Study of reactivity of tannins of leaves and barks of Barbatimão Stryphnodendron adstringens (Mart.) Coville

The work aimed at evaluating the reactivity of tannins extracted from leaves and bark of barbatimão (Stryphnodendron adstringens Mart. Coville) with par formaldehyde. Studies of this reactivity become important in the search and production of new natural wood adhesives. The Influence of pH on the gel time and reactivity were analyzed at different concentrations of par formaldehyde (8, 10 and 12%), by comparing them with the commercial black-wattle-tannin-based adhesive (Acacia molissima). The tannins of barbatimão leaves and bark were extracted in hot water with 3g of sodium sulfite by mixing these tannins with water and the binding agent par formaldehyde. The Influence of pH on the gel time was obtained by the determination of time required for gel formation at different pH's and the peak temperature and enthalpy were determined by DSC Technique (differential calorimetric scanning). The blend presented acid character, elevated viscosity, solids content of about 50% and very low gel time. An increase of reactivity with the increasing par formaldehyde concentration and adhesive pH was found. The peak temperature stayed around 130°C and enthalpy values were high compared to those reported for Eucalyptus tannins, but they were lower than the values found for the commercial black wattle adhesive.