木材湿热软化压缩技术及其机制研究进展

Research Development of Wood Compression Technology and Its Mechanism under Hydro-Thermal Condition

木材压缩是提高软质木材密度、强度和硬度,改善木材物理力学性能,扩大木材应用范围的有效方法。本文针对湿热软化下的木材压缩问题,从木材软化机制、软化特性、软化点的确定、热板加热下的传热传质特性、层状压缩的形成和压缩变形固定等方面分析木材压缩技术的研究现状、进展以及存在的问题。木材细胞壁的成分和组织构造是影响木材软化和压缩变形的主要内在因素,而湿和热则是影响木材压缩变形的外在因素。木材是一种具有弹塑性的天然高聚物。干燥木材缺乏塑性,水分和热量都能对木材组分起到增塑作用,特别是在湿热共同作用下增塑作用更加显著。木材细胞壁主要成分纤维素、半纤维素和木质素的特性及所占比例直接影响木材的可塑性,其中木质素的含量和软化特性是木材软化的主要影响因素。玻璃化转变温度和应力屈服点是表征木材软化最常用的参数。在木材弹塑性分析中,应力屈服点控制了木材在塑性区域的应力-应变关系,同时也决定了塑性变形潜能,但由于木材成分和结构非常复杂,应力-应变关系的拐点并不明显,因此应力屈服点和屈服应力的确定是木材塑性变形表征的关键点,也是一个难点。木材的组织构造主要影响木材的传热传质过程。利用木材3个断面渗透性的显著差异,通过干燥、浸水、放置、热板加热等处理,可使木材内部各个层面上形成差异显著的含水率梯度分布和屈服应力差,压缩后形成层状压缩木材。层状压缩木材压缩层的密度可达0.8 g·cm-3以上,未压缩层仍然保持原有的密度,而且压缩层的形成部位是可控的。层状压缩技术可以解决整体压缩木材损失大的问题,但目前木材压缩变形机制的研究都是围绕木材整体压缩开展的,缺乏木材软化点和屈服应力随含水率变化规律以及热板加热下木材内部屈服应力差变化规律的基础研究。要实现层状压缩的可控性,还需要在热板加热下的传热传质规律及木材湿热梯度分布的形成与调控等方面开展深入研究。

Wood compression is an effective method of enhancing the plantation fast-growing wood density,strength and hardness and improving its physical and mechanical properties,which broadens its application fields. In this paper,wood compression research status,development and problems were analyzed from the aspects of wood softening mechanism and characteristics,softening point,heat and mass transfer characteristics under hot plate heating,formation of sandwich compression and fixation of compression deformation. Composition and organizational structure of wood cellular wall is the main internal factors and hydro-thermal condition is external factors affecting wood softening and compression deformation.Wood is a natural macromolecular composite and an elastic-plastic material. Dry wood with low plasticity is plasticized by moisture and heat,especially the combined action of them was more significant. As the main components of wood cellular wall,cellulose,hemicellulose and lignin characteristics and their proportions directly impact on wood plasticity,especially the lignin. The glass transition temperature and stress yield point are the most common parameters to represent wood softening. In the elastoplastic analysis of wood,the stress yield point is used for determining the plastic potential which dominates the stress-strain relationships of material in the plastic region. However,due to the complexity of wood composition and structure,the change point of stress-strain is not obvious,so the stress yield point and the yield stress isthe key and difficulty point to determine the wood plastic deformation characterization. Organizational structure of wood mainly affects its heat and mass transfer. Because of the permeability differences among wood three sections,significant moisture distribution gradient and yield stress difference can be formed by conducting the treatments of drying,soaking,setting and hot plate heating. Then sandwich compressed wood can be obtained by hot press. Density of compressed layer reached above 0. 8 g·cm-3,and the density of uncompressed layer still maintained. However,the compressed layer can be formed on the surface layer or any part of the interior. Sandwich compression can reduce volumetric loss of wood during compression. By now,the deformation mechanism study of wood compression mainly focus on the overall compression,lacking of fundamental investigation on softening point and yield stress change with moisture content variation and yield stress difference under hot plate heating. To achieve the controllability of sandwich compression,heat and mass transfer under hot plate heating status and hydro-thermal gradient distribution need to further research.