Modification of lignin by hexamethylene diisocyanate to synthesize lignin-based polyurethane as an organic polymer for marine polyurethane anticorrosive coatings

A green and facile preparation was used to synthesize modified lignin (ML) from alkaline lignin by using hexamethylene diisocyanate (HDI) as the modifier and tetrahydrofuran as the solvent without a catalyst. Then, the as-synthesized ML was dispersed in hydroxy acrylic resin and used to prepare modified lignin-based polyurethane composites by using HDI trimer as the curing agent and dibutyltin dilauric acid as the catalyst at room temperature. Fourier transform infrared (FTIR) and elemental analysis confirmed that the main chemical in ML was lignin urethane when the amount of HDI was 0.2 g. The tensile strength of the modified lignin-based polyurethane (MLPU-0.2) film using ML-0.2 reached 12.07 MPa, which was improved by 23.2% and 68.3% compared with that of the pure polyurethane (PU) film (9.79 MPa) and lignin-based polyurethane (LPU) film prepared using unmodified lignin (7.17 MPa), respectively. After the addition of lignin or ML, the elongation at break of the polyurethane film increased. The water contact angle of MLPU-0.2 increased up to 96°, which was higher than that of PU (74°) and LPU (68°). The pencil hardness of MLPU-0.2 (3 H) was higher than that of PU (H) and LPU (2 H). Modified lignin-based polyurethane shows promise for marine anticorrosive coatings.