Rheological studies of sterically stabilised concentrated polystyrene latex dispersions under conditions of incipient flocculation

Abstract

Measurements of shear stress-shear rate curves were made on concentrated aqueous polystyrene latex dispersions, stabilised with physically adsorbed poly(vinyl alcohol). Measurements were carried out under conditions of incipient flocculation, produced by addition of KCl or Na₂SO₄ at constant temperature (25°C) or by raising the temperature of the dispersion, at constant electrolyte concentration. The results showed a change in the rheological behaviour of the dispersion from Newtonian to pseudoplastic (with hysteresis i.e. thixotropy) at a critical electrolyte concentration or at a critical temperature at constant electrolyte concentration. From plots of the extrapolated yield value versus electrolyte concentratioin and temperature (at constant electrolyte concentration) the critical electrolyte flocculation concentration (CFC) and the critical flocculation temperature (CFT) were established for the concentrated dispersion. The CFC of the concentrated dispersion was significantly lower than the corresponding value for a dilute dispersion. Above at CFC and CFT, the yield value τ_ß showed an increase with increase of electrolyte concentration and with temperature. The results were quantitatively analysed using the elastic floc model proposed by Hunter and coworkers. Good agreement between the experimental yield values and those calculated from theory was obtained, thus confirming an applicability of the elastic floc model to the case of flocculating sterically stabilised dispersions.