Polyacrylonitrile fiber, flame-retardant polyacrylonitrile pre-oxidized fiber and preparation methods of polyacrylonitrile fiber and flame-retardant polyacrylonitrile pre-oxidized fiber

Abstract

The invention relates to a polyacrylonitrile fiber, a flame-retardant polyacrylonitrile pre-oxidized fiber and a preparation method thereof, the polyacrylonitrile fiber is obtained by taking acrylonitrile, organic silicon modified acrylic acid and ammonium p-styrenesulfonate as raw materials, carrying out homogeneous solution copolymerization and carrying out dry jet wet spinning; polyacrylonitrile fibers are subjected to pre-oxidation treatment, and the flame-retardant polyacrylonitrile pre-oxidized fibers are obtained. The prepared flame-retardant polyacrylonitrile pre-oxidized fiber is good in flame-retardant effect and excellent in mechanical property.

Description

Technical Field

[0001] The invention belongs to the field of flame-retardant fibers, and in particular relates to polyacrylonitrile fiber, flame-retardant polyacrylonitrile pre-oxidized fiber and a preparation method thereof. Background Art

[0002] Due to its unique properties, flame retardant fibers can be used not only for flame retardant textiles for cars, trains, and airplanes, decorative textiles for public places such as hotels and restaurants, protective clothing and home textile products for hospitals, the military, and forest firefighting, but also for flame retardant composite materials for aerospace, etc. At present, flame retardant fibers mainly include flame retardant polyester fibers, flame retardant polyamide fibers, and flame retardant regenerated cellulose fibers. Polyacrylonitrile pre-oxidized fibers have good flexibility and high temperature resistance, and will not burn, melt, soften or drip. Its limiting oxygen index LOI value is greater than 45%, which is much higher than the LOI value of other fibers. The flame retardant grade is SA, and it does not release toxic gases in flames or under strong radiant heat. It is a fire retardant material with excellent performance and has broad application prospects in civil, military and industrial fields.

[0003] Chinese patent CN115478365B discloses a high temperature resistant flame retardant fiber nonwoven fabric and a preparation method thereof, wherein the high temperature resistant flame retardant fiber is a modified polyacrylonitrile pre-oxidized fiber, and the method is to ammoniate the polyacrylonitrile fiber, and then prepare the aminated polyacrylonitrile fiber under catalyst conditions to obtain an organic polysilazane composite polyacrylonitrile fiber, and after pre-oxidation, obtain the modified polyacrylonitrile pre-oxidized fiber. The advantage of this method is that it enhances the strength and fiber cohesion of the polyacrylonitrile pre-oxidized fiber to a certain extent, but the disadvantages are that the process is complicated, the cost is high, the component distribution is uneven, and the use of a variety of dimethyl sulfoxides in the preparation process is not conducive to environmental protection.

[0004] Chinese patent CN116770580A discloses a method for manufacturing post-processed modified flame-retardant fibers, which is to spray a flame retardant solution on the fibers in a spraying manner, and after drying and evaporating excess water, the flame retardant is attached to the fibers to obtain flame-retardant fibers. The advantages of this method are that it uses a one-time spraying method, does not require soaking or circulating leaching, and the power required for flame retardant preparation, fiber drying, and equipment operation are all lower than other existing technologies. The disadvantages are that the flame retardant is unevenly distributed on the fibers, is not washable, and does not have permanent flame retardancy.

[0005] Chinese patent CN110078861B discloses a polyacrylonitrile spinning solution and a preparation method thereof and a flame-retardant polyacrylonitrile fiber. The comonomer in the method is a halogen-containing compound, which can make the prepared fiber have flame-retardant properties, and improve the rate and conversion rate of the polymerization reaction during the preparation of the polyacrylonitrile spinning solution. The disadvantages are that the halogen-containing comonomer is easy to release toxic gas at high temperature, the fiber prepared by this method has poor high temperature resistance, and the monomer feed amount is large. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a polyacrylonitrile fiber, a flame-retardant polyacrylonitrile pre-oxidized fiber and a preparation method, so as to avoid the compatibility problem between the flame retardant and the polyacrylonitrile polymerization liquid in the flame-retardant fiber currently prepared by the post-treatment method, the poor high-temperature stability of the comonomer in the blending method and the poor flame retardancy of the fiber, and overcome the problems of complex preparation process of the existing flame-retardant fiber, environmental pollution by toxic substances, high raw material cost, short service life of the flame-retardant fiber, uneven composition and the like.

[0007] In order to solve the above technical problems, the present invention provides a polyacrylonitrile fiber. The polyacrylonitrile fiber is prepared by homogeneous solution copolymerization of acrylonitrile, silicone-modified acrylic acid and styrene sulfonate amide as raw materials and then obtained by dry-jet wet spinning.

[0008] The present invention provides a method for preparing polyacrylonitrile fiber, comprising the following steps:

[0009] (1) Synthesizing silicone-modified acrylic acid using dichlorotetramethyldisiloxane and hydroxy acrylate as raw materials;

[0010] (2) dissolving three monomers of acrylonitrile, silicone-modified acrylic acid and styrene sulfonate amine in dimethyl sulfoxide to prepare a homogeneous solution, and then degassing and filtering;

[0011] (3) passing the filtered homogeneous solution into a polymerization kettle for polymerization to prepare a polymerization solution;

[0012] (4) The polymer solution is filtered, degassed, filtered, concentrated, and filtered to obtain a spinning solution;

[0013] (5) The spinning solution is wet-spinned to obtain polyacrylonitrile fiber.

[0014] Preferably, the organosilicon-modified acrylic acid in step (1) is prepared by the following method: adding a dichlorotetramethyldisiloxane mixture to an acrylic acid hydroxy ester mixture at 2-8° C. under nitrogen protection; heating the obtained mixed solution to 22-30° C. for reaction, and after the reaction is completed for 10-12 hours, filtering to finally obtain the organosilicon-modified acrylic acid.

[0015] Preferably, in the hydroxy acrylate mixed solution, the molar ratio of hydroxy acrylate, pyridine and dimethyl sulfoxide is 1:1:3-7; in the dichlorotetramethyldisiloxane mixed solution, the molar ratio of dichlorotetramethyldisiloxane and dimethyl sulfoxide is 1:4-7; in the mixed solution obtained after the addition, the molar ratio of hydroxy acrylate and dichlorotetramethyldisiloxane is 2-2.3:1.

[0016] Preferably, the hydroxy acrylate in step (1) comprises one of hydroxyethyl acrylate, hydroxypropyl acrylate or hydroxybutyl acrylate.

[0017] Preferably, the molar fractions of acrylonitrile, silicone-modified acrylic acid and styrene sulfonate in step (2) are 96.4% to 98.4%, 0.8% to 1.8% and 0.8% to 1.8% respectively; and the mass percentage concentration of the monomers in the homogeneous solution prepared in step (2) is 18% to 28%.

[0018] Preferably, the polymerization temperature in step (3) is 55-73° C., the reaction time is 1-8 h, and the stirring speed is 100-350 r / min.

[0019] The invention provides a flame retardant polyacrylonitrile preoxidized fiber. The flame retardant polyacrylonitrile preoxidized fiber is obtained by subjecting the polyacrylonitrile fiber to a preoxidation treatment.

[0020] The present invention also provides a method for preparing flame-retardant polyacrylonitrile preoxidized fiber, comprising placing the above-mentioned polyacrylonitrile fiber in a preoxidation furnace, setting four temperature zones with a temperature rise gradient, and applying multiple stages of drafting to perform preoxidation to obtain flame-retardant polyacrylonitrile preoxidized fiber.

[0021] Preferably, the atmosphere in the pre-oxidation furnace is air, and the air flow rate is 1-1.5 m / s; the multi-stage stretching includes one temperature zone stretching, two temperature zone stretching, three temperature zone stretching and four temperature zone stretching, and the applied stretching is 1%-4%; the pre-oxidation temperature is: 220-250°C for the first temperature zone, 235-260°C for the second temperature zone, 250-270°C for the third temperature zone, and 260-280°C for the fourth temperature zone, and the insulation time of each temperature zone is 10-35 min.

[0022] Beneficial Effects

[0023] The process of the invention is simple and has good repeatability; the polymerization reaction conversion rate is high, the spinnability is good, and the production cost is low; the prepared flame-retardant polyacrylonitrile pre-oxidized fiber has good flame-retardant effect, and the obtained fiber has excellent mechanical properties and has both strength and heat resistance. DETAILED DESCRIPTION

[0024] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0025] Example 1

[0026] The polyacrylonitrile fiber and the flame-retardant polyacrylonitrile pre-oxidized fiber in this embodiment are prepared according to the following method:

[0027] (1) preparing a solution of hydroxybutyl acrylate, pyridine and dimethyl sulfoxide in a molar ratio of 2:2:9; preparing a solution of dichlorotetramethyldisiloxane and dimethyl sulfoxide in a molar ratio of 1:6;

[0028] (2) adding the prepared tetramethyl disiloxane dichloride mixture to the hydroxybutyl acrylate mixture at 5° C. under nitrogen protection, wherein the molar ratio of hydroxybutyl acrylate to tetramethyl disiloxane dichloride in the resulting mixed solution is 2:1;

[0029] (3) heating the mixed solution obtained in step (2) to 25° C. for reaction, and after the reaction is completed for 10 hours, filtering to finally obtain organosilicon-modified acrylic acid;

[0030] (4) dissolving three monomers of acrylonitrile (96.4%, molar fraction), silicone-modified acrylic acid (1.8%, molar fraction) and styrene sulfonate amine (1.8%, molar fraction) in dimethyl sulfoxide to prepare a homogeneous solution, wherein the mass percentage concentration of the monomers is 20%, and degassing and filtering are performed, and then the filtered homogeneous solution is introduced into a polymerization kettle for polymerization, and the temperature is maintained at 58° C. and the stirring speed is 150 r / min during the polymerization, and the polymerization time is 6 hours to prepare a polymerization solution; filtering, degassing, filtering, concentrating and filtering the polymerization solution to obtain a spinning solution; and obtaining polyacrylonitrile fibers by dry-jet wet spinning;

[0031] (5) The prepared polyacrylonitrile fiber is oxidized in a pre-oxidation furnace, the atmosphere in the furnace is air, and the air flow rate is 1.2 m / s; the temperature in the furnace is divided into four temperature zones, and various levels of drafting are applied. Specifically, the temperature of the first temperature zone is 230-240°C, kept warm for 30 minutes, and the drafting applied is 1%; the temperature of the second temperature zone is 240-255°C, kept warm for 30 minutes, and the drafting applied is 3%; the temperature of the third temperature zone is 255-265°C, kept warm for 25 minutes, and the drafting applied is 2.5%; the temperature of the fourth temperature zone is 265-280°C, kept warm for 25 minutes, and the drafting applied is 1.5%; finally, flame-retardant polyacrylonitrile pre-oxidation fiber is obtained.

[0032] Example 2

[0033] The polyacrylonitrile fiber and the flame-retardant polyacrylonitrile pre-oxidized fiber in this embodiment are prepared according to the following method:

[0034] (1) preparing a solution of hydroxypropyl acrylate, pyridine and dimethyl sulfoxide in a molar ratio of 2:2:10; preparing a solution of dichlorotetramethyldisiloxane and dimethyl sulfoxide in a molar ratio of 1:6;

[0035] (2) adding the prepared tetramethyl disiloxane dichloroate to the hydroxypropyl acrylate mixed solution at 5° C. under nitrogen protection, wherein the molar ratio of hydroxypropyl acrylate to tetramethyl disiloxane dichloroate in the obtained mixed solution is 2:1;

[0036] (3) heating the mixed solution obtained in step (2) to 25° C. for reaction, and after the reaction is completed for 11 hours, filtering is performed to finally obtain organosilicon-modified acrylic acid;

[0037] (4) dissolving three monomers of acrylonitrile (97.4%, molar fraction), silicone-modified acrylic acid (1.3%, molar fraction) and styrene sulfonate amine (1.3%, molar fraction) in dimethyl sulfoxide to prepare a homogeneous solution, wherein the mass percentage concentration of the monomers is 22%, and degassing and filtering are performed, and then the filtered homogeneous solution is introduced into a polymerization kettle for polymerization, and the temperature is maintained at 65° C. and the stirring speed is 200 r / min during the polymerization, and the polymerization time is 5 hours to prepare a polymerization solution; filtering, degassing, filtering, concentrating and filtering the polymerization solution to obtain a spinning solution; and obtaining polyacrylonitrile fibers by dry-jet wet spinning;

[0038] (5) The prepared polyacrylonitrile fiber is oxidized in a pre-oxidation furnace, the atmosphere in the furnace is air, and the air flow rate is 1.5 m / s; the temperature in the furnace is divided into four temperature zones, and various levels of drafting are applied. Specifically, the temperature of the first temperature zone is 230-240°C, kept warm for 25 minutes, and the drafting is 1.5%; the temperature of the second temperature zone is 240-250°C, kept warm for 25 minutes, and the drafting is 2.5%; the temperature of the third temperature zone is 250-260°C, kept warm for 25 minutes, and the drafting is 4%; the temperature of the fourth temperature zone is 260-280°C, kept warm for 25 minutes, and the drafting is 2%; finally, flame-retardant polyacrylonitrile pre-oxidation fiber is obtained.

[0039] Example 3

[0040] The polyacrylonitrile fiber and the flame-retardant polyacrylonitrile pre-oxidized fiber in this embodiment are prepared according to the following method:

[0041] (1) preparing a solution of hydroxyethyl acrylate, pyridine and dimethyl sulfoxide in a molar ratio of 2:2:10; preparing a solution of dichlorotetramethyldisiloxane and dimethyl sulfoxide in a molar ratio of 1:6;

[0042] (2) adding the prepared dichlorotetramethyldisiloxane mixed solution to the hydroxyethyl acrylate mixed solution at 2° C. under nitrogen protection, wherein the molar ratio of hydroxyethyl acrylate to dichlorotetramethyldisiloxane in the resulting mixed solution is 2:1;

[0043] (3) heating the mixed solution obtained in step (2) to 28° C. for reaction for 12 h. After the reaction is completed, filtering is performed to finally obtain organosilicon-modified acrylic acid;

[0044] (4) dissolving three monomers of acrylonitrile (98.4%, molar fraction), silicone-modified acrylic acid (0.8%, molar fraction) and styrene sulfonate amine (0.8%, molar fraction) in dimethyl sulfoxide to prepare a homogeneous solution, wherein the mass percentage concentration of the monomers is 25%, and degassing and filtering are performed, and then the filtered homogeneous solution is introduced into a polymerization kettle for polymerization, and the temperature is maintained at 68° C. and the stirring speed is 150 r / min during the polymerization, and the polymerization time is 6 hours to prepare a polymerization solution; filtering, degassing, filtering, concentrating and filtering the polymerization solution to obtain a spinning solution; and obtaining polyacrylonitrile fibers by dry-jet wet spinning;

[0045] (5) The prepared polyacrylonitrile fiber is oxidized in a pre-oxidation furnace, the atmosphere in the furnace is air, and the air flow rate is 1.3 m / s; the temperature in the furnace is divided into four temperature zones, and various levels of drafting are applied. Specifically, the temperature of the first temperature zone is 230-250°C, kept warm for 28 minutes, and the drafting is 1.5%; the temperature of the second temperature zone is 250-260°C, kept warm for 28 minutes, and the drafting is 2.5%; the temperature of the third temperature zone is 260-270°C, kept warm for 28 minutes, and the drafting is 4%; the temperature of the fourth temperature zone is 270-280°C, kept warm for 28 minutes, and the drafting is 2%; finally, flame-retardant polyacrylonitrile pre-oxidation fiber is obtained.

[0046] Example 4

[0047] The polyacrylonitrile fiber and the flame-retardant polyacrylonitrile pre-oxidized fiber in this embodiment are prepared according to the following method:

[0048] (1) preparing a solution of hydroxyethyl acrylate, pyridine and dimethyl sulfoxide in a molar ratio of 2:2:9; preparing a solution of dichlorotetramethyldisiloxane and dimethyl sulfoxide in a molar ratio of 1:4.5;

[0049] (2) adding the prepared tetramethyl disiloxane dichlorohydrate mixed solution into the hydroxyethyl acrylate mixed solution at 2° C. under nitrogen protection, wherein the molar ratio of hydroxyethyl acrylate to tetramethyl disiloxane dichlorohydrate in the obtained mixed solution is 2:1.

[0050] (3) heating the mixed solution obtained in step (2) to 30° C. for reaction for 12 h. After the reaction is completed, filtering is performed to finally obtain organosilicon-modified acrylic acid;

[0051] (4) dissolving three monomers of acrylonitrile (98%, molar fraction), silicone-modified acrylic acid (1%, molar fraction) and styrene sulfonate (1%, molar fraction) in dimethyl sulfoxide to prepare a homogeneous solution, wherein the mass percentage concentration of the monomers is 28%, and degassing and filtering are performed, and then the filtered homogeneous solution is introduced into a polymerization kettle for polymerization, and the temperature is maintained at 60° C. and the stirring speed is 250 r / min during the polymerization, and the polymerization time is 4 hours to prepare a polymerization solution; filtering, degassing, filtering, concentrating and filtering the polymerization solution to obtain a spinning solution; and obtaining polyacrylonitrile fibers by dry-jet wet spinning;

[0052] (5) The polyacrylonitrile fiber is placed in a pre-oxidation furnace for oxidation. The atmosphere in the furnace is air and the air flow rate is 1.3 m / s. The temperature in the furnace is divided into four temperature zones, and various levels of drafting are applied. Specifically, the temperature of the first temperature zone is 220-240°C, kept warm for 35 minutes, and the drafting is 1.5%; the temperature of the second temperature zone is 240-250°C, kept warm for 35 minutes, and the drafting is 2.5%; the temperature of the third temperature zone is 250-265°C, kept warm for 22 minutes, and the drafting is 4%; the temperature of the fourth temperature zone is 265-280°C, kept warm for 22 minutes, and the drafting is 2%; finally, flame-retardant polyacrylonitrile pre-oxidation fiber is obtained.

[0053] Comparative Example 1

[0054] To further illustrate that the present invention uses silicone-modified acrylic acid and styrene sulfonate amine as comonomers to effectively improve the flame retardant properties of the fiber, the comonomers are replaced to carry out polymerization reaction, and the steps are as follows:

[0055] (1) acrylonitrile (96.4%, molar fraction), methyl acrylate (1.8%, molar fraction) and styrene sulfonamide (1.8%, molar fraction) are dissolved in dimethyl sulfoxide to prepare a homogeneous solution, wherein the mass percentage concentration of the monomers is 20%, and degassing and filtering are performed, and then the filtered homogeneous solution is introduced into a polymerization kettle for polymerization, and the temperature is maintained at 65° C., the stirring speed is 300 r / min, and the polymerization time is 8 hours during the polymerization to prepare a polymerization solution; the polymerization solution is filtered, degassed, filtered, concentrated, and filtered to obtain a spinning solution; and polyacrylonitrile fibers are obtained by dry-jet wet spinning;

[0056] (2) The polyacrylonitrile fiber is placed in a pre-oxidation furnace for oxidation. The oxidation conditions are the same as those in Example 1. The atmosphere in the furnace is air and the air flow rate is 1.2 m / s. The temperature in the furnace is divided into four temperature zones, and various levels of drafting are applied. Specifically, the temperature of the first temperature zone is 230-240° C., which is kept warm for 30 min, and the drafting is 1%; the temperature of the second temperature zone is 240-255° C., which is kept warm for 30 min, and the drafting is 3%; the temperature of the third temperature zone is 255-265° C., which is kept warm for 25 min, and the drafting is 2.5%; the temperature of the fourth temperature zone is 265-280° C., which is kept warm for 25 min, and the drafting is 1.5%; and finally, the polyacrylonitrile pre-oxidized fiber is obtained.

[0057] After testing, the polyacrylonitrile pre-oxidized fiber prepared in this comparative example was measured by LOI 901 limiting oxygen index instrument, and its limiting oxygen index test value was 47, while the LOI values ​​of Examples 1 to 4 were all greater than 53.

[0058] Comparative Example 2

[0059] In order to further illustrate that the pre-oxidation process adopted in the present invention can effectively improve the flame retardant properties of the flame retardant fiber, polyacrylonitrile fiber without pre-oxidation was used for comparison.

[0060] The manufacturing steps of this polyacrylonitrile fiber are as follows:

[0061] (1) preparing a solution of hydroxyethyl acrylate, pyridine and dimethyl sulfoxide in a molar ratio of 2:2:9; preparing a solution of dichlorotetramethyldisiloxane and dimethyl sulfoxide in a molar ratio of 1:4.5;

[0062] (2) adding the prepared dichlorotetramethyldisiloxane mixed solution to the hydroxyethyl acrylate mixed solution at 5° C. under nitrogen protection, wherein the molar ratio of hydroxyethyl acrylate to dichlorotetramethyldisiloxane in the resulting mixed solution is 2:1;

[0063] (3) heating the mixed solution obtained in step (2) to 28° C. for reaction for 12 h. After the reaction is completed, filtering is performed to finally obtain organosilicon-modified acrylic acid;

[0064] (4) The obtained acrylonitrile (97.4%, molar fraction), silicone-modified acrylic acid (1.3%, molar fraction) and styrene sulfonate (1.3%, molar fraction) three monomers are dissolved in dimethyl sulfoxide to prepare a homogeneous solution, wherein the mass percentage concentration of the monomers is 23%, and degassing and filtering are performed. Then, the filtered homogeneous solution is introduced into a polymerization kettle for polymerization. During the polymerization, the temperature is maintained at 58°C, the stirring speed is 150 r / min, and the polymerization time is 6 hours to prepare a polymerization solution; the polymerization solution is filtered, degassed, filtered, concentrated, and filtered to obtain a spinning solution; and polyacrylonitrile fibers are obtained by dry-jet wet spinning.

[0065] After testing, it was found that the polyacrylonitrile fiber prepared in this comparative example burned out immediately after being ignited, and had no flame retardancy.

[0066] By comparing Comparative Example 1 and Examples 1 to 4, it can be seen that the present invention uses silicone-modified acrylic acid and styrene sulfonamide as copolymer monomers to effectively improve the flame retardant properties of the fiber.

[0067] By comparing Comparative Example 2 and Examples 1 to 4, it can be seen that the pre-oxidation process of the present invention can effectively improve the flame retardant properties of the fiber.

Claims

1. A polyacrylonitrile fiber, characterized in that: The polyacrylonitrile fiber is obtained by using acrylonitrile, silicone-modified acrylic acid and styrene sulfonate amine as raw materials through homogeneous solution copolymerization and dry-jet wet spinning.

2. A method for preparing polyacrylonitrile fiber as claimed in claim 1, comprising the following steps: (1) Synthesizing silicone-modified acrylic acid using dichlorotetramethyldisiloxane and hydroxy acrylate as raw materials; (2) dissolving three monomers of acrylonitrile, silicone-modified acrylic acid and styrene sulfonate amine in dimethyl sulfoxide to prepare a homogeneous solution, and then degassing and filtering; (3) passing the filtered homogeneous solution into a polymerization kettle for polymerization to prepare a polymerization solution; (4) The polymer solution is filtered, degassed, filtered, concentrated, and filtered to obtain a spinning solution; (5) The spinning solution is wet-spinned to obtain polyacrylonitrile fiber.

3. The method for preparing polyacrylonitrile fiber according to claim 2, characterized in that: The organosilicon-modified acrylic acid in step (1) is prepared by the following method: adding a dichlorotetramethyldisiloxane mixture to an acrylic acid hydroxy ester mixture at 2-8° C. under nitrogen protection; heating the obtained mixed solution to 22-30° C. for reaction, and after the reaction is completed for 10-12 hours, filtering to finally obtain the organosilicon-modified acrylic acid.

4. The method for preparing polyacrylonitrile fiber according to claim 3, characterized in that: In the hydroxy acrylate mixed solution, the molar ratio of hydroxy acrylate, pyridine and dimethyl sulfoxide is 1:1:3-7; in the dichlorotetramethyldisiloxane mixed solution, the molar ratio of dichlorotetramethyldisiloxane and dimethyl sulfoxide is 1:4-7; in the mixed solution obtained after the addition, the molar ratio of hydroxy acrylate and dichlorotetramethyldisiloxane is 2-2.3:

1.

5. The method for preparing polyacrylonitrile fiber according to claim 2, characterized in that: The hydroxy acrylate in step (1) includes one of hydroxyethyl acrylate, hydroxypropyl acrylate or hydroxybutyl acrylate.

6. The method for preparing polyacrylonitrile fiber according to claim 2, characterized in that: The molar fractions of acrylonitrile, silicone-modified acrylic acid and styrene sulfonate in step (2) are 96.4% to 98.4%, 0.8% to 1.8% and 0.8% to 1.8% respectively; the mass percentage concentration of the monomers in the homogeneous solution prepared in step (2) is 18% to 28%.

7. The method for preparing polyacrylonitrile fiber according to claim 2, characterized in that: The polymerization temperature in step (3) is 55-73° C., the reaction time is 1-8 hours, and the stirring speed is 100-350 r / min.

8. A flame-retardant polyacrylonitrile pre-oxidized fiber, characterized in that: The flame-retardant polyacrylonitrile pre-oxidized fiber is obtained by subjecting the polyacrylonitrile fiber as claimed in claim 1 to a pre-oxidation treatment.

9. A method for preparing flame-retardant polyacrylonitrile pre-oxidized fiber as described in claim 8, comprising placing the polyacrylonitrile fiber in a pre-oxidation furnace, setting four temperature zones with a temperature rise gradient, and applying multiple stages of drawing to perform pre-oxidation to obtain flame-retardant polyacrylonitrile pre-oxidized fiber.

10. The method for preparing flame-retardant polyacrylonitrile pre-oxidized fiber according to claim 9, characterized in that: The atmosphere in the pre-oxidation furnace is air, and the air flow rate is 1-1.5 m / s; the multi-stage stretching includes one-temperature zone stretching, two-temperature zone stretching, three-temperature zone stretching and four-temperature zone stretching, and the applied stretching is 1%-4%; the pre-oxidation temperature is: 220-250°C for the first temperature zone, 235-260°C for the second temperature zone, 250-270°C for the third temperature zone, and 260-280°C for the fourth temperature zone, and the insulation time of each temperature zone is 10-35 minutes.