Rheological Behavior of Environmentally Friendly Castor Oil-Based Waterborne Polyurethane Dispersions

Novel biorenewable, waterborne, castor oil-based polyurethane dispersions (PUDs) were successfully synthesized via homogeneous solution polymerization in methyl ethyl ketone followed by solvent exchange with water. Small-amplitude oscillatory shear flow experiments were used to systematically investigate the rheological behavior of these environmentally friendly, biorenewable, aqueous dispersions as a function of angular frequency, solid content, and temperature. In addition, the morphology of the dispersions was investigated at 60 °C for different time intervals using transmission electron microscopy (TEM). The solid content and temperature were found to significantly affect the rheological behavior of the PUDs. The composition dependency of the complex viscosity (η*) was found to be well described by the Krieger–Dougherty equation. Thermally induced gelation was observed for PUDs with a solid content ≥27 wt %. Although the viscoelastic behavior of the PUDs was well described by the time–temperature superposition (TTS) principle in a temperature range lower than the gel point, TTS failed to represent the behavior of the PUDs at temperatures near the critical gel point. The real time gelation behavior was also studied for different solid contents of PUDs under isothermal conditions over a wide range of angular frequencies. Furthermore, both G ′ and G ″ showed a power law relationship with the angular frequency at the gel point, with critical power law exponents similar to those predicted theoretically by percolation theory. Aggregation and interconnection of the nano-PU particles caused the formation of fractal gels at a critical temperature, as confirmed by TEM.