Influence of chalk and its surface treatment on crystallization of filled polypropylene

Isotactic polypropylene has been shown to crystallize from the melt spherulitically at intermediate undercoolings (100–130°C) in the presence of a high amount (40 wt %) of chalk (calcium carbonate). Linear growth rate is not remarkably affected by the presence of a filler. Both nonisothermal and isothermal crystallization DSC experiments indicate that chalk acts as a weak nucleant. Introduction of liquid [oligomer of ethylene oxide (OEO), Mw = 300] at polymer–filler interface lowers the nucleating activity of chalk. Analysis of crystallization kinetics in Avrami coordinates, followed by dimensional‐analysis correction for the case of nonintegral values of exponents, indicates that the amount of extra “chalk nuclei” is approximately constant in the range of 100–130°C while the amount of OEO‐modified “chalk nuclei” decreases with the increase of the crystallization temperature. Comparison of the melting endotherms indicates that filled melt‐crystallized material contains higher fraction of lower melting crystallites than unfilled polypropylene. This fraction is also higher in the case of polypropylene filled with unmodified chalk. Nucleating activity of chalk has been shown to depend upon its crystal structure (calcite or aragonite), surface topography, and aggregation.