Water treeing and polymer oxidation

The influence of oxidation on water tree degradation is examined. Studies of the thermal oxidation of cable-grade PE (polyethylene) and XLPE (cross-linked PE) and also of PE and XLPE immersed in hot aqueous solutions in the absence of an electric field are reviewed. The data are compared with the results of oxidation studies of water trees in PE and XLPE. It is proposed that the surfaces of the cavities in the trees become oxidized in the presence of oxygen during cavity formation. However, this oxidation is initially confined to the cavity surfaces. Eventual oxidation of the treed regions can occur by the same mechanism as for polymers in aqueous solutions without an electric field. The oxidation is initiated at the cavity surfaces and spreads through the volume of the tree slowly at a rate depending on the contaminants (type and concentration), availability of oxygen, and temperature. Water trees are more susceptible to oxidation at higher temperature, probably due to contaminants acting as catalysts for oxidation and/or autoxidation from the cavity surfaces. Tree initiation and growth appear to be independent of moderate levels of oxidation typical of those found in field-aged XLPE cables