Enhanced water resistance of welded wood joints

A multidisciplinary research team has recently developed an original method for assembling pieces of lignocellulosic materials. This method requires to apply a specific welding compound, inexpensive, non toxic, natural, abundant and totally environmentally friendly, on one or both faces of the materials to be joined, and then to apply a mechanically-induced friction welding. This compound is not a binder, welding involving exclusively the wood alone. Scientists have carried out and exemplified their invention by welding together two wood pieces either by linear vibration welding or rotational dowel welding.
In terms of strength, the joint obtained after application of the welding composition is comparable to the one obtained by a sole mechanical friction welding. Moreover, it has been shown that immersion in water of these novel assemblies does not alter the waterproof property of the welded joint, an original feature of this invention. In other words, welding of wood pieces by mechanical friction including a prior step of coating of the contact surface of at least one of the wood pieces with a specific welding composition is significantly enhancing the water resistance of welded joints of the wood assembly. The method of the invention particularly improves the water resistance properties of the rotational dowel welding joints, which are already much higher than that of linear vibration welding joints thanks to the geometry of welding joint itself.

Afterwards, X-ray micro-densitometry analysis was performed to measure the density of assembling samples obtained by linear mechanical friction welding according to increased welding time. It was observed that the area of greater density around the weldline is broader than the one observed when the linear vibration welding is performed without prior coating of the contact surfaces of the wood pieces with the welding composition.
Moreover, analysis by solid-phase CP-MAS (Cross Polarization - Magic Angle Spinning) 13C-NMR of the interfaces welded with the welding composition also showed an accumulation of lignin in the welding area. Advantages of this novel method - a 100% wood-based welding method - are robustness, waterproof, rapidity of welding (few seconds only!), and the low cost and environmental impact. The compound used is inexpensive, non toxic, bio-sourced, abundant, environmentally friendly, and not a binder. Industrial applications concern the wood and lignocellulosic industry, especially construction, civil engineering, furniture, etc. (e.g. construction of frames, framing, barrels, coffins, etc.).
 

This method of assembling wood pieces - solid wood or lignocellulosic materials - characterized by an enhanced water resistance of welded wood joints, is protected under the French Patent Application No. 1054460 filed by INRA on June 7, 2010 and its extension PCT/FR2011/051285 filed on June 7, 2011, entitled "Method for assembling lignocellulosic pieces."
INRA Transfert is in charge of the technology transfer of this innovation thanks to non-exclusive licenses for commercial applications towards the wood processing Industry.