Effects of formaldehyde/urea mole ratio and melamine content on the hydrolytic stability of cured urea-melamine-formaldehyde resin

The influence of formaldehyde/urea (F/U) mole ratio and melamine content on the hydrolytic stability of urea-melamine-formaldehyde (UMF) resin was investigated. The hydrolytic stability of cured UMF resin was determined by measuring the mass loss and the liberated formaldehyde concentration after acid hydrolysis. A higher F/U mole ratio and greater melamine content of UMF resins resulted in lower hydrolytic stability. These results indicated that higher F/U mole ratio and greater melamine content resulted in more branched network structure, which subsequently increases the susceptibility of cured UMF resin toward acid hydrolysis.     

Effects of formaldehyde/urea mole ratio and melamine content on the hydrolytic stability of cured urea-melamine-formaldehyde resin

The influence of formaldehyde/urea (F/U) mole ratio and melamine content on the hydrolytic stability of urea-melamine-formaldehyde (UMF) resin was investigated. The hydrolytic stability of cured UMF resin was determined by measuring the mass loss and the liberated formaldehyde concentration after acid hydrolysis. A higher F/U mole ratio and greater melamine content of UMF resins resulted in lower hydrolytic stability. These results indicated that higher F/U mole ratio and greater melamine content resulted in more branched network structure, which subsequently increases the susceptibility of cured UMF resin toward acid hydrolysis.     

The compatibility of structural adhesives with wood

An evaluation procedure, based on simple test methods, was developed in order to assess the compatibility with wood adherents of some new types of structural adhesives.Zusammenfassung Ein Bewertungsprozess wurde aufgrund einfacher Testmethoden entwickelt, um die Kompatibilität einiger neuer Typen von Klebern für Bauholz einzuschätzen.

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Die Kompatibilität von Klebern für Bauholz

     

The compatibility of structural adhesives with wood

An evaluation procedure, based on simple test methods, was developed in order to assess the compatibility with wood adherents of some new types of structural adhesives.     

Lignin-based cold setting wood adhesives structural fingerjoints and glulam

A traditional-type cold-setting wood adhesive as well as separate applications fast-set wood adhesives for exterior-grade structural fingerjoints and glulam based on soda bagasse lignin were developed and tested. Their results satisfy the requirements of the relevant international specifications. The soda bagasse lignin used composed approximately 75% of the total adhesive solids while resorcinol content as low as 13.6% on liquid resin was used, with good results. On top of their novelty and of their excellent performance these adhesives appear to be economically very attractive.     

Durability of structural wood adhesives after 30 years ageing

Eight urea-formaldehyde (UF) glues and one acid phenolic (PF) glue were compared with “established” glues of the casein and resorcinol (RF) types as for long-term durability, by use of accelerated and natural ageing. The purpose was to evaluate these glues for structural softwood bonding. After unprotected outdoor exposure, only the phenolresorcinols (PRF) still meet the delamination requirements of the Norwegian Glulam Control (Norsk Treteknisk Institutt, 1975) after 10 years. In the case of indoor (30 years) and protected outdoor exposure (22 years), all glues except acid PF still have sufficient dry strength left when judged by Norwegian Standard 3470 (NSF 1979). The wet strength after 30 years (not tested for caseins) is still satisfactory for the RF/PRF adhesives and for the modified UFs, but not for the straight UFs and the acid PF. The accelerated ageing distinguished between glue types in the same order as natural ageing. Conclusions: The caseins and the modified UFs are considered suitable for indoor and protected outdoor exposure. The RF and PRF adhesives are considered suitable for indoor as well as outdoor exposure. The acid PF is considered unsuitable, and the straight UFs with inert fillers should be regarded with suspicion due to their steadily decreasing wet strength. This experiment was supported by The Royal Norweigian Council for Scientific and Industrial Research, and by Casco Nobel AB and Dyno Industrier A/S.     

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.     

Fracture energy testing of thick joints with structural wood adhesives

The test method described by Gagliano and Frazier (2001) was used for the measurement of fracture energy of wood adhesives applied in thick joints for on-site assembling or repair of timber structures.Zusammenfassung Die von Gagliano und Frazier (2001) beschriebene Testmethode wurde verwendet um die Bruchenergie von Klebern für Bauholz zu messen, die für breite Verbindungen zur einseitigen Verstärkung oder Restauration von Holzkonstruktionen angewendet wurden.

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Testen der Bruchenergie breiter Klebeverbindungen für Bauholzkleber

     

On the formaldehyde release of wood particles

The emission of formaldehyde from softwood particles, as measured by the flask method (EN 717-3), depends highly on the particle size. Therefore, no definite value for the formaldehyde release from wood can be given.     

Increased pine tannins extraction and wood adhesives development by phlobaphenes minimization

The phlobaphenes formation and precipation during sulphite/water extraction of pine tannins from pine bark was minimized by blocking tannin self-condensation by the addition of small amounts of a strong nucleophile such as phloroglucinol, m-phenylenediamine and urea, the latter due to its low cost for industrial application. These inhibitors reacted preferentially with the reaction intermediates formed in the tannins self-condensation mechanism, stopping the reaction reaching the molecular rearranged precipates known as phlobaphenes. The principle of inhibition appears to be effective at both the laboratory and industrial level. The results of laboratory and industrial extractions and the results of the adhesives from the modified pine tannin extract/urea adducts obtained are presented. The pine tannin/urea extracts proved to yield good thermosetting wood adhesives for panel products. The minimization of phlobaphenes precipitation increased the yield from 19% to 25% industrial scale.     

Evaluation of formaldehyde emission of pine and wattle tannin-based adhesives by gas chromatography

Formaldehyde emissions from wood particleboards bonded with pine and wattle tannin-based adhesives, using hardeners paraformaldehyde, hexamethylenetetramine and TN (tris(hydroxyl)nitromethan), were measured by the perforator method (DIN EN 120—European Committee For Standardization 1991). All particleboards made using the wattle tannin systems with the three different hardeners were satisfied grade E₁, while in the case of pine tannin only the use of the hexamine hardener led to grade E₁ being satisfied. This tendency was attributed to the curing mechanism of the hardener, the reactivity of the tannin molecule toward formaldehyde and the fast reactivity toward formaldehyde of pine tannin, due to the inclusion of phloroglucinolic A-rings within its molecular structure. Hexamine was effective at reducing formaldehyde emission in tannin-based adhesives when used as the hardener. Being a more sensitive and advanced method, it was also used. The samples used for gas chromatography were gathered during the perforator method. The formaldehyde emission levels obtained from gas chromatography were similar to those obtained from the perforator method.     

Artificial weathering of wood surfaces modified by melamine formaldehyde resins

Spruce and poplar samples were treated with different melamine formaldehyde resins. A long term artificial weathering experiment was performed in order to clarify the resistance to weathering regarding wood colour and surface hardness. The increase in hardness due to melamine treatment was well preserved after simulated long term weathering. The treated samples also showed advantages compared to untreated reference samples regarding discolouration and crack formation. Low molar mass and low degree of methylolation of the melamine resin used was found to be favourable for a successful treatment.     

Comparison of thermal expansion of wood and epoxy adhesives

and Iroko, Clorophora excelsa) and four different epoxy resins have been compared using a modified test method that allows for nearly iso-hygric conditions of the wood specimens. Minor differences in TEC have been observed between wood in the transversal-to-the-grain direction and an experimental epoxy adhesive, which is considered highly compatible with wood on the basis of mechanical and ageing tests. Other commercial epoxy adhesives show greater differences in terms of TEC and a proportionally decreasing mechanical compatibility. According to these results, the evaluation of thermal properties can be considered very useful for a modelling approach to predict the long term behaviour of wood-epoxy adhesive interface.     

非常规蛋白基木材胶黏剂研究现状及发展前景

介绍了动物和植物型非常规蛋白资源的种类及其制备蛋白基木材胶黏剂的研究现状和存在的问题,展望了非常规蛋白胶黏剂的发展前景。