Flame-retardant spandex fiber and preparation method thereof

Abstract

The invention relates to a flame-retardant spandex fiber and a preparation method thereof, and belongs to the technical field of spandex fibers. The preparation method comprises the following steps: S1, under the action of a photoinitiator, 1, 4-dimercapto-2, 3-butanediol and dimethyl vinyl phosphate are subjected to a photochemical reaction in a solvent, and a phosphorus / sulfur flame retardant containing hydroxyl is obtained; s2, under the action of a catalyst, a hydroxyl-containing phosphorus / sulfur flame retardant, diisocyanate and polyether glycol react in a solvent, and a flame-retardant polyurethane prepolymer is obtained; s3, carrying out chain extension reaction and neutralization reaction on the flame-retardant polyurethane prepolymer to obtain a flame-retardant polyurethane fiber-forming polymer; and S4, preparing the flame-retardant polyurethane fiber-forming polymer into a spinning solution, and carrying out dry spinning to obtain the flame-retardant polyurethane fiber. Phosphorus / sulfur-containing flame-retardant groups in the flame-retardant spandex fiber are combined with macromolecular chain segments of the spandex fiber through covalent bonds, so that the flame-retardant spandex fiber is good in stability and good in washing resistance.

Description

Technical Field

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

[0002] Spandex, with its excellent elasticity, combines the comfort, functionality and beauty of textiles, and has become an important raw material to support the development of the textile industry. The application of spandex has rapidly expanded from traditional textiles to emerging fields such as sporting goods, medical and sanitary products, and military supplies. However, spandex fiber is flammable, with a limiting oxygen index of about 17%-18%, and produces melt droplets during combustion, and has poor flame retardant properties, which seriously limits the application of spandex fiber in clothing textiles, interior decoration fabrics, protective and industrial textiles, etc. Therefore, the development of spandex fiber with flame retardant function is particularly important to expand its application areas.

[0003] At present, there are two main methods for flame retardant modification of spandex fibers: one is fiber post-treatment, which is simple and convenient to operate, but has defects such as poor functional durability; the other is to blend flame retardants with other additives and add them to a polyurethane matrix, and then obtain flame retardant spandex fibers by spinning. For example, invention patent CN 105420844 A discloses a method for preparing flame retardant spandex fibers, wherein halogen-free phosphorus flame retardants of phenylphosphonate, synergistic flame retardants of non-metallic oxides, dyeing aids and other additives are added to a polyurethane polymer solution, mixed evenly, and spun to obtain flame retardant spandex fibers. Invention patent CN113773347 A discloses a method for preparing a spirocyclic phosphate flame retardant for spandex fibers and flame retardant spandex fibers based thereon, wherein a spirocyclic phosphate flame retardant is synthesized and blended into polyurethane to prepare flame retardant spandex fibers. However, the above-mentioned additive flame retardant is to add the flame retardant directly to the polyurethane matrix by physical blending, which has the problem of poor compatibility between the flame retardant and the polyurethane matrix, resulting in a decrease in the physical and mechanical properties of the spandex fiber, and the flame retardant is easy to migrate and lose. The flame retardant performance of the material may decay with the extension of service time, resulting in poor durability of the flame retardant function.

[0004] Reactive flame retardant is to introduce flame retardant functional groups into the molecular structure of polyurethane by chemical grafting. This method overcomes the shortcomings of easy migration of additive flame retardants and large loss of mechanical properties of materials. The addition amount is small, and it has a more lasting flame retardant effect and better stability. Therefore, it is a technical problem that urgently needs to be solved to introduce flame retardant groups into the polyurethane matrix by covalent bonds and obtain flame retardant spandex fibers by spinning. Invention patent CN 109295518 A discloses a method for preparing flame retardant modified spandex, in which uronic acid oligomers are copolymerized with diisocyanate and treated with calcium chloride to obtain a polyurethane solution containing a hybrid block of calcium uronic acid oligomers, and flame retardant spandex fibers are obtained after spinning. However, the preparation method is complicated, the process flow is complicated, and the practical application value is low.

[0005] Therefore, developing a flame-retardant spandex fiber through reactive flame retardancy is of great significance, but there are great challenges. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a flame-retardant spandex fiber and a preparation method thereof. First, a hydroxyl-containing phosphorus / sulfur flame retardant is prepared by photochemical reaction of 1,4-dimercapto-2,3-butanediol and vinyl dimethyl phosphate; then, in the polyurethane polymerization process, ethylene glycol is used as a chain extender to react with an isocyanate group, and the phosphorus / sulfur flame retardant group is introduced into the side end of the polyurethane macromolecular segment through a covalent bond, and is tightly combined with the polyurethane macromolecular skeleton to synthesize a flame-retardant polyurethane prepolymer; finally, the flame-retardant spandex fiber is prepared by dry spinning. The phosphorus-containing flame retardant group, the sulfur-containing flame retardant group and the polyurethane carbon-rich molecular segment are organically combined to form an expansion flame retardant system, and the phosphorus-containing flame retardant group and the sulfur-containing flame retardant group effectively improve the flame retardant properties of the polyester fiber through a synergistic flame retardant effect; in addition, the phosphorus / sulfur flame retardant group is grafted onto the side end of the spandex macromolecular segment through a polymerization reaction, and is tightly combined with the spandex macromolecular skeleton, so that the modified spandex fiber has durable flame retardant properties and has important practical application value.

[0007] The first object of the present invention is to provide a method for preparing a flame retardant spandex fiber, comprising the following steps: S1. Under the action of a photoinitiator, 1,4-dimercapto-2,3-butanediol and vinyl dimethyl phosphate undergo a photochemical reaction in a first solvent to obtain a hydroxyl-containing phosphorus / sulfur flame retardant; the structural formula of the hydroxyl-containing phosphorus / sulfur flame retardant is ; S2. Under the action of a catalyst, the hydroxyl-containing phosphorus / sulfur flame retardant, diisocyanate and polyether diol described in S1 react in a second solvent to obtain a flame-retardant polyurethane prepolymer; S3, performing chain extension reaction and neutralization reaction on the flame retardant polyurethane prepolymer described in S2 to obtain a flame retardant polyurethane fiber-forming polymer; S4. The flame-retardant polyurethane fiber-forming polymer described in S3 is prepared into a spinning solution, and the flame-retardant spandex fiber is obtained by dry spinning.

[0008] In one embodiment of the present invention, in S1, the photoinitiator is 2,2-dimethoxy-2-phenylacetophenone.

[0009] In one embodiment of the present invention, in S1, the molar ratio of 1,4-dimercapto-2,3-butanediol to vinyl dimethyl phosphate is (1-1.1):2; two thiols in each mole of 1,4-dimercapto-2,3-butanediol react with two moles of vinyl dimethyl phosphate to synthesize a hydroxyl-containing phosphorus / sulfur flame retardant monomer product or a hydroxyl-containing flame retardant monomer product, wherein 1,4-dimercapto-2,3-butanediol is slightly excessive to ensure complete reaction of vinyl dimethyl phosphate; The molar ratio of the 1,4-dimercapto-2,3-butanediol to the photoinitiator is 2:(1-1.3).

[0010] In one embodiment of the present invention, in S1, the UV light wavelength of the photochemical reaction is 365nm and the light intensity is 200mW / cm 2 -300mW / cm 2 , the reaction temperature is 70℃-80℃, and the reaction time is 2h-3h. Under this condition, the reaction rate is relatively high.

[0011] In one embodiment of the present invention, in S2, the catalyst is dibutyltin dilaurate; The diisocyanate is selected from diphenylmethane-4,4-diisocyanate and / or 2,4-toluene diisocyanate; The polyether diol is selected from polyoxyethylene and / or polypropylene glycol.

[0012] In one embodiment of the present invention, in S2, the molar ratio of the hydroxyl-containing phosphorus / sulfur flame retardant, the hydroxyl-containing phosphorus / sulfur flame retardant and the total amount of polyether diol is (0.3-0.5):1; the higher the amount of the hydroxyl-containing phosphorus / sulfur flame retardant, the better the flame retardant properties of the polyurethane fiber. However, if the amount is too high, it will be a waste and will easily affect the properties of the polyurethane itself. The molar ratio of the hydroxyl-containing phosphorus / sulfur flame retardant, the total amount of polyether diol, and the diisocyanate is 1:(2-2.2), and a slight excess of diisocyanate helps to promote the formation of the polyurethane prepolymer; The two hydroxyl groups of the polyether diol and the hydroxyl-containing phosphorus / sulfur flame retardant react with the isocyanate group at the same time, so that the phosphorus / sulfur flame retardant group is grafted onto the side end of the polyurethane macromolecular chain segment through a covalent bond.

[0013] In one embodiment of the present invention, in S2, the reaction temperature is 80°C-90°C, and the reaction time is 3h-5h.

[0014] In one embodiment of the present invention, in S1 and S2, the first solvent and the second solvent are both N,N-dimethylformamide.

[0015] In one embodiment of the present invention, in S3, the chain extension reaction is carried out under the action of ethylene glycol at 90° C.-100° C. for 3 h-4 h; The neutralization reaction is carried out under the action of triethanolamine at 30° C.-40° C. for 1 h-2 h.

[0016] In one embodiment of the present invention, in S4, the mass fraction of the flame-retardant polyurethane fiber-forming polymer in the spinning solution is 25%-35%, and the solvent is dimethylacetamide; The dry spinning temperature is 200° C.-240° C., under which the volatilization and recovery of dimethylacetamide are guaranteed.

[0017] The second object of the present invention is to provide a flame retardant spandex fiber prepared by the preparation method.

[0018] The technical solution of the present invention has the following advantages compared with the prior art: (1) The preparation method of the present invention utilizes the mercapto group of 1,4-dimercapto-2,3-butanediol and the unsaturated double bond of vinyl dimethyl phosphate to synthesize a hydroxyl-containing phosphorus / sulfur flame retardant through a click chemistry reaction; during the polyurethane polymerization reaction, ethylene glycol is used as a chain extender to react with the isocyanate group, and the phosphorus-containing flame retardant group and the sulfur-containing flame retardant group are simultaneously introduced into the side end of the polyurethane macromolecular segment through a covalent bond to synthesize a flame-retardant polyurethane. The phosphorus-containing flame retardant group and the sulfur-containing flame retardant group improve the flame retardant effect of the spandex fiber through a synergistic flame retardant effect. The phosphorus-containing flame retardant group at the side end acts as an acid source and decomposes in advance during the pyrolysis process, which has the effect of catalyzing the carbonization of the matrix; the sulfur-containing flame retardant group at the side end acts as a foaming agent and releases non-flammable sulfur-containing gas during the thermal decomposition process; and the carbon chain in the middle spandex macromolecular segment can act as a carbon source, and generates an expanded carbon layer under the catalysis of compounds such as phosphoric acid, which covers the surface of the spandex fiber. Therefore, the phosphorus-containing flame retardant group and the sulfur-containing flame retardant group play a synergistic flame retardant effect and have excellent flame retardant properties. In addition, the phosphorus / sulfur flame retardant groups form stable covalent bonds with the macromolecular segments of the spandex fiber, so the spandex fiber has good flame retardant properties and water washability.

[0019] (2) The preparation method described in the present invention grafts the phosphorus-containing flame retardant groups of vinyl dimethyl phosphate and the sulfur-containing flame retardant groups of 1,4-dimercapto-2,3-butanediol onto the macromolecular chain segments of spandex fibers through a copolymerization reaction, and the hydroxyl-containing phosphorus / sulfur flame retardant has good compatibility with the spandex fiber macromolecules, so the prepared spandex fibers have excellent flame retardant properties.

[0020] (3) The phosphorus / sulfur flame retardant groups in the flame retardant spandex fiber of the present invention are bonded to the macromolecular segments of the spandex fiber through covalent bonds, and have good stability. Therefore, the flame retardant spandex fiber has good water washing resistance and has broad application prospects and important practical application value. DETAILED DESCRIPTION

[0021] The present invention is further described below in conjunction with specific embodiments so that those skilled in the art can better understand the present invention and implement it. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. It should be understood that the specific embodiments are only used to explain the present invention, but the embodiments are not intended to limit the present invention.

[0022] In the present invention, unless otherwise explained, technical and scientific terms used in the present invention have the same meanings as commonly understood by those skilled in the art to which the present invention belongs.

[0023] In the present invention, unless otherwise stated, the term "and / or" used in the present invention includes any and all combinations of one or more of the associated listed items.

[0024] In the present invention, unless otherwise stated, the experimental methods used in the embodiments of the present invention are conventional methods unless otherwise stated, and the materials, reagents, etc. used are all commercially available unless otherwise stated.

[0025] In the present invention, unless otherwise specified, the polyoxyethylene used in the examples of the present invention is purchased from Shanghai MacLean Biochemical Technology Co., Ltd., with an average molecular weight Mv of about 8000 and a product number of P831923.

[0026] In the present invention, unless otherwise specified, the polypropylene glycol used in the examples of the present invention is purchased from Shanghai MacLean Biochemical Technology Co., Ltd., with an average molecular weight Mv of about 4000 and a product number of P815575. Example 1

[0027] The flame-retardant spandex fiber and the preparation method thereof of the present invention specifically comprise the following steps:

[0028] S1. Add 0.1 mol of 1,4-dimercapto-2,3-butanediol, 0.2 mol of dimethyl vinyl phosphate, 0.05 mol of 2,2-dimethoxy-2-phenylacetophenone, and 50 mL of N,N-dimethylformamide into a round-bottom flask, heat to 80 °C, and use UV light (365 nm, 300 mW / cm 2 ) irradiate and react for 2 hours to obtain a hydroxyl-containing phosphorus / sulfur flame retardant; S2, dissolving 0.05 mol of polyoxyethylene, 0.05 mol of a hydroxyl-containing phosphorus / sulfur flame retardant, 0.2 mol of diphenylmethane-4,4-diisocyanate and 5 drops of dibutyltin dilaurate in 50 mL of N,N-dimethylformamide, raising the temperature to 90° C. and reacting for 3 hours to obtain a flame-retardant polyurethane prepolymer; S3, adding 0.1 mol of ethylene glycol and continuing to react at 90° C. for 4 h to extend the chain of the flame-retardant polyurethane prepolymer, then slowly adding triethanolamine dropwise for neutralization, reacting at 40° C. for 1 h, adding deionized water with high-speed stirring, emulsifying for 1 h, and removing the solvent by reduced pressure distillation to obtain a flame-retardant polyurethane fiber-forming polymer; S4. The flame-retardant polyurethane fiber-forming polymer is dissolved in dimethylacetamide to prepare a spinning solution with a mass fraction of 35%, and then the flame-retardant spandex fiber is prepared by dry spinning; the temperature of the spinning tunnel is 200°C. Example 2

[0029] The flame-retardant spandex fiber and the preparation method thereof of the present invention specifically comprise the following steps: S1. Add 0.11 mol of 1,4-dimercapto-2,3-butanediol, 0.2 mol of dimethyl vinyl phosphate, 0.07 mol of 2,2-dimethoxy-2-phenylacetophenone, and 50 mL of N,N-dimethylformamide into a round-bottom flask, heat to 70 °C, and use UV light (365 nm, 200 mW / cm 2 ) irradiate and react for 3 hours to obtain a hydroxyl-containing phosphorus / sulfur flame retardant; S2, dissolving 0.07 mol of polyoxyethylene, 0.03 mol of hydroxyl-containing phosphorus / sulfur flame retardant, 0.22 mol of diphenylmethane-4,4-diisocyanate and 5 drops of dibutyltin dilaurate in 50 mL of N,N-dimethylformamide, raising the temperature to 80° C. and reacting for 5 hours to obtain a flame-retardant polyurethane prepolymer; S3, adding 0.1 mol of ethylene glycol and continuing to react at 90° C. for 4 h to extend the chain of the flame-retardant polyurethane prepolymer, then slowly adding triethanolamine dropwise for neutralization, reacting at 30° C. for 2 h, adding deionized water with high-speed stirring, emulsifying for 1 h, and removing the solvent by reduced pressure distillation to obtain a flame-retardant polyurethane fiber-forming polymer; S4. The flame-retardant polyurethane fiber-forming polymer is dissolved in dimethylacetamide to prepare a spinning solution with a mass fraction of 25%, and then the flame-retardant spandex fiber is prepared by dry spinning; the temperature of the spinning tunnel is 240°C. Example 3

[0030] The flame-retardant spandex fiber and the preparation method thereof of the present invention specifically comprise the following steps: S1. Add 0.1 mol of 1,4-dimercapto-2,3-butanediol, 0.2 mol of dimethyl vinyl phosphate, 0.06 mol of 2,2-dimethoxy-2-phenylacetophenone, and 50 mL of N,N-dimethylformamide into a round-bottom flask, heat to 75 °C, and use UV light (365 nm, 250 mW / cm 2 ) irradiate and react for 2.5 hours to obtain a hydroxyl-containing phosphorus / sulfur flame retardant; S2, dissolving 0.06 mol of polyoxyethylene, 0.04 mol of a hydroxyl-containing phosphorus / sulfur flame retardant, 0.21 mol of diphenylmethane-4,4-diisocyanate and 5 drops of dibutyltin dilaurate in 50 mL of N,N-dimethylformamide, raising the temperature to 85° C. and reacting for 4 hours to obtain a flame-retardant polyurethane prepolymer; S3, adding 0.1 mol of ethylene glycol and continuing to react at 90° C. for 4 h to extend the chain of the flame-retardant polyurethane prepolymer, then slowly adding triethanolamine dropwise for neutralization, reacting at 35° C. for 1.5 h, adding deionized water with high-speed stirring, emulsifying for 1 h, and removing the solvent by reduced pressure distillation to obtain a flame-retardant polyurethane fiber-forming polymer; S4. The flame-retardant polyurethane fiber-forming polymer is dissolved in dimethylacetamide to prepare a spinning solution with a mass fraction of 30%, and then the flame-retardant spandex fiber is prepared by dry spinning; the temperature of the spinning tunnel is 220°C. Example 4

[0031] The flame-retardant spandex fiber and the preparation method thereof of the present invention specifically comprise the following steps: S1. Add 0.11 mol of 1,4-dimercapto-2,3-butanediol, 0.2 mol of dimethyl vinyl phosphate, 0.06 mol of 2,2-dimethoxy-2-phenylacetophenone, and 50 mL of N,N-dimethylformamide into a round-bottom flask, raise the temperature to 77 °C, and use UV light (365 nm, 250 mW / cm 2 ) irradiate and react for 2.5 hours to obtain a hydroxyl-containing phosphorus / sulfur flame retardant; S2, dissolving 0.055 mol of polypropylene glycol, 0.045 mol of a hydroxyl-containing phosphorus / sulfur flame retardant, 0.21 mol of 2,4-toluene diisocyanate and 5 drops of dibutyltin dilaurate in 50 mL of N,N-dimethylformamide, raising the temperature to 90° C. and reacting for 4 hours to obtain a flame-retardant polyurethane prepolymer; S3, adding 0.1 mol of ethylene glycol and continuing to react at 100° C. for 3 h to chain-extend the flame-retardant polyurethane prepolymer, then slowly adding triethanolamine dropwise for neutralization, reacting at 40° C. for 1.5 h, adding deionized water with high-speed stirring, emulsifying for 1 h, and removing the solvent by reduced pressure distillation to obtain a flame-retardant polyurethane fiber-forming polymer; S4. The flame-retardant polyurethane fiber-forming polymer is dissolved in dimethylacetamide to prepare a spinning solution with a mass fraction of 33%, and then the flame-retardant spandex fiber is prepared by dry spinning; the temperature of the spinning tunnel is 210°C.

[0032] Comparative Example 1 (unmodified spandex fiber) The method is basically the same as Example 1, except that no modification with the hydroxyl-containing phosphorus / sulfur flame retardant is performed. Comparative Example 2

[0033] The process is basically the same as Example 1, except that 1,4-dimercapto-2,3-butanediol is replaced with 1,4-dimercaptobutane. Comparative Example 3

[0034] The method is basically the same as Example 1, except that only 1,4-dimercapto-2,3-butanediol is used for modification. Comparative Example 4

[0035] The method is basically the same as Example 1, except that only dimethyl vinyl phosphate is used for modification. Test Example 1

[0036] The physical properties, flame retardant properties and water washability of the flame retardant spandex fibers (modified spandex fibers) and unmodified spandex fibers of Examples 1-4 and Comparative Examples 1-4 were tested.

[0037] Breaking strength: The breaking strength of modified spandex fiber and unmodified spandex fiber was measured according to the standard FZ / T 50006-2013 "Test method for tensile properties of spandex yarn"; Limiting oxygen index: The limiting oxygen index of modified spandex fiber and unmodified spandex fiber is measured according to the standard FZ / T 50017-2011 "Test method for flame retardancy of polyester fiber - oxygen index method"; Washing resistance: Wash according to the standard of AATCC 61-2006 "Accelerated Test for Color Fastness to Washing for Household and Commercial Use".

[0038] Table 1 shows the relevant properties of the modified spandex fiber and the unmodified spandex fiber finally measured: Table 1

[0039] As can be seen from Table 1, the limiting oxygen index of the modified spandex fiber of the embodiment is significantly higher, all higher than 27.5%, and increases with the increase of the content of the phosphorus / sulfur flame retardant segment, indicating that the modified spandex fiber of the embodiment has good flame retardant properties. In addition, after 20 washings, the limiting oxygen index of the modified spandex fiber of the embodiment is still higher than 27.0%, having good flame retardant properties, indicating that the modified spandex fiber has good water washing resistance. In addition, the breaking strength of the modified spandex fiber of the embodiment is similar to the breaking strength of the unmodified modified spandex fiber in Comparative Example 1, indicating that the embedding of the phosphorus / sulfur flame retardant segment does not significantly affect the strength properties of the spandex fiber.

[0040] By comparing Example 1 and Comparative Example 1, it can be seen that the limiting oxygen index of the unmodified spandex fiber after 0 and 20 times of washing is 18.1%, indicating that the flame retardant property and water washing resistance of the unmodified spandex fiber are poor.

[0041] By comparing Example 1 and Comparative Example 2, it can be seen that when 1,4-dimercapto-2,3-butanediol is replaced with 1,4-dimercaptobutane, the flame retardancy, water washability and mechanical properties of the modified spandex fiber are reduced. This is because the synthetic phosphorus / sulfur flame retardant does not contain reactive hydroxyl groups, and the phosphorus / sulfur flame retardant is only blended into the polyurethane fiber and cannot be combined with the polyurethane chain segments.

[0042] By comparing Example 1 and Comparative Example 3, it can be seen that when only 1,4-dimercapto-2,3-butanediol is used for modification, the flame retardant properties of the modified spandex fiber are reduced and the water washing resistance is reduced. This is because the flame retardant efficiency of the sulfur-containing group of 1,4-dimercapto-2,3-butanediol alone is low.

[0043] By comparing Example 1 and Comparative Example 4, it can be seen that when only vinyl dimethyl phosphate is used for modification, the flame retardant properties, water wash resistance and mechanical properties of the modified spandex fiber are reduced. This is because vinyl dimethyl phosphate cannot react with the polyurethane chain segment and is only doped inside the polyurethane fiber through blending and cannot combine with the polyurethane chain segment.

[0044] In summary, the flame-retardant spandex fiber prepared by the present invention has good flame retardant properties and water washability.

[0045] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A method for preparing a flame retardant spandex fiber, characterized in that: The following steps are involved: S1. Under the action of a photoinitiator, 1,4-dimercapto-2,3-butanediol and vinyl dimethyl phosphate undergo a photochemical reaction in a first solvent to obtain a hydroxyl-containing phosphorus / sulfur flame retardant; the structural formula of the hydroxyl-containing phosphorus / sulfur flame retardant is ; S2. Under the action of a catalyst, the hydroxyl-containing phosphorus / sulfur flame retardant, diisocyanate and polyether diol described in S1 react in a second solvent to obtain a flame-retardant polyurethane prepolymer; S3, performing chain extension reaction and neutralization reaction on the flame retardant polyurethane prepolymer described in S2 to obtain a flame retardant polyurethane fiber-forming polymer; S4. The flame-retardant polyurethane fiber-forming polymer described in S3 is prepared into a spinning solution, and the flame-retardant spandex fiber is obtained by dry spinning.

2. The method for preparing the flame retardant spandex fiber according to claim 1, characterized in that: In S1, the photoinitiator is 2,2-dimethoxy-2-phenylacetophenone.

3. The method for preparing the flame retardant spandex fiber according to claim 1, characterized in that: In S1, the molar ratio of 1,4-dimercapto-2,3-butanediol to dimethyl vinyl phosphate is (1-1.1):2; The molar ratio of the 1,4-dimercapto-2,3-butanediol to the photoinitiator is 2:(1-1.3).

4. The method for preparing the flame retardant spandex fiber according to claim 1, characterized in that: In S1, the UV light wavelength of the photochemical reaction is 365nm and the light intensity is 200mW / cm 2 -300mW / cm 2 , the reaction temperature is 70℃-80℃, and the reaction time is 2h-3h.

5. The method for preparing the flame retardant spandex fiber according to claim 1, characterized in that: In S2, the catalyst is dibutyltin dilaurate; The diisocyanate is selected from diphenylmethane-4,4-diisocyanate and / or 2,4-toluene diisocyanate; The polyether diol is selected from polyoxyethylene and / or polypropylene glycol.

6. The method for preparing the flame retardant spandex fiber according to claim 1, characterized in that: In S2, the molar ratio of the hydroxyl-containing phosphorus / sulfur flame retardant, the hydroxyl-containing phosphorus / sulfur flame retardant and the total amount of the polyether diol is (0.3-0.5):1; The molar ratio of the hydroxyl-containing phosphorus / sulfur flame retardant to the total amount of polyether diol and diisocyanate is 1:(2-2.2).

7. The method for preparing the flame-retardant spandex fiber according to claim 1, characterized in that: In S2, the reaction temperature is 80°C-90°C, and the reaction time is 3h-5h.

8. The method for preparing the flame retardant spandex fiber according to claim 1, characterized in that: In S3, the chain extension reaction is carried out under the action of ethylene glycol at 90° C.-100° C. for 3 h-4 h; The neutralization reaction is carried out under the action of triethanolamine at 30° C.-40° C. for 1 h-2 h.

9. The method for preparing the flame retardant spandex fiber according to claim 1, characterized in that: In S4, the mass fraction of the flame-retardant polyurethane fiber-forming polymer in the spinning solution is 25%-35%, and the solvent is dimethylacetamide; The temperature of the dry spinning is 200°C-240°C.

10. The flame retardant spandex fiber prepared by the preparation method according to any one of claims 1 to 9.

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