Effect of impregnation, compression and temperature on electric strength of polythene and polypropylene

Abstract

The effect of impregnation and compression on the electric strengths of polythene and polypropylene has been investigated. Measurements have been made over the temperature range ?195 to + 20°C. Four impregnants were used: The dipolar liquids hexachiorodiphenyl, trichiorodiphenyl and dibutyl phthalate and nonpolar transformer oil. At room temperature, the impulse strengths of polythene and polypropylene are substantially increased by impregnation with certain of the dipolar liquids. Similar, although smaller, increases are found in the DC strength of polypropylene impregnated with any of the dipolar liquids. Transformer oil produces no improvements in the DC or impulse strengths of either of the two polymers. Using either of the chlorinated diphenyls as the impregnant, the DC strength of polythene is increased for all temperatures below the freezing points of the impregnants. Both the DC and impulse strengths are increased at liquid-air temperatures for polypropylene impregnated with trichlorodiphenyl. Compression improves the DC and impulse electric strengths of polythene. A mechanism of breakdown has been proposed, based on the presence of defects in the polymers. The defects initiate breakdown, and improvement in the electric strength results from their partial removal or filling by pressure or impregnation.