Electrosteric Stabilization of Al2O3, ZrO2, and 3Y–ZrO2 Suspensions: Effect of Dissociation and Type of Polyelectrolyte

Abstract

The mechanisms of eight anionic polyelectrolytes stabilizing colloidal sized α-Al2O3, pure ZrO2, and Y2O3-doped ZrO2 particles in aqueous solution are discussed. The polyelectrolytes studied were the Na+ and NH4+ salts of polyacrylic acid and polymethacrylic acid having different molecular weights. The particle–dispersant interactions were studied by measuring adsorption isotherms, particle size, thickness of adsorbed layer, and zeta potentials by elektrokinetic sonic analysis at different powder volume fractions (φ=0.01–0.3), pH, and electrolyte (KCl) content. The dissociation of the polyelectrolytes was studied by potentiometric titrations. The dissociation constant of the polymethacrylates was found to be 0.6 pH unit higher than that for the polyacrylates. High-affinity adsorption isotherms were observed over the pH range when the polyelectrolytes were fully ionized. The results show good correlation between adsorption isotherms and zeta potential data in systems of dispersed, dilute alumina particles. When particles and polymers were of equal charge (the same sign of charge) the polymer shell was thicker. At higher volume fractions (φ=0.3), and when alumina particles/added ammonium polyelectrolyte were of equal charge, a maximum in the absolute value of zeta potential resulted. Due to adsorption all the anionic polyelectrolytes studied provided electrosteric stabilization of the α-Al2O3, and Y2O3-doped ZrO2 suspensions by enhancing the zeta potential to 40 mV or over and by shifting the isoelectric point to lower pH, the low-molecular-weight polyelectrolytes decreasing the isoelectric point more than the polyelectrolytes having higher molecular weight. The polyelectrolytes studied failed to stabilize pure monoclinic ZrO2 particles. Due to the shortness of the chain of polyelectrolytes studied, no bridging was observed between oppositely charged polyelectrolyte/alumina particles.