Method for recovering carbon fiber from waste carbon composite material containing thermosetting resin

Abstract

The present invention aims to improve the technology disclosed in Korean Patent No. 2530557, issued to the present applicant and relates to a method for recovering carbon fibers from a waste carbon composite material containing a thermosetting resin, which can reduce the environmental pollution load of waste liquid by lowering the concentration of an aqueous hydrogen peroxide solution while simultaneously improving the decomposition efficiency of the thermosetting resin. According to the present invention, the method for recovering carbon fibers from a waste carbon composite material containing a thermosetting resin comprises the steps of: impregnating a waste carbon composite material containing a thermosetting resin in an aqueous formic acid solution to induce swelling of the waste carbon composite material and delamination of carbon fibers; and decomposing the thermosetting resin present on the surface of the carbon fibers by introducing the delaminated carbon fibers into an aqueous hydrogen peroxide solution and applying ultraviolet light, wherein hydroxyl radicals (OH·) are generated from hydrogen peroxide by ultraviolet irradiation and the polymer chains of the thermosetting resin are cleaved by the ultraviolet light and the hydroxyl radicals (OH·), thereby reducing the thermosetting resin to a low-molecular-weight form.

Description

Method for recovering carbon fiber from waste carbon composite containing thermosetting resin

[0001] Cross-reference to related applications: This application claims priority to Korean Patent Application No. 10-2024-0120651, filed September 5, 2024.

[0002] The present invention was made under the support of the Ministry of Trade, Industry and Energy of the Republic of Korea under the project identification number 1415188793 and project number 20024956, and the management expert organization of the said project is the Korea Institute of Industrial Technology Planning and Evaluation, the research project name is “Material and component technology development”, the research project name is “Development of low-temperature (130℃) fast-curing (curing time within 2 minutes) eco-friendly resin intermediate material for green SMC and development of future mobility parts using the same”, and the research period is from 2023.07.01 to 2027.12.31.

[0003] The present invention relates to a method for recovering carbon fibers from waste carbon composites containing thermosetting resins, and more particularly, to a method for recovering carbon fibers from waste carbon composites containing thermosetting resins, which is intended to improve the technology disclosed in Korean Patent No. 2530557, which is the registered patent of the present applicant, and which can reduce the environmental pollution load of waste liquid by lowering the concentration of an aqueous hydrogen peroxide solution and at the same time improve the decomposition efficiency of the thermosetting resin.

[0004] Carbon composites, which combine thermosetting resins and carbon fibers, offer lightweight, high strength, and high elasticity, and are therefore used in a variety of fields. A representative example of a carbon composite combining thermosetting resins and carbon fibers is carbon fiber-reinforced plastic (CFRP), which is attracting attention in various fields such as automobiles, ships, wind power generation, oilfields, architecture / civil engineering, sports, biomedical engineering, and environmental industries.

[0005] However, carbon composites based on thermosetting resins such as epoxy present challenges in disposal due to the inherent recalcitrance of the thermosetting resin. For this reason, waste carbon composites have, until recently, been largely disposed of through landfill or incineration.

[0006] Meanwhile, attempts have been made to recycle waste carbon composites, as they contain expensive carbon fiber. Recycling processes for waste carbon composites include thermal decomposition (pyrolysis) and chemical treatment (solvolysis). Pyrolysis suffers from the disadvantages of high heat, which degrades the quality of the regenerated carbon fiber and yields low yields. Chemical treatment involves decomposing thermosetting resins using chemical solutions to recover carbon fibers from waste carbon composites.

[0007] As an example of a chemical treatment method, a method utilizing acetic acid and sodium hypochlorite has been proposed. However, due to its strong corrosiveness, the safety of the reaction equipment is reduced. To address this issue, the applicant has proposed a chemical treatment method using an aqueous formic acid solution and an aqueous hydrogen peroxide solution in Korean Patent No. 2530557. The applicant has demonstrated that this method can achieve superior decomposition efficiency even at low temperatures and atmospheric pressures, compared to the chemical treatment method using acetic acid and sodium hypochlorite.

[0008] For reference, companies such as ELG Carbon Fibre, Karborex, CFK Valley, Material Innovation Technologies, and Adherent Technologies are developing CFRP recycling businesses using thermal decomposition methods, while Hitachi is developing CFRP recycling technology based on chemical treatment methods.

[0009] [Prior Art Literature]

[0010] [Patent Document]

[0011] (Patent Document 1) Korean Patent Publication No. 2530557 (Published on May 10, 2023)

[0012] The present invention is intended to improve upon the technology disclosed in Korean Patent No. 2530557, which is the registered patent of the applicant, and to provide a method for recovering carbon fibers from waste carbon composites containing thermosetting resins, which can reduce the environmental pollution load of waste liquid by lowering the concentration of hydrogen peroxide aqueous solution and at the same time improve the decomposition efficiency of thermosetting resins.

[0013] In order to achieve the above object, a method for recovering carbon fibers from waste carbon composites containing a thermosetting resin according to the present invention comprises the steps of: impregnating waste carbon composites containing a thermosetting resin in an aqueous formic acid solution to induce swelling of the waste carbon composite and exfoliation of carbon fibers; and putting the exfoliated carbon fibers into an aqueous hydrogen peroxide solution and irradiating them with ultraviolet rays to decompose the thermosetting resin present on the surface of the carbon fibers; wherein hydroxyl radicals (OH•) are generated from hydrogen peroxide by the ultraviolet irradiation, and polymer chains of the thermosetting resin are broken by the ultraviolet ray and the hydroxyl radicals (OH•), thereby reducing the molecular weight of the thermosetting resin.

[0014] A step of immersing the peeled carbon fiber in a hydrogen peroxide aqueous solution and irradiating it with ultraviolet rays to decompose the thermosetting resin present on the surface of the carbon fiber; and a step of impregnating the carbon fiber in a formic acid aqueous solution to remove residual synthetic organic substances on the surface of the carbon fiber can be performed.

[0015] The hydrogen peroxide concentration of the hydrogen peroxide solution is 9-15%.

[0016] The formic acid concentration of the formic acid aqueous solution is 50-90%.

[0017] The method for recovering carbon fibers from waste carbon composites containing a thermosetting resin according to the present invention has the following effects.

[0018] By inducing the decomposition of thermosetting resins by hydroxyl radicals (OH·), regenerated carbon fibers with excellent physical properties can be recovered. Furthermore, the concentration of hydrogen peroxide in aqueous hydrogen peroxide solutions can be lowered, thereby reducing the pollutant load of wastewater. Furthermore, the decomposition efficiency of thermosetting resins can be improved without the use of radical initiators. Furthermore, the shortened reaction time can enhance the production efficiency of regenerated carbon fibers.

[0019] FIG. 1 is a photograph of the results of each process step of a method for recovering carbon fibers from a waste carbon composite including a thermosetting resin according to one embodiment of the present invention.

[0020] Figure 2 is an experimental result showing the decomposition characteristics of a thermosetting resin according to Experimental Example 1.

[0021] Figures 3a to 3c are experimental results showing the properties of regenerated carbon fiber according to Experimental Example 2.

[0022] Figure 4 is an SEM image showing the surface characteristics of regenerated carbon fiber according to Experimental Example 3.

[0023] The present invention is an improvement on the technology disclosed in Korean Patent No. 2530557, which is the registered patent of the present applicant.

[0024] In the present invention, "carbon composite" refers to a composite material of a thermosetting resin and carbon fiber, and "waste carbon composite" refers to a used carbon composite. An example of a carbon composite includes, but is not limited to, carbon fiber reinforced plastic.

[0025] The applicant of this invention has proposed a chemical treatment method using an aqueous formic acid solution and an aqueous hydrogen peroxide solution through Korean Patent No. 2530557, and has demonstrated that the decomposition efficiency of waste carbon composites can be improved through a combination of an aqueous formic acid solution, an aqueous hydrogen peroxide solution, and a radical initiator.

[0026] The inventors of the present invention set forth the following as matters requiring improvement in Korean Patent No. 2530557: 1) reduction of hydrogen peroxide concentration in hydrogen peroxide aqueous solution, 2) elimination of radical initiator, and 3) improvement of decomposition efficiency and shortening of reaction time of thermosetting resin in a state where the hydrogen peroxide concentration is lowered and the radical initiator is excluded.

[0027] 1) Regarding the reduction of the hydrogen peroxide concentration in the aqueous hydrogen peroxide solution, the concentration of the aqueous hydrogen peroxide solution disclosed in Korean Patent No. 2530557 is 34.5%. As the concentration of hydrogen peroxide increases, the pollutant load increases, which complicates the process for treating the used aqueous hydrogen peroxide solution, i.e., wastewater, and inevitably increases the treatment time and the amount of chemicals for wastewater treatment. The present invention proposes a process for reducing the concentration of the aqueous hydrogen peroxide solution to 9-15%.

[0028] 2) With regard to the exclusion of radical initiators, in Korean Patent No. 2530557, the radical initiator was involved in the decomposition of thermosetting resins, and played a role in improving the decomposition efficiency of the thermosetting resin by about 10% compared to when the radical initiator was not added. However, since such radical initiators act as difficult-to-decompose organic pollutants after the regeneration process, separate treatment is required during the water treatment of the waste liquid. In addition, if a radical initiator is applied to the regeneration process of the present invention, it can act as a scavenger for hydroxyl radicals (OH·), which can act as a factor that reduces the decomposition efficiency of the thermosetting resin. Therefore, the radical initiator is excluded in the present invention.

[0029] 3) With regard to improving the decomposition efficiency of a thermosetting resin by lowering the concentration of hydrogen peroxide and excluding the radical initiator, the present invention can improve the decomposition efficiency of a thermosetting resin by lowering the concentration of hydrogen peroxide and excluding the radical initiator, and can also shorten the reaction time, thereby increasing the production efficiency of regenerated carbon fiber.

[0030] In order to recover carbon fibers from waste carbon composites containing thermosetting resins, chemical additives such as sizing agents, reaction retardants, and curing agents contained in the thermosetting resins must be removed, and the thermosetting resins must be decomposed.

[0031] In Korean Patent No. 2530557, hydrogen peroxide oxidizes and removes chemical additives contained in a thermosetting resin, and also decomposes the thermosetting resin by oxidizing the hydroxyl group (-OH) contained in the chemical bonding structure of the thermosetting resin. The hydroxyl group (-OH) present in the chemical bonding structure of the thermosetting resin is involved in the adhesiveness with carbon fiber, and the oxidation of the hydroxyl group (-OH) can induce the decomposition of the thermosetting resin. Meanwhile, thermosetting resins such as epoxy are high molecular weight polymers, and in order to effectively decompose the thermosetting resin, the polymer chain of the thermosetting resin must be broken. For example, in the case of epoxy resin, when the phenolic ether chain is broken, the epoxy resin becomes low molecular weight, and the low molecular weight epoxy resin can be easily decomposed. However, the phenolic ether chain is not broken by hydrogen peroxide.

[0032] The present invention proposes a technology for more effectively decomposing a thermosetting resin by inducing a low molecular weight of the thermosetting resin through simultaneous application of ultraviolet rays and hydrogen peroxide, in addition to the effect of using hydrogen peroxide alone.

[0033] When ultraviolet rays are irradiated, hydroxyl radicals (OH·) are generated from hydrogen peroxide (H2O2), and since hydroxyl radicals (OH·) have a stronger oxidizing power than hydrogen peroxide, they can easily oxidize the chemical additives contained in thermosetting resins and the hydroxyl groups (-OH) contained in the chemical bonding structure of thermosetting resins in preparation for the application of hydrogen peroxide. In addition, the thermosetting resin is reduced to a low molecular weight by ultraviolet rays. For example, the phenol ether chain of an epoxy resin is broken by ultraviolet rays, thereby reducing the molecular weight of the epoxy resin. Low-molecular-weight thermosetting resins are more easily oxidized by hydroxyl radicals (OH·) (and hydrogen peroxide) than high-molecular-weight thermosetting resins. Therefore, the simultaneous application of ultraviolet rays and hydrogen peroxide can effectively remove chemical additives present in thermosetting resins and improve the decomposition efficiency of the thermosetting resin.

[0034] In addition, the combination of ultraviolet rays and hydroxyl radicals (OH·) acts as a factor that increases the recovery efficiency of carbon fibers by weakening the physical bond between thermosetting resin and carbon fibers.

[0035] The method for recovering carbon fibers from waste carbon composites containing thermosetting resins according to the present invention, which reflects the improvements of Korean Patent No. 2530557, sequentially performs the following steps: a swelling and exfoliation step of the waste carbon composites using a formic acid aqueous solution; a decomposition step of the thermosetting resin using ultraviolet rays and an aqueous hydrogen peroxide solution; and a removal step of residual synthetic organic substances using an aqueous formic acid solution.

[0036] Swelling and exfoliation steps of waste carbon composites using aqueous formic acid solution

[0037] By impregnating waste carbon composites in a formic acid solution, the physical bond between the thermosetting resin and carbon fibers within the waste carbon composite is weakened. As the physical bond between the thermosetting resin and carbon fibers is weakened, the waste carbon composite swells and the carbon fibers delaminate from the thermosetting resin. Figure 1 (a) illustrates the waste carbon composite, (b) illustrates the swollen waste carbon composite, and (c) illustrates the delaminated carbon fibers.

[0038] In the swelling and peeling step, the temperature of the formic acid aqueous solution is preferably maintained at 80°C or higher and less than 100°C, and the impregnation time can be selected within the range of 1 to 24 hours. In addition, the formic acid concentration in the formic acid aqueous solution can be 50% or higher and less than 100%, but from an environmental standpoint, it is preferably 90% or lower or less than 85%, and from a reactivity standpoint, it is used at 50% or higher. This swelling and peeling step can be performed within a predetermined reactor. That is, the waste carbon composite can be impregnated in a reactor filled with a formic acid aqueous solution to perform the swelling and peeling step.

[0039] Decomposition stage of thermosetting resin using ultraviolet rays and hydrogen peroxide solution

[0040] Carbon fibers that have been exfoliated by the swelling and exfoliation steps are collected and placed in a reactor filled with a hydrogen peroxide aqueous solution to induce the decomposition of the thermosetting resin. At this time, an ultraviolet lamp that irradiates ultraviolet rays is installed inside or outside the reactor. That is, the decomposition of the thermosetting resin proceeds under an environment in which the ultraviolet rays are irradiated in the reactor filled with the hydrogen peroxide aqueous solution while the carbon fibers are placed in the reactor. Here, since only the carbon fibers that have been exfoliated and separated from the waste carbon composite are placed in the hydrogen peroxide aqueous solution, the decomposition of the thermosetting resin refers to the decomposition of the thermosetting resin present on the surface of the carbon fiber.

[0041] As ultraviolet rays are irradiated, hydroxyl radicals (OH·) are generated from hydrogen peroxide (H2O2) (see Equation 1 below), and decomposition of the thermosetting resin by hydroxyl radicals (OH·) and ultraviolet rays proceeds.

[0042] (Equation 1) H2O2+ hv → 2OH·

[0043] Specifically, hydroxyl radicals (OH·) oxidize and remove chemical additives such as sizing agents, reaction retardants, and curing agents contained in thermosetting resins. In addition, hydroxyl radicals (OH·) oxidize hydroxyl groups (-OH) contained in the chemical bonding structure of thermosetting resins, thereby weakening the bonding strength of the thermosetting resin and causing some decomposition of the thermosetting resin. In addition, ultraviolet rays and hydroxyl radicals (OH·) induce low molecular weight of the thermosetting resin. As an example, thermosetting resins such as epoxy resins include phenol ether chains, and the phenol ether chains are broken by ultraviolet irradiation. As the phenol ether chains are broken, the thermosetting resin becomes low molecular weight, and the low molecular weight thermosetting resin is easily decomposed by hydroxyl radicals (OH·) (and hydrogen peroxide).

[0044] Chemical formula 1 below shows the chemical formula of bisphenol A epoxy resin. In chemical formula 1, A indicates a phenol ether chain and B indicates a hydroxyl group (-OH).

[0045] [Chemical Formula 1]

[0046]

[0047] Meanwhile, it is a well-known fact that hydrogen peroxide is converted into hydroxyl radicals (OH·) by ultraviolet irradiation, and this is widely used as an advanced oxidation process (AOP) in water treatment processes. In other words, in the field of water treatment, a technology is being utilized to convert hydrogen peroxide into hydroxyl radicals (OH·) under ultraviolet irradiation, thereby oxidizing and decomposing organic pollutants such as microorganisms through these hydroxyl radicals (OH·). In this case, the reaction between organic pollutants and hydroxyl radicals (OH·) can be considered a selective reaction.

[0048] Taking note of this, the present inventors applied the combination of ultraviolet light and hydrogen peroxide to a process for recovering carbon fibers from waste carbon composites. However, in the field of water treatment, hydroxyl radicals (OH·) are applied to oxidize and decompose organic pollutants such as microorganisms. However, in the process for recovering carbon fibers from waste carbon composites, the target of decomposition is a high molecular weight polymer such as a thermosetting resin, and the effect of hydroxyl radicals (OH·) on the decomposition of a thermosetting resin is not known. In addition, a technology for reducing the molecular weight of a thermosetting resin through a combination of ultraviolet light and hydrogen peroxide has not been presented.

[0049] As described above, the inventors of the present invention experimentally confirmed that the polymer chains of thermosetting resins are broken by ultraviolet rays and hydroxyl radicals (OH·) to reduce the molecular weight, and that the thermosetting resin with reduced molecular weight is oxidized and decomposed by hydroxyl radicals (OH·) and hydrogen peroxide, ultimately resulting in the decomposition of the thermosetting resin of the waste carbon composite by the process of the present invention. In addition, the experimental results confirmed that the combination of ultraviolet rays and hydroxyl radicals (OH·) acts as a factor to increase the recovery efficiency of carbon fibers by weakening the physical bond between the thermosetting resin and carbon fibers.

[0050] Meanwhile, as can be seen from the reaction formula of Equation 1, since 2 mol of hydroxyl radicals (OH·) are generated from 1 mol of hydrogen peroxide (H2O2), the decomposition efficiency of thermosetting resin can be improved even if the concentration of the hydrogen peroxide aqueous solution is lowered compared to Korean Patent Registration No. 2530557.

[0051] The concentration of hydrogen peroxide in the aqueous hydrogen peroxide solution can be selected within the range of 9 to 15%, and this concentration of the aqueous hydrogen peroxide solution is significantly lowered to less than half of the concentration of the aqueous hydrogen peroxide solution of Korean Patent No. 2530557, which is 34.5%. The reason why the decomposition efficiency of the thermosetting resin is equivalent to or higher than that of Korean Patent No. 2530557 despite the significantly lower concentration of the aqueous hydrogen peroxide solution is that, as described above, the polymer chains of the thermosetting resin are broken by ultraviolet rays and hydroxyl radicals (OH·), resulting in a low molecular weight of the thermosetting resin, and oxidation and decomposition of the low molecular weight thermosetting resin progresses due to the high oxidizing power of the hydroxyl radicals (OH·), and at the same time, 2 mol of hydroxyl radicals (OH·) are generated from 1 mol of hydrogen peroxide (H2O2). Here, if the hydrogen peroxide concentration is less than 9%, the amount of hydroxyl radicals (OH·) generated is small, so the reaction time and external energy increase, which reduces the decomposition efficiency of the thermosetting resin. Since sufficient hydroxyl radicals (OH·) are generated at a hydrogen peroxide concentration of 15% or less, the hydrogen peroxide concentration does not need to exceed 15%.

[0052] Meanwhile, the method of generating hydroxyl radicals (OH·) through a combination of ultraviolet rays and hydrogen peroxide and decomposing thermosetting resins through these radicals has the additional advantage of not generating any significant reaction byproducts. In other words, ultraviolet irradiation only converts hydrogen peroxide (H2O2) into hydroxyl radicals (OH·), and no significant reaction byproducts are generated. On the other hand, combining ultraviolet rays and ozone (O3) generates harmful byproducts.

[0053] In the decomposition step of a thermosetting resin using ultraviolet rays and an aqueous hydrogen peroxide solution, a reaction time of 2 to 4 hours and a temperature of 80 to 95°C of the aqueous hydrogen peroxide solution can be applied. Figure 1 (d) is a photograph of carbon fibers collected after the decomposition step of a thermosetting resin using ultraviolet rays and an aqueous hydrogen peroxide solution is completed.

[0054] Step for removing residual synthetic organic substances using aqueous formic acid solution

[0055] Through the above-described 'thermosetting resin decomposition step using ultraviolet rays and hydrogen peroxide aqueous solution', the thermosetting resin present on the surface of the carbon fiber is decomposed and removed, but the synthetic organic substances generated during the decomposition process of the thermosetting resin may remain on the surface of the carbon fiber. Here, the synthetic organic substances are byproducts generated during the decomposition process of the thermosetting resin by hydroxyl radicals (OH·) and hydrogen peroxide, and are the opposite concept of inorganic substances, and have a different meaning from organic pollutants such as microorganisms in the water treatment field.

[0056] To remove these residual synthetic organic substances from the carbon fibers, a second impregnation process with an aqueous formic acid solution is performed. Specifically, after the "thermosetting resin decomposition step using UV light and an aqueous hydrogen peroxide solution" is completed, the carbon fibers are collected and impregnated with an aqueous formic acid solution to remove any residual synthetic organic substances present on the carbon fiber surface. During this process, the carbon fibers can be softened by the formic acid.

[0057] In the step of removing residual synthetic organic substances using a formic acid aqueous solution, the temperature of the formic acid aqueous solution is preferably maintained at 80°C or higher and less than 100°C, and the impregnation time can be selected within the range of 1 to 24 hours. In addition, the formic acid concentration in the formic acid aqueous solution is preferably set to 50% or higher and less than 100%.

[0058] After removing residual synthetic organic substances, the carbon fibers are collected and dried, thereby completing the method for recovering carbon fibers from waste carbon composites containing a thermosetting resin according to the present invention. Fig. 1 (e) illustrates carbon fibers recovered after removing residual synthetic organic substances, and Fig. 1 (f) is a photograph of the recovered carbon fibers after drying is completed.

[0059] Meanwhile, the 'step of removing residual synthetic organic substances using a formic acid aqueous solution' can be applied selectively. In one embodiment, a method for recovering carbon fibers from a waste carbon composite including a thermosetting resin according to the present invention can be configured by combining only the steps of swelling and exfoliating the waste carbon composite using a formic acid aqueous solution; and the step of decomposing the thermosetting resin using ultraviolet rays and a hydrogen peroxide aqueous solution; and the step of removing residual synthetic organic substances using a formic acid aqueous solution can be applied selectively as needed. In addition, each of the above-described steps is performed under normal pressure.

[0060] Above, a method for recovering carbon fibers from waste carbon composites containing a thermosetting resin according to the present invention has been described. Below, the present invention will be described in more detail through experimental examples.

[0061] Experimental Example 1: Decomposition Characteristics of Thermosetting Resins

[0062] The decomposition efficiency of thermosetting resins was analyzed by measuring the amount of residual organic substances remaining in the recovered carbon fibers after applying processes with different conditions. The processes with different conditions are divided into Example 1, Example 2, Example 3, and Comparative Example 1. In Comparative Example 1, CFRP was impregnated in a 90°C formic acid aqueous solution (concentration 80%) for 2 hours, and then the peeled carbon fibers were placed in a 90°C hydrogen peroxide aqueous solution (concentration 34.5%) and reacted for 4 hours, and then the carbon fibers were impregnated in a 90°C formic acid aqueous solution (concentration 80%) for 2 hours. Example 1 applied the same experimental conditions as Comparative Example 1, but during the reaction in the hydrogen peroxide aqueous solution, a UV lamp was installed in the hydrogen peroxide aqueous solution (concentration 34.5%) and UV rays were irradiated through the UV lamp. Example 2 applied the same experimental conditions as Example 1, but applied UV irradiation for 2 hours to a hydrogen peroxide aqueous solution having a concentration of 13 to 15%. Example 3 applied the same experimental conditions as Example 1, but applied UV irradiation to a hydrogen peroxide aqueous solution having a concentration of 9 to 10%. In addition, for the reliability of the analysis, the experiments of Comparative Example 1, Example 1, Example 2, and Example 3 were repeated three times.

[0063] After each of the processes of Comparative Example 1, Example 1, Example 2, and Example 3 was performed, the amount of residual organic matter remaining in the carbon fiber was measured through thermogravimetric analysis (TGA).

[0064] Referring to the results shown in Fig. 2, in the case of Comparative Example 1, an average of 0.87 to 0.89% of residual organic substances remained on the surface of the carbon fiber after the reaction in each sample, whereas in the case of Example 1, where ultraviolet rays were irradiated together with the same concentration of hydrogen peroxide, the high concentration of hydrogen peroxide acted as a scavenger that inhibited the formation of hydroxyl radicals (OH·), resulting in a residual organic substance of more than 1% being measured.

[0065] On the other hand, in the case of Example 2, even though the hydrogen peroxide concentration was reduced by 50% and the reaction time was reduced to 2 hours, it was confirmed that the residual organic matter was similar to or slightly reduced compared to Comparative Example 1 due to the continuous activation of hydroxyl radicals (OH·) by ultraviolet irradiation. In addition, in the case of Example 3, even though the hydrogen peroxide concentration was reduced by 75% compared to Comparative Example 1, the decomposition efficiency was similar due to the hydroxyl radicals (OH·) continuously formed during the reaction time.

[0066] Experimental Example 2: Physical Properties of Regenerated Carbon Fiber

[0067] Tensile strength, elongation, and elastic modulus were measured for carbon fibers, i.e., regenerated carbon fibers, recovered through each of the processes of Comparative Example 1, Example 1, Example 2, and Example 3.

[0068] As a result of the measurement, there was no significant difference in the properties of the recovered carbon fibers through Comparative Example 1, Example 1, Example 2, and Example 3, respectively. Looking at the average values ​​of Comparative Example 1, Example 1, Example 2, and Example 3 for the three experiments, in the case of tensile strength (see Fig. 3a), Comparative Example 1 had 5426.3 MPa, Example 1 had 5270.3 MPa, Example 2 had 4991.3 MPa, and Example 3 had 5206.5 MPa, showing that Comparative Example 1 had the best tensile strength characteristics, but Examples 1, 2, and 3 had tensile strength characteristics that were 3.4%, 3.2%, and 2.4% lower than that of Comparative Example 1, showing no significant difference from Comparative Example 1.

[0069] In the case of elongation (see Fig. 3b), Comparative Example 1 showed similar results with an average of 1.75%, while Examples 1, 2, and 3 showed similar results with 1.72, 1.71, and 1.72, respectively. Meanwhile, in the case of elastic modulus, which shows an inversely proportional characteristic with elongation (see Fig. 3c), a tendency for Examples 1, 2, and 3 to slightly increase compared to Comparative Example 1 was observed, and the average values ​​were 3.4%, 1.6%, and 2.2%, respectively.

[0070] Experimental Example 3: Surface Characteristics of Regenerated Carbon Fibers

[0071] SEM analysis was performed on the surfaces of the regenerated carbon fibers recovered through Examples 2 and 3. As shown in Fig. 4, it can be confirmed that the regenerated carbon fibers recovered through Examples 2 and 3, respectively, have smooth surfaces and are free of impurities.

Claims

1. A step of impregnating a waste carbon composite including a thermosetting resin into a formic acid aqueous solution to induce swelling of the waste carbon composite and exfoliation of carbon fibers; and It comprises a step of putting the peeled carbon fiber into a hydrogen peroxide solution and irradiating it with ultraviolet rays to decompose the thermosetting resin present on the surface of the carbon fiber; A method for recovering carbon fibers from a waste carbon composite material including a thermosetting resin, characterized in that hydroxyl radicals (OH·) are generated from hydrogen peroxide by the above ultraviolet irradiation, and the polymer chains of the thermosetting resin are broken by the ultraviolet irradiation and the hydroxyl radicals (OH·), thereby reducing the molecular weight of the thermosetting resin.

2. In the first paragraph, a step of putting the peeled carbon fiber into a hydrogen peroxide aqueous solution and irradiating it with ultraviolet rays to decompose the thermosetting resin present on the surface of the carbon fiber; thereafter, A method for recovering carbon fibers from a waste carbon composite material containing a thermosetting resin, characterized by carrying out a step of removing residual synthetic organic substances on the surface of the carbon fibers by impregnating the carbon fibers in a formic acid aqueous solution.

3. A method for recovering carbon fibers from a waste carbon composite material containing a thermosetting resin, characterized in that in the first paragraph, the hydrogen peroxide concentration of the hydrogen peroxide aqueous solution is 9 to 15%.

4. A method for recovering carbon fibers from a waste carbon composite material containing a thermosetting resin, characterized in that in the first paragraph, the formic acid concentration of the formic acid aqueous solution is 50 to 90%.

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