Probing into double crystallisation behaviour of polypropylene/CaCO 3 composites

A series of isotactic polypropylene (iPP)/calcium carbonate (CaCO₃) composites are prepared by melt blending, and the crystallisation behaviours of these composites are investigated by differential scanning calorimetry (DSC). The results show that the crystallisation exotherms displayed double peaks as the concentration of CaCO₃ reaches a critical value of typically ∼8 wt-% for nano-CaCO₃ and ∼20 wt-% for micron-CaCO₃. With increasing content of CaCO₃, the intensity of the higher temperature crystallisation peak increases, while that of the lower one decreases. Modifying the surface of CaCO₃ with stearic acid leads to the increase in the critical concentration to trigger double crystallisation peaks, which is 50 wt-% for nano-CaCO₃ filled iPP. The results of isothermal crystallisation, transmission electron microscopy (TEM) and polarised light optical microscope (POM) experiments indicate that the different nucleating abilities of CaCO₃ composites are mainly responsible for the occurrence of the double crystallisation peaks in the composites.