Effect of Frequency on the Growth of Water Trees in Polyethylene

The effect of frequency on the growth of water trees in polyethylene has been investigated using the water needle test. An accelerating effect was observed up to a maximum between 4 and 8 kHz with both NaCl solution and deionized water. This phenomenon cannot be ascribed to a frequency dependence of the electrical behavior of the trees. It results from the mechanical properties of the polymer itself and can be interpreted by analogy with environmental stress cracking. The frequency dependence of the growth rate appears to be a consequence of the existence of a mechanical absorption peak in polyethylene located at about 30 kHz at room temperature.