Enhancing electrical properties of impact polypropylene copolymer for eco-friendly power cable insulation by manipulating the multiphase structure through molten-state annealing

Composite insulating materials with high electrical properties are integral for high voltage power cable. In this paper, we achieved significant increases by about 17% and 30% in breakdown strength and charge injecting transition field of impact polypropylene copolymer (IPC), a promising composite for power cable insulation, through 60 min molten-state annealing (MSA). To unravel the influencing mechanism of MSA on electrical properties, the multiphase structure evolution were investigated. Results showed that ethylene-propylene random copolymer (EPR) was separated from matrix with ongoing MSA. Meanwhile, two melting peaks were found in DSC curves. The one at lower temperature was raised from 142.2 C to 144.6 C after 60 min MSA, while the higher one remained unchanged, indicating the increase in thickness of partial lamellas, which was proved to be the reason for the increased deep trap level from 1.01 eV to 1.05 eV. Under 50 Hz AC electric field, electrohole recombination was proposed to be the main origins of high-energy electrons, rather than acceleration under electric field. The greater the energy level of the deep trap, the smaller the energy released by recombination, resulting in reduced probability of collision ionization and thus increased breakdown strength. This work provides a new comprehending into the electrical breakdown and its enhancement of IPC eco-friendly power cable insulation.