Method for preparing acrylonitrile copolymer, carbon fiber and preparation method of carbon fiber

Abstract

The invention discloses a method for preparing an acrylonitrile copolymer, a carbon fiber and a preparation method of the carbon fiber. The molecular weight distribution index of the acrylonitrile copolymer is 1.5-2.4; the method for preparing the acrylonitrile copolymer comprises the following steps: putting a mixed solution containing an acrylonitrile monomer, a comonomer, a free radical initiator, a solvent and water into a supergravity field of 1000G-4000G, and carrying out polymerization reaction to obtain a reaction product containing the acrylonitrile copolymer. By adopting the method, the acrylonitrile copolymer with narrower molecular weight distribution can be prepared, so that the high-performance carbon fiber can be prepared.

Description

Technical Field

[0001] This invention belongs to the field of fibers. Specifically, this invention relates to a method for preparing acrylonitrile copolymers, carbon fibers, and the preparation method thereof. Background Technology

[0002] Acrylonitrile copolymers are raw materials for the production of carbon fibers and their precursors, and their molecular weight distribution has a significant impact on the properties of carbon fibers and precursors. A narrower molecular weight distribution in acrylonitrile copolymers is beneficial for reducing the viscosity of polyacrylonitrile spinning solutions, minimizing outlet bulging during spinning, and facilitating the orientation of polyacrylonitrile macromolecular chains, which is fundamental for preparing high-quality polyacrylonitrile precursors. Precursors prepared using acrylonitrile copolymers with a narrower molecular weight distribution can reduce defects during the pre-oxidation and carbonization process, thereby improving the mechanical properties of the carbon fibers. Acrylonitrile copolymers for carbon fiber production are typically obtained through free radical polymerization. Conventional free radical polymerization yields polymers with a wide molecular weight distribution, generally with a molecular weight distribution index (MDI) above 3.0. While anionic polymerization or atom transfer radical polymerization can achieve MDIs below 1.5, the polymerization reaction conditions are harsh and difficult to control. Furthermore, the polymerization initiators often contain metal ions, making them difficult to apply to the production of high-performance carbon fiber acrylonitrile copolymers. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a method for preparing acrylonitrile copolymers, carbon fibers, and the same method. This method can produce acrylonitrile copolymers with a narrow molecular weight distribution, thereby enabling the preparation of high-performance carbon fibers.

[0004] In one aspect of the present invention, a method for preparing an acrylonitrile copolymer having a molecular weight distribution index of 1.5-2.4 is provided. The method comprises: placing a mixed solution comprising acrylonitrile monomer, comonomer, free radical initiator, solvent and water in a hypergravity field of 1000G to 4000G to carry out a polymerization reaction to obtain a reaction product comprising the acrylonitrile copolymer.

[0005] Furthermore, the mass ratio of the acrylonitrile monomer, comonomer, free radical initiator, solvent and water is 10-30:0.2-2:0.1-1:10-50:100.

[0006] Furthermore, the comonomer includes at least one of itaconic acid, methyl acrylate, acrylic acid, acrylamide, and monobutyl itaconic acid.

[0007] Furthermore, the solvent includes at least one of cyclobutanol, dimethyl sulfoxide, ethylene carbonate, dimethylformamide, and dimethylacetamide.

[0008] Furthermore, the free radical initiator includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, and ammonium persulfate.

[0009] Furthermore, the polymerization reaction is carried out at a temperature of 40-65°C for 1-10 hours.

[0010] Furthermore, the method further includes washing and drying the reaction product comprising the acrylonitrile copolymer.

[0011] In a second aspect, the present invention provides a method for preparing carbon fibers, comprising: Polyacrylonitrile precursor fibers are obtained by dissolving, degassing, spinning, coagulating and stretching, boiling water stretching, washing, oiling, drying and densifying, steam stretching and heat setting of acrylonitrile copolymer. The polyacrylonitrile precursor fiber is pre-oxidized and carbonized to obtain carbon fiber. The acrylonitrile copolymer is prepared using the method described in the first aspect of this invention.

[0012] Furthermore, the pre-oxidation temperature is 220-250℃.

[0013] Furthermore, the carbonization treatment temperature is 700-1500℃.

[0014] In a third aspect, the present invention provides a carbon fiber prepared by means of the method described in the second aspect of the present invention.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: The present invention places a mixed solution including acrylonitrile monomer, comonomer, free radical initiator, solvent and water in a hypergravity field of 1000-4000G for polymerization reaction, that is, free radical aqueous precipitation polymerization in a hypergravity field, which solves the problem of wide molecular weight distribution of acrylonitrile copolymer obtained by existing free radical aqueous precipitation polymerization of acrylonitrile. The resulting acrylonitrile copolymer has a narrow molecular weight distribution. The carbon fiber prepared using this acrylonitrile copolymer has a tensile strength of 6600-8130MPa and a tensile modulus of 298-325GPa. The tensile strength is more than 14% higher than that of carbon fiber obtained by polymerization under conventional conditions. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. The following examples are merely descriptive and not limiting, and should not be used to limit the scope of protection of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0017] In one aspect of the present invention, a method for preparing an acrylonitrile copolymer having a molecular weight distribution index of 1.5-2.4 is provided. The method comprises: placing a mixed solution comprising acrylonitrile monomer, comonomer, free radical initiator, solvent and water in a hypergravity field of 1000G to 4000G to carry out a polymerization reaction to obtain a reaction product comprising the acrylonitrile copolymer.

[0018] In conventional free radical precipitation polymerization, acrylonitrile monomers undergo polymerization after being initiated by free radicals generated from the decomposition of an initiator, yielding polyacrylonitrile macromolecules. Since polyacrylonitrile is insoluble in water, it gradually precipitates from the reaction system during polymerization. Due to the randomness of this precipitation process, the resulting polymer has a wide molecular weight distribution. However, in a hypergravity field, the polymer generated during polymerization rapidly separates from the reaction system under the influence of the hypergravity field, thereby achieving effective control over the polymer molecular weight and its distribution. Furthermore, the inventors of this invention have discovered that if the hypergravity field is too low, the effect of controlling the molecular weight distribution cannot be achieved; while if the hypergravity field is too high, the resulting polymer molecular weight distribution is too narrow, which is detrimental to subsequent spinning and carbon fiber performance improvement. Therefore, this invention places a mixed solution comprising acrylonitrile monomer, comonomer, free radical initiator, solvent and water in a hypergravity field of 1000-4000G for polymerization reaction, i.e., free radical aqueous precipitation polymerization in a hypergravity field. This solves the problem of wide molecular weight distribution of acrylonitrile copolymers obtained by existing free radical aqueous precipitation polymerization of acrylonitrile. The resulting acrylonitrile copolymer has a molecular weight distribution index of 1.5-2.4. The carbon fibers prepared using this acrylonitrile copolymer have a tensile strength of 6600-8130MPa and a tensile modulus of 298-325GPa. The tensile strength is more than 14% higher than that of carbon fibers polymerized under conventional conditions.

[0019] It should be noted that the hypergravity field conditions of this invention can be achieved using a centrifuge.

[0020] In this invention, the method for testing the molecular weight distribution of acrylonitrile copolymer includes: testing with a gel permeation chromatograph (GPC) manufactured by Waters Corporation, using dimethylformamide as a solvent, and adding an appropriate amount of lithium chloride when preparing the polymer solution.

[0021] Further, in the above-mentioned mixed solution, the mass ratio of the acrylonitrile monomer, comonomer, free radical initiator, solvent, and water is 10-30:0.2-2:0.1-1:10-50:100, for example 10:0.2-2:0.1-1:10-50:100, 20:0.2-2:0.1-1:10-50:100, 30:0.2-2:0.1-1 : 10-50: 100, 10-30: 0.2: 0.1-1: 10-50: 100, 10-30: 0.5: 0.1-1: 10-50: 100, 10-30: 1: 0.1-1: 10-50: 100, 10-30: 1.5: 0.1-1: 10-50: 100, 10-30: 2: 0.1-1: 10-50: 100 The following are examples of acrylonitrile copolymers with molecular weight distribution indices: 10-30:0.2-2:0.1:10-50:100, 10-30:0.2-2:0.5:10-50:100, 10-30:0.2-2:1:10-50:100, 10-30:0.2-2:0.1-1:10:100, 10-30:0.2-2:0.1-1:20:100, 10-30:0.2-2:0.1-1:30:100, 10-30:0.2-2:0.1-1:40:100, 10-30:0.2-2:0.1-1:50:100, etc. Therefore, this invention uses a mixed solution with these compositions to carry out a polymerization reaction under the above-mentioned gravity field conditions, which can yield acrylonitrile copolymers with a molecular weight distribution index of 1.5-2.4, thereby enabling the preparation of high-performance carbon fibers.

[0022] Furthermore, the comonomer includes at least one of itaconic acid, methyl acrylate, acrylic acid, acrylamide, and monobutyl itaconic acid. This composition of comonomers can improve the spinnability and thermal properties of polyacrylonitrile, thereby enhancing the processability of polyacrylonitrile precursor fibers and their carbon fibers.

[0023] Furthermore, the solvent includes at least one of cyclobutanol, dimethyl sulfoxide, ethylene carbonate, dimethylformamide, and dimethylacetamide. Since the solubility of acrylonitrile monomer in water is only about 7%, adding this solvent to the polymerization system can increase the solubility of acrylonitrile monomer, thereby improving production efficiency.

[0024] Furthermore, the free radical initiator includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, and ammonium persulfate. This type of free radical initiator can decompose at a certain temperature to generate free radicals, thereby initiating a free radical polymerization reaction of acrylonitrile and comonomers.

[0025] Furthermore, the polymerization reaction is carried out at a temperature of 40-65°C for a time of 1-10 hours, for example, at temperatures of 40°C, 50°C, 60°C, and 65°C for times of 1 hour, 5 hours, and 10 hours. Thus, under these polymerization conditions, acrylonitrile copolymers with a molecular weight distribution index of 1.5-2.4 can be prepared, thereby enabling the production of high-performance carbon fibers.

[0026] Furthermore, the method further includes: washing and drying the reaction product comprising the acrylonitrile copolymer to obtain acrylonitrile copolymer powder.

[0027] It should be noted that washing and drying of the reaction products are standard procedures in the field and will not be described in detail here.

[0028] In a second aspect, the present invention provides a method for preparing carbon fibers, comprising: S10: Polyacrylonitrile precursor fibers are obtained by dissolving, degassing, spinning, coagulating and stretching, boiling water stretching, washing, oiling, drying and densifying, steam stretching and heat setting of acrylonitrile copolymer. In this step, the acrylonitrile copolymer obtained by the above method is dissolved in dimethyl sulfoxide (DMSO). The resulting spinning solution, after degassing under vacuum, is metered and extruded from the spinneret to form a fine stream, which then enters the primary coagulation bath. The coagulation process includes primary, secondary, and tertiary coagulation. The temperature of the primary coagulation bath in the primary process is 0-40°C, and the primary coagulation bath uses a dimethyl sulfoxide aqueous solution with a volume content of 65-75% and a primary coagulation draw ratio of 0.7-3.0. The coagulated filament exits... After the primary coagulation bath, the fibers enter the secondary coagulation bath at a temperature of 50-70℃. The secondary coagulation bath uses a dimethyl sulfoxide aqueous solution with a volume content of 30-45% and a draw ratio of 1.0-2.0. After exiting the secondary coagulation bath, the coagulated filaments enter the tertiary coagulation bath at a temperature of 55-75℃. The tertiary coagulation bath also uses a dimethyl sulfoxide aqueous solution with a volume content of 10-20% and a draw ratio of 1.0-2.0, yielding nascent fibers.

[0029] Furthermore, the nascent fibers obtained above are subjected to boiling water stretching at a temperature of 95-100℃ with a stretching ratio of 2.0-4.5. 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 undergo drying and densification treatment at a temperature of 110-140℃ for 30-45 seconds. Next, they are subjected to steam stretching at a temperature of 100-140℃ with a stretching ratio of 2.6-3.5. Finally, they undergo heat setting treatment to obtain polyacrylonitrile precursor fibers at a temperature of 140-160℃ with a stretching ratio of 0.9-1.1.

[0030] S20: The polyacrylonitrile precursor fiber is pre-oxidized and carbonized to obtain carbon fiber. 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: 220-250℃, 3-6 stage gradient heating for 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.

[0031] Therefore, the carbon fibers prepared from the acrylonitrile copolymer obtained by the above method have a tensile strength of 6600-8130 MPa and a tensile modulus of 298-325 GPa, which is more than 14% higher than that of carbon fibers obtained by polymerization under conventional conditions.

[0032] It should be noted that the features and advantages described above for the method of preparing acrylonitrile copolymers also apply to this method of preparing carbon fibers, and will not be repeated here.

[0033] In a third aspect, the present invention provides a carbon fiber prepared by means of the method described in the second aspect of the present invention.

[0034] 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.

[0035] 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.

[0036] Example 1 (1) Acrylonitrile monomer, methyl acrylate, ammonium persulfate, dimethylformamide and deionized water were mixed evenly at room temperature in a mass ratio of 25:0.3:0.2:15:100. The mixture was heated to 50°C under a 1000 G hypergravity field generated by a high-speed centrifuge and reacted for 4 hours to obtain a reaction product including acrylonitrile copolymer. The product was washed with water and dried to obtain acrylonitrile copolymer powder. (2) Under stirring, acrylonitrile copolymer powder was dissolved in dimethyl sulfoxide at 66°C. After 8 hours, stirring was stopped, and the solution was degassed under vacuum at 60°C to obtain a spinning solution with a solid content of 20.6 wt%. The spinning solution was then sprayed through a spinneret with a 70 μm aperture at a heating temperature of 45°C and entered a primary coagulation bath (dimethyl sulfoxide aqueous solution). The primary coagulation bath temperature was 3°C, the volume concentration of the primary coagulation bath was 72%, and the draw ratio was 0.75. Then it entered a secondary coagulation bath (dimethyl sulfoxide aqueous solution). The secondary coagulation bath temperature was 55°C, the volume concentration of the secondary coagulation bath was 40%, and the draw ratio was 1.2. After exiting the secondary coagulation bath, it entered a tertiary coagulation bath. The tertiary coagulation bath temperature was 60°C. The primary coagulation bath uses a dimethyl sulfoxide aqueous solution, while the volume content of dimethyl sulfoxide in the tertiary coagulation bath is 15%, and the tertiary coagulation draw ratio is 1.5, resulting in nascent fibers. The nascent fibers are then subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing, and heat setting. The boiling water draw ratio is 4.2, and the temperature is 100℃. The washing is done in 60℃ hot water for 100 seconds, with a washing draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification is carried out using hot rollers at a densification temperature of 140℃. The dried and densified fiber bundles are then subjected to steam drawing at a temperature of 120℃ with a draw ratio of 2.8. Finally, they undergo heat setting at a temperature of 160℃ with a draw ratio of 0.95. After heat setting, the fibers are wound to obtain polyacrylonitrile precursor fibers. (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 220℃, 230℃, 240℃ and 250℃. 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.

[0037] Example 2 (1) Acrylonitrile monomer, itaconic acid monobutyl ester, benzoyl peroxide, dimethylacetamide and deionized water were mixed at room temperature in a mass ratio of 23:1.0:0.4:20:100 and heated to 60°C under a 2000 G hypergravity field generated by a high-speed centrifuge for 5 hours to obtain a reaction product including acrylonitrile copolymer. The product was washed with water and dried to obtain acrylonitrile copolymer powder. (2) Under stirring, acrylonitrile copolymer powder was dissolved in dimethyl sulfoxide at 66°C. After 8 hours, stirring was stopped, and the solution was degassed under vacuum at 60°C to obtain a spinning solution with a solid content of 20.6 wt%. The spinning solution was then sprayed through a spinneret with a 65 μm aperture at a heating temperature of 45°C and entered a primary coagulation bath (dimethyl sulfoxide aqueous solution). The primary coagulation bath temperature was 3°C, the volume concentration of the primary coagulation bath was 72%, and the draw ratio was 0.75. Then it entered a secondary coagulation bath (dimethyl sulfoxide aqueous solution). The secondary coagulation bath temperature was 55°C, the volume concentration of the secondary coagulation bath was 40%, and the draw ratio was 1.2. After exiting the secondary coagulation bath, it entered a tertiary coagulation bath. The tertiary coagulation bath temperature was 60°C. The primary coagulation bath uses a dimethyl sulfoxide aqueous solution, while the volume content of dimethyl sulfoxide in the tertiary coagulation bath is 15%, and the tertiary coagulation draw ratio is 1.5, resulting in nascent fibers. The nascent fibers are then subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing, and heat setting. The boiling water draw ratio is 4.2, and the temperature is 100℃. The washing is done in 60℃ hot water for 100 seconds, with a washing draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification is carried out using hot rollers at a densification temperature of 140℃. The dried and densified fiber bundles are then subjected to steam drawing at a temperature of 120℃ with a draw ratio of 2.8. Finally, they undergo heat setting at a temperature of 160℃ with a draw ratio of 0.95. After heat setting, the fibers are wound to obtain polyacrylonitrile precursor fibers. (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 220℃, 230℃, 240℃ and 250℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each range was 12 minutes to obtain pre-oxidized fibers. Then the pre-oxidized fibers were carbonized in three temperature ranges 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.

[0038] Example 3 (1) Acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, dimethyl sulfoxide and deionized water were mixed evenly at room temperature in a mass ratio of 20:0.5:0.5:20:100. The mixture was heated to 63°C under a hypergravity field of 2500 G generated by a high-speed centrifuge and reacted for 4 hours to obtain a reaction product including acrylonitrile copolymer. The product was washed with water and dried to obtain acrylonitrile copolymer powder. (2) Under stirring, acrylonitrile copolymer powder was dissolved in dimethyl sulfoxide at 66°C. After 8 hours, stirring was stopped, and the solution was degassed under vacuum at 60°C to obtain a spinning solution with a solid content of 20.6 wt%. The spinning solution was then sprayed through a spinneret with a 60 μm aperture at a heating temperature of 45°C and entered a primary coagulation bath (dimethyl sulfoxide aqueous solution). The primary coagulation bath temperature was 3°C, the volume concentration of the primary coagulation bath was 72%, and the draw ratio was 2.6. Then it entered a secondary coagulation bath (dimethyl sulfoxide aqueous solution). The secondary coagulation bath temperature was 55°C, the volume concentration of the secondary coagulation bath was 40%, and the draw ratio was 1.2. After exiting the secondary coagulation bath, it entered a tertiary coagulation bath. The tertiary coagulation bath temperature was 60°C. The primary coagulation bath uses a dimethyl sulfoxide aqueous solution, while the volume content of dimethyl sulfoxide in the tertiary coagulation bath is 15%, and the tertiary coagulation draw ratio is 1.5, resulting in nascent fibers. The nascent fibers are then subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing, and heat setting. The boiling water draw ratio is 4.2, and the temperature is 100℃. The washing is done in 60℃ hot water for 100 seconds, with a washing draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification is carried out using hot rollers at a densification temperature of 140℃. The dried and densified fiber bundles are then subjected to steam drawing at a temperature of 120℃ with a draw ratio of 2.8. Finally, they undergo heat setting at a temperature of 160℃ with a draw ratio of 0.95. After heat setting, the fibers are wound to obtain polyacrylonitrile precursor fibers. (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 220℃, 230℃, 240℃ and 250℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each range was 12 minutes to obtain pre-oxidized fibers. Then the pre-oxidized fibers were carbonized in three temperature ranges 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.

[0039] Example 4 (1) Acrylonitrile monomer, acrylamide, ammonium persulfate, cyclobutanol and deionized water were mixed evenly at room temperature in a mass ratio of 15:0.3:0.2:15:100. The mixture was heated to 50°C under a hypergravity field of 3500 G generated by a high-speed centrifuge and reacted for 6 hours to obtain a reaction product including acrylonitrile copolymer. The product was washed with water and dried to obtain acrylonitrile copolymer powder. (2) Under stirring, acrylonitrile copolymer powder was dissolved in dimethyl sulfoxide at 66°C. After 8 hours, stirring was stopped, and the solution was degassed under vacuum at 60°C to obtain a spinning solution with a solid content of 20.6 wt%. The spinning solution was then sprayed through a spinneret with a 55 μm aperture at a heating temperature of 45°C and entered a primary coagulation bath (dimethyl sulfoxide aqueous solution). The primary coagulation bath temperature was 3°C, the volume concentration of the primary coagulation bath was 72%, and the draw ratio was 2.6. Then it entered a secondary coagulation bath (dimethyl sulfoxide aqueous solution). The secondary coagulation bath temperature was 55°C, the volume concentration of the secondary coagulation bath was 40%, and the draw ratio was 1.0. After exiting the secondary coagulation bath, it entered a tertiary coagulation bath. The tertiary coagulation bath temperature was 60°C. The primary coagulation bath uses a dimethyl sulfoxide aqueous solution, while the volume content of dimethyl sulfoxide in the tertiary coagulation bath is 15%, and the tertiary coagulation draw ratio is 1.5, resulting in nascent fibers. The nascent fibers are then subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing, and heat setting. The boiling water draw ratio is 4.2, and the temperature is 100℃. The washing is done in 60℃ hot water for 100 seconds, with a washing draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification is carried out using hot rollers at a densification temperature of 140℃. The dried and densified fiber bundles are then subjected to steam drawing at a temperature of 120℃ with a draw ratio of 2.8. Finally, they undergo heat setting at a temperature of 160℃ with a draw ratio of 0.95. After heat setting, the fibers are wound to obtain polyacrylonitrile precursor fibers. (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 220℃, 230℃, 240℃ and 250℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each range was 12 minutes to obtain pre-oxidized fibers. Then the pre-oxidized fibers were carbonized in three temperature ranges 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.

[0040] Example 5 (1) Acrylonitrile monomer, acrylamide, azobisisobutyronitrile, dimethyl sulfoxide and deionized water were mixed evenly at room temperature in a mass ratio of 27:1.5:0.8:40:100. The mixture was heated to 45°C under a hypergravity field of 4000 G generated by a high-speed centrifuge and reacted for 8 hours to obtain a reaction product including acrylonitrile copolymer. The product was washed with water and dried to obtain acrylonitrile copolymer powder. (2) Under stirring, acrylonitrile copolymer powder was dissolved in dimethyl sulfoxide at 66°C. After 8 hours, stirring was stopped, and the solution was degassed under vacuum at 60°C to obtain a spinning solution with a solid content of 20.6 wt%. The spinning solution was then sprayed through a spinneret with a 50 μm aperture at a heating temperature of 45°C and entered a primary coagulation bath (dimethyl sulfoxide aqueous solution). The primary coagulation bath temperature was 3°C, the volume concentration of the primary coagulation bath was 72%, and the draw ratio was 2.6. Then it entered a secondary coagulation bath (dimethyl sulfoxide aqueous solution). The secondary coagulation bath temperature was 55°C, the volume concentration of the secondary coagulation bath was 40%, and the draw ratio was 1.0. After exiting the secondary coagulation bath, it entered a tertiary coagulation bath. The tertiary coagulation bath temperature was 60°C. The primary coagulation bath uses a dimethyl sulfoxide aqueous solution, while the volume content of dimethyl sulfoxide in the tertiary coagulation bath is 15%, and the tertiary coagulation draw ratio is 1.5, resulting in nascent fibers. The nascent fibers are then subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing, and heat setting. The boiling water draw ratio is 4.2, and the temperature is 100℃. The washing is done in 60℃ hot water for 100 seconds, with a washing draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification is carried out using hot rollers at a densification temperature of 140℃. The dried and densified fiber bundles are then subjected to steam drawing at a temperature of 120℃ with a draw ratio of 2.8. Finally, they undergo heat setting at a temperature of 160℃ with a draw ratio of 0.95. After heat setting, the fibers are wound to obtain polyacrylonitrile precursor fibers. (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 220℃, 230℃, 240℃ and 250℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each range was 12 minutes to obtain pre-oxidized fibers. Then the pre-oxidized fibers were carbonized in three temperature ranges 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.

[0041] Comparative Example 1 (1) Acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, dimethyl sulfoxide and deionized water were mixed evenly at room temperature in a mass ratio of 20:0.5:0.5:20:100, and then heated to 63°C and reacted for 4 hours to obtain a reaction product including acrylonitrile copolymer. The product was washed with water and dried to obtain acrylonitrile copolymer powder. (2) Under stirring, acrylonitrile copolymer powder was dissolved in dimethyl sulfoxide at 66°C. After 8 hours, stirring was stopped, and the solution was degassed under vacuum at 60°C to obtain a spinning solution with a solid content of 20.6 wt%. The spinning solution was then sprayed through a spinneret with a 70 μm aperture at a heating temperature of 45°C and entered a primary coagulation bath (dimethyl sulfoxide aqueous solution). The primary coagulation bath temperature was 3°C, the volume concentration of the primary coagulation bath was 72%, and the draw ratio was 0.75. Then it entered a secondary coagulation bath (dimethyl sulfoxide aqueous solution). The secondary coagulation bath temperature was 55°C, the volume concentration of the secondary coagulation bath was 40%, and the draw ratio was 1.2. After exiting the secondary coagulation bath, it entered a tertiary coagulation bath. The tertiary coagulation bath temperature was 60°C. The primary coagulation bath uses a dimethyl sulfoxide aqueous solution, while the volume content of dimethyl sulfoxide in the tertiary coagulation bath is 15%, and the tertiary coagulation draw ratio is 1.5, resulting in nascent fibers. The nascent fibers are then subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing, and heat setting. The boiling water draw ratio is 4.2, and the temperature is 100℃. The washing is done in 60℃ hot water for 100 seconds, with a washing draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification is carried out using hot rollers at a densification temperature of 140℃. The dried and densified fiber bundles are then subjected to steam drawing at a temperature of 120℃ with a draw ratio of 2.8. Finally, they undergo heat setting at a temperature of 160℃ with a draw ratio of 0.95. After heat setting, the fibers are wound to obtain polyacrylonitrile precursor fibers. (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 220℃, 230℃, 240℃ and 250℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each range was 12 minutes to obtain pre-oxidized fibers. Then the pre-oxidized fibers were carbonized in three temperature ranges 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.

[0042] Comparative Example 2 (1) Acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, dimethyl sulfoxide and deionized water were mixed evenly at room temperature in a mass ratio of 20:0.5:0.5:20:100. The mixture was heated to 63°C under a hypergravity field of 4500 G generated by a high-speed centrifuge and reacted for 8 hours to obtain a reaction product including acrylonitrile copolymer. The product was washed with water and dried to obtain acrylonitrile copolymer powder. (2) Under stirring, acrylonitrile copolymer powder was dissolved in dimethyl sulfoxide at 66°C. After 8 hours, stirring was stopped, and the solution was degassed under vacuum at 60°C to obtain a spinning solution with a solid content of 20.6 wt%. The spinning solution was then sprayed through a spinneret with a 70 μm aperture at a heating temperature of 45°C and entered a primary coagulation bath (dimethyl sulfoxide aqueous solution). The primary coagulation bath temperature was 3°C, the volume concentration of the primary coagulation bath was 72%, and the draw ratio was 2.6. Then it entered a secondary coagulation bath (dimethyl sulfoxide aqueous solution). The secondary coagulation bath temperature was 55°C, the volume concentration of the secondary coagulation bath was 40%, and the draw ratio was 1.0. After exiting the secondary coagulation bath, it entered a tertiary coagulation bath. The tertiary coagulation bath temperature was 60°C. The primary coagulation bath uses a dimethyl sulfoxide aqueous solution, while the volume content of dimethyl sulfoxide in the tertiary coagulation bath is 15%, and the tertiary coagulation draw ratio is 1.5, resulting in nascent fibers. The nascent fibers are then subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing, and heat setting. The boiling water draw ratio is 4.2, and the temperature is 100℃. The washing is done in 60℃ hot water for 100 seconds, with a washing draw ratio of 0.99. The oiling agent is silicone oil. Drying and densification is carried out using hot rollers at a densification temperature of 140℃. The dried and densified fiber bundles are then subjected to steam drawing at a temperature of 120℃ with a draw ratio of 2.8. Finally, they undergo heat setting at a temperature of 160℃ with a draw ratio of 0.95. After heat setting, the fibers are wound to obtain polyacrylonitrile precursor fibers. (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 220℃, 230℃, 240℃ and 250℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each range was 12 minutes to obtain pre-oxidized fibers. Then the pre-oxidized fibers were carbonized in three temperature ranges 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.

[0043] Comparative Example 3 (1) Acrylonitrile monomer, itaconic acid, azobisisobutyronitrile, dimethyl sulfoxide and deionized water were mixed evenly at room temperature in a mass ratio of 20:0.5:0.5:20:100. The mixture was heated to 63°C under a 500 G hypergravity field generated by a high-speed centrifuge and reacted for 2 hours to obtain a reaction product including acrylonitrile copolymer. The product was washed with water and dried to obtain acrylonitrile copolymer powder. (2) Under stirring, acrylonitrile copolymer powder was dissolved in dimethyl sulfoxide at 66°C. After 8 hours, stirring was stopped, and the solution was degassed under vacuum at 60°C to obtain a spinning solution with a solid content of 20.6 wt%. The spinning solution was sprayed out through a spinneret with a diameter of 50 μm at a heating temperature of 45°C and entered the first-stage coagulation bath (dimethyl sulfoxide aqueous solution). The temperature of the first-stage coagulation bath was 3°C, the volume concentration of the first-stage coagulation bath was 72%, and the draw ratio was 2.6. Then it entered the second-stage coagulation bath (dimethyl sulfoxide aqueous solution). The temperature of the second-stage coagulation bath was 55°C, the volume concentration of the second-stage coagulation bath was 40%, and the draw ratio was 1.0. After exiting the second-stage coagulation bath, it entered the third-stage coagulation bath. The temperature of the third-stage coagulation bath was 60°C. The coagulation bath uses a dimethyl sulfoxide aqueous solution, with the dimethyl sulfoxide volume content in the tertiary coagulation bath being 15% and the tertiary coagulation draw ratio being 1.5, to obtain nascent fibers. The nascent fibers are then subjected to boiling water drawing, washing, oiling, drying and densification, steam drawing, and heat setting in sequence. The boiling water draw ratio is 4.2 at a temperature of 100℃, the washing is done in 60℃ hot water for 100 seconds with a wash draw ratio of 0.99, and the oiling agent is silicone oil. Drying and densification is carried out using hot rollers at a densification temperature of 140℃. The dried and densified fiber bundles are then subjected to steam drawing at a temperature of 120℃ with a draw ratio of 2.8. Finally, they undergo heat setting treatment at a temperature of 160℃ with a draw ratio of 0.95. After heat setting, the fibers are wound to obtain polyacrylonitrile precursor fibers. (3) Polyacrylonitrile precursor fibers were pre-oxidized at four temperature ranges of 220℃, 230℃, 240℃ and 250℃. Each pre-oxidation was carried out in an air atmosphere and the dwell time of each range was 12 minutes to obtain pre-oxidized fibers. Then the pre-oxidized fibers were carbonized in three temperature ranges 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.

[0044] Test method: The acrylonitrile copolymer powders obtained in step (1) of Examples 1-5 and Comparative Examples 1-3 were tested by GPC method to obtain the molecular weight distribution index of acrylonitrile copolymers. The test results are shown in Table 1. The carbon fibers of Examples 1-5 and Comparative Examples 1-3 were tested using GB3362 "Test Standard for Carbon Fiber" and GB / T 3364 "Test Method for Diameter and Number of Carbon Fibers". The test results are shown in Table 1.

[0045] Table 1

[0046] Conclusion: As shown in Table 1, the molecular weight distribution index of the acrylonitrile copolymers obtained in Examples 1-5 is between 1.5 and 2.4, while the molecular weight distribution index of the acrylonitrile copolymers obtained in Comparative Examples 1 and 3 is relatively high, and the molecular weight distribution index of the acrylonitrile copolymer obtained in Comparative Example 2 is only 1.3. Furthermore, the tensile strength and tensile modulus of the carbon fibers obtained in Examples 1-5 are higher than those of the carbon fibers in Comparative Examples 1 and 3. Although the carbon fiber in Comparative Example 2 has a high tensile modulus, its tensile strength is low. This indicates that the present invention can obtain acrylonitrile copolymers with a molecular weight distribution index between 1.5 and 2.4, thereby enabling the preparation of high-performance carbon fibers.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing an acrylonitrile copolymer, wherein the acrylonitrile copolymer has a molecular weight distribution index of 1.5-2.4, characterized in that, The method includes: placing a mixed solution comprising acrylonitrile monomer, comonomer, free radical initiator, solvent and water in a hypergravity field of 1000G~4000G to carry out a polymerization reaction to obtain a reaction product comprising acrylonitrile copolymer.

2. The method according to claim 1, characterized in that, The mass ratio of acrylonitrile monomer, comonomer, free radical initiator, solvent and water is 10-30:0.2-2:0.1-1:10-50:

100.

3. The method according to claim 1 or 2, characterized in that, The comonomer includes at least one of itaconic acid, methyl acrylate, acrylic acid, acrylamide, and monobutyl itaconic acid.

4. The method according to claim 1 or 2, characterized in that, The solvent includes at least one of cyclobutanol, dimethyl sulfoxide, ethylene carbonate, dimethylformamide, and dimethylacetamide; Optionally, the free radical initiator includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, and ammonium persulfate.

5. The method according to claim 1, characterized in that, The polymerization reaction is carried out at a temperature of 40-65℃ for 1-10 hours.

6. The method according to claim 1, characterized in that, Also includes: The reaction product, which includes the acrylonitrile copolymer, is washed with water and dried.

7. A method for preparing carbon fiber, characterized in that, include: Polyacrylonitrile precursor fibers are obtained by dissolving, degassing, spinning, coagulating and stretching, boiling water stretching, washing, oiling, drying and densifying, steam stretching and heat setting of acrylonitrile copolymer. The polyacrylonitrile precursor fiber is pre-oxidized and carbonized to obtain carbon fiber. The acrylonitrile copolymer is prepared by the method described in any one of claims 1-6.

8. The method according to claim 1 or 6, characterized in that, The pre-oxidation temperature is 220-250℃.

9. The method according to claim 1 or 6, characterized in that, The carbonization process is carried out at a temperature of 700-1500℃.

10. A carbon fiber, characterized in that, The carbon fiber is prepared by any one of claims 1-9.

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