Rheological Behavior of Aqueous Polyurethane Dispersions:  Effects of Solid Content, Degree of Neutralization, Chain Extension, and Temperature

Rheological behavior of waterborne polyurethane dispersions was investigated with small-amplitude oscillatory shear flow experiments over wide ranges of concentration, degree of neutralization, chain extension, and temperatures to accelerate efforts to understand their film formation characteristics. The rheological properties of these environmentally friendly dispersions were found to be dependent on composition and degree of postneutralization. But the chain extension and degree of preneutralization were observed to have little effect on the rheological behavior of the dispersions at a constant polyurethane (PU) concentration. The complex viscosity of the polyurethane dispersions increased dramatically at a critical concentration of polyurethane (φ = 0.43), below which the viscosity increased slightly with composition. At this critical concentration the particles are crowded, and the observed viscosity increase is ascribed to the hydrodynamic interaction between the different particles. Furthermore, both G‘ ‘ and G‘ ‘ are strongly increased with increasing PU wt % in the dispersions (i.e., the higher the concentration of PU, the higher the values of G ‘ and G‘ ‘). At 46 wt % PU the values of G ‘ and G‘ ‘ are no longer frequency dependent, and G ‘ is almost 1 order of magnitude higher than G‘ ‘, indicating formation of a fractal-type gel. The viscoelastic material functions were well described by simple power law equations and a Maxwellian (Hookean) model with 2−3 relaxation modes based on PU concentration and degrees of postneutralization at 30 °C. Time−temperature superposition of the dynamic moduli was good at temperatures and PU concentrations below that of the critical gel, and the temperature dependence of the shift factors conformed well to predictions from an Arrhenius-type relation, enabling calculation of the flow activation energy of 45 kJ mol-1 for the PUDs. As expected, time−temperature superposition failed to represent the behavior of the PUDs near the critical gel point. While the results of this study indicate a number of similarities to critical gelling systems, observed deviations from the viscoelastic behavior of Brownian suspensions of hard spheres were obtained, indicating that a more complicated theory that explicitly takes the intrinsic interactions, concentrations, and size distributions of the PU particles into account may be necessary for a more accurate quantitative description of these special model PUDs with enhanced benefits.