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1.
Changes in physical properties of tropical and temperate hardwoods below and above the fiber saturation point 总被引:1,自引:0,他引:1
Changes in physical and mechanical properties of wood were analyzed using sorption tests combined with dimensional measurements and perpendicular-to-the-grain tangential compression tests. In order to determine the influence of wood structure on these changes, three hardwood species (Fagus grandifolia, Brosimum alicastrum and Cariniana domestica) presenting different anatomical structures were studied. Two experimental techniques were used to perform moisture sorption tests at 25°C. The first technique used saturated salt solutions (from 33 to 90% relative humidity) and the second used the pressure membrane method (above 96% relative humidity). Special attention was given to the “fiber saturation region”, where changes in wood properties started to take place. Results showed that at equilibrium moisture content (EMC), radial, tangential and volumetric shrinkage, as well as changes in transverse strength occurred above the fiber saturation point (FSP). This behavior can be explained by the effect of hysteresis at saturation on wood properties. This hysteresis indicates that loss of bound water takes place in the presence of liquid or capillary water, which contradicts the concept of FSP. The initial EMC at which bound water starts to be removed varied largely among the wood species. 相似文献
2.
This study focuses on liquid water transport in wood above the fiber saturation point in the nonhygroscopic region. The liquid
water transport of hygroscopic porous materials including wood is usually described by Darcy’s law. It requires knowledge
of capillarity and intrinsic and relative permeabilities. In this study, the capillary pressure-water relation and relative
permeability were investigated using experimental data for wood available in the literature. The performance of three models
(Brooks-Corey model, van Genuchten model, and Durner’s bimodal pore-size distribution model) was investigated for the capillary
pressure-water relation. These models have advantages in that each shape parameter has qualitative physical meaning for the
pore-size distribution. Most species had unimodal pore distributions except for aspen, which had a bimodal pore distribution.
The van Genuchten model represented the capillary pressure-water relation better than the Brooks-Corey model. Durner’s bimodal
model was found to be the most appropriate for the capillary pressure-moisture relation of aspen. The relative permeability
was calculated by using Mualem’s model, which was compared with the value from the Couture model. From the results, the liquid
water diffusivity divided by intrinsic permeability of wood was estimated. This approach may be promising for adopting the
liquid water diffusivity for the numerical simulation of drying and sorption, although there might be considerable variation
within and between trees. 相似文献
3.
Some considerations in heterogeneous nonisothermal transport models for wood: a numerical study 总被引:1,自引:1,他引:0
This study compares a number of coupled heat and mass transfer models and presents numerical comparisons of phenomenological
coefficients between the four models (Stanish, Perre, Pang, and Avramidis) that are most frequently used in the literature
to describe wood-drying processes. The USDA sorption isotherm, the Hailwood-Horrobin model, was adopted to calculate the relations
between moisture content in wood and water vapor pressure at any temperature. Due to different assumptions about the driving
forces of heat and mass transfer, coefficients in each model represent different values for moisture content and temperature
and are closely related to each other. In the case of isothermal mass transfer, the moisture diffusion coefficient in the
transverse directions from the Stanish and Pang models increased with decreasing moisture content. This contradicts the Avramidis
and Perre models and numerous experimental results. Thermal diffusion effects on the drying process may not be predominant
because the nonisothermal state is relatively short. Therefore, the Perre model, which does not consider the thermal diffusion
effect, has been used successfully in the drying simulation. However, it may be erroneous in certain cases when the nonisothermal
state prevails over the system, such as building physics. The Pang model cannot explain the phenomena of thermal diffusion
and moisture thermodiffusion. It might be reasonable to modify the thermal diffusion of the Avramidis model, which is lower
than that of the Stanish model. The apparent heat diffusivity was higher than the true heat diffusivity. 相似文献
4.
《Wood material science & engineering》2013,8(2):95-110
Abstract Wood moisture content of coated panels of Scots pine sapwood was recorded during 24 months natural weathering in Vienna by logging electric resistance and temperature near the surface. Eight coating systems with various film thicknesses were used, including three solvent borne alkyd stains, three water borne acrylic stains and two water borne acrylic paints. At five sites in Europe wood moisture content of panels coated with three solvent borne alkyd stains, a brown acrylic stain and a white opaque acrylic paint was recorded weekly by changes in panel mass. Fluctuations in wood moisture content were influenced by the film thickness, water permeability and colour of the coating systems used. Degradation phenomena led to decreasing moisture protection of less durable coating systems over time of exposure. The exposure site clearly affected wood moisture content in the panels. 相似文献
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【目的】研究高温热处理对人工林樟子松、杉木、美洲黑杨木材平衡含水率和吸湿特性的影响,为科学评价热处理木材吸湿特性提供理论基础,为人工林木材高附加值利用和实际高温热处理木材生产提供参考。【方法】以水蒸气为保护介质,设定180、200和220 ℃3个温度进行高温热处理,采用双室温、湿度控制法,在25 ℃环境中以8种不同类型饱和盐溶液精确控制水蒸气相对湿度进行等温吸附试验,运用Hailwood-Horrobin模型拟合等温吸附曲线,分析高温热处理对木材水蒸气等温吸附曲线线形、平衡含水率、单层分子吸附水和多层分子吸附水的影响。【结果】 180、200和220 ℃处理后,试样吸湿平衡含水率均值相当于素材含水率均值的80%、70%和50%左右;3个树种素材试样和高温热处理材试样均表现为第2类等温吸附曲线形态特征,Hailwood-Horrobin模型能够较好拟合不同树种素材和高温热处理材等温吸附曲线,不同热处理条件试样等温吸附曲线的拟合度均高于0.980 0,处理温度越高,等温吸附曲线越接近直线;高温热处理后代表含有单位摩尔数吸附位的绝干木材质量参数( W )显著增加,不同相对湿度下高温热处理材的单层分子吸附水和多层分子吸附水含量也随之降低;180、200和 220 ℃处理后,木材试样单层分子吸附水含量相较于素材下降20%、30%和50%左右,高温热处理对多层分子吸附水含量影响规律与之相近;高温热处理后单层分子吸附水、多层分子吸附水和吸附水总量的最大值相较于素材明显下降,且处理温度越高,下降幅度越大。【结论】高温热处理可明显降低3个树种试样的吸湿平衡含水率,且处理温度越高,平衡含水率下降幅度越大;高温热处理会一定程度影响木材等温吸附曲线线形,Hailwood-Horrobin模型可用于描述高温热处理材等温吸附曲线,且拟合度较高;高温热处理可明显降低3个树种试样等温吸附过程单层分子吸附水和多层分子吸附水含量,且处理温度越高影响越明显,单层分子吸附水和多层分子吸附水最大含量均明显降低,进而影响吸附水总量最大值。 相似文献
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H. Mounji Prof. Dr. J. Bouzon Prof. Dr. J. M. Vergnaud 《Wood Science and Technology》1991,26(1):23-37
Summary The process of absorption and desorption of water by two-dimensional transport in wood was studied at a moisture content beyond the fiber saturation point. The transverse directions through a square wood beam were considered. A numerical model capable of describing the process was constructed and successfully tested. It can predict by calculation the kinetics of absorption when wood is immersed into water and the kinetics of desorption when it is exposed to air. The model gives a fuller insight into the process with the profiles of moisture content developed through the cross-section of the wood at any time.This work was carried out with the help and support of the French MRT and Wood Technical Center (CTBA) 相似文献
9.
In this research, acoustic emission technology was used to estimate the moisture excluding effectiveness of coatings on wood.
Moisture interaction with wood causes damage, and coatings such as paints and varnishes can effectively limit this interaction.
Over time, the moisture excluding effectiveness of wood is reduced by the actions of weathering and the resulting wood-water
interaction leads to problems due to the dimensional instability of the wood. Five coating types, at two spreading levels,
were applied to samples of southern yellow pine. The samples were exposed to three weathering conditions to simulate the effects
of exposure then immersed in water while the moisture interaction was monitored using acoustic emission monitoring equipment.
Cumulative counts as well as changes in size and moisture content were measured or calculated. The results show that acoustic
emission technology is an effective and sensitive estimator of the moisture excluding effectiveness of coatings on wood. The
acoustic emission method is sensitive to local changes and coating fractures whereas the more traditional methods are sensitive
to gross changes in sample characteristics. Of five coatings tested, the most effective one was found to be an oil-based paint,
and the worst performance was by a water-based, clear poly-urethane.
Received: 10 September 1998 相似文献
10.
Julian Moreno Chan John C. Walker Carolyn A. Raymond 《Wood Science and Technology》2011,45(4):609-626
The effects of moisture content from 17 to 159% MC and temperatures from −71°C to +58°C on resonance-based acoustic velocity
and dynamic modulus of elasticity (DMOE) were investigated using 36 boards of radiata pine sapwood. Acoustic velocity decreased
with increasing moisture content and temperature, although effects differed noticeably below and above fibre saturation point
(FSP). Below FSP, acoustic velocity decreased rapidly and linearly with increasing moisture; whereas above FSP changes in
velocity were rather gradual and curvilinear with marked differences in velocity patterns between temperatures below and above
freezing. Acoustic velocity decreased linearly with increasing temperature but there was an abrupt discontinuity at the freezing
point for wood above FSP. Changes in moisture content and temperature affecting wood density and acoustic velocity led to
changes in DMOE, although this relationship was not straightforward because changes of velocity with moisture were linear
or curvilinear and depended on whether wood was frozen or unfrozen, whereas changes of density with moisture content were
invariably linear. For practical work, it is important to estimate the relative effect of changing moisture content or temperature
with respect to standard conditions on DMOE, thus general guidelines were devised to account for significant changes. 相似文献
11.
Asghar Tarmian Romain Remond Hadi Dashti Patrick Perré 《Wood Science and Technology》2012,46(1-3):405-417
The moisture diffusion coefficient of compression wood in spruce (P. abies) and tension wood in beech (F. sylvatica) was examined. The results indicated that the diffusion coefficient measured under steady-state condition (cup method) could well characterize the drying kinetics of the reaction woods. The compression wood offered more resistance to the moisture diffusivity when compared with the corresponding normal wood. The thick cell wall rich in lignin explains the small mass diffusivity in compression wood. In contrast, the mass diffusivity in beech is almost always higher in tension wood than in normal wood, in spite of similar density values. The high moisture diffusion in tension wood can be explained by the ease of bound water diffusion in the gelatinous layers (G-layers). 相似文献
12.
以白橡热压干燥材为研究对象,利用动态水分吸附仪研究了不同热压温度干燥处理后白橡木材和未处理对照材的等温吸湿特性,并采用H-H模型拟合;分析热压干燥对木材吸湿特性的降低机理。结果表明:白橡木材等温吸湿线皆为IUPAC Ⅱ型等温吸湿线。在任意相对湿度下,热压干燥材平衡含水率均明显低于对照材,且热压温度越高,平衡含水率降低越明显。H-H模型对白橡木材等温吸湿数据表现出良好的拟合效果。单分子层和多分子层含水率降低共同作用使得热压干燥材吸湿性降低,且相对湿度越高,多分子层水的减少对吸湿性的降低作用越大。与对照材相比,热压干燥材(140、150 ℃和160 ℃)的纤维饱和点推测值分别降低8.89%、11.76%和13.62%。白橡热压干燥材吸湿性降低机理主要为游离羟基等亲水基团含量减少和细胞壁刚度增加等。 相似文献
13.
《Wood material science & engineering》2007,2(1):45-53
Samples of nine tropical hardwoods from Peru and sugar maple wood from Quebec were selected for moisture sorption and swelling tests at 25°C. These tests evaluated the fiber saturation point (FSP) by two methods: following adsorption over distilled water, and from the volumetric swelling intersection point. Cold-water and hot-water extractives, sequential cyclohexane, acetone and methanol extracts, ash content, wood density and interlocked grain were also determined on matched samples. The results indicated that adsorption tests over distilled water were not applicable for determining FSP in all wood species. Condensation of water vapor apparently occurred, even though temperature during adsorption was controlled to the nearest 0.01°C. The volumetric swelling intersection point method was judged more appropriate. FSP ranged from 15 to 25% for tropical hardwoods and was 30% for sugar maple wood. FSP was negatively correlated with wood density, acetone extracted fraction, interlocked grain and ash content. These parameters each exerted similar effects on variability in FSP. 相似文献
14.
《Wood material science & engineering》2013,8(1):45-53
Abstract Samples of nine tropical hardwoods from Peru and sugar maple wood from Quebec were selected for moisture sorption and swelling tests at 25°C. These tests evaluated the fiber saturation point (FSP) by two methods: following adsorption over distilled water, and from the volumetric swelling intersection point. Cold-water and hot-water extractives, sequential cyclohexane, acetone and methanol extracts, ash content, wood density and interlocked grain were also determined on matched samples. The results indicated that adsorption tests over distilled water were not applicable for determining FSP in all wood species. Condensation of water vapor apparently occurred, even though temperature during adsorption was controlled to the nearest 0.01°C. The volumetric swelling intersection point method was judged more appropriate. FSP ranged from 15 to 25% for tropical hardwoods and was 30% for sugar maple wood. FSP was negatively correlated with wood density, acetone extracted fraction, interlocked grain and ash content. These parameters each exerted similar effects on variability in FSP. 相似文献
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泡桐木材流体渗透性与扩散性的研究 总被引:11,自引:1,他引:11
本文论述了泡桐木材流体渗透性,扩散性,吸着水与自由水的吸着与吸收动力学特性,密度和干缩性以及电镜观察结果;阐明了泡桐木材具有流体可渗透及可扩散路径,有着输导流体的天然机制,水、热处理难易一般;泡桐材本身并不具有隔潮耐湿性,但由于它密度低,胀缩小,变形和开裂的可能性亦小,用它干燥后制成的箱盒容器的密封性、隔潮耐湿性较好。 相似文献
17.
Masahiro Takatani Osamu Kato Takashi Kitayama Tadashi Okamoto Mitsuhiko Tanahashi 《Journal of Wood Science》2000,46(3):210-214
The effect of steam-exploded wood flour (SE) added to wood flour/plastic composite was examined using SE from beech, Japanese cedar, and red meranti and three kinds of thermoplastic polymer: polymethylmethacrylate, polyvinyl chloride, and polystyrene. Addition of SE increased the fracture strength and water resistance of the composite board to an extent dependent on the polymer species and the composition of wood/SE/polymer. However, water resistance decreased with the increasing proportion of SE when SE meranti was added. Effects of the wood species of SE on the properties of resulting board were small. An increased moisture content of wood flour or SE (or both) increased the variation of board performance. 相似文献
18.
A drying experiment with 36 mm thick softwood boards having an average initial moisture content of approximately 1.2 (dry basis) was performed. Drying temperatures of 40, 60 and 80°C were used. Relative humidity and superficial air velocity were maintained at 40% and 3.0 m s?1, respectively. Internal moisture content was monitored along the process in the single direction of the internal flux of water. Loss in mass of the entire timber board was also determined. An effective coefficient of mass transfer was tuned to internal experimental profiles of moisture content by involving the Fick’s second law. An explicit finite difference method for the numerical solution of the mass balance represented by the Fick’s equation was combined with the simplex method of optimization to obtain a mass transport parameter in the magnitude of 1.5–3.5 × 10?9 m2 s?1. A positive and significant effect of temperature on the effective diffusion coefficient, which was well described by an Arrhenius type expression, was deduced from this investigation. Although a negative effect of the average moisture content on the internal resistance to mass transfer was also observed, it was much less evident; mainly above the wood fiber saturation point. A negligible influence of the local moisture content on the investigated transport parameter was noticed when either a linear or a nonlinear model correlating these variables was adopted. 相似文献
19.
《Wood material science & engineering》2013,8(3-4):194-203
Abstract Two Malaysian hardwoods, acacia (Acacia mangium) and sesendok (Endospermum malaccense), that had been subjected to oleo-thermal modification were studied to determine their sorption isotherm behaviour using a dynamic vapour sorption apparatus. All the specimens were thermally modified using palm oil at three different temperatures (180, 200 and 220°C) and three different times (1, 2 and 3 h). The results showed that there was a reduction in equilibrium moisture content at each target relative humidity due to the heat treatment, but that the two wood species showed different behaviour in this respect. The adsorption isotherms were analysed using the Hailwood and Horrobin model, with excellent fits to the experimental data. The monolayer water and polylayer water were both reduced at a range of relative humidity values of the treated samples, although behaviour between the two wood species differed. Heat treatment resulted in an increase in hysteresis ratio, which was probably due to the increase in matrix stiffness of the cell walls. 相似文献
20.
木材含水率测定方法的比较研究 总被引:1,自引:0,他引:1
为修订国家标准“CB1931—80木材含水率测定方法”提供依据,本文对含树脂和挥发性物质较多的马尾松等5种木材,采用烘干法、真空干燥法和蒸馏法进行含水率测定的比较。结果表明,烘干法简便实用,但所测得的含水率结果比真空干燥法和蒸馏法为高,如果试样含有较多的挥发性物质,应用烘干法测定含水率误差偏大时,可采用真空干燥法。 相似文献