Nitrogen and phosphorus co-doped metal organic framework flame-retardant material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of functional materials, and particularly discloses a nitrogen-phosphorus co-doped metal organic framework material and a preparation method and application thereof. The composite material is constructed by taking phosphonitrilic chloride trimer and adenine as precursors and performing hydrothermal reaction on the precursors and metal salt. Adenine shows more excellent flame-retardant performance due to a unique double-ring structure and multiple coordination sites, and the flame-retardant mechanism is a synergistic effect which takes condensed phase flame retardance as a main component and takes gas phase flame retardance as an auxiliary component. A phosphoric acid-containing substance generated by thermal decomposition of the composite material can catalyze and promote a dehydration carbonization reaction of a polymer molecular chain to realize condensed phase flame retardance; generated phosphorus-containing free radicals (PO., HPO. And the like) can be subjected to a binding reaction with active free radicals such as H., HO. And the like, a combustion chain reaction is effectively inhibited through a free radical quenching effect, gas-phase flame retardance is realized, and the combustion rate of the material is remarkably reduced. The composite material is particularly suitable for flame-retardant modification of epoxy resin, and the flame-retardant property of the epoxy resin can be remarkably improved under the condition of low addition amount.

Description

Technical Field

[0001] The present invention relates to the technical field of functional materials, and in particular to a nitrogen-phosphorus co-doped metal organic framework flame retardant material and a preparation method and application thereof. Background Art

[0002] In recent years, with the development of industry, the requirements for material safety performance have been increasing, especially in the field of flame retardant materials. Epoxy resin, as a widely used polymer material, has excellent mechanical properties, electrical properties and bonding properties, and is widely used in many fields such as electronics, aerospace, and construction. However, epoxy resin itself is flammable and will produce a large amount of heat and toxic smoke during combustion, which greatly limits its application in some scenarios with high fire safety requirements. Therefore, improving the flame retardant properties of epoxy resin is of great practical significance.

[0003] Metal organic framework materials (MOFs) are a new type of porous materials. Due to their outstanding features such as strong designability and environmental friendliness, they have gradually shown important application value in the field of epoxy resin flame retardancy. At present, researchers have experimentally explored the use of different ligands to construct MOFs flame retardant systems to improve the flame retardant properties of epoxy resins. For example, the MOFs flame retardant system constructed using 2-methylimidazole or melamine as ligands has been proven to have a certain flame retardant effect.

[0004] The study found that the MOFs flame retardant system constructed using 2-methylimidazole or melamine as ligands does have a certain flame retardant effect, but there are obvious shortcomings: although the 2-methylimidazole ligand has good compatibility with epoxy resin, the synergistic effect of flame retardant elements is insufficient; although the melamine ligand has a significant gas phase flame retardant effect, its charring performance is poor, and the existing preparation method makes it difficult to achieve efficient doping of phosphorus elements, resulting in difficulty in taking into account both gas phase flame retardancy and condensed phase flame retardancy of the material.

[0005] In view of this, there is an urgent need to develop a new MOFs flame retardant system to overcome the shortcomings of the existing system and achieve a more efficient and comprehensive flame retardant effect on epoxy resin. Summary of the invention

[0006] The purpose of the present invention is to provide a nitrogen-phosphorus co-doped metal organic framework flame retardant material and its preparation method and application in view of the above-mentioned deficiencies in the prior art, and to achieve a dual flame retardant mechanism of gas phase free radical capture and condensed phase carbonization at the initial stage of combustion through the synergistic effect of the highly active phosphorus element in hexachlorocyclotriphosphazene and the nitrogen-containing heterocyclic ligand.

[0007] The first object of the present invention is to provide a method for preparing a nitrogen-phosphorus co-doped metal organic framework material, comprising the following steps:

[0008] S1. After ultrasonically dissolving hexachlorocyclotriphosphazene and adenine in an organic solvent, an organic base is added and the mixture is heated for reaction. After cooling, filtration, washing, and drying, Compound 1 is obtained.

[0009] S2. React Compound 1 with a metal salt under hydrothermal conditions to form Compound 2, which is the nitrogen and phosphorus co-doped metal-organic framework material.

[0010] Further, the organic solvent includes any one of tetrahydrofuran, methanol, or chlorobenzene.

[0011] Further, the metal salt is any one of ferric chloride hexahydrate, zinc nitrate hexahydrate, cobalt nitrate hexahydrate, zirconium nitrate pentahydrate, copper nitrate hexahydrate, or nickel nitrate hexahydrate.

[0012] Further, the organic base is triethylamine.

[0013] Further, the mass ratio of the hexachlorocyclotriphosphazene to the adenine is 1:(2 - 3).

[0014] Further, in step S1, the temperature of the heating reaction is 60 - 80 °C, and the reaction time is 22 - 26 h.

[0015] Further, the solvents used for washing in step S1 are acetone and water.

[0016] Further, in step S2, the solvent for the hydrothermal reaction is a mixed solution of N,N-dimethylacetamide and water with a volume ratio of (3 - 5):1, the reaction temperature is 90 - 110 °C, and the reaction time is 45 - 50 h.

[0017] Further, the mass ratio of Compound 1 to the metal salt is 1:(2 - 3).

[0018] Further, the solvents used for washing in step S2 are N,N-dimethylacetamide and water.

[0019] The second object of the present invention is to provide the nitrogen and phosphorus co-doped metal-organic framework material prepared by the above preparation method.

[0020] The third object of the present invention is to provide the application of the nitrogen and phosphorus co-doped metal-organic framework material in the preparation of flame-retardant products.

[0021] The fourth object of the present invention is to provide a polymer flame-retardant material. The above nitrogen and phosphorus co-doped metal-organic framework material is added to epoxy resin, and the addition amount is 1 - 5 wt%.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) The present invention provides a method for preparing a nitrogen and phosphorus co-doped metal-organic framework material and its flame retardant application. This composite material uses hexachlorocyclotriphosphazene (HCCP) and the nitrogen-containing heterocyclic ligand adenine as precursors, and constructs a MOFs system with metal salts through a hydrothermal reaction. Due to its unique bicyclic structure and multiple coordination sites, adenine enables the obtained composite material to exhibit more excellent flame retardant properties. Its flame retardant mechanism mainly shows a synergistic mechanism dominated by condensed-phase flame retardancy and supplemented by gas-phase flame retardancy. In terms of condensed-phase flame retardancy, the phosphoric acid-containing substances generated by the thermal decomposition of the composite material can catalyze and promote the dehydration and carbonization reaction of the epoxy resin molecular chain. At the same time, the metal oxides generated by the thermal decomposition of the composite material not only act as catalysts to accelerate the decomposition and carbonization process of the epoxy resin molecular chain, but also serve as important components of the carbon layer, significantly enhancing the thermal stability of the carbon layer. Experimental results show that the synergistic effect between the catalytic dehydration and carbonization effect of phosphorus elements and the catalytic carbonization effect of metal oxides jointly promotes the formation of a dense and stable carbon layer, effectively blocking the transmission of heat and combustible gases, thereby significantly reducing the combustion rate of the material.

[0024] (2) At the same time, the 3D nanoflower sphere structure formed by the self-assembly of 2D nanosheets constructs a unique "zigzag path" effect in the polymer matrix, and this special microstructure effectively delays the release rate of combustible gases. In terms of gas-phase flame retardancy, the composite material generates phosphorus-containing free radicals (PO·, HPO·, etc.) during thermal decomposition, and these free radicals can undergo binding reactions with active free radicals such as H· and HO·, effectively inhibiting the combustion chain reaction through the free radical quenching effect and significantly reducing the combustion rate of the material. In addition, the components of the composite material release CO 2 、H 2 O、N 2 and other non-combustible gases during the thermal decomposition process, and these gases act as diluents to reduce the concentration and reaction temperature of combustible gases around the combustion area, thereby further inhibiting the progress of the combustion process.

[0025] (3) The design of the present invention breaks through the technical limitations of traditional MOFs flame retardants, and can achieve the synergistic enhancement of gas-phase free radical capture and condensed-phase carbonization at extremely low addition amounts, providing a new solution for the development of high-efficiency and environmentally friendly epoxy resin flame retardants. Description of the Drawings

[0026] Figure 1 is the XRD spectrum of the Bio-PMOFs material provided by the present invention;

[0027] Figure 2 is the FTIR spectrum of the Bio-PMOFs material provided by the present invention;

[0028] Figure 3XPS full spectrum of the Bio-PMOFs material provided by the present invention. In the figure, (a) is the XPS full spectrum; (b) is the high-resolution XPS spectrum of N element, (c) is the high-resolution XPS spectrum of P element, and (d) is the high-resolution XPS spectrum of Co element;

[0029] Figure 4 Figure showing the results of microscopic morphology characterization analysis of Bio-HCCP and Bio-PMOFs materials by SEM. In the figure, (a): Bio-HCCP; (b)-(d): Bio-PMOFs; (e): EDS element distribution map;

[0030] Figure 5 Figure showing the results of flame retardancy evaluation of the composite material. In the figure, (a) is the vertical burning result of the composite material, and (b) is the limiting oxygen index result;

[0031] Figure 6 Figure showing the results of morphological characterization of the char residue of the composite material. In the figure, (a) and (a1) are the digital photo and SEM map of EP respectively; (b) and (b1) are the digital photo and SEM map of EP / Bio-HCCP respectively; (c) and (c1) are the digital photo and SEM map of EP / Bio-PMOFs 0.5 respectively; (d) and (d1) are the digital photo and SEM map of EP / Bio-PMOFs 1 respectively; (e) and (e1) are the digital photo and SEM map of EP / Bio-PMOFs 2 respectively;

[0032] Figure 7 Raman spectrum of the char residue of EP;

[0033] Figure 8 Raman spectrum of the char residue of EP / Bio-PMOFs 2;

[0034] Figure 9 FTIR spectrum of the char residue of EP / Bio-PMOF 2. Detailed implementation manners

[0035] In order to more clearly and comprehensively explain the technical solutions and beneficial effects of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and embodiments. It should be clear that the described reference drawings are only partial embodiments of the present invention, which are only used to explain the present invention and should not be construed as a limitation to the present invention. Unless otherwise specified, the equipment and reagents used in the present invention are conventional commercially available products in the technical field.

[0036] An embodiment of the present invention provides a method for preparing a nitrogen and phosphorus co-doped metal-organic framework material, which uses hexachlorocyclotriphosphazene (HCCP) and a nitrogen-containing heterocyclic ligand as precursors, and reacts with metal salts through a hydrothermal reaction to construct a MOFs system. The nitrogen-containing heterocyclic ligand is adenine. The following formula 1 structure is hexachlorocyclotriphosphazene (HCCP), and the method includes the following steps:

[0037] Step 1: After ultrasonic dissolution of the compound of formula 1 structure and the compound of formula 2 structure in a tetrahydrofuran organic solvent, add the compound of formula 3 structure and heat and stir for reaction, and obtain the compound of formula 4 structure after cooling, suction filtration, washing, and drying. Specifically, hexachlorocyclotriphosphazene (HCCP) and adenine are dissolved in tetrahydrofuran and then heated. The mass ratio of hexachlorocyclotriphosphazene (HCCP) to adenine is 1:(2-3), the reaction temperature is 60-80 °C, and the reaction time is 22-26 h.

[0038]

[0039]

[0040] Step 2: Dissolve the compound of formula 4 structure and metal salts in a mixed solvent of N,N-dimethylacetamide (DMA) and deionized water respectively, stir well to obtain two solutions A and B, then fully mix solutions A and B, and obtain the compound of formula 5 structure after hydrothermal reaction, cooling, suction filtration, washing, and drying.

[0041]

[0042] In the embodiment of the present invention, the compound of formula 4 structure reacts with metal salts under hydrothermal conditions to form the compound of formula 5 structure, Bio-PMOF. The reaction temperature between the compound of formula 4 structure and metal salts is preferably 90-110 °C, and the reaction time is preferably 40-45 h; the metal salt is selected from any one of ferric chloride hexahydrate, zinc nitrate hexahydrate, cobalt nitrate hexahydrate, zirconium nitrate pentahydrate, copper nitrate hexahydrate, or nickel nitrate hexahydrate.

[0043] The embodiment of the present application also provides the application of the above nitrogen and phosphorus co-doped metal-organic framework material as a flame retardant.

[0044] According to the embodiment of the present application, the nitrogen and phosphorus co-doped metal-organic framework material is used as a flame retardant in polymer materials such as epoxy resin, polypropylene, and polyethylene materials. The addition ratio of the nitrogen and phosphorus co-doped metal-organic framework material in the polymer material is preferably 1-5 wt%. As a preferred technical solution of this application, taking epoxy resin as an example, the preparation steps for preparing an epoxy resin / nitrogen and phosphorus co-doped metal-organic framework composite material are as follows:

[0045] Dissolve the nitrogen and phosphorus co-doped metal-organic framework material in a solvent. Preferably, the solvent is an acetone solution. Stir ultrasonically for 1 to 3 hours, add the preheated epoxy resin, and continue to stir ultrasonically for 1 to 3 hours to obtain a mixture. Transfer the mixture to a heating and stirring apparatus at 60 to 100 °C for 10 to 14 hours, add the pre-melted 4,4-diaminodiphenylmethane solid, and then transfer it to a mold and heat at 80 to 120 °C for 1 to 3 hours and then continue to heat at 130 to 170 °C for 1 to 3 hours.

[0046] The following further describes the present invention in detail with specific embodiments. All raw materials involved in the embodiments are commercially available.

[0047] Example 1

[0048] This example provides a nitrogen and phosphorus co-doped metal-organic framework material and a preparation method, and the specific steps are as follows:

[0049] (1) Dissolve 1.74 g of the structure of Formula 1 and 4.05 g of adenine after drying treatment in 100 mL of tetrahydrofuran (THF) organic solvent, and fully dissolve it by ultrasonic stirring. Subsequently, transfer the mixed solution to an oil bath at 70 °C, and slowly dropwise add 6.06 g of triethylamine (TEA) solution thereto. After the dropping is completed, keep the mixed solution reacting at a constant temperature of 70 °C for 24 hours.

[0050] Then, after the reaction is completed, cool the reaction system to room temperature, and separate the solid precipitate by suction filtration. To remove impurities, wash the solid precipitate three times repeatedly with acetone and deionized water, and finally obtain a white solid precipitate. Place the white solid precipitate in an oven at 60 °C for drying, and finally prepare the compound of Formula 4, denoted as Bio-HCCP.

[0051] (2) Stir 0.837 g of the compound of Formula 4 in a mixed solution of 25 mL of N,N-dimethylacetamide (DMA) and water for 60 minutes, marked as Component A, with a volume ratio of 4:1, a reaction temperature of 100 °C, and a reaction time of 48 hours. Dissolve 0.291 g of Co(NO 3 ) 2 ·6H 2 O in the same ratio of DMA / water mixed solvent, marked as Component B. Quickly pour Component B into Component A and mix evenly. The mass ratio of the compound of Formula 4 to Co(NO 3 ) 2 ·6H 2 O is 1:2.8, the reaction temperature is 95 °C, and the reaction time is 48 h. The product is denoted as Bio-PMOFs.

[0052] Example 2

[0053] This embodiment provides a nitrogen and phosphorus co-doped metal-organic framework material and a preparation method, and the specific steps are as follows:

[0054] (1) Dissolve 1.74 g of the structure of formula 1 and 4.35 g of adenine after drying treatment in 100 mL of tetrahydrofuran (THF) organic solvent, and fully dissolve it by ultrasonic stirring. Subsequently, transfer the mixed solution to an oil bath at 70 °C, and slowly dropwise add 6.06 g of triethylamine (TEA) solution thereto. After the dropping is completed, keep the mixed solution reacting at a constant temperature of 60 °C for 26 hours.

[0055] Then, after the reaction is completed, cool the reaction system to room temperature, and separate the solid precipitate by suction filtration. To remove impurities, wash the solid precipitate three times repeatedly with acetone and deionized water, and finally obtain a white solid precipitate. Place the white solid precipitate in an oven at 60 °C for drying to finally obtain the compound of formula 4 structure.

[0056] After the reaction is completed, lower the temperature of the reaction system to room temperature, obtain a solid precipitate by suction filtration, and wash it three times repeatedly with acetone and deionized water to obtain a white solid precipitate. Then, place the white solid precipitate in an oven at 60 °C for drying for 24 h to obtain the compound of formula 4 structure.

[0057] (2) Stir 0.735 g of the compound of formula 4 structure in a mixed solution of 25 mL of N,N-dimethylacetamide (DMA) and water for 60 minutes, label it as component A, with a volume ratio of 3:1, a reaction temperature of 100 °C, and a reaction time of 48 hours. Dissolve 0.287 g of Co(NO 3 ) 2 ·6H 2 O in the same proportion of DMA / water mixed solvent, label it as component B, quickly pour component B into component A and mix evenly. The mass ratio of the compound of formula 4 structure to Co(NO 3 ) 2 ·6H 2 O is 1:2.5, the reaction temperature is 100 °C, and the reaction time is 45 h.

[0058] Example 3

[0059] This embodiment provides a nitrogen and phosphorus co-doped metal-organic framework material and a preparation method, and the specific steps are as follows:

[0060] (1) Dissolve 1.74 g of the structure of formula 1 and 5.22 g of adenine after drying treatment in 100 mL of tetrahydrofuran (THF) organic solvent, and fully dissolve it by ultrasonic stirring. Subsequently, transfer the mixed solution to an oil bath at 70 °C, and slowly dropwise add 6.06 g of triethylamine (TEA) solution thereto. After the dropping is completed, keep the mixed solution reacting at a constant temperature of 80 °C for 22 hours.

[0061] Next, after the reaction is completed, the reaction system is cooled to room temperature, and the solid precipitate is separated by suction filtration. To remove impurities, the solid precipitate is washed three times repeatedly with acetone and deionized water, and finally a white solid precipitate is obtained. The white solid precipitate is placed in an oven at 60 °C for drying, and finally the compound of formula 4 is prepared.

[0062] (2) 0.689 g of the compound of formula 4 is stirred well in a mixed solution of 25 mL of N,N-dimethylacetamide (DMA) and water for 60 minutes, marked as component A, with a volume ratio of 5:1, a reaction temperature of 100 °C, and a reaction time of 48 hours. 0.265 g of Co(NO 3 ) 2 ·6H 2 O is dissolved in the same proportion of DMA / water mixed solvent, marked as component B. Component B is quickly poured into component A and mixed evenly. The mass ratio of the compound of formula 4 to Co(NO 3 ) 2 ·6H 2 O is 1:2.6, the reaction temperature is 90 °C, and the reaction time is 50 h.

[0063] The Bio-PMOFs prepared in Examples 1-3 all have similar structures and properties. Taking the Bio-PMOF prepared in Example 1 as an example, the following is a detailed description:

[0064] Referring to Figure 1 , the crystal structure of the Bio-PMOFs material was studied by XRD. It can be seen from Figure 1 that a strong diffraction peak appears at 2θ = 9.4° for Bio-PMOFs, indicating that the material has good crystallinity. In addition, the intensity of the diffraction peak is relatively high, further indicating that Bio-PMOFs is a material with a porous structure, and there may be a large number of pores or cavities inside. To further verify the crystal structure characteristics of Bio-PMOFs, its XRD pattern was compared and analyzed with the XRD data of the adenine-based MOFs material. The results show that the diffraction peaks of BioPMOFs at 2θ = 14.4°, 17.1°, 19.1°, 20.4°, 26.6°, 27.5° and 29.6° are highly consistent with the characteristic peaks of the adenine-based MOFs, which further confirms that Bio-PMOFs has the typical crystal structure characteristics of MOFs materials. The above results fully indicate that a porous and highly crystalline MOFs structure was successfully formed during the preparation of Bio-PMOFs.

[0065] Referring to Figure 2, which is the FTIR test result of Bio-PMOFs. In the FTIR spectrum of Bio-HCCP, the characteristic peaks at 1673 cm-1 and 1604 cm-1 correspond to the stretching vibration of the C=N bond in the adenine structure, and the characteristic peaks at 1211 cm-1 and 883 cm-1 belong to P=N and P-N in the HCCP structure. In addition, the intensity of the stretching vibration peaks of the P-Cl bond in the HCCP structure at 520 cm-1 and 602 cm-1 is significantly weakened. This proves the successful synthesis of the Bio-HCCP ligand. In the FTIR spectrum of Bio-PMOFs, in addition to the typical characteristic peaks of the Bio-HCCP ligand, a new characteristic peak appears at 483 cm-1, mainly attributed to the vibration of the Co-N bond, confirming the successful preparation of Bio-PMOFs.

[0066] Reference Figure 3 , which is the XPS characterization result of Bio-PMOFs. In the figure, a is the full XPS spectrum of Bio-PMOFs, and the characteristic peaks of C1s, N 1s, P 2p, and Co 2p can be clearly seen. This result corroborates with the infrared spectroscopy analysis result, further confirming the elemental composition of the material. Figure b is the high-resolution XPS spectrum of N1s. After peak fitting, five characteristic peaks can be observed at 400.9 eV, 399.8 eV, 399.2 eV, 398.5 eV, and 397.9 eV, corresponding to pyrrole-N, N-Co, N-P, N=P, and pyridine-N bonds respectively. Figure c is the P 2p spectrum. Bio-PMOFs show two characteristic peaks at 133.9 eV and 132.7 eV, which are attributed to P=N and P-N bonds respectively, consistent with the valence bond state of the P element in the Bio-PMOFs nanoflowers. Figure d shows the high-resolution XPS spectrum of Co 2p, and four characteristic peaks are observed at 797.8 eV, 781.4 eV, 802.6 eV, and 787.3 eV, corresponding to Co 2p3 / 2, Co 2p1 / 2, and their satellite peaks respectively. The XPS test results fully indicate that the Bio-PMOFs nanoflowers have been successfully prepared, and their elemental composition and chemical bonding state are consistent with the expected structure.

[0067] Reference Figure 5, SEM was used to characterize the microscopic morphology of the Bio-HCCP ligand and Bio-PMOFs materials. As shown in Figure a, the Bio-HCCP ligand presents a uniform granular nanostructure with a diameter distribution in the range of dozens of nanometers. After binding the Bio-HCCP ligand with cobalt ions (Co2+) through a coordination reaction, the Bio-PMOFs material was successfully prepared. As shown in Figures b-d, the Bio-PMOFs presents a typical disordered stacked nanostructure of nanosheets, and the overall morphology is nanospherical. To further confirm the elemental composition of the material, EDS was used to analyze the elemental distribution of the Bio-PMOFs nanospheres. The results in Figure e show that the C, N, P, and Co elements in the material are uniformly distributed, and the distribution regions of each element highly coincide with the structural morphology of the nanospheres. Based on the above characterization results, it can be confirmed that the adenine-based Bio-PMOFs material with a nanospherical morphology was successfully prepared.

[0068] Example 4

[0069] This example provides an application of a nitrogen and phosphorus co-doped metal-organic framework material.

[0070] Dissolve and disperse 0.2 g of the prepared Bio-PMOF flame retardant in acetone and stir ultrasonically for 3 h. Then add 40 g of preheated epoxy resin to the above solution and continue to stir ultrasonically for 3 h. Transfer the mixed solution to an oil bath and stir at 80 °C for 12 h. After adding a certain amount of molten 4,4-diaminodiphenylmethane solid, quickly pour it into a mold and transfer it to an oven. Cure at 100 °C and 150 °C for 3 h respectively to prepare the epoxy resin composite EP / Bio-PMOF 0.5, and the addition amount of the Bio-PMOF flame retardant is 0.5 wt%.

[0071] Example 5

[0072] It is basically the same as Example 4, except that the addition amount of the Bio-PMOF flame retardant is 2.0 wt%, and other process conditions are the same, and the epoxy resin composite EP / Bio-PMOF 2 is prepared.

[0073] Example 7

[0074] It is basically the same as Example 4, except that the addition amount of the Bio-PMOF flame retardant is 1.0 wt%, and other process conditions are the same, and the epoxy resin composite EP / Bio-PMOF 1 is prepared.

[0075] The flame retardant properties of the composites prepared in Examples 4-7 and pure EP were first evaluated by LOI and UL-94 tests.

[0076] The results are as Figure 6As shown, the LOI value of pure EP is 23.5%, and it fails to obtain any rating in the UL-94 test, indicating its flammable characteristics. However, by introducing hybrid materials, the LOI value of the composite material is significantly improved. Specifically, the LOI values of the EP / Bio-HCCP 2, EP / Bio-PMOFs 0.5, EP / Bio-PMOFs 1, and EP / Bio-PMOFs 2 composite materials are increased to 26.5%, 25.8%, 26.6%, and 27.3% respectively. As shown in a of the figure, compared with pure EP, the EP / Bio-PMOFs 2 composite material shows only a weak unstable flame after two ignitions, and its total combustion duration is significantly shortened, reaching the V-1 level in the UL-94 test. Based on the above experimental results, it can be concluded that Bio-PMOFs nanoflowers, as an efficient flame retardant additive, have great potential in improving the flame retardant properties of EP-based composite materials.

[0077] Furthermore, the char residue analysis and flame retardant mechanism study of the composite materials were carried out. The details are as follows:

[0078] The morphological characteristics of the char residues of the composite materials were characterized by digital photos and SEM images, and the results are shown in a-e and a1-e1 of the figure respectively. It can be observed from a and a1 of the figure that the pure EP matrix is almost completely decomposed after combustion, and its residual char layer shows significant discontinuous characteristics, with a large number of macroscopic crack structures on the surface. When 2.0 wt% of Bio-HCCP is introduced into the system (shown in b and b1 of the figure), the integrity of the char residue of the composite material is improved, but the char layer still shows the microscopic structural characteristics of being porous and loose, and these pore structures may become channels for heat transfer and combustible gas diffusion during combustion. It is worth noting that the char residue layer of the EP / Bio-PMOFs 0.5 composite material (shown in c and c1 of the figure) shows a complete and continuous microscopic structure, and no obvious crack or pore defects are observed. With the increase in the content of Bio-PMOFs nanoflowers (shown in d, d1, e, and e1 of the figure), the densification degree of the char layer surface is significantly improved. The significant improvement in this char layer structure is mainly attributed to the synergistic effect of the following two factors: one is the catalytic effect of transition metal oxides, and the other is the catalytic effect of phosphoric acid and pyrophosphate generated by Bio-MOFs during combustion. The synergistic effect of the two promotes the formation of a dense char layer.

[0079] As Figures 7 - 8As shown, the Raman spectroscopy analysis is the result graph for evaluating the graphitization degree of the char residue. The Raman spectra of the char residues of pure EP and its composites are presented. The characteristic peaks observed at 1589 cm-1 and 1353 cm-1 correspond to the vibration of the G band of graphitic carbon and the D band of disordered carbon, respectively. The graphitization degree of the material can be quantitatively evaluated by calculating the intensity ratio of the D band to the G band (ID / IG), where a lower ID / IG value indicates a higher graphitization degree of the char residue. As Figure 7 and Figure 8 shown, the ID / IG values of the composites show the following results: EP / Bio-PMOFs 2 (2.01) < EP (3.12). Among them, the EP / Bio-PMOFs 2 composite shows the lowest ID / IG value, which confirms its highest graphitization degree, mainly attributed to the synergistic catalytic carbonization effect of phosphorus elements and metal oxides in Bio-PMOFs. This highly graphitized char residue layer can not only serve as an effective thermal insulation layer but also as a diffusion barrier for combustible gases, thus significantly inhibiting the combustion process and ultimately enhancing the flame retardancy of the composite.

[0080] The FTIR analysis results of the char residue of the EP / Bio-PMOFs composite are as Figure 9 shown. In the FTIR spectrum, the strong absorption peak observed at 3415 cm-1 is attributed to the stretching vibration of the hydroxyl (-OH) structure in the char residue, and the absorption peaks at 1593 cm-1 and 1430 cm-1 correspond to the C=C stretching vibration mode of the aromatic ring structure. It is worth noting that due to the presence of the -P=N- structure in Bio-PMOFs, characteristic absorption peaks appear at 1174 cm-1, 1058 cm-1, and 875 cm-1 after the composite is burned. These characteristic peaks are attributed to the bending vibrations of P=O, P-O-C, and P-O-P bonds, respectively, indicating that the phosphorus-containing derivatives generated by the thermal decomposition of Bio-PMOFs enter the char layer and can effectively enhance the strength of the char layer. In addition, with the addition of the Bio-PMOFs filler, the vibration absorption peak of the Me-O bond of the metal oxide generated by the oxidation of the filler appears at 586 cm-1. The presence of these phosphorus-containing structures and metal oxides promotes the formation of a stable char layer, effectively enhancing the shielding effect of the char layer and thus improving the flame retardancy of the composite.

[0081] Where not covered above, the prior art shall apply.

[0082] Although some specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are only for illustration purposes and not for limiting the scope of the present invention. Those skilled in the technical field to which the present invention pertains can make various modifications, supplements, or use similar methods of substitution to the specific embodiments described, but will not deviate from the direction of the present invention or exceed the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent substitutions, improvements, etc. made to the above embodiments based on the technical essence of the present invention should all be included within the protection scope of the present invention.

Claims

1. A method for preparing a nitrogen-phosphorus co-doped metal-organic framework material, characterized in that: The following steps are involved: S1, dissolving hexachlorocyclotriphosphazene and adenine in an organic solvent by ultrasonication, adding an organic base and heating for reaction, cooling, filtering, washing and drying to obtain compound 1; S2. Reacting compound 1 with a metal salt under hydrothermal conditions to form compound 2, which is a nitrogen-phosphorus co-doped metal organic framework material.

2. The preparation method according to claim 1, characterized in that The organic solvent includes any one of tetrahydrofuran, methanol or chlorobenzene.

3. The preparation method according to claim 2, characterized in that: The organic base is triethylamine.

4. The preparation method according to claim 3, characterized in that: The mass ratio of the hexachlorocyclotriphosphazene to the adenine is 1:(2-3), the temperature of the heating reaction is 60-80° C., and the reaction time is 22-26 hours.

5. The preparation method according to claim 1, characterized in that: The metal salt is any one of ferric chloride hexahydrate, zinc nitrate hexahydrate, cobalt nitrate hexahydrate, zirconium nitrate pentahydrate, copper nitrate hexahydrate or nickel nitrate hexahydrate.

6. The preparation method according to claim 1, characterized in that: In step S2, the solvent of the hydrothermal reaction is a mixed solution of N,N-dimethylacetamide and water, the volume ratio is (3-5):1, the reaction temperature is 90-110°C, and the reaction time is 45-50h.

7. The preparation method according to claim 6, characterized in that: The mass ratio of compound 1 to metal salt is 1:(2-3).

8. The nitrogen-phosphorus co-doped metal organic framework material is prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the nitrogen-phosphorus co-doped metal organic framework material as claimed in claim 8 in the preparation of flame retardant products.

10. A polymer flame retardant material, characterized in that: The nitrogen-phosphorus co-doped metal organic framework material as claimed in claim 8 is added to the epoxy resin in an amount of 1 to 5 wt %.

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