聚氨酯预聚体的合成及其在木材粘接领域的应用

本文研究了聚氨酯预聚体的合成工艺，测试了固化后的聚氨酯胶粘剂胶片的性能，讨论了分子结构对拉伸强度和伸长率的影响，同时进行了聚氨酯粘剂用于含水率木材粘接的试验，结果表明，具有线型结构的，高分子量的胶粘剂延伸性好，但拉伸强度稍低，聚氨酯胶粘剂可以用于高含水率木材的粘接。     

日本落叶松无性系管胞力学性质的遗传变异

对4个12年生的日本落叶松无性系幼龄材早材单根管胞进行了拉伸实验。结果表明:日本落叶松无性系幼龄材早材单根管胞拉伸弹性模量、拉伸强度和断裂伸长率的均值分别为11.44 GPa、616.59 MPa和6.54%,相应的变异系数为26.97%、26.26%和23.17%。方差分析表明日本落叶松幼龄材早材单根管胞拉伸弹性模量和管胞断裂伸长率在无性系间差异极显著(0.01水平),拉伸强度在无性系间差异不显著;株内早材单根管胞拉伸弹性模量、拉伸强度和管胞断裂伸长率在年轮间差异显著。对日本落叶松无性系幼龄材早材单根管胞拉伸力学性能进行遗传参数估计,单根管胞弹性模量和断裂伸长率的重复力分别是0.79和0.57,属于中度到强度遗传控制,无性系早期选择及材性遗传改良潜力较大。     

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.     

Tensile properties of Moso bamboo (Phyllostachys pubescens) and its components with respect to its fiber-reinforced composite structure

Bamboo is a fiber-reinforced bio-composite since its culm wall is mainly composed of parenchymatous ground tissue in which vascular bundles are embedded. In order to analyze the mechanical properties of bamboo as a function of its components, tensile tests were performed on bamboo blocks and the corresponding volume fractions of fiber and parenchymatous ground tissue were measured. More significant linear relationships were found between tensile properties and volume fractions of the bamboo components. The tensile strength and modulus of elasticity of bamboo fiber and parenchymatous tissue were estimated according to the linear equations obtained by regression analysis. The macrographs of fractured bamboo blocks and the micrographs of fracture surfaces obtained by scanning electron microscope were also analyzed. Further tensile tests on separated bamboo fiber bundles were analyzed. Results show that the tensile strength of bamboo fiber obtained from the tests on bamboo blocks was higher than that on separated fiber bundles. This might be due to the interaction between components in bamboo in which parenchymatous ground tissue can pass loads and distribute the stresses loaded on fibers.     

Multivariate modeling of acoustomechanical response of 14-year-old suppressed loblolly pine (<Emphasis Type="Italic">Pinus taeda</Emphasis>) to variation in wood chemistry,microfibril angle and density

The polymeric angle and concentration within the S₂ layer of the softwood fiber cell wall are very critical for molecular and microscopic properties that influence strength, stiffness and acoustic velocity of wood at the macroscopic level. The main objective of this study was to elucidate the effect of cellulose, hemicellulose, lignin, microfibril angle and density on acoustic velocity and material mechanical properties of 14-year-old suppressed loblolly pine. Cellulose, hemicellulose and density are consistently the most important drivers of strength, stiffness and velocity. Cellulose and lignin are the highest and lowest contributor to velocity, respectively, with lignin acting as a sound wave dispersant, while cellulose is the most important conductor of sound wave at the molecular level, while hemicellulose acts as a special coupling agent between these components. The polymeric constituents are thus important drivers of sound wave propagation at the molecular level, while density played a subsequent role at the macroscale.     

超声辅助提取对竹纤维结构和机械性能的影响

文章以超声波辅助提取竹纤维,研究超声波处理以及超声波处理方式（前超声和后超声）对低试剂量条件下提取竹纤维结构和热性能的影响。结果表明：超声波处理应用于低试剂量竹纤维提取,可降低竹纤维胶质含量,改善竹纤维细度和细度均匀性。用后超声辅助提取竹纤维,竹纤维胶质含量可降低13.2%,纤维直径由489±247 μm降至224±52 μm,平均直径降低率达54.2%。超声辅助提取对竹纤维机械性能有一定程度的影响,对竹纤维拉伸强度略有降低,拉伸模量下降明显,断裂伸长率略有提高,机械强度均匀性增加;后超声辅助提取较前超声辅助提取对竹纤维的机械性能降低程度小。经提取工艺,竹纤维结晶度增加,但超声辅助提取竹纤维结晶度又略有下降,未超声竹纤维结晶度为56.50%,后超声竹纤维结晶度为55.89%,前超声竹纤维结晶度为56.25%;傅里叶红外光谱分析表明,竹纤维化学结构变化主要由纤维提取工艺引起,超声处理对纤维结构影响不明显;通过电镜观察,经纤维提取工艺,原竹材结构中被胶质包覆的单纤维形态暴露,单丝状明显,超声辅助提取增加了纤维表面粗糙度,后超声辅助提取单丝分散性增强;总体上,超声辅助提取可促进低试剂量条件下的长竹纤维提取,对纤维细度有明显改善效果,但对纤维结构和机械性能影响不大。     

Characterization of raffia palm fiber for use in polymer composites

Raffia palm fibers are potential reinforcement materials for making cost-effective polymer-based composite. This paper presents the results obtained from a study of physical, chemical, thermal and mechanical properties of raffia palm fibers (RPFs) derived from the raffia palm tree (Raphia farinifera). The as-received RPFs had their remnant binders manually removed and was subsequently cleaned in a 2% detergent solution before drying in an air oven at 70 °C for 24 h. Evaluation of the properties of the dried samples was carried out using a combination of characterization techniques including chemical composition determination, density measurement, moisture adsorption and water absorption measurements, tensile testing, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Raman spectroscopy, X-ray diffractometry, and Fourier transform infrared spectromicroscopy. The main constituents of RPFs were found to be cellulose, hemicellulose and lignin. The average diameter and average density were 1.53?±?0.29 mm and 1.50?±?0.01 g/cm³, respectively. The average breaking strength of the fibers ranged from 152?±?22 to 270?±?39 MPa; it did not vary significantly with fiber length and cross-head speed during tensile testing. The results of scanning electron microscopic investigation of the fibers showed that they comprise several elemental fibers which are tightly packed together with each having its own lumen. Synchrotron-based Fourier-transform infrared spectromicroscopy of a cross-section of the fiber showed that lignin is concentrated mostly on the outside while cellulose and pectin are concentrated in the mid-section. A two-stage water sorption behavior was observed for the fibers.     

Mechanical properties of natural fiber composites produced using dynamic sheet former

ABSTRACT

Composites formed from wood fibers and man-made cellulosic fibers in PLA (polylactic acid) matrix, manufactured using sheet forming technique and hot pressing, are studied. The composites have very low density (due to high porosity) and rather good elastic modulus and tensile strength. As expected, these properties for the four types of wood fiber composites studied here improve with increasing weight fraction of fibers, even if porosity is also increasing. On the contrary, for man-made cellulosic fiber composites with circular fiber cross-section, the increasing fiber weight fraction (accompanied by increasing void content) has detrimental effect on stiffness and strength. The differences in behavior are discussed attributing them to fiber/ fiber interaction in wood fiber composites which does not happen in man-made fiber composites, and by rather weak fiber/matrix interface for man-made fibers leading to macro-crack formation in large porosity regions.     

The influence of high defibration temperature on the properties of medium-density fiberboard (MDF) made from laccase-treated softwood fibers

This study was carried out to elucidate the effect of defibration temperature in the range 171–202°C on the properties of 12-mm thick MDF boards made without synthetic resins from softwood fibers activated by laccase treatment for the generation of phenoxy radicals on the fiber surfaces. Laccase treatment generated radicals in the fibers. An increase in defibration temperature improved the reactivity of fibers during laccase-catalyzed oxidation. The number of radicals detected in the fibers after laccase treatment in water suspension and the fiber oxygen consumption during the treatments increased with an increase in defibration temperature, while a concurrent improvement was observed in the mechanical strength and thickness swell of dry-process MDF boards made from fibers refined at different temperatures and treated with laccase in the refiner blowline. The different fiber reactivities or board properties were not due to a presence of different amounts of lignin remaining on the fiber surfaces after acetone extraction. The probable reason for them was the fact that the amount of low-molecular weight lignin, a reactive substrate for laccase, increases with increasing defibration temperature. The adhesion occurring during pressing is thus likely to involve coupling or other reactions of radicals located on adjacent fibers, whereby interfiber covalent bonds are formed.     

Effect of Wood Variables on the Properties of Wood Fiber-Polypropylene Composites

The effect of wood species (Chinese fir and Poplar), wood fiber content (10%, 25%, 40%) and wood fiber sizes (16 to 32 mesh, 32-65 mesh, above 65 mesh) on the properties of the wood fiber-Polypropylene composites were studied in this paper. The results indicate that the effect of wood fiber content and size in composite were more important than that of chosen wood species. Compared with polypropylene without wood fiber, the flexural strength of the composites increased when adding wood fiber into polypropylene, but the tensile and unnotched charpy impact strength decreased. And the above strength decreased with the wood fiber content increasing. When the wood fiber size becoming smaller (in higher mesh), the strength increased. In the comparison of wood species, the properties of composite using Chinese fir wood were better than that of Poplar, but not significant. The dynamic mechanical properties of the composites and PP were also tested and analyzed in this paper.     

纳米CaCO3对木纤维增强生物可降解复合材料力学性能的影响

为探索纳米CaCO₃对增强生物可降解复合材料力学性能的影响,采用混炼、注射成型工艺制备纳米CaCO₃改性木纤维/聚乳酸复合材料,研究了纳米粒子添加量（1wt%,2wt%,3wt%,4wt%）及粒子预处理（偶联剂,硬脂酸,偶联剂-硬脂酸）对材料拉伸性能与冲击性能的影响。随着CaCO₃添加量增加,复合材料力学强度先增大后减小,质量分数2%时材料拉伸强度和冲击强度分别提高8%与20%,粒子的增韧效果明显。预处理不仅能增强木纤维与聚乳酸的结合,也提高了纳米粒子分散性,增强材料整体力学性能。纳米粒子在聚合物基体中的分散性及其与聚合物界面结合是影响材料性能的关键。     

竹节对竹材力学强度影响的研究

本文对毛竹和刚竹的带节材与不带节竹材的主要力学性质研究表明,带节竹材的抗弯强度、顺纹抗压、抗剪、抗拉强度和冲击韧性都有一定程度降低的趋向,但抗劈开强度和横纹抗拉强度却有明显提高。影响竹材力学强度的主要因素是维管束数量、维管束排列方向及维管束中纤维的力学强度,了解这些规律,对复合材料的结构仿生有重要参考作用。     

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.     

Effects of laccase incubated from white rot fungi on the mechanical properties of fiberboard

White rot fungi were optimized to cultivate highly active laccase. The characteristics of laccase incubated by continuous culture were compared with those of direct culture. The enzyme activity of laccase incubated by continuous culture technology reached a higher value on the fifth day of the growth. The optimization incubation time of high activity laccase was the eleventh day. A large amount of highly active laccase can be obtained in a relatively short time by continuous culture to replace traditional laccase. After laccase treatment, the lignin composition of wood fibers were oxidation-catalyzed by laccase. The number of chemical-bonding points between the wood fibers was increased. The wood fibers treated by laccase were fabricated into boards and their mechanical properties improved with the laccase-incubation times.Compared with the fiberboards made from fibers that were pre-treated by laccase of incubation 5 days, the static bending strength of those that were pre-treated by laccase of incubation 11 days was increased by 18.95%, the elastic modulus was increased by 35.49%, and the internal bond strength was increased by 44.11%.     

Surface analysis of explosion pulps by ESCA Part 1. Carbon (1s) spectra and oxygen-to-carbon ratios

Summary The surface composition of explosion pulp and conventional CMP and CTMP underwent ESCA spectroscopy. Explosion pulps exhibited a higher oxygen-to-carbon ratio and less C1 peak areas compared to conventional CMP/CTMP. Based on the theoretical O/C ratios and C1 contents for the main components of wood fibers (i.e., carbohydrates, lignin and extractives), a triangular graph was used to illustrate the relative amounts of the three components on the surface. This analysis indicated that explosion pulps had more carbohydrates exposed on their fiber surface, which may explain their high physical strength. A bulk analysis indicated that these differences could not depend on their bulk compositions, since the latter were similar.     

Strength properties of black spruce wood under different treatment

Summary The effect of various forms of treatment — chemical, thermal and pressure — on the tensile properties of wood was investigated. Spruce was impregnated in water, sodium sulfite and/or sodium bicarbonate, and heated at temperatures ranging from 20 to 190 °C. At the end of cooking (190 °C), decompression was applied both slowly and suddenly.A rise in temperature, an increase in heating time, from 4 to 10 min. at 190 °C, as well as fast pressure release influenced the tensile strength. The chemical treatment resulted in lignin sulfonation while carboxylation produced fiber swelling and, consequently, tensile strength decreased.The authors wish to thank the FCAR (Québec), NSERC (Canada) and Stake Tech. Co. for their financial support     

干法无胶纤维板粘合机理的研究Ⅲ．木材主要化学成分的影响

分别用氢氧化钾和亚氯酸钠处理木材原料，以移去木材原料中的一部分半纤维素和木素。用这些特制浆料压制的无胶纤维板，无论是板的强度性能还是板的耐水性能都比未处理木材原料制造的无胶纤维板的性能有明显的下降。这一结果表明，木材原料中的半纤维素和木素都对纤维间自生胶粘因素的形成具有重要的影响     

Micromechanics of wood subjected to axial tension

Summary The behaviour of a small group of wood fibers of Sitka spruce during tensile loading is investigated. The load-extension curves for both early and late wood fibers consist of three distinct segments. The first segment is almost a straight line, at some stage of loading a yield point is observed. Beyond this point the specimen becomes less stiff and undergoes a large, mainly irreversible deformation. As the load is increased further, the curve exhibits the third segment showed by a significant change in slope. These curves look different from those obtained on thick specimens. In this respect, the behaviour of a thin wood specimen subjected to cyclic type tensile loading along its longitudinal direction is also illustrated. Based on wood microstructure, a model is presented to interpret the evolution of the Young's modulus of a wood fiber during tensile loading. The model considers wood as an assembly of cylindrical fibers pasted together in a longitudinal direction. We have assumed the cell wall to comprise only an S₂ layer made of a composite material consisting of a lignin and hemicellulose matrix reinforced by helical microfibrils along the fiber. Furthermore, it is assumed that the microfibril angle a in the S₂ layer is not uniform along the fiber axis and matrix degradation occurs in the zones where the microfibril angles are bigger. The validity of this assumption is verified by using holographic interferometry to visualize the displacement field of the specimen's surface under tension.The work reported in this paper is supported by a grant from the Swiss National Science Foundation, and its support is gratefully acknowledged     

Field test of resistance of modified wood to marine borers

Abstract

Five mineral fillers were tested for wood–plastic composites (WPCs): calcium carbonate, two different types of wollastonite, soapstone and talc. The impact of the fillers on the mechanical properties of the composites was studied. The experiments included bending tests, tensile tests, Brinell hardness and scanning electron microscopy experiments. The amount of wood, mineral and plastic (polypropylene) was kept steady. Only the mineral type was changed during the tests. A control sample without any mineral added was also manufactured. The mineral addition improved the tensile strength of the WPCs. The hardness of the composite was also improved when the minerals were added, and along with the increasing mineral hardness, the hardness of the composite increased. The wollastonite acicular shape was crushed during the manufacturing process, so the phase of the process in which the minerals are added requires careful consideration.     

The ambient aging of wood fiber and its effect on mechanical properties of MDF panels

Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) were used to study ambient-aged wood fibers and their effects on the mechanical properties of medium-density fiberboard (MDF). It was found that MDF made with ambient-aged fibers had poorer mechanical properties than MDF made with fresh fibers; this difference resulted from the alterations of surface characteristics of wood fibers after ambient aging, which led to poor wettability of the urea–formaldehyde (UF) resin applied to the aged wood fibers. After 6 months of ambient aging, the concentration of carbonyl groups in the fibers increased by 144%, while the pH value of wood fiber decreased from 5.2 to 4.7. SEM showed that much more UF resin agglomerated on the surface of ambient-aged fibers and the breakage of MDF made with aged fiber frequently occurred at the resin-fiber interfaces, indicated the poorer wettability of UF resin to fibers due to the decrease in surface energy after aging.