Low-molecular weight silicone fluid in RTV silicone rubber coatings

Coatings of room-temperature vulcanized (RTV) silicone rubber (SIR) are used increasingly on outdoor HV porcelain and glass insulators and bushings when subjected to heavy pollution. An important property of the RTV-SIR coating is its ability to restore the hydrophobicity of the surface after a layer of contamination has built up on it, and thus continues to suppress the development of leakage current, dry band arcing and flashover. This is accomplished by the diffusion of low molecular weight (LMW) silicone fluid from the bulk to the surface of the coating and then to the top of the contamination layer The presence of sufficient LMW silicone fluid in the bulk of the coating is a critical factor in both the recovery of hydrophobicity and its long term maintenance under wet and polluted conditions. This paper reports on several factors which affect the level of LMW silicone fluid present in RTV-SIR coatings. Different formulations of the RTV-SIR were used, The percentage weight of the LMW silicone fluid was determined in RTV-SIR coatings as a function of the size of the particles of the alumina trihydrate (ATH) filler which was varied from 1 to 75 mu m, the concentration of the ATH filler from 50 to 150 pph of the base silicone rubber, the addition of 10%wt silicone fluid to the formulation of RTV-SIR, the carrier solvents of 1,1,1 trichloroethane and naphtha, exposure to continuous wind at a speed of 7.45+/-0.25 km/h for 22 months, exposure to stationary air for 14 months,immersion in low salinity water simulating rain (70 mu S/cm) for 14 months and tests in energized low conductivity salt-fog. The mechanisms responsible for the observed phenomena are suggested and discussed.