The effect of environmental stress cracking on water tree growth

Water tree ageing experiments have been performed on a low-density grade of polyethylene and two high-density grades. The materials were chosen for their relative susceptibilities to environmental stress cracking (ESC). It was shown that the threshold field for water tree initiation is greater in the high-density co-polymer (41±6 kV/mm) than in the homo-polymer (35±5 kV/mm). The threshold fields agreed well with a theoretical prediction based on the electrokinetic model; the model shows that there is a transition from dielectric to mechanical power loss at ~40 kV/mm. Failure of a polymer due to ESC is a synergistic affair involving a mechanical stress and a wet environment; if the environment includes a surfactant, then ESC is greatly accelerated. Similar synergies do not exist during wet electrical time-to-failure experiments. The use of a surfactant solution during wet electrical ageing greatly increased the density of water trees, but these trees were relatively small and slow growing. This resulted in a substantial endurance time relative to wet electrical ageing in plain tap water. When ageing in tap water only, it was found that of the two grades of linear high-density polyethylene the one engineered to be more resistant to failure due to ESC was also found to be resistant to failure due to WTG