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Material hierarchy: Polymer - Thermoset
These polymers are based on silicon rather than carbon and are partly inorganic. They have unparalleled long-term heat resistance, excellent electrical insulating properties (stable under both frequency and temperature changes), and good moisture and chemical resistance. Silicone polymers come in liquid, elastomeric, and rigid forms, depending on their molecular weight and degree of crosslinking. In this discussion, only the elastomeric and rigid forms are considered.
Elastomeric silicones usually are based on dimethylsiloxane and come in one- or two-part systems. Room temperature vulcanizing (RTV) silicone rubbers are used to make molds for casting various plastics and as adhesives and sealants for glass and metal structures. Two-component systems are used for potting electronic circuits and can be processed by liquid injection molding (LIM) into flexible parts such as O-rings and other seals. Ultraviolet light (UV)-curing silicones are used to coat printed circuit boards.
The much less familiar rigid silicones are polysiloxane resins with thermal stability well over 500°F. One growing application area is the hard-coating of softer plastics such as acrylics and polycarbonates to provide scratch resistance for lenses and windows.