Electrical, chemical and mechanical processes in water treeing

Water treeing is a complex phenomenon involving several processes with many synergistic effects. Although a huge number of papers on the subject have been published over the last 25 years, there is no comprehensive theory able to describe the often contradictory experimental results. However, there are some tendencies that are always observed, whatever the experimental conditions. A critical review of some electrical, chemical and mechanical processes is made. In fact, water treeing is likely to be due to a mixture of all these processes. When existing data is examined carefully, it appears that very few theories can explain the results. It is concluded that voltage and frequency are major aging factors, and the role of oxidation may be less important than sometime was suggested. The energy of ion reduction is pointed out as a factor that may possibly influence water tree initiation and growth. The solubility parameter of additives and solutions is another factor that deserves some attention. Finally, the polymer morphology appears to affect water treeing greatly through modifications of its mechanical properties. Before a detailed model could be proposed, several studies remain to be done and thorough investigations devoted to the initiation phase and to synergistic effects are required. The development of a reliable and significant accelerated aging test simulating field conditions is also suggested.