Preparation method of hexagonal flake-shaped magnesium hydroxide nanoparticles

Abstract

This invention provides a method for preparing hexagonal sheet-like nano-magnesium hydroxide, which solves the technical problem that it is difficult to stably control the particle size of existing hexagonal sheet-like nano-magnesium hydroxide to within 100 nm. The preparation method includes: Step S1. Dispersing a silane coupling agent in water to prepare a base solution, adding a magnesium salt solution and a liquid alkali solution dropwise to the base solution using a double-dropping method, controlling the system to be in a low supersaturation state during the dropwise addition process; after the magnesium salt solution is added, increasing the dropwise addition rate, and continuing to add the remaining liquid alkali solution until the system reaches a high supersaturation state, stirring the reaction to obtain a primary magnesium hydroxide slurry; Step S2. Statically aging the primary magnesium hydroxide slurry under heating conditions to induce directional growth of crystal nuclei, obtaining an aged slurry; Step S3. Transferring the aged slurry to a high-pressure reactor for hydrothermal reaction, washing and drying the product after the reaction to obtain hexagonal sheet-like nano-magnesium hydroxide; This method can be widely applied in the field of inorganic material preparation technology.

Description

Technical Field

[0001] This application relates to the field of inorganic material preparation technology, and in particular to a method for preparing hexagonal sheet-like nano-magnesium hydroxide. Background Technology

[0002] Magnesium hydroxide, as a green and environmentally friendly inorganic chemical product, is hailed as a "green flame retardant" due to its high thermal decomposition temperature, large heat absorption, significant smoke suppression effect, and non-toxicity and non-corrosiveness. With the rapid development of 5G communication technology, new energy vehicles, aerospace, and high-end electronic packaging, the market has placed extremely high demands on material performance. Magnesium hydroxide is not only widely used in the flame-retardant modification of general-purpose plastics such as polypropylene, polyethylene, and ethylene-vinyl acetate copolymers, but also plays an irreplaceable role in copper-clad laminate substrates, lithium battery separator coating materials, high-voltage cable insulation layers, and high-end rubber products. In these high-end applications, fillers must possess extremely high purity, specific crystal morphology, and nanoscale size effects to meet the stringent standards for dielectric properties, mechanical strength, and flame-retardant efficiency.

[0003] Chinese invention patent CN 107804863 A discloses a method for preparing uniform hexagonal plate-like nano-magnesium hydroxide using a hydration process. This method uses highly reactive magnesium oxide as a raw material and performs a hydration reaction under ultrasonic assistance. Although the prepared product exhibits a good hexagonal plate-like crystal structure, its core process relies on the activity control of the magnesium oxide precursor and ultrasonic physical dispersion. This method is essentially a solid-liquid conversion mechanism, making it difficult to stably control the particle size of the final product within 100 nm. This technical problem urgently needs to be solved. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the above-mentioned technology and provide a method for preparing hexagonal sheet-like nano-magnesium hydroxide, which can stably control the particle size and has good control over the crystal form of the particles.

[0005] Therefore, the present invention provides a method for preparing hexagonal sheet-like nano-magnesium hydroxide, comprising the following steps: Step S1. Disperse the silane coupling agent in water to prepare a base solution. Add magnesium salt solution and liquid alkali solution dropwise to the base solution using a double-dropping method, maintaining the system at a low supersaturation state (pH 7-9) during the dropwise addition process. After the magnesium salt solution is added, increase the dropping rate and continue adding the remaining liquid alkali solution until the system reaches a high supersaturation state (pH 12-14). Stir the reaction to obtain a primary magnesium hydroxide slurry. The ionic equation for this step is as follows: Mg 2+ +2OH - =Mg(OH)2↓ Step S2. The primary magnesium hydroxide slurry obtained in step S1 is statically aged under heating conditions to induce directional growth of crystal nuclei, resulting in an aged slurry.

[0006] Step S3. The aged slurry obtained in step S2 is transferred to a high-pressure reactor for hydrothermal reaction. After the reaction is completed, the product is washed and dried to obtain hexagonal sheet-like nano magnesium hydroxide.

[0007] Preferably, in step S1, the concentration of the magnesium salt solution is 1-2 mol / L; the liquid alkali solution is a sodium hydroxide solution with a concentration of 1-4 mol / L.

[0008] Preferably, in step S1, the magnesium salt solution is selected from one or a mixture of magnesium chloride solution, magnesium sulfate solution, and magnesium nitrate solution.

[0009] Preferably, in step S1, the amount of silane coupling agent added is 4.2 to 5.3% of the mass of magnesium salt.

[0010] Preferably, in step S1, the silane coupling agent is selected from one or more of vinyltriethoxysilane, γ-aminopropyltriethoxysilane, and γ-epoxypropyltrimethoxysilane.

[0011] Preferably, in step S1, the double dropping rate is 1.25-15 mL / min, the amount of liquid alkali solution added is 71-91% of the total amount of liquid alkali solution, and the system is controlled to be in a low supersaturation state during the dropping process; the dropping rate of the remaining liquid alkali solution is 25-100 mL / min until the system reaches a high supersaturation state.

[0012] Preferably, in step S2, the aging temperature is 80℃~85℃ and the aging time is 20h~24h.

[0013] Preferably, in step S3, the hydrothermal reaction temperature is 180–220°C and the hydrothermal time is 10–14 h.

[0014] Preferably, in step S3, the washing method includes: washing the product with water until the washing solution is neutral.

[0015] Preferably, in step S3, the drying temperature is 120–140°C and the drying time is 10–14 hours.

[0016] This invention provides a method for preparing hexagonal plate-like nano-magnesium hydroxide. By precisely controlling the nucleation kinetics and crystallization thermodynamics, using a silane coupling agent as the reaction substrate, and controlling the dropping rate and solution saturation, nano-sized magnesium hydroxide is prepared through a hydrothermal reaction. This method can stably control the particle size within 100 nm and effectively maintain the hexagonal plate-like crystal structure of the particles, resulting in regular morphology and high dispersibility. The main advantages are as follows: (1) This invention abandons the traditional single-rate feeding method and innovatively adopts a "slow first, fast later" alkali addition process. The slow dropping stage controls the supersaturation of the system, so that the system is kept in a weakly alkaline low supersaturation state with a pH of 7 to 9 during the dropping process, avoiding excessively small crystals and agglomeration caused by explosive nucleation; the subsequent rapid addition of liquid alkali solution to the system to a high supersaturation state with a pH of 12 to 14 causes the environment to quickly become strongly alkaline, locking in the number of crystal nuclei and providing the driving force for anisotropic growth.

[0017] (2) In the early stage of nucleation, the addition of a silane coupling agent in this invention solves the problem of agglomeration between magnesium hydroxide molecules. The specific effects are as follows: By introducing silane coupling agents for in-situ coating during the precipitation stage, the reaction mechanism is as follows: the silane coupling agent molecule contains both easily hydrolyzable alkoxy groups (such as methoxy and ethoxy groups) and target organic functional groups (such as long-chain alkyl groups). In the aqueous phase system of the precipitation reaction, the silane coupling agent first undergoes a hydrolysis reaction, and the alkoxy groups are replaced by hydroxyl groups (-Si-OH) to generate active silanol intermediates. Meanwhile, the hexagonal plate-shaped nano-magnesium hydroxide crystal nuclei and the surface of the growing particles are rich in hydroxyl groups (-Mg-OH). The silanol intermediates undergo a dehydration condensation reaction with the hydroxyl groups on the surface of magnesium hydroxide to form stable Si-O-Mg covalent bonds, thereby grafting organic functional groups onto the surface of the prepared hexagonal plate-shaped nano-magnesium hydroxide. The long-chain alkyl groups of the silane coupling agent construct a steric barrier between the particles, physically isolating the contact between the particles. In addition, it effectively reduces the surface energy of magnesium hydroxide, transforming the hydrophilic surface into a hydrophobic surface, cutting off the aggregation path at the root, and improving the dispersibility of the material. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application.

[0019] Figure 1 This is a scanning electron microscope (SEM) image of the hexagonal sheet-like nano-magnesium hydroxide prepared in Example 1 of this invention; Figure 2 This is a scanning electron microscope (SEM) image of the hexagonal sheet-like nano-magnesium hydroxide prepared in Example 2 of this invention; Figure 3 This is a scanning electron microscope (SEM) image of the hexagonal sheet-like nano-magnesium hydroxide prepared in Example 3 of the present invention; Figure 4 This is a scanning electron microscope (SEM) image of magnesium hydroxide prepared in Comparative Example 1 of this invention; Figure 5This is a scanning electron microscope (SEM) image of magnesium hydroxide prepared in Comparative Example 2 of this invention; Figure 6 This is a scanning electron microscope (SEM) image of magnesium hydroxide prepared in Comparative Example 3 of this invention; Figure 7 This is a scanning electron microscope (SEM) image of the magnesium hydroxide prepared in Comparative Example 4 of this invention. Detailed Implementation

[0020] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this application.

[0021] Unless otherwise specified, the methods used in this invention are conventional methods; the raw materials and apparatus used, unless otherwise specified, are conventional commercially available products. Wherein: Magnesium chloride hexahydrate was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. Sodium hydroxide was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. Vinyltriethoxysilane was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. γ-aminopropyltriethoxysilane (KH550) was purchased from Hangzhou Jessica Chemical Co., Ltd. γ-glycidyltrimethoxysilane (KH560) was purchased from Zhongjie New Materials Co., Ltd.

[0022] Example Example 1: Example 1 provides a method for preparing hexagonal sheet-like nano-magnesium hydroxide, comprising the following steps: Step S1. Dissolve 3.5g of vinyltriethoxysilane in 200ml of deionized water and stir magnetically at 30℃. Separately prepare a 1.5mol / L magnesium chloride solution and a 1mol / L sodium hydroxide solution for later use. Using a double-dropping method, add 500ml of magnesium chloride solution at a rate of 5ml / min, and simultaneously add 1500ml of sodium hydroxide solution at a rate of 15ml / min to bring the system to a low supersaturation state with a pH of 7-9. After the addition is complete, add 200ml of sodium hydroxide solution rapidly to the reactor at a rate of 100ml / min to bring the system to a high supersaturation state with a pH of 12-14.

[0023] Step S2. The slurry obtained in step S1 is statically aged at 80°C for 20 hours. Step S3. The aged slurry is transferred to a high-pressure reactor for hydrothermal reaction. The reaction temperature is controlled at 200℃ and the time is controlled at 12h. After the reaction is completed, the product is washed with deionized water until the washing solution is neutral. It is then dried at 130℃ for 10h to finally obtain hexagonal flake nano magnesium hydroxide flame retardant.

[0024] In step S1 above, the amount of silane coupling agent added is 4.9% of the mass of magnesium chloride, and the amount of liquid alkali solution added is 88% of the total amount of liquid alkali solution.

[0025] Example 2: Example 2 provides a method for preparing hexagonal sheet-like nano-magnesium hydroxide, comprising the following steps: Step S1. Dissolve 5.0 g of vinyltriethoxysilane in 200 ml of deionized water and stir magnetically at 35 °C. Separately prepare 2 mol / L magnesium chloride solution and 2 mol / L sodium hydroxide solution for later use. Using a double dropwise addition method, add 500 ml of magnesium chloride solution at a rate of 5 ml / min, and simultaneously add 1000 ml of sodium hydroxide solution at a rate of 10 ml / min to bring the system to a low supersaturation state with a pH of 7-9. After the dropwise addition is complete, add 100 ml of sodium hydroxide solution rapidly to the reactor at a rate of 50 ml / min to bring the system to a high supersaturation state with a pH of 12-14.

[0026] Step S2. The slurry obtained in step S1 is statically aged at 85°C for 24 hours.

[0027] Step S3. Transfer the aged slurry to a high-pressure reactor for hydrothermal reaction. The reaction temperature is controlled at 180℃ and the time is controlled at 10h. After the reaction is completed, the product is washed with deionized water until the washing solution is neutral. It is then dried at 120℃ for 12h to finally obtain hexagonal sheet-like nano magnesium hydroxide.

[0028] In step S1 above, the amount of silane coupling agent added is 5.3% of the mass of magnesium chloride, and the amount of liquid alkali solution added is 91% of the total amount of liquid alkali solution.

[0029] Example 3: Example 3 provides a method for preparing hexagonal sheet-like nano-magnesium hydroxide, comprising the following steps: Step S1. Dissolve 2g of vinyltriethoxysilane in 200ml of deionized water and stir magnetically at 25℃. Separately prepare 1mol / L magnesium chloride solution and 4mol / L sodium hydroxide solution for later use. Using a double dropwise addition method, add 500ml of magnesium chloride solution at a rate of 5ml / min, and simultaneously add 125ml of sodium hydroxide solution at a rate of 1.25ml / min to bring the system to a low supersaturation state with a pH of 7-9. After the addition is complete, add 50ml of sodium hydroxide solution rapidly to the reactor at a rate of 25ml / min to bring the system to a high supersaturation state with a pH of 12-14.

[0030] Step S2. The slurry obtained in step S1 is statically aged at 85°C for 24 hours.

[0031] Step S3. The aged slurry is transferred to a high-pressure reactor for hydrothermal reaction. The reaction temperature is controlled at 220℃ and the time is controlled at 14h. After the reaction is completed, the product is washed with deionized water until the washing solution is neutral. It is then dried at 140℃ for 14h to finally obtain hexagonal sheet-like nano magnesium hydroxide.

[0032] In step S1 above, the amount of silane coupling agent added is 4.2% of the mass of magnesium chloride, and the amount of liquid alkali solution added is 71% of the total amount of liquid alkali solution.

[0033] Comparative Example Comparative Example 1: The technical solution of Comparative Example 1 differs from that of Example 1 in that, after the magnesium chloride solution and sodium hydroxide solution are added dropwise in step S1, 100 ml of sodium hydroxide solution is added to the reactor at the original rate of 10 ml / min instead of being added rapidly. The rest of the contents are the same and will not be described again.

[0034] Comparative Example 2: The technical solution of Comparative Example 2 differs from that of Example 1 in that step S2 is omitted. That is, after completing step S1, the hydrothermal reaction of step S3 is carried out directly without heating and aging treatment. The other contents are the same and will not be described again.

[0035] Comparative Example 3: The technical solution of Comparative Example 3 differs from that of Example 1 in that no silane coupling agent is added in step S1. The other contents are the same and will not be repeated.

[0036] Comparative Example 4: The technical solution of Comparative Example 4 differs from that of Example 1 in that the order of adding the silane coupling agent in step S1 is different. The silane coupling agent is added after magnesium chloride and all liquid alkali are added. The other contents are the same and will not be repeated.

[0037] The hexagonal sheet-like nano-magnesium hydroxide prepared in Examples 1-3 of this invention and the products prepared in Comparative Examples 1-4 were used as samples for SEM characterization tests.

[0038] To verify the effectiveness of the preparation process of this invention, a comparative analysis of the microstructure of the samples obtained in Examples 1-3 and Comparative Examples 1-4 was conducted: Depend on Figures 1-3 As shown, the nano-magnesium hydroxide exhibits a regular hexagonal plate-like crystal structure and excellent dispersibility. Specifically: Depend on Figure 1 As shown, the sample obtained in Example 1 is clearly visible under a 50nm scale, with a uniform particle size distribution between 40-60nm and a particle size controlled within 100nm. The lamellae are extremely thin, achieving a significant nanoscale effect, which proves that the high supersaturation brought about by rapid impact effectively locks the crystal nucleus size.

[0039] Depend on Figure 2 As shown, the sample obtained in Example 2 is clearly visible under a 100nm scale, with a uniform particle size distribution within 100nm. This indicates that the product still maintains excellent monodispersity at higher reaction concentrations, with no obvious hard agglomeration. This further proves that the product maintains a good monodisperse state at different concentrations and temperatures.

[0040] Depend on Figure 3 As shown, the sample obtained in Example 3 is clearly visible under a 100nm scale, with a uniform particle size distribution within 100nm, indicating that the crystals are fully developed and have sharp edges under high-temperature hydrothermal conditions. This further proves that the product of this process always maintains a good monodisperse state under different concentrations and temperatures.

[0041] In contrast, by Figure 4 As shown, compared with the technical solution of Example 1, in Comparative Example 1, sodium hydroxide solution was added at the original rate in step S1, but the rapid addition was cancelled. The resulting sample showed product morphology on a 1μm scale, with obvious crystal stacking growth and irregular morphology. At the same time, the particle size increased significantly to more than 100nm, and the particle size coarsened to the micrometer level. This proves that the sodium hydroxide solution did not adopt a stepwise rate control process, and the reaction system was always in a low supersaturation, which caused the product to stack in layers and made it impossible to obtain nanoscale products.

[0042] Depend on Figure 5As shown, compared with the technical solution of Example 1, Comparative Example 2 omits the heating and static aging step S2. The resulting sample shows the product morphology under a 1μm scale. The particle edges are rounded and blurry, and the morphology is extremely irregular. At the same time, the particle size is significantly increased to more than 100nm, and the particle size is coarsened to the micrometer level. This proves that the product without heating and static aging treatment has an irregular morphology, blurry edges and a large number of defects, and it is impossible to obtain nanoscale products.

[0043] Depend on Figure 6 As shown, compared with the technical solution of Example 1, Comparative Example 3 omits the addition of silane coupling agent in step S1. The resulting sample shows product morphology under a 1μm scale, with severe particle agglomeration, uneven particle size, and irregular morphology. This proves that the product without silane coupling agent treatment has severe particle agglomeration and irregular morphology, making it impossible to obtain nanoscale products.

[0044] Depend on Figure 7 As shown, compared with the technical solution of Example 1, Comparative Example 3 changed the order of adding silane coupling agent. After magnesium chloride and all liquid alkali were added, silane coupling agent was added. The resulting sample showed product morphology under a 1μm scale. The particles were severely agglomerated and the particle size was uneven. This proved that the products treated with silane coupling agent added at different reaction stages had severe particle agglomeration and uneven particle size, and could not obtain nanoscale products.

[0045] In summary, this invention provides a method for preparing hexagonal sheet-like nano-magnesium hydroxide, employing a synergistic process of "stepwise controlled-rate precipitation-high-temperature aging-hydrothermal crystallization." During the precipitation stage, a silane coupling agent is introduced for in-situ coating, and the formation and growth kinetics of crystal nuclei are controlled using a feeding method combining slow dripping and rapid impact. Subsequently, high-temperature aging and hydrothermal treatment guide directional crystal growth, ultimately obtaining a product with regular development along specific crystal lines. The nano-magnesium hydroxide prepared by this method exhibits regular hexagonal sheet-like structures with a narrow particle size distribution and good dispersibility. Furthermore, the surface is grafted with organic functional groups, significantly improving its dispersibility and flame-retardant efficiency as a flame retardant in polymer matrices.

[0046] This invention provides a method for preparing hexagonal sheet-like nano-magnesium hydroxide, resulting in nano-magnesium hydroxide with regular morphology and high dispersibility. Due to its regular hexagonal sheet structure, the nano-magnesium hydroxide exhibits excellent compatibility and flowability, enabling high filling amounts without significantly reducing the mechanical properties of the matrix. Simultaneously, the sheet-like structure facilitates the formation of a dense char layer, significantly improving flame retardancy and smoke suppression efficiency, thus possessing broad market application prospects.

[0047] It should be noted that: (1) In step S1, in addition to using magnesium chloride solution, one or a mixture of one or more salt solutions such as magnesium sulfate solution and magnesium nitrate solution can be used instead.

[0048] (2) In step S1, in addition to using deionized water to dissolve the silane coupling agent, conventional water such as distilled water can also be used instead.

[0049] (3) In step S3, in addition to using deionized water to wash the hydrothermal reaction product, conventional water such as distilled water can also be used instead.

[0050] (4) The “low supersaturation state” mentioned in this invention refers to adding an excessive amount of liquid alkali and controlling the system to a pH value of 7 to 9.

[0051] (5) The “high supersaturation state” mentioned in this invention refers to the state in which an excess of liquid alkali is rapidly added to make the pH value of the system reach 12 to 14.

[0052] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing hexagonal sheet-like nano-magnesium hydroxide, characterized in that, Includes the following steps: Step S1. Disperse the silane coupling agent in water to prepare a base solution. Add magnesium salt solution and liquid alkali solution dropwise to the base solution using a double dropwise addition method. The double dropwise addition rate is 1.25-15 mL / min. During the dropwise addition process, control the system to a low supersaturation state with a pH value of 7-9. After the magnesium salt solution is added, increase the dropwise addition rate. The remaining liquid alkali solution is added at a dropwise addition rate of 25-100 mL / min until the system reaches a high supersaturation state with a pH value of 12-14. Stir the reaction to obtain a primary magnesium hydroxide slurry. Step S2. The primary magnesium hydroxide slurry obtained in step S1 is statically aged under heating conditions to induce directional growth of crystal nuclei, thereby obtaining an aged slurry. Step S3. The aged slurry obtained in step S2 is transferred to a high-pressure reactor for hydrothermal reaction. After the reaction is completed, the product is washed and dried to obtain hexagonal sheet-like nano magnesium hydroxide.

2. The method for preparing hexagonal sheet-like nano-magnesium hydroxide according to claim 1, characterized in that, In step S1, the concentration of the magnesium salt solution is 1-2 mol / L; the liquid alkali solution is a sodium hydroxide solution with a concentration of 1-4 mol / L.

3. The method for preparing hexagonal sheet-like nano-magnesium hydroxide according to claim 1, characterized in that, In step S1, the magnesium salt solution is selected from one or a mixture of magnesium chloride solution, magnesium sulfate solution, and magnesium nitrate solution.

4. The method for preparing hexagonal sheet-like nano-magnesium hydroxide according to claim 1, characterized in that, In step S1, the amount of silane coupling agent added is 4.2 to 5.3% of the mass of magnesium salt.

5. The method for preparing hexagonal sheet-like nano-magnesium hydroxide according to claim 1, characterized in that, In step S1, the silane coupling agent is selected from one or more of vinyltriethoxysilane, γ-aminopropyltriethoxysilane, and γ-epoxypropyltrimethoxysilane.

6. The method for preparing hexagonal sheet-like nano-magnesium hydroxide according to claim 1, characterized in that, In step S1, the amount of liquid alkali solution added is 71-91% of the total amount of liquid alkali solution, and the system is controlled to be in a low supersaturation state during the adding process.

7. The method for preparing hexagonal sheet-like nano-magnesium hydroxide according to claim 1, characterized in that, In step S2, the aging temperature is 80℃~85℃ and the aging time is 20h~24h.

8. The method for preparing hexagonal sheet-like nano-magnesium hydroxide according to claim 1, characterized in that, In step S3, the hydrothermal reaction temperature is 180–220°C, and the hydrothermal time is 10–14 hours.

9. The method for preparing hexagonal sheet-like nano-magnesium hydroxide according to claim 1, characterized in that, In step S3, the washing method includes washing the product with water until the washing solution is neutral.

10. The method for preparing hexagonal sheet-like nano-magnesium hydroxide according to claim 1, characterized in that, In step S3, the drying temperature is 120-140℃ and the drying time is 10-14h.

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