Tracking and Erosion Resistant Cellulose-Silica Co-doped Silicone Rubber Composites for Oil Filled Transformer Bushing Applications

Silica doped room temperature vulcanized (RTV) silicone rubber microcomposites are widely used as surface coatings for porcelain based transformer bushings due to their excellent hydrophobicity and anti-pollution performance. However, prolonged exposure to transformer oil degrades these coatings making them susceptible tracking and erosion. Incorporation of cellulose fillers suppresses oil penetration by creating physical entanglements and tortuous diffusion pathways within the silicone rubber matrix. In this context, the present study investigates cellulose-silica co-doped RTV silicone rubber composites designed to mitigate tracking after aging in natural ester oil. Composite formulations were prepared with a fixed silica microparticle content of 30 wt.% and varying micro size cellulose loadings of 10 wt.%, 15 wt.% and 20 wt.% along with unfilled silicone rubber as a reference. Accelerated aging was carried out in natural ester oil at 80 °C for 200 hours. Post aging mass analysis and Fourier transform infrared spectroscopy revealed a progressive reduction in oil absorption and chain scission in both the silicone rubber backbone (Si-O-Si) and side chains (Si-CH₃) as the cellulose content increased from 10 to 15 wt.%. The composite containing 15 wt.% cellulose exhibited the lowest weight gain and minimal chemical degradation. Thermogravimetric analysis and pyrolysis studies further demonstrated enhanced residual mass and smoother pyrolyzed surfaces with fewer cracks indicating improved thermal stability after aging. IEC 60587 inclined plane tracking and erosion testing confirmed that the aged composite co-doped with 30 wt.% SiO₂ and 15 wt.% cellulose successfully passed the test, demonstrating its suitability for oil-exposed transformer bushing insulation applications.