Study on the structure and mechanical and electrical properties of β-crystal blended polypropylene

Copolymer polypropylene has good insulating properties and mechanical properties and has the potential to become a thermoplastic DC cable insulation material. However, its low breakdown strength limits its application in high-voltage DC cables. In this paper, ethylene propylene block copolymer polypropylene (EPC) was selected as the matrix, and isotactic polypropylene (iPP) and a β nucleating agent were added for blending modification. The crystal structure was characterized by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and metallographic microscopy (POM). The results showed that flower-like β crystals were formed in the β-crystal blended polypropylene. The mechanical properties, included space charge, and DC breakdown properties of the composite were tested. Compared with that of pure iPP, the mechanical tensile strength of the β-crystal blended polypropylene material was greatly improved, the elongation at break reached 666.4%, the normal temperature impact performance reached 39.1 kJ/m2 at the same time, and the low-temperature embrittlement temperature point also greatly decreased to −39.8 °C, meaning that the material can completely meet the requirements of working at low temperatures. After applying a field strength of 50 kV/mm for 30 min, there was almost no space charge accumulation in the β-crystal blended polypropylene; the DC breakdown strength at 25 °C reached 267.6 kV/mm.