Aging Resistance Properties of Poplar Plywood Bonded by Soy Protein-Based Adhesive

The aging resistance properties of poplar plywood prepared with soy protein-based adhesives were investigated. The shear strength of soybean meal/bisphenol epoxy resin (SM/EP) adhesive increased by 197.5% (surface layer) to 1.19 MPa and 153.5% (core layer) to 1.09 MPa compared to soybean meal (SM) adhesive. Wet-dry cycles of 25 ± 3 °C, 63 ± 2 °C, and 95 ± 2 °C accelerated the aging of poplar plywood with soy protein-based adhesive. After eight 25 ± 3 °C wet-dry cycles, the shear strength of plywood bonded with SM/EP adhesive was reduced to 0.88 MPa (surface layer) and 0.71 MPa (core layer). Furthermore, the shear strength of SM adhesive gradually decreased to 0 (surface and core layer) after six and five 25 ± 3 °C wet-dry cycles. The shear strength of SM/EP adhesives was reduced to 0.96 MPa and 0.79 MPa (surface and core layer) after eight 63 ± 2 °C wet-dry cycles, and 0.53 MPa and 0.27 MPa (surface and core layer) after eight 95 ± 2 °C wet-dry cycles. Vertical density profiles indicated that the decrease of shear strength could be attributed to several factors: The small molecules were dissolved, the molecular chains of the adhesives were hydrolyzed by water, and the interior and thermal stress destroyed the bonding structure.