Heat-cured silicone

The compression set of silicone mbber is similar to organic types of mbber at low (0—50°C) temperatures, ranging from 5 to 15% (380). Above 50°C, silicone mbber is superior, but compression set increases with time and temperature. Silicone mbber is more tear-sensitive than butyl mbber, and the degree of sensitivity is a function of filler size and dispersion, cross-link density, and curing conditions. The electrical properties of silicone mbber are generally superior to organic mbbers and are retained over a temperature range from —50 to 250°C (51). Typical electrical values for a heat-cured silicone mbber are shown in Table 9. 

Silicone elastomers are either room-temperature vulcanization (RTV) or heat-cured silicone rubbers, depending on whether cross-linking is accomplished at ambient or elevated temperature. [The term vulcanization (see Chapter 1 and Chapter 2) is a synonym for cross-linking. While curing is also a synonym for cross-linking, it often refers to a combination of additional polymerization plus cross-linking.] RTV and heat-cured silicone rubbers typically involve polysiloxanes with degrees of polymerizations of 200-1500 and 2500-11,000, respectively. 

Heat-cured silicone 3-7% dimethyl methylhydrogen siloxane with methylsilsequioxane, 1-6% glycidoxypropyl trimethylsiloxy silane Dow Corning EA-6052 (Dow Corning)... 

The electrical properties, especially for the high-purity semiconductor-grade silicones, are excellent even at temperature extremes of —80 °C and 200 °C and in high humidity. Silicone adhesives are available as electrically conductive and as electrically insulative types. Typical electrical properties for moisture- and heat-cured silicone adhesives are given in Table 3.12. 

Heat cured silicone Low and high viscosity chloroplatanic or organo platinum catalyst (1-5 ppm), vinyl resin blend (Polymer VS-50, 100, 90,000 Anders) Dow Coming 3-6605, and 3-6265... 

TABLE 3.39 Properties of Heat-Cured Silicone Rubbers... 

Heat-cured silicone rubber is commercially available in several forms as gum stock, reinforced gum, or partially filled gum. These compoimds can be im-catalyzed, catalyzed, or can contain catalyst dispersions. Catalyzed compounds are ready for use without additional processing. Silicone rubber is often freshened, ie, the compound is freshly worked on a rubber mill until it is a smooth continuous sheet. The freshening process eliminates the structuring problems associated with pol5uner-filler interactions. Rubber can be extruded on wire and cured in place (479). 

Electromagnetic/radio-frequency interference shielding materials have to meet much lower demands in terms of overall electrical conductivity (typically 4-5 orders of magnitude lower than a silver-flake-filled adhesive). This means that cheaper conductive fillers can be employed, for example, silver-coated copper flake, nickel flake, and carbon black. Typically the adhesive has to form a compliant joint between two mating surfaces, and hence room temperature vulcanizing or heat-cure silicone is often a convenient choice of matrix material. 

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