A method for producing polyacrylonitrile-based carbon fibers and the resulting carbon fibers

Abstract

The application relates to a preparation method of polyacrylonitrile (PAN) based carbon fiber and the obtained carbon fiber. A certain amount of ammonium polyphosphate (APP) is added in a polymerization system to carry out a polymerization reaction, a spinning solution is obtained, PAN protofilaments are prepared through a wet spinning method or a dry-wet spinning method, and the carbon fiber is prepared after pre-oxidation and carbonization. The carbon fiber prepared through the method provided by the application has a tensile strength of 5.58 GPa, an elastic modulus of more than 260 GPa and a carbon yield of not less than 53% under the premise that the fineness is not less than 0.66 dtex.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber technology, specifically relating to a method for preparing polyacrylonitrile-based carbon fiber and the resulting carbon fiber. The method involves adding ammonium polyphosphate (APP) as a modifier to the polyacrylonitrile polymerization system to prepare high-strength, high-modulus, and high-carbon-yield polyacrylonitrile-based carbon fiber. Background Technology

[0002] Polyacrylonitrile-based carbon fibers have been widely used in various fields due to their high strength and high modulus. The industry consensus is that high-quality precursor fibers are fundamental to the preparation of high-performance carbon fibers. A crucial step in precursor fiber preparation is the hydrophilization treatment of the spinning solution, also known as ammoniation modification. This hydrophilization treatment slows down the dual diffusion rate of solvent in the spinning solution and water in the coagulation bath, preventing the formation of macropores and thus producing high-quality precursor fibers with a dense structure.

[0003] Currently, domestic and international carbon fiber manufacturers employ various methods to hydrophilize spinning solutions to prevent the formation of large pores during coagulation. These methods include adding hydrophilic copolymers such as ammonium itaconic acid and acrylamide during polymerization; ammonifying the polymerization solution with ammonia after polymerization; and adding ammonia water to the coagulation bath to regulate the structure of the nascent fibers. Each of these main technical routes has its own drawbacks. Using ammonia to treat the spinning solution presents challenges due to the significant viscosity difference between ammonia and the polymerization solution, making uniform mixing at the microscopic level difficult. The large difference in polymerization reactivity between the hydrophilic comonomers and acrylonitrile results in irregular distribution of the copolymer components within the molecular chain, affecting the performance of the final product. Adding ammonia water to the coagulation bath is cumbersome, and the strong ammonia odor near the spinneret poses a health risk to workers. Chinese patent CN102277629A discloses an ammonification method using an air layer in dry-jet wet spinning. While this process allows for uniform contact between the polymerization solution and ammonia, the ammonia gas flow disturbs the fine stream of the spinning solution, hindering smooth spinning.

[0004] Polyacrylonitrile (PA) precursor fibers are pre-oxidized and carbonized to produce finished carbon fibers. The pre-oxidation process, taking 40-60 minutes, is a major bottleneck restricting the improvement of carbon fiber production efficiency. Simply increasing the pre-oxidation temperature to shorten the residence time results in severe core-sheath structure, leading to a decrease in both carbon yield and performance. Improving the production efficiency of the pre-oxidation stage without reducing carbon yield has been a research hotspot. Chinese patent CN112553711A uses ammonium dihydrogen phosphate aqueous solution to impregnate and modify PA precursor fibers; Chinese patent CN101956253A uses hydrogen peroxide of a certain concentration for pretreatment of PA precursor fibers; and Chinese patent CN111910291A discloses a method for pre-treatment of PA precursor fibers using nitrogen- and phosphorus-containing compounds, including ammonium polyphosphate (APP). All these methods are believed to promote cyano cyclization during pre-oxidation and reduce the residence time. However, due to the high density of PA precursor fibers, impregnation treatment makes it difficult for the modifier to penetrate the fiber interior, thus limiting the modification effect. After pre-oxidation, polyacrylonitrile precursor fibers require a carbonization process to obtain finished carbon fibers. During carbonization, non-carbon elements are gradually removed, resulting in a carbon content exceeding 95% in the final carbon fibers. The pre-oxidation process affects the carbon yield; insufficient or excessive pre-oxidation leads to a low carbon yield and decreased performance. Under current process conditions, the carbon yield in the production of polyacrylonitrile-based carbon fibers is between 46% and 50%. Chinese patent CN112760752A discloses a method for improving the carbon yield of polyacrylonitrile precursor fibers by adding phosphoric acid to the polymerization solution. However, because phosphoric acid is a small molecule, it is easily displaced during coagulation and washing, resulting in a low residual content in the finished precursor fibers. Therefore, its modification effect in subsequent pre-oxidation and carbonization processes is limited, and its contribution to improving the carbon yield is also limited. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention provides a method for preparing polyacrylonitrile-based carbon fiber and the carbon fiber obtained by the method. Specifically, ammonium polyphosphate is added during the polyacrylonitrile polymerization process, which can conveniently improve the hydrophilicity of the polymerization solution and simultaneously solve the problems of long pre-oxidation and carbonization time and low carbon yield in the preparation process of polyacrylonitrile-based carbon fiber.

[0006] One of the objectives of this invention is to provide a method for preparing polyacrylonitrile-based carbon fiber, comprising: filtering a polyacrylonitrile solution and then obtaining polyacrylonitrile precursor fibers by wet spinning or dry-wet spinning, followed by pre-oxidation and carbonization treatment to obtain the polyacrylonitrile-based carbon fiber, wherein the polyacrylonitrile solution is a copolymer solution obtained by copolymerization of components including acrylonitrile, itaconic acid, and ammonium polyphosphate.

[0007] According to an embodiment of the present invention, the method for preparing the polyacrylonitrile solution includes: adding components including acrylonitrile, itaconic acid, and ammonium polyphosphate into a solvent to obtain a reaction system, and obtaining the polyacrylonitrile solution after polymerization.

[0008] Specifically, in the method for preparing the polyacrylonitrile solution:

[0009] The degree of polymerization of the ammonium polyphosphate is 5 to 50, preferably 5 to 20; for example, the degree of polymerization of the ammonium polyphosphate can be 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50, and can be any of the above values ​​or a range between any two values.

[0010] The solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, aqueous solution of sodium thiocyanate, aqueous solution of zinc chloride, and aqueous solution of nitric acid, wherein the mass percentage concentration of the aqueous solution of sodium thiocyanate is 50-52%, the mass percentage concentration of the aqueous solution of zinc chloride is 25-30%, and the mass percentage concentration of the aqueous solution of nitric acid is 10-15%; preferably, the solvent is selected from at least one of N,N-dimethylformamide and aqueous solution of sodium thiocyanate.

[0011] At least one of methyl acrylate and methyl methacrylate may be added to the reaction system. The addition of methyl acrylate and / or methyl methacrylate may be selected according to actual needs. There is no particular limitation on the amount of methyl acrylate and / or methyl methacrylate added. It can be added according to product requirements.

[0012] In terms of mass percentage, the components including acrylonitrile, itaconic acid, and ammonium polyphosphate account for 17-24% of the total weight of the reaction system, preferably 20-24%; for example, it can be 17%, 18%, 19%, 20%, 21%, 22%, 23%, or 24%, and can be any of the above values ​​or any range between two values.

[0013] In terms of mass percentage, the acrylonitrile accounts for 95-99% of the total weight of the components including acrylonitrile, itaconic acid, and ammonium polyphosphate, preferably 97-98.5%; for example, it can be 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, or 99%, and can be any of the above values ​​or a range between any two values;

[0014] The ammonium polyphosphate accounts for 0.5% to 5% of the mass of acrylonitrile, preferably 0.5% to 3%; for example, it can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%, and can be any of the above values ​​or any range between two values;

[0015] The polymerization reaction also includes an initiator. Preferably, the initiator can be a commonly used initiator in polyacrylonitrile polymerization, such as an azo initiator, preferably azobisisobutyronitrile. The amount of the initiator is 0.3 to 0.5 wt% of the acrylonitrile, for example, it can be 0.3%, 0.35%, 0.4%, 0.45% or 0.5%, and can be any of the above values ​​or any range between two values.

[0016] In the preparation method of the polyacrylonitrile solution, the polymerization reaction conditions are: reaction temperature of 60-65℃ and reaction time of 10-20h; in the polymerization reaction, the monomer conversion rate is 40-60%, preferably 45-55%, and the monomer conversion rate is the total conversion rate of all monomers; the polyacrylonitrile solution obtained by the polymerization reaction is optionally further deactivated by removing unreacted monomers and part of the solvent.

[0017] According to an embodiment of the present invention, the preparation method of the polyacrylonitrile-based carbon fiber specifically includes: after the polyacrylonitrile solution is spun out through a spinneret, it is sequentially subjected to solidification molding, hot water stretching, water washing, oiling, drying densification, and steam stretching to obtain polyacrylonitrile precursor fiber, and then subjected to pre-oxidation and carbonization treatment to obtain the polyacrylonitrile-based carbon fiber.

[0018] Specifically, the wet spinning or dry-wet spinning process can be adopted using commonly used wet spinning or dry-wet spinning processes in the field. The operation steps such as spinning, coagulation and molding, hot water drawing, washing, oiling, drying and densification, and steam drawing can all use commonly used equipment and process parameters in the field.

[0019] For example, wet spinning can be performed using the following process:

[0020] A three-stage gradient coagulation method was adopted, with the following coagulation conditions: Stage 1: coagulation bath concentration 65%, temperature 45℃; Stage 2: coagulation bath concentration 40%, temperature 50℃; Stage 3: coagulation bath concentration 20%, temperature 60℃, with a total draw of 1 to 2 times during coagulation; hot water drawing conditions: 2 to 2.5 times draw at 90 to 95℃; water washing temperature: 50 to 80℃; drying and densification temperature: 140 to 160℃; steam drawing: 2 to 2.5 times draw at 140 to 160℃; and total draw of 9 to 11 times during wet spinning (excluding spinneret draw).

[0021] The dry and wet spinning process can be carried out using the following steps:

[0022] Single-stage coagulation is adopted, and the coagulation and forming conditions are as follows: coagulation bath concentration 30%, temperature 3-5℃; hot water drawing: 2.5-4 times drawing at 45-75℃; water washing temperature 60-85℃; drying and densification temperature 140-160℃; steam drawing: 2.5-3.5 times drawing at 140-160℃; total drawing of dry and wet spinning is 9-11 times (excluding spinneret drawing).

[0023] According to a specific embodiment of the present invention, in the method for preparing the polyacrylonitrile-based carbon fiber:

[0024] The solid content of the polyacrylonitrile solution is 15-19%, for example, it can be 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, or 19%, and can be any of the above values ​​or a range between any two values; the polyacrylonitrile solution obtained by polymerizing components such as acrylonitrile, itaconic acid, and ammonium polyphosphate in a solvent is used to remove unreacted monomers and part of the solvent by controlling the vacuum degree of the system, so that the solid content in the polyacrylonitrile solution is 15-19%;

[0025] The total draw ratio of the fibers extruded through the spinneret in the subsequent preparation process is 9 to 11, for example, it can be 9, 9.5, 10, 10.5 or 11, and it can be any of the above values ​​or any range between two values;

[0026] The pre-oxidation can adopt the pre-oxidation process conditions commonly used in the prior art. Preferably, the pre-oxidation conditions are: temperature 220-280℃; residence time 30-60 min, preferably 30-40 min; and zero stretching throughout the pre-oxidation process.

[0027] The carbonization process can employ commonly used carbonization conditions in the prior art. For example, the carbonization includes low-temperature carbonization and high-temperature carbonization. Preferably, the conditions for low-temperature carbonization are: temperature 450–750°C, residence time 40–60 s, and draw ratio -0.2–+0.5%; and / or, the conditions for high-temperature carbonization are: temperature 1100–1400°C, residence time 40–60 s, and draw ratio -3.5–-5%.

[0028] According to a specific embodiment of the present invention, in the method for preparing the polyacrylonitrile-based carbon fiber, the carbon yield is >53%.

[0029] The second objective of this invention is to provide a polyacrylonitrile-based carbon fiber, which is prepared by the above-mentioned preparation method. After carbonization, the carbon fiber is surface treated and sized to obtain the finished carbon fiber. Preferably, the carbon fiber has a fineness greater than 0.65 dtex, a tensile strength greater than 5.3 GPa, and an elastic modulus greater than 260 GPa.

[0030] The advantages of this invention lie in the fact that the added ammonium polyphosphate (APP) in the system can simultaneously improve the hydrophilicity of the spinning solution, increase the pre-oxidation rate, and improve the carbon yield. Regarding improving hydrophilicity, the ammonium ions in APP have the same effect as conventional ammonia-modified spinning solutions, and compared to ammonia modification, they offer advantages such as accurate metering, ease of operation, environmental friendliness, and uniform mixing. Furthermore, because ammonium polyphosphate has a certain degree of polymerization, it is not easily removed during subsequent washing after polymer fiberization, and most of it remains in the PAN precursor yarn. In addition, during the pre-oxidation process, APP decomposes thermally to generate polyphosphoric acid, where the carboxyl groups catalyze cyano cyclization via an ionic mechanism, effectively shortening the oxidation residence time and forming a more regular and complete conjugated cyclic structure. This structure allows more carbon elements to be retained during subsequent low-temperature and high-temperature carbonization, forming a disordered graphite structure, resulting in a significant improvement in strength, modulus, and carbon yield compared to traditional production processes.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. The APP added to the polymerization formula of this invention can regulate the hydrophilicity of the spinning solution, which helps to slow down the double diffusion process during solidification and avoid the formation of macropores, thereby producing high-quality precursor fibers.

[0033] 2. During the pre-oxidation process, APP can promote the cyclization reaction, form a more complete conjugated cyclic structure, shorten the oxidation time, reduce the burn-off rate in the low-temperature carbonization stage, and significantly improve the carbon yield of the finished fiber.

[0034] 3. The preparation method provided by this invention is simple, easy to implement in industrial production, and has broad application prospects. Detailed Implementation

[0035] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0036] The testing instruments and conditions used in this embodiment are as follows:

[0037] Tensile strength and modulus were tested in accordance with GB / T3362-2005 using a universal testing machine.

[0038] Carbon fiber fineness testing first involves testing the fiber linear density according to GB / T7690.1-2013, and then converting it into monofilament fineness.

[0039] Carbon yield was determined by weighing method: first, the linear density of the precursor filament was determined according to GB / T3325-2008, and the carbon yield = carbon filament linear density × total draw ratio of oxidation and carbonization / linear density of precursor filament.

[0040] All raw materials used in the examples are commercially available products.

[0041] Comparative Example 1

[0042] Acrylonitrile (AN) and itaconic acid (IA) were sequentially added to N,N-dimethylformamide (DMF) and mixed, with AN and IA accounting for 22% of the total mass of the polymerization system and an IA / AN mass ratio of 1.1:100. After homogenization, the mixture was heated to 65°C, and 0.6% (by mass) of azobisisobutyronitrile (AIBN) was added to initiate polymerization. After 18 hours of polymerization, the monomers were removed and some solvent was removed to increase the solids content of the polymer liquid to 16%.

[0043] Polyacrylonitrile (PAN) precursor fibers were prepared by wet spinning after filtration of the polymer solution, with a total draw ratio of 9.5 times (excluding spinneret draw) and a fiber fineness of 1.22 dtex. The specific steps of the wet spinning process are as follows: a three-stage gradient coagulation process was adopted, with coagulation bath concentrations and temperatures of 65% and 45℃ for stage 1, 40% and 50℃ for stage 2, and 20% and 60℃ for stage 3, and a total draw ratio of 2 times in the coagulation stage; water washing temperature of 80℃; hot water drawing at 95℃ with a draw ratio of 2.5 times; drying and densification temperature of 150℃; and steam drawing at 145℃ with a draw ratio of 2 times.

[0044] The obtained PAN precursor fibers were pre-oxidized at 220–270℃ for 30 minutes with zero stretching throughout. The fibers were then sequentially passed through a low-temperature carbonization furnace and a high-temperature carbonization furnace. The low-temperature carbonization temperature was 450–750℃ with a residence time of 60 seconds and a stretching of +0.5%; the high-temperature carbonization temperature was 1150–1350℃ with a residence time of 40 seconds and a stretching of -3.5%. After surface treatment and sizing, the finished carbon fiber was obtained. This carbon fiber has a fineness of 0.61 dtex, a tensile strength of 4.58 Ga, a tensile modulus of elasticity of 242 GPa, and a carbon yield of 48.2%.

[0045] Comparative Example 2

[0046] AN and IA were sequentially added to DMF for mixing, with AN and IA accounting for 22% of the total mass of the polymerization system and the IA / AN mass ratio being 1.1:100. After uniform mixing, the temperature was raised to 65°C, and 0.6% (by mass) of azobisisobutyronitrile (AIBN) was added to initiate polymerization. After 18 hours of polymerization, the monomers were removed and some solvent was removed to increase the solid content of the polymerization solution to 16%. The spinning solution was hydrophilized using an ammonia bubbling method, with a molar ratio of ammonia to IA of 1:1. The polymer solution was then filtered and PAN precursor fibers were prepared using a wet spinning process (same as Comparative Example 1) with a total draw ratio of 9.5 times and a fiber fineness of 1.22 dtex. The PAN precursor fibers were pre-oxidized at 220–270°C for 40 minutes, with zero draw throughout the process. The fibers are then passed sequentially through a low-temperature carbonization furnace and a high-temperature carbonization furnace. The low-temperature carbonization temperature is 450–750℃, the residence time is 60s, and the stretch is +0.5%. The high-temperature carbonization temperature is 1150–1350℃, the residence time is 40s, and the stretch is -3.5%. After surface treatment and sizing, the finished carbon fiber is obtained. This carbon fiber has a fineness of 0.63 dtex, a tensile strength of 5.52 Ga, a tensile modulus of elasticity of 248 GPa, and a carbon yield of 49.8%.

[0047] Comparative Example 3

[0048] The spinning process was changed from wet to dry-wet, with the remaining process parameters remaining the same as in Comparative Example 2. The specific steps of the dry-wet spinning process were as follows: single-stage coagulation was used, with a coagulation bath concentration of 30% and a temperature of 3°C; hot water drawing at 70°C resulted in a 3-fold draw; water washing temperature was 75°C; drying and densification temperature was 150°C; steam drawing was performed at 145°C at a 3.5-fold draw, for a total draw of 10.5 times (excluding spinneret drawing).

[0049] The prepared carbon fiber has a fineness of 0.62 dtex, a tensile strength of 5.55 GPa, a tensile modulus of elasticity of 245 GPa, and a carbon yield of 49.0%.

[0050] Comparative Example 4

[0051] The precursor fiber prepared in Comparative Example 1 was impregnated in a 5% (w / w) aqueous solution of ammonium polyphosphate (APP) with a degree of polymerization of 5 for 10 s, followed by pre-oxidation and carbonization. The resulting carbon fiber had a fineness of 0.64 dtex, a tensile strength of 4.58 GPa, a tensile modulus of elasticity of 244 GPa, and a carbon yield of 50.6%.

[0052] Example 1

[0053] AN, IA, and APP were sequentially added to DMF for dissolution and mixing. AN and IA accounted for 22% of the total mass of the polymerization system, and the mass ratio of IA / APP / AN was 1.1:2:100. The degree of polymerization of APP was 10. After uniform mixing, the temperature was raised to 65°C, and 0.6% (by mass) of azobisisobutyronitrile (AIBN) was added to initiate polymerization. After 18 hours of polymerization, the monomers were removed and some solvent was removed to increase the solid content of the polymer solution to 16%. The polymer solution was then filtered and PAN precursor fibers were prepared using a wet spinning process (same as Comparative Example 1). The total draw ratio of the wet spinning process was 9.5 times (excluding spinneret draw), and the fiber fineness was 1.22 dtex. The PAN precursor fibers were pre-oxidized at 220–270°C for 30 minutes, with zero draw throughout the process. The fibers were then passed sequentially through a low-temperature carbonization furnace and a high-temperature carbonization furnace. The low-temperature carbonization temperature was 450–750℃, the residence time was 60s, and the stretch was +0.5%. The high-temperature carbonization temperature was 1150–1350℃, the residence time was 40s, and the stretch was -3.5%. After surface treatment and sizing, the finished carbon fiber was obtained. Testing showed that the carbon fiber had a fineness of 0.669 dtex, a tensile strength of 5.58 Ga, a tensile modulus of elasticity of 262 GPa, and a carbon yield of 53.2%.

[0054] Example 2

[0055] AN, IA, and APP were sequentially added to DMF for mixing, with AN and IA accounting for 22% of the total mass of the polymerization system. The mass ratio of IA / APP / AN was 1.1:3:100, and the degree of polymerization of APP was 5. After uniform mixing, the temperature was raised to 65°C, and 0.6% (by mass) of azobisisobutyronitrile (AIBN) was added to initiate polymerization. After 18 hours of polymerization, the monomers were removed and some solvent was removed to increase the solid content of the polymerization liquid to 17%. The polymer solution was then filtered and PAN precursor fibers were prepared using a wet spinning process (same as Comparative Example 1). The total draw ratio of the wet spinning process was 9.5 times (excluding spinneret draw), and the fiber fineness was 1.22 dtex. The PAN precursor fibers were pre-oxidized at 220–270°C for 30 minutes, with zero draw throughout the process. The fibers were then passed sequentially through a low-temperature carbonization furnace and a high-temperature carbonization furnace. The low-temperature carbonization temperature was 450–750℃, the residence time was 60s, and the stretch was +0.5%. The high-temperature carbonization temperature was 1150–1350℃, the residence time was 40s, and the stretch was -3.5%. After surface treatment and sizing, the finished carbon fiber was obtained. Testing showed that the carbon fiber had a fineness of 0.680 dtex, a tensile strength of 5.48 Ga, a tensile modulus of elasticity of 265 GPa, and a carbon yield of 54.1%.

[0056] Example 3

[0057] AN, IA, methyl acrylate (MA), and APP were sequentially added to DMF and mixed. AN, IA, and MA accounted for 22% of the total mass of the polymerization system, and the mass ratio of IA / MA / APP / AN was 1.1:1.2:3:100. The degree of polymerization of APP was 5. After uniform mixing, the mixture was heated to 65°C, and 0.6% (by mass) of azobisisobutyronitrile (AIBN) was added to initiate polymerization. After 18 hours of polymerization, the monomers were removed and some solvent was removed to increase the solid content of the polymerization liquid to 17%. The polymer solution was then filtered and PAN precursor fibers were prepared using a wet spinning process (same as Comparative Example 1). The total draw ratio of the wet spinning process was 11 times (excluding spinneret draw), and the fiber fineness was 1.22 dtex. The PAN precursor fibers were pre-oxidized at 220–270°C for 30 minutes, with zero draw throughout the process. The fibers are then passed sequentially through a low-temperature carbonization furnace and a high-temperature carbonization furnace. The low-temperature carbonization temperature is 450–750℃, the residence time is 60s, and the stretch is +0.5%. The high-temperature carbonization temperature is 1150–1350℃, the residence time is 40s, and the stretch is -3.5%. After surface treatment and sizing, the finished carbon fiber is obtained. This carbon fiber has a fineness of 0.677 dtex, a tensile strength of 5.42 Ga, a tensile modulus of elasticity of 262 GPa, and a carbon yield of 53.8%.

[0058] Example 4

[0059] The spinning process was changed from wet to dry-wet (same as Comparative Example 3), and the other process parameters were the same as in Example 3. The resulting fiber had a fineness of 0.667 dtex, a tensile strength of 5.51 GPa, a tensile modulus of elasticity of 260 GPa, and a carbon yield of 53%.

[0060] Example 5

[0061] In Example 2, the mass ratio of IA / APP / AN was changed to 1.1:0.8:100, while keeping the other conditions unchanged. The final fiber produced had a fineness of 0.657 dtex, a tensile strength of 5.37 GPa, a tensile modulus of elasticity of 260 GPa, and a carbon yield of 53%.

[0062] Example 6

[0063] AN, IA, and APP were sequentially added to a 51.8% (w / w) NaSCN aqueous solution for mixing. AN and IA accounted for 22% of the total mass of the polymerization system, and the mass ratio of IA / APP / AN was 1.1:3:100. The degree of polymerization of APP was 5. After uniform mixing, the mixture was heated to 65°C, and 0.55% (w / w) of azobisisobutyronitrile (AIBN) was added to initiate polymerization. After 17 hours of polymerization, the monomers were removed and some solvent was removed to increase the solid content of the polymerization solution to 18%. The polymer solution was then filtered and PAN precursor fibers were prepared using a wet spinning process (same as Comparative Example 1). The total draw ratio of the wet spinning process was 9.5 times (excluding spinneret draw), and the fiber fineness was 1.22 dtex. The PAN precursor fibers were pre-oxidized at 220–270°C for 30 minutes, with zero draw throughout the process. The fibers are then passed sequentially through a low-temperature carbonization furnace and a high-temperature carbonization furnace. The low-temperature carbonization temperature is 450–750℃, the residence time is 60s, and the stretch is +0.5%. The high-temperature carbonization temperature is 1150–1350℃, the residence time is 40s, and the stretch is -3.5%. After surface treatment and sizing, the finished carbon fiber is obtained. This carbon fiber has a fineness of 0.670 dtex, a tensile strength of 5.50 Ga, a tensile modulus of elasticity of 265 GPa, and a carbon yield of 53.3%.

Claims

1. A method for preparing polyacrylonitrile-based carbon fiber, comprising: After filtering the polyacrylonitrile solution, polyacrylonitrile precursor fibers are obtained through wet spinning or dry-wet spinning. These precursor fibers are then subjected to pre-oxidation and carbonization treatments to obtain the polyacrylonitrile-based carbon fibers. A reaction system is formed by adding components including acrylonitrile, itaconic acid, and ammonium polyphosphate to a solvent. After polymerization, the polyacrylonitrile solution is obtained. By mass percentage, the ammonium polyphosphate accounts for 0.5-3% of the acrylonitrile mass, and the components including acrylonitrile, itaconic acid, and ammonium polyphosphate account for 17-24% of the total weight of the reaction system. Acrylonitrile accounts for 95-99% of the total weight of the components including acrylonitrile, itaconic acid, and ammonium polyphosphate. The pre-oxidation conditions are: temperature 220-280℃, residence time 30-40 min.

2. The preparation method according to claim 1, characterized in that, The degree of polymerization of the ammonium polyphosphate is 5 to 50; and / or, The solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, aqueous sodium thiocyanate solution, aqueous zinc chloride solution, and aqueous nitric acid solution; and / or, Optionally, at least one of methyl acrylate and methyl methacrylate is added to the reaction system.

3. The preparation method according to claim 2, characterized in that, The degree of polymerization of the ammonium polyphosphate is 5 to 20.

4. The preparation method according to claim 1, characterized in that, An initiator is also added to the polymerization reaction.

5. The preparation method according to claim 4, characterized in that, By mass percentage, the components including acrylonitrile, itaconic acid, and ammonium polyphosphate account for 20-24% of the total weight of the reaction system; and / or, By mass percentage, the acrylonitrile accounts for 97-98.5% of the total weight of the components including acrylonitrile, itaconic acid, and ammonium polyphosphate; and / or, The initiator is an azo initiator, and the amount of the initiator is 0.3~0.5wt% of the acrylonitrile.

6. The preparation method according to claim 1, characterized in that, The polymerization reaction conditions are: a reaction temperature of 60-65℃ and a reaction time of 10-20 h; and / or, In the polymerization reaction, the monomer conversion rate is 40-60%; and / or, The polyacrylonitrile solution obtained from the polymerization reaction may also optionally have unreacted monomers and some solvent removed.

7. The preparation method according to claim 6, characterized in that, In the polymerization reaction, the monomer conversion rate is 45-55%.

8. The preparation method according to claim 1, characterized in that, The preparation method of the polyacrylonitrile-based carbon fiber specifically includes: after the polyacrylonitrile solution is spun out through a spinneret, it is sequentially subjected to solidification molding, hot water stretching, water washing, oiling, drying and densification, and steam stretching to obtain polyacrylonitrile precursor fiber, and then subjected to pre-oxidation and carbonization treatment to obtain the polyacrylonitrile-based carbon fiber.

9. The preparation method according to claim 8, characterized in that, The solid content of the polyacrylonitrile solution is 15-19%; and / or, The total draw ratio of the fibers ejected from the spinneret in the subsequent preparation process is 9~11.

10. The preparation method according to claim 8, characterized in that, The pre-oxidation conditions are as follows: The pre-oxidation process involves zero stretching throughout.

11. The preparation method according to claim 8, characterized in that, The carbonization includes low-temperature carbonization and high-temperature carbonization.

12. The preparation method according to claim 11, characterized in that, The conditions for low-temperature carbonization are: temperature of 450~750℃, residence time of 40~60s, and draw ratio of -0.2~+0.5%; and / or the conditions for high-temperature carbonization are: temperature of 1100~1400℃, residence time of 40~60s, and draw ratio of -3.5~-5%.

13. The preparation method according to any one of claims 1 to 12, characterized in that, In the preparation method of the polyacrylonitrile-based carbon fiber, the carbon yield is >53%.

14. A polyacrylonitrile-based carbon fiber, prepared by the preparation method according to any one of claims 1 to 13.

15. The polyacrylonitrile-based carbon fiber according to claim 14, characterized in that, The carbon fiber has a fineness greater than 0.65 dtex, a tensile strength greater than 5.3 GPa, and an elastic modulus greater than 260 GPa.