Rheological behaviour of stabilised aqueous alumina dispersions in presence of hydroxyethyl cellulose

The flocculation behaviour of sterically stabilised aqueous alpha-Al2O3 dispersions in presence of hydroxyethyl cellulose (HEC) of various molecular weights has been studied by means of theology. Steady state shear stress-shear rate measurements as well as oscillatory measurements suggest the existence of a critical flocculation polymer volume fraction (CFF) Phi(p)(+) above which the rheological properties of the dispersions deviate from those observed when the dispersion contains no added polymer. The ceramic powder had an average diameter of 0.4 mu m and was stabilised in the dispersion by the polyelectrolyte ammonium polymethylmethacrylate. 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 value of K, which can be seen as an effective yield stress of the suspension, 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 insensitive to Phi(p) above the critical flocculation fraction. This value decreased 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 HEC, which is non adsorbing on the alumina particles (Liang, W., Tadros, Th.F. and Luckham, P., Rheological properties of concentrated sterically stabilized latex dispersions in the presence of hydroxyethyl cellulose. J. Colloid Interface Sci., 1993, 160, 183-189), is depleted during a particle collision, giving rise to an osmotic gradient, is suggested to explain the behaviour of the system. (C) 1997 Elsevier Science Limited. All rights reserved.