Three mechanisms affecting the mechanical properties of spruce wood dried at high temperatures

Wood drying experiments are conducted in which the temperature and the drying rate are controlled independently. In relationship to drying processes, at least three mechanisms are believed to contribute to the properties of dried wood. However, only two of these are found to affect the properties of macroscopic specimens, the third mechanism being observable in microtomed earlywood sections, and possibly in specimens loaded in the radial direction. Degradation of structural components and irreversible hydrogen bonding (hornification) are found to contribute to both the hygroscopicity and the mechanical properties of macroscopic wood specimens. Mass loss from thermal degradation occurs predominantly in slow high-temperature drying processes. Irreversible hydrogen bonding takes place in high-temperature drying, in particular with high ultimate dryness. Regarding the effect on strength and stiffness, mass loss and hornification appear to compete. The third identified mechanism, microscopic cell wall damage caused by incompatible drying shrinkage of cell wall elements, does not seem to affect the mechanical properties of macroscopic wood specimens. Consequently, slow high-temperature drying processes do not provide much benefit regarding the mechanical behavior of dried wood. The reasons for this are discussed.     

Effects of basic density,annual growth and drying schedule on selected quality factors of dried Siberian larch (Larix sibirica) timber

Abstract

Siberian larch (Larix sibirica Ledeb.) timber is suitable for many uses in the mechanical wood industry. Drying it without any decrease in value, however, is difficult and slow. The purpose of this research was to compare the drying quality of Siberian larch timber dried with three different conventional schedules taking into account the density and annual growth of wood. Five drying tests were performed. The final moisture content (MC), MC gradient, cracks, deformations (bow, crook, twist and cup) and case hardening were measured from the dried timber. The basic density particularly affected the MC, as shown in the differences regarding dried wood, with the denser wood having higher final MC and MC gradient. It was also found that large annual growth increased some deformations. Most of the measured factors were best after drying at the highest temperatures used; however, a slightly different trend was observed for bow, twist and cup. MC factors and twisting were the most problematic properties in drying according to this study. Sorting Siberian larch timber, particularly according to density, would improve the MC properties of dried timber by ensuring sufficient drying time, as economically as possible, for each timber piece.     

Microstructure,strength and structural integrity of heat-treated beech and spruce wood

Abstract

Heat treatment of wood is an effective method by which to improve the dimensional stability and biological durability, but the mechanical strength is decreased at the same time. Besides chemical modification of cell-wall constituents, physical weakening of the microstructure owing to heat-induced defects may also contribute to strength loss. Therefore, anatomical properties of heat-treated beech (Fagus sylvatica L.) and spruce (Picea abies Karst.), studied by light microscopic and scanning electron microscopic analysis, and their interrelation with strength properties and structural integrity were investigated. For determination of structural integrity, the high-energy–multiple-impact (HEMI) test was applied. Microscopic analyses showed frequent formation of radial cracks in heat-treated beech close to the rays as well as tangential cracks in the latewood of spruce. In addition, the modulus of rupture was more affected by the heat treatments than the resistance to impact milling (RIM) determined by the HEMI test, because RIM is based on multiple fractures on the microlevel that are not affected by the formation of intercellular cracks or other defects due to the heat treatment. It was concluded that heat-induced defects in the wood microstructure contribute to the substantial strength loss of thermally modified timber.     

Superheated steam treatment of rubberwood to enhance its mechanical,physiochemical, and biological properties

ABSTRACT

This research was aimed to investigate mechanical properties, color and cell-wall components changes, and durability of pre-dried rubberwood (Hevea brasiliensis) after superheated steam (SS) treatment. Wood samples were treated at different SS temperatures (140–180°C) for 1–3?h. The highest compression strength parallel-to-grain, hardness and impact strength were found for samples treated at 160°C for 3?h (30.7% higher than untreated), at 150°C for 1?h (26.6% higher than untreated) and at 150°C for 2?h (52.6% higher than untreated), respectively. The surface color became darker after each treatment in comparison with the untreated wood. The number of accessible hydroxyl groups decreased and the relative cellulose crystallinity increased with SS temperature, indicating decreased hygroscopicity of the treated wood. Also, SEM micrographs of wood surface showed consistent decrease in starch particles with treatment temperature. Both decay and termite resistances of treated rubberwood improved with treatment temperature. All the analyzes showed that dried rubberwood treated with SS had some improvements in the mechanical properties, decreased hygroscopicity, and increase resistance to decay.     

Changes of decay and termite durabilities of Japanese larch (Larix leptolepis) wood due to high-temperature kiln drying processes

We investigated the effects of high-temperature drying schedules (120°–130°C) on decay and termite feeding of Japanese larch timbers. Thermogravimetric analysis was conducted to investigate changes of the wood components. Decay and termite feeding tests showed that specimens dried under high-temperature schedules were susceptible against a decaying fungus Fomitopsis palustris and attacks from termites Coptotermes formosanus and Reticulitermes speratus. These drying schedules changed chemical components, which were suggested by the thermal analytical result compared to the control sample. The results of this study indicated that the acceleration of termite feeding takes place even under temperatures that are comparatively lower than that used in our previous research in which 170°C steaming treatment was applied to Japanese larch wood. Decay durability against a brown rot fungus also decreased, possibly from production of low molecular weight fragments when hemicellulose decreased during the high-temperature drying processes.     

High temperature and chemical effects on wood stability

Summary The increasing supplies of fast-grown woods grown on short roations contain significantly higher proportions of juvenile wood with properties different from those of mature wood. Improved processes will be required to produce dried wood that is satisfactorily stable with few distortional or dimensional changes. The basic wood features affecting different forms of instability are considered. Variations in the amounts of cellulose, hemicelluloses and lignins in wood, the changes in them and the degradation products formed under different conditions are discussed. Changes in the nature of hemicelluloses appear to play an important role in conveying stability. The different volumes of heartwood in green timber have an effect on moisture levels and other properties. As industry is increasing kiln temperatures for high-temperature drying, the effect of time-temperature-moisture relationships on stability and degradation are discussed. The effect of ammonia and other chemicals on stability is considered.     

Influence of steam heating on the properties of pine (<Emphasis Type="Italic">Pinus pinaster</Emphasis>) and eucalypt (<Emphasis Type="Italic">Eucalyptus globulus</Emphasis>) wood

Heat treatment of Pinus pinaster and Eucalyptus globulus woods, two important species in Portugal, was performed in the absence of air by steaming, inside an autoclave, for 2–12 h at 190–210°C. Mass losses increased with treatment time and temperature reaching 7.3% for pine and 14.5% for eucalypt wood. The wood behaviour with moisture was improved. The equilibrium moisture content decreased by 46% for pine and 61% for eucalypt, the dimensional stability increased (maximum anti-shrinking efficiency in the radial direction of 57 and 90% for pine and eucalypt, respectively) and the surface wettability was lowered. In relation to mechanical properties, the modulus of elasticity was little affected (maximum decrease of 5% for pine and 15% for eucalypt) but the bending strength was reduced (by 40% at 8% mass loss for pine and 50% at 9% mass loss for eucalypt wood). The variation of properties was related to treatment intensity and mass loss but significant improvements could already be obtained for a 3–4% mass loss without impairing the mechanical resistance. The response of eucalypt was higher than that of pinewood. Heat treatment of eucalypt wood shows an interesting potential to improve the wood quality for solid timber products.     

Mechanical behaviour of Eucalyptus wood modified by heat

Summary  Eucalyptus wood (Blue gum) shows very high mechanical performances, presents very few knots and gives strong glued joints, reasons that justify its interest for structural uses, carpentry and furniture components or even building construction. The inconveniences of this species are its slow and difficult drying process and its low dimensional stability – with very high swelling and shrinkage coefficients. The drying process has been studied at INETI and the problem revealed to be solved with appropriate drying schedules and the stresses released by a steam treatment. Heat treatment of Eucalyptus wood has been identified as one of the most promising techniques to increase its dimensional stability. Studies in this area still proceed. This paper presents some test results that show the influence of heat treatment in the strength properties of this wood, namely the bending modulus of elasticity and tensile strength perpendicular to grain, in addition to the explanation of dimensional stability evaluation and some results of dimensional stability treatments. Received 15 June 1998     

Wood drying process: impact on Scots pine lumber durability

There are indications that the drying process may have negative effects on the natural durability of wood. The impact of various drying processes on the durability of Scots pine lumber has been evaluated with mass loss in a decay test with brown rot fungus, Coniophora puteana, as measure of the decay resistance of sapwood and inner and outer heartwood. Drying with or without steam conditioning was performed in six different series: air drying, kiln drying at temperature ranges commonly used in Swedish sawmills at 70°C and 90°C with two different regulation principles, and one high-temperature drying at 110°C. Durability varied considerably both between and within boards. Sapwood showed considerable less durability than heartwood. No difference in durability was found between inner heartwood and outer heartwood. Air-dried heartwood showed the highest durability compared to other drying series. The lowest durability in sapwood and heartwood was found for series dried at the 90°C temperature level with high material temperature early in drying. The interpretation is that the duration of high material temperature at high moisture content (MC) is the critical combination for decay resistance in heartwood. Steam conditioning after drying decreased durability in sapwood.     

三倍体毛白杨新无性系木材气干密度和力学性质性状的遗传控制及对单板材育种的意义

该文利用 9个 9年生三倍体毛白杨无性系木材试样 ,研究了木材气干密度组成及力学性质的遗传变异规律 .结果表明 ,木材气干密度组成及力学性质在无性系间存在显著或极显著差异 ,并受到中等强度的遗传控制 ;气干密度的径向和纵向变异与多数研究结果一致 ;力学性质除抗弯弹性模量和端面硬度外其株内变异趋势也基本符合木材学理论 .在力学性质指标中 ,抗弯弹性模量和弦面硬度是遗传性很强的性状 ,其无性系重复力分别为 0 90和 0 80 ,抗弯强度、顺纹抗压强度和硬度的无性系重复力稍低 .遗传相关表明对单板材可通过木材密度与干形等形质指标进行优良无性系选择     

Compression behaviors of acetylated wood in organic liquids. Part II. Drying-set and its recovery

Unmodified and acetylated cedar wood specimens were swollen in various liquids and dried under radial compression. Two stress relaxation processes were observed during drying, and the second process observed below the fiber saturation point was responsible for the drying-set and the temporary fixation of compressive deformation. The fixed shape of acetylated wood was partly recovered by soaking it in water and toluene and completely recovered in acetone. The effective shape fixation and recovery of toluene-swollen samples implied that the intermolecular hydrogen bonding was not necessary for the drying-set of acetylated wood. The degree of shape recovery was not explained by initial softening, while the acetylated wood always exhibited greater recoverability than unmodified wood. Although 85% stiffness was lost after large compression set and recovery of unmodified wood, such a stiffness loss was limited to 39% when the acetylated wood was processed with organic liquids. This indicated that the swelling of the hydrophobic region in the acetylated wood was effective in preventing mechanical damage due to large compressive deformation.     

Poplar wood heat treatment: effect of air ventilation rate and initial moisture content on reaction kinetics,physical and mechanical properties

The kinetics of heat treatment as well as its effect on some physical and mechanical properties of poplar wood (Populus alba L.) were analysed in this research. Kinetic tests were performed at different treatment temperatures and two different air ventilation settings [low and high air exchange rate (AER) with the exterior]. The treatment kinetics was studied, starting from the oven-dry condition, according to the mass loss during time. The time–temperature equivalency was checked, the mass loss versus time formalised through a master curve. The analysis clearly showed how the heat treatment at low and high AER presents different degradation kinetics even if similar activation energy values were found. Some physical and mechanical properties of wood after treatments up to a mass loss of 7 and 10 % starting from oven-dry or standard environmental conditions state were also studied. All of the treated samples showed statistically significant differences compared to the untreated one. The treatments performed at 7 or 10 % of dry mass loss showed homogeneous behaviour. The same tendency was observed for the treatments starting at oven-dry or standard environmental conditions with the exception of Young’s modulus, which resulted in smaller reductions in wet starting condition when compared to dry samples.     

Finger-jointing green softwood: Evaluation of the interaction between polyurethane adhesive and wood

Abstract

Existing European standards for finger-jointing of load-bearing lumber require the wood to be dried before gluing. This article presents a study on the properties of green-glued finger joints, wet wood being bonded prior to drying. Issues to consider, in comparison to finger-jointing of dry wood, are mechanical performance of the joint, absorption of the polymer by the wood in its natural/wet state, and the chemical reactions of the adhesive on contact with water. Finger-jointed samples were tested in bending, and the glue joints analysed by optical microscopy, scanning electron microscopy and microdensitometry. A patented one-component polyurethane adhesive developed for gluing-green wood which has a moisture content usually higher than 70% was used in the study. The resulting green-glued joints showed improved strength properties in comparison to dry-jointed joints. The results confirm that green-glued joints provide a wide, continuous wood/adhesive interface from one substrate to the other. The adhesive penetrates several cells deep and the density of the wood adjacent to the joint surfaces is increased. The results also indicate that the patented adhesive forms covalent bonds to the wood substrate.     

Dynamic viscoelastic behavior of wood under drying conditions

In this study heartwood from a Chinese fir [Cunninghamia lanceolata (Lamb.) Hook] plantation was treated using a high-temperature drying (HTD) method at 115°C, a low-temperature drying (LTD) method at 65°C, and freeze vacuum drying (FVD), respectively. The dynamic viscoelastic properties of dried wood specimens were investigated. The measurements were carried out at a temperature range of −120 to 250°C at four different frequencies (1, 2, 5, and 10 Hz) using dynamic mechanical analysis (DMA). We have drawn the following conclusions: 1) the storage modulus E′ and loss modulus E″ are the highest for HTD wood and the lowest for FVD wood; 2) three relaxation processes were detected in HTD and LTD wood, attributed to the micro-Brownian motion of cell wall polymers in the non-crystalline region, the oscillations of the torso of cell wall polymers, and the motions of the methyl groups of cell wall polymers in the non-crystalline region in a decreasing order of temperatures at which they occurred; and 3) in FVD wood, four relaxation processes were observed. A newly added relaxation is attributed to the micro-Brownian motions of lignin molecules. This study suggests that both the HTD and the LTD methods restrict the micro-Brownian motion of lignin molecules somewhat by the cross-linking of chains due to their heating history. __________ Translated from Journal of Beijing Forestry University, 2008, 30(3): 96–100 [译自: 北京林业大学学报]     

Influence of heating history on dynamic viscoelastic properties and dimensions of dry wood

To obtain new information about the mechanical and physical properties of dry wood in unstable states, the influence of heating history on viscoelastic properties and dimensional changes of dry wood in the radial, tangential, and longitudinal directions was studied between 100° and 200°C. Unstable states of dry wood still existed after heating at 105°C for 30 min and were modified by activated molecular motion in the first heating process to temperatures above 105°C. This phenomenon is thought to be caused by the unstable states reappearing after wetting and drying again. Dry wood components did not completely approach the stable state in the temperature range tested, because they did not entirely surpass the glass transition temperatures in most of the temperature range. In constant temperature processes at 135° and 165°C, E′ increased and E″ decreased with time regardless of the direction. This indicated that the unstable states of dry wood components were gradually modified with time at constant temperatures. On the other hand, anisotropy of dimensional change existed and dimension increased in the longitudinal direction, was unchanged in the radial direction, and decreased in the tangential direction with time at constant temperatures. Part of this report was presented at the 13th Annual Meeting of the Chubu Branch of the Japan Wood Research Society, Shizuoka, August 2003     

Hornification—its origin and interpretation in wood pulps

Although perfectly diagnosed in terms of the occurrence of physical changes, the hornification phenomenon, in its origin, has frequently been associated with the formation of irreversible or partially reversible hydrogen bonding in wood pulps or paper upon drying or water removal. Its characterisation has therefore been confusing and unsatisfactory. The authors propose that a sufficiently varied source of experimental data already exists to show that hornification is only a particular case of lactone bridge formation in lignocellulosic materials.     

长岭岗林场日本落叶松幼龄材干缩性能研究

测定了建始县长岭岗林场引种栽培的12年生日本落叶松标准木干缩性能指标，并分析了来自两个样地的20株样木株同差异，其结果表明：干缩性能指标问差异不显著；径向全干干缩率为4．64，弦向全干干缩率为6．81，体积全干干缩率为10．81。全干差畀干缩为1．47；径向气干干缩率为2．41，弦向气干干缩率为3．93，体积气干干缩率为6．59，气干差异干缩为1．63。在此基础上，讨论了日本落叶松的栽培与木材利用中的有关问题和新旧标准下测定的干缩性能指标的换算与利用。     

Dependence of shear strength on wood properties in cultivated Larix sibirica

Abstract

Mechanical properties are very important in structural uses of wood. In the case of larch wood, shear strength (SS) may be a limiting property, because the wood is brittle and it splinters easily. In this study, SS of Siberian larch (Larix sibirica) wood was studied in relation to its axial and radial location and to some other wood properties in the part of the trunk that produces logs of good carpentry quality. SS increased by 21–23% from the pith to the outer heartwood at each studied height (butt, 4.5 and 9 m), and it was at its most approximately 7% higher at the butt than at the other studied heights. The main reason for the radial increase in SS was the increase in density caused by wood maturation, but other factors also affected SS, depending on the grouping mode (combined material, radial/axial location). An exception was sapwood, for which no factors affecting SS could be identified. Thus, sapwood may behave differently from heartwood regarding use, even unexpectedly. The results suggest that mature heartwood, but not juvenile wood and sapwood, of Larix sibirica should primarily be used in products that require high SS.     

Aging law of spruce wood

Abstract

Accelerated aging of spruce wood samples have been carried out by thermo-hydro (TH) treatments. These treatments were applied to accelerate the chemical reactions that take place during the natural aging of wood. In order to avoid dissimilar chemical reactions between the TH treatments and the natural aging, mild temperatures (between 100 and 150°C) have been selected at low relative humidity (RH). The mechanical properties of non-aged, natural aged and accelerated aged spruce wood have been compared. It is apparent that longitudinal Young's modulus of accelerated aged wood increase slightly at the beginning of the treatment and is followed by a reduction. Along the radial direction, Young's modulus remains almost unchanged. On the other hand the radial strength is severely reduced. From these results, the relative radial strength has been fitted on the chemical kinetic law. The rate constant of this law has been calculated and the treatment temperature and wood moisture content have been integrated. Finally this law has been extrapolated to standard climatic conditions in order to predict the loss of strength of old wood by knowing its age and its mean climate history of temperature and RH (ambiance condition).     

Dielectric properties of softwood species at microwave frequencies

Summary Dielectric measurements at 3 GHz were made for three softwoods, European pine, spruce and hemlock. The longitudinal, radial and tangential grain directions of the wood were considered as well as moisture contents ranging from 6% to 35%. The positive effect of the moisture content on the loss factor illustrates the selectivity of microwave drying techniques, while the observations also show that the longitudinal dielectric properties are substantially higher than the transverse ones. The specific effect of the wood species on the dielectric behaviour has to be ascribed mainly to those intrinsic characteristics of the species which influence the sorptive capacity of the wood.