Water penetration and initial ion location during water treeing in low density polyethylene

Measurements of water tree growth have been made using test cells in which a platinum electrode was embedded into low-density, nominally additive free polyethylene immersed in deionized water or LiCl solution. In some specimens the platinum electrode was contaminated with LiCl prior to insertion into the polyethylene. A comparison has been made between the tree growth obtained using contaminated and clean electrodes. Tree growth was found to be much more extensive for the LiCl contaminated electrode, the polymer in this case being in contact with deionized water. Measurements of water penetration through a 1-mm thick sheet of low-density polyethylene have shown that there is no effect from a uniform applied field of 2kV/mm at 3kHz but that penetration is increased when temperature is raised from 30°C to 50°C. These observations suggest that the presence of water in the region surrounding the metal electrode leads to the disassociation of any ionic materials present at the interface. The resultant ionic penetration into the polymer may then be responsible for the ultimate development of the water trees