Meta-aramid fibrid and preparation method thereof

Abstract

The invention relates to the technical field of aramid fibers, in particular to a meta-aramid fibrid and a preparation method thereof. According to the invention, the problem that the meta-aramid fibrid is poor in heat resistance and mechanical property is solved. 4, 4 '-diaminobenzene imide and m-phenylenediamine are polymerized according to a specific molar ratio, so that a molecular chain is promoted to form a more regular and compact structure, and the mechanical property of the fiber is improved; by introducing a compounding system of a DOPO derivative and a surface modified ZrO2 nanosheet, a phosphorus-zirconium dual heat-resistant barrier is constructed in a polymer molecular chain, the heat resistance of the fiber is improved, and the silane modified ZrO2 nanosheet inhibits microcrack propagation through a pinning effect, so that the mechanical property of the fiber is improved; through a series of optimization processes from polymerization to spinning and stretching, a highly-crystallized and oriented fiber structure is constructed, the high-temperature stability of the fiber is improved, fiber molecular chains are orderly arranged in the stress direction, and the external force resistance of the fiber is greatly enhanced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of aramid fibers, in particular to a meta-aramid fiber and a preparation method thereof. BACKGROUND

[0002] As a special high-performance fiber material, meta-aramid fiber has an irreplaceable position in modern industry, mainly used in the manufacture of aramid paper, which plays a key role in filling and bonding in paper, usually accounting for more than 50%, and has a decisive influence on the final performance of the paper. In the fields of electrical insulation, high-temperature filtration, protective clothing, etc., meta-aramid fiber also plays an important role with its high-temperature resistance, flame retardance, insulation, etc. However, the preparation and application of meta-aramid fiber still face many challenges.

[0003] In terms of heat resistance, although it has good thermal stability, meta-aramid fiber is prone to thermal oxidative aging in high-temperature environments, especially after high-temperature heat pressing. This not only reduces the appearance quality of aramid paper, but also affects its insulation performance and mechanical properties, limiting its application in higher temperature environments. In addition, the halogen-based flame retardants added in the prior art to improve heat resistance have problems such as easy loss, low utilization rate, environmental risk, etc., which are difficult to meet the increasingly stringent environmental requirements and high-performance demands; in terms of mechanical properties, meta-aramid fiber needs to be closely combined with short fibers during the wet laying process to form high-strength, high-uniformity paper. However, the existing meta-aramid fiber has problems such as uneven length distribution and uneven film structure, resulting in poor combination ability of the formed paper and poor paper uniformity. In addition, the proportion of long fibers and fine fibers is difficult to accurately control, and long fibers are easy to entangle with short fibers to form slurry, affecting the uniformity of the paper; while a high proportion of fine fibers will reduce the combination ability and air permeability of the paper, which is difficult to meet the strict requirements of high-end application fields on paper performance. Therefore, it is urgent to develop a new preparation method to effectively improve the heat resistance and mechanical properties of meta-aramid fiber.

[0004] Therefore, a meta-aramid fiber and a preparation method thereof are provided. SUMMARY

[0005] The present application aims to provide a meta-aramid fiber and a preparation method thereof. By adopting 4,4'-diaminobenzamide and m-phenylenediamine for polymerization in a specific molar ratio, the molecular chain is prompted to form a more regular and compact structure, and the mechanical properties of the fiber are improved. By introducing a DOPO derivative and a surface modified ZrO2 nanosheet compound system, a phosphorus-zirconium double heat-resistant barrier is constructed on the polymer molecular chain, the heat-resistant performance of the fiber is improved, and the silane modified ZrO2 nanosheet suppresses the expansion of micro-cracks through pinning effect, and the mechanical properties of the fiber are improved. By optimizing a series of processes from polymerization to spinning and stretching, a highly crystalline and oriented fiber structure is constructed, the high temperature stability of the fiber is improved, the fiber molecular chain is arranged in order along the stress direction, and the ability of the fiber to resist external force is greatly enhanced.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical scheme:

[0007] The present application provides a preparation method of a meta-aramid fiber. The preparation method is as follows: 4,4'-diaminobenzamide and m-phenylenediamine are added to a pretreated N-methylpyrrolidone solution in a molar ratio of 6-8:3, nitrogen is used for three times of air replacement, and stirring is performed at 40 DEG C until transparency; after adding a heat-resistant synergist, ultrasonic dispersion is performed at 20 kHz for 40 min, and then stepwise temperature rising polymerization is performed to obtain a polymer solution; the polymer solution is vacuum degassed at-0.095 MPa for 1 h and filtered through a 5 mu m sintered metal filter to obtain a spinning solution; dry-wet spinning is performed on the spinning solution, and then four-stage stretching is performed to obtain a fiber precursor; the fiber precursor is washed with water at 40 DEG C for 20 min, and then dried at 120 DEG C for 3 h to obtain a meta-aramid fiber.

[0008] The heat-resistant synergist is obtained by compounding a DOPO derivative and a modified ZrO2 nanosheet in a mass ratio of 1-3:1; the DOPO derivative is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide;

[0009] The modified ZrO2 nanosheet is obtained by modifying ZrO2 nanosheets with a silane coupling agent.

[0010] Preferably, the preparation method of the pretreated N-methylpyrrolidone solution is as follows: N-methylpyrrolidone is pre-dehydrated at 80 DEG C for 4 h, and then a vacuum thin film evaporator is used for cyclic treatment at 80 DEG C and 0.3 kPa until the water content is less than or equal to 80 ppm; calcium chloride powder is added, the concentration is 3.5-6.0 wt%, and stirring is performed at 40 DEG C for 1 h to obtain the pretreated N-methylpyrrolidone solution; the amount of N-methylpyrrolidone is 4 times the total amount of 4,4'-diaminobenzamide and m-phenylenediamine.

[0011] Preferably, the modified ZrO2 nanosheet is prepared by the following method: adding a silane coupling agent KH550 into a mixed solution of ethanol and deionized water, the amount of the silane coupling agent added being 3-5% of the volume of the mixed solution, stirring at a speed of 300 rpm during the dropping process, continuing to stir for 50 min after the dropping is completed, to obtain a hydrolysis solution; adding ZrO2 nanosheets into the hydrolysis solution, the mass-volume ratio of the ZrO2 nanosheets to the hydrolysis solution being 1g:50-100mL, reacting at a speed of 400 rpm for 2-4h while the temperature is raised to 70℃, centrifuging and washing with ethanol for 3 times after the reaction is completed, and vacuum drying at 70℃ for 7h to obtain the modified ZrO2 nanosheet.

[0012] Preferably, the step of temperature programming is as follows: first, raising the temperature to 65℃ at a speed of 300 rpm, dropping isophthaloyl chloride, and pre-polymerizing at a low temperature for 1.2-2h; then, raising the temperature to 95℃, adding isophthaloyl chloride to compensate for the loss of hydrolysis, and completing the polymerization by constant temperature reaction for 3.5-4.5h.

[0013] Preferably, the dry-wet spinning step is as follows: the spinning solution is extruded through a metering pump at a spinning speed of 15-20m / min, enters the coagulation bath through the spinneret, and the diameter of the spinneret hole is 0.1-0.4mm; the coagulation bath is composed of 90% of a 50wt% calcium chloride aqueous solution and 10% of N-methyl pyrrolidone; and the temperature of the coagulation bath is 5℃.

[0014] Preferably, the step of four-stage stretching is as follows: first, pre-drawing in the coagulation bath, the draw ratio being 1.2 times; then, main drawing in a glycerol bath at 60℃, the draw ratio being 3 times; then, heat treatment using a hot roller at 260℃, the draw ratio being 0.8 times; and finally, relaxation in air at 25℃.

[0015] Another aspect of the present application provides a meta-aramid fibrid, which is prepared by any one of the above preparation methods; and the preparation raw materials of the meta-aramid fibrid include 4,4'-diaminobenzamide, m-phenylenediamine, N-methyl pyrrolidone and a heat-resistant synergist.

[0016] Compared with the prior art, the present application has the following beneficial effects:

[0017] 1、The application adopts 4,4'-diaminobenzamide and m-phenylenediamine to polymerize in a molar ratio of 7:3, and this special raw material ratio can promote the formation of more regular and compact structure of molecular chain. The introduction of 4,4'-diaminobenzamide increases the rigid groups and hydrogen bonding sites between molecular chains, so that the interaction force between molecular chains is significantly enhanced. In a high temperature environment, this stable molecular structure can effectively inhibit the thermal motion of molecular chain, reduce the molecular chain rupture and decomposition caused by high temperature, thereby greatly improving the heat resistance of the fiber; by pretreating the N-methyl pyrrolidone solution, the anhydrous and oxygen-free environment eliminates the unreacted end groups and microgel, and reduces the stress concentration point from the source; at the same time, the regular and compact molecular structure also provides stronger bearing capacity for the fiber, so that the fiber can more evenly disperse stress when stretched by external force, thereby improving the tensile strength and modulus of the fiber and enhancing the mechanical properties.

[0018] 2、The application builds a phosphorus-zirconium double heat-resistant barrier in the polymer molecular chain by introducing a complex system of DOPO derivatives and surface modified ZrO2 nanosheets. The phosphorus hetero-fused structure of DOPO decomposes to form a free radical capture layer at high temperature, inhibiting fiber thermal cracking; at the same time, the modified ZrO2 nanosheet forms a covalently bonded dense network with the aramid molecular chain through silane coupling agent, effectively reflecting heat radiation and blocking oxygen diffusion, both of which synergistically improve the thermal decomposition threshold and flame retardant durability of the fiber, overcoming the high temperature embrittlement defect of traditional precipitation fibers, so that the fiber still maintains structural integrity under extreme thermal shock. In addition, the silane modified ZrO2 nanosheet realizes molecular level dispersion in the polymer matrix, and this nano-enhancing effect induces the directional arrangement of polymer chains during fiber stretching and forming, so that stress is efficiently transmitted along the nanosheet interface. At the same time, the rigid particles of ZrO2 inhibit the expansion of micro-cracks through pinning effect, significantly improving the mechanical properties of the fiber.

[0019] 3、The application plays a key role in improving the performance of the fiber through a series of optimized processes from polymerization to spinning and stretching. The segmented temperature rising polymerization can effectively improve the molecular weight and uniformity of the molecular weight distribution of the polymer, thereby improving the tensile strength and modulus of the fiber, and optimizing the molecular structure to enhance the thermal stability of the fiber. Dry-wet spinning combined with a specific coagulation bath promotes rapid solidification of the fiber and forms a good initial structure; the four-stage stretching process controls the fiber supramolecular structure in stages through a gradient temperature field and a variable draw ratio: the coagulation bath forms a high-porosity skin layer at low temperature, the glycerol bath releases molecular chain entanglement and improves crystallinity during high-temperature main stretching, the heat roller treatment locks the oriented structure while eliminating residual stress, and finally air relaxation gives a moderate rebound, improving the heat resistance and mechanical strength of the fiber. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The figure is a test result diagram of the heat resistance and mechanical properties of the application comparative examples 1-12. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative work are within the protection scope of the present application.

[0022] Please refer to Figure 1 The present application provides a kind of meta aramid fiber and its preparation method, technical scheme as follows:

[0023] The substance information involved in the present application is as follows:

[0024] M-phenylenediamine CAS: 108-45-2; 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide CAS: 35948-25-5; N-methylpyrrolidone CAS: 872-50-4; calcium chloride CAS: 10043-52-4; silane coupling agent KH550 CAS: 919-30-2; isophthaloyl chloride CAS: 99-63-8; ZrO2 nanosheet CAS: 1314-23-4.

[0025] Example 1

[0026] N-methylpyrrolidone is pre-dehydrated at 80°C for 4h (molecular sieve: N-methylpyrrolidone (NMP) was used as solvent, the dosage was 20 wt% of N-methylpyrrolidone (NMP), then vacuum thin film evaporator was used for cyclic treatment at 80 °C, 0.3 kPa until the water content was ≤80 ppm; calcium chloride powder was added with a concentration of 3.5 wt%, stirred at 40 °C for 1 h to obtain a pretreated N-methylpyrrolidone solution; the dosage of N-methylpyrrolidone was 4 times of the total amount of 4,4'-diaminobenzanilide and m-phenylenediamine; 4,4'-diaminobenzanilide and m-phenylenediamine were added to the pretreated N-methylpyrrolidone solution according to a molar ratio of 6:3, and the gas was replaced three times using nitrogen, and stirred at 40 °C until transparent; after adding the heat-resistant synergist (5 wt% of the total amount of 4,4'-diaminobenzanilide and m-phenylenediamine), ultrasonic dispersion was performed at 20 kHz for 40 min, then the temperature was raised to 65 °C at a speed of 300 rpm, m-phthaloyl chloride was added dropwise, the molar ratio of m-phthaloyl chloride to amine monomer was 1:1, and the reaction was carried out for 1.2 h to perform low-temperature pre-polycondensation; then the temperature was raised to 95 °C, 0.5 mol% of m-phthaloyl chloride was added to compensate for the loss of hydrolysis, and the polymerization was completed by constant temperature reaction for 3.5 h to obtain a polymer solution; the polymer solution was degassed under a vacuum of -0.095 MPa for 1 h and filtered through a 5 μm sintered metal filter to obtain a spinning solution with an intrinsic viscosity of 4.5 dL / g; the spinning solution was extruded through a metering pump at a spinning speed of 15 m / min, passed through a spinneret into a coagulation bath, and the spinneret hole diameter was 0.1 mm; the coagulation bath was composed of 90% calcium chloride aqueous solution with a concentration of 50 wt% and 10% N-methylpyrrolidone; the coagulation bath temperature was 5 °C; then pre-drawing was performed in the coagulation bath, and the draw ratio was 1.2 times; then main drawing was performed in a glycerol bath at 60 °C, and the draw ratio was 3 times; then heat treatment was performed using a hot roller at 260 °C, and the draw ratio was 0.8 times; finally, relaxation was performed in air at 25 °C to obtain a fiber precursor; the fiber precursor was washed in water at 40 °C for 20 min, and then dried at 120 °C for 3 h to obtain m-aramid precipitation fiber; the heat-resistant synergist was compounded from a DOPO derivative and modified ZrO2 nanosheets according to a mass ratio of 1:1; the DOPO derivative was 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide;

[0027] The preparation method of the modified ZrO2 nanosheet is as follows: silane coupling agent KH550 was added to a mixed solution of ethanol and deionized water (ethanol and deionized water in a volume ratio of 4:1), and the silane coupling agent was added in an amount of 3% of the volume of the mixed solution; during the dropping process, stirring was continuously performed at a speed of 300 rpm; after the dropping was completed, stirring was continued for 50 min to obtain a hydrolysis solution; ZrO2 nanosheets were added to the hydrolysis solution, and the mass-volume ratio of ZrO2 nanosheets to hydrolysis solution was 1 g:50 mL; the temperature was raised to 70 °C at a speed of 400 rpm for 2 h; after the reaction was completed, centrifugal separation was performed (10000 rpm, 15 min) and washing was performed three times with ethanol; and vacuum drying was performed at 70 °C for 7 h to obtain modified ZrO2 nanosheets.

[0028] Example 2

[0029] Referring to the preparation method and parameter conditions of Example 1, except that the concentration of calcium chloride powder added during the pretreatment of N-methylpyrrolidone was 4wt%; the molar ratio of 4,4'-diaminobenzamide and m-phenylenediamine was 6.5:3; the low-temperature pre-polycondensation was performed for 1.5h, and the polymerization was completed after constant-temperature reaction for 3.8h; the spinning solution was extruded through a metering pump at a spinning speed of 18m / min, and the diameter of the spinneret hole was 0.2mm; the heat-resistant synergist was obtained by compounding DOPO derivatives and modified Zr02nanosheets according to a mass ratio of 1.5:1; during the preparation of the modified Zr02nanosheets, the amount of silane coupling agent added was 3.5% of the volume of the mixed solution, the mass-volume ratio of Zr02nanosheets to hydrolysis solution was 1g:65mL, and the reaction was performed at a rotation speed of 400rpm and a temperature of 70°C for 2.5h.

[0030] Example 3

[0031] Referring to the preparation method and parameter conditions of Example 1, except that the concentration of calcium chloride powder added during the pretreatment of N-methylpyrrolidone was 5wt%; the molar ratio of 4,4'-diaminobenzamide and m-phenylenediamine was 7:3; the low-temperature pre-polycondensation was performed for 1.8h, and the polymerization was completed after constant-temperature reaction for 4.2h; the spinning solution was extruded through a metering pump at a spinning speed of 20m / min, and the diameter of the spinneret hole was 0.3mm; the heat-resistant synergist was obtained by compounding DOPO derivatives and modified Zr02nanosheets according to a mass ratio of 2:1; during the preparation of the modified Zr02nanosheets, the amount of silane coupling agent added was 4% of the volume of the mixed solution, the mass-volume ratio of Zr02nanosheets to hydrolysis solution was 1g:80mL, and the reaction was performed at a rotation speed of 400rpm and a temperature of 70°C for 3h.

[0032] Example 4

[0033] Referring to the preparation method and parameter conditions of Example 1, except that the concentration of calcium chloride powder added during the pretreatment of N-methylpyrrolidone was 6wt%; the molar ratio of 4,4'-diaminobenzamide and m-phenylenediamine was 8:3; the low-temperature pre-polycondensation was performed for 2h, and the polymerization was completed after constant-temperature reaction for 4.5h; the spinning solution was extruded through a metering pump at a spinning speed of 16m / min, and the diameter of the spinneret hole was 0.4mm; the heat-resistant synergist was obtained by compounding DOPO derivatives and modified Zr02nanosheets according to a mass ratio of 3:1; during the preparation of the modified Zr02nanosheets, the amount of silane coupling agent added was 5% of the volume of the mixed solution, the mass-volume ratio of Zr02nanosheets to hydrolysis solution was 1g:100mL, and the reaction was performed at a rotation speed of 400rpm and a temperature of 70°C for 4h.

[0034] Comparative Example 1

[0035] The preparation method and parameters of Example 1 were referred to, except that the N-methylpyrrolidone was not pretreated.

[0036] Comparative Example 2

[0037] The preparation method and parameters of Example 1 were referred to, except that calcium chloride powder was not added during the pretreatment of N-methylpyrrolidone.

[0038] Comparative Example 3

[0039] The preparation method and parameters of Example 1 were referred to, except that no heat-resistant synergist was added.

[0040] Comparative Example 4

[0041] The preparation method and parameters of Example 1 were referred to, except that the ZrO2 nanosheet was not modified.

[0042] Comparative Example 5

[0043] The preparation method and parameters of Example 1 were referred to, except that only the DOPO derivative was used as the heat-resistant synergist.

[0044] Comparative Example 6

[0045] The preparation method and parameters of Example 1 were referred to, except that only the modified ZrO2 nanosheet was used as the heat-resistant synergist.

[0046] Comparative Example 7

[0047] The preparation method and parameters of Example 1 were referred to, except that the heat-resistant synergist was not subjected to ultrasonic dispersion after being added.

[0048] Comparative Example 8

[0049] The preparation method and parameters of Example 1 were referred to, except that the stepwise temperature increase polymerization was not performed, but the temperature was directly increased to 65°C for 6h.

[0050] Comparative Example 9

[0051] The preparation method and parameters of Example 1 were referred to, except that the stepwise temperature increase polymerization was not performed, but the temperature was directly increased to 95°C for 5h.

[0052] Comparative Example 10

[0053] The preparation method and parameters of Example 1 were referred to, except that during the stepwise temperature increase polymerization, the isophthaloyl chloride was not supplemented after the temperature was increased to 95°C.

[0054] Comparative Example 11

[0055] The preparation method and parameters of Example 1 were referred to, except that the coagulation bath was 15wt% DMAc aqueous solution.

[0056] Comparative Example 12

[0057] The preparation method and parameters of Example 1 were referred to, except that no four-stage stretching was performed, but after the spinning solution was solidified into nascent fibers in the coagulation bath, stretching treatment was immediately performed, the stretching temperature was 160℃, and the stretching multiple was 3 times.

[0058] Heat resistance and mechanical property test

[0059] Tensile strength: tested according to the standard test of GB / T12914-2018;

[0060] Heat damage test method: the product was treated at 285℃, 2MPa, and then tear resistance was tested according to the test method of GB / T455-2002;

[0061] Limiting oxygen index test method: tested according to the test method of GB / T17591-2006;

[0062] The results are shown in Table 1 and Figure 1 .

[0063] Table 1 Heat resistance and mechanical property test of Examples 1-4 and Comparative Examples 1-12

[0064]

[0065]

[0066] As can be seen from Table 1, in Examples 1-4, the performance of meta-aramid fiber is significantly improved by multi-component synergy and process optimization. The pretreatment N-methyl pyrrolidone solution provides a high-purity environment for polymerization by deep dehydration and calcium chloride addition, ensuring that the amine monomer is fully dissolved and reacted; the special ratio of 4,4'-diaminobenzamide and m-phenylenediamine forms a regular molecular chain structure, enhancing the intermolecular force; among the heat-resistant synergists, the DOPO derivative forms a carbon layer to insulate heat at high temperatures, and the modified ZrO2 nanosheet disperses stress and enhances the skeleton structure, and the two components play a synergistic flame-retardant and reinforcing effect; the segmented temperature rising polymerization process controls the reaction process, forming a polymer with uniform molecular weight distribution; dry-wet spinning and four-stage stretching optimize the fiber orientation and crystallinity, further improving the mechanical properties; each component and process link cooperates with each other, from molecular structure to macroscopic performance, realizing all-round improvement, so that the fiber has excellent heat resistance and high strength. The prepared meta-aramid fiber has improved heat resistance and mechanical properties, the tensile strength is 2.49-2.67kN / m, the limiting oxygen index is greater than 35, and the heat damage resistance is 40-47cN.

[0067] As can be seen from Table 1 and Figure 1 It can be seen that in Comparative Example 1, N-methylpyrrolidone is not pretreated, and the water content of N-methylpyrrolidone is high, which leads to monomer hydrolysis, insufficient polymerization reaction, molecular chain breakage, reduced molecular weight, loose fiber structure, and significantly decreased heat resistance and mechanical properties. In Comparative Example 2, calcium chloride powder is not added during the pretreatment of N-methylpyrrolidone, which cannot effectively remove trace amounts of water and impurities, and the polymerization reaction is disturbed, resulting in many molecular chain defects and reduced fiber performance. In Comparative Example 3, no heat-resistant synergist is added, and the fiber is easily decomposed at high temperatures, with poor molecular chain stability and reduced mechanical properties due to ineffective stress dispersion. In Comparative Example 4, the ZrO2 nanosheet is not modified, and its dispersion in the fiber is poor, which cannot effectively transfer stress and improve heat resistance, with limited performance improvement. In Comparative Example 5, only DOPO derivatives are used as heat-resistant synergists, which can improve flame retardancy but lack the reinforcing skeleton effect of modified ZrO2 nanosheets, with poorer overall performance than the combination of the two. In Comparative Example 6, only modified ZrO2 nanosheets are used as heat-resistant synergists, which cannot form an efficient heat-resistant and reinforcing system with DOPO derivatives, with insufficient performance improvement. In Comparative Example 7, the heat-resistant synergist is not dispersed by ultrasonic dispersion after being added, and the heat-resistant synergist is not uniformly dispersed, which cannot fully play its role and affect the overall performance improvement of the fiber. In Comparative Example 8, instead of stepwise temperature increase polymerization, the temperature is directly increased to 65°C for 6h, which is low in temperature and long in reaction time, resulting in incomplete polymerization reaction, low molecular weight, uneven molecular weight distribution, and poor fiber performance. In Comparative Example 9, instead of stepwise temperature increase polymerization, the temperature is directly increased to 95°C for 5h, which is high in initial temperature and intense in reaction, easily leading to side reactions, irregular molecular chain structure, and affected performance. In Comparative Example 10, during stepwise temperature increase polymerization, no isophthaloyl chloride is added after the temperature is increased to 95°C, which cannot compensate for the loss of hydrolysis, resulting in incomplete polymerization reaction, insufficient molecular weight, and reduced fiber performance. In Comparative Example 11, the coagulation bath is 15wt% DMAc aqueous solution, which, although relatively fast in coagulation speed, leads to wide fiber diameter distribution and insufficiently dense internal structure, affecting the heat resistance and mechanical properties of the fiber, while the CaCl2-containing system makes the fiber diameter distribution more uniform. Four-stage stretching can optimize the crystallinity and orientation of the fiber, and the highly crystalline and oriented fiber structure not only improves the dimensional stability and chemical stability of the fiber at high temperatures, but also orderly arranges the fiber molecular chains along the stress direction, greatly enhancing the ability of the fiber to resist external force, thereby achieving double improvement in heat resistance and mechanical properties. In Comparative Example 12, no four-stage stretching is performed, and instead, the spinning solution is directly stretched after being solidified in the coagulation bath to form the as-spun fiber, with a stretching temperature of 160°C and a stretching ratio of 3, which cannot optimize the crystallinity and orientation of the fiber, and the molecular chains are arranged disorderly, resulting in reduced fiber performance.

[0068] While embodiments of the application have been shown and described, it is to be understood that the embodiments described are merely exemplary of the principles and application of the present application. Numerous modifications and adaptions can be effected without departing from the spirit and scope of the present application, which is not limited to the exact construction and arrangement described. It is intended, therefore, to cover all modifications and adaptions that fall within the scope of the claims and their equivalents.

Claims

1. A method for preparing meta-aramid fibrid, characterized in that: The preparation method is as follows: 4,4'-diaminobenzimide and m-phenylenediamine are added to a pretreated N-methylpyrrolidone solution, ventilated three times with nitrogen, and stirred until transparent; a heat-resistant synergist is added, ultrasonically dispersed, and then staged temperature polymerization is performed to obtain a polymer solution; the polymer solution is vacuum degassed and filtered to obtain a spinning solution; the spinning solution is subjected to dry-wet spinning, and then four-stage stretching is performed to obtain a fiber precursor; washing and drying the fiber precursor to obtain the meta-aramid fibrid; The heat-resistant synergist is obtained by compounding a DOPO derivative and modified ZrO2 nanosheets; The modified ZrO2 nanosheets are obtained by modifying the ZrO2 nanosheets with a silane coupling agent.

2. The method for preparing meta-aramid fibrid according to claim 1, wherein: The preparation method of the pretreated N-methylpyrrolidone solution is as follows: pre-dehydrating N-methylpyrrolidone, then cyclically treating it using a vacuum thin film evaporator; adding calcium chloride powder, and stirring to obtain the pretreated N-methylpyrrolidone solution.

3. The method for preparing meta-aramid fibrid according to claim 1, wherein: The preparation method of the modified ZrO2 nanosheets is as follows: adding the silane coupling agent to a mixed solution of ethanol and deionized water, stirring continuously during the dropwise addition process, and stirring after the dropwise addition is completed to obtain a hydrolyzed solution; adding the ZrO2 nanosheets to the hydrolyzed solution, heating the solution to react, and obtaining the modified ZrO2 nanosheets.

4. The method for preparing meta-aramid fibrid according to claim 1, wherein: The staged temperature-raising polymerization steps are as follows: after heating, isophthaloyl chloride is added dropwise to perform low-temperature pre-condensation; then the temperature is further raised, the isophthaloyl chloride is added to compensate for the hydrolysis loss, and the polymerization is completed by constant temperature reaction.

5. The method for preparing meta-aramid fibrid according to claim 1, wherein: The dry-wet spinning process comprises the following steps: extruding the spinning solution through a metering pump, passing the solution through a spinneret and entering a coagulation bath; the coagulation bath comprises a calcium chloride aqueous solution and N-methylpyrrolidone.

6. The method for preparing meta-aramid fibrid according to claim 1, wherein: The four-stage stretching steps are as follows: first, pre-stretching in a coagulation bath; then main stretching in a glycerin bath; then heat treatment using a hot roller; and finally relaxation in air.

7. A meta-aramid fibrid, characterized by: The meta-aramid fibrid is prepared by the preparation method according to any one of claims 1 to 6; the raw materials for preparing the meta-aramid fibrid include 4,4'-diaminobenzimide, metaphenylenediamine and a heat-resistant synergist.

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