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

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 flow problem during hot curing of this adhesive/hardener system and its consequences on adhesive performance is identified. The solution of the tannin/hexamine problem confirmed and gave a clear theoretical justification for the applied finding that under many application conditions hexamine is not a formaldehyde-yielding compound yielding extremely low formaldehyde emissions in bonded joints. C-13 NMR evidence is presented confirming that the main decomposition land 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 re-active 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.