Binder Distribution in Spray-Dried Alumina Agglomerates

Abstract

Organic binders, especially those which are water-soluble, tend to migrate with the solvent flow which occurs during the spray-drying process of ceramic suspensions. The adsorption isotherm of PVA on the surface of alumina particles 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 weight basis of alumina) was adsorbed onto the alumina surface, and the main fraction of ‘free’, non-adsorbed, binder was able to migrate during drying. Images of the distribution of the polyvinyl alcohol in the alumina agglomerates and in the green compacts indicate an inhomogeneous distribution of the binder. The PVA segregates to the surface of the granules and leads to the formation of a polymer-rich layer. The thickness of this layer, which contains about 50% of the binder introduced, was estimated to be a few percent of the granule radius.