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EPOXY RESINS Introduction
Epoxy resins are an important class of polymeric materials, characterized by the presence of more than one three-membered ring known as the epoxy, epoxide, oxirane, or ethoxyline group.
The word “epoxy” is derived from the Greek preﬁx “ep,” which means over and between, and “oxy,” the combining form of oxygen (1). By strict deﬁnition, epoxy resins refer only to uncross-linked monomers or oligomers containing epoxy groups. However, in practice, the term epoxy resins is loosely used to include cured epoxy systems. It should be noted that very high molecular weight epoxy resins and cured epoxy resins contain very little or no epoxide groups. The vast majority of industrially important epoxy resins are bi- or multifunctional epoxides. The monofunctional epoxides are primarily used as reactive diluents, viscosity modiﬁers, or adhesion promoters, but they are included here because of their relevance in the ﬁeld of epoxy polymers.
Epoxies are one of the most versatile classes of polymers with diverse applications such as metal can coatings, automotive primer, printed circuit boards, semiconductor encapsulants, adhesives, and aerospace composites. Most cured epoxy resins provide amorphous thermosets with excellent mechanical strength and toughness; outstanding chemical, moisture, and corrosion resistance; good thermal, adhesive, and electrical properties; no volatiles emission and low shrinkage upon cure; and dimensional stability—a unique combination of properties generally not found in any other plastic material. These superior performance characteristics, coupled with outstanding formulating versatility and reasonable costs, have gained epoxy resins wide acceptance as materials of choice for a multitude of bonding, structural, and protective coatings applications.
Commercial epoxy resins contain aliphatic, cycloaliphatic, or aromatic backbones and are available in a wide range of molecular weights from several hundreds to tens of thousands. The most widely used epoxies are the glycidyl ether derivatives of bisphenol A (>75% of resin sales volume). The capability of the highly strained epoxy ring to react with a wide variety of curing agents under diverse conditions and temperatures imparts additional versatility to the epoxies. The major industrial utility of epoxy resins is in thermosetting applications. Treatment with curing agents gives insoluble and intractable thermoset polymers. In order to facilitate processing and to modify cured resin properties, other constituents may be included in the compositions: ﬁllers, solvents, diluents, plasticizers, catalysts, accelerators, and tougheners.
Epoxy resins were ﬁrst offered commercially in the late 1940s and are now used in a number of industries, often in demanding applications where their performance attributes are needed and their modestly high prices are justiﬁed. However, aromatic epoxies ﬁnd limited uses in exterior applications because of their
Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
Vol. 9 EPOXY RESINS 679 poor ultraviolet (UV) light resistance. Highly cross-linked epoxy thermosets sometimes suffer from brittleness and are often modiﬁed with tougheners for improved impact resistance.
The largest use of epoxy resins is in protective coatings (>50%), with the remainder being in structural applications such as printed circuit board (PCB) laminates, semiconductor encapsulants, and structural composites; tooling, molding, and casting; ﬂooring; and adhesives. New, growing applications include lithographic inks and photoresists for the electronics industry.
The patent literature indicates that the synthesis of epoxy compounds was discovered as early as the late 1890s (2). In 1934, Schlack of I.G. Farbenindustrie AG in Germany ﬁled a patent application for the preparation of reaction products of amines with epoxies, including one epoxy based on bisphenol A and epichlorohydrin (3). However, the commercial possibilities for epoxy resins were only recognized a few years later, simultaneously and independently, by the DeTrey Freres Co. in Switzerland (4) and by the DeVoe and Raynolds Co. (5) in the United States.
In 1936, Pierre Castan of DeTrey Freres Co. produced a low melting epoxy resin from bisphenol A and epichlorohydrin that gave a thermoset composition with phthalic anhydride. Application of the hardened composition was foreseen in dental products, but initial attempts to market the resin were unsuccessful. The patents were licensed to Ciba AG of Basel, Switzerland, and in 1946 the ﬁrst epoxy adhesive was shown at the Swiss Industries Fair, and samples of casting resin were offered to the electrical industry.
Immediately after World War I, Sylvan Greenlee of DeVoe and Raynolds
Co. patented a series of high molecular weight (MW) epoxy resin compositions for coating applications. These resins were based on the reaction of bisphenol A and epichlorohydrin, and were marketed through the subsidiary Jones-Dabney Co. as polyhydroxy ethers used for esteriﬁcation with drying oil fatty acids to produce alkyd-type epoxy ester coatings. Protective surface coatings were the ﬁrst major commercial application of epoxy resins, and they remain a major outlet for epoxy resin consumption today. Concurrently, epoxidation of polyoleﬁns with peroxy acids was studied by Daniel Swern as an alternative route to epoxy resins (6). Meanwhile, Ciba AG, under license from DeTrey Freres, further developed epoxy resins for casting, laminating, and adhesive applications, and the Ciba Products Co. was established in the United States.
In the late 1940s, two U.S. companies, Shell Chemical Co. and Union Carbide
Corp. (then Bakelite Co.), began research on bisphenol A based epoxy resins. At that time, Shell was the only supplier of epichlorohydrin, and Bakelite was a leading supplier of phenolic resins and bisphenol A. In 1955, the four U.S. epoxy resin manufacturers entered into a cross-licensing agreement. Subsequently, The DowChemicalCo.andReichholdChemicals,Inc.joinedthepatentpoolandbegan manufacturing epoxy resins.
In the 1960s, a number of multifunctional epoxy resins were developed for higher temperature applications. Ciba Products Co. manufactured and marketed o-cresol epoxy novolac resins, which had been developed by Koppers Co.
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Dow developed the phenol novolac epoxy resins, Shell introduced polyglycidyl ethers of tetrafunctional phenols, and Union Carbide developed a triglycidyl paminophenol resin. These products continue to ﬁnd uses today in highly demanding applications such as semiconductor encapsulants and aerospace composites where their performance justiﬁes their higher costs relative to bisphenol A based epoxies.
Carbide in the United States and by Ciba AG in Europe for cycloaliphatic structures. Ciba Products marketed cycloaliphatic epoxy resins in 1963 and licensed several multifunctional resins from Union Carbide in 1965. The ensuing years witnessed the development of general-purpose epoxy resins with improved weathering characteristics based on the ﬁve-membered hydantoin ring and also on hydrogenated bisphenol A, but their commercial success has been limited because of their higher costs. Flame-retardant epoxy resins based on tetrabromobisphenol A were developed and commercialized by Dow Chemical for electrical laminate and composite applications in the late 1960s.
In the 1970s, the development of two breakthrough waterborne coating technologies based on epoxy resins helped establish the dominant position of epoxies in these markets: PPG’s cathodic electrodeposition automotive primer and ICIGlidden’s epoxy acrylic interior can coatings.
While epoxy resins are known for excellent chemical resistance properties, the development and commercialization of epoxy vinyl ester resins in the 1970s by Shell and Dow offered enhanced resistance properties for hard-to-hold, corrosive chemicals such as acids, bases, and organic solvents. In conjunction with the development of the structural composites industry, epoxy vinyl ester resin composites found applications in demanding environments such as tanks, pipes and ancillary equipment for petrochemical plants and oil reﬁneries, automotive valve covers, and oil pans. More recently, epoxy and vinyl esters are used in the construction of windmill blades for wind energy farms. Increasing requirements in the composite industries for aerospace and defense applications in the 1980s led to the development of new, high performance multifunctional epoxy resins based on complex amine and phenolic structures. Examples of those products are the trisphenol epoxy novolacs developed by Dow Chemical and now marketed by Huntsman (formerly Ciba).
The development of the electronics and computer industries in the 1980s demanded higher performance epoxy resins. Faster speeds and more densely packed semiconductors required epoxy encapsulants with higher thermal stability, better moisture resistance, and higher device reliability. Signiﬁcant advancees in the manufacturing processes of epoxy resins led to the development of electronicgrade materials with lower ionic and chloride impurities and improved electrical properties. Dow Chemical introduced a number of new, high performance products such as hydrocarbon epoxy novolacs based on dicyclopentadiene. The 1980s also witnessed the development of the Japanese epoxy resin industry with focus on specialty, high performing and high purity resins for the electronics industry. These include the commercialization of crystalline resins such as biphenol diglycidyl ether.
More recently, in order to comply with more stringent environmental regulations, there has been increased attention to the development of epoxy resins
Vol. 9 EPOXY RESINS 681 for high solids, powder, and waterborne and radiation-curable coatings. Powder coatings based on epoxy–polyester and epoxy–acrylate hybrids have continued to grow in the global markets, including new applications such as primer-surfacer and topcoats for automotive coatings. Radiation-curable epoxy–acrylates and cycloaliphatic epoxies showed tremendous growth in the 1990s in radiation-curable applications. These include important and new uses of epoxy resins such as the photoresists and lithographic inks for the electronics industry. Waterborne epoxy coatings are projected to grow substantially.
Thecontinuingtrendofdeviceminiaturizationinthecomputerindustry,and the explosive growth of portable electronics and communications devices such as wireless cellular telephones in the 1990s demanded new, high performance resins for the PCB market. This has led to the development of new epoxies and epoxy hybridsystemshavinglowerdielectricconstants(Dk),higherglass-transitiontem- peratures (Tg), and higher thermal decomposition temperatures (Td) for electrical laminates. Environmental pressures in the PCB industry have fueled the develop- ment of a number of new bromine-free resin systems, but their commercialization is limited because of higher costs.
Signiﬁcant efforts have been directed toward performance enhancements of epoxy structural composites. Advances have been made in the epoxy-toughening area. Epoxy nanocomposites and nanotube systems have been studied and are claimedtobringexceptionalthermal,chemical,andmechanicalpropertyimprovements. However, commercialization has not yet materialized.
In 1999, Dow Chemical introduced a new epoxy-based thermoplastic resin, BLOX∗, for gas barrier, adhesives, and coatings applications.
FromtheﬁrstcommercialintroductionofdiglycidyletherofbisphenolA(DGEBA) resins in the 1940s, epoxy resins have gradually established their position as an important class of industrial polymers. Epoxy resin sales increased rapidly in the 1970s and continued to rise into the 1980s as new applications were developed (annual growth rate >10% in the U.S. market, Table 1). More recently, the slower growth rates (3–4%) of the U.S., Japanese, and European markets in the 1990s were made up for by the higher growth rate (5–10%) in the Asia-Paciﬁc markets outsideofJapan,particularlyinTaiwanandChina.Epoxyresingrowthhashistorically tracked well with economic developments and demands for durable goods, and so the growth of the epoxy markets in Asia-Paciﬁc is expected to continue into the next decade.
The global market for epoxy resins is estimated at approximately 1.15 million metric tons (MT) for the year 2000 (8). This is an increase of 5% over 1999 demands.TheNorthAmericanmarketconsumedover330,000MTofepoxyresins, the European market is estimated at more than 370,0 MT, and the Asian market has surpassed both the North American and European markets by consuming 400,0 MT of epoxy resins. About 50,0 MT of epoxies were consumed in the South American markets. Imports of epoxy resins from Asia into North America hassteadilygrowntoabout120,000MTin2000.Epoxyresinswereusedwithover
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Table 1. History of U.S. Epoxy Resin Annual Productiona
Year Production, 103 MT
aData from U.S. International Trade Commission, Synthetic Organic Chemicals. Data include modiﬁed and unmodiﬁed epoxy resins. Modiﬁed epoxy resins include solid epoxy resin (SER), vinyl ester resins, epoxy acrylates, etc. There appear to be some discrepancies in epoxy resin production and market data as reported by different publications and organizations (7). This is primarily due to the fact that some epoxy resinssuchasliquidDGEBAresinsandepoxynovolacsareusedasraw materials to produce modiﬁed or advanced epoxy resins, which may be further converted to end-use products. Some publications report only unmodiﬁed epoxies.
400,0 MT of curing agents to produce an estimated 3 million MT of formulated compounds, worth over $20 billion.
Up until the mid-1990s, the major worldwide producers of epoxy resins were
Dow Chemical, Shell, and Ciba-Geigy. However, both Shell and Ciba-Geigy have recently divested their epoxy resins businesses. Shell sold their epoxy business to Apollo Management LP (based in New York City) in the year 2000 and the company was renamed Resolution Performance Products. Similarly, Ciba’s epoxy business was sold in 2000 to Morgan Grenfell, a London (U.K.)-based private equity ﬁrm, and the new company name was Vantico. More recently, in June 2003, the Vantico group of companies joined Huntsman. The Vantico business units are now named Huntsman Advanced Materials. The cycloaliphatic epoxy business of Union Carbide became part of The Dow Chemical Company after their merger in the year 2001. Together, these three producers continue to dominate the world market for epoxy resins, accounting for almost 65% of the global market. However, this is a reduction from over 70% of market shares owned by the three largest producers in the 1980s. Smaller producers of epoxy resins for the NorthAmericanmarketsareReichhold(ownedbyDainipponInkandChemicals), CVC Specialty Chemicals, Paciﬁc Epoxy Polymers, and InChem (phenoxy thermoplastic resins). Suppliers of epoxy derivatives include Ashland Specialty Chemical, UCB Chemicals (Radcure), AOC LLC, Eastman Chemical, and Interplastic Corp.
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