Study Of The Effect Of Voltage Stabilizer Modified Nanoparticles On The Property Of Thermoplastic Polyolefin Composites

Due to the demand for long-distance and high-power transmission systems,high voltage direct current(HVDC)transmission network has been greatly developed,and the insulation layer performance of HVDC transmission lines is one of the keys to guarantee the long-term stable operation of the transmission system.Usually,under high transmission voltage,space charge tends to accumulate inside the insulating polymer,which causes the internal electric field distortion,accelerates the aging of the material and even results in cable breakdown.Therefore,suppressing space charges and enhancing the breakdown strength is a necessary research topic.Voltage stabilizers are organic molecules that enable to capture high-energy electrons and reduce the collision between the matrix and charges,thereby increasing the breakdown strength.However,low-weight organic molecules are easy to migrate towards the polymer's surface,so voltage stabilizers are grafted onto the surface of the nanoparticles.In this way,apart from ensuring the long-term effect,the compatibility of nanoparticles and polymer become better,and space charge accumulation and injection are suppressed more strictly.First of all,p-dimethoxybenzil was regarded as the reactant,and after hydroxylation and esterification,a new type of voltage stabilizer molecule named BVA was synthesized.BVA has better voltage stability and stronger reactivity.Subsequently,BVA was grafted onto the surface of nano-Si O₂ via click chemistry and modified nanoparticle(Si O₂-BVA)was obtained.Finally,the low-density polyethylene nanocomposite(LDPE/Si O₂-BVA) was prepared,and its properties were tested and the reasons for performance improvement were explained.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)showed that the dispersion of Si O₂-BVA in polymer was improved.The breakdown strength of LDPE/Si O₂-BVA was enhanced because the accumulation and injection of space charge were suppressed and the density of space charge was declined.The distribution of the electron traps in the composites was studied,which indicated that the density of the traps was increased.The thermal stability,mechanical strength and electrical properties of LDPE cannot meet the requirements of cable.In addition,it is difficult to tailor the supermolecular structure of LDPE by adding nanoparticles.Polypropylene(PP),as a commonly used thermoplastic,can be recycled and reused,and its thermal,mechanical and electrical properties are better than LDPE.Therefore,Si O₂-BVA prepared in chapter 2 was added into PP and PP/Si O₂-BVA nanocomposite was prepared.The dispersion of Si O₂-BVA in matrix was improved shown by SEM and TEM.Both polarized optical microscope(POM)and differential scanning calorimetry(DSC)showed that the spherulite size and crystallinity of PP were obviously decreased after introducing Si O₂-BVA,which contributed to the enhancement of breakdown strength.It was found that the initial decomposition temperature and maximum decomposition temperature of PP/Si O₂-BVA were increased through thermogravimetric analysis(TGA).In terms of electrical properties,maximum breakdown strength of PP/Si O₂-BVA increased about 46.2%compared with PP,and space charge density decreased apparently.Through the surface potential decay test,it shown that the density of shallow traps in PP/Si O₂-BVA was greatly increased,which can accelerate the migration of space charge and prevent accumulation,thus improving breakdown strength.At last,taking the advantages of the two-dimension property of boron nitride nanosheet(BNNS),the voltage stabilizer modified boron nitride nanosheet(BNNS-BVA)was prepared,and polypropylene nanocomposite(PP/BNNS-BVA)was obtained.The compatibility of BNNS-BVA and matrix were enhanced,and the size and crystallinity of PP/BNNS-BVA were reduced.The initial decomposition temperature and maximum decomposition temperature of PP/BNNS-BVA have risen.In terms of electrical properties,breakdown strength of PP/BNNS-BVA increased and space charge density decreased.It was demonstrated that increased trap density in PP/BNNS-BVA could inhibit the accumulation and injection of space charge,thus enhancing its breakdown strength.Above all,it is reported that voltage stabilizer modified nanoparticles can significantly enhance the thermal property and breakdown strength of thermoplastic polyolefin.Thus,this investigation paved a new way for developing environmental-friendly HVDC cable insulation.