Effect Of Acid Anhydride Grafting Agent On Trapping Energy Level And Trap Density Of Al₂O₃/LDPE System

As nano-polyethylene composite materials with excellent electrical properties,which are widely used in high-voltage and ultra-high voltage direct current transmission cable.However there is accumulation of space charge in the polymer dielectric under the long-term DC high voltage electric field,which will cause the distortion of the electric field and further cause higher local field strength than the breakdown field,then leading to destroy of polymer dielectric.Some nanoparticle doping can significantly suppress the space charge injection,but the mechanism of action is unclear exactly.In this paper,the mechanism of the trapping energy level of the Al₂O₃/LDPE system modified by the anhydride grafting agent was studied to complement the above mechanism and to investigate the physical and chemical properties of inhibiting the space charge in the Al₂O₃/LDPE system.The photo-stimulated discharge method was used to measure the trapping energy level and trap density of nano-polyethylene system.And the polymer space charge theory and interface theory were introduced to explain that the structure of anhydride can effectively suppress the space charge of Al₂O₃/LDPE system.The influence of anhydride grafting agent on the trapping energy levels of Al₂O₃/LDPE system with different components was quantified.Firstly,the anhydride-grafting LDPE?MAH/LDPE?and anhydride-modified Al₂O₃/LDPE?MAH/Al₂O₃/LDPE?nanocomposites were prepared by graft copolymerization and ultrasonic blending,and then the sample film was obtained by hot-pressing film composite process.The MAH/LDPE system and anhydride-modified Al₂O₃/LDPE system were characterized by Fourier transform infrared spectroscopy?FTIR?,and the grafted product and oxidation products of hightemperature were confirmed.The results show that the grafting ratio reach the maximum when the anhydride grafting accounted for 16.7% of the total mass fraction of MAH / LDPE.Secondly,the trap energy levels and trap densities of MAH/LDPE and MAH/Al₂O₃/LDPE systems were studied by using PSD.In MAH/LDPE system without nano-Al₂O₃ particles,the different concentration of anhydrides grafting will affect the trap energy level of LDPE,and with the increase of anhydride content the number of shallow traps in LDPE increases.When the anhydride grafts are destroyed,the number of shallow traps in LDPE decreases with the increase of the degree of destruction of anhydride grafts,and a few deep traps appear.This structural damage of LDPE can't be restored to the original state,indicating that the shallow traps in LDPE are related to the interfacial properties of LDPE as well.In the MAH/Al₂O₃/LDPE system with nano-Al₂O₃ particles,the anhydride grafting agent can provide a large number of shallow traps for the polyethylene system.These shallow traps can inhibit the formation of new traps caused by the addition of nano-particles,and can effectively reduce the trap density of the nano-Al₂O₃/LDPE system.Especially this inhibition effect is more obvious to the nano-Al₂O₃/LDPE composite doped with high-quality fraction.In the end,polymer space charge theory and polymer interface theory were used to explain the change of trap level and trap density of the Al₂O₃/PE system modified by acid anhydride.In addition,anhydride grafting agent can have an effect on suppressing the injection of space charge.