Influence of dispersant, storage time and temperature on the rheological properties of zirconia-paraffin feedstocks for LPIM

The dependence of the rheological properties of zirconia-paraffin feedstocks for low-pressure injection moulding (LPIM) on binder composition, storage time and temperature was investigated. Feedstocks with varying amounts of dispersant were fabricated to work out the required quantity for a monolayer of dispersant molecules on zirconia particles. Experimental results revealed that a shear rate dependent characteristic of the viscosity against the amount of dispersant exists. The observations were compared with calculated values according to an adsorption model, which overestimated the required quantity of dispersant to form a monolayer. Feedstocks stored at elevated temperature for several days exhibited a time-dependent decrease of the yield stress and viscosity, which is supposed to be caused by physical or chemical interactions among zirconia and the dispersant. Increasing working temperature and decreasing solids loading were found to significantly decrease the yield stress as well as the viscosity. Flow activation energies and flow indices were calculated and compared with literature. This study shows that the dispersant used in this investigation has a remarkable influence especially on the time-dependent flow behaviour of zirconia-paraffin feedstocks that affects further processing and reproducibility. (C) 2009 Elsevier Ltd. All rights reserved.