A wash-resistant flame-retardant cotton fabric and its preparation method

Abstract

The present invention provides a water-washable flame-retardant cotton fabric and a preparation method thereof. It is characterized in that the water-washable flame-retardant cotton fabric is obtained by treating a cotton fabric with a flame-retardant finishing solution containing an N-P-Si type flame retardant; the N-P-Si type flame retardant contains multiple flame-retardant groups based on N, P, and Si, has excellent thermal stability, char-forming ability, and flame-retardant efficiency, and when used for finishing cotton fabrics, imparts high limiting oxygen index and flame-retardant effect to the cotton fabrics; the flame retardant can bond with the cotton fabric fiber molecules through strong hydrogen bonds, so that the flame-retardant cotton fabric with the N-P-Si type flame retardant loaded on the surface has excellent water-washable performance.

Description

Technical Field

[0001] This application belongs to the technical field of functional textiles, and particularly relates to a water-washable flame-retardant cotton fabric and a preparation method thereof. Background Art

[0002] Flame-retardant fibers, also known as non-combustible fibers, refer to fibers that only smolder in a flame, do not produce a flame themselves, and self-extinguish when removed from the flame and the smoldering stops. They are widely used in clothing, home furnishings, decoration, non-woven fabrics, and fillers, etc. Compared with ordinary fibers, flame-retardant fibers have significantly reduced flammability, with a significantly slower burning rate during combustion, can quickly self-extinguish after leaving the fire source, and release less toxic smoke. Generally speaking, flame-retardant fibers are mainly concentrated in the field of synthetic fibers. For example, inorganic polymer flame retardants exist in the organic macromolecules of viscose fibers in a nano state or an interpenetrating network state to produce fiber fabrics with flame-retardant effects, which are suitable for applications in fields such as fire-fighting suits and work clothes. However, bedding such as mattresses, bed sheets, and quilt covers in home life mainly consists of cotton fabrics, and the risk of fire caused by cigarette butts, electrical failures, etc. in these places is also relatively high. Therefore, the development and application of flame-retardant cotton fabrics are of great significance.

[0003] In the prior art, flame-retardant cotton fabrics are usually prepared by physically adsorbing or forming covalent bonds to introduce or fix flame retardants on the surface of cotton fabrics. Among common flame retardants, halogen-based flame retardants have been prohibited in some countries and regions because they produce toxic and harmful substances such as dioxins and hydrogen halide gases during combustion, which endanger human life and health as well as the environment; the chemical bonds formed by organophosphorus-based flame retardants on the surface of cotton fabrics are unstable, with poor thermal stability and easy volatility; intumescent flame retardants are highly efficient but contain many water-soluble groups and have poor water-washability; the flame-retardant effect of silicon-based flame retardants is relatively poor, and it will affect the comfort and strength of cotton fabrics.

[0004] Therefore, it is urgent to develop new flame-retardant cotton fabrics and flame retardants to solve problems such as low flame-retardant efficiency and weak water-washability of cotton fabrics. Summary of the Invention

[0005] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a water-washable flame-retardant cotton fabric and a preparation method thereof. By treating cotton textiles with a self-made N-P-Si type flame retardant, a cotton fabric with water-washable flame-retardant effects can be obtained.

[0006] As one aspect of the present invention, the present invention provides a water-washable flame-retardant cotton fabric, which is prepared by treating a cotton fabric with a flame-retardant finishing solution containing an N-P-Si type flame retardant; the N-P-Si type flame retardant has a molecular structure as shown in Formula I:

[0007]

[0008] Further, the N-P-Si type flame retardant is synthesized by the following steps:

[0009] S1. Under nitrogen protection, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is dissolved in tetrahydrofuran. 4-Bromocinnamic acid is added to the reaction system within 0.5 h at 80-90 °C, and then the temperature is raised to 130-150 °C for reflux reaction for 4-10 h. After the reaction is completed, the system is cooled to room temperature, filtered, washed with water, recrystallized with tetrahydrofuran, and dried in vacuum to obtain Intermediate 1.

[0010] S2. Intermediate 1 is dissolved in tetrahydrofuran, and a tetrahydrofuran solution of a carbodiimide condensing agent and a coupling agent is added under an ice-water bath, and stirred for 0.5-2 h. Subsequently, while slowly adding a tetrahydrofuran solution containing diphenylphosphate diamine, stirring is continued, and ultrasonic stirring reaction is carried out at room temperature for 20-24 h. After the reaction is completed, suction filtration, washing with water, and drying in vacuum are carried out to obtain Intermediate 2.

[0011] S3. Under an anhydrous and anaerobic environment, 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, cesium carbonate, palladium acetate, and 1,1'-binaphthalene-2,2'-bis(diphenylphosphine) are added to 1,4-dioxane and stirred evenly. The temperature of the mixture system is raised to 70-90 °C, Intermediate 2 is added, and stirring reaction is carried out for 20-24 h. Filtration is carried out, and the solid is precipitated with deionized water. The solid is washed at least twice with a mixed solution of ethyl acetate and petroleum ether to obtain the N-P-Si type flame retardant.

[0012] Further, the carbodiimide condensing agent in S2 is one of 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), diisopropylcarbodiimide (DIC), and N,N'-dicyclohexylcarbodiimide (DCC); the coupling agent is one of N-hydroxysuccinimide (NHS) or N-hydroxythiosuccinimide (Sulfo-NHS). In some embodiments of the specification, the substances used are EDC and NHS, and the two are used in combination to promote the coupling of carboxyl and amino groups and reduce the generation of by-products.

[0013] Further, the vacuum drying conditions in S1-S2 are the same: drying at 100-120 °C for 6-8 h.

[0014] Further, the molar ratio of DOPO to 4-bromocinnamic acid in S1 is (1.1-1.3):1.

[0015] Further, the molar ratio of Intermediate 1 to diphenylphosphate diamine in S2 is (2.2-2.5):1; a slight excess of Intermediate 1 can ensure the coupling effect of carboxyl and amino groups.

[0016] Further, the molar ratio of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane to Intermediate 2 described in S3 is (1.1 - 2.0):1.

[0017] Further, the average molecular weight of the N-P-Si type flame retardant is 3000 - 30000 Da.

[0018] On the other hand, the present invention provides a method for preparing a water-washable flame-retardant cotton fabric, which is characterized by comprising the following steps:

[0019] (1) Pretreatment of the cotton fabric surface: The cotton fabric is rinsed in a sodium hydroxide solution to activate the surface of the cotton fabric; then it is repeatedly washed with deionized water until the wash liquor is nearly neutral, and dried for later use;

[0020] (2) Preparation of the flame-retardant finishing liquor: Weigh the synthesized N-P-Si type flame retardant described above and dissolve it in an ethanol solution with a volume fraction of 75% - 95% to obtain a flame-retardant finishing liquor with a mass concentration of 20% - 40%;

[0021] (3) The flame-retardant finishing liquor obtained in (2) is used to carry out flame-retardant finishing on the cotton fabric by means of padding, baking, and drying to obtain a water-washable flame-retardant cotton fabric.

[0022] Further, in step (1), the concentration of the sodium hydroxide solution is 1.0 - 2.5 mol / L, and the rinsing time is 12 - 18 h; in step (3), the padding parameters are: two-bath two-roll padding, the padding pressure is 0.1 - 0.4 Pa, and the liquor pickup rate is 90% - 120%; the baking temperature is 140 - 170 °C, and the baking time is 2 - 5 min; the drying temperature is 90 - 100 °C.

[0023] Advantages of the present invention:

[0024] In the present invention, different flame-retardant groups are bonded by chemical bonds to the same molecule to form an N-P-Si type flame retardant containing multi-group flame-retardant molecules, endowing it with excellent thermal stability, char-forming ability, and flame-retardant efficiency. On the one hand, the interaction between different flame-retardant groups generates a more efficient group synergistic flame-retardant mechanism; on the other hand, through chemical bonding, the phosphaphenanthrene, amino, and siloxane groups form an aggregated structure at the molecular scale, which is conducive to obtaining a more efficient group aggregation flame-retardant effect.

[0025] The synthesized N-P-Si type flame retardant is applied to cotton textiles in the present invention, and a cotton fabric with water-resistant flame retardant effect can be obtained. The results show that when the cotton fiber is loaded with a flame retardant containing Si, P, and N elements, both its flame retardant effect and thermal stability are greatly improved. The cotton fabric fibers finished with 20 g / L flame retardant can pass the UL94 vertical burning test, and the afterflame time and smoldering time are both 0. The finished cotton fibers maintain the whiteness and mechanical properties of the cotton fabric; at the same time, hydrogen bonding can occur between the amino groups in the flame retardant system and the cotton fiber molecules, realizing the flame retardant performance and wash resistance of the cotton fabric synchronously: the cotton fabric still has good flame retardant performance after 50 washes. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below.

[0027] Figure 1 is the thermogravimetric curve of three N-P-Si type flame retardants;

[0028] Figure 2 is the infrared absorption spectrum of N-P-Si type flame retardant 3;

[0029] Figure 3 is the schematic diagram of the synthesis route of N-P-Si type flame retardant. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the specific embodiments of the present invention will be described in detail below in conjunction with specific embodiments.

[0031] Many specific details are set forth in the following description in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways different from those described herein. Those skilled in the art can make similar generalizations without departing from the spirit of the present invention, so the present invention is not limited by the specific embodiments disclosed below.

[0032] Secondly, the so-called "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation manner of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment that excludes other embodiments.

[0033] Cotton fabric: The warp and weft yarn densities are 330 per 10 cm and 315 per 10 cm respectively, and the limiting oxygen index is 18.2%, Shanghai Lingda Textile Co., Ltd.;

[0034] DOPO (97% AR), EDC (C 8 H18 N 3 Cl, 98%), N-hydroxysuccinimide NHS (C 4 H 5 NO 3 , 98% AR) were all purchased from Shanghai Macklin Biochemical Co., Ltd.;

[0035] 4-Bromocinnamic acid (C 9 H 7 O 2 Br, 99% AR), Shanghai Huayuan Century Trading Co., Ltd.;

[0036] Phenyl diaminophosphate (C 6 H 9 N 2 O 2 P, 98% AR), tetrahydrofuran (THF), ethyl acetate and dimethylformamide (DMF), cesium carbonate, palladium acetate, 1,1'-binaphthalene-2,2'-bis(diphenylphosphine), 1,4-dioxane were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0037] 1,3-Bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (C 10 H 28 N 2 OSi 2 , BTMDS), AR, Beijing MRD Technology Co., Ltd.

[0038] Tetrahydrofuran and DMF were pre-dried to remove moisture before the experiment, and the remaining reagents could be used without further purification.

[0039] Preparation Example 1

[0040] An N-P-Si type flame retardant was synthesized by the following steps:

[0041] S1: Charge nitrogen into a four-necked round-bottom flask equipped with a strong stirrer, a condenser and a thermometer. Then, under a nitrogen atmosphere, add 0.55 molar parts of DOPO to tetrahydrofuran to a concentration of 0.275 mol / L. After it is completely dissolved, heat it up to 90 °C. Add 0.5 molar parts of 4-bromocinnamic acid to the reaction system in several portions within 0.5 h, and then raise the temperature to 140 °C and reflux for 8 h. After the reaction is completed, cool the system to room temperature and filter. Recrystallize with tetrahydrofuran and dry under vacuum at 100 °C for 8 h to obtain Intermediate 1. The yield of Intermediate 1 is 95.8%.

[0042] S2: Dissolve 0.3 mol portion of Intermediate 1 in tetrahydrofuran to prepare a solution with a concentration of 2 mol / L, and add a tetrahydrofuran solution (concentration: 0.05 mol / L) of 0.05 mol portion of EDC condensing agent and a tetrahydrofuran solution (concentration: 0.01 mol / L) of 0.01 mol portion of NHS coupling agent under an ice-water bath, activate and stir for 1 h; then slowly add a tetrahydrofuran solution (concentration: 1 mol / L) containing 0.5 mol portion of phenyl diphosphate diamine while stirring, and perform ultrasonic stirring reaction at room temperature for 24 h; after the reaction ends, perform suction filtration, wash with water, and then vacuum dry at 100 °C for 8 h to obtain Intermediate 2; the yield is 89.5%.

[0043] S3: Under an anhydrous and anaerobic environment, add 0.2 mol portion of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, cesium carbonate, 0.01 mol portion of palladium acetate, and 0.01 mol portion of 1,1'-binaphthalene-2,2'-bis(diphenylphosphine) to 1,4-dioxane and stir evenly (the concentration of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane is 2 mol / L); raise the temperature of the mixture system to 80 °C, add Intermediate 2, and stir and react for 24 h; filter, precipitate the solid with deionized water, and wash the solid 3 times with a mixed solution of ethyl acetate and petroleum ether with a volume ratio of 1:1 to obtain the N-P-Si type flame retardant.

[0044] Preparation Examples 2 - 3

[0045] N-P-Si type flame retardant 2, N-P-Si type flame retardant 3: Compared with the preparation method of N-P-Si type flame retardant 1, the difference lies in the different ratios of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane and Intermediate 2; the specific ratios are shown in Table 1.

[0046] Furthermore, perform test experiments on the yield, molecular weight, and thermal weight loss of the N-P-Si type flame retardant.

[0047] Molecular weight: Determined by GPC, HLC-8420GPC gel permeation chromatography, TSKgel GMHXL (9 μm, 7.8 mm I.D.×30 cm×2); mobile phase: DMF (containing 0.02 mol / L LiBr); flow rate: 1.0 mL / min; temperature: 35 °C; injection volume: 100 μL.

[0048] Thermal weight loss test: PYRIS type thermogravimetric analyzer, temperature range from room temperature to 800 °C, heating rate: 10 °C / min, gas flow rate: 20 mL / min. Take 10 mg of the sample for each test; weighing accuracy ±0.1%; weighing precision ±0.01%; weighing sensitivity <0.1 μg. The thermal decomposition curves of different N-P-Si type flame retardants are shown in Figure 1, the corresponding thermal decomposition characteristic parameters are shown in Table 1.

[0049] Table 1 Test results of yield, molecular weight and thermal weight loss of N-P-Si type flame retardant

[0050]

[0051] From the test results of the molecular weight in Table 1, it can be seen that the higher the concentration of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, the faster the rate of the alkylation reaction and the higher the yield. However, the degree of polymerization decreases and the molecular weight decreases; when the concentration of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane is relatively low, the reaction rate decreases, which can promote the elongation of the polymer chain and increase the molecular weight.

[0052] From Figure 1 It can be seen that the three N-P-Si type flame retardants start thermal decomposition at 300 °C - 400 °C. The initial decomposition temperatures of N-P-Si type flame retardants 1, 2, and 3 are 336 °C, 353 °C, and 382 °C respectively. In this stage, it is because the C-P bond (305 kJ / mol), C-N bond (305 kJ / mol) with relatively low bond energy in the flame retardant and the long aliphatic chain (C-C bond energy 332 kJ / mol) are broken by heat; the weight loss after 400 °C mainly comes from the destruction of the benzene ring or biphenyl structure contained and the formation of coke residues at the same time. The thermal weight loss curves of N-P-Si type flame retardants 1, 2, and 3 become flat after 530 °C, 552 °C, and 567 °C respectively, and no longer continue to degrade with the increase of temperature. The residual carbon mass retention rates reach 30.83%, 33.41%, and 35.83% in turn. This shows that the introduction of P, N, and Si flame retardant elements and the presence of benzene ring and biphenyl flame retardant functional groups can significantly improve the thermal stability and char-forming ability of N-P-Si type flame retardants.

[0053] Furthermore, N-P-Si type flame retardant 3 was selected for infrared spectrum analysis. Test conditions: NicoltAvatar370(is10) Fourier transform infrared spectroscopy instrument, using KBr tablet pressing method for sample preparation, scanning 16 times, resolution 4 cm -1 , scanning range 400 - 4000 cm -1 . The infrared spectrum is shown in Figure 2 .

[0054] Among them, the broad absorption peak near 3453 cm- 1 is the N-H stretching vibration in secondary amine and primary amine, and 3207 cm- 1 is the absorption of N-H bond in amide; the stronger absorption peaks at 2833 cm- 1 , 2916 cm- 1 are the -CH 2 , -CH3 Stretching characteristic peak; 1596 cm- 1 and 1455 cm- 1 are the framework vibration peaks of the benzene ring; 1236 cm -1 and 1220 cm- 1 show P=O and P-N bonds respectively; 980 cm- 1 is attributed to the stretching vibration of P=O; 1089 cm -1 Nearby are the antisymmetric stretching vibration of Si-O-Si and the absorption vibration peak of P-O-Ph, 836 cm- 1 is the stretching vibration peak of Si-C, 807 cm -1 is the stretching vibration peak of Si-O, 755 cm- 1 is the absorption vibration peak of P-C. The above infrared absorption spectrum analysis proves the existence of each flame retardant group in the flame retardant.

[0055] Example 1

[0056] A washable flame retardant cotton fabric, the preparation steps are as follows:

[0057] (1) Rinse the cotton fabric in 2 mol / L sodium hydroxide solution for 12 h to activate the surface of the cotton fabric; then repeatedly wash with deionized water until the washing liquid is nearly neutral, and dry at 90 °C for later use; weigh the mass m of the initial cotton fabric 1 .

[0058] (2) Dissolve the N-P-Si type flame retardant 1 obtained in Preparation Example 1 in 75% ethanol solution to obtain a flame retardant finishing solution with a mass concentration of 20%.

[0059] (3) Use the flame retardant finishing solution obtained in (2) to carry out flame retardant finishing on the cotton fabric by means of impregnation, padding, drying, and baking: impregnate with the flame retardant finishing solution obtained in (2) for 30 min; then pad with a small padding mangle at 0.2 Pa (two-dip two-roll), with a liquor pickup of 90%; then bake at 160 °C for 5 min; finally, wash three times to remove the unfixed flame retardant, and dry the fabric at 90 °C. Weigh the mass of the obtained washable flame retardant cotton fabric (m 2 ). The weight percentage gain of cotton fiber (WPG, %) is calculated according to the following formula:

[0060]

[0061] Examples 2-3

[0062] A washable flame retardant cotton fabric, with the same preparation process as in Example 1, the difference is that the flame retardants are N-P-Si type flame retardant 2 and N-P-Si type flame retardant 3 obtained in Preparation Examples 2-3 respectively.

[0063] Examples 4-7

[0064] A wash-resistant flame-retardant cotton fabric, compared with Example 1 in the preparation process, is different in that the mass concentrations of the flame-retardant finishing liquor are 25%, 30%, 35%, and 40% respectively.

[0065] Comparative Example 1

[0066] A cotton fabric, omitting the flame-retardant finishing.

[0067] Comparative Examples 2-3

[0068] A wash-resistant flame-retardant cotton fabric, which is different from Example 1 in that the mass concentrations of the used flame-retardant finishing liquor are 5% and 15% respectively.

[0069] Test Example

[0070] The following performance tests were carried out on the cotton fabrics of the examples and comparative examples, and the test results are shown in Table 2.

[0071] 1) Limiting oxygen index (LOI). Using a JE-5 type full-automatic oxygen index measuring instrument (Nanjing Jionglei Instrument Equipment Co., Ltd.), the limiting oxygen index test was carried out in accordance with GB / T 5454-1997 "Textiles - Burning performance - Oxygen index method". The sample size was 110 mm × 6 mm × 2 mm; the higher the limiting oxygen index value, the higher the oxygen concentration required for the sample to burn, that is, the more difficult it is for the sample to burn; when the fabric LOI ≥ 26, it belongs to a flame-retardant material.

[0072] 2) Vertical burning performance. Using a CZF-3 type horizontal and vertical burning tester from Nanjing Jiangning Analytical Instrument Factory, the vertical burning UL-94 grade of the sample was determined in accordance with the national standard GB / T 8746-2009 "Textiles - Burning performance - Vertical method"; and the damage length of the cotton fabric was determined in accordance with the standard of GB / T5455-2014 "Textiles - Determination of the damage length, smoldering and afterflaming time in the vertical direction of burning performance". The sample size was 130 mm × 13 mm × 3.2 mm.

[0073] 3) Fabric washability test. The cotton fabric was washed 50 times in accordance with the standard of GB / T 17591-1998 "Domestic washing procedure before fabric burning test", taken out and dried, and then the LOI was tested according to GB / T5454-1997 "Textiles - Burning performance - Oxygen index method" to analyze the wash-resistant ability of the flame-retardant cotton fabric.

[0074] Table 2 Performance test results of examples and comparative examples

[0075]

[0076]

[0077] For Examples 1 to 3 and Comparative Example 1 in Table 2, the LOI value of the cotton fabric without flame retardant finishing is 18.2, belonging to extremely flammable materials. The flame retardant finishing liquid formulated with the N-P-Si type flame retardant provided by the present invention can significantly improve the flame retardant performance of cotton fabrics; and the large molecular weight flame retardant can improve the flame retardant performance of cotton fabrics more efficiently.

[0078] Combined with the test results of the cotton fabrics in Example 1, Examples 4 to 7 and Comparative Examples 2 to 3, with the increase of the flame retardant finishing agent, the damage length, afterflame time and smoldering time are continuously shortened, and the vertical burning effect of the cotton fabric is continuously improved; and when the concentration reaches 25% or more, the cotton fabric has excellent flame retardant effect: when the cotton fabric burns vertically, it quickly carbonizes, the afterflame and smoldering times are both 0, the damage length < 80 mm, and the lowest can be reduced to 55 mm. After being washed 50 times, the damage length of the cotton fabric increases slightly; the fabric still self-extinguishes and has certain flame retardant performance; even the washable flame retardant cotton fabric with the smallest LOI value has an LOI of 26.2 after being washed, and still belongs to the difficult-to-burn materials, indicating that the thermal stability and durability of the coated cotton fabric are both very good.

[0079] It should be noted that the above examples are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A washable flame-retardant cotton fabric, characterized in that: The washable flame-retardant cotton fabric is prepared by treating the cotton fabric with a flame-retardant finishing liquid containing an NP-Si type flame retardant; the molecular structure of the NP-Si type flame retardant is shown in Formula I:

2. The washable flame-retardant cotton fabric according to claim 1, characterized in that: The NP-Si type flame retardant is synthesized by the following steps: S1. Under nitrogen protection, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide was dissolved in tetrahydrofuran, 4-bromocinnamic acid was added to the reaction system within 0.5 h at 80-90 °C, and then the temperature was raised to 130-150 °C and refluxed for 4-10 h. After the reaction, the system was cooled to room temperature, filtered, washed with water, recrystallized from tetrahydrofuran, and dried in vacuo to obtain intermediate 1. S2. Dissolve the intermediate 1 in tetrahydrofuran, add a tetrahydrofuran solution of a carbodiimide condensing agent and a coupling agent in an ice-water bath, and stir for 0.5 to 2 hours; then slowly add a tetrahydrofuran solution containing phenyl diaminophosphorate while stirring continuously, and react under ultrasonic stirring at room temperature for 20 to 24 hours; after the reaction, filter, wash with water, and vacuum dry to obtain the intermediate 2; S3. In an anhydrous and oxygen-free environment, add 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, cesium carbonate, palladium acetate, and 1,1'-binaphthyl-2,2'-bisdiphenylphosphine to 1,4-dioxane and stir evenly; raise the temperature of the mixture system to 70-90°C, add intermediate 2, and stir the reaction for 20-24 hours; filter, precipitate the solid with deionized water, and wash the solid with a mixed solution of ethyl acetate and petroleum ether at least twice to obtain an NP-Si type flame retardant.

3. A washable flame-retardant cotton fabric as claimed in claim 2, characterized in that: The vacuum drying conditions in S1-S2 are drying at a temperature of 100-120° C. for 6-8 hours.

4. The washable flame-retardant cotton fabric according to claim 2, characterized in that: The carbodiimide condensation agent described in S2 is one of 1-ethyl-(3-dimethylaminopropyl)-carbodiimide hydrochloride, diisopropylcarbodiimide, and N,N-dicyclohexylcarbodiimide; the coupling agent is one of N-hydroxysuccinimide or N-hydroxysulfosuccinimide.

5. The washable flame-retardant cotton fabric according to claim 2, characterized in that: The molar ratio of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to 4-bromocinnamic acid in S1 is (1.1-1.3):

1.

6. The washable flame-retardant cotton fabric according to claim 2, characterized in that: The molar ratio of the intermediate 1 to phenyl diaminophosphorate in S2 is (2.2-2.5):

1.

7. The washable flame-retardant cotton fabric according to claim 2, characterized in that: The molar ratio of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane described in S3 to intermediate 2 is (1.1-2.0):

1.

8. The washable flame-retardant cotton fabric according to claim 2, characterized in that: The average molecular weight of the NP-Si type flame retardant is 3000 to 30000 Da.

9. The method for preparing a washable flame-retardant cotton fabric according to any one of claims 1 to 8, characterized in that: The following steps are involved: (1) Pretreatment of the cotton fabric surface: rinsing the cotton fabric in a sodium hydroxide solution to activate the cotton fabric surface; then repeatedly washing with deionized water until the washing solution is nearly neutral, and drying for later use; (2) Preparation of flame retardant finishing liquid: Weigh NP-Si type flame retardant and dissolve it in 75% to 95% by volume of ethanol solution to obtain a flame retardant finishing liquid with a mass concentration of 20% to 40%; (3) The flame retardant finishing liquid obtained in (2) is used to flame retardantly finish cotton fabric by the process of padding, baking and drying to obtain washable flame retardant cotton fabric.

10. The method for preparing a washable flame-retardant cotton fabric according to claim 9, characterized in that: The concentration of the sodium hydroxide solution in step (1) is 1.0-2.5 mol / L, and the rinsing time is 12-18 h; the padding parameters in step (3) are: two dips and two pads, the padding pressure is 0.1-0.4 Pa, and the liquid carrying rate is 90%-120%; the baking temperature is 150-170° C., the baking time is 2-5 min; and the drying temperature is 90-100° C.

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