EXPERIMENTAL AND NUMERICAL STUDIES OF BUBBLE DEVELOPMENT PROCESS IN ROTATIONAL FOAM MOLDING

Abstract

<p>Commercial interests in polymeric foams continue to increase due to their unique physical characters and the new emerging applications for foamed materials. This thesis investigates the foam development process under non-pressurized conditions as applicable to rotational molding to elucidate the underlying mechanisms in the bubble transformation process and provide an accurate basis for predicting the morphological structure and macroscopic properties of the foamed materials. It was found that the foaming mechanism is comprised of four distinct stages: two stages of bubble nucleation, primary and secondary nucleation, followed by bubble growth and bubble coalescence/shrinkage. Following the nucleated bubbles during the foaming process revealed that primary nucleation was the controlling stage in determining the final cellular structure. Growth and coalescence mechanisms were dynamically active and competed during both heating and cooling cycles.</p> <p>The influence of the rheological properties on the rate of nucleation and the bubble growth mechanism were investigated. Morphological analysis was used to determine the rheological processing window in terms of shear viscosity, elastic modulus, melt strength and strain-hardening, intended for the production of foams with greater foam expansion, increased bubble density and reduced bubble size. Visualization experiments and theoretical predictions showed that higher viscosity could impede the number of nuclei generated in the foaming system. A bubble growth model and simulation scheme was also developed to describe the bubble growth phenomena that occurred in non-pressurized foaming systems. It was verified that the viscous bubble growth model was capable of depicting the growth behaviors of bubbles under various processing conditions.</p>