Crystallinity and microstructure in injection moldings of isotactic polypropylenes. Part 1: A new approach to modeling and model parameters

The present study is an attempt to formulate a new approach to the modeling of the injection moldings of semicrystalline polymers. A phenomenological approach is employed to develop the qualitative description of crystallizat ion phenomena under nonisothermal flow occurring in various polymer process ing operations. The unified crystallization model, which is suitable to des cribe the multilayer microstructure arising in the injection moldings of se micrystalline polymers, is proposed. The "competing mechanisms" of crystall ization in terms of the induction time indices for introducing various micr ostructure layers are utilized to quantitatively distinguish between the hi ghly oriented skin layer and spherulitic core in the moldings. The applicat ion of the equations of continuity, momentum and energy, along with the rhe ological model, the equation of state and the unified crystallization model , to the injection molding process leads to the modeling of crystallinity a nd microstructure development in the moldings. The material parameters to b e used in the modeling of the crystallinity and microstructure development in the injection moldings of isotactic polypropylenes (i-PP's) with various molecular weights are determined. The melt viscosity as a function of shea r rate and temperature is measured and fitted to the modified Cross model. Quiescent crystallization kinetics, including the induction time and the he at release due to crystallization, is characterized using differential scan ning calorimetry. The spherulite growth rate of i-PP's under isothermal and nonisothermal quiescent crystallization conditions is measured by optical microscopy. The molecular deformation factor during the induction period of shear-induced crystallization is obtained from the extrusion-quenching and extrusion-relaxation-quenching experiments with a slit die. The Variation of the induction time with shear rate obtained by Lagasse and Maxwell from the shearing-crystallization experiments is used to determine the shear enh ancement coefficient of crystallization.