Lignin Caprolactone-Derived Wood Coatings

The variation in the characteristics of lignin can influence its potential use for coating applications. In this work, we examined the polymerization of three lignin types, i.e., birch alkaline (BL), wheat straw alkaline (WL), and softwood kraft lignin (SL), with caprolactone (CL) before and after ethanol fractionation. The molecular weight, hydroxyl group, and thermal stability of lignin were significantly altered after ethanol fractionation, affecting its reactivity to ring-opening polymerization with CL and, consequently, the characteristics of the final polymers. Due to more significant changes in the functional groups of SL and WL compared to BL, the polymerization of SL and WL with CL was affected more intensely than that of BL with CL. The wood coating performance (e.g., water contact angle (WCA) and flame-retardant properties) of fractionated lignin-CL polymers was superior to that of their unfractionated counterparts. The wheat straw fractionated lignin (WLE)-CL polymer (WLEP) exhibited a superior WCA (125°) and a limiting oxygen index (27.5%) to other lignin-CL polymers, which were also stable after sand abrasion and knifing. The superior coating performance of WLEP on wood surfaces is attributable to the heightened reactivity of WLE with CL and an increased polymerization of PCL on the lignin backbone. The preliminary cost estimation and robustness of a proposed process confirmed windows of opportunity for fabricating lignin-CL polymers to replace oil-derived polymers for coating applications.