Effects of nucleating agents on the crystallization and melting behavior of isotactic polypropylene

Abstract

Effects of nucleating agents on the crystallization and melting behavior isotactic polypropylene (iPP) were studied by differential scanning calorimetry (DSC). Dibenzylidene sorbitol(EC-1), (p-chloro, p-``methyl)-dibenzylidene sorbitol (EC-4), and bis (p-ethylbenzylidene) sorbitol (NC-4) were used as nucleating agents. A nonisothermal crystallization kinetic equation as well as the simple Avrami equation was employed to analyze the crystallization characteristics of iPP with or without the nucleating agents from DSC crystallization thermograms. In the isothermal crystallization kinetic study, the simple Avrami equation may describe the crystallization behavior of the unnucleated iPP fairly well. However, the Avrami equation has a limit to follow the whole crystallization process of the nucleated samples. Regression to determine the kinetic parameters for the nucleated iPP was made using the DSC data obtained below the relative crystallinity of 30%. With the values of rate constants corrected assuming the three dimensional spherulite growth, the number of effective nuclei in the nucleated iPP was calculated to be $10^2$~$10^5$ time higher than that in the virgin sample. It was observed that the number of effective nuclei decreased with increasing crystallization temperature for both the virgin and the nucleated iPPs. The temperature dependence of the nucleation activity varied with the nucleating agent incorperated and was presented in terms of the deactivation factor defined in this study. The nonisothermal crystallization kinetic equation in the study took into account the growth processes initiated by either heterogeneous or homogeneous nucleation as well as the growth site impingement. The equilibrium melting temperature of iPP necessary for the nonisothermal kinetic study was obtained by the conventional extrapolation method to be 209℃. The nonisothermal crystallization kinetic analysis for the unnucleated iPP at different cooling rates was possible by ...