Mechanical properties of high density polyurethane foams: II Effect of the filler size

This article presents the mechanical behaviour of rigid polyurethane reinforced by mineral fillers, for a density range (ρf/ρs) > 0.27. The influence of the particle size on the foam microstructure and its mechanical behaviour has been studied using calcium carbonate with two different sizes (with average diameter of 1 and 30 μm, respectively) and crystallised silica particles with an intermediate size. The microstructural characterisation has been performed by electron microscopy. It has shown closed spherical cells whose size is decreased when the fillers are added. The results of mechanical characterisation by large and small deformation (mechanical spectrometry) tests have been compared to different modelling approaches. It appeared that a correct description of the viscoelastic properties and of the yield stress needs to take into account the filler size compared to the wall size. Two modelling hypothesis which consider the composite foam as either a filler dispersion in a foam or a void dispersion in a filled polymer have been made. The first one is adapted to the description of PU filled with the big carbonate fillers, when the second one is better to describe the PU filled with the small carbonate fillers. The use of the crystallized silica particles gives composite foams with properties in between that of the two calcium carbonate composites, showing that the filler size in the range studied is a key parameter to take into account in the reinforcement level.