Thanks to the performance improvement introduced by micro sized functional fillers, application of epoxy composites for electrical insulation purposes has become popular. This paper investigates the dielectric properties of epoxy micro-composites filled with alumina (Al₂O₃). In particular, measurements of relative permittivity, dissipation factor, and electrical breakdown are performed, and a comprehensive statistical analysis on dielectric properties was conducted. AC breakdown strength (AC-BDS) was analyzed for normal distribution using four methods (Anderson–Darling, Shapiro–Wilk, Ryan–Joiner, and Kolmogorov–Smirnov). In addition, the AC-BDS was analyzed at risk probabilities of 1%, 5%, 10%, and 50% using Weibull distribution functions. Both normal and Weibull distributions were evaluated using the Anderson–Darling (A-D) statistic and p-value. Additionally, the log-normal, gamma, and exponential distributions of AC-BDS were examined by A-D goodness-of-fit test. The hypothesis test results of AC-BDS were fit by normal and Weibull distributions, and the compliance was evaluated by p-value and each method statistics. In addition, the experimental results of AC-BDS were fit by log-normal and gamma distributions, and the goodness-of-fit was evaluated by p-value and A-D testing. On the other hand, exponential distribution was not suitable for p-value and A-D testing. The results showed that the distributions of AC-BDS were the best using log-normal distribution. Meanwhile, statistical analysis results verified the apparent effect of temperature on dielectric properties using a paired t-test. The analysis results of this paper not only contribute to better characterization of epoxy/Al₂O₃ micro-composites but also introduce a comprehensive approach for performing statistical analysis for electrical insulation materials.

This study examined the thermomechanical properties and electrical insulation performance of bio-epoxy composites filled with micro-sized SiO ${}_{\mathbf {2}}$ under 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 SiO ${}_{\mathbf {2}}$ /bio-epoxy composites through post-curing effects, whereas prolonged aging causes structural breakdown and deterioration of the electrical insulation performance.