The structure of electrical trees in semi-crystalline polymers

The growth of electrical trees within insulating polymers is both a major technological problem and an area of considerable academic interest. Consequently much research effort has been expended in attempting to understand the phenomenon and many models have been proposed. However, few attempts have been made to explore material factors in detail, despite the potential this offers both for improved basic understanding of dielectric breakdown and for optimized materials for technological exploitation. A recent investigation of short-term failure within polyethylene employed specifically designed blends of linear and branched polyethylenes, in which the morphology could be varied systematically and independently of other factors. This work indicates that morphology plays an important role in such failure processes and suggests that, by appropriate structural control, enhanced materials can be produced. However, the absolute significance of alternating voltage ramp breakdown tests is limited in that the derived data reflect both the sample and the chosen test conditions and, even then, only relate to rapid puncture breakdown. Nevertheless, such data are useful for comparative purposes. The work described here set out to use similar material design concepts to study tree growth in semi-crystalline polymers and investigate how growth is influenced by the microstructure of the surrounding material.