Preparation method of composite modified magnesium-based flame retardant

Abstract

This invention relates to the field of flame retardant technology, specifically to a method for preparing a composite modified magnesium-based flame retardant. The method comprises: 40-70 parts by weight of modified magnesium hydroxide composite material, 10-30 parts by weight of aluminum hydroxide, 3-10 parts by weight of zinc oxide, and 10-20 parts by weight of microencapsulated flame retardant. This invention combines attapulgite-templated modified magnesium hydroxide, iron tannate-coated aluminum hypophosphite microencapsulated flame retardant, and aluminum hydroxide and zinc oxide, with mechanical activation to achieve synergistic flame retardancy in both the gas and condensed phases. The thermal decomposition temperature gradients of each component are complementary, synergistically endothermic, flame-suppressing, and catalytic char formation to suppress smoke. Simultaneously, particle size is controlled to form a multi-level compact packing structure, reducing particle agglomeration, improving filling density and processability, and rapidly generating a dense and stable char layer during combustion, significantly improving flame retardancy, smoke suppression, and thermal stability, reducing the amount of flame retardant used, and taking into account the material's mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of flame retardant technology, and specifically to a method for preparing a composite modified magnesium-based flame retardant. Background Technology

[0002] Polyolefin materials are widely used in construction, transportation, and electronics due to their excellent physical and mechanical properties, processability, and cost advantages. However, their low limiting oxygen index, flammability, and dripping properties necessitate the addition of flame retardants to ensure safe use. Magnesium hydroxide is a halogen-free, environmentally friendly flame retardant with good smoke suppression properties, playing a crucial role in polyolefin flame retardancy. However, the flame retardant efficiency of magnesium hydroxide alone is limited. To achieve flame retardant ratings such as UL94V-0, a high filler content is typically required. This high filler content severely degrades the mechanical properties and processing flowability of polyolefin materials, becoming a key bottleneck restricting its widespread application.

[0003] To reduce the amount of magnesium hydroxide filler and improve its dispersibility, existing technologies often employ surface modification treatment and physical compounding with phosphorus-containing flame retardants, aiming to enhance overall flame retardant efficiency by leveraging the combined functions of different components during thermal decomposition, such as endothermic dehydration, dilution of combustible gases, and promotion of char formation. However, currently available commercially available simple physical compounding products remain at the powder mixing level, mainly exhibiting the following problems: First, the particle size distribution of each component lacks design, failing to form efficient and dense packing within the polymer matrix. This results in large free volumes between particles, numerous interface defects, and uneven dispersion, which not only reduces flame retardant efficiency but also introduces stress concentration under material stress, further impairing mechanical properties. Second, the thermal decomposition temperature range and flame retardant action modes of different flame retardant components differ, failing to utilize their gradient thermal decomposition synergistic effect and thus failing to form a coherent and synergistic condensed phase and gas phase flame retardant barrier. Third, powdered compounding is highly susceptible to stratification, bridging, and agglomeration during storage, transportation, and processing due to differences in particle density and size, leading to uneven distribution of the flame retardant composition and poor stability of flame retardant performance.

[0004] Therefore, this application develops a method for preparing a composite modified magnesium-based flame retardant that utilizes surface modification and compounding effects, and can also significantly improve flame retardant efficiency and reduce filler content through component particle size distribution, synergistic thermal decomposition gradient, and efficient catalytic char formation, in order to solve the problems in the prior art. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a method for preparing a composite modified magnesium-based flame retardant.

[0006] This invention provides a method for preparing a composite modified magnesium-based flame retardant, comprising: S1: Preparation of modified magnesium hydroxide composite material; First, attapulgite, magnesium chloride, and sodium hydroxide were used as raw materials to prepare magnesium hydroxide composite material. Then, the modified magnesium hydroxide composite material was prepared by ball milling with stearic acid and silane coupling agent. S2: Preparation of microencapsulated flame retardants; Microcapsule flame retardants were prepared by in-situ polymerization using aluminum hypophosphite as the core material and a chelate formed by tannic acid and ferric chloride hexahydrate as the shell material. S3: Preparation of composite modified magnesium-based flame retardant; A composite modified magnesium-based flame retardant was prepared by premixing 40-70 parts by weight of modified magnesium hydroxide composite material, 10-30 parts by weight of aluminum hydroxide, 3-10 parts by weight of zinc oxide and 10-20 parts by weight of microencapsulated flame retardant, and then adding an activator.

[0007] Among them, the median particle size D50 of the modified magnesium hydroxide composite material is 5-15 μm, the median particle size D50 of aluminum hydroxide is 1-5 μm, the median particle size D50 of zinc oxide is 0.1-1 μm, and the median particle size D50 of the microencapsulated flame retardant is 2-8 μm.

[0008] As a preferred aspect, S1: The preparation of the modified magnesium hydroxide composite material specifically includes the following steps: S1.1: Add 20-30 parts by weight of attapulgite to 200-300 parts by weight of deionized water, then stir and mix at 300-500 rpm for 20-30 min at 75-80℃. Then add 320-350 parts by weight of 1M magnesium chloride solution and continue stirring and mixing for 1-2 h. Then raise the temperature to 83-85℃, add 300-320 parts by weight of 3M sodium hydroxide solution, and continue stirring and mixing for 2-3 h. After the reaction is complete, centrifuge and wash with deionized water 3-5 times. Finally, dry at 70℃ to constant weight to obtain magnesium hydroxide composite material. S1.2: Place the magnesium hydroxide composite material in a high-speed mixer and then treat it at 90-92℃ for 20-30 min. Then add stearic acid and silane coupling agent and stir for 30-40 min. After stirring, transfer it to a ball mill jar, add zirconium balls and mix. The ball-to-material ratio is 5-6:1. Then ball mill for 1-2 h. After ball milling, sieve to obtain the modified magnesium hydroxide composite material.

[0009] As a preferred aspect, in step S1.2, the amount of stearic acid added is 4-5 wt% of magnesium hydroxide, and the amount of silane coupling agent added is 2-3 wt% of magnesium hydroxide.

[0010] As a preferred aspect, the silane coupling agent in step S1.2 is one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, and vinyltriethoxysilane.

[0011] As a preferred aspect, S2: the preparation of the microencapsulated flame retardant specifically includes the following steps: S2.1: Add 5-10 parts by weight of aluminum hypophosphite to 20-30 parts by weight of deionized water, and ultrasonically disperse for 20-30 minutes to obtain an aluminum hypophosphite dispersion. Add 0.5-0.8 parts by weight of tannic acid and 0.02-0.03 parts by weight of ferric chloride hexahydrate to 25-30 parts by weight of deionized water, and stir and mix at 200-300 rpm for 20-30 minutes to obtain a mixed solution. S2.2: Mix the aluminum hypophosphite dispersion and the mixed solution, then add 1-2 mol / L sodium hydroxide solution to adjust the pH to 8-9. Then stir at 400-500 rpm for 10-20 min at room temperature, ultrasonically disperse for 20-30 min, centrifuge, and wash the precipitate 3-5 times with deionized water. Finally, dry at 60-70℃ to constant weight to obtain the microencapsulated flame retardant.

[0012] As a preferred aspect, S3: the preparation of the composite modified magnesium-based flame retardant specifically includes the following steps: S3.1: Mix 40-70 parts by weight of modified magnesium hydroxide composite material, 10-30 parts by weight of aluminum hydroxide, 3-10 parts by weight of zinc oxide and 10-20 parts by weight of microencapsulated flame retardant at 300-500 rpm for 20-30 min to obtain a premix. S3.2: Add an activator to the premix at 25-30℃, and then stir at 2000-3000 rpm for 2-3 hours to obtain a composite modified magnesium-based flame retardant.

[0013] As a preferred aspect, the amount of activator added in step S3.2 is 1-2% of the amount of premixed material.

[0014] As a preferred aspect, the activator in step S3.2 is one of titanate coupling agent or aluminate coupling agent.

[0015] The present invention has the following advantages: 1. This invention employs a modified magnesium hydroxide composite material and a microencapsulated flame retardant, and introduces aluminum hydroxide and zinc oxide to achieve a synergistic effect of flame retardancy in both the condensed and gas phases. The modified magnesium hydroxide composite material uses attapulgite as a template to load magnesium hydroxide, and is surface-modified with stearic acid and silane coupling agents. This significantly improves the dispersibility and interfacial compatibility of the inorganic flame retardant in the polymer, and the nanofiber structure of attapulgite provides skeletal support during combustion, enhancing the char layer strength. The microencapsulated flame retardant uses a tannic acid-iron ion complex as a wall material to encapsulate aluminum hypophosphite, inhibiting premature decomposition and moisture absorption of aluminum hypophosphite, and utilizing the polyphenolic structure of tannic acid and the catalytic effect of iron ions. The process promotes polymer crosslinking to form char and captures free radicals. After compounding, aluminum hydroxide and zinc oxide are introduced, and the components form a gradient complementarity in the thermal decomposition temperature range: aluminum hydroxide first absorbs heat and dehydrates in the low temperature range, magnesium hydroxide releases water vapor in the medium temperature range to dilute combustible gases, microencapsulated flame retardant releases phosphorus-containing active species and promotes char formation in a wide temperature range, and zinc oxide further catalyzes char formation, inhibits smoke generation, and enhances the density of the char layer. With the help of mechanochemical activation by activator, the surface activity of each component is improved and the interparticle binding force is enhanced. While significantly improving flame retardant efficiency, reducing heat release rate and smoke density, it can effectively reduce the total amount of flame retardant added, taking into account both the flame retardant performance and mechanical properties of the material.

[0016] 2. This invention achieves a multi-level, tightly packed structure by precisely controlling the particle size distribution of each component. Larger modified magnesium hydroxide particles form the main framework, while aluminum hydroxide and microencapsulated flame retardant particles fill the gaps. Ultrafine zinc oxide particles further fill the micropores, effectively reducing the free volume and agglomeration defects of the flame retardant particles in the polymer matrix. This particle size distribution design significantly improves the dispersion uniformity and maximum filling density of the flame retardant in the resin, reduces the system viscosity, and improves processing performance. Simultaneously, the tight packing facilitates the rapid formation of a continuous, dense, and non-detachable protective char layer during combustion, enhancing the heat insulation, oxygen barrier, and smoke suppression effects. In particular, the ultrafine zinc oxide powder has a high specific surface area and surface activity, allowing it to fully contact polymer degradation products, efficiently catalyze cross-linking into char, and work with the multi-level particles to enhance the density of the char layer. This synergistically improves the limiting oxygen index, vertical burning rating, and thermal stability of the flame retardant material while maintaining good mechanical properties. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the preparation method of the composite modified magnesium-based flame retardant used in the embodiments of the present invention. Detailed Implementation

[0018] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of this invention.

[0019] Example 1: A method for preparing a composite modified magnesium-based flame retardant, referring to... Figure 1 ,include: S1: Preparation of modified magnesium hydroxide composite material S1.1: Add 20 parts by weight of attapulgite to 200 parts by weight of deionized water, then stir and mix at 300 rpm for 20 min at 75 °C. Then add 320 parts by weight of 1M magnesium chloride solution and continue stirring and mixing for 1 h. Then raise the temperature to 83 °C, add 300 parts by weight of 3M sodium hydroxide solution, and continue stirring and mixing for 2 h. After the reaction is complete, centrifuge and wash with deionized water 3 times. Finally, dry at 70 °C to constant weight to obtain magnesium hydroxide composite material. S1.2: The magnesium hydroxide composite material was placed in a high-speed mixer and then treated at 90°C for 20 min. Then, 4 wt% stearic acid and 2 wt% γ-aminopropyltriethoxysilane were added and stirred for 30 min. After stirring, the mixture was transferred to a ball mill jar, and zirconium balls were added and mixed. The ball-to-material mass ratio was 5:1. The mixture was then ball-milled for 1 h. After ball milling, the mixture was sieved to obtain the modified magnesium hydroxide composite material. S2: Preparation of microencapsulated flame retardants S2.1: Add 5 parts by weight of aluminum hypophosphite to 20 parts by weight of deionized water and sonicate for 20 min to obtain aluminum hypophosphite dispersion. Add 0.5 parts by weight of tannic acid and 0.02 parts by weight of ferric chloride hexahydrate to 25 parts by weight of deionized water and stir at 200 rpm for 20 min to obtain mixed solution. S2.2: Mix the aluminum hypophosphite dispersion and the mixed solution, then add 1 mol / L sodium hydroxide solution to adjust the pH to 8, then stir at 400 rpm for 10 min at room temperature, ultrasonically disperse for 20 min, then centrifuge, wash the precipitate three times with deionized water, and finally dry at 60℃ to constant weight to obtain the microencapsulated flame retardant. S3: Preparation of Composite Modified Magnesium-Based Flame Retardants S3.1: Mix 40 parts by weight of modified magnesium hydroxide composite material, 10 parts by weight of aluminum hydroxide, 3 parts by weight of zinc oxide and 10 parts by weight of microencapsulated flame retardant at 300 rpm for 20 min to obtain a premix. Among them, the median particle size D50 of the modified magnesium hydroxide composite material is 5 μm, the median particle size D50 of aluminum hydroxide is 1 μm, the median particle size D50 of zinc oxide is 0.1 μm, and the median particle size D50 of the microencapsulated flame retardant is 2 μm. S3.2: At 25°C, add titanate coupling agent to the premix at 1% of the premix mass, and then stir at 2000 rpm for 2 hours to obtain a composite modified magnesium-based flame retardant.

[0020] Example 2, a method for preparing a composite modified magnesium-based flame retardant, see [link to example]. Figure 1 ,include: S1: Preparation of modified magnesium hydroxide composite material S1.1: Add 30 parts by weight of attapulgite to 300 parts by weight of deionized water, then stir and mix at 500 rpm for 30 min at 80 °C. Then add 350 parts by weight of 1M magnesium chloride solution and continue stirring and mixing for 2 h. Then raise the temperature to 85 °C, add 320 parts by weight of 3M sodium hydroxide solution, and continue stirring and mixing for 3 h. After the reaction is complete, centrifuge and wash with deionized water 5 times. Finally, dry at 70 °C to constant weight to obtain magnesium hydroxide composite material. S1.2: The magnesium hydroxide composite material was placed in a high-speed mixer and then treated at 92°C for 30 min. Then, 5 wt% stearic acid and 3 wt% γ-glycidyl etheroxypropyltrimethoxysilane were added and stirred for 40 min. After stirring, the mixture was transferred to a ball mill jar, and zirconium balls were added and mixed. The ball-to-material ratio was 6:1. The mixture was then ball-milled for 2 h. After ball milling, the mixture was sieved to obtain the modified magnesium hydroxide composite material. S2: Preparation of microencapsulated flame retardants S2.1: Add 10 parts by weight of aluminum hypophosphite to 30 parts by weight of deionized water and sonicate for 30 min to obtain aluminum hypophosphite dispersion. Add 0.8 parts by weight of tannic acid and 0.03 parts by weight of ferric chloride hexahydrate to 30 parts by weight of deionized water and stir at 300 rpm for 30 min to obtain mixed solution. S2.2: Mix the aluminum hypophosphite dispersion and the mixed solution, then add 2 mol / L sodium hydroxide solution to adjust the pH to 9, then stir at 500 rpm for 20 min at room temperature, ultrasonically disperse for 30 min, then centrifuge, wash the precipitate 5 times with deionized water, and finally dry at 70℃ to constant weight to obtain the microencapsulated flame retardant. S3: Preparation of Composite Modified Magnesium-Based Flame Retardants S3.1: Mix 70 parts by weight of modified magnesium hydroxide composite material, 30 parts by weight of aluminum hydroxide, 10 parts by weight of zinc oxide and 20 parts by weight of microencapsulated flame retardant at 500 rpm for 30 min to obtain a premix. Among them, the median particle size D50 of the modified magnesium hydroxide composite material is 15 μm, the median particle size D50 of aluminum hydroxide is 5 μm, the median particle size D50 of zinc oxide is 1 μm, and the median particle size D50 of the microencapsulated flame retardant is 8 μm. S3.2: At 30°C, an aluminate coupling agent is added to the premix at 2% of the premix mass. The mixture is then stirred at 3000 rpm for 3 hours to obtain a composite modified magnesium-based flame retardant.

[0021] Example 3, a method for preparing a composite modified magnesium-based flame retardant, see [link to example]. Figure 1 ,include: S1: Preparation of modified magnesium hydroxide composite material S1.1: 25 parts by weight of attapulgite were added to 250 parts by weight of deionized water, and then stirred at 400 rpm for 25 min at 77 °C. Then, 336 parts by weight of 1M magnesium chloride solution were added, and stirring was continued for 1.5 h. After that, the temperature was raised to 84 °C, and 310 parts by weight of 3M sodium hydroxide solution were added. Stirring was continued for 2.5 h. After the reaction was completed, the mixture was centrifuged and washed 4 times with deionized water. Finally, it was dried at 70 °C to constant weight to obtain magnesium hydroxide composite material. S1.2: The magnesium hydroxide composite material was placed in a high-speed mixer and then treated at 91°C for 25 min. Then, 4.5 wt% stearic acid and 2.5 wt% vinyltriethoxysilane were added and stirred for 35 min. After stirring, the mixture was transferred to a ball mill jar, and zirconium balls were added and mixed. The ball-to-material ratio was 5.5:1. The mixture was then ball-milled for 1.5 h. After ball milling, the mixture was sieved to obtain the modified magnesium hydroxide composite material. S2: Preparation of microencapsulated flame retardants S2.1: Add 7 parts by weight of aluminum hypophosphite to 25 parts by weight of deionized water and sonicate for 25 min to obtain aluminum hypophosphite dispersion. Add 0.7 parts by weight of tannic acid and 0.025 parts by weight of ferric chloride hexahydrate to 27 parts by weight of deionized water and stir at 250 rpm for 25 min to obtain mixed solution. S2.2: Mix the aluminum hypophosphite dispersion and the mixed solution, then add 1.5 mol / L sodium hydroxide solution to adjust the pH to 8.5. Then stir at 450 rpm for 15 min at room temperature, sonicate for 25 min, centrifuge, wash the precipitate 4 times with deionized water, and finally dry at 65℃ to constant weight to obtain the microencapsulated flame retardant. S3: Preparation of Composite Modified Magnesium-Based Flame Retardants S3.1: Mix 55 parts by weight of modified magnesium hydroxide composite material, 20 parts by weight of aluminum hydroxide, 7 parts by weight of zinc oxide and 15 parts by weight of microencapsulated flame retardant at 400 rpm for 25 min to obtain a premix. Among them, the median particle size D50 of the modified magnesium hydroxide composite material is 10 μm, the median particle size D50 of aluminum hydroxide is 3 μm, the median particle size D50 of zinc oxide is 0.5 μm, and the median particle size D50 of the microencapsulated flame retardant is 5 μm. S3.2: Add titanate coupling agent to the premix at 25-30℃, the amount of titanate coupling agent added is 1.5% of the mass of the premix, and then stir at 2500rpm for 2.5h to obtain composite modified magnesium-based flame retardant.

[0022] Comparative Example 1 differs from Example 1 in that step S1 is removed, and the modified magnesium hydroxide composite material in step S3 is replaced with an equal amount of microencapsulated flame retardant, while the remaining steps remain unchanged to prepare a composite modified magnesium-based flame retardant. This is referred to as Comparative Example 1.

[0023] Comparative Example 2 differs from Example 1 in that step S2 is removed, and the microencapsulated flame retardant in step S3 is replaced with an equal amount of modified magnesium hydroxide composite material, while the remaining steps remain unchanged to prepare a composite modified magnesium-based flame retardant. This is referred to as Comparative Example 2.

[0024] Comparative Example 3 differs from Example 1 in that aluminum hydroxide and zinc oxide are removed in step S3, and only modified magnesium hydroxide composite material and microencapsulated flame retardant are compounded together with a weight ratio of 5:1. The total number of parts is the same as that of the premix in Example 1. The remaining steps are unchanged to prepare the composite modified magnesium-based flame retardant, and it is referred to as Comparative Example 3.

[0025] Comparative Example 4 differs from Example 1 in that the median particle size D50 of the raw materials used in Comparative Example 4 is uniformly controlled to 2 μm, while the other steps remain unchanged in the preparation of the composite modified magnesium-based flame retardant.

[0026] Comparative Example 5 differs from Example 1 in that step S3.2 is removed in Comparative Example 5, while the remaining steps remain unchanged to prepare the premix, which is a composite modified magnesium-based flame retardant, and is referred to as Comparative Example 5.

[0027] 100g of polypropylene, 20g of composite modified magnesium-based flame retardant, and 1g of antioxidant 1010 were premixed in a high-speed mixer for 5 minutes. Then, the mixture was melt-blended and granulated using a twin-screw extruder at an extrusion temperature of 200℃ and a screw speed of 80rpm. After drying the resulting granules at 80℃ for 4 hours, standard test specimens were prepared using an injection molding machine at an injection temperature of 200℃.

[0028] Polypropylene (PP) with a melt flow index (MFI) of 3.0 g / 10 min (230℃, 2.16 kg).

[0029] Limiting Oxygen Index (LOI): Tested according to ASTM D2863 standard, specimen size 130×6.5×3.2mm. 3 .

[0030] Vertical flammability rating: Tested according to UL-94 standard, with a sample thickness of 3.2 mm.

[0031] Mechanical properties: Tensile strength and elongation at break were tested according to ASTM D638, with a tensile rate of 50 mm / min.

[0032] The flame retardant properties of the standard test strips prepared in Examples 1-3 and Comparative Examples 1-3 were determined. The tests were conducted three times, and the average value was taken. The test results are shown in Table 1.

[0033] Table 1. Flame retardant performance test results of Examples 1-3 and Comparative Examples 1-3

[0034] Example 1 38.5 V-0 Example 2 39.2 V-0 Example 3 38.8 V-0 Comparative Example 1 27.2 V-2 Comparative Example 2 28.3 V-2 Comparative Example 3 31.5 V-1

[0035] As can be seen from the data in Table 1, Comparative Example 1 shows that the lack of cooling and dilution by magnesium hydroxide and the charring effect of the attapulgite framework leads to the failure of flame retardancy in both the gas phase and condensed phase. Comparative Example 2 shows that the lack of phosphorus-containing active species catalyzing charring and free radical capture makes it difficult for the heat-insulating and oxygen-barrier char layer to form quickly, resulting in a poorer flame retardant effect. Comparative Example 3 confirms the necessity of low-temperature heat absorption by aluminum hydroxide and catalytic charring and smoke suppression by zinc oxide. The data from Comparative Examples 1-3 show that the multi-component temperature gradient complementarity adopted in this invention significantly improves the flame retardant performance of the material.

[0036] Flame retardant and mechanical properties of Examples 1-3 and Comparative Examples 4-5 were tested three times, and the average value was taken. The test results are shown in Table 2.

[0037] Table 2. Results of flame retardancy and mechanical properties of Examples 1-3 and Comparative Examples 4-5

[0038]

[0039] As can be seen from the data in Table 2, Example 4, which uniformizes the particle size of all components to 2 μm, disrupts the dense packing structure of multi-level particles, resulting in a significant decrease in its LOI. This indicates that uniform particle size cannot form an efficient "skeleton-filler-bridging" packing, leading to an increase in the free volume between flame retardant particles and between the flame retardant particles and the resin. During combustion, it is difficult to quickly form a continuous and dense protective char layer, resulting in poor heat insulation and oxygen barrier effects. Furthermore, the flame retardant system without particle size distribution will experience severe agglomeration in the polymer matrix, forming stress concentration points and causing a decline in the mechanical properties of the material. Comparative Example 5, which removes the final high-speed activation step of the coupling agent and only performs simple premixing, also has a significantly lower LOI than Examples 1-3. This indicates that the activation process enhances the surface activity of each component, improves the bonding force between particles and the dispersibility in the resin, and allows the flame retardant components to exert a more complete synergistic effect. The lack of activation treatment leads to uneven dispersion and local agglomeration of the flame retardant, hindering the improvement of flame retardant efficiency. Moreover, simple physical mixing cannot form a strong interfacial bond between the flame retardant and the resin matrix, resulting in voids at the filler-matrix interface and deterioration of mechanical properties.

[0040] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims. Parts not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A method for preparing a composite modified magnesium-based flame retardant, characterized in that, include: S1: Preparation of modified magnesium hydroxide composite material; First, attapulgite, magnesium chloride, and sodium hydroxide were used as raw materials to prepare magnesium hydroxide composite material. Then, the modified magnesium hydroxide composite material was prepared by ball milling with stearic acid and silane coupling agent. S2: Preparation of microencapsulated flame retardants; Microcapsule flame retardants were prepared by in-situ polymerization using aluminum hypophosphite as the core material and a chelate formed by tannic acid and ferric chloride hexahydrate as the shell material. S3: Preparation of composite modified magnesium-based flame retardant; A composite modified magnesium-based flame retardant was prepared by premixing 40-70 parts by weight of modified magnesium hydroxide composite material, 10-30 parts by weight of aluminum hydroxide, 3-10 parts by weight of zinc oxide and 10-20 parts by weight of microencapsulated flame retardant, and then adding an activator. Among them, the median particle size D50 of the modified magnesium hydroxide composite material is 5-15 μm, the median particle size D50 of aluminum hydroxide is 1-5 μm, the median particle size D50 of zinc oxide is 0.1-1 μm, and the median particle size D50 of the microencapsulated flame retardant is 2-8 μm.

2. The preparation method of a composite modified magnesium-based flame retardant according to claim 1, characterized in that, S1: The preparation of the modified magnesium hydroxide composite material includes the following steps: S1.1: Add 20-30 parts by weight of attapulgite to 200-300 parts by weight of deionized water, then stir and mix at 300-500 rpm for 20-30 min at 75-80℃. Then add 320-350 parts by weight of 1M magnesium chloride solution and continue stirring and mixing for 1-2 h. Then raise the temperature to 83-85℃, add 300-320 parts by weight of 3M sodium hydroxide solution, and continue stirring and mixing for 2-3 h. After the reaction is complete, centrifuge and wash with deionized water 3-5 times. Finally, dry at 70℃ to constant weight to obtain magnesium hydroxide composite material. S1.2: Place the magnesium hydroxide composite material in a high-speed mixer and then treat it at 90-92℃ for 20-30 min. Then add stearic acid and silane coupling agent and stir for 30-40 min. After stirring, transfer it to a ball mill jar, add zirconium balls and mix. The ball-to-material ratio is 5-6:

1. Then ball mill for 1-2 h. After ball milling, sieve to obtain the modified magnesium hydroxide composite material.

3. The preparation method of a composite modified magnesium-based flame retardant according to claim 2, characterized in that, In step S1.2, the amount of stearic acid added is 4-5 wt% of magnesium hydroxide, and the amount of silane coupling agent added is 2-3 wt% of magnesium hydroxide.

4. The method for preparing a composite modified magnesium-based flame retardant according to claim 2, characterized in that, In step S1.2, the silane coupling agent is one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, and vinyltriethoxysilane.

5. The method for preparing a composite modified magnesium-based flame retardant according to claim 1, characterized in that, S2: the preparation of the microencapsulated flame retardant specifically includes the following steps: S2.1: Add 5-10 parts by weight of aluminum hypophosphite to 20-30 parts by weight of deionized water, and ultrasonically disperse for 20-30 minutes to obtain an aluminum hypophosphite dispersion. Add 0.5-0.8 parts by weight of tannic acid and 0.02-0.03 parts by weight of ferric chloride hexahydrate to 25-30 parts by weight of deionized water, and stir and mix at 200-300 rpm for 20-30 minutes to obtain a mixed solution. S2.2: Mix the aluminum hypophosphite dispersion and the mixed solution, then add 1-2 mol / L sodium hydroxide solution to adjust the pH to 8-9. Then stir at 400-500 rpm for 10-20 min at room temperature, ultrasonically disperse for 20-30 min, centrifuge, and wash the precipitate 3-5 times with deionized water. Finally, dry at 60-70℃ to constant weight to obtain the microencapsulated flame retardant.

6. The method for preparing a composite modified magnesium-based flame retardant according to claim 1, characterized in that, S3: The preparation of the composite modified magnesium-based flame retardant includes the following steps: S3.1: Mix 40-70 parts by weight of modified magnesium hydroxide composite material, 10-30 parts by weight of aluminum hydroxide, 3-10 parts by weight of zinc oxide and 10-20 parts by weight of microencapsulated flame retardant at 300-500 rpm for 20-30 min to obtain a premix. S3.2: Add an activator to the premix at 25-30℃, and then stir at 2000-3000 rpm for 2-3 hours to obtain a composite modified magnesium-based flame retardant.

7. The method for preparing a composite modified magnesium-based flame retardant according to claim 6, characterized in that, The amount of activator added in step S3.2 is 1-2% of the mass of the premix.

8. The method for preparing a composite modified magnesium-based flame retardant according to claim 6, characterized in that, The activator in step S3.2 is one of titanate coupling agent or aluminate coupling agent.

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