Study On Preparation And Electrical Properties Of Acetophenone-Polyethylene Insulating Material Showing Migration Resistance

As a voltage stabilizer,acetophenone(AP)can considerably improve the breakdown strength and suppress the electric tree growth of polyethylene(PE)insulating materials.However,AP has poor compatibility with the PE matrix and thus can easily migrate,which affects the dielectric properties and long-term stability of the materials.In this study,two schemes of grafting and modifying AP onto polymer molecular chains are designed to solve the problem of AP easily migrating from PE.It also suggests ideas for other aromatic voltage stabilizers to migrate from PE and for solving the problem of migration.The research conducted is described as follows.A grafted AP voltage stabilizer 4-vinyloxyacetophenone(VPE)was prepared by introducing a double bond reactive group at the AP para position.VPE structures were characterized by Fourier transform infrared spectroscopy(FTIR)and nuclear magnetic resonance.The effects of reaction time and temperature on the yield of VPE were analyzed and VPE was grafted onto polyethylene(PE)molecule chain during the radiation cross-linking process.Grafted AP cross-linked polyethylene(VPE-g-XLPE)was then prepared.By comparing the FTIR spectra of VPE/PE and VPE-g-XLPE samples before and after the extraction experiment,we determined that VPE was successfully grafted onto the molecular chain of XLPE and effectively inhibited the migration of VPE from XLPE.The grafting efficiency of VPE and the loss rate of VPE during blending and moulding were measured by TG.The results showed that the grafting efficiency of VPE was 75.2% and the loss rate was 40.5% during blending and moulding.The electrical properties of XLPE,AP/XLPE,and VPE-g-XLPE were studied.The results show that both VPE grafted onto the XLPE molecular chainand AP free in XLPE material could considerably improve the AC and DC breakdown strengths as well as the tree initiation voltage of XLPE.The effect of VPE was even better than that of AP because VPE has para-electron-donating alkoxy groups,which make the conversion from ketone to an enol isomerization reaction energy barrier lower than that of AP from ketone to an enol isomerization reaction energy barrier.This can more effectively consume the energy of high-energy electrons in XLPE material and reduce damage to the PE molecular chain.Compared with AP/XLPE and VPE-g-XLPE before and after high temperature degassing tests,VPE-g-XLPE exhibited better thermal stability than did AP/XLPE.The breakdown field strength and starting voltage of VPE-g-XLPE before and after high temperature degassing tests were significantly higher than XLPE.VPE-g-XLPE showed a good suppression effect on the initiation and growth of electric branches and space charge accumulation.Studies have shown that this is mainly related to the consumption of high electron energy by VPE in the XLPE matrix and the introduction of uniform and stable deep traps.The functional units of AP were modified onto molecular chains of polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene(SEBS)by an acetylation reaction.Acetylated SEBS(Ac-SEBS)containing AP was prepared.Ac-SEBS/LDPE composites were then prepared using a blending method.A thermal weightlessness test showed that AP was stable in the matrix of the Ac-SEBS/LDPE composite.Scanning electron microscopy was used to observe the dispersion of Ac-SEBS in the LDPE matrix with different degrees of acetylation.The results showed that Ac-SEBS was uniformly dispersed in the LDPE matrix at a nanometer scale by adjusting the degree of acetylation of SEBS.The dielectric properties of Ac-SEBS/LDPE showed that Ac-SEBS could significantly improve the DC breakdown strength and suppress space charge accumulation of LDPE.In addition,our study showed that the starting voltage of LDPE was increased,whereas the AC breakdown field strength was not reduced.The improved dielectric properties of Ac-SEBS/LDPE composites was mainly related to the introduction of AP into the LDPE matrix by Ac-SEBS.AP can consume high electron energy through keto-enol isomerization reaction during high-energy electron collision and can also reduce the damage to the LDPEmolecular chain.It can also be uniformly and stably distributed in the LDPE matrix as a deep well,which can inhibit space charge injection and migration.The results of this study show that whether the AP was grafted onto the XLPE molecular chain or modified by blending as a part of the polymer molecular chain to alter the LDPE,the dielectric properties could be significantly improved,thus effectively inhibiting the migration of AP from the PE matrix.