New nucleating agents for shear-induced crystallization of polypropylene

Abstract

The effect of shear on the flow-induced crystallization of a polypropylene with and without nucleating agents was studied using shear rheometry. Generally, an increase in strain and strain rate or decrease in temperature is found to decrease the thermodynamic barrier for crystal formation, thus enhancing crystallization kinetics at temperatures between the melting and crystallization points. Second, the use of nucleating agents dramatically increases the crystallization and melting point of polypropylene, thus enhancing the kinetics of crystallization. Herein, we report the quiescent isothermal and shear-induced crystallization (rheology) behavior of a random copolymer polypropylene with ethylene as a co-monomer containing sorbitol nucleating agents (NA) with different degrees of polarity. The presence of sorbitol NA increases the Tm and Tc by 5 °C and 15 °C, respectively. By performing steady-shear experiments at shear rates varying from 0.001 s−1 to 1 s−1, “quiescent crystallization” and “shear-induced crystallization” regions could be identified. From both isothermal and shear-induced crystallization experiments, sorbitol-based NA with the lowest degree of polarity was found to cause the highest crystallization kinetics.