The role of ammonium polyacrylate in dispersing concentrated alumina suspensions

The role of low molecular weight ammonium polyacrylate (NHPA) has been char acterised as a dispersant for the colloidal powder processing of high solid s content alumina suspensions. It has been found to be a very effective dis persant under the correct conditions, yielding fluid, highly homogeneous aq ueous suspensions. Minimum suspension viscosities, one measure of the degre e of suspension dispersion, were observed at two different NHPA. concentrat ions for each solids loading examined. The first viscosity minimum was inde pendent of the solids loading; however it produced bodies of relatively low green density. The second viscosity minimum increased with increasing soli ds loading and yielded components of the highest green density. Adsorption data revealed that maximum adsorption of NHPA onto the alumina surface occu rred at this secondary minimum concentration. From the evidence presented i t is concluded that the first minimum is rather unstable, decaying as the s ample is dried. The second minimum is the more stable, yielding the most fa vourable properties expected for a dispersed suspension. Predictions of the adsorbed polymer configuration are offered as an explanation for the prese nce of the two minima. (C) 2000 Elsevier Science Ltd. All rights reserved.