Some Dielectric and Mechanical Properties of Heat Treated Biaxially Drawn Pet Films

The dc dielectric strength of heat-treated biaxially drawn polyethylene terephthalate (PET) is studied under mechanical stresses. The sample is prepared on films with a draw ratio of 3.3x3.9 obtained by a continuous process. To study the effect of crystallinity on the dielectric strength, the sample that is held to constant size is annealed for durations of 1 and 60 min in a silicone oil bath maintained at a fixed temperature within ±2°C. The annealing temperature range is from 100 to 2200C. The maximum dielectric strength of the sample under no mechanical stress is found to occur at an annealing temperature of about 1400C. But the compressive stress showing maximum dielectric strength under compressive stress increases with the annealing temperature, and its value is not significantly varied by the cooling process after annealing. Compared with undrawn and uniaxially drawn PET, the biaxially drawn sample has good mechanical properties and dielectric strength characteristics under mechanical stresses. However, it is believed that the best annealing condition should be decided by the requirements for the electrical and mechanical characters of the sample.