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

Abstract

In order to restrain electric-stress impacts of water micro-droplets in insulation defects under alternating current (AC) electric fields in crosslinked polyethylene (XLPE) material, the present study represents chemical graft modifications of introducing chloroacetic acid allyl ester (CAAE) and maleic anhydride (MAH) individually as two specific polar-group molecules into XLPE material with peroxide melting approach. The accelerated water-tree aging experiments are implemented by means of a water-blade electrode to measure the improved water resistance and the affording mechanism of the graft-modified XLPE material in reference to benchmark XLPE. Melting–crystallization process, dynamic viscoelasticity and stress-strain characteristics are tested utilizing differential scanning calorimeter (DSC), dynamic thermomechanical analyzer (DMA) and electronic tension machine, respectively. Water-tree morphology is observed for various aging times to evaluate dimension characteristics in water-tree developing processes. Monte Carlo molecular simulations are performed to calculate free-energy, thermodynamic phase diagram, interaction parameter and mixing energy of binary mixing systems consisting of CAAE or MAH and water molecules to evaluate their thermodynamic miscibility. Water-tree experiments indicate that water-tree resistance to XLPE can be significantly improved by grafting CAAE or MAH, as indicated by reducing the characteristic length of water-trees from 120 to 80 μm. Heterogeneous nucleation centers of polyethylene crystallization are rendered by the grafted polar-group molecules to ameliorate crystalline microstructures, as manifested by crystallinity increment from 33.5 to 36.2, which favors improving water-tree resistance and mechanical performances. The highly hydrophilic nature of CAAE can evidently inhibit water molecules from aggregating into water micro-droplets in amorphous regions between crystal lamellae, thus acquiring a significant promotion in water-tree resistance of CAAE-modified XLPE. In contrast, the grafted MAH molecules can enhance van der Waals forces between polyethylene molecular chains in amorphous regions much greater than the grafted CAAE and simultaneously act as more efficient crystallization nucleation centers to ameliorate crystalline microstructures of XLPE, resulting in a greater improvement (relaxation peak magnitude increases by >10%) of mechanical toughness in amorphous phase, which primarily accounts for water-tree resistance promotion.

Electrical-Tree Resistant Characteristics of SEBS and Nano-SiO2 Modified Polypropylene Composites

“…17 To determine the specific temperature at which in the thermal elongation test of PP and its composites are performed, the differential scanning calorimetry (DSC) is employed to test heat-flow from and out-off samples when being gradually heated/cooled by a rate of 5 °C min −1 in nitrogen atmosphere, as implemented in differential scanning calorimeter (DSC-3, METTLER TOLEDO, Zurich, Switzerland). 18 The results from DSC indicate that the melting points of PP cable material and its two composites are approximately 160 °C, as shown in Fig. 1.…”

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.…”

Molecular Dynamics Simulations on Epoxy Resin Composite via Grafting Acryloyl-chloride to Inhibit Electron Transport and Improve Thermal-mechanical Properties

“…Organic compounds with polar functional groups used for grafting modification, such as carbonyl (C=O), can improve space charge characteristics, reduce charge carrier mobility, and act as non-z E-mail: zhliu@ytu.edu.cn; wang_lei@btbu.edu.cn ECS Journal of Solid State Science and Technology, 2024 13 053007 uniform nucleation centers to increase the density of polyethylene spherulites, which respectively demonstrate the overall improvement of water tree aging resistance, electrical tree resistance and insulation strength. [21][22][23][24] This is a method of improving the electrical performance of power cables under polarity reversal by chemically introducing polar groups (such as hydroxy,lethyl, cyano,nitro, or aromatic rings), thereby suppressing ethylene/α-A comprehensive solution for the accumulation of space charge in olefin copolymers and the improvement of their resistance. In the crosslinking reaction initiated by polyethylene photons, the modification of 4-vinyloxyphenylacetone onto polyethylene molecular chains was successfully achieved, which can effectively prevent functional group compounds from migrating from the polymer matrix and significantly improve the DC dielectric properties of crosslinked polyethylene (XLPE).…”