Effect Of Orgnic Motmorillonite On Electrical Properties Of XLPE/OMMT Nanocomposite Dielectries

Crosslinked polyethylene(XLPE)cable is widely used in the insulation of medium and high voltage transmission cables because of its excellent mechanical property,thermal stability,electrical properties and etc.But with the continuous improvement of voltage level and capacity of transmission line,in order to meet the transmission demand of high voltage and large capacity of cable,higher requirements are put forward for XLPE cable.Nanoparticles in polymer based nanocomposite dielectrics have the characteristics of surface effect and volume effect.They can significantly improve the physical and chemical properties and electrical properties of compound material without changing the original properties of polymers.Therefore,nano fillers have always been the research hotpot in the field of polymer modification.Among them,the unique layered structure of layered silicate is outstanding in improving the mechanical and electrical properties of materials.However,there are few studies on the microstructure of nanocomposite dielectrics by layered silicate,and the relationship between the microstructure and electrical properties of nanocomposites still needs to be further discussed.In this paper,cross-linked polyethylene/organic montmorillonite(XLPE/OMMT)nanocomposite dielectrics were prepared by melt blending using maleic anhydride grafted polyethylene(MAH-g-PE)as compatibilizer.The effects of the content,type and particle size of organic montmorillonite on the microstructure of nanocomposites were studied.The relationship between the polymer microstructure and the tensile and electrical properties of the materials was discussed,and its mechanism was explained.The result of small angle X-ray diffraction(XRD)shows that in the nanocomposites,with the synergistic action of compatibilizers,the XLPE molecular chain is successfully inserted into the interlayer of OMMT,which significantly increases the interlayer spacing of montmorillonite and realizes the stripping and dispersion of OMMT in the polymer.The melting and crystallization processes of XLPE/OMMT nanocomposites were analyzed by differential scanning calorimetry(DSC).The result shows that OMMT can accelerate the crystallization rate,improve the crystallinity and reduce the crystal size.The content,type and particle size of OMMT have different effects on the crystallinity of the composites.When the mass fraction of OMMT is 0.5%,the crystallinity of the nanocomposites is the highest.The crystal size and distribution are the smallest.Affected by the polarity matching effect of organic intercalator and compatibilizer,OMMT2 lamellae is more evenly dispersed in the matrix,which is conducive to the formation of crystals and improve the crystallinity of the material.With the decrease of MMT particle size,the crystallinity of the polymer increases and the microstructure is more perfect.The crystallinity of XLPE/OMMT3-0.5% is the highest.The crystal morphology of nanocomposites was observed by polarizing microscope(PLM)and scanning electron microscope(SEM).It is found that the addition of OMMT reduced the crystal size and crystal size distribution of the polymer,which is consistent with the experimental result of DSC.At the same time,it is found that the mechanical properties of nanocomposite dielectrics are mainly affected by the crystallization behavior of polymer.Appropriate doping of OMMT greatly reduces the flexibility and stiffness of XLPE molecular chain,improves the tensile strength and elastic modulus of the material,and reduces the elongation at break.When the amount of organic montmorillonite is too high,it is easy to disperse unevenly and form local defects,the mechanical properties of polymers decreased.The electrical performance test shows that when the doping amount of OMMT is0.5%,the volume resistivity and breakdown field strength of XLPE/OMMT nanocomposite dielectric are the largest and the dielectric constant is the smallest.This is mainly related to the dispersion of OMMT in the matrix and the interface between organic phase(OMMT)and inorganic phase(XLPE).OMMT is evenly dispersed in the matrix,and the interface area formed with XLPE can introduce more interface structure to enhance the interface force between OMMT sheet and XLPE.Under the synergistic action of compatibilizer,OMMT2 is more evenly dispersed in XLPE,the formed interface area introduces more interface structure.And with the decrease of OMMT particle size,the interface effect between OMMT and XLPE increases.However,the deep trap energy level formed at XLPE OMMT interface is deep.With the action of electric field,the interface polarization in nanocomposite dielectric forms dipole moment,resulting in the increase of dielectric loss of nanocomposite dielectric.Excessive doping of OMMT destroys the interface structure between XLPE and OMMT,resulting in the decrease of breakdown field strength,activation energy,polarizability and loss of polar molecules.