赤桉材干燥皱缩的热处理调控

以人工林赤桉材为研究对象,根据皱缩评价指标对比研究了热水浸泡和汽蒸预处理对皱缩发生程度的影响,以及干燥中的热处理对皱缩回复的影响。研究表明：包括4 h常压预汽蒸处理和80℃热水处理在内的预热处理可以有效地预防皱缩;在板材平均含水率18%时进行3 h的汽蒸处理可以有效回复皱缩材。     

预冻处理减少木材皱缩的研究

以大青杨为试验材料，研究了预冻处理对木材干燥皱缩特性的影响，分析了木材在冰冻处理过程中木材细胞腔和细胞壁尺寸的变化规律，理论上计算了细胞壁强度的增加范围。并通过电子显微镜分析冰冻过程对木材细胞壁结构的影响，进而深入探讨预冻处理减小木材皱缩的机理，为减少木材皱缩提供理论基础。结果表明：预冻处理可以减少大青杨木材的皱缩。     

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…     

木材干燥皱缩机理及其特性研究

本文从理论上探讨了木材干燥皱缩的机理．并通过对红柳桉和多枝桉进行干缩性试验，阐述了木材干燥皱缩的特征、特性及其影响因子．揭示了木材干燥皱缩受外界环境影响的规律，对在生产中克服和防止木材干燥皱缩有一定的参考价值．     

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。     

人工林杨木皱缩恢复工艺参数优化研究北大核心

人工林杨木由于密度低、易皱缩的缺陷限制了其进一步利用。利用不同温度的饱和蒸汽对皱缩杨木试样进行不同时间的皱缩恢复处理,同时测试皱缩试材的径弦向恢复指标。试验数据采用SAS软件进行分析。结果表明:在温度对恢复指标的影响中,试样在100℃处理温度下其弦径向达到最大恢复率,分别为1.650%和1.501%;在时间对恢复指标的影响中,6 h处理试样的弦径向尺寸恢复率达到1.670%和1.411%;根据交互影响得到杨树皱缩恢复最佳工艺条件为100℃,6 h较优;80℃,6 h次之。     

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。     

热水处理对王桉木材干缩率和皱缩的影响

研究了热水处理的温度和时间对王桉（Eucalyptus regnans）木材全干干缩率和皱缩的影响。结果表明：随着处理温度的升高,王桉木材的全干干缩率呈增大的趋势,70℃和80℃时,处理时间对王桉木材的各向干缩率的影响不大;90℃和100℃时,处理时间对王桉木材的各向干缩率影响明显。经100℃热水处理6 h后王桉木材体积、弦向和径向全干干缩率分别增大了78.3%、116.1%和44.4%,分别达到25.5%,18.8%,7.8%。王桉木材的皱缩深度随处理温度的升高显著增大,处理时间对王桉皱缩深度的影响不明显。     

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。     

杨木干燥基准及其皱缩特性的研究

用百度试验法研究了大青杨、北京杨和山杨等三种杨木的干燥基准和皱缩特性，并对百度试验法中的截面变形评定分级标准进行了修订；采用百度试验法研制的基准对北京杨进行了室干试验，同时研究了北京杨室干过程中的皱缩特性。     

平武县清漪江沟泥石流物源现存量分析

清漪江沟为涪江上游平通河的一条支沟,位于四川省平武县境内,曾多次发生泥石流灾害.2010年8月12日清漪江流域降雨112 mm,由此引发山洪和泥石流.泥石流发生后的第16天,调查了宽坝林场场部至锁江乡立石沟19.73 km长的两岸汇水沟、侵蚀沟和不同类型崩塌体的发生数量及其泥石流物源的现存量.结果表明,调查区段内,两岸有大小汇水沟54条,密度2.7条·km-1,在林地和耕地上新形成了65条侵蚀沟,密度3.3条·km-1,沟口的物源现存量达1 264 m3,密度为64 m3·km-1;有394处大小不同的崩塌体,数量密度为20处·km-1;现存的崩塌体量共7 324 m3,现存量密度达371 m3·km-1.南坡和北坡的崩塌数量和现存量存在很大的差异,南坡崩塌了286处,数量密度为14.5处·km-1,占河流两岸崩塌数的72.6％;崩塌现存量5 486 m3,现存量密度为278.1 m3·km-1,占崩塌现存总量的74.9％;北坡崩塌了108处,数量密度为5.5处·km-1,占河流两岸崩塌数的27.4％;崩塌体现存量1 838 m3,现存量密度为93.2 m3·km-1,占崩塌体现存总量的25.1％.分析还表明,人为活动是泥石流物源的主要来源,尤其是耕地.因此,在泥石流灾害治理中,应重视和引导农村调整产业结构,改善生态环境,提高防灾减灾的综合治理能力.     

Effect of pre-drying schedule ramping on collapse recovery and internal checking with Victorian Ash eucalypts

Matched sample boards from 20 quarter-sawn boards of Victorian Ash (Eucalyptus regnans F. Muell and E. delegatensis R.T. Baker) were dried using three different levels of ramped pre-drying schedules to investigate the effects of moisture gradients on collapse recovery and internal checking. Prior to reconditioning, most wet cores were found in highly collapsed boards with low density. Reducing the gradients in these boards is crucial for recovering collapse and closing internal checks. If time allows the boards to be equilibrated prior to steam reconditioning, a target mean moisture content of ≤20% with a moisture gradient of close to 5% (core to surface moisture content) is likely to recover slightly more collapse than targeting a mean moisture content close to ≥15%. However, if time or kiln restraints limit equilibration it is likely to be better to target a percentage moisture content of closer to 15% in order to ensure that the core to surface moisture gradients are below 8–10%. The slight reduction in collapse recovery with this second approach is less important than the possibility that collapse and internal checks in the centre of boards with wet cores will not be closed. Care needs to be used with this latter approach not to over-dry some boards, since moisture contents below 15% will progressively reduce collapse recovery. For boards within these moisture content guidelines, the application of heat, rather than moisture pick-up, appears to be the most important component of the steaming reconditioning process. Hence, steaming only needs to be undertaken for long enough to heat the core of the board close to the target temperature of 100°C. A simple method for estimating this heat-up time for different thicknesses and species was demonstrated based on a key dimensionless group for heat transfer, the heat-transfer Fourier number.     

Shrinkage and collapse in thin sections and blocks of Tasmanian mountain ash regrowth

Summary Total collapse values from the green condition to various equilibrium moisture contents (EMC) were obtained by comparing shrinkage before reconditioning in matched blocks and sections of the wood of Eucalyptus regnans. It was shown that while collapse to 17% EMC comprised the largest portion of total collapse, a significant component, apparently caused by drying stress, developed below 17% EMC. Between 5% EMC and 0% moisture content a collapse recovery of some 1% occurred, seemingly because of changes in drying temperature. After reconditioning, collapse recovery was only partial and a high level of residual collapse remained. It was shown that collapse recovery was highest near the sapwood-heartwood boundary, while residual collapse was highest near the heartwood centre. While residual collapse was weakly negatively correlated with specific gravity, this relationship was not significant after adjustment was made for change in shrinkage after reconditioning in sections. All types of collapse were positively correlated with R-ratios calculated for blocks. However, relationships were not as well defined when R-ratios for sections were employed, the exception being for collapse below 17% EMC which was highly negatively correlated with R. The difference between shrinkage before reconditioning and shrinkage after reconditioning in sections, while totalling near zero, was positively correlated with specific gravity. It was demonstrated that this quantity could not constitute collapse in the traditional sense of collapse of the cell lumens. A possible association with moisture content of the material was discussed.     

The distribution and interrelationship of collapse,volumetric shrinkage,moisture content and density in trees of Eucalyptus regnans F. Muell.

Summary The relationships among collapse, volumetric shrinkage, moisture content and basic density and their distribution within the stem were examined for trees of E. regnans. It was found that collapse and volumetric shrinkage each were significantly related to moisture content (positively), basic density (negatively), and (positively) to the derived values P (per cent of theoretical saturation) and Q (per cent of cell cavity volume containing water). Notwithstanding the inverse association evident between moisture content and density in the living tree, moisture content was shown to be independently related to each of collapse and volumetric shrinkage.Within the stem, collapse and volumetric shrinkage were negatively correlated with height in the tree although the significance of this relationship was qualified by the adjustment for other variables. Samples containing sapwood displayed collapse values significantly lower than heartwood samples. This was reflected in the presence of a positive relationship between collapse and distance from the periphery when all material was considered, but a negative relationship when sapwood-containing samples were omitted. Basic density was positively correlated with height in the tree and negatively correlated with distance from the periphery. Moisture content, P and Q were negatively correlated with both height in the tree and distance from the periphery although the latter relationship for moisture content was dependent on adjustment for density.The highly significant relationship between volumetric shrinkage and collapse suggested that the former could confidently be used in assessing collapse severity. Moisture content was determined to be an independent indirect indicator of collapse whereas basic density, P, Q and green density could all be of indicatory value, especially the last in view of its ease of measurement.     

What factors influence the collapse of trees retained on logged sites?: A case-control study

Living trees (green trees) are intentionally retained on logged sites for many purposes such as wildlife conservation, future wood potential and aesthetics. Minimizing the risk that these trees collapse in the short-term will improve the chances that green-tree retention meets its objectives. In a retrospective case-control study, we identified factors significantly associated with the collapse of living trees retained on logged and slash-burnt sites in south-eastern Australia within 8 years of harvesting. Trees with incomplete crowns or trees with at least one visible cavity were, on average, at three times greater risk of collapse relative to trees with complete crowns or no visible cavities, respectively. Trees with fire-scarring, trees retained greater than 50 m from intact forest or trees retained in isolation were, on average, around twice the risk of collapse relative to trees without fire-scarring, trees retained within 50 m of intact forest or trees retained among other trees, respectively. When considered jointly, the significant factors in a model predicting the collapse of retained trees – tree height, tree diameter, crown form – were all associated with the health of trees. Choosing the healthiest trees possible for fulfilling their function, protecting trees from damage (e.g. fire) and retaining trees near, or among, other trees are likely to reduce the risk of collapse among retained trees and thus contribute to the effectiveness of green-tree retention in logged eucalypt forests.     

The effect of boiling on shrinkage,collapse and other wood-water properties in core segments of Eucalyptus regnans F. Muell

Summary Preboiling and drying wood samples of Eucalyptus regnans has shown that while in the heartwood shrinkage before reconditioning increased as a linear function of the logarithm of boiling time (B_L), in the sapwood it decreased. These changes were interpreted as reflecting changes in green permeability through extractive modification which, for the sapwood, were sufficient to overcome any weakening of the cell walls through heat effects; in the heartwood, however, cell wall degradation apparently overshadowed any collateral increase in permeability, thus causing an increase in collapse.Shrinkage after reconditioning described a cubic function with respect to B_L and showed an overall reduction which was greatest after 16 minutes boiling time. Correspondingly, recoverable collapse was greatly increased through preboiling; it was shown that per cent collapse recovery in the heartwood could be increased by more than one third that of controls by preboiling for 8 minutes. The possibilities for industry application are considered.Moisture content after reconditioning (M_r) was greatest after about two minutes preboiling, then declined to be less than the control after 16 minutes. Change in shrinkage after reconditioning and collapse were shown to be significantly related to change in M_r. It was suggested that while M_r may reflect relative permeabilities after reconditioning, this may not apply to permeability in the green condition.Additional parameters derived from the data, viz. the intersection point, unit shrinkage, the R-ratio, collapse-free shrinkage, total collapse and residual collapse described mostly significant, cubic relationships with the logarithm of the boiling time. Generally, the major inflection point of regression curves occurred after 8 or 16 minutes boiling.The author expresses his appreciation to Margaret Walker for technical assistance