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

Growth of electrical trees under 50 Hz high electric stress has been studied in a clarified propylene/ethylene copolymer, to explore the effects of the applied field and the material microstructure. Crystallization of the copolymer at low temperatures (<128<degrees>C) produces a continuous lamellar texture and the material consequently is optically transparent At higher crystallization temperatures (134 degreesC), more sporadic nucleation occurs and, as a result of the larger scale structural features that are able to develop, 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. In the low temperature crystallized copolymer, electrically conducting tree structures were found to develop with a growth rate that increased monotonically with increasing applied voltage. Conversely, non-conducting tree structures formed in the 134 degreesC crystallized copolymer that mimic the well documented decreasing tree growth rate with increasing applied voltage behavior of both low density polyethylene and a flexible epoxy resin.