Preparation Of Polyethylene Based Nanocomposite Dielectrics And Their Application In Electrical Insulation

In order to solve the problem of uneven energy distribution in China and realize the grand goal of "peak carbon dioxide emissions before 2030 and achieve carbon neutrality before 2060",a number of high-voltage,large capacity and long-distance DC transmission projects are being built one after another in China.However,the low electrical properties of polyethylene based insulating material limit the development of high voltage direct current transmission system in China.In this paper,polyethylene(PE)was used as the matrix,and the electrical properties of PE were improved based on pre-irradiation grafting technology and filled nano-particles modification technology.The main research works are as follows:(1)Polyethylene grafted polystyrene(PS)graft copolymers(PES)were synthesized by pre-irradiation suspension grafting.It is found that the PS side chains connect PE and PS macromolecular chains in PE/PES blends,and there is no interfacial debonding.When the PS content is 15 wt%,the DC breakdown strength(Eb)of PE/PES was up to 408.5 kV/mm,which is 1.45 times than that of PE.(2)In order to investigate the effect of ungrafted polar monomers on electrical properties,polyethylene grafted acrylic acid(AA)graft copolymers(PEA)were prepared by pre-irradiation melt grafting technology.By designing comparative experiments,it was found that the addition of polyacrylic acid(PAA)branched chains significantly inhibite the accumulation of space charge and increase Ebby 32.1%.The ungrafted AA monomer(0.0195?0.1560 wt%)had no obvious effect on the space charge distribution,but could enhance the dielectric strength.(3)In order to achieve uniform dispersion of nanoparticles and obtain nanocomposite dielectric insulating materials with excellent electrical properties,two methods were used to assist the dispersion of nanoparticles.One is blending with functionalized PE.PES,PEA and silica nanoparticles(SiO2)were successively introduced into PE to prepare a series of composites.Transmission electron microscopy(TEM)showed that SiO2 in PE/PES/PEA/SiO2 was uniformly dispersed in nano scale.The electrical properties of PE/PES/PEA composites showed that Eb is significantly enhanced,which is 57.8%higher than that of PE.Moreover,the introduction of SiO2 could significantly inhibit the accumulation of space charge in the material.Another method is coating nanoparticles by in-situ polymerization.The surface of SiO2 was coated with a layer of PS polymer.It was found that the introduction of 0.5 wt%unmodified SiO2 improves the Eb and effectively inhibites the space charge accumulation.Compared with the modified SiO2,the unmodified SiO2 can improve the electrical properties more effectively.It was found that the dispersion of the two nanoparticles in the polymer was similar.The difference of electrical properties can be attributed to the different dispersion position of SiO2 in nanocomposites.In terms of process complexity and efficiency,blending with functionalized polymer is more efficient and suitable for industrial production.(4)The effect of hydrogenated polystyrene-b-butadiene-b-polystyrene(SEB S)on the electrical properties of PE/high density polyethylene blends(BPE)and BPE/PES is investigated.SEBS was not compatible with BPE in molecular chain level,but enhanced the compatibility between BPE and PS,and significantly improved the flexibility of BPE/PES.It is found that the introduction of SEBS has little effect on Eb of BPE.But in BPE/PES/SEBS,Eb decreased significantly with the increase of SEBS content.(5)The influence of the modified h-BN dispersed and positioned in BPE phase or SEBS of the BPE/SEBS(30/70 wt/wt)blends on the electrical properties was studied.H-BN was first etched by dichlorosulfoxide,and then suffered from alkylation reaction to give A-BN.Scanning electron microscope(SEM)observation showed that A-BN was successfully dispersed in different phases through the pre-paration of master batch.The dispersion of A-BN in different phases has little effect on the mechanical,thermal and thermal conductivity properties of BPE/SEBS.However,the electrical properties tests showed that A-BN located in BPE has better DC breakdown resistance than in SEBS.