Enhancing Environmentally Friendly Tannin Adhesive for Plywood through Hyperbranched Polyamide

Tannin-based adhesives, as an environmentally friendly alternative, have gained significant attention for addressing indoor pollution issues. However, limitations in shear strength and water resistance have hindered their widespread application. This study aims to enhance tannin adhesives by incorporating hyperbranched polyamide (HBPA): a multifunctional polymer characterized by its highly branched structure and abundant amino functional groups. The characterizations of HBPA were performed by Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (1H NMR), and thermogravimetric analysis (TG). The results showed that HBPA has a unique structure and reactive amino functional groups when the molar ratio of succinic anhydride to diethylenetriamine (S/D) stands at 0.8. By incorporating HBPA as a cross-linking agent into the tannin adhesive formulation, the adhesive exhibited notable enhancements in viscosity, shear strength, and water resistance. Specifically, the result of viscosity indicated that the optimal addition of HBPA should be less than 5%. With a 3% incorporation of HBPA, the tannin adhesive exhibited a 41.2% rise in dry shear strength, a 52.5% elevation in wet shear strength, and a 39.3% improvement in water resistance. Furthermore, the FTIR analysis of the cured adhesive layer indicated that HBPA facilitates the cross-linking with tannin molecules, resulting in a denser three-dimensional network.