应拉木轴向湿热回复的形成机制研究

At early period of green wood drying process, an irreversible viscoelastic recovery originated from release of locked-in component of growth stress in cell wall, known as hydrothermal recovery, will form and cause several types of dry defect consequently. Because hydrothermal recovery in tension wood is extremely large and anisotropic, green board contains tension wood will tend to be distortion or splitting during seasoning process. However, the formation of longitudinal hydrothermal recovery in tension wood is still unclear for now. Therefore, based on published research results and related fundamental theories, it is proposed in our application that two different mechanisms executing for formation longitudinal shrinkage of hydrothermal recovery in tension wood sorted by processing temperature, namely, softening of matrix and movement of cellulose molecular chain in paracrystalline region. Several types of materials are going to be prepared from tension wood and normal wood at first. And then, series of advanced techniques, including polarized Raman microscopy, thermal mechanical analysis, dynamic thermal mechanical analysis, wide-angle X-ray diffraction, NMR and HPAEC, will be implemented to testify the propose. If all preset targets in the application are achieved, it will clarify the formation of longitudinal shrinkage in tension wood during heating in water or vapor and further explain the difference of hydrothermal recovery between normal wood and tension wood at one side. At another side, useful information and instruction will be supported to reduce wood dry defects and optimize procedures of wood physical treatment, and benefit to comprehensive utilization of tension wood finally.

生材在干燥初期会出现一种来源于细胞壁中残余生长应力释放的不可逆粘弹性回复，即湿热回复，它是产生木材干燥缺陷的主要原因之一。应拉木湿热回复程度和各向异性明显，极易造成生材干燥后出现变形和开裂, 但是相关形成机制目前尚不明确。申请人基于前人相关研究结论和基础理论，提出了应拉木轴向收缩在不同温度区间形成了两个不同机制的假设，即基质软化机制和纤维素准晶区域分子链段运动机制。本项目将选择不同类型正常木和应拉木为研究材料，采用偏正拉曼光谱显微技术、热机械分析、动态热力机械、广角X射线衍射、NMR和HPAEC等技术，逐步实现对上述推论和假设的验证。如果此项研究能顺利完成，则一方面将全面揭示应拉木在水/湿热处理过程中轴向收缩的形成机制，进一步可以解释应拉木与正常木湿热回复行为差异性以及胶质层在其中作用机理；另一方面，可以为降低人工林木材干燥缺陷和木材物理改性提供理论依据，进而为应拉木综合利用提供理论指导。

应拉木作为倾斜生长树木中一种常见木材品质缺陷极大地影响了人工林木材加工利用，尤其是应拉木在湿热环境中会产生了比正常木数倍的轴向收缩，造成了含有应拉木的板材在使用和干燥过程中发现扭曲和开裂，但是目前学术界对其形成机理存在一定的争议。为此，本项目按照计划任务书具体要求，利用SAXS, XRD, 13C-NMR，1H-NMR，DMA，DSC，FT-IR，FT-Raman等多种先进技术手段对杨树应拉木和正常木经过湿热处理之后的尺寸稳定性、结晶结构、含水能力、动态力学行为以及对从应拉木中分离纯化胶质层颗粒的理化性能分析，发现了胶质层中纤维素分子链在湿热处理中发生再结晶重排构成了轴向收缩引发机制，而应拉木细胞壁其它壁层在水分有效增塑下跟随胶质层发生收缩，从而形成了应拉木轴向收缩。研究证实了应拉木吸湿性高于正常木，且应拉木细胞壁中存水能力高于正常木，胶质层的存在是形成这种差异的主要原因。湿热处理之后应拉木存水能力显著下降，预示着胶质层多孔结构的改变和吸湿位点之间形成了不可逆的氢键。与此同时还发现了应拉木木质素比正常木木质素含有更多的酚羟基含量，可能会导致应拉木细胞壁其它壁层更容易被水塑化发生变形。上述研究结论基本解释了应拉木在湿热处理中发生轴向收缩的原因，为合理利用加工应拉木提供了理论支持，为后续制定木材干燥基准和木材水热处理提供了信息。基于以上成果项目组成员共发表标注项目资助的SCI收录研究论文6篇，其中中科院一区期刊4篇，TOP期刊3篇，培养研究生4名。