Materials and methods Two types of tannins were used, 47% water solution of the tannin extracted was prepared by dissolving the spray dried powder of the tannins, extracted from the bark of a Sudanese Acacias, namely Acacia nilotica subspecies nilotica (Ann) and 40% water solution of commercial tannin of Pinus radiata.The pH of both tannins was adjusted to pH4 using a 33% solution of NaOH, then the two tannins were tested dynamically by TMA on a Mettler apparatus using a method already reported (Garcia, Pizzi, 1998). Small beach wood plies each of 0.6 mm thick and total dimensions of 21 · 6 · 1.4 mm were tested between 25°C-250°C using a heating rate of 10°C/min. and the exercising rate of fore cycle of 0.1/0.5 N on the specimen. Results and discussions The reactivity of A-ring of tannin has been recognized and well proven by former studies on tannin based adhesives.The TMA curve in Fig. 1 for Ann shows a series of peaks starting at lower temperature. The first peak at 94°C producing a moderate value of Young modulus (1500 M) is due to the formation of an entanglement network, due to the progressive loss of water from the tannins to the wood, and it is directly proportional to the concentration of the tannins solution.The second peak at 131°C is a peak of tannin autocondensation which gives the highest value of Young modulus (3050 Mpa), due to the further reactivity of a phloroglucinolic A-ring of Ann procyanidin.However, the TMA curves show a raise of another peak at a higher temperature of 217°C. This peak is due to the start of lower reactivity of catecholic B-rings of the tannins, which are more noticeable at higher curing temperature and which then participate in the process of the autocondensation of tannin nuclei and lead to a formation of another peak, as a result of the outocondensation of catecholic B-ring nuclei. The curve shows that the B-rings of catecholic tannin reactivate with the increase in the temperature of hardening and lead to a separation of the curve of MOE, after the A-ring nuclei were exhausting their reactivity by formation of the first peak and before the process of the degradation has reached its max. point, as shown in Fig. 1.The TMA curves for Pine tannin in the Fig. 1 is similar to that for Ann, but there are some major differences. The first peak for pine tannin appears at relatively higher temperature (107°C) than in Ann and gives a higher MOE value (3200 Mpa). This peak is followed by another one at 109°C with a much higher MOE value (3600 Mpa) both of these peaks can be considered as entanglement and autocondensation peaks respectively. The curve shows at higher temperature small peaks at 169°C, are due to the tannin degradation.The tannin of Ann was found to be very reactive. It is clear now that its high reactivity is due to the activation of B-rings which take place at higher temperature, making an apparent contribution to the reactivity of the system as whole, tannin and wood and/or tannin and a hardener. Fig. 1. TMA curves of bark tannins from Acacia (Ann) and Pine species...
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