Carbon fiber and its preparation method

Abstract

The present invention discloses a carbon fiber and a preparation method thereof. The tensile strength of the carbon fiber is 7.7 - 8.3 GPa, and the tensile modulus is not less than 290 GPa.

Description

Technical Field

[0001] The present invention belongs to the field of carbon fibers, and particularly relates to a carbon fiber and a preparation method thereof. Background Art

[0002] Carbon fiber has the characteristics of high specific strength and high specific modulus. As an important reinforcing body of composite materials, it can meet the requirements of lightweight, high structural stiffness, structural dimensional stability, and function / structure integration for equipment such as aviation and aerospace. It is the core key material for the primary and secondary structures of high-end equipment and is also the focus of the future development of aerospace materials. With the continuous upgrading of equipment, new requirements are continuously put forward for carbon fibers. In particular, the improvement of tensile strength is the key to realizing the lightweight of equipment. At present, the highest tensile strength of the marketized carbon fiber product is the T1100 grade carbon fiber of Toray in Japan, with a tensile strength reaching 7 GPa. The demand for the upgrading of high-end equipment continues to grow, and higher-performance carbon fibers will be the key to improving the level of high-end equipment in China.

[0003] The key index for the high performance of carbon fiber is ultra-high strength, which has great potential application value in the structural parts of high-end equipment with strict lightweight requirements and is an ideal reinforcing body for the future new generation of advanced composite materials. Carbon fiber is a typical brittle material, and its strength is controlled by defects, which include both microscopic holes, grooves, microcracks, etc. and mesoscopic radial inhomogeneity, including both the number of defects and the type and size of defects. Fineness reduction, radial homogenization, high orientation, high densification, etc. are important measures for carbon fiber to improve strength, and polymerization and precursor are the decisive factors for the improvement of carbon fiber tensile strength. Although certain progress has been made in the improvement of carbon fiber tensile strength in recent years, there are still technical problems that need to be solved urgently in the systematic preparation of new carbon fiber products with ultra-high strength. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a carbon fiber and a preparation method thereof. By using this method, carbon fibers with a tensile strength of 7.7 - 8.3 GPa, a tensile modulus ≥ 290 GPa, and a single filament fineness of 0.25 - 0.35 dtex can be prepared.

[0005] In one aspect of the present invention, a carbon fiber is proposed, and the carbon fiber has a tensile strength of 7.7 - 8.3 GPa and a tensile modulus not less than 290 GPa.

[0006] Furthermore, the single filament fineness of the carbon fiber is 0.25 - 0.35 dtex.

[0007] In yet another aspect of the present invention, the present invention provides a method for preparing carbon fibers, comprising: (1) mixing acrylonitrile, itaconic acid and azobisisobutyronitrile for polymerization, and introducing ammonia after the polymerization to prepare a spinning dope. The solid content of the spinning dope is 19-21%, the rotational viscosity at 45 °C is 60-80 Pa·s, the intrinsic viscosity is 1.65-1.95 dL / g, and the molecular weight distribution is 2.15-2.45; (2) passing the spinning dope through a precision filter and then entering a primary coagulation bath through an air layer to form a nascent fiber, and then through a secondary coagulation bath, atmospheric steam drawing, washing, oiling, drying and densification, high-pressure saturated steam drawing and heat setting to obtain a polyacrylonitrile precursor fiber with a fineness of 0.5-0.7 dtex; (3) pre-oxidizing and carbonizing the polyacrylonitrile precursor fiber to obtain carbon fibers.

[0008] Further, in step (1), the molar ratio of itaconic acid to acrylonitrile is 0.5-0.8:100.

[0009] Further, in step (1), the amount of ammonia introduced is carried out according to the molar ratio of ammonia to itaconic acid of 1:3-2:3.

[0010] Further, in step (2), the spinning dope is precisely filtered through two-stage filters. The pore size of the first-stage filter is 3-5 μm, the temperature of the spinning solution at the first-stage filter is 60-70 °C, the ratio of the pore size of the second-stage filter to the pore size of the spinneret is 0.003-0.006:1, and the temperature of the spinning solution at the second-stage filter is 60-70 °C.

[0011] Further, in step (2), the spinning dope is ejected from the spinneret under the condition of heat tracing at 35-50 °C after being precisely filtered.

[0012] Further, the height of the air layer is 3-9 mm, the draw ratio of the air layer is 2.6-3.0, the primary coagulation bath includes dimethyl sulfoxide, and the temperature of the primary coagulation bath is 2-10 °C and the concentration is 65-75 wt%;

[0013] Further, the secondary coagulation bath includes dimethyl sulfoxide, the temperature of the secondary coagulation bath is 50-70 °C, the concentration is 30-45 wt%, and the draw ratio in the secondary coagulation bath is 1.0-1.2.

[0014] Further, in step (2), the temperature of the atmospheric steam drawing is 95-100 °C and the draw ratio is 3.0-4.5.

[0015] Further, in step (2), the pressure of the high-pressure saturated steam drawing is 0.4-0.8 MPa and the draw ratio is 2.6-3.5.

[0016] Further, in step (3), the pre-oxidation treatment conditions include: treatment at 210-260°C in a 3-6-stage gradient heating manner for 30-80 minutes, with a draw ratio of 0.9-1.2.

[0017] Further, the carbonization includes low-temperature carbonization and high-temperature carbonization. The temperature of the low-temperature carbonization treatment is 350°C - 700°C, the treatment time is 2-4 minutes, and the draw ratio is 1.05-1.20; the starting temperature of the high-temperature carbonization treatment is 1000-1200°C, the termination temperature is 1400-1700°C, the treatment time is 2-4 minutes, and the draw ratio is 0.96-1.02.

[0018] Compared with the prior art, the present invention has the following effects:

[0019] In the method of the present invention, acrylonitrile, itaconic acid, and azobisisobutyronitrile are mixed for polymerization. After the polymerization is completed, ammonia is introduced to prepare a spinning dope. Ammonia can carry out amidation treatment on itaconic acid to regulate the hydrophilicity of the acrylonitrile copolymer, and control the solid content and intrinsic viscosity of the spinning dope, which is beneficial to achieving high densification and high orientation of the raw silk; at the same time, controlling the molecular weight distribution of the spinning dope can reduce the number of small molecules in the polymer, which is beneficial to reducing hairiness and achieving high draw ratio, thereby improving the strength; controlling the rotational viscosity of the spinning dope can achieve continuous and stable spinning of low-denier raw silk. Then, dry-jet wet spinning of the spinning dope can obtain raw silk with a smooth surface and few defects. Moreover, dry-jet wet spinning is easy to achieve high draw ratio, improve the orientation, and control the fineness of the raw silk to 0.5-0.7 dtex. The raw silk with this fineness has a high degree of radial homogeneity and is easy to obtain radially homogeneous carbon fiber. Then, through pre-oxidation and carbonization, the carbon fiber with a tensile strength of 7.7-8.3 GPa and a tensile modulus of not less than 290 GPa can be obtained. Compared with the currently highest marketable product T1100-grade carbon fiber with the highest tensile strength at home and abroad, the tensile strength is increased by more than 10%, and it is expected to meet the upgrading needs of high-end equipment in China. Specific Embodiments

[0020] The present invention will be further described in detail below by combining examples. The following examples are only descriptive and not restrictive, and the protection scope of the present invention cannot be limited thereby.

[0021] It should be noted that the fineness unit dtex in this article, decitex, is defined as the mass (g) of a single fiber with a length of 10000 m.

[0022] In one aspect of the present invention, a carbon fiber is proposed. The tensile strength of the carbon fiber is 7.7-8.3 GPa, and the tensile modulus is not less than 290 GPa.

[0023] Further, the denier per filament of the carbon fiber is 0.25 - 0.35 dtex.

[0024] In another aspect of the present invention, the present invention provides a method for preparing carbon fiber. According to an embodiment of the present invention, the method includes:

[0025] S1: Mix acrylonitrile, itaconic acid, and azobisisobutyronitrile for polymerization. After the polymerization is completed, introduce ammonia gas to prepare a spinning dope.

[0026] In this step, acrylonitrile is used as the first monomer, itaconic acid as the second monomer, dimethyl sulfoxide (DMSO) as the solvent, and azobisisobutyronitrile as the initiator for polymerization. The polymerization temperature is 60 - 70 °C, and the reaction lasts for 12 - 36 hours. After the polymerization is completed, introduce ammonia gas. The ammonia gas can perform amidation treatment on itaconic acid to regulate the hydrophilicity of the acrylonitrile copolymer. Then, under stirring, at 60 - 70 °C and a vacuum degree greater than 0.095 MPa, remove the unreacted monomers in the polymer spinning solution. Stop stirring after 7 - 9 hours, and degas under vacuum conditions at 55 - 65 °C. The obtained spinning dope has a solid content of 19 - 21%, a rotational viscosity of 60 - 80 Pa·s at 45 °C, an intrinsic viscosity of 1.65 - 1.95 dL / g, and a molecular weight distribution of 2.15 - 2.45.

[0027] The inventors found that controlling the solid content and intrinsic viscosity of the spinning dope is beneficial to achieving high densification and high orientation of the precursor filaments; simultaneously controlling the molecular weight distribution of the spinning dope can reduce the number of small molecules in the polymer, which is beneficial to reducing hairiness and achieving high - multiple drawing, thereby enhancing the strength; controlling the rotational viscosity of the spinning dope can achieve continuous and stable spinning of low - denier precursor filaments. Then, dry - jet wet - spinning the spinning dope can obtain precursor filaments with a smooth surface and few defects. Moreover, dry - jet wet - spinning is easy to achieve high - multiple drawing, improve the orientation, and control the denier of the precursor filaments to be 0.5 - 0.7 dtex. The precursor filaments with this denier have a high degree of radial homogeneity and are easy to obtain radially homogeneous carbon fibers.

[0028] According to an embodiment of the present invention, the molar ratio of itaconic acid to acrylonitrile is 0.5 - 0.8:100.

[0029] According to an embodiment of the present invention, the amount of ammonia gas introduced is carried out according to the molar ratio of ammonia gas to itaconic acid of 1:3 - 2:3. The inventors found that using ammonia gas to perform amidation treatment on itaconic acid can regulate the hydrophilicity of the acrylonitrile copolymer. If the degree of amidation is too high, it is not conducive to the pre - oxidation process; if it is too low, the hydrophilicity is poor. By using the ratio of ammonia gas to itaconic acid of the present invention, the hydrophilicity and the pre - oxidation process can be balanced, thereby enhancing the radial homogeneity and densification of the precursor filaments and pre - oxidized filaments.

[0030] S2: After precisely filtering the spinning dope, it enters the first coagulation bath through an air layer to form nascent fibers, and then passes through a second coagulation bath, atmospheric steam drawing, water washing, oiling, drying densification, high-pressure saturated steam drawing, and heat setting.

[0031] The inventor found that dry-jet wet spinning of the spinning dope can obtain raw filaments with a smooth surface and few defects. Moreover, dry-jet wet spinning is easy to achieve high-draft drawing, improve orientation, and control the fineness of the raw filaments to be 0.5 - 0.7 dtex. The raw filaments with this fineness have a high degree of radial homogeneity and are easy to obtain radially homogeneous carbon fibers.

[0032] According to an embodiment of the present invention, the spinning dope is precisely filtered through two-stage filters. The pore size of the first-stage filter is 3 - 5 μm, the temperature of the spinning solution at the first-stage filter is 60 - 70 °C, the ratio of the pore size of the second-stage filter to the pore size of the spinneret is 0.003 - 0.006:1, and the temperature of the spinning solution at the second-stage filter is 60 - 70 °C. Specifically, the filtration accuracy is defined by the pore size of the first-stage filter and the ratio of the pore size of the second-stage filter to the pore size of the spinneret, effectively removing large-size impurities and microgels in the spinning dope, reducing the number of defects, and reducing the defect size.

[0033] According to an embodiment of the present invention, after being precisely filtered, the spinning dope is ejected from the spinneret under the condition of 35 - 50 °C heat tracing. The height of the air layer is 3 - 9 mm, the draft ratio of the air layer is 2.6 - 3.0. The first coagulation bath includes dimethyl sulfoxide, the temperature of the first coagulation bath is 2 - 10 °C, and the concentration is 65 - 75 wt%. The second coagulation bath includes dimethyl sulfoxide, the temperature of the second coagulation bath is 50 - 70 °C, and the concentration is 30 - 45 wt%. The draft ratio in the second coagulation bath is 1.0 - 1.2. The inventor found that if the heat tracing temperature of the spinning solution is too low, the filaments are prone to break at the spinneret. If the heat tracing temperature of the spinning solution is too high, the spinneret is prone to overflow and caking. The spinning dope of the present invention can achieve stable preparation of low-fineness raw filaments at a heat tracing temperature of 35 - 50 °C.

[0034] According to an embodiment of the present invention, the temperature of the atmospheric steam drawing is 95 - 100 °C, and the draft ratio is 3.0 - 4.5. The inventor found that if the draft ratio in this process is too low, the crystallinity and orientation of the raw filaments are low. If the draft ratio in this process is too high, the filaments are prone to break. Therefore, by controlling the draft ratio of the atmospheric steam drawing within the above range in the present invention, the crystallinity and orientation of the raw filaments can be improved, which is beneficial to the improvement of the tensile strength and tensile modulus of the carbon fibers.

[0035] According to an embodiment of the present invention, for the water washing in the spinning process, a gradient temperature rise is adopted, and the water washing is carried out in hot water at 50 - 85 °C for 60 - 150 s. Specifically, first, the water washing is carried out in a water bath at 50 °C for 7 - 15 s, and then the water washing is carried out in water baths at 55, 60, 65, 70, 75, 80, and 85 °C for 7 - 15 s respectively. During the water washing process, the solvent in the filament diffuses outwards. The solvent content decreases from high to low, and the driving force for its outward diffusion comes from the concentration difference with the outside. As the water washing progresses, the concentration difference gradually decreases. It is necessary to increase the molecular motion ability by a higher water washing temperature, and through the cooperation of the filament diameter control during water washing and the gradient water washing temperature, the residual solvent content in the fiber is made lower than five ten-thousandths.

[0036] According to an embodiment of the present invention, the temperature of the drying densification process is 110 - 140 °C, and the time is 30 - 45 s. Specifically, after the water-washed fiber is oiled, it is dried on a hot roller. The filament is in direct contact with the heat source of the hot roller, and the filament is heated by heat conduction. The water in the fiber evaporates and the pores close. Since the outermost layer of the fiber first contacts the hot roller and the component and structure transformation occur prior to the core layer, and heat transfer causes a temperature difference in the radial direction of the fiber, a gradient temperature is adopted for drying densification.

[0037] According to an embodiment of the present invention, the pressure of the high-pressure saturated steam drawing is 0.4 - 0.8 MPa, and the drawing ratio is 2.6 - 3.5. The inventors found that if the drawing ratio in this process is too low, the orientation degree of the raw silk is low; if the drawing ratio in this process is too high, it is easy to break the wire and fluff. At the same time, high pressure can promote the penetration of water molecules into the dense fiber interior, the fiber is fully plasticized, the drawability is effectively improved, and the orientation degree is increased. Therefore, by controlling the drawing ratio and pressure of the high-pressure steam drawing within the above range in the present invention, the orientation degree of the raw silk can be improved, which is beneficial to the improvement of the tensile strength and tensile modulus of the carbon fiber.

[0038] It should be noted that other conditions in the spinning process are conventional conditions in the art and will not be elaborated here.

[0039] S3: Subject the polyacrylonitrile raw silk to pre-oxidation and carbonization

[0040] In this step, the obtained polyacrylonitrile raw silk is successively subjected to pre-oxidation and carbonization treatments to obtain carbon fiber. Specifically, the pre-oxidation treatment conditions include: treatment at 210 - 260 °C in a 3 - 6-stage gradient temperature rise manner for 30 - 80 minutes, and a drawing ratio of 0.9 - 1.2. The carbonization includes low-temperature carbonization and high-temperature carbonization. The temperature of the low-temperature carbonization treatment is 350 °C - 700 °C, the treatment time is 2 - 4 minutes, and the drawing ratio is 1.05 - 1.20. The starting temperature of the high-temperature carbonization treatment is 1000 - 1200 °C, the termination temperature is 1400 - 1700 °C, the treatment time is 2 - 4 minutes, and the drawing ratio is 0.96 - 1.02.

[0041] Therefore, the method can be used to prepare carbon fibers with a tensile strength of 7.7-8.3 GPa, a tensile modulus of ≥290 GPa, and a single fiber fineness of 0.25-0.35 dtex.

[0042] The present invention is described below with reference to specific embodiments. It should be noted that these embodiments are merely illustrative and should not limit the present invention in any way.

[0043] Example 1

[0044] (1) The molar ratio of acrylonitrile to itaconic acid is 100:0.5, and polymerization is carried out with azobisisobutyronitrile as an initiator. After the polymerization, ammonia is introduced to obtain a spinning solution, and the molar ratio of ammonia to itaconic acid is 1:2. Then, under stirring, unreacted monomers in the polymer spinning solution are removed at 66° C. and a vacuum degree greater than 0.096 MPa. After 8 hours, stirring is stopped, and degassing is carried out under vacuum conditions at 60° C. The obtained spinning solution has a solid content of 20.6wt%, a rotational viscosity of 71Pa.s at 45° C., an intrinsic viscosity of 1.78dL / g, and a molecular weight distribution of 2.33;

[0045] (2) The spinning solution is spun after secondary filtration, the aperture of the primary filter is 5 μm, the temperature of the spinning solution at the primary filter is 65°C, the aperture of the secondary filter is 0.04 μm, the temperature of the spinning solution at the secondary filter is 65°C, the spinning solution is ejected through a spinneret with an aperture of 90 μm at a heating temperature of 45°C, passes through a 6 mm air layer, and enters a primary coagulation bath to obtain nascent fibers, the temperature of the primary coagulation bath is 3°C, the concentration is 72%, and the draft ratio is 2.6, the nascent fibers enter a secondary coagulation bath, the temperature of the secondary coagulation bath is 55°C, the concentration is 40%, and the draft ratio is 2.6. The ratio is 1.0. After the tow comes out of the secondary coagulation bath, it is sequentially subjected to normal pressure steam drawing, water washing, oiling, drying and densification. The normal pressure steam drawing ratio is 4.2, the temperature is 100°C, the water washing drawing ratio is 0.99, the oil agent is silicone oil, and the drying and densification is carried out by hot rollers, and the densification temperature is 140°C. The dried and densified tow is subjected to high-pressure steam drawing and heat setting, the high-pressure saturated water vapor pressure is 0.59MPa, the drawing ratio is 2.8, the heat setting temperature is 160°C, and the raw silk is obtained by winding after heat setting, and the raw silk fineness is 0.51dtex.

[0046] (3) The precursor fibers were pre-oxidized at four temperatures of 202 °C, 219 °C, 231 °C, and 249 °C in an air atmosphere for a total treatment time of 60 minutes; the obtained pre-oxidized fibers were subjected to low-temperature carbonization at three temperatures of 350 °C, 430 °C, and 650 °C in a nitrogen atmosphere for a total treatment time of 3 minutes; then they were subjected to high-temperature carbonization at two temperatures of 1200 °C and 1600 °C in a nitrogen atmosphere for a total treatment time of 4 minutes to produce high-strength carbon fibers with low fineness. The performance of the prepared carbon fibers was tested using GB3362-2017 "Test Method for Tensile Properties of Carbon Fiber Multifilament". The results were: tensile strength 8.28 GPa, tensile modulus 325 GPa, and fineness 0.26 dtex.

[0047] Example 2

[0048] (1) The molar ratio of the monomer acrylonitrile to itaconic acid in the formulation was 100:0.8. Polymerization was carried out using azobisisobutyronitrile as an initiator. After the polymerization was completed, ammonia gas was introduced to obtain a spinning solution. The molar ratio of ammonia gas to itaconic acid was 1:2. Then, under stirring, unreacted monomers in the polymer spinning solution were removed at 66 °C with a vacuum degree greater than 0.096 MPa. After 8 hours, stirring was stopped, and degassing was carried out under vacuum conditions at 60 °C. The obtained spinning dope had a solid content of 20.1 wt%, a rotational viscosity of 63 Pa·s at 45 °C, an intrinsic viscosity of 1.72 dL / g, and a molecular weight distribution of 2.26;

[0049] (2) The spinning dope was spun after passing through a two-stage filter. The pore size of the first-stage filter was 5 μm, and the temperature of the spinning solution at the first-stage filter was 65 °C. The pore size of the second-stage filter was 0.05 μm, and the temperature of the spinning solution at the second-stage filter was 65 °C. The spinning dope was extruded through a spinneret with a pore size of 110 μm at an accompanying temperature of 45 °C, passed through a 7-mm air layer, and entered the first coagulation bath to obtain nascent fibers. The temperature of the first coagulation bath was 3 °C, the concentration was 72%, and the draw ratio was 3.0. The nascent fibers entered the second coagulation bath. The temperature of the second coagulation bath was 55 °C, the concentration was 40%, and the draw ratio was 1.1. After the tow exited the second coagulation bath, it was successively subjected to atmospheric steam drawing, water washing, oiling, and drying densification. The atmospheric steam drawing ratio was 3.5, and the temperature was 95 °C. The water washing draw ratio was 0.99, the oiling agent was silicone oil, and drying densification was carried out using a hot roll. The densification temperature was 140 °C. The tow after drying densification was subjected to high-pressure saturated steam drawing and heat setting. The pressure of the high-pressure saturated steam was 0.45 MPa, the draw ratio was 2.8, and the heat setting temperature was 160 °C. After heat setting, the precursor fibers were obtained through winding, and the fineness of the precursor fibers was 0.70 dtex.

[0050] (3) The precursor fibers are pre-oxidized at four temperatures of 202 °C, 219 °C, 231 °C, and 243 °C in an air atmosphere for a total treatment time of 80 minutes; the obtained pre-oxidized fibers are subjected to low-temperature carbonization at three temperatures of 350 °C, 430 °C, and 650 °C in a nitrogen atmosphere for a total treatment time of 3 minutes; then they are subjected to high-temperature carbonization at two temperatures of 1200 °C and 1650 °C in a nitrogen atmosphere for a total treatment time of 3 minutes to obtain low-denier high-strength carbon fibers. The properties of the prepared carbon fibers are tested using GB3362-2017 "Test Method for Tensile Properties of Carbon Fiber Multifilaments". The results are as follows: tensile strength 7.74 GPa, tensile modulus 318 GPa, and fineness 0.33 dtex.

[0051] Example 3

[0052] (1) The molar ratio of the monomer acrylonitrile to itaconic acid in the formulation is 100:0.6. Polymerization is carried out using azobisisobutyronitrile as an initiator. After the polymerization is completed, ammonia gas is introduced to obtain a spinning solution. The molar ratio of ammonia gas to itaconic acid is 1:2. Then, under stirring, unreacted monomers in the polymer spinning solution are removed at 66 °C under a vacuum greater than 0.96 MPa. Stirring is stopped after 8 hours, and degassing is carried out under vacuum conditions at 60 °C. The obtained spinning dope has a solid content of 20.8 wt%, a rotational viscosity of 78 Pa·s at 45 °C, an intrinsic viscosity of 1.88 dL / g, and a molecular weight distribution of 2.41;

[0053] (2) The spinning dope is spun after passing through a two-stage filter. The pore size of the first-stage filter is 5 μm, and the pore size of the second-stage filter is 0.04 μm. The temperature of the spinning solution at the second-stage filter is 65 °C. The spinning dope is ejected through a spinneret with a pore size of 90 μm at an accompanying heat temperature of 45 °C, passes through a 6-mm air layer, and enters the first coagulation bath to obtain nascent fibers. The temperature of the first coagulation bath is 5 °C, the concentration is 72%, and the draw ratio is 2.6. The nascent fibers enter the second coagulation bath. The temperature of the second coagulation bath is 45 °C, the concentration is 40%, and the draw ratio is 1.0. After the filament bundle exits the second coagulation bath, it is successively subjected to atmospheric steam drawing, water washing, oiling, and drying densification. The atmospheric steam draw ratio is 3.4, and the temperature is 98 °C. The water washing draw ratio is 0.98, the oiling agent is silicone oil, and drying densification is carried out using a hot roll. The densification temperature is 140 °C. The dried and densified filament bundle is subjected to high-pressure saturated steam drawing and heat setting. The high-pressure saturated steam pressure is 0.49 MPa, the draw ratio is 2.8, and the heat setting temperature is 160 °C. After heat setting, the precursor fibers are obtained through winding, and the fineness of the precursor fibers is 0.62 dtex;

[0054] (3) The precursor fibers were pre-oxidized at four temperatures of 210 °C, 221 °C, 231 °C, and 243 °C in an air atmosphere for a total treatment time of 70 minutes; the obtained pre-oxidized fibers were then subjected to low-temperature carbonization at three temperatures of 410 °C, 430 °C, and 650 °C in a nitrogen atmosphere for a total treatment time of 3 minutes; and then high-temperature carbonization was carried out at two temperatures of 1200 °C and 1550 °C in a nitrogen atmosphere for a total treatment time of 4 minutes to obtain low-denier high-strength carbon fibers. The prepared carbon fibers were tested for performance using GB3362-2017 "Test Method for Tensile Properties of Carbon Fiber Multifilaments". The results were: tensile strength 8.02 GPa, tensile modulus 310 GPa, and denier 0.29 dtex.

[0055] Comparative Example 1

[0056] (1) The molar ratio of the monomer acrylonitrile to itaconic acid in the formulation was 100:0.6. Polymerization was carried out using azobisisobutyronitrile as an initiator. After the polymerization was completed, ammonia was introduced to obtain a spinning solution. The molar ratio of ammonia to itaconic acid was 1:4. Then, under stirring, the unreacted monomers in the polymer spinning solution were removed at 66 °C under a vacuum greater than 0.096 MPa. Stirring was stopped after 8 hours, and degassing was carried out under vacuum at 60 °C. The obtained spinning dope had a solid content of 18.6 wt%, a rotational viscosity of 56 Pa·s at 45 °C, an intrinsic viscosity of 1.59 dL / g, and a molecular weight distribution of 2.68;

[0057] (2) The spinning dope was filtered through two stages and then spun. The pore size of the first-stage filter was 5 μm, and the temperature of the spinning solution at the first-stage filter was 65 °C. The pore size of the second-stage filter was 0.04 μm, and the temperature of the spinning solution at the second-stage filter was 65 °C. The spinning dope was extruded through a spinneret with a pore size of 90 μm at an accompanying temperature of 45 °C, passed through a 6-mm air layer, and entered the first coagulation bath to obtain nascent fibers. The temperature of the first coagulation bath was 5 °C, the concentration was 72%, and the draw ratio was 2.6. The nascent fibers entered the second coagulation bath. The temperature of the second coagulation bath was 45 °C, the concentration was 40%, and the draw ratio was 1.0. After the tow exited the second coagulation bath, it was successively subjected to atmospheric steam drawing, washing, oiling, and drying densification. The atmospheric steam draw ratio was 3.4, and the temperature was 98 °C. The washing draw ratio was 0.98, the oiling agent was silicone oil, and drying densification was carried out using a hot roller. The densification temperature was 140 °C. The dried and densified tow was subjected to high-pressure saturated steam drawing and heat setting. The pressure of the high-pressure saturated steam was 0.49 MPa, the draw ratio was 2.8, and the heat setting temperature was 160 °C. After heat setting, the precursor fibers were obtained by winding. The denier of the precursor fibers was 0.55 dtex;

[0058] (3) The precursor fibers were pre-oxidized at four temperatures of 210 °C, 221 °C, 231 °C, and 243 °C in an air atmosphere for a total treatment time of 70 minutes; the obtained pre-oxidized fibers were subjected to low-temperature carbonization at three temperatures of 410 °C, 430 °C, and 650 °C in a nitrogen atmosphere for a total treatment time of 3 minutes; then they were subjected to high-temperature carbonization at two temperatures of 1200 °C and 1550 °C in a nitrogen atmosphere for a total treatment time of 4 minutes to obtain low-denier high-strength carbon fibers. The prepared carbon fibers were tested for performance using GB3362-2017 "Test Method for Tensile Properties of Carbon Fiber Multifilaments". The results were: tensile strength 6.72 GPa, tensile modulus 312 GPa, and fineness 0.28 dtex.

[0059] Comparative Example 2

[0060] (1) The molar ratio of the monomer acrylonitrile to itaconic acid in the formulation was 100:0.6. Polymerization was carried out using azobisisobutyronitrile as an initiator. After the polymerization was completed, ammonia gas was introduced to obtain a spinning solution. The molar ratio of ammonia gas to itaconic acid was 1:2. Then, under stirring, the unreacted monomers in the polymer spinning solution were removed at 66 °C with a vacuum degree greater than 0.096 MPa. After 8 hours, stirring was stopped, and degassing was carried out under vacuum conditions at 60 °C. The obtained spinning dope had a solid content of 20.7 wt%, a rotational viscosity of 82 Pa·s at 45 °C, an intrinsic viscosity of 2.16 dL / g, and a molecular weight distribution of 2.92;

[0061] (2) The spinning dope was spun after passing through two-stage filtration. The pore size of the first-stage filter was 5 μm, and the pore size of the second-stage filter was 0.04 μm. The temperature of the spinning solution at the filter was 65 °C. The spinning dope was ejected through a spinneret with a pore size of 90 μm at an accompanying temperature of 45 °C, passed through a 6-mm air layer, and entered the first coagulation bath to obtain nascent fibers. The temperature of the first coagulation bath was 5 °C, the concentration was 72%, and the draw ratio was 2.6. The nascent fibers entered the second coagulation bath. The temperature of the second coagulation bath was 45 °C, the concentration was 40%, and the draw ratio was 1.0. After the filament bundle exited the second coagulation bath, atmospheric steam drawing, washing, oiling, and drying densification were carried out in sequence. The atmospheric steam drawing ratio was 3.4, and the temperature was 98 °C. The washing draw ratio was 0.98. The oiling agent was silicone oil. Drying densification was carried out using a hot roller, and the densification temperature was 140 °C. The dried and densified filament bundle was subjected to high-pressure saturated steam drawing and heat setting. The high-pressure saturated steam pressure was 0.49 MPa, the draw ratio was 2.8, and the heat setting temperature was 160 °C. After heat setting, the precursor fibers were obtained by winding, and the fineness of the precursor fibers was 0.62 dtex;

[0062] (3) The precursor fibers are pre-oxidized at four temperatures of 210°C, 221°C, 231°C, and 243°C in an air atmosphere for a total treatment time of 70 minutes; the obtained pre-oxidized fibers are subjected to low-temperature carbonization at three temperatures of 410°C, 430°C, and 650°C in a nitrogen atmosphere for a total treatment time of 3 minutes; then they are subjected to high-temperature carbonization at two temperatures of 1200°C and 1550°C in a nitrogen atmosphere for a total treatment time of 4 minutes to obtain low-denier high-strength carbon fibers. The prepared carbon fibers are tested for performance using GB3362-2017 "Test Method for Tensile Properties of Carbon Fiber Multifilament". The results are as follows: tensile strength 7.32 GPa, tensile modulus 309 GPa, and fineness 0.26 dtex.

[0063] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art of this technology, without departing from the technical principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as within the protection scope of the present invention.

Claims

1. A method for preparing carbon fiber, characterized in that, Comprising: (1) Acrylonitrile, itaconic acid and azobisisobutyronitrile are mixed for polymerization. After the polymerization is completed, ammonia is introduced to prepare a spinning dope. The solid content of the spinning dope is 19 - 21%, the rotational viscosity at 45°C is 60 - 80 Pa·s, the intrinsic viscosity is 1.65 - 1.95 dL / g, and the molecular weight distribution is 2.15 - 2.45; (2) The spinning dope is precisely filtered and then enters a primary coagulation bath through an air layer to form nascent fibers, and then undergoes a secondary coagulation bath, atmospheric steam drawing, washing, oiling, drying densification, high-pressure saturated steam drawing and heat setting to obtain polyacrylonitrile precursor filaments with a fineness of 0.5 - 0.7 dtex; (3) The polyacrylonitrile precursor filaments are pre-oxidized and carbonized to obtain carbon fibers, wherein, in step (1), the amount of ammonia introduced is carried out according to the molar ratio of ammonia to itaconic acid being 1:3 - 2:3; in step (2), after the spinning dope is precisely filtered, it is ejected from a spinneret under heating at 35 - 50°C; the temperature of the atmospheric steam drawing is 95 - 100°C, and the drawing ratio is 3.0 - 4.5; the pressure of the high-pressure saturated steam drawing is 0.4 - 0.8 MPa, and the drawing ratio is 2.6 - 3.

5.

2. The method according to claim 1, wherein In step (1), the molar ratio of itaconic acid to acrylonitrile is 0.5 - 0.8:

100.

3. The method according to claim 1, wherein In step (2), the spinning dope is precisely filtered through two-stage filters. The pore size of the first-stage filter is 3 - 5 μm, the temperature of the spinning solution at the first-stage filter is 60 - 70°C, the ratio of the pore size of the second-stage filter to the pore size of the spinneret is 0.003 - 0.006:1, and the temperature of the spinning solution at the second-stage filter is 60 - 70°C.

4. The method according to claim 1, characterized in that, In step (2), the height of the air layer is 3 - 9 mm, the air layer drawing ratio is 2.6 - 3.0, the primary coagulation bath includes dimethyl sulfoxide, and the temperature of the primary coagulation bath is 2 - 10°C and the concentration is 65 - 75 wt%.

5. The method according to claim 1 or 4, characterized in that, In step (2), the secondary coagulation bath includes dimethyl sulfoxide, the temperature of the secondary coagulation bath is 50 - 70°C, the concentration is 30 - 45 wt%, and the drawing ratio in the secondary coagulation bath is 1.0 - 1.

2.

6. The method according to claim 1, wherein In step (3), the pre-oxidation treatment conditions include: treatment at 210 - 260°C in a 3 - 6-stage gradient heating mode for 30 - 80 minutes, and the drawing ratio is 0.9 - 1.

2.

7. The method according to claim 1, wherein The carbonization includes low-temperature carbonization and high-temperature carbonization. The temperature of the low-temperature carbonization treatment is 350°C - 700°C, the treatment time is 2 - 4 minutes, and the drawing ratio is 1.05 - 1.

20. The starting temperature of the high-temperature carbonization treatment is 1000 - 1200°C, the ending temperature is 1400 - 1700°C, the treatment time is 2 - 4 minutes, and the drawing ratio is 0.96 - 1.

02.

8. The method according to claim 1, characterized in that, The tensile strength of the carbon fiber is 7.7 - 8.3 GPa, and the tensile modulus is not less than 290 GPa.

9. The method according to claim 1 or 8, characterized in that, The single-filament fineness of the carbon fiber is 0.25 - 0.35 dtex.