The effect of electrolyte content on synthetic latex flow behavior

Usually a synthetic latex contains inorganic salts from the water-soluble initiator used in polymerization and from the impurities in the surfactant used to stabilize the latex. Upon removal of this electrolyte, by dialysis, the viscosity of the latex increases and becomes markedly shear thinning, i.e., non-Newtonian. When an electrolyte such as sodium chloride is added, the viscosity decreases immediately and may even become less than the viscosity of the original latex, and the flow behavior becomes Newtonian.

Measurements show that the only significant change in the latex system is in the gegenion concentration. The particle size distribution measured on diluted latices does not change through all these viscosity changes and only a small fraction of the adsorbed surfactant is desorbed and lost upon dialysis. Upon addition of sufficient electrolyte a minimum viscosity is obtained which is that which would have been predicted by Mooney's equation.(J. Colloid Sci. 6, 162 (1951))

The use of Mooney's equation to obtain effective volume fractions from viscosities shows that the experimental data are consistent with the viewpoint that the effective volume fraction is given by the polymer volume plus the volume excluded to particles by the interaction of the charged double layers. Thus, the observed phenomenon is also consistent with a large body of colloidal theory and experiment.