Preparation method and application of an ultra-low addition amount MOF-derived LDH composite flame retardant

Abstract

This invention relates to the field of flame retardant materials technology, and discloses a method for preparing and applying an ultra-low addition amount MOF-derived LDH composite flame retardant. First, MOF-based materials are loaded onto a phosphorus-nitrogen flame retardant, and then etched and modified using an acidic salt solution to form a hollow, layered surface structure of a double hydroxide, resulting in LDH@MOF@APP. The synthesized MOF flame retardant, containing LDH, reduces its impact on the physical and mechanical properties of polymer materials, while also reducing the amount of flame retardant required. The introduction of MOF-derived LDH material and the flame-retardant element nitrogen into the flame retardant imparts excellent compatibility with the polylactic acid matrix. Only extremely low addition amounts are required to achieve a UL-94 vertical burning V-0 rating with a limiting oxygen index greater than 26%. The preparation process of this invention is mature, convenient to operate, and operates under mild conditions, making it easy to promote and apply on a large scale.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant materials technology, and particularly relates to LDH composite flame retardant and its preparation method and flame retardant polymer materials. Background Technology

[0002] PLA, as a low-cost and biodegradable material, is hampered in practical applications due to its high flammability. Metal-Organic Frameworks (MOFs) are a class of crystalline porous materials formed by the interconnection of metal ions or clusters with organic ligands. They possess tunable structures, allowing for adjustments and modifications to meet flame-retardant requirements. Furthermore, MOFs, as organic-inorganic porous hybrid materials, have attracted significant attention due to their superior performance, particularly in the field of flame retardancy. The combination of MOFs with functional polymers has become an important platform for constructing polymer-based nanocomposites, encompassing aspects such as controlling polymer polymerization, enhancing polymer corrosion resistance, reducing polymer dielectric constant, and constructing polymer / MOF hybrid films. MOF materials overcome the shortcomings of inorganic flame retardants (poor compatibility with polymer matrices) and organic flame retardants (uneven molecular weight distribution and poor thermal stability), combining the potential advantages of both organic and inorganic materials. Simultaneously, MOFs, as flame retardants or synergists, exhibit excellent flame-retardant properties and can improve other performance characteristics. Currently, highly efficient flame retardants for flame-retardant polylactic acid (PLA) are scarce in the market, and their synthesis is mostly hydrothermal. This method is not only complex, requiring high temperature, high pressure, and various conditions, but also results in low yield. CN118515958A discloses a flame-retardant PLA masterbatch obtained by sequentially melt-blending a modified aluminum diethylphosphinate flame retardant with PLA masterbatch and then extruding and granulating it. Although it achieves a V-0 flame retardant rating, the synthesis process is complex and requires a large amount of different types of organic solvents, which will severely burden production costs and solvent recovery. CN202310580886.0 describes the preparation and application of a novel flower-shaped composite flame retardant based on Co / Cu-MOF, which requires reaction in a high-pressure reactor, resulting in higher energy consumption and lower yield.

[0003] This invention utilizes zeolite imidazole ester framework materials (ZIFs) as templates and employs etching techniques to prepare halogen-free flame retardants with highly efficient flame-retardant properties. The MOF precursor can prevent accidental layer aggregation that may occur during the conventional synthesis of LDHs. Therefore, the preparation of MOF-derived LDHs can provide advanced properties and promotes mass diffusion due to the formation of layered channels within the layers. This may also lead to an increase in the number of metal active sites for various applications, such as energy and environmental applications. Furthermore, MOFs and LDHs are composed using different preparation methods, introducing a heterogeneous interface between MOFs and LDHs to optimize the material. The resulting MOF / LDH hybrid possesses important properties that promote ion and mass transport. Currently, most MOF flame retardants are added only as synergists, i.e., added in two steps simultaneously with the traditional flame retardant. This invention loads the MOF component onto the traditional flame retardant, which is not only more convenient and simpler to add, but also utilizes the good compatibility between the MOF organic components and polymer materials, reducing the deterioration of the mechanical properties and transparency of plastic products. It is especially suitable for plastics such as polylactic acid, and can achieve flame retardant effect with extremely low addition amount. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide an LDH-based composite flame retardant, its preparation method, and flame-retardant polymer material, so as to at least solve the problem that existing halogen-free flame retardants require large amounts of additives to achieve the desired flame-retardant effect.

[0005] The present invention solves the above-mentioned technical problems through the following technical means:

[0006] First, MOF-based materials are loaded onto phosphorus-nitrogen flame retardants, and then etched and modified using an acidic salt solution to derive a double hydroxide structure with a hollow surface layered structure, namely LDH@MOF@APP.

[0007] The preparation method of LDH@MOF@APP composite flame retardant is as follows:

[0008] (1) Disperse the phosphorus and nitrogen flame retardant in methanol solvent to obtain a mixed solution. Add metal salt 1 to the mixed solution, stir, and then add dimethylimidazolium to it. After the reaction is complete, centrifuge to obtain a solid, dry it, and obtain MOF composite flame retardant, which is named ZIF@APP. Activate ZIF@APP by heating.

[0009] (2) After heating and activating ZIF@APP and metal salt 2, they were dissolved in solvent and mixed. The mixture was stirred thoroughly, and the product was collected, washed, and dried to obtain MOF-derived composite flame retardant, which was named LDH@MOF@APP.

[0010] Furthermore, the phosphorus-nitrogen flame retardant in step (1) is one or more of ammonium polyphosphate (APP), melamine-ammonium polyphosphate (MPP), 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives.

[0011] Furthermore, metal salt 1 and metal salt 2 are any one or more of cobalt nitrate hexahydrate, nickel nitrate hexahydrate, zinc nitrate hexahydrate, copper nitrate trihydrate, and aluminum nitrate nonahydrate. Preferably, metal salt 1 is cobalt nitrate hexahydrate and metal salt 2 is nickel nitrate hexahydrate.

[0012] Furthermore, the mass ratio of metal salt 1, dimethylimidazole and phosphorus-nitrogen flame retardant is 1:2~5:(1~5).

[0013] Further, the full reaction is carried out by reacting at 25℃~35℃ for 8~12 hours, followed by static aging for 24~48 hours. The heating activation is performed by heating ZIF@APP in a vacuum oven at 130℃~150℃ for 2~4 hours, followed by grinding.

[0014] Furthermore, the mass ratio of metal salt 2 to ZIF@APP is 1:(1.5~3).

[0015] Furthermore, thoroughly stir at a speed of 800-1000 rpm for 4-8 hours.

[0016] The above-mentioned ultra-low addition amount MOF-derived LDH composite flame retardant is used in the flame retardant application of polymer matrix.

[0017] The MOF-derived composite flame retardant (LDH@MOF@APP) prepared in this invention incorporates MOF-derived LDH materials and nitrogen as a flame-retardant element, imparting excellent compatibility with the polylactic acid matrix. Only extremely low addition amounts are required to achieve a UL-94 vertical burning V-0 rating with a limiting oxygen index greater than 27%. The preparation process of this invention is mature, convenient to operate, and operates under mild conditions, making it easy to promote and apply on a large scale. Attached Figure Description

[0018] Figure 1 This is a scanning electron microscope image of ammonium polyphosphate, the raw material in Example 2 of Embodiment 1 of the present invention.

[0019] Figure 2 This is a scanning electron microscope image of MOF-coated ammonium polyphosphate in Comparative Example 1 of this invention.

[0020] Figure 3 This is a scanning electron microscope image of the product of Example 1 of the present invention.

[0021] Figure 4 This is a scanning electron microscope image of the product of Example 1 of the present invention.

[0022] Figure 5The XPS spectra of Example 1, Comparative Example 2, and Comparative Example 5 of this invention are shown.

[0023] Figure 6 The images show the XRD patterns of Example 1, Comparative Example 1, Comparative Example 5, and ammonium polyphosphate of the present invention.

[0024] Figure 7 The thermogravimetric curves of pure polylactic acid of the present invention, Example 1, Comparative Example 1 and Comparative Example 3 are shown. Detailed Implementation

[0025] Example 1: Ammonium polyphosphate (5 g) was dispersed in methanol solvent to obtain a mixed solution. Co(NO3)2·6H2O (1.44 g) was added to the mixed solution and stirred at 300 rpm for 30 min at room temperature. Then, dimethylimidazole (4.10 g) was added and reacted at room temperature for 12 h. After standing and aging for 24 h, a light purple solid was obtained by centrifugation. The solid was further washed and purified with methanol and dried in a vacuum oven at 80 °C for 12 h. It was named ZIF67@APP.

[0026] ZIF67@APP was activated by heating at 130℃~150℃ for 2~4h in a vacuum oven, then ground. ZIF67@APP (5.0 g) and Ni(NO3)2·6H2O (2.25 g) were then dissolved separately in 100 mL of methanol. After the two solutions formed a homogeneous solution, nickel nitrate solution was poured into the ZIF@APP solution, and the mixture was stirred at 300 rpm for 1.5 h at room temperature. The desired LDH@ZIF@APP was obtained. The sample was then washed three times with ethanol and placed in a vacuum oven at room temperature.

[0027] Example 2: Ammonium polyphosphate (5 g) was dispersed in methanol solvent to obtain a mixed solution. Zn(NO3)2·6H2O (1.56 g) was added to the mixed solution and stirred at 300 rpm for 30 min at room temperature. Then, dimethylimidazole (4.10 g) was added and reacted at room temperature for 12 h. After standing and aging for 24 h, a white solid was obtained by centrifugation. The solid was further washed and purified with methanol and dried in a vacuum oven at 80 °C for 12 h. The solid was named ZIF8@APP.

[0028] ZIF8@APP was activated by heating at 130℃~150℃ for 2~4h in a vacuum oven, then ground. ZIF8@APP (5.0 g) and Al(NO3)2·9H2O (2.25 g) were then dissolved separately in 100 mL of methanol. After the two solutions formed a homogeneous solution, nickel nitrate solution was poured into the ZIF8@APP solution, and the mixture was stirred at 300 rpm for 1.5 h at room temperature to obtain the desired LDH@ZIF8@APP. The sample was then washed three times with ethanol and placed in a vacuum oven at room temperature.

[0029] Comparative Example 1: Ammonium polyphosphate (5 g) was dispersed in methanol solvent to obtain a mixed solution. Co(NO3)2·6H2O (1.44 g) was added to the mixed solution, and the mixture was stirred at 300 rpm for 30 min at room temperature. Then, dimethylimidazole (4.10 g) was added, and the reaction was allowed to proceed for 12 h. After standing and aging for 24 h, a light purple solid was obtained by centrifugation. The solid was further purified by washing with methanol and dried in a vacuum oven at 80 ℃ for 12 h, and named ZIF67@APP.

[0030] Comparative Example 2: Co(NO3)2·6H2O (1.44 g) was completely dissolved in methanol solvent and stirred at 300 rpm for 30 min at room temperature. Then, dimethylimidazole (4.10 g) was added and reacted at room temperature for 12 h. After standing and aging for 24 h, a light purple solid was obtained by centrifugation. The solid was further washed and purified with methanol and dried in a vacuum oven at 80 ℃ for 12 h, and named ZIF67.

[0031] ZIF67 (1g) was activated by heating at 130℃~150℃ in a vacuum oven for 2~4h, then ground and dispersed in 100 mL of methanol. Ni(NO3)2·6H2O (0.5g) was dissolved in 100 mL of methanol. After the two solutions formed a homogeneous solution, nickel nitrate solution was poured into the ZIF solution and stirred at 300 rpm for 1.5 h at room temperature. The desired LDH@ZIF was obtained. Then, the sample was washed three times with ethanol and placed in a vacuum oven at room temperature, named LDH@ZIF.

[0032] Comparative Example 3: ZIF67 (1g) was activated by heating at 130℃~150℃ in a vacuum oven for 2~4h, then ground and dispersed in 100 mL of methanol. Ni(NO3)2·6H2O (0.5 g) was added to the methanol, and the reaction was carried out at room temperature for 30 min. After filtration, washing, and centrifugation, a light purple solid was obtained, named LDH@ZIF. LDH@ZIF and APP were mixed according to the loading ratio of Example 1, that is, the calculated loading rate was 9.2%, named LDH@ZIF+APP(cal.).

[0033] Comparative Example 4: Commercially available ammonium polyphosphate was added directly.

[0034] Comparative Example 5: ZIF67 was used as a flame retardant. Similar to ZIF67 in Comparative Example 2, Co(NO3)2·6H2O (1.44 g) was completely dissolved in methanol solvent. The mixture was stirred at 300 rpm for 30 min at room temperature, and then dimethylimidazole (4.10 g) was added. The reaction was allowed to proceed for 12 h, followed by aging for 24 h. The resulting light purple solid was obtained by centrifugation, further purified by washing with methanol, and dried in a vacuum oven at 80 ℃ for 12 h.

[0035] Application Example 1: The flame retardants prepared in the above examples and comparative examples are used in the preparation of polylactic acid (PLA) materials: Flame retardants prepared in different percentage weights (0.5wt%~2wt%) are blended and added to PLA products. The thermoplastic temperature is 180℃, the rotation speed is 50rpm, and the processing time is 5min. The resulting material is a flame-retardant modified plastic additive-modified PLA material. Then, dumbbell-shaped samples for tensile testing, oxygen index testing, and strip samples for vertical burning are prepared using an injection molding machine. The tensile test is performed according to ASTM D638-2003, the oxygen index test is performed according to ASTM D2863, the UL-94 vertical burning performance test is performed according to ASTM D3801, and the transparency test is performed according to GB / T 2410-2008. The test results are shown in Table 1.

[0036] Table 1 Performance of modified polylactic acid products

[0037]

[0038] As shown in Table 1, the polylactic acid (PLA) products modified with plastic additives in Example 1 and Comparative Example 1, prepared by this invention, exhibit highly efficient flame-retardant properties. When the addition amount reaches 1 wt% or higher, their flame-retardant rating is V-0. Although Comparative Example 1 shows good flame-retardant effect, its mechanical properties are severely compromised once the addition exceeds 1%. Adding 4% causes the PLA to crumble directly and even become unformable. Therefore, the modified product with an LDH layered hydroxide structure demonstrates superior mechanical properties. Good mechanical properties greatly promote practical applications.

[0039] In-depth analysis of Comparative Example 2 (LDH@ZIF) and Comparative Example 3 (LDH@ZIF + APP (cal.)) reveals that LDH@ZIF alone did not exhibit a significant flame-retardant effect under extremely low addition conditions. However, when mixed with APP, even at extremely low addition levels, the flame-retardant performance of the material was significantly improved compared to LDH@ZIF alone. It should be noted, however, that the flame-retardant effect exhibited in Comparative Example 3 still lags behind that of the material loaded with APP in Example 1.

[0040] In stark contrast, other material systems using flame retardant coatings did not exhibit similar premature decomposition. This indicates that the material system in Comparative Example 3 has significant shortcomings in thermal stability during the processing of polylactic acid (PLA) materials, while the flame retardant coating helps maintain the stability of the material system within the processing temperature range to some extent, providing more favorable conditions for the effective processing of PLA materials.

[0041] Compared to pure polylactic acid (PLA), which struggles to achieve a good flame retardant rating, adding just 1% LDH@APP achieves a V-0 rating. In contrast to the unmodified version, different percentages (0.5wt% ~ 2wt%) of LDH flame retardant were blended into PLA products, and modified PLA materials were prepared using a specific process. Tests were conducted on the tensile strength, elastic modulus, oxygen index, UL-94, and other properties. The results showed that the modified PLA products exhibited highly efficient flame retardant properties, achieving a V-0 rating, demonstrating a significant advantage over other comparative methods.

[0042] Figure 1 This is a scanning electron microscope image of unmodified commercial ammonium polyphosphate feedstock. Figure 2 It was found that by simply coating MOF-modified ammonium polyphosphate, it grew well on the surface of ammonium polyphosphate. Due to the organic ligand structure of MOF, it can improve the compatibility with polylactic acid to a certain extent. However, due to the limited exposure of active sites, it is difficult to achieve a better catalytic effect. Figure 3 This is a product image etched with an acidic salt solution, which exposes more reaction sites and a good layered structure of double hydroxides, allowing for the exposure of more hydrogen bonds and resulting in improved mechanical properties of the composite material. Figure 4 The study demonstrates an etch-modified MOF with a hollow structure, and the many layered nanosheets on the surface have better compatibility with the plastic matrix.

[0043] Figure 5 The XPS images of the examples and comparative examples show and demonstrate the composition of the bimetallic layered hydroxide, which is NiCo bimetallic layered hydroxide in Example 1. Both electron microscopy and XPS images confirm the successful preparation of the flame retardant.

[0044] In the field of flame-retardant materials research, the type and content of flame-retardant elements play a crucial role in material performance. When etching ZIF67 (a metal-organic framework material) with metal salt solutions to construct specific structures, nickel salt solutions exhibit unique advantages compared to other common metal salt solutions, making them more suitable for etching ZIF67. This is because nickel salt solutions can more effectively promote the formation of hollow structures in ZIF67, which is of great significance for improving the subsequent performance of the material.

[0045] From the perspective of flame retardants, phosphorus-nitrogen synergistic flame retardants containing cobalt and nickel exhibit superior flame retardant performance and higher flame retardant efficiency. This is due to the synergistic effect between cobalt and nickel and the phosphorus-nitrogen system, which can more effectively suppress flame spread and reduce heat release during combustion.

[0046] In the application of flame retardants to plastic matrices, different layered double hydroxides (LDHs) exhibit varying dispersion properties. CoNi-LDH (cobalt-nickel layered double hydroxide) shows better dispersion in the plastic matrix compared to ZnAl-LDH (zinc-aluminum layered double hydroxide). This is primarily because ZnAl-LDH metal salts contain a greater amount of water of crystallization. During preparation, this water of crystallization enhances the interaction between particles, making agglomeration more likely. Agglomerated ZnAl-LDH is not conducive to uniform dispersion in the plastic matrix, thus affecting the overall flame retardant effect. CoNi-LDH, on the other hand, does not have this problem and can more effectively exert its flame retardant properties.

[0047] As shown in Figure 7, under specific experimental conditions, only the material system of Comparative Example 3 exhibits a rather unique thermal decomposition behavior. When the temperature reaches 230℃, the system begins to decompose prematurely. This decomposition temperature is significantly lower than the suitable temperature range required for the processing of polylactic acid materials, which is extremely detrimental to the processing of polylactic acid materials.

[0048] The above findings indicate that flame retardants (FRs) based on metal-organic frameworks (MOFs) do not offer significant advantages. However, MOFs play a crucial synergistic role. Traditional synthesis methods are complex and energy-intensive, and cannot achieve a favorable ratio between MOFs and traditional flame retardants, leading to increased dosages. The method proposed in this invention not only allows for clever control of MOF dosage through encapsulation but also further enhances compatibility with plastics and the stability of the flame retardant through MOF encapsulation.

[0049] 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 low-addition MOF-derived LDH composite flame retardant, characterized in that: First, MOF-based materials are loaded onto phosphorus-nitrogen flame retardants, and then etched and modified using acidic salt solutions to form a double-layer hydroxide structure with a hollow surface layered structure, which is the LDH composite flame retardant derived from ultra-low addition amount MOF. (1) Disperse the phosphorus-nitrogen flame retardant in methanol solvent to obtain a mixed solution. Add metal salt 1 to the mixed solution, stir, and then add dimethylimidazole to it. After the reaction is complete, centrifuge to obtain a solid, dry it, and obtain MOF composite flame retardant. (2) The MOF composite flame retardant obtained in step (1) is heated and activated. The activated MOF composite flame retardant and metal salt 2 are dissolved in solvent and then mixed. The mixture is stirred thoroughly, and the product is collected, washed, and dried to obtain the MOF-derived composite flame retardant.

2. The preparation method of the ultra-low addition amount MOF-derived LDH composite flame retardant according to claim 1, characterized in that: (1) Disperse the phosphorus and nitrogen flame retardant in methanol solvent to obtain a mixed solution. Add metal salt 1 to the mixed solution, stir, and then add dimethylimidazolium to it. After the reaction is complete, centrifuge to obtain a solid, dry it, and obtain MOF composite flame retardant. (2) The MOF composite flame retardant obtained in step (1) is heated and activated. The activated MOF composite flame retardant and metal salt 2 are dissolved in solvent and then mixed. The mixture is stirred thoroughly, and the product is collected, washed, and dried to obtain the MOF-derived composite flame retardant.

3. The preparation method of the ultra-low addition amount MOF-derived LDH composite flame retardant according to claim 2, characterized in that: The phosphorus-nitrogen flame retardant in step (1) is one or more of ammonium polyphosphate, melamine-ammonium polyphosphate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and its derivatives.

4. The preparation method of the ultra-low addition amount MOF-derived LDH composite flame retardant according to claim 2, characterized in that: Metal salt 1 and metal salt 2 are each independently selected from one or more of the following: cobalt nitrate hexahydrate, nickel nitrate hexahydrate, zinc nitrate hexahydrate, copper nitrate trihydrate, and aluminum nitrate nonahydrate.

5. The preparation method of the ultra-low addition amount MOF-derived LDH composite flame retardant according to claim 2, characterized in that: Metal salt 1 is cobalt nitrate hexahydrate, and metal salt 2 is nickel nitrate hexahydrate.

6. The method for preparing the ultra-low addition amount MOF-derived LDH composite flame retardant according to claim 2, characterized in that; The mass ratio of metal salt 1, dimethylimidazole and phosphorus nitrogen flame retardant is 1:4:(1-5).

7. The method for preparing the ultra-low addition amount MOF-derived LDH composite flame retardant according to claim 2, characterized in that; A full reaction refers to reacting at 25℃~35℃ for 8~12 hours and then aging at rest for 24~48 hours; activation by heating at 130℃~150℃ for 2~4 hours.

8. The method for preparing the ultra-low addition amount MOF-derived LDH composite flame retardant according to claim 2, characterized in that; In step (2), the mass ratio of metal salt 2 and MOF composite flame retardant is 1:(1.5~3).

9. The method for preparing the ultra-low addition amount MOF-derived LDH composite flame retardant according to claim 2, characterized in that; Mix thoroughly at 800-1000 rpm for 4-8 hours.

10. The flame retardant application of the ultra-low addition amount MOF-derived LDH composite flame retardant according to claim 1 in a polymer matrix.

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