Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 69 of 331

fresh water, sea water, and hot water. (6) Some formulations are more flexible than others, depending on the choice of curing agent. (7) They can be air-dried at ambient temperatures within 3 to 5 hours and force-dried at 150°F within 30 min- utes. (8) They are primarily used in military, marine, offshore, and chemical plant applications. (9) Some high-build formulations allow for thick films in excess of 5 mils in one application. Disadvantages are the following: (1) Usually they are two-component systems comprising component A (clear or colored epoxy resin) and component B (cur- ing agent); therefore, they must be accurately mixed. (2) Any unused, mixed coating must be disposed of as hazardous waste. (3) They offer poor resistance when exposed directly to UV light (sunlight). (4) High-solids materials are difficult to apply to achieve dry films less than 1.5 mils, particularly when coating com- plex shapes. (5) They are generally not available in small quantities of custom col- ors. (6) Some formulations require an induction period of 20–30 minutes after the two components have been mixed before coating can be applied. (7) Pot-life limitations of 4 to 6 hours or less at ambient temperatures are common. (8) Ap- plication equipment must be cleaned before coating starts to set. (9) They are sen- sitive to cleanliness of the substrate. (10) It is difficult to strip coating from damaged, coated parts. (11) Some formulations, particularly those based on the more chemically resistant polyamine resins, can cause severe dermatitis and oth- er health effects. They must be used with caution. CATALYZED POLYURETHANES Polyurethanes are a type of coating formed by the reaction of a polyisocyanate with a polymer that contains hydroxyl functionality. Two-component polyurethanes are supplied in two separate containers, of which the first is usu- ally labeled component A and the second component B. Component A can either be clear or pigmented, offering a wide range of colors and gloss levels. The primary resin (polyol) is usually an acrylic, polyester or polyether, each of which contains more than one hydroxyl group. The second container, component B, contains a multifunctional, prepoly- merized isocyanate. When components A and B are mixed according to the man- ufacturers' prescribed ratios, the polymers react to form a highly cross-linked polyurethane. Figure 2 shows the simplified chemistry of the two components; Eqs. (5) and (6) depict the results of mixing the two components: Typically, polyfunctional polyisocyanates used in two-component polyurethanes are homopolymers or copolymers of toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), dicy- clomethane diisocyanate (HMDI). Components A and B can be batch mixed by manually mixing immediately be- fore the coating is applied. Alternatively, special proportioning equipment can 68

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