Method for dry spinning of polyimide nascent fiber

Abstract

The invention relates to a method for dry spinning of polyimide nascent fibers. The method mainly comprises the following steps: mixing and stirring a polyimide A solution and a polyamide acid B solution to obtain a spinning solution; the spinning solution is subjected to vacuum defoaming and filtering and extruded into a dry spinning channel through a spinneret plate, and the spinning solution is cured into polyimide nascent fibers under the conditions of flash evaporation and microwave-assisted heating; the solvent is quickly and uniformly evaporated from inside to outside through a flash evaporation and microwave-assisted heating method, so that the skin-core structure and section irregularization caused by non-uniform diffusion of the solvent is avoided; polyamide acid can be converted into polyimide to a higher degree under microwave heating, the proportion of a complexing solvent in the fiber is reduced, and therefore the content of the solvent in the fiber is reduced; the nascent fiber prepared by the method is low in section profile degree, good in mechanical property and low in energy consumption, and industrialization can be realized.

Description

Technical Field

[0001] The invention belongs to the field of high-performance organic fibers, and in particular relates to a method for dry-spinning polyimide primary fibers. Background Art

[0002] As a high-performance organic fiber, polyimide fiber, with its excellent high-temperature and chemical resistance and good mechanical properties, is widely used in high-temperature filtration materials, protective clothing, aerospace, and electrical insulation. Dry spinning is one of the main methods for producing polyimide fibers. The process involves dissolving a polyamic acid precursor in a polar solvent, extruding it through a spinneret, and evaporating the solvent in a high-temperature channel to produce the fibers. However, during the solvent evaporation process, uneven heating of the spinning solution inside and on the surface of the fiber leads to differences in solvent evaporation rate, molecular chain orientation, and degree of cyclization, resulting in a skin-core structure. Furthermore, under the action of drafting forces, the fiber cross-section deforms, affecting its mechanical properties and stability during subsequent processing. This phenomenon is common not only in polyimide fibers but also in dry spinning of materials such as acrylonitrile and cellulose acetate fibers, resulting in "dumbbell" or "C-shaped" cross-sections. While these shaped fibers have unique advantages in certain applications, they often compromise the mechanical properties and consistency of the fibers. Especially when the polymer molecular chain is rigid or the intermolecular force is strong, the rapidly solidified rigid skin will collapse when the core layer solvent continues to evaporate, forming a "dumbbell-shaped" or "C-shaped" cross-section with internal pores, which seriously reduces the mechanical strength of the fiber.

[0003] Currently, some patents have proposed solutions for improving fiber cross-sectional shape by adjusting spinning process parameters or introducing auxiliary devices. For example, Chinese patent CN101543722B proposes a method for reducing fiber cross-sectional irregularity by adjusting the spinning solution temperature and spinneret speed; while US Patent US8968553B2 describes a process for optimizing fiber morphology by improving spinneret design. While these methods improve fiber cross-sectional shape to a certain extent, they still face issues such as uneven heating and difficulty in precisely controlling solvent evaporation rates. In particular, the spinning process is prone to localized overheating of the fiber or solvent residue, which can affect 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 primary fibers. This method uses flash evaporation and microwave-assisted heating to quickly and evenly evaporate the solvent from the inside to the outside, effectively avoiding the skin-core structure and cross-sectional deformation caused by uneven diffusion of the solvent; at the same time, by blending a certain proportion of flexible polyimide segments, the flexibility and collapse resistance of the fiber are increased, thereby solving the problem of "dumbbell-shaped" or "C-shaped" fibers caused by the collapse of the outer cortex.

[0005] The present invention provides a method for dry-spinning polyimide spun fibers, comprising the following steps:

[0006] (1) An anhydride monomer, a flexible diamine monomer, and a polar solvent in equal molar ratios are reacted at 0-60°C for 6-12 hours, and then a cyclization catalyst is added. The temperature is raised to 160-200°C and the reaction is continued for 8-16 hours to obtain a polyimide A solution;

[0007] (2) reacting anhydride monomer, rigid diamine monomer and polar solvent in equal molar ratio at 0-60°C for 6-12 hours, then adding Lewis base and stirring to obtain polyamic acid B solution;

[0008] (3) Mixing the polyimide A solution and the polyamic acid B solution at 25-80°C and stirring for 3-6 hours to obtain a spinning solution;

[0009] (4) The spinning solution is vacuum degassed and filtered, and then squeezed into a dry spinning tunnel through a spinneret. The spinning solution is solidified into polyimide primary fibers under the conditions of flash evaporation and microwave-assisted heating.

[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, the amount of which is 10-20% of the anhydride monomer, and 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 the molar ratio of the Lewis base to the rigid diamine monomer is 0.5 to 5:1. The Lewis base has a strong microwave absorption effect and mainly complexes with the polyamic acid unit, which can promote the absorption of microwaves by the spinning solution and catalyze the cyclization reaction of the 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 number of holes in the spinneret in step (4) is 20 to 50, and the hole spacing is 5 to 20 mm.

[0021] Preferably, the spinneret has a diameter of 0.1 to 0.3 mm and an aspect ratio of 3 to 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° C., and the assembly pressure is 2-5 MPa, so as to improve the flash evaporation effect.

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

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

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

[0027] Preferably, the solvent content of the polyimide spun fibers 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 spun fiber obtained in step (4) is 1.5-8 cN / dtex, and the elongation at break is 50-90%.

[0029] Beneficial effects

[0030] (1) Eliminating the skin-core structure and achieving in-situ cyclization: The present invention introduces a Lewis base and uses microwave heating to target the microwave energy to the interior of the fiber, achieving uniform diffusion of the solvent. This solves the problem of the skin-core structure formed by the rapid volatilization of the outer layer solvent caused by traditional hot air heating, and avoids the formation of irregular cross-sections during subsequent drawing. At the same time, the Lewis base complexes with the carboxyl groups of the 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 reaction rate. The cyclization rate of the primary fiber is as high as 90% or more. This simplifies the multiple heat treatment steps required in traditional processes, significantly improves production efficiency, reduces production costs, and can meet the needs of industrial large-scale production.

[0031] (2) Inhibition of rigid shrinkage defects and pore formation: To address the problem of "dumbbell-shaped" or "C-shaped" cross-sectional wrinkles and internal pores caused by the high volume shrinkage rate of the rigid polyimide main chain when the solvent evaporates, the present invention introduces a soluble polyimide flexible chain segment. Due to the high ductility and solvent complexation effect of the flexible chain segment, the molecular chains can be tightly stacked when the solvent evaporates, achieving densification inside the fiber, solving the shrinkage defects of the rigid outer layer, and inhibiting the formation of pores inside the fiber.

[0032] (3) Synergistically improving processability and mechanical properties by combining rigidity and flexibility: By introducing flexible, soluble polyimide segments (soft segments) into the rigid polyamide acid spinning solution, the solvation layer can effectively solve the problems of broken fibers and dripping caused by premature cyclization of the rigid segments. At the same time, in the dry spinning tunnel, the cyclized hard segments build a mechanical skeleton, the solvated soft segments increase the fiber draft ratio, and the slippage of the flexible segments induces a high degree of orientation of the hard segments, achieving a synergistic improvement in the tensile strength and modulus of the fiber.

[0033] (4) The present invention uses flash evaporation and microwave-assisted heating methods to allow the solvent to evaporate quickly and evenly from the inside to the outside, avoiding the skin-core structure and cross-sectional deformation caused by uneven diffusion of the solvent; polyamic acid can be converted into polyimide to a higher degree under microwave heating, reducing the proportion of complexing 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. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0035] Figure 2 This is the thermogravimetric analysis diagram of the polyimide as-spun fiber in Example 2.

[0036] Figure 3 This is a scanning electron microscope image of the cross section of the polyimide as-spun fiber in Comparative Example 1.

[0037] Figure 4 This is a scanning electron microscope image of the cross section of the polyimide as-spun fiber in Comparative Example 2.

[0038] Figure 5 This is the thermogravimetric analysis diagram of the polyimide as-spun fiber in Comparative Example 2.

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

[0040] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0041] Example 1

[0042] Step 1: In a 20L polymerization reactor, first add 3000g of dimethylacetamide (DMAc) as the initial solvent. Add 1.228mol of , and stirred for 12h to make it uniformly dispersed. Then slowly add 1.228mol of , and then add 2000g of DMAc solvent to adjust the solid content to 15%, stir and carry out polymerization reaction at 5°C for 10 hours, then add 0.1228mol of isoquinoline to catalyze the cyclization reaction, raise the temperature to 180°C and continue the reaction 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 , and stirred for 12h to make it uniformly dispersed. Then slowly add 1.7488mol of 2000 g of DMAc solvent was added to adjust the solid content to 15%. The polymerization reaction was continued at 5°C with stirring for 10 hours, and then 1.7488 mol of isoquinoline was added and stirred to obtain a polyamic acid B solution.

[0044] Step 3: Mix the solutions prepared in steps 1 and 2 in equal proportions by mass and stir thoroughly to achieve a uniform mixture, thereby obtaining a spinning solution. The solution was allowed to stand at room temperature for 2 hours to degas, ensuring that no bubbles remained within the solution. After filtering through a metal mesh, the spinning solution was pumped into the spinning equipment via a spinning pump. It was extruded through a circular spinneret and then into the spinning tunnel. The spinneret assembly was made of tantalum metal (grade TaNb3). The assembly pressure was 3 MPa and the assembly temperature was 205°C. The tunnel was 10 meters long, with a hot air temperature of 205°C and a nitrogen atmosphere. A 20-cm-long annular microwave-assisted heater was installed 5 cm from the spinneret outlet. The microwave frequency was set to 2.45 GHz and the power was 30 kW. Microwave heating promoted uniform evaporation of the solvent from the fiber interior, minimizing the skin-core effect.

[0045] Sample characterization: The obtained polyimide as-spun fiber has an approximately elliptical cross section. The electron microscope photo of its cross section is shown in the figure below. Figure 1 As shown, the irregularity is 12%, the breaking strength is 7.33 cN / dtex, and the elongation at break is 74.3%. The residual solvent rate (including high-temperature dehydration of polyamic acid) calculated by thermogravimetric analysis at a temperature range of 200°C to 400°C is 9.6%, and the degree of cyclization is 76.4%.

[0046] Example 2

[0047] Step 1 and step 2: the same as step 1 and step 2 of embodiment 1.

[0048] Step 3: Mix the solutions prepared in Step 1 and Step 2 in a mass ratio of 2:1 and stir thoroughly to ensure uniform mixing. This results in a spinning solution. The solution is allowed to stand at room temperature for 2 hours to degas, ensuring that no bubbles remain. After filtering through a metal mesh, the spinning solution is pumped into the spinning equipment via a spinning pump. It is extruded through a spinneret with a circular cross-section and enters the spinning tunnel. The spinneret assembly is made of tantalum metal (grade TaNb3), with an assembly pressure of 3 MPa and an assembly temperature of 205°C. The tunnel is 10 meters long, the hot air temperature in the tunnel is 205°C, and the atmosphere is nitrogen. A 20-cm-long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet. The microwave frequency is set to 2.45 GHz and the power is 30 kW.

[0049] Sample characterization: The obtained polyimide raw fiber has an approximately elliptical cross section, a profile of 17%, a breaking strength of 6.55 cN / dtex, and an elongation at break of 66.7%. Figure 2 ) The calculated solvent residual rate (including high-temperature dehydration of polyamic acid) was 12.5%, and the cyclization degree was 68.6%.

[0050] Example 3

[0051] Step 1: Same as step 1 of Example 1.

[0052] Step 2: The isoquinoline in step 2 of Example 1 was replaced with triethylenediamine, which has a higher alkalinity, to improve the microwave absorption rate. The other steps remained unchanged.

[0053] Step 3: Mix the solutions prepared in Steps 1 and 2 in equal proportions by mass and stir thoroughly to ensure uniform mixing to obtain a spinning solution. Allow the solution to stand at room temperature for 2 hours to degas to ensure that no bubbles remain within the solution. After filtering through a metal mesh, the spinning solution is pumped into the spinning equipment via a spinning pump, extruded through a spinneret with a circular cross-section, and then into the spinning tunnel. The spinneret assembly is made of tantalum metal with the grade TaNb3, the assembly pressure is 3 MPa, and the assembly temperature is 205°C. The tunnel is 10 meters long, the hot air temperature in the tunnel is 205°C, and the atmosphere is nitrogen. A 20 cm long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet. The microwave frequency is set to 2.45 GHz and the power is 30 kW.

[0054] Sample Characterization: The resulting polyimide as-spun fibers had a nearly elliptical cross-section, a profile of 8%, a breaking strength of 7.93 cN / dtex, and an elongation at break of 85.2%. The residual solvent content (including high-temperature dehydration of the polyamic acid) was 5.5%, and the degree of cyclization was 84.7%, calculated by thermogravimetric analysis at temperatures between 200°C and 400°C.

[0055] Example 4

[0056] Step 1 and step 2: the same as step 1 and step 2 of embodiment 1.

[0057] Step 3: Mix the solutions prepared in steps 1 and 2 in equal proportions by mass and stir thoroughly to ensure uniform mixing of the solutions to obtain a spinning solution. Allow the solution to stand at room temperature for 2 hours to degas to ensure that no bubbles remain inside the solution. After filtering through a metal filter, the spinning solution is pumped into the spinning equipment via a spinning pump, extruded through a spinneret with a circular cross-section, and enters the spinning tunnel. The spinneret assembly is made of tantalum metal with the grade TaNb3, the assembly pressure is 3 MPa, and the assembly temperature is 205°C. The tunnel is 10 meters long, the hot air temperature of the tunnel is 205°C, and the atmosphere is nitrogen. A 20 cm long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet. The microwave frequency is set to 2.45 GHz and the power is 45 kW.

[0058] Sample Characterization: The resulting polyimide as-spun fibers had a nearly elliptical cross-section, a profile of 6%, a breaking strength of 7.95 cN / dtex, and an elongation at break of 85.6%. The residual solvent content (including high-temperature dehydration of the polyamic acid) was 7.6%, and the degree of cyclization was 87.3%, calculated by thermogravimetric analysis at temperatures between 200°C and 400°C.

[0059] Example 5

[0060] Step 1: Replace the flexible diamine in step 1 of Example 1 with , 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: The resulting polyimide as-spun fibers had a nearly elliptical cross-section, a profile of 21%, a breaking strength of 6.97 cN / dtex, and an elongation at break of 68.8%. The residual solvent content (including high-temperature dehydration of the polyamic acid) was 10.3%, and the degree of cyclization was 71.2%, calculated by thermogravimetric analysis at temperatures between 200°C and 400°C.

[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 , other conditions remain unchanged.

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

[0068] Sample Characterization: The resulting polyimide as-spun fibers had a nearly elliptical cross-section, a profile of 13%, a breaking strength of 7.59 cN / dtex, and an elongation at break of 76.7%. The residual solvent content (including high-temperature dehydration of the polyamic acid) was 8.7%, and the degree of cyclization was 80.1%, calculated by thermogravimetric analysis over a temperature range of 200°C to 400°C.

[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 , and stirred for 12h to make it uniformly dispersed. Then slowly add 1.7488mol of 2000 g of DMAc solvent was added to adjust the solid content to 15%. The polymerization reaction was carried out at 5°C with stirring for 10 hours, and then the mixture was stirred evenly to obtain a polyamic acid B solution.

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

[0073] Sample characterization: The obtained polyimide spun fibers have a C-shaped or dumbbell-shaped cross section. The electron microscope photos of the cross section are as follows: Figure 3 As shown, the profile is 85%, the breaking strength is 3.12 cN / dtex, and the elongation at break is 32.2%. The residual solvent content (including high-temperature dehydration of polyamic acid) calculated by thermogravimetric analysis at temperatures between 200°C and 400°C is 32%, and the cyclization degree is 52.3%. In the absence of a catalyst, the optimized cyclization reaction has a high energy barrier and a slow reaction rate, resulting in a low cyclization degree and high residual solvent in the spun fiber, which in turn reduces the mechanical properties of the spun fiber.

[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: The microwave heater is not turned on during the spinning process, and the other steps are consistent with Step 3 of Example 1.

[0078] Sample characterization: The polyimide spun fibers with C-shaped or dumbbell-shaped cross-sections were obtained ( Figure 4 ), the degree of irregularity is 68%, the breaking strength is 4.25cN / dtex, and the breaking elongation is 43.1%. Through the thermal gravimetric analysis at a temperature range of 200℃~400℃ ( Figure 5 ) calculated the residual solvent rate (including high-temperature dehydration of polyamic acid) to be 24% and the degree of cyclization to be 75.4%. Without microwave heating, although the cyclization degree of the as-spun fibers was also high under the action of a catalyst, the uneven diffusion of solvent within the fibers resulted in a high degree of irregularity, which in turn reduced their mechanical properties.

[0079] Comparative Example 3 Step 1: In a 20L polymerization reactor, first add 3000g of dimethylacetamide (DMAc) as the initial solvent. Add 1.228mol of , and stirred for 12h to make it uniformly dispersed. Then slowly add 1.228mol of , and then add 2000g of DMAc solvent to adjust the solid content to 15%, stir and carry out polymerization reaction at 5°C for 10 hours, then add 0.1228mol of isoquinoline to catalyze the cyclization reaction, raise the temperature to 180°C and continue the reaction for 10 hours to obtain polyimide A solution.

[0080] Step 2: The solution prepared in Step 1 is used as the spinning solution. It is allowed to stand at room temperature for 2 hours to degas. This ensures that no bubbles remain inside the liquid. After filtering through a metal filter, the spinning solution is pumped into the spinning equipment via a spinning pump. It is extruded through a spinneret with a circular cross-section and enters the spinning tunnel. The spinneret assembly is made of tantalum metal with the grade TaNb3. The assembly pressure is 3 MPa and the assembly temperature is 205°C. The tunnel is 10 meters long, the hot air temperature in the tunnel is 205°C, and the atmosphere is nitrogen. A 20 cm long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet. The microwave frequency is set to 2.45 GHz and the power is 30 kW.

[0081] Sample Characterization: The resulting polyimide fibers had a circular cross-section, a profile of 2%, a breaking strength of 3.31 cN / dtex, and an elongation at break of 96%. The resulting polyimide fibers are based on a soluble polyimide matrix with flexible molecular chains, resulting in low mechanical properties and insufficient heat resistance.

[0082] Comparative Example 4

[0083] Step 1: In a 20L polymerization reactor, first add 3000g of dimethylacetamide (DMAc) as the initial solvent. Add 1.7488mol of , and stirred for 12h to make it uniformly dispersed. Then slowly add 1.7488mol of 2000 g of DMAc solvent was added to adjust the solid content to 15%. The polymerization reaction was continued at 5°C with stirring for 10 hours, and then 1.7488 mol of isoquinoline was added and stirred to obtain a polyamic acid B solution.

[0084] Step 2: The solution prepared in Step 1 is used as the spinning solution. It is allowed to stand at room temperature for 2 hours to degas. This ensures that no bubbles remain inside the liquid. After filtering through a metal filter, the spinning solution is pumped into the spinning equipment via a spinning pump. It is extruded through a spinneret with a circular cross-section and enters the spinning tunnel. The spinneret assembly is made of tantalum metal with the grade TaNb3. The assembly pressure is 3 MPa and the assembly temperature is 205°C. The tunnel is 10 meters long, the hot air temperature in the tunnel is 205°C, and the atmosphere is nitrogen. A 20 cm long annular microwave auxiliary heater is installed 5 cm from the spinneret outlet. The microwave frequency is set to 2.45 GHz and the power is 30 kW.

[0085] Sample characterization: During the spinning process, e.g. Figure 6 As shown in the figure: Due to the low spinnability of the rigid polyamic acid solution, the fiber forms a diffuse flow state on the spinneret under the same spinning process and cannot be stably formed into fibers.

[0086] Example 7

[0087] Step 1: In a 20L polymerization reactor, first add 3000g of N-methylpyrrolidone (NMP) as the initial solvent. Add 1.215mol of , and stirred for 12h to make it uniformly dispersed. Then slowly add 1.215mol of , and then add 2000g of NMP solvent to adjust the solid content to 15%, stir and carry out polymerization reaction at 5°C for 10 hours, then add 0.1215mol of isoquinoline to catalyze the cyclization reaction, raise the temperature to 180°C and continue the reaction for 10 hours to obtain polyimide A solution.

[0088] Step 2: In a 20L polymerization reactor, first add 3000g of N-methylpyrrolidone (NMP) as the initial solvent. Add 1.3912mol of , and stirred for 12h to make it uniformly dispersed. Then slowly add 1.3912mol of 2000 g of NMP solvent was added to adjust the solid content to 15%. The polymerization reaction was continued at 5°C with stirring for 10 hours, and then 1.3912 mol of isoquinoline was added and stirred to obtain a polyamic acid B solution.

[0089] Step 3: Mix the solutions prepared in steps 1 and 2 in equal proportions by mass and stir thoroughly to obtain a spinning solution. The solution was allowed to stand at room temperature for 2 hours to degas, ensuring that no bubbles remained. After filtering through a metal mesh, the spinning solution was pumped into the spinning equipment, extruded through a circular spinneret, and then into the spinning tunnel. The spinneret assembly was made of tantalum metal (grade TaNb3), with an assembly pressure of 3 MPa and an assembly temperature of 205°C. The tunnel was 10 meters long, with a hot air temperature of 205°C and a nitrogen atmosphere. A 20-cm-long annular microwave-assisted heater was installed 5 cm from the spinneret outlet. The microwave frequency was set to 2.45 GHz and the power was 30 kW. Microwave heating promoted uniform evaporation of the solvent from the fiber interior, minimizing the skin-core effect.

[0090] Sample Characterization: The resulting polyimide as-spun fibers had a nearly elliptical cross-section, a profile of 17%, a breaking strength of 7.11 cN / dtex, and an elongation at break of 68.2%. The residual solvent content (including high-temperature dehydration of the polyamic acid) was 11.5%, and the degree of cyclization was 79.5%, calculated by thermogravimetric analysis at temperatures between 200°C and 400°C.

[0091] Example 8

[0092] Step 1: Same as step 1 in Example 7.

[0093] Step 2: Same as step 2 in Example 7.

[0094] Step 3: Change the hot air temperature in step 3 of Example 7 to 235°C, and keep other conditions unchanged.

[0095] Sample Characterization: The resulting polyimide as-spun fibers had a nearly elliptical cross-section, an 11% profile, a breaking strength of 7.86 cN / dtex, and an elongation at break of 54.1%. The residual solvent content (including high-temperature dehydration of the polyamic acid) was 7.6%, and the degree of cyclization was 88.2%, calculated by thermogravimetric analysis at temperatures between 200°C and 400°C.

[0096] Comparing Example 1 with Comparative Example 1 shows that the Lewis base introduced in the present invention can increase the cyclization degree of the spun fiber, thereby increasing the strength of the spun fiber and reducing the degree of irregularity. Comparing Example 1 with Comparative Example 2 shows that the microwave heater introduced in the present invention can efficiently thermally cyclize PAA to PI, thereby increasing the strength and cyclization degree of the spun fiber. Comparing Example 1 with Comparative Example 3 shows that although the fiber spun from pure soluble PI has a low degree of irregularity, the flexible structure of the soluble PI molecules determines its low strength. Comparing Example 1 with Comparative Example 4 shows that the spinnability of the rigid PAA solution is poor, so it is necessary to add a PI solution with flexible molecules to regulate its spinnability. Comparing Example 1 with Example 3 shows that the more alkaline Lewis base small molecule has a higher microwave absorptivity, which can further increase the cyclization degree of the spun fiber, thereby improving the mechanical properties of the fiber.

Claims

1. A method for dry spinning polyimide spun fibers, comprising the following steps: (1) An anhydride monomer, a flexible diamine monomer, and a polar solvent in equal molar ratios are reacted at 0-60°C for 6-12 hours, and then a cyclization catalyst is added. The temperature is raised to 160-200°C and the reaction is continued for 8-16 hours to obtain a polyimide A solution; (2) reacting anhydride monomer, rigid diamine monomer and polar solvent in equal molar ratio at 0-60°C for 6-12 hours, then adding Lewis base and stirring to obtain polyamic acid B solution; (3) Mixing the polyimide A solution and the polyamic acid B solution at 25-80°C and stirring for 3-6 hours to obtain a spinning solution; (4) The spinning solution is vacuum degassed and filtered, and then squeezed into a dry spinning tunnel through a spinneret. The spinning solution is solidified into polyimide primary fibers under the conditions of flash evaporation and microwave-assisted heating.

2. The method according to claim 1, wherein: The anhydride monomer in steps (1) and (2) is at least one of the following structures: ， ， ， 。 3. The method according to claim 1, wherein: The flexible diamine monomer in step (1) is at least one of the following structures: ， ， ， ， ， ； The rigid diamine monomer in step (2) is at least one of the following structures: ， ， ， ， ， ， 。 4. The method according to claim 1, wherein: 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.

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

1.

6. The method according to claim 1, wherein: 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%.

7. The method according to claim 1, wherein: The tunnel atmosphere in step (4) is one of nitrogen, carbon dioxide, and argon, and the tunnel temperature is 120°C to 350°C.

8. The method according to claim 1, wherein: The microwave heater used in the microwave-assisted heating in step (4) covers the entire tunnel, the distance between the microwave heater and the spinneret outlet is 1 cm to 50 cm, and the microwave heater covers 1 to 50% of the tunnel length.

9. The method according to claim 1, wherein: The solvent content of the polyimide spun fibers obtained in step (4) is 5% to 30%, and the degree of cyclization is 20% to 90%.

10. The method according to claim 1, wherein: The tensile strength of the polyimide spun fibers obtained in step (4) is 1.5-8 cN / dtex, and the elongation at break is 50-90%.

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