Hydrophobically associating polymer shear rate effects

In HEC-thickened formulations, low-shear-rate viscosities increase with decreasing latex particle size. This effect has been a major limitation in formulating small-particle latices. The phenomenon appears to arise from electro viscous, hydration, or flocculation effects, not a depletion layer mechanism. Associative thickeners achieve efficient viscosity in coating formulations via participation in synthesis and formulation surfactant micelles to form pseudo macromolecules and via an ion-dipole interaction between the cations of surface carboxylate groups on the latex and the ether linkages of the associative thickener. Generally, an excess of synthesis surfactant is found in the production of small-particle latices. The achievement of lower viscosities in small-particle ( 100 nm) latex coatings thickened with associative thickener appears to occur by extensive disruption of the polymer hydrophobe s participation in intermicellar networks. 

Itmay be expected that aqueous solutions of such hydrophobically "crosslinked" polymers show a shearaqueous solution of the 5 mol % Chol-Cs-MA polymer at 30 C is independent of Ae shear rate up to 250 s. This observation suggests that hydrophobic association between cMesterol pendants is tight enough not to be dissociated by mechanical shear. 

Additional evidence for intermolecular hydrophobic association is provided by the effects on the Brookfield viscosity by addition of NaCLThus, the viscosity of the copolymer solution increases with increasing NaCl concentration, especially at lower shear rate (0.4 s ) (Fig. 7.6). At polymer concentrations above 2000 ppm, the viscosity increases are especially pronounced between 1.0 and 5.0... 

---