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Prediction of Spherulite Size in Rotationally Molded Polypropylene

Rotational molding is used to manufacture hollow plastic parts. It is characterized by relatively slow cooling rates, which leads to large spherulites and brittleness in rotomolded polypropylene parts. Using both theoretical and experimental methods, this article assesses the factors that control spherulite size so that the properties of rotationally molded polypropylene parts can be improved. The approach taken is to predict the average density of the nuclei of isothermally crystallized polypropylene as a function of the crystallization temperature, using data on the half-time of crystallization (determined by differential scanning calorimetry) and the spherulite growth rate (measured by optical microscopy). The prediction method is then extended to nonisothermal quiescent crystallization, such as occurs in rotational molding, by determining the temperature corresponding to half of the phase change and its relationship with the cooling rate. To establish the average true sample temperature on cooling, experimental data are corrected for the temperature calibration at a particular cooling rate, the thermal resistance of the sample, and the release of the heat of crystallization. The surface nuclei density of polypropylene specimens, as crystallized isothermally and nonisothermally in differential scanning calorimetry, and also as processed by rotational molding, was determined by optical microscopy and converted to volume density using the Voronoy relationship. A good agreement was found to exist between the experimental results and the predictions.