Wood bonding by mechanically‐induced in situ welding of polymeric structural wood constituents

Abstract

Mechanically induced wood fusion welding, without any adhesive, is shown here to yield rapidly bonding wood joints satisfying the relevant requirements for structural application. The mechanism of mechanically induced vibrational wood fusion welding is shown to be due mostly to the melting and flowing of amorphous cells-interconnecting polymer material in the structure of wood, mainly lignin, but also some hemicelluloses. This causes a partial detachment, the "ungluing," of long wood cells and wood fibers and the formation of an entanglement network drowned in a matrix of melted material which then solidifies, thus forming a wood cell/fiber entanglement network composite with a molten lignin polymer matrix. During the welding period some of the detached wood fibers which are no longer being held by the interconnecting material are pushed out of the joint as excess fiber. Crosslinking chemical reactions of lignin and carbohydrate-derived furfural also occur. Their presence has been identified by CP-MAS 13 C-NMR. These reactions, however, are relatively minor contributors during the very short welding period. Their contribution increases after welding has finished, which explains why relatively longer holding times under pressure after the end of welding contribute strongly to obtaining a good bond.

Surface modification of wood using friction

“…Linear vibration friction technology (LVFT) is widely used in the plastics and automotive industries (Gfeller et al 2004) and has been investigated as a means of bonding wood without adhesives (Gfeller et al 2003). With LVFT, wood is joined together by the vibrational movement of one wood surface against another resulting in localised heating and softening of the wood material and as such it has the potential to induce local densification of wood surfaces.…”

Tribological mechanisms involved in friction wood welding

“…A significant quantity of molten material should nonetheless be present in the contact. As already mentioned in the literature 3 , one part of this fluid-like material cannot be kept trapped any longer into the contact and is pushed out of the contact. It is then quickly cured to generate large platelets as depicted in Figure 7k.…”

“…The Friction Wood Welding (FWW) process was first discovered and then patented by Sutthoff 1 in 1996. The first studies seeking to understand the physical phenomena involved in FWW were then published since 2003 by Gfeller and Pizzi et al 2,3 FWW is tribologically driven since it is achieved by the relative displacement of two wooden parts under normal contact pressure. The frictional heat generated at the contact interface induces local transformation of wood into a molten material which then solidifies to form a strong joint.…”

Welding of thermally modified wood and thermal modification of the welded wood: Effects on the shear strength under climatic conditions

“…The appropriate range of parameters that could result in the maximum shear strength of the welded joints was defined by a screening test. Notwithstanding the earlier studies (Gfeller et al 2003;Boonstra et al 2006), using a longer welding time and/or higher pressure for the welding of thermally modified wood caused severe degradation of the wood and delamination of the welded joint just after welding. Thus, the pressures and welding times needed to be less extreme than the conditions of welding optimized for non-treated wood.…”

“…Wood welding is an environmentally compatible assembling and manufacturing process that has a remarkable potential as an alternative to synthetic adhesives in timber engineering. The adhesion mechanism of welded wood is based on the softening and flowing of some amorphous, cells-interconnecting polymer material, mainly lignin and hemicelluloses (Gfeller et al 2003). However, there have been few studies related to structural applications, which is partially due to the vulnerability of the welded joint to damage from moisture.…”