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.     

Reversible volumetric changes of acetylated wood with after-treatments

Dimensional changes of acetylated wood were measured during wet–dry cycling and heating to clarify the reversible and irreversible effects of those after-treatments. During wet–dry cycling, the acetylated wood showed slight swelling in its completely dry condition, while its weight decreased slightly. Similar swelling was recognized when the acetylated wood was heated dry above 140°C. On the other hand, the anomalous effects of wet–dry cycling and heating disappeared after soaking in acetone. It was considered that the rearrangement of hydrophobic wood polymers during wet–dry cycling and heating induced the expansion of voids, whereas the wood polymers recovered their initial conformation after soaking in organic liquids. The wet volume of acetylated wood also exhibited reversible changes during wet–dry cycling. Just after the acetylation, the wood was highly swollen in acetic anhydride. The swollen volume of acetylated wood was reduced by leaching in water, and additional decrease in the wet volume resulted from the following drying or boiling. However, the reduced wet volume was recovered almost completely after acetone-soaking. These results suggested that the water molecules remaining in the hydrophobic region expanded the wet volume of acetylated wood, while such weakly bound water was easily removable by drying or boiling.     

Mechanism of partial fixation of compressed wood based on a matrix non-softening method

Abstract

Three different mechanisms to explain the partial fixation of the compressive deformation of wood are postulated: non-softening, cross-linking and stress relaxation. This study attempted to fix the compressive deformation of wood by the non-softening mechanism of the cell-wall matrix using acetylation of the cell wall making it more hydrophobic. In this method, partial recovery of compressive deformation by wetting decreased at room temperature as the acetyl content increased. However, almost complete recovery occurred by boiling the compressed wood in water or soaking in acetone. This is due to the ability of boiling water or acetone to soften the cell-wall matrix of acetylated wood enough to enable recovery from compression. It is, therefore, possible to partially fix the compressive deformation of wood, preventing the resoftening of the cell-wall matrix in water.     

Effects of previous solvent exchange on acetylation of wood

Wood specimens were prepared in a swollen state using solvent exchange (PS) treatment. The swollen wood specimens were acetylated using acetic anhydride by heating at 80–120°C. At the beginning of heating, the weight percent gain (WPG) of PS-treated wood was greater than that of conventionally acetylated wood. This acceleration effect of the PS treatment was explained by the introduction of treating reagent into the wood polymers where the intermolecular hydrogen bonds were previously broken. On the other hand, the PS treatment had no influence on the final WPG and moisture sorption characteristics of acetylated wood. This indicated that the intrinsic reactivity of wood constituents was unaffected by the PS treatment. The acetylation of PS-treated wood produced greater bulking and slightly higher dimensional stability than that in the case of conventional acetylation at the same WPG. It was speculated that the expansion of cell lumina due to the PS treatment resulted in greater bulking on acetylation and lesser swelling of acetylated wood with moisture sorption.     

Dynamic viscoelastic properties of wood acetylated with acetic anhydride solution of glucose pentaacetate

 Spruce wood specimens were acetylated with acetic anhydride (AA) solutions of glucose pentaacetate (GPA), and their viscoelastic properties along the radial direction were compared to those of the untreated and the normally acetylated specimens at various relative humidities and temperatures. Higher concentrations of the GPA/AA solution resulted in more swelling of wood when GPA was introducted into the wood cell wall. At room temperature the dynamic Young's modulus (E′) of the acetylated wood was enhanced by 10% with the introduction of GPA, whereas its mechanical loss tangent (tan δ) remained almost unchanged. These changes were interpreted to be an antiplasticizing effect of the bulky GPA molecules in the wood cell wall. On heating in the absence of moisture, the GPA-acetylated wood exhibited a marked drop in E′ and a clear tan δ peak above 150°C, whereas the E′ and tan δ of the untreated wood were relatively stable up to 200°C. The tan δ peak of the GPA-acetylated wood shifted to lower temperatures with increasing GPA content, and there was no tan δ peak due to the melting of GPA itself. Thus the marked thermal softening of the GPA-acetylated wood was attributed to the softening of wood components plasticized with GPA. Received: March 29, 2002 / Accepted: May 21, 2002 Correspondence to:E. Obataya     

Micromorphology,moisture sorption and mechanical properties of a biocomposite based on acetylated wood particles and cellulose ester

Abstract

One of the major issues in a long-term perspective for the use of wood–plastic composites (WPCs) in outdoor applications is the moisture sensitivity of the wood component and the consequent dimensional instability and susceptibility to biological degradation of the composite. In this work, the effects of using an acetylated wood component and a cellulose ester as matrix on the micromorphology, mechanical performance and moisture uptake of injection-moulded WPCs have been studied. Composites based on unmodified and acetylated wood particles, specially designed with a length-to-width ratio of about 5–7, combined with both cellulose acetate propionate (CAP) and polypropylene (PP) matrices were studied. The size and shape of the wood particles were studied before and after the processing using light microscopy, and the micromorphology of the composites was studied using a newly developed surface preparation technique based on ultraviolet laser irradiation combined with low-vacuum scanning electron microscopy (LV-SEM). The water vapour sorption in the composites and the effect of accelerated weathering were measured using thin samples which were allowed to reach equilibrium moisture content (EMC). The length-to-diameter ratio was only slightly decreased for the acetylated particles after compounding and injection moulding, although both the unmodified and the acetylated particles were smaller in size after the processing steps. The tensile strength was about 40% higher for the composite based on acetylated wood than for the composite with unmodified wood using either CAP or PP as matrix, whereas the notched impact strength of the composite based on acetylated wood was about 20% lower than those of the corresponding unmodified composites. The sorption experiments showed that the EMC was 50% lower in the composites with an acetylated wood component than in the composites with an unmodified wood component. The choice of matrix material strongly affected the moisture absorptivity of the WPC. The composites with CAP as matrix gained moisture more rapidly than the composites with PP as matrix. It was also found that accelerated ageing in a Weather-Ometer® significantly increased the moisture sensitivity of the PP-based composites.     

Preparation of acetylated wood meal and polypropylene composites II: mechanical properties and dimensional stability of the composites

Acetylated wood meals of Sugi (Cryptomeria japonica D.Don) wood were prepared by mechanochemical processing using a high-speed vibration rod mill. Weight percent gain (WPG) of the acetylated wood meals ranged from 7.0 to 35.5 %. Wood–plastic composites (WPCs) containing 50 % acetylated woods were produced by an injection molding technique. The polymer matrix used was polypropylene homopolymer. Maleic anhydride-grafted polypropylene (MAPP) was also used as a compatibilizing agent. The mechanical properties of WPCs in bending and tensile tests were independent of WPG of acetylated wood meals, and the test values for WPCs containing acetylated wood meals were lower than that of unmodified wood meal. The use of MAPP increased bending and tensile strength, but no effect on bending modulus was found. An increase in WPG significantly decreased water absorbability and thickness swelling of WPCs as measured by dimensional stability tests. These results demonstrated that mechanochemical processing is a promising technique for preparing WPC material with improved dimensional stability. The future challenge is to inhibit the decreases in mechanical properties of WPCs containing acetylated wood meals.     

Swelling of acetylated wood II: effects of delignification on solvent adsorption of acetylated wood

To clarify the role of lignin in the affinities of acetylated wood for organic solvents, the effects of delignification on the solvent adsorption of acetylated wood were investigated. Acetylated wood meals rapidly adsorbed organic solvents that were hardly adsorbed by unmodified wood. For nonpolar and low-polarity organic solvents, a clear positive correlation was observed between the amount of adsorption and the lignin content. This indicated that acetylated lignin was responsible for the excellent affinities of acetylated wood for hydrophobic organic solvents. On the other hand, for lower alcohols and water, the amount of adsorption reduced with an increase in the lignin content. It was suggested that the adsorption of such polar solvents was dominated by insufficiently acetylated hydrophilic polysaccharides.     

Erratum to: Mechanical properties of wood in an unstable state due to temperature changes,and analysis of the relevant mechanism III: Effect of quenching on stress relaxation of chemically modified woods

In order to understand the mechanism of destabilization occurring when wood is quenched, we applied chemical modifications, and controlled the number of moisture adsorption sites in wood. The degree of destabilization was evaluated according to the fluidity (1-E _t/E ₀), increase in fluidity, and relative fluidity in relation to the nonmodified wood, and was discussed by comparing these quantities with the hygroscopicity or swelling of wood. We found that destabilization of chemically modified wood was lower than that in nonmodified wood, and the amount of adsorbed water controlled the magnitude of flow of wood. Moreover, according to the analysis of water state by the Hailwood-Horrobin equation, it was shown that the function of dissolved water to the fluidity is almost identical for both chemical modifications, whereas hydrated water has more effect on acetylated wood than on formaldehydetreated wood. We speculate that the motion of water molecules due to quenching accompanied with the redistribution of energy resulting from the exchange of their potential energy and movement to attain a new balance, and the introduced acetyl groups and cross-linking restrict the water molecule movement. An erratum to this article is available at .     

木材顺纹压缩率与PDR的影响机制研究

木材顺纹压缩弹塑性和PDR(永久变形率)变化规律及机理研究是攻克木材顺纹压缩与多维弯曲技术的核心.从木材的树种、密度、软化处理条件、化学组分和相对结晶度以及压缩速度几个方面,分析了木材顺纹压缩过程中这些因素对木材顺纹压缩率和PDR的影响机制.     

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

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

Swelling of acetylated wood I. Swelling in organic liquids

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

Corrosion of metal fasteners embedded in acetylated and untreated wood at different moisture contents

ABSTRACT

Acetylated wood is now commercially available and designed to be used in certain outdoor applications as an alternative to preservative-treated wood. Fastener corrosion can be a concern in preservative treated wood when the wood remains wet for long periods. However, little data on the corrosiveness of acetylated wood exists beyond the product literature. Here we examine the corrosiveness of commercially obtained acetylated wood and compare it against unmodified (untreated) southern pine (Pinus spp.). Corrosion rates of plain carbon steel, hot dip galvanized steel, and stainless steel were calculated gravimetrically after a one year exposed in the wood. Four different moisture conditions were examined: 90% relative humidity (RH), 95% RH, 100% RH, and a fully water saturated condition. When compared to literature data on the corrosion of fasteners in preservative treated wood at 100% RH, the acetylated wood had much lower steel corrosion rates than all preservatives examined; the measured corrosion rates for galvanized steel were lower than all preservatives except chromated copper arsenate. These measured corrosion rates across a range of moisture conditions can be used to inform the selection of appropriate corrosion resistant fasteners when building with acetylated wood.     

刨花板厚度方向变形研究Ⅱ.刨花板厚度方向变形模型的建立及释因

本文基于我们以往所做的众多的实验事实 ,从刨花板厚度方向的结构出发 ,全面考虑影响刨花板厚度膨胀率的因素 ,经过论证和筛选 ,建立了刨花板厚度膨胀率的方程模型 :TS(t) =F(t) f(M .C ,t) V(t) ,即刨花板的厚度膨胀率主要取决于人造板内木材的粘弹变形恢复、人造板内木材的吸湿膨胀和胶接点破坏引起的变形 ;当刨花板的含水率存在较大变化时 ,需再加上机械吸附蠕变项A(M .C ,t)     

木材湿热软化径向压缩变形特征研究进展

压缩变形特征是压缩木研究的重要内容。文中针对木材湿热软化径向压缩变形,从压缩变形的影响因素、压缩变形位置对其力学性能的影响和压缩变形固定3个方面综述国内外研究进展,包括树种、生长轮构造和压缩工艺对木材压缩变形的影响,压缩变形位置对力学性能的影响,以及热处理对压缩变形的影响,指出木材湿热软化径向压缩研究中关于压缩变形需要继续解决的主要问题以及发展方向,以期为湿热软化径向压缩木材的进一步研究和应用提供参考。     

人工林软质木材表面密实化新技术

采用一种新型木材改性处理剂，分别以改性异氰酸酯浓度5％、10％、15％、20％，对美国人工林火炬松（Pinus taeda）进行表面密实化处理。结果表明，随着树脂浓度的增加，无论是冷水浸泡还是煮沸，木材的吸水厚度膨胀率和压缩变形恢复率明显降低。表面密实化后，火炬松处理材的MOR和MOE值分别比素材提高43．9％和30．1％；水浸24h和煮沸2h后的湿状抗弯性能比素材略低，干状抗弯性能明显比素材高，MOR分别高28．0％和25．76％；MOE分别高22．55％和27．79％。改性异氰酸酯浸渍处理后的表面密实化木材，具有一定的阻燃效果；表面耐磨耗性能和表面硬度亦明显改善。     

压缩工艺对毛白杨脲醛树脂胶浸注改性效果的影响研究

对毛白杨木材浸注脲醛树脂胶制备压缩改性木材中的主要影响因素及相关工艺参数进行初步探索与试验,并在实验结果基础上讨论了各因素对制作工艺及其性能的影响。结果表明:①影响板材性能由主到次因素的顺序为压缩率-热压时间-热压温度;②在试验参数范围内较好的工艺参数为热压温度140℃、热压时间20min、压缩率50%;③在试验参数范围内热压时间对试件增重率、含水率、树脂留存率影响显著,而热压温度对试件增重率影响显著,压缩率对试件密度、变形回复率、吸水厚度膨胀率影响显著。     

Phenol–formaldehyde impregnation of densified wood for improved dimensional stability

To produce a highly stable wood-based product with increased mechanical properties, phenol formaldehyde (PF) resin impregnation was combined with the viscoelastic thermal compression (VTC) process. Dimensional stability and bending stiffness were evaluated. Two PF resins with weight average molecular weights of 172 and 780 were studied at three different concentrations, 5, 10 and 20%. After 24-h room temperature water soak and 2-h boil, both PF treatments at all concentration levels showed high levels of dimensional stability compared to non-impregnated VTC processed controls. The higher molecular weight PF provided greater stability with an average thickness swell value of 12% compared to 20 and 37% for the lower molecular weight PF resin treatment and control, respectively. High anti-swelling efficiency values were recorded for both low and high molecular weight resins, implying these modifications were effective at reducing the volumetric swelling which occurred in the unmodified control. PF treatments were also extremely effective at reducing irreversible swelling. The low and high molecular weight resin treatments had 1/5th and 1/7th the irreversible swelling than the unmodified VTC processed controls, respectively. All dimensional stability values improved as resin concentrations increased. Both resin types at all concentration levels reduced Young’s modulus.     

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.     

Bending creep behavior of hot-pressed wood under cyclic moisture change conditions

This study examined the bending creep behavior of hot-pressed wood during cyclic moisture changes. Sugi (Cryptomerica japonica D. Don) specimens were pressed in the radial direction under six combinations of nominal compressive strain (33% and 50%) and press temperatures (140°C, 170°C, 200°C). Creep tests were conducted at 20°C with three cyclic relative humidity changes between 65% and 95% under 25% of short-breaking stress. The effect of moisture content (MC) change on elastic compliance and mechanosorptive (MS) compliance was investigated. The relation between MS compliance and thickness swelling was studied. The results indicated that total compliance increased over the history of cyclic moisture changes; and its behavior was closely related to the changes in MC and thickness swelling. The total compliance increased during adsorption and decreased during desorption. Elastic compliance increased linearly with MC and was dependent on press temperature and compression. With increasing MC change, MS compliance increased during adsorption and decreased during desorption. The first adsorption led to greater MS compliance than did the subsequent adsorption with the same amount of MC change. In general, the elastic parameterK _E and the MS parameterK _Mincreased with compression and decreased as the press temperature increased. The MS parameterK _M was apparently greater than the elastic parameterK _E. The MS parameterK _M increased with swelling coefficient K_SW of the hot-pressed specimen during adsorption and decreased with an increasing shrinkage coefficientK _SH during desorption.