Influence of the first normal stress differences on model hydrophobically modified ethoxylated urethane-thickened waterborne paints brush drag

Abstract

Commercial paints are complex formulations and their application properties are typically explained based on their steady shear viscosity profiles. Hydrophobically modified ethoxylated urethane (HEUR) polymers are widely used as rheology modifiers (RM) in waterborne paints. In this work we formulated a series of HEUR-based model paints with matching high-shear viscosity (0.12 Pa-s at 10,000 s−1). Compositionally these paints were similar, differing only in the HEUR end-hydrophobe length (Cn, n = 10–18) and the amount used to obtain the targeted high-shear viscosity. Their linear viscoelastic response was dominated by a transient network of HEUR-bridged latex particles as revealed from oscillatory shear measurements. The low (steady) shear viscosities were related to the HEUR end-hydrophobe length while the high-shear viscosities tend to approach the targeted value. The first normal stress difference (N1) at high shear changed sign from negative for low Cn (n = 10, 12) to positive for high Cn (n = 16, 18) paints. Further, brush drag was measured in real time where the typical application shear rate was ≈ 10,000 s−1. Paint brush drag was found to increase monotonically with HEUR Cn and this variation/trend could not be explained through their high-shear viscosity. It is proposed that the observed trend can be explained based on the paint N1 sign, where a negative N1 provides a compressive force pulling the brush towards the surface and facilitates paint transfer at relatively lower drag. In contrast, a positive N1 exerts a tensile force tending to pull the brush out from the surface and extra force is required to overcome this effect. This work emphasizes the importance of normal stress (along with viscosity) to understand paint properties, especially for high-shear applications.