Improved suppression of electrical breakdown strength variation during melting annealing by enhanced multiphase stability in long-chain branching polypropylene high-voltage insulation

As a promising high-voltage insulating material with recyclability, energy-saving fabrication, and excellent electrical performance, polypropylene (PP) has attracted increasing attentions. However, the significant changes of breakdown strength are observed in PP insulation after extrusion, originating from the chain migration and structural evolutions. To address the problem, in this paper, the impact polypropylene copolymer (IPC) with long-chain branching (LCB) structures is synthesized by grafting polymerization. The LCB structures are obtained by hydrolyzing Si–Cl bonds on grafting groups and the physical interactions between chains were replaced with chemical bonds. In comparison with IPC, 10.2 % higher AC breakdown strength is achieved in LCBIPC. In addition, 4.0 % lower variation in breakdown strength is also observed in LCBIPC than IPC after the 200 °C melting annealing. Further results revealed that the variation in breakdown strength originates from the elastomer evolution and the changes in the crystalline structures. 3.9 % increase in crystallinity is found in IPC while only 2.0 % is presented in LCBIPC with the annealing time of 45 min. At this annealing time, the best compatibility between elastomer and PP matrix is achieved, leading to the highest breakdown strengths as 148.5 kV/mm and 158.8 kV/mm of IPC and LCBIPC, respectively. Subsequently, the elastomer begins to coarsen and a 22.0 % lower coarsening rate is achieved in LCBIPC, leading to a slighter decline in crystallinity and breakdown strength. Evidently, the breakdown strength variation is suppressed and multiphase stability is enhanced by LCB structures, providing insights in improvement of PP insulating materials with processing stability and excellent insulating performance.