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RESEARCH ARTICLE | Updated:2024-09-23
    • Recyclable High-performance Carbon Fiber Reinforced Epoxy Composites Based on Dithioacetal Covalent Adaptive Network

    • Shi Gui-Lian

      a ,  

      Li Ting-Cheng

      a ,  

      Zhang Dao-Hong

      a ,  

      Zhang Jun-Heng

      abc ,  
    • Chinese Journal of Polymer Science   Vol. 42, Issue 10, Pages: 1514-1524(2024)
    • DOI:10.1007/s10118-024-3191-8    

      CLC:
    • Published:01 October 2024

      Published Online:27 August 2024

      Received:30 April 2024

      Revised:10 June 2024

      Accepted:12 June 2024

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  • Gui-Lian Shi, Ting-Cheng Li, Dao-Hong Zhang, et al. Recyclable High-performance Carbon Fiber Reinforced Epoxy Composites Based on Dithioacetal Covalent Adaptive Network. [J]. Chinese Journal of Polymer Science 42(10):1514-1524(2024) DOI: 10.1007/s10118-024-3191-8.

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    Abstract

    Recycling of carbon fiber reinforced composites is important for sustainable development and the circular economy. Despite the use of dynamic chemistry, developing high-strength recyclable CFRPs remains a major challenge due to the mutual exclusivity between the dynamic and mechanical properties of materials. Here, we developed a high-strength recyclable epoxy resin (HREP) based on dynamic dithioacetal covalent adaptive network using diglycidyl ether bisphenol A (DGEBA), pentaerythritol tetra(3-mercapto-propionate) (PETMP), and vanillin epoxy resin (VEPR). At high temperatures, the exchange reaction of thermally activated dithioacetals accelerated the rearrangement of the network, giving it significant reprocessing ability. Moreover, HREP exhibited excellent solvent resistance due to the increased cross-linking density. Using this high-strength recyclable epoxy resin as the matrix and carbon fiber modified with hyperbranched ionic liquids (HBP-AMIM+PF6) as the reinforcing agent, high performance CFRPs were successfully prepared. The tensile strength, interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of the optimized formulation (HREP20/CF-HBPPF6) were 1016.1, 70.8 and 76.0 MPa, respectively. In addition, the CFRPs demonstrated excellent solvent and acid/alkali-resistance. The CFRPs could completely degrade within 24 h in DMSO at 140 °C, and the recycled CF still maintained the same tensile strength and ILSS as the original after multiple degradation cycles.

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    Keywords

    Epoxy resin; Hyperbranched ionic liquid; Recycling; Carbon fiber; Composites

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    INTRODUCTION

    Carbon fiber reinforced epoxy resin composites (CFRPs) feature high strength, lightweight, easy manufacture, and high resistance to heat and corrosion, and are extensively used in many fields, including construction, aviation, automobiles, and wind energy.[

    1−3] However, the irreversible three-dimensional crosslinking which furnishes the material with desired properties also makes recycling and reprocessing difficult.[4−8] To solve this problem, CFRPs with covalent adaptive networks (CANs) have been exploited. Such materials can be reprocessed in ambient conditions, owing to the reversible association and dissociation with the CANs.
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    Recently, CANs based on imines,[

    9−11] ester bond,[1, 12−14] disulfide bond,[15] borate ester bond,[16,17] polyacylsemicarbazide,[18,19] and Diels-Alder bond[20−22] have been successfully used to produce recyclable CFRPs. Although CANs improved the recyclability of CFRPs, some challenges, such as poor tolerance to acidic and alkaline environments, and decreased mechanical strength still remained. It is therefore critical to develop high-performance acid/alkali-resistant CFRPs to meet the demanding requirements. Among various CANs, dithioacetal (also called S,S-acetal or thioacetal) is a typical dynamic covalent bond with relatively high stability, and has been employed to produce a variety of recyclable polymer materials, such as commercial diene rubber,[19] phenolic resins[20] and polyurethane.[21] For instance, Tang et al.[23] developed a novel crosslinking agent containing dithioacetal and used it to prepare rubber with excellent reprocessability. Yuan et al.[24] prepared phenolic resins containing dithioacetal which exhibited outstanding reprocessing and degradation properties. Yuan et al.[25] exploited a trifunctional compound containing dithioacetal and used it to prepare a polyurethane network with good acid resistance, recyclability, and photodegradability. However, despite the extensive research of dithioacetals for such applications, designing and manufacturing easily recyclable and environmentally resistant CNAs with excellent mechanical performance remains a huge challenge.
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    The interface interaction of CFRPs also plays an important role in its performance.[

    26,27] Various methods such as oxidation, plasma treatment,[28] coating,[29] and chemical grafting[30] have been applied to improve the interface interaction of CFRPs.[31] Recently, significant progress has been made in the modification of CF surface with hyperbranched polymers (HBPs). Through the thiol-ene click reaction, hyperbranched polymers were successfully grafted on the CF surface in our previous work.[32−35] Due to the copious functional groups at the end of hyperbranched polymers (HBPs), a great deal with chemically active sites were introduced on the CF surface. Therefore, the cross-linking between CF and the matrix resin was significantly enhanced, leading to the creation of a tight mechanical interlock at the interface. In addition, the deformable topology of hyperbranched polymers can facilitate load and stress transfer,[32,33] thus serving as excellent modifiers to reinforce interface interaction.[12,32−36]
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    As is well known, hyperbranched ionic liquids (HBPILs) have been widely applied due to their multifunctional terminals, low viscosity, high solubility and intramolecular cavities.[

    36−39] HBPILs containing electron deficient aromatic imidazole rings can further interact with CF (sp2 hybridized carbon atoms) surface through π-π stacking,[40−42] therefore can better mingle with the matrix resin.[41,43] Herein, we synthesized a high-strength recyclable epoxy resin (HREP) based on dynamic dithioacetal covalent adaptive network. The HREP exhibited excellent self-healing and reprocessing ability, as well as recyclability. In addition, it showed excellent solvent and acid/alkaline resistance. HBP-AMIM+PF6 modified CF (hereby designated CF-HBPPF6) and the optimized formulation HREP20 (containing 20phr VEPR) were then used to prepare CFPR composites (Scheme 1). Compared with HREP20/CF, the HREP20/CF-HBPPF6 exhibited superior tensile strength, ILSS and IFSS. The HBP-AMIM+PF6 modified CF significantly improved the compatibility and the load transfer capability of CFRPs.
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    Fig 1  Schematic illustration of the preparation of HREP, HREP20/CF-HBPPF6 and the mechanism of interface strengthening of HREP20/CF-HBPPF6.

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    EXPERIMENTAL

    Synthesis, characterizations, preparation processes and recycling of carbon fiber composites are included in electronic supplementary information (ESI).

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    RESULTS AND DISCUSSION

    Mechanical and Thermal Properties of HREP

    The mechanical properties of HREP were presented in Figs. 1(a) and 1(b). The addition of VEPR significantly increased the mechanical performance of HREP. With increasing the content of VEPR to 20 phr, the tensile strength, impact strength and flexural strength of HREP significantly increased, but further increase the VEPR content to 30 phr only changed these properties slightly (Table S2 in ESI, Figs. 1a and 1b). When 20phr VEPR was added, the composite exhibited the highest mechanical properties, with the tensile, impact, flexural strengths and toughness increased by 62.6 %, 63.5 %, 67.9% and 174.0%, respectively. The same trend was also observed in the flexural strength modulus and Young's modulus. The strength and modulus of HREP increased due to the increased cross-linking between VEPR and PETMP. The increased cross-linking can regulate and dissipate external forces to prevent crack propagation. However, excessive crosslinking may constrain the network, and lead to a decrease in elongation at break.

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    Fig 1  (a) The stress-strain curves, (b) impact strength and flexural strength, (c) storage modulus and tanδ, (d) TG thermograms of HREP.

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    Fig. 1(c) displays the thermomechanical properties of HREP, and the storage modulus and Tg data are listed in Table S3 (in ESI). As the VEPR content increased, the modulus first increased and then plateaued owing to the rigid structure of VEPR, which constrained the mobility of chain segments; the cross-linking density calculated by Eq. S(2) (in ESI) also showed an upward trend (Table S3 in ESI), thereby enhancing the overall network rigidity and elevating the Tg. Fig. 1(d) and Table S4 (in ESI) present the TGA analysis results of HREP under N2 atmosphere. The HREP5−HREP30 displayed similar T5% (temperature with a weight loss of 5%) and Tmax (temperature at which the maximum degradation rate was reached). Due to the easy cleavage of methoxy groups from the benzene ring,[

    44] HREP began to weight loss at lower temperatures than the HREP0. It is interesting that the residual carbon of HREP5−HREP30 was higher than that of HREP0 at 700 °C, suggesting that the benzene ring in the HREP chain segment is beneficial for carbonization during thermal degradation.
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    Dynamic Properties of HREP

    To further characterize the dynamic cross-linking of HREP, stress relaxation experiments were conducted at different temperatures. HREP were found to undergo dynamic exchange to reconstruct the cross-linking network. This reconstruction led to the release of internal stresses and exhibited dynamic characteristics. In the meantime, the cross-linking density remained almost the same. Fig. 2(a) shows that the relaxation time (τ) of HREP20 diminished from 1018 s to 57 s when the temperature increased from 80 °C to 110 °C. This is because the network of HREP20 is frozen at low temperatures, resulting in a slower exchange rate of dithioacetals. As the temperature increased, the exchange of dithioacetals was activated, thus the network rearrangement was accelerated and the relaxation time gradually shortened. Based on Arrhenius law (Eq. S3 in ESI), the activation energy of HREP20 was found to be 107.6 kJ/mol. Fig. 2(c) displayed the relaxation time of HREP composites with different content of VEPR at 110 °C. The relaxation time of HREP5-HREP30 decreased from 210 s to 34 s. The concentration of dithioacetals increased as the VEPR content increased, causing the dithioacetals in the network get closer to each other and the chance of collision increased.[

    45,46] Therefore, the relaxation time of HREP was shortened with the increase of the content of VEPR.
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    Fig 2  (a) Stress relaxation curves of HREP20 at different temperatures; (b) Arrhenius plots and activation energy according to Arrhenius equation of HREP20; (c) Stress relaxation curves with different content of VEPR at 110 °C; (d) Schematic of dynamic exchange mechanism of HREP; (e) Schematic diagram of repeated processing; (f) The stress-strain curves, (g) storage modulus, (h) tanδ of HREP20 and reprocessed HREP20.

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    Dithioacetal model compounds (ESI, BAB and CAC, Fig. S4 in ESI) were synthesized to explore the dynamic exchange mechanism of dithioacetal dynamic networks. Under thermal induction, dithioacetals follow an associative mechanism, which is the exchange between two thioacetale structures. [

    23] After mixing the two model compounds in proportion and reacting at 140 °C for 1h (Fig. S5 in ESI), the mixture was measured by 1H-NMR. The new peak of the CH of BAC (Fig. S5 in ESI, 5.18 ppm) indicated that dithioacetal compounds can exchange C―S bond connections at high temperatures through associative mechanism (Fig. 2d), leading to network rearrangement.
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    The recovery of surface scratches at 190 °C was evaluated using an optical microscope to estimate the self-healing performance of HREP20. Fig. S6 (in ESI) illustrates that the scratch width significantly decreased at 190 °C after 5 min, as a result of thermally activated chain rearrangement and the rapid recombination of dithioacetals at the damaged interface.[

    20] Owing to dithioacetal exchange reaction, HREP20 can be reprocessed under hot pressing (Fig. 2e). The mechanical properties of HREP20 after 3 reprocessing cycles are presented in Figs. 2(f)−2(h) and Table S5 (in ESI), which showed almost the same tensile strength as the original ones. Similarly, the Tg, modulus, cross-linking density, as well as the tensile and thermal properties of the reprocessed samples were also well retained after multiple cycles of reprocessing.
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    The swelling and gel content of HREP20 are shown in Table S6 (in ESI). HREP20 showed lower swelling ratio (3.7%) and higher gel content (98.5%) compared with HREP0, suggesting the creation of well cross-linking network. In order to further evaluate the solvent resistance, HREP20 was placed in H2O, ethanol (EtOH), dimethylformamide (DMF), tetrahydrofuran (THF), acetonitrile (ACN), HCI (1 mol/L), NaOH (1 mol/L) and dimethyl sulfoxide (DMSO) at room temperature for 168 h (Fig. S7 in ESI). As seen, HREP had great solvent resistance except DMSO. The excellent solvent resistance of HREP is due to the considerable stability of dithioacetals in harsh environments.[

    47]
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    Characterization of Virgin CF and CF-HBPPF6

    The hyperbranched polymer ionic liquid (HBP-AMIM+PF6) functionalized CF (CF-HBPPF6) was developed to prepare CFRPs (Scheme 1). The imidazole ring of HBP-AMIM+PF6 can adhere to the CF surface through π-π interaction and also react with the epoxy matrix resin. As shown in the FTIR spectra of CF, CF-HBPPF6 and HBP-AMIM+PF6 (Fig. 3a),[

    39,48] the characteristic bands at 1735 cm−1 (―COO―) and 843 cm−1 (P―F) appeared in CF-HBPPF6 indicated the presence of hyperbranched ionic liquids (HBP-AMIM+PF6) on the virgin CF surface. Fig. 3(b) displays the Raman spectra of virgin CF and CF-HBPPF6. The characteristic bands of D-band (1364 cm−1) and G-band (1590 cm−1) corresponded to the amorphous graphite structure and graphite carbon structure, respectively. The ID/IG ratios of CF-HBPPF6 (2.38) was high than that of virgin CF (2.29), suggesting the sp2-C (graphite carbon) structure decreases and the sp3-C (disordered carbon) structure increases.[32,33]
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    Fig 3  (a) FTIR spectra of virgin CF, CF-HBPPF6 and HBP-AMIM+PF6; (b) Raman spectra of virgin CF and CF-HBPPF6; (c) XPS spectra of virgin CF and CF-HBPPF6, (d) C1s of virgin CF, (e) C1s of CF-HBPPF6 and (f) S2p of CF-HBPPF6; SEM images of (g) virgin CF and (h) CF-HBPPF6; (i) Stress-strain curves of virgin CF and CF-HBPPF6.

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    Fig. 3(c) displays the XPS spectra of virgin CF and CF-HBPPF6. The peaks at 284.8, 400.3 V, 532.4, 163.6, 136.4 and 686.5 eV were ascribed to C1s, N1s, O1s, S2p, P2p and F1s, respectively. The fitting curves of C1s peaks of virgin CF show C―C (284.7 eV), C―O (285.7 eV), C=O (286.8 eV) and π-π (291.0 eV) species (Figs. 3d−3f). Evident characteristic peaks of C―N (285.4 eV) and C=N (285.2 eV) from imidazole ring appear in the C1s fitting curve of CF-HBPPF6, while in the S2p spectrum, two characteristic S2p peaks (C―S―C, 164.9 eV, and C―S, 163.6 eV) are observed, indicating the successful functionalization of virgin CF with HBP-AMIM+PF6. The SEM image clearly shows a smooth CF surface (Fig. 3g), while HBP-AMIM+PF6 coated CF appeared much rougher (Fig. 3h), the latter of which was essential for enhancing mechanical entanglement,[

    39] energy dissipation and stress transfer,[32,35] thereby achieving high strength.
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    Fig. S8 (in ESI) shows the test results of the contact angle between CF and matrix resin. As seen, the contact angle decreased from 86° (virgin CF) to 53° (CF-HBPPF6), indicating a significant increase in surface energy. This improved wettability and compatibility between CF and epoxy resins were crucial for the interfacial enhancement of the composites.[

    49] Additionally, the tensile properties of virgin CF and CF-HBPPF6 monofilaments were studied (Fig. 3i and Table S7 in ESI). In contrast to virgin CF, the tensile strength and modulus of CF-HBPPF6 increased by 97.4% and 79.8%, respectively, as a result of the efficient deformation-enabled load transfer achieved by HBP-AMIM+PF6.[32,50] The π-π stacking, molecular chain entanglement, and topological structure of HBP-AMIM+PF6 all contributed to CF/polymer interlocking and thus increase of the strength and modulus of carbon fibers. Specifically, the deformable hyperbranched topological structures of HBP-AMIM+PF6 are highly efficient to transfer the load, thus exhibited much improved mechanical properties.
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    The Mechanical Properties and Enhancement Mechanism of CFRPs

    Fig. S9 (in ESI) presents the influence of the content of hyperbranched ionic liquids and the volume fraction of the matrix resin on the tensile properties of CFRPs. As seen, the tensile strength first increased and then decreased when the HBP-AMIM+PF6 content increased, and reached maximum (993 MPa) at a content of 8.5 g/mL (ESI, Fig. S9a). This is due to improved HBP-AMIM+PF6 penetration to the CF fabric as HBP-AMIM+PF6 content increased, increasing the compatibility and wettability between CF and the matrix resin, thereby increasing the interface interaction of the composites and increasing its tensile strength. However, the efficacious area between CF and the matrix resin was reduced when the HBP-AMIM+PF6 content was too high, and might also result in a decrease in tensile strength.[

    51] As the volume fraction of resin increased, the mechanical performance of CF composites showed a similar trend, with the maximum (998 MPa and 52.5 GPa) obtained at a resin volume of 50 vol% (Fig. S9b in ESI).[19] When the resin content was too high, a decrease in CF ratio and localized high resin content resulted in a weakening of the mechanical strength.[32,33,52]
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    The mechanical properties of optimized CFRPs with an HBP-AMIM+PF6- content of 8.5 g/ml and a resin volume fraction of 50 vol% were studied and illustrated in Fig. 4 and Table S8 (in ESI). Compared with HREP20/CF, the tensile strength and modulus of HREP20/CF-HBPPF6 (Fig. 4a and Table S8 in ESI) improved by 24.6% and 18.5%, respectively. ILSS and IFSS were key parameters for evaluating the interfacial properties. As illustrated in Fig. 4(b), the ILSS and IFSS values of HREP20/CF-HBPPF6 composites were 76.0 MPa and 70.8 MPa, respectively, increasing 16.0% and 19.2%, respectively, as compared with HREP20/CF. This was because on the one hand, the stacking of π-π bond between the imidazole ring in HBP-AMIM+PF6 and the sp2 carbon atom on CF facilitated the adsorption of HBP-AMIM+PF6 on CF, therefore improved the interface interaction;[

    53,54] on the other hand, the deformation of HBP-AMIM+PF6 was able to transfer loads and dissipate energy at the interface, thereby effectively suppressing crack propagation.[32,54−56] HREP20/CF-HBPPF6 exhibited higher tensile strength and ILSS values than most degradable and recyclable CFRP composites (Fig. 4c).[12,18,32,33,57−62]
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    Fig 4  (a) The stress-strain curves, (b) ILSS and IFSS of HREP20/CF and HREP20/CF-HBPPF6; (c) Comparison of tensile strength and ILSS with previously reported CF composites; (d) Tensile strength of HREP20/CF-HBPPF6 immersing in 3 mol/L HCI and 3 mol/L NaOH solutions at different time; SEM micrographs of (e) HREP20/CF and (f) HREP20/CF-HBPPF6; (g) Schematic diagram of the failure between CF and epoxy and enhancement mechanism.

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    The fracture surfaces of CFRPs were studied by SEM to further assess the interaction. There were some cracks on the fracture surfaces (Fig. 4e), indicating weak interface interaction. However, the dense layer between CF-HBPPF6 and matrix resin indicated good adhesion (Fig. 4f). Fig. 4(g) shows the schematic diagram of interface interactions and reinforcement mechanisms of CFRPs. Compared with HREP20/CF-HBPPF6, the weak wettability and absence of chemical interaction at the HREP20/CF interface resulted in debonding of CF and matrix resin at the interface. For HREP20/CF-HBPPF6, Fig. 4(f) shows a tight bonding between CF-HBPPF6 and the matrix resin, indicating good interface interaction, as a result of the π-π stacking between imidazole ring in HBP-AMIM+PF6 and CF and the sufficient anchoring points associated with the branched structure.[

    57] In addition, the hyperbranched topology of HBP-AMIM+PF6 resulted in shorter sliding displacement and more effective load transfer, which was conducive to energy dissipation and inhibiting crack propagation, resulting in a transition from adhesive failure to cohesive failure model.[63,64]
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    To evaluate the acid and alkaline resistance of HREP20/CF-HBPPF6 composites, samples were immersed in 3 mol/L HCI and 3 mol/L NaOH solutions at 60 °C for 7 days, with weight and tensile strength measured at different time intervals. Fig. S10 (in ESI) shows weight retention rates of 99.41% and 99.64% in HCI and NaOH solutions after 7 days, respectively. Similarly, the tensile strength of the samples remained basically unchanged (Fig. 4d), even after soaking for 7 days.

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    Degradation of HREP20/CF-HBPPF6 Composite and Recovery of CF

    The influence of temperature, time, and the solvent type/amount on the degree of degradation of HREP20/CF-HBPPF6were systematically investigated, as illustrated in Fig. S11 (in ESI). Figs. S11(a) and S11(b) (in ESI) indicate that the HREP20/CF-HBPPF6 can be fully degraded in DMSO at 140 for 24 h (Fig. 5a). As seen in Figs. 6(b) and 6(d), the new peaks at 9.85 ppm (―CHO) in 1H-NMR and at 480 cm−1 (disulfide bonds) gradually increased in intensity during degradation. The degradation of HREP20/CF-HBPPF6 was related to the oxidative degradation of dithioacetal and the cleavage of C―S bonds by DMSO and the supposed degradation mechanism was illustrated in Fig. 5(d).

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    Fig 5  (a) The degradation process of HREP20/CF-HBPPF6; Real-time (b) 1H-NMR and (c) Raman spectra of HREP20/CF-HBPPF6 degradation solution during degradation; (d) Postulated degradation mechanism of HREP20/CF-HBPPF6.

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    Fig 6  (a−d) SEM images pristine and recycled carbon fibers; (e) XPS spectra, (f) Raman spectra, (g) XRD patterns and (h) the stress-strain curves of virgin and recycled carbon fibers; (i) The stress-strain curves, tensile modulus and (j) ILSS of original and recycled HREP20/CF-HBPPF6.

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    As shown in Figs. 6(a)6(g), the surface morphology and chemical compositions of the recycled carbon fibers are very close to virgin samples, indicating that the matrix and hyperbranched ionic liquids have degraded completely. As seen, the tensile strength of the 1st, 2nd and 3rd recycled CFs changed little compared with the virgin CF, the stress-strain curves of the recycled CF and CF (Fig. 6h and Table S7 in ESI) almost overlapped. The breaking elongation and Young's modulus remained consistent with the virgin CF. Figs. 6(i) and 6(j) show the tensile strength and ILSS of original and regenerated HREP20/CF-HBPPF6 composites. Even after three cycles of regeneration, the tensile properties, and ILSS (Table S9 in ESI) of the regenerated carbon fiber composites retained 99.6%, 89.0% and 99.3% retention rates, respectively.

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    CONCLUSIONS

    In summary, we prepared a high-strength recyclable epoxy resin (HREP) with dynamic dithioacetal covalent adaptive network, which exhibited high mechanical performance and excellent solvent resistance, as well as excellent reprocessing/recycling properties. Subsequently, CFRPs were prepared with this high-strength recyclable epoxy resin and HBP-AMIM+PF6 modified CF. The tensile strength, IFSS and ILSS of modified CF composites (HREP20/CF-HBPPF6) reached 1016.1, 70.8 and 76.0 MPa, respectively. The increased mechanical strength and interfacial properties was due to the topological deformation of HBP-AMIM+PF6, which effectively transferred stress. In addition, CF in CFRPs could achieve non-destructive recovery. The tensile property and ILSS retention rate of the reclaimed carbon fiber composites after 3 cycles were 99.6%, 89.0% and 99.3%, respectively.

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