Shear-thickening in aqueous surfactant-associative thickener mixtures

Associative thickeners represent an important class of rheology modifiers used in waterborne coatings. Understanding molecular level interactions between associative thickeners and surfactants has been the subject of a number of prior studies. Our recent studies focused on the behavior of a hydrophobically modified, aminoplast ether (HEAT) associative thickener and a highly hydrophobic ethoxylated octylphenol surfactant in aqueous solution. Aqueous blends of these two materials exhibit shear-thinning, as well as rarely reported, transient, shear-induced thickening behavior. In addition, the same compositions exhibit both thixotropy and antithixotropy. The shear-induced thickening is shown to be the result of transient aggregated structures formed under shear. Addition of a third component, β-cyclodextrin—a molecule known to disrupt hydrophobic associations—to the mixture helped us advance the understanding of the nature of associative thickener–surfactant interactions that cause the transient shear-thickening behavior. Results indicate that, while overall viscosity of the HEAT/surfactant mixtures is decreased by β-cyclodextrin, the shear-induced thickening is unaffected. An intermolecular interaction model to describe the transient thickening mechanism is presented.