Effect of Particles on the Flow-Induced Crystallization of Polypropylene at Processing Speeds

The effect of particles with different diameters on flow-induced crystallization from low shear rates to processing speeds is studied using mixtures of polypropylene and zinc oxide with diameters between 35 nm and 1 mu m. The nucleating efficiency of the particles was characterized by means of the Lotz efficiency scale using DSC. Flow-induced crystallization at processing speeds was studied by means of the birefringence and transmitted intensity measurements using a specially designed sliding-plate flow cell. For crystallization under quiescent conditions and after shear steps at low shear rates, there are significant differences between the kinetics of the neat polymer, the mixture with the 1 mu m zinc oxide, and the mixtures with the midrange particles, which are in agreement with the nucleating efficiency of the particles. As the shear rate is increased, the crystallization kinetics of the mixtures and neat polymer become similar, independent of particle diameter. The transition to highly oriented shish-kebab structures occurs somewhat earlier as the nucleating efficiency is higher, but at the highest shear rates, the kinetics of all materials are equal. This suggests a gradual shift in dominance from the influence of particle addition on the crystallization process at low speeds to the influence of flow at high speeds.