Melamine hydrobromide flame retardant and method for preparing the same

Abstract

The application belongs to the technical field of flame retardants, and particularly relates to a melamine hydrobromide flame retardant and a preparation method thereof. The flame retardant has a core-shell structure, the inner core is hydroxylated melamine hydrobromide surface-modified by a silane coupling agent, and the outer shell is a melamine resin synergistically enhanced by ammonium polyphosphate and oxidized cellulose nanofilament. The interface combination of the core-shell is strengthened through silanization treatment, and the spatial and temporal synergy of gas phase and condensed phase flame retardation is realized by using the composite shell. The obtained flame retardant has high thermal stability in a polymer matrix, can significantly improve the limiting oxygen index of the composite material after being added, and passes the UL-94 V-0 level test, and has a wide application prospect in the field of high polymer material flame retardation.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant technology, specifically relating to a melamine hydrobromide flame retardant and its preparation method. Background Technology

[0002] Melamine hydrobromide (MHB), a typical nitrogen-bromine synergistic flame retardant, has attracted widespread attention due to its high efficiency, low smoke, and environmental friendliness. Its flame-retardant mechanism is mainly based on the combined action of the gas and condensed phases: upon thermal decomposition, MHB simultaneously releases non-flammable ammonia, nitrogen, and hydrogen bromide free radical scavengers. Hydrogen bromide effectively interrupts the combustion chain reaction, while melamine residues promote the formation of an expanded char layer, acting as a barrier against heat and oxygen. This "gas-solid" two-phase flame-retardant mode makes it superior to single bromine or nitrogen-based flame retardants in polymers such as epoxy resins, polypropylene, and polyurethane.

[0003] Despite the aforementioned advantages, MHB still faces several key technical bottlenecks in practical applications, limiting its full performance and high-value-added applications. First, interfacial compatibility and dispersibility issues are prominent. As a polar inorganic salt, MHB has poor compatibility with most non-polar or weakly polar polymer matrices, leading to easy agglomeration and uneven dispersion within the matrix during processing. Second, the thermal decomposition initiation temperature of MHB needs to be precisely matched with the processing temperature window of the polymer matrix; too low a temperature leads to premature decomposition and failure, while too high a temperature results in a delayed flame-retardant effect. Finally, the flame-retardant efficacy of MHB alone still has room for improvement. Achieving higher flame-retardant ratings often requires larger addition amounts, which further exacerbates the negative impact on the material's mechanical properties. Therefore, overcoming these shortcomings is crucial to improving the overall performance of MHB flame retardants. Summary of the Invention

[0004] The purpose of this invention is to provide a melamine hydrobromide flame retardant and its preparation method to solve the above-mentioned technical problems.

[0005] To achieve the above-mentioned technical objectives, the technical solution of the present invention is as follows:

[0006] A melamine hydrobromide flame retardant and its preparation method, the specific steps of which are as follows:

[0007] S1. Disperse hydroxylated melamine in anhydrous ethanol, place at 10~20℃, add hydrobromic acid solution dropwise, after the addition is complete, stir at 5~10℃ for 2h, filter, collect filter cake, wash filter cake 3 times with pre-cooled anhydrous ethanol, and vacuum dry to obtain hydroxylated melamine hydrobromide.

[0008] S2. Add 3-aminopropyltriethoxysilane to ethanol-water, adjust the pH to 4-5, and stir at room temperature for 30 min to obtain a silanol solution; disperse hydroxylated melamine hydrobromide in anhydrous ethanol and sonicate for 30-40 min to obtain a dispersion; add the silanol solution to the dispersion and stir at 50-60℃ for 0.5-1 h; adjust the pH to 7.0-7.5 and stir at 70-75℃ for 1-2 h; after the reaction is complete, centrifuge, collect the precipitate, wash three times with ethanol, and vacuum dry to obtain silanized core powder;

[0009] S3. Disperse the silanized core powder in an OCNF / APP-melamine resin prepolymer solution and stir at 50-65℃ and 800-1200rpm for 0.5-1h to obtain a mixed dispersion. Add hydrochloric acid solution at a constant low speed to adjust the pH to 4.0-5.0. Stir at 90-95℃ for 3-4h, cool to room temperature, centrifuge, and wash the precipitate three times each with deionized water and ethanol. Disperse the washed precipitate in deionized water and spray dry to obtain melamine hydrobromide flame retardant.

[0010] As a further improvement, in step S1, the volume concentration of hydrobromic acid in the hydrobromic acid solution is 30-40%; the mass-to-volume ratio of the hydroxylated melamine to the hydrobromic acid solution is 1g:1.5-2.5mL.

[0011] As a further improvement, in step S2, the mass ratio of 3-aminopropyltriethoxysilane to hydroxylated melamine hydrobromide is 1:10~20; and the volume percentage of ethanol in the ethanol-water mixture is 80~90%.

[0012] As a further improvement, in step S3, the mass ratio of the silanized core powder to the solids in the OCNF / APP-melamine resin prepolymer solution is 1:0.2~0.5, wherein the solids in the OCNF / APP-melamine resin prepolymer solution is the total mass of melamine, paraformaldehyde, APP and OCNF contained therein; the constant low speed is specifically 0.5~1.0 mL / min.

[0013] As a further improvement, in step S3, the parameters of the spray drying are: inlet air temperature of 120~130℃, outlet air temperature of 60~70℃, and atomization pressure of 0.3MPa.

[0014] As a further improvement, the preparation method of the hydroxylated melamine is as follows: melamine and hydroxyl-containing haloalkanes are added to a mixed solvent of ethanol and water, mixed well, and the pH is adjusted to 8.5-9.5 with triethylamine under stirring. The mixture is reacted at 50-60℃ for 6-8 hours to obtain a reaction solution. After cooling the reaction solution to room temperature, it is poured into a large amount of ice water, filtered, and the precipitate is collected. The precipitate is washed three times each with deionized water and ice-cold ethanol, and then dried under vacuum to obtain hydroxylated melamine.

[0015] As a further improvement, the hydroxyl-containing haloalkane is bromoethanol or chloroethanol; the molar ratio of melamine to the hydroxyl-containing haloalkane is 1:1.2~1.5.

[0016] As a further improvement, the preparation method of the OCNF / APP-melamine resin prepolymer solution is as follows: melamine and paraformaldehyde are added to deionized water, the pH is adjusted to 8.5-9.0 with triethylamine, the temperature is raised to 85℃, and the mixture is stirred for 30-60 min to obtain a low-polymerization prepolymer solution; OCNF and APP are added to the low-polymerization prepolymer solution, the pH is adjusted to 7.0-7.5, and the mixture is stirred at 80-85℃ for 1.5-2.5 h to obtain the OCNF / APP-melamine resin prepolymer solution.

[0017] As a further improvement, the mass ratio of melamine, paraformaldehyde, APP and OCNF is 1:1.5~1.8:0.5~0.8:0.05~0.15.

[0018] The present invention also provides a melamine hydrobromide flame retardant.

[0019] Due to the adoption of the above technical solution, the beneficial effects of the present invention are as follows:

[0020] 1. The use of silanization to treat melamine hydrobromide enhances the interfacial bonding and compatibility between the core and shell: silane undergoes dehydration condensation with the hydroxyl groups on the surface of hydroxylated melamine hydrobromide to form strong covalent bonds, introducing an organosilane molecular layer on the core surface. The amino groups at the silane terminals generate strong electrostatic interactions or hydrogen bonds with the carboxyl groups on the OCNF surface during subsequent acidic coating, thereby strengthening the core-shell interfacial bonding. The introduced long organic silane chains improve the compatibility between the core and the organic polymer shell and enhance network interlocking, promoting uniform coating of the shell material and laying a chemical foundation for constructing a complete and dense core-shell structure.

[0021] 2. The modified design of the composite shell enhances and synergizes the flame-retardant function: The melamine resin shell is modified with oxidized cellulose nanofibers (OCNF) and ammonium polyphosphate (APP), creating a composite barrier with enhanced mechanical properties and flame-retardant function. The nanofiber network of OCNF plays a reinforcing and toughening role in the resin, making the shell denser and less prone to cracking. When heated, APP not only releases flame-retardant gases, but also catalyzes the dehydration and cross-linking reaction of the polymer substrate and itself, forming a continuous, strong, and expanding char layer. The two work synergistically with the melamine resin matrix, enabling the shell to effectively insulate heat and oxygen in the early stages of a fire, and inhibit dripping, significantly improving the flame-retardant performance of the condensed phase.

[0022] 3. The core-shell structure design achieves spatiotemporal synergy and efficient utilization of flame-retardant components: This invention combines a melamine hydrobromide core with a condensation phase mechanism as its main component with a composite resin shell with a condensation phase mechanism as its main component, constructing a hierarchical, multifunctional synergistic flame-retardant system. When heated, the outer composite shell first plays its role, forming a physical barrier. As the temperature rises, the core gradually decomposes, and the released hydrogen bromide gas can accurately enter the gas phase combustion zone, efficiently capturing free radicals. This achieves seamless connection and spatial synergy of the flame-retardant effect in the gas phase and condensation phase, avoiding the limitations of a single flame-retardant mechanism. Attached Figure Description

[0023] Figure 1 These are ATR-FTIR images of the silanized core powder prepared in Example 1;

[0024] Figure 2 Figure A shows the SEM results of the melamine hydrobromide flame retardant prepared in Example 1, Comparative Example 3, and Comparative Example 4. Figure B shows the SEM results of the melamine hydrobromide flame retardant prepared in Example 1, Figure C shows the SEM results of the melamine hydrobromide flame retardant prepared in Comparative Example 4, and Figure D shows the SEM results of the melamine hydrobromide flame retardant prepared in Comparative Example 4. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or manufacturer's conditions shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0026] Example 1: A method for preparing a melamine hydrobromide flame retardant, the specific steps of which are as follows:

[0027] 1. Melamine and bromoethanol were added to an ethanol-water solution (ethanol volume concentration of 80%), wherein the molar ratio of melamine to bromoethanol was 1:1.2, and the mixture was stirred until homogeneous. Triethylamine was added dropwise under continuous stirring to adjust the pH of the reaction solution to 9.0. The reaction system was heated to 55°C and stirred for 7 hours to obtain the reaction solution. The reaction solution was cooled to room temperature and slowly poured into 500 mL of ice water, resulting in the precipitation of a white precipitate. The precipitate was filtered and collected. The precipitate was washed three times each with deionized water and pre-cooled anhydrous ethanol. The washed solid was dried in a vacuum drying oven at 55°C for 18 hours to obtain hydroxylated melamine. The equation is shown below:

[0028] ;

[0029] 2. Take 60g of the hydroxylated melamine obtained above, disperse it in 300mL of anhydrous ethanol, and place it in a 15℃ water bath; while stirring, slowly add 120mL of hydrobromic acid solution (hydrobromic acid volume concentration of 30%); after the addition is complete, transfer the reaction system to a 7℃ environment and continue stirring for 2h; after the reaction is complete, filter, collect the filter cake, and wash it three times with pre-cooled anhydrous ethanol; place the filter cake in a 60℃ vacuum drying oven and dry for 15h to obtain hydroxylated melamine hydrobromide; the equation is shown below:

[0030] ;

[0031] 3. Add 2.64 g of 3-aminopropyltriethoxysilane (KH-550) to 36 mL of 85% (v / v) ethanol aqueous solution, adjust the pH of the mixture to 4.5 with dilute hydrochloric acid, and stir at 25 °C for 30 min to obtain a clear silanol solution; separately, disperse 40 g of hydroxylated melamine hydrobromide in 200 mL of anhydrous ethanol, and sonicate for 35 min to obtain a uniform dispersion; slowly add the silanol solution dropwise to the dispersion while stirring; after the addition is complete, stir at 50 °C for 1 h; adjust the pH of the mixture to 7.2 with dilute ammonia, transfer to a 75 °C oil bath, and stir for 1 h; after the reaction is complete, centrifuge, collect the precipitate, wash the precipitate three times with anhydrous ethanol, and vacuum dry at 60 °C for 12 h to obtain silanized core powder; the equation is shown below:

[0032]

[0033] ;

[0034] 4. Add melamine and paraformaldehyde to 100 mL of deionized water, and adjust the pH to 8.8 with triethylamine; heat to 85 °C, stir for 45 min until the solution changes from turbid to clear, to obtain a low-polymer prepolymer solution; add ammonium polyphosphate (APP, degree of polymerization 20~50) and oxidized cellulose nanofibers (OCNF) to the low-polymer prepolymer solution, wherein the mass ratio of melamine, paraformaldehyde, APP and OCNF is 1:1.6:0.6:0.1, adjust the pH to 7.5, and continue stirring at 85 °C for 2 h to obtain an OCNF / APP-melamine resin prepolymer solution;

[0035] The oxidized cellulose nanofibers are nanofibers of natural cellulose that have been oxidized to introduce carboxyl groups into the molecular chain. The carboxyl group content is 1.0 mmol / g and the fiber diameter is 15~30 nm.

[0036] In this step, APP is coated by a rapidly forming melamine-formaldehyde prepolymer network, and the pH of the system is adjusted to neutral. Under these conditions, the hydrolysis tendency of APP is significantly reduced, and its structure and flame retardant function can be kept stable in the prepolymer solution.

[0037] 5. Take the silanized core powder and disperse it in the OCNF / APP-melamine resin prepolymer solution. Stir at 65℃ and 1000rpm for 1h to obtain a mixed dispersion. Add 1.0mol / L hydrochloric acid solution dropwise at a rate of 0.75mL / min to adjust the pH of the system to 4.5. Heat the reaction system to 90℃ and stir for 3.5h. After the reaction is completed, cool to room temperature, centrifuge, and collect the precipitate. Wash the precipitate three times each with deionized water and ethanol alternately. Redisperse the final product in deionized water and spray dry it. The spray drying conditions are: inlet air temperature 125℃, outlet air temperature 65℃, and atomization pressure 0.3MPa. Melamine hydrobromide flame retardant is obtained.

[0038] In this step, the mass ratio of the silanized core powder to the solids in the OCNF / APP-melamine resin prepolymer solution is 1:0.3; the solids in the OCNF / APP-melamine resin prepolymer solution is the total mass of melamine, paraformaldehyde, APP and OCNF contained therein.

[0039] Example 2: A method for preparing a melamine hydrobromide flame retardant, the specific steps of which are as follows:

[0040] 1. Melamine and chloroethanol were added to an ethanol-water solution (ethanol volume concentration of 80%), wherein the molar ratio of melamine to chloroethanol was 1:1.5, and the mixture was stirred until homogeneous. Triethylamine was added dropwise under continuous stirring to adjust the pH of the reaction solution to 8.5. The reaction system was heated to 50°C and stirred for 8 hours to obtain a reaction solution. The reaction solution was cooled to room temperature and slowly poured into 500 mL of ice water, resulting in the precipitation of a white precipitate. The precipitate was filtered and collected. The precipitate was washed three times each with deionized water and pre-cooled anhydrous ethanol. The washed solid was dried in a vacuum drying oven at 50°C for 24 hours to obtain hydroxylated melamine.

[0041] 2. Take 60g of the hydroxylated melamine prepared above, disperse it in 200mL of anhydrous ethanol, and place it in a 10℃ water bath; under stirring, slowly add 150mL of hydrobromic acid solution (hydrobromic acid volume concentration of 40%); after the addition is complete, transfer the reaction system to a 5℃ environment and continue stirring for 2h; after the reaction is completed, filter, collect the filter cake, and wash it three times with pre-cooled anhydrous ethanol; place the filter cake in a 65℃ vacuum drying oven and dry for 20h to obtain hydroxylated melamine hydrobromide;

[0042] 3. Add 4.0 g of 3-aminopropyltriethoxysilane (KH-550) to 36 mL of 90% (v / v) aqueous ethanol solution, adjust the pH of the mixture to 5 with dilute hydrochloric acid, and stir at 25 °C for 30 min to obtain a clear silanol solution; separately, disperse 40 g of hydroxylated melamine hydrobromide in 200 mL of anhydrous ethanol, and sonicate for 40 min to obtain a uniform dispersion; slowly add the silanol solution dropwise to the dispersion while stirring; after the addition is complete, stir at 60 °C for 0.5 h; adjust the pH of the mixture to 7.0 with dilute ammonia, transfer to a 73 °C oil bath, and stir for 2 h; after the reaction is complete, centrifuge to separate the precipitate, wash the precipitate three times with anhydrous ethanol, and vacuum dry at 65 °C for 20 h to obtain silanized core powder;

[0043] 4. Add melamine and paraformaldehyde to 100 mL of deionized water, and adjust the pH to 8.5 with triethylamine; heat to 85℃ and stir for 30 min until the solution changes from turbid to clear to obtain a low-polymer prepolymer solution; add ammonium polyphosphate (APP, degree of polymerization 20~50) and oxidized cellulose nanofibers (OCNF) to the low-polymer prepolymer solution, wherein the mass ratio of melamine, paraformaldehyde, APP and OCNF is 1:1.8:0.8:0.15, adjust the pH to 7.0, and continue stirring at 80℃ for 2.5 h to obtain an OCNF / APP-melamine resin prepolymer solution;

[0044] The oxidized cellulose nanofibers are nanofibers of natural cellulose that have been oxidized to introduce carboxyl groups into the molecular chain. The carboxyl group content is 1.5 mmol / g and the fiber diameter is 15~30 nm.

[0045] 5. Take the silanized core powder and disperse it in the OCNF / APP-melamine resin prepolymer solution. Stir at 50℃ and 1200rpm for 0.5h to obtain a mixed dispersion. Add 1.0mol / L hydrochloric acid solution dropwise at a rate of 1.0mL / min to adjust the pH of the system to 5.0. Heat the reaction system to 95℃ and stir for 4h. After the reaction is complete, cool to room temperature, centrifuge, and collect the precipitate. Wash the precipitate three times each with deionized water and ethanol alternately. Redisperse the final product in deionized water and spray dry it. The spray drying conditions are: inlet air temperature 120℃, outlet air temperature 70℃, and atomization pressure 0.3MPa. Melamine hydrobromide flame retardant is obtained.

[0046] In this step, the mass ratio of the silanized core powder to the solids in the OCNF / APP-melamine resin prepolymer solution is 1:0.2; the solids in the OCNF / APP-melamine resin prepolymer solution is the total mass of melamine, paraformaldehyde, APP and OCNF contained therein.

[0047] Example 3: A method for preparing a melamine hydrobromide flame retardant, the specific steps of which are as follows:

[0048] 1. Melamine and bromoethanol were added to an ethanol-water solution (ethanol volume concentration of 80%), wherein the molar ratio of melamine to bromoethanol was 1:1.3, and the mixture was stirred until homogeneous. Triethylamine was added dropwise under continuous stirring to adjust the pH of the reaction solution to 9.5. The reaction system was heated to 60°C and stirred for 6 hours to obtain a reaction solution. The reaction solution was cooled to room temperature and slowly poured into 500 mL of ice water, resulting in the precipitation of a white precipitate. The precipitate was filtered and collected. The precipitate was washed three times each with deionized water and pre-cooled anhydrous ethanol. The washed solid was dried in a vacuum drying oven at 60°C for 12 hours to obtain hydroxylated melamine.

[0049] 2. Take 60g of the hydroxylated melamine prepared above, disperse it in 200mL of anhydrous ethanol, and place it in a 20℃ water bath; under stirring, slowly add 90mL of hydrobromic acid solution (hydrobromic acid volume concentration of 35%); after the addition is complete, transfer the reaction system to a 10℃ environment and continue stirring for 2h; after the reaction is completed, filter, collect the filter cake, and wash it three times with pre-cooled anhydrous ethanol; place the filter cake in a 50℃ vacuum drying oven and dry it for 10h to obtain hydroxylated melamine hydrobromide;

[0050] 3. Add 2.0 g of 3-aminopropyltriethoxysilane (KH-550) to 20 mL of 80% (v / v) ethanol aqueous solution, adjust the pH of the mixture to 4 with dilute hydrochloric acid, and stir at room temperature for 30 min to obtain a clear silanol solution; separately, disperse 40 g of hydroxylated melamine hydrobromide in 200 mL of anhydrous ethanol, sonicate for 30 min to obtain a uniform dispersion; slowly add the silanol solution dropwise to the dispersion while stirring; after the addition is complete, stir at 55 °C for 1 h; adjust the pH of the mixture to 7.5 with dilute ammonia, transfer to a 70 °C oil bath, and stir for 1.5 h; after the reaction is complete, centrifuge to separate the precipitate, wash the precipitate three times with anhydrous ethanol, and vacuum dry at 50 °C for 20 h to obtain silanized core powder;

[0051] 4. Add melamine and paraformaldehyde to 100 mL of deionized water, and adjust the pH to 9.0 with triethylamine; heat to 85℃ and stir for 60 min until the solution changes from turbid to clear to obtain a low-polymer prepolymer solution; add ammonium polyphosphate (APP, degree of polymerization 20~50) and oxidized cellulose nanofibers (OCNF) to the low-polymer prepolymer solution, wherein the mass ratio of melamine, paraformaldehyde, APP and OCNF is 1:1.5:0.5:0.05, adjust the pH to 7.0, and continue stirring at 80℃ for 1.5 h to obtain an OCNF / APP-melamine resin prepolymer solution;

[0052] The oxidized cellulose nanofibers are nanofibers of natural cellulose that have been oxidized to introduce carboxyl groups into the molecular chain. The carboxyl group content is 1.0 mmol / g and the fiber diameter is 15~30 nm.

[0053] 5. Take the silanized core powder and disperse it in the OCNF / APP-melamine resin prepolymer solution. Stir at 60℃ and 800rpm for 1h to obtain a mixed dispersion. Add 1.0mol / L hydrochloric acid solution dropwise at a rate of 0.5mL / min to adjust the pH of the system to 4.0. Heat the reaction system to 93℃ and stir for 3h. After the reaction is complete, cool to room temperature, centrifuge, and collect the precipitate. Wash the precipitate three times each with deionized water and ethanol alternately. Redisperse the final product in deionized water and spray dry it. The spray drying conditions are: inlet air temperature 130℃, outlet air temperature 60℃, and atomization pressure 0.3MPa. Melamine hydrobromide flame retardant is obtained.

[0054] In this step, the mass ratio of the silanized core powder to the solids in the OCNF / APP-melamine resin prepolymer solution is 1:0.5; the solids in the OCNF / APP-melamine resin prepolymer solution is the total mass of melamine, paraformaldehyde, APP and OCNF contained therein.

[0055] Comparative Example 1: A method for preparing a melamine hydrobromide flame retardant, differing from Example 1 in that the melamine used to prepare the core powder was not modified by hydroxylation and silanization. The specific steps are as follows:

[0056] 1. Take 60g of melamine, disperse it in 300mL of anhydrous ethanol, and place it in a 15℃ water bath; while stirring, slowly add 120mL of hydrobromic acid solution (hydrobromic acid volume concentration of 30%); after the addition is complete, transfer the reaction system to a 7℃ environment and continue stirring for 2h; after the reaction is complete, filter, collect the filter cake, and wash it three times with pre-cooled anhydrous ethanol; place the filter cake in a 60℃ vacuum drying oven and dry it for 15h to obtain melamine hydrobromide;

[0057] 2. Melamine and paraformaldehyde were added to 100 mL of deionized water, and the pH was adjusted to 8.8 with triethylamine. The temperature was raised to 85 °C, and the mixture was stirred for 45 min until the solution changed from turbid to clear, thus obtaining a low-polymer prepolymer solution. Ammonium polyphosphate (APP, degree of polymerization 20~50) and oxidized cellulose nanofibers (OCNF) were added to the low-polymer prepolymer solution, wherein the mass ratio of melamine, paraformaldehyde, APP and OCNF was 1:1.6:0.6:0.1. The pH was adjusted to 7.5, and the mixture was stirred at 85 °C for 2 h to obtain an OCNF / APP-melamine resin prepolymer solution.

[0058] The oxidized cellulose nanofibers are nanofibers of natural cellulose that have been oxidized to introduce carboxyl groups into the molecular chain. The carboxyl group content is 1.0 mmol / g and the fiber diameter is 15~30 nm.

[0059] 3. Melamine hydrobromide was dispersed in an OCNF / APP-melamine resin prepolymer solution and stirred at 65℃ and 1000 rpm for 1 h to obtain a mixed dispersion. 1.0 mol / L hydrochloric acid solution was added dropwise at a rate of 0.75 mL / min to adjust the pH of the system to 4.5. The reaction system was heated to 90℃ and stirred for 3.5 h. After the reaction was completed, the mixture was cooled to room temperature, centrifuged, and the precipitate was collected. The precipitate was washed three times each with deionized water and ethanol, alternating between the two. The final product was redispersed in deionized water and spray-dried. The spray-drying conditions were: inlet air temperature 125℃, outlet air temperature 65℃, and atomization pressure 0.3 MPa. Melamine hydrobromide flame retardant was obtained.

[0060] In this step, the mass ratio of melamine hydrobromide to the solids in the OCNF / APP-melamine resin prepolymer solution is 1:0.3; the solids in the OCNF / APP-melamine resin prepolymer solution is the total mass of melamine, paraformaldehyde, APP and OCNF contained therein.

[0061] Comparative Example 2: A method for preparing a melamine hydrobromide flame retardant, differing from Example 1 in that the melamine used to prepare the core powder was not silanized. The specific steps are as follows:

[0062] 1. Melamine and bromoethanol were added to an ethanol-water solution (ethanol volume concentration of 80%), wherein the molar ratio of melamine to bromoethanol was 1:1.2, and the mixture was stirred until homogeneous. Triethylamine was added dropwise under continuous stirring to adjust the pH of the reaction solution to 9.0. The reaction system was heated to 55°C and stirred for 7 hours to obtain a reaction solution. The reaction solution was cooled to room temperature and slowly poured into 500 mL of ice water, resulting in the precipitation of a white precipitate. The precipitate was filtered and collected. The precipitate was washed three times each with deionized water and pre-cooled anhydrous ethanol. The washed solid was dried in a vacuum drying oven at 55°C for 18 hours to obtain hydroxylated melamine.

[0063] 2. Take 60g of the hydroxylated melamine prepared above, disperse it in 300mL of anhydrous ethanol, and place it in a 15℃ water bath; under stirring, slowly add 120mL of hydrobromic acid solution (hydrobromic acid volume concentration of 30%); after the addition is complete, transfer the reaction system to a 7℃ environment and continue stirring for 2h; after the reaction is completed, filter, collect the filter cake, and wash it three times with pre-cooled anhydrous ethanol; place the filter cake in a 60℃ vacuum drying oven and dry for 15h to obtain hydroxylated melamine hydrobromide;

[0064] 3. Add melamine and paraformaldehyde to 100 mL of deionized water, and adjust the pH to 8.8 with triethylamine; heat to 85℃ and stir for 45 min until the solution changes from turbid to clear to obtain a low-polymer prepolymer solution; add ammonium polyphosphate (APP, degree of polymerization 20~50) and oxidized cellulose nanofibers (OCNF) to the low-polymer prepolymer solution, wherein the mass ratio of melamine, paraformaldehyde, APP and OCNF is 1:1.6:0.6:0.1, adjust the pH to 7.5, and continue stirring at 85℃ for 2 h to obtain an OCNF / APP-melamine resin prepolymer solution;

[0065] The oxidized cellulose nanofibers are nanofibers of natural cellulose that have been oxidized to introduce carboxyl groups into the molecular chain. The carboxyl group content is 1.0 mmol / g and the fiber diameter is 15~30 nm.

[0066] 4. Hydroxylated melamine hydrobromide was dispersed in an OCNF / APP-melamine resin prepolymer solution and stirred at 65℃ and 1000 rpm for 1 h to obtain a mixed dispersion. 1.0 mol / L hydrochloric acid solution was added dropwise at a rate of 0.75 mL / min to adjust the pH of the system to 4.5. The reaction system was heated to 90℃ and stirred for 3.5 h. After the reaction was completed, the mixture was cooled to room temperature, centrifuged, and the precipitate was collected. The precipitate was washed three times each with deionized water and ethanol, alternating between the two. The final product was redispersed in deionized water and spray-dried. The spray-drying conditions were: inlet air temperature 125℃, outlet air temperature 65℃, and atomization pressure 0.3 MPa. Melamine hydrobromide flame retardant was obtained.

[0067] In this step, the mass ratio of hydroxylated melamine hydrobromide to the solids in the OCNF / APP-melamine resin prepolymer solution is 1:0.3; the solids in the OCNF / APP-melamine resin prepolymer solution is the total mass of melamine, paraformaldehyde, APP and OCNF contained therein.

[0068] Comparative Example 3: A method for preparing a melamine hydrobromide flame retardant, differing from Example 1 in that OCNF was not used in the preparation of the melamine resin shell. The specific steps are as follows:

[0069] 1. Melamine and bromoethanol were added to an ethanol-water solution (ethanol volume concentration of 80%), wherein the molar ratio of melamine to bromoethanol was 1:1.2, and the mixture was stirred until homogeneous. Triethylamine was added dropwise under continuous stirring to adjust the pH of the reaction solution to 9.0. The reaction system was heated to 55°C and stirred for 7 hours to obtain a reaction solution. The reaction solution was cooled to room temperature and slowly poured into 500 mL of ice water, resulting in the precipitation of a white precipitate. The precipitate was filtered and collected. The precipitate was washed three times each with deionized water and pre-cooled anhydrous ethanol. The washed solid was dried in a vacuum drying oven at 55°C for 18 hours to obtain hydroxylated melamine.

[0070] 2. Take 60g of the hydroxylated melamine prepared above, disperse it in 300mL of anhydrous ethanol, and place it in a 15℃ water bath; under stirring, slowly add 120mL of hydrobromic acid solution (hydrobromic acid volume concentration of 30%); after the addition is complete, transfer the reaction system to a 7℃ environment and continue stirring for 2h; after the reaction is completed, filter, collect the filter cake, and wash it three times with pre-cooled anhydrous ethanol; place the filter cake in a 60℃ vacuum drying oven and dry for 15h to obtain hydroxylated melamine hydrobromide;

[0071] 3. Add 2.64 g of 3-aminopropyltriethoxysilane (KH-550) to 36 mL of 85% (v / v) ethanol aqueous solution, adjust the pH of the mixture to 4.5 with dilute hydrochloric acid, and stir at 25 °C for 30 min to obtain a clear silanol solution; separately, disperse 40 g of hydroxylated melamine hydrobromide in 200 mL of anhydrous ethanol, and sonicate for 35 min to obtain a uniform dispersion; slowly add the silanol solution dropwise to the dispersion while stirring; after the addition is complete, stir at 50 °C for 1 h; adjust the pH of the mixture to 7.2 with dilute ammonia, transfer to a 75 °C oil bath, and stir for 1 h; after the reaction is complete, centrifuge to separate the precipitate, collect the precipitate, wash the precipitate three times with anhydrous ethanol, and vacuum dry at 60 °C for 12 h to obtain silanized core powder;

[0072] 4. Add melamine and paraformaldehyde to 100 mL of deionized water, and adjust the pH to 8.8 with triethylamine; heat to 85℃ and stir for 45 min until the solution changes from turbid to clear to obtain a low-polymer prepolymer solution; add ammonium polyphosphate (APP, degree of polymerization 20~50) to the low-polymer prepolymer solution, wherein the mass ratio of melamine, paraformaldehyde and APP is 1:1.6:0.6, adjust the pH to 7.5, and continue stirring at 85℃ for 2 h to obtain an APP-melamine resin prepolymer solution;

[0073] 5. Take the silanized core powder and disperse it in the APP-melamine resin prepolymer solution. Stir at 65℃ and 1000rpm for 1h to obtain a mixed dispersion. Add 1.0mol / L hydrochloric acid solution dropwise at a rate of 0.75mL / min to adjust the pH of the system to 4.5. Heat the reaction system to 90℃ and stir for 3.5h. After the reaction is complete, cool to room temperature, centrifuge, and collect the precipitate. Wash the precipitate three times each with deionized water and ethanol alternately. Redisperse the final product in deionized water and spray dry it. The spray drying conditions are: inlet air temperature 125℃, outlet air temperature 65℃, and atomization pressure 0.3MPa. Melamine hydrobromide flame retardant is obtained.

[0074] In this step, the mass ratio of the silanized core powder to the solids in the APP-melamine resin prepolymer solution is 1:0.3; the solids in the APP-melamine resin prepolymer solution is the total mass of melamine, paraformaldehyde and APP contained therein.

[0075] Comparative Example 4: A method for preparing a melamine hydrobromide flame retardant, differing from Example 1 in that APP was not used in the preparation of the melamine resin shell. The specific steps are as follows:

[0076] 1. Melamine and bromoethanol were added to an ethanol-water solution (ethanol volume concentration of 80%), wherein the molar ratio of melamine to bromoethanol was 1:1.2, and the mixture was stirred until homogeneous. Triethylamine was added dropwise under continuous stirring to adjust the pH of the reaction solution to 9.0. The reaction system was heated to 55°C and stirred for 7 hours to obtain a reaction solution. The reaction solution was cooled to room temperature and slowly poured into 500 mL of ice water, resulting in the precipitation of a white precipitate. The precipitate was filtered and collected. The precipitate was washed three times each with deionized water and pre-cooled anhydrous ethanol. The washed solid was dried in a vacuum drying oven at 55°C for 18 hours to obtain hydroxylated melamine.

[0077] 2. Take 60g of the hydroxylated melamine prepared above, disperse it in 300mL of anhydrous ethanol, and place it in a 15℃ water bath; under stirring, slowly add 120mL of hydrobromic acid solution (hydrobromic acid volume concentration of 30%); after the addition is complete, transfer the reaction system to a 7℃ environment and continue stirring for 2h; after the reaction is completed, filter, collect the filter cake, and wash it three times with pre-cooled anhydrous ethanol; place the filter cake in a 60℃ vacuum drying oven and dry for 15h to obtain hydroxylated melamine hydrobromide;

[0078] 3. Add 2.64 g of 3-aminopropyltriethoxysilane (KH-550) to 36 mL of 85% (v / v) ethanol aqueous solution, adjust the pH of the mixture to 4.5 with dilute hydrochloric acid, and stir at 25 °C for 30 min to obtain a clear silanol solution; separately, disperse 40 g of hydroxylated melamine hydrobromide in 200 mL of anhydrous ethanol, and sonicate for 35 min to obtain a uniform dispersion; slowly add the silanol solution dropwise to the dispersion while stirring; after the addition is complete, stir at 50 °C for 1 h; adjust the pH of the mixture to 7.2 with dilute ammonia, transfer to a 75 °C oil bath, and stir for 1 h; after the reaction is complete, centrifuge to separate the precipitate, collect the precipitate, wash the precipitate three times with anhydrous ethanol, and vacuum dry at 60 °C for 12 h to obtain silanized core powder;

[0079] 4. Add melamine and paraformaldehyde to 100 mL of deionized water, and adjust the pH to 8.8 with triethylamine; heat to 85 °C, stir for 45 min until the solution changes from turbid to clear, to obtain a low-polymer prepolymer solution; add oxidized cellulose nanofibers (OCNF) to the low-polymer prepolymer solution, wherein the mass ratio of melamine, paraformaldehyde and OCNF is 1:1.6:0.1, adjust the pH to 7.5, and continue stirring at 85 °C for 2 h to obtain an OCNF-melamine resin prepolymer solution;

[0080] The oxidized cellulose nanofibers are nanofibers of natural cellulose that have been oxidized to introduce carboxyl groups into the molecular chain. The carboxyl group content is 1.0 mmol / g and the fiber diameter is 15~30 nm.

[0081] 5. Take the silanized core powder and disperse it in the OCNF-melamine resin prepolymer solution. Stir at 65℃ and 1000rpm for 1h to obtain a mixed dispersion. Add 1.0mol / L hydrochloric acid solution dropwise at a rate of 0.75mL / min to adjust the pH of the system to 4.5. Heat the reaction system to 90℃ and stir for 3.5h. After the reaction is complete, cool to room temperature, centrifuge, and collect the precipitate. Wash the precipitate three times each with deionized water and ethanol alternately. Redisperse the final product in deionized water and spray dry it. The spray drying conditions are: inlet air temperature 125℃, outlet air temperature 65℃, and atomization pressure 0.3MPa. Melamine hydrobromide flame retardant is obtained.

[0082] In this step, the mass ratio of the silanized core powder to the solids in the OCNF-melamine resin prepolymer solution is 1:0.3; the solids in the OCNF-melamine resin prepolymer solution is the total mass of melamine, paraformaldehyde, and OCNF contained therein.

[0083] Comparative Example 5: A method for preparing a melamine hydrobromide flame retardant, differing from Example 1 in that melamine hydrobromide is used as the flame retardant in this method. The specific steps are as follows:

[0084] Take 10g of melamine, disperse it in 50mL of anhydrous ethanol, and place it in a 15℃ water bath; while stirring, slowly add 35mL of hydrobromic acid solution (hydrobromic acid volume concentration of 30%); after the addition is complete, transfer the reaction system to a 7℃ environment and continue stirring for 2h; after the reaction is complete, filter and collect the filter cake, wash it three times with pre-cooled anhydrous ethanol; place the filter cake in a 60℃ vacuum drying oven and dry it for 15h to obtain melamine hydrobromide.

[0085] Performance testing

[0086] 1. ATR-FTIR

[0087] The silanized core powder prepared in Example 1 was subjected to ATR-FTIR testing: as follows Figure 1 As shown, at 3400cm -1 The nearby peaks are attributed to the stretching vibrations of the OH and NH bonds; at 2950 cm⁻¹ -1 The nearby peaks represent stretching vibrations of the CH bonds, which originate from the introduction of the silane coupling agent molecular chain; at 1500 cm⁻¹... -1 The absorption peak corresponds to the bending vibration of the NH bond in the melamine skeleton and the silane amino group; at 1100 cm⁻¹ -1 The absorption peaks that appear are characteristic vibrational peaks of Si-OC or Si-O-Si bonds, providing direct evidence of a chemical grafting reaction between the silane coupling agent and the surface of hydroxylated melamine hydrobromide; 810 cm⁻¹ -1The peak at the point originates from the triazine ring of melamine; the results show that 3-aminopropyltriethoxysilane has been successfully grafted onto the surface of melamine hydrobromide via chemical bonds.

[0088] 2. SEM detection

[0089] The melamine hydrobromide flame retardants prepared in Example 1, Comparative Example 3 and Comparative Example 4 were subjected to SEM analysis to observe the core-shell structure and integrity of each sample.

[0090] Experimental results are as follows Figure 2 As shown in the figure; Figure A is the SEM detection result of the melamine hydrobromide flame retardant prepared in Example 1; Figure B is the SEM detection result of the melamine hydrobromide flame retardant prepared in Comparative Example 3; Figure C is the SEM detection result of the melamine hydrobromide flame retardant prepared in Comparative Example 4.

[0091] from Figure 2 As can be seen, the flame retardant particles prepared in Example 1 are complete and uniform spherical, with the particle surface covered by a continuous and dense resin shell, and the shell surface is generally flat. However, the flame retardant particles prepared in Comparative Example 3 are broken and incomplete, with some areas exposing the internal silanized core, indicating that the shell of Comparative Example 3 is more brittle. This result shows that without the fiber network reinforcement of OCNF, the single APP-melamine resin composite shell is prone to cracking under curing or stress, making it difficult to form a complete and dense coating layer. The flame retardant particles prepared in Comparative Example 4 can form a basic coating structure; however, its shell is thinner, and the density and mechanical strength of the shell may be lower.

[0092] 3. Thermogravimetric analysis (TGA)

[0093] Thermogravimetric analysis (TGA) was performed on the flame retardants prepared in Examples 1-3 and Comparative Examples 1-5. Thermogravimetric analysis was conducted under a nitrogen atmosphere using a thermogravimetric analyzer. The samples were placed in an alumina crucible and heated from 50°C to 800°C at a heating rate of 10°C / min. The initial decomposition temperature (T0) of the samples was recorded and analyzed. 5% ), temperature of maximum weight loss rate (T) max ) and the residual mass fraction (carbon residue) at 700℃;

[0094] The test results are shown in Table 1:

[0095] Table 1 Thermogravimetric Analysis Test Results

[0096] Test sample <![CDATA[T 5% (℃)]]> <![CDATA[T max (℃)]]> Carbon residue at 700℃ (wt%) Example 1 285 320 35.6 Example 2 282 318 35.5 Example 3 280 315 34.0 Comparative Example 1 255 295 28.1 Comparative Example 2 260 300 28.5 Comparative Example 3 240 310 26.0 Comparative Example 4 265 305 26.4 Comparative Example 5 240 282 22.0

[0097] As can be seen from Table 1, the flame retardants prepared in Examples 1-3 of the present invention have the highest initial decomposition temperature and the maximum weight loss rate temperature, indicating that their thermal stability is optimal; Example 1 has the highest char residue at 700℃, which is significantly higher than all comparative examples.

[0098] Comparative Examples 1 and 5 exhibited poor thermal stability and the lowest char residue, demonstrating the insufficient thermal stability of hydrobromide alone. While Comparative Example 2 showed better performance than Comparative Example 1, it was significantly lower than Example 1, indicating that silanization improved the interfacial bonding between the core and shell, promoting the formation of a more stable char layer. The char residue of Comparative Examples 3 and 4 was also lower than that of the Example, demonstrating that the OCNF fiber network contributes to the formation of a more robust char layer skeleton. The char formation reaction promoted by the decomposition of APP at high temperatures is crucial for increasing the char residue. The synergy between these two factors and the melamine resin matrix is ​​key to achieving optimal thermal stability and char formation performance. In summary, the complete core-shell structure and the synergistic effect of OCNF and APP are crucial for endowing the flame retardant of this invention with excellent thermal stability and high char formation ability.

[0099] 4. Limiting Oxygen Index (LOI)

[0100] Referring to the national standard GB / T 2406.2-2009 "Determination of Combustion Behavior by Oxygen Index Method for Plastics - Part 2: Room Temperature Test", the limiting oxygen index (LOI) of the flame retardants prepared in Examples 1-3 and Comparative Examples 1-5 was tested. Each sample was melt-blended with polypropylene resin (PP) at a dosage of 15 wt% on a two-roll mill, pressed into sheets, and prepared into strips of the same size (100 mm long, 10 mm wide, and 4 mm thick). The strips were vertically fixed in a combustion chamber, and a mixed gas with a controllable oxygen-nitrogen ratio was supplied from the bottom. The top of the strip was ignited using a top igniter. The minimum oxygen concentration required for the sample to maintain candle-like combustion in the nitrogen-oxygen mixed gas flow was determined by the "lifting method" and expressed as a volume percentage; this is the limiting oxygen index (LOI). The test results are shown in Table 2.

[0101] Table 2 Limiting Oxygen Index Test Results

[0102] Test sample LOI value (%) Example 1 31.8 Example 2 31.5 Example 3 31.0 Comparative Example 1 27.0 Comparative Example 2 27.5 Comparative Example 3 25.5 Comparative Example 4 25.0 Comparative Example 5 23.5 Pure PP base material 18.9

[0103] As shown in Table 2, the LOI values ​​of the PP composite materials with added flame retardants from Examples 1-3 of this invention all exceeded 30%, while the LOI values ​​of all comparative sample samples were significantly lower than those of the examples: the LOI value of Comparative Example 5 was only 23.5%, proving that the flame retardant effect of melamine hydrobromide alone was limited. The results of Comparative Examples 1 and 2 revealed the key role of silanization modification on the core surface in forming a strong and complete core-shell structure. Without effective modification of the core, the interfacial bonding will be weakened, the outer shell will be incomplete, and the flame retardant components will not achieve effective synergy. The results of Comparative Examples 3 and 4 showed that OCNF and APP have a clear synergistic effect in condensed phase flame retardancy: OCNF promotes the formation of a dense char layer, physically isolating heat and oxygen; APP catalyzes dehydration to form char and forms a glassy protective layer. The absence of either one will lead to a decrease in the quality of the char layer and a reduction in the flame retardant efficiency.

[0104] 5. Vertical Burning Test (UL-94)

[0105] According to the national standard GB / T 2408-2021 "Determination of Burning Performance of Plastics - Horizontal and Vertical Methods", the flame retardants prepared in Examples 1-3 and Comparative Examples 1-5 were subjected to vertical burning tests. Each sample was melt-blended with polypropylene resin (PP) at the same addition amount of 15 wt% on a two-roll mill, pressed into sheets, and prepared into strips of the same size (125 mm long, 13 mm wide, and 3.2 mm thick). Vertical burning tests were then conducted to determine the V-0, V-1, V-2, or NR (no rating) rating of each sample. V-0 is the highest flame retardant rating. The test results are shown in Table 3.

[0106] Table 3 Vertical Combustion Test Results

[0107] Test sample Did it drip and ignite? UL-94 rating Example 1 no V-0 Example 2 no V-0 Example 3 no V-0 Comparative Example 1 no V-1 Comparative Example 2 no V-1 Comparative Example 3 no V-1 Comparative Example 4 yes V-1 Comparative Example 5 yes V-2 Pure PP base material yes NR

[0108] As can be seen from Table 3, Examples 1-3 of the present invention all achieved the highest V-0 rating and showed no ignition dripping, demonstrating excellent resistance to flame spread and dripping. Comparative Example 5 showed severe dripping, confirming the limitations of melamine hydrobromide alone in vertical burning tests. Comparative Examples 1 and 2 achieved a V-1 rating, indicating that the unmodified melamine hydrobromide had weak interfacial bonding with the resin shell, making the shell prone to breakage or detachment in the early stages of combustion, leading to premature exposure and decomposition of the core and failure to achieve effective flame retardant synergy. Comparative Example 3 achieved V-1 but not V-0, indicating that the char layer without OCNF reinforcement was insufficient in strength and might crack under flame impact, resulting in a decrease in flame retardant protection. Comparative Example 4 was V-1 and showed dripping, proving that APP plays an important role in promoting the formation of a stable, drip-resistant expanded char layer.

[0109] In summary, this invention achieves excellent flame retardant performance of UL-94 V-0 rating by constructing a complete core-shell synergistic flame retardant system that works efficiently in both the gas and condensed phases.

[0110] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a melamine hydrobromide flame retardant, characterized in that, The specific steps are as follows: S1. Disperse hydroxylated melamine in anhydrous ethanol, place at 10~20℃, add hydrobromic acid solution dropwise, after the addition is complete, stir at 5~10℃ for 2h, filter, collect filter cake, wash filter cake 3 times with pre-cooled anhydrous ethanol, and vacuum dry to obtain hydroxylated melamine hydrobromide. S2. Add 3-aminopropyltriethoxysilane to ethanol-water, adjust the pH to 4-5, and stir at room temperature for 30 min to obtain a silanol solution. Hydroxylated melamine hydrobromide was dispersed in anhydrous ethanol and sonicated for 30-40 min to obtain a dispersion. A silanol solution was added to the dispersion and stirred at 50-60℃ for 0.5-1 h. The pH was adjusted to 7.0-7.5 and the reaction was stirred at 70-75℃ for 1-2 h. After the reaction was completed, the precipitate was collected by centrifugation, washed three times with ethanol, and dried under vacuum to obtain silanized core powder. S3. Disperse the silanized core powder in an OCNF / APP-melamine resin prepolymer solution and stir at 50-65℃ and 800-1200rpm for 0.5-1h to obtain a mixed dispersion. Add hydrochloric acid solution at a constant low speed to adjust the pH to 4.0-5.

0. Stir at 90-95℃ for 3-4h, cool to room temperature, centrifuge, and wash the precipitate three times each with deionized water and ethanol. Disperse the washed precipitate in deionized water and spray dry to obtain melamine hydrobromide flame retardant. The preparation method of the OCNF / APP-melamine resin prepolymer solution is as follows: melamine and paraformaldehyde are added to deionized water, the pH is adjusted to 8.5-9.0 with triethylamine, the temperature is raised to 85℃, and the mixture is stirred for 30-60 min to obtain a low-polymerization prepolymer solution; OCNF and APP are added to the low-polymerization prepolymer solution, the pH is adjusted to 7.0-7.5, and the mixture is stirred at 80-85℃ for 1.5-2.5 h to obtain the OCNF / APP-melamine resin prepolymer solution.

2. The method for preparing a melamine hydrobromide flame retardant according to claim 1, characterized in that, In step S1, the volume concentration of hydrobromic acid in the hydrobromic acid solution is 30-40%; the mass-to-volume ratio of the hydroxylated melamine to the hydrobromic acid solution is 1g:1.5-2.5mL.

3. The method for preparing a melamine hydrobromide flame retardant according to claim 1, characterized in that, In step S2, the mass ratio of 3-aminopropyltriethoxysilane to hydroxylated melamine hydrobromide is 1:10~20; the volume percentage of ethanol in the ethanol-water mixture is 80~90%.

4. The method for preparing a melamine hydrobromide flame retardant according to claim 1, characterized in that, In step S3, the mass ratio of the silanized core powder to the solids in the OCNF / APP-melamine resin prepolymer solution is 1:0.2~0.5, wherein the mass of the solids in the OCNF / APP-melamine resin prepolymer solution is the total mass of melamine, paraformaldehyde, APP and OCNF contained therein; the constant low speed is specifically 0.5~1.0 mL / min.

5. The method for preparing a melamine hydrobromide flame retardant according to claim 1, characterized in that, In step S3, the parameters for spray drying are: inlet air temperature of 120~130℃, outlet air temperature of 60~70℃, and atomization pressure of 0.3MPa.

6. The method for preparing a melamine hydrobromide flame retardant according to claim 1, characterized in that, The preparation method of the hydroxylated melamine is as follows: melamine and hydroxyl-containing haloalkanes are added to a mixed solvent of ethanol and water, mixed well, and the pH is adjusted to 8.5-9.5 with triethylamine under stirring. The mixture is reacted at 50-60℃ for 6-8 hours to obtain a reaction solution. After cooling the reaction solution to room temperature, it is poured into a large amount of ice water, filtered, and the precipitate is collected. The precipitate is washed three times each with deionized water and ice-cold ethanol, and then dried under vacuum to obtain hydroxylated melamine.

7. The method for preparing a melamine hydrobromide flame retardant according to claim 6, characterized in that, The hydroxyl-containing haloalkane is bromoethanol or chloroethanol; the molar ratio of melamine to the hydroxyl-containing haloalkane is 1:1.2~1.

5.

8. The method for preparing a melamine hydrobromide flame retardant according to claim 1, characterized in that, The mass ratio of melamine, paraformaldehyde, APP and OCNF is 1:1.5~1.8:0.5~0.8:0.05~0.

15.

9. The melamine hydrobromide flame retardant prepared by the preparation method according to any one of claims 1 to 8.

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