Induced accelerated autocondensation of polyflavonoid tannins for phenolic polycondensates. I. ¹³C‐NMR, ²⁹Si‐NMR, X‐ray, and polarimetry studies and mechanism

Abstract

SYNOPSISPolyflavonoid tannins have been found to autocondense and harden when in presence of small amounts of SiO, at high pH. Small amounts of boric acid and AlC13 were found to have the same effect. The mechanism of tannin autocondensation to hardening was found to depend on the Lewis acid behaviour of the additives used. Such mechanism involves Lewis acid acceptance of electrons from the ether oxygen of the flavonoid unit pyran ring with subsequent facilitation of base-induced heterocycle opening. The reactive C2 site created by heterocycle opening proceeds to autocondense with the reactive A-ring of a flavonoid unit on another chain denying to the flavonoid the possibility of intramolecular rearrangements to catechinic acid and phlobatannin which occur in model compounds. Si has been shown to pass through a coordination state of 5, by CP-MAS "Si-NMR. The portion of Si, which has not been able to complete the reaction due to premature hardening, remains attached to the flavonoids, in this coordination state, in the hardened network. The Si portion, which has completed the reaction and caused the hardening, reverts instead to SiOz and is detached from the flavonoid. The Si02, H3B03, and A1C13 mechanism shows that their presence also reverses the relative facility of cleavage between interflavonoid bond and heterocycle opening in mainly procyanidin and profisetinidin tannins. 0 1994

A no-aldehyde emission hardener for tannin-based wood adhesives for exterior panels

“…In Table 2 are reported the results of laboratory particleboard prepared when using TN as a hardener alone or in combination with methylolureas hardener (Trosa 1997(Trosa , 1999 and with silica accelerator of tannin autocondensation (Meikleham et al 1994;Pizzi et al 1995). The result indicates that at least for the more reactive mimosa tannin the use of TN alone is capable of giving dry and after boiling internal bond (I.B.)…”

Tannins: Major Sources, Properties and Applications

“…The self-condensation reactions characteristic of polyfl avonoid tannins have only recently been used to prepare adhesive polycondensates hardening in the absence of aldehydes [85] . This self-condensation reaction is based on the opening under alkaline and acid conditions of the O1ᎏC2 bond of the fl avonoid repeating unit and the subsequent condensation of the reactive centre formed at C2 with the free C6 or C8 sites of a fl avonoid unit on another tannin chain [85][86][87][88][89] . Although this reaction may lead to considerable increases in viscosity, gelling does not generally occur.…”

Adhesives for wood composites

“…Tannin can be used as a pure resin, and the auto-condensation of the polyflavonoids in acid or base conditions will lead to a tannin chain formed through the free C6 and C8 sites on the molecule (Meikleham et al, 1994;Pizzi and Stephanou, 1993). When pure tannins are auto-condensed the reaction leads to a rapid increase of viscosity but no gelling occurs.…”