Measurements of water vapour sorption in wood

Summary Results from sorption measurements at 23 °C on Picea abies in the intervals 54 to 75% RH and 75 to 84% RH are reported. In the higher interval non-Fickian sorption is clearly seen.This work was supported by the Swedish Building Research Council     

The water vapour sorption properties of Sitka spruce determined using a dynamic vapour sorption apparatus

The water vapour sorption properties of Sitka spruce (Picea abies) have been investigated over a range of temperatures (14.2–43.8°C) using a dynamic vapour sorption apparatus. The sorption kinetics behaviour was evaluated using the parallel exponential kinetics model which has been found to give very accurate fits to the data in studies of foodstuffs or plant fibres, but has not been previously applied to sorption studies with wood. Both the adsorption and desorption kinetics curve can be deconvoluted into a fast and slow exponential process. Under conditions of adsorption, the fast process appears to be associated with the formation of monolayer water (determined using the Hailwood Horrobin model) up to a relative humidity of 20%. Under desorption, there is no clear differentiation between fast and slow processes. The area bounded by the sorption hysteresis loop reduced as the temperature at which the isotherm was measured increased, due to movement of the desorption curve only, with the adsorption curve remaining the same at all temperatures. This behaviour is consistent with sorption processes taking place on nanoporous glassy solids below the glass transition temperature. The heat of wetting was determined from the temperature dependence of the desorption isotherms by using the Clausius–Clapeyron equation, yielding results that are consistent with literature values. However, doubts are raised in this paper as to the applicability of using the Clausius–Clapeyron equation for analyses of this type.     

The sorption of water vapour by anhydride modified softwood

A comprehensive investigation into the effect of molecular size of the substituent group of softwood modified with linear chain carboxylic acid anhydrides, namely acetic, propionic, butyric, valeric, hexanoic, upon the sorption of water vapour has been performed. The sorption isotherms for untreated and chemically modified wood were analysed using the Hailwood–Horrobin model. The experimental analysis of the sorption isotherms showed that esterification affects the total, polymolecular and monomolecular sorption. The effect of molecular size of the substituent group on site accessibility was addressed by comparing the effect on water vapour sorption produced by adducts with differences in molecular size. Similar levels of cell-wall bulking were produced at different levels of hydroxyl substitution. Analysis of the sorption isotherms at comparable weight percentage gain revealed that the five anhydrides used show similar effectiveness in both total, polymolecular and monomolecular sorption, despite the substantial difference in the proportion of hydroxyl groups reacted. It is concluded that the reduction in total, polymolecular and monomolecular sorption produced by the linear chain anhydrides is primarily determined by the volume of adduct deposited in the cell wall (bulking) rather than by the number of hydroxyl groups that have been substituted. The validity of the Hailwood–Horrobin model is questioned.     

Dynamic water vapour sorption properties of wood treated with glutaraldehyde

The dynamic water vapour sorption properties of Scots pine (Pinus sylvestris L.) wood samples were studied to investigate the modifying effects of glutaraldehyde. Pine sapwood was treated with solutions of glutaraldehyde and a catalyst (magnesium chloride) to obtain weight per cent gains of 0.5, 8.6, 15.5, and 21.0%, respectively. The sorption behaviour of untreated and treated wood was measured using a Dynamic Vapour Sorption apparatus. The results showed considerable reduction in equilibrium moisture content of wood and the corresponding equilibrium time at each target relative humidity (RH) due to glutaraldehyde treatment. The moisture adsorption and desorption rates of modified and unmodified wood were generally faster in the low RH range (up to approximate 20%) than in the high range. Modification primarily reduced the adsorption and desorption rates over the high RH range of 20–95%. Glutaraldehyde modification resulted in a reduction in sorption hysteresis due to the loss of elasticity of cell walls.     

The sorption of water vapour by chemically modified softwood: analysis using various sorption models

The sorption data for Corsican pine wood chemically modified with a homologous series of linear chain carboxylic acid anhydrides was analysed using various models, namely, BET, Dent, Le and Ly, Hailwood and Horrobin, Nelson, and Henderson. Hailwood and Horrobin resulted in the more appropriate model to represent the equilibrium data of chemically modified wood. In the present study, the parameter representing the molecular weight of the dry cell wall in the Hailwood and Horrobin isotherm was perfectly correlated with the weight percentage gain of anhydride. A linear relation was proposed between these two parameters in order to fit all experimental isotherms (the results showed an r²=0.993).     

The sorption of water vapour of wood treated with a nanotechnology compound

The potential of improving the hygroscopicity of solid wood by applying a new nanotechnology compound was investigated. The sorption isotherms were analysed using the Hailwood–Horrobin model. The experimental analysis of the sorption isotherms showed that the treatment affected total, polymolecular and monomolecular sorption. The application of the compound reduced the total sorption by 26.5%, polymolecular sorption by 23.9% and monomolecular sorption by 33.1% at saturation.     

Analysis of the water vapour sorption isotherms of thermally modified acacia and sesendok

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.     

Wood fracture, acoustic emission, and the drying process Part 1. Acoustic emission associated with fracture

Summary Acoustic emission (AE) signals were collected during fracture tests in order to analyze them for characteristics that could be used as parameters in a reactive control system for the wood drying process. Ponderosa pine and California black oak single edge notch tensile specimens were tested in the TR, TL, and mixed mode configurations, at 12 and 18 percent moisture content and at temperatures of 20, 40 and 60 °C. AE was observed in both opening mode and mixed mode tests, but cumulative events to maximum load in mixed mode were 7.4 times greater than in opening mode for oak and 3.4 times greater in pine. It was concluded that mixed mode AE signals were most promising for pattern recognition analysis, which will be the subject of Part II of this study.This research was supported in part by USDA Grant No. 90-37291-5762     

大木竹纤维形态与组织比量的研究

通过对浙江南部大木竹的纤维形态和组织比量测定,并与造纸性能良好的青皮竹和当地分布较广的绿竹进行比较,结果表明：大木竹竹材属于长纤维原料,纤维长2．2,4mm,比青皮竹(2．29m)略小,而比绿竹(1．87mm)为大;纤维长宽比达166,较青皮竹和绿竹的相应值147和131均大,其中与绿竹的差异达显著水平;壁腔比为3．5;纤维组织比量达48．68％,比青皮竹(50．19％)略小,而比绿竹(45．78％)为大。大木竹纤维形态和组织比量各指标在竹秆的纵向和径向均有一定变异,主要表现为,纤维壁腔比和纤维组织比量随秆的高度增加而增大;在径向部位,纤维长度和长宽比均为竹壁中部较大,而壁腔比则为竹壁外侧较大。竹龄主要对壁腔比产生影响,1年生、3年生大木竹的壁腔比明显大于当年生竹。     

Changes of chemical properties and the water vapour sorption of <Emphasis Type="Italic">Eucalyptus pellita</Emphasis> wood thermally modified in vacuum

The aim of this study was to evaluate the chemical composition and the dynamic water vapour sorption properties of Eucalyptus pellita wood thermally modified in vacuum. For this purpose, wood samples were thermally modified in a vacuum oven at 160–240 °C for 4 h. Chemical composition were investigated by wet chemical analysis, elemental analysis, as well as Fourier transform infrared (FTIR) analysis, and dynamic water vapour sorption properties were evaluated by dynamic vapour sorption apparatus. The results showed that holocellulose and alpha-cellulose contents decreased and lignin and extractives contents relatively increased during the heat process. Elemental analysis showed a reduction in hydrogen content and an increase in carbon content. FTIR analysis indicated that the degradation of hemicellulose and condensation reactions of lignin occurred. In addition, the thermo-vacuum resulted in a reduction in the equilibrium moisture content of wood during the adsorption or desorption process. And the sorption hysteresis had a decreasing trend with increasing treatment temperature. The development of the hygroscopicity was related to the increase in the relative content of lignin, the degradation of the carbonyl groups in xylan and the loss of carbonyl group linked to the aromatic skeleton in lignin after heat treatment.     

Effects of water vapour heating on structure and properties of cork

Summary The effects of heating cork in water vapour at 100°C and 300°C are studied and compared with those produced by heating in air at the same temperatures. Dimensional and mass changes were measured and radial compression curves were obtained following various treatments. The 300°C treatments originate a straightening of the originally corrugated cell walls, while the cell wall material undergoes thermal degradation. Larger swellings and larger mass changes are observed in the treatments in water vapour as compared to those in air. The 100°C treatments do not induce degradation and produce reversible changes in technological properties which are simply due to differences in the water content.     

Current and Future Status of Wood Structure in China (Part I)

回顾我国木结构的发展历程,总结我国木结构产业的现状,分析现存问题并展望未来的发展方向.本篇为连载的第一部分,重点根据我国木结构产业区域发展特点,将我国木结构产业划分为8个区域,分析各区域的产业规模、原料来源、产品类型特征;总结木结构产业的宏观发展特征,指出我国木结构产业正处于发展速度快、市场需求潜力大的时期,存在主要使用进口原材料的问题.     

Wood in the stationary state: Zero entropy gradients for water vapor and bound water

Summary According to nonequilibrium thermodynamics, the local rate of entropy production is minimized for moist wood in the stationary state. Furthermore, the rate of entropy production due to moisture flow must be zero in this state. Conservation of energy applied to the steady flow of water vapor through an arbitrarily selected volume element of wood shows that the vapor entropy gradient is zero. Because the entropy production due to moisture flow must be balanced by a corresponding entropy flow away from the element, entropy gradients for bound water and water vapor are equal and the bound water entropy gradient also is zero.     

Wettability and the hysteresis effect in the sorption of water vapor by wood

Summary The hysteresis effect in the adsorption and desorption of water vapor by wood has been variously explained as a consequence of differences in (1) the availability of bonding sites for sorption on molecular surfaces, (2) the degree of aggregation of a swelling or shrinking cellulosic gel system, and (3) the wettability of submicroscopic capillaries within the cell wall.The wettability hysteresis of 28 tropical woods, calculated as the ratio of cosines of advancing and receding contact angles made by water, has been determined by the inclined plate method.For 13 of these species the availability of complete sorption isotherms permitted analysis by means of the Hailwood-Horrobin model to differentiate between monomolecular and polymolecular sorbed moisture. In the upper range of relative humidities, total sorption hysteresis is primarily the result of hysteresis in polymolecular sorption.Positive relationships found in this study between polymolecular sorption hysteresis and wettability hysteresis are consistent with the Kelvin equation with respect to the effect of varying contact angle and give at least partial support to Zsigmondy's explanation of hysteresis as a phenomenon of capillary condensation.Total sorption hysteresis for all 28 species in the upper range of relative humidities was also positively correlated with wettability hysteresis due to the predominant effect of polymolecular sorption hysteresis. It may be concluded that in the range of relative humidity above 60 percent, hysteresis shown by typical sigmoid isotherms is to a considerable degree a phenomenon of capillary condensation explainable by the Kelvin equation in its complete form including cosine of contact angle.

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Zusammenfassung Die Hysterese bei Adsorption und Desorption von Wasserdampf in Holz wird in der Regel als Folge von Unterschieden bei 1. der Zugänglichkeit von Bindungsstellen für die Sorption an molekulare Schichten, 2. des Aggregatzustandes eines quellenden oder schwindenden Cellulose-Gel-Systems und 3. der Benetzbarkeit der submikroskopischen Kapillaren innerhalb der Zellwand erklärt. Die Benetzbarkeits-Hysterese bei 28 tropischen Holzarten wurde durch das Verfahren mit geneigter Ebene bestimmt durch die Berechnung des Verhältnisses der cos-Werte des vorderen und hinteren Kontaktwinkels von Wasser.Bei 13 der geprüften Holzarten erlaubte das Vorhandensein der vollständigen Sorptionsisothermen eine Analyse mit Hilfe des Hailwood-Horrobin-Modells zur Unterscheidung zwischen monomolekular und polymolekular sorbierter Feuchtigkeit. In den höheren Bereichen der relativen Feuchtigkeit ist die Gesamtsorptionshysterese vorwiegend das Ergebnis der polymolekularen Sorption.Die in dieser Untersuchung gefundenen positiven Zusammenhänge zwischen der Hysterese der polymolekularen Sorption und der Benetzungshysterese stehen in Übereinstimmung mit der Kelvinschen Gleichung hinsichtlich des Einflusses des variierenden Kontaktwinkels und sie unterstützen, zumindest teilweise, die Theorie von Zsigmondy über die Hysterese als einer Erscheinung der Kapillar-Kondensation.Im oberen Bereich der relativen Feuchtigkeiten korrelierte die Gesamt-Sorptionshysterese für alle 28 Holzarten ebenfalls positiv mit der Benetzungshysterese infolge des überwiegenden Einflusses der polymolekularen Sorptionshysterese. Hieraus kann geschlossen werden, daß im Bereich der relativen Feuchtigkeit über 60% die Hysterese, die sich in einem typischen S-förmigen Verlauf der Sorptionsisothermen zeigt, zu einem wesentlichen Grade eine Erscheinung der Kapillarkondensation ist und durch die gesamte Kelvin-Gleichung, einschließlich des cos-Kontaktwinkels, erklärt werden kann.

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This research is part of a comprehensive study being conducted at the Yale School of Forestry in cooperation with the Office of Naval Research, U.S. Navy, under Contract No. 609(13), Project NR 330-001, Properties of Tropical Woods.     

The water vapor sorption mechanism and its hysteresis in wood: the water/void mixture postulate

A new method of wood moisture sorption analysis is presented using sorption isotherms of a series of mildly heat-treated specimens with varied and known elemental composition. This method allows the determination of the occupancy of accessible sorption sites in wood as a function of relative humidity h, θ(h) ≈ h, found in agreement with the literature data on the non-freezing water occupancy of hydroxyl groups for h < 0.9. Complementary sorption isotherm shape analysis identifies an empirical power law occupancy function, θ(h) = h ^α , α ≈ 0.73, which is close to the former two determinations in the same humidity range. The validity of widely accepted surface sorption theories for wood with a strongly bound primary layer and loosely bound secondary layers is disproven. To explain the found occupancy function, θ(h) ≈ h, a near-ideal liquid mixture of moisture and polar dynamic microvoids in the cell wall substance is postulated. The power law occupancy function is used to calculate the humidity-dependent number of sorption sites in adsorption/desorption isotherms to show that (1) the number of sorption sites from the adsorption line monotonically increases with increasing humidity—argued to represent the equilibrium number of sorption sites at each humidity, and (2) the number of sorption sites from the desorption line fails to fully return to that of the (equilibrium) adsorption line. Hysteresis is quantitatively explained as the result of non-equilibrium excess sorption sites being occupied according to the occupancy law. The relaxation of non-equilibrium excess sorption sites is satisfactorily modeled by a first-order rate equation. Applying the analysis to study mild thermal modification of moisture sorption isotherms revealed that (1) moisture contents decrease directly linear to the removed amount of sorption sites at all humidity <0.95, and (2) the absolute hysteresis is nearly unaffected as a result of counter-acting effects of the reduced number of sorption sites and reduced amount of relaxation.     

Statics and kinetics of water vapor sorption of small loblolly pine samples

A better understanding of the sorption behavior of different wood structures could be useful in protecting wood against wood deterioration and fungal attack. The purpose of this study was to investigate the effect of differences among earlywood, latewood, and tree ring location within the stem cross-section of loblolly pine (Pinus taeda) on the sorption kinetics and statics of water vapor under ambient conditions. The water vapor sorption of earlywood and latewood in different tree rings was recorded using a dynamic contact angle analyzer under relative humidity changes from 11 to 89%, as provided by saturated salt solutions. Earlywood had higher sorption rates and diffusion coefficients than latewood, while outer tree rings had higher sorption rates and diffusion coefficients than inner tree rings. The sorption isotherms of earlywood, latewood, and different tree ring locations within the stem cross-section were fitted very well by a Hailwood–Horrobin model.     

Wood fracture, acoustic emission, and the drying process Part 2. Acoustic emission pattern recognition analysis

Summary Signal features were extracted from AE signals collected during mixed mode fracture tests of ponderosa pine and California black oak conducted at 12 and 18 percent moisture content and temperatures of 20, 40 and 60 °C. Five features in the time domain and three in the frequency domain were selected for further analysis. Signal features were found to be significantly affected by both temperature and moisture content. Cluster analysis showed that while signals could be successfully classified, the resulting patterns showed little relationship to wood fracture. Only load levels well beyond maximum load could be consistently distinguished from lower load levels. It was concluded that pattern recognition analysis would have only limited application to control the wood drying process.This study was supported in part by USDA Grant No. 90-37291-5762     

Winter water relations of New England conifers and factors influencing their upper elevational limits. I. Measurements

The upper elevational limits of tree species are thought to be controlled by abiotic factors such as temperature and the soil and atmospheric conditions affecting plant water status. We measured relative water contents (RWC), water potentials (Psi) and cuticular conductances (g(c)) of shoots of four conifer species-eastern hemlock (Tsuga canadensis (L.) Carr.), eastern white pine (Pinus strobus L.), red pine (P. resinosa Ait.) and red spruce (Picea rubens Sarg.)-during two winters on Mt. Ascutney, Vermont, USA. Some micrometeorological measurements are also reported. Eastern hemlock and white pine were studied near their upper elevational limits at a 640-m site, and red pine was studied near its upper elevational limit at 715 m. Red spruce was also studied at the 715-m site, which is in the middle of its elevational range on this mountain. There was no evidence of winter desiccation stress in any species. The observed distribution of seedlings suggested that the upper elevational limits on shade-intolerant eastern white pine and red pine are set by the absence of suitable seed beds after 100 years without fire. Eastern hemlock is able to reproduce in deep shade on organic substrates, but germination at high elevations may be restricted by low temperatures.