Physiochemical and Thermal Assessment of Mineral oil Aged Silicone Rubber Nano/Micro Composites

Room temperature vulcanized (RTV) silicone rubber coatings are commonly used as an external insulating material for transformer oil immersed ceramic-based porcelain bushings. Their superior hydrophobicity reduces dry band arcing, resulting in lower leakage current on the material's surface. However, these coatings can undergo significant degradation when exposed to transformer mineral oil. This paper examines the effect of aged mineral oil on silicone rubber composites doped with micro and nano fillers of Alumina Trihydrate (ATH) and Boron Nitride (BN). Silicone rubber composites of 40 wt.% of micro ATH and 5 wt.% of nano BN were synthesized and thermally aged in mineral oil at 40oC and 80oC for 200 hours. Performance evaluation was conducted using the Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and inclined plane tracking (IPT) test as per IEC 60587. The results showed that specimens aged at 80°C underwent more significant degradation compared to those aged at 40°C. FTIR analysis showed a significant breaking of chemical bonds in the micro ATH silicone rubber composite than the nano BN composite. The TGA results also indicated that the nano BN silicone rubber composite demonstrated higher thermal stability, which decreased as the ageing temperature increased from 40°C to 80°C. Further, the IPT test results showed a decline in tracking and erosion resistance for the specimens aged at 80°C. The results suggest that the properties of silicone rubber composites degraded more as the ageing temperature increased from 40oC to 80oC. Furthermore, the BN doped filler with silicone rubber demonstrated enhanced resistance degradation caused by mineral oil ageing.