Influences of Bi-axial Orientation on the Crystallization and DC Breakdown Properties of Polypropylene Films

Bi-axial oriented polypropylene film (BOPP) has been widely used in power capacitors as dielectric material for decades. However, the study on its properties evolution and the mechanism brought by the orientation process is rare to see. Besides, most of current researches on the modification of polypropylene (PP) films are based on the pressed film or cast film, so the feasibility of these modification methods for BOPP films is still unclear. In the present study, BOPP films with increasing orientation ratio of 4 × 4, 5 × 5 and 4 × 7 were prepared. Morphology tests and DC breakdown test were performed on BOPP sample films, together with the pressed PP as the reference sample. Results showed that crater-like crystalline were formed in BOPP films which was totally different from spherulites of pressed PP film. X-ray diffraction (XRD) test indicated that the β-crystal was suppressed in BOPP films. Besides, the crystallinity was enlarged with the increase of orientation ratio measured by differential scanning calorimetry (DSC). As a result, the DC breakdown strength was raised on the increasing crystallinity, that the highest DC breakdown strength of 627.6 kV/mm was achieved from the BOPP film with the orientation ratio of 4 × 7. This study may provide experimental supports for the further researches on the development of PP based dielectrics.