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     

Viscoelastic properties of wood in swelling systems

Summary The torsion modulus and the mechanical damping were investigated on wood swollen with formamide and a series of glycols, at frequencies of 0.5 and 0.02 Hz as a function of temperature. In wood swollen with formamide to the same extent as it would swell when saturated with water, the temperature of maximum damping was about 48° and above 100°C for wood swollen with polyethylene glycols, while that of water saturated wood was 80°C. For more highly formamide swollen wood (1.2 times the swelling in water) the temperature at which maximum damping develops decreased to 30°C. With regard to the influence of swelling and temperature on the torsion modulus of wood, three regions of viscoelastic behavior were recognized in these swelling systems. They are the glassy region in non-swollen wood, where the torsion modulus decreases gradually with increasing temperature, the transition region where the torsion modulus decreases abruptly with increasing temperature and swelling, and a plateau region appearing at high temperatures for highly swollen wood where the torsion modulus remains fairly constant with temperature with a value of about one tenth the modulus for non-swollen wood.     

Laminar sorption and swelling theory for wood and cellulose

Summary The known requirements for the sorption and swelling of wood are reviewed. These are shown to be compatible, in the case of softwoods, with the following simplified model. The fibers are continuous with either rectangular or circular cross sections and lumen of the same shape with a constant size. The fiber walls consist of concentric lamina made up of small repeating units 100 Å by 100 Å, consisting of a microcrystalline core surrounded by an amorphous sheath. All sorption and swelling occurs at the surfaces of or within the amorphous sheath. The major portion of the sorption and swelling is inter-laminar with just sufficient intra-laminar sorption and swelling to avoid laminar distortion. Calculations give the generally accepted internal sorption surface for water of about 200 square meters per gram. The portion of the total sorption that is intra-laminar varies from 5 ... 20 percent in going from wood with a specific gravity of 0.3 ... 1.0. Thickness of sorption in water molecules per sorption site vary from 6.1 ... 4.9 for inter-laminar sorption and 0.35 ... 1.35 for intra-laminar sorption in going from wood with a specific gravity of 0.3 to one of 1.0. Similar values are obtained from experimental swelling data where lumen cross sections change. Similar calculations for super swelling of wood and pulps show that laminar separations may become sufficiently great to be microscopically visible. The calculations show that bound water fiber saturation points for wood normally fall in the range of 25 ... 40 percent. Super swollen wood, chemically isolated fibers and beaten fibers may as a result of reduced restraints have fiber saturation points greater than 150 percent. The latter are attributed to dispersion or diffusion forces rather than the conventional bound water forces of hydrogen bonding for intact wood.

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Zusammenfassung Die bekannten Bedingungen für die Sorption und Quellung von Holz werden erörtert. Sie sind auf Nadelholz unter Anwendung des folgenden, vereinfachten Modells anwendbar. Die Fasern sind durchgehend und haben rechteckigen oder kreisförmigen Querschnitt, mit Zellhohlräumen gleicher Form und konstanter Größe. Die Faserwände bestehen aus konzentrisch angeordneten Schichten, die wiederum aus kleinen, sich wiederholenden 100 ×100 großen Einheiten zusammengesetzt sind; diese wiederum bestehen aus einem mikrokristallinen Kern, umgeben von einer amorphen Auflagerung. Alle Sorptions- und Quellungsvorgänge finden an der Oberfläche oder innerhalb dieser amorphen Auflagerung statt; sie spielen sich in der Hauptsache in der Schicht selbst ab (inter-laminar), jedoch findet genügend Zwischenschicht-Sorption und-Quellung (intra-laminar) statt, um Verformungen der Schichten zu vermeiden. Durch Berechnung erhält man die allgemein anerkannte Größe der inneren Sorptionsfläche für Wasser von etwa 200 m²/g. Der Anteil der intra-laminaren Sorption an der Gesamtsorption schwankt zwischen 5 und 20% bei Holz mit Rohdichten von 0.3 bis 1.0. Die Schichtdicke der sorbierten Wassermoleküle je Sorptionsstelle liegt für inter-laminare Sorption zwischen 6.1 und 4.9, und für intra-laminare Sorption zwischen 0.35 und 1.35 bei Holz mit Rohdichten zwischen 0.3 bis 1.0. Ähnliche Werte ergaben sich aus experimentell ermittelten Quellungsdaten bei Zellhohlräumen mit sich änderndem Querschnitt. Vergleichbare Berechnungen der Super-Quellung des Holzes und des Zellstoffes zeigen, daß laminare Abtrennungen so groß werden können, daß sie mikroskopisch sichtbar werden. Sie zeigen ferner, daß die Fasersättigungspunkte bei gebundenem Wasser meist zwischen 25 und 40% liegen. Extrem gequollenes Holz, chemisch herausgelöste und gemahlene Fasern können aufgrund verringerter Behinderung Fasersättigungspunkte über 150% erreichen. Diese letztere Erscheinung ist eher den Dispersions- oder Diffusionskräften zuzuschreiben als den Kräften aus Wasserstoffbrücken des gebundenen Wassers im intakten Holz.

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Paper No. 2743 of the Journal Series of the N. C. State University Agricultural Experiment Station, Raleigh, North Carolina.

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Anselme Payen award paper of the Cellulose, Wood and Fiber Div., Am. Chem. Soc. presented at their 156th National Meeting in Atlantic City, New Jersey, September, 1968.     

Polyelectrolyte swelling behavior of chlorite delignified spruce wood fibers

Summary A coarse thermomechanical Asplund pulp was prepared from Norway spruce (Picea abies). The pulp was delignified to different degrees using acidified sodium chlorite. The swelling behavior (measured as water retention value=WRV) of the resulting pulps was studied under various chemical conditions (pH and conc. of NaCl). It is shown that chlorite-delignified pulps have an appreciable polyelectrolytic character. Whereas the WRV of an Asplund pulp does not respond to changes in the chemical environment, the delignified pulp has a WRV of 155 at pH 3 and 250 at pH 9. Compared under the same chemical conditions, the WRV increases with increasing degree of delignification (<70%) and is approximately constant at high degrees of delignification (>70%).The authors are indebted to Mr. Claes Beer at Sund-Defibrator, who assisted with the preparation of the Asplund pulp     

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 hygro-mechanical analysis of poplar wood along the tangential direction by restrained swelling test

A better understanding of the hygro-mechanical behaviour of poplar wood (Populus alba L.) is proposed by means of restrained swelling tests. Small clear specimens were tested along their tangential direction in controlled climatic conditions; a dry climate (30 % relative humidity and 30 °C temperature) and a humid one (80 % relative humidity and 30 °C temperature) were cyclically set and kept constant each during 7 days. In these conditions, three matched specimens were tested at the same time along the tangential direction: specimen A free to shrink and swell, specimen B free to shrink and prevented from swelling beyond its initial dimension, specimen C prevented from shrinking and swelling. A specific device was set up to monitor strain and/or stress on each specimen. During the adsorption phase, compression forces (on specimens B and C) were measured, and during the desorption step, compression set shrinkage (compression strain at zero load) and tension stress (on specimens B and C, respectively) were measured. In this paper, the dedicated experimental device is presented and a deep analysis of experimental curves is then proposed by means of a rheological approach. By the analysis, some basic data are obtained (swelling coefficient, swelling pressure and compression set shrinkage), and a complex behaviour of wood made of reversible and irreversible phenomena is characterized.     

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).     

建筑材防腐处理工艺及其质量的研究

用高浓度防腐剂以浸渍-扩散法、满细胞法和双真空法对木结构房屋用材进行处理研究,确定了经济、实用的处理工艺。此外,按照国家标准《木材物理力学试验方法》(GB1927～1943—80)对鱼鳞云杉防腐处理材和对照材的主要力学性质进行对比测定,分析防腐处理对材质的影响,并用扫描电镜能谱仪分析测定CCA三组分在木材内部的分布,从微观角度评价处理工艺与防腐材的质量关系。     

谈木家具生产成本的控制

就木家具生产的全过程,从产品设计、材料供应和产品加工3个方面论述木家具生产成本的控制。     

Effect of structural direction and initial moisture content on swelling rate of wood

Summary The swelling rate of Engelmann spruce (Picea engelmannii Parry) in water was determined on cross sectional, quartersawn, and flatsawn samples. The rate of swelling from six initial moisture content levels to saturation was measured in both the tangential and radial directions.Internal restraints were found to influence the swelling rate of the quartersawn and flatsawn samples in their radial and tangential directions, respectively. These restraints occurred when swelling commenced from a low initial moisture content and were attributed to the compressive forces imposed on the outer swelling surface by the dry unswollen interior of the wood.The restraining effect was also obvious in the change with time of the ratio of tangential-to-radial swelling at the lower initial moisture content levels. In the case of the quartersawn samples, swelling from oven-dry to saturation, the ratio was at first approximately 3.5: 1.0, but decreased with time to an equilibrium ratio 2.2: 1.0. The ratio for the flatsawn samples, swelling from zero to saturation, was at first approximately 0.58: 1.0, but increased with time to about 2.0: 1.0. No obvious effects of internal swelling restraints were observed during the swelling of the cross sectional samples.The tangential-to-radial swelling ratios for all three types of samples increased with increasing initial moisture content. This indicated that swelling at the higher levels of moisture content may have been largely in the tangential direction.

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Zusammenfassung Die Quellungsgeschwindigkeit von Engelmannsfichte (Picea engelmannii Parry) in Wasser wurde an Querschnitten sowie an radial und tangential geschnittenen Proben dieses Holzes bestimmt. Ausgehend von sechs verschiedenen Anfangs-Holzfeuchtigkeiten wurde die Quellungsgeschwindigkeit bis zur Sättigung in tangentialer und in radialer Richtung gemessen.Innere Behinderungskräfte beeinflußten, wie festgestellt werden konnte, die Quellungsgeschwindigkeit der radial bzw. tangential geschnittenen Proben in tangentialer oder radialer Richtung. Diese Quellungsbehinderung trat vor allem im Falle niedriger Anfangs-Holzfeuchtigkeiten in Erscheinung und wurde jenen Druckkräften zugeschrieben, die den äußeren quellenden Schichten durch die noch nicht gequollenen inneren Holzteile entgegenwirken. Die Wirkung der Behinderung war ebenso an der änderung der Zeitkomponente des Quellungsverhältnisses tangential/radial bei niedrigen Anfangs-Holzfeuchtigkeiten deutlich zu erkennen. Bei der Quellung der Radialproben vom Darrzustand bis zur Sättigung betrug dieses Verhältnis zunächst 3,5: 1,0 und sank dann mit der Zeit bis zu einem Ausgleichsverhältnis von 2,2: 1,0. Bei der entsprechenden Quellung der Tangentialproben von Darrtrockenheit bis zur Sättigung waren die Werte zuerst 0,58: 1,0, stiegen dann aber mit der Zeit auf etwa 2,0: 1,0 an. Bei den quer geschnittenen Proben war dagegen keine Auswirkung behindernder, innerer Kräfte auf die Quellung festzustellen.Bei allen drei Probenarten wurde das tangential/radial-Quellungsverhältnis mit ansteigender Anfangs-Holzfeuchtigkeit größer. Dies ist ein Hinweis dafür, daß bei höheren Anfangs-Holzfeuchtigkeiten die Quellung überwiegend in tangentialer Richtung erfolgt.

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Prepared for the meeting of International Union of Forestry Research Organizations, Section 41 Working Group on Wood Physics, Paris, France, June 1, 1966.

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Contribution from the Department of Wood Science and Technology, North Carolina Agricultural Experiment Station, Raleigh, N. C. Published with the approval of the Director of Research as Paper No. 2186 of the Journal Series. This research was done by the author at North Carolina State University (under the direction of Alfred Joaquin Stamm) as part of a National Science Foundation research grant.

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Maintained at Madison, Wis., in cooperation with the Univeristy of Wisconsin.     

Review of the sufficiency of research on the swelling pressure of wood

Summary Terms used in relation to swelling pressure are first defined. The present status of knowledge of the swelling pressure of wood under various conditions is then briefly discussed.Methods are outlined for the determination of swelling pressure with restraint in one direction only and their limitations discussed. The method developed by Tarkow and Turner for the determination of swelling pressure when restraint is present in two directions is also referred to and outlined.Conclusions are then drawn from the present status of knowledge of swelling pressure in wood and gaps in this knowledge are indicated.     

Identification of anisotropic vibrational properties of Padauk wood with interlocked grain

Grain deviations and high extractives content are common features of many tropical woods. This study aimed at clarifying their respective impact on vibrational properties, referring to African Padauk (Pterocarpus soyauxii Taub.), a species selected for its interlocked grain, high extractives content and uses in xylophones. Specimens were cut parallel to the trunk axis (L), and local variations in grain angle (GA), microfibril angle (MFA), specific Young’s modulus (E′ _L /ρ, where ρ stands for the density) and damping coefficient (tanδ_L) were measured. GA dependence was analysed by a mechanical model which allowed to identify the specific Young’s modulus (E′₃/ρ) and shear modulus (G′/ρ) along the grain (3) as well as their corresponding damping coefficients (tanδ₃, tanδ_G). This analysis was done for native and then for extracted wood. Interlocked grain resulted in 0–25° GA and in variations of a factor 2 in E′_L/ρ and tanδ_L. Along the grain, Padauk wood was characterized, when compared to typical hardwoods, by a somewhat lower E′₃/ρ and elastic anisotropy (E′/G′), due to a wide microfibril angle plus a small weight effect of extracts, and a very low tanδ₃ and moderate damping anisotropy (tanδ_G/tanδ₃). Extraction affected mechanical parameters in the order: tanδ₃ ≈ tanδ_G > G′/ρ > > E′₃/ρ. That is, extractives’ effects were nearly isotropic on damping but clearly anisotropic on storage moduli.     

Solute diffusion into cell walls in solution-impregnated wood under conditioning process III: effect of relative humidity schedule on solute diffusion into shrinking cell walls

This study has focused on solute diffusing into cell walls in solution-impregnated wood under conditioning, process of evaporating solvent. The purpose of this paper was to clarify the RH- (relative humidity-) schedule that promotes the solute diffusion into shrinking cell walls during conditioning. The wood samples impregnated with a 20 mass% aqueous solution of polyethylene glycol (PEG1540) was conditioned with a temperature of 40?°C to the equilibrium point at the RH where the samples swelled maximally. The samples were subsequently conditioned at 40?°C under the schedules including four ways of RH-decrease steps where the cell walls shrunk. The amount of solute (PEGs) diffused into cell walls during the conditioning logarithmically increased with increasing the number of the RH-decrease steps. This was well explained by the theoretical model that describes the solute diffusion into shrinking cell walls. It is clarified from the model that the RH, or moisture content of the sample, should be decreased as gradually as possible to increase the total amount of diffused solute into shrinking cell walls, and that the amount of diffused solute is smaller for the lower moisture content. The model also suggests that effect of change in RH schedule on change in total amount of diffused solute does not depend on solute diffusivity in the sample under drying in a vacuum over phosphorous pentoxide, and that impregnated wood should be conditioned under natural convection rather than forced convection for promoting the diffusion into shrinking cell walls.     

Shrinkage and swelling behavior of archaeological waterlogged wood preserved with slightly crosslinked sodium polyacrylate

In this article, the conservation of archaeological waterlogged wood (WW) of Afzelia sp. (medium degraded: U_max = 385%) was impregnated in vacuum with an aqueous solution of acrylic acid (AA), sodium acrylate monomer (AANa), crosslinking agent (MBA) and catalyst (V-501). The simultaneous in situ polymerization and crosslinking resulted in slightly crosslinked sodium polyacrylate (PAANa) in the wood structure. The results showed that untreated WW had only a very limited ability to re-swell to recover its original dimensions from a collapsed condition, while WW protected by crosslinked PAANa could almost fully recover its original shape and size even after several drying–rewetting cycles. From microscopic observations, treated wood was found to maintain its original cell structure, form and shape even after repeated drying–rewetting cycles. PAANa was observed to be densely localized near the middle lamella, the cell corners, and the cell lumina by transmission electron microscopy observation. These observations suggest that our PAANa treatment provides reasonable strength as well as favorable hydrophilicity to avoid hornification of the cell wall upon drying, thus providing unique shape recovery properties.     

The effects of cell wall swelling on the permeability of grand fir wood

Summary The longitudinal permeability of Abies grandis wood swollen to varying degree by water vapour sorption has been measured using a non-swelling liquid. The results obtained show that cell wall swelling causes a reversible decrease in permeability in both air dried and solvent exchange dried specimens.Direct microscopic measurements have shown that change in lumen diameter cannot explain the observed effect.Mathematical analysis of the data suggests that the permeability decrease may be due to increase in thickness of bordered pit membrane fibrils resulting from sorption of water.