High flame retardant aramid fiber and preparation method thereof

Abstract

The application relates to the technical field of fiber materials, and provides high-flame-retardant aramid fibers and a preparation method. An inorganic salt is dissolved in a solvent, and p-phenylenediamine is added to obtain a main monomer solution; a phosphorus-containing aromatic difunctional monomer and a silicon-containing aromatic difunctional monomer are dissolved in a solvent to obtain a flame-retardant comonomer solution; p-phthaloyl chloride is added to the main monomer solution to perform a prepolymerization reaction, and a prepolymer solution is obtained; the flame-retardant comonomer solution is added to the prepolymer solution, and p-phthaloyl chloride is added to perform a polycondensation reaction, and a copolymerized p-aramid polymer is obtained; the copolymerized p-aramid polymer is separated, washed, dried, and dissolved in concentrated sulfuric acid to obtain a spinning solution, and the high-flame-retardant p-aramid fibers are obtained after the spinning solution is post-treated. The phosphorus-containing aromatic difunctional monomer and the silicon-containing aromatic difunctional monomer are introduced in the aramid polymerization process, and the copolymerized p-aramid fibers with flame retardance and fiber-forming property are obtained.

Description

Technical Field

[0001] This invention relates to the field of fiber materials technology, and more specifically, to a highly flame-retardant aramid fiber and its preparation method. Background Technology

[0002] Para-aramid fiber is a typical high-performance fiber material with high strength, modulus, and heat resistance, and has been widely used in flame retardant protection, electrical insulation and heat resistance, high-temperature filtration, and aerospace fields. Current para-aramid fibers are typically produced by the low-temperature solution polycondensation of p-phenylenediamine and terephthaloyl chloride to obtain a polymer, which is then dissolved in concentrated sulfuric acid and spun into fibers. While this type of fiber inherently possesses certain heat resistance, its flame retardant properties still need further improvement to meet the requirements of higher-end applications or more demanding flame environments.

[0003] Among existing technical approaches to improving the flame retardancy of aramid fibers, one approach involves adding flame retardants, surface finishing agents, or blending modifiers, while another involves modifying the polymer structure by introducing functional units. However, for polymers like para-aramid, which are characterized by high linearity, rigidity, and regularity, improper introduction of flame-retardant modification units can easily interfere with the regular growth of the para-aramid bulk chain segments and subsequent orientation and fiber formation processes. This can lead to disruption of the polymer molecular structure, decreased spinnability of the spinning solution, and even adverse effects on the mechanical properties and fiber formation stability of the resulting fibers. Therefore, how to improve the flame retardancy of para-aramid fibers while maintaining their original fiber-forming properties has become a pressing technical problem to be solved in this field. Summary of the Invention

[0004] The present invention aims to provide a high flame retardant aramid fiber and its preparation method, which incorporates phosphorus- and silicon-containing flame retardant structural units into the para-aramid backbone through reactive copolymerization. While preserving the rigid segments and fiber-forming properties of the PPTA backbone as much as possible, the invention improves the fiber's long-lasting flame retardancy, charring ability, and dense char layer insulation ability.

[0005] To address the above problems, this invention provides a method for preparing highly flame-retardant aramid fibers, comprising the following steps: S100. Dissolve the inorganic salt in a solvent, add p-phenylenediamine, and obtain the main monomer solution; S200. Dissolve phosphorus-containing aromatic difunctional monomers and silicon-containing aromatic difunctional monomers in a solvent to obtain a flame-retardant comonomer solution. S300. Add the first part of terephthaloyl chloride to the main monomer solution to carry out a prepolymerization reaction to obtain a prepolymerization solution; S400. Add flame-retardant comonomer solution to prepolymer solution, and add the remaining terephthaloyl chloride to carry out polycondensation reaction to obtain copolymerized para-aramid polymer. S500: Separate, wash and dry the copolymerized para-aramid to obtain the copolymerized para-aramid resin; S600: Copolymerized para-aramid resin is dissolved in concentrated sulfuric acid to prepare a spinning solution. The spinning solution is then subjected to dry-jet wet spinning, coagulation, washing, stretching, drying and heat setting to obtain highly flame-retardant para-aramid fiber.

[0006] In the above technical solution, in S100, the solvent includes at least one of N-methylpyrrolidone, dimethylacetamide, tetramethylurea, and tetraethylurea; and / or the inorganic salt is selected from at least one of calcium chloride and lithium chloride.

[0007] In the above technical solution, in S200, the phosphorus-containing aromatic difunctional monomer includes at least one of bis(4-aminophenyl)arylphosphine oxide and bis(4-aminophenoxyaryl)phosphine oxide; and / or the silicon-containing aromatic difunctional monomer includes at least one of bis(4-aminophenyl)diarylsilane, bis(4-aminophenoxy)diarylsilane, and a bis-para-aminoaryl compound bridged by a short-chain disiloxane.

[0008] In the above technical solution, based on the total molar amount of diamine, the amount of phosphorus-containing aromatic difunctional monomer is 0.5~6 mol%, the amount of silicon-containing aromatic difunctional monomer is 0.5~5 mol%, and p-phenylenediamine is the balance.

[0009] In the above technical solution, the molar ratio of phosphorus-containing aromatic difunctional monomer to silicon-containing aromatic difunctional monomer is 1:(0.3~3).

[0010] In the above technical solution, the molar ratio of inorganic salt to total diamine is (0.1~0.8):1.

[0011] In the above technical solution, in S300, the temperature of the prepolymerization reaction is -15~10℃; and / or in S400, the temperature of the polycondensation reaction is -15~10℃, and the time is 10~120min.

[0012] In the above technical solution, in S300, the amount of the first part of terephthaloyl chloride added accounts for 85~98 mol of the total molar amount of terephthaloyl chloride.

[0013] In the above technical solution, in S600, the polymer mass fraction of the spinning solution is 15~22wt%, and / or the mass fraction of concentrated sulfuric acid is 98.0~100wt%.

[0014] The present invention also provides a highly flame-retardant aramid fiber, wherein the highly flame-retardant aramid fiber is prepared by any of the above preparation methods.

[0015] Beneficial effects (1) This application introduces phosphorus-containing aromatic difunctional monomers and silicon-containing aromatic difunctional monomers into the aramid main chain through reactive copolymerization, so that the flame-retardant structural unit becomes part of the polymer body structure, rather than an added external flame-retardant component. Therefore, it is beneficial to improve the stability and durability of the flame-retardant effect and reduce the problems of migration, precipitation or loss of flame-retardant components. (2) This application adopts a stepwise polycondensation method in which p-phenylenediamine and the first part of terephthaloyl chloride are prepolymerized first, and then phosphorus-containing aromatic difunctional monomers and silicon-containing aromatic difunctional monomers are introduced in the later stage. This is conducive to forming a main skeleton dominated by para-aramid segments first, and then introducing flame-retardant functional structural units, thereby reducing the impact of functional monomers participating in polymerization too early on the regularity and fiber-forming properties of the main chain, and is more conducive to taking into account both flame retardant performance and fiber mechanical properties. (3) In this application, phosphorus-containing structural units are conducive to improving the tendency of polymer to form char when heated, and silicon-containing structural units are conducive to improving the density and thermal stability of the char layer or surface protective layer. When the two work together, they are conducive to forming a continuous and stable barrier layer during the heating or combustion of the fiber, thereby slowing down the transfer of heat, oxygen and pyrolysis products, and improving the flame retardant performance and high temperature protection performance of the obtained high flame retardant aramid fiber. Detailed Implementation

[0016] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, a detailed description of specific embodiments of the present invention will be provided below.

[0017] Unless otherwise specified, all reagents and raw materials used in this invention are commercially available. Experimental methods in the following examples that do not specify particular conditions should be performed according to conventional methods and conditions, or as selected in the product instructions.

[0018] This invention provides a highly flame-retardant aramid fiber and its preparation method, addressing the issue of improving the flame retardancy, charring ability, and high-temperature thermal insulation barrier capacity of the fiber while maintaining the fiber-forming and mechanical properties of para-aramid. A method for preparing a highly flame-retardant aramid fiber includes the following steps: S100. Dissolve the inorganic salt in a solvent, add p-phenylenediamine, and obtain the main monomer solution; S200. Dissolve phosphorus-containing aromatic difunctional monomers and silicon-containing aromatic difunctional monomers in a solvent to obtain a flame-retardant comonomer solution. S300. Add the first part of terephthaloyl chloride to the main monomer solution to carry out a prepolymerization reaction to obtain a prepolymerization solution; S400. Add flame-retardant comonomer solution to prepolymer solution, and add the remaining terephthaloyl chloride to carry out polycondensation reaction to obtain copolymerized para-aramid polymer. S500: Separate, wash and dry the copolymerized para-aramid to obtain the copolymerized para-aramid resin; S600: Copolymerized para-aramid resin is dissolved in concentrated sulfuric acid to prepare a spinning solution. The spinning solution is then subjected to dry-jet wet spinning, coagulation, washing, stretching, drying and heat setting to obtain highly flame-retardant para-aramid fiber.

[0019] In S100, inorganic salts are dissolved in a solvent before p-phenylenediamine is added to construct a homogeneous host monomer system suitable for the low-temperature polycondensation of para-aramid fibers. This provides a stable reaction medium for the subsequent addition of terephthaloyl chloride and the prepolymerization reaction. This improves the solubility and dispersion uniformity of p-phenylenediamine in the system, reduces the risk of uneven reaction, local thickening, or local gelation caused by local concentration differences during the subsequent polycondensation process, and facilitates the formation of more uniform prepolymer segments. The solvent provides a highly polar liquid-phase environment, and the inorganic salts and solvent form a solvated salt system, which jointly regulates the interaction between the monomers and nascent polymeric segments in the reaction medium, thereby improving the system homogeneity and enhancing the stability of the low-temperature polycondensation.

[0020] Furthermore, the solvent includes at least one of N-methylpyrrolidone, dimethylacetamide, tetramethylurea, and tetraethylurea. All of these solvents have high polarity and good compatibility with the low-temperature polycondensation system of aromatic polyamides. N-methylpyrrolidone and dimethylacetamide are commonly used solvents in the polymerization of para-aramid fibers, while tetramethylurea and tetraethylurea have also been used in low-temperature polycondensation systems of aramid fibers. The inorganic salt includes at least one of calcium chloride and lithium chloride, which can form a solvated salt system with the solvent, improving the solution state of p-phenylenediamine and primary polymeric segments in the reaction medium, inhibiting premature aggregation during polymerization, thereby improving the uniformity and controllability of the prepolymerization reaction. In S200, phosphorus-containing aromatic difunctional monomers and silicon-containing aromatic difunctional monomers are pre-dissolved in a solvent to obtain a flame-retardant comonomer solution. This allows the functional monomers to participate in the subsequent polycondensation reaction in S400 in a more uniform and stable state, which is beneficial for introducing flame-retardant structural units into the para-aramid backbone in a covalent manner. By pre-preparing the flame-retardant comonomer solution, the problems of excessively high local concentrations and uneven distribution caused by direct feeding of functional monomers can be reduced, and it is beneficial to achieve backbone-type flame-retardant modification while maintaining the continuous growth of the PPTA backbone segments. Phosphorus-containing structural units are beneficial for increasing the polymer's char-forming tendency when heated, while silicon-containing structural units are beneficial for improving the density and thermal stability of the char layer or surface protective layer. The synergy of the two is more conducive to forming a continuous and stable barrier layer, thereby improving the flame-retardant properties of the resulting fibers.

[0021] Preferably, the phosphorus-containing aromatic difunctional monomer includes at least one of bis(4-aminophenyl)arylphosphine oxide and bis(4-aminophenoxyaryl)phosphine oxide, and the silicon-containing aromatic difunctional monomer includes at least one of bis(4-aminophenyl)diarylsilane, bis(4-aminophenoxy)diarylsilane, and a bis-para-aminoaryl compound bridged by a short-chain disiloxane. All of the above monomers possess functional groups capable of participating in amidation reactions while retaining the aromatic skeleton, which is beneficial for reducing damage to the rigidity and fiber-forming properties of the para-aramid backbone while introducing phosphorus and silicon flame-retardant elements. Among them, the phosphorus-containing monomer is more inclined to play a char-promoting role, while the silicon-containing monomer is more inclined to play a char-stabilizing and barrier layer strengthening role.

[0022] Furthermore, based on the total molar amount of diamine, the amount of phosphorus-containing aromatic difunctional monomers is 0.5~6 mol%, the amount of silicon-containing aromatic difunctional monomers is 0.5~5 mol%, and p-phenylenediamine is the balance. This range is beneficial for achieving a balance between flame retardant effect and fiber-forming performance: when the amount of phosphorus- or silicon-containing functional monomers is too low, the number of effective flame-retardant structural units in the main chain is insufficient, making it difficult to fully realize the synergistic flame-retardant advantages of phosphorus-silicon; when the amount is too high, it can easily cause significant interference with the regularity of the para-aramid main chain segments, interchain interactions, and subsequent orientation fiber-forming processes.

[0023] Furthermore, the molar ratio of phosphorus-containing aromatic difunctional monomers to silicon-containing aromatic difunctional monomers is 1:(0.3~3). Within this range, the phosphorus-based char-promoting effect and the silicon-based carbon layer densification effect can be at a relatively coordinated level, which is more conducive to the formation of a protective layer with both char-forming and barrier properties. If the proportion of silicon-containing monomers is too low, the synergistic effect of silicon-based monomers is not obvious; if the proportion of silicon-containing monomers is too high, it is easy to weaken the phosphorus-dominated char-promoting effect and may have an adverse effect on the rigidity and fiber-forming properties of the main chain.

[0024] Furthermore, the molar ratio of inorganic salt to total diamine is (0.1~0.8):1. In the low-temperature solution polycondensation system of para-aramid, the inorganic salt mainly functions as a solubilizer and system regulator, forming a salt-solvent environment with the solvent to improve the solution state of p-phenylenediamine and the nascent polymerized chain segments. When the dosage is below this range, its solubilizing and stabilizing effects are insufficient; when the dosage is above this range, it increases the ionic strength of the system and the burden of post-processing, which is detrimental to polymer purification and subsequent spinning stability.

[0025] In S300, a prepolymerization reaction is initiated by adding the first portion of terephthaloyl chloride to the main monomer solution. This establishes a prepolymer system dominated by para-aramid segments, providing the main framework for the subsequent introduction of functional monomers. Prepolymerization facilitates the preferential formation of longer PPTA main segments, reducing the adverse effects of premature functional monomer participation in polycondensation on the regular growth of the main chain. It also helps to balance flame retardant modification with fiber-forming properties. The high strength and high orientation fiber-forming properties of para-aramid rely on the rigid rod-like chains and high regularity resulting from its para-aromatic structure. Therefore, the preferential reaction between p-phenylenediamine and the first portion of terephthaloyl chloride is more conducive to the formation of PPTA-type main segments.

[0026] Furthermore, the preferred prepolymerization temperature in S300 is -15 to 10°C. Traditional synthesis of para-aramid involves low-temperature solution polycondensation. As the molecular weight increases, the system viscosity rises rapidly with significant exothermic activity, necessitating control of the reaction rate and system viscosity at a relatively low temperature. Excessive temperature can lead to localized overheating and thickening, hindering the formation of uniform prepolymer segments; conversely, excessively low temperature impedes mass transfer and industrial operation. Maintaining the prepolymerization reaction at -15 to 10°C helps achieve a balance between reaction rate, exothermic control, and system uniformity.

[0027] In S400, a flame-retardant comonomer solution is added to the prepolymer solution, followed by the addition of remaining terephthaloyl chloride for a polycondensation reaction. This covalently incorporates phosphorus-containing aromatic difunctional monomers and silicon-containing aromatic difunctional monomers into the para-aramid backbone, resulting in a copolymerized para-aramid polymer possessing both backbone and flame-retardant functional segments. This step helps impart backbone-type flame-retardant functionality to the material and reduces the migration and precipitation of added flame retardants. Specifically, the phosphorus-containing structural units are more conducive to promoting char formation, while the silicon-containing structural units are more conducive to improving the density and thermal stability of the char layer or surface protective layer. The synergistic effect of these two components contributes to the formation of a more continuous and stable barrier layer.

[0028] Furthermore, the polycondensation reaction temperature in S400 is -15 to 10°C, and the time is 10 to 120 minutes. Continuing to use low-temperature polycondensation on the basis of the already formed prepolymer system helps control the reaction rate and system viscosity during the introduction of functional monomers, avoiding local overheating and uneven copolymerization. Too short a reaction time is not conducive to the full incorporation of functional monomers into the main chain, while too long a reaction time increases the residence time of the system in a high-viscosity state, which is detrimental to process stability. Controlling the polycondensation time within 10 to 120 minutes helps to achieve a balance between effective introduction of functional monomers and stable system operation.

[0029] Furthermore, this invention employs a stepwise addition of terephthaloyl chloride, controlling the amount of the first portion of terephthaloyl chloride added to 85-98 mol% of the total molar amount of terephthaloyl chloride. This stepwise addition method helps divide the polymerization process into a primary growth stage of the main chain segment and a later-stage introduction stage of the functional monomer, thereby reducing the adverse interference of the functional monomer on the formation of the PPTA main chain segment in the early stages of polymerization. If the amount of the first portion of terephthaloyl chloride added is too low, the main backbone will not be sufficiently established in the prepolymerization stage; if the amount added is too high, there will be insufficient acyl chloride reaction sites left for the later-stage functional monomer to integrate into the main chain. Controlling it at 85-98 mol% helps to balance the primary growth of the main chain segment and the effective introduction of the later-stage functional monomer.

[0030] In S500, the copolymerized para-aramid polymer is separated, washed, and dried to obtain the copolymerized para-aramid resin. This process removes residual solvents, inorganic salts, oligomers, and other small-molecule impurities from the polymerization system, providing a high-purity and stable resin precursor for subsequent concentrated sulfuric acid dissolution and spinning solution preparation. This step improves resin purity and uniformity, reduces the interference of residual salts and solvents on subsequent spinning solution formation and spinneret stability, and lowers the risk of gelation, localized insoluble substances, or pore blockage during spinning. Washing and drying remove low-molecular-weight components that affect concentrated sulfuric acid dissolution and spinning solution uniformity, bringing the polymer to a state suitable for subsequent spinning solution preparation.

[0031] In S600, a spinning solution is prepared by dissolving copolymerized para-aramid resin in concentrated sulfuric acid. This solution is then subjected to dry-jet wet spinning, coagulation, washing, drawing, drying, and heat setting to obtain highly flame-retardant para-aramid fibers. This process transforms the copolymerized para-aramid resin into fibers with a highly oriented structure and superior final performance. Dissolving in concentrated sulfuric acid helps weaken the strong hydrogen bonds and aromatic aggregation between aramid chains, forming a highly ordered spinning solution. Dry-jet wet spinning and subsequent drawing improve the orientation of the molecular chains along the fiber axis. Coagulation, washing, and heat setting help stabilize the fiber structure and properties. Concentrated sulfuric acid allows the copolymerized para-aramid segments to enter a flowable, spinnable solution state. The spinning and drawing processes promote molecular chain orientation, while coagulation and post-treatment fix the oriented structure.

[0032] Furthermore, the polymer mass fraction of the spinning solution is 15~22wt%; this range is conducive to achieving a balance between the stability and spinnability of the spinning solution: when the polymer mass fraction is too low, it is not conducive to forming a stable and continuous highly ordered spinning solution; when the polymer mass fraction is too high, the viscosity of the spinning solution increases significantly, the rheological stability of the spinneret decreases, and the spinnability is easily affected.

[0033] Furthermore, the mass fraction of concentrated sulfuric acid is 98.0~100wt%; this range is beneficial to ensure the full dissolution of the copolymerized para-aramid resin and the formation of a uniform and stable spinning solution; if the sulfuric acid concentration is too low, the water content in the system will increase, which is not conducive to breaking the strong interaction between aramid chains, thus affecting the resin dissolution and the stability of the spinning solution.

[0034] This invention also provides a highly flame-retardant aramid fiber, which is prepared by the above-described method. The polymer backbone of the highly flame-retardant aramid fiber comprises a first repeating structural unit formed by terephthaloyl and p-phenylenediamine residues, a second repeating structural unit formed by terephthaloyl and phosphorus-containing aromatic difunctional monomer residues, and a third repeating structural unit formed by terephthaloyl and silicon-containing aromatic difunctional monomer residues. The highly flame-retardant aramid fiber is not formed by a simple blend of aramid resin and flame retardant, but rather by covalently introducing phosphorus-containing and silicon-containing structural units into the aramid backbone in a reactive manner, forming a main-chain type flame-retardant aramid fiber.

[0035] High flame-retardant aramid fibers, while maintaining the rigid segments and fiber-forming properties of para-aramid, incorporate main-chain phosphorus-containing and silicon-containing structural units, thus giving the fibers both good flame-retardant properties and structural stability. Since the phosphorus-containing and silicon-containing structural units are covalently bonded into the polymer backbone, they are less prone to migration, precipitation, or loss, which helps improve the stability and durability of the flame-retardant effect. Specifically, the phosphorus-containing structural units enhance the polymer's char-forming tendency when heated, while the silicon-containing structural units improve the density and thermal stability of the char layer or surface protective layer. Together, they facilitate the formation of a continuous and stable barrier layer during fiber heating or combustion, thereby slowing the transfer of heat, oxygen, and pyrolysis products and improving the fiber's flame-retardant performance. Simultaneously, because the fiber matrix remains dominated by rigid para-aramid segments, it is even more advantageous to balance flame retardancy and fiber-forming properties. Example 1

[0036] This embodiment provides a high flame-retardant aramid fiber and its preparation method, including the following steps: S100. In a 3L four-necked reactor equipped with mechanical stirring, low-temperature circulation and nitrogen protection, add 1200g of N-methylpyrrolidone and 33.3g of anhydrous calcium chloride, stir until completely dissolved, and then cool to 0℃; then add 103.82g of p-phenylenediamine, and continue stirring until completely dissolved to obtain the main monomer solution. S200. Add 200g of N-methylpyrrolidone to a clean flask, and add 6.16g of bis(4-aminophenyl)phenylphosphine oxide and 7.32g of bis(4-aminophenyl)diphenylsilane sequentially at 45℃. Stir until dissolved and then cool to 0℃ to obtain a flame-retardant comonomer solution. In this solution, the amount of phosphorus-containing monomer is 2mol%, the amount of silicon-containing monomer is 2mol%, the molar ratio of the two is 1:1, and p-phenylenediamine is the balance. S300, under nitrogen protection at 0~5℃, 186.78g of terephthaloyl chloride was added in batches to the main monomer solution, with the addition time controlled at 20min, and the reaction continued for 15min to obtain the prepolymer solution; S400: Add the flame-retardant comonomer solution to the prepolymer solution within 10 min, then add the remaining 16.24 g of terephthaloyl chloride, and continue the polycondensation reaction at 0~5℃ for 60 min to obtain the copolymerized para-aramid polymer slurry. S500. The copolymerized para-aramid polymer slurry is slowly poured into 10L of deionized water for precipitation. After filtration, it is repeatedly washed with deionized water until the chloride ion detection in the filtrate is basically negative. Then, it is replaced and washed with anhydrous ethanol. Finally, it is vacuum dried at 90℃ for 12h to obtain the copolymerized para-aramid resin. S600. Weigh 180g of the copolymerized para-aramid resin and slowly add it to 820g of concentrated sulfuric acid with a mass fraction of 99.5wt%. Stir and dissolve at 5°C to obtain a spinning solution with a polymer mass fraction of 18wt%. After degassing and filtration, the spinning solution is subjected to dry-jet wet spinning. The spinneret has 100 holes with a diameter of 0.08mm and an air gap of 8mm. The coagulation bath is 5°C deionized water. Subsequently, the solution is subjected to water washing, 2.8 times stretching in hot water at 90°C, drying at 150°C, and heat setting at 430°C for 15s to obtain highly flame-retardant aramid fiber. Example 2

[0037] This embodiment provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Embodiment 1, except that: In S100, 1200g of dimethylacetamide was used as a solvent, 4.24g of anhydrous lithium chloride was added and stirred to dissolve, and then 107.06g of p-phenylenediamine was added to obtain the main monomer solution. In S200, 200g of dimethylacetamide was added to 1.54g of bis(4-aminophenyl)phenylphosphine oxide and 1.83g of bis(4-aminophenyl)diphenylsilane to obtain a flame-retardant comonomer solution; wherein, based on the total molar amount of diamine, the amount of phosphorus-containing monomer and silicon-containing monomer was 0.5mol%, and the molar ratio was 1:1; In S300, 172.57 g of terephthaloyl chloride was added to the main monomer solution at -15℃ for 20 min to carry out a prepolymerization reaction, and a prepolymerization solution was obtained. In S400, a flame-retardant comonomer solution was added to the prepolymer solution, followed by the addition of the remaining 30.45 g of terephthaloyl chloride. Polycondensation was continued at -15 °C for 120 min to obtain a copolymerized para-aramid polymer. In S600, 150g of copolymerized para-aramid resin is weighed and dissolved in 850g of concentrated sulfuric acid with a mass fraction of 98.0wt% to obtain a spinning solution with a polymer mass fraction of 15wt%. After dry-jet wet spinning, coagulation, washing, stretching, drying and heat setting, high flame-retardant aramid fiber is obtained. Example 3

[0038] This embodiment provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Embodiment 1, except that: In S100, 1300g of N-methylpyrrolidone and 55.5g of anhydrous calcium chloride were used to prepare the main reaction system, and then 99.70g of p-phenylenediamine was added to obtain the main monomer solution. In S200, 250g of N-methylpyrrolidone was added to 18.48g of bis(4-aminophenyl)phenylphosphine oxide and 6.59g of bis(4-aminophenyl)diphenylsilane, and stirred to dissolve, thus obtaining a flame-retardant comonomer solution; wherein, based on the total molar amount of diamine, the amount of phosphorus-containing monomer was 6mol%, the amount of silicon-containing monomer was 1.8mol%, and the molar ratio of the two was 1:0.3; In S300, 192.87 g of terephthaloyl chloride was added to the main monomer solution at 0 °C to carry out a prepolymerization reaction; After adding the flame-retardant comonomer solution to S400, the remaining 10.15g of terephthaloyl chloride is added, and polycondensation is carried out at 0℃ for 90min to obtain the copolymerized para-aramid polymer. In S600, a spinning solution with a polymer mass fraction of 20 wt% is prepared using 99.5 wt% concentrated sulfuric acid. After dry-jet wet spinning, coagulation, washing, stretching, drying and heat setting, high flame-retardant aramid fiber is obtained. Example 4

[0039] This embodiment provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Embodiment 1, except that: In S100, 1500g of N-methylpyrrolidone and 88.8g of anhydrous calcium chloride were used to prepare the main reaction system, and then 101.65g of p-phenylenediamine was added to obtain the main monomer solution. In S200, 300g of N-methylpyrrolidone was added to 4.62g of bis(4-aminophenyl)phenylphosphine oxide and 16.47g of bis(4-aminophenyl)diphenylsilane to obtain a flame-retardant comonomer solution; wherein, based on the total molar amount of diamine, the amount of phosphorus-containing monomer was 1.5mol%, the amount of silicon-containing monomer was 4.5mol%, and the molar ratio of the two was 1:3; In S300, 198.96g of terephthaloyl chloride was added at 10℃ to carry out a prepolymerization reaction; In S400, after adding the flame-retardant comonomer solution, the remaining 4.06 g of terephthaloyl chloride is added, and polycondensation is carried out at 10°C for 10 min to obtain the copolymerized para-aramid polymer. In S600, 220g of copolymerized para-aramid resin was weighed and dissolved in 780g of concentrated sulfuric acid with a mass fraction of 100wt% to obtain a spinning solution with a polymer mass fraction of 22wt%. After dry-jet wet spinning, coagulation, washing, stretching, drying and heat setting, high flame-retardant aramid fiber was obtained. Example 5

[0040] This embodiment provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Embodiment 1, except that: In S100, 1100g of tetramethylurea and 21.20g of anhydrous lithium chloride were used to prepare the main reaction system, and then 104.90g of p-phenylenediamine was added to obtain the main monomer solution. In S200, 250g of tetramethylurea was added to 0.020mol of bis(4-aminophenoxyaryl)phosphine oxide and 0.010mol of bis(4-aminophenoxy)diarylsilane, and stirred until completely dissolved to obtain a flame-retardant comonomer solution. The amount of phosphorus-containing monomer was 2mol% and the amount of silicon-containing monomer was 1mol% based on the total molar amount of diamine. The molar ratio of the two was 1:0.5. In S300, 182.72 g of terephthaloyl chloride was added at 5 °C to carry out a prepolymerization reaction; After adding the flame-retardant comonomer solution to S400, the remaining 20.30 g of terephthaloyl chloride is added, and polycondensation is continued at 5°C for 45 min to obtain the copolymerized para-aramid polymer. In S600, a spinning solution with a polymer mass fraction of 18.5 wt% is prepared using 99.0 wt% concentrated sulfuric acid. After dry-jet wet spinning, coagulation, washing, stretching, drying and heat setting, highly flame-retardant aramid fiber is obtained. Example 6

[0041] This embodiment provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Embodiment 1, except that: In S100, 1000g of N-methylpyrrolidone and 200g of tetraethylurea were used as a mixed solvent. 44.4g of anhydrous calcium chloride was added and stirred to dissolve the solution. Then, 101.65g of p-phenylenediamine was added to obtain the main monomer solution. In S200, 250g of a mixed solvent with the same proportion was mixed with 9.24g of bis(4-aminophenyl)phenylphosphine oxide and 0.030mol of 1,3-bis(4-aminophenyl)tetramethyldisiloxane, and stirred to dissolve, to obtain a flame-retardant comonomer solution; wherein, based on the total molar amount of diamine, the amount of phosphorus-containing monomer and silicon-containing monomer was 3mol%, and their molar ratio was 1:1; In S300, 178.66g of terephthaloyl chloride was added at -5℃ to carry out a prepolymerization reaction; In S400, after adding the flame-retardant comonomer solution, the remaining 24.36 g of terephthaloyl chloride was added, and polycondensation was carried out at -5℃ for 75 min to obtain the copolymerized para-aramid polymer. In S600, a spinning solution with a polymer mass fraction of 19 wt% is prepared using 98.5 wt% concentrated sulfuric acid. After dry-jet wet spinning, coagulation, washing, stretching, drying and heat setting, highly flame-retardant aramid fiber is obtained.

[0042] Comparative Example 1 This comparative example provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Example 1, except that bis(4-aminophenyl)phenylphosphine oxide and bis(4-aminophenyl)diphenylsilane are not added in S200, that is, only p-phenylenediamine and terephthaloyl chloride are used for polymerization.

[0043] Comparative Example 2 This comparative example provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Example 1, except that in S200, only 0.040 mol of bis(4-aminophenyl)phenylphosphine oxide is added, that is, bis(4-aminophenyl)diphenylsilane is not added.

[0044] Comparative Example 3 This comparative example provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Example 1, except that in S200, only 0.040 mol of bis(4-aminophenyl)diphenylsilane is added, that is, bis(4-aminophenyl)phenylphosphine oxide is not added.

[0045] Comparative Example 4 This comparative example provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Example 1, except that: p-phenylenediamine, bis(4-aminophenyl)diphenylsilane, and bis(4-aminophenyl)phenylphosphine oxide are all added to the main reaction system at one time, and all terephthaloyl chloride is added at one time for polycondensation, i.e., without S300.

[0046] Comparative Example 5 This comparative example provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Example 1, except that: based on the total molar amount of diamine, the amount of bis(4-aminophenyl)phenylphosphine oxide is 8 mol%, and the amount of bis(4-aminophenyl)diphenylsilane is 6 mol.

[0047] Comparative Example 6 This comparative example provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Example 1, except that in S300, the amount of the first part of terephthaloyl chloride added accounts for only 70 mol of the total molar amount.

[0048] Comparative Example 7 This comparative example provides a high flame-retardant aramid fiber and its preparation method. The preparation method is the same as that shown in Example 1, except that in S600, concentrated sulfuric acid with a mass fraction of 96.5 wt% is used to prepare the spinning solution, and the polymer mass fraction in the spinning solution is 24 wt%.

[0049] Performance testing Examples 1-6 and Comparative Examples 1-7 were tested using the following methods, and the test results are shown in Table 1. Limiting oxygen index: The obtained fibers are made into specimens of specified size and their limiting oxygen index is tested in accordance with GB / T 2406.2; Thermogravimetric analysis of carbon residue: Thermogravimetric analysis was performed under nitrogen atmosphere, with the temperature increased to 800℃ at 10℃ / min, and the carbon residue at 800℃ was recorded. Fiber breaking strength: The fiber breaking strength is tested according to the corresponding fiber mechanical property testing standards; Spinning stability: Record the occurrence of filament breakage, pore blockage, and spinning fluctuations during the spinning process, and evaluate them as excellent, good, average, or poor. Table 1 As shown in Table 1, the high flame-retardant aramid fibers prepared in Examples 1-6 of this application are superior to Comparative Example 1 in terms of limiting oxygen index and char residue. This indicates that introducing phosphorus-containing aromatic difunctional monomers and silicon-containing aromatic difunctional monomers into the aramid backbone via reactive copolymerization can effectively improve the flame-retardant properties and high-temperature charring ability of the fibers. Comparative Examples 2 and 3 introduced only phosphorus-containing monomers or silicon-containing monomers, respectively. Although they improved the flame-retardant properties to some extent compared to Comparative Example 1, the overall effect was still lower than that of Examples 1-6, which introduced both phosphorus-containing and silicon-containing monomers. This indicates that there is a synergistic effect between the phosphorus-containing structural units and the silicon-containing structural units. Among them, the phosphorus-containing structural units are more conducive to promoting charring, while the silicon-containing structural units are more conducive to improving the density and thermal stability of the char layer. The synergistic effect of the two is even better. The first method is beneficial for forming a continuous and stable barrier layer. Comparative Example 4, which did not employ a stepwise polycondensation method of prepolymerization followed by the introduction of functional monomers, resulted in a significant decrease in fiber breaking strength. This indicates that the stepwise polycondensation process used in this application is beneficial for preferentially forming a main skeleton dominated by PPTA segments, thereby reducing the impact of premature participation of functional monomers in polymerization on the regularity and fiber-forming properties of the main chain. In Comparative Example 5, the amount of functional monomers was too high. Although the flame retardant index was further improved, the fiber breaking strength and spinning stability significantly deteriorated. This indicates that more functional monomers are not necessarily better; controlling them within the limits specified in this application is more conducive to balancing flame retardant performance and fiber mechanical properties. Comparative Examples 6 and 7 respectively illustrate the importance of the amount of terephthaloyl chloride added in the first part and the spinning solution parameters to the technical effect of this application. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of this invention. Therefore, the scope of protection of this invention should be determined by the scope defined in the claims.

Claims

1. A method for preparing highly flame-retardant aramid fiber, characterized in that, Includes the following steps: S100. Dissolve the inorganic salt in a solvent, add p-phenylenediamine, and obtain the main monomer solution; S200. Dissolve phosphorus-containing aromatic difunctional monomers and silicon-containing aromatic difunctional monomers in a solvent to obtain a flame-retardant comonomer solution. S300. Add the first portion of terephthaloyl chloride to the main monomer solution to carry out a prepolymerization reaction to obtain a prepolymerization solution; S400. Add the flame-retardant comonomer solution to the prepolymer solution, and add the remaining terephthaloyl chloride to carry out a polycondensation reaction to obtain a copolymerized para-aramid polymer. S500: Separate, wash and dry the copolymerized para-aramid polymer to obtain the copolymerized para-aramid resin; S600. The copolymerized para-aramid resin is dissolved in concentrated sulfuric acid to prepare a spinning solution. The spinning solution is subjected to dry-jet wet spinning, coagulation, water washing, stretching, drying and heat setting to obtain the high flame-retardant para-aramid fiber.

2. The preparation method according to claim 1, characterized in that, In S100, The solvent includes at least one of N-methylpyrrolidone, dimethylacetamide, tetramethylurea, and tetraethylurea; and / or The inorganic salt is selected from at least one of calcium chloride and lithium chloride.

3. The preparation method according to claim 1, characterized in that, In S200, The phosphorus-containing aromatic difunctional monomer includes at least one of bis(4-aminophenyl)arylphosphine oxide and bis(4-aminophenoxyaryl)phosphine oxide; and / or The silicon-containing aromatic difunctional monomer includes at least one of bis(4-aminophenyl)diarylsilane, bis(4-aminophenoxy)diarylsilane, and bis-para-aminoaryl compounds bridged by short-chain disiloxane.

4. The preparation method according to claim 1, characterized in that, The amount of phosphorus-containing aromatic difunctional monomer used is 0.5-6 mol% based on the total molar amount of diamine, the amount of silicon-containing aromatic difunctional monomer used is 0.5-5 mol%, and the p-phenylenediamine is the balance.

5. The preparation method according to claim 4, characterized in that, The molar ratio of the phosphorus-containing aromatic difunctional monomer to the silicon-containing aromatic difunctional monomer is 1:(0.3~3).

6. The preparation method according to claim 1, characterized in that, The molar ratio of the inorganic salt to the total diamine is (0.1~0.8):

1.

7. The preparation method according to claim 1, characterized in that, In S300, the temperature of the prepolymerization reaction is -15 to 10°C; and / or In S400, the polycondensation reaction is carried out at a temperature of -15 to 10°C for a time of 10 to 120 minutes.

8. The preparation method according to claim 1, characterized in that, In S300, the amount of the first portion of terephthaloyl chloride added accounts for 85-98 mol of the total molar amount of terephthaloyl chloride.

9. The preparation method according to claim 1, characterized in that, In the S600, The polymer mass fraction of the spinning solution is 15~22wt%, and / or The concentrated sulfuric acid has a mass fraction of 98.0~100wt%.

10. A highly flame-retardant aramid fiber, characterized in that, The highly flame-retardant aramid fiber comprises that prepared by the method described in any one of claims 1 to 9.

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