Surface modification of wood flour and its effect on the properties of PP/wood composites

The surface of wood flour used as reinforcement in PP/wood composites was successfully modified by benzylation in NaOH solution of 20 wt% concentration at 105 °C. The time of the reaction was changed between 5 and 360 min in several steps. The progress of modification was followed by the measurement of weight increase and by diffuse reflectance infrared spectroscopy (DRIFT). The structure of the wood was characterized by X-ray diffraction (XRD) and its surface tension was determined by inverse gas chromatography (IGC). PP composites containing 20 wt% filler were prepared from a PP block copolymer and the modified wood flour. The mechanical behavior of the composites was characterized by tensile testing. The majority of the active hydroxyl groups at the surface were replaced by benzyl groups in about 2 h under the conditions used. Further increase in reaction time did not influence the properties of the filler. Both the structure of the wood flour and its surface tension changed as an effect of modification. The reduction of surface tension led to significant changes in all interactions between the wood flour and other substances resulting in a considerable decrease of water absorption, which is the major benefit of this modification. All measured mechanical properties of the composites decreased slightly with increasing degree of modification. A detailed analysis of the results proved that the dominating micromechanical deformation process of these PP/wood composites is debonding, which is further facilitated by the decrease in the surface tension of the filler. Chemical modification of wood flour slightly improved processability and the surface appearance of the composites prepared with them and considerably decreased the water absorption of these latter.     

Fatigue properties of wood and wood composites

An overview of the state of fatigue research for wood and wood products is presented. The extreme lack of a satisfactory data base and the persistent neglect of fatigue characteristics in wood material design is emphasized. Some theoretical considerations are presented to point out the difficulties met when attempting to model the behavior of wood products exposed to cyclic loading phenomena.A major conclusion of the work is the need for the entire materials science community to consider wood as a valuable and rewarding material upon which to focus research efforts. As wood is the world's primary renewable structural resource, a concerted effort must be made to understand its fatigue performance and the degree of accuracy in predicting life expectancy.

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Résumé On présente une revue générale des recherches dans le domaine de la fatigue du bois et des produits à base de bois. On fait remarques l'extrème pauvreté de bases de données satisfaisantes et la négligence persistante avec laquelle les caractéristiques de fatigue sont prises en considération dans la conception à partir de matériaux en bois. On présente quelques considérations théoriques exprimant les difficultés rencontrées lorsqu'il s'agit de modéliser le comportement des produits ligneux exposés au phénomène de mise en charge cyclique.Une conclusion importante du travail vise à sensibiliser la communauté des sciences des matériaux à considérer le bois comme un matériau valable et prometteur sur lequel un effort de recherche doit être concentré. Comme le bois est la ressource structurelle première entièrement renouvelable de ce monde, un effort concerté doit être entrepris afin de comprendre ses performances à la fatigue et le degré d'exactitude des prévisions de vie.

     

Dielectric properties of macro-defect-free (MDF) cements

Macro-defect-free cements, based mostly on Secar-71 and using polyvinyl acetate as a plasticizer, have been prepared. Dielectric constant and loss factor at 1 MHz and d.c. resistivity of these cements have been measured as a function of their composition, processing conditions and humidity. For some of the samples, flexural strengths have been measured in order to correlate with their electrical properties. Results of efforts to inhibit the effects of higher humidities on the electrical properties are also reported.     

高温中和高温后LVL木纤维复合材的抗弯性能

为研究LVL木纤维复合材在高温中和高温后的抗弯性能,对104个试样进行了温度在20~225℃下的三点静态抗弯测试。研究结果表明,随着温度的升高,高温中和高温后的抗弯强度、抗弯弹性模量、延性系数均明显减小;相对于高温中的试样,高温后的抗弯强度和抗弯弹性模量均明显较高,而延性系数则较低。根据回归分析,提出了LVL在高温中和高温后的抗弯强度与温度的关系模型,该模型预测结果与实测值吻合良好。     

Changes of wood properties treated with aqueous amine solution,bending tests and X-ray analysis of wood after amine treatment

Bending tests were conducted on oven-dried wood samples (Picea jezoensis Carr.) following treatment with various concentrations of aqueous ethylenediamine (EDA) to investigate the influence of amine treatment on the mechanical properties of wood. Under oven-drying conditions following EDA treatment and a methanol rinse, the densities of wood samples increased at concentrations above 50%, and the Young’s modulus decreased at concentrations above 60%. The specific Young’s modulus of wood samples decreased at concentrations above 60%, and stress- and strain-at-yield changed slightly at EDA concentrations in the range of 60–70%. X-ray analysis showed that the structures of cellulose changed at concentrations above 60% EDA and confirmed the transformation into cellulose III_I at 70% EDA. These results indicate the possibility that changes in the structure of the cell wall, accompanied by changes in the structures of cellulose microfibrils, contributed to changes in the specific Young’s modulus of the treated wood samples. In the same concentration range, changes in the Young’s modulus of wood samples increased with increasing relative humidity (RH). This also suggests that changes in the cell wall structure during the treatment contributed to changes in the Young’s modulus of wood at different RHs.     

Dielectric properties of chemically treated wood

Dielectric properties along the grain for absolutely dried untreated and seven kinds of chemically treated Sitka spruce (Picea sitchensis Carr.) woods were measured. Cole-Cole's circular arc law was applied to the results of the relaxation due to the motions of methylol groups. The following changes were caused by chemical treatments. In polyethylene glycol (PEG) impregnation, the distribution of relaxation times became very narrow, the generalized relaxation time (_m) was considerably decreased, and the relaxation magnitude (₀ – ) was slightly increased. In acetylation, the distribution of relaxation times became very broad, _m was considerably increased, and (₀ – ) was remarkably decreased. In propylene oxide treatment, the distribution of relaxation times became slightly narrow and _m was decreased. _m was slightly decreased in formalization, phenol-formaldehyde (PF) resin treatment and wood methyl methacrylate (MMA) composite. (₀ – ) was decreased in formalization and PF-resin treatment and was hardly changed in wood-MMA composite and heat treatment. The distribution of relaxation times was almost unchanged in formalization, PF-resin treatment, wood-MMA composite and heat treatment.     

Terahertz birefringence and attenuation properties of wood and paper

The far-infrared properties of spruce wood are examined with a terahertz time-domain spectrometer. The solid wood is shown to exhibit both birefringence and diattenuation. The birefringence properties are sufficient for construction of a quarter-wave plate operating at 0.36 THz, and a half-wave plate operating at 0.71 THz. The origin of the birefringence is attributed to preferential fiber orientation within the wood. Similar birefringence is observed in lens paper in which the fibers are preferentially oriented in one direction.     

线切割单晶硅表面损伤的研究

利用台阶仪、扫描电镜（ＳＥＭ）和Ｘ射线双晶衍射仪,研究了线切割硅片和内圆切割硅片的表面切割损伤和损伤层厚度。实验指出线切割硅片表面粗糙度大,外表面损伤大,但损伤层的厚度要小于常规内圆切割硅片。初步讨论了影响线切割硅片表面损伤的原因     

木材表面化学镀镍工艺的研究

实验研究了普通木材表面化学镀镍的操作方法、工艺条件,讨论了各种操作因素对镀层质量的影响,试验了化学镀层与镀件基体的结合力,确定了化学镀镍的工艺流程和工艺条件。实验结果表明,木材经过预处理及敏化、活化处理后,化学镀镍溶液的pH值控制在8．0,镀镍温度控制在65℃,反应时间40min,木材表面可以得到较好的化学镀层。     

Vibrational properties of wood along the grain

The dynamic Young's modulus (E_L) and loss tangent (tan _L ) along the grain, dynamic shear modulus (G_L) and loss tangent (tan _S) in the vertical section, and density () of a hundred spruce wood specimens used for the soundboards of musical instruments were determined. The relative acoustic conversion efficiency ( ) and a ratio reflecting the anisotropy of wood (, (E_L/G_L)(tan _S/tan _L)) were defined in order to evaluate the acoustic quality of wood along the grain. There was a positive correlation between and , and the variation in was larger than that in . It seemed logical to evaluate the acoustic quality of spruce wood by a measure of . By using a cell wall model, those acoustic factors were expressed with the physical properties of the cell wall constituents. This model predicted that the essential requirement for an excellent soundboard is smaller fibril angle of the cell wall, which yields higher and higher . On the other hand, the effects of chemical treatments on the and of wood were clarified experimentally and analyzed theoretically. It was suggested that the and of wood cannot be improved at the same time by chemical treatment.     

Mechanical and interfacial properties of wood and bio-based thermoplastic composite

The aim of this investigation was to study a new family of wood polymer composites with thermoplastic elastomer matrix (pebax® copolymers) instead of commonly used WPC matrices. These copolymers are polyether-b-amide thermoplastic elastomers which present an important elongation at break and a melting point below 200 °C to prevent wood fibers degradation during processing. Moreover these polymers are synthesized from renewable resources and they present a hydrophilic character which allow them to interact with wood fibers. We have used two pebax® grade with different hardness and three types of wood fibers, so the influence of the matrix and wood fibers characteristics were evaluated. Composites were produced using a laboratory-size twin screw extruder to obtain composite pellets prior to injection moulding into tensile test samples. We have evaluated fibers/matrix interaction by differential scanning calorimetry (DSC), infrared spectroscopy (IRTF) and scanning electron microscopy (SEM). Then, the mechanical properties, through tensile test, were assessed. We also observed fibers dispersion into the matrix by tomography X. DSC, IRTF and SEM measurements confirmed the presence of strong interface interactions between polymer and wood. These interactions lead to good mechanical properties of the composites with a reinforcement effect of wood fibers due also to a good dispersion of fibers into the matrix without agglomerate.     

金属镍木材复合材料的制备及其性能研究

以速生林木材为基体,丁二酸C4H6O4和苹果酸C4H6O5为络合剂,N2H4.H2O为还原剂,采用水热法在木材基质内将醋酸镍原位还原得到金属Ni木材复合材料。通过光学显微镜、X射线衍射仪(XRD)和扫描电子显微镜(SEM)对复合材料的形貌结构进行表征。光学显微结果显示金属镍在木材细胞壁内分布均匀;XRD结果显示在44.5°出现了Ni(111)的特征峰;SEM结果表明金属镍为片状结构,粒径分布较窄,微粒尺寸约为0.5～1μm。力学性能测试结果表明,复合材料的力学性能得到提高,当金属镍的质量分数为2%,反应时间为12h时,比例极限和抗压强度相对于生材可增加83.42%和20.65%。导电性能测试结果表明,复合材料的电导率显著增加,在Ni的质量分数为3%,反应时间为6h时电导率达到最大为1.25×10-2S/m。     

Cell-wall recovery after irreversible deformation of wood

The remarkable mechanical properties of biological materials reside in their complex hierarchical architecture and in specific molecular mechanistic phenomena. The fundamental importance of molecular interactions and bond recovery has been suggested by studies on deformation and fracture of bone and nacre. Like these mineral-based materials, wood also represents a complex nanocomposite with excellent mechanical performance, despite the fact that it is mainly based on polymers. In wood, however, the mechanistic contribution of processes in the cell wall is not fully understood. Here we have combined tensile tests on individual wood cells and on wood foils with simultaneous synchrotron X-ray diffraction analysis in order to separate deformation mechanisms inside the cell wall from those mediated by cell-cell interactions. We show that tensile deformation beyond the yield point does not deteriorate the stiffness of either individual cells or foils. This indicates that there is a dominant recovery mechanism that re-forms the amorphous matrix between the cellulose microfibrils within the cell wall, maintaining its mechanical properties. This stick-slip mechanism, rather like Velcro operating at the nanometre level, provides a 'plastic response' similar to that effected by moving dislocations in metals. We suggest that the molecular recovery mechanism in the cell matrix is a universal phenomenon dominating the tensile deformation of different wood tissue types.     

Manufacture and properties of ceramic cutting tools: a review

Abstract

The past 15 years have seen major advances in the development of ceramic tool materials to give high cutting speeds and long tool lifetimes. These developments require rigid machine tools and high-power motors, and a change in the way the tool tips are used. The main areas of application of these new tool materials are in the aerospace industries and, to a somewhat lesser extent, in cast iron production. There is an ever increasing number of grades (and tradenames) available, and in this paper the background and development of the materials are described. Briefly, the materials may be classified as alumina or silicon nitride based; these base compositions, with alloying additions, form two main families of materials. The manufacturing routes are discussed as these influence both the grain size and the porosity of the finished material, and both these properties affect the behaviour of the materials in use. Finally, the wear of these tools is examined in the light of present understanding of the microstructures.

MST/713     

Mechanical properties of wood examined by semi-destructive devices

This study presents a comparative evaluation of semi-destructive and destructive testing of wood. Semi-destructive methods are represented by screw-withdrawal test, microdrilling, pin penetration and a new construction of a diagnostic device for in situ evaluation of timber based on the principle of measuring the mechanical resistance to pin being pushed gradually into wood. In order to ascertain the differences between the methods, measuring of three basic softwood species was conducted. One of the aims was to capture the variability of properties brought about by the distribution of properties along the diameter and along the trunk (section) length; therefore, an entire trunk of spruce, fir and pine were used for testing. The accuracy of the prediction of mechanical properties provided by the semi-destructive devices was verified using correlations with mechanical properties established by standard pressure tests in compliance with European norms. The results show a very good correlation of the mean force necessary to push the pin in the timber with wood density and strength in compression along the grain found in standard specimens.     

Physical and mechanical properties of injection-molded wood powder thermoplastic composites

In the last decades, filler-reinforced thermoplastics especially natural filler-reinforced plastics have been frequently used to improve the physical and thermal properties of polymer materials in plastic industry due to their low density, fully degradable, helpful to reduce the CO₂ emission, free from health hazard and low cost. At current study, wood powder/polypropylene composites (wood/PP) with different filler contents were molded by injection molding process to investigate the effect of filler content on the physical and mechanical properties of the composites. Additionally, the comparison of physical and mechanical properties between talc/PP (which has been widely used in the automotive products) and wood/PP has been carried out based on the tensile, bending, Izod impact tests and the scanning electron microscope observation on the fracture surfaces. Results showed that the highest mechanical property of wood/PP was determined at 30?wt.% wood content. More interesting is that, at the same composite density up to 30?wt.%, the mechanical property of wood/PP was much higher than that of talc/PP. Theoretically, Nielsen equation is often used to predict the elastic modulus of filler reinforced plastics, and in this study, the predicted values were in good agreement with experimental values up to 30?wt.%, after that, they were higher to the experimental values of wood/PP composite. It is considered that is due to the discounted of the distribution and orientation of the filler in the matrix in Nielsen equation.     

Fatigue properties of wood in tension,compression and shear

The fatigue properties of wood laminates have been investigated in tension, compression and shear. Fatigue lives in compression are significantly less than in tension, and S-N data at five R ratios has yielded a set of constant-life lines, the form of which is related to the failure mode of the wood observed by scanning electron microscopy. A point of inflection in the constant-life lines at the transition between all compressive and partially tensile fatigue loading is a new observation. S-N curves for wood laminates have been produced for shear across the radial-longitudinal and tangential-longitudinal planes, and the latter plane is observed to be more resistant to fatigue loads. Samples with four times the cross-sectional area of standard-sized samples have been fatigued at R=−1 and no significant difference in fatigue life is apparent. It appears that the absence of a size effect in tension-compression results from the orthotropic structure of wood which is insensitive to variations in the density of surface flaws.     

二氧化硅/木材复合材料的微观结构与物理性能

为了明确二氧化硅/ 木材复合材料的微观结构与物理性能, 通过EDAX 及XRD 的测定分析了溶胶2凝胶法制备的二氧化硅/ 木材复合材料的微观结构, 通过应力松弛、介电性质、热重、表面显微硬度的测定分析了该材料的物理性能。结果表明: 生成的二氧化硅与增重率呈正相关, 在纤维饱和点以下, 生成的二氧化硅存在于细胞壁中。XRD 衍射峰位置没有改变, 增重率增大, 峰强减弱, 结晶度减小。应力松弛量变小, 材料内部的结合力增强, 分子间产生了交联结合。介电常数值增大, 介电损耗随着频率的增加呈先增加后减小的趋势, 室温下, 出现最大峰值的频率均在log f = 6. 5 Hz 附近。热失重过程中, 快速失重的起始温度提高, 残余质量增加, 表面显微硬度提高。