Electrical and environmental stress and the hydrophobic stability of SIR, EVA and their blends

This paper treats the effects of electrical and environmental stresses on the hydrophobicity of polymers intended for use in nonceramic insulators. We investigated the effect of UV radiation, corona discharges, dry-band arcing, acid rain and water absorption on hydrophobic transfers in, and the aging of, room temperature vulcanized (RTV)-silicone rubber (SIR), high temperature vulcanized (HTV)-silicone rubber (SIR), ethylene vinyl acetate (EVA) rubber and the blends made from HTV-SIR and EVA. Acid rain had little influence on hydrophobicity in any of the samples. EVA and the blends with low silicone ratios to EVA allowed much larger reduction in hydrophobicity after or while subjected to the above stresses (except for acid rain) than did the silicone rubbers and a blend with a high silicone ratio to EVA. Salt-fog tests were conducted to correlate the hydrophobic stabilities of materials with leakage current levels thereon. The hydrophobic stability of the silicone rubbers was attributed to low-molecular-weight silicone components that were able to migrate and finally cover the aged surface layers. Because EVA possessed less fluid components and less chemical stability under electrical and environmental stresses, its hydrophobicity was readily reduced and hardly recovered, which led to higher current leakage and early material aging. For each material, the critical hydrophobic level permitting leakage current to develop was determined by adding a non-ionic wetting agent to saline water.