Determination of formic-acid and acetic acid concentrations formed during hydrothermal treatment of birch wood and its relation to colour, strength and hardness

Formation of benzyl esters from acetic and formic acids during heat treatment of birch at 160–200°C has been studied by gas chromatography. High concentrations of formic and acetic acids formed by the wood itself during hydrothermal treatment were found. The concentrations of acids increased with both treatment time and temperature. The maximum formic- and acetic acid concentrations found at 180°C and after 4 h of treatment performed in this work were 1.1 and 7.2%, based on dry-weight wood, respectively. The treated wood material was characterised by mechanical testing [bending tests perpendicular to the grain, modulus of rupture, modulus of elasticity, Brinell hardness, impact bending and colour measurements (CIE colour space)]. The experiments, where high concentration of acids was formed, showed severe losses in mass and mechanical strength. Indications of possible enhanced mechanical properties for the treated, compared with untreated birch wood were found around 180–200°C at short treatment times. This paper discusses possible degradation reactions coupled with the colour and mechanical properties in relation to acid formation, and suggestions for process optimisations.     

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     

Comparative study on the durability of heat-treated White Birch (Betula papyrifera) subjected to the attack of brown and white rot fungi

Abstract

The effect of heat treatment on decay resistance of white birch was evaluated for different incubation periods ranging from 2 to 12 weeks using three species of brown rot and one species of white rot fungus. The results of weight loss tests showed that the white rot fungus, Trametes versicolor, effectively degraded the untreated wood (73.5%). While the degradation of untreated wood by brown rot fungi species, Gloephyllum trabeum (11.6%) and Conifora puteana (6.2%), was considerably less compared to T. versicolor, the third brown rot fungi studied, Poria placenta, caused an appreciable degradation of the same species (52.4%). The results clearly showed that the heat treatment reduced the effect of fungi attack on white birch. Increasing the heat treatment temperature from 195 to 215°C resulted in reduction of weight loss, consequently, reduction in fungal attack. As an example, the weight loss reductions due to T. versicolor, P. placenta, G. trabeum and C. puteana attack was 62.2%, 71.3%, 89.6% and 100%, respectively, compared to the weight loss of untreated wood when it is heat treated at 215°C. Thus, these results confirmed that the heat treatment increased the biological resistance of white birch.     

Optimisation of a two-stage heat treatment process: durability aspects

Heat treatment of wood at relatively high temperatures (in the range of 150–280°C) is an effective method to improve biological durability of wood. This study was performed to investigate the effect of heat treatment process optimisation on the resistance against fungal attack, including basidiomycetes, molds and blue stain fungi. An industrially used two-stage heat treatment method under relatively mild conditions (<200°C) was used to treat the boards. Heat treatment of radiata pine sapwood revealed a clear improvement of the resistance against the brown rot fungi Coniophora puteana and Poria placenta. Increasing process temperature and/or effective process time during the first process stage, the hydro thermolysis, appeared to affect the resistance against C. puteana attack, but the effect on the resistance against P. placenta was rather limited. Heat treated radiata pine showed a limited resistance against the white rot fungus Coriolus versicolor and process variations during the hydro thermolysis stage appeared not to affect this resistance. A clear difference between the resistance of heat treated Scots pine sapwood and heartwood against fungal attack is observed. Scots pine heartwood showed a higher resistance against C. puteana and P. placenta but also against the white rot fungus C. versicolor. Similar results were obtained when heat treated birch was exposed to brown and white rot fungi. Heat treatment showed an improved resistance against C. puteana attack, especially at higher temperatures during the hydro thermolysis stage. A clear improvement of the durability was also observed after exposure to the white rot fungus C. versicolor and especially Stereum hirsutum. Increasing the process temperature or process time during the hydro thermolysis stage appeared to have a limited effect on the resistance against C. versicolor attack. Heat treated radiata pine and Norway spruce were still susceptible to mold growth on the wood surface, probably due to the formation of hemicelluloses degradation products (e.g. sugars) during heat treatment. Remarkable is the absence of blue stain fungi on heat treated wood specimen, also because the abandant blue stain fungi were observed on untreated specimen. Molecular reasons for the resistance of heat treated wood against fungal attack are discussed in detail contributing to a better understanding of heat treatment methods.     

Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents

Thermal modification at relatively high temperatures (ranging from 150 to 260 °C) is an effective method to improve the dimensional stability and resistance against fungal attack. This study was performed to investigate the impact of heat treatment on the mechanical properties of wood. An industrially-used two-stage heat treatment method under relative mild conditions (< 200 °C) was used to treat the boards. Heat treatment revealed a clear effect on the mechanical properties of softwood species. The tensile strength parallel to the grain showed a rather large decrease, whereas the compressive strength parallel to the fibre increased after heat treatment. The bending strength, which is a combination of the tensile stress, compressive stress and shear stress, was lower after heat treatment. This decrease was less than the decrease of only the tensile strength. The impact strength showed a rather large decrease after heat treatment. An increase of the modulus of elasticity during the bending test has been noticed after heat treatment. Changes and/or modifications of the main wood components appear to be involved in the effects of heat treatment on the mechanical properties. The possible effect of degradation and modification of hemicelluloses, degradation and/or crystallization of amorphous cellulose, and polycondensation reactions of lignin on the mechanical properties of heat treated wood have been discussed. The effect of natural defects, such as knots, resin pockets, abnormal slope of grain and reaction wood, on the strength properties of wood appeared to be affected by heat treatment. Nevertheless, heat treated timber shows potential for use in constructions, but it is important to carefully consider the stresses that occur in a construction and some practical consequences when heat treated timber is used.     

热处理对木材力学性能的影响综述

热处理可有效提高木材尺寸稳定性,同时对木材力学性能也有明显的影响。文中按照力学指标分别综述热处理工艺对静曲强度/弹性模量、抗压强度、脆性/冲击韧性及其他力学性能的影响,进而总结热处理对木材力学性能影响机理的研究进展;提出可以从以下方面深化热处理技术的研发:一是热处理技术研发时宜根据应用领域科学选择合适的力学指标,二是进一步拓展热处理介质以控制力学性能损失,三是进一步探索热处理对木材不同力学指标的影响机理。     

Genetic variation in wood stiffness and strength properties of hybrid larch (Larix gmelinii var. japonica × L. kaempferi)

Genetic parameters for wood stiffness and strength properties were estimated in a 29-year-old hybrid larch stand (Larix gmelinii var. japonica × Larix kaempferi). The study included 19 full-sib larch families from Hokkaido, northern Japan. Implications of these genetic parameters in wood quality improvement are subsequently discussed. Traits included in the analyses were the dynamic modulus of elasticity of green logs (E _log), the modulus of elasticity (MOE), the modulus of rupture (MOR), compression strength parallel to the grain (CS) in small clear specimens, wood density (DEN), and diameter at breast height (DBH). DEN had the lowest coefficients of variation and MOE the highest. The narrow-sense heritability estimates of E _log, MOE, MOR, and CS were 0.61, 0.44, 0.60, and 0.43, respectively, and those of DEN and all mechanical properties increased from an inner to outer position within the stem. E _log and DEN had high positive phenotypic (0.52–0.83) and genetic (0.70–0.92) correlations with MOE, MOR, and CS. The mechanical properties of the inner position of the stem had rather high phenotypic and genetic correlations with those of the outer position and overall mean. The predicted gains in wood stiffness (E _log and MOE) were higher than those of the strength properties (MOR and CS). The predicted correlated responses in MOE, MOR, and CS when selecting for E _log and DEN were 72.6%–97.8% of a gain achievable from direct selection of these traits. DBH showed an insignificant correlation with all mechanical properties, although selection of this trait had a slightly negative effect on the mechanical properties.     

Physical properties and molecular mobility of the new wood composite plastic “thermobalite”

At the Altai State University a new method of synthesizing composite materials from wood without using traditional chemical components has been developed. The foundation of the method is the physical and chemical treatment of cheap wood waste by hot water steam under pressure. The paper presents comparative results of an investigation into the structure and properties of the composite material “thermobalite” synthesized on the basis of birch wood. The investigation has been carried out using dynamic mechanical analysis, linear dilatometry, wide-angle X-ray diffraction and other physical methods. It has been shown that “thermobalite” is a partially crystalline composite polymer, whose glass transition temperature is 80 °C lower compared to the initial wood. It has been established that preliminary moistening affects viscoelastic properties of “thermobalite”. Certain indicators of “thermobalite” as a structural material have been correlated with similar indicators of standard wood chip plates on the basis of phenolformaldehyde binders. Received 6 March 1997     

介质对木材热处理的影响

文中总结了不同介质热处理的技术特点,介质对热处理材性能的影响,以及预处理和复合介质热处理的研究进展,认为采用油、预处理或复合介质等热处理可以进一步提高热处理材的尺寸稳定性,减小木材强度降幅,还能显著增强防腐性能等其他单纯蒸汽热处理无法实现的功能,建议今后进一步拓展热处理介质的范围,深化预处理和复合介质热处理研究,赋予热处理材更多、更好的性能,研发出更具有竞争力的热处理技术。     

基于纳米压痕技术的木材胶合界面力学行为

【目的】研究木材胶合界面的静态和动态力学行为,探讨树脂渗透对木材管胞壁层力学性能的影响,为木质复合材料制造工艺优化和增强改性提供理论依据。【方法】采用纳米压痕静态和动态力学测试技术(Nano-DMA),对针叶材火炬松与酚醛树脂(PF)、脲醛树脂(UF)胶黏剂所形成胶合界面区域各相材料的静态弹性模量、硬度、蠕变性能以及储能模量和损耗模量等力学行为进行分析。【结果】静态力学行为方面,在界面区域,PF和UF渗透进入管胞壁层后,木材管胞壁的弹性模量( E r)和硬度( H )提高;经PF渗透后,木材管胞壁的 E r和 H 分别增加7%和26%;Burgers蠕变力学模型可有效描述胶合界面区域管胞壁的纳米压痕蠕变特性,经树脂渗透后,木材管胞壁的瞬时弹性模量增加,黏弹性模量和黏性系数减小;在保载初期,PF界面区域木材管胞壁的蠕变柔量约下降60%,UF界面区域木材管胞壁的蠕变柔量约下降58%。动态力学行为方面,随着加载频率增加,界面材料的储能模量( E ′ r)逐渐增大,而损耗模量( E ″ r)和损耗因子(tan δ)呈减小趋势;当加载频率为10 Hz时,PF和UF树脂渗透使得管胞壁层的储能模量分别增加16%和29%。【结论】胶合界面区域胶黏剂进入管胞壁层,对木材管胞的静态力学性能具有增强作用,同时胶黏剂可提高管胞壁的短期抗蠕变能力;木材管胞壁具有较高的储能模量和损耗模量,而树脂的储能模量和损耗模量较低,经树脂渗透后,木材管胞壁的储能模量增加,但损耗模量和损耗因子呈下降趋势,可能对界面传递和分散应力产生不利影响。     

Effects of heat treatment and impregnation with zinc-oxide nanoparticles on physical,mechanical, and biological properties of beech wood

Effects of zinc-oxide nanoparticles on physical and mechanical properties, as well as biological resistance of untreated and heat-treated beech wood were investigated in this study. Test specimens were prepared from sapwood and impregnated with a 5,000-ppm nano-zinc-oxide (NZ) suspension with a size ranging from 10 to 80 nm at 2.5 bars of pressure and using the Rueping process for 20 min. Control (C) and nano-zinc-oxide-impregnated specimens after (NZA) and before (NZB) heat treatment were divided into four subgroups of unheated (C and CNZ), heated at 50, 145 and 185 °C. Heat treatment resulted in a significant decrease in mechanical strength at temperatures of 145 and 185 °C. Heat-treated specimens showed less dimensional instability and fungal degradation. Impregnation with nano-zinc resulted in a slight and significant increase in weight loss and biological resistance against Trametes versicolor. The results showed that the impregnation significantly decreased the water absorption of the specimens. Impregnation before heat treatment showed considerable effect on the properties of wood compared to that of untreated ones.     

Characterisation of juvenile wood in teak

Juvenile wood properties are studied in a ring-porous tropical hardwood – teak (Tectona grandis L. F), to assess the utilisation potential of short rotation timber. Compared to mature wood, it is characterised by wide rings, short fibres, small diameter, low vessel percentage, high cell wall, wide microfibrillar angle and relatively low or almost similar mechanical properties. While the average modulus of elasticity and modulus of rupture in juvenile wood are 85% and 82% respectively of the mature wood value, the longitudinal compression strength is similar. With relatively small fibrillar angle of 15° and the scope for genetic selection of individual trees, teak juvenile wood has potential for desired dimensional stability. The segmented regression models and visual interpretation of radial patterns of variation in anatomical properties reveal that juvenility in plantation grown teak extends up to 15, 20–25 years depending on the property, growth rate and individual tree and plantation site. The fitted regression models, to explain the age-related variations in juvenile wood properties range from simple, linear to exponential, reciprocal and quadratic equations. Fibre length, microfibrillar angle, vessel diameter/percentage and ring width appear to be the best anatomical indicators of age demarcation between juvenile and mature wood, although maturation age often varies among the properties. The projected figures for proportion of juvenile wood in plantation grown teak at breast height are 80–100% and 25% at ages 20 and 60 years respectively. Received 3 November 1998     

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.     

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.     

思茅松热处理木材的研究

热处理对于提高木材的尺寸稳定性和耐久性、抵抗生物破坏等性能来说是一种非常有效的方法。作者采用油浴法对思茅松木材进行热处理工艺的探讨,分别对热处理木材的失重率、吸湿率、线性胀缩率、以及微观构造等进行了分析,研究结果表明：（1）思茅松热处理材随着处理温度和处理时间的增加,失重率逐渐增加;（2）思茅松热处理材随着处理温度和处理时间的增加,吸湿率逐渐降低;（3）思茅松热处理材随着处理温度和处理时间的增加,线性胀缩率逐渐降低;（4）在微观构造观察方面,由于早材腔大壁薄,材质较松软,经过热处理后早材部分容易引起径向开裂,早材管胞形态的变形程度要比晚材要大,早材轴向管胞壁的弯曲变形较晚材的要严重。随着热处理温度升高,热处理时间的延长,炭化程度越来越严重,为不使木材物理力学以及微观构造方面遭到严重破坏,建议木材热处理温度为160℃、热处理时间为6～12h或木材热处理温度为180℃、热处理时间为6～8h为宜。     

Determination of optimum borate concentration for alleviating strength loss during heat treatment of wood

Based on the already established mitigating potentials of borate salt on the negative effects of heat treatment on the strength properties of wood (Awoyemi and Westermark 2005), the optimum concentration of the alkali buffer solution required to minimize strength loss was determined. Wood samples were impregnated with 0.1, 0.3 and 0.5 M sodium borate solution and exposed to heat treatment for 4 h at 200°C. The mitigating effect of borate salt on the degree of strength loss during heat treatment increases significantly with increasing concentration from 0.1 to 0.3 M. Increasing the concentration of sodium borate from 0.3 to 0.5 M did not produce significant differences in the degree of strength loss during heat treatment. The increase in the buffering effect observed with increasing concentration of the sodium borate preservative is more pronounced on the modulus of rupture than on the modulus of elasticity. It is evidenced therefore that the buffering effect of borate salt on the modulus of elasticity of wood exposed to heat treatment did not start significantly until the 0.3 M concentration is reached and further increase in concentration beyond this point did not produce any appreciable improvement in strength properties.     

Vibrational properties of heat-treated green wood

To investigate the influence of water on heat treatment, green wood was heat-treated. Sitka spruce (Picea sitchensis Carr.) with about 60% moisture content (MC) was used. Young's modulus and loss tangent were measured by the free-free flexural vibration test. The specimens were heated in nitrogen at 160°C for 0.5h. The results were as follows. (1) Recognizing that the effects of heat treatment are mild and that the same specimens cannot be used for both heat treatment and as controls, it was necessary to investigate the effects of the heat treatment based on the variations of properties in the whole of the test lumber. (2) Young's modulus increased and the loss tangent decreased due to heat treatment. When the vibrational properties were measured at various MCs, the MCs at the maximum value of Young's modulus and the minimum value of the loss tangent were lower in heat-treated specimens than in controls. The effects of heat treatment in green wood were similar to those in air-dried wood. (3) The loss tangents of heat-treated specimens were smaller than those of controls at about 0% MC but were larger than those of controls at about 10% MC. We thought that this resulted from the decreased MC at the minimum loss tangent after the heat treatment mentioned above. (4) The properties measured at several MCs were more useful than those at only one moisture content for investigating the effects of heat treatment.This study was presented in part at the 46th annual meeting of the Japan Wood Research Society, Kumamoto, April 3–5, 1996; and at the 47th annual meeting of the Japan Wood Research Society, Kochi, April 3–5, 1997     

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]     

培育措施对人工林樟子松木材力学性质的影响

研究了不同培育措施(初植密度、间伐强度、坡向、坡位)对人工林樟子松(Pinus sylvestris var.mongolica)木材的密度和力学性质(横纹抗弯强度、抗弯弹性模量、顺纹抗压强度、冲击韧性、端面硬度)的影响。初植密度(1.5m×1.0m,1.5m×2.0m和1.5m×2.5m)对木材密度和抗弯弹性模量有显著的影响。初植密度为1.5m×1.0m时,木材主要力学指标值最大。适当间伐可提高木材的抗弯弹性模量和顺纹抗压强度,但重度间伐则会降低木材的力学强度。坡向(阳坡和阴坡)对木材的抗弯弹性模量影响显著,阳坡高于阴坡。坡位(上坡位和下坡位)对除端面硬度外的木材力学性质指标影响都显著,各项力学指标值均是坡下高于坡上。表5参9。