Synthesis of lignin-based polyols via thiol-ene chemistry for high-performance polyurethane anticorrosive coating

Sustainable thermosetting polyurethane (PU) anticorrosive coatings based on renewable biomass resources with long-lasting corrosion protection is highly desired. Herein, thermosetting lignin-based polyurethanes (LPU) coatings with superior corrosion resistance and high content of lignin were prepared by polymerization of lignin-based polyol (OH-EL) with hexamethylene diisocyanate (HDI) in absence of catalysts. In order to increase the reactivity and solubility of OH-EL when reacting with isocyanates, the phenolic hydroxyls of enzymatic hydrolysis lignin were selectively converted to primary aliphatic hydroxyls by sequential alkylation and thiol-ene reaction. Due to the well dispersion and crosslinking reactivity of OH-EL in the polyurethane networks, the resulting LPUs presented high thermal stability and excellent mechanical properties. Remarkably, LPU-3 with the content of OH-EL of 54.8 wt%, exhibited high Tg of 112 °C, high char residue percentage of 26.3% at 600 °C, high tensile strength up to 81.6 MPa and tensile modulus of 1.4 GPa. Moreover, the LPU-3 coating on carbon steel substrate exhibited low corrosion current density Icorr of 7.58 × 10−11 A cm−2, positive corrosion voltage Ecorr of 118 mV, and high impedance modulus |Z|0.01Hz of 8.3 × 1010 Ω cm2 after immersing in 3.5 wt% NaCl solution for 0.5 h, and the |Z|0.01Hz remained at 3.9 × 1010 Ω cm2 after 40 days immersion, displaying superior corrosion resistance than other reported bio-based polymer coatings. These results demonstrate that the synthesis of highly reactive lignin-based polyols based on thiol-ene chemistry is an effective and facile strategy, which is vital for the development of high-performance bio-based polyurethane anticorrosive coatings.