Phosphorus-nitrogen-boron synergistic smoke suppression flame retardant and preparation method thereof

Abstract

The invention belongs to the field of functional finishing of textiles, and particularly relates to a phosphorus-nitrogen-boron synergistic smoke suppression flame retardant and a preparation method thereof. The preparation method of the flame retardant comprises the following steps: firstly, adding cyanuric chloride, phosphoric acid and a solvent into a reaction device together, heating and stirring, adjusting the pH value by using ammonia water until the reaction is complete, and then carrying out reduced pressure distillation, purification and drying to obtain an intermediate product 1; the method comprises the following steps: adding boric acid, mannitol and a solvent into a reaction flask, heating and stirring, and after reaction, distilling and drying to obtain an intermediate product 2; and adding the intermediates 1 and 2 into a reaction flask containing a solvent, heating and stirring, adjusting the pH value by using ammonia water, and after the reaction is completed, distilling, purifying and drying to obtain the target flame retardant. The flame retardant contains three flame-retardant elements of phosphorus, nitrogen and boron which can play a synergistic flame-retardant role, so that the synthesized target flame retardant has an excellent flame-retardant effect.

Description

Technical Field

[0001] This invention belongs to the field of functional finishing of textiles, and specifically relates to a phosphorus-nitrogen-boron durable flame retardant and its preparation method. Background Technology

[0002] Cotton fabrics are widely chosen by shoppers due to their softness and comfort. However, because cotton fabrics have a very low limiting oxygen index, they are easily ignited when exposed to a source of ignition, which can lead to fires, causing personal injury and property damage. With increasing demands for material safety and environmental friendliness, the research and development of synthetic flame retardants has become increasingly important.

[0003] Currently, researchers have synthesized various series of flame retardants, such as halogen-based, nitrogen-based, phosphorus-based, and silicon-based flame retardants. However, because halogen-based flame retardants (such as bromine-based and chlorine-based flame retardants) release toxic hydrogen halide gases and persistent organic pollutants during combustion, they pose potential risks to the environment and human health. Therefore, the use of halogen-based flame retardants has been restricted or banned in many countries and regions. Single phosphorus-containing flame retardants often fail to achieve excellent flame retardant effects. In recent years, a large amount of research has focused on improving flame retardant efficiency through the synergistic effect of phosphorus with other environmentally friendly flame retardant elements (such as nitrogen, silicon, and boron). Currently, commonly used flame retardants are mainly phosphorus- and nitrogen-based. Phosphorus-nitrogen flame retardants have shown significant effectiveness in the field of cotton fabric flame retardancy, but these flame retardants have the drawback of releasing a large amount of smoke during the flame retardant process, which has not yet been effectively resolved. To address the problem of high smoke content released by conventional phosphorus-containing flame retardants, commonly used cotton flame retardants incorporate metal ions, organosilicon, organoboron, etc., into the system. While metal-based flame retardants effectively reduce heat release and smoke emissions, their application in cotton fabrics is often limited by fabric hardening, discoloration, and environmental issues related to metal ions. Silicon-based flame retardants reduce smoke release by promoting the formation of a dense char layer, but often sacrifice fabric softness, breathability, and wearing comfort, limiting their practical application in cotton textiles. Among commonly used low-smoke flame retardants, boron-based flame retardants are highly regarded for their superior performance. These flame retardants are popular not only for their high efficiency but also because they produce very little smoke and toxic substances during combustion, perfectly meeting the current market demand for green, environmentally friendly, and energy-efficient solutions. This invention synergistically combines phosphorus, nitrogen, and boron. Through the synergistic effect of boron on the phosphorus-nitrogen flame retardant system, it ensures flame retardant efficiency while significantly reducing smoke release, exhibiting excellent char formation and various physical properties. The flame retardant molecule has multiple reactive groups, enabling it to form stable covalent bonds with cotton fibers, ensuring reliable flame retardant performance even after multiple washes. Flame retardant properties can be achieved in fabrics using conventional finishing methods, shortening the process, reducing costs and energy consumption, and producing no toxic byproducts. This flame retardant does not release harmful formaldehyde during use, produces low smoke content during combustion, and exhibits good flame retardant performance and thermal stability, ensuring user health and safety while reducing environmental impact. This flame retardant has potential for industrialization research and has significant application value and social benefits. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a phosphorus-nitrogen-boron durable flame retardant and its preparation method. Based on the boron-containing structure, groups containing phosphorus and nitrogen elements are grafted onto it respectively. The synthesized flame retardant has high flame retardant performance, is green and pollution-free, and has high product purity.

[0005] The technical solution adopted is as follows: A phosphorus-nitrogen-boron durable flame retardant, the chemical structure of which is as follows:

[0006] The preparation method of the above-mentioned phosphorus-nitrogen-boron durable flame retardant includes the following synthesis steps: (1) Add phosphoric acid and cyanuric chloride in a certain molar ratio together with solvent into the reaction apparatus, heat to 70-110℃ and stir until dissolved, adjust pH=6-7 with ammonia water, react for 9-12h, remove solvent by vacuum distillation, wash and purify and dry to obtain intermediate product 1; (2) Add boric acid and mannitol in a certain molar ratio together with the solvent into the reaction apparatus, stir until dissolved, heat to 90-110℃, react for 9-12 hours, and after the reaction is complete, distill and dry to obtain intermediate product 2; (3) Add intermediate 1, intermediate 2 and solvent into the reaction apparatus, heat to 110-120 °C and continue the reaction for 5-8 h, adjust the pH to 6-7 with ammonia water, after the reaction is complete, perform vacuum distillation and dry to constant weight to obtain the target flame retardant.

[0007] Preferably, the molar ratio of boric acid to mannitol in step (1) is 3:1.

[0008] Preferably, the solvent in step (1) is any one of toluene, acetonitrile, tetrahydrofuran, dichloromethane and 1,4-dioxane or a mixture thereof in any proportion.

[0009] Preferably, the molar ratio of phosphoric acid and cyanuric chloride in step (2) is 2:1.

[0010] Preferably, the solvent in step (2) is any one of water, toluene, ethanol, methanol, dichloromethane and N,N-dimethylformamide or a mixture thereof in any proportion.

[0011] Preferably, the molar ratio of intermediate 1 to intermediate 2 in step (3) is 3:1.

[0012] Preferably, the solvent in step (3) is any one of water, ethanol, acetonitrile, dichloromethane and N,N-dimethylformamide or a mixture thereof in any proportion.

[0013] Preferably, the method for synthesizing intermediate product 1 is as follows: phosphoric acid and cyanuric chloride are added together with a solvent into a reaction apparatus, heated to 90-110°C and stirred until dissolved, the pH is adjusted to 6-7 with ammonia water, the reaction is carried out for 9-12 hours, the solvent is removed by vacuum distillation, and then washed, purified and dried to obtain intermediate product 1.

[0014] Preferably, the method for synthesizing intermediate product 2 is as follows: boric acid and mannitol are added together with solvent into a reaction apparatus, stirred until dissolved, heated to 90-110°C, reacted for 9-12 hours, and after the reaction is complete, the intermediate product 2 is obtained by distillation and drying. Preferably, the method for synthesizing the target flame retardant is as follows: intermediate 1, intermediate 2 and solvent are added to a reaction apparatus, heated to 110-120 °C and reacted for 5-8 h, the pH is adjusted to 6-7 with ammonia water, after the reaction is complete, vacuum distillation is performed and dried to constant weight to obtain the target flame retardant.

[0015] Compared with the prior art, the advantages of the present invention are: This invention provides a phosphorus-nitrogen-boron durable flame retardant, which, as can be seen from its chemical structure, contains more phosphorus. On one hand, the boron content can reduce smoke release during the combustion of cotton fabrics, giving them a certain degree of smoke suppression; on the other hand, the excess reactive groups will act during combustion, resulting in a better and more efficient flame retardant effect. The preparation method provided by this invention has the advantages of simple synthesis process, mild conditions, no formaldehyde release, green and environmentally friendly, and high product purity. Attached Figure Description

[0016] Figure 1 SEM images of cotton fabrics and flame-retardant cotton fabrics before and after combustion: pure cotton, treated cotton, and char residue of treated cotton. Figure 2 Schematic diagram of vertical combustion of cotton fabrics under different flame retardant concentrations; Detailed Implementation

[0017] The accompanying drawings are for illustrative purposes only; it should be understood that common knowledge or prior art in the embodiments may be omitted; the examples mentioned below are only used to explain the present invention, in order to facilitate the description of the present invention and simplify the description, and therefore should not be construed as limiting the present invention.

[0018] The present invention will now be described in detail with reference to embodiments and comparative examples.

[0019] Example 1: In a 500 mL three-necked flask, add 27.66 g (0.15 mol) of cyanuric chloride, 29.4 g (0.3 mol) of phosphoric acid, and 200 mL of tetrahydrofuran. Heat the mixture to 75 °C. oC The reaction was stirred, and the pH of the system was adjusted to 6-7 with ammonia during the reaction. After the reaction was completed, the solvent was removed by rotary evaporation, the product was washed with ethyl acetate, and then dried to obtain intermediate product 1, with a yield of 80%.

[0020] In a 100 mL three-necked flask equipped with an oil-water separator and a spherical condenser, add 9.11 g (0.05 mol) of mannitol, 9.27 g (0.15 mol) of boric acid, and 40 mL of toluene, and heat to 110 °C. o C. Stir and reflux until no more water is generated, then stop the reaction. Remove the solvent toluene by vacuum distillation, wash with acetone, and dry under vacuum at 45 °C to give a white solid with a yield of about 86%, which is intermediate product 2.

[0021] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of water were added. The temperature was raised to 110 °C, and the pH of the system was adjusted to 6-7 with ammonia. The reaction was stirred for 5 h to obtain the target flame retardant with a yield of 81%.

[0022] The reaction apparatus at each stage is standard equipment used in actual operation, such as a three-necked flask, and therefore will not be described in detail.

[0023] Example 2: In a 500 mL three-necked flask, add 27.66 g (0.15 mol) of cyanuric chloride, 29.4 g (0.3 mol) of phosphoric acid, and 200 mL of toluene. Heat the mixture to 75 °C. oC The reaction was stirred, and the pH of the system was adjusted to 6-7 with ammonia during the reaction. After the reaction was completed, the solvent was removed by rotary evaporation, the product was washed with ethyl acetate, and then dried to obtain intermediate product 1, with a yield of 75%.

[0024] In a 100 mL three-necked flask equipped with an oil-water separator and a spherical condenser, add 9.11 g (0.05 mol) of mannitol, 9.27 g (0.15 mol) of boric acid, and 40 mL of toluene, and heat to 110 °C. o C. Stir and reflux until no more water is generated, then stop the reaction. Remove the solvent toluene by vacuum distillation, wash with acetone, and dry under vacuum at 45 °C to give a white solid with a yield of about 86%, which is intermediate product 2.

[0025] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of water were added. The temperature was raised to 110 °C, and the pH of the system was adjusted to 6-7 with ammonia. The reaction was stirred for 5 h to obtain the target flame retardant with a yield of 81%.

[0026] Example 3: In a 500 mL three-necked flask, add 27.66 g (0.15 mol) of cyanuric chloride, 29.4 g (0.3 mol) of phosphoric acid, and 200 mL of tetrahydrofuran. Heat the mixture to 75 °C. oC The reaction was stirred, and the pH of the system was adjusted to 6-7 with ammonia during the reaction. After the reaction was completed, the solvent was removed by rotary evaporation, the product was washed with ethyl acetate, and then dried to obtain intermediate product 1, with a yield of 80%.

[0027] In a 100 mL three-necked flask equipped with an oil-water separator and a spherical condenser, add 9.11 g (0.05 mol) of mannitol, 9.27 g (0.15 mol) of boric acid, and 40 mL of water, and heat to 110 °C. o C. Stir and reflux until no more water is generated, then stop the reaction. Remove the solvent toluene by vacuum distillation, wash with acetone, and dry under vacuum at 45 °C to give a white solid with a yield of about 76%, which is intermediate product 2.

[0028] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of water were added. The temperature was raised to 110 °C, and the pH of the system was adjusted to 6-7 with ammonia. The reaction was stirred for 5 h to obtain the target flame retardant with a yield of 81%.

[0029] Example 4: In a 500 mL three-necked flask, add 27.66 g (0.15 mol) of cyanuric chloride, 29.4 g (0.3 mol) of phosphoric acid, and 200 mL of tetrahydrofuran. Heat the mixture to 75 °C. oCThe reaction was stirred, and the pH of the system was adjusted to 6-7 with ammonia during the reaction. After the reaction was completed, the solvent was removed by rotary evaporation, the product was washed with ethyl acetate, and then dried to obtain intermediate product 1, with a yield of 80%.

[0030] In a 100 mL three-necked flask equipped with an oil-water separator and a spherical condenser, add 9.11 g (0.05 mol) of mannitol, 9.27 g (0.15 mol) of boric acid, and 40 mL of toluene, and heat to 110 °C. o C. Stir and reflux until no more water is generated, then stop the reaction. Remove the solvent toluene by vacuum distillation, wash with acetone, and dry under vacuum at 45 °C to give a white solid with a yield of about 86%, which is intermediate product 2.

[0031] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of ethanol were added. The temperature was raised to 110 °C, and the pH of the system was adjusted to 6-7 with ammonia. The reaction was stirred for 5 h to obtain the target flame retardant with a yield of 80%.

[0032] Example 5: In a 500 mL three-necked flask, add 27.66 g (0.15 mol) of cyanuric chloride, 29.4 g (0.3 mol) of phosphoric acid, and 200 mL of tetrahydrofuran. Heat the mixture to 90 °C. oC The reaction was stirred, and the pH of the system was adjusted to 6-7 with ammonia during the reaction. After the reaction was completed, the solvent was removed by rotary evaporation, the product was washed with ethyl acetate, and then dried to obtain intermediate product 1, with a yield of 83%.

[0033] In a 100 mL three-necked flask equipped with an oil-water separator and a spherical condenser, add 9.11 g (0.05 mol) of mannitol, 9.27 g (0.15 mol) of boric acid, and 40 mL of toluene, and heat to 110 °C. o C. Stir and reflux until no more water is generated, then stop the reaction. Remove the solvent toluene by vacuum distillation, wash with acetone, and dry under vacuum at 45 °C to give a white solid with a yield of about 86%, which is intermediate product 2.

[0034] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of water were added. The temperature was raised to 110 °C, and the pH of the system was adjusted to 6-7 with ammonia. The reaction was stirred for 5 h to obtain the target flame retardant with a yield of 81%.

[0035] Example 6: In a 500 mL three-necked flask, add 27.66 g (0.15 mol) of cyanuric chloride, 29.4 g (0.3 mol) of phosphoric acid, and 200 mL of tetrahydrofuran. Heat the mixture to 75 °C. oC The reaction was stirred, and the pH of the system was adjusted to 6-7 with ammonia during the reaction. After the reaction was completed, the solvent was removed by rotary evaporation, the product was washed with ethyl acetate, and then dried to obtain intermediate product 1, with a yield of 80%.

[0036] In a 100 mL three-necked flask equipped with an oil-water separator and a spherical condenser, add 9.11 g (0.05 mol) of mannitol, 9.27 g (0.15 mol) of boric acid, and 40 mL of N,N-dimethylformamide, and heat to 110 °C. o C. Stir and reflux until no more water is generated, then stop the reaction. Remove the solvent toluene by vacuum distillation, wash with acetone, and dry under vacuum at 45 °C to give a white solid with a yield of about 76%, which is intermediate product 2.

[0037] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of water were added. The temperature was raised to 110 °C, and the pH of the system was adjusted to 6-7 with ammonia. The reaction was stirred for 5 h to obtain the target flame retardant with a yield of 81%.

[0038] Example 7: In a 500 mL three-necked flask, add 27.66 g (0.15 mol) of cyanuric chloride, 29.4 g (0.3 mol) of phosphoric acid, and 200 mL of tetrahydrofuran. Heat the mixture to 75 °C. oC The reaction was stirred, and the pH of the system was adjusted to 6-7 with ammonia during the reaction. After the reaction was completed, the solvent was removed by rotary evaporation, the product was washed with ethyl acetate, and then dried to obtain intermediate product 1, with a yield of 80%.

[0039] In a 100 mL three-necked flask equipped with an oil-water separator and a spherical condenser, add 9.11 g (0.05 mol) of mannitol, 9.27 g (0.15 mol) of boric acid, and 40 mL of toluene, and heat to 110 °C. o C. Stir and reflux until no more water is generated, then stop the reaction. Remove the solvent toluene by vacuum distillation, wash with acetone, and dry under vacuum at 45 °C to give a white solid with a yield of about 86%, which is intermediate product 2.

[0040] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of water were added. The temperature was raised to 120 °C, and the pH of the system was adjusted to 6-7 with ammonia. The reaction was stirred for 5 h to obtain the target flame retardant with a yield of 82%.

[0041] Example 8: In a 500 mL three-necked flask, add 27.66 g (0.15 mol) of cyanuric chloride, 29.4 g (0.3 mol) of phosphoric acid, and 200 mL of tetrahydrofuran. Heat the mixture to 75 °C. oC The reaction was stirred, and the pH of the system was adjusted to 6-7 with ammonia during the reaction. After the reaction was completed, the solvent was removed by rotary evaporation, the product was washed with ethyl acetate, and then dried to obtain intermediate product 1, with a yield of 80%.

[0042] In a 100 mL three-necked flask equipped with an oil-water separator and a spherical condenser, add 9.11 g (0.05 mol) of mannitol, 9.27 g (0.15 mol) of boric acid, and 40 mL of toluene, and heat to 110 °C. o C. Stir and reflux until no more water is generated, then stop the reaction. Remove the solvent toluene by vacuum distillation, wash with acetone, and dry under vacuum at 45 °C to give a white solid with a yield of about 86%, which is intermediate product 2.

[0043] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of acetonitrile were added. The temperature was raised to 110 °C, and the pH of the system was adjusted to 6-7 with ammonia. The reaction was stirred for 5 h to obtain the target flame retardant with a yield of 60%.

[0044] Comparative Example 1: The synthesis methods of intermediate product 1 and intermediate product 2 are the same as those in Example 1.

[0045] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of water were added. The temperature was raised to 60 °C, and the pH of the system was adjusted to 6-7 with ammonia. The reaction was stirred for 5 h, but the final flame retardant was not obtained.

[0046] Comparative Example 2: The synthesis methods of intermediate product 1 and intermediate product 2 are the same as those in Example 1.

[0047] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of acetonitrile were added. The temperature was raised to 80 °C, the pH of the system was adjusted to 6-7 with ammonia, and the reaction was stirred for 5 h. The final flame retardant was not obtained.

[0048] Comparative Example 3: The synthesis methods of intermediate product 1 and intermediate product 2 are the same as those in Example 1.

[0049] In a 500 mL three-necked flask equipped with a thermometer, magnetic stirrer and spherical condenser, 18.45 g of intermediate 1, 5.19 g of intermediate 2 and 200 mL of toluene were added. The temperature was raised to 110 °C, the pH of the system was adjusted to 6-7 with ammonia, and the reaction was stirred for 5 h. The final flame retardant was not obtained.

[0050] like Figure 1 As shown, the left image is a scanning electron microscope (SEM) image of untreated pure cotton fabric, the middle image is a SEM image of cotton fabric treated with the flame retardant TBMP, and the right image is a SEM image of the char residue remaining after burning the TBMP-treated cotton fabric. The raw cotton SEM image shows a low impurity content, a relatively smooth fiber surface, and naturally curled cotton fibers. Figure 1 It can be seen that the surface smoothness of the cotton fabric decreased after flame retardant treatment, and a large number of particles were observed adhering to it, indicating that the flame retardant effectively treated the cotton fabric. Furthermore, from... Figure 1 It can be seen that the fabric structure of the fabric treated with flame retardant was not significantly damaged after burning, and the original morphology of the cotton fibers was basically preserved. The amount of char residue after burning the cotton fabric was small, leaving only a small amount of white ash. After burning, the fiber surface of the cotton fabric treated with flame retardant was covered with a dense char layer, indicating that the flame retardant has good char-forming properties.

[0051] like Figure 2As shown, the fabric exhibited good charring properties after flame-retardant treatment. At a weight gain of 8.31%, the char length decreased from 30 cm to 8.8 cm. With further increases in flame retardant concentration, reaching a weight gain of 24.04%, the char length shortened to 6.3 cm, significantly reducing the damage to the cotton fabric. These data indicate that the flame retardant has a good catalytic charring effect, giving the cotton fabric excellent flame retardancy. Table 1 shows that when the weight gain was only 8.31%, the LOI value had already increased to 32.2%, a significant improvement compared to cotton's 18.0%, proving that the flame retardant exhibited a considerably high flame-retardant ability. As the flame retardant concentration gradually increased, the LOI value of the cotton fabric continued to rise. At a weight gain of 24.04%, the limiting oxygen index reached 46.9%, which greatly reduced the flammability of the cotton fabric. The flame retardant TBMP involved in this invention is a highly efficient flame retardant that can significantly improve the flame-retardant performance of cotton fabrics even at low concentrations.

[0052] Table 1 Flame retardant performance at different flame retardant concentrations Flame retardant concentration (g / L) Weight gain rate (%) LOI value (%) Afterburn time (s) Smoldering time (s) Length of charcoal residue (cm) 0 0 18.0 17.5 18.2 30 100 8.31 32.2 0 0 8.8 200 17.89 38.5 0 0 8.5 300 24.04 46.9 0 0 6.3

[0053] Those skilled in the art will recognize that the experimental examples shown herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various modifications to other aspects of the invention without departing from its spirit, based on the technical teachings disclosed herein, and these modifications are still within the scope of protection of the invention.

Claims

1. A phosphorus-nitrogen-boron durable flame retardant, characterized in that, The chemical structure of the phosphorus-nitrogen-boron durable flame retardant is as follows: 。 2. The method for preparing a phosphorus-nitrogen-boron durable flame retardant as described in claim 1, characterized in that, The synthesis steps are as follows: (1) Add phosphoric acid and cyanuric chloride in a certain molar ratio together with solvent into the reaction apparatus, heat to 70-110℃ and stir until dissolved, adjust pH=6-7 with ammonia water, react for 9-12h, remove solvent by vacuum distillation, wash and purify and dry to obtain intermediate product 1; (2) Add boric acid and mannitol in a certain molar ratio together with the solvent into the reaction apparatus, stir until dissolved, heat to 90-110℃, react for 9-12 hours, and after the reaction is complete, distill and dry to obtain intermediate product 2; (3) Add intermediate 1, intermediate 2 and solvent into the reaction apparatus, heat to 110-120 °C and continue to react for 5-8 h, adjust pH to 6-7 with ammonia water, after the reaction is complete, perform vacuum distillation and dry to constant weight to obtain the target flame retardant.

3. The method for preparing a phosphorus-nitrogen-boron durable flame retardant according to claim 2, characterized in that, In step (1), the molar ratio of phosphoric acid and cyanuric chloride is 2:

1.

4. The method for preparing a phosphorus-nitrogen-boron durable flame retardant according to claim 2, characterized in that, The solvent in step (1) is any one of toluene, acetonitrile, tetrahydrofuran, dichloromethane and 1,4-dioxane or a mixture thereof in any proportion.

5. The method for preparing a phosphorus-nitrogen-boron durable flame retardant according to claim 2, characterized in that, In step (2), the molar ratio of boric acid to mannitol is 3:

1.

6. The method for preparing a phosphorus-nitrogen-boron durable flame retardant according to claim 2, characterized in that, The solvent in step (2) is any one of water, toluene, ethanol, methanol, dichloromethane and N,N-dimethylformamide or a mixture thereof in any proportion.

7. The method for preparing a phosphorus-nitrogen-boron durable flame retardant according to claim 2, characterized in that, In step (3), the molar ratio of intermediate product 1 to intermediate product 2 is 3:

1.

8. The method for preparing a phosphorus-nitrogen-boron durable flame retardant according to claim 2, characterized in that, The solvent in step (3) is any one of water, ethanol, acetonitrile, dichloromethane and N,N-dimethylformamide or a mixture thereof in any proportion.

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