Rheology of highly filled natural CaCO3 composites:: IV.: Effect of surface treatment

The rheology of poly(dimethyl siloxane) (PDMS) suspensions containing natural CaCO3 particles, treated with a homologous series of fatty acids, was studied using capillary and dynamic rheometry. Suspensions containing the maximal possible solid loading of raw CaCO3 were investigated, emphasizing the unique behavior of high filler loaded compositions. Surface treatment with fatty acids caused profound decrease of viscosity in the capillary flow, enabling a 4 vol% increase in the possible maximal solid loading of the suspension. The surface treatment leads to significant shear thinning opposed to the shear thickening exhibited by the untreated suspensions. Yet this effect was observed only at shear rates higher than 10 sec(-1), while at lower shear rates and also under oscillatory low shear stresses, the opposite occurs: the viscosity of the surface treated systems increased. The contradictory effect of surface treatment at different shear rates regimes was ascribed to yield phenomenon of the treated suspensions, which is absent in the untreated ones. The combined, chemically and physically adsorbed acids caused higher increase in viscosity under oscillatory stresses, than the chemically adsorbed acids; thus, it can be concluded that thicker interface layer increased the yield effect reflected in higher viscosity at low shear rates. The oscillatory flow of the treated fine suspensions revealed also a consistent effect of the fatty acid length, the viscosity increased with increasing chain length; it correlated with higher layer thickness and higher surface energy of the treated filler, probably due to a skin friction effect. Copyright (c) 2007 John Wiley & Sons, Ltd.