Effect of Lignin Removal on the Hygroscopicity of PMMA/Wood Composites

Wood delignification can provide a wood-based template with sufficient pore volume for polymer/wood composites. At the same time, delignification is conducive to the penetration of polymer into the wood cell wall, which is of great significance to improve the function and performance of composites. However, lignin is the main chemical component in wood. The removal of lignin will inevitably lead to the change of the wood's physical properties, including the hygroscopicity of the wood. In this study, prepolymerized methyl methacrylate (MMA) impregnated delignified wood (DW) was used to obtain polymerized methyl methacrylate/delignified wood (PMMA/DW) composites with different lignin removal. The effect of lignin removal on the hygroscopicity of the composites is discussed. The results of nitrogen adsorption showed that the DW could adsorb more nitrogen than the original wood, and the amount of nitrogen adsorption gradually increased with the improvement of the processing degree. After filling with PMMA, the adsorption amount of nitrogen was greatly reduced. The results of the BET analysis showed that delignification promoted the distribution of PMMA in the pores of the wood cell wall. When lignin was almost completely removed, all mesopores in the cell wall were filled with PMMA. The results of the moisture absorption analysis isotherm curve showed that the moisture absorption content of the wood was positively correlated with the amount of lignin removed, and the moisture absorption content of the PMMA/DW composite was negatively correlated with the amount of lignin removed. The hygroscopic data were further analyzed using the Hailwood-Horrobin model. The results showed that the mole number of adsorbable or hydratable sites of the DW increased with the increase of lignin removal, and the situation of the PMMA/DW composites was just the opposite. In addition, after delignification, the dissolved water content and hydrated water content of the DW increased, and the increase was related to the delignification strength. The increase of dissolved water content indicates that the removal of lignin promotes the production of more volume in the cell wall, which provides space for the adsorption of multilayer water. After MMA in situ polymerization, the hydration and dissolved water content of the PMMA/DW decreased significantly, and the dissolved water content decreased even more significantly. The fitting curves of the H-H model and GDW model for the experimental data points of the differently treated samples were similar. The removal of lignin led to the increase of the w value, indicating that the ratio of water molecules adsorbed in the first layer of DW to the second layer increased, and the removal of lignin resulted in the enhancement of wood hygroscopicity; the opposite was true for the PMMA/DW.