Flame-retardant, recyclable, and hydrothermally degradable epoxy resins and their degradation products for high-strength adhesives

Abstract

To date, sustainable thermosetting polymers and their composites have emerged to address recyclability issues. However, achieving mild degradation of these polymers compromises their comprehensive properties such as flame retardancy and glass transition temperature (T_g). Moreover, the reuse of degradation products after recycling for upcycling remains a significant challenge. This study introduces phosphorus-containing anhydride into tetraglycidyl methylene diphenylamine via a facile anhydride-epoxy curing equilibrium with triethanolamine as a transesterification modifier to successfully prepare flame-retardant, malleable, reprocessable, and easily hydrothermally degradable epoxy vitrimers and recyclable carbon fiber-reinforced epoxy composites (CFRECs). The composite exhibited excellent flame retardancy and a high T_g of 192 °C, while the presence of stoichiometric primary hydroxyl groups along the ester-bonding crosslinks enabled environmentally friendly degradation (in H₂O) at 200 °C without any external catalyst. Under mild degradation conditions, the fibers of the composite material were successfully recycled without being damaged, and the degradation products were reused to create a recyclable adhesive with a peel strength of 3.5 MPa. This work presents a method to produce flame retardants and sustainable CFRECs for maximizing the value of degradation products, offering a new upcycling method for high-end applications.

Graphical abstract

Keywords

epoxy vitrimer / carbon fiber composites / flame retardancy / upcycling

1 Introduction

The significance of carbon fiber-reinforced epoxy composites (CFRECs) cannot be overstated in various industries, such as aerospace, automotive manufacturing, and wind power generation, due to their thermal, chemical and dimensional stability, and exceptional strength-to-weight ratio [1]. Despite the numerous advantages of CFRECs, the accumulation of a large amount of waste (including unused prepreg, offcuts from manufacturing processes, and end-of-life components) has posed urgent economic and environmental challenges because highly crosslinked structures and flame-retardant treatments complicate the process [2]. With 175 million tons of plastic waste entering the environment, there is a pressing need to develop sustainable thermosetting polymers and composites for a circular carbon economy, not only to reduce pollution but also to minimize resource waste [3].

The concept of covalent adaptive networks (CANs) [4−7] has inspired the development of a new class of sustainable thermosetting polymers that combine chemical crosslinking structures and the corresponding performance of conventional thermosetting polymers, and the reprocessing capacity of thermoplastics [8]. During the last decade, they have received increasing attention and have experienced rapid growth [9,10]. These distinctive characteristics make CANs a promising option for repair, reprocessing, and recycling [11,12], potentially addressing the recycling challenges associated with thermosets [13]. Despite their potential, the industrial use of CANs faces obstacles such as low recovery efficiency, inadequate mechanical/thermal properties, and high flammability [10]. To address these challenges, Zhang et al. [14] reported an epoxy vitrimer (EV) by integrating small-molecule polyols as transesterification modifiers into an epoxy-anhydride curing system [14−16]. These EVs, as the thermosetting matrix for sustainable CFRECs, exhibited outstanding mechanical properties, self-healing capabilities, degradability, and tunable recyclability [17]. Furthermore, in response to the fire hazards associated with CFRECs in industries requiring flame retardancy, such as transportation, aerospace, and wind power [18], phosphorus-containing compounds have been integrated into CANs as effective flame-retardants to create a range of recyclable materials [19−21]. In our latest studies, Ren et al. [22] introduced stoichiometric polyols containing diphenylphosphinate/tertiary amine groups into an ester-bonded dynamic network to construct sustainable-yet-flame-retardant EVs and their CFRECs, which maintained excellent flame retardancy (LOI of 38%) and mechanical strength (flexural strength of 90 GPa). Moreover, the chemical degradation of CFRECs is achieved at a mild temperature (180 °C) in ethylene glycol, leading to complete resin degradation and nondestructive recovery of carbon fibers (CFs). Zhang et al. [23] also developed phosphorus-containing anhydride-containing compounds to participate in the curing process of EVs. Even after multiple reprocessing steps, the EV retained its outstanding flame-retardancy (at the UL-94 V-0 level), and the relevant composite remained resistant to ethanolamine at 160 °C. Furthermore, it can be fully degraded in a solvent within 3 h. Despite recent advances, there are still some shortcomings in the field. (1) Meeting flame-retardant requirements in industries such as aerospace and wind power requires a T_g higher than 180 °C, but most EVs have T_g lower than 130 °C [19,22,24]. (2) The predominant use of nonenvironmentally friendly organic solvents for degradation is a concern [25−28]. (3) There is a lack of reports on the high-value utilization of degradation products from flame-retardant CFRECs [18,29].

Among different dynamic covalent exchange reactions, the dynamic transesterification reaction (DTER) of carboxylate esters is recognized as a promising option among various dynamic covalent exchange reactions for recyclable thermosets with a high T_g (> 180 °C). These thermosets, which are based on the DTER and are cured with carboxylic acids/anhydrides, have been widely utilized in industrial applications [30,31]. DTER typically occurs at elevated temperatures (> 120 °C) [14,32], allowing for the production of materials with high service temperatures. Research indicates that tertiary amines present in carboxylate-crosslinked EVs are crucial for facilitating the DTER and promoting chemical degradation (hydrolysis) under mild hydrothermal conditions [33−36]. Therefore, integrating tertiary amines into an EP matrix with a high T_g is a promising strategy for developing CFRECs with improved recyclability [20,22,23].

This work presents a straightforward approach for the fabrication of flame-retardant and hydrothermally recyclable CFRECs with high T_g by using tetraglycidyl methylene diphenylamine (TGDDM) as the starting resin, phosphorus-containing anhydride (dihydro-3-[(6-oxide-6h-dibenz[c,e] [1,2]oxaphosphorin-6-yl)methyl]-2,5-furandi-one, DPI) as the curing agent and triethanolamine (TEOA) as the transesterification modifier and internal catalyst. The outstanding flame-retardancy (heat release rate (HRR) reduced by 51%), high T_g (191.7 °C), and tensile strength (71.2 MPa) of EV were successfully developed. Notably, the resulting EV degrades rapidly in pure water under mild conditions, resulting in nearly undamaged recycled CFs. Furthermore, the degradation products reconstituted the adhesive with high peel strength. This study offers innovative insights into the fabrication of flame-retardant and hydrothermally recyclable CFRECs, promoting their potential high-value utilization.

2 Experimental

2.1 Materials

TGDDM (epoxy value = 0.88 mol·100 g^–1), 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO, 97%), itaconic anhydride (95%), TEOA (AR) and tetrahydrofuran (THF, 99.5%) were acquired from Shanghai Aladdin. DPI was synthesized according to a previously reported procedure [37]. No additional dehydration or purification steps were needed.

2.2 Preparation of DT/TGDDM vitrimers and DT/TGDDM/CF composites

The DT/TGDDM mixtures containing TGDDM, MHHPA, TEOA, and DPI were prepared in specific proportions (Fig.1), as detailed in Table S1 (cf. Electronic Supplementary Material, ESM). The mixture was agitated at 80 °C for 30 min to ensure an even distribution of the components. Subsequently, the air pockets were removed using a vacuum for 10 min, and the homogeneous mixture was then poured into a polytetrafluoroethylene mold that had been preheated and placed into a flat vulcanizer for curing. The curing procedure involved heating at 120 °C/2 h, 160 °C/3 h, 200 °C/3 h to yield the epoxy resins D₀T₁₀/TGDDM and D₂₅T₁₀/TGDDM. The differential scanning calorimetry nonisothermal curing curves of DT/TGDDMs with varying DPI and TEOA contents were analyzed at different heating rates (Fig. S1, cf. ESM). The DT/TGDDM matrix was used to fabricate CFRECs using a hand lay-up process followed by vacuum-assisted hot-pressing molding. The heating curing procedure was the same as that for DT/TGDDMs, followed by cooling and demolding to obtain D₀T₁₀/TGDDM/CF and D₂₅T₁₀/TGDDM/CF, and the carbon fiber content in D₂₅T₁₀/TGDDM/CFs was 61.0 wt %.

Fig.1 Chemical structures of DPI, TEOA, TGDDM, and MHHPA, and schematic diagram of the curing network.

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2.3 Hydrothermal degradation of D₂₅T₁₀/TGDDM/CF

First, 5 g of D₂₅T₁₀/TGDDM/CF was combined with 40 mL of pure water in a 100 mL pressure reactor and allowed to react for 5 h at 200 °C. Upon completion of the reaction, the reactor was cooled to room temperature. Subsequently, an aqueous solution containing DT/TGDDM dissolved degradation polymer (DTP) was obtained through rotary evaporation and then dried in a vacuum oven at 80 °C for 12 h, after which the recycled CFs were washed and dried.

3 Results and discussion

3.1 Thermal properties

The thermal stability of DT/TGDDM vitrimers with different contents of DPI was evaluated using thermogravimetric analysis (TGA). Fig.2(a) and Fig.2(b) show the TG and derivative thermogravimetry (DTG) curves of DT/TGDDMs under a nitrogen atmosphere. The introduction of DPI resulted in a lower initial decomposition temperature of 264.2 °C for D₂₅T₁₀/TGDDM compared to 273.3 °C for D₀T₁₀/TGDDM, which was attributed to the easier breakage of P–O–C bonds in DPI [38]. Furthermore, D₂₅T₁₀/TGDDM exhibited increased residual mass compared to D₀T₁₀/TGDDM, indicating that DPI promoted the dehydration, aromatization, and further carbonization of the epoxy resins [39−42]. Following the introduction of DPI, the storage modulus at 50 °C increased from 1925 MPa to 2682 MPa (Table S2, cf. ESM). The T_α of D₂₅T₁₀/TGDDM was 191.7 °C, which was less than that of D₀T₁₀/TGDDM (202.7 °C). Additionally, the crosslinking density of D₂₅T₁₀/TGDDM decreased, indicating greater resistance to DOPO groups in the curing networks [43].

Fig.2 (a) TG and (b) DTG curves of DT/TGDDM under a nitrogen atmosphere; (c) storage modulus and (d) tan δ versus temperature for D0T10/TGDDM and D25T10/TGDDM with different compositions.

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3.2 Flame retardancy and mechanical properties

The impact of introducing the phosphorus-containing curing agent DPI on flame retardancy of the DT/TGDDM vitrimers was evaluated using UL-94 and LOI tests, as shown in Fig.3. The results indicated that the EVs containing DPI were able to extinguish rapidly after initial ignition, and exhibited excellent self-extinguishing behavior. Notably, D₂₅T₅/TGDDM and D₂₅T₁₀/TGDDM achieved a UL-94 V-0 rating. Additionally, the LOI of the EVs notably increased in the presence of DPI, from 22.6% of the reference D₀T₁₀/TGDDM to 33.2% of D₂₅T₅/TGDDM and 32.5% of D₂₅T₁₀/TGDDM, underscoring the crucial role of DPI in enhancing flame retardancy [44].

Fig.3 UL-94 vertical burning and LOI tests of the DT/TGDDMs.

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The burning behavior of DT/TGDDM was investigated via cone calorimetry. Cone calorimetry is a bench-scale fire testing tool based on the principle that the amount of heat released from a burning sample is directly related to the amount of oxygen consumed during combustion (Fig.4), to analyze the time-dependent HRR and total heat release (THR). The introduction of DPI led to a gradual decrease in the HRR, from 1162 kW∙m^–2 for the reference D₀T₁₀/TGDDM to 567 kW∙m^–2 for D₂₅T₁₀/TGDDM, representing a 51% reduction. The THR curve displayed in Fig.4(b) demonstrated that the THR for D₀T₁₀/TGDDM was 93 MJ∙m^–2, while the THR for D₂₅T₁₀/TGDDM decreased by 33%. This lower THR was 62% less than that of D₀T₁₀/TGDDM, indicating a significant inhibitory effect of DPI on heat release [45]. Additionally, the residual mass [46] increased from 2.4 wt % (D₀T₁₀/TGDDM) to 9.0 wt % (D₂₅T₁₀/TGDDM), as shown in Fig. S2 (cf. ESM), suggesting that DPI enhances carbonization and positively impacts the flame retardancy of DT/TGDDM in the condensed phase, which is in accordance with the results of TGA [47].

Fig.4 (a) HRR, (b) THR curves of DT/TGDDMs as a function of burning time, (c) tensile properties, and (d) flexural properties of DT/TGDDMs.

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Introducing DPI as a co-curing agent into DT/TGDDM did not change the brittleness of characteristics of the highly crosslinked EVs, even though the rigid DOPO pendent group further deteriorated the segmental mobility between the crosslinking points. Consequently, typical brittle fracture was observed (Fig.4(c, d)). With increasing TEOA, the tensile strength of D₂₅T₅/TGDDM slightly decreased compared to that of D₂₅T₁₀/TGDDM (from 78.6 to 71.2 MPa). This decrease is attributed to the increase in the hydroxyl content and a slight decrease in the crosslinking density [48]. Notably, D₂₅T₁₀/TGDDM exhibited a flexural strength of 106.0 MPa, surpassing the strength of recyclable flame-retardant epoxy resin reported in the literature [19−21,24].

3.3 Shape memory and stress relaxation behaviors

Next, the shape memory performance of D₂₅T₁₀/TGDDM was investigated, and photos of the original, deformed, and recovered rectangular samples were recorded, as illustrated in Fig.5. The D₂₅T₁₀/TGDDM vitrimer underwent a transition from its initial linear permanent shape to temporary curved shapes such as “S”, “C”, or “U” when subjected to an external force and heated to 180 °C within the T_g range, followed by cooling to room temperature. The curved temporary shape of D₂₅T₁₀/TGDDM can be retained, and upon reheating to 180 °C, the material reverts to its original linear permanent shape. By following the fabrication steps outlined for D₂₅T₁₀/TGDDM, it was possible to strategically create a variety of shape configurations, whether in a permanent or temporary state. Fig.5(b) shows the stress relaxation curves of D₂₅T₁₀/TGDDM at different temperatures, revealing an increase in the stress relaxation rate with increasing temperature. This phenomenon was attributed to the accelerated rate of transesterification within the D₂₅T₁₀/TGDDM structure as the temperature increased. By applying the Arrhenius equation, the E_a of the transesterification for D₂₅T₁₀/TGDDM was found to be relatively high (Fig.5(c)). This was a result of the high crosslinking density and rigid structures of the thermoset [34], which restricted the movement of segments within the crosslinking points.

Fig.5 (a) Multiple shape memory behavior, (b) normalized stress relaxation curves of (c) Arrhenius analysis of the characteristic relaxation time lnτ versus 1000/T for D25T10/TGDDM.

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3.4 Carbon fiber composites

The aforementioned EVs containing DPI demonstrate excellent flame retardancy and adequate viscosity, making them a suitable matrix material for preparing a CFREC. To model the burning behavior of the CFREC in real fire scenarios, a cone calorimetric test was also used. Compared with the reference composite D₀T₁₀/TGDDM/CF, D₂₅T₁₀/TGDDM/CF exhibited 18% suppression in the PHRR and 49% reduction in the THR, which indicated enhanced fire safety due to the incorporation of DPI (Fig.6(a, b)), showing the same trend as that of the vitrimer. Moreover, D₂₅T₁₀/TGDDM/CF exhibited improved mechanical properties, such as flexural strength, modulus and impact strength, compared to D₀T₁₀/TGDDM/CF (Fig.6(c) and Table S3 (cf. ESM)). Specifically, the flexural strength of D₂₅T₁₀/TGDDM/CF increased to 791 MPa, representing a 16% increase. These results highlight the dual benefits of using DPI as a co-curing agent in either the vitrimer, or the CFREC, leading to advancements in both flame-retardancy and mechanical properties.

Fig.6 (a) HRR, (b) THR curves of DT/TGDDM/CFs as a function of burning time, and (c) flexural strength and modulus of DT/TGDDM/CFs.

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3.5 Recyclability of CFREC

The chemical recycling process of D₂₅T₁₀/TGDDM/CFs and its corresponding mode-of-action were investigated. The D₂₅T₁₀/TGDDM/CF sample underwent a hydrothermal reaction in distilled water at 200 °C for 5 h, followed by washing, drying, and other necessary procedures to obtain clean degradation products and CFs. Fourier transform infrared (FTIR) analysis (Fig.7(b)) was conducted on the samples before and after degradation to analyze the degradation process and the corresponding mode-of-action of the EVs. Before degradation, the sample exhibited a characteristic peak of carbonyl ester (carboxylate) at 1735 cm^–1, which almost disappeared after degradation, indicating complete ester bond breakage. Furthermore, an increase in the intensity of the peak attributed to the hydroxyl group at 3300 cm^–1 after degradation suggested the formation of new free hydroxyl groups [49]. Liquid chromatography-mass spectrometry was employed to further characterize the chemical structure of the degradation products (Fig. S3, cf. ESM), confirming the presence of degraded oligomers with hydroxyl end groups. Therefore, the recycling mode-of-action likely involves tertiary molecules reacting under high temperature and pressure conditions, with tertiary amines in TGDDM acting as catalysts to facilitate the attack of hydroxyl groups on ester bonds, leading to ester bond cleavage and the generation of new hydroxyl groups. This process results in the rapid dissociation of the thermosetting networks into diverse oligomers with hydroxyl terminal groups. Subsequently, the morphology of the recycled CFs was analyzed. Scanning electron microscopy (SEM) images revealed no resin residue on the surface (Fig.8(a, b)), with a morphology closely resembling that of the pristine CFs. Furthermore, Raman spectroscopy confirmed the structural similarity between the recycled CFs from D₂₅T₁₀/TGDDM/CF and the pristine CFs (Fig.8(c)), with no alteration in the signal peak intensity. The tensile strength of the monofilament for the recycled CFs was also almost identical to that of the pristine CFs (Fig.8(d)), and the residual mass of the recycled CFs at 700 °C was 98% of the initial mass (Fig.8(e)). Overall, the chemical structure of the CFs remained intact throughout the degradation process, enabling successful recycling without any damage [50].

Fig.7 (a) The recycling process for D25T10/TGDDM/CF; (b) FTIR spectra of H2O before and after degradation.

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Fig.8 (a), (b) SEM and (c) Raman spectra of the pristine and recycled D25T10/TGDDM/CFs; (d) tensile strength of the recycled CFs; (e) TG and DTG curves of the recycled CFs.

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3.6 Upcycling of the degradation products

For chemical degradation of the thermosetting matrix in CFRECs, the most crucial aspect lies not only in achieving nondestructive regeneration of the CFs but also in addressing how to avoid transforming degraded resin from solid waste into liquid waste. The latter is even more unacceptable for industrial circles. Furthermore, from an environmental and resource perspective, avoiding futile waste of nonrenewable resources is a key focus of today’s circular economy development. Therefore, this article specifically investigates the reuse of a dissolved DTP. Herein, the DTP was formulated into an adhesive that effectively bonded various substrates, including different sheets made of wood, aluminum, and glass, at room temperature (Fig.9(a, b)). The shear adhesion strength of the DTP with aluminum sheets was notably greater at 3.5 MPa than that of the commercially available 502 glue (aluminum sheets), which was tested at 2.0 MPa for comparison (Fig.9(c)). When the amount of ethanol used increased, DTP dissolved in ethanol, allowing easy removal and enabling rebonding cycles. Even after being rebonded to the wood sheets five times, DTP maintained a bonding strength of approximately 1.2 MPa (Fig.9(d)), suggesting that the DTP was fully recovered in the ethanol solvent and reversibly recycled on the substrate surface.

Fig.9 (a) Schematic diagram of the adhesion process, (b) adhesion test of DTP adhesive to different substrates, (c) adhesion strength of DTP adhesive to different substrates, for comparison, the adhesion strength of the commercially available 502 glue (aluminum sheet) was recorded, and (d) DTP cycling test of bonded wood sheets for 5 cycles.

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4 Conclusions

In this work, a feasible and facile approach was introduced for developing flame-retardant yet hydrothermally recyclable CFRECs through a catalyst-free transesterification. Cured via the phosphorus-containing anhydride DPI, the DT/TGDDM vitrimer achieved a V-0 level in the UL-94 test and reduced the THR by 62% in cone calorimetry, demonstrating excellent flame retardancy. Additionally, the rigid structure of TGDDM and DPI contributed to a high T_g (> 180 °C). The presence of tertiary amines from TGDM and hydroxyl groups in TEOA led to thermally induced dynamic transesterification at high temperatures (200 °C), facilitating the hydrolysis of ester bonds in the networks. This allowed for the separation of thermosetting matrix and CFs in the composite, with the recycled CFs exhibiting a tensile strength (3.1 GPa) consistent with that of the pristine CFs. This suggests efficient degradation of the CFREC and nondestructive recycling of CFs. Moreover, the degradation product DTP was converted into a versatile adhesive with strong bonding strength (3.5 MPa at the Al sheet) that was easily removed with ethanol for repeated use. This strategy involving the use of an environmentally friendly degradation solvent (pure water) and the upcycling of the degradation products provides valuable insights for designing sustainable flame-retardant thermosets and their composites.

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Competing interests

The authors declare that they have no competing interests.

Acknowledgements

Financial supports by the National Key Research and Development Program of China (Grant No. 2021YFB3700201), the National Science Foundation of China (Grant Nos. 21975166, 51991351, 51991350), the 111 Project (Grant No. B20001) and the Fundamental Research Funds for the Central Universities are sincerely acknowledged.

Electronic Supplementary Material

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