Ionic liquids as enhancers of urea-glyoxal panel adhesives as substitutes for urea–formaldehyde resins

Acid ionic liquids are used as hardeners of urea-glyoxal adhesives to enhance them to a level sufficient enough for substituting urea–formaldehyde adhesives used in wood panels.     

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。     

Characterization of a formaldehyde-free cornstarch-tannin wood adhesive for interior plywood

This study investigated the physical properties (rheological and thermogravimetric analysis) of cornstarch-tannin adhesives and the mechanical properties (dry tensile strength and 3-point bending strength) of plywood made using cornstarch-tannin adhesives. This adhesive was evaluated for its utility in interior plywood manufacture. The optimum cure temperature and cure time of cornstarch-tannin adhesives were 170°C and 4?min, respectively. Plywood bonded with formaldehyde-free cornstarch-tannin adhesive exhibited excellent mechanical properties comparable to commercially available phenol-formaldehyde plywood adhesives. It was found that cornstarch-tannin panels which do not contain formaldehyde and with an emission equal to that of heated but unbound wood can be obtained by the use of hexamethylenetetramine (hexamine) as hardener. The work has indicated that an environmentally friendly wood adhesive can be prepared from a natural renewable resource (cornstarch and wattle tannin) for bonding interior-type plywood.     

木材工业用胶黏剂低毒化研究进展

从合成树脂型胶黏剂的改进和生物质胶黏剂开发利用两方面,总结了木材工业用胶黏剂的低毒化研究进展,并提出根据我国国情着力发展生物质胶黏剂的建议。     

Use of tannin adhesive from Stryphnodendron adstringens (Mart.) Coville in the production of OSB panels

Stryphnodendron adstringens (Mart.) Coville is a species from the Cerrado (Brazilian tropical savanna) popularly known as barbatimão. Its main use of economic value lies in the extraction of the tannins found in its bark and leaves. The purpose of this study was to assess the technical viability of using a tannin-based adhesive coming from the bark of Stryphnodendron adstringens (Mart.) Coville for the production of Oriented Strand Board (OSB) panels. Tannin-based adhesives were produced from tannins extracted from the bark of barbatimão, and these adhesives were used in proportions of 0, 25, 50, 75 and 100 % with commercial urea–formaldehyde adhesive. The properties of the pure adhesives and the mixtures were determined. Subsequently, OSB panels were produced with urea–formaldehyde adhesives, tannin–formaldehyde adhesives and their mixtures. The panels were produced with wood from Pinus oocarpa, with 8 % adhesive, a press cycle of 40 kgf cm^?2, and temperature of 160 °C for a period of 8 min. Only the OSB panels produced with 100 % urea–formaldehyde adhesive, 100 % barbatimão tannin adhesive and 25 % barbatimão adhesive combined with 75 % urea–formaldehyde fulfilled all the pre-requisites stipulated in the standard EN 300 (2006) for type 1 panels, thus proving to be technically viable for application in OSB panels.     

Glue-line wood cell walls increased viscoelastic flow by high moisture tolerance thermoset adhesives

Glue-lines in boards made of wood particles using high moisture tolerance tannin-based adhesives were observed to present the unusual behaviour of producing a melting effect of the wood cell walls directly in contact with the adhesive. The high moisture content of up to 29% used in the preparation of these exterior grade particleboard and OSB industrial panels at standard pressing times causes considerable flow of the amorphous lignin and hemicelluloses in the wood directly in contact with the adhesive leading to a composite, compact interphase in which almost loose wood fibers are drowned in a hardened adhesive matrix. In this interphase the anatomical characteristics of the wood cell walls appear to be lost. This considerable flow is apparently due to the marked decrease of the glass transition temperature of hemicelluloses and lignin at the very high moisture contents used. This effect was replicated in boards prepared with a synthetic MUF adhesive using also a moisture content level much higher than what normally this resin is capable of tolerating by just lengthening considerably the pressing time used.     

Tannin–resorcinol–aldehyde cold-set wood adhesives with only formaldehyde as hardener

Based on standard tannin adhesive formulations, tannin-resorcinol-glyoxal and tannin-resorcinol-glutaraldehyde cold-setting adhesives were prepared for wood glulam in this paper. Only less para-formaldehyde was added as hardener of adhesives. Both of them showed a rival tensile strength with tannin-resorcinol–formaldehyde adhesive. However they exhibited lower wood failure. The proportion of formaldehyde of the two are drastically decreased.     

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.     

Non-toxic, zero emission tannin-glyoxal adhesives for wood panels

In this study different tannin-glyoxal glue mixes for particleboard were studied. Gel times at 100 °C of 45% water solutions of commercial pine tannin extract (Pinus radiata ex Diteco Ltda, Chile) with respectively 8% paraformaldehyde and 9% glyoxal were done at different pHs. Duplicate one layer laboratory particleboards were prepared by adding 12% total resin solids of adhesives composed of commercial pine tannin extract (Pinus radiata ex Diteco Ltda, Chile) with paraformaldehyde, or glyoxal or polymeric 4,4’ diphenylmethanne diisocianate (pMDI), respectively. It was found that tannin-glyoxal panels which do not contain formaldehyde and with an emission equal to that of unbounded wood can be obtained by the use of glyoxal as hardener.     

Bonding of densified beech wood using adhesives based on thermally modified soy proteins

The bondability/glueability of aged and sanded thermo-hydro-mechanically (THM) densified beech wood (Fagus sylvatica L.) was tested and compared with undensified sanded beech wood as a control. THM and control specimens were bonded with five different soy protein isolate (SPI) based adhesives. Commercial SPI powder was thermally modified in the vacuum chamber at 50 or 100?°C and pH adjusted (to pH 10.0) dispersions in water prepared at 24, 50 or 90?°C. Wettability was determined with measuring the sessile drop contact angles of water. Effective penetrations (EPs) and tensile shear strengths of THM and control specimens were determined. THM and control wood had similar wettability. Although THM wood had lower moisture content than control wood, it absorbed the water more slowly than control wood. THM specimens showed lower EPs than control specimens when comparing individual adhesives due to increased density of THM wood. Adhesives prepared with SPI thermally modified at 50?°C showed statistically significantly lower tensile shear strength of bonded THM specimens than that of bonded control specimens. THM densification had no significant effect on the bonding strengths of adhesives prepared with non-modified SPI and SPI thermally modified at 100?°C.     

The mechanical properties of densified VTC wood relevant for structural composites

The mechanical properties of densified wood relevant for structural composites were studied. Low density hybrid poplar (Populus deltoides × Populus trichocarpa) was densified using the viscoelastic thermal compression (VTC) process to three different degrees of densification (63, 98, and 132%). The modulus of rupture (MOR) and the modulus of elasticity (MOE) of the control (undensified) wood and of the VTC wood were determined. The bonding performance of the control and VTC wood, using two phenol-formaldehyde (PF) adhesives, was studied. Four different 3-layer composites were also prepared from undensified and VTC wood, and tested in four-point bending. The results showed that the bending properties of the VTC wood (MOR and MOE) were significantly improved due to the increased density. The bonding performance of VTC wood with PF adhesives was comparable with or better than in the case of the control wood. Increased density of the face layers in the 3-layer VTC composites was advantageous for their mechanical performance.     

Colourless formaldehyde-free urea resin adhesives for wood panels

Amino resin precursors prepared by the addition of a new, colourless, non-volatile and non-toxic aldehyde, dimethoxyethanal (DME), to urea gave resins for boards that while able to harden were underperforming due to the lower reactivity of DME in relation to formaldehyde. Urea reacts with one and even two molecules of DME to form UDME and U(DME)₂ (called DU) but the subsequent cross-linking reaction to form bridges between two ureas, although existing as observed by CP MAS ¹³C NMR, was too slow at temperatures lower than 140 °C to be of significance for wood panel adhesives. However, addition of 20% isocyanate (pMDI) contributed to cross-linking of DU by its reaction with pMDI to also form urethane bridges, their existence being confirmed by CP-MAS ¹³C NMR. The adhesive resins so formed had excellent performance, were colourless, and produced boards that satisfied well the requirements of the relevant norms for interior panels (EN 120 and EN 312). The results were good enough to decrease the proportion of pMDI to 14% at pressing times starting to be of significance for industrial panel products. Formaldehyde emission, by perforator method was down exclusively to the formaldehyde produced by heating the wood chips. The panel emission was sufficiently low to even satisfy the relevant F^**** JIS A 5908 Japanese standard (JIS A 5908, 1994 ). These adhesives are colourless, as UF resins.     

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.     

Volatile organic compounds and formaldehyde in nature, wood and wood based panels

The emission of terpene compounds from vegetation is subject to seasonal and diurnal variations. Due to oxidation of terpene compounds simple aldehydes like formaldehyde can be formed. Insofar formaldehyde is an ubiquitous chemical. Due to its high reactivity it has a short half-life time. Wood and wood-based panels emit a low, but still detectable, amount of formaldehyde. The emission depends on exogenic (temperature, relative humidity, air exchange level) and endogenic (wood species, binder level, binder type, production conditions, etc.) factors. With the aging of boards formaldehyde release declines tremendously to reach very low level. Nevertheless, with low fuming binders wood-based panels with formaldehyde emission close to that of untreated wood can be prepared. Non-formaldehyde volatile organic compounds (VOCs) can be released from wood and wood-based panels. The emission rate depends on the wood species as well as on the boundary conditions (drying, storage, etc.). In the case of wood-based panels it depends also on the production factors as well as on the storage conditions. Methods to assess VOCs have been developed and regulations regarding the limits of emission values are under way.     

Chinese wattle tannin adhesives suitable for producing exterior grade plywood in China

Wattle tannin adhesives were successfully formulated from tannin extracts, which were commercially produced from black wattle (Acacia mearnsii) bark in China. Using the Chinese wattle tannin adhesives a total of 153 plywood panels were prepared in order to study effects of pH, paraformaldehyde, glue spread, hot pressing temperature and time, and closed assembly time on bonding strength and their bonding quality was assessed according to the Chinese Standard for exterior grade plywood. Statistical analysis results revealed that Chinese wattle tannin can be used for producing exterior grade plywood under Chinese plywood factory conditions. One of the specific characteristics of these Chinese wattle tannin adhesives was that their use resulted in plywood of high quality bonding over the wide range of closed assembly time from 30 minutes to 16 hours. These Chinese wattle tannin adhesives have a great potential for commercialization in China.     

A comparative study on some physical and mechanical properties of Laminated Veneer Lumber (LVL) produced from Beech (Fagus orientalis Lipsky) and Eucalyptus (Eucalyptus camaldulensis Dehn.) veneers

Laminated Veneer Lumber (LVL) panels made from eucalyptus (Eucalyptus camaldulensis Dehn.) and beech (Fagus orientalis Lipsky.) veneers were tested for physical and mechanical strength properties in this study. Urea formaldehyde (UF) and Polyvinyl acetate (PVA) adhesives were used for eucalyptus LVL panels and UF adhesive for beech LVL panels. The effect of veneer wood species on some physical and mechanical properties was found statistically significant. Also, different glue species caused the differences in strength properties of LVL panels.

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Über einige Eigenschaften von Furnierschichtholzplatten hergestellt aus Buchen- und Eukalyptusfurnieren

Zusammenfassung In diesem Beitrag werden einige physikalische und mechanische Eigenschaften von Schichtholzplatten, hergestellt aus Buchen- (Fagus orientalis Lipsky.) und Eukalyptusfurnieren (Eucalyptus camaldulensis Dehn.) untersucht. Für die Herstellung von Schichtholzplatten wurden die Buchenfurniere mit UF und die Eukalyptusfurniere mit UF und PVA verleimt. Die Ergebnisse zeigen, dass sowohl die Art des Bindemittels als auch die Holzart der Furniere die Eigenschaften der Schichtholzplatten signifikant beeinflussen.

     

Improving physical and mechanical properties of new lignin- urea-glyoxal resin by nanoclay

To eliminate toxic formaldehyde from wood based panels, glyoxal, a low volatility and nontoxic aldehyde, was used to react with urea and lignin to prepare a glyoxalated lignin- urea-glyoxal (LUG) wood adhesive resin. Moreover, another objective of this research work was to improve the physical and mechanical properties of the new LUG resins by nanoclay addition. For the preparation of LUG resin, glyoxalated lignin (15 mass%) was added instead of second urea to the urea-glyoxal resin synthesis under acid conditions. The LUG resin so prepared was mixed with 1, 2 and 3 mass% nanoclay by mechanically stirring for 5 min at room temperature. Then, the physicochemical and structural properties of the prepared resins as well as the water absorption and the mechanical properties of the plywood panels bonded with it were measured according to standard methods. The physicochemical test results indicated that the gel time of the LUG resin was markedly slower than that of the UF resin. Plywood panels prepared with the LUG resin also presented lower water absorption as well as weaker shear strength than those prepared with the UF resin. Addition of nanoclay changed the physicochemical properties of the resins as the gelation time of the LUG resin was shorter when adding sodium montmorillonite (NaMMT). Higher shear strength values and lower water absorption were achieved by continuously increasing nanoclay proportion from 1 to 3 mass%. Furthermore, addition of nanoclay had more influence on panels bending strength than their flexural modulus. X-ray diffraction (XRD) analysis also indicated that NaMMT exfoliated completely when mixed with LUG resin.     

生物质胶粘剂在人造板中的应用

分析了生物质胶粘剂在人造板中的应用现状,综述了近年来大豆蛋白基胶粘剂、淀粉基胶粘剂和木质素胶粘剂的研究进展.针对目前生物质胶粘剂在胶合强度、耐水性和改性工艺以及在人造板应用等方面存在的问题,笔者认为:应从生物质胶粘剂的宏观粘结特性和微观结构变化出发,深入研究其胶合机理并采用更有效的改性方法和生产工艺,使之广泛应用于人造板工业.     

再识木材胶粘剂

近几十年,以再生资源为原料的胶粘剂逐步被以化石资源为主体的胶粘剂所取代。然而,由于后者在生产与使用中对环保的负面影响,如今以再生资源为原料的胶粘剂如大豆蛋白胶等又被重新重视,国外已再度开展对大豆蛋白胶的研究应用。以生命周期分析木材胶粘剂,既要应用胶粘剂增强木材,又要减少其造成的污染。     

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.