Adsorption of hydrophobically-modified, ethoxylated urethane thickeners on latex and titanium dioxide disperse phases

Abstract

Early developments in the use of water-soluble polymers as rheology modifiers in water-borne latex coatings are discussed, with an emphasis on the adsorption of hydrophobically-modified, ethoxylated urethane (HEUR) on the disperse phases, latex and TiO 2. The surfaces of the latex and pigment are distinctly different and this influences the nature of the adsorption. The amount of latex surface acid stabilizer, pH of the dispersion, neutralization cation, and the median size of the latex are important parameters in the adsorption process. In addition to the associative thickener, there will be various types of surfactants in a coating formulation, and their influence on the adsorption of HEURs in aqueous dispersions are considered. When the traditional HEUR thickener with only terminally active hydrophobes is used as a thickener and stabilizer, the size and concentration of its terminal hydrophobes relative to that of the free surfactant are important to the amount of HEUR adsorbed. Adsorption on TiO 2 offers a significantly different set of variables. The adsorption is influenced by the dispersion's pH and the p K a, of the metal oxide treatment of the TiO 2 surface. Adsorption on these surfaces is determined by the acid–base or hydrogen-bonding interactions. When the traditional HEUR thickener with only terminally active hydrophobes is used as a thickener and stabilizer, interaction with the TiO 2 particle requires a preadsorbed co-oligomerized dispersant that contains hydrophobic groups. This is not a requirement for adsorption when HEURs have hydrophobes placed pendant to the chain (i.e. as teeth in a comb). The comb HEURs also appear to destabilize the small particle latex dispersions.