| Crosslinked polyethylene(XLPE)is widely used in the insulation of medium and high voltage power cables due to its excellent electrical properties,heat resistance,and environmental stress cracking resistance.However,power cables are subjected to the combined effects of electricity,heat,and force during operation.The insulation of XLPE cables laid in a humid environment is prone to water tree,which will cause further electric tree generation and insulation breakdown failures,which will seriously affect reliability of electricity supply.Therefore,improving the water tree resistance of XLPE insulation material is of great significance to the safe operation of the power system.In this paper,polyethylene/montmorillonite(PE/OMMT)and crosslinked polyethylene/montmorillonite(XLPE/OMMT)nanocomposites with different mass fractions were prepared by melt blending method,after which the accelerated water tree ageing experiment were conducted by using needle electrode and real-time observation system.Starting from the micro structure,study the effect of crosslinking behavior on the mechanical properties of nanocomposites and the resistance to water tree.According to the test results of tensile mechanical properties of nanocomposites,it is found that the XLPE with OMMT plays a role of physical crosslinked point and stress dispersion.OMMT and the three-dimensional network structure formed by crosslinked effectively limits the movement of molecular segments and improve the tensile strength of XLPE/OMMT nanocomposite increases the toughness and fracture energy.The dynamic mechanical analysis(DMA)shows that the storage modulus of nanocomposites decreases with increasing temperature.The OMMT of dispersing uniformly,which double functions of lamellar heat resistance and physical support improves the storage modulus of the nanocomposite at high temperature.The accelerated water tree ageing of nanocomposite were conducted by using needle electrode and real-time observation system,and to study the initiation probability,growth morphology and initiation time of the nanocomposite water tree.The experimental results show that the initiation time of water tree is shorter in nanocomposite,the initiation probability and the length of water-tree in XLPE/OMMT decrease comparing to PE/OMMT.When the mass fraction of OMMT is 2%,the performance of inhibiting water tree is the best.The synergistic effect of the inhibition of the crosslinked mesh structure on molecular chain movement and the lamellar barrier effect of OMMT significantly inhibited the growth of water tree.Differential scanning calorimetry(DSC)was used to analyze the melting behavior and crystallization behavior of the nanocomposite before and after aging,and it was found that the crystallization ability of the water tree area became worse,the crystallinity decreased,and the cross-linking structure restricted the amorphous area and the molecular chain.The delay of segment relaxation restricts the diffusion of water molecules in the nanocomposite.Through continuous self-nucleation annealing(SSA)analysis,it is found that the crystal size distribution of the XLPE/OMMT composite material with an OMMT content of 2% is uniform,the structure is more perfect,the crosslinked makes the microstructure dense,the water tree channel narrows,and the barrier effect is enhanced.The crystallizability of the water-tree region degrades,decreasing the crystallinity and the lamellae thickness.It can be seen that the growth of water-tree causes the destruction of molecular chains and crystal structures resulting in local cracks,indicating that the ageing of water-tree is a process of microscopic degradation.The results of infrared spectroscopy(FTIR)showed that the carbonyl index in the water tree region of the two nanocomposites increases,indicating that the water tree ageing was the result of electrochemical degradation. |
