Mechanical Behavior of Anisotropic Polypropylene Nanocomposite Foams

Polymer foams from rigid polymers can be used for structural applications, for instance in the core of sandwich panels. For these specific applications the density of the foam should be as small as possible, however the loss of mechanical properties when density is reduced is very strong, with normally a square dependency with density. The present work is focused on the study of the structure-property relationship of anisotropic medium density polypropylene (PP) foams. Medium density foams (100-600 kg/m3) are seldom encountered in the market and in the literature because of the strong loss of mechanical properties when density is reduced. The most common approaches followed to face this limitation rely, on the one hand, on reinforcing the polymer matrix to increase its stiffness, and on the other hand on modifying the cellular structure of the foam somehow to improve the mechanical behavior. In this study both strategies are investigated: the effect of anisotropic cellular structure, obtained by a specific foaming technique, (anisotropy ratio, cell size distribution) on mechanical properties was analysed and the matrix reinforcement effect was evaluated by preparing PP foams filled with nanoclays and by comparing the results obtained for non-filled PP foams. The modification of the process parameters allowed obtaining different structure morphology maintaining constant foam density. Elastic modulus in plane and in the expansion direction was evaluated by means of quasi-static compression tests. Nanoclays reinforced foams displayed better mechanical properties than pure PP foams prepared in the same process conditions. Microstructure characterization, performed by means of SEM images analysis, revealed significant differences between pure PP foams and filled PP foams, and a strong dependence of structure morphology on the process conditions. Finally, a part of the study was devoted to assess the applicability of existing models, already validated for low density foams, in order to predict the dependence of mechanical behavior on the anisotropy of the cellular structure.