Mild condition lignin modification enabled high-performance anticorrosive polyurethane coating

The diverse active hydroxyl groups of lignin pose challenges in the preparation of lignin-based polyurethane coatings with exceptional long-term anticorrosive properties. Here, the dense and defect-free lignin-based polyurethane coating with a thickness of 25 ± 5 μm was successfully synthesized using a mild hydroxypropyl lignin modification approach, exhibiting outstanding barrier properties (|Z| > 10⁹ Ω cm²) and long-term anti-corrosion performance exceeding 120 d. Under ambient conditions (i.e., 25 °C and atmospheric pressure), propylene oxide was directly blended with the alkali solution of lignin to effectively convert phenolic hydroxyl groups into more reactive aliphatic hydroxyl groups, while also minimizing the significant increase in molecular weight caused by lignin condensation. As a result, the high crosslinking density of lignin polyurethane coatings effectively prevented the penetration of corrosive media and enhanced the long-term corrosion resistance of the coatings. Overall, the results demonstrate that a mild hydroxypropyl modification process is an effective and facile strategy to prepare highly reactive lignin-based polyols, which is crucial for the development of high-performance bio-based polyurethane anticorrosive coatings.