Structure–Property Relationship of Polypropylene-Modified EPDM Composites for High-Performance Cable Accessory Insulation

Cable accessories are weak points in high-voltage systems, imposing stringent requirements on insulation materials. In this work, EPDM was modified with polypropylene (PP) of different molecular structures and contents to clarify structure–property relationships. Among random-copolymer PP (R-PP), block-copolymer PP (B-PP), and homopolymer PP (H-PP) compared at the same content, H-PP exhibited the highest crystallinity and showed superior electrical performance, particularly at elevated temperatures. Meanwhile, H-PP also enhanced tensile strength, elastic modulus, and thermal conductivity while retaining a high elongation at break. Increasing the H-PP content from 0 to 15 wt% further increased the crystallinity from approximately 0.3% to 6.1%, with only minor changes in crosslinking density. As a result, the AC breakdown strength, volume resistivity, tensile strength, and thermal conductivity were progressively improved, while the relative permittivity and dielectric loss remained comparatively stable. However, high H-PP contents markedly increased the elastic modulus and reduced the elongation at break, revealing a trade-off between reinforcement and mechanical compliance. Among the investigated formulations, 5 wt% H-PP provided the most balanced combination of electrical performance, mechanical strength, and deformability. These results demonstrate that optimizing PP structure and content provides an effective approach for improving the comprehensive properties of EPDM for cable accessory insulation.