A meta-aramid fiber, its molecular weight regulation method and application

Abstract

The present invention discloses a meta-aramid fiber, its molecular weight regulation method and application, belonging to the technical field of the preparation of poly(m-phthaloyl-m-phenylenediamine) fibers. Using m-phenylenediamine and m-phthaloyl chloride as raw materials, through process steps such as amino protection, pre-polycondensation, polycondensation, and spinning, meta-aramid fibers with a relatively high molecular weight are prepared. An organosilane is used to implement an amino protection strategy on DMP, replacing the active hydrogen atoms on the amino group, which not only effectively avoids the occurrence of oxidation side reactions, but also increases the solubility of the protected monomer in the solvent; the polymerized DMP after amino protection and IPC avoid the generation of HCl during the low-temperature solution polycondensation process, greatly improving the stability of the reaction system. At the same time, the post-reaction neutralization step is omitted, and a nitrogen circulation cooling device is introduced into the reaction system to ensure that nitrogen always reacts and protects at a low temperature state, thereby quickly reducing the temperature of the polymerization reaction system and avoiding the violent release of reaction heat.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of poly(m-phenylene isophthalamide) fibers, and particularly relates to a meta-aramid fiber, a method for regulating its molecular weight, and applications thereof. Background Art

[0002] Meta-aramid fiber, namely poly(m-phenylene isophthalamide) (PMIA) fiber, is a high-performance fiber prepared by two main steps: obtaining a polymer through a polycondensation reaction using m-phenylenediamine (DMP) and isophthaloyl chloride (IPC) as monomers, and then spinning the polymer. Due to the large number of hydrogen bond interactions in the meta-aramid molecule, PMIA fibers exhibit excellent performance in terms of heat resistance, flame retardancy, electrical insulation, chemical stability, and mechanical properties. Based on the above advantages, PMIA fibers have become important basic materials in many fields such as aerospace, high-speed rail locomotives, military fire protection, petrochemical industry, and electronic communication. In practical applications, there are usually certain requirements for the molecular weight of meta-aramid. Generally, as the molecular weight increases, various properties of the high-molecular fiber will be improved. Therefore, in the production process, strict requirements are imposed on the molecular weight and molecular weight distribution. The control of the molecular weight of aramid fiber needs to be achieved in the polymer synthesis step. If the molecular weight of the polymer solution is too low, filament breakage is likely to occur during the spinning process, resulting in a decline in the quality of the fiber product. The molecular weight of the polymer solution can be indirectly characterized by the viscosity of the solution. Usually, the logarithmic viscosity of the polymer solution required for the wet spinning process of aramid is about 2.2 dL·g -1 or so.

[0003] At present, the preparation methods of poly(m-phenylene isophthalamide) include interfacial polycondensation, emulsion polycondensation and low-temperature solution polycondensation. However, the PMIA resin obtained by the interfacial polycondensation method has a low molecular weight and a large dispersity; although the molecular weight of the resin generated by the emulsion polycondensation method is higher than that of the interfacial polycondensation method, the molecular weight dispersity is still large and the product quality is unstable. At present, the main method used in the industrial production of meta-aramid is the low-temperature solution polycondensation method, which refers to the homogeneous reaction of DMP and IPC in a low-temperature and anhydrous organic solvent system, and by-products hydrogen chloride are generated during this process. The advantage of low-temperature solution polycondensation is that the obtained resin has a small molecular weight dispersity and relatively stable operation. However, at present, the problem of too high temperature is still likely to occur during the synthesis of PMIA by low-temperature solution polycondensation. A higher temperature will accelerate the side reaction of acyl chloride hydrolysis, increase the by-products, and seriously affect the generation of PMIA with a higher molecular weight and a more uniform distribution. In addition, the activities of both monomers are relatively high. Among them, the amino terminal group in m-phenylenediamine is easily oxidized, which affects the molar ratio of the monomers, resulting in a decrease in the viscosity of the obtained polymer; moreover, the hydrogen chloride generated during the polycondensation of IPC and DMP is extremely easy to react with DMP to form m-phenylenediamine salt, and it cannot further react with IPC, resulting in the termination of the polycondensation process, an increase in small molecule polymers, a decrease in the molecular weight of the PMIA resin and a wide distribution. When acyl chloride and diamine are polycondensed, hydrogen chloride (HCl) is generated. If HCl is not absorbed or discharged from the reaction zone in time, chain termination will occur. Although patents such as Chinese patents CN 1265034C, CN 101876092B, CN 102534840B, CN 110983475A, etc. have provided improvements to the equipment, there is currently no relevant patent that pays attention to controlling the reaction system temperature by adjusting the temperature of the protective gas. Chinese patents CN 110983475A, CN 101285214B, CN 1162572C, etc. have all provided synthesis methods for preparing aramid resin polymers by the low-temperature method, but there are still no relevant patents on controlling the product molecular weight by the protection strategy of monomers during the preparation process of meta-aramid resin polymerization.

[0004] Aiming at the problems that the existing DMP monomer is easily oxidized, affecting the low-temperature solution polycondensation process, resulting in a low molecular weight of the final meta-aramid fiber and poor fiber-forming quality, the by-product HCl in the traditional PMIA low-temperature solution polycondensation process is easy to cause chain termination in the system, resulting in a lower polymer molecular weight, and the reaction heat is relatively high, causing an increase in side reactions and affecting the viscosity of the reaction system and the molecular weight of the product, it is urgent to find a new preparation method for meta-aramid fibers, control the product molecular weight through the protection strategy of monomers, and then control the reaction temperature to improve the molecular weight of meta-aramid fibers and the fiber-forming quality. Summary of the Invention

[0005] To overcome the above-mentioned drawbacks of the prior art, the object of the present invention is to provide a meta-aramid fiber, a method for regulating its molecular weight, and an application thereof, so as to solve the technical problems that the m-phenylenediamine monomer in the existing meta-aramid fiber is easily oxidized, the by-product HCl in the low-temperature solution polycondensation process of traditional meta-aramid fiber is likely to cause chain termination in the system, and the reaction heat is relatively high.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention discloses a method for regulating the molecular weight of a meta-aramid fiber, comprising the following steps:

[0008] 1) Dissolve m-phenylenediamine in an organic solvent completely; add an organosilane protecting agent and an acid-binding agent, stir and react, distill and collect the distillate, and obtain substance A after drying;

[0009] 2) Dissolve the substance A obtained in step 1) in a polar organic solvent completely to obtain solution A; introduce cooled nitrogen into solution A, add m-phthaloyl chloride under stirring, and filter to obtain a prepolymer after the pre-polycondensation reaction ends;

[0010] 3) Add m-phthaloyl chloride to the prepolymer prepared in step 2), and after the polycondensation reaction ends, filter, wash with water and dry to obtain a poly(m-phenylene isophthalamide) resin;

[0011] 4) Dissolve the poly(m-phenylene isophthalamide) resin prepared in step 3) in a polar solvent to obtain solution B, and then add a co-solvent to obtain a spinning dope; adopt a dry-wet spinning process, pass the spinning dope through a spinneret, enter a first coagulation bath to obtain a nascent fiber, stretch it, then pass it through a second coagulation bath, and then wash with water, dry, dry-heat stretch, heat-set and wind up to obtain a meta-aramid fiber.

[0012] Preferably, in step 1), the conditions for the stirring reaction are continuous stirring reaction at 150-155 °C for 2-2.5 h; the concentration of the solution A is 0.3-0.8 mol·L -1 ; the organic solvent is benzene, toluene, ethylbenzene or xylene; the mass ratio of the organic solvent to m-phenylenediamine is (4.0-4.5):1.

[0013] Preferably, in step 1), the organosilane protecting agent is trimethylchlorosilane; the molar ratio of m-phenylenediamine to the organosilane protecting agent is 1:(2.2-2.6).

[0014] Preferably, in step 1), the acid-binding agent is triethylamine, diethylamine, diisopropylethylamine, tetramethylethylenediamine, pyridine or 4-dimethylaminopyridine; the molar ratio of the acid-binding agent to m-phenylenediamine is (3.0-3.5):1.

[0015] Preferably, in step 2), the molar ratio of the substance A to isophthaloyl chloride is 1:(1.01 - 1.05); the stirring speed needs to be controlled at 500 - 1000 rpm during the dissolution process; the temperature of the prepolycondensation reaction is -25 to -10 °C; the stirring speed is 2000 - 3000 rpm; the addition amount of isophthaloyl chloride is 60% - 80% of the total amount of isophthaloyl chloride.

[0016] Preferably, in step 2), the temperature of the cooled nitrogen is -7 to -2 °C; the polar organic solvent is N-methylpyrrolidone, N,N-dimethylacetamide, chloroform or carbon tetrachloride; the feeding method of isophthaloyl chloride is powder feeding or molten state feeding.

[0017] Preferably, in step 3), the addition amount of isophthaloyl chloride is 20% - 40% of the total amount of isophthaloyl chloride; the intrinsic viscosity of the poly(m-phenylene isophthalamide) resin is 1.9 - 2.3 dL·g -1 .

[0018] Preferably, in step 4), the polar solvent is N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide or N-methylpyrrolidone; the concentration of the solution B is 16 - 20 wt.%; the co-solvent is one or more of calcium chloride and lithium chloride; the addition amount of the co-solvent is 3 - 5 wt.% of the polar solvent.

[0019] The present invention also discloses the meta-aramid fiber prepared by the above preparation method.

[0020] The present invention also discloses the application of the above meta-aramid fiber in the preparation of basic materials for aerospace, high-speed rail locomotives, military fire protection, petrochemical industry and electronic communication.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention discloses a method for regulating the molecular weight of meta-aramid fibers. By using organosilane to implement an amino protection strategy on DMP, the active hydrogen atoms on the amino group are replaced, which not only effectively avoids the occurrence of oxidation side reactions but also increases the solubility of the protected monomer in the solvent. It effectively solves the problem that the DMP monomer is easily oxidized, affecting the low-temperature solution polycondensation process and resulting in low molecular weight and poor fiber-forming quality of the final meta-aramid fibers. Through the polymerization of the amino-protected DMP with IPC, the generation of HCl during the low-temperature solution polycondensation process is avoided, greatly improving the stability of the reaction system. At the same time, the post-reaction neutralization step is omitted, effectively solving the problem that the by-product HCl in the traditional PMIA low-temperature solution polycondensation process easily causes chain termination in the system, resulting in a relatively low molecular weight of the polymer. By adopting the strategy of introducing a nitrogen circulation cooling device into the reaction system, nitrogen is controlled to be introduced into the polymerization system in a low-temperature state, and the high-temperature nitrogen escaping from the system is collected. After the gas is rapidly cooled in the device, it is introduced into the reaction system again as a protective atmosphere to achieve the recycling of the gas, ensuring that nitrogen always reacts and protects in a low-temperature state, thereby rapidly reducing the temperature of the polymerization reaction system, avoiding the violent release of reaction heat, and effectively solving the problem that the high reaction heat in the low-temperature solution polycondensation process causes an increase in side reactions, affecting the viscosity of the reaction system and the molecular weight of the product.

[0023] The present invention also discloses the meta-aramid fibers prepared by the above method. Since the monomers for preparing this kind of meta-aramid fibers are well protected, and the reaction heat and by-products in the polymerization process are well controlled, this kind of meta-aramid fibers has the advantages of higher molecular weight, high fiber-forming quality, and good strength performance.

[0024] The present invention also discloses the application of the above meta-aramid fibers in the preparation of basic materials for aerospace, high-speed rail locomotives, military fire protection, petrochemical industry, and electronic communication. The materials prepared from this kind of meta-aramid fibers exhibit higher strength, insulation, flame retardancy, and heat resistance in these application fields. Detailed implementation manners

[0025] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0026] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention are used to distinguish similar objects and do not necessarily describe a specific order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances so that the embodiments of the present invention described herein can be implemented in an order other than those described herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or units need not be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0027] The following further describes the present invention in detail with reference to embodiments:

[0028] The present invention provides a method for regulating the molecular weight of meta-aramid fibers, which includes using m-phenylenediamine and isophthaloyl chloride as raw materials, and through process steps such as amino protection, pre-polycondensation, polycondensation, and spinning, preparing meta-aramid fibers with a relatively high molecular weight.

[0029] A method for regulating the molecular weight of meta-aramid fibers specifically includes the following steps:

[0030] 1) Amino protection: Dissolve m-phenylenediamine with a purity greater than 99% in a dry organic solvent, and the mass ratio of the organic solvent to m-phenylenediamine is (4.0 - 4.5):1; then add an organosilane protecting agent, and the molar ratio of m-phenylenediamine to the organosilane protecting agent is 1:(2.2 - 2.6); at the same time, slowly add a certain proportion of an acid-binding agent to timely remove the by-product hydrogen chloride to ensure complete reaction, and the molar ratio of the acid-binding agent to m-phenylenediamine is (3.0 - 3.5):1; react at 150 - 155 °C with continuous stirring for 2 - 2.5 hours, collect the distillate through a distillation device, and obtain the protected substance A after drying;

[0031] In step 1), the organic solvent includes benzene, toluene, ethylbenzene, xylene, etc.; the organosilane protecting agent is trimethylchlorosilane (TMSCl); the acid-binding agent includes triethylamine, diethylamine, diisopropylethylamine, tetramethylethylenediamine, pyridine, or 4-dimethylaminopyridine;

[0032] 2) Pre-polycondensation: Prepare the substance A prepared in step 1) and isophthaloyl chloride (IPC) according to a certain molar ratio, and the molar ratio of the substance A to isophthaloyl chloride is 1:(1.01 - 1.05); first dissolve the substance A prepared in step 1) in a dry polar organic solvent with a water content ≤ 0.2% at room temperature, so that the concentration of the obtained solution A is 0.3 - 0.8 mol·L -1Within this range, the dissolution process requires controlling the stirring speed at 500 - 1000 rpm; subsequently, dry nitrogen gas that has passed through a cooling device is introduced into the system, controlling the gas temperature at -7 to -2 °C, and at the same time, controlling the reaction system temperature at -25 to -10 °C through the cooling jacket of the reaction kettle; under high-speed stirring, IPC with a purity greater than 99% is added and dissolved in the system, the stirring speed is 200 - 3000 rpm, the IPC addition amount is 60% - 80% of the total IPC amount, and the feeding method is powder feeding or molten state feeding; then filtration is carried out to obtain a prepolymer;

[0033] In step 2), the polar organic solvent is N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), chloroform or carbon tetrachloride;

[0034] 3) Polycondensation: The remaining 20% - 40% of IPC and the prepolymer in step 2) are continuously subjected to polycondensation reaction in a reactive twin-screw extruder, and are also fully mixed under the protection of dry and cooled nitrogen gas. The reaction system temperature is the same as that in step 2), still controlled within -5 to 0 °C; after the reaction ends, filtration is carried out, the reaction product is washed with water and dried to obtain poly(m-phenylene isophthalamide) resin, and the logarithmic viscosity of the poly(m-phenylene isophthalamide) resin is 1.9 - 2.3 dL·g -1 , and the logarithmic viscosity is measured using an Ubbelohde viscometer, with concentrated sulfuric acid as the solvent, prepared into a solution of 0.5 g per 100 mL, and measured at 30 °C;

[0035] 4) Spinning: First, a spinning dope is prepared by dissolving the poly(m-phenylene isophthalamide) resin obtained in step 3) in a polar solvent. The concentration of the poly(m-phenylene isophthalamide) resin in the polar solvent is 16 - 20 wt.%, and at the same time, a cosolvent is added to the system, and the addition amount of the cosolvent is 3 - 5 wt% of the polar solvent; using the dry-jet wet spinning process, the spinning dope passes through a spinneret and enters the first coagulation bath to obtain a nascent fiber. After drawing, it passes through the second coagulation bath, and then is washed with water, dried, dry heat stretched, heat set and wound to obtain meta-aramid fiber;

[0036] In step 4), the polar solvent includes N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide or N-methylpyrrolidone; the cosolvent is one or more of calcium chloride and lithium chloride.

[0037] Example 1

[0038] A method for regulating the molecular weight in the synthesis of meta-aramid fiber, comprising the following steps:

[0039] 1) Amino protection: Add 400.0 kg of dry benzene into a 600 L stainless steel reactor with a jacket, and add 96.0 kg of high-purity m-phenylenediamine to dissolve it completely. Subsequently, add 230.4 kg of trimethylchlorosilane, and slowly add 269.5 kg of triethylamine as an acid-binding agent at the same time. React under continuous stirring at 150 °C for 2 hours, collect the distillate through a distillation device, and after drying, obtain the protected substance A, which is stored under dry conditions;

[0040] 2) Pre-polycondensation: Take 50.4 kg of the substance A prepared in step 1), and dissolve it completely in dry N,N-dimethylacetamide at room temperature and 800 rpm to make the concentration of solution A after dissolution 0.5 mol·L -1 ; Subsequently, introduce dry nitrogen at -2 °C into the system, and control the temperature of the reaction system at -25 °C through the cooling jacket of the reactor. Add 28.7 kg of high-purity isophthaloyl chloride powder under high-speed stirring at 3000 rpm to dissolve it in the system, and then filter to obtain the prepolymer;

[0041] 3) Polycondensation: Transfer 12.3 kg of high-purity isophthaloyl chloride powder and the prepolymer in step 2) to a reactive twin-screw extruder, introduce dry nitrogen at -2 °C for protection, and control the temperature of the whole system at -25 °C; After the reaction is completed, filter, wash with water, and dry to obtain poly(m-phenylene isophthalamide) resin; The logarithmic viscosity of the poly(m-phenylene isophthalamide) resin is 2.0 dL·g -1 ;

[0042] 4) Spinning: Take 1 part of the poly(m-phenylene isophthalamide) resin obtained in step 3), dissolve it in 5 parts of N-methylpyrrolidone, and add 0.25 part of lithium chloride to the system at the same time to obtain a spinning dope; Adopt the dry-wet spinning process, and pass the prepared dope through a spinneret, a first coagulation bath, stretching, a second coagulation bath, washing, drying, dry heat drawing, heat setting and winding in sequence to finally obtain a meta-aramid fiber product.

[0043] Example 2

[0044] A method for regulating the molecular weight in the synthesis of meta-aramid fibers, comprising the following steps:

[0045] 1) Amino protection: Add 400.0 kg of dry benzene into a 600 L stainless steel reactor with a jacket, and add 96.0 kg of high-purity m-phenylenediamine to dissolve it completely. Subsequently, add 230.4 kg of trimethylchlorosilane, and slowly add 345.1 kg of diisopropylethylamine as an acid-binding agent at the same time. React under continuous stirring at 150 °C for 2 hours, collect the distillate through a distillation device, and after drying, obtain the protected substance A, which is stored under dry conditions;

[0046] 2) Pre - condensation: Take 50.4 kg of the substance A prepared in step 1), and dissolve it fully in dry N,N - dimethylacetamide at room temperature and 800 rpm, so that the concentration of the resulting solution A is 0.5 mol·L -1 ; Subsequently, introduce dry nitrogen at - 2°C into the system, and control the temperature of the reaction system at - 10°C through the cooling jacket of the reaction kettle; Add 25.5 kg of high - purity isophthaloyl chloride powder under high - speed stirring at 3000 rpm to dissolve it in the system, and then filter to obtain the prepolymer;

[0047] 3) Polycondensation: Transfer 17.1 kg of high - purity isophthaloyl chloride powder and the prepolymer in step 2) to a reactive twin - screw extruder, introduce dry nitrogen at - 2°C for protection, and control the temperature of the whole system at - 10°C; After the reaction, filter, wash with water, and dry to obtain the poly(m - phenylene isophthalamide) resin; The logarithmic viscosity of the poly(m - phenylene isophthalamide) resin is 2.2 dL·g -1 ;

[0048] 4) Spinning: Take 1 part of the poly(m - phenylene isophthalamide) resin obtained in step 3), dissolve it in 5 parts of N - methylpyrrolidone, and add 0.25 part of calcium chloride to the system at the same time to obtain the spinning dope; Adopt the dry - wet spinning process, and pass the prepared dope through a spinneret, a first coagulation bath, stretching, a second coagulation bath, washing, drying, dry - heat stretching, heat setting, and winding in sequence to finally obtain the meta - aramid fiber product.

[0049] Example 3

[0050] A method for regulating the molecular weight in the synthesis of meta - aramid fibers, comprising the following steps:

[0051] 1) Amino protection: Add 400.0 kg of dry toluene to a 600 L stainless - steel reaction kettle with a jacket, and add 91.4 kg of high - purity m - phenylenediamine to dissolve it fully; Subsequently, add 220.4 kg of trimethylchlorosilane, and slowly add 343.8 kg of tetramethylethylenediamine as an acid - binding agent; React for 2.2 hours under continuous stirring at 155°C, collect the distillate through a distillation device, and after drying, obtain the substance A protected by amino groups, and store it under dry conditions;

[0052] 2) Pre - condensation: Take 50.4 kg of the substance A prepared in step 1), and dissolve it fully in dry N - methylpyrrolidone at room temperature and 800 rpm, so that the concentration of the resulting solution A is 0.6 mol·L -1; Subsequently, dry nitrogen gas at -5°C was introduced into the system, and the temperature of the reaction system was controlled at -25°C through the cooling jacket of the reaction kettle; 32.8 kg of high-purity isophthaloyl chloride powder was added under high-speed stirring at 2500 rpm to dissolve it in the system, and then the prepolymer was obtained by filtration;

[0053] 3) Polycondensation: 8.2 kg of high-purity isophthaloyl chloride powder and the prepolymer in step 2) were transferred to a reactive twin-screw extruder, and dry nitrogen gas at -5°C was introduced for protection. The temperature of the whole system was controlled at -25°C; after the reaction, filtration, washing with water, and drying were carried out to obtain poly(m-phenylene isophthalamide) resin; the logarithmic viscosity of the poly(m-phenylene isophthalamide) resin was 2.3 dL·g -1 ;

[0054] 4) Spinning: Take 1 part of the resin obtained in step 3), dissolve it in 6 parts of N,N-dimethylformamide, and at the same time add 0.25 part of a mixture of lithium chloride and calcium chloride (mass ratio 1:1) to the system to obtain a spinning dope; adopt the dry-wet spinning process, and the prepared dope was successively passed through a spinneret, a first coagulation bath, drawing, a second coagulation bath, washing with water, drying, dry heat drawing, heat setting, and winding to finally obtain a meta-aramid fiber product.

[0055] Example 4

[0056] A method for regulating the molecular weight in the synthesis of meta-aramid fibers, comprising the following steps:

[0057] 1) Amino protection: 400.0 kg of dry ethylbenzene was added to a 600 L stainless steel reaction kettle with a jacket, and 100.0 kg of high-purity m-phenylenediamine was added and fully dissolved; subsequently, 221.0 kg of trimethylchlorosilane was added, and at the same time, 256.0 kg of pyridine was slowly added as an acid-binding agent; the reaction was carried out under continuous stirring at 155°C for 2.5 hours, and the distillate was collected through a distillation device. After drying, the amino-protected substance A was obtained and stored under dry conditions;

[0058] 2) Prepolycondensation: Take 50.4 kg of the substance A prepared in step 1), and fully dissolve it in dry chloroform at room temperature and 500 rpm to make the concentration of the solution A obtained after dissolution 0.3 mol·L -1 ; Subsequently, dry nitrogen gas at -5°C was introduced into the system, and the temperature of the reaction system was controlled at -15°C through the cooling jacket of the reaction kettle; 29.0 kg of high-purity molten isophthaloyl chloride was added under high-speed stirring at 2500 rpm to dissolve it in the system, and then the prepolymer was obtained by filtration;

[0059] 3) Polycondensation: Transfer 12.4 kg of high-purity molten isophthaloyl chloride and the prepolymer in step 2) to a reactive twin-screw extruder, introduce dry nitrogen at -5°C for protection, and control the temperature of the whole system at -15°C; after the reaction is completed, filter, wash with water, and dry to obtain poly(m-phenylene isophthalamide) resin; the logarithmic viscosity of the poly(m-phenylene isophthalamide) resin is 1.9 dL·g -1 ;

[0060] 4) Spinning: Take 1 part of the resin obtained in step 3), dissolve it in 6.25 parts of N,N-dimethylacetamide, and at the same time add 0.19 part of a mixture of lithium chloride and calcium chloride (mass ratio 1:1) to the system to obtain a spinning dope; adopt the dry-wet spinning process, and pass the prepared dope through a spinneret, a first coagulation bath, stretching, a second coagulation bath, washing with water, drying, dry heat drawing, heat setting and winding in sequence, and finally obtain a meta-aramid fiber product.

[0061] Example 5

[0062] A method for regulating the molecular weight in the synthesis of meta-aramid fibers, comprising the following steps:

[0063] 1) Amino protection: Add 400.0 kg of dry xylene to a 600 L stainless steel reaction kettle with a jacket, add 88.9 kg of high-purity m-phenylenediamine and dissolve it fully; then add 232.2 kg of trimethylchlorosilane, and at the same time slowly add 321.4 kg of 4-dimethylaminopyridine as an acid-binding agent; react at 152°C with continuous stirring for 2.5 hours, collect the distillate through a distillation device, and obtain substance A protected by amino group after drying, and store it under dry conditions;

[0064] 2) Pre-polycondensation: Take 50.4 kg of substance A prepared in step 1), dissolve it fully in dry N-methylpyrrolidone at room temperature and 1000 rpm, so that the concentration of solution A obtained after dissolution is 0.8 mol·L -1 ; then introduce dry nitrogen at -7°C into the system, and control the temperature of the reaction system at -20°C through the cooling jacket of the reaction kettle; add 33.1 kg of high-purity molten isophthaloyl chloride under high-speed stirring at 2000 rpm to dissolve it in the system, and then filter to obtain a prepolymer;

[0065] 3) Polycondensation: Transfer 8.3 kg of high-purity molten isophthaloyl chloride and the prepolymer in step 2) to a reactive twin-screw extruder, introduce dry nitrogen at -7°C for protection, and control the temperature of the whole system at -20°C; after the reaction is completed, filter, wash with water, and dry to obtain poly(m-phenylene isophthalamide) resin; the logarithmic viscosity of the poly(m-phenylene isophthalamide) resin is 2.1 dL·g -1 ;

[0066] 4) Spinning: Take 1 part of the resin obtained in step 3), dissolve it in 6.25 parts of dimethyl sulfoxide, and at the same time add 0.25 part of a mixture of lithium chloride and calcium chloride (mass ratio 1:1) to the system to obtain a spinning dope; adopt the wet-dry spinning process, and pass the prepared dope through a spinneret, a first coagulation bath, stretching, a second coagulation bath, water washing, drying, dry heat drawing, heat setting and winding in sequence to finally obtain a meta-aramid fiber product.

[0067] The above content is only to illustrate the technical idea of the present invention, and the protection scope of the present invention cannot be limited thereby. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A method for regulating the molecular weight of meta-aramid fiber, characterized in that, It includes the following steps: 1) Add m-phenylenediamine into an organic solvent and dissolve it fully; add an organosilane protecting agent and an acid-binding agent, stir and react, distill and collect the distillate, and obtain substance A after drying; the organosilane protecting agent is trimethylchlorosilane; the acid-binding agent is triethylamine, diethylamine, diisopropylethylamine, tetramethylethylenediamine, pyridine or 4-dimethylaminopyridine; 2) Dissolve substance A obtained in step 1) fully in a polar organic solvent to obtain solution A; introduce cooled nitrogen into solution A, add isophthaloyl chloride under stirring, and after the prepolymerization reaction ends, filter to obtain a prepolymer; 3) Add isophthaloyl chloride into the prepolymer prepared in step 2), and after the polycondensation reaction ends, filter, wash with water and dry to obtain poly(m-phenylene isophthalamide) resin; 4) Dissolve the poly(m-phenylene isophthalamide) resin prepared in step 3) in a polar solvent to obtain solution B, and then add a cosolvent to obtain a spinning dope; adopt a dry-wet spinning process, pass the spinning dope through a spinneret, enter a first coagulation bath to obtain a nascent fiber, draw it and then pass it through a second coagulation bath, and then wash with water, dry, dry-heat stretch, heat-set and wind to obtain meta-aramid fiber.

2. The method for regulating the molecular weight of meta-aramid fiber according to claim 1, characterized in that, In step 1), the conditions for the stirring reaction are 150~155 o C with continuous stirring for 2~2.5 h; the concentration of solution A is 0.3~0.8 mol·L -1 ; the organic solvent is benzene, toluene, ethylbenzene or xylene; the mass ratio of the organic solvent to m-phenylenediamine is (4.0~4.5):

1.

3. The method for regulating the molecular weight of meta-aramid fiber according to claim 1, characterized in that, In step 1), the molar ratio of m-phenylenediamine to the organosilane protecting agent is 1:(2.2~2.6).

4. The method for regulating the molecular weight of meta-aramid fiber according to claim 1, characterized in that, In step 1), the molar ratio of the acid-binding agent to m-phenylenediamine is (3.0~3.5):

1.

5. The method for regulating the molecular weight of meta-aramid fiber according to claim 1, characterized in that, In step 2), the molar ratio of the substance A to isophthaloyl chloride is 1:(1.01 - 1.05); the stirring speed needs to be controlled at 500 - 1000 rpm during the dissolution process; the temperature of the pre-polycondensation reaction is -25~-10 o °C; the stirring speed is 2000 - 3000 rpm; the addition amount of isophthaloyl chloride is 60% - 80% of the total molar amount of isophthaloyl chloride.

6. The method for regulating the molecular weight of meta-aramid fiber according to claim 1, characterized in that, In step 2), the temperature of the cooled nitrogen gas is -7~ -2 o °C; the polar organic solvent is N-methylpyrrolidone, N,N-dimethylacetamide, chloroform or carbon tetrachloride; the feeding method of isophthaloyl chloride is powder feeding or molten state feeding.

7. The method for regulating the molecular weight of meta-aramid fiber according to claim 1, characterized in that, In step 3), the addition amount of isophthaloyl chloride is 20% to 40% of the total molar amount of isophthaloyl chloride; the inherent viscosity of the poly(m-phenylene isophthalamide) resin is 1.9 to 2.3 dL·g -1 .

8. The method for regulating the molecular weight of meta-aramid fiber according to claim 1, characterized in that, In step 4), the polar solvent is N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide or N-methylpyrrolidone; the concentration of solution B is 16~20 wt.%; the cosolvent is one or more of calcium chloride and lithium chloride; the addition amount of the cosolvent is 3~5 wt.% of the polar solvent.

9. Meta-aramid fiber prepared by the method according to any one of claims 1 to 8.

10. Application of the meta-aramid fiber according to claim 9 in preparing basic materials for aerospace, high-speed rail locomotives, military fire protection, petrochemical industry and electronic communication.