Enhancement in Electrical Properties of Impact Polypropylene Copolymer for Cable Insulation Induced by Rapid Cooling

Impact polypropylene copolymer (IPC) is a promising eco-friendly power cable insulating material with great potential to replace the thermosetting cross-linked polyethylene (XLPE). This article compares the breakdown strength and water tree resistance of IPC and isotactic polypropylene (iPP) after different cooling rates. Unexpectedly, an abnormal enhancement of IPC's insulating property is discovered. The breakdown strength and water tree resistance of IPC are both increased after rapid cooling, which is opposite to the electrical property deterioration generally found on iPP and XLPE with similar treatment. To reveal the mechanism of abnormal enhancement of IPC, the crystalline structure, phase morphology, and trap distribution are characterized. The results indicate that rapid cooling induces an abnormal increase in the crystallinity of IPC due to the formation of $\beta $ crystals, which in turn increases the deep trap level and density and leads to enhanced breakdown strength. Moreover, the formation of $\beta $ crystal can also enhance the inter-lamellar coupling and result in higher tensile yield strength, further, decreasing the water tree initiation probability. These results reveal that rapid cooling can significantly regulate the performance of IPC, which should be given serious consideration for the application of IPC in cable insulation.