Carbon fiber recovery method and carbon fiber

Abstract

The invention provides a carbon fiber recovery method and a carbon fiber. The carbon fiber recovery method comprises the following steps: treating a carbon fiber prepreg into a chopped material with a preset size; placing the chopped material in an acidic medium, and carrying out first treatment under the action of a phase transfer catalyst to obtain an intermediate fiber product; and placing the intermediate fiber product in an alkaline medium, and carrying out second treatment under the action of a phase transfer catalyst to obtain the carbon fiber. According to the carbon fiber recovery method provided by the invention, through sequential design of acidolysis (first treatment)-alkali washing (second treatment), the problem of interface pollutant residues in carbon fiber recovery is solved. A first treatment process in an acidic medium exfoliates the polymer matrix, clearing obstacles for a subsequent second treatment in an alkaline medium. In the second treatment process in the alkaline medium, the interface of the intermediate fiber product is cleaned, the sizing agent is thoroughly removed, the performance of the carbon fibers can be reserved through the recycling method, and the clean carbon fibers can be obtained.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of material recycling, and particularly relates to a carbon fiber recycling method and carbon fiber. BACKGROUND

[0002] Carbon fiber-reinforced polymer (CFRP) has an irreplaceable application position in the fields of aerospace, automobile manufacturing, wind power generation and other cutting-edge fields due to its excellent lightweight and high-strength characteristics. However, as its application range expands, the offcuts generated in the production process, unqualified products, and prepreg waste formed after the product reaches the service life have constituted a serious environmental and resource challenge. These waste materials are difficult to naturally degrade, and traditional landfill disposal not only wastes valuable land resources, but also causes permanent loss of high-value carbon fibers. SUMMARY

[0003] To solve the problems in the related art, the present disclosure provides a carbon fiber recycling method and carbon fiber.

[0004] According to a first aspect of an embodiment of the present disclosure, a carbon fiber recycling method is provided, and the recycling method comprises: processing carbon fiber prepreg into chopped material of a preset size; placing the chopped material in an acidic medium and performing first processing under the action of a phase transfer catalyst to obtain an intermediate fiber product; placing the intermediate fiber product in an alkaline medium and performing second processing under the action of the phase transfer catalyst to obtain the carbon fiber.

[0005] In some embodiments of the present disclosure, the placing the chopped material in an acidic medium and performing first processing under the action of a phase transfer catalyst to obtain an intermediate fiber product comprises: placing the chopped material in the acidic medium, and decomposing a polymer matrix in the chopped material under the action of the phase transfer catalyst to obtain the intermediate fiber product.

[0006] In some embodiments of the present disclosure, the acidic medium comprises at least one of sulfuric acid, nitric acid, phosphoric acid, formic acid or acetic acid.

[0007] In some embodiments of the present disclosure, the acidic medium comprises the sulfuric acid, the formic acid and the acetic acid, and a mass ratio of the sulfuric acid, the formic acid and the acetic acid is (5.4-8.3):(0.8-1.5):(1.2-2.7).

[0008] In some embodiments of the present disclosure, the placing the intermediate fiber product in the alkaline medium and performing a second treatment on the intermediate fiber product under the action of the phase transfer catalyst to obtain the carbon fiber comprises: The placing the intermediate fiber product in the alkaline medium and hydrolyzing the sizing agent on the surface of the intermediate fiber product under the action of the phase transfer catalyst to obtain the carbon fiber.

[0009] In some embodiments of the present disclosure, the alkaline medium comprises at least one of sodium hydroxide or potassium hydroxide.

[0010] In some embodiments of the present disclosure, the phase transfer catalyst comprises at least one of quaternary ammonium salt or crown ether.

[0011] In some embodiments of the present disclosure, the processing the carbon fiber prepreg into the short-cut material of a preset size comprises: cooling the carbon fiber prepreg to make the polymer matrix in the carbon fiber prepreg brittle, and mechanically crushing the carbon fiber prepreg to obtain the short-cut material of a preset size; or impacting the carbon fiber prepreg by a fluid jet to cut the carbon fiber prepreg and strip the polymer matrix in the carbon fiber prepreg, to obtain the short-cut material of a preset size.

[0012] In some embodiments of the present disclosure, the recycling method further comprises: repeatedly washing the carbon fiber by a neutral solvent; monitoring the cleanliness of the washing liquid at a preset time interval, and stopping the washing until the cleanliness reaches a preset threshold; and drying the washed carbon fiber; or cleaning and drying the carbon fiber by a supercritical carbon dioxide extraction method.

[0013] According to a second aspect of the embodiments of the present disclosure, a carbon fiber is provided, which is obtained by the above-mentioned recycling method of carbon fiber.

[0014] The beneficial effects of the present disclosure include but are not limited to: the recycling method of carbon fiber provided by the present disclosure solves the problem of residual interface contaminants (especially sizing agent) in carbon fiber recycling through a unique sequential design of “acidolysis (first treatment)-alkali washing (second treatment)”. The first treatment process in the acidic medium gently strips the polymer matrix, removing the obstacle for the subsequent second treatment process in the alkaline medium. The second treatment process in the alkaline medium precisely cleans the interface of the exposed intermediate fiber product, completely removing the sizing agent. This unique sequential design of “acidolysis (first treatment)-alkali washing (second treatment)” avoids damage to the carbon fiber by pyrolysis, thereby preserving the performance of the carbon fiber and obtaining clean carbon fiber.

[0015] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0016] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the principles of the embodiments of the present disclosure. In the drawings:

[0017] Figure 1 Flow chart of a carbon fiber recycling method according to an example embodiment of the present disclosure. DETAILED DESCRIPTION

[0018] In order to make the objects, technical solutions and advantages of the present disclosure clearer, the technical solutions of the present disclosure will be described clearly and completely below in conjunction with the embodiments of the present disclosure. Obviously, the described embodiments are only some, but not all, of the embodiments of the present disclosure. Based on the embodiments in the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of the present disclosure. It should be noted that the embodiments in the present disclosure and the features in the embodiments can be combined with each other arbitrarily without conflict.

[0019] At present, the recycling technology for CFRP waste mainly includes pyrolysis method. However, the pyrolysis method has inherent and difficult-to-overcome defects. First, the process has huge energy consumption; second, high temperature inevitably causes thermal damage to the microstructure of carbon fibers, resulting in a decrease of 10% to 25% in mechanical strength.

[0020] Based on this, the present disclosure provides a carbon fiber recycling method, which solves the problem of residual interface pollutants (especially sizing agents) in carbon fiber recycling through a unique "acidolysis (first treatment)-alkali washing (second treatment)" sequential design. The first treatment process in the acid medium gently peels off the polymer matrix, removing obstacles for the subsequent second treatment process in the alkaline medium. The second treatment process in the alkaline medium precisely cleans the interface of the exposed intermediate fiber product, completely removing the sizing agent. This unique "acidolysis (first treatment)-alkali washing (second treatment)" sequential design avoids the damage to carbon fibers by pyrolysis method, thereby preserving the performance of carbon fibers and obtaining clean carbon fibers.

[0021] An example embodiment of the present disclosure provides a carbon fiber recycling method, which includes: Figure 1 The recycling method includes: S100, treating the carbon fiber prepreg into a preset size of chopped material.

[0022] Exemplarily, the carbon fiber prepreg can be carbon fiber prepreg waste.

[0023] Exemplarily, the preset size of the chopped material can be 0.5-30 mm in length along the extension direction of the carbon fiber. For example, the preset size of the chopped material can be 0.5 mm, 5 mm, 10 mm, 20 mm or 30 mm in length along the extension direction of the carbon fiber, and the preset size of the chopped material can also be any value between the exemplary sizes, for example, the preset size of the chopped material can be any value between 5-25 mm in length along the extension direction of the carbon fiber. In this embodiment, the carbon fiber prepreg is treated into a chopped material with a length of 0.5-30 mm along the extension direction of the carbon fiber, which can increase the contact area of the carbon fiber prepreg with the acidic medium or the alkaline medium in the subsequent treatment process, so that the acidic medium, the alkaline medium and the phase transfer catalyst can penetrate into the carbon fiber prepreg more quickly, shorten the processing time of the recovery process and improve the recovery efficiency.

[0024] S200, placing the chopped material in an acidic medium and performing a first treatment under the action of a phase transfer catalyst to obtain an intermediate fiber product.

[0025] The first treatment of the chopped material under the action of the acidic medium and the phase transfer catalyst can effectively decompose the polymer matrix in the chopped material without damaging the fiber structure to obtain the intermediate fiber product.

[0026] S300, placing the intermediate fiber product in an alkaline medium and performing a second treatment under the action of a phase transfer catalyst to obtain carbon fiber.

[0027] The second treatment of the intermediate fiber product under the action of the alkaline medium and the phase transfer catalyst can remove the sizing agent from the surface of the intermediate fiber product to obtain clean carbon fiber.

[0028] The carbon fiber recovery method provided in this embodiment solves the problem of residual interfacial contaminants (especially sizing agent) in carbon fiber recovery through a unique sequential design of "acidolysis (first treatment)-alkali washing (second treatment)". The first treatment process in the acidic medium gently peels off the polymer matrix, removing the obstacle for the subsequent second treatment process in the alkaline medium. The second treatment process in the alkaline medium precisely cleans the exposed interface of the intermediate fiber product and completely removes the sizing agent. This unique sequential design of "acidolysis (first treatment)-alkali washing (second treatment)" avoids damage to the carbon fiber by pyrolysis, thereby preserving the performance of the carbon fiber (mechanical performance retention rate > 95%) and obtaining clean carbon fiber.

[0029] In an exemplary embodiment, the chopped material is placed in an acidic medium and subjected to a first treatment under the action of a phase transfer catalyst to obtain an intermediate fiber product, comprising: placing the chopped material in an acidic medium, and decomposing the polymer matrix in the chopped material under the action of the phase transfer catalyst to obtain an intermediate fiber product.

[0030] The chopped material is immersed in an acidic medium and subjected to a first treatment under the action of a phase transfer catalyst, which can efficiently transport acidic protons in the acidic medium to the interior of the hydrophobic polymer matrix in the chopped material, thereby significantly accelerating the catalytic hydrolysis of the main chemical bonds (such as ether bonds, imine bonds) in the bulk polymer matrix network under milder conditions (such as lower temperature or shorter time), achieving rapid and complete decomposition of the polymer matrix, and exposing the intermediate fiber product interface with the sizing agent, laying the foundation for the subsequent second treatment process.

[0031] Exemplarily, the polymer matrix may, for example, be a thermosetting resin such as epoxy resin, bismaleimide resin, or vinyl ester resin.

[0032] In the first treatment process, the chopped material is immersed in an acidic medium and subjected to a first treatment under the action of a phase transfer catalyst, and the temperature is 60-80°C, and the ultrasonic oscillation and immersion time is 1.5-3h. After the first treatment, the intermediate fiber product is separated by filtration. The temperature of the first treatment process may, for example, be 60°C, 70°C, or 80°C, and the temperature of the first treatment process may also be any value between the exemplary temperatures, for example, the temperature of the first treatment process may be any value between 65-75°C. The duration of the first treatment process may, for example, be 1.5h, 2h, or 3h, and the duration of the first treatment process may also be any value between the exemplary durations, for example, the duration of the first treatment process may be any value between 2-2.5h.

[0033] In an exemplary embodiment, the acidic medium comprises at least one of sulfuric acid, nitric acid, phosphoric acid, formic acid, or acetic acid.

[0034] The acidic medium in this embodiment includes a variety of acidic medium options, which can be selected according to the type of polymer matrix and the type of carbon fiber in the recovered carbon fiber prepreg during the specific recovery process. For example, strong acids (such as sulfuric acid, nitric acid) can be used for strong chemical resistance polymer matrix, while weak acids (such as phosphoric acid, formic acid, acetic acid) can be used for sensitive carbon fiber to reduce the risk of corrosion of carbon fiber during the recovery process. It should be noted that strong and weak acids can also be used in combination to adjust the acidolysis reaction speed and depth in the first treatment process.

[0035] In an exemplary embodiment, the acidic medium comprises sulfuric acid, formic acid and acetic acid, wherein the mass ratio of sulfuric acid, formic acid and acetic acid is (5.4~8.3):(0.8~1.5):(1.2~2.7).

[0036] In this embodiment, the acidic medium comprising sulfuric acid, formic acid and acetic acid in a mass ratio of (5.4~8.3):(0.8~1.5):(1.2~2.7) is used to acidize the chopped material, wherein the sulfuric acid in a large proportion provides strong decomposition ability, so that the acidic medium can be used for polymer matrix with strong chemical resistance, and has high acidolysis efficiency, and the small amount of formic acid and acetic acid can enhance the permeability of the acidic medium to the polymer matrix, and the three together can quickly decompose the polymer matrix while minimizing the erosion of the carbon fiber. For example, the mass ratio of sulfuric acid, formic acid and acetic acid can be 5.4:0.8:1.2, 8.3:1.5:2.7, 7.0:1.0:2.0 or 6.5:1.2:2.3, and the mass ratio of sulfuric acid, formic acid and acetic acid can also be any value between the exemplary mass ratios, for example, the mass ratio of sulfuric acid, formic acid and acetic acid can be any value between (6.4~7.3):(1.0~1.2):(1.5~2.5).

[0037] In an exemplary embodiment, the intermediate fiber product is placed in an alkaline medium, and a second treatment is performed under the action of a phase transfer catalyst to obtain carbon fibers, comprising: placing the intermediate fiber product in an alkaline medium, and hydrolyzing the sizing agent on the surface of the intermediate fiber product under the action of a phase transfer catalyst to obtain carbon fibers.

[0038] The intermediate fiber product is placed in an alkaline medium, and a second treatment is performed under the action of a phase transfer catalyst, which can efficiently transport hydroxyl ions in the alkaline medium to the hydrophobic sizing agent layer on the surface of the intermediate fiber product, greatly enhancing the nucleophilic attack efficiency on the functional groups such as ester bonds and urethane bonds in the sizing agent, realizing rapid and complete saponification hydrolysis of the interfacial contaminants, realizing cleaning of the intermediate fiber product, and thus obtaining carbon fibers with clean surfaces.

[0039] Exemplarily, the sizing agent can include polymers of epoxy resin, polyester resin, polyurethane resin, polyether resin and combinations thereof.

[0040] The second treatment process can be that the intermediate fiber product is put into an alkaline medium, and under the action of a phase transfer catalyst, ultrasonic oscillation and soaking are performed at a temperature of 40-70°C for 0.5-1.5h. After the second treatment process is completed, the carbon fibers are separated by filtration. The temperature of the second treatment process can be, for example, 40°C, 55°C or 70°C. The temperature of the second treatment process can also be any value between the example temperatures, for example, the temperature of the second treatment process can be any value between 45-60°C. The duration of the second treatment process can be, for example, 0.5h, 1h or 1.5h. The duration of the second treatment process can also be any value between the example durations, for example, the duration of the second treatment process can be any value between 0.75-1h.

[0041] In an example embodiment, the alkaline medium comprises at least one of sodium hydroxide or potassium hydroxide.

[0042] In this embodiment, the sodium hydroxide or potassium hydroxide alkaline medium used is a strong base with strong hydrolysis ability, which can quickly hydrolyze functional groups such as ester bonds and carbamate bonds in the sizing agent, ensuring that the sizing agent on the surface of the intermediate fiber product is completely removed and ensuring the cleanliness of the carbon fibers. The solvent of the alkaline medium can be water or alcohol.

[0043] In an example embodiment, the phase transfer catalyst comprises at least one of a quaternary ammonium salt or a crown ether.

[0044] The quaternary ammonium salt (such as tetrabutylammonium hydrogen sulfate, benzyl triethyl ammonium chloride) or the crown ether as the phase transfer catalyst can promote the mass transfer between the aqueous phase (acidic medium and alkaline medium) and the organic phase (polymer matrix and sizing agent), for example, efficiently transporting acidic protons into the hydrophobic polymer matrix in the first treatment process, and efficiently transporting hydroxide ions to the hydrophobic sizing agent layer on the surface of the intermediate fiber product in the second treatment process, thereby improving the reaction rate and uniformity of the reaction.

[0045] In an example embodiment, the carbon fiber prepreg is processed into a short-cut material of a preset size, which comprises: cooling the carbon fiber prepreg to make the polymer matrix in the carbon fiber prepreg brittle, and mechanically crushing the carbon fiber prepreg to obtain a short-cut material of a preset size.

[0046] The carbon fiber prepreg is cooled by a low-temperature medium such as liquid nitrogen to a level far below the glass transition temperature of the polymer matrix, so that the carbon fiber prepreg is brittle. Then, a low-impact mechanical action is applied to break the carbon fiber prepreg into short-cut materials of a preset size, and induce brittle fracture of the polymer matrix, thereby forming a micro-crack network in the polymer matrix of the short-cut materials, which can enhance the permeability of the acidic medium and the alkaline medium in the subsequent processing. The breaking treatment of the short-cut materials of the carbon fiber prepreg in this embodiment is essentially different from the conventional crushing or grinding method which can cause serious degradation and thermal damage of the carbon fiber. The breaking treatment provided in this embodiment can generate a microstructure with enhanced permeability in the polymer matrix.

[0047] The carbon fiber prepreg is cooled by a low-temperature medium such as liquid nitrogen to a level far below the glass transition temperature of the polymer matrix, so that the carbon fiber prepreg is brittle. Then, a low-impact mechanical action is applied to break the carbon fiber prepreg into short-cut materials of a preset size, and induce brittle fracture of the polymer matrix, thereby forming a micro-crack network in the polymer matrix of the short-cut materials, which can enhance the permeability of the acidic medium and the alkaline medium in the subsequent processing. The breaking treatment of the short-cut materials of the carbon fiber prepreg in this embodiment is essentially different from the conventional crushing or grinding method which can cause serious degradation and thermal damage of the carbon fiber. The breaking treatment provided in this embodiment can generate a microstructure with enhanced permeability in the polymer matrix.

[0048] In an exemplary embodiment, the carbon fiber prepreg is processed into short-cut materials of a preset size, including: performing impact treatment on the carbon fiber prepreg by a fluid jet to cut the carbon fiber prepreg and strip the polymer matrix in the carbon fiber prepreg, to obtain short-cut materials of a preset size.

[0049] In this embodiment, the impact treatment on the carbon fiber prepreg by the fluid jet, for example, can use a high-pressure water jet carrying abrasive (pressure set to 200-300 MPa) to impact the carbon fiber prepreg at room temperature. The high-speed fluid uses the internal defects and weak carbon fiber-polymer matrix interface of the carbon fiber prepreg as a breakthrough point to remove the polymer matrix by erosion and stripping, and cuts the carbon fiber prepreg to obtain short-cut materials of a preset size. The impact treatment provided in this embodiment is a "cold working" process, which can maximize the original strength of the carbon fiber. The pressure of the jet fluid, for example, can be set to 200 MPa, 250 MPa or 300 MPa, and the pressure of the jet fluid can also be set to any value between the example pressures, for example, the pressure of the jet fluid can be set to any value between 220-280 MPa.

[0050] In an exemplary embodiment, the recycling method further includes: repeatedly washing the carbon fiber with a neutral solvent; monitoring the cleanliness of the washing liquid at a preset time interval, and stopping the washing when the cleanliness reaches a preset threshold; and drying the washed carbon fiber.

[0051] The recovered carbon fibers are repeatedly washed by a neutral solvent (such as deionized water or ethanol), which can effectively remove the residual acidic medium, basic medium, phase transfer catalyst, salt and degradation product, and prevent the chemical residues on the surface of the carbon fibers from affecting the performance or subsequent application of the recovered carbon fibers. During the repeated washing process, the cleanliness of the washing liquid is monitored at preset time intervals or in real time, for example, the characteristic peak intensity of the residual acidic medium, basic medium, phase transfer catalyst, salt and degradation product in the washing liquid flowing out of the washing process is monitored at preset time intervals or in real time by using an online Raman spectrum probe. The washing continues until the cleanliness reading of the online sensor is lower than a preset threshold, for example, the preset threshold can be set as that no characteristic peak of the residual acidic medium, basic medium, phase transfer catalyst, salt and degradation product is detected in the washing liquid, which indicates the arrival of the washing endpoint, and the washing is stopped. The washed carbon fibers are dried at 70-90°C. The washing method provided in the embodiment changes the traditional fixed-time-based rough cleaning into a data-driven and result-verified precise purification process. When the cleanliness of the washing liquid is monitored at preset time intervals, the preset time interval can be 1-5 minutes, for example, the preset time interval can be 1 minute, 2 minutes, 3 minutes or 5 minutes, and the preset time interval can also be any value between the example time intervals, for example, the preset time interval can be any value between 2-4 minutes. The drying temperature of the washed carbon fibers can be, for example, 70°C, 80°C or 90°C, and the drying temperature of the washed carbon fibers can also be any value between the example temperatures, for example, the drying temperature of the washed carbon fibers can be any value between 75-85°C.

[0052] In an example embodiment, the recovery method further comprises: cleaning and drying the carbon fibers by a supercritical carbon dioxide extraction method. The recovered carbon fibers are washed and dried by the supercritical carbon dioxide extraction method, which combines the washing and drying of the carbon fibers into one process without solvent residue, and can obtain carbon fibers with extremely high cleanliness.

[0053] An example embodiment of the present disclosure provides a carbon fiber obtained by the above-mentioned carbon fiber recovery method.

[0054] The carbon fiber obtained by the above-mentioned carbon fiber recovery method provided in the example embodiments of the present disclosure has high cleanliness and retains good performance and surface characteristics, and can be directly used in the preparation of new high-performance carbon fiber composites, thereby improving economic benefits and reducing pollution and waste of discarded prepreg.

[0055] Exemplarily, the recycling method of carbon fibers provided by the exemplary embodiments of the present disclosure recycles the obtained carbon fibers, which can be subjected to surface activation treatment to further increase the surface energy and / or introduce reactive functional groups on the surface thereof. The surface-activated carbon fibers are used as reinforcing bodies, blended with new high-performance polymer matrix materials, and molded by injection molding, compression molding, or extrusion to prepare regenerated composite material parts with excellent mechanical properties.

[0056] In order to more clearly explain the technical solutions provided by the exemplary embodiments of the present disclosure, two specific examples of the recycling method of carbon fibers provided by the exemplary embodiments of the present disclosure are given.

[0057] Embodiment One Take 2100 g of carbon fiber reinforced bismaleimide prepreg waste, immerse in liquid nitrogen for low-temperature embrittlement treatment for 5 minutes. Subsequently, the embrittled prepreg waste is sent to a low-impact extrusion crusher to obtain a chopped material with a size of 1-15 mm and rich in microcracks.

[0058] The above chopped material is put into a 2L mixed acid solution (mass fraction: 70% sulfuric acid, 10% formic acid, and 20% acetic acid) and tetrabutylammonium hydrogen sulfate is added as a phase transfer catalyst, ultrasonic oscillation and immersion are carried out at 60°C water bath for 1.5 hours. After treatment, the intermediate fiber product is separated by filtration.

[0059] The above intermediate fiber product is put into a 1L mixed alkaline solution (mass fraction: 6% sodium hydroxide and 4% potassium hydroxide aqueous solution) and benzyltriethylammonium chloride is added as a phase transfer catalyst, ultrasonic oscillation and immersion are carried out at 50°C for 45 minutes. After treatment, the carbon fibers are separated by filtration.

[0060] The above carbon fibers are repeatedly washed with deionized water in a circulating washing system with an online Raman spectrum probe until no characteristic peaks of residual acidic medium, basic medium, phase transfer catalyst, salt, and degradation product are detected in the effluent washing liquid, and the washing endpoint is confirmed. Then dried in an 80°C oven to obtain clean carbon fibers 681 g. According to the calculation of the resin content of 38% in the original prepreg, the fiber recovery rate reaches 90.2%.

[0061] In addition, the above recycled carbon fibers are placed in a plasma reaction chamber and treated in an oxygen atmosphere, with a power of 10W and a treatment time of 5 minutes. Since the surface of the carbon fibers is highly clean, the plasma can directly act on the carbon fibers to introduce oxygen-containing functional groups, significantly improving the surface activity.

[0062] The activated carbon fibers (25% of the total mass) are dispersed in a solution of polyamide acid in N-methylpyrrolidone, and after uniform stirring, they are placed in a mold, and are cured by heating to form a recycled carbon fiber reinforced polyimide composite material. Tests show that the tensile strength of the carbon fiber reinforced polyimide composite material reaches 147 MPa, which is comparable to the performance of a similar composite material (153 MPa) prepared using original chopped carbon fibers, proving that the carbon fibers recovered by the carbon fiber recovery method provided in the example embodiments of the present disclosure retain good performance.

[0063] Example Two 1000g of T800 grade carbon fiber / epoxy resin prepreg scraps are fixed on a workbench, and a high-pressure water jet system carrying garnet abrasive (pressure set to 250 MPa) is used for cutting and peeling, and by controlling the jet path and speed, chopped materials with an average length of about 10 mm are obtained. The process is carried out at room temperature, effectively avoiding thermal damage.

[0064] The chopped materials are put into a 1L mixed acid solution (mass fraction: 50% sulfuric acid, 30% acetic acid, 20% deionized water), and crown ether is added as a phase transfer catalyst, and stirred and ultrasonically oscillated at 80°C for 2 hours. After filtration and drying, an intermediate fiber product is obtained.

[0065] Alkaline washing (interface purification): The product is put into a 10% sodium hydroxide aqueous solution, and crown ether is added as a phase transfer catalyst, and stirred and ultrasonically oscillated at 60°C for 1 hour. After treatment, the carbon fibers are separated by filtration.

[0066] The above carbon fibers are subjected to final cleaning and drying using supercritical carbon dioxide (sc-CO2) extraction technology, which combines cleaning and drying into one process without leaving any solvent residue, and high-chemical-purity carbon fibers are obtained. Measurement shows that the single-fiber tensile strength retention rate of the carbon fibers is as high as 97%, proving that the carbon fibers recovered by the carbon fiber recovery method provided in the example embodiments of the present disclosure retain good performance.

[0067] In addition, the recovered carbon fibers are subjected to plasma treatment in a nitrogen / ammonia mixed atmosphere, with a power of 15W and a time of 8 minutes, to introduce nitrogen-containing functional groups on the surface of the carbon fibers and enhance the compatibility with the polymer matrix.

[0068] The activated carbon fibers are blended with polyether ether ketone (PEEK) powder at a mass ratio of 30:70, and composite material granules are prepared by a twin-screw extruder, and standard mechanical test samples are prepared by injection molding. Tests show that the tensile strength of the recycled composite material reaches 185 MPa, showing excellent mechanical properties.

[0069] In addition, the present disclosure also adopts a conventional pyrolysis recovery process as Comparative Example 1. Comparative Example 1 The same 1000g of T800 grade carbon fiber / epoxy resin prepreg scrap as in Example 2 was placed in a tube furnace and heated to 550°C at a rate of 10°C / min under nitrogen protection and maintained for 60 minutes. After cooling, carbon fibers with a small amount of carbonized residues on the surface were obtained. The recovered fibers were subjected to the same surface activation and re-composite, molding as in Example 2. The final recycled composite material had a tensile strength of only 145 MPa, which was significantly lower than the recycled composite material prepared from the recovered carbon fibers in Example 2.

[0070] It can be seen that the carbon fibers obtained by the carbon fiber recovery method provided by the exemplary embodiments of the present disclosure have high cleanliness and retain good performance and surface properties, and can be directly used in the preparation of new high-performance carbon fiber composites, thereby improving economic benefits and reducing pollution and waste of waste composites.

[0071] The above-described content can be implemented individually or in various combinations, and these variations are within the protection scope of the present disclosure.

[0072] Finally, it should be noted that in this document, the terms "comprise", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or equipment containing a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or equipment.

[0073] The above examples are only used to illustrate the technical solutions of the present disclosure, but not to limit it. Although the present disclosure has been described in detail with reference to the foregoing examples, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacements to part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present disclosure.

Claims

1. A method for recycling carbon fibers, characterized by, The recycling method comprises: processing carbon fiber prepreg into short-cut material of preset size; placing the short-cut material in an acidic medium and performing a first treatment under the action of a phase transfer catalyst to obtain an intermediate fiber product; placing the intermediate fiber product in an alkaline medium and performing a second treatment under the action of the phase transfer catalyst to obtain the carbon fiber.

2. The carbon fiber recovery method according to claim 1, characterized by, The placing of the short-cut material in an acidic medium and the first treatment under the action of a phase transfer catalyst to obtain an intermediate fiber product comprises: placing the short-cut material in the acidic medium, and decomposing the polymer matrix in the short-cut material under the action of the phase transfer catalyst to obtain the intermediate fiber product.

3. The carbon fiber recovery method according to claim 2, characterized by, The acidic medium comprises at least one of sulfuric acid, nitric acid, phosphoric acid, formic acid or acetic acid.

4. The carbon fiber recovery method according to claim 3, characterized by, The acidic medium comprises the sulfuric acid, the formic acid and the acetic acid, and the mass ratio of the sulfuric acid, the formic acid and the acetic acid is (5.4~8.3):(0.8~1.5):(1.2~2.7).

5. The carbon fiber recovery method according to claim 1, characterized by, The placing of the intermediate fiber product in an alkaline medium and the second treatment under the action of the phase transfer catalyst to obtain the carbon fiber comprises: placing the intermediate fiber product in the alkaline medium, and hydrolyzing the sizing agent on the surface of the intermediate fiber product under the action of the phase transfer catalyst to obtain the carbon fiber.

6. The carbon fiber recovery method according to claim 5, characterized by, The alkaline medium comprises at least one of sodium hydroxide or potassium hydroxide.

7. The carbon fiber recovery method according to any one of claims 1 to 6, characterized by, The phase transfer catalyst comprises at least one of quaternary ammonium salt or crown ether.

8. The carbon fiber recovery method according to claim 1, characterized by, The processing of carbon fiber prepreg into short-cut material of preset size comprises: cooling the carbon fiber prepreg to make the polymer matrix in the carbon fiber prepreg brittle, mechanically crushing the carbon fiber prepreg to obtain the short-cut material of preset size; or impacting the carbon fiber prepreg by fluid jet to cut the carbon fiber prepreg and strip the polymer matrix in the carbon fiber prepreg to obtain the short-cut material of preset size.

9. The carbon fiber recovery method according to claim 1, characterized by, The recycling method further comprises: repeatedly washing the carbon fiber by a neutral solvent; monitoring the cleanliness of the washing liquid at preset time intervals until the cleanliness reaches a preset threshold, and then stopping the washing; drying the washed carbon fiber; or cleaning and drying the carbon fiber by a supercritical carbon dioxide extraction method.

10. A carbon fiber, characterized by, The carbon fiber is obtained via the recycling method of carbon fiber according to any one of claims 1~9.

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