The Effect Of Molding Methods On The Properties Of XLPE/OMMT Nanocomposites Dielectric

Crosslinked polyethylene(XLPE)cables have a simple structure,convenient laying,excellent electrical insulation performance,stress impact resistance,and heat resistance,and have been widely used in electrical insulation.However,cables are prone to mechanical vibration and external force impacts during manufacturing,transportation,installation,and other processes.After being put into operation,they are also difficult to avoid the combined effects of electricity,heat,force,and external environmental factors(such as humid environments),which may cause changes in the dielectric and physicochemical properties of insulation materials.The changes in macroscopic properties are closely related to changes in microstructure.Nanometer doped polymer can make composite materials superior to traditional polymers in electrical,thermal and mechanical properties.The molding and preparation process of polymer will change the orientation and dispersion state of nanoparticles,thus affecting the interaction between nanoparticles and polymer matrix,resulting in differences in macro performance.In the current material preparation methods used,the factory production of cable insulation cores generally adopts a continuous three-layer co extrusion method,while laboratory research on insulation materials generally adopts the molding method of a flat vulcanizing machine.The equivalence or difference in performance between the two molding methods is worth exploring.In order to compare the differences in material properties between the two molding methods,this article prepared cross-linked polyethylene/organically modified montmorillonite nanocomposite dielectrics using compression molding method and extrusion molding method,respectively.The mechanical properties,resistance temperature characteristics,relative dielectric constant,dielectric loss tangent,power frequency breakdown field strength,and water tree aging characteristics of the material were tested.The microstructure and chemical composition of the nanocomposite dielectric were analyzed and characterized using X-ray diffraction(XRD),scanning electron microscopy(SEM),Fourier transform infrared spectroscopy(FTIR),respectively.A study on the mechanical properties of nanocomposite dielectrics found that moderate doping with Organic montmorillonite(OMMT)can improve the elasticity and toughness of the composite material.The extrusion effect causes the molecular chain segments and OMMT particles to be arranged in a regular manner along the stress direction,enhancing the impact of external forces on the sample.When the OMMT doping amount is 0.5%,the elastic modulus,tensile strength,and elongation at break of the extruded sample all reach their maximum values,resulting in the best mechanical properties.The dielectric properties of nanocomposite dielectrics were tested.The tensile stress during the extrusion molding process oriented the OMMT layers in the sample along the tensile direction,forming a regular arrangement unit that hindered the migration of charge carriers,thereby improving the volume resistivity of the sample;The movement of polymer molecular segments is limited by the interlayer orientation,resulting in a decrease in the polarization of the dipole and a decrease in the dielectric constant in the sample;At the same time,the effective intercalation of OMMT and the hybrid structure formed by the polymer result in diffuse reflection of electrons,prolonging the electron motion path,and making the breakdown field strength of the extruded sample better than that of the pressed sample.The interlayer spacing of OMMT in Montmorillonite,OMMT,masterbatch,and nanocomposite dielectrics was measured using small angle X-ray diffraction.The results showed that the OMMT layers reached a stripping dispersion state in the polymer matrix,and the interlayer spacing of OMMT in the extruded sample was slightly higher than that in the pressed sample;Scanning electron microscopy observation of the crystal morphology of the sample revealed that the heterogeneous nucleation of OMMT particles increased the crystallinity of the sample,and the order of the extruded polymer molecular chains along the tensile direction was enhanced,further increasing the crystallinity.Conduct accelerated water tree aging experiments on nanocomposite dielectrics,observe the growth status and microstructure of water trees,and calculate the probability of water tree initiation and growth length.The results showed that the growth morphology of water trees in the sample changed significantly after the addition of OMMT,and the growth length of water trees decreased significantly along the electric field direction and perpendicular to the main electric field direction.When the OMMT doping amount was0.5%,the best effect was to suppress water trees.Compared with compressed samples,the growth length of water tree branches in extruded samples is shorter,and the blocking effect is influenced by the molding method.The extrusion molding method arranges the OMMT layers perpendicular to the main electric field direction,which can significantly block the growth of water tree branches.The chemical composition of samples prepared by different molding methods is similar;After the aging of the sample,the carbonyl index increases,the methylene index decreases,and the chemical composition of the nanocomposite dielectric changes significantly,indicating that electrochemical degradation occurs during the water tree aging process.