Effect of nano-calcium carbonate/pimelic acid bicomponent nucleating agent on polypropylene microporous membranes prepared via thermally induced phase separation

Polypropylene (PP) microporous membranes were prepared with the composite diluents of soybean oil and di-n-butyl-phthalate (DBP) via thermally induced phase separation (TIPS). The effects of nano-CaCO₃ nucleating agent and nano-CaCO₃/pimelic acid bicomponent nucleating agent on the crystalline morphology and melting behaviors of the PP/soybean oil/DBP system, and on the microstructure of PP membrane were investigated. The experimental results showed that with addition of 0% - 4% single nano-CaCO₃ nucleating agent the peak temperature of melting curve of PP in PP/soybean oil/DBP system varied from 150.7°C to 151.3°C, while the peak melting temperature of pure PP is 165°C. The differential scanning calorimetry (DSC) showed that the addition of bicomponent nucleating agent nano-CaCO₃ (1% - 4%) and pimelic acid (0.5%) induced two peaks on the melting curve. Wide-angle X-ray diffraction (WAXD) measurements proved that PP formed β-phase and α-phase with addition of bicomponent nucleating agent nano-CaCO₃ (1% - 4%) and pimelic acid (0.5%), while the addition of single nano-CaCO₃ nucleating agent resulted in only PP crystals of α-phase, which was consistent with the results of DSC. The addition of single nano-CaCO₃ nucleating agent in PP/soybean oil/DBP system had unremarkable effect on the morphology of PP membrane. While nano-CaCO ₃/pimelic acid (1%/0.5%) bicomponent nucleating agent was used, the PP microporous membrane showed smaller and regular pores. When the nano-CaCO₃/pimelic acid content was 2%/0.5% and 4%/0.5%, PP microporous membrane showed more tiny irregular spherulites, as well as larger pores, which were related to the formation of β phase.