Metal Finishing Guide Book


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the most popular systems recommended for general-purpose topcoats. When high-performance properties are required, such as resistance to strong chemicals or solvents or color and gloss retention when the coating is exposed to sunlight over the long term, other resin systems are usually more appropriate. Alkyds and modified alkyds are also among the least expensive of the VOC-compliant coating systems. BAKED ALKYDS Baked alkyds include melamine formaldehyde, urea formaldehyde or phenolic modifications as well as polyester, oil-free and acrylic coatings. The primary difference between air- and force-dried alkyds and baked alkyds is that they do not dry at ambient (room) temperatures but must be cured at elevated temperatures, usually in the range110���176��C (230���350��F) for 45���10 minutes, respectively. Although the coatings may feel touch dry after air-drying, they can only achieve their optimum chemical and physical resistance properties after they have been fully cured at their specified baking schedules. Non-air-dried (baked) alkyds are cross-linked with stabilized ���aminoplast��� resins because the cross-linking is initiated when the high temperatures are attained. The most frequently used aminoplast resins are urea formaldehyde and melamine formaldehyde. In the white baking enamels that are used for metal shelving, metal furniPolytester Polyhydric alcohol + Coconut oil = Alkyd (3) + Polybasic acid Alkyd + Urea formaldehyde or = Modified alkyd melamine formaldehyde (4) ture, computer cabinets, etc., urea formaldehyde provides excellent initial color, color retention and resistance to heat, soap,water and fatty acids. Alkyd baking enamels based on melamine formaldehyde are harder, more chemically resistant and faster drying. They are used to coat refrigerators, washing machines, highquality fluorescent light fixtures, and automotive components. A typical sequence of reactions for these types of coatings is shown in Eqs. (3) and (4): These coatings are available at 360 g/L (3.0 lb/gal), as required in most states. Some formulations are as low as 275 g/L (2.3 lb/gal). The advantages of the baked alkyds are the following: (1) They are available at VOC levels [275 g/L (2.3 lb/gal)] to meet most regulations and offer excellent high performance properties. (2) They are single-component coatings, available in wide range of colors and gloss levels, and can be applied directly to metal substrates. (3) They are excellent for appliances, such as washing machines, driers, dishwashers, refrigerators, metal shelving and cabinets and lighting fixtures. (4) With proper controls, they can achieve uniform, thin film thick171

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