Electrical surface resistance, hydrophobicity and diffusion phenomena in PVC

Polymeric materials are used increasingly in both outdoor and indoor insulation and it has become imperative to find a convenient and a practical method to evaluate the performance of the insulation in situ. The hydrophobicity of a polluted surface particularly in the presence of moisture, determines the level of the leakage current which may result in a flashover and an outage of the power system. However, it is difficult to measure the hydrophobicity of insulators in the field, and therefore the measurement of the electrical surface resistance in situ has been suggested as an alternative method that might give information on the surface state. In the present study, polyvinylchloride (PVC) was used to study the characteristics of the surface resistance, the change of the hydrophobicity and the relationship between both of them in the presence of salt-fog. The dependence of the surface resistance on factors such as the duration of the wetting in salt-fog, the recovery during drying time, the length of the specimens, the level of the applied dc stress used to measure the resistance and the ac stress is reported. The surface free energy per unit area of PVC, during exposure to salt-fog, was calculated using the harmonic-mean method and was found to be consistent with the changes in both the surface resistance and the hydrophobicity of the surface. The surface tension γ_s, increased from 43.1×10^-3 J/m² for the virgin specimen to 76.8×10^-3 J/m² after complete wetting in un-energized salt-fog. The diffusion coefficients of a saline solution having a conductivity of 1 mS/cm into the pvc were found to increase from 2.8×10^-15 m²/s at 74°C to 1.6×10^-14 m² /s at 98°C