"Green" bio-thermoset resins derived from soy protein isolate and condensed tannins

The condensed tannins (CTs) have a similar catechol structure compared with mussel protein's high water resistance structure, thus, CTs was used to mix with soy protein isolate (SPI) to develop a thermoset resin. Different pH condition was applied to catalyze the reaction of CTs and SPI. The chemical structure, thermal degradation behavior, fracture morphology, and physical characteristics of the resulting resins were tested. The resultant resin was applied on plywood fabrication and it wet shear strength was measured. Results showed that, in the acid condition, the catechol structure on B ring was protected and the active sites (C6 and C8) of A-ring reacted with functions and formed a crosslinking network with SPI, which effectively brought catechol into SPI. In the alkaline condition, the catechol structure on B ring of CTs was oxidized and formed an ortho-quinone to crosslink SPI and then formed a denser structure. This crosslinked structure formed by both ways improved the water resistance of the resultant resin, also, the structure led to a better thermostability and created a homogeneous fracture surface of the resin, which further increased the resin's water resistance. The wet shear strength of the plywood bonded by the ac ST 2.5 and al ST 5 resin reached at 1.07 and 0.95 MPa, which was a 75.4% and 55.7% higher than that of SPI resin. The resultant resins had favorable water resistance, suitable viscosity, cost-effective, and easy to handle, which were potential for industry application.