Epoxy Resins

Chemistry and Technology, Second Edition,

Clayton May

• Publisher: CRC Press
• Year: 2018
• eBook ISBN: 9781351449953
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Materials Science

Featuring new techniques of physicochemical analysis and broader coverage of textile applications, the thoroughly rewritten and enlarged Second Edition provides hands-on assistance inthe use, formulation, synthesis, processing, and handling of epoxy resins.Epoxy Resins, Second Edition, Revised and Expanded documents available commercialproducts, including rarer species of epoxides... shows how to achieve qualityassurance through analytical methods... discusses toxicity, hazards, and safe handling...looks closely at elastomer modification of resins as well as adhesives, coatings, electrical andelectronic applications, fiber-reinforced composites, and the use of epoxy resins in thestabilization of polymers, plasticizers, and textiles... and assists in the more efficientselection and application of epoxy resins.Complete with nearly 300 pages of tables for quick references, plus over 300 diagrams andphotographs, and more than 4, 400 bibliographic references, this volume will proveindispensable to polymer, physical, and organic chemists, rheologists, materials scientists andengineers, and chemical, plastics, aerospace, automotive, and electrical and electronicsengineers.

1

Introduction to Epoxy Resins

Clayton A. May

Arroyo Research and Consulting Corp. Watsonville, California

I. INTRODUCTION

Since their conception, epoxy resins have been the subject of a multitude of patents and technical publications. Undoubtedly, there has been more written about these products per pound of sales than any of the other commercially available thermosetting resins. This immediately compels us to pose the question, why? In the chapters that follow, the answers become increasingly apparent.

This broad interest in epoxy resins originates from the extremely wide variety of chemical reactions and materials that can be used for the curing and the many different properties that result. The chemistry is unique among the thermosetting resins. In contrast to the formaldehyde resins, no volatiles are given off during the cure. This means that minimum pressures are required for the fabrication techniques normally used on these materials. The shrinkage is much less than that encountered in the vinyl polymerizations used to cure unsaturated polyester resins. This means reduced stresses in the cured product. Furthermore, a knowledge of the chemistry involved permits the user to cure over a wide range of temperatures and to control the degree of crosslinking. The latter plays an important role in the physical properties.

Considering the range of attainable properties, the versatility of epoxy resins becomes even more apparent. Depending on the chemical structure of the curing agent and the curing conditions, it is possible to obtain toughness, chemical resistance, mechanical properties ranging from extreme flexibility to high strength and hardness, high adhesive strength, good heat resistance, and high electrical resistance. Uncured, the resins have a variety of physical forms, ranging from low-viscosity liquids to tack-free solids, that, along with the curing agents, afford the fabricator a wide range of processing conditions. As a result of this versatility, these products have found use in protective coatings, adhesives for most substrates, body solders and caulking compounds, flooring, tooling compounds for molds, low- pressure molding resins, textiles, and fiber reinforced plastics. In the absence of curing agents, the epoxies are also useful as plasticizers and stabilizers for vinyl resins.

Since this book was originally published in 1973, numerous changes have occurred both in the chemistry and application of epoxy resins. Perhaps the most significant has been the intense interest in the use of these resins as the matrices for fiber reinforced plastics (FRP) or composites. The cause of this interest has stemmed primarily from two events. The first was the commercial availability of new fiber forms. Most important among these were the polyaramid or polyaromatic nylons (KEVLAR) and graphite or carbon fibers produced by the pyrolysis of rayon or polyacrylonitrile. The second important event was a developing awareness for the utility of FRP in aircraft and aerospace structures. Much of the credit for the latter belongs to the various Department of Defense (DoD) agencies and NASA. Now in use are both Air Force and Navy aircraft that contain well in excess of 1000 pounds of FRP per aircraft. The civil aircraft fields are close behind. An “all graphite/epoxy composite aircraft will shortly be available in the marketplace, and the Boeing Airplane Company is using substantial quantities of composite structure on the 757/67 series aircraft. More recently, the value of FRP to the automotive field has become increasingly apparent. Once this latter application reaches the production stage, not only epoxies, but the entire reinforced plastics and adhesive bonding industries as well, will expand at a rapid rate.

This intensified activity in the composites field has resulted in secondary rewards. Because large structures are expensive and human lives are becoming involved, product quality assurance has gained in importance. When the first edition of this book was published, composite hardware fabrication and adhesive bonding were more an art than a science. The intervening years have seen the evolution of chemical and physical test procedures that can be used to assure proper starting formulations and to verify that these materials have been properly processed. Thus the manufacture of products based on epoxy resins is changing from an engineering-oriented data sheet approach to one recognizing that these are chemical processes which can be controlled using chemical technology.

Another new area of development is the use of epoxy resins as chemical intermediates in the synthesis of other products of value in applications technology. The chapters on polymer stabilizers and textiles are cases in point. Further, although not discussed in this book, a variety of useful structural products can be manufactured from the “bis-GMA” resins. These latter materials, which are primarily the reaction products of various epoxy resins with the rapid cure technology of the unsaturated polyesters. Their main use is in conjunction with other vinyl monomers as the matrix resin for chemically resistant FRP structures. They are also the basis for a variety of commercial, fast-curing, plastic tooth filling compounds.

A growing need in recent years has been that of increased toughness. This is particularly true in the aerospace field as a composite matrix material. Simple plasticization will help to some extent, but only at the expense of valuable elevated temperature performance. The use of a second, discrete rubbery phase as a toughening agent is one approach toward this goal, which has minimal effect on elevated-temperature properties. In principle, it is of sufficient interest to form a new chapter (6) for this book. This is an area of continuing research, and, undoubtedly, other techniques of even greater significance will evolve in the years ahead.

II. HISTORY

A brief history of these products suffices because expanded reviews can be found in several earlier books on the subject.

Epoxy resins made their significant commercial debut around 1947. In the United States, the first product was made by the Devoe-Raynolds Company. It was essentially considered as a polyol for the preparation of synthetic drying oils and correspond to the approximate chemical structure:

At the end of World War II, the research vogue was synthetic drying oils made from the various fatty acids and resinous, polyfunctional alcohols. Functionality was a relatively new word, alkyd resins had become an important synthetic coating material, and the new approach was to place all of the hydroxyl functionality in a single molecule, such as shown in Structure 1.

Our laboratories (Shell Development) were busily engaged in this type of research. Shell Chemical Company became the first commercial producer of synthetic glycerin. Epichlorohydrin is an intermediate in the synthetic glycerol process and one of the key ingredients in the manufacture of epoxy resins. Thus the reaction products of epichlorohydrin with bisphenol A, when brought to our attention by Devoe-Raynolds, were of obvious interest. Through the properties of adhesion, toughness, and chemical resistance, epoxy resins of the structure shown (1) rapidly proved their value to the surface coatings industry. The use of epoxy resins as intermediates for protective coatings was the first major commercial application of this family of resins and still remains one of the most important.

Although 1947 marked the commercial debut of epoxy resins as we know them today, the true beginnings were much earlier. In 1909, the Russian chemist Prileschajew [1] discovered that olefins would react with peroxybenzoic acid to form epoxides. Peroxy acid epoxidations currently play an important role in epoxy resin production. In 1934, Schlack [2] of I. G.

Farbenindustrie AG in Germany applied for a patent on the preparation of high-molecular-weight polyamines by the reaction of amines with epoxide compounds containing more than one epoxide group. Among the various materials was a product prepared from epichlorohydrin and bisphenol A. It was disclosed that this resin could be hardened with equivalent amounts of amine. However, Schlack and/or I. G. Farben failed to recognize of the significance of the latter part of this invention.

The realization of the true value of these products came a few years later. Almost simultaneously and independently, two inventors, Pierre Castan in Switzerland and Sylvan Greenlee in the United States, recognized the value of epoxy resins as we know them today. Castan [3] was conducting research on new denture materials and in 1938 filled a patent that describes the preparation of the diglycidyl ether of bisphenol A from epichlorohydrin and bisphenol A. The Castan disclosure revealed that the resin, when cured with phthalic anhydride, had excellent adhesion to a variety of substances. It is interesting to note that Castan’s dental application never attained commercial fruition, but the bis-GMA resins mentioned earlier are the primary materials in tooth filling applications. Commercial epoxy resin production today utilizes preparative procedures similar to those of Castan. However, most of the lower-molecular-weight epoxy resins are now made by a continuous process.

The approach taken by Greenlee was different. His first patent, filled in 1943 [4], describes very similar resins made by the same process as Castan’s but higher in molecular weight. Today many of the higher-molecular-weight resins are made by condensation of the diglycidyl ether of bisphenol A and bisphenol A. Greenlee’s objective was primarily the preparation of a polyol for esterification with drying oil fatty acids to be used in surface coatings. It was this approach that led to successful commercialization in the United States.

During this same period, in the early 1940s, Daniel Swern was studying epoxidation by another route, the reaction between peroxy acids and olefins. Among his many publications is an excellent review on this subject [5]. The current commercial process, which is based on peroxyacetic acid, is used for the production of epoxidized drying oil plasticizer-stabilizers for vinyl chloride polymers and several cycloaliphatic epoxy resins. The peracid process is discussed in detail in Chapter 2, Section IV.C. Although not clearly delineated in the federal statistics, the epoxide resin plasticizer market is substantial. Sales of epoxidized oils are estimated to be well in excess of 100 million pounds annually [6].

III. COMMERCIAL GROWTH AND END-USE PATTERNS

The use of epoxy resins has grown steadily since their commercialization in the late 1940s. Data on their consumption from 1954 to date are shown in Figure 1. The early information was obtained from the U. S. Tariff Commission Reports [7] and more recently from Modern Plastics [8].

The data in Figure 1 show that the consumption of these products increased by about 20% per year until 1978. Since 1978, consumption has been fairly steady. This probably reflects the stagnation of business generally during this period. Increased consumption in the future is thus anticipated, particularly in light of recent aerospace and automotive activities. Overall, although the growth of these materials has not been spectacular, it has been quite healthy. Undoubtedly, resin prices have been a major deterrent to more rapid acceptance. For example, the liquid diglycidyl ether of bisphenol A cost one dollar per pound as late as 1953. Subsequently the price declined steadily until around 1973 (80 cents in 1954, 65 cents in 1958, 56 cents in 1965 and 41 cents in 1973). Prices have subsequently risen to a current level (1986) of around $1.25 because of inflationary pressures and raw material costs. Their major competition, particularly in the structural fields, comes from the unsaturated polyester resins and phenolics. Thus the choice of an epoxy resin is a compromise of increased performance for a higher price.

Figure 1. Consumption of bisphenol-type epoxy resins in the United States since 1954.

The end use pattern of epoxy resin consumption covers a broad range of applications as cited earlier. Figure 2 shows the annual consumption of these materials subdivided into five categories: coatings, structural applications, bonding and adhesives, exports, and miscellaneous. Since their commercialization, surface coatings have remained the largest single application and account for around 40–45% of the annual sales. The recent spurt in the growth pattern has been in the field of structural applications which includes FRP hardware. Undoubtedly, aerospace applications of these materials is the underlying reason. Adhesives, which include flooring, paving, and aggregate materials, have also shown steady, but less spectacular growth.

Figure 2. Pattern of epoxy resin consumption since 1962.

A more complete picture of epoxy resin consumption by end use is given for 1980 in Figure 3. The broad range of applications explains why these materials are discussed so widely in the technical literature. No single application exceeds 12% of the total consumption. If we compare these data with a similar analysis made 10 years ago, it is of interest to note...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Contributors
• 1. Introduction to Epoxy Resins
• 2. Synthesis and Characteristics of Epoxides
• 3. Curing” Reactions
• 4. Curing Agents and Modifiers
• 5. Elastomer-Modified Epoxy Resins
• 6. Physical and Mechanical Properties of Cured Resins
• 7. Epoxy Resin Adhesives
• 8. Epoxy Resin Coatings
• 9. Electrical and Electronic Applications
• 10. Fiber-Reinforced Epoxy Composites
• 11. Epoxides as Polymer Stabilizers and Plasticizers
• 12. Textile Finishing with Epoxides
• 13. Chemical Analysis of Epoxides and Epoxy Resins
• 14. Physical and Chemical Characterization of Epoxy Resins
• 15. Toxicity, Hazards, and Safe Handling
• Index