Review of conventional and emerging recycling methods for carbon fiber-reinforced thermoset polymer composites: challenges and future perspectives

Reza Eslami-Farsani; Saba Noorsina; Amir Hossein Sadeghi; Abbas Tcharkhtchi

doi:10.1039/d6ra03142k

. 2026 Jul 3;16(35):33690–33734. doi: 10.1039/d6ra03142k

Review of conventional and emerging recycling methods for carbon fiber-reinforced thermoset polymer composites: challenges and future perspectives

Reza Eslami-Farsani ^a,^✉, Saba Noorsina ^a, Amir Hossein Sadeghi ^a, Abbas Tcharkhtchi ^b

PMCID: PMC13330985 PMID: 42405085

Abstract

Carbon fiber-reinforced polymer (CFRP) composites play critical roles in advanced industries due to their high strength-to-weight ratio and durability; however, CFRP waste management and recycling have become an environmental and economic necessity. Landfilling these composites, after their service life, wastes valuable resources of reinforcing fibers (such as carbon fibers) and may release toxic byproducts. Despite significant advances in this field, recycling CFRP composites remains a challenge. Thus, it is crucial to develop efficient, clean, and cost-effective methods for recycling CFRP, especially to preserve the value of both the fibers and thermoset polymer matrix components. This review study covers current CFRP recycling technologies, including the chemical, thermal, and mechanical recovery of CFRP composites across various matrix types (such as epoxy and phenolic, and other thermoset matrices). Also, recent developments have introduced advanced techniques such as carbonization, solvothermal, electrochemical, sonochemical, and biological recycling. These methods aim to improve fiber recovery while reducing energy consumption and preserving material quality. Future advancements are expected to focus on hybrid and multifunctional recycling systems integrated within circular economy frameworks, enabling closed-loop processes and reducing reliance on virgin resources. Such innovations will transform CFRP composites from waste into valuable sustainable resources.

1. Introduction

Carbon fiber is a high-performance material widely recognized for its exceptional properties, including low density (1700–1800 kg m⁻³), high specific strength and stiffness, chemical inertness under most conditions, excellent damping, and favorable thermophysical performance. These characteristics enhance fatigue resistance and reduce creep in composite materials.^1,2

Carbon fibers were initially produced from natural cellulosic materials such as cotton and linen. Modern carbon fibers are then produced from polyacrylonitrile (PAN) precursors, bitumen, and rayon, and PAN-based fibers are the preferred reinforcement for structural composites due to their superior mechanical properties and low weight. However, the production cost of high-performance carbon fibers with superior mechanical properties remains high. This elevated cost, particularly for advanced grades, represents a significant limitation and provides a strong economic incentive for the development and implementation of efficient carbon fiber recycling technologies.^3–5

Given these properties, carbon fibers are extensively used as reinforcements in various composite matrices, including polymers, ceramics, and metals, in forms such as short fibers, continuous filaments, and woven fabrics. These reinforcements enhance the mechanical and thermal behavior of composites by improving stiffness, strength, and dimensional stability. In general, approximately 83, 10, 1, and 6 percent of the total carbon fibers used as reinforcements are incorporated into polymer matrix composites, ceramic, metal, and other matrix composites, respectively.^1,6,7 The reason for the widespread use of carbon fibers in the polymer matrix as an alternative to metal matrix composites is that metal-embedded composite materials have a higher density and are more susceptible to corrosion, which limits their wider applications compared to epoxy-based carbon fibers reinforced polymer (CFRP) composites.^8,9 Consequently, their predominant application is in polymer matrix composites, particularly thermosets such as epoxy, phenolic, and polyester, which dominate the market due to their strong adhesion and thermal stability, accounting for 69% of the market, whereas thermoplastics constitute 28.8%, and other matrices represent a smaller fraction.^1,6,7

The exceptional mechanical and physical properties of CFRP composites render them highly suitable for advanced, high-performance applications. These composites demonstrate outstanding strength-to-weight ratios, high stiffness, corrosion resistance, and dimensional stability, along with low thermal expansion and superior fatigue performance. Consequently, CFRP composites are increasingly employed in demanding sectors, including automotive, aerospace, sports equipment, wind energy, electronics, and construction.^1,6,7

Industrial production of CFRP began in the early 20th century, but its commercial use expanded significantly during the 1980s, especially in the aviation industry. The historical evolution of composite materials and their utilization across various sectors including infrastructure, transportation, and communication from as early as 3400 B.C. is illustrated in Fig. 1.^10,11 Despite over a century of manufacturing, an efficient global recycling system for these materials has yet to be developed.¹¹ The current methods for recycling reinforcing fibers on an industrial scale remain limited and technically challenging. One of the important challenges is that thermoset polymers cannot be melted and reused.¹² Other challenges in recycling polymer composites include maintaining the quality of the recycled polymers, extracting long fibers with suitable mechanical properties, high recycling costs, and environmental impacts. Moreover, the four main factors that affect the quality of recycling are: polymer cross-contamination in landfills, additives used in polymer composites, non-polymer impurities, and the gradual degradation of these materials over time. Sources of contamination can come from direct additives such as dyes and adhesives in post-production stages, or from waste collection and management processes.¹³

In parallel with market expansion, the generation of CFRP scrap has increased substantially worldwide. The global market value of carbon fibers used in composite applications is projected to grow from USD 21.95 billion in 2024 to approximately USD 39.39 billion by 2034, corresponding to a compound annual growth rate (CAGR) of 6.02% (Fig. 2a).¹⁴ The current global demand for carbon fibers is estimated at around 115 kilotons per year as of 2023 and is expected to reach nearly 280 kilotons per year by 2030.¹⁵ In 2022 alone, the aviation and wind energy sectors accounted for over 56% of total carbon fiber consumption, corresponding to approximately 68 760 tons, while the inclusion of other industries increased this figure to 122 780 tons.^6,16 Consequently, annual carbon fiber waste generation has been estimated at 50 000–62 000 tons and is projected to exceed 100 000 tons per year after 2029, with the majority originating from industrial scrap and end-of-life (EoL) components in aerospace and wind turbine applications (Fig. 2b).¹⁷

The absence of large-scale recycling initiatives poses significant environmental and economic risks. Without effective recycling strategies, the aviation industry alone is expected to generate approximately 500 000 tons of CFRP waste by 2050.¹⁸ The accumulation of non-biodegradable composite waste in landfills raises serious environmental and human health concerns, also the high cost of virgin carbon fibers further underscores the urgency of developing efficient recycling and reuse pathways.^6,16 In response, considerable research efforts have focused on recovering carbon fibers for reuse as fillers or reinforcing agents in new composite systems. Reflecting this trend, the global market for recycled carbon fibers (rCFs) was valued at approximately USD 234.43 million in 2024 and is expected to grow from USD 264.79 million in 2025 to USD 701.43 million by 2033, with a notable CAGR of 12.95% over the forecast period (Fig. 2c).¹⁹ Recycled carbon fibers offer a cost-effective alternative to virgin fibers and conventional metallic materials and are increasingly being adopted in automotive, aerospace, wind energy, and sports equipment applications to achieve weight reduction, improved fuel efficiency, and enhanced energy performance. Table 1 summarizes the most recent growth trends, and industrial drivers of CFRP composites based on updated market reports.

Table 1. Recent global market trends of CFRP composites (2023–2030)^a.

Market parameter	Reported value/trend	Description
CFRP segment growth rate	∼6.6% CAGR	CFRP expected to grow faster than the overall FRP market
Dominant CFRP end-use sectors	Aerospace, automotive, construction	Key industries driving CFRP demand
Main growth drivers for CFRP	Lightweighting, high strength-to-weight ratio	Functional advantages replacing metals
CFRP market maturity	Emerging–expanding	CFRP transitioning from niche to broader industrial adoption
Technology trend	Shift toward recyclable and sustainable CFRP systems	Increasing focus on recycling and circular economy

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Based on Grand View Research, 2023–2024.

In general, the framework of waste management strategies includes methods such as landfill, reuse, and recycling. As Fig. 3 shows, the CFRP waste management cycle from the production stage to final routes such as landfill, incineration, and recycling. Selecting the most appropriate method requires a comprehensive assessment in terms of environmental impacts, technical efficiency, and commercialization potential.^20,21

Landfilling remains one of the most common methods of waste disposal, but it faces challenges such as space constraints in some countries, high disposal costs, the risk of contamination of soil and groundwater resources, and the difficulty of transporting waste. These problems have led many governments to enact stricter regulations and impose taxes on landfills to encourage the use of more sustainable methods, such as recycling and reuse.^22,23

One of the simplest alternative solutions is the reuse of waste polymer composite materials, which can be reused in the same or similar applications without spending much energy and costs (Fig. 4).^20,24 This method has several advantages, including preventing materials from entering the waste cycle, saving natural resources, reducing pollution, and conserving the energy spent on primary production. However, reuse is relatively uncommon for polymer composites; therefore, these materials are frequently discarded after a single use, especially in hygiene- and food-related applications, due to safety concerns.²⁰

One practical approach is the recycling of CFRP composites, but the process still faces challenges. One of the most important challenges in this field is the type of polymer matrix used in the manufacture of these composites. In most cases, this matrix is made of thermosetting resins, which, due to their three-dimensional network structure and irreversible cross-linking, resist melting or modification. This characteristic makes the recycling process of these composites more complex and costly than composites with thermoplastic matrices.²⁰

Traditionally, recycling technologies for CFRP composites have been broadly classified into three main categories, including mechanical, thermal, and chemical recycling. Mechanical recycling involves crushing and reusing in secondary applications. Chemical recycling concerns breaking down the polymer structure through solvents or chemical processes to separate the fibers and resin. Finally, thermal recycling engages using heat to decompose the resin and recover the fibers. Each of these conventional methods has advantages and limitations that are selected depending on the type of product, cost, quality of recycled fibers, and environmental considerations.

In recent years, however, a number of advanced recycling technologies have emerged that do not fit neatly within a single conventional category. Processes such as solvothermal combine thermal and chemical effects, while carbonization involves high-temperature transformation of the matrix under controlled atmospheres. Similarly, electrochemical and sonochemical recycling methods introduce external energy fields to promote selective bond cleavage within the polymer matrix under comparatively mild conditions. Biological recycling has also recently emerged as a potential sustainable route, utilizing microorganisms or enzymes to degrade polymer matrices under mild conditions, although its application to composite recycling remains largely at the laboratory stage. These advanced approaches often blur the boundaries between mechanical, thermal, and chemical recycling and are therefore discussed separately in this review as complementary, next-generation recycling strategies.

In this review, recycling strategies for CFRP composites are systematically examined using a structured and complementary classification framework. The discussion first follows the conventional categorization of recycling technologies into mechanical, thermal, and chemical approaches, providing an overview of their fundamental principles, advantages, and limitations. Particular emphasis is placed on chemical and thermal recycling routes, as these methods offer the greatest potential for high-quality fiber recovery from thermoset-based composites.

Subsequently, advanced and emerging recycling technologies that extend beyond the boundaries of traditional classifications are discussed separately. These include solvothermal processes, carbonization, as well as electrochemical, sonochemical, and biological recycling methods, which integrate thermal, chemical, or field-assisted mechanisms, or in the case of biological systems, biochemical pathways mediated by microorganisms or enzymes, to enhance resin degradation and fiber preservation.

In addition, the influence of different thermoset matrices, such as epoxy and phenolic systems, on recycling efficiency and process selection is critically analyzed. Finally, the review concludes with a discussion of current challenges, future perspectives, and emerging trends in the development of sustainable and industrially viable CFRP recycling technologies.

2. Fiber reinforced polymer (FRP) composites recycling methods

In general, the recycling methods for FRP composite waste are commonly classified into three conventional categories, including mechanical, thermal, and chemical. These three common methods for recycling CFRP can be seen in Fig. 5.²⁵

Mechanical recycling (Fig. 5a) is considered the simplest method in terms of technology, as this process is carried out in two stages, including the use of low-speed grinders and crushers and the use of high-speed hammer mills. However, it typically results in recycled products in powder form, with no separate recovery of fibers. Consequently, the quality of the recycled material is reduced, and the resulting products are of low value. This method is applied at an industrial scale primarily for glass fibers-reinforced plastic (GFRP) composites. For CFRP, mechanical recycling is mainly used as a pre-treatment step to crush the composite before applying thermal or chemical recycling methods.^26,27

Thermal recycling (Fig. 5b) involves separating fibers from the resin matrix by applying heat, typically through pyrolysis. This process includes three main methods, namely fluidized bed, conventional pyrolysis, and microwave-assisted pyrolysis. While it enables fiber recovery, it may also result in fiber degradation depending on the thermal conditions.²⁸

Chemical recycling (Fig. 5c) uses specific chemical agents to break down the polymer matrix, enabling the simultaneous recovery of fibers and energy. This method is particularly suitable for CFRP composites, as it better preserves the mechanical properties of the carbon fibers. However, the high cost of implementation poses a significant barrier to large-scale adoption. Solvolysis is a major subset of chemical recycling and can be broadly categorized into two main methods, including low-temperature solvolysis and sub/supercritical solvolysis.^27,29

Currently, thermal and chemical recycling methods that preserve the mechanical properties of carbon fibers are more commonly applied to CFRP composites than to GFRP systems. This preference is primarily driven by economic considerations, as carbon fibers have a higher market value and their recovery is therefore more cost-effective. In addition, from a materials perspective, glass fibers are more susceptible to degradation at elevated temperatures, whereas carbon fibers exhibit greater thermal stability under the conditions typically employed in thermal and chemical recycling processes. However, the high cost associated with these recycling methods remains a significant barrier to their widespread implementation. Scaling these techniques from laboratory-scale research to commercial deployment continues to represent a major challenge. Among the available approaches, chemical recycling generally yields the highest-quality recycled fibers; nevertheless, despite ongoing advancements, it has not yet been widely commercialized.^6,27

Beyond the conventional mechanical, thermal, and chemical recycling routes, a growing body of research has focused on advanced and hybrid strategies aimed at improving fiber quality, process selectivity, and overall sustainability in FRP recycling. These emerging approaches are primarily motivated by the limitations of traditional methods, particularly with respect to fiber degradation, high energy consumption, and limited value recovery. Among these advanced approaches, carbonization, solvothermal processing, electrochemical, sonochemical, and biological recycling have attracted particular attention.

In the carbonization process, the polymer matrix in CFRP composites is thermochemically decomposed under controlled high-temperature conditions. During this process, the polymer matrix is primarily volatilized, producing gaseous and oil products, while a fraction may decompose into residual pyrolytic carbon. The structural integrity of the carbon fibers is largely preserved; however, carbonaceous char can remain on the fiber surface and is often considered a contaminant that requires a subsequent oxidation step for removal. Beyond the efficient recovery of these fibers, this method also enables the fabrication of carbon–carbon (C–C) composites, which exhibit favorable mechanical properties and possess significant added value.³⁰

Also, the solvothermal method, considered one of the more recent hybrid recycling approaches, integrates key characteristics of both chemical and thermal processes. In this technique, the polymer resin is decomposed in the presence of specific solvents under controlled temperature and pressure conditions, enabling efficient fiber recovery while preserving their structural integrity and mechanical properties.³¹

Furthermore, electrochemical recycling has been introduced as an advanced alternative that employs electrochemical reactions to break down the polymer matrix. This approach is particularly effective in cases where precise control over recycling parameters is required. One of its primary advantages lies in its ability to operate under comparatively milder conditions than conventional thermal techniques.³²

Moreover, sonochemical recycling employs high-frequency ultrasound to enhance chemical reactions through acoustic cavitation in liquid media. The collapse of cavitation bubbles generates localized high-energy conditions that improve mass transfer, surface erosion, and radical formation, thereby accelerating the degradation of cross-linked thermoset resins. In the context of CFRP recycling, this approach has been shown to enable effective epoxy resin decomposition under relatively mild bulk temperatures and reduced chemical severity, while preserving the mechanical integrity of recovered carbon fibers. As a result, sonochemical recycling is considered a promising complementary technique to conventional chemical methods, although further optimization is required to improve its scalability and industrial applicability.³³

Furthermore, biological recycling has recently emerged as a sustainable alternative that utilizes microorganisms or enzymes to degrade polymer matrices under mild conditions. This approach offers advantages such as reduced energy demand and limited reliance on hazardous chemicals. However, its application to FRP recycling remains at an early stage, as epoxy degradation is typically slow and incomplete, and scalable processes for efficient resin removal and high-quality fiber recovery have not yet been established. Despite these limitations, biological recycling is regarded as a promising long-term strategy for environmentally friendly composite waste management.^34,35

Collectively, these emerging methods have been developed to enhance recycling efficiency, minimize fiber degradation, reduce overall processing costs, and improve the quality of the recovered materials. An overview of recycling technologies and their general process is illustrated in Fig. 6.

2.1. Mechanical recycling

Mechanical recycling, also known as physical recycling, is a widely adopted method for processing FRP composites waste due to its simplicity and cost-effectiveness. This approach typically involves the use of specialized equipment, such as multi-shaft shredders, as depicted in Fig. 7. In this type of recycling, composite waste is progressively crushed and sorted by particle size through multiple stages to produce raw materials suitable for the manufacture of new products. In the initial stage, the waste is reduced to a particle size of approximately 50–100 mm using low-speed grinders and shredders, a process that also facilitates the separation of any embedded metallic components. In the subsequent stage, high-speed hammer mills are employed to further reduce the material to a size range of 10–50 mm. Finally, the processed material is classified into various size fractions using cyclones and screening systems.^20,26,36

In the context of CFRP waste, mechanical recycling involves the reuse of EoL composites as fillers or short fibers reinforcements in new composite materials after crushing, grinding, milling, and sieving. The resulting recycled products consist of mixed polymer powder and short fibers³⁸ Compared with thermal recycling routes, mechanical recycling is generally more cost-effective and environmentally benign, primarily due to its lower energy requirements and absence of chemical reagents.³⁹ However, despite these advantages, the industrial applicability of this method remains limited, particularly for high-performance composites.

The main drawbacks of mechanical recycling are associated with the inferior quality of the recovered fibers. The recycled fibers are typically short and often retain residual polymer matrix on their surfaces, which significantly compromises their mechanical properties compared to virgin fibers.⁴⁰ In addition, the allowable filler loading in secondary composites is limited. Incorporating mechanically recycled fibers beyond approximately 10 wt% in epoxy matrices can lead to agglomeration, increased internal stresses, and a consequent reduction in mechanical performance.^41,42 Poor interfacial bonding between the recycled fibers and the polymer matrix, along with stress concentrations arising from irregular fiber geometry and low-density recycled powders, further limits their suitability for high-performance thermoset applications.^6,20 As a result, mechanical recycling is primarily applied to GFRP, where performance requirements are less stringent, while its use for CFRP remains largely confined to downgraded, low-value applications. The method performs optimally when the input material is clean and free of contaminants.

Despite these limitations, mechanical recycling remains attractive for certain applications due to its technological simplicity and lower energy consumption. Recycled FRP materials are commonly repurposed in cement and concrete, formwork, roof waterproofing, drainage systems, and industrial boxes.⁴³ compared to other recycling techniques, such as pyrolysis (3–30 MJ kg⁻¹), chemical recycling (63–91 MJ kg⁻¹), and high-volatility delamination (4 MJ kg⁻¹), mechanical recycling offers a more energy-efficient and straightforward solution.⁴⁴ Next, we review the literature on mechanical recycling methods for polymer composites.

Thomas et al.⁴⁵ employed a mechanical process involving cutting and grinding laminate CFRP waste, followed by sieving to remove oversized particles. This method produced carbon powder with an average particle size exceeding 1.25 mm, which was incorporated into an epoxy resin matrix. The incorporation of carbon powder wastes to the epoxy resin provides a loss of fluid viscosity. Also, the resulting composites demonstrated improved mechanical properties, including enhanced compressive, flexural, and impact strengths.

In a similar study, Song et al.⁴⁶ investigated the mechanical recycling of CFRP using jet milling. The recovered carbon fibers, separated from the polymer matrix without any chemical treatment, were utilized as reinforcing agents in new composite materials. These fibers, predominantly in powder form with an average length of approximately 59 µm, were added at up to 10 wt% into an epoxy foam matrix. This incorporation led to significant improvements in mechanical performance, 98% in specific compressive modulus, and 75% in specific compressive strength. These results demonstrate that the CFRP recycles have excellent reinforcing ability for epoxy foams.

Furthermore, Mamanpush et al.⁴⁷ developed recycled carbon fibers-reinforced polyether ether ketone (PEEK) composites using carbon fibers recovered from carbon fibers-epoxy and carbon fibers-vinyl ester composites wastes. The recycling process involved shredding and hammer milling to reduce the particle size to below 3.4 mm, and those that passed through a 1.2 mm screen. The shorter recycled fibers improved the flexural performance of the composite materials.

In another study, Ogi et al.⁴⁸ employed ball milling to mechanically process CFRP composite waste, yielding particles primarily in the range of 1–10 µm. The recycled material, comprising numerous separated fibers and large bundles, was blended with acrylonitrile butadiene-styrene (ABS) to fabricate new composites. Notably, the mechanical strength and modulus of the resulting materials improved considerably with increasing recyclate fiber content, up to 50 wt%.

Moreover, Li & Englund⁴⁹ conducted a related study focused on recycling aerospace-grade CFRP (carbon fibers/PEEK) waste using a combination of milling and shredding techniques. The recycled material was compression-molded into flat panels and subjected to mechanical testing. Although the recycled composites showed a reduction in mechanical properties compared to their virgin counterparts, smaller recyclate particle sizes were found to enhance the mechanical performance of the new composites.

Research conducted on the mechanical recycling of FRP composites is summarized in Table 2. In general, mechanical recycling emerges as the most feasible and widely applied method for composite waste recovery, both in academia and industry. The substitution of virgin carbon fibers with mechanically recycled alternatives presents a promising pathway toward improved environmental sustainability and cost-effectiveness. While some concerns persist regarding potential degradation in mechanical properties, experimental findings have not consistently supported this assumption. Significantly, all carbon fiber-containing waste materials can be effectively processed using mechanical methods and reused as fillers or reinforcements in the development of purpose-designed composite systems.⁵⁰

Table 2. A summary of mechanical recycling of FRP composites.

Study	Type of waste	Mechanical method	Final particle size	New polymer matrix	Mechanical performance
Thomas et al.⁴⁵	CFRP laminate	Cutting, grinding, sieving	>1.25 mm (powder)	Epoxy resin	Flexural modulus increased by ∼20–100% depending on rCFC content and processing temperature
Song et al.⁴⁶	CFRP	Jet milling	∼59 µm (powder)	Epoxy foam	98% increase in specific compressive modulus; 75% increase in specific compressive strength
Mamanpush et al.⁴⁷	Carbon fibers/epoxy and carbon fibers/vinyl ester	Shredding, hammer milling	<3.4 mm	PEEK	20% increase in compressive strength, 30% increase in flexural strength, and up to 28% improvement in impact resistance (at 20 wt% CFRP powder loading)
Ogi et al.⁴⁸	CFRP	Ball milling	1–10 µm	ABS	Considerable improvement in strength and modulus up to 50 wt% fiber content
Li & Englund⁴⁹	Aerospace-grade CFRP (carbon fiber/PEEK)	Milling and shredding	Variable (smaller = better)	Compression-molded flat panels	50–60% lower tensile strength vs. virgin CFRP, but acceptable performance

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2.2. Chemical recycling

Chemical recycling, also known as raw material recycling, is a process in which solvents are used to decompose the polymer matrix and recover valuable fibers such as carbon fiber. In this method, polymer waste is broken down into monomers or partially depolymerised to oligomers or other basic chemicals.⁵¹ These products can be used as feedstock in the petrochemical industry to produce new polymers, other chemicals, or as alternative fuels. It also allows the separation of the polymer as low molecular weight hydrocarbons from the valuable fibers. These fibers can be used as fillers for other composite materials, and the hydrocarbons can be used as chemicals or fuels.^6,20

Despite the high potential for recycling various polymer composites, this process faces challenges such as severe environmental pollution, serious health risks for workers, the need for high temperatures and pressures, high cost, and technological complexity. As a result, the industrial-scale implementation of chemical recycling for CFRP waste remains limited.^52,53 Solvolysis is a major subset of chemical recycling and can be broadly categorized into two main methods, including low-temperature solvolysis and sub/supercritical solvolysis.^6,20

Low-temperature solvolysis

Low-temperature solvolysis refers to recycling conducted below 200 °C and under atmospheric pressure, using solvents or acidic media to disintegrate the polymer matrix in CFRP composites. Common solvents include water, alcohols, ammonia, nitric acid, sulfuric acid, and acetic acid. Due to the relatively mild thermal conditions, catalysts, additives, and mechanical stirring are often used to enhance resin degradation. Pretreatment significantly improves the process efficiency. For example, an eight-layer FRP composite can be recycled in one hour with pretreatment, whereas it may take up to 15 hours without it.⁵⁴

The quality of the recovered fibers strongly depends on their chemical composition; fibers containing Al₂O₃ and CaO, for example, may lose up to 60% of their weight over time, reducing mechanical strength. Overall, the primary advantage of low-temperature solvolysis lies in its precise control over reaction conditions, which minimizes undesirable secondary reactions and facilitates effective fiber recovery.⁵⁵

In the following, experimental studies on low-temperature solvolysis are discussed. In summary, these studies have extensively investigated the chemical recycling of CFRP wastes via low-temperature solvolysis, usually using nitric acid as the reaction medium to decompose epoxy resin matrices.^56–58 These studies have shown that the mechanical properties of recycled carbon fibers and recovered resin are largely comparable to those of the raw materials. In practical applications, recovered carbon fibers are usually reused in the form of shredded or ground fibers, while the degraded resin fraction can be valorized as a chemical feedstock or used as a fuel.⁵⁹

Liu et al.⁵⁶ employed an 8 M nitric acid solution within a glass vessel (100 mm diameter × 300 mm length) to decompose an epoxy resin matrix composed of bisphenol A cured with isophorone diamine (IPDA) under thermal conditions of 80 °C for 2 hours, followed by 150 °C for 4 hours. The recovery of carbon fibers from the epoxy matrix was carried out at a temperature of 70–90 °C. The optimum temperature was 90 °C, at which the epoxy resin matrix was decomposed into low molecular weight compounds, and an insoluble carbon fiber was recovered. The carbon fibers recovered from the epoxy matrix retained favorable mechanical properties, showing only a 1.1% reduction in strength compared to virgin fibers. Similarly, (Lee et al.⁶⁰ developed a hexahedral circulating flow reactor system made of quartz, Teflon supporter, acid-resistant pump, and auxiliaries for CFRP recycling utilizing a 12 M nitric acid solution at 90 °C for 6 hours. This method yielded clean carbon fibers with a minimal 2.91% reduction in tensile strength.

Feraboli et al.⁵⁷ dissolved the epoxy matrix in sulfuric acid at 110 °C. When the solution starts to boil, hydrogen peroxide is added to accelerate the reaction and oxidize the matrix. Also, mechanical stirring throughout the duration of the entire process is essential to recover carbon fibers. They reused the same resin and recycled fibers to manufacture new CFRP composites. Mechanical characterization revealed that the recycled composite exhibited comparable properties to virgin material, with minimal differences observed in tensile strength and modulus, compressive strength and modulus, short beam shear strength, and three-point flexural bending strength.

In summary, based on low-temperature solvolysis studies, nitric acid emerges as the most effective solvent for recycling CFRP composites due to its strong ability to decompose epoxy matrices while largely preserving carbon fiber integrity. Optimal processing conditions are generally reported within a moderate temperature window of approximately 70–110 °C and reaction times of 4–6 h, with 90 °C identified as an optimal operating temperature that balances efficient resin degradation and minimal fiber damage. The use of auxiliary measures such as mechanical stirring, continuous flow circulation, or the addition of mild oxidizing agents further enhances reaction kinetics under atmospheric pressure. Under these conditions, high-quality carbon fibers can be recovered with minimal surface damage and tensile strength losses typically below 3%, demonstrating the technical feasibility of low-temperature solvolysis. However, industrial-scale implementation remains constrained by challenges related to acid consumption and recovery, corrosion-resistant reactor requirements, process throughput, and environmental and safety considerations associated with handling concentrated acids.

Sub/supercritical solvolysis

Sub/Supercritical solvolysis is an advanced chemical recycling technique that employs fluids under extreme thermodynamic conditions to break down FRP composites. Among these, supercritical fluids offer unique advantages due to their distinctive physicochemical properties. The supercritical state represents a separate thermodynamic phase that exists independently of the three conventional states of matter. When a substance reaches its supercritical state, two fundamental characteristics emerge. On the one hand, the gas phase cannot be fully liquefied through pressure increase alone, and on the other hand, the liquid phase cannot be completely vaporized through temperature elevation alone. So, at this point, the substance becomes a supercritical fluid, where both temperature and pressure exceed critical points, resulting in a phase that combines the density of liquids with the diffusivity and viscosity of gases, so that their surface tension becomes negligible (see Fig. 8). In this state, supercritical fluids can effectively penetrate and dissolve polymer matrices, making them ideal for recycling applications.⁶¹

The supercritical fluid technique for recycling FRP composites was first developed in Japan during the mid-1990s. Over time, this method has attracted significant research interest due to its excellent properties and its ability to operate across a continuum from subcritical to supercritical conditions. Supercritical fluids represent a prominent method in polymer recycling, offering optimal conditions within the reaction medium to facilitate the decomposition of polymers through rapid and selective chemical reactions. Condensation polymerization plastics, addition polymerization plastics, and composite plastics can be efficiently broken down into high-yield monomeric components, individual monomers, and smaller monomeric units, along with recovered fiber materials, respectively.⁶³

Supercritical solvolysis typically involves the use of supercritical fluids, such as water or alcohol (methanol, ethanol, propanol, acetone, or glycols), at temperatures above 374 °C and pressures exceeding 22.1 MPa, which correspond to the supercritical point of water. This concept is clarified in Fig. 8.⁶² Under these conditions, water exhibits unique physicochemical properties, including low viscosity, high diffusivity, and densities that are intermediate between those of gases and liquids. These properties create an ideal reaction medium for the rapid and selective decomposition of polymer matrices. To reduce the extreme temperature and pressure requirements of supercritical water, alternative solvents or solvent mixtures, often in combination with additives or catalysts, can be employed. These co-solvents not only serve as the primary reactants in the solvolysis process but also facilitate the breakdown of polymer chains under milder conditions. In some cases, the mechanical properties of the recycled fibers are nearly indistinguishable from those of virgin fibers, with minimal loss in tensile strength and mass, some studies reporting weight losses as low as 0.08%. Despite these promising outcomes, supercritical solvolysis has not yet proven to be economically viable for the large-scale recycling of composite waste. The optimal combination of process parameters, including fluid selection, temperature, pressure, and reactor design, remains under investigation, and consensus on standardized conditions has yet to be reached. Experimental studies have explored a broad temperature range from 230–500 °C, yielding diverse results that range from ineffective decomposition to the recovery of high-performance fibers. However, these investigations have primarily been limited to laboratory-scale experiments using small FRP samples. Due to variations in composite formulations, experimental conditions, and methodologies, the scalability and generalizability of these findings to industrial applications remain uncertain.^6,64,65 The following discusses research conducted on Sub-supercritical water solvolysis and Sub-Supercritical alcohol solvolysis.

(a) Sub-supercritical water solvolysis

Studies on the CFRP recycling via sub/supercritical water solvolysis were first conducted by Okajima et al.⁶⁶ They successfully recovered carbon fiber from CFRP waste at temperatures between 300–450 °C. Optimal conditions were identified at 380 °C and 25 MPa pressure, yielding clean, high-quality recycled carbon fibers. Furthermore, the same author successfully recovered carbon fibers using subcritical water at a temperature of 400 °C and a pressure of 20 MPa for a duration of 45 minutes, in the presence of a catalyst (specifically, 2.5 wt% potassium carbonate). The tensile strength of the recovered fibers was reduced by approximately 15% compared to virgin carbon fibers.⁶⁷

Also, Raul Piñero-Hernanz et al.⁶⁸ conducted a series of experiments using a batch-type reactor with supercritical water at temperatures ranging from 523 to 673 K and pressures between 4-27 MPa, with reaction times varying from 1–30 minutes and without mechanical stirring. The primary objective of the study was to determine the optimal operating conditions for the recovery of carbon fibers. The highest recovery yield, approximately 79.3 wt%, was achieved at a temperature of 673 K and a pressure of 28 MPa. The tensile strength of the recovered fibers ranged from 90% to 98% of that of virgin fibers. Additionally, the study evaluated the effect of a catalyst, specifically potassium hydroxide, on recovery efficiency. The recovery yield was found to be 79.3% without the catalyst and increased to 95.3% when the catalyst was used.

Further, Yuyan et al.⁶⁹ conducted experiments under conditions similar to previous studies, with the exception of a lower temperature range (250–290 °C) and a specific resin-to-water ratio of 1 : 5 g mL⁻¹. Complete resin decomposition was achieved at 260 °C after 105 minutes and at 290 °C after 75 minutes. The tensile strength of the recovered carbon fibers was found to be 98.2% of that of virgin fibers, indicating minimal degradation in mechanical performance.

In another study, Bai et al.⁷⁰ explored the use of oxygen in supercritical water for carbon fibers recovery, operating at temperatures between 430-450 °C and pressures ranging from 29–31 MPa for durations of 25–35 minutes. Their results demonstrated high tensile strength in the recovered fibers and epoxy resin decomposition rates ranging from 94–97 wt%. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) analyses were employed to examine the surface morphology of the recycled fibers, revealing a clean surface and well-preserved fiber structure.

In addition, Knight et al.⁷¹ employed supercritical water in combination with a 0.05 M solution of potassium hydroxide (KOH) as a catalytic agent to recover carbon fibers from the waste aerospace epoxy CFRP (Hexcel 8552/IM7). The tensile strength of the recovered carbon fibers was evaluated and found to be equivalent to that of virgin fibers, measuring 5.25 GPa. SEM analysis of the fiber surface revealed clean and unblemished carbon fibers. Additionally, a resin removal efficiency of 99.2 wt% was achieved, indicating the effectiveness of the process.

Liu et al.⁷² investigated the decomposition of CFRP resin cured with 4,4′-diaminodiphenylmethane using subcritical water at temperatures ranging from 310–325 °C and reaction times between 15–60 minutes. The process incorporated phenol (0–5 g) and potassium hydroxide (0–0.5 g) as a catalyst to enhance resin degradation. The tensile strength of the recovered carbon fibers was not damaged during the recycling process. Similarly, the surface properties of carbon fibers showed little change during the recycling process. The highest decomposition efficiency was achieved at 315 °C for 30 minutes with the addition of 5 g phenol and 0.5 g potassium hydroxide.

Overall, studies on carbon fibers recycling via sub-/supercritical water solvolysis indicate that effective fibers recovery is generally achieved at temperatures ranging from approximately 300–450 °C, pressures of about 20–28 MPa, and reaction times between 25 and 105 minutes. The use of alkaline catalysts, most notably potassium hydroxide, and in some cases potassium carbonate, has been shown to significantly enhance epoxy resin degradation and improve fiber recovery efficiency. Under optimized catalytic conditions, high recovery yields of up to 95.3% and tensile strength retention reaching approximately 98% of virgin fibers have been reported. Common trends across the reviewed studies include the beneficial role of catalysts in accelerating resin removal and the critical importance of carefully balancing temperature and pressure to maximize fiber quality. Despite these promising results, challenges remain for industrial-scale implementation, including energy consumption, ensuring consistent fiber quality, and achieving cost-effective processes.

(b) Sub–supercritical alcohol solvolysis

Raúl Piñero-Hernanz et al.⁷³ investigated subcritical and supercritical alcohol solvolysis for the recycling of CFRP waste using both batch-type and semi-continuous reactor configurations. Fig. 9 (ref. 28) illustrates a semi-continuous flow reactor employed for FRP recycling via subcritical alcohol techniques. In these studies, alcohols such as methanol, ethanol, 1-propanol, and acetone were used as reactive solvents over a temperature range of 200–450 °C to effectively degrade epoxy resin matrices, resulting in the recovery of carbon fibers retaining 85–99% of the tensile strength of virgin fibers. To enhance resin degradation and reduce temperature requirements, alkaline catalysts including NaOH, KOH, and CsOH were applied at concentrations ranging from 0.016 to 0.50 mol L⁻¹. The incorporation of these catalysts, particularly in combination with flow-assisted mass transfer in the semi-continuous mode, significantly improved resin removal efficiency. Optimal performance was achieved using 0.02 mol L⁻¹ KOH at 275 °C and a flow rate of 1.1 kg alcohol per kg fiber per min, yielding 96.5% resin removal, while near-complete decomposition (up to 98%) was obtained at 350 °C without damaging the fibers. SEM confirmed clean fiber surfaces, and thermogravimetric analysis (TGA) verified complete resin removal. The recycled fibers exhibited diameters of 6.95–7.18 µm, tensile strength of 3.44–4.07 GPa, Young's modulus of 181.2–207.9 GPa, and carbon purity of 79.8–97.5%. Overall, this approach represents an efficient route for recovering high-quality carbon fibers and addresses the growing demand for sustainable recycling of advanced composite materials.

Also, Xu et al.⁷⁴ developed a method for carbon fibers recovery using a solution of hydrogen peroxide (H₂O₂) and N,N-dimethylformamide (DMF) at 90 °C for 30 minutes, following pretreatment with acetic acid. This approach achieved a decomposition rate exceeding 90% while retaining over 95% of the original tensile strength in the recovered fibers.

Moreover, Mizuguchi et al.⁷⁵ introduced an innovative method based on thermal activation of semiconductors (TASC) for the recovery of fibers from CFRP and GFRP. TASC is a technology characterized by the use of the oxidative properties of defect electrons (i.e., holes) of semiconductors. In this technique, waste FRP was placed between two Cr₂O₃-coated honeycomb substrates and thermally activated in air at 400–500 °C for 10–20 minutes, leading to its decomposition into water and carbon dioxide, thereby enabling the separation of fibers from the matrix. Characterization of the recovered fibers was conducted using SEM, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and optical microscopy. The results indicated no discernible difference between the recovered fibers and virgin fibers, but the sizing agent had also totally been removed. Therefore, it is necessary to coat a sizing material again when the recovered fibers are reused for recycling.

In addition, Okajima et al.⁷⁶ investigated the recycling of CFRP using a supercritical methanol process in a batch reactor (Fig. 10) at 270 °C and 8 MPa for 1.5 hours. The decomposed resin extracts were blended with virgin epoxy resin, a promoter, and a curing agent to produce recycled CFRP, which exhibited comparable shear strength to that of virgin CFRP. However, the mechanical properties of the reclaimed fibers, i.e., tensile strength and interfacial strength, were reduced by 9% and 20%, respectively, compared to pristine fibers.

Furthermore, Okajima et al.⁷⁷ successfully recovered carbon fibers through a supercritical alcohol/ketone treatment (methanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, tert-butanol, acetone, or methyl ethyl ketone) at 320 °C for 6–120 minutes. In a batch-type reactor. The rate of decomposition depends on the type of solvent, with acetone providing better performance. Remarkably, the reclaimed fibers exhibited no reduction in tensile strength compared to the original fibers.

Overall, studies on CFRP recycling via sub-/supercritical alcohol solvolysis indicate that effective fibers recovery is typically achieved at temperatures in the range of approximately 275–350 °C, although broader operational windows extending from about 200 to 450 °C have also been reported depending on the solvent system and reactor configuration. Reaction times generally fall within tens of minutes to a few hours, with relatively short residence times often sufficient to achieve high resin removal efficiencies. Alkaline catalysts such as KOH, NaOH, and CsOH play a critical role in enhancing epoxy resin decomposition, with up to 96.5% resin removal achieved at 275 °C using 0.02 mol L⁻¹ KOH, and near-complete decomposition reported at higher temperatures even in the absence of catalysts. Commonly employed solvents include methanol, ethanol, and acetone, which facilitate effective matrix degradation and mass transfer. In most studies, the recovered carbon fibers retain approximately 85–99% of their original tensile strength, indicating limited degradation of mechanical properties, although some processes report moderate strength losses depending on treatment severity. Common trends across the reviewed literature include the strong influence of catalysts on reaction efficiency and the need to carefully optimize temperature and reaction time to balance resin removal with fiber quality preservation. Despite these promising laboratory-scale outcomes, challenges associated with solvent toxicity, energy consumption, process cost, and maintaining consistent fiber quality remain key obstacles to large-scale industrial implementation.

A comparative evaluation of sub-supercritical water and sub-supercritical alcohol solvolysis^66–77 indicates that both approaches operate within broadly comparable temperature and pressure ranges and employ similar alkaline catalytic strategies to promote epoxy resin decomposition. In both systems, effective recycling is typically achieved at temperatures between approximately 250 and 450 °C, resulting in high resin removal efficiencies and the recovery of carbon fibers with tensile strength retention generally exceeding 85% and, in many cases, approaching that of virgin fibers.

The underlying reaction mechanisms differ primarily in the nature of the reactive medium, with hydrolysis dominating in water-based systems and alcoholysis governing alcohol-based solvolysis. Despite this distinction, the practical outcomes in terms of fiber cleanliness, surface morphology, and mechanical performance are largely comparable. Alcohol-based solvolysis often benefits from enhanced resin solubility and improved mass transfer, enabling high resin removal at slightly lower temperatures or shorter reaction times in certain cases. In contrast, water-based solvolysis offers advantages related to the use of a non-toxic, non-flammable, and environmentally benign solvent, as well as simpler solvent recovery and post-treatment procedures.

From an industrial perspective, both approaches face common barriers to large-scale implementation, including high energy demand, high-pressure reactor design, catalyst management, and the need to ensure consistent fiber quality. The choice between water- and alcohol-based solvolysis therefore depends on a balance between process efficiency, environmental considerations, and economic feasibility.

While the preceding sections classified chemical recycling strategies primarily according to the reaction medium and process conditions, it is important to note that the effectiveness, reaction pathways, and degradation kinetics of chemical solvolysis are also strongly governed by the chemical nature of the polymer matrix. Thermoset resins such as epoxy and phenolic systems exhibit distinct crosslink densities, network bond chemistries, and chemical stabilities,⁷⁸ which directly influence their thermal resistance, degradation behavior, and suitability for sub-/supercritical water or alcohol-based recycling processes. Accordingly, to provide a clearer and more application-oriented perspective and to avoid ambiguity in classification, the following sections reorganize the discussion of chemical recycling methods based on the type of thermoset matrix. This complementary framework enables a more systematic comparison of recycling performance, reaction efficiency, and recovered fiber quality across different resin systems, while remaining consistent with the solvolysis mechanisms discussed earlier. In this context, chemical recovery routes for carbon fiber reinforced polymer composites are examined separately for epoxy, phenolic, and other thermoset substrates.

2.2.1. Chemical recycling of epoxy matrix

Cross-linked epoxy resin is a thermosetting resin that do not dissolve in an organic solvent. Therefore, it is difficult to recycle materials containing epoxy resins, especially CFRP. In the following, studies on the chemical recycling of these composites under different conditions are reviewed.

Hyde et al.,⁷⁹ supercritical propanol (scPrOH) was used as an extractant to recover carbon fibers from epoxy resin. The raw material consisted of a composite containing carbon fibers and epoxy resin, and the recovery process was conducted at temperatures above 450 °C and pressures exceeding 50 bar. This process resulted in the extraction of resin by breaking the amide cross-links in the polymer structure, and the recycled carbon fibers remained almost unchanged in tensile strength, similar to the original raw fibers. As can be seen in Fig. 11, the surface of the recovered fibers is largely free of residual epoxy resin deposits and therefore are comparable in distribution to other extraction methodologies. The final properties obtained, including the tensile strength of the recycled fibers and the residual chemical composition of the extracted resin, were investigated by tensile testing and chemical analysis, respectively. This method is simple, rapid, and applicable in a semi-continuous manner, and has been introduced as an acceptable method for the recovery of composites due to its complete ability to extract and analyze the resin and maintain the mechanical properties of the fibers.

Also, Li et al.,⁸⁰ used a combination of acetone and hydrogen peroxide (H₂O₂) as a mild oxidizing system for the chemical recycling of carbon fibers/epoxy composites. The starting materials consisted of composites containing carbon fibers and epoxy resin, and recycling was carried out at 60 °C for 30 minutes (Fig. 12). This process resulted in the decomposition of more than 90% of the epoxy resin and the recycling of carbon fibers with more than 95% of their initial strength. The obtained properties were characterized by tensile testing to evaluate the residual strength of the fibers and tests such as SEM, gas chromatography (GC), and Raman to investigate the composition of the residual organic products. This method not only effectively separates the resin from the fibers, but also allows the production of useful organic compounds under green and mild conditions.

Addition, Yang et al.⁸¹ performed chemical recycling of fiber-reinforced composites using a polyethylene glycol (PEG) and NaOH system, which consisted of epoxy composites reinforced with two types of carbon fibers (Torry T300 and T700S) and two types of glass fibers (alkali-free glass and medium-alkali glass). The recycling process was carried out by solvolysis at atmospheric pressure, 200 °C, for 4 h, and with a ratio of 0.1 g NaOH per gram of composite. The resin decomposition efficiency under these conditions was reported to be between 84.1–93%. The results show that the carbon fibers and alkali-free glass fibers retained up to 94–96% of their initial strength, while the medium-alkali glass fibers experienced a strength reduction below 90%. To examine the final properties, tensile testing was used to evaluate mechanical strength, SEM to observe the fiber surface and the amount of residual resin, as well as XPS and XRD to examine the surface chemical composition and the degree of graphitization of the recycled fibers.

Moreover, Wang et al.⁸² chose an efficient strategy for chemical recycling of cured epoxy resin from its CFRP using AlCl₃/CH₃COOH as the degradation system. Acetic acid swells the dense structures of cured epoxy resin, facilitating the penetration of the aluminum ion catalyst into the polymer matrix. The weakly coordinating aluminum ions in CH₃COOH solution selectively cleave the C–N bond while leaving the C–C, C–O (aryl alkyl ether) bonds intact. This process recovers valuable oligomers and carbon fibers from CFRP. The tertiary carbon–nitrogen bond cleavage was evidenced by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), elemental analysis, and gel permeation chromatography (GPC). These characterizations also showed that the C–C and C–O bonds remained intact (Fig. 13). Under the optimal degradation conditions (15 wt%, 180 °C, 6 h), the recovery yield of cured epoxy resin reaches up to 97.43 wt%, and the clean carbon fibers recovered conserved about 97.77% of their tensile strength compared with that of virgin carbon fibers. Especially, oligomers of cured epoxy resin can be preserved and recycled, which facilitates their reuse in resin manufacturing. This promising novel strategy is an environmentally benign and economically feasible method for chemical recycling of the CFRP composite.

In a study by Yu et al.,⁸³ the starting material was a carbon fiber reinforced epoxy composite. The recycling method involved immersing the composite in ethylene glycol, which acted as a solvent and triggered bond exchange reactions (BERs) within the covalent adaptable network (CAN) of the epoxy matrix. The process was conducted under heat, allowing the long polymer chains to break down into smaller segments, fully dissolving the matrix. After immersing the composite in ethylene glycol (EG) at 180 °C for 4 hours, the polymer matrix was fully dissolved, enabling the recovery of clean carbon fibers. Continued heating of the resulting polymer solution for an additional 3 hours led to the evaporation of excess EG solvent (see Fig. 14). The polymer solution was then transferred into a mold, with the reclaimed carbon fibers positioned at the center. Subsequent heating at 180 °C for approximately 10 hours resulted in the repolymerization of the epoxy and the fabrication of the first-generation recycled CFRP composite. The recovered carbon fibers remained clean and retained 97% of their original modulus and 95% of their tensile strength. Since the dissolved solution could be re-polymerized to form a new epoxy matrix, establishing a closed-loop recycling system with nearly 100% material recovery. Characterization of the recovered materials was performed using tensile strength and modulus testing to confirm mechanical performance, and solubility/dissolution rate analysis to evaluate the efficiency of matrix breakdown. The method proved to be environmentally friendly and maintained both fiber and matrix integrity for reuse.

La Rosa et al.⁸⁴ produced carbon fiber/epoxy composites via high-pressure resin transfer molding (HP-RTM) and chemically treated them to recover both clean carbon fibers and an epoxy-based thermoplastic polymer as recycled materials. The recycling process involved chemical solvolysis, where a sample of carbon fiber/epoxy (CF/epoxy) composite was treated with an acetic acid solution (25 vol% acetic acid) at 80 °C for 1.5 hours. The resulting mixture was then filtered, and the carbon fibers were separated from the liquid phase. The recovered fibers were dried and weighed. The acetic acid solution was subsequently neutralized using a sodium hydroxide (NaOH) solution (pH = 10), leading to the formation of a solid precipitate. After cooling, the mixture was filtered again, and the solid was washed with distilled water at approximately 40 °C. This solid redissolved during the washing process. Upon the addition of a few drops of NaOH solution, a white solid (an epoxy thermoplastic polymer) precipitated from the solution. So, this method enabled the recovery of clean carbon fibers and an epoxy-derived thermoplastic polymer. The study focused on developing a reactor to preserve the carbon fibers shape and mechanical properties during recovery, though further treatment is required to make them suitable for reuse.

Thermomechanical characterization confirmed the recycled materials' suitability for automotive structural components, emphasizing their recyclability as a key innovation. Life cycle assessment highlighted environmental benefits, particularly from avoiding virgin polyacrylonitrile (PAN)-based carbon fibers production. The recovered epoxy thermoplastic's chemical structure was validated through polymer characterization, though its reuse requires further analysis of thermomechanical properties. Future work aims to scale the process to plant level and integrate life cycle cost analysis, pending additional data on the thermoplastic's properties and reprocessing feasibility.

Furthermore, Jiang et al.⁸⁵ offered a green and efficient chemical recycling method was developed for thermosetting carbon fibers/epoxy resin composites. The raw materials included high-performance carbon fibers/epoxy composites, which were first pretreated with nitric acid to induce partial decomposition and layering of the material. This pretreatment improved solvent penetration in the subsequent step. The layered composites were then subjected to a solution of macrogol 400 (PEG-400) and KOH at 160 °C for 200 minutes under atmospheric pressure. This process enabled the complete dissolution of the cross-linked epoxy network and successful separation of clean carbon fibers, with a resin removal rate exceeding 95 wt%. The process operated under significantly milder conditions (lower temperature and shorter time) compared to conventional recycling techniques, contributing to lower energy consumption and minimal fiber damage. The recovered carbon fibers retained approximately 95% of their original tensile strength, and their surface appeared smooth and undamaged, free from residual resin or visible corrosion. This was confirmed using SEM and XPS, which showed surface characteristics similar to virgin fibers. Additionally, a dynamic contact angle test indicated improved surface wettability, suggesting enhanced interfacial bonding potential in future composite reapplications. While a slight decrease in mechanical strength was observed, the increase in surface polarity may be advantageous for resin infiltration. The liquid by-products retained key molecular structures such as bisphenol F-epichlorohydrin monomers or dimers, and future work will focus on repolymerizing these liquids to formulate reclaimed resin. Overall, this method offers a highly promising, waste-free, and industrially viable solution for the closed-loop recycling of thermosetting carbon fibers/epoxy composites.

In a study by Liu et al.⁸⁶ a mild and efficient chemical recycling method for aerospace-grade carbon fibers/epoxy composites with a high glass transition temperature (T_g ∼ 210 °C). The recycling approach used a ZnCl₂/ethanol catalyst system, where ZnCl₂ selectively coordinated with aliphatic C–N bonds in the epoxy matrix, and ethanol served as a swelling agent to facilitate penetration and degradation. The process was conducted at a relatively low temperature of 190 °C, enabling the cleavage of the epoxy matrix into oligomeric decomposed matrix polymer (DMP) while preserving the integrity of the carbon fibers. The recovered fibers were found to have smooth, clean surfaces with minimal damage, as confirmed by SEM. The DMP contains amine groups and hydroxyl groups and can serve as a reactive ingredient in the preparation of new epoxy materials without scarifying the mechanical properties at a certain level of incorporation. When up to 15 wt% of DMP was added to the formulation of new cross-linked epoxy materials, the resulting composites retained high mechanical strength and modulus, comparable to those made from virgin resin. The effectiveness of the recycling was validated through mechanical property tests and chemical characterization, showing that both the recovered fibers and recycled matrix are viable for reuse. Additionally, the ZnCl₂ catalyst could be recovered and reused with sustained efficiency, making the process not only technically effective but also economically and environmentally sustainable. This approach offers strong potential for closed-loop recycling in aerospace composite waste management.

In a study by Das et al.,⁸⁷ CFRP waste was recycled using peracetic acid (PAA), generated in situ from a mixture of 95 vol% 14 M acetic acid and 5 vol% 9 M hydrogen peroxide. The one-step oxidative degradation process, conducted under mild conditions at elevated temperature, achieved a 97% epoxy resin decomposition rate. Over 90% of the solvents were recovered through simple distillation, with 94% of the acetic acid retaining its original strength. The reclaimed carbon fibers were clean, long, and exhibited tensile strength comparable to virgin fibers. The recovered epoxy was reused along with an adhesive-grade epoxy (2 wt%) with no significant loss of tensile strength. Analytical techniques (ATR-FTIR, GPC, NMR, etc.) confirmed the formation of both low- and high-molecular-weight oxidation products. The solvent recovery process further decomposed the polymer matrix, generating phenolic compounds through combined oxidative and thermal degradation, without producing hazardous byproducts. This environmentally friendly approach enables the near-complete recycling of materials and solvents under mild conditions, offering a sustainable solution for CFRP waste management.

Also, Cheng et al.⁸⁸ worked on recycling CFRP composed of diglycidyl ether of bisphenol-A epoxy resin (DGEBA/7901) cured with amine and reinforced with 12K-A42 carbon fibers. The recycling process utilized supercritical fluids, including n-butanol, n-propanol, acetone, ethanol, methanol, and isopropanol. The degradation was performed by batch reactor (Fig. 15) at temperatures ranging from 280–360 °C, under pressures exceeding the critical pressure of each solvent, typically above 4.8 MPa, and reaction times ranging from 30–360 minutes. Among the solvents, supercritical n-butanol and n-propanol showed the most effective degradation, achieving a resin removal efficiency of over 82% at 360 °C for 240 minutes. The degradation occurred via scission of C–C, C–O, –O–, and C–N bonds in the epoxy matrix. The reaction order was determined to be 2 or 2.5, and kinetic parameters such as activation energy and rate constants were derived using Arrhenius equations and nonlinear fitting. The recycled carbon fibers retained about 98% of their original tensile strength, showing only ∼2% loss. Their surface morphology and composition were analyzed using SEM, AFM, and XPS, confirming a decrease in surface elements such as N, O, and Si, and an increase in C, indicating successful resin removal. Tensile strength and Weibull modulus were assessed per ISO 11566/11569 standards, demonstrating minimal mechanical degradation and suitability for reuse.

In the study of Keith et al.⁸⁹ investigated the recycling of CFRP composed of an RTM6 thermoset epoxy resin and Toray T700 6k carbon fibers using a supercritical solvent mixture of acetone and water in an 80 : 20 volume ratio. The recycling was conducted in a non-stirred batch reactor under temperatures ranging from 300–380 °C, pressures from 15.8–30.0 MPa, and reaction times up to 150 minutes. The optimal conditions for effective fiber recovery were determined to be 320 °C, 20 MPa, and 120 minutes, under which up to 95 wt% of the resin was decomposed, and the original fiber weave architecture was preserved. At higher temperatures (340–360 °C), complete degradation was achieved faster (15–45 min), but excessive heat damaged the fiber structure. The kinetics of degradation followed a first-order reaction, confirmed through modeling with both an Arrhenius equation and a reaction-rate-limited shrinking core model (SCM). Activation energies of 222.3 and 164.3 kJ mol⁻¹ were calculated for the Arrhenius and SCM models, respectively. Characterization techniques included TGA to measure resin content, X-ray computed tomography (XRCT) for studying solvent diffusion and structural integrity, and mass loss measurements before and after processing, ensuring precise determination of resin degradation.

In the study by Kim et al.,⁹⁰ CFRP composed of a cross-linked epoxy resin, were successfully recycled using an aqueous sodium hypochlorite (NaOCl) solution under mild and eco-friendly conditions, specifically at atmospheric pressure, a reaction temperature of 100 °C, and a reaction time of 2 hours, with an initial pH of 12.05. Fig. 16 outlines the complete recycling process followed in this study. The recycling process relied on the generation of hydroxyl radicals from NaOCl in water, which effectively depolymerized the epoxy resin, enabling the separation and recovery of carbon fibers. The reclaimed carbon fibers (r-CFs) retained physical and mechanical properties comparable to virgin carbon fibers (v-CFs). Additionally, both the recovered carbon fibers and resin decomposition products were reused to create value-added products, such as polyurethane foams (PUFs) and polypropylene (PP)/r-CF composite materials, demonstrating a high degree of recyclability. So that, the compounded r-CF/PP chips were placed in a stainless mold and hot-pressed at 130 °C under 5 MPa for 30 min to form a carbon fiber-reinforced thermoplastic (CFRTP). The characterization of the recovered materials included mechanical testing to compare physical properties of r-CFs with v-CFs and functional assessments of the new composite products to confirm usability and performance.

Liu et al.⁹¹ recycled CFRP waste, composed of epoxy resin and carbon fibers, using supercritical n-butanol in a process optimized at 330 °C, 6.5 MPa pressure, and a reaction time of 60 minutes. The feed ratio was 0.1 kg CFRP per liter of n-butanol, and KOH was used as a catalyst at a concentration of 0.0538 mol L⁻¹. The process was carried out in a pilot-scale supercritical reactor, resulting in a resin decomposition rate of over 99%. The recycled carbon fibers retained 95.17% of the tensile strength (2.76 GPa) and 93.17% of the tensile modulus (144.41 GPa) compared to virgin fibers. The fibers were analyzed using SEM to confirm clean surfaces without residue. The overall energy consumption for the recycling process was 49.21 MJ kg⁻¹, significantly lower than virgin carbon fiber production (229 MJ kg⁻¹), and the method demonstrated environmental benefits in seven life cycle categories, such as global warming potential and acidification. Furthermore, acetone cleaning and distillation were employed post-recycling to remove oily residues and recover solvents, while the liquid by-products were incinerated to generate electricity, contributing to the sustainability of the process.

Kim et al.⁹² presented a mild and eco-friendly method for recycling CFRP composed of ∼70% carbon fibers and 30% epoxy resin made from DGEBA and a Jeffamine hardener. The recycling process involved oxidative depolymerization using a 1 M aqueous potassium permanganate (KMnO₄) solution under ambient pressure, at a reaction temperature of 100 °C, and a reaction time of 2 hours. Before depolymerization, CFRP chips underwent pretreatment in acetic acid at 120 °C for 1 hour. The process achieved a maximum depolymerization degree of 94.38%, with a pseudo-first-order reaction rate constant of 2.45 × 10⁻² min⁻¹. The recovered carbon fibers maintained 90.65% of the tensile strength of virgin fibers (3.20 vs. 3.53 GPa), demonstrating minimal performance loss. The reclaimed fibers were characterized using SEM and energy-dispersive spectroscopy (EDS) to confirm clean surfaces and elemental composition, while TGA verified effective resin removal. Mechanical properties were tested per ASTM C1557-03 standard using single fiber tensile strength tests. This mild method avoids high temperatures and pressures, offering industrial feasibility and sustainability for CFRP recycling.

Wu et al.⁹³ made a recyclable CFRP using diglycidyl ester of aliphatic cyclo (DGEAC) and methylhexahydrophthalic anhydride (MHHPA) as the epoxy matrix, and T300 woven carbon fibers as reinforcement. The cross-linked matrix, formed by epoxy-anhydride curing, exhibited excellent thermal and mechanical stability with T_g of 126 °C and decomposition temperature (T_d5) of 300 °C. The recycling process involved transesterification reactions (TERs) in ethylene glycol using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a catalyst under mild conditions of 180 °C for 6–10 hours, achieving complete matrix degradation and 100% recovery of carbon fibers. The mechanism involved catalytic cleavage of ester bonds in the matrix by hydroxyl groups from EG, facilitated by TBD. Characterization techniques included SEM for observing fiber surface morphology, Raman spectroscopy to confirm chemical structure similarity with virgin carbon fibers, TGA and differential scanning calorimetry (DSC) for thermal behavior, and FT-IR for chemical analysis. The recycled carbon fibers maintained nearly 100% similarity to surface microscopic morphology and chemical structure compared with virgin carbon fibers, validating the process for efficient, non-destructive fiber recovery.

Torkaman et al.⁹⁴ developed a catalytic solvolysis method to recycle CFRP using Al(NO₃)₃·9H₂O or Fe(NO₃)₃·9H₂O as Lewis acid catalysts and dimethylacetamide (DMAc) as a solvent. The process involved swelling the epoxy matrix with DMAc, followed by catalytic cleavage of C–N bonds in the cured epoxy at 160 °C for 5 hours, achieving a 100% degradation ratio (Fig. 17). The Al(NO₃)₃·9H₂O catalyst proved superior, preserving the carbon fibers' mechanical and chemical properties, with 99.71% retention of tensile strength compared to virgin fibers. The degraded epoxy resin was repurposed as a co-hardener for new composites, maintaining 96.5% tensile strength when blended at 5 wt%. Recycled solvents retained their properties post-distillation. Characterization techniques (TGA, XRD, Raman spectroscopy, XPS, SEM-EDX, tensile testing, and DSC) confirmed the fibers' structural integrity, surface cleanliness, and thermal stability, while NMR and FT-IR analyzed the degraded epoxy's reusable components. The method offers an eco-friendly, energy-efficient alternative to conventional recycling, enabling near-complete recovery of fibers, resin, and solvents.

This research on the chemical recovery of carbon fibers reinforced epoxy matrix composites is summarized in Table 3.

Table 3. Summary of chemical recycling of carbon fibers reinforced epoxy matrix^a.

No.	Authors	Solvents	Temp./time	Pressure	Resin removal (%)	Fiber strength retention (%)
1	Hyde et al.⁷⁹	Supercritical propanol	>450 °C/—	>50 bar	High	∼Unchanged in tensile strength
2	Li et al.⁸⁰	Acetone + H₂O₂	60 °C/30 min	—	>90%	>95%
3	Yang et al.⁸¹	PEG + NaOH	200 °C/4 h	Atmospheric	84.1–93%	94–96% (CF)
4	Wang et al.⁸²	AlCl₃ + CH₃COOH	180 °C/6 h	Atmospheric	97.43%	97.77%
5	Yu et al.⁸³	Ethylene glycol	180 °C/4 h	—	Nearly 100%	95–97%
6	La Rosa et al.⁸⁴	Acetic acid	80 °C/1.5 h	—	High	High
7	Jiang et al.⁸⁵	PEG-400 + KOH	160 °C/200 min	Atmospheric	>95%	∼95%
8	Liu et al.⁸⁶	ZnCl₂ + ethanol	190 °C/—	—	High	High
9	Das et al.⁸⁷	Peracetic acid (in situ)	High/—	—	97%	Comparable to virgin CFs
10	Cheng et al.⁸⁸	Alcohols (e.g., n-butanol and etc.)	280–360 °C/30–360 min	>4.8 MPa	>82%	∼98%
11	Keith et al.⁸⁹	Acetone + H₂O (80 : 20)	320 °C/120 min	20 MPa	95%	Preserved architecture
12	Kim et al.⁹⁰	NaOCl (aq)	100 °C/2 h	Atmospheric	High	Comparable to virgin CFs
13	Liu et al.⁹¹	n-Butanol + KOH	330 °C/60 min	6.5 MPa	>99%	93.17–95.17%
14	Kim et al.⁹²	Pretreatment: acetic acid	120 °C/1 h	Atmospheric	94.38%	90.65%
14	Kim et al.⁹²	KMnO₄	100 °C/2 h	Atmospheric	94.38%	90.65%
15	Wu et al.⁹³	EG + TBD	180 °C/6–10 h	—	∼100%	∼100%
16	Torkaman et al.⁹⁴	DMAc + Al(NO₃)₃/Fe(NO₃)₃	160 °C/5 h	—	100%	Al(NO₃)₃ 99.71%
16	Torkaman et al.⁹⁴	DMAc + Al(NO₃)₃/Fe(NO₃)₃	160 °C/5 h	—	100%	Fe(NO₃)₃ 82.75%

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PEG = polyethylene glycol, EG = ethylene glycol, TBD = 1,5,7-triazabicyclo[4.4.0]dec-5-ene, DMAc = dimethylacetamide.

2.2.2. Chemical recycling of phenolic matrix

Carbon–carbon (C–C) composites are ceramic in nature and exhibit mechanical behavior ranging from brittle to pseudoplastic. These composites are produced by embedding carbon fibers into a carbon-based matrix. The primary goal, as with most composite systems, is to integrate the exceptional specific strength and stiffness of carbon fibers with the high thermal resistance of the carbon matrix. When the fibers are arranged in near-net shapes with multidirectional reinforcements, the resulting structure is ideally suited for high-temperature applications.⁹⁵ In the future, the recycling of C–C composites is explored, with particular emphasis on chemical treatment methods.

Autoclave-based thermal treatment has been demonstrated as a preservation-focused recycling route for carbon/phenolic composites. Oumam et al.⁹⁶ carbon fibers/phenolic resin composites were processed in a stainless-steel autoclave. The autoclave was heated to 390 °C at a heating rate of 16 °C min⁻¹, it was held at this temperature for 4 hours to almost completely extract the organic matrix from the composite, and the maximum pressure reached during the entire process was <0.5 MPa. Cooling occurred passively in the inert atmosphere of the furnace. The resulting fibers were washed with chloroform and dried at 60 °C after the thermal treatment. Both pristine and recovered carbon fibers were characterized using SEM. The analysis revealed that the diameter of the recovered fibers was greater than that of the original fibers (6.98 versus 6.80 µm), a difference attributed to the residual matrix deposited on the fibers. The mechanical properties of pristine and recovered fibers were also determined. The median values of fiber diameter, tensile strength, and Young's modulus were similar for both sets, confirming that the fibers were not significantly altered during the reaction. The tensile strength of recovered fibers was slightly lower than that of the pristine fibers (3494 vs. 3688 MPa). This method thus yields fibers with mechanical properties nearly identical to virgin material.

In addition, supercritical alcohol solvolysis offers a versatile pathway for recycling CFRP wastes and producing 3D-printable composite filaments. Huang et al.⁹⁷ examined a novel recycling process for CFRP waste. An additional 500 mL of n-butanol solution (at a concentration of 0.05 M) with potassium hydroxide as a catalyst was also added to the reactor containing 50 g of CFRP waste in their experimental setup, producing a supercritical phase reactor of 1000 mL. Later, the reactor was heated to 330 °C, where the n-butanol became endothermically supercritical to allow complete and effective decomposition of the trapped epoxy matrix within the pre-polymer waste. The reactor temperature was kept for 1 hour for total degradation of the matrix. The recovered carbon fibers were then reduced in size by ball milling after the thermal treatment. These fibers were then mixed with PEEK powder to create composite filaments via an extrusion process. Finally, the filament prepared in this work was used as feedstock in an FDM 3D printer for the fabrication of three-dimensional components. These filaments were successfully used as feedstock in FDM 3D printing, showcasing a closed-loop route from waste to functional components.

In the another study, Cho & Lim⁹⁸ developed a novel recycling method to convert waste phenol resin typically used in electric insulation boards and adhesives into carbon nanoparticles using subcritical and supercritical solvents such as methanol, ethanol, acetone, and water. Given the resin's insoluble and infusible nature, it is often incinerated or buried, contributing to environmental pollution. In this method synthesized carbon particles at much lower temperatures (350–380 °C) with a high-pressure batch reactor (up to 500 °C and 700 MPa), and the reaction time was defined such that the temperature inside the reactor reached the desired reaction temperature, and then cooling began. This work systematically investigated the characteristics of the recycled carbon particles as a function of the reactive solvent using FT-IR, XRD, TGA, SEM, and EDS analyses. The TGA and EDS results confirmed higher carbon content and thermal stability at this temperature, while FT-IR and XRD showed that the resulting carbon particles, regardless of solvent, had similar amorphous structures and chemical compositions. Importantly, the process enabled graphitization at significantly lower temperatures compared to traditional methods (>1200 °C), with methanol yielding the highest carbon content. This technique demonstrates that supercritical fluid treatment is a viable, energy-efficient method for producing high-quality carbon particles from phenol resin waste.

Furthermore, solvothermal degradation of carbon fibers-reinforced boron phenolic resin composites (CF/BPR) has proven effective for resin removal and performance enhancement. Deng et al.³¹ studied the recycling of carbon fibers by the solvothermal degradation of CF/BPR. From 1-propanol, water, and acetone were used as solvents for the degradation reaction, which was performed in a 50 mL Teflon-lined autoclave at 225–280 °C for 1–4 hours. KOH was used as an additive to speed up the degradation process (Fig. 18). The degradation rate (D_r) was as high as 98.3% and there existed few resins on the surface of the recycled carbon fibers. The removal of phenolic resin from the surface of recycled carbon fibers in this process was supported by SEM observation, which showed little residue of the phenolic and no obvious voids on the surface of the recycled carbon fibers. FT-IR analyses confirmed these results. The XPS data also suggested that the phenolic resin was almost completely degraded and removed, which resulted in a significant increase in the contact angle between the fibers and the deionized water. In addition, the XRD and Raman spectra indicated that the recovery fibers exhibited a slightly reduced degree of graphitization. Bending tests showed that composites produced from recycled fibers had better mechanical properties than those built up from virgin fibers. This superior performance can be ascribed to better interfacial adhesion between the fibers and the matrix due to the decrease in the oxygen-to-carbon ratio present on the fiber surfaces. This suggests that surface modifications during recycling may improve composite performance rather than compromise it.

Acidic oxidizing solvolysis achieves high-purity carbon fibers recovery with beneficial surface morphologies. Kulikova et al.⁹⁹ studied carbon composites with phenol-formaldehyde resin and showed that solvolysis in the presence of oxidizing agents in acidic media allows for an effective extraction of fibers. They used a setup consisting of a glass reactor with a stirrer and an external heating to the solvolysis process. The conditions of this research include operating temperatures from room temperature to 220 °C, reaction times from 1 to 24 hours, and atmospheric pressure up to 4 bar. Promising results have been obtained with a 1 : 1 volumetric mixture of nitric acid and orthophosphoric acid, along with the addition of 10 g of potassium chromate per 100 mL of the acidic solution. Nitric acid was used both as a strong acid and a strong oxidizing agent (acting as an electron acceptor in redox reactions). Furthermore, a few of potassium chromate and chromium oxide promoted the formation of peroxides, which hydrolyzed to produce active oxygen and water. The same mechanism of degradation was noticed when sulfuric and nitric acids were used in combination with chromium oxide, and this combination showed reasonably good carbon fiber extraction. Specifically, the optimal fiber liberation was accomplished through the application of a mixture of hydrogen peroxide, sulfuric acid, and water (in volumetric ratio 1 : 1 : 1.3). SEM microstructural analysis indicated trace amounts of residual elements on the fiber surfaces. These residues were dispersed and showed low adhesion, allowing easy removal by distilled water washing. The collected secondary carbon fibers showed high purity with resin removal efficiencies from 95–99.5%. Additionally, the diameter of the fibers was decreased by around 20% on average, which ranged from 9.49 µm for the primary fibers to 7.36 µm for the secondary fibers. It was also concluded that the roughness generated in the fiber surfaces could have an important impact on the mechanical performance of the composites using these recycled carbon fibers as reinforcement. These findings reinforce the role of oxidizing solvolysis in generating clean, functional fibers suitable for reuse in high-performance applications. This research on the chemical recovery of carbon fibers reinforced phenolic matrix composites is summarized in Table 4.

Table 4. Summary of chemical recycling of carbon fibers reinforced phenolic matrix.

No.	Authors	Composite type	Recycling method	Conditions (Temp./time/pressure)	Solvent	Resin removal (%)	Fiber strength retention (%)	Key outcomes
1	Oumam et al.⁹⁶	Carbon/phenolic	Autoclave thermal treatment	390 °C/4 h/<0.5 MPa	—	∼Completely	∼95% (3494 vs. 3688 MPa)	Minimal fiber degradation; slight matrix residue
2	Huang et al.⁹⁷	CFRP	Supercritical alcohol solvolysis + ball milling	330 °C/1 h/—	n-Butanol + KOH	Full matrix degradation	—	Filament made with PEEK; used for 3D printing
3	Cho & Lim⁹⁸	Phenolic resin	Sub/supercritical solvolysis	350–380 °C/–/—	Acetone, ethanol, methanol, water	Synthesized carbon particles	—	All solvents produced carbon particles of similar quality
4	Deng et al.³¹	CF/BPR (boron phenolic resin)	Solvothermal degradation	225–280 °C/1–4 h/—	1-Propanol, water, acetone + KOH	Degradation ratio (Dr) of the resin was as high as 98.3%	Higher flexural strength of RFs/BPR than VFs/BPR due to better interfaces resulting from reduced O/C ratio of fibers	Improved fiber–matrix adhesion due to surface chemistry
5	Kulikova et al.⁹⁹	CF/phenol–formaldehyde	Acidic oxidative solvolysis	25–220 °C/1–24 h/up to 4 bar	HNO₃, H₃PO₄, K₂CrO₄, H₂SO₄	95–99.5%	Diameter reduced by ∼20%	CFs with roughened surfaces; good for reinforcement

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2.2.3. Chemical recycling in other composite matrices

Yuan et al.¹⁰⁰ developed a recyclable carbon fibers-reinforced polyhexahydrotriazine (PHT) thermosetting composite using 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) and formaldehyde as raw materials. The recycling process involved depolymerizing the composite under mild and eco-friendly conditions using a 1 M HCl/THF aqueous-organic solution (H₂O/THF = 2/8) at room temperature (∼25 °C). Resin depolymerization occurred in two stages, which included dissolution into soluble oligomers in 3.1 hours and further hydrolysis into the original monomers (BAPP and formaldehyde) in 14.9 hours. BAPP was recovered via neutralization with NaHCO₃, filtration, and drying, achieving a 94.7% yield, while formaldehyde was confirmed using acetylacetone spectrophotometry (HJ 601-2011 standard). Simultaneously, fibers were recovered from the carbon fibers/PHT composite after 36 hours in the same solution, showing no structural or chemical degradation. SEM validated the integrity of the fiber surface, while XPS and XRD confirmed that the chemical composition remained unchanged. Monofilament tensile tests demonstrated full retention of mechanical strength, and the carbon fibers could be recycled through three cycles without performance loss. The recovered carbon fibers and BAPP were reused to fabricate new composites, which exhibited mechanical and thermal properties comparable to the original material. The flexural strength reached 829.7 MPa, and the flexural modulus was 54.8 GPa, as measured by ASTM D790, while short-beam strength was evaluated using ASTM D2344. The recycled composite showed high thermal stability with a decomposition temperature (T_d) of 376.3 °C, matching the performance of commercial epoxy and bismaleimide (BMI) composites. Additionally, chemical resistance to solvents, acids, and bases was preserved. Characterization techniques included TGA for thermal stability, DSC for T_g behavior, NMR for confirming chemical structure, and mechanical testing for assessing material strength. The study demonstrated a scalable, low-temperature, and pressure-free approach for the simultaneous recovery and reuse of both the resin and carbon fibers, offering a highly sustainable and circular solution for advanced composite recycling.

Building upon low-temperature, solvent-based recovery, Olivar et al.¹⁰¹ introduced an integrated chemical-biocatalytic upcycling strategy recycling method developed to recover both the carbon fibers and the polymer matrix from CFRP containing polystyrene-based thermoset matrices, crosslinked with unsaturated polyesters or vinyl esters. In step one, composite panels were pretreated in benzyl alcohol and tripotassium phosphate trihydrate (K₃PO₄·3H₂O) at 180 °C for 24 hours, facilitating swelling and partial depolymerization of the resin. Step two, the partially digested composites were subjected to chemical oxidation with manganese(ii) and cobalt(ii) nitrate salts, under mild thermal conditions, yielding benzoic acid as the primary degradation product. The isolated benzoic acid was subsequently upgraded via biocatalysis using an engineered strain of Aspergillus nidulans (strain LO11055) into high-value fine chemicals such as (2Z, 4Z, 6E)-octa-2,4,6-trienoic acid (OTA). Recovered carbon fibers were thoroughly characterized by SEM and XPS, which confirmed clean, structurally intact fibers with >97% tensile strength and >99% modulus retention relative to virgin fibers. XPS further revealed minimal disruption to sizing functionalities, which could be fully restored by a nitric acid treatment. The recovered fibers were successfully remanufactured into second-generation composites using aerospace-grade resin films and a vacuum-bag-only (VBO) curing process, maintaining structural integrity and mechanical performance. This method represents one of the integrated systems to recover full material value from both the fiber and matrix of CFRP through a tandem chemical-biological upcycling strategy. This research on the chemical recovery of other carbon fibers reinforced thermoset matrices is summarized in Table 5.

Table 5. Summary of chemical recycling of other carbon fibers reinforced thermoset matrix^a.

No.	Authors	Matrix type	Recycling method	Conditions	Resin recovery (%)	Fiber strength retention (%)	Key outcomes
1	Yuan et al.¹⁰⁰	PHT thermoset	Depolymerization by mild acid (HCl/THF)	∼25 °C, 36 h total	94.7% BAPP recovery; formaldehyde recovered; complete monomer regeneration	Full retention of mechanical strength (on 3 cycles)	Fully recyclable CFs and resin; new composites remanufactured with flexural strength of 829.7 MPa and T_d = 376.3 °C
2	Olivar et al.¹⁰¹	Polystyrene-based thermoset (crosslinked with UPR/VE)	Two-step chemical oxidation + biocatalysis	180 °C, 24 h (pretreatment in K₃PO₄·3H₂O) + mild oxidation with cobalt(ii) nitrate and manganese(ii) salts	Benzoic acid → upgraded to OTA via biocatalysis	>97% tensile, >99% modulus retention	Closed-loop system; recovery of CFs and conversion of matrix into high-value chemicals

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PHT = poly(hexahydrotriazine), BAPP = 2,2-bis[4-(4-aminophenoxy)phenyl]propane, UPR = unsaturated polyester, VE = vinyl ester, OTA= (2Z, 4Z, 6E)-octa-2,4,6-trienoic acid.

2.3. Thermal recycling

Thermal recycling of fiber-reinforced composites encompasses three primary methods fluidized-bed, pyrolysis, and microwave, which all aimed at recovering glass or carbon fibers, mineral fillers, and the polymer matrix.¹⁰² In these processes, the polymer matrix is decomposed into light gases (CO₂, H₂, CH₄), coal, oil, and grease (containing toluene, benzene, and ethylbenzene).^103,104 The coal by-product can be used as agricultural fertilizer, and the produced gases can be combusted for energy recovery, and also, the oil obtained can be used to produce new polymer resins.^6,20 Consequently, thermal recycling not only reclaims critical fiber reinforcements but also valorizes matrix decomposition products into useful fuels and feedstocks.

The efficiency of recycling and the quality of recovered fibers are strongly temperature-dependent, typically operating between 450–700 °C. At lower temperatures, more solids are produced, while at higher temperatures, the share of gas and oil increases. The operating temperature of the process also varies depending on the molecular weight of the polymer matrix. Polyesters decompose at lower temperatures, and epoxies or thermoplastics decompose at higher temperatures. This temperature–property relationship enables tailoring of the pyrolysis conditions to maximize fiber integrity or desired by-product yields.¹⁰⁵ Thus, precise temperature control is essential to balance fiber quality with efficient matrix conversion.

Sometimes, high-temperature thermal recycling is conducted directly in cement kilns, where the polymer composite waste is converted into energy, and other raw materials (fillers and fibers) are used to produce cement. However, only 10% of the fuel input to a cement kiln can be replaced by GFRP composites. This restriction underscores the need to optimize upstream recycling methods for broader material recirculation.^6,20 Therefore, while cement-kiln co-processing offers a practical disposal route, its potential for large-scale composite recovery remains limited. The thermal recovery method, as can be seen in Fig. 6, is divided into three methods Fluidized bed pyrolysis, Pyrolysis combustion, and Microwave pyrolysis, each of which is examined separately below.

(a) Fluidized-bed technique

Fluidized-bed recycling offers a thermally efficient route to reclaim both fibers and energy from composite waste. The recycling of FRP composite materials through the fluidized-bed technique was pioneered in the mid-1990s at the University of Nottingham. This method enables the recovery of high-grade fibers from both GFRP and CFRP, whether in the form of scrap or production waste.^106,107 In this process, the composite waste is initially reduced to particles approximately 25 mm in size. These particles are then introduced into a fluidized bed, typically consisting of silica sand. The sand is fluidized by a stream of hot air or nitrogen, maintained at a temperature range of 450–550 °C and a velocity of 0.4–1.0 m s⁻¹. Under these conditions, the polymer matrix undergoes thermal degradation and volatilisation, thereby liberating the embedded fibers and fillers. These liberated components are entrained in the gas stream and subsequently separated via a cyclone system. The separated materials are then conveyed into a secondary combustion chamber operating at a temperature of approximately 1000 °C. This stage ensures complete oxidation of any residual polymer, allowing for the recovery of thermal energy.²⁸ A schematic of this process can be seen in Fig. 19. This method valorizes both reinforcement recovery and energy production.¹⁰⁸

Surface analysis by Jiang et al. underscores the importance of texture in determining recycled fibers performance. Jiang et al.¹⁰⁹ examined the surface characteristics of various types of carbon fibers (T600S, T700S, and MR60H) recovered via the fluidized-bed process at a higher operating temperature of 550 °C and the same velocity of 1.0 m s⁻¹. Their findings emphasized the critical role of surface texture in determining the interfacial bonding performance of recycled fibers. Also, the surface properties of the recovered fibers were found to be comparable to those of virgin fibers. In this recycling process, due to the effect of heat in the air atmosphere, some surface hydroxyl groups are converted to carbonyl and carboxyl groups, and overall, the O/C ratio did not change significantly. It was also shown that the surface texture may play a dominant role in the interfacial bonding performance.

Investigations by Yip et al. reveal the balance between fiber cleanliness and mechanical retention at moderate processing conditions. Yip et al.¹⁰⁷ investigated the recycling of CFRP composites using the fluidized-bed technique. Their study focused on the recovery of discrete, randomly oriented fibers at a processing temperature of 450 °C and a fluidization velocity of 1.0 m s⁻¹. A range of analytical techniques, including SEM, XPS, image analysis, and tensile strength testing, were employed to characterize the recovered fibers. The results revealed that the fibers were clean, with an average length ranging between 5.9-9.5 mm. However, the tensile strength of the recovered fibers was reduced by approximately 75% compared to virgin fibers, while Young's modulus remained largely unaffected. As a result, low-temperature fluidized-bed recycling yields clean fibers with maintained stiffness but substantial strength loss.

(b) Pyrolysis technique

The pyrolysis technique enables comprehensive decomposition of polymer matrices in CFRP to recover fibers and valuable byproducts under various atmospheres and temperatures. The recycling of CFRP through pyrolysis involves thermal degradation of the organic matrix at temperatures ranging from 450–700 °C, conducted either in the presence or absence of oxygen, nitrogen, or, more recently, steam within a static pyrolysis reactor, as shown in Fig. 20.^28,59,110 During this process, the thermal decomposition of the matrix yields oily compounds, solid residues (char), and gaseous byproducts. The solid and oily products primarily consist of low molecular weight substances, while the gaseous fraction includes carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), hydrogen, and other hydrocarbons. The recovered fibers are typically contaminated with char residues; thus, to obtain clean fibers, a post-treatment step is required, involving combustion in a furnace at temperatures between 450–550 °C, as demonstrated in the case of glass fibers.¹¹¹ While most research has focused on glass fiber-reinforced plastics, this method is also applicable to recycling carbon fiber-reinforced plastics.¹¹² Therefore, pyrolysis offers a versatile route for decomposing CFRP matrices into recoverable materials while yielding energy-rich byproducts.

Early studies on the recycling of CFRP from waste materials using pyrolysis were first conducted by Ushikoshi et al.¹¹³ The researchers performed experiments at varying temperatures (400, 500, and 600 °C) and for 120 minutes. Their findings demonstrated that the recovered fibers retained favorable mechanical properties and surface morphology comparable to those of virgin fibers. Thus, early pyrolysis research validated the process's ability to recover CFRP fibers with near-virgin properties.

Temperature-dependent product distributions and strength retention in the pyrolysis process for recycling composites were quantified by Cunliffe et al.¹¹¹ In this study, the composite is made of a polymer matrix (including polyester, phenolic, epoxy, polypropylene, and vinyl ester) reinforced with carbon/glass fibers and calcium carbonate filler. Pyrolysis of recycled waste composites was investigated in a fixed-bed reactor in the temperature range of 350–800 °C. Results showed solid, liquid (oil/wax), and gaseous product yields for the epoxy matrix of 65.3–81.7 wt%, 18–31.7 wt%, and 0.2–3 wt%, respectively, depending on the pyrolysis temperature. The tensile strength of the recovered fibers retained 50–60% of the virgin fibers' strength at 400–650 °C but further degraded at higher temperatures (650–800 °C). Additionally, the composition of the gaseous products was found to be highly dependent on both temperature and the composite matrix. Consequently, optimizing pyrolysis temperature is critical for balancing yield distribution with fiber strength retention.

Meyer et al.¹¹⁴ optimized pyrolysis parameters to achieve high-purity fiber recovery in different atmospheres. They conducted a comprehensive study to optimize critical pyrolysis process parameters, including temperature variation, isothermal dwell time, and oven atmosphere (nitrogen versus synthetic air), utilizing TGA. The recovered fibers were characterized using SEM and Raman spectroscopy. Their findings revealed that in a nitrogen atmosphere, fiber decomposition was primarily temperature-dependent rather than time-dependent, with optimal carbon fibers recovery achieved at 700 °C. In contrast, under synthetic air conditions, decomposition exhibited dependence on both temperature and exposure time, with effective fiber recovery occurring at 600 °C after 120 minutes. Notably, the researchers also investigated a hybrid approach involving sequential pyrolysis at 550 °C for 2 hours in nitrogen, followed by 550 °C in air. This combined process yielded carbon fibers of exceptional purity, with surface characteristics comparable to virgin fibers.

Product yield profiles and organic compound generation from recycling composite waste composed of carbon fibers and polybenzoxazines resin were detailed by Nahil and Williams.¹¹⁵ They conducted pyrolysis experiments across a temperature range of 350–700 °C at a heating rate of 5 °C min⁻¹ for 1 hour. Their results demonstrated product yields of 70–83.6 wt% solids, 14–24.6 wt% liquids, and 0.7–3.8 wt% gases across the tested temperature spectrum. The liquid products comprised a mixture of organic compounds, predominantly aniline (63.9–66.8%). Gas-phase analysis revealed the presence of CO, CO₂, CH₄, hydrogen, and various hydrocarbon species. Thus, pyrolysis not only recovers fibers (retaining 90% of the mechanical properties of virgin carbon fibers) but also generates valuable chemical feedstocks and fuels.

Combined thermal-chemical treatments can further enhance surface adhesion without sacrificing fiber mechanics. Greco et al.¹¹⁶ employed a combined thermal-chemical treatment approach to enhance the adhesion properties and surface morphology of recovered carbon fibers. The fibers thermal treatment in air at either 450 °C or 600 °C for 90 minutes, followed by underwent chemical treatment in 5% nitric acid (HNO₃) for 30 minutes at 100 °C. Characterization using SEM and energy dispersive spectrometry revealed significant differences in fiber properties based on treatment temperature. The 600 °C thermal treatment resulted in substantial surface degradation, including fiber diameter reduction and diminished mechanical properties, compared to the 450 °C treatment. In contrast, the nitric acid treatment induced only partial surface modification while simultaneously improving fiber-matrix adhesion characteristics without compromising mechanical performance (tensile strength and modulus). Hence, optimized thermal pre-treatment combined with mild chemical steps yields recovered fibers with superior interfacial properties and preserved mechanics.

Therefore, it was observed that pyrolysis stands out as an effective method for recycling CFRP by balancing fiber integrity with resource and energy recovery. Also, pyrolysis is considered a more suitable recycling technique for CFRP due to its ability to preserve mechanical properties that are comparable to those of virgin fibers, with only a 5–20% reduction in tensile strength. While it is also applicable to GFRP, its effectiveness is relatively limited in comparison. One key advantage of pyrolysis over mechanical recycling is its capacity to retain long fibers, which contributes to the quality of the recovered material. Additionally, the process generates fuels, chemical compounds, and gases, the latter of which are typically reused to supply the energy required for the recycling process itself. The fibers recovered through this thermal recycling method can be repurposed for the manufacturing of new composite materials or products.²⁸ Consequently, pyrolysis remains a versatile and scalable solution for CFRP recycling, combining high-quality fiber recovery with sustainable byproduct utilization.

(c) Microwave technique

Microwave pyrolysis harnesses electromagnetic radiation for efficient thermal recycling of FRP composites. Microwaves are part of the electromagnetic spectrum and are located between infrared and radio waves, with 915 MHz and 2.45 GHz being the most commonly used frequencies in industrial applications.¹¹⁷ In microwave systems, a magnetron converts electrical energy into high-power microwave radiation and directs it into the processing chamber through a waveguide. One of the main advantages of microwave technology is that it heats materials from the inside, which allows for faster heat transfer and reduces energy loss to the surroundings, resulting in better energy efficiency.¹¹⁸ Over the past 15 years, microwave-based pyrolysis has proven to be an effective method for recovering both glass and carbon fibers from composite waste. This process breaks down the polymer matrix into gases and oils while recovering the fibers. It also offers economic benefits, as it consumes relatively low energy, approximately 10 MJ kg⁻¹, which is less than traditional recycling methods.²⁸ Thus, microwave pyrolysis offers an energy-efficient alternative for thermal composite recycling.

Optimizing the processing atmosphere is critical for defect-free fiber recovery in microwave recycling. Obunai et al.¹¹⁹ conducted experiments under three different atmospheric conditions, including air, nitrogen, and argon, using a gas flow rate of 2.5 L min⁻¹ to investigate the effectiveness of microwave irradiation in fiber recovery. Carbon fiber extraction using microwave can be considered in three stages. First, the carbon fibers in CFRP were joule heated by induced current by microwave irradiation. Second, the gasification of resin was promoted by the heated carbon fibers. Third, the gasified resin was decomposed by spark glow plasma induced by spark glow discharge between carbon fibers. Their results indicated that microwave treatment under an argon atmosphere was the most effective for recovering carbon fibers from composite waste without introducing structural defects. Moreover, the tensile strength of the fibers recovered using this technique was found to be comparable to that of virgin fibers or those obtained through conventional recycling methods. Hence, inert atmospheres like argon are essential for preserving fiber integrity during microwave pyrolysis.

Mechanical testing confirms high performance of microwave-recycled fibers despite minor strength losses. Lester et al.¹²⁰ successfully recovered carbon fibers from an epoxy resin matrix using the microwave technique and compared the properties and surface morphology of the recycled fibers with those of virgin fibers. The results showed that the recycled fibers exhibited a good surface topology, characterized by a smooth and homogeneous texture. Although there was a slight reduction in mechanical properties, the recycled fibers still retained relatively high performance, with a tensile strength of 3.26 GPa compared to 4.5 GPa for virgin fibers and a Young's modulus of 210 GPa compared to 234 GPa for the virgin counterparts. Therefore, microwave pyrolysis yields recycled fibers with near-virgin acceptable strength for reuse.

In short, thermal recycling is less sensitive to contaminated waste than chemical recycling. Among the thermal methods, fluidized-bed produces a cleaner product, but its quality may not reach that of virgin fibers. In pyrolysis, the presence of varying amounts of carbon on the recycled fibers may limit their reuse, thus requiring purification and treatment. Pyrolysis is a relatively more complex process than the fluidized-bed process, but it produces more useful organic products from the polymer. Finally, in microwave pyrolysis, the organic components of composite materials are decomposed through rapid microwave heating and low energy consumption in an inert atmosphere, producing recycled fibers with higher strength. The only drawback of this method is that this process is only available on a laboratory scale. Hence, while microwave pyrolysis offers superior energy efficiency and fiber quality, its current laboratory-scale status limits widespread industrial adoption.²⁰

2.3.1. Thermal recycling of epoxy matrix

Kim et al.¹²¹ developed a low-energy, two-step pyrolysis technique to optimize for recycling CFRP. The process first decomposed the epoxy matrix at 400 °C under a CO₂ atmosphere (200 mL min⁻¹ flow rate) for rapid resin-to-char conversion, followed by superheated steam treatment at 700 °C (1.2 mL min⁻¹ H₂O flow rate) for >40 minutes to remove carbonaceous residues. The conditions were optimized through activation energy analysis (9.60 kJ mol⁻¹ for CO₂ step, lowest among tested gases) and integral procedure decomposition temperature (IPDT) calculations. SEM confirmed complete resin removal after 60 minutes of steam treatment, while single-fiber tensile tests showed recycled fibers retained 90% of virgin tensile strength (3.95 GPa) and 102.5% modulus (211.1 GPa). Characterization included TGA for decomposition kinetics, SEM for surface cleanliness, and ASTM D1577-compliant tensile testing. This method reduced energy use by leveraging CO₂'s low-temperature efficiency and steam's oxidative power, offering a viable solution for CFRP waste recovery.

Moreover, Limburg et al.¹²² explored the thermal recycling of CFRP using a two-stage strategy, pyrolysis followed by oxidative treatment in CO₂ or water vapor to remove carbon residues while preserving fiber integrity. The process involved decomposing epoxy-based matrices (e.g., Araldite LY 556, Biresin CR141) in nitrogen until the desired temperature was reached, followed by residue removal in CO₂ or H₂O at 600–1000 °C for 5–15 minutes. The atmosphere was then switched back to nitrogen while cooling the sample. Optimal conditions were 600–700 °C, with higher temperatures reducing dwell times but risking fiber damage above 750 °C (e.g., surface degradation at 1000 °C in CO₂, likely due to production impurities). Reclaimed fibers exhibited no residues (confirmed via microscopy) and retained flexibility, unlike brittle nitrogen-treated fibers. Characterization methods included TGA to determine decomposition profiles and microscopy to assess surface cleanliness and structural integrity. While effective for most matrices, challenges included fiber shortening over cycles and damage variability, necessitating further research for heterogeneous waste streams. This process highlighted the critical role of temperature and atmosphere control in maximizing fiber recovery.

Furthermore, the method proved to be universally applicable, with CO₂ and water vapor outperforming oxygen-based treatments in minimizing thermal damage to carbon fibers. Key findings emphasized the influence of matrix type, treatment temperature, and weave structure on the efficiency of residue removal. Despite its success, some fibers demonstrated sensitivity to extreme temperatures, underscoring the importance of tailored processing. The study concluded that this approach enables high-quality fiber recovery.

Microwave thermolysis emerges as a rapid, energy-efficient alternative for carbon fiber recovery. In the study by Deng et al.,¹²³ carbon fibers were recovered CFRP waste composed of Formosa Plastics TC33-3K carbon fibers and E51 epoxy resin using microwave thermolysis and compared to traditional thermolysis (Fig. 21). The recycling process involved heating CFRP waste under an oxygen atmosphere using microwave energy at a frequency of 2.45 GHz. Optimal conditions for microwave treatment were identified at 450 °C for 13 minutes, achieving a recovery ratio of ∼85%, significantly higher than the 70% achieved by the traditional method. Compared to the traditional method, the reaction time was reduced by 56.67%, and the recovery ratio was increased by 15%. The method eliminated the need for high pressure, relying solely on controlled temperature and time. The recovered fibers were cleaner, showed minimal surface damage, and retained an average diameter close to virgin fibers (∼7.02 µm). Their structural and chemical properties were evaluated using TGA for thermal decomposition, field emission scanning electron microscopy (FESEM) for surface morphology, FT-IR for functional group analysis, XRD for crystallinity, and Raman spectroscopy for graphitization degree. Results showed the microwave method preserved the carbon fibers' structure better and led to a lower degree of oxidation and damage, making it a faster, more energy-efficient, and effective recycling approach. Also, it was observed that microwave-assisted thermolysis can significantly improve recovery yield and fiber integrity compared to conventional techniques.

A fast steam pyrolysis process using superheated steam enables efficient recovery of carbon fibers from epoxy-based CFRP. For this purpose, in the study by Jeong et al.,¹²⁴ carbon fibers were efficiently recovered from epoxy-based CFRP using a fast steam pyrolysis process in which superheated steam (H₂O) acted as the oxidizing agent. The raw materials included CFRP made from Toray T700 PAN-based carbon fibers and DGEBA epoxy resin cured with diaminodiphenylmethane (DDM). The pyrolysis was conducted in a home-made SiC tube furnace at temperatures ranging from 600–800 °C, with a heating rate of 10 °C min⁻¹, holding time (decomposition time) of 60 minutes, and a steam flow rate of 1 mL min⁻¹. At an optimal condition, the recovered fibers retained 66% of the original tensile strength (2.68 GPa) and 100% of the tensile modulus (197.05 GPa), with a 100% resin removal achieved in a total process time of 140 minutes. Characterization of the recovered fibers included SEM for surface morphology, Raman spectroscopy for defect and amorphous content analysis, XRD for crystallite structure, and XPS for chemical surface composition. Also, mechanical tests included tensile testing according to ASTM D1577, interfacial shear strength testing via micro-droplet method. Results revealed that steam pyrolysis effectively decomposed the epoxy matrix. Moreover, removed amorphous structures, and enhanced oxygen- and nitrogen-containing functional groups on the fiber surface. Therefore, improving interfacial shear strength to 34.2 MPa (120% of the original value). This technique can help the fast recycling of the CFRP and accelerate to reduce the CFRP landfilling because it has a good advantage from a commercial point of view.

Further, an optimized two-stage nitrogen–oxygen pyrolysis effectively removes resin while preserving carbon fiber morphology. In the study by Yatim et al.,¹²⁵ CFRP prepreg waste, consisting of approximately 60% carbon fibers and 40% epoxy resin, was recycled using a pyrolysis process. Recycling was carried out in a tube furnace under two atmospheric conditions of pure nitrogen and nitrogen followed by oxygen, both with a gas pressure of 5 cc/mm³. Based on the TGA results, there was a heating profile of nitrogen atmosphere from 250 to 420 °C followed by oxygen from 420 °C to a final temperature of 800 °C. Among the tested temperatures, 540 °C in the combination atmosphere was identified as optimal, allowing complete removal of resin while preserving the morphology of the recovered carbon fibers without significant degradation. The recovered carbon fibers obtained under these conditions showed clean surfaces, smooth cross-sectional ends, and a uniform spherical shape. These properties were characterized through SEM for morphological analysis, and Raman spectroscopy for structural analysis for monitoring decomposition stages. Raman spectra confirmed the presence of characteristic D and G bands of carbon, while additional peaks indicated minor residual resin, suggesting potential benefit from further purification. Hence, combined nitrogen–oxygen pyrolysis method achieved clean fiber morphology while minimizing degradation.

In addition, oxidative pyrolysis under varied oxygen concentrations models epoxy degradation kinetics and guides fiber recovery optimization. In the study by Ma et al.¹²⁶ DGEBA, cured with methylhexahydrophthalic anhydride and a curing accelerator, was subjected to oxidative pyrolysis to investigate its degradation behavior and support the recovery of carbon fibers from composites. Recycling experiments were conducted using TGA and a fixed bed reactor. First, the samples were heated at atmospheric pressure from room temperature to 800 °C at a rate of 10 °C min⁻¹ and then held at this temperature for 60 min. Also, different atmospheres containing 0% (pure nitrogen), 5%, 10% and air (21%) oxygen were used. The results showed that increasing O₂ concentrations significantly altered the thermal behavior of the epoxy resin by accelerating initial decomposition and shifting the oxidation of residue to lower temperatures. Major decomposition occurred between 368-405 °C, depending on the oxygen level. Product distribution included gases (CO₂, CO, CH₄), liquids (primarily methylcyclohexene and phenolic derivatives), and solid residue, with reduced liquid yield and increased gas formation under higher oxygen concentrations. The products were characterized using SEM for morphology, FT-IR for functional groups, gas chromatography-mass spectrometry (GC-MS) for liquid-phase composition, and micro-GC for gas analysis. Kinetic modeling was performed using a multi-distribution activation energy model (multi-DAEM) with six reaction steps. This modeling successfully simulated the complex thermal degradation mechanisms in different atmospheres. Furthermore, it provided a reliable framework for optimizing the thermal recycling of fiber/epoxy composites.

Moreover, microwave pyrolysis followed by thermal oxidation recovers high-performance carbon fibers with minimal damage and enhanced surface chemistry. In the study by Hao et al.,¹²⁷ carbon fibers were successfully recovered from cured carbon fibers/epoxy prepreg using a microwave pyrolysis process, followed by thermal oxidation to remove residual char. The raw material comprised unidirectional carbon fibers/epoxy prepregs made from Toray® T700 carbon fibers (63 wt%) and Bisphenol-A epoxy resin (37 wt%), pre-cured at 120 °C for 2 hours. The microwave pyrolysis was conducted in a nitrogen atmosphere at three different temperatures, 450, 550, and 650 °C, with each sample held isothermally for 15 minutes (Fig. 22). Post-pyrolysis oxidation was performed in air at 550 °C for 30 minutes. The recovered fibers were characterized through single fiber tensile testing (SFTT), SEM, XPS, GC, and GC-MS. The fibers showed a strength retention of 87.3% (at 450 °C), 82.9% (at 550 °C), and 80.6% (at 650 °C) relative to virgin fibers, with negligible change in tensile modulus. SEM images confirmed that higher pyrolysis temperatures led to reduced char on the fiber surface, while XPS analysis showed an increase in the O/C ratio from 0.253–0.331, indicating the formation of oxygen-containing functional groups. Pyrolytic gas products were rich in CO, H₂, CO₂, and CH₄, and the major liquid products included phenols and aromatics, which increased in yield with temperature. The microwave pyrolysis process investigated in this study demonstrated significant advantages over conventional thermal recycling methods by enabling selective and rapid heating of carbon fibers, resulting in improved time efficiency and reduced energy consumption. This technique effectively recovers high-quality carbon fibers with minimal mechanical degradation and enhanced surface chemistry, making them suitable for composite re-manufacturing. However, scaling up the process presents challenges, particularly in maintaining a controlled, closed environment for pyrolysis and efficiently transferring materials for subsequent oxidation.

In the energy sector, carbon fibers are known as good energy storage materials due to their adequate conducting and capacitive properties as supercapacitor electrodes. In this sense, the purpose of this work is to enable the use of recovered carbon fibers from the pyrolysis of carbon fiber epoxy composites as electrode material for supercapacitors and to evaluate the influence of thermochemical and chemical treatments on their capacitive properties. Thus, Rodrigues et al.¹²⁸ employed thermal pyrolysis as the recycling method to recover carbon fibers from structural epoxy matrix composites, targeting their application as electrode materials in supercapacitors. The methodology enabled effective degradation of the polymer matrix, resulting in carbon fibers that were largely free of resin residue, as confirmed by Field Emission SEM imaging. Following recovery, the carbon fibers underwent thermochemical and chemical surface treatments to evaluate their influence on electrochemical performance. Characterization techniques included Brunauer, Emmett, Teeler (BET) surface area analysis, FT-IR, Raman spectroscopy, and cyclic voltammetry. The BET area and FT-IR, Raman, and cyclic voltammetry analyses showed that thermochemical and chemical treatments carried out at the pyrolyzed carbon fiber did not contribute to improving the charge storage capacity at the electrode/electrolyte interface. On the contrary, the chemical treatment led to a great reduction in energy storage capacity due to the incorporation of OH and COOH groups. This proved that pyrolyzed carbon fiber can be used as an electrode material in supercapacitors without the need for further treatments. The charge/discharge curves show good cycling, and the specific capacitance values for the recycled carbon fiber of 34 Fg⁻¹, power density of 37.6 Wkg⁻¹, and energy density of 4.7 Whkg⁻¹ are within range for applications such as supercapacitor electrodes. The main values that enhance its application, such as specific capacitance, energy density, and power, are within the range presented by conventional supercapacitors. These values depended on the efficiency of epoxy resin removal in the carbon fibers recovery process by pyrolysis. The pyrolysis treatments affected the surface condition of the fibers, leading to the formation of functional groups. These groups altered the surface energy and, consequently, influenced the conductive and capacitive behavior of the recovered carbon fibers. Consequently, the study concluded that carbon fibers recovered from the pyrolysis process of CERFC waste showed potential as electrodes in supercapacitors.

Furthermore, Chen et al.¹²⁹ optimized oxidative pyrolysis parameters using design of experiments and genetic algorithms to maximize tensile strength retention in recycled fibers. They recycled carbon fibers reinforced epoxy composites using oxidative pyrolysis in air, employing T700SC-12K carbon fibers and 5313A epoxy resin as raw materials. The recycling process was designed using pyrolysis conducted at temperatures ranging from 375–545 °C and durations from 2.48–6.01 hours. Optimal mechanical performance was achieved at 428 °C for 4.0 hours, resulting in a matrix residue of 5.6% and a tensile strength retention of up to 80.11%, surpassing the industrial benchmark of 70% of the original fiber strength. Characterization of the recycled fibers involved tensile testing, and SEM, which revealed that moderate resin residue contributed positively to strength retention, while excessive oxidation degraded fiber integrity. The results suggest that temperature plays the most significant role in determining the properties of the fibers during pyrolysis and oxidation. Also, a small amount of resin residue can improve the tensile strength of the recycled carbon fibers. Moreover, when the temperature is kept constant between 400–520 °C, the pyrolysis time shows a parabolic relationship with the tensile strength of the recycled fibers. These findings highlight the critical role of controlled oxidative pyrolysis conditions in optimizing the mechanical performance of recycled carbon fibers. This research on the thermal recovery of carbon fiber reinforced epoxy matrix is summarized in Table 6.

Table 6. Summary of thermal recycling of carbon fibers reinforced epoxy matrix.

No.	Authors	Recycling method	Temp./time/atmosphere	Resin removal (%)	Fiber strength retention (%)	Key outcomes
1	Kim et al.¹²¹	Two-step pyrolysis: CO₂ + steam	400 °C (CO₂) + 700 °C (steam), >40 min	100%	90% strength, 102.5% modulus	Low energy, clean fibers, effective char removal
2	Limburg et al.¹²²	Two-stage pyrolysis + CO₂/H₂O oxidation	(N₂) + (oxidation) 500–1000 °C/5–15 min	High	—	Fiber integrity preserved; temperature-dependent damage
3	Deng et al.¹²³	Microwave thermolysis	450 °C/13 min/oxygen	∼85%	High (close to virgin)	Fast, low-damage process with superior surface quality
4	Jeong et al.¹²⁴	Fast steam pyrolysis	600–800 °C/140 min/superheated steam	100% (in a total process time of 140 min)	66% strength, 100% modulus	—
5	Yatim et al.¹²⁵	Nitrogen–oxygen pyrolysis	250–800 °C/dual atmosphere	Complete	High (no major degradation)	Clean morphology; optimal at 540 °C
6	Ma et al.¹²⁶	Oxidative pyrolysis under O₂ gradients	RT–800 °C/60 min/0–21% O₂/Pressure atmospheric	—	—	Kinetic model developed; resin oxidized faster at higher O₂
7	Hao et al.¹²⁷	Microwave pyrolysis + thermal oxidation	450, 550, 650 °C (N₂)/15min + 550 °C (air)/30 min	High	87.3% (450 °C), 82.9% (550 °C), 80.6% (650 °C)	Enhanced surface functionality (increase in the O/C), fast
8	Rodrigues et al.¹³⁰	Pyrolysis for energy storage	—	High	—	Fibers used as supercapacitor electrodes
9	Chen et al.¹²⁹	Oxidative pyrolysis	375–545 °C/2.48–6.01 h/air	Residue = 5.6% (optimum)	Up to 80%	Controlled residue improves fiber strength; time/temp optimized via genetic algorithms

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2.3.2. Thermal recycling of phenolic matrix

Meyer et al.¹¹⁴ have touched on different process parameters through the investigation and optimization of pyrolysis to recycle carbon fibers with properties similar to virgin ones. TGA was used to conduct laboratory-scale pyrolysis experiments while consciously varying pyrolysis temperature, isothermal residence time, and there, atmospheric conditions of the furnace. The heating rate was 10 °C min⁻¹, and the flow rate of gas used was 60 mL min⁻¹. TGA, SEM for surface visualization, and Raman spectroscopy provided valuable information on both the pyrolysis process and the resultant effect on the fibers. Furthermore, partial oxidation of the pyrolyzed carbon on the fiber surface was demonstrated while maintaining the integrity of the fibers. The recovered carbon fibers showed excellent mechanical properties comparable to virgin fibers and had a clean fiber surface free of residual pyrolytic carbon. Therefore, the study demonstrated the high potential of pyrolysis as a carbon fiber recovery method for production waste. Finally, optimized process parameters were determined from the experiments and an optimized pyrolysis test run in a semi-industrial pyrolysis plant was performed.

Furthermore, Giorgini et al.¹³¹ put CFRC waste in a reactor. The chamber was purged with nitrogen to remove any atmospheric air after the reactor was sealed. The system was heated at 8 °C min⁻¹⁻¹ until the desired temperature was achieved, and then held at this temperature for 150 min. The CFRCs pyrolysis experiments have been carried out in a batch pilot plant which is able to treat up to 70 kg of CFRCs materials per pyrolysis cycle. The pyrolysis batches have been performed isothermally at different temperatures, namely 450, 500, 550, and 600 °C, investigation. The oxidation step was implemented on the solid residue, which included carbon fibers coated with a carbonaceous layer, obtained from the pyrolysis of processed CFRC waste at 500 and 600 °C. The fibers (∼1 kg), after pyrolysis, were placed in a muffle furnace and held isothermally at either 500 or 600 °C for different residence times (10, 20, 30, 40, 50, and 60 minutes) to ensure complete removal of the organic residue that remained after pyrolysis. The morphology and chemical characteristics of the recycled fibers were analyzed using SEM and Raman spectroscopy. In addition, the reinforcing activity of the pyrolyzed fibers, as well as those obtained by the combined pyrolysis/oxidation process, was assessed with respect to virgin fibers in the preparation of new composites. The optimized two-stage pyrolysis/oxidation process confirmed that the fiber recovered at optimum conditions showed a similar performance to virgin fibers in composite use.

Moreover, Deng et al.¹³² developed the traditional two-step thermal decomposition and microwave process for the recovery of high-value carbon fibers from CFRP waste (Fig. 23). Various techniques, such as TGA, FESEM, XRD, FT-IR, and XPS, were used to appraise the properties of the recycled carbon fibers. In the oxidation by traditional two-step thermolysis method, the CFRP panels were heated in the argon atmosphere to 500 °C and kept at that temperature for 30 min. In the second step, the samples were subjected to oxidation at a temperature of 500 °C and an air atmosphere (with an approximate oxygen volume fraction of 21%, gas flow rate of ca. 200 mL min⁻¹) for 40 minutes. After cooling, the yield of clean carbon fibers recovered was 93.47%. The yield of recovery reached 94.49% when the a microwave-based method was used, and the oxidation temperature decreased by 50 °C compared to the conventional method that is, it reached 450 °C. In addition, the microwave method required only 12.70% heating time compared to the traditional technique. Thus, microwave heating was a more efficient process with higher recovery yield and lower energy consumption. Heating with a microwave had a modest effect on reducing the level of graphitization for the carbon fibers, but it did not change the chemical bonding in the fibers. The advantages of the microwave-based procedure in terms of processing speed, recovery efficiency, and energy requirements were significant when compared to the conventional thermal method.

Thermochemical recycling techniques like pyrolysis also require size reduction of composite waste through chopping and milling in the first step. Before processing in a thermal reactor, it is a critical step for homogenizing the waste size and for the specific surface area. A high-value approach was provided by Tan et al.¹³³ about the recycling of carbon/carbon friction composite materials (CFCM). This study harnesses the potential of solid-state shear milling (S³M) technology, which facilitates the ultrafine milling of the carbon matrix in C–C composites while maintaining the mechanical integrity and functionality of the carbon fibers. The CFCM waste was initially size-reduced with a powerful shredder. Thereafter, they further milled the material with an S³M device made by their research team. After 20 milling cycles, carbon fibers and carbon matrix were mostly separated. The composite powder obtained showed multifunctionality, including electrical conductivity (decreased from 332.31 S cm⁻¹ to 109.38 S cm⁻¹), thermal conductivity (decreased from 0.30 W mk⁻¹ to 0.21 W mk⁻¹), reinforcement, and wear resistance. SEM was employed to monitor the morphological changes during the S³M process. Post-milling analysis indicated that the carbon fibers became rougher with their surface having a micro-appearance due to the formation of longitudinal grooves distributed along the fiber body. Such surface modulations are thought to increase the interfacial interaction between the carbon fibers and the polymer matrix when the composite is being fabricated, which increases the roughness and adhesion of the material. Additionally, EDS results showed that the chopping and milling processes led to a higher surface oxygen content of the obtained CFCM. Such a surface oxidation could benefit the future application of the recycled composite. The tensile strength of the CFCM/PA6 composite increased from 64.35 MPa to 72.79 MPa, while impact strength increased by 93.43%. The thermal conductivity increased from 0.211 W mk⁻¹ to 0.611 W mk⁻¹. The friction coefficient was reduced from 0.51 to 0.36, a 25.4% reduction. The tensile elongation rate dropped from 39.37% to 8.22%, and impact energy decreased from 7.87 kJ m⁻² to 2.13 kJ m⁻². The tensile strength and modulus of the carbon fibers are still very high, but the resistance to shear forces is lower than on other materials. Such S³M-enabled surface modulations are poised to improve fiber-matrix adhesion in next-generation composite recycling. This research on the thermal recovery of carbon fiber reinforced phenolic matrix is summarized in Table 7.

Table 7. Summary of thermal recycling of carbon fibers reinforced phenolic matrix.

No.	Authors	Recycling method	Temp./time/atmosphere	Fiber recovery yield/retention
1	Meyer et al.¹¹⁴	Optimized pyrolysis via TGA	∼10 °C min⁻¹ rate; 60 mL min⁻¹ gas flow; multiple temps/times	Comparable to virgin
2	Giorgini et al.¹³¹	Two-stage pyrolysis + oxidation	Pyrolysis at 450–600 °C (N₂); oxidation up to 500 or 600 °C for 10–60 min	Comparable to virgin
3	Deng et al.¹³²	Traditional two-step thermolysis method: pyrolysis + oxidation	500 °C (Ar) for 30min + 500 °C (air) for 40 min	93.47%
3	Deng et al.¹³²	Microwave	500 °C (Ar) for 30 min + 450 °C (air) for 40 min	94.49%
4	Tan et al.¹³³	Solid-state shear milling (S³M)	—	High retention of strength/modulus

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2.3.3. Thermal recycling in other composite matrices

Gastelu et al.¹³⁴ presented the possibility of recovering chemical compounds through the thermo-catalytic treatment of the gases and vapors produced from the decomposition of the polymeric resin that takes place in the recycling of CFRP by pyrolysis. In this study, carbon fibers were recovered from waste poly(benzoxazine)-based CFRP using a pyrolysis process carried out at 500 °C for 30 minutes in a nitrogen atmosphere, followed by a thermo-catalytic treatment of the pyrolysis vapors at 900 °C. The raw materials consisted of expired pre-impregnated CFRP sheets reinforced with Toray T300/3K carbon fibers (61 wt%) and a poly(benzoxazine) resin (39 wt%). Two types of catalysts were used during vapor treatment: a commercial Ni on alumina catalyst and a lab-prepared Ni on ZSM-5 zeolite catalyst, with the latter offering higher acidity and surface area. The pyrolysis and catalytic experiments were performed in a lab-scale system consisting of two reactors in series, supported by condensation and gas analysis systems (Fig. 24). The recovered carbon fibers were characterized by TGA, SEM, AFM, elemental and proximate analysis, and surface roughness profiling, while pyrolysis vapors were analyzed using GC-MS and GC with thermal conductivity and flame ionization detectors (GC-TCD/FID). SEM and AFM confirmed partial resin removal at 15 °C min⁻¹ heating rates and improved surface cleanliness at 3 °C min⁻¹, with residual char observed on the fibers. The pyrolysis yielded solid carbon fibers with a carbon content of ∼91%, but some undecomposed resin and char remained, necessitating an oxidation step for full fiber purification. The thermo-catalytic treatment significantly reduced liquid by-products (by up to 90%) and produced hydrogen-rich gas fractions (exceeding 50 vol% H₂), particularly with zeolite-supported catalysts, highlighting both fibers recovery and gas valorization as co-benefits of the process. These results confirm the technical feasibility of combining pyrolysis with catalytic vapor treatment for efficient CFRP recycling and hydrogen production.

2.4. Comparison of traditional recycling methods

2.4.1. The trade-offs of traditional recycling methods

As summarized in Table 8, each recycling route involves important trade-offs between fiber quality retention, energy demand, process complexity, and environmental considerations. Mechanical recycling remains the most established approach due to its relatively simple operation and low energy consumption; however, the substantial reduction in fiber length and deterioration of mechanical properties restrict the performance of the recovered materials. In contrast, thermal recycling methods such as pyrolysis and fluidized-bed treatment can preserve a larger fraction of the fiber properties, although they generally require higher processing temperatures and may generate gaseous by-products. Chemical recycling offers the highest potential for maintaining fiber integrity and recovering resin-derived compounds, but in these processes, solvent recovery and purification are essential to improve economic feasibility and reduce environmental impacts. For this reason, additional separation and recycling units increase process complexity and operating costs compared to mechanical and thermal recovery methods that do not require solvent management systems.

Table 8. Comparative analysis of traditional recycling methods: trade-offs in mechanical, thermal, and chemical processes.

Comparative assessment		Traditional recycling methods
		Mechanical Recycling⁵⁹	Chemical recycling		Thermal recycling
			Sub/dupercritical fluid		Fluidized bed^{28,107,109,118}	Pyrolysis^{111,121,124,131,132}	Microwave^{119,123,127,132}
			Water^67–71	Alcohol^28,73,97,99	Fluidized bed^{28,107,109,118}	Pyrolysis^{111,121,124,131,132}	Microwave^{119,123,127,132}
Temperature		25–27 °C	∼250–300 °C/>374 °C	200–450 °C	450–550 °C	450–700 °C	450–650 °C
Pressure		0.5–1 MPa	∼5–15 MPa/10–30 MPa	5–15 MPa	—	Normal pressure	Atmospheric pressure
Fiber quality retention	Tensile strength retention	50–60%	∼90–98%	85–99%	75% reduction	50–90%	∼81–87%
	Recovered fiber length¹³⁵	5–10 mm	∼30 mm	10–50 mm	∼6–9.5 mm	∼5.6 mm	Not reported
	Resin removal rate	Not applicable	∼94–99%	∼98%	Clean fibers with maintained stiffness	100%	∼85–94%
Energy consumption^7,44,136		0.1–4.8 MJ kg⁻¹	21–91 MJ kg⁻¹		5–40 MJ kg⁻¹	24–30 MJ kg⁻¹	10 MJ kg⁻¹
Environment impact^135,137		Dust generation	Organic solvent generation, energy demand	Organic solvent generation, energy demand	Energy demand, organic solvent generation, poisonous gas	Energy demand, gaseous emissions, oil products	Low energy demand, gaseous products, oil
Solvent recovery¹³⁸		Not required	Essential for economic feasibility and reducing environmental impacts	Essential for economic feasibility and reducing environmental impacts	Not required	Not required	Not required

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Overall, the comparison highlights that no single recycling strategy can simultaneously maximize recovery efficiency, fiber performance, environmental compatibility, and economic viability. Therefore, selecting an appropriate recycling pathway requires balancing these competing trade-offs according to the targeted application and recovery objectives. Despite significant progress in recent years, important challenges remain, particularly in achieving high-strength recovery through milder and more sustainable processing routes. These considerations also underline the importance of evaluating recycling technologies not only from a technical perspective but also in terms of their technological maturity, which is further discussed in the 2.4.2 section on technology readiness level (TRL) scale assessment.

2.4.2. Technology readiness level (TRL) scale assessment

The TRL framework is widely employed across various industrial sectors to assess the maturity of emerging technologies, ranging from the initial observation of fundamental scientific principles to full-scale commercial deployment. In addition to evaluating technological maturity, the TRL concept provides a useful basis for identifying the capabilities, infrastructure, and resources required at different stages of technology development. In the present study, the classification proposed by Yang¹³⁹ was adopted to categorize recycling technologies according to their development status. Technologies at TRL 1–3 were considered laboratory-scale, those at TRL 4–6 were classified as pilot-scale, and technologies at TRL 7–9 were regarded as commercial-scale. This categorization facilitates a clearer comparison of the industrial readiness and deployment potential of different recycling approaches. The overall classification methodology is illustrated in Table 9.

Table 9. Yang TRL scale allocation: L-Lab, P- Pilot, C-Commercial¹⁴⁰.

TRL level	1	2	3	4	5	6	7	8	9
Scale	L	L	L	P	P	P	C	C	C

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The industrial deployment of composite recycling technologies depends not only on their technical performance but also on their technological maturity, scalability, and economic feasibility. As summarized in Table 10, mechanical recycling represents the most mature recycling route, having achieved the highest TRL and widespread commercial implementation due to its simple processing requirements, established infrastructure, and relatively low capital investment. In comparison, thermal recycling technologies are divided into experiment, pilot, and commercial scales.

Table 10. Comparison of the current TRL, scalability limitations, and commercialization status of traditional recycling methods.

Industrial analysis	Traditional recycling methods
	Mechanical recycling	Chemical recycling		Thermal recycling
		Sub/supercritical fluid		Fluidized bed	Pyrolysis	Microwave
		Water	Alcohol	Fluidized bed	Pyrolysis	Microwave
Industrial maturity¹³⁵	Commercial	Pilot	Pilot	Pilot	Commercial	Experiment
Scalability limitation	Reduced fiber quality and limited high-value applications	Environmental factors, high energy demand	Environmental factors, high energy demand	Reduction in the mechanical properties of fibers	High energy demand and emission control requirements	Difficulty designing large reactors, controlling heating at large scales
TRL	8	4	4	6	8	1–3
Representative implementation examples¹⁰⁴	University of Manchester	Hitachi chemical		Pilot plant at university of Nottingham	ELG carbon fiber (UK)	—
Processing capacity (tons per year)¹⁰⁴	_∼20 (calculated from lab capacity)	12		100	2000	—

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Among thermal recycling approaches, pyrolysis remains the most established method for carbon fibers recovery. Its operational flexibility and applicability to waste generated throughout the composite manufacturing chain have facilitated its commercialization, with a reported TRL of approximately 8, indicating near-commercial maturity and large-scale applicability.¹⁰⁴ Nevertheless, energy consumption and emission management continue to represent important economic and environmental challenges for broader deployment. Despite these limitations, pyrolysis currently offers one of the most practical pathways for industrial-scale carbon fiber recycling.

Chemical recycling technologies, particularly solvolysis, have attracted growing interest because of their ability to recover carbon fibers with minimal damage while operating with lower environmental impacts and, in some cases, lower energy requirements.¹⁴¹ However, most solvolysis-based processes remain at the laboratory or pilot stage, with a typical TRL of around 4.^40,140 Their industrial implementation is constrained by challenges associated with solvent recovery, corrosion-resistant equipment, process optimization, and overall economic viability. Although significant progress has been achieved in improving fiber recovery quality, further technological advancements are required to overcome scale-up barriers and enhance process sustainability. These limitations have stimulated increasing research efforts toward advanced recycling technologies that combine high recovery efficiency with improved environmental and economic performance. The 2.5 section reviews these emerging approaches and discusses their potential to address the shortcomings of conventional mechanical, thermal, and chemical recycling methods.

2.5. Advanced recycling methods

Advanced recycling methods for CFRP, including solvothermal, carbonization, electrochemical, sonochemical, and biological techniques, offer significant advantages over traditional thermal, chemical, and mechanical approaches. Unlike conventional methods that often involve harsh conditions, high energy consumption, or fiber damage, these novel techniques enable efficient resin decomposition while preserving the structural integrity and mechanical performance of recovered carbon fibers. The solvothermal method utilizes moderate temperatures and organic solvents to break down cross-linked thermoset matrices through a synergistic combination of swelling, delamination, and catalytic degradation, achieving over 98% decomposition with minimal fiber damage. Carbonization, on the other hand, uses high temperatures under inert atmospheres to convert the polymer matrix into pyrolytic carbon, enabling clean fiber recovery for high-performance applications such as C–C composites. Electrochemical recycling introduces a low-temperature, catalyst-assisted process that promotes selective bond cleavage in the resin via electrical current, allowing for nearly complete resin removal and high-quality fiber recovery. Sonochemical recycling, on the other hand, employs ultrasonic irradiation to generate acoustic cavitation, which enhances mass transfer and promotes resin degradation, typically in combination with chemical oxidants. Although often used as an assisting technique rather than a standalone process, sonochemical methods have demonstrated effective resin removal and excellent fiber quality under mild operating conditions. Also, biological recycling is an emerging and less explored approach for waste FRP composites that use microorganisms or enzymes to degrade the polymer matrix under mild conditions and has the potential for low energy consumption and negligible environmental impact. Collectively, these advanced methods not only enhance environmental sustainability by reducing toxic waste and energy use, but also improve scalability, cost-effectiveness, and the commercial value of recycled fibers, overcoming major limitations of conventional mechanical, thermal, and chemical recycling strategies.

2.5.1. Solvothermal recycling

The solvothermal method is an advanced chemical recycling technique for decomposing thermoset composites, particularly CFRP, under moderate temperatures and pressures using organic solvents (e.g., 1-propanol, acetone, benzyl alcohol) and alkaline catalysts (e.g., KOH). This process leverages synergistic effects of physical swelling (solvent penetration), mechanical impact (stirring-induced delamination), and chemical degradation (hydrolysis/oxidation) to break down cross-linked matrices like boron phenolic resin (BPR) without damaging the embedded carbon fibers. Optimal conditions typically range between 225–280 °C and 1–4 hours, achieving decomposition ratios exceeding 98%. This method preserves fiber integrity, which can be confirmed by characterization techniques (SEM, XPS, Raman spectroscopy), and also allows for the recovery of high-quality fibers with minimal resin residue. Recycled fibers exhibit enhanced interfacial bonding in reprocessed composites, improving mechanical properties (e.g., flexural/punch shear strength) and functional performance (e.g., electromagnetic shielding). By combining efficiency, scalability, and environmental benefits, solvothermal recycling offers a promising pathway for sustainable CFRP waste management and closed-loop material reuse.^31,142

Deng et al.³¹ first recovered high-quality carbon fibers from carbon fibers-reinforced boron phenolic resin (CF/BPR) composites using a solvothermal decomposition method. The raw materials included CF/BPR composites, with 1-propanol, water, and acetone used as solvents, and potassium hydroxide (KOH) as an additive to enhance degradation. The recycling was performed in a 50 mL Teflon-lined autoclave at temperatures ranging from 225–280 °C for 1–4 hours, with optimal conditions being 3 M KOH, 4 hours of reaction time, and 280 °C, yielding a high degradation ratio of 98.3%. Surface analyses showed few resin residues and no visible damage on the RFs. Characterization techniques such as SEM, FT-IR, XPS, contact angle measurements, XRD, and Raman spectroscopy were employed to assess the microstructure, surface chemistry, and graphitization degree. FT-IR and GC-MS were used to analyze degradation products, revealing the breakdown of B–O bonds and benzene rings, as well as the occurrence of the Guerbet reaction involving 1-propanol. Second, the recycled fibers were reused to fabricate new composites (recycled CF/BPR), which exhibited superior flexural strength compared to virgin fiber composites, attributed to improved fiber-matrix interfacial bonding due to the reduced O/C ratio on the recycled CF surfaces.

In a follow-up investigation, the same authors, Deng et al.,¹⁴² explored the solvothermal method for recycling carbon fiber-reinforced boron phenolic resin (CF/BPR) composites. This material is known for its high-performance applications and strong resistance to degradation due to its highly cross-linked network. Because of these properties, recycling CF/BPR composites presents particular challenges. To address this, the study examined the recycling process under different experimental conditions. Raw materials included CF/BPR composites, with solvents such as benzyl alcohol, acetone, and water, and potassium hydroxide (KOH) as the alkaline additive. The recycling was performed in a Teflon-lined autoclave at 260 °C for 3 hours with 0.5 g KOH, achieving a decomposition ratio as high as 99.78% (Fig. 25). Decomposition occurred through a synergistic mechanism involving physical swelling, mechanical impact, and chemical degradation, aided by the release of reactive species like hydrogen free radicals. Characterization of the recycled carbon fibers using SEM, AFM, FT-IR, XPS, XRD, Raman spectroscopy, and GC-MS confirmed nearly complete resin removal, no structural damage, reduced O/C ratio, and increased surface roughness. The recycled carbon fibers were reused to fabricate new composites (recycled CF/BPR) via compression molding. These new composites exhibited superior mechanical properties, such as punch-type shear strength. They also showed enhanced electromagnetic shielding performance in the X-band frequency. These improvements were attributed to better fiber-matrix interfaces and more effective conductive networks. This work demonstrates a scalable, eco-friendly approach to recycling CF/BPR waste and reusing recycled carbon fibers in high-performance structural and functional applications.

2.5.2. Carbonization recycling

Carbonization is a thermal recycling technique in which organic materials such as polymer resins are heated in the presence of an inert gas (usually argon or nitrogen) at high temperatures (usually between 600–1000 °C) to decompose volatile compounds and leave only solid carbon. In this process, the polymer matrix decomposes and becomes pyrolytic carbon, while reinforcing fibers such as carbon fibers do not oxidize due to the absence of oxygen and maintain their structure. The purpose of carbonization in composite recycling is to effectively separate or convert the polymer matrix into a usable form and recover the fibers with the least possible damage. This method has less waste and better energy efficiency than chemical or mechanical methods and is suitable for the production of high-value-added products such as C–C composites.³⁰

A novel recycling strategy for CFRP has been developed by Guo et al.,³⁰ offering a sustainable solution for repurposing waste materials into high-performance composites. Specifically, the method converts carbon fiber/epoxy (CF/EP) composites into C–C composites using a thermal carbonization process. The raw materials consisted of CF/EP composites containing 43.51% PAN-based T300 carbon fibers and 56.49% epoxy resin, with hexaphenoxy-cyclotriphosphazene (HPCTP) as a charring agent. The recycling method involved thermal carbonization through controlled pyrolysis in an inert argon atmosphere. The process included several steps, starting with pre-pyrolysis at 350 °C for 5 minutes, followed by HPCTP impregnation at 150 °C for 10 minutes. This was then followed by pyrolysis at 800 °C for 60 minutes and densification through four cycles up to 1000 °C using furfural-acetone resin. The process flowsheet of recycling CF/EP by carbonization and preparation of rCF/C composites are shown in Fig. 26. The results demonstrated that the recycled carbon fibers experienced minimal damage and retained properties comparable to those of virgin carbon fibers. The resulting recycled carbon fibers/C composites demonstrated high density, strong interfacial bonding (12.6 MPa vs. 13.0 MPa for virgin carbon fibers/C), similar bending strength (106.4 MPa vs. 111.5 MPa), and nearly equivalent ablation resistance. Characterization techniques, including TGA, SEM, mechanical testing, and ablation analysis, confirmed that recycled carbon fibers/C composites are viable substitutes for virgin carbon fibers/C composites, providing a sustainable, high-value recycling solution for discarded CFRP.

2.5.3. Electrochemical recycling

The electrochemical recycling method for recovering carbon fibers from polymer matrix composites involves the selective decomposition of the epoxy resin matrix through an electrically assisted catalytic reactions. In this technique, CFRP are immersed in a nontoxic aqueous electrolyte and subjected to an applied electrical current under ambient temperature and pressure. The electrical stimulation promotes heterogeneous catalytic reactions that cleave the C–N and other bonds in the epoxy resin without harming the carbon fibers. This results in efficient resin degradation and the recovery of clean, structurally intact carbon fibers. The process maintains up to 90% of the original tensile strength and improves the interfacial bonding capacity of the fibers, making them suitable for reuse. Compared to conventional methods, this electrochemical approach is simpler, environmentally friendly, and scalable for industrial applications.³²

For example, Sun et al.¹⁴³ developed a notable electrochemical approach for carbon fibers recovery. A schematic representation of the electrochemical recycling system for CFRP is shown in Fig. 27.²⁸ The experimental investigation evaluated various sodium chloride (NaCl) concentrations (3, 10, and 20) at different current levels (4, 10, 20, and 25 mA). The recycled carbon fibers were characterized by using the single fiber tensile strength test, SEM, XRD, and XPS techniques. The results demonstrated that optimal tensile strength retention (80% of the original value) was achieved using a 3% NaCl solution at 25 mA current.

Furthermore, Zhu et al.³² introduced an innovative electrically driven heterogeneous catalytic (EHD) method for recycling CFRP by decomposing the epoxy resin while preserving the carbon fibers. The process uses a nontoxic aqueous NaCl-based electrolyte and a KOH catalyst under ambient temperature and pressure, eliminating the need for harsh conditions. By applying an electrical current, the C–N bonds in the epoxy resin break via catalytic degradation, enabling nearly 100% resin removal without damaging the fibers. The reclaimed carbon fibers retained 90% of the tensile strength and 121% of the interfacial shear strength (IFSS) compared to virgin carbon fibers, as confirmed by mechanical testing (tensile and IFSS measurements). Characterization techniques such as spectroscopic analysis (XPS for bond cleavage verification) and microscopy (SEM for fiber integrity) validated the efficiency of the method. Unlike conventional recycling techniques, this approach is simple, scalable, and environmentally friendly, as it avoids high temperatures, toxic chemicals, and pressure reactors. The method also removes size restrictions on CFRP waste, enhancing the commercial viability of reclaimed carbon fibers for large-scale applications.

In comparison with other advanced recycling methods such as solvothermal processing and carbonization, electrochemical recycling offers several advantages, including mild operating conditions, reduced environmental impact, and effective preservation of fiber properties. However, each method presents inherent limitations. Solvothermal methods require high-pressure equipment and solvents, raising safety and cost concerns. Carbonization consumes a lot of energy due to high temperatures and may affect fiber surfaces. Electrochemical recycling, while eco-friendly, can be slow and sensitive to operating conditions. All methods may need additional post-processing, which can impact scalability and cost-effectiveness.

2.5.4. Sonochemical recycling

Sonochemical recycling has attracted attention as an alternative and assisting approach for the recovery of carbon fibers from CFRP waste. Unlike conventional thermal or high-pressure chemical recycling methods, sonochemical processes rely on ultrasonic irradiation to induce acoustic cavitation in liquid media. The collapse of cavitation bubbles generates localized extreme conditions of temperature and pressure at the microscale, which significantly enhance mass transfer, radical formation, and reaction kinetics while maintaining relatively mild bulk operating conditions.³³

These unique characteristics enable efficient degradation of thermoset polymer matrices when combined with mild oxidizing agents, without subjecting the fibers to severe thermal or mechanical damage. As a result, sonochemical recycling offers a promising route for resin removal with high fiber quality retention and reduced chemical severity. Although the technique requires electrical energy input for ultrasound generation, its ability to operate at ambient pressure and moderate temperatures makes it an attractive complementary strategy for sustainable CFRP recycling.³³ The sonochemical technique as an auxiliary approach involves the application of ultrasound to the traditional chemical recycling method, which is schematically depicted in Fig. 28.²⁵

In this context, Das and Varughese³³ investigated a sonochemical approach utilizing a combination of dilute nitric acid and H₂O₂ under ultrasonic irradiation for carbon fibers reclamation. Their results demonstrated excellent retention of tensile strength in the recovered fibers. Comprehensive characterization using SEM and TGA confirmed the effectiveness of the method, revealing clean, defect-free fibers with complete removal of epoxy resin residues. The maximum decomposition ratio of epoxy resin was 95%.

2.5.5. Biological recycling

Biological recycling has emerged as a promising alternative to conventional thermal and chemical recycling methods for CFRPs, owing to its potential to selectively degrade the polymer matrix while preserving the integrity of the reinforcing fibers. This approach exploits the metabolic and enzymatic capabilities of microorganisms to decompose polymeric materials under mild operating conditions, thereby reducing energy consumption and avoiding the use of environmentally hazardous chemicals.^144,145 Numerous bacteria, fungi, and algae have been reported to degrade a wide range of synthetic polymers.¹⁴⁶ However, the biodegradation of thermoset epoxy resins remains particularly challenging due to their highly crosslinked network structure and the absence of readily hydrolysable ester or amide bonds, despite the presence of heteroatoms in the polymer backbone.^147,148 Although research on the biodegradation of epoxy resins and CFRPs is still in its infancy, recent studies have demonstrated the potential of microorganisms and isolated enzymes to interact with and modify epoxy matrices, highlighting biological recycling as a potentially sustainable and resource-efficient route for future CFRP waste management. The following reviews several representative studies that have explored microbial and enzymatic approaches for the degradation and recycling of epoxy-based CFRP composites.

The first evidence of biological degradation of fiber-reinforced polymer composites (FRPCs) was reported by Gu et al.³⁵ in the 1990s. They investigated the biodegradation of a graphite-fiber-reinforced epoxy composite using a fungal consortium consisting of Aspergillus versicolor, Cladosporium cladosporioides, and Chaetomium sp. After 30 days of incubation, the fungi successfully colonized the composite surface, formed biofilms, and penetrated the material. SEM analysis confirmed degradation at the fiber–matrix interface, while electrochemical impedance spectroscopy (EIS) measurements revealed a decrease in impedance and an increase in pore size and density, indicating structural deterioration of the composite. Mechanical testing further showed reduced interfacial bonding strength, leading to fiber debonding and partial separation. The authors also found that resin constituents and fiber sizing agents could serve as carbon and energy sources for fungal growth. These findings provided the first clear evidence that FRPCs are susceptible to microbial attack and demonstrated the potential of biological processes for composite waste recycling.

Moeser et al.¹⁴⁵ investigated microbial and enzymatic degradation of bisphenol-A-based epoxy resins as a potential recycling route for CFRPs. Fig. 29 illustrates the mechanism of epoxy resin degradation by microorganisms. Although no detectable degradation was observed during enzyme incubation, microbial treatment of uncured epoxy resin resulted in significant structural modifications, with large epoxy droplets fragmenting into smaller droplets after several days of incubation. These findings provided the first evidence of microbial attack on epoxy resins and demonstrated the feasibility of biologically assisted recycling of CFRPs. However, complete resin degradation and carbon fibers recovery were not achieved, and the mechanical properties of the recovered fibers could not be assessed. The authors concluded that enzymatic recycling represents a promising environmentally friendly alternative that operates without high temperatures, high pressures, or aggressive chemicals, while offering potential for the future recovery of high-quality carbon fibers.

Dolz et al.¹⁴⁹ reported the first systematic investigation of enzyme-engineering approaches for the degradation of aerospace-grade epoxy resin. The authors developed a directed-evolution platform to design enzyme variants with enhanced composite-degrading capabilities, highlighting the future potential of biocatalytic recycling, although direct degradation of the epoxy matrix has not yet been experimentally demonstrated.

Overall, biological recycling of CFRPs represents a promising yet still immature strategy for sustainable composite waste management. Current studies demonstrate that microorganisms and enzymes can interact with epoxy-based materials, causing surface colonization, interfacial degradation, and in some cases partial structural modification, thereby confirming the feasibility of biologically assisted recycling. However, the technology remains constrained by incomplete resin removal, slow degradation rates, limited understanding of degradation mechanisms, and the lack of large-scale implementation. Despite these challenges, biological recycling offers important theoretical advantages, particularly its very low energy demand, minimal reliance on hazardous chemicals, and reduced greenhouse gas emissions compared with conventional thermal and chemical recycling methods. From a mechanistic perspective, findings suggest that although epoxy thermosets exhibit high resistance to biodegradation, they are not completely inert to biological attack under specific conditions. Consequently, while practical application remains limited at present, the approach holds significant potential for the development of environmentally sustainable and resource-efficient CFRP recycling technologies in the future.

3. Key challenges of carbon fibers recycling

Despite considerable progress in carbon fiber recycling technologies, several critical challenges continue to limit their effective and large-scale industrial implementation. These challenges are shared, to varying extents, by both conventional recycling routes (mechanical, thermal, and chemical) and emerging or hybrid approaches such as solvothermal, electrochemical, sonochemical, biological, and carbonization-based methods. One of the primary concerns across all recycling routes is preserving the intrinsic quality and mechanical properties of recycled carbon fibers. In mechanical recycling, shredding and grinding processes cause fiber fragmentation, leading to a significant reduction in tensile strength. Thermal recycling methods, such as pyrolysis and carbonization, can preserve the overall fiber structure; however, exposure to elevated temperatures may induce surface oxidation, char deposition, or defect formation if temperature and atmosphere are not carefully controlled. Chemical recycling techniques, including solvolysis, generally offer better retention of fiber mechanical properties, although improper solvent selection or reaction conditions may still lead to degradation of the fibers.

Another major challenge lies in the recovery of long, continuous carbon fibers suitable for high-value reuse. Mechanical recycling inherently produces discontinuous fibers, restricting their application to non-structural or filler materials. In contrast, thermal and chemical recycling approaches have the potential to recover longer fibers; nevertheless, thermal processes may require additional post-treatment to remove residual char, while chemical and electrochemical approaches demand precise process control to ensure effective matrix removal without excessive fiber damage. Achieving an optimal balance between resin decomposition efficiency and fiber preservation therefore remains a key technical challenge.

Economic feasibility also represents a substantial barrier to widespread adoption of carbon fibers recycling. Mechanical recycling is generally the least costly option but yields low-value recycled products due to fiber degradation. Thermal and carbonization-based methods involve high energy consumption and require advanced emission control systems, increasing operational costs. Chemical and solvothermal recycling processes, while capable of producing higher-quality fibers, is often the most expensive approach because of the need for high-pressure reactors, specialized solvents, and complex solvent recovery systems. Although biological recycling may potentially reduce energy consumption and eliminate the need for harsh chemicals, its economic viability remains uncertain due to slow processing rates, low throughput, and the current lack of industrial-scale implementation. This trade-off between cost and material quality continues to hinder large-scale commercialization.

Finally, environmental and health concerns further complicate the recycling of CFRP composites. Chemical recycling processes may involve use of toxic solvents and create hazardous byproducts that require careful treatment and disposal. Thermal processes can emit greenhouse gases and other pollutants if not properly managed, necessitating additional pollution control measures. For this reason, emerging techniques aim to mitigate these impacts through the use of greener solvents, lower operating temperatures, and improved process selectivity.

4. Future prediction

Looking ahead, the development of carbon fiber recycling technologies is expected to play a pivotal role in advancing both sustainability and resource efficiency across industries. As demand for lightweight, high-performance materials continues to rise, particularly in aerospace, automotive, and renewable energy sectors, the responsible management of carbon fiber waste will become increasingly critical. Although conventional mechanical, thermal, and chemical recycling methods have demonstrated technical viability at the laboratory scale, persistent limitations such as high solvent costs, inefficient resin decomposition, and fiber quality degradation continue to hinder widespread industrial adoption. The development of multifunctional recycling platforms that combine low-energy consumption with high-yield material recovery will be essential. Advanced methods such as carbonization, electrochemical, sonochemical, solvothermal, and biological recycling hold great promise for improving fiber quality recovery, reducing energy consumption, and enabling closed-loop systems. By investing in the optimization and scaling of these techniques, the industry can significantly lower its environmental footprint while easing the pressure on virgin carbon fiber production.

From a long-term perspective, the integration of advanced recycling technologies into circular economy frameworks represents a key opportunity for the composites industry. By enabling closed-loop reuse of recovered fibers and resin-derived products, these approaches can significantly reduce landfill disposal, lower greenhouse gas emissions associated with virgin carbon fibers production, and decrease dependence on fossil-based raw materials. Future research should therefore prioritize the scale-up of these innovative methods, the development of environmentally benign processing routes, and the establishment of standardized quality metrics for recycled carbon fibers.

5. Conclusion

With the increasing demand for carbon fibers-reinforced polymer composites, recycling processes for these materials have attracted significant attention from researchers in recent years. Each of the three conventional recycling methods, which include mechanical, thermal, and chemical recycling, offers distinct advantages and limitations. Mechanical recycling is environmentally benign and cost-effective, but due to severe fiber damage, it severely reduces the mechanical properties of the material and produces low-value materials. In contrast, fibers recovery from the polymer matrix is only possible in chemical and thermal methods, which almost preserve the strength of the raw materials, but these two methods are more expensive than the mechanical method. Also, the chemical method, due to the use of hazardous solvents, severely pollutes the environment and also seriously harms the health of workers. On the other hand, thermal recycling is also dangerous for the environment by releasing CO₂, CO, CH₄, etc., gases.

Given these trade-offs, the selection of an appropriate recycling strategy should be guided by the type of composite material, targeted reuse application, economic considerations, and environmental constraints. Typically, mechanical recycling can be used to process CFRP waste with environmental benefits. However, chemical and thermal recycling are generally more suitable for CFRP. The choice between chemical and thermal methods depends on the specific industrial conditions, costs, and required properties.

Also, in response to the limitations of conventional recycling routes, a new generation of advanced and hybrid recycling techniques, including solvothermal processing, sonochemically assisted methods, electrochemical, biological recycling, and carbonization, have emerged. These approaches aim to overcome the trade-offs between fiber quality, energy demand, and environmental impact by enabling more controlled matrix degradation and improving fiber preservation. Although many of these technologies remain at the laboratory or pilot scale, they show great potential to significantly enhance the value and performance of recycled carbon fibers. Future research should prioritize the optimization of these innovative techniques for large-scale applications while integrating environmentally friendly solvents, low-energy processes, and strict adherence to environmental requirements to develop a truly sustainable recycling ecosystem for CFRP waste.

Author contributions

Saba Noorsina: writing – original draft, investigation, data curation. Amir Hossein Sadeghi: writing – original draft, investigation. Reza Eslami-Farsani: review and editing, supervision. Abbas Tcharkhtchi: review and editing, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work is partially supported by the Iran National Science Foundation (INSF).

Biographies

Biography

Reza Eslami-Farsani has been a Professor of mechanical engineering at K. N. Toosi University of Technology (Tehran, Iran) since 2017. His research focuses on smart and advanced materials and structures (self-healing materials, FML, and composites), smart coatings, biomaterials, nanomaterials and nanotechnology, advanced fibers, and Li-ion batteries.

Biography

Saba Noorsina obtained her master's degree in Materials Science and Engineering at Sharif University of Technology in 2023. She is studying for her PhD degree in the same field at K. N. Toosi University of Technology (Tehran, Iran) under the supervision of Prof. Reza Eslami-Farsani. Her research interests include fiber reinforced polymer composites and their recycling, and batteries.

Biography

Amirhossein Sadeghi received his Bachelor's degree in Materials Science and Engineering at Shahrood University of Technology in 2022. He is currently pursuing a Master's degree in the same field at K. N. Toosi University of Technology (Tehran, Iran), under the supervision of Prof. Reza Eslami-Farsani. His research focus is on composites, fibers, recycling, and coatings.

Biography

Abbas Tcharkhtchi has been a full Professor at ENSAM (Arts et Metiers ParisTech) Paris since 2008. His research focuses on processing, rheology, mechanical properties, and structure–property–relationships of polymers and bio-polymers, composites and nano-composites.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

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PERMALINK

Review of conventional and emerging recycling methods for carbon fiber-reinforced thermoset polymer composites: challenges and future perspectives

Reza Eslami-Farsani

Saba Noorsina

Amir Hossein Sadeghi

Abbas Tcharkhtchi

Abstract

1. Introduction

Fig. 1. Timeline of the development of composite manufacturing across various industries (reproduced from ref. 10, © The Authors 2022, published under a Creative Commons CC BY 4.0 license).

Fig. 2. (a) Global market size for CFRP composites (redrawn based on data reported in ref. 14), (b) CF demand and estimated waste (2019–2024) (redrawn based on data reported in ref. 17), (c) global market size of recycled CFs (redrawn based on data reported in ref. 19).

Table 1. Recent global market trends of CFRP composites (2023–2030)a.

Fig. 3. Flow chart of the CFRP waste management process, from waste generation in the manufacturing sector, end-of-life of the material, recycling possibilities, and development of new materials (this figure has been adapted from ref. 21 with permission from Sage journal, copyright 2023).

2. Fiber reinforced polymer (FRP) composites recycling methods

Fig. 5. Waste FRP composite recycling methods, (a) mechanical recycling, (b) thermal recycling, (c) chemical methods (this figure has been reproduced from ref. 25 with permission from Sage journal, copyright 2025).

Fig. 6. Different types of FRP composites recycling methods.

2.1. Mechanical recycling

Table 2. A summary of mechanical recycling of FRP composites.

2.2. Chemical recycling

Low-temperature solvolysis

Sub/supercritical solvolysis

Fig. 8. Schematic diagram of the phase diagram of the supercritical state (reproduced from ref. 62, © The Authors 2020, published by MDPI under a Creative Commons Attribution (CC BY) license).

(a) Sub-supercritical water solvolysis

(b) Sub–supercritical alcohol solvolysis

Fig. 9. A semi-continuous flow reactor to recycle FRP using sub-supercritical alcohol techniques (this figure has been reproduced from ref. 28 with permission from Elsevier, copyright 2019).

Fig. 10. Experimental setup of batch-type reactors for CFRP recycling using the supercritical methanol process (this figure has been reproduced from ref. 76 with permission from Elsevier, copyright 2014).

2.2.1. Chemical recycling of epoxy matrix

Fig. 11. SEM micrographs of carbon fibers obtained after scPrOH-treatment, (a) overview of fibers, (b) close-up image of a single fiber. The fibers are 7–8 µm in diameter. Accelerating potential 20 keV (this figure has been reproduced from ref. 79 with permission from Elsevier, copyright 2006).

Fig. 12. Processing of the decomposition of carbon fibers/epoxy resin composites in the H2O2/acetone mixed solution (this figure has been reproduced from ref. 80 with permission from Royal Society of Chemistry, copyright 2012).

Fig. 13. Schematic of chemical recycling of carbon fibers reinforced epoxy composites via selective cleavage of the carbon–nitrogen bond (this figure has been reproduced from ref. 82 with permission from ACS Publications, copyright 2015).

Fig. 15. Schematic of the reaction apparatus for CFRP recycling by supercritical (this figure has been reproduced from ref. 88 with permission from Springer, copyright 2018).

Fig. 16. Complete recycling and reuse process flow of CFRP using aqueous NaOCl solution (this figure has been reproduced from ref. 90 with permission from ACS Publisher, copyright 2019).

Table 3. Summary of chemical recycling of carbon fibers reinforced epoxy matrixa.

2.2.2. Chemical recycling of phenolic matrix

Fig. 18. Diagram of the recycling of carbon fibers by the solvothermal treatment of CF/BPR (this figure has been reproduced from ref. 31 with permission from Elsevier, copyright 2021).

Table 4. Summary of chemical recycling of carbon fibers reinforced phenolic matrix.

2.2.3. Chemical recycling in other composite matrices

Table 5. Summary of chemical recycling of other carbon fibers reinforced thermoset matrixa.

2.3. Thermal recycling

(a) Fluidized-bed technique

Fig. 19. Fluidized-bed thermal recycling method (this figure has been reproduced from ref. 108 with permission from Elsevier, copyright 2022).

(b) Pyrolysis technique

Fig. 20. Static bed pyrolysis reactor (this figure has been reproduced from ref. 28 with permission from Elsevier, copyright 2019).

(c) Microwave technique

2.3.1. Thermal recycling of epoxy matrix

Fig. 21. Schematic of CFRP waste treatment by microwave thermolysis (reproduced from ref. 123, © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license).

Fig. 22. The experimental setup for the microwave pyrolysis system (reproduced from ref. 127, © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license).

Table 6. Summary of thermal recycling of carbon fibers reinforced epoxy matrix.

2.3.2. Thermal recycling of phenolic matrix

Fig. 23. Schematic of recycling carbon fiber (CF) by the two-step microwave thermolysis (this figure has been reproduced from ref. 132 with permission from Elsevier, copyright 2020).

Table 7. Summary of thermal recycling of carbon fibers reinforced phenolic matrix.

2.3.3. Thermal recycling in other composite matrices

Fig. 24. Lab-scale plant for the pyrolysis and vapor treatment of waste CFRP (reproduced from ref. 134, © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license).

2.4. Comparison of traditional recycling methods

2.4.1. The trade-offs of traditional recycling methods

Table 8. Comparative analysis of traditional recycling methods: trade-offs in mechanical, thermal, and chemical processes.

2.4.2. Technology readiness level (TRL) scale assessment

Table 9. Yang TRL scale allocation: L-Lab, P- Pilot, C-Commercial140.

Table 10. Comparison of the current TRL, scalability limitations, and commercialization status of traditional recycling methods.

2.5. Advanced recycling methods

2.5.1. Solvothermal recycling

Fig. 25. Solvothermal decomposition of CF/BPR (reproduced from ref. 142, © 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license).

2.5.2. Carbonization recycling

Fig. 26. Process flow diagram of recycling CF/EP by carbonization and being reused to prepare rCF/C composites (this figure has been reproduced from ref. 30 with permission from Sage journal, copyright 2021).

2.5.3. Electrochemical recycling

Fig. 27. Electrochemical recycling system for CFRP (this figure has been reproduced from ref. 28 with permission from Elsevier, copyright 2019).

2.5.4. Sonochemical recycling

Fig. 28. Sonochemical recycling for CFRP (this figure has been reproduced from ref. 25 with permission from Sage journal, copyright 2025).

2.5.5. Biological recycling

Fig. 29. Scheme of epoxy resin degradation caused by microorganisms (this figure has been reproduced from ref. 145 with permission from https://Scientific.Net, copyright 2014).

3. Key challenges of carbon fibers recycling

4. Future prediction

5. Conclusion

Author contributions

Conflicts of interest

Acknowledgments

Biographies

Biography

Reza Eslami-Farsani.

Biography

Saba Noorsina.

Table 1. Recent global market trends of CFRP composites (2023–2030)^a.

Fig. 12. Processing of the decomposition of carbon fibers/epoxy resin composites in the H₂O₂/acetone mixed solution (this figure has been reproduced from ref. 80 with permission from Royal Society of Chemistry, copyright 2012).

Table 3. Summary of chemical recycling of carbon fibers reinforced epoxy matrix^a.

Table 5. Summary of chemical recycling of other carbon fibers reinforced thermoset matrix^a.

Table 9. Yang TRL scale allocation: L-Lab, P- Pilot, C-Commercial¹⁴⁰.