Binder distribution in spray-dried alumina agglomerates

Organic binders, especially those which are water-soluble, tend to migrate with the solvent flow which occurs during the spray-drying process of ceram ic suspensions. The adsorption isotherm of PVA on the surface of alumina pa rticles shows a rather low affinity of the polymer when compared to that of the ammonium polymethacrylate. Only 6% of the added PVA (3wt% on a dry wei ght basis of alumina) was adsorbed onto the alumina surface, and the main f raction of 'free', non-adsorbed, binder was able to migrate during drying. Images of the distribution of the polyvinyl alcohol in the alumina agglomer ates and in the green compacts indicate an inhomogeneous distribution of th e binder. The PVA segregates to the surface of the granules and leads to th e formation of a polymer-rich layer. The thickness of this layer, which con tains about 50% of the binder introduced, was estimated to be a few percent of the granule radius. (C) 1998 Elsevier Science Limited. All rights reser ved.