Photocatalytic Oxidation of Technical Lignin to Potent Modifiers for Improving the Performance of Urea-Formaldehyde Resin

Modifications of urea-formaldehyde (UF) resin to achieve better bonding performance and lower formaldehyde emission are still an alluring task in the wooden composite industry. Herein, by strategically constructing a photocatalytic oxidation system relying on carbon dot-catalyzed peroxymonosulfate (PMS) activation, we confirmed that Kraft lignin can be readily oxidized to potent modifiers for improving the bonding performance of low-molar-ratio UF resin (F/U = 0.95). We found that the oxidation system consisting of carbon dots and PMS in a mass ratio of 1:20 revealed optimal efficiency in the oxidative valorization of lignin by primary cleavage of β–O–4 linkages, resulting in a remarkable reduction of molecular weight from 2026 to 679 Da. Meanwhile, lignin side-chain oxidation triggered by radicals (mainly involving sulfate radicals) also contributed largely to the transformation of S units to G and H units via demethoxylation, and the reaction patterns conferred the oxidation products an increased amount of aliphatic OH and phenolic OH. In comparison with untreated lignin, the lignin-degraded fragments promised relatively higher hydroxymethylation efficiency and enhanced chemical compatibility in the UF resin synthesis system, fostering the formation of copolymers containing phenolic-like moieties and branched polymer structures. Profiting from this structurally homogeneous modification of oxidized lignin, the copolymer resin not only revealed comparable storage stability to the UF resin but also possessed enhanced bonding performance on the premise of rather low formaldehyde emission, and plywood panels bonded with the lignin-modified resin exhibited a higher tensile shear strength of 0.81 MPa, which was a 58.8% increase with reference to the UF resin-bonded panels, and as a result complied with the relevant national standard (≥0.7 MPa). Overall, the lignin photocatalytic oxidation strategy proposed in our study would open a new avenue for providing biobased and potent modifiers in the production of wood adhesives.