Polyacrylonitrile fibers and methods of making, polyacrylonitrile-based carbon fibers and methods of making

Abstract

The application relates to a polyacrylonitrile fiber and a preparation method, and a polyacrylonitrile-based carbon fiber and a preparation method, and mainly adopts the technical scheme that a polyacrylonitrile fiber is prepared by the following steps: polyacrylonitrile spinning solution is transmitted to a spinning device through a conveying pipeline to be spun, and then is sequentially subjected to coagulation forming, washing treatment and post-treatment to obtain the polyacrylonitrile fiber; the inner wall surface of the conveying pipeline is a polishing surface, and the roughness Ra is less than or equal to 0.40 mu m; and the transmission speed of the polyacrylonitrile spinning solution in the conveying pipeline is controlled to be 1-10 cm / s. The application mainly aims at eliminating or controlling defects generated in the production process of the polyacrylonitrile fiber, reasonably matching and finely controlling process conditions of carbonization treatment and graphitization treatment, effectively controlling fiber structure characteristics such as microcrystalline orientation, multidimensional grain size and pore defect size of the fiber, and further improving tensile strength and tensile modulus indexes of the fiber, so that the problem that high tensile strength and high tensile modulus are difficult to coexist is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon fibers, and particularly relates to a polyacrylonitrile fiber and a preparation method, and a polyacrylonitrile-based carbon fiber and a preparation method. BACKGROUND

[0002] The polyacrylonitrile-based carbon fiber is an inorganic fiber material prepared by taking the polyacrylonitrile fiber (i.e., PAN precursor) as a precursor and performing high-temperature treatment. The polyacrylonitrile-based carbon fiber has been widely applied to aerospace and military fields such as space shuttles, space probes, artificial satellites, rockets and missiles, fighter jets, civil passenger aircrafts, automobile, wind power generation, sports and leisure, rail transportation, medical devices, civil engineering and building, and the like, due to its excellent characteristics such as high specific strength, high specific modulus, heat resistance, corrosion resistance, fatigue resistance, creep resistance, radiation resistance, and small relative density.

[0003] The polyacrylonitrile-based carbon fiber includes high-strength standard model carbon fiber, high-strength medium model carbon fiber, and high-strength high model carbon fiber (e.g., graphite fiber). The highest tensile strength and the highest tensile modulus of the currently prepared carbon fiber are far from the theoretical values, and there is also a problem that the tensile strength and the tensile modulus cannot be simultaneously considered.

[0004] Defects are the main factors restricting the improvement of the quality of the carbon fiber, and the larger the size and the more the number of the defects, the lower the performance of the fiber. Most of the defects of the carbon fiber are derived from the polyacrylonitrile fiber, and the defects of the polyacrylonitrile fiber can be “inherited” by the carbon fiber. Therefore, high-quality polyacrylonitrile fiber is the basis for preparing high-strength and high-modulus polyacrylonitrile-based carbon fiber. The defects of the polyacrylonitrile fiber have a great influence on the implementation of the subsequent process and the performance of the fiber, and these defects mainly include broken filaments, filament adhesion, holes, impurities, and the like. The preparation of the polyacrylonitrile fiber and the carbon fiber belongs to a systematic engineering, and there are many control points. Although the existing method for controlling defects can reduce the generation of some defects, it cannot finely and systematically eliminate or control the fiber defects, and cannot meet the preparation requirements of the carbon fiber with higher performance, thereby seriously restricting the improvement of the fiber performance.

[0005] In summary, how to systematically eliminate or control the defects generated in the production process of the polyacrylonitrile fiber by improving the production equipment, optimizing the process parameters, and improving the quality of the polyacrylonitrile fiber needs further research. SUMMARY

[0006] Therefore, the present application provides a polyacrylonitrile fiber and a preparation method, and a polyacrylonitrile-based carbon fiber and a preparation method, and the main purpose is to eliminate or control the defects generated in the production process of the polyacrylonitrile fiber, so as to further improve the tensile strength and the tensile modulus of the fiber.

[0007] In order to achieve the above-mentioned purpose, the present application mainly provides the following technical scheme:

[0008] In one aspect, the embodiments of the present application provide a method for preparing polyacrylonitrile fiber, comprising the following steps: transporting polyacrylonitrile spinning solution to a spinning device through a delivery pipe for spinning, and then sequentially performing coagulation forming, washing treatment and post-treatment to obtain polyacrylonitrile fiber; wherein the inner wall surface of the delivery pipe is a polished surface with a roughness Ra≤0.40 μm, preferably 0.10-0.40 μm; and the transportation speed of the polyacrylonitrile spinning solution in the delivery pipe is controlled to be 1-10 cm / s.

[0009] Preferably, the roughness Ra of the inner wall surface of the delivery pipe is ≤0.20 μm.

[0010] Preferably, the transportation speed of the polyacrylonitrile spinning solution in the delivery pipe is controlled to be 3-7 cm / s.

[0011] Preferably, the polyacrylonitrile spinning solution is first transported to a filtering device through the delivery pipe, and then transported to the spinning device for spinning; wherein the filtering device comprises a leaf disc filter; wherein the filtering precision is 1-5 μm, the filter material is a metal fiber mesh, and the thickness of the filter material is 0.4-3.0 mm.

[0012] Preferably, the filtering precision is 1-2 μm; and / or the material of the metal fiber on the metal fiber mesh is 316L; and / or the diameter of the metal fiber on the metal fiber mesh is <25 μm, preferably 3-15 μm; and / or the thickness of the filter material is 0.5-2 mm; and / or the filtering speed of the polyacrylonitrile spinning solution in the filtering device is 2-13 cm / h, preferably 3-5 cm / h.

[0013] Preferably, the spinning device comprises a wet spinning spinneret or a dry-wet spinning spinneret.

[0014] When wet spinning is adopted, a wet spinning spinneret is used; wherein the pore size of the wet spinning spinneret is 0.050-0.075 mm, the aspect ratio is 1-3, the hole density is 0.5-5 pieces / mm 2 , and the number of holes is 500-50000; preferably, the pore size of the wet spinning spinneret is 0.055-0.065 mm; preferably, the aspect ratio is 1.5-2; preferably, the hole density is 1.2-3.7 pieces / mm 2 ; preferably, the number of holes is 1000-24000; and preferably, the wet spinning spinneret is a circular spinneret.

[0015] When dry-wet spinning is adopted, a dry-wet spinning spinneret is used; wherein the pore size of the dry-wet spinning spinneret is 0.1-0.3 mm, the aspect ratio is 2-5, the hole density is 0.1-0.9 pieces / mm 2, the number of holes is 500-8000; preferably, the hole diameter of the dry-jet wet spinning spinneret is 0.1-0.2 mm; preferably, the aspect ratio is 2.5-4; preferably, the hole density is 0.12-0.50 per mm 2 ; preferably, the number of holes is 1000-6000; preferably, the dry-jet wet spinning spinneret is a circular spinneret.

[0016] Preferably, in the step of the water washing treatment: the water used in the water washing treatment is deionized water; wherein the conductivity of the deionized water is ≤3.0 μS / cm; and / or the fibers after the coagulation forming treatment are sequentially subjected to water washing treatment in 3-9 water washing temperature zones, to obtain the fibers after the water washing treatment; wherein the temperature of a later water washing temperature zone is higher than that of an earlier water washing temperature zone; the temperature of the first water washing temperature zone is 30-50℃, and the temperature of the last water washing temperature zone is 40-60℃; wherein the water washing temperature gradually increases along the running direction of the fibers; the water flow direction is opposite to the running direction of the fibers; and / or the total effective residence time of the fibers in all the water washing temperature zones is 30-200 s, preferably 40-120 s; and / or the ratio of the water used in the water washing treatment to the weight of the running water washing fibers is 10-20, preferably 10-15; and / or the residual amount of the solvent in the fibers after the water washing treatment is 0.01-0.03%; and / or the residual amount of the solvent in the deionized waste water after the water washing treatment is 5-8%.

[0017] Preferably, the post-treatment comprises hot water drawing treatment, oiling treatment, drying densification treatment, steam drawing treatment.

[0018] In another aspect, the embodiments of the present application provide a polyacrylonitrile fiber, wherein the cross section of the polyacrylonitrile fiber is substantially oval or circular; the batch-to-batch linear density dispersion degree CV of the polyacrylonitrile fiber is <0.50%, the batch-to-batch linear density dispersion degree CV is <0.55, and the long-range linear density dispersion degree CV is <0.50% (here, the dispersion degree is a comprehensive index, and a low dispersion degree can comprehensively indicate that there are less fluffs, broken filaments, and fiber adhesion, especially the linear density dispersion degree. The batch-to-batch, batch-to-batch, and long-range dispersion can better indicate that there are fewer defects; in addition, the "batch-to-batch dispersion" refers to the quality difference of products in the same batch; the "batch-to-batch dispersion" refers to the quality difference of products between different batches; and the "long-range dispersion" refers to the quality difference of products on the same axis); the brightness difference ΔL of the polyacrylonitrile fiber is 5-15; and the total content of metal ions in the polyacrylonitrile fiber is ≤50 ppm; preferably, the circular degree coefficient of the cross section of the polyacrylonitrile fiber is greater than 0.85; preferably, the brightness difference ΔL of the polyacrylonitrile fiber is 5-10; preferably, the total content of metal ions in the polyacrylonitrile fiber is ≤30 ppm; and preferably, the polyacrylonitrile fiber is prepared by the preparation method of the polyacrylonitrile fiber described in any one of the above.

[0019] In still another aspect, the embodiments of the present application provide a method for preparing polyacrylonitrile-based carbon fiber, wherein the polyacrylonitrile fiber is subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment and post-treatment to obtain the polyacrylonitrile-based carbon fiber; preferably, in the step of high-temperature carbonization treatment, the fiber after low-temperature carbonization treatment is subjected to high-temperature carbonization treatment in 3-8 high-temperature carbonization temperature zones successively; wherein the temperature of the next high-temperature carbonization temperature zone is higher than that of the previous high-temperature carbonization temperature zone; the temperature of the first high-temperature carbonization temperature zone is 950-1100°C; the temperature of the last high-temperature carbonization temperature zone is 1300-1900°C, preferably 1500-1900°C; the time of high-temperature carbonization treatment is 10-180s, preferably 30-90s; preferably, in the step of high-temperature carbonization treatment, the draft ratio applied to the fiber is -4.0-20.0%, preferably 7-15%; preferably, in the step of high-temperature carbonization treatment, the temperature difference between two adjacent high-temperature carbonization temperature zones is 100-280°C; preferably, in the step of high-temperature carbonization treatment, the sealing gas and carrier gas of the high-temperature carbonization furnace are selected from one or more of argon, nitrogen and helium; the oxygen content in the sealing gas and carrier gas of the high-temperature carbonization furnace is controlled to be 2ppm or less, and the dew point is not higher than -65°C; preferably, in the step of high-temperature carbonization treatment, the oxygen content in the high-temperature carbonization furnace is 0-5ppm, and the dew point is not higher than -65°C; preferably, the oxygen content in the high-temperature carbonization furnace is not higher than 2ppm; preferably, the pressure in the high-temperature carbonization furnace is 3-15Pa. It is to be noted that the dew point refers to the temperature at which the gaseous water contained in the gas condenses into liquid water.

[0020] In still another aspect, the embodiments of the present application provide a polyacrylonitrile-based carbon fiber, wherein the bulk density of the polyacrylonitrile-based carbon fiber is 1.78-1.81g / cm3, the tensile strength is 5.7-7.1GPa, and the tensile modulus is 320-390GPa; preferably, the orientation degree, multi-dimensional grain size, pore defect size and fiber performance index of the polyacrylonitrile-based carbon fiber satisfy the following conditions: 3

[0021] 82%≤Q≤88%;

[0022] 20nm 3 ≤Lc×La ⊥ ×La ∥ ≤30nm 3 ;

[0023] 7nm 2 ≤Lc×La ⊥ ≤10nm 2 ;

[0024] ​(30 x Lc + 230) GPa≤ M≤ (30 x Lc + 290) GPa;

[0025] 1 nm≤ X≤ 7 nm;

[0026] wherein Q represents the orientation degree of the (002) crystal plane of the fiber along the fiber axis, in %; Lc, La ⊥ , La ∥ respectively represent the grain size of the (002) crystal plane in equatorial direction, the (100) crystal plane in equatorial direction, and the (100) crystal plane in meridian direction, in nm; M represents the modulus of the polyacrylonitrile-based carbon fiber, in GPa; X represents the pore defect size of the fiber surface layer, in nm.

[0027] Preferably, the cross section of the polyacrylonitrile-based carbon fiber is substantially elliptical or circular;

[0028] Preferably, the circularity coefficient of the polyacrylonitrile-based carbon fiber is > 0.85;

[0029] Preferably, the in-batch linear density dispersion degree CV of the polyacrylonitrile-based carbon fiber is < 0.60%, the inter-batch linear density dispersion degree CV is < 0.65%, and the long-range linear density dispersion degree CV is < 0.60%; further preferably, the in-batch strength dispersion degree CV of the polyacrylonitrile-based carbon fiber is < 3.0%, the inter-batch strength dispersion degree CV is < 3.5%, and the long-range strength dispersion degree CV is < 3.0%;

[0030] Preferably, the total content of metal ions in the polyacrylonitrile-based carbon fiber is ≤ 50 ppm, preferably ≤ 30 ppm;

[0031] Preferably, the polyacrylonitrile-based carbon fiber is prepared by the above-mentioned method for preparing a polyacrylonitrile-based carbon fiber.

[0032] In another aspect, the embodiment of the present application provides a method for preparing a graphite fiber, wherein the above-mentioned polyacrylonitrile fiber is subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, graphitization treatment, and post-treatment to obtain a graphite fiber; wherein

[0033] In the step of the graphitization treatment: the temperature of the graphitization treatment is 2200-3000 ℃, preferably 2500-2800 ℃; the draft ratio applied to the fiber is 4.0-25.0%, preferably 10.0%-20.0%; the graphitization treatment time is 10-180 s, preferably the graphitization treatment time is 30-120 s;

[0034] Preferably, in the step of the graphitization treatment: the sealing gas and the carrier gas of the graphitization furnace are selected from one or more of argon, nitrogen, and helium; preferably, the oxygen content of the sealing gas and the carrier gas is controlled to be ≤ 2 ppm, and the dew point is not greater than -65 ℃.

[0035] Preferably, in the step of the graphitization treatment, the oxygen content in the graphitization furnace is 0-5ppm, and the dew point is not more than -65℃; preferably, the oxygen content in the graphitization furnace is not more than 2ppm; the pressure in the graphitization furnace is 1-15Kg / cm 2 , preferably 2-10Kg / cm 2 .

[0036] Preferably, the post-treatment includes surface treatment, water washing treatment, sizing treatment, and drying treatment.

[0037] In still another aspect, the embodiment of the present application provides a graphite fiber, wherein the bulk density of the graphite fiber is 1.91-1.96g / cm 3 , the tensile strength is 3.6-4.2GPa, and the tensile modulus is 580-650Gpa.

[0038] Preferably, the orientation degree, multi-dimensional grain size, pore defect size, and fiber performance of the graphite fiber satisfy the following conditions:

[0039] 92nm≤Q≤97nm;

[0040] 120nm 3 ≤Lc×La ⊥ ×La ∥ ≤195nm 3 ;

[0041] 24nm 2 ≤Lc×La ⊥ ≤34nm 2 ;

[0042] (50×Lc+300)GPa≤M≤(50×Lc+370)GPa;

[0043] 1nm≤X≤7nm;

[0044] Wherein, Q represents the orientation degree of the (002) crystal plane of the fiber along the fiber axis, in %; Lc, La ⊥ , La ∥ respectively represent the grain size of the (002) crystal plane in the equatorial direction, the (100) crystal plane in the equatorial direction, and the (100) crystal plane in the meridian direction, in nm; M represents the modulus of the graphite fiber, in GPa; and X represents the pore defect size of the surface layer of the fiber, in nm.

[0045] Compared with the prior art, the polyacrylonitrile fiber and the preparation method, and the polyacrylonitrile-based carbon fiber and the preparation method at least have the following beneficial effects:

[0046] In one aspect, the present application provides a method for preparing polyacrylonitrile fiber. The polyacrylonitrile spinning solution is transmitted to a spinning device through a delivery pipeline for spinning. After spinning, the polyacrylonitrile fiber is obtained by sequentially performing coagulation molding, washing treatment and post-treatment. The inner wall surface of the delivery pipeline is a polished surface with a roughness Ra of 0.04 μm or less, preferably 0.10-0.40 μm. The transmission speed of the polyacrylonitrile spinning solution in the delivery pipeline is controlled to be 1-10 cm / s, preferably 3-7 cm / s. In this application, the roughness of the inner wall surface of the delivery pipeline and the transmission speed of the polyacrylonitrile spinning solution in the delivery pipeline are controlled to reduce the spinning solution transmission resistance, reduce the generation and growth of bubbles and gels, ensure the stability during fiber production, and further reduce the defects of the polyacrylonitrile fiber. (It should be noted that the faster the transmission speed of the spinning solution, the greater the pressure drop, which causes the spinning solution to be insufficient in the transmission process, and bubbles are generated, which reduces the stability during fiber production. The slower the transmission speed, the longer the residence time of the spinning solution in the transmission process, and the spinning solution delivery pipeline is prone to gelation. The gel particles not only block the spinneret holes, but also have a direct relationship with the uniformity of the microstructure of the nascent fiber and the quality of the polyacrylonitrile fiber. After polishing treatment of the inner wall of the delivery pipeline, the surface roughness is small, which can effectively prevent the retention of the spinning solution and cause gelation and gelation. If the roughness is large, the spinning solution is prone to retention and gelation, which further generates new gels, and the original gels hanging on the rough points grow and fall off at any time. The newly generated gels and the falling gels enter the spinneret through the transmission system, causing the spinneret to be blocked. At the same time, the smaller the roughness of the inner wall surface of the delivery pipeline, the smaller the spinning solution transmission resistance, the lower the pressure loss in the transmission process, the sufficient supply of the spinning solution, the stability of the fiber production process and the difficulty of generating bubbles. The insufficient supply of the spinning solution, bubbles and gels have adverse effects on fiber production, resulting in problems such as fiber hair, broken fiber, fiber adhesion, and hole.

[0047] Further, the embodiment of the present application provides a preparation method of polyacrylonitrile fiber. The polyacrylonitrile spinning solution is transmitted to a filtering device through a conveying pipeline and then transmitted to a spinning device for spinning; wherein the filtering device comprises a leaf disc filter; wherein the filtering precision is 1-5 μm, the filter material is a metal fiber mesh, and the thickness of the filter material is 0.4-3.0 mm. Preferably, the material of the metal fiber on the metal fiber mesh is 316L; preferably, the diameter of the metal fiber on the metal fiber mesh is <25 μm, and preferably, the filtering speed of the polyacrylonitrile spinning solution in the filtering device is 2-13 cm / h. Through the above setting, the foreign matters in the spinning solution can be effectively removed and the filter clogging can be inhibited, and then the polyacrylonitrile fiber production is stable and the high-performance, high-quality and stable polyacrylonitrile fiber is prepared (detailed description is as follows: the gel particles and mechanical impurities can cause the polyacrylonitrile fiber to produce hair, broken yarn, fiber adhesion, hole and other defects. Therefore, precise filtration is needed to remove the gel particles, impurities and other foreign matters, and the uniformity and spinnability of the spinning solution are ensured. The lower the filtering precision value is, the easier the gel particles, mechanical impurities and other foreign matters in the spinning solution are removed, but the filter is more likely to be clogged; with the increase of the thickness of the filter, the foreign matters in the spinning solution are more easily removed, but the pressure loss of the spinning solution through the filter increases, which causes insufficient supply and easy production of bubbles, and the stability in the fiber production process is reduced; the faster the filtering speed is, the greater the pressure drop is, and the production is unstable. The slower the filtering speed is, the longer the residence time of the spinning solution in the filter is, and the gel is easily produced. Therefore, the filtering conditions need to be further optimized to effectively remove the foreign matters in the spinning solution and inhibit the filter clogging, and then the polyacrylonitrile fiber production is stable and the high-performance, high-quality and stable carbon fiber is prepared.

[0048] Further, the embodiment of the present application provides a preparation method of polyacrylonitrile fiber. The wet spinning spinneret is adopted when the wet spinning is used; the pore size of the wet spinning spinneret is 0.050-0.075 mm, the length-diameter ratio is 1-3, the pore density is 0.5-5 pieces / mm 2 , and the number of pores is 500-50000. The dry-wet spinning spinneret is adopted when the dry-wet spinning is used; the pore size of the dry-wet spinning spinneret is 0.10-0.30 mm, the length-diameter ratio is 2-5, the pore density is 0.1-0.9 pieces / mm 2 , and the number of pores is 500-8000. Here, through the above design, the fineness uniformity between the fiber filaments can be effectively improved and the fiber performance discrete coefficient can be reduced, so that the fiber production is stable and the fiber quality is high.

[0049] Further, the embodiment of the present application provides a preparation method of polyacrylonitrile fiber, the residual amount of dimethyl sulfoxide in the washed fiber after water washing treatment is 0.01-0.03%, which can effectively avoid the problems such as filament fusion in the subsequent processing of the fiber, and avoid structural defects of the fiber. At the same time, the introduction of metal impurities such as potassium, sodium, calcium, magnesium and iron in the fiber production process is reduced, the fiber performance and fiber quality are improved, the content of metal impurities is low, which can meet the needs of high-end fields such as ablation-resistant heat-resistant composite materials, and prevent the ablation performance of the ablation-resistant heat-resistant composite materials from decreasing.

[0050] In summary, the embodiment of the present application provides a preparation method of polyacrylonitrile fiber, through the synergistic effect of the above conditions, the defects generated in the production process of polyacrylonitrile fiber can be effectively eliminated or controlled, high-quality polyacrylonitrile fiber is prepared, the roundness coefficient of the fiber cross section is >0.85, the batch line density dispersion degree CV is <0.50%, the batch line density dispersion degree CV is <0.55%, the long-range line density dispersion degree CV is <0.50%, the fiber brightness difference ΔL is 5-10, and the total content of metal ions is ≤30ppm. It can be seen that the polyacrylonitrile fiber has excellent performance and good stability, and the polyacrylonitrile-based carbon fiber and graphite fiber with excellent mechanical properties can be prepared by using the polyacrylonitrile fiber.

[0051] On the other hand, the embodiment of the present application provides a polyacrylonitrile-based carbon fiber and a preparation method thereof, wherein the polyacrylonitrile-based carbon fiber is obtained after pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment and post-treatment of the above-prepared polyacrylonitrile fiber. Herein, based on the above-mentioned polyacrylonitrile fiber with few defects, the carbonization treatment process is finely controlled, the draw ratio of high-temperature carbonization treatment (the draw ratio is improved compared with the conventional scheme) and the temperature of high-temperature carbonization treatment (the temperature of high-temperature carbonization is appropriately increased) are further controlled, the fine control of the multi-dimensional grain size, orientation degree and pore defect size of the carbon fiber is realized, and finally the high strength and high modulus are realized at the same time. The polyacrylonitrile-based carbon fiber has the following characteristics: the bulk density of the polyacrylonitrile-based carbon fiber is 1.78-1.81g / cm 3, the tensile strength is 5.7-7.1 GPa, the tensile modulus is 320-390 GPa, the roundness coefficient of the fiber section is greater than 0.85, the batch linear density dispersion degree CV is less than 0.60%, the batch-to-batch linear density dispersion degree CV is less than 0.65%, the long-range linear density dispersion degree CV is less than 0.60%, the polyacrylonitrile-based carbon fiber batch strength dispersion degree CV is less than 3.0%, the batch-to-batch strength dispersion degree CV is less than 3.5%, the long-range strength dispersion degree CV is less than 3.0%, and the total content of metal ions is less than or equal to 30 ppm. As can be seen, the polyacrylonitrile-based carbon fiber has excellent mechanical properties and good stability. Here, the polyacrylonitrile-based carbon fiber performance index takes into account the fiber tensile strength and tensile modulus, ensuring that the fiber has a high tensile modulus without reducing the tensile strength. The tensile modulus of the polyacrylonitrile-based carbon fiber is 320-390 GPa, and the tensile strength is 5.7-7.1 GPa; in particular, when the tensile modulus of the polyacrylonitrile-based carbon fiber is 330-390 GPa, no graphitization treatment is required to achieve the modulus index of graphite fiber, and the tensile strength is greater than 5.7.

[0052] In another aspect, the present application also provides a graphite fiber and a preparation method thereof. The polyacrylonitrile-based carbon fiber (especially, the tensile modulus is 330-390 GPa and the tensile strength is greater than 5.7) is subjected to graphite treatment, which can effectively reduce the graphitization temperature. As a key component of the graphitization furnace, the graphite heating body is subjected to surface carbon atom sublimation under the action of high-temperature heat energy, and the number of sublimated carbon atoms increases continuously as the graphitization temperature increases, resulting in serious graphite heating body loss. In the actual graphite fiber preparation process, as the graphitization temperature increases, especially when the graphitization temperature exceeds 2800°C, the graphite heating body loss increases sharply, and the service life of the heating body decreases significantly; at the same time, due to the certain width of the graphite heating body in the actual industrial production process, the graphite heating body may be broken due to the excessive width as the graphitization temperature rises. Therefore, in order to prepare ultra-high modulus graphite fiber, continuously increasing the graphitization temperature is not suitable for continuous high-modulus graphite fiber production and preparation. Here, the present application finely controls the graphitization process, matches the pressure, graphitization temperature, and draw ratio in the graphite furnace, and uses a high-temperature, high-pressure, and high-draw ratio preparation method to continuously improve the crystal region of the graphite fiber and further improve the tensile strength and tensile modulus, thereby obtaining a graphite fiber with excellent performance. The high pressure in the graphite furnace not only improves the crystal growth effect, but also inhibits the sublimation of carbon in the graphite heating body, which helps to prolong the service life of the graphite heating body. In addition, the graphite fiber provided by the present application has the following characteristics: the bulk density of the graphite fiber is 1.91-1.96 g / cm 3The tensile strength is 3.6-4.2 GPa, the tensile modulus is 580-650 GPa, and the roundness coefficient of the fiber section is greater than 0.85. It can be seen that the mechanical properties of the graphite fiber are significantly excellent.

[0053] In summary, the scheme of the present application is suitable for the preparation of 1K-24K high-performance polyacrylonitrile fibers, polyacrylonitrile-based carbon fibers and graphite fibers, and can realize the preparation of higher polyacrylonitrile-based carbon fibers and graphite fibers. The related technical path is suitable for the preparation of high-performance polyacrylonitrile fibers, polyacrylonitrile-based carbon fibers and graphite fibers by wet method and dry-wet method.

[0054] The above description is only a summary of the technical scheme of the present application. In order to more clearly understand the technical means of the present application, and to implement the content of the description, the following will be described in detail with the preferred embodiments of the present application and with the aid of the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0055] Figure 1 is the X-ray two-dimensional diffraction pattern of the polyacrylonitrile-based carbon fiber prepared in Example 4 of the present application.

[0056] Figure 2 is the TEM photo of the cross section of the polyacrylonitrile-based carbon fiber prepared in Example 5 of the present application.

[0057] Figure 3 is the X-ray two-dimensional diffraction pattern of the polyacrylonitrile-based carbon fiber prepared in Example 5 of the present application.

[0058] Figure 4 is the TEM photo of the cross section of the polyacrylonitrile-based carbon fiber prepared in Comparative Example 9 of the present application.

[0059] Figure 5 is the TEM photo of the cross section of the graphite fiber prepared in Example 6 of the present application.

[0060] Figure 6 is the X-ray two-dimensional diffraction pattern of the graphite fiber prepared in Example 6 of the present application. DETAILED DESCRIPTION

[0061] In order to further illustrate the technical means and effects adopted by the present application to achieve the predetermined invention purpose, the following will describe the specific implementation, structure, features and effects according to the present application in combination with the drawings and preferred embodiments. In the following description, different "an embodiment" or "embodiments" do not necessarily refer to the same embodiment. In addition, the specific features, structures or characteristics in one or more embodiments can be combined in any suitable form.

[0062] The spinning process of polyacrylonitrile fiber includes spinning solution transmission, metering, filtering, jetting, coagulation molding, washing, heat drawing, drying densification, steam drawing and the like. The present inventors have found through a large number of researches that in the preparation of polyacrylonitrile fiber, the main processes and factors introducing defects mainly include the following aspects:

[0063] (1) The spinning solution after polymerization, single removal and defoaming is transmitted to the spinning jet plate through the conveying pipeline. In the transmission process, selecting appropriate transmission speed is the key to the stability of the supply amount and supply pressure. The faster the transmission speed of the spinning solution, the greater the pressure drop increases, and the stability of the fiber production process decreases. The slower the transmission speed, the easier the gel is produced in the spinning solution conveying pipeline. The gel particles not only block the jet orifice, but also have a direct relationship with the uniformity of the microstructure of the nascent fiber and the quality of the polyacrylonitrile fiber. In addition, in order to reduce the conveying resistance of the spinning solution and reduce the generation of bubbles and gels, the inner wall of the conveying pipeline needs to be polished.

[0064] (2) The gel particles and insoluble mechanical impurities are produced in the process of polymerization, single removal, defoaming and the like, and introduced in the conveying process. These gel particles and mechanical impurities will also cause defects in the polyacrylonitrile fiber. Therefore, precise filtration is required to remove gel particles, impurities and the like to ensure the uniformity and spinnability of the spinning solution. The lower the filtering precision value, the easier it is to remove gel particles, mechanical impurities and the like in the spinning solution, but the filter is more likely to be blocked. With the increase of the thickness of the filter, it is easier to remove impurities in the spinning solution, but the pressure loss of the spinning solution through the filter increases, and the stability of the fiber production process decreases. The faster the filtering speed, the greater the pressure drop increases, and the production is unstable. The slower the filtering speed, the easier the gel is produced. Therefore, the filtering conditions need to be further optimized to effectively remove impurities in the spinning solution and inhibit the blocking of the filter, and then the polyacrylonitrile fiber production is stable and the high-performance, high-quality and stable carbon fiber is prepared.

[0065] (3) The jet plate is the core component of the spinning equipment, and the material, hole density, hole size distribution, hole length-diameter ratio and the like are the key parameters for the uniformity of the single yarn and the reduction of the fiber dispersion coefficient. When designing the jet plate, the coagulation bath should be able to flow rapidly and completely between each single yarn, so the hole density cannot be too high. However, when the hole density is relatively low, the flow channel of the spinning solution is too large, the temperature at the flow channel will decrease, the viscosity of the spinning solution will increase, and problems such as broken yarns and the like are easy to occur, causing the quality of the fiber to decrease. At the same time, in the actual production process, when the hole density is low, the spinning solution will also be gathered on the surface of the jet plate, affecting the normal production.

[0066] (4) After the coagulation bath, the pre-drawing fiber contains a certain amount of residual dimethyl sulfoxide solvent (DMSO), which can cause the fiber to fuse and other problems in the subsequent processing process, so that the fiber forms a certain structural defect, which seriously reduces the fiber performance index. Therefore, in order to obtain high-quality polyacrylonitrile fiber, the residual amount of solvent in the fiber needs to be reduced to a very low level through the washing process. At the same time, if the water consumption is continuously increased in order to reduce the residual amount of dimethyl sulfoxide in the tow, it will cause waste of water resources and increase the cost of fiber production. In addition, metal impurities (potassium, sodium, calcium, magnesium, iron, etc.) in the fiber escape during high-temperature carbonization, not only leaving holes, but also being carbon oxidation catalysts, causing the strength and hairiness level of the fiber to decrease; at the same time, the metal impurity content of the carbon fiber used for ablation-resistant heat-resistant composite materials should be strictly controlled to prevent the ablation performance of the ablation-resistant heat-resistant composite materials from decreasing. Therefore, how to effectively control the residual amount of solvent in the tow under the premise of saving production cost and not introducing metal ion impurities is a problem that carbon fiber production enterprises must consider.

[0067] In addition, carbonization treatment is an important process in the preparation of carbon fiber that determines the performance of carbon fiber. In addition to carbonization treatment temperature, carbonization treatment time, the drawing ratio applied to the fiber is also very important. The application of drawing force is beneficial to the arrangement of graphite layers along the fiber axis, and improves the mechanical properties of the fiber. During graphitization treatment, the non-carbon elements remaining in the fiber are further removed, the graphite crystallites not only grow, but also are arranged along the fiber axis, and the structure gradually approaches the ideal graphite. With the increase of graphitization temperature, the tensile modulus of the fiber gradually increases, and the tensile strength decreases. Therefore, whether it is carbonization treatment or graphitization treatment, how to precisely control the carbonization treatment and graphitization treatment, effectively control the orientation degree, multi-dimensional grain size, pore defect size and other fiber structure characteristics, and how to consider the fiber tensile strength and tensile modulus index are particularly important.

[0068] Here, the inventors of the present application propose the scheme of the present application according to the above research, as follows:

[0069] On the one hand, the present application provides a preparation method of polyacrylonitrile fiber, wherein the polyacrylonitrile spinning solution is transmitted to the spinning device through the conveying pipeline to be spun, and then coagulation molding, washing treatment and post-treatment are sequentially performed to obtain polyacrylonitrile fiber; wherein the inner wall surface of the conveying pipeline is a polished surface, and the roughness Ra is 0.10-0.40 μm; the transmission speed of the polyacrylonitrile spinning solution in the conveying pipeline is controlled to be 1-10 cm / s. Preferably, the roughness Ra of the inner wall surface of the conveying pipeline is ≤0.20 μm. Preferably, the transmission speed of the polyacrylonitrile spinning solution in the conveying pipeline is 3-7 cm / s.

[0070] With regard to the above-mentioned scheme, it is to be noted that the present application controls the roughness Ra of the inner wall surface of the conveying pipeline to be 0.10-0.40 μm, preferably Ra < 0.20 μm; at the same time, the transmission speed of the polyacrylonitrile spinning solution in the conveying pipeline is controlled to be 1-10 cm / s, preferably 3-7 cm / s; after the above-mentioned conditions are met, the polyacrylonitrile spinning solution can be effectively prevented from being insufficient in flow during transmission, the polyacrylonitrile spinning solution can be effectively prevented from being caused to be hung with glue and gelled due to the residence of the spinning solution, the bubbles and gels in the spinning solution can be effectively reduced, the stability of fiber production can be ensured, and the microstructure uniformity of the as-spun fiber and the quality (defect reduction) of the polyacrylonitrile fiber can be improved.

[0071] Further, the polyacrylonitrile spinning solution is transmitted to the filter device through the conveying pipeline first, and then transmitted to the spinning device for spinning; wherein the filter device comprises a vane disc filter; wherein the filtering precision is 1-5 μm, and the filter material is a metal fiber mesh. Preferably, the filtering precision is 1-2 μm; the material of the metal fiber on the metal fiber mesh is 316L; the diameter of the metal fiber on the metal fiber mesh is < 25 μm, preferably 3-15 μm; the thickness of the filter material is 0.4-3.0 mm, preferably 0.5-2 mm; the filtering speed of the polyacrylonitrile spinning solution in the filter device is 2-13 cm / h, preferably 3-5 cm / h.

[0072] It is to be noted that the filter device is designed in the present application, and a filter device with high filtering precision can be prepared, the filter device can effectively remove the foreign matters in the spinning solution and inhibit the filter from being blocked, and further, the polyacrylonitrile fiber production is stable, and the high-performance, high-quality and stable carbon fiber can be prepared.

[0073] Further, the spinning device of the present application comprises a wet spinning spinneret or a dry-wet spinning spinneret; the wet spinning spinneret is used when wet spinning; the aperture of the wet spinning spinneret is 0.050-0.075 mm (preferably 0.055-0.065 mm), the length-diameter ratio is 1-3 (preferably 1.5-2), the hole density is 0.5-5 pieces / mm 2 (preferably 1.2-3.7 pieces / mm 2 ), and the number of holes is 500-50000 (preferably 1000-24000). Preferably, the wet spinning spinneret is a circular spinneret.

[0074] The dry-wet spinning spinneret is used when dry-wet spinning; the aperture of the dry-wet spinning spinneret is 0.1-0.3 mm (preferably 0.1-0.2 mm), the length-diameter ratio is 2-5 (preferably 2.5-4), the hole density is 0.1-0.9 pieces / mm 2 (preferably 0.12-0.50 pieces / mm 2), and the number of holes is 500-8000 (preferably 1000-6000); preferably, the dry-wet spinning spinneret is a circular spinneret.

[0075] It should be noted that: the present application can effectively improve the uniformity of the fiber single filament and reduce the fiber performance dispersion coefficient by controlling the spinning device to the above conditions, so that the fiber production is stable and the fiber quality is high.

[0076] Further, in the water washing process: the fiber after the coagulation forming treatment is sequentially subjected to water washing treatment in 3-9 water washing temperature zones to obtain the water washing treated fiber; wherein, the temperature of the later water washing temperature zone is higher than that of the former water washing temperature zone; the temperature of the first water washing temperature zone is 30-50℃, and the temperature of the last water washing temperature zone is 40-60℃; the water washing temperature gradually increases along the running direction of the fiber; the water flow direction is opposite to the running direction of the fiber; the total effective residence time of the fiber in all water washing temperature zones is 30-200s; the ratio of the water quantity used for water washing to the weight of the running water washing fiber is 10-20; preferably, the effective residence time of the fiber in the water washing tank is 40-120s; preferably, the ratio of the water quantity used for water washing to the weight of the running water washing fiber is 10-15; further preferably, the conductivity of the deionized water is ≤3.0 μS / cm; preferably, the residual solvent content in the water washing treated fiber is 0.01-0.03%; preferably, the residual solvent content in the deionized waste water after water washing treatment is 5-8%.

[0077] It should be noted that: by controlling the above washing conditions, the residual amount of solvent in the fiber can be effectively controlled under the premise of saving production cost, not introducing potassium, sodium, calcium, magnesium, iron and other impurities, solving the problem of single fiber fusion and other problems, and improving the performance index of the fiber. The detailed analysis is as follows: After the coagulation bath and pre-drawing, the fiber contains a certain amount of residual dimethyl sulfoxide solvent (DMSO), which will cause single fiber adhesion and other problems in the subsequent processing process, form a certain structure defect of the fiber, and seriously reduce the performance index of the fiber. In the washing process, the more deionized water used, the less the residual amount of solvent in the deionized water waste after washing treatment, the easier to prepare the fiber with less dimethyl sulfoxide content, but too much deionized water will increase the production cost. The residual amount of solvent in the deionized water waste after washing treatment is 5-8%, which can reduce the use of deionized water and save production cost while meeting the requirement that the residual amount of solvent in the fiber after washing treatment is 0.01-0.03%. The metal impurities (potassium, sodium, calcium, magnesium, iron, etc.) in the fiber escape during high-temperature carbonization, not only leaving holes, but also being carbon oxidation catalysts, causing the decrease of fiber strength and hairiness grade; at the same time, as the carbon fiber used for ablation-resistant heat-resistant composite material, the content of metal impurities should be strictly controlled to prevent the decrease of ablation-resistant performance of the ablation-resistant heat-resistant composite material. The conductivity of deionized water is ≤3.0 μS / cm, which can effectively reduce the potassium, sodium, calcium, magnesium, iron and other impurities in the deionized water, and the total content of metal ions in the finally prepared fiber is ≤30 ppm.

[0078] In another aspect, the embodiments of the present application provide a polyacrylonitrile fiber, wherein the cross section of the polyacrylonitrile fiber is substantially oval or circular; the batch-to-batch linear density dispersion degree CV of the polyacrylonitrile fiber is less than 0.50%, the batch-to-batch linear density dispersion degree CV is less than 0.55, and the long-range linear density dispersion degree CV is less than 0.50% (preferably, the same axis polyacrylonitrile fiber is sampled at intervals of 10 meters, 30 samples are tested to characterize the dispersion degree of fiber performance). The circularity coefficient of the cross section of the polyacrylonitrile fiber is greater than 0.85 (here, the circularity coefficient is the ratio of the minimum diameter to the maximum diameter of the same cross section). The brightness difference ΔL of the polyacrylonitrile fiber is 5-15 (iodine solution adsorption method, which characterizes the compactness of the fiber), preferably 5-10; the total content of metal ions (potassium, sodium, calcium, magnesium, iron) in the polyacrylonitrile fiber is ≤50 ppm, preferably ≤30 ppm; wherein the polyacrylonitrile fiber is prepared by the preparation method of the polyacrylonitrile fiber described in any one of the above.

[0079] In still another aspect, the embodiments of the present application provide a preparation method of polyacrylonitrile-based carbon fiber, wherein the polyacrylonitrile fiber is subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment and post-treatment to obtain polyacrylonitrile-based carbon fiber.

[0080] It is to be noted that the polyacrylonitrile fiber prepared by the above method has less defects, and on this basis, the present application further controls the draft ratio of high-temperature carbonization treatment (the present application increases the draft ratio compared with the conventional scheme) and the high-temperature carbonization treatment temperature (appropriately increases the temperature of high-temperature carbonization) and other process conditions, realizes fine control of the carbonization treatment process, effectively controls the multi-dimensional grain size, orientation degree, pore defect size and other fiber structure characteristics of the carbon fiber, and finally realizes high strength and high modulus while taking into account both.

[0081] Specifically, in the step of high-temperature carbonization treatment, the fiber after low-temperature carbonization treatment is sequentially subjected to high-temperature carbonization treatment in 3-8 high-temperature carbonization temperature zones.

[0082] Among them, the temperature of the last high-temperature carbonization temperature zone is higher than that of the previous high-temperature carbonization temperature zone; the temperature of the first high-temperature carbonization temperature zone is 950-1100℃; the temperature of the last high-temperature carbonization temperature zone is 1300-1900℃, preferably 1500-1900℃; preferably, in the step of high-temperature carbonization treatment, the draft ratio applied to the fiber is -4.0-20.0%, preferably 7-15%; preferably, in the step of high-temperature carbonization treatment, the temperature difference between adjacent two high-temperature carbonization temperature zones is 100-280℃, and the high-temperature carbonization treatment time is 10-180s, preferably 30-90s.

[0083] Preferably, in the step of high-temperature carbonization treatment, the sealing gas and carrier gas of the high-temperature carbonization furnace are selected from one or more of argon, nitrogen and helium; the oxygen content in the sealing gas and carrier gas of the high-temperature carbonization furnace is controlled to be 2ppm or less, and the dew point is not more than -65℃;

[0084] Preferably, in the step of high-temperature carbonization treatment, the oxygen content in the high-temperature carbonization furnace is 0-5ppm, and the dew point is not more than -65℃; preferably, the oxygen content in the high-temperature carbonization furnace is not more than 2ppm; preferably, the pressure in the high-temperature carbonization furnace is 3-15Pa.

[0085] It is to be noted that the high-purity inert gas is not only used for the gas sealing of the high-temperature carbonization furnace, but also directly enters the high-temperature carbonization furnace as a carrier gas to increase the pressure in the high-temperature carbonization furnace, instantaneously discharge the exhaust gas in the furnace, and continuously and stably produce the fiber. The presence of trace oxygen not only oxidizes the running fiber, but also oxidizes the heating body in the high-temperature carbonization furnace, causing the performance of the carbon fiber to decrease and the service life of the heating body to decrease. Trace water is an oxidation activator of carbon at high temperature, and plays an activation role in oxidation etching in the high-temperature carbonization environment, resulting in a decrease in the performance of the carbon fiber. Therefore, strictly controlling the oxygen content and dew point can improve the performance of the carbon fiber, and at the same time, can prolong the service life of the heating body and continuously and stably produce the fiber.

[0086] In another aspect, embodiments of the present invention provide a polyacrylonitrile-based carbon fiber, wherein the bulk density of the polyacrylonitrile-based carbon fiber is 1.78–1.81 g / cm³. 3 Tensile strength is 5.7–7.1 GPa, and tensile modulus is 320–390 GPa.

[0087] It should be noted here that the performance of the polyacrylonitrile-based carbon fiber of the present invention is comparable to that of existing graphitized fibers.

[0088] Preferably, the orientation degree, multidimensional grain size, pore defect size, and fiber performance indicators of the polyacrylonitrile-based carbon fiber meet the following conditions:

[0089] 82% ≤ Q ≤ 88%;

[0090] 20nm 3 ≤Lc×La ⊥ ×La ∥ ≤30nm 3 ;

[0091] 7nm 2 ≤Lc×La ⊥ ≤10nm 2 ;

[0092] (30×Lc+230)GPa≤M≤(30×Lc+290)GPa;

[0093] 1nm≤X≤7nm;

[0094] Where Q represents the orientation degree of the (002) crystal plane of the fiber along the fiber axis, in percentage; Lc, La ⊥ La ∥ The crystal sizes of the (002) crystal plane, (100) crystal plane, and (100) crystal plane in the equatorial direction are respectively represented in nm; M represents the modulus of polypropylene-based carbon fiber in GPa; and X represents the size of pore defects on the fiber surface in nm. It should be noted that the structure of carbon fiber includes crystalline structure and pore structure. Crystalline structure refers to the structure of carbon microcrystals, including microcrystal size, orientation, and other structural characteristics. Pore structure refers to the pore structure characteristics between carbon microcrystals. Pore defect size is one of the methods for characterizing pore structure. The better the crystalline structure of carbon fiber (larger microcrystal size, better orientation along the fiber axis) and the smaller the pore defect size, the higher the modulus of the carbon fiber and the better its performance. For the Lc×La of carbon fiber... ⊥ ×La ∥ 、Lc×La ⊥ Characterization can better reveal its multidimensional crystalline structure characteristics.

[0095] It can be seen from the above formula that the microstructure and performance indexes of the polyacrylonitrile-based carbon fiber are significantly improved.

[0096] Preferably, the cross section of the polyacrylonitrile-based carbon fiber is approximately oval or circular; preferably, the circularity coefficient of the polyacrylonitrile-based carbon fiber is > 0.85; preferably, the in-batch linear density dispersion degree CV of the polyacrylonitrile-based carbon fiber is < 0.60%, the inter-batch linear density dispersion degree CV is < 0.65%, and the long-range linear density dispersion degree CV is < 0.60%; further preferably, the in-batch strength dispersion degree CV of the polyacrylonitrile-based carbon fiber is < 3.0%, the inter-batch strength dispersion degree CV is < 3.5%, and the long-range strength dispersion degree CV is < 3.0%; preferably, the total content of metal ions in the polyacrylonitrile-based carbon fiber is ≤ 50 ppm, preferably ≤ 30 ppm; and preferably, the polyacrylonitrile-based carbon fiber is prepared by the above-mentioned method for preparing polyacrylonitrile-based carbon fiber.

[0097] In still another aspect, the embodiments of the present application provide a method for preparing a graphite fiber, wherein the high-temperature carbonized fiber corresponding to the above-mentioned polyacrylonitrile-based carbon fiber is subjected to graphitization treatment and post-treatment to obtain a graphite fiber (i.e., the above-mentioned polyacrylonitrile fiber is subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, graphitization treatment, and post-treatment to obtain a graphite fiber).

[0098] It should be noted that: on the basis of the above-mentioned polyacrylonitrile-based carbon fiber (the polyacrylonitrile-based carbon fiber takes into account both strength and modulus, the tensile modulus of the polyacrylonitrile-based carbon fiber is 320-390 GPa, and the tensile strength is 5.7-7.1 GPa; especially when the tensile modulus of the polyacrylonitrile-based carbon fiber is 330-390 GPa, no graphitization treatment is needed to achieve the modulus index of the graphite fiber, and the tensile strength is > 5.7), especially when the polyacrylonitrile-based carbon fiber with a tensile modulus of 330-390 GPa and a tensile strength of > 5.7 is used to perform graphitization treatment on the high-temperature carbonized fiber corresponding to the polyacrylonitrile-based carbon fiber, it is easier to improve the fiber modulus. Therefore, by using the high-temperature carbonized fiber corresponding to the polyacrylonitrile-based carbon fiber provided by the present application, and by fine control of the graphitization process and reasonable matching of the pressure, drawing ratio, and other conditions in the graphitization furnace, the graphitization temperature can be effectively reduced, and the preparation of ultra-high modulus graphite fiber can be achieved without too high a temperature.

[0099] In the step of graphitization treatment, the temperature of graphitization treatment is 2200-3000 ℃, preferably 2500-2800 ℃; the drawing ratio applied to the fiber is 4.0-25.0%, preferably 10.0%-20.0%, and the drawing ratio increases with the increase of the graphitization temperature; and the graphitization treatment time is 10-180 s, preferably 30-120 s.

[0100] Preferably, in the step of the graphitization treatment, the sealing gas and carrier gas of the graphitization furnace is one or more of argon, nitrogen and helium; preferably, the oxygen content of the sealing gas and carrier gas is controlled to be less than 2 ppm, and the dew point is not more than -65℃; preferably, in the step of the graphitization treatment, the oxygen content in the graphitization furnace is 0-5 ppm, and the dew point is not more than -65℃; preferably, the oxygen content in the graphitization furnace is not more than 2 ppm; and the pressure in the graphitization furnace is 1-15 Kg / cm 2 , preferably 2-10 Kg / cm 2 . It is to be noted that the high-purity inert gas is used not only for the gas sealing of the graphitization furnace, but also as the carrier gas directly entering the graphitization furnace to increase the pressure in the graphitization furnace, and the exhaust gas in the furnace is instantaneously discharged to realize the stable and continuous production of the fiber. The existence of trace oxygen not only oxidizes the running fiber, but also oxidizes the heating body in the graphitization furnace, resulting in the decrease of the performance of the graphite fiber and the decrease of the service life of the heating body. Trace water is an oxidation activator of carbon at high temperature, and plays an activation role in the oxidation etching in the high-temperature graphitization environment, resulting in the decrease of the performance of the graphite fiber. Therefore, the strict control of the oxygen content and dew point can improve the performance of the graphite fiber, and at the same time, can prolong the service life of the heating body and realize the stable and continuous production of the fiber. The high pressure in the graphitization furnace can improve the crystal growth effect, continuously improve the crystal region of the fiber, further improve the tensile modulus performance, and obtain the graphite fiber with excellent performance; in addition, the high pressure in the graphitization furnace can help to prolong the service life of the heating body in the graphitization furnace.

[0101] Preferably, the post-treatment includes surface treatment, water washing treatment, sizing treatment and drying treatment.

[0102] In another aspect, the embodiment of the present application provides a graphite fiber, wherein the bulk density of the graphite fiber is 1.91-1.96 g / cm 3 , the tensile strength is 3.6-4.2 GPa, and the tensile modulus is 580-650 GPa.

[0103] Here, the performance of the graphite fiber of the present application has far exceeded the performance of the existing graphite fiber.

[0104] Preferably, the orientation degree, multi-dimensional grain size, pore defect size and fiber performance of the graphite fiber satisfy the following conditions:

[0105] 92nm≤Q≤97nm;

[0106] 120nm 3 ≤Lc×La ⊥ ×La ∥ ≤195nm 3 ;

[0107] 24nm 2 ≤Lc×La⊥ ≤ 34 nm 2 ;

[0108] (50 x Lc + 300) GPa ≤ M ≤ (50 x Lc + 370) GPa;

[0109] 1 nm ≤ X ≤ 7 nm;

[0110] wherein Q represents the orientation degree of the (002) crystal plane of the fiber along the fiber axis, in %; Lc, La ⊥ , La ∥ respectively represent the grain size of the (002) crystal plane in the equatorial direction, the (100) crystal plane in the equatorial direction, and the (100) crystal plane in the meridian direction, in nm; M represents the modulus of the graphite fiber, in GPa; and X represents the pore defect size of the fiber surface layer, in nm. It should be noted that the structure of the graphite fiber includes a crystal structure and a pore structure, wherein the crystal structure refers to the structure of the carbon microcrystal, including structural characteristics such as microcrystal size and orientation, and the pore structure refers to the pore structure characteristics between carbon microcrystals, and the pore defect size is one of the characterization methods of the pore structure. The better the crystal structure of the graphite fiber (the larger the microcrystal size and the better the orientation degree along the fiber axis), and the smaller the pore defect size, the higher the modulus of the carbon fiber, and the better the performance of the fiber. The L c x La ⊥ x La ∥ x La ⊥ of the graphite fiber are characterized, which can better reflect the multi-dimensional crystal structure characteristics.

[0111] The present application is suitable for the preparation of 1K-24K high-performance polyacrylonitrile fibers, polyacrylonitrile-based carbon fibers, and graphite fibers, and can realize the preparation of higher polyacrylonitrile-based carbon fibers and graphite fibers. The related technical path is also suitable for the preparation of high-performance polyacrylonitrile fibers, polyacrylonitrile-based carbon fibers, and graphite fibers by wet method or dry-wet method.

[0112] Preferably, the present application provides a preparation method of polyacrylonitrile fibers, polyacrylonitrile-based carbon fibers, and graphite fibers. The spinning solution is subjected to transmission, filtration, and spinning, and then subjected to coagulation molding treatment, washing treatment, hot water drawing treatment, oiling treatment, drying densification treatment, and steam drawing treatment in sequence to obtain polyacrylonitrile fibers. Specifically, the method comprises the following steps:

[0113] 1) transmission, filtration, and spinning

[0114] The spinning solution is sprayed through the wet spinning nozzle to form a spinning stream. The polyacrylonitrile fiber preparation method provided by the embodiment of the application is suitable for any type of polyacrylonitrile spinning solution, and preferably uses a spinning solution with a solid content of 18-22%, a viscosity of 80-120 Pa.s, and a specific viscosity of 1.75-1.93 dL / g.

[0115] In this case, the transmission, filtration and spinning need to meet the conditions described above, such as the roughness of the inner wall of the transmission pipeline, the transmission speed, the filtration device and the conditions of the spinning nozzle.

[0116] 2) Coagulation forming treatment

[0117] The spinning stream sequentially enters the first-stage coagulation forming treatment, the second-stage coagulation forming treatment, the third-stage coagulation forming treatment and the fourth-stage coagulation forming treatment to obtain a coagulation forming fiber. The composition of the first-stage coagulation bath liquid in the first-stage coagulation forming treatment is dimethyl sulfoxide, water and a hydrophilic agent, and the compositions of the coagulation bath liquids of other stages are dimethyl sulfoxide and water.

[0118] a) First-stage coagulation forming treatment

[0119] The temperature of the first-stage coagulation bath liquid is 45-62°C; the coagulation draft ratio is 0.5-1.0 times; the residence time is 0.2-2 min; the mass fraction of dimethyl sulfoxide in the first-stage coagulation bath liquid is 58-70%; and the mass concentration of the hydrophilic agent in the first-stage coagulation bath liquid is 0-0.1 mol / L.

[0120] b) Second-stage coagulation forming treatment

[0121] The temperature of the second-stage coagulation bath liquid is 50-70°C; the coagulation draft ratio is 1.0-2.0 times; the residence time is 0.2-1.5 min; and the mass fraction of dimethyl sulfoxide in the second-stage coagulation bath liquid is 25-45%.

[0122] c) Third-stage coagulation forming treatment

[0123] The temperature of the third-stage coagulation bath liquid is 55-75°C; the coagulation draft ratio is 1.0-2.0 times; the residence time is 0.3-1.5 min; and the mass fraction of dimethyl sulfoxide in the third-stage coagulation bath liquid is 10-30%.

[0124] d) Fourth-stage coagulation forming treatment

[0125] The temperature of the fourth-stage coagulation bath liquid is 70-89°C; the coagulation draft ratio is 1.0-2.0 times; the residence time is 0.3-1.5 min; and the mass fraction of dimethyl sulfoxide in the fourth-stage coagulation bath liquid is 2-10%.

[0126] 3) Washing treatment

[0127] The fiber after the coagulation forming treatment is sequentially passed through 3-9 water washing temperature zones to obtain a water washed fiber; wherein, the temperature of the later water washing temperature zone is higher than that of the former water washing temperature zone; the temperature of the first water washing temperature zone is 30-50°C, and the temperature of the last water washing temperature zone is 40-60°C. The water washed fiber has a dimethyl sulfoxide residue of 0.01-0.03%. In the water washing treatment step, the water used for water washing is deionized water; preferably, the deionized water has an electric conductivity of ≤3.0 μS / cm, and the dimethyl sulfoxide residue in the deionized waste water after the water washing treatment is 5%-8%.

[0128] 4) Hot water drawing treatment

[0129] The fiber after the water washing treatment is subjected to a hot water drawing treatment to obtain a hot drawn fiber. The hot water drawing temperature is 80-95°C; and the drawing ratio is 1-3.

[0130] 5) Oiling treatment

[0131] The fiber after the hot water drawing treatment is subjected to an oiling treatment, and the oiling agent is used at a concentration of 1.0-3.0%.

[0132] 6) Drying and densification treatment

[0133] The fiber after the oiling treatment is sequentially passed through 10-20 drying and densification temperature zones to obtain a drying and densification fiber. Wherein, the temperature of the later drying and densification temperature zone is higher than that of the former drying and densification temperature zone; the temperature of the first drying and densification temperature zone is 80-100°C, and the temperature of the last drying and densification temperature zone is 115-140°C. The temperature difference between two adjacent drying and densification temperature zones is 0-10°C, and the drying and densification treatment time of each stage is 4-9 s.

[0134] 7) Steam drawing treatment

[0135] The fiber is subjected to a high ratio drawing treatment using saturated water vapor or superheated steam as the medium. The steam pressure is 0.15-0.4 MPa; the drawing ratio is 2-4; and the steam drawing residence time is 2-7 s.

[0136] The polyacrylonitrile fiber is obtained through the steps 1) -7).

[0137] In still another aspect, the polyacrylonitrile fiber prepared above is used to prepare a polyacrylonitrile-based carbon fiber according to the following steps:

[0138] 8) Pre-oxidation treatment

[0139] The polyacrylonitrile fiber obtained through the steps 1) -7) is subjected to a pre-oxidation treatment to obtain a pre-oxidized fiber.

[0140] The pre-oxidized fiber is subjected to low-temperature carbonization treatment to obtain a low-temperature carbonized fiber. The low-temperature carbonization temperature is 300-900°C; and the low-temperature carbonization treatment time is 1-6 min.

[0141] 9) Low-temperature carbonization treatment

[0142] The pre-oxidized fiber is subjected to low-temperature carbonization treatment to obtain a low-temperature carbonized fiber. The low-temperature carbonization temperature is 300-900°C; and the low-temperature carbonization treatment time is 1-6 min.

[0143] 10) High-temperature carbonization treatment

[0144] The low-temperature carbonized fiber is subjected to high-temperature carbonization treatment in 3-8 high-temperature carbonization temperature zones in sequence to obtain a high-temperature carbonized fiber; wherein the temperature of a later high-temperature carbonization temperature zone is higher than that of an earlier high-temperature carbonization temperature zone; the temperature of the first high-temperature carbonization temperature zone is 950-1100°C, and the temperature of the last high-temperature carbonization temperature zone is 1300-1900°C. In the high-temperature carbonization treatment step, the fiber is subjected to a draft ratio of -4.0-20.0%; preferably, the draft ratio is 7-15%. The temperature difference between two adjacent high-temperature carbonization temperature zones is 100-280°C, and the high-temperature carbonization treatment time is 10-180 s. Preferably, the high-temperature carbonization treatment time is 30-90 s. The sealing gas and the carrier gas are selected from one or more of argon, nitrogen and helium; preferably, the sealing gas and the carrier gas are high-purity nitrogen, and the oxygen content of the purified sealing gas and carrier gas should be controlled to be less than 2 ppm, and the dew point should be not more than -65°C. The oxygen content in the high-temperature carbonization furnace is 0-5 ppm, and the dew point is not more than -65°C; preferably, the oxygen content in the high-temperature carbonization furnace is not more than 2 ppm. The pressure in the high-temperature carbonization furnace is 3-15 Pa.

[0145] The high-temperature carbonized fiber obtained in steps 1) to 10) is subjected to surface treatment, water washing, sizing, drying and other post-treatment to obtain a polyacrylonitrile-based carbon fiber.

[0146] In still another aspect, the high-temperature carbonized fiber prepared by the above steps (i.e., the fiber after the high-temperature carbonization treatment in step 10) is subjected to post-treatment, and then the following steps are used to prepare a graphite fiber:

[0147] 11) Graphitization treatment

[0148] The high-temperature carbonized fiber obtained in the steps 1) to 10) is subjected to graphitization treatment to obtain a graphitized fiber. In the graphitization treatment step, the graphitization treatment temperature is 2200 to 3000°C; preferably, the graphitization treatment temperature is 2500 to 2800°C. The drawing ratio applied to the fiber is 4.0 to 25.0%; preferably, the drawing ratio is 10.0% to 20.0%; the graphitization treatment time is 10 to 180 s; preferably, the graphitization treatment time is 30 to 120 s. The sealing gas and the carrier gas are selected from one or more of argon, nitrogen, and helium; preferably, the sealing gas and the carrier gas are high-purity argon. The oxygen content of the purified sealing gas and carrier gas should be controlled to be less than or equal to 2 ppm, and the dew point is not greater than -65°C. The oxygen content in the graphitization furnace is 0 to 5 ppm, and the dew point is not greater than -65°C; preferably, the oxygen content in the graphitization furnace is not greater than 2 ppm. The pressure in the graphitization furnace is 1 to 15 Kg / cm 2 ; preferably, the pressure in the graphitization furnace is 2 to 10 Kg / cm 2 .

[0149] The graphitized fiber obtained in the steps 1) to 11) is subjected to surface treatment, washing, sizing, drying, and other post-treatment to obtain a graphitic fiber.

[0150] The above steps are the preparation steps of the polyacrylonitrile fiber, the polyacrylonitrile-based carbon fiber, and the graphitic fiber.

[0151] The main difference between the preparation steps of the polyacrylonitrile fiber, the polyacrylonitrile-based carbon fiber, and the graphitic fiber prepared by the dry-wet method and the above steps is the preparation of the polyacrylonitrile fiber.

[0152] The preparation of the polyacrylonitrile fiber by the dry-wet method comprises the following steps:

[0153] 1) transmission, filtration, and spinning

[0154] The spinning solution is filtered and spun through the dry-wet spinning spinneret to form a spinning stream. Here, the preparation method of the polyacrylonitrile fiber provided by the embodiments of the present application is suitable for any type of polyacrylonitrile spinning solution, and preferably a spinning solution with a solid content of 18 to 22%, a viscosity of 80 to 120 Pa.s, and a specific viscosity of 1.75 to 1.93 dL / g is used.

[0155] Here, the transmission, filtration, and spinning need to meet the conditions described above, such as the roughness of the inner wall of the transmission pipeline, the transmission speed, the filtration device, and the conditions of the spinneret.

[0156] 2) coagulation forming treatment

[0157] The spinning stream enters the primary coagulation forming treatment to obtain coagulation forming fibers; the composition of the coagulation bath is dimethyl sulfoxide and water. The temperature of the primary coagulation bath is 1-30°C; the coagulation draft ratio is 2-5 times; the residence time is 0.2-2 min; the mass fraction of dimethyl sulfoxide in the primary coagulation bath is 20-60%.

[0158] 3) water washing treatment

[0159] The coagulation forming fibers are sequentially subjected to 3-9 water washing temperature zones to obtain water washed fibers; the temperature of a later water washing temperature zone is higher than that of an earlier water washing temperature zone; the temperature of the first water washing temperature zone is 30-50°C, and the temperature of the last water washing temperature zone is 40-60°C. The residual amount of dimethyl sulfoxide in the water washed fibers is 0.01-0.03%. In the water washing treatment, the water used is deionized water; preferably, the conductivity of the deionized water is ≤3.0 μS / cm, and the residual amount of dimethyl sulfoxide in the deionized wastewater after the water washing treatment is 5-8%.

[0160] 4) hot water draft treatment

[0161] The water washed fibers are subjected to hot water draft treatment to obtain hot drafted fibers. The hot water draft temperature is 60-80°C; the draft ratio is 1-3 times.

[0162] 5) oiling treatment

[0163] The hot drafted fibers are subjected to oiling treatment, and the concentration of the oiling agent used is 1.0-3.0%.

[0164] 6) drying and densification treatment

[0165] The oiled fibers are sequentially subjected to 10-20 drying and densification temperature zones to obtain dried and densified fibers. The temperature of a later drying and densification temperature zone is higher than that of an earlier drying and densification temperature zone; the temperature of the first drying and densification temperature zone is 100-120°C, and the temperature of the last drying and densification temperature zone is 130-160°C. The temperature difference between two adjacent drying and densification temperature zones is 0-10°C, and the time for each drying and densification treatment is 3-5 s.

[0166] 7) steam draft treatment

[0167] The fibers are subjected to high draft with saturated water vapor or superheated steam as the medium. The steam pressure is 0.2-0.6 MPa; the draft ratio is 3-5 times; and the residence time for the steam draft is 2-7 s.

[0168] The polyacrylonitrile fibers are obtained through processes 1)-7).

[0169] It should be noted that the number of spinneret holes is preferably 1000-6000 for dry-wet spinning. If polyacrylonitrile fibers of 6K or more are to be prepared, the fibers need to be cabled during the preparation of the polyacrylonitrile fibers. The cabled polyacrylonitrile fibers are subjected to subsequent process treatment to prepare polyacrylonitrile-based carbon fibers or graphite fibers.

[0170] The scheme of the present application is further illustrated below through specific experimental examples:

[0171] Example 1

[0172] This example prepares a polyacrylonitrile fiber, which mainly includes the following steps:

[0173] The polyacrylonitrile spinning solution is subjected to transmission, filtration and spinning, followed by coagulation forming treatment, washing treatment, hot water drawing treatment, oiling treatment, drying densification treatment and steam drawing treatment to obtain a polyacrylonitrile fiber. The specific steps include:

[0174] 1) Transmission, filtration and spinning

[0175] Transmission: The transmission speed of the polyacrylonitrile spinning solution during transmission in the conveying pipeline is 5 cm / s, and the roughness Ra of the inner wall surface of the conveying pipeline is 0.2 μm.

[0176] Filtration: A vane disc filter is used, and the filtration accuracy is 2 μm; the filter material is a metal fiber mesh, the material of the metal fiber is 316L, the diameter of the metal fiber is 10 μm; the thickness of the filter material is 1 mm; and the filtration speed of the spinning solution in the filter is 3 cm / h.

[0177] Spinneret: A circular spinneret is used, the hole diameter of the spinneret is 0.055 mm, the length-diameter ratio is 1.5, the hole density is 2.5 per mm2, and the number of holes is 6000.

[0178] Wet spinning is used, and the polyacrylonitrile spinning solution is formed into a spinning stream after being spun out through the spinneret after the above transmission and filtration. The polyacrylonitrile spinning solution is a dimethyl sulfoxide solution of polyacrylonitrile, the solid content is 20%, the viscosity is 110 Pa.s, and the intrinsic viscosity is 1.89 dL / g.

[0179] 2) Coagulation forming treatment

[0180] The spinning stream is sequentially subjected to first-stage coagulation forming treatment, second-stage coagulation forming treatment, third-stage coagulation forming treatment and fourth-stage coagulation forming treatment to obtain a coagulation formed fiber. The composition of the first-stage coagulation bath in the first-stage coagulation forming treatment is dimethyl sulfoxide, water and a hydrophilic agent, and the compositions of the coagulation baths in the other stages are dimethyl sulfoxide and water.

[0181] a) First-stage coagulation forming treatment

[0182] The temperature of the primary coagulation bath was 60℃; the coagulation stretch ratio was 0.70 times; the residence time was 0.7 min; the mass fraction of dimethyl sulfoxide in the primary coagulation bath was 60%; and the mass concentration of the hydrophilic agent in the primary coagulation bath was 0.04 mol / L.

[0183] b) Secondary solidification and molding process

[0184] The temperature of the secondary coagulation bath was 65℃; the coagulation stretch ratio was 1.2 times; the residence time was 0.5 min; and the mass fraction of dimethyl sulfoxide in the secondary coagulation bath was 30%.

[0185] c) Three-stage solidification and molding process

[0186] The temperature of the tertiary coagulation bath is 70℃; the coagulation stretch ratio is 1.3 times; the residence time is 0.5 min; and the mass fraction of dimethyl sulfoxide in the tertiary coagulation bath is 20%.

[0187] d) Four-stage solidification and molding treatment

[0188] The temperature of the fourth-stage coagulation bath is 85℃; the coagulation stretch ratio is 1.4 times; the residence time is 0.4 min; and the mass fraction of dimethyl sulfoxide in the fourth-stage coagulation bath is 3%.

[0189] 3) Water washing treatment

[0190] The coagulated fibers were sequentially passed through nine washing zones to obtain washed fibers. The washing temperatures were 45℃, 47℃, 49℃, 51℃, 53℃, 55℃, 57℃, 59℃, and 60℃; the deionized water used for washing had a conductivity of 1.6 μS / cm; the residual dimethyl sulfoxide in the washing wastewater was 6%; and the residual dimethyl sulfoxide in the washed fibers was 0.02%.

[0191] 4) Hot water stretching treatment

[0192] Washed fibers are subjected to hot water stretching treatment to obtain hot-stretched fibers. The hot water stretching temperature is 80-95℃; the stretching ratio is 2.

[0193] 5) Oiling treatment

[0194] The heat-drawn fibers are oiled with an oil concentration of 1.5%.

[0195] 6) Drying and densification treatment

[0196] The oiled fibers were sequentially passed through 12 drying and densification temperature zones to obtain dried and densified fibers. The temperature of the drying and densification zones increased sequentially, with the first zone at 90℃ and the last at 130℃. Each drying and densification process lasted 6 seconds.

[0197] 7) Steam drawing treatment

[0198] The fiber is subjected to high draw ratio by using saturated steam as medium. The steam pressure is 0.30 MPa; the draw ratio is 2.3 times; the steam drawing residence time is 5 s.

[0199] The polyacrylonitrile fiber is obtained after the above processes 1) to 7).

[0200] The preparation conditions of the polyacrylonitrile fiber prepared in Example 1 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0201] Example 2

[0202] Example 2 prepared a kind of polyacrylonitrile fiber, the main difference from Example 1 is that:

[0203] Transportation: the transportation speed of the polyacrylonitrile spinning solution in the transportation pipeline is 7 cm / s, and the roughness Ra of the inner wall surface of the pipeline is 0.1 μm.

[0204] Filtration: a leaf disc type filter is used, the filtration accuracy is 1 μm; the thickness of the filter material is 2 mm; the filtration speed of the spinning solution in the filter is 5 cm / h.

[0205] Spinneret: the hole density is 1.2 pieces / ㎜2.

[0206] Water washing treatment: the conductivity of the deionized water used for water washing is 1.6 μS / cm; the residual amount of dimethyl sulfoxide in the water washing wastewater is 5%; the residual amount of dimethyl sulfoxide in the fiber after water washing treatment is 0.02%.

[0207] Other preparation conditions are the same as those in Example 1.

[0208] The preparation conditions of the polyacrylonitrile fiber prepared in Example 2 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0209] Example 3

[0210] Example 3 prepared a kind of polyacrylonitrile fiber, the main difference from Example 1 is that:

[0211] Transportation: the transportation speed of the polyacrylonitrile spinning solution in the transportation pipeline is 3 cm / s.

[0212] Filtration: the thickness of the filter material is 0.5 mm.

[0213] Spinneret: the hole density is 3.7 pieces / ㎜2.

[0214] Water washing treatment: the conductivity of the deionized water used for water washing is 3 μS / cm; the residual amount of dimethyl sulfoxide in the water washing wastewater is 8%; the residual amount of dimethyl sulfoxide in the fiber after water washing treatment is 0.03%.

[0215] Other preparation conditions are the same as those in Example 1.

[0216] The preparation conditions of the polyacrylonitrile fiber prepared in Example 3 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0217] Comparative Example 1

[0218] The polyacrylonitrile fiber prepared in Comparative Example 1 is different from that in Example 1 in that:

[0219] Transportation: the transportation speed of the polyacrylonitrile spinning solution in the transportation process of the conveying pipeline is 0.5 cm / s, and the roughness Ra of the inner wall surface of the conveying pipeline is 0.6 μm.

[0220] Filtration: a vane disc filter is used, the filtration precision is 6 μm, the thickness of the filter material is 4 mm, and the filtration speed of the spinning solution in the filter is 0.7 cm / h.

[0221] Spinneret: the hole density is 0.4 pieces / ㎜2.

[0222] Water washing treatment: the conductivity of the deionized water used for water washing is 10 μS / cm, the residual amount of dimethyl sulfoxide in the water washing wastewater is 11%, and the residual amount of dimethyl sulfoxide in the fiber after the water washing treatment is 0.09%.

[0223] Other preparation conditions are the same as those in Example 1.

[0224] The preparation conditions of the polyacrylonitrile fiber prepared in Comparative Example 1 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0225] Comparative Example 2

[0226] The polyacrylonitrile fiber prepared in Comparative Example 2 is different from that in Example 1 in that:

[0227] Transportation: the transportation speed of the polyacrylonitrile spinning solution in the transportation process of the conveying pipeline is 0.5 cm / s.

[0228] Filtration: a vane disc filter is used, the filtration precision is 6 μm, the thickness of the filter material is 4 mm, and the filtration speed of the spinning solution in the filter is 0.7 cm / h.

[0229] Spinneret: the hole density is 0.4 pieces / ㎜2.

[0230] Water washing treatment: the conductivity of the deionized water used for water washing is 10 μS / cm, the residual amount of dimethyl sulfoxide in the water washing wastewater is 11%, and the residual amount of dimethyl sulfoxide in the fiber after the water washing treatment is 0.11%.

[0231] Other preparation conditions are the same as those in Example 1.

[0232] The preparation conditions of the polyacrylonitrile fiber prepared in Comparative Example 2 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0233] Comparative Example 3

[0234] Comparative Example 3 prepared a kind of polyacrylonitrile fiber, and the difference from Example 1 is that:

[0235] Filtering: a leaf disc filter is used, the filtering precision is 6 μm; the thickness of the filter material is 4 mm; the filtering speed of the spinning solution in the filter is 0.7 cm / h.

[0236] Spinneret: the hole density is 0.4 per mm2.

[0237] Water washing treatment: the conductivity of deionized water used for water washing is 10 μS / cm; the residual amount of dimethyl sulfoxide in the water washing wastewater is 11%; the residual amount of dimethyl sulfoxide in the fiber after water washing treatment is 0.10%.

[0238] Other preparation conditions are the same as those in Example 1.

[0239] The preparation conditions of the polyacrylonitrile fiber prepared in Comparative Example 3 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0240] Comparative Example 4

[0241] Comparative Example 4 prepared a kind of polyacrylonitrile fiber, and the difference from Example 1 is that:

[0242] Spinneret: the hole density is 0.4 per mm2.

[0243] Water washing treatment: the conductivity of deionized water used for water washing is 10 μS / cm; the residual amount of dimethyl sulfoxide in the water washing wastewater is 11%; the residual amount of dimethyl sulfoxide in the fiber after water washing treatment is 0.09%.

[0244] Other preparation conditions are the same as those in Example 1.

[0245] The preparation conditions of the polyacrylonitrile fiber prepared in Comparative Example 4 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0246] Comparative Example 5

[0247] Comparative Example 5 prepared a kind of polyacrylonitrile fiber, and the difference from Example 1 is that:

[0248] Water washing treatment: the conductivity of deionized water used for water washing is 10 μS / cm; the residual amount of dimethyl sulfoxide in the water washing wastewater is 11%; the residual amount of dimethyl sulfoxide in the fiber after water washing treatment is 0.10%.

[0249] Other preparation conditions are the same as those in Example 1.

[0250] The preparation conditions of the polyacrylonitrile fiber prepared in Comparative Example 6 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0251] Comparative Example 6

[0252] The polyacrylonitrile fiber prepared in Comparative Example 6 is different from that in Example 1 in that:

[0253] Spinneret: hole density is 0.2 / ㎜2.

[0254] Other preparation conditions are the same as those in Example 1.

[0255] The preparation conditions of the polyacrylonitrile fiber prepared in Comparative Example 6 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0256] Comparative Example 7

[0257] The polyacrylonitrile fiber prepared in Comparative Example 7 is different from that in Example 1 in that:

[0258] Water washing treatment: the residual amount of dimethyl sulfoxide in the water washing wastewater is 1%, and the residual amount of dimethyl sulfoxide in the fiber after the water washing treatment is 0.02%.

[0259] Other preparation conditions are the same as those in Example 1.

[0260] The preparation conditions of the polyacrylonitrile fiber prepared in Comparative Example 7 are shown in Table 1, and the performance indexes of the polyacrylonitrile fiber are shown in Table 2.

[0261] Table 1 shows the preparation conditions of the polyacrylonitrile fiber

[0262]

[0263] Table 2 shows the performance indexes of the polyacrylonitrile fiber

[0264]

[0265] From Example 1 to Example 3, Comparative Example 1 to Comparative Example 7, Table 1 and Table 2, it can be seen that the polyacrylonitrile fibers prepared in Example 1, Example 2 and Example 3 have higher grades by eliminating or controlling the defects generated in the production process of the polyacrylonitrile fibers.

[0266] The preparation method of Comparative Example 6 has the phenomenon of spinning solution aggregation on the spinneret surface in the spinning process due to the too low hole density, and the yarn collection is difficult, which affects the normal production.

[0267] The preparation method of Comparative Example 7 has the phenomenon of water waste in the water washing treatment process due to the low residual amount of dimethyl sulfoxide in the water washing wastewater, which causes the waste of water resources.

[0268] Example 4

[0269] Example 4 produces a polyacrylonitrile-based carbon fiber; mainly pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, post-treatment of polyacrylonitrile fiber produced in Example 1 to obtain polyacrylonitrile-based carbon fiber. The main steps are as follows:

[0270] 8) Pre-oxidation treatment

[0271] The polyacrylonitrile fiber obtained in Example 1 is subjected to pre-oxidation treatment to obtain pre-oxidized fiber. Four-stage hot air medium pre-oxidation treatment is adopted, and the pre-oxidation temperature is 200°C, 219°C, 234°C and 250°C in turn; the draft ratio applied to the fiber is 1.9%; the pre-oxidation treatment time is 50 min.

[0272] 9) Low-temperature carbonization treatment

[0273] The pre-oxidized fiber is subjected to low-temperature carbonization treatment to obtain low-temperature carbonized fiber. The low-temperature carbonization temperature is 300-900°C; the low-temperature carbonization treatment time is 1.5 min.

[0274] 10) High-temperature carbonization treatment

[0275] The low-temperature carbonized fiber is subjected to high-temperature carbonization treatment in four high-temperature carbonization zones in turn to produce high-temperature carbonized fiber; the high-temperature carbonization treatment temperature is 960°C, 1100°C, 1370°C and 1580°C in turn; the draft ratio applied to the fiber is 7.2%; the high-temperature carbonization treatment time is 70 s; the sealing gas and carrier gas are high-purity nitrogen, and the oxygen content of the purified nitrogen should be controlled below 1.5 ppm and the dew point is -65°C; the oxygen content in the high-temperature carbonization furnace is 1.9 ppm and the dew point is -65°C; the pressure in the high-temperature carbonization furnace is 9 Pa.

[0276] The high-temperature carbonized fiber obtained in steps 8) to 10) is subjected to surface treatment, washing, sizing, drying and other post-treatment to obtain polyacrylonitrile-based carbon fiber.

[0277] The performance indicators of the polyacrylonitrile-based carbon fiber produced in this example are shown in Tables 3 and 4, and the X-ray two-dimensional diffraction pattern is shown in Figure 1 In addition, the orientation degree, multi-dimensional grain size, pore defect size and fiber performance indicators of the polyacrylonitrile-based carbon fiber produced in this example meet the following conditions:

[0278] 82%≤Q≤88%;

[0279] 20nm 3 ≤Lc×La ⊥ ×La ∥ ≤30nm 3 ;

[0280] 7nm 2≤ Lc x La ⊥ ≤ 10 nm 2 ;

[0281] (30 x Lc + 230) GPa ≤ M ≤ (30 x Lc + 290) GPa;

[0282] 1 nm ≤ X ≤ 7 nm;

[0283] wherein Q represents the orientation degree of the (002) crystal plane of the fiber along the fiber axis, in %; Lc, La ⊥ , La ∥ respectively represent the grain size of the (002) crystal plane in the equatorial direction, the (100) crystal plane in the equatorial direction, and the (100) crystal plane in the meridian direction, in nm; M represents the modulus of the polypropylene-based carbon fiber, in GPa; and X represents the pore defect size of the fiber surface layer, in nm.

[0284] Comparative Example 8

[0285] Comparative Example 8 prepared a polyacrylonitrile-based carbon fiber, mainly by pre-oxidizing, low-temperature carbonizing, high-temperature carbonizing, and post-treating the polyacrylonitrile fiber prepared in Comparative Example 1 to obtain a polyacrylonitrile-based carbon fiber.

[0286] wherein the specific pre-oxidizing, low-temperature carbonizing, high-temperature carbonizing, and post-treating conditions are consistent with those in Example 4.

[0287] The performance indicators of the polyacrylonitrile-based carbon fiber prepared in Comparative Example 8 are shown in Tables 3 and 4.

[0288] As can be seen from Comparative Example 4 and Comparative Example 8, Example 4 eliminates or controls the defects generated in the production process of the polyacrylonitrile fiber, reasonably matches and finely controls the carbonization process conditions, effectively controls the microcrystalline orientation, multi-dimensional grain size, pore defect size, and other fiber structure characteristics of the fiber, and can further improve the tensile strength and tensile modulus indicators of the fiber to prepare an ultra-high-strength carbon fiber.

[0289] Example 5

[0290] Example 5 prepared a polyacrylonitrile-based carbon fiber; mainly by pre-oxidizing, low-temperature carbonizing, high-temperature carbonizing, and post-treating the polyacrylonitrile fiber prepared in Example 1 to obtain a polyacrylonitrile-based carbon fiber. Wherein the main difference between Example 5 and Example 4 is:

[0291] High-temperature carbonization treatment: the low-temperature carbonized fiber successively passes through four high-temperature carbonization temperature zones for high-temperature carbonization treatment to prepare a high-temperature carbonized fiber; the high-temperature carbonization treatment temperature is 1060℃, 1205℃, 1480℃, and 1730℃; the draft ratio applied to the fiber is 14%.

[0292] Other preparation conditions are consistent with those of Example 4.

[0293] The performance indexes of the polyacrylonitrile-based carbon fibers prepared in this example are shown in Tables 3 and 4, the TEM photos of the cross sections are shown in Figure 2 , and the X-ray two-dimensional diffraction patterns are shown in Figure 3 In addition, the orientation degree, multi-dimensional grain size, pore defect size, and fiber performance indexes of the polyacrylonitrile-based carbon fibers prepared in this example meet the following conditions:

[0294] 82%≤Q≤88%;

[0295] 20nm 3 ≤Lc×La ⊥ ×La ∥ ≤30nm 3 ;

[0296] 7nm 2 ≤Lc×La ⊥ ≤10nm 2 ;

[0297] (30×Lc+230)GPa≤M≤(30×Lc+290)GPa;

[0298] 1nm≤X≤7nm;

[0299] wherein Q represents the orientation degree of the (002) crystal plane of the fiber along the fiber axis, in %; Lc, La ⊥ , La ∥ respectively represent the grain sizes of the (002) crystal plane in the equatorial direction, the (100) crystal plane in the equatorial direction, and the (100) crystal plane in the meridian direction, in nm; M represents the modulus of the polyacrylonitrile-based carbon fiber, in GPa; and X represents the pore defect size of the surface layer of the fiber, in nm.

[0300] Comparative Example 9

[0301] The polyacrylonitrile-based carbon fiber prepared in Comparative Example 9 is prepared by performing pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment on the polyacrylonitrile fiber prepared in Comparative Example 1.

[0302] The specific pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment conditions are consistent with those of Example 5.

[0303] The performance indexes of the polyacrylonitrile-based carbon fiber prepared in Comparative Example 9 are shown in Tables 3 and 4, and the TEM photo of the cross section is shown in Figure 4 .

[0304] It can be seen from Comparative Example 5 and Comparative Example 9 that Example 5 eliminates or controls the defects generated in the production process of the polyacrylonitrile fiber, reasonably matches and finely controls the carbonization process conditions, effectively controls the microcrystalline orientation, multidimensional grain size, pore defect size and other fiber structure characteristics of the fiber, and can further improve the tensile strength and tensile modulus indexes of the fiber. The carbon fiber of the prior art needs graphitization treatment to reach high modulus (330-390 GPa). The polyacrylonitrile-based carbon fiber prepared by the present application reaches high modulus without graphitization treatment. The high-strength and high-modulus carbon fiber can be prepared by the polyacrylonitrile fiber prepared by the present application through pre-oxidation, low-temperature carbonization, high-temperature carbonization and other processes. The prepared fiber has high tensile strength and high tensile modulus, meets the needs of high-end fields such as aerospace, and lays a foundation for the optimization of the performance of composite materials.

[0305] Table 3 is the performance index of the carbon fiber

[0306]

[0307] Table 4 is the performance index of the carbon fiber

[0308]

[0309] Example 6

[0310] In this embodiment, a graphite fiber is prepared, which mainly uses the high-temperature carbonized fiber prepared in Example 5 (i.e., the high-temperature carbonized fiber before post-treatment in Example 5) to perform graphitization treatment and post-treatment to obtain a graphite fiber; wherein the main steps are as follows:

[0311] 11) Graphitization treatment

[0312] The high-temperature carbonized fiber prepared in Example 5 is subjected to graphitization treatment to obtain a graphitized fiber. The temperature of the graphitization treatment is 2750℃, and the draft ratio applied to the fiber is 19%; the time of the graphitization treatment is 80s. High-purity argon is used as the sealing gas and the carrier gas. After purification, the oxygen content of the sealing gas and the carrier gas should be controlled to be 1.3ppm, and the dew point is-65℃. The oxygen content in the graphitization furnace is 1.8ppm, and the dew point is-65℃. The pressure in the graphitization furnace is 4Kg / cm 2 .

[0313] Finally, the graphitized fiber is subjected to surface treatment, washing, sizing and drying and other post-treatment to obtain a graphite fiber.

[0314] The performance index of the graphite fiber prepared in this embodiment is shown in Table 5, the TEM photo of the cross section is shown in Figure 5 , and the X-ray two-dimensional diffraction pattern is shown in Figure 6The orientation degree, multi-dimensional grain size, pore defect size, and fiber performance of the graphite fiber prepared in this embodiment satisfy the following conditions:

[0315] 92nm≤Q≤97nm;

[0316] 120nm 3 ≤Lc×La ⊥ ×La ∥ ≤195nm 3 ;

[0317] 24nm 2 ≤Lc×La ⊥ ≤34nm 2 ;

[0318] (50×Lc+300)GPa≤M≤(50×Lc+370)GPa;

[0319] 1nm≤X≤7nm;

[0320] wherein Q represents the orientation degree of the (002) crystal plane of the fiber along the fiber axis, in %; Lc, La ⊥ , La ∥ respectively represent the grain size of the (002) crystal plane in the equatorial direction, the (100) crystal plane in the equatorial direction, and the (100) crystal plane in the meridian direction, in nm; M represents the modulus of the graphite fiber, in GPa; and X represents the pore defect size of the fiber surface, in nm.

[0321] Example 7

[0322] In this embodiment, a graphite fiber is prepared by performing graphitization treatment and post-treatment on the high-temperature carbonized fiber prepared in Example 5 (i.e., the high-temperature carbonized fiber before post-treatment in Example 5); wherein the main steps are as follows:

[0323] 11) Graphitization treatment

[0324] The high-temperature carbonized fiber prepared in Example 5 is subjected to graphitization treatment to obtain a graphitized fiber. The temperature of the graphitization treatment is 2650°C, and the draft ratio applied to the fiber is 11.0%; the time of the graphitization treatment is 80s. High-purity argon is used as the sealing gas and the carrier gas. After purification, the oxygen content of the sealing gas and the carrier gas should be controlled at 1.3ppm, and the dew point is -65°C. The oxygen content in the graphitization furnace is 1.8ppm, and the dew point is -65°C. The pressure in the graphitization furnace is 2Kg / cm 2 .

[0325] Finally, the graphitized fiber is subjected to surface treatment, water washing, sizing, drying, and other post-treatment to obtain a graphite fiber.

[0326] The performance indexes of the graphite fibers prepared in this embodiment are shown in Table 5.

[0327] For Example 6, Example 7: the high-temperature carbonized fibers (prepared in Example 5) provided by the application are subjected to subsequent graphitization treatment and the like, and at the same time, the graphite fiber preparation method provided by the application is used to reasonably match and finely control the graphitization process conditions, effectively control the fiber structure characteristics such as crystallite orientation, multi-dimensional grain size, and pore defect size of the fiber, realize precise adjustment of the fiber performance, more easily prepare graphite fibers with higher modulus, and the graphite fibers have high strength. In addition, by using the graphite fiber preparation method provided by the application, the graphitization temperature is low, which can effectively prolong the service life of the graphite heating body in the graphitization furnace, and ensure stable preparation for a long time.

[0328] Table 5 is the performance index of the graphite fiber

[0329]

[0330] Example 8

[0331] In this embodiment, polyacrylonitrile fibers and polyacrylonitrile-based carbon fibers are prepared.

[0332] The prepared polyacrylonitrile fibers are dry-wet polyacrylonitrile fibers, and the main difference from Example 1 is that:

[0333] Spinneret: circular spinneret, the pore diameter of the spinneret is 0.012 mm, the aspect ratio is 3, the hole density is 2.5 per mm2, and the number of holes is 6000.

[0334] The spinning solution is filtered through the above transmission and filtration, and is sprayed out through the dry-wet spinning spinneret to form a spinning stream.

[0335] Solidification forming treatment: the spinning stream enters the first-stage solidification forming treatment to obtain a solidification forming fiber; the composition of the coagulation bath is dimethyl sulfoxide and water. The temperature of the first-stage coagulation bath is 10℃; the coagulation draft ratio is 3.5 times; the residence time is 0.5 min; and the mass fraction of dimethyl sulfoxide in the first-stage coagulation bath is 40%.

[0336] Hot water drafting treatment: the hot water drafting temperature is 75℃; and the drafting ratio is 2.8 times.

[0337] Drying and densification treatment: the fiber after the oiling treatment is sequentially subjected to 20 drying and densification temperature zones to obtain a drying and densification fiber. The temperature of the drying and densification temperature zones increases in sequence, the temperature of the first drying and densification temperature zone is 110℃, and the temperature of the last drying and densification temperature zone is 160℃. The time for each stage of drying and densification treatment is 3 s.

[0338] Steam drawing treatment: steam pressure: 0.50 MPa; draw ratio: 3.5 times; steam drawing residence time: 3 s.

[0339] The polyacrylonitrile fiber obtained through the above processes is subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment to obtain a polyacrylonitrile-based carbon fiber. The pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment are prepared according to the method of Example 4.

[0340] The polyacrylonitrile fiber obtained through the above processes is subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment to obtain a polyacrylonitrile-based carbon fiber. The pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment are prepared according to the method of Example 4.

[0341] The polyacrylonitrile fiber obtained through the above processes is subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment to obtain a polyacrylonitrile-based carbon fiber. The pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment are prepared according to the method of Example 4.

[0342] Example 9

[0343] The polyacrylonitrile fiber obtained through the above processes is subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment to obtain a polyacrylonitrile-based carbon fiber. The pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment are prepared according to the method of Example 4.

[0344] The polyacrylonitrile fiber obtained through the above processes is subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment to obtain a polyacrylonitrile-based carbon fiber. The pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment are prepared according to the method of Example 4.

[0345] The above-described, only the preferred embodiments of the present application, not any form of the application on the limit, according to the technical essence of the application on the above examples of any simple modification, equivalent changes and modifications, still belong to the scope of the present application technical solutions.

Claims

1. A polyacrylonitrile-based carbon fiber, characterized in that, The bulk density of the polyacrylonitrile-based carbon fiber is 1.78–1.81 g / cm³. 3 Tensile strength is 5.7–7.1 GPa, and tensile modulus is 320–390 GPa; The orientation degree, multidimensional grain size, pore defect size, and fiber performance indicators of the polyacrylonitrile-based carbon fiber shall meet the following conditions: 82％≤Q≤88％； 20nm 3 ≤Lc×La ⊥ ×La ∥ ≤30nm 3 ; 7nm 2 ≤Lc×La ⊥ ≤10nm 2 ; (30×Lc+230)GPa≤M≤(30×Lc+290)GPa; 1nm≤X≤7nm; Where Q represents the orientation degree of the (002) crystal plane of the fiber along the fiber axis, in percentage; Lc, La ⊥ La ∥ The values ​​represent the grain size of the (002) crystal plane in the equatorial direction, the (100) crystal plane in the equatorial direction, and the (100) crystal plane in the meridional direction, respectively, in nm; M represents the modulus of polypropylene-based carbon fiber, in GPa; X represents the size of the surface pore defects of the fiber, in nm. Among them, polyacrylonitrile fibers are subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, and post-treatment to obtain polyacrylonitrile-based carbon fibers. The cross-section of the polyacrylonitrile fiber is approximately elliptical or circular; the intra-batch linear density dispersion CV of the polyacrylonitrile fiber is <0.50%, the inter-batch linear density dispersion CV is <0.55, and the long-range linear density dispersion CV is <0.50%; the brightness difference ΔL of the polyacrylonitrile fiber is 5-15; and the total content of metal ions in the polyacrylonitrile fiber is ≤50ppm. The preparation method of polyacrylonitrile fiber includes the following steps: Polyacrylonitrile spinning solution is transported to a spinneret via a pipeline for spinning. After spinning, the solution undergoes coagulation, washing, and post-treatment to obtain polyacrylonitrile fibers. The inner wall of the conveying pipe is polished and has a roughness Ra≤0.40μm; the conveying speed of the polyacrylonitrile spinning solution in the conveying pipe is controlled to be 1~10cm / s.

2. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The roughness of the inner wall surface of the conveying pipeline is 0.10 to 0.40 μm.

3. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The roughness Ra of the inner wall surface of the conveying pipeline is ≤0.20μm.

4. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The polyacrylonitrile spinning solution is transported at a speed of 3-7 cm / s in the conveying pipe.

5. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The polyacrylonitrile spinning solution is first transported to a filtration device for filtration via the aforementioned conveying pipe, and then transported to a spinneret for spinning; wherein, The filtration device includes a disc filter; wherein the filtration accuracy is 1-5 μm, the filter media is a metal fiber mesh, and the thickness of the filter media is 0.4-3.0 mm.

6. The polyacrylonitrile-based carbon fiber according to claim 5, characterized in that, The filtration accuracy is 1–2 μm.

7. The polyacrylonitrile-based carbon fiber according to claim 5, characterized in that, The metal fibers on the metal fiber mesh are made of 316L.

8. The polyacrylonitrile-based carbon fiber according to claim 5, characterized in that, The diameter of the metal fibers on the metal fiber mesh is <25μm.

9. The polyacrylonitrile-based carbon fiber according to claim 8, characterized in that, The diameter of the metal fibers on the metal fiber mesh is 3 to 15 μm.

10. The polyacrylonitrile-based carbon fiber according to claim 5, characterized in that, The thickness of the filter material is 0.5 to 2 mm.

11. The polyacrylonitrile-based carbon fiber according to claim 5, characterized in that, The filtration rate of the polyacrylonitrile spinning solution in the filtration device is 2-13 cm / h.

12. The polyacrylonitrile-based carbon fiber according to claim 11, characterized in that, The filtration rate of the polyacrylonitrile spinning solution in the filtration device is 3-5 cm / h.

13. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The spinneret includes a wet spinning spinneret or a dry-wet spinning spinneret. The wet spinning spinneret has an aperture of 0.050–0.075 mm, an aspect ratio of 1–3, and a pore density of 0.5–5 pores / mm. 2 The number of holes ranges from 500 to 50,000. The spinneret used in the dry-wet spinning process has an aperture of 0.1–0.3 mm, an aspect ratio of 2–5, and a pore density of 0.1–0.9 pores / mm. 2 The number of holes ranges from 500 to 8000.

14. The polyacrylonitrile-based carbon fiber according to claim 13, characterized in that, The wet spinning spinneret has an orifice diameter of 0.055–0.065 mm, an aspect ratio of 1.5–2, and an orifice density of 1.2–3.7 orifices / mm². 2 The number of holes is 1,000-24,000; the wet spinning spinneret is a circular spinneret.

15. The polyacrylonitrile-based carbon fiber according to claim 13, characterized in that, The dry / wet spinneret has an aperture of 0.1–0.2 mm, an aspect ratio of 2.5–4, and a pore density of 0.12–0.5 pores / mm². 2 The number of holes is 1000-6000; the dry-wet spinning spinneret is a circular spinneret.

16. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the water washing process: the water used for the water washing process is deionized water; wherein, the conductivity of the deionized water is ≤3.0μS / cm.

17. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the water washing process: the coagulated fibers are sequentially washed through 3 to 9 water washing temperature zones to obtain washed fibers; wherein, the temperature of each subsequent water washing temperature zone is higher than that of the previous water washing temperature zone; the temperature of the first water washing temperature zone is 30 to 50°C, and the temperature of the last water washing temperature zone is 40 to 60°C; wherein, the temperature of the water washing temperature zones gradually increases along the fiber running direction; and the water flow direction is opposite to the fiber running direction.

18. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the washing process, the total effective residence time of the fiber in all washing temperature zones is 30 to 200 seconds.

19. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the water washing process, the ratio of water used in the water washing process to the weight of the washed fibers is 10 to 20.

20. The polyacrylonitrile-based carbon fiber according to claim 19, characterized in that, In the water washing process, the ratio of water used in the water washing process to the weight of the washed fibers is 10 to 15.

21. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the water washing process, the residual solvent content in the fiber after water washing is 0.01% to 0.03%.

22. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the water washing treatment step: the solvent residue in the deionized wastewater after water washing treatment is 5-8%.

23. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The post-processing includes hot water stretching, oiling, drying and densification, and steam stretching.

24. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The roundness coefficient of the cross section of the polyacrylonitrile fiber is greater than 0.

85.

25. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The brightness difference ΔL of the polyacrylonitrile fiber is 5 to 10.

26. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The total content of metal ions in the polyacrylonitrile fiber is ≤30ppm.

27. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the high-temperature carbonization process: the fibers after low-temperature carbonization are sequentially passed through 3 to 8 high-temperature carbonization zones for high-temperature carbonization treatment; wherein the temperature of each subsequent high-temperature carbonization zone is higher than that of the previous high-temperature carbonization zone; the temperature of the first high-temperature carbonization zone is 950 to 1100°C; the temperature of the last high-temperature carbonization zone is 1300 to 1900°C; and the high-temperature carbonization treatment time is 10 to 180 seconds.

28. The polyacrylonitrile-based carbon fiber according to claim 27, characterized in that, In the high-temperature carbonization process: the temperature of the last high-temperature carbonization zone is 1500-1900℃.

29. The polyacrylonitrile-based carbon fiber according to claim 27, characterized in that, In the high-temperature carbonization process, the high-temperature carbonization time is 30 to 90 seconds.

30. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the high-temperature carbonization process, the draw ratio applied to the fiber is -4.0 to 20.0%.

31. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the high-temperature carbonization process, the draw ratio applied to the fiber is 7 to 15%.

32. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the high-temperature carbonization process, the temperature difference between two adjacent high-temperature carbonization zones is 100–280°C.

33. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the high-temperature carbonization process: the sealing gas and carrier gas of the high-temperature carbonization furnace are selected from one or more of argon, nitrogen and helium; the oxygen content in the sealing gas and carrier gas of the high-temperature carbonization furnace is controlled to be below 2 ppm and the dew point is not greater than -65℃.

34. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, In the high-temperature carbonization process, the oxygen content in the high-temperature carbonization furnace is 0-5 ppm and the dew point is no greater than -65°C.

35. The polyacrylonitrile-based carbon fiber according to claim 34, characterized in that, The oxygen content in the high-temperature carbonization furnace shall not exceed 2 ppm.

36. The polyacrylonitrile-based carbon fiber according to claim 34, characterized in that, The pressure inside the high-temperature carbonization furnace is 3–15 Pa.

37. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The cross-section of the polyacrylonitrile-based carbon fiber is approximately elliptical or circular.

38. The polyacrylonitrile-based carbon fiber according to claim 37, characterized in that, The roundness factor of the polyacrylonitrile-based carbon fiber is >0.

85.

39. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The intra-batch linear density dispersion (CV) of the polyacrylonitrile-based carbon fiber is <0.60%, the inter-batch linear density dispersion (CV) is <0.65%, and the long-range linear density dispersion (CV) is <0.60%; the intra-batch strength dispersion (CV) of the polyacrylonitrile-based carbon fiber is <3.0%, the inter-batch strength dispersion (CV) is <3.5%, and the long-range strength dispersion (CV) is <3.0%.

40. The polyacrylonitrile-based carbon fiber according to claim 1, characterized in that, The total metal ion content in the polyacrylonitrile-based carbon fiber is ≤50ppm.

41. The polyacrylonitrile-based carbon fiber according to claim 40, characterized in that, The total metal ion content in the polyacrylonitrile-based carbon fiber is ≤30ppm.

42. A method for preparing graphite fibers, characterized in that, Polyacrylonitrile fibers are subjected to pre-oxidation treatment, low-temperature carbonization treatment, high-temperature carbonization treatment, graphitization treatment, and post-treatment to obtain graphite fibers. in, The cross-section of the polyacrylonitrile fiber is approximately elliptical or circular; the intra-batch linear density dispersion CV of the polyacrylonitrile fiber is <0.50%, the inter-batch linear density dispersion CV is <0.55, and the long-range linear density dispersion CV is <0.50%; the brightness difference ΔL of the polyacrylonitrile fiber is 5-15; and the total content of metal ions in the polyacrylonitrile fiber is ≤50ppm. The preparation method of polyacrylonitrile fiber includes the following steps: Polyacrylonitrile spinning solution is transported to a spinneret via a pipeline for spinning. After spinning, the solution undergoes coagulation, washing, and post-treatment to obtain polyacrylonitrile fibers. The inner wall surface of the conveying pipe is polished and the roughness Ra≤0.40μm; the conveying speed of the polyacrylonitrile spinning solution in the conveying pipe is controlled to be 1~10cm / s; In the graphitization process: the graphitization temperature is 2200–3000℃; the draw ratio applied to the fiber is 4.0–25.0%; and the graphitization time is 10–180 s.

43. The method for preparing graphite fibers according to claim 42, characterized in that, The graphitization temperature is 2500–2800℃; the draw ratio applied to the fiber is 10.0%–20.0%; and the graphitization time is 30–120 s.

44. The method for preparing graphite fibers according to claim 42, characterized in that, In the graphitization process, the sealing gas and carrier gas of the graphitization furnace are selected from one or more of argon, nitrogen, and helium.

45. The method for preparing graphite fibers according to claim 44, characterized in that, The oxygen content of the sealing gas and carrier gas is controlled below 2 ppm, and the dew point is no greater than -65℃.

46. ​​The method for preparing graphite fibers according to claim 42, characterized in that, In the graphitization process: the oxygen content in the graphitization furnace is 0-5 ppm, the dew point is no greater than -65°C; the oxygen content in the graphitization furnace does not exceed 2 ppm; the pressure in the graphitization furnace is 1-15 kg / cm². 2 .

47. The method for preparing graphite fibers according to claim 46, characterized in that, The pressure inside the graphitization furnace is 2-10 kg / cm². 2 .

48. The method for preparing graphite fibers according to claim 42, characterized in that, The post-processing includes surface treatment, washing, sizing, and drying.

49. A graphite fiber, characterized in that, The bulk density of the graphite fibers is 1.91–1.96 g / cm³. 3 The tensile strength is 3.6–4.2 GPa and the tensile modulus is 580–650 GPa; wherein the graphite fiber is prepared by the preparation method of graphite fiber according to any one of claims 42-48.

50. The graphite fiber according to claim 49, characterized in that, The orientation, multidimensional grain size, pore defect size, and fiber properties of the graphite fibers meet the following conditions: 92nm≤Q≤97nm; 120nm 3 ≤Lc×La ⊥ ×La ∥ ≤195nm 3 ; 24nm 2 ≤Lc×La ⊥ ≤34nm 2 ; (50×Lc+300)GPa≤M≤(50×Lc+370)GPa; 1nm≤X≤7nm; Where Q represents the orientation degree of the fiber (002) crystal plane along the fiber axis, in percentage; Lc, La ⊥ La ∥ The values ​​represent the grain size of the (002) crystal plane in the equatorial direction, the (100) crystal plane in the equatorial direction, and the (100) crystal plane in the meridional direction, respectively, in nm; M represents the graphite fiber modulus in GPa; and X represents the size of the surface pore defects of the fiber in nm.

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