Structural beams from thick wood panels bonded industrially with formaldehyde-free tannin adhesives

Mimosa tannin hardened with hexamine at pH 10 has shown both at the laboratory and industrial level to be a formaldehyde-free system, within the limits of sensitivity of the method of Japanese standard JIS A 5908. This useful effect is based on the double mechanism of slow hexamine decomposition to reactive imino-amino methylene bases and their immediately subsequent very rapid reaction with the tannin. Decomposition to formaldehyde can never be reached under the conditions used. This yielded a long ambient temperature pot-life coupled with the fast hardening of the adhesive and fast pressing times of the thick panels by introducing a two-step steam-injection sequence during panel pressing. No formaldehyde emission was found in the panels bonded with such an adhesive system once tested according to the relevant Japanese standard. No free formaldehyde was detected by solid state C-13 NMR, nor residual hexamine, in the hardened tannin-hexamine adhesive, although these spectra have to be taken with caution due to the usual peak enlargement and relative lack of sensitivity in solid state NMR spectra. The reactions involved were explained by C-13 NMR. The panels obtained satisfied the relevant Japanese standard specification for both internal bond strength and formaldehyde emission.