Construction of thermally-modified wood surface with superstrong UV-resistance by in-situ modification of lignin

Ultraviolet (UV)-induced discoloration and cracking pose formidable challenges for thermally modified wood (TMW) when used in outdoor applications. Herein, we propose a novel pretreatment strategy using deep eutectic solvent (DES) to enhance the weathering resistance of TMW. DES-assisted TMW (DES-TMW) was prepared by coating wood with choline chloride/citric acid (CA) DES, followed by thermal modification. During the DES-assisted thermal modification process, the cross-linking of the lignin network in DES-TMW was increased as a result of lignin self-polymerization, cross-linking with autologous molecules and furfural, as well as esterified connections caused by CA. This in-situ modification reduces the quantity of photosensitive phenolic hydroxyl groups within lignin by forming cross-linked structures, reducing phenoxy free radical generation during the weathering and obstructing chromophores production pathways, thus enhancing the photostability of lignin. Consequently, the DES-TMW exhibited remarkable resistance to discoloration and cracking compared to TMW. These findings provide evidence that our strategy can effectively enhance the photostability of wood, thereby contributing to the development and production of sustainable outdoor wood products capable of withstanding changing weather conditions.