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Page 185 of 903

Unlike the unmodified silicone coatings, the silicone-organic copolymers, such as silicone-alkyds, -acrylics, -phenolics, -melamines, can be cured at ambient temperatures or typical baking temperatures. They are also commonly used as coil coatings. The silicone-modified alkyds and other organic copolymers are available in lowVOC formulations, typically with VOC contents of approximately 420 g/L (3.5 lb/gal). Advantages of silicone-based coatings are as follows: (1) They are generally available as single-component coatings. (2) They can be air-dried or baked, depending on formulation. (3) Depending on the formulation and pigmentation, silicone resins can withstand temperatures 260���360��C (500���600��F) up to 540���650��C (1,000���1,200��F). Although their temperature resistance is excellent, color stability will depend on the choice of pigments and their resistance to high temperatures. (4) Copolymers, such as silicone-alkyds exhibit excellent resistance to exterior weathering .(5) Nonchalking coatings are available. (6) They have excellent resistance to thermal shock and corrosion and excellent water repellency. (Clear, thin coatings are commonly used as water sealers on the exterior of buildings.) (7) They can be formulated as copolymers with organic resins to obtain the advantages of each resin. (8) They have excellent electrical insulating properties; therefore they are used in resistors, capacitors, rectifiers, etc. (9) They have good adhesion, but require excellent surface preparation of substrate. Disadvantages are as follows: (1) High-temperature-resistant coatings must usually be cured at temperatures above 260��C (500��F). This can be a problem when coating large furnaces, stacks, etc. (2) If high-temperature-resistant coatings are not fully cured before being placed in service, corrosion of the substrate may take place. (3) Silicone resin-based coatings are sensitive to surface preparation and must be applied to properly prepared substrates. (4) They exhibit generally poor hardness and poor abrasion resistance. (5) Low-VOC, high-temperature coatings are not readily available. 180

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