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.

Colloidal particle formation followed by their clustering has been shown to be the normal way of ageing of aminoplastic resins, in particular melamine-ureaformaldehyde (MUF) resins. Ageing (or further advancement of the resin by other means such as longer condensation times) causes whitening of the resin. This is a macroscopic indication both of the formation of colloidal particles and of their clustering. Some clustering appears rather early in this process, even when the great majority of the resin does visually appear to be in colloidal state, being transparent. However, it eventually progresses to resins which are mostly in colloidal, clustered state, followed much later by a supercluster formation starting to involve the whole resin. There appears to be clear correspondence between molecular mass increases as obtained by gel permeation chromatography (GPC), low-angle laser light scattering (LALLS) analysis, and observation by polarizing optical microscopy. LALLS, however, appears to indicate the dimensions of the colloidal particles themselves when the level of colloidal aggregation is rather low, but it indicates the dimensions of the clusters once these are mostly aggregated. The smaller visible colloidal particles, already aggregates, were found by polarizing optical microscopy to be of a mostly elongated, rodlike shape, the length of which was shown to grow much further than their width with resin advancement and ageing. As their dimensions indicate, these are already clusters; this implies that the mainly linear increase of the polycondensate chains influences also the simpler colloidal clusters' growth direction, possibly explaining the resins' lack of tridimensional hardening while still in storage. It also explains why molecules such as free urea and acetals, by disrupting these colloidal aggregation mechanisms, allow both a much longer shelf life of the resin and its better performance in hardening. These findings explained the considerable difference in the behavior and performance of different MUF resin formulations. The ageing of the MUF resins of different preparation procedures appeared then to proceed from (1) clear resin (molecular colloidal aggregation) to (2) superclusters of a whitened, heavily thixotropic resin, which is the beginning of physical gelation to (3) liquid/cluster separation, which is the terminal stage of physical gelation.

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Iminoamino methylene base intermediates obtained by the decomposition of hexamethylenetetramine (hexamine) stabilized by the presence of strong anions such as SO 4 2Ϫ and HSO 4 Ϫ , or hexamine sulfate, were shown to markedly improve the water and weather resistance of hardened melamine-urea-formaldehyde (MUF) resins used as wood adhesives and of the wet internal bond strength performance of wood boards bonded with them. The effect was shown to be induced by very small amounts, between 1 and 5 wt % of this material on resin solid content. This strong effect allowed the use of MUF resins of much lower melamine content and also provided good performance of the bonded joints. Because the main effect was also present at the smaller proportion of hexamine as hexamine sulfate, it was not due at all to any increase in the molar ratio of the resin as a consequence of hexamine sulfate addition.

ABSTRACT:The strength improvement induced by addition of acetals such as methylal and ethylal in melamineurea-formaldehyde (MUF) resins could be mostly ascribed to the increased effectiveness and participation of the melamine to resin cross-linking. This phenomenon has been shown here, by matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectroscopy, resin aging time stability, and mainly by laser light scattering, to be due to the following: (i) the increased solubility in water afforded by the acetals cosolvents of both the unreacted melamine and of the normally very much lower solubility, higher molecular weight, lower methylolated oligomers fraction, this leading to preferentially homogeneous and hence more effective reaction rather than heterogeneous reactions; and (ii) the effect that such acetals have on the size distribution of the resin colloidal particles, with the presence of acetals such as methylals markedly decreasing the average colloidal particles diameter of the resin. This latter effect appears to be due to the disruption of the molecular clustering of the MUF resin colloidal particles, but rearrangements in the size of the colloidal particles due to the decrease in surface tension of the system, which has also been noted, cannot be excluded.

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

No abstract