蒸汽介质热处理木材性质及其强度损失控制原理
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摘要
蒸汽介质热处理是改善木材尺寸稳定性和耐久性的有效方法之一。热处理后木材的颜色变深,尺寸稳定性和耐久性得到显著提高,但是力学强度降低,这一缺陷严重阻碍了热处理木材的广泛应用。因此研究热处理材的性能变化规律及其力学强度损失机理,对进一步拓宽热处理材的应用领域具有非常重要的意义。本研究采用完全随机区组设计方法,以蒸汽为介质兼作保护气体,在氧气含量低于2%的密闭干燥箱内分别对杉木心材、杉木边材、毛白杨木材进行处理,处理条件为温度170℃～230℃、时间1h～5hrs。采用方差分析、多元回归分析等数据处理方法对热处理木材的性质及其力学性能进行分析,重点考察了温度和时间两个工艺因子对木材性质变化的影响,通过对热处理木材物理性质和化学性质的研究,揭示了热处理过程中木材性质变化规律及其力学强度损失的机理,分别建立了热处理材抗弯强度损失率和弹性模量损失率与其他性能之间的数学回归模型以及温度和时间与热处理材各个性质变化率之间的数学回归模型。依据相关回归模型,可以分别预测出在不同热处理条件下木材各个性能的变化程度。在此基础上进一步提出了控制热处理材力学强度损失的方法。本论文的研究意义在于将蒸汽介质热处理方法推广应用到我国人工林木材的加工利用领域,为更好地利用我国人工林资源提供理论依据。
本论文的主要研究结论如下:
1、温度和时间是决定热处理材性能的两个重要因子,其中尤以温度更为重要。热处理材性能发生显著变化的临界温度为200℃,时间为2h。在温度低于200℃时,热处理材的全干密度、综纤维素和α-纤维素的含量、体积干缩率和湿胀率、木材颜色等均有不同程度的缓慢降低,而失重率、耐腐性能、硬度、抗弯强度和弹性模量等均有不同程度的缓慢提高;在温度高于200℃时,除了耐腐性能急剧增强之外,以上其他性能均开始急剧下降。
2、傅立叶变换红外光谱分析表明,热处理材中表征木材纤维素和半纤维素的官能团的特征吸收峰的强度减弱,而表征木素的官能团的特征吸收峰的强度增加,证实了热处理过程中综纤维素和α-纤维素的含量降低,而木质素含量升高的变化规律。本试验条件下,杉木心材综纤维素、α-纤维素的含量损失率分别为2.87%～21.40%、0.33%～35.30%,木素含量的提高率为0.53%～22.63%;杉木边材综纤维素、α-纤维素的含量损失率分别为2.91%～22.71%、0.34%～50.32%,木素含量的提高率为0.47%～37.09%;毛白杨木材综纤维素、α-纤维素的含量损失率分别为2.52%～23.72%、0.94%～41.44%,木素含量的提高率为9.06%～123.64%。方差分析表明,在α=0.01水平上,温度和时间对木材化学组分含量变化的影响极显著。热处理过程中,纤维素和半纤维素的热降解反应可能是造成热处理材综纤维素和α-纤维素的含量降低的主要原因,缩聚反应可能是造成木质素含量升高的主要原因。
3、木质素是木材产生颜色的主要来源,热处理过程中木材中木质素含量的增加是造成木材颜色加深的主要原因。热处理后,木材的颜色变深,逐渐变为褐色至深褐色。本试验条件下,杉木心材的色饱和度差值△C*、总体色差△E*和色相差△H*的变化范围分别为3.67～-7.73、6.61～43.46、0.60～6.02;杉木边材的△C*、△E*和△H*的变化范围分别为4.46～-8.31、11.18～57.49、1.76～7.11;毛白杨的△C*、△E*和△H*的变化范围分别为6.87～-5.14、13.98～62.00、3.63～10.46。表明热处理能够显著改善木材的颜色。实际生产中,可根据不同颜色的需求来设置温度和时间,将木材颜色调控至预期的颜色。
4、热处理过程中,木材化学组分综纤维素和α-纤维素的热降解反应和木材内无机物质以及可挥发性物质的流失,可能是导致了木材全干密度降低的主要原因。本试验条件下,杉木心材、杉木边材、毛白杨木材的全干密度损失率分别为0.62%～15.07%、0.77%～15.80%、0.52%～13.63%。方差分析表明,在α=0.01水平上,温度和时间对热处理材全干密度变化的影响极显著。
5、热处理显著提高了木材的尺寸稳定性。热处理过程中,随着温度的升高和时间的延长,木材的尺寸稳定性稳步提高。本试验条件下,杉木心材、杉木边材、毛白杨的尺寸稳定性最大提高率分别为72.63%、67.21%、70.71%。方差分析表明,在α=0.01水平上,温度和时间对热处理材体积变化的影响极显著。热处理可能造成了木材中大量的亲水性基团羟基(-OH)流失,同时生成了憎水性新物质,因此大大减少了木材的吸湿性,提高了木材的尺寸稳定性。
6、在温度200℃左右,时间少于3h的热处理可以提高木材的硬度,这是由于低温时木材的热降解反应并未占据主导地位,而此时无定形区内水分的流失导致相邻纤维素之间形成了新的氢键,结果导致木材的硬度有所提高。其中,杉木心材、杉木边材、毛白杨木材的硬度最大提高率分别为12.67% (200℃,1h)、26.82% (200℃,2h)、15.82% (200℃,3h)。当温度高于200℃后,热降解反应逐渐占据了主导地位,导致木材硬度开始急剧下降。在温度230℃,5h时,杉木心材、杉木边材、毛白杨木材的硬度的损失率分别为26.07%、24.36%、22.09%。方差分析表明,在α=0.01水平上,温度和时间对热处理材硬度变化的影响极显著。
7、热处理能够显著提高毛白杨木材的耐腐性能,即从不耐腐等级提高至强耐腐等级。未处理毛白杨木材的失重率为55.746%,其耐腐性为不耐腐等级;在温度230℃、5h的热处理条件下,其失重率仅为2.052%,其耐腐性已为强耐腐等级。这可能是由于在热处理过程中,木材中可供木腐菌食用的营养物质如多糖类物质和无机类物质等大量流失,导致木腐菌无法存活,从而使得热处理材少受或免于腐朽。杉木心材和杉木边材的耐腐性能在热处理前后均为强耐腐等级,因此对杉木进行热处理的意义在于提高其尺寸稳定性以及改善其其他性能。
8、在温度200℃以下时,2h左右的热处理可以增加杉木边材、毛白杨木材的抗弯强度和弹性模量,这可能是由于木材内无定形区内水分的流失导致相邻纤维素之间形成了新的氢键,使得纤维排列更加紧密,而此时热降解反应并未占据主导地位,因此热处理材的抗弯强度和弹性模量有所增加。其中,杉木边材抗弯强度和弹性模量的最大提高率分别为6.36%和2.84%;毛白杨抗弯强度和弹性模量的最大提高率分别为11.28%和15.80%。当温度等于或高于200℃时,热降解反应开始占据主导地位,致使纤维素大分子链断裂形成小分子,严重破坏了木材的骨架结构,因此降低了木材的力学强度。本试验条件下,在230℃、5h时,杉木心材、杉木边材、毛白杨的抗弯强度和弹性模量的损失率均达到最大值,分别为49.39%和21.93%;49.72%和22.42%;54.20%和-2.73%。方差分析表明,在α=0.01水平上,温度和时间对热处理材抗弯强度和弹性模量的变化影响均为极显著。
9、以y代表抗弯强度损失率,x_1为处理温度、x_2为处理时间。那么,杉木心材、杉木边材、毛白杨木材的抗弯强度损失率的回归模型分别为y = 0.558x_1+2.806x_2-99.975 (R~2=0.943)、y = 0.693x_1+5.566x_2-137.897 (R~2=0.909)、y =0.961x_1+4.218x_2-183.832 (R~2=0.953)。以y代表弹性模量损失率,则回归模型分别为y = 0.247x_1+1.235x_2-44.865 (R~2=0.874)、y = 0.222x_1+3.512x_2-47.676 (R~2=0.927)、y =0.089x_1+1.544x_2-32.172 (R~2=0.777)。根据上述模型,可分别推测出不同热处理条件下木材抗弯强度或弹性模量的损失率。
10、控制木材力学强度损失的根本方法是控制热处理温度。本试验中,将温度控制在临界温度即200℃以下,能够保证木材的抗弯强度和弹性模量基本上没有或仅有轻微的损失,甚至短时间的热处理还能够增强木材的抗弯强度和弹性模量。依据相关数学回归模型,实际生产中应根据热处理材的最终用途来选择合适的处理温度和时间。
Steam heat treatment has been known for long time as one of a number of effective methods to improve the dimensional stability and durability of wood. The main effect obtained by the heat-treatment of wood is a reduced wood hygroscopicity. Foremost advantages of wood treated in this manner are the wood’s increased resistance to different types of biodegradation and an improved dimensional stability. Additionally, the treatment leads to a darkening of the wood color which can also be a beneficial aspect of the treatment.
Unfortunately, undesired side effects, in particular the loss of strength and increased brittleness of the heat-treated wood, have prevented the commercial utilization of thermal modification. In order to expand the industrial applicability of heat-treated wood, it is important to study the reasons for the adverse effects and devise methods to control mechanical strength loss and the property changes of heat-treated wood during the heating process.
The Random Complete Block Design was developed to arrange each experiment unit in this paper. Three wood specimens of China-Fir heartwood, sapwood and Chinese White Poplar were heat treated at temperatures from 170℃to 230℃for times from 1h to 5hrs in airtight equipment with an atmosphere within comprising less than 2 per cent of oxygen content. Saturated steam was used as a heating medium and a shielding gas. The properties and mechanical strength of the heat-treated wood were analyzed by statistical methods of variance and multiple regression analysis. Mathematical regression models relating to the treatment temperature, treatment time and the properties of the heat-treated wood were established, along with mathematical regression models comparing loss ratio of bending strength or modulus of elasticity and other properties of the heat-treated wood.
A method of controlling mechanical strength loss was devised based on results of experiments in this study to provide a mechanism for, and justification of, increasing the utilization of heat-treated wood so as to make better use of the plantation forests of China and other countries where similar problems with timber use may occur.
The main research results are summarized as follows:
1. Temperature and time are two crucial factors affecting the final quality of heat-treated wood, with temperature having a greater effect than time. Analysis shows 200℃and 2hrs are the critical temperature and time factors for the heat-treated process in this study. When the temperature is below 200℃factors such as the absolute-dry density, the content of holocellulose andα-cellulose, the shrinking ratio and swelling ratio of volumes, wood color were slowly decreased, while the weight loss ratio, decay resistance, hardness, bending strength and modulus of elasticity were slowly increased. However, when the temperature is over 200℃, except for decay resistance which is still increased, the all above mentioned other properties are decreased rapidly.
2. Fourier Transform Infrared spectroscopy indicates the intensity of the characteristic absorption peak of holocellulose and hemicelluloses become weaker and weaker with the increase of temperature and lengthening time of the heat treatment. However, the density of the characteristic absorption peak of lignin becomes stronger and stronger. It indicates that the heat treatment decreases the contents of holocellulose andα-cellulose respectively, while enhancing the content of wood lignin. In this study, the loss ratios of holocellulose,α-cellulose and the increased ratio of lignin in China-Fir heartwood, sapwood and Chinese White Poplar respectively, are 2.87%～21.40%, 0.33%～35.30%, 0.53%～22.63% 2.91%～22.71%、0.34%～50.32%, 0.47%～37.09%;2.52%～23.72%、0.94%～41.44%, 9.06%～123.64%. There is a highly significant difference at the 0.01 level between the chemical components content of the wood and temperature with time. It is likely that the thermal degradation leads to a decrease in the content of holocellulose andα-cellulose respectively, while the condensation reaction leads to an increase in the lignin content of the heat-treated wood.
3. Lignin level is the main factor relating to wood color, with the increase of lignin content of the wood being the essential reason for the treatment making the wood color become darker and darker. The color of heat-treated wood becomes brown and puce with the increasing of temperature and time during the heating process. In this study, the range of variance value of△C*,△E*,△H* in China-Fir heartwood, sapwood and Chinese White Poplar respectively, are 3.67～-7.73, 6.61～43.46, 0.60～6.02;4.46～-8.31, 11.18～57.49, 1.76～7.11;6.87～-5.14, 13.98～62.00, 3.63～10.46. There is a significant difference at the 0.01 level between the color of the wood and the temperature and time respectively. This data indicates that the heat treatment changes the color of wood significantly and that the desired timber color will be able to be obtained by setting relative and selected temperatures and times in manufacturing operations.
4. The thermal degradation of holocellulose andα-cellulose and the disappearance of inorganic and volatile materials are the most likely reason for the decrease of absolute-dry density of the heat-treated wood. In this study, the loss ratios of densities in China-Fir heartwood, sapwood and Chinese White Poplar respectively, are 0.62%～15.07%、0.77%～15.80%、0.52%～13.63%。There is a significant difference at the 0.01 level between the absolute-dry density of the heat-treated wood and the temperature and time respectively.
5. Heat treatment enhances the dimensional stability of heat-treated wood significantly. The dimensional stability was improved step by step with the increase of temperature and time during the heat treatment process. In this study, the highest improving ratios of China-Fir heartwood, sapwood and Chinese White Poplar respectively are 72.63%, 67.21 % and 70.71 %. There is a significant difference between the volume change of the heat-treated wood and the temperature and time respectively. Heat treatment reduces large numbers of carbonyl while apparently generating significant amounts of new hydrophobic materials, resulting in the hygroscopic property of the wood being reduced substantially, thereby significantly improving the wood’s dimensional stability.
6. The hardness of treated wood is increased by heat-treating at around 200℃for about 3 hours. It is possible that the water in the amorphous region in the wood disappears causing the production of the new hydrogen bonds between fibres of wood, further improving the hardness of the heat-treated wood. In this study, the highest improving ratios of China-Fir heartwood, sapwood and Chinese White Poplar respectively are 12.67% at 200℃for 1hr, 26.82% at 200℃for 2hrs and 15.82% at 200℃for 3hrs. When temperatures are above 200℃, thermal degradation becomes the main effect during the heat-treatment so that the hardness of the heat-treated wood is decreased rapidly. The loss ratio of the three heat-treated wood species, at 230℃for 5hrs, are 26.07%, 24.36% and 22.09% respectively. There is a highly significant difference at the 0.01 level between hardness of wood and temperature with time.
7. Heat treatment enhances the decay resistance of Chinese White Poplar fromⅣGrade toⅠGrade. The untreated and the heat-treated Chinese White Poplar respectively belong to ⅣGrade with a weight loss ratio of 55.746% andⅠGrade with a weight loss ratio of 2.052% at 230℃for 5hrs. The nutrient materials such as polysaccharide and the inorganic materials and the like in the wood that generally provide food for bacteria were likely eliminated so that the bacteria have not enough food to survive. Therefore, the decay resistance of heat-treated wood is improved remarkably. The decay resistance of China-Fir heartwood and sapwood are bothⅠGrade, whether heat-treated or not heat-treated. The aim of the heat-treated of China-Fir is to improve the other properties such as dimensional stability.
8. The bending strength and the modulus of elasticity of the China-Fir sapwood and the Chinese White Poplar are improved with treatment below 200℃for around 2hrs. This is likely caused because of the disappearance of the water of the amorphous region in the wood resulting in the production of new hydrogen bonds between fibers of wood, with the lower temperatures not causing thermal degradation in the wood. The highest improving ratios of the bending strength and the modulus of elasticity respectively are 6.38% and 2.84% for the China-Fir sapwood, with 11.28% and 15.80% for the Chinese White Poplar. When the temperature is above 200℃, the thermal degradation of cellulose and hemicellulose is the main reaction in which the fiber chains become shorter and shorter along with the increase of temperature and time during the heat-treated process. Therefore, the mechanical properties of the heat-treated wood are decreased. The loss ratios of the bending strength and the modulus of elasticity of the China-Fir heartwood, sapwood and Chinese White Poplar at 230℃for 5hrs respectively are 49.39% and 21.93%, 49.72% and 22.42%, 54.20% and -2.73%. There is a highly significant difference at the 0.01 level between the bending strength and the modulus of elasticity of the wood and the temperature and time respectively.
9. In this study, y means loss ratio of bending strength of heat-treated wood, x_1 means treatment temperature, x_2 means treatment time, so the regression models between bending strength and x_1with x_2 of the China-Fir heartwood, the sapwood and the Chinese White Poplar respectively, are y = 0.558x_1+2.806x_2-99.975 (R~2=0.943)、y = 0.693x_1+5.566x_2-137.897 (R~2=0.909)、y =0.961x_1+4.218x_2-183.832 (R~2=0.953). If y means the modulus of elasticity, the regression models between the modulus of elasticity and x_1with x_2 of above the three species wood respectively, are y = 0.247x_1+1.235x_2-44.865 (R~2=0.874)、y = 0.222x_1+3.512x_2-47.676 (R2=0.927)、y =0.089x_1+1.544x_2-32.172 (R~2=0.777). Using above mathematical regression models, the loss ratios of the bending strength and the modulus of elasticity of heat-treated wood will be predicted under different treatment temperatures and times respectively.
10. The best method of controlling the loss of the mechanical strength of the wood is by the controlling of the temperature. Based on the results from the above three wood species experiments the temperature should be controlled at below 200℃so that there is no loss or a minimum loss in the mechanical strength of the wood, with an enhancement of the hardness, bending strength and modulus of elasticity properties of the heat-treated wood within a short time. With the correlative mathematic regression models, the optimum temperature and time used in the heat treatment can be determined to accord with the final use of the heat-treated wood.
本论文的主要研究结论如下:
1、温度和时间是决定热处理材性能的两个重要因子,其中尤以温度更为重要。热处理材性能发生显著变化的临界温度为200℃,时间为2h。在温度低于200℃时,热处理材的全干密度、综纤维素和α-纤维素的含量、体积干缩率和湿胀率、木材颜色等均有不同程度的缓慢降低,而失重率、耐腐性能、硬度、抗弯强度和弹性模量等均有不同程度的缓慢提高;在温度高于200℃时,除了耐腐性能急剧增强之外,以上其他性能均开始急剧下降。
2、傅立叶变换红外光谱分析表明,热处理材中表征木材纤维素和半纤维素的官能团的特征吸收峰的强度减弱,而表征木素的官能团的特征吸收峰的强度增加,证实了热处理过程中综纤维素和α-纤维素的含量降低,而木质素含量升高的变化规律。本试验条件下,杉木心材综纤维素、α-纤维素的含量损失率分别为2.87%～21.40%、0.33%～35.30%,木素含量的提高率为0.53%～22.63%;杉木边材综纤维素、α-纤维素的含量损失率分别为2.91%～22.71%、0.34%～50.32%,木素含量的提高率为0.47%～37.09%;毛白杨木材综纤维素、α-纤维素的含量损失率分别为2.52%～23.72%、0.94%～41.44%,木素含量的提高率为9.06%～123.64%。方差分析表明,在α=0.01水平上,温度和时间对木材化学组分含量变化的影响极显著。热处理过程中,纤维素和半纤维素的热降解反应可能是造成热处理材综纤维素和α-纤维素的含量降低的主要原因,缩聚反应可能是造成木质素含量升高的主要原因。
3、木质素是木材产生颜色的主要来源,热处理过程中木材中木质素含量的增加是造成木材颜色加深的主要原因。热处理后,木材的颜色变深,逐渐变为褐色至深褐色。本试验条件下,杉木心材的色饱和度差值△C*、总体色差△E*和色相差△H*的变化范围分别为3.67～-7.73、6.61～43.46、0.60～6.02;杉木边材的△C*、△E*和△H*的变化范围分别为4.46～-8.31、11.18～57.49、1.76～7.11;毛白杨的△C*、△E*和△H*的变化范围分别为6.87～-5.14、13.98～62.00、3.63～10.46。表明热处理能够显著改善木材的颜色。实际生产中,可根据不同颜色的需求来设置温度和时间,将木材颜色调控至预期的颜色。
4、热处理过程中,木材化学组分综纤维素和α-纤维素的热降解反应和木材内无机物质以及可挥发性物质的流失,可能是导致了木材全干密度降低的主要原因。本试验条件下,杉木心材、杉木边材、毛白杨木材的全干密度损失率分别为0.62%～15.07%、0.77%～15.80%、0.52%～13.63%。方差分析表明,在α=0.01水平上,温度和时间对热处理材全干密度变化的影响极显著。
5、热处理显著提高了木材的尺寸稳定性。热处理过程中,随着温度的升高和时间的延长,木材的尺寸稳定性稳步提高。本试验条件下,杉木心材、杉木边材、毛白杨的尺寸稳定性最大提高率分别为72.63%、67.21%、70.71%。方差分析表明,在α=0.01水平上,温度和时间对热处理材体积变化的影响极显著。热处理可能造成了木材中大量的亲水性基团羟基(-OH)流失,同时生成了憎水性新物质,因此大大减少了木材的吸湿性,提高了木材的尺寸稳定性。
6、在温度200℃左右,时间少于3h的热处理可以提高木材的硬度,这是由于低温时木材的热降解反应并未占据主导地位,而此时无定形区内水分的流失导致相邻纤维素之间形成了新的氢键,结果导致木材的硬度有所提高。其中,杉木心材、杉木边材、毛白杨木材的硬度最大提高率分别为12.67% (200℃,1h)、26.82% (200℃,2h)、15.82% (200℃,3h)。当温度高于200℃后,热降解反应逐渐占据了主导地位,导致木材硬度开始急剧下降。在温度230℃,5h时,杉木心材、杉木边材、毛白杨木材的硬度的损失率分别为26.07%、24.36%、22.09%。方差分析表明,在α=0.01水平上,温度和时间对热处理材硬度变化的影响极显著。
7、热处理能够显著提高毛白杨木材的耐腐性能,即从不耐腐等级提高至强耐腐等级。未处理毛白杨木材的失重率为55.746%,其耐腐性为不耐腐等级;在温度230℃、5h的热处理条件下,其失重率仅为2.052%,其耐腐性已为强耐腐等级。这可能是由于在热处理过程中,木材中可供木腐菌食用的营养物质如多糖类物质和无机类物质等大量流失,导致木腐菌无法存活,从而使得热处理材少受或免于腐朽。杉木心材和杉木边材的耐腐性能在热处理前后均为强耐腐等级,因此对杉木进行热处理的意义在于提高其尺寸稳定性以及改善其其他性能。
8、在温度200℃以下时,2h左右的热处理可以增加杉木边材、毛白杨木材的抗弯强度和弹性模量,这可能是由于木材内无定形区内水分的流失导致相邻纤维素之间形成了新的氢键,使得纤维排列更加紧密,而此时热降解反应并未占据主导地位,因此热处理材的抗弯强度和弹性模量有所增加。其中,杉木边材抗弯强度和弹性模量的最大提高率分别为6.36%和2.84%;毛白杨抗弯强度和弹性模量的最大提高率分别为11.28%和15.80%。当温度等于或高于200℃时,热降解反应开始占据主导地位,致使纤维素大分子链断裂形成小分子,严重破坏了木材的骨架结构,因此降低了木材的力学强度。本试验条件下,在230℃、5h时,杉木心材、杉木边材、毛白杨的抗弯强度和弹性模量的损失率均达到最大值,分别为49.39%和21.93%;49.72%和22.42%;54.20%和-2.73%。方差分析表明,在α=0.01水平上,温度和时间对热处理材抗弯强度和弹性模量的变化影响均为极显著。
9、以y代表抗弯强度损失率,x_1为处理温度、x_2为处理时间。那么,杉木心材、杉木边材、毛白杨木材的抗弯强度损失率的回归模型分别为y = 0.558x_1+2.806x_2-99.975 (R~2=0.943)、y = 0.693x_1+5.566x_2-137.897 (R~2=0.909)、y =0.961x_1+4.218x_2-183.832 (R~2=0.953)。以y代表弹性模量损失率,则回归模型分别为y = 0.247x_1+1.235x_2-44.865 (R~2=0.874)、y = 0.222x_1+3.512x_2-47.676 (R~2=0.927)、y =0.089x_1+1.544x_2-32.172 (R~2=0.777)。根据上述模型,可分别推测出不同热处理条件下木材抗弯强度或弹性模量的损失率。
10、控制木材力学强度损失的根本方法是控制热处理温度。本试验中,将温度控制在临界温度即200℃以下,能够保证木材的抗弯强度和弹性模量基本上没有或仅有轻微的损失,甚至短时间的热处理还能够增强木材的抗弯强度和弹性模量。依据相关数学回归模型,实际生产中应根据热处理材的最终用途来选择合适的处理温度和时间。
Steam heat treatment has been known for long time as one of a number of effective methods to improve the dimensional stability and durability of wood. The main effect obtained by the heat-treatment of wood is a reduced wood hygroscopicity. Foremost advantages of wood treated in this manner are the wood’s increased resistance to different types of biodegradation and an improved dimensional stability. Additionally, the treatment leads to a darkening of the wood color which can also be a beneficial aspect of the treatment.
Unfortunately, undesired side effects, in particular the loss of strength and increased brittleness of the heat-treated wood, have prevented the commercial utilization of thermal modification. In order to expand the industrial applicability of heat-treated wood, it is important to study the reasons for the adverse effects and devise methods to control mechanical strength loss and the property changes of heat-treated wood during the heating process.
The Random Complete Block Design was developed to arrange each experiment unit in this paper. Three wood specimens of China-Fir heartwood, sapwood and Chinese White Poplar were heat treated at temperatures from 170℃to 230℃for times from 1h to 5hrs in airtight equipment with an atmosphere within comprising less than 2 per cent of oxygen content. Saturated steam was used as a heating medium and a shielding gas. The properties and mechanical strength of the heat-treated wood were analyzed by statistical methods of variance and multiple regression analysis. Mathematical regression models relating to the treatment temperature, treatment time and the properties of the heat-treated wood were established, along with mathematical regression models comparing loss ratio of bending strength or modulus of elasticity and other properties of the heat-treated wood.
A method of controlling mechanical strength loss was devised based on results of experiments in this study to provide a mechanism for, and justification of, increasing the utilization of heat-treated wood so as to make better use of the plantation forests of China and other countries where similar problems with timber use may occur.
The main research results are summarized as follows:
1. Temperature and time are two crucial factors affecting the final quality of heat-treated wood, with temperature having a greater effect than time. Analysis shows 200℃and 2hrs are the critical temperature and time factors for the heat-treated process in this study. When the temperature is below 200℃factors such as the absolute-dry density, the content of holocellulose andα-cellulose, the shrinking ratio and swelling ratio of volumes, wood color were slowly decreased, while the weight loss ratio, decay resistance, hardness, bending strength and modulus of elasticity were slowly increased. However, when the temperature is over 200℃, except for decay resistance which is still increased, the all above mentioned other properties are decreased rapidly.
2. Fourier Transform Infrared spectroscopy indicates the intensity of the characteristic absorption peak of holocellulose and hemicelluloses become weaker and weaker with the increase of temperature and lengthening time of the heat treatment. However, the density of the characteristic absorption peak of lignin becomes stronger and stronger. It indicates that the heat treatment decreases the contents of holocellulose andα-cellulose respectively, while enhancing the content of wood lignin. In this study, the loss ratios of holocellulose,α-cellulose and the increased ratio of lignin in China-Fir heartwood, sapwood and Chinese White Poplar respectively, are 2.87%～21.40%, 0.33%～35.30%, 0.53%～22.63% 2.91%～22.71%、0.34%～50.32%, 0.47%～37.09%;2.52%～23.72%、0.94%～41.44%, 9.06%～123.64%. There is a highly significant difference at the 0.01 level between the chemical components content of the wood and temperature with time. It is likely that the thermal degradation leads to a decrease in the content of holocellulose andα-cellulose respectively, while the condensation reaction leads to an increase in the lignin content of the heat-treated wood.
3. Lignin level is the main factor relating to wood color, with the increase of lignin content of the wood being the essential reason for the treatment making the wood color become darker and darker. The color of heat-treated wood becomes brown and puce with the increasing of temperature and time during the heating process. In this study, the range of variance value of△C*,△E*,△H* in China-Fir heartwood, sapwood and Chinese White Poplar respectively, are 3.67～-7.73, 6.61～43.46, 0.60～6.02;4.46～-8.31, 11.18～57.49, 1.76～7.11;6.87～-5.14, 13.98～62.00, 3.63～10.46. There is a significant difference at the 0.01 level between the color of the wood and the temperature and time respectively. This data indicates that the heat treatment changes the color of wood significantly and that the desired timber color will be able to be obtained by setting relative and selected temperatures and times in manufacturing operations.
4. The thermal degradation of holocellulose andα-cellulose and the disappearance of inorganic and volatile materials are the most likely reason for the decrease of absolute-dry density of the heat-treated wood. In this study, the loss ratios of densities in China-Fir heartwood, sapwood and Chinese White Poplar respectively, are 0.62%～15.07%、0.77%～15.80%、0.52%～13.63%。There is a significant difference at the 0.01 level between the absolute-dry density of the heat-treated wood and the temperature and time respectively.
5. Heat treatment enhances the dimensional stability of heat-treated wood significantly. The dimensional stability was improved step by step with the increase of temperature and time during the heat treatment process. In this study, the highest improving ratios of China-Fir heartwood, sapwood and Chinese White Poplar respectively are 72.63%, 67.21 % and 70.71 %. There is a significant difference between the volume change of the heat-treated wood and the temperature and time respectively. Heat treatment reduces large numbers of carbonyl while apparently generating significant amounts of new hydrophobic materials, resulting in the hygroscopic property of the wood being reduced substantially, thereby significantly improving the wood’s dimensional stability.
6. The hardness of treated wood is increased by heat-treating at around 200℃for about 3 hours. It is possible that the water in the amorphous region in the wood disappears causing the production of the new hydrogen bonds between fibres of wood, further improving the hardness of the heat-treated wood. In this study, the highest improving ratios of China-Fir heartwood, sapwood and Chinese White Poplar respectively are 12.67% at 200℃for 1hr, 26.82% at 200℃for 2hrs and 15.82% at 200℃for 3hrs. When temperatures are above 200℃, thermal degradation becomes the main effect during the heat-treatment so that the hardness of the heat-treated wood is decreased rapidly. The loss ratio of the three heat-treated wood species, at 230℃for 5hrs, are 26.07%, 24.36% and 22.09% respectively. There is a highly significant difference at the 0.01 level between hardness of wood and temperature with time.
7. Heat treatment enhances the decay resistance of Chinese White Poplar fromⅣGrade toⅠGrade. The untreated and the heat-treated Chinese White Poplar respectively belong to ⅣGrade with a weight loss ratio of 55.746% andⅠGrade with a weight loss ratio of 2.052% at 230℃for 5hrs. The nutrient materials such as polysaccharide and the inorganic materials and the like in the wood that generally provide food for bacteria were likely eliminated so that the bacteria have not enough food to survive. Therefore, the decay resistance of heat-treated wood is improved remarkably. The decay resistance of China-Fir heartwood and sapwood are bothⅠGrade, whether heat-treated or not heat-treated. The aim of the heat-treated of China-Fir is to improve the other properties such as dimensional stability.
8. The bending strength and the modulus of elasticity of the China-Fir sapwood and the Chinese White Poplar are improved with treatment below 200℃for around 2hrs. This is likely caused because of the disappearance of the water of the amorphous region in the wood resulting in the production of new hydrogen bonds between fibers of wood, with the lower temperatures not causing thermal degradation in the wood. The highest improving ratios of the bending strength and the modulus of elasticity respectively are 6.38% and 2.84% for the China-Fir sapwood, with 11.28% and 15.80% for the Chinese White Poplar. When the temperature is above 200℃, the thermal degradation of cellulose and hemicellulose is the main reaction in which the fiber chains become shorter and shorter along with the increase of temperature and time during the heat-treated process. Therefore, the mechanical properties of the heat-treated wood are decreased. The loss ratios of the bending strength and the modulus of elasticity of the China-Fir heartwood, sapwood and Chinese White Poplar at 230℃for 5hrs respectively are 49.39% and 21.93%, 49.72% and 22.42%, 54.20% and -2.73%. There is a highly significant difference at the 0.01 level between the bending strength and the modulus of elasticity of the wood and the temperature and time respectively.
9. In this study, y means loss ratio of bending strength of heat-treated wood, x_1 means treatment temperature, x_2 means treatment time, so the regression models between bending strength and x_1with x_2 of the China-Fir heartwood, the sapwood and the Chinese White Poplar respectively, are y = 0.558x_1+2.806x_2-99.975 (R~2=0.943)、y = 0.693x_1+5.566x_2-137.897 (R~2=0.909)、y =0.961x_1+4.218x_2-183.832 (R~2=0.953). If y means the modulus of elasticity, the regression models between the modulus of elasticity and x_1with x_2 of above the three species wood respectively, are y = 0.247x_1+1.235x_2-44.865 (R~2=0.874)、y = 0.222x_1+3.512x_2-47.676 (R2=0.927)、y =0.089x_1+1.544x_2-32.172 (R~2=0.777). Using above mathematical regression models, the loss ratios of the bending strength and the modulus of elasticity of heat-treated wood will be predicted under different treatment temperatures and times respectively.
10. The best method of controlling the loss of the mechanical strength of the wood is by the controlling of the temperature. Based on the results from the above three wood species experiments the temperature should be controlled at below 200℃so that there is no loss or a minimum loss in the mechanical strength of the wood, with an enhancement of the hardness, bending strength and modulus of elasticity properties of the heat-treated wood within a short time. With the correlative mathematic regression models, the optimum temperature and time used in the heat treatment can be determined to accord with the final use of the heat-treated wood.
引文
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