Mechanism of water tree generation and propagation in XLPE

The authors studied the chemical stabilities of XLPE (cross-linked polyethylene) with and without water trees by oxidation induction period, the Fourier-transform infrared technique, scanning electron microscopy, and transmission electron microscopy to clarify the mechanism of inception and propagation of water trees. It was found that the part of XLPE with a water tree was oxidized and the voids constituting a water tree in XLPE were enlarged and increased after treatment with mixed mineral acids or xylene. This suggests that XLPE around the voids may deteriorate during water treeing and the voids must be formed through deterioration of XLPE. Furthermore, the possibility of new types of treeing by aprotic, polar organic liquids such as acetonitrile or propylene carbonate was investigated. Vented trees without `water´ were detected in XLPE and the tree formation was accelerated by incorporation of electrolyte such as lithium perchlorate. Penetration of water molecules into XLPE and chemical degradation under electric field as the theoretical mechanism of water treeing are also discussed