethylamine, 156, 419 4-Dimethylaminobenzaldehyde, 193, 666 3-Dimethylaminobenzoic acid, 193 4-Dimethylaminobenzoic acid, 193 4-Dimethylaminobenzoin, 193 4-Dimethylamino-1-butanol, 119 2(Di-methylamino)ethanol, 104 2-(Dimethylamino)ethyl methacrylate, 82, 83 2-Dimethylaminoethyl urea, 105 2-(Dimethylamino)-2-(hydroxymethyl)-1,3-propanediol, 253, 265 5-Dimethylamino-3-methyl-1-pentanol, 99 2-Dimethylamino-2-methyl-1-propanol, 253, 264, 265 5-Dimethylaminonaphthalene-1-(2-aminoethyl)sulfonamide, 197 4-Dimethylaminophenethanol, 666 2-[4-(Dimethylamino)phenyl] ethanol, 669 3-Dimethylamino-1,2-propanediol, 110 1-(3-Dimethylaminopropoxy)-2-butanol, 104, 105 N-(3-Dimethylaminopropyl)-2-ethylhexanoic acid amide, 99 4-Dimethylaminopyridine, 199 Dimethyl ammonium methyl sulfate, 695 N,N-Dimethylaniline, 39, 192 N,N-Dimethylbenzylamine, 99, 180 2,5-Dimethyl-2,5-bis(benzoylperoxy)hexane, 182 N,N-Dimethylcyclohexylamine, 99 2,6-Dimethyl-3,5-diacetyl-1,4-dihydropyridine, 284 2,5-Dimethyl-2,5-di(benzoylperoxy)hexane, 592 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane, 516, 546, 591, 592, 594, 607, 621 2,5-Dimethyl-2,5-di-tert-butylperoxyhexane, 595 2,4-Dimethyl-2,5-di(tert-butylperoxy)-3-hexyne, 573 2,5-Dimethyl-2,5-di(tert-butylperoxy)-3-hexyne, 516, 557, 592, 595, 597 Dimethyldichlorosilane, 327 Dimethyldiethoxysilane, 325 2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, 592 2,5-Dimethyl-2,5-di(hydroperoxy)hexane, 591 N,N-Dimethyl-N′ ,N ′ -di(2-hydroxypropyl)-1,3-propane diamine, 483 Dimethyldimethoxysilane, 325, 331 2,3-Dimethyl-2,3-diphenylbutane, 627 N,N-Dimethylethanolamine, 99, 100 N,N-Dimethylethylamine, 99 N,N-Dimethylethylethanolamine, 119 N,N-Dimethylformamide, 544, 623 2,5-Dimethylhexene-2,5-diperoxyisononanoate, 596

Index

737

1,1-Dimethyl-3-hydroxybutyl-3-(carboxy)-5-norbornene-2-ylperoxycarboxylate, 624 1,1-Dimethyl-3-hydroxybutyl-2-(carboxy)perbenzoate, 624, 626 1,1-Dimethyl-3-hydroxybutyl-2-(carboxy)peroxycyclohexanecarboxylate, 624 1,1-Dimethyl-3-hydroxybutyl hydroperoxide, 626 1,1-Dimethyl-3-hydroxybutyl-6-(hydroxy)peroxyhexanoate, 624 1,1-Dimethyl-3-hydroxypropyl-3-(carboxy)peroxypropanoate, 624 1,2-Dimethylimidazole, 101 Dimethyl itaconate, 630 Dimethylmelamine, 300 Dimethylol butanoic acid, 93, 121 α,α′ -Dimethylol propionic acid, 124 Dimethylol propionic acid, 120 4,5-Dimethyl-2-oxo-1,3,2-dioxathiolane, 482 N,N-Dimethyl-p-phenylene diamine, 6 N′ ,N′ -Dimethylpiperazine, 99 3,5-Dimethylpyrazole, 80 Dimethyl sebacate, 476 Dimethyl terephthalate, 35, 697 3,5-Dimethylthio-toluene diamine, 93 N,N-Dimethyl-p-toluidine, 479, 669, 672 2,4-Dinitrotoluene, 74 Dinonylphenol cyanate, 374 Dioctyl adipate, 476 Dioctyl glutarate, 476 Dioctyl phthalate, 476 7,26-Dioxatrispirobicyclo[4.1.0]heptane-4,5′ -1,3-dioxane-2′ ,2′′ 1,3-dioxane-5′′ ,4′′ -bicyclo[4.1.0]heptane, 665, 667 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], 185 5-(2,5-Dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, 180 Dipentaerythritol pentaacrylate monophosphate, 684 Dipentaerythritol pentamethacrylate monophosphate, 684 Diphenylchlorosilane, 325 Diphenyldimethoxysilane, 325 N,N ′ -Diphenylethane-1,2-diamine, 6 N,N-4,4-Diphenyl ether bismaleimide, 398 o,o-Di-1-phenylethyl phenylphosphonate, 186 N,N ′ -Diphenylhexane-1,6-diamine, 6 Diphenyliodoniumhexafluorantimonate, 666 Diphenyl iodonium hexafluoroantimonate, 192 Diphenylmelamine, 300

738


N,N-4,4-Diphenylmethanebismaleimide, 398, 412 4,4′ -Diphenylmethane bismaleimide, 547 4,4′ -Diphenylmethane carbodiimide, 547 4′ ,4′ -Diphenylmethane diamine, 87 Diphenylmethane-4,4′ -diisocyanate, 69 4,4′ -Diphenylmethanedimaleimide, 410 Diphenylphosphine, 334 3,3′ ,4,4′ -Diphenylsulfone tetracarboxylic dianhydride, 342 1,3-Diphenyltetramethyldisilazane, 338 Dipropylene glycol, 44 2,2′ -Dipyridyl disulfide, 479 2,3,8,9-Di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane, 141 2,3,8,9-Di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane spiroorthocarbonate, 185 2,14-Dithiacalix[4]arene, 248 4,4′ -Dithiodianiline, 176 6,6′ -Dithiodinicotinic acid, 479 1,3-Divinyl-1,1,3,3-tetramethyldisiloxane, 333 Dodecanoyl peroxide, 592, 597 1-Dodecene, 147, 638 2-Dodecen-1-yl succinic anhydride, 544 Dodecenyl succinic anhydride, 180 Dodecyl acrylate, 615 Dodecyl aldehyde, 484 Dodecyl diacid, 516 Dodecyl mercaptane, 641 Dodecylphenol, 381 Eosine, 673 Epichlorohydrin, 139–141, 148, 150, 173 Epoxy allyl soyate, 141 Epoxychlorotriazine, 565 2-(3,4-Epoxycyclohexyl)ethyl(methyl)dimethoxysilane, 326 2-(3,4-Epoxycyclohexyl)ethyl(phenyl)diethoxysilane, 326 2-(3,4-Epoxycyclohexyl)ethyltriethoxysilane, 326 2-(3,4-Epoxycyclohexyl)ethyltrimethoxysilane, 326 3,4-Epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate, 141 3,4-Epoxycyclohexylmethyl-3′ ,4′ -epoxycyclohexane carboxylate, 187 Epoxy methyl soyate, 141 E-12,13-Epoxyoctadeca-E-9-enoic acid ester, 141 1,2-Epoxy-3-phenoxypropane, 159, 167 2,3-Epoxypropoxy methacrylate, 663 2,3-Epoxypropyl(methyl)dimethoxysilane, 326 2,3-Epoxypropyl(phenyl)dimethoxysilane, 326

Index 2,3-Epoxypropyltriethoxysilane, 326 2,3-Epoxypropyltrimethoxysilane, 326 2,3-Epoxypropyltrimethylammonium chloride, 523 exo-3,6-Epoxy-1,2,3,6-tetrahydrophthalic anhydride, 142 exo-3,6-Epoxy-1,2,3,6-tetrahydrophthalimidocaproic acid, 142 17-β-Estradiol, 366 Ethanolamine, 289, 561 2-Ethenylisopropanol, 333 Ethenylphenyloxirane, 141 2-Ethoxyethyl cyanoacrylate, 473 Ethyl acrylate, 91, 213, 214, 290, 351 Ethylal, 286 Ethylaluminum dichloride, 451 Ethylamine, 289, 315 Ethyl-(4,4′ -bismaleimidophenyl)phosphonate, 402, 422 Ethyl α-(bromomethyl)acrylate, 189 Ethyl cyanoacetate, 471 Ethyl 2-cyanoacrylate, 472 Ethyl cyanoacrylate, 473, 489 Ethyl-3,3-di(tert-amylperoxy)butyrate, 596 Ethyl diazoacetate, 631 Ethyl-3,3-di(tert-butylperoxy)butyrate, 596 N-Ethyldiisopropanolamine, 675 Ethyl-4-dimethylamino benzoate, 193, 662, 666, 669, 673, 675 N,N ′ -Ethylene-bisstearamide, 513 Ethylene carbonate, 84 Ethylene diamine, 86, 88, 93, 121, 454 3,4-Ethylenedioxythiophene, 697 Ethylene glycol, 3, 11, 89, 93, 117, 363, 369, 407, 495, 517, 697 Ethylene glycol antimonite, 34 Ethylene glycol dimethacrylate, 351, 662 Ethylene oxide, 6, 85, 88, 173, 353, 566, 571 Ethylene propylene diene monomer, 606 Ethylene/propylene rubber, 557, 606 Ethyl formate, 264 2-Ethylhexyl acrylate, 151, 349, 351, 642 2-Ethylhexyl alcohol, 352 2-Ethylhexyl N-methacryloylcarbamate, 351, 352 Ethyl hydroxymethyl oxazoline, 557 Ethylmelamine, 300 Ethyl methacrylate, 351, 365, 366 Ethyl N-methacryloylcarbamate, 352 2-Ethyl-4-methylimidazole, 176

739

740


Ethylmethylphosphinic anhydride, 379 N-Ethylmorpholine, 99–101 Ethyltriethoxysilane, 325 Ethyltrimethoxysilane, 325 Ethynyl cyclohexanol, 334 Ferric acetylacetonate, 107, 182 9-Fluorenyl tetramethylene sulfonium hexafluoroantimonate, 187 Fluorohectorite, 163 Formaldehyde, 243, 245, 248, 310, 400, 416, 471 Formic acid, 95, 304 Fumaric acid, 3, 699 Furan, 308, 407 2,5-Furandicarboxylic acid, 308 2-Furan formaldehyde, 308 Furfural, 243, 308 Furfuraldehyde, 308 Furfuryl alcohol, 308–310, 407 2-Furfurylmethacrylate, 308 Glutamic acid, 147 Glutaric acid, 44, 90 Glutaric anhydride, 180, 205, 626 Glycerol, 3, 86, 89, 93, 363, 412, 522, 523, 559, 638 Glycerol diglycidyl ether, 141 Glycerol dimethacrylate, 688 Glycerophosphate dimethacrylate, 684 Glycerophosphoric acid, 684 Glyceryl dimethacrylate phosphate, 684 Glyceryl-2-phosphate, 684 Glyceryl triacetate, 476 Glyceryl tributyrate, 476 3-Glycidoxypropyl(methyl)dibutoxysilane, 326 3-Glycidoxypropyl(methyl)diethoxysilane, 326 3-Glycidoxypropyl(methyl)dimethoxysilane, 326 3-Glycidoxypropyltributoxysilane, 326 3-Glycidoxypropyltriethoxysilane, 326 (3-Glycidoxypropyl)trimethoxysilane, 210 3-Glycidoxypropyltrimethoxysilane, 205, 326, 391 Glycidyl acrylate, 12, 369 Glycidyl methacrylate, 12, 141, 213, 536, 537, 554, 616, 635, 648, 660, 661 Glycidyl phenyl ether, 186 Glycol trilinoleate, 638 Glyoxal, 243, 286 Guttapercha, 449

Index Hectorite, 18 3,3,4,4,5,5,5-Heptafluoro-1-pentene, 345 Heptamethyltrisiloxane, 328 Heptanoic anhydride, 639 2-Heptanone, 268 HET acid, 3, 5, 32–34 HET anhydride, 180 Hexa(allylamino)cyclotriphosphonitrile, 548 Hexabromocyclododecane, 627 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-Hexadecafluoro-decane-1,10-diol, 150 2,4-Hexadienedioic acid, 573 1,5-Hexadiyne, 493 Hexafluoroisobutene, 148 Hexafluoropropene, 366 4,4′ -Hexafluoropropylidenebisphthalic dianhydride, 342 Hexafunctional methacrylate ester, 660 Hexahydro-4-methylphthalic anhydride, 180 Hexahydrophthalic acid, 142 Hexahydrophthalic anhydride, 163, 180 Hexakis(methoxymethyl)melamine, 164 Hexakis(methylol)melamine, 263 Hexa(methoxymethyl)melamine, 263 1,1,4,4,7,7-Hexamethylcyclo-4,7-diperoxynonane, 592 Hexamethyldisilazane, 338 Hexamethyldisiloxane, 331 1,6-Hexamethylene-bis(2-furanylmethylcarbamate), 412 N,N ′ -Hexamethylenebismaleimide, 410 Hexamethylene diamine, 175, 177, 289 1,6-Hexamethylenediisocyanate, 659 Hexamethylene diisocyanate, 73, 119, 407 Hexamethylenetetramine, 262–264 Hexamethylol melamine, 300, 302 Hexamethylphosphoramide, 623 3,3,6,6,9,9-Hexamethylcyclo-1,2,4,5-tetraoxanonane, 592 1,6-Hexane bismaleimide, 398 1,6-Hexane diamine, 84 Hexane-1,6-diamine, 69 Hexane-1,6-diisocyanate, 69 Hexanediol diglycidyl ether, 150 1,6-Hexanediol dimethacrylate, 660, 662 n-Hexanol, 12 1-Hexene, 643 tert-Hexyl hydroperoxide, 671

741

742


n-Hexyl isocyanate, 82 tert-Hexylperoxybenzoate, 36 High density poly(ethylene), 542 Hydrazine, 88, 93 Hydrazine monohydrate, 121 Hydroquinone, 16, 17, 22, 481, 498, 555 Hydroquinone monomethyl ether, 672, 677 4-Hydroxyacetanilide, 159 α-Hydroxy-acetophenone, 37 3-Hydroxy-1-azabicyclo[2.2.2]octane, 99, 105 Hydroxybenzoic acid, 498 2-Hydroxybenzoquinone, 481 2-[2-Hydroxy-3,5-bis(α, α-dimethylbenzyl)phenyl]-2H-benzotriazole, 356 4-Hydroxybutyl acrylate, 91 4-Hydroxybutylvinyl ether, 428 3-Hydroxy-1,1-dimethylbutylperoxyneodecanoate, 596 2-Hydroxy-4(2,3-epoxypropoxy)benzophenone, 142 2-Hydroxyethyl acrylate, 27, 91 2-Hydroxyethyl methacrylate, 123, 206, 351, 518, 558 Hydroxyethyl methacrylate, 660 Hydroxyethyl methacrylate monophosphate, 684 (6-Hydroxy)hexyl-2-(trimethylaminonio)ethyl phosphate, 119 2-Hydroxy-4-methoxybenzophenone, 356 2-Hydroxy-4-methoxy-4′ -chlorobenzophenone, 356 2-Hydroxymethyl-4,6-dimethylphenol, 260 5-Hydroxymethylfurfural, 308, 309 2-Hydroxy-2-methyl-1-phenyl-1-propane, 485 2-Hydroxy-2-methylphenylpropane-1-one, 11, 37 2-Hydroxy-2-methyl-1-phenyl-propan-1-one, 37, 38 Hydroxy-2-methyl-1-phenyl-propanone, 193 3-Hydroxymethyl quinuclidine, 105 Hydroxynaphthoic acid, 498 2-Hydroxy-4-octoxybenzophenone, 356, 683 2-(2′ -Hydroxy-5′ -tert-octylphenyl)benzotriazole, 356 N-(4-Hydroxyphenyl)maleimide, 402, 414, 416 N-(p-Hydroxy)phenylmaleimide, 389 2-Hydroxypropyl acrylate, 20, 210, 642 1-(2-Hydroxypropyl)imidazole, 99 p-Hydroxystyrene, 151 6-Hydroxy-5-[(4-sulfophenyl)azo]-2-naphthalenesulfonic acid, 481 (4-(2-Hydroxytetradecyloxyphenyl))phenyliodoniumhexafluoroantimonate, 192 4-Hydroxy-2,2,6,6-tetramethyl-piperidin-1-oxyl, 615 4-Hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy, 602

Index 1-(3′ -(Imidazolinyl)propyl)urea, 99 4-(3-Iodopropyl)benzocyclobutene, 643 Isobutenylsuccinic anhydride, 672 Isobutyl cyanoacrylate, 473 Isobutyl vinyl ether, 190 2-Isocyanatoethyl methacrylate, 351, 363, 660, 661 Isophorone diamine, 93, 175 Isophorone diisocyanate, 73, 76, 110, 118, 120, 121, 124 Isophthalic acid, 3, 5, 35, 45, 89, 154, 697 Isophthaloyl bis-4-benzocyclobutene, 493, 496 Isophthaloyl dichloride, 516 3-Isopropenyl-α,α-dimethylbenzene isocyanate, 546 2-Isopropenyl-2-oxazoline, 573, 630 1-(Isopropenylphenyl)-1,1-dimethylmethyl isocyanate, 82, 83 Isopropoxybenzoin, 673 Isopropyl alcohol, 167, 369 Isopropyl chloride, 324 Isopropyl myristate, 476 Isosebacic acid, 44 Itaconic acid, 3, 27, 620, 629 Itaconic anhydride, 672 3-Ketocoumarin, 209 Lauric acid, 476 Laurolactam, 572 Lauroyl peroxide, 359, 362, 592 Lauryllactam, 513 Lead naphthenate, 107 Lead octoate, 107 Limonene, 142, 448, 449 Linear low density poly(ethylene), 542, 635 Linoleic acid, 638 Lithium stearate, 515 Longifolene, 449 Low density poly(ethylene), 542, 620 Lupeol, 449 Lysine-diisocyanate, 73, 120 Magnesium hydroxide, 539 Magnesium oxide, 564 Maleated poly(propylene), 541 Maleic anhydride, 3, 15, 35, 113, 494, 541, 612, 616, 637 p-Maleimidobenzoic anhydride, 424 4-(N-Maleimidophenyl)glycidyl ether, 398, 402, 404 4-Maleimidophenyl isocyanate, 407

743

744


Manganese(III)acetylacetonate, 483 Manganese octoate, 38 Melam, 299 Melamine cyanurate, 32, 111–113 Melamine phosphate, 169, 526 Melem, 299 Melon, 299 Menthane diamine, 175, 177 2-Mercaptoethanol, 82, 641 3-Mercapto-1,2-propanediol, 108 Mercaptopropyltrimethoxysilane, 343 Methacrylamide, 369 2-Methacrylamide-2-methylpropenesulfonic acid, 571 Methacrylate-terminated phosphoric acid ester, 684 Methacrylic acid, 11 Methacrylic anhydride, 84 Methacryloxyethyl phosphate, 684 4-Methacryloxyethyl trimellitate, 684 3-Methacryloxypropoxytrimethoxysilane, 681 (3-Methacryloxypropyl)trimethoxysilane, 23, 24 3-Methacryloxypropyl-trimethoxysilane, 351, 365 Methacryloyl-β-alanine, 660, 663 Methacryloyl chloride, 82, 87 Methacryloyl glutamic acid, 660 Methacryloyl isocyanate, 351, 352 Methacryloyloxyethane-1,1-diphosphonic acid, 684 2-Methacryloyloxyethyldihydrogen phosphate, 671 2-Methacryloyloxyethyl isocyanate, 351 2-(Methacryloyloxy)ethyl phosphate, 665 11-Methacryloyloxy-1,1-undecanedicarboxylic acid, 671 2-Methoxybenzyl-1,3-propanediol, 184 2-Methoxyethyl cyanoacrylate, 473 Methoxyethylmorpholine, 103 2-Methoxy-1-methylethyl cyanoacrylate, 473 m-Methoxyphenol, 243 p-Methoxyphenol, 481 1-(2-Methoxyphenyl)piperazine, 116 1-Methoxypoly(oxyethylene)benzocyclobutene, 493, 495 Methoxypropyl cyanoacrylate, 488, 489 4-Methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy, 602 Methyl acrylate, 8, 349, 351, 365, 637, 639 Methylal, 286 Methylamine, 289

Index

745

9-Methylanthracene, 385 2-Methyl-2,4-bis(2,3-epoxycyclohexyloxy)pentane, 202 Methylbutynol, 334 p-Methylcalix[6]arene, 189 Methyl cyanoacrylate, 473 4-Methylcyclohexylmethyl methacrylate, 350, 351 Methyldichlorosilane, 327 N-Methyldiethanolamine, 675 Methyldiethanolamine, 359 2-Methyl-2,5-dioxo-1-oxa-2-phospholane, 32, 33 3-Methyl-1-dodecyn-3-ol, 334 4,4′ -Methylene bis(2-chloroaniline), 93 4,4′ -Methylene bis(3-chloro-2,6-diethylaniline), 93, 158 4,4′ -Methylene bis[3-chloro-2,6-diethylaniline], 176 4,4-Methylene biscyclohexyl diisocyanate, 73 4,4′ -Methylene bis(cyclohexyl isocyanate), 111 Methylene bis(4-phenyl isocyanate), 245 α-Methylene-γ-butyrolactone, 660, 665 4,4′ -Methylenedianiline, 158, 175, 402 p,p′ -Methylene diphenyl diisocyanate, 113 4,4′ -Methylene diphenyl diisocyanate, 73 1,1′ -(Methylene di-4,1-phenylene)bismaleimide, 416, 423 N-(1-Methylethyl)-1-cyclohexyl-1-(diethylphosphono)nitroxide, 602 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, 169 1,1′ -(1-Methylethylidene)bis(4-(1-(2-furanylmethoxy)2-propanolyloxy))benzene, 410 2,2′ -[(1-Methylethylidene)bis(4,1-phenyleneoxymethylene)]bis(oxirane), 150 Methylethylketone, 593 Methylethylketone peroxide, 28, 36, 597 Methylethylketoxime, 80 (4-(1-Methylethyl)phenyl)(4-methylphenyl)iodonium tetrakis pentafluorophenylborate, 192 2-Methylfuran, 313, 433 5-Methylfurfural, 308 5-[2-(5-Methyl furylene vinylene)]furancarboxyaldehyde, 317, 318 3-Methyl-3-hydroxymethyl quinuclidine, 105 2-(5-Methyl-2-hydroxyphenyl)benzotriazole, 356 1-Methylimidazole, 101, 102, 176 Methylisobutylketone, 593 Methylisobutylketone peroxide, 593, 597 Methylisopropylketone, 593 Methyl-p-maleimidobenzoate, 424

746


Methylmelamine, 300 Methyl methacrylate, 8, 27, 351, 352, 354, 362, 365, 507, 634, 670 Methyl N-methacryloylcarbamate, 351–353 2-Methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, 193 N-Methylmorpholine, 99, 101, 479 Methyl nadic anhydride, 180 7-Methyl-1,6-octadiene, 643 2-Methylolphenol, 267 4-Methylolphenol, 267 2-Methylphenol, 259 4-Methylphenol, 259 Methylphenylchlorosilane, 325 Methylphenyldichlorosilane, 328 Methylphenyldiethoxysilane, 325 Methylphenyldimethoxysilane, 325 1-Methylpiperazine, 104 2-Methyl-2-(4-piperidyl)-1,3-propanediol, 105 2-Methyl-1,3-propanediol, 6, 14 1-Methyl-3-propyl-5-butylmelamine, 300 2-Methyl-1,3-propylene diol, 93 Methylpropylketone peroxide, 40 2-Methyl-2-(4-pyridyl)-1,3-propanediol, 105 N-Methyl-2-pyrrolidone, 412, 421 Methyl salicylate, 515 α-Methylstyrene, 8, 328, 358 3-Methyl-1,2,3,6-tetrahydrophthalic anhydride, 180 Methyltetrahydrophthalic anhydride, 163, 180 Methyltricaprylylammonium chloride, 148 Methyltrichlorosilane, 327 Methyltriethoxysilane, 325 Methyltrimethoxysilane, 325, 331 Methyltris(methylethylketoxime)silane, 331 Methylvinyldimethoxysilane, 325 Mica, 19, 109 Monomethyl itaconate, 630 Montmorillonite, 20 cis,cis-Muconic acid, 573 cis,trans-Muconic acid, 573 Myrcene, 449, 465 Myristic acid, 366 Nadic anhydride, 400, 401, 637 1,5-Naphthalene diamine, 175, 178 1,5-Naphthalene diisocyanate, 73

Index Naphthalene-1,5-diisocyanate, 69 β-Naphthol, 243 2-Naphthol, 419 1,2-Naphthoquinonediazide, 268 Naphthoquinonediazidesulfonic acid, 268 1-(α-Naphthyl)-3,3-di(2-hydroxyethyl)-triazene-1, 93 Natural rubber, 456, 461, 568 Neopentyl(diallyl)oxy tri(dioctyl)pyrophosphatotitanate, 25 Neopentyl glycol, 3, 5, 44, 90, 245, 362 Nitrokonjac glucomannan, 117 3-Nitroperoxybenzoic acid, 148 4-Nitroperoxybenzoic acid, 148 4,4′ -Nitrophenylazoaniline, 210 o-Nitrotoluene, 74 p-Nitrotoluene, 73 Nonafluorohexene, 345 Nonylphenol, 381 Norbornane diketimine, 176 5-Norbornene-2,3-dicarboxylic anhydride, 400 1-Octadecanethiol, 514 2,2,3,3,4,4,5,5-Octafluoro-hexane-1,6-diol, 150 Octamethylcyclotetraoxysilane, 540 1-Octene, 147 2-Octen-1-ylsuccinic anhydride, 671 n-Octylamine, 483 2-Octyl cyanoacrylate, 473 Octyl mercaptane, 641 (4-Octyloxyphenyl)phenyliodonium hexafluoroantimonate, 192 p-Octylphenyl salicylate, 356 Oxalic acid, 242 2-(6-Oxid-6H-dibenz[c,e][1,2]oxa-phoshorin-6-yl)-1,4-benzenediol, 141 2-(6-Oxid-6H-dibenz[c,e][1,2]oxa-phosphorin-6-yl)-1,4-benzenediol, 168 2-(6-Oxid-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)methanol, 168 4-Oxo-2,2,6,6-tetramethyl-1-piperidinyloxy, 602 2,2′ -Oxybis(N,N-dimethylethanamine), 103 4,4′ -Oxydianiline, 407 3,3′ -(Oxydi-p-phenylene)bis(2,4,5-triphenylcyclopentadienone), 410 4,4′ -Oxydiphthalic anhydride, 498 Palmitic anhydride, 639 Paraformaldehyde, 243, 245, 257, 262, 472 Pentabromobenzyl acrylate, 501 Pentaerythritol, 86, 90, 245, 291, 526, 622 Pentaerythritol triacrylate, 516

747

748


Pentaerythritol triacrylate monophosphate, 684 Pentaerythritol trimethacrylate monophosphate, 665, 684 1,2,3,4,5-Pentamethylcyclopenta-1,3-dienide, 428 N,N,N ′ ,N ′ ,N ′′ -Pentamethyldiethylene triamine, 99 3,6,6,9,9-Pentamethyl-3-(ethyl acetate)1,2,4,5-tetraoxacyclononane, 592 3,6,6,9,9-Pentamethyl-3-n-propyl-1,2,4,5-tetraoxacyclononane, 592 2,4-Pentandione, 27 Perfluoro (alkyl vinyl ether), 467 Peroxyacetic acid, 147 Perylene(peri-dinaphthalene), 384 Phellandrene, 449 Phenol, 243, 259, 295, 307, 344, 452, 640 Phenolphthalein, 412 Phenolphthalein poly(ether ether ketone), 155 N-(1-Phenylbenzyl)-((1-diethylphosphono)-1-methyl ethyl)nitroxide, 602 Phenyl-(4,4′ -bismaleimidophenyl)phosphonate, 402, 422 3-Phenyl-3-tert-butylperoxyphthalide, 592 p-Phenyl diamine, 400 N-Phenyldiethanolamine, 699 N-Phenyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide, 602 N-Phenyl-1-diethylphosphono-1-methyl ethyl nitroxide, 602 1-Phenyl-3,3-di(2-hydroxyethyl)-triazene-1, 93 1,3-Phenylene-bis(2-oxazoline), 514 2,2′ -(1,3-Phenylene)bis(2-oxazoline), 564 2,2′ -(1,4-Phenylene)bisoxazoline, 547 3,3′ -(p-Phenylene)bis(2,4,5-triphenylcyclopentadienone), 410 m-Phenylene diamine, 175, 178, 198, 272, 414 N,N ′ -m-Phenylenedimaleimide, 410, 547 N,N ′ -o-Phenylenedimaleimide, 410 N,N ′ -p-Phenylenedimaleimide, 410 4,4′ -[1,3-Phenylene(1-methyl ethylidene)]bisaniline, 498 Phenyl glycidyl ether, 142, 194 N-Phenylglycine, 193 2-Phenyl-2-imidazoline, 483 N-Phenylmaleimide, 182, 416, 428 Phenylmelamine, 300 Phenyl N-methacryloylcarbamate, 351–353 N-(1-Phenyl-2-methylpropyl)-1-diethylphosphono-1-methyl ethyl nitroxide, 602 o-Phenylphenol, 168 Phenylphosphonic acid, 186 3-Phenyl-1-propanol, 567 Phenyltrimethoxysilane, 30, 325 Phlogopite, 291

Index Phosgene, 70 Phosphorus oxychloride, 186 Phthalic anhydride, 3, 5, 14, 35, 44, 89, 103, 113, 154, 180, 556, 626, 697 Phthaloyl chloride, 390 Picric acid, 17 Pimaric acid, 449 Pimelic acid, 44 Pinane hydroperoxide, 591 α-Pinene, 449 β-Pinene, 449 Piperidine, 186 Polyamide 6, 629 Poly(benzo[1,2-d4,5-d′ ]bisthiazole-2,6-diyl)-1,4-phenylene, 495 Poly(butylene terephthalate), 214, 525, 541 Poly(ε-caprolactam), 550 Poly(ε-caprolactone), 480, 517, 518, 522, 523, 525 Poly(carbonate)dimethacrylate, 660 Poly(2,6-dichloro-1,4-phenylene oxide), 555 Poly(2,6-dimethyl-1,4-phenylene ether), 565 Poly(2,6-dimethyl-1,4-phenylene oxide), 549, 555 Polydioxanone, 488 Poly(2,2-di(4-phenylene)propane phthalate), 390 Poly(2,2-di(4-phenylene)propane phthalate-co-2,2-di(4-phenylene)propane isophthalate), 390 Poly(2,2-di(4-phenylene)propane phthalate-co-2,2-di(4-phenylene)propane terephthalate), 390 Poly(epichlorohydrin), 111, 112 Poly(ether ether ketone), 154 Polyetherimide, 417 Poly(ethylene-co-glycidyl methacrylate), 552 Poly(ethylene glycol)dimethacrylate, 351, 363, 662 Poly(ethylene glycol)methacrylate, 518 Poly(ethylene 2,6-naphthalate), 555 Poly(ethylene-octene) copolymer, 551 Poly(ethylene oxide), 153, 206, 533 Poly(ethylene terephthalate), 342 Poly(glycolic acid), 480 Polyglycolic acid, 488 Poly(p-hydroxybenzoate), 555 Poly(β-hydroxybutyrate-co-β-hydroxyvalerate), 517 Poly(3-hydroxybutyric acid), 480 Polyhydroxy fullerene, 144 3,4-Poly(isoprene), 459

749

750


Poly(lactic acid), 480 Polylactic acid, 488, 519 Poly(lactide), 521 Poly(methyl methacrylate), 153, 533 Poly(oxypropylene)diamine, 205 Polyoxypropylene glycol, 389, 390 Poly(p-phenylene benzobisthiazole), 498 Poly(phenylene ether), 214 Poly(phenylene sulfide), 559 Poly(phthaloyl diphenyl ether), 418 Poly(phthaloyl diphenyl ether-co-isophthaloyl diphenyl ether), 418 Poly(propylene), 607 Poly(propylene oxide)diamine, 176 Poly(styrene), 152, 557 Poly(styrene-co-acrylonitrile), 153 Poly(styrene-b-(ethylene-co-butylene)), 542 Polysulfone, 390 Poly(tetramethylene ether), 513 Polytetramethylene glycol, 119 Poly(thiophene), 645 Polyurethane dimethacrylate, 659, 660 Poly(vinyl acetate), 26 Poly(vinyl chloride), 98 Poly(vinyl chloride-co-vinyl acetate), 26 Poly(vinyl chloride-co-vinyl acetate-co-maleic anhydride), 26 Poly(vinylidene difluoride-co-hexafluoropropylene), 435 Poly(vinylidene fluoride), 561 Potassium hydroxide, 16 Pristine, 20 1,2-Propanediol, 44 Propanephosphonic anhydride, 379 1,3-Propanesulfone, 119 2-Propenylphenol, 418 β-Propiolactone, 119 Propionaldehyde, 243 Propionamide, 263 4-Propoxy-2,2,6,6-tetramethyl-piperidin-1-oxyl, 615 Propylamine, 289 N-Propyldiethanolamine, 675 1,2-Propylene glycol, 3, 11, 14 Propylene glycol, 89, 93, 363, 366, 697 Propylene oxide, 6, 85, 86, 88, 545, 571 1-Pyrenesulfonyl chloride, 196

Index Pyrogallol, 481 2,2′ -Pyromellitdiimidodisuccinic anhydride, 92 Pyromellitic dianhydride, 36, 166, 180, 524 Pyrophosphoric acid, 489 Quinacridone, 695 o-Quinodimethane, 495 3-Quinuclidinol, 105 Rectorite, 109 Resin-modified glass ionomer cements, 663 Resorcinol, 243 Resorcinol dicyanate, 377 Retinol, 449 Rice starch, 629 Ricinoloxazoline maleate, 634 Rosin, 450 Rutile, 365 Sago starch, 544 Salicylaldehyde, 147 Salicylic acid, 103 Sebacic acid, 3, 45 Silicon oxycarbide, 338 Sisal, 25, 553 Sodium ascorbate, 334 Sodium dodecyl sulfate, 644 Sodium dodecylsulfate, 697 Sodium hypochlorite, 148 Sodium stearate, 565 Sodium sulfoisophthalate, 697 Sorbitol, 522 Sorbitol monoethoxylate, 522 Soy flour, 121 Squalene, 449 Stannous octoate, 107, 525 Starch, 121 Strychnine, 344 Styrene, 616 Styrene butadiene rubber, 465 Styrene-ethylene/butylene-styrene triblock copolymer, 541 Styrene-b-(ethylene-co-1-butene)-b-styrene triblock copolymer, 542 Styrene oxide, 141, 142 p-Styrenesulfonic acid, 571, 647 Succinic acid, 90 Sucrose, 86, 205

751

752


Sulfanilamide, 146, 176 3-Sulfolene, 481 5-Sulfonatoisophthalic acid, 11 Sulfonic acid, 119, 324, 481, 644 2-Sulfonyl(meth)acrylate, 571 Sulfur dioxide, 472, 481, 482 Tapioca starch, 545 Tartaric acid, 120, 311 Terephthalic acid, 3, 5, 11, 35, 45, 89, 498, 555, 697 Terephthaloylbis(4-oxybenzoic) acid, 141 Terephthaloyl dichloride, 498, 516 Terpinene, 449 α-Terpineol, 201 Terpinolene, 449, 454 2,6,2′ ,6′ -Tetrabromobisphenol A, 32 Tetrabromobisphenol A, 3, 142, 168, 169, 357 Tetrabromophthalic anhydride, 3, 32, 33 α, α, α′ , α′ -Tetrabromo-o-xylene, 493 Tetra-n-butylammonium chloroacetate, 103 Tetra-n-butylammonium cyanoborohydride, 484 Tetra-n-butyl ammonium fluoride, 483 Tetrabutylphosphonium acetate, 515 Tetrabutyltitanate, 421 Tetrachloromethane, 623 Tetrachlorophthalic anhydride, 27 1-Tetradecene, 147 Tetraethoxysilane, 110, 167, 325, 345, 681 3,3′ ,5,5′ -Tetraethyl-4,4′ -diaminodiphenylmethane, 175 Tetraethylene glycol dimethacrylate, 662 N,N,N ′ ,N ′ -Tetraethylethylene diamine, 483 Tetrafluoroethylene, 467 Tetraglycidyl diaminodiphenylmethane, 524 Tetraglycidyl-4,4′ -diaminodiphenylmethane, 144, 153 Tetrahydrofuran, 85, 309, 366, 414, 666 Tetrahydrofurfuryl cyclohexene dimethacrylate, 660 Tetrahydrofurfuryl methacrylate, 660, 661 Tetrahydrophthalic anhydride, 11, 180, 697 Tetrahydrophthalimide, 309 Tetrakis(4-hydroxyphenyl)ethane, 142 1,1,2,2-Tetramethoxyethane, 286 Tetramethoxysilane, 30, 325 Tetramethylammonium pivalate, 106 N,N,N ′ ,N ′ -Tetramethyl-1,3-butane diamine, 483

Index Tetramethyldivinyldisiloxane, 331 N,N,N ′ ,N ′ -Tetramethylethylene diamine, 483 2,2,6,6-Tetramethyl-4-hydroxypiperidine-1-oxyl monophosphonate, 602 2,2,6,6-Tetramethyl-1-piperidinyloxy, 602, 603, 643 Tetramethyl-1-piperidinyloxy, 568 2,2,5,5-Tetramethyl-1-pyrrolidinyloxy, 602, 603 m-Tetramethylxylene diisocyanate, 75, 76 Tetrapropoxysilane, 325 Thermoplastic starch, 559 2,2′ -Thiobis[4-tert-butylphenol], 248 Thiourea, 299 Thiuram disulfide, 608 Tin-di-n-butyl-di-3,5-amino benzoate, 109 Tin oxide, 357 Titanium n-butoxide, 200 Titanocene, 673 Toluene diamine, 86 2,4-Toluene diisocyanate, 73, 123 2,6-Toluene diisocyanate, 73 Toluene diisocyanate, 407 p-Toluenesulfonic acid, 15, 79, 242, 311, 312, 324, 400 Tosyl isocyanate, 108 Triallyl cyanurate, 8, 10 Triallyl isocyanurate, 211, 548 2,5,8-Triamino-1,3,4,6,7,9,9b-heptaazaphenalene, 299 Triarylsulfonium hexafluoroantimonate, 208 1,5,7-Triazabicyclo[4.4.0]dec-5-ene, 483 1,3,5-Triazine-2,4,6-triamine, 299 Tributylamine, 675 Tri-n-butylborane, 668 Tributylborane, 672 Tributylphosphine, 483 1,2,4-Trichlorobenzene, 567 Tri(p-chloro phenyl)phosphine, 253 Tri(p-cresyl)phosphate, 476 Tricresyl phosphate, 357 Triethanolamine, 86, 104, 208, 284, 293, 669, 675 Triethylamine, 99, 101, 121, 253, 264, 265, 514, 544 Triethylborane, 670 Triethyl citrate, 366 Triethylene diamine, 101 Triethylene glycol diacrylate, 369 Triethylene glycol dimethacrylate, 659–661, 675

753

754


Triethylene glycol divinyl ether, 10 Triethylene glycol methylvinyl ether, 187 Triethylenetetramine, 175 Tri(2-ethylhexyl)phosphate, 476 Triethyl phosphate, 111, 112, 476 Triethyl phosphite, 623 3,6,9-Triethyl-3,6,9-trimethyl-1,4,7-triperoxonane, 594, 607 Triflic acid, 185, 322 Trifluoromethanesulfonic acid, 185 3,3,3-Trifluoropropyltrimethoxysilane, 325 Triglycidyl isocyanurate, 141 Triglycidyloxy phenyl silane, 141, 168, 169 Triisopropanolamine, 675 Trimellitic anhydride, 11 Trimercaptothioethylamine, 176 Trimercaptotriethylamine, 208 1,2,4-Trimethoxybenzene, 193 Trimethoxysilane, 391 3-(Trimethoxysilyl)propyl methacrylate, 631 Trimethylamine, 268 2,4,6-Trimethylbenzoyldiphenylphosphine oxide, 37, 39, 662 Trimethylborate, 311 Trimethylchlorosilane, 327 3,7,11-Trimethyl-1-dodecyn-3-ol, 334 Trimethylene glycol-di-p-aminobenzoate, 93 3,5,5-Trimethylhexanoyl peroxide, 592 Trimethylmelamine, 300 Trimethylolethane, 245 1,1,1-Trimethylolpropane, 76, 89, 93 Trimethylolpropane, 3, 86, 89, 245 Trimethylolpropane diallyl ether, 11 1,1,1-Trimethylolpropane dipropenyl ether, 10 Trimethylolpropane mono allyl ether, 3 1,1,1-Trimethylolpropane triacrylate, 473, 632 Trimethylolpropane triacrylate, 351, 513, 638 1,1,1-Trimethylolpropane trimethacrylate, 660 Trimethylolpropane trimethacrylate, 27 Trimethylolpropyl trimethacrylate, 662 2,4,6-Trimethylphenol, 259 4-Trimethylsiloxybenzocyclobutene, 493 Trioctyl trimellitate, 476 Trioxane, 243 Triphenylphosphine, 253

Index Triphenylphosphine oxide, 428 Triphenyl phosphite, 565 2,4,6-Triphenylpyrylium tetrafluoroborate, 485 Tripropylamine, 675 Tripropylene glycol diacrylate, 638 Tris(2-allylphenoxy)-s-triazine, 435 Tris(2-allylphenoxy)triphenoxy cyclotriphosphazene, 435 Tris(2-aminoethyl)amine, 484 Tris(2-chloroethyl)phosphate, 357 2,3-Tris(dibromopropylene)phosphate, 627 2,4,6-Tris(dimethylaminomethyl)phenol, 106, 179 1,3,5-Tris(3-dimethylaminopropyl)-s-hexahydrotriazine, 101 N,N ′ ,N ′′ -Tris(5-hydroxy-3-oxapentyl)melamine, 300 1,1,1-Tris(4-hydroxyphenyl)ethane, 210 Tris(2-hydroxyphenyl)phosphine oxide, 169 Trismercaptopropionate, 427 Tris(p-toluenesulfonato)iron(III), 697 2,4,6-Tris(2,4,6-tribromophenoxy)-1,3,5-triazine, 169 Trypsin, 647 Turpentine, 450 10-Undecene-1-ol, 644 10-Undecenyl sulfate, 644 Urethane dimethacrylate, 675 Vanadium acetylacetonate, 38 Vermiculite, 273, 291 Vernonia oil, 141 Vinyl acetate, 290, 634 Vinylbenzyldodecyldimethyl ammonium chloride, 20 Vinylbenzyloctadecyldimethyl ammonium chloride, 20 Vinyl-4-tert-butylbenzoate, 638 N-Vinyl carbazole, 190 4-Vinyl-1-cyclohexene, 328, 329 Vinylcyclohexene epoxide, 141 N-Vinylformamide, 285 Vinylidene fluoride, 366 Vinyloxazoline, 573 Vinylpyridine, 8 1-Vinyl-2-pyrrolidinone, 290 N-Vinyl pyrrolidone, 641 Vinylpyrrolidone, 52 p-Vinyltoluene, 8 Vinyltriethoxysilane, 631 Vinyltriethylsilane, 631

755

756


Vinyltrimethoxysilane, 325, 631 Vinyltrimethylsilane, 631 Wollastonite, 23, 24 Zeolite, 109 Zinc chloride, 311 Zinc 2-ethylhexanoate, 360 Zinc hexacyanocobaltate, 87 Zinc hydroxystannate, 32 Zinc naphthenate, 381 Zinc octoate, 334, 381 Zinc stearate, 25, 695 Zirconium hydroxide, 357 Zirconium oxide, 677, 678 Zirconium tetrachloride, 179 Zirconyl chloride, 681 Zirconyl dimethacrylate, 681

757

GENERAL INDEX

Index Terms

Links

Ablative properties allyl boron compounds AC-calorimeters Acceleration gelation polymerization Accelerators crosslinking crown ethers cyanoacrylates dental polymers ester-type Acetals cosolvents for UF cyclic dichlorobenzaldehyde melamine/urea/formaldehyde resins Acetylation sisal starch Acid-base reactions Acidolysis reaction poly(carbonate) polyester Acoustic ceiling tiles Acrylic resins films Adhesion amino-functionalized polysiloxane coupling agent coupling sites cyanate ester resin dental polymer graphite/bismaleimide composite hybrid resins interfacial interlaminar plasma activation self-assembling polymers sisal fibers tackifiers terpene phenol resin

419 194 168 42 38 331 478 477 669 264 286 91 206 286

477

658

553 639 697 561 555 303 365 345 391 269 391 682 435 160 523 19 648 343 553 456 463

This page has been reformatted by Knovel to provide easier navigation.

758 Index Terms

Links

to glass fibers to Kevlar fiber to nonpolar substrates to oily surfaces Aerobic degradation Aerodams Aerospace applications Aerospaceapplications Agave sisalana Agricultural applications Alder-ene reaction Alkenoylcarbamates Allophanate Amalgam replacement Amphiphilic polymers Anti-punk properties Antibodies Anticarcinogenic activity Antifoaming agents Antifouling compositions Antioxidants chewing gum grafted Antiplasticizers hyperbranched polymers Antistatic formulations Aryl cyanates hydrolysis Autocatalysis isocyanate phenol Autocatalytic curing Autocatalytic polyol Automotive applications Backbiting Backcoats Bagasse Base-catalyzed equilibration polymerization inorganic bases Batch cell method Batteries lithium Benzene yellow Beverage containers Binders abrasive friction

8 434 483 156 519 517 24 373 552 245 400 352 92 657 141 255 433 142 340 344 338 466 632 159 144 167

23

197

322

295 642

308 643

360

375 114 179 379 88 98 423 343 24 325 253 353 167 122 695 525

382

427

160 520

203

241

322

307

363

266 266


517

759 Index Terms

Links

glass fibers petroleum recovery sand Bioabsorbable polymers Biocomposites Biodegradability poly(lactide) starch Biodegradable composites compositions epoxy-polyester resins grafted polymer Lysine-diisocyanate poly(lactide) polyesters terpene resins Biofibers Biuret Blow molding Blowing chemical epoxy resins formic acid physical Blowmolding Bonding adhesive adhesives chemical covalent hydrogen interfacial interphase polyolefin substrates primers Bone cement Bookbindery Boron trifluoride complexes Bottles Bragg reflector mirrors Brake composites Bridges dental ether methylene methylene-ether phenoxy

251 266 266 488 363

266

294

316

316

521 522 161 517 205 518 73 521 52 459 161 92 524

96

94 203 95 95 517 648 475 24 648 122 22 152 485 482 49 462 175 46 504 273 672 286 248 310 261

154

254

488 161 535

639 212

52

688

179 519

251

286

312


760 Index Terms

Links

silicon groups Brittleness Brunauer Emmett Teller equation Bubbles Building materials Bumpers Cables Calcification Capacitors Capillary flow microreactor Carbon glass-like Carbon black Carbonylation propyne urethane Carboxybetaine grafting Carboxylation Caries Cashew nut shell liquid Cassava Cast elastomers Casting polymerization sand binders steel syrups Catalysis acidolysis copolyesterification enzymatic degradation isomerization latent titanium dioxide urethane zwitterions Catalysts addition-fragmentation delayed-action latent organometallic Cavitation shear banding ultrasonic curing Ceiling temperature Cement

402 1 519 543 38 47 517 341 120 164 304 271 694

155 700

255

397

466

501

273 204

356 340

462 360

365

368

116

182

204

179 176

184

311

386

510

511

617

620

631 363

313

353 70 119 272 682 25 544 69 47 358 313 294 349 561 14 488 6 107 428 103 103 190 103 107 106 152 312 482


761 Index Terms

Links

bone dental furan resins Ceramics aluminum nitride microcellular synthesis by pyrolysis Chain branching entanglements reversible termination Chain extenders diols for polyesters glycols photosensitive waterborne Chain extension BCB Diels-Alder reaction diglycidyl compound Michael addition polyaddition Chain scission shear induced UV Chain stoppers undecanol Chain transfer mercaptan Charge carrier Charge control toner Charring agents aromatic polyesters Chelates Chemoreceptors Chewing gums Chitin N-deacetylation Chlorodioxins Chlorofluorocarbons blowing Chlorosulfonation Chromatography packing materials stationary phases

52 669 313 421 338 336 608 26 567 110 31 52 92 92 93 498 407 525 407 524 534 614 365 330 3 184 82 51

166 121

156

400

514

343 641

514

587

631

122

123

182

590

498

51 112 89 38 189 465 118 292 95 434 314 189


524

762 Index Terms

Links

Chrome yellow Chromophore conjugated furans Disperse Orange 3 maleimide phenylazo-benzothiazoles Clay, see also Oganoclays Hectorite macroporous nanocomposites organo organophilic Rectorite Cloud point Co-condensation melamine and urea urea resins Co-continuous phase Coagulation Coal-tar pitches Coalescence dispersed droplets dispersed phase prevention viscosity dependence Coatings waterborne Coconut shells Coefficient diffusion extinction friction heat transfer thermal conductivity thermal expansion Cohesion Cohesion energy density Colloidal silica Colorant Colorants Coloration Coloring agents toners Comb-like polymers Combustion Comonomer assisted grafting Compatibilizers

695 317 209 427 210 162 18 271 540 162 20 109 152 302 286 27 304 315 211 538 546 211 1 203 203 270

157

195

454

458

167

389

390

417

543

3 322

48 365

86 422

120 463

180

336

666

540

687 192 50 509 509 146 352 621 688 51 694 15

199

465

375 461

400 701

694 82 30 633

87 116

695 466

484 292


763 Index Terms

Links

block copolymers epoxy resins grafted polymers UP resin Composites hydrophobic rigid rod-like wood flour Condensation base-catalyzed Calixarenes crosslinking hydrolytic intermolecular Knoevenagel resols Conducting glues Conducting paints Conductivity electric ionic thermal Consolidation restoration materials sand Controlled-release drugs fertilizers Coordination catalysts Copolymerization cationic Corona discharge Corrosion problems resistance Cotackifiers Coupling agent for compatibilization for sisal fibers silane Crack front Crack front bowing Crack propagation Crack trapping mechanism Crazing Critical solution temperature Crosslinkers Crosslinking inhibitor

157 212 615 49 397 665 498 541

53

471 189 334 322 330 472 247 167 167 207 123 19

207

377

386

421

103 51

379 203

313

378

161

343

658

205 317 304 206 182 187 482 96 12 456 328 548 25 24 158 158 158 23 358 461 472

646

501


509

764 Index Terms

Links

N,N-dimethylformamide Crown ethers accelerators polymerization retarder Crystallinity reduction Curing fluorescence response microwave ultrasonic Cycloaddition anisotropic Cyclocondensation phenols Cyclotrimerisation Cyclotrimerization Cytocompatibility polyurethane Cytotoxicity HEMA monomer spiroorthocarbonates Deep-drawing Degradation acid competitive crosslinking controlled rheology enzymatic glycolytic hydrolytic mechanism microbial photo thermal Dehydrobromination Dehydrochloration Dehydrochlorination Dehydrodecarboxylation Dendrimers Depolycondensation polyurethane Depolymerizable systems Depolymerization Devices electronic electrophotography medical optical photocopying Diacyl peroxides

623 478 187 89 196 198 312 317 208 248 379 379

122

213

543

381 427

498

676

410

495

564

382 388

120 687 663 185 559 115 53 618 366 200 118 601 459 365 21 645 148 101 410 80 116 488 45 164 693 341 209 341 36

431

116

387

204

637

144

484

459

526

167

211

645

593


765 Index Terms

Links

Diatomaceous earth DiBenedetto equation Dicarboxylic acid a,b-unsaturated Dielectric analysis a-relaxation Dielectric loss factor Diels-Alder polymerization Diels-Alder reaction o-xylylene retro Diisocyanates blocked Dimer a-methyl styrene Dimerization isocyanates Dinnerware Dip-coating Discoloration Dispersive mixing Disproportionation termination Diterpenes Donor-acceptor complex Drug release Dual initiator system Dynamic fracture toughness Elasticity improvement melt Electrochromic devices Electrochromic windows Electrodeposition Electroless plating Electrolytes gel-type photosensitive solid Electronparamagnetic resonance Electrophotography Emission suppressants End groups amino carboxyl functionalization Enzymatic synthesis Epoxidation reaction Estrogenic activity

539 424 19 195 195 410 15 495 260

428 309

400

411

313

410

433

98

669

434

79 641 106 303 270 87 518 605 448 144 366 28 22 269 517 122 207 203 215 207 317 122 646 693 11 565 214 569 84 146

300 538

207

363

694 18

255

147


643

766 Index Terms

Links

dental resins Exchanger cation Exfoliation nanoclays nanocomposites Expandable graphite Explosive polymerization Fermentation Ferroelectric film Fibers Aramid binder biodegradable carbon coupling agent cure inhibiting fused-silica glass graphite insulation jute Kevlar mineral wool natural poly(ethylene) dyeing polyester sisal strength improvement Fillers dental flame retardant natural fibers plant-based Film blowing Flame retardants Flammability Flash point Flexibility enhancers Flip-chip manufacturing Flocculation sewage treatment Flory-Huggins interaction parameter Flow improvers Fluorescence response Fluorocarbons blowing Flyash

687 481 111 163 33 190 517 362 161 310 257 521 24 161 24 96 23 385 267 25 434 317 24 646 89 552 315 19 84 378 24 21 524 31 33 336 154 206

111 521 332 294

321

678

259

331

677

111 303 600

168 378

291 501

356

265 533 52 196 95 21


767 Index Terms

Links

Foams ceramic flexible floral applications microcellular PET rigid Footwear Formaldehyde dispersions exothermic hazards free hydroxylamine titration low emission types ratio to phenol reduction resol resins scavengers Foundries furan binders Foundry sands Fragrant oil encapsulation Friedel-Crafts alkylation catalysts reaction Fries rearrangement Furan photosensitive polymer electrolyte Functionalization block copolymers macromonomer montmorillonite of nitrile rubber poly(lactide) poly(propylene) star branched polymers terminating reagents Furniture coatings Gardner scale Gel coat Gel point thermal mechanical analysis Gelling catalysts hybride resins

109 122 86 266 338 524 76 560

266

273

86

257 254 275 275 255 250 256 257 256 316 294 304 569 311 405 514

452

317 542 82 20 560 520 546 163 567 37 458 5 41 42 96 94 30

617

47 10 194

12 388

18 424


768 Index Terms

Links

inhibition preliminary reduced contraction viz. blowing Gels drug delivery system thermoreversible Glass transition temperature IPN modelling structure properties relationships Gloss polyesters Glue resins Graphite Gratings Grignard reagents synthesis Halomethylation Hantz reaction Hanza yellow Hardeners Hemp Hildebrand solubility parameter

338 16 676 101

Himalaya pine Hindered amine light stabilizers Hock process Holography Hot-melt adhesive reactive Hot-melt extrusion adhesives Household applications Hydrocarbonylation ethene Hydrogels Hydroperoxides Hydrosilylation aromatic compounds crosslinking inhibitors platinum complexes silicones Hydrosylilation Ignition point peroxides Image

449 36 140 209 459 114

322

206 435 29 153 44 19 283 694 504 321 328 272 284 695 176 23 452

460 203 353 205 590 328 328 335 333 333 199 391

644

263 455

644 462

341

360

616

643

361

368

591


769 Index Terms

Links

latent Indene resins Inhibitor coloring silicones trimerization Inhibitors anionic polymerization crosslinking grafting hydrosilylation radical polymerization Iniferter Method Initiation ultrasonic Initiator systems dual Initiators anionic atom transfer radical polymerization cationic controlled rheology dental polymers dual encapsulated functionalized iniferter method latent peroxide radical redox UV-sensitive Insertion carbene epoxide epoxy intercalation vinyl monomer Intercalation melt processing Interfacial slip Interlayers charged Intramolecular cyclization Intumescence Ionomers acrylic modified polyolefins

693 462 17 327 106 480 623 623 333 16 567

17

328

352

677

628 676 428 461 183 589 669 28 384 624 567 186 35 358 658 48 631 182 386 162 568 162 109 534

485

387

540

162 423 113

526

554

555


770 Index Terms

Links

polyurethane Iron black Isocyanates comb-like phosgene-free synthesis Isomerization allyl ether oxazoline propylene oxide unsaturated polyester Jute Karstedt catalysts Keratinization Ketone Peroxides Kinetics autocatalytic curing crosslinking curing cyclotrimerization grafting infrared spectroscopy intragallery curing isomerization monitoring peroxide decomposition photopolymerization polyesterification polymerization self-catalyzed reaction water content Knoevenagel reaction Laminates continuous fibers hyperbranched polymers printed circuit boards Lenses Lignin Low-profile additives Lubricants Macroradicals poly(propylene) Mandioca Manicure compositions Manihot Manioc Mannich bases reaction

110 694 82 70 10 386 87 6 23 328 487 36 153 117 40 389 623 423 163 6 193 599 675 14 249 16 262 472 161 145 385 208 523 21 160 516 547 544 486 544 544

25 333

259 391

257 114

166

669

618

621

26 612

624

179 264


633

771 Index Terms

Links

Marble artificial conservation Masterbatches peroxides Mechanochemistry Melt condensation Melt phase boundaries Membranes carbon dialysis drug release molecular sieves polyurethane reactive thermally stable Mercury porosimetry Mesomorphic phases Metallocene catalyst terminal unsaturation Metallocene salts Methylene blue Methylolation Michler’s ketone Microcapsules controlled-release of drugs Microcracking Microfibrils Microfiltration membranes Microgels Microvoids Microwave curing Mixer batch Brabender cavity transfer extruder Plasti-Corder static Modifiers alkenyl conductivity epoxy adhesives epoxy resins impact strength interphase liquid rubber melt strength

361 365 595 612 14 531 270 636 366 270 109 206 435 543 553 642 569 485 695 286 189 296 304 44 555 270 151 26 197 53 550 612 513 555 612

16

159

698 44

699

566

531

374 161 203 112 606 537 156 524


772 Index Terms

Links

polyterpene resins tiazone toughness unaturated polyester urea resins Modulus compression elastic flexural shear storage tensile Moisture barriers Molecular sieve Monomer reactivity ratios Monoterpenes Multi-initiator systems Multiring monomers Multivariate analysis Müller-Rochow process Nail chapping Nanocomposites clay intercalation layered silicate montmorillonite rectorite silica silicate Nanofibers Nanofillers silsesquioxane Nanoparticles layered silicate metal oxide titanium dioxide Nanotubes Natural rubber epoxidized Nematic acrylics film network Neoprene rubber Network breaking hybrid interlock

462 289 212 6 300 21 121 20 434 388 20 460 270 28 448 676 405 196 327 486 22 540 20 111 111 109 110 377 678

290

389

417

199 21

377 378

389

405 45

643 544

553

522 40

635

590

432

532

540

162

680

421 164 678 22 161 366 22

449

663 208 146 461 201 31 29

313 167


773 Index Terms

Links

interpenetrating liquid crystalline nonazeotropic composition porous reduction of crosslink density reinforcement reversible silicone unsaturated polyester Nigrosine Nitroxides cyclic Nitroxyl radicals Nonlinear optics Novolak bismaleimide-modified cyanate ester diallyl bisphenol A epoxy resin resin thermoplastic toughener for Nucleation Number acid Avogadro hydroxyl Sherwood Optical applications benzocyclobutene Optical resins Organisms aquatic Organoclays Oxamides Ozone depletion potential resistance treatment Paintability Paper release agents Paraffin wax Particle collisions Peresters Peroxides flash points half-lifes

28 429 146 40 435 374 247 201 331 1 694 568 602 209 142 421 377 400 168 241 640 466 304 16 621 89 510 141 504 207 344 18 103 95 340 503 571 341 19 538 36

29

117

164

210

435

387 247

390

551

176

20

162

461

560

455 595

600 600


388

774 Index Terms

Links

Phase transfer catalyst Phosgenation Phosgene reaction of bisphenol Phosphorylation Photo curing Photoalignment method Photochemical bromination chlorination generation of dienes reaction Photodimerization chalcone Photoinitiators cationic radical visible light Photopolymerization cationic postpolymerization radical Photoresist gratings negative positive Photostability Photostabilizers Phytotoxicity Pine resin Plasma treatment Plasticizers cyanoacrylate esters Plasticizersepoxy resins Polyamide 6 g-crystals Polyesterimides Polymerization anionic coordinative emulsion Enthalpy Entropy furfuryl alcohol living metathesis multibranching suspension

148 73 514 272 197 208 645 645 411 209 208 36 188 193 190 30 186 187 206 504 92 268 209 681 317 450 644

514 74

485

317

52

186

384

673

675 189

384

475 167 551 418 485 182 154 511 511 316 82 512 144 340

555

696


775 Index Terms

Links

Polymers heat-resistant hyperbranched self-healing telechelic Polyphosphazenes Polyurea resins Polyurethane waterborne Postcure treatment Postpolymerization photo curing Pot life Pour point depressants Powder coatings Prepregs Pressure-sensitive adhesive hot-melt Primers Printed circuit boards Printing inks Printing medium ink-jet toner Promoters adhesion amine dental polymers redox silicone synthesis visbreaking Pulse-cure method Pyrones cycloadduct Quinoline yellow Radical b-scission branching chlorination copolymerization coupling diffusion control grafting grafting kinetics induced decomposition inhibitor initiator photoinitiator

110 144 296 568 208 92 122 197

325 212

342 412

414 484

414

425

675

40 52

42

179

182

342

385

187 17 49 48 386

397

642 482 162 186

483 204 454

688 332

663

682

428

368 693 615 29 668 37 327 274 676 410 695 593 516 645 40 534 29 521 623 627 670 34 186

608

541

611

614


384

776 Index Terms

Links

polymerization kinetics scavenger stable telomerization Radical polymerization atom transfer bismaleimides chain transfer cyanoacrylate living ring opening Reactive solvents Reactors bent loop Recombination termination Reductive amination Reinforcing materials epoxides unsaturated polyesters Relaxation dipol viscoelastic Renewable resources vegetable cellulose Residence time Resistance hydrolytic impact thermal Reworkable resins Rigidity control Ring and ball method Rubber tackifier Rutherford back-scattering technique Salicylates ultraviolet absorbers Sanitary products Sawdust Scaffold Scavengers acid formaldehyde Schiff bases Schotten-Baumann reaction Schulz-Flory distribution Scission homolytic

42 643 601 82 567 412 343 485 567 665 210 509 618 92

614

568

569

246

104

161 23 195 389 307 507 613 3 22 20 201 8 458 6 646 356 47 21 119 90 251 147 663 88

508 698

513

571

90 31 50

554

571

44

89

341 24

361

462

590

558

256

601


608

777 Index Terms

Links

Sealants Self-extinguishing unsaturated polyesters Sequence length reactivity Sesquiterpenes Sewage treatment phenol/formaldehyde resins Shrinkage cationic polymerization control cyclic monomers low profile additive measurements strain Silacrown compounds preparation promoter Silaferrocenophanes Silly putty Silsequioxane resins Sisal Sizing agents Softening point Soil amendment Solubility parameters peroxides solvents and polymers Solvents aprotic Solvolysis Spin-coating Spoilers Stabilizers acid efficiency foam polyvinyl chloride storage time UV Starch acetylated amylose blend cassava corn esterification grafted

341

460

32 14 448 265 35 666 27 665 28 28 676 478 478 325 321 322 23 463 458 295

42

107

198

662

462

621 457 412 37 210 517 472 622 340 204 79 573 639 639 522 545 248 639 517

114

200

480

523 639 637

639


778 Index Terms

Links

modified rice sago tapioca thermoplastic Steam activation distillation hydrolysis treatment Stereolithography Steric hindrance Storage time inhibitors phenolic resin unsaturated polyester resin Strength bond cohesive compressive flexural peel tensile

Sulfonation Supercritical carbon dioxide Superglues Surface metallization Swelling chromatography support drug delivery system jute Syrups casting Tackifier Tackifying resin waterborne Tapes adhesive Tapioca Tempera paintings Tetraterpenes Thermal cracking Thermal transfer ribbons Thermochromic dyes

523 629 544 545 559 272 450 526 259 367 14 639

101

166

424

425

427

19 21 388 199 29 389 630

267 24 389 385 111 431

498 25

663 322

332

456 117 435

121 484

159 544

17 268 18 475 472 13 20 384 156 22 291 563 272 618 471 215 315 206 259 349 366

456

460 203 544 366 448 274 342 686

342

462


779 Index Terms

Links

Thermolabile linkages Thermolysis peroxyketals recycling Thermotropic polymers Thickeners Thixotropic additives resins Tint agents dental polymers Tissue adhesives Toners bisphenol A fumarate low fix temperature styrene-acrylic resin textile printing Top coat Tougheners dendrimers rubbers Toxicity isocyanates maleic anhydride silacrown ethers tissue adhesives Transesterification zinc acetate Trimerization Triterpenes Trommsdorff effect Ultrasonic assisted extrusion curing initiation reactor Ultraviolet absorbers Ultraviolet stabilizers Unsaturated polyesters Π-interactions waterborne Urdiol Urethane dimethacrylates Uretonimine Vector fluids Vinyl offset Vinyl staining

201 184 626 200 498 480

553

18 13 659 487 51 698 698 700 5 212 144 156 79 622 478 487 5 515 45 381 448 29 532 312 628 115 36 36 30 10 84 84 77 549 698 98

682 488

417

89 525

200 554

264

472

42

356

105


478

780 Index Terms

Links

Visbreaking Viscosity branched polymers chain stopper controlled rheology grafting hot-melt adhesives interfacial slip intrinsic liquid crystalline polymer reactive diluents thickeners vis-breaking processes Visible light sensitizer Vitrification Wagner-Jauregg reaction Wastewater treatment plants Water sorption Wetting agent Whiskers Xanthens Yucca Zwitterionic salts

274 525 668 590 618 462 534 524 554 400 480 606 190 166 425 519 431 317 161 259 544 103

573 663

195

367

678

686


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