Chemical nature of water trees in cross-linked ethylene vinyl silane copolymer

Water tree growth behaviour of ethylene vinyl silane copolymer cross-linked in the presence of moisture and an organic tin catalyst was studied as a function of cure-time of the copolymer. During cross-linking the silane groups in the copolymer were hydrolysed first and subsequently condensed to form Si-O-Si cross-links. The size of water trees grown at any given condition depended upon the total number of available silane groups to react with the ingressed moisture during the accelerated water tree test. At higher test temperature, there appeared to be a preferential reaction of the ingressed moisture with the residual silane groups contributing to an increased cross-link density within the already existing gel in the treed region. This was accompanied by only a moderate increase in oxidation in water trees compared to what was observed for a peroxide cross-linked polyethylene. The increased cross-link density in the treed regions is proposed to be responsible for the microscopic structural differences observed between water trees in cross-linked silane copolymer and peroxide cross-linked polyethylene and for the better retention of a.c. breakdown strength in the former