A delamination test for structural wood adhesives used in thick joints

Subject A test method was developed for the measurement of delamination of structural wood adhesives applied in thick joints. Large differences were observed within a range of 8 commercial epoxy-based products.     

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

     

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.     

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.     

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.     

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.     

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.     

One-shot cold-set,urethane adhesives for wet wood

Ohne Zusammenfassung     

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.