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

Abstract

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.

CHT and TTT curing diagrams of polyflavonoid tannin resins

“…The gelation and vitrification curves are the most characteristic features of both TTT and CHT diagrams; the S‐shape of the CHT diagram, in particular, defines the region at which the reaction rate is greatly reduced because diffusion control becomes more pronounced in the glass‐transition region. Although the majority of the CHT and TTT diagrams studies reported to date have mostly focused on the curing of epoxy resins on glass fiber braid,1–5 CHT and TTT diagrams of other polycondensation resins used in considerable amounts as wood adhesives [i.e., urea–formaldehyde (UF), melamine–urea–formaldehyde (MUF) resins, phenol–formaldehyde (PF), and phenol–resorcinol–formaldehyde (PRF) cold‐set resins, on lignocellulosic substrates (i.e., wood)] have recently been reported 6, 7. Such studies have been carried out because these resins are the most used (by volume) polycondensation resins to date, of which their main use is as thermosetting wood adhesives 8.…”

Rheology of polyflavonoid tannin–Formaldehyde reactions before and after gelling. I. Methods

“…Water solutions of commercial polyflavonoid tannin extracts of mimosa (Acacia mearnsii) bark tannin extract, natural quebracho (Schinopsis balansae) wood tannin extract, pine (Pinus radiata) bark tannin extract, and pecan (Carya illinoensis) nut membranes tannin extract and, furthermore, a quebracho tannin extract modification from which all carbohydrates had been eliminated (quebracho QS) by organic solvent extraction and a quebracho tannin adhesive intermediate treated according to viscosity-reducing and reactivityenhancing techniques already reported, the origins of which were, respectively, from Brazil, Argentina, Chile, United States, Italy, and, again, Argentina, were prepared, respectively, at a 50, 42, 35,36,45, and 45% extract solids concentration in water. The tannin extracts are produced industrially by countercurrent extraction with just water at 95°C in the case of mimosa, by countercurrent extraction with, respectively, 5 and 2% sodium sulfite and at 100 and 70°C for quebracho and pine tannin extracts, respectively, and by countercurrent extraction with 2-4% sodium sulfite and 0.4% sodium carbonate for pecan nut tannin extract.…”

“…The curves based on the equation without the 1/3 exponent appear to be closer to reality with still a very rapid but more moderate increase of p, followed by a slowing down of its growth due to the appearance of diffusional problems at degrees of conversion higher than 0.9, confirming a limit already determined by TTT diagrams built using different techniques. 6,34,35 In the case of the reaction of a polyflavonoid tannin and formaldehyde, it is then eq. (13) which needs to be used for the calculation of the degree of conversion p after the gel point:…”

“…The trends observed, however, confirm that the reaction phases studied after the gel point are still those where, while diffusion problems do clearly already occur and are not negligible, they are still relatively small. The high degree of conversion phase, where diffusion problems are predominant and very marked, 35,38,39 appears then to occur well beyond the physical limits imposed by the equipment used, a fact confirmed, using different tech- Figure 7 Determination by graphic methods according to the diffusion model of Jender of the rate constants k 4 and k 5 of the two reaction zones for the system mimosa tannin extract (prorobinetinidin tannin) ϩ 5% paraformaldehyde at 65°C, characterizing the polycondensation reaction after the gel point if the reaction is considered to be diffusion-dependent (see Table IV).…”

Influence of preparation parameters on the CHT curing diagrams of MUF polycondensation resins

“…Temperature–time–transformation (TTT) curing diagrams and continuous heating transformation (CHT) curing diagrams of melamine–urea–formaldehyde (MUF) polycondensation resins, cured on a wood substrate were recently described 1. While the general trends of these diagrams have been clearly identified and described, what is needed for the CHT diagrams, more technologically significant for wood‐adhesive application, is to determine which manufacturing parameters influence the trends and the values of the time and temperature of the diagram.…”