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 suspensions of zirconia in kerosene. Rheological measurements and sediment packing 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) surfactant molecule. The amount of dispersant can be optimized to reduce viscosity and yield point of the suspension, and to eliminate thixotropic hysteresis. The use of a dispersion medium of lower viscosity than the dispersant makes it easy to detect when complete monolayer coverage has been achieved. The loss of pseudoplasticity, brought about by a higher degree of deflocculation, can be recovered by increasing the volume fraction of solids of a suspension and this is beneficial in the plastic forming of ceramics. Rheological measurements showed that these suspensions reach a “critical state” above a critical shear stress (τc). This critical state is described by several parameters, i.e., the Bingham yield stress (τb), the plastic viscosity (νPl), and the critical shear rate (γc), which are dependent on the volume fraction of solids.