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

The peculiar behaviour noticed in tannin-based and other wood adhesives whe n hexamethylenetetramine (hexamine) is used as a hardener is described. Whe n using hexamine as hardener in tannin adhesives, a flow problem during hot curing of this adhesive/hardener system and its consequences on adhesive p erformance is identified. The solution of the tannin/hexamine problem confi rmed and gave a clear theoretical justification for the applied finding tha t 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 r ecomposition) 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 t hat any species with strong real or nominal negative charge under alkaline conditions, be it a tannin, resorcinol or other highly re-active phenols, b e it melamine or another highly reactive amine or amide, or an organic or i norganic anion, it is capable of reacting with the intermediate species for med by decomposition (or recomposition) of hexamine far more readily than f ormaldehyde explaining the capability of wood adhesives formulations based on hexamine to give bonded panels of extremely low formaldehyde emission. I f no highly reactive species with strong real or nominal negative charge is present, then decomposition of hexamine proceeds rapidly to formaldehyde f ormation as reported in previous literature. The elucidation of the hexamin e decomposition mechanism, which is presented, and a scanning electron micr oscopy (SEM) investigation also allowed to advance a reason for the without -curing-formation of ambient temperature stiff gels in tannin/ hexamine glu e mixes and to propose chemical structures for the ionic coordination linea r polymers formed.