Factors affecting water absorption by polyethylene

There have been many reports in recent years of the premature failure of extruded polyethylene insulated cables. In seeking reasons for this it has become apparent that polyethylene under electrical stress will absorb water when in contact with it. Many accounts have been given of the deleterious effect of water on the electrical properties of the insulation, but little has been established about the mechanism of water ingress. However, it is believed¹ that the crystallinity of the polymer is important and there are reports² that the rate of deterioration in electrical properties is dependent on the conductivity of the aqueous solution, which indicates an ionic phenomenon. We have previously shown³ that both the rate of water absorption, and the amount of water absorbed by specimens of low density polythene are decreased by heat treatments causing the density, and hence the crystallinity, to increase. Experiments are now described which clearly demonstrate the influence of the ions in the water.