CHT and TTT curing diagrams of polyflavonoid tannin resins

TTT and CHT curing diagrams for tannin-based adhesives were built by thermo mechanical analysis (TMA) by following the in situ hardening directly in a wood joint, and the curve trends observed were similar to those previously observed for synthetic polycondensation resins on lignocellulosic substrate s. Of the parameters that most influence the relative position of vitrifica tion and gel curves on the diagrams (i.e., where the influence has been qua ntified), chief among them is the reactivity of the tannin with formaldehyd e and any factor influencing it: thus, the inherent higher reactivity of th e A-ring of the tannin (such as in procyanidins versus prorobinetinidins) a nd the pH of the tannin solution. The percentage formaldehyde hardener has some influence in CHT diagrams, especially for the slower-reacting tannins, but practically no influence in TTT diagrams within the 4-10% formaldehyde range used. As in the case of synthetic polycondensation adhesive resins, regression equations relating the internal bond strength of a wood particle board, prepared under controlled conditions, with the inverse of the minimu m deflection, obtained by constant heating rate TMA of a wood joint during resin cure, have been obtained for the two types of tannins of lower reacti vity (profisetinidins/prorobinetinidins) but not for the faster-reacting pr ocyanidin and prodelphinidin tannins. (C) 2001 John Wiley & Sons, Inc.