High Bonding Strength and Reassemble Ability of Epoxidized Soybean Oil-Based Adhesives with Dynamic Diels-Alder Bonds

Synthetic adhesives rely on interfacial interactions to hold together different solid materials. Their molecular structure is highly designable, enabling the preparation of adhesives with diverse properties by adjusting components. This versatility has led to widespread use and increasing demand for synthetic adhesives. Thermoplastic and thermosetting adhesives play crucial roles. Thermoplastics, made from resins like polymethacrylate or polyvinyl, undergo reversible physical changes with temperature without altering their chemical characteristics. They can be reassembled but have limited creep resistance and mechanical strength. Thermosetting adhesives, such as phenolic or epoxy resins, exhibit high adhesive strength and creep resistance due to cross-linked networks formed under specific conditions. However, these networks render them insoluble and non-melting, making reassembly difficult. Combining the advantages of both types of synthetic adhesives to achieve high bonding strength and reassembly capability is highly desirable.

Researchers have explored various approaches, including the introduction of dynamic covalent bonds into adhesive cross-linked networks. These bonds undergo reversible exchange reactions under external stimulation, such as Diels-Alder bonds, imines, or disulfides. Optimizing the selection and utilization of dynamic covalent bonds is critical.

Dynamic covalent bonds are classified into associative and dissociative types based on their exchange mechanism. Associative bonds react slowly at room temperature, and external stimulation accelerates the reaction rate without affecting activation energy. The networks containing associative bonds maintain a constant cross-linking density during network topology rearrangement, resulting in viscous fluidity determined by the exchange rate. Achieving low viscosity like thermoplastics is challenging, hindering rapid reassembly in practical applications. Dissociative bonds, on the other hand, break and then form during network topology rearrangement, exhibiting good fluidity. Diels-Alder bonds are a prime example of dissociative bonds, with well-studied dissociation mechanisms.

Moreover, the shift towards sustainable materials has spurred research on bio-based alternatives for synthetic adhesives. Among promising vegetable oils, soybean oil stands out due to its large-scale production, consistent supply throughout the year, and cost-effectiveness.

This study employed epoxidized soybean oil (ESO) as the substrate and incorporated Diels-Alder bonds to impart reassemble ability to the prepared adhesives. The synthesis route of ESOF-Ix was successfully achieved, and the mechanical properties of the adhesive were regulated by adjusting the quantity of bismaleimide (BMI) cross-linker. ESOF-I1.0 exhibited higher stress at break (18.45 MPa) and toughness (2.4 MJ m-3) compared to ESOF-I0.6 (10.19 MPa and 1.9 MJ m-3). The self-healing capability of ESOF-I1.0 was demonstrated through the gradual reduction of a surface crack under heating. Additionally, the adhesive exhibited recyclability for up to three cycles without significant changes in properties. The adhesives exhibited hydrophilicity, and the viscosity of the cross-linked network topology decreased rapidly during rearrangement, facilitating reassembly. These results highlight the potential of incorporating dynamic covalent bonds into synthetic adhesives to achieve high bonding strength and reassemble ability.

In summary, this study successfully developed a high bonding strength, reassembled adhesive by introducing dynamic covalent bonds into the cross-linked networks of synthetic adhesives. Epoxidized soybean oil (ESO) was used as the substrate, and Diels-Alder bonds were incorporated to enable quick reassembly. The synthesis route of the modified adhesive, ESOF-Ix, was successfully achieved. The mechanical properties of the adhesive could be regulated by adjusting the quantity of bismaleimide (BMI) cross-linker, with ESOF-I1.0 exhibiting higher stress at break and toughness compared to ESOF-I0.6. The self-healing capability of ESOF-I1.0 was demonstrated through the gradual reduction of a surface crack under heating, and the adhesive also showed the ability to undergo recycling for up to three cycles without significant changes in properties. The adhesives exhibited hydrophilicity, and the viscosity of the cross-linked network topology decreased rapidly during rearrangement, facilitating reassembly. These results indicate the potential of incorporating dynamic covalent bonds into synthetic adhesives to achieve high bonding strength and reassemble ability.