Plastic compositions for dielectric applications

Abstract

HYDROCARBON RESINS which possess ideal dielectric properties generally fall into two broad classes: (a) Materials like polyisobutylene with a second-order transition temperature below -50 °C. At normal temperatures of usage these plasties are rubberlike, weak mechanically, and lacking in dimensional stability. (b) Materials such as polystyrene with a secondorder transition temperature above -f 50°C. At normal temperatures of usage (i,e., below the transition point) such materials show good dimensional stability and high tensile strength, but tend to be brittle.The term "second-order transition point" is used to designate the temperature, or temperature interval, below which high polymers behave as brittle, dimensionally stable materials, and above which they exhibit cold flow and rubberlike behavior. For many plastics the second-order transition point, the heat distor-. tion temperature, and the brittle point may be within a few de-grees of one another. There are exceptions to this generalization.A recent review article (2) discusses this subject more completely and cites pertinent literature references. Polyethylene (10, 16) actually is a combination of both types a and b since it is a partially crystalline plastic whose amorphous regions have a transition temperature of -00'°C ., while its crystal-