In situ evidence of the nanoparticle nucleating effect in polyurethane-nanoclay foamed systems

This paper presents a comparative study on the nanoadditives (nanoclays) effect during the foaming process of rigid polyurethane (PU) foams. A non-standard technique, high-resolution microfocus X-ray radioscopy, has been applied for the first time on these materials to study the mechanisms (nucleation and coalescence) occurring during the reactive foaming process. Using this technique in combination with image analysis it has been possible to determine the evolution of density and cell size throughout the whole foaming process. Thus, cell density values have been calculated from the beginning of the expansion and, using the cell density data, it has been possible to demonstrate that nucleation at initial foaming stages strongly dominates over the coalescence mechanism - not observed - in the nanocomposite systems under study. These results confirm the theories assuming that nanoparticles act as heterogeneous nucleation sites for this type of materials, which were not undoubtedly confirmed via ex situ studies.