In situ modification of cellulose and lignin using deep eutectic solvents for high-strength wood with negligible set recovery.
Abstract
Cellulose, hemicelluloses, and lignin in wood cell walls form a biological macromolecular network whose organization and intermolecular interactions determine the compressibility and moisture stability of densified wood. This study presents a deep eutectic solvent (DES)-retention strategy that enables in situ plasticization and modification of this network during densification. The retained DES lowered the softening temperature from 100 to 55 °C, promoted the viscoelastic deformation of cell-wall polymers, and substantially relieved internal stress during compression. The resulting densified wood reached a density of 1.01 g cm-3, a modulus of rupture (MOR) of 197.04 ± 9.88 MPa, a modulus of elasticity (MOE) of 10.17 ± 0.71 GPa, and a Shore D hardness of 91, while exhibiting only 1.5% set recovery after soaking-boiling-drying cycles. Multiscale characterization indicated that DES pretreatment induced matrix depolymerization and cellulose swelling/disordering, whereas hot pressing promoted cellulose realignment and lignin recondensation, thereby stabilizing the compressed structure. The resulting material also showed improved resistance to fungal decay and mold growth, as well as improved flame-exposure behavior. A cradle-to-gate life cycle assessment further indicated lower environmental impacts than steel under the evaluated conditions. These results demonstrate that retaining DES to regulate cell-wall macromolecules provides an effective route to high-strength densified wood with negligible set recovery.