Shear-Dependent Interactions in Hydrophobically Modified Ethylene Oxide Urethane (HEUR) Based Coatings: Mesoscale Structure and Viscosity

We have investigated the in situ mesoscale structure of paint formulations under shear using ultra small-angle neutron scattering (rheo-USANS). Contrast match conditions were utilized to independently probe the latex binder particle aggregates and the TiO₂ pigment particle aggregates. Two different latex chemistries and two different hydrophobically modified ethylene oxide urethane (HEUR) rheology modifiers were studied. The rheo-USANS data reveal that both the latex particles and the TiO₂ particles form transient aggregates which are fractal in nature. The structures depend on the chemistry of the binder particles, the type of rheology modifier present and the shear stress imposed upon the formulation. The aggregate size of both the latex and pigment generally decreases with increasing shear stress. In two of the formulations studied, the latex and TiO₂ correlation lengths remain large even at high shear stress and are characteristic of TiO₂ crowding. In a third formulation, shear induces string-like aggregate structures of TiO₂, and a further increase in shear leads to pigment particles becoming more uniformly dispersed. The changes in the latex and pigment transient aggregate structures correlate with the changes observed in their viscosity flow curve profiles. We have used this correlation to develop an elementary viscosity prediction model based on the structural parameters extracted from the rheo-USANS data. Using a single fitting parameter and only the latex transient fractal aggregate structural parameters, good agreement between the measured and calculated viscosity is obtained. This implies that the structural parameters extracted from the scattering data are representative of the colloidal structure under shear and that energy dissipation from transient fractal aggregates of latex is the predominant mechanism of viscosity creation in HEUR thickened latex paints