Metal Finishing Guide Book

2012-2013

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Fig. 1. Reaction for typical 2-component epoxy. R��� = bisphenol A derivative; R��� = polyamide chain. Component A = epoxy resin, which is the product of reaction of epichlorhydrin and bisphenol A; component B = solution of multifunctional polyamide. have poor color retention. (2) For exterior service, they can be successfully used only as primers and must be top-coated as soon as possible after being applied on an exterior exposed surface. (3) Because of their poor exterior durability they should only be used as top coats for interior exposure. (4) Yellowing can be a problem depending on the epoxy and fatty acid from which the epoxy ester polymer was reacted. CATALYZED EPOXY Epoxy resins are the reaction products that result when epichlorohydrin is reacted with bisphenol A. For the coating to form a cured, useful film, the epoxy resin must be further reacted with yet another resin. The unique features of an epoxy resin are due to the epoxy groups in the molecule as well as reactive hydroxyl (OH) groups. A typical reaction is shown in Fig. 1. Typical Properties In general, epoxy coatings are known for their toughness, flexibility and excellent adhesion to a wide range of substrates, including most metals, plastics, wood, ceramics, masonry, and glass. It is understandable therefore that epoxies are a popular choice as primers. They are commonly used where resistance to many chemicals, solvents and alkalies, such as soaps and detergents, is required. In addition, they have excellent resistance to fresh water, salt (sea) water, and hot water. For these reasons, they are a popular choice for protecting structures, such as offshore drilling platforms, ships, and bridges, where resistance to marine environments is critical. They are also used to coat industrial and potable water tanks and pipelines. One of the most notable weaknesses of epoxy coatings is their relatively poor resistance to UV light. For instance, when exposed to sunlight, many epoxy coatings tend to chalk readily, causing them to lose gloss and color. Although chalking takes place primarily at the surface of the film, it does not significantly affect the chemical properties of the coating. When a decorative, corrosion-resistant or chemically resistant coating system is desired, such as on bridges, in chemical refineries, or on offshore drilling 173

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