Structure effect of thin film polypropylene view by dielectric spectroscopy and X-ray diffraction: Application to dry type power capacitors

This work reports on the relationship between structure and dielectric properties of biaxially oriented polypropylene. The morphology of semicrystalline bioriented isotactic polypropylene films is investigated using wide angle X-ray diffraction and Polarized Optical Microscopy. A b-orthorhombic structure, with a crystallinity ratio of about 46%, and “Crater” morphology of the b-form is identified. Dielectric properties are measured by Broadband Dielectric Spectroscopy over a wide temperature range (2150 to 1258C). Since the dissipation factor of the PP is very low, special care was taken to obtain valid data. Two main relaxation processes are observed: a a-relaxation peak associated to the glass transition temperature (Tg) at temperature about 278C, and a broad b*-relaxation at about 2608C, partly attributed to CH orientation. The variation of the dissipation factor versus sample thickness (from 3.8 to 11.8 mm) is correlated and partly explained by the increase of crystallinity ratio and lamella size at larger thicknesses. It comes out that the thinnest film seems perfectly meet the application requesting, namely lowest dissipation factor and highest permittivity.