Improved Wide-Temperature-Range Insulation Properties of Block Polypropylene by UV-Irradiated Cografting of Maleic Anhydride and 4-tert-Butylstyrene

Polypropylene (PP), as a recyclable thermoplastic polymer, is a promising insulating material for application as an environmentally friendly insulation layer in high-voltage-direct-current (HVDC) cables. However, the lack of toughness and the poor wide-temperature-range electrical properties under a DC electric field seriously restrict its development and application. Herein, a block polypropylene with a low elastic modulus is selected and grafted with organic molecules of maleic anhydride (MAH) and 4-tert-butylstyrene (St*) as a masterbatch (PP-g-(St*-co-MAH)) by an ultraviolet-irradiated radical reaction. The modified b-PP with different grafting contents was prepared by diluting the masterbatch through melt blending, and their DC electrical properties at a wide temperature range of 40–100 °C were tested. The results indicate that the introduction of a comonomer St* significantly improves the grafting rate of MAH and suppresses the degradation of PP chains during the grafting reaction; thus, the PP-g-(St*-co-MAH) possesses improved thermogravimetric performance and maintains the original mechanical properties and workability. The grafted functional groups introduce deep charge traps into the matrix and thus effectively suppress the space charge injection and accumulation, significantly reduce the conductivity, and enhance the breakdown strength of PP at wide temperature ranges. Among the tested materials, the PP grafted with 0.57 wt % St*-co-MAH exhibits the best electrical properties, while inadequate or excessive grafting content of St*-co-MAH relatively weakens the positive effects of grafting modification. The mechanism is analyzed based on calculation of the trap level and the three-dimensional (3D) electric potential.