High-Strength and UV-Shielding Transparent Thin Films from Hot-Pressed Sulfonated Wood

An in situ sulfonated woodveneer is densified into strong,transparent, UV-shielding, and recyclable thin wood films withoutadditional binder or resin. Wood is a high-strength lightweight material owing toits orthotropiccellular structure and composite-like constitution. In conventionalfabrication of wood-derived functional materials, the removal of thepotentially beneficial components, such as lignin and hemicellulose,often leads to the disruption of the native hierarchical wood structure.Herein, we developed a facile method of in situ wood sulfonation followedby hot pressing for pine veneers to prepare high-density transparentthin films with preserved wood components and the natural fiber alignment.An optimum lignin content of the hot-pressed films was found to be20.6% where both mechanical and optical properties were significantlyenhanced with a more dense and compact structure. The hot-pressedtransparent wood films also showed UV-blocking capability and couldbe recycled into discrete wood fibers owing to the sulfonate groupsendowed by the in situ sulfonation step. The unique combination ofproperties achieved for thin wood films marks an important step inengineering functional wood-based materials that utilize both thestructure of aligned fibers and the complex components of naturalwood.