EFFECTS OF POLYMER ADSORPTION AND DESORPTION ON RHEOLOGICAL PROPERTIES OF SILICA SUSPENSIONS

Shear stresses and dynamic moduli of silica suspensions in aqueous hydroxypropyl methyl cellulose (HPMC) solutions were measured by using a cone-plate geometry as functions of the concentrations of silica, HPMC, isopropyl alcohol, acetic acid and pyridine. Isopropyl alcohol and pyridine behaved as displacers and they cause desorption of HPMC from the silica surfaces. The amounts of HPMC adsorbed on the silica surfaces decrease with an increase in the displacer concentration. In plots of steady state shear stress against shear rate, there was a shear rate of about 0.1 s-1 due to aggregation of the silica particles and this did not disappear on addition of isopropyl alcohol or acetic acid. However, the silica suspensions with pyridine must show some stress responses, although these may be outside the measurement range for the instrument. Pyridine increases the pH in the dispersion, leading to stable and unaggregated silica particles. By adding isopropyl alcohol to the silica suspensions, the storage modulus decreases with the amount of HPMC desorbed from the silica surfaces. The larger amounts of HPMC desorbed resulted in no second plateau in the dynamic moduli, showing the weaker aggregated structures in the silica suspensions. In contrast, pyridine had no detectable effect on the dynamic moduli of the silica suspensions.