A high-uniformity polyacrylonitrile primary fiber and preparation method thereof

Abstract

The present invention relates to the field of polyacrylonitrile nascent fiber preparation, and in particular to a method for preparing high-uniformity polyacrylonitrile nascent fiber, which comprises extruding, stretching, and coagulating a polyacrylonitrile stock solution; wherein the intrinsic viscosity of the polyacrylonitrile copolymer in the stock solution is 2.5 to 4.3; and the theoretical tensile deformation rate in the air bath stretching process is 1500 to 10000 s. ‑1 The proposed coagulation bath temperature of -20 to 5°C allows for a coagulation process that begins with gelation and then undergoes macroscopic phase separation, under the combined effects of stretching induction and a low-temperature coagulation bath. This overcomes the homogeneity differences that arise from differences in internal and external diffusion coefficients during diffusion-controlled curing. This invention produces highly uniform polyacrylonitrile as-spun fibers with no significant skin-core structure and an average pore size of less than 15 nm.

Description

Technical Field

[0001] The invention belongs to the field of polyacrylonitrile primary fiber preparation, and particularly relates to high-uniformity polyacrylonitrile primary fiber and a preparation method thereof. Background Art

[0002] Polyacrylonitrile-based carbon fiber precursor is an important intermediate product in the preparation of carbon fiber. The characteristics of the precursor largely determine the upper limit of carbon fiber performance. The process from raw liquid to solidification to obtain primary fiber determines the initial aggregation structure of the fiber. The subsequent processes in the spinning section must be based on this to formulate supporting processes. The structure of the primary fiber determines the initial conditions of the fiber structure evolution path. In order to obtain high-performance precursor, the primary fiber must be stretched at a high multiple in the process. Super-stretching requires the primary fiber to have a high degree of uniformity. This requirement for uniformity covers all levels, including molecular-level microstructure and nano- to micron-scale microstructure.

[0003] According to current analysis, the main defect level that restricts carbon fiber strength is still microstructural defects (Jayanan Bhatt, Structure and Properties of High-Performance Fibers, translated by Zhu Zhiguo, Ma Tao, and Wang Bin, China Textile Press). The method of obtaining primary fibers with uniform microstructures requires continued research. Summary of the Invention

[0004] To address the above technical problems, the present invention provides a highly uniform polyacrylonitrile spun fiber and a method for preparing the same. The present invention utilizes a spinning solution, which is sprayed into a coagulation bath via a dry-jet wet spinning process, for coagulation to produce the spun fiber. The spinning solution is selected to have a relatively high molecular weight and low solids content, and the thin stream of the solution undergoes substantial deformation at a high stretching rate in an air section before entering the low-temperature coagulation bath. The resulting spun fiber exhibits a highly uniform structure.

[0005] One of the purposes of the present invention is to provide a method for preparing high-uniformity polyacrylonitrile spun fibers, comprising the steps of extruding a spinning solution through a spinneret into an air bath for drawing, and then passing through a coagulation bath to obtain the high-uniformity polyacrylonitrile spun fibers, wherein the tensile deformation rate of the air bath drawing is 1500 to 10000 s -1 .

[0006] Wherein, the spinning solution comprises polyacrylonitrile copolymer and solvent;

[0007] The polyacrylonitrile copolymer is obtained by copolymerizing acrylonitrile with at least one of itaconic acid, acrylic acid, methyl acrylate, methacrylic acid, acrylamide, and styrene sulfonamide, and is preferably selected from an acrylonitrile-itaconic acid copolymer system (IA-AN copolymer system);

[0008] The intrinsic viscosity of the polyacrylonitrile copolymer is 2.5 to 4.3, preferably 2.8 to 3.8. For polymers, the larger the molecular weight of the polymer, the greater the intrinsic viscosity. The polyacrylonitrile copolymer used in the present invention has an intrinsic viscosity of 2.5 to 4.3, which is a high molecular weight polyacrylonitrile copolymer. When the molecular weight of the copolymer is increased, the rheological properties of the spinning solution are changed, and gelation is easily induced under large deformation at high stretching rates. The nascent fibers obtained after phase separation have fewer defects.

[0009] Calculated by mass percentage, the acrylonitrile monomer content in the polyacrylonitrile copolymer is not less than 97%;

[0010] The solvent of the spinning solution is selected from polar aprotic solvents, preferably at least one selected from dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0011] In the above spinning solution, the content of the polyacrylonitrile copolymer is 5-20% by mass, preferably 6-15% by mass.

[0012] In the above preparation method, the spinneret aperture, spinneret hole distribution and extrusion process conditions are selected according to the rheological properties of the stock solution. Preferably, the spinneret aperture is 0.075-0.3 mm, and the spinning stock solution extrusion temperature is 25-90°C.

[0013] In the above preparation method, the air bath can be completed on an air bath device commonly used in the art. Preferably, the air bath draft ratio is 1.8 to 3.2;

[0014] The tensile deformation rate of the air bath drawing is 2000~8000s -1 , preferably 2400~5500s -1 ;

[0015] The air bath height is 2 to 10 mm, preferably 2 to 5 mm.

[0016] The calculation formula for the air bath tensile deformation rate is as follows:

[0017] Air bath tensile deformation rate = (curling speed V2-average extrusion speed V1) / air bath height H

[0018] Wherein, V1 is the average extrusion speed of spinning solution, unit is m / s;

[0019] V2 is the curling speed, in m / s;

[0020] H is the air bath height, in m.

[0021] In the above preparation method, the solvent of the coagulation bath is a mixed solvent of water and a polar aprotic solvent; the water content of the coagulation bath solvent is 25-85% by mass, preferably 30-75% by mass; the polar aprotic solvent in the coagulation bath solvent is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide;

[0022] The coagulation bath temperature is -20 to 5°C.

[0023] The present invention adopts a polyacrylonitrile spinning solution extrusion, stretching and coagulation method, the intrinsic viscosity of the copolymer in the spinning solution is 2.5-4.3, and the theoretical stretching deformation rate during the stretching process is 1500s -1 ~10,000s -1 The coagulation bath temperature is -20 to 5°C. The polyacrylonitrile spinning solution with an intrinsic viscosity of 2.5 to 4.3 is subjected to a coagulation process of gelation followed by macroscopic phase separation under the combined effects of stretching induction and a low-temperature coagulation bath. This overcomes the homogeneity differences caused by the different internal and external diffusion coefficients in the diffusion-controlled solidification process. The resulting polyacrylonitrile primary fibers are highly uniform in microstructure and lack a significant skin-core structure. The resulting primary fibers are uniform in size and have fewer defects.

[0024] The second object of the present invention is to provide a highly uniform polyacrylonitrile spun fiber prepared by the above-mentioned preparation method. The average pore size of the obtained polyacrylonitrile spun fiber is less than 15 nm, preferably less than 10 nm.

[0025] The present invention utilizes a dry-wet spinning process to extrude a spinning solution through a spinneret into an air bath for stretching. The extruded stream is then drawn through the air bath and then enters a coagulation bath for coagulation to produce nascent fibers. A relatively high molecular weight, low-solids spinning solution is selected, and the stream undergoes substantial deformation at a high stretching rate in the air section before entering the low-temperature coagulation bath. For polymers, a higher molecular weight results in a higher intrinsic viscosity. The present invention utilizes a high-molecular-weight polyacrylonitrile copolymer with an intrinsic viscosity of 2.5 to 4.3. This high-molecular-weight polyacrylonitrile copolymer changes the rheological properties of the spinning solution as the molecular weight of the copolymer increases, making it easier to induce gelation under high stretching rates and substantial deformation. The resulting nascent fibers, after phase separation, have fewer defects.

[0026] The present invention uses a high-molecular-weight copolymer solution as the spinning solution. Under the induction of large strain and high tensile deformation rate, the macromolecules produce entropic elastic orientation in the air segment. In this state, a thin stream enters the spinning coagulation bath. Under the dual effects of tensile induction and low-temperature coagulation bath, the coagulation process is first gelated and then macrophase separated. This overcomes the homogeneity difference caused by the difference in internal and external diffusion coefficients during the diffusion-controlled solidification process, resulting in a fine-grid gel structure. During further diffusion phase separation, the size of the fibrils is controlled below the gel grid, thereby obtaining highly uniform primary fibers at the microscale. The technical solution provided by the present invention produces a primary fiber with an average pore size of less than 15nm, and no obvious skin-core structure is observed under an electron microscope.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The present invention uses a polyacrylonitrile copolymer with an intrinsic viscosity of 2.5 to 4.3, which is a relatively high molecular weight polyacrylonitrile copolymer. The stock solution begins to solidify after a large deformation at a high tensile rate, and the resulting nascent fibers have fewer defects.

[0029] 2. The main control parameter in the present invention is the tensile deformation rate during air bath drawing. Process parameters such as the crimping rate and average extrusion speed can be selected under the condition of ensuring the tensile deformation rate. Therefore, there will be no significant restrictions on the selection of process conditions. It can be implemented in general industrial equipment and has great value for promotion and application.

[0030] 3. The polyacrylonitrile primary fiber prepared by the preparation method provided by the present invention has a highly uniform microstructure, few defects, and no obvious skin-core structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The scanning electron microscope image of the cross section of the as-spun fiber prepared in Example 3 of the present invention is Figure 1 It can be seen that the cross section of the as-spun fiber prepared by the present invention has almost no obvious macroporous defects and the structure is highly uniform. DETAILED DESCRIPTION

[0032] The present invention provides a high-uniformity polyacrylonitrile spun fiber and a preparation method thereof. The high-uniformity polyacrylonitrile spun fiber is obtained by extruding a spinning solution through a spinneret into an air bath for drawing and then passing through a coagulation bath.

[0033] Among them, the air bath draft ratio is 1.8 to 3.2;

[0034] Air bath tensile deformation rate is 1500~10000s -1 , preferably 2000~8000s -1 , more preferably 2400~5500s -1 ;

[0035] The air bath height is 2 to 10 mm, preferably 2 to 5 mm.

[0036] The calculation formula of air bath tensile deformation rate is as follows:

[0037] Air bath tensile deformation rate = (V2-V1) / H

[0038] Wherein, V1 is the average extrusion speed of spinning solution, unit is m / s;

[0039] V2 is the curling speed, in m / s;

[0040] H is the air bath height, in m.

[0041] In the above preparation method, the spinning solution includes polyacrylonitrile copolymer and solvent;

[0042] The content of polyacrylonitrile copolymer in the spinning solution is 5-20%, preferably 6-15%;

[0043] The polyacrylonitrile copolymer is obtained by copolymerizing acrylonitrile with at least one of itaconic acid, acrylic acid, methyl acrylate, methacrylic acid, acrylamide, and styrene sulfonamide, preferably an acrylonitrile-itaconic acid copolymer system;

[0044] The content of acrylonitrile monomer in the polyacrylonitrile copolymer is not less than 97%;

[0045] The intrinsic viscosity of the polyacrylonitrile copolymer is 2.5 to 4.3, preferably 2.8 to 3.8;

[0046] The solvent of the spinning solution is selected from polar aprotic solvents, preferably at least one selected from dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0047] In the above preparation method, the solvent of the coagulation bath is a mixed solvent of water and a polar aprotic solvent;

[0048] Calculated by mass percentage, the water content in the coagulation bath solvent is 25 to 85%, preferably 30 to 75%;

[0049] The polar aprotic solvent in the coagulation bath solvent is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide;

[0050] The coagulation bath temperature is -20 to 5°C.

[0051] The present invention will be described in detail below with reference to specific embodiments. It is necessary to point out that the following embodiments are only used to further illustrate the present invention and are not to be construed as limiting the scope of protection of the present invention. Non-essential improvements and adjustments made by those skilled in the art to the present invention based on the contents of the present invention still fall within the scope of protection of the present invention.

[0052] The test method adopted in the embodiment is as follows:

[0053] Copolymer intrinsic viscosity test method:

[0054] Refer to "Carbon Fiber and Graphite Fiber" (edited by He Fu)

[0055] Average pore size test method:

[0056] BET N2 adsorption method

[0057] Primary fiber cross section:

[0058] Scanning electron microscopy observation

[0059] The sources of raw materials used in the embodiments and comparative examples are as follows:

[0060] Polyacrylonitrile copolymer, dimethyl sulfoxide, acrylonitrile, and itaconic acid are all commercially available products.

[0061] Example 1

[0062] A 6% by mass polyacrylonitrile copolymer dimethyl sulfoxide solution was used. The copolymer monomers in the copolymer were acrylonitrile and itaconic acid, with acrylonitrile mass fraction of 98% and an intrinsic viscosity of 3.8. The stock solution was extruded from a 0.18mm aperture spinneret at 65°C, with the air bath height controlled at 5mm. The spinning solution was sprayed into the coagulation bath at a speed of 8m / min, with an air bath stretch ratio of 2.5 times and a controlled theoretical tensile deformation rate of 2400. -1 s. The coagulation bath consisted of a 25% by mass aqueous solution of dimethyl sulfoxide at a temperature of -5°C. After coagulation of the spinning solution, spun fibers were obtained. BET N₂ adsorption analysis of freeze-dried spun fibers revealed an average pore size of 8.2 nm, with pores larger than 40 nm accounting for less than 5% of the total fiber volume. Electron microscopy of the spun fiber cross-section revealed few obvious macroporous defects.

[0063] Example 2

[0064] A 12% by mass polyacrylonitrile copolymer dimethyl sulfoxide solution was used. The copolymer monomers in the copolymer were acrylonitrile and itaconic acid, with acrylonitrile mass fraction of 98% and an intrinsic viscosity of 3.0. The stock solution was extruded from a 0.15mm aperture spinneret at 80°C, with the air bath height controlled at 2mm. The spinning solution was sprayed into the coagulation bath at a speed of 6m / min, with an air bath stretch ratio of 2x and a controlled theoretical tensile deformation rate of 3000. -1 The coagulation bath consisted of a 25% by mass aqueous solution of dimethyl sulfoxide at a temperature of -5°C. After coagulation of the spinning solution, spun fibers were obtained. After freeze-drying, the spun fibers, as measured by BET N₂ adsorption, had an average pore size of 9.8 nm. Pores larger than 40 nm accounted for less than 5% of the total fiber volume. Electron microscopy observation of the spun fiber cross-section revealed few obvious macroporous defects.

[0065] Example 3

[0066] A 15% by mass polyacrylonitrile copolymer dimethyl sulfoxide solution was used. The copolymer monomers in the copolymer were acrylonitrile and itaconic acid, with acrylonitrile mass fraction of 98% and an intrinsic viscosity of 2.8. The stock solution was extruded from a 0.12 mm aperture spinneret at 60°C, with the air bath height controlled at 2 mm. The spinning solution was sprayed into the coagulation bath at a speed of 7 m / min, with an air bath stretch ratio of 2.5 times and a controlled theoretical tensile deformation rate of 5250 -1 The coagulation bath consisted of a 25% by mass aqueous solution of dimethyl sulfoxide at a temperature of -10°C. After coagulation of the spinning solution, nascent fibers were obtained. After freeze-drying, the nascent fibers, as measured by the BET N2 adsorption method, had an average pore diameter of 8.4 nm. Pores larger than 40 nm accounted for less than 5% of the total fiber volume. Electron microscopy observation of the nascent fiber cross-section revealed few obvious macroporous defects.

[0067] Comparative Example 1

[0068] A 22% by mass polyacrylonitrile copolymer dimethyl sulfoxide solution was used. The copolymer monomers in the copolymer were acrylonitrile and itaconic acid, with a 97% by mass fraction of acrylonitrile and a copolymer intrinsic viscosity of 1.9. The stock solution was extruded from a 0.12mm aperture spinneret at 45°C, with the air bath height controlled at 8mm. The spinning solution was sprayed into the coagulation bath at a speed of 8m / min, with an air bath stretch ratio of 2x, and a control theoretical tensile deformation rate of 1000. -1 s. The coagulation bath consisted of a 25% by mass aqueous solution of dimethyl sulfoxide at a temperature of 5°C. After coagulation of the spinning solution, spun fibers were obtained. After freeze-drying, the spun fibers, as measured by the BET N2 adsorption method, had an average pore diameter of 13.7 nm. Pores larger than 40 nm accounted for 15.7% of the total volume. Electron microscopic observation of the cross-section of the spun fibers revealed the presence of micropores.

[0069] Comparative Example 2

[0070] A 22% by mass polyacrylonitrile copolymer dimethyl sulfoxide solution was used. The copolymer monomers in the copolymer were acrylonitrile and itaconic acid, with a 97% by mass fraction of acrylonitrile and a copolymer intrinsic viscosity of 1.9. The stock solution was extruded from a 0.12mm aperture spinneret at 25°C, with the air bath height controlled at 2mm. The spinning solution was sprayed into the coagulation bath at a speed of 12m / min, with an air bath stretch ratio of 3x and a controlled theoretical tensile deformation rate of 12,000. -1 During the spinning process, there are many broken ends and it is impossible to obtain highly uniform nascent fibers.

[0071] Comparative Example 3

[0072] A 14% by mass polyacrylonitrile copolymer dimethyl sulfoxide solution was used. The copolymer monomers in the copolymer were acrylonitrile and itaconic acid, with a mass fraction of acrylonitrile of 98.5% and an intrinsic viscosity of 3.9. The stock solution was extruded from a 0.15mm aperture spinneret at 40°C, with the air bath height controlled at 15mm. The spinning solution was sprayed into the coagulation bath at a speed of 12m / min, and the air bath stretching ratio was 2.4 times. After coagulation, the spinning solution produced nascent fibers. After freeze-drying the nascent fibers, the average pore size was measured by BET N2 adsorption to be 27nm. The volume of pores larger than 40nm accounted for 37% of the total pore volume. Electron microscopy observation revealed the presence of micropores in the cross-section of the nascent fibers.

[0073] Comparative Example 4

[0074] A 12% by mass polyacrylonitrile copolymer dimethyl sulfoxide solution was used. The copolymer monomers in the copolymer were acrylonitrile and itaconic acid, with acrylonitrile accounting for 98.5% by mass and a copolymer intrinsic viscosity of 4.0. The stock solution was extruded from a 0.15 mm aperture spinneret at 70°C, with the air bath height controlled at 6 mm. The spinning solution was sprayed into the coagulation bath at a speed of 25 m / min, and the air bath stretching ratio was 1.5 times. After coagulation, the spinning solution was obtained as a nascent fiber. After freeze-drying the nascent fiber, the average pore size was measured by BET N2 adsorption method to be 32.2 nm, and the volume of pores larger than 40 nm accounted for 26.7% of the total pore volume. Electron microscopy observation revealed the presence of micropores in the cross-section of the nascent fiber.

[0075] It can be seen from the above examples and comparative examples that the present invention uses a polyacrylonitrile copolymer with a higher specific viscosity and performs spinning at a set air bath tensile deformation rate to obtain an average pore size of the as-spun fiber less than 10 nm, and the fiber is highly uniform in microstructure.

Claims

1. A method for preparing high-uniformity polyacrylonitrile spun fibers, comprising: extruding a spinning solution through a spinneret into an air bath for drawing, and then passing through a coagulation bath to obtain the high-uniformity polyacrylonitrile spun fibers, wherein: The intrinsic viscosity of the polyacrylonitrile copolymer is 2.5 to 4.3; the content of the polyacrylonitrile copolymer in the spinning solution is 5 to 20% by mass; the tensile deformation rate of the air bath drawing is 1500 to 10000 s -1 , air bath tensile deformation rate = (curling speed V2-average extrusion speed V1) / air bath height H.

2. The preparation method according to claim 1, characterized in that The spinning solution comprises polyacrylonitrile copolymer and solvent.

3. The preparation method according to claim 2, characterized in that The polyacrylonitrile copolymer is obtained by copolymerizing acrylonitrile with at least one of itaconic acid, acrylic acid, methyl acrylate, methacrylic acid, acrylamide, and styrene sulfonamide; and / or, The content of acrylonitrile monomer in the polyacrylonitrile copolymer is not less than 97% by mass; and / or The solvent of the spinning solution is selected from polar aprotic solvents.

4. The preparation method according to claim 3, characterized in that The polyacrylonitrile copolymer is selected from acrylonitrile-itaconic acid copolymer system; and / or, The intrinsic viscosity of the polyacrylonitrile copolymer is 2.8 to 3.8; and / or, The solvent in the spinning solution is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide and N,N-dimethylacetamide.

5. The preparation method according to claim 2, characterized in that Calculated by mass percentage, the content of polyacrylonitrile copolymer in the spinning solution is 6-15%.

6. The preparation method according to claim 1, characterized in that The spinneret aperture is 0.075 to 0.3 mm; and / or, The extrusion temperature of the spinning solution is 25-90°C.

7. The preparation method according to claim 1, characterized in that The air bath draft ratio is 1.8 to 3.2; and / or, The tensile deformation rate of the air bath drawing is 2000~8000s -1 and / or, The air bath height is 2 to 10 mm.

8. The preparation method according to claim 7, characterized in that The tensile deformation rate of the air bath drawing is 2400~5500s -1 and / or, The air bath height is 2 to 5 mm.

9. The preparation method according to claim 1, characterized in that The solvent of the coagulation bath is a mixed solvent of water and a polar aprotic solvent; and / or, The temperature of the coagulation bath is -20 to 5°C.

10. The preparation method according to claim 9, characterized in that Calculated by mass percentage, the water content in the solvent of the coagulation bath is 25 to 85%.

11. The preparation method according to claim 10, characterized in that: Calculated by mass percentage, the water content in the solvent of the coagulation bath is 30-75%.

12. The preparation method according to claim 9, characterized in that The polar aprotic solvent in the coagulation bath solvent is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide.

13. A high-uniformity polyacrylonitrile primary fiber prepared by the preparation method according to any one of claims 1 to 12.

14. The polyacrylonitrile primary fiber according to claim 13, characterized in that: The average pore size of the polyacrylonitrile primary fiber is less than 15nm.

15. The polyacrylonitrile primary fiber according to claim 14, characterized in that: The average pore size of the polyacrylonitrile primary fiber is less than 10 nm.

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