Dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fiber and method of making the same

Abstract

The application relates to a dry-jet wet-spun ultra-high-performance polyacrylonitrile-based carbon fiber and a preparation method thereof, and belongs to the technical field of carbon fibers. The preparation method of the dry-jet wet-spun ultra-high-performance polyacrylonitrile-based carbon fiber comprises the following steps: S1, mixing acrylonitrile, a comonomer, a chain transfer agent, an initiator and a solvent, performing a polymerization reaction through a solvent dropping method, and preparing a spinning stock solution; wherein the comonomer comprises a first comonomer, a second comonomer and a third comonomer; S2, preparing a precursor fiber through coagulation, water washing, water drawing, oiling, drying, and steam drawing of the spinning stock solution; S3, preparing an ultra-high-performance polyacrylonitrile-based carbon fiber through pre-oxidation, low-temperature carbonization, high-temperature carbonization, graphitization, surface treatment, sizing and drying of the precursor fiber in sequence; wherein the pre-oxidation adopts four-stage pre-oxidation treatment. The strength and modulus of the carbon fiber can be effectively and synergistically optimized, and the carbon fiber with high strength and high modulus can be prepared.

Description

Technical Field

[0001] This application relates to the field of carbon fiber technology, and in particular to a dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fiber and its preparation method. Background Technology

[0002] Carbon fiber, with its superior specific strength and high modulus, has become a key strategic material driving modern industrial innovation. It demonstrates extremely broad and far-reaching application prospects in aerospace, enabling aircraft to soar through the skies; in new energy, empowering lightweight batteries and wind turbine blades; in infrastructure, reinforcing bridges and buildings; and in transportation, enabling nimble automobiles and rail transit. Among these applications, carbon fiber with its ultra-high mechanical properties is not only the core foundation for achieving the ultimate lightweighting and ultra-high structural rigidity requirements of next-generation aerospace equipment (such as hypersonic aircraft, next-generation fighter jets, large civilian airliners, and advanced satellites), but also an indispensable "material engine" for equipment performance breakthroughs and iterative upgrades, having a decisive impact on aerospace capabilities and the competitiveness of cutting-edge equipment.

[0003] Currently, most high-performance carbon fibers are produced by binary copolymerization of acrylonitrile (AN) and itaconic acid (ITA). The carboxyl groups in ITA initiate intramolecular cyclization, transforming the free radical cyclization of acrylonitrile into ionic cyclization, thus lowering the initial exothermic temperature and increasing the degree of fiber pre-oxidation. However, ITA as a comonomer has at least two drawbacks. On the one hand, in polyacrylonitrile (PAN) formed by AN-ITA copolymerization, the carboxyl groups easily form numerous hydrogen bonds with electronegative atoms (such as N and O), hindering the stretching and extension of the molecular chain. On the other hand, an accelerated pre-oxidation rate is more likely to lead to a core-sheath structure characterized by excessive surface cyclization and insufficient internal cyclization. Furthermore, the molecular chain structure defects of PAN formed by the binary copolymerization of acrylonitrile and itaconic acid can induce the generation of new defects during spinning and carbonization, which are then inherited and amplified into the carbon fiber. Simultaneously, the defects introduced externally by PAN are not eliminated during subsequent spinning and carbonization processes.

[0004] Therefore, it is difficult to achieve both high strength and high modulus in carbon fibers prepared by existing technologies. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the objective of this application is to provide a dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fiber and its preparation method. This method can effectively synergistically optimize the strength and modulus of the carbon fiber, achieving the preparation of carbon fibers with both high strength and high modulus.

[0006] In a first aspect, embodiments of this application provide a method for preparing dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fibers, comprising:

[0007] S1, a mixed solution is obtained by mixing acrylonitrile, comonomer, chain transfer agent, initiator and solvent. Solution polymerization reaction is carried out at the polymerization temperature by solvent dropwise to prepare the polymerization stock solution. The polymerization stock solution is then subjected to monomer removal, degassing and amination to prepare the spinning stock solution.

[0008] The comonomers include a first comonomer, a second comonomer, and a third comonomer; the first comonomer is selected from vinyl acetate, the second comonomer is selected from at least one of acrylic acid and methacrylic acid, and the third comonomer is selected from at least one of isobutyl acrylate and isobutyl methacrylate.

[0009] S2, the spinning solution is solidified to form nascent fibers, and the nascent fibers are successively washed with water, water-drawn, oiled, dried and steam-drawn to prepare raw yarn fibers;

[0010] S3, ultra-high performance polyacrylonitrile-based carbon fiber is prepared by sequentially subjecting the original fiber to pre-oxidation, low-temperature carbonization, high-temperature carbonization, graphitization, surface treatment, sizing and drying.

[0011] The pre-oxidation process employs a four-stage pre-oxidation treatment. The temperature of the first stage pre-oxidation is determined based on the DSC of the original fiber, and the time of the first stage pre-oxidation is determined based on the cyclization index of the fiber at the first stage pre-oxidation temperature. The temperature and time of the next stage pre-oxidation are determined according to the fiber after the previous stage pre-oxidation in the aforementioned manner.

[0012] This application employs a solvent dropwise copolymerization method with acrylonitrile, which combines the advantages of a first comonomer (favorable for spinning and drawing), a second comonomer (favorable for cyclization and spinning), and a third comonomer (favorable for homogeneous oxidation and spinning). This method facilitates the production of high molecular weight, long molecular chain precursors, effectively improves molecular chain orientation and extension, and reduces fiber defects. Simultaneously, a four-stage pre-oxidation treatment is used. The pre-oxidation temperature of the next stage fiber is determined based on the DSC of the previous stage fiber, and the pre-oxidation time is determined based on the cyclization index of the fiber at the pre-oxidation temperature. This improves the exothermic reaction caused by high temperatures and promotes efficient and uniform reactions, thereby facilitating the preparation of carbon fibers with both high strength and high modulus.

[0013] In some embodiments of this application, an initial draw ratio for the pre-oxidation treatment is set, the crystal orientation degree of the raw fiber is L1, and the crystal orientation degree of the fiber after the fourth-stage pre-oxidation treatment is L2. When (L1-L2) / L1≤0.1, the initial draw ratio is the target draw ratio; when (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment is increased to obtain the crystal orientation degree of the fiber after the fourth-stage pre-oxidation treatment as L2', until (L1-L2') / L1≤0.1, the target draw ratio is obtained.

[0014] This application determines the target draw ratio by measuring the crystal orientation degree of the precursor fiber and the fiber after four-stage pre-oxidation treatment, and calculating the relative reduction of the crystal orientation degree of the fiber after four-stage pre-oxidation treatment compared to the crystal orientation degree of the precursor fiber. This is to help maintain a high crystal orientation degree of the fiber, thereby facilitating the preparation of carbon fibers with both high strength and high modulus.

[0015] In some embodiments of this application, the mass ratio of the first comonomer, the second comonomer, and the third comonomer is (1.0~1.5):(2.0~2.5):(0.5~1.0).

[0016] This application utilizes a suitable mass ratio of a first comonomer, a second comonomer, and a third comonomer to copolymerize with acrylonitrile, which facilitates the preparation of high molecular weight, long molecular chain precursors, effectively improves molecular chain orientation and extension, and reduces defects in fiber formation.

[0017] In some embodiments of this application, the mass ratio of acrylonitrile, comonomer, chain transfer agent, initiator, and solvent is (19.20~21.62):(0.80~1.38):(0.004~0.011):(0.02~0.03):(76.96~79.98).

[0018] This application utilizes a suitable mass ratio of acrylonitrile, comonomer, chain transfer agent, initiator, and solvent to conduct a copolymerization reaction, which facilitates the preparation of high molecular weight, long-chain polyacrylonitrile.

[0019] In some embodiments of this application, the method for determining the first-stage pre-oxidation temperature includes: obtaining the DSC curve of the raw fiber by differential scanning calorimetry, and taking the temperature corresponding to the intersection of the tangent line of the curve before the peak and the bottom line of the peak as the first-stage pre-oxidation temperature.

[0020] This application uses differential scanning calorimetry to determine the DSC curve of the raw fiber, and uses the temperature corresponding to the intersection of the tangent line of the curve before the peak and the bottom line of the peak as the first-stage pre-oxidation temperature. This can make the first-stage pre-oxidation temperature more reasonable and can effectively improve the problems of high-temperature violent reaction exothermic reaction and low-temperature pre-oxidation efficiency.

[0021] In some embodiments of this application, the method for determining the first-stage pre-oxidation time includes: pre-oxidizing the raw fiber at the first-stage pre-oxidation temperature, and using in-situ infrared spectroscopy to determine the curve of the cyclization index of the raw fiber versus the pre-oxidation time. The time corresponding to the intersection of the tangents on both sides of the curve at the inflection point of the curve is taken as the first-stage pre-oxidation time.

[0022] This application uses in-situ infrared spectroscopy to determine the cyclization index of the precursor fiber at the first-stage pre-oxidation temperature, and obtains a curve of the fiber cyclization index versus pre-oxidation time. The time corresponding to the intersection of the two baselines on both sides of the curve at the inflection point is determined as the first-stage pre-oxidation time, which can promote efficient and uniform reaction.

[0023] In some embodiments of this application, the method for determining the orientation degree of the crystal region includes: using two-dimensional wide-angle X-ray diffraction to measure the curve of the diffraction intensity versus the azimuth angle of the fiber, and calculating the orientation degree of the crystal region as (360-FWHM1-FWHM2) / 360; wherein, FWHM1 is the full width at half maximum (FWHM1) of the first peak, and FWHM2 is the full width at half maximum (FWHM2) of the second peak.

[0024] This application uses two-dimensional wide-angle X-ray diffraction to determine the diffraction intensity versus azimuth curve of the fiber, thereby calculating the fiber's crystal orientation degree. By calculating the relative reduction in the crystal orientation degree of the fiber after four-stage pre-oxidation treatment compared to the original fiber, the target draw ratio is determined to maintain a high crystal orientation degree of the fiber, thus facilitating the preparation of carbon fibers with both high strength and high modulus.

[0025] In some embodiments of this application, the weight-average molecular weight M of the polyacrylonitrile in the spinning solution prepared in step S1 is... w (45~65)×10 4 g / mol, number-average molecular weight M n (26~36)×10 4 g / mol, radius of gyration R g The wavelength is 50~70 nm.

[0026] This application obtains high molecular weight, long molecular chain polyacrylonitrile through copolymerization, which can provide pre-oxidized single molecular structures with large-area cyclic formation, so that the single molecular structures are carbonized and crystallized into large-area graphene sheets, thereby improving the strength of the fiber.

[0027] In some embodiments of this application, in step S1, the mass ratio of the solvent in the mixed solution to the added solvent is (1.5~3):1, the reaction time for adding the solvent is 0.5~1.5 h, the stirring speed is 30~50 rpm, the pressure is 5.0~6.0 kPa, the polymerization temperature is 60~70 ℃, and the total polymerization time is 15~20 h.

[0028] This application achieves a balance between increasing the molecular weight of the polymerized polyacrylonitrile by using an appropriate solvent mass ratio and a suitable reaction time for adding the solvent, while also making the polymerization rate more uniform, improving the polymerization effect, and reducing the occurrence of explosive polymerization.

[0029] In some embodiments of this application, the crystal orientation degree of the precursor fiber before pre-oxidation in step S3 is 91%~93%, the crystallinity is 52%~58%, and the grain size is 7.5~8.5 nm.

[0030] This application uses precursor fibers with the above-mentioned crystal orientation, crystallinity and grain size range for pre-oxidation to determine that the precursor fibers undergoing pre-oxidation have a better crystal structure, thereby helping to maintain the carbon fibers prepared by subsequent processing such as pre-oxidation to have both good strength and modulus.

[0031] In some embodiments of this application, the temperatures of water washing, water drawing, and oiling in step S2 are 30~40 ℃, 65~75 ℃, and 20~22 ℃, respectively.

[0032] This application employs appropriate temperatures to wash, draw, and oil the nascent fibers, which helps to ensure that the prepared raw fibers have a low residual solvent content and an appropriate oil content.

[0033] In some embodiments of this application, the drying temperature is 160~180 ℃, the drying pressure is 0.30~0.55 MPa, and the drying residence time is 5~10 s; the steaming pressure is 0.30-0.55 MPa, and the steaming residence time is 800-3000 ms.

[0034] This application reduces the cyclization reaction of PAN during drying by using appropriate temperature and pressure to dry the nascent fibers for a suitable time, which is beneficial for subsequent high-ratio steam drawing. The subsequent use of high-ratio and high-pressure steam drawing is conducive to obtaining raw fibers with high molecular chain extension and orientation, giving full play to the long molecular chain effect of high molecular weight, thereby improving the strength of the fibers.

[0035] In some embodiments of this application, the total draw ratio of the raw fiber prepared in step S2 is 10 to 18 times, wherein each draw ratio is allocated according to the ratio of empty draw: water washing: water draw: oiling: drying: steaming draw = (10~14):(7~11):(15~21):(5~8):(5~8):(22~25).

[0036] This application uses a higher draw ratio for steam drawing, which is beneficial to obtaining raw silk fibers with high molecular chain extension and orientation, and fully utilizes the long molecular chain effect of high molecular weight to improve the strength of the fiber.

[0037] In some embodiments of this application, the raw fiber prepared in step S2 has a residual solvent content of ≤200 ppm, an oil content of 0.9 wt%~1.6 wt%, and a fineness of 0.55~0.75 dtex.

[0038] The precursor fibers prepared in this application have a low residual solvent content, which can effectively reduce fiber defects caused by solvent evaporation; and have a suitable oil content, which can reduce the friction coefficient of fiber bundle spinning and carbonization residue.

[0039] In some embodiments of this application, the high-temperature carbonization temperature in step S3 is 1000~1500 ℃, the high-temperature carbonization time is 2~6 min, and gradient heating is performed using 2~6 temperature zones with a temperature difference of 50~250 ℃ between the temperature zones.

[0040] This application employs appropriate temperatures and multi-temperature gradient heating for high-temperature carbonization, which facilitates efficient and stable carbonization of fibers.

[0041] In some embodiments of this application, the graphitization temperature is 1500~2500 ℃, the graphitization time is 3~5 min, and gradient heating is performed using 4~6 temperature zones with a temperature difference of 50~200 ℃ between the temperature zones.

[0042] This application employs appropriate temperature and multi-temperature gradient heating for graphitization, which facilitates efficient and stable graphitization of fibers to obtain high-performance and stable carbon fiber products.

[0043] In some embodiments of this application, the microscopic defects and surface element distribution of fibers at the tail of the carbonization furnace are monitored during high-temperature carbonization. When obvious etching defects are observed in the fibers or oxygen is detected on the fiber surface, the flow rate of inert gas is increased to reduce the oxygen content in the carbonization furnace. Fiber samples at the tail of the carbonization furnace are collected at intervals to detect the carbon content. When the carbon content of the fibers is lower than the standard value, the temperature of the carbonization furnace is increased.

[0044] This application uses the method of monitoring the microscopic defects, surface element distribution, and carbon content of the fibers at the tail of the carbonization furnace to adjust the oxygen content and temperature in the carbonization furnace in real time, thereby maintaining the carbon fiber with a high carbon content and reducing surface defects.

[0045] In some embodiments of this application, graphitization is carried out in the same manner as the high-temperature carbonization described above, adjusting the oxygen content and temperature in the graphite furnace.

[0046] This application maintains a high carbon content in carbon fibers and reduces surface defects by using the same method as high-temperature carbonization to adjust the oxygen content and temperature in the graphite furnace in real time.

[0047] In some embodiments of this application, the surface treatment in step S3 is of level 2 to 4, with a total charge of 30 to 150 C / g; after surface treatment, the surface is washed in 4 to 6 stages to ensure that the conductivity of the water after washing is less than 10 μS / cm; and after washing, the surface is dried at 130 to 150°C.

[0048] This application employs a slightly lower electrical charge for multi-stage surface treatment, which not only improves the etching degree of the surface treatment but also mitigates the problem of reduced carbon fiber strength caused by excessive electrical charge. After surface treatment, multiple water washes are performed, and the conductivity of the water after washing is monitored to be less than 10 μS / cm, minimizing the negative impact of residues on the carbon fiber performance.

[0049] In some embodiments of this application, the mass concentration of the sizing agent used for sizing is 1.0% to 1.5%; after sizing, it is dried at 130 to 210 °C.

[0050] This application utilizes a suitable sizing agent for sizing and dries the material at a suitable temperature to facilitate the preparation of carbon fiber products with good and stable performance.

[0051] Secondly, embodiments of this application provide a dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fiber prepared by the preparation method of the first aspect.

[0052] In some embodiments of this application, the dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fibers have a stacking layer number of 4.8~5.5, an interlayer spacing of 0.33~0.38 nm, a grain thickness of 1.75~1.95 nm, a grain length of 5.25~5.40 nm, a grain width of 4.20~4.50 nm, a surface oxygen-to-carbon atom ratio of 15%~20%, a tensile strength of 7100~8300 MPa, a tensile modulus of 320~350 GPa, a tensile strength Cv value ≤2.0%, a tensile modulus Cv value ≤1.0%, and an interlaminar shear strength of 100~120 MPa.

[0053] The polyacrylonitrile-based carbon fibers prepared by the above-described preparation method in this application possess high strength, modulus, and interlaminar shear strength. Attached Figure Description

[0054] Figure 1 This is a process flow diagram for this application.

[0055] Figure 2 This is an XRD pattern of the precursor fiber in Example 1 of this application.

[0056] Figure 3 This is a graph showing the diffraction intensity versus azimuth angle of the original fiber in Example 1 of this application.

[0057] Figure 4 This is a DSC curve of the fibers in the pre-oxidation process of Example 1 of this application.

[0058] Figure 5 This is a graph showing the cyclization index of the fibers versus the pre-oxidation time in the first stage of pre-oxidation in Example 1 of this application.

[0059] Figure 6 This is a graph showing the cyclization index of the fibers versus the pre-oxidation time during the second-stage pre-oxidation in Example 1 of this application.

[0060] Figure 7 This is a graph showing the cyclization index of the fiber versus the pre-oxidation time in the third-stage pre-oxidation process of Example 1 of this application.

[0061] Figure 8 This is a graph showing the cyclization index of the fiber versus the pre-oxidation time in the fourth-stage pre-oxidation process of Example 1 of this application.

[0062] Figure 9 This is an XRD pattern of the carbon fiber prepared in Example 1 of this application. Detailed Implementation

[0063] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0064] This application provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fibers by dry-jet wet spinning, such as... Figure 1 As shown, it includes:

[0065] S1, a mixed solution is prepared by mixing acrylonitrile, comonomer, chain transfer agent, initiator and solvent, and solution polymerization is carried out by solvent dropwise addition at the polymerization temperature to prepare a polymerization stock solution. The polymerization stock solution is then subjected to monomer removal, degassing and amination to prepare a spinning stock solution. The comonomer includes a first comonomer, a second comonomer and a third comonomer. The first comonomer is selected from vinyl acetate, the second comonomer is selected from at least one of acrylic acid and methacrylic acid, and the third comonomer is selected from at least one of isobutyl acrylate and isobutyl methacrylate.

[0066] Further, acrylonitrile, comonomer, chain transfer agent, initiator, and solvent are mixed to obtain a mixed solution. At a polymerization temperature of 60-70°C, solvent is added dropwise at a mass ratio of (1.5-3):1 (solvent in the mixed solution to added solvent) for 0.5-1.5 h. The mixture is stirred at 30-50 rpm, and nitrogen gas is introduced to maintain the pressure inside the reactor at 5.0-6.0 kPa. The solution polymerization reaction is carried out for 15-20 h to obtain a polymerization stock solution. The polymerization stock solution is then subjected to monomer removal, degassing, and amination to prepare a spinning stock solution.

[0067] In some embodiments of this application, the mass ratio of the first comonomer, the second comonomer, and the third comonomer is (1.0~1.5):(2.0~2.5):(0.5~1.0). Further, the mass ratio of the first comonomer, the second comonomer, and the third comonomer can be (1.2~1.5):(2.2~2.5):(0.5~0.9). Using a suitable mass ratio of the first comonomer, the second comonomer, and the third comonomer in the copolymerization reaction with acrylonitrile is beneficial for obtaining high molecular weight, long molecular chain precursors, effectively improving the molecular chain orientation and extension, and reducing defects in fiber formation.

[0068] In some embodiments of this application, the mass ratio of acrylonitrile, comonomer, chain transfer agent, initiator, and solvent is (19.20~21.62):(0.80~1.38):(0.004~0.011):(0.02~0.03):(76.96~79.98). It should be noted that the solvent here refers to the total solvent in the mixed solution and the solvent added dropwise. Using a suitable mass ratio of acrylonitrile, comonomer, chain transfer agent, initiator, and solvent for the copolymerization reaction is beneficial for preparing high molecular weight, long-chain polyacrylonitrile.

[0069] In some embodiments of this application, in step S1, the chain transfer agent may be, but is not limited to, α-methylstyrene dimer (AMSD); the initiator may be, but is not limited to, azobisisobutyronitrile (AIBN); and the solvent may be, but is not limited to, dimethyl sulfoxide (DMSO). AMSD is sulfur-free, which can reduce impurity elements and is environmentally friendly, while also improving the uniformity of molecular weight distribution.

[0070] In some embodiments of this application, in step S1, the mass ratio of the solvent in the mixed solution to the added solvent may be, but is not limited to, 1.5:1, 1.55:1, 1.6:1, 1.65:1, 1.7:1, 1.75:1, 1.8:1, 1.85:1, 1.875:1, 1.9:1, 1.95:1, 2:1, 2.3:1, 2.33:1, 2.35:1, 2.4:1, 2.45:1, 2.5:1, 2.55:1, 2.6:1, 2.65:1, 2.7:1, 2.75:1, 2.8:1, 2.85:1, 2.9:1, 2.95. The solvent mass ratio is 1:3:1; using an appropriate solvent mass ratio ensures an suitable initial monomer concentration, facilitating the increase of the molecular weight of the polymerized polyacrylonitrile while simultaneously making the polymerization rate more uniform, improving the polymerization effect, and reducing the risk of explosive polymerization. Specifically, the reaction time for adding the solvent can be, but is not limited to, 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h, 1.0 h, 1.1 h, 1.2 h, 1.3 h, 1.4 h, and 1.5 h; further, the reaction time for adding the solvent can be 0.5~1.3 h. Using an appropriate reaction time for adding the solvent ensures a more uniform polymerization rate, improves the polymerization effect, facilitates the increase of the molecular weight of the polymerized polyacrylonitrile, and reduces the risk of explosive polymerization. The stirring speed can be, but is not limited to, 30 rpm, 35 rpm, 40 rpm, 45 rpm, and 50 rpm. Nitrogen gas can be introduced to maintain the pressure inside the reactor at, but is not limited to, 5.0 kPa, 5.5 kPa, or 6.0 kPa. Introducing nitrogen to maintain a positive pressure for the polymerization reaction mitigates the problem of oxygen reducing free radical activity and inhibiting the polymerization reaction. The polymerization temperature can be, but is not limited to, 60 ℃, 65 ℃, or 70 ℃. The total polymerization time can be, but is not limited to, 15 h, 16 h, 18 h, 19 h, or 20 h.

[0071] In some embodiments of this application, in step S1, the monomer removal process is carried out under a vacuum pressure of 2.00~5.00 KPa and a temperature of 80~90 ℃; to remove unreacted monomers from the polymerization solution, reduce defects caused by subsequent volatilization, and ensure that the residual acrylonitrile content in the spinning solution is ≤5 ppm. The degassing process is carried out under a vacuum pressure of 0.50~1.50 KPa and a temperature of 50~70 ℃; to remove bubbles from the polymerization solution, and improve the problem of filament breakage caused by discontinuous spinning solution. In the ammoniation process, ammonia gas at 30-40 °C is introduced into the polymerization solution, and the volume flow ratio of ammonia gas to polymerization solution is (0.01-0.03):1. The ammonia gas reacts with the carboxyl groups in the second comonomer, which improves the hydrophilicity of PAN and facilitates the uniform diffusion in the coagulation bath. At the same time, it neutralizes the acidic groups in the carboxyl groups, reducing their adverse effects on stretchability and spinnability, so as to achieve an ammoniation modification degree of 25%-40%.

[0072] In some embodiments of this application, the spinning solution prepared in step S1 has a solid content of 15%~19%, an apparent viscosity of 60~80 Pa·s at 45℃, a residual acrylonitrile content ≤5 ppm, an amination modification degree of 25%~40%, a physical gel relative content of 100~200, and a chemical gel relative content of 3%~10%. A spinning solution with a suitable solid content is more stable and more conducive to subsequent stable spinning. At the same time, the residual acrylonitrile content is satisfied, reducing defects caused by subsequent residual volatilization. A suitable amination modification degree, physical gel relative content, and chemical gel relative content can benefit the hydrophilicity, interaction force, and uniformity of the spinning solution, effectively improving the stability of the spinning operation.

[0073] In some embodiments of this application, the weight-average molecular weight M of the polyacrylonitrile in the spinning solution prepared in step S1 is... w (45~65)×10 4 g / mol, number-average molecular weight M n (26~36)×10 4 g / mol, radius of gyration R g The wavelength is 50~70 nm. Additionally, M... w / M n It can be used to determine the uniformity of molecular weight distribution. A uniform molecular weight distribution can reduce the proportion of small molecular weight segments and defects caused by short chain carbonization; R g / M w It can be used to determine the extensibility of molecular chains; high molecular chain extensibility is beneficial for spinning and drawing to prepare fibers. High molecular weight, long-chain polyacrylonitrile can be obtained through copolymerization, which can provide pre-oxidation of individual molecular structures into large-area rings, allowing the individual molecular structures to carbonize and crystallize into large-area graphene sheets, thereby improving fiber strength.

[0074] S2, the spinning solution is solidified to form nascent fibers, and the nascent fibers are then successively washed, drawn, oiled, dried and steamed to prepare raw yarn fibers.

[0075] Furthermore, the spinning solution obtained by dry-jet wet extrusion is passed through an air layer of 5-10 mm and then enters a coagulation bath at 0-15 ℃ and a concentration of 25%-35%. The solute in the coagulation bath is dimethyl sulfoxide (DMSO) and the solvent is water. After coagulation, nascent fibers are formed. The nascent fibers are then sequentially washed with water at 30-40 ℃, water-drawn at 65-75 ℃, oiled at 20-22 ℃, dried at 160-180 ℃, and steam-drawn to obtain the precursor fiber.

[0076] In some embodiments of this application, in step S2, the oiling agent used is a high heat-resistant oiling agent. Specifically, the high heat-resistant oiling agent is the one disclosed in patent application number CN201911369493.5, which involves uniformly mixing amino-modified polydimethylsiloxane, epoxy-modified polydimethylsiloxane, and surfactant at a mass ratio of 100:(35~60):(20~50), and slowly adding deionized water while stirring at high speed to obtain an emulsion with a mass concentration of 25~40%. The oil tank concentration (wt) is 1.5%~2.5%.

[0077] In some embodiments of this application, in step S2, the nascent fibers are fused together after a water washing or water drawing process to prepare fibers with a filament count of 9K~24K, and then subjected to subsequent processing steps.

[0078] In some embodiments of this application, in step S2, the drying pressure is 0.30~0.55 MPa, and the drying residence time is 5~10 s; specifically, the drying pressure may be, but is not limited to, 0.30 MPa, 0.35 MPa, 0.40 MPa, 0.45 MPa, 0.5 MPa, or 0.55 MPa; further, the drying pressure may be 0.30~0.5 MPa. The steam drawing pressure is 0.30-0.55 MPa, and the residence time is 800-3000 ms. Specifically, the steam drawing pressure can be, but is not limited to, 0.30 MPa, 0.32 MPa, 0.35 MPa, 0.36 MPa, 0.38 MPa, 0.40 MPa, 0.42 MPa, 0.45 MPa, 0.46 MPa, 0.48 MPa, 0.50 MPa, 0.52 MPa, and 0.55 MPa; furthermore, the steam drawing pressure can be 0.46~0.55 MPa. Using appropriate temperature and pressure to dry the nascent fibers for an appropriate time can reduce the cyclization reaction of PAN during drying, which is beneficial for subsequent high-ratio steam drawing. Subsequent high-ratio, high-pressure steam drawing is used to obtain precursor fibers with high molecular chain extension and orientation, fully utilizing the long molecular chain effect of high molecular weight to improve fiber strength.

[0079] In some embodiments of this application, the total draw ratio of the precursor fibers prepared in step S2 is 10 to 18 times, wherein each draw ratio is allocated according to the ratio of air drawing: water washing: water drawing: oiling: drying: steam drawing = (10~14):(7~11):(15~21):(5~8):(5~8):(22~25). Using a higher draw ratio for steam drawing is beneficial to obtaining precursor fibers with high molecular chain extension and orientation, and to fully utilize the long molecular chain effect of high molecular weight to improve fiber strength.

[0080] In some embodiments of this application, the precursor fibers prepared in step S2 have a residual solvent content ≤200 ppm, an oil content of 0.9 wt%~1.6 wt%, a fineness of 0.55~0.75 dtex, a cyclization index of 0.05~0.15, and a dehydrogenation index of 0.35~0.45. The prepared precursor fibers have a low residual solvent content, which can effectively reduce fiber defects caused by solvent evaporation; they have a suitable oil content, which can balance reducing the friction coefficient and carbonization residue during fiber tow spinning; at the same time, the control of the cyclization index and dehydrogenation index can improve the problem of significantly premature cyclization reactions that are detrimental to spinning and drawing.

[0081] In some embodiments of this application, if the precursor fibers obtained in step S2 need to be stored, they must be stored in an environment with a temperature of 20~30 ℃ and a humidity of ≤60%, and the storage time must be ≤50 days. This is to reduce the changes in crystal structure caused by the plasticization of molecular chains under humid heat, so as to meet the requirements of crystal orientation, crystallinity and grain size of the precursor fibers for entering the pre-oxidation process as much as possible.

[0082] S3, ultra-high performance polyacrylonitrile-based carbon fibers are prepared by sequentially subjecting the precursor fibers to pre-oxidation, low-temperature carbonization, high-temperature carbonization, graphitization, surface treatment, sizing, and drying. Among them, the pre-oxidation adopts a four-stage pre-oxidation treatment. The temperature of the first stage pre-oxidation is determined based on the DSC of the precursor fibers, the time of the first stage pre-oxidation is determined based on the cyclization index of the fibers at the first stage pre-oxidation temperature, and the temperature and time of the next stage pre-oxidation are determined according to the fibers after the previous stage pre-oxidation in the aforementioned manner.

[0083] In some embodiments of this application, the method for determining the first-stage pre-oxidation temperature includes: obtaining the DSC curve of the precursor fiber using differential scanning calorimetry (DSC), and taking the temperature corresponding to the intersection of the tangent line to the pre-peak curve (the vertex of the first peak) and the pre-peak baseline as the first-stage pre-oxidation temperature. This can effectively improve the problems of exothermic reactions at high temperatures and low pre-oxidation efficiency at low temperatures. It should be noted that when the pre-peak baseline and post-peak baseline are on a straight line, the line connecting the pre-peak and post-peak baselines is taken as the pre-peak baseline; when the pre-peak baseline and post-peak baseline are not on a straight line, the line connecting the intersection points of the extended pre-peak and post-peak baselines with the pre-peak and post-peak curves, respectively, is taken as the pre-peak baseline. Here, the baseline refers to the heat flow curve as a function of temperature when both the sample cell and the reference cell are empty samples.

[0084] In some embodiments of this application, the method for determining the first-stage pre-oxidation time includes: pre-oxidizing the precursor fiber at the first-stage pre-oxidation temperature, and using in-situ infrared spectroscopy to determine the curve of the cyclization index of the precursor fiber versus the pre-oxidation time. The time corresponding to the intersection of the tangents on both sides of the curve at the inflection point is taken as the first-stage pre-oxidation time. This can promote a highly efficient and uniform reaction.

[0085] In some embodiments of this application, the atmosphere for the first to third stages of pre-oxidation is air, and the atmosphere for the fourth stage of pre-oxidation is nitrogen. The first three stages of pre-oxidation in an air atmosphere can gently achieve a high degree of oxidation and a relatively high degree of cyclization, while the final stage of pre-oxidation in a nitrogen atmosphere stops increasing the degree of oxidation and continues to increase the degree of cyclization. It should be noted that the testing for each stage of pre-oxidation, with its specific temperature and time determined, also uses the atmosphere corresponding to that stage.

[0086] Further, in an air atmosphere, the DSC curve of the precursor fiber (PF) was first determined using differential scanning calorimetry. The temperature corresponding to the intersection of the tangent line to the curve before the first peak and the bottom line of the peak was taken as the first-stage pre-oxidation temperature. The PF was pre-oxidized in air at the first-stage pre-oxidation temperature. In air, the cyclization index of the fiber and the pre-oxidation time of the PF at the first-stage pre-oxidation temperature were determined using in-situ infrared spectroscopy. The time corresponding to the intersection of the tangent lines to the curves on both sides of the inflection point was taken as the first-stage pre-oxidation time, thus preparing the first-stage pre-oxidized fiber OF1. In air, the DSC curve of OF1 was determined using the aforementioned method to determine the second-stage pre-oxidation temperature. The OF1 cyclization index was then determined using the aforementioned method in an air atmosphere. 1. By plotting the cyclization index of the fiber against the pre-oxidation time at the second-stage pre-oxidation temperature, the second-stage pre-oxidation time is determined, and the second-stage pre-oxidized fiber OF2 is prepared. 2. In an air atmosphere, the DSC curve of OF2 is measured using the aforementioned method to determine the third-stage pre-oxidation temperature. 3. In an air atmosphere, the cyclization index of OF2 at the third-stage pre-oxidation temperature is measured using the aforementioned method to determine the third-stage pre-oxidation time, and the third-stage pre-oxidation fiber OF3 is prepared. 4. In a nitrogen atmosphere, the DSC curve of OF3 is measured using the aforementioned method to determine the fourth-stage pre-oxidation temperature. 5. In a nitrogen atmosphere, the cyclization index of OF3 at the fourth-stage pre-oxidation temperature is measured using the aforementioned method to determine the fourth-stage pre-oxidation time, and the fourth-stage pre-oxidation fiber OF4 is prepared.

[0087] In some embodiments of this application, an initial draw ratio for the pre-oxidation treatment is set, the crystal orientation degree of the precursor fiber is L1, and the crystal orientation degree of the fiber after four-stage pre-oxidation treatment is L2. When (L1-L2) / L1≤0.1, the initial draw ratio is the target draw ratio; when (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment is increased to obtain the crystal orientation degree of the fiber after four-stage pre-oxidation treatment as L2', until (L1-L2') / L1≤0.1, the target draw ratio is obtained. The crystal orientation degrees of the precursor fiber and the fiber after four-stage pre-oxidation treatment are measured, and the relative reduction in the crystal orientation degree of the fiber after four-stage pre-oxidation treatment compared to the crystal orientation degree of the precursor fiber is calculated to determine the target draw ratio, which is beneficial for maintaining a high crystal orientation degree of the fiber, thereby facilitating the preparation of carbon fibers with both high strength and high modulus. Specifically, the initial draw ratio for the pre-oxidation treatment is set to 0.86~0.92.

[0088] In some embodiments of this application, the method for determining the crystal region orientation degree includes: measuring the diffraction intensity versus azimuth angle curve of the fiber using two-dimensional wide-angle X-ray diffraction, and calculating the crystal region orientation degree as (360-FWHM1-FWHM2) / 360; where FWHM1 is the full width at half maximum (FWHM1) of the first peak and FWHM2 is the FWHM2 of the second peak. It should be noted that when determining the crystal region orientation degree of the precursor fiber, the diffraction intensity versus azimuth angle curve of the precursor fiber is measured; when determining the crystal region orientation degree of the fiber after pre-oxidation treatment, the diffraction intensity versus azimuth angle curve of the fiber after pre-oxidation treatment is measured. Furthermore, the elution positions of the two peaks are the standard elution positions of polyacrylonitrile. This helps maintain a high crystal region orientation degree of the fiber, thereby facilitating the preparation of carbon fibers with both high strength and high modulus.

[0089] Furthermore, the diffraction intensity versus azimuth curve of the precursor fiber (PF) was obtained using two-dimensional wide-angle X-ray diffraction. The crystal orientation degree was calculated as L1 using (360-FWHM1-FWHM2) / 360. Then, the crystal orientation degree of the OF4 fiber after four stages of pre-oxidation was measured and calculated as L2 using the aforementioned method. The value was then calculated using (L1-L2) / L1. If (L1-L2) / L1 ≤ 0.1, the initial draw ratio is the target draw ratio; if (L1-L2) / L1 > 0.1, the draw ratio of the pre-oxidation treatment is increased. The crystal orientation degree of the OF4 fiber after four stages of pre-oxidation is then measured and calculated as L2' using the aforementioned method. The value was again calculated using (L1-L2') / L1. If (L1-L2') / L1 ≤ 0.1, the draw ratio is not adjusted further; if (L1-L2') / L1 ≤ 0.1, the draw ratio is adjusted further. When (L2') / L1>0.1, the draw ratio of the pre-oxidation treatment is increased, and the crystal orientation degree of the OF4 fiber after the fourth-stage pre-oxidation is measured and calculated according to the aforementioned method. The above steps are repeated until (L1-L2') / L1≤0.1 to obtain the target draw ratio.

[0090] In some embodiments of this application, the crystal orientation degree of the precursor fibers before pre-oxidation in step S3 is 91%~93%, the crystallinity is 52%~58%, and the grain size is 7.5~8.5 nm. Using precursor fibers with the above-mentioned crystal orientation degree, crystallinity, and grain size range for pre-oxidation ensures that the pre-oxidized precursor fibers have a better crystal structure, thereby facilitating the preparation of carbon fibers with higher strength and modulus through subsequent processing such as pre-oxidation.

[0091] In some embodiments of this application, the temperature for low-temperature carbonization in step S3 is 300~780 ℃, and gradient heating is performed using 6~8 temperature zones with a heating rate of 40~100 ℃ / min.

[0092] In some embodiments of this application, the high-temperature carbonization temperature in step S3 is 1000~1500 ℃, the high-temperature carbonization time is 2~6 min, and gradient heating is performed using 2~6 temperature zones with a temperature difference of 50~250℃ between the temperature zones. Using suitable temperatures and multi-temperature zone gradient heating for high-temperature carbonization facilitates efficient and stable carbonization of the fibers.

[0093] Furthermore, the microscopic defects and surface element distribution of the fibers at the tail end of the high-temperature carbonization furnace are monitored. When obvious etching defects are observed in the fibers or oxygen is detected on the fiber surface, the inert gas flow rate is increased to reduce the oxygen content in the carbonization furnace. It should be noted that monitoring the microscopic defects of the fibers allows for direct observation of whether the fiber surface morphology has been oxidized by oxygen. The inert gas can be, but is not limited to, nitrogen or argon. Fiber samples are collected at the tail end of the carbonization furnace every 20-50 seconds to test the carbon content. When the carbon content of the fibers is lower than the standard value, the temperature of the carbonization furnace is increased, specifically by adjusting the temperature within 5°C. By monitoring the microscopic defects, surface element distribution, and carbon content of the fibers at the tail end of the carbonization furnace, the oxygen quantity and temperature in the carbonization furnace can be adjusted in real time, thereby helping to maintain a high carbon content in the carbon fibers, reduce surface defects, and achieve efficient and stable conversion of precursor fibers into carbon fibers. It should be noted that the monitoring-feedback-control process is automated.

[0094] In some embodiments of this application, the graphitization temperature is 1500~2500 ℃, the graphitization time is 3~5 min, and gradient heating is performed using 4~6 temperature zones with a temperature difference of 50~200 ℃ between the zones. Using suitable temperatures and multi-zone gradient heating for graphitization facilitates efficient and stable graphitization of the fibers, resulting in high-performance and stable carbon fiber products.

[0095] Furthermore, graphitization employs the same method as high-temperature carbonization to adjust the oxygen content and temperature in the graphite furnace. Using the same method as high-temperature carbonization to adjust the oxygen content and temperature in the graphite furnace in real time also helps maintain a high carbon content in the carbon fibers, reduces surface defects, and achieves efficient and stable conversion from precursor fibers to carbon fibers.

[0096] In some embodiments of this application, the surface treatment in step S3 involves 2-4 stages, with a total charge of 30-150 C / g. After surface treatment, 4-6 stages of water washing are performed, ensuring the water conductivity after washing is less than 10 μS / cm. The surface is then dried at 130-150°C. Using a slightly lower charge for multi-stage surface treatment benefits the etching degree and mitigates the problem of reduced carbon fiber strength due to excessive charge. Multiple stages of water washing after surface treatment, with monitoring the water conductivity after washing to ensure it is less than 10 μS / cm, minimizes the negative impact of residues on fiber performance. After surface treatment, the oxygen-to-carbon ratio (O / C) of the carbon fiber surface reaches 15%-20%, effectively increasing the carboxyl content on the carbon fiber surface, thereby enhancing the interaction force between the carbon fiber surface and the resin, and improving interlaminar shear strength.

[0097] In some embodiments of this application, the mass concentration of the sizing agent used for sizing is 1.0%~1.5%; after sizing, it is dried at 130~210 °C. The sizing agent is selected as a highly versatile sizing agent with switchable functional groups, allowing for performance switching by controlling the activity of the functional groups, thereby broadening its application range. Using a suitable sizing agent and drying at a suitable temperature facilitates the preparation of carbon fiber products with good and stable performance. Surface treatment and sizing are used to ensure that the interlaminar shear strength of the carbon fiber is ≥100 MPa.

[0098] This application employs a first comonomer that balances spinning and drawing, a second comonomer that balances cyclization and spinning, and a third comonomer that balances homogeneous oxidation and spinning, to copolymerize with acrylonitrile using a solvent dropwise method. The first comonomer, vinyl acetate, has a small molecular size and contains highly mobile side groups connected to the main chain via oxygen, significantly improving molecular chain mobility and extension, resulting in highly oriented and extended molecular chains that fully utilize the long molecular chain effect of high molecular weight. The second comonomer, acrylic acid or methacrylic acid, has a small molecular size and a higher carboxyl group molar concentration, improving cyclization ability while maintaining spinnability. The linear molecular chain structure enhances fiber heat resistance, and the suitable oxygen-containing structure further... Improved heat resistance; the third comonomer, isobutyl acrylonitrile or isobutyl methacrylate, with isobutyl side groups, provides molecular-level oxygen permeation channels due to its large volume, while also being free of acidic groups and maintaining spinnability, promoting homogeneous pre-oxidation reaction to improve the radial structure consistency of carbon fibers; comprehensively beneficial for obtaining high molecular weight, long molecular chain precursors, effectively improving molecular chain orientation and extension, and reducing fiber formation defects; at the same time, the four-stage pre-oxidation treatment, based on the DSC of the previous stage fiber to determine the pre-oxidation temperature of the next stage fiber, and based on the cyclization index of the fiber at the pre-oxidation temperature to determine the pre-oxidation time, can improve the high-temperature violent reaction exothermic and promote efficient uniform reaction, thus facilitating the preparation of carbon fibers with both high strength and high modulus.

[0099] This application provides a dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fiber.

[0100] In some embodiments of this application, the dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fibers have a stacking layer count of 4.8–5.5, an interlayer spacing of 0.33–0.38 nm, a grain thickness of 1.75–1.95 nm, a grain length of 5.25–5.40 nm, a grain width of 4.20–4.50 nm, a surface oxygen-to-carbon atomic ratio of 15%–20%, a tensile strength of 7100–8300 MPa, a tensile modulus of 320–350 GPa, a tensile strength Cv value ≤2.0%, a tensile modulus Cv value ≤1.0%, and an interlaminar shear strength of 100–120 MPa. The polyacrylonitrile-based carbon fibers prepared using the above method possess high strength, modulus, and interlaminar shear strength.

[0101] The testing methods used in this application are as follows:

[0102] 1. Solid content of spinning solution: Take a certain mass of spinning solution, weigh it, and record its mass as _____. m 0. The spinning solution was evenly applied to a dry glass plate, rinsed with deionized water, and then boiled in water at 80 ℃ for 2 h to remove residual dimethyl sulfoxide (DMSO). The membrane surface was then wiped dry and placed in a drying oven at 105 ℃ for 1 h. The mass of the membrane was then recorded as follows. m 1; The solid content of the spinning solution is: Solid content (%) = m 1 / m 0×100%.

[0103] 2.45 ℃ Apparent viscosity: After the spinning solution was placed in a 45 ℃ water bath for 1 h to eliminate thermal history, the apparent viscosity of the spinning solution was tested by rotational viscometer method using a corresponding rotor to measure the interaction between the rotor and the sample.

[0104] 3. Degree of amination modification: The method disclosed in the patent application number CN202311013967.9 was used for testing.

[0105] 4. Relative content of physical gel and relative content of chemical gel: The method disclosed in the patent application number CN202311675264.2 was used for testing.

[0106] 5. Residual acrylonitrile content: Weigh 0.5 g of the polymerization solution after monomer removal into a 20 mL headspace vial, add 5 mL of methanol, seal and vortex to mix, equilibrate at 80 ℃ for 30 min to allow AN to evaporate into the gas phase; prepare a series of standard solutions and establish a standard curve of GC peak area versus concentration; after the AN gas phase sample is injected and tested, substitute the sample peak area into the standard curve equation, calculate the concentration and multiply by the dilution factor to obtain the residual acrylonitrile monomer concentration.

[0107] 6. Determination of weight-average and number-average molecular weight: Using 0.025 mol / L LiBr / DMF as the mobile phase and porous monodisperse cross-linked polystyrene as the stationary phase, a 3-5 mg / mL dilute solution (heating required) was prepared using the dried PAN film as the solvent and the mobile phase. The sample solution filtered through a pinhole filter was then injected into the system. The molecular weight and distribution of the sample were determined by gel permeation chromatography (GPC; Waters), multi-angle laser light scattering (SLS; Brookhaven), and differential refractive index detector (RI; Waters). Before use, the system was calibrated with a standard (e.g., PS; Mw = 13.6 kDa, PDI = Mw / Mn = 1.055). The refractive index increment dn / dc = 0.065 mL / g (measured by RI) was input into the BICParSEC Chromatography software for calculation.

[0108] 7. Residual solvent content in raw fibers: Determination of DMSO residue by ultraviolet absorption spectroscopy: Prepare DMSO solutions of different concentrations, establish a standard curve for ultraviolet absorption of DMSO solutions, take samples of raw fibers for ultraviolet absorption spectroscopy testing, and calculate the residual amount of DMSO in the raw fibers by referring to the standard curve for ultraviolet absorption of DMSO.

[0109] 8. Oil content: Take a 4-6 g sample of raw fiber, remove the moisture from the raw fiber, weigh the raw fiber after removing the moisture, and record the weight. m 1. Place the sample in a Soxhlet extractor, connected to a flat-bottomed flask containing 150 mL of a mixed solution of petroleum ether and ethanol (volume ratio 3:1). Assemble the reflux apparatus, turn on the cooling water for reflux, and turn on the standard thermostatic multi-unit heating mantle. Maintain the temperature at 60-90 °C and the reflux rate at 7-8 times / h. Start timing from the first drop of reflux and extract for 180 min. After extraction, use tweezers to squeeze out as much solvent as possible from the sample, allowing the solvent to reflux into the flat-bottomed flask. Then, transfer the extracted raw fibers to a 120 °C oven and bake for 60 min. Remove and place in a desiccator to cool for 30 min. Weigh and record the weight. m 2; Oil content = ( m 2- m 1) / m 1×100.

[0110] 9. Cycloning Index and Dehydrogenation Index: During the pre-oxidation process, as the pre-oxidation reaction proceeds, the cyano group (CN) gradually transforms into a C=N structure through cyclization; -CH2- transforms into a C=C structure through dehydrogenation. Therefore, tracking the continuous changes in its characteristic absorption peaks using infrared spectroscopy (IR) can reflect the degree of transformation. The formulas for quantitatively calculating the cyclization index and dehydrogenation index are: Cycloning Index = I 1590 / (I 1590 +I 2240 Dehydrogenation index = I 1360 / I 1454 In the formula: 2240 cm -1 It is the characteristic infrared peak of cyano, 1590 cm⁻¹ -1 It is a characteristic peak of C=N, at 1360 cm⁻¹. -1 It is a C=C characteristic peak, 1454 cm⁻¹ -1 It is a characteristic peak of -CH2-.

[0111] 10. Crystallinity, crystal orientation, and grain size of the raw fiber: (1) such as Figure 2 As shown, the X-ray diffraction (XRD) test of the original fiber was performed using a Bruker D2 PHASER instrument. Cu-K (wavelength = 0.154 nm) was used as the radiation source, and diffraction angle scanning (2 q: 5~50) was performed. The XRD curves were analyzed with diffraction angles (2 q) ca. 16.8° and 29.3° as the diffraction peaks of the crystal regions of the

[100] and

[110] crystal planes, respectively, and with diffraction angles ca. 16.9° and 27.6° as the diffraction peaks of the amorphous regions. Gaussian function was used for peak fitting, and crystallinity = integral area of ​​crystal region / total integral area × 100%; (2) as Figure 3 As shown, two-dimensional wide-angle X-rays are used to fix the diffraction angle (using... Figure 2 The diffraction angle of the peak

[100] was measured and scanned at an azimuth angle of 360° (-90°~270°) to obtain the curve of diffraction intensity versus azimuth angle. The crystal orientation degree (φ) was calculated: φ(%) = [360-(FWHM1+FWHM2)] / 360×100%, where FMWH is the half-peak width of the peak; the crystal orientation degree of the pre-oxidized fiber was also determined by the same method; (3) The grain size was calculated using the Scherrer Equation: L=Kλ / (β cosθ).

[0112] 11. DSC curve of fiber: Differential scanning calorimetry (DSC) test was performed on fiber samples using a DSC instrument (Netschle). The sample mass was 4.0 mg, and the sample was heated from 30 ℃ to 450 ℃ at a rate of 10 ℃ / min in an air atmosphere of 60 mL / min. The change of heat flux with temperature was recorded to obtain the DSC curve.

[0113] 12. In-situ infrared spectroscopy test: The fiber samples were tested using a Fourier transform infrared spectrometer (Summit X) in variable temperature ATR diamond crystal mode; the detector was DTGS and the software was OMNIC Series to collect data and obtain the curve of fiber cyclization index versus pre-oxidation time.

[0114] 13. SEM and EDS Testing: The surface morphology of the fiber samples was observed using a scanning electron microscope (HITACHI Regulus 8100) at an operating voltage of 15 kV to monitor microscopic defects in the fibers. The elemental composition of the regions was analyzed using a matching energy dispersive spectroscopy (EDS) instrument, covering elements such as C, N, O, and Si, with a detection accuracy ≤0.1 at%. For oxygen content analysis, five micro-regions were selected from both the defect-intensive and normal areas observed during SEM, and the average value was taken.

[0115] 14. Carbon content determination: Carbon fiber samples are collected at the tail of the furnace every 30 seconds. The carbon fiber samples are completely oxidized by combustion and converted into gases such as CO2. The carbon content is accurately determined by gas chromatography or thermal conductivity detector. Each test is ≥5mg, and three parallel tests are performed and the average value is taken to obtain the real-time carbon content (C%).

[0116] 15. XRD curve of carbon fiber: as shown Figure 9 As shown, an X-ray diffractometer (model Bruker D2 PHASER) was used to perform diffraction angle scanning (2 q: 10~80) under Cu Kα radiation (λ=0.15406 nm) to obtain the XRD curves; (1) Interlayer spacing (d 002 The position of peak 002 is calculated using the Bragg equation: d 002 =λ / (2sinθ); d 002 The smaller the value, the higher the orderliness; (2) Calculation of grain size (Lc, La): Scherrer Equation is used: L=Kλ / (βcosθ); Lc (along the c-axis) is calculated using the half-width at half maximum (WHM) of the (002) peak, and K is taken as 0.89; La (along the a-axis) is calculated using the half-width at half maximum (WHM) of the (100) peak, and K is taken as 1.84; β is the instrument broadening that needs to be deducted (using standard silicon standard for calibration). (3) Estimation of the number of stacked layers (N): N≈L C / d 002 (N is an integer); N reflects the number of quasi-graphite crystal sheets stacked along the longitudinal direction. The larger the value, the higher the degree of ordered stacking of the crystals.

[0117] 16. Surface oxygen-carbon atomic ratio: The carbon fiber samples were tested using an X-ray photoelectron spectroscopy (Nexsa G2) instrument to determine the elemental composition of the samples and to perform peak separation on the XPS spectra to obtain the oxygen-carbon atomic ratio (O / C) of the carbon fiber surface phase.

[0118] 17. Interlaminar Shear Strength (ILSS) Test: According to the national standard GB / T1450.1, the interlaminar shear strength of carbon fiber composites was tested using a Shimadzu AG-X plus electronic tensile testing machine; the calculation formula is: ILSS=Pmax / bh; where Pmax is the maximum load when the specimen breaks in shear, N; h is the height of the shear surface of the specimen, mm; b is the width of the shear surface of the specimen, mm.

[0119] The features and performance of this application will be further described in detail below with reference to the embodiments.

[0120] Example 1

[0121] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fibers by dry-jet wet spinning, including:

[0122] S1, a mixed solution was prepared by mixing 20.50 parts by weight of acrylonitrile, 1.25 parts by weight of comonomers (including 0.331 parts by weight of vinyl acetate, 0.685 parts by weight of methacrylic acid, and 0.234 parts by weight of isobutyl methacrylate), 0.008 parts by weight of α-methylstyrene dimer (AMSD), 0.025 parts by weight of azobisisobutyronitrile (AIBN), and 54.25 parts by weight of dimethyl sulfoxide (DMSO) solvent. At a polymerization temperature of 68°C, 23.25 parts by weight of dimethyl sulfoxide solvent were added dropwise from the start of the reaction. The reaction time was 1.0 h. The mixture was stirred at 35 rpm, and nitrogen gas was introduced to maintain the pressure inside the reactor at 5.5 ± 0.2 kPa. The solution polymerization reaction was carried out for 18 h to prepare the polymerization stock solution.

[0123] The polymerization solution was discharged into a de-acrylonitrile removal tower at a vacuum pressure of 3.50±0.05 KPa and a temperature of 85 ℃ for a de-acrylonitrile removal process. It then entered a degassing process at a vacuum pressure of 1.05±0.05 KPa and a temperature of 60 ℃. Subsequently, 0.02 parts by volume of ammonia gas at 35±2 ℃ was introduced into 1.0 part by volume of the polymerization solution for an ammoniation process, yielding a spinning solution with a solid content of 18.5%, an apparent viscosity of 75 Pa·s at 45 ℃, a residual acrylonitrile content of 2 ppm, an ammoniation modification degree of 33%, a physical gel content of 112%, and a chemical gel content of 5.6%. The weight-average molecular weight of the polyacrylonitrile in the spinning solution was 58 × 10⁻⁶. 4 g / mol, number-average molecular weight 33×10 4 g / mol, radius of gyration is 53 nm;

[0124] S2, the spinning solution obtained by dry-jet wet extrusion is passed through a 6 mm air layer, with a draw ratio of 2.430, and then enters a coagulation bath at 7 ℃ with a concentration of 32%. The solute in the coagulation bath is dimethyl sulfoxide (DMSO) and the solvent is water. After coagulation, nascent fibers are formed. The nascent fibers are washed with water at 35 ℃ with a draw ratio of 2.025 and then spun into 12K fibers. They are then sequentially water-drawn at 72 ℃ with a draw ratio of 3.848, oiled at 20 ℃ with a draw ratio of 1.418 (the oiling agent is the high heat-resistant oiling agent disclosed in patent application number CN201911369493.5), dried at 175 ℃ for 10 s with a draw ratio of 1.418 and a pressure of 0.52 MPa, and steam-drawn at a pressure of 0.52 MPa for 2000 ms. The resulting product has a residual DMSO content of 130 ppm and an oil content of 1.1%. The precursor fiber has a wt% cyclization index of 0.0079, a dehydrogenation index of 0.394, and a fineness of 0.58 dtex.

[0125] The raw fibers were stored at a temperature of 25±2 ℃ and a humidity of ≤50% for 10 days. XRD patterns and diffraction intensity versus azimuth curves were then measured from the raw fibers. Figure 2 and Figure 3 As shown, the crystal orientation degree is calculated to be 93%, the crystallinity is 56%, and the grain size is 8.1 nm, which meets the requirements for the precursor fiber before pre-oxidation. Therefore, this precursor fiber is used for the subsequent pre-oxidation process.

[0126] S3, four oxidation furnaces are set up for pre-oxidation treatment, namely OX1, OX2, OX3 and OX4, as follows: Figures 4-8As shown, the DSC curve of the precursor fiber (PF) was obtained by differential scanning calorimetry in air. The extended baseline of the first peak in the PF DSC curve is represented by dashed line 1, which intersects the pre-peak curve. The extended baseline of the post-peak curve is represented by dashed line 2, which intersects the post-peak curve. Connecting the pre-peak and post-peak intersections forms the peak-to-bottom line 3. A tangent line 4 is drawn through the apex of the first peak to the pre-peak curve, intersecting the peak-to-bottom line 3 at point A. The temperature corresponding to intersection point A, 232 ℃, is the first-stage pre-oxidation temperature. PF is pre-oxidized at 232 ℃. In air, the cyclization index of PF at 232 ℃ is measured as a function of pre-oxidation time using in-situ infrared spectroscopy. The time corresponding to the intersection of the tangent lines drawn from the inflection point of the curve on both sides is the first-stage pre-oxidation time, which is 31 minutes. First-stage pre-oxidized fiber OF1 was prepared. In air, the DSC curve of OF1 was measured using the aforementioned method, determining the second-stage pre-oxidation temperature to be 241 °C. In air, the cyclization index of OF1 at 241 °C was measured against the pre-oxidation time, determining the second-stage pre-oxidation time to be 34 min, thus preparing second-stage pre-oxidized fiber OF2. In air, the DSC curve of OF2 was measured using the aforementioned method, determining the third-stage pre-oxidation temperature to be 256 °C. In air, the cyclization index of OF2 at 256 °C was measured against the pre-oxidation time, determining the third-stage pre-oxidation time to be 46 min, thus preparing third-stage pre-oxidized fiber OF3. In nitrogen atmosphere, the DSC curve of OF3 was measured using the aforementioned method, determining the fourth-stage pre-oxidation temperature to be 278 °C. In nitrogen atmosphere, the cyclization index of OF3 at 278 °C was measured using the aforementioned method. The cyclization index versus pre-oxidation time curve at ℃ was used to determine the fourth-stage pre-oxidation time as 28 min, and OF4 fiber after four-stage pre-oxidation was prepared. The atmosphere for OX1, OX2, and OX3 was air, and the atmosphere for OX4 was nitrogen. The initial draw ratio for the pre-oxidation treatment was set to 0.90 times. The crystal orientation degree of PF was measured to be 93%, and the crystal orientation degree of OF4 was measured to be 85.67%, which met the requirement of (L1-L2) / L1≤0.1. The initial draw ratio of 0.90 times was adopted as the target draw ratio.

[0127] OF4 undergoes low-temperature carbonization, comprising eight temperature zones: 360 °C, 420 °C, 480 °C, 540 °C, 600 °C, 660 °C, 720 °C, and 780 °C. This is followed by a series carbonization process using a high-temperature furnace and a graphite furnace. The high-temperature carbonization process comprises six temperature zones: 1050 °C, 1100 °C, 1200 °C, 1350 °C, 1400 °C, and 1450 °C, with a residence time of 4 minutes. The graphitization process comprises six temperature zones: 1500 °C, 1550 °C, 1650 °C, 1800 °C, 1950 °C, and 2000 °C, with a residence time of 5 minutes. The nitrogen flow rate in both the high-temperature carbonization furnace and the graphite furnace is 200 Nm³. 3 / h; and real-time control of parameters in the carbonization furnace and graphite furnace, specifically: real-time monitoring of microscopic defects and surface element distribution of fibers at the tail of the carbonization furnace; if no obvious etching defects are observed in the fibers and no oxygen element is detected on the fiber surface, then the nitrogen flow rate is not adjusted; fiber samples are collected at the tail of the carbonization furnace every 30 seconds to detect carbon content; when the carbon content of the fiber is lower than 96% (the standard value for the fiber), the temperature of the carbonization furnace is increased by up to 5℃; similarly, real-time monitoring of microscopic defects and surface element distribution of fibers at the tail of the graphite furnace; if no obvious etching defects are observed in the fibers and no oxygen element is detected on the fiber surface, then the nitrogen flow rate is not adjusted; and the temperature in the graphite furnace is adjusted using the same method;

[0128] Subsequently, a four-stage surface treatment process was carried out with a charge of 30 C / g per stage and a total charge of 120 C / g. After surface treatment, the carbon fiber was washed in six stages, and the conductivity of the water after washing was measured to be 6 μS / cm. The washed carbon fiber was first dried at 150 ℃, and the O / C ratio of the carbon fiber surface was measured to be 17%. The dried carbon fiber was then sized using the sizing agent prepared in Example 1 of the patent application number CN202510543058.9, with a sizing agent mass concentration of 1.2%. After sizing, the carbon fiber was dried again at 160 ℃.

[0129] The polyacrylonitrile-based carbon fibers prepared in this embodiment have the following XRD curves: Figure 9 As shown, the number of stacked layers is 5.2, and the interlayer spacing (d) is calculated. 002 0.3568 nm, grain thickness (Lc) 1.76 nm, grain length (La) ∥ 5.31 nm, grain width (La) ⊥ ) 4.27 nm, K number is 12K, tensile strength is 7579 MPa, tensile modulus is 336 GPa, tensile strength Cv value is 1.1%, tensile modulus Cv value is 0.6%, interlaminar shear strength is 107 MPa.

[0130] Example 2

[0131] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning, which differs from Example 1 in that 1.199 parts of comonomer (including 0.280 parts of vinyl acetate, 0.685 parts of methacrylic acid, and 0.234 parts of isobutyl methacrylate) are added.

[0132] Example 3

[0133] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning, which differs from Example 1 in that 1.315 parts of comonomer (including 0.331 parts of vinyl acetate, 0.750 parts of methacrylic acid, and 0.234 parts of isobutyl methacrylate) are added.

[0134] Example 4

[0135] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning, which differs from Example 1 in that 1.316 parts of comonomer (including 0.331 parts of vinyl acetate, 0.685 parts of methacrylic acid, and 0.300 parts of isobutyl methacrylate) are added.

[0136] Example 5

[0137] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning. The difference from Example 1 is that the dimethyl sulfoxide solvent in the mixed solution is 45.25 parts, and 32.25 parts of dimethyl sulfoxide solvent are added dropwise.

[0138] Example 6

[0139] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning. The difference from Example 1 is that the reaction time for adding dimethyl sulfoxide solvent is 1.5 h.

[0140] Example 7

[0141] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning. The difference from Embodiment 1 is that the drying pressure in step S2 is 0.55 MPa.

[0142] Example 8

[0143] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning. The difference from Embodiment 1 is that the drying pressure in step S2 is 0.80 MPa.

[0144] Example 9

[0145] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning. The difference from Embodiment 1 is that the steam drawing pressure in step S2 is 0.45 MPa.

[0146] Example 10

[0147] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning. The difference from Embodiment 1 is that the precursor fiber prepared in step S2 is stored for 90 days.

[0148] Example 11

[0149] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning. The difference from Example 1 is that the pre-oxidation draw ratio is always 0.90 times and is not adjusted in real time.

[0150] Example 12

[0151] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fibers by dry-jet wet spinning. The difference from Embodiment 1 is that the temperatures of high-temperature carbonization and graphitization are not adjusted in real time.

[0152] Example 13

[0153] This embodiment provides a method for preparing ultra-high performance polyacrylonitrile-based carbon fiber by dry-jet wet spinning. The difference from Embodiment 1 is that the surface treatment process uses a charge of 120 C / g for Level 1 surface treatment.

[0154] Comparative Example 1

[0155] This comparative example provides a method for preparing polyacrylonitrile-based carbon fiber, which differs from Example 1 in that the total solvent is used in a one-step polymerization reaction.

[0156] Comparative Example 2

[0157] This comparative example provides a method for preparing polyacrylonitrile-based carbon fiber, which differs from Example 1 in that the comonomer is replaced with a single itaconic acid.

[0158] Comparative Example 3

[0159] This comparative example provides a method for preparing polyacrylonitrile-based carbon fiber, which differs from Example 1 in that: the pre-oxidation does not dynamically adjust the temperature and time, but adopts a conventional procedure to perform pre-oxidation at 230 ℃, 240 ℃, 250 ℃, and 260 ℃ respectively, with each pre-oxidation segment lasting 30 min.

[0160] Comparative Example 4

[0161] This comparative example provides a method for preparing polyacrylonitrile-based carbon fiber, which differs from Example 1 in that the comonomer is replaced with a single itaconic acid, and the pre-oxidation is carried out using a conventional procedure at 230 ℃, 240 ℃, 250 ℃, and 260 ℃, with each pre-oxidation stage lasting 30 min.

[0162] Experimental Example 1

[0163] In this experiment, the carbon fibers prepared by the methods of Examples 1-13 and Comparative Examples 1-4 were tested for tensile strength and tensile modulus according to the GB / T 3362-2017 method. The results are shown in Table 1.

[0164] Table 1 Carbon fiber properties

[0165]

[0166] As shown in Table 1, Example 1 uses three comonomers in a suitable mass ratio for copolymerization and adjusts the temperature and time of the four-stage pre-oxidation in real time. The carbon fibers prepared by this method have high tensile strength and tensile modulus, as well as high interlaminar shear strength. At the same time, the tensile strength Cv value and tensile modulus Cv value are low. That is, the preparation method of dry-jet wet-spinning ultra-high performance polyacrylonitrile-based carbon fiber of this application has been used to prepare polyacrylonitrile-based carbon fiber with high strength, modulus and interlaminar shear strength.

[0167] Compared with Example 1 and Comparative Example 1, the tensile strength and tensile modulus of the carbon fibers prepared in Comparative Example 1 were significantly reduced; this is because the one-step polymerization method is not conducive to the synthesis of high molecular weight polyacrylonitrile. Compared with Example 1 and Comparative Example 2, the tensile strength and tensile modulus of the carbon fibers prepared in Comparative Example 2 were significantly reduced, and the interlaminar shear strength was also somewhat reduced, while the tensile strength Cv value and tensile modulus Cv value were significantly increased; this is because the use of a single itaconic acid as a comonomer is not conducive to improving the molecular chain orientation and extension, and cannot effectively reduce defects in fiber formation. Compared with Example 1 and Comparative Example 3, the tensile strength and tensile modulus of the carbon fibers prepared in Comparative Example 3 were significantly reduced; this is because the pre-oxidation does not dynamically adjust the temperature and time, and is prone to high-temperature violent exothermic reaction and non-uniform reaction. Compared with Example 1 and Comparative Example 4, the tensile strength and tensile modulus of the carbon fiber prepared in Comparative Example 4 were significantly reduced, and the interlaminar shear strength was also reduced to a certain extent. At the same time, the tensile strength Cv value and tensile modulus Cv value were increased to a certain extent. This was because itaconic acid was used as a single comonomer, and the temperature and time were not dynamically adjusted during pre-oxidation.

[0168] Compared with Examples 1 and 2, the carbon fibers prepared in Example 2 showed a certain decrease in tensile strength, tensile modulus, and interlaminar shear strength, while the tensile strength Cv value and tensile modulus Cv value both increased to some extent. This was because the reduced mass of the first comonomer, vinyl acetate, decreased the fiber's spinnability. Compared with Examples 1 and 3, the carbon fibers prepared in Example 3 showed a certain increase in tensile strength. This was because the increased mass of the second comonomer, methacrylic acid, promoted the cyclization effect, which helped reduce fiber formation defects. Compared with Examples 1 and 4, the carbon fibers prepared in Example 4 showed a significant increase in both tensile strength and tensile modulus, but the tensile strength Cv value and tensile modulus Cv value both increased to some extent. Although the increased mass of the third comonomer, isobutyl methacrylate, promoted the homogeneous oxidation effect, which helped reduce fiber formation defects, its large molecular volume and excessive mass were detrimental to spinning. Compared with Examples 1 and 5, the carbon fibers prepared in Example 5 showed a significant increase in tensile strength, but a certain decrease in tensile modulus. This was because reducing the mass ratio of solvent to added solvent in the mixed solution increased the weight-average molecular weight of PAN. Compared with Examples 1 and 6, the tensile strength of the carbon fiber prepared in Example 6 was slightly reduced; the weight-average molecular weight of PAN was reduced due to the increased reaction time of the added solvent. Compared with Examples 1 and 7, the tensile strength of the carbon fiber prepared in Example 7 was improved to some extent, but the tensile modulus was reduced to some extent, and the tensile strength Cv value was also increased to some extent; the increased drying pressure promotes the cyclization reaction of PAN during drying, which is not conducive to subsequent high-ratio evaporation. Compared with Examples 1 and 8, the tensile strength, tensile modulus, and interlaminar shear strength of the carbon fiber prepared in Example 8 were all significantly reduced, while the tensile strength Cv value and tensile modulus Cv value were significantly increased; the significantly increased drying pressure promotes the cyclization reaction of PAN during drying, which is not conducive to subsequent high-ratio evaporation. Compared with Examples 1 and 9, the tensile strength and tensile modulus of the carbon fiber prepared in Example 9 were reduced to some extent; the reduced evaporation pressure is not conducive to obtaining precursor fibers with high molecular chain extension and orientation. Compared with Examples 1 and 10, the tensile strength of the carbon fibers prepared in Example 10 was reduced to some extent; this was because prolonged storage of the precursor fibers could cause plasticization of the fiber molecular chains, leading to changes in the crystal structure. Compared with Examples 1 and 11, the tensile strength of the carbon fibers prepared in Example 11 was reduced to some extent, while both the tensile strength Cv value and the tensile modulus Cv value increased significantly; this was because the pre-oxidation draw ratio was not adjusted in real time, which was detrimental to maintaining a high degree of crystal orientation in the fibers. Compared with Examples 1 and 12, the tensile strength Cv value and the tensile modulus Cv value of the carbon fibers prepared in Example 12 were significantly increased; this was because the high-temperature carbonization and graphitization temperatures were not adjusted in real time, which was detrimental to maintaining a high carbon content in the carbon fibers.Compared with Example 1 and Example 13, the interlaminar shear strength of the carbon fiber prepared in Example 13 was significantly reduced. This is because the use of Level 1 surface treatment may reduce the interaction force between the carbon fiber surface and the resin due to insufficient etching of the fiber surface.

[0169] The embodiments described above are some, but not all, of the embodiments of this application. The detailed description of the embodiments of this application is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

Claims

1. A process for the production of dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fibers, characterized in that, The method comprises the following steps: S1, mixing acrylonitrile, comonomer, chain transfer agent, initiator and solvent to obtain a mixed solution, and performing solution polymerization reaction at a polymerization temperature by solvent dropping method to prepare a polymerization stock solution, and the polymerization stock solution is sequentially subjected to monomer removal, defoaming and ammoniation to prepare a spinning stock solution; wherein the comonomer comprises a first comonomer, a second comonomer and a third comonomer; the first comonomer is selected from vinyl acetate, the second comonomer is selected from at least one of acrylic acid and methacrylic acid, and the third comonomer is selected from at least one of isobutyl acrylate and isobutyl methacrylate; S2, forming a nascent fiber by coagulation from the spinning stock solution, and sequentially performing water washing, water drawing, oiling, drying and steam drawing on the nascent fiber to prepare a precursor fiber; S3, preparing an ultra-high performance polyacrylonitrile-based carbon fiber by sequentially performing pre-oxidation, low-temperature carbonization, high-temperature carbonization, graphitization, surface treatment, sizing and drying on the precursor fiber; wherein the pre-oxidation adopts four-stage pre-oxidation treatment, the first-stage pre-oxidation temperature is determined based on the DSC of the precursor fiber, the first-stage pre-oxidation time is determined based on the cyclization index of the fiber at the first-stage pre-oxidation temperature, the pre-oxidation temperature of the next stage is determined according to the DSC of the fiber after pre-oxidation of the previous stage, and the pre-oxidation time of the next stage is determined based on the cyclization index of the fiber at the pre-oxidation temperature of the stage.

2. The production method according to claim 1, characterized by, An initial draw ratio of the pre-oxidation treatment is set, the crystal region orientation degree of the precursor fiber is L1, the crystal region orientation degree of the fiber after the four-stage pre-oxidation treatment is L2, when (L1-L2) / L1≤0.1, the initial draw ratio is a target draw ratio; when (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment is increased to obtain the crystal region orientation degree of the fiber after the four-stage pre-oxidation treatment as L2', until (L1-L2') / L1≤0.1, the target draw ratio is obtained.

3. The preparation method according to claim 1, characterized in that, The mass ratio of the first comonomer, the second comonomer and the third comonomer is (1.0-1.5):(2.0-2.5):(0.5-1.0).

4. The method of claim 1, wherein, The mass ratio of the acrylonitrile, the comonomer, the chain transfer agent, the initiator, the solvent is (19.20-21.62):(0.80-1.38):(0.004-0.011):(0.02-0.03):(76.96-79.98).

5. The preparation method according to claim 1, characterized in that, The determination method of the first-stage pre-oxidation temperature comprises: determining the DSC curve of the precursor fiber by differential scanning calorimetry, and taking the temperature corresponding to the intersection point of the tangent line of the peak front curve and the peak bottom line at the top of the first peak in the DSC curve as the first-stage pre-oxidation temperature; and / or, the determination method of the first-stage pre-oxidation time comprises: pre-oxidation treatment of the precursor fiber at the first-stage pre-oxidation temperature, and determining the cyclization index and pre-oxidation time curve of the precursor fiber by in-situ infrared spectroscopy, and taking the time corresponding to the intersection point of the tangent lines of the curves at the inflection point of the curve as the first-stage pre-oxidation time.

6. The preparation method according to claim 2, characterized in that, The determination method of the crystal region orientation degree comprises: The fiber diffraction intensity and azimuth angle curve is obtained by two-dimensional wide-angle X-ray diffraction method, and the crystal orientation degree is calculated as (360-FWHM1-FWHM2) / 360, wherein FWHM1 is the half-height width of the first peak, and FWHM2 is the half-height width of the second peak.

7. The method of any one of claims 1 to 6, wherein the method further comprises the step of: The weight average molecular weight M of the polyacrylonitrile in the spinning dope prepared by the S1 w is (45~65)×10 4 g / mol, the number average molecular weight M n is (26~36)×10 4 g / mol, and the radius of gyration R g is 50~70 nm. ​ In S1, the mass ratio of the solvent in the mixed solution to the dropwise solvent is (1.5-3):1, the reaction time of the dropwise solvent is 0.5-1.5 h, the stirring speed is 30-50 rpm, and the pressure is 5.0-6.0 KPa; the temperature of the polymerization reaction is 60-70 ℃, and the total time of the polymerization reaction is 15-20 h.

8. The production method according to any one of claims 1 to 6, characterized by, The crystal orientation degree of the precursor fiber before pre-oxidation in S3 is 91%-93%, the crystallinity is 52%-58%, and the grain size is 7.5-8.5 nm.

9. The production method according to any one of claims 1 to 6, characterized by, The temperatures of washing, water drawing, and oiling in S2 are 30-40 ℃, 65-75 ℃, and 20-22 ℃, respectively. And / or, the temperature of drying is 160-180 ℃, the pressure of drying is 0.30-0.55 MPa, and the residence time of drying is 5-10 s; the pressure of steaming is 0.30-0.55 MPa, and the residence time of steaming is 800-3000 ms. And / or, the total draw ratio of the precursor fiber prepared in S2 is 10-18, wherein each draw ratio is distributed in the ratio of air drawing: washing: water drawing: oiling: drying: steaming = (10-14):(7-11):(15-21):(5-8):(5-8):(22-25).

10. The method of claim 9, wherein The residual solvent content of the precursor fiber prepared in S2 is ≤200 ppm, the oil content is 0.9 wt%-1.6 wt%, and the fineness is 0.55-0.75 dtex.

11. The production method according to any one of claims 1 to 6, characterized by, In S3, the temperature of high-temperature carbonization is 1000-1500 ℃, the time of high-temperature carbonization is 2-6 min, and 2-6 temperature zones are used for gradient heating with a temperature difference of 50-250 ℃ between temperature zones. And / or, the temperature of graphitization is 1500-2500 ℃, the time of graphitization is 3-5 min, and 4-6 temperature zones are used for gradient heating with a temperature difference of 50-200 ℃ between temperature zones. And / or, the temperature of low-temperature carbonization is 300-780 ℃, and 6-8 temperature zones are used for gradient heating with a heating rate of 40-100 ℃ / min.

12. The method of claim 11, wherein, The micro-defects and surface element distribution of the fiber at the tail of the carbonization furnace are monitored during high-temperature carbonization, and when obvious etching defects are observed or oxygen elements are detected on the surface of the fiber, the flow rate of inert gas is increased to reduce the oxygen content in the carbonization furnace; The carbon content of the fiber sample at the tail of the carbonization furnace is detected at intervals, and when the carbon content of the fiber is lower than the standard value, the temperature of the carbonization furnace is increased; And / or, the oxygen content and temperature in the graphite furnace are adjusted in the same way as the high-temperature carbonization described above.

13. The production method according to any one of claims 1 to 6, characterized by, The number of surface treatment in the S3 is 2-4, and the total electric quantity is 30-150 C / g; after the surface treatment, 4-6 stages of water washing are carried out, and the electric conductivity of water after the water washing is less than 10 μS / cm; after the water washing, drying is carried out at 130-150 ℃; And / or, the mass concentration of the sizing agent used in the sizing is 1.0%-1.5%; after the sizing, drying is carried out at 130-210 ℃. 14.A dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fiber prepared by the preparation method in any one of claims 1-13.

15. The dry-jet-wet-spun ultra-high-performance polyacrylonitrile-based carbon fiber according to claim 14, wherein, The dry-jet wet-spun ultra-high performance polyacrylonitrile-based carbon fiber has a stacking layer number of 4.8-5.5, an interlayer spacing of 0.33-0.38 nm, a grain thickness of 1.75-1.95 nm, a grain length of 5.25-5.40 nm, a grain width of 4.20-4.50 nm, a surface oxygen-carbon atomic ratio of 15%-20%, a tensile strength of 7100-8300 MPa, a tensile modulus of 320-350 GPa, a tensile strength Cv value of ≤2.0%, a tensile modulus Cv value of ≤1.0%, and an interlayer shear strength of 100-120 MPa.

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