A bio-based core-shell flame retardant, a preparation method thereof and a preparation method of a composite material

Abstract

The application discloses a kind of bio-based core-shell flame retardant and preparation method and the preparation method of composite material thereof.The application is based on the condensation reaction of biomass resveratrol and hexachlorocyclotriphosphazene to prepare novel biomass-based polyphosphazene flame-retardant microspheres (PHRB), and further adopts self-sacrificial template method, in-situ growth three-dimensional (3D) cobalt layered double hydroxide (Co-LDH) on the surface of microspheres, successfully build 3D bio-based Co-LDH@PHR flame retardant with core-shell structure.The bio-based core-shell flame retardant obtained by the application has good thermal stability and high residual carbon content, high flame-retardant efficiency, good compatibility with the base material, and the flame-retardant polyurea composite material prepared by using the bio-based core-shell flame retardant as raw material still has excellent mechanical properties.The bio-based core-shell flame retardant described in the application has wide raw material sources, low cost, simple synthesis method, easy operation and easy large-scale production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer materials, in particular to a bio-based core-shell flame retardant, a preparation method thereof and a preparation method of a composite material. BACKGROUND

[0002] Under the background of global low-carbon development, the building and building material field pays more and more attention to the development of low-carbon energy-saving building materials. The promotion of green, low-carbon and high-quality buildings is in full swing. Polyurea (PUA) has many excellent properties such as wear resistance, waterproofness, corrosion resistance, strong adhesion, heat preservation and insulation due to its unique molecular chain structure. These properties make PUA an ideal energy-saving and low-carbon building material, which is widely used in various building and industrial scenes such as building waterproofing, chemical corrosion protection, floor wear resistance and surface decoration. However, PUA is inherently flammable, and its combustion can produce a large amount of smoke and toxic gases, posing a huge risk to human life and property. Therefore, it is necessary to use various flame retardant technologies to improve the flame retardant performance of PUA composite materials. Among them, how to develop green, low-cost and efficient flame retardants has become a major problem in the field of PUA flame retardants.

[0003] In recent years, various biomass compounds have been widely used in the field of flame retardants due to their advantages such as large yield, low cost, green environmental protection and strong renewability. Resveratrol (REV) is a non-flavonoid polyphenol compound that is abundant in nature. The mature industrialized extraction also makes it cost-effective. The unique polyhydroxy structure allows REV to covalently bond with other molecules and coordinate with various metal ions. The diversified synergistic complexation makes REV a potential biomass flame retardant raw material.

[0004] Hexachlorocyclotriphosphazene (HCCP) is a cyclic compound with alternating arrangement of phosphorus and nitrogen atoms in the backbone. The two flame-retardant elements of phosphorus and nitrogen enable it to play a flame-retardant role in both condensed and gaseous phases, so it is considered as a high-efficiency flame-retardant raw material integrating acid source and gas source. As a key precursor for the synthesis of phosphazene-based macromolecules, HCCP can be substituted with two highly active chlorine atoms on phosphorus by various nucleophilic reagents such as hydroxyl and amino compounds to broaden its functional applications. Patent CN 115433361A discloses a polyphosphazene derivative flame retardant for toughening flame-retardant polylactic acid and a preparation method. The prepared polyphosphazene derivative flame retardant has good thermal stability, high carbon residue content and self-extinguishing properties, which can significantly improve the flame-retardant performance and toughness of polylactic acid.

[0005] Layered bimetallic hydroxides (LDHs) are a newly emerging class of metal flame retardants in recent years, possessing advantages such as non-toxicity, low smoke, environmental friendliness, and low cost. In flame retardant applications, LDHs typically require high concentrations to effectively enhance the flame retardant properties of the matrix material. However, the high surface energy of LDH nanosheets increases their tendency to aggregate; excessive addition can reduce the thermal stability and mechanical properties of the composite material. Patent CN 103333366 A discloses a layered bimetallic hydroxide-based flame retardant and smoke suppressant, its preparation method, and flame retardant and smoke suppressant composite materials modified with it. The method uses hydrolysis products such as spirocyclic phosphate salts to modify LDH, improving its flame retardant and smoke suppressant properties while simultaneously improving the compatibility of LDH with the polymer matrix. Patent CN113150440A discloses a method for preparing flame-retardant polypropylene, using a co-precipitation method to modify β-cyclodextrin-intercalated LDH, imparting good dispersibility and flame retardant ability to LDH. Currently, there are few reports on improving the dispersibility and flame retardancy of LDH derivatives by constructing 3D functionalized LDH, especially the lack of reports on the effective use of biomass resveratrol and simple self-sacrifice method to construct efficient bio-based core-shell LDH flame retardants. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a bio-based core-shell flame retardant and its preparation method, as well as a method for preparing a flame-retardant polyurea composite material with the addition of the bio-based core-shell flame retardant. This significantly improves the flame-retardant and smoke-suppressing properties of the composite material, and also enhances its mechanical properties.

[0007] The present invention is achieved as follows: a bio-based core-shell flame retardant comprising resveratrol, hexachlorocyclotriphosphazene, 4,4'-dihydroxydiphenyl sulfone (BPS) and acetonitrile as raw materials.

[0008] The molar ratio of resveratrol, hexachlorocyclotriphosphazene, and 4,4'-dihydroxydiphenyl sulfone is 1:1.5:0.5; the mass ratio of the total mass of resveratrol, hexachlorocyclotriphosphazene, and 4,4'-dihydroxydiphenyl sulfone to acetonitrile is 2.5:78-79.

[0009] The preparation method of bio-based core-shell flame retardants is carried out according to the following steps:

[0010] (1) Dissolve resveratrol in acetonitrile, then add hexachlorocyclotriphosphazene and 4,4'-dihydroxydiphenyl sulfone to the above solution, and then sonicate the mixed solution at room temperature;

[0011] (2) After ultrasonic dispersion, triethylamine is slowly injected into the above solution to initiate a condensation reaction. The ultrasonic conditions and temperature are kept constant until the reaction ends. After the reaction ends, the precipitate is collected by centrifugation and washed repeatedly with acetonitrile, ethanol and deionized water respectively. Finally, the white product is freeze-dried to obtain biomass-based polyphosphazene flame retardant microspheres.

[0012] (3) Place cobalt nitrate hexahydrate in a glass beaker, pour in methanol to dissolve it completely, then add biomass-based polyphosphazene flame retardant microspheres to the beaker to obtain mixed solution A. Perform ultrasonic treatment on mixed solution A, and then stir the uniformly dispersed solution A under constant temperature oil bath conditions.

[0013] (4) Quickly pour the methanol solution B containing 2-methylimidazole into the final mixed solution A in step (3), and continue the reaction while keeping the stirring speed and temperature constant; then keep the reaction temperature constant and adjust the stirring speed to continue the reaction; after the reaction is completed, collect the precipitate by centrifugation and wash it several times with methanol, ethanol and deionized water respectively to obtain the purple product; finally, freeze-dry the purple product to obtain the bio-based core-shell flame retardant.

[0014] In step (1), the ultrasonic treatment conditions are: reaction temperature 20-25℃, ultrasonic power 100W, frequency 45KHz, and treatment time 15-30 minutes.

[0015] In step (2), the amount of triethylamine added is 5% of the mass of the acetonitrile solution; the reaction time is 5-7 hours; and the freeze-drying time is 48 hours.

[0016] In step (3), the mass ratio of biomass-based polyphosphazene flame retardant microspheres to cobalt nitrate hexahydrate is 1:0.4-0.5, and the mass ratio of the total mass of biomass-based polyphosphazene flame retardant microspheres and cobalt nitrate hexahydrate to the mass of methanol is 1:38-40.

[0017] In step (3), the ultrasonic treatment conditions are: reaction temperature 20-25℃, ultrasonic power 100W, frequency 45KHz) for 10-20 minutes; oil bath temperature of mixed solution A is 50-60℃; stirring conditions are 500RPM for 1 hour.

[0018] In step (4), the mass ratio of methanol solution B to mixed solution A is 1:0.9-1.1; the molar ratio of cobalt nitrate hexahydrate to 2-methylimidazolium is 3:8.

[0019] In step (4), after reacting for a period of time, the stirring speed is adjusted to 190-220 RPM, and the reaction is carried out for 6-7 hours.

[0020] A method for preparing flame-retardant polyurea composite materials using a bio-based core-shell flame retardant includes the following steps:

[0021] 1) Add polyetheramine to isophorone diisocyanate and carry out the prepolymerization reaction under nitrogen protection atmosphere while keeping the mixture stirred;

[0022] 2) Dissolve dimethylthiotoluene diamine and polyetheramine thoroughly in N,N-dimethylacetamide;

[0023] 3) Add the bio-based core-shell flame retardant to the N,N-dimethylacetamide mixed solution in step 2) and disperse it ultrasonically for 30-40 minutes;

[0024] 4) Add the ultrasonically treated N,N-dimethylacetamide mixed solution to the prepolymer reactant in step 1) to carry out the polymerization reaction. The reaction is completed in 5-10 minutes to obtain a polyurea solution.

[0025] 5) Finally, pour the reacted polyurea solution into a polytetrafluoroethylene mold and cure it in an oven at 70-90℃ for 24 hours to obtain the polyurea composite material.

[0026] The volume ratio of polyetheramine to isophorone diisocyanate in step 1) is 10-15:9-11; the mechanical stirring speed is 280-320 RPM; the prepolymerization reaction time is 60-70 min; and the reaction temperature is 60-70℃.

[0027] In step 2), the volume ratio of dimethylthiotoluene diamine, polyetheramine and N,N-dimethylacetamide is 5-6:15-20:20-25.

[0028] The bio-based core-shell flame retardant added in step 3) is 0.5%-5% of the total mass of the polyurea reactants.

[0029] Compared with existing technologies, this invention uses biomass resveratrol as the polymer monomer, which is green and environmentally friendly. The PHRB microspheres prepared by combining hexachlorocyclotriphosphazene and 4,4'-dihydroxydiphenyl sulfone are non-toxic and harmless, and have a low cost. Using PHRB microspheres as a precursor, a self-sacrificial template strategy is employed to generate 3D Co-LDH in situ on its surface, preparing a bio-based core-shell flame retardant with a core-shell structure. This method of constructing a 3D structure from two-dimensional LDHs improves the dispersibility of LDHs in the polymer matrix. An addition of approximately 3 wt% can significantly improve the flame retardant and smoke suppression properties of the composite material, and the mechanical properties of the composite material are also improved. The bio-based core-shell flame retardant obtained by this invention has good thermal stability and high residual carbon content, high flame retardant efficiency, and good compatibility with the substrate. The flame-retardant polyurea composite material prepared using this bio-based core-shell flame retardant as a raw material still exhibits excellent mechanical properties. The bio-based core-shell flame retardant described in this invention has widely available and low-cost raw materials, and the synthesis method is simple, convenient, and easy to mass-produce. Attached Figure Description

[0030] Figure 1 The infrared spectrum of the Co-LDH@PHRB flame retardant obtained in Example 1;

[0031] Figure 2 The XRD pattern of the Co-LDH@PHRB flame retardant obtained in Example 1 is shown below.

[0032] Figure 3 The infrared spectrum of the PHRB flame retardant obtained in Comparative Example 1 is shown. Detailed Implementation

[0033] Preparation of bio-based core-shell flame retardants:

[0034] (1) Dissolve 0.428 g of resveratrol in 100 mL of acetonitrile. Then add 0.435 g of hexachlorocyclotriphosphazene and 0.234 g of 4,4'-dihydroxydiphenyl sulfone to the above solution (molar ratio: HCCP:REV:BPS = 1:1.5:0.5). Then sonicate the mixture at room temperature (25 °C) (100 W, 45 kHz) for 30 minutes.

[0035] (2) After uniform ultrasonic dispersion, 5 mL of triethylamine was slowly injected into the above solution to initiate a condensation reaction. The ultrasonic conditions and temperature were kept constant, and the reaction was carried out for 6 hours. After the reaction was completed, the precipitate was collected by centrifugation (8000 rpm) for 3 minutes and washed multiple times with acetonitrile, ethanol and deionized water, respectively. Finally, the white product was freeze-dried for 48 hours.

[0036] (3) Place 0.873 g of cobalt nitrate hexahydrate in a 200 mL glass beaker and dissolve it in 50 mL of methanol. Then, add 0.2 g of PHRB to the beaker and sonicate for 15 minutes. Place the well-dispersed solution in an oil bath at 50 °C and 500 r / min and stir for 1 h.

[0037] (4) Subsequently, under the same stirring speed and temperature, 50 mL of a methanol solution containing 1.968 g of 2-methylimidazole was rapidly added to the PHRB dispersion. The reaction was continued for 1 h under these conditions. Then, the rotor speed was reduced to 210 RPM, and the reaction was continued for 7 h while maintaining the temperature. After the reaction was completed, the precipitate was collected by centrifugation at 8000 rpm for 3 minutes and washed several times with methanol, ethanol, and deionized water. The resulting bio-based core-shell flame retardant (Co-LDH@PHRB) was freeze-dried for 48 h.

[0038] The obtained bio-based core-shell flame retardant was tested, and the test results are as follows:

[0039] Analysis of infrared spectra Figure 1The absorption peaks at 2813 cm⁻¹ and 2723 cm⁻¹ are mainly due to the asymmetric stretching vibration of CH. Combined with the CO stretching vibration at 1139 cm⁻¹, these peaks indicate characteristic absorption caused by methanol solvent intercalation. Furthermore, the peak at 1591 cm⁻¹ is attributed to the bending vibration of water molecules, indicating the presence of water in the Co-LDH interlayer. The absorption bands related to Co are mainly concentrated in the 400–800 cm⁻¹ range, with a significant absorption peak at 429 cm⁻¹, which originates from the Co-O or Co-O-Co stretching and bending modes in the Co-LDH lattice.

[0040] In addition, from Figure 2 XRD analysis of Co-LDH@PHRB revealed that, unlike the amorphous structure of PHRB and the characteristic diffraction peaks of ZIF-67, the XRD pattern of Co-LDH@PHRB microspheres, after converting ZIF-67 to Co-LDH using a self-sacrificial template method, exhibited typical LDH characteristics. The diffraction peaks at 11.2°, 23.2°, 34.5°, 39.1°, and 60.0° corresponded to the (003), (006), (012), (015), and (110) crystal planes of cobalt-based LDHs, respectively. This pattern was consistent with that of Co-LDH (JCPDS 46-0605), and no characteristic diffraction peaks of ZIF-67 were observed, confirming the complete conversion of ZIF-67 on the surface of the PHRB microspheres to Co-LDH. These results confirm the successful preparation of a bio-based core-shell flame retardant.

[0041] Example 2

[0042] Preparation of flame-retardant polyurea composite materials:

[0043] Measure 10 mL of isophorone diisocyanate (IPDI) into a four-necked flask using a graduated cylinder. Then, measure 15 mL of polyetheramine into a constant-pressure separatory funnel. Slowly add the polyetheramine dropwise into the four-necked flask and perform a prepolymerization reaction at 60°C with mechanical stirring (300 RPM) for 1 hour under nitrogen protection. Measure 5 mL of dimethylthiotoluene diamine and 15 mL of polyetheramine into a beaker. Then, measure 20 mL of N,N-dimethylacetamide solvent into the beaker to dissolve the dimethylthiotoluene diamine and polyetheramine evenly. Weigh 1 wt% of Co-LDH@PHRB powder and add it to the above N,N-dimethylacetamide mixed solution for ultrasonic dispersion for 30 minutes. After the prepolymerization reaction is complete, slowly add the ultrasonicated solution dropwise into the prepolymer through a constant-pressure separatory funnel for post-polymerization reaction, which is completed after 10 minutes. Finally, the reacted polyurea solution was poured into a polytetrafluoroethylene mold and cured in an oven at 80°C for 24 hours to obtain the polyurea composite material (PUA / Co-LDH@PHRB 1.0).

[0044] Example 3

[0045] Adjusting the amount of flame retardant in Example 2 and adding 2wt% Co-LDH@PHRB, a polyurea composite material (PUA / Co-LDH@PHRB 2.0) was obtained.

[0046] Example 4

[0047] Adjusting the amount of flame retardant in Example 2 and adding 3wt% Co-LDH@PHRB, a polyurea composite material (PUA / Co-LDH@PHRB 3.0) was obtained.

[0048] Comparative Example 1

[0049] Biomass-based resveratrol was used as a green flame retardant raw material. Combined with hexachlorocyclotriphosphazene and 4,4'-dihydroxydiphenyl sulfone, three flame retardant elements (P, N, and S) were introduced. Biomass-based polyphosphazene flame retardant microspheres (PHRB) were prepared via condensation polymerization.

[0050] (1) Dissolve 0.428 g of resveratrol in 100 mL of acetonitrile. Then add 0.435 g of hexachlorocyclotriphosphazene and 0.234 g of 4,4'-dihydroxydiphenyl sulfone to the above solution (molar ratio: HCCP:REV:BPS = 1:1.5:0.5). Then sonicate the mixture at room temperature (25 °C) (100 W, 45 kHz) for 30 minutes.

[0051] (2) After uniform ultrasonic dispersion, 5 mL of triethylamine was slowly injected into the above solution to initiate a condensation reaction. The ultrasonic conditions and temperature were kept constant, and the reaction was carried out for 6 hours. After the reaction was completed, the precipitate was collected by centrifugation (8000 rpm) for 3 minutes and washed multiple times with acetonitrile, ethanol and deionized water, respectively. Finally, the white product was freeze-dried for 48 hours.

[0052] The obtained PHRB flame-retardant microspheres were characterized, and the test results are as follows:

[0053] Infrared spectroscopy analysis Figure 3The absorption peaks at 1260-1210 cm⁻¹ and 875 cm⁻¹ correspond to the stretching vibrations of the PN and P=N bonds on the polyphosphazene ring, respectively. The absorption peaks of REV and BPS at 1260-1210 cm⁻¹ are complex and cannot be identified, but a significant enhancement of the corresponding PHRB peak at this location is still observed. The peaks of REV and BPS at 1500-1600 cm⁻¹ are attributed to the stretching vibrations of the aromatic ring skeleton, which are fully reflected in the PHRB spectrum. The new peak at 960 cm⁻¹ is generated by the resonance absorption of PO₄⁻(Ph). The peak at 611 cm⁻¹ in the HCCP spectrum is attributed to the stretching vibrations of P-Cl, while the P-Cl peak completely disappears at this point in the PHRB spectrum, indicating that most of the P-Cl in HCCP participated in the polymerization reaction, forming condensation products. The figure also shows that the hydroxyl vibration peaks of REV and BPS appear at 3200-3370 cm⁻¹ and 3300-3450 cm⁻¹, respectively, while in the PHRB band, these peaks show a significant shift and contraction, indicating that most of the hydroxyl groups on REV and BPS have reacted with HCCP. Therefore, the FTIR results demonstrate the successful preparation of PHRB microspheres.

[0054] Comparative Example 2

[0055] Preparation method of Co-LDH:

[0056] (1) Place 0.873 g of cobalt nitrate hexahydrate in a 200 mL glass beaker, add 50 mL of methanol to dissolve it, and sonicate for 15 minutes. Place the well-dispersed solution in an oil bath at 50 °C and 500 r / min and stir for 1 h.

[0057] (2) Subsequently, under the same stirring speed and temperature, 50 mL of a methanol solution containing 1.968 g of 2-methylimidazole was rapidly added to the above solution. The reaction was continued for 1 h under these conditions. Then, the rotor speed was reduced to 210 RPM, and the reaction was continued for 7 h while maintaining the temperature. After the reaction was completed, the precipitate was collected by centrifugation at 8000 rpm for 3 minutes and washed several times with methanol, ethanol, and deionized water. The resulting purple product was freeze-dried for 48 hours to obtain Co-LDH.

[0058] Comparative Example 3

[0059] Preparation method of pure polyurea material without flame retardants:

[0060] No flame retardant was added, and other conditions were the same as in Example 2.

[0061] Comparative Example 4

[0062] Preparation method of polyurea composite material containing PHRB flame retardant:

[0063] Other conditions were the same as in Example 2, except that 1 wt% of PUA / Co-LDH@PHRB powder was replaced with 3 wt% of PHRB flame retardant.

[0064] Comparative Example 5

[0065] Preparation method of polyurea composite material containing Co-LDH:

[0066] Other conditions were the same as in Example 2, except that 1 wt% of PUA / Co-LDH@PHRB powder was replaced with 3 wt% Co-LDH.

[0067] The performance of the polyurea composite materials of Examples 2-4 and Comparative Examples 3-5 was tested and analyzed. The test data are shown in Table 1.

[0068] Table 1

[0069]

[0070] Table 1 shows that the LOI of pure PUA is 21.10%, classifying it as a flammable material. Adding three different flame retardants improved the LOI of the composite materials, with the PUA / Co-LDH@PHRB composite material with 3wt% flame retardant showing the best improvement. Cone calorimetry analysis revealed that Comparative Example 4 confirmed that PHRB has excellent heat release suppression capabilities, significantly reducing the peak heat release rate (PHRR) and total heat release (THR) of the composite material, but its smoke suppression effect needs further improvement. Comparative Example 5 confirmed that the addition of Co-LDH alone cannot effectively improve the heat release of the composite material, but it does have a certain effect on suppressing smoke release. Therefore, combining the data from Examples 2-4, it can be found that Co-LDH@PHRB effectively integrates the advantages of PHRB and Co-LDH, significantly enhancing the flame retardant properties of the PUA composite material while reducing smoke release during combustion. Furthermore, comparing Example 4 and Comparative Example 5, it can be seen that the addition of pure Co-LDH leads to a decrease in the mechanical properties of the composite material. However, the 3D structure of Co-LDH@PHRB created using the self-sacrificial template method minimizes the aggregation tendency of LDH nanosheets, promotes the uniform dispersion of Co-LDH@PHRB in the PUA matrix, and further improves the mechanical properties of the composite material.

[0071] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a bio-based core-shell flame retardant, characterized in that: Follow these steps: (1) Resveratrol is dissolved in acetonitrile, and then hexachlorocyclotriphosphazene and 4,4'-dihydroxydiphenyl sulfone are added to the above solution. The mixed solution is then sonicated at room temperature. The molar ratio of resveratrol, hexachlorocyclotriphosphazene and 4,4'-dihydroxydiphenyl sulfone is 1:1.5:0.

5. (2) After ultrasonic dispersion, triethylamine is slowly injected into the above solution to initiate a condensation reaction. The ultrasonic conditions and temperature are kept constant until the reaction ends. After the reaction ends, the precipitate is collected by centrifugation and washed repeatedly with acetonitrile, ethanol and deionized water respectively. Finally, the white product is freeze-dried to obtain biomass-based polyphosphazene flame retardant microspheres. (3) Place cobalt nitrate hexahydrate in a glass beaker, pour in methanol to dissolve it completely, then add biomass-based polyphosphazene flame retardant microspheres to the beaker to obtain mixed solution A. Sonicate mixed solution A, and then stir the uniformly dispersed solution A under constant temperature oil bath conditions; the mass ratio of biomass-based polyphosphazene flame retardant microspheres to cobalt nitrate hexahydrate is 1:0.4-0.

5. (4) Quickly pour the methanol solution B containing 2-methylimidazole into the final mixed solution A in step (3), and continue the reaction while keeping the stirring speed and temperature constant; then keep the reaction temperature constant and adjust the stirring speed to continue the reaction; after the reaction is completed, collect the precipitate by centrifugation and wash it several times with methanol, ethanol and deionized water respectively to obtain a purple product; finally, freeze dry the purple product to obtain a bio-based core-shell flame retardant; the molar ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 3:

8.

2. The preparation method according to claim 1, characterized in that: In step (1), the ultrasonic treatment conditions are: reaction temperature 20-25℃, ultrasonic power 100 W, frequency 45 KHz, and treatment time 15-30 minutes.

3. The preparation method according to claim 1, characterized in that: In step (2), the amount of triethylamine added is 5% of the mass of the acetonitrile solution; the reaction time is 5-7 hours; and the freeze-drying time is 48 hours.

4. The preparation method according to claim 1, characterized in that: The total mass ratio of biomass-based polyphosphazene flame-retardant microspheres and cobalt nitrate hexahydrate to methanol is 1:38-40.

5. The preparation method according to claim 1, characterized in that: In step (3), the ultrasonic treatment conditions are as follows: reaction temperature 20-25℃, ultrasonic power 100 W, frequency 45 KHz for 10-20 minutes; oil bath temperature of mixed solution A is 50-60℃; stirring conditions are 500 RPM for 1 hour.

6. The preparation method according to claim 1, characterized in that: In step (4), the mass ratio of methanol solution B to mixed solution A is 1:0.9-1.

1.

7. The preparation method according to claim 1, characterized in that: In step (4), after reacting for a period of time, the stirring speed is adjusted to 190-220 RPM, and the reaction is carried out for 6-7 hours.

8. A method for preparing flame-retardant polyurea composite materials using a bio-based core-shell flame retardant obtained by the preparation method described in claim 1, characterized in that: Includes the following steps 1) Add polyetheramine to isophorone diisocyanate and carry out the prepolymerization reaction under nitrogen protection atmosphere while keeping the mixture stirred; 2) Dissolve dimethylthiotoluene diamine and polyetheramine thoroughly in N,N-dimethylacetamide; 3) Add the bio-based core-shell flame retardant to the N,N-dimethylacetamide mixed solution in step 2) and disperse it ultrasonically for 30-40 min; 4) Add the ultrasonically treated N,N-dimethylacetamide mixture to the prepolymer of step 1) for polymerization reaction. The reaction is completed in 5-10 minutes to obtain polyurea solution. 5) Finally, pour the reacted polyurea solution into a polytetrafluoroethylene mold and cure it in an oven at 70-90 ℃ for 24 hours to obtain the polyurea composite material.

9. The method according to claim 8, characterized in that: The volume ratio of polyetheramine to isophorone diisocyanate in step 1) is 10-15:9-11; the mechanical stirring speed is 280-320 RPM; the prepolymerization reaction time is 60-70 min; and the reaction temperature is 60-70 ℃. In step 2), the volume ratio of dimethylthiotoluene diamine, polyetheramine and N,N-dimethylacetamide is 5-6:15-20:20-25. The bio-based core-shell flame retardant added in step 3) is 0.5%-5% of the total mass of the polyurea reactants.

10. A bio-based core-shell flame retardant obtained by the preparation method according to claim 1, characterized in that: The preparation raw materials include resveratrol, hexachlorocyclotriphosphazene, 4,4'-dihydroxydiphenyl sulfone and acetonitrile.

11. The bio-based core-shell flame retardant according to claim 10, characterized in that: The total mass ratio of resveratrol, hexachlorocyclotriphosphazene, and 4,4'-dihydroxydiphenyl sulfone to acetonitrile is 2.5:78-79.

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