Synergistic enhancement of thermal-oxidative stability and DC electrical properties in impact polypropylene copolymer via maleimide-functionalized antioxidant grafting

Grafting modification represents a critical method for augmenting the electrical properties of polypropylene (PP), as an insulating material in high-voltage direct current (HVDC) cables. Additionally, the aging performance requires further enhancement since cables are the power equipment that operates over the long term. Grafting antioxidants that contain charge trapping functional groups has the potential to enhance both the electrical and aging properties simultaneously. Nevertheless, direct grafting remains a formidable challenge due to the low grafting reactivity of antioxidants and the high susceptibility to degradation of PP. Drawing inspiration from click chemistry, this paper selects PP grafted with maleic anhydride (PP-g-MAH) as the reaction platform. N-phenyl-p-phenylenediamine (AD) is covalently attached to the MAH side groups through the maleimide-functionalized reaction. This results in forming a functionalized material, PP-g-(MAH-co-AD). Compared with PP and PP-g-MAH blended with traditional antioxidants, PP-g-(MAH-co-AD)s demonstrate enhanced thermo-oxidative aging resistance and more remarkable improvement in DC electrical properties. Before and after aging, it sustains a robust suppression of space charge and conduction current. Moreover, it displays a lower decline rate in DC breakdown strength and reveals a substantial reduction in the sensitivity of conductivity to temperature. The observed improvement can be ascribed to the (MAH-co-AD) groups. These groups introduce not only electron deep traps but also hole deep traps and maintain localized deep traps even after aging. In summary, compared with cross-linked polyethylene (XLPE) and PP modified by molecular grafting with single functional properties, the functionalized PP demonstrates superior aging resistance and more pronounced enhancement of electrical properties.