A method for preparing high-performance polyimide fibers

Abstract

This invention discloses a method for preparing high-performance polyimide fibers, belonging to the technical field of polyimide fibers. The method includes: reacting PPD with TFAA to prepare F-PPD; polymerizing F-PPD, ODA, and PMDA at low temperature in DMAc / THF to form a PAA solution; obtaining nascent PAA fibers through wet spinning and double coagulation bath stretching; removing residual solvent using supercritical CO2 extraction; and obtaining fibers through multi-stage thermal imidization. The resulting fibers have less than 0.005 wt% residual solvent, a breaking strength of 4.20–4.90 cN / dtex, excellent heat resistance, and are suitable for high-performance applications.

Description

Technical Field

[0001] This invention relates to the technical field of polyimide fibers, and in particular to a method for preparing high-performance polyimide fibers. Background Technology

[0002] Polyimide (PI) fiber, as a high-performance polymer fiber, exhibits broad application prospects in aerospace, microelectronics, and industrial filtration fields due to its excellent high-temperature resistance, mechanical strength, and electrical insulation properties. Initially, PI fibers were prepared by a two-step process using polyamic acid (PAA) solution followed by thermal imidization. Subsequently, with advancements in materials science, processes such as wet spinning, dry-wet spinning, and electrospinning have been gradually optimized, significantly improving the mechanical properties and morphological stability of the fibers. In the 21st century, research has shifted towards functional modification, such as introducing flame retardants or conductive fillers to expand applications. Simultaneously, by precisely controlling polymerization conditions and solidification parameters, the breaking strength of PI fibers reported in the literature has reached 4.0–4.5 cN / dtex, and the heat resistance temperature can be stably maintained above 400℃. Furthermore, the application of nanotechnology and composite materials has further enhanced the uniformity of the fiber's microstructure; for example, doping with nano-oxides or employing multi-stage heat treatment processes has improved the fiber's density and thermal stability.

[0003] Although existing technologies have made some progress in the preparation of high-performance polyimide fibers, for example, Chinese patent (CN117230543 A) provides a method for preparing polyimide fibers, including: dissolving PPD, ODA, and PMDA in an aprotic polar solvent, polymerizing under nitrogen protection with low-temperature stirring; after filtration and degassing, the PAA solution is sprayed out through a wet spinning machine, and then drawn, wound, and dried successively through a first and second coagulation bath; the PAA fibers are thermally imidized to obtain polyimide fibers. While this invention provides corresponding polyimide fibers suitable for high-performance applications, several key shortcomings still exist. First, the aprotic polar solvents (such as DMAc, NMP, or DMF) commonly used in the preparation process are added as the reaction medium for PAA solutions. Due to their high boiling point and strong polarity, they are difficult to completely remove during the coagulation bath and drying stages. Although thermal imidization (370-400℃) can partially volatilize, the dense structure inside the fiber may encapsulate the residual solvent, leading to a decrease in Tg and breaking strength, and making the fiber prone to deformation or breakage during high-temperature use. Second, the residual solvent interferes with the imidization reaction of PAA to PI, inhibits the formation of imide rings or causes uneven local crosslinking, resulting in micropores and defects, reducing fiber density and heat resistance. Summary of the Invention

[0004] This application provides a method for preparing high-performance polyimide fibers, comprising the following steps: Step S1. Reacting PPD and TFAA in DMF, adding TEA as a catalyst, reacting at 0℃~5℃ for 6~8 hours, and purifying by recrystallization from ethanol to obtain F-PPD; Step S2. Dissolving F-PPD, ODA, and PMDA in a molar ratio of (0.4 to 0.6):(0.4 to 0.6):(1.8 to 2.2) in a mixed solvent of DMAc and THF, with a solid content of 16wt%~18wt%, under nitrogen protection, PAA solution was prepared by stirring and polymerization at -5℃ to 0℃ for 6 to 10 hours; Step S3. The PAA solution was filtered, degassed, and then sprayed out through a wet spinning machine, passing through a first coagulation bath and a second coagulation bath in sequence, with a draw ratio of 1.5 to 2.5, to obtain PAA nascent fibers; Step S4. The residual solvent in the PAA nascent fibers was removed by supercritical carbon dioxide extraction, and PEG-400 was added as an extraction aid at a concentration of 0.01wt% to 0.03wt% of the extraction medium; Step S5. The extracted PAA fibers were subjected to multi-stage thermal imidization to obtain polyimide fibers.

[0005] It should be noted that in step S1, PPD and TFAA react in DMF at 0℃~5℃ for 6~8 hours, and under TEA catalysis, -CF3 groups are introduced to generate F-PPD, which is purified by recrystallization from ethanol to reduce the hydrogen bond affinity with DMAc and promote solvent evaporation. In step S2, F-PPD, ODA, and PMDA are dissolved in a DMAc / THF mixed solvent (solid content 16%~18%) at a molar ratio of 0.4~0.6:0.4~0.6:1.8~2.2, and polymerized at -5℃~0℃ for 6~10 hours to obtain PAA solution. The low boiling point of THF accelerates solvent diffusion. In step S3, PAA nascent fibers are formed by wet spinning, with the first and second coagulation baths at a draw ratio of 1.5~2.5. Nano-TiO2 improves fiber density. Step S4 uses supercritical CO2 extraction (with 0.01wt%–0.03wt% PEG-400 as an extraction aid) to remove residual solvent, reduce fiber surface tension, and ensure residual solvent content is <0.005 wt% to avoid microporous defects. Step S5 promotes imide ring formation through multi-stage thermal imidization to obtain PI fibers with significantly better performance than those produced by traditional processes.

[0006] In a preferred embodiment of a method for preparing high-performance polyimide fibers, in step S1, the molar ratio of PPD to TFAA is 1:(0.2 to 0.4), and the amount of TEA is 0.5 wt% to 1 wt% of the mass of PPD.

[0007] It should be noted that the molar ratio of PPD to TFAA is 1:0.2–0.4 to control the degree of substitution of the -CF3 group. An appropriate amount of -CF3 is introduced onto the amino group of the PPD benzene ring via electrophilic substitution, reducing the hydrogen bonding energy between F-PPD and aprotic polar solvents (such as DMAc), promoting solvent evaporation, and maintaining the reactivity of the molecular chain to ensure the efficiency of subsequent polyimide polymerization. The amount of TEA used is 0.5 wt%–1 wt% of the PPD mass, serving as a catalyst to neutralize the strong acid byproduct trifluoroacetic acid generated in the TFAA reaction, maintaining the pH of the reaction system at 7–8 to avoid an acidic environment inhibiting the reaction process. An appropriate amount of TEA ensures catalytic efficiency and prevents excessive amounts from initiating side reactions.

[0008] In a preferred embodiment of a method for preparing high-performance polyimide fibers, in step S2, the volume ratio of DMAc to THF is 4:1 to 5:1, and the viscosity of the PAA solution is 3000 to 5000 mPa·s.

[0009] It should be noted that DMAc, as the main solvent, provides excellent solubility to disperse F-PPD, ODA, and PMDA, ensuring uniform polymerization. THF, as a low-boiling-point co-solvent, lowers the overall boiling point and surface tension of the mixed solvent system, increasing the solvent diffusion rate in the coagulation bath, thereby reducing solvent residue in the nascent PAA fibers. The volume ratio of DMAc to THF of 4:1 to 5:1 balances solubility and volatility, avoiding excessively high THF ratios that could lead to insufficient PAA solubility or excessively low THF ratios that would affect solvent removal efficiency. The PAA solution viscosity of 3000–5000 mPa·s is achieved by controlling the solid content (16%–18%) and polymerization conditions (-5℃–0℃, 6–10 hours), ensuring a suitable molecular weight for wet spinning to form uniform fibers, while also guaranteeing the fiber's mechanical properties and subsequent thermal imidization efficiency, laying the foundation for the preparation of high-performance polyimide fibers.

[0010] In a preferred embodiment of a method for preparing high-performance polyimide fibers, in step S3, the first coagulation bath is a mixture of THF and water in a volume ratio of 1:4 to 1:5, and the temperature is 20℃ to 30℃; the second coagulation bath is an aqueous solution containing 0.1wt% to 0.3wt% nano-TiO2, and the temperature is 40℃ to 50℃.

[0011] It should be noted that in the first coagulation bath, the low boiling point and high volatility of THF accelerate the diffusion of DMAc from the PAA solution into the aqueous phase, forming uniform PAA nascent fibers. The THF / water ratio of 1:4 to 1:5 balances the coagulation rate and fiber structural stability, and the temperature of 20℃ to 30℃ avoids fiber surface defects caused by excessively rapid coagulation. In the second coagulation bath, nano-TiO2 forms weak hydrogen bonds with the carboxyl groups (-COOH) of PAA through surface hydroxyl groups (-OH), depositing on the fiber surface and in the micropores, filling the pores, and improving fiber density. A TiO2 concentration of 0.1% to 0.3% ensures uniform dispersion without clogging the spinning pores, and a temperature of 40℃ to 50℃ promotes the adsorption of TiO2 and PAA and optimizes the draw ratio (draw ratio 1.5 to 2.5). This gradient coagulation bath design, through rapid solvent exchange and nano-reinforcement, significantly reduces solvent residue and micropore defects, providing a structural basis for high-performance polyimide fibers.

[0012] In a preferred technical solution for the preparation of high-performance polyimide fibers, in step S4, the supercritical carbon dioxide extraction process is carried out at a pressure of 10-15 MPa and a temperature of 35℃-45℃ for an extraction time of 0.5-1.5 hours.

[0013] It should be noted that supercritical CO2 extraction (10-15 MPa, 35℃-45℃, 0.5-1.5 hours) utilizes high permeability and low surface tension, combined with PEG-400 extraction aid and F-PPD hydrophobicity, to efficiently remove DMAc from PAA fibers, avoid microporous defects, and improve fiber density and performance.

[0014] As a preferred technical solution for the preparation method of high-performance polyimide fibers, in step S5, the multi-stage thermal imidization includes: preheating at 150℃~200℃ for 0.5~1 hour, holding at 280℃~320℃ for 1~2 hours and introducing a nitrogen atmosphere containing 0.1wt%~0.3wt% oxygen, and high-temperature imidization at 400℃~430℃ for 0.5~1 hour.

[0015] It should be noted that preheating at 150℃~200℃ gradually removes moisture and volatile impurities from the PAA fiber, stabilizing the fiber structure; the 280℃~320℃ stage involves slight oxidation with trace amounts of oxygen, promoting the dehydration and cyclization of PAA carboxyl groups to form imide rings, reducing uneven crosslinking and microporous defects; high-temperature imidization at 400℃~430℃ completes the closure of the imide rings, ensuring molecular chain regularity and thermal stability. This staged heating and micro-oxygen atmosphere synergistically optimize the imidization reaction, improving fiber density, breaking strength, and heat resistance, significantly outperforming the traditional pure nitrogen imidization process.

[0016] In addition, the polyimide fiber prepared by the above preparation method has a residual solvent content of less than 0.005 wt%.

[0017] The beneficial effects of this application are that the present invention provides a method for preparing high-performance polyimide fibers. By introducing fluorinated diamine monomer F-PPD, DMAc / THF mixed solvent, a dual coagulation bath system, and supercritical CO2 extraction, the residual solvent content is significantly reduced to 0.0020wt%~0.0040wt%, thereby improving fiber density and imidization degree. The resulting fibers exhibit a breaking strength of 4.20~4.90 cN / dtex, and mass retention rates at 400℃ and 500℃ are 92.5%~95.1% and 86.0%~90.0%, respectively, demonstrating excellent mechanical properties and heat resistance, making them suitable for aerospace, microelectronics, and other fields. Attached Figure Description

[0018] Figure 1 The infrared spectrum of the F-PPD prepared in step S1 of Example 1. Detailed Implementation

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0020] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0021] Secondly, the term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places throughout this specification does not necessarily refer to the same embodiment, nor is it a single embodiment or an embodiment selectively excluded from other embodiments.

[0022] Example 1

[0023] Step S1. Preparation of F-PPD: Under nitrogen protection, p-phenylenediamine (PPD) was dissolved in 500 mL of N,N-dimethylformamide (DMF), and triethylamine (TEA, 0.5 wt% of PPD) was added, stirring until completely dissolved. Trifluoroacetic anhydride (TFAA, PPD:TFAA molar ratio 1:0.2) was slowly added dropwise over an ice bath at 0°C for 1 hour. After the addition was complete, stirring was continued at 0°C for 6 hours, followed by heating to 5°C and stirring for 1 hour. The reaction solution was distilled under reduced pressure at 60°C and 0.01 MPa to remove DMF, recrystallized with 300 mL of ethanol, filtered, and dried to obtain F-PPD.

[0024] Step S2. PAA Solution Preparation: Under nitrogen protection, F-PPD and 4,4'-diaminodiphenyl ether (ODA) were dissolved in a mixed solvent of 400 mL DMAc and 100 mL THF (volume ratio 4:1) at a molar ratio of 0.4:0.6, and stirred until homogeneous. Pyromellitic dianhydride (PMDA, F-PPD+ODA:PMDA molar ratio 1:1.8) was added in portions at -5℃, with each addition being 0.3 mol at 10-minute intervals. Polymerization was carried out by stirring for 6 hours to obtain a PAA solution with a solid content of 16 wt% and a viscosity of 3000 mPa·s. The solution was filtered through a 200-mesh filter and allowed to stand for 12 hours to remove bubbles before use.

[0025] Step S3. Wet spinning: The PAA solution is spun out at a speed of 15 m / min through a wet spinning machine (spinneret diameter 0.05 mm). The fibers are sequentially passed through a first coagulation bath (THF / water volume ratio 1:4, 20℃) with a draw ratio of 1.5; and a second coagulation bath (containing 0.1 wt% nano TiO2, aqueous solution, 40℃) with a draw ratio of 2.0. After winding and washing (deionized water, 25℃), the fibers are vacuum dried at 40℃ for 4 hours to obtain PAA nascent fibers.

[0026] Step S4. Supercritical CO2 extraction: Place the PAA nascent fibers in a supercritical CO2 extraction apparatus, set the pressure to 10 MPa and the temperature to 35°C, add 0.01 wt% PEG-400 extraction aid, and extract for 0.5 hours. After extraction, dry the fibers at 40°C for 2 hours.

[0027] Step S5. Multi-stage thermal imidization: The extracted PAA fibers are placed in a heat treatment furnace and preheated at 150°C for 0.5 hours under a nitrogen atmosphere; the temperature is then raised to 280°C and held for 1 hour, while a nitrogen atmosphere containing 0.1 wt% oxygen is introduced; finally, imidization is performed at 400°C for 0.5 hours. After cooling to room temperature, PI fibers are obtained.

[0028] Example 2

[0029] Step S1. Preparation of F-PPD: Under nitrogen protection, PPD was dissolved in 600 mL of DMF, and TEA (0.65 wt% of PPD) was added and stirred until dissolved. TFAA (PPD:TFAA molar ratio 1:0.257) was slowly added dropwise at 0℃ over 1 hour. After reacting for 6.5 hours, the temperature was raised to 5℃ and stirred for 0.5 hours. The reaction solution was distilled under reduced pressure at 60℃ and 0.01 MPa to remove DMF, recrystallized with 350 mL of ethanol, filtered, and dried to obtain F-PPD.

[0030] Step S2. PAA Solution Preparation: Under nitrogen protection, F-PPD and ODA (molar ratio 0.45:0.55) were dissolved in a mixed solvent of 450 mL DMAc and 100 mL THF (volume ratio 4.5:1), and stirred until homogeneous. PMDA (F-PPD+ODA:PMDA molar ratio 1:1.9) was added in portions at -3℃, 0.3 mol each time, with 10-minute intervals. Polymerization was carried out with stirring for 7 hours to obtain a PAA solution with a solid content of 16.5 wt% and a viscosity of 3500 mPa·s. The solution was filtered through a 200-mesh filter and allowed to stand for 12 hours to remove bubbles.

[0031] Step S3. Wet spinning: The PAA solution is spun out at 16 m / min using a wet spinning machine (spinneret diameter 0.06 mm). The fibers pass through a first coagulation bath (THF / water volume ratio 1:4.5, 23℃) with a draw ratio of 1.7; and a second coagulation bath (containing 0.15 wt% nano TiO2, 43℃) with a draw ratio of 2.1. After winding, the fibers are washed with deionized water and vacuum dried at 40℃ for 4 hours to obtain PAA nascent fibers.

[0032] Step S4. Supercritical CO2 extraction: Place the PAA nascent fibers in a supercritical CO2 extraction apparatus at a pressure of 11 MPa and a temperature of 38°C. Add 0.015 wt% PEG-400 as an extraction aid and extract for 0.8 hours. After extraction, dry the fibers at 40°C for 2 hours.

[0033] Step S5. Multistage thermal imidization:

[0034] PAA fibers were preheated at 150°C for 0.7 hours (in a nitrogen atmosphere); then heated to 290°C and held for 1.2 hours, while nitrogen containing 0.15 wt% oxygen was introduced; finally, imidization was performed at 410°C for 0.6 hours. After cooling, PI fibers were obtained.

[0035] Example 3

[0036] Step S1. Preparation of F-PPD: Under nitrogen protection, PPD was dissolved in 700 mL of DMF, and TEA (0.8 wt% of PPD) was added and stirred until dissolved. TFAA (PPD:TFAA molar ratio 1:0.314) was slowly added dropwise at 2℃ over 1 hour. After reacting for 7 hours, the temperature was raised to 5℃ and stirred for 1 hour. The reaction solution was distilled under reduced pressure at 60℃ and 0.01 MPa to remove DMF, recrystallized with 400 mL of ethanol, filtered, and dried to obtain F-PPD.

[0037] Step S2. PAA Solution Preparation: Under nitrogen protection, F-PPD and ODA (molar ratio 0.5:0.5) were dissolved in a mixed solvent of 480 mL DMAc and 100 mL THF (volume ratio 4.8:1), and stirred until homogeneous. PMDA (F-PPD+ODA:PMDA molar ratio 1:2.0) was added in portions at -2℃, 0.4 mol each time, with 10-minute intervals. Polymerization was carried out by stirring for 8 hours to obtain a PAA solution with a solid content of 17 wt% and a viscosity of 4000 mPa·s. The solution was filtered through a 200-mesh filter and allowed to stand for 12 hours to remove bubbles.

[0038] Step S3. Wet spinning: The PAA solution is spun out at 18 m / min using a wet spinning machine (spinneret diameter 0.08 mm). The fibers pass through a first coagulation bath (THF / water volume ratio 1:4.8, 25℃) with a draw ratio of 1.9; and a second coagulation bath (containing 0.2 wt% nano TiO2, 45℃) with a draw ratio of 2.3. After winding, the fibers are washed with deionized water and vacuum dried at 40℃ for 4 hours to obtain PAA nascent fibers.

[0039] Step S4. Supercritical CO2 extraction: Place the PAA nascent fibers in a supercritical CO2 extraction apparatus at a pressure of 13 MPa and a temperature of 40°C. Add 0.02 wt% PEG-400 as an extraction aid and extract for 1 hour. After extraction, dry the fibers at 40°C for 2 hours.

[0040] Step S5. Multistage thermal imidization:

[0041] PAA fibers were preheated at 180°C for 0.8 hours (in a nitrogen atmosphere); then heated to 300°C and held for 1.5 hours, followed by the introduction of nitrogen containing 0.2 wt% oxygen; and finally imidized at 420°C for 0.8 hours. After cooling, PI fibers were obtained.

[0042] Example 4

[0043] Step S1. Preparation of F-PPD: Under nitrogen protection, PPD was dissolved in 800 mL of DMF, and TEA (1 wt% of PPD mass) was added and stirred until dissolved. TFAA (PPD:TFAA molar ratio 1:0.4) was slowly added dropwise at 5℃ over 1 hour. After reacting for 8 hours, DMF was removed by vacuum distillation (60℃, 0.01 MPa), and the mixture was recrystallized from 450 mL of ethanol. After filtration and drying, F-PPD was obtained.

[0044] Step S2. PAA Solution Preparation: Under nitrogen protection, F-PPD and ODA (molar ratio 0.6:0.4) were dissolved in a mixed solvent of 500 mL DMAc and 100 mL THF (volume ratio 5:1), and stirred until homogeneous. PMDA (F-PPD+ODA:PMDA molar ratio 1:2.2) was added in portions at 0°C, 0.4 mol at 10-minute intervals. Polymerization was carried out by stirring for 10 hours to obtain a PAA solution with a solid content of 18 wt% and a viscosity of 5000 mPa·s. The solution was filtered through a 200-mesh filter and allowed to stand for 12 hours to remove bubbles.

[0045] Step S3. Wet spinning: The PAA solution is spun out at 20 m / min using a wet spinning machine (spinneret diameter 0.1 mm). The fibers pass through a first coagulation bath (THF / water volume ratio 1:5, 30℃) with a draw ratio of 2.0; and a second coagulation bath (containing 0.3 wt% nano TiO2, 50℃) with a draw ratio of 2.5. After winding, the fibers are washed with deionized water and vacuum dried at 40℃ for 4 hours to obtain PAA nascent fibers.

[0046] Step S4. Supercritical CO2 extraction: Place the PAA nascent fibers in a supercritical CO2 extraction apparatus at a pressure of 15 MPa and a temperature of 45°C. Add 0.03 wt% PEG-400 as an extraction aid and extract for 1.5 hours. After extraction, dry the fibers at 40°C for 2 hours.

[0047] Step S5. Multi-stage thermal imidization: PAA fibers are preheated at 200℃ for 1 hour (nitrogen atmosphere); heated to 320℃ and held for 2 hours, while nitrogen containing 0.3wt% oxygen is introduced; imidization is then performed at 430℃ for 1 hour. After cooling, PI fibers are obtained.

[0048] Compare with Example 1

[0049] The difference between this comparative example and Example 1 is that F-PPD is not introduced; PPD is used directly.

[0050] Specific procedures: In step S1, the reaction between PPD and TFAA is not performed; instead, PPD (the same molar amount as in Example 1) is directly used for the preparation of the PAA solution in step S2. The parameters for the remaining steps (S2-S5) are completely consistent with those in Example 1.

[0051] Compare with Example 2

[0052] The difference between this comparative example and Example 1 is that it uses a single DMAc solvent and does not use THF.

[0053] Specific procedures: In step S2, F-PPD and ODA (molar ratio 0.4:0.6) were dissolved in 500 mL of DMAc, and PMDA (molar ratio 1:1.8) was added, with a solid content of 16 wt%. Polymerization was carried out at -5℃ for 6 hours, with a viscosity of 3000 mPa·s. The remaining steps (S1, S3-S5) were the same as in Example 1.

[0054] Compare with Example 3

[0055] The difference between this comparative example and Example 1 is that it uses a single coagulation bath to remove nano-TiO2 from the second coagulation bath.

[0056] Specific operation: In step S3, the PAA solution is sprayed out through a wet spinning machine (spinneret diameter 0.05 mm, 15 m / min), passing only through the first coagulation bath (THF / water, 1:4, 20°C), with a draw ratio of 1.5. The remaining steps (S1, S2, S4, S5) are the same as in Example 1.

[0057] Compare with Example 4

[0058] The difference between this comparative example and Example 1 is that conventional drying is used instead of supercritical CO2 extraction.

[0059] Specific procedures: In step S4, the nascent PAA fibers are placed in a vacuum oven and dried at 80°C for 6 hours. The remaining steps (S1-S3, S5) are the same as in Example 1.

[0060] Performance testing:

[0061] 1. Residual Solvent Content: Take a certain mass of polyimide fiber sample, cut it into small pieces, and place it in a sealed glass container. Add an appropriate amount of deionized water as the extraction medium. Place the container in a constant temperature water bath and heat it to 80℃ for 2 hours to allow residual solvents (such as DMAc) to be released from the fibers into the water. After cooling, take the extract and analyze it using gas chromatography-mass spectrometry (GC-MS) to detect the characteristic peaks of the solvent and quantify its content. Repeat the test three times and take the average value to ensure the accuracy of the results. The sample needs to be vacuum dried at 50℃ for 4 hours before the test to remove surface adsorbed moisture and avoid interference.

[0062] 2. Tensile Strength: A single fiber, approximately 50 mm in length, was randomly selected from the polyimide fiber sample and subjected to tensile testing using an electronic single-fiber strength tester. Both ends of the fiber were fixed to clamps with a clamp spacing of 20 mm. The testing environment was controlled at 23°C and 50% relative humidity. The fiber was slowly stretched at a constant tensile speed until it broke, and the maximum force at break was recorded. At least 20 fibers were tested for each sample, and the average value was taken to ensure data representativeness. Before testing, the fiber surface should be checked for uniformity to avoid defects affecting the results.

[0063] 3. Heat Resistance: Weigh approximately 10 mg of polyimide fiber and place it in the sample pan of a thermogravimetric analyzer (TGA). Under a nitrogen protective atmosphere, heat the fiber from room temperature to 600°C at a constant heating rate, and observe the mass loss. Record the mass retention rate of the fiber at 400°C and 500°C to assess its stability under high-temperature conditions. Maintain a stable nitrogen flow during the test to avoid oxidation interference. Repeat the test three times for each sample, take the average value, and ensure the sample is dry to eliminate the influence of moisture.

[0064] Table 1 shows the experimental data of Examples 1 to 4 and Control Examples 1 to 4.

[0065]

[0066] In conjunction with Example 1 and Figure 1 It can be seen that at 3350 cm -1 At 3050 cm⁻¹, the strong peak is attributed to the NH stretching vibration of the primary amine group (-NH₂), indicating that some amino groups in PPD are retained, supporting the subsequent polycondensation with PMDA to form PAA. -1 The moderate intensity peak corresponds to the CH stretching vibration of the aromatic ring, 1550 cm⁻¹. -1 The peaks represent C=C skeletal vibrations, collectively confirming that the aromatic ring (C6H4) provides molecular rigidity and thermal stability. (1625 cm⁻¹) -1 The weaker peak corresponds to the NH bending vibration, further confirming the presence of the primary amine group. (1250 cm⁻¹) -1 The strong peak indicates the CF stretching vibration of trifluoromethyl (-CF3), proving that TFAA successfully introduced -CF3. Its strong electronegativity reduces the hydrogen bond energy with DMAc, promoting solvent evaporation. (850 cm⁻¹) -1 The weak peak is due to the CH bending vibration of aromatic hydrogen, reflecting the unsubstituted hydrogen on the benzene ring, which maintains the reactivity.

[0067] As can be seen from Examples 1 to 4 and Table 1, the residual solvent content of the polyimide fiber obtained by the present invention is in the range of 0.0020wt% to 0.0040wt%, the breaking strength is between 4.20cN / dtex and 4.90cN / dtex, and the heat resistance has a mass retention rate of 92.5% to 95.1% at 400℃ and a mass retention rate of 86.0% to 90.0% at 500℃.

[0068] As can be seen from Example 1, Comparative Example 1, and Table 1, the residual solvent content of the polyimide fiber obtained in Example 1 is approximately 0.0040 wt%, significantly lower than the 0.0065 wt% of Comparative Example 1; its breaking strength reaches 4.20 cN / dtex, higher than the 3.80 cN / dtex of Comparative Example 1; and its mass retention rates at 400℃ and 500℃ are 92.5% and 86.0%, respectively, both superior to the 89.0% and 82.0% of Comparative Example 1. This is because the introduction of a fluorinated diamine monomer (F-PPD) in Example 1 improves the solvent compatibility and film-forming properties of the polyamic acid molecular chain. Combined with the supercritical CO2 extraction process, this significantly reduces the residue of high-boiling-point solvents, thereby reducing the formation of micropores and defects. This results in a more compact molecular chain arrangement and a higher degree of imidization in the fiber, thus exhibiting lower residual solvent, higher mechanical properties, and better heat resistance.

[0069] As can be seen from Example 1, Comparative Example 2, and Table 1, the residual solvent content of Example 1 was 0.0040 wt%, significantly lower than that of Comparative Example 2 (0.0090 wt); its breaking strength reached 4.20 cN / dtex, higher than that of Comparative Example 2 (3.60 cN / dtex); and its mass retention rates at 400℃ and 500℃ were 92.5% and 86.0%, respectively, also better than those of Comparative Example 2 (87.0% and 80.0%). This is because Example 1 used a mixed solvent system of DMAc / THF. THF, as a low-boiling-point co-solvent, helps to form a more uniform phase separation structure and facilitates subsequent solvent removal. In contrast, Comparative Example 2 used DMAc alone, resulting in a higher solvent residue rate and insufficient density of the fiber's internal structure, thus exhibiting performance degradation during high-temperature treatment and mechanical testing.

[0070] Based on Example 1, Comparative Example 3, and Table 1, it can be seen that the residual solvent content of Example 1 is 0.0040 wt%, slightly lower than that of Comparative Example 3 (0.0055 wt); its tensile strength is 4.20 cN / dtex, higher than that of Comparative Example 3 (3.90 cN / dtex); and its mass retention rates at 400℃ and 500℃ are 92.5% and 86.0%, respectively, which are also better than those of Comparative Example 3 (90.0% and 83.0%). The mechanism is that Example 1 introduced nano-TiO2 particles into the secondary coagulation bath, which can promote the orientation and crystallization of polymer chains and enhance the stability of fiber structure, while improving solvent exchange efficiency and effectively reducing solvent residue; while Comparative Example 3 did not use a TiO2 secondary coagulation bath, resulting in insufficient density and stability of fiber microstructure, thus exhibiting poor mechanical properties and heat resistance.

[0071] As can be seen from Example 1, Comparative Example 4, and Table 1, the residual solvent content of the polyimide fiber obtained in Example 1 is 0.0040 wt%, which is significantly lower than the 0.0150 wt% of Comparative Example 4; its breaking strength reaches 4.20 cN / dtex, which is higher than the 3.20 cN / dtex of Comparative Example 4; and its mass retention rates at 400℃ and 500℃ are 92.5% and 86.0%, respectively, which are also better than the 82.0% and 74.0% of Comparative Example 4. The reason is that the supercritical CO2 extraction process used in Example 1 utilizes the high permeability and low surface tension of CO2, combined with PEG-400 extraction aid, to efficiently remove the high-boiling-point solvent DMAc deep into the fiber, avoiding damage to the fiber structure and the formation of micropores, thereby improving the fiber density, molecular chain regularity and imidization degree. In contrast, the conventional drying of Control Example 4 only removes the surface solvent through thermal evaporation, leaving a high level of residual solvent inside, resulting in incomplete imidization, increased defects, and a significant decrease in performance under high temperature and mechanical stress.

[0072] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing high-performance polyimide fibers, characterized in that, Includes the following steps: Step S1. Reaction of p-phenylenediamine and TFAA in DMF, with triethylamine added as a catalyst, reaction temperature 0℃~5℃, reaction time 6~8 hours, purification by recrystallization from ethanol to obtain F-PPD; Step S2. F-PPD, 4,4'-diaminodiphenyl ether and PMDA are dissolved in a mixed solvent of DMAc and THF at a molar ratio of (0.4 to 0.6):(0.4 to 0.6):(1.8 to 2.2), with a solid content of 16wt% to 18wt%. The mixture is stirred and polymerized at -5℃ to 0℃ for 6 to 10 hours under nitrogen protection to obtain a PAA solution. Step S3. After filtering and degassing the PAA solution, it is spun out through a wet spinning machine and passed sequentially through a first coagulation bath and a second coagulation bath at a draw ratio of 1.5 to 2.5 to obtain PAA nascent fibers. The first coagulation bath is a mixture of THF and water in a volume ratio of 1:4 to 1:5 at a temperature of 20°C to 30°C. The second coagulation bath is an aqueous solution containing 0.1 wt% to 0.3 wt% nano-TiO2 at a temperature of 40°C to 50°C. Step S4. Use supercritical carbon dioxide extraction to remove residual solvent from PAA nascent fibers, and add PEG-400 as an extraction aid at a concentration of 0.01wt% to 0.03wt% of the extraction medium. Step S5. The extracted PAA fibers are subjected to multi-stage thermal imidization to obtain polyimide fibers.

2. The method for preparing high-performance polyimide fibers according to claim 1, characterized in that, In step S1, the molar ratio of p-phenylenediamine to TFAA is 1:(0.2 to 0.4), and the amount of triethylamine used is 0.5 wt% to 1 wt% of the mass of p-phenylenediamine.

3. The method for preparing high-performance polyimide fibers according to claim 1, characterized in that, In step S2, the volume ratio of DMAc to THF is 4:1 to 5:1, and the viscosity of the PAA solution is 3000 to 5000 mPa·s.

4. The method for preparing high-performance polyimide fibers according to claim 1, characterized in that, In step S4, the supercritical carbon dioxide extraction process is carried out at a pressure of 10-15 MPa and a temperature of 35℃-45℃ for an extraction time of 0.5-1.5 hours.

5. The method for preparing high-performance polyimide fibers according to claim 1, characterized in that, In step S5, the multi-stage thermal imidization includes: preheating at 150℃~200℃ for 0.5~1 hour, holding at 280℃~320℃ for 1~2 hours while introducing a nitrogen atmosphere containing 0.1wt%~0.3wt% oxygen, and high-temperature imidization at 400℃~430℃ for 0.5~1 hour.

6. The polyimide fiber prepared by the preparation method according to any one of claims 1 to 5, characterized in that, The residual solvent content of the fiber is less than 0.005 wt%.

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