Epoxy–Aminated Lignin Impregnation Combined with Densification for Enhanced Mechanical Properties and Deformation Fixation of Wood

Hot-pressing densification is an effective method to enhance the mechanical properties of wood; however, excessively high pressing temperatures can cause thermal degradation of wood components, compromising these improvements. In this study, aminated lignin (AL), with improved water solubility and reactive amino groups facilitating crosslinking, was utilized as a bio-based amine curing agent for the water-soluble, low-molecular-weight epoxy compound polyethylene glycol diglycidyl ether (PEGDGE). The PEGDGE-AL modifier was applied for wood impregnation, followed by hot-pressing densification at a relatively low temperature of 120 °C, to enhance the mechanical properties of wood. The chemical composition of AL was analyzed using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and X-ray photoelectron spectroscopy (XPS). The gelation and curing behavior of the PEGDGE-AL modifier demonstrated its ability to readily form a network structure at both room temperature and elevated temperatures. The impact strength of densified wood (DW) modified with 12 wt% PEGDGE and 8 wt% AL, denoted as 12PEGDGE+8AL-DW, exhibited an impact strength of 15.2 kJ/m², representing a 72% increase compared to untreated wood (UW). The modulus of rupture (MOR) and modulus of elasticity (MOE) reached 241.1 MPa and 14.6 GPa, respectively, corresponding to 60% and 75% improvements over UW. Furthermore, the 24 h water uptake and thickness swelling of 12PEGDGE+8AL-DW were 45.2% and 24.7%, which were 11% and 43% lower than those of water-impregnated and hot-pressed densified wood (W-DW), respectively. This study provides a low-temperature route for wood densification while contributing to the valorization of lignin in high-performance material applications.