Polyacrylonitrile as-spun fiber for carbon fiber and preparation method and application thereof

Abstract

The present application relates to a kind of polyacrylonitrile primary fibers for carbon fiber and its preparation method, mainly solve the problem of low light transmittance of primary fiber in prior art, the quality of carbon fiber product is not high.The present application is by providing a kind of polyacrylonitrile primary fibers for carbon fiber, the primary fiber is not less than 35% at 550nm monochromatic light transmittance, after subsequent oxidation carbonization processing, can obtain the carbon fiber product of tensile strength 5.0~5.5GPa, Young's modulus 450~550GPa.The present application can be used in the industrial production in the field of carbon fiber precursor.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of carbon fibers, and particularly relates to a polyacrylonitrile as-spun fiber for carbon fibers and a preparation method and application thereof. BACKGROUND

[0002] Carbon fibers have high modulus and fatigue resistance, and are widely used in aerospace, automobile lightweight, wind power and other fields as special materials, and are the representatives of modern high-performance materials. Although carbon fibers are inorganic materials, they are obtained by oxidizing and carbonizing various organic carbon-containing precursors. Therefore, a common classification method of carbon fibers is to divide them according to the components of the precursors before oxidation and carbonization. Under this classification method, carbon fibers are mainly divided into polyacrylonitrile, viscose-based and pitch-based three types, and among them, polyacrylonitrile-based carbon fibers have the largest production scale and the most use in the current production industry.

[0003] Since the melting point of polyacrylonitrile is higher than its decomposition temperature, there are insurmountable technical difficulties in using melt method to process polyacrylonitrile. Therefore, the production of polyacrylonitrile-based carbon fiber precursor is still mainly adopted by solution process route. In this process route, polyacrylonitrile is dissolved by a solvent to obtain a spinning solution, and the spinning solution is extruded through a spinneret and then coagulated in a coagulation bath to obtain an as-spun fiber. The as-spun fiber is then subjected to multiple processes such as washing, stretching and drying to obtain a polyacrylonitrile precursor for preparing carbon fibers. Since the whole process flow is long, defects will be generated in the precursor at each link, and the generated defects will be added and inherited. It is also due to the existence of these defects that there is a large gap between the actual strength and the theoretical strength of carbon fibers. So far, the mechanical strength of the best quality carbon fiber product on the market is still less than 5% of the theoretical strength. In order to obtain carbon fibers with more excellent mechanical properties, how to obtain a precursor with fewer defects has been an important research direction.

[0004] Throughout the entire polyacrylonitrile precursor fiber spinning process, the formation process of the nascent fiber is extremely important. A more intuitive case is the performance difference between the products produced by wet spinning and dry-jet wet spinning. In the early days of polyacrylonitrile-based carbon fiber, the wet spinning method used in the production of acrylic fiber was used for the preparation of the precursor fiber. At that time, the strength of the carbon fiber product was about 3.0 GPa. With the development of technology and the innovation of the original process, the strength of the highest grade wet spinning carbon fiber product on the market can now reach 5.5 GPa. However, with the advent of dry-jet wet spinning technology, the highest strength carbon fiber product has reached 7.0 GPa. According to Japanese patent JP2006257580A, the highest strength carbon fiber product obtained by dry-jet wet spinning technology is greater than 7.4 GPa. The biggest difference between wet spinning and dry-jet wet spinning is the difference in the formation process of the nascent fiber. Therefore, the change in the structure of the nascent fiber caused by the change in the process has a significant impact on the performance of the final product, carbon fiber.

[0005] There have been a considerable number of reports on the research of structure improvement and defect inhibition of nascent fibers. In general, current work is mainly focused on two aspects. The first is to study how to reduce the swelling degree of the nascent fiber to improve the density of the fiber. According to Chinese patent CN109137117A, the minimum swelling degree of the nascent fiber can be less than 105%. However, although the swelling degree is an important evaluation index, in fact, the swelling degree has no strong correlation with the size of the defect. There may still be large hole defects in the nascent fiber at a small swelling degree. The second direction is to improve the crystallinity of the polymer in the nascent fiber. For example, according to Chinese patents CN111304768A and CN103614801A, the crystallinity of the nascent fiber can be increased to 88%. However, the crystalline structure formed in the nascent fiber is not the same as that in the precursor fiber, and in addition, too high a crystallinity of the nascent fiber is not conducive to the subsequent drawing.

[0006] At present, in actual industrial production, the quality of the nascent fiber is mainly determined by subjective observation. If the nascent fiber is relatively transparent, it means that the nascent fiber is relatively ideal. If the nascent fiber appears obvious white and opaque, it means that the fiber structure is not ideal. However, such experience still remains at the stage of subjective judgment and still lacks reasonable theoretical explanation and objective judgment standard. At present, it is still necessary to quantitatively describe the nascent fiber with excellent structure characteristics, and to solve the problem of low light transmittance of the nascent fiber and low quality of the obtained carbon fiber product. SUMMARY

[0007] To solve the above technical problems, the present application provides a polyacrylonitrile primary fiber for preparing carbon fiber and a preparation method thereof. The present application provides a primary fiber with excellent structural characteristics, limits the monochromatic light transmittance thereof at 550 nm, and controls the swelling degree of the primary fiber, so that a primary fiber with more regular structure can be obtained, thereby solving the problems of low light transmittance of the primary fiber and low quality of the obtained carbon fiber product.

[0008] One of the objects of the present application is to provide a polyacrylonitrile primary fiber for carbon fiber, wherein the monochromatic light transmittance at 550 nm is not less than 35%, preferably 40-55%.

[0009] Preferably,

[0010] The linear density of the primary fiber is 6-12 Dtex, preferably 8-10 Dtex, on a dry basis.

[0011] The swelling degree of the primary fiber is 160-250%, and the swelling degree after relaxation treatment in hot water at 95°C is 0.4-0.8 times the initial swelling degree, preferably, the swelling degree of the primary fiber is 170-240%, and the swelling degree after relaxation treatment in hot water at 95°C is 0.45-0.75 times the initial swelling degree.

[0012] The above-mentioned polyacrylonitrile primary fiber for carbon fiber is prepared by dry-jet wet spinning process from polyacrylonitrile solution.

[0013] The second object of the present application is to provide a preparation method of the above-mentioned polyacrylonitrile primary fiber for carbon fiber, which comprises extruding a polyacrylonitrile solution containing cations through a spinneret into an air bath draft, and obtaining the polyacrylonitrile primary fiber after a coagulation bath.

[0014] In the above-mentioned preparation method, the cations are selected from at least one of alkali metal cations and organic cation groups, preferably selected from Na + , K + , NH4 + , [RNH3] + , [R2NH2] + , [R3NH] + , wherein R is a linear aliphatic hydrocarbon with 1-3 carbon atoms, and more preferably selected from NH4 + The cations in the polyacrylonitrile solution can be obtained by adding a compound containing the above-mentioned cations or a compound capable of generating the above-mentioned cations, for example, NH4 + can be obtained by neutralization in ammonia.

[0015] In the above-mentioned preparation method, the content of cations in the polyacrylonitrile solution is 0.02-0.05 mmol / g.

[0016] In the above preparation method, the zero shear viscosity of the polyacrylonitrile solution is 20-120 Pa-s at 60°C.

[0017] In the above preparation method, the polyacrylonitrile solution comprises acrylonitrile copolymer and solvent, and the solvent is selected from at least one of polar aprotic solvents, preferably selected from dimethyl sulfoxide, N, N dimethylformamide, N, N dimethylacetamide.

[0018] In the above preparation method, the acrylonitrile copolymer is obtained by copolymerization of acrylonitrile and vinyl-based comonomer;

[0019] The content of acrylonitrile copolymer in the polyacrylonitrile solution is 14-24% by mass percentage;

[0020] The content of acrylonitrile monomer in the acrylonitrile copolymer is not less than 97% by mass percentage.

[0021] In the above preparation method, the process conditions of the air bath drawing process have no specific requirements, and the commonly used air bath drawing process conditions in the art can be used, as long as the extruded spinning dope can be drawn in the air bath.

[0022] In the above preparation method, the process conditions of the coagulation bath process have no specific requirements, and the commonly used coagulation bath process conditions in the art can be used, preferably, the solvent of the coagulation bath is a mixed solvent of water and polar aprotic solvent, and the polar aprotic solvent is selected from at least one of dimethyl sulfoxide, N, N dimethylformamide, N, N dimethylacetamide; the temperature of the coagulation bath is -15-5°C.

[0023] The as-spun fiber prepared by the as-spun fiber preparation method provided by the present application can obtain high-strength and high-modulus carbon fiber after the processes of washing, hot water drawing, steam drawing, pre-oxidation, carbonization and graphitization.

[0024] The third object of the present application is to provide a carbon fiber prepared from the above polyacrylonitrile as-spun fiber or the polyacrylonitrile as-spun fiber prepared by the above preparation method. Preferably, the tensile strength of the carbon fiber is 5.0-5.5 GPa, and the Young's modulus is 450-550 GPa.

[0025] The as-spun fiber of the present application is obtained by dissolving and spinning polyacrylonitrile-based polymer. The as-spun fiber involved in the present application has a transmittance of not less than 35% at 550 nm monochromatic light, which is controlled from the more subtle structure of the as-spun fiber. By subsequent oxidation and carbonization processing of the as-spun fiber with the above characteristics, a carbon fiber product with a tensile strength of 5.0-5.5 GPa and a Young's modulus of 450-550 GPa can be obtained.

[0026] The present application finds that the difference of the transmittance of the nascent fiber at different wavelengths is caused by the difference of the micro-phase structure. If the phase separation scale is large and the phase size is in the range of the visible light wavelength, the nascent fiber will show an opaque appearance. If the phase structure is small after the phase separation, the nascent fiber will show a transparent feature in general. The difference of the transmittance at the visible wavelength can directly reflect the structural feature of the nascent fiber. Therefore, the higher the transmittance at a certain wavelength, the more uniform and delicate the structure is, and the carbon fiber precursor can be considered to be better.

[0027] In the present application, the light transmittance of the nascent fiber is regulated to meet the requirement that the monochromatic light transmittance at 550 nm is not less than 35%, preferably 40-55%. The carbon fiber prepared by using the nascent fiber as the precursor can have good mechanical properties.

[0028] Compared with the prior art, the present application has the following beneficial effects:

[0029] 1. The present application uses the polyacrylonitrile solution with a specific viscosity, and the cation is added in the solution, so that the defects of the nascent fiber in the preparation process can be avoided;

[0030] 2. The nascent fiber with a specific light transmittance range and swelling degree is obtained by adjusting the process conditions in the present application, and the structure of the nascent fiber is more uniform;

[0031] 3. The preparation method of the present application is simple and easy to operate, and the structure of the nascent fiber is controllable, so that the carbon fiber with excellent performance can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The electron microscope image of the nascent fiber in Example 2 of the present application. DETAILED DESCRIPTION

[0033] The present application will be specifically described below in combination with specific examples. It is necessary to point out here that the following examples are only used to further illustrate the present application, and cannot be understood as limiting the protection scope of the present application. Some non-essential improvements and adjustments of the present application made by the person skilled in the art according to the content of the present application still belong to the protection scope of the present application.

[0034] The test instruments and test conditions used in the examples are as follows:

[0035] The swelling degree test method in the present application is as follows:

[0036] Swelling degree:

[0037] Take 5-8 g of the fiber sample, wash it with water, then centrifuge it at 3000 RPM for 8 min, weigh it as W1, and weigh it after drying as W2.

[0038] Swelling degree = (W1-W2 / W2)*100%

[0039] Transmittance:

[0040] The fiber sample was spread into a 8000-10000 Dtex / cm thickness of the filament, and the transmittance was tested on-line using a three-wavelength transmittance instrument.

[0041] Mechanical properties of carbon fiber:

[0042] The test was performed according to GB / T 3362-2005.

[0043] The raw materials used in the examples are all commercially available.

[0044] Example 1

[0045] Preparation of polyacrylonitrile as-spun fiber:

[0046] A dimethyl sulfoxide solution of polyacrylonitrile copolymer was used, the solid content of the solution was 22.5% by mass ratio, the copolymer monomers in the copolymer were acrylonitrile and itaconic acid, and the mass fraction of acrylonitrile was 98.5% and that of itaconic acid was 1.5%. The original solution was neutralized by passing in ammonia gas, and the final original solution contained 0.03 mmol / L of NH4 + The viscosity of the original solution was 100 Pa·s at 60°C; a 1k0.12 mm spinneret was used for extrusion, the air bath draw ratio was 2.1 times, and the coagulation bath was a mixture of dimethyl sulfoxide and water, in which the mass ratio of dimethyl sulfoxide was 30%, and the coagulation bath temperature was -5°C. The as-spun fiber with a fineness of 9 Dtex was obtained, the swelling degree was 195%, and the transmittance of 550 nm monochromatic light was 44%. After hot water treatment at 95°C, the fiber swelling degree was 100%.

[0047] Preparation of carbon fiber:

[0048] The above as-spun fiber was drawn 2 times in hot water, drawn 4.3 times in steam, pre-oxidized at 225°C, carbonized at 900-1400°C, and graphitized at 2600°C, to obtain a graphite fiber with a strength of 5.05 GPa and a Young's modulus of 460 GPa.

[0049] Example 2

[0050] Preparation of polyacrylonitrile as-spun fiber:

[0051] A dimethyl sulfoxide solution of polyacrylonitrile copolymer was used, the solid content of the solution was 23% by mass ratio, the copolymer monomers in the copolymer were acrylonitrile and itaconic acid, and the mass fraction of acrylonitrile was 99% and that of itaconic acid was 1.0%. The original solution was neutralized by passing in ammonia gas, and the final original solution contained 0.02 mmol / L of NH4 +, the viscosity of the dope is 115 Pa-s at 60°C, extrusion is performed using a 1k 0.10 m spinneret, the air bath draw ratio is 2.0, and the coagulation bath is a mixture of dimethyl sulfoxide and water, in which the mass ratio of dimethyl sulfoxide is 30%, and the coagulation bath temperature is -2°C. The as-spun fiber obtained has a fineness of 11 Dtex, a swelling degree of 190%, and a 550 nm monochromatic light transmittance of 41%. After hot water treatment at 95°C, the fiber has a swelling degree of 102%.

[0052] Preparation of carbon fiber:

[0053] The as-spun fiber is drawn 2.5 times in hot water, 4.3 times in steam, pre-oxidized at 225°C, carbonized at 900-1400°C, and graphitized at 2600°C, to obtain a graphite fiber with a strength of 5.15 GPa and a Young's modulus of 455 GPa.

[0054] Example 3

[0055] Preparation of polyacrylonitrile as-spun fiber:

[0056] A dimethyl sulfoxide solution of a polyacrylonitrile copolymer is used, the solid content of the solution is 19.5% by mass, the copolymer monomers in the copolymer are acrylonitrile and itaconic acid, the mass fraction of acrylonitrile is 98.5%, and the mass fraction of itaconic acid is 1.5%, the original solution contains 0.03 mmol / L of NH4 + after neutralization by ammonia gas, the viscosity at 60°C is 65 Pa-s, extrusion is performed using a 1k 0.12 m spinneret, the air bath draw ratio is 2.1 times, and the coagulation bath is a mixture of dimethyl sulfoxide and water, in which the mass ratio of dimethyl sulfoxide is 30%, and the coagulation bath temperature is -10°C. The as-spun fiber obtained has a fineness of 8 Dtex, a swelling degree of 180%, and a 550 nm monochromatic light transmittance of 47%. After hot water treatment at 95°C, the fiber has a swelling degree of 80%.

[0057] Preparation of carbon fiber:

[0058] The as-spun fiber is drawn 2 times in hot water, 4.6 times in steam, pre-oxidized at 225°C, carbonized at 900-1400°C, and graphitized at 2600°C, to obtain a graphite fiber with a strength of 5.42 GPa and a Young's modulus of 451 GPa.

[0059] Example 4

[0060] Preparation of polyacrylonitrile as-spun fiber:

[0061] A dimethyl sulfoxide solution of a polyacrylonitrile copolymer is used, the solid content of the solution is 22% by mass, the copolymer monomers in the copolymer are acrylonitrile and itaconic acid, the mass fraction of acrylonitrile is 99.5%, and the mass fraction of itaconic acid is 0.5%, the original solution contains 0.05 mmol / L of NH4+ The solution has a solid content of 22.5% by mass, and the copolymer monomers in the copolymer are acrylonitrile and itaconic acid, wherein the mass fraction of acrylonitrile is 98.5% and the mass fraction of itaconic acid is 1.5%. The dope has a viscosity of 100 Pa-s at 60°C. The solution is extruded through a 1k 0.12 mm spinneret, the air bath draw ratio is 2.1, and the coagulation bath is a mixture of dimethyl sulfoxide and water, wherein the mass ratio of dimethyl sulfoxide is 30%, and the coagulation bath temperature is -5°C. The as-spun fiber obtained has a fineness of 9Dtex, a swelling degree of 190%, and a 550 nm monochromatic light transmittance of 25%. The fiber has a swelling degree of 95% after hot water treatment at 95°C.

[0062] Preparation of carbon fiber:

[0063] The as-spun fiber is drawn 2 times in hot water and 4.3 times in steam, pre-oxidized at 225°C, carbonized at 900-1400°C, and graphitized at 2600°C to obtain a graphite fiber with a strength of 4.01 GPa and a Young's modulus of 425 GPa.

[0064] Comparative Example 1

[0065] Preparation of polyacrylonitrile as-spun fiber:

[0066] The solution has a solid content of 22.5% by mass, and the copolymer monomers in the copolymer are acrylonitrile and itaconic acid, wherein the mass fraction of acrylonitrile is 98.5% and the mass fraction of itaconic acid is 1.5%. The dope has a viscosity of 100 Pa-s at 60°C. The solution is extruded through a 1k 0.12 mm spinneret, the air bath draw ratio is 2.1, and the coagulation bath is a mixture of dimethyl sulfoxide and water, wherein the mass ratio of dimethyl sulfoxide is 30%, and the coagulation bath temperature is -5°C. The as-spun fiber obtained has a fineness of 9Dtex, a swelling degree of 190%, and a 550 nm monochromatic light transmittance of 25%. The fiber has a swelling degree of 95% after hot water treatment at 95°C.

[0067] Preparation of carbon fiber:

[0068] The as-spun fiber is drawn 2 times in hot water and 4.3 times in steam, pre-oxidized at 225°C, carbonized at 900-1400°C, and graphitized at 2600°C to obtain a graphite fiber with a strength of 4.01 GPa and a Young's modulus of 425 GPa.

[0069] Comparative Example 2

[0070] Preparation of polyacrylonitrile as-spun fiber:

[0071] The solution has a solid content of 22.5% by mass, and the copolymer monomers in the copolymer are acrylonitrile and itaconic acid, wherein the mass fraction of acrylonitrile is 98.5% and the mass fraction of itaconic acid is 1.5%. The dope has a viscosity of 100 Pa-s at 60°C. The solution is extruded through a 1k 0.12 mm spinneret, the air bath draw ratio is 2.1, and the coagulation bath is a mixture of dimethyl sulfoxide and water, wherein the mass ratio of dimethyl sulfoxide is 30%, and the coagulation bath temperature is -5°C. The as-spun fiber obtained has a fineness of 9Dtex, a swelling degree of 190%, and a 550 nm monochromatic light transmittance of 25%. The fiber has a swelling degree of 95% after hot water treatment at 95°C. +, the dope has a viscosity of 100 Pa-s at 60°C, extruded using a 1 k 0.12 mm spinneret, air bath draw ratio 2.1 times, coagulation bath is a mixture of dimethyl sulfoxide and water, the dimethyl sulfoxide is 30% by mass, coagulation bath temperature 12°C. The as-spun fiber has a fineness of 9 Dtex, swellability of 210%, and a 550 nm monochromatic light transmittance of 24%. After hot water treatment at 95°C, the fiber has a swellability of 130%.

[0072] Preparation of carbon fiber:

[0073] The as-spun fiber is drawn 2 times in hot water, 3.8 times in steam, pre-oxidized at 225°C, carbonized at 900-1400°C, and graphitized at 2600°C, to obtain a graphite fiber with a strength of 3.88 GPa and a Young's modulus of 455 GPa.

Claims

1. A type of nascent polyacrylonitrile fiber for carbon fiber, characterized in that, The nascent fiber has a transmittance of not less than 35% for monochromatic light at 550 nm, and the swelling degree of the nascent fiber is 160-250%, and the swelling degree after relaxation treatment in hot water at 95°C is 0.4-0.8 times the initial swelling degree; the polyacrylonitrile nascent fiber is prepared by dry-jet wet spinning process of polyacrylonitrile solution, the acrylonitrile copolymer in the polyacrylonitrile solution is obtained by copolymerization of acrylonitrile and vinyl comonomers, and the cationic content in the polyacrylonitrile solution is 0.02-0.05 mmol / g.

2. The nascent fiber according to claim 1, characterized in that, The nascent fiber has a transmittance of 40-55% for monochromatic light at 550 nm.

3. The nascent fiber according to claim 1, characterized in that, The linear density of the nascent fibers, on a dry basis, is 6–12 Dtex.

4. The nascent fiber according to claim 3, characterized in that, The linear density of the nascent fibers, on a dry basis, is 8–10 Dtex.

5. The nascent fiber according to claim 1, characterized in that, The swelling degree of the nascent fiber is 170-240%, and the swelling degree after relaxation treatment in hot water at 95°C is 0.45-0.75 times the initial swelling degree.

6. A method for preparing nascent polyacrylonitrile fibers for carbon fiber according to any one of claims 1 to 5, comprising extruding a polyacrylonitrile solution containing cationic ions through a spinneret into an air bath for stretching, and obtaining the nascent polyacrylonitrile fibers after passing through a coagulation bath.

7. The preparation method according to claim 6, characterized in that, The cation is selected from at least one of alkali metal cations and organic cationic groups; and / or, The cation content in the polyacrylonitrile solution is 0.02–0.05 mmol / g.

8. The preparation method according to claim 7, characterized in that, The cation is selected from Na. + K + NH4 + [RNH3] + [R2NH2] + [R3NH] + , where R is a straight chain of aliphatic hydrocarbons with 1 to 3 carbon atoms.

9. The preparation method according to claim 8, characterized in that, The cation is selected from NH4 + .

10. The preparation method according to claim 6, characterized in that, The zero-shear viscosity of the polyacrylonitrile solution is 20–120 Pa·s at 60°C.

11. The preparation method according to claim 6, characterized in that, The polyacrylonitrile solution comprises an acrylonitrile copolymer and a solvent.

12. The preparation method according to claim 11, characterized in that, The acrylonitrile copolymer is obtained by copolymerization of acrylonitrile and vinyl comonomers; and / or, The polyacrylonitrile solution contains 14-24% acrylonitrile copolymer by mass percentage; and / or, By weight percentage, the acrylonitrile monomer content in the acrylonitrile copolymer is not less than 97%; and / or, The solvent is selected from polar aprotic solvents.

13. The preparation method according to claim 12, characterized in that, The solvent is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide.

14. The preparation method according to claim 6, characterized in that, The solvent in the coagulation bath is a mixture of water and a polar aprotic solvent; and / or, The temperature of the coagulation bath is -15 to 5°C.

15. The preparation method according to claim 14, characterized in that, The polar aprotic solvent is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide.

16. A carbon fiber, prepared from the nascent polyacrylonitrile fiber according to any one of claims 1 to 5 or from the nascent polyacrylonitrile fiber obtained by the preparation method according to any one of claims 6 to 15.

17. The carbon fiber according to claim 16, characterized in that, The carbon fiber has a tensile strength of 5.0–5.5 GPa and a Young's modulus of 450–550 GPa.

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