Deposits in heartshakes in wood

Summary Crystalline or almost pure organic compounds can be formed either alone or with two or three related compounds in juxtaposition in shakes in central heartwood. The compounds may not always be present in the more complex extractives formed at the heartwood periphery. The largest amounts of deposits are in the innermost or widest portion of the shake. No evidence could be found of a source of substrate, energy or specific enzymes necessary for the selective biosynthesis of these deposits. They are considered to be formedin situ by an undetermined mechanism during enlargement of the shake.     

Differences in wood properties between juvenile wood and mature wood in 10 species grown in China

 This study examined the intrinsic differences in various wood properties between juvenile wood and mature wood in 10 major reforestation species in China. Comparisons between juvenile wood and mature wood were made in both plantation- and naturally-grown trees. Considerable differences in most wood properties were found both between plantation-grown juvenile wood and mature wood, and between naturally-grown juvenile wood and mature wood. This suggests that wood properties of plantation-grown trees, to a large extent, depends on their juvenile wood contents, and can thus be manipulated effectively through rotation age. In general, the longer the rotation age, the lower the juvenile wood content, and the stronger the mechanical properties of the plantation-grown woods. However, the differences between juvenile wood and mature wood vary with wood properties and species. In general, juvenile wood and mature wood have less difference in chemical composition than in anatomical and physico-mechanical properties. Compared to the softwoods studied, the hardwoods appear to have less difference between juvenile wood and mature wood. Received 25 June 1999     

Thermo-directional movement of wood and wood composites

Summary Thermal movement of wood and wood composites was studied and compared with that of random and oriented phenolic foams. Cell orientation was a dominate factor determining the thermal response of these structures. In dried wood, thermal movement in the direction parallel to the cells (longitudinal) decreased in dimension during heating while the radial and tangential directions expanded under similar heating condition. Oriented foams showed more restraint in thermal movement in the parallel to orientation direction. These findings indicate constraining forces act on the direction parallel to the cells while more flexibility exists in perpendicular directions. Wood element size and orientation and the manufacturing process also influence the thermal response in wood composites. The experimental results also revealed the potential fire hazard of waferboard type of composites.     

传统木材生产与效益木材生产

本探讨了传统木材生产和效益木材生产的区别，并从生产方式和经营模式上具体论述了传统木材生产和效益木材生产。     

木材干燥产生缺陷的预防措施及补救方法

缺陷对木材的加工及使用影响很大 ,本文对木材干燥产生的缺陷及产生的原因进行了探讨 ,提出干燥引起缺陷的预防措施。     

Compressive fatigue in wood

Summary  An investigation of fatigue failure in wood subjected to load cycles in compression parallel to grain is presented. Small clear specimens of spruce are taken to failure in square wave formed fatigue loading at a stress excitation level corresponding to 80% of the short term strength. Four frequencies ranging from 0.01 Hz to 10 Hz are used. The number of cycles to failure is found to be a poor measure of the fatigue performance of wood. Creep, maximum strain, stiffness and work are monitored throughout the fatigue tests. Accumulated creep is suggested identified with damage and a correlation is observed between stiffness reduction and accumulated creep. A failure model based on the total work during the fatigue life is rejected, and a modified work model based on elastic, viscous and non-recovered viscoelastic work is experimentally supported, and an explanation at a microstructural level is attempted. The outline of a model explaining the interaction of the effect of load duration and the effect of the loading sequences is presented. Received 8 December 1997     

酯化、醚化木材及其应用

通过适当的化学改性反应如酯化、醚化等,可以使木材转化为热塑性高分子材料。这些热塑性材料可单独或与合成高聚物按比例混合热压加工成型为各种板材或其他成型产品,这对扩大木材的加工利用途径、充分利用木材加工剩余物、提高木材利用率等都具有十分重要的意义。本文介绍了酯化、醚化木材的主要方法,产品的特点,以及今后木材及纤维素酯化、醚化的发展方向。     

Mode III fracture energy of wood composites in comparison to solid wood

A simple experimental setup for mode III and mixed mode (I?+?III) fracture tests with anisotropic materials under steady state crack propagation has been developed. Load-displacement curves can be recorded up to the complete separation of the specimen. From the load-displacement curves several mechanical material parameters can be derived. The tests have been performed for solid wood and different wood composites, being PARALLAM^® PSL in different orientations, particleboard and INTRALLAM^® LSL, and the fracture behaviour is characterised by the specific fracture energy.     

Differential calometric analysis of wood and wood components

Summary The prolysis of cellulose, hemicelluloses, lignin preparations, and wood was studied by differential calorimetric analysis (DCA) for the range of 25° to 800° C. The test samples included powdered and filter paper celluloses; hardwood xylan; softwood galactoglucomannans, compression wood galactan, and arabinogalactan; a synthetic (DHP), sulfuric acid, Björkman, Brownell, and cellulase lignins; and unextracted and extracted hardwoods and softwoods. Heats of reaction were determined from the DCA thermal transition areas. Distinct differences were found between the thermograms of each hemicellulose and lignin sample. Although wood species could not be separated thermally, hardwood and softwood thermograms differed because of the hemicellulose degradation pattern.Trade names and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment of the product by the U.S. Department of Agriculture.Formerly Research Technologist, Forest Products Laboratory, Forest Service, U.S. Department of Agriculture. The Laboratory is maintained at Madison, Wis. 53705, in cooperation with the University of Wisconsin. Present address: The Pennsylvania State University, University Park, Pennsylvania 16802.     

Translucent wood

Summary Wet cross-sections up to several centimetres thick of young trees show marked translucency. Various research applications are suggested by the observation of this phenomenon.     

Improved interaction between wood and synthetic polymers in wood/polymer composites

Summary This article describes the properties of wood polymer composites consisting of linear low density polyethylene (LLDPE) and wood flour (WF). In an attempt to improve the interfacial adhesion between the matrix and the filler, different compatibilizers were used. The interaction between polymer and wood were studied by comparing LLDPE/WF composites with composites when compatibilizer was added. The experimental measurements were conducted by impact and tensile strength testing and Scanning Electron Microscopy (SEM). The mechanical properties of the composites were improved with SEBS triblock copolymer modified with maleic anhydride and with the ionomer polymer, Surlyn, as compatibilizers. SEM fractography confirmed better adhesion between wood particles and LLDPE matrix when SEBS was present.This study was financed by the Swedish National Board for Industrial and Technical Development (NUTEK) which is gratefully acknowledged     

Movement of fluids in wood — Part I: Flow of fluids in wood

Summary The flow of fluids and diffusion through wood follow different laws and vary in effectiveness through different structures. For this reason this review has been divided into two parts, Part I covers flow of fluids and part II diffusion. The conclusions drawn here involve Part I only.Voids in wood vary in size from vessels in hardwoods, which are visible under very low magnification, down to spaces of molecular size. Voids in dry unbulked cell walls of wood cannot exceed a few per cent of the Volume. Reported findings of much higher values are in error due to the fact that the contained moisture and any bulking material in the cell walls was not taken into account. Only polar fluids can penetrate the cell walls where they are held in solid solution by an attractive force greater than that of wood for itself. Flow of this bound liquid through the cell walls is negligible compared to that through the permanent openings in the pit membranes. This fine pit structure controls the rate of flow of fluids through softwoods, the pressure drop occurring in the fiber cavities being negligible compared to that occurring across the pit membrane openings. In the case of hardwoods the pits share this resistance to flow with fine openings in tyloses in the vessels. Flow is 100 to 200 times greater in the fiber direction than transversely for softwoods under the same pressure because about that many more pits have to be traversed per unit distance. From various flow considerations the average effective pit membrane openings range from 10 to 200 millimicrons in radius, the smaller values being for impervious heartwood and the larger values for pervious sapwood.The rate of flow of fluids through wood is highly affected by the presence of air or other gases. Only when great precautions are taken to remove dissolved air can reproducable constant rates of flow be obtained. Considerably more pressure has to be applied to force a gas-liquid interface through wood than to cause flow of the liquid alone. The pressure to cause the first bubble of gas to appear through a liquid saturated specimen of wood as a result of displacement of the liquid can, together with the surface tension, be used to calculate the largest effective radius of all of the paths in parallel, where the effective radius is the smallest radius of each path in series. The maximum radius of the fiber cavities, the maximum effective radius of the pit membrane openings for passage through one pit in each path, and the approximate average maximum effective radius of the pit membrane openings for passage through a large number of pits in series can be calculated from displacement measurements on softwood cross sections varying from the thinnest possible sections to sections many fiber lengths thick. These values for a white cedar sapwood are 30 microns, 2 microns and 0.1 to 0.2 microns respectively. The latter values are 3 to 6 times the most probable pit membrane opening sizes obtained from measurements of the reduction in flow of humidified air through wood as a result of condensation occurring in the communicating openings. The combined data show that the most effective pit membrane openings may range from 0.01 to 2.0 m or more in radius. Considerable resistance to impregnation of wood is afforded by the small openings in resistant species due to the fact that the surface tension effect in the fine communicating openings has to be overcome. This is true even for the impregnation of dry wood, as vapor may condense ahead of the advance of liquid. In order to avoid these surface tension effects, gas phase treatments should be tried.Movement of free water in the drying of water saturated wood is restriced to [1] movement created by an internal hydrostatic head resulting from heating above the boiling point of water or to [2] drying of completely watersaturated wood under conditions such that the drying tension set up in the largest pit membrane opening of a fiber exceeds the proportional limit in compression perpendicular to the grain of the fiber. In this case the fiber collapses as water flows under tension from the fiber cavity. When the resistance to collapse exceeds the drying tension evaporation of water will occur from the largest pit opening and then recede into the fiber cavity. The wet line of the specimen will hence move inwards without internal loss of moisture above the wetline. A normal diffusion controlled drying gradient extends inwards to the fiber saturation point followed by an abrupt increase in moisture content to the original value.Usually the fiber cavities of wood contain some air in bubbles larger than the largest pit membrane openings. In this case free water moves under the drying tension without causing collapse due to the relief of internal stress because of the expansion of the air. Under these conditions the moisture distribution above the fiber-saturation point is a smooth continuation of the portion below the fiber-saturation point. This liquid movement of free water is not a diffusion, but it is controlled by the diffusion below the fiber-saturation point.It is thus evident that the movement of free liquids in wood is quite complex and affected by a number of different factors, the most important of which are to be considered in this paper.

---

Zusammenfassung Die Strömung von Flüssigkeiten durch das Holz einerseits und die Diffusion andererseits folgen jeweils verschiedenen Gesetzen und unterscheiden sich in ihrer Auswirkung je nach dem betroffenen Teil des Holzgefüges. Die vorliegende Arbeit wurde deshalb in zwei Teilen abgefaßt. Der vorliegende Teil I behandelt die Strömung von Flüssigkeiten, Teil II die Diffusion. Die Ergebnisse des ersten Teiles können wie folgt zusammengefaßt werden.Die Hohlräume im Holz variieren in Form und Größe von großen Gefäßen in Laubhölzern, die man sehon bei geringen Vergrößerungen erkennen kann, bis hinab zu kleinsten Zwischenräumen von molekularer Größenordnung. Die Hohlräume in trockenem ungequollenem Holz können einen Anteil von weingen Prozent nicht überschreiten. Berichte über wesentlich höhere Werte sind insofern falsch, als hierbei die in den Zellwänden enthaltene Feuchtigkeit und sämtliche füllenden Stoffe nicht mit in Rechnung gestellt werden. Nur polare Flüssigkeiten können in die Zellwände eindringen, wo sie in fester Lösung durch eine Kraft festgehalten werden, die größer ist als die Kohäsionskraft im Holze selbst. Der Fluß dieser gebundenen Flüssigkeit durch die Zellwände ist vernachlässigbar gering im Vergleich zu dem, der ständig durch die Öffnungen der Tüpfelmembranen stattfindet. Diese Elemente des Tüpfelaufbaues regeln die Strömungsgeschwindigkeit der Flüssigkeiten in Weichhölzern, da der Druckabfall in den Faserhohlräumen im Vergleich zu demjenigen, der durch die Membranöffnungen bedingt wird, vernachlässigbar klein ist. Bei den Harthölzern teilen die Tüpfel diesen Strömungswiderstand zusammen mit feinen Öffnungen in den Thyllen, die sich in den Gefäßen befinden. Die Strömung ist in Längsrichtung 100 bis 200 mal größer als bei Weichhölzern in Querrichtung bei gleichem Druck, da bei diesen wesentlich mehr Tüpfel je Längeneinheit durchströmt werden müssen.Auf Grund verschiedener Beobachtungen läßt sich feststellen, daß die effektive mittlere Weite der Tüpfelmembranöffnungen einen Radius zwischen 10 und 200 m haben; die kleineren Werte gelten für das wenig durchlässige Kernholz, die größeren für das durchlässigere Splintholz.Der Flüssigkeitsstrom durch das Holz wird weiterhin in hohem Maße von der Gegenwart von Luft oder anderen Gasen beeinflußt. Nur unter Anwendung verhältnismäßig aufwendiger Vorkehrungen zur Entfernung der in Lösung gegangenen Luft ist es möglich, reproduzierbar gleichmäßige Strömungsgeschwindigkeiten zu erhalten. Gegenüber einem reinen Flüssigkeitsstrom benötigt man für ein Flüssigkeits-Gasgemisch einen wesentlich höheren Druck, um es durch das Holz zu führen. Der Druck, der notwendig ist, um die erste Gasblase als Ergebnis einer Flüssigkeitsverdrängung in einem flüssigkeitsgesättigten Holz zu erzeugen, kann zusammen mit der Oberflächenspannung zur Berechnung des größten wirksamen Radius aller parallel laufenden Durchflußwege verwendet werden, wobei dieser wirksame Radius gleichzeitig auch der kleinste Radius aller in Serie, d. h. hintereinander liegenden Durchflußwege ist. Der größte Radius der Faserhohlräume, der größte wirksame Radius der Tüpfelmembranöffnungen für den Durchfluß durch einen Tüpfel jedes Durchflußweges und der mittlere größte wirksame Radius der Tüpfelmembranöffnungen für den Durchfluß durch eine größere Anzahl hintereinander liegender Tüpfel kann mit Hilfe von Verdrängungsmessungen an Weichholzquerschnitten, deren Dicke vom Mikrotomschnitt bis zum mehrere Faserlängen dicken Stück reicht, berechnet werden. Diese Dicken betragen für White cedar Splintholz 30 m, 2 m bzw. 0,1... 0,2 m. Die letztgenannten Zahlen sind das drei- bis sechsfache der am häufigsten auftretenden Größe der membranöffnungen, Sie wurden durch Messung des Abfalles der Durchflußmenge feuchter Luft durch Holz, der durch Kondensationserscheinungen in den zusammenhängenden Öffnungen zustande kam, ermittelt. Die errechneten Daten lassen erkennen, daß der Radius der am häufigsten auftretenden wirksamen Tüpfelmembranöffnungen zwischen 0,01 und 0,02 m liegt. Der große Widerstand gegen die Imprägneirung von Holz muß auf die sehr kleinen Membranöffnungen bei den schwer zu imprägnierenden Holzarten zurückgeführt werden, und zwar auf Grund der Tatsache, daß die Oberflächenspannung in den jeweiligen öffnungen der Feinstruktur überwunden werden muß. Dies gilt auch für die Imprägnierung von trockenem Holz, da die dampfförmige Phase schon vor der vordringenden Flüssigkeit kondensieren kann. Um also diese Oberflächenspannungseffekte zu umgehen, erscheint es sinnvoll, Behandlungsverfahren mit gasförmigen Mitteln zu entwickeln.Die Bewegung von freiem Wasser während der Trocknung wassergesättigten Holzes ist beschränkt 1. auf eine Bewegung, die durch ein inneres hydrostatisches Druckgefälle infolge der Erwärmung über den Siedepunkt des Wassers herbeigeführt wird, oder 2. auf die Trocknung von wassergesättigtem Holz unter der Bedingung, daß die Trocknungsspannung, die sich in der größten Tüpfelmembranöffnung einer Faser ausbildet, die Proportionalitätsgrenze für den Druck senkrecht zur Faserrichtung überschreitet. In diesem Falle kollabiert die Faser, da das Wasser unter Zugspannung aus dem Faserhohlraum ausfließt. Ist jedoch der Widerstand gegen den Zellkollaps größer als die Trocknungsspannung, so tritt an der größten Tüpfelöffnung Verdampfung ein und anschließend der Rückfluß in den Faserhohlraum. Die Feuchtigkeitszone in einer Holzprobe wird also in Richtung auf das Zentrum zu immer kleiner, ohne daß die Feuchtigkeit innerhalb der Zone selbst absinkt. Ein gewöhnliches diffusionsgesteuertes Feuchtigkeitsgefälle erstreckt sich nach innen bis zum Fasersättigungspunkt, gefolgt von einem plötzlichen Feuchtigkeitsanstieg bis zum Ausgangswert.Im Normalfalle enthalten aber die Faserhohlräume des Holzes einige Luftblasen, die größer sind als die größte Tüpfelmembranöffnung. Dabei fließt das freie Wasser unter der Trocknungsspannung ab, ohne daß ein Kollaps eintritt, da die innere Spannung auf Grund der Ausdehnung der Luft herabgemindert wird. Unter diesen Bedingungen bildet die Feuchtigkeitsverteilung oberhalb des Fasersättigungspunktes einen ziemlich glatten Übergang zu dem Teil unterhalb des Fasersättigungspunktes. Diese Art der Feuchtigkeitsbewegung des freien Wassers ist zwar keine Diffusion, aber sie wird durch die Diffusion unterhalb des Fasersättigungspunktes gesteuert. Aus all dem geht klar hervor, daß die Bewegung freier Flüssigkeiten in Holz sehr komplex ist und von einer ganzen Reihe verschiedener Faktoren beeinflußt wird, deren wichtigste hier besprochen werden sollen.

---

---

Contribution from the School of Forestry, North Carolina Agricultural Experiment Station, Raleigh, North Carolina, published with the approval of the Director of Research as Paper No. 2141 of the Journal Series.     

Knot dinteraction in wood poles

Summary Superposition of the longitudinal stress distributions associated with neighboring knots was used to determine the interaction distances between different knot sizes. The interaction between two knots was considered significant when the superposition of the stress distribution caused an increase in the longitudinal stress level of ten percent or more. Out of 4,374 knots measured in 44 Douglas-fir poles only 55 knots showed significant interactions with adjacent knots. However, in more than half of the poles at least one interacting knot pairs existed. Therefore, it has been concluded that stress interaction between knots could play a major role in controlling the failure of poles in bending.     

The thermoosmotic effect in wood

Summary Water vapor pressures computed from stationary state data for moist wood (Choong 1963) are used to infer changes in the logarithm of vapor pressure per unit change in Kelvin temperature, d ln p/dT, for comparison with corresponding values from a thermodynamic model. The model evaluates the overall heat of transfer associated with passage of vapor through wood, and hence quantifies the thermoosmotic effect. Results of the comparison verify the existence of a constant vapor entropy for wood in the stationary state.     

Friction mechanisms in wood cutting

The variation of the coefficient of friction with normal load and tool surface finish in slow speed orthogonal cutting of white fir was investigated to determine the relative importance of different sources of friction. Cutting forces were measured as a function of tool-chip contact length and tool surface roughness. The coefficient of friction was independent of tool roughness for roughness in the range of typically well-finished cutting tools but did become a factor for large values of tool surface roughness. The data indicate that for well-finished cutting tools the primary friction mechanism is adhesion between the tool and work and that for rough tool surfaces the effect of tool surface asperities (unevennesses) as they advance through the chip becomes an important source of friction.     

Mechano-sorptive creep in wood fibres

In order to investigate the way in which fibre properties affect the mechano-sorptive creep phenomenon in paper, single wood fibres were exposed to tensile stresses at a constant humidity of 80% relative humidity (RH) and in a cyclic humidity environment varying between 80 and 30% RH. Contrary to earlier claims, it was demonstrated that single wood fibres exposed to a cyclic RH show a considerably higher creep than that corresponding to the highest RH experienced in the cycle, i.e., a mechano-sorptive behaviour. The creep strain rate at cyclic humidity was shown to be a function of the creep rate at constant climate, and to be an apparent linear function of the applied stress.     

Nonisothermal moisture movement in wood

In order to analyze the effect of temperature gradient on moisture movement during highly intensive drying, such as microwave-vacuum drying, the profile of the temperature and moisture content in sealed wood whose opposite faces were subjected to temperature gradient for a short time was measured. The ratio of the moisture content (MC) gradient to the temperature gradient (dM/dT) was calculated and the factors influencing moisture movement under nonisothermal conditions were discussed. The results indicate that moisture moved in wood from the warm surface to the cold one even if opposite faces of the sealed wood assembly were exposed continuously to different but constant temperatures for a short period. The moisture content on the cold surface was higher than that on the warm surface. The moisture content gradient opposite to the temperature gradient was established, and the dM/dT was below 0.9%/°C. The temperature in the sample and the distance from the hot surface of the sample was strongly linearly correlated. With an increase in temperature, initial moisture content and experimental time, the dM/dT was significantly increased. __________ Translated from Journal of Beijing Forestry University, 2005, 27(2): 96–100 [译自: 北京林业大学学报, 2005, 27(2): 96–100]