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

Abstract

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 TiO 2 dispersions was investigated using particle size (determined by static light scattering), zeta potential and rheological measurements. Diluted dispersions as well as those containing 20% TiO 2 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 TiO 2 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 TiO 2 dispersions at pH = 3 showed neutralization of the positive charge on TiO 2 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 TiO 2 dispersion causes a dramatic reduction in yield value when PA concentration is ≥1%. Oscillatory measurements were obtained for 30% TiO 2 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 TiO 2 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.