Effects of stearic acid coated talc, CaCO3, and mixed Talc/CaCO3 particles on the rheological properties of polypropylene compounds

The effect of the stearic acid coated filters and their geometry on the shear/dynamic viscosity and complex viscosity has been investigated using polypropylene (PP) compounds filled with stearic acid uncoated and coated talc, calcite, and mixed talc/calcite particles. The viscosity was measured over a wide range of shear rates (10(-8) to 10(3)) using a capillary, cone-plate and sandwich rheometer. Overall, the rheological properties of the compounds exhibited different behavior upon different filler systems, stearic acid involvement, shear stress or strain, and frequencies due to stearic acid involvement. This implies that the stearic acid lowers the interfacial force between the filler surface and the resin matrix, followed by a favorable processing. In addition, at very low shear stresses, the viscosity of talc(un) compounds was higher than calcite(un) ones; at very high shear stresses, on the other hand, talc compounds became lower than calcite(un) compounds. This is interpreted as due to the different geometry between talc and calcite. The yield value as a function of shear stress was observed for all filler systems and exhibited lower than that obtained from the extrapolation. Furthermore, the Cox-Merz relation between the complex and shear viscosity for both the stearic acid uncoated and coated compounds is found not valid. (C) 2004 Wiley Periodicals, Inc.