Electrical surface resistance, hydrophobicity and diffusion phenomena in PVC

Polymeric materials are used increasingly in both outdoor and indoor insula tion and it has become imperative to find a convenient and a practical meth od to evaluate the performance of the insulation in situ. The hydrophobicit y of a polluted surface particularly in the presence of moisture, determine s the level of the leakage current which may result in a flashover and an o utage of the power system. However, it is difficult to measure the hydropho bicity of insulators in the field, and therefore the measurement of the ele ctrical surface resistance in situ has been suggested as an alternative met hod that might give information on the surface state. In the present study, polyvinylchloride (pvc) was used to study the characteristics of the surfa ce resistance, the change of the hydrophobicity and the relationship betwee n both of them in the presence of salt-fog. The dependence of the surface r esistance on factors such as the duration of the wetting in salt-fog, the r ecovery during drying time, the length of the specimens, the level of the a pplied de stress used to measure the resistance and the ac stress is report ed. The surface free energy per unit area of pvc, during exposure to salt-f og, was calculated using the harmonic-mean method and was found to be consi stent with the changes in both the surface resistance and the hydrophobicit y of the surface. The surface tension gamma(s), increased from 43.1x10(-3) J/m(2) for the virgin specimen to 76.8x10(-3) J/m(2) 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 .8x10(-15) m(2)/s at 74 degrees C to 1.6x10(-14) m2/s at 98 degrees C.