The electric strength of some synthetic polymers

The intrinsic electric strengths of polythene, polystyrene, polyisobutene, polymethyl methacrylate, chlorinated polythene and oxidized polythene have been measured over the temperature range ¿200° to + 110°C. At low temperatures the electric strengths of the non-polar polymers, polythene, polystyrene, and polyisobutene, do not vary much with temperature; for each of them there is a characteristic temperature above which the electric strength falls rapidly with further temperature rise. This temperature is about 40° C for polythene, 90° C for polystyrene, and ¿50° C for polyisobutene. The electric strength of the polar polymer, polymethyl methacrylate, is much higher than those of the non-polar polymers at low temperatures, and falls with increasing temperature throughout the temperature range investigated; there is an increased rate of change of electric strength with temperature above 20° C. Mild chlorination of polythene leads to an increase in the electric strength at low temperatures, a decrease at high temperatures, and a lowering of the temperature at which the temperature gradient changes. At low temperatures the electric strength of oxidized polythene is greater than the corresponding strength of unoxidized polythene. The results are discussed in the light of Froöhlich´s electronic theory of breakdown.