Effect of Crosslinking Degree on Water-Tree Aging Characteristics of XLPE/OMMT Nanocomposites

The increasing voltage level and complex operating environment of power systems have placed more and higher demands on cable insulation materials. The insulation materials should have superior electrical, thermal and mechanical properties when working in extreme environments with moisture and strong electric fields. The polymer-based nanocomposite dielectric has gained much attention for its unique structure and excellent electrical, mechanical and chemical properties. The performance of nanocomposites depends on the stability of the microstructure morphology formed by nanoparticles and crosslinking. In order to investigate the effect of crosslinking degree on the water-tree aging characteristics of nanocomposites from the microstructure aspect, two kinds of crosslinked polyethylene/montmorillonite (XLPE/OMMT) nanocomposites with different crosslinking degrees were prepared in this paper by regulating the crosslinking time, and the accelerated water-tree aging experiment was conducted. Firstly, the water-tree morphology was observed by the polarized light microscopy. The length and initiation probability of water trees were calculated, and the gel content was tested to characterize the crosslinking degree of the nanocomposites. Secondly, the changes of the chemical composition of the specimens before and after aging were analyzed by the Fourier Infrared Spectroscopy (FTIR). The carbonyl index and methylene index that characterize the aging degree of the nanocomposite specimens were calculated. Finally，the scanning electron microscope (SEM) was used to compare the changes of the crystal morphology in the water-tree area and the nonwater-tree area before and after aging. The damage of the water-tree growth to the crystal structure was also studied. The experimental results show that the crosslinking degree of nanocomposites affects the initiation probability of water trees. The water-tree morphology of the specimens in the positive crosslinking state is sparse, whose fractal dimension and duty cycle is small. The electrochemical degradation phenomenon exists during the aging process of the specimens, and the carbonyl group increases. An appropriate crosslinking degree can produce a more perfect three-dimensional mesh structure, with smaller variability of the crystal size of the specimens, more uniform crystal distribution, and lower degree of the crystal damage after aging. The organic montmorillonite (OMMT) forms a solid interfacial force between the layers and the matrix, which enhances the molecular chain toughness. The barrier effect of OMMT and the perfect crosslinked network work together to hinder the diffusion and aggregation of water molecules, inhibit the generation of microscopic water gaps, and improve the water trees resistance of nanocomposites. © 2023 Editorial Department of Journal of Sichuan University. All rights reserved.