Interactions between polymeric dispersants and colloidal ceramic surfaces
Increased use of environmentally acceptable chemicals and reduced energy consumption are two foreseeable demands within the ceramics industry. This will require the replacement of organic solvents and lowering of sintering temperatures. To meet these demands, new dispersants will be needed for colloidal processing of highly sinterable ceramic nanopowders. When selecting new dispersants, it is essential to understand the relations between molecular structures and adsorption properties of present dispersants. A deeper understanding of the properties of adsorbed layers, such as viscoelasticity, is also necessary to design dispersants for improved fluid properties and consolidation behavior of highly concentrated systems. The present work has been devoted to increasing the knowledge in this area.
Interactions between polymeric dispersants and ceramic surfaces have been studied with a combination of surface characterization techniques (quartz crystal microbalance with dissipation (QCM-D), atomic force microscopy (AFM) and zeta potential) and rheological evaluations. More specifically, the efficiency of polyelectrolyte type dispersants represented by a poly(acrylic acid) and a lignosulphonate were compared to that of graft copolymers with nonionic chains of different molecular weights. A novel approach of this research work has been to use hydrophilic block and graft copolymers in aqueous systems and in mixtures of water and alcohols. The water content of adsorbed dispersant layers was determined by combining mass adsorption data from QCM-D measurements with mass optimization by viscosity measurements of alumina suspensions. Water was found to constitute up to 90% of the total adsorbed layer of a comb copolymer with long polyoxyethylene chains. Viscoelastic modeling also showed that these layers were viscous and thick, about 6 nm. It was also found that plastic behavior of wet consolidated bodies was obtained when a block copolymer with shorter polyoxyethylene chains was used, probably due to a lubrication effect from bound water. In contrast to this, both polyelectrolytes adsorbed in thin, highly rigid layers with relatively low water content, about 30%. Brittle behavior of wet bodies stabilized by poly(acrylic acid) indicated lack of lubrication effect with the polyelectrolyte dispersant. AFM showed that both polyelectrolytes gave efficient electrostatic stabilization, while the long comb copolymer provided steric stabilization.
Moderate additions (25%) of isopropanol to water systems were shown to improve long-term stability of ceramic dispersions with slightly soluble species. This was demonstrated for an alumina suspension with magnesia added as a grain growth inhibitor.