Effects of Bi-axial Orientation on the Crystallization Properties and DC Breakdown Strength of Film

Bi-axial oriented polypropylene film (BOPP) is the most widely used dielectric material for power capacitors. At present, few studies have ever been performed on film properties evolution and the mechanism after biaxially stretched. Besides, most of current researches on the modification of polypropylene (PP) films are based on the pressed film or cast film. In the present study, Therefore, pure PP, PP-g-MAH and nano-PP modified films were first prepared, and different tensile ratio films with tensile ratios of 4 × 4 and 5 × 5 were prepared respectively. At the same time, the crystal morphology, crystallinity and DC breakdown strength of the film samples and the hot-pressed polypropylene film as a comparative specimen are tested and characterized. X-ray diffraction (XRD) test showed that the crystal structure of different modified films was different, and compared with pure polypropylene films, the β crystals in the modified films were suppressed, and the β crystals were converted into α crystals during the stretching process. The measurement results of differential scanning calorimetry (DSC) show that the crystallinity of different films increases with the increase of tensile ratio. The DC breakdown strength of different films increases with the increase of the tensile ratio, and the DC breakdown strength of nano-modified PP films reaches up to 638.0 kV/mm when the tensile ratio is 5 × 5, which is 36.5% higher than that of hot-pressed PP films.