Volume 37 Issue 4
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XING Dong, HU Jianpeng, YAO Lihong. Research review of material prediction and quality control of heat-treated wood[J]. Journal of Zhejiang A&F University, 2020, 37(4): 793-800. doi: 10.11833/j.issn.2095-0756.20190449
Citation: XING Dong, HU Jianpeng, YAO Lihong. Research review of material prediction and quality control of heat-treated wood[J]. Journal of Zhejiang A&F University, 2020, 37(4): 793-800. doi: 10.11833/j.issn.2095-0756.20190449

Research review of material prediction and quality control of heat-treated wood

doi: 10.11833/j.issn.2095-0756.20190449
  • Received Date: 2019-06-29
  • Rev Recd Date: 2020-02-10
  • Available Online: 2020-07-21
  • Publish Date: 2020-07-21
  • As an environmental-friendly wood physical modification method, wood heat treatment can not only improve the dimensional stability and biological durability but also effectively improve wood color, and is widely used in the functional improvement of fast-growing wood. Some representative research achievements on heat treatment technology are discussed. The traditional processes of wood heat treatment are summarized, and the future research is prospected. The present research on wood heat treatment mainly focuses on the following aspects: (1) the influence mechanism of high temperature heat treatment on such properties as wood dimensional stability, color and crystallization; (2) effects of high temperature environment on the content of the main chemical components, the volatile evaporations and degradation process of wood extraction; (3) changes and reactivity of phenolic hydroxyl and surface free radicals in lignin; (4) effects of heat treatment on wood permeability, paint film adhesion and durability. On this basis, the correlation between changes of wood characteristics (mass or surface chromaticity index) before and after heat treatment and heat treatment intensity is further analyzed. The characteristic parameters of wood such as mass loss rate, color difference of treated wood and the ratio of oxygen to carbon elements are important parameters to predict the properties of heat-treated wood. The future research on wood heat treatment should focus on the relation of micromechanical properties at the cell level and the macro-mechanical properties, and the development of catalysts to reduce the energy consumption of wood heat treatment. The biological durability, environmental and mechanical properties of heat-treated wood can be predicted rapidly and accurately based on the content or proportion of the main elements, surface color and other characteristics. On the other hand, the main parameters of heat treatment process are determined by using the prediction model according to the application environment and requirements, which can provide reference for the follow-up study. [Ch, 48 ref.]
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Research review of material prediction and quality control of heat-treated wood

doi: 10.11833/j.issn.2095-0756.20190449

Abstract: As an environmental-friendly wood physical modification method, wood heat treatment can not only improve the dimensional stability and biological durability but also effectively improve wood color, and is widely used in the functional improvement of fast-growing wood. Some representative research achievements on heat treatment technology are discussed. The traditional processes of wood heat treatment are summarized, and the future research is prospected. The present research on wood heat treatment mainly focuses on the following aspects: (1) the influence mechanism of high temperature heat treatment on such properties as wood dimensional stability, color and crystallization; (2) effects of high temperature environment on the content of the main chemical components, the volatile evaporations and degradation process of wood extraction; (3) changes and reactivity of phenolic hydroxyl and surface free radicals in lignin; (4) effects of heat treatment on wood permeability, paint film adhesion and durability. On this basis, the correlation between changes of wood characteristics (mass or surface chromaticity index) before and after heat treatment and heat treatment intensity is further analyzed. The characteristic parameters of wood such as mass loss rate, color difference of treated wood and the ratio of oxygen to carbon elements are important parameters to predict the properties of heat-treated wood. The future research on wood heat treatment should focus on the relation of micromechanical properties at the cell level and the macro-mechanical properties, and the development of catalysts to reduce the energy consumption of wood heat treatment. The biological durability, environmental and mechanical properties of heat-treated wood can be predicted rapidly and accurately based on the content or proportion of the main elements, surface color and other characteristics. On the other hand, the main parameters of heat treatment process are determined by using the prediction model according to the application environment and requirements, which can provide reference for the follow-up study. [Ch, 48 ref.]

XING Dong, HU Jianpeng, YAO Lihong. Research review of material prediction and quality control of heat-treated wood[J]. Journal of Zhejiang A&F University, 2020, 37(4): 793-800. doi: 10.11833/j.issn.2095-0756.20190449
Citation: XING Dong, HU Jianpeng, YAO Lihong. Research review of material prediction and quality control of heat-treated wood[J]. Journal of Zhejiang A&F University, 2020, 37(4): 793-800. doi: 10.11833/j.issn.2095-0756.20190449
  • 木材作为一种具有高强重比/可再生的材料,在不同领域得到广泛应用。然而,由于人工林速生树材生物耐久性较弱,尺寸稳定性差,极大的限制了其使用范围[1]。在众多木材改性手段中,热处理作为环境友好型改性方法受到广泛关注。热处理木材是通过蒸汽、氮气等气体或导热油作为保护介质和传热介质,将木材加热至150~260 ℃并保持数小时所制得,木材中的半纤维素和无定型区部分纤维素发生热降解,木质素在热处理过程中发生交联反应,最终木材形成了新的纤维-木质素网络-交联结构[2-4]。热处理使木材化学成分和结构发生热降解和交联反应等造成处理材材色更深沉优雅,其尺寸稳定性得到有效提升,并且抗微生物能力有所提高。热处理木材以其环境友好特性、沉稳典雅的木材材色、优异的尺寸稳定性和耐久性等受到市场广泛青睐[5-8],因此,热处理木制品被广泛用于家庭装潢、桑拿房、家具、室外用栅栏、建筑物的外墙板以及海港码头建材等[7-9]。1920年TIEMANN[10]通过高温干燥方式降低了木材的平衡含水率;1937年,STAMM等[11]利用多种气体加热木材以降低其干缩湿胀性,1945年进一步研究了热处理对木材尺寸稳定性的影响[1, 3]。近年来关于热处理对木材性能影响的研究较多,大多集中于不同热处理工艺对木材尺寸稳定性[2-5]、木材材色[12-13]、化学组分变化[14-15]或力学性能等的影响[16-18],而探究热处理工艺与处理材质量损失率、结构构成、力学性能和漆膜附着力等的相关关系是优化热处理工艺和预测热处理材产品质量的重要途径和手段之一。本文详述了热处理对木材结晶性、化学组分、力学性能和漆膜性能等的影响,以期为今后热处理木材的材性预测和产品质量控制提供详尽的理论支持。

    • 20世纪末,木材热改性处理在多国实现了全面的工业化,热处理木材产品迅速进入市场并得到广泛关注。在1990年,芬兰技术研究中心实现了木材热处理的工业化,同时将其命名为Thermowood®工艺。该工艺采用蒸汽作为导热介质,处理温度为180~250 ℃[1-2]。Thermowood®按照热处理强度划分为Thermo-S和Thermo-D,其中Thermo-S热处理材用于有良好尺寸稳定性的场合,而Thermo-D热处理材用于需要良好耐久性的场合。芬兰技术研究中心还对不同树材分别制定了针叶材和阔叶材的Thermo-S和Thermo-D处理工艺。

    • Plato Wood®热处理工艺由SHELL创建,主要利用水、蒸汽及空气作为导热介质[8-10]。其热处理工艺包括:①高温高压饱和蒸汽环境下将木材加热至150 ℃以上(具体温度需结合热处理树种、处理材用途等确定)保持4 h左右;②降温至常规干燥温度,直至木材含水率低于8%;③升温至150 ℃以上加热14 h左右;⑤冷却和调整过程保持2 d左右,调湿并释放处理材内部应力,同样该过程需要考虑树种和板材厚度等因素进行微调。

    • 法国的Retification®热处理工艺采用氮气作为保护和导热介质,其阔叶材的热处理技术发展较早。Retification®工艺一般要求初始木材平衡含水率在12%左右,在处理箱内通入氮气作为保护和导热介质(其中要求处理箱内氧气体积分数低于2%),将木材加热至190 ℃以上,保持6 h左右并进行调温调湿过程[10]

    • 德国将木材浸入充满工业用油或植物油的处理罐内,将木材加热至180 ℃以上保持数小时,该工艺被命名为德国油热处理工艺(Oil-heat Treatment)。在该热处理工艺之前要求进行恰当的木材预干燥处理。同时要求所选用传热油的沸点高于木材热处理温度。因导热油可快速均匀地加热木材,同时可将木材与空气隔绝保证处理材质量。油热处理过程所涉及的控制参数全部由计算机辅助远程控制,其中参数包括导热油消耗量、油温、内腔压力和处理时间等。基本上所有的树种均可进行油热处理,处理材主要应用于室内装饰、游乐场、景观结构以及其他室外场合等[2, 10-11]

      另外,近年来还形成了法国的波依斯佩杜尔(Le Bois Perdure®)工艺、丹麦的木材处理技术(Wood Treatment Technology, WTT®)和奥地利的哈勃霍尔茨(Huber Holz®)工艺等[4-5]。这些工艺主要的区别体现在不同的导热介质、加热温度及对木材初始含水率的要求有所不同。

    • 目前,已有大量热处理材干缩湿胀特性的研究。孙伟伦等[1]研究表明:热处理后木材的尺寸稳定性显著提高40.0%~63.9%。同时,在180~220 ℃范围内,随热处理温度的提高和处理时间的延长,尺寸稳定性逐渐增大,并且热处理温度对其影响更显著[2-4]。高温环境时木材中相邻纤维素链段-链段间的羟基脱水形成氢键架桥结合;同时,高温造成半纤维全部或部分发生裂解,其中的亲水基团被破坏,从而降低了木材的亲水性,最终使其尺寸稳定性提升[3]。NAVICKAS等[5]认为:高温环境下木材内部的生长和干燥应力得到释放,进一步改善和提高了木材的尺寸稳定性。

    • 通过调整热处理的升温曲线以获得不同性能的处理材,实现产品性能的多样化。对浅色甚至惨白的木材进行热处理可有效改善其材色,从而提高其使用价值[6-7]。HUANG等[8]研究了短叶松Pinus banksiana、美洲杨Populus tremuloides木和桦木Betula papyrifera热处理所造成的颜色变化,结果表明:热处理后木材明度L*显著下降,即木材颜色变暗。黄蓝色品指数b*有所下降,即木材颜色轻微变黄。BEKHTA等[9]和吴再兴等[4]认为:热处理材材色加深的主要原因是热处理过程中木材内的水分向外迁移时部分水溶性抽提物酚类、黄酮类等化合物随水分迁移到木材表面,使木材褪去原有的色彩,最终造成材色的改变。木材中半纤维素、木质素的降解和氧化,以及低分子物质缩聚反应生成的有色物质也造成木材颜色变化。STAMM等[11]对热处理木材进行了紫外老化试验,结果表明:热处理后木材抵抗紫外老化能力略强于未处理材,但热处理材材色暴露在日光下并不稳定,随时间而逐渐褪色至灰白色。FAN等[12]认为:热处理后木材材色变化主要是由高温下抽提物的化学反应造成的。另外,SALCA等[13]认为:热处理后木材变色与高温过程新生成的醌类氧化物有关,醌类氧化物的增加使更多可见光被吸收,最终造成木材材色加深。

    • INARI等[14]研究表明:热处理使木材纤维素、半纤维素和木质素均发生不同程度的热降解反应和结构变化,从而导致其机械强度降低。BORREGA等[15]认为:热处理木材力学性能的变化与质量损失率和保护介质的相对湿度有关,当质量损失率低于3%时,热处理材力学性能的略微提高与其平衡含水率较低有关。KUBOJIMA等[16]将云杉Picea asperata置于氮气和空气环境下加热至160 ℃,静态杨氏模量在热处理初期有所增加,随后减小;热处理木材脆性也有所增加;同时空气为保护介质下木材力学性能变化幅度更大。LEE等[17]将50、100、150和200 ℃热处理橡胶木Hevea brasiliensis颗粒制得刨花板,结果表明:对原料橡胶木颗粒的热处理能显著改善刨花板的尺寸稳定性,但对机械性能产生了一定的不利影响。JAMALIRAD等[18]研究了高温热处理材人工老化后木材的性能变化,结果表明:180 ℃热处理基本不会影响木材剪切和弯曲强度,而长期紫外线老化则会影响木材的剪切和弯曲强度。

    • BHUIYAN等[19]研究了热处理云杉的结晶性能,结果表明:热处理有水分时木材组分发生部分降解,同时减弱了结晶区纤维素的内部应力,而木材内部应力在绝干热处理过程中受到的影响则较小。GUO等[20]利用原位micro-FTIR研究了热处理木材中吸附水的化学位点,结果表明:不同相对湿度水平下热处理材形成强、中和弱的氢键结合水。

    • MOHAREB等[21]研究了热处理材化学组分变化,结果表明:随热处理材质量损失率的提高(5%~15%),处理材综纤维素质量分数由57.8%降至48.8%,木质素的相对含量则由38.5%提高到44.9%,同时木材氧碳元素质量比(O/C)的降低表明热处理过程中发生了大量的脱水反应。热处理后木材中原有抽提物蒸发及裂解,同时处理材形成新的抽提物,包括无水糖、甘露聚糖、半乳糖、左旋葡萄糖和2个C5无水糖,其中木质素衍生物为丁香醛、丁香酸和辛醛[22]。OKON等[23]利用硅油对梧桐Firmiana platanifolia木进行热处理,研究表明:处理材化学组分变化主要是由于半纤维素和纤维素的降解。MENG等[24]利用X光电子能谱(XPS)和红外光谱(FTIR)对干热空气热处理竹进行研究,处理过程中氧气体积分数低于2.5%,结果表明:热处理竹材化学组分变化与木材相似。BRANDT等[25]对高温热解的木材细胞壁结构进行研究,透射电镜(TEM)和原子力显微镜(AFM)观测结果显示:200 ℃处理时细胞壁半纤维素发生部分降解,225 ℃处理时细胞壁内部半纤维素完全发生降解,而250 ℃处理时包括纤维素在内木材组分出现大规模裂解。

    • INARI等[26]将热处理后的松木和榉木Zelkova schneideriana木粉与吡啶中的不同羧基酸酐或者与二甲基甲酰胺中的苯基异氰酸酯反应,结果表明:热处理木粉反应后增重率明显小于未处理材,即热处理后木材反应活性有所降低,并将热处理后木粉反应活性的降低归因于木材综纤维素中游离羟基数量的减少,特别是半纤维素的热裂解。GARROTE等[27]对蓝桉Eucalyptus globulus进行了高温(145~190 ℃)水热处理,造成抽提物的析出、半纤维素的降解以及半纤维素和乙酰化低聚糖的脱乙酰化作用,同时建立起木聚糖降解和脱乙酰化作用的联系,木材降解生成的乙酸类物质进一步促进木质-纤维素物质的降解速度。BROSSE等[28]研究了热处理榉木木质素变化,结果表明:木质素化学结构在热处理过程中发生较大变化。通过核磁共振观测到木质素分子质量减小,酚羟基数量的增加和侧链碳原子(Cα、Cβ和Cγ)的减少均证明大规模α或β-芳基-醚键的断裂。丁涛等[29]利用拉曼光谱研究了热处理木材吸湿性,结果表明:木质素是改变木材吸湿性的重要因素之一,在细胞壁水平木质素含量的增加既加固了木材细胞壁,又对木材吸湿解吸起到阻碍作用。王喆等[30]对日本落叶松Larix kaempferi进行了真空-常压热处理,研究结果表明:热处理材木质素结构单元增加,同时其表面自由基也有所增加。

    • 顾炼百等[31]研究了热处理樟子松Pinus sylvestris var. mongolica的耐腐性,结果表明:热处理后生成的乙酸等酸类物质使木材pH值降低至4.0~4.5,同时半纤维素大量降解,最终造成樟子松由不耐腐提升为强耐腐。热处理后木材抵抗腐朽菌和软腐菌能力有所提高,部分处理材耐腐性基本可以和铬化砷酸铜(CCA)处理材相媲美[32]。对于天然耐腐性差(4~5级)的针叶树材(如欧洲云杉Picea abies、欧洲赤松Pinus sylvestris和海岸松 Pinus pinaster等)经过恰当的热处理后,其耐腐等级可显著提高[33-34]。但是,目前热处理木材仍不能有效防护白蚁Coptotermes formosanus等昆虫及海生钻孔动物的危害[35-37]。尚需进一步深入研究。

    • 于家豪[38]研究了热处理木材紫外线(UV)固化涂料的漆膜性能,结果表明:热处理后UV涂料的渗透性降低,附着力有所下降。邓邵平等[39]利用聚氨酯和醇酸清漆对热处理木材进行涂饰处理,结果表明:涂饰后处理材光泽度和明度降低,同时色差有所增大。SAHA等[40]利用二氧化钛混合UV光稳定剂对热处理木材表面进行涂饰处理,结果表明:此方法可有效提升木材抗紫外能力。核磁共振研究表明:热处理造成木材半纤维素的脱乙酰化、木质素的脱甲氧基化和木质-纤维素结构的重新构建,同时还建立了解聚/降解温度与化学组分转变的关系[41]

    • 热处理造成木材主要组分降解,同时裂解产物中的挥发性气体溢出最终造成木材质量减小。ESTEVES等[42]对海岸松和桉树材进行热处理,结果表明:松木与桉木的质量损失率与热处理温度和热处理时间均呈现正相关关系。

      WANG等[43]对南方松Pinus spp. 进行150~225 ℃热处理,建立起热处理材质量损失率与抗弯弹性模量、抗弯强度之间的关系,同时还构建了处理材电阻抗特性、介电常数与其对应抗弯弹性模量、抗弯强度的关系,结果表明:拟合模型相关系数(R2)均大于0.95,实现了通过质量损失率预测热处理材材性的目的。

    • BEKHTA等[9]对热处理云杉的色度与力学性能进行研究,建立了处理材不同湿度环境下抗弯强度与色差之间的良好线型相关关系,结果表明:处理材色度参数中的色差可用于预测处理材的力学强度。

      ESTEVES等[44]建立了热处理海岸松和桉树的近红外光谱(NIR)模型,对其质量损失率交叉验证的协同系数达96%以上;对于处理材平衡含水率的协同系数达到78%以上;对其色度指数La*和b*的协同系数为66%以上。BRISCHKE等[45]对欧洲云杉、樟子松Pinus sylvestris和山毛榉Fagus sylvatica木粉进行热处理,建立了处理材色度指数L*、a*、b*与热处理强度的关系,并利用处理材材色对产品质量进行控制。

    • CHAOUCH等[46]研究了氮气热处理木材在白腐菌Poria placenta作用下木材的质量损失率,以及热处理材主成分的变化,结果表明:热处理松木质量损失率由0增加到14.5%时,其处理材氧碳元素质量比由0.7降低至0.5。同时,松木热处理过程中的质量损失率由0增加到14.5%时,与腐朽过程的质量损失率则由25%降低至0%,即热处理后木材O/C可作为预测处理材耐久性和抗白腐性能的重要参数之一。

      丁涛等[47]对白蜡木Fraxinus chinensis进行蒸汽热处理,结果表明:热处理后木材中的碳(C)和氧(O)元素比与处理材色差值有较高的相关度,同时处理材O/C随热处理温度提高而持续降低。ŠUŠTERŠIC等[48]对热处理松木边材进行X射线光电子能谱分析(XPS),结果表明:热处理材O/C与其腐朽过程的质量损失率呈高度相关关系,即热处理材主元素可预测其抗腐朽能力。

    • 木材热处理是一种高温乏氧环境下的高效木材改性手段。热处理后木材的尺寸稳定性显著提升,木材沉稳优雅同时耐久性有所改善,但不恰当的热处理工艺也会造成木材开裂、脆性增加和力学性能损失严重等问题。当前,木材热处理研究应关注以下焦点问题。

    • 热处理造成木材物理性能显著变化,这与木材的微观构造变化息息相关。从细胞壁水平研究并探讨热处理材微观力学性能变化对宏观力学性能的影响,并建立与宏观力学强度损失的关系具有重要意义。

    • 高温热处理过程中木材组分发生复杂的热裂解和氧化反应,这对处理材性能均产生显著影响。为进一步优化和促进热处理工艺升级,降低热处理工艺的综合能耗,尝试对木材热处理过程中加入有利与木材热裂解的催化剂。这同时为木材热处理设备设计提供理论支持。

    • 在高温下,木材半纤维素降解产生乙酸等酸性物质,对处理材力学性能影响较大。对中性和碱性环境下的热处理木材进行研究,以期控制处理材机械性能的损失。另外,热处理材较好的热绝缘性和电绝缘性,仍需发掘其特殊的应用场合。

    • 对工业化生产的热处理木材,通过数学模型建立市场常用的热处理树种的化学成分(如碳氧比和碳含量等),木材表面色度学指数与处理材性能的关系,侧重于热处理材主要目标性能的预测,包括:生物耐久性、环境学性质(视觉特性和润湿性)、尺寸稳定性和力学性能等。同时,根据热处理木材具体的使用场合和要求,使用恰当的处理工艺。根据预测模型为木材热处理建立不同的处理等级,以及高效合理地使用热处理木制品。

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