Growth of Water Trees in Polyethylene and Silicone Rubber by Water Electrodes

In this paper, various experimental results are described of water trees obtained in polyethylene and silicone rubber, using the water electrode method. The mechanisms of initiation and growth of water trees are discussed. The growth of water trees depends upon the applied voltage, the frequency, and the water resistivity below 10 kΩ·cm. No differences in the growth of water trees were found among NaCl, KC1 and BaC12 solutions having equal resistivity. It is concluded that Maxwell stress, exerted on polyethylene or silicone rubber by applying an alternating voltage, forms micro-cracks in the material. Water permeates into the polymer, because the dielectric constant of water is higher than that of polyethylene or silicone rubber. With a high electric field and a high frequency, vaporization of water due to dielectric heating may occur: vapors may permeate into the amorphous region, and micro-cracks are formed by the vapor pressure.