The space charge distribution, conductivity characteristic and breakdown strength of cross-linked polyethylene (XLPE) insulation materials under DC voltage are the key issues in the development of HVDC cable materials. The introduction of deep charge traps in polyolefin matrix by appropriately grafting polar groups onto polyolefin molecular chains can efficiently improve the dielectric performances of HVDC cable insulation materials. It is of great significance for developing HVDC cable materials to study the modifying schemes and mechanisms of chemically grafting polar molecules in polyethylene. With the thermochemical method based on peroxide initiated radical reaction, three kinds of polar molecules acrylic acid (AA), acrylate chloroacetate (CAAE) and maleic anhydride (MAH) were individually grafted onto XLPE molecular chains. It is indicated by infrared spectra that AA, CAAE and MAH molecules have been successfully grafted onto polyethylene molecular chains in the crosslinking networks of XLPE by radical addition reactions on their unsaturated double-bonds. Space charge distribution, charge trap characteristics, DC conductance and breakdown strength of the modified XLPE materials were analyzed to elucidate the effective mechanisms of polar-groups on the comprehensive amelioration in dielectric performances. The study found that grafting polar molecules introduces deep charge traps in XLPE, which remarkably impedes the electrode injection and internal transport of charge carriers under high electric field, thus alleviating the space charge accumulation inside, abating conductance currents and increasing breakdown strengths. Among them, MAH graft-modified XLPE material shows relatively superior comprehensive properties.

cross-linked polyethylene;polar molecule;charge trap characteristic;electrical conductance