Study on Temporal Variation in Shape of Water Droplet on Silicone Rubber Surface Treated by Surface Discharge

Silicone rubber (SiR) is used for housing material of polymer insulators. This study focuses on the temporal variation in the shape of a water droplet on the SiR surface treated by surface discharge (SD). The SD treatment is used as a controlled electrical stress to investigate how the surface condition and the hydrophobicity of SiR change with aging. A 7 μL distilled water droplet is dropped on unused, 18-year-aged, and 20-year-aged specimens, and the contact angles (θ) were recorded for 600 s to obtain θ –tcurves. The obtained curve is divided into four stages: primary maintenance, primary reduction, secondary maintenance, and secondary reduction. The unused specimens show a clear plateau in a secondary maintenance stage because low-molecular-weight (LMW) siloxanes may be dispersed from the SiR bulk to the surface and temporarily restore hydrophobicity. In aged specimens, θ decreases continuously without recovery. The continuous droplet observation after the SD treatment is a simple and effective method to evaluate the degradation of SiR.