【作者】 王雪花；

【导师】 刘君良；

【作者基本信息】 中国林业科学研究院 ， 木材科学与技术， 2012， 博士

【摘要】 热处理是改善木材尺寸稳定性的重要手段之一，相比其它热处理方式，真空热处理具有热处理材力学强度损失低、材色变化小等优点。目前，有关木材蒸汽、热油等热处理方式、木材真空干燥的相关报道很多，而关于真空热处理的报道很少。本研究以我国南方常见的速生树种粗皮桉木材为研究对象，将其在真空条件下进行热处理，考察热处理过程中木材内部的热量传递规律及处理材的物理力学特征，并选用不同分析手段分析热处理后木材性质的变化机理，得出以下结论：1真空热处理过程的时温曲线符合hill1函数模型，且模型的决定系数在0.99以上，故可选用hill1函数作为真空热处理过程的时温曲线模型；除自变量时间t、因变量木材内部的温度T，hill1函数包括四个参数Tstart、Tend、k、n，其中，Tstart是热处理前环境的温度，Tend为设定的热处理温度，当热处理环境温度及热处理温度确定后，便可视为常数；k&n是与板材厚度密切相关的量，两者之间的关系可以开口向上的二次抛物线描述；木材升至特定温度所需的时间与板材厚度有关，也与该温度所处的热处理阶段相关。热处理前期（T<(Tstart~+Tend)/2），板材越厚，达到特定温度所需的时间越长；热处理后期（T>(Tstart~+Tend)/2），板材厚度变化时k&n也发生变化，但两者对t影响的方向及程度不同，故不能简单做出判断。2以木材单位质量吸热量、最大及平均升温速度表征指标，以热处理温度、板材厚度、端距及侧距为参数，以方差分析及多重比较分析考察其对热处理过程中传热均匀性的主要影响因素及影响程度，发现：热处理温度对单位质量吸热量、最大及平均升温速度的影响在0.05水平上均差异显著；板材厚度对单位质量吸热量及平均升温速度的影响在0.05水平上差异显著；端距对最大升温速度的影响在0.05水平上差异显著，而侧距对单位质量吸热量、最大及平均升温速度的影响在0.05水平上差异均不显著。在木材尺寸上，除端部外，木材在长度和宽度方向上的受热均比较均匀，只需考虑因厚度造成的传热差异。若以木材单位质量吸热量来衡量木材的热处理效果，120℃~240℃的温度段进行热处理，可满足对热处理效果差异性的需求。3随热处理温度升高，真空热处理后木材的力学强度先升后降，尺寸稳定性提高，全干密度减小，失重率增加。木材的全干密度减小，失重率增加，变化幅度先缓后急，在200℃以下时，二者的变化程度较为一致；当热处理温度达200℃以上时，失重率较全干密度变化率大，这可能是由于在高温下木材的体积发生皱缩引起的。真空高温热处理对尺寸稳定性改善的程度随热处理温度升高呈先慢后快再慢的趋势。当热处理温度在160℃及以下时，变化幅度较小，吸水后体积湿胀率及全干体积干缩率约可降低10%左右；在240℃时可达到65%左右，但在此之后若继续提高热处理温度，则对尺寸稳定性的改善不再显著，故当在200℃~240℃范围热处理时，可获得比较理想的尺寸稳定性效果。随热处理温度升高，处理材的抗弯弹性模量和抗弯强度先有所升高，后急剧降低，二者出现最大值的温度，抗弯弹性模量为200℃，较未处理材提高约25%；抗弯强度为160℃，较未处理材提高约6.5%，从木材力学强度的角度说，160℃~200℃是较为合适的热处理温度。4随热处理温度升高，热处理材全干密度降低，失重率增加，全干密度降低幅度低于失重率增加的幅度的原因：随热处理温度升高，木材内成分分解加剧，质量损失严重，失重率增加，全干密度下降，但由于木材在热量作用下细胞出现皱缩，体积也有所减小，从而使全干密度降低的幅度低于失重率增加的幅度。热处理材尺寸稳定性增强的原因：①随热处理温度升高，木材中吸湿性强的半纤维素由于耐热性差，容易分解，从而可降低木材的吸湿性；②在热处理过程中，氧氢键发生氢键缔合作用，从而减少了木材中亲水性羟基的数量，从而提高了木材的尺寸稳定性（3423cm-1处吸收峰向低频位移）；③纤维素羟基之间形成新的氢键结合，使纤维素非晶区部分结晶化。热处理材力学强度先增强后下降的原因：①在热量作用下，木材细胞皱缩，木材细胞的形态变化使木材在受外力作用时应力的分布状态发生变化，从而引起木材力学强度的改变；②随热处理温度升高，在热量作用下，纤维素分子的链段运动增强，部分分子间距离缩小而有机会形成新的氢键缔合，从而使非结晶区部分结晶化，结晶度增大，分子间作用力增强，木材的力学强度增强；热处理温度继续升高（240℃以上），由于纤维素分子链上的氢键因链段运动过于剧烈受到破坏，木材缔合的氢键数量减少，分子间作用力降低，结晶度下降，木材的力学强度降低；③在热处理过程中，木质素发生玻璃化转变，木质素软化，产生粘着力，对木材内高温下部分降解或易挥发成分产生固着，减弱了木材内成分的损失速度，同时对木材内的其他成分固化胶着，冷却后使细胞壁层结构硬固，从而使木材的抗弯强度和抗弯弹性模量得到了部分提高，但当热处理温度继续增大，纤维素和半纤维分解加剧，单靠木质素对木材细胞的硬固作用已无意义，木材强度急剧下降。更多还原

【Abstract】 Heat treatment is an imporant method for improving dimensional stability of wood.Compared with other ways, vacuum heat-treatment is better in retainning wood strength andcolor. However, research about steaming heat hreatment and vaccum drying were commonwhile vacuum heat treatment was rarely reported. Taken small pieces of Eucalyptus pellitawood, which is a usual fast-growing tree in south china, as research object, treated it withvaccum heat treatment, heat transfer law during vaccum heat treatment and change of woodproperties after vaccum heat treatment werestudied in this reach, mechanism of woodproperties chang was discussed. Main conclusions were as follows:1Time-temperature curve during vaccum heat treatment could be fitted well by hill1function, in which R2was above0.99, so hill1function could be used as time-temperaturecurve fitting model. Except independent variable time (t), dependent variable temperature (T),there were for parameter in the function: Tstart, Tend, k, and n. Tstartwas defined as theenvironment temperature, and Tendas the heat treatment temperature, both of the two could beviewed as constant. Close connection existed between parameter k&n and thickness of testpiece, the relationship could be described by upper second-degree parabolic function; Timeneeded of test piece climbing up to a given temperature was related with thickness of test piece,but also with the stage of the temperature lied in. In prophase (T <(Tstart+Tend)/2), as thetestpiece thickness augmenting, the time needed extended, in anaphase(T<(Tstart+Tend)/2), both kand n changed, which had different affects on time needed, so, the time needed could not bejudged under this condition.2Taken heat absorption per unit mass (Qm), max rate of temperature increase(Vmax), meanrate of temperature increase(Vmean) as detecting index, heat treatment temperature(Taim),thickness of test piece(ts), distence between test point to test piece top(lth), distence betweentest point to side of test piece(wth) as experimental parameters, adopted analysis of varianceand mulitiple comparions to analysis the impact factors and influence degree of heat transfer uniformity during heat treatment, results shows that: influence of Taimto Qm, Vmaxand Vmeanhad significant difference at0.05level, influence of tsto Qm, Vmeanhad significant difference at0.05level, influence of lthto Vmaxhad significant difference at0.05level, while influence ofwthto Qm, Vmaxand Vmeanhad no significant difference at0.05level. On length and width oftest piece, except the front end, heat treasfer ranged uniformly, heat transfer diversity shouldonly be considered in thickness. If measured by Qm, wood treated by120℃~240℃wouldsatify the need of multiplity of thermal effectiveness.3After treated under vaccum heat treatment, strength was enchanced firstly then reduced,dimensional stability and weight loss ratio were augmented, oven-dry density was decreased asheat treatment temperature increasing. Rangeability of weight loss ratio and oven-dry densitywere similar when the heat treatment was under200℃, while over200℃, change rate ofweight loss ratio was larger than oven-dry density, which might caused by cell crimpling underthermal. Improvemeng degree of dimensional stability showed a trend of growing slow at firstthen get fast. Volume shrinkage ratio from wet to dry and volume swelling ratio from dry towet could be reduced about10%under160℃, while reached about65%under240℃, ifcontinue to increase heat treatment temperature after240℃, improvement of dimensionalstability would be faint.Heat treatment during200℃~240℃would be suitable for dimensionalstability. Bending strength got top value at200℃, improved by about25%, while modulus ofelasticity got top value at160℃, improved by6.5%, so heat treatment during160℃~200℃would be suitable considering from wood strength.4Reasons for change of oven-dry density and weight loss ratio might be as follows: astemperature increasing, wood component decomposing deepened, weight loss ratio increased,oven-dry density reduced, as wood cell crimple under thermal, volume reduced to a certaindegree, which caused oven-dry density changed slower than weight loss ratio.Reasons for dimensional stability improvement:①as heat hreatment temperatureincreasing, hemicellulose was easy to be decomposed as its poor heat tolerance, which washydrophility component in wood, hygroscopicity of wood was reduced consequently;② during heat treatment, association of hydrogen bond was formed between O-H bond, whichreduced O-H bond quantity in wood, and improved its dimensional stability(it could be provedby the absorption peak lied in3423cm-1moved to low frequency in FTIR);③amorphousregion turned into crystalline region in cellulose.Reasons for wood strength changing:①as wood cell crimpling under heat treatmentmade cell shape changed, stress distribution was some different between treated and untreatedwood when wood suffered with force, which caused wood strength change;②when heattreatment temperature was under200℃, as heat treatment temperature increasing, amorphousregion turned into crystalline region in cellulose, intermolecular force boost up, strength ofwood largened; if continuing to increase heat treatment temperature(above240℃), associationof hydrogen bond in cellulose reduced because of the thermal motion on molecular chain,intermolecular force decrescence, strength of wood reduced;③nearby the heat treatmenttemperature200℃, lignin became soft and ropy as glass transition took place, which couldplay a cementation part for wood and strengthen the cellular construction, and then improvewood strength, but when heat treatment above240℃， decomposition of hemicellulose andcellulose was so severe that lignin strengthen to wood could be negligible, wood strengthreduced seriously.更多还原