From Wastes to Functions: A New Soybean Meal and Bark-Based Adhesive

Due to the increased awareness of utilization of waste biomass utilization for higher sustainability and concern for indoor air pollution associated with formaldehyde emission by wood adhesives, there are strong interests in developing biobased formaldehyde-free wood adhesives for plywood production. In this study, bark and soybean meal were used as raw materials to develop a cost-effective and formaldehyde-free wood adhesive through epoxidation of bark extractives to enhance performance of soybean meal-based (SM) adhesive. The bark-based bioepoxy resin (BEP) reacted with functional groups on the protein and formed a cross-linked structure, which improved the water resistance, mechanical strength, and thermal stability of the cured adhesive. Plywood bonded by the adhesive with 10 wt % of BEP showed a wet shear bonding strength of 0.89 MPa, exceeding that of the SM-only adhesive by 187.1%, which met requirements for interior-use plywood. Meanwhile, the dry shear bonding strength and cold water soaking bonding strength were improved by 56.9% and 81.1%, respectively. Further, the viscosity of the adhesive reduced to 10700 mPa·s and solids content of the adhesive increased to 32.05%, which improved the technological applicability of the biobased adhesive for plywood production. Thus, this novel waste-biomass-derived formaldehyde-free adhesive has excellent potential to be applied as an environmental-friendly commercial wood adhesive for plywood production.