Dielectric, Mechanical, and Breakdown Properties of Liquid Silicone Rubber Under Different Thermal Aging Conditions

To investigate the effect of thermal aging on the liquid silicone rubber (LSR) used for dry-type transformers, various LSR samples are aged at different temperatures of $225~^{\circ }$ C, $250~^{\circ }$ C, $275~^{\circ }$ C, and $300~^{\circ }$ C, with different aging times of 24, 48, 96, 336, and 504 h. The experimental results show that the aging process of LSR can be roughly divided into two stages depending on the result of competition between crosslinking reaction and fracture reaction, as evidenced by the FTIR. In the early stage, dc conductivity and integral charge Q(t) decrease, while the breakdown strength increases with the aging time increases, which are attributed to the more compact network structure caused by crosslinking reaction. The later stage is dominated by facture reaction with the deepening of thermal aging. Dielectric properties begin to show a decreasing trend. The breakdown strength could be reduced by 16.5% at $300~^{\circ }$ C with 504 h. Meanwhile, the mechanical properties of the tensile strength and the elongation at break are decreased by 73.2% and 85.2% after 504 h thermal aging at $504~^{\circ }$ C, respectively. Dielectric spectra are investigated under different aging conditions, and differential processing is performed to distinguish the different aging state of the LSR. The results show that the loss peak shifts toward the low frequency as the aging degrees increase. This article aims to provide a theoretical basis for the application of LSR for dry-type transformers.