Core-shell flame-retardant composite material, preparation method therefor and use thereof

Abstract

The present invention relates to the technical field of flame-retardant materials, and relates to a core-shell flame-retardant composite material, a preparation method therefor and a use thereof. The present invention explores the design and synthesis of a novel core-shell flame retardant, so as to significantly improve the flame retardancy of TPU. The flame retardant is formed by using ammonium polyphosphate graft-modified lignin as a core and using a metal organic framework as a shell. The unique design not only makes full use of the good char-forming capability of lignin, but also further enhances the flame retardancy of the flame retardant by means of ammonium polyphosphate.

Description

A core-shell type flame-retardant composite material, its preparation method and application Technical Field

[0001] This invention belongs to the field of flame retardant materials technology, and relates to a core-shell type flame retardant composite material, its preparation method and application. Background Technology

[0002] Thermoplastic polyurethane elastomer (TPU) plays a vital role in electronics, automotive, medical, and textile industries due to its excellent mechanical properties, abrasion resistance, and processing performance. However, the inherent flammability of TPU materials poses a fire hazard in the event of electrical faults or high-temperature exposure, limiting its application in fields with extremely high safety requirements, such as aerospace and intelligent building systems. Therefore, improving the fire safety of TPU materials has become a key research focus.

[0003] Traditionally, flame retardant strategies mainly include the use of halogenated flame retardants, phosphorus-based flame retardants, and metal hydroxides. While these methods can improve the fire resistance of materials to some extent, they also introduce new problems. For example, halogenated flame retardants may release harmful gases during combustion, posing a potential threat to the environment and human health; and conventional inorganic flame retardants such as Al(OH)3 and Mg(OH)2 usually require large addition amounts to achieve the desired flame retardant effect, but this often reduces the mechanical properties of the material. In view of these problems, core-shell structured flame retardants have received widespread attention in recent years as a novel solution. This structure not only helps optimize the dispersion of the flame retardant in the TPU matrix, but also maximizes the flame retardant function of the core and shell layers through synergistic effects, thereby improving its flame retardant efficiency without affecting or even improving the overall performance of the material.

[0004] Meanwhile, lignin, as a natural aromatic polymer, is emerging as a promising environmentally friendly flame retardant due to its abundant benzene ring structure and excellent char-forming ability. During combustion, lignin forms a protective char layer, effectively preventing oxygen and heat from penetrating the material. Simultaneously, its pyrolysis releases inert gases such as CO2, H2O, and CO, which dilute the concentration of combustible gases in the combustion zone, lower the flame temperature, and inhibit the combustion reaction. However, pure lignin has relatively low flame retardant efficiency; its flame retardant properties can be further improved through chemical modification. This method not only solves the environmental pollution and mechanical property degradation problems caused by traditional flame retardants but also provides a new approach for developing efficient and environmentally friendly TPU flame-retardant materials. In conclusion, with the development of science and technology, the exploration of TPU flame-retardant modification continues to deepen, and wider safe applications are expected in the future.

[0005] An article published in the *Chemical Engineering Journal* ("Functionalizing lignin by in situ solid-phase grafting ammonium polyphosphate for enhancing thermal, flame-retardant, mechanical, and UV-resistant properties of polylactic acid," https: / / doi.org / 10.1016 / j.cej.2024.153429) describes the functionalization of lignin by in-situ solid-phase grafting of ammonium polyphosphate to enhance the thermal stability, flame retardancy, mechanical properties, and UV resistance of polylactic acid. This research employed an innovative in-situ solid-phase grafting method to react lignin with APP, thereby achieving lignin functionalization. This functionalized lignin, when added to a PLA matrix, not only effectively improved the flame retardant properties of PLA but also enhanced its thermal stability, mechanical strength, and UV resistance. Experimental results show that this method can effectively overcome some inherent defects of traditional PLA materials, such as flammability and poor weather resistance. Despite significant progress, there is still room for further optimization in terms of thermal stability when relying solely on ammonium polyphosphate grafting to modify lignin. This means that although existing modification strategies have greatly improved the performance of PLA, in certain specific applications, it may be necessary to explore more efficient modification techniques or additives to further enhance the thermal stability of the materials.

[0006] Chinese patent document (CN116715867A) discloses a lignin flame retardant, its preparation method, and flame-retardant composite material. When applied to PLA flame retardancy, only a 3% addition is needed to achieve the UL-94 V-0 rating. However, when this flame-retardant system is applied to TPU, the flame-retardant performance decreases significantly, requiring an addition of 15% to achieve the UL-94 V-0 rating. Excessive addition of the flame retardant severely affects the mechanical properties of the TPU composite material. Therefore, modifying the lignin flame retardant is essential for achieving high-efficiency flame retardancy in TPU, enabling excellent flame-retardant effects with minimal addition and reducing the impact on the mechanical properties of the composite material. Technical issues

[0007] This invention proposes a core-shell type flame-retardant composite material, which uses ammonium polyphosphate-grafted modified lignin as the core and a metal-organic framework as the shell. It can improve the flame-retardant performance of TPU through a synergistic flame-retardant mechanism, thus solving the technical problem of poor flame-retardant effect in current products. Technical solutions

[0008] The purpose of this invention is to provide a core-shell type flame-retardant composite material, its preparation method and application, wherein the core-shell type flame-retardant composite material can improve the flame-retardant performance of TPU through a synergistic flame-retardant mechanism.

[0009] The first objective of this invention can be achieved through the following technical solutions:

[0010] A core-shell type flame-retardant composite material, wherein the flame-retardant composite material utilizes ammonium polyphosphate-grafted modified lignin as the core and a metal-organic framework as the shell, and the flame-retardant composite material comprises the following raw materials in parts by mass: metal source, ammonium polyphosphate-grafted modified lignin, and organic ligand.

[0011] Preferably, the metal-organic framework includes one of ZIF-7, ZIF-8, and ZIF-9.

[0012] Further optimization yielded a metal-organic framework of ZIF-7.

[0013] The metal source and organic ligand are components for preparing the metal-organic framework. Preferably, the metal source includes at least one of zinc nitrate and cobalt nitrate. Zinc nitrate and cobalt nitrate can be hydrated or anhydrous.

[0014] Preferably, the organic ligand includes at least one of benzimidazole and 2-methylimidazole.

[0015] Preferably, the metal source is zinc nitrate and the organic ligand is benzimidazole.

[0016] Preferably, the mass ratio of the metal source, ammonium polyphosphate-grafted modified lignin, and organic ligand is 1:(2-3):(0.8-1.2).

[0017] Zinc nitrate, as the primary zinc source for generating metal-organic frameworks (such as ZIF-7), plays a crucial role in providing zinc ions in the reaction system. The specific mass ratio between zinc nitrate, ammonium polyphosphate-grafted modified lignin, and benzimidazole ensures that zinc ions can fully participate in subsequent coordination reactions, forming a stable and uniformly coated metal-organic framework layer. The ammonium polyphosphate-grafted modified lignin, acting as the core, not only forms a protective char layer during combustion due to its excellent char-forming ability, reducing heat and oxygen transfer, but also enhances its flame-retardant efficiency due to its combination with ammonium polyphosphate. Benzimidazole, as an organic ligand, participates in the coordination reaction with zinc ions, promoting the formation of the metal-organic framework. This invention, by finely adjusting the mass ratio of the three components, not only optimizes the proportions of each component in the final core-shell flame retardant but also enables precise control of the material's flame-retardant performance for different application scenarios.

[0018] The flame-retardant composite material is obtained by reacting ammonium polyphosphate-grafted modified lignin dispersed in a solution containing a metal source and an organic ligand. The solution includes N,N-dimethylformamide, and at least one of methanol, ethanol, n-propanol, isopropanol, and ethylene glycol. The reaction is carried out at 10–40°C for 10–100 min.

[0019] The second objective of this invention can be achieved through the following technical solutions:

[0020] A method for preparing a core-shell type flame-retardant composite material includes the following steps:

[0021] S1. Ammonium polyphosphate-grafted modified lignin is uniformly dispersed in N,N-dimethylformamide solution to obtain solution A;

[0022] S2. Disperse the metal source uniformly in N,N-dimethylformamide to obtain solution B;

[0023] S3. A solution A containing ammonium polyphosphate grafted modified lignin and a solution B containing a metal source are uniformly mixed, and then an organic solvent containing organic ligands is added to react. Finally, a core-shell flame retardant is obtained by solid-liquid separation.

[0024] The preparation process of this invention first utilizes an ion exchange reaction between zinc ions and ammonium ions in ammonium polyphosphate-grafted modified lignin. During this process, zinc ions are precisely positioned on the surface of the ammonium polyphosphate-grafted modified lignin. Subsequently, benzimidazole organic ligands are added to the system, and the zinc ions immediately undergo a coordination reaction with these organic ligands. This specific coordination effect promotes the formation of a metal-organic framework (MOF) that tightly coats the surface of the ammonium polyphosphate-grafted modified lignin. The final product formed through the above process is a unique core-shell flame retardant, with the ammonium polyphosphate-grafted modified lignin as the core and the MOF as the outer shell. As an effective intumescent flame retardant component, the ammonium polyphosphate-grafted modified lignin's core position facilitates the formation of a continuous and stable char layer at high temperatures, effectively preventing the transfer of heat and oxygen, thereby enhancing the flame retardant effect. The MOF, as the outer shell, not only provides an additional thermal barrier but also, due to its special porous structure, can adsorb or capture some harmful substances generated during combustion, further reducing the fire risk. Furthermore, this core-shell structure design helps improve the dispersion of flame retardants in the polymer matrix, thereby enhancing the overall performance of the composite material.

[0025] In the above-mentioned method for preparing a core-shell type flame-retardant composite material, preferably, the modified lignin grafted with ammonium polyphosphate is prepared by adding phosphoric acid and urea to phosphoric acid and then heating and reacting.

[0026] In the preparation of the above-mentioned ammonium polyphosphate grafted modified lignin, preferably, the mass ratio of phosphorylated lignin, urea, and phosphoric acid is (8-10):(10-15):(15-25). Preferably, the mass fraction of phosphoric acid is 10-80 wt%. Preferably, the heating reaction specifically includes: reacting at 100-150℃ for 10-30 min, and then continuing the reaction at 210-250℃ for 2-3 h. Optionally, the preparation method of phosphorylated lignin includes: dispersing lignin, urea, and ammonium dihydrogen phosphate in a mass ratio of 1:(2-3):(2-3) in water, reacting, drying, and then further reacting, followed by soaking in an acid solution, and finally obtaining phosphorylated lignin after centrifugation, washing, and drying.

[0027] The specific method for preparing phosphorylated lignin includes: dispersing lignin, urea and ammonium dihydrogen phosphate in water at a mass ratio of 1:(2-3):(2-3), reacting at 50-100℃ for 0.5-2h, drying, then reacting at 120-200℃ for 0.5-2h, then soaking in an acid solution, and finally obtaining phosphorylated lignin after centrifugation, washing and drying.

[0028] In the above-mentioned method for preparing a core-shell type flame-retardant composite material, preferably, the N,N-dimethylformamide solution is a mixture of N,N-dimethylformamide and an organic solvent in a volume ratio of 1:(0.5-1.5).

[0029] Preferably, the organic solvent includes at least one of methanol, ethanol, n-propanol, isopropanol, ethylene glycol, and glycerol.

[0030] In the above-mentioned method for preparing a core-shell type flame-retardant composite material, preferably, the reaction in step S3 is carried out at 10~40℃ for 10~100min.

[0031] The third objective of this invention can be achieved through the following technical solutions:

[0032] The application of the above-mentioned core-shell type flame-retardant composite material in flame-retardant thermoplastic polyurethane elastomer (TPU). Beneficial effects

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. This invention explores the design and synthesis of a novel core-shell flame retardant, aiming to significantly improve the flame retardant properties of TPU. This flame retardant is constructed with ammonium polyphosphate-grafted modified lignin (Lig-APP) as the core and a metal-organic framework (MOF, preferably ZIF-7) as the shell. This unique design not only fully utilizes the excellent char-forming ability of lignin itself but also further enhances its flame retardant effect through ammonium polyphosphate.

[0035] 2. This invention utilizes a metal-organic framework (preferably ZIF-7) as an outer shell, which effectively protects the core material and forms a stable thermal barrier at high temperatures, thereby inhibiting the diffusion of heat and oxygen into the material and achieving a synergistic flame-retardant effect. Furthermore, due to the use of environmentally friendly materials as raw materials, this flame retardant has lower toxicity and environmental impact compared to traditional halogenated flame retardants, making it suitable for applications in fields with strict environmental protection requirements.

[0036] 3. This invention further explores how this core-shell structure optimizes the dispersion of flame retardants in the TPU matrix and how to maximize the synergistic effect between the layers. Experimental results show that by adding Lig-APP as the core and combining it with ZIF-7 to form a core-shell flame retardant in TPU, it can not only effectively improve the limiting oxygen index of the composite material and reduce the amount of smoke released during combustion, but also have little impact on the mechanical properties of the material itself.

[0037] 4. This invention provides a novel method for the flame-retardant modification of TPU, and lays a theoretical foundation and accumulates practical experience for the development of efficient, low-toxicity, and environmentally friendly flame retardants. With the continued in-depth research on this novel core-shell flame retardant, it is expected that more high-performance, safe, and environmentally friendly polymer materials suitable for different fields will be developed in the future, promoting technological progress and development in related industries. Attached Figure Description

[0038] Figure 1 is a SEM image of the ammonium polyphosphate-grafted modified lignin in Example 1;

[0039] Figure 2 is a SEM image of the core-shell flame-retardant composite material prepared in Example 1. Embodiments of the present invention

[0040] In the description of this invention, the values ​​in parentheses include endpoint values, such as ratios of (1~5):1, which include 1:1 and 5:1.

[0041] The technical solution of the present invention will be further described and illustrated below with reference to specific embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are only for the purpose of helping to understand the present invention and are not intended to limit the specific scope of the present invention. Furthermore, the accompanying drawings used herein are merely for better illustrating the content disclosed in the present invention and do not limit the scope of protection. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commonly used in the art, and the methods used in the embodiments are all conventional methods in the art.

[0042] Example 1

[0043] The preparation method of the flame retardant in Example 1 is as follows:

[0044] S1. 10.0 g lignin, 24.0 g urea and 23.0 g ammonium dihydrogen phosphate were dispersed in 360 mL of deionized water and reacted at 70 °C for 1.0 h under magnetic stirring. After the reaction was completed, the reaction solution was dried and then reacted at 170 °C for 1.0 h. Subsequently, the product was soaked in 1.5 mol / L HCl solution for 4.0 h, centrifuged and washed 3 times, and dried to obtain phosphorylated lignin.

[0045] S2. Take 9.9 g of phosphorylated lignin and 12.0 g of urea and add them to 11.5 mL of 85 wt% phosphoric acid solution. React at 120℃ for 20 min, and then continue to react at 230℃ for 2.5 h. After centrifugation and washing until neutral, dry to obtain ammonium polyphosphate grafted modified lignin (Lig-APP).

[0046] S3. Mix 30 mL of methanol and 30 mL of DMF, add 3.4 g of Lig-APP, and stir for 0.5 h until it is evenly dispersed to obtain solution A;

[0047] S4. Disperse 1.36g of zinc nitrate hexahydrate evenly in 20ml of DMF to obtain solution B;

[0048] S5. Mix solution A containing ammonium polyphosphate grafted modified lignin and solution B containing zinc nitrate evenly, then add 20 ml of methanol containing 1.2 g benzimidazole and react at room temperature (25 °C) for 30 min. Finally, obtain the core-shell flame retardant by solid-liquid separation.

[0049] Figure 1 is an SEM image of the ammonium polyphosphate-grafted modified lignin in Example 1; as can be seen from the figure, the surface of the ammonium polyphosphate-grafted modified lignin is smooth.

[0050] Figure 2 is a SEM image of the core-shell flame-retardant composite material prepared in Example 1. It can be clearly seen from the figure that the surface of the modified lignin grafted with ammonium polyphosphate becomes rough and there are many ZIF-7 small particles, which proves the formation of the core-shell flame retardant.

[0051] Example 2

[0052] The preparation method of the flame retardant in Example 2 is as follows:

[0053] S1, the same as step S1 in Example 1.

[0054] S2. Mix 30 mL of methanol and 30 mL of DMF, add 3.4 g of phosphorylated lignin prepared in step S1, and stir for 0.5 h until it is evenly dispersed to obtain solution A;

[0055] S3. Disperse 1.36g of zinc nitrate hexahydrate evenly in 20ml of DMF to obtain solution B;

[0056] S4. Mix solution A containing phosphorylated lignin and solution B containing zinc nitrate evenly, then add 20 ml of methanol containing 1.2 g benzimidazole and react at room temperature (25 °C) for 30 min. Finally, obtain the core-shell flame retardant by solid-liquid separation.

[0057] Example 3

[0058] The only difference between Example 3 and Example 1 is that the amount of benzimidazole added in step S5 of Example 3 is 2.5g.

[0059] Example 4

[0060] The only difference between Example 4 and Example 1 is that the amount of benzimidazole added in step S5 of Example 4 is 0.5g.

[0061] Example 5

[0062] The only difference between Example 5 and Example 1 is that in step S3 of Example 5, 1.36g of Lig-APP is added.

[0063] Example 6

[0064] The only difference between Example 6 and Example 1 is that in step S3 of Example 6, 5.5g of Lig-APP is added.

[0065] Comparative Example 1

[0066] The flame retardant in Comparative Example 1 was ZIF-7, and its preparation method is as follows:

[0067] 1.36g of zinc nitrate hexahydrate was uniformly dispersed in 20ml of DMF, and then 20ml of methanol containing 1.2g of benzimidazole was added and reacted at room temperature (25℃) for 30min. Finally, the flame retardant ZIF-7 was obtained by solid-liquid separation.

[0068] Comparative Example 2

[0069] The flame retardant in Comparative Example 2 was Lig-APP, and its preparation method is as follows:

[0070] S1. 10.0 g lignin, 24.0 g urea and 23.0 g ammonium dihydrogen phosphate were dispersed in 360 mL of deionized water and reacted at 70 °C for 1.0 h under magnetic stirring. After the reaction was completed, the reaction solution was dried and then reacted at 170 °C for 1.0 h. Subsequently, the product was soaked in 1.5 mol / L HCl solution for 4.0 h, centrifuged and washed 3 times, and dried to obtain phosphorylated lignin.

[0071] S2. Take 9.9 g of phosphorylated lignin and 12.0 g of urea and add them to 11.5 mL of 85 wt% phosphoric acid. React at 120℃ for 20 min, and then continue to react at 230℃ for 2.5 h. After centrifugation and washing until neutral, dry to obtain ammonium polyphosphate grafted modified lignin (Lig-APP).

[0072] Comparative Example 3

[0073] The flame retardant in Comparative Example 3 was a physical mixture of ZIF-7 and Lig-APP, and its preparation method is as follows:

[0074] S1. 10.0 g lignin, 24.0 g urea and 23.0 g ammonium dihydrogen phosphate were dispersed in 360 mL of deionized water and reacted at 70 °C for 1.0 h under magnetic stirring. After the reaction was completed, the reaction solution was dried and then reacted at 170 °C for 1.0 h. Subsequently, the product was soaked in 1.5 mol / L HCl solution for 4.0 h, centrifuged and washed 3 times, and dried to obtain phosphorylated lignin.

[0075] S2. Take 9.9 g of phosphorylated lignin and 12.0 g of urea and add them to 11.5 mL of 85 wt% phosphoric acid. React at 120℃ for 20 min, and then continue to react at 230℃ for 2.5 h. After centrifugation and washing until neutral, dry to obtain ammonium polyphosphate grafted modified lignin (Lig-APP).

[0076] S3. Disperse 1.36g of zinc nitrate hexahydrate evenly in 20ml of DMF, then add 20ml of methanol containing 1.2g of benzimidazole and react at room temperature (25℃) for 30min. Finally, ZIF-7 is obtained by solid-liquid separation.

[0077] S4. The prepared Lig-APP and ZIF-7 are directly physically mixed to obtain a flame retardant.

[0078] The flame-retardant materials of Examples 1-6 and Comparative Examples 1-3 were thoroughly dried in a vacuum oven at 100°C for 6 hours. Following the formulations in Table 1, the TPU and materials of Examples 1-6 and Comparative Examples 1-3 were melt-blended on a Haake Polylab torque rheometer (Thermo Scientific, Germany) at 180°C for 6 minutes at a speed of 60 rpm. All samples were pressed to the required dimensions using an R3201 hot press (Wuhan Qianen Co., Ltd., China) to meet the corresponding testing standards: the TPU composite material was preheated at 2 MPa and 180°C for 5 minutes, then hot-pressed at 180°C and 30 MPa for 3 minutes, and finally compressed at the same pressure at room temperature for 3 minutes to obtain the final molded sample.

[0079] Table 1: Proportions of TPU and flame retardant materials from Examples 1-6 and Comparative Examples 1-3

[0080]

[0081] An oxygen index analyzer of type HC-2 from Jiangning was used to analyze oxygen indexes of 150×6×3mm. 3 Application Examples 1-6 and Comparative Examples 1-6 were used to determine the Limiting Oxygen Index (LOI). Five UL-94 vertical burning tests were conducted on a Jiangning CZF-Ⅲ vertical burning tester according to ASTM D3801 standard. The experimental results are shown in Table 2.

[0082] Table 2: Flame retardant performance test results of molded TPU composite material samples from Application Examples 1-6 and Comparative Examples 1-5

[0083]

[0084] The results above indicate that pure TPU has an LOI value of only 21.5%, and upon ignition, it drips heavily and ignites the cotton below, with no vertical burning rating. Even with the addition of 7% Lig-APP (Comparative Example 3), 7% ZIF-7 (Comparative Example 1), or a physical mixture of Lig-APP and ZIF-7 (Comparative Example 5), there is still a certain probability of igniting the cotton, with a vertical burning rating of V-2. Only when the addition amount of Lig-APP reaches 15% can the vertical burning rating reach V-0, at which point the LOI value increases to 25.6. In contrast, the core-shell flame retardant Lig-APP@ZIF-7 prepared in Example 1 only requires 7% addition to achieve a vertical burning rating of V-0, with an LOI value of 27.2, which is far superior to the same addition amount of Lig-APP alone, ZIF-7 alone, or the physical mixture. This indicates that the core-shell structure of Lig-APP@ZIF-7 fully leverages the synergistic effect of Lig-APP and ZIF-7, improving flame retardant efficiency, reducing the required amount of additives, and has the potential to improve the mechanical and processing properties of the material.

[0085] A comparison of Application Examples 3-6 with Application Example 1 shows that controlling the specific ratio of modified lignin and benzimidazole grafted with zinc nitrate and ammonium polyphosphate can optimize the proportion of each component in the final core-shell flame retardant, ensuring that zinc ions can fully participate in the subsequent coordination reaction to form a stable and uniformly coated metal-organic framework layer, thereby improving the flame retardant performance of the material.

[0086] In summary, this invention explores the design and synthesis of a novel core-shell flame retardant, aiming to significantly improve the flame retardant properties of thermoplastic polyurethane elastomers (TPU). This flame retardant is constructed with ammonium polyphosphate-grafted modified lignin (Lig-APP) as the core and a metal-organic framework (MOF, specifically ZIF-7) as the shell. This unique design not only fully utilizes the excellent char-forming ability of lignin itself but also further enhances its flame retardant effect through ammonium polyphosphate and, synergistically with the metal-organic framework, improves flame retardant efficiency.

[0087] The embodiments herein cover any points not exhaustively within the scope of the technical claims of this invention, as well as new technical solutions formed by equivalent substitutions of one or more technical features in the embodiments. These are all within the scope of the claims of this invention. Furthermore, in all listed or unlisted embodiments of this invention, each parameter in the same embodiment merely represents an instance (i.e., a feasible solution) of its technical solution, and there is no strict coordination or limitation relationship between the parameters. The parameters can be substituted for each other without violating axioms and the claims of this invention, unless otherwise stated.

[0088] The technical means disclosed in this invention are not limited to those described above, but also include technical solutions composed of any combination of the above technical features. The above descriptions are specific embodiments of this invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention.

[0089] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A core-shell type flame-retardant composite material, characterized in that, The flame-retardant composite material uses ammonium polyphosphate-grafted modified lignin as the core and a metal-organic framework as the shell; the flame-retardant composite material includes the following raw materials: metal source, ammonium polyphosphate-grafted modified lignin, and organic ligands.

2. The core-shell type flame-retardant composite material according to claim 1, characterized in that, The flame-retardant composite material is obtained by reacting ammonium polyphosphate-grafted modified lignin dispersed in a solution containing a metal source and organic ligands.

3. The core-shell type flame-retardant composite material according to claim 1, characterized in that, The metal source includes at least one of zinc nitrate and cobalt nitrate.

4. A core-shell type flame-retardant composite material according to claim 1, characterized in that, Organic ligands include at least one of benzimidazole and 2-methylimidazole.

5. A core-shell type flame-retardant composite material according to claim 1, characterized in that, The metal source is zinc nitrate, and the organic ligand is benzimidazole.

6. A core-shell type flame-retardant composite material according to any one of claims 1-5, characterized in that, The mass ratio of the metal source, ammonium polyphosphate-grafted modified lignin, and organic ligand is 1:(2-3):(0.8-1.2).

7. A method for preparing a core-shell type flame-retardant composite material as described in any one of claims 1-5, characterized in that, Includes the following steps: Ammonium polyphosphate-grafted modified lignin was uniformly dispersed in N,N-dimethylformamide solution to obtain solution A, and a metal source was uniformly dispersed in N,N-dimethylformamide to obtain solution B; Solution A containing ammonium polyphosphate-grafted modified lignin and solution B containing a metal source were uniformly mixed, and then an organic solvent containing organic ligands was added to react. Finally, the core-shell flame retardant was obtained by solid-liquid separation.

8. A method for preparing a core-shell type flame-retardant composite material according to claim 7, characterized in that, Ammonium polyphosphate-grafted modified lignin is prepared by adding phosphoric acid and urea to phosphoric acid and then heating the mixture.

9. A method for preparing a core-shell type flame-retardant composite material according to claim 8, characterized in that, The mass ratio of phosphorylated lignin, urea and phosphoric acid is (8-10):(10-15):(15-25).

10. A method for preparing a core-shell type flame-retardant composite material according to claim 8, characterized in that, The heating reaction specifically includes: first reacting at 100-150℃ for 10-30 minutes, and then continuing the reaction at 210-250℃ for 2-3 hours.

11. A method for preparing a core-shell type flame-retardant composite material according to claim 8, characterized in that, The preparation method of phosphorylated lignin includes: dispersing lignin, urea and ammonium dihydrogen phosphate in water, reacting and drying, then reacting further, soaking in an acid solution, and finally centrifuging, washing and drying to obtain phosphorylated lignin.

12. A method for preparing a core-shell type flame-retardant composite material according to claim 11, characterized in that, The mass ratio of lignin, urea and ammonium dihydrogen phosphate is 1:(2-3):(2-3).

13. A method for preparing a core-shell type flame-retardant composite material according to claim 8, characterized in that, The N,N-dimethylformamide solution is a mixture of N,N-dimethylformamide and an organic solvent in a volume ratio of 1:(0.5-1.5).

14. A method for preparing a core-shell type flame-retardant composite material according to claim 13, characterized in that, Organic solvents include at least one of methanol, ethanol, n-propanol, ethylene glycol, and glycerol.

15. The application of a core-shell type flame-retardant composite material as described in claim 1 in a flame-retardant thermoplastic polyurethane elastomer.

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