Interactions in associative thickener/220 nm methyl methacrylate latex dispersions

Abstract

The rheology of commercial associative thickener dispersions containing 220 nm ‘model’ methyl methacrylate (MMA) latices have been examined. These high T g MMA latices were synthesized by a semicontinuous process with variable percentages of methacrylic acid (MAA). The latex surface without monomer acid segments was only 70% saturated. The latices stabilized with neutralized methacrylic acid segments provided surfactant-saturated surfaces and uniform dispersions when thickened. With the acid-monomer-free latex, the addition of surfactant to saturate the surface of the latex and to increase the free surfactant concentration in the aqueous phase to the level needed to optimize viscosity by intermicellar participation did not effect viscosities equivalent to those noted in associative thickener dispersions containing acid-stabilized latices. These observations support the ion—dipole associative thickener mechanism previously proposed. The viscoelastic behavior of associative thickener dispersions containing stabilized MMA/MAA latices neutralized with ammonium and alkali metal hydroxides has also been examined. The viscosities of all dispersions containing associative thickeners were highest with NH 4OH-neutralized latices. These results are interpreted in terms of the greater solubilizing power and lower p K a value of the ammonium salt. The viscosity of hydroxy-ethylcellulose (HEC) thickened dispersions was found to be insensitive to the free surfactant concentration and the nature of the base cation, with only minimal changes in low shear rate viscosities (LSVs) being observed with increasing latex surface acids. These variables notably influence the LSV of the associative thickeners (AT), the magnitude of the latter variations being specific to the AT employed. In oscillatory experiments, dispersions containing commercial associative thickeners, used predominately with acrylic latices in the U.S. and Australian markets, were observed to exhibit large viscous components. A large elastic component, reflecting thickening via network formation, was noted with commercial associative thickener used largely with commercial vinyl/acrylic latices.