Effects of High-Temperature Thermal Aging on the Thermomechanical and Surface Flashover Performance of Bio-Epoxy Composites

This study examined the thermomechanical properties and electrical insulation performance of bio-epoxy composites filled with micro-sized SiO2under long-term thermal aging. The composites were prepared from a mixture of petroleum-based diglycidyl ether of bisphenol-A (DGEBA) and bio-based epoxidized soybean oil (ESO) with 65 wt.% untreated micro-silica filler. Accelerated thermal aging was performed at 200 °C for 1000, 1500, and 2000 h, and the structural, thermal, and electrical properties were analyzed systematically. Fourier transform infrared spectroscopy revealed the degradation of epoxy backbones and hydroxyl groups with prolonged aging. Thermogravimetric and dynamic mechanical analyses showed that the decomposition temperature and crosslink density increased up to 1500 h, indicating post-curing and network densification, but decreased after 2000 h because of thermal degradation. Surface contact angle (SCA) measurements revealed sustained hydrophobicity, peaking at 1500 h before declining, which is consistent with the bulk degradation trend. The AC flashover strength showed a similar pattern, increasing by approximately 2% after 1000–1500 h but decreasing by 4% at 2000 h. A statistical evaluation confirmed that the AC flashover data followed normal and Weibull distributions. These findings suggest that short-term thermal exposure enhances the reliability of SiO2/bio-epoxy composites through post-curing effects, whereas prolonged aging causes structural breakdown and deterioration of the electrical insulation performance.