Tuning the Lignin-Caprolactone Copolymer for Coating Metal Surfaces

Coating materials based on biodegradable and natural polymers exhibit considerable promise for applications in the paint industry, but these polymers are rarely used as primary ingredients for paint formulations. Lignin is a natural polymer that is able to adhere onto many oxide surfaces because of its phenolic/catecholic groups, but its film forming ability has been a major challenge. Herein, we report the synthesis and characterization of a lignin copolymer with aliphatic polyester oligomers that display excellent film forming capabilities and adhesion toward stainless-steel substrates. The strategy to synthesize the target copolymer is based on tuning the degree of polymerization (DP) of epsilon-caprolactone (CL) onto the lignin-OH groups via ring opening polymerization and the grafting-from method. This allows copolymers with polyester oligomers below a DP of 25 to be prepared in gram quantities and characterized by size exclusion chromatography and nuclear magnetic resonance spectroscopy. Physicochemical analysis of the copolymers provides evidence that the aliphatic polyester oligomers were mostly reacted on the aliphatic lignin alcohol, leaving the phenolic alcohol unmodified, which favors adhesion properties. Films prepared using copolymers with the DP of CL below 5 have strong adhesion to metals and considerable potential to act as a binder in metal primers/base coatings. We anticipate that this approach will provide important value addition for lignin in coating applications.