Morphological changes of microvoids in steam-cured XLPE cable insulation by heat treatment were investigated using a scanning electron microscope. The observations showed that the voids are filled with low molecular weight material composed of crystalline and amorphous polyethylene which exuded from the surrounding bulk polymer. The filling rate and the properties of the LMWPE are dependent on the temperature and the heat treatment. Void filling, especially partial filling, tended to accelerate bow-tie tree generation. On the other hand, the microvoids in steam-cured XLPE insulation, which contained some special additives, did not change by heat treatment even at temperatures higher than the bulk polyethylene melting point, and furthermore, few bow-tie trees were observed in this cable insulation.
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Namiki et al. (1980) studied this question.
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