Core-shell type flame-retardant composite material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of flame-retardant materials, and relates to a core-shell type flame-retardant composite material as well as a preparation method and application thereof. The invention explores the design and synthesis of a novel core-shell flame retardant, and aims at remarkably improving the flame retardant property of TPU (Thermoplastic Polyurethane). The flame retardant is constructed by taking ammonium polyphosphate grafted and modified lignin as a core and adopting a metal organic framework as a shell. The unique design not only gives full play to the good char forming ability of the lignin, but also can further enhance the flame retardant effect through the synergistic effect of the metal organic framework.

Description

Technical Field

[0001] The present invention belongs to the technical field of flame retardant materials, and relates to a core-shell type flame retardant composite material, a preparation method thereof and an application thereof. Background Art

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

[0003] Traditionally, flame retardant strategies mainly include the use of halogen-based flame retardants, phosphorus-based flame retardants, and metal hydroxides, etc. Although these methods can improve the fire resistance of materials to a certain extent, they also bring new problems. For example, halogen-based flame retardants may release harmful gases during combustion, posing a potential threat to the environment and human health; while conventional inorganic flame retardants such as Al(OH)3 and Mg(OH)2 usually require a large addition amount to achieve an ideal flame retardant effect, but this often reduces the mechanical properties of the materials. In view of these problems, in recent years, core-shell structure flame retardants have received extensive attention as a new type of solution. This structure not only helps to optimize the dispersion of the flame retardant in the TPU matrix, but also can maximize the flame retardant functions of the core layer and the shell layer through synergistic effects, thereby improving the flame retardant efficiency without affecting or even enhancing the comprehensive properties of the materials.

[0004] Meanwhile, lignin, as a natural aromatic polymer, is becoming a potential environmentally friendly flame retardant due to its rich benzene ring structure and good char-forming ability. Lignin can form a protective coke layer during combustion, effectively preventing the transmission of oxygen and heat into the material interior. At the same time, during its pyrolysis process, inert gases such as CO2, H2O, and CO are released, and these gases will dilute the concentration of combustible gases in the combustion zone, reduce the flame temperature, and inhibit the combustion reaction. However, the flame retardant efficiency of pure lignin is relatively low, and its flame retardant performance can be further improved through chemical modification. This method can not only solve the problems of environmental pollution and mechanical property degradation brought by traditional flame retardants, but also provide new ideas for the development of high-efficiency and environmentally friendly TPU flame retardant materials. In short, with the development of science and technology, people's exploration of TPU flame retardant modification is continuously deepening, and more extensive safe applications are expected in the future.

[0005] An article was published in the journal 《Chemical Engineering Journal》, which functionalized 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 study adopted an innovative in-situ solid-phase grafting method to react lignin with APP, thus realizing the functionalization of lignin. This functionalized lignin was added to the PLA matrix, not only effectively improving the flame retardancy of PLA, but also enhancing its thermal stability, mechanical strength, and UV resistance. The experimental results show that this method can effectively overcome some inherent defects of traditional PLA materials, such as flammability and poor weather resistance. Although significant progress has been made, there is still room for further optimization in the thermal stability of lignin grafted and modified solely with ammonium polyphosphate. This means that although the existing modification strategies have greatly improved the performance of PLA, in some specific application fields, more efficient modification technologies or additives may still need to be explored to further improve the thermal stability of the material.

[0006] Chinese patent document (CN116715867A) discloses a lignin flame retardant, its preparation method, and a flame retardant composite material. When it is applied to the flame retardancy of PLA, only 3% addition amount is required to reach the UL-94 V-0 grade. However, when this flame retardant system is applied to TPU, the flame retardancy significantly decreases, and the addition amount needs to be increased to 15% to reach the UL-94 V-0 grade. But too much addition of the flame retardant will seriously affect the mechanical properties of the TPU composite material. Therefore, for the efficient flame retardancy of TPU, it is extremely necessary to modify the lignin flame retardant to achieve excellent flame retardant effects under the premise of extremely low addition amounts and reduce the impact on the mechanical properties of the composite material. Summary of the Invention

[0007] The purpose of the present invention is to address the above-mentioned problems existing in the prior art and propose a core-shell type flame retardant composite material, which uses ammonium polyphosphate grafted modified lignin as the core and metal-organic framework as the shell, and can improve the flame retardancy of TPU through a synergistic flame retardant mechanism.

[0008] The purpose of the present invention can be achieved by the following technical solutions:

[0009] A core-shell type flame retardant composite material, the flame retardant composite material uses ammonium polyphosphate grafted modified lignin as the core and metal-organic framework as the shell, and the flame retardant composite material includes the following raw materials in parts by mass: metal source, ammonium polyphosphate grafted modified lignin, organic ligand.

[0010] Preferably, the metal-organic framework includes one of ZIF-7, ZIF-8, and ZIF-9.

[0011] Further preferably, the metal-organic framework is ZIF-7.

[0012] In the above-mentioned core-shell flame retardant composite material, the metal source includes at least one of zinc nitrate and cobalt nitrate.

[0013] In the above-mentioned core-shell flame retardant composite material, the organic ligand includes at least one of benzimidazole and 2-methylimidazole.

[0014] In the above-mentioned core-shell flame retardant composite material, the mass ratio of the metal source, ammonium polyphosphate grafted modified lignin, and organic ligand is 1:(2 - 3):(0.8 - 1.2).

[0015] Preferably, the metal source is zinc nitrate.

[0016] Preferably, the organic ligand is benzimidazole.

[0017] Zinc nitrate, as the main zinc source for generating metal-organic frameworks (such as ZIF-7), plays a role of providing zinc ions in the reaction system. Its specific mass ratio with ammonium polyphosphate grafted modified lignin and benzimidazole ensures that zinc ions can fully participate in the subsequent coordination reaction to form a stable and uniformly coated metal-organic framework layer. And ammonium polyphosphate grafted modified lignin, as the core, not only forms a protective coke layer during combustion due to its good char-forming ability, reducing the transfer of heat and oxygen, but also improves its own flame retardancy efficiency due to its combination with ammonium polyphosphate. As for benzimidazole, it participates in the coordination reaction with zinc ions as an organic ligand to promote the formation of metal-organic frameworks. By finely adjusting the mass ratio among the three, the present invention can not only optimize the proportion of each component in the finally formed core-shell flame retardant, but also achieve precise control of the flame retardant performance of the material for different application scenarios.

[0018] A preparation method of a core-shell flame retardant composite material, the method comprising the following steps:

[0019] S1. Uniformly disperse ammonium polyphosphate grafted modified lignin in N,N-dimethylformamide solution to obtain solution A;

[0020] S2. Uniformly disperse the metal source in N,N-dimethylformamide solution to obtain solution B;

[0021] S3. Uniformly mix solution A containing ammonium polyphosphate grafted modified lignin and solution B containing the metal source, then add an organic solution containing an organic ligand for reaction, and finally obtain a core-shell flame retardant through solid-liquid separation.

[0022] In the preparation process of the present invention, an ion exchange reaction is first carried out between zinc ions and ammonium ions in the ammonium polyphosphate-grafted modified lignin. In this process, zinc ions are precisely located on the surface of the ammonium polyphosphate-grafted modified lignin. Subsequently, a benzimidazole organic ligand is added to the system, and the zinc ions immediately undergo a coordination reaction with these organic ligands. This specific coordination effect promotes the formed MOF metal-organic framework to tightly coat the surface of the ammonium polyphosphate-grafted modified lignin. The final product formed through the above process is a unique core-shell type flame retardant, in which the ammonium polyphosphate-grafted modified lignin serves as the core and the metal-organic framework serves as the shell. The ammonium polyphosphate-grafted modified lignin, as an effective intumescent flame retardant component, its core position is conducive to promoting the formation of a continuous and stable carbon layer at high temperatures, effectively blocking the transfer of heat and oxygen, thereby enhancing the flame retardant effect. The metal-organic framework, as the shell, not only provides an additional heat insulation barrier, but also can adsorb or capture some harmful substances generated during the combustion process due to its special pore structure, further reducing the fire risk. In addition, the design of this core-shell structure also helps to improve the dispersibility of the flame retardant in the polymer matrix and enhance the overall performance of the composite material.

[0023] In the preparation method of the above-mentioned core-shell type flame retardant composite material, the ammonium polyphosphate-grafted modified lignin is prepared by adding phosphorylated lignin and urea to phosphoric acid and carrying out a heating reaction.

[0024] In the preparation method of the above-mentioned core-shell type flame retardant composite material, the mass ratio of phosphorylated lignin, urea and phosphoric acid is (8 - 10):(10 - 15):(15 - 25).

[0025] In the preparation method of the above-mentioned core-shell type flame retardant composite material, the preparation method of phosphorylated lignin includes: dispersing lignin, urea and ammonium dihydrogen phosphate with a mass ratio of 1:(2 - 3):(2 - 3) in water, soaking in an acid solution, and finally obtaining phosphorylated lignin after centrifugation, washing and drying.

[0026] In the preparation method of the above-mentioned core-shell type flame retardant composite material, the heating reaction specifically includes: first reacting at 100 - 150 °C for 10 - 30 min, and then continuing to react at 210 - 250 °C for 2 - 3 h.

[0027] In the preparation method of the above-mentioned core-shell type flame retardant composite material, the N,N-dimethylformamide solution is a mixed solution of N,N-dimethylformamide and an organic solvent with a volume ratio of 1:(0.5 - 1.5).

[0028] In the preparation method of the above-mentioned core-shell type flame retardant composite material, the organic solvent includes at least one of methanol, ethanol, n-propanol, ethylene glycol, and glycerol.

[0029] The present invention also provides an application of the above-mentioned core-shell flame retardant composite material in flame-retardant thermoplastic polyurethane elastomer (TPU).

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The present invention explores the design and synthesis of a novel core-shell flame retardant, aiming to significantly improve the flame retardancy of TPU. This flame retardant is constructed with ammonium polyphosphate grafted lignin (Lig-APP) as the core and metal-organic framework (MOF, preferably ZIF-7) as the shell. This unique design not only fully utilizes the good char-forming ability of lignin itself but also further enhances its flame retardant effect through ammonium polyphosphate.

[0032] 2. By using metal-organic framework (preferably ZIF-7) as the shell, the present invention can effectively protect the core material and form a stable heat insulation barrier at high temperatures, thereby inhibiting the diffusion of heat and oxygen into the material interior and achieving a synergistic flame retardant effect. In addition, since environmentally friendly materials are used as raw materials, this flame retardant has lower toxicity and environmental impact compared with traditional halogen-based flame retardants, which provides the possibility for its application in fields with strict environmental protection requirements.

[0033] 3. The present invention also deeply explores how this core-shell structure optimizes the dispersion of the flame retardant in the TPU matrix and maximizes the synergistic effect between layers. Experimental results show that by adding the core-shell structure flame retardant formed by combining Lig-APP as the core with ZIF-7 to TPU, not only can the limiting oxygen index of the composite material be effectively increased and the smoke release amount during combustion be reduced, but also the mechanical properties of the material itself are less affected.

[0034] 4. The present invention provides a brand-new method for the flame retardant modification of TPU, and at the same time lays a theoretical foundation and accumulates practical experience for the development of high-efficiency, low-toxic and environmentally friendly flame retardants. With the continuous 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 the technological progress and development of related industries. Description of the Drawings

[0035] Figure 1 SEM image of ammonium polyphosphate grafted modified lignin in Example 1;

[0036] Figure 2 SEM image of the core-shell flame retardant composite material prepared in Example 1. Detailed Embodiments

[0037] The following are specific embodiments of the present invention, which further describe the technical solutions of the present invention, but the present invention is not limited to these embodiments.

[0038] Unless otherwise specified, the raw materials and equipment used in the present invention can be purchased from the market or are commonly used in the art. The methods in the embodiments are conventional methods in the art unless otherwise specified.

[0039] Example 1:

[0040] S1. Disperse 10.0 g of lignin, 24.0 g of urea, and 23.0 g of ammonium dihydrogen phosphate in 360 mL of deionized water, react at 70 °C for 1.0 h under magnetic stirring. After the reaction, dry the reaction solution and react at 170 °C for 1.0 h. Then, immerse the product in 1.5 mol / L HCl solution for 4.0 h, and centrifuge and wash it 3 times. After drying, phosphorylated lignin is obtained.

[0041] S2. Take 9.9 g of phosphorylated lignin and 12.0 g of urea, add them to 11.5 mL of phosphoric acid, react at 120 °C for 20 min, and then continue to react at 230 °C for 2.5 h. After centrifuging and washing until neutral and drying, ammonium polyphosphate-grafted modified lignin (Lig-APP) is obtained.

[0042] 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.

[0043] S4. Uniformly disperse 1.36 g of zinc nitrate hexahydrate in 20 mL of DMF to obtain solution B.

[0044] S5. Uniformly mix solution A containing ammonium polyphosphate-grafted modified lignin and solution B containing zinc nitrate, then add it to 20 mL of methanol containing 1.2 g of benzimidazole for reaction for 30 min. Finally, perform solid-liquid separation to obtain a core-shell flame retardant.

[0045] Example 2:

[0046] The difference from Example 1 is only that Lig-APP in step S3 is replaced with the phosphorylated lignin in step S1.

[0047] Example 3:

[0048] The difference from Example 1 is only that the addition amount of benzimidazole in step S5 is 2.5 g.

[0049] Example 4:

[0050] The difference from Example 1 is only that the addition amount of benzimidazole in step S5 is 0.5 g.

[0051] Example 5:

[0052] The difference from Example 1 is only that the addition amount of ammonium polyphosphate grafted modified lignin in step S5 is 1.36 g.

[0053] Example 6:

[0054] The difference from Example 1 is only that the addition amount of ammonium polyphosphate grafted modified lignin in step S5 is 5.5 g.

[0055] Comparative Example 1:

[0056] 1.36 g of zinc nitrate hexahydrate was uniformly dispersed in 20 ml of DMF, then added to 20 ml of methanol containing 1.2 g of benzimidazole for reaction for 30 min, and finally the flame retardant was obtained by solid-liquid separation.

[0057] Comparative Example 2:

[0058] S1. 10.0 g of lignin, 24.0 g of urea and 23.0 g of ammonium dihydrogen phosphate were dispersed in 360 mL of deionized water, reacted at 70 °C for 1.0 h under magnetic stirring. After the reaction, the reaction solution was dried and reacted at 170 °C for 1.0 h. Subsequently, the product was soaked in 1.5 mol / L HCl solution for 4.0 h and centrifuged and washed 3 times, and phosphorylated lignin was obtained after drying.

[0059] S2. 9.9 g of phosphorylated lignin and 12.0 g of urea were added to 11.5 mL of phosphoric acid, reacted at 120 °C for 20 min, and then continued to react at 230 °C for 2.5 h. After centrifuging and washing to neutrality and drying, ammonium polyphosphate grafted modified lignin (Lig-APP) was obtained.

[0060] Comparative Example 3:

[0061] S1. 10.0 g of lignin, 24.0 g of urea and 23.0 g of ammonium dihydrogen phosphate were dispersed in 360 mL of deionized water, reacted at 70 °C for 1.0 h under magnetic stirring. After the reaction, the reaction solution was dried and reacted at 170 °C for 1.0 h. Subsequently, the product was soaked in 1.5 mol / L HCl solution for 4.0 h and centrifuged and washed 3 times, and phosphorylated lignin was obtained after drying.

[0062] S2. 9.9 g of phosphorylated lignin and 12.0 g of urea were added to 11.5 mL of phosphoric acid, reacted at 120 °C for 20 min, and then continued to react at 230 °C for 2.5 h. After centrifuging and washing to neutrality and drying, ammonium polyphosphate grafted modified lignin (Lig-APP) was obtained.

[0063] S3. Disperse 1.36 g zinc nitrate hexahydrate evenly in 20 ml DMF, then add 20 ml methanol containing 1.2 g benzimidazole to react for 30 min, and finally obtain ZIF-7 by solid-liquid separation.

[0064] S4. Directly physically mix the prepared Lig-APP and ZIF-7 to obtain a flame retardant.

[0065] The flame retardant materials of Examples 1-6 and Comparative Examples 1-3 were fully dried in a vacuum oven at 100°C for 6 hours. According to the formula in Table 1, TPU and the materials of Examples 1-6 and Comparative Examples 1-2 were melt blended at a temperature of 180°C on a Haake Polylab torque rheometer (Thermo Scientific, Germany), with a blending time of 6 minutes and a rotation speed of 60rpm. All samples were pressed to the required size using an R3201 hot press (Wuhan Qianen Co., Ltd., China) to meet the corresponding test standards. The TPU composite material was first preheated at 2MPa and 180°C for 5 minutes, then hot pressed at 180°C and 30MPa for 3 minutes, and compressed at the same pressure for 3 minutes at room temperature to obtain the final molded sample.

[0066] Table 1: Proportions of TPU and flame retardant materials of Examples 1-6 and Comparative Examples 1-3

[0067] Application Example TPU Flame Retardant Material Application Example 1 93wt% 7wt% Flame Retardant Material Prepared in Example 1 Application Example 2 93wt% 7wt% Flame Retardant Material Prepared in Example 2 Application Example 3 93wt% 7wt% Flame Retardant Material Prepared in Example 3 Application Example 4 93wt% 7wt% Flame Retardant Material Prepared in Example 4 Application Example 5 93wt% 7wt% Flame Retardant Material Prepared in Example 5 Application Example 6 93wt% 7wt% Flame Retardant Material Prepared in Example 6 Application Comparative Example 1 93wt% 7wt% Flame Retardant Material Prepared in Comparative Example 1 Application Comparative Example 2 85wt% 15wt% Flame Retardant Material Prepared in Comparative Example 2 Application Comparative Example 3 93wt% 7wt% Flame Retardant Material Prepared in Comparative Example 2 Application Comparative Example 4 100wt% / Application Comparative Example 5 93wt% 7wt% Flame Retardant Material Prepared in Comparative Example 3

[0068] Figure 1 This is a SEM image of the modified lignin grafted with ammonium polyphosphate in Example 1; it can be seen from the image that the surface of the modified lignin grafted with ammonium polyphosphate is smooth.

[0069] Figure 2 This is a SEM image of the core-shell flame retardant composite material prepared in Example 1; it can be clearly seen from the image that the surface of the modified lignin grafted with ammonium polyphosphate becomes rough, and there are many small ZIF-7 particles, proving the formation of the core-shell flame retardant.

[0070] The Jiangning HC-2 oxygen index analyzer was used to analyze the 3 The samples prepared in the application examples 1-6 and the comparative examples 1-6 were subjected to the limiting oxygen index (LOI) determination. According to the ASTM D3801 standard, 5 UL-94 vertical combustion tests were conducted on a Jiangning CZF-Ⅲ vertical combustion test machine.

[0071] Table 2: Performance test results of molded TPU composite material samples prepared in application examples 1-6 and comparative examples 1-5

[0072]

[0073]

[0074] The above results show that the LOI value of pure TPU is only 21.5%. When ignited, it is accompanied by severe dripping and ignites the cotton below, and there is no rating for vertical burning. Only when the addition amount of Lig-APP reaches 15%, the vertical burning rating can reach V-0, and the LOI value increases to 25.6 at this time. When the addition amount of the core-shell flame retardant Lig-APP@ZIF-7 is 7%, the vertical burning rating can reach V-0, and the LOI value is 27.2. In contrast, when only 7% of Lig-APP and 7% of ZIF-7 are added, there is still a certain probability of igniting the cotton, and the vertical burning rating is V-2.

[0075] In summary, the present invention explores the design and synthesis of a novel core-shell flame retardant, aiming to significantly improve the flame retardancy of thermoplastic polyurethane elastomer (TPU). The flame retardant is constructed with lignin grafted with ammonium polyphosphate (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 good char-forming ability of lignin itself but also further enhances its flame retardant effect through ammonium polyphosphate.

[0076] For the points not exhausted by the midpoint values in the technical scope claimed by the present invention in the embodiments herein and the new technical solutions formed by the equivalent replacement of single or multiple technical features in the technical solutions of the embodiments, they are also within the scope claimed by the present invention; at the same time, in all the listed or unlisted embodiments of the present invention, the various parameters in the same embodiment only represent an example of its technical solution (i.e., a feasible solution), and there is no strict cooperation and limitation relationship between the various parameters. Among them, the various parameters can be replaced with each other when not violating the axiom and the requirements of the present invention, except as otherwise specifically stated.

[0077] The technical means disclosed in the present invention is not limited to the technical means disclosed by the above technical means, but also includes the technical solutions formed by any combination of the above technical features. The above is the specific implementation manner of the present invention. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements are also regarded as the protection scope of the present invention.

[0078] The specific embodiments described herein are only illustrative of the spirit of the present invention. Those skilled in the art of the present invention can make various modifications or supplements to the described specific embodiments or use similar ways to replace them, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

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

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

3. The core-shell flame-retardant composite material according to claim 1, characterized in that: The organic ligand includes at least one of benzimidazole and 2-methylimidazole.

4. The core-shell flame-retardant composite material according to claim 1, characterized in that: The mass ratio of the metal source, the modified lignin grafted with ammonium polyphosphate, and the organic ligand is 1:(2-3):(0.8-1.2).

5. The method for preparing a core-shell flame-retardant composite material according to claim 1, characterized in that: The modified lignin grafted with ammonium polyphosphate is prepared by adding phosphorylated lignin and urea into phosphoric acid and then heating the mixture for reaction.

6. The method for preparing a core-shell flame-retardant composite material according to claim 5, characterized in that: The mass ratio of phosphorylated lignin, urea and phosphoric acid is (8-10):(10-15):(15-25).

7. The method for preparing a core-shell flame-retardant composite material according to claim 5, characterized in that: The heating reaction specifically includes: first reacting at 100-150° C. for 10-30 minutes, and then continuing to react at 210-250° C. for 2-3 hours.

8. A method for preparing the core-shell flame-retardant composite material according to any one of claims 1 to 7, characterized in that: The method comprises the following steps: S1, uniformly dispersing the modified lignin grafted with ammonium polyphosphate in an N,N-dimethylformamide solution to obtain a solution A; S2, uniformly dispersing the metal source in the N,N-dimethylformamide solution to obtain solution B; S3, uniformly mixing solution A containing modified lignin grafted with ammonium polyphosphate and solution B containing a metal source, then adding an organic solution containing an organic ligand to react, and finally obtaining a core-shell flame retardant by solid-liquid separation.

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

10. Use of the core-shell flame-retardant composite material as claimed in claim 1 in a flame-retardant thermoplastic polyurethane elastomer.

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