Enhanced Electrical Tree Resistance of Polypropylene Cable Insulation by Introducing β-Crystals

Polypropylene (PP) is regarded as a recyclable material for high-voltage direct current (HVDC) cable insulation due to its high melting point and electrical resistivity. This work focuses on the effect of the β-nucleating agent content on the electrical tree growth characteristics in isotactic PP (iPP) insulation. The results demonstrate that adding β-nucleating agents promotes the growth of β-crystals while limiting the α-crystal content. The crystallinity improves with the reduction in the average size of spherulites due to the addition of a β-nucleating agent with 0.1 wt% content. Electrical tree experiments show that the electrical tree growth rate declines as the nucleating agent content rises from 0 to 0.1 wt%. Meanwhile, the expansion coefficient increases with higher nucleating agent content. Continuous increases in the nucleate agent content result in the upward growth rate of electrical trees. When the nucleating agent content is below 0.1 wt%, the α–β-crystal interface introduced by the agent suppresses carrier migration and limits impact ionization, leading to the slower growth rate of the electrical tree. Further addition of the nucleate agent induces a β–β-crystal interface with weak coupling in carriers. It is concluded that β-nucleating agent-modified PP with 0.1 wt% content has potential application in HVDC cable insulation.