Preparation And Properties Of Non-crosslinked High Voltage Direct Current Cable Material By Modified Polypropylene

High voltage direct current cable transmission is the focus of power system development due to its large capacity,small loss,high reliability and good stability.Plastic insulated cable is widely used due to its high operating temperature,high strength,light weight and other characteristics.At present,most of the plastic insulated cables use cross-linked polyethylene(XLPE)as insulation material,but the thermosetting property of XLPE makes it unable to recycle at the end of its service life,resulting in environmental pollution and waste of resources.Polypropylene(PP)is considered as the most promising alternative material to XLPE insulation because of its excellent insulation and heat resistance.However,PP is short of flexibility at room temperature,and easy to accumulate space charge under high voltage direct current electric field,causing local electric field distortion,and eventually insulation failure,which limits the development of PP in the field of HVDC transmission.The shielding material of HVDC cable should match the insulating material.However,the modification of PP is mostly focused on the research of insulation materials,and its research of shielding materials for HVDC cable is rarely reported.In this paper,PP was chemically modified by melt grafting method to prepare polypropylene grafted styrene(PP-g-St)and polypropylene grafted 4-acetoxy styrene(PP-g-AOS),and the factors affecting the grafting degree were investigated to obtain the optimum preparation conditions.The microstructure of the modified PP was characterized by infrared spectroscopy(FTIR)and the crystallization by X-ray diffraction(XRD).The thermal properties of the modified PP were studied by differential scanning calorimetry(DSC)and thermogravimetry(TG).The electrical properties of the modified PP were studied by electroacoustic pulse method(PEA),breakdown field strength and volume resistivity.The results showed that the optimum reaction condition of PP-g-St was 200 ^oC(temperature),70 r/min(speed),7 min(time),50:8:1.4(mass ratio of PP,St to initiator),in which the grafting degree was the highest,up to 6.9%.Meanwhile,the optimum reaction condition of PP-g-AOS was 190 ^oC(temperature),80 r/min(speed),10 min(time),50:1.4:0.08(mass ratio of PP,AOS to initiator),in which the grafting degree was the highest,up to 1.31%.The grafting of St or AOS modified PP can inhibit the space charge accumulation and improve the breakdown field strength and volume resistivity.It was found that the improvement of the electrical properties of PP-g-AOS was more obvious.When the grafting degree was 1.14%,the space charge density in PP-g-AOS was only 0.076 C/m³,and the breakdown field strength and volume resistivity were the highest,up to 289 k V·mm^-1and 5.1×10¹⁵?·m,respectively.Secondly,block copolymer polypropylene(BPP)/PP-g-AOS/styrene-ethylene-butene-styrene block copolymer(SEBS)cable insulation material was prepared by melt blending,and the microstructure and the crystallization were characterized by SEM,XRD and hot stage polarizing microanalysis(POM).The mechanical,thermal and electrical properties were studied as well.The results showed that SEBS was uniformly distributed in the core-shell structure of BPP/PP-g-AOS system,which can refine the grain size during the crystallization process.SEBS can improve the flexibility of BPP/PP-g-AOS system while maintaining its excellent heat resistance and mechanical strength at high temperature.Moreover,SEBS can further improve the ability to inhibit space charge.The thermal,mechanical and electrical properties of BPP/PP-g-AOS/SEBS blend with 20 wt%SEBS content were the best.Finally,BPP/polypropylene grafted maleic anhydride(PP-g-MAH)/SEBS/CB cable shielding materials were prepared by melt blending with BPP and SEBS blends as matrix and conductive carbon black(CB)as filler.The changes of tensile strength,thermal elongation,volume resistivity and processability with different component contents were studied.The results showed that when the mass ratio of PP-g-MAH to BPP was 2/3,the bulk resistivity,tensile strength,elongation at break and thermal elongation of the semi-conductive shielding composites met the industry standards,and the adhesion between the shielding layer and conductor copper was the largest.With the increasing of CB content,the volume resistance of the semi-conductive shielding composites decreased rapidly at first and then decreased slowly,and the high temperature deformation resistance gradually increases.This paper provides some theoretical and experimental references for the research on insulating materials and shielding materials of non-cross-linked HVDC cables.