Modified nylon 66 fiber and preparation method thereof

Abstract

The invention discloses a modified nylon 66 fiber and a preparation method thereof, and belongs to the field of chemical fibers. The fiber is prepared by the following steps: copolymerizing STI6 salt obtained by salifying STPA / SIPA and hexamethylenediamine with nylon 66 salt to prepare a modified nylon 66 polymer, and spinning. The STI6 salt accounts for 3-10 wt% of the mass of the nylon 66 salt, and the mixing ratio of STPA to SIPA is 65: 35-85: 15. The preparation method comprises the steps of salifying reaction, high-pressure reaction, normal-pressure reaction, vacuum reaction and spinning, namely respectively preparing nylon 66 salt and STI6 salt, mixing, adding a catalyst, performing high-pressure polycondensation, normal-pressure reaction and vacuum reaction to obtain a modified polymer, and performing melt extrusion, spinning and the like to obtain a finished product. Through copolymerization modification, the melting point of a polymer is reduced, cross-linking reaction is inhibited to reduce generation of gel, and the mechanical property and spinnability of the fiber are improved; due to the introduced sulfonic acid radical group, the fiber can be dyed by adopting cationic dye, and the structure is loose, so that the dyeability is improved. The method is simple in process, and existing equipment does not need to be transformed.

Description

Technical Field

[0001] This invention relates to the field of chemical fibers, and in particular to a modified nylon 66 fiber and its preparation method. Background Technology

[0002] Nylon fiber is one of the earliest synthetic fibers to be industrially produced, ranking second in production volume among synthetic fibers. Due to its soft hand feel, lightweight texture, good drape, breathability, moisture absorption, elasticity, and ease of processing, nylon fiber is widely used in clothing and home textiles. Among them, nylon 66 fiber, compared to traditional nylon 6 fiber, is widely used in the manufacture of high-end clothing, carpets, tire cord fabric, airbags, parachutes, and tarpaulins due to its high strength, abrasion resistance, high temperature resistance, and corrosion resistance.

[0003] However, Nylon 66 has a high melting point (260–265°C), and its spinning temperature is generally above 285°C, making it prone to thermal degradation. Simultaneously, due to its structure, it is susceptible to cross-linking during melt spinning, forming insoluble and infusible gels. Besides gels introduced from the raw material, gel deposits and wall-forming phenomena can also occur within the melt channel during production, potentially leading to blockages, increased filament breakage and fuzzing, and decreased spinnability, with the problems being more pronounced when spinning fine denier filaments. Regarding fiber quality, dyeing uniformity is difficult to control. Furthermore, Nylon 66's dense structure makes dye penetration difficult, requiring high-temperature dyeing, resulting in lower color fastness and a tendency for color variations.

[0004] In existing technologies, nylon 66 with low gel content can be obtained by improving polymerization equipment and control methods. For example, patent CN108602227A discloses a customizable polyamide polymer with high molecular weight, excellent color, and low gel content. This patent increases the molecular weight of nylon 66 and reduces gel formation by adding a vacuum process in the final polymerization stage and using a twin-screw extruder to remove volatile components. The resulting nylon 66 has a gel size greater than 10 micrometers of less than 50 ppm. However, only gels larger than 10 micrometers were measured, and the inconsistent material residence time in the vacuum devolatilization twin-screw extruder leads to a wide molecular weight distribution. The die temperature also has a significant impact on the gel content. In addition, introducing this equipment increases production costs and technical difficulty. Patent CN114106319A discloses a method for the continuous preparation of polyamides with low gel content and narrow molecular weight distribution. The method uses an online potentiometric titration system to precisely control the concentration of nylon salt solution and the amino acid ratio during the salt formation stage, ultimately controlling the gel content in the polyamide resin to 0.20-0.40%. However, the reduction in gel content is limited, and the online potentiometric titration system has high technical requirements and is expensive, significantly increasing production costs.

[0005] Regarding process formulation improvement, patent CN118126522A discloses a copolymer modified nylon 66 with low gel content, its preparation method and application. It is prepared by a co-salt reaction of adipic acid, hexamethylenediamine and aminosiloxane, with methyl silicone oil added in the prepolymerization stage. The gel content of the obtained nylon 66 does not exceed 1wt‰. However, this method has high control requirements, reduced production stability, and high cost of aminosiloxane modifier. Summary of the Invention

[0006] To overcome the problems and shortcomings of existing nylon 66 fibers and their preparation process, this invention provides a modified nylon 66 fiber and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: Modified Nylon 66 Fiber

[0008] The modified nylon 66 polymer is prepared by copolymerization, that is, by copolymerizing the STI6 salt obtained by the salting of STPA / SIPA and hexamethylenediamine with nylon 66 salt. The polymer is then directly transported through pipelines or granulated underwater and melt-extruded into a spinning box, where it is processed into modified nylon 66 fibers through cooling, oiling, stretching, and setting.

[0009] Among them, the modified monomer STI6 salt (sodium terephthalate-5-sulfonate (STPA) / sodium isophthalate-5-sulfonate (SIPA) salt with hexamethylenediamine) accounts for 3 to 10 wt% of the nylon 66 salt; the mixing ratio of STPA and SIPA is 65:35 to 85:15.

[0010] The main quality indicators of the modified nylon 66 fiber are as follows: polymer relative viscosity of 2.05–2.85, melting point of 245–260℃, gel content ≤1wt‰, black particle content of 0.3–1mm diameter ≤8 particles / kg, fiber monofilament fineness of 0.8–2.5dtex, breaking strength of 4.5–6.5cN / dtex, and breaking elongation of 10–35%. The resulting textiles can be dyed with cationic dyes. Preparation method of modified nylon 66 fiber

[0011] Includes the following steps: Salt formation reaction: Adipic acid and hexamethylenediamine were added to water to carry out a salt formation reaction to obtain Nylon 66 salt; STPA / SIPA and hexamethylenediamine were added to water to carry out a salt formation reaction to obtain STI6 salt.

[0012] The concentration of the nylon 66 salt solution was 40–55 wt%, the molar ratio of adipic acid to hexamethylenediamine was 1:1, the reaction temperature was 45–60 °C, the reaction time was 2–3 hours, and the pH of the solution at the end of the reaction was 7.3–7.5. The concentration of the STI6 salt solution was 25–40 wt%, the molar ratio of STPA / SIPA to hexamethylenediamine was 1:1.05, the reaction temperature was 60–80 °C, the reaction time was 2–3 hours, and the pH of the solution at the end of the reaction was 7.5–7.8.

[0013] High pressure reaction: After adding the Nylon 66 salt and STI6 salt obtained in step (1) to the catalyst, they are thoroughly stirred and mixed evenly, and a high compression polymerization reaction is carried out under nitrogen protection.

[0014] The catalyst is sodium hypophosphite, and the mass of the catalyst is 0.15–0.45 wt% of the total salt mass. The reaction temperature is 235–250℃, the reaction pressure is 1.5–1.8 MPa, and the pressure holding time is 1.5–2.5 Hr.

[0015] Atmospheric pressure reaction: After the high-pressure reaction is completed, the pressure is slowly reduced to atmospheric pressure within 30 minutes, while the temperature is continuously increased at a constant rate to 265-275℃. The reaction is carried out for 1-2 hours, and more than 95% water is collected.

[0016] Vacuum reaction: After reacting at atmospheric pressure for a certain time, vacuum is applied to an absolute pressure of 0.02-0.08 MPa, the reaction endpoint temperature is 277-285℃, and the reaction time is 0.5-1.5 Hr. After the reaction reaches the target degree of polymerization, nitrogen is applied to atmospheric pressure, and the modified nylon 66 polymer is obtained by discharging.

[0017] Spinning: Modified nylon 66 polymer is directly transported through pipelines or granulated underwater, then dried, melt-extruded, and fed into the spinning box. The spinning temperature is controlled at 270-290℃ and the spinning speed is 3500-6000m / min. After cooling, oiling, stretching, and setting, it is processed into modified nylon 66 fiber. Beneficial effects

[0018] Compared with the prior art, the present invention has the following beneficial effects: This invention modifies nylon 66 by adding para- and meta-dicarboxylic acids (STPA / SIPA) with sulfonate groups attached to the benzene ring, which then form salts with hexamethylenediamine, thereby lowering the polymer's melting point. Furthermore, the steric hindrance of the benzene ring and the synergistic effect of the para- and meta-positions inhibit the crosslinking reaction of nylon 66, reducing the formation of gels.

[0019] The introduction of STI6 salt disrupts the regularity of the nylon 66 molecular chain, improves crystallinity, increases orientation during spinning, enhances the mechanical properties of the fiber, and also helps to improve spinnability.

[0020] The sulfonic acid anion group contained in STI6 salt allows the textiles made from it to be dyed with cationic dyes; and the STI6 structure makes the structure of nylon 66 looser, making it easier for dyes to enter the fiber during dyeing, thus improving its dyeability.

[0021] The preparation method of the present invention is simple and easy to implement, and does not require changes to existing production equipment. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] The test methods used in the embodiments and comparative examples of this invention are as follows: Relative viscosity of polymer: tested according to the method of national standard GB / T 12006.1-2009.

[0024] Gel content: The formic acid dissolution method was used for testing: Nylon 66 samples were dissolved in a 90% formic acid solution, and the undissolved part was separated by filtration through a 0.22-micron pore size filter membrane. The gel was then dried and weighed for calculation.

[0025] Black particle content: According to the method of HG / T 4182-2012, weigh 1000g of sample and pour it into the funnel of the impurity particle sorter. Set the detection range of black particle diameter to 0.3mm~1.0mm, start the impurity particle sorter, automatically sort out the particles with black spots, and record the number of particles, in units of particles per kilogram (particles / kg).

[0026] Fiber physical properties, such as monofilament fineness, breaking strength and breaking elongation, are tested according to GB / T 14343-2008 Test Method for Linear Density of Chemical Fiber Filaments.

[0027] Washing fastness: According to the test standard A-2 in TRS 2006A: soap 5g / L, liquid volume 100ml, capacity 550±50ml, temperature 50±2℃, rotation time 30min. The treated samples were dried in an environment not exceeding 60℃. The color change of the test piece before and after the test, and the staining of the lining fabric, were compared with the gray grading card to distinguish the grades. Example 1

[0028] A method for preparing modified nylon 66 fiber includes the following steps: Salt formation reaction: Adipic acid and hexamethylenediamine were added to water at a molar ratio of 1:1 to prepare a 50 wt% solution. The reaction temperature was controlled at 50℃, and the reaction time was 2.5 hours. The reaction was terminated when the pH value of the solution reached 7.4 to obtain Nylon 66 salt. In addition, STPA / SIPA (70:30) and hexamethylenediamine were added to water at a molar ratio of 1:1.05 to prepare a 35 wt% STI6 salt solution. The reaction temperature was controlled at 55℃, and the reaction time was 2.4 hours. The reaction was terminated when the pH value of the solution reached 7.5 to obtain STI6 salt.

[0029] High-pressure reaction: Nylon 66 salt and STI6 salt accounting for 5 wt% of the mass of Nylon 66 salt are added to the polycondensation reactor, and then sodium hypophosphite accounting for 0.35 wt% of the total salt mass is added. The mixture is stirred thoroughly and mixed evenly. Nitrogen gas is purged to a reaction pressure of 1.7 MPa, the reaction temperature is controlled at 245℃, and the reaction is carried out under pressure for 2 hours.

[0030] Atmospheric pressure reaction: After the high-pressure reaction is completed, the pressure is slowly reduced to atmospheric pressure within 30 minutes, while the temperature is uniformly increased to 270°C. The reaction is carried out for 1.5 hours, and more than 95% water is collected.

[0031] Vacuum reaction: After the reaction at atmospheric pressure is completed, the vacuum is uniformly drawn to an absolute pressure of 0.05 MPa, the reaction endpoint temperature is controlled at 280℃, and the reaction is carried out for 1 hour; nitrogen is then purged to atmospheric pressure, and the modified nylon 66 polymer is obtained by discharging.

[0032] Spinning: The modified nylon 66 polymer is directly piped to the spinning box, the spinning temperature is controlled at 280℃ and the spinning speed is 4500m / min. After cooling, oiling, stretching and setting, it is processed into modified nylon 66 fiber.

[0033] The quality indicators of the modified nylon 66 fiber prepared by the above method are as follows: polymer relative viscosity 2.55, melting point 255℃, gel content 0.8wt‰, black particle content of 0.3~1mm diameter 6 particles / kg, fiber monofilament fineness 1.5dtex, breaking strength 5.5cN / dtex, and breaking elongation 20%. Example 2

[0034] A method for preparing modified nylon 66 fiber includes the following steps: Salt formation reaction: Adipic acid and hexamethylenediamine were added to water at a molar ratio of 1:1 to prepare a 40wt% solution. The reaction temperature was controlled at 45℃, and the reaction time was 3 hours. The solution was terminated when the pH value reached 7.3 to obtain Nylon 66 salt. In addition, STPA / SIPA (65:35) and hexamethylenediamine were added to water at a molar ratio of 1:1.05 to prepare a 25wt% STI6 salt solution. The reaction temperature was controlled at 60℃, and the reaction time was 2 hours. The solution was terminated when the pH value reached 7.5 to obtain STI6 salt.

[0035] High-pressure reaction: Nylon 66 salt and STI6 salt accounting for 3wt% of the mass of Nylon 66 salt are added to the polycondensation reactor, and then sodium hypophosphite accounting for 0.15wt% of the total salt mass is added. The mixture is stirred thoroughly and mixed evenly. Nitrogen gas is purged to a reaction pressure of 1.5MPa, the reaction temperature is controlled at 250℃, and the reaction is carried out under pressure for 1.5 hours.

[0036] Atmospheric pressure reaction: After the high-pressure reaction is completed, the pressure is slowly reduced to atmospheric pressure within 30 minutes, while the temperature is uniformly increased to 265°C. The reaction is carried out for 2 hours, and more than 95% water is collected.

[0037] Vacuum reaction: After the reaction at atmospheric pressure is completed, the vacuum is uniformly drawn to an absolute pressure of 0.02 MPa, the reaction endpoint temperature is controlled at 285℃, and the reaction is carried out for 0.5 hours; nitrogen is then purged to atmospheric pressure, and the modified nylon 66 polymer is obtained by discharging.

[0038] Spinning: The modified nylon 66 polymer is directly piped to the spinning box, the spinning temperature is controlled at 290℃ and the spinning speed is 6000m / min. After cooling, oiling, stretching and setting, it is processed into modified nylon 66 fiber.

[0039] The quality indicators of the modified nylon 66 fiber prepared by the above method are as follows: polymer relative viscosity 2.85, melting point 260℃, gel content 0.9wt‰, black particle content of 0.3~1mm diameter 8 particles / kg, fiber monofilament fineness 0.8dtex, breaking strength 6.5cN / dtex, and breaking elongation 12%. Example 3

[0040] A method for preparing modified nylon 66 fiber includes the following steps: Salt formation reaction: Adipic acid and hexamethylenediamine were added to water at a molar ratio of 1:1 to prepare a 55 wt% solution. The reaction temperature was controlled at 60℃, and the reaction time was 2 hours. The reaction was terminated when the pH value of the solution reached 7.5 to obtain Nylon 66 salt. In addition, STPA / SIPA (85:15) and hexamethylenediamine were added to water at a molar ratio of 1:1.05 to prepare a 40 wt% STI6 salt solution. The reaction temperature was controlled at 45℃, and the reaction time was 3 hours. The reaction was terminated when the pH value of the solution reached 7.3 to obtain STI6 salt.

[0041] High-pressure reaction: Nylon 66 salt and STI6 salt accounting for 10 wt% of the mass of Nylon 66 salt are added to the polycondensation reactor, and then sodium hypophosphite accounting for 0.45 wt% of the total salt mass is added. The mixture is stirred thoroughly and mixed evenly. Nitrogen gas is purged to a reaction pressure of 1.8 MPa, the reaction temperature is controlled at 235℃, and the reaction is carried out under pressure for 2.5 hours.

[0042] Atmospheric pressure reaction: After the high-pressure reaction is completed, the pressure is slowly reduced to atmospheric pressure within 30 minutes, while the temperature is uniformly increased to 275°C. The reaction is carried out for 1 hour, and more than 95% water is collected.

[0043] Vacuum reaction: After the atmospheric pressure reaction is completed, the vacuum is uniformly drawn to an absolute pressure of 0.08 MPa, the reaction endpoint temperature is controlled at 277℃, and the reaction is carried out for 1.5 hours; nitrogen is then purged to atmospheric pressure, and the modified nylon 66 polymer is obtained by discharging.

[0044] Spinning: The modified nylon 66 polymer is directly piped to the spinning box, the spinning temperature is controlled at 270℃ and the spinning speed is 3500m / min. After cooling, oiling, stretching and setting, it is processed into modified nylon 66 fiber.

[0045] The quality indicators of the modified nylon 66 fiber prepared by the above method are as follows: polymer relative viscosity 2.05, melting point 245℃, gel content 0.3wt‰, black particle content of 0.3~1mm in diameter 3 particles / kg, fiber monofilament fineness 2.5dtex, breaking strength 4.5cN / dtex, and breaking elongation 35%. Example 4

[0046] A method for preparing modified nylon 66 fiber includes the following steps: Salt formation reaction: Adipic acid and hexamethylenediamine were added to water at a molar ratio of 1:1 to prepare a 50wt% solution. The reaction temperature was controlled at 50℃, and the reaction time was 2 hours. The solution was terminated when the pH value reached 7.3 to obtain Nylon 66 salt. In addition, STPA / SIPA (75:25) and hexamethylenediamine were added to water at a molar ratio of 1:1.05 to prepare a 30wt% STI6 salt solution. The reaction temperature was controlled at 55℃, and the reaction time was 2 hours. The solution was terminated when the pH value reached 7.4 to obtain STI6 salt.

[0047] High-pressure reaction: Nylon 66 salt and STI6 salt accounting for 6 wt% of the mass of Nylon 66 salt are added to the polycondensation reactor, and then sodium hypophosphite accounting for 0.20 wt% of the total salt mass is added. The mixture is stirred thoroughly and mixed evenly. Nitrogen gas is purged to a reaction pressure of 1.6 MPa, the reaction temperature is controlled at 245℃, and the reaction is carried out under pressure for 2.2 hours.

[0048] Atmospheric pressure reaction: After the high-pressure reaction is completed, the pressure is slowly reduced to atmospheric pressure within 30 minutes, while the temperature is uniformly increased to 272°C. The reaction is carried out for 1.2 hours, and more than 95% water is collected.

[0049] Vacuum reaction: After the atmospheric pressure reaction is completed, the vacuum is uniformly drawn to an absolute pressure of 0.06 MPa, the reaction endpoint temperature is controlled at 279℃, and the reaction is carried out for 1.2 hours; nitrogen is then purged to atmospheric pressure, and the modified nylon 66 polymer is obtained by discharging.

[0050] Spinning: The modified nylon 66 polymer is granulated underwater, dried, melt-extruded, and then transported to the spinning box. The spinning temperature is controlled at 285℃ and the spinning speed is 5500m / min. After cooling, oiling, stretching, and setting, it is processed into modified nylon 66 fiber.

[0051] The quality indicators of the modified nylon 66 fiber prepared by the above method are as follows: polymer relative viscosity 2.65, melting point 254℃, gel content 1wt‰, black particle content of 0.3~1mm in diameter 4 particles / kg, fiber monofilament fineness 1.2dtex, breaking strength 5.6cN / dtex, and breaking elongation 20%. Example 5

[0052] The modified nylon 66 fiber prepared in Example 1 was made into garters. After degreasing, cationic red 5BL dye (2% owf) was added, the pH was adjusted to 4.6, the temperature was raised to 105°C, and the temperature was kept for 50 minutes. Then, the garters were washed and dried.

[0053] The dyed garter belts are brightly colored and evenly dyed without any color difference spots, and the washing color fastness value is 4.5. Example 6

[0054] The modified nylon 66 fiber prepared in Example 1 was made into garters. After degreasing, weak acid red B dye (1.5% owf), ammonium sulfate (1.2 g / L) and penetrant JFC (0.8 g / L) were added to adjust the pH to 4.8. The temperature was raised to 95°C and kept at that temperature for 30 minutes. Then the garters were washed and dried.

[0055] The dyed garter belts were evenly dyed without any color difference spots, and the color fastness to washing was 4.5. Comparative Example 1

[0056] A method for preparing modified nylon 66 fiber is the same as in Example 1, except that STI6 salt is not prepared in step (1) and STI6 salt is not added in step (2).

[0057] The quality indicators of the modified nylon 66 fiber prepared by the above method are as follows: polymer relative viscosity 2.95, melting point 262℃, gel content 6wt‰, black particle content of 0.3~1mm diameter 25 particles / kg, fiber monofilament fineness 1.5dtex, breaking strength 4.5cN / dtex, and breaking elongation 14%.

[0058] Comparing Example 1 with Comparative Example 1, it can be seen that without the addition of STI6 salt, the gel content and black particle content in the nylon 66 fiber polymer both increased significantly, while the fiber's breaking strength and elongation at break both decreased. Comparative Example 2

[0059] The modified nylon 66 fiber prepared in Comparative Example 1 was made into a glove belt. It was dyed with cationic dye according to the method of Example 5, and it was observed that the glove belt did not dye. It was dyed with acid dye according to the method of Example 6, and its washing fastness value was 4.5, but the color was 1 degree lighter than that of the glove belt of Example 6.

[0060] The above description is merely a preferred embodiment of the present invention, and therefore should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made in accordance with the scope of the patent and the contents of the specification should still fall within the scope of the present invention.

Claims

1. A modified nylon 66 fiber and its preparation method, characterized in that, The modified nylon 66 polymer is prepared by copolymerization modification. Specifically, the modified nylon 66 polymer is prepared by copolymerization of STI6 salt obtained by salting STPA / SIPA with hexamethylenediamine and nylon 66 salt. The modified nylon 66 polymer is obtained by direct pipeline transportation or underwater pelletizing and melt extrusion, followed by processing in a spinning box, cooling, oiling, stretching and shaping steps. The STI6 salt accounts for 3 to 10 wt% of the nylon 66 salt mass, and the mixing ratio of STPA / SIPA is 65:35 to 85:

15.

2. The modified nylon 66 fiber according to claim 1, characterized in that, The modified nylon 66 polymer has a relative viscosity of 2.05–2.85, a melting point of 245–260°C, a gel content of ≤1 wt‰, and a black particle content of ≤8 particles / kg with a diameter of 0.3–1 mm.

3. The modified nylon 66 fiber according to claim 1, characterized in that, The fiber has a single filament fineness of 0.8–2.5 dtex, a breaking strength of 4.5–6.5 cN / dtex, and a breaking elongation of 10–35%.

4. The modified nylon 66 fiber according to claim 1, characterized in that, The resulting textiles can be dyed with cationic dyes.

5. A method for preparing modified nylon 66 fiber as described in any one of claims 1-4, characterized in that, The process includes the following steps: (1) Salt formation reaction: Adipic acid and hexamethylenediamine are added to water to form a salt to obtain Nylon 66 salt; STPA / SIPA and hexamethylenediamine are added to water to form a salt to obtain STI6 salt; (2) High pressure reaction: Nylon 66 salt and STI6 salt obtained in step (1) are added to the catalyst and stirred to mix evenly, and high compression polymerization is carried out under nitrogen protection; (3) Atmospheric pressure reaction: After the high pressure reaction is completed, the pressure of the reactor is adjusted to atmospheric pressure and the reaction continues; (4) Vacuum reaction: After the atmospheric pressure reaction is carried out for a certain period of time, a vacuum is drawn, and the material is discharged after the reaction reaches the target degree of polymerization to obtain modified Nylon 66 polymer; (5) Spinning: Modified Nylon 66 polymer is directly transported by pipeline or granulated underwater, dried and melted and extruded, and then fed into the spinning box. After cooling, oiling, stretching and shaping steps, it is processed into modified Nylon 66 fiber.

6. The preparation method according to claim 5, characterized in that, In step (1): the concentration of nylon 66 salt solution is 40-55 wt%, the molar ratio of adipic acid to hexamethylenediamine is 1:1, the reaction temperature is 45-60℃, the reaction time is 2-3 hours, and the pH value of the solution at the end of the reaction is 7.3-7.5; the concentration of STI6 salt solution is 25-40 wt%, the molar ratio of STPA / SIPA to hexamethylenediamine is 1:1.05, the reaction temperature is 60-80℃, the reaction time is 2-3 hours, and the pH value of the solution at the end of the reaction is 7.5-7.

8.

7. The preparation method according to claim 5, characterized in that, In step (2): the catalyst is sodium hypophosphite, the mass of the catalyst is 0.15 to 0.45 wt% of the total salt mass, the reaction temperature is 235 to 250 °C, the reaction pressure is 1.5 to 1.8 MPa, and the pressure holding reaction lasts for 1.5 to 2.5 Hr.

8. The preparation method according to claim 5, characterized in that, In step (3): After the high-pressure reaction is completed, the pressure is slowly reduced to normal pressure within 30 minutes, while the temperature is raised to 265-275℃ at a constant rate. The reaction is carried out for 1-2 hours, and more than 95% of the water is collected.

9. The preparation method according to claim 5, characterized in that, In step (4): the absolute pressure of the vacuum reaction is 0.02-0.08 MPa, the reaction endpoint temperature is 277-285℃, the reaction time is 0.5-1.5 Hr, and the material is discharged after being purged with nitrogen to atmospheric pressure.

10. The preparation method according to claim 5, characterized in that, In step (5): the spinning temperature is 270-290℃ and the spinning speed is 3500-6000m / min.