A. Despres

Parameter interactions in two-block welding and the wood nail concept in wood dowel welding

The interactions between parameters found to be determinant in wood dowel welding by high speed rotation have been evaluated. Of these, the interactions that proved to be the most significant, in descending order, were rotation rate/dowel moisture content, followed by rotation rate/ethylene glycol, and finally, at a lower level of significance, the interactions rotation rate/dowel temperature, wood grain direction/wood species and dowel temperature/wood species. Of the individual factors, once the most determinant factor already optimized in previous studies, namely the dowel/hole diameter difference, was fixed, the most significant were wood grain direction, dowel moisture content (dryness) and wood species. The optimized process yielded excellent strength results. The regression equations developed were able to predict the strength obtainable. The torque for insertion of the dowel in the substrate hole has been measured for several cases and the results are presented. In no cases the value of the torque needed for insertion was excessive and insertion was, therefore, easy. Wood joints composed of two pieces of timber held together by a dowel welded to both of them were assembled for the first time. Two further new concepts have also been advanced and tested: (i) the conical dowel, to maximize welding area and (ii) the concept of the wood nail in which a slightly conical fast-rotating hardwood dowel is inserted rapidly into a softwood substrate into which no hole has been pre-drilled. X-ray densitometry of the samples prepared with the latter approach showed some interesting mechanical interlocking features that might contribute to dowel bonding in softwoods.

Wood joints by through-dowel rotation welding: microstructure, 13C-NMR and water resistance

Two-block wood joints were obtained by insertion and welding without adhesives of dowels by high-speed rotation. Their strengths were better than that obtained by poly(vinyl acetate) (PVAc) gluing. X-ray-microdensitometry analysis showed that a complete welding of the dowel to the substrate occurred and that a perfectly tight joint was formed. Isolation of the flow material allowed CP-MAS 13C-NMR analysis of its composition with possibly low interference from the constituents from the substrate. The flow material appeared to be composed of hemicelluloses, apparently xylans, and lignin. Scanning electron microscopy coupled with the NMR analysis results showed that microfibrils of cellulose, in both amorphous and crystalline states, torn from the wood surface during welding, as well as very small proportions possibly of recrystallized xylans and furanic compounds formed by heat transformation of the carbohydrates, were present. The geometry of the dowel joint allowed the joint to maintain up to 88% of its initial tensile strength after 24-h immersion in cold water.

Wood joints and laminated wood beams assembled by mechanically-welded wood dowels

Dowel welding by high-speed rotation was used to join two wood blocks and strong joints were obtained. Dowel angle to the surface of the wood blocks to be joined had a marked influence on the mechanical performance of the joint. When the dowel was inserted at 90° to the substrate, the dowel was subjected to and resisted a shear force only. When introduced at an angle such as 30° or 45°, the dowel was subjected to and resisted both shear and tension forces, resulting in better joint strength. The joint almost always failed by dowel fracture. The dowel/substrate interface was almost always stronger and did not break. Short two-layer beams joined exclusively by a series of welded dowels were prepared, tested in shear according to structural standards, and their performance was compared to those of solid wood and of glued laminated beams (glulam) of the same dimensions. The short two-layer beams prepared for testing met the Eurocode 5 standard requirements when the optimum dowel insertion angle was used. Then 2-m-long two-layer wood beams were prepared, with the two layers connected exclusively by a series of welded dowels, and tested in bending. Their maximum failure strength and stiffness in bending were determined. These beams outperformed both nailed beams and glued-dowel beams. All the beams had the same length and conformation. The number of nails necessary was double the number of dowels used.

Welding-through doweling of wood panels

Panel products have been joined to levels satisfying the requirements of the relevant standards for metallic connector assemblies by rotation welding of beech dowels through them. The wood panels substrates such as particleboard, OSB and MDF welded to the dowels almost as well as solid timber. Welded-through dowel assemblies presented better stiffness of the test joints than similar assemblies of dowelled solid timber. ZusammenfassungMittels Reibschweissen hergestellte Verbindungen von Holzwerkstoffen mit Buchendübeln entsprechen den Anforderungen der Normen für Verbindungen mit metallischen Verbindungsmitteln. Die Holzwerkstoffe wie zum Beispiel Spanplatten, OSB- und MDF-Platten liessen sich mit Dübeln nahezu so gut wie Massivholz verschweissen. Die Reibschweiss-Dübelverbindung der Holzwerkstoffe wiesen eine höhere Steifigkeit auf als vergleichbare Massivholzverbindungen.

Formaldehyde-Free Dimethoxyethanal-Derived Resins for Wood-Based Panels

Here we report on the results of a research project focused on the reduction of formaldehyde emissions from wood-based panels, by using a novel set of adhesive formulations based on dimethoxyethanal (DME)-derived resins. The investigated adhesives were evaluated on laboratory scale in order to study their technical performances, their gluing parameters, their reactivity as well as their formaldehyde emissions. It was found that all formulations met the requirements of the current standards EN 319:1993-08 and that for class P2 particleboards for general uses. From the technical point of view, major advantages of the resin systems tested were found to be: colorless, low toxicity, easy handling, and high stability at room temperature (long shelf-life, pot-life and open-time). The formaldehyde emissions of the boards produced were found to be comparable to those of natural wood (F****JIS A 1460:2001 standard). The laboratory results obtained with these formulations were validated on industrial scale. The technical properties as well as the formaldehyde emissions were measured. The new formulations were shown to be able to satisfy the requirements of standards with very low levels of formaldehyde emissions. However, to fulfill the requirements of the wood industry, the reactivity of the adhesive needs to be enhanced.