The applicability of orifice flow and drag theory to the axial flow of gases through conifer wood

Summary Orifice flow and drag theory are found theoretically to be generally inapplicable to axial gas flow through conifer wood under the conditions normally used for gas permeability studies of the material. However, orifice flow theory might be applicable at higher mean pressures or at higher applied pressure drops. Methods of testing this possibility experimentally are discussed. The authors wish to acknowledge the encouragement of Professor L. Roche. Paper presented at the seminar on “Flow and Permeability in Wood” of the Royal Microscopical Society, Wood Anatomy Group, held in Princes Risborough, 17. October 1979     

Laminar flow of fluids through short capillaries in conifer wood

Summary The viscous and slip flow terms of the Hanks-Weissberg and Petty-Puritch gas flow equations are compared. A Couette correction of 3/8 x radius must be added to the capillary length in the viscous term of the latter equation. The slip flow terms in the two equations are equivalent to within ±10%. A modified method is presented for calculating the radius and number of short capillaries from gas flow measurements. The applicability of this method was confirmed by measurements of gas flow through artificial membranes containing short capillaries. Earlier published values for the number and radius of pit membrane pores are recalculated using the modified method. The possible effects of very close spacing of pores in a membrane are discussed and it is suggested that the Couette correction is probably still applicable in this case.This work was supported in part by a Research Grant from the Science Research Council.     

A model of axial flow of organic liquids in two softwoods

Summary The observed axial flow of an inert fluid (heptance, octane, etc.) in Scots pine and spruce over lengths between 0.05 m and 0.4 m was correctly represented by a phenomenological model consisting of series of elements in which flow takes place, each element comprising a stagnant zone. Moreover, there is partial recycling of the fluid from the final to the initial element. The volume fraction occupied by the stagnant zone increases sharply with the length of the sample and this variation is probably the reason for the increased lineic pressure drop with the length of the samples. Test using pentachlorophenol as a tracer showed that there is no irreversible fixation of this compound on sapwood of Pinus sylvestris.     

Diffusion of non-swelling gases through dry conifer wood

Summary The conditions under which mutual, Knudsen and transition-range diffusion occur are described, and the corresponding gaseous diffusion coefficients are stated. The coefficient of hindered diffusion through pit membrane pores is shown to be about one third of the mutual diffusion coefficient and not about one thirtieth as postulated by earlier workers. A new longitudinal diffusion equation is developed, and results of a new gas permeability technique are used to derive coefficients for Picea sitchensis and Abies grandis. Only a small proportion of the tracheids is assumed to be conducting, owing to pit aspiration. About 98% of the resistance to longitudinal diffusion is in the tracheid lumina, 1% in the pit apertures and 0.2% in the pit membrane pores. From similar considerations a new tangential diffusion equation is developed, and coefficients are calculated. Roughly 90% of the resistance to tangential diffusion occurs in the pit apertures, 10% in the pit membrane pores and only 0.3% in the tracheid lumina. The agreement between the theoretical and published experimental values is satisfactory. For tangential diffusion the agreement is closer by an order of magnitude than that obtained by earlier workers. The experimentally determined temperature dependence of the diffusion coefficients is shown to be consistent with the above results. A hindered diffusion coefficient of one thirtieth of the mutual coefficient would lead to a temperature dependence not observed experimentally.     

A model for unsteady-state gas flow in the longitudinal direction of wood

The longitudinal unsteady-state flow of compressible fluids in wood is modeled on a basis of series elements of differing permeabilities and three zones acting in parallel. Two of these zones having widely differing permeabilities are capable of varying degrees of intermixing while a third low-permeability zone is isolated from the other two. A good agreement between theoretical and experimental results was achieved by the appropriate selection of the parameters.This research was supported by the National Science Foundation, Grant No. ENG 69-0056-A03.     

Growth of Phlebiopsis gigantea in wood of seven conifer species

Heterobasidion parviporum and Heterobasidion annosum are widely distributed root‐rot fungi that infect conifers throughout Europe. Infection of conifer stumps by spores of these pathogens can be controlled by treating fresh stumps with a competing non‐pathogenic fungus, Phlebiopsis gigantea. In this study, growth of three Latvian strains of P. gigantea and the biological control agent ‘Rotstop’ strain was evaluated in stem pieces of Norway spruce, Scots pine, lodgepole pine, Douglas‐fir, Weymouth pine, Siberian larch and Sitka spruce. The growth rates of one H. parviporum and one H. annosum isolate were also measured in the same stem pieces. The growth rate of P. gigantea varied greatly in wood of different conifer species. It was higher in the three pine species, lower in Norway spruce and lowest in Sitka spruce and Siberian larch, and in Douglas‐fir, this fungus did not grow. The largest area of wood occupied by P. gigantea was in lodgepole pine. Growth of Latvian isolates of P. gigantea in the wood of Pinus and Picea species was comparable to that of the Rotstop isolate. Consequently, stump treatment with local P. gigantea isolates should be recommended. However, our results suggest that Douglas‐fir stump treatment against Heterobasidion by P. gigantea may be ineffective and other stump treatment methods should be considered.     

Micromechanics of wood subjected to axial tension

Summary The behaviour of a small group of wood fibers of Sitka spruce during tensile loading is investigated. The load-extension curves for both early and late wood fibers consist of three distinct segments. The first segment is almost a straight line, at some stage of loading a yield point is observed. Beyond this point the specimen becomes less stiff and undergoes a large, mainly irreversible deformation. As the load is increased further, the curve exhibits the third segment showed by a significant change in slope. These curves look different from those obtained on thick specimens. In this respect, the behaviour of a thin wood specimen subjected to cyclic type tensile loading along its longitudinal direction is also illustrated. Based on wood microstructure, a model is presented to interpret the evolution of the Young's modulus of a wood fiber during tensile loading. The model considers wood as an assembly of cylindrical fibers pasted together in a longitudinal direction. We have assumed the cell wall to comprise only an S₂ layer made of a composite material consisting of a lignin and hemicellulose matrix reinforced by helical microfibrils along the fiber. Furthermore, it is assumed that the microfibril angle a in the S₂ layer is not uniform along the fiber axis and matrix degradation occurs in the zones where the microfibril angles are bigger. The validity of this assumption is verified by using holographic interferometry to visualize the displacement field of the specimen's surface under tension.The work reported in this paper is supported by a grant from the Swiss National Science Foundation, and its support is gratefully acknowledged     

Preparation of synthetic wood composites using ionic liquids

Synthetic wood composite films containing cellulose, hemicelluloses, and lignin, the three major components of natural wood, were prepared in a room temperature ionic liquid solvent, 1-ethyl-3-methylimidazolium acetate, [EMIM][Ac]. Various synthetic wood composites were obtained by dissolution of individual wood components together with additives, including polyethylene glycol (PEG), chitosan, and multi-wall carbon nanotubes (MWNTs) in [EMIM][Ac]. The addition of water affords a gel that was dried in either a low humidity environment or under vacuum. Synthetic wood films showed smoother surface textures, higher water resistance, and higher tensile strengths than cellulose films formed by the same methods. Tailor-made synthetic wood composites were also prepared having a variety of desirable properties, including antimicrobial activities, controlled hydro-phobicity/philicity, high relative dielectric constant, and a high degree of cohesiveness.     

A global rheological model of wood cantilever as applied to wood drying

In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep. The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood, the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time. The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process.     

A note on a possible source of error in maximum effective pore radius determination in conifer wood

Summary With coniferous timbers, if the fluid system selected for use in maximum effective pore radius determinations has too high an interfacial surface tension, the pressure differential required for the determination can be large enough to cause partial or total pit membrane displacement. Under these circumstances, the results of the method must be interpreted with care.The authors wish to acknowledge the encouragement of Professors Roche and Matthews     

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.     

A model of supplying poplar wood for Iranian paper ＆ wood factories

The objective of this research is to design a method for an appropriate alternative to supply poplar wood as raw material for wood and paper factories in Iran.Due to the destruction of the forests and lack of proper plantation,replacement of the forest wood by the fast growing wood is vital to satisfy all requirements of these factories.A hierarchy was used to prioritize benefits,opportunities,costs and risks (BOCR) using the Analytic Hierarchy Process (AHP) ratings approach.To evaluate the "control criteria" of the system,a control hierarchy was also created and prioritized by applying the Analytic Network Process (ANP).A total of four major control criteria in the system are prioritized where each one controls a network structure evaluated by using ANP.The final synthesis results of the system showed that internal poplar tree farming supplied by the forest product factories was the best choice among three potential alternatives (factory procurement,external procurement and mix procurement).     

Swelling of acetylated wood I. Swelling in organic liquids

To investigate the affinity of acetylated wood for organic liquids, acetylated yezo spruce wood specimens were soaked in various liquids, and their swellings were compared to those of untreated specimens. The acetylated wood was rapidly and remarkably swollen in liquids having low hydrogen bonding power such as benzene and toluene in which the untreated wood was swollen only slightly or very slowly. On the other hand, the swollen volume of wood in water, ethylene glycol, and alcohols remained unchanged or slightly decreased after the acetylation. The effect of acetylation was greater in liquids having smaller solubility parameters. The easier penetration of aprotic organic liquids into the acetylated wood was considered to be due to the reduction of polarity and the scission of hydrogen bonds in the amorphous wood constituents where the hydrophilic hydroxyl groups were substituted by hydrophobic acetyl groups.     

Temperature dependences of the dynamic viscoelastic properties of wood and acetylated wood swollen by water or organic liquids

In this study, to summarize the changes of thermal-softening behaviors of wood and acetylated wood due to differences in the kinds of swelling liquids, the following measurements were conducted. Untreated and acetylated wood samples were swollen by various liquids and the temperature dependences of the dynamic viscoelastic properties were measured after the heating and cooling histories were unified among the samples. The results obtained are as follows. Untreated samples swollen by high-polarity liquid had lower peak temperature of tanδ, however acetylated samples had higher peak temperature of tanδ than those of untreated wood. On the other hand, untreated wood samples swollen by low-polarity liquid had higher peak temperature of tanδ, however acetylated samples had lower peak temperature of tanδ than those of untreated wood. The amount of swelling is determined by interaction between wood and liquid due to proton-accepting power and molar volumes of liquid and so on, therefore the peak temperature of tanδ and degree of reduction in dynamic elastic modulus (E´) with increasing temperature were corresponded to the amount of swelling.     

A model of the anisotropic swelling and shrinking process of wood. Part 1. Generalization of Barber's wood fiber model

Summary The anisotropic mechanism of wood swelling and shrinkage was investigated theoretically. The reinforced-matrix hypothesis offered by Barber and Meylan (1964) was reformulated by using a multi-layered wood fiber model, and the formula describing the dynamical behavior of swelling and shrinking wood fiber was derived. For modelling, the moisture content changes were taken into consideration as an explicit parameter. It is expected to predict the anisotropic swelling and shrinking process of wood dynamically and quantitatively, as well as to initiate elucidating the interaction between the moisture and cell wall components on the basis of the present model. Some concrete calculations will be demonstrated in the following report, and the results will be compared with the experimental ones. Received 2 September 1997     

A model of anisotropic swelling and shrinking process of wood

To elucidate the origin of the shrinking anisotropy of wood during the drying process, as well as to begin to gain an understanding of the interaction between the moisture and the cell wall components, the shrinking process of a single wood fiber regarding water desorption was simulated by using an analytical model which was developed in the previous report (Part 1). Resulting data were compared with the experimental ones in this paper. The following conclusions were obtained: (1) The matrix substance, as a skeleton in the secondary wall, tends to shrink isotropically. However, the cellulose microfibrils, as a rigid framework of the cell wall, almost did not shrink at all due to the water desorption. As result, wood shrinks anisotropically during a drying process. The microfibril angle in the S2 layer is one of the most important factors related to the degree of shrinking anisotropy of the wood while drying. (2) According to the simulation, the expansive strain caused in the matrix skeleton by the water sorption increases by 15% (= 150,000 micro-strains) from the oven-dried condition to the green condition. Based on this value, the moisture content at the fiber saturation point is calculated to be about 35%, which is close to the experimentally obtained one. These results give quantitative evidences that the hygroexpansion of the wood cell wall is controlled by the mechanism of the reinforced matrix hypothesis. Received: 28 July 1998     

Improvement of capillary penetration of liquids into wood by use of supersonic waves

Summary The effect of supersonic waves on the capillary penetration of liquids into wood was found to be positive or negative depending upon a number of factors. The penetration of non-polar compounds like paraffins and aromatic hydrocarbons was not improved by using supersonic waves. The effect of supersonic waves, however, became positive by introducing one or several hydrophilic groups into straight hydrocarbon chains.Diluting hydrocarbons containing hydrophilic groups with non-polar solvents reduced the positive effect of supersonic waves until it became negative below a certain concentration. Polyfunctional alcohols exhibited an optimum effect at certain concentrations when diluted with water. These concentrations correspond to alcohol-water blends of maximum affinity to the surface of wood.Supersonic waves improved the capillary penetration of water when the surface tension was reduced by using surface active agents.In fields were the rapid and thorough impregnation of wood by various liquids is important, supersonic waves can under certain conditions aid this process and be of practical use.