The influence of structure on water treeing in crosslinked polyethylene: crystallization and annealing effects in bow-tie tree growth

Crosslinked polyethylene cable insulation is subjected to two types of thermal treatment. The first occurs during manufacture and is largely a result of the cooling process applied after curing. The second occurs during use, when the temperature of the conductor rises due to eddy currents. Miniature cables have been produced using a laboratory-size extrusion-curing-quenching facility designed to duplicate the manufacturing process. They have been subjected to heat treatments designed to resemble both types of thermal treatment encountered in practice. The effects of these thermal treatments have been evaluated using accelerated aging experiments, the nucleation and growth rates of bow-tie-type water trees have been determined. Major increases are observed in the nucleation rates of trees for annealing in the temperature range just below the peak melting temperature. Smaller but clear increases in the rate of growth of trees also occur. For higher annealing temperatures and higher crystallization temperatures, slight improvements in susceptibility to treeing occur in the quenched cables