Closed-Loop Recycling of Tough Epoxy Supramolecular Thermosets Constructed by Hyperbranched Topological Structure

doi:10.21203/rs.3.rs-3696589/v1

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Closed-Loop Recycling of Tough Epoxy Supramolecular Thermosets Constructed by Hyperbranched Topological Structure

https://doi.org/10.21203/rs.3.rs-3696589/v1

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published 07 Jun, 2024

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Covalent adaptable networks are critical for the recycling and reuse of cross-linked epoxy thermosets. However, a major challenge is to develop efficient recyclable strategies while maintaining the high-performance of epoxy thermosets. Here, we synthesized vanillin-based hyperbranched epoxy resin (VanEHBP) to prepare tough epoxy supramolecular thermosets. The supramolecular structures were constructed with VanEHBP via intermolecular hydrogen bonds, intramolecular and intermolecular cavities, dual dynamic covalent bonds (imine exchange and transesterification). The epoxy thermosets exhibited excellent mechanical properties, as well as fast reprocessability, which can be reprocessed at 120°C within 30 sec and maintain about 100% of tensile strength. Importantly, the epoxy thermosets can be easily fully recycled under room temperature and the recovered materials can preserve 93.5% of mechanical properties of the original samples. This wok represents a unique strategy for developing room-temperature closed-loop recyclable epoxy thermosets with superior comprehensive performance and promising practical application prospects.

Physical sciences/Chemistry/Green chemistry/Sustainability

Physical sciences/Materials science/Techniques and instrumentation/Design, synthesis and processing

Epoxy thermosetsthat exhibit excellent mechanical and thermal performancehave gained significant attention in the fields of coatings, adhesives, and structural components^1,2. However, the covalent crosslinked structure of epoxy thermosets hinder reprocessing and recycling, posing great economic and environmental problems^3,4. To tackle this problem, covalent adaptable networks (CANs) provide a pragmatic solution to fabricate cross-linked healable and recyclable epoxy thermosets, which can dissociateor reversibly crosslink under certain conditions^5,6. Commonly used reversible covalent linkages includeurethane exchange⁷, disulfide exchange^8,9, imine exchange^10,11, boronic ester exchange¹², and transesterification reaction^13,14. In the majority of cases, the reprocessing process needs high temperature andhigh pressure, which sacrifice their mechanical properties or cause thermal degradation^15,16. Dynamic covalent crosslinking enables the crosslinked epoxy thermosets to depolymerize into initial monomers or soluble oligomers to prepare regenerated thermosets^17,18. However, the chemical recycling processes usually require a combination of complex pre-treatment, separation, purification, andthere is some degree of material deterioration at elevated temperatures^19,20. To circumvent such problems, it remains a challenge to reprocess or depolymerize epoxy thermosets at low temperatures, in short reaction times under mild conditions.

Recently, several studies have revealed that polymer materials rely on dynamically reversible bonds can be recycledat room-temperature, demonstrating the simple and energy-saving recycling methods^21,22. For instance, a supramolecular polysaccharides was designed with sodium alginate (SA) and cetyltrimethylammonium bromide (CTAB), which can be recycled at room temperature via water-induced plasticization²³. A supramolecular polyimine with high tensile strength and toughness was developed by introducing dynamic imine bonds and hydrogen bonds, which can depolymerized at room temperature in THF/HCl mixture solution. Cross-linked polymers based on reversible amidation chemistry with maleic anhydride and secondary amine monomers can be depolymerized in HCl solution to realize recovery of monomers with high purity and yield. Nevertheless, the above room-temperature chemically recyclable polymers exhibited relatively low T_g. To improve T_g, a recyclable poly (diketoenamine)s with high T_g was synthesized from triketones and aromatic or aliphatic amines, which can be depolymerized at room temperature in strong acidic conditions (5.0 M H₂SO₄)²¹. However, it is difficult to be recycled at low temperature without using strong acidic, oxidants and catalyst.

In most cases, epoxy thermosets based on CANs usually exhibit unsatisfied mechanical strength and creep resistance due to the weak dynamic bonding²⁴. To address this limitation, high-density cross-linking and rigid structures were widely used for reversible reinforcement. High-density cross-linking can improve the mechanical strength of CANs, but usually sacrifices the reprocessing and recyclability²⁵. The incorporation of reversible noncovalent interactions such as multiple hydrogen bonds into CANs can not only endow polymers with excellent mechanical performance, but also provide recyclability and self-healing capacity²⁶. Besides hydrogen bonds, hyperbranched topological structures have also been applied to effectively improve the strength, toughness, solvent resistance and dimensional stability of CANs^27,28. Hyperbranched polymers have attracted great interests due to their unique properties and structural diversity, and have subsequently been used for various applications. Compared with conventional linear polymers, hyperbranched polymers feature intramolecular cavitiesand abundant functional groups, thus demonstrating great potential instrengthening and toughening materials^8,18. Rencently, we have designed a series of hyperbranched polymers including hyperbranched ionic liquids, hyperbranched epoxy resins and hyperbranched polyesters to effectively improve the mechanical strength of epoxy thermosets^17,29–31. The strength and toughness is strongly influenced by the hyperbranched topological structures of hyperbranched polymers^17,29,30. Thus, construction of supramolecular polymer networks through reconstruction of hyperbranched topological structure may offer exciting properties.

Thus in this work we designed a class of epoxy thermosets with supramolecular networks, which was capable of rapid reprocessing and room temperature closed-loop chemical recycling. As shown in Fig. 1, vanillin derivatives bearing phenolic hydroxyl (Van-TTE) was formed by conjugating vanillin with trimethylolpropane tris[poly(propylene glycol), amine terminated] ether. The Van-TTE then reacted with epichlorohydrin to obtain vanillin-based epoxy resin (VanEP). The vanillin-based hyperbranched epoxy resin (VanEHBP) was synthesiszed from VanEP and bisphenol A via a proton transfer polymerization. The curing agent CBA-TTE was synthesiszed with 4-formylbenzoic acid and trimethylolpropane tris[poly(propylene glycol), amine terminated] ether. Then VanEP and VanEHBP were cured with CBA-TTE without catalyst to form epoxy thermosets, which were synergistically crosslinked by transesterification and hydrogen bonds. The resultant epoxy thermosets took a supramolecular network structure, exhibited excelent mechanical properties, improved creep and chemical resistance, and demonstrated fast reprocessability. More importantly, these epoxy thermosets can be converted into the soluble oligomers at room temperature, and then completely regenerated into crosslinked networks without compromising performance.

Synthesis and structural characterization

To study the hydrogen bonds, the samples were examined by FT-IR, and the results were shown in Figs. 2a-f. The peaks of 1684 and 1641 cm^− 1 were attributed to the stretching vibration of disordered hydrogen-bonded -C = O and ordered hydrogen bonded -C = O respectively, while the peaks at 1570 cm^− 1 were assigned to the hydrogen bonded in-plane bending vibration. In the other region, 3342 cm^− 1 was the characteristic peak of -OH. In the FT-IR spectra of VanEHBP from 20 to 150 ^oC (Fig. 2a-b), the disordered hydrogen-bonded -C = O (1684 cm^− 1) and ordered hydrogen bonded-C = O (1641 cm^− 1) gradually decreased while the H-bond intensities of the free -C = O group at 1717 cm^− 1 gradually increased. At the same time, the -OH characteristic peak shifted to higher wavenumber. Two-dimensional (2D) correlation analysis of FTIR spectra from 20 to 200°C were further performed, and the synchronous and asynchronous maps were shown in Figs. 2c-f. Four correlation cross peaks, that is, Φ(1728, 3364), Φ(1728, 3306), Φ(1652, 3364), and Φ(1652, 3306) were apparent in the synchronous spectrum and disappeared in the asynchronous spectrum, proving the existence of H-bonds in the dynamic networks³².

From the chemical structure of VanEHBP and CBA-TTE, the C = O groups of CBA-TTE and -OH of VanEHBP can form intermolecular hydrogen bonding (Fig. 3a). In order to further reveal thesupramolecular network structure of epoxy thermosets, molecular dynamics (MD) simulations were conducted to elucidate the cross-linking structures of EN-VanEP and EN-VanEHBP (Fig. 3b and c). The average cohesive energy per polymer chain for EN-VanEP and EN-VanEHBP are calculated to be 956.9 kJ·mol^− 1 and 914.2 kJ·mol^− 1 respectively. More specifically, the simulated revealed the number of H-bonds in EN-VanEHBP (151 per 2480 CBA-TTE units) are much higher than that of EN-VanEP (74 per 2480 CBA-TTE units), indicating the intermolecular interactions in EN-VanEHBP are much stronger than that of EN-VanEP. The hyperbranched topological structure of VanEHBP was favorable to the formation of H-bonds to induce higher-efficiency H-bonding interactions and denser H-bonds, as compared to the VanEHBP. Taken together, the hydrogen-bonding in supramolecular networks endows EN-VanEHBP with excellent mechanical properties, creep and chemical resistance.

Mechanical and thermal properties

Figure 4a displayed the tensile curves of epoxy thermosets, with the relevant data presented in Table S4. Obviously, EN-VanEP exhibited comparable mechanical properties as EN-DGEBA due to the similar structure of VanEP and DGEBA. The incorporation of VanEHBP simultaneously improved the mechanical properties and T_g of epoxy thermosets. With increase of the content of VanEHBP, the mechanical performance of epoxy thermosets first increased, reached maximum at 7 wt% VanEHBP, and then decrease. The tensile strength and toughness of EN-VanEHBP were 103.9 MPa and 3.58 MJ·m^− 3, increased 43.1% and 132.5% respectively over the cured EN-VanEP (72.6 MPa and 1.54 MJ·m^− 3). The effect of VanEHBP on the free volume (f_v) of epoxy thermosets were studied by positron annihilation lifetime spectroscopy (PALS) and the results were presented in Table S5. Theincrease of f_v in epoxy network was achieved through the incorporation of VanEHBP with large numbers of intramolecular cavities. In addition, epoxy chain segments crosslinked with VanEHBP extended in all directions, forming intermolecular cavities, which also helped to increase the f_v. But the intermolecular hydrogen bonds restricted the chain mobility andtended to reduce the f_v^30,33,34_, thus the f_v of epoxy thermosets first decreased and then increased with increasing content of VanEHBP. The DMA curves of epoxy thermosets were shown in Figs. 4b and c, the storage modulus (E_c), T_g and crosslinking density (ρ) were listed in Table S4. The DGEBA has higher reactivity with curing agent than VanEP, thus EN-DGEBA exhibited higher T_g. The introduction of trimethylolpropane tris[poly(propylene glycol), amine terminated] ether as flexible aliphatic segment in VanEP decreased the rigidity of epoxy thermosets, thereby reduced the T_g. With the addition of VanEHBP, the E_c and crosslinking density increased first and then decreased. When 7 wt% VanEHBP was added, the E_c reached 2.8 GPa, and when the amount of VanEHBP was increased to 10 wt%, the E_c decreased to 2.1 GPa. The intermolecular hydrogen bonding with VanEHBP and FAT increased the crosslinking density of epoxy thermosets. However, the presence of non-crosslinkable cavities and flexible chain segment decreased the crosslinking density^34,35. Thus, the T_g of EN-VanEHBP firstly increased and then decreased with gradually increasing content of VanEHBP. The SEM images of fracture surfaces of EN-DGEBA, EN-VanEP and EN-VanEHBP7 were presented in Figs. 4d-f. The smooth surfaces in the micrographs indicated brittle fracture of EN-DGEBA and EN-VanEP, while the rough, irregular surfaces of EN-VanEHBP7 suggest significant plastic deformation in ductile fracture. Figures 4g showed the AFM micrographs of EN-VanEHBP7 fracture surface, including height image, phase image, and three-dimensional image at a size of 5 µm × 5 µm. Only one phase was observed in the AFM topographical image (Fig. 4g). The mapping of phase image and three-dimensional image showed no phase separation.

The increased mechanical performance of EN-VanEHBP can be explained through supramolecular networks: (i) The hydrogen bonding interactions effectively stiffened and strengthened the materials. The reversible hydrogen bonds dissipated energy and redistributed stress before the failure, thus endowed EN-VanEHBP with high ductility and toughness. (ii) The deformation capability of intramolecular and intermolecular cavities provided efficient pathway to dissipate energy, leading to the stiff yet tough EN-VanEHBP. Meanwhile, the local free volume associated with hyperbranched crosslinks increased space for kink motions, and the crankshaft in strands made secondary relaxations possible. (ⅲ) The introduction of soft segments reduced the internal stress, and the arms of hyperbranched crosslinks could easily redistribute the forces reduce stress concentration. VanEHBP provided greater flexibility between crosslinks and increased the amount of conformational rearrangement during fracturing. Thereby, the as-developed supramolecular structural epoxy thermosets displayed outstanding mechanical performance.

Reprocessing and creep resistance

There were dual dynamic exchange reactions including imine exchange and transesterification in EN-VanEHBP (Fig. 5a). At elevated temperature, the exchange reactions of imine bonds and transesterification reaction promoted the topological rearrangement of EN-VanEHBP. Although the addition of VanEHBP increased the crosslinking density, which was unfavourable for the exchange reaction, the hydroxyl groups, high content of ester groups and the close-by statistical distance between dynamic exchange groups contributed to the mobility and rearrangement abilities of network segments. Thus, owing to the supramolecular networks, EN-VanEHBP could be easily reprocessed by compression moulding. Figures 5b-d depicted the time-dependent relaxation stress of EN-DGEBA, EN-VanEP and EN-VanEHBP (Fig. 5b) at different temperatures from 100 to 130 ℃. High temperature facilitated the movement of molecular segments, thus the relaxation rate increased as the temperature increased. The relaxation time of stress reached 1/e of the initial stress in stress relaxation curves treated with Maxwell model. As shown in the Fig. 5c, absolute stress relaxations were observed for EN-DGEBA, EN-VanEP and EN-VanEHBP7 at high temperature. For instance, the relaxation time of EN-VanEHBP7, EN-VanEP, EN-DGEBA reduced from 15 s, 47 s and 52 s to 10 s, 14 s and 18 s, respectively, when the temperature was raised from 100 to 130 ℃.

The values of activation energy (E_a) of exchange reaction were calculated using the following Eq. 3⁶:

$$\text{ln}\tau =\text{ln}{\tau }_{0}+{E}_{a}/RT$$

where ${\tau }_{0}$ is the relaxation time, R is the gas constant (8.314 J·mol^− 1K^− 1). As shown in Fig. 5c, the curve of ln$t$ vs. 1000/T was fitted to Arrhenius law. The E_a of EN-DGEBA, EN-VanEP, and EN-VanEHBP7 were calculated to be 44.8 K·Jmol^− 1, 40.6 KJ·mol^− 1 and 17.3 KJ·mol^− 1, respectively. The value of E_a exhibited the same trend as the relaxation rate. As expected, the high content of dynamic bonds of EN-VanEP were conducive to the rearrangement of the molecular segments, and resulted in lower E_a and relaxation time than that of EN-DGEBA. The E_a of EN-VanEHBP further decreased with the addition of VanEHBP. The topology freezing transition temperatures (T_v) of epoxy thermosets were measured by DMA and the results were shown in Fig. 5d. If the samples were heated above T_v, the viscosity significantly decreased and exhibited Arrhenius-like viscosity variations like thermoplastic materials. So when the temperature exceeded T_v, samples featured weldability and malleability by rearranging the network topology³⁷. The T_v of EN-VanEP was lower than that of EN-DGEBA, due to the rearrangement of EN-VanEP combined of imine exchange and transesterification. With the addition of VanEHBP, the exchange reactions were further promoted and the T_v of EN-VanEHBP gradually decreased with increasing content of VanEHBP. The exchange reaction was accelerated due to the following factors: (1) catalytic effect of hydroxyl groups from VanEHBP on transesterification, (2) higher concentration of ester groups of VanEHBPwhich was favourable for transesterification, (3) reduction of the statistical distance between reactive groups, (4) dissociation ofthe intermolecular hydrogen bonding at high temperatures which released chain mobility.As seen, EN-VanEHBP exhibited high dynamic exchange rate, excellent reprocessability and high T_g^11,38–62. As a result of the fast exchange reactions, the fragments of EN-VanEHBP7 could be reprocessed into an integral film through hot press at 120 ℃ for 30 s under a pressure of 5 MPa (Fig. 5a). The mechanical and dynamic mechanical properties of the reprocessed samples were shown in Figs. 5e-f, Figure S10 and Tables S6-S9. The recover efficiency in terms of tensile strength was almost 100%. Similarly, the storage modulus and T_g were closed to those of the original samples after multiple reprocessing cycles, indicating that EN-VanEHBP7 could be well reprocessed and retain their original properties (Figs. 5e-f, Figure S10 and Table S6-9).

In order to study the dynamic imine exchange reactions, small model compounds were utilized. Firstly, model compounds A₁A₂ and B₁B₂ were synthesised with vanillin, m-toluidine, p-hydroxybenzaldehyde and aniline. Then the dynamic exchange between A₁A₂ and B₁B₂ were evaluated using GC-MS (Fig. 6). New species of A₁B₂ and B₁A₂ were obtained (within 30 min) through the associative and dissociative imine metathesis reactions between A₁A₂ and B₁B₂, which confirmed that the network rearrangement of imine exchange could readily proceed at relatively low temperatures.

Creep resistance was a key factor for EN-VanEHBP as engineering plastic and structure material. Creep performances of epoxy thermosets were tested under constant stress at different temperatures (Figs. 7a-c). After removing the stress, the deformation of EN-VanEP and EN-DGEBA could fully recover at temperature below 60 ℃. There was practically no creeping for EN-VanEHBP7 until the temperature was increased to 80 ℃ (Fig. 7d). Therefore, EN-VanEHBP showed better dimensional stability than EN-VanEP and EN-DGEBA. The cured EN-DGEBA, EN-VanEP and EN-VanEHBP7 samples were immersed into different solvents for 72 h at room temperature to further examine their solvent resistance (Figure S12 and Table S10). All the samples remained unchanged after being immersed in C₅H₁₂, CCl₄, PhMe, H₂O solution at 25 ℃ for 72 h. The better deformation resistance and chemical resistance of EN-VanEHBP is owing to the hydrogen bonds induced by VanEHBP.Thecross-linking from polymer chains by hydrogen bonds allowed EN-VanEHBP to be relatively immobile to withhold external forces in various environments, thus leading to improved creep and chemical resistance. Nevertheless, the network arrangement ability and dynamic properties were not affected at elevated temperatures due to the reversible dissociation of hydrogen bonds at higher temperatures.

Closed-Loop Recycling

As a special reversible chemical bond, the dynamic imine bond-based CANs can be degraded into oligomers containing aldehyde and amine groups under mild acidic conditions (Fig. 8a). As seen from Table S10, the swelling of epoxy thermosets in DMF were more pronounced than in other solvents (H₂O, EtOH and THF). Therefore, we chose DMF as solvent for HCl aqueous solution degradation. The EN-VanEHBP7 samples were immersed into DMF with 0.1 M HCl aqueous solution (HCl aqueous/DMF = 1/10, v/v) at room temperature. After 180 min, the samples were depolymerised to orange and transparent degradation solution, then regenerated oligmers were obtained after removing the solvent. In order to further study the degradation of EN-VanEHBP7, the degradation solution, original and chemically recycled EN-VanEHBP7 were measured by FT-IR and NMR. As shown in Fig. 8b, depolymerized EN-VanEHBP7 had obvious aldehyde peak at 192 ppm in the ¹³C NMR spectrum, and the aldehyde peak at 9.83 ppm was also found in ¹H NMR spectrum (Figure S13). Moreover, the solid-state NMR spectra of original and chemically recycled EN-VanEHBP7 exhibited similar patterns, and the imine bond peaks at 150 ppm in solid-state ¹³C NMR spectra were observed (Figure S13, 14), indicating that the recycled samples were of similar chemical structure as the original samples. To further verify the imine depolymerization, 600 mg EN-VanEHBP7 was immersed in a solution containing 9 mL DMSO-D₆ and 0.9 mL 0.1 M HCl/DMSO-D₆ solution (HCl/DMSO-D₆ = 1/50, v/v) at room temperature and the degradation process was monitored by real-time ¹H NMR (Fig. 8e). As shown in Fig. 8e, the CH = N proton peak (δ = 8.43 ppm) gradually decreased and new peak of CH = O (δ = 9.83 ppm) increased, indicating the successful dissociation of EN-VanEHBP7 at room temperature. By Calculating the integral area of the CH = O peak in real-time ¹H NMR, it could be seen that the content of aldehyde increased and reached equilibrium after 100 min (Figure S15). The FT-IR spectra (S16, S17) also showed that the CH = N at 1643 cm^-1 in the original samples disappeared and CH = O at 1683 cm^-1 appeared in the degradation solution. The original sample and chemically recycled sample exhibited similar structure, demonstrating successful room temperature closed-loop recycling of EN-VanEHBP7. The tensile curves, storage modulus and tan δ vs temperature curves of original and regenerated epoxy thermosets were shown in Figs. 8c-f and Figure S18. The tensile curves of chemically recycled EN-VanEHBP7 were very close to that of the original sample, with a strength recovery efficiency of 93.5%. The storage modulus and T_g of the chemically recycled EN-VanEHBP7 were also almost the same as the original samples, exhibiting excellent room temperature closed-loop recyclability. Figure 8f showed that EN-VanEHBP7 exhibited the highest tensile strength among samples which can be recycled at room temperature^{2,36,37,63–70}.

In summary, a novel method for preparing strong and tough epoxy thermosets with supramolecular networks was developed by introducing of vanillin-based hyperbranched epoxy resin (VanEHBP). The unique supramolecular networks enabled excellent mechanical properties and room-temperature closed-loop chemical recyclability of epoxy thermosets. The tensile strength and toughness of epoxy thermosets reached 103.9 MPa and 3.58 MJ·m^-3, respectively, much higher than those of EN-DGEBA and EN-VanEP. The supramolecular networks rendered fast stress relaxation with a relatively low activation energy of 17.3 kJ·mol^-1, resulting in fast reprocessability, excellent weldability and self-healing properties. The mechanical properties remained almost 100% of the original sample after multiple reprocessing cycles. The supramolecular networks enhanced the resistance to deformation and thus improved the creep and chemical resistances during service. In addition, EN-VanEHBP could be close-loop recycled at room temperature, with over 93.5% of the tensile strength be recovered for the regenerated epoxy thermosets.

Synthesis of vanillin-based hyperbranched epoxy resin

The synthetic route of vanillin-based hyperbranched epoxy resin (VanEHBP) was shown in Fig. 1. Firstly, vanillin (22.82 g, 0.15 mol), TTE (22.00 g, 0.05 mol) and 100 mL ethanol were poured into a 250 mL three-necked bottle and mixed evenly under magnetic stirring at room temperature for 30 min. Ethanol was removed using rotary evaporator and an orange solid Van-TTE was obtained. Then, Van-TTE (42.12 g, 0.05 mol) and epichlorohydrin (238.80 g, 2.50 mol) were mixed and heated at 110 ℃ with stirring for 3 h. The above mixture was dissolved in 600 mL ethyl acetate and reacted for 4 h in ice bath by a dropwise addition of 120 g of 40 wt% NaOH solution. Finally, the mixture was filtered, washed with water, dried with anhydrous sodium sulfate, and a yellow liquid (VanEP) was obtained with a yield of 76.0% after rotary evaporator.VanEP (31.92 g, 0.03 mol), BPA (6.84 g, 0.03 mol), tetrabutylammonium bromide (0.55 g, 0.0017 mol) and 50 mL THF were mixed and stirred at 50 ℃ for 6 h. The resultant product, a yellow liquid VanEHBP with yield of 98.2%, was obtained after THF was removed using rotary evaporator (Mn = 5271 g·mol^-1 and PDI = 1.74).

Synthesis of carboxyl terminated epoxy curing agent containing dynamic imine bonds

4-Formylbenzoic acid (45.04 g, 0.30 mol), TTE (48.00 g, 0.10 mol) and 100 mL methanol were placed into a 250 mL three-neck flask and magnetic stirred for 30 min. After that, methanol was removed using a rotary evaporator and a yellow liquid 4-formylbenzoic acid-TTE (CBA-TTE) was obtain with a yield of 98.9%.

Preparation of epoxy thermosets

Epoxy thermosets containing different contents of VanEHBP were prepared according to the following process:stoichiometric amounts of VanEP, VanEHBP and CBA-TTE (the molar ration of epoxy group to -COOH is 1:1) were mixed with vigorous stirring at room temperature for 5 min. The obtained viscous uniform mixture was poured into molds and cured at 100 ℃ for 3 h and 120 ℃ for 3h. The control samples of DGEBA with CBA-TTE were prepared through the same method for comparison. The different formulations are summarized in Table S1.

Reprocessing of epoxy thermosets

The reprocessed recycling testswere carried out with a plate vulcanizer. The samples of epoxy thermosets were cut into small pieces and pressed at 120 ℃ under a pressure of 5 MPa. After cooling to room temperature, the reprocessed epoxy thermosets were obtained.

Chemical recycling of epoxy thermosets

The dynamic imine bonds, commonly known as Schiff base linkage, in the covalent cross-linking epoxy thermosets could be easily hydrolyzed under mild acidic conditions. The reversible equilibrium of dynamic imine bonds in an acidic solution enabled the reversibly crosslinking epoxy thermosets to reconstruct on the interface and thus made the epoxy network closed-loop recyclable by de-crosslinking and re-crosslinking. Typically, 10 g epoxy thermosets samples were immersed in 150 mL of DMF solvent. Next, 15 mL 0.1 M HCl aqueous solution was added, and the mixture was mildly stirred at room temperature. When the samples were completely dissolved, the degradation solution was orange red and transparent. The degradation solution was transferred to single port round bottom flask. The DMF and HCl were removed by a rotary evaporator at 100 ℃ and then heated at 100 ℃ for 6 h. After that, the recycled epoxy thermosets were obtained.

Competing interests

The authors declare no competing interests.

Author contributions

J.H.Z. and H.J.Z. conceived the project and designed the experiments. C.J. and G.Y.D. carried out experiments and data analysis. M.L., B.J.Y., M.H.M. and T.C.L. participated in the discussion of the manuscript and provided constructive suggestions. D.H.Z. supervised the whole work. All authors commented on the manuscript.

Acknowledgements

This work was supported by the National Natural Science Foundation of China [grant numbers 11975225, 12275270 and 12175232], National Natural Science Foundation of China (U23A20691), the Fundamental Research Funds for the Central Universities, South-Central Minzu University [grant number CZY23017], the Innovation Group of National Ethnic Affairs Commission of China [grant numbers MZR20006].

Data availability

The data that support the findings of this study are available from the corresponding author upon request.

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published 07 Jun, 2024

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Closed-Loop Recycling of Tough Epoxy Supramolecular Thermosets Constructed by Hyperbranched Topological Structure

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Abstract

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Introduction

Results

Synthesis and structural characterization

Mechanical and thermal properties

Reprocessing and creep resistance

Closed-Loop Recycling

Discussion

Methods

Synthesis of vanillin-based hyperbranched epoxy resin

Synthesis of carboxyl terminated epoxy curing agent containing dynamic imine bonds

Preparation of epoxy thermosets

Reprocessing of epoxy thermosets

Chemical recycling of epoxy thermosets

Declarations

Competing interests

Author contributions

Acknowledgements

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References

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