M. Zanetti

X-ray microdensitometry analysis of vibration-welded wood

X-ray microdensitometry tests on vibration-welded hardwoods (beech and oak) and a softwood (spruce) showed that adhesion of wood surfaces by vibration welding was accompanied by a considerable increase in the density of the wood at the bonded interface. This is due to the loss of the intercellular structure of the wood at the interface and considerable decrease of empty spaces in its cellular structure. The sharper and more regular is the increase in density at the interface the better is the mechanical performance of the joint. This was ascribed to the marked difference in wood density between earlywood and latewood. This intra-ring wood heterogeneity is a limiting factor for reaching high local compression rates during the welding process. In the case of spruce wood it causes cells collapse and, consequently, poorer bonding. Absolute and relative density maps of well-bonded, poorly-bonded and unbonded joints are reported. X-ray microdensitometry proved to be a valuable technique to determine the extent of wood welding.

Solid wood joints by in situ welding of structural wood constituents

Abstract 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 13C 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.

Upgrading of MUF particleboard adhesives and decrease of melamine content by buffer and additives

Iminoamino methylene bases intermediates obtained by the decomposition of hexamethylenetetramine (hexamine) stabilized by the presence of strong anions such as SO - 4 and HSO - 4 , or hexamine sulphate, have been shown to markedly improve the water and weather resistance of hardened MUF resins used as wood adhesives and of the wet internal bond (IB) strength performance of wood boards bonded with them. The effect has been shown to be induced by very small amounts, between 1% and 5% by weight of this material on resin solids content. This strong effect also allows the use of MUF resin of much lower melamine content while still allowing good performance of the bonded joints. As the effect is present also at the smaller proportion of hexamine as hexamine sulphate, it was shown that it was closely linked to the strong buffering action that hexamine sulphate has on MUF resins in the correct pH range for their hardening and not due at all to any increase in molar ratio of the resin as a consequence of hexamine sulphate addition. The reasons for the hexamine sulphate behaviour linked to its buffering action appeared to be due mainly to the shifting of the polycondensation ↔ degradation equilibrium of the resin to the left as a consequence of the increased pH induced by the buffering action. The presence of the buffer induces an increase in pH of between 25% and 50%, yielding faster hardening as a consequence of the higher SO - 4 ions content, but a much lesser extent of the simultaneous degradation reaction as a consequence of the higher pH. The network formed is then tighter and less touched by degradation resulting in lower thickness swelling of the board and a much greater strength after water attack of the panel. Some alternate systems presenting the same buffer action within the wanted parameters ranges were also shown to have a comparable effect.

Recycling melamine-impregnated paper waste as board adhesives

Melamine impregnated paper offcuts and waste has been shown to be usable both directly, in finely powdered form, as a binder for particle board and as a melamine substitute during the formulation and preparation of liquid MUF resins. Both approaches were based on the residual activity of the MF resin present in the resin. Amounts of waste melamine paper of 22% on dry wood, equivalent to 10% melamine resin solids on total dry panel weight, were shown necessary for water resistant particleboard, and 18.5% for interior particleboard. Due to problems connected to excessive viscosity increase only a maximum of 25% of the total melamine in the formulated melamine resin could be substituted to good effect in the formulation of liquid MUF resin adhesives.ZusammenfassungEs wurde gezeigt, dass sowohl melamin-imprägnierte Papierabfälle als auch Altpapier in feiner pulverisierter Form direkt als Bindemittel für Spanplatten und als Melaminersatz während der Mischung und Herstellung von flüssigen MUF-Harzen verwendbar sind. Beide Ansätze basierten auf der Restaktivität des MF-Harzes. Es wurde gezeigt, dass Zusätze von 22% Melaminaltpapier zu trockenem Holz, äquivalent zu 10% Melaminfestharz bezogen auf das gesamte Trockengewicht der Platten für die Herstellung von beständigen Platten notwendig sind sowie 18,5% für Platten für Innenverwendung. Aufgrund der Probleme, die mit exzessivem Viskositätsanstieg verbunden sind, konnten nur maximal 25% des Gesamtmelamins effektiv in der flüssigen MUF-Harzmischung substituiert werden.

Dependance on the adhesive formulation of the upgrading of MUF particleboard adhesives and decrease of melamine content by buffer and additives

The type of MUF adhesive formulation has been shown to have a determinant effect in the performance of MUF adhesives for wood panels upgraded with iminoamino methylene basis of the “hexamine sulphate” type. The high performance sequential MUF formulations are upgraded little or not at all by these additives. Much weaker non-sequential MUF formulations instead are so strongly upgraded for IB boiled strength by the use of these additives to markedly outperform the normally stronger sequential type formulations and even phenolic resins. To the buffering action leading to optimization of the hardening ⇄ degradation equilibrium described previously as the main cause of the functioning of these additives must then be added the effect of the type of formulation. This is quantified and described. It is partly related to the presence of higher or lower levels of unreacted primary amine groups of the melamine and their consequent level of contribution to buffering. It is also partly related to the level of plasticization of the hardened glue line by unreacted urea and the consequent viscoelastic dissipation of energy that would derive from this.ZusammenfassungDie Art der MUF-Zusammensetzung erwies sich als bestimmender Einfluss auf die Leistungsfähigkeit von MUF-Harzen für Holzwerkstoffplatten, die mit Iminoamino-Methylen vom Typ „Hexaminsulfat“ aufgebessert wurden. Die Hochleistungs-Harze mit stufenweiser Zumischung werden wenig oder gar nicht durch diese Zusätze verbessert. Bei schwächeren Harzmischungen ohne stufenweise Aushärtung wird die Querzugfestigkeit nach dem Kochtest erheblich verbessert, so dass sie die sequentiellen zum Teil sogar die Phenolharze übertreffen. Zusätzlich zu dem früher beschriebenen Puffer-Effekt, der das Gleichgewicht zwischen Aushärtung und Abbau optimiert, muss nun noch der Einfluss der Art der Zusammensetzung beachtet werden. Dieser Einfluss wird hier beschrieben und quantifiziert. Er steht zum Teil in Zusammenhang mit mehr oder weniger hohen Anteilen von nicht regierenden primären Aminogruppen des Melanins und daraus folgend deren Beitrag zum Puffersystem. Ein anderer Teil des Einflusses ergibt sich aus dem Plastifizieren der Klebefuge durch nicht regierenden Harnstoff und die daraus folgende viscoelastische Energiedämpfung.

Time delay effect in TMA methods for MUF resins testing

Materials and methods Melamine-urea-formaldehyde (MUF) resins of molar ratios variable between 1:1.2 and 1:1.9 to which was added 1.5% ammonium sulphate hardener were tested by thermomechanical analysis (TMA) on sandwich joints of beech wood forming specimens of 21 · 6 · 1.2 mm dimension in three points bending and at a constant heating rate of 10� C/minute according to techniques already reported (Kamoun and Pizzi 2000). The resins were tested at time intervals of 15 minutes, starting immediately after the composition of the glue mix. It was observed that the results obtained improved in approximately the first 30 minutes after the composition of the glue-mix and then stabilized. Typical TMA thermograms describing the modulus of elasticity (MOE) as a function of temperature for the resin tested immediately (t ¼ 0, thin curve) and after 30 minutes standing (t ¼ 30 minutes, thick curve) are shown in the figure. Results and discussion The indication from the figure are that the max value of the MOE obtained by testing the same MUF and hardener glue-mix increases for up to approximately 30 minutes after the preparation of the glue-mix. It does stabilize afterwards and does not increase anymore after longer time periods. To eliminate the possibility that the effect was a fabrication of the method or of the equipment such as the presence of residual heat from the first run in the TMA equipment the same experiment was repeated on two glue-mixes of the same type one of which was aged first and then tested while the second one was tested second but immediately after preparation. The results were the same showing that the effect is real and not due to a fabrication of the equipment or method used. This effect must always be taken into account when testing aminoplastic resins glue-mixes by TMA. The reason to which this effect may be ascribed to is the relatively slow rate of dissociation of the ammonium sulphate salt when added to a very concentrated resin solution such as the MUF resin here, where the amount of water available for this purpose is relatively low. The result obtained is then maximized at approximately 30 minutes glue-mix ageing, this being approximately the time at ambient temperature that the ammonium sulphate hardener takes to reach its dissociation equilibrium. Thus, immediately after composition of the glue-mix the low rate of salt dissociation hardly affects the alkaline pH of the resin at ambient temperature and this is indeed the starting pH of the resin for hardening once the application of higher temperature causes complete hardener dissociation. After the hardener dissociation equilibrium has been reached at ambient temperature, approximately 30 minutes after preparation of the glue-mix, the resin starting pH at ambient temperature is sensibly lower, resulting in more complete hardening and higher MOE in these acid-setting resins once the temperature is increased.