Hydrophobically modified urethane—ethoxylate (HEUR) associative thickeners 1. Rheology of aqueous solutions and interactions with surfactants

Abstract

The rheological properties of aqueous solutions of hydrophobically modified ethylene oxide—urethane block copolymers, or HEUR thickeners, of different chemical compositions have been studied. A steep increase in viscosity with increasing thickener concentration is obtained at a concentration that increases with decreasing polymer molecular weight as predicted by a simple network model. However, this model does not predict the efficiency of the thickener, which is greatly affected by the hydrophobicity of the terminal hydrocarbon chains and decreases in the order octadecyl / pentadecyl / 9-heptadecenyl. The more hydrophobic the end groups, the stronger are the cross-link points in the network, and as a consequence the more elastic is the solution. In HEUR solutions adjusted to the low shear viscosity of 10 Pa s, 6–12% of the thickener molecules are elastically effective in the network and the upper limit of the aggregation number is 40–60. The addition of surfactant increases the viscosity of the HEUR solutions, until, for most systems, a maximum is reached, after which the viscosity decreases. The magnitude of the variation in viscosity is dependent on the chemical structures of both the polymer and the surfactant. The observed differences can be explained qualitatively by stoichiometric considerations of the mixed aggregates that are assumed to be formed between the hydrophobic chains of the polymer and the surfactant. Additional information relating to the HEUR-surfactant interaction was obtained from surface tension measurements. A non-ionic surfactant is bound by a non-cooperative process only to the hydrophobic aggregates of the HEUR. Sodium dodecyl sulphate is bound both to the hydrophobic domains and to the ethylene oxide blocks of the HEUR polymers.