Study of the Cellular Structure Heterogeneity and Anisotropy of Polypropylene and Polypropylene Nanocomposite Foams

This article presents the analysis of the processing parameters influence on the foaming behavior and cellular structure of PP-montmorillonite foams. Polypropylene nanocomposites containing 5.0 phr of an organically-modified montmorillonite (MMT) were initially melt-compounded in a twin-screw extruder with azodicarbonamide (ADC) and later foamed using a one-step compression-molding process. The cellular structure and morphology of the foams was assessed using both scanning and transmission electron microscopies. A time-dependant double-effect was observed during foaming: (1) first of all, the melt strength of the polymer, too high for shorter times, not allowing full cell growth, and too low for high foaming times due to thermal oxidation; (2) and polymer degradation, clearly observed for very high foaming times, directly affecting polymer's melt resistance. Comparatively, PP-MMT foams exhibited a broader foaming time processing window, a more isometric type of cellular structure and decreased open-cell contents, indicating an effective nucleation and cell wall stabilization induced by the exfoliated MMT particles. POLYM. ENG. SCI., 49:2400-2413, 2009. (C) 2009 Society of Plastics Engineers