Composition, Structure and Growth of Water Trees in Polyethene

摘要

The present study deals with the composition, structure and growth of water trees in polyethylene. Water trees to be used for analysis were obtained from field- and lab-aged cables and from material-test objects ('platelets'). The hydrophilic nature of water trees was regarded as a key to understanding their chemical composition. The structure of water trees is detailed down to the nanometer scale. Water trees consist of hydrophilic tracks running through the amorphous phase of polyethene. These tracks cannot be regarded as simple channels because they are semi-permeable. Water-tree growth processes are discussed on the basis of data obtained from off-line analysis. Oxidation, chain scission and ion diffusion occur during water treeing. These processes cooperate in the process of water-tree growth but at the same time compete with respect to the degradation products. The role of antioxidants and some consequences for testing are discussed. Preliminary work regarding on-line (or dynamic) analysis of water-tree growth is presented next. Two techniques were developed: dielectric measurements at high voltages, and dynamic Fourier transform infrared (DFTIR) spectrometry.