Sustainable Polyurethane–Lignin Aqueous Dispersions and Thin Films: Rheological Behavior and Thermomechanical Properties

Sustainable aqueous polyurethane (PU) dispersions derived from castor oil and lignosulfonate (LS) have been successfully synthesized with stable nanoscale morphology and average particle sizes as small as 35 nm with no organic solvent. Incorporation of a rigid structure of LS into the castor oil-based PU reinforces the resulting cast films of the PU–LS nanocomposites and increases the cross-link density, as determined by dynamic mechanical analysis measurements and the theory of rubber elasticity. In addition, the LS plays a significant role in enhancing the ultimate mechanical and thermal properties of the resulting nanocomposite films. The TEM morphologies of the PU–LS dispersions confirmed that the average particle sizes are constant (approximately 35 nm) for all different LS concentrations. The effect of LS on the dynamic viscosity of PU–LS aqueous dispersions at different angular frequencies in the linear viscoelastic regime has also been investigated, and the experimental data were analyzed based on the Cross model. The dynamic viscosity increases significantly with increasing LS content while holding a constant 15 wt % of the solid content in all dispersions. The nature of LS as a water-soluble material suggests that PU–LS nanoparticles adsorb more water onto the particle surface with increasing LS content, which leads to a considerable decrease in the concentration of free water in the dispersion, and consequently, the viscosity increases dramatically. The influence of different concentrations of LS on the reduced viscosity of the dispersions was investigated using the Krieger–Dougherty viscosity model with a critical concentration of LS (ϕc = 0.08) above which the viscosity of the dispersions increased strongly. The current study provides an effective and promising approach to synthesize high-performance, biorenewable nanocomposites for coatings and adhesives.