ELECTRIC-FIELD-INDUCED DEGRADATION OF POLYMERS

The electric-field-induced degradation of polymers can occur in two ways, namely, 'treeing' (electrical treeing and water treeing) and 'non-treeing' degradation. Electrical treeing, which results in a pattern of dendritic defects in 'dry' insulation, is attributed to the combination of hot-electron- or UV-photon-induced damage (which weakens the material) combined with electromechanical forces (which fracture the weakened material). Initiation of an electrical tree generally requires a local electric field in excess of 100 kV mm(-1). In the presence of moisture, a second phenomenon can cause numerous water-rich dendritic defects, known as 'water trees'. These arise after a period of electrical aging at a modest average field typically in the range 1-6 kV mm(-1). Such defects usually grow along the direction of the electric field and typically range in size from ten to several thousands of micrometers. The growth of water trees is a major cause of electrical failure for polyolefin-insulated distribution cables(1-3). The mechanism of water treeing has remained unclear for decades. Only recently have researchers explained that the growth of water trees in polyolefins may be dominated by a process of field-induced electrochemical degradation, although other processes are active and may contribute to degradation at different stages. The understanding of both electrical treeing and water treeing is evolving from microscopic theories into a derailed theory at the molecular chemistry level.