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 t he hydrophobicity of polymers intended for use in nonceramic insulators. We investigated the effect of uv radiation, corona discharges, dry-band arcin g, acid rain and water absorption on hydrophobic transfers in, and the agin g of, room temperature vulcanized (RTV)-silicone rubber (SIR), high tempera ture vulcanized (HTV)-silicone rubber (SIR), ethylene vinyl acetate (EVA) r ubber and the blends made from HTV-SIR and EVA. Acid rain had little influe nce on hydrophobicity in any of the samples. EVA and the blends with low si licone ratios to EVA allowed much larger reduction in hydrophobicity after or while subjected to the above stresses (except for acid rain) than did th e 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 silic one rubbers was attributed to low-molecular-weight silicone components that were able to migrate and finally cover the aged surface layers. Because EV A possessed less fluid components and less chemical stability under electri cal and environmental stresses, its hydrophobicity was readily reduced and hardly recovered, which led to higher current leakage and early material ag ing. For each material, the critical hydrophobic level permitting leakage c urrent to develop was determined by adding a non-ionic wetting agent to sal ine water.