A method for preparing flame-retardant liquid crystal polymer modified nylon fiber

Abstract

The invention belongs to the technical field of nylon fibers, and discloses a method for preparing a flame-retardant liquid crystal polymer modified nylon fiber, wherein terephthalic acid, DOPO succinamide diphenol monomer, and 4-hydroxybenzoic acid are subjected to polycondensation reaction to obtain a phosphorus-containing polyarylate liquid crystal polymer. Then, the polymer is melt-spun with nylon 6 to obtain a flame-retardant liquid crystal polymer modified nylon fiber; the phosphorus-containing polyarylate liquid crystal polymer has strong compatibility and interaction with the amide bond in nylon 6, which enhances the compatibility between the liquid crystal polymer and nylon 6, and is beneficial to improving the breaking strength and elongation at break of the nylon 6 fiber. At the same time, the polymer contains a DOPO flame-retardant group, has gas-phase flame-retardant and condensed-phase flame-retardant effects, reduces the peak value of the combustion heat release rate and the total heat release of the nylon fiber, and exhibits better flame-retardant properties.

Description

Technical Field

[0001] The invention belongs to the technical field of nylon fibers, and in particular relates to a method for preparing flame-retardant liquid crystal polymer modified nylon fibers. Background Art

[0002] Polyarylate liquid crystal polymers have excellent mechanical properties, high temperature resistance and processability, and are widely used in polymer materials such as fibers and plastics to improve the strength and heat resistance of materials. The current preparation methods of polyarylate liquid crystal polymers mainly include melt transesterification and polycondensation. Adding functional monomers during the preparation process can give liquid crystal polymers unique properties.

[0003] Nylon and its fiber products have good wear resistance, elastic recovery rate, and excellent moisture absorption performance. They are widely used in textiles, automotive interiors, protective clothing, safety nets, etc. Improving the mechanical strength of nylon and its fibers is conducive to expanding the practical application field of nylon materials; and nylon and its fibers have poor flame retardancy and are easy to burn, which is not conducive to their practical application in the field of fire retardancy. Adding high-performance liquid crystal polymers to nylon materials can improve the comprehensive properties of nylon materials. Patent CN104448814B discloses that thermotropic liquid crystal polyester PET / PHB, talcum powder, etc. are compounded with nylon 66 resin to obtain a nylon composite material with high dimensional stability, excellent heat resistance and low-temperature impact strength, but the patent does not improve the flame retardant properties of nylon materials. Summary of the invention

[0004] The invention solves the problem that nylon fiber has low strength and poor flame retardancy.

[0005] The technical solution of the present invention is: a method for preparing flame-retardant liquid crystal polymer modified nylon fiber:

[0006] Step S1, nitrogen is introduced into a flask, benzenesulfonyl chloride, N,N-dimethylformamide, and pyridine are added, and after stirring and mixing, a pyridine solution containing terephthalic acid is added dropwise, the temperature is raised to 115-120° C., and the reaction is condensed and refluxed for 10-20 minutes, and then a pyridine solution containing DOPO succinamide diphenol monomer and 4-hydroxybenzoic acid is added dropwise, and the reaction is continued for 4-6 hours, cooled, diluted with ethanol, filtered, the filter cake is washed with ethanol, and dried to obtain a phosphorus-containing polyarylate liquid crystal polymer.

[0007] The structural formula of DOPO succinamide diphenol monomer is as follows:

[0008]

[0009] Step S2, adding nylon 6 and phosphorus-containing polyarylate liquid crystal polymer into a twin-screw extruder, melt blending, extruding and granulating; then melt spinning in a melt spinning machine, and finally stretching through a hot roller stretching machine to obtain flame-retardant liquid crystal polymer modified nylon fiber.

[0010] Preferably, in step S1, the molar ratio of benzenesulfonyl chloride, N,N-dimethylformamide, terephthalic acid, DOPO succinamide diphenol, and 4-hydroxybenzoic acid is (320-360):(45-60):100:(100-110):(98-102).

[0011] Preferably, in step S2, the mass ratio of nylon 6 to phosphorus-containing polyarylate liquid crystal polymer is 100:(5-25).

[0012] Preferably, in step S2, the melt blending temperature is 230-245° C.; the melt spinning temperature is 250-260° C.; the temperature of the hot roller drawing machine is 100-120° C., and the drawing ratio is 4-5 times.

[0013] Preferably, the preparation method of DOPO succinamide diphenol monomer is:

[0014] Step 1, add dichloromethane solvent, 4-methoxyaniline, triethylamine, and trans-butylene dichloride in a molar ratio of (190-210): (200-240): 100 into a flask, react at room temperature for 8-10 hours, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and gradient elute with ethyl acetate and petroleum ether solution to obtain succinamide dimethoxy monomer.

[0015] Step 2: Add toluene or xylene solvent, succinamide dimethoxy monomer and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide in a molar ratio of 100:(110-130) to a flask, heat to 110-130° C., stir and react for 18-24 hours, and after the reaction, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain DOPO succinamide dimethoxy monomer.

[0016] Step 3: Add pyridine hydrochloride to a flask, heat to 150-160° C., add DOPO succinamide dimethoxy monomer, stir to dissolve, then heat to 190-200° C., react for 2-3 hours, add water to dilute, extract with ethyl acetate, collect the ethyl acetate organic phase, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain DOPO succinamide diphenol monomer.

[0017] Preferably, in step 3, the molar ratio of pyridine hydrochloride to DOPO succinamide dimethoxy monomer is (1600-2000):100.

[0018] Technical effect: The present invention uses 4-methoxyaniline, trans-butylene dichloride, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and the like as reactants, and obtains DOPO succinamide diphenol monomer containing amide bonds through acyl chloride esterification reaction, phosphorus-hydrogen bond addition reaction and demethylation reaction. Then, in a catalyst system of benzenesulfonyl chloride, N,N-dimethylformamide and pyridine, DOPO succinamide diphenol monomer, terephthalic acid and p-hydroxybenzoic acid are subjected to condensation reaction to obtain a novel phosphorus-containing polyarylate liquid crystal polymer.

[0019] The present invention melt-blends and spins nylon 6 and phosphorus-containing polyarylate liquid crystal polymer. The phosphorus-containing polyarylate liquid crystal polymer contains a large number of amide bonds, has strong compatibility with the amide bonds in nylon 6, and can form interactions such as hydrogen bonds, thereby enhancing the compatibility between the liquid crystal polymer and nylon 6, which is beneficial to improving the breaking strength and breaking elongation of nylon 6 fiber.

[0020] The phosphorus-containing polyarylate liquid crystal polymer of the present invention contains DOPO flame retardant groups, which have gas phase flame retardant and condensed phase flame retardant effects. When DOPO burns, it can generate phosphorus-containing free radicals, which can capture free radicals in the combustion reaction, thereby interrupting the combustion chain reaction and inhibiting the combustion process. In addition, the DOPO flame retardant group thermally decomposes to produce phosphoric acid compounds, which can dehydrate the nylon fiber and form a stable carbon layer, which is conducive to isolating oxygen, preventing the material from burning and oxidative decomposition, and can reduce heat transfer, reduce smoke emission, and significantly reduce the peak value of the combustion heat release rate and the total heat release, so that the nylon 6 fiber exhibits better flame retardant properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the reaction formula for preparing DOPO succinamide diphenol monomer.

[0022] Figure 2 It is the reaction formula for preparing phosphorus-containing polyarylate liquid crystal polymer. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0024] (1) Add dichloromethane solvent, 4-methoxyaniline, triethylamine, and trans-butylene dichloride (CAS No. 627-63-4) in a molar ratio of (190-210):(200-240):100 to a flask, react at room temperature for 8-10 hours, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain succinamide dimethoxy monomer.

[0025] (2) Add toluene or xylene solvent, succinamide dimethoxy monomer and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (CAS No. 35948-25-5) in a molar ratio of 100:(110-130) to a flask, heat to 110-130° C., stir and react for 18-24 hours, and after the reaction, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain DOPO succinamide dimethoxy monomer.

[0026] (3) Add pyridine hydrochloride to a flask, heat to 150-160° C., add DOPO succinamide dimethoxy monomer, control the molar ratio of pyridine hydrochloride and DOPO succinamide dimethoxy monomer to be (1600-2000):100; stir to dissolve, then heat to 190-200° C., react for 2-3 hours, add water to dilute, extract with ethyl acetate, collect the ethyl acetate organic phase, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and gradient elute with ethyl acetate and petroleum ether solution to obtain DOPO succinamide diphenol monomer.

[0027] (4) nitrogen is introduced into the flask, benzenesulfonyl chloride, N,N-dimethylformamide, and pyridine are added, stirred and mixed, and then a pyridine solution containing terephthalic acid is added dropwise, the temperature is raised to 115-120° C., and the reaction is condensed and refluxed for 10-20 minutes, and then a pyridine solution containing DOPO succinamide diphenol monomer and 4-hydroxybenzoic acid is added dropwise, and the molar ratio of benzenesulfonyl chloride, N,N-dimethylformamide, terephthalic acid, DOPO succinamide diphenol, and 4-hydroxybenzoic acid is controlled to be (320-360):(45-60):100:(100-110):(98-102); the reaction is continued for 4-6 hours, cooled, ethanol is added to dilute, filtered, the filter cake is washed with ethanol, and dried to obtain a phosphorus-containing polyarylate liquid crystal polymer.

[0028] (5) Adding nylon 6 and phosphorus-containing polyarylate liquid crystal polymer in a mass ratio of 100:(5-25) into a twin-screw extruder, melt blending, and controlling the melt blending temperature to 230-245°C; extruding granulation; then melt spinning in a melt spinning machine, controlling the melt spinning temperature to 250-260°C; finally stretching through a hot roller stretching machine at a temperature of 100-120°C and a stretching multiple of 4-5 times; and obtaining flame-retardant liquid crystal polymer modified nylon fiber.

[0029] Example 1

[0030] (1) Add 100 mL of dichloromethane solvent, 40 mmol of 4-methoxyaniline, 40 mmol of triethylamine, and 20 mmol of trans-butylene dichloride into a flask, react at room temperature for 10 hours, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain succinamide dimethoxy monomer.

[0031] (2) Add 150 mL of toluene solvent, 30 mmol of succinamide dimethoxy monomer, and 39 mmol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide into a flask, heat to 110° C., stir and condense under reflux for 24 hours, and after the reaction, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain DOPO succinamide dimethoxy monomer.

[0032] (3) Add 600 mmol of pyridine hydrochloride to a flask, heat to 150° C., add 30 mmol of DOPO succinamide dimethoxy monomer, stir to dissolve, then heat to 200° C., react for 2 h, add water to dilute, extract with ethyl acetate, collect the ethyl acetate organic phase, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain DOPO succinamide diphenol monomer.

[0033] (4) Nitrogen was introduced into the flask, and 360 mmol of benzenesulfonyl chloride, 45 mmol of N,N-dimethylformamide, and 40 mL of pyridine were added. After stirring and mixing, 40 mL of a pyridine solution containing 100 mmol of terephthalic acid was added dropwise, and the temperature was raised to 115° C., and the reaction was condensed and refluxed for 10 min. Then, 60 mL of a pyridine solution containing 100 mmol of DOPO succinamide diphenol monomer and 100 mmol of 4-hydroxybenzoic acid was added dropwise, and the reaction was continued for 4 h. The mixture was cooled, diluted with ethanol, filtered, the filter cake was washed with ethanol, and dried to obtain a phosphorus-containing polyarylate liquid crystal polymer.

[0034] (5) Add 1000 g of nylon 6 and 50 g of phosphorus-containing polyarylate liquid crystal polymer into a twin-screw extruder, melt blend, and control the temperature during melt blending to be 230°C; extrusion granulation; then melt spinning in a melt spinning machine, and control the temperature during melt spinning to be 260°C; finally, stretching through a hot roller stretching machine, the stretching temperature is 120°C, and the stretching ratio is 4 times; to obtain flame-retardant liquid crystal polymer modified nylon fiber.

[0035] Example 2

[0036] (1) Add 120 mL of dichloromethane solvent, 42 mmol of 4-methoxyaniline, 48 mmol of triethylamine, and 20 mmol of trans-butylene dichloride to a flask, react at room temperature for 8 hours, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain succinamide dimethoxy monomer.

[0037] (2) Add 150 mL of xylene solvent, 30 mmol of succinamide dimethoxy monomer, and 33 mmol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide into a flask, heat to 130° C., stir and react for 18 hours, and after the reaction, rotary evaporate to remove the solvent, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain DOPO succinamide dimethoxy monomer.

[0038] (3) Add 480 mmol of pyridine hydrochloride to a flask, heat to 150° C., add 30 mmol of DOPO succinamide dimethoxy monomer, stir to dissolve, then heat to 190° C., react for 3 h, add water to dilute, extract with ethyl acetate, collect the ethyl acetate organic phase, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain DOPO succinamide diphenol monomer.

[0039] (4) Nitrogen was introduced into the flask, and 320 mmol of benzenesulfonyl chloride, 60 mmol of N,N-dimethylformamide, and 40 mL of pyridine were added. After stirring and mixing, 40 mL of a pyridine solution containing 100 mmol of terephthalic acid was added dropwise, and the temperature was raised to 120° C., and the reaction was condensed and refluxed for 20 min. Then, 60 mL of a pyridine solution containing 100 mmol of DOPO succinamide diphenol monomer and 100 mmol of 4-hydroxybenzoic acid was added dropwise, and the reaction was continued for 6 h. The mixture was cooled, diluted with ethanol, filtered, the filter cake was washed with ethanol, and dried to obtain a phosphorus-containing polyarylate liquid crystal polymer.

[0040] (5) Add 1000 g of nylon 6 and 50 g of phosphorus-containing polyarylate liquid crystal polymer into a twin-screw extruder, melt blend, and control the temperature during melt blending to be 240°C; extrusion granulation; then melt spinning in a melt spinning machine, and control the temperature during melt spinning to be 260°C; finally, stretching through a hot roller stretching machine, the stretching temperature is 120°C, and the stretching ratio is 4 times; to obtain flame-retardant liquid crystal polymer modified nylon fiber.

[0041] Comparative Example 1

[0042] (1) 1000 g of nylon 6 was melt-spun in a melt spinning machine, and the temperature during melt spinning was controlled to be 260° C.; finally, it was stretched by a hot roller stretching machine, and the stretching temperature was 120° C. and the stretching ratio was 4 times; and nylon fiber was obtained.

[0043] Comparative Example 2

[0044] (1) Nitrogen was introduced into a flask, and 360 mmol of benzenesulfonyl chloride, 45 mmol of N,N-dimethylformamide, and 40 mL of pyridine were added. After stirring and mixing, 40 mL of a pyridine solution containing 100 mmol of terephthalic acid was added dropwise. The temperature was raised to 115° C., and the mixture was refluxed under condensation for 10 min. Then, 60 mL of a 100 mmol 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phosphaphenanthrene-10-oxide (CAS No. 99208-50-1, structural formula: ), 100mmol 4-hydroxybenzoic acid in pyridine solution, continue the reaction for 4h, cool, add ethanol to dilute, filter with suction, wash the filter cake with ethanol, and dry to obtain a phosphorus-containing polyarylate liquid crystal polymer.

[0045] (2) Add 1000 g of nylon 6 and 50 g of phosphorus-containing polyarylate liquid crystal polymer into a twin-screw extruder, melt blend, and control the temperature during melt blending to be 230°C; extrusion granulation; then melt spinning in a melt spinning machine, and control the temperature during melt spinning to be 260°C; finally, stretching through a hot roller stretching machine, the stretching temperature is 120°C, and the stretching ratio is 4 times; to obtain flame-retardant liquid crystal polymer modified nylon fiber.

[0046] Comparative Example 3

[0047] (1) Add 100 mL of dichloromethane solvent, 40 mmol of 4-methoxyaniline, 40 mmol of triethylamine, and 20 mmol of trans-butylene dichloride to a flask, react at room temperature for 10 hours, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and perform gradient elution with ethyl acetate and petroleum ether solution to obtain succinamide dimethoxy monomer. The structural formula is

[0048] (2) Add 600 mmol of pyridine hydrochloride to a flask, heat to 150°C, add 30 mmol of succinamide dimethoxy monomer, stir to dissolve, then heat to 200°C, react for 2 hours, add water to dilute, extract with ethyl acetate, collect the ethyl acetate organic phase, remove the solvent by rotary evaporation, separate by silica gel column chromatography, and gradient elute with ethyl acetate and petroleum ether solution to obtain succinamide diphenol monomer. The structural formula is:

[0049] (3) Nitrogen was introduced into the flask, and 360 mmol of benzenesulfonyl chloride, 45 mmol of N,N-dimethylformamide, and 40 mL of pyridine were added. After stirring and mixing, 40 mL of a pyridine solution containing 100 mmol of terephthalic acid was added dropwise, and the temperature was raised to 115° C., and the reaction was refluxed under condensation for 10 min. Then, 60 mL of a pyridine solution containing 100 mmol of succinamide diphenol monomer and 100 mmol of 4-hydroxybenzoic acid was added dropwise, and the reaction was continued for 4 h. The mixture was cooled, diluted with ethanol, filtered, the filter cake was washed with ethanol, and dried to obtain a polyarylate liquid crystal polymer.

[0050] (4) Add 1000 g of nylon 6 and 50 g of polyarylate liquid crystal polymer into a twin-screw extruder, melt blend, and control the temperature during melt blending to be 230°C; extrusion granulation; then melt spinning in a melt spinning machine, and control the temperature during melt spinning to be 260°C; finally, stretching through a hot roller stretching machine, the stretching temperature is 120°C, and the stretching ratio is 4 times; to obtain liquid crystal polymer modified nylon fiber.

[0051] According to the method of GB / T 14344-2022, the tensile breaking properties of nylon fibers were tested using an electronic yarn strength and elongation tester.

[0052] The combustion performance of nylon fiber was tested using a microcalorimeter with a combustion zone temperature of 900°C and a heating rate of 1°C / min.

[0053] Table 1 Performance test of nylon fiber

[0054]

[0055] As shown in Table 1, compared with the nylon fiber of Comparative Example 1, the nylon 6 and the phosphorus-containing polyarylate liquid crystal polymer are melt-blended and spun in Example 1 and Example 2, and the breaking strength and breaking elongation of the flame-retardant liquid crystal polymer-modified nylon fiber obtained are significantly improved, the mechanical properties are excellent, and the peak value of the combustion heat release rate and the total heat release amount decrease greatly, and the flame retardancy is good. This is attributed to the large amount of amide bonds contained in the phosphorus-containing polyarylate liquid crystal polymer, which has a strong compatibility with the amide bonds in nylon 6, and can form hydrogen bonds and other interactions at the same time, which enhances the compatibility between the liquid crystal polymer and nylon 6, and is conducive to improving the breaking strength and breaking elongation of nylon 6 fiber. In addition, the liquid crystal polymer contains DOPO flame retardant groups, which have gas phase flame retardant and condensed phase flame retardant effects. When DOPO burns, it can generate phosphorus-containing free radicals, which can capture free radicals in the combustion reaction, thereby interrupting the combustion chain reaction and inhibiting the combustion process. In addition, the thermal decomposition of the DOPO flame retardant group produces phosphate compounds, which can dehydrate the nylon fiber and form a stable carbon layer, which is beneficial for isolating oxygen, preventing the material from burning and oxidative decomposition, and can reduce heat transfer and smoke emission, significantly reducing the peak value of the combustion heat release rate and the total heat release, showing better flame retardant properties.

[0056] Comparative Example 2: 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phosphaphenanthrene-10-oxide, terephthalic acid and 4-hydroxybenzoic acid are subjected to a condensation reaction to obtain a phosphorus-containing polyarylate liquid crystal polymer, which also contains a DOPO flame retardant group and has a good flame retardant effect. It can reduce the peak value of the combustion heat release rate and the total heat release of the nylon fiber. However, it does not contain an amide bond, and the interaction force between it and nylon 6 is low. The compatibility of the two is poor. When added to nylon 6, the two are prone to phase separation to form defects, which reduces the breaking strength and elongation at break of the nylon 6 fiber, and the mechanical properties are affected.

[0057] Comparative Example 3: Terephthalic acid, succinamide diphenol monomer, and 4-hydroxybenzoic acid are subjected to polycondensation reaction, and the obtained polyarylate liquid crystal polymer contains a large number of amide bonds, which has strong compatibility with the amide bonds in nylon 6, and can form interactions such as hydrogen bonds, thereby enhancing the compatibility between the liquid crystal polymer and nylon 6, which is beneficial to improving the breaking strength and elongation at break of nylon 6 fiber. However, the polyarylate liquid crystal polymer does not contain DOPO flame retardant groups, and cannot improve the flame retardancy of nylon 6 fiber, and the peak value of the combustion heat release rate and the total heat release are large.

[0058] Taking Example 1 as an example, the amount of phosphorus-containing polyarylate liquid crystal polymer is controlled to be 100g, 150g, 200g, and 250g.

[0059] (1) 1000 g of nylon 6 and 100-250 g of phosphorus-containing polyarylate liquid crystal polymer are added to a twin-screw extruder for melt blending, and the temperature during melt blending is controlled to be 230° C.; extrusion granulation is performed; then melt spinning is performed in a melt spinning machine, and the temperature during melt spinning is controlled to be 260° C.; finally, stretching is performed on a hot roller stretching machine, and the stretching temperature is 120° C. and the stretching ratio is 4 times; thereby obtaining a flame-retardant liquid crystal polymer modified nylon fiber.

[0060] The tensile breaking properties and flame retardant properties tests are shown in the following table.

[0061] Table 2 Flame retardant liquid crystal polymer modified nylon fiber performance test.

[0062]

[0063] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make modifications to the present embodiment without any creative contribution as needed. However, as long as it is within the scope of the claims of the present application, it shall be protected by the patent law.

Claims

1. A method for preparing flame-retardant liquid crystal polymer modified nylon fiber, characterized in that: The preparation method comprises the following steps: Step S1, nitrogen is introduced into a flask, benzenesulfonyl chloride, N,N-dimethylformamide, and pyridine are added, and after stirring and mixing, a pyridine solution containing terephthalic acid is added dropwise, the temperature is raised to 115-120° C., and condensation reflux reaction is performed for 10-20 minutes, and then a pyridine solution containing DOPO succinamide diphenol monomer and 4-hydroxybenzoic acid is added dropwise, and the reaction is continued for 4-6 hours, and then cooled, ethanol is added to dilute, and suction is filtered, the filter cake is washed with ethanol, and dried to obtain a phosphorus-containing polyarylate liquid crystal polymer; The structural formula of the DOPO succinamide diphenol monomer is as follows: Step S2, adding nylon 6 and phosphorus-containing polyarylate liquid crystal polymer into a twin-screw extruder, melt blending, extruding and granulating; then melt spinning in a melt spinning machine, and finally stretching through a hot roller stretching machine to obtain flame-retardant liquid crystal polymer modified nylon fiber.

2. The method for preparing the flame-retardant liquid crystal polymer modified nylon fiber according to claim 1, characterized in that: In the step S1, the molar ratio of benzenesulfonyl chloride, N,N-dimethylformamide, terephthalic acid, DOPO succinamide diphenol, and 4-hydroxybenzoic acid is (320-360):(45-60):100:(100-110):(98-102).

3. The method for preparing the flame-retardant liquid crystal polymer modified nylon fiber according to claim 1, characterized in that: In the step S2, the mass ratio of nylon 6 to phosphorus-containing polyarylate liquid crystal polymer is 100:(5-25).

4. The method for preparing the flame-retardant liquid crystal polymer modified nylon fiber according to claim 1, characterized in that: In step S2, the melt blending temperature is 230-245° C.; the melt spinning temperature is 250-260° C.; the temperature of the hot roller drawing machine is 100-120° C., and the drawing multiple is 4-5 times.

5. The method for preparing the flame-retardant liquid crystal polymer modified nylon fiber according to claim 1, characterized in that: The preparation method of the DOPO succinamide diphenol monomer is as follows: Step 1, add dichloromethane solvent, 4-methoxyaniline, triethylamine, and trans-butylene dichloride in a molar ratio of (190-210): (200-240): 100 to a flask, react at room temperature for 8-10 hours, rotary evaporate, and separate by column chromatography to obtain succinamide dimethoxy monomer; Step 2, adding a solvent, succinamide dimethoxy monomer, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide into a flask, performing rotary evaporation after the reaction, and performing column chromatography separation to obtain DOPO succinamide dimethoxy monomer; Step 3: Add pyridine hydrochloride to a flask, heat to 150-160° C., add DOPO succinamide dimethoxy monomer, stir to dissolve, then heat to 190-200° C., react for 2-3 hours, extract, separate by column chromatography, and obtain DOPO succinamide diphenol monomer.

6. The method for preparing the flame-retardant liquid crystal polymer modified nylon fiber according to claim 5, characterized in that: In the step 2, the solvent is toluene or xylene.

7. The method for preparing the flame-retardant liquid crystal polymer modified nylon fiber according to claim 5, characterized in that: In the step 2, the molar ratio of the succinamide dimethoxy monomer to 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 100:(110-130).

8. The method for preparing the flame-retardant liquid crystal polymer modified nylon fiber according to claim 5, characterized in that: In the step 2, the reaction is carried out at 110-130° C. for 18-24 hours.

9. The method for preparing the flame-retardant liquid crystal polymer modified nylon fiber according to claim 5, characterized in that: In step 3, the molar ratio of pyridine hydrochloride to DOPO succinamide dimethoxy monomer is (1600-2000):100.

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