Effect Of Allyl Chloroacetate And Maleic Anhydride On Water Treeing Resistance Of Cross Linked Polyethylene

It is inevitable to produce defects,interfuse impurities and infiltrate moisture into cable insulation materials during the processes of cable production,transportation and installation,which may initiate water-tree and exacerbate its growth that will further evolve into electric-trees,resulting in electrical breakdowns and insulation failures.In high interest of restraining the water micro-beads aggregated by water molecules from bearing electric-stress impacts on the insulation defects under AC electric field in XLPE insulation materials,the present study performs the chemical grafting modifications with the peroxide melting approach to individually introduce two specific polar-group molecules of acrylene chloride（CAAE）and maleic anhydride（MAH）into XLPE materials,which could render heterogeneous nucleation centers of polyethylene crystallization to ameliorate crystalline structures and thus accounts for the improvements of water-tree resistance and mechanical performances.Further,hydrophilia of the grafted polar-groups can evidently inhibit the water-molecules from aggregating into water micro-beads in the amorphous regions between crystal lamellae,thus acquiring a significant promotion in water-tree resistance.Besides,the grafted polar-groups can provide densely distributed charge-traps to appreciably improve dielectric performances.The accelerated water-tree aging experiments are implemented by means of a water-blade electrode to measure the acquired water-resistances and the affording mechanisms of the graft-modified XLPE materials in reference to the benchmark XLPE without grafting.Water-trees are initiated and growing under high AC electric fields with a high frequency,and the water-tree morphology is observed for various aging time to evaluate the dimension characteristics in water-tree developing processes.Melting-crystallization features,dynamic viscoelastic properties and stress-strain characteristics of the modified materials are tested respectively utilizing differential scanning calorimeter（DSC）,scanning dynamic thermomechanical analyzer（DMA）and electronic tension machine.Monte Carlo molecular simulations are performed to calculate free-energy,thermodynamic phase diagram,interaction parameter and mixing energy of the binary mixing systems consisting polar-group molecules and water molecules,which is competent of evaluating thermodynamic mixing compatibility between the grafted molecules with water molecules.Water-tree experiments indicate that water-tree resistance of XLPE can be effectively improved by grafting CAAE and MAH.It is consistently manifested by DSC spectra,DMA peaks and stress-strain characteristics that the grafted polar-groups can enhance Van der Waals’force between polyethylene molecules and are available as heterogeneous nucleation center for polyethylene crystallization,which lead to the increased densities of lamellae and spherulites with the reduced-volume and increased-tenacity amorphous regions between lamellae,accounting for the considerable improvement in water-tree resistance.It is indicated from Monte Carlo molecular simulations that CAAE/H₂O binary system possesses negative interaction parameters and mixing energy throughout a large temperature range,implying that water molecules will be dispersed without aggregating into water micro-beads in the modified XLPE with grafting CAAE,which alleviates electric-stress impacts on XLPE amorphous regions.The improved water-tree resistance of CAAE-modified XLPE derives dominantly from the highly hydrophilic nature of CAAE.In contrast,the grafted MAH can much greater enhance van der Waals forces between polyethylene molecules,and simultaneously act as the center of crystallizing nucleation to ameliorate crystalline micro-structures of polyethylene molecules,which result in greater improvement of mechanical toughness in the reduced amorphous regions that primarily accounting for the higher water-tree resistance than XLPE benchmark.Complex dielectric spectra,AC dielectric breakdown strengths and DC conductance of two graft-modified XLPE materials are tested in combinations of the charge-trap level distributions obtained from thermally stimulated currents（TSC）and the first-principles electronic-states calculations to investigate the dielectric polarization behaviour and the physical mechanism of inhibiting charge injection,impeding electric conduction and improving AC breakdown field.It is suggested for grafting polar-group molecules,especially for MAH,that charge traps with much deeper energy levels have been introduced in to XLPE materials,which can efficiently shield out charge injections from electrodes and scatter charge carriers in transports,as manifested by the abatement in electrical conductance and the elevation in dielectric breakdown strength.