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 owing 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 bres, and the formation of a bre entanglement network in the matrix of molten material which then solidi es. Thus, a wood cells/ bre entanglement network composite having a molten lignin polymer matrix is formed. During the welding period some of the detached wood bres which are no longer held by the interconnecting material are pushed out of the joint as excess bres. Crosslinking chemical reactions also have shown to occur. The most likely one of these identi ed by NMR appears to be a cross-linking reaction of lignin with carbohydrate-derived furfural. The presence of these reactions has been identi ed 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 nished, which explains why long holding times under pressure after the end of welding contribute strongly to obtaining a good bond.

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The peculiar behaviour noticed in tannin-based and other wood adhesives when hexamethylenetetramine (hexamine) is used as a hardener is described. When using hexamine as hardener in tannin adhesives, a¯ow problem during hot curing of this adhesive/hardener system and its consequences on adhesive performance is identi®ed. The solution of the tannin/hexamine problem con®rmed and gave a clear theoretical justi®cation for the applied ®nding that under many application conditions hexamine is not a formaldehyde-yielding compound yielding extremely low formaldehyde emissions in bonded joints. 13 C NMR evidence is presented con®rming that the main decomposition (and recomposition) mechanism of hexamine is not directly due to formaldehyde but rather proceeds through now-identi®ed intermediates, i.e. mainly through the formation of reactive imines rather than methylene bases, possibly also forming a very slight amount of iminomethylene bases. This also con®rms that any species with strong real or nominal negative charge under alkaline conditions, be it a tannin, resorcinol or other highly reactive phenols, be it melamine or another highly reactive amine or amide, or an organic or inorganic anion, it is capable of reacting with the intermediate species formed by decomposition (or recomposition) of hexamine far more readily than formaldehyde explaining the capability of wood adhesives formulations based on hexamine to give bonded panels of extremely low formaldehyde emission. If no highly reactive species with strong real or nominal negative charge is present, then decomposition of hexamine proceeds rapidly to formaldehyde formation as reported in previous literature. The elucidation of the hexamine decomposition mechanism, which is presented, and a scanning electron microscopy (SEM) investigation also allowed to advance a reason for the without-curingformation of ambient temperature stiff gels in tannin/ hexamine glue mixes and to propose chemical structures for the ionic coordination linear polymers formed.