木材皱缩的研究现状及发展趋势

通过总结和分析国内外关于木材皱缩的研究现状，指出皱缩深度、皱缩因子、体积收缩率、干缩率等4项指标能够合理地评价木材皱缩的程度；采用预冻处理、热处理、加压或拉伸处理以及后期调湿处理能够在一定程度上减少皱缩的产生；并讨论了木材皱缩研究的发展趋势。     

按树木材干燥特性与工艺及其皱缩研究现状

本文回顾和分析了桉树木材与干燥有关的材性和干燥特性，指出按木的密度、含水率和干缩率变异性大、渗透性差，在干燥过程中易产生皱缩、开裂等干燥缺陷，若采用预冻处理和前后期调温处理可减少木材的皱缩；同时建议研究各种桉木不同规格板材的干燥基准和两段式干燥工艺。     

桉树木材皱缩形成机制与调控研究进展

文章综述了近年来国内外学者对桉树木材皱缩形成机制的研究进展, 分析了可有效防止木材皱缩的后期调湿处理、热处理、加压或拉伸处理以及预冻处理等主要预处理方法对减少木材皱缩的作用, 展望了今后木材皱缩研究的发展趋势, 以期为我国桉树人工林资源高附加值利用提供参考。     

预冻及压缩预处理对尾巨桉干燥特性的影响

桉木在干燥过程中极易发生皱缩,使木材降等严重甚至报废,有效地解决桉木干燥皱缩问题是桉木资源高附加值实木化利用的重大难题之一。对桉木进行适度的预处理能够改变其内部细胞的微观结构,形成新的水分迁移通道。以尾巨桉(Eucalyptus urophylla×E.grandis)为试材,对其进行预冻、压缩及预冻-压缩预处理,然后进行常规干燥,研究预处理条件对桉木干燥速率、干燥应力应变及皱缩特性的影响。结果表明:3种预处理条件都能有效提高桉木的干燥速率,其中,预冻-压缩预处理后试件的干燥速率提高20%,幅度最大。预处理材与未处理材残余应力指标变化趋势一致,干燥后期预处理材指标值小于未处理材,残余应力小。3种预处理方式都不同程度地改变了细胞壁微观构造,破坏了皱缩发生的条件,抑制了木材的干燥皱缩。其中,预冻-压缩预处理材全干缩率最大减少15.8%,抑制木材皱缩效果最为显著。预冻-压缩预处理能够改变木材的微观结构,改善了木材的干燥特性,是一种有效抑制木材干燥皱缩的预处理技术。     

木材皱缩评价指标的研究

近年来 ,国内外学者对木材皱缩缺陷开始关注 ,澳大利亚学者Ｔ .ＣＩｎｎｅｓ( 1 995,1 996)、中国学者王喜明 ( 1 989)、刘　元 ( 1 994)等分别研究了澳大利亚产桉木如多枝桉、细叶桉和赤杨、山杨等木材的干燥皱缩缺陷。他们从调湿处理对皱缩的作用 ;皱缩材组织结构的变化 ;皱缩机理 ;皱缩极限温度 ;预冻处理对皱缩的作用等方面做了大量深入而细致的工作。所取得的成果公开发表后得到同行学者的认可 ,对进一步研究避免皱缩的干燥工艺打下了较好的基础。但在众多有关木材皱缩的研究报告中 ,普遍遇到一个同样的问题就是如何定量地去评价和测量…     

细胞壁空隙对木材性能及加工利用的影响

细胞壁空隙是木材水分横向传输的重要通道,是影响木材干燥速率、木材改性效果的重要因素。针对细胞壁空隙的研究不仅是对木材本身构造和性能的进一步了解,更是对木材物理和化学加工基础的深入诠释。文中概述木材细胞壁空隙的组成、分类、具体尺寸等,归纳其对木材物理力学性能及加工利用的影响,总结目前木材细胞壁空隙构造研究尚存在的问题并提出建议,以为实际生产和科学研究提供参考。     

干燥工艺条件对木材皱缩特性的影响

以内蒙古西部产大青杨为研究树种,研究了干燥工艺条件对木材皱缩特性的影响,通过正交试验分析了温度、湿度对皱缩材的皱缩深度,体积收缩率和皱缩因子的影响,并探讨了影响的原因。     

桉树木材干燥特性与工艺及其皱缩研究现状

本文回顾和分析了桉树木材与干燥有关的材性和干燥特性 ,指出桉木的密度、含水率和干缩率变异性大、渗透性差 ,在干燥过程中易产生皱缩、开裂等干燥缺陷 ,若采用预冻处理和前后期调湿处理可减少木材的皱缩 ;同时建议研究各种桉木不同规格板材的干燥基准和两段式干燥工艺     

纳米氧化铜木材防腐剂的防腐性能和抗流失性研究

以大青杨为试材,进行室内木材防腐试验和实验室水流失试验,研究纳米氧化铜木材防腐剂效果和抗流失性能.研究结果表明:经过纳米氧化铜防腐剂处理后,大青杨试样耐腐能力大大提高,失重率明显减少,当药剂浓度为1.6％时已经达到强耐腐等级.该防腐剂的耐褐腐能力与耐白腐能力相比稍差.不同浓度防腐剂处理后,大青杨试样中防腐剂的保持量随防腐剂浓度的增加而增加.另外,纳米氧化铜防腐剂具有很好的抗流失性能.     

三木材尺寸稳定的处理

木材细胞壁内吸着水的增减，会使木材产生湿胀和干缩，由此而来发生变形、翘曲和开裂等木材尺寸的不稳定，此外伴随水分的增加，使木材强度降低和易发生腐朽现象等。改善木材吸湿性，使木材尺寸稳定的同时，防止材质劣化，在木材利用方面是极其重要的，也是木材改性的主要内容。1尺寸稳定处理分类及主要检验指标1．1分类尺寸稳定处理方法可分物理和化学两类。除选择干缩性小的树种使用，制人造板和细木工板减少木材干缩外，物理方法尚包括：①覆盖处理；②细胞内腔充填；③细胞壁充胀等。化学方法包括：①亲水基的减少（热处理）；②亲水基…     

Advantages of prefreezing for reducing shrinkage-related degrade in eucalypts: General considerations and review of the literature

Summary Difficulties associated with the drying of ash eucalypts including collapse and internal checking, are discussed briefly. Prefreezing is one method that has been used successfully as a pretreatment for the drying of both hardwoods and softwoods from temperate and tropical regions.Prefreezing has produced marked reductions in shrinkage, collapse and drying degrade of the heartwood in the following species: California redwood, black walnut, black cherry, tanoak, toon, bamboo, and eucalypts. Little or no collapse reduction has been observed in New Zealand red beech, Pacific madrone, white birch, sitka spruce, and white ash. Limited response has been observed for numerous other species notably red oak and white oak.Reduced drying time in response to prefreezing has been observed in jarrah, karri, black walnut, Asian oak, toon, and California redwood; in Pacific madrone and tanoak the drying time increased. Not all species which respond with a reduction in shrinkage show reduced drying rates.Prefreezing wood at -20°C appears to be the most practicable temperature, although some species respond better at lower temperatures. However, in all cases, it is critical to ensure that the wood freezes and remains frozen for a number of hours. Indications are that the effect is retained for days to weeks and that the length of time of freezing need not exceed 12–24 hours.A number of explanations have been put forward to explain the behaviour of prefrozen wood. It is suggested that the main mechanism responsible for reduced shrinkage is due to the migration of moisture from the cell wall onto frozen lumen water. The moisture loss from the cell wall produces a cold shrinkage; water to ice transformation leads to an expansion of liquid water in the lumen, thus imparting a compressive stress to the cell wall, which together with the moisture loss, make the cell more rigid, and therefore likely to shrink less. There is some evidence that certain types of wood extractives migrate into the cell wall during freezing and may play a role in the reinforcement of the wall. Reduced shrinkage after prefreezing has also been attributed to a reduction of the plasticising effect of wood extractives in wood dried at higher temperatures and low humidities; this effect does not occur at low temperatures.Many suggestions and discussion from Dr. W. E. Hillis are gratefully acknowledged     

桉树木材超微结构及其对干燥皱缩的影响

以多枝桉等４种桉树木材为材料，采用扫描电镜观察并分析了其导管、纤维和管胞等胞壁上的纹孔和瘤层等超微结构．结果表明，富含侵填体的桉材因其堵塞了导管间的轴向通道，并部分覆盖了管间纹孔及导管—射线间纹孔，严重影响其渗透性，是其干燥皱缩的主要原因．通过热处理后，能使多枝桉和细叶桉木材皱缩发生改善．但对赤桉和红桉无效．     

山杨小径木超微结构及其与干燥皱缩的关系

前言山杨(Populus Dividiana Dode)小径木是黑龙江省尚志具帽儿山次生林区主要的间伐树种之一。该树种在干燥时易产生皱缩等缺陷,我们设想与其结构有关,故利用扫描电镜观察其超微结构,以期洞悉其中的奥秘。 W.R.Perng研究颤杨(Populus tremuloides)解剖构造与液体传导的关系,在电镜下观察时,指出颤杨心材中存在侵填体,这些侵填体使导管闭塞,因而液体流动困难。他又对此木材进行聚合物渗透,而心材导管中几乎无聚合物沉积,说明心材导管全部被侵填体堵塞,聚合物难以在心材沉积。     

The fluorescence characteristics of furfurylated wood studied by fluorescence spectroscopy and confocal laser scanning microscopy

The fluorescence of furfurylated wood was studied. Furfurylation of wood introduced fluorescent curing products into the cell walls. Fluorescent products were also found in the lumina if high loadings were used and/or if the wood contained cells with small radii. More intense fluorescence was obtained from lignin-rich parts of the compound cell wall (i.e., the middle lamella and the cell corners) than from the secondary cell wall if the wood was impregnated using furfuryl alcohol (FA), while impregnation using small poly(FA) oligomers showed no or only minor differences between different regions. The study also showed that when higher amounts of catalyst were added to the impregnation liquid, a red-shift in the fluorescence from the furfurylated wood was seen, corresponding to an increased conjugation length of conjugated poly(FA) formed within the wood cell wall. Longer conjugation length than within the cell wall was observed for the poly(FA) formed within the lumina. This indicates that the cell wall polymers restrict poly(FA) formation.     

Stress model of a wood fibre in relation to collapse

A wood fibre cell from a Tasmanian Eucalypt is typically cylindrical in shape with a length to diameter ratio of approximately 501. Early in the process of seasoning for solid timber, when the fibre lumens are still saturated, internal tension within a fibre can rise to a value high enough to cause it to physically flatten, or collapse. A stress model of a fibre cell has been developed which predicts the stress and strain distributions within the fibre wall as a function of temperature, moisture content, and fibre wall strength properties and size in the early stages of drying. This model will be used together with measurement of the behaviour of collapse prone timbers to determine conditions which will avoid collapse during seasoning.The author is pleased to acknowledge the assistance of Emeritus Professor A. R. Oliver, Associate Professor P. E. Doe, University of Tasmania, and the Australian Furniture Research and Development Institute     

Compressive deformation of wood impregnated with low molecular weight phenol formaldehyde (PF) resin II: effects of processing parameters

To obtain high-strength phenol formaldehyde (PF) resin-impregnated compressed wood at low pressing pressure, the effects of resin content, preheating temperature, pressing temperature, and pressing speed on the compressive deformation of oven-dried low molecular weight PF resin-impregnated wood was investigated. With an increase of PF resin content, the Youngs modulus of the cell wall perpendicular to the fiber direction decreases, and collapse-initiating pressure decreases linearly with the Youngs modulus. This indicates that the occurrence of cell wall collapse is strain-dependent. By increasing preheating temperatures, the collapse-initiating pressure increases due to the increment of the Youngs modulus of the cell wall. An increase in pressing temperature results in the thermal softening of the cell wall and causes collapse at a lower pressure. The wood is compressed effectively despite accelerated resin curing. The pressing speed significantly affects the viscoelastic deformation of the cell wall and the wood is well deformed with decreasing pressing speed, although the differences in density and mechanical properties are relatively small after a pressure-holding period of 30min. In all the parameters examined in this study, the Youngs modulus and bending strength increase with increasing density.     

Compressive deformation of phenol formaldehyde (PF) resin-impregnated wood related to the molecular weight of resin

The effects of molecular weight of PF resin on the deformation behaviour of NaClO₂ treated resin-impregnated wood during compression were investigated. Blocks of Japanese cedar were subjected to 2% NaClO₂ aqueous solution. This was repeated up to four times resulting in a weight loss of 28%. Treated and untreated samples were impregnated with PF resin having different molecular weight. With increasing molecular weight, weight gain and volume gain decreased for untreated PF resin-impregnated wood, while NaClO₂ treated wood impregnated with high molecular weight PF resin showed almost double the weight gain compared to untreated condition. NaClO₂ treatment has shown considerable potential for high compression of PF resin-impregnated wood at lower pressing pressure regardless of the molecular weight of the resin. Low to high molecular weight resin was shown to penetrate into NaClO₂ treated wood as estimated by weight gain contributing to the plasticization of cell wall considerably and thus resulting in cell wall collapse at low pressing pressure. The density of NaClO₂ treated wood impregnated with high molecular weight resin attained a value of over 0.8 g/cm³ which is close to the density of untreated wood impregnated with low molecular weight resin. Such compressed wood exhibited high dimensional stability after boiling for 3 h. Thus, the penetration of resin into wood contributes to highly compressed dimensional stable resin-impregnated wood at low pressing pressure.