Effects of TiO2, ZnO, and Fe3O4 nanofillers on rheological behavior, microstructure, and reaction kinetics of rigid polyurethane foams

Effects of different types and shapes of titanium dioxide, zinc oxide, and magnetite nanofillers on the rheological behavior of polyol/nanofiller suspensions, on the rigid polyurethane foam formation reaction, and hence on the final microstructure were investigated. The rheological percolation threshold of polyol/nanofiller suspensions decreased as the aspect ratio of nonspherical nanoparticles (platelet or rod) increased, regardless of the nanofiller type. The results of reaction kinetics showed that above a critical surface area (approximate to 30 m(2)), independently of nanofiller type, the reaction rate increased as the surface area increased. The introduction of oxide surfaces reduced the final cell size until a critical surface area (approximate to 30 m(2)). However, above this critical value cell size distribution gets wider and the cell size can no longer be correlated with the surface area. In the latter case, an increase of the reaction rate and the polymerization reaction being exothermic may facilitate uncontrolled cell nucleation, growth, and hence coalescence which results in an uncontrolled foam structure. (c) 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43658.