Rheokinetics of Thermal-Induced Gelation of Waterborne Polyurethane Dispersions

Thermal-induced gelation for waterborne polyurethane dispersion has been studied rheologically under isothermal condition over a wide range of frequencies at different constant temperatures (55, 60, 65, and 70 °C). The elastic storage modulus, G‘, at a constant temperature in the vicinity of the gel point increases abruptly, and the magnitude of the elevation in G‘ was found to be temperature dependent. Similar behavior has been observed for both the viscous loss modulus, G‘‘, and the complex dynamic viscosity, η*. The gel point, tgel, was determined from the point of intersection in tan δ vs gelation time for different constant shear frequencies, where tan δ is frequency independent and all curves cross over, indicating the validity of the Winter−Chambon criterion. The value of tgel obtained from the coincidence of G‘ and G‘‘ was in excellent agreement with that obtained from tan δ vs t. At the gel point, G‘ and G‘‘ showed a power law with shear frequency, i.e., G‘ ∼ G‘‘ ∼ ωn with critical exponents n‘ a...