The Effect of Hardeners on the Dry and Wet Bondline Strength of Quebracho Tannin Resin in Three-Layer Plywood

This study investigates the tensile shear strength of maple plywood bond lines bonded with quebracho tannin adhesive, modified using different hardeners: hexamine, paraformaldehyde, and methylene diphenyl diisocyanate. Adhesives were prepared by dispersing tannin powder in water, followed by the incorporation of each hardener, and applied to three-layer veneer panels cured under hot pressing. Bond line performance was evaluated under dry and wet conditions using tensile shear tests, with wet testing incorporating an accelerated cyclic-boil aging protocol. The results revealed that paraformaldehyde-modified tannin adhesives achieved the highest dry strength, 3.5 MPa, significantly exceeding the performance of both other tannin-based systems and synthetic adhesives. In contrast, under wet conditions, no significant differences were observed among tannin-based and synthetic adhesives, underscoring the strong influence of moisture on performance.