Influence of silica aerogel particles on the foaming process and cellular structure of rigid polyurethane foams

Water blown rigid polyurethane (RPU) composite foams were produced using different concentrations of nanoporous silica aerogel micrometric powder (0.5, 1 and 3 wt%). The effect of these particles on the foaming kinetics was analysed from a physical and chemical viewpoint. On the physical side, the foaming process was studied by in-situ X-ray radioscopy. The inclusion of aerogel particles in the system delays the foam expansion and enhances the nucleation of cells. However, high amounts of these particles (3 wt%) lead to intense cell coalesce during foam evolution. On the chemical side, the reaction kinetics was investigated by in-situ FTIR spectroscopy and reaction temperature measurements. The addition of low contents of aerogel (below 3 wt%) reduces the conversion of isocyanate while favouring the generation of urethane groups, which explains the higher density of the foams with low aerogel contents. However, the foam with high contents of aerogel (3 wt%) does not change the reaction balance in comparison to the reference. Therefore, this foam presents similar expansion and density to those of the unfilled foam (Reference). Furthermore, higher reaction temperatures were reached by the reference foam during the foaming process, and higher dissipation speeds of these temperatures were detected for the foams containing aerogel with respect to those of the reference foam.