Rheological behavior of waterborne polyurethane/starch aqueous dispersions during cure

Abstract Isothermal rheological behaviors of waterborne polyurethane (WPU)/starch aqueous dispersions during cure were investigated with a small-amplitude oscillatory shear flow experiment to evaluate their crosslinked structure and to predict their mechanical properties, for the first time. An abrupt increase in the elastic storage modulus ( G ′), the viscous loss modulus ( G ″), the complex dynamic viscosity ( η ∗ ) and the loss tangent (tan  δ ) was observed during the curing process of the dispersions, as a result of the formation of a fractal polymer gel. The gel point ( t gel ) was determined from the intersection in tan  δ vs curing time for different constant shear frequencies, where tan  δ was frequency independent and all curves crossed over, indicating the validity of the Winter–Chambon criterion for the complex system. The values of the power law exponent ( n ) and the gel strength ( S g ) at the gel point indicated that with an increase of starch content the crosslinked WPU/starch gels underwent a transition from weak fractal to strong elastic ones. Moreover, the WPU/starch composite sheets, obtained from the aqueous dispersions with relatively high S g values, also exhibited the increased tensile strength ( σ b ) and Young's modulus ( E ). Their structure–mechanical properties relationship and the phase transitions of dispersed starch–dual-phase continuity–starch matrix were revealed. This work confirmed that the rheological characters could be used to predict the mechanical properties of the WPU materials blended with natural polymer.