Polyacrylonitrile precursor fiber and method for producing the same, carbon fiber and method for producing the same

Abstract

The application discloses a polyacrylonitrile precursor fiber and a preparation method thereof, and carbon fiber and a preparation method thereof. The preparation method of the polyacrylonitrile precursor fiber comprises the following steps: mixing acrylonitrile, a comonomer, an initiator, ammonia water and a solvent to perform a polymerization reaction, so as to obtain a spinning solution; after the spinning solution is spun, the spinning solution enters a coagulation bath comprising a solvent, an organic acid and water, and is coagulated and formed; after boiling water drafting, washing, oiling, drying and densification, steam drafting and heat setting, a polyacrylonitrile precursor fiber is obtained. The mass ratio of the acrylonitrile, the comonomer, the initiator, the ammonia water and the solvent is 20-40:0.1-2:0.1-1:1-10:100. The method can not only prepare a large-diameter precursor fiber with a radial homogeneity, but also has a relatively high coagulation speed.

Description

Technical Field

[0001] This invention belongs to the field of fibers, specifically relating to a polyacrylonitrile precursor fiber and its preparation method, and carbon fiber and its preparation method. Background Technology

[0002] Polyacrylonitrile (PAC) precursor fiber is the raw material for carbon fiber production, and its properties determine the final carbon fiber performance. Coagulation forming typically refers to the phase transition that occurs after the spinning solution is extruded through a spinneret and enters a coagulation bath composed of solvent and precipitant, transforming from a liquid state to a solid or quasi-solid fiber. This is the most critical step in the preparation of PAC precursor fiber. The coagulation process of PAC fiber is a double diffusion process: the precipitant in the coagulation bath diffuses into the fiber, while the solvent inside the fiber diffuses into the coagulation bath. Because the double diffusion process starts from the fiber, the outer layer of the fiber solidifies first upon entering the coagulation bath, forming a sheath layer that hinders subsequent double diffusion. This results in a slow coagulation process and a tendency to form a core-sheath structure, severely affecting the quality of the precursor fiber and the final carbon fiber performance. As the fiber diameter increases, the core-sheath difference becomes even more pronounced due to the increased double diffusion distance. Adding a certain amount of ammonia to the polymer solution or coagulation bath can effectively improve the core-sheath difference of PAC fibers formed during the coagulation process, but it usually causes a decrease in coagulation rate and a prolonged coagulation time, which is detrimental to improving production efficiency and reducing manufacturing costs. Summary of the Invention

[0003] This invention provides a polyacrylonitrile precursor fiber and its preparation method, as well as a carbon fiber and its preparation method. The method can not only prepare radially homogeneous large-diameter precursor fibers, but also achieve a faster solidification rate.

[0004] In one aspect of the present invention, a method for preparing polyacrylonitrile precursor fibers is provided, comprising: Acrylonitrile, comonomer, initiator, ammonia and solvent are mixed and polymerized to obtain a spinning solution. After spinning, the spinning solution is introduced into a coagulation bath containing solvent, organic acid and water. After coagulation and shaping, it is subjected to boiling water stretching, water washing, oiling, drying and densification, steam stretching and heat setting to obtain polyacrylonitrile precursor fiber. The mass ratio of acrylonitrile, comonomer, initiator, ammonia and solvent is 20-40:0.1-2:0.1-1:1-10:100.

[0005] In some embodiments of the present invention, the comonomer includes at least one of itaconic acid, methyl acrylate, acrylic acid, acrylamide, and monobutyl itaconic acid.

[0006] In some embodiments of the present invention, the solvent is dimethyl sulfoxide.

[0007] In some embodiments of the present invention, the initiator includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, and ammonium persulfate.

[0008] In some embodiments of the present invention, the polymerization reaction is carried out at a temperature of 40-70°C for 10-20 hours.

[0009] In some embodiments of the present invention, the mass ratio of dimethyl sulfoxide, organic acid and water in the coagulation bath is 100:1-10:30-100.

[0010] In some embodiments of the present invention, the organic acid includes at least one of acetic acid, succinic acid, benzoic acid, citric acid, and benzenesulfonic acid.

[0011] In some embodiments of the present invention, the diameter of the polyacrylonitrile precursor fiber is 12-20 micrometers.

[0012] In a second aspect of the invention, a polyacrylonitrile precursor fiber is provided, which is prepared by the method described in the first aspect of the invention.

[0013] In a third aspect of the present invention, a method for preparing carbon fibers is provided, comprising: pre-oxidizing and carbonizing the polyacrylonitrile precursor fiber to obtain carbon fibers, wherein the polyacrylonitrile precursor fiber includes polyacrylonitrile precursor fiber obtained by the method of the first aspect of the present invention or polyacrylonitrile precursor fiber of the second aspect of the present invention.

[0014] In some embodiments of the present invention, the pre-oxidation temperature is 210-270°C.

[0015] In some embodiments of the present invention, the carbonization temperature is 700-1500°C.

[0016] In a fourth aspect, the present invention provides a carbon fiber prepared by means of the method described in the third aspect of the present invention.

[0017] In some embodiments of the present invention, the carbon fiber monofilament has a diameter of 7.5-12 micrometers, a tensile strength of 4000MPa-5500MPa, and a tensile modulus of 220 GPa-280GPa.

[0018] This invention involves polymerizing acrylonitrile, comonomer, initiator, ammonia, and solvent, controlling the mass ratio of acrylonitrile, comonomer, initiator, ammonia, and solvent to 20-40:0.1-2:0.1-1:1-10:100. This ammonia content effectively prevents polyacrylonitrile precipitation in the polymer solution without altering the polymerization process. After spinning, the spinning solution is introduced into a coagulation bath containing solvent, organic acid, and water. The organic acid in the coagulation bath dissociates into hydrogen ions, which rapidly enter the fiber and neutralize the ammonia as the polymer stream enters the coagulation bath, accelerating the coagulation rate within the fiber. Furthermore, the larger anions in the organic acid complex with the ammonia on the fiber surface, preventing rapid coagulation and the formation of a skin layer. This improves the core-skin structure often formed by conventional coagulation methods, achieving homogeneous coagulation. This process not only produces radially homogeneous large-diameter precursor fibers but also achieves rapid coagulation. Attached Figure Description

[0019] Figure 1 This is a radial SEM image of the polyacrylonitrile precursor obtained in Example 1 of the present invention.

[0020] Figure 2 This is a radial SEM image of the polyacrylonitrile precursor obtained in Example 2 of the present invention.

[0021] Figure 3 This is a radial SEM image of the polyacrylonitrile precursor fiber obtained in Example 3 of the present invention.

[0022] Figure 4 This is a radial SEM image of the polyacrylonitrile precursor fiber obtained in Example 4 of the present invention.

[0023] Figure 5 This is a radial SEM image of the polyacrylonitrile precursor fiber obtained in Comparative Example 1 of this invention.

[0024] Figure 6 This is a radial SEM image of the polyacrylonitrile precursor obtained in Comparative Example 2 of this invention.

[0025] Figure 7 This is a radial SEM image of the polyacrylonitrile precursor fiber obtained in Comparative Example 3 of the present invention.

[0026] Figure 8 This is a radial SEM image of the polyacrylonitrile precursor fiber obtained in Comparative Example 6 of this invention. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0028] The "range" disclosed in this invention is defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for specific parameters, it is understood that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers, and ranges defined in this way can include endpoints a and b. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0029] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0030] In one aspect of the present invention, a method for preparing polyacrylonitrile precursor fibers is provided, the method comprising: S100: Polymerization reaction is carried out by mixing acrylonitrile, comonomer, initiator, ammonia and solvent. In this step, acrylonitrile, comonomer, initiator, ammonia, and solvent are mixed and polymerized to obtain a spinning solution. The mass ratio of acrylonitrile, comonomer, initiator, ammonia, and solvent is 20-40:0.1-2:0.1-1:1-10:100. The inventors discovered that if the amount of ammonia added is too high, it will affect the acrylonitrile polymerization, resulting in a significant decrease in polymerization conversion rate, low viscosity of the spinning solution, and inability to perform normal spinning. If the amount of ammonia added is too low, it will not play a role in regulating the solidification rate.

[0031] As an example, the concentration of ammonia is 25%-35%; the comonomer includes at least one of itaconic acid, methyl acrylate, acrylic acid, acrylamide, and monobutyl itaconic acid, which can improve the spinnability and thermal properties of polyacrylonitrile, thereby improving the processability of polyacrylonitrile precursor and its carbon fiber preparation; the solvent is dimethyl sulfoxide; the initiator includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, and ammonium persulfate; the polymerization reaction temperature is 40-70°C, and the time is 10-20 hours.

[0032] S200: After spinning, the spinning solution is introduced into a coagulation bath containing solvent, organic acid and water. After coagulation and shaping, it undergoes boiling water drawing, water washing, oiling, drying and densification, steam drawing and heat setting. In this step, the spinning solution obtained in step S100 is de-bubbled and de-isolated under vacuum conditions to obtain spinning dope. The spinning dope is metered and extruded into a spinneret with a diameter of 70-100 micrometers to form a fine stream, which enters a coagulation bath to solidify and form a coagulation bath including solvent, organic acid and water. The mass ratio of dimethyl sulfoxide, organic acid and water in the coagulation bath is 100:1-10:30-100, the coagulation draw ratio is 0.7-3.0, the coagulation bath temperature is 5-70℃, and the coagulation time is 30-150 seconds to obtain nascent fibers.

[0033] Furthermore, the nascent fibers obtained above are subjected to boiling water stretching at a temperature of 95-100℃ and a stretching ratio of 2.0-6.0. Then, they are washed in hot water at 50-85℃ for 60-150 seconds using a gradient heating method. After being oiled (e.g., with silicone oil), they are dried and densified at a temperature of 110-140℃ for 30-45 seconds. Finally, they are steam stretched at a temperature of 100-140℃ with a stretching ratio of 2.5-4.0. Finally, they are heat-set to obtain polyacrylonitrile precursor fibers at a temperature of 140-160℃ with a stretching ratio of 0.9-1.1, resulting in polyacrylonitrile precursor fibers with a diameter of 12-20 micrometers.

[0034] The inventors discovered that by adding organic acids to the coagulation bath, hydrogen ions are released. These hydrogen ions rapidly enter the filaments as the polymer streams enter the coagulation bath and undergo an acid-base neutralization reaction with the ammonia, thereby accelerating the coagulation rate inside the filaments. On the other hand, the larger anions in the organic acids complex with the ammonia on the filament surface, preventing the rapid coagulation of the filament surface and the formation of a skin layer. This improves the core-skin structure of the fibers that often forms with conventional coagulation methods, thus achieving homogeneous coagulation.

[0035] In a second aspect, the present invention provides a polyacrylonitrile precursor fiber, which is prepared by the method described in the first aspect of the present invention.

[0036] It should be noted that the features and advantages described above for the preparation of polyacrylonitrile precursor fibers also apply to these polyacrylonitrile precursor fibers, and will not be repeated here.

[0037] In a third aspect of the present invention, a method for preparing carbon fibers is provided, comprising: pre-oxidizing and carbonizing the polyacrylonitrile precursor to obtain carbon fibers, wherein the polyacrylonitrile precursor includes polyacrylonitrile precursor obtained by the method described in the first aspect of the present invention or polyacrylonitrile precursor described in the second aspect of the present invention.

[0038] In this step, the obtained polyacrylonitrile precursor fibers are subjected to pre-oxidation and carbonization treatments sequentially to obtain carbon fibers. Specifically, the pre-oxidation treatment conditions include: 210-270℃, a 3-6 stage gradient heating method, a total treatment time of 30-80 minutes, and a draw ratio of 0.9-1.2. The carbonization treatment temperature is 700℃-1500℃, the treatment time is 2-4 minutes, and the draw ratio is 0.95-1.20.

[0039] Thus, using the above method, carbon fiber monofilaments with diameters of 7.5-15 micrometers, tensile strengths of 4000-5500 MPa, and tensile moduli of 220-260 GPa were obtained.

[0040] It should be noted that the features and advantages described above for polyacrylonitrile precursor fibers and their preparation methods also apply to this method for preparing carbon fibers, and will not be repeated here.

[0041] In a fourth aspect, the present invention provides a carbon fiber prepared by means of the method described in the fourth aspect of the present invention.

[0042] It should be noted that the features and advantages described above for the method of preparing carbon fiber also apply to this carbon fiber, and will not be repeated here.

[0043] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and should not be construed as limiting the present invention in any way.

[0044] Example 1 (1) Acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, ammonia and dimethyl sulfoxide were mixed evenly at room temperature in a mass ratio of 30:0.3:0.2:5:100 and polymerized at 63°C for 18 hours to obtain a polyacrylonitrile solution; (2) The polyacrylonitrile solution was de-bubbled and de-isolated under vacuum at 60°C to obtain the spinning solution; the spinning solution was sprayed out through a spinneret with an aperture of 85μm at a heating temperature of 45°C and entered a coagulation bath composed of dimethyl sulfoxide, acetic acid and water in a mass ratio of 100:5:35. The coagulation bath temperature was 23°C, the coagulation time was 60 seconds, and the draw ratio was 0.64 to obtain nascent fibers; the nascent fibers were subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing and heat setting in sequence, with a boiling water drawing ratio of 4. 2. The temperature is 100℃, and the washing is done in 60℃ hot water for 100 seconds with a draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification are carried out using hot rollers at a densification temperature of 140℃. The dried and densified filaments are then steam-drawn at 140℃ with a draw ratio of 2.8. Finally, they undergo heat setting at 160℃ with a draw ratio of 0.95. After heat setting, they are wound to obtain polyacrylonitrile precursor fibers with a single filament diameter of 13.7μm. The radial SEM image is shown below. Figure 1 As shown, the obtained precursor fiber is radially homogeneous; (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 210℃, 230℃, 250℃ and 270℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each segment was 12 minutes to obtain pre-oxidized fibers. Then, the pre-oxidized fibers were carbonized sequentially at temperatures of 700℃, 1200℃ and 1500℃. Each carbonization was carried out in a high-purity nitrogen atmosphere and the dwell time of each carbonization temperature was 1 minute to obtain carbon fibers.

[0045] Example 2 (1) Acrylonitrile monomer, methyl acrylate, azobisisobutyronitrile, ammonia and dimethyl sulfoxide were mixed evenly at room temperature in a mass ratio of 25:0.2:0.5:3:100 and polymerized at 45°C for 15 hours to obtain a polyacrylonitrile solution. (2) The polyacrylonitrile solution was de-bubbled and de-isolated under vacuum at 60°C to obtain the spinning solution; the spinning solution was sprayed out through a spinneret with a 75μm aperture at a heating temperature of 45°C and entered a coagulation bath composed of dimethyl sulfoxide, citric acid and water in a mass ratio of 100:3:50. The coagulation bath temperature was 23°C, the coagulation time was 60 seconds, and the draw ratio was 0.64 to obtain nascent fibers; the nascent fibers were subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing and heat setting in sequence, with a boiling water drawing ratio of 3. 2. The temperature was 100℃, and the washing was done in 60℃ hot water for 100 seconds with a draw ratio of 0.99. The oiling agent was silicone oil. Drying and densification were carried out using hot rollers at a densification temperature of 140℃. The dried and densified filaments were then steam-drawn at 140℃ with a draw ratio of 2.5. Finally, they underwent heat setting at 160℃ with a draw ratio of 0.95. After heat setting, they were wound to obtain polyacrylonitrile precursor fibers with a single filament diameter of 15.7μm. The radial SEM image is shown below. Figure 2 As shown, the obtained precursor fiber is radially homogeneous; (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 210℃, 230℃, 250℃ and 270℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each segment was 12 minutes to obtain pre-oxidized fibers. Then, the pre-oxidized fibers were carbonized sequentially at temperatures of 700℃, 1200℃ and 1400℃. Each carbonization was carried out in a high-purity nitrogen atmosphere and the dwell time of each carbonization temperature was 1 minute to obtain carbon fibers.

[0046] Example 3 (1) Acrylonitrile monomer, acrylamide, benzoyl peroxide, ammonia and dimethyl sulfoxide were mixed evenly at room temperature in a mass ratio of 35:1.0:0.2:8:100 and polymerized at 50°C for 18 hours to obtain a polyacrylonitrile solution; (2) The polyacrylonitrile solution was de-bubbled and de-isolated under vacuum at 60°C to obtain the spinning solution; the spinning solution was sprayed out through a spinneret with a diameter of 90μm at a heating temperature of 45°C and entered a coagulation bath composed of dimethyl sulfoxide, benzoic acid and water in a mass ratio of 100:8:70. The coagulation bath temperature was 3°C, the coagulation time was 60 seconds, and the draw ratio was 0.70 to obtain nascent fibers; the nascent fibers were subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing and heat setting in sequence, with a boiling water drawing ratio of 4. 8. The temperature is 100℃, and the washing is done in 60℃ hot water for 100 seconds with a draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification are carried out using hot rollers at a densification temperature of 140℃. The dried and densified filaments are then steam-drawn at 140℃ with a draw ratio of 3.0. Finally, they undergo heat setting at 160℃ with a draw ratio of 0.95. After heat setting, they are wound to obtain polyacrylonitrile precursor fibers with a single filament diameter of 17.6μm. The radial SEM image is shown below. Figure 3 As shown, the obtained precursor fiber is radially homogeneous; (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 210℃, 230℃, 250℃ and 270℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each segment was 12 minutes to obtain pre-oxidized fibers. Then the pre-oxidized fibers were carbonized sequentially at temperatures of 700℃, 1200℃ and 1300℃. Each carbonization was carried out in a high-purity nitrogen atmosphere and the dwell time of each carbonization temperature was 1 minute to obtain carbon fibers.

[0047] Example 4 (1) Acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, ammonia and dimethyl sulfoxide were mixed evenly at room temperature in a mass ratio of 28:0.3:0.5:8:100 and polymerized at 65°C for 12 hours to obtain a polyacrylonitrile solution; (2) The polyacrylonitrile solution was de-bubbled and de-isolated under vacuum at 60°C to obtain the spinning solution; the spinning solution was sprayed out through a spinneret with an aperture of 85μm at a heating temperature of 45°C and entered a coagulation bath composed of dimethyl sulfoxide, benzoic acid and water in a mass ratio of 100:8:35. The coagulation bath temperature was 23°C, the coagulation time was 60 seconds, and the draw ratio was 0.64 to obtain nascent fibers; the nascent fibers were subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing and heat setting in sequence, with a boiling water drawing ratio of 3. The yarn was washed at 100℃ in 60℃ hot water for 100 seconds with a draw ratio of 0.99. Silicone oil was used as the oiling agent. Drying and densification were performed using hot rollers at 140℃. The dried and densified yarn was then steam-drawn at 140℃ with a draw ratio of 2.5. Finally, it underwent heat setting at 160℃ with a draw ratio of 0.95. After heat setting, it was wound to obtain polyacrylonitrile precursor yarn with a single filament diameter of 12.4μm. Its radial SEM image is shown below. Figure 4 As shown, the obtained precursor fiber is radially homogeneous; (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 210℃, 230℃, 250℃ and 270℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each segment was 12 minutes to obtain pre-oxidized fibers. Then, the pre-oxidized fibers were carbonized sequentially at temperatures of 700℃, 1200℃ and 1500℃. Each carbonization was carried out in a high-purity nitrogen atmosphere and the dwell time of each carbonization temperature was 1 minute to obtain carbon fibers.

[0048] Comparative Example 1 (1) Acrylonitrile monomer, itaconic acid, azobisisobutyronitrile and dimethyl sulfoxide were mixed evenly at room temperature in a mass ratio of 30:0.3:0.2:100 and polymerized at 63°C for 18 hours to obtain a polyacrylonitrile solution; (2) The polyacrylonitrile solution was de-bubbled and de-isolated under vacuum at 60°C to obtain the spinning solution; the spinning solution was sprayed out through a spinneret with an aperture of 85μm at a heating temperature of 45°C and entered a coagulation bath composed of dimethyl sulfoxide and water with a mass ratio of 100:35. The coagulation bath temperature was 23°C, the coagulation time was 180 seconds, and the draw ratio was 0.64 to obtain nascent fibers; the nascent fibers were then subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing, and heat setting in sequence, with a boiling water draw ratio of 4.2. The temperature was 100℃, and the washing was performed in 60℃ hot water for 100 seconds with a draw ratio of 0.99. Silicone oil was used as the oiling agent. Drying and densification were carried out using hot rollers at 140℃. The dried and densified filaments were then steam-drawn at 140℃ with a draw ratio of 2.8. Finally, they underwent heat setting at 160℃ with a draw ratio of 0.95. After heat setting, the filaments were wound to obtain polyacrylonitrile precursor fibers with a single filament diameter of 12.3 μm. The radial SEM image is shown below. Figure 5As shown, it can be seen that there is a distinct core-skin structure in its radial direction; (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 210℃, 230℃, 250℃ and 270℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each segment was 12 minutes to obtain pre-oxidized fibers. Then, the pre-oxidized fibers were carbonized sequentially at temperatures of 700℃, 1200℃ and 1500℃. Each carbonization was carried out in a high-purity nitrogen atmosphere and the dwell time of each carbonization temperature was 1 minute to obtain carbon fibers.

[0049] Comparative Example 2 (1) Acrylonitrile monomer, itaconic acid, azobisisobutyronitrile and dimethyl sulfoxide were mixed evenly at room temperature in a mass ratio of 30:0.3:0.2:100 and polymerized at 63°C for 18 hours to obtain a polyacrylonitrile solution; (2) The polyacrylonitrile solution was de-bubbled and de-isolated under vacuum at 60°C to obtain the spinning solution; the spinning solution was sprayed out through a spinneret with an aperture of 85μm at a heating temperature of 45°C and entered a coagulation bath composed of dimethyl sulfoxide, acetic acid and water in a mass ratio of 100:5:35. The coagulation bath temperature was 23°C, the coagulation time was 180 seconds, and the draw ratio was 0.64 to obtain nascent fibers; the nascent fibers were subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing and heat setting in sequence, with a boiling water drawing ratio of 4. 2. The temperature is 100℃, and the washing is done in 60℃ hot water for 100 seconds with a draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification are carried out using hot rollers at a densification temperature of 140℃. The dried and densified filaments are then steam-drawn at 140℃ with a draw ratio of 2.8. Finally, they undergo heat setting at 160℃ with a draw ratio of 0.95. After heat setting, they are wound to obtain polyacrylonitrile precursor fibers with a single filament diameter of 12.4μm. The radial SEM image is shown below. Figure 6 As shown, it can be seen that there is a distinct core-skin structure in its radial direction; (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 210℃, 230℃, 250℃ and 270℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each segment was 12 minutes to obtain pre-oxidized fibers. Then, the pre-oxidized fibers were carbonized sequentially at temperatures of 700℃, 1200℃ and 1500℃. Each carbonization was carried out in a high-purity nitrogen atmosphere and the dwell time of each carbonization temperature was 1 minute to obtain carbon fibers.

[0050] Comparative Example 3 The difference between this and Example 1 lies in the preparation process of the polyacrylonitrile solution. Acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, ammonia, and dimethyl sulfoxide were mixed uniformly at room temperature in a mass ratio of 30:0.3:0.2:0.6:100, and polymerized at 63°C for 18 hours, with a solidification time of 180 seconds in the coagulation bath. The resulting precursor fiber had a diameter of 12.4 μm, and its radial SEM image is shown below. Figure 7 As shown, it can be seen that there is a distinct core-skin structure in its radial direction.

[0051] Comparative Example 4 The difference between this and Example 1 is that in the process of preparing the polyacrylonitrile solution, acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, ammonia and dimethyl sulfoxide are mixed evenly at room temperature in a mass ratio of 30:0.3:0.2:12:100, and polymerized at 63°C for 18 hours. The resulting spinning solution has low viscosity and cannot be spun normally.

[0052] Comparative Example 5 The difference between this and Example 1 is that the coagulation bath used in the coagulation process is composed of dimethyl sulfoxide and water in a mass ratio of 100:35, which makes coagulation difficult during the spinning process and prevents normal spinning.

[0053] Comparative Example 6 The difference between this and Example 1 is that hydrochloric acid was used instead of acetic acid in the coagulation bath during the coagulation process, and the coagulation time in the coagulation bath was 180 seconds. The diameter of the resulting precursor fiber monofilament was 12.4 μm, and its radial SEM image is shown below. Figure 8 As shown, it can be seen that there is a distinct core-skin structure in its radial direction.

[0054] Test method: The carbon fibers of Examples 1-4 and Comparative Examples 1-6 were tested according to GB3362 "Mechanical Properties Test Standard for Carbon Fiber Bundles" and GB / T 3364 "Test Methods for Diameter and Number of Carbon Fibers". The test results are shown in Table 1.

[0055] Table 1

[0056] Conclusion: As shown in Table 1, the coagulation time used in Examples 1-4 was shorter than that in Comparative Examples 1 and 2, and the resulting precursor fibers had a larger diameter and a more uniform radial distribution. Compared with Example 1, Comparative Example 3 differed in that the acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, ammonia, and dimethyl sulfoxide were mixed uniformly at room temperature in a mass ratio of 30:0.3:0.2:0.6:100, and the coagulation time was 180 seconds. Comparative Example 4 differed in that the acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, ammonia, and dimethyl sulfoxide were mixed uniformly at room temperature in a mass ratio of 30:0.3:0.2:12:100. Comparative Example 5 differed from Example 1 in that the coagulation bath used a coagulation bath composed of dimethyl sulfoxide and water in a mass ratio of 100:35. Comparative Example 6 differed from Example 1 in that hydrochloric acid was used instead of acetic acid in the coagulation bath, and the coagulation time in the coagulation bath was 180 seconds. As shown in Table 1, the polyacrylonitrile precursor obtained in Example 1 has a larger diameter and a more uniform radial distribution, while the spinning solutions obtained in Comparative Examples 4 and 5 cannot be spun normally. The polyacrylonitrile precursors obtained in Comparative Examples 3 and 6 have a core-sheath structure in the radial direction. Furthermore, the tensile strength and tensile modulus of the carbon fiber obtained in Example 1 are higher than those in Comparative Examples 3 and 6. This indicates that by using the spinning conditions and coagulation bath of the present invention, not only is the coagulation speed faster, but also large-diameter polyacrylonitrile precursors with a uniform radial distribution can be obtained, and the resulting carbon fibers have superior mechanical properties.

[0057] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing polyacrylonitrile precursor fibers, characterized in that, include: Acrylonitrile, comonomer, initiator, ammonia and solvent are mixed and polymerized to obtain a spinning solution. After spinning, the spinning solution is introduced into a coagulation bath containing solvent, organic acid, and water. After coagulation and shaping, it undergoes boiling water drawing, water washing, oiling, drying and densification, steam drawing, and heat setting to obtain polyacrylonitrile precursor fibers. The mass ratio of acrylonitrile, comonomer, initiator, ammonia and solvent is 20-40:0.1-2:0.1-1:1-10:

100.

2. The method according to claim 1, characterized in that, The comonomer includes at least one of itaconic acid, methyl acrylate, acrylic acid, acrylamide, and monobutyl itaconic acid; Optionally, the solvent is dimethyl sulfoxide; Optionally, the initiator includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, and ammonium persulfate.

3. The method according to claim 1 or 2, characterized in that, The polymerization reaction is carried out at a temperature of 40-70℃ for 10-20 hours.

4. The method according to claim 1, characterized in that, The mass ratio of dimethyl sulfoxide, organic acid and water in the coagulation bath is 100:1-10:30-100; Optionally, the organic acid includes at least one of acetic acid, succinic acid, benzoic acid, citric acid, and benzenesulfonic acid.

5. The method according to claim 1, characterized in that, The diameter of the polyacrylonitrile precursor fiber is 12-20 micrometers.

6. A polyacrylonitrile precursor fiber, characterized in that, It is prepared by any one of claims 1-5.

7. A method for preparing carbon fiber, characterized in that, include: The polyacrylonitrile precursor is subjected to pre-oxidation and carbonization treatment to obtain carbon fiber, wherein the polyacrylonitrile precursor includes polyacrylonitrile precursor obtained by any one of claims 1-5 or polyacrylonitrile precursor as described in claim 6.

8. The method according to claim 7, characterized in that, The pre-oxidation temperature is 210-270℃; Optionally, the carbonization treatment temperature is 700-1500℃.

9. A carbon fiber, characterized in that, It is prepared by the method described in claim 7 or 8.

10. The carbon fiber according to claim 9, characterized in that, The carbon fiber monofilament has a diameter of 7.5-12 micrometers, a tensile strength of 4000 MPa-5500 MPa, and a tensile modulus of 220 GPa-280 GPa.

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