The Preparation And Properties Study Of Polyolefin Based Nanodielectrics

Insulation materials play a significant role in the field of power cable,transformer and electric machine.The service life of power equipment may be decided by the properties of insulation materials.With the increasing demand of power and the developing of power system,higher requirements were proposed to the insulation materials of cable.Recently,high voltage direct current（HVDC）cable with polymer material as insulation has gained more and more attentions.However,the space charge is easy to be accumulated in the polymer insulation materials and lead to disordered electric field distribution and accelerated aging of the cable.Besides,the oxidation reaction of polymer insulation materials in the air is inevitable,which would initiate the degradation and crosslinking of molecular chain.Thus how to improve the electrical and thermal（oxidative）properties of power cable insulation is so important for the enhancement of the reliability and service life of HVDC cable.Based on these,polyethylene and polypropylene which are used widely in the power cable insulation were used to prepare for various composites.The electrical,thermal and mechanical properties of the proposed composites were investigated and evaluated.In chapter 2,a novel hindered phenol antioxidant was prepared by using graphene oxide（GO）as solid support to immobilize reactive antioxidant.Methyl 3-（3,5-di-tert-butyl-4-hydroxyphenyl）propionate（AO）was firstly reacted with ethylenediamine（EDA）to prepare reactive antioxidant.Then AO-functional reduced graphene oxide（RGO-AO）was obtained by surface modified of reactive antioxidant and reduction with dopamine.Fourier-transform infrared（FT-IR）spectra,X-Ray Diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）measurements confirmed that reactive antioxidant was covalently grafting onto the graphene oxide surface.The low density polyethylene based AO-functional reduced graphene oxide composites with different filler content were fabricated.The thermal oxidative stability of RGO-AO/LDPE composites was investigated with oxidative induction time（OIT）and isothermal TGA curves.The results revealed that the thermal oxidative stability of RGO-AO/LDPE composites was much better than pure LDPE.The filler of RGO-AO/LDPE could not only improve the OIT valve of LDPE,but also retard the degradation of LDPE at high temperature.Besides,RGO-AO/LDPE composites exhibited excellent electrical insulating properties for dielectric applications.In chapter 3,the surface modification of MgO nanoparticles was accomplished by introducing trialkoxysilanes with different terminal alkyl chain lengths.Fourier-transform infrared（FT-IR）spectra,solid-state ¹³C nuclear magnetic resonance（NMR）spectra and X-ray photoelectron spectroscopy（XPS）were employed to confirm the successful attachment of these trialkoxysilanes on the MgO nanoparticles.The Low density polyethylene（LDPE）and cross-linked polyethylene（XLPE）based nanocomposites with 1 wt.%MgO nanoparticles（as-received and silane functionalized）were fabricated by melt blending and the thermal,mechanical properties,space charge distribution and DC breakdown strength of the proposed nanocomposites were investigated and evaluated.The results showed that the initial degradation temperature(T_ini)and the maximum thermal decomposition temperature(T_max)of polyethylene based MgO nanocomposites were higher than pure polymer.Besides,MgO nanoparticles could significantly suppress the space charge of polyethylene especially for the XLPE.In chapter 4,the propylene-ethylene copolymer based nanocomposites with unmodified and modified MgO nanoparticles were fabricated by melt blending through two screw extruder.SEM and TEM images showed that the surface modification with trialkoxysilanes of MgO could improve the dispersibility of nanopariticles in the polymer matrix.The elongation at break and tensile strength of propylene-ethylene copolymer and its nanocomposites are all above 1000%and 40 MPa,respectively.These results indicate that the proposed nanocomposites are all flexible and strong enough for power cable insulation application.Besides,the modified MgO could suppress the space charge of propylene-ethylene copolymer to some extent.However,the influence of the alkyl chain lengths of the silane coupling agents seems ambiguous.With excellent thermal,mechanical and electrical properties,the propylene-ethylene copolymer and its nanocomposites are promising to be used as thermoplastic HVDC cable insulation.