Morphology and the growth of electrical trees in a propylene/ethylene copolymer

Growth of electrical trees under 50 Hz high electric stress has been studie d in a clarified propylene/ethylene copolymer, to explore the effects of th e applied field and the material microstructure. Crystallization of the cop olymer at low temperatures (<128<degrees>C) produces a continuous lamellar texture and the material consequently is optically transparent At higher cr ystallization temperatures (134 degreesC), more sporadic nucleation occurs and, as a result of the larger scale structural features that are able to d evelop, the material becomes optically scattering. Nevertheless, CCD images of evolving tree structures could be obtained in both systems. Electrical treeing was found to occur reproducibly, but in a markedly different manner in the two morphologically different but chemically identical materials. I n the low temperature crystallized copolymer, electrically conducting tree structures were found to develop with a growth rate that increased monotoni cally with increasing applied voltage. Conversely, non-conducting tree stru ctures formed in the 134 degreesC crystallized copolymer that mimic the wel l documented decreasing tree growth rate with increasing applied voltage be havior of both low density polyethylene and a flexible epoxy resin.