Bio-based core-shell flame retardant, preparation method thereof and preparation method of composite material

Abstract

The invention discloses a bio-based core-shell flame retardant, a preparation method thereof and a preparation method of a composite material. A novel biomass-based polyphosphazene flame-retardant microsphere (PHRB) is prepared based on a condensation reaction of biomass resveratrol and phosphonitrilic chloride trimer, a self-sacrifice template method is further adopted, three-dimensional (3D) cobalt layered double hydroxide (Co-LDH) grows on the surface of the microsphere in situ, and the 3D bio-based Co-LDH-PHR flame retardant with a core-shell structure is successfully constructed. The bio-based core-shell flame retardant obtained in the invention has good thermal stability, high residual carbon content, high flame retardant efficiency and good compatibility with a base material, and the flame-retardant polyurea composite material prepared from the bio-based core-shell flame retardant as a raw material still has excellent mechanical properties. The bio-based core-shell flame retardant disclosed by the invention is wide in raw material source, relatively low in cost, simple in synthesis mode, convenient to operate and easy for large-scale production.

Description

Technical Field

[0001] The invention relates to the technical field of polymer materials, and in particular to a bio-based core-shell flame retardant and a preparation method thereof, as well as a preparation method of a composite material. Background Art

[0002] In the context of global low-carbon development, the construction and building materials fields pay more and more attention to the development of low-carbon and energy-saving building materials. The promotion of green, low-carbon and high-quality buildings is gaining momentum. Polyurea (PUA) has many excellent properties such as wear resistance, waterproofness, corrosion resistance, strong adhesion, thermal insulation, etc. due to its unique molecular chain structure. These properties make PUA an ideal energy-saving and low-carbon building material, and it is widely used in various construction and industrial scenarios such as building waterproofing, chemical anti-corrosion, floor wear resistance, surface decoration, etc. However, PUA is inherently flammable, and its combustion will produce a large amount of smoke and toxic gases, posing a huge risk to human life and property. Therefore, it is necessary to adopt various flame retardant technologies to improve the flame retardant properties of PUA composites. Among them, how to develop green, low-cost and efficient flame retardants has become the main problem in the field of PUA flame retardants.

[0003] In recent years, various biomass compounds have been widely used in the flame retardant field due to their advantages of large output, low cost, green environmental protection, and strong renewability. Resveratrol (REV), as a non-flavonoid polyphenol compound, is abundant in nature, and mature industrial extraction also makes it low-cost. The unique polyhydroxy structure allows REV to act as an active site and covalently bond with other molecules, and coordinate with a variety of metal ions. The diverse synergistic compounding methods make REV a potential biomass flame retardant raw material.

[0004] Hexachlorocyclotriphosphazene (HCCP) is a cyclic compound with alternating phosphorus and nitrogen atoms in its skeleton. The two flame retardant elements of phosphorus and nitrogen enable it to play a flame retardant role in both the condensed phase and the gas phase. Therefore, it is considered to be a highly efficient flame retardant raw material that integrates acid source and gas source. As a key precursor for the synthesis of phosphazene-based macromolecules, HCCP can replace the two highly active chlorine atoms on phosphorus with 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 the characteristics of good thermal stability, high residual carbon content, self-extinguishing, etc., and can significantly improve the flame retardant properties and toughness of polylactic acid.

[0005] Layered double hydroxides (LDHs) are a new type of metal flame retardant that has emerged in recent years. They have the advantages of being non-toxic, low smoke, environmentally friendly and low cost. In flame retardant applications, LDHs usually require a higher addition concentration 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 will reduce the thermal stability and mechanical properties of the composite material. Patent CN 103333366 A discloses a layered double hydroxide-based flame retardant and smoke suppressant, a preparation method thereof, and a flame retardant and smoke suppressant composite material modified therewith. The hydrolysis product of spirocyclic phosphate salts is used to modify LDH to improve the flame retardant and smoke suppression properties of LDH, and at the same time improve the compatibility of LDH with the polymer matrix. Patent CN113150440A discloses a method for preparing flame retardant polypropylene. LDH intercalated with β-cyclodextrin is prepared by coprecipitation, which gives LDH good dispersibility and flame retardancy. At present, there are still few reports on improving the dispersibility and flame retardant ability of LDH derivatives by constructing 3D functionalized LDH, especially the effective use of biomass resveratrol and a simple self-sacrificial method to construct efficient bio-based core-shell LDH flame retardants. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a bio-based core-shell flame retardant and a preparation method thereof, as well as a preparation method of a flame-retardant polyurea composite material with the bio-based core-shell flame retardant added thereto. The flame retardant and smoke suppression properties of the composite material are significantly improved, and the mechanical properties of the composite material are also improved.

[0007] The invention is achieved in this way: a bio-based core-shell flame retardant, comprising resveratrol, hexachlorocyclotriphosphazene, 4,4'-dihydroxydiphenyl sulfone (BPS) and acetonitrile as preparation 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 the bio-based core-shell flame retardant is carried out according to the following steps:

[0010] (1) dissolving resveratrol in acetonitrile, adding hexachlorocyclotriphosphazene and 4,4'-dihydroxydiphenyl sulfone to the solution, and then ultrasonically treating the mixed solution at room temperature;

[0011] (2) After ultrasonic dispersion is uniform, triethylamine is slowly injected into the above solution to initiate a polycondensation reaction; the ultrasonic conditions and temperature are kept unchanged until the reaction is completed, and after the reaction is completed, the precipitate is collected by centrifugation and washed with acetonitrile, ethanol and deionized water in turn for multiple times; finally, the white product is freeze-dried to obtain biomass-based polyphosphazene flame-retardant microspheres.

[0012] (3) placing cobalt nitrate hexahydrate in a glass beaker, pouring methanol into it to fully dissolve it, then adding biomass-based polyphosphazene flame retardant microspheres into the beaker to obtain a mixed solution A, ultrasonically treating the mixed solution A, and then stirring the uniformly dispersed solution A in a constant temperature oil bath;

[0013] (4) quickly pouring the methanol solution B containing 2-methylimidazole into the mixed solution A at the end of step (3), maintaining the stirring speed and temperature unchanged to continue the reaction; then maintaining the reaction temperature unchanged, adjusting the stirring speed to continue the reaction; after the reaction, collecting the precipitate by centrifugation and washing it with methanol, ethanol and deionized water in turn for multiple times to obtain a purple product; finally, freeze-drying the purple product to obtain a bio-based core-shell flame retardant.

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

[0015] In the 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 the step (3), the mass ratio of the 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 the biomass-based polyphosphazene flame retardant microspheres and cobalt nitrate hexahydrate to methanol is 1:38-40.

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

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

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

[0020] A method for preparing a flame-retardant polyurea composite material using a bio-based core-shell flame retardant, comprising the following steps

[0021] 1) adding polyetheramine to isophorone diisocyanate, and carrying out a prepolymerization reaction under stirring in a nitrogen atmosphere;

[0022] 2) fully dissolving dimethylthiotoluenediamine and polyether amine in N,N-dimethylacetamide;

[0023] 3) adding the bio-based core-shell flame retardant to the N,N-dimethylacetamide mixed solution of step 2) and performing ultrasonic dispersion for 30-40 minutes;

[0024] 4) adding the ultrasonically treated N,N-dimethylacetamide mixed solution to the prepolymerization reactant in step 1) to carry out polymerization reaction, and the reaction is completed for 5-10 minutes to obtain a polyurea solution;

[0025] 5) Finally, the reacted polyurea solution is poured into a polytetrafluoroethylene mold and cured in an oven at 70-90° C. for 24 hours to obtain a polyurea composite material.

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

[0027] In the step 2), the volume ratio of dimethylthiotoluenediamine, 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 the prior art, the present invention uses biomass resveratrol as a polymerization 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 low cost. PHRB microspheres are used as precursors, and 3D Co-LDH is generated in situ on their surface by a self-sacrificial template strategy to prepare 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. The addition amount of about 3wt% 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 the present invention has good thermal stability and high residual carbon content, high flame retardant efficiency, good compatibility with the substrate, and the flame-retardant polyurea composite material prepared with the bio-based core-shell flame retardant as a raw material still has excellent mechanical properties. The bio-based core-shell flame retardant described in the present invention has a wide range of raw materials, low cost, simple synthesis method, convenient operation, and easy large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0031] Figure 2 The XRD spectrum of the Co-LDH@PHRB flame retardant obtained in Example 1;

[0032] Figure 3 This is the infrared spectrum of the PHRB flame retardant obtained in Comparative Example 1. DETAILED DESCRIPTION

[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 treat the mixed solution with ultrasound (100 W, 45 KHz) at room temperature (25°C) for 30 minutes.

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

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

[0037] (4) Subsequently, 50 mL of a methanol solution containing 1.968 g of 2-methylimidazole was quickly added to the PHRB dispersion at the same stirring speed and temperature. The reaction was continued for 1 h under this condition. The rotor speed was then 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 hours.

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

[0039] By analyzing infrared spectroscopy Figure 1It can be seen that the absorption peaks at 2813cm-1 and 2723cm-1 are mainly caused by the asymmetric stretching vibration of CH. Combined with the CO stretching vibration at 1139cm-1, these peaks indicate the characteristic absorption caused by methanol solvent intercalation. In addition, the peak at 1591cm-1 is attributed to the bending vibration of water molecules, indicating the presence of water between the Co-LDH layers. The absorption bands related to the Co element are mainly concentrated in the range of 400 to 800cm-1, with a significant absorption peak at 429cm-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 shows that, unlike the amorphous structure of PHRB and the characteristic diffraction peaks of ZIF-67, after converting ZIF-67 into Co-LDH by the self-sacrificial template method, the XRD spectrum of Co-LDH@PHRB microspheres shows the typical characteristics of LDH. The diffraction peaks at 11.2°, 23.2°, 34.5°, 39.1° and 60.0° correspond to the (003), (006), (012), (015) and (110) crystal planes of cobalt-based LDHs, respectively. The spectrum is consistent with that of Co-LDH (JCPDS 46-0605), and no characteristic diffraction peaks of ZIF-67 appear, confirming that ZIF-67 on the surface of PHRB microspheres is completely converted into Co-LDH. The above results confirm the successful preparation of bio-based core-shell flame retardants.

[0041] Example 2

[0042] Preparation of flame retardant polyurea composite materials:

[0043] 10mL of isophorone diisocyanate (IPDI) was measured with a measuring cylinder and added to a four-necked flask. Then 15mL of polyetheramine was measured with a measuring cylinder and added to a constant pressure separatory funnel. The polyetheramine was slowly added dropwise to the four-necked flask and prepolymerized for 1h under mechanical stirring (300RPM). The reaction temperature was 60°C and nitrogen was introduced for protection. 5mL of dimethylthiotoluenediamine and 15mL of polyetheramine were measured and added to a beaker. Then 20mL of N,N-dimethylacetamide solvent was measured and added to the beaker to dissolve dimethylthiotoluenediamine and polyetheramine evenly. 1wt% of Co-LDH@PHRB powder was weighed and added to the above N,N-dimethylacetamide mixed solution for ultrasonic dispersion for 30min. After the prepolymerization was completed, the ultrasonic solution was added to a constant pressure separatory funnel and slowly added dropwise to the prepolymer for postpolymerization. The reaction was completed for 10min. Finally, the reacted polyurea solution was poured into a polytetrafluoroethylene mold and cured in an oven at 80°C for 24 hours to obtain a polyurea composite material (PUA / Co-LDH@PHRB 1.0).

[0044] Example 3

[0045] The amount of flame retardant in Example 2 was adjusted, and 2 wt % of Co-LDH@PHRB was added to obtain a polyurea composite material (PUA / Co-LDH@PHRB 2.0).

[0046] Example 4

[0047] The amount of flame retardant in Example 2 was adjusted, and 3 wt % of Co-LDH@PHRB was added to obtain a polyurea composite material (PUA / Co-LDH@PHRB 3.0).

[0048] Comparative Example 1

[0049] Biomass resveratrol is used as a green flame retardant raw material, combined with hexachlorocyclotriphosphazene and 4,4'-dihydroxydiphenyl sulfone to introduce three flame retardant elements P, N and S, and biomass-based polyphosphazene flame retardant microspheres (PHRB) are prepared through 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 treat the mixed solution with ultrasound (100 W, 45 KHz) at room temperature (25°C) for 30 minutes.

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

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

[0053] By infrared spectroscopy analysis Figure 3It can be seen that the absorption peaks at 1260-1210cm-1 and 875cm-1 correspond to the stretching vibrations of PN and P=N bonds on the polyphosphazene ring, respectively. The absorption peaks of REV and BPS at 1260-1210cm-1 are more complex and cannot be identified, but it can still be seen that the peaks corresponding to PHRB at this location are significantly enhanced. The peaks of REV and BPS at 1500-1600cm-1 are attributed to the stretching vibrations of the aromatic ring skeleton, which are fully reflected in the spectrum of PHRB, while the new peak at 960cm-1 is generated by the resonance absorption of PO-(Ph). The peak at 611cm-1 on the HCCP spectrum is attributed to the stretching vibration of P-Cl, while the peak of P-Cl on the PHRB spectrum completely disappears here, indicating that most of the P-Cl in HCCP participates in the polymerization reaction to form a polycondensation product. It can also be seen from the figure that the hydroxyl vibration peaks of REV and BPS appear at 3200-3370cm-1 and 3300-3450cm-1, respectively, while in the PHRB band, the peaks are significantly shifted and shrunk, indicating that most of the hydroxyl groups on REV and BPS reacted with HCCP. Therefore, the FTIR results show that PHRB microspheres were successfully prepared.

[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, and disperse by ultrasonic for 15 minutes. Place the evenly dispersed solution in an oil bath at 50°C and 500 r / min and stir for 1 hour.

[0057] (2) Subsequently, 50 mL of a methanol solution containing 1.968 g of 2-methylimidazole was quickly added to the above solution at the same stirring speed and temperature. The reaction was continued for 1 h under this condition. The rotor speed was then 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 obtained 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 retardant:

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

[0067] The performance test and analysis of the polyurea composite materials of Examples 2 to 4 and Comparative Examples 3 to 5 are performed, and the test data are shown in Table 1.

[0068] Table 1

[0069]

[0070] It can be seen from Table 1 that the LOI of pure PUA is 21.10%, which is a flammable material. After adding three different flame retardants, the LOI values ​​of the composite materials are improved, among which the PUA / Co-LDH@PHRB composite material with a flame retardant addition of 3wt% has the best improvement effect. Cone calorimetry analysis shows that Comparative Example 4 confirms that PHRB has a good ability to inhibit heat release, and can significantly reduce the peak heat release rate (PHRR) and total heat release (THR) of the composite material, but the smoke suppression effect needs to be further improved; Comparative Example 5 confirms that the addition of a single Co-LDH cannot effectively improve the heat release of the composite material, but it has a certain effect on inhibiting smoke release. Therefore, based on the data of Examples 2 to 4, it can be found that Co-LDH@PHRB effectively integrates the advantages of PHRB and Co-LDH, can significantly enhance the flame retardant properties of the PUA composite material, and at the same time reduces the smoke release during combustion. In addition, by comparing Example 4 with Comparative Example 5, it can be seen that the addition of pure Co-LDH will lead to a decrease in the mechanical properties of the composite material, while 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 above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A bio-based core-shell flame retardant, characterized in that: The method comprises taking resveratrol, hexachlorocyclotriphosphazene, 4,4'-dihydroxydiphenyl sulfone and acetonitrile as preparation raw materials.

2. The bio-based core-shell flame retardant according to claim 1, characterized in that: 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.

3. The method for preparing a bio-based core-shell flame retardant according to claim 1 or 2, characterized in that: Follow these steps: (1) dissolving resveratrol in acetonitrile, adding hexachlorocyclotriphosphazene and 4,4'-dihydroxydiphenyl sulfone to the solution, and then ultrasonically treating the mixed solution at room temperature; (2) After ultrasonic dispersion is uniform, triethylamine is slowly injected into the above solution to initiate a polycondensation reaction; the ultrasonic conditions and temperature are kept unchanged until the reaction is completed, and after the reaction is completed, the precipitate is collected by centrifugation and washed with acetonitrile, ethanol and deionized water in turn for multiple times; finally, the white product is freeze-dried to obtain biomass-based polyphosphazene flame-retardant microspheres. (3) placing cobalt nitrate hexahydrate in a glass beaker, pouring methanol into it to fully dissolve it, then adding biomass-based polyphosphazene flame retardant microspheres into the beaker to obtain a mixed solution A, ultrasonically treating the mixed solution A, and then stirring the uniformly dispersed solution A in a constant temperature oil bath; (4) quickly pouring the methanol solution B containing 2-methylimidazole into the mixed solution A at the end of step (3), maintaining the stirring speed and temperature unchanged to continue the reaction; then maintaining the reaction temperature unchanged, adjusting the stirring speed to continue the reaction; after the reaction, collecting the precipitate by centrifugation and washing it with methanol, ethanol and deionized water in turn for multiple times to obtain a purple product; finally, freeze-drying the purple product to obtain a bio-based core-shell flame retardant.

4. The preparation method according to claim 3, characterized in that: In the step (1), the ultrasonic treatment conditions are: reaction temperature 20-25°C, ultrasonic power 100W, frequency 45KHz, and treatment time 15-30 minutes.

5. The preparation method according to claim 3, characterized in that: In the 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.

6. The preparation method according to claim 3, characterized in that: In the step (3), the mass ratio of the 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 the biomass-based polyphosphazene flame retardant microspheres and cobalt nitrate hexahydrate to methanol is 1:38-40.

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

8. The preparation method according to claim 3, characterized in that: In the step (4), the mass ratio of the methanol solution B to the mixed solution A is 1:0.9-1.1; and the molar ratio of the cobalt nitrate hexahydrate to the 2-methylimidazole is 3:

8.

9. The preparation method according to claim 3, characterized in that: In the step (4), after a period of reaction, the stirring speed is adjusted to 190-220 RPM and the reaction is continued for 6-7 hours.

10. A method for preparing a flame-retardant polyurea composite material using the bio-based core-shell flame retardant according to claim 1, characterized in that: The following steps are included 1) adding polyetheramine to isophorone diisocyanate, and carrying out a prepolymerization reaction under stirring in a nitrogen atmosphere; 2) fully dissolving dimethylthiotoluenediamine and polyether amine in N,N-dimethylacetamide; 3) adding the bio-based core-shell flame retardant to the N,N-dimethylacetamide mixed solution of step 2) and performing ultrasonic dispersion for 30-40 minutes; 4) adding the ultrasonically treated N,N-dimethylacetamide mixed solution to the prepolymerization reactant in step 1) to carry out polymerization reaction, and the reaction is completed for 5-10 minutes to obtain a polyurea solution; 5) Finally, the reacted polyurea solution is poured into a polytetrafluoroethylene mold and cured in an oven at 70-90° C. for 24 hours to obtain a polyurea composite material.

11. The preparation method according to claim 9, characterized in that: The volume ratio of the polyetheramine to isophorone diisocyanate in step 1) is 10-15:9-11; the speed of the mechanical stirring is 280-320RPM, the prepolymerization time is 60-70min, and the reaction temperature is 60-70°C; In the step 2), the volume ratio of dimethylthiotoluenediamine, 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.

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