木材热处理研究及产业化进展

Development of wood heat treatment research and industrialization

热处理是在160℃以上低氧环境下进行的木材改性技术,可以改善木材的尺寸稳定性、生物耐久性和机械振动性能,同时能调节木材的材色,并且具有良好的环境友好性。但是高温处理也降低了木材的力学性能,对抗弯强度和冲击韧性的影响尤为显著。半纤维素在高温下的热解是热处理材改性的主要原因,它减少了木材中亲水基团和营养物质的数量,降低了木材的韧性,同时也影响了木材的明度指标。纤维素和木素在热处理过程中的变化相对较小,纤维素的结晶度有所上升,进一步降低了木材的亲水性,并提高了木材刚度;木素的结构在处理过程中发生了重组,形成了更加稳固的网状结构,对细胞壁的膨胀具有抑制作用,同时也是木材材色变化的主要原因。热处理过程中产生的抽提物主要是半纤维素的降解产物,它们对木材的吸湿性和声学特性都有影响。热处理对木材性能的调节受诸多工艺参数影响,温度是对木材改性深度影响最大的因子,处理环境的压力和在最高处理温度下的处理时间也对改性效果产生影响。木材热处理目前已获得较为广泛的产业化应用,这一方面是得益于系统性的基础研究,另一方面是因为它具有环保、低成本、操作简便等优点。木材热处理目前已逐渐形成了一个较为完整的技术和理论体系,但在平衡性能提升与强度损失、保持材色稳定性等方面的研究仍具有持续深入和拓展的空间。

Wood heat treatment is a wood modification technology performed in an inert atmosphere at temperatures above 160℃,which can significantly improve dimensional stability,biological durability and vibrational properties of wood.Besides,it can adjust wood color and modify wood environmentally friendly.However,wood mechanical properties,especially bending strength and impact strength are weakened by the high temperature.The degradation of hemicelluloses is the factor dominating the properties of heat-treated wood,which reduces the hydrophilic groups and nutrient substances in the wood;lowers wood toughness and influences its brightness index.Cellulose and lignin undergo less pronounced changes during the treatment.The increase of cellulose crystallinity makes wood more hydrophobic and rigid.A more rigid cross-linked structure of lignin was formed during the treatment,which not only prohibits the swelling of cell wall but also is the main reason for the color change of the heat-treated wood.The heat-induced extractives are mainly degradation products of hemicelluloses and may affect hygroscopicity and vibrational properties of the wood.The modification degree of the heat-treated wood is affected by various processing factors,among which,temperature ranks No.1 followed by the pressure of treatment environment and remaining time at the maximum temperature.The treatment temperature of 200℃is an altering point for the properties of the treated wood,above which the dimensional stability and biological durability can be significantly improved with the cost of great strength loss.Wood heat treatment has been widely used in the wood industries,which is resulted from the systematic fundamental studies on the one hand,and due to its good environmental,economic and technological advantages on the other hand.Although the theories and technologies of wood heat treatment have been well established,there still exist many unknowns need to be further explored.One challenge is to balance the property improvement and strength loss so that the application of the heat-treated wood can be further broadened.Keeping the color of the treated wood stable is another work worthy of considering especially for those trying to use it as an alternative to tropical species.The treatment equipment and its auxiliaries still have room to be updated to guarantee the long-term service of the system.