1.4 Propargylated softwood Kraft Lignin for thermally curable bio-based anticorrosion coatings

Considering the enormous scale of various end-uses of metals, there is a strong and growing need for fully biobased protective coatings based on renewable non-fossil recourses. In this work, we alkylated softwood kraft lignin with propargyl halide to introduce reactive triple bond functionalities. The obtained product was found to be self-reactive under elevated temperatures due to so-called Claisen rearrangement reactions. Thus, the propargylated lignin was deposited over the copper surface and thermally cured at 150°C. The thus obtained lignin coating was stable in 3.5% sodium chloride solution, providing efficient protection against corrosion with 84% lower penetration of corrosion compared to unprotected copper. However, the coating was not stable under acidic conditions (0.5 M sulfuric acid). According to SEM observation, the acid-instability of the coating was caused by cracking due to the rigid nature of the lignin molecules and its increased stiffness after the curing step. To increase the flexibility of the coating, we added 10 wt. % of epoxidized soybean oil, which acts as low molecular weight and flexible cross-linker. Such thermally cured lignin coatings containing soybean epoxy additive turned out to be efficient up to 98% in model anticorrosion test (3.5% sodium chloride) and displayed improved stability in the sulfuric acid medium. The developed approach paves the way for fully biobased lignin-based coatings for metals, with possible applications also in other materials such as wood.