TTT and CHT curing diagrams of water-borne polycondensation resins on lignocellulosic substrates

Lignocellulosic substrates such as wood were found to have a marked modifying influence on both lower temperature and higher temperature zones of TTT and CHT diagrams during hardening of phenol-resorcinol-formaldehyde (PRF) and melamime-urea-formaldehyde (MUF) polycondensates. Although the modifying influence of the substrate on the higher temperature zone of CHT diagrams presented the same trend of what was already reported for phenol-formaldehyde (PF) and ureaformaldehyde (UF) polycondensates, marked differences from what reported in the literature were recorded for TTT diagrams of all these polycondensates as well for the lower temperature zones of the CHT diagrams on lignocellulosic substrates, both of which had not been investigated previously. The chemical and physical mechanisms of the interactions of the resins, the substrate, and the water carrier causing such marked variations are presented and discussed. Although in the higher temperature zones both substrate and water carrier play an important role, in the lower temperature zone the presence of water appears to be the dominant factor causing the observed variations. The generalized modified CHT and TTT diagrams characteristic of the behavior of these water-borne polycondensates on lignocellulosic substrates can be used to describe the behavior and complex changes of phase the formaldehyde-based polycondensation resins undergo when used as wood adhesives during their curing directly in the wood joint. The results also show that diagrams obtained with pure resin cannot be used to predict the behavior of the polycondensate when this is markedly modified by the presence of interacting solvents and substrates. (C) 2001 John Wiley & Sons, Inc.