Chemical Recycling of Epoxy Thermosets: From Sources to Wastes

Abstract

As one of the most widely used thermosets due to its excellent performances, epoxy resin (EP) is widely used in various fields and often employed as a component of composite actuator devices, strengthening their mechanical properties. However, the expanding production of EP inevitably leads to the accumulation of waste end-of-life equipment and the corresponding increasingly serious environmental problems. This review summarizes the recycling strategies of EP, divided into two perspectives: recycling from wastes and sources. Chemical recycling is expected to be the future of waste EP treatment, and we discuss the chemical recycling methods of existing waste EP based on different mechanisms, including the selective cleavage of ester bonds, C–N bonds, and C–O bonds. On the other hand, epoxy vitrimer networks based on various dynamic covalent linkages are also outlined, which can respond to multiple external stimuli and provide materials with recyclability from the origin. Therefore, the use of epoxy vitrimer actuators can prevent waste generation throughout the whole lifecycle. We present some issues of concern in both waste-based and source-based recycling strategies and emphasize the significance of scaling-up. Finally, we summarized the current situation and present some future perspectives with the aim of making practical contributions to environmental issues.

1. Introduction

Since the 1950s, the world’s production of plastic has rapidly expanded. From 2000 to 2019, the amount of plastic used worldwide increased by 96.6%, from 234 million to 460 million tons annually, while the amount of waste plastic generated climbed by 126.3%, from 156 million to 353 million tons annually [1]. As a result, the quantity of carelessly handled and dumped waste plastic has been rising, reaching 82 million tons per year. However, fewer than 4% of the total of waste (3 million tons) were gathered for appropriate disposal. The main cause of environmental problems was incorrectly disposed of waste plastics, which imposed an unprecedented amount of strain on the whole ecosystem [2,3]. Among the numerous types of plastics, thermoset plastics play an important role in many fields due to the excellent mechanical and thermal properties originated from the three-dimensional crosslinking network structure. The use of thermosets has been steadily expanding, accounting for about 20% of all plastics [4,5]. Epoxy resin (EP), one of the most significant thermosetting resins, is used extensively in a variety of industries, including the electrical and electronic industry, the automotive industry, the aerospace industry, and the manufacturing of wind turbine blade materials [6,7,8,9,10,11,12,13], which results from its excellent mechanical properties, heat resistance, adhesion, chemical resistance, electrical insulation, and thermal stability [14,15,16,17,18,19,20]. Due to these outstanding properties, EP and its composites have been widely employed to construct composite actuator devices as thin carrier layers, while epoxy-based actuators have also been developed and applicated [21,22,23,24,25]. Those actuators were usually impressive in terms of strength, insulation, stability, and durability and originated from epoxy components.

However, excellent performance makes recycling more difficult. End-of-life rigid products like EPs have typically become serious obstacles in waste plastic recycling, and proper treatment of them is crucial to preserving the environment and achieving sustainable development. As the market for EP and its composite materials has kept growing recently, there must be increased end-of-life equipment used during their updating process. Recycling and treatment of them are under tremendous pressure, and accumulating waste EP products have been posing major environmental issues [2]. For actuators, their recyclability and retainment of stimuli responsiveness have been attracting attention as the recyclable economy is being discussed more frequently nowadays.

The balance of strengthening mechanical performance and loss of actuation as well as recyclability is a critical issue to consider in actuators. The recently reported development of epoxy-based vitrimer actuators [26,27,28] was expected to be more promising than those based on other types of materials. Jiang and coworkers [26] constructed an asymmetric bilayer-structured actuator with commercially available carbon fiber tows to provide strength and epoxy vitrimer to provide responsiveness, which showed significant deformability (210°/7 s) and efficient self-healing under a variety of stimuli such as heat (60–160 °C), light (0.4–1.0 W·cm⁻²), electricity (2–4 V), and solvent (acetone). The scratches could be rapidly repaired under light, electrical, and solvent stimuli (within 3 min). To demonstrate its potential in aerospace applications, a quadruped crawling robot with the capacity to lift objects 45 times its weight under light stimuli and a flap actuator with an angle change of 63° within 10 s under an electric stimulus were successfully constructed. They also validated the excellent mechanical performance of this composite and its retention in multiple recycling loops. Those studies showed the great potential of EP-based actuators or EP-contained composites used as stimuli-responsive devices. Therefore, the recycling of actuators and recovery of the EP polymer matrix have become increasingly necessary.

Nowadays, the most common methods for the centralized treatment of waste EP are incineration or landfilling [29,30,31], with the convenience for one-time, large-scale treatment. However, there were significant issues with resource loss, energy consumption, and environmental contamination brought about by these simple and crude treatments [32,33,34]. Moreover, since EP products usually contain complex additives, the soil, atmosphere, and water bodies will be gravely endangered by release of those additives during the treatment of waste EP [30]. Therefore, a sustainable and effective recycling technique must be found in order to preserve the ecosystem and global resource base, as landfills and incinerators are not long-term fixes.

The recycling of waste EPs is largely limited by their stable rigid crosslinking structure. Besides the chemical recycling of waste EP, researchers have recently started to focus on attaining recyclability to EP from the origin production, where the appropriate selection of raw materials could construct a reversible EP network [35,36,37]. This greatly facilitates the recycling of waste end-of-life materials. Introducing dynamic covalent bonds into the crosslinked network to prepare epoxy vitrimer materials could provide the materials with structural reversibility, bringing the responsiveness and reparability under external stimuli such as heating, energization, light, humidity, and solvents (Figure 1). These properties of vitrimer also allow it to act as an actuator that can be driven by multiple stimuli, expanding the possible application of epoxy vitrimer. Additionally, there has been a growing interest in bio-based sources [38,39,40] as a potential replacement for the widely used EP products originally synthesized by fossil-based feedstocks as a means of promoting ecological sustainability. The use of bio-based raw materials is frequently combined with the construction of vitrimer networks, resulting in a more optimal cleanliness of the production process and the lifecycle of novel products.

The development of chemical recycling methods for end-of-life products and the design of a recyclable EP network have jointly enhanced the recyclability of EP. This review summarizes the current status of recycling strategies for EP. Recycling from wastes offers potential ways to improve the current system of the centralized treatment of end-of-life wastes, while recycling from sources generates fresh concepts of trying to create recyclable EP products in the future. Finally, challenges and future perspectives are presented.

2. Recycling Strategies of Epoxy Thermosets

Depending on whether the chemical structure of EP differs throughout the process, recycling strategies of EP could be categorized into two types: physical/mechanical recycling and chemical recycling. Physical/mechanical recycling usually involves cutting, grinding, and other operations to change the external shape and size of EP. The treated material is usually used for other purposes, such as to prepare new composites as a reinforcing filler [41,42,43]. Although the chemical structure of EP remains unchanged after physical treatment, there is still a reduction in properties compared to the original product due to dimensional changes caused by mechanical forces. As a result, the majority of physical recycling belongs to downcycling [44].

In contrast, chemical recycling focuses on the chemical structure of EP and tries to depolymerize the crosslinking network by chemical methods. This approach somewhat permits control of the structure of depolymerization products according to the reaction mechanism. Selectivity is a crucial concept in chemical recycling, where the cleavage of particular bonds occurs to obtain depolymerization products with desirable functional groups. Typically, oligomers or small-molecule chemicals could be recycled after chemical recycling, which could usually be utilized in the synthesis of new materials. Some of them could also be used to reprepare original EP and thus accomplish close-loop recycling, or the depolymerization product itself is considered a high-value chemical [45]. As for the recycling of EP composites, mild chemical recycling could usually better preserve the integrity of fibers while effectively dissolving the resin matrix. Therefore, chemical recycling significantly prolongs and extends the value of waste EP through an upcycling way, which will greatly support the circular economy and resource efficiency. When “recycling” is mentioned below, this all refers to chemical recycling.

3. Recycling from Wastes

Today, the most practical and serious challenge in the recycling of EP is to properly handle the massive heaps of waste end-of-life EP equipment produced during the process of rapidly renewing products as their demand is rapidly extending. The non-selective recycling of EP, like pyrolysis, does not take the composition and structure of EP into consideration, where EP is usually directly cracked into carbon dioxide, methane, and hydrogen, as well as light oils and residual carbon solids under extreme temperature and pressure [46,47,48,49]. During a non-selective process, the polymer network randomly breaks into fragments, leading to the complicated compositions of products and difficulty in separation and reuse. Additionally, harsh reaction conditions require large amounts of energy and high-quality equipment. When it comes to the treatment of composites, the preservation of fibers might be challenging, as their original integrity and high performance would get lost under violent conditions.

A total of 90% of EP products at present are synthesized from the diglycidyl ether of bisphenol A (DGEBA) [50], and the robust aromatic hydrocarbon backbone would not be considered selective cleavage sites. Thus, the structure of the curing agent and the curing reaction frequently dictate the design of a chemical recycling system. Polyamines and acid/anhydrides are the most widely used curing agents for EP [51,52], and EPs are mainly classified into amine-cured and acid/anhydride-cured varieties correspondingly. The crosslinking points of amine-cured EP consist of C–N bonds formed during the curing process [53,54,55,56,57,58,59], while those in acid/anhydride-cured EP consist of ester bonds [60,61,62,63] (Figure 2). Compared to other parts of the structure, they are usually targeted as selective cleavage sites for the depolymerization of EP into high-value products with practical functionalities.

Chemical recycling with selectivity, which has received extensive attention from researchers in recent years, has great potential to increase product applicability since it is anticipated to realize upcycling. The development of selective and mild degradation EP favors the non-destructive recovery of other components of composite actuators. Some strategies could achieve closed-loop recycling and the feasibility of reconstructing messed-up devices. This section reviews recent chemical recycling strategies of EP with different structural features, such as alcoholysis, ammonolysis, hydrolysis, oxidation, Lewis acid catalysis, and C–O cleavage.

3.1. Alcoholysis

Alcoholysis is one of the classical methods used for the chemical recycling of polyethylene terephthalate (PET) [64,65,66,67,68,69,70,71,72]. Similarly, acid/anhydride-cured EP contains lots of ester bonds in the network and thus is also suitable for alcoholysis, cracked into small-molecule ester through the transesterification mechanism. The use of alcohol with a high boiling point, such as ethylene glycol (EG) and diethylene glycol (DEG), allows for a milder depolymerization at atmospheric pressure [73,74,75,76]. Qi and coworkers [73] established a method to mildly depolymerize anhydride-cured EP in a solution of 1,5,7-triazabicyclo [4.4.0]dec-5-ene (TBD)–alcohols below 180 °C under atmospheric pressure. Additionally, in their other work [74], they investigated the effect of different types of solvents and catalysts on the depolymerization efficiency. It was discovered that a mixture of EG and N-methyl-2-pyrrolidone (NMP) with TBD as a catalyst led to the highest dissolution rate of EP, allowing for 50% decomposition in 28 min and 95% decomposition in 70 min. This method effectively broke down the EP matrix from its composites into epoxy oligomers, which could be used as active ingredients to prepare new EP materials. Furthermore, high-value carbon fibers and metal-based parts were successfully recovered with no loss of properties (Figure 3).

Supercritical fluid is a unique state between gas and liquid when the temperature and pressure are higher than that at the critical point of the chemical. Among the common liquid reagents, alcohols with low carbon numbers and lower critical temperatures, such as methanol, ethanol, and propanol, are often used in the alcoholysis of epoxy resins [76,77,78,79,80,81]. However, alcoholysis would lose its selectivity for ester bonds’ cleavage due to the high temperature and pressure conditions required to maintain the fluid in a supercritical state. The harsh conditions lead to the random crack of the EP network, where theoretically unsuitable amine-cured EPs could also break down into complicated small pieces in supercritical alcohol fluids.

3.2. Ammonolysis

Similarly to the alcoholysis of ester bonds, amines can also dissociate ester bonds of EP through an ammonolysis process [82,83], terminating epoxy chains with small-molecule amines and generating an amide structure. Different functional structures could be introduced into ammonolysis products to achieve certain uses depending on the employed amine solvent’s chemical structure at the opposite site to the terminal amino group that reacted to generate amide. For example, Zhao and coworkers [82] reported a mild and efficient closed-loop recovery method of anhydride-cured EP using tri-functional diethylenetriamine (DETA) as the ammonolysis reagent, which acted as both solvent and catalyst and significantly simplified and facilitated the process (Figure 4a). Additionally, the pretreatment of dichloromethane (DCM) with the assistance of microwave heating obtained a porous structure, thus greatly enhancing the mass transfer and heat transfer (Figure 4b). This avoided the limitation of T_g and allowed for the degradation to proceed effectively at a lower temperature, where 99% of the degradation rate of EP was realized at 130 °C within 50 min. Since multifunctional DETA was used for ammonolysis, the degradation products possessed a terminal amino group. This allowed the products to be used as curing agents together with excess DETA for the preparation of re-cured EP materials without any isolation and purification.

Considering alcoholysis and ammonolysis together, the employed reagent would be linked onto the molecules of the degradation product by cleaving ester bonds of EP through hydroxyl and amino groups to terminate the widely extended chains, forming other ester and amide structures, respectively. Therefore, the remaining structures of alcoholysis and ammonolysis reagents determine the functional structures and thus the application routes of depolymerization products. Accordingly, we could design the reuse pathway of products in order to achieve full recovery and obtain the desired economic benefits.

3.3. Hydrolysis

Water is a highly eco-friendly solvent, and the hydrolysis reactivity of ester bonds also makes water a viable reagent for the depolymerization of EP [84,85,86,87]. Similarly to alcohol, supercritical water has also been used to decompose EP, but there are a number of drawbacks, like the lack of selectivity, high equipment requirements and energy consumption, difficulty in the recovery and reuse of products, and poor economic benefits. The selective hydrolysis of ester bonds in EP is usually carried out under milder conditions. However, it is difficult to directly hydrolyze ester bonds with pure water in a non-supercritical state. Auxiliary reagents such as acids, bases, and Lewis acids [88,89,90,91] that are added to activate ester bonds are usually necessary. Zhang and coworkers [88] reported an environmentally benign phosphotungstic acid (HPW) aqueous solution with an excellent selectivity of ester bonds’ cleavage of anhydride-cured EP. Within 5 h at 190 °C, EP was depolymerized into soluble oligomers with carboxylic and hydroxyl functional groups. The products could be recovered after a series of simple treatments of filtration, washing, and drying and were then utilized as additives in the synthesis of new EP materials. The newly cured EP had excellent properties with the addition of depolymerization products less than 40 wt%, including high T_g, storage modulus, and mechanical strength. The remaining catalyst solution after filtration could be directly used in the next catalysis of depolymerization with good stability and reusability, without an obvious decrease in catalytic efficiency after recycling six times. Meanwhile, this recycling process produced no secondary waste liquid, which was consistent with the concepts of green chemistry and circular economy.

Compared to ester bonds in anhydride-cured EP, a C–N crosslinked structure of amine-cured EP is more chemically stable but could also be selectively hydrolyzed with the assistance of Lewis acid catalysts [92]. Hou’s group [92] degraded the polymer matrix in carbon fiber/epoxy resin composites (CF/EP) using a 60 wt% ZnCl₂ aqueous solution at 220 °C and recovered the carbon fibers with the integrity of the structure and mechanical properties. The selective cleavage of the R₂CH–N bond (the secondary carbon–nitrogen bond) was realized by the incompletely coordinated Zn²⁺ ions without breaking the RCH₂–N bond (the primary carbon–nitrogen bond) (Figure 5), as well as the aromatic ether bond and C–C bond, decomposing EP into a basic carbon unit and curing agent part. Since the number of Zn²⁺ ions consumed in the degradation was much less than that of the concentrated solution, the aqueous ZnCl₂ solution could be directly recycled and used for the next degradation and remains high in terms of reactivity after six cycles of catalysis. Therefore, the whole process is described as green and sustainable.

3.4. Oxidation

Due to the chemical stability of C–N bond, the crosslinking network of amine-cured EP is usually not easy to deconstruct in mild conditions in the absence of high temperatures or other activation methods. One typical way to achieve selective C–N cleavage is to add oxidizing reagents to modify and activate it, allowing for the depolymerization of amine-cured EP through the selective oxidative cleavage of C–N bond at a mild temperature. Commonly employed oxidizing reagents include nitric acid (HNO₃) [93,94,95,96,97,98], hydrogen peroxide (H₂O₂) [99,100,101,102,103,104], sodium hypochlorite (NaClO) [105], and high-valent metal salts [106,107,108,109,110,111].

In the strategies of HNO₃ oxidation, the concentration of HNO₃ was a crucial parameter. An increase in concentration within a certain range could greatly improve the efficiency of degradation. Lee and coworkers [97] treated amine-cured EP composites with a 12 mol/L HNO₃ for 6 h at 90 °C and achieved complete depolymerization of the polymer matrix. Carbon fibers with desirable performance retainment were recovered with only a 2.91% loss of tensile strength. This was almost the highest concentration of HNO₃ reported in the recycling of composites while maintaining the major properties of recovered fibers. Further increasing of concentration did promote oxidation degradation, but harsh oxidative conditions would damage the properties of recycled fibers from the composites [98].

H₂O₂ is a green oxidizing reagent with water as its reduction product, which does not produce environmentally harmful by-products and complies with the principles of green chemistry. The H₂O₂ oxidation system usually needed the assistance of other additives to mildly and efficiently depolymerize EP and its composites within a few minutes to several hours, such as acetic acid [99,100,101,102], acetone [103], and DMF [104]. The properties of recycled fibers from composites were well maintained due to the low temperature.

In order to achieve controllable and selective oxidation degradation, novel oxidizing reagents have been designed and investigated recently. For example, ammonium ceric nitrate (CAN) is one of the strong oxidizing reagents that originated from the high-valent cerium element. Long and coworkers [111] depolymerized amine-cured EP with CAN aqueous solution, achieving efficient decomposition at a mild temperature of 60 °C within 1 h. In contrast to traditional oxidation degradation, this method was controllable due to the selectivity of CAN to cleave hydroxyethyl ether unit and C–N bond while retaining the valuable main skeleton structure of EP (Figure 6). In addition, all of the materials used or produced in the process could be completely recovered. A total of 99% of the degradation products were recovered, while the carbon fibers from the composites were also recycled with no resin residue and 97.09% of tensile strength retainment, meaning that this method is characterized by notable economic benefits.

Generally, although oxidation degradation is mild and effective, it requires the frequent use of dangerous reagents, and the reaction is usually difficult to control, which might lead to potential safety hazards. In addition, the complicated composition of degradation products that results from random mechanisms and difficulty in separation and purification make it hard to realize a circular economy.

3.5. Lewis Acid Catalysis

The Lewis acid catalysis strategy is another way to activate the C–N bond to cleave it in amine-cured EP [92,112,113]. In addition to the selective cleavage of the R₂CH–N bond by the ZnCl₂ aqueous solution mentioned before [92], this research group reported another depolymerization system for carbon fiber-reinforced amine-cured EP composites (CFREP) by the AlCl₃/CH₃COOH system [112], resulting in the complete decomposition of the resin matrix at 180 °C for 6 h. Besides its role as solvent, CH₃COOH also exerted the swelling of the densely crosslinked network of EP, facilitating the mass transfer process that Al³⁺ penetrated into the polymer matrix to catalyze the bond cleavage reaction. The tertiary carbon nitrogen bond was selectively disconnected due to weakly coordinating Al³⁺ while retaining C–C and C–O bonds, recovering valuable epoxy oligomers and carbon fibers with 97.77% retainment of tensile strength compared with the virgin ones. Another Lewis acid degradation system of ZnCl₂/ethanol reported by Liu and coworkers [113] realized the complete recycling of the EP matrix from commercially available carbon fiber-reinforced polymers (CFRPs) at 180–190 °C for 5 h, and carbon fibers were recovered with minimal damage. During the depolymerization process, ethanol also acted as a swelling reagent, which helped the penetration and activation effect of the catalyst. The EP matrix converted into oligomers with reactive functional groups, including amino and hydroxyl groups, allowing them to be used as additives for the synthesis of new EP materials. When the added amount of degradation products was up to 15 wt%, excellent mechanical properties of newly prepared EP could still maintain. After the reaction, the catalyst underwent concentration and recovery, meaning that it was directly employed in subsequent catalytic degradation repeatedly. In all, Lewis acid catalysis strategies are ideal in terms of selectivity and recyclability, which are conductive to the realization of upcycling and circular economy.

3.6. C–O Cleavage

In the most recent two years, researchers have gradually turned their attention to the selective cleavage of the C–O bond in EP to recover high-value chemicals like bisphenol A (BPA) or its derivatives [114,115,116,117]. This approach eschews the conventional concept of designing different degradation strategies depending on the type of curing agent and structure of EP network, expected to be a potential generic method for the decomposition of various EP types. The currently reported methods mainly include transition metal-catalyzed reductive cleavage [114,115] and base-mediated disconnection [116,117].

Skrydstrup and coworkers [114] developed a catalytic disconnection of C–O bond in amine-cured EP and its composites with ideal selectivity by a homogeneous Ru catalyst (Figure 7). The highest BPA yield of 56% was obtained from amine-cured EP as 6 wt% catalyst was added, with high purity and the potential for direct reuse. Furthermore, the method was applied in the recycling of EP composites for the recovery of BPA as well as the regeneration of integral carbon fibers, applicable for a wide range of rigid amine-cured EP and commercial composites. However, the high costs of precious metal catalysts constrain the popularization and scaling-up of this method.

Some common inorganic bases also exhibited unique reactivity towards the selective cleavage of the C–O bond in EP [116,117] with low-cost reagents. Skrydstrup’s group [116] also reported a solvent–base mismatch system of NaOH/toluene, where the alkali mismatched with the apolar solvent showed high reactivity instead. This method could be used in both amine-cured and anhydride-cured EP, as well as various commercial products and composites. Rebecca and coworkers [117] also used toluene as the solvent and potassium tert-butoxide (KOtBu) as the base to mediate the simultaneous cleavage of C–O and C–N bonds in amine-cured EP, recovering BPA and amine products. Various inorganic bases, including hydroxide and alkoxide bases, exhibited reactivity, among which KOtBu showed the best effect for the cleavage of the C–O bond in the model compound experiments. Compared to the transition metal catalysis, the degradation system in the base-mediated strategy has simpler components and lower costs, making it a better option for scaling-up and industrial applications.

Generally, degradation based on selective C–O cleavage is suitable for BPA-based EP cured by various types of curing agents to recover BPA. Therefore, it is expected to be a promising method for the one-step treatment of mixtures of waste EP. Usually, the structures of EP components in composite actuator devices are unclear, making it hard to realize the full degradation of them based on the disconnection of specific linkages like esters. But as C–O is a general motif of EP material, degradation based on C–O cleavage exhibits unique advantages when treating existing EP wastes with unknown structures and their hybrids.

4. Recycling from Sources

As the economy and society keep developing quickly, the demand for EP products will increase, which will also increase the amount of waste produced year by year. However, this will put increasing pressure on both production and waste disposal, potentially leading to an endless cycle of rising production and increasing waste accumulation. Problems about the consumption of sources and energy consumption in the lifecycle of EP still need more effort.

The introduction of noncovalent interactions and the construction of a covalent adaptable network (CAN) could confer intrinsic recyclability onto polymer materials [118,119,120,121,122]. Vitrimers refer to materials based on associative CANs (Figure 8b) whose crosslinking density remains a constant throughout the reversible exchange reactions of dynamic covalent bonds; thus, no significant property drop occurs [123,124]. In 2011, Leibler’s group [125] firstly developed the epoxy vitrimer based on a transesterification reaction and drew the attention of more researchers to this field. The structures of existing EP products are mainly irreversible crosslinked structures and lack intrinsic recyclability. Therefore, the development of epoxy vitrimer materials with comparable mechanical properties to present products is expected to be a promising recycling strategy of EP from sources. While it is urgent to deal with the serious problem of the accumulation of waste EP, it is expected that the generation of wastes will be minimized if recyclable EP materials like epoxy vitrimers are designed from sources to replace the present irreversible products. Furthermore, the responsiveness to a wide range of stimuli creates potential new applications for novel EP products with a dynamic network, such as smart actuators, as mentioned in previous sections.

Epoxy vitrimers exhibit stable performance at service temperature and qualify various application fields while obtaining fluidity, processability, shape memory, and stress relaxation under external stimuli like heating [126]. Therefore, epoxy vitrimers are usually convenient to recycle, with low energy and material consumption generated in their repeatable lifecycle [127,128]. Heat is the most common stimuli for the responsiveness of vitrimers, and healing or repairing induced by heating is a general recycling method of epoxy vitrimer materials with the goal of achieving reuse and performance restoration. In addition, the reactivity of reversible bonds and their exchange reactions bring the feasibility for chemical recycling under the stimuli of external solvents, though not all of the reported work has included it. Furthermore, increased numbers of researchers have recently focused on bio-based EP to substitute traditional fossil-based EP [129,130,131,132], which introduces recyclability from sources from another perspective and helps resource conservation and sustainability.

While it is important to reduce the continuous accumulation of existing waste EP, such actions are only palliatives. A completely clean EP lifecycle is expected to be realized in the future when all the produced and used products are reversible, preventing the generation of waste at the source. This section provides an overview of the current status of the development of epoxy vitrimers based on different bond exchanges, as well as the efforts researchers have made to reduce the distance between experiments and practical applications.

4.1. Transesterification

A transesterification reaction is the most common mechanism in epoxy vitrimers. This is because there are abundant ester bonds and hydroxyl groups in the network of EP cured by acid/anhydride, one of the most frequently used curing agent types, making the transesterification reaction able to occur intrinsically. The addition of catalysts and heating facilitate the transesterification process, thus constructing a dynamic crosslinked network with an efficient exchange and imparting processability to various EP materials.

Due to the slow rate of catalyst-free transesterification, catalysts, especially some Lewis acids containing Zn, were added to accelerate the transesterification process, such as zinc acetate and zinc acetyl acetonate [75,125,133,134,135]. In addition, organic basic reagents also served as catalysts, some of which could induce the curing process, such as TBD [136,137] and imidazole reagents [138,139,140]. In addition to the malleability under heating that originated from the efficient transesterification reaction, solvents with hydroxyl groups like alcohols could participate in the transesterification reaction and cut the network into pieces at the ester sites, which could dissolve epoxy vitrimer into soluble fragments and realize its chemical recycling. Compared to repairing and recycling by heating, the chemical recycling of epoxy vitrimer might be an easier and more energy-efficient method and is promising for the achievement of closed-loop recycling. Yu and coworkers [133] synthesized an epoxy vitrimer by a curing reaction between DGEBA and a mixture of multifunctional fatty acids with the addition of Zn(Ac)₂ catalyst. Any scratch on the surface of the epoxy matrix could be repaired by dropping EG around the wounded area and heating to 180 °C. A glass slide was necessary to prevent the evaporation of EG, and the volume of drops should not be too much in the repairing process; otherwise, excessive epoxy matrix would be dissolved unnecessarily. When the epoxy matrix near the scratch becomes homogeneous, EG should be evaporated to allow for the repolymerization of the epoxy matrix through reverse transesterification, finally obtaining the repaired material. This reversible process originated from the equilibrium between alcohol and ester through transesterification (Figure 9). The equilibrium moved towards the direction of depolymerization as excessive EG was dropped and heated with the restriction of its evaporation in order to degrade epoxy vitrimer. Contrarily, epoxy fragments dissolved in EG were repolymerized when EG was evaporated under continuous heating, where the equilibrium moved to the opposite direction. The storage modules, tensile strength, and stress relaxation properties of the regenerated vitrimer were very close to those of the original material. Therefore, the chemical recycling method based on a reversible transesterification reaction made epoxy vitrimer easy to be recycled, fundamentally preventing the generation of wastes. However, the low T_g (about 30 °C) of the synthesized epoxy vitrimer led to a lack of stability at room temperature, let alone various service environments, thus limiting the application of this material. Therefore, a more stable network with higher T_g needs to be designed, which is expected to combine stable excellent performance in different service environments and recyclability under process conditions. Generally, this method provided a valuable idea for a network for the recycling of epoxy vitrimers that could be depolymerized through the addition of a solvent to participate in transesterification and be reconstructed by the evaporation of it. This was not only applicable to epoxy vitrimers based on transesterification [75,134] but also one of the strategies worth considering for those based on other CANs [141,142,143].

However, the addition of external catalysts would sometimes cause problems in terms of compatibility and material stability, and the reproducibility of recycling could also be limited due to the loss of catalysts. Therefore, it is necessary to develop catalyst-free epoxy vitrimer, which was usually achieved by increasing the density of functional groups like carboxyl and hydroxyl groups in the network to enhance the proximity effect and facilitate an exchange reaction. The neighboring group participation (NGP) effect is an important reaction pathway in organic chemistry that usually plays a role in the stabilization of intermediates [144]. It has been gradually applied in epoxy vitrimer to construct catalyst-free networks in recent years [145,146,147,148]. Inspired by the phenomenon that the hydrolysis rate of phthalate monomethyl ester (PME) was more than 10 times that of methyl benzoate, Maarten and coworkers [147] investigated the kinetics of the transesterification reaction for PME and methyl benzoate under the same conditions; the only difference was the possession of a free neighboring carboxyl group in substances (Figure 10). The result showed that products of the transesterification reaction at 110 °C for PME were obviously detected, whereas those of methyl benzoate were not detected even at 140 °C by 1H NMR. This demonstrated the necessity of the NGP effect for efficient catalyst-free transesterification, which was almost prohibited in the absence of the neighboring carboxyl group. Therefore, a series of catalyst-free dynamic epoxy networks were constructed using different bisphthalic anhydrides and multifunctional alcohols. The dynamic properties of the synthesized materials were characterized by FTIR, where the anhydride signal disappeared and reappeared during the curing process and the annealing process, respectively. The reversible signal suggested that the CAN was based on the dissociative mechanism. However, the dissociative CAN did not bring about a significant drop in the viscosity of materials due to the almost negligible decrease in crosslinking density. In the experimental temperature range (100–180 °C), the majority of reformed anhydride tended to quickly undergo further ester formation, and only a trace amount was accumulated. The materials could be remolded by hot pressing at 150 °C for 60 min, obtaining recycled materials with no obvious difference in T_g compared to the original ones. But the tensile strength and elongation at break received a significant drop after two cycles, which was assumed to be related to the heterogeneity caused by repetitive grinding and pressing. The application of the NGP effect in a dynamic network of epoxy thermosets is generally promising, by which recyclable smart materials with a wider range of properties and functionalities could be synthesized with adjustments for the structural design and formulations of monomers.

The construction of unique topologies like a hyperbranched network is another frequently used strategy. Multifunctional terminal groups and intramolecular cavities could be introduced to facilitate a transesterification reaction in a hyperbranched epoxy network [149]. Meanwhile, such topology also allows for the optimization of material characteristics for more effective energy dissipation, simultaneously improving strength and toughness as well as properties like flame retardancy and recyclability of epoxy thermosets [150,151,152]. Zhang’s group [152] reported synthesized tough epoxy supramolecular thermosets through a transesterification reaction with rapid reprocessability and closed-loop recyclability at room temperature. The material was based on vanillin-based hyperbranched epoxy resin (VanEHBP) with a dual dynamic exchange system of both dynamic imine exchange and transesterification. In addition, intermolecular noncovalent hydrogen bonding synergistic crosslinks were also introduced, resulting in a supramolecular network with excellent mechanical properties as well as creep resistance and chemical stability. Specifically, this bio-based EP with hyperbranched topology exhibited higher tensile strength, toughness, and impact strength than traditional fossil-based products. The reversibility of the material was also remarkable due to the dual dynamic exchange system and noncovalent crosslinks, allowing for reprocessing at 120 °C within only 30 s in many cycles with almost complete recovery of the mechanical properties. It could also be recovered by chemical recycling at room temperature, fully degraded based on the cleavage of imines by immersion in 0.1 M HCl for 3 h. Repolymerization could be realized by the evaporation of solvent at 100 °C, regenerating the hyperbranched supramolecular epoxy network and comparable mechanical properties to those of the origin material. Transesterification in the hyperbranched network also occurred without the need of a catalyst, the essence of which is similar to the NGP effect since they all increase the chances for exchange to occur by increasing the content of functional groups and their distance between each other. This topology design gave materials both outstanding dynamic and mechanical properties, providing valuable inspiration for the development of high-strength and closed-loop EP materials.

4.2. Imine Exchange

Imine is another type of reversible covalent bond formed by the dehydration between an aldehyde or ketone and an amino group. It is able to undergo hydrolysis under acidic conditions, amine–imine exchange, and imine metathesis [153]. Imine bonds are also widely used in dynamic responsive materials due to these reactivities, giving materials the capability to be repaired and recycled [154,155,156,157,158]. Wang and coworkers [157] synthesized a vanillin-based epoxy network containing the Schiff base structure through imine formation between vanillin-derived epoxy and diamine hardener. The epoxy group was introduced in the vanillin-based precursor molecule by the modification of phenolic hydroxyl with epichlorohydrin; thus, it could be attacked by diamine through ring opening during the curing process while the aldehyde group at the para site underwent in situ imine formation with diamine simultaneously. This Schiff base EP had excellent mechanical and thermal properties, with 81 MPa of tensile strength, 172 °C of T_g, and 323 °C of T_d5% specifically, making it better than that of BPA-based EP. The presence of an exchangeable Schiff base structure provided the material with thermally induced malleability, reprocessed into films at 180 °C and 15 MPa in 20 min. The time- and temperature-dependent stress relaxation was characterized by DMA, showing a decrease in relaxation time (τ*) with an increase in temperature in the range above T_g, which was in accordance with Arrhenius’ law. The material could undergo chemical recycling through the hydrolysis reactivity of imine under acidic conditions. They achieved the chemical recycling of composites by immersing the composites into 0.1 HCl solution at room temperature, resulting in the complete dissolution of epoxy matrix and recovery of carbon fibers with the virgin surfaces and mechanical properties after 12 h.

Zhao and coworkers [158] firstly synthesized bisphenol monomers bridged by imine bonds followed by the modification of phenolic hydroxyl to connect with an epoxy group, also starting with vanillin, and using amine hardener to produce the epoxy vitrimer containing reversible imine bonds (Figure 11). This bio-based epoxy vitrimer had comparable properties to those of traditional high-performance epoxy thermosets that originated from BPA, with excellent dynamic properties at the same time, exhibiting a short relaxation time under a low temperature. They classified different properties of the material based on the reaction types of the imine bond. For example, the degradability and recyclability originated from the reversible hydrolysis and reformation of imine, and the complete degradation of material could be realized in 0.17 mol/L HCl at 25 °C. After drying the degradation solution, the formed polymer gel was heated at 120 °C for 24 h to evaporate the residual solvent and promote the reconstruction of the network without an obvious loss of strength by imine reformation between aldehyde and amine. The origin of the malleability and weldability was the associative amine–imine exchange mechanism, giving fast stress relaxation under low temperatures to the material. The relaxation time at 30 °C was 251 s and reduced to 11 s as the temperature rose to 60 °C. They cut the dog-bone-shaped sample into two pieces from the middle and overlapped them and welded them at 120 °C for 4 h, resulting in the welded sample with comparable tensile strength to that of the virgin sample. To conclude, alternative reaction types of imines broaden their use in CANs, which is a highly promising kind of dynamic bond.

4.3. Disulfide Exchange

Disulfide bonds are relatively weak covalent bonds. The interconversion between disulfides and thiols occurs readily at mild temperatures and even in vivo of living organisms. Therefore, dynamic properties under low temperatures of epoxy vitrimers could be achieved by introducing a dynamic disulfide structure into the network. The common strategy to construct a disulfide-containing epoxy network is to employ a curing agent with disulfide linkages, including disulfide-bridged aromatic diamine [143], acid or anhydride [159], and polysulfide [160].

Luzuriaga and coworkers [143] prepared a novel epoxy vitrimer using 4-aminophenyl disulfide (AFD) as the dynamic hardener of DGEBA while preparing an amine-cured EP as the reference material at the same time with diethyltoluenediamine (DETDA) lacking a disulfide linkage. Thermal and mechanical tests showed the comparable performances of the dynamic network to traditional thermosets, making the dynamic material a potentially ideal substitute for current products. The essential role of disulfide in the structure was demonstrated by comparative experiments. The network containing dynamic disulfide linkages realized complete stress relaxation at the temperature above T_g, with a relaxation time of 3 h at 130 °C and only 20 s at 200 °C, a property nod found in that without disulfide bonds. In addition, the epoxy vitrimer could be healed at 200 °C by hot pressing, and small scratches on the surface were easily repaired under the heat of just a household iron. In contrast, the reference material was crushed into powder during hot pressing and lost its original shape and properties. Furthermore, the presence of disulfide linkages provided the dynamic material with the possibility of dissolving in mercaptan solvents, achieving chemical recycling through a reversible disulfide formation reaction. They applied epoxy vitrimer into the preparation of composites for the first time, endowing the composites with reprocessability, reparability, and recyclability. The 3D shapes of fiber-reinforced polymer composites (FRPCs) could be fast programming and reprocessing at high temperatures without breaking and reshaping after cooling, which is of great significance for massive production industries such as the automotive industry. In addition to thermal recycling, chemical recycling could also be realized by immersing FRPCs in a solution of 2-mercaptoethanol in DMF at room temperature for 24 h, where the epoxy matrix was completely dissolved and carbon fibers were recovered undamaged.

The introduction of disulfide linkages into acid/anhydride curing agents helps construct the dual dynamic network based on both disulfide exchange and transesterification. Chen and coworkers [159] realized the dual dynamic epoxy network using 4,4′-dithiodibutyric acid (DTDA) as a curing agent in the presence of TBD as a catalyst for transesterification (Figure 12). The synthesized material exhibited fast stress relaxation with a relaxation time of only 1.5 s at 200 °C due to highly efficient exchange reactions in a dual dynamic network, whose relaxation rate was about 28 times faster than that of the single-disulfide vitrimer and 122 times faster than that of the single-ester vitrimer. The reprocessing of the material could be realized within 1 h at 100 °C, and the recovered samples after several cycles retained the same tensile strength as the virgin ones. Moreover, phosphines were reported as catalysts for disulfide exchange reactions. Lei and coworkers [160] synthesized an epoxy network with a high content of disulfide linkages by using polysulfide as the hardener and polysulfide diglycidyl ether as the epoxy precursor. With the addition of the efficient catalyst tri-n-butylphosphine (TBP) combined with the high content of disulfide linkages, the disulfide exchange reactions were highly active in the network. As a result, this material could be remolded and reprocessed by pressing at room temperature, and the reprocessed ones exhibited comparable mechanical properties and self-healing ability to the virgin one. The crosslinking density showed no loss in the reprocessing, indicating the excellent recyclability of the material.

4.4. Siloxane Equilibration

The stress relaxation properties of polysiloxanes were reported as early as 1954 [161]. McCarthy’s group [162] focused again on the siloxane equilibrium reaction and prepared a dynamic network of polysiloxanes by anionic polymerization and emphasized the use of silicones in self-healing materials. The crosslinking density and density of the reactive tetramethylammonium dimethylsilanolate end groups could be controlled by the adjustment of the ratio of monomers and the amount of added initiator. The synthesized material cut into pieces underwent a heat-induced welding process, revealing the self-healing mechanism based on siloxane equilibrium. Wu and coworkers [163] constructed an epoxy vitrimer network with siloxane equilibrium by introducing the silicon oxygen bond into the structure of the oligo-aminopropylmethylsiloxanediolate (PAMS) curing agent, the structure of which included aminopropyl side groups, catalytically active potassium silanolate end groups, and polysiloxane units (Figure 13). This network exhibited fast stress relaxation with a relaxation time of 376.8 s at 90 °C and 40.2 s at 170 °C. Additionally, the material could be fully recycled through reprocessing within 40 min at 130 °C. However, current research on epoxy vitrimers based on siloxane equilibrium is still relatively scarce compared to ones based on other dynamic bond types, and the chemical recycling methods need further investigation. It might be a viable strategy to depolymerize the network by small-molecule siloxane solvents, learning from experiences in other dynamic systems.

4.5. Bio-Based Sources

In addition to the introduction of dynamic covalent bonds into epoxy crosslinking networks to construct the recyclable vitrimer network, the source of raw materials has also attracted the interest of researchers in recent years. Epoxy thermosets synthesized from bio-based monomers as substitute for traditional fossil-based ones have been receiving increasing amounts of attention. With the continuous consumption of the finite fossil resources on earth, substances extracted from biomass exhibit the advantages of being green, environmentally friendly, and sustainable, and the epoxy network constructed from them contributes to sustainable development. Renewable substances such as vanillin, soybean oils, ferulic acid, naringenin, and 2,5-furan dicarboxylic acid (Figure 14) have been used as building blocks for a dynamic epoxy network containing reversible structures of imines, acetals, and ester bonds [151,158,164,165,166,167].

According to the structure and functional groups of the bio-based substances, different dynamic covalent bonds could form, determined by the choice of modification methods. For example, vanillin, the lignin derivative, is a phenol with an aldehyde group; thus, it could graft an epoxy group onto the phenolic hydroxyl by reacting it with epichlorohydrin or forming imine and acetal based on the aldehyde group [158,164]. Wang and coworkers [164] firstly modified the aldehyde group in vanillin by two hydroxyl groups on glycerol through a solvent-free acetal formation, followed by grafting epoxy groups onto the phenolic hydroxyl from vanillin and the last unreacted hydroxyl from glycerol. This difunctional bio-based epoxy precursor exhibited the advantages of being non-toxic and environmentally friendly compared to the commonly used BPA-based one. The comparison between the thermosets after curing with DDM showed that the bio-based one received comparable thermal performances and even better mechanical properties than the fossil-based one. The degradation of the bio-based network into soluble oligomers with formyl and hydroxyl groups was achieved in the solution of 0.1 M HCl in THF or acetone at 50 °C based on the reversible deformation of acetal under acidic conditions, indicating the recyclability of this rigid and flexible material. This material provided a practical way for producing both high-performance and degradable epoxy thermosets. In another work, Zhang and coworkers [167] synthesized a bio-based diacetal monomer by vanillin and tetra functional erythritol in the molar ratio of 2:1 (Figure 15). The two phenolic hydroxyl groups at two terminals were reacted with the epoxy groups in soybean oil to construct the crosslinking network. The reversible acetal structure provided the material with dynamic properties like stress relaxation and self-healing under hot pressing. It could also easily undergo chemical recycling in the acidic condition of 0.1 M HCl, while the polymer films could be regenerated by direct hot pressing after solvent evaporation of the degradation solution. Although it showed closed-loop recyclability of the bio-based network, a 76% conservation of tensile strength of the regenerated film was not ideal enough for repeating its use.

Dinu and coworkers [166] synthesized a high-performance network with high crosslinking density based on phloroglucinol and naringenin, the renewable flavonoid, and phlorotannin extracts. All the anhydride curing agents were bio-based and employed to investigate the influence of the structure of anhydride on the network. These EP materials received high T_g (134–199 °C), storage modulus at 25 °C (2.7–3.5 GPa), mechanical strength and toughness (Young’s moduli ~1.26–1.68 GPa), stability under heating (T_5%~335–355 °C), and heat-resistant performances (LOI~28–33%), making them comprehensive materials and promising environmentally friendly alternatives to petroleum-based EPs. At the same time, the materials could be depolymerized by the treatment of EG at 170 °C through the transesterification mechanism. Moreover, the epoxy matrix and fibers could be separately recovered and reused in the recycling of its composites.

Generally, bio-based molecules and related derivatives have emerged as part of a new perspective for conferring environmental friendliness and recyclability to the epoxy network as its building blocks, which have been arousing widespread research interest and optimistic expectations. The problem of resource depletion is expected to be alleviated, and the emission of CO₂ will be reduced. In the future, we may even be able to see biodegradable bio-based materials that are entirely safe for the environment and the human body, leading to a positive outlook everywhere in daily life and in various fields like biology and healthcare.

5. Conclusions and Perspectives

The recycling strategies of epoxy thermosets are generally divided into two categories: recycling from wastes and recycling from sources. The most urgent problem at present is to address the problem of growing waste accumulation, properly treating and recycling the existing end-of-life products. Large-scale chemical recycling is expected to maximize the value of end-of-life products through upcycling. It is of great value to develop the application of recovered chemical degradation products, such as converting them into high-value chemicals or being employed as building blocks of new materials. However, the primary impediment to the development of chemical recycling is the difficulty of being applied in industrialized processes. Chemical recycling methods that are more ideal in terms of safety, energy saving, and environmental friendliness should be further developed, and more scaling-up attempts are also needed. Additionally, many of the current methods have some restrictions and requirements on the chemical structure of EPs, which will lead to the need for extra classification based on their structure, thus bringing extra requirements of manpower and resources. Furthermore, the composition of wastes may be complex and unclear, resulting in incomplete degradation and multi-batch treatments.

Another strategy is based on the design of the chemical structure of EP from sources. The products with recyclability are expected to be produced by introducing dynamic covalent linkages during production, the initial stage of the EP lifecycle. The development of epoxy vitrimers with excellent performances to replace the traditional irreversible EP products contributes to realizing their full recyclability in the whole lifecycle of EP and preventing the generation of wastes from the origin. However, the introduction of dynamic bonds often brings about problems in terms of the decrease in T_g and loss of mechanical properties. The heat resistance and stability of the materials may be damaged, and the balance between performance loss and dynamic properties should be seriously considered. The recycling of epoxy vitrimers usually includes thermal recycling and chemical recycling. Thermal recycling is mainly used for the reuse of the materials and recovery of their performances, in which the key is to strengthen the efficiency of the dynamic exchange reactions in the network under the premise of guaranteeing excellent performances, improving convenience and energy saving during repairing and healing. Chemical recycling usually aims to realize a closed-loop process based on the reversible reactions of dynamic bonds, depolymerizing the network under mild conditions and carrying out subsequent repolymerization. Another way to consider the enhancement of the recyclability of EP from source-based design is to synthesize sustainable EP materials by replacing traditional fossil-based feedstocks with bio-based ones, which will reduce the consumption of non-renewable fossil sources and CO₂ emissions and make contributions to sustainable development. This is often combined with the introduction of dynamic linkages to develop environmentally friendly recyclable EP materials.

Generally speaking, both recycling from wastes and sources requires ongoing optimization by taking multifaceted factors into consideration. There is still an unbridgeable divide between experiments and industries, requiring more effort to be paid to promoting the practical application of experimental methods to make real contributions to the current urgent environmental problems.