RHEOLOGICAL BEHAVIOR OF STABILIZED AQUEOUS ALUMINA DISPERSIONS IN PRESENCE OF HYDROXYETHYL CELLULOSE

The flocculation behaviour of sterically stabilised aqueous alpha-Al2O 3 dispersions in presence of hydroxyethyl cellulose (HEC) of various m olecular weights has been studied by means of theology. Steady state s hear stress-shear rate measurements as well as oscillatory measurement s suggest the existence of a critical flocculation polymer volume frac tion (CFF) Phi(p)(+) above which the rheological properties of the dis persions deviate from those observed when the dispersion contains no a dded polymer. The ceramic powder had an average diameter of 0.4 mu m a nd was stabilised in the dispersion by the polyelectrolyte ammonium po lymethylmethacrylate. Both Phi(p) (polymer volume fraction) and Phi(s) (solids volume fraction) had a positive effect on the storage modulus G'. A Sisko type equation of the form eta = eta(infinity) + K gamma(n -1) was found to closely Jit the shear stress-shear rate data. The val ue of K, which can be seen as an effective yield stress of the suspens ion, was also found to increase with Phi(p) and Phi(s). The exponent n was found to strongly depend on Phi(s) and it was essentially insensi tive to Phi(p) above the critical flocculation fraction. This value de creased with increasing volume fraction. The polymer molecular weight was found to affect positively both G' and K although the effect on K was more pronounced. A depletion flocculation mechanism, whereby the H EC, which is non adsorbing on the alumina particles (Liang, W., Tadros , Th.F. and Luckham, P., Rheological properties of concentrated steric ally stabilized latex dispersions in the presence of hydroxyethyl cell ulose. J. Colloid Interface Sci., 1993, 160, 183-189), is depleted dur ing a particle collision, giving rise to an osmotic gradient, is sugge sted to explain the behaviour of the system. (C) 1997 Elsevier Science Limited. All rights reserved.