The role of ammonium polyacrylate in dispersing concentrated alumina suspensions

The role of low molecular weight ammonium polyacrylate (NHPA) has been characterised as a dispersant for the colloidal powder processing of high solids content alumina suspensions. It has been found to be a very effective dispersant under the correct conditions, yielding fluid, highly homogeneous aqueous suspensions. Minimum suspension viscosities, one measure of the degree of suspension dispersion, were observed at two different NHPA concentrations for each solids loading examined. The first viscosity minimum was independent of the solids loading; however it produced bodies of relatively low green density. The second viscosity minimum increased with increasing solids loading and yielded components of the highest green density. Adsorption data revealed that maximum adsorption of NHPA onto the alumina surface occurred at this secondary minimum concentration. From the evidence presented it is concluded that the first minimum is rather unstable, decaying as the sample is dried. The second minimum is the more stable, yielding the most favourable properties expected for a dispersed suspension. Predictions of the adsorbed polymer configuration are offered as an explanation for the presence of the two minima.