F. Pichelin

Hexamine hardener behaviour: effects on wood glueing, tannin and other wood adhesives

13C NMR evidence is presented confirming that the main decomposition (and recomposition) mechanism of hexamine is not directly due to formaldehyde but rather proceeds through now-identified 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 confirms 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-curing-formation of ambient temperature stiff gels in tannin/hexamine glue mixes and to propose chemical structures for the ionic coordination linear polymers formed.13C NMR-Spektren machten deutlich, daß der hauptsächliche Zersetzungs- und Wiederaufbaumechanismus des Hexamins nicht unmittelbar über Formaldehyd, sondern über jetzt identifizierte Zwischenverbindungen verläuft, d.h. überwiegend über die Bildung von reaktiven Iminen anstatt über Methylenbasen; nur ein sehr geringer Anteil an Iminomethylenbasen kann dabei ebenfalls entstehen. Die Untersuchungen zeigten auch, daß jede Komponente mit starker realer oder nomineller negativer Ladung unter alkalischen Bedingungen in der Lage ist, mit den Zwischenprodukten beim Abbau und Wiederaufbau des Hexamins viel schneller zu reagieren als Formaldehyd. Das gilt sowohl für Tanrin, Resorcin oder andere hochreaktive Phenole, als auch für Melanin oder andere hochreaktive Amine oder Amide, und allgemein für organische oder anorganische Anionen. Dies erklärt die Besonderheit, daß mit Holzklebern unter Hexaminzusatz Platten mit extrem niedriger Formaldehydemission hergestellt werden können. Wenn keine Komponente mit starker negativer Ladung im Harzgemisch vorhanden ist, dann führt der Hexaminabbau rasch zu Formaldehyd wie es bisher in der Literatur beschrieben wurde. Die Aufklärung des Mechanismus des Hexaminabbaus sowie Rasterelektronenmikroskopische Untersuchungen lieferten zudem eine Begründung für die Bildung fester Gele (ohne Aushärtung) bei Umgebungstemperatur. Daraufhin konnte auch eine chemische Struktur für die gebildeten ionischen linearen Polymere vorschlagen werden.

Wood dowel bonding by high-speed rotation welding

High-speed rotation-induced wood dowel welding, without any adhesive, is shown here to rapidly yield wood joints of considerable strength. The mechanism of mechanically-induced highspeed rotation wood welding is shown here to be due, as already observed in vibration welding, to the temperature-induced softening and flowing of some amorphous, cells-interconnecting polymer material in the structure of wood, mainly lignin, but also of hemicelluloses and consequent high densification of the bonded interface. Wood species, relative diameter differences between the dowel and the receiving hole, and pressing time were shown to be parameters yielding significant strength differences; while relative orientation of the fibre grain of the dowel in relation to the fibre grain of the substrate, relative rate of rotation within a limited range and the use of rough or smooth dowels did not have any significant influence. X-ray microdensitometry and scanning electron microscopy were used to determine the limits of wood dowel welding by high-speed rotation. The type of parameters that had an influence on strength indicated that the strength values obtained, although often rather high, were often due to welding of only a limited part of the dowel to the substrate. This is due to the forcing of the dowel into a truncated conical shape by the pressure of insertion and the consequent disruption of bonding in some areas. Notwithstanding this effect, the welded contact area is sufficient to yield strength results comparable to or even slightly higher than that obtained by PVAc adhesive bonding. The use of dry dowels inserted hot in the substrate after preheating them at high temperature (100°C) yielded consistently better results than that obtained with PVAc gluing.

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.

Parameters influencing wood-dowel welding by high-speed rotation

Oven-dry dowels, insertion of hot dowels, cross-cut dowels, substrate holes of step-decreasing diameter as a function of depth, use of ethylene glycol or other compounds able to decrease the glass transition temperature of wood components have all been shown to contribute to improving weld joint strengths in a variety of less drastic conditions than the 10 mm/8 mm dowel/substrate hole diameter difference. The results show that once the depth of the dowel is much greater than 15 mm, then almost all the conditions used improve the weld strength. This means that the proportion of area welded in relation to the tensile strength of the dowel itself is a determining factor. The greater this area the higher the strength, irrespective of the application conditions used. Thus, over a certain welded area the dowel breaks when tested in tensile, i.e., the joint is stronger than the dowel. Temperatures > 180°C are reached during the quick welding step with the temperature decreasing in less than 1 min to 60–70°C. The same chemical reactions as occurring in vibrational welding have been shown by solid-state 13C-NMR analysis to also occur in dowel rotation welding. In dowel rotation welding the production of carbohydrate-derived furanic aldehydes is higher (a) from the wood material of the substrate in which the hole is pre-drilled rather than from the material of the wood dowel itself, (b) when the weld joint strength is good, and (c) when the rate of dowel insertion is higher.

Properties and set-recovery of surface densified Norway spruce and European beech

The chemistry and wetting behaviour of surface densified wood were investigated using FT-IR spectroscopy and contact angle analyses. Furthermore, set-recovery of the surface under conditions of fluctuating humidity was measured and quantitative microscopy analyses were undertaken. FT-IR indicated that no significant chemical changes took place during the densification process. However, the wettability of the densified surfaces was significantly lower than unmodified surfaces. Following several high humidity-oven dry cycles, it was found that this densification process was almost completely reversible, i.e., there was full set-recovery.

Vibration welding of heat-treated wood

Vibration welding of wood that has been preheated according to an industrial two-step process indicates that such wood can be welded and can yield welded joints of good strength. The joint strength is, however, markedly lower than obtained when welding non-heat-treated timber. In general, weld strength of the timber is poor if welding is done on hydrothermolyzed wood. The strength results are instead much better if welding is done at the end of the complete heat treatment process, i.e., after the dry heat step. The weld lines of heat-treated wood show entangled cells where there is none or very little of the molten matrix intercellular material usually observed in welded timber. Furthermore, in weldlines obtained after hydrothermolysis an increase in rigidity and brittleness of the wood cells is observed. Hence, the wood cells are not entangled at all or very little. Both observations indicate that heat treatment has affected the main melting region of the wood, namely the intercellular material. As most of this material is already either lost or heavily cross-linked during heat treatment, only little of it is now available to melt and bind the wood surfaces during vibrational 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.

Influence of grain direction in vibrational wood welding

Abstract Wood grain orientation differences in the two surfaces to be bonded yield bondlines of different strength in no-adhesives wood welding. Longitudinal wood grain bonding of tangential and radial wood sections yields an approximately 10% difference in strength results of the joint. Cross-grain (±90°) bonding yields instead a much lower strength result, roughly half that observed for pieces bonded with the grain parallel to each other. These differences can be explained by the very marked effect that homogeneity of fibre orientation is known to have on fibre–matrix composites. Oak yields lower results than beech and maple and is more sensitive to welding conditions. Differences in both anatomical and wood constituent composition can account for this difference in performance. Contrary to the other wood species, oak always presents joint bondlines where little or no increase in density at the interface is noticed. This explains its somewhat lower strength results. This is based on the different mode of bonding predominant in this species, while the other species present two different modes of bonding. Thus, two types of bondlines are observed by scanning electron microscopy (SEM): (i) bondlines where entangled fibre–matrix composites are formed at the interface and (ii) bondlines in which direct welding of the cell walls occurs, just by fused intercellular material or cell surface material. In this latter case the cells remain flat, without an entangled fibre–matrix composite being formed. This is the almost exclusively predominant case for oak. Both cases and even hybrid cases between the two have also been observed in beech.

Wood Welded Connections: Energy Release Rate Measurement

The energy release rates of beech specimens bonded by linear friction welding were determined using double cantilever beam (DCB) tests. The analysis of the results was carried out with the experimental compliance method, which is based on the linear-elastic fracture mechanics. The compliance relation was approximated to a third-order polynomial equation for smoothing and followed by calculation of least squares. This analysis method had not been previously used for glued connections in mode I. It proved to be an ideal method for the results obtained from the study of the energy release rates of the welded joints. The value of the energy release rate G Ic obtained is, on average, 106 J/m2. The variation in the results is less than for most energy release rates of the wood–adhesive couples using beech studied previously. This proved that this method of measurement of the energy release rate adapts well to DCB wood welded specimens. It is the first time that fracture mechanics is applied to the study of linear friction welded wood joints.