Improving soy-based adhesives for wood particleboard by tannins addition

Two different types of commercial tannins, namely chestnut (a hydrolysable tannin) and mimosa (a condensed flavonoid tannin), were used with two different types of hardener, namely hexamine and glyoxal, to upgrade soy-based adhesives for bonding of wood particleboard. Different soy-based adhesive formulations were tested by differential scanning calorimetry (DSC) and by thermomechanical analysis (TMA). The physical properties of different adhesive formulations such as viscosity, solid contents, acidity and density were also measured. One-layer laboratory particleboards bonded with the experimental adhesive formulations were prepared and tested. Based on the results obtained, tannins appear to decrease effectively soy-based adhesives’ viscosity. The TMA analysis showed that chestnut tannin extract appeared to react well with soy flour and to improve the soy flour adhesion properties. The TMA results also showed that both tannin types shorten the adhesives gel time and gel temperature. The DSC analysis also showed that tannin changes the thermal properties of soy-based adhesive and decreases both its glass transition temperature (Tg) and its denaturation temperature (Td). Moreover, particleboard bonded with tannin-modified adhesives showed internal bond strength results higher than particleboard bonded with unmodified soy adhesive. Thickness swelling and water absorption of the boards decreased when using tannins in soy-based adhesive formulations.