Crystallization and Dielectric Properties of Polypropylene Film during Sequential Biaxial Stretching

Biaxially oriented polypropylene (BOPP) films used for capacitors need to be stretched in longitudinal and transverse directions. Stretching causes orientation and rearrangement of polymer molecular chains, resulting in variation of crystallinity. In addition, the crystal size and β-crystal content change while stretching. Crystallization morphology affects dielectric properties, which will influence the practical application of capacitors. After pretreatment and melting, the polypropylene resin was cooled and shaped by the casting system to produce cast film. After machine direction (MD) stretching, the MD-stretched film was obtained, and further, BOPP film was obtained after MD and transverse direction (TD) stretching. In this paper, the differential scanning calorimetry (DSC) method was used to measure the heat flow curves of three kinds of films, from which the crystallinity and β-crystal content were obtained. The relative dielectric constant ε r and the dielectric dissipation factor tanϑ were obtained by the frequency-domain spectroscopy (FDS) method, from which the polarization and dielectric loss characteristics of the films during stretching were analyzed. The results show that the crystallinity increases by 3.8% and the β-crystal content decreases by 14.6% after MD stretching, and TD stretching does not change crystallinity. In addition, the results of FDS show that ε r and tanδ of cast film are the largest and most sensitive to frequency, ε r and tanδ of BOPP film are the stablest, and tanδ of MD-stretched film shows an obvious relaxation peak. In this paper, the crystallization and dielectric properties of polypropylene film during sequential biaxial stretching were studied, which to some extent provided theoretical guidance for the stretching process of polypropylene film for capacitors.