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.     

Curing behavior and bonding strength of two-component fast-setting adhesives prepared with phenol-liquefied <Emphasis Type="Italic">Cryptomeria japonica</Emphasis> wood

Honeymoon adhesive is a special adhesive system that contains two components which are applied onto two substrates separately for glulam manufacturing to provide a fast-setting capability. It has been confirmed that phenol-liquefied wood can be used as a raw material for preparing phenolic resins. In this study, liquefied wood (LW) was obtained by liquefying Cryptomeria japonica wood in phenol with sulfuric acid as a catalyst. Resorcinol–formaldehyde resin (RF), phenol-resorcinol–formaldehyde resins (PRF), LW–PF and LW–PRF (synthesized using LW as a raw material), and RF/LW and RF/LW–PF (blended RF with LW and LW–PF) were prepared. The properties such as non-volatile content, pH, viscosity gelation time and DSC curing behavior of these resins were measured. The results show all resins can be cured at room temperature when paraformaldehyde is added as a hardener, except for the LW–PF. The honeymoon adhesive was prepared by using RF and PRF that contains 10% paraformaldehyde as the A-component and LW–PRF, RF/LW and RF/LW–PF without paraformaldehyde as the B-component. After mixing the A-component and B-component, the gelation time of RF and PRF can be shortened. The glued lumber was prepared by separately applying the A-component and B-component on the surface of two wood strips and combining them. The result shows using RF/LW–PF as the B-component has faster bonding strength development than others. When PRF and RF are used as the A-component, they have a bonding strength of over 5.4 MPa, the requirement of CNS 11031 standard for the glued lumber of C. japonica, at pressing times of 2 and 4 h, respectively.     

Potential of tannin extracts as resorcinol replacements in cold cure thermoset adhesives

Blends of phenol resorcinol formaldehyde (PRF) resin with tannin obtained from bark offers the potential to minimise the quantity of resorcinol required in conventional two pot, cold set PRF wood adhesives. In the presence of ammonia, both quebracho and radiata pine bark extracts, resorcinolic and phloroglucinolic type tannins respectively, showed accelerated adhesive cure behaviour with formaldehyde, in a similar manner to PRF adhesive Greenweld technology for modifying glued timber processing rates. Segregation of pine bark components established that only tannin-containing fractions exhibited accelerated adhesive cure behaviour. Results from adhesive testing indicated adhesive blends of quebracho extract and PRF resin failed to attain satisfactory bond strength at full cure. Of the pine bark components, only a pure tannin fraction produced acceptable bond strength when blended with PRF (3:1). The study indicated pine bark extract can be substituted into PRF adhesives and adhesive cure rate accelerated using ammonia. However, extracts containing crude tannin or carbohydrate materials in adhesive blends led to inferior adhesives.     

The wettability and bonding performance of densified VTC beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) bonded with phenol–formaldehyde adhesive and liquefied wood

The influence of viscoelastic thermal compression (VTC) on surface wettability and bonding performance of wood was evaluated. Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with the VTC process to different degrees of densification. Control and densified strips were bonded with phenol–formaldehyde (PF) adhesive and liquefied wood (LW). Shear strength of bonded assemblies was determined after 1 week of conditioning at 20 °C and relative humidity of 65 %. Wettability was determined on the basis of the contact angle of water, PF adhesive, and LW using the Wilhelmy method. Results showed that densification of beech and spruce wood did not significantly affect the shear strength of specimens bonded with PF adhesive. In beech assemblies bonded with LW shear strength decreased significantly with increased density, whereas in bonded spruce specimens decrease of shear strength was not significant. It was found that degree of densification and bonding process used in the study were not appropriately chosen for spruce wood specimens, since major deformations after the bonding process occurred. Wettability changed significantly after densification. Contact angle of water and LW increased after densification, whereas contact angle of PF showed inverse trend and decreased after VTC process. Furthermore, the degree of densification had a minor effect on the wettability.     

Finger jointing of green Black pine wood (<Emphasis Type="Italic">Pinus nigra L.</Emphasis>)

This research work presents a study on the finger jointing of green Black pine wood (Pinus nigra L.) using a phenol resorcinol formaldehyde 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 green gluing of Black pine wood is feasible.     

Dynamic mechanical thermal analysis (DMTA) of aqueous phenol formaldehyde (PF) resin modified by nano copper oxide (CuO)

To improve the bio-resistance of engineered wood composites products via gluing process, aqueous phenol formaldehyde (PF) resin was modified using nano CuO containing alkane surfactant and polyvinyl alcohol (PVA) 17-99. The modified PF system was analyzed by dynamic mechanical thermal analysis, and the mechanical properties of the bonded plywood panels including tensile strength, modulus of rupture (MOR), modulus of elasticity (MOE) and shear strength under five test conditions were also evaluated. The results indicated that the addition of nano CuO incorporating PVA 17-99 separated the gel point and vitrification point in the curve of tan δ, which is related to the delaying of moisture loss in modified PF resin during the curing process. The modification showed adverse effect on tensile strength but only a minimal influence on MOR and MOE. Additionally, PVA 17-99 reduced the water resistance of cured PF resins. However, with the test conditions of dipping in 100 °C water for 6 h, then drying for 20 h at 63 °C in air, followed by dipping in 100 °C water for 4 h, PVA consolidated the re-curing effect on the PF resin and compensated the strength loss from hydrolysis. Thus, the modified PF system not only guaranteed bio-resistance of glued wood composites via CuO, but also has the potential for developing self-curing wood composites being applied as structural construction materials.     

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.     

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.     

Hydroxymethylated resorcinol (HMR) priming agent for improved bondability of silicone modified wood glued with a polyvinyl acetate adhesive

A hydroxymethylated resorcinol (HMR) was used as a coupling agent to improve glueability of silicone (aminofunctional polydimethylsiloxane) modified pine sapwood (Pinus sylvestris) with polyvinyl acetate (PVAc). Priming with HMR did not significantly increase tensile shear strength of unmodified and silicone treated wood under dry conditions. Under wet conditions, tensile shear strength of the control specimens was increased by 37% and of the silicone modified specimens by 13%. Despite of higher shear strength under wet conditions, the primed silicone treated specimens displayed lower degree of wood failure than the unprimed treated specimens.     

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.     

柞木弦／径切面胶合强度的试验研究

对柞木木材自身弦／径面强度及其水性高分子异氰酸酯(API)胶粘剂胶合试件弦／径面胶接强度进行了研究。结果表明:柞木木材API胶粘剂弦／径面胶合试件常态压缩剪切强度存在着差异;柞木径切面API胶合试件的常态压缩剪切强度比弦切面胶合试件的常态压缩剪切强度高;径切面API胶合试件的常态压缩剪切强度是弦切面的1.22倍;而柞木弦、径面API胶合试件的反复煮沸压缩剪切强度没有显著性的差别。柞木木材本身弦／径面顺纹抗剪强度试验结果表明:柞木木材径向的顺纹抗剪强度为14.41MPa,柞木木材弦向的顺纹抗剪强度为14.97MPa,尽管弦径向间差别不大,但经t-检验证明:柞木的弦／径向间顺纹抗剪强度有显著性的差别,柞木的径向顺纹抗剪强度比弦向顺纹抗剪强度小。     

Bending performance of 3-layer beech (Fagus sylvativa L.) and Norway spruce (Picea abies (L.) Karst.) VTC composites bonded with phenol–formaldehyde adhesive and liquefied wood

Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with viscoelastic thermal compression (VTC) process to two different degrees of densification, and lamellas were used to manufacture different types of 3-layer VTC composites. Bending properties of 3-layer VTC composites bonded with phenol formaldehyde (PF) adhesive and liquefied wood (LW) were determined and compared to 3-layer composites produced with undensified beech or spruce wood lamellas. Morphology of VTC spruce wood of higher density was analysed with fluorescent microscopic technique. All composites produced with densified beech lamellas and bonded with PF adhesive had significantly higher values of modulus of rupture (MOR) and modulus of elasticity (MOE) than composites produced with undensified lamellas. Densified spruce lamellas contributed to better bending performance of 3-layer VTC composites bonded with PF adhesive to some extent. Furthermore, composites bonded with LW had significantly lower MOR and MOE values compared to composites bonded with PF adhesive. Study of VTC spruce wood microstructure showed that densification caused non-uniform deformation of cell wall structure, in which cell wall fractures were observed.     

Bonding surface-modified Karri and Jarrah with resorcinol formaldehyde

A series of abrasive papers were used to sand machined regrowth karri (Eucalyptus diversicolor) and regrowth jarrah (E. marginata) wood surfaces. Shear test specimens were then prepared by gluing sanded wood blocks with resorcinol-formaldehyde and pressing them at 1500 kPa and 20°C for 7 hours. Wettability measurements on sanded wood surfaces and strength tests on shear test specimens were used to assess the ability of the sanding treatment to improve adhesive performance. The effect of sanding on wettability and shear strength varied according to the abrasiveness of the sanding treatment. Application of the coarsest abrasive (80 grit) significantly improved wettability and shear (adhesive) strength in both dry and wet conditions. A positive correlation between wettability and shear strength occurred only after sanding. SEM observations revealed different morphological structures of the glueline in sanded and non-sanded surfaces.     

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.     

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.     

The strength of press-glued and screw-glued wood-plywood joints

The effects of screw-gluing and gap-filling phenol resorcinol formaldehyde (GPRF) adhesive using different glue line thicknesses on the shear strength of wood-plywood joints were studied to determine the use of screw-gluing fastening (SG) method and GPRF adhesive. The specimens were manufactured using either SG method with GPRF adhesive and fine threaded drywall screws or press-gluing (PG) method with GPRF adhesive. The Analysis of Covariance (ANCOVA) was used for statistical analyses. The results showed that the strength among fastening methods was significantly affected by the glue line thickness, so the strength decreased as the glue line thickness increased. The SG fastening method was found as effective as the PG method and more effective when thick glue lines present. The method can be successfully used to bond wood to plywood for wooden panels i.e. stressed skin and sandwich panels.     

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.

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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.

     

Modification of beech veneers with N-methylol-melamine compounds for the production of plywood

Rotary cut beech (Fagus sylvatica L.) veneers were treated with two formulations based on N-methylol-melamine (NMM): (1) NMM solution (NMM-1, 10% solid content), (2) fatty acid modified NMM dispersions containing paraffin (with an aluminium salt as catalyst, mNMM-2, 5% solid content). Five treated veneers were bonded with phenol formaldehyde (PF) adhesive to produce plywood in two different processes. The plywood treated with NMM-1 displayed increased shear strength in comparison to the controls, whereas the treatments with mNMM-2 diminished the shear strength of the plywood to different extents. Treatment with NMM-1 also enhanced other mechanical properties of the plywood as compared to water-treated control plywood such as Brinell hardness, modulus of elasticity and bending strength. The plywood treated with mNMM-2 showed only slight changes of these mechanical properties compared to the control plywood. The plywood specimens were subjected to cyclic water submersion and oven-drying. The treatment with mNMM-2 caused greater reduction in water uptake and thickness swelling than NMM-1; the degree of delamination depended on the process. The plywood treated with NMM-1 exhibited lower equilibrium moisture content than that treated with mNMM-2. The formaldehyde emission of NMM-1-treated plywood was much higher than that of the control and the mNMM-2-treated plywood.     

Bestimmung von natriumhaltigen Zusatzstoffen in Holzwerkstoffen

Methods for the determination of sodium or sodium-containing components in wood and wood-based products are described. Sodium contents are shown for wood, urea formaldehyde (UF) resins, resol-type phenol formaldehyde (PF) resins, and particleboards produced with these resins.     

乳化剂十二烷基硫酸钠(SDS)对淀粉基木材胶特性的影响

研究了乳化剂十二烷基硫酸钠(SDS)对淀粉胶胶接强度和低温储藏稳定性的影响。对比没有加入SDS的淀粉胶,加入SDS后胶的粘结强度均有所提高,最佳添加量为淀粉用量的1%,此时干强度提高了60.1%,湿强度提高了211.5%。此外,SDS可以有效地提高反应的接枝参数,接枝率提高了93.7%,接枝百分率提高了83.8%。耐低温稳定性实验证明,加入乳化剂可以增加淀粉胶通过冷藏-解冻循环的次数。动态时间扫描实验和TEM的结果证明,加入SDS有利于削弱淀粉分子之间的氢键作用力,促进淀粉与单体之间发生接枝反应,从而改善了淀粉胶乳液的粘结性能及稳定性。