HYDROPHOBICALLY-MODIFIED URETHANE ETHOXYLATE (HEUR) ASSOCIATIVE THICKENERS .2. INTERACTION WITH LATEX

The rheological behaviour of model latexes thickened with hydrophobica lly 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 thickeni ng efficiency of the HEUR. Measurements of the electrophoretic mobilit y of the latex particles at different HEUR concentrations indicate ads orption of the thickener in the absence of surfactant and in the prese nce of nonylphenol ethoxylate, NPE10. However, the HEUR thickeners do not adsorb to the latex particles in the presence of sodium dodecyl su lphate (SDS) at levels above the CMC of the surfactant. This can be ex plained by SDS-HEUR interactions in the solution. Both adsorption and rheological measurements support a latex-HEUR association mechanism, a ccording to which the particles are incorporated through adsorption of their hydrophobic end-groups into a three-dimensional transient thick ener network. At a given volume fraction, latexes with a small particl e size present a larger number of potential crosslink points at shorte r distances from each other. This leads to a synergistic increase in v iscosity 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-rat e viscosity. This may be explained by the assumption that the latex pa rticles introduce stronger crosslink points in the network in addition to the micelle-like hydrophobic aggregates which act as crosslink poi nts in aqueous solutions of HEUR. Variations in the chemistry of the H EUR affect both the level of the low-shear-rate viscosity and the shea r thinning behaviour of the thickened latex. Octadecyl-terminated HEUR s 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 i n increasing the viscosity at low and medium shear rates. The effect o f the molecular weight of the HEUR is mainly to increase the length of the network junctions between the crosslink points. As a result the g reatest thickener efficiency is obtained with medium molecular weights . The effects on the rheology of thickened latex obtained by varying t he HEUR composition parallel the effects seen in aqueous solution. Thi s does not mean that the solution properties dominate in the rheologic al behaviour, but rather that both types of crosslinks present in the HEUR-latex network are affected in similar ways by variations in the H EUR chemistry. This is reasonable, as both crosslinks are a product of hydrophobic interactions.