Reassembleable Synthetic Adhesives Based on Epoxidized Soybean Oil and Dynamic Diels-Alder Bonds

Synthetic adhesives can rely on interfacial interactions to hold together the same or different kinds of solid materials. The molecular structure of synthetic adhesives is highly designable and adhesives with different properties can be prepared by adjusting the types and proportions of components to meet the needs of different fields. Therefore, the synthetic adhesives have been widely used and the demand is increasing. Thermoplastic and thermosetting adhesives play an important role in synthetic adhesives. Thermoplastic adhesives are made of thermoplastic resin such as polymethacrylate, polyvinyl, etc. In a certain temperature range, its physical state changes with temperature while the chemical characteristics remain unchanged. The curing process does not include any chemical reaction, only through the solvent volatilization or melting cooling. Therefore, thermoplastic adhesives could be reassembled according to the intention in actual use. However, its creep resistance and mechanical strength are limited.

Compared with thermoplastic, thermosetting adhesives (such as phenolic resin and epoxy resin) exhibit high adhesive strength and creep resistance owing to the existence of cross-linked networks. But the cross-linked networks can only be formed under specified conditions and difficult to reassemble because the networks resulted in an insoluble, non-melting adhesive layer. Therefore, how to combine the advantages of the two kinds of synthetic adhesives and prepare high bonding strength, reassembled adhesives is of great significance to the development of the field of bonding.

In order to achieve this objective, researchers have made many meaningful attempts. Among these, introducing dynamic covalent bonds into adhesives cross-linked networks is a very potential approach. Dynamic covalent bonds generally refer to a class of chemical bonds which can undergo reversible exchange reaction under external stimulation, including Diels-Alder bonds, imines, disulfides, transeserifications, transaminations, hindered urea bonds, boroxines and so forth. Although there are various dynamic covalent bonds, how to take advantage of appropriate dynamic covalent bonds is particularly important.

On the basis of exchange mechanism, dynamic covalent bonds could be divided into associative and dissociative bonds. The exchange reaction of associative bonds could occur at room temperature with a very slow rate. The external stimulation could accelerate the reaction rate, but activation energy of the reaction is not affected. The networks containing associative bonds have a constant cross-linking density during the rearrangement process of the network topology. Therefore, the viscous fluidity is determined by the exchange rate and difficult to achieve a low viscosity as thermoplastics. This is unfavorable to the requirement of fast reassemble of adhesives in a short time in practical application. In contrast, the dissociative bonds are first broken and then formed in the rearrangement process of the network topology, so the network exhibits good fluidity. Diels-Alder is a typical representative of the dissociative bonds, and its dissociation mechanism has been well studied.

Moreover, the raw materials of synthetic adhesives are mostly petroleum-based. With the depletion of petroleum resources and the aggravation of environmental problems, the preparation of synthetic adhesives from biomass resources has attracted great attentions. In terms of price and production scale, palm oil, castor oil and soybean oil are the most promising vegetable oils. Palm oil is mainly produced in Southeast Asia, and has the largest yield. However its production has a certain seasonality and palm oil-based polyols are not resistant to low temperature which is inconvenient for transportation and storage, limiting its application. Because the global production of castor oil less than 1.5 million tons, the cost advantage is not obvious. The global production of soybean oil is over 60 million tons. The harvest is not seasonal and the supply is stable throughout the year. Therefore, soybean oil will remain the most important raw material in the future development of vegetable oil industry.

Here, epoxidized soybean oil (ESO) was used as the synthetic adhesives substrate, and Diels-Alder bonds were introduced to endow the prepared adhesives with reassemble ability. Owing to the fact that Diels-Alder bonds are dissociatived dynamic covalent bonds, the viscosity of the cross-linked network topology decreased rapidly during rearrangement, endowing the adhesive with the ability to quickly reassemble.

The synthesis route of ESOF was depicted in the Scheme 1. On the basis of the reaction between thiol and epoxy group, furyl group could be grafted onto the ESO chains. Therefore, the thermal performance of ESO and ESOF would be different. Their DSC curves were presented in Figure 1a. In the ESO curve, a distinct endothermic peak was observed at approximately -3.3 ℃, indicating the melting points of ESO at this temperature. In contrast, ESOF exhibited a melting point at around xx ℃, suggesting that the introduction of furyl group increased the distance among chains and the weakened intermolecular interactions reduced the melting temperature. This shift in melting temperature is an evidence of successful synthesis of ESOF. Compared to ESO, the unsaturated double bonds in the furyl group of ESOF can undergo π→π transitions, resulting in a redshift of the ultraviolet absorption peak. This can be confirmed through ultraviolet-visible spectroscopy tests as shown in Figure 1b, indicating the incorporation of the furyl group into the main chain of ESO. The chemical structures of ESO and ESOF were analyzed by 1H NMR. In Figure 1c, the peaks around 3.0 ppm indicated the existence of epoxy group. Moreover, it could be calculated that there were four epoxy groups of each chain based on the integral of these peaks. In comparison, characterization peaks of furyl group were shown in Figure 1d at 6.2 ppm, 6.3 ppm and 7.4 ppm. The peaks at 3.0 ppm mostly disappeared indicating epoxy groups had successfully undergone ring-opening reaction. According to the comprehensive test results, it can be concluded that the ESOF has been successfully prepared according to the designed modification route.

Scheme 1. The synthesis route of ESOF-Ix. x is the mole ratio of imide groups and furyl groups. DA adducts加上。这个放到Scheme 1. 加上环氧大豆油改性的化学式

Figure 1 (a) The comparison DSC curves, (b) ultraviolet-visible spectra of ESO and ESOF. The 1H NMR spectra of (c) ESO and (d) ESOF.

The construction of the covalent polymer network was carried out according to the procedure depicted in Scheme 1. Bismaleimide (BMI) was employed as the cross-linker and mixed with ESOF initially. Subsequently, the polymer networks, designated as ESOF-Ix, were gradually formed upon heating at 80℃ due to the formation of Diels-Alder adducts between the furyl and imide groups. To confirm the formation of covalent cross-links, swelling tests were conducted. The resistance of the obtained films to solvents was assessed using toluene. The swelling ratio (SR) and gel fraction (GF) were determined and depicted in Figure 2a. The results revealed that the GF and SR of ESOF-I0.6 were approximately 60% and 621%, respectively. However, when the amount of BMI used was increased, the GF of ESOF-I1.0 increased to 92%. This observation suggests that a higher amount of BMI resulted in an elevated cross-linking density, thereby enhancing the solvent resistance of the resulting cross-linked network.

The quantity of cross-linker plays a significant role in determining the properties of the adhesive, which is crucial for practical applications. Consequently, the obtained films were subjected to tensile tests, and the resulting curves are illustrated in Figure 2b. Evidently, by adjusting the quantity of BMI utilized, the mechanical properties of the adhesive could be regulated. For instance, ESOF-I1.0 exhibited a higher stress at break (18.45 MPa) compared to ESOF-I0.6 (10.19 MPa) due to the increased utilization of BMI during the preparation process. Furthermore, the toughness of ESOF-I1.0, as determined by the integral area of the curves, was approximately 2.4 MJ m-3, whereas ESOF-I0.6 only exhibited 1.9 MJ m-3. The tensile results indicated that the mechanical properties of the adhesive could be customized to meet specific requirements. Additionally, the glass transition temperature (Tg) gradually increased with the quantity of BMI, as observed from the DSC results (Figure 4d). For example, ESOF-I0.6 transitioned from a glass state to a rubber state at around 16.8℃, whereas the Tg of ESOF-I1.0 was approximately 31.2℃, attributed to the strengthened interaction between molecular chains with the increased amount of BMI.

Figure 2. (a) The swelling results, (b) the stress strain curves, (c) toughness and elastic modulus, (d) DSC curves of ESOF-Ix,

The decline in adhesive performance is often caused by internal cracks, which are difficult to detect and repair in a timely manner. This leads to a reduction in mechanical performance and a shortened service life of materials, restricting their application range. Therefore, it is crucial to repair cracks promptly and achieve self-healing of materials, as it significantly improves the utilization efficiency of materials. Self-healing materials are capable of perceiving changes in the external environment and responding accordingly, ultimately restoring their own performance. They are a widely applicable and highly demanded type of intelligent material. In the case of ESOF-Ix, intrinsic self-healing properties are expected due to the incorporation of Diels-Alder bonds. To evaluate the self-healing capability, ESOF-I1.0 was intentionally scratched to create a surface crack, and then subjected to heating at 130℃ for different durations. As illustrated in Figure 3, the crack gradually diminished as the heating time increased, demonstrating remarkable self-healing performance. This phenomenon is attributed to the ability of Diels-Alder bonds to be disconnected at high temperatures, thereby disrupting the cross-linked network structure and increasing the mobility of the chains. Consequently, the chains are able to move between the two interfaces of the scratch. Upon cooling, the Diels-Alder bonds can reform; however, due to the altered relative position of the chains, the structure of the cross-linked network is reconstructed, effectively repairing the scratch.

Figure 3. The digital images of cracked ESOF-I1.0 under heat at different time. 加时间标注

In addition, the inclusion of dynamic Diels-Alder bonds bestowed ESOF-Ix with the ability to undergo recycling. As illustrated in Figure 4a, a fragment of ESOF-I1.0 film was fragmented and subsequently subjected to hot pressing at 130℃ and 10 MPa for 1 hour. As a result, an intact film without any visible defects was obtained. This recycling process could be repeated up to three times, and subsequent tensile tests were conducted to evaluate the efficacy of the recycling. As depicted in Figure 4a, the stress-strain curve of ESOF-I1.0 displayed a similar pattern after each cycle. After subjecting the material to three cycles of hot pressing, DSC tests revealed that the Tg did not undergo significant changes in comparison to the original sample, indicating that the crosslinking density remained constant while the network structure was adjusted (Figure 4b). The ability to retain properties in the recycled networks can be attributed to the abundance of dynamic Diels-Alder bonds, which facilitate the reorganization of the network topology.

Figure 4 (a) The stress strain curves and (b) DSC curves of original and recycled ESOF-I1.0.

The interaction between interfaces is crucial for the adhesive effect as adhesion occurs between interfaces. It is generally required for adhesives to possess a certain polarity, which facilitates the binding of interfaces through supramolecular interactions. Here, the polarity of the prepared adhesives was analyzed using water contact angle experiments. As shown in the Figure 5, the water contact angles of the prepared adhesives are all less than 90 degrees, indicating their hydrophilicity. This is mainly due to the presence of polar groups such as hydroxyl groups, esters, and imides in the networks. Furthermore, as the cross-linking density increases, the water contact angle also slightly increases, which may be attributed to the tighter cross-linked network structure.

Figure 5. The digital photos depicting water contact angle experiments of the prepared ESOF-Ix.

To evaluate the flow characteristics of adhesives during the reassembly process, a vulcanization analyzer was utilized to monitor the changes in torque of the adhesive over time at a specific temperature. The findings, as shown in Figure 6, indicated that the torque initially increases rapidly, which can be attributed to the contact between the adhesive and the rotating plate. Subsequently, the torque decreases quickly due to the softening of the adhesive upon heating. Finally, the thermal dissociation of Diels-Alder bonds leads to the gradual collapse of the cross-linked network, resulting in the transformation into linear molecular chains. The torque continued to decrease until it reaches a stable state. Since torque is indicative of viscosity, these experimental results demonstrated that the viscosity of the cross-linked network topology can rapidly decrease during rearrangement at elevated temperatures, thus facilitating the reassembly of adhesives. This phenomenon could be attributed to the dissociation of dynamic covalent bonds.

In summary, this study focused on the development of synthetic adhesives with high bonding strength and reassemble ability by introducing dynamic covalent bonds into the adhesive network. Epoxidized soybean oil (ESO) was used as the substrate and Diels-Alder bonds were incorporated to impart reassemble ability. The synthesis of ESOF was successfully achieved and characterized through various tests. The cross-linking density and mechanical properties of the adhesive were regulated by adjusting the amount of cross-linker. The adhesive also exhibited self-healing properties and the ability to undergo recycling. The adhesives were found to possess hydrophilicity and showed a decrease in viscosity during the reassembly process. These findings demonstrate the potential of using dynamic covalent bonds in the development of synthetic adhesives with improved properties.