Correlations Between Morphology and Insulating Properties of Polypropylene Film

The increasing demand of compact capacitors requires higher energy storage density of the dielectric material-polypropylene (PP) films. The improvement of breakdown strength of PP films, is key to the energy storage density enhancement. In this paper, the comparative studies between normal $\alpha$-crystal dominated PP and PP with $\beta$ nucleating agent introduced were carried out, to investigate the relations between the morphology and insulation performances. The polarized light microscope and X-ray diffraction measurement were applied for the crystalline structure of raw granules and the surface structure of corresponded bi-axial oriented PP (BOPP) films. The DC breakdown tests were performed both on the hot-pressed films and BOPP films, and the evolution of breakdown strength (BDS) under elevated temperatures from $20^{\circ} \mathrm{C}$ to $80^{\circ} \mathrm{C}$ were obtained. The results proved that the $\beta$-nucleating agent could lead to the higher crystallinity of the resin, while $\alpha$-crystal could contribute to the better morphology of BOPP films. In consequence, the BDS of hot-pressed film of $\beta$-crystal was higher than the $\alpha$-crystal dominated film, but the BDS of BOPP of each resin were on the contrary. With the increase of temperature, the BDS of both sample BOPP films decreased but films with $\beta$-nucleating agent had smaller decrease rate due to the optimization of molecular chains alignment.