THE EFFECT OF LATEX PARTICLE-SIZE ON THE RHEOLOGY OF THICKENED DISPERSIONS

The rheological behavior of model 100-nm poly(methyl methacrylate) (PMMA) latices thickened with hydroxyethyl cellulose (HEC) and with commercial Hydrophobe-modified Ethoxylated URethane (HEUR) associative polymers are compared with thickened 220-nm model PMMA latices. This study also includes a model film-forming (i.e., low glass transition temperature) methyl acrylate copolymer latex and two commercial 100-nm latices. In HEC-thickened pigmented and unpigmented dispersions, the smaller latices (at 0.32 volume fraction) are higher in viscosity due to a greater surface area and the related effect of hydration on the effective volume fraction of the dispersion. In HEUR associative thickeners, the viscosity of the smaller particle size latex is also influenced by the hydrophobicity of the synthesis surfactant, its concentration in the aqueous phase, particle separation distances and ion-dipole interactions. The viscosity is also dependent on the ratio of free surfactant to associative thickener concentration, the amount of formulation surfactant added with titanium dioxide in a pigmented coating, and on the adsorption of HEUR thickener on the latex, as determined by the relative size of the HEUR hydrophobes to that of the conventional surfactants. The studies indicate that HEUR associative thickeners facilitate lower viscosities at low shear rates (not attainable with cellulose ethers) in 100-nm latices, due to network disruption related to an excess of conventional surfactant in the total formulation.