Лако-красочные материалы — производство

Temperature-time-transformation (TTT) and continuous-heating-transformation (CHT) curing diagrams for polycondensation resins are starting to acquire more importance in the deductions of the behavior of different resins during hardening. They are a type of state diagram. TTT and CHT diagrams of resins by themselves or on noninteracting substrates show similar trends as exemplified by the case of epoxy resins on glass fibers [42,43] (Fig. 8a). Different trends than those for TTT and CHT diagrams of epoxy resins reported in the literature (Fig. 8a) occur, however, in the higher and lower temperature zones of the diagrams of waterborne formaldehyde-based resins hardening on wood. CHT and TTT diagrams have already been reported for PF, UF, MUF, phenol-resorcinol — formaldehyde (PRF), and tannin-formaldehyde thermosetting resins [31,44-46] (Figs. 8b and c). The higher temperature zone of the CHT diagrams for MUF resins in a wood joint, reported in Fig. 8c, shows the same trends (and for the same reasons, namely the interactive nature of the substrate and movement of water from resin to substrate and vice versa) observed for UF and PF resins.

However, the experimental TTT diagram in Fig. 8b shows quite a different trend from the CHT diagram for the same resins and for the TTT diagrams reported in the literature for epoxies on noninteracting substrates such as glass fiber. To start to under­stand the trend shown in Figs. 8b and c it is first necessary to observe what happens to the modulus of the wood substrate alone (without a resin being present) when examined under

the same conditions of a wood joint during bonding. No significant degradation occurs up to a temperature of 180°C as shown by the relative stability of the value of the elastic modulus as a function of time. Some slight degradation starts to occur at 200° C, but after some initial degradation the elastic modulus again settles to a steady value as a function of time and at a value rather comparable to the steady value obtained at lower temperatures. Evident degradation starts to be noticeable in the 220-240° C range and this becomes even more noticeable at higher temperatures. The effect of substrate degradation on the TTT diagram in Fig. 8b can then only start to influence the trends in gel and vitrification curves at temperatures higher than 200° C and it is for this reason that the region of the curves higher than 200°C is indicated by dashed lines in Fig. 8b. At a temperature < 200°C the trends observed are due to the resin only. In this range of temperature the eventual turning to longer time and stable temperature of the vitrification curve, characteristic of the TTT diagrams of epoxy resins, becomes also evident for the TTT diagrams of the waterborne PRF and MUF resins on lignocellulosic substrates indicating that diffusion hindrance at a higher degree of conversion becomes for these resins too the determinant parameter defining reaction rate. What differs, however, from previous diagrams is that the trend of all the curves, namely the gelation curve, initial pseudogel (entanglement) curve, and start and end of vitrification curve is the same. In epoxy resin TTT diagrams the trend of the gelation curve is completely different from that reported here. The result shown in Fig. 8b is, however, rather logical because if diffusion problems alter the trend of the vitrification curve, then the same diffusional problem should also alter the gelation and pseudogel curves. This is indeed what the experimental results in Fig. 8b indicate. It may well be that in waterborne resins the effect is more noticeable than in epoxy resins. This is the reason why it is possible to observe it for PF, UF, PRF, and MUF resins. With the data available and with the limitation imposed by the start of wood substrate degradation of higher temperatures it is not really possible to say if the gelation curve and the vitrifica­tion curve run asymptotically towards the same value of temperature at time = 1 although the indications are that this is quite likely to be the case. What is also evident in the trend of the two curves is the turn to the left, hence the inverse trend of their asymptotic tendency towards Tg1. This turn cannot be ascribed to substrate degradation because for very reactive resins, such as PRFs, such a turn already occurs at a temperature lower than 150° C, hence much lower than the temperature at which substrate degradation becomes significant. This inverse trend can only be attributed to movements of water coming from the substrate towards the resin layer as the trend of the curves indicates an easing of the diffusional problem already proven to occur at such a high degree of conversion [30,39].

Two other aspects of the TTT diagrams in Figs. 8b and c must be discussed, these being the trend of the curves at temperatures higher than 200°C and the trend of the devitrification (or resin degradation) curve. The dashed line trend and experimental points of all the curves at temperatures higher than 200°C are clearly only an effect caused by the ever more severe degradation of the substrate: degradation of the substrate implies a greater mobility of the polymer network constituting the substrate, hence the continuation of the curves as shown in their segmented part. That this is the case is also supported by the virtual negative times yielded by the TMA equipment when the temperature becomes extreme, as well as by the trend of the resin’s higher degradation curve which tends to intersect the vitrification curve at about 200-220°C or higher, this being a clear indication that one is measuring the changes in the reference system, the substrate itself, and that these are at this stage so much more important than the small changes occurring in the resin as to be able to dominate the whole complex system which is the bonded joint.

The CHT and TTT diagrams pertaining to waterborne formaldehyde-based poly­condensation resins on a lignocellulosic substrate should then appear in their entirety as shown in Figs. 8b (TTT) and 8c (CHT) rather than as the classical diagrams of epoxies on noninterfering substrates such as glass fiber shown in Fig. 8a.