Comparative Thermodynamic Study on the Contribution of the Autocondensation and Copolymerization Reactions for the Tannins of the Subspecies of Acacia nilotica

Autocondensation and copolymerization reactions of the Acacia nilotica subspecies tomentosa (Ant) and the subspecies adansonii (Ana) tannins extracts solutions have been studied at several pH values by thermomechanical analyzer. Results of chemical analysis of these tannins revealed that the studied tannins, Ant and Ana contained high percentages of extractable tannins (54 and 57 %) for and polyphenolic materials (78 and 80 %) respectively. Different hardeners such as paraformaldehyde, Urea and pMDI were added at different ratios and their polycondensation reactions was studied and compared with their autocondensation ones. The aim was to evaluate the tannins suitability for the production of commercially and technically viable tannin adhesives with reduced Formaldehyde emission for wood products and to study the interference between the autocondensation and the copolymerization reaction. The obtained results of autocondensation reaction for both of the tannins studied showed that the best Young’s modulus values for Ant (3,500 and 2,750 MPa) and Ana (2,650 and 2,620 MPa) were obtained at pH 5 and 7. The Young’s modulus values obtained by the tannins Ant were higher than those achieved by Ana. This indicates that the Ant is more reactive than Ana. These results were also in line with results achieved by the gel time for both of the tannins. Gel time results indicate that the reactivity of both tannins increased towards alkalinity with Ana being more reactive at alkaline pH. Addition of 8 % of paraformaldehyde was adversely affecting the autocondensation reactions, as the best Young’s modulus values were achieved at pH 4 for Ant tannins. As for Ana the higher Young’s modulus values (2,000 and 2,310 MPa) were achieved at pH 5 and 7. This indicates that autocondensation reaction was contributed to the final network of the copolymerization reaction. When smaller ratio of paraformaldehyde and Urea (5 %) was added to Ant tannins it favors the autocondensation reaction and the best Young’s modulus values were obtained at pH 5 and 7. Addition of pMDI (10–30 %) was found to decrease the temperature of copolymerization and the obtained Young’s modulus values by Ant were lower than those obtained by autocodensation reaction. Best Young’ modulus values were obtained by Ant at pH 5 and 7. Ana gave the best Young’s modulus values at pH 4 and 5 indicating that the autocondensation appears to depress the copolymerization reactions. The obtained results by both reactions were very important from technical and economical point of view as they concluded that it is very possible to produce adhesives system with zero emission depending on the tannins autocondensation reaction and pH values. Reduction of formaldehyde emission was also possible upon addition of smaller amount of paraformaldehyde and Urea.