The rheological behavior of pastes was characterized by employing capillary flow. The wall slip properties of pastes are very significant in the extrusion process. All pastes were found to exhibit non-Newtonian, pseudoplastic, shear thinning behavior under all experimental conditions in a wide apparent shear rate range of 1-4000 1/s. It was found that the corrected shear stress values increased with an increase in the solid:liquid ratio and particle size, and with a decrease in temperature at above 100 kPa. The results confirm that the computed shear stress depends upon both the diameter and the length of the capillary tube; and also show that the influence of the wall slip is significant and that the wall slip velocities increase with an increase in shear stress. The Mooney method was applied to examine the nature of the shear flow. It was concluded that the slip effects dominate the flow of kaolinite-silicon oil pastes at a high shear stress (above 100 kPa) and true flow and deformation characteristics of the pastes may be overshadowed by slip at the walls. The contribution of the slip of the paste at the wall to the volumetric flow rate in capillary flow was found to increase with an increase in temperature, particle size and amount of solid.

Keywords: Kaolinite; silicon oil; paste; wall slip analysis; Mooney method