Coating formulation, thickener-latex associations

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. 

HMHEC polymers are improved rheological modifiers for latex paints. They retain the benefits of traditional cellulosic ethers (compatibility with a wide range of coatings ingredients, storage stability, better sag resistance, etc.) but overcome many deficiencies of synthetic associative thickeners (sensitivity to latex systems and pH of paint formulations, poor sag resistance, etc.). The combined advantages offered by HMHEC are best suited for interior and exterior flat paints (5). 

The studies just discussed do not address one of the significant contributions associative thickeners make to coatings rheology minimizing LSVs in small-particle latex formulations. 

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