Non-steady state water adsorption of wood

Summary A rate equation of non-steady state adsorption for wood is theoretically discussed, and an adsorption equation is solved. This will clarify the mechanism of the mechano-sorptive process.Non-steady state water adsorption of wood cannot be represented by an equation which is derived from Fick's law, because the equation by Fick's law is similar to that of Langmuir's adsorption theory. Therefore, the adsorption process in non-steady state was discussed, based on the assumption that water molecules create a new secondary internal surface as the water adsorption proceeds. That is, the process of water adsorption was regarded as an auto-catalyzed reaction. Additionally, the effect of the ultra-structure of wood was also considered. Consequently, a rate equation of adsorption and an adsorption equation could be theoretically obtained which satisfactorily represented the process of water adsorption.     

Analysis of creep of wood during water adsorption based on the excitation response theory

The creep behavior of wood during water adsorption was mathematically analyzed based on the excitation response theory. The creep change _u (t), obtained by subtracting an instantaneous complianceJ _u (0) from a creep complianceJ _u (t), was linear in terms of moisture content at a steady state of moisture and was separable into two functions of time and moisture content. The creep compliance, however, was nonlinear. The creep change during water adsorption was obtained by applying the excitation-response theory to the creep change in a steady state of moisture. The equation was formally equal to the results reported so far. By using the derived equations, it was theoretically proved that the change in creep compliance during water adsorption from moisture contentu ₀ tou ₁ is always greater than the difference between creep compliance atu ₀ and that at u, in the steady state.     

Mechanical stresses generated by water adsorption in wood and their determination by tension creep measurements

Summary The creep of beech wood in tension perpendicular to the grain with a simultaneous increase in its moisture content was investigated. It was found that the value of increased swelling of the wood is proportional to the acting stresses. Such a relation facilitates indirect determination of the stresses necessary to complete swelling of wood under restraint. The course of adsorption stresses obtained in this way is consistent with the course of swelling pressure measured traditionally during moistening in humid air.Slightly altered version of a report presented at the 2nd Symp. Rheology of wood and wood structures at Rydzyna (Poland) 16 and 17 Oct. 1986     

Modeling the kinetics of moisture adsorption by wood

Summary Modeling of the kinetic of moisture adsorption by wood has been studied by using cubic samples. The model is based on an explicit numerical method with finite differences. Experiments have been carried out either for determining the data necessary for calculations (diffusivity, amount adsorbed at equilibrium) and for testing the validity of the model. Two different experiments have been done in case of the longitudinal adsorption: the one by increasing the relative humidity of the atmosphere following a discontinuous step by step process. The other by determining the kinetic adsorption of moisture by samples previously equilibrated under the same conditions when they are contacted with atmosphere at various R. H. Good correlations are obtained between calculated values and experiments in both cases. Although the actual paper is concerned with constant diffusivity, the model is capable to use concentration-dependent diffusivities.This work has been done with the help and support of the French C.T.B. (Wood Technical Center), 10 Av. St.-Mandé, Paris     

Swelling of acetylated wood II: effects of delignification on solvent adsorption of acetylated wood

To clarify the role of lignin in the affinities of acetylated wood for organic solvents, the effects of delignification on the solvent adsorption of acetylated wood were investigated. Acetylated wood meals rapidly adsorbed organic solvents that were hardly adsorbed by unmodified wood. For nonpolar and low-polarity organic solvents, a clear positive correlation was observed between the amount of adsorption and the lignin content. This indicated that acetylated lignin was responsible for the excellent affinities of acetylated wood for hydrophobic organic solvents. On the other hand, for lower alcohols and water, the amount of adsorption reduced with an increase in the lignin content. It was suggested that the adsorption of such polar solvents was dominated by insufficiently acetylated hydrophilic polysaccharides.     

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.     

Stress-strain state of wood at kiln drying

Summary A model has been suggested which allows the calculation of stresses arising in kiln drying and humidification of wood, as well as the total residual strain, i.e. set strain _s, consisting of purely residual strain _r and the so-called frozen strain _f. Frozen strains arise under the operating influence of loading when the stiffness is increased because of a decrease in moisture content (or temperature) of the wood. The process of formation of set strains _s has been shown to depend on the history of loading, variations of the moisture content of the wood, as well as cooling of the section before the measurement of _s. The possibility of using set strain as a parameter of the state of stress of wood in kiln drying has been noted.Presented at the VIII. International Symposium on Fundamental Research of Wood. Warsaw, Poland, October 1990     

Modelling water absorption in wood

Abstract

A fibre-level model for the longitudinal absorption of liquid water in wood has been developed. The model is primarily intended for simulation of absorption in softwoods. Capillary suction is based on the lumen radius, which is a stochastic parameter. The average lumen volume (and thus radius) is assumed to vary linearly across the annual ring and thus account for the difference in earlywood/latewood. The number of open bordered pits between fibres is also a stochastic parameter. The water flow rate is determined by the capillary suction and the flow resistance between fibres, i.e. the number of unaspirated bordered pits between fibres. The resulting pressure field in the liquid phase is calculated and the calculation has to be updated each time a new fibre has been filled with water. In this way the absorption is determined in a stepwise manner. Some general results are presented and the results resemble those obtained in experiments. It is shown that the model can predict features that a pure diffusion-based model cannot predict. Finally, the model is used for the simulation of an experiment that showed some unexpected results and the model gives a reasonable explanation.     

Diffusion of bound water in wood

Summary A model for isothermal bound water diffusion in wood is derived from Babbitt's (1950) analysis of diffusion in adsorbing solids. Calculations of the energy required for water molecules to become dissociated from their sites is identified as one component of the activation energy for diffusion. Consideration of the resistance to diffusion leads to a second component of activation energy associated with overcoming the attraction of water molecules for themselves. Also, an approximate expression for the resistance coefficient is developed. These results are combined into a transport model for bound water. The model shows that equations for bound water movement based on fluid mechanics (Babbitt 1950) and thermodynamics (Katchalsky, Curran 1965) are identical when the driving force for diffusion is defined as the moisture flux per unit transport coefficient. Activation energies and diffusion coefficients derived from the model compare favorably with literature values.The author wishes to thank Dr. Christen Skaar for his interest and advice during preparation of this paper for publication     

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     

Diffusion of bound water in wood

Summary Fick's law states that during steady-state diffusion, moisture flux is proportional to the gradient in moisture concentration, making concentration gradient the driving force for diffusion. Although Fick's law permits convenient comparisons between predicted and measured moisture contents, fundamental and experimental difficulties with it have been reported. In this paper, the dependence of spreading pressure on vapor pressure and temperature is developed. This information is used to show that gradients in spreading pressure and chemical potential are not proportional. Spreading pressure gradient is considered the correct driving force because the associated transport law can be derived from first principles.The author wishes to thank Dr. Christen Skaar for his interest and advice during preparation of this paper for publication     

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     

Thermal diffusion of bound water in wood

Summary There are few references in the wood science literature to nonisothermal moisture movement. Some experiments by Voight, Babbitt, and Choong indicate that thermal diffusion in wood may be very significant.Three equations are presented to represent nonisothermal moisture movement through wood in the transverse direction. The first, described in detail in a previous paper, is based upon two driving forces: Soret potential which results in thermal diffusion and chemical potential which results from a gradient of equilibrium relative humidity. All three equations include the same term for Soret potential and, in the second two equations, the thermal-diffusion term was derived by the application of activation theory. The isothermal term in these two equations utilizes moisture content gradient as the driving force and therefore a knowledge of the sorption isotherm is not necessary. The third equation contains an additional term for moisture-content activation to account for the increase in flux with an increased moisture content. All three equations give approximately the same result with the same input data at low moisture contents. The results diverge at high moisture contents and experimental data are required to determine which equation is most representative of the physical phenomenon of nonisothermal moisture movement.     

Phase transformations of wood cell wall water

The amount of apparently nonfreezing water per dry mass unit significantly differs between earlywood and latewood, and drying changes the nonfreezing water content of earlywood cell walls in a time-dependent manner. However, the equilibrium moisture content of spruce wood is not affected by drying and rewetting. The results indicate that different mechanisms govern these two types of phase transformations of cell wall water. The nonfreezing water content, as determined using differential scanning calorimetry, appears to be a nonequilibrium property. It is hypothesized that the measured changes in nonfreezing water content mostly reflect changes in the porous cell wall structure, on a scale well above the molecular scale, rather than the abundance of chemical adsorption sites.     

Simulation of effects of wood microstructure on water transport

A tracheid-level model was used to quantify the effects of differences in wood microstructure between coastal and interior Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii and var. glauca) wood on larger scale properties like hydraulic conductivity. The model showed that tracheid length, the ease of flow through a bordered pit and effective tracheid diameter can all limit maximum hydraulic conductivity. Among the model parameters tested, increasing bordered pit conductivity and tracheid length resulted in the greatest increase in maximum conductivity in both the inland and coastal ecotypes. A sensitivity analysis of the uncertainty between parameters governing flow through the bordered pit and air-seeding potential showed that, although decreased pit flow resistance increased maximum hydraulic conductivity, increased cavitation led to lower conductivity over time. The benefits of increasing the number of bordered pits depended on the intensity of the meteorological driving function: in drier environmental conditions, wood with fewer pits was more conductive over time than wood with more pits. Switching the bordered pit characteristics between coastal and interior wood indicated that the conductivity time course of coastal and interior wood was primarily governed by differences in the number of bordered pits and not differences in tracheid dimensions. The rate at which tracheids refilled had little effect on the conductivity time course of either coastal or interior wood during the first two summers when the wood was highly saturated, but had a marked influence in subsequent years once the cavitation profile stabilized. Our work highlights the need for more empirical work on bordered pits to determine whether variation in their number and properties is related to changing environmental conditions. In addition, a detailed simulation model of a bordered pit is needed to understand how variation in pit properties affects the relationship between ease of flow through a bordered pit and its potential for facilitating air-seeding.