Research On Melt-grafting Antioxidant AMD Onto LDPE And Its DC Dielectric Properties

Over the past ten years,the scale of China’s power grid has nearly doubled,and the length of 66 kV and above high-voltage transmission lines has exceeded 1.14 million kilometers.The UHV project has achieved a leap from scratch to more than30 alternating current(AC)/direct current(DC)ultra-high voltage(UHV)lines up to now,among which polyethylene(PE)insulated high-voltage cables occupy a pivotal position.However,with the extensive application of PE insulation materials in power transmission system,the problems of space charge accumulation and conductivity loss of PE insulated cables in long-term operation need to be solved urgently.Therefore,the preparation of PE insulation materials with excellent DC dielectric properties is of great significance to the industrial application of cable insulation materials and social and economic benefits.Melt grafting of polarity-containing compounds initiated by dicumyl peroxide(DCP)is a practical and effective method for PE insulation modification,and the amount of grafting and the gel produced in the grafting process have great influence on the dielectric properties of PE.In this thesis,the antioxidant AMD was grafted onto the molecular chain of low-density polyethylene(LDPE)by melt grafting technology,and the antioxidant AMD grafted LDPE(LDPE-g-AMD)insulation materials with different contents of AMD grafted were prepared by controlling the amount of DCP.The effects of the amount of AMD grafted and the gel contents on the physical and chemical properties and DC dielectric properties of LDPE were studied.The successful grafting of AMD onto LDPE macromolecular chains was verified by Fourier transform infra-red(FT-IR)absorption spectroscopy.The calculation of grafting rate and the test of gel contents show that with the increase of DCP contents,the grafting rate increased rapidly and then tended to be flat,while the gel contents changed very slowly at first and then increased rapidly.When the mass fraction of DCP reaches 0.2phr,adding 0.5phr AMD to LDPE-g-AMD can achieve the maximum grafting rate.When the addition of DCP increased to 0.3phr,the gel contents of LDPE-g-AMD increased significantly,which show that the grafting of antioxidant AMD is prior to the crosslinking process of PE.The results of differential scanning calorimetry(DSC)tests show that grafting AMD can slightly improve the crystallinity of LDPE,but has no significant effect on the sheet crystal thickness.The oxidative inductive time(OIT)test results show that grafting AMD can effectively improve the oxidation resistance of LDPE insulation materials.The test results of space charge characteristics show that grafting AMD can inhibit the space charge accumulation in LDPE.With the increase of grafting rate of AMD,the improvement effect of space charge accumulation in LDPE-g-AMD becomes more and more excellent.Among them,LDPE-g-AMD(LDPE-g-AMD-0.2DCP)initiated by 0.2phr DCP has a better comprehensive effect and can effectively improve the accumulation of space charge.Thermally stimulated depolarization current(TSDC)tests and electronic density of states(DOS)calculations show that grafting AMD can introduce deep traps with deeper trap energy levels in LDPE,which can effectively trap charge carriers and suppress space charge accumulation,thus improving the DC dielectric properties of the material.The results of DC conductivity characteristics tests show that the conductivity currents of LDPE-g-AMD insulation grafted with AMD are significantly lower than those of LDPE,and with the increase of grafting rate,LDPE-g-AMD shows more excellent suppression of conductivity currents.The results of DC breakdown strength test show that at 30 ℃,a small amount of AMD grafting can improve the DC breakdown strength of LDPE,but too high grafting rate will decrease the breakdown strength of LDPE.However,the DC breakdown strength of LDPE-g-AMD is higher than that of LDPE at higher temperatures and increases with the increase of grafting rate.