Nanoindentation of juvenile and mature loblolly pine (<Emphasis Type="Italic">Pinus taeda</Emphasis> L.) wood fibers as affected by thermomechanical refining pressure

The effects of thermomechanical refining pressures, varying from 2 to 18 bars, on the cell-wall properties of refined wood fibers of a 54-year-old loblolly pine (Pinus taeda L.) with reference to both juvenile (JW) and mature wood (MW) were investigated using nanoindentation and atomic force microscopy. The results of this study indicate that refining pressure plays a significant role in the physical damage sustained by refined wood fibers. No obvious damage was observed in the cell walls of MW fibers refined at 2 and 4 bar. Nanocracks (<500 nm in width) were found in fibers refined at pressures in the range of 2–12 bar for JW and 6–12 bar for MW, and micro cracks (>3,000 nm in width) were found in both MW and JW fibers subjected to a refining pressure of 14 and 18 bar. The micro damage to the fibers refined at higher pressures was more severe inside the lumen than on the surface of the fibers, and the lumen or S₃ layer was significantly damaged. The elastic modulus, hardness, and creep resistance of MW fibers were higher than those of the JW fibers subjected to the same refining-pressure conditions. The elastic modulus and hardness decreased, whereas nanoindentation creep increased, with increasing refining pressure. This study also suggests that lower refining pressures (<4 bar) and higher pressures (>14 bar for MW and >12 bar for JW) should be avoided in the manufacture of fiberboards and wood fiber–polymer composites, because of the lower aspect ratio of the fiber bundles, shorter length of the fibers and fines, and severe damages to the fiber cell walls.     

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.     

Study on the changes in surface characteristics of <Emphasis Type="Italic">Populus tomentosa</Emphasis> due to thermo-hydro-process

The effects of high temperature, moisture, and mechanical action during the thermo-hydro-mechanical (THM) processing, on the changes in surface properties of poplar, namely, surface color, roughness, wettability, and microstructure, were investigated in this study. The correlation between observed changes in surface properties and chemical characteristics was also analyzed. Poplar woods with high moisture content were compressed using different pressures at temperature of 160 °C for four different periods. The wood surfaces became darker and smoother, and their surface free energy decreased significantly after the THM process. THM process markedly reduced surface hygroscopicity compared to the control wood. The cell lumens of THM wood became narrow with increasing compression ratio due to the enhancing high pressure. Collapse and fractures of cell walls developed during THM treatment. Furthermore, results indicated that a series of chemical reactions in different components of wood took place during THM process, such as degradation of hemicelluloses, condensation of lignin, and decomposition of extractives. In turn, these chemical modifications contributed to the darkening of color as well as the reduction of wettability and surface free energy of THM wood.     

Ultrastructural features affecting mechanical properties of wood fibres

Abstract

The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure–property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure–property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.     

Analysis of the water vapour sorption isotherms of thermally modified acacia and sesendok

Abstract

Two Malaysian hardwoods, acacia (Acacia mangium) and sesendok (Endospermum malaccense), that had been subjected to oleo-thermal modification were studied to determine their sorption isotherm behaviour using a dynamic vapour sorption apparatus. All the specimens were thermally modified using palm oil at three different temperatures (180, 200 and 220°C) and three different times (1, 2 and 3 h). The results showed that there was a reduction in equilibrium moisture content at each target relative humidity due to the heat treatment, but that the two wood species showed different behaviour in this respect. The adsorption isotherms were analysed using the Hailwood and Horrobin model, with excellent fits to the experimental data. The monolayer water and polylayer water were both reduced at a range of relative humidity values of the treated samples, although behaviour between the two wood species differed. Heat treatment resulted in an increase in hysteresis ratio, which was probably due to the increase in matrix stiffness of the cell walls.     

Determination of the cellulose and lignin content on wood fibre surfaces of eucalypts as a function of genotype and site

We compared the chemical composition of wood fibres and fibre surfaces of several eucalypt species and hybrids originating from various growth sites in South Africa. The objective was to test for differences in chemical surface composition due to genetics or site with the ultimate aim to facilitate a tailor-made supply of wood for pulping that results in an optimal blend of fibres that can be pulped together with similar yields. This, however, requires a sound knowledge of the fibre properties. The surface functionality on the single fibre level is a key property, because it determines how good inter-fibre bonding will be when paper is formed, which depends amongst other fibre properties on the amount of free hydroxyl groups that are available and therefore on the cellulose content on the fibre surface. The cellulose and lignin content on the fibre surface were determined with chemical force microscopy, a variation of atomic force microscopy. Since the general bulk composition of the fibre and the surface composition might differ, both parameters were determined. We found significant differences in the cellulose and lignin content on fibre surfaces, with regard to genotype and site, respectively. In some, but not all, cases, the surface composition of wood fibres followed the bulk composition, and differences were generally more pronounced. Differences due to genotype were significant, especially with regard to the surface lignin content—but variation due to site was also distinctly recognisable. This variation in surface functionality could be the reason why some pulpwood blends result in a lower pulp yield and different quality.     

Changes of chemical properties and the water vapour sorption of <Emphasis Type="Italic">Eucalyptus pellita</Emphasis> wood thermally modified in vacuum

The aim of this study was to evaluate the chemical composition and the dynamic water vapour sorption properties of Eucalyptus pellita wood thermally modified in vacuum. For this purpose, wood samples were thermally modified in a vacuum oven at 160–240 °C for 4 h. Chemical composition were investigated by wet chemical analysis, elemental analysis, as well as Fourier transform infrared (FTIR) analysis, and dynamic water vapour sorption properties were evaluated by dynamic vapour sorption apparatus. The results showed that holocellulose and alpha-cellulose contents decreased and lignin and extractives contents relatively increased during the heat process. Elemental analysis showed a reduction in hydrogen content and an increase in carbon content. FTIR analysis indicated that the degradation of hemicellulose and condensation reactions of lignin occurred. In addition, the thermo-vacuum resulted in a reduction in the equilibrium moisture content of wood during the adsorption or desorption process. And the sorption hysteresis had a decreasing trend with increasing treatment temperature. The development of the hygroscopicity was related to the increase in the relative content of lignin, the degradation of the carbonyl groups in xylan and the loss of carbonyl group linked to the aromatic skeleton in lignin after heat treatment.     

Ammonia sorption isotherms of wood and cotton cellulose

Summary Sorption isotherms of ammonia were measured on cellulosic materials, such as beech and birch wood, as well as on cotton cellulose, the object being to obtain information on the nature of interaction between the wood and the ammonia by means of application of various sorption theories. As a result several analogies between the sorption of ammonia vapour and water vapour could be observed.—The isotherms displayed the typical S-shape and developed a hysteresis along the adsorption and desorption lines. The sorbate films reached a thickness of 4–6 molecular layers. The lowering of enthalpy of the ammonia vapour appeared to be the propelling force of the sorption process, as in the case with the uptake of water vapour.—In contrast to water isotherms, however, ammonia isotherms cannot be reproduced with the same specimen. Each sorption cycle brought a loss of substance and a reduction of the fibre saturation capacities of the adsorbent. In ammonia vapour the fibre saturation points were found at sorbate concentrations twice as high as in water vapour. The hysteresis between the adsorption and desorption processes also appeared with ammonia vapour; however, the continuous changes of the adsorbent, caused by chemical interaction with ammonia, produced heavy displacements. The specific surface area of the samples in ammonia was approximately twice the size of that in water, but the extensions varied greatly during the sorption cycles.These investigations have been supported by the Zentenarfonds of the Swiss Federal Institute of Technology.     

木质材料激光表面处理研究进展

激光以其能量密度高、运行轨迹自如、方向性好等优点,被广泛用于木质材料切削加工以及表面处理等领域.其中,木质材料激光表面处理,即利用激光热/光电子效应促进材料发生物理、化学变化,以实现改性的目的.笔者对木质材料激光表面处理原理、激光类型与用途以及激光表面处理技术特点与应用领域等内容的研究现状进行了综述与分析.现有研究表明...     

Ultrastructural features affecting mechanical properties of wood fibres

The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure-property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure-property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.     

热磨条件对麦秸中密度纤维板性能的影响

在不同的热磨条件下(蒸汽压力为0.4、0.6、0.8、1.0MPa,磨盘转数为2 500r/min和3 000r/min),将麦秸原料分离成纤维,并试制了相应的麦秸中密度纤维板,对比分析了不同的热磨条件对麦秸中密度纤维板的内结合强度、弯曲性能、吸水性能、表面硬度和边部握螺钉力的影响。结果表明,不同的热磨条件下分离得到的麦秸纤维形态存在差异,纤维的强度和形态对麦秸中密度纤维板的物理力学性能影响较大。当蒸汽压力为0.8MPa,磨盘转数为3 000r/min,麦秸中密度纤维板的各项性能得刮改善。     

Water vapour diffusion and adsorption characteristics of sugar maple (Acer saccharum,Marsh.) wood polymer composites

Summary Water vapour diffusion characteristics and adsorption isotherms were determined for cell-lumen and cell-wall treated wood polymer composites (WPC). The diffusion coefficients of the cell-lumen WPC were lower than untreated wood and the cell-wall WPC coefficients were lower than cell-lumen. Using the Hailwood and Horrobin sorption model, it was found that the unimolecular layer is formed at lower moisture contents in WPC than in wood. The amount of free dissolved water was reduced only in the cell-wall WPC. The polymer reduces the water vapour accessibility in both types of WPC.     

Moisture storage and transport properties of preservative treated and untreated southern pine wood

Moisture storage and transport properties of southern pine (Pinus spp.) wood were measured for implementation into hygrothermal models. Specimens were untreated or pressure-treated with alkaline copper quaternary (ACQ) preservative. Moisture storage was characterized with sorption isotherms in the hygroscopic region (high capillary pressures) and documented with mercury intrusion porosimetry in the overhygroscopic region (low capillary pressures). The data were then combined into a single moisture retention curve as a function of capillary pressure. Moisture transport was evaluated from steady-state water vapor transmission and dynamic capillary water absorption experiments. These data were used to calculate the moisture permeability over the entire range of capillary pressures using the diffusivity approach of Carmeliet et al. Moisture storage and transport properties were similar for the untreated and ACQ-treated southern pine, except for the permeability of the treated wood which was lower in the radial direction. The data presented here can be used to improve the accuracy of hygrothermal and combined hygrothermal–corrosion modeling simulations.     

Water vapour diffusion through historically relevant glutin-based wood adhesives with sorption measurements and neutron radiography

In this work, the sorption and moisture diffusion behaviour of historically relevant glutin-based adhesives (i.e. bone glue, hide glue, fish glue) is characterized. The adhesive’s sorption isotherms were assessed on thin film samples revealing fundamental differences between the glutin-based adhesives and the synthetic reference adhesive (polyurethane). Furthermore, the water vapour diffusion parallel to the fibre was examined by means of neutron imaging on bonded two-layer samples of Norway spruce wood. In contrast to previous studies using neutron imaging, a new evaluation approach is presented, which allows for nonzero initial moisture conditions and takes into account and compensates for the geometry changes in the sample caused by swelling and shrinkage, thus allowing for a characterization of the diffusion behaviour within the glue line. The diffusion coefficients determined with neutron imaging were interpreted in terms of a theoretical model which takes into account the glue line microstructure. Although the diffusion coefficients were on average larger values for the glutin-based adhesives compared to the reference polyurethane adhesive, the significant variation observed in the sorption measurement is not reflected. This can partially be ascribed to excessive penetration of the adhesives into the wood substrate in fibre direction, which impedes a continuous adhesive layer. Furthermore, deformation and densification of the wood structure was assessed in the vicinity of the adhesive joint. This effect can be ascribed to the surface roughness, which results in very high local stresses leading to buckling and deformation of the tracheids. This situation is similar to that found for adhesive joints in or close to the fibre direction such as finger or butt joints.     

Photodegradation of wood flour/polypropylene composites incorporated with carbon materials with different morphologies

ABSTRACT

In this study, wood flour/polypropylene (WF/PP) composites were prepared by incorporating 2?wt% carbon materials with different morphologies, including carbon black (CB), graphite (G), and multi-walled carbon nanotubes (CNTs). WF/PP composite without any additives was included as a reference. All composites were placed in the accelerated UV weathering tester for a total of 960 h. The physical and mechanical properties of composites were compared. The changes in surface morphology and surface chemistry were characterized by SEM AFM, and ATR-FTIR. The results showed that: (1) the addition of CNTs improved both flexural and impact properties of composites; (2) composites containing CNTs exhibited better color stability and less severe surface cracking during weathering; and (3) ATR-FTIR results revealed that all the additives alleviated surface photo-oxidation of composites due to their UV-screening effect. Overall, due to the improved interfacial bonding between WF and PP, CNTs were more effective in protecting wood–plastic composites from photodegradation.     

木材表面非极性化原理的研究 I.木材乙酰化过程中化学官能团的变化

对杉木和“三北”一号杨两种木材的各主要组成进行了酯化处理,研究其在乙酰化过程中木材各主要组成的化学官能团的变化特征和结构的稳定性。结果表明：磨木木质素、纤维素、半纤维素经乙酸酐处理后,红外光谱上的１７４２ｃｍ－１附近均出现了表示酯类ＣＯ伸缩振动的新的吸收峰,表明在乙酰化过程中这些组分的化学结构都有新的非极性酯类化学官能团产生;而极性的羟基官能团的数量均有不同程度的减少;芳香基和脂肪基的结构上均有乙酸酯生成。纤维素酯化程度较低,半纤维素化学结构在乙酰化过程中会发生降解,而木质素和纤维素较为稳定。乙酰化处理是降低木材表面极性的一种有效方法。     

USING HARALICK METHOD TO MEASURE WOOD SURFACE ROUGHNESS

INTRODUCTIoNWoodsurfaceroughnessisaveryim-portantparameterforevaluatingsurfacemanufacturingquality.Sometimes,thellmitsofwoodsurfaceroughnessmustbegivenbeforemachined,becausewoodsur-faceroughnessdirectlyaffectspaintingandgluingqualityofwoodproductsandtheconsumptionofpaintmaterialsandgluingmaterials.Thesurfaceroughnessalsocanbeusedtoevaluatethequalityofma-chiningdeviceandtoolsandthesuitabilityofworkingprocesses.C0NSTRUCTIONOFMEASUREMENTSYSTEMConstructionof'themeasurementsys-temis…     

白橡热压干燥材等温吸湿特性研究

以白橡热压干燥材为研究对象,利用动态水分吸附仪研究了不同热压温度干燥处理后白橡木材和未处理对照材的等温吸湿特性,并采用H-H模型拟合;分析热压干燥对木材吸湿特性的降低机理。结果表明:白橡木材等温吸湿线皆为IUPAC Ⅱ型等温吸湿线。在任意相对湿度下,热压干燥材平衡含水率均明显低于对照材,且热压温度越高,平衡含水率降低越明显。H-H模型对白橡木材等温吸湿数据表现出良好的拟合效果。单分子层和多分子层含水率降低共同作用使得热压干燥材吸湿性降低,且相对湿度越高,多分子层水的减少对吸湿性的降低作用越大。与对照材相比,热压干燥材(140、150 ℃和160 ℃)的纤维饱和点推测值分别降低8.89%、11.76%和13.62%。白橡热压干燥材吸湿性降低机理主要为游离羟基等亲水基团含量减少和细胞壁刚度增加等。     

Sorption of acetylated pine wood decayed by brown rot, white rot and soft rot: different fungi—different behaviours

Acetylated wood was exposed to decay by brown, white and soft rot in order to address the sorption behaviour of decayed wood. The sorption isotherms were analysed using the Hailwood–Horrobin model. It was found that the different types of decay exhibit different sorption properties. Brown rot decayed wood exhibits significantly lower hygroscopicity in both total, monomolecular and polymolecular sorption. Soft rot decayed wood followed the same pattern as the brown rot decayed wood; however, the reduction in hygroscopicity was not significant. White rot decayed wood showed a different behaviour, with increased hygroscopicity in total and polymolecular sorption and decreased hygroscopicity in monomolecular sorption.     

Analysis of the temperature dependence of water sorption for wood on the basis of dual mode theory

Isotherm curves of water sorption for wood at various temperatures were analyzed based on the dual mode theory where the total coverage was represented by a linear combination of the Langmuir and Henry equations. The saturation concentration and affinity constant of the Langmuir equation and the parameter of Henry’s law had a transition point near 60°C. The analysis based on the dual mode theory found that the constants for whole wood were related to those of wood components and depended more on their glass transition temperatures. That is, it was theoretically demonstrated that the characteristic tem-perature dependence of water sorption for wood occurs because wood consists of three components (cellulose, hemicellulose, and lignin) with different glass transition temperatures.