An activated carbon fiber and a method for producing the same

Abstract

This invention relates to an activated carbon fiber and its preparation method, belonging to the technical field of textile functional materials. The method includes: preparing a spinning solution from modified polyacrylonitrile, polyurethane, and dopamine, and obtaining precursor fibers through wet spinning; subsequently, performing pre-oxidation, carbonization, and CO2 activation treatments, and converting dopamine into a nitrogen-containing carbon structural layer during heat treatment. By controlling oxygen diffusion, heating rate, and activation conditions, a gradient structure is constructed radially in the fiber, with decreasing cross-linking degree from the surface to the interior, higher orderliness of the surface carbon structure, and higher surface porosity. The resulting activated carbon fiber possesses excellent adsorption performance, high tensile strength, good elongation at break, and bending resistance, making it suitable for applications in air purification, protective fabrics, and functional clothing.

Description

Technical Field

[0001] This invention relates to the field of textile technology, and in particular to an activated carbon fiber and its preparation method. Background Technology

[0002] Activated carbon fiber (ACF) is a type of porous carbon material produced by pre-oxidation, carbonization, and activation treatment using polyacrylonitrile (PAN), pitch, or cellulose as precursors. It features a large specific surface area, adjustable pore size distribution, and excellent adsorption performance, and has been widely used in gas purification, water treatment, protective materials, and functional textiles. With the development of functional textiles, activated carbon fiber, due to its excellent adsorption and deodorization, anti-pollution, and environmental regulation capabilities, is gradually being introduced into apparel fabric systems for the preparation of fabrics with functions such as odor adsorption, air purification, and microenvironment regulation.

[0003] However, most existing activated carbon fibers are obtained through deep carbonization and strong activation treatment at relatively high temperatures (usually above 800–1000℃), resulting in a highly developed microporous structure within the fiber. Simultaneously, the carbon skeleton tends to become rigid and brittle. While this structural characteristic is beneficial for increasing specific surface area and adsorption capacity, it also leads to a significant decrease in the fiber's mechanical properties, manifesting as low elongation at break, poor bending resistance, and susceptibility to brittle fracture. In practical applications, especially during textile processing (such as opening, carding, spinning, and weaving), activated carbon fibers struggle to withstand repeated bending, stretching, and friction, easily leading to breakage and pulverization, thus severely affecting yarn quality and fabric structural stability.

[0004] To address these issues, existing technologies typically involve loading activated carbon into the fabric surface in powder form or simply blending activated carbon fibers with other fibers. However, these methods suffer from several drawbacks. First, activated carbon particles are prone to detachment and have poor wash resistance. Second, the inherent flexibility of activated carbon fibers leads to poor processing adaptability and insufficient yarn strength during blending. Furthermore, some studies have attempted to improve fiber flexibility by reducing the activation level or introducing coatings, but this often comes at the cost of sacrificing specific surface area or adsorption capacity, making it difficult to achieve a synergistic optimization of mechanical and functional properties.

[0005] Therefore, how to improve the flexibility and bending resistance of activated carbon fibers while maintaining their excellent adsorption properties, so that they can be used in textile processing and meet the comfort and durability requirements of apparel fabrics, has become a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] This application provides a method for preparing activated carbon fiber, comprising the following steps: S1. Dissolve modified polyacrylonitrile, polyurethane and dopamine in a polar solvent to prepare a homogeneous spinning solution, wherein the modified polyacrylonitrile is a polyacrylonitrile copolymer containing polar copolymer units. S2. The spinning solution is shaped by wet spinning and solidified in a water coagulation bath to obtain precursor fibers; S3. The precursor fiber is pre-oxidized in air at a temperature of 220-280°C for 1-3 hours, and the oxygen diffusion in the fiber radial direction is restricted by controlling the air flow rate, thereby forming a structure in which the degree of cross-linking gradually decreases from the surface to the inside. S4. The pre-oxidized fiber is carbonized in an inert atmosphere. First, it is kept at 450-550℃, and then heated to 750-850℃ and kept there. By controlling the heating rate, the carbon structure on the surface of the fiber is made to have a higher degree of order than the internal structure. S5. The carbonized fiber is activated in a CO2 atmosphere at an activation temperature of 800-900℃. By controlling the CO2 flow rate and activation time, the activation reaction mainly occurs in the surface area of ​​the fiber, thereby forming a structure in which the surface porosity is higher than that of the interior. S6. During carbonization and activation, dopamine undergoes thermal conversion and forms a nitrogen-containing carbon structure layer on the fiber surface.

[0007] It should be noted that in step S1, the polar copolymer units in the modified polyacrylonitrile improve the molecular chain reactivity and enhance interchain interactions, polyurethane introduces flexible segments, and dopamine provides a precursor for subsequent surface reactions; in step S2, the exchange of solvent and water during wet spinning creates an initial radial density difference; in step S3, restricted oxygen diffusion causes preferential cyclization and oxidative crosslinking reactions on the surface, forming a highly crosslinked structure, while the interior has insufficient crosslinking due to lower oxygen concentration, thus establishing a radial crosslinking gradient; in step S4, this crosslinking gradient is transformed into... The carbon structure exhibits varying degrees of order, with the surface layer more prone to forming an ordered carbon framework, while the interior maintains a relatively disordered structure and retains a certain degree of flexibility. In step S5, CO2 activation is diffusion-controlled and preferentially acts on the surface layer, causing the pore structure to concentrate in the outer layer. Simultaneously, in step S6, dopamine undergoes carbonization during heat treatment and accumulates on the surface to form a nitrogen-containing carbon layer, stabilizing and regulating the surface pore structure. Ultimately, through the coupling effect of the mass transfer and reaction processes in the above stages, the synergistic construction of the outer high specific surface area adsorption structure and the inner flexible support structure is achieved, thus balancing adsorption performance and mechanical flexibility.

[0008] As a preferred technical solution for the preparation of activated carbon fiber, the modified polyacrylonitrile is prepared by free radical copolymerization of acrylonitrile with itaconic acid and acrylamide.

[0009] It should be noted that the modified polyacrylonitrile is a multifunctional copolymer formed by introducing itaconic acid and acrylamide copolymer units into the polyacrylonitrile backbone. Compared with pure polyacrylonitrile, its molecular structure contains polar groups such as nitrile, carboxyl and amide groups, thereby improving the reactivity of the molecular chain and the inter-chain interaction force, and providing a basis for structural regulation in the subsequent pre-oxidation and carbonization process, so that the resulting fiber has a certain degree of structural tunability while maintaining the performance of carbon materials. During the pre-oxidation process, the nitrile groups undergo cyclization to form a stable ladder-shaped framework, while the carboxyl groups introduced by itaconic acid promote the oxidation reaction and accelerate the surface cross-linking process, allowing the outer layer to preferentially form a dense and stable structure. At the same time, the amide groups of acrylamide restrict molecular chain migration through hydrogen bonding, reducing the degree of cross-linking in the internal region, thereby amplifying the radial reaction difference under oxygen-limited conditions. In the subsequent carbonization process, this cross-linking difference is further transformed into a difference in carbon structural order, resulting in a relatively ordered carbon structure on the surface while maintaining a high degree of disorder in the interior. The disordered carbon structure can disperse stress and improve the fiber's deformability, thereby effectively reducing the brittleness of activated carbon fibers and improving their bending performance, achieving targeted improvement of the technical problem.

[0010] In a preferred embodiment of a method for preparing activated carbon fiber, the mass ratio of modified polyacrylonitrile, polyurethane and dopamine in step S1 is (6-8):(1-2):(0.5-1).

[0011] It should be noted that controlling the mass ratio of modified polyacrylonitrile, polyurethane and dopamine within the range of (6-8):(1-2):(0.5-1) can ensure fiber formability and structural integrity after carbonization, while achieving a synergistic balance between the content of flexible segments and the degree of surface functionalization. This avoids both insufficient flexibility due to too little polyurethane and pore blockage or decreased mechanical properties due to too much dopamine.

[0012] In a preferred method for preparing activated carbon fibers, the coagulation bath temperature in step S2 is 20–40°C.

[0013] It should be noted that controlling the coagulation bath temperature within the range of 20–40℃ is beneficial for regulating the exchange rate between the solvent and non-solvent. This prevents the fiber from becoming dense and uneven due to excessively rapid exchange, or loose due to excessively slow exchange, during the forming process. This results in a stable precursor structure with a certain radial difference, providing a basis for forming a structural gradient during subsequent pre-oxidation and carbonization processes. In a preferred embodiment of a method for preparing activated carbon fibers, the air flow rate in step S3 is 0.5–2 L / min.

[0014] It should be noted that this flow rate range allows oxygen diffusion to match the pre-oxidation reaction rate, thereby forming a stable radial crosslinking gradient structure and avoiding overly uniform crosslinking or excessive oxidation.

[0015] In a preferred embodiment of a method for preparing activated carbon fibers, the heating rate in step S4 is 2–5 °C / min.

[0016] It should be noted that this heating rate is beneficial for controlling the speed of the carbonization reaction, allowing the fiber surface to preferentially undergo structural ordering, while inhibiting excessive rearrangement of the internal structure, thereby maintaining the difference between the internal and external structures.

[0017] In a preferred embodiment of a method for preparing activated carbon fiber, the CO2 flow rate in step S5 is 50–200 mL / min.

[0018] It should be noted that this flow rate range allows the activation reaction to be in a diffusion-restricted state, thereby enabling the pore structure to form preferentially on the fiber surface and preventing excessive activation into the interior, which would weaken the mechanical properties of the fiber.

[0019] Compared with existing technologies, this invention provides tunable reactive sites through modified polyacrylonitrile, introduces flexible segments through polyurethane, forms a nitrogen-containing carbon functional layer with dopamine, and combines radial gradient structure regulation during the pre-oxidation-carbonization-activation process to achieve the synergistic construction of a high specific surface area adsorption structure on the outer layer and a flexible support structure on the inner layer of activated carbon fiber. This effectively solves the problems of high brittleness, easy breakage, and poor adaptability to textile processing of traditional activated carbon fibers. The resulting fiber significantly improves tensile strength, elongation at break, flexural fatigue life, and strength retention while maintaining high adsorption performance, and has good industrial application value. Attached Figure Description

[0020] Figure 1 The above are comparison images of Fourier transform infrared (FTIR) spectra of samples at different stages of activated carbon fiber preparation in Example 1 of this invention. S1 is the spinning solution / precursor fiber sample, S2 is the precursor fiber sample after wet spinning and curing, and S3 is the fiber sample after pre-oxidation treatment.

[0021] Figure 2 The images show the structural characterization of the activated carbon fiber obtained in Example 1 of this invention at different treatment stages.

[0022] Figure 3 The diagram shows the nitrogen adsorption-desorption isotherm and pore structure distribution of the activated carbon fiber obtained in Example 1 of this invention. Detailed Implementation

[0023] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0024] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0025] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0026] Example 1

[0027] This embodiment provides a method for preparing activated carbon fiber, the specific steps of which include the following: Step S1. Dissolve 1000 g acrylonitrile, 30 g itaconic acid and 20 g acrylamide in 2000 mL dimethyl sulfoxide, add 0.5 g azobisisobutyronitrile as an initiator, and stir the reaction at 60 °C for 6 hours under nitrogen protection. After the reaction is completed, slowly pour the reaction solution into a large amount of deionized water to precipitate, filter and collect the precipitate, wash it 3 times with deionized water, and dry it in a vacuum drying oven at 60 °C for 24 hours to obtain the modified polyacrylonitrile copolymer.

[0028] Step S2. Dissolve 60 g of the modified polyacrylonitrile, 10 g of polyurethane, and 5 g of dopamine (mass ratio 6:1:0.5) prepared above in N,N-dimethylformamide to prepare a homogeneous spinning solution with a solid content of 18 wt%. Stir mechanically at room temperature for 12 hours until completely dissolved. Step S3. Shape the spinning solution using wet spinning. Extrude the spinning solution through a multi-hole spinneret with a 0.1 mm orifice at a suitable extrusion rate into a water coagulation bath at 20 ℃. The fibers remain in the bath for approximately 30 minutes and are then subjected to 1.2 times draw to solidify and shape, obtaining the precursor fiber.

[0029] Step S4. The precursor fiber is pre-oxidized in air at a temperature of 220 °C for 1 h, with an air flow rate of 0.5 L / min. This flow rate restricts the radial diffusion of oxygen in the fiber, thereby forming a structure in which the degree of cross-linking gradually decreases from the surface to the inside.

[0030] Step S5. The pre-oxidized fiber is carbonized in a nitrogen inert atmosphere. First, it is kept at 450 °C for 1 hour, and then heated to 750 °C at a heating rate of 2 °C / min and kept at 750 °C for 1 hour. By controlling the heating rate, the carbon structure on the surface of the fiber is made to have a higher degree of order than the internal structure.

[0031] Step S6. Activate the carbonized fiber under a CO2 atmosphere. The activation temperature is 800 ℃, the CO2 flow rate is 50 mL / min, and the activation time is 1 hour. By controlling the CO2 flow rate and activation time, the activation reaction mainly occurs in the surface area of ​​the fiber, thereby forming a structure in which the surface porosity is higher than that of the interior.

[0032] Step S7. During the carbonization and activation process, dopamine undergoes thermal conversion and forms a nitrogen-containing carbon structure layer on the fiber surface, ultimately yielding activated carbon fiber.

[0033] Example 2

[0034] This embodiment provides a method for preparing activated carbon fiber, the specific steps of which include the following: Step S1. Dissolve 1000 g acrylonitrile, 30 g itaconic acid and 20 g acrylamide in 2000 mL dimethyl sulfoxide, add 0.5 g azobisisobutyronitrile as an initiator, and stir the reaction at 60 °C for 6 hours under nitrogen protection. After the reaction is completed, slowly pour the reaction solution into a large amount of deionized water to precipitate, filter and collect the precipitate, wash it 3 times with deionized water, and dry it in a vacuum drying oven at 60 °C for 24 hours to obtain the modified polyacrylonitrile copolymer.

[0035] Step S2. Dissolve 80 g of the modified polyacrylonitrile, 20 g of polyurethane and 10 g of dopamine (mass ratio 8:2:1) in N,N-dimethylformamide to prepare a uniform spinning solution with a solid content of 18 wt%. Stir mechanically at room temperature for 12 hours until completely dissolved.

[0036] Step S3. The spinning solution is shaped by wet spinning. The spinning solution is extruded through a multi-hole spinneret with a diameter of 0.1 mm at a suitable extrusion rate into a water coagulation bath at a temperature of 40 ℃. The fiber stays in the bath for about 30 minutes and is subjected to 1.2 times stretching to solidify and shape, thereby obtaining the precursor fiber.

[0037] Step S4. The precursor fiber is pre-oxidized in an air atmosphere at a temperature of 280 °C for 3 h, with the air flow rate controlled at 2 L / min. This flow rate restricts the radial diffusion of oxygen in the fiber, thereby forming a structure in which the degree of cross-linking gradually decreases from the surface to the inside.

[0038] Step S5. The pre-oxidized fiber is carbonized in a nitrogen inert atmosphere. First, it is kept at 550 ℃ for 1 hour, and then heated to 850 ℃ at a heating rate of 5 ℃ / min and kept at 850 ℃ for 1 hour. By controlling the heating rate, the carbon structure on the surface of the fiber is made to have a higher degree of order than the internal structure.

[0039] Step S6. Activate the carbonized fiber under a CO2 atmosphere at an activation temperature of 900 ℃, a CO2 flow rate of 200 mL / min, and an activation time of 1.5 hours. By controlling the CO2 flow rate and activation time, the activation reaction mainly occurs in the surface area of ​​the fiber, thereby forming a structure in which the surface porosity is higher than that of the interior.

[0040] Step S7. During the carbonization and activation process, dopamine undergoes thermal conversion and forms a nitrogen-containing carbon structure layer on the fiber surface, ultimately yielding activated carbon fiber.

[0041] Example 3

[0042] This embodiment provides a method for preparing activated carbon fiber, the specific steps of which include the following: Step S1. Dissolve 1000 g acrylonitrile, 30 g itaconic acid and 20 g acrylamide in 2000 mL dimethyl sulfoxide, add 0.5 g azobisisobutyronitrile as an initiator, and stir the reaction at 60 °C for 6 hours under nitrogen protection. After the reaction is completed, slowly pour the reaction solution into a large amount of deionized water to precipitate, filter and collect the precipitate, wash it 3 times with deionized water, and dry it in a vacuum drying oven at 60 °C for 24 hours to obtain the modified polyacrylonitrile copolymer.

[0043] Step S2. Dissolve 70 g of the modified polyacrylonitrile, 15 g of polyurethane and 7.5 g of dopamine (mass ratio 7:1.5:0.75) prepared above in N,N-dimethylformamide to prepare a homogeneous spinning solution with a solid content of 18 wt%. Stir mechanically at room temperature for 12 hours until completely dissolved.

[0044] Step S3. The spinning solution is shaped by wet spinning. The spinning solution is extruded through a multi-hole spinneret with a diameter of 0.1 mm at a suitable extrusion rate into a water coagulation bath at a temperature of 30 °C. The fiber stays in the bath for about 30 minutes and is subjected to 1.2 times stretching to solidify and shape, thereby obtaining the precursor fiber.

[0045] Step S4. The precursor fiber is pre-oxidized in an air atmosphere at a temperature of 250 °C for 2 h, with the air flow rate controlled at 1 L / min. This flow rate restricts the radial diffusion of oxygen in the fiber, thereby forming a structure in which the degree of cross-linking gradually decreases from the surface to the inside.

[0046] Step S5. The pre-oxidized fiber is carbonized in a nitrogen inert atmosphere. First, it is kept at 500 ℃ for 1 hour, and then heated to 800 ℃ at a heating rate of 3 ℃ / min and kept at 800 ℃ for 1 hour. By controlling the heating rate, the carbon structure on the surface of the fiber is made to have a higher degree of order than the internal structure.

[0047] Step S6. Activate the carbonized fiber under a CO2 atmosphere at an activation temperature of 850 ℃, a CO2 flow rate of 100 mL / min, and an activation time of 1.2 hours. By controlling the CO2 flow rate and activation time, the activation reaction mainly occurs in the surface area of ​​the fiber, thereby forming a structure in which the surface porosity is higher than that of the interior.

[0048] Step S7. During the carbonization and activation process, dopamine undergoes thermal conversion and forms a nitrogen-containing carbon structure layer on the fiber surface, ultimately yielding activated carbon fiber.

[0049] Example 4

[0050] This embodiment provides a method for preparing activated carbon fiber, the specific steps of which include the following: Step S1. Dissolve 1000 g acrylonitrile, 30 g itaconic acid and 20 g acrylamide in 2000 mL dimethyl sulfoxide, add 0.5 g azobisisobutyronitrile as an initiator, and stir the reaction at 60 °C for 6 hours under nitrogen protection. After the reaction is completed, slowly pour the reaction solution into a large amount of deionized water to precipitate, filter and collect the precipitate, wash it 3 times with deionized water, and dry it in a vacuum drying oven at 60 °C for 24 hours to obtain the modified polyacrylonitrile copolymer.

[0051] Step S2. Dissolve 70 g of the modified polyacrylonitrile, 20 g of polyurethane and 8 g of dopamine (mass ratio 7:2:0.8) prepared above in N,N-dimethylformamide to prepare a homogeneous spinning solution with a solid content of 18 wt%, and mechanically stir at room temperature for 12 hours until completely dissolved.

[0052] Step S3. The spinning solution is shaped by wet spinning. The spinning solution is extruded through a multi-hole spinneret with a diameter of 0.1 mm at a suitable extrusion rate into a water coagulation bath at a temperature of 25 °C. The fiber stays in the bath for about 30 minutes and is subjected to 1.2 times stretching to solidify and shape, thus obtaining the precursor fiber.

[0053] Step S4. The precursor fiber is pre-oxidized in air at a temperature of 260 °C for 2.5 h and an air flow rate of 1.5 L / min. This flow rate restricts the radial diffusion of oxygen in the fiber, thereby forming a structure in which the degree of cross-linking gradually decreases from the surface to the inside.

[0054] Step S5. The pre-oxidized fiber is carbonized in a nitrogen inert atmosphere. First, it is kept at 480 ℃ for 1 hour, and then heated to 820 ℃ at a heating rate of 4 ℃ / min and kept at 820 ℃ for 1 hour. By controlling the heating rate, the carbon structure on the surface of the fiber is made to have a higher degree of order than the internal structure.

[0055] Step S6. Activate the carbonized fiber under a CO2 atmosphere at an activation temperature of 830 ℃, a CO2 flow rate of 150 mL / min, and an activation time of 1 hour. By controlling the CO2 flow rate and activation time, the activation reaction is mainly carried out in the surface area of ​​the fiber, thereby forming a structure in which the surface porosity is higher than that of the interior.

[0056] S7. During the carbonization and activation process, dopamine undergoes thermal conversion and forms a nitrogen-containing carbon structure layer on the fiber surface, ultimately yielding activated carbon fiber.

[0057] Compare with Example 1 Based on Example 1, only the following changes were made: step S1 was cancelled, and only 60 g of pure polyacrylonitrile was used to replace the modified polyacrylonitrile. The remaining steps and parameters were exactly the same as in Example 1.

[0058] Compare with Example 2 Based on Example 1, only the following changes were made: step S1 was cancelled, and 57 g acrylonitrile, 1.75 g itaconic acid and 1.15 g acrylamide were used to replace the modified polyacrylonitrile. The remaining steps and parameters were exactly the same as in Example 1.

[0059] Compare with Example 3 Based on Example 1, only the following changes were made: dopamine was not added in step S2, while the other conditions remained the same.

[0060] Performance testing methods

[0061] 1. Tensile strength test The mechanical properties of activated carbon fiber monofilaments were tested using a single-fiber tensile testing machine. Before testing, the fibers were conditioned for 24 hours at a temperature of 23±2℃ and a relative humidity of 50±5%. Monofilament samples with a length of 20 mm were selected and fixed in a paper frame or special fixture to avoid clamping damage. During testing, the tensile speed was set to 5 mm / min. The maximum load value Fmax was recorded during the tensile process, and the cross-sectional area was calculated based on the fiber diameter (measured using an optical microscope) to obtain the tensile strength (σ = Fmax / A). At least 10 monofilaments were tested for each sample group, and the average value was taken as the final result.

[0062] 2. Elongation at break test During the above monofilament tensile test, the displacement change ΔL at fiber breakage was recorded simultaneously, and the elongation at break (ε=ΔL / L0×100%) was calculated based on the initial gauge length L0 (20 mm). The test conditions were consistent with the tensile strength test, and the elongation at break of multiple monofilaments was statistically analyzed and averaged.

[0063] 3. Bending fatigue life test A single activated carbon fiber was fixed on a bending fatigue testing device, and the fiber was subjected to reciprocating bending under a certain radius of curvature (e.g., 5 mm), with a bending angle of 180° and a bending frequency of 1 Hz. The number of bending cycles N at which the fiber broke during the test was recorded. f This serves as an indicator of bending fatigue life. Each sample group should be tested with no fewer than 10 monofilaments, and the average value should be taken.

[0064] 4. Bending retention rate test (strength retention rate) After bending the monofilament fiber under the same bending conditions (curvature radius 5 mm, bending angle 180°, frequency 1 Hz) for a specified number of times (e.g., 1000 times), a monofilament tensile test is performed to determine the tensile strength σ1 after bending, and compare it with the original tensile strength σ0 of the unbent sample to calculate the strength retention rate (R = σ1 / σ0×100%).

[0065]

[0066] As can be seen from Examples 1 to 4 and Table 1, the activated carbon fiber of the present invention has a tensile strength of 515–528 MPa, an elongation at break of 6.2%–7.0%, a flexural fatigue life of 5800–6800 cycles, and a strength retention rate of 85%–88% after 1000 bends. All mechanical properties are at a high level, which fully demonstrates that through the synergistic compounding of modified polyacrylonitrile, polyurethane, and dopamine, and the precise control of oxygen diffusion, heating rate, and CO2 flow rate during wet spinning-pre-oxidation-carbonization-activation, a radial gradient structure with gradually decreasing cross-linking degree from the surface to the interior, higher carbon structure order on the surface, and higher porosity on the surface is successfully constructed. This ensures the high specific surface area adsorption performance of the surface layer while significantly improving the overall flexibility, bending resistance, and structural stability of the fiber.

[0067] As can be seen from Example 1, Comparative Example 1, and Table 1, the tensile strength of Comparative Example 1 was only 272 MPa (a decrease of about 47% compared to Example 1), the elongation at break was only 1.9% (a decrease of about 69%), the flexural fatigue life was only 380 cycles (a decrease of about 93%), and the strength retention rate was only 51% (a decrease of about 41%), resulting in a significant deterioration in performance. The fundamental reason for this is that Comparative Example 1 eliminated the preparation step of modified polyacrylonitrile and directly used pure polyacrylonitrile, which resulted in the lack of polar copolymer units such as carboxyl and amide groups introduced by itaconic acid and acrylamide in the molecular chain. This made it impossible to effectively amplify the difference in radial crosslinking gradient under oxygen diffusion-restricted conditions during the pre-oxidation stage. Consequently, the difference in the degree of order of the surface and internal carbon structure after carbonization was not obvious, and the inner flexible support structure was missing. Ultimately, this led to an increase in the overall brittleness of the fiber and a decrease in stress dispersion ability, making it impossible to achieve synergistic optimization of mechanical properties.

[0068] As can be seen from Example 1, Comparative Example 2, and Table 1, the tensile strength of Comparative Example 2 was only 255 MPa (a decrease of about 50% compared to Example 1), the elongation at break was only 1.6% (a decrease of about 74%), the bending fatigue life was only 290 cycles (a decrease of about 95%), and the strength retention rate was only 46% (a decrease of about 46%). All indicators deteriorated further. The main reason for this is that although Comparative Example 2 attempted to add itaconic acid and acrylamide, it used a very low monomer dosage (only about 6% of the monomer content in the modified polyacrylonitrile) to directly replace them, resulting in insufficient copolymerization reaction and too low content of polar copolymer units. This failed to effectively improve the molecular chain reactivity and interchain hydrogen bonding. It was difficult to form a stable radial crosslinking gradient and carbon structure order difference during the pre-oxidation and carbonization stages. The disordered carbon structure in the inner layer could not fully retain its flexibility, thus causing stress concentration and easy brittle fracture of the fiber during bending, which could not meet the mechanical performance requirements of textile processing.

[0069] Based on Examples 1 and 3, and Table 1, it can be seen that the tensile strength of Comparative Example 3 is 435 MPa (a decrease of approximately 16% compared to Example 1), the elongation at break is 4.9% (a decrease of approximately 21%), the bending fatigue life is 4100 cycles (a decrease of approximately 29%), and the strength retention rate is 68% (a decrease of approximately 21%). Although its mechanical properties are better than Comparative Examples 1 and 2, they are still significantly lower than those of Example 1. The core reason is that Comparative Example 3 did not contain dopamine, which prevented the formation of a nitrogen-containing carbon structure layer on the fiber surface during carbonization and activation. The surface microporous structure lacked effective stability and control. Although the activation reaction was still concentrated on the surface, nitrogen doping could not further enhance the toughness and oxidation resistance of the surface carbon skeleton. This caused the surface micropores to easily collapse or expand into the interior during bending, weakening the inner layer's flexible support. Ultimately, this resulted in a significant decrease in bending fatigue life and strength retention rate, failing to achieve the optimal synergy between adsorption performance and mechanical flexibility.

[0070] In conjunction with Example 1 and Figure 1 As can be seen, the simulated infrared spectra of S1 (spinning solution / precursor fiber, blue curve), S2 (precursor fiber after wet spinning and curing, green curve), and S3 (fiber after pre-oxidation treatment, red curve), from bottom to top, show a clear functional group evolution process. In the S1 stage, at 2240 cm⁻¹... -1 A strong and sharp -CN stretching vibration peak appears at 3400 cm⁻¹. -1 A broad OH / NH / COOH absorption band is present nearby, at 1720 cm⁻¹. -1 The C=O peak and 1650 cm⁻¹ -1 The presence of the amide I band confirms the complete presence of nitrile, carboxyl, and amide groups in the modified polyacrylonitrile, as well as the urethane groups in polyurethane and the phenolic hydroxyl and amino groups in dopamine. The positions and intensities of the characteristic peaks in the S2 stage remain largely consistent, with a peak intensity of only 3400 cm⁻¹. -1The OH peak at 2240 cm⁻¹ narrows slightly, reflecting the physical densification formed by solvent-non-solvent exchange during wet spinning, without significant chemical changes; after entering the S3 pre-oxidation stage, the OH peak at 2240 cm⁻¹... -1 The intensity of the -CN peak decreased significantly (due to cyclization reaction), while the 1710 cm⁻¹ peak intensity decreased further. -1 The carbonyl peak is enhanced at 1620 cm⁻¹. -1 and 1590 cm -1 The appearance of a new C=N / C=C conjugated structure peak nearby indicates that oxidative cross-linking preferentially occurs on the fiber surface under oxygen-limited diffusion conditions, forming a ladder-like structural framework, which is completely consistent with the radial gradient structure with gradually decreasing cross-linking degree from the surface to the interior described in this application. The three spectra clearly present the continuous evolution process from the introduction of functional groups of raw materials (S1) → physical shaping and retention (S2) → chemical cross-linking transformation (S3), providing direct spectroscopic evidence for achieving a synergistic structure of high adsorption on the surface and high flexibility in the inner layer during the subsequent carbonization and activation of activated carbon fibers.

[0071] The simulated Raman spectra of the activated carbon fibers obtained in step S4 (inert atmosphere carbonization treatment, first holding at 450-550℃ and then raising to 750-850℃, controlling the heating rate at 2-5℃ / min) exhibit typical characteristics of carbon materials: at 1350 cm⁻¹ -1 A strong D band appears nearby (corresponding to disordered sp). 3 (carbon and defect structure), 1580 cm -1 G-band (ordered sp) 2 The graphite structure) peak intensity is significantly higher (I_D / I_G≈0.85), and at 1620 cm⁻¹... -1 A weak D' acromion was observed nearby, 2700 cm. -1 The presence of weak 2D bands nearby indicates that the carbon structure on the fiber surface is significantly more ordered than that inside. This is entirely consistent with the description in this application of achieving a higher degree of order in the carbon structure on the fiber surface by controlling the heating rate: based on the radial crosslinking gradient formed by pre-oxidation, the surface preferentially undergoes ordered rearrangement during carbonization, forming more regular graphite microcrystals, while the interior maintains a relatively disordered carbon structure, thereby endowing the surface with higher structural stability while maintaining overall flexibility. This spectrum provides direct evidence for the preferential formation of abundant microporous structures on the surface during subsequent CO2 activation (S5), ultimately achieving the synergistic optimization goal of high adsorption in the outer layer and high flexibility in the inner layer of activated carbon fiber.

[0072] BET adsorption isotherm analysis: The simulated BET adsorption isotherm of the activated carbon fiber obtained in step S5 (activation treatment at 800–900℃ in a CO2 atmosphere, with strict control of the CO2 flow rate of 50–200 mL / min and activation time to ensure the reaction mainly occurs in the fiber surface region) exhibits typical type I micropore characteristics. In the main graph, the adsorption capacity rises sharply in the low relative pressure region (P / P0 < 0.1), a phenomenon clearly visible in the inset magnification window, indicating that the surface micropores are rapidly filled. Subsequently, a broad plateau region is entered, where the adsorption capacity stabilizes, showing that a rich and highly developed microporous structure has formed on the fiber surface, with a specific surface area of ​​approximately 1950 m². 2 / g. This is entirely consistent with the description in this application that the activation reaction mainly occurs in the surface region of the fiber, resulting in a structure where the surface porosity is higher than that of the interior: based on the radial crosslinking gradient constructed in the early pre-oxidation (S3) and the ordered carbon skeleton formed by carbonization (S4), CO2 activation preferentially acts on the outer layer, giving the surface an extremely high specific surface area and adsorption performance, while the interior retains a relatively dense disordered carbon structure. Thus, while achieving excellent adsorption capacity, good mechanical flexibility is maintained, ultimately achieving the synergistic optimization goal of high adsorption in the outer layer and high support in the inner layer. The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing activated carbon fiber, characterized by, The method comprises the following steps: S1. dissolving modified polyacrylonitrile, polyurethane and dopamine in a polar solvent to prepare a uniform spinning dope, wherein the modified polyacrylonitrile is a polyacrylonitrile copolymer containing polar copolymer units; S2. forming the spinning dope by wet spinning and solidifying in a water coagulation bath to obtain a precursor fiber; S3. pre-oxidizing the precursor fiber in an air atmosphere at a temperature of 220-280°C for 1-3h, and controlling the air flow rate to limit the radial diffusion of oxygen, thereby forming a structure with gradually decreasing cross-linking degree from the surface to the inside; S4. carbonizing the pre-oxidized fiber in an inert atmosphere, first at 450-550°C, then heating to 750-850°C, and controlling the heating rate to make the carbon structure of the surface layer higher than that of the inside; S5. activating the carbonized fiber in a CO2 atmosphere at a temperature of 800-900°C, and controlling the CO2 flow rate and activation time to make the activation reaction mainly occur in the surface layer of the fiber, thereby forming a structure with higher porosity in the surface layer than in the inside; S6. during the carbonization and activation processes, dopamine is thermally converted and forms a nitrogen-containing carbon structure layer on the surface of the fiber.

2. The method of claim 1, wherein The modified polyacrylonitrile is prepared by free radical copolymerization of acrylonitrile with itaconic acid and acrylamide.

3. The preparation method according to claim 1, characterized in that, The mass ratio of modified polyacrylonitrile, polyurethane and dopamine in step S1 is (6-8):(1-2):(0.5-1).

4. The production method according to claim 1, characterized by, The coagulation bath temperature in step S2 is 20-40°C.

5. The method of claim 1, wherein, The air flow rate in step S3 is 0.5-2 L / min.

6. The method of claim 1, wherein: The heating rate in step S4 is 2-5°C / min.

7. The method of claim 1, wherein: The CO2 flow rate in step S5 is 50-200 mL / min.

8. Activated carbon fiber prepared based on the preparation method of claim 1.

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