Study of interactions between polyelectrolyte dispersants, alumina and latex binders by rheological characterisation

The influence of varied polyelectrolyte concentrations in alumina suspensions with and without the addition of a latex binder has been studied using rheological measurements. Two types of Eater binders (one anionic and another one nonionic) and a polyacrylic dispersant were used, and these systems have earlier been successfully used for tape casting of alumina. The stability of the suspensions was characterised using continuous shear measurements for viscosity data and oscillatory procedures for viscoelastic properties. Excess polyelectrolyte dispersant, even with a low molecular weight of about 5000, leads to increased viscosity and elasticity in highly loaded suspensions, volume fraction 0.55. The systems, however, are stable as no thixotropy effects were observed. The mechanism causing this phenomenon was found to be a combination of three mechanisms; (i) increased rigidity from the additional, charged particles which the polyelectrolyte at excess addition can be considered as, (ii) decreased electrostatic repulsion as a result of the increased ionic strength from the counterions of the polyelectrolyte, and (iii) increased viscosity of the liquid phase from free polymer. It was found that the optimum dispersant concentration in the mixed systems with latex added was higher than in the pure alumina system. It was also found that a correct dispersant level is critical as interactions were observed both for the anionic and the nonionic latex. Adsorption of the dispersant onto the latex-particles, both the anionic and nonionic ones, results in bridging flocculation at a low dispersant level. (C) 1998 Elsevier Science Limited. All rights reserved.