A polyimide as-spun fiber, fiber and method of making and use thereof

Abstract

The present application relates to a kind of polyimide primary fibers, fiber and its preparation method and application, comprising: polyamide acid spinning solution and chemical cyclization auxiliary are mixed at 0 ℃ below, spinning solution is obtained, then the spinning solution is first treated by stepwise temperature rising and then extruded into spinning channel, winding, the preparation of primary fiber is obtained after drawing.This preparation method can simplify process flow, and the polyimide fiber prepared has excellent drawability, orientation degree and mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of advanced functional materials, and specifically relates to a polyimide nascent fiber, the fiber, its preparation method, and its application. Background Technology

[0002] Polyimide fiber is a high-performance fiber with a rigid main chain containing a large number of imide and benzene rings. Due to its rigid molecular chain characteristics, this fiber possesses excellent comprehensive properties, including excellent heat resistance, flame retardancy, high strength and high modulus, and excellent radiation resistance. These properties make it a key material for extreme environments and it is widely used in special protective clothing, aerospace structural components, high-temperature filter materials, and thermal insulation for new energy vehicle batteries.

[0003] Polyimide fibers can be produced from polyamic acid via a two-step process or from polyimide via a one-step process. The one-step process involves the direct polymerization of monomers into polyimide without passing through polyamic acid. Its main disadvantages are the use of phenolic solvents, which are highly toxic, and the limited variety of monomers available. Currently, the main method for industrial production of polyimide fibers is two-step solution spinning. This involves first synthesizing a polyamic acid solution, then spinning it to obtain nascent polyamic acid fibers, and finally subjecting these fibers to high-temperature thermal cyclization and thermal stretching treatments to convert them into polyimide fibers.

[0004] To date, nascent fibers produced using the traditional two-step spinning process generally suffer from low cyclization. The cyclization degree of nascent fibers is closely related to their molecular chain structure; generally, the greater the rigidity of the chain segments, the lower the cyclization degree of the resulting nascent fibers. To achieve full imidization, the traditional process requires additional thermal cyclization treatment of the nascent fibers. However, this not only increases the number of process steps and affects production efficiency, but more seriously, the high temperature may induce depolymerization, multibranching, and inter-chain crosslinking of uncyclic polyamic acid units, thus significantly impacting the performance of the final polyimide fiber.

[0005] Compared to high-temperature thermal cyclization, chemical cyclization occurs under milder conditions. Chemical cyclization refers to the process where polyamic acid achieves a high degree of cyclization at relatively low temperatures under the action of a chemical cyclization aid composed of a dehydrating agent and a catalyst. Chinese patent CN 118028999 A uses wet spinning, adding a chemical cyclization aid to the spinning solution and coagulation bath to prepare partially chemically imidized nascent fibers. This technology not only requires a long residence time in the coagulation bath, resulting in low efficiency, but also leads to incomplete imidization of the nascent fibers, still requiring high-ratio hot drawing after thermal cyclization. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a polyimide nascent fiber, the fiber, its preparation method and application, which overcomes the technical defects of the traditional two-step method, which requires the nascent fiber to rely on subsequent thermal cyclization treatment, resulting in increased process flow, increased energy consumption, reduced production efficiency and increased production costs; and the high-temperature thermal cyclization process is prone to side reactions that damage the final mechanical properties of the fiber. The preparation method of this invention simplifies the process flow while enabling the prepared polyimide nascent fiber and polyimide fiber to have excellent stretchability, orientation and mechanical properties.

[0007] This invention provides a method for preparing polyimide nascent fibers, comprising:

[0008] Step (1) Under inert gas conditions, the diamine monomer, solvent and dianhydride monomer are mixed and reacted to obtain polyamic acid spinning solution;

[0009] Step (2) Mix the polyamic acid spinning solution and the chemical cyclization aid at below 0°C to obtain the spinning solution; wherein the chemical cyclization aid is a dehydrating agent and a catalyst;

[0010] Step (3) The spinning solution is first heated and then squeezed into the spinning channel and wound to obtain polyimide nascent fibers;

[0011] Preferably, the diamine monomer in step (1) includes one or more of 4,4'-diaminodiphenyl ether (ODA), p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), 2-(4-aminophenyl)-5-aminobenzimidazole (BIA), and 4,4'-diaminobenzoyl aniline (DABA).

[0012] Preferably, the dianhydride monomer in step (1) includes one or more of the following: pyromellitic dianhydride PMDA, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride BPDA, 4,4'-(hexafluoroisopropylidene) phthalic anhydride 6FDA, and 4,4'-oxobisphthalic anhydride ODPA.

[0013] Preferably, the solvent in step (1) includes one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-vinylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO).

[0014] Preferably, in step (1), the molar ratio of dianhydride monomer to diamine monomer is 0.98:1-1.02:1; the solid content of the polyamic acid spinning solution is 15-25%, and the apparent viscosity is 1500-3000 poise.

[0015] Preferably, the reaction temperature in step (1) is 0-10℃ and the time is 2-24 h.

[0016] The reaction in step (1) is carried out in a reaction vessel.

[0017] Further, in step (1) mixing and reaction: under inert gas conditions, the diamine monomer and solvent are mixed and dissolved, and then dianhydride is added, and the reaction is carried out at 0-10℃ for 2-24 h.

[0018] The inert gas includes, but is not limited to, nitrogen.

[0019] Preferably, the dehydrating agent in step (2) is one or more of acetic anhydride, propionic anhydride, benzoic anhydride, and thionyl chloride.

[0020] Preferably, the catalyst in step (2) is one or more of piperidine, imidazole, isoquinoline, pyridine, 1,2-dimethylimidazolium, and triethanolamine.

[0021] Preferably, in step (2), the molar ratio of dianhydride monomer, dehydrating agent and catalyst is 1:(0.6-2.5):(0.5-2).

[0022] Preferably, step (2) involves mixing at -10 to 0°C for 0.5 to 1 h.

[0023] The heating process in step (3) is either constant temperature treatment or stepped heating treatment; the constant temperature treatment temperature is 10-50℃; the stepped heating treatment is to pass through 0-10℃, 15-30℃ and 35-50℃ in sequence.

[0024] Preferred step-up heating treatment.

[0025] Further, in step (3), the spinning solution is first subjected to a step heating treatment and then squeezed into the spinning channel: the spinning solution first passes through the pipe set by the step heating program, and then is squeezed into the spinning channel through the spinneret; wherein the step heating program is set to 0-10℃, 15-30℃ and 35-50℃.

[0026] The spinneret is a circular spinneret with perforated holes.

[0027] The spinning channel is a passageway.

[0028] Preferably, in step (3), the material is extruded into the spinning channel, wherein the spinning channel is provided with three heating sections in sequence, namely 60-100℃, 140-160℃ and 180-230℃.

[0029] The total residence time of the spinning solution in the spinning channel is 3-7 seconds.

[0030] Preferably, the spinning speed in step (3) is 300-600 m / min.

[0031] In step (3), the degree of cyclization of the polyimide nascent fiber is ≥60%, and further, the degree of cyclization is ≥90%.

[0032] This invention provides a polyimide fiber, which is obtained by stretching the polyimide nascent fiber prepared by any of the methods described above.

[0033] The drawing temperature is 350-500℃, and the drawing ratio is 2.0-6.0 times.

[0034] The polyimide fiber has a tensile strength of 1.5-5.5 GPa, a modulus of 10-210 GPa, and an elongation at break of 3%-15%.

[0035] This invention provides a method for preparing polyimide fibers, comprising: stretching polyimide nascent fibers prepared by any of the methods to obtain polyimide fibers.

[0036] Preferably, the stretching is 350-500℃ and the stretching is 2.0-6.0 times.

[0037] This invention provides an application of the aforementioned polyimide fiber in the fields of high-temperature filtration and composite materials. For example, in the field of high-temperature filtration, it can be used to manufacture high-efficiency dust removal filter bags for industries such as steel and power. In the field of composite materials, it can be used as a high-performance reinforcing material, combined with a resin matrix, to manufacture lightweight, high-temperature resistant components required for high-end equipment such as aerospace and rail transportation.

[0038] This invention prepares a polyamic acid spinning solution by reacting raw materials containing diamine monomer, solvent, and dianhydride monomer in a reaction vessel. A chemical cyclization aid, composed of a dehydrating agent and a catalyst, is added to the spinning solution at low temperature. The spinning solution containing the chemical cyclization aid is then extruded into a high-temperature channel (tunnel) through a stepped heating pipe. At this point, the spinning solution has undergone microgelation. Upon entering the high-temperature channel (tunnel), the solvent evaporates, the fiber solidifies, and the cyclization aid simultaneously promotes the imidization reaction, ultimately producing fully imidized nascent fibers. These nascent fibers are then subjected to high-ratio hot drawing and wound to obtain polyimide fibers.

[0039] Beneficial effects

[0040] (1) Compared with traditional technologies, this invention first employs a low-temperature mixing process to ensure that the chemical cyclization aid is uniformly dispersed without reacting when mixed into the polyamic acid solution. Then, through a stepped heating program in the extrusion line, the chemical imidization reaction kinetics are efficiently promoted while ensuring processability. More importantly, this technology uses a dry spinning process. In the high-temperature spinning channel (tunnel), the chemical cyclization aid not only promotes the precursor polymer to undergo further rapid chemical imidization before volatilization, but the weakly alkaline catalyst contained therein also simultaneously promotes the cyclization reaction, thereby significantly increasing the degree of cyclization of the nascent fiber to over 90%. This technology eliminates the need for subsequent high-temperature thermal cyclization (>300℃) of the nascent fiber, enabling a spinning rate of over 400 m / min, significantly improving spinning efficiency, and reducing energy consumption and production costs.

[0041] (2) This invention achieves the preparation of non-circular cross-section fibers by adjusting the amount of chemical cyclization agent and using a circular spinneret orifice shape. This allows for precise control of the fiber cross-sectional shape, enabling a controllable transformation from a circular to a flat structure. This is significant for enhancing the application of polyimide fibers in baghouse dust collectors and other composite materials. The mechanism is as follows: after the fiber enters the spinning channel (tunnel), the surface solvent rapidly evaporates to form a skin layer. As the amount of chemical cyclization agent increases, the glass transition temperature (Tg) of the formed skin layer rises, the molecular chain mobility decreases, and internal solvent diffusion is hindered. At this point, the skin layer is basically solidified with a low degree of shrinkage; while the core layer, due to the large amount of residual solvent, continues to shrink slowly. This difference in shrinkage between the core layer and the skin layer causes the skin layer to be pulled inward by the core layer, resulting in a radial force imbalance in the fiber, ultimately forming a flat cross-section. The flat structure significantly increases the specific surface area of ​​the fiber. This characteristic demonstrates key advantages in two major application scenarios: when used as a reinforcement in composite materials, it can effectively enhance the interfacial bonding performance between fibers and the matrix; when applied in the field of high-temperature filtration, it provides more sites for capturing particulate matter, thereby simultaneously improving the filtration accuracy and dust holding capacity of the material.

[0042] (3) The partial chemical imidization in the stepped heating extrusion pipeline of the present invention brings about microgelation of the system, which helps the spinning solution to be stretched by a high ratio in the channel, realizes highly oriented crystallization of molecular chains, reduces internal defects of fibers, and greatly improves the mechanical properties of fibers, with tensile strength reaching 1.5-5.5 GPa and modulus of 10-210 GPa. Attached Figure Description

[0043] Figure 1In Example 1, image a is a cross-sectional SEM image of the polyimide nascent fiber prepared without the addition of a chemical cyclizing agent; image b is a cross-sectional SEM image of the polyimide nascent fiber prepared in Example 1; image c is a cross-sectional SEM image of the polyimide nascent fiber prepared in Example 2; and image d is a cross-sectional SEM image of the polyimide nascent fiber prepared in Example 3. Detailed Implementation

[0044] 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.

[0045] Raw material source:

[0046] 4,4'-Diaminodiphenyl ether, 99.5%, Tianjin Zhongtai Materials Technology Co., Ltd.;

[0047] p-Phenylenediamine, 99.5%, Tianjin Zhongtai Materials Technology Co., Ltd.;

[0048] m-Phenylenediamine, 99%, Shanghai Maclean Biochemical Technology Co., Ltd.;

[0049] 2-(4-Aminophenyl)-5-aminobenzimidazole, 99%, Tianjin Zhongtai Materials Technology Co., Ltd.;

[0050] 3,4,4'-Diaminobenzoyl aniline, 99%, Tianjin Zhongtai Materials Technology Co., Ltd.;

[0051] Pyromellitic dianhydride, 99.5%, Tianjin Zhongtai Materials Technology Co., Ltd.

[0052] 3,3',4,4'-Biphenyltetracarboxylic acid dianhydride, 99.5%, Tianjin Zhongtai Materials Technology Co., Ltd.

[0053] 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 99.5%, Tianjin Zhongtai Materials Technology Co., Ltd.;

[0054] 4,4'-Oxybisphthalic anhydride, 99.5%, Tianjin Zhongtai Materials Technology Co., Ltd.

[0055] N,N-Dimethylacetamide, AR, Shanghai Jingwei Chemical Co., Ltd.;

[0056] Acetic anhydride, AR, Shanghai Titan Co., Ltd.;

[0057] Pyridine, AR, Shanghai Mairui Chemical Technology Co., Ltd.;

[0058] Isoquinoline, RG, Shanghai Titan Co., Ltd.

[0059] Related tests:

[0060] Mechanical property testing: The mechanical properties of the fibers were tested using an XD-1 fineness tester and an XQ-1 single fiber strength tester from Shanghai Xinxian Instrument Co., Ltd., with a tensile speed of 10 mm / min. Each sample was tested more than 15 times, and the average value was taken.

[0061] Wide-angle X-ray diffraction (WAXD): The wide-angle X-ray diffraction (WAXD) experiment was conducted at beamline 16B1 of the Shanghai Synchrotron Radiation Facility (SSRF) to study the orientation of fibers. The X-ray wavelength (λ) used was 0.124 nm, and the distance between the sample and the detector was 279.32 mm.

[0062] Fourier transform infrared spectroscopy (FTIR): A Nicolet 8700 Fourier transform infrared spectrometer from Nicolet Corporation, USA, was used in attenuated total reflectance (ATR) mode, with a scan wavenumber range of 4000-500 cm⁻¹. -1 The number of scans was 32, and the resolution was 4 cm. -1 The characteristic absorption peak of the benzene ring in the infrared spectrum (1500 cm⁻¹) is used to determine its absorption. -1 () was used as an internal standard to correct the characteristic absorption peak of the imide ring (1710 cm⁻¹) under different conditions. -1 and 738 cm -1 The absorbance of the fiber. The formula for calculating the fiber circumsaturation degree (ID) is shown below:

[0063]

[0064] In the formula, A1 is the absorbance of the characteristic absorption peak of the fibrous imide ring being calibrated; B1 is the absorbance of the characteristic absorption peak of the fibrous benzene ring being calibrated; A ∞ B represents the absorbance of the characteristic absorption peak of the imide ring when fully cyclized; ∞ The absorbance is the characteristic absorption peak of the benzene ring when it is fully cyclized.

[0065] Peel strength: The peel strength of the specimens was tested according to GB / T 2791—1995 "Adhesives - Test Method for Peel Strength of Flexible Materials - Flexible Materials". The specimen specifications were 200 mm in length and 25 mm in width, and the tensile speed was 100 mm / min.

[0066] Fiber irregularity: The irregularity of fiber cross-section can be expressed by the following formula:

[0067]

[0068] In the formula r N It is the short axis of the fiber cross-section, r W It is the major axis of the fiber cross-section.

[0069] Note: The total residence time of the spinning solution in the high-temperature tunnel involved in Examples 1-6 and Comparative Examples 1-2 is 3-7 seconds.

[0070] Example 1

[0071] (1) Dissolve 4,4'-diaminodiphenyl ether (ODA) in DMAc and stir until completely dissolved under nitrogen protection. Then, add pyromellitic dianhydride (PMDA) in equimolar amounts to ODA. After the addition is complete, react at 0°C for 4-6 h to finally obtain a polyamic acid (PAA) spinning solution with a solid content of 20%.

[0072] (2) Acetic anhydride and pyridine were added to the spinning solution treated at -5℃, wherein the molar ratio of PMDA, acetic anhydride and pyridine was 1:1.6:0.8. The mixture was then stirred for 0.5 h to ensure uniform mixing and to obtain the spinning solution.

[0073] (3) The spinning solution with added cyclization agent is squeezed into a pipeline equipped with a stepped heating system. The spinning solution is induced to undergo partial chemical imidization rapidly through stepped heating treatment at 10℃, 30℃ and 50℃, thereby forming a spinning solution containing microgels in the pipeline.

[0074] (4) Connect the above-mentioned pipeline to the spinneret and squeeze the spinning solution into the high-temperature channel. The channel is divided into three sections for heating from the top, with the specific temperature program as follows: 80℃, 150℃, and 220℃. Control the spinning speed at 500 m / min to finally obtain polyimide nascent fibers.

[0075] Example 2

[0076] (1) Dissolve ODA in DMAc and stir until completely dissolved under nitrogen protection. Then, add PMDA in equimolar amounts to ODA. After the addition is complete, react at 0°C for 4-6 h to finally obtain a polyamic acid (PAA) spinning solution with a solid content of 20%.

[0077] (2) Acetic anhydride and pyridine were added to the spinning solution treated at -5℃, wherein the molar ratio of PMDA, acetic anhydride and pyridine was 1:2:1. The mixture was then stirred for 0.5 h to ensure uniform mixing and to obtain the spinning solution.

[0078] (3) The spinning solution with added cyclization agent is squeezed into a pipeline equipped with a stepped heating system. The spinning solution is induced to undergo partial chemical imidization rapidly through stepped heating treatment at 10℃, 30℃ and 50℃, thereby forming a spinning solution containing microgels in the pipeline.

[0079] (4) Connect the above-mentioned pipeline to the spinneret and squeeze the spinning solution into the high-temperature channel. The channel is divided into three sections for heating from the top, with the specific temperature program as follows: 80℃, 150℃, and 220℃. Control the spinning speed at 500 m / min to finally obtain polyimide nascent fibers.

[0080] Example 3

[0081] (1) Dissolve ODA in DMAc and stir until completely dissolved under nitrogen protection. Then, add PMDA in equimolar amounts to ODA. After the addition is complete, react at 0°C for 4-6 h to finally obtain a polyamic acid (PAA) spinning solution with a solid content of 20%.

[0082] (2) Acetic anhydride and pyridine were added to the spinning solution treated at -5℃, wherein the molar ratio of PMDA, acetic anhydride and pyridine was 1:2.4:1.2. The mixture was then stirred for 0.5 h to ensure uniform mixing and to obtain the spinning solution.

[0083] (3) The spinning solution with added cyclization agent is squeezed into a pipeline equipped with a stepped heating system. The spinning solution is induced to undergo partial chemical imidization rapidly through stepped heating treatment at 10℃, 30℃ and 50℃, thereby forming a spinning solution containing microgels in the pipeline.

[0084] (4) Connect the above-mentioned pipeline to the spinneret and squeeze the spinning solution into the high-temperature channel. The channel is divided into three sections for heating from the top, with the specific temperature program as follows: 80℃, 150℃, and 220℃. Control the spinning speed at 500 m / min to finally obtain polyimide nascent fibers.

[0085] Example 4

[0086] (1) Dissolve ODA in DMAc and stir until completely dissolved under nitrogen protection. Then, add PMDA in equimolar amounts to ODA. After the addition is complete, react at 0°C for 4-6 h to finally obtain a polyamic acid (PAA) spinning solution with a solid content of 20%.

[0087] (2) Acetic anhydride and isoquinoline were added to the spinning solution treated at -5℃, wherein the molar ratio of PMDA, acetic anhydride and isoquinoline was 1:2.4:1.2. The mixture was then stirred for 0.5 h to ensure uniform mixing and to obtain the spinning solution.

[0088] (3) The spinning solution with added cyclization agent is squeezed into a pipeline equipped with a stepped heating system. The spinning solution is induced to undergo partial chemical imidization rapidly through stepped heating treatment at 10℃, 30℃ and 50℃, thereby forming a spinning solution containing microgels in the pipeline.

[0089] (4) Connect the above-mentioned pipeline to the spinneret and squeeze the spinning solution into the high-temperature channel. The channel is divided into three sections for heating from the top, with the specific temperature program as follows: 80℃, 150℃, and 220℃. Control the spinning speed at 500 m / min to finally obtain polyimide nascent fibers.

[0090] Example 5

[0091] (1) Dissolve ODA in DMAc and stir until completely dissolved under nitrogen protection. Then, add PMDA in equimolar amounts to ODA. After the addition is complete, react at 0°C for 4-6 h to finally obtain a polyamic acid (PAA) spinning solution with a solid content of 20%.

[0092] (2) Acetic anhydride and pyridine were added to the spinning solution treated at -5℃, wherein the molar ratio of PMDA, acetic anhydride and pyridine was 1:2.4:1.2. The mixture was then stirred for 0.5 h to ensure uniform mixing and to obtain the spinning solution.

[0093] (3) The spinning solution with added cyclization agent is squeezed into a pipeline equipped with a constant temperature system. The temperature is kept constant at 50°C to induce partial chemical imidization of the spinning solution, thereby forming a spinning solution containing microgel in the pipeline.

[0094] (4) Connect the above-mentioned pipeline to the spinneret and squeeze the spinning solution into the high-temperature channel. The channel is divided into three sections for heating from the top, with the specific temperature program as follows: 80℃, 150℃, and 220℃. Control the spinning speed at 500 m / min to finally obtain polyimide nascent fibers.

[0095] Example 6

[0096] (1) p-phenylenediamine (p-PDA) and 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) were dissolved in DMAc, with a molar ratio of PDA to BIA of 7:3. The mixture was stirred until homogeneous under nitrogen protection. Subsequently, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) was added in equimolar amounts to the diamine. After the addition was complete, the mixture was reacted at 0°C for 10-24 h to finally obtain a polyamic acid (PAA) spinning solution with a solid content of 12%.

[0097] (2) Acetic anhydride and pyridine were added to the spinning solution treated at -5℃, wherein the molar ratio of BPDA, acetic anhydride and pyridine was 1:2.4:1.2. The mixture was then stirred for 0.5 h to ensure uniform mixing and to obtain the spinning solution.

[0098] (3) The spinning solution with added cyclization agent is squeezed into a pipeline equipped with a stepped heating system. The spinning solution is induced to undergo partial chemical imidization rapidly through stepped heating treatment at 10℃, 30℃ and 50℃, thereby forming a spinning solution containing microgels in the pipeline.

[0099] (4) Connect the above-mentioned pipeline to the spinneret and squeeze the spinning solution into the high-temperature channel. The channel is divided into three sections for heating from the top, with the specific temperature program as follows: 80℃, 150℃, and 220℃. Control the spinning speed at 400 m / min to finally obtain polyimide nascent fibers.

[0100] Example 7

[0101] The polyimide nascent fibers prepared in Example 3 were subjected to high-ratio hot stretching at 480°C.

[0102] Example 8

[0103] The polyimide nascent fibers prepared in Example 6 were subjected to high-ratio hot stretching at 450°C.

[0104] Comparative Example 1

[0105] (1) Dissolve ODA in DMAc and stir until completely dissolved under nitrogen protection. Then, add PMDA in equimolar amounts to ODA. After the addition is complete, react at 0°C for 4-6 h to finally obtain a polyamic acid (PAA) spinning solution with a solid content of 20%.

[0106] (2) The PAA spinning solution is squeezed into a pipeline equipped with a stepped heating system and subjected to stepped heating treatment at 10℃, 30℃ and 50℃.

[0107] (3) The spinning solution is extruded into a high-temperature channel, which is divided into three sections from the top for heating. The specific temperature program is as follows: 80℃, 150℃, and 220℃. The spinning speed is controlled at 500 m / min to finally obtain polyimide nascent fibers.

[0108] Comparative Example 2

[0109] (1) Dissolve p-PDA and BIA in DMAc, wherein the molar ratio of PDA to BIA is 7:3, and stir evenly under nitrogen protection. Then, add BPDA in equimolar amounts to diamine. After the addition is complete, react at 0°C for 10-24 h to finally obtain polyamic acid (PAA) spinning solution with a solid content of 12%.

[0110] (2) The PAA spinning solution is squeezed into a pipeline equipped with a stepped heating system and subjected to stepped heating treatment at 10℃, 30℃ and 50℃.

[0111] (3) The spinning solution is extruded into a high-temperature channel, which is divided into three sections from the top for heating. The specific temperature program is as follows: 80℃, 150℃, and 220℃. The spinning speed is controlled at 400 m / min to finally obtain polyimide nascent fibers.

[0112] Comparative Example 3

[0113] The polyimide nascent fibers prepared in Comparative Example 1 were subjected to high-ratio hot stretching at 480℃.

[0114] Comparative Example 4

[0115] The polyimide nascent fibers prepared in Comparative Example 2 were subjected to high-ratio hot stretching at 450°C.

[0116] Table 1 Comparison of various properties of polyimide nascent fibers

[0117]

[0118] Compared with Comparative Example 1, Examples 1-4, due to the introduction of chemical cyclization aids, have improved the regular orientation and imidization cyclization of their polyimide nascent fibers during the molding process, resulting in significantly better mechanical properties and cyclization degree than Comparative Example 1.

[0119] Examples 1-3 demonstrate polyimide nascent fibers obtained by adding different amounts of chemical cyclizing agents. The results show that the degree of cyclization of the polyimide nascent fibers gradually increases with increasing agent dosage, even exceeding 90% at the dosage in Example 3. Simultaneously, the fiber cross-sectional irregularity continuously improves.

[0120] Compared with Example 1, Example 4 used a catalyst with a higher boiling point, and the polyimide nascent fibers obtained had a cyclization degree of 100%.

[0121] Compared with Example 3, Example 5 did not use a stepped heating extrusion pipeline, which resulted in the organic weak base catalyst pyridine having a greater catalytic degradation effect than catalytic dehydration and cyclization effect at this constant temperature. As a result, it was inferior to Example 3 in terms of both mechanical properties and degree of cyclization.

[0122] Table 2 Comparison of various properties of hot-drawn fibers

[0123]

[0124] Compared to Comparative Example 3, Example 7, with the addition of a chemical cyclization agent, achieved a maximum hot draw ratio of 5.8 times, significantly higher than the 4.5 times of Comparative Example 3. This is because the polyimide nascent fibers of Example 7 were fully imidized before high-temperature hot drawing. This not only effectively avoided side reactions such as depolymerization of polyamic acid units and cross-linking between molecular chains during hot drawing, but also resulted in better regularity of the molecular chains themselves, thus jointly ensuring a higher draw ratio. Compared to Comparative Example 4, the ternary fiber system prepared in Example 8 also achieved the same promoting effect after the addition of the chemical cyclization agent.

[0125] like Figure 1 As shown in Figures a and b, the cross-sections of the nascent polyimide fibers from Comparative Example 1 and Examples 1-3 are shown sequentially, corresponding to the increasing amounts of acetic anhydride and pyridine. During spinning, the solvent on the fiber surface evaporates rapidly to form a skin layer; as the amounts of acetic anhydride and pyridine increase, the Tg of the skin layer increases, the molecular chain mobility decreases, and internal solvent diffusion is hindered. At this point, the skin layer is basically solidified and shrinks little; while the core layer has more residual solvent and continues to shrink slowly. The difference in shrinkage between the core layer and the skin layer causes the skin layer to be pulled inward, resulting in radial force imbalance and ultimately forming a flat cross-section.

Claims

1. A method for preparing polyimide nascent fibers, characterized in that, include: Step (1) Under inert gas conditions, the diamine monomer, solvent and dianhydride monomer are mixed and reacted to obtain polyamic acid spinning solution; Step (2) Mix the polyamic acid spinning solution and the chemical cyclization aid at below 0°C to obtain the spinning solution; wherein the chemical cyclization aid is a dehydrating agent and a catalyst; Step (3) The spinning solution is first heated and then squeezed into the spinning channel and wound to obtain polyimide nascent fibers.

2. The preparation method according to claim 1, characterized in that, The diamine monomer in step (1) includes one or more of 4,4'-diaminodiphenyl ether ODA, p-phenylenediamine p-PDA, m-phenylenediamine m-PDA, 2-(4-aminophenyl)-5-aminobenzimidazole BIA, and 4,4'-diaminobenzoyl aniline DABA. The dianhydride monomers include one or more of the following: pyromellitic dianhydride PMDA, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride BPDA, 4,4'-(hexafluoroisopropylidene) phthalic anhydride 6FDA, and 4,4'-oxobisphthalic anhydride ODPA. The solvent includes one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-vinylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO).

3. The preparation method according to claim 1, characterized in that, In step (1), the molar ratio of dianhydride monomer to diamine monomer is 0.98:1-1.02:1; the solid content of the polyamic acid spinning solution is 15-25%.

4. The preparation method according to claim 1, characterized in that, The reaction temperature in step (1) is 0-10℃ and the time is 2-24 h.

5. The preparation method according to claim 1, characterized in that, The dehydrating agent in step (2) is one or more of acetic anhydride, propionic anhydride, benzoic anhydride, and thionyl chloride; the catalyst is one or more of piperidine, imidazole, isoquinoline, pyridine, 1,2-dimethylimidazolium, and triethanolamine. In step (2), the molar ratio of dianhydride monomer, dehydrating agent, and catalyst is 1:(0.6-2.5):(0.5-2). The mixing step (2) involves stirring at -10~0℃ for 0.5-1 h.

6. The preparation method according to claim 1, characterized in that, The heating process in step (3) is either constant temperature treatment or stepped heating treatment; wherein the stepped heating treatment is to sequentially pass through 0-10℃, 15-30℃ and 35-50℃. In step (3), the material is extruded into the spinning channel, where three heating sections are sequentially set in the spinning channel, namely 60-100℃, 140-160℃ and 180-230℃; Step (3) The spinning speed is 300-600m / min.

7. A polyimide fiber, characterized in that, The polyimide nascent fibers prepared by any one of the methods described in claims 1-6 are obtained by stretching.

8. The polyimide fiber according to claim 7, characterized in that, The drawing temperature is 350-500℃, and the drawing ratio is 2.0-6.0 times.

9. A method for preparing polyimide fibers, characterized in that, include: The polyimide nascent fibers prepared by any one of the methods described in claims 1-6 are stretched to obtain polyimide fibers.

10. The application of the polyimide fiber of claim 7 in the field of high-temperature filtration and composite materials.

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