Degradation Dynamics of Mineral Oil Aged Silica Doped Silicone Rubber Nanocomposites

Room temperature vulcanized silicone rubber (RTVSR) is widely used as a coating material for transformer bushings to provide anti-pollution resistance. However, mineral oil leakage from power transformers can significantly degrade the performance of these coatings. This study investigates the degradation dynamics of RTVSR coatings exposed to mineral oil and evaluates the potential of nano silica (SiO₂) doping to mitigate this issue, leveraging its molecular compatibility with silicone rubber. Silicone rubber (SR) doped with 1%, 3%, and 5% by weight of silica was subjected to thermal ageing in transformer mineral oil for 6 hours at 80°C. The physicochemical, surface morphological, and electrical properties of the aged specimens were analyzed using Fourier Transform Infrared Spectroscopy (FTIR), contact angle measurements, 3D microscopy, and breakdown voltage (BDV) testing. FTIR revealed molecular side-chain and backbone scission in all samples, with silica-doped specimens showing a less pronounced extent of degradation. This degradation caused reduced surface hydrophobicity, lower BDV, and increased surface roughness post-ageing. Thermal stability was further assessed using thermogravimetric analysis (TG-DTA) and infrared thermography. Residual weight, indicative of thermal stability, significantly decreased post-ageing for all specimens. Infrared thermography revealed lower heat accumulation in silica-doped specimens compared to undoped samples, although heat retention increased in all samples after ageing. Overall, the results confirm that mineral oil exposure has a detrimental effect on RTVSR coatings. However, 3 wt.% nano-SiO₂ doping significantly enhances degradation resistance, making it a promising composition for transformer bushings exposed to mineral oil leakage.