Low-molecular weight silicone fluid in RTV silicone rubber coatings

Coatings of room-temperature vulcanized (RTV) silicone rubber (SIR) are use d increasingly on outdoor HV porcelain and glass insulators and bushings wh en subjected to heavy pollution. An important property of the RTV-SIR coati ng is its ability to restore the hydrophobicity of the surface after a laye r of contamination has built up on it, and thus continues to suppress the d evelopment of leakage current, dry band arcing and flashover. This is accom plished 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 contamina tion 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 it s long term maintenance under wet and polluted conditions. This paper repor ts 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 p ercentage weight of the LMW silicone fluid was determined in RTV-SIR coatin gs as a function of the size of the particles of the alumina trihydrate (AT H) 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%w t 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,imm ersion in low salinity water simulating rain (70 mu S/cm) for 14 months and tests in energized low conductivity salt-fog. The mechanisms responsible f or the observed phenomena are suggested and discussed.