Quiesent Crystallization Kinetics of Nucleated Metallocene and ZN Isotactic Polypropylenes

Abstract

Crystallization kinetics and thermodynamic properties of nucleated isotactic polypropylene (PP) are analyzed using Hoffman—Lauritzen crystallization theory to determine the mechanistic effects of the nucleators. Calorimetric data provides quantitative comparisons between nucleating efficiences of the α (Millad) and β (NJSTAR) nucleator in Metallocene (M) and Ziegler—Natta (ZN) PP. The two types of PP without nucleators showed similar crystallization behavior though the T°_m for ZN-iPP was about 10°C higher than M-iPP. Both nucleators show significant improvement in crystallization rate in both types of PP. In addition, Millad outperforms NJSTAR. The magnitude of the kinetic response is,however, different and both the nucleators appear to function better in ZN than in Metallocene PP. β nucleated PP shows predominantly the β form. The amount of the β form is thermal history dependent and changes with supercooling (ΔT=T°_m−T_c). Similar equilibrium melting temperature (T°_m) in the α nucleated and control PPs indicates the lack of any thermodynamic effect of the α nucleator. All nucleated PPs show a much lower secondary nucleation rate constant, K_g.