Synergistically Enhanced Electrical and Mechanical Properties of Long-Chain Branched Polypropylene Insulation with Propylene–Butene Copolymerization

Polypropylene (PP) is a promising insulating material for high-voltage cables, while the synergistic enhancement of its mechanical and insulating properties has been a challenge. In this paper, long-chain branched (LCB) polypropylene copolymers were prepared via a two-stage in situ polymerization, where a propylene–butene copolymer and an ethylene–propylene copolymer (EPC) were subsequently formed. Meanwhile, H-shape LCB structures were introduced by hydrolysis of the Si–Cl bonds in branched chains of comonomers obtained by in situ polymerization. Synergistic improvement in both mechanical and electrical properties was finally achieved. The rubber phase from ethylene–propylene copolymerization in the PP matrix, combined with highly flexible ethylene chains, contributed to an ∼50% decrease in elastic modulus and an ∼100% increase in the elongation at break. Concurrently, the overall electrical performance was significantly improved. The dielectric constant and tan δ both decreased, while the threshold field strength and the breakdown field strength increased by 49.2 and 18.65%, respectively. It was attributed to the deep-level traps introduced by the copolymerization of propylene and butene and the LCB structures, which increased the probability of carrier capture and thereby suppressed carrier migration. It is indicated that long-chain branched polypropylene copolymers provide a potential approach to high-performance, eco-friendly, high-voltage cable insulation.