Effect of adhesive type and glue-line thickness on the fatigue strength of mortise and tenon furniture joints

The fatigue strength of rectangular mortise and tenon rubberwood furniture joints made from polyvinyl-acetate (PVAc) and urea-formaldehyde (UF) adhesives were compared. The results showed that the allowable design stress for PVAc adhesive joints could be set at 30 % of its ultimate bending moment as compared to 25 % for the UF adhesive joint. Further, PVAc adhesive joints have large working tolerances for glue-line thicknesses which make it suitable for mass production furniture manufacturing.     

Gross adhesive penetration in furfurylated, <Emphasis Type="Italic">N</Emphasis>-methylol melamine-modified and heat-treated wood examined by fluorescence microscopy

This study investigated the radial penetration of three conventional cold-set wood adhesives [emulsion polymer isocyanate (EPI), poly (vinyl acetate) (PVAc), one-component polyurethane (PU)] into various degrees of furfurylated and N-methylol melamine-modified (NMM) Scots pine, and heat-treated Scots pine and beech based on measurements of effective (EP) and maximum penetration (MP) from microscopic observations. EP of EPI adhesive decreased after modification with higher concentration of furfuryl alcohol while an improved penetration was recorded for PVAc into furfurylated wood. A deeper penetration was observed for all adhesives into wood treated with lower concentration of furfuryl alcohol. The EP of EPI and PU adhesives reduced after NMM treatment but it increased in the case of PVAc. In spite of reduction of EP of PU after NMM treatment, it represented a deeper penetration among all adhesives possibly due to its lower molecular weight. For Scots pine, increasing the treatment temperature improved EP of all adhesives while for beech, the EP of PU and PVAc increased largely in the case of samples treated at 195 °C. Visual analysis of fluorescence microscopy pictures provided more detailed information on modality of penetration. The results are useful for understanding the interaction among common adhesives and modified materials, and can be used in future research to explain the bonding behavior of modified wood.     

Artificial ageing of softwood joints and its effect on internal bond strength with special consideration of flat-to-end grain joints

The effect of ageing processes on internal bond strength of three-part specimens produced from Norway spruce wood (Picea abies Karst.) bonded with one-component polyurethane (PUR), two-component emulsion polymer isocyanate (EPI), melamine-urea-formaldehyde (MUF) or phenol-resorcinol-formaldehyde (PRF) adhesive was studied. By varying the grain angle of the middle part from 0° (parallel to grain of the surface layers) to 90° (perpendicular to grain of the surface layers) in incremental steps of 10° the effect of grain direction was investigated. The specimens were exposed to a three-step ageing cycle lasting for seven days (50°C/95% relative humidity (rH), ?20°C/65?C70% rH and 75°C/15% rH) which was repeated twelve and twenty-four times, respectively. In general, a decrease in internal bond strength of the exposed specimens was observed. For all tested adhesive systems, influence of ageing processes was more pronounced for flat-to-end grain joints compared to longitudinal joints. For samples bonded with PUR adhesive strength reduction was lower compared to the other adhesives used.     

胶黏剂种类对人造板内结合强度检测结果的影响

使用聚乙酸乙烯酯乳液(PVAc)、单组份聚氨酯(PUR)和热熔胶(EVA)3种胶黏剂粘结金属卡头与试件,检测并分析它们对强化木地板和地板基材用纤维板内结合强度检测结果的影响。结果表明:使用PUR粘结时检测的内结合强度值比使用PVAc、EVA高,其中强化木地板内结合强度高出0.47MPa,地板基材用纤维板内结合强度高出约0.40MPa。     

胶黏剂种类对人造板内结合强度检测结果的影响

使用聚乙酸乙烯酯乳液（PVAc）、单组份聚氨酯（PUR）和热熔胶（EVA）3种胶黏剂粘结金属卡头与试件,检测并分析它们对强化木地板和地板基材用纤维板内结合强度检测结果的影响。结果表明：使用PUR粘结时检测的内结合强度值比使用PVAc、EVA高,其中强化木地板内结合强度高出0.47MPa,地板基材用纤维板内结合强度高出约0.40MPa。     

我国传统木工胶粘剂的研究进展与发展趋势

介绍了木工用胶粘剂发展的历史和研究现状及改性方法,重点是综合了传统木工用三种甲醛类胶粘剂和人造板二次加工及家具生产用聚乙酸乙烯酯乳液胶的研究进展,同时提出了木工胶粘剂的发展趋势。     

Emissions during combustion of particleboard and glued veneer

The combustion of particleboard and glued veneer was studied in order to evaluate if there are any negative effects on the environment from incineration of waste with adhesive. The particleboard was made with urea formaldehyde (UF) resin and the veneers were glued with different types of adhesives, UF, polyvinyl acetate, emulsion polymer isocyanate (EPI), melamine urea formaldehyde (MUF) and phenol resorcinol formaldehyde. The combustion tests were carried out in a fluidised sand bed reactor with a good oxygen supply at temperatures between 500°C and 1000°C for particleboard and at 750°C and 850°C for glued veneer. The emissions were compared with the emissions from combustion of pure wood and pellets made from wood. The results show that the emissions from both particleboard and glued veneer are similar to the emissions from pure wood. The only main difference is that the nitrogen oxide (NO_x) is increased when particleboard and nitrogen-containing adhesives, like UF, EPI and MUF, are combusted. The nitrogen from the adhesive is only to a minor extent converted to NO_x, e.g. only 4% of the nitrogen in particleboard gives NO_x.     

Thermal stability of glued wood joints measured by shear tests

The thermal stability of glued wood joints is an important criterion to determine the suitability of adhesives in the field of engineered wood. During their product life, glued wood joints can be exposed to high temperatures in various ways (direct exposure to the sun, fire, etc.). Thereby the cohesiveness of the adhesive must not degrade. This raises the question of how the strength of bonding changes under thermal load. The current investigation covers the influence of temperature (T=20 to 220°C) on the shear strength of glued wood joints. Different adhesive systems were investigated. With increasing temperature, the shear strength of solid wood and also of glued wood joints decreased. There were big differences in thermal stability and failure behaviour between the adhesive systems as well as within the polyurethane group. The thermal stability of one-component polyurethane systems can be greatly varied by modifying their chemical structure. Well adapted one-component polyurethane adhesives reach a strength similar to that of phenol resorcinol resin.     

Bonding performance of heat treated wood with structural adhesives

Bonding of untreated, intermediate (hydro-thermolysed) and heat treated wood with melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF) and polyurethane (PUR) adhesives was studied. An industrial heat treatment process (Plato ^®) was used, which included two separate heat treatment stages and a drying stage in-between. Laminated beams having four lamellas were prepared from untreated and treated timber for mechanical testing of the bond lines. The results of the tests showed that heat treatment affected the shear strength and the delamination of the laminated wood depending on the adhesive system used for bonding. The PUR and MUF adhesives performed in a rather similar way, and better than the PRF adhesive. The shear strength of laminated wood bonded with the waterborne MUF and PRF adhesives decreased for the specimens made of hydro-thermolysed timber and decreased further for the specimens made of fully heat treated timber. The difference in adhesive bond shear strength between untreated, intermediate and fully treated wood was less obvious in the case of the PUR adhesive. Delamination of the PRF bond line decreased drastically for all the specimens made of heat treated timber.     

Utilization of bark flours as additive in plywood manufacturing

Increasing demand for wood based panel products and shortage of wood as raw material have triggered many efforts to utilize residues generated annually by the forest industries including a large portion of bark in panel production. In this study, the effects of using bark flours as additives obtained from different wood species (walnut, chestnut, fir and spruce), having much polyphenol content, on some physical and mechanical properties and formaldehyde emission of plywood panels were examined. Wheat flour, which has been used widely as additive in plywood manufacturing, served as control. Plane tree (Platanus orientalis) logs were obtained for veneer manufacturing. Urea formaldehyde (UF) resin with 55 % solids content was used as adhesive. The bonding shear strength, bending strength, modulus of elasticity (MOE), density, equilibrium moisture content and formaldehyde emission of plywood panels were determined according to related standards. It was found that the use of flours obtained from the barks of chestnut and fir trees in the glue mixture decreased the formaldehyde emission of panels. The bonding strength values of the test panels made using the glue mixture including the flour of walnut and spruce barks as additive were lower than those of the panels with adhesive containing the flour of fir and chestnut barks. The panels manufactured with adhesives including the flour of fir bark gave the highest bending strength and modulus of elasticity values.     

Failure mechanisms in wood joints bonded with urea-formaldehyde adhesives

Wood joints bonded with urea-formaldehyde (UF) are weakened by cyclic swelling and shrinking. To study the failure mechanisms in UF-bonded joints, specimens were bonded with unmodified, modified (amine), or phenol formaldehyde adhesive and subjected to accelerated aging. Modification of the adhesive properties increased the cleavage fracture toughness and shear strength of bonded joints and improved the resistance of joints to cyclic swell-shrink treatment and accelerated moist-heat aging. Joints bonded with some modified urea-formaldehyde adhesives were as resistant to these treatments as joints bonded with phenol formaldehyde. Physical and mechanical origins of the improved adhesive performance were determined by microscopic analysis. Cure-shrinkage stresses precracked unmodified adhesive layers and damaged the wood interphase. The damaged interphase was especially susceptible to the effects of cyclic swelling and shrinking stresses. Certain modifiers reduced or eliminated cure-shrinkage cracking and damage to the wood. Moist-heat aging caused molecular scission in the bulk unmodified adhesive layer as revealed by the onset of shear cracking in the adhesive layer and erosion of exposed surfaces. Certain modifiers reduced or eliminated molecular scission and erosion responsible for adhesive weakening in moist-heat aging. We conclude that incorporating flexible amines in the adhesive structure improves the durability and stability of UF-bonded joints.     

Comparing dry bond strength of spruce and beech wood glued with different adhesives by means of scarf- and lap joint testing method

Dry bond strength of different adhesives on spruce and beech wood was tested by means of the former DIN standard 53253 (1964)scarf joint and the operative standard lap joint testing method according to EN 302-1 (2004). The failure mode in standard lap joint tests was mainly wood failure, whereas scarf joints failed in the bond line. Due to the high proportion of wood failure, no clear effect of the different adhesives, but a clear effect of wood species on the bond strength was observed for lap joint samples. By contrast, scarf joint testing was far less dependent on wood species and delivered clear differences in the strength of different adhesive systems.     

Untersuchungen zu Diffusionsvorgängen in mehrschichtigen Massivholzplatten

Multilayer solid wood spruce panels with different glueing and structure type were tested for water vapor diffusion for a wet climate (relative humidity ranging from 100% to 65% at 20°C) and for a dry climate (relative humidity ranging from 0% and 65% at 20°C). Results showed that the water vapor diffusion resistance for the dry climate is distinctly higher in comparison to the wet climate. The water vapor diffusion resistance is stronger related to the number of glue layers per panel thickness than to the type of adhesive. The average moisture content of the panels in a wet climate was 20%. If one considers a water saturation point of 28% (spruce wood) on the wet side, and 12% moisture content on the dry side (normal climate) of the panel a linear moisture gradient can be expected. Since the average moisture content of the solid wood panels was about 11.3% in a dry climate, and thus corresponding approximately to the equilibrium moisture content of spruce wood exposed to normal climate, the partial pressure gradient on equilibrium caused water uptake corresponding to the normal moisture content for spruce wood.     

Properties of laboratory made yellow poplar (Liriodendron tulipifera) laminated veneer lumber: effect of the adhesives

Four different commercial resin formulations namely cross-linked polyvinyl acetate (XPVAc), melamine urea formaldehyde (MUF), urea formaldehyde (UF) and melamine formaldehyde (MF) were used to produce laminated veneer lumber (LVL) from yellow poplar veneers in the laboratory. Physical and mechanical properties were evaluated using ASTM (D 1037, D 906, D 5456) standards and compared. Internal bond, tensile shear and block shear strengths of specimens subjected to accelerated (boiled water) and cyclic (repeated cold water) conditions were also determined. Strength properties of yellow poplar LVL were improved compared with the solid wood. The properties of XPVAc bonded LVL were superior or comparable to the thermosetting adhesives. This study clearly indicated that cross-linked PVAc can be used to manufacture LVL with acceptable properties.     

六种人工林木材胶合性能初步研究

初步研究了杉木、杨树、巨桉、尾巨桉、柠檬桉和窿缘桉等六种人工林木材的胶合性能,结果表明:胶合剪切强度沿杉木、杨树、巨桉、尾巨桉、柠檬桉和窿缘桉顺序增大;六种木材胶合性能差异显著,其中杉木、杨树、巨桉和尾巨桉胶合性能较好,表现为胶合剪切强度能达到标准要求、且木破率高;柠檬桉和窿缘桉木破率低,难于胶合,胶合性能有待改进;胶粘剂类型影响木材胶合性能,其中API和PF剪切强度和木破率平均值高于PVAc和UF;热压胶合的PF和UF的离散性比冷压胶合的API和PVAc要差。     

Hydroxymethylated resorcinol (HMR) priming agent for improved bondability of wax-treated wood

The effects of hydroxymethylated resorcinol (HMR) on the tensile shear strength of wood joints treated with montan ester wax were studied with polyvinyl acetate (PVAc) and melamine formaldehyde (MF) adhesives. Untreated specimens which were bonded with MF and PVAc had similar bond strength properties under dry conditions, but MF bonded specimens’ tensile shear strength values were higher than that of PVAc glued specimens under wet conditions. With both adhesives tested, dry tensile shear strengths of wax-treated specimens were in a comparable range to those of untreated wood, while under wet conditions the strength values of wax-treated specimens were significantly lower than those of the controls. HMR priming increased the shear strength of PVAc bonded specimens under wet conditions and of MF-bonded specimens under dry and wet conditions. The effect on MF-bonded specimens, however, was much more pronounced under wet conditions.     

Recycled micronized polyurethane powders as active extenders of UF and PF wood panel adhesives

The addition of micronized polyurethane powders obtained from waste flexible polyurethanes to urea-formaldehyde (UF) resins and to phenol-formaldehyde (PF) resins improves markedly the performance of the panels prepared with these resins, namely plywood and particleboard. Infrared spectrophotometry (FT-IR) indicates that to some limited extent a reaction does appear to occur between micronized PUR waste powder and PF resin when the mix is cured under alkaline conditions. There appears to be no reaction at all instead between acid-setting UF resins and PUR powder. In both cases an active filler or extender effect appears to occur, otherwise the improvement in dry strength of UF–bonded joints cannot be explained. Even when reaction does evidently not occur, the addition of PUR powder improves markedly the water resistance of UF and PF resins. This active extender/filler effect is not due to any isocyanate group being re-generated on heating as both FT-IR and NMR confirm the absence of these groups.     

Visualisation of UF resin on MDF fibre by XPS imaging

Using X-ray photoelectron spectroscopy (XPS) will distinguish urea formaldehyde (UF) resin on the surface of medium density fibreboard (MDF) fibre. High spatial resolution XPS imaging was employed to 2-dimensionally map the presence and concentration of carbon and nitrogen on pressed MDF fibre. Using image analysis routines, composite XPS images of fibre and UF resin were produced for laboratory and commercially prepared MDF panels using the C1s and N1s peaks respectively. Results showed that MDF could be analysed directly, and the presence of UF resin features imaged on fibre was comparable to UF resin characterised by confocal microscopy. Due to low surface nitrogen contents, the XPS imaging instrument was necessarily operated at a low resolution, and with a relatively small field of view. This may make the XPS imaging method less attractive for visualising UF resin on wood fibre compared to other microscopy techniques.     

Emissions during combustion of particleboard and glued veneer

The combustion of particleboard and glued veneer was studied in order to evaluate if there are any negative effects on the environment from incineration of waste with adhesive. The particleboard was made with urea formaldehyde (UF) resin and the veneers were glued with different types of adhesives, UF, polyvinyl acetate, emulsion polymer isocyanate (EPI), melamine urea formaldehyde (MUF) and phenol resorcinol formaldehyde. The combustion tests were carried out in a fluidised sand bed reactor with a good oxygen supply at temperatures between 500°C and 1000°C for particleboard and at 750°C and 850°C for glued veneer. The emissions were compared with the emissions from combustion of pure wood and pellets made from wood. The results show that the emissions from both particleboard and glued veneer are similar to the emissions from pure wood. The only main difference is that the nitrogen oxide (NO_x) is increased when particleboard and nitrogen-containing adhesives, like UF, EPI and MUF, are combusted. The nitrogen from the adhesive is only to a minor extent converted to NO_x, e.g. only 4% of the nitrogen in particleboard gives NO_x.

---

Emissionen während der Verbrennung von Spanplatten und verleimtem Furnier

Zusammenfassung Untersucht wurde die Verbrennung von Holzspanplatten und verleimtem Furnier, um zu beurteilen, ob durch Verbrennung von Äbfällen mit Klebstoffen negative Auswirkungen auf die Umwelt entstehen. Die Holzspanplatte war mit Harnstoff- Formaldehyd – Harz (UF) hergestellt, und die Furniere waren mit verschiedenen Typen von Klebstoffen verleimt: UF, PVAC, EPI, MUF und PRF. Die Verbrennungsprüfungen wurden in einem Sandbett Reaktor durchgeführt, mit großzügiger Sauerstoffzufuhr bei Temperaturen zwischen 500°C und 1,000°C für die Holzspanplatte, sowie bei 750°C und 850°C für das verleimte Furnier. Die Emissionen wurden verglichen mit denjenigen der Verbrennung von reinem Holz und Holzpellets. Die Ergebnisse zeigten, dass beide Emissionen, von Holzspanplatten und verleimtem Furnier, denjenigen von reinem Holz glichen. Der einzige deutliche Unterschied besteht darin, dass NO_x erhöht ist, wenn Spanplatten und stickstoffhaltige Klebstoffe, wie UF, EPI und MUT verbrannt werden. Der Stickstoff in den Klebstoffen wird nur in geringem Maße in NO_x umgewandelt, z. B. nur 4% des Stickstoffs in Holzspanplatten ergibt NO_x.

     

Hydroxymethyl furfural-modified urea–formaldehyde resin: synthesis and properties

Considering the importance of urea–formaldehyde (UF) resins in the wood industry, this work reports on a new bio-based modification of UF resins. The use of 5-hydroxymethyl furfural (HMF) is motivated by the current concerns about the effects of formaldehyde on human health. UF and urea–HMF–formaldehyde (UHF) resins were synthesized by an alkaline-acid method and characterized by FTIR, thermogravimetric analysis, and differential scanning calorimetry. The UHF, as a newly modified polymeric resin, was thermally characterized, and it was found that its thermo-stability and char yield was improved. In order to investigate the performance of the UHF, the preparation of particleboards with the UHF as adhesive, as well as its film formation ability have been studied. The UHF films formed on wood panels were uniform without any crack. Film formation ability of the UHF resin was improved due to the presence of more hydroxyl groups as well as furan rings of the HMF moieties resulting in more activated groups to be bonded by wood. Furthermore, formaldehyde release of the particleboards bonded by UHF was significantly lower than that of which bonded by the UF resin. Lab particleboards using the UHF resins showed higher modulus of rupture, modulus of elasticity, and internal bond compared to boards with UF resins, as well as lower water absorption and thickness swelling. Based on these results UHF resin can be considered as a possible candidate as adhesive for wood and wood based panels.