Effects of temperature and crystallinity on partial discharge resistance of poly-L-lactic acid

Partial discharge (PD) resistance of biodegradable poly-L-lactic acid (PLLA) was examined at temperatures between room temperature and 70°C, namely at around the glass transition temperature of 57°C. The PD activity becomes more pronounced at temperatures above 60°C, reflecting that the permittivity increases in the rubber state. Furthermore, we have changed the crystallinity of PLLA by drawing or annealing thermally, and have examined its effect on the PD resistance. Although the depth eroded by a fixed period of PD degradation becomes shallower when the sample was crystallized by the drawing or the heat treatment, its reduction ratio is almost equal to the reduction ratio of PD activities due to the decrease in permittivity. Therefore, it is hard to assume that crystallization of PLLA improves its PD resistance to a unit amount of charge.