Effects of cooling rate on space charge characteristics of polypropylene

Abstract

Polypropylene (PP) is considered as a candidate for high-voltage direct current (HVDC) cable insulation owing to its excellent thermal stability, insulation properties, and environmental friendliness. It is a semi-crystalline polymer, and the difference in cooling rate during processing affects its crystallisation morphology and characteristics, which may change its insulation properties. In this study, PP film samples with varying cooling rates of 10, 20, 40, 70, 120, and 170 °C min−1 were prepared. The crystal morphology was observed, crystal characteristics were analysed, and the space charge distribution and direct current (DC) breakdown strength were measured. It can be concluded that with an increase in the cooling rate, the spherulite size of PP decreased, the spherulite number increased, and the characteristic breakdown strength increased. Notably, the variation trends of the space charge distortion factor and carrier mobility of PP are opposite to that of crystallinity. When the cooling rate was 40 °C min−1, the number of space charges in PP was small, the distortion of the electric field was not obvious, the apparent mobility of carriers was high, the charge decay was rapid in the depolarisation process, and the average breakdown strength was acceptable. Therefore, during the processing of the HVDC cable, the space charge characteristics of PP can be optimized by adjusting the cooling rate.