“Electrical property modifications of polypropylene and polyethylene foils stressed with water electrodes”

Water does not wet polymers such as polypropylene or polyethylene; the measurement of the capacitance of thin foils with water electrodes shows that a gaseous layer exists at the liquid-solid interface; in this layer, partial discharges appear under -certains conditions- The addition of a wetting agent, or a long period of rest, reduce the layer mean thickness and the partial discharges. When the A.C. voltage is raised quickly, the breakdown voltage is much higher than with a slow rise of the voltage: most of the very low breakdown voltages observed are presumably caused by the field-enhanced penetration of liquid and ions into the polymer; the mechanism of charge migration cannot be described by a simple model of injection from the liquid into the solid. Localized regions of high field could induce deformation of the material and help the propagation of conducting regions. A high voltage bridge allowing the analysis of non linear losses of small capacities and the partial discharges has been built. Some results with D.C. acting on polymers in contact with swelling or non swelling liquids are given.