Continuous carbon fiber recycling technology using the electrolytic sulfuric acid method

New technology: Electrolytic sulfuric acid method

The emphasis was placed on employing the electrolytic sulfuric acid method (ESAM) as a novel technology to accomplish this program. The utilization of this method enables the regeneration of carbon fibers with the same strength and surface functional groups as continuous fibers similar to that in new products. Therefore, the recycled carbon fibers obtained by this method can be used in new products, unlike the conventionally short-recycled carbon fibers. The ESAM technology recycles carbon fibers by decomposing only the resin component of CFRP/CFRTP into CO₂ and water using oxidative active species, generated by electrolytic sulfuric acid (Figure 2).^4–6

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Figure 2.

New technology: Electrolytic sulfuric acid method.

Resin Decomposition Mechanism by Sulfuric Acid-Derived Oxidative Active Species.^7,8 The oxidative active species generated by the electrolysis of sulfuric acid are peroxymonosulfuric acid and peroxodisulfuric acid. These two oxidative active species contribute to the decomposition of the resin, each through a different mechanism.

Firstly, the decomposition reaction by peroxymonosulfuric acid is believed to proceed by degrading the resin via hydroxyl radicals.

Next, the decomposition reaction by peroxodisulfuric acid is considered to involve oxidation decomposition via sulfuric acid radicals. Unlike the reaction involving hydroxyl radicals via peroxymonosulfuric acid, this reaction is believed to be an oxidative decomposition reaction that requires oxygen.

This technology has the following three characteristics: (1) its applicability to all resins, (2) maintenance of fiber strength, and (3) regeneration of carbon fiber as a continuous fiber (Figure 3).

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Figure 3.

Characteristics of the electrolytic sulfuric acid method.

Table 1 shows a comparison with existing carbon fiber recycling technology. These three points are explained below (Table 1).

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Table 1.

Comparison with existing carbon fiber recycling technologies.^9–11

	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)

Materials

Sulfuric acid (95% reagent special grade) manufactured by Fujifilm Wako Pure Chemical Industries was used. HTA-6K manufactured by Toho Tenax was used as carbon fiber. Carbon Fiber Cylinder 0.36 L manufactured by Hexger Official Store was used as the pressure tank.

Methods

Strength measurement

Measurements of the above-mentioned carbon fiber sample were performed as follows: A test sample was prepared by adhering a carbon fiber to a sample piece mount (approximately 0.1 mm thick) with a hole of 15 mm ± 0.5 mm and allowing it to stand in the measurement environment for 24 h.

Test standard: JIS R 7606

“Carbon fiber−Determination of the tensile properties of the single-filament specimen” was used as a reference.

Measurement: The number of measurements was n = 20, and the average value is reported.

Tester: Small physical property tester EZ-graph (Shimadzu Corp.)

Test speed: 1 mm/min

Load cell: 1 N (load resolution: 0.004 N)

Measurement environment: 23 ± 2°C/50% ± 5% RH

Comparisons and evaluations were conducted on the physical properties of a carbon fiber (HTA-6K, manufactured by Toho Tenax). The term “new carbon fiber” refers to the carbon fiber from which the sizing agent has been removed using a solvent. Comparisons and evaluations were conducted on the physical properties of a carbon fiber (HTA-6K, manufactured by Toho Tenax). The “Electrolytic sulfuric acid method-based carbon fiber” refers to the new carbon fiber from which the sizing agent has been removed using the ESAM. The “Pyrolysis method-based carbon fiber” is the carbon fiber from the sizing agent was removed by the pyrolysis method (500°C, under nitrogen). The carbon fiber obtained by the ESAM and the new carbon fiber had almost the same single-fiber strength, but the carbon fiber obtained by the pyrolysis method had a strength reduction of approximately 10% even though it had the lowest pyrolysis temperature under standard conditions (Figure 4).

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Figure 4.

Tensile strength of a single fiber (n = 20).

Functional group concentrations measurement

Measurements of total and strong acidic functional group concentrations were performed using the following protocols:

(1) Measurement of total acidic functional group concentration

A 30 mL volume of 0.05 mol/L aqueous sodium hydroxide solution was added to 1 g of sample and sealed. After homogenizing for 4 h using a shaker, the solution was allowed to stand for at least 8 h. A 15 mL volume of the supernatant was titrated with 0.05 mol/L hydrochloric acid. A blank test was conducted with the same procedure, and the quantity of total acidic functional groups was calculated from the difference in the titration quantity from the blank test.

(2) Measurement of strongly acidic functional group concentration

A 30 mL volume of 0.05 mol/L aqueous sodium hydroxide solution was added to 1 g of sample and sealed. After homogenizing for 4 h using a shaker, the solution was allowed to stand for at least 8 h. A 15 mL volume of the supernatant was titrated with 0.05 mol/L hydrochloric acid. A blank test was conducted with the same procedure, and the quantity of total acidic functional groups was calculated from the difference in the titration quantity from the blank test.

The number of functional groups on the surface of the carbon fiber obtained by the ESAM had more than twice as many acidic functional groups as that of the new carbon fiber, whereas no surface functional groups were detected in carbon fibers obtained by the pyrolysis method (Figure 5).

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Figure 5.

Functional groups on the fiber surface (Boehm method).

Resin applicability

Automobiles, which constitute the largest application of CFRP/CFRTP, are comprised of approximately 30,000 parts. CFRP/CFRTP are composed of the most suitable resins and molding methods for generating various parts, such as the body, propeller shaft, monocoque, and hydrogen tank. Additionally, the resin used for composites differs depending on the automobile manufacturer. Therefore, from a practical perspective, a technology that can be applied to all composite resins is of a paramount requirement.

The pyrolysis method is widely studied in Japan, Europe, and the United States. In this method, the resin component is thermally decomposed at a high temperature of 500°C–900°C in a hypoxic environment, and only carbon fibers are extracted. The “superheated steam method” is basically the same as that of the pyrolysis method, but steam is heated to high temperatures in a special furnace to improve the efficiency of heat transfer. In both the cases, energy consumption is reduced by recovering the combustible gas generated by the pyrolysis of resin components and is used for heating. Therefore, while using pyrolysis and superheated steam methods, special attention must be given to the toxic gases generated by the pyrolysis of the resin. Epoxy resin, the main resin involved in the thermosetting of CFRP, generates bisphenol A during pyrolysis, which is suspected to be a carcinogenic substance. ⁹

In the ambient-pressure dissolution method, only carbon fibers are extracted by dissolving the resin component in a specific organic solvent. The processing temperature is low at 100°C–150°C; therefore, the strength of the extracted carbon fiber does not decrease. Furthermore, it is easy to scale up the size of the dissolution process. However, only selected resins (e.g., polyethylene terephthalate (PET)) can be dissolved in organic solvents using the ambient-pressure dissolution method, which limits the processing of CFRP/CFRTP. CFRTP, which uses thermoplastic resins, has a wide variety of composite resins, including PET, polypropylene (PP), polyether ether ketone (PEEK), and polyamide (PA). However, it is difficult to distinguish between them at a glance. Although it is possible to analyze these using specialized instruments, it is not practical to analyze and sort the collected CFRTP wastes on an individual basis. Furthermore, in the ambient-pressure dissolution method, the thermosetting resins that form a crosslinked structure (such as epoxy resin) do not dissolve easily, and the processing time is over 10 h, making it practically difficult. ¹⁰

In the supercritical method, methanol is put into a high-pressure apparatus of at least 8 MPa to create a supercritical state and dissolve resins; however, the resins that can be processed are limited to some esters. Furthermore, high-pressure equipment that can withstand supercritical conditions are extremely expensive, and it is difficult to increase the scale of the process. This method is characterized by the fact that the raw material monomer can be recovered from the resin, which is interesting from an academic perspective, but unsuitable for practical use from an economic perspective. ¹¹

The ESAM generates reactive oxygen species called peroxydisulfuric acid by electrolyzing sulfuric acid and uses this solution to decompose the resin component of CFRP/CFRTP into CO₂ and water to recycle carbon fibers. To date, this method has been confirmed to decompose various composite resins as CFRP/CFRTP, and epoxy resins such as PET, PP, polyethylene, polystyrene (PS), PEEK, PA, and polyimide. The origin of this technique is a mixture of sulfuric acid and hydrogen peroxide, commonly known as the piranha solution, which is used to remove residues of organic resist components from semiconductor wafers. The piranha solution decomposes most organic components; however, being highly hazardous, care must be taken during solution preparing, and hydrogen peroxide must be added each time. Piranha solution can be handled with care if it is used in a small-scale, such as in cleaning semiconductor wafers. However, for decomposing large parts such as CFRP/CFRTP used in automobiles and aircraft, mixing of large quantities of sulfuric acid and hydrogen peroxide is extremely dangerous as it generates a large quantity of heat.

Maintenance of strength of carbon fiber

High strength is a characteristic of carbon fibers. CFRP/CFRTP with strength and reliability cannot be produced from low strength carbon fibers. In other words, even if a carbon fiber with reduced strength is processed, it disqualifies as a product and cannot be termed as “recycled”.

In the pyrolysis and superheated steam methods the strength of the extracted carbon fibers decreases, due to formation of cracks on the surface of the carbon fiber processed at 500°C–900°C high-temperature. Although the electrolytic oxidation method has a similar name to the ESAM used here, they are technically entirely different methods. First, CFRP/CFRTP is chopped into pieces measuring several centimeters, later heated and sintered at 400°C–500°C. Next, the CFRP/CFRTP is electrolyzed by placing it in a furnace composed of electrodes and passing electricity directly through it in an aqueous alkaline solution. Like the pyrolysis method, this method results in the generation of cracks on the surface of the carbon fiber due to the high temperature processing, which results in decreased strength. Later, when the carbon fiber is directly energized in an aqueous alkaline solution, the current load causes further cracking, resulting in a stark decrease in strength. ¹²

In contrast, in the ESAM processes, the CFRP/CFRTP is treated with a solution of electrolytic sulfuric acid at a low temperature of 150°C in a duration of 2–4 h. The resin can be decomposed at a relatively low temperature, thus maintaining the strength of the recycled carbon fiber. Additionally, the sulfuric acid electrolysis process and the CFRP/CFRTP decomposition process are completely separated, thus carbon fibers are not directly energized and degraded as in the electrolytic oxidation method.

Surface processing is required when the carbon fiber is composited with resin, and the presence of more functional groups on the surface of the carbon fiber enables easier surface processing. The interface between the composite carbon fiber and resin delaminates in the absences of proper surface processing. Hence, the strength of the CFRP/CFRTP product cannot be obtained, thus disrupting the performance improvement effect of the carbon fiber. In other words, the carbon fiber obtained by the ESAM maintains its strength and has many surface functional groups, which are advantageous for composites, and can recycle high-strength CFRP/CFRTP products. The pyrolysis method reduces the strength of carbon fibers and further reduces their compatibility with the resin, and increases the probability of further reduction of the strength of the CFRP/CFRTP product.

Carbon fiber recycled as a continuous fiber

Physically chopped carbon fibers lose their function as fibers. Pulverized carbon fibers cannot be made into fibers again, and can only be used as carbon fillers, but its applications are limited, and even from an economic perspective, it is more favorable to manufacture carbon fillers by usual method. Furthermore, pulverized waste may remain permanently as highly stable microplastics and become a source of marine plastic pollution. It is desirable to “recycle” carbon “fibers,” in the form of “fibers”.

To improve thermal conductivity in the pyrolysis and superheated steam methods, it is advantageous to conduct pyrolysis after the maximum possible reduction of size of CFRP/CFRTP through crushing. The development of a huge pyrolytic furnace has enabled the implementation of pyrolysis even with coarse pulverization, enabling the extraction of chopped carbon fibers with a length of several centimeters to several dozens of centimeters. However, it is difficult to handle chopped carbon fibers of large lengths, and as a result, difficult to composite. Furthermore, once the fibers are shortened, they cannot be restored to their original length, so the form is limited to pellets or non-woven fabrics. CFRP/CFRTP using non-woven fabric is difficult to composite, and there is still no established market for them. CFRTP pellets have an established market, but its future growth potential is uncertain. The ends of the fibers need to be detected and extracted to recycle them as a continuous fiber, thus it is difficult to use the pyrolysis and superheated steam methods to conduct atmospheric decomposition.

The electrolytic oxidation method requires chopping the CFRP/CFRTP into lengths of several centimeters to put them into the electrode furnace. The extracted chopped carbon fibers are obtained as a slurry with a length of several centimeters, but its application as a non-woven fabric is very limited. The CFRP/CFRTP decomposition tank itself consists of electrodes, so it is difficult to increase the size of the decomposition tank to extract the long fibers. Furthermore, there is high electrode consumption due to the adhesion of decomposition products.

As mentioned above, the supercritical method requires high-pressure equipment that can withstand supercritical conditions at a very high cost, and it is difficult to increase the scale of this equipment. Therefore, the CFRP/CFRTP needs to be pulverized to compress the size. As with the existing recycling technology, the extracted chopped carbon fiber is only a few centimeters in length, and that makes its application limited.

In contrast, in the ESAM, the sulfuric acid electrolysis process and the CFRP/CFRTP decomposition process are separate processes. The CFRP/CFRTP decomposition tank can be easily scaled up as long as it has a container that can be heated in the same way as a normal chemical reactor. Furthermore, as the active species life of electrolytic sulfuric is several months, it can be stored in a pool tank. In other words, by scaling up the decomposition tank, it is possible to process large quantities of CFRP/CFRTP without the need for pulverization, and it is possible to recycle them in the form of fibers. Additionally, because the process is conducted in a solution, it is possible to float the ends of the fiber in the liquid, detect them, and extract them, where they can be rewound into continuous fiber.

Continuous carbon fiber recycling technology

Automotive pressure tanks (hydrogen and CNG tanks) are fabricated using CFRP instead of metal to reduce their weights and comply with fuel efficiency regulations. However, no effective recycling method for CFRP pressure tanks is available, and automobile manufacturers recognize this as a major issue. CFRP pressure tank recycling is difficult because they are composed of composite materials such as glass fiber reinforced plastic and aluminum (Figure 6). ¹³

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Figure 6.

Hydrogen tank structure.

Conventional reproduction technologies such as pyrolysis and superheated steam methods, which thermally decompose resin components at higher temperatures, require processing temperatures higher than the melting points of glass and aluminum. As a result, these are heat-sealed, and carbon fibers cannot be taken out (E-glass softening point: 840°C and aluminum melting point: 660°C). The ambient-pressure dissolution method, wherein processing at low temperatures is possible, cannot decompose the epoxy resin used in pressure tanks. The electrolytic oxidation method and supercritical method cannot process pressure tanks as the decomposition tank cannot be scaled up. The pressure tank is made in a very sturdy manner with large quantities of carbon fiber, thus making it difficult to pulverize. In contrast, the ESAM is the only technology that can recycle carbon fiber from CFRP pressure tanks because it is a low temperature process (i.e., at 150°C or less), where the heat fusion of glass and aluminum does not occur, and epoxy resin can be decomposed. This was confirmed through a basic experiment using a sample of a cut hydrogen tank (Figure 7).

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Figure 7.

Result of basic examination using hydrogen tank scraps.

The CFRP in the pressure tank is manufactured by winding 3000–4000 m of carbon fiber using filament winding molding, but the end of the carbon fiber (winding end) always exists on the tank surface. The end of the carbon fiber detected and extracted can be used to rewind the fiber into a continuous fiber from that point (Figure 8). ¹⁴

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Figure 8.

Filament winder.

In the ESAM, the decomposition of the resin progresses from the surface layer, so the ends of carbon fibers can be found at the initial stage of decomposition. Therefore, Asahi Kasei Engineering Co. Ltd prepared a sample of filament winding molding in which the carbon fiber was wrapped around a Teflon rod. After decomposition, it was observed that the ends of carbon fibers peeled off, confirming that they can be recycled as continuous fibers (Figure 9).

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Figure 9.

Basic experiment using the filament winding molding sample.

This principle was used to develop a continuous fiber recycling process technology from commercially available small CFRP tanks for scuba diving. In the early stages of development, there were problems such as clumping and fluffing due to the recycled carbon fiber; however, it is currently possible to recycle all carbon fibers >70 m (Figure 10).

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Figure 10.

Small pressure tank and recycled continuous carbon fiber.

Utilization of recycled continuous carbon fiber

Unlike existing short carbon fibers, carbon fibers recycled by this method can be handled in the same manner as new carbon fibers. For example, a tank-to-tank circular economy becomes possible by conducting filament winding again (Figure 11).

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Figure 11.

Filament-wound molded product using recycled continuous carbon fiber and tank-to-tank circular economy.

Furthermore, it is possible to process the CFRTP-unidirectional (UD) tape using our Leona^TM film. The CFRTP-UD tape is stronger than metal and is expected to be applied to automobile frames and bodies in the future, making it possible to recycle automobile parts from other automobile parts (Figure 12).

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Figure 12.

Carbon fiber reinforced thermoplastics-unidirectional (CFRTP-UD) tape using recycled continuous carbon fiber and Leona^TM film.