【作者】 蒋佳荔；

【导师】 吕建雄；

【作者基本信息】 中国林业科学研究院 ， 木材科学与技术， 2009， 博士

【摘要】 为深入探讨木材动态黏弹特性,以期为人工林木材高效合理的干燥技术、加工技术以及高性能、高附加值材料的制备与应用提供理论依据和科学指导。本研究采用杉木(Cunninghamia lanceolata [Lamb.] Hook.)人工林木材作为试验材料,系统研究了温度-时间-频率作用下木材动态黏弹性的响应机理。本论文包括4个部分的研究内容:①通过测定不同温度下木材线性黏弹区域的临界应变值,得到临界应变值随温度和测量频率变化的信息,并以此作为动态黏弹性能测试参数的选择依据。②通过测定木材在轴向、径向和弦向上的动态黏弹性,考察了木材动态黏弹性的各向异性行为。③系统讨论并分析了木材贮存模量、损耗模量与损耗因子的温度谱、时间谱和频率谱。其中,通过测定动态黏弹性能温度谱,获取木材力学状态与松弛转变行为方面的相关信息。对3种干燥材的动态黏弹行为进行考察,探讨了不同干燥历程对木材结构与性能的影响及其机理;通过测定木材动态黏弹性能时间谱,得到木材的力学状态与松弛转变行为在热作用过程中的经时变化规律;通过测定木材动态黏弹性能频率谱,得到木材的动态刚度与阻尼性质随温度、测量频率的变化情况,并探讨了经历温度急剧变化后木材的动态黏弹行为。④验证并分析了木材时-温等效原理的适用性,通过合成贮存模量和损耗因子主曲线、对水平移动因子与温度的关系曲线进行WLF与Arrhenius模型拟合,讨论了时-温等效原理描述木材动态刚度与阻尼性能的适用性。本论文的主要研究结论归纳如下:1.在-120 ~ 220℃范围内的不同恒定温度水平下,木材线性黏弹区域的临界应变值为0.03% ~ 0.19%,临界应变值随温度的升高呈现出减小的变化趋势。在-80、-20、40、120和220℃时,临界应变值的降幅明显增大。临界应变值对温度的响应规律与木材的力学松弛过程有关,且随测量频率的增加而略有减小。2.木材轴向试样的贮存模量显著高于横向试样的贮存模量,其中,弦向试样的贮存模量最低。木材轴向、径向和弦向试样出现相似的力学松弛过程,力学损耗峰温度存在明显差异。木材轴向试样的刚度高,但其松弛过程的损耗峰温度却低于刚度较低的横向试样的损耗峰温度,这与许多高聚物复合材料的情况是相悖的。木材的动态黏弹行为依赖于试验所选取的形变模式。3.在-120 ~ 280℃范围内的匀速升温过程中,木材的贮存模量随温度的升高呈现出减小的变化趋势。在温度升高的方向上依次出现了4个力学松弛过程:①木材细胞壁无定形区中伯醇羟基的回转取向运动与吸着水分子的回转取向运动叠加而成的松弛过程(出现在-100 ~ -80℃温度域);②低分子量的半纤维素发生玻璃化转变引起的松弛过程(出现在0 ~ 40℃温度域);③木质素发生玻璃化转变引起的松弛过程(出现在90℃附近)以及④木材细胞壁无定形聚合物的微布朗运动引起的松弛过程(出现在240℃附近)。力学损耗峰温度随着含水率的增加向低温方向移动。松弛转变行为出现的温度越高,其表观活化能值越大。4.不同干燥材之间以及干燥材与对照材之间的动态刚度性质、力学损耗峰温度、力学损耗峰强度以及力学松弛过程的表观活化能均存在明显差异。115℃干燥材动态刚度相对较高,是因为在该干燥过程中,木材细胞壁无定形区发生了结晶化或交联化反应,使得刚度增加。真空冷冻干燥材的动态刚度相对较低,是由于在干燥过程中木材细胞壁在负压作用下发生了一定程度的破坏所致。干燥历程改变了木材对水分子的吸着能力和吸着状态。3种干燥材之间、干燥材与对照材之间动态黏弹行为的差异随含水率的增加逐渐缩小。5.在25 ~ 220℃范围内的不同恒定温度水平下,温度和热作用时间主要引起木材出现动态刚度降低、木质素热软化和细胞壁无定形聚合物热降解等一系列现象。升高温度与延长热作用时间对引起木材动态刚度降低是等效的。在140、160和180℃恒温过程的不同时间域内均可观察到木质素的热软化现象,升高温度可以缩短木质素发生玻璃化转变的松弛时间。当温度高于180℃时,木材细胞壁无定形聚合物发生热降解是导致木材黏弹性能发生变化的一个主要因素。6.在25 ~ 220℃范围内的不同恒定温度水平下,木材的贮存模量随温度的升高而降低,随测量频率(0.1 ~ 100 Hz)的增加而增大。随着温度的升高,损耗因子的极小值所对应的特征频率值移向高频方向。经历温度急剧变化后,木材的动态刚度降低、阻尼性能增大,木材细胞壁无定形区内形成了不稳定结构,从而加快分子链段运动的松弛过程。7.在25 ~ 150℃温度范围内,利用时-温等效原理描述低含水率(约为0.6%)木材的动态刚度性质是适用的,但无法用来预测木材在宽阔频率范围内的松弛转变行为。更多还原

【Abstract】 In order to investigate the dynamic viscoelasticities of wood, and provide theoretical basis and scientific instructions for drying technology, manufacture processing and high value-added materials of plantation wood, the dynamic viscoelasticity of Chinese fir (Cunninghamia lanceolata [Lamb.]Hook) wood under temperature - time - frequency coupling was systematically investigated in this current study.Firstly, the critical strain of wood linear viscoelastic region was determined at different temperatures, therefore, some valuable information concerning the changes of critical strain with temperature and measurement frequency could be obtained. Secondly, the dynamic viscoelastic properties in longitudinal, radial and tangential directions were also investigated, and the differences in wood anisotropic behaviour of viscoelasticity were discussed. Then, this dissertation systematically described and analysed the dynamic mechanical temperature/ time/frequency spectrum of wood. In the investigation of dynamic viscoelastic properties during temperature ramping process, the storage modulus, loss modulus and loss factor of specimens were measured. Thus, the changes of mechanical relaxation behavior during temperature ramping process can be clarified. Furthermore, the dynamic viscoelastic behavior of three kinds of dried wood was also investigated. As a result, the effects of heating/drying history on wood structure and properties were obtained. With respect to the dynamic viscoelastic properties under different constant temperatures, the dynamic mechanical behavior during isothermal processes at various constant temperatures was analyzed. Furthermore, the relationship between dynamic viscoelastic properties of wood over temperature and heating time were obtained. Moreover, the influence of frequency on wood viscoelasticity under two types of heating conditions was also investigated.In the investigation of the validity of wood TTSP (Time-Temperature Superposition Principle), a master curve was generated, and the shift factors were analyzed as a function of temperature to evaluate the fit of the data to the WLF equation and Arrhenius relation. As a result, the applicability of TTSP that predicts the stiffness and damping of wood over broad frequency scales were verificated and analyzed. The major achievements of this study were summarized as follows:1. Under constant temperatures ranged from -120 to 220oC, the critical strain value of linear viscoelastic region are between 0.03% and 0.19%, and it generally reduced with increasing temperature except temperatures of -80, -20, 40, 120 and 220oC. These five exceptions were reasoned by the occurrence of relaxation processes. With the increasing of applied frequency, the critical strain presented a smoothly decreasing tendency.2. The specimens oriented parallel to the grain presented the highest storage modulus, and the storage modulus was much lower in the tangential direction than that in the radial direction. Two relaxation processes were observed for all of longitudinal sample (L), radial sample (R) and tangential sample (T). While L, R and T samples differed in loss peak temperatures. The L sample showed a lower loss peak temperature than that for the R and T sample, and it was in conflict with polymer composites where the higher loss peak temperatures were found in the stiffer direction. The rheological properties of wood showed a dependence upon the mechanical modes used during experiments.3. During the temperature ramping process at temperature range from -120 to 280oC, the storage modulus decreased with the increase of temperature. Four relaxation processes were detected in this temperature range:①the relaxation process at about -100 to -80oC, which was most probably attributed to the motions of methyl groups in amorphous region of wood cell wall (absolutely dry state) and the motions of absorbed water molecules in wood (water existence state);②the relaxation process at about 0 to 40oC, which was presumably assigned to glass transition of hemicellulose with low molecular weight;③the relaxation process at around 90oC, which was attributed to molecular motion of lignin, and④the relaxation process at around 240oC, which was assigned to micro-Brownian motion of cell-wall polymers in the non-crystalline region. With the increase of moisture content, the loss peak temperatures of relaxation processes shifted to lower temperature range. The apparent activation energy of relaxation process at higher temperature location showed a higher value than that at lower temperature location.4. Three kinds of dried wood and control wood differed in the dynamic stiffness properties, the loss peak temperatures, the loss peak density and the apparent activation energy of relaxation processes. 115oC dried wood displayed more stiffness than the other two kinds of dried specimens as well as the untreated wood. It was probably due to the crosslinking action or cellulose crystallization during drying process. Damage to the wood cell walls during the freeze vacuum drying process probably caused the lowest stiffness of vacuum-freeze dried wood. Furthermore, the moisture adsorption capability and adsorption state were altered by different heating/drying history. With increasing moisture content, the differnence in dynamic viscoelastic properties was reduced among dried and control wood.5. Under constant temperatures ranged from 25 to 220oC, the temperature and heating time mainly resulted in the reduction of wood stiffness, thermal softening and thermal degradation of wood. Elevation of temperature or prolongation of heating time has the equal effects on wood stiffness. Lignin softening was suggested to be the cause of the relaxation process that appeared at temperatures of 140, 160 and 180oC. The relaxation time for glass transition of lignin could be shortened by elevating temperature. At temperatures above 180oC, the loss of amorphous polysaccharides due to degradation was considered to be the main factor affecting wood viscoelasticity.6. Under constant temperatures ranged from 25 to 220oC, storage modulus exhibited lower values at higher temperatures and decreased gradually as frequencies decreased from 100 to 0.1 Hz for specimens acclimated at constant temperatures. The minimum value of the loss factor slightly shifted towards higher frequencies at higher temperatures. After suffered rapid heating process, the dynamic stiffness and damping of wood showed decreasing and increasing, respectively. It was suggested that the unstable structures were formed in the cell walls because of rapid heating and consequently increased the mobility of molecular chains.7. The time-temperarture superposition principle (TTSP) could be used effectively to demonstrate frequency-temperature equivalence for the dynamic stiffness property of wood with minor moisture content (about 0.6%) over a temperature range of 25 to 150oC. While it was failed in predicting the relaxation transition behavior of wood over broad frequency scales.更多还原