Effect Of Crosslinking Degree On Properties Of XLPE/OMMT Nanocomposite Dielectric

Crosslinked polyethylene(XLPE)is widely used in the insulation layer of power cables because of its excellent electrical,mechanical and heat-resistant properties.However,the actual operating environment of cables is complex and changeable,which will be influenced by various external factors,and some physical and chemical changes will occur,which will lead to the decline of insulation performance and threaten the safe operation of power systems.Therefore,higher requirements are put forward for the insulation performance of cables.The appearance of polymer-based nanocomposites provides a new idea for the development and research of electrical insulation materials.Polymer matrix determines the intrinsic properties of composite dielectric,and good dispersion of nanoparticles is beneficial to improve the properties of composite dielectric.However,the degree of cross-linking of polymer will lead to the uncertainty of nano-particle dispersion,which will affect the combination of phase interface between nano-particles and polymer matrix,resulting in differences in macro-properties of nano-composite dielectrics.The organic montmorillonite(OMMT)with mass fraction of 0.5% was selected to modify polyethylene,and XLPE/OMMT nanocomposites with different crosslinking degrees were prepared by adjusting the crosslinking time.Combined with the interface characteristics formed by nano-dispersed OMMT,the relationship between the crosslinking process and the microstructure of nano-composite dielectric,and studies the influence of crosslinking degree on the mechanical properties and dielectric properties of nano-composite dielectric.Through accelerated water tree aging test,the differences of water tree morphology,changes of chemical composition and crystal morphology between samples with different crosslinking degrees were analyzed,and the inhibition mechanism of crosslinking degree on water tree growth of nano-composite dielectric was discussed.According to the results of small angle X-ray diffraction(XRD),the interlayer spacing of OMMT is as high as 5.73 nm,which indicates that OMMT has been exfoliated and dispersed in the matrix.The results of wide-angle XRD,gel content and scanning electron microscope(SEM)show that with the increase of crosslinking time,the crosslinking degree of composite media first increases and then decreases.When the cross-linking time is 15～20 min,the composite dielectric is in a positive cross-linking state.At this time,the gel content of the medium exceeds 80%,which meets the standard requirements.In this state,the crystal distribution is the most uniform.When the cross-linking time exceeds 20 min,the sample is in a cross-linking state.At this time,the crystallinity and crystal size of the sample are obviously decreased,and the number of crystals is greatly increased.The mechanical properties show that the exfoliated and dispersed OMMT acts as a physical cross-linking point in XLPE/OMMT,which works together with the continuously formed cross-linking bonds to make the cross-linking network inside the medium more perfect,improve the tensile strength and elastic modulus of the nanocomposite dielectric,and enhance its toughness.The dielectric properties were tested,and the results showed that the conductivity of nanocomposite dielectric increased with the increase of temperature.In the positive cross-linking state,the larger interface area formed between OMMT and matrix increases the barrier energy level and the activation energy of composite dielectric.With the increase of crosslinking degree,strong intermolecular force is formed,which hinders the steering polarization of dipole and side group,and reduces the dielectric constant and dielectric loss tangent.The addition of OMMT and the uniform and compact cross-linked structure improve the breakdown field strength of the composite dielectric,but in the state of over-cross-linking,the micro-defects increase and the breakdown field strength of the dielectric decreases.The accelerated aging test of water treeing shows that the crosslinking degree of nano-composite dielectric affects the initiation probability of water treeing,and the water treeing in the positive crosslinking state is short in length,sparse in morphology and small in fractal dimension and duty cycle.Fourier infrared spectrum analysis shows that there is electrochemical degradation and carbonyl groups increase during the aging process of the sample.Comparing the crystal structure before and after aging,it can be seen that the proper degree of crosslinking makes the three-dimensional network structure more perfect,the crystal size difference of the sample is small,the crystal distribution is uniform,and the degree of crystal damage after aging is low.