MICROWAVE-ENHANCED POLYMER

CHEMISTRY AND TECHNOLOGY

Dariusz Bogdal and Aleksander Prociak

MICROWAVE-ENHANCED POLYMER

CHEMISTRY AND TECHNOLOGY

MICROWAVE-ENHANCED POLYMER

CHEMISTRY AND TECHNOLOGY

Dariusz Bogdal and Aleksander Prociak

Dariusz Bogdal, PhD, Professor, Institute of Polymer Science and Technology, CracowUniversity of Technology in Kracow, Poland.

Aleksander Prociak, PhD, Asst. Professor, Institute of Polymer Science and Tech-nology, Cracow University of Technology in Kracow, Poland.

© 2007 Dariusz Bogdal and Aleksander ProciakAll rights reserved

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First edition, 2007

Library of Congress Cataloging-in-Publication Data

Bogdal, Dariusz.Microwave-enhanced polymer chemistry and technology / Dariusz (Darek)

Bogdal, Aleksander Prociak. — 1st ed.p. cm.

Includes index.ISBN-13: 978-0-8138-2537-3 (alk. paper)ISBN-10: 0-8138-2537-7 (alk. paper)1. Polymers. 2. Polymerization. 3. Microwaves—Environmental aspects. 4. Green technology. I. Prociak, Aleksander. II. Title.

QD281.P6B6 2007668.9—dc22

2006100548

The last digit is the print number: 9 8 7 6 5 4 3 2 1

DEDICATION

v

This book is dedicated to my wife Malgorzata; and to our sons Mikolaj andGrzegorz; and to their grandmother and my mother Antonina.

—Dariusz Bogda/l

This book is dedicated to my wife Ewa and to our children Tomasz andAgnieszka.

—Aleksander Prociak

TABLE OF CONTENTS

vii

Preface ix

1 Fundamentals of Microwaves 3Interaction of Microwaves with Materials 3Microwave Equipment 18

Microwave Generators 18Transmission Lines (Waveguides) 20Microwave Applicators (Cavities) 21Microwave Reactors 27Temperature Monitoring 28

Methods for Performing Reactions Under Microwave Irradiation 29

2 Overview of Polymerization Processes Under MicrowaveConditions in Comparison with Conventional Conditions 33Brief Story of the Application of Microwaves in Polymer

Chemistry 34Characterization of Dielectric Properties of Polymers 35Temperature Control 41Suspension Polymerization 42Emulsion Polymerization 45Acceleration of Polymerization Reactions Under

Microwave Conditions 50Solid State Polymerization 54Resin Transfer Molding 56Pulse Microwave Irradiation and Temperature Control 58

3 Thermoplastic Polymers 63Chain Polymerizations 63

Free Radical Polymerization 63Controlled “Living” Radical Polymerization 76Ring-Opening Polymerization 87

Step-Growth Polymerization 93Polyethers and Polyesters 93Polyamides 104Miscellaneous Polymers 110

4 Thermosetting Resins 121Epoxy Resins 121Polyurethanes 143Polyimides 152

5 Polymer Composites and Blends 173Composites of Selected Resins 173Nanocomposites 184Effects of Different Fillers 187

6 Renewable Resources for the Preparation of Polymeric Materials and Polymer Modification 199Vegetable Oils 199Cellulose 205Chitosan 211Guar Gum 215Starch 217

7 Recycling of Plastics 229Poly(vinyl chloride) 230Polystyrene 235Poly(ethylene oxide) 237Polyamide 238Polyesters and Polyurethanes 240

Poly(ethylene terephthalate) 240Polyurethanes (PUR) 245

Rubber 252Carbon Fiber Composites with Epoxy Resins 253

8 Commercialization and Scaling-Up 257

Index 271

viii Contents

PREFACE

ix

The emergence of more efficient and environmentally benign methods ofperforming chemical synthesis has created microwave-enhanced chemistry,one of the most significant events of the 1990s with tremendous potentialfor the 21st century. Recently, it was realized that such features of process-ing under microwave irradiation—short processing time, energy efficiency,high yield, low waste, and use of alternative solvents or solvent-free con-ditions—could play an important role in the development of “green chem-istry” methods in the future.

Polymer chemistry and technology, aside from ceramics processing,forms what is probably the largest single discipline in microwave technol-ogy, and the methods and procedures used are certainly among the mostdeveloped. For example, a remarkable achievement of the late 1960s wasthe application of microwave irradiation for the continuous vulcanization ofextruded rubber and the discontinuous vulcanization of molded rubber ar-ticles. The microwave-assisted vulcanization of rubber compounds used in-dustrially since the 1970s is the most important application of microwaveheating to polymeric materials in terms of number of installed plants.

Although a great deal of papers have been published on the processingand curing of polymers and polymeric materials under microwave irradia-tion, and the application of microwaves to polymer synthesis is still grow-ing. There exists only a limited number of full resources on the microwave-assisted polymer synthesis and processing; therefore, this subject was takenas the main objective of the book.

This book is intended for polymer chemists and engineers in both indus-try and academia to relate novel approaches of polymer chemistry and pro-cessing under microwave irradiation and thus spread ideas of microwavetechnologies. It is hoped that the book will also serve as an introduction tothe field for industrial chemists without prior training in microwave-assisted synthesis and processing of polymers. The book covers back-ground and scientific data, discussions of processes and product properties

in comparison with existing technology, and the status of current research.The book should also prove interesting to chemistry students whoseknowledge of the subject is more advanced.

The first chapter provides basic knowledge about the interaction of ma-terials with microwaves and explains the dielectric heating mechanism and,therefore, why some materials can be effectively heated under microwaveirradiation. Also, a short description of equipment (i.e., generators, wave-guides, and applicators) required for microwave processing is presented.

The second chapter deals with an overview of polymerization processesunder microwave conditions. The intention of this chapter is to emphasizedifferences and limitations and to provide examples of the most spectacu-lar applications of microwave irradiation in comparison to conventionalprocesses.

In the third and fourth chapters, the synthesis of thermoplastic polymersand thermosetting resins is described, respectively. Examples include mostsyntheses in addition to microwave systems that are used by different re-search groups, and the scale of the experiments will give readers a deeperfeeling about the microwave-assisted processes.

The fifth chapter describes interactions during the bonding of polymers,joining of thermoplastics (welding), thick composites, and higher perform-ance polymeric composite reinforced with carbon and glass fibers undermicrowave irradiation.

The sixth and seventh chapters present the application of microwave ir-radiation for modification of natural polymers (polysaccharides) and syn-thesis of raw materials based on vegetable oils for thermosetting resins, to-gether with degradation processes of plastic waste and chemical recyclingof polyurethanes and polyesters.

The eighth chapter discusses aspects of commercialisation. Microwavesdo not provide a universal solution to all problems but should be consid-ered whenever all other processes fail to solve an industrial problem, inwhich case the advantages of microwaves become unique and offer con-siderable savings compared to other existing processes.

During the past four decades, the overriding need for close cooperationbetween the user and the manufacturer of microwave equipment has be-come apparent. Thus, a real cross-disciplinary approach has to be consid-ered to fully understand all the limitations and advantages of microwaveprocessing. Improper application of microwave irradiation will usually leadto disappointment, whereas proper understanding and use of microwavepower can bring even greater benefits than expected.

Dariusz Bogdal and Aleksander ProciakKraków, 2006

x Preface

MICROWAVE-ENHANCED POLYMER

CHEMISTRY AND TECHNOLOGY

1

FUNDAMENTALS OF MICROWAVES

3

Microwave frequencies occupy the electromagnetic spectrum between radiofrequencies and infrared radiation with the frequencies of 300 GHz to 300MHz, which corresponds to the wavelengths of 1 mm to 1 m, respectively(Fig. 1.1). Their major applications fall into two categories, depending onwhether they are used for transmission of information (telecommunication)or transmission of energy. However, the extensive application of micro-waves in the field of telecommunication (e.g., most of the wavelengths inthe range of 1 cm to 25 cm are used for mobile phones, radar, and radarline transmissions) has caused only specially assigned frequencies to be al-located for energy transmission (i.e., for industrial, scientific, or medical ap-plications). Currently, to minimize interferences with telecommunication de-vices, these household and industrial microwave applicators are operatedonly at a few precise frequencies with narrow tolerance that are allocatedunder international regulations. For example, the most common microwaveapplicators (i.e., domestic microwave ovens) use the frequency of 2.45 GHz.This is probably why most commercially available microwave reactors de-voted for chemical use operate at the same frequency; however, some otherfrequencies are also available for heating (Thuery, 1992).

Interaction of Microwaves with Materials

When a piece material is exposed to microwave irradiation, microwavescan be reflected from its surface if it is an electrical conductor (e.g., metals,graphite, etc.), can penetrate the material without absorption in the case ofgood insulators with good dielectric properties (e.g., quartz glass, porce-lain, ceramics), and can be absorbed by the material if it is a lossy dielec-tric (i.e., a material that exhibits so-called dielectric losses, which in turn re-sults in heat generation in a quickly oscillating electromagnetic field, suchas water) (Fig. 1.2).

3