Strengthen and toughen bio-based adhesive by the synergy of hierarchical structure and dynamic covalent network

Abstract

The plant protein meal-based adhesives have shown promise as eco-friendly alternatives to replace formaldehyde-based resins because of their economics and sustainability. However, there are still challenges in achieving satisfactory water-resistant bonding strength and toughness of these adhesives, due to the high polysaccharide content of raw materials and irrational energy dissipation paths. This study leverages the chemical selectivity of borate towards polysaccharides to develop a robust and tough soybean meal (SM)-based adhesive with hierarchical structure by constructing a graphene oxide-supported dynamic covalent network, labeled SM/BGO/BCS. Benefitting from hierarchical structure and dynamic covalent network, the SM/BGO/BCS adhesive exhibited favorable mechanical strength (dry shear strength of 2.35 MPa), and the water-resistance bonding strength (63 °C) is 1.62 MPa, to our knowledge, surpassing most of the existing reported SM-based adhesives. The work of debonding of SM/BGO/BCS adhesive is 1148.2 N/m, an increase of 234 % compared to the unmodified SM adhesive, which is toughened by stress transfer between the two phases (BGO and SM matrix), breaking the traditional approach of reinforcing polymers at the expense of toughness. In addition, the SM/BGO/BCS adhesive showed promising mildew resistance in both dry and wet states. According to the cost estimate, the cost of SM/BGO/BCS adhesive is comparable to that of commercially available urea-formaldehyde (UF) resin, making this strategy a viable insight for advancing the industrialization of plant protein adhesives.