水溶性乙烯基单体改性木材尺寸稳定性提高机制

The Mechanism to Improve the Dimensional Stability of Wood with Modification of Water-Soluble Vinyl Monomers

【目的】分析水溶性乙烯基单体改性前后木材极性基团数量和细胞壁结构变化,揭示水溶性甲基丙烯酸-2-羟乙酯(HEMA)和N-羟甲基丙烯酰胺(NMA)原位共聚改性木材尺寸稳定性提高机制,为该改性技术优化发展提供理论基础。【方法】采用动态水蒸气吸附和接触角表征水溶性乙烯基单体改性前后木材极性基团数量和表面极性变化,利用扫描电镜、拉曼光谱、X射线衍射、压汞法和氮气吸附系统研究改性剂在木材中的分布、细胞壁润胀、细胞壁两相结构以及孔隙变化情况。【结果】在相对湿度0%~95%环境下,HEMA和NMA改性材的平衡含水率明显低于未改性材;当相对湿度达95%及以上时,改性材的平衡含水率超过未改性材。利用H-H模型拟合分析浸水处理后改性材与未改性材的吸湿曲线发现,改性材原有极性基团数量有所下降但并不显著。接触角测试表明,改性材的表面极性大于未改性材,残留单体去除后改性材的表面极性弱于未改性材,残留单体可抑制木材疏水性能的改善。SEM观察结果显示,改性后细胞壁显著增厚,且细胞间隙减少。拉曼光谱分析得出,改性剂均匀分布于细胞壁中,结合SEM结果可知改性剂能够顺利进入细胞壁并润胀细胞壁。X射线衍射分析表明,改性材未出现新的晶体结构,且改性材的结晶度相对于未改性材变化较小。压汞法和氮气吸附测试表明,改性材的孔隙率相对于未改性材出现较显著下降,改性剂可成功填充细胞壁孔隙。【结论】HEMA和NMA改性木材可有效提高其尺寸稳定性,改性剂对细胞壁的充分润胀、加固及对细胞壁孔隙的填充作用是木材尺寸稳定性提高的主要原因。

【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.