The Colloidal Properties and Rheological Behavior of Precipitated Calcium Carbonate Suspensions Dispersed with Sodium Polyacrylate

The colloidal properties and the rheological behavior of precipitated calcium carbonate suspensions were investigated by changing the ionic group density, molecular weight, and dose level of sodium polyacrylate dispersants and by adding various electrolytes. The amount of dispersant in the aqueous phase, adsorbed onto the pigment particles, and the concentrations of soluble carbonate and bicarbonate were determined using attenuated total reflection infrared spectroscopy. The total divalent cation concentration in the aqueous phase of the suspensions was measured by complexometric titrations with Na2H2EDTA. The divalent cation concentration correlated linearly with the solution concentration of dispersant for suspensions without electrolytes. The concentration of divalent cations relative to the dispersant concentration varied with the addition of electrolytes and it was observed that increased association of a dispersant with divalent cations increased the adsorbed amount of dispersant. Negative zeta potentials were seen for all suspensions, with similar values and trends for suspensions dispersed with dispersants of the same ionic group density. The addition of electrolytes altered zeta potentials. Most notably high doses of salts of divalent cations, significantly decreased the magnitudes of zeta potentials. Further, the addition of electrolytes decreased the Debye length of slurries. For all suspensions, shear thinning occurred at low shear rates and shear thickening was observed at medium-high shear rates. Viscosity curves for suspensions prepared with the low and medium ionic group density dispersants correlated well with zeta potential. The elastic modulus curves as a function of strain increased as the degree of shear thinning of the suspensions increased. Measurement of the viscosity at a constant shear rate revealed that shear induced flocculation was apparent for suspensions with low doses of dispersant. Two different batches of calcium carbonate pigment were used and, while differences in the absolute values were observed between suspensions containing the two, relative trends remained the same. The results indicate that the rheological behavior of suspensions can be improved by using a dispersant that would provide surface charge similar to the dispersants used while providing less unassociated counterions to the aqueous phase