Investigation of an ambient temperature-curable soy-based adhesive for wood composites

Soy-based bio-adhesives are considered as potential substitutes for formaldehyde-based wood adhesives due to their environmental safety and broad availability. However, high curing temperatures (120–180 °C) makes them less energy efficient for wood processing. In the present study, an ambient temperature-curable soy-based adhesive was prepared via the emulsion polymerization of soy flour and styrene. The results of Fourier-transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1HNMR) confirmed that N-methylolacrylamide had grafted onto the soy flour components, and subsequently copolymerized with styrene. The developed soy-based adhesive achieved a wet shear strength of 1.04 MPa with 5% styrene and 8% pMDI, which exceeded the bonding strength requirements of plywood for exterior applications. Soy flour components are essential for this ambient temperature-curable adhesive system. The emulsion soy-based adhesive facilitates moisture evaporation at ambient temperature, and polymeric methylenediphenyl diisocyanate (pMDI) was introduced to produce a crosslinked structure with high water resistance and thermal stability. The soy-based adhesive developed in this study showed great potential in wood composites thanks to its high performance and low curing temperature.