Dynamic Covalent Bonds in Synthetic Adhesives: A Review of Recent Advancements and Challenges

Synthetic adhesives play a crucial role in holding together solid materials through interfacial interactions. The designable molecular structure of synthetic adhesives allows for the preparation of adhesives with different properties by adjusting the types and proportions of components. This versatility has led to their wide usage and increasing demand. Thermoplastic and thermosetting adhesives are important types of synthetic adhesives, with thermoplastic adhesives being reassemblable and thermosetting adhesives exhibiting high adhesive strength and creep resistance. However, there is a need to combine the advantages of both types and develop high bonding strength, reassemblable adhesives.

To address this challenge, researchers have explored the introduction of dynamic covalent bonds into adhesives' cross-linked networks. Dynamic covalent bonds are reversible and can undergo exchange reactions under external stimulation. Two types of dynamic covalent bonds, associative and dissociative bonds, have been studied. Associative bonds have a slow exchange rate at room temperature, making them unsuitable for fast reassembly. In contrast, dissociative bonds exhibit good fluidity during rearrangement due to their breakage and reformation mechanism.

Additionally, the depletion of petroleum resources and environmental concerns have led to the exploration of biomass-based synthetic adhesives. Vegetable oils such as palm oil, castor oil, and soybean oil have shown promise as raw materials. Among them, soybean oil is the most important due to its stable supply throughout the year. Epoxidized soybean oil (ESO) has been used as a substrate for synthetic adhesives, and the introduction of Diels-Alder bonds has endowed the adhesives with reassemble ability.

The synthesis of ESOF was achieved by grafting furyl group onto ESO chains. The successful synthesis was confirmed through various analyses such as DSC, ultraviolet-visible spectroscopy, and 1H NMR. The construction of covalent polymer networks was carried out using bismaleimide (BMI) as the cross-linker. The mechanical properties of the resulting adhesives could be regulated by adjusting the quantity of BMI, with higher amounts resulting in increased stress at break and toughness. The inclusion of Diels-Alder bonds also provided the adhesives with self-healing capabilities and the ability to undergo recycling.

The polarity and flow characteristics of the adhesives were also analyzed. The adhesives exhibited hydrophilicity, which facilitated binding between interfaces through supramolecular interactions. The viscosity of the cross-linked network topology decreased rapidly during rearrangement, as demonstrated by the changes in torque over time at elevated temperatures.

In conclusion, the development of synthetic adhesives with high bonding strength and reassemble ability is of great significance. The introduction of dynamic covalent bonds, such as Diels-Alder bonds, has shown promise in achieving this objective. Furthermore, the use of biomass-based raw materials, particularly soybean oil, is a sustainable approach to address the depletion of petroleum resources and environmental concerns. Future research should focus on optimizing the properties and performance of these adhesives to meet the needs of different applications.