Water vapour diffusion and adsorption characteristics of sugar maple (Acer saccharum,Marsh.) wood polymer composites

Summary Water vapour diffusion characteristics and adsorption isotherms were determined for cell-lumen and cell-wall treated wood polymer composites (WPC). The diffusion coefficients of the cell-lumen WPC were lower than untreated wood and the cell-wall WPC coefficients were lower than cell-lumen. Using the Hailwood and Horrobin sorption model, it was found that the unimolecular layer is formed at lower moisture contents in WPC than in wood. The amount of free dissolved water was reduced only in the cell-wall WPC. The polymer reduces the water vapour accessibility in both types of WPC.     

Short hold times in dynamic vapor sorption measurements mischaracterize the equilibrium moisture content of wood

Recently, the dynamic vapor sorption (DVS) technique has been used to measure sorption isotherms and develop moisture-mechanics models for wood and cellulosic materials. This method typically involves measuring the time-dependent mass response of a sample following step changes in relative humidity (RH), fitting a kinetic model to the data, and extrapolating the asymptotic mass. A series of steps covering the full RH range is used to generate the sorption isotherm. The majority of prior DVS data were taken with hold times of either 60 min or until the change in moisture content was <0.002% per minute over a 10-min period. Here, DVS measurements on wood and isolated wood polymers are presented where the hold times at certain relative humidity steps were much longer, ranging between 24 and 50 h. The data clearly show that the criteria for hold time in previous DVS measurements result in significant errors in prediction of the asymptotic mass. Although the data at short times are consistent with previous measurements, the data exhibit slow sorption behavior with characteristic times on the order of 500–2000 min that cannot be identified with shorter hold times. The results suggest that new hold time criteria need to be developed for dynamic vapor sorption measurements in wood.     

Gluability of thermally modified beech (Fagus silvatica L.) and birch (Betula pubescens Ehrh.) wood

Abstract

One of the main disadvantages of wood is hygroscopicity resulting from its polar character. The sorption–desorption of water causes unwanted swelling and shrinkage in wood. Thermal modification substantially reduces this inconvenient feature. Unfortunately, the same chemical changes that reduce water sorption alter the polar character of the material and result in poorer wetting of thermally treated wood by waterborne adhesives. Gluability of thermally modified beech (Fagus silvatica L.) and birch (Betula pubescens Ehrh.) wood with two commercial amino resins, melamine–urea–formaldehyde (MUF) and melamine–formaldehyde (MF), and a two-component polyurethane (PUR) adhesive was investigated. Both wood species were modified according to two temperature regimes: 160°C and 190°C. Shear strengths of the joints were then determined according to EN 205:2003 standard. The results showed that thermally modified beech and birch wood can be effectively glued not only with commercially available PUR adhesives, but also with aqueous MF and MUF resins. The resultant shear strengths of the joints were limited by the strength of the thermally modified substrate.     

白橡热压干燥材等温吸湿特性研究

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。     

Mid-infrared absorption properties of green wood

There is a lack of quantitative data on the penetration depth and the amount of energy absorbed by green wood under infrared (IR) radiation. This lack of knowledge is a potential barrier to the development of IR heating as an alternative to soaking as a means of warming logs prior to peeling in the manufacture of plywood. Experimental measurements of normal hemispherical spectral reflectance and transmittance over the range 550–5,500 cm^?1 wavenumbers on four wood species, beech, birch, Douglas-fir and spruce have brought new knowledge on mid-infrared absorption properties of green wood and removed some uncertainties. For instance, it is not possible to deliver energy deeper than up to 0.3 mm below the wood surface because 70–90 % of all incident IR radiation on the wood surface is absorbed in this layer. Some wood features, such as surface quality, the presence of knots and of free water in wood (the latter two having a more significant effect) influence the amount of energy absorbed. These results illustrate that IR radiation can heat the surface layers, but then heat penetrates deeper into the inside layers of wood by conduction.     

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.     

Preparation of acetylated wood meal and polypropylene composites I: acetylation of wood meal by mechanochemical processing and its characteristics

Wood meals of Sugi (Cryptomeria japonica D.Don) passing 2.0 mm and retained on 1.0 mm mesh screens were milled along with acetic anhydride (AA) and pyridine as a catalyst in a high-speed vibration rod mill at ambient temperature. The weight percent gain (WPG) of the chemically modified wood was calculated based on the yield after washing with deionized water. The effects of amounts of AA and catalyst added, pulverization time, and saponification of the acetylated wood on WPG were examined. In addition, FT-IR analysis, and water vapor adsorption and desorption tests were performed as functions of the WPG. Increases in WPG, the acetyl contents of the acetylated wood after saponification, changes in the FT-IR spectra after pulverization, and the water vapor sorption isotherms showed that the one-step acetylation systematically modified the hydroxyl groups of the wood into acetyl groups. Up to 38 % WPG was obtained at 100 phr AA and 15 phr catalyst, and 120 min pulverization. Pulverization time and the amounts of AA and catalyst added to the wood meals could be adjusted to obtain acetylated wood meal with the desired WPG. These demonstrated that the mechanochemical acetylation is a method to prepare acetylated wood meals with high WPG at less reaction time and required AA addition.     

A model for bound-water transport in wood

Summary A model for the isothermal transport of bound water through the cell wall of wood is developed, based on the assumption that the driving force for moisture movement is the gradient of spreading pressure , as first proposed by Babbitt (1950). This pressure is a surface phenomenon, derivable from the surface sorption theory of Dent (1977), a modification of the BET sorption theory. The force resisting moisture transport is assumed to be inversely proportional to moisture content and directly proportional to the equivalent viscosity of the sorbed water, calculated to be orders of magnitude larger than that of free water. The coefficients normally used to describe isothermal moisture transport in wood are derived from the model, and their predicted behavior as functions of the relative vapor pressure h of the cell wall are described graphically. An attempt is made to calculate a quantitative magnitude for the diffusion coefficient D, based on an assumed relationship between viscosity and the activation energy for water diffusion.     

Diffusion of bound water in wood

Summary Choong's (1963) data for isothermal sorption of water vapor by wood are used to compute pressures, chemical potentials, and entropies of water in the wood specimens of his nonisothermal mass equilibrium experiments. Entropies of both the bound water and water vapor were reasonably constant. A balance existed between thermal diffusion and mass diffusion, as indicated by gradients in temperature and chemical potential. This balance also is suggested by opposing gradients in spreading pressure and vapor pressure. Equal chemical potentials showed that the vapor and bound water were in equilibrium. The model proposed by Siau (1980) for nonisothermal diffusion is consistent with these results. Expressions are given for the two unknown parameters in this model: moisture conductivity and heat of transfer. The constant entropy of water vapor is used to show that the heat of transfer exceeds the activation energy for bound water diffusion by about 25 percent.The author wishes to thank Dr. Christen Skaar for his helpful comments during preparation of this paper for publication     

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.     

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.     

Effects of extraneous substances,wood density and interlocked grain on fiber saturation point of hardwoods

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.     

Vapor phase reaction of wood with maleic anhydride (I): dimensional stability and durability of treated wood

Reaction between maleic anhydride (MA) and wood specimens was carried out in a vapor phase reaction system. Reaction conditions such as the ratio of supplied MA to wood, initial moisture content, and reaction temperature were optimized. The MA supplied to the reaction system was effectively absorbed by the wood, and a satisfactorily high dimensional stability was achieved even at a low MA/wood ratio. The dimensional stability increased with rising initial moisture content. When the reaction was conducted at an elevated temperature (180°C), high dimensional stability was attained without remarkable weight increase and bulking. The mechanism of dimensional stabilization was discussed on the basis of the dimensional changes at high humidity and during repeated water soaking and drying. It was shown that the dimensional stabilization arises mainly from a decrease of hygroscopicity. When the reaction was conducted at 180°C, the formation of cross-links in the cell wall was apparent. Following the MA treatment, the antifungal property was remarkably enhanced and met the Japanese Industrial Standard K1571. Therefore, MA treatment in the vapor phase is an effective method to attain antifungal properties as well as high dimensional stability with a small amount of nontoxic reagent.     

A new method for nondestructive evaluation of solid wood moisture content based on dual-energy X-ray absorptiometry

This paper presents a new method to determine the moisture content of solid wood based on the principle of dual-energy X-ray absorptiometry. The study investigates the theoretical relationship between X-ray wavelength and mass attenuation coefficients of wood, water, and reference substance. In accordance with this relationship, a theoretically obtained equation is proposed to calculate the moisture content in wood. The proposed equation is compared to experimental results using small blocks of sugi wood, which showed that the change in mass attenuation coefficient of wood with X-ray tube voltage increased with increasing moisture content as expected from the theoretical equation. A regression equation for moisture content estimation was determined based on the experimental results, and the standard error of estimate in the 0–120 % dry-basis moisture content range using the regression equation was determined to be 21.9 % with the most appropriate pair of tube voltages, 15 and 40 kVp. The accuracy of the method will be improved by reducing the duration of X-ray radiography and by increasing the disparity between paired tube voltages. This method has the potential to determine moisture content of solid wood using X-ray without oven-drying or assuming oven-dry density.     

Moisture storage and transport properties of preservative treated and untreated southern pine wood

Moisture storage and transport properties of southern pine (Pinus spp.) wood were measured for implementation into hygrothermal models. Specimens were untreated or pressure-treated with alkaline copper quaternary (ACQ) preservative. Moisture storage was characterized with sorption isotherms in the hygroscopic region (high capillary pressures) and documented with mercury intrusion porosimetry in the overhygroscopic region (low capillary pressures). The data were then combined into a single moisture retention curve as a function of capillary pressure. Moisture transport was evaluated from steady-state water vapor transmission and dynamic capillary water absorption experiments. These data were used to calculate the moisture permeability over the entire range of capillary pressures using the diffusivity approach of Carmeliet et al. Moisture storage and transport properties were similar for the untreated and ACQ-treated southern pine, except for the permeability of the treated wood which was lower in the radial direction. The data presented here can be used to improve the accuracy of hygrothermal and combined hygrothermal–corrosion modeling simulations.     

The effect of openings on combined bound water and water vapor diffusion in wood

This study was undertaken to estimate the effect of openings between cell walls on combined bound water and water vapor diffusion in wood. Using a newly developed model, the radial and tangential moisture diffusion coefficients can be predicted depending on the opening area. The new model explicitly involves a term for water vapor diffusion through the openings, as well as a term for the combined diffusion of bound water and water vapor. A classical model developed by Stamm and Choong had higher longitudinal moisture diffusion coefficients than that in the parallel model at higher moisture content, which is inconsistent with the Wiener bound rule. The new model suggested in this article is useful for analyzing the experimental results and understanding the variability of the diffusion coefficients.     

Micromorphology,moisture sorption and mechanical properties of a biocomposite based on acetylated wood particles and cellulose ester

Abstract

One of the major issues in a long-term perspective for the use of wood–plastic composites (WPCs) in outdoor applications is the moisture sensitivity of the wood component and the consequent dimensional instability and susceptibility to biological degradation of the composite. In this work, the effects of using an acetylated wood component and a cellulose ester as matrix on the micromorphology, mechanical performance and moisture uptake of injection-moulded WPCs have been studied. Composites based on unmodified and acetylated wood particles, specially designed with a length-to-width ratio of about 5–7, combined with both cellulose acetate propionate (CAP) and polypropylene (PP) matrices were studied. The size and shape of the wood particles were studied before and after the processing using light microscopy, and the micromorphology of the composites was studied using a newly developed surface preparation technique based on ultraviolet laser irradiation combined with low-vacuum scanning electron microscopy (LV-SEM). The water vapour sorption in the composites and the effect of accelerated weathering were measured using thin samples which were allowed to reach equilibrium moisture content (EMC). The length-to-diameter ratio was only slightly decreased for the acetylated particles after compounding and injection moulding, although both the unmodified and the acetylated particles were smaller in size after the processing steps. The tensile strength was about 40% higher for the composite based on acetylated wood than for the composite with unmodified wood using either CAP or PP as matrix, whereas the notched impact strength of the composite based on acetylated wood was about 20% lower than those of the corresponding unmodified composites. The sorption experiments showed that the EMC was 50% lower in the composites with an acetylated wood component than in the composites with an unmodified wood component. The choice of matrix material strongly affected the moisture absorptivity of the WPC. The composites with CAP as matrix gained moisture more rapidly than the composites with PP as matrix. It was also found that accelerated ageing in a Weather-Ometer® significantly increased the moisture sensitivity of the PP-based composites.     

高温热处理木材吸湿特性

【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。     

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.     

Some considerations in heterogeneous nonisothermal transport models for wood: a numerical study

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.