RHEOLOGY OF ZIRCONIA SUSPENSIONS IN A NONPOLAR ORGANIC MEDIUM

Three dispersants (stearic acid, oleic acid,and poly(12-hydroxystearic acid)) are compared for their ability to produce low-viscosity suspen sions of zirconia in kerosene, Rheological measurements and sediment p acking density measurements show that poly(12-hydroxystearic acid) is a better dispersant than stearic acid or oleic acid; this is explained in terms of the longer tail of the poly(12-hydroxystearic acid) surfa ctant molecule. The amount of dispersant can be optimized to reduce vi scosity and yield point of the suspension, and to eliminate thixotropi c hysteresis. The use of a dispersion medium of lower viscosity than t he dispersant makes it easy to detect when complete monolayer coverage has been achieved. The loss of pseudoplasticity, brought about by a h igher degree of deflocculation, can be recovered by increasing the vol ume fraction of solids of a suspension and this is beneficial in the p lastic forming of ceramics. Rheological measurements showed that these suspensions reach a ''critical state'' above a critical shear stress (tau(c)). This critical state is described by severaI parameters, i.e. , the Bingham yield stress (tau(B)), the plastic viscosity (eta(PL)), and the critical shear rate (gamma(c)), which are dependent on the vol ume fraction of solids.