Method for controlling molecular weight distribution of meta-aramid resin through gradient extraction and elution and application

Abstract

The invention discloses a method for controlling molecular weight distribution of meta-aramid resin through gradient extraction and elution and application. According to the method, N, N-dimethylacetamide aqueous solutions with different mass concentrations are used, multi-stage sequential extraction is carried out on resin powder according to the concentration from high to low, insoluble substances are separated, washed and dried after each stage of extraction, and a plurality of resin fractions with narrower molecular weight distribution and smaller PDI are obtained. The technology not only narrows the overall molecular weight distribution, but also can realize grading according to molecular sizes. The high molecular weight component is suitable for high-strength fibers and high-performance insulation paper, the middle molecular weight component can be used for conventional fibers and insulation paper, and the low molecular weight component is suitable for being used as a bonding layer or a functional additive, so that high-value utilization of the raw materials is realized. The obtained resin has better processing stability and product performance uniformity, and has important application value in the fields of preparation of high-strength and high-modulus fibers, high-performance insulation paper, films, composite materials and the like.

Description

Technical Field

[0001] This invention relates to the field of high-performance polymer material preparation technology, specifically to a method for precisely controlling and narrowing the molecular weight distribution (PDI) of meta-aramid (poly(m-phenylene isophthalamide, PMIA)) resin through multi-level gradient solvent extraction technology, and meta-aramid resin fractions with different molecular weight distribution characteristics obtained by this method. Background Technology

[0002] Meta-aramid is an important high-performance aromatic polyamide, renowned for its excellent high-temperature resistance, flame retardancy, chemical stability, and electrical insulation properties. It is widely used in protective clothing, electrical insulation, high-temperature filtration, and advanced composite materials. Its properties, particularly its mechanical properties, thermal stability, and solution processing performance, largely depend on the polymer's molecular weight and its distribution. In the field of electrical insulation, meta-aramid insulating paper is widely used in power equipment such as transformers and motors due to its excellent heat resistance and dielectric properties; its performance stability is closely related to the resin's molecular weight distribution.

[0003] Currently, the low-temperature solution polycondensation method is mainly used for the industrial and laboratory preparation of meta-aramid resins. Although high molecular weight resins can be obtained by controlling the reaction temperature, monomer ratio, and feeding method, the polymers obtained by this method usually have a wide molecular weight distribution. Extensive literature and practical experience show that the PDI value of meta-aramid resins synthesized by conventional low-temperature polycondensation methods is generally around 2.0 or even wider. A wide molecular weight distribution means that the resin contains both extremely long and short molecular chains. This inhomogeneity leads to a series of problems: 1) During subsequent spinning, molecular chains of different lengths behave differently during stretching, orientation, and crystallization, resulting in an uneven internal fiber structure, affecting the stability and consistency of its final mechanical properties (such as strength and modulus), and causing a higher strength CV value; 2) When preparing films or composite materials, the wide distribution of resin may lead to unstable processing rheological behavior, resulting in internal stress concentration points in the product and reducing its overall reliability; 3) It limits the application of the resin in high-end fields (such as high-strength fibers for aerospace, ultra-thin insulating films, and transformer insulating paper).

[0004] To obtain materials with superior performance, the control and purification of the molecular weight distribution of meta-aramid resins is particularly important. Traditional polymer fractionation methods, such as preparative separation using gel permeation chromatography (GPC), while highly accurate, suffer from low throughput, high cost, and high solvent consumption, failing to meet the needs of industrial or large-scale laboratory preparations. Other methods, such as precipitation fractionation, are simple to operate, but typically have low fractionation efficiency, making it difficult to achieve precise and progressively narrowed molecular weight distributions, and their effectiveness is limited for meta-aramids, a polymer with complex solubility behavior in common solvent / non-solvent systems.

[0005] Therefore, developing a method that is relatively simple to operate, highly controllable, can effectively narrow the molecular weight distribution of meta-aramid resin, and can separate resins in different molecular weight ranges is of great significance for improving the overall performance of meta-aramid materials and expanding their applications in high-end fields, including high-performance insulating paper. Summary of the Invention

[0006] The purpose of this invention is to overcome the aforementioned shortcomings in the prior art and provide a method that is controllable in operation and can effectively narrow and regulate the molecular weight distribution of meta-aramid resins. This method is based on the gradient extraction elution principle, utilizing the difference in solubility of meta-aramid in aqueous solutions of different concentrations of good solvents (DMAc) to achieve selective dissolution and separation of polymer chains with different molecular weights.

[0007] To achieve the above objectives, the present invention provides a method for controlling the molecular weight distribution of meta-aramid resin through gradient extraction and elution, characterized by comprising the following steps: S1. Using a low-temperature solution polycondensation process, m-phenylenediamine, isophthaloyl chloride, and N,N-dimethylacetamide (DMAc) are mixed and polymerized to obtain a meta-aramid resin solution. S2. Add the meta-aramid resin solution obtained in step S1 to a non-solvent to precipitate the resin. After separation, washing and drying, a meta-aramid resin solid powder with a wide molecular weight distribution is obtained. S3. Select at least one of N,N-dimethylacetamide (DMAc), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP) as a good solvent, and at least one of water, ethanol, and ethylene glycol as a poor solvent. Prepare extraction solutions of different mass concentrations by mixing the good and poor solvents in different proportions. Perform multi-stage sequential extraction on the meta-aramid resin solid powder obtained in step S2 in order of decreasing concentration. After each stage of extraction, separate, wash, and dry the insoluble matter to obtain meta-aramid resin fractions with different molecular weight distributions. The mass concentration gradient range of the extract is 85% to 20%, and the concentration difference between adjacent stages is 10% to 25%.

[0008] As a further preferred technical solution of the present invention, in step S1, the low-temperature solution polycondensation process specifically involves: under nitrogen protection, dissolving m-phenylenediamine in dry N,N-dimethylacetamide, and cooling the system to -5°C to 5°C; adding isophthaloyl chloride in batches while stirring, controlling the reaction temperature to not exceed 10°C, and performing initial prepolymerization; subsequently, slowly raising the temperature to 25°C to 35°C and continuing the reaction for 2-4 hours; after the reaction is completed, adding a neutralizing agent to adjust the pH to 7.0 to 8.0 to obtain a meta-aramid resin solution.

[0009] As a further preferred embodiment of the present invention, the molar ratio of m-phenylenediamine to isophthaloyl chloride is 1:1.00 to 1:1.02; the neutralizing agent is one or a combination of magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, and aluminum oxide.

[0010] As a further preferred technical solution of the present invention, the apparent viscosity of the meta-aramid resin solution obtained in step S1 is in the range of 500~2000 poise (25°C), and the molecular weight distribution index (PDI) is 1.8~2.5.

[0011] As a further preferred technical solution of the present invention, in step S2, the non-solvent is deionized water or an alcohol-water mixture; the stirring speed during precipitation is 200~400 rpm; the precipitate is washed sequentially with deionized water and anhydrous ethanol; and the drying temperature is 70~85℃.

[0012] As a further preferred technical solution of the present invention, in step S3, the conditions for each stage of extraction are as follows: extraction temperature 20℃ ± 5℃; solid content of meta-aramid resin powder in the extract is 4~10wt%; stirring speed during extraction is 600 rpm ± 50 rpm; extraction time is 2 hours ± 0.5 hours. If the extraction temperature is too high (e.g., above 30℃), on the one hand, the solubility of good solvents such as DMAc, DMF, DMSO, and NMP solutions is enhanced, causing some originally insoluble high molecular weight segments to enter the solution, resulting in a decrease in the molecular weight differentiation between fractions and affecting the accuracy of fractionation; on the other hand, excessively high temperatures increase solvent evaporation, changing the actual concentration of the extract and thus disrupting the established equilibrium system. Conversely, if the extraction temperature is too low (e.g., below 10℃), the movement of polymer segments is restricted, the dissolution rate decreases significantly, and the extraction process may not achieve true dissolution equilibrium, resulting in some soluble fractions remaining in the insoluble matter. This not only reduces the yield of the target fraction but also broadens the molecular weight distribution of the remaining insoluble matter, weakening the cumulative effect of multi-stage extraction. Therefore, strictly controlling the extraction temperature within the range of 20℃±5℃ is the key to ensuring stable dissolution behavior and optimal fractionation efficiency.

[0013] As a further preferred technical solution of the present invention, after each stage of extraction, the resulting mixture is centrifuged at a speed of 5000 rpm ± 500 rpm to collect the supernatant (containing soluble fractions) and precipitate (insoluble matter); the precipitate is washed multiple times with deionized water at the same temperature, centrifuged after each washing, and then dried in a vacuum drying oven to obtain the residual solid of this stage of extraction (i.e., the extraction raw material for the next stage) or the final fraction.

[0014] As a further preferred technical solution of the present invention, in step S3, the multi-stage sequential extraction includes at least three stages of extraction, namely: a first stage extraction using an aqueous solution / ethanol or ethylene glycol solution of N,N-dimethylacetamide / dimethylformamide / dimethyl sulfoxide and N-methylpyrrolidone with a mass concentration of 70%-85%; a second stage extraction using an aqueous solution / ethanol or ethylene glycol solution of N,N-dimethylacetamide / dimethylformamide / dimethyl sulfoxide and N-methylpyrrolidone with a mass concentration of 50%-65%; and a third stage extraction using an aqueous solution / ethanol or ethylene glycol solution of N,N-dimethylacetamide / dimethylformamide / dimethyl sulfoxide and N-methylpyrrolidone with a mass concentration of 20%-40%.

[0015] Furthermore, the resin fraction includes, for example, a mixture of a good solvent DMAc and a poor solvent water: First fraction (high molecular weight narrow distribution fraction): obtained by extraction from the highest concentration (e.g., 85%-90%) DMAc aqueous solution. This fraction dissolves the highest molecular weight portion, PDI ≤ 2.0, Mw ≥ 70000 g / mol; Second fraction (medium molecular weight narrow distribution fraction): obtained by extraction with medium concentration (e.g., 60%-70%) DMAc aqueous solution, PDI ≤ 1.8, Mw range 40000 ~ 70000 g / mol; The third fraction (low molecular weight narrow distribution fraction or final residue): obtained by extraction with low concentration (e.g., 25%-35%) DMAc aqueous solution, or the final insoluble matter after the first two extraction stages. This fraction has the lowest molecular weight, PDI ≤ 1.6, Mw ≤ 40000 g / mol.

[0016] As a further preferred embodiment of the present invention, the molecular weight distribution index (PDI) of the meta-aramid resin fraction is less than 1.8, and the weight-average molecular weight (Mw) ranges from 10,000 to 100,000 g / mol.

[0017] According to another aspect of the present invention, the present invention also provides the use of the above-described meta-aramid resin fraction in the preparation of meta-aramid fibers, films, composite materials or insulating materials.

[0018] Compared with the prior art, the present invention can achieve the following beneficial effects: 1. Effective control and narrowing of PDI: Through a carefully designed gradient extraction process, this invention can systematically reduce the PDI of meta-aramid resin from around 2.0 to below 1.6, significantly improving the molecular weight uniformity of the resin.

[0019] 2. Achieved resin grading and resource utilization: The method not only narrows the overall distribution but also grades the resin according to molecular weight, obtaining multiple fractions with smaller PDI and more concentrated molecular weight ranges. Different fractions can be used in targeted applications with different performance requirements, realizing high-value utilization of raw materials.

[0020] 3. Strong process controllability and clear parameters: This invention provides clear operable ranges for every step of the extraction process, including solvent concentration, solid-liquid ratio, temperature, time, stirring, centrifugation, drying, etc., with good repeatability, which is convenient for implementation in the laboratory and pilot-scale.

[0021] 4. It is beneficial to improve the performance of downstream products: When the narrow distribution resin fraction obtained by this invention is used for spinning or film formation, due to the more uniform molecular chain length, it exhibits more consistent rheological behavior and orientation crystallization characteristics during processing, which is expected to produce a final product with more uniform structure, more stable mechanical properties (such as lower strength CV value) and better overall performance.

[0022] 5. The method is relatively simple and the solvent can be recovered: Compared with preparative GPC, this method uses conventional equipment, has a larger processing capacity, and the extraction solvent DMAc can be recovered and reused through distillation and other methods, making it more economical and environmentally friendly. Attached Figure Description

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0024] Figure 1 This is a process flow diagram of the gradient extraction and elution method for controlling the molecular weight distribution of meta-aramid resin in Example 1. The left side is the "crude resin preparation" module, which includes monomer dissolution, low-temperature polycondensation, neutralization, filtration and degassing, precipitation washing and drying, producing "broad distribution crude resin (PDI≈2.0)". The right side is the core module of "gradient extraction and separation". The broad distribution crude resin enters a multi-stage series block: the first stage (high concentration DMAc aqueous solution) → separation → F1 (high Mw, low PDI fraction) and intermediate precipitate; the intermediate precipitate enters the second stage (medium concentration DMAc) → separation → F2 fraction and intermediate precipitate; the intermediate precipitate enters the third stage (low concentration DMAc) → separation → F3 fraction and final residue R, finally producing multiple "narrow distribution resin fractions (PDI<1.6)".

[0025] Figure 2The graph shows a comparison of gel permeation chromatography (GPC) curves of the crude resin obtained in Comparative Example 1 with the resin fractions F1-3 and R obtained in Example 1. The horizontal axis represents retention time (or Log M), and the vertical axis represents the differential detector response. The graph contains 5 curves: one wider and shorter peak represents CR-1 (crude resin); four narrower and taller peaks represent F1, F2, F3, and R, respectively. They appear in the high molecular weight to low molecular weight region, and their peak widths are significantly narrower than those of CR-1.

[0026] Figure 3 The figure shows a comparison of gel permeation chromatography (GPC) curves of the crude resin obtained in Comparative Example 2 with the resin fractions E1-4 and T obtained in Example 2. The figure contains 6 curves: one wider and shorter peak represents CR-1 (crude resin); four narrower and taller peaks represent E1, E2, E3, E4, E5 and T, respectively. They appear in the high molecular weight to low molecular weight region, and their peak widths are significantly narrower than those of CR-1.

[0027] Figure 4 The graph shows the molecular weight distribution histogram (schematic diagram) of the crude resin obtained in Comparative Example 1 and the four main fractions in Example 1. The horizontal axis represents molecular weight (logarithmic coordinates), and the vertical axis represents the normalized mass fraction. The distribution curve of CR-1 is broad and flat, covering a wide range. The distribution curves of F1, F2, F3 and R are narrower and higher peaks, located in the high, medium and low molecular weight regions, respectively.

[0028] Figure 5 The graph shows the molecular weight distribution histograms (schematic diagrams) of the crude resin obtained in Comparative Example 1 and the five main fractions in Example 2; the horizontal axis represents molecular weight (logarithmic scale), and the vertical axis represents the normalized mass fraction. The distribution curve of CR-1 is broad and flat, covering a wide range; the distribution curves of E1, E2, E3, E4, E5, and T show narrower and higher peaks, located in the high, medium, and low molecular weight regions, respectively.

[0029] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0030] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0031] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0032] The raw materials and equipment used in the following examples are all known products.

[0033] The raw materials were obtained by purchasing commercially available products: m-phenylenediamine (MPD, purity ≥99.5%) was purchased from Zhejiang Shouerfu Chemical Co., Ltd.; isophthaloyl chloride (IPC, purity ≥99.0%) was purchased from Shandong Kaisheng New Material Co., Ltd.; N,N-dimethylacetamide (DMAc, analytical grade, dried for one week by 4A molecular sieve before use) was purchased from Sinopharm Chemical Reagent Co., Ltd.; magnesium oxide (analytical grade) was purchased from Chengdu Kelong Chemical Co., Ltd. All experimental water was deionized water.

[0034] Main equipment: Low-temperature constant temperature reaction bath (Shanghai Bilang, BL-1000A); Top-mounted mechanical stirrer (IKA, RW 20digital); Precision electronic balance (Mettler Toledo, ME204); High-speed centrifuge (Shanghai Luxiangyi, H1850R); Vacuum drying oven (Shanghai Yiheng, DZF-6050); Gel permeation chromatograph (Waters, 1515 series, using DMAc containing 0.05M LiBr as the mobile phase and polystyrene as the standard substance).

[0035] Example 1: Three-stage gradient extraction control of PDI (process as follows) Figure 1 (As shown) Step S1. Preparation of crude meta-aramid resin In a dry 1L four-necked reaction flask, connect a mechanical stirrer, thermometer, nitrogen inlet / outlet, and feed port. Purge the air with high-purity nitrogen for 30 minutes. Add 450 g of dried DMAc and start stirring. Weigh 32.42 g (0.30 mol) of m-phenylenediamine (MPD) and add it to the reaction flask, stirring until completely dissolved. Place the reaction flask in an ice-salt bath and cool to an internal temperature of 0℃ ± 1℃.

[0036] Weigh 60.93 g (0.30 mol) of isophthaloyl chloride (IPC) and place it in a dry feeder. Add the IPC in four batches, approximately 5 minutes apart, while stirring vigorously (about 500 rpm), ensuring the reaction temperature does not exceed 5°C after each batch. After all the IPC has been added, the system becomes viscous. Remove the ice-salt bath and allow the reaction system to warm naturally to room temperature. Then place it in a constant temperature water bath at 50°C ± 1°C and continue stirring at 300 rpm for 3 hours. At the end of the reaction, the apparent viscosity of the solution was measured to be 680 poise (25°C) using an Ubbelohde viscometer.

[0037] After the reaction was complete, 8.9 g of magnesium oxide powder was slowly added to the system, and stirring was continued for 2 hours to neutralize the generated HCl. The pH of the system was measured to be approximately 7.5 using precision pH paper. The resulting viscous solution was filtered through a 400-mesh stainless steel filter and degassed under vacuum at 40°C for 2 hours to obtain a white, clear, and transparent meta-aramid resin solution (solution A). A small amount of solution A was precipitated with deionized water, washed, and dried. The weight-average molecular weight (Mw) was measured by GPC to be 57,450 g / mol, the number-average molecular weight (Mn) was 25,170 g / mol, and the PDI was 2.28.

[0038] Step S2. Precipitation and pretreatment of crude resin All of the above solution A was added dropwise to 5 L of vigorously stirred (300 rpm) deionized water at a rate of approximately 2 mL / min using a constant flow pump while stirring at 400 rpm. The resin completely precipitated as a fine flocculent. After standing for 2 hours, most of the supernatant was discarded. The remaining slurry was transferred to a centrifuge cup and centrifuged at 5000 rpm for 10 minutes, discarding the supernatant. The precipitate was redispersed with approximately 2 L of deionized water and centrifuged again. This washing process was repeated 3 times until no white precipitate was found in the last wash (indicating Cl) when tested with 0.1 M AgNO3 solution. - (Already washed). The precipitate was then washed once with 500 mL of anhydrous ethanol and centrifuged. The wet filter cake was transferred to a watch glass and dried in a vacuum drying oven at 80°C for 36 hours until constant weight. A pale yellow powdery crude resin (denoted as CR-1) was obtained. The mass of CR-1 was 86.5 g (yield approximately 95% based on total monomer content).

[0039] Step S3. Gradient extraction and elution (three stages) First-stage extraction (high molecular weight fractionation): Prepare 500 mL of 80 wt.% DMAc aqueous solution. Accurately weigh 10.00 g of CR-1 powder and place it in a 500 mL jacketed glass extraction flask. Add 200 mL of 85% DMAc aqueous solution (solid-liquid ratio = 1:20 g / mL). Connect the extraction flask to a constant temperature water bath and maintain the system temperature at 20℃ ± 0.5℃. Mechanically stir at a constant speed of 300 rpm for 2 hours.

[0040] After extraction, the entire mixture was transferred to a centrifuge tube and centrifuged at 5000 rpm for 15 minutes. The supernatant (S1-supernatant) was carefully removed, and the precipitate was retained. The S1-supernatant was transferred to another container, and 4 times the volume of deionized water was added while stirring to allow the resin to reprecipitate. The precipitate was centrifuged, washed with water (3 times), washed with ethanol (1 time), and dried under vacuum at 80°C for 24 hours to obtain white flake resin, weighing 4.36 g. This fraction is designated F1 (first fraction).

[0041] The precipitate obtained by centrifugation was redispersed with 200 mL of deionized water at 20°C, washed with stirring at 300 rpm for 10 minutes, and then centrifuged at 5000 rpm for 10 minutes. The washing solution was discarded. The washing was repeated once. The insoluble matter (wet state) after the first stage extraction was obtained, ready for the next stage extraction.

[0042] Second-stage extraction (medium molecular weight fractionation): Prepare 500 mL of a 60 wt.% DMAc aqueous solution. Transfer the washed wet precipitate (equivalent to approximately 10 - 4.36 = 5.64 g dry basis; in actual operation, it was used directly without drying, estimated according to the law of conservation of mass) back to the extraction flask. Add 112.8 mL of 60% DMAc aqueous solution (maintaining a solid-liquid ratio of approximately 1:20 on the estimated dry basis). Extract at 20℃ ± 0.5℃ with stirring at 300 rpm for 2 hours.

[0043] Following the same procedure, centrifuge to separate the components. The supernatant (S2-supernatant) was precipitated, washed, and dried using the method described above to obtain 3.15 g of resin, designated as F2 (second fraction). The precipitate was also washed twice with water to obtain the insoluble residue after the second extraction.

[0044] Third-stage extraction (low molecular weight fractionation) and final residue: Prepare 300 mL of a 30 wt.% DMAc aqueous solution. Transfer the wet precipitate from the second-stage extraction wash back to the extraction flask, and add 63 mL of a 30% DMAc aqueous solution (maintaining an estimated solid-liquid ratio of 1:20). Extract at 20℃ ± 0.5℃ with stirring at 300 rpm for 2 hours.

[0045] Centrifugation was performed. The supernatant (S3-supernatant) was treated to obtain 1.61 g of resin, denoted as F3 (third fraction). The remaining precipitate was washed with water and ethanol, then vacuum dried at 80°C for 24 hours to obtain 0.47 g of white powder, denoted as R (final residue).

[0046] The total weight of each component is: F1 (8.72g) + F2 (6.15g) + F3 (3.41g) + R (1.87g) = 9.59g, which is basically consistent with the input amount of 10.00g, and the error is within an acceptable range.

[0047] The fractions obtained in Example 1 were characterized as follows: Samples F1, F2, F3, and the original CR-1 were taken and their molecular weight and distribution were determined using GPC (DMAc + 0.05 M LiBr as the mobile phase). The results are shown in [Figure number missing]. Figure 2 , Figure 4 And Table 1.

[0048] Table 1. Molecular weight data of crude resin and each extraction fraction in Example 1

[0049] Results Analysis: As shown in Table 1, after three-stage gradient extraction, the broadly distributed crude resin CR-1 with a PDI of 2359 was successfully separated into four fractions with significantly narrowed molecular weight distributions. Fraction F1 had the highest molecular weight, but its PDI had decreased to 1.866; the PDIs of fractions F2 and F3 decreased to 1.762 and 1.626, respectively; even the lowest molecular weight residue R had a PDI of only 1.582. This indicates that gradient extraction is extremely effective in narrowing the molecular weight distribution of meta-aramid resins.

[0050] Example 2: Five-stage gradient extraction further optimizes separation Steps S1 and S2 are the same as in Example 1, i.e., the same batch of crude resin CR-1 is used for the test.

[0051] Step S3. Gradient extraction and elution (five stages): Weigh 20.00 g of CR-1 powder.

[0052] First stage: Extraction was performed using 80% DMAc aqueous solution (400 mL, 20℃, 300 rpm, 2h) to obtain fraction F1 (high molecular weight end).

[0053] Second stage: Extract the first stage insoluble matter using a 70% DMAc aqueous solution (at an estimated dry basis ratio of 1:20) to obtain fraction E2 (medium to high molecular weight).

[0054] Third stage: Extract the second stage insoluble matter with 50% DMAc aqueous solution to obtain fraction E3 (medium and low molecular weight).

[0055] Fourth stage: The insoluble matter from the third stage was extracted with a 30% DMAc aqueous solution to obtain fraction E4 (low molecular weight). Fifth stage: The insoluble matter from the fourth stage was extracted with a 20% DMAc aqueous solution to obtain fraction E5 (low molecular weight), and the remainder was residue T.

[0056] The molecular weight data of each fraction in Example 2 are shown below. Figure 3 , Figure 5 and Table 2 Table 2

[0057] Results analysis: Compared with three-stage extraction, four-stage extraction increased the cut-off points at 70% and 50% concentrations, resulting in finer separation. The PDI of each fraction was further reduced, especially in the T-stage fraction, where the PDI reached 1.451, and the molecular weight distribution was more concentrated.

[0058] Comparative Example 1: Traditional Single Precipitation / Washing Method Take 20.00 g of the crude resin CR-1 sample prepared in Example 1, without gradient extraction. Use this CR-1 resin directly for subsequent performance comparison. Its molecular weight data is the same as CR-1 in Table 1 (Mw=58657, PDI=2.359).

[0059] Comparative Example 2: Single-Concentration Solvent Extraction Method 10.00 g of the crude CR-1 resin from Example 1 was extracted once using only 200 mL of 65% DMAc aqueous solution (under the same conditions as in Example 1). After centrifugation, the supernatant was treated to obtain the "soluble fraction," and the precipitate was treated to obtain the "insoluble fraction." The molecular weights were determined as follows: soluble fraction Mw = 58813, PDI = 2.030; insoluble fraction Mw = 86622, PDI = 2.172.

[0060] Results analysis: Although a single extraction can roughly divide the resin into two parts, the improvement of the PDI of the two parts (2.030 and 2.172) compared with the raw material (2.359) is limited, and it is far from achieving the narrowing effect that gradient extraction can achieve (such as PDI < 1.6).

[0061] Example 3: Solvent Replacement Extraction Take 38.00 g of F from Example 1 and perform one extraction using 100 mL of 80% DMF ethanol solution, 80% DMSO ethanol solution, and NMP ethylene glycol solution, respectively, under the same conditions as in Example 1. After centrifugation, the supernatant was treated to obtain the "soluble fraction," and the precipitate was treated to obtain the "insoluble fraction." The molecular weights were measured, as shown in Table 3.

[0062] Table 3

[0063] Results analysis: Changing the combination of good and bad solvents can also significantly reduce PDI, proving that the extraction method is universal and the effect after replacement is comparable, with the lowest PDI reaching 1.413.

[0064] Example 3: Comparison of mechanical properties between wet spinning and film formation Ten mL each of the CR-1 crude resin and the F3 extracted resin from Example 1 were taken and wet-spun at room temperature using an aqueous solution as the spinning solution. The mechanical properties of the meta-aramid fibers spun from CR-1 and F3 resins are shown in Table 4.

[0065] Table 4

[0066] The comparative experiments described above confirm that the present invention obtains narrow-distribution meta-aramid resin fractions through a gradient extraction and elution method, effectively solving a series of technical problems existing in the spinning process of wide-distribution resins, such as uncoordinated molecular chain behavior, uneven internal fiber structure, and unstable mechanical properties. A single fiber spun using narrow-distribution resin (F13 fraction) with a PDI of 1.62 exhibits a tensile modulus approximately 1.645 times higher than that of wide-distribution resin (PDI=2.36), a breaking strength increased by 216.4%, and a strength CV value decreased by 37.7%. This demonstrates that the method of the present invention improves the comprehensive mechanical properties and quality stability of meta-aramid fibers by narrowing the molecular weight distribution.

[0067] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and various changes or modifications can be made to these embodiments without departing from the principles and essence of the present invention. The scope of protection of the present invention is defined only by the appended claims.

Claims

1. A method for controlling the molecular weight distribution of meta-aramid resin by gradient extraction and elution, characterized in that, Includes the following steps: S1. Using a low-temperature solution polycondensation process, m-phenylenediamine, isophthaloyl chloride, and N,N-dimethylacetamide are mixed and polymerized to obtain a meta-aramid resin solution. S2. Add the meta-aramid resin solution obtained in step S1 to a non-solvent to precipitate the resin. After separation, washing and drying, a meta-aramid resin solid powder with a wide molecular weight distribution is obtained. S3. Select at least one of N,N-dimethylacetamide, dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone as a good solvent, and at least one of water, ethanol, and ethylene glycol as a poor solvent. Prepare extracts of different mass concentrations by mixing the good solvent and the poor solvent in different proportions. The extracts are used to perform multi-stage sequential extraction on the meta-aramid resin solid powder obtained in step S2 in order of decreasing concentration. After each stage of extraction, the insoluble matter is separated, washed, and dried to obtain meta-aramid resin fractions with different molecular weight distributions. The mass concentration gradient range of the extract is 85% to 20%, and the concentration difference between adjacent levels is 10% to 25%.

2. The method for controlling the molecular weight distribution of meta-aramid resin by gradient extraction and elution according to claim 1, characterized in that, In step S1, the low-temperature solution polycondensation process is as follows: under nitrogen protection, m-phenylenediamine is dissolved in dry N,N-dimethylacetamide, and the system is cooled to -5℃~5℃; under stirring, isophthaloyl chloride is added in batches, and the reaction temperature is controlled not to exceed 10℃ for initial prepolymerization; then, the temperature is slowly raised to 25℃~35℃, and the reaction continues for 2-4 hours; after the reaction is completed, a neutralizing agent is added to adjust the pH to 7.0~8.0 to obtain a meta-aramid resin solution.

3. The method for controlling the molecular weight distribution of meta-aramid resin by gradient extraction and elution according to claim 2, characterized in that, The molar ratio of m-phenylenediamine to isophthaloyl chloride is 1:1.00 to 1:1.02; the neutralizing agent is one or a combination of magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, and aluminum oxide.

4. The method for controlling the molecular weight distribution of meta-aramid resin by gradient extraction and elution according to claim 3, characterized in that, The apparent viscosity of the meta-aramid resin solution obtained in step S1 ranges from 500 to 2000 poise, and the molecular weight distribution index ranges from 1.8 to 2.

5.

5. The method for controlling the molecular weight distribution of meta-aramid resin by gradient extraction and elution according to claim 1, characterized in that, In step S2, the non-solvent is deionized water or an alcohol-water mixture; the stirring speed during precipitation is 200-400 rpm; the precipitate is washed sequentially with deionized water and anhydrous ethanol; and the drying temperature is 70-85℃.

6. The method for controlling the molecular weight distribution of meta-aramid resin by gradient extraction and elution according to claim 1, characterized in that, In step S3, the conditions for each stage of extraction are as follows: extraction temperature 20℃ ± 5℃; solid content of meta-aramid resin powder in the extract is 4~10wt%; stirring speed during extraction is 600 rpm ± 50 rpm; extraction time is 2 hours ± 0.5 hours.

7. The method for controlling the molecular weight distribution of meta-aramid resin by gradient extraction and elution according to claim 1, characterized in that, After each extraction stage, the resulting mixture was centrifuged at 5000 rpm ± 500 rpm, and the supernatant and precipitate were collected. The precipitate was washed multiple times with deionized water at the same temperature, centrifuged after each wash, and then dried in a vacuum drying oven to obtain the residual solid or final fraction of this extraction stage.

8. The method for controlling the molecular weight distribution of meta-aramid resin by gradient extraction and elution according to claim 1, characterized in that, In step S3, the multi-stage sequential extraction includes at least three stages of extraction, namely: the first stage of extraction using an extractant with a mass concentration of 70%-85%, the second stage of extraction using an extractant with a mass concentration of 50%-65%, and the third stage of extraction using an extractant with a mass concentration of 20%-40%.

9. The method for controlling the molecular weight distribution of meta-aramid resin by gradient extraction and elution according to claim 1, characterized in that, The meta-aramid resin fraction has a molecular weight distribution index of less than 1.8 and a weight-average molecular weight range of 10,000 to 100,000 g / mol.

10. The use of the meta-aramid resin fraction obtained by the method of controlling the molecular weight distribution of meta-aramid resin by gradient extraction elution according to any one of claims 1-9 in the preparation of meta-aramid fibers, films, composite materials or insulating paper.

Images