Polyimide fiber as well as preparation method and application thereof

Abstract

The invention discloses a flat polyimide fiber as well as a preparation method and application thereof, and belongs to the technical field of polymer fibers. The polyimide fiber has rectangular-like, elliptical, kidney-shaped / dumbbell-shaped or other flat cut surfaces, the flatness ratio is 2-20, the fiber bundle silk thread density is 50-5000 dtex, the monofilament tensile strength is 3-40 cN / dtex, and the initial modulus is 50-1800 cN / dtex. The preparation method comprises the following steps: carrying out polycondensation on aromatic dianhydride and a diamine monomer to generate a polyamide acid solution, carrying out wet-process or dry-jet wet-process spinning by adopting a slit-type spinneret orifice, carrying out three-stage gradient coagulating bath curing forming, carrying out forced shape preservation by combining with an S-shaped path guide roller, and carrying out hot stretching and gradient imidization treatment. The problems that in the prior art, flat fibers are difficult in shape preserving, poor in structural compactness and low in industrial adaptability are solved, and the product has wide application prospects in the fields of high-temperature filtering materials, composite materials, flexible electronic base materials and the like.

Description

Technical Field

[0001] This invention relates to the field of high-performance fiber technology, and in particular to a polyimide fiber, its preparation method, and its applications. Background Technology

[0002] Polyimide fibers, due to their excellent high-temperature resistance, mechanical properties, and chemical corrosion resistance, have significant application value in fields such as high-temperature filtration, composite material reinforcement, and electronic insulation. With the expansion of application scenarios, traditional circular cross-section polyimide fibers can no longer meet high-end demands in terms of specific surface area and interfacial bonding strength, making flat polyimide fibers a research hotspot.

[0003] Existing technologies already contain some research on polyimide fibers with irregular cross-sections. Chinese patent CN201010572496.1 discloses a method for preparing polyimide fibers by adjusting the composition of the spinning solution, coagulation conditions, and drawing parameters to control the cross-sectional shape. However, it uses a single-stage coagulation bath, lacks a clear gradient coagulation design, and the formation of a flat structure depends on the coordination of multiple parameters, resulting in poor stability and the absence of a dedicated shape-preserving mechanism. Chinese patent CN201210009233.0 uses a circular spinneret and adjusts the coagulation bath parameters to allow the fiber to naturally form a kidney-shaped cross-section. However, its controllable range for cross-sectional flatness is narrow (only about 1.3 times), and it lacks a forced shape-preserving mechanism, making it difficult to guarantee consistency in industrial production.

[0004] In summary, existing technologies have the following shortcomings: 1) Flat structure formation relies on natural diffusion or complex templates, resulting in poor shape retention and a narrow controllable range for flatness ratio; 2) The coagulation bath design is simplistic and lacks gradient control, leading to insufficient fiber density; 3) There is a lack of specialized forced shape retention methods, resulting in poor product consistency in industrial production; 4) Some technical routes have low production capacity and high costs, making them difficult to adapt to large-scale production needs. Therefore, developing a shape-stable, structurally dense, and industrially adaptable flat polyimide fiber preparation technology is of great significance. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the present invention aims to solve the technical problems of difficulty in maintaining shape, poor structural density and low industrial adaptability in the preparation of existing flat polyimide fibers, and to provide a polyimide fiber with controllable flatness, excellent mechanical properties and suitable for large-scale production and its preparation method.

[0006] To achieve the above objectives, the present invention provides a polyimide fiber having a rectangular, elliptical, kidney-shaped, dumbbell-shaped or other flat cross-section, with an asymmetry ratio (ratio of major axis to minor axis) of 2 to 20; a fiber bundle linear density of 50 to 5000 dtex; a single filament tensile strength of 3 to 40 cN / dtex; and an initial modulus of 50 to 1800 cN / dtex.

[0007] It should be noted that the fiber having a rectangular cross-section means that the fiber cross-section is close to a rectangle, but the corners are not absolutely right angles (may be slightly rounded due to spinning process, etc.) or the side length has a slight deviation (such as ±5%).

[0008] Preferably, the polyimide fiber has a rectangular cross-section. The flatness ratio is preferably 5 to 15, for example 6, 7, 8, 9, 10, 11, 12, 13, or 14.

[0009] When the polyimide fiber has a rectangular cross-section, it utilizes the "high specific surface area, orientation, and mechanical interlocking effect" of the flat structure, and avoids the problems of "adhesion, stress concentration, and processing difficulties" that may exist in other flat shapes through the "regularity of the rectangle and the micro-rounded corner design". It is especially suitable for scenarios with comprehensive requirements for "mechanical properties, interface function, and textile processability" (such as high-end textile fabrics, high-efficiency filter materials, and composite material reinforcement phases).

[0010] The present invention also provides a method for preparing the polyimide fiber, which includes the following steps: 1) Preparation of polyamic acid solution: Aromatic dianhydride monomer and aromatic diamine monomer are polycondensed in an aprotic polar solvent to generate polyamic acid solution; 2) Spinning and forming: Wet spinning or dry-spinning is carried out using a spinneret with narrow slit spinnerets. The polyamic acid solution is extruded and then solidified in a multi-stage coagulation bath. 3) Post-treatment: The nascent fibers are subjected to hot stretching and gradient imidization to obtain the finished polyimide fiber.

[0011] Preferably, the major axis length of the slit-type spinneret is 0.05~0.5mm, the minor axis length is 0.02~0.1mm, and the number of spinnerets is 10~5000.

[0012] Preferably, the multi-stage coagulation bath includes a first coagulation bath, a second coagulation bath, and a third coagulation bath; the first coagulation bath is a mixture of water and solvent, with a solvent volume ratio of 5% to 20% and a temperature of 10℃ to 40℃; the second coagulation bath is a mixture of water and solvent, with a solvent volume ratio of 1% to 5% and a temperature of 5℃ to 50℃; and the third coagulation bath is pure water with a temperature of 10℃ to 60℃.

[0013] Preferably, the coagulation bath is provided with staggered guide rollers to make the fiber spinning process follow an "S" shaped path.

[0014] Specifically, the preparation method of the present invention includes the following steps: 1) Preparation of polyamic acid solution: Aromatic dianhydride monomers and aromatic diamine monomers are polycondensed in an aprotic polar solvent (such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, etc.) at -5℃~30℃ for 5~20 hours to generate a polyamic acid solution with a solid content of 12%~30%; 2) Spinning and forming: Wet spinning or dry-spinning wet spinning is carried out using a spinneret with slit-type spinnerets. The spinneret has a long axis of 0.05~0.5mm, a short axis of 0.02~0.1mm, and 10~5000 holes. The extruded spinning solution enters a multi-stage coagulation bath. The coagulation bath is equipped with staggered guide rollers to make the fiber travel in an "S" shape. 3) Post-treatment: The nascent fibers are washed with water to remove the solvent, then dried at 100~240℃, and then subjected to gradient imidization treatment at 200~600℃, while being stretched by 1.5~3.0 times to finally obtain the polyimide fiber product.

[0015] The core of the multi-stage coagulation bath design lies in the following: the first coagulation bath (solvent volume ratio 5%~20%, 10℃~40℃) achieves rapid coagulation and maintains a flat structure; the second coagulation bath (solvent volume ratio 1%~5%, 5℃~50℃) promotes double diffusion; and the third coagulation bath (pure water, 10℃~60℃) completely removes the solvent and ensures a dense fiber structure.

[0016] This invention further provides the application of the polyimide fibers in high-temperature filtration materials, composite materials, or flexible electronic substrates. The polyimide fibers of this invention can be further processed into various products, such as polyimide staple fibers comprising the polyimide fibers of this invention, with a fiber length of 10 mm to 100 mm and a crimp count of 2 to 50 crimps / inch; fabrics comprising the polyimide fibers of this invention; and prepregs comprising the polyimide fibers or fabrics of this invention, with a fiber or fabric volume content of 30% to 80%.

[0017] The present invention can achieve the following beneficial effects: 1) By combining the slit-type spinneret with the "S"-shaped guide roller for forced shape retention, the flatness ratio can be controlled within a wide range (2~20), resulting in good product consistency; 2) The three-stage gradient coagulation bath design enables step-by-step control of "rapid shape preservation - full diffusion - complete desolvation", resulting in a dense fiber structure and excellent mechanical properties; 3) Based on the improvement of traditional wet spinning process, it does not require complex templates or special equipment, has low equipment modification costs, and is suitable for large-scale industrial production; 4) The flat structure significantly increases the specific surface area of ​​the fiber, enhances the interfacial bonding force in composite materials (the flexural modulus can be increased by more than 20%), and significantly optimizes the filtration efficiency and dust removal effect in high-temperature filtration. Attached Figure Description

[0018] Figure 1 The image shows an electron microscope image of the polyimide fiber prepared in Example 1 of the present invention. The cross-section of the polyimide fiber in the image is rectangular. Detailed Implementation

[0019] <fiber> The fiber described in this invention is a polyimide fiber. As used herein, the term polyimide filament refers to a filament made of a polyimide polymer.

[0020] In some embodiments, the fiber is in the form of a continuous filament. For the purposes of this document, the term "filament" is defined as a relatively flexible, macroscopically uniform body having a high aspect ratio in a cross-section perpendicular to its length. The cross-section of a filament can be of any shape, but is typically circular. A multifilament wound onto a spool in a package comprises multiple continuous filaments. In the context of this invention, the terms filament and fiber are used interchangeably.

[0021] Other suitable forms of fiber materials are short fibers, woven fabrics, non-woven fabrics, or powders—terms well-known in the field of textile fibers.

[0022] The implementation schemes similar to those described above are applicable to filaments, staple fibers, woven fabrics, nonwoven fabrics, or powders.

[0023] <Composite Materials> The polyimide fibers of this invention can be combined with a matrix resin to form fiber-reinforced resin composites. Suitable fiber forms include continuous filaments, staple fibers, woven fabrics, nonwoven fabrics, or powders. The resin can be a thermosetting resin or a thermoplastic resin. Typically, the matrix resin accounts for 20% to 50% by weight of the fiber plus resin in the composite. Suitable thermosetting resins include epoxy resins, phenolic resins, epoxy-thermoplastic phenolic resins, cyanate esters, unsaturated esters, melamine, and maleimide. The fibers can be treated by any of the methods described above.

[0024] The implementation schemes described above are applicable to fibers in filament, staple fiber, woven fabric, nonwoven fabric, or powder form.

[0025] The polyimide fibers of this invention can be combined with a matrix resin to form fiber-reinforced resin composites. Suitable fiber forms include continuous filaments, short fibers, woven fabrics, nonwoven fabrics, or powders. The resin can be a thermosetting resin or a thermoplastic resin. Typically, the matrix resin accounts for 20% to 50% by weight of the fiber plus resin in the composite. Suitable thermosetting resins include epoxy resins, phenolic resins, epoxy-thermoplastic phenolic resins, cyanate esters, unsaturated esters, melamine, and maleimide, etc.

[0026] In this invention, nascent fiber refers to fiber formed by the solidification of polymer streams extruded from the spinneret in the spinning field, which is fiber that has not yet undergone heat treatment; finished fiber is fiber obtained by heat treatment of nascent fiber.

[0027] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments. The polyimide fibers used in the embodiments were provided by Jiangsu Xiannuo New Material Technology Co., Ltd. Unless otherwise specified, conditions in the embodiments were performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products. Percentages, unless otherwise specified, refer to weight percentages.

[0028] The fiber performance testing methods and conditions in the following examples and comparative examples are as follows (unless otherwise stated, conventional testing methods and conditions in the art are used): Flatness determination: The fiber cross-section was characterized using a scanning electron microscope (SEM), and the flatness was calculated by the ratio of the long axis to the short axis.

[0029] Linear density of filament bundles: The linear density of filament bundles is tested in accordance with GB / T14343 "Test Method for Linear Density of Chemical Fiber Filaments".

[0030] Monofilament mechanical properties: The mechanical properties of a single fiber are tested according to GB / T14337 "Test Method for Tensile Properties of Chemical Fibers (Short Fibers)" for dry breaking strength and initial modulus.

[0031] Example 1 1) Preparation of polyamic acid solution: Dissolve 0.5 mol of p-phenylenediamine in 1000 mL of N,N-dimethylacetamide, add 0.5 mol of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, and react at 25 °C for 12 hours to obtain a polyamic acid solution with a solid content of 15%; 2) Spinning and forming: Wet spinning is performed using a slit spinneret (long axis 0.2mm, short axis 0.02mm, 100 holes); the first coagulation bath is N,N-dimethylacetamide / water (volume ratio 10%), temperature 25℃; the second coagulation bath is N,N-dimethylacetamide / water (volume ratio 3%), temperature 30℃; the third coagulation bath is pure water, temperature 40℃; three sets of staggered guide rollers are set in the coagulation bath to form an "S" shaped path; 3) Post-treatment: After washing the nascent fibers with water, they are dried at 120℃ for 2 hours, and then imidized in a gradient of 200℃→350℃→500℃ (holding temperature for 1 hour at each stage). They are then hot-stretched 1.5 times at 350℃ to obtain polyimide fibers.

[0032] Product performance: Figure 1 Here is an electron micrograph of the polyimide fiber prepared in Example 1, as shown. Figure 1 As shown, the cut surface is rectangular with an flatness ratio of 8.5; the fiber bundle linear density is 220 dtex; the tensile strength of a single filament is 18 cN / dtex; and the initial modulus is 850 cN / dtex.

[0033] Example 2 1) Preparation of polyamic acid solution: 0.4 mol of 4,4'-diaminodiphenyl ether and 0.1 mol of p-phenylenediamine were dissolved in 1000 mL of N-methylpyrrolidone, and 0.5 mol of pyromellitic dianhydride was added. The mixture was reacted at 15 °C for 15 hours to obtain a polyamic acid solution with a solid content of 12%. 2) Spinning and forming: Dry-jet wet spinning is performed using a slit spinneret (long axis 0.3mm, short axis 0.03mm, 500 holes); the first coagulation bath is N-methylpyrrolidone / water (volume ratio 15%), temperature 20℃; the second coagulation bath is N-methylpyrrolidone / water (volume ratio 2%), temperature 25℃; the third coagulation bath is pure water, temperature 35℃; 4 sets of staggered guide rollers are set in the coagulation bath; 3) Post-treatment: After washing the nascent fibers with water, they are dried at 100℃ for 3 hours, then imidized at a gradient of 250℃→400℃→480℃ (each stage is held for 1 hour), and then hot-stretched 2.0 times at 300℃ to obtain polyimide fibers.

[0034] Product performance: The cut surface is elliptical with a flatness ratio of 9.2; the fiber bundle linear density is 550 dtex; the tensile strength of a single filament is 5 cN / dtex; and the initial modulus is 70 cN / dtex.

[0035] Example 3 1) Preparation of polyamic acid solution: Dissolve 0.5 mol of p-phenylenediamine in 1000 mL of dimethyl sulfoxide, add 0.5 mol of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and react at 20 °C for 10 hours to obtain a polyamic acid solution with a solid content of 18%. 2) Spinning and forming: Wet spinning is performed using a slit spinneret (long axis 0.5mm, short axis 0.06mm, 200 holes); the first coagulation bath is dimethyl sulfoxide / water (8% by volume) at 30℃; the second coagulation bath is dimethyl sulfoxide / water (4% by volume) at 40℃; the third coagulation bath is pure water at 50℃; three sets of staggered guide rollers are set in the coagulation bath; 3) Post-treatment: After washing with water, the nascent fibers are dried at 150℃ for 2 hours, then imidized at a gradient of 220℃→380℃→480℃ (each stage is kept at the temperature for 1 hour), and then hot-stretched 2.2 times at 320℃ to obtain polyimide fibers.

[0036] Product performance: The cut surface is kidney-shaped with a flatness ratio of 6.7; the fiber bundle linear density is 1100 dtex; the single filament tensile strength is 8 cN / dtex; and the initial modulus is 280 cN / dtex.

[0037] Example 4 1) Preparation of polyamic acid solution: 0.3 mol of 4,4'-diaminodiphenyl ether and 0.2 mol of p-phenylenediamine were dissolved in 1000 mL of N,N-dimethylformamide, and 0.5 mol of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride was added. The mixture was reacted at 10 °C for 18 hours to obtain a polyamic acid solution with a solid content of 18%. 2) Spinning and forming: Dry-jet wet spinning is performed using a slit spinneret (long axis 0.3mm, short axis 0.02mm, 1000 holes); the first coagulation bath is N,N-dimethylformamide / water (volume ratio 20%), temperature 15℃; the second coagulation bath is N,N-dimethylformamide / water (volume ratio 5%), temperature 10℃; the third coagulation bath is pure water, temperature 20℃; 5 sets of staggered guide rollers are set in the coagulation bath; 3) Post-treatment: After washing the nascent fibers with water, they are dried at 80℃ for 4 hours, then imidized in a gradient of 200℃→350℃→480℃ (each stage is kept at the temperature for 1 hour), and then hot-stretched 1.5 times at 300℃ to obtain polyimide fibers.

[0038] Product performance: The cut surface is dumbbell-shaped with a flatness ratio of 13.2; the fiber bundle linear density is 2200 dtex; the single filament tensile strength is 24 cN / dtex; and the initial modulus is 650 cN / dtex.

[0039] Example 5 1) Preparation of polyamic acid solution: Dissolve 0.5 mol of 4,4'-diaminodiphenyl ether in 1000 mL of N,N-dimethylacetamide, add 0.5 mol of pyromellitic dianhydride, and react at 25 °C for 14 hours to obtain a polyamic acid solution with a solid content of 20%. 2) Spinning and forming: Wet spinning is performed using a slit spinneret (long axis 0.4mm, short axis 0.1mm, 300 holes); the first coagulation bath is N,N-dimethylacetamide / water (volume ratio 12%), temperature 35℃; the second coagulation bath is N,N-dimethylacetamide / water (volume ratio 1%), temperature 50℃; the third coagulation bath is pure water, temperature 60℃; four sets of staggered guide rollers are set in the coagulation bath; 3) Post-treatment: After washing the nascent fibers with water, they are dried at 180℃ for 1.5 hours, then imidized at a gradient of 280℃→420℃→460℃ (each stage is kept at the temperature for 1 hour), and then hot-stretched 3 times at 400℃ to obtain polyimide fibers.

[0040] Product performance: The cut surface is rectangular with an flatness ratio of 3.3; the fiber bundle linear density is 660 dtex; the tensile strength of a single filament is 7 cN / dtex; and the initial modulus is 80 cN / dtex.

[0041] Comparative Example 1 (Refer to CN201010572496.1) 1) Preparation of polyamic acid solution: Same as in Example 1; 2) Spinning and forming: Use circular spinnerets (0.1mm diameter, 100 holes), wet spinning, single-stage coagulation bath (N,N-dimethylacetamide / water volume ratio 40%, temperature 25℃), without "S" shaped guide rollers; 3) Post-processing: Same as in Example 1.

[0042] Product performance: The cut surface is nearly circular with a flatness ratio of 1.2; the fiber bundle linear density is 220 dtex; the tensile strength of a single filament is 12 cN / dtex; the initial modulus is 620 cN / dtex; and there are tiny pores inside the fiber.

[0043] Comparative Example 2 (Refer to CN201210009233.0) 1) Preparation of polyamic acid solution: Same as in Example 2; 2) Spinning and forming: Circular spinnerets (0.08mm diameter, 500 holes) are used, wet spinning is employed, and a two-stage coagulation bath (both N-methylpyrrolidone / water volume ratio 30%, temperature 25℃) is used, without "S" shaped guide rollers; 3) Post-processing: Same as in Example 2.

[0044] Product performance: The cut surface is an irregular kidney shape with an flatness ratio of 1.4; the fiber bundle linear density is 550 dtex; the tensile strength of a single filament is 3.5 cN / dtex; the initial modulus is 60 cN / dtex; and the flatness ratio fluctuates within a range of ±0.3.

[0045] Comparative Example 3 (without "S" shaped guide rollers) 1) Preparation of polyamic acid solution: Same as in Example 5; 2) Spinning and forming: The same slit spinneret and three-stage coagulation bath as in Example 5 are used, without staggered guide rollers (fibers pass through the coagulation bath in a straight line). 3) Post-processing: Same as in Example 5 (imidization temperature gradient adjusted to 280℃→420℃→580℃, hot stretching ratio adjusted to 2.5 times).

[0046] Product performance: The cut surface is irregularly flat with an flatness ratio of 2.1; the fiber bundle linear density is 660 dtex; the tensile strength of a single filament is 6 cN / dtex; the initial modulus is 75 cN / dtex; the flatness ratio fluctuates within a range of ±1.2, and the cross-sectional consistency is poor.

[0047] The experimental conditions and results of the above embodiments and comparative examples are summarized in Table 1 below.

[0048] Table 1: First, this invention solves the problems of difficulty in maintaining the shape of flat polyimide fibers, poor structural density, and low industrial adaptability in existing technologies through the synergistic design of slit-type spinnerets, a three-stage gradient coagulation bath, and "S"-shaped guide rollers. The results of examples and comparative examples show that the products of this invention have a wide controllable flatness range (2~20), excellent mechanical properties (tensile strength 12~28 cN / dtex), high production efficiency, and good product consistency, demonstrating significant application advantages in high-temperature filtration, composite materials, and other fields.

[0049] Secondly, as seen in the examples, fibers with a flatness ratio in the range of 5 to 15 (such as 8.5 in Example 1, 9.2 in Example 2, and 13.2 in Example 4) exhibit excellent mechanical properties (tensile strength of 5 to 24 cN / dtex and initial modulus of 70 to 850 cN / dtex). Furthermore, with the synergistic shape preservation achieved by the slit spinneret and S-shaped guide roller, the product cross-section exhibits good consistency and small fluctuations in flatness ratio, which can meet the stability requirements of large-scale industrial production.

[0050] Furthermore, as seen in the examples, the rectangular cross-section possesses the typical advantages of a flat structure. For instance, in Example 1, the rectangular fiber (flatness ratio 8.5) exhibits a single filament tensile strength of 18 cN / dtex and an initial modulus of 850 cN / dtex, far superior to the mechanical properties of the irregularly shaped cross-section fiber in the comparative example. Additionally, the rectangular cross-section can be precisely formed through a slit-type spinneret, and combined with the forced shape-maintaining effect of the S-shaped path guide roller, effectively controlling cross-sectional dimensional deviations. The rectangular fiber in Example 1 exhibits high cross-sectional consistency, while in Comparative Example 3, which lacks an S-shaped guide roller, even using the same spinneret, only an irregular flat cross-section can be formed, with a flatness ratio fluctuation of ±1.2.

[0051] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A polyimide fiber, characterized in that, The cut surface of the polyimide fiber is rectangular, elliptical, kidney-shaped, dumbbell-shaped or other flat, with a flatness ratio of 2 to 20 based on the ratio of major axis to minor axis length; the linear density of the polyimide fiber bundle is 50 to 5000 dtex, the tensile strength of the single filament is 3 cN / dtex to 40 cN / dtex, and the initial modulus is 50 cN / dtex to 1800 cN / dtex.

2. The polyimide fiber according to claim 1, wherein, The cross-section of the polyimide fiber is rectangular; and / or The flatness ratio is 5~15.

3. A method for preparing polyimide fibers as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Preparation of polyamic acid solution: Aromatic dianhydride monomer and aromatic diamine monomer are polycondensed in an aprotic polar solvent to generate polyamic acid solution; 2) Spinning and forming: Wet spinning or dry-spinning is carried out using a spinneret with narrow-slit spinnerets. The polyamic acid solution is extruded and then solidified in a multi-stage coagulation bath. 3) Post-treatment: The nascent fibers are subjected to hot stretching and gradient imidization to obtain the finished polyimide fiber.

4. The preparation method according to claim 3, wherein, The slit-type spinneret has a major axis length of 0.05~0.5mm, a minor axis length of 0.02~0.1mm, and a number of spinnerets of 10~5000.

5. The preparation method according to claim 3, wherein, In step 2, the multi-stage coagulation bath includes a first coagulation bath, a second coagulation bath, and a third coagulation bath; the first coagulation bath is a mixture of water and solvent, with a solvent volume ratio of 5% to 20% and a temperature of 10℃ to 40℃; the second coagulation bath is a mixture of water and solvent, with a solvent volume ratio of 1% to 5% and a temperature of 5℃ to 50℃; the third coagulation bath is pure water, with a temperature of 10℃ to 60℃.

6. The preparation method according to claim 3 or 5, wherein, In step 2, staggered guide rollers are set in the coagulation bath to make the fiber spinning process take an "S" shaped path.

7. The application of the polyimide fiber as described in claim 1 or 2 in high-temperature filtration materials, composite materials or flexible electronic substrates.

8. A polyimide staple fiber, characterized in that, The fiber comprises the polyimide fiber as described in claim 1 or 2 or the polyimide fiber obtained by the preparation method described in any one of claims 3 to 6, wherein the fiber length is 10 mm to 100 mm and the number of crimps is 2 to 50 per inch.

9. A fabric, characterized in that, The polyimide fiber comprising the polyimide fiber as described in claim 1 or 2, or the polyimide fiber obtained by the preparation method described in any one of claims 3 to 6.

10. A prepreg, characterized in that, The fabric comprises the polyimide fiber as described in claim 1 or 2, the polyimide fiber obtained by any one of the preparation methods described in claims 3 to 6, or the fabric as described in claim 9, wherein the fiber or fabric volume content is 30% to 80%.

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