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

Abstract

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¯ow problem during hot curing of this adhesive/hardener system and its consequences on adhesive performance is identi®ed. The solution of the tannin/hexamine problem con®rmed and gave a clear theoretical justi®cation for the applied ®nding that under many application conditions hexamine is not a formaldehyde-yielding compound yielding extremely low formaldehyde emissions in bonded joints. 13 C NMR evidence is presented con®rming that the main decomposition (and recomposition) mechanism of hexamine is not directly due to formaldehyde but rather proceeds through now-identi®ed 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 con®rms 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-curingformation of ambient temperature stiff gels in tannin/ hexamine glue mixes and to propose chemical structures for the ionic coordination linear polymers formed.

Tannin-caprolactam and Tannin-PEG formulations as outdoor wood preservatives: weathering properties

“…The inclusion of the monomeric aminocaproic acid or its oligomers in the tannin-hexamine network can be figured out in many ways. According to the signal of R-CH 2 -O, the more probable hypothesis is that the reactive imines produced from hexamine in alkaline environment react with the carboxylic group of the caprolactam (Pichelin et al 1999). Here, a possible polymer structure is observed (Fig.…”

“…A significant breakthrough in the chemistry of flavonoid tannins was that their condensation reactions occurred with similar extent as phenol and resorcinol (Pizzi 1980;Pichelin et al 1999), and therefore, it was possible to oppose the long-standing leaching problem of tannin in the wood preservation field and hence raise this approach to a new level.…”

Environmental effects and economic analysis of adhesives: a review of life cycle assessment (LCA) and techno-economic analysis (TEA)

“…Due to the increasing restrictions on the release of volatile organic compounds (VOCs) to the atmosphere, there are decreasing trends from using organic solvents and dispersing media. As indicated in Pichelin et al (1999), any species with a strong negative charge under alkaline condition is capable to render bonded panels of extremely low formaldehyde emission, in other words, the adhesives based on hexamine could reduce the impact of environment of formaldehyde. The results (Pichelin et al, 1999) showed that tannin-hexamine adhesive is a very environmentally-friendly product.…”

Wood production, wood technology, and biotechnological impacts

“…Hexamine (see Fig. 9B in Chapter 15 of this book) or the cheap methylolated nitroparaffins for example can functionally replace formaldehyde as hardener , Pichelin et al 1999, Trosa & Pizzi 2001, Kim & Kim 2003, Kim et al 2003. In such way, panel boards satisfying the Japanese standard JIS A 5908 (2003) were obtained in laboratory and in industrial trials (Pichelin et al 2006).…”