ii CHARACTERIZATION OF FOAMING PHENOMENA OF POLYPROPYLENE FILLED WITH AZODICARBONAMIDE AND SILICA

iii Mixtures of polypropylene, azodicarbonamide, and precipitated silica were investigated and compared for their thermal and optical properties and their densities. Dispersion of filler and its influence on the crystallinity and bubble shape and size was studied using DSC and optical microscopy. Since two competing effects are occurring, mainly nucleation and growth of polymer crystals in the presence of a nucleating agent as well as nucleation and growth of bubbles, two competing mechanisms are suggested to influence the overall crystallinity, number, size, and shape of bubbles as well as the density of the mixtures. As expected, an increase in filler concentration increases the viscosities when added into the polymer matrix, which should in turn, influence the amount of bubbles observed and the densities. Interestingly enough, optical microscopy reveals that loading of filler and blowing agent and mixing temperatures influences the number, shape and size of bubbles observed in addition to the resulting densities of the mixtures. Density measurements also revealed that at 10 weight percent filler concentration, densities were greatly affected based on the mixing temperature and whether or not subsequent compression was done. i