Rheological investigations on the influence of addition of sodium polyacrylate to titanium dioxide suspensions

The influence of addition of sodium polyacrylate (with 3 different molecular weights PA20 = 2500 g/mol, PA30 = 8000 g/mol and PA40 = 15,000 g/mol) on the stability of TiO2 dispersions was investigated using particle size (determined by static light scattering), zeta potential and rheological measurements. Diluted dispersions as well as those containing 20% TiO2 were dispersed by sonication for 5 min. The results show that without sonication and in the absence of NaPAA large aggregates with mean volume diameter of 3.89 μm are produced. On sonication for 5 min, the aggregates are broken down giving a mean volume diameter of 0.239 μm. Addition of NaPAA at low concentration (0.2 and 0.4%) resulted in a large increase in mean volume diameter indicating aggregation of the initially formed small aggregates on sonication. When the concentration of NaPAA was ≥1%, the aggregates became dispersed giving mean volume diameter of about 0.2 μm. These particle size measurements were confirmed by using SEM and TEM measurements. The zeta potential of diluted TiO2 dispersion in the absence of NaPAA gave an isoelectric point of pH ∼ 6. The effect of addition of sodium polyacrylate PA20, PA30 and PA40 on the zeta potential of TiO2 dispersions at pH = 3 showed neutralization of the positive charge on TiO2 particles reaching a zero charge at PAA concentration of 0.3–0.4% above which the particles acquired a negative charge that increase with further increase of PAA concentration reaching a plateau value of −45 mV when the PAA concentration was ≥1%. Steady state measurement showed that addition of PA20 to the TiO2 dispersion causes a dramatic reduction in yield value when PA concentration is ≥1%. Oscillatory measurements were obtained for 30% TiO2 suspension without any sonication. The frequency was kept constant at 1 Hz, and the stress was gradually increased till a critical value was reached above which the modulus showed a rapid decrease with further increase of the stress. In this way both elastic modulus G′ and critical stress were measured as a function of PAA concentration. The results showed an initial increase in the elastic modulus reaching a maximum at critical PAA concentration above which there was a rapid reduction in both rheological parameters. These results indicated flocculation of TiO2 dispersion at low PAA concentration which was accounted for/by charge neutralization and/or bridging. When the PAA concentration was ≥1% all dispersions showed low G′ and critical stress indicating a highly deflocculated system. An attempt was made to correlate the rheological results with zeta potential measurements.

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► By combining rheological tests, zeta potential and particle size distribution measurements, a clear picture was obtained on the effect of sodium polyacrylate on the stability of TiO2 suspensions.
► The initial flocculation can be accounted for by charge neutralization and/or bridging. The high stability obtained at high NaPAA concentration results from a combination of both electrostatic and steric stabilization.
► An attempt was made to correlate the rheological results with zeta potential measurements.