Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 67 of 331

Fig. 1. Reaction for typical 2-component epoxy. R' = bisphenol A derivative; R" = polyamide chain. Com- ponent A = epoxy resin, which is the product of reaction of epichlorhydrin and bisphenol A; compo- nent B = solution of multifunctional polyamide. ter 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 mol- ecule 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, ce- ramics, 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 al- kalies, such as soaps and detergents, is required. In addition, they have excel- lent 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 criti- cal. 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 re- sistance to UV light. For instance, when exposed to sunlight, many epoxy coat- ings tend to chalk readily, causing them to lose gloss and color. Although chalk- ing 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 sys- tem is desired, such as on bridges, in chemical refineries, or on offshore drilling equipment, it is customary to use epoxy coatings as the primer and undercoat and then apply a more UV-resistant top coat, such as an acrylic or polyurethane. Epoxies should not be applied at low ambient temperatures, usually less than 50–60°F (10–15°C), because they will not cure properly. The common air- or force-dried, two-component epoxies that are used in the general metals, plas- 66

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