Silicone Varnishes

varnishes based on silicones— polyorganosiloxanes (mainly polymethylphenylsiloxanes). Aromatic hydrocarbons and their mixtures with ethers, alcohols, and ketones are used as solvents in silicone varnishes. Organic film-forming materials, such as alkyd or epoxy resins, are introduced into some silicone varnishes to lower the drying temperature, as well as to improve the mechanical properties and adhesion to the substrate and to increase the oil and gasoline resistance of the materials. Inorganic pigments such as titanium dioxide, iron oxide, and aluminum powder are usually used in producing enamels based on silicone varnishes. Chalk, talc, or ground mica is usually used as a filler. The solids content of silicone varnishes may be 30–70 percent, and the drying time may vary from 24 hr at room temperature to 0.5-2.0 hr at 150°-200°C. Silicone varnish films usually have better service characteristics when dried at high temperatures. The thickness of the coatings, which are most frequently applied by a paint sprayer, is 45–55 microns. The varnish films are capable of prolonged service at 180°-200°C and of short-term service (500-1,000 hr) at 250°-300°C; films of some enamels may be used at 400°-500°C. The lower temperature limit for the use of such coating is —50° to — 60°C. The electric strength of the films at ordinary temperatures is 50–120 kilovolts per mm, and the volume electrical resistance is 1–10² teraohm-meters (Tohm . m), or 10¹⁴-10¹⁶ ohm • cm. The coatings are resistant to atmospheric effects, including tropical moisture and mold fungi.

Silicone varnishes and enamels are mainly used as insulating materials for electrical equipment in high-temperature service, as well as for the protection of various devices and structures from the effects of high temperatures, solar radiation, and other environmental influences.