Лако-красочные материалы — производство

The use of TMA techniques has allowed for the first time the determination of time- temperature-transformation (TTT) and continuous-heating-transformation (CHT) dia­grams for UF and PF [59,60] adhesives and for MUF and phenol-resorcinol-formalde­hyde (PRF) [41] adhesives hardening directly within wood joints and allowed the quantification of the wood/adhesive interface characteristics and parameter limits. Until then, such diagrams had been obtained only for epoxy resins on glassfiber braid. In Figs. 7, 8, and 9 are shown PF resins on wood isothermal TTT and constant heating rate CHT diagrams as compared to what was previously known about the same diagrams of epoxy resins on glassfiber (a noninterfering substrate) [61,62].

Different trends to those reported in the literature for TTT and CHT diagrams of epoxy resins in the literature, occur in the higher and lower temperature zones of the diagrams of waterborne formaldehyde-based resins hardening on wood. CHT and TTT diagrams have been reported for PF, UF, MUF, PRF, and tannin-formaldehyde thermo­setting resins [59,60]. The experimental TTT diagram in Fig. 8 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 glassfiber. To start to understand the trend shown in Fig. 8 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. 8 can then only start to influence the trends in the gel and vitrification curves at temperatures higher than 200° C and it is for this reason that the regions of the curves above 200° C are indicated by dashed lines in Fig. 8. At a temperature < 200° C the trends observed are only due to the resin. In this temperature range the eventual change to longer time and more stable temperature of the vitrification curve, characteristic of the TTT diagrams of epoxy resins (Fig. 7), becomes also evident for the TTT diagrams of the waterborne PF and other formaldehyde resins on lignocellulosic substrates, indicating that diffusion hin­drance at a higher degree of conversion becomes for these resins too the determinant parameter defining the reaction rate. However, what differs from previous diagrams is that the trend of all the curves, namely the gel curve, initial pseudogel (entanglement) curve, and start and end of the 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. 8 is, however, rather logical because if diffusion problems alter the trend of the vitrification curve, then the same diffusional problem should also alter the gel and pseudogel curves. This is indeed what the experimental results in Fig. 8 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 at higher temperatures it is not really possible to say if the gel curve and the vitrifica­tion curve run asymptotically towards the same value of temperature at time of infinity, 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 [59,60].

Two other aspects of the TTT diagrams in Fig. 7, 8, and 9 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 trend indicated by the dashed lines and the experimental points of all the curves at temperatures above 200°C is 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 dashed 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 much more important than the small changes occurring in the resin and so dominate the whole complex system of 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. 8 (TTT) and 9 (CHT).