Foam microstructure including cell size, cell wall thickness and foam density has a prominent role on physical and mechanical response of PU foams. In some researches, relation between some microstructural parameters on mechanical properties were modeled and investigated [1, 2]. Saint-Michel et al. [3] studied the effect of density on mechanical properties of high density PU foams. They succeeded in modeling the yield strength by assuming that foam density controls the microstructure. Owing to the fact that specific area of nanoparticles is very high, they are prone to be attracted to each other by intermolecular van der Waals forces. This attraction causes clusters of agglomerated particles which play as defect in the composite. In order to avoid agglomeration, a focused external energy like ultra sound wave is applied to separate the particles in one of the liquid constituents of neat PU foam. One of the most prominent mechanisms of particle separation is acoustic cavitation. By this mechanism tiny bubbles are successively generated, grow and collapse which result into production of transient micro hot spots. These local areas with high temperature and pressure are favorable sites to overcome the attracting forces between particles and promote the wettability between nanoparticles and liquid polymer.

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