Induced Tannin Adhesive by Boric Acid Addition and Its Effect on Bonding Quality and Biological Performance of Poplar Plywood

Abstract

Quebracho tannin wood adhesive in a nonformaldehyde approach has a low hardening rate and needs more pressing time than formaldehyde-based wood adhesives during wood composites production. In this study, boric acid (BA) was used to increase autocondensation rate of tannin to make poplar plywood. The main ingredients of the adhesives included quebracho tannin, NaOH, hexamine, BA and polymeric isocyanate (PMDI). Thermomechanical analysis of without BA adhesives showed that maximum modulus of elasticity increased with increasing the initial tannin concentration from 40% to 50% (w/w), whereas no significant effect has been found by adding PMDI. Moreover, the addition of BA lowered time and temperature of hardening and increased modulus of elasticity of the adhesive. Also, tensile shear strength increased significantly with increasing the tannin concentration and the BA content (from 2% to 4%). Interestingly, applied BA in the glueline not only improved bonding quality of the panels but also provided good pres...