Silicone Elastomers

silicones having rubberlike properties. Industrial silicone elastomers are of the polyorganosiloxane type. The macromolecules of silicone elastomers have the structure

where R and R’ are alkyl, alkenyl, or aryl and R” is hydrogen, an alkyl, or —Si(R)₃.

The main industrial silicone elastomers are thermostable dimethylmethylvinylsiloxanes (Soviet brand name, SKTV). The macromolecules of these materials contain more than 99 percent dimethylsiloxane (R = R’ = CH₃—) and up to 1 percent methylvinylsiloxane units (R = CH₃—; R’ = CH₂= CH —). Silicone elastomers of the phenylsiloxane type (SKTF, SKTFV) for low-temperature uses and the oil-resistant fluorosi-loxane type (SKTFT) are also produced. Silicone elastomers are produced by the hydrolysis of diorganodichlorosilanes (for example, dimethyldichlorosilane) and subsequent polymerization of the resulting cyclosiloxanes in the presence of catalysts, such as alkalies or sulfuric acid.

Silicone elastomers are colorless, tasteless, and odorless gellike products. Molecular weight, (3-8) × 10⁵; density, 960–980 kg/m³ (0.96-0.98 g/cm3); second-order transition temperature, about — 130°C. The SKTV elastomers are soluble in hydrocarbons, esters, and ethers but insoluble in alcohols and ketones. They are vulcanized mainly by organic peroxides (for example, by cumyl and tertiary butyl peroxides). Vulcanization by radiation is also used.

Silicone elastomers are special-purpose elastomers. Cured rubber produced from these raw elastomers is distinguished by high atmospheric and thermal stability and has the greatest frost resistance and best electrical insulation properties of all cured elastomers (phenyl-substituted silicone elastomers are the most frost-resistant). The temperature limits for use of silicone elastomers are —100° to 250°C, the specific volume electric resistance at 20° and 250°C is 10 teraohm-meters (TΩ • m) and 1 gigaohm-meter (GΩ · m), or 1 × 10¹⁵ and 1 × 10¹¹ Ω · cm), respectively. The tensile strength of silicone elastomers does not exceed 10 meganewtons per sq m (MN/m²), or 100 kilograms-force per sq cm (kgf/cm²).

The main areas of use of silicone elastomers are in the production of various electrical insulation materials. They are also used in the aircraft industry—for example, in the production of insulating layers and high-temperature air ducts. Some silicone elastomers are used in the production of sealants. The physiological inertness of silicone elastomers makes possible their use in medicine (for tubes for blood transfusions, artificial heart valves, and various prosthetic devices). The main foreign producers of silicone elastomers are Great Britain (E-301, E-360, and LS-53), France (RP-35 and Silastene), West Germany (Silopren), the USA, and Japan.