Environmental storage conditions-induced variations in the mechanical strength of soy protein-based adhesive-bonded plywood: Unraveling the underlying mechanisms

Bio-based adhesives, particularly soybean meal (SM)-based adhesives, have gained attention for their renewability, biodegradability, and nontoxicity, with polyamide-epoxy (PAE) resin cross-linking enabling SM-based adhesives to meet industrial standards for water-resistant plywood. However, their bonding performance stability under varying service temperatures remains to be investigated. This study conducted a systematic investigation into the 180-day evolution of bonding strength in PAE-modified soy protein-based plywood adhesives under six distinct temperature storage conditions (−80 ℃, −40 ℃, −18 ℃, 4 ℃, ambient temperature, and 55 ℃), accompanied by an in-depth examination of the underlying aging mechanisms. The storage environments induced varying degrees of bonding strength deterioration, with a mechanistic analysis revealing that oxidation and hydrolysis-driven chemical degradation of the adhesive predominated at temperatures above zero, whereas the formation of ice crystals at subzero temperatures led to localized stress, disrupting the protein network structure and inducing irreversible micro-defects that accelerated adhesive failure. These findings elucidate the microstructural changes and durability mechanisms underlying bio-based adhesives, thereby informing their broader industrial implementation in sustainable wood bonding.