Epoxy polymers are cured to form thermoset resins by either homopolymerization of epoxy groups with themselves, or reaction with curing agents such as anhydrides, amines, and novolacs. Because the curing agent contributes significantly to the cured properties of the resin, this is called an addition reaction. Shrinkage during polymerization of epoxy resins is extremely low.
The most widely used epoxy resins are based on the reaction of epichlorohydrin with bisphenol-A. The reaction ratios of these two constituents can be varied to produce products ranging from low-viscosity liquids to high-molecular-weight solids.
The novolacs are another important class of epoxy resins, particularly the ECNs (epoxy cresol novolacs) and EPNs (epoxy phenol novolacs). These polyfunctional resins offer higher thermal properties and improved chemical resistance over bis-A derivatives. The cycloaliphatics comprise a third group of epoxy resins that are particularly important for applications requiring resistance to arc tracking and weathering. A fourth class of epoxy resins is based on the epoxidation of aromatic amines. These resins have good mechanical properties and high thermal capabilities and fatigue resistance.
Properties: Epoxies have excellent electrical, thermal, and chemical resistance. Their strength can be further increased with fibrous reinforcement or mineral fillers. The variety of combinations of epoxy resins and reinforcements provides a wide latitude in properties obtainable in molded parts.
Molded epoxy parts are hard, rigid, relatively brittle, and have excellent dimensional stability over a broad temperature range. Some fiber-reinforced formulations can withstand service temperatures above 500 °F for brief periods. Their excellent electrical properties, in combination with high mechanical strength, qualify them for electrostructural applications. Resins based on bisphenol-A are adequate for most services. However, cycloaliphatics are recommended for parts subjected to arcing conditions or those requiring outdoor weatherability.
Excellent adhesion in structural applications is another outstanding property of epoxy systems. Epoxy adhesives for bonding many dissimilar materials can be supplied either as one or two-part systems. One-part systems require heat for curing; two-part systems usually cure at room temperature, but properties are improved when the materials are heat cured. Some epoxy adhesive systems can withstand temperatures to 450 °F, although properties at such temperatures are considerably lower than at room temperature.
Applications: Epoxy resin systems are used with various reinforcements: glass, graphite and aramid fibers, asbestos, cotton, and metal foils. Epoxy laminates are important because of their excellent electrical properties over a wide temperature range, as well as their dimensional stability and chemical resistance. The amine-based resins are used in conjunction with graphite fiber to make structural composites for commercial and military aircraft and components for space equipment. These composites offer significant advantages over metals in the areas of weight savings and corrosion resistance. Printed-circuit boards, consisting of a glass-fabric-reinforced flame-retardant epoxy resin are another major use.
Filled and unfilled liquid systems are used for potting and encapsulating electrical/electronic components ranging from semiconductor devices and miniature coils and switches to large motors and generators. The epoxy resins provide excellent adhesion, and low shrinkage (volume shrinkage is only 2 to 3%) needed for such applications. The compounds cure in 1 to 4 hr, and they do not outgas during the cure or in service. The casting cycle can be significantly accelerated by using one of the newer processes: liquid injection molding or pressure gelation. The latter method can reduce the molding cycle of a 15-lb casting from 4 hr to 10 min. Applications being evaluated for pressure-gelation casting include high-voltage electrical components and pump and valve housings.
Another important use for epoxy resins is in coatings, both as liquids and powders. These finishes have excellent flexibility, impact and abrasion resistance, and adhesion to most substrates. They can be formulated to resist many industrial chemicals and other corrosive materials. Epoxy coatings are typically used as automotive and appliance primers, in industrial maintenance paints, marine finishes, pipe coatings, and decorative topcoats for various products. Epoxy resins are also furnished as molding powders, usually incorporating glass or mineral-fiber reinforcement. These compounds can be molded by conventional thermoset methods. The outstanding electrical and thermal properties qualify the epoxies for industrial switchgear applications and for encapsulating semiconductor devices.