利用红外成像和纳米压痕测试技术研究热处理落叶松管胞性能

Study on properties of heat-treated larch tracheid using infrared imaging and nanoindentation test

以我国主要用材树种日本落叶松木材为研究对象,采用物理改性方法真空热处理技术对其材质进行改良。利用场发射扫描电子显微镜和傅里叶变换红外成像显微镜研究不同热处理条件(160~240℃温度范围,处理4 h)下落叶松木材晚材管胞细胞壁形态、化学组分结构和分布的变化规律,并结合纳米压痕测试系统研究热处理木材细胞壁力学性能变化趋势。结果表明:木材热处理过程中,随着热处理温度的升高,晚材管胞细胞壁出现裂纹并增加;半纤维素发生降解,细胞壁红外成像中半纤维素相对高浓度分布区域逐渐减少,木质素高浓度分布区域呈现先增加后降低的趋势,表明该过程中木质素发生了热解并进行了重新分布。木材细胞壁压痕弹性模量和硬度随着真空热处理温度的增加呈先增大后降低的趋势,其中当热处理温度为200℃时,压痕弹性模量和硬度相比素材分别增大54.57%和45.69%,达到最大值,该变化规律与木材细胞壁中木质素分布变化趋势相吻合。以上研究结果有助于深入理解真空热处理木材性能的变化,为真空热处理改性木材工艺优化提供理论依据。

Larch wood is one of the main economic forest species in northern China,which has the advantages of hard surface,high mechanical strength,beautiful texture,and strong natural decay resistance,while it has the disadvantages of poor dimensional stability.The vacuum heat treatment is an effective and environmentally friendly way to improve wood properties.The purpose of this study was to investigate the changes in latewood tracheid cell wall morphology,chemical composition structure and distribution,as well as mechanical properties of larch wood under different heat treatment conditions ranging from 160 to 240 ℃ for 4 h by the field emission scanning electron microscope,Fourier transform infrared spectroscopy(FT-IR) imaging microscope,and nanoindentation test system.The results showed that cracks appeared in the tracheid cell walls and increased with the increase of the treatment temperature.According to the FT-IR imaging microscope,as the heat treatment temperature increased,the hemicellulose degraded,and there was a decreasing trend in the high concentration distribution area of hemicellulose in the cell walls.For the high concentration distribution area of lignin in the cell walls,it expanded from the middle lamella for the untreated wood to the cell walls and middle lamella at 200 ℃,but the distribution area became smaller when the temperature reached 240 ℃.This change trend indicated that lignin was also pyrolyzed,but the relative degradation ratio was lower compared with polysaccharides.Therefore,the relative content of lignin increased.The indentation modulus of elasticity and the hardness of tracheid cell walls increased with the increase of the treatment temperature.In comparison with the untreated wood,the change in different temperature heat-treated wood samples increased firstly and then decreased,when the treatment temperature was 200 ℃,the indentation modulus of elasticity and the hardness increased by 54.57% and 45.69%,respectively,reaching the maximum values.The change trend of mechanical properties of the heat-treated larch cell wall was consistent with that of the high concentration distribution area of lignin in the cell wall.This research would be helpful to understand the changes in the macroscopic properties of the heat-treated wood,providing a theoretical basis for the vacuum heat treatment process optimization and wood performance control.Furthermore,it would offer a new method for modification and high value utilization of plantation wood.