A method of dry spinning polyimide nascent fibers

Abstract

The present application relates to a kind of dry method polyimide nascent fiber spinning method, mainly including the following steps: polyimide A solution, polyamide acid B solution is mixed after stirring to obtain spinning solution;Above-mentioned spinning solution is filtered by vacuum degassing, extruded into dry spinning duct from spinneret, under the condition of flash evaporation and microwave auxiliary heating, spinning solution is solidified into polyimide nascent fiber.The present application is by flash evaporation and microwave auxiliary heating method, so that solvent is evaporated from inside to outside quickly and uniformly, avoid the skin-core structure and cross section heteromorphy caused by uneven diffusion of solvent;Polyamide acid can be converted into polyimide to a higher degree under microwave heating, reduce the proportion of complex solvent in fiber, thereby reducing the solvent content in fiber;The nascent fiber prepared by the method, cross section heteromorphy is low, mechanical property is good, energy consumption is low, industrialization can be realized.

Description

Technical Field

[0001] This invention belongs to the field of high-performance organic fibers, and specifically relates to a method for dry spinning polyimide nascent fibers. Background Technology

[0002] As a type of high-performance organic fiber, polyimide fiber is widely used in high-temperature filtration materials, protective clothing, aerospace, and electrical insulation due to its excellent high-temperature resistance, chemical resistance, and good mechanical properties. Dry spinning is one of the main methods for preparing polyimide fibers. It involves dissolving a polyamic acid precursor in a polar solvent, extruding it through a spinneret, and evaporating the solvent in a high-temperature tunnel to obtain the fiber. However, during solvent evaporation, uneven heating of the spinning solution inside and on the surface leads to differences in solvent evaporation rates, molecular chain orientation, and cyclization degrees between the fiber's interior and surface, resulting in a core-sheath structure. Under stretching force, this causes deformation of the fiber cross-section, affecting its mechanical properties and the stability of subsequent processing. This phenomenon is not only prevalent in polyimide fibers but also common in the dry spinning processes of acrylonitrile and cellulose acetate fibers, forming "dumbbell-shaped" or "C-shaped" cross-sections. While these irregularly shaped fibers have unique advantages in certain applications, they generally affect the fiber's mechanical properties and consistency. Especially when the polymer molecular chains are rigid or the intermolecular forces are strong, the rapidly curing rigid skin will collapse as the core solvent continues to evaporate, forming a "dumbbell-shaped" or "C-shaped" cross section with internal pores, which severely reduces the mechanical strength of the fiber.

[0003] Currently, several patents have proposed solutions to improve fiber cross-sectional shape by adjusting spinning process parameters or introducing auxiliary devices. For example, Chinese patent CN101543722B proposes a method to reduce fiber cross-sectional irregularities by adjusting the spinning solution temperature and spinneret speed; while US patent US8968553B2 describes a process to optimize fiber morphology by improving the spinneret design. These methods improve the fiber cross-sectional shape to some extent, but problems such as uneven heating and difficulty in precisely controlling solvent evaporation rates still exist. In particular, localized overheating or solvent residue can easily occur during spinning, thus affecting fiber quality. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for dry spinning polyimide nascent fibers. This method uses flash evaporation and microwave-assisted heating to make the solvent evaporate rapidly and uniformly from the inside to the outside, effectively avoiding the deformation of the core-sheath structure and cross-section caused by uneven diffusion of the solvent. At the same time, by blending a certain proportion of flexible polyimide segments, the flexibility and anti-collapse ability of the fiber are increased, thereby solving the problem of "dumbbell-shaped" or "C-shaped" fibers caused by the collapse of the outer skin.

[0005] This invention provides a method for dry spinning polyimide nascent fibers, comprising the following steps:

[0006] (1) After reacting an anhydride monomer, a flexible diamine monomer, and a polar solvent in an equimolar ratio at 0-60°C for 6-12 hours, a cyclization catalyst is added, and the temperature is raised to 160-200°C to continue the reaction for 8-16 hours to obtain a polyimide A solution.

[0007] (2) After reacting an anhydride monomer, a rigid diamine monomer, and a polar solvent in an equimolar ratio at 0-60°C for 6-12 hours, a Lewis base is added and the mixture is stirred until homogeneous to obtain a polyamic acid B solution.

[0008] (3) Mix and stir polyimide A solution and polyamic acid B solution at 25~80℃ for 3~6 hours to obtain spinning solution;

[0009] (4) The above spinning solution is degassed and filtered under vacuum, and then extruded into the dry spinning channel through the spinneret. Under the conditions of flash evaporation and microwave-assisted heating, the spinning solution is solidified into polyimide nascent fibers.

[0010] Preferably, the anhydride monomer in steps (1) and (2) is at least one of the following structures:

[0011] , , , .

[0012] Preferably, the flexible diamine monomer in step (1) is at least one of the following structures:

[0013] , , , , , , , , .

[0014] Preferably, the cyclization catalyst in step (1) is isoquinoline, and its amount is 10-20% of the anhydride monomer. Its function is to catalyze the conversion of polyamic acid into polyimide.

[0015] Preferably, the rigid diamine monomer in step (2) is at least one of the following structures:

[0016] , , , , , , .

[0017] Preferably, the polar solvent in steps (1) and (2) is at least one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, γ-butyrolactone, p-chlorophenol, sulfolane, cyclohexane, and m-cresol.

[0018] Preferably, the Lewis base in step (2) is at least one of 4-dimethylaminopyridine, isoquinoline, triethylenediamine, and N,N-diisopropylethylamine; and its molar ratio with the rigid diamine monomer is 0.5~5:1. Lewis bases have a strong absorption effect on microwaves, and they mainly complex with polyamic acid units, which can promote the absorption of microwaves by the spinning solution and catalyze the cyclization reaction of polyamic acid.

[0019] Preferably, the solid content of the polyimide A solution is 5 wt% to 15 wt%; the solid content of the polyamic acid B solution is 15 wt% to 30 wt%; and the solid content of the spinning solution is 10 wt% to 30 wt%.

[0020] Preferably, the spinneret in step (4) has 20 to 50 holes and the hole spacing is 5 to 20 mm.

[0021] Preferably, the diameter of the spinneret orifice is 0.1~0.3mm and the length-to-diameter ratio is 3~5:1.

[0022] Preferably, the spinneret assembly in step (4) is made of tantalum metal or titanium alloy; the tantalum metal grade is one of Ta2, TaNb3, and TaNb20; the titanium alloy grade is one of TA1, TA2, and TA3.

[0023] Preferably, the temperature of the spinneret assembly in step (4) is 120~250℃ and the assembly pressure is 2~5MPa to improve the flash evaporation effect.

[0024] Preferably, in step (4), the microwave-assisted heating uses a microwave heater that covers the circumference of the tunnel, the distance between the microwave heater and the spinneret outlet is 1cm to 50cm, and the microwave heater covers 1 to 50% of the length of the tunnel.

[0025] Preferably, the power of the microwave heater is 1~45kW and the microwave frequency is set to 1~5 GHz.

[0026] Preferably, the tunnel atmosphere in step (4) is one of nitrogen, carbon dioxide, or argon, and the tunnel temperature is 120℃~350℃, preferably 160℃~200℃.

[0027] Preferably, the solvent content of the polyimide nascent fiber obtained in step (4) is 5% to 30%, and the degree of cyclization is 20% to 90%.

[0028] Preferably, the tensile strength of the polyimide nascent fiber obtained in step (4) is 1.5~8 cN / dtex, and the elongation at break is 50~90%.

[0029] Beneficial effects

[0030] (1) Elimination of core-sheath structure and realization of in-situ cyclization: This invention introduces a Lewis base and uses microwave heating to target microwave energy to the inside of the fiber, achieving uniform solvent diffusion. This solves the core-sheath structure formed by the rapid evaporation of the outer solvent caused by traditional hot air heating, and avoids the generation of irregular cross-sections during subsequent stretching. At the same time, the Lewis base complexes with the carboxyl groups of polyamic acid to form strong microwave absorption sites. Microwaves stimulate the catalytic activity of the Lewis base, reducing the activation energy of the polyamic acid cyclization reaction and increasing the cyclization rate. This results in a cyclization rate of over 90% for the nascent fiber, simplifying the multiple heat treatment steps required in traditional processes, significantly improving production efficiency, reducing production costs, and meeting the needs of large-scale industrial production.

[0031] (2) Suppressing rigid shrinkage defects and pore formation: In response to the problem of "dumbbell-shaped" or "C-shaped" cross-section wrinkles and internal pores caused by the high volume shrinkage rate of rigid polyimide main chain during solvent evaporation, this invention introduces soluble polyimide flexible chain segments. Due to the high extensibility and solvent complexation effect of the flexible chain segments, the molecular chains can be tightly packed when the solvent evaporates, thereby achieving internal densification of the fiber, solving the shrinkage defects of the rigid outer layer and suppressing the formation of internal pores in the fiber.

[0032] (3) Combining rigidity and flexibility to synergistically improve processability and mechanical properties: Introducing flexible soluble polyimide segments (soft segments) into rigid polyamic acid spinning solution can effectively solve the problems of fiber breakage and dripping caused by premature cyclization of rigid segments. At the same time, in the dry spinning channel, the cyclized hard segments form a mechanical skeleton, the solubilized soft segments increase the fiber draw ratio, and the slippage of flexible segments induces the high orientation of hard segments, thereby achieving a synergistic improvement in fiber tensile strength and modulus.

[0033] (4) The present invention uses flash evaporation and microwave-assisted heating to make the solvent evaporate quickly and uniformly from the inside to the outside, avoiding the skin-core structure and cross-sectional deformation caused by uneven diffusion of solvent; polyamic acid can be converted into polyimide to a higher extent under microwave heating, reducing the proportion of complexed solvent in the fiber, thereby reducing the solvent content in the fiber; the nascent fiber prepared by this method has low cross-sectional deformation, good mechanical properties, low energy consumption, and can be industrialized. Attached Figure Description

[0034] Figure 1This is a cross-sectional scanning electron microscope image of the polyimide nascent fiber in Example 1.

[0035] Figure 2 This is a thermogravimetric analysis diagram of the polyimide nascent fibers in Example 2.

[0036] Figure 3 This is a cross-sectional scanning electron microscope image of the polyimide nascent fibers in Comparative Example 1.

[0037] Figure 4 This is a cross-sectional scanning electron microscope image of the polyimide nascent fibers in Comparative Example 2.

[0038] Figure 5 Thermogravimetric analysis diagram of polyimide nascent fibers in Comparative Example 2.

[0039] Figure 6 This is a digital image of the polyamic acid spinning assembly in Comparative Example 4. Detailed Implementation

[0040] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0041] Example 1

[0042] Step 1: In a 20L polymerization reactor, first add 3000g of dimethylacetamide (DMAc) as the initial solvent. Then add 1.228mol of... The mixture was stirred for 12 hours to ensure uniform dispersion. Then, 1.228 mol of [a specific ingredient] was slowly added while stirring. Then, 2000g of DMAc solvent was added to adjust the solid content to 15%. The polymerization reaction was carried out by stirring at 5°C for 10 hours. Then, 0.1228mol of isoquinoline was added to catalyze the cyclization reaction. The temperature was raised to 180°C and the reaction was continued for 10 hours to obtain polyimide A solution.

[0043] Step 2: In a 20L polymerization reactor, first add 3000g of dimethylacetamide (DMAc) as the initial solvent. Then add 1.7488mol of... The mixture was stirred for 12 hours to ensure uniform dispersion. Then, 1.7488 mol of [a specific ingredient] was slowly added under stirring. Then, 2000g of DMAc solvent was added to adjust the solid content to 15%. The polymerization reaction was carried out by stirring at 5°C for 10 hours. Then, 1.7488mol of isoquinoline was added and stirred evenly to obtain polyamic acid B solution.

[0044] Step 3: Mix the solutions prepared in Step 1 and Step 2 in equal mass ratio and stir thoroughly to obtain a uniform spinning solution. Allow the solution to stand at room temperature for 2 hours to remove bubbles, ensuring no residual bubbles remain. After filtration through a metal screen, the spinning solution is pumped into the spinning equipment and extruded through a circular spinneret into the spinning tunnel. The spinneret assembly is made of tantalum metal (TaNb3), with a pressure of 3 MPa and a temperature of 205°C. The tunnel is 10 meters long, with a hot air temperature of 205°C and a nitrogen atmosphere. A 20-cm-long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet, with a microwave frequency set to 2.45 GHz and a power of 30 kW. Microwave heating promotes uniform evaporation of the solvent from within the fiber, reducing the core-sheath effect.

[0045] Sample characterization: Primitive polyimide fibers with an approximately elliptical cross-section were obtained, as shown in the electron microscope image of their cross-section. Figure 1 As shown, the anisotropy is 12%, the tensile strength is 7.33 cN / dtex, and the elongation at break is 74.3%. The solvent residue (including high-temperature dehydration of polyamic acid) was calculated to be 9.6% and the degree of cyclization was 76.4% based on thermogravimetric analysis within the temperature range of 200℃ to 400℃.

[0046] Example 2

[0047] Step 1 and Step 2: Same as Step 1 and Step 2 in Example 1.

[0048] Step 3: Mix the solutions prepared in Step 1 and Step 2 at a mass ratio of 2:1 and stir thoroughly to ensure homogeneity. This yields the spinning solution. Allow it to stand at room temperature for 2 hours to remove air bubbles, ensuring no residual bubbles remain. After filtration through a metal screen, the spinning solution is pumped into the spinning equipment, extruded through a circular spinneret, and enters the spinning tunnel. The spinneret is made of tantalum metal (TaNb3), with a pressure of 3 MPa and a temperature of 205°C. The tunnel is 10 meters long, with a hot air temperature of 205°C and a nitrogen atmosphere. A 20-cm-long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet, with a microwave frequency set to 2.45 GHz and a power of 30 kW.

[0049] Sample characterization: Primary polyimide fibers with an approximately elliptical cross-section were obtained, with an anisotropy of 17%, a breaking strength of 6.55 cN / dtex, and an elongation at break of 66.7%. Thermogravimetric analysis was conducted within the temperature range of 200℃ to 400℃. Figure 2 The calculated solvent residue rate (including high-temperature dehydration of polyamic acid) was 12.5%, and the degree of cyclization was 68.6%.

[0050] Example 3

[0051] Step 1: Same as Step 1 in Example 1.

[0052] Step 2: Replace isoquinoline in Step 2 of Example 1 with triethylenediamine, which has a higher basicity, to improve microwave absorption rate, while keeping other steps unchanged.

[0053] Step 3: Mix the solutions prepared in Step 1 and Step 2 in equal mass ratio and stir thoroughly to obtain a uniform spinning solution. Allow the solution to stand at room temperature for 2 hours to remove air bubbles, ensuring no residual bubbles remain. After filtration through a metal screen, the spinning solution is pumped into the spinning equipment and extruded through a circular spinneret into the spinning tunnel. The spinneret is made of tantalum metal (TaNb3), with a pressure of 3 MPa and a temperature of 205°C. The tunnel is 10 meters long, with a hot air temperature of 205°C and a nitrogen atmosphere. A 20-cm-long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet, with a microwave frequency set to 2.45 GHz and a power of 30 kW.

[0054] Sample characterization: Approximately elliptical polyimide nascent fibers were obtained, with an anisotropy of 8%, a tensile strength of 7.93 cN / dtex, and an elongation at break of 85.2%. Thermogravimetric analysis within the temperature range of 200℃ to 400℃ yielded a solvent residue (including high-temperature dehydration of polyamic acid) of 5.5% and a degree of cyclization of 84.7%.

[0055] Example 4

[0056] Step 1 and Step 2: Same as Step 1 and Step 2 in Example 1.

[0057] Step 3: Mix the solutions prepared in Step 1 and Step 2 in equal mass ratio and stir thoroughly to obtain a uniform spinning solution. Allow the solution to stand at room temperature for 2 hours to remove air bubbles, ensuring no residual bubbles remain. After filtration through a metal screen, the spinning solution is pumped into the spinning equipment, extruded through a circular spinneret, and enters the spinning tunnel. The spinneret is made of tantalum metal (TaNb3), with a pressure of 3 MPa and a temperature of 205°C. The tunnel is 10 meters long, with a hot air temperature of 205°C and a nitrogen atmosphere. A 20-cm-long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet, with a microwave frequency set to 2.45 GHz and a power of 45 kW.

[0058] Sample characterization: Approximately elliptical polyimide nascent fibers were obtained, with an anisotropy of 6%, a tensile strength of 7.95 cN / dtex, and an elongation at break of 85.6%. Thermogravimetric analysis within the temperature range of 200℃ to 400℃ yielded a solvent residue (including high-temperature dehydration of polyamic acid) of 7.6% and a degree of cyclization of 87.3%.

[0059] Example 5

[0060] Step 1: Replace the flexible diamine in Step 1 of Example 1 with... All other conditions remain unchanged.

[0061] Step 2: Same as Step 2 in Example 1.

[0062] Step 3: Same as step 3 in Example 1.

[0063] Sample characterization: Approximately elliptical polyimide nascent fibers were obtained, with an anisotropy of 21%, a tensile strength of 6.97 cN / dtex, and an elongation at break of 68.8%. Thermogravimetric analysis within the temperature range of 200℃ to 400℃ yielded a solvent residue (including high-temperature dehydration of polyamic acid) of 10.3% and a degree of cyclization of 71.2%.

[0064] Example 6

[0065] Step 1: Same as Step 1 in Example 1.

[0066] Step 2: Replace the rigid diamine in Step 2 of Example 1 with... All other conditions remain unchanged.

[0067] Step 3: Same as step 3 in Example 1.

[0068] Sample characterization: Approximately elliptical polyimide nascent fibers were obtained, with an anisotropy of 13%, a tensile strength of 7.59 cN / dtex, and an elongation at break of 76.7%. Thermogravimetric analysis within the temperature range of 200℃ to 400℃ yielded a solvent residue (including high-temperature dehydration of polyamic acid) of 8.7% and a degree of cyclization of 80.1%.

[0069] Comparative Example 1

[0070] Step 1: Same as Step 1 in Example 1.

[0071] Step 2: In a 20L polymerization reactor, first add 3000g of dimethylacetamide (DMAc) as the initial solvent. Then add 1.7488mol of... The mixture was stirred for 12 hours to ensure uniform dispersion. Then, 1.7488 mol of [a specific ingredient] was slowly added under stirring. Then, 2000g of DMAc solvent was added to adjust the solid content to 15%. The polymerization reaction was carried out by stirring at 5°C for 10 hours, and then the mixture was stirred evenly to obtain polyamic acid B solution.

[0072] Step 3: Same as step 3 in Example 1.

[0073] Sample characterization: Primitive polyimide fibers with C-shaped or dumbbell-shaped cross-sections were obtained, and electron microscope images of their cross-sections are shown below. Figure 3 As shown, the anisotropy is 85%, the tensile strength is 3.12 cN / dtex, and the elongation at break is 32.2%. Thermogravimetric analysis (TGA) within the temperature range of 200℃ to 400℃ yielded a solvent residue rate (including high-temperature dehydration of polyamic acid) of 32% and a cyclization degree of 52.3%. Without a catalyst, the optimized cyclization reaction has a high energy barrier and a slow rate, resulting in a low cyclization degree and high solvent residue in the nascent fibers, thus reducing their mechanical properties.

[0074] Comparative Example 2

[0075] Step 1: Same as Step 1 in Example 1.

[0076] Step 2: Same as Step 2 in Example 1.

[0077] Step 3: Do not turn on the microwave heater during the spinning process. Other steps are the same as Step 3 in Example 1.

[0078] Sample characterization: Primary polyimide fibers with C-shaped or dumbbell-shaped cross-sections were obtained. Figure 4 The anisotropy was 68%, the breaking strength was 4.25 cN / dtex, and the elongation at break was 43.1%. The thermal weight loss was measured within the temperature range of 200℃ to 400℃. Figure 5 The calculated solvent residue rate (including high-temperature dehydration of polyamic acid) was 24%, and the degree of cyclization was 75.4%. Even without microwave heating, although the nascent fibers exhibited a high degree of cyclization under the influence of a catalyst, the uneven diffusion of solvent within the fibers resulted in high irregularity, which in turn reduced their mechanical properties.

[0079] Comparative Example 3

[0080] Step 1: In a 20L polymerization reactor, first add 3000g of dimethylacetamide (DMAc) as the initial solvent. Then add 1.228mol of... The mixture was stirred for 12 hours to ensure uniform dispersion. Then, 1.228 mol of [a specific ingredient] was slowly added while stirring. Then, 2000g of DMAc solvent was added to adjust the solid content to 15%. The polymerization reaction was carried out by stirring at 5°C for 10 hours. Then, 0.1228mol of isoquinoline was added to catalyze the cyclization reaction. The temperature was raised to 180°C and the reaction was continued for 10 hours to obtain polyimide A solution.

[0081] Step 2: Using the solution prepared in Step 1 as the spinning solution, allow it to stand at room temperature for 2 hours to degas, ensuring no air bubbles remain inside the liquid. After filtration through a metal screen, the spinning solution is pumped into the spinning equipment, extruded through a circular spinneret, and enters the spinning tunnel. The spinneret assembly is made of tantalum metal with the grade TaNb3, with an assembly pressure of 3 MPa and an assembly temperature of 205°C. The tunnel is 10 meters long, with a hot air temperature of 205°C and a nitrogen atmosphere. A 20-cm-long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet, with the microwave frequency set to 2.45 GHz and the power to 30 kW.

[0082] Sample characterization: Circular cross-section polyimide nascent fibers were obtained, with an anisotropy of 2%, a breaking strength of 3.31 cN / dtex, and an elongation at break of 96%. The prepared polyimide fiber matrix is ​​soluble polyimide, whose molecular chains are flexible, resulting in low mechanical properties and insufficient heat resistance.

[0083] Comparative Example 4

[0084] Step 1: In a 20L polymerization reactor, first add 3000g of dimethylacetamide (DMAc) as the initial solvent. Then add 1.7488mol of... The mixture was stirred for 12 hours to ensure uniform dispersion. Then, 1.7488 mol of [a specific ingredient] was slowly added under stirring. Then, 2000g of DMAc solvent was added to adjust the solid content to 15%. The polymerization reaction was carried out by stirring at 5°C for 10 hours. Then, 1.7488mol of isoquinoline was added and stirred evenly to obtain polyamic acid B solution.

[0085] Step 2: Using the solution prepared in Step 1 as the spinning solution, allow it to stand at room temperature for 2 hours to degas, ensuring no air bubbles remain inside the liquid. After filtration through a metal screen, the spinning solution is pumped into the spinning equipment, extruded through a circular spinneret, and enters the spinning tunnel. The spinneret assembly is made of tantalum metal with the grade TaNb3, with an assembly pressure of 3 MPa and an assembly temperature of 205°C. The tunnel is 10 meters long, with a hot air temperature of 205°C and a nitrogen atmosphere. A 20-cm-long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet, with the microwave frequency set to 2.45 GHz and the power to 30 kW.

[0086] Sample characterization: During the spinning process, such as Figure 6 As shown: Due to the low spinnability of rigid polyamic acid solutions, the spinning solution forms a diffuse state on the spinneret under the same spinning process, making it impossible to stably form fibers.

[0087] Example 7

[0088] Step 1: In a 20L polymerization reactor, first add 3000g of N-methylpyrrolidone (NMP) as the initial solvent. Then add 1.215mol of... The mixture was stirred for 12 hours to ensure uniform dispersion. Then, 1.215 mol of [a specific ingredient] was slowly added while stirring. Then, 2000g of NMP solvent was added to adjust the solid content to 15%. The polymerization reaction was carried out by stirring at 5°C for 10 hours. Then, 0.1215mol of isoquinoline was added to catalyze the cyclization reaction. The temperature was raised to 180°C and the reaction was continued for 10 hours to obtain polyimide A solution.

[0089] Step 2: In a 20L polymerization reactor, first add 3000g of N-methylpyrrolidone (NMP) as the initial solvent. Then add 1.3912mol of... The mixture was stirred for 12 hours to ensure uniform dispersion. Then, 1.3912 mol of [a specific compound / component] was slowly added under stirring. Then, 2000g of NMP solvent was added to adjust the solid content to 15%. The polymerization reaction was carried out by stirring at 5°C for 10 hours. Then, 1.3912mol of isoquinoline was added and stirred evenly to obtain polyamic acid B solution.

[0090] Step 3: Mix the solutions prepared in Step 1 and Step 2 in equal mass ratio and stir thoroughly to obtain a uniform spinning solution. Allow the solution to stand at room temperature for 2 hours to remove air bubbles, ensuring no residual bubbles remain. After filtration through a metal screen, the spinning solution is pumped into the spinning equipment, extruded through a circular spinneret, and enters the spinning tunnel. The spinneret is made of tantalum metal (TaNb3), with a pressure of 3 MPa and a temperature of 205°C. The tunnel is 10 meters long, with a hot air temperature of 205°C and a nitrogen atmosphere. A 20-cm-long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet, with a microwave frequency set to 2.45 GHz and a power of 30 kW. Microwave heating promotes uniform evaporation of the solvent from within the fiber, reducing the core-sheath effect.

[0091] Sample characterization: Approximately elliptical polyimide nascent fibers were obtained, with an anisotropy of 17%, a tensile strength of 7.11 cN / dtex, and an elongation at break of 68.2%. Thermogravimetric analysis within the temperature range of 200℃ to 400℃ yielded a solvent residue (including high-temperature dehydration of polyamic acid) of 11.5% and a degree of cyclization of 79.5%.

[0092] Example 8

[0093] Step 1: Same as Step 1 in Example 7.

[0094] Step 2: Same as Step 2 in Example 7.

[0095] Step 3: Change the hot air temperature in Step 3 of Example 7 to 235°C, while keeping other conditions unchanged.

[0096] Sample characterization: Approximately elliptical polyimide nascent fibers were obtained, with an anisotropy of 11%, a tensile strength of 7.86 cN / dtex, and an elongation at break of 54.1%. Thermogravimetric analysis within the temperature range of 200℃ to 400℃ yielded a solvent residue (including high-temperature dehydration of polyamic acid) of 7.6% and a degree of cyclization of 88.2%.

[0097] A comparison of Example 1 and Comparative Example 1 shows that the Lewis base introduced in this invention can increase the cyclization degree of nascent fibers, thereby increasing the strength of nascent fibers and reducing their irregularity. A comparison of Example 1 and Comparative Example 2 shows that the microwave heater introduced in this invention can efficiently thermally cyclize PAA into PI, thereby increasing the strength and cyclization degree of nascent fibers. A comparison of Example 1 and Comparative Example 3 shows that although fibers spun from pure soluble PI have low irregularity, the flexible structure of soluble PI molecules determines their low strength. A comparison of Example 1 and Comparative Example 4 shows that rigid PAA solutions have poor spinnability, so it is necessary to add a PI solution with flexible molecules to control its spinnability. A comparison of Example 1 and Example 3 shows that smaller Lewis base molecules with stronger alkalinity have higher microwave absorption rates, which can further increase the cyclization degree of nascent fibers, thereby improving the fiber's mechanical properties.

Claims

1. A method for dry spinning polyimide nascent fibers, comprising the following steps: (1) An equimolar ratio of anhydride monomer, flexible diamine monomer, and polar solvent are reacted at 0-60°C for 6-12 hours, followed by the addition of a cyclization catalyst. The temperature is then raised to 160-200°C and the reaction continues for 8-16 hours to obtain a polyimide A solution. The flexible diamine monomer is at least one of the following structures: ， ， ， ， ， ； (2) After reacting an anhydride monomer, a rigid diamine monomer, and a polar solvent in an equimolar ratio at 0-60°C for 6-12 hours, a Lewis base is added, and the mixture is stirred until homogeneous to obtain a polyamic acid B solution; the rigid diamine monomer is at least one of the following structures: ， ， ， ， ， ， ； The anhydride monomer in steps (1) and (2) is at least one of the following structures: ， ， ， ； (3) The polyimide A solution and the polyamic acid B solution are mixed and stirred at 25~80℃ for 3~6 hours to obtain the spinning solution; (4) The above spinning solution is degassed and filtered under vacuum, and then extruded into the dry spinning channel through a spinneret. Under flash evaporation and microwave-assisted heating conditions, the spinning solution is solidified into polyimide nascent fibers; wherein, The microwave-assisted heating uses a microwave heater that covers the circumference of the tunnel. The distance between the microwave heater and the spinneret outlet is 1cm to 50cm, and the microwave heater covers 1% to 50% of the tunnel length.

2. The method according to claim 1, characterized in that: The polar solvent in steps (1) and (2) is at least one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, γ-butyrolactone, p-chlorophenol, sulfolane, cyclohexane, and m-cresol.

3. The method according to claim 1, characterized in that: The Lewis base in step (2) is at least one of 4-dimethylaminopyridine, isoquinoline, triethylenediamine, and N,N-diisopropylethylamine; and its molar ratio with the rigid diamine monomer is 0.5 to 5:

1.

4. The method according to claim 1, characterized in that: The solid content of the polyimide A solution is 5 wt% to 15 wt%; the solid content of the polyamic acid B solution is 15 wt% to 30 wt%; and the solid content of the spinning solution is 10 wt% to 30 wt%.

5. The method according to claim 1, characterized in that: The tunnel atmosphere in step (4) is one of nitrogen, carbon dioxide, or argon, and the tunnel temperature is 120℃~350℃.

6. The method according to claim 1, characterized in that: The solvent content of the polyimide nascent fiber obtained in step (4) is 5%~30%, and the degree of cyclization is 20%~90%.

7. The method according to claim 1, characterized in that: The tensile strength of the polyimide nascent fiber obtained in step (4) is 1.5~8 cN / dtex, and the elongation at break is 50~90%.

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