Carbon Black Distribution Optimization Method And Its Effect On The Properties Of Polypropylene-based Semi-conductive Shielding Materials

Polypropylene(PP)cable with the advantages of environmental protection and energy-saving has become a topic research subject.At present,most of the research focuses on PP insulating materials,and less attention is paid to the semi-conductive shielding materials.Different from the cross-linked shielding material,PP shielding material is a thermoplastic multiphase blend consisting of PP,elastomer and conductive carbon black(CB).Comparing with PP,CB is selectively dispersed in the elastomer with relative strong polarity.The non-uniform distribution of CB will lead to the local electric field concentration on the shielding material surface,and then lead to the insulation failure.The co-optimization of CB distribution in PP shielding material and the conductivity of PP shielding material is an urgent problem to be solved in the research and development of PP high voltage AC cable and high voltage DC cable.In order to improve the dispersion of CB in the shielding materials,UV initiated grafting technique was employed to synthetic polypropylene grafted maleic anhydride(PP-g-MAH),which selected maleic anhydride(MAH)as the graft monomer and styrene derivatives as the anti-degradation monomer.The effects of grafting modification on the molecular structure and physicochemical properties of PP were tested.The CB distribution and electrical properties of PP-based semi-conductive shielding materials before and after grafting were investigated,and the effect of CB distribution on the conductivity and charge emission behavior of PP-based semi-conductive shielding materials was studied.By comparing the infrared spectra,grafting ratio,mechanical properties,melt index and gel content tests of different content grafting materials,it was found that the grafted material with 3 wt% grafting monomer exhibited higher grafting efficiency,retained the original mechanical and melt flow properties of PP,and no gel was produced,so this was the best grafting process ratio.Further,the molecular weight distribution and thermogravimetric test of PP-g-MAH grafting modified material were investigated.The results showed that there was little difference of molecular weight between PP and grafted PP,which was proved that there was no side reaction of cross-linking and degradation in the grafting process,and the grafting modification of polar molecules had no significant effect on the thermal stability of PP.Four kinds of shielding materials with different CB dispersion were prepared by using PP,PP-g-MAH,POE/PP and POE/PP-g-MAH as matrix,respectively,and the mesoscopic structure and surface potential were performed.SEM and KPFM results showed that the graft modification significantly improved the dispersion of CB in PP matrix,and Kelvin microscopy results further confirmed that CB dispersion was optimized,and the potential distribution on the surface of the shielding material becomes uniform.The electrical and thermal conductivity test show that the homogeneous dispersion of CB has a positive effect on the temperature sensitivity of electrical and thermal conductivity.The results of mechanical properties show that tensile strength and elongation at break are positively correlated with CB dispersion.In order to investigate the influence of CB dispersion on the DC performance of shielding/insulating structures,the space charge,DC conductivity and DC breakdown characteristics of different shielding/insulating structures at different temperatures were measured.The results show that the CB dispersion improves the insulation field uniformity,increases the space charge threshold,reduces the conductivity current and improves the breakdown field strength.