Decomposition of carbon fiber-reinforced plastic resins using persulfate for carbon fiber recycling

Abstract

Carbon fibers are valued for their high strength, low weight, and chemical stability, making them essential in the aerospace, automotive, and energy sectors. However, current recycling methods often degrade fiber quality or limit reuse, particularly for continuous fibers. To address this, we developed the “electrolytic sulfuric acid method,” which uses oxidative active species—peroxomonosulfate and peroxodisulfate—generated by sulfuric acid electrolysis to selectively decompose resin in waste CFRP into CO₂ and water, thereby enabling the recovery of continuous carbon fibers. This study investigates changes in the concentrations of oxidative species during epoxy resin decomposition and analyzes the reaction using Arrhenius plots. The activation energy was determined to be 52.9 kJ/mol for electrolytic sulfuric acid, 40.9 kJ/mol for a 4:1 mixture of 60 wt% electrolytic sulfuric acid and 30 wt% hydrogen peroxide, and 63.1 kJ/mol for a similar mixture prepared with non-electrolyzed sulfuric acid. All these values were significantly lower than those for combustion-based decomposition. These findings demonstrate that the electrolytic sulfuric acid method enables efficient, low-energy decomposition of CFRP resin. Furthermore, the highly oxidative liquid produced by sulfuric acid electrolysis is already widely used in industrial applications. Continued research on resin decomposition using these oxidative active species is expected to advance sustainable carbon fiber recycling technologies and facilitate their implementation in key industries. This method offers a promising solution to the growing demand for environmentally responsible recycling of high-performance composite materials.

Keywords

electrolysis oxidative active species carbon fiber recycling sulfuric acid

Introduction

Challenges of carbon fiber sustainability

Carbon fibers have numerous applications owing to their light weight, mechanical strength, and chemical stability, with production expected to increase from 71350000 kg in 2017 to approximately 279010000 kg by 2030.¹ Although carbon fibers can be used in their pure form, they are more commonly combined with various resins (plastics) to produce carbon fiber-reinforced plastic (CFRP) composites. CFRP is increasingly used in automobiles, aircraft, and wind turbine blades, where its relatively low weight considerably improves energy efficiency. While the inherent properties of CFRPs facilitate their application, these same properties pose major challenges in the disposal of CFRP waste. Unlike conventional plastics that can be easily incinerated, the highly graphitized structure of carbon fibers makes them fire resistant. CFRP waste is therefore often crushed and landfilled as industrial waste. Landfilled carbon fibers do not biodegrade and thus may leach into the environment and potentially contribute to marine plastic pollution. Considering the product lifecycle, CFRP waste is expected to become widespread approximately 10 and 30 years after its initial use in automobiles and aircraft, respectively. Therefore, the development of carbon fiber recycling technologies is becoming increasingly urgent.

However, recycling carbon fibers differs fundamentally from conventional plastic recycling. This is due to the chemical and physical stability of carbon fibers, which makes them unsuitable for material recycling methods such as melting and reshaping. While physical cutting of carbon fibers is possible, their mechanical strength is derived from their continuous fibrous structure, and once cut, they lose their functional properties as fibers. Furthermore, unlike chemical recycling of plastics, carbon fibers cannot be depolymerized into monomers or chemical feedstocks. Additionally, as previously mentioned, carbon fibers cannot be incinerated, rendering thermal recycling infeasible.

To address this issue, we have developed a Carbon Fiber Circular Economy Program to recycle waste CFRP into continuous carbon fibers that can be used in automobiles (Figure 1).²

New technology: The electrolytic sulfuric acid method

Several technologies have been developed to recycle carbon fibers. Among them, “Pyrolysis method” is the most extensively studied method. This technique involves pyrolyzing the resin components of CFRP at high temperatures (500 °C–900 °C) under low-oxygen conditions to recover only the carbon fibers. To reduce energy consumption, combustible gases generated during resin pyrolysis are collected and reused as heating fuel. However, caution is required regarding the toxicity of gases produced during this process. For example, epoxy resin, a major thermosetting resin used in CFRP, can generate bisphenol A—a suspected carcinogen—upon thermal decomposition.

To improve thermal conductivity, CFRP is typically crushed into fine particles before pyrolysis. However, the development of large-scale pyrolysis furnaces has enabled processing of coarsely crushed CFRP, allowing recovery of chopped carbon fibers ranging from a few centimeters to several tens of centimeters in length. Despite this advancement, chopped carbon fibers of such lengths are difficult to handle and challenging to reprocess into composites. Once shortened, carbon fibers cannot be restored to their original length, limiting their reuse to forms such as pellets or nonwoven fabrics. CFRP made from nonwoven fabrics is difficult to composite and has not yet gained market traction. While pellets have entered the market, their future growth potential remains limited. Furthermore, recycling carbon fibers as continuous fibers requires detection and extraction of fiber ends, which is not feasible with pyrolysis due to its airborne decomposition nature.³

“Solvent dissolution method” is another method, which involves dissolving the resin components in specific organic solvents to recover carbon fibers. This process operates at relatively low temperatures (100 °C–150 °C), preserving the mechanical strength of the recovered fibers and allowing for easy scale-up. However, its applicability is limited to ester-based resins such as epoxy and PET (polyester), restricting the types of CFRP that can be processed. CFRP using thermoplastic resins include a wide variety of polymers such as PET, PP (polypropylene), PEEK (polyether ether ketone), and PA (polyamide), making visual identification difficult. Although analytical instruments can be used to distinguish resin types, it is impractical to analyze and sort each piece of recovered CFRP waste individually. Additionally, atmospheric dissolution is not well-suited for thermosetting resins like epoxy, which form cross-linked structures and require over 10 h of processing time, posing challenges for practical implementation.⁴

Supercritical method involves dissolving resin components by placing methanol under high pressure (above 8 MPa) to achieve a supercritical state. One limitation of this method is that it can only process certain ester-based resins. Moreover, the high-pressure equipment required to withstand supercritical conditions is extremely costly and difficult to scale up. Although this method allows for the recovery of monomers from resins and is academically intriguing, it is not economically viable. Due to the small size of the equipment, CFRP must be crushed before processing. As with other recycling methods, the recovered chopped carbon fibers are only a few centimeters long, limiting their applications.⁵

To realize this program, we employed the “Electrolytic sulfuric acid method”, which uses oxidative active species generated through the electrolysis of sulfuric acid, including peroxodisulfate, peroxomonosulfate, and hydrogen peroxide, to selectively decompose the resin components of CFRP into CO₂ and water, enabling the remaining carbon fibers to be recycled. Hereafter, the term “electrolytic sulfuric acid” is used to describe a solution containing oxidative active species produced by the electrolysis of sulfuric acid (Figure 2).²

This technology is broadly applicable to all types of resins, preserves the strength of the recycled carbon fibers, and produces continuous fibers (Figure 3), thereby facilitating “tank-to-tank” recycling (Figure 4).²

The addition of hydrogen peroxide to sulfuric acid forms oxidative active species such as peroxomonosulfate. (Table 1). This sulfuric acid/hydrogen peroxide mixture is commonly used in semiconductor manufacturing to remove photoresist after lithography; however, the addition of hydrogen peroxide reduces the oxidative power of the mixture. This can be overcome by electrolyzing sulfuric acid to continuously generate oxidative species in the absence of hydrogen peroxide.^6–8 This process produces oxidative active peroxomonosulfate and peroxodisulfate species.^9–13 Peroxodisulfate is a highly potent oxidizing agent with widespread industrial applications as a food additive, free radical initiator, bleaching agent, and an etching agent for plating resins.^14,15

Table 1.

Comparison with existing carbon fiber recycling technologies.^2–5

	Electrolytic sulfuric acid method	Pyrolysis method	Solvent dissolution method	Supercritical method
Resin type	All	Resin with no decomposition gas toxicity	Ester resin	Ester resin
Decreased strength of carbon fiber	No	Yes	No	No
Recycling of continuous fiber	Possible	Not possible	Possible	Not possible
Processing temperature (°C)	100∼150	500∼900	100∼200	250∼300 (8∼10 MPa)

Resin decomposition mechanism of sulfuric-acid-derived oxidative species

The oxidative active species formed by the electrolysis of sulfuric acid include peroxodisulfate, peroxomonosulfate, and hydrogen peroxide, which promote the decomposition of organic resins, including the epoxy resin component of CFRP. However, each oxidative species follows a distinct decomposition mechanism.

At elevated temperatures, peroxodisulfate forms sulfate radicals (Equation (1)). The decomposition of the epoxy resin is thought to be an oxidative degradation process catalyzed by peroxodisulfate-derived sulfate radicals (Equation (2)).¹⁶

S_{2} O_{8}^{2 -} \to {2 SO}_{4}^{- ・}

(1)

(C_{18} H_{20} O_{3}) n + 43 n (O) \overset{{SO}_{4}^{-}}{\to} ・ {18 nCO}_{2} + 10 n H_{2} O

(2)

In contrast, peroxomonosulfate reacts with water to produce hydroxyl radicals (Equation (3)),^7,8 while the thermal decomposition of hydrogen peroxide produces two radical species (Equations (4) and (5)).¹⁷ Hydroxyl radicals directly decompose the epoxy resin even in the absence of oxygen (Equation (6)).² Thus, the mechanism of epoxy resin degradation by hydroxyl radicals differs from that induced by sulfate radicals.

H_{2} {SO}_{5} + H_{2} O \to H_{2} {SO}_{4} + 2 OH ・

(3)

H_{2} O_{2} \to 2 OH ・

(4)

H_{2} O_{2} \to H ・ + {HO}_{2} ・

(5)

(C_{18} H_{20} O_{3}) n + 86 nOH ・ \to {18 nCO}_{2} + {53 nH}_{2} O

(6)

Both peroxomonosulfate and peroxodisulfate coexist in electrolytic sulfuric acid, resulting in the decomposition of CFRP resin components via a complex process involving two simultaneous reaction mechanisms: one driven by sulfate radicals derived from peroxodisulfate and the other by hydroxyl radicals derived from peroxomonosulfate.

The decomposition behavior of resins by sulfuric acid-derived oxidative active species and the effect of changes in the concentration of these species have not been investigated. Accordingly, this study analyzes the decomposition rate and apparent activation energy to determine the decomposition behavior of resins in the presence of sulfuric-acid-derived oxidative active species. In addition, changes in the concentrations of oxidative active species and oxygen during the decomposition of CFRP resin components are investigated.

Methodology

Materials

Reagents

Concentrated sulfuric acid (95 wt.%, special-grade) and dilute sulfuric acid (0.1 mol/L, analytical grade), hydrogen peroxide solution (30 wt.%, special-grade), potassium iodide (special grade), sodium thiosulfate (0.02 mol/L, analytical grade), and potassium permanganate solution (0.01 mol/L, analytical grade) were obtained from Fujifilm Wako Pure Chemical Corporation.

Preparation of electrolytic sulfuric acid

Electrolytic sulfuric acid was produced using an electrolytic sulfuric acid supply system (DMS-100103 YB, De Nora Permelec Ltd). Concentrated sulfuric acid was diluted to 50 wt.% and electrolyzed in a membrane-type electrolytic cell equipped with a 20 cm diamond electrode. In this apparatus, sulfuric acid (8 L) was circulated on both the anode and cathode sides, and electrolysis was performed at a rated current of 220 A for 120 min. The sulfuric acid concentration of the resulting electrolytic sulfuric acid (60 wt.%) was confirmed using the specific gravity method.

Carbon fiber-reinforced plastics

A unidirectional laminate material (Niwakasofuto Co., Ltd) measuring 10 mm × 10 mm × 2 mm and having a resin content of 32.5% was used as the CFRP.

Methods

Decomposition of CFRP

CFRP samples (∼0.3 g) were decomposed in a decomposition solution (10 ml) containing oxidative active species by heating using an Ace Reactor equipped with a reflux cooling function (Saitem Co., Ltd), to decompose the resin. After heating, the CFRP pieces were removed, washed with water, dried, and the change in weight was measured. Figure 5 illustrates the state of the composite material before and after decomposition, as well as the experimental procedure.

Measurement of the total oxidative species concentration

The total concentration of oxidative active species (the sum of the concentrations of peroxomonosulfate, peroxodisulfate, and hydrogen peroxide) was determined using the iodometric titration method with minimal dilution. The sample (0.2 mL) was added to ultrapure water (5 mL), followed by potassium iodide solution (5 mL, 200 g/L). The flask was then purged with nitrogen, sealed, and allowed to stand for 2 h. Subsequently, the solution was titrated with a sodium thiosulfate solution (0.02 M) until the color of the solution changed from yellow to transparent. Titration was performed three times to confirm reproducibility.

T o t a l C o n c e n t r a t i o n o f O x i d a t i v e A c t i v e S p e c i e s (\frac{m o l}{L}) = \frac{T i t r a t i o n V o l u m e (m L) \times {N a}_{2} S_{2} O_{3} (0.02 m o l / L)}{S a m p l e V o l u m e (m L) \times M o l a r C o e f f i c i e n t o f {N a}_{2} S_{2} O_{3} (2)}

(7)

Measurement of the peroxomonosulfuric acid concentration

The concentration of peroxomonosulfate was determined using the iodometric titration method, which relies on the difference in the reaction rate of potassium iodide with peroxomonosulfate and peroxodisulfate to enable the selective measurement of the peroxomonosulfate concentration.

The sample (0.6 mL) was added to ultrapure water (300 mL) along with sulfuric acid (1 mL, 95 wt.%). The solution was titrated with 0.1 N potassium permanganate until a light pink color formed, and was then cooled to below 10°C. Subsequently, potassium iodide solution (5 mL, adjusted to 200 g/L) was added and the solution was cooled to below 10°C before being immediately titrated with a 0.02 M sodium thiosulfate solution, maintaining the temperature below 10°C using an ice-water bath, until the solution changed from yellow to transparent. Titration was performed three times to confirm reproducibility.

C o n c e n t r a t i o n o f P e r o x o m o n o s u l f u r i c A c i d (m o l / L) = \frac{T i t r a t i o n V o l u m e (m L) \times {N a}_{2} S_{2} O_{3} (0.02 m o l / L)}{S a m p l e V o l u m e (m L) \times M o l a r C o e f f i c i e n t o f {N a}_{2} S_{2} O_{3} (2)}

(8)

Measurement of the hydrogen peroxide concentration

The concentration of hydrogen peroxide was determined by titration with potassium permanganate. The sample (0.6 mL) was added to ultrapure water (12 mL) along with sulfuric acid (5 mL, 0.1 mol/L). The solution was then titrated with a 0.1 N potassium permanganate solution until a light pink color was observed. Titration was performed three times to confirm reproducibility.

C o n c e n t r a t i o n o f H y d r o g e n P e r o x i d e (m o l / L) = \frac{T i t r a t i o n V o l u m e (m L) \times K M n O_{4} (0.02 m o l / L)}{S a m p l e V o l u m e (m L) \times M o l a r C o e f f i c i e n t o f K M n O_{4} (2 / 5)}

(9)

Peroxodisulfuric acid concentration

The concentration of peroxodisulfate was calculated by subtracting the concentrations of peroxomonosulfate and hydrogen peroxide from the total concentration of the oxidative active species.

Measurement of the dissolved oxygen concentration

The dissolved oxygen concentration in the solution was measured at 20°C using a portable optical dissolved oxygen meter (HI 98198, Hanna Instruments, Japan). Measurement was performed three times to confirm reproducibility.

Results and discussion

Decomposition of epoxy resin in CFRPs by electrolytic sulfuric acid

Changes in the concentrations of oxidative species

Figure 6(a)–(c) shows the concentrations of the oxidative active species in electrolyzed sulfate heated to 80, 100, and 120°C, as well as that in the solution after the decomposition of the epoxy resin in CFRP.

The reduction in the concentration of oxidative active species after the decomposition of epoxy resin from CFRP (C_B) relative to the initial concentration (C_B0) is denoted by ΔC_B. ΔC_B is assumed to be the cumulative reduction in the concentrations of the oxidative active species owing to self-decomposition from heating alone (ΔC_A), which was attributed to the decomposition reaction of the epoxy resin in CFRP (ΔC_C). Accordingly, ΔC_C was calculated by subtracting ΔC_A from ΔC_B.

{Δ C}_{C} = {Δ C}_{B} - {Δ C}_{A}

(10)

{Δ C}_{A} = C_{A 0} - C_{A}

(11)

{Δ C}_{B} = C_{B 0} - C_{B}

(12)

where C_A0 and C_A are the initial and time-evolved concentrations of oxidative active species under Condition A, respectively, C_B0 and C_B are those under Condition B. Figure 7(a)–(c) show the concentrations of the oxidative active species that participated in the decomposition reaction of epoxy resin in CFRP at 80, 100, and 120°C.

The electrolysis of sulfuric acid generates oxidative active peroxodisulfate, peroxomonosulfate, and hydrogen peroxide, which initiate the decomposition of epoxy resin; however, these species also undergo self-decomposition reactions, leading to the partial loss of these species (Equations (1) and (13)).

2 {SO}_{4}^{- ・} + {2 H}_{2} O \to 2 {SO}_{4}^{2 -} + {2 O H}^{・} + {2 H}^{+}

(13)

The loss arising from the self-decomposition of peroxomonosulfate is described by Equation (3), while that from the self-decomposition of hydrogen peroxide is described by Equations (4) and (5), and the reaction yields water and oxygen (Equation (14)).¹⁸

2 H_{2} O_{2} \to 2 H_{2} O + O_{2}

(14)

The self-decomposition rates and lifetimes of the oxidative active species in electrolysis sulfate have been thoroughly investigated.¹⁹ The total concentration of oxidative active species participating in the decomposition of epoxy resin in CFRP via electrolysis sulfate is highest at 120°C, which is consistent with the decomposition rates of the epoxy resin (Section 3.1.3); however, extensive self-decomposition and thus consumption of peroxodisulfate (Equation (13)) was also observed at 120°C (Figure 6(c)). The decomposition of epoxy resin by peroxodisulfate requires oxygen (Equation (2)); therefore, insufficient oxygen in the system will likely result in self-decomposition of peroxodisulfate.

Changes in the dissolved oxygen concentration

The concentrations of dissolved oxygen in the electrolyzed sulfate solution at 80, 100, and 120°C and after the decomposition of the epoxy resin in CFRP are shown in Figure 8(a)–(c).

The change in concentration of oxygen Δ[O₂]_C, which contributed to the decomposition of the epoxy resin in CFRP, was calculated based on the the data presented in Figure 8. Δ[O₂]_C was calculated in the same way as ΔCc, as described in Equations (15)-(17)

{Δ [O_{2}]}_{C} = {Δ [O_{2}]}_{B} - {Δ [O_{2}]}_{A}

(15)

{Δ [O_{2}]}_{A} = {[O_{2}]}_{A} - {[O_{2}]}_{A 0}

(16)

{Δ [O_{2}]}_{B} = {[O_{2}]}_{B} - {[O_{2}]}_{B 0}

(17)

where [O₂]_A0 and [O₂]_A are the initial and postreaction dissolved oxygen concentrations under Condition A, respectively, while [O₂]_B0 and [O₂]_B are the initial and postreaction dissolved oxygen concentrations under Condition B, respectively.

The changes in the concentration of dissolved oxygen, which contributed to the decomposition reaction of the epoxy resin in CFRP at 80, 100, and 120°C, are shown in Figure 9.

The decomposition of the resin at 100 and 120°C was accompanied by a significant reduction in the oxygen concentration, likely due to the consumption of oxygen by the peroxodisulfate-driven decomposition of the resin. In contrast, at 80°C, the oxygen concentration increased owing to the decomposition of the resin, which was attributed to the higher solubility of gases in the solution at lower temperatures. In addition, the rate of oxygen-consuming reactions (Equation (2)) driven by externally applied thermal energy was relatively low. Concurrently, the heat generated by the resin decomposition may have promoted the formation of oxygen by the decomposition of hydrogen peroxide (Equation (14)) on the CFRP surface, resulting in the observed increase in the oxygen concentration under Condition B.

Decomposition behavior of epoxy resin in CFRPs

The decomposition of CFRP can be expressed by Equation (18).

R + O_{2} \overset{k}{\to} {CO}_{2} + H_{2} O

(18)

The rate equation for the oxidative decomposition of epoxy resin in an oxygen-deficient environment is expressed by Equation (19):²⁰

\begin{array}{l} \frac{d α}{d t} = {k (1 - α)}^{0.4} & α = \frac{W_{0} - W_{t}}{W_{0} - W_{f}} \end{array}

(19)

where k is the reaction rate constant, W₀ and W_f are the initial and final masses of the resin, and W_t is the mass of the resin at time t. Integrating Equation (19) with respect to time t gives Equation (20):

\frac{5}{3} {(1 - α)}^{0.6} = k t + C

(20)

Substituting the initial conditions t = 0 and α = 0 into Equation (20), the integration constant C can be derived (equation (21)).

C = \frac{5}{3}

(21)

Table 2 summarizes the decomposition results of CFRP resin at temperatures of 80°C, 100°C, and 120°C, as well as the values calculated using Equations (20) and (21). The relationship between reaction time and decomposition rate is illustrated in Figure 10.

Table 2.

Decomposition results of carbon fiber-reinforced plastic (CFRP) resin by electrolyzed sulfate.

Temperature (°C)	Time (h)	Pre-decomposition			Post-decomposition
Temperature (°C)	Time (h)	CFRP (g)	Carbon fiber (g)	Resin W_o (g)	CFRP (g)	Resin W_t (g)	Resin decomposition rate α (%)	$\frac{5}{3} {(1 - α)}^{0.6}$
80	1	0.324	0.219	0.105	0.321	0.102	2.80	1.64
	3	0.341	0.230	0.111	0.333	0.103	7.20	1.59
	6	0.361	0.244	0.117	0.348	0.104	11.1	1.55
100	1	0.335	0.226	0.109	0.320	0.094	13.8	1.53
	3	0.338	0.228	0.110	0.310	0.082	25.5	1.40
	6	0.329	0.222	0.107	0.295	0.073	31.8	1.33
120	1	0.323	0.218	0.105	0.298	0.080	23.8	1.42
	3	0.326	0.220	0.106	0.282	0.062	41.5	1.21
	6	0.328	0.221	0.107	0.265	0.044	59.1	9.75 × 10^-1

The decomposition rates of CFRP resin at 80, 100, and 120°C as a function of time t are shown in Figure 11(a)–(c).

The reaction rate constants at decomposition temperatures of 80, 100, and 120°C were 0.0202, 0.0653, and 0.125 h^-1, respectively. Their Arrhenius plot are shown in Figure 12.

The activation energy associated with the decomposition of CFRP resin using electrolytic sulfuric acid (calculated from the Arrhenius plot) was 52.9 kJ/mol, with a frequency factor of 1.46 × 10^-6 h^-1. This activation energy is lower than that of the decomposition of CFRP resin under low-oxygen combustion conditions (173 kJ/mol), indicating a more efficient decomposition.²⁰ Although the decomposition rate increased with temperature, higher temperatures also facilitated the self-decomposition of the oxidative active species. The consumption of these species was attributed to an oxygen concentration deficiency in the system. Hydrogen peroxide was therefore added to supply oxygen to the reaction system, thereby improving the efficiency of the oxidative decomposition reaction.

Decomposition of epoxy resin in CFRPs using an electrolytic sulfuric acid/hydrogen peroxide mixture

Changes in the oxidative species concentration

The decomposition of resin by oxidative active species, particularly peroxodisulfate, in electrolytic sulfuric acid was strongly influenced by the concentration of dissolved oxygen in the system. Hydrogen peroxide, which decomposes to produce oxygen (Equation (14)), was therefore added to the electrolytic sulfuric acid to enhance the oxidative process and facilitate the investigation of the resin decomposition reaction. Figure 13(a)–(c) shows the concentrations of various oxidative active species in the solution after the decomposition of epoxy resin in CFRP using a mixture of 60 wt.% electrolyzed sulfuric acid and 30 wt.% hydrogen peroxide at a 4:1 ratio under various temperatures.

The concentration of oxidative species contributing to the decomposition reaction of epoxy resin in CFRP (Condition C) was determined by the cumulative reduction in the concentration of the individual oxidative species arising from self-decomposition from heating alone (Condition A) and that from the reduction in concentration observed after the decomposition of epoxy resin in CFRP (Condition B).

Figure 14(a)–(c) shows the concentrations of the oxidative species that contributed to the decomposition of epoxy resin in CFRP at various temperatures.

The total amount of oxidative active species contributing to the decomposition of the CFRP epoxy resin using a 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide peaked at 120°C, which is consistent with the decomposition rate of epoxy resin (Section 3.2.3). Furthermore, the amount of peroxodisulfate in this mixed system that contributed to the decomposition of the CFRP epoxy resin was greater than that in the electrolyzed sulfuric acid owing to the added oxygen. This is because the decomposition reaction of the epoxy resin is considered to be an oxidative degradation reaction that requires oxygen and is catalyzed by sulfate radicals generated from peroxydisulfate (Equation (2)).¹⁶ However, a major part of the oxidative species was lost through self-decomposition reactions, suggesting that further optimization of the conditions may improve the decomposition efficiency. The disappearance of peroxydisulfate due to its self-decomposition reaction follows Equation (13), subsequent to Equation (1).¹⁹

Changes in the dissolved oxygen concentration

Figure 15(a)–(c) show the dissolved oxygen concentration in the solution after heating the 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen during heating and after the decomposition of the CFRP epoxy resin.

The changes in dissolved oxygen concentration contributing to the decomposition reaction of epoxy resin in CFRP (Condition C) were determined by subtracting the change in dissolved oxygen concentration after self-decomposition due to heating alone (Condition A) from that after the decomposition of the epoxy resin in CFRP (Condition B). Thus, the change in oxygen concentration contributing to the decomposition of epoxy resin in CFRP was calculated.

Figure 16 shows the change in the concentration of dissolved oxygen contributing to the decomposition of epoxy resin in CFRP at 80, 100, and 120°C.

A considerable reduction in the oxygen concentration was observed as the resin decomposed at 100 and 120°C, likely due to the extensive progression of decomposition reactions involving oxygen consumption by peroxodisulfate, similar to that observed in electrolytic sulfuric acid alone. Conversely, the oxygen concentration increased during the decomposition of the resin at 80°C, which was attributed to the higher gas solubility in the solution at lower temperatures, similar to that in electrolytic sulfuric acid alone. This increase in oxygen concentration is also facilitated by the lower rate of thermal-energy-driven oxygen-consuming reactions (Equation (2)). Concurrently, the exothermic resin decomposition likely induced oxygen-forming reactions on the CFRP surface (Equation (14)), that led to the observed increase in the oxygen concentration under Condition B.

The 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide elevated the oxygen concentration, which in turn enhanced the consumption of peroxodisulfate. This is because, in addition to the reactions described by Equations (4) and (5), the disappearance of hydrogen peroxide due to its self-decomposition involves a reaction that produces water and oxygen, as shown in Equation (14).¹⁸ However, the hydrogen peroxide in the system was depleted after approximately 3 h at 100 and 120°C, thereby inhibiting oxygen generation and reducing the oxygen concentration. Introducing oxygen from external sources to increase the oxygen concentration in the system may therefore further improve the decomposition efficiency.

Decomposition dynamics of epoxy resin in CFRPs

The decomposition rate of CFRP resin was estimated using Equations (20) and (21).

Table 3 summarizes the decomposition results of CFRP resin at temperatures of 80°C, 100°C, and 120°C, as well as the values calculated using Equations (20) and (21). The relationship between reaction time and decomposition rate is illustrated in Figure 17.

Table 3.

Decomposition results of CFRP resin using a 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide.

Temperature (°C)	Time (h)	Pre-decomposition			Post-decomposition
Temperature (°C)	Time (h)	CFRP (g)	Carbon fiber (g)	Resin W_o (g)	CFRP (g)	Resin W_t (g)	Resin decomposition rate α (%)	$\frac{5}{3} {(1 - α)}^{0.6}$
80	1	0.328	0.221	0.107	0.320	0.099	7.50	1.59
	3	0.327	0.221	0.106	0.306	0.085	19.8	1.46
	6	0.349	0.236	0.113	0.316	0.080	29.1	1.36
100	1	0.321	0.217	0.104	0.303	0.086	17.3	1.49
	3	0.317	0.214	0.103	0.278	0.064	37.9	1.25
	6	0.323	0.218	0.105	0.265	0.047	55.3	1.03
120	1	0.321	0.217	0.104	0.288	0.071	31.6	1.33
	3	0.320	0.216	0.104	0.249	0.033	68.3	8.37 × 10^-1
	6	0.322	0.217	0.105	0.226	0.009	91.7	3.73 × 10^-1

The decomposition rate of CFRP resin at 80, 100, and 120°C was analyzed with respect to time (t) using Equations (20) and (21) (Figure 18(a)–(c)).

The decomposition of CFRP resin at 80, 100, and 120°C exhibited rate constants of 0.0556, 0.114, and 0.230 h⁻¹, respectively. The corresponding Arrhenius plots are shown in Figure 19.

The 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide significantly accelerated the decomposition rate of CFRP resin relative to that obtained using electrolytic sulfuric acid alone, primarily owing to the addition of hydrogen peroxide, which increased the oxygen concentration and thus improved the rate of the oxidative decomposition reaction.

Conversely, the activation energy of CFRP resin decomposition using the 4:1 mixture (40.9 kJ/mol, with a frequency factor of 6.29 × 10⁻⁴ h⁻¹) is significantly lower than that of CFRP resin decomposition using electrolytic sulfuric acid (52.9 kJ). Unexpectedly, the addition of hydrogen peroxide lowered the activation energy to 77% of the original value in addition to increasing the oxidation reaction rate.

The reduction in activation energy in the 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide may have several possible explanations. The addition of hydrogen peroxide promotes the reaction formation of peroxomonosulfuric acid (equation (22)) through the mechanism described in Equation (4), which facilitates the decomposition process. The oxidative decomposition reactions involving peroxodisulfuric acid and peroxomonosulfuric acid proceed via a competitive reaction mechanism in which both active species facilitate oxidation via different pathways. Thus, the apparent reduction in the activation energy was attributed to the increased concentration of peroxomonosulfuric acid resulting from the addition of hydrogen peroxide.

H_{2} {SO}_{4} + 2 OH ・ \to H_{2} {SO}_{5} + H_{2} O

(22)

Decomposition of epoxy resin in CFRPs using a sulfuric acid/hydrogen peroxide mixture

Changes in the concentrations of oxidative species

The decomposition of CFRP resin in electrolytic sulfuric acid and a 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide is thought to be governed by a competitive reaction mechanism, involving peroxodisulfuric acid and peroxomonosulfuric acid, which promote the oxidation reaction via distinct pathways. Accordingly, while the experimentally measured and calculated decomposition rate constants and activation energies have practical applications, they do not provide a comprehensive academic understanding of the underlying mechanisms.

To address this issue, the decomposition behavior of CFRP resin was investigated using a 4:1 mixture of 60 wt.% sulfuric acid and 30 wt.% hydrogen peroxide. This mixture uses sulfuric acid without electrolysis and thus lacks peroxodisulfuric acid, enabling the analysis of oxidative decomposition reactions involving only peroxomonosulfuric acid.

Figure 20(a)-(c) show the concentrations of oxidative actively species in the solution at various temperatures after heating and following the decomposition of the epoxy resin in CFRP using the 4:1 mixture of 60 wt.% sulfuric acid and 30 wt.% hydrogen peroxide.

The concentrations of oxidative active species contributing to the decomposition of the epoxy resin in CFRP (Condition C) were calculated by subtracting the reduction in the concentration of oxidative active species due to self-decomposition by heating alone (Condition A) from that observed after epoxy resin decomposition (Condition B). The concentrations of oxidative active species contributing to the decomposition reaction of the epoxy resin in CFRP at 80, 100, and 120°C are shown in Figure 21(a)–(c).

The total concentration of oxidative active species contributing to the decomposition of epoxy resin in CFRP using a 4:1 mixture of 60 wt.% sulfuric acid and 30 wt.% hydrogen peroxide peaked at 120°C, which is consistent with the decomposition rate of epoxy resin (Section 3.3.3).

Changes in the dissolved oxygen concentration

The decomposition of CFRP epoxy resin by peroxomonosulfuric acid is thought to occur in the absence of oxygen (Equation (6)). To investigate this further, the dissolved oxygen concentration in the 4:1 mixture of 60 wt.% sulfuric acid and 30 wt.% hydrogen peroxide was measured after heating and after the decomposition of epoxy resin in CFRP at 80, 100, and 120°C (Figure 22(a)–(c)).

The change in oxygen concentration contributing to the decomposition of CFRP epoxy resin (Condition C) was calculated by subtracting the change in dissolved oxygen concentration due to self-decomposition induced by heating alone (Condition A) from that during CFRP epoxy resin decomposition (Condition B).

Figure 23 shows the changes in the concentration of dissolved oxygen that contributes to the decomposition reaction at 80, 100, and 120°C.

The oxygen concentration decreased considerably during resin decomposition at 100 and 120°C in electrolytic sulfuric acid with a 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide. In contrast, a slight increase in the oxygen concentration was observed with the 4:1 mixture of 60 wt.% sulfuric acid and 30 wt.% hydrogen peroxide. This suggests that the resin decomposition reaction involving peroxydisulfuric acid, which consumes oxygen, did not occur in the non-electrolytic sulfuric acid/hydrogen peroxide mixture. In contrast, the peroxomonosulfuric acid-driven decomposition proceeded without oxygen (Equation (6)). The higher oxygen concentration observed at 80°C was attributed to the same factors observed in reactions using electrolytic sulfuric acid and its hydrogen peroxide mixture. In addition, the slight increase in oxygen concentration upon increasing the temperature from 100°C to 120°C was attributed to the increased consumption of hydrogen peroxide (Figures 22(b) and (c)) as described by Equation (14).

Decomposition dynamics of epoxy resin in CFRPs

Table 4 summarizes the decomposition results of CFRP resin at temperatures of 80°C, 100°C, and 120°C, as well as the values calculated using Equations (20) and (21). The relationship between reaction time and decomposition rate is illustrated in Figure 24.

Table 4.

Decomposition results of CFRP resin using a 4:1 mixture of 60 wt.% sulfuric acid and 30 wt.% hydrogen peroxide.

Temperature (°C)	Time (h)	Pre-decomposition			Post-decomposition
Temperature (°C)	Time (h)	CFRP (g)	Carbon fiber (g)	Resin W_o (g)	CFRP (g)	Resin W_t (g)	Resin decomposition rate α (%)	$\frac{5}{3} {(1 - α)}^{0.6}$
80	1	0.318	0.215	0.103	0.317	0.102	1.00	1.66
	3	0.293	0.198	0.095	0.290	0.092	3.20	1.64
	6	0.322	0.227	0.105	0.316	0.099	5.70	1.61
100	1	0.330	0.223	0.107	0.326	0.103	3.70	1.63
	3	0.322	0.217	0.105	0.12	0.095	9.60	1.57
	6	0.319	0.215	0.104	0.302	0.087	16.4	1.50
120	1	0.319	0.215	0.104	0.310	0.095	8.70	1.58
	3	0.317	0.214	0.103	0.290	0.076	26.2	1.39
	6	0.316	0.213	0.103	0.274	0.061	40.9	1.22

The decomposition rates of CFRP resin at 80, 100, and 120°C were calculated with respect to time t using Equations (20) and (21) (Figure 25(a)–(c)).

The reaction rate constants at decomposition temperatures of 80, 100, and 120°C were 0.00990, 0.0293, and 0.0885 h^-1, respectively, as illustrated in the Arrhenius plot shown (Figure 26).

The decomposition rate of CFRP resin components in the presence of a 4:1 mixture of 60 wt.% sulfuric acid and 30 wt.% hydrogen peroxide was significantly lower than those observed using a 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide or with electrolytic sulfuric acid alone, likely because no decomposition reactions involving the highly active peroxodisulfuric acid occurred. The activation energy of the decomposition of CFRP resin using the 4:1 mixture of 60 wt.% sulfuric acid and 30 wt.% hydrogen peroxide, calculated from the Arrhenius plot, was 63.1 kJ/mol, with a pre-exponential factor of 2.10 × 10^-7 h^-1.

Conclusion

The decomposition rates of CFRP epoxy resin in electrolytic sulfuric acid, electrolytic sulfuric acid/hydrogen peroxide mixtures, and sulfuric acid/hydrogen peroxide mixtures were investigated, and the activation energy and pre-exponential factors of all the reactions were calculated. The activation energy for the decomposition of CFRP resin using electrolytic sulfuric acid alone was 52.9 kJ/mol, whereas that for the 4:1 mixture of 60 wt% electrolytic sulfuric acid and 30 wt% hydrogen peroxide was reduced by 77% to 40.9 kJ/mol. This reduction was attributed to the addition of hydrogen peroxide, which facilitated the production of peroxomonosulfuric acid (equation (22)) via the dissociation of hydrogen peroxide (Equation (4)), thereby enhancing the oxidative decomposition reaction. The oxidative decomposition by peroxodisulfuric acid and peroxomonosulfuric acid likely occurred via competitive reaction pathways: thus, the increase in the peroxomonosulfuric acid concentration may have lowered the average activation energy. In contrast, the activation energy of the decomposition of CFRP epoxy resin using only peroxomonosulfuric acid, as in the 4:1 mixture of 60 wt.% sulfuric acid and 30 wt.% hydrogen peroxide, remained relatively high at 63.1 kJ/mol, indicating that the apparent reduction in activation energy was limited.

The 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide increased the concentrations of both peroxomonosulfuric acid and peroxodisulfuric acid. This mixed system likely favored the more efficient peroxodisulfuric acid-driven decomposition of CFRP epoxy resin, resulting in a lower activation energy. Furthermore, the activation energy of the decomposition of CFRP resin under low-oxygen combustion conditions (173 kJ/mol) was considerably higher than that of the process using electrolytic sulfuric acid (52.9 kJ/mol), and that using a 4:1 mixture of 60 wt.% electrolytic sulfuric acid and 30 wt.% hydrogen peroxide (40.9 kJ/mol). This indicated that the use of electrolytic sulfuric acid enabled the efficient decomposition of CFRP epoxy resin at a lower energy cost, thereby demonstrating its potential application in the recycling of carbon fibers from CFRP. In addition, the use of oxidative active species in research on resin decomposition is also expected to promote innovations in related industrial sectors, including the aerospace, automotive, and energy industries. These efforts also align with the recent government regulations introduced to achieve global sustainable development goals.

Footnotes

Acknowledgements

We would like to thank Editage () for English language editing and journal submission support. The authors have authorized the submission of this manuscript through Editage.

ORCID iD

Yuji Okada

Ethical Considerations

This article does not contain any studies with human or animal participants.

Author Contributions

Yuji Okada: Conceptualization, Methodology, Data Collection, Data Analysis, Writing – Original Draft, Funding Acquisition. Takeshi Kondo: Conceptualization, Data Analysis, Writing – Review & Editing.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This paper presents findings from Project JPNP21005, supported by a subsidy from the New Energy and Industrial Technology Development Organization.

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.

Data Availability Statement

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.*

References

Fuji Keizai Co. Ltd.

Outlook for the Market of Carbon Fiber Composite Materials (CFRP/CFRTP) Related Technologies and Applications. 2022.

Okada

. Continuous carbon fiber recycling technology using the electrolytic sulfuric acid method. J Compos Mater 2024; 58(10): 1279–1287.

Itazu

Kanki

Moritomi

. Recycling of Carbon Fiber using Two-stage Thermal Treatment System. Mater. Cycles Waste Manag. Res. 2013; 24(5): 371–378.

Shibata

Mitsutoshi

. CFRP recycling technology using depolymerization under ordinary pressure. Hitachi Chemical Technical Report 2015; 42: 6–11.

Okajima

Sako

. Recycling of CFRP using supercritical and subcritical fluids. Mater. Cycles Waste Manag. Res. 2013; 24: 364–370.

Hayamizu

Tange

. Environmentally conscious semiconductor resist stripping technology. Toshiba Rev 2009; 64(5): 38–41.

Kawashima

. Caro acid generator, resist remover and resist remover. Japanese Patent Disclosure. 2002; 53312.

Kato

. Sulfuric acid electrolysis method and sulfuric acid electrolysis device. Japanese Patent Granted. 2014; 5649948.

Balej

Kaderavek

. Influence of sulphuric acid concentration on yield of peroxodisulphate. Collect Czech Chem Commun 1979; 44: 1510.

10.

Kolthoff

Miller

. The chemistry of persulfate I. The kinetics and mechanism of the decomposition of the persulfate ion in aqueous medium. J Am Chem Soc 1951; 73(7): 3055–3059.

11.

Michaud

Mahe

Haenni

, et al. Preparation of peroxodisulfuric acid using boron-doped diamond thin film electrodes. Electrochem Solid State Lett 2000; 3(2): 77–79.

12.

Oloman

Watkinson

. The electroreduction of oxygen to hydrogen peroxide on fixed bed cathodes. Can J Chem Eng 1976; 54(4): 312–318.

13.

Spiro

. The standard potential of the Peroxosulphate/Sulphate couple. Electrochim Acta 1979; 24(3): 313–314.

14.

Yamakawa

Iseri

Yamamoto

. Etching technology for ABS resin using electrolyzed sulfuric acid. J Surf Finish Soc Jpn 2020; 71(12): 791–794.

15.

Nagai

. Application of electrolyzed sulfuric acid for cleaning silicon wafers. J Surf Finish Soc Jpn 2016; 67(8): 421–426.

16.

Matzek

Carter

. Activated persulfate for organic chemical degradation: a review. Chemosphere 2016; 151: 178–188.

17.

Ogata

Tabusi

. The hydrogen peroxide oxidation of organic compounds. J Synth Org Chem, Jpn 1960; 18(6): 368–387.

18.

Nakashita

Suzuki

Touch

, et al. Characteristics of organic matter in sediments decomposed by hydrogen peroxide treatment. Coast Eng 2018; 74(2): I_1171–I_1176.

19.

Okada

Kondo

. Stability prediction of oxidative active species in electrolytic sulfuric acid. Kagaku Kogaku Ronbunshu 2024; 50(5): 116–121.

20.

Chen

Yeh

. Pyrolysis kinetics of epoxy resin in a nitrogen atmosphere. J Hazard Mater 1996; 49(2–3): 105–113.