Comparative study of single-step and two-step thermal recycling for near 100% carbon fiber recovery from T800 CFRP waste

Abstract

The study presents a comprehensive analysis of the high-temperature pyrolysis process of CFRP, along with a comparative analysis between single-step and two-step methods during thermal recycling. Before thermal recycling through a customized muffle furnace, optimized pyrolysis process parameters were determined using thermogravimetric analysis (TGA), optical microscopy, scanning electron microscopy (SEM), and EDS to evaluate degradation characteristics and surface morphology of reclaimed carbon fibers (CFs). The results show near 100% recyclability of CFs through a single-step and two-step in 1.25 h and 2 h, respectively, achieving more than 81% tensile strength and 83% modulus. The optimized parameters in the single-step process are heating CFRP waste at 25°C/min till 490°C under an air atmosphere. CFRP is first pyrolyzed under argon and nitrogen atmospheres at 650°C separately, followed by oxidation under air atmosphere until 490°C using a two-step method. Analysis of surface morphology shows a slight improvement in the quality of reclaimed CFs with two-step E1 than that of two-step E2 and single-step E3. During thermal recycling, the influence of isothermal time on CFRP under air atmosphere is highly significant. The findings emphasize the critical impact of heating ranges, temperature limits, and atmosphere, providing insights into the thermal recycling process to reclaim CFs for remanufacturing advanced composite materials.

Keywords

CFRP thermal recycling pyrolysis single-step recycling reclaimed carbon fiber optimized parameters

Introduction

Carbon fiber-reinforced polymer (CFRP) composites are widely used in various aerospace, automobile, maritime, construction, sports, and household industries due to their high thermal resistance, mechanical strength, stiffness, low toxicity, and low weight.¹ The growing usage of CFRP has led to enormous waste, including scraps from the manufacturing process, end-of-life (EoL), and damaged components. It highlights the significance of recycling CFRP waste for environmental and economic benefits.

The global CFRP market, valued at over USD 8.6 billion by 2020, is anticipated to triple by 2030,² and waste disposal will increase accordingly. The global waste of CFRP composite has reached 62 kilotons by 2020,³ and this output is assumed to rise to approximately 483 kilotons by 2030.⁴ The manufacturing costs of carbon fiber precursors consume 50% of CFRP production costs, and equipment consumes one-third,⁵ which is nearly 83% of the total CFRP production cost to produce only carbon fibers.^6,7 The manufacturing process of CFRP components produces almost 40 percent of CFRP waste.⁸ For instance, the CFRP makes up about half of the body of the Airbus A350 and Boeing 787 Dreamliner aircraft,⁹ and the construction process of the Boeing 787 aircraft’s main wings only produces nearly 1000 tons of CFRP waste annually.¹⁰ Consequently, it is required to respond to the waste recycling issues of CFRP. The CFRP waste comes from two primary sources: the components made from CFRP that are damaged or reached EoL, and virgin prepregs or CFRP composite scraps that are not required in manufacturing.

Figure 1 demonstrates the source of CFRP waste: the products or components reach expiration or are damaged, and scraps from the production process are leftover composites, hybrid yarns, and nonwovens from the dry-laid or wet-laid process.¹¹ The first category is divided into large and small-to-medium CFRP waste. The primary sources of large CFRP waste are large pieces of equipment such as rockets, satellites, missiles, wind turbine blades, space shuttles, and ships. These pieces of waste equipment exhibit good mechanical properties after they are scrapped. On the other hand, small-and-medium-sized CFRP waste is generated from automobile bodies, interior parts, sports equipment, household scraps, etc. These scraps are generated from damage to automobile bodies caused by collisions, high-degree heat exposure that causes material deformation, and leftover after EoL. Recent studies have addressed the CFRP recycling issue by adopting chemical,² thermal,^12,13 and mechanical recycling¹⁴ along with repair and recovery mechanisms of CFRPs.¹⁵

Figure 1.

CFRP waste generation sources and types.

Figure 2 categorizes the prominent waste recycling methods. Thermal recycling is an effective and environmentally friendly method that includes high-temperature pyrolysis, microwave pyrolysis, and fluidized bed recycling methods.^16–18 In contrast, chemical recycling breaks down the thermoset matrix and includes dissolution repolymerization, electrochemical, and solvolysis to recover non-defective carbon fibers. However, this method is less recommended as it requires costly chemicals to decompose the epoxy matrix, such as tetralin, sulfuric acid, nitric acid, and super/subcritical alcohols and water.^19,20 In addition, mechanical recycling involves shredding material into smaller pieces, followed by ball milling or fluidized bed reactor grinding. However, carbon fibers are typically reclaimed as powder or randomly sized fragments from CFRP and the fibers’ mechanical strength deteriorates, and the surface is damaged during the crushing or grinding. Although this approach is cost-effective, the recycling proportion and quality of recyclates are comparatively less.

Figure 2.

CFRP waste recycling methods.

Moreover, extensive machining and thermal operations deteriorate the mechanical properties of recyclates.²¹ Few studies adopted mechanical treatment for recycling CFRP composites.^22–24 Contrary to mechanical recycling, physical recycling encompasses traditional disposal methods like landfilling and incineration, which contribute to waste accumulation and fail to capitalize on the valuable properties of composite materials. Incineration for energy recovery has a lower ecological impact than landfilling due to avoiding long-term methane gas release, which is 25 times stronger than carbon dioxide.²⁵ Therefore, the physical recycling approach is not an environmentally friendly and sustainable solution.^26,27

Carbon fibers undergo high-temperature treatments during manufacturing, such as oxidation at around 300°C under air atmosphere, pre-carbonization at 1100°C, and carbonization at around 1800°C under nitrogen atmosphere.²⁸ Therefore, thermal recycling is adopted by utilizing an oven to perform the pyrolysis process in nitrogen and oxygen atmospheres; the parameters selected to heat CFRP at 550°C for 2 h in a nitrogen atmosphere to eliminate the resin matrix led to a slight variation of residues as pyrolytic carbon. Afterward, removing the residues in an oxygen-containing atmosphere at 550°C; however, increasing the temperature to more than 600°C under an air atmosphere may deteriorate the quality of recovered CFs.¹⁸ On the contrary, the fluidized bed CFRP recycling involves heating the CFRP at 550°C for the CF and matrix decoupling processes and 750°C for oxidation.¹⁶ The microwave furnace reached 500°C in 7 min, sustained at 500°C for the next 23 min under an argon atmosphere, and then performed the second step of oxidation under 450°C for 40 min in an air atmosphere, claiming 94.49% of CF recovery.¹⁷ Another study observed the weight loss of carbon fibers from 530°C in an air atmosphere, and the CF content was assumed to be 64% based on the experiment as there were no theoretical values available.²⁹ However, 0.38 ± 0.07% weight loss of CF occurred at 500°C, and at 600°C, 5.34 ± 0.07% of weight loss and 0.58 ± 0.19 μm diameter reduction were recorded under air atmosphere.³⁰ In addition, comprehensive parameters are listed in the Table 1 from the recent studies.

Table 1.

Thermal recycling parameters from recent studies.

Equipment	Recycling steps	Recycling process	Heating rate (°C/min)	Temperature limit	Atmos-phere	Retention time mins	Retention reason	Recovery rate (CF)	Ref.
TGA analyzer	Two-step	Pyrolysis	10	550°C	N₂	120	Dec. resin	—	2009¹⁸
TGA analyzer	Two-step	Oxidation	10	550°C	Air	120	E. residues	—	2009¹⁸
Microwave	Single step	Thermolysis	—	450°C-500W	O₂	30	E. resin	115%	2019³¹
Microwave	Two-step	Pyrolysis	—	500°C-1000W	Ar	30	Dec. resin	94.49%	2020¹⁷
Microwave	Two-step	Oxidation	—	450°C	Air	40	E. residues	94.49%	2020¹⁷
Electrical furnace	Two-step	Pyrolysis	—	500°C	N₂	—	E. resin	—	2020³⁰
Electrical furnace	Two-step	Oxidation		500°C	Air	—	Dec. resin	—	2020³⁰
Muffle furnace	Single step	Oxidative pyrolysis	20	550°C	Air	60	E. resin	—	2021²⁹
Tube furnace (vacuum)	Two-step	Pyrolysis	—	600°C	Ar	30	Dec. resin	95%	2022³²
Tube furnace (vacuum)	Two-step	Oxidation	—	450°C	Air	20	E. residues	95%	2022³²
Horizontal furnace	Two-step	Pyrolysis	25	550°C	N₂	300	Dec. resin	—	2023¹²
Horizontal furnace	Two-step	Oxidation	25	500°C	Air	50	E. residues	—	2023¹²
Furnace	Two-step	Pyrolysis	15	425°C	N₂	—	Dec. resin	—	2024¹³
Furnace	Two-step	Oxidation	15	550°C	Air	45	E. residues	—	2024¹³
Customized muffle furnace	Two-step	Pyrolysis	25	650°C	Ar & N₂	—	Dec. resin	100%	This study
	Two-step	Oxidation	25	490°C	Air	10	E. residues	100%	This study
	Single step	Oxidative pyrolysis	25	490°C	Air	10	E. resin	100%	This study

“Dec. resin” is denoted as decomposed resin, “E. residues” is denoted as eliminating residues, “E. resin” denoted as Eliminating resin.

The thermal recycling parameters vary in previous studies, and CFRP exposure to heat in the oxygen-containing atmosphere without optimized parameters may reduce the quality of recyclates. Therefore, this study aims to improve the quality of recycled carbon fibers with increased efficiency. To this aim, pilot experiments are carried out before thermal recycling to determine the optimized recycling parameters, such as variation of heating rates, temperature limits, gaseous atmospheres, kinetic behavior, and mass change. Furthermore, pyrolysis with two steps under an argon/nitrogen atmosphere and a single step under an air atmosphere is presented and compared regarding recyclability ratio and the quality of recyclates.

Materials and methods

Materials

The CFRP composite used in the study consists of epoxy resin, 4,4′-Diaminodiphenylsulfone (DDS) curing agent, and T800-3K carbon fiber. The E-51 bisphenol A-type epoxy resin was used with the molecular formula given in Figure 3(a), having flash point and boiling point as 273.8 ± 30.1°C and 529.0 ± 50.0°C respectively. DDS curing agent was used with flash point and boiling points of 263.2 ± 25.9°C and 511.7 ± 35.0°C, respectively, the molecular formula is shown in Figure 3(b). While, T800-3K carbon fiber was used in composites with 72 wt% and a diameter of ∼7 µm, provided by Shandong Zhuoliou Carbon Fiber Composite Materials Co., Ltd.

Figure 3.

Molecular formula of (a) E-51 epoxy resin and (b) curing agent DDS.

The CFRP composite have been manufactured from the fiber winding molding process as illustrated in Figure 4. First, E-51 epoxy resin is heated to 110°C, and the curing agent DDS is added to the heated epoxy resin to form a mixture of resin and curing agent. The ratio of epoxy resin to curing agent is 100:30. After immersing the reinforced carbon fiber in the prepared mixture, wrap it onto the core mold and place it in a preheated mold. Cure the carbon fiber and resin mixture after molding at a temperature and time of 100°C/30 min + 130°C/120 min + 180°C/180 min. After curing, wait for natural cooling to room temperature before demolding, and finally obtain CFRP.

Figure 4.

Fiber winding molding process diagram.

Methodology

The main obstacle in the thermal recycling method is the process parameters to achieve the reclaimed CFs quality near to virgin. Therefore, some pilot experiments are performed to identify the effective thermal degradation parameters that preserve the quality and mechanical properties of recycled CFs. Figure 5 demonstrates the structure of this study.

Figure 5.

Overview of the research workflow for CF recycling.

The pilot experiment was conducted to analyze the quality of CFs during the thermal degradation process and the effects of heating rates on thermal decomposition. The samples were extracted from the waste material, using a DS610 slicing machine to cut into dimensions with a width of 3 mm, length of 2 mm, and thickness of 2 mm; the weight of the samples was between 12 ± 0.5 mg. The samples were put in an air atmosphere using a Hitachi STA300 (thermogravimetric analyzer), record the data, and analyze the optimized process parameters for further experiments, as shown in Figure 5(a).

After pilot-experimental analysis, the samples from the unidirectional layered same CFRP waste were used and cut into random dimensional pieces of 23 ± 3 mm (length) × 14 ± 2 mm (width) × 2 mm (thickness), with a weight of between 1.9 ± 2 g. The author performed comparative experiments such as the single-step and two-step thermal recycling processes for the first time. The temperature range varies from 350°C to 700°C with an isothermal time of 10 min in nitrogen, argon, and air atmosphere; proceeded with the customized SX2-4-10A integrated intelligent muffle furnace. Later, measure its weight loss, observe the surface morphology through a scanning electron microscope (SEM) and optical microscopy, and examine the elemental composition of fibers by using an EDS energy spectrometer.

Pilot experiments

The pilot experiments were carried out to investigate the high-temperature thermal decomposition performance of CFRP at various heating rates of 10°C/min, 15°C/min, 20°C/min, and 25°C/min, and gaseous atmospheres like argon, nitrogen, and air. The temperature range of each group varies between 0°C and 700°C, which reveals one weight loss stage under an argon and nitrogen atmosphere, and three weight loss stages in an air atmosphere. The initial finding shows almost the same pattern of material degradation concerning temperature limits with minor changes in weight loss percentage.

However, significantly varying time durations are shown in Tables 2 and 3, and Figure 6. A higher heating rate results in lower reaction times, with a 10°C/min rate taking 67.5 min, 15°C/min taking 45.5 min, and a 20°C/min rate reducing reaction time to 34.2 min. A 25°C/min rate results in a reaction ending in 27.7 min, indicating a relationship between speed and heating rate. The temperature rate of 25°C/min not only speeds up the process but also makes the reaction pattern smooth, which may be related to the pore formation, water vapor, or other gaseous products produced during the resin melting reaction. Therefore, further experimentation continued at a heating rate of 25°C/min.

Table 2.

High-temperature thermal decomposition experiments at different rates.

Experiment ID	Thermal degradation parameters	Weight loss (%)	Duration (mins)	Atmosphere
A	10°C/min till 700°C	36.8	67.5	Air
B	15°C/min till 700°C	34	45.5	Air
C	20°C/min till 700°C	32.5	34.2	Air
D	25°C/min till 700°C	34.6	27.7	Air

Table 3.

High-temperature thermal decomposition experiments at different atmospheres.

Experiment ID	Thermal degradation parameters	Weight loss (%)	Atmosphere
D	25°C/min till 700°C	34.6	Air
E	25°C/min till 700°C	25.97	Argon
F	25°C/min till 700°C	27.33	Nitrogen

Figure 6.

Influence of (a) heating rates under air atmosphere and (b) different atmospheres.

Consequently, investigations at a 25°C/min heating rate under the Argon and Nitrogen environment show only one stage of thermal weight loss in between the 300°C–490°C range as shown in Figure 6(b), continuing the heating does not even exceed 1.8% of weight loss till 700°C. However, oxidative pyrolysis under an air atmosphere shows three reaction segments, heating CFRP more than 490°C will cause the quality degradation of recycled CFs and affect the mechanical properties, these results are consistent with the findings of recent study.³³

Pyrolysis recycling is being explored by UK-based ELG Carbon Fiber and German-based CFK Valley Recycling, which produce clean, dry CF products for automotive applications.³⁴ The advanced CFRP thermoplastic composites with the matrix such as polypropylene (PP), polycarbonate (PC), and Polyetheretherketone (PEEK) have decomposition temperatures in the range of 400°C–500°C, 420°C–600°C, and 550°C–640°C respectively.³⁵ In addition, the full decomposition temperature of most of the thermoplastic lies under 650°C; this range fulfills the two-step recycling parameter reported in Table 4. During the investigation of finding fiber content of carbon/polyamide410 (C-P410), after reaching the temperature of 500°C under a nitrogen atmosphere, the polymer residues ring was found on CFs³³; a similar effect was observed during this preliminary analysis.

Table 4.

Detailed information of both the single-step and two-step thermal recycling.

ID	Recycling process	Process steps	Temp. Limit at heating rate	Retention time (min)	Temperature limit at cooling rate	Atmosphere	Gas flow rate (ml/min)
E1	Two-step	Pyrolysis	650°C at 25°C/min		50°C/min till 300°C	Argon	100
E1	Two-step	Oxidation	490°C at 25°C/min	10	5°C/min till 300C then 50°C/min till room temperature	Air	200
E2	Two-step	Pyrolysis	650°C at 25°C/min		50°C/min till 300°C	Nitrogen	100
E2	Two-step	Oxidation	490°C at 25°C/min	10	5°C/min till 300°C then 50°C/min till room temperature	Air	200
E3	Single step	Oxidative pyrolysis	490°C at 25°C/min	10	5°C/min till 300°C then 50°C/min till room temperature	Air	200

The variation of resin-decomposed residues is calculated through the equation (1).

R = (\frac{M_{f}}{M_{i} \cdot {C F}_{w t %}} \times 100 %) - 100

(1)

where

R

represents the resulting value from the thermal degradation concerning carbon fibers. If

R > 0

, the residues are equal to

R

, on the other hand, if

R < 0

, the pyrolytic residues might not exist, and the value

| R |

in percentage is reduced from the carbon fiber proportion which was initially 100%. The

M_{i}

is the initial mass of the sample in grams,

M_{f}

is mass after thermal degradation in grams,

{C F}_{w t %}

is the carbon fibers weight ratio in CFRP which was 0.72.

Following the findings of pilot experiments, the furnace was customized by integrating a gaseous atmosphere and embedding the chamber vessel inside. To ensure the complete displacement of nitrogen gas, the authors calculated the chamber volume as 78.5 mL. During the transition from an argon atmosphere to air, the mid-atmosphere of nitrogen was used to prevent combustion, then changing the atmosphere into air. The air is inlet at a controlled flow rate of 200 mL/min with a four-volume exchange, ensuring the complete displacement of N₂ and minimizing the residual. At the same time, one cycle took approximately 2.5 min. The gas exchange was according to standard purging calculations. The concluded experimental IDs are listed in Table 4 as E1, which is two-step thermal recycling under argon atmosphere and oxidation under air atmosphere Ar650°C-Air490°C; E2, which corresponds to two-step thermal recycling under nitrogen atmosphere and oxidation in an air N₂650°C-Air490°C; and E3 is single-step oxidative pyrolysis in an air atmosphere Air490°C.

In addition, to prevent the effects of temperature on gas density, especially during nitrogen and air atmospheric conditions. The authors replicated the gas flow rate of TGA during pilot experiments, which was 100 mL/min for argon and nitrogen atmosphere and 200 mL/min for air atmosphere, which supports the preliminary findings during the analysis. The gas flow meter for continuous gas flow maintenance was used, and detailed information is presented in Table 4, while the schematic of the setup is demonstrated in Figure 7.

Figure 7.

Schematic of customized muffle furnace.

Mechanical properties investigation of recycled fibers

The tensile strength and modulus of single carbon fiber was tested by using the XQ-1C Fiber Tensile Tester from Shanghai New Fiber Instrument Co., Ltd. with a load range of 0 to 200 cN the speed was 1 mm/min. The fiber diameter was measured with SEM analysis and CF was carefully placed on a piece of paper (circle cut from the center) maintaining the gauge length and were attached with AB adhesive glue, paper-tape was attached on both end of specimen for the grip, the gauge length of 20 mm, and the length of specimen was 30 mm, the sample preparation is shown in Figure 8; after calibration and placing the sample in grips, testing began, and failure load was recorded, the average of 10 different fibers of each experimental approach is calculated through ASTM C1557-20 guidelines.³⁶

Figure 8.

(a) Specimen demonstration for single-fiber tensile testing, (b) the actual specimen, (c) magnified image of the implanted CF, and (d) magnified image of the glue area bonding the CF and coupon.

Results and discussion

Mechanism of thermal decomposition

During the thermal degradation process, the resin matrix undergoes a chemical reaction. Figure 9(a) is a schematic diagram of the molecular structure of the resin matrix after cross-linking; the red part shows carbon and nitrogen, where the chain is breaking. The reaction process then forms five types of small molecular chains, as shown in Figure 9(b); these small molecules combine again by absorbing energy during the thermal degradation process and finally form new substances. The five different small molecular chains formed after the carbon-nitrogen chain scission reaction react again to create a variety of substances by absorbing the energy generated by the chain scission. The energy this reaction provides increases as the pyrolysis temperature rises, and the higher the temperature during the thermal degradation process, the more complex the molecular structure of the new substances produced.

Figure 9.

(a) Reaction principle during thermal degradation process and (b) five main types of small molecules.

The thermal recycling method utilizes the high-temperature resistance of reinforced fibers. The epoxy resin in the crucible is melted at high temperatures to extract carbon fibers. The resin matrix is decomposed into low-molecular-weight molecules, resulting in small-molecule products such as tar, CO₂, H₂, CH₄, etc. The solid residues are char, inorganic fillers, reinforced fibers, carbonaceous layer, and a small residual carbon.³⁷ This recycling approach can recycle the vast majority of waste, including CFRP waste and GFRP composite waste materials,³⁸ and maintain the good mechanical properties of carbon fiber, which can recycle a large amount of waste at once.

Influence of heating rates on thermal degradation process

Carbon fiber exposure to high temperatures in an oxygen-containing atmosphere shows three segments of reaction: resin degradation, oxidative resin residue elimination along with oxidation of carbon fibers, and further oxidation of carbon fibers to mass loss. To investigate the kinetic behavior of material degradation in the context of TGA, the Arrhenius kinetics analysis assists in analyzing activation energy for opposing processes, kinetic behavior, and thermal transitions that occur during thermal decomposition process, assisting in determining the yield point of opposite reaction,¹³ as shown in equation (2).

k (T) = A \cdot e^{\frac{{- E}_{a}}{R T}}

(2)

in the equation,

k (T)

is the reaction rate constant at temperature

T

A

is a pre-exponential factor,

e

is Euler’s number (a constant approximately equal to 2.718),

E_{a}

is the activation energy and

R

is the gas constant (8.314 J/mol K), T is the absolute temperature in Kelvin,

a

is the degree of conversion which could be calculated via equation (3).

a = \frac{m_{i} - m_{t}}{m_{i} - m_{f}}

(3)

where,

m_{i}

represents initial mass,

m_{t}

is transient mass at the time (

t

), and

m_{f}

is the final mass of pyrolyzed CFRP.

Figure 10 shows two thermal weight losses and two peaks in the DTG curve, corresponding to two different oxidation reactions. The first weight loss occurs when the resin matrix melts between 300°C and 460°C, breaking chemical bonds in the air atmosphere, while the second weight loss occurs when pyrolytic carbon and oxygen react between 460°C and 580°C, causing CF’s diameter and structural integrity degradation. The temperature limit above 564°C∼607°C continues the oxidation reaction and mass decline of CFs; that trend line stopped due to the temperature limit of 700°C, leading to a further weight loss of CFs. The CFRP thermal degradation at 10°C/min decomposes resin matrix at the rate of mass loss is dm/dt = 0.303 mg/min at the peak around 38 min; at 15°C/min, the rate of mass loss is dm/dt = 0.463 mg/min at around 27 min, at 20°C/min it is dm/dt = 0.565 mg/min at around 20 min, and at 25°C/min is dm/dt = 0.814 mg/min at around 17 min. However, the temperature between 300°C and 460°C causes a significant drop due to the resin degradation; this reaction slightly varies against heating rates. The negative activation energy (DSC curve) was observed at 464°C under 10°C/min in Figure 10(a), 475°C at 15°C/min in Figure 10(b), 481°C at the rate of 20°C/min in Figure 10(c), and 491°C at the rate of 25°C/min in Figure 10(d), which means endothermic reaction had stopped and increasing the temperature may cause the quality degradation of CFs, reducing diameter, and produce pits on the surface.

Figure 10.

TG, DTG, and DSC of CFRP at different heating rates, (a) 10°C/min, (b) 15°C/min, (c) 20°C/min, and (d) 25°C/min.

Regardless of the heating rate used for thermal recycling, the CFRP mass decreases in a similar pattern proportional to the temperature, with a variation of 27°C.

Comparing the data in Figure 6 shows that atmospheric conditions have minimum effects on thermal decomposition below 490°C regardless of heating rates. Oxidative pyrolysis of CFRP under air atmosphere section demonstrates the impact of heating limit ranging from 440°C to 490°C with an isothermal time of 10 min in an air atmosphere.

Influence of gaseous atmospheres during the thermal degradation process

The influence of variable atmosphere is analyzed as gas atmospheres affect the quality and surface of CFs. For instance, argon atmospheres reveal a carbonaceous layer, while nitrogen atmospheres produce a carbonaceous layer and minor residues. However, the air atmosphere continuously reacts and oxidizes with increasing temperature, reducing the quality. The thermal degradation process parameters are selected based on pilot experiments, as described in pilot experiments section.

Oxidative pyrolysis of CFRP under air atmosphere

The optimum temperature range for oxidative pyrolysis of CFRP in an air atmosphere is 480–490°C at the rate of 25°C/min. Increasing the temperature limit may cause the quality degradation of CFs and produce craters or shallow pits on the surface of CFs due to the oxidation of stuck pyrolytic carbon and reducing the diameters of CFs. To avoid such damages, the author set the isothermal time of 10 min to clean the surface of CFs from pyrolytic residues with minimum damage, as it assists in burning off the residues and oxidizing them into gaseous products such as carbon dioxide and carbon monoxide. The cooling rate of 5°C per minute is essential to reduce the temperature of recyclates to 300°C. Then, adjust the cooling rate to 50°C per minute until room temperature. This process helps to preserve the quality of carbon fibers by mitigating thermal shocks, which can cause shallow pits and surface damage. Additionally, it facilitates the smooth rearrangement of the carbon chains. Figure 11 illustrates the decomposed resin residue removal trend, showing significant changes within the temperature range of 440°C to 490°C, as depicted in the SEM images.

Figure 11.

SEM surface morphology of CFRP in an air atmosphere.

Pyrolyzing the CFRP with isothermal time continues to oxidize the residues without increasing the temperature. The 440°C limit shows a significant amount of pyrolytic residues on the surface of carbon fibers that is 4.134 wt%, as shown in Figure 11(a) and Table 5; oxidation at 450°C further reduces the number of residues in Figure 11(b). Increasing the temperature to 460°C continues to lessen the pyrolytic residues and maintain the quality of CFs in Figure 11(c), while at 470°C oxidative residues on the surface are minimal, and partially separated clean fibers are also visible in Figure 11(d). On the contrary, oxidation at 480°C and 490°C fully removes the oxidative residues, separating the carbon fibers from each other, and is found to be cleaned without any damage, as shown in Figure 11(e) and (f).

Table 5.

Comparison of weight loss% with/without isothermal time and cooling rate (air).

Temperature limit	Initial weight of CFRP sample (g)	Furnace pyrolyzed weight with isothermal (g)	Furnace weight loss (%)	TGA weight loss (%)	Isothermal and cooling influence on weight loss (%)	Residues wt%
440	2.11	1.582	25.02	18.75	6.27	4.134
450	2.01	1.49	25.87	20.1	5.77	2.957
460	1.9	1.39	26.84	20.86	5.98	1.608
470	1.76	1.28	27.27	21.37	5.9	1.01
480	1.752	1.27	27.51	21.76	5.75	0.679
490	1.86	1.34	27.96	22.11	5.85	0.06

Table 5 shows the CFRP weight loss % comparison between the furnace with a holding time of 10 min and the Hitachi STA300 TGA without a holding time during thermal recycling. Carbon fibers at 490°C achieved 100% reclamation with negligible pyrolytic residue contamination of 0.06%. The amount of CFs in used CFRP samples is approximately 100% because the carbon fibers don’t degrade below the temperature of 490°C at 25°C/min. The variation of resin-decomposed residues is further calculated, as shown in equation (1).

High-temperature pyrolysis of CFRP under argon atmosphere

The thermal degradation of CFRP under an argon atmosphere shows only one reaction: resin decomposition. Significant weight loss has occurred from the temperature range of 308°C – 490°C. The range decomposes the resin matrix and forms the carbonaceous layer on the surface of CFs. The DTG graph shows the peak of 0.965 mg/min rate of weight loss at 410°C. At the same time, the amount of heat (energy) in this endothermic reaction is consumed 11.35 mJ/mg in the range of 384°C to 460°C then heat absorption is uniform till around 600°C; after this limit, the smooth pattern of heat absorption continues at the same rate. CFRP pyrolysis under argon atmosphere in the range of 490°C–700°C gradually reduces the carbonaceous layer and pyrolytic residues. This range shows only 1.8% weight loss and the activation energy shows minor thermal degradation as shown in Figures 6(b) and 12. Continuing the heat increase till 630°C reduces the 25.66% weight of CFRP, which does not affect the quality of carbon fiber due to the protective layer.

Figure 12.

Thermal gravimetric analysis of CFRP under argon atmosphere.

Figure 13 depicts the SEM and optical visuals at different temperature limits: 550°C, 600°C, 650°C, and 700°C. The surface morphology analyzed through optical and SEM microscopy provides better insights for understating the surface quality and damage. During thermal pyrolysis in an argon atmosphere till 490°C, CFs are stuck together due to the carbonaceous layer; this layer further decomposes and produces char as temperature increases and partially separates the fibers. For instance, at 550°C, Figure 13(a) shows a significant amount of char residues and sticking of carbon fibers due to the carbonaceous layer. At 600°C, it reduces the amount of char and partially decomposes the layer, separating the carbon fibers, as seen in Figure 13(b). Further increasing the temperature to 650°C and 700°C follow the same phenomenon and peel off the layer besides residue decomposition, as shown in Figure 13(c) and (d). This residue elimination correlates with a minor weight loss of 1.8% within this range, as mentioned above.

Figure 13.

SEM and optical microscopy of CFRP thermal degradation (Ar), (a) till 550°C, (b) till 600°C, (c) till 650°C, and (d) till 700°C.

Table 6 shows the comparative data that CFRP pyrolysis under an argon atmosphere exhibits a minor weight loss difference comparing the samples from the furnace and TGA. The weight loss at a heating range of 550°C is 25.25% with an isothermal time of 10 min, while without isothermal, it was 25.06%. Similarly, at 600°C and 650°C, the samples from the furnace show a 0.18% weight loss increase; at 700°C, it was 0.12% compared to the TGA sample, which shows minor changes in weight loss as temperature increases. Therefore, increasing the temperature above 650°C does not significantly influence the positive outcomes during the pyrolysis of CFRP. The amount of CFs is approximately 100% because the carbon fibers don’t degrade in an argon atmosphere below 630°C. Variation in pyrolytic residues wt% is further computed.

Table 6.

Comparison of weight loss% with/without isothermal time and cooling rate (argon).

Temperature limit	Initial weight of CFRP sample (g)	Furnace pyrolyzed weight with isothermal (g)	Furnace weight loss (%)	TGA weight loss (%)	Isothermal and cooling influence on weight loss (%)	Residues wt%
550	1.96	1.465	25.27	25.06	0.21	3.792
600	2.08	1.546	25.66	25.48	0.18	3.25
650	1.85	1.37	25.93	25.75	0.18	2.875
700	1.79	1.323	26.09	25.97	0.12	2.653

High-temperature pyrolysis of CFRP under nitrogen atmosphere

The thermal degradation of CFRP under a nitrogen atmosphere shows resin decomposition like argon atmosphere. However, the reaction begins a little earlier at 302°C and ends at 490°C as compared to Figures 6(b) and 14. This temperature range brings significant weight loss while decomposing the resin matrix, forming the carbonaceous layer on the surface of CFs. While producing minor pyrolytic residues containing “O” and “Si” elements. The DTG graph shows the peak of 1.039 mg/min rate of weight loss at 412°C, which is a little faster than the argon atmosphere. In comparison, this endothermic reaction’s amount of heat (energy) is also increased to 17.01 mJ/mg in the range of 385°C to 466°C. Further exposure of CFRP with increasing temperatures like 490°C–700°C gradually reduces the amount of carbonaceous layer and pyrolytic residues. This range is similar to the argon atmosphere and shows only 1.8% weight loss. The activation energy shows a smooth thermal degradation pattern, which means the oxidation of residues is still required in both atmospheres, continuing the heat increase till 625°C does not affect the quality of carbon fibers.

Figure 14.

Thermal weight loss of CFRP in a nitrogen atmosphere.

Figure 15 illustrates the SEM and optical visuals at different temperature limits such as 550°C, 600°C, 650°C, and 700°C. As shown in Figure 15(a) and (b), the surface morphology of CFs is slightly different from that of pyrolysis in an argon atmosphere; the carbonaceous layer comparatively forms a strong stickiness of fibers until 600°C. After this limit, the fibers are partially separated, with some residues from the carbonaceous layer peeling off as shown in Figure 15(c) and (d).

Figure 15.

SEM and optical microscopy of CFRP thermal degradation (N₂), (a) till 550°C, (b) till 600°C, (c) till 650°C, and (d) till 700°C.

Table 7 presents the comparative data under the nitrogen atmosphere; CFRP pyrolysis under the nitrogen atmosphere also shows a minor difference in weight loss comparing the samples from the furnace and TGA. However, CFRP reduces the weight under a nitrogen atmosphere slightly more than in an argon atmosphere. The weight loss at a heating range of 550°C is 26.69% with an isothermal time of 10 min, while it was 26.44% without isothermal. Similarly, at 600°C and 650°C, the samples from the furnace show a 0.2% and 0.17% weight loss increase; at 700°C, it was 0.14% compared to the TGA sample. This trend reveals that isothermal time does not significantly influence the CFRP under argon and nitrogen atmospheres during thermal recycling. Still, in an air atmosphere, this element plays a significant role in preserving the quality of recyclates at certain temperature limits. Therefore, isothermal time and increasing the temperature above 650°C under an argon and nitrogen atmosphere could be a non-value-added factor.

Table 7.

Comparison of weight loss% with/without isothermal time and cooling rate (nitrogen).

Temperature limit	Initial weight of CFRP sample (g)	Furnace pyrolyzed weight with isothermal (g)	Furnace weight loss (%)	TGA weight loss (%)	Isothermal and cooling influence on weight loss (%)	Residues wt%
550	2.11	1.547	26.69	26.44	0.25	1.819
600	2.04	1.488	27.05	26.85	0.2	1.319
650	1.98	1.44	27.29	27.12	0.17	0.986
700	1.81	1.313	27.47	27.33	0.14	0.736

Considering the quality of recyclates under an argon and nitrogen atmosphere, the second oxidation step is required to achieve clean CFs. However, only a single step of oxidative pyrolysis can achieve near to similar results in an air atmosphere compared to two-step that is pyrolysis and oxidation. Consequently, the author attempted to recycle CFRP through both processes while comparing the data to optimize parameters for better results.

Comparison and characterization of two-step and single-step thermal recycling

A two-step method was adopted for the thermal recycling of CFRP that starts with first-step of pyrolysis in an N₂/Ar atmosphere followed by second-step that involves oxidation in an air atmosphere to burn off the carbonaceous and char residues followed by their oxidation into gaseous products. Analysis of Figures 12 and 13 shows that there is still pyrolytic carbon even when heated to 650°C in an argon or nitrogen atmosphere; by comparing the change trends in Figure 11, the carbon fibers are separated after the removal of epoxy resin and pyrolytic carbon when heated to 490°C in an air atmosphere with an isothermal time. Therefore, CFRP is first pyrolyzed in an argon and nitrogen atmosphere to 650°C to remove the resin matrix without isothermal time, followed by CFRP oxidation in an air atmosphere at 490°C with isothermal and cooling rate factors to remove the residues from the surface of CFs. Alternatively, the analysis of section 3.3 shows positive results for thermal recycling through a single-step process, the comparative visuals of two-step thermal recycling process under both environments and single-step oxidative pyrolysis under air atmosphere are shown in Figure 16.

Figure 16.

SEM and optical microscopy comparison of two-step and single-step thermal recycling process, (a) two-step E1, (b) two-step E2, and (c) single-step E3.

Table 8 exhibits the data of elements composition; the surface characterization with the help of EDS elemental analysis of recyclates shows that the “O” element may be due to the functional groups like carbonyl and carboxylic, which better form the bonding while applying the resin matrix to form a composite and a negligible proportion of “Si” may be due to the minor reaction with the crucible. The surface of CFs is highly active to conceive the contamination, so careful handling is necessary after thermal recycling. The percentage by weight (Wt%) and the percentage by atom (At%) of carbon and oxygen slightly vary in reclaimed CFs from both processes. In addition, the surface morphology of E1 in Figure 16(a) is comparatively better than that of E2 in Figure 16(b) and single-step E3 in Figure 16(c).

Table 8.

Elements of CFRP after thermal recycling under both conditions.

Main elements	E1		E2		E3
Main elements	Wt%	At%	Wt%	At%	Wt%	At%
C K	97.79	98.45	96.72	97.63	98.59	98.93
O K	1.83	1.39	2.21	1.68	1.41	1.07
SiK	0.38	0.16	0.54	0.46	—	—
N K	—	—	0.53	0.23	—	—

Decomposition reactions in an argon and nitrogen gas atmosphere are influenced by process temperature. While in an air atmosphere, oxidation processes depend on time, temperature, and isothermal dwelling time. Isothermal dwelling time after reaching the temperature limit in an oxygen-containing atmosphere plays a significant role in preserving the carbon fiber’s quality and cooling rate parameters that stabilize the reaction and protect the surface from thermal shocks. The reclaimed carbon fibers are near to 100%. However, a minor proportion of pyrolytic residues still exist, such as 0.075% after the two-step Ar-650°C-Air-490°C, 0.168% after the two-step N₂-650°C-Air-490°C, and 0.059% after the single-step Air-490°C process, as shown in Table 9. It indicates that a further increase in temperature may degrade the carbon fibers and can impact the mechanical properties and overall performance of remanufactured CFRP in specific applications.

Table 9.

The comparative results of recyclates from two-step and single-step thermal recycling.

Experiment ID	Initial weight of CFRP sample (g)	Weight after 1^st-step (g)	Weight after 2nd-step oxidation (g)	Oxidation influence on weight loss %	Carbon fibers + residues %
Experiment ID	Initial weight of CFRP sample (g)	Weight after 1^st-step (g)	Weight after 2nd-step oxidation (g)	Oxidation influence on weight loss %	Residues	CFs
(E1) Ar650°C-Air490°C	1.85	1.370	1.333	2.02	0.075	100.00
(E2) N2650°C-Air490°C	1.98	1.440	1.428	0.59	0.168	100.00
(E3) Air490°C	1.86	1.34	—	—	0.060	100.00

The mechanical properties vary in each category, with the least of 81.03% and 83.65% of tensile strength and modulus, respectively, which may be used to remanufacture advanced composites where structural and flexural properties are sensitive. Figure 17 shows the average tensile strength of virgin CF samples that is 2882.1 MPa. While, E1, E2, and E3 were 2409.37 MPa, 2363.66 MPa, and 2335.39 MPa, respectively. The average tensile modulus of virgin carbon fibers was 222.02 GPa, while, E1, E2, and E3 were 191.99 GPa, 185.87 GPa, and 185.7 GPa, respectively.

Figure 17.

Comparison of single fiber tensile testing (a) tensile strength and (b) tensile modulus.

Figure 18 depicts the final appearance of CFRP after thermal recycling; reclaimed carbon fibers’ chemical composition and surface morphology are similar to those of virgin fibers. The process parameters used in thermal recycling do not significantly affect the fibers’ diameter and length. The variations of virgin and recycled (from E1, E2, E3) CF diameters are reported in Table 10, with a sample size of 10 from each category.

Figure 18.

Appearance of reclaimed CFs through optimized process parameters: 25°C/min, isothermal 10 min, colling 5°C/min till 300°C then 50°C/min cooling rate. (a) Two-step E1 (b) two-step E2, and (c) single-step E3.

Table 10.

Comparing the variations of virgin and recycled fiber diameters.

Sample ID	Mean	Standard deviation	Variance	Minimum	Maximum
Virgin	7.076	0.06703	0.00449	6.98	7.19
E1	7.029	0.06757	0.00457	6.93	7.13
E2	7.007	0.08433	0.00711	6.87	7.13
E3	7.005	0.05039	0.00254	6.93	7.09

This study successfully recycled carbon fibers while retaining their original lengths from the waste component. This advancement paves the way for additive manufacturing research to utilize these recyclates in producing new CFRP components. Alternatively, these CFs might couple with virgin epoxy resin solution to form CFRP composite components through traditional manufacturing methods such as forging or resin transfer molding (RTM).

Conclusion

This study investigated the high-temperature thermal recycling process under optimized parameters along with a comparative experiment between single-step and two-step methods during thermal recycling. Pilot experimental analysis was performed using TGA to evaluate the effect of heating rate and different atmospheres on thermal degradation, a customized muffle furnace was used for reclaiming CFs from CFRP waste followed by surface morphology analysis, chemical composition, and mechanical properties testing. The main findings are as follows:

• A single-step and two-step thermal recycling processes both achieved 100% reclamation of clean CFs having surface quality near to virgin with a fiber’s diameter difference of ±0.12 μm. The process parameters for single-step oxidative pyrolysis (E3) are heating the CFRP waste at 25°C/min till 480–490°C, holding the limits for 10 min, followed by sequential cooling, first at 5°C/min till 300°C, then at 50°C/min till room temperature preserving CFs from surface damages that is, thermal shocks and pits. The whole process finishes in almost 1.25 h. Although heating rates below 25°C/min had little effect on the weight loss of CFRP during recycling, the reaction completion time decreases as the heating rate increases. However, isothermal dwelling time after reaching the temperature limit and sequential cooling in an oxygen-containing atmosphere significantly influences the degradation process of CFRP.

• On the contrary, the optimized parameters for the two-step include pyrolyzing the CFRP under an argon and nitrogen atmosphere (E2 and E3) till 650°C at 25°C/min heating rate, then cooling at 50°C/min till 300°C. Then change the atmosphere into the air for oxidation at 25°C/min heating rate till 490°C, leaving it for 10 min followed by sequential cooling, first at 5°C/min till 300°C then at 50°C/min till room temperature. The entire recycling process takes less than 2 h. However, isothermal time does not influence the CFRP under argon and nitrogen atmospheres.

• Regarding the quality of reclaimed carbon fibers, a two-step E1 have better properties than those of two-step E2 and a single-step E3. However, the CFs reclaimed through E3 are cost effective. While, CFs obtained from E2 and E3 can be treated and improved chemically, like acid etching or applying a suitable sizing agent before applying the resin matrix and remanufacturing the composite component. The thermal recycling parameters used in this study are applicable for composites made from A-stage materials (resin in raw/uncured state and dry CFs). In contrast, a two-step thermal recycling process can be applied to recycle thermoplastic composites. However, the recycling output of thermoset composites made from B-stage prepreg (partially cured resin with CFs, laminate prepreg) may differ.

• The surface of reclaimed carbon fibers from both processes shows functional groups, which are beneficial for adhesive properties. The concluded tensile strengths are E1-83.6%, E2-82.01%, E3-81.03% and tensile modulus are E1-86.48%, E2-83.74%, E3-83.65%. Although, the difference in reclaimed CFs is not big, E1 process is recommended for CFRP based on thermoset composite due to the rationality between process simplicity and recyclates output. In addition, CFRP based on thermoplastics, the E1 process is recommended due to the quality preferences.

Further studies will continue to recycle CFRP using the optimized parameters and remanufacture composite material using virgin epoxy and CF recyclates through traditional RTM and additive manufacturing approaches.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This research was funded by National Natural Science Foundation of China (No. 52275455), Science and Technology Plan Project of Liaoning Province, China (grant number 2022TH1/10800081).

ORCID iD

Bilal Shabbir Chohan

Data Availability Statement

Data available on request from the authors.*

References

Almushaikeh

Alaswad

Alsuhybani

, et al. Manufacturing of carbon fiber reinforced thermoplastics and its recovery of carbon fiber: a review. Polym Test 2023; 122: 108029. DOI: 10.1016/j.polymertesting.2023.108029.

Sakai

Kurniawan

Kubouchi

. Chemical recycling of CFRP in an environmentally friendly approach. Polymers 2024; 16: 143. DOI: 10.3390/polym16010143.

Wei

Hadigheh

. Cost benefit and life cycle analysis of CFRP and GFRP waste treatment methods. Constr Build Mater 2022; 348: 128654. DOI: 10.1016/j.conbuildmat.2022.128654.

Karuppannan Gopalraj

Kärki

. A review on the recycling of waste carbon fibre/glass fibre-reinforced composites: fibre recovery, properties and life-cycle analysis. SN Appl Sci 2020; 2: 1–21. DOI: 10.1007/s42452-020-2195-4.

Singh

Visotsky

Mears

, et al. Cost estimation model for polyacrylonitrile-based carbon fiber manufacturing process. J Manuf Sci Eng 2017; 139: 041011. DOI: 10.1115/1.4034713.

Nunna

Blanchard

Buckmaster

, et al. Development of a cost model for the production of carbon fibres. Heliyon 2019; 5: e02698. DOI: 10.1016/j.heliyon.2019.e02698.

Baker

Rials

. Recent advances in low-cost carbon fiber manufacture from lignin. J Appl Polym Sci 2013; 130: 713–728. DOI: 10.1002/app.39273.

Pimenta

Pinho

. Recycling carbon fibre reinforced polymers for structural applications: technology review and market outlook. Waste Manag 2011; 31: 378–392. DOI: 10.1016/j.wasman.2010.09.019.

Katunin

Krukiewicz

Herega

, et al. Concept of a conducting composite material for lightning strike protection. Adv Mater Sci 2016; 16: 32–46. DOI: 10.1515/adms-2016-0007.

10.

Kanemasu

Nishimura

Yoshikawa

, et al. Reduction of environmental impact by recycling waste composite material for aircraft. Mitsubishi Heavy Indus Techn Rep 2018; 55(2): 1–5.

11.

Pakdel

Kashi

Varley

, et al. Recent progress in recycling carbon fibre reinforced composites and dry carbon fibre wastes. Resour Conserv Recycl 2021; 166: 105340. DOI: 10.1016/j.resconrec.2020.105340.

12.

Termine

Naxaki

Semitekolos

, et al. Investigation of carbon fibres reclamation by pyrolysis process for their reuse potential. Polymers 2023; 15: 30768. DOI: 10.3390/polym15030768.

13.

Wei

Hadigheh

. Enhancing carbon fibre recovery through optimised thermal recycling: kinetic analysis and operational parameter investigation. Mater Today Sustain 2024; 25: 100661. DOI: 10.1016/j.mtsust.2023.100661.

14.

Bai

McKechnie

. Environmental and financial performance of mechanical recycling of carbon fibre reinforced polymers and comparison with conventional disposal routes. J Clean Prod 2016; 127: 451–460. DOI: 10.1016/j.jclepro.2016.03.139.

15.

Shi

Dunn

, et al. Carbon fiber reinforced thermoset composite with near 100% recyclability. Adv Funct Mater 2016; 26: 6098–6106. DOI: 10.1002/adfm.201602056.

16.

Meng

McKechnie

Turner

, et al. Energy and environmental assessment and reuse of fluidised bed recycled carbon fibres. Compos Appl Sci Manuf 2017; 100: 206–214. DOI: 10.1016/j.compositesa.2017.05.008.

17.

Deng

Liu

, et al. Efficient method of recycling carbon fiber from the waste of carbon fiber reinforced polymer composites. Polym Degrad Stabil 2020; 182: 109419. DOI: 10.1016/j.polymdegradstab.2020.109419.

18.

Meyer

Schulte

Grove-Nielsen

. CFRP-recycling following a pyrolysis route: process optimization and potentials. J Compos Mater 2009; 43: 1121–1132. DOI: 10.1177/002199830809773.

19.

Jiang

Ulven

Gutschmidt

, et al. Recycling carbon fiber composites using microwave irradiation: reinforcement study of the recycled fiber in new composites. J Appl Polym Sci 2015; 132: 42658. DOI: 10.1002/app.42658.

20.

Zhang

Chevali

Wang

, et al. Current status of carbon fibre and carbon fibre composites recycling. Compos B Eng 2020; 193: 108053. DOI: 10.1016/j.compositesb.2020.108053.

21.

Colucci

Ostrovskaya

Frache

, et al. The effect of mechanical recycling on the microstructure and properties of PA66 composites reinforced with carbon fibers. J Appl Polym Sci 2015; 132: 42275. DOI: 10.1002/app.42275.

22.

Verma

Balasubramaniam

Gupta

. Recycling, reclamation and re-manufacturing of carbon fibres. Curr Opin Green Sustainable Chem 2018; 13: 86–90. DOI: 10.1016/j.cogsc.2018.05.011.

23.

Howarth

Mareddy

SSR

Mativenga

. Energy intensity and environmental analysis of mechanical recycling of carbon fibre composite. J Clean Prod 2014; 81: 46–50. DOI: 10.1016/j.jclepro.2014.06.023.

24.

Demski

Misiak

Majchrowicz

, et al. Mechanical recycling of CFRPs based on thermoplastic acrylic resin with the addition of carbon nanotubes. Sci Rep 2024; 14: 11550. DOI: 10.1038/s41598-024-62594-y.

25.

Witik

Teuscher

Michaud

, et al. Carbon fibre reinforced composite waste: an environmental assessment of recycling, energy recovery and landfilling. Compos Part A: Appl Sci Manuf Rev 2013; 49: 89–99. DOI: 10.1016/j.compositesa.2013.02.009.

26.

Khalil

. Comparative environmental and human health evaluations of thermolysis and solvolysis recycling technologies of carbon fiber reinforced polymer waste. Waste Manag 2018; 76: 767–778. DOI: 10.1016/j.wasman.2018.03.026.

27.

Rybicka

Tiwari

Leeke

. Technology readiness level assessment of composites recycling technologies. J Clean Prod 2016; 112: 1001–1012. DOI: 10.1016/j.jclepro.2015.08.104.

28.

Park

S-J

. Precursors and manufacturing of carbon fibers. In: Park

S-J

(ed) Carbon fibers. Singapore: Springer Singapore, 2018, pp. 31–67.

29.

Irisawa

Aratake

Hanai

, et al. Elucidation of damage factors to recycled carbon fibers recovered from CFRPs by pyrolysis for finding optimal recovery conditions. Compos B Eng 2021; 218: 108939. DOI: 10.1016/j.compositesb.2021.108939.

30.

Sánchez-Rodríguez

Kamo

. Influence of thermal treatment on the properties of carbon fiber reinforced plastics under various conditions. Polym Degrad Stabil 2020; 178: 109199. DOI: 10.1016/j.polymdegradstab.2020.109199.

31.

Deng

Zhang

, et al. Recycling of carbon fibers from CFRP waste by microwave thermolysis. Processes 2019; 7: 7040207. DOI: 10.3390/pr7040207.

32.

Guo

Ren

, et al. Research on a two-step pyrolysis-oxidation process of carbon fiber-reinforced epoxy resin-based composites and analysis of product properties. J Environ Chem Eng 2022; 10: 107510. DOI: 10.1016/j.jece.2022.107510.

33.

Asfew

Moens

Ivens

. Investigation of fiber content evaluation methods of carbon fiber reinforced thermoplastic composites. J Thermoplast Compos Mater 2023; 37: 128–145. DOI: 10.1177/08927057231171524.

34.

Stieven Montagna

Ferreira de Melo Morgado

Lemes

, et al. Recycling of carbon fiber-reinforced thermoplastic and thermoset composites: a review. J Thermoplast Compos Mater 2022; 36: 3455–3480. DOI: 10.1177/08927057221108912.

35.

Zeyrek

Aydogan

Dilekcan

, et al. Experimental investigation of the recycling of carbon fiber polyetherketoneketone thermoplastic composite. J Thermoplast Compos Mater 2023; 37: 1859–1876. DOI: 10.1177/08927057231200011.

36.

ASTM-Standard . Standard test method for tensile strength and young’s modulus of fibers. West Conshohocken, PA: ASTM Standards, 2020.

37.

Palmer

Ghita

Savage

, et al. Successful closed-loop recycling of thermoset composites. Compos Part A: Appl Sci Manu Rev 2009; 40: 490–498. DOI: 10.1016/j.compositesa.2009.02.002.

38.

Onwudili

Miskolczi

Nagy

, et al. Recovery of glass fibre and carbon fibres from reinforced thermosets by batch pyrolysis and investigation of fibre re-using as reinforcement in LDPE matrix. Compos B Eng 2016; 91: 154–161. DOI: 10.1016/j.compositesb.2016.01.055.