Importance of Epoxy Resins

Epoxy resins are high-performance thermosetting resins, which display a unique combination of properties. Epoxy resins have been commercially available for almost a half-century. Epoxy resins are arguably one of the most versatile polymers with uses across an enormously wide variety of industries. The outstanding physical properties exhibited by epoxy resins include 

Furthermore, epoxy resin systems are capable of curing at either ambient or elevated temperatures, and they require only minimal pressure during the cure. Thus, epoxies can be applied and cured under many adverse conditions including outdoors. These properties provide great added value in many industries engaged in product assembly. Epoxy resins have been commercially available for almost a half-century. 

One of the major advantages of epoxy chemistry is the wide latitude it provides the for-mulator for solving technical problems. Epoxies can be designed to be flexible or rigid high or low modulus homogeneous, filled, or foamed conductive or insulative fire-retardant and resistant to heat and chemicals. 

The number of raw materials that the formulator has to work with is enormous. These include epoxy resins and modified epoxy resins of all types and forms, fillers and additives, 

Structural parts of aircraft, spacecraft, Switchgear construction and insulation 

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Hot-Curing Epoxy Resin Esters. Solid epoxy resins with a molecular mass of ca. 2000 can be esterified with semidrying and/or nondrying fatty acids. These resins are used in combination with amino resins for stoving primers, can coatings, and industrial finishes. The importance of epoxy resin esters has, however, decreased in recent years. 

The global market for epoxy resins is estimated at approximately 1.15 mil-hon metric tons (MT) for the year 2000 (8). This is an increase of 5% over 1999 demands. The North American market consumed over 330,000 MT of epoxy resins, the European market is estimated at more than 370,000 MT, and the Asian market has surpassed both the North American and European markets by consuming 400,000 MT of epoxy resins. About 50,000 MT of epoxies were consumed in the South American markets. Imports of epoxy resins from Asia into North America has steadily grown to about 120,000 MT in 2000. Epoxy resins were used with over... 

The domestic production of phenolic plastics can meet the market demand in China. The country exports over 11 000 tonnes of phenolic resins each year. However, imports of epoxy resins and unsaturated polyester resins are increasing swiftly due to insufficient supply by the Chinese producers. 

Because of the importance of epoxy resins, it is not surprising that they received early attention from researchers using the soUd-state NMR techitique [43,44]. A number of other crosslinked systems have been investigated, including acetylene-terminated polyimide resins [45,46], poly(p-phenylene) [47], 2-propenenitrile polymer with 1,3-butadiene and ethenylbenzene (ABS) resins [48], furfuryl alcohol resins [49], phenolic resins [50], acrylic resins [51,52], melamine-formaldehyde resins [53], polynuclear hydroxymethyl phenol (resol)-formaldehyde resins [54], and plasma-polymerized materials [55]. 

Specialty Epoxy Resins. In addition to bisphenol, other polyols such as aUphatic glycols and novolaks are used to produce specialty resins. Epoxy resins may also include compounds based on aUphatic, cycloaUphatic, aromatic, and heterocycHc backbones. Glycidylation of active hydrogen-containing stmctures with epichlorohydrin and epoxidation of olefins with peracetic acid remain the important commercial procedures for introducing the oxirane group into various precursors of epoxy resins. 

Important properties of epoxy resins include their ability to adhere strongly to metal surfaces, their resistance to chemicals, and their high dimensional stability. They can also withstand temperatures up to 500°C. 

Phenol is also used to manufacture several important monomers. Bisphenol A, a phenol derivative, is used to make very strong polycarbonate plastics and epoxy resins (the kind you buy in two tubes and mix to make glue). Ocher applications of epoxy resins include paints, fiberglass binder, and construction adhesives. 

Dispersion forces57 , which result from temporary variations in the distribution of electron density in atoms, can account for up to 90 per cent58 of the adhesion forces between non-polar polymers and metal substrates (bond energy 0.5-5 Kcal/ mole)50 . However, for the adhesion of epoxy resins and other polar polymers to metals, dispersion forces are of secondary importance when compared to the electromagnetic and mechanical interactions discussed above. 

This article summarizes and analyzes the results obtained for the anionic copolymerization of cyclic ethers with cyclic anhydrides. This reaction is of great practical importance, especially as curing reaction of epoxy resins and is also used for the preparation of linear polyesters with special functional pendant groups. 

The inception of epoxy adhesives occurred almost simultaneously with the commercialization of epoxy resins. This is due to the fact that all the unique handling, application, and performance properties of epoxy resins are especially important when it comes to adhesive bonding. 

Epoxy resin can have varying amounts of inorganic material remaining in the product after its synthesis. One of these is sodium chloride, which is formed by the reaction of sodium hydroxide with epichlorohydrin molecules, as described in Chap. 2. This is important in electrical applications because the sodium salt can be hydrolyzed and can degrade electrical properties. Some epoxy resin manufacturers will offer special ultrapure electrical grades or low-chlorine grades of epoxy resins for application where this is an important factor. 

Since the epoxy resin cures primarily by a ring-opening mechanism, it exhibits a smaller degree of cure shrinkage than other thermosetting resins. In these reaction processes, the epoxy group may react in one of two different ways anionically and cationically. Both are of importance in epoxy resin chemistry. In the anionic mechanism, the epoxy group may be opened in various fashions to produce an anion, as shown in Fig. 2.10. 

The polyaddition reaction is the most commonly used type of reaction for the cure of epoxy resins. The curing agents used in this type of reaction have an active hydrogen compound, and they include amines, amides, and mercaptans. With this reaction mechanism, the most important curing agents for adhesives are primary and secondary amines containing at least three active hydrogen atoms and various di- or polyfunctional carboxylic acids and their anhydrides. 

Refractive index is defined as the ratio of the speed of light of a given wavelength in vacuum to the speed of light at the same wavelength in the substance measured. The refractive index of epoxy resins is seldom reported. However, the property is important for cured epoxy adhesives that are used in the assembly of optical devices. 

We demonstrated previously that the elastic character of the rubbery domain is an important factor (4) in toughening epoxy resins. However the moduli of these liquid polymers when cured with a stoichiometric amount of epoxy resin (Epon 828) all have similar values (2.8-4.1 MPa at 300-500% elongation). 

Curing of epoxy resins by polycarboxylic acids and cyclic anhydrides is also important in applications, but it is much less understood due to more complex reaction mechanism. Also, the statistical treatment is less developed and partly requires a revision. In this section, the statistics of curing of epoxy resins with polycarboxylic acids and cyclic anhydrides is discussed. 

Hydroxyl equivalent. The hydroxyl equivalent is the weight of epoxy resin containing one equivalent of hydroxyl group. It is important in calculating average molecular weights. It is determined by various methods (Lee and Nivelle, 1967), such as esterification with acids, reaction with lithium aluminium hydride, reaction with acetyl chloride and near-infra-red spectroscopy. 

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