Epoxy Varnish

any of a number of varnishes based on epoxy resins (mainly bisphenol A/epichlorohydrin resins) or on modifications thereof. Ethylene glycol monoethyl ether, ketones, alcohols, and aromatic hydrocarbons serve as solvents for epoxy varnishes. The varnishes may contain curing agents (such as diamines, polyamines, polyamides, isocyanates, and synthetic resins), accelerators for the curing process (such as tertiary amines and phosphoric acids), and additives that improve the way a varnish spreads over a surface (such as organosilicon liquids).

Certain epoxy varnishes are cured with substances that react with the epoxy resin at room temperature. These varnishes are unstable upon storage and are therefore supplied as two solutions—one containing the curing agent and the other containing the rest of the components. The two solutions are mixed immediately before applying the varnish. Such epoxy varnishes are called double-pack types. Their curing agents are polyamides of low molecular weight, diamines, and polyamines; the curing process takes from four to six hours at 18°–20°C or one-and-a-half to two hours at 120°C. Epoxy-urethane varnishes are based on epoxy resins modified by alkyd resins; they have Thiokol as a plasticizer and isocyanate as a curing agent and may be cured at temperatures below 0°C.

Other epoxy varnishes are stable upon storage and contain all the necessary components in one mixture; these are known as single-pack varnishes. They are cured with phenol-formaldehyde, urea-formaldehyde, and melamine-formaldehyde resins by heating at temperatures ranging from 160°C to 200°C for 25 to 30 minutes. Epoxy esters (the products of the reaction of epoxy resins with the fatty acids from drying vegetable oils, such as linseed oil) may serve as bases for single-pack varnishes. These varnishes can be cured in 24 hours at 18°–20°C with the addition of a drier or in one hour at 150°C with the addition of melamine-formaldehyde resin.

Epoxy varnishes are applied to protected surfaces mainly by spraying (seePAINT AND VARNISH COATINGS). Epoxy varnishes and pigmented materials that are based on these varnishes (enamel paints, primers, and spackles) form coatings that have a high adhesion to metal and other surfaces, are resistant to bases and dilute acids, and can withstand ionizing radiation and temperatures of 150°–160°C. These materials are used for protecting tin cans, storage and transport tanks that contain corrosive substances, and chemical and medical equipment and instruments that are used indoors in tropical climates. (The coatings turn yellow in direct sunlight.) Materials made from compositions of epoxy resin with coal tar pitch or with divinyl acetate varnish form water-resistant coatings and are applied to underwater parts of ships.