CN111101240B

CN111101240B - Polyacrylonitrile carbon fiber with low defects and high strength and preparation method thereof

Info

Publication number: CN111101240B
Application number: CN201811247005.9A
Authority: CN
Inventors: 宋文迪; 顾文兰; 季春晓; 袁玉红; 吴嵩义; 黄翔宇
Original assignee: China Petroleum and Chemical Corp; Sinopec Shanghai Petrochemical Co Ltd
Current assignee: China Petroleum and Chemical Corp; Sinopec Shanghai Petrochemical Co Ltd
Priority date: 2018-10-25
Filing date: 2018-10-25
Publication date: 2023-07-25
Anticipated expiration: 2038-10-25
Also published as: CN111101240A

Abstract

The invention discloses a polyacrylonitrile carbon fiber with low defects and high strength and a preparation method thereof. According to the invention, through a reversible addition-fragmentation chain transfer (RAFT) polymerization technology and adopting trithiocarbonate (TRIT) with carboxyl functional groups as a RAFT reagent, the molecular weight and the distribution of the polyacrylonitrile are controlled on one hand, and the polyacrylonitrile with carboxyl at the tail end is prepared, and meanwhile, a second monomer with double functional groups and similar to the reactivity ratio of the acrylonitrile is adopted for copolymerization reaction, so that the polyacrylonitrile with high chain structure regularity and uniform distribution of copolymerization units is obtained, a high-quality precursor is prepared, and further, the polyacrylonitrile carbon fiber with low defects and high strength is prepared through pre-oxidation, carbonization and post-treatment.

Description

Polyacrylonitrile carbon fiber with low defects and high strength and preparation method thereof

Technical Field

The invention relates to a polyacrylonitrile carbon fiber with low defects and high strength and a preparation method thereof. In particular to a method for preparing polyacrylonitrile solution with high molecular weight and narrow distribution by a reversible addition/fragmentation chain transfer (RAFT) free radical polymerization process, preparing polyacrylonitrile with carboxyl at the tail end by adopting a RAFT reagent with carboxyl functional groups, and simultaneously carrying out copolymerization reaction by adopting a second monomer with difunctional groups and similar reactivity ratio with acrylonitrile, thereby obtaining polyacrylonitrile with higher chain structure regularity and uniform distribution of copolymerized units, preparing high-quality precursor, and further preparing the polyacrylonitrile carbon fiber with low defects and high strength by pre-oxidation, carbonization and post-treatment.

Background

The preparation process of the polyacrylonitrile carbon fiber is a system engineering which relates to a plurality of complex processes such as polymerization, spinning, pre-oxidation, carbonization, post-treatment and the like and has high correlation between front and back. How to control the molecular structure of polyacrylonitrile by the polymerization process is a prerequisite for preparing high-performance filaments and carbon fibers. The polyacrylonitrile currently used as a carbon fiber precursor is mostly prepared by binary copolymerization of acrylonitrile with itaconic acid or ternary copolymerization of acrylonitrile with itaconic acid, unsaturated carboxylic acid esters (unsaturated amides), etc. Wherein, carboxyl contained in the polymerized monomer is used for reducing the cyclization temperature of the precursor in the preoxidation cyclization process, widening the exothermic peak and relieving the exothermic rate; the ester groups contained in the polymerized monomers serve to improve the spinnability and the proto-drawdown of the polymer product.

Since radical copolymerization is a very complex process, the resulting acrylonitrile copolymers tend to have the following disadvantages: (1) lower molecular weight and broader distribution; (2) The difference of the reactivity ratios of the comonomers is large, so that the composition deviation of the obtained copolymer is large and the distribution of the copolymerization units is uneven; (3) The acrylonitrile sequence has low regularity and many defects of macromolecular structure. The composition and structure defects can cause the quality of the precursor to be low, so that the defects are inherited to the carbon fiber, and the performance improvement of the carbon fiber is severely restricted.

The cleavage-addition chain transfer radical polymerization (RAFT) is a polymerization method in which a chain transfer agent is added to a polymerization system to form a dormant intermediate with a growing chain radical, and irreversible chain termination is limited, thereby effectively controlling reactivity. Compared with the traditional free radical polymerization method, the RAFT free radical polymerization can prepare polyacrylonitrile with ultrahigh molecular weight and narrow distribution, has simple process, less impurities and high product purity, and the polymerization system adopts a homogeneous environment, and can directly obtain spinning solution after removing monomers and bubbles so as to further prepare the precursor.

Disclosure of Invention

The invention aims to overcome the defects in the prior art and provide a polyacrylonitrile carbon fiber with low defects and high strength and a preparation method thereof. The preparation method is simple in process, few in impurities and high in product purity, and can directly obtain spinning solution after removing comonomers and bubbles. In the preparation method, the RAFT reagent with carboxyl functional groups is adopted to prepare polyacrylonitrile with carboxyl at the tail end; the invention also adopts a second monomer with double functional groups and similar reactivity ratio with acrylonitrile to carry out copolymerization reaction, thereby obtaining polyacrylonitrile with higher chain structure regularity and uniform distribution of copolymerization units. After the polyacrylonitrile is prepared, the polyacrylonitrile carbon fiber with low defects and high strength is finally prepared by a proper oxidization carbonization process.

The specific technical scheme of the invention is as follows:

the invention provides a preparation method of polyacrylonitrile carbon fiber with low defects and high strength, which mainly comprises the following steps:

1) Adding acrylonitrile, comonomer, initiator, RAFT reagent and solvent into a reactor, stirring and mixing uniformly, and introducing inert gas to perform deoxidization treatment. Then, the polymerization reaction is carried out, the polymerization reaction temperature is 60-85 ℃, and the polymerization reaction time is 6-24 hours. Residual monomers and bubbles are removed, and a polyacrylonitrile solution with the polyacrylonitrile content of 10-18 wt percent is obtained. Wherein the RAFT agent is S, S '-bis (α, α' -dimethyl- α "-acetic acid) trithiocarbonate (TRIT); the acrylonitrile adding amount is 15-30wt% based on the weight of the whole polymerization system; based on the total polymerized monomer weight, the adding amount of the comonomer is 0.5-5.0 wt%, the adding amount of the initiator is 0.2-1.5 wt%, the adding amount of the RAFT reagent is 0.05-0.5 wt%, and the balance is solvent;

2) And (3) filtering the polyacrylonitrile solution obtained in the step (1), spinning, and solidifying by a coagulating bath to obtain the nascent fiber. Wherein, the solution used in the coagulating bath is sodium thiocyanate (NaSCN) aqueous solution with the concentration of 8-15 wt%, and the temperature of the coagulating bath is-5-1 ℃;

3) Washing the primary fiber obtained in the step 2) with water, drafting, oiling, drying and densifying to obtain polyacrylonitrile precursor;

4) And 3) performing pre-oxidation, carbonization and post-treatment on the polyacrylonitrile precursor obtained in the step 3) to obtain the polyacrylonitrile carbon fiber. Wherein the post-treatment process comprises surface treatment and sizing.

In the step 1), the acrylonitrile addition amount is preferably 20 to 25wt% based on the weight of the whole polymerization system; based on the total polymerized monomer weight, the adding amount of the comonomer is 1.0-2.5 wt%, the adding amount of the initiator is 0.5-1.0 wt%, the adding amount of the RAFT reagent is 0.1-0.2 wt%, and the balance is solvent.

In the step 1), the comonomer is any one of monomethyl itaconate, monoethyl itaconate, monopropyl itaconate and monoamide itaconate. More preferably, the comonomer is monomethyl itaconate.

In the step 1), the initiator is any one or more of Azobisisobutyronitrile (AIBN) and Azobisisoheptonitrile (ABVN). More preferably, the initiator is Azobisisobutyronitrile (AIBN).

In the step 1), the solvent is 45-58% NaSCN water solution by mass fraction. The aqueous NaSCN solution has a lower chain transfer constant than organic solvents, and is more advantageous for preparing polyacrylonitrile with high molecular weight and narrow distribution. The solvent is preferably an aqueous solution of NaSCN with a mass fraction of 52-55%.

In the step 1), the inert gas adopted in the deoxidization treatment is any one of nitrogen, helium and neon. Preferably, the inert gas is nitrogen.

In the above step 1), the polymerization temperature is preferably 70 to 80 ℃.

In the above step 1), the polymerization time is preferably 15 to 20 hours.

In the step 1), the molecular weight of the polyacrylonitrile contained in the polyacrylonitrile solution is 9 to 25 ten thousand, preferably 13 to 18 ten thousand; the molecular weight distribution is 1.2 to 1.7, preferably 1.2 to 1.4; the isotacticity is 33.8-40%, preferably 36-40%.

In the step 1), the polyacrylonitrile content in the polyacrylonitrile solution is preferably 11 to 13wt%.

In the above step 2), the solution used in the coagulation bath is preferably a 10 to 13wt% NaSCN aqueous solution, and the coagulation bath temperature is preferably-3 to 0 ℃.

In the step 4), the pre-oxidation process comprises 4 temperature interval control stages, wherein the temperature is 170-200 ℃, 210-230 ℃, 235-255 ℃ and 260-275 ℃ respectively; more preferably, the temperatures of the 4 temperature interval control stages are 175-190 ℃, 215-225 ℃, 240-252 ℃ and 262-268 ℃, respectively; the carbonization comprises two processes of low-temperature carbonization and high-temperature carbonization, wherein the low-temperature carbonization adopts 4-zone gradient temperature rise control of 340-400 ℃, 420-500 ℃, 520-600 ℃ and 640-720 ℃, and preferably 4-zone gradient temperature rise control of 360-380 ℃, 450-480 ℃, 560-580 ℃ and 660-690 ℃; the high temperature carbonization adopts 3-zone gradient temperature rise control at 800-1050 ℃, 1050-1200 ℃ and 1200-1350 ℃, preferably 900-1000 ℃, 1080-1150 ℃ and 1250-1320 ℃.

The invention also provides the polyacrylonitrile carbon fiber obtained by the preparation method.

Compared with the prior art, the technical scheme of the invention has the following beneficial effects:

(1) The method adopts a reversible addition/fragmentation chain transfer (RAFT) free radical polymerization method to realize the activity controllable polymerization of the acrylonitrile, and compared with the traditional free radical polymerization method, the polyacrylonitrile with controllable molecular weight and narrower molecular weight distribution can be obtained.

(2) The polyacrylonitrile with carboxyl at the end group is prepared by adopting S, S ' -di (alpha, alpha ' -dimethyl-alpha ' -acetic acid) trithiocarbonate (TRIT) with carboxyl functional groups at two ends as a RAFT reagent through an activity controllable free radical polymerization method.

(3) The second polymerization monomer with double functional groups and similar reactivity ratio to acrylonitrile is adopted for copolymerization reaction, and the obtained acrylonitrile copolymer has higher chain regularity, uniform distribution of copolymerization units, good consistency of molecular composition and few structural defects.

(4) When NaSCN aqueous solution is used as a polymerization solvent, the catalyst has a lower chain transfer constant than an organic solvent, is more beneficial to control of molecular weight and distribution thereof, and is beneficial to improvement of chain regularity.

(5) The precursor prepared from the polyacrylonitrile solution obtained by the preparation method provided by the application has the advantages of less impurity, high pre-oxidation cyclization reaction activity, fast process, less defects of molecular structure and high strength.

Detailed Description

The invention will be further illustrated with reference to specific examples.

1) Preparation of Polyacrylonitrile solution

Polyacrylonitrile solutions were prepared according to the polymerization conditions shown in Table 1.

Examples 1 to 19

Sodium thiocyanate (NaSCN) water solution with a certain mass fraction is used as a reaction solvent, azodiisobutyronitrile (AIBN) or Azodiisoheptonitrile (ABVN) is used as an initiator, S, S ' -di (alpha, alpha ' -dimethyl-alpha ' -acetic acid) trithiocarbonate (TRIT) is used as a RAFT reagent, acrylonitrile, a comonomer, the initiator, the RAFT reagent and the solvent are added into a reactor, and nitrogen is introduced for deoxidization treatment. Then RAFT controllable free radical polymerization reaction is carried out at 60-85 ℃ for 6-24 hours. The acrylonitrile addition amounts are shown in Table 1 based on the weight of the whole polymerization system; the comonomer addition amount, the initiator addition amount and the RAFT reagent addition amount based on the total polymerization monomer weight are shown in Table 1; naSCN concentrations are shown in Table 1; the reaction temperatures are shown in Table 1; the reaction times are shown in Table 1.

[ comparative example 1 ]

Acrylonitrile and monomethyl itaconate in example 3 were replaced with acrylonitrile, with the other conditions unchanged.

[ comparative example 2 ]

The RAFT agent addition was omitted under the conditions of example 3, the others remaining unchanged.

[ comparative example 3 ]

The NaSCN solution in example 3 was replaced with DMSO, the others remaining unchanged.

[ comparative example 4 ]

The NaSCN solution in example 3 was replaced with DMF, the others remaining unchanged.

TABLE 1

2) Preparation of Polyacrylonitrile precursor

The polyacrylonitrile solution obtained in the above 1) in part of the examples and all the comparative examples is treated by removing residual comonomer and bubbles, the content of polyacrylonitrile is controlled to be 10 to 18 weight percent, and after filtration, spinning is carried out, and the primary fiber is formed by coagulation bath coagulation, wherein the coagulation bath is 8 to 15 weight percent of NaSCN aqueous solution, and the temperature of the coagulation bath is-5 to 1 ℃; washing the obtained nascent fiber with deionized water at 55 ℃ to remove residual solvent; the spun fiber after washing is drafted in hot water at 98 ℃ for 4-5 times and is oiled, and the concentration of an oil bath is 1%. Drying and densification are carried out on the fibers after oiling, wherein the drying and densification temperature is 130 ℃ and the time is 45s; carrying out steam drafting on the dried and densified fiber, wherein the drafting multiple is 1.5-2.0 times; oiling and drying again to obtain the polyacrylonitrile precursor. In the specific examples, the content of raw liquid polyacrylonitrile is shown in table 2, the mass concentration of NaSCN in the coagulation bath is shown in table 2, the temperature of the coagulation bath is shown in table 2, the hot water draft ratio is shown in table 2, the steam draft ratio is shown in table 2, and the used polymer sample is shown in table 2.

TABLE 2

3) Preparation of polyacrylonitrile carbon fiber

The polyacrylonitrile precursor obtained in the above 2) and all of the comparative examples was subjected to a pre-oxidation, carbonization, and post-treatment process to prepare a high-performance carbon fiber, and the post-treatment process includes surface treatment and sizing. The pre-oxidation process comprises 4 temperature interval controls of 170-200 ℃, 210-230 ℃, 235-255 ℃ and 260-275 ℃ respectively, and the time is 45min; carbonization is divided into two processes of low-temperature carbonization and high-temperature carbonization, wherein low carbon is controlled by adopting 4-zone gradient heating at 340-400 ℃, 420-500 ℃, 520-600 ℃ and 640-720 ℃ for 90s; the high carbon is controlled by adopting 3-zone gradient temperature rise at 800-1050 ℃, 1050-1200 ℃ and 1200-1350 ℃ for 90s; the surface treatment adopts an anodic oxidation surface treatment mode. The temperature of the pre-oxidized 4 area is shown in Table 3, the temperature of the low-temperature carbonization 4 area is shown in Table 3, the temperature of the high-temperature carbonization 3 area is shown in Table 3, and the used precursor sample is shown in Table 3.

TABLE 3 Table 3

To examine the structure and properties of the polyacrylonitrile obtained in examples and comparative examples, the polyacrylonitrile solutions obtained in each example and comparative example in 1) above were precipitated in deionized water, immersed for a certain period of time, boiled, and then the obtained polymer solids were baked to constant weight in a vacuum oven at 60 ℃ for testing.

Test instrument and conditions：

Gel Permeation Chromatography (GPC) determines molecular weight and distribution thereof: a Waters 1525/2414 gel permeation chromatograph was used. PMMA is used as a standard sample, and 0.065mol/L NaNO ₃ DMF was taken as a flow sample at a flow rate of 1.5ml/min and at a temperature of 40 ℃.

A20 contact angle meter test polyacrylonitrile contact angle: and under the room temperature condition, the contact angle is measured by taking water as titration liquid. The polymer molecular weight and distribution and contact angle were measured and shown in Table 4.

Measurement of Polypropylene by Nuclear Magnetic Resonance (NMR)Tacticity of nitrile: AV600 type high resolution liquid nuclear magnetic resonance spectrometer of Bruker company is adopted for measurement ¹³ C-NMR spectrum. ¹³ 2.6 to 2.8X10 of C-NMR spectrum ^-5 The peaks in the range are carbon peaks of tertiary carbons on the PAN backbone, which are split into triplets. The lower field to the higher field are respectively syndiotactic (rr), hetero-syndiotactic (mr) and isotactic (mm) in triad assignment. The integral intensity of 3 peaks of tertiary carbon atoms can be obtained by integrating 3 peaks through Nuts software, the proportion of the 3 structural units mm, mr and rr in the molecular weight is calculated indirectly, and therefore the tacticity of the polyacrylonitrile molecular chain is characterized, and the test result is shown in Table 5.

Differential Scanning Calorimeter (DSC) measurement of polyacrylonitrile filament thermal properties: differential scanning calorimeter manufactured by METTLER company, switzerland was used. And washing a sample to be tested by acetone, putting the sample into an oven, drying the sample at 110 ℃ for 2 hours, taking out the sample, and cooling the sample in a dryer for 15 minutes. About 1.50mg of a crude yarn sample was taken, chopped and pressed into a sheet of a predetermined shape in an aluminum crucible, and DSC was performed. The scanning range is 100-400 ℃, the heating rate is 5 ℃/min, the ambient atmosphere is air, and the atmosphere flow is 20ml/min. Data are available from DSC exotherms: exothermic Peak onset temperature (T) _i ) Termination temperature (T) _f ) Peak of exothermic heat (T _pi ) Heat release amount (Δh), heat release peak width (Δt=t) _f —T _i ) And heat release (. DELTA.H), and the results of the thermal properties of the polyacrylonitrile precursor are shown in Table 6.

And measuring the mechanical properties of the polyacrylonitrile carbon fiber by a tension meter: after a carbon fiber sample is impregnated and solidified by glue, a breaking experiment is carried out by using an Shimadzu HT-9112 type tensile tester, and the average value is obtained after repeating 10 times, so that the strength, the strength CV, the modulus and the modulus CV of the fiber are obtained, and are shown in Table 7.

And (3) measuring the density of the polyacrylonitrile carbon fiber body by a floating sedimentation method: a mixture of the two liquids was prepared and placed in a beaker with a density of the mixture less than the sample density. The liquid was thoroughly mixed and the mixture was maintained in an environment of 23 ℃ ± 0.1 ℃ and at that temperature. The sample to be tested is tied into a knot, placed in a liquid mixture, defoamed at a vacuum of 60hPa, and maintained at this vacuum for at least 2 minutes. Wait for 5 minutes, if the sample is sinking, add a few drops of high density liquid, if it is floating, add a few drops of low density liquid until the sample stays in the middle for 2 hours. The liquid mixture was filtered and the liquid density, i.e. the carbon fiber bulk density, was measured by the method of ISO1675 using a pycnometer or pycnometer, as shown in table 7.

TABLE 4 Table 4

TABLE 5

	Isotactic mm%	Hetero-mr%	Syndiotactic rr%
				Example 1	38.2	20.9	40.9
Example 2	39.1	20.9	41.3
				Example 3	39.7	20.1	40.2
Example 4	37.8	20.5	41.7
				Example 5	38.4	20.2	41.4
Example 6	36.6	21.1	42.3
				Example 7	35.3	22.8	41.9
Example 8	35.1	23.1	41.8
				Example 9	36.5	21.7	41.8
Example 10	35.7	22.2	42.1
				Example 11	33.8	23.9	42.3
Example 12	35.6	21.7	42.7
				Example 13	34.5	21.6	43.9
Example 14	35.2	23.1	41.7
				Example 15	38.9	20.1	41.0
Example 16	34.7	22.8	42.5
				Example 17	38.7	20.2	41.1
Example 18	38.4	21.4	40.2
				Example 19	38.6	20.9	40.5
Comparative example 1	38.8	20.5	40.7
				Comparative example 2	25.9	23.3	50.8
Comparative example 3	28.3	22.3	49.4
				Comparative example 4	27.1	25.7	47.2

TABLE 6

TABLE 7

The test results in tables 4 to 7 show that:

examples 1 to 19 used a proper mass fraction of NaSCN solution as a solvent, and the molecular weight distribution of the polyacrylonitrile obtained by adding RAFT agent and comonomer was narrower, and was Mw/mn=1.23 to 1.74, and the molecular weight distribution of the polyacrylonitrile obtained when the solvent was changed to an organic solvent was broadened, and the Mw/Mn of comparative examples 3 and 4 was >2.0, indicating that the polymerization process in this example was in the active controlled polymerization category. In the polymerization process, the polymerization reaction rate can be adjusted by changing the polymerization reaction temperature, the initiator dosage and the proportion of the comonomer and the RAFT reagent, and the designability of the molecular weight of the polymer can be realized according to the principle that the comonomer conversion rate and the polymerization time form a first-order linear relation.

Because the special RAFT reagent with carboxyl functional groups at two ends is selected, compared with the common acrylonitrile copolymer precursor or (comparative example 1), the DSC exothermic initiation temperature is reduced, the exothermic width is increased, and the polyacrylonitrile precursor has better oxidative cyclization activity.

In addition, the carbon fiber performance test results show that the polyacrylonitrile carbon fibers obtained in examples 3 to 5 are obviously improved in the aspects of tensile strength, tensile modulus and fiber density.

Claims

1. The preparation method of the polyacrylonitrile carbon fiber with low defects and high strength is characterized by comprising the following steps:

1) Adding acrylonitrile, a comonomer, an initiator, a reversible addition/fragmentation chain transfer (RAFT) reagent and a solvent into a reactor, stirring and mixing uniformly, and introducing inert gas to perform deoxidization treatment; then carrying out polymerization reaction, wherein the polymerization reaction temperature is 60-85 ℃ and the polymerization reaction time is 6-24 hours; removing residual comonomer and bubbles to obtain a polyacrylonitrile solution with the polyacrylonitrile content of 10-18 wt%; wherein the RAFT agent is S, S '-bis (α, α' -dimethyl- α "-acetic acid) trithiocarbonate (TRIT); based on the weight of the whole polymerization system, the total monomer adding amount is 15-30wt%; based on the total polymerized monomer weight, the adding amount of the comonomer is 0.5-5.0 wt%, the adding amount of the initiator is 0.2-1.5 wt%, the adding amount of the RAFT reagent is 0.05-0.5 wt%, and the balance is solvent; the comonomer is monomethyl itaconate; the solvent is sodium thiocyanate (NaSCN) aqueous solution with the mass concentration of 45-58%;

2) Filtering the polyacrylonitrile solution obtained in the step 1), spinning, and solidifying by a coagulating bath to obtain nascent fibers; wherein, the concentration of the solution used in the coagulating bath is 8-15 wt% of sodium thiocyanate (NaSCN) aqueous solution, and the temperature of the coagulating bath is-5-1 ℃;

4) The polyacrylonitrile precursor obtained in the step 3) is subjected to pre-oxidation, carbonization and post-treatment to obtain polyacrylonitrile carbon fiber; wherein the post-treatment process comprises surface treatment and sizing.

2. The method for preparing the low-defect and high-strength polyacrylonitrile carbon fiber according to claim 1, wherein in the step 1), the initiator is any one or more of Azobisisobutyronitrile (AIBN) and Azobisisoheptonitrile (ABVN).

3. The method for producing a low-defect, high-strength polyacrylonitrile carbon fiber according to claim 2, wherein in step 1), the initiator is Azobisisobutyronitrile (AIBN).

4. The method for preparing the low-defect and high-strength polyacrylonitrile carbon fiber according to claim 1, wherein the solvent is a sodium thiocyanate (NaSCN) aqueous solution with a mass concentration of 52-55%.

5. The method for preparing low-defect and high-strength polyacrylonitrile carbon fiber according to claim 1, wherein in the step 1), the inert gas is any one of nitrogen, helium and neon.

6. The method for producing a low-defect, high-strength polyacrylonitrile carbon fiber as claimed in claim 5, wherein said inert gas is nitrogen.

7. The method for preparing the low-defect and high-strength polyacrylonitrile carbon fiber according to claim 1, wherein in the step 1), the polymerization temperature is 70-80 ℃ and the polymerization time is 15-20 hours; the acrylonitrile adding amount is 20 to 25 weight percent based on the weight of the whole polymerization system; the adding amount of the comonomer is 1.0 to 2.5 weight percent based on the total monomer weight; the initiator is added in an amount of 0.5 to 1.0 weight percent, the RAFT is added in an amount of 0.1 to 0.2 weight percent, and the balance is solvent.

8. The method for preparing a low-defect and high-strength polyacrylonitrile carbon fiber according to claim 1, wherein in the step 1), the polyacrylonitrile content in the polyacrylonitrile solution is 11-13 wt%.

9. The method for preparing low-defect and high-strength polyacrylonitrile carbon fiber according to claim 1, wherein in the step 2), the solution used in the coagulating bath is a NaSCN aqueous solution with the concentration of 10-13 wt%, and the coagulating bath temperature is-3-0 ℃.

10. The method according to claim 1, wherein in the step 4), the pre-oxidation process comprises 4 temperature interval control stages, the temperatures are respectively 170-200 ℃, 210-230 ℃, 235-255 ℃ and 260-275 ℃, carbonization comprises two processes of low-temperature carbonization and high-temperature carbonization, wherein the low-temperature carbonization adopts 4-zone gradient temperature rise control of 340-400 ℃, 420-500 ℃, 520-600 ℃ and 640-720 ℃, and the high-temperature carbonization adopts 3-zone gradient temperature rise control of 800-1050 ℃, 1050-1200 ℃ and 1200-1350 ℃.

11. The method according to claim 1 or 10, wherein in the step 4), the pre-oxidation process comprises 4 temperature interval control stages, the temperatures are 175-190 ℃, 215-225 ℃, 240-252 ℃ and 262-268 ℃, carbonization comprises two processes of low-temperature carbonization and high-temperature carbonization, wherein the low-temperature carbonization adopts 4-zone gradient temperature rise control of 360-380 ℃, 450-480 ℃, 560-580 ℃ and 660-690 ℃, and the high-temperature carbonization adopts 3-zone gradient temperature rise control of 900-1000 ℃, 1080-1150 ℃ and 1250-1320 ℃.

12. A polyacrylonitrile contained in a polyacrylonitrile solution obtained by the process for producing a low-defect, high-strength polyacrylonitrile carbon fiber according to any one of claims 1 to 11, characterized in that the molecular weight of the polyacrylonitrile is 9 to 25 ten thousand, the molecular weight distribution is 1.2 to 1.7, and the isotacticity is 33.8 to 40%.

13. The polyacrylonitrile according to claim 12, wherein the molecular weight of the polyacrylonitrile is 13 to 18 ten thousand, the molecular weight distribution is 1.2 to 1.4, and the isotacticity is 36 to 40%.

14. A low-defect, high-strength polyacrylonitrile carbon fiber obtained by the process for producing a low-defect, high-strength polyacrylonitrile carbon fiber according to any one of claims 1 to 11.