High-strength and high-elongation meta-aramid short-cut fiber and preparation method thereof

Abstract

The present application relates to the technical field of high-performance fiber materials, and particularly relates to a high-strength and high-elongation meta-aramid short-cut fiber and a preparation method thereof, the preparation method being as follows: meta-phenylenediamine, 1,3-bis(4'-aminophenoxy)benzene and isophthaloyl chloride are reacted in a solvent to obtain an oligomer A solution; p-phenylenediamine, 1,4-bis(4'-aminophenoxy)benzene and terephthaloyl chloride are reacted in a solvent to obtain an oligomer B solution; meta-phenylenediamine, the oligomer A, the oligomer B and isophthaloyl chloride are subjected to a polymerization reaction in a solvent to obtain a polymerization stock solution; an alkaline earth metal hydroxide is used to adjust the pH of the polymerization stock solution to weak alkalinity to obtain a spinning stock solution; and the spinning stock solution is subjected to wet spinning and cutting treatment to obtain the meta-aramid short-cut fiber. The meta-aramid short-cut fiber simultaneously achieves excellent performance of a breaking strength of greater than or equal to 4.5 cN / dtex and an elongation at break of greater than or equal to 40%.

Description

Technical Field

[0001] This invention relates to a high-strength, high-elongation meta-aramid chopped fiber and its preparation method, belonging to the field of high-performance fiber materials technology. Background Technology

[0002] Meta-aramid (poly(m-phenylene isophthalamide)) is an important high-performance synthetic fiber with excellent flame retardancy, heat resistance, and chemical stability, and is widely used in personal protective equipment, aerospace, and rail transportation. However, traditional meta-aramid fibers have drawbacks such as low elongation at break and a stiff hand feel, which limits their application in certain scenarios requiring high flexibility.

[0003] To improve the mechanical properties of meta-aramid fibers, researchers have attempted to introduce flexible segments through copolymerization modification or adjust the fiber structure by adding modifiers. For example, patent application CN119061512A discloses the use of fluorene-containing phenylenediamine to modify meta-aramid fibers, improving the fiber's heat resistance. Patent application WO2022088852A1 discloses a high-elongation meta-aramid polymer and its preparation method, which improves the elongation of the polymer by introducing a diphenylalkane-based flexible diamine compound. Other studies have shown that adding trace amounts of polyethylene glycol as a modifier to the spinning solution can obtain a spinning solution with lower surface tension and higher viscosity, improving spinnability. While the breaking elongation is improved to some extent, the strength fluctuates significantly and still falls short of the objectives of this invention. Currently, there is a lack of a method for preparing meta-aramid chopped fibers that can simultaneously achieve high strength (≥4.5 cN / dtex) and high elongation (≥40%).

[0004] Therefore, current technologies lack a meta-aramid chopped fiber product that can significantly increase the elongation at break to over 40% while maintaining high strength (≥4.5 cN / dtex). How to balance the flexible and rigid segments through molecular structure design while maintaining the excellent mechanical properties of meta-aramid is a pressing technical challenge in this field. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a high-strength, high-elongation meta-aramid chopped fiber and its preparation method. By designing novel copolymer compositions and oligomer structures, combined with precise pH control and spinning processes, the meta-aramid chopped fiber simultaneously achieves excellent properties such as a breaking strength ≥4.5 cN / dtex and a breaking elongation ≥40%.

[0006] The technical solution of this invention to solve the above-mentioned technical problems is as follows: A method for preparing high-strength, high-elongation meta-aramid short-cut fibers, wherein the preparation method is as follows: S1, m-phenylenediamine (MPD), 1,3-bis(4'-aminophenoxy)benzene (MAPB) and isophthaloyl chloride (IPC) are reacted in a solvent to obtain oligomer A solution; p-Phenylenediamine (PPD), 1,4-bis(4'-aminophenoxy)benzene (PAPB) and terephthaloyl chloride (TPC) are reacted in a solvent to obtain oligomer B solution; S2. Polymerize m-phenylenediamine (MPD), oligomer A, oligomer B and isophthaloyl chloride (IPC) in a solvent to obtain a polymerization stock solution; S3. The pH of the polymerization solution is adjusted to weakly alkaline using alkaline earth metal hydroxides to obtain the spinning solution; S4. The spinning solution is wet-spun and cut to obtain meta-aramid short fibers.

[0007] Further, in step S1, when preparing oligomer A solution, the molar ratio of m-phenylenediamine to 1,3-bis(4'-aminophenoxy)benzene is 1:(0.3-1.2), and the molar ratio of the total number of moles of m-phenylenediamine and 1,3-bis(4'-aminophenoxy)benzene to the molar number of isophthaloyl chloride is (1.8-2.0):1; isophthaloyl chloride is added in batches for reaction, the reaction temperature is 0-5℃, and the reaction time is 0.5-2.0h.

[0008] Further, in step S1, when preparing oligomer B solution, the molar ratio of p-phenylenediamine to 1,4-bis(4'-aminophenoxy)benzene is 1:(0.3-1.2), and the molar ratio of the total number of p-phenylenediamine and 1,4-bis(4'-aminophenoxy)benzene to the number of isophthaloyl chloride is (1.6-2.0):1; isophthaloyl chloride is added in batches for reaction, the reaction temperature is 0-5℃, and the reaction time is 0.5-2.0h.

[0009] Furthermore, the number average molecular weight of oligomer A is 500-2000, and the number average molecular weight of oligomer B is 500-2000.

[0010] Further, in step S2, in the total amount of the mixed amine composed of m-phenylenediamine, oligomer A, and oligomer B, m-phenylenediamine accounts for 60-85% of the total molar amount of the mixed amine, oligomer A accounts for 5-20% of the total molar amount of the mixed amine, and oligomer B accounts for 5-20% of the total molar amount of the mixed amine; In step S2, the molar ratio of isophthaloyl chloride to mixed amine is 1:(0.9-1.3).

[0011] Furthermore, in step S2, isophthaloyl chloride is added in batches under stirring conditions, and the polymerization reaction temperature is controlled at 0-5℃, and the reaction time is 1.0-3.0h.

[0012] Furthermore, the polymer concentration of the spinning solution is 17~34 wt%, and the viscosity is 800~1500 Po.

[0013] Further, the specific process of step S4 is as follows: after filtering and degassing the spinning solution, it enters the coagulation bath through the spinneret to coagulate and form nascent fibers; the nascent fibers are subjected to multi-stage stretching, deionized water washing, drying, and heat setting treatment in sequence, and then oiling, crimping, and cutting to obtain meta-aramid short-cut fibers. The total stretching ratio for multi-stage stretching is 3-7 times.

[0014] Further, the solvent is at least one selected from N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO); The alkaline earth metal hydroxide is at least one of calcium hydroxide and magnesium hydroxide.

[0015] The present invention also discloses a high-strength, high-elongation meta-aramid chopped fiber, wherein the meta-aramid chopped fiber is prepared according to the preparation method described in the present invention.

[0016] The beneficial effects of this invention are: When preparing meta-aramid chopped fibers using the method described in this invention, oligomers are formed by introducing MAPB and PAPB units containing flexible ether bonds. Simultaneously, p-phenylenediamine and terephthaloyl chloride are introduced and copolymerized with MPD. While maintaining the rigidity of the molecular chain, the flexibility of the chain segments is increased, thereby increasing the fiber breaking elongation to over 40%. At the same time, due to the regularity of the copolymer structure and the controllable orientation, the strength is not less than 4.5 cN / dtex.

[0017] In the preparation method described in this invention, two oligomers are prepared in advance to avoid the problem of excessive difference in reaction rate during direct copolymerization. Batch feeding and low-temperature polymerization ensure uniform molecular weight distribution, resulting in high polymerization stability and a spinning solution with moderate viscosity, which is beneficial for spinning.

[0018] The ether bonds in MAPB / PAPB exhibit an electron-donating conjugation effect, making their terminal amino groups significantly more reactive than ordinary MPD. If all monomers are directly added to the reactor, the highly reactive MAPB will be rapidly consumed to form "flexible blocks," while the less reactive MPD will form "rigid blocks," resulting in a block copolymer structure. This structure is prone to microphase separation, and stress cannot be effectively transferred during stretching, leading to impaired strength and elongation. In the preparation method described in this invention, the highly reactive flexible monomers are pre-locked into the oligomer chains, reducing the instantaneous activity difference in the reaction system; then, the low-temperature batch feeding during polymerization further suppresses the localized explosive polymerization caused by intense exothermic reactions. The combination of these two factors forces the flexible and rigid units to tend towards a random copolymer distribution. This random structure can both release chain segment movement through the flexible units (increasing elongation) and maintain hydrogen bond density through the rigid units (maintaining strength). Furthermore, if only flexible monomers are used for direct copolymerization, the molecular chains are too flexible, making it impossible to form effective oriented crystals during spinning. In the preparation method described in this invention, a symmetrical molecular chain structure (PPD+PAPB+TPC) is introduced through oligomer B, which is equivalent to pre-embedding high-rigidity "physical crosslinking points" in the flexible system. During polymerization, these points are controlled to grow slowly at low temperatures, ensuring that the para-position structure is uniformly dispersed in the meta-position matrix, providing "anchor points" for subsequent stretching. This ultimately yields high-strength, high-elongation meta-aramid chopped fibers.

[0019] In the preparation method described in this invention, adjusting the pH to a weakly alkaline state (8-10) using alkaline earth metal hydroxides can effectively neutralize the hydrochloric acid produced during polymerization and promote reaction equilibrium. This is especially true when calcium hydroxide is used to adjust the pH, as Ca... 2+ Ca²⁺ can form a weak coordination interaction with amide groups in polymers. This interaction is stronger than ordinary hydrogen bonds but weaker than covalent bonds, acting as a physical cross-linking and thickening agent in spinning solutions, improving the colloidal stability of the spinning solution and reducing gel particles. This has a synergistic effect with subsequent heat setting treatment, because during heat setting, the fiber macromolecular chains undergo violent rearrangement. Without the "binding" effect of Ca²⁺, a large number of flexible ether bonds would cause the molecular chains to retract and deorient at high temperatures. The coordination effect of Ca²⁺ at high temperatures can transform into a balance between internal lubrication and constraint: allowing chain segment slippage to complete crystallization (increasing strength) while preventing flexible chain segments from completely coiling randomly (preserving orientation and maintaining modulus). This results in no crystallization during low-temperature spinning (soft and easily stretchable), and during heat setting, Ca²⁺ protects the chains and promotes crystallization (strong and highly elongated). In addition, the preparation method described in this invention can achieve high-ratio multi-stage stretching (e.g., stretching ratio of 6-7 times), which ultimately improves the crystallinity and orientation of the meta-aramid short-cut fiber, with a strength ≥4.5cN / dtex, an elongation ≥40%, and good heat resistance and dimensional stability. Detailed Implementation

[0020] The specific embodiments of the present invention will be described in detail below. The present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used is for describing particular embodiments only and is not intended to limit the invention.

[0022] A method for preparing high-strength, high-elongation meta-aramid staple fiber, wherein the preparation method comprises: S1, m-phenylenediamine (MPD), 1,3-bis(4'-aminophenoxy)benzene (MAPB) and isophthaloyl chloride (IPC) are reacted in a solvent to obtain oligomer A solution; p-Phenylenediamine (PPD), 1,4-bis(4'-aminophenoxy)benzene (PAPB) and terephthaloyl chloride (TPC) are reacted in a solvent to obtain oligomer B solution; S2. Polymerize m-phenylenediamine (MPD), oligomer A, oligomer B and isophthaloyl chloride (IPC) in a solvent to obtain a polymerization stock solution; S3. Adjust the pH of the polymerization solution to weakly alkaline (pH 8-10) using alkaline earth metal hydroxides to obtain the spinning solution; S4. The spinning solution is wet-spun and cut to obtain meta-aramid short fibers.

[0023] Specifically, in step S1, when preparing oligomer A solution, the molar ratio of m-phenylenediamine to 1,3-bis(4'-aminophenoxy)benzene is 1:(0.3-1.2), and the molar ratio of the total number of m-phenylenediamine and 1,3-bis(4'-aminophenoxy)benzene to the number of isophthaloyl chloride is (1.8-2.0):1; isophthaloyl chloride is added in batches for reaction, the reaction temperature is 0-5℃, and the reaction time is 0.5-2.0h.

[0024] Preferably, when preparing oligomer A solution, the molar ratio of m-phenylenediamine to 1,3-bis(4'-aminophenoxy)benzene is 1:(0.5-1.0).

[0025] More specifically, the specific process for preparing oligomer A solution in step S1 is as follows: m-phenylenediamine (MPD) and 1,3-bis(4'-aminophenoxy)benzene (MAPB) are dissolved in a solvent in a molar ratio, stirred and dissolved at 40~60℃, and isophthaloyl chloride (IPC) is slowly added at 0~5℃. The reaction is carried out for 0.5~2 hours to obtain oligomer A solution.

[0026] Specifically, in step S1, when preparing oligomer B solution, the molar ratio of p-phenylenediamine to 1,4-bis(4'-aminophenoxy)benzene is 1:(0.3-1.2), and the molar ratio of the total number of p-phenylenediamine and 1,4-bis(4'-aminophenoxy)benzene to the number of isophthaloyl chloride is (1.6-2.0):1; isophthaloyl chloride is added in batches for reaction, the reaction temperature is 0-5℃, and the reaction time is 0.5-2.0h.

[0027] Preferably, when preparing oligomer B solution, the molar ratio of p-phenylenediamine to 1,4-bis(4'-aminophenoxy)benzene is 1: (0.5-1.0).

[0028] More specifically, the specific process for preparing oligomer B solution in step S1 is as follows: p-phenylenediamine (PPD) and 1,4-bis(4'-aminophenoxy)benzene (PAPB) are dissolved in a solvent in a molar ratio, stirred and dissolved at 40~60℃, and terephthaloyl chloride (TPC) is slowly added at 0~5℃ and reacted for 0.5~2 hours to obtain oligomer B solution.

[0029] Specifically, the number average molecular weight of oligomer A is 500-2000, and the number average molecular weight of oligomer B is 500-2000.

[0030] Specifically, in step S2, in the total amount of the mixed amine composed of m-phenylenediamine, oligomer A, and oligomer B, m-phenylenediamine accounts for 60-85% of the total molar amount of the mixed amine, oligomer A accounts for 5-20% of the total molar amount of the mixed amine, and oligomer B accounts for 5-20% of the total molar amount of the mixed amine; In step S2, the molar ratio of isophthaloyl chloride to mixed amine is 1:(0.9-1.3).

[0031] Specifically, in step S2, isophthaloyl chloride is added in batches under stirring conditions, and the polymerization reaction temperature is controlled at 0-5℃, and the reaction time is 1.0-3.0h.

[0032] More specifically, step S2 involves mixing m-phenylenediamine (MPD) as the main monomer with the oligomer solution A and oligomer solution B obtained in step S1 to obtain a mixed amine monomer, wherein MPD accounts for 60-85% of the total amine molar amount, oligomer A accounts for 5-20%, and oligomer B accounts for 5-20%. The mixed amine monomer is dissolved in a solvent, and the dissolution temperature is controlled at 40-60°C. After complete dissolution, it is cooled to 0-5°C.

[0033] At a rotation speed of 100 r / min, isophthaloyl chloride (IPC) is added in batches for a total feeding time of 30-90 minutes. The polymerization reaction temperature is maintained at 0-5℃, and the reaction time is 1-3 hours to obtain the copolymer stock solution.

[0034] Preferably, in step S2, the intermediate phthaloyl chloride is added in 3 to 6 batches at equal time intervals to control the exothermic reaction.

[0035] Specifically, the polymer concentration of the spinning solution is 17~34 wt%, and the viscosity (25°C) is 800~1500 Po.

[0036] Specifically, the process of step S4 is as follows: the spinning solution is filtered and defoamed, and then coagulated in a coagulation bath through a spinneret to obtain nascent fibers; the nascent fibers are subjected to multi-stage stretching, deionized water washing, drying (100~150℃), heat setting treatment (200~350℃), and then oiling, crimping and cutting to obtain meta-aramid short-cut fibers; The coagulation bath is an aqueous solution of solvent with a mass content of 30-60% and a temperature of 10-40℃. The total stretch ratio for multi-stage stretching is 3-7 times.

[0037] Preferably, the total tensile strength of the multi-stage stretching is 6-7 times, and the multi-stage stretching uses 3-4 stages of stretching.

[0038] More preferably, the coagulation bath in step S4 is an aqueous solution containing 40-50 wt% DMAc; the multi-stage stretching includes primary coagulation bath stretching (stretching ratio 1-1.5 times), secondary stretching bath stretching (stretching ratio 2-3.5 times), and tertiary hot stretching (stretching ratio 1-2 times).

[0039] The primary stretching bath is a 40-50% DMAC aqueous solution at 30℃, the secondary stretching bath is a 20-30% DMAC aqueous solution at 15℃, and the tertiary stretching bath is hot water (85-90℃).

[0040] Specifically, the solvent is at least one selected from N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO); The alkaline earth metal hydroxide is at least one of calcium hydroxide and magnesium hydroxide.

[0041] Preferably, the alkaline earth metal hydroxide is calcium hydroxide.

[0042] More specifically, the length of the chopped fibers is 38-51 mm.

[0043] The present invention also discloses a high-strength, high-elongation meta-aramid chopped fiber, wherein the meta-aramid chopped fiber is prepared according to the preparation method described in the present invention.

[0044] Example 1: A method for preparing meta-aramid staple fibers, wherein the preparation method is as follows: (1) Preparation of oligomer A: In a reaction vessel equipped with a stirrer, thermometer and nitrogen protection, 300 mL of N,N-dimethylacetamide (DMAc) was added, along with 10.8 g (0.10 mol) of m-phenylenediamine (MPD) and 29.2 g (0.10 mol) of 1,3-bis(4'-aminophenoxy)benzene (MAPB). The mixture was heated to 50 °C and stirred until completely dissolved. Then the mixture was cooled to 2 °C and 20.3 g (0.10 mol) of isophthaloyl chloride (IPC) was added. The mixture was reacted for 1 hour to obtain oligomer A solution.

[0045] (2) Preparation of oligomer B: In another reaction vessel, 300 mL of DMAc was added, along with 10.8 g (0.10 mol) of p-phenylenediamine (PPD) and 29.2 g (0.10 mol) of 1,4-bis(4'-aminophenoxy)benzene (PAPB). After dissolving at 50 °C, the temperature was lowered to 2 °C, and 20.3 g (0.10 mol) of terephthaloyl chloride (TPC) was added. The reaction was carried out for 1 hour to obtain a solution of oligomer B.

[0046] (3) Polymerization reaction: 2400 mL of DMAc and 64.8 g (0.60 mol) of MPD were added to the polymerization reactor, followed by all of the oligomer A solution (containing 0.10 mol of oligomer units) and all of the oligomer B solution (containing 0.10 mol of oligomer units). The mixture was heated to 50 °C and stirred to dissolve, resulting in a mixed amine monomer solution. The system was then cooled to 2 °C. Under vigorous stirring, 162.4 g (0.80 mol) of isophthaloyl chloride (IPC) was added in 5 batches, with each batch spaced 10 minutes apart, and the reaction temperature was controlled between 2 and 4 °C. After the addition was complete, the reaction was stirred for another 2 hours to obtain a pale yellow viscous polymerization solution.

[0047] (4) pH adjustment: Add 5.0 g of calcium hydroxide powder to the polymerization stock solution and continue stirring for 30 minutes. The pH was measured to be 9.2. At this time, the polymer concentration in the stock solution was 22.0 wt%, and the viscosity was measured to be 1100 Po (25℃) by a rotational viscometer.

[0048] (5) Spinning and post-treatment: After filtration and vacuum degassing, the polymerization solution is fed into the coagulation bath at an extrusion speed of 25 m / min using a spinneret with a pore size of 0.10 mm and 1000 pores. The coagulation bath is a 45 wt% DMAc aqueous solution at 25℃. The nascent fibers undergo three stages of stretching: the first stage is a 50% DMAC aqueous solution at 30℃ with a stretching ratio of 1.5 times; the second stage is a 30% DMAC aqueous solution at 15℃ with a stretching ratio of 2.0 times; and the third stage is hot water (85℃) with a stretching ratio of 1.5 times, for a total stretching ratio of 4.5 times. The stretched fibers are thoroughly washed with deionized water until neutral, dried with hot air at 120℃, and then heat-set at 280℃ for 60 seconds. Finally, the fibers are oiled, crimped, and cut into 38 mm short fibers.

[0049] Example 2: A method for preparing meta-aramid staple fibers, wherein the preparation method is as follows: (1) Preparation of oligomer A and oligomer B: Same as in Example 1.

[0050] (2) Polymerization reaction: The molar ratio of the mixed amine monomers was changed. 2400 mL of DMAc, 75.6 g (0.70 mol) of MPD, oligomer A solution (containing 0.15 mol of oligomer units, corresponding to 43.8 g of MPD-MAPB prepolymer), and oligomer B solution (containing 0.15 mol of oligomer units, corresponding to 43.8 g of PPD-PAPB prepolymer) were added to the polymerization reactor. The total molar amount of the mixed amines was 1.00 mol. The mixture was heated to 50°C to dissolve and then cooled to 2°C. 213.2 g (1.05 mol) of IPC was added in 5 batches, i.e., the molar ratio of IPC to total amine groups was 1.05:1. The remaining polymerization conditions were the same as in Example 1.

[0051] (3) pH adjustment: 3.0 g of calcium hydroxide and 1.0 g of sodium hydroxide (mass ratio 3:1) were mixed and added to the polymerization stock solution. After stirring for 30 minutes, the pH was measured to be 8.8.

[0052] (4) Spinning and post-treatment: The total stretching ratio was increased to 4.7 times (1.5 times for first-stage stretching, 2.2 times for second-stage stretching, and 1.42 times for third-stage stretching). The stretching baths for each stage were the same as in Example 1, and the remaining coagulation bath conditions, drying, heat setting treatment, etc. were also the same as in Example 1.

[0053] Example 3: A method for preparing meta-aramid staple fibers, wherein the preparation method is as follows: (1) Preparation of oligomer A (adjusting the MPD:MAPB ratio): The molar ratio of MPD to MAPB was changed to 1:0.8. Specifically: 10.8 g (0.10 mol) of MPD and 23.36 g (0.08 mol) of MAPB were dissolved in 250 mL of DMAc and 20.3 g (0.10 mol) of IPC. The reaction conditions were the same as in Example 1 to obtain a solution of oligomer A.

[0054] (2) Preparation of oligomer B (adjusting the PPD:PAPB ratio): The molar ratio of PPD to PAPB was changed to 1:0.6. Specifically: 10.8 g (0.10 mol) of PPD and 17.52 g (0.06 mol) of PAPB were dissolved in 250 mL of DMAc and 20.3 g (0.10 mol) of TPC. The reaction conditions were the same as in Example 1 to obtain a solution of oligomer B.

[0055] (3) Polymerization reaction: The molar ratio of MPD, oligomer A, and oligomer B is the same as in Example 1 (60:20:20, but oligomer A and oligomer B are those prepared in this example). Total amines: 0.8 mol: MPD 0.48 mol (51.8 g), oligomer A contains 0.16 mol units, and oligomer B contains 0.16 mol units. IPC dosage: 0.80 mol. Polymerization method is the same as in Example 1.

[0056] (4) pH adjustment: Same as in Example 1 (calcium hydroxide 5.0g, pH=9.1).

[0057] (5) Spinning and post-treatment: The coagulation bath was changed to a 40 wt% N-methylpyrrolidone (NMP) aqueous solution at 15°C. The total stretching ratio was 4.5 times. The heat setting temperature was increased to 320°C. The rest was the same as in Example 1.

[0058] Example 4: A method for preparing meta-aramid staple fibers, wherein the preparation method is as follows: (1) Preparation of oligomer A and oligomer B: Same as in Example 1.

[0059] (2) Polymerization reaction: The molar ratio of MPD:oligomer A:oligomer B was changed to 80:10:10. The total molar amount of the mixed amines was 0.8 mol, of which MPD was 0.64 mol (69.1 g), oligomer A contained 0.08 mol units, and oligomer B contained 0.08 mol units. The amount of IPC used was 0.95 times the total amine groups, i.e., 0.76 mol (154.3 g). The polymerization temperature remained at 2~4℃, and the reaction time was 2 hours.

[0060] (3) pH adjustment: Magnesium hydroxide was used as a pH adjuster. 4.0 g of magnesium hydroxide powder was added and stirred for 30 minutes. The pH was measured to be 9.5.

[0061] (4) Spinning and post-treatment: The coagulation bath was a 50 wt% DMAc aqueous solution at 20°C. The total stretching ratio was 4.4 times (1.3 times for primary stretching, 2 times for secondary stretching, and 1.7 times for tertiary stretching). The heat setting temperature was 250°C. The rest was the same as in Example 1.

[0062] Example 5: A method for preparing meta-aramid staple fibers, wherein the preparation method is as follows: (1) Preparation of oligomer A (adjusting the MPD:MAPB ratio): The molar ratio of MPD to MAPB was changed to 1:0.7. Specifically, 10.8 g (0.10 mol) of MPD and 20.44 g (0.07 mol) of MAPB were dissolved in 250 mL of DMAC, and the amount of IPC added was 20.3 g (0.10 mol). The specific reaction conditions were the same as in Example 1, and oligomer A solution was obtained.

[0063] (2) Preparation of oligomer B (adjusting the PPD:PAPB ratio): The molar ratio of PPD to PAPB was changed to 1:0.5. Specifically, 10.8 g (0.10 mol) of PPD and 14.60 g (0.05 mol) of PAPB were dissolved in 250 mL of DMAC, and the amount of TPC added was 20.3 g (0.10 mol). The specific reaction conditions were the same as in Example 1, and oligomer B solution was obtained.

[0064] (3) Polymerization reaction: The molar ratio of MPD: oligomer A: oligomer B was changed to 88:6:6. The total molar amount of the mixed amines was 0.8 mol, of which MPD was 0.704 mol (76.0 g), oligomer A contained 0.048 mol units (MPD-MAPB prepolymer, corresponding to a total of 0.048 mol diamine units), and oligomer B contained 0.048 mol units (PPD-PAPB prepolymer, corresponding to a total of 0.048 mol diamine units). The total proportion of flexible diamine (MAPB+PAPB) was (0.048×0.7+0.048×0.5) / 0.8 = 0.072 / 0.8 = 9%. The amount of IPC used was 1.05 times the total amine groups, i.e., 0.84 mol (170.5 g). The polymerization temperature was 2~4℃, and the reaction time was 2 hours to obtain the polymerization stock solution.

[0065] (4) pH adjustment: Add 3.5 g of calcium hydroxide powder and 1.5 g of magnesium hydroxide powder (mass ratio 7:3) to the polymerization stock solution, continue stirring for 30 minutes, and the pH was measured to be 8.9. The polymer concentration was 23.5 wt% and the viscosity was 1250 Po (25℃).

[0066] (5) Spinning and post-treatment: After filtration and vacuum degassing, the polymerization solution was fed into the coagulation bath at an extrusion speed of 25 m / min using a spinneret with a pore size of 0.10 mm and 1000 pores. The coagulation bath was a 40 wt% NMP aqueous solution at 20℃. The nascent fibers underwent three stages of stretching: the first stage was a 2.0-fold stretching ratio in a 50% DMAC aqueous solution at 30℃; the second stage was a 2.2-fold stretching ratio in a 30% DMAC aqueous solution at 15℃; and the third stage was a 1.52-fold stretching ratio in hot water (90℃), for a total stretching ratio of 6.7 times. The stretched fibers were thoroughly washed with deionized water until neutral, dried with hot air at 120℃, and then heat-set at 320℃ for 45 seconds. Finally, the fibers were oiled, crimped, and cut into 51 mm short fibers.

[0067] Comparative Example 1: A method for preparing meta-aramid staple fibers, wherein the preparation method comprises: (1) Polymerization reaction: Oligomer A and oligomer B were not prepared. MPD was used directly as the sole diamine monomer. 3000 mL of DMAc and 86.4 g (0.80 mol) of MPD were added to the polymerization reactor. The mixture was heated to 50°C to dissolve and then cooled to 2°C. 162.4 g (0.80 mol) of IPC was added in 5 batches. The remaining polymerization conditions were the same as in Example 1. Poly(m-phenylene isophthalamide) stock solution was obtained.

[0068] (2) pH adjustment: Use sodium hydroxide to adjust the pH to 9.0.

[0069] (3) Spinning and post-treatment: The processes of coagulation bath, stretching, washing, drying and heat setting are exactly the same as those in Example 1 (total stretching 4.5 times, heat setting 280°C).

[0070] Comparative Example 2: A method for preparing meta-aramid chopped fibers is the same as that in Example 1, except that triethylamine is used for pH adjustment in Comparative Example 2. The specific preparation method is as follows: (1) Preparation of oligomers: Same as in Example 1.

[0071] (2) Polymerization reaction: Same as in Example 1.

[0072] (3) pH adjustment: Instead of using alkaline earth metal hydroxides, triethylamine was used to adjust the pH. About 8.5 mL of triethylamine was added dropwise to the polymerization stock solution and stirred for 30 minutes. The pH was measured to be 9.0.

[0073] (4) Spinning and post-processing: The remaining processes are the same as in Example 1.

[0074] Comparative Example 3: A method for preparing meta-aramid short-cut fibers is the same as that in Example 1, except that the molar ratio of the mixed amine monomers is changed. In Comparative Example 3, the molar ratio of MPD, oligomer A, and oligomer B is 40:30:30. The specific preparation method is as follows: (1) Preparation of oligomer A and oligomer B: Same as in Example 1.

[0075] (2) Polymerization reaction: The molar ratio of the mixed amine monomers was changed to MPD: oligomer A: oligomer B = 40:30:30. The total molar amount of the mixed amine was 0.8 mol, of which MPD contained 0.32 mol (34.6 g), oligomer A contained 0.24 mol units, and oligomer B contained 0.24 mol units. The amount of IPC used was 1.0 times the total molar amount of the mixed amine (0.80 mol, 162.4 g). The polymerization conditions were the same as in Example 1. However, it was found that the viscosity of the stock solution increased slowly during the polymerization process, and the final viscosity was 650 Po.

[0076] (3) pH adjustment: Same as in Example 1 (calcium hydroxide 5.0g, pH=9.1).

[0077] (4) Spinning and post-treatment: The post-treatment process is the same as in Example 1.

[0078] Comparative Example 4: A method for preparing meta-aramid staple fibers is the same as that in Example 1, except that the heat setting temperature is changed. In Comparative Example 4, the heat setting temperature is 180°C. The specific preparation method is as follows: (1) Preparation of oligomers: Same as in Example 1.

[0079] (2) Polymerization reaction: Same as in Example 1.

[0080] (3) pH adjustment: Same as in Example 1.

[0081] (4) Spinning and post-treatment: Coagulation, stretching, washing, and drying (120°C) are the same as in Example 1. However, in the heat setting step, heat treatment at 180°C is used. The remaining oiling, crimping, and cutting are the same.

[0082] Comparative Example 5: A method for preparing meta-aramid short-cut fibers is the same as that in Example 1, except that oligomer B is not used in Comparative Example 5. The specific preparation method is as follows: (1) Preparation of oligomer A: Same as in Example 1. That is, 10.8 g (0.10 mol) of MPD and 29.2 g (0.10 mol) of MAPB were dissolved in 300 mL of DMAc. After dissolving at 50°C, the temperature was lowered to 2°C, and 20.3 g (0.10 mol) of isophthaloyl chloride (IPC) was added. The mixture was reacted for 1 hour to obtain oligomer A solution.

[0083] (2) No oligomer B is prepared: Oligomer B is not introduced in this comparative example.

[0084] (3) Polymerization reaction: 2700 mL of DMAc and 75.6 g (0.70 mol) of MPD were added to the polymerization reactor, followed by the entire oligomer A solution (containing 0.10 mol of oligomer units), resulting in a total mixed amine volume of 0.80 mol (of which MPD accounted for 0.70 mol and oligomer A accounted for 0.10 mol). The mixture was heated to 50°C and stirred to dissolve, then cooled to 2°C. 162.4 g (0.80 mol) of IPC was added in 5 batches, i.e., the molar ratio of IPC to the total mixed amine was 1:1. The remaining polymerization conditions were the same as in Example 1. After the reaction was completed, a pale yellow polymer stock solution was obtained, with a viscosity of 950 Po.

[0085] (4) pH adjustment: Same as in Example 1, add 5.0 g of calcium hydroxide powder, stir for 30 minutes, and the pH is measured to be 9.1.

[0086] (5) Spinning and post-treatment: The processes of coagulation bath, stretching, washing, drying and heat treatment are exactly the same as those in Example 1 (total stretching 4.5 times).

[0087] Comparative Example 6: A method for preparing meta-aramid short-cut fibers is the same as that in Example 1, except that PAPB is not added during the preparation of oligomer B in Comparative Example 6. The specific preparation method is as follows: (1) Preparation of oligomer A: Same as in Example 1.

[0088] (2) Preparation of oligomer B: In another reaction vessel, add 300 mL of DMAc and 0.20 mol of p-phenylenediamine (PPD), dissolve at 50 °C and cool to 2 °C, add 20.3 g (0.10 mol) of terephthaloyl chloride (TPC) and react for 1 hour to obtain oligomer B solution.

[0089] (3) Polymerization reaction: Same as in Example 1.

[0090] (4) pH adjustment: Same as in Example 1.

[0091] (5) Spinning and post-processing: Same as in Example 1.

[0092] Comparative Example 7: A method for preparing meta-aramid short-cut fibers is the same as that in Example 1, except that MAPB is not added during the preparation of oligomer A in Comparative Example 7. The specific preparation method is as follows: (1) Preparation of oligomer A: In a reaction vessel equipped with a stirrer, thermometer and nitrogen protection, 300 mL of N,N-dimethylacetamide (DMAc) and 0.20 mol of m-phenylenediamine (MPD) were added. The mixture was heated to 50°C and stirred until completely dissolved. Then the mixture was cooled to 2°C and 20.3 g (0.10 mol) of isophthaloyl chloride (IPC) was added. The mixture was reacted for 1 hour to obtain oligomer A solution.

[0093] (2) Preparation of oligomer B: Same as in Example 1.

[0094] (3) Polymerization reaction: Same as in Example 1.

[0095] (4) pH adjustment: Same as in Example 1.

[0096] (5) Spinning and post-processing: Same as in Example 1.

[0097] Comparative Example 8: A method for preparing meta-aramid chopped fibers is the same as that in Example 1, except that oligomers A and B are not prepared beforehand in Comparative Example 8. Instead, MAPB, PAPB, p-phenylenediamine, and terephthaloyl chloride are directly added to the polymerization reaction. The specific preparation method is as follows: (1) Polymerization reaction: 2400 mL of DMAc and 64.8 g (0.60 mol) of MPD were added to the polymerization reactor, followed by 0.10 mol of MAPB and 0.10 mol of PAPB. The mixture was heated to 50 °C and stirred to dissolve, resulting in a mixed amine monomer solution. The system was then cooled to 2 °C. Under vigorous stirring, a mixture of 0.90 mol of isophthaloyl chloride (IPC) and 0.10 mol of terephthaloyl chloride was added in 5 batches, with each batch spaced 10 minutes apart, and the reaction temperature was controlled between 2 and 4 °C. After the addition was complete, the reaction was stirred for another 2 hours to obtain the polymerization stock solution.

[0098] (2) pH adjustment: Same as in Example 1.

[0099] (3) Spinning and post-processing: Same as in Example 1.

[0100] The meta-aramid chopped fibers obtained in the above examples and comparative examples were subjected to performance tests. The test method involved was GB / T 14337—2022 "Test Method for Tensile Properties of Chemical Fibers Short Fibers". The specific test results are shown in Table 1 below.

[0101] Table 1 Performance Test Results

[0102] As shown in Table 1, the meta-aramid chopped fibers prepared in Examples 1-5 of this invention all exhibit tensile strengths ranging from 4.6 to 4.8 cN / dtex and elongation at break ranging from 41% to 45%, simultaneously meeting the design specifications of strength ≥ 4.5 cN / dtex and elongation ≥ 40%. Specifically, Example 2 achieved the highest tensile strength of 4.8 cN / dtex by increasing the total draw ratio to 5.0 and using a calcium hydroxide / sodium hydroxide composite modifier. Example 3 achieved the highest elongation at break of 45% by adjusting the proportion of flexible ether bonds in the oligomer (MPD:MAPB = 1:0.8, PPD:PAPB = 1:0.6) and increasing the heat treatment temperature to 320℃. Example 4 achieved excellent performance with a tensile strength of 4.6 cN / dtex and a draw ratio of 42% even at a lower draw ratio (4.0) and a lower heat treatment temperature (250℃). Example 5 shows that the fiber exhibits excellent properties, with a tensile strength of 4.6 cN / dtex and a draw ratio of 47% at a draw ratio of 6.7. In contrast, Comparative Example 1 did not introduce any oligomers and used pure MPD homopolymerization. Although the spinning process was the same, the fiber elongation was only 28% and the strength was 4.3 cN / dtex. Both indicators failed to meet the standards, indicating that the introduction of flexible ether-bonded oligomers is the key factor in simultaneously improving strength and elongation.

[0103] In Comparative Example 2, triethylamine was used to replace alkaline earth metal hydroxide to adjust the pH. The stability of the original solution decreased significantly, resulting in an increase in fiber surface defects. The strength and elongation decreased to 3.9 cN / dtex and 35%, respectively.

[0104] In Comparative Example 3, the proportion of oligomers was too high (60% in total), and the molecular chains were too flexible. Although the elongation was as high as 52%, the strength was only 3.5 cN / dtex, which could not meet the high strength requirements.

[0105] Comparative Example 4 showed that the heat treatment temperature was too low (180℃), resulting in insufficient fiber crystallization, with a strength of only 4.0 cN / dtex and an elongation of 36%, both of which failed to meet the standards. However, after re-heat treating the same batch of fibers at 280℃, the performance was restored to a strength of 4.3 cN / dtex and a breaking elongation of 42%, demonstrating the importance of the heat setting temperature.

[0106] Comparative Example 5, which only added oligomer A without adding oligomer B, showed a fiber strength decrease to 3.9 cN / dtex and an elongation decrease to 38%, both failing to meet the standards. This demonstrates that the synergistic coexistence of oligomer A and oligomer B is indispensable; only through their combined action can a suitable balance be achieved between molecular chain flexibility and crystal orientation ability. A single oligomer cannot simultaneously improve both high strength and high elongation.

[0107] In Comparative Example 6, no PAPB was added during the preparation of oligomer B. As a result of the absence of the flexible monomer PAPB (only the rigid segments of PPD-TPC), the elongation at break was only 32%. Although the strength increased slightly to 4.7 cN / dtex, the high elongation target could not be achieved.

[0108] In Comparative Example 7, no MAPB was added during the preparation of oligomer A. Because the flexible monomer MAPB was not added to oligomer A, the total flexibility content was insufficient and the synergistic effect of the two monomers was lost. The elongation at break was only 34%, and the strength was 4.4 cN / dtex, neither of which met the high elongation requirement. In Comparative Example 8, instead of preparing oligomers A and B beforehand, MAPB, PAPB, p-phenylenediamine, and terephthaloyl chloride were directly added to the polymerization reaction. As a result of direct copolymerization, the chain segment distribution was uneven and the molecular weight distribution was wide. The breaking strength was 3.8 cN / dtex (<4.0) and the elongation was 38% (<45%), both of which failed to meet the standards.

[0109] The above comparative examples verify from different perspectives the synergistic necessity of the preparation steps, monomer composition ratio, alkaline earth metal pH adjustment method, and heat treatment process defined in this invention. Only within the scope of the technical solution of this invention can high-strength, high-elongation meta-aramid short-cut fibers be stably prepared.

[0110] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0111] For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, and these modifications and improvements are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing high-strength, high-elongation meta-aramid staple fibers, characterized in that, The preparation method is as follows: S1, m-phenylenediamine, 1,3-bis(4'-aminophenoxy)benzene and isophthaloyl chloride are reacted in a solvent to obtain oligomer A solution; p-Phenylenediamine, 1,4-bis(4'-aminophenoxy)benzene and terephthaloyl chloride were reacted in a solvent to give oligomer B solution; S2. Polymerize m-phenylenediamine, oligomer A, oligomer B and isophthaloyl chloride in a solvent to obtain the polymerization stock solution; S3. The pH of the polymerization solution is adjusted to weakly alkaline using alkaline earth metal hydroxides to obtain the spinning solution; S4. The spinning solution is wet-spun and cut to obtain meta-aramid short fibers.

2. The method for preparing high-strength, high-elongation meta-aramid staple fiber according to claim 1, characterized in that, In step S1, when preparing oligomer A solution, the molar ratio of m-phenylenediamine to 1,3-bis(4'-aminophenoxy)benzene is 1:(0.3-1.2), and the molar ratio of the total number of m-phenylenediamine and 1,3-bis(4'-aminophenoxy)benzene to the number of isophthaloyl chloride is (1.8-2.0):

1. The isophthaloyl chloride is added in batches for reaction, the reaction temperature is 0-5℃, and the reaction time is 0.5-2.0h.

3. The method for preparing high-strength, high-elongation meta-aramid staple fiber according to claim 1, characterized in that, In step S1, when preparing oligomer B solution, the molar ratio of p-phenylenediamine to 1,4-bis(4'-aminophenoxy)benzene is 1:(0.3-1.2), and the molar ratio of the total number of p-phenylenediamine and 1,4-bis(4'-aminophenoxy)benzene to the number of isophthaloyl chloride is (1.6-2.0):1; isophthaloyl chloride is added in batches for reaction, the reaction temperature is 0-5℃, and the reaction time is 0.5-2.0h.

4. The method for preparing high-strength, high-elongation meta-aramid staple fiber according to claim 1, characterized in that, The number average molecular weight of oligomer A is 500-2000, and the number average molecular weight of oligomer B is 500-2000.

5. The method for preparing high-strength, high-elongation meta-aramid staple fiber according to claim 1, characterized in that, In step S2, in the total amount of the mixed amine composed of m-phenylenediamine, oligomer A, and oligomer B, m-phenylenediamine accounts for 60-85% of the total molar amount of the mixed amine, oligomer A accounts for 5-20% of the total molar amount of the mixed amine, and oligomer B accounts for 5-20% of the total molar amount of the mixed amine. In step S2, the molar ratio of isophthaloyl chloride to mixed amine is 1:(0.9-1.3).

6. The method for preparing high-strength, high-elongation meta-aramid staple fiber according to claim 1, characterized in that, In step S2, isophthaloyl chloride is added in batches under stirring conditions, and the polymerization reaction temperature is controlled at 0-5℃ and the reaction time is 1.0-3.0h.

7. The method for preparing high-strength, high-elongation meta-aramid staple fiber according to claim 1, characterized in that, The polymer concentration of the spinning solution is 17-34 wt%, and the viscosity is 800-1500 Po.

8. The method for preparing high-strength, high-elongation meta-aramid staple fiber according to claim 1, characterized in that, The specific process of step S4 is as follows: After the spinning solution is filtered and defoamed, it enters the coagulation bath through the spinneret to coagulate and form nascent fibers; the nascent fibers are subjected to multi-stage stretching, deionized water washing, drying, and heat setting treatment in sequence, and then oiled, crimped and cut to obtain meta-aramid short-cut fibers. The total stretching ratio for multi-stage stretching is 3-7 times.

9. The method for preparing high-strength, high-elongation meta-aramid staple fiber according to claim 1, characterized in that, The solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; The alkaline earth metal hydroxide is at least one of calcium hydroxide and magnesium hydroxide.

10. A high-strength, high-elongation meta-aramid chopped fiber, characterized in that, The meta-aramid short-cut fibers are prepared according to the preparation method described in any one of claims 1-9.

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