Abstract

Polymeric foams appear in diverse fields from automotive to medical applications due to their unique characteristics. The morphology of the polymer foams has a huge impact on their mechanical and physical properties such as strength and density. In this thesis, nucleation and growth of bubbles in foaming of Polypropylene through the extrusion process have been investigated both experimentally and numerically. Both cell nucleation and cell growth phenomenon should be controlled through the process in order to have a customized foam. Several models have been proposed for many years and theoretical and experimental investigations have been performed on both bubble nucleation and bubble growth phenomenon. In the nucleation step, a semi-experimental model has been adopted in our studies after reconciling between the accuracy and complexity of the models. This model uses the heterogeneous nucleation mechanism, proposed by classical nucleation theory, as the main mechanism of nucleation in the foaming process and modifies it by introducing energy reduction and frequency factors. For the growth stage, a two-dimensional model that accounts for the diffusion-driven growth of bubbles in the viscoelastic fluid (PP melt) has been chosen. The two-dimensional models have a higher accuracy in predicting the final bubble radius compared to the one-dimensional models (e.g. cell model) and is capable of predicting the exact shape of non-spherical bubbles in the polymer. Using a mini-extruder, the effect of processing parameters on the final foam morphology has been studied qualitatively. Then the governing equations for the diffusion-driven bubble growth have been solved by the finite element method. The sensitivity of the growth dynamic to the processing parameters has been studied by simulating the evolution of a single bubble enclosed inside an influence volume under the assumptions of the cell model. The interactions between the bubbles, which cannot be thoroughly considered in the cell model, have been simulated by the two-dimensional model and the predictions of the bubble shapes at different processing conditions have been performed. Finally, the ability of the cell model and the two-dimensional model in predicting the final foam density have been compared with the experimental data. The results show that the accuracy of the cell model decays as the cell density and the bubble-bubble interactions increases while the 2D model has the capability of predicting foam density and the average bubble size with acceptable accuracy in nearly all experimental conditions.