Wood bonding by vibrational welding

Abstract

—Mechanically-induced wood welding, without any adhesive, is shown here to rapidly yield wood joints satisfying the relevent 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 some amorphous, cells-interconnecting polymer material in the structure of wood, mainly lignin, but also hemicelluloses. This causes partial detachment, the 'unglueing' of long wood cells, wood fibres, and the formation of a fibre entanglement network in the matrix of molten material which then solidifies. Thus, a wood cells/fibre entanglement network composite having a molten lignin polymer matrix is formed. During the welding period some of the detached wood fibres which are no longer held by the interconnecting material are pushed out of the joint as excess fibres. Crosslinking chemical reactions also have shown to occur. The most likely one of these identified by NMR appears to be a cross-linking reaction of lignin with carbohydrate-derived furfural. The presence of these reactions has been identified by CP-MAS 13C-NMR. These reactions, however, are relatively minor contributors during the very short welding period. Their contribution increases after welding has finished, which explains why long holding times under pressure after the end of welding contribute strongly to obtaining a good bond.

Keywords:

• WOOD WELDING
• WOOD FUSION
• POLYMER MELTING
• ADHESIVES
• LIGNIN
• JOINTS