Rheology of highly filled natural CaCO3 composites.: I.: Effects of solid loading and particle size distribution on capillary rheometry

The rheology of poly(di-methyl siloxane) suspensions containing various loadings of natural CaCO3 particles, of several particle size distributions, was studied using capillary rheometry. Highly loaded suspensions were investigated, emphasizing the unique behavior of high filler loaded compositions. Mild shear thinning was observed for most suspensions, whereas shear thickening was observed for suspensions containing nearly maximal possible solid loadings. The dominancy of particle size distribution was demonstrated by studying the relative suspensions viscosity as function of shear rate. It was further illustrated by the influence of the filler specific surface area on the flow properties. A seemingly slight changes in the particles-specific surface area caused significant changes in the viscosity of maximal solid loaded unimodal suspensions. Fair agreement with the Chong and Krieger and Dougherty models was found for unimodal suspensions, while the combined Farris-Chong model showed an improved fit for the bimodal suspensions. Evidence for wall slip was demonstrated, using scanning electron microscopy and energy dispersive spectrometry, as a clear difference in the extrudate outer and bulk compositions; although no influence on the flow data was observed.