Water-Tree Resistability of UV-XLPE from Hydrophilicity of Auxiliary Crosslinkers

Abstract

The water-resistant characteristics of ultraviolet crosslinked polyethylene (UV-XLPE) are investigated specially for the dependence on the hydrophilicities of auxiliary crosslinkers, which is significant to develop high-voltage insulating cable materials. As auxiliary crosslinking agents of polyethylene, triallyl isocyanurate (TAIC), trimethylolpropane trimethacrylate (TMPTMA), and N,N′-m-phenylenedimaleimide (HAV2) are individually adopted to prepared XLPE materials with the UV-initiation crosslinking technique, for the study of water-tree resistance through the accelerating aging experiments with water blade electrode. The stress–strain characteristics and dynamic viscoelastic properties of UV-XLPE are tested by the electronic tension machine and dynamic thermomechanical analyzer. Monte Carlo molecular simulation is used to calculate the interaction parameters and mixing energy of crosslinker/water binary systems to analyze the compatibility between water and crosslinker molecules. Water-tree experiments verify that XLPE-TAIC represents the highest ability to inhibit the growth of water-trees, while XLPE-HAV2 shows the lowest resistance to water-trees. The stress–strain and viscoelastic properties show that the concentration of molecular chains connecting the adjacent lamellae in amorphous phase of XLPE-HAV2 is significantly higher than that of XLPE-TAIC and XLPE-TMPTMA. The molecular simulation results demonstrate that TAIC/water and TMPTMA/water binary systems possess a higher hydrophilicity than that of HAV2/water, as manifested by their lower interaction parameters and mixing free energies. The auxiliary crosslinkers can not only increase the molecular density of amorphous polyethylene between lamellae to inhibit water-tree growth, but also prevent water molecules at insulation defects from agglomerating into micro-water beads by increasing the hydrophilicity of auxiliary crosslinkers, which will evidently reduce the damage of micro-water beads on the amorphous phase in UV-XLPE. The better compatibility of TAIC and water molecules is the dominant reason accounting for the excellent water resistance of XLPE-TAIC.

Water-Tree Resistant Characteristics of Crosslinker-Modified-SiO2/XLPE Nanocomposites

“…The molecular chains in amorphous phase of XLPE matrix bear mechanical stress from water micro-beads, as manifested by the macroscopic material tenacity, which should be enhanced to restrain water-tree development. The preferable compatibility of TMPTA with water can restrict water molecules from accumulating into micro-beads at polymer structure defects, thus reducing the damage from the impact of water micro-beads under AC electric field [ 25 ]. As schematically illustrated in Figure 8 , TMPTA groups on surfaces of TMPTA-s-SiO 2 nanofillers provide many chemical cross-linking points, which make TMPTA-s-SiO 2 nanoparticles as central nodes being chemically introduced into XLPE crosslinking network.…”

Dielectric Characteristics of Crosslinked Polyethylene Modified by Grafting Polar-Group Molecules

“…Graft modification is a molecular-level technology of chemically introducing specific functional groups into polymer backbone to promote intrinsic electrical properties of insulation materials used for fabricating HV power cables. Organic compounds with polar groups, such as carbonyl (C=O), used for graft modifications, are competent in reducing charge carrier mobility, ameliorating space charge characteristics, and acting as inhomogeneous nucleation centers to increase polyethylene spherulite density, which account for the holistic improvements in electric-tree resistant performance, insulation strength, and water-tree aging resistance, respectively [ 29 , 30 , 31 , 32 , 33 ]. It is a comprehensive scheme of suppressing the space charge accumulations and enhancing electric resistance of ethylene/α-alkenes copolymers by chemically introducing polar groups, such as ethyl, hydroxyl, nitro, cyanyl, or aromatic ring to improve the electrical performances of power cables under a polar reversal.…”

Water-Tree Characteristics and Its Mechanical Mechanism of Crosslinked Polyethylene Grafted with Polar-Group Molecules

“…Accordingly, the amorphous molecular-chains of XLPE suffer continuous impacts of water micro-droplets filled in micro-cracks under AC electric-stress to be further broken, leading to the enlargement of micro-cracks full of water micro-droplets and forming water-fill channels, which will randomly extend along electric field [ 7 ]. Compared to amorphous regions, the polyethylene molecular-chains assemble into condensed periodic lattices with much higher stability in crystalline regions, so water-trees are only initiated and propagate in amorphous regions of XLPE material [ 8 ]. Water-trees consist of tree-shaped channels of water-filling defects produced inside XLPE insulation under electric field in water-bath environments, which is classified into bow-tie and tube types according to their macroscopic shape [ 9 ].…”

“…It has been reported that the larger size and fewer grains of XLPE spherulites conduce to a higher speed of water-tree propagation [ 14 ]. Employing elastomer (SEBS and EVA), inorganic nanofiller (nano MgO) or auxiliary crosslinker (HAV2) for XLPE modifications can ameliorate crystallization morphology, increase crystallinity or crosslink-degree so as to improve water-tree resistance and electrical insulation performances of XLPE material [ 8 , 15 , 16 , 17 ].…”