Effects of calcium carbonate and its purity on crystallization and melting behavior, mechanical properties, and processability of syndiotactic polypropylene

Calcium carbonate-filled syndiotactic poly(propylene) (CaCO₃-filled s-PP) was prepared in a self-wiping, co-rotating twin-screw extruder. The effects of CaCO₃ of varying particle size (1.9, 2.8 and 10.5 μm), content (0–40 wt %), and type of surface modification (uncoated, stearic acid-coated, and paraffin-coated) on the crystallization and melting behavior, mechanical properties, and processability of CaCO₃-filled s-PP were investigated. Non-isothermal crystallization studies indicate that CaCO₃ acts as a good nucleating agent for s-PP. The nucleating efficiency of CaCO₃ for s-PP was found to depend strongly on its purity, type of surface treatment, and average particle size. Tensile strength was found to decrease, while Young's modulus increased, with increasing CaCO₃ content. Both types of surface treatment on CaCO₃ particles reduced tensile strength and Young's modulus, but improved impact resistance. Scanning electron microscopy (SEM) observations of the fracture surfaces for selected CaCO₃-filled s-PP samples revealed an improvement in CaCO₃ dispersion as a result of surface treatment. Finally, steady-state shear viscosity of CaCO₃-filled s-PP was found to increase with increasing CaCO₃ content and decreasing particle size. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 201–212, 2004