Abstract:

Objective: The purpose of this paper was to study the changes of the number of polar groups and cell wall structure for wood modification, and to clarify the mechanisms of enhancing the dimensional stability of wood modified by in situ copolymerization of water-soluble hydroxyethyl methacrylate (HEMA) and N-hydroxymethylacrylamide (NMA). Method: The polar group lumber and wood surface polarity changes were determinated by dynamic vapour sorption (DVS) and the contact angle using the wood before and after modified with water-soluble vinyl monomers as materials. The distribution of modifier in timber, swelling of cell wall, cell wall two phase variation and the changes of pore structure were in vestigated using scanning electron microscope (SEM), Raman spectroscopy, X-ray diffraction, mercury intrusion method and the nitrogen adsorption system. Result: The equilibrium moisture content of HEMA and NMA modified wood was significantly lower than that of unmodified wood under the condition of 0%-95% relative humidity. When relative humidity reached 95% or above, the equilibrium moisture content of modified woods exceeded that of unmodified woods. The hygroscopic curves of modified and unmodified wood after water soaking were analyzed by H-H model fitting, it was found that the number of polar groups of the modified wood decreased but was not significant. The contact angle test showed that the surface polarity of the modified wood was greater than that of the unmodified wood, but weaker than that of the unmodified wood after the removal of the residual monomer, which indicated that the residual monomer inhibited the improvement of the hydrophobic property of the wood. The cell wall was thickened and intercellular space was decreased by SEM. Raman spectrum analysis showed that the modifier was evenly distributed in the cell wall. Combined with SEM results, it could be found that the modifier entered the cell wall smoothly and moistened the cell wall. X-ray diffraction test showed that no new crystal structure has been found and the crystallinity of modified wood was less than that of the unmodified wood. The results of mercury injection and nitrogen adsorption showed that the porosity of the modified wood was significantly lower than that of the unmodified wood, indicating that the modified agent successfully filled the pores of the cell wall. Conclusion: HEMA and NMA modified wood could effectively improve its dimensional stability. The main reason for improving the dimensional stability of wood might be that the modifier could fully expand and reinforce the cell wall and fill the pores of the cell wall.

Key words: vinyl monomer, dimensional stability, polar groups, cell wall structure