Comparative analysis of a wood: adhesive bondline

The wood–adhesive interface was analyzed using five methods with the objective of quantitatively assessing penetration of adhesive into the porous wood network. Methods included fluorescence microscopy, scanning electron microscopy, backscatter electron imaging, wavelength dispersive spectroscopy, and X-ray microtomography (XMT). Each method provided a visual inspection, and all of the analysis methods were applied to the same field of view. XMT was the primary technique of interest. Rubidium hydroxide was used in place of sodium hydroxide in the formulation of phenol–formaldehyde adhesive. Rubidium was found to increase the X-ray attenuation of the adhesive. However, rubidium migrated beyond the adhesive interphase during specimen preparation, thus reducing its effectiveness for image contrast enhancement. The wood species studied included red oak (Quercus rubra), Douglas-fir (Pseudotsuga menziesii), and hybrid poplar (Populus deltoides × Populus trichocarpa). All techniques used for this study were useful, but each presented some limitations for bondline analysis. Despite the problem with rubidium migration, XMT for this application was promising.     

New approaches to wood bonding A base-activated lignin adhesive system

Current knowledge of wood surface characteristics and surface modification are briefly reviewed and the postulated effects of chemical activation are summarized. It was found that aqueous sodium hydroxide can effectively activate wood surfaces to give strong dry autohesive bonds, but only low wet strength was obtained. However, excellent dry and wet wood bond strengths, equivalent to phenol-formaldehyde bonded samples, were obtained when methylolated lignin was used in combination with 3N sodium hydroxide activation. Several mechanisms of base activation are suggested, including enhanced wood surface contact and reactivity.     

Water sorption mechanisms of commercial wood adhesive films

As water moves inside the wood, the gluelines might act as barriers that are potentially leading to local moistening. Even low amounts of water may influence the mechanical performance of glued wood products. Dynamic vapor sorption analysis was performed to assess the sorption processes of six commercial wood adhesives. Vapor sorption isotherms were compared with vapor uptake dynamics. Phenol–resorcinol–formaldehyde showed high moisture uptake of 18 %, while vapor diffusion speed was low. Fish glue showed a water uptake of 45 % at otherwise moderate vapor uptake speed. Melamine–formaldehyde resin gained 22 % water, and polyvinyl acetate absorbed 10 %. The latter was also the fastest vapor absorbing adhesive. Polyurethane only absorbed 3.5 % of moisture at medium uptake speed. Mechanisms of water diffusion seem to be driven by (1) the available free volume in the polymer and (2) the interacting ionic groups of the polymer chain. While the free volume could be linked to the accumulated moisture, the ionic group interaction might determine the measured vapor diffusion dynamics.     

Technological heterogeneity of adhesive bonds in wood joints

 Gluing of wood is among the most effective methods for the permanent joining of furniture elements or building woodwork manufactured from wood. Technological errors occurring during the preparation process of the glue material may lead to variations in the strength of adhesive/wood joints. The purpose of the described research project was to investigate the effect of the heterogeneity of the glue bond on the distribution of tangential stresses in furniture joints, especially the effect of gas cavities, faulty glue bonds and glue outflows on the distribution of tangential stresses in adhesive bonded overlap, cross and angle wood joints. Using developed numerical models, it was shown that shear stresses in bonds of cross and angle joints reach their maximum values in corners of joints. The torsion center of cross joints is situated in the geometrical center of the bond, while in angle joints – it is found half-way through the length of one of the perpendicular edges of the joint. It was also proven that gas cavities present in the glue bond contribute to increased stresses in the neighbourhood of the source of heterogeneity. This phenomenon initiates a process of de-cohesion and, hence, reduces the overall strength of the joint. Faulty gluing, similar to gas cavities, constitutes a potential source of stress-breaking processes and reduces the strength of joints. On the other hand, glue outflows present in wood bonds increase their strength by expanding the initiation threshold of fractures even in situations where technological heterogeneity of the glue bond occurs. In furniture constructions as well as in large-size building woodwork constructions or, wherever grace and elegance of the finished product is of lesser importance, glue outflows can be treated as a positive and desirable phenomenon. Received 13 March 1999     

Comparison of protein-based adhesive resins for wood composites

The search for new value-added uses for oilseed and animal proteins led us to develop protein-based wood adhesives. Low-fat soy and peanut flours and blood meal were hydrolyzed in an alkaline state, and PF-cross-linked protein resins were formulated by reacting the protein hydrolyzates with phenol-formaldehyde (PF) in solid-tosolid ratios ranging from 70% to 50% hydrolyzates and 30% to 50% PF. Physical properties of medium density fiberboard (MDF) bonded with protein-based phenolic resins were compared to those of boards bonded with ureaformaldehyde (UF) and PF resins, and flakeboard bonded with soy protein-based phenolic resin was compared to PF-bonded board. As MDF binders, adhesive properties of protein-based phenolic resins depended upon protein content of proteinacious materials. MDF board bonded with blood-based phenolic resin was comparable to PF-bonded board and met the requirements for exterior MDF. Boards bonded with soy-protein-based phenolic resin met requirements for interior MDF, while peanut-based phenolic failed to meet some of the requirements. Flakeboard bonded with soy-protein-based phenolic resins was inferior to PF-bonded board but outperformed PF-bonded board in accelerated aging tests. Although they exhibit a slow curing rate, the cost effectiveness and superior dimensional stability of protein-based phenolic resins may make them attractive for some uses.     

乙酸酐对大豆基木材胶粘剂的改性研究

用乙酸酐改性大豆基木材胶粘剂,研究了乙酸酐为不同浓度时,双因素(pH值和温度)对耐水胶合强度的影响。试验结果得到4个不同浓度下pH值和温度的最佳组合值为:乙酸酐体积分数为1.2%、1.8%、2.4%、3.0%时对应的pH值/温度是8/55℃、8/55℃、7/70℃、7/70℃,最佳耐水胶合强度可提高到0.72 MPa。以乙酸酐体积分数为1.8%为例,采用茚三酮法测得大豆蛋白质的酰化程度,分析不同温度下pH值对酰化程度的影响,结果显示:耐水胶合强度与酰化程度不成正相关。由红外光谱分析可知,乙酸酐与大豆蛋白的游离氨基发生了交联反应,从而提高了大豆基木材胶粘剂的耐水胶合强度。     

A global rheological model of wood cantilever as applied to wood drying

In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep. The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood, the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time. The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process.     

Evaluation of decay resistance of wood products made from borax-impregnated wood and bonded with a formaldehyde-free cornstarch and tannin adhesive

? At present, the production of wood composites mainly relies on the petrochemical-based and formaldehyde-based adhesives such as phenol-formaldehyde (PF) resins and urea-formaldehyde (UF) resins, which are non-renewable and therefore ultimately limited in supply.

? This paper concerns the decay resistance of wood products bonded with a new, environment-friendly adhesive derived from abundant and renewable cornstarch and tannin. To improve the total resistance of the composite against both Coriolus versicolor and Coniophora puteana rot fungi, borax (di-sodium tetraborate) was added in proportions of 0.5%, 1% and 2% (w/w) to the cornstarch-tannin adhesives.

? The results show that increasing the concentration of borax in the adhesive decreased the mechanical properties of the composite. The best way to avoid this problem was to use wood impregnated with borax.

? Biodegradation studies were conducted on new composites, first without any treatment, followed by borax at 0.5% aqueous solution treatment. The results show that wood impregnated with borax, in the presence of tannin and sodium hydroxide in the adhesive improves the total resistance of the wood composite against both Coriolus versicolor and Coniophora puteana rot fungi.

     

一种新型竹木集成材粘着剂问世

CU—3型粘着剂为日本株式会社生产,日本竹协会特别推荐的具有超高水平,超高安全性能的水基乙烯聚氨酯型粘着剂。最适合于竹子集成材及各种木质集成材的粘合。作为竹材特选专用粘着剂,CU—3型粘着剂已经在日本国内广泛使用,并博得用户很高的评价。凡使用CU—3型粘着剂的竹木制品能够顺     

Piezoelectricity as a fundamental property of wood

Summary The piezoelectric effect in wood, i.e. the occurrence of electric polarization under mechanical stress and also of mechanical strain in an electric field, was accounted for by considering the uniaxial orientation of cellulose crystallites in fibers and their monoclinic symmetry. A shear stress in one plane, including the grain direction, produced electrical polarization perpendicular to it. The value of the piezoelectric modulus for wood was approximately one twentieth of that of a quartz crystal.The chemical treatments which transform the lattice structure from cellulose I to II or III, increased the piezoelectric modulus. However, gamma-ray irradiation up to a dose sufficiently high to decrease the molecular weight had only little influence on the piezoelectric modulus.The variation with temperature of the phase angle between sinusoidal stress and polarization showed a maximum of advanced phase around room temperature and a maximum of delayed phase at about-100°C. Dielectric and viscoelastic measurements indicated that the former was caused by the dielectric loss due to water at a temperature above freezing and the latter by the viscoelastic loss due to local vibrations of cellulose molecules.The piezoelectric polarization in wood can be utilized in technical problems such as the measurement of shock velocity in timber. The physiological meaning of the piezoelectrical effect in plants has not been investigated.

---

Zusammenfassung Der piezoelektrische Effekt in Holz, d. h. das Auftreten einer elektrischen Polarisation unter mechanischer Spannung und ebenso das Auftreten mechanischer Verformungen in einem elektrischen Feld wird als Folgeerscheinung der einachsigen Orientierung der Cellulosekristallite in den Holzfasern und durch deren monokline Symmetrie erklärt. Es wurde beobachtet, daß eine Scherspannung in einer Ebene, welche in Faserrichtung liegt, eine elektrische Polarisation senkrecht dazu hervorruft. Die Größe des piezoelektrischen Moduls für Holz betrug etwa 1/20 des piezoelektrischen Moduls eines Quarzkristalls.Chemische Behandlungen, welche die Gitterstruktur der Cellulose I in diejenige von Cellulose II und III umformen, erhöhen gleichzeitig den piezoelektrischen Modul. Dagegen zeigte eine Behandlung mit -Strahlen selbst bis zu einer Dosisleistung, die ausreichte, um das Molekulargewicht zu erniedrigen, nur geringen Einfluß auf den piezoelektrischen Modul.Die Temperaturabhängigkeit des Phasenwinkels zwischen einer S-förmig verlaufenden Spannung und der Polarisation zeigt ein Maximum der vorauseilenden Phase etwa bei Raumtemperatur und ein Maximum der nachlaufenden Phase bei etwa-100°C. Dielektrische und viskoelastische Messungen ließen erkennen, daß die Dielektrizität auf Grund dielektrischer Verluste von Wasser über 0°C zustande kommt und daß die Viskoelastizität durch viskoelastische Verluste auf Grund örtlicher Schwingungen von Cellulosemolekülen entsteht.Die piezoelektrische Polarisation bei Holz kann für die Lösung technischer Probleme, wie z. B. bei der Messung der Schallgeschwindigkeit in Holz praktisch eingesetzt werden. Die physiologische Bedeutung des piezoelektrischen Effekts in lebenden Pflanzen ist bisher noch unbekannt.

     

一种脲醛树脂胶粘剂的设计

鉴于对文献的分析和实验验证,本文提出一种脲醛树脂胶粘剂的设计原则。即:1)脲醛树脂最终摩尔比应取1.40～1.20:1,这样可以使制成的板材具有足够的胶接强度;2)为了使人造板的甲醛释放量达到10mg/100g(穿孔法)以下,树脂中的游离尿素应在10%以上;3)在脲醛树脂的分子中引入尿素的环状衍生物,如三嗪环或Uron。环状化合物含量最好在10%左右。按此原则合成了三种树脂,在试验室按标准刨花板和中密度纤维板工艺制成板材,其甲醛释放量在10mg/100g左右,板材物理力学性能均超过国家标准和企业标准。     

Friction coefficient as a guide to optimum rake angle in wood machining

Summary In spite of the obviously important role of friction in the wood cutting process, there has been little application in wood machining practice of true friction coefficients obtained from rubbing tests. Friction coefficients were measured during slow rubbing of incense-cedar blocks at two moisture contents on surfaces of saw steel, high speed steel and tungsten carbide. The coefficients were used in an orthogonal model of cutting to estimate the optimum rake angle for the planing of incense-cedar, and some evaluation of the estimate was made using data from cutting experiments reported elsewhere.The author wishes to thank Charles Berolzheimer, Director, The California Cedar Products Company Research Department, for his support of this work, and Richard Lemaster, University of California Forest Products Laboratory, for provision of facilities and assistance in that laboratory     

Green gluing of oak wood (Quercus conferta L.) with a one-component polyurethane adhesive

Abstract

This research work presents a study on the properties of finger jointing green oak wood (Quercus conferta L.) using a one-component polyurethane adhesive. The effect of finger-joint orientation (vertical or horizontal fingers) was also examined. In general, the results from the measurements of modulus of rupture and modulus of elasticity of green-glued finger-jointed specimens indicated that the green gluing of a high-density species such as oak wood is feasible.     

脲醛树脂胶粘剂添加剂不同配比研究*

为降低生产成本、开发新型木材胶粘剂添加剂,选用羧甲基纤维素钠和细沙为主要原料,面粉为辅料,并研究其成分配比对胶粘剂人造板产品质量及加工成本的影响。结果表明：当添加剂中羧甲基纤维素钠、面粉、细沙质量百分比为3∶30∶67时,成本比传统添加剂降低了33%,面粉使用量减少了70%。     

Scientific and economic aspects of wood research as we look to the future of wood

Summary As the world's supply of non-renewable resources becomes increasingly critical, wood with its many attributes, but also with some inherent problems, has the opportunity of playing a more and more important role in meeting a wide variety of man's needs. Research and the development and utilization of research findings will be most important in determining how well wood fares in the years ahead. Those involved in the research effort must be aware of the need to select problems which if successful solutions are found can aid wood in better meeting man's needs. Also, there need be the recognition that the solution will only find use, if in addition to the problem being deemed of importance, both technical and economic feasibility to a significant degree can be demonstrated. The industrial rather than the scientific community is the final judge.Paper presented at the IAWS International Conference, September 9–11, 1975, in Banská Bystrica, Czechoslowakia.     

Microspectroscopy as applied to the study of wood molecular structure

Microspectroscopy gives access to spatially resolved information on the molecular structure and chemical composition of a material. For a highly heterogeneous and anisotropic material like wood, such information is essential when assessing structure/property relationships such as moisture-induced dimensional changes, decay resistance or mechanical properties. It is, however, important to choose the right technique for the purpose at hand and to apply it in a suitable way if any new insights are to be gained. This review presents and compares three different microspectroscopic techniques: infrared, Raman and ultraviolet. Issues such as sample preparation, spatial resolution, data acquisition and extraction of knowledge from the spectral data are discussed. Additionally, an overview of applications in wood science is given for each method. Lastly, current trends and challenges within microspectroscopy of wood are discussed.     

Moisture content—water potential relationship of wood from saturated to dry conditions

Summary The water potential concept as applied to wood-water relations is presented. The gradient in water potential can be used as the driving force of moisture in wood in a model of drying in isothermal conditions provided the moisture content — water potential relationship is known. This relationship is established for aspen sapwood in desorption from saturated to dry conditions at 20, 35 and 50 °C for two specimen orientations. The tension plate, pressure plate and pressure membrane methods were used at high moisture contents and equilibration over saturated salt solutions was used at low moisture contents. The results obtained demonstrate that these methods can be used in combination in order to establish the relationship within the whole range of moisture contents. The equilibrium moisture contents obtained by the tension plate, the pressure plate and the pressure membrane methods for tangential desorption were slightly higher than those measured for radial desorption. The water potential increased with temperature at a given moisture content. This effect cannot be solely explained by the variation of surface tension of water with temperature.This research was supported by the Fonds pour la Formation de Chercheurs et l'Aide à la Recherche, Gouvernement du Québec, and by the Natural Sciences and Engineering Research Council of Canada     

Finger-jointing green softwood: Evaluation of the interaction between polyurethane adhesive and wood

Abstract

Existing European standards for finger-jointing of load-bearing lumber require the wood to be dried before gluing. This article presents a study on the properties of green-glued finger joints, wet wood being bonded prior to drying. Issues to consider, in comparison to finger-jointing of dry wood, are mechanical performance of the joint, absorption of the polymer by the wood in its natural/wet state, and the chemical reactions of the adhesive on contact with water. Finger-jointed samples were tested in bending, and the glue joints analysed by optical microscopy, scanning electron microscopy and microdensitometry. A patented one-component polyurethane adhesive developed for gluing-green wood which has a moisture content usually higher than 70% was used in the study. The resulting green-glued joints showed improved strength properties in comparison to dry-jointed joints. The results confirm that green-glued joints provide a wide, continuous wood/adhesive interface from one substrate to the other. The adhesive penetrates several cells deep and the density of the wood adjacent to the joint surfaces is increased. The results also indicate that the patented adhesive forms covalent bonds to the wood substrate.     

Finger-jointing green softwood: Evaluation of the interaction between polyurethane adhesive and wood

Existing European standards for finger-jointing of load-bearing lumber require the wood to be dried before gluing. This article presents a study on the properties of green-glued finger joints, wet wood being bonded prior to drying. Issues to consider, in comparison to finger-jointing of dry wood, are mechanical performance of the joint, absorption of the polymer by the wood in its natural/wet state, and the chemical reactions of the adhesive on contact with water. Finger-jointed samples were tested in bending, and the glue joints analysed by optical microscopy, scanning electron microscopy and microdensitometry. A patented one-component polyurethane adhesive developed for gluing-green wood which has a moisture content usually higher than 70% was used in the study. The resulting green-glued joints showed improved strength properties in comparison to dry-jointed joints. The results confirm that green-glued joints provide a wide, continuous wood/adhesive interface from one substrate to the other. The adhesive penetrates several cells deep and the density of the wood adjacent to the joint surfaces is increased. The results also indicate that the patented adhesive forms covalent bonds to the wood substrate.