Evidently Retarded Degradation in Breakdown Strength of Polypropylene Eco-Friendly Cable Insulation from Trace Self-Grown γ-Crystals during Thermal Aging

Recyclable thermoplastic polypropylene (PP) cable insulation with excellent performances is expected to represent traditional thermoset cross-linked polyethylene (XLPE) insulation. However, the poor resistance to thermal aging of PP-based cable insulation is generally considered as an obstacle limiting their practical applications. In this article, trace self-grown γ-crystals of PP were accidentally found in impact PP copolymer (IPC) cable insulation during thermal aging, which evidently retarded degradation in breakdown strength caused by decline in crystallinity. The unexpected growth of γ-crystals is further proven to be facilitated by crystalline ethylene segments from rubber phases and those blocked in PP chains. As a result, more deep traps are introduced by γ-crystals that are presented as crystalline structure defects. It makes carriers in IPC more difficult to migrate and detrap under external electric field. Therefore, although the crystallinity, which determines the breakdown strength of semi-crystalline polymers, decreases at day 12 of thermal aging, the degradation in breakdown strength is delayed until day 24. The self-grown γ-crystals in IPC insulation thus achieve the self-protective resistance against thermal aging. This work provides a new understanding to the effect of elastomers on thermal-aging stability in PP-based eco-friendly power cable insulation.