Thermal-Softening Behavior of Softwood with Lignin Modified In Situ Using Diols of Different Chain Lengths

In situ lignin modification via organosolv treatment enhances the thermal softening of wood. However, the influence of diol solvent structures on this process remains unclear. We examined the effects of various diol types on the lignin structure and thermal-softening behavior of organosolv-treated wood using HSQC nuclear magnetic resonance (NMR), thermogravimetry, thermomechanical analysis, and dynamic mechanical analysis. NMR analyses confirmed the grafting of all tested diols onto lignin, while thermal analyses revealed that lignin mobility depended on diol type. Notably, for 1,4-butanediol- and 1,5-pentanediol-treated wood, dynamic elastic modulus (E') was similar to that of untreated wood at an ambient temperature but decreased sharply upon heating under oven-dry conditions. In contrast, the other diols reduced E' at an ambient temperature because of a substantial mass loss. These findings indicate that the thermal-softening behavior of wood depends on the balance between lignin modification and component removal and can be controlled by diol selection and treatment optimization.