Bending creep behavior of wood under cyclic moisture changes

This study examined the bending creep behavior in the longitudinal direction of six species under cyclic moisture content (MC) changes. For each species, tests were made at 20°C with five cyclic relative humidity changes between 65% and 95%, beginning from moisture adsorption. A load corresponding to 25% of short-term breaking load of the species was applied to the radial section of each specimen with four-point bending. The effect of MC change on instantaneous compliance was also investigated under the same condition. The quantitative relation between mechano-sorptive (MS) compliance and MC change was examined, and the material parameter KM for the relation in specific sorption was determined. Results indicated that the total compliance in the six species with different behavior increases with sorption time. As an integral part of total compliance, instantaneous compliance changes linearly with MC and influences to a greater or lesser extent the total compliance behavior. In general, with increasing MC change, the MS compliance linearly increases during the first adsorption and all desorption and decreases slightly during subsequent adsorption. The material parameterK _M varies markedly not only with species but also with specific sorption. The first adsorption causes the largest deformation, followed by desorption.This paper was presented at the 48th annual meeting of the Japan Wood Research Society in Shizuoka, Japan. April 1998     

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.     

Bending creep behavior of hot-pressed wood under cyclic moisture change conditions

This study examined the bending creep behavior of hot-pressed wood during cyclic moisture changes. Sugi (Cryptomerica japonica D. Don) specimens were pressed in the radial direction under six combinations of nominal compressive strain (33% and 50%) and press temperatures (140°C, 170°C, 200°C). Creep tests were conducted at 20°C with three cyclic relative humidity changes between 65% and 95% under 25% of short-breaking stress. The effect of moisture content (MC) change on elastic compliance and mechanosorptive (MS) compliance was investigated. The relation between MS compliance and thickness swelling was studied. The results indicated that total compliance increased over the history of cyclic moisture changes; and its behavior was closely related to the changes in MC and thickness swelling. The total compliance increased during adsorption and decreased during desorption. Elastic compliance increased linearly with MC and was dependent on press temperature and compression. With increasing MC change, MS compliance increased during adsorption and decreased during desorption. The first adsorption led to greater MS compliance than did the subsequent adsorption with the same amount of MC change. In general, the elastic parameterK _E and the MS parameterK _Mincreased with compression and decreased as the press temperature increased. The MS parameterK _M was apparently greater than the elastic parameterK _E. The MS parameterK _M increased with swelling coefficient K_SW of the hot-pressed specimen during adsorption and decreased with an increasing shrinkage coefficientK _SH during desorption.     

Selected physical and mechanical properties of the Chilean wood species roble,lingue, manio and alerce

Abstract

Data sets with mechanical and physical properties of wood species can aid the recognition of biomechanical correlations, and they can also be valuable tools to assist decision making regarding the practical industrial employment of the species. However, such data sets are still lacking for many species, particularly non-European and non-North-American species. Therefore, the goal of this study was to determine the mechanical properties of four Chilean species (the softwoods alerce and manio, and the hardwoods lingue and roble), including the three Young's moduli and the three shear moduli. Dynamic methods were applied as well as static ones. The results suggest that Brinell hardness, compression strength and Young's moduli are clearly correlated with the density and can partly be related to anatomical details. Since the investigated species cover only a fraction of the Chilean species that are suitable for construction purposes, for example, further future examination of relevant species would be beneficial for basic research and industry.     

Viscoelastic properties of wood from Chinese-fir and poplar plantations

Elastic and strength properties(proportional-limit stress(σ prop ),Young's modulus(E),breaking stress(σ max )in static bending parallel to grain in a longitudinal direction),as well as stress relaxation in air-dried condition and water-saturated conditions at seven different constant temperatures and increasing and decreasing temperatures were investigated for wood from Chinese-fir and poplar plantations.The results show that hygrothermal conditions considerably affect these mechanical properties.The higher the moisture content(MC)or temperature,the lower the strength of wood.Further investigation of the effects of constant temperature on stress relaxation indicates that high temperature specimens have low relaxation moduli and high fluidity.In the case of increasing temperature the range of the modulus of relaxation is larger than in the case of a reduction in temperature,while the residual moduli do not show large differences.This is because the modulus at high temperatures decreases more than that at low temperatures.The fluidity of specimens in a state of water desorption increases slowly at the beginning,increases quickly until the MC reaches an equilibrium moisture content(EMC)and then becomes stable,which is quite different from that in a water-saturated state.Fluidity in a desorption state is much higher than in a water-saturated state.This is probably due to the fact that the former is in an unstable state which can be interpreted as a state with internal strain and has therefore a greater potential to release strain.     

Cantilever experimental setup for rheological parameter identification in relation to wood drying

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.     

Influence of moisture sorption on the tangential compression strength of Mahogany wood (<Emphasis Type="Italic">Swietenia macrophylla</Emphasis> King)

Moisture sorption tests and compression tangential tests at 25°C were carried out on specimens of mahogany (Swietenia macrophylla King) wood from Peru. The tests were performed over seven adsorption and five desorption moisture conditions, and differences in strength between adsorption and desorption curves at a given equilibrium moisture content were evaluated. The results showed that second-order effects in mahogany wood were not discernible in either the tangential compliance coefficient or for the stress at the proportional limit in tangential compression.     

Physical and mechanical properties of wood after moisture conditioning

Some properties of wood (hinoki:Chamaecyparis obtusa) moisture-conditioned by an adsorption process from a dry state and by two desorption processes (from a water-saturated state and from a state with a moisture content slightly below the fiber saturation point) were investigated. The moisture contents of wood conditioned by the adsorption process and by the desorption process continued to approach to one another for the moisture-conditioning period of over 50 weeks. Accordingly, sorption hysteresis should be regarded as a transitional phenomenon that occurs during the process of approaching the true equilibrium, which requires a long time. The wood conditioned by the desorption process beginning from a water-saturated state showed slightly smaller dimensions than those conditioned by the adsorption process with the same moisture content; however, the wood conditioned by the desorption process from a moisture content below the fiber saturation point showed slightly larger dimensions than those conditioned by the adsorption process. The wood conditioned by the adsorption process from a dry state showed a higher modulus of elasticity and modulus of rupture than did the wood conditioned from a water-saturated state with the same moisture content. The mechanical properties of the wood also varied based on the states at which the desorption process was started. This is a notable characteristic of the relation between the drying condition and the mechanical properties of wood.     

Physical properties of two tropical wood species from Mozambique

Abstract

The aim of this study was to reveal some important physical properties of two lesser used wood species from Mozambique. Density of wood, green moisture content (MC), shrinkage, swelling, sorption–desorption behaviour and quantitative colour analyses were carried out to facilitate the potential use of Icuria dunensis (ncurri) and Pseudolachnostylis maprounaefolia (ntholo). The study found that the average densities at 12% MC were 907.1 kg m^?3 for ncurri and 1023.4 kg m^?3 for ntholo. The average values of green MC were 31.4% for ncurri and 39.2% for ntholo. Ncurri and ntholo wood showed low coefficients of anisotropy for heartwood, 1.3 and 1.4, respectively. The colour measurements described the patterns of radial and longitudinal variations in wood colour. In conclusion, ntholo and ncurri are characterized by high density and dimensional stability. Ntholo can be used where small dimensional changes are required, e.g. in joinery, flooring and furniture.     

Determining moisture-dependent elastic characteristics of beech wood by means of ultrasonic waves

The present study investigates the influence of moisture content on the elastic characteristics of beech wood (Fagus sylvatica L.) by means of ultrasonic waves. A set of elastic engineering parameters (i.e. three Young’s moduli, three shear moduli and six Poisson’s ratios) is determined at four specific moisture contents. The results reveal the significant influence of the moisture content on the elastic behaviour of beech wood. With the exception of some Poisson’s ratios, the engineering parameters decrease with increasing moisture content, indicating a decline in stiffness at higher moisture contents. At the same time, wood anisotropy, displayed by the two-dimensional representation of the velocity surface, remains almost unchanged. The results prove that the ultrasonic technique is suitable for determining the elastic moduli. However, non-diagonal terms of the stiffness matrix must be considered when calculating the Young’s moduli. This is shown experimentally by comparing the ultrasonic Young’s moduli calculated without, and allowing for, the non-diagonal terms. While the ultrasonic technique is found to be reliable to measure the elastic moduli, based on the measured values, its eligibility to measure the Poisson’s ratios remains uncertain.     

Modeling of the sorption hysteresis for wood

This paper deals with an adsorption/desorption model in order to predict evolutions of boundary conditions during the mass transfer process versus time in timber elements. This model is derived from a thermodynamic balance between the free water and its saturated vapor pressure, and is generalized for the bound water phase. It allows describing a realistic adsorption and desorption phenomena characterized by a moisture content hysteresis induced by cyclic variations of the relative humidity and the temperature. The sorption isotherm explains the equilibrium between the bound water phase in wood and the vapor pressure in the environment. The model includes different latent heats for the adsorption and desorption process. An analytic explanation allows to model partial variations in terms of relative humidity domain.     

Effects of fire-retardant treatment and wood grain on three-dimensional changes of sandwich panels made from bubinga decorative veneer

ABSTRACT

The effects of a fire-retardant treatment (FRT) and wood grain on three-dimensional changes of aircraft sandwich panels were evaluated. Unvarnished and varnished panels having the outer decorative layer made with bubinga (Guibourtia spp.) were studied. Half of the samples from each type of panel received an FRT (phosphate-based) on all three layers of the decorative plywood. The other half had the two inner layers treated and the outer layer untreated. Three different figures formed by the rotary cutting and grain orientation were identified and separately studied on veneer surfaces. Samples pre-conditioned to 20°C and 40% relative humidity (RH) underwent an adsorption (25°C, 90% RH) and then a desorption (25°C, 40% RH) treatments. Changes in moisture content (MC), swelling, shrinkage, roughness, and waviness were measured after each moisture exposure condition. The results showed that the FRT increased significantly MC, swelling, and shrinkage of unvarnished and varnished panels. This treatment as well as the type of wood figure affected roughness and waviness variations of unvarnished panels. However, the effects of these two factors were not noticeable once panels were varnished.     

Hygroexpansion of wood during moisture adsorption and desorption processes

In order to investigate the shrinking and swelling behavior of wood at a non-equilibrium state, the moisture sorptlon processes of wood under constant and changing conditions were studied. For the static sorption experiment, Chinese fir （Cunninghamia lanceolata） specimens were subjected to the adsorption processes at 25℃, 10 different relative humidity environments and the moisture contents were measured at distinct time intervals of adsorption processes. For the dynamic sorption experiment, the specimens were exposed to periodically and linearly varying relative humidity between 45% and 75% at 25℃. Moisture content as well as radial and tangential dimensional changes in response to the changing relative humidity were measured. The main results from the experiments indicated that： the moisture sorption isotherms of Chinese fir at equilibrium state and different stages of adsorption processes could be characterized by S-shape curves. From the non-equilibrium state to the equilibrium state, the sigmoid moisture sorption isotherms changed from smooth, gradually increasing values to a steep rise at 100% humidity. Furthermore, under dynamic conditions with a constant temperature and a linearly and periodically varying relative humidity, the moisture content as well as radial and tangential dimensional changes of the specimens generally waved but lagged behind the relative humidity change.     

Aging law of spruce wood

Abstract

Accelerated aging of spruce wood samples have been carried out by thermo-hydro (TH) treatments. These treatments were applied to accelerate the chemical reactions that take place during the natural aging of wood. In order to avoid dissimilar chemical reactions between the TH treatments and the natural aging, mild temperatures (between 100 and 150°C) have been selected at low relative humidity (RH). The mechanical properties of non-aged, natural aged and accelerated aged spruce wood have been compared. It is apparent that longitudinal Young's modulus of accelerated aged wood increase slightly at the beginning of the treatment and is followed by a reduction. Along the radial direction, Young's modulus remains almost unchanged. On the other hand the radial strength is severely reduced. From these results, the relative radial strength has been fitted on the chemical kinetic law. The rate constant of this law has been calculated and the treatment temperature and wood moisture content have been integrated. Finally this law has been extrapolated to standard climatic conditions in order to predict the loss of strength of old wood by knowing its age and its mean climate history of temperature and RH (ambiance condition).     

Influence of wood planing on the second-order effects of moisture sorption in sugar maple

Summary Two types of machines, a conventional planer, and a fixed-knife pressure-bar planer were used to prepare matched specimens of sugar maple wood. After adsorption and desorption, both experiments at 21 °C, the EMC, swelling in all principal directions as well as compliance coefficient in radial compression were measured. Two specimen sizes were used for these expe‐riments. For a given equilibrium moisture content, tangential and radial dimensions were greater after desorption than after adsorption, as previously described. When equilibrium was reached by gaining moisture, the wood was stiffer in radial compression compared to when the equilibrium was reached after losing moisture. The magnitude of this phenomenon, second-order effects of moisture sorption, was slightly affected by the type of planing. These effects on swelling were greater for large specimens prepared by conventional planing compared to fixed-knife pressure-bar planing. Small specimens showed similar magnitudes of this phenomenon with both planing methods. No differences between planing methods were found for the radial compliance coefficient measured on either specimen size. Therefore, the second-order effects of moisture sorption appeared to be a bulk phenomenon and not restricted to the superficial layers of wood. Received 9 December 1997     

Changes in physical properties of tropical and temperate hardwoods below and above the fiber saturation point

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.     

The effect of extractives on water-vapor sorption by wood

Summary The woods of different species vary widely in the nature and amount of their extractive content with a resultant variation in their sorption characteristics. This is particularly evident in the case of tropical woods, many of which are characterized by high extractive content. The nine species included in this study ranged from 3... 17 percent in extractive content. As determined from their desorption isotherms these species ranged from 20.5... 32.8 percent in fiber saturation point prior to extraction, and from 30.4... 38.0 percent after the removal of extractives with a series of neutral solvents. An analysis of variance showed that the variation in fiber saturation point among these species was reduced by about one-half following extraction in desorption and by approximately 7/8 in adsorption.The results of this study lead to several conclusions.The principal effect of extractives is to depress the sigmoid isotherm in the upper range of relative humidity, typically above 60 ... 70 percent. Isotherms of extracted and unextracted wood coincide very closely at relative humidities below about 60 percent.Based on these nine species, the increase in fiber saturation point resulting from the removal of extractives is greatest for a species with a low initial fiber saturation point and least for a species with a high initial fiber saturation point. Model equations accounted for 68 percent of the increase in desorption and 78 percent of the increase in adsorption. The remaining variance is presumed to be due to differences in chemical composition and physical organization of the skeletal cell wall.By employing the Hailwood-Horrobin model equation in analyzing the data, it was possible to partition the sorbed moisture as monomolecular and polymolecular sorption. The presence of extractives has very little effect on the amount of moisture sorbed as a monolayer on the internal surfaces of the cell wall (Langmuir sorption). In contrast, the sorption of moisture in polymolecular layers is reduced in greatly varying degree consistent with the theory of bulking action whereby the extractive substance within the cell wall precludes moisture from occuping the same space.By means of a regression analysis of monomolecular moisture vs. the constant M (a measure of the availability of bonding sites for sorption) in the Hailwood-Horrobin equation, it is evident that monolayer sorption increases with increased availability of bonding sites in both unextracted and extracted wood. By similar analysis polymolecular sorption also increases with increased surface activity in the case of extracted wood, but this effect is completely masked in the analysis of unextracted wood by the predominant bulking action. Total sorption, dominated by polymolecular sorption, shows similar relationships. Only in the case of essentially extractive-free wood can total sorption be expected to reflect the availability of molecular sorption sites and hence reveal variations in degree of crystallinity, cell wall compactness, or chemical composition to a significant degree.

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Zusammenfassung Die verschiedenen Holzarten schwanken hinsichtlich der Art und der Menge ihrer Extraktstoffe in weiten Bereichen, was sich in gleicher Weise in einer Schwankung ihres sorptions verhaltens ausdrückt. Diese Erscheinung tritt bei tropischen Hölzern besonders deutlich hervor, von denen eine ganze Reihe durch hohe Extraktstoffgehalte gekennzeichnet sind. Die in der vorliegenden Arbeit untersuchten neun Holzarten besaßen zwischen 3 und 17 Prozent Extraktstoffgehalt. Anhand der Desorptionsisothermen könnten bei diesen Holzarten die Fasersättigungspunkte vor der Extraktion zwischen 20,5 und 32,8 Prozent und nach der Extraktion mit einer Reihe von neutralen Lösungsmitteln, zwischen 30,4 und 38,0 Prozent liegend festgestellt werden. Eine Varianzanalyse zeigte, daß die Streuung der Fasersättigungspunkte der genannten Holzarten im Anschluß an die Extraktion für die Desorption auf etwa 1/2 und für die Adsorption auf rund 7/8 zurückging. Die Ergebnisse der vorliegenden Untersuchungen lassen folgende Schlüsse zu.Die Wirkung der Extraktstoffe zeigt sich allgemein in einer Abflachung der Isothermenkurve im oberen Bereich der relativen Luftfeuchtigkeit, besonders oberhalb 60 ... 70 Prozent. Die Isothermen der extrahierten und nichtextrahierten Hölzer liegen bei relativen Luftfeuchtigkeiten unterhalb von 60 Prozent verhältnismäßig nahe beieinander. Die mit den gewählten 9 Holzarten erzielten Ergebnisse zeigen, daßdie aufgrund der Extraktion eingetretene Erhöhung des Fasersättigungspunktes für die Holzarten mit ursprünglich niedrigem Fasersättigungspunkt am größten und für die Holzarten mit ursprünglich hohem Fasersättigungspunkt am geringsten war. Mit Hilfe entsprechender Gleichungen wurde für die Desorption ein Anstieg von 68 Prozent und für die Adsorption ein solcher von 78 Prozent errechnet. Die verbleibende Restvarianz wird auf Unterschiede im chemischen Aufbau und in der physikalischen Anordnung der Zellwände des Stützgewebes zurückgeführt.Die Anwendung der Hailwood-Horrobin-Modellgleichung zur Analyse der gefundenen Werte ermöglicht eine Unterscheidung der sorbierten Feuchtigkeit in eine monomolekulare und eine polymolekulare Sorption. Danach zeigt sich, daß das Vorhandensein von Extraktstoffen einen nur geringen Einfluß auf jene Feuchtigkeitsmenge ausübt, die als Monoschicht an die inneren Oberflächen der Zellwand sorbiert wird (Langmuir-Sorption). Im Gegensatz hierzu wird die Feuchtigkeitssorption in polymolekularen Schichten in allerdings sehr unterschiedlich hohem Grade vermindert. Dies steht in Übereinstimmung mit der Bulking-Action-Theorie, die besagt, daß die Extraktstoffe in der Zellwand die Feuchtigkeit daran hindern den gleichen Raum einzunehmen.Mit Hilfe einer Regression von der monomolekular adsorbierten Feuchtigkeit auf die Konstante M der Hailwood-Horrobin-Gleichung (=Maß für das Vorhandensein von freien Bindungsstellen für die Sorption) kann deutlich gemacht werden, daß die monomolekulare Sorption mit dem zunehmenden Vorhandensein von freien Bindungen sowchl im nichtextrahierten als auch im extrahierten Holz ansteigt. Mit Hilfe eines ähnlichen Analysenganges läßt sich zeigen, daß die polymolekulare Sorption bei extrahiertem Holz mit seiner gesteigerten Oberflächenaktivität ebenfalls zunimmt. Dieser Einfluß ist aber bei nichtextrahiertem Holz durch das Überwiegen der Bulking-Action vollständig überdeckt und daher nicht erkennbar. Die Gesamtsorption, die durch die polymolekulare Sorption wesentlich bestimmt wird, zeigt ähnliche Beziehungen. Allein im Falle vollständig extraktstoffreien Holzes kann man annehmen, daß die Gesamtsorption ein adäquates Bild von der Zugänglichkeit freier Bindungen für die molekulare Sorption wiedergibt, so daß hierdurch Schwankungen des Kristallinitätsgrades, der Zellwanddichte oder der chemischen Zusammensetzung in signifikanter Weise zu erkennen sind.

     

Behavior of the cellulose microfibril in shrinking woods

We measured the longitudinal and tangential shrinking processes in wood specimens from Chamaecyparis obtuse Endl. with different microfibril angles (MFAs). The shape of the shrinking curve was compared with the MFA. Only the longitudinal shrinking process of specimens with a small MFA clearly showed nonlinearity, and the degree of nonlinearity increased as the MFA decreased. In contrast, the tangential shrinking process and the longitudinal shrinking process of compression wood with a large MFA were linear. The nonlinearity is probably caused by the longitudinal shrinkage of the noncrystalline region of the cellulose microfibril (CMF) in regions of low moisture content during water desorption. When the moisture content is high, the matrix substance in the cell wall begins to dry; however, the shrinkage in the chain direction is restrained by the rigid CMF. As the wood dries further, the noncrystalline region of the CMF embedded in the matrix substance begins to shrink. Because the longitudinal mechanical behavior of wood with a small MFA is greatly affected by a rigid CMF, longitudinal shrinkage increases suddenly at about 10% moisture content; as a result, the shrinking process shows nonlinearity.     

稻秸/淀粉胶复合材料性能研究

为充分利用农作物废弃物,研究环境友好材料,通过热压成型方法制备稻秸/淀粉胶全降解复合材料。研究了预处理、胶黏剂、增塑剂和阻湿剂含量对复合材料内结合强度(IB)、弯曲强度(FS)、弯曲弹性模量(MOE)、冲击韧性(IR)、拉伸强度(TS)、拉伸模量(MOR)、含水率及2h吸水厚度膨胀率的影响,并观察了复合材料细观表面。结果表明:水热处理复合材料力学性能除IB外均较高;含胶量为10%时力学性能除MOR外均较高;1%增塑剂提高了IB、FS及TS,2%增塑剂提高了IS,增塑剂对MOE和MOR有负面影响;阻湿剂降低了复合材料力学性能。增塑剂、阻湿剂均能不同程度的增大复合材料MC。复合材料2h吸水厚度膨胀率大,防水性能差。复合材料基体和增强体两相界面结合存在明显缝隙、空洞及分层现象。     

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.