RHEOLOGICAL BEHAVIOR OF HIGHLY CONCENTRATED AQUEOUS SILICA SUSPENSIONS IN THE PRESENCE OF SODIUM-NITRATE AND POLYETHYLENE OXIDE

The shear flow properties of aqueous silica suspensions at different S olids loading in the presence of polyethylene oxide (PEG) and added sa lt (NaNO3) have been investigated. Particular attention is given to th e evaluation of the role of electrolyte concentration, pH, and polymer concentration in viscosity reduction. The variation of the rheologica l properties with shear rate, solids loading, particle size, pH, elect rolyte concentration, PEO concentration, and its molecular weight are discussed. The viscosity of the silica suspension was determined to be a decreasing function of the particle diameter and at 54% volume, the effect of particle size was scaled using Peclet number and relative v iscosity of the suspension (hard sphere scaling). The flow properties of silica suspensions are highly affected by the pH of the suspending media and concentration of the added salt. At a fixed electrolyte conc entration, the viscosity decreases by increasing pH, reaches a minimum , and then increases with further increase in pH. At a fixed pH level, the same behavior was observed for viscosity as a function of electro lyte concentration. The effect of the electrolyte concentration is mor e significant at lower shear rates. It is shown that the rheological p roperties of silica suspensions are strongly affected by the concentra tion of the added polymer and its molecular weight. For each sample, t here is a critical amount of polymer that must be added before low vis cosities result. There is a nearly linear relation between the critica l polymer concentration and volume fraction of the particles. The resu lts of this work provide useful information for selection of a proper pH range, electrolyte concentration, and optimum polymer concentration to control the viscosity behavior of highly concentrated silica slurr ies. (C) 1996 Society of Rheology.