Accelerated Autocondensation of Quebracho Tannin Wood Adhesive by Boric Acid

Quebracho tannin wood adhesive in non formaldehyde approach has low autocondensation rate and needs more press time than formaldehyde-based wood adhesives to produce wood composites. In this study, boric acid was used to increase autocondensation rate of quebracho tannin adhesive to make poplar plywood. The main ingredients of the adhesives include quebracho tannin, NaOH, hexamine, boric acid and polymeric isocyanate (PMDI). Thermomechanical analysis of control adhesives (without boric acid) showed maximum Young’s modulus values increased with increasing tannin initial concentration from 40% to 50% (m/m), while no significant effect has been found by adding 20% PMDI based on the tannin solid content. The addition of the boric acid not only (1) lowered time and temperature of hardening, (2) but also increased Young’s modulus values of the adhesive by adding boric acid from 2% to 4% based on the solid content of the tannin.. Tensile shear strength of the plywoods confirmed thermomechanical previous results. Tensile shear values did not meet EN 314-2 requirements for interior plywood classification without boric acid systems. While boric acid significantly increased tensile shear values which had ascending trend with increasing boric acid and tannin concentration. The current study indicates that plywoods were made by 50% quebracho tannin and 2-4% boric acid meet the relevant performance requirements for interior applications.