Aqueous-Only, Green Route to Self-Healable, UV-Resistant, and Electrically Conductive Polyurethane/Graphene/Lignin Nanocomposite Coatings

Lignin is an undervalued sustainable commodity with many potential applications. In this work, we used lignin to produce noncovalently modified graphene from pristine graphite. Lignin- modified graphene (LMG) was added to waterborne polyurethane (WPU) to produce nanocomposites through a facile, aqueous-only route. Dynamic mechanical analysis shows increased storage modulus, i.e., as high as 171%, for WPU/LMG nanocomposites. Unlike unloaded WPU, WPU/LMG nanocomposite coatings show complete self-healing after only 150 s of infrared (IR) irradiation because of entropic and enthalpic contributions of LMG to self-healing. Moreover, it is revealed for the first time that although the nanocomposite coatings display two-dimensional healing behavior, at each phase the healing progresses uniaxially, which can be attributed to the interplay among viscoelastic recovery response, surface-tension-driven viscoelastic restoration, and polymer diffusion. Furthermore, WPU/LMG nanocomposites exhibit substantial improvement in UV stability owing to LMG's preventive antioxidant activities such as UV absorption and gas impermeability, its radical scavenging properties, as well as the synergy between lignin and LMG. Finally, WPU/LMG shows decent conductance, up to 0.276 S/m, which means that our self-healable, UV-resistant films can find applications as durable corrosion-preventive or antistatic coatings.