Influence of supramolecular additives on foam morphology of injection-molded i-PP

Foaming isotactic polypropylene (i-PP) by foam injection molding usually results in inhomogeneous, large cell structures. Possibilities to realize more homogeneous and finer foam morphologies are adjusting processing parameters or adding nucleating agents. Often, inorganic nucleating agents such as talc in concentrations of about 2 wt% are used to influence the foam morphology. This article discusses the use of two benzene trisamide-based nucleating agents to control cell nucleation during foaming of i-PP. These additives form supramolecular nanostructures in the polymer melt acting first as nucleating sites for foam formation and second as nuclei for the polymer crystallization. Foaming was performed by foam injection molding with nitrogen as physical blowing agent. A specially designed variotherm mold technology was utilized to exactly control the foaming temperature, foaming pressure, and expansion ratio. Foamed i-PP samples were prepared with a density reduction of 50% and analyzed with respect to foam structure and mechanical properties. We demonstrate that the benzene trisamide additives have a strong influence on the foam morphology at very low additive concentrations. Only 0.02 wt% of an additive is sufficient to obtain a remarkable reduction of the cell sizes. It appears that the cell struts, those dimensions can be influenced by the additives as well, leads to a significant improvement of the mechanical properties.