Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 73 of 331

Garden lawn mowers, snow blowers, tractors Plastic housings, keyboards, computer monitors, etc. Computer and business machines Medical and laboratory equipment SILICONE COATINGS Silicone resins may be considered hybrids of glass and organic resins. Because of their inertness they can be used beneficially in conjunction with many other or- ganic resins. Silicone resins with methyl and phenyl groups have been found to confer the most desirable properties; Table II lists their most important advan- tages. Most silicone resins used in the coatings industry comprise combinations of methyl and phenyl groups, and the resulting properties are dependent on the ra- tio of one to the other. High-Temperature Coatings High-temperature-resistant coatings cure only when exposed to high tempera- tures, such as 260°C (500°F), but formulations are available in which coatings can cure at lower temperatures. In addition, inorganic pigments are commonly used. Colored high-temperature coatings are available, but to retain their color, the exposure temperature should not exceed 204°C (400°F). Weatherability Although temperature resistance is one of the major benefits of silicone resins, they also exhibit excellent weatherability. This implies that they have improved color and gloss retention and exhibit less chalking. They may be incorporated into coatings, such as alkyds, phenolics, epoxies, polyesters, and other oleoresinous materials containing hydroxyl groups. Generally, the organic resins that are modified with silicone resins provide better properties than if the unmodified resins were to be exposed to the same environments. Typical Applications Unmodified silicone resin coatings are used on steel and aluminum substrates that will be subjected to high temperatures. These include such items as exhaust manifolds, mufflers, heat stacks, furnaces, boilers, ovens, heat exchangers, and aerospace and engine components. Colored silicone coatings generally withstand lower temperatures but are used on items such as space heaters, clothes driers, stoves, and similar applications. Copolymers, such as silicone-alkyds, are popular as industrial maintenance coat- ings where excellent exterior durability and chemical resistance are required. Applications include outdoor storage tanks, highway guard rails, railway tank cars, bridges, and aluminum siding. Unlike the unmodified silicone coatings, the silicone-organic copolymers, such as silicone-alkyds, -acrylics, -phenolics, -melamines, can be cured at ambient tem- peratures or typical baking temperatures. They are also commonly used as coil coatings. The silicone-modified alkyds and other organic copolymers are available in low- VOC formulations, typically with VOC contents of approximately 420 g/L (3.5 lb/gal). 72

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