Preparation method of spheroidal aluminum hydroxide micropowder

Abstract

A method for preparing spherical aluminum hydroxide micropowder includes the following steps: A sodium aluminate solution A is prepared by mixing aluminum hydroxide and sodium hydroxide; after heating, a self-made polyvinylpyrrolidone-g-polysiloxane additive is added, and the mixture is continuously heated and stirred to form solution B; sodium bicarbonate is then added and the reaction is continued with stirring; after the reaction is complete, the mixture is filtered while hot to obtain the filtrate, which is solution C; solution C is heated under stirring conditions, and after the reaction is complete, the temperature is lowered at a certain rate to obtain a mixed product; the mixed product is filtered, washed, and dried to obtain spherical aluminum hydroxide. The spherical aluminum hydroxide prepared by this method has a regular morphology and concentrated particle size distribution, with a D50 of approximately 7-10 micrometers. Compared with irregularly shaped aluminum hydroxide of the same particle size, it has better filling performance, effectively improving the problems of poor dispersibility and high viscosity caused by high filling of aluminum hydroxide in polymer materials, and improving the subsequent processing performance of the material.

Description

Technical Field

[0001] This invention relates to the field of aluminum hydroxide powder preparation technology, and specifically to a method for preparing spherical aluminum hydroxide micropowder. Background Technology

[0002] Aluminum hydroxide possesses multiple functions, including filling, smoke suppression, and flame retardancy. It is a widely used inorganic non-metallic material, found in various fields such as solid waste treatment, pharmaceuticals, and inorganic flame retardants. It holds a particularly important position in the field of inorganic flame retardants. As a flame-retardant filler, aluminum hydroxide exhibits excellent flame-retardant compatibility with polymers such as rubber, plastics, and epoxy resins. Aluminum hydroxide undergoes dehydration and endothermic reaction at 200-350℃, delaying the combustion of polymeric substrates. Furthermore, using aluminum hydroxide as a filler in flame-retardant materials reduces the amount of flammable polymeric substrate required. The water vapor generated during the endothermic dehydration of aluminum hydroxide dilutes the concentration of flammable gases produced by the decomposition of polymers and the surrounding oxygen, preventing combustion and ignition. Simultaneously, the dehydration of aluminum hydroxide forms an alumina protective film on the surface of polymers, isolating them from oxygen. Finally, under combustion conditions, aluminum hydroxide generates a strong dehydrating substance that carbonizes the polymer, making it less prone to generating flammable volatiles, thus preventing the spread of flames.

[0003] Due to these unique properties, aluminum hydroxide flame retardants have become one of the most widely used and consumed inorganic flame retardants globally. Since the beginning of the 21st century, the global annual demand for aluminum hydroxide flame retardants has exceeded 600,000 tons.

[0004] As a filler powder, aluminum hydroxide exhibits advantages such as smaller specific surface area and better particle dispersibility when spherical aluminum hydroxide is used, especially for particles of the same size. Furthermore, spherical aluminum hydroxide fillers in polymers show better flowability and processing performance at the same filler content. Additionally, spherical aluminum hydroxide can serve as a precursor for the preparation of spherical alumina. Therefore, spherical aluminum hydroxide has broad application prospects.

[0005] Chinese patent document CN110963516A discloses a "method for preparing spherical α-alumina powder," which obtains spherical aluminum hydroxide through the hydrolysis of aluminum alkoxide. The alkoxide method can produce aluminum hydroxide with high sphericity and is simple to implement, but alkoxides are expensive, and the organic solvents used in the process are somewhat toxic, hindering large-scale production. Chinese patent document CN115490253A discloses a "method for preparing aluminum hydroxide microspheres," which uses diluted sodium aluminate mother liquor and urea as a precipitant to prepare micron-sized spherical aluminum hydroxide via hydrothermal treatment. Chinese patent document CN103318932A discloses a "method for preparing uniformly dispersed spherical aluminum hydroxide powder," which uses aluminum sulfate as the aluminum source and weakly alkaline urea as a precipitant to prepare spherical aluminum hydroxide powder via hydrothermal treatment. Both methods use hydrothermal methods to prepare spherical aluminum hydroxide, which can produce well-crystallized powders. However, the hydrothermal method requires sophisticated equipment, consumes high energy, and is costly, also hindering large-scale production.

[0006] Currently, most industries require aluminum hydroxide with smooth surfaces and high sphericity for flame retardant filling to increase its addition amount and uniform dispersion in organic polymers. Therefore, exploring the preparation process of spherical or near-spherical aluminum hydroxide with simple technology, low energy consumption, low raw material cost, and ease of large-scale production is an urgent problem to be solved in the future. Summary of the Invention

[0007] To address the shortcomings of the existing technology, this invention provides a method for preparing near-spherical aluminum hydroxide micropowder. Using industrial-grade aluminum hydroxide, sodium hydroxide, sodium bicarbonate, and a self-made polyvinylpyrrolidone-g-polysiloxane additive as raw materials, near-spherical aluminum hydroxide micropowder is prepared through a low-temperature reaction. This method uses readily available raw materials, requires minimal equipment, has low production costs, and is easily scalable for industrial production.

[0008] The preparation method of the spherical aluminum hydroxide micro powder of the present invention includes the following steps: (1) Dissolve aluminum hydroxide in sodium hydroxide solution to prepare sodium aluminate solution A of a certain concentration; (2) Heat sodium aluminate solution A to a certain reaction temperature, then add self-made polyvinylpyrrolidone-g-polysiloxane additive to sodium aluminate solution A, and then continue to stir for a certain time under continuous heating to form solution B. (3) Add sodium bicarbonate to solution B under continuous heating and continue stirring for a certain period of time. After the reaction is completed, filter while hot to obtain the filtrate, which is solution C. (4) The solution C is heated to a certain temperature under stirring to carry out the reaction. After the reaction is completed, the temperature is lowered at a certain rate to obtain the mixed product. The mixed product is filtered, washed and dried to obtain spherical aluminum hydroxide.

[0009] The structural formula of the self-made additive is: Where n = 40 - 60.

[0010] The preparation method of the self-made polyvinylpyrrolidone-g-polysiloxane additive is as follows: (1) Add 6.4 mL of N-vinylpyrrolidone and 27.2 mg of azobisisobutyronitrile to a 50 mL reaction flask, then add 136 μL of RAFT reagent (1-(O-ethylxanthanyl)ethyl)benzene and mix well. After three cycles of liquid nitrogen freezing-thawing to remove oxygen, place it in a 70 °C constant temperature oil bath and react for 5 h under stirring at 350 r / min. After the reaction is stopped, take out the reactants and add 20 mL of dichloromethane to dissolve them completely. Then slowly pour the solution into 200 mL of anhydrous diethyl ether to precipitate. The obtained solid is washed with diethyl ether to obtain PVP-CTA; (2) Add 30 mL of deionized water and 2 g of azobisisobutyronitrile to a 250 mL flask. After stirring PVP-CTA evenly, place it in a light-protected environment, quickly add 0.1g of sodium borohydride and react for 24 hours, then add 2g of 50cp single-ended vinyl silicone oil, photocatalyst benzoin dimethyl ether (2wt%) and 30mL of tetrahydrofuran, purge with nitrogen gas, and after the reactants are completely dissolved, irradiate with ultraviolet light for 10 minutes while stirring, react for 24 hours and remove excess solvent by rotary evaporation. Finally, precipitate with n-hexane three times to remove unreacted reactants and catalyst, thus obtaining polyvinylpyrrolidone-g-polysiloxane.

[0011] The median diameter D50 of the aluminum hydroxide is preferably 1-120 micrometers, and there are no requirements for the particle size and morphology of the powder.

[0012] The sodium aluminate solution A has a caustic ratio of 1.30-1.50 and an alumina concentration of 130-150 g / L.

[0013] The self-made polyvinylpyrrolidone-g-polysiloxane additive accounts for 0.1‰-0.5‰ of the mass of sodium aluminate solution A.

[0014] The heating reaction time is 0.5-1 h, the reaction temperature is 40-55 °C, and the stirring rate is 400-600 r / min.

[0015] The sodium bicarbonate accounts for 5.0%-7.0% of the mass of solution B, the reaction temperature is 40-55℃, the reaction time is 15-20 min, and the stirring rate is 800 r / min.

[0016] The solution C is heated to 60°C, stirred at a rate of 200-300 r / min, cooled at a rate of 4°C / h, and cooled to an end of 32°C.

[0017] The drying time is 20-24 hours, and the drying temperature is 100-110℃.

[0018] The washing method involves washing three times with double-distilled water, followed by washing twice with anhydrous ethanol.

[0019] The formation mechanism and process of the spherical aluminum hydroxide micropowder described in this invention are as follows: (1) Sodium bicarbonate is added to react with sodium aluminate to generate aluminum hydroxide and sodium carbonate, that is, aluminum hydroxide crystal nuclei are generated in step 2. The coarse crystal nuclei are filtered out, so that only nano-sized ultrafine aluminum hydroxide crystal nuclei exist in solution C. (2) The interfacial tension between the sodium aluminate solution and the seed crystals is as high as 1.25 Nm. -1 Self-formation of crystal nuclei is difficult. The addition of polyvinylpyrrolidone-g-polysiloxane reduces the surface tension between the solution and the seed crystals, making nucleus formation easier and thus facilitating the generation of a large number of ultrafine crystal nuclei in the early stages of the reaction. During cooling seeding, as the temperature decreases, the supersaturation of the sodium aluminate solution increases, and the polyvinylpyrrolidone chains adsorb onto the seed crystal surface, reacting with Al(OH)4. - The hydroxyl groups in the polyvinylpyrrolidone (PVP) form hydrogen bonds, acting as a bridge and increasing the probability of agglomeration during the aluminum hydroxide precipitation process. Furthermore, the PPVP chains increase the wettability of the solution, further reducing its stability and making aluminum hydroxide precipitation easier. Agglomeration causes the ultrafine crystal nuclei to cluster together, forming micropowder. The subsequently precipitated aluminum hydroxide acts as a binder, making the agglomeration of the ultrafine crystal nuclei even tighter. Simultaneously, the polysiloxane chains in PPVP-g-polysiloxane are highly hydrophobic, resulting in less tight contact with aluminum hydroxide, which increases steric hindrance, reduces the degree of aluminum hydroxide micropowder agglomeration, and leads to a final aluminum hydroxide product with uniform particle size.

[0020] The key technical points and beneficial effects of this invention are as follows: (1) Compared with ordinary aluminum hydroxide, the aluminum hydroxide prepared by the present invention has a more regular morphology and is approximately spherical, which has better filling properties in polymer resin matrix, and the material can still maintain excellent processability after high filling. (2) The overall process of the technical solution of the present invention is simple, does not use high temperature and high pressure conditions, has low equipment requirements, low energy consumption in the production process, and is easier to industrialize. However, hydrothermal method and salt water hydrolysis method have problems such as complex process, high price, and high toxicity, making it difficult to achieve large-scale industrialization. (3) The present invention uses self-made polyvinylpyrrolidone-g-polysiloxane as an additive to make the prepared aluminum hydroxide micro powder have high sphericity and low degree of agglomeration between micro powders. (4) The raw materials selected in this invention are low in cost and widely available, such as the common sodium aluminate aluminum source; secondly, the sodium aluminate solution after aluminum hydroxide precipitation can be recycled in the sintering process, which further reduces the cost. Attached Figure Description

[0021] Figure 1 Scanning electron microscope (SEM) image of the spherical aluminum hydroxide micropowder prepared in Example 1 of this method. Figure 2 Scanning electron microscope (SEM) image of the spherical aluminum hydroxide micropowder prepared in Example 2 of this method. Figure 3 Scanning electron microscope (SEM) image of the spherical aluminum hydroxide micropowder prepared in Example 3 of this method. Figure 4 Scanning electron microscope (SEM) image of the spherical aluminum hydroxide micropowder prepared in Example 4 of this method. Figure 5 This is a scanning electron microscope (SEM) image of the aluminum hydroxide micropowder prepared by Comparative Example 1 using this method. Figure 6 This is a scanning electron microscope (SEM) image of the aluminum hydroxide micropowder prepared by Comparative Example 2 using this method. Figure 7 This is a scanning electron microscope (SEM) image of the aluminum hydroxide micropowder prepared by Comparative Example 3 using this method. Detailed Implementation

[0022] The present invention will be further described below with reference to the embodiments. It is worth noting that the following description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications and substitutions based on the technical solutions and inventive concepts provided by the present invention should be covered within the protection scope of the present invention.

[0023] The specific implementation steps of this invention are as follows: (1) Add 6.4 mL of N-vinylpyrrolidone and 27.2 mg of azobisisobutyronitrile to a 50 mL reaction flask, then add 136 μL of RAFT reagent (1-(O-ethylxanthanyl)ethyl)benzene and mix well. After three cycles of liquid nitrogen freezing-thawing to remove oxygen, place the mixture in a 70 °C constant temperature oil bath and react for 5 h with stirring at 350 r / min. After the reaction is stopped, remove the reactants and add 20 mL of dichloromethane to dissolve them completely. Then slowly pour the solution into 200 mL of anhydrous diethyl ether to precipitate the solid. Wash the solid obtained with diethyl ether to obtain PVP-CTA. Add 30 mL of deionized water and 2 g of [unspecified substance] to a 250 mL flask. After stirring PVP-CTA evenly, place it in a light-protected environment, quickly add 0.1g of sodium borohydride and react for 24 hours, then add 2g of 50cp single-ended vinyl silicone oil, photocatalyst benzoin dimethyl ether (2wt%) and 30mL of tetrahydrofuran, purge with nitrogen gas, and after the reactants are completely dissolved, irradiate with ultraviolet light for 10 minutes while stirring, react for 24 hours and remove excess solvent by rotary evaporation. Finally, precipitate with n-hexane three times to remove unreacted reactants and catalyst, thus obtaining polyvinylpyrrolidone-g-polysiloxane. (2) Preparation of spherical aluminum hydroxide: Aluminum hydroxide is dissolved in sodium hydroxide solution to obtain sodium aluminate solution A with a caustic ratio of 1.30-1.50 and an alumina concentration of 130-150 g / L; then the sodium aluminate solution A is heated to 40-55℃, and a self-made polyvinylpyrrolidone-g-polysiloxane additive is added to it. The self-made polyvinylpyrrolidone-g-polysiloxane additive accounts for 0.1‰-0.5‰ of the mass of sodium aluminate solution A, and the mixture is stirred at a rate of 400-600 r / min for 0.5-1 hour. Solution B is obtained. While maintaining heating, sodium bicarbonate is added to solution B, with sodium bicarbonate accounting for 5.0%-7.0% of the mass of solution B. The mixture is stirred at a stirring rate of 800 r / min for 15-20 min. The mixture is then filtered while hot to obtain solution C. Solution C is heated to 60℃ and then cooled to 32℃ at a stirring rate of 200-300 r / min for 4℃ / h. The mixture is filtered, washed with deionized water, then washed with anhydrous ethanol, and dried in an oven at 100-110℃ for 20-24 hours.

[0024] Example 1 The preparation method of the self-made polyvinylpyrrolidone-g-polysiloxane additive is as follows: 6.4 mL of N-vinylpyrrolidone and 27.2 mg of azobisisobutyronitrile (AIB) were added to a 50 mL reaction flask, followed by 136 μL of RAFT reagent (1-(O-ethylxanthanyl)ethyl)benzene. After thorough mixing and three cycles of liquid nitrogen freezing-thawing to remove oxygen, the mixture was placed in a 70°C constant temperature oil bath and reacted for 5 hours with stirring at 350 rpm. The reaction was then stopped, and 20 mL of dichloromethane was added to dissolve the reactants completely. The solution was then slowly poured into 200 mL of anhydrous diethyl ether to precipitate the solid. The resulting solid was washed with diethyl ether to obtain PVP-CTA. 30 mL of deionized water and 2 g of... were added to a 250 mL flask... After stirring PVP-CTA evenly, place it in a light-protected environment, quickly add 0.1g of sodium borohydride and react for 24 hours, then add 2g of 50cp single-ended vinyl silicone oil, photocatalyst benzoin dimethyl ether (2wt%) and 30mL of tetrahydrofuran, purge with nitrogen gas, and after the reactants are completely dissolved, irradiate with ultraviolet light for 10 minutes while stirring, react for 24 hours and remove excess solvent by rotary evaporation. Finally, precipitate with n-hexane three times to remove unreacted reactants and catalyst, thus obtaining polyvinylpyrrolidone-g-polysiloxane. Preparation of spherical aluminum hydroxide micropowder: Aluminum hydroxide was dissolved in an aqueous sodium hydroxide solution to prepare sodium aluminate solution A with a caustic ratio of 1.30 and an alumina concentration of 130 g / L. Sodium aluminate solution A was then heated to 40°C, and 0.0609 g of a self-made polyvinylpyrrolidone-g-polysiloxane additive was added. The mixture was stirred at 600 r / min for 0.5 hours to obtain solution B. Then, under the same reaction conditions at 40°C, 34.71 g of sodium bicarbonate was added to solution B, and the mixture was stirred at 800 r / min for 15 minutes. The mixture was then filtered while hot to obtain solution C. Solution C was then heated to 60°C and cooled to 32°C at a rate of 4°C / h with stirring at 200 r / min. The solution was filtered, washed three times with double-distilled water, then washed twice with anhydrous ethanol, and finally dried in an oven at 100°C for 24 hours to obtain spherical aluminum hydroxide micropowder.

[0025] Example 2 The preparation method of the self-made polyvinylpyrrolidone-g-polysiloxane additive is as follows: 6.4 mL of N-vinylpyrrolidone and 27.2 mg of azobisisobutyronitrile (AIB) were added to a 50 mL reaction flask, followed by 136 μL of RAFT reagent (1-(O-ethylxanthanyl)ethyl)benzene. After thorough mixing and three cycles of liquid nitrogen freezing-thawing to remove oxygen, the mixture was placed in a 70°C constant temperature oil bath and reacted for 5 hours with stirring at 350 rpm. The reaction was then stopped, and 20 mL of dichloromethane was added to dissolve the reactants completely. The solution was then slowly poured into 200 mL of anhydrous diethyl ether to precipitate the solid. The resulting solid was washed with diethyl ether to obtain PVP-CTA. 30 mL of deionized water and 2 g of... were added to a 250 mL flask... After stirring PVP-CTA evenly, place it in a light-protected environment, quickly add 0.1g of sodium borohydride and react for 24 hours, then add 2g of 50cp single-ended vinyl silicone oil, photocatalyst benzoin dimethyl ether (2wt%) and 30mL of tetrahydrofuran, purge with nitrogen gas, and after the reactants are completely dissolved, irradiate with ultraviolet light for 10 minutes while stirring, react for 24 hours and remove excess solvent by rotary evaporation. Finally, precipitate with n-hexane three times to remove unreacted reactants and catalyst, thus obtaining polyvinylpyrrolidone-g-polysiloxane. Preparation of spherical aluminum hydroxide micropowder: Aluminum hydroxide was dissolved in an aqueous sodium hydroxide solution to prepare sodium aluminate solution A with a caustic ratio of 1.41 and an alumina concentration of 140 g / L. Next, at 45°C, 0.1575 g of a self-made polyvinylpyrrolidone-g-polysiloxane additive was added to sodium aluminate solution A, and the mixture was stirred at 400 r / min for 0.5 hours to obtain solution B. Then, under the same reaction conditions at 45°C, 40.95 g of sodium bicarbonate was added to solution A, and the mixture was stirred at 800 r / min for 15 minutes. The mixture was then filtered while hot to obtain solution C. Solution C was then heated to 60°C, stirred at 200 r / min, and cooled to 32°C at a rate of 4°C / h. The solution was filtered, washed three times with double-distilled water, then washed twice with anhydrous ethanol, and finally dried in an oven at 100°C for 24 hours to obtain spherical aluminum hydroxide micropowder.

[0026] Example 3 Add 6.4 mL of N-vinylpyrrolidone and 27.2 mg of azobisisobutyronitrile to a 50 mL reaction flask, then add 136 μL of RAFT reagent (1-(O-ethylxanthanyl)ethyl)benzene and mix thoroughly. After three cycles of liquid nitrogen freezing-thawing to remove oxygen, place the mixture in a 70 °C constant temperature oil bath and react for 5 h with stirring at 350 r / min. After the reaction is complete, remove the reactants and add 20 mL of dichloromethane to dissolve them completely. Then, slowly pour the solution into 200 mL of anhydrous diethyl ether to precipitate the solid. Wash the resulting solid with diethyl ether to obtain PVP-CTA. Add 30 mL of deionized water and 2 g of [unspecified substance] to a 250 mL flask. After stirring PVP-CTA evenly, place it in a light-protected environment, quickly add 0.1g of sodium borohydride and react for 24 hours, then add 2g of 50cp single-ended vinyl silicone oil, photocatalyst benzoin dimethyl ether (2wt%) and 30mL of tetrahydrofuran, purge with nitrogen gas, and after the reactants are completely dissolved, irradiate with ultraviolet light for 10 minutes while stirring, react for 24 hours and remove excess solvent by rotary evaporation. Finally, precipitate with n-hexane three times to remove unreacted reactants and catalyst, thus obtaining polyvinylpyrrolidone-g-polysiloxane. Preparation of spherical aluminum hydroxide micropowder: Aluminum hydroxide was dissolved in an aqueous sodium hydroxide solution to prepare sodium aluminate solution A with a caustic ratio of 1.43 and an alumina concentration of 145 g / L. Then, at 45°C, 0.2560 g of a self-made polyvinylpyrrolidone-g-polysiloxane additive was added to sodium aluminate solution A, and the mixture was stirred at 600 r / min for 0.5 hours to obtain solution B. Then, under the same reaction conditions at 45°C, 44.80 g of sodium bicarbonate was added to solution B, and the mixture was stirred at 800 r / min for 20 minutes. The mixture was then filtered while hot to obtain solution C. Solution C was then heated to 60°C, stirred at 300 r / min, and cooled to 32°C at a rate of 4°C / h. The solution was filtered, washed three times with double-distilled water, then washed twice with anhydrous ethanol, and finally dried in an oven at 110°C for 24 hours to obtain spherical aluminum hydroxide micropowder.

[0027] Example 4 The preparation method of the self-made polyvinylpyrrolidone-g-polysiloxane additive is as follows: 6.4 mL of N-vinylpyrrolidone and 27.2 mg of azobisisobutyronitrile (AIB) were added to a 50 mL reaction flask, followed by 136 μL of RAFT reagent (1-(O-ethylxanthanyl)ethyl)benzene. After thorough mixing and three cycles of liquid nitrogen freezing-thawing to remove oxygen, the mixture was placed in a 70°C constant temperature oil bath and reacted for 5 hours with stirring at 350 rpm. The reaction was then stopped, and 20 mL of dichloromethane was added to dissolve the reactants completely. The solution was then slowly poured into 200 mL of anhydrous diethyl ether to precipitate the solid. The resulting solid was washed with diethyl ether to obtain PVP-CTA. 30 mL of deionized water and 2 g of... were added to a 250 mL flask... After stirring PVP-CTA evenly, place it in a light-protected environment, quickly add 0.1g of sodium borohydride and react for 24 hours, then add 2g of 50cp single-ended vinyl silicone oil, photocatalyst benzoin dimethyl ether (2wt%) and 30mL of tetrahydrofuran, purge with nitrogen gas, and after the reactants are completely dissolved, irradiate with ultraviolet light for 10 minutes while stirring, react for 24 hours and remove excess solvent by rotary evaporation. Finally, precipitate with n-hexane three times to remove unreacted reactants and catalyst, thus obtaining polyvinylpyrrolidone-g-polysiloxane. Preparation of spherical aluminum hydroxide micropowder: Aluminum hydroxide was dissolved in an aqueous sodium hydroxide solution to prepare sodium aluminate solution A with a caustic ratio of 1.50 and an alumina concentration of 150 g / L. Then, at 55°C, 0.3250 g of a self-made polyvinylpyrrolidone-g-polysiloxane additive was added to sodium aluminate solution A, and the mixture was stirred at 600 r / min for 0.75 hours to obtain solution B. Then, under the same reaction conditions at 55°C, 45.50 g of sodium bicarbonate was added to solution B, and the mixture was stirred at 800 r / min for 20 minutes. The mixture was filtered while hot to obtain solution C. Solution C was then heated to 60°C, stirred at 200 r / min, and cooled to 32°C at a rate of 4°C / h. It was filtered, washed three times with double-distilled water, then washed twice with anhydrous ethanol, and finally dried in an oven at 100°C for 24 hours to obtain spherical aluminum hydroxide micropowder.

[0028] Comparative Example 1 Aluminum hydroxide was dissolved in an aqueous sodium hydroxide solution to prepare sodium aluminate solution A with a caustic ratio of 1.30 and an alumina concentration of 130 g / L. The solution was then stirred at 600 r / min for 0.5 hours at 40°C to obtain solution B. Under the same 40°C conditions, 34.71 g of sodium bicarbonate was added to solution B, and the mixture was stirred at 800 r / min for 15 minutes. The solution was then filtered while hot to obtain solution C. Solution C was then heated to 60°C and cooled to 32°C at a rate of 4°C / h with stirring at 200 r / min. The solution was filtered, washed three times with double-distilled water, then twice with anhydrous ethanol, and finally dried in an oven at 100°C for 24 hours to obtain comparative aluminum hydroxide micropowder.

[0029] Comparative Example 2 Aluminum hydroxide was dissolved in an aqueous sodium hydroxide solution to prepare sodium aluminate solution A with a caustic ratio of 1.30 and an alumina concentration of 130 g / L. 0.0609 g of N-vinylpyrrolidone and 50 cp single-terminated vinyl silicone oil (mass ratio of N-vinylpyrrolidone to 50 cp single-terminated vinyl silicone oil was 1:1) were added. The mixture was then stirred at 600 r / min at 40 °C for 0.5 hours to obtain solution B. The same reaction was then carried out at 40 °C. Under the specified conditions, 34.71 g of sodium bicarbonate was added to solution B, and the mixture was stirred at a rate of 800 r / min for 15 min. The mixture was then filtered while hot to obtain solution C. Solution C was then heated to 60 °C and cooled to 32 °C at a rate of 4 °C / h while stirring at 200 r / min. The solution was then filtered, washed three times with double-distilled water, and then washed twice with anhydrous ethanol. Finally, it was placed in an oven at 100 °C and dried for 24 hours to obtain comparative aluminum hydroxide micro powder.

[0030] Comparative Example 3 The preparation method of the self-made polyvinylpyrrolidone-g-polysiloxane additive is as follows: 6.4 mL of N-vinylpyrrolidone and 27.2 mg of azobisisobutyronitrile (AIB) were added to a 50 mL reaction flask, followed by 136 μL of RAFT reagent (1-(O-ethylxanthanyl)ethyl)benzene. After thorough mixing and three cycles of liquid nitrogen freezing-thawing to remove oxygen, the mixture was placed in a 70°C constant temperature oil bath and reacted for 5 hours with stirring at 350 rpm. The reaction was then stopped, and 20 mL of dichloromethane was added to dissolve the reactants completely. The solution was then slowly poured into 200 mL of anhydrous diethyl ether to precipitate the solid. The resulting solid was washed with diethyl ether to obtain PVP-CTA. 30 mL of deionized water and 2 g of... were added to a 250 mL flask... After stirring PVP-CTA evenly, place it in a light-protected environment, quickly add 0.1g of sodium borohydride and react for 24 hours, then add 2g of 50cp single-ended vinyl silicone oil, photocatalyst benzoin dimethyl ether (2wt%) and 30mL of tetrahydrofuran, purge with nitrogen gas, and after the reactants are completely dissolved, irradiate with ultraviolet light for 10 minutes while stirring, react for 24 hours and remove excess solvent by rotary evaporation. Finally, precipitate with n-hexane three times to remove unreacted reactants and catalyst, thus obtaining polyvinylpyrrolidone-g-polysiloxane. Preparation of aluminum hydroxide micro powder: Aluminum hydroxide was dissolved in an aqueous sodium hydroxide solution to prepare sodium aluminate solution A with a caustic ratio of 1.30 and an alumina concentration of 130 g / L; 0.0609 g of self-made polyvinylpyrrolidone-g-polysiloxane additive was added; then, under the condition of 40℃, the mixture was stirred at a rate of 600 r / min for 0.5 hours to obtain solution B; then, under the same reaction conditions of 40℃, 34.71 g of sodium bicarbonate was added to solution B, and the mixture was stirred at a rate of 800 r / min for 15 min; then, the temperature was raised to 60℃, and the temperature was lowered to 32℃ at a rate of 4℃ / h under stirring at 200 r / min. The mixture was filtered, washed three times with double-distilled water, then washed twice with anhydrous ethanol, and then dried in an oven at 100℃ for 24 hours to obtain comparative aluminum hydroxide micro powder.

[0031] Blank example The blank example is commercially available 8-9μm irregularly shaped aluminum hydroxide.

[0032] The products obtained from Examples 1-4, Comparative Examples 1-3, and the blank example were subjected to performance tests and characterization (Table 1 and figures). The performance test and characterization methods are as follows: The maximum filling capacity test method is as follows: aluminum hydroxide powder is continuously added to 100 parts of 350cp vinyl silicone oil. After initial dispersion, the mixture is placed in a vacuum stirrer and degasser. The speed is set to 600r / min and the stirring time is 2min until the colloid forms a hard lump that is difficult to fill. The maximum filling capacity can be obtained from this. Powder oil absorption value test method: Weigh 2g of powder sample and 4g of dioctyl phthalate (DOP) into a centrifuge tube, sonicate for 15 minutes to fully impregnate the powder, then place the centrifuge tube in a centrifuge, set the speed to 3000r / min, and centrifuge for 1 hour. After centrifugation, remove the centrifuge tube and invert it to allow excess DOP to flow out. Wipe away any residual DOP in the centrifuge tube, weigh the centrifuge tube, and calculate the powder oil absorption value (g / 100g). Morphology testing method: The micromorphology of the samples was observed using a COXEM desktop scanning electron microscope. Particle size testing method: The particle size of the sample was tested using an LS-609 laser particle size analyzer, with a focus on changes in D50. Specific surface area testing method: The specific surface area of ​​the sample was tested by nitrogen adsorption method using a JW-TB200 specific surface area and pore size simultaneous analyzer. Viscosity testing method: Fill the same amount of aluminum hydroxide powder into the same type of silicone oil, disperse it using the same process, and test the viscosity using an NDJ-8S rotational viscometer.

[0033] Table 1

[0034] (1) Based on the particle size characterization and scanning electron microscopy results, in Examples 1-4, due to the addition of the self-made polyvinylpyrrolidone-g-polysiloxane additive, the precipitated aluminum hydroxide can be well aggregated into spherical shapes and dispersed, with basically no agglomeration. The particle size distribution is relatively uniform, and the D50 is around 9 micrometers. In Comparative Example 1, which did not add the self-made polyvinylpyrrolidone-g-polysiloxane additive, the prepared aluminum hydroxide is irregular in shape and has a wider particle size distribution. In Comparative Example 2, which added the self-made polyvinylpyrrolidone-g-polysiloxane additive in monomer form, the degree of agglomeration of the prepared aluminum hydroxide is slightly lower than that of Comparative Example 1, but its sphericality is lower as observed by electron microscopy. Although Comparative Example 3 added the self-made polyvinylpyrrolidone-g-polysiloxane additive, the non-filtration step in step (3) resulted in the failure to screen out the large-sized, non-uniform crystals, resulting in the sphericality of the finally prepared aluminum hydroxide being lower than that of Example 1, and the particle size uniformity was also lower than that of Example 1. (2) From the characterization results of oil absorption rate, specific surface area, viscosity, and maximum filling, the particle size of Examples 1-4 is not much different from that of the blank example. However, due to the highly spherical morphology of the examples, the oil absorption rate is significantly reduced, the specific surface area is significantly reduced, the viscosity is reduced from 9600 to about 3100, and the maximum filling amount is increased from 500 parts to about 700. This shows that the spheroidization of aluminum hydroxide by the technical solution of the present invention can effectively improve the filling properties of aluminum hydroxide and reduce its viscosity in polymer resin, which is beneficial to the actual processing of subsequent materials. In contrast, Comparative Example 1 is mostly irregular aluminum hydroxide. Compared with the examples, the particle size is smaller, and the oil absorption rate, specific surface area, and viscosity are significantly increased, with a maximum filling amount of only 260 parts. Compared with the examples, Comparative Example 2 has a lower degree of spheroidization, a higher degree of agglomeration, a larger particle size, increased oil absorption rate and viscosity, decreased specific surface area, and the maximum filling amount is reduced from 700 to 320. Compared to the examples, Comparative Example 3 showed irregular product morphology, lower spheroidization, larger particle size, and increased oil absorption rate, specific surface area, and viscosity. The maximum filler content decreased from 700 to 450.

Claims

1. A method for preparing spherical aluminum hydroxide micropowder, characterized in that, Includes the following steps: (1) Dissolve aluminum hydroxide in sodium hydroxide solution to prepare sodium aluminate solution A of a certain concentration; (2) Heat sodium aluminate solution A to a certain reaction temperature, then add self-made polyvinylpyrrolidone-g-polysiloxane additive to sodium aluminate solution A, and then continue to stir for a certain time under continuous heating to form solution B. (3) Add sodium bicarbonate to solution B under continuous heating and continue stirring for a certain period of time. After the reaction is completed, filter while hot to obtain the filtrate, which is solution C. (4) The solution C is heated to a certain temperature under stirring to carry out the reaction. After the reaction is completed, the temperature is lowered at a certain cooling rate to obtain the mixed product. The mixed product is filtered, washed and dried to obtain spherical aluminum hydroxide. The structural formula of the self-made polyvinylpyrrolidone-g-polysiloxane additive is as follows: , where n = 40-60.

2. The method for preparing spherical aluminum hydroxide micropowder according to claim 1, characterized in that, The preparation method of the self-made polyvinylpyrrolidone-g-polysiloxane additive mentioned in step (2) is as follows: (1) Add 6.4 mL of N-vinylpyrrolidone and 27.2 mg of azobisisobutyronitrile to a 50 mL reaction flask, then add 136 μL of RAFT reagent (1-(O-ethylxanthanyl)ethyl)benzene and mix well. After three cycles of liquid nitrogen freezing-thawing to remove oxygen, place it in a 70 °C constant temperature oil bath and react for 5 h under stirring at 350 r / min. After the reaction is stopped, take out the reactant and add 20 mL of dichloromethane to dissolve it completely. Then slowly pour the solution into 200 mL of anhydrous diethyl ether to precipitate. The obtained solid is washed with diethyl ether to obtain PVP-CTA. (2) Add 30 mL of deionized water and 2 g of PVP-CTA to a 250 mL flask, stir well, place in a dark environment, quickly add 0.1 g of sodium borohydride and react for 24 hours, then add 2 g of 50 cp single-ended vinyl silicone oil, 2 wt% of photocatalyst benzoin dimethyl ether and 30 mL of tetrahydrofuran, purge with nitrogen, and after the reactants are completely dissolved, irradiate with ultraviolet light for 10 min while stirring, and after reacting for 24 hours, remove excess solvent by rotary evaporation. Finally, precipitate with n-hexane three times to remove unreacted reactants and catalyst, and obtain polyvinylpyrrolidone-g-polysiloxane.

3. The method for preparing spherical aluminum hydroxide micropowder according to claim 1, characterized in that, The caustic ratio of sodium aluminate solution A in step (1) is 1.30-1.50, and the alumina concentration is 130-150 g / L.

4. The method for preparing spherical aluminum hydroxide micropowder according to claim 1, characterized in that, The amount of the self-made polyvinylpyrrolidone-g-polysiloxane additive mentioned in step (2) is 0.1‰-0.5‰ of the mass of sodium aluminate solution A.

5. The method for preparing spherical aluminum hydroxide micropowder according to claim 1, characterized in that, The heating and stirring time in step (2) is 0.5-1h, the reaction temperature is 40-55℃, and the stirring rate is 400-600r / min.

6. The method for preparing near-spherical aluminum hydroxide micropowder according to claim 1, characterized in that, In step (3), the sodium bicarbonate accounts for 5.0%-7.0% of the mass of solution B, the reaction temperature is 40-55℃, the reaction time is 15-20 min, and the stirring rate is 800 r / min.

7. The method for preparing near-spherical aluminum hydroxide micropowder according to claim 1, characterized in that, In step (4), the solution C is heated to 60°C, stirred at 200-300 r / min, cooled at 4°C / h, and cooled to 32°C.

8. The method for preparing spherical aluminum hydroxide micropowder according to claim 1, characterized in that, The drying time in step (4) is 20-24 hours and the drying temperature is 100-110℃.

9. The method for preparing spherical aluminum hydroxide micropowder according to claim 1, characterized in that, The washing method described in step (4) is to wash three times with double-distilled water and then wash twice with anhydrous ethanol.

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