The rheological behaviour of model latexes thickened with hydrophobically modified ethylene oxide-urethane block copolymers, i.e. HEUR type associative thickeners, is described. The latex particle size, as well as the type and level of surfactant used, greatly affect the thickening efficiency of the HEUR. Measurements of the electrophoretic mobility of the latex particles at different HEUR concentrations indicate adsorption of the thickener in the absence of surfactant and in the presence of nonylphenol ethoxylate, NPE10. However, the HEUR thickeners do not adsorb to the latex particles in the presence of sodium dodecyl sulphate (SDS) at levels above the CMC of the surfactant. This can be explained by SDS-HEUR interactions in the solution. Both adsorption and rheological measurements support a latex-HEUR association mechanism, according to which the particles are incorporated through adsorption of their hydrophobic end-groups into a three-dimensional transient thickener network. At a given volume fraction, latexes with a small particle size present a larger number of potential crosslink points at shorter distances from each other. This leads to a synergistic increase in viscosity on addition of HEUR to the latex. A HEUR-thickened latex that exhibits such a synergistic viscosity increase is more shear thinning and elastic than an aqueous HEUR solution of comparable low-shear-rate viscosity. This may be explained by the assumption that the latex particles introduce stronger crosslink points in the network in addition to the micelle-like hydrophobic aggregates which act as crosslink points in aqueous solutions of HEUR. Variations in the chemistry of the HEUR affect both the level of the low-shear-rate viscosity and the shear thinning behaviour of the thickened latex. Octadecyl-terminated HEURs produce latex systems that are more shear thinning and elastic than HEURs with shorter hydrophobic modifications (i.e. with pentadecyl or 9-heptadecenyl terminations). However, the latter are less efficient in increasing the viscosity at low and medium shear rates. The effect of the molecular weight of the HEUR is mainly to increase the length of the network junctions between the crosslink points. As a result the greatest thickener efficiency is obtained with medium molecular weights. The effects on the rheology of thickened latex obtained by varying the HEUR composition parallel the effects seen in aqueous solution. This does not mean that the solution properties dominate in the rheological behaviour, but rather that both types of crosslinks present in the HEUR-latex network are affected in similar ways by variations in the HEUR chemistry. This is reasonable, as both crosslinks are a product of hydrophobic interactions.
