Method for producing porous aggregates

Abstract

To provide a production method for a porous aggregate of metal oxide, few in number of processes, simple in unit operation and controllable in collapse strength.SOLUTION: A porous aggregate production method in this invention includes: a dispersion process of obtaining dispersion liquid by mixing hydrophilic metal oxide particles with a hydrophobic solvent; an extraction process of obtaining a liquid mixture comprising the hydrophobic solvent of an upper layer and the ammonia aqueous solution of a lower layer including metal oxide particles by adding 0.1-1 pts.wt. of the alkaline aqueous solution of pH 12 or more to 1 pt.wt. of the hydrophobic solvent to extract the metal oxide particles from the above solvent into the ammonia aqueous solution; an emulsification process of preparing a W / O emulsion by adding a surfactant to the liquid mixture; an aggregation process of obtaining an aggregate by removing water from the W / O emulsion; a cleaning process of obtaining a surfactant-removed clean material by cleaning the aggregate; and a drying process of obtaining the porous aggregate by drying the clean material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a porous aggregate. [Background technology]

[0002] Porous aggregates are useful as various additives and carriers because they have a high specific surface area, a large pore volume, and good packing properties and slip properties.

[0003] For example, when a grinding wheel made of porous aggregates dispersed in a resin is used for precision polishing, the aggregates break down and release the nano-sized primary particles that make up the aggregates. This polishing method is attracting attention because it does not require the use of chemical abrasives, as opposed to conventional chemical mechanical polishing (CMP polishing).

[0004] Furthermore, in cosmetic material applications, the porous structure can absorb sebum well due to its high pore volume, and its high specific surface area can cause diffused reflection on the skin surface, preventing shine.

[0005] Other applications that utilize the internal voids of porous aggregates include drug carriers and chromatography carriers.

[0006] Furthermore, in order to obtain good dispersibility, a smooth feel, and a high packing density when used in the above applications, it is desirable that these porous aggregates have a particle diameter of 1 to several tens of μm and a spherical shape. The following methods have been proposed as methods for producing such spherical porous aggregates with an appropriate particle diameter: One method involves spray-drying a dispersion of nano-sized primary particles (Patent Document 1); another method involves dispersing a W phase consisting of an aqueous metal oxide sol or colloidal solution in an organic solvent phase to form a W / O emulsion, and then gelling the spherical W phase to obtain porous aggregates of metal oxides (Patent Document 2).

[0007] The disadvantage of the method described in Patent Document 1 (spray drying method) is that it requires a step of separating coarse particles from the raw material dispersion to prevent damage to the spray nozzle. Furthermore, porous aggregates of metal oxides are formed quickly at high temperatures, which tends to result in high cohesion. On the other hand, lowering the temperature and dispersion concentration to weaken the cohesion makes it difficult to form porous aggregates of metal oxides with high circularity.

[0008] In contrast, the porous aggregates of metal oxides produced by the method described in Patent Document 2 (emulsion method) are almost perfectly spherical, but because a low-viscosity metal oxide sol or colloidal solution (W phase) is required to generate the emulsion, the primary particle concentration in the raw material is limited to a narrow range.

[0009] It is known that when a colloidal solution of a metal oxide comes into contact with an organic solvent with a certain degree of water solubility, the metal oxide aggregates due to a change in surface potential. Although there are methods for producing metal oxide aggregates based on this principle, it is known that the aggregation is weak and the aggregates are easily redispersed in water (Patent Document 3). [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent Publication No. 2010-138021 [Patent Document 2] Patent Publication No. 2014-210671 [Patent Document 3] WO2012 / 096172 Summary of the Invention [Problem to be solved by the invention]

[0011] In the methods described in Patent Documents 1 to 3, it is necessary to ultimately increase the cohesive force through a baking step or high-temperature heating to prevent re-dispersion of the primary particles and collapse of the spheres.

[0012] Porous aggregates of metal oxides are widely used in abrasives, cosmetics, carriers, etc., but current manufacturing methods have the drawback of requiring cumbersome pretreatment and a heating step to improve the agglomeration force.

[0013] Furthermore, due to the complicated number of steps, there remains the problem that many efforts are required to prevent the inclusion of impurities.

[0014] In light of the above background, an object of the present invention is to provide a method for producing porous aggregates of metal oxides with a reduced number of steps, simple unit operations, and controllable crushing strength. [Means for solving the problem]

[0015] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that porous aggregate particles made of silica nanoparticles with crushing strength controlled within a specific range can be prepared by using the following original techniques in a process for producing porous aggregates made of silica nanoparticles, thereby completing the present invention.

[0016] That is, the present invention provides: a dispersing step of mixing hydrophilic metal oxide particles with a hydrophobic solvent to obtain a dispersion; an extraction step of adding 0.1 to 1 part by weight of an alkaline aqueous solution having a pH of 12 or higher to the dispersion liquid relative to 1 part by weight of the hydrophobic solvent, thereby extracting the metal oxide particles from the hydrophobic solvent into the alkaline aqueous solution, thereby obtaining a mixed liquid consisting of an upper layer of the hydrophobic solvent and a lower layer of the alkaline aqueous solution containing the metal oxide particles; an emulsification step of adding a surfactant to the mixed liquid to prepare a W / O emulsion; an aggregation step of removing water from the W / O emulsion to obtain an aggregate; a washing step of washing the aggregate to obtain a washed product from which the surfactant has been removed; a drying step of drying the washed material to obtain a porous aggregate; The method for producing a porous aggregate is characterized by comprising:

[0017] In a first aspect of the aggregation step, water is distilled off from the W / O emulsion at a temperature of 150° C. or lower without stirring under a pressure of 1 atmosphere or lower to obtain an aggregate. In a second invention of the aggregation step, 1 part by weight of the W / O emulsion is supplied to 0.5 to 5 parts by weight of a liquid composed of at least one alcohol selected from alcohols having 4 or less carbon atoms to form an upper O phase and a lower W phase, and water-insoluble aggregates are formed in the W phase, followed by solid-liquid separation to obtain the aggregates. [Effects of the Invention]

[0018] According to the manufacturing method of the present invention, in the process of manufacturing a porous aggregate, a mechanical pretreatment step of the dispersion (such as filtration or classification to prevent nozzle clogging) is not required. Furthermore, a calcination step for insolubilizing the aggregate by heating at a high temperature is also not required. This makes it possible to easily manufacture a porous aggregate. DETAILED DESCRIPTION OF THE INVENTION

[0019] The following embodiments are examples of the present invention, and the present invention is not limited to these embodiments. Furthermore, unless otherwise specified, the expression "A to B" for a numerical range means "A or more and B or less." In such expressions, when a unit is assigned only to the numerical value B, the unit also applies to the numerical value A. Below, the manufacturing method of the present invention will be explained step by step.

[0020] First Embodiment (Dispersion process) In the dispersion step, hydrophilic metal oxide particles and a hydrophobic solvent are mixed to obtain a dispersion.

[0021] As the hydrophilic metal oxide particles for preparing the dispersion, metal oxide nanoparticles such as silica, titania, zirconia, zinc oxide, aluminum oxide, and gallium oxide can be used as long as they are insoluble in alcohols having four or fewer carbon atoms, poorly soluble in water, and not hydrolyzed. The hydrophilic metal oxide particles are preferably non-spherical particles composed of multiple bonded silica fine particles with an average particle diameter of 5 to 150 nm, more preferably 5 to 50 nm. If the average particle diameter is 150 nm or more, the specific surface area is small, which may result in a long time until the dissolution-precipitation equilibrium is reached in the extraction step described below, and may prevent aggregation in the aggregation step described below.

[0022] The metal oxide particles are ultrafine particles produced by a gas phase method, and those with an average particle size of several nanometers (several nm) to several tens of nanometers (several tens of nm) are preferably used. For example, metal oxide nanoparticles SiO2, TiO2, ZnO, and Al2O3 manufactured by Kanto Chemical Co., Ltd., various hydrophilic grades of Reolosil manufactured by Tokuyama Corporation, various hydrophilic grades of Aerosil manufactured by Nippon Aerosil Co., Ltd., various hydrophilic grades of dry silica HDK manufactured by Wacker Asahi Kasei Silicone Co., Ltd., zirconia powders Zirconeo-Cp and Zirconeo-Rp manufactured by ITEC Co., Ltd., and gallium oxide nanoparticles manufactured by Rare Metal Materials Research Institute Co., Ltd. can be used. Furthermore, fumed silica is highly pure and contains almost no impurities such as alkali metals, and is therefore preferred because it allows the produced porous aggregate to have an extremely low alkali metal content.

[0023] The hydrophobic solvent used to prepare the dispersion preferably has a solubility in water at 20°C of 20 g / L or less, more preferably 5 g / L or less. Any solvent may be used as long as it is hydrophobic enough to form an emulsion with the alkaline aqueous solution. Examples of such solvents include organic solvents such as saturated or unsaturated hydrocarbons with linear or cyclic skeletons, and halogenated hydrocarbons. More specific examples include pentane, hexane, cyclohexane, heptane, octane, nonane, decane, dichloromethane, dichloroethane, chloroform, carbon tetrachloride, dichloropropane, liquid paraffin, toluene, xylene, and mesitylene. Among these, hexane, heptane, and decane, which have appropriate viscosities, are preferred. If necessary, multiple solvents may be used in combination.

[0024] The dispersion method for the dispersion is not particularly limited, and can be carried out before or after adding the particles and solvent (particles to liquid, particles to liquid, or simultaneous addition), as long as no powdery particles remain with moderate stirring or vibration. Commercially available powder-liquid mixers and dispersers can also be used. The amount of hydrophobic solvent used is preferably 4.5 to 45 parts by weight, and more preferably 8 to 25 parts, per part by weight of hydrophilic metal oxide particles. If the amount is less than 4.5 parts by weight, the dispersion will lose fluidity and may not proceed to subsequent processes. If more than 45 parts by weight is used, production efficiency will be poor.

[0025] (extraction process) In the extraction step, an alkaline aqueous solution having a pH of 12 or higher is added to the dispersion in an amount of 0.1 to 1 part by weight per 1 part by weight of the hydrophobic solvent to obtain a mixed solution.

[0026] The concentration of hydrophilic metal oxide particles in the mixed solution is preferably 2 to 10% by mass, and more preferably 3 to 7% by mass. If the particle concentration is 2% by mass or less, the dissolution-precipitation equilibrium concentration is low, and aggregates may not be obtained in the aggregation step described below. If the particle concentration is 10% by mass or more, the dissolution-precipitation equilibrium concentration is high, and the W phase of the W / O emulsion thickens quickly in the emulsification step described below. This results in a large difference in surface tension between the W phase and the O phase, and even if a surfactant is added, a W / O emulsion may not be formed.

[0027] The solute of the alkaline aqueous solution with a pH of 12 or higher used to prepare the above-mentioned mixed solution may be at least one selected from salts (such as alkali metal hydroxides) of metal elements belonging to Group I (Li, Na, K, Rb, and Cs) and those that cannot be extracted with hydrophobic organic solvents, such as ammonia. In addition, when a commercially available product such as ammonia water (aqueous ammonia solution) is used, it may be used as is or diluted.

[0028] The mixed solution can be prepared by adding an alkaline aqueous solution having a pH of 12 or higher to the dispersion and mixing using any suitable method, including a stirrer, disperser, or static mixer. The amount of alkaline aqueous solution having a pH of 12 or higher used is preferably 0.1 to 1 part by weight, more preferably 0.3 to 0.6 parts by weight, per 1 part by weight of the hydrophobic solvent. If the amount is less than 0.1 part by weight, the hydrophilic metal oxide particles will easily solidify, causing the mixed solution to lose fluidity and preventing it from proceeding to subsequent steps. If more than 1 part by weight is used, production efficiency will be poor.

[0029] The above mixing is carried out by adding an alkaline aqueous solution and stirring until the upper layer (hydrophobic solvent) is transparent and the lower layer is a slurry (uniform in color, fluid, and can be picked up with a dropper) when the stirring is stopped, and the concentration of metal oxide particles in the hydrophobic solvent is 2 mass% or less, which can be used to determine the end point of the extraction process.

[0030] Here, we will describe the mechanism and advantages of this process. Whether spray-drying a colloidal solution of metal oxides or agglomerating and then calcining the resulting aggregates, the colloidal solution of metal oxides is an essential starting material. However, concerns about the stability of colloidal solutions (agglomeration stability) are known. Commercially available products contain alkali to prevent agglomeration, and the silica concentration is limited to a low range. However, there are concerns about the quality of the products varying over time.

[0031] On the other hand, in dispersions of hydrophilic metal oxide particles prepared from starting materials, the hydrophilic metal oxide particles undergo aggregation during production and storage. However, to control the quality of the aggregated products, a classification process must be carried out after the dispersion is produced. Furthermore, when the primary particle size of hydrophilic metal oxide particles is smaller, the bulk density is lower, resulting in a large particle volume relative to the liquid. This necessitates a long dispersion production process in which the particles are added little by little while stirring the liquid.

[0032] The manufacturing method of the present invention is a solvent extraction method (liquid-liquid extraction), i.e., a dispersion method that utilizes the phenomenon of solute partitioning (which one is more soluble in which) between two immiscible liquids, such as water and oil. Specifically, the metal oxide particles dispersed in the hydrophobic solvent (oil phase) in the dispersion process described above migrate to the aqueous phase due to their overwhelmingly high affinity (solubility) with the alkaline aqueous solution (aqueous phase) with a pH of 12 or higher added in this process. As a result, because the particles migrate between liquids, the process time is extremely fast, regardless of the volume of the particles (solutes).

[0033] The reason why the pH of the alkaline aqueous solution used to prepare the above-mentioned mixed solution must be 12 or higher is that, based on the potential-pH diagram, in an alkaline environment of 12 or higher, the aforementioned elements Si, Ti, Zr, Zn, Al, and Ga are thought to become their hydroxides or ions. In other words, in a strongly alkaline environment, the oxides of these metals are thought to be able to maintain an equilibrium between dissolution and precipitation. At pH levels below 12, some of the metals are not in a dissociated ionic state, but rather their oxides or oxide hydrates are stable. Therefore, it is thought that the conversion to the oxides or oxide hydrates (gelation) of an aqueous dispersion with a pH below 12 begins shortly after preparation. This causes the entire solution to become a single mass, making it impossible to proceed to subsequent processes.

[0034] Furthermore, because the dissolution-precipitation equilibrium occurs on the surface of the primary particles, it is believed that weak aggregation of hydrophilic metal oxide particle solids during storage is resolved by dissolution. Moreover, because the degree of dissolution, or the imbalance in the dissolution-precipitation equilibrium, depends on pH and concentration, quality control becomes easier by controlling pH and particle concentration. For these reasons, adopting the process of the present invention eliminates the need for mechanical force in the dispersion and classification processes, simplifying the process and reducing the risk of contamination.

[0035] (emulsification process) A nonionic surfactant is added to the mixture to prepare a W / O emulsion.

[0036] In the production method of the present invention, a surfactant is added when forming the above-mentioned W / O emulsion. The surfactant to be used may be any of anionic surfactants, cationic surfactants, and nonionic surfactants. Among these, nonionic surfactants are preferred because they are easy to form a W / O emulsion.

[0037] The amount of surfactant used is the same as the amount typically used to form a W / O emulsion. Specifically, a range of 0.025 g to 10 g per 100 g of hydrophobic solvent can be suitably used. If a large amount of surfactant is used, the W / O emulsion droplets tend to become finer; conversely, if a small amount of surfactant is used, the W / O emulsion droplets tend to become larger. Therefore, the average particle size of the porous aggregates can be adjusted by increasing or decreasing the amount of surfactant used.

[0038] When forming a W / O emulsion, known methods for forming W / O emulsions can be used to disperse alkaline water containing dispersed hydrophilic metal oxide particles in a hydrophobic solvent. From the standpoint of ease of industrial production, emulsion formation by mechanical emulsification is preferred, and specific examples include methods using a mixer, homogenizer, etc. Preferably, a homogenizer can be used. Since the particle size of the dispersed metal oxide sol droplets is related to the particle size of the porous aggregates, it is preferable to adjust the particle size of the sol droplets to achieve the desired particle size.

[0039] In the above production method, the particle size of the obtained porous spherical silica is approximately the same as the droplet size (W phase) of the dispersion liquid in the W / O emulsion prepared in the emulsification step. Therefore, it is necessary to set the dispersion conditions so that the droplet size falls within the desired range. Various methods for controlling the droplet size in W / O emulsions are known, and these techniques can be appropriately selected and applied.

[0040] The purpose of the emulsification step is to form an emulsion using the hydrophobic solvent in the above-mentioned mixed solution as the dispersion medium and the alkaline water in which the hydrophilic metal oxide particles are dispersed as the W phase of the W / O emulsion. By forming such a W / O emulsion, the dispersoids become spherical due to surface tension, etc., and by gelling them in this spherical shape, spherical gelled bodies can be obtained. Thus, by going through the W / O emulsion preparation step of forming a W / O emulsion, it is possible to produce porous aggregates with a high degree of circularity.

[0041] (Agglutination process) The solvent is removed from the W / O emulsion to obtain a coagulate. The solvent is distilled off from the W / O emulsion at a pressure of 1 standard atmospheric pressure or less under non-stirring conditions at a temperature of 150° C. or less to obtain a coagulate.

[0042] Regarding the pressure conditions in this step, pressurized conditions are disadvantageous for distilling off the solvent, and a pressure of 1 standard atmosphere or less is preferred, with 1 standard atmosphere being more preferred in terms of process costs.

[0043] Regarding the temperature conditions for this step, temperatures higher than 150°C are poor in thermal efficiency, so 150°C or less is preferable. Also, temperatures lower than the ambient temperature are cost-inefficient and there are concerns about volume change when water freezes, so a temperature of 0 to 150°C is more preferable.

[0044] The embodiment of this step is not particularly limited, but from the viewpoint that non-stirring conditions are essential, examples include a method using a commercially available general box-type tray-type constant temperature (vacuum) dryer, a dryer with a belt conveyor, etc.

[0045] As mentioned above, it is believed that metal oxides reach an equilibrium state between dissolution and precipitation in a strongly alkaline environment with a pH of 12 or higher. In this process, the equilibrium state is broken by distilling off the solvent (mainly water), and the metal species in an ion-dissociated state present in the aqueous solution of the W phase of the W / O emulsion precipitate as their oxides, forming new cross-linked structures between the primary particles and resulting in the formation of aggregates.

[0046] In this process, the non-stirring condition means that the W / O emulsion is left to stand without applying any external shear force. Stirring destroys the shape of the unsolidified W phase during precipitation, and the resulting aggregates become non-spherical.

[0047] The aggregates can also be aged at 10 to 90°C before solid-liquid separation. Aging tends to reduce the crushing strength of the final porous aggregate. The higher the temperature and the longer the aging time, the more pronounced the effect. However, the aggregates will not dissolve and disappear even after aging for at least several days.

[0048] (Cleaning process) The aggregate is washed to obtain a washed product from which the nonionic surfactant has been removed.

[0049] After the above-mentioned aggregates are obtained, the aggregates are preferably washed with at least one selected from water, a hydrophobic solvent, and an alcohol having 4 or less carbon atoms in order to remove salts, surfactants, and the like contained in the aggregates. This washing operation can be carried out by a known method. In order to improve the washing efficiency, it is preferable to use an aqueous solution of about several tens of mass % of isopropyl alcohol after washing with the hydrophobic solvent. Furthermore, raising the temperature to a level not exceeding the boiling point of the hydrophobic organic solvent can sometimes improve the washing efficiency. Typically, the washing can be carried out at a temperature in the range of 30 to 70°C.

[0050] (drying process) The washed material is dried to obtain a porous aggregate.

[0051] In the production method of the present invention, if the above washing step has been carried out, this step can be carried out subsequently. That is, the washed product dispersed in an aqueous solution of about several tens of mass % isopropyl alcohol is filtered, and the solvent is removed (dried). The temperature during drying is preferably above the boiling point of the solvent and below 150°C, and the pressure is preferably normal pressure or reduced pressure.

[0052] <Second embodiment> The second embodiment differs from the first embodiment in the aggregation step as follows. (Agglutination process) The solvent is removed from the W / O emulsion to obtain a coagulate. One part by weight of the W / O emulsion is fed to 0.5 to 5 parts by weight of a liquid composed of at least one alcohol selected from alcohols having 4 or less carbon atoms to form an upper O phase and a lower W phase, and water-insoluble aggregates are formed in the W phase, followed by solid-liquid separation to obtain the aggregates.

[0053] The alcohol used in this step may be any alcohol having four or fewer carbon atoms, as long as it is insoluble in the metal oxide. Examples of such alcohols include methanol, ethanol, and propanol. Among these, 2-propanol is preferred because of its ease of handling.

[0054] The amount of alcohol used in this step is preferably 0.5 to 5 parts by weight per 1 part by weight of the W / O emulsion. If the amount is less than 0.5 parts by weight, sufficient dilution may not be achieved, and spherical aggregates may not be obtained. If more than 5 parts by weight is used, the solvent is wasted and subsequent steps become a burden.

[0055] The embodiment of this step is not particularly limited, and any mixing method, such as a stirrer or static mixer, can be used, in which the W / O emulsion is added dropwise or poured while stirring the alcohol. The solid-liquid separation is preferably carried out by filtration or centrifugation, from the viewpoint of preventing aggregates from being broken.

[0056] First, let us explain the mechanism of this process. As mentioned above, in a strongly alkaline environment with a pH of 12 or higher, the surface of the metal oxide reaches an equilibrium state between dissolution and precipitation. If this equilibrium state is broken, i.e., if the water in the W phase of the W / O emulsion is removed, the ion-dissociated metal species present in the aqueous solution will precipitate as their oxide, forming new cross-linked structures between the primary particles and resulting in aggregates.

[0057] Therefore, in this process, we focused on the dilution technique. When the W / O emulsion is fed into alcohol, the W / O emulsion breaks down, and the water in the micron-order W-phase micelles is rapidly diluted by the alcohol. Taking advantage of this decrease in water content, dissociated metal species precipitate as their oxides, forming new cross-linked structures between primary particles, resulting in the formation of aggregates.

[0058] The upper O phase and the lower W phase containing the aggregates are subjected to solid-liquid separation to obtain the aggregates. The solid-liquid separation is preferably carried out by filtration or centrifugation, from the viewpoint of preventing the aggregates from being broken down.

[0059] Here, before the solid-liquid separation of the produced aggregates, an aging step may be performed in which the W phase is heated to 10 to 90°C to age the aggregates. Aging tends to reduce the average crushing strength of the final porous aggregates. The higher the temperature and the longer the aging time, the more pronounced the effect. To obtain the desired average crushing strength, the aging time is preferably 0 to 180 minutes, more preferably 0 to 60 minutes. The aging temperature is preferably 10 to 90°C, more preferably 30 to 80°C. However, the aggregates will not dissolve and disappear even after aging for at least several days.

[0060] The aging process involves a chemical equilibrium between silica and hydroxide ions on the surface of the silica, which is the primary particle that makes up the precipitated aggregates (secondary particles). The higher the aging temperature, the shorter the time it takes to reach equilibrium. On the other hand, the higher the hydroxide ion concentration, the more favorable the equilibrium becomes for the reaction to proceed. Therefore, in the case of the present invention, it is believed that as aging progresses, more silica reacts to become ions, the skeleton of the primary particles becomes thinner, and the average crushing strength decreases.

[0061] Although the above description of the present invention has mainly exemplified a method for producing a porous aggregate in which the hydrophilic metal oxide particles are mainly silica, the present invention is not limited to this form.

[0062] For example, when titania is to be obtained by a method using the aforementioned hydrophilic metal oxide particles as a raw material, non-spherical particles in which multiple titania particles having an average particle diameter of 5 to 50 nm are bonded together can be produced by the same procedure as above.

[0063] <Porous aggregate> The porous aggregate obtained by the production method of the present invention is in a powder form after drying. The individual independent particles (secondary particles) constituting the powder usually have an average circularity of 0.8 or more, more preferably 0.85 or more. The "average circularity" is a value obtained by image analysis of an SEM image observed using a scanning electron microscope (SEM) to determine the value C (circularity) defined by the following formula (10) for each particle, and calculating this circularity C as the arithmetic mean value for, for example, 1,500 to 2,000 particles (image analysis method). The area S (projected area) occupied by each of the 1,500 to 2,000 particles in the image was converted into the diameter of a circle equivalent to the area S. From the obtained particle size distribution, the cumulative 50% diameter based on the number of particles is defined as the equivalent circle diameter. In this case, a group of particles forming a single aggregate particle is counted as one particle. C=4πS / L 2 (1) [In formula (1), S represents the area (projected area) that the particle occupies in the image, and L represents the length (perimeter) of the outer periphery of the particle in the image.] As the average circularity increases from 0.8 and approaches 1, the individual particles constituting the porous aggregate become closer to a perfect sphere and the number of aggregated particles decreases. Therefore, if the average circularity is high, the rolling property improves when used as a filler, and excellent packing properties can be obtained.

[0064] The porous aggregate obtained by the production method of the present invention preferably has a pore volume measured by the BJH method of typically 0.5 to 5 mL / g, and a median diameter in the range of 1 to 50 μm in the particle size distribution measured by laser diffraction. The mode of pore diameter is the pore diameter value at which the cumulative pore volume (volume distribution curve) expressed as the logarithm of the pore diameter takes the most frequent peak value. The porous aggregate of the present invention preferably has a mode of pore diameter of 5 to 50 nm, more preferably 15 to 50 nm. [Example]

[0065] Examples are given below to specifically explain the present invention, but the present invention is not limited to these examples.

[0066] <Evaluation method> The produced porous aggregates were evaluated for the following items.

[0067] (Measurement of average circularity and equivalent circle diameter by SEM image analysis) First, a scanning electron microscope (SEM: Hitachi High-Technologies Corporation, SU-3500) was used to obtain SEM images at a secondary electron detection, low acceleration voltage (20 kV), and magnification of 270x. The obtained SEM images were analyzed using image analysis software (Mitani Corporation, WINROOF2018) to calculate the average circularity and equivalent circle diameter. The average circularity is the arithmetic mean value of the value C (circularity) defined by the following formula (1) obtained for 1500 to 2000 particles. C=4πS / L 2 (1) [In formula (1), S represents the area (projected area) that the particle occupies in the image, and L represents the length (perimeter) of the outer periphery of the particle in the image.] The equivalent circle diameter is the cumulative 50% diameter based on the number of particles, based on the particle size distribution evaluated by evaluating the diameter of a circle equivalent to the area S (projected area) occupied by 1500 to 2000 particles in the image. In this case, a group of particles forming one aggregate particle is counted as one particle. The number of particles sampled for calculating the average circularity and the equivalent circle diameter is preferably 500 to 5,000, more preferably 1,500 to 2,000, in order to achieve both measurement accuracy and calculation efficiency.

[0068] (Measurement of BJH pore volume and mode of pore diameter) Measurements of the BJH pore volume and the mode (peak) of pore diameter were performed using a specific surface area / pore distribution analyzer (BELSORP-mini, manufactured by BEL Japan Co., Ltd.). As a pretreatment for the measurement, the sample to be measured was dried at 150°C for at least 2 hours under a vacuum of 1 kPa or less. Then, an adsorption isotherm was obtained for only the nitrogen adsorption side at liquid nitrogen temperature. Analysis was then performed using the BJH method. The mode of pore diameter is the pore diameter value at which the cumulative pore volume (volume distribution curve) of the logarithm of the pore diameter reaches its maximum peak value.

[0069] (crushing strength) The "crushing strength" was measured as the arithmetic mean value of 10 particles, determined according to the method specified in JIS Z8844:2019. Following the definition above, the measurement was performed using a microcompression testing machine (Shimadzu Corporation, MCT-W510-J). The measurement was performed under the following conditions: test force 9.81 mN, load rate 9.81 mN / sec, and load holding time 0 sec. An indenter with a diameter of 200 μm was used for the measurement.

[0070] Example 1 (Dispersion process) Rheoloseal QS-30 (Tokuyama Corporation, specific surface area 300±30m 2 40 g of decane was added to 4.0 g of cellulose acetate (0.1% w / g, average primary particle size 7 nm, SiO2 purity >99.9%, Cl <50 ppm, Fe <20 ppm, Al <20 ppm), and the mixture was stirred with a spoon until no powdery particles remained, yielding a creamy dispersion.

[0071] (extraction process) 16.0 g of 9.38% by mass ammonia water, pH 13.0 (prepared by diluting 25% by mass ammonia water manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the above dispersion, and then the mixture was stirred for 30 seconds at 7000 rpm using a homogenizer (manufactured by IKA, T25BS1) to obtain a mixed solution.

[0072] (emulsification process) 3 g of decane containing 0.3 g of sorbitan monooleate (Kao Corporation, Rheodor SP-010V) dispersed therein was added all at once, and the mixture was stirred at 7000 rpm for 15 seconds using a homogenizer (IKA, T25BS1) in the same manner as in the previous step, to obtain a W / O emulsion.

[0073] (Agglutination process) The obtained W / O emulsion was transferred to a shallow tray and dried in a vacuum dryer at 150°C for 2 hours to obtain an aggregate.

[0074] (Cleaning process) The obtained aggregates were dispersed in 40 g of 2-propanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) for 30 minutes with stirring, then filtered from the solvent using a suction filter and washed with decane, water, and 2-propanol while suctioning, respectively, to obtain washed products.

[0075] (drying process) The washed product was transferred to a beaker and dried in a vacuum dryer for 3 hours at 150° C. The physical properties of the porous aggregate thus obtained are shown in Table 1.

[0076] <Example 2> (Dispersion process) 40 g of decane was added to 4.0 g of Reolosil QS-30, and the mixture was stirred with a spoon until no powdery particles remained, to obtain a creamy dispersion.

[0077] (extraction process) To the dispersion was added 16.0 g of 9.38% by mass aqueous ammonia at pH 13.0, and the mixture was stirred for 30 seconds at 7000 rpm using a homogenizer to obtain a mixed liquid.

[0078] (emulsification process) 3 g of decane in which 0.3 g of sorbitan monooleate had been dispersed was added all at once, and the mixture was stirred for 15 seconds at 7,000 rpm using the homogenizer as in the previous step to obtain a W / O emulsion.

[0079] (Agglutination process) The W / O emulsion obtained in the previous step was immediately added to 40 g of 2-propanol while stirring with a magnetic stirrer. After that, the mixture was allowed to stand, and separated into two layers: an upper O phase and a lower W phase containing aggregates.

[0080] (Cleaning process) The resulting aggregate was filtered off from the W phase using a suction filter, and washed with decane, water, and 2-propanol while being suctioned to obtain a washed product.

[0081] (drying process) The washed product was transferred to a beaker and dried in a vacuum dryer for 3 hours at 120° C. The physical properties of the porous aggregate thus obtained are shown in Table 1.

[0082] Example 3 After the aggregation step and before the washing step, the two-layer liquid consisting of an upper O phase and a lower W phase containing the aggregates was allowed to stand at room temperature for 40 hours. Except for this, the procedure was the same as in Example 2. The physical properties of the resulting porous aggregates are shown in Table 1.

[0083] Example 4 In the dispersion step, 3.0 g of Reolosil QS-30 was used. Following the aggregation step and before the washing step, a two-layer liquid consisting of an upper O phase and a lower W phase containing aggregates was allowed to stand at 70°C for 20 minutes. Other than that, the procedure was the same as in Example 2. The physical properties of the resulting porous aggregates are shown in Table 1.

[0084] <Example 5> In the dispersion step, 3.0 g of Reolosil QS-30 was used. In addition, in the aggregation step, 40 g of ethanol was used instead of 2-propanol. Except for these, the same operations as in Example 2 were carried out. The physical properties of the obtained porous aggregate are shown in Table 1.

[0085] Example 6 In the dispersion process, Rheoloseal QS-102 (manufactured by Tokuyama Corporation, specific surface area 200±20 m 2 The porous aggregate (SiO2 purity >99.9%, Cl <50 ppm, Fe <20 ppm, Al <20 ppm) was used in an amount of 4.0 g. The other conditions were the same as in Example 2. The physical properties of the resulting porous aggregate are shown in Table 1.

[0086] Example 7 In the dispersion step, a mixture of 2.0 g of Reolosil QS-102 and 2.0 g of Reolosil QS-30 was used. Except for this, the procedure was the same as in Example 2. The physical properties of the resulting porous aggregate are shown in Table 1.

[0087] <Comparative Example 1> (Dispersion process) 16.0 g of 9.38% by mass aqueous ammonia, pH 13.0, was added to 4.0 g of Reolosil QS-30, and the mixture was stirred with a spoon until no powdery particles remained, to obtain a slurry dispersion.

[0088] (extraction process) To the above dispersion, 43 g of decane in which 0.3 g of sorbitan monooleate had been dispersed was added, and the mixture was stirred for 30 seconds at 7,000 rpm using a homogenizer. However, a mixed liquid was not obtained, and the mixture became a mayonnaise-like cream with no fluidity. After that, the process was no longer possible.

[0089] <Comparative Example 2> In the dispersion step, 43 g of decane in which 0.3 g of sorbitan monooleate had been dispersed was used instead of 40 g of decane. The second step was carried out in the same manner as in Example 2. The desired W / O emulsion was not obtained, and the mixture contained amorphous gel in the decane. Further work was no longer possible.

[0090] <Comparative Example 3> The dispersion, extraction, and emulsification steps were carried out in the same manner as in Example 2. In the aggregation step, 40 g of 2-propanol was added to the W / O emulsion while stirring with a magnetic stirrer. As a result, the W / O emulsion was demulsified, the lower W phase was a cloudy liquid, and no aggregates were obtained. Further processing was no longer possible.

[0091] <Comparative Example 4> The dispersion step and extraction step were carried out in the same manner as in Example 2, except that 21.2 g of 0.12% by mass aqueous ammonia, pH 11.8, was used in the extraction step. The W phase thickened rapidly, and a mixed liquid could not be obtained. Further work was no longer possible.

[0092] [Table 1]

Claims

1. a dispersing step of mixing hydrophilic metal oxide particles with a hydrophobic solvent to obtain a dispersion; an extraction step of adding 0.1 to 1 part by weight of an alkaline aqueous solution having a pH of 12 or higher to the dispersion liquid per 1 part by weight of the hydrophobic solvent to extract the metal oxide particles from the hydrophobic solvent into the alkaline aqueous solution, thereby obtaining a mixed liquid consisting of an upper layer of hydrophobic solvent and a lower layer of alkaline aqueous solution containing the metal oxide particles; an emulsification step of adding a surfactant to the mixed solution to prepare a W / O emulsion; an aggregation step of removing water from the W / O emulsion to obtain an aggregate; a washing step of washing the aggregate to obtain a washed product from which the surfactant has been removed; a drying step of drying the washed material to obtain a porous aggregate; A method for producing a porous aggregate, comprising:

2. 2. The method for producing a porous aggregate according to claim 1, wherein in the aggregation step, the W / O emulsion is distilled at a temperature of 150°C or less under non-stirring conditions to remove water, thereby obtaining an aggregate.

3. 2. A method for producing a porous aggregate as described in claim 1, wherein in the aggregation step, 1 part by weight of the W / O emulsion is supplied to 0.5 to 5 parts by weight of a liquid composed of at least one alcohol selected from alcohols having 4 or less carbon atoms to form an upper O phase and a lower W phase, and water-insoluble aggregates are generated in the W phase, followed by solid-liquid separation to obtain the aggregates.

4. The method for producing a porous aggregate according to claim 3, further comprising a maturation step of heating the W phase to 10 to 90°C to mature the aggregate before subjecting the generated aggregate to solid-liquid separation.

5. The method for producing a porous aggregate described in any one of claims 1 to 4, characterized in that the hydrophilic metal oxide particles are non-spherical particles formed by bonding together multiple silica microparticles having an average particle diameter of 5 to 150 nm.

6. The method for producing a porous aggregate according to any one of claims 1 to 5, wherein the hydrophobic solvent has a solubility in water at 20°C of 20 g / L or less.

7. The method for producing a porous aggregate according to any one of claims 1 to 6, wherein the alkaline aqueous solution is at least one selected from a solution of a salt of a metal element belonging to Group I and ammonia water.

8. The method for producing a porous aggregate according to any one of claims 1 to 7, wherein in the extraction step, the concentration of the hydrophilic metal oxide in the mixed solution is 2 to 10 mass%.

9. The method for producing a porous aggregate according to any one of claims 1 to 8, wherein in the extraction step, the particles are stirred and mixed until the particle concentration in the hydrophobic solvent becomes 2 mass % or less.

10. The method for producing a porous aggregate according to any one of claims 1 to 9, wherein in the washing step, washing is carried out with at least one washing liquid selected from water, a hydrophobic solvent, and an alcohol having 4 or less carbon atoms.

11. The method for producing a porous aggregate according to any one of claims 1 to 10, wherein in the solid-liquid separation step, solid-liquid separation is carried out by filtration or centrifugation.

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