Pyrogallol-formaldehyde thermosetting adhesives

Abstract

A new thermosetting wood adhesive system from pyrogallol has been developed. Pyrogallol can be easily obtained from tara pods (Caesalpinia spinosa) a native leguminosae of low cost widely distributed in Peru. In this work, polymerization of formaldehyde with pyrogallol was carried out at different pH values and optimal conditions were determined to establish the adhesive formulation. The reactivity of this resin was characterized by differential scanning calorimetry (DSC) and the results were compared with those obtained with resins made with tara tannin, gallic acid, and phenol. The results show that tara tannin and gallic acid are less reactive due to the presence of deactivating groups (i.e., carboxylates) in the phenolic moieties while their polymerization is limited to that of a bidimensional network upon curing. In contrast, pyrogallol–formaldehyde kinetic parameters (Ea and ΔH) were determined and they are comparable with those of phenol-formaldehyde adhesives. In addition, mechanical property values (MOR, MOE, and IB) of particleboards prepared with pyrogallol–formaldehyde compare favorably to those of Canadian standard requirements (CSA). Main assets of the new thermosetting adhesive is lower pressing times and temperatures than those currently used in the industry. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:399–408, 1997