Silica particle and method for producing the same

Abstract

Provided is a method for producing silica particles having a micrometer size, no fusion, and a low silanol group content.　This method for producing silica particles includes the steps of: preparing silica gel containing metals having a melting point of 1650°C or higher in an amount of 5 mass ppm or more in terms of metal elements, and having a median diameter of 10 μm or less; and firing the silica gel.

Description

Silica particles and method for producing the same

[0001] The present invention relates to silica particles and a method for producing them.

[0002] Silica particles are industrial materials used in a wide range of fields, including desiccants, adsorbents, fillers for resins and rubber materials, catalysts, paints, adhesives, fire retardants, and glass materials. Among these, micrometer-sized silica particles have high dispersibility and stability when mixed with other materials, and are used in various applications.

[0003] Silica particles are known to be produced by various methods, including the thermal decomposition of silicon tetrachloride, the deionization of alkali silicates such as water glass, and the hydrolysis and condensation reactions of alkoxysilanes.

[0004] Generally, silica particles are composed of a network of siloxane bonds, with hydroxyl groups called silanol groups at their ends. Silanol groups are known to affect various physical properties of silica particles; for example, they can cause bubbles during vitrification and contribute to the absorption of infrared light, thus affecting light transmittance.

[0005] Many studies have been conducted on methods for producing silica particles. For example, Patent Document 1 discloses a method for obtaining spherical silica powder by firing spherical silica particles produced by a wet process, followed by pulverization or crushing. In addition, Patent Document 2 discloses a method for improving the strength when combined with a resin composition by performing an operation to generate silanol groups on the surface of silica particles.

[0006] Japanese Patent Publication No. 2022-153911 Japanese Patent Publication No. 2015-078105

[0007] The method disclosed in Patent Document 1 yields micrometer-sized silica particles, but because the firing temperature is below 1100°C, the reduction in silanol group content is insufficient. Furthermore, attempting to increase the firing temperature to further reduce the silanol group content results in sintering or fusion of the silica particles, which presents a problem.

[0008] As a result of diligent research to solve the above problems, the inventors have found that the above problems can be solved by controlling the particle size of silica gel, the silica particle raw material, and then firing it while appropriately adding metals with a melting point of 1650°C or higher.

[0009] In other words, the present invention has the following aspects: [1] A method for producing silica particles, comprising the steps of: preparing silica gel containing 5 ppm or more by mass in terms of metallic elements of metals with a melting point of 1650°C or higher, and having a median diameter of 10 μm or less; and calcining the silica gel. [2] The method for producing silica particles according to [1], wherein the temperature of the calcination step is 1100 to 1280°C. [3] The method for producing silica particles according to [1] or [2], wherein the median diameter of the silica particles after calcination is 10 μm or less. [4] The method for producing silica particles according to any one of [1] to [3], wherein the ratio of the median diameter of the silica particles after calcination to the median diameter of the silica gel before calcination (after calcination / before calcination) is 1 or less. [5] The BET specific surface area of ​​the silica particles is 3 m². ２ [1] to [4] A method for producing silica particles according to any one of the following, wherein the amount is less than / g. [6] A method for producing silica particles according to any one of the following, wherein the alkali metal content of the silica particles is 100 ppm by mass or less. [7] Silica particles containing 5 ppm by mass or more of metals with a melting point of 1650°C or higher in terms of metal elements, having a median diameter of 10 μm or less, and a silanol group content of 150 ppm by mass or less. [8] Silica particles according to [7], wherein the silica particles have an amorphous structure. [9] Silica particles according to [7] or [8], wherein the metals are at least one metal selected from the group consisting of metals, metal oxides, and metal salts.

[10] Silica particles according to any one of the following, wherein the metal species of the metals are at least one metal selected from the group consisting of Group 4 elements, Group 6 elements, and aluminum.

[11] Silica particles with a BET specific surface area of ​​3 m² ２ Silica particles according to any of [7] to

[10] , wherein the amount is less than / g.

[12] Silica particles according to any of [7] to

[11] , wherein the alkali metal content is 100 ppm by mass or less.

[0010] According to the present invention, it is possible to provide silica particles that are micrometer-sized, non-fused, and have a low silanol group content, as well as a method for producing the same.

[0011] This is an explanatory diagram showing one embodiment of the preparation and preparation steps for silica gel used in the silica particle manufacturing method of the present invention. This is a diagram showing a secondary electron image obtained by observing the silica particles obtained in Example 3 with a scanning electron microscope. This is a diagram showing a secondary electron image obtained by observing the silica particles obtained in Comparative Example 1 with a scanning electron microscope.

[0012] The embodiments of the present invention will be described in detail below, but the present invention is not limited to the embodiments described below and can be implemented in various ways within the scope of its gist.

[0013] In this specification, "x and / or y (where x and y are any combination)" means at least one of x and y, and can mean x only, y only, or x and y. In this specification, when expressed as "X to Y" (where X and Y are any numbers), unless otherwise specified, it includes the meaning of "greater than or equal to X and less than or equal to Y," as well as "preferably greater than X" or "preferably less than Y." In this specification, when expressed as "greater than or equal to X" (where X is any number) or "less than or equal to Y" (where Y is any number), it also includes the meaning of "preferably greater than X" or "preferably less than Y." In this specification, for numerical ranges described in stages, the upper or lower limit of a numerical range in one stage can be arbitrarily combined with the upper or lower limit of a numerical range in another stage. Also, in numerical ranges described in this specification, the upper or lower limit of that numerical range can be replaced with the value shown in the example. In this specification, a preferred combination of embodiments is a more preferred embodiment. In this specification, all physical properties and characteristic values ​​are given at 23°C unless otherwise specified.

[0014] A method for producing silica particles according to one embodiment of the present invention (hereinafter referred to as "this method") comprises the steps of preparing a specific silica gel and calcining the silica gel. These steps will be described in order below.

[0015] <<Preparation Process for Silica Gel>> In order to produce silica particles using this method, first, silica gel containing at least 5 ppm by mass of metals with a melting point of 1650°C or higher, and with a median diameter of 10 μm or less, is prepared. The following are the details of the silica gel preparation process.

[0016] <Preparation of Silica Gel> The method for preparing silica gel is not particularly limited and can be any known method, such as adding acid to an aqueous sodium silicate solution to gel it, the sol-gel method, or the gas phase method. However, an example of a silica gel preparation method will be described below with reference to Figure 1. First, the process includes a hydrolysis step S1 in which an alkoxysilane is hydrolyzed to obtain a wet gel, a pulverization step S2 in which the wet gel obtained in the hydrolysis step S1 is pulverized to obtain a pulverized wet gel, and a drying step S3 in which the pulverized wet gel obtained in the pulverization step S2 is dried. Each step will be described below.

[0017] (Hydrolysis Step S1) Hydrolysis step S1 involves reacting the raw material, tetraalkoxysilane, with water to perform hydrolysis. This step can be carried out according to known methods and is not particularly limited, but examples include the sol-gel method and the deflagration method. From the viewpoint of reducing the amount of impurities, silica gel obtained by the sol-gel method is preferred. The sol-gel method is described in literature, for example (Sumio Sakka, "The Science of the Sol-Gel Method"), and specific manufacturing methods are described in Japanese Patent Publication No. 5-246708 and Japanese Patent Publication No. 8-91822.

[0018] The sol-gel method basically produces silica gel by hydrolyzing an alkoxysilane, as shown in the following reaction equation (a). While the type of alkoxysilane is not particularly limited, reaction equation (a) illustrates the case using tetraalkoxysilane. (RO) ４ Si + 2H ２ O → SiO ２ +4ROH (a)

[0019] In the above reaction formula (a), R represents an alkyl group, preferably having 1 to 4 carbon atoms, and particularly preferred is a methyl group, which undergoes a rapid hydrolysis reaction and leaves little residue of alkoxy groups in the resulting silica.

[0020] The sol-gel method is carried out in a reactor. Reactors made of stainless steel, glass, Teflon®, or conical rotary reactors may be used, but from the perspective of reactor strength and cost advantages, a stainless steel reactor is preferred. Furthermore, the produced silica gel may contain foreign particles. Therefore, in this embodiment, the silica gel produced by the sol-gel method may be passed through a magnetic separator to remove foreign particles as needed.

[0021] Here, the hydrolysis product of tetraalkoxysilane produced by hydrolysis step S1 is called the "wet gel." The obtained wet gel is transferred to the hopper.

[0022] In the hydrolysis step S1, organic solvents such as alcohols, ethers, or ketones that are compatible with the by-product alcohol may be mixed as needed. Examples of alcohols include methanol, ethanol, and propanol; examples of ethers include diethyl ether; and examples of ketones include acetone. Among these, methanol or ethanol is preferred because it does not accelerate the reaction rate too much and makes it easy to control the physical properties.

[0023] However, if the reaction solution becomes homogeneous before gelation (formation of a wet gel) occurs, as the alkoxy groups bonded to silicon are released as alcohol as the hydrolysis reaction progresses, that is, if the raw material contains alkoxy groups with a high hydrolysis rate (e.g., methoxy groups), then production can be carried out without practical problems even without adding alcohol or other substances.

[0024] A catalyst is not essential in this reaction, and it is generally preferable not to use one in order to obtain higher purity silica particles. However, in some cases, acids such as hydrochloric acid, acetic acid, hydrofluoric acid, or sulfuric acid, or alkalis such as aqueous ammonia, may be used as catalysts.

[0025] For hydrolysis, it is preferable to use ultrapure water or similar to achieve higher purity of the target silica particles. While there are no particular restrictions on the amount of water used, as long as it is sufficient for the hydrolysis reaction to proceed, in practice, it is common to add an excess amount greater than the theoretical amount (twice the molar amount of tetraalkoxysilane).

[0026] Furthermore, in order to set the time required for gelation and the time required for coarse grinding within an appropriate range, it is practical to set the molar ratio of tetraalkoxysilane to water in the range of 1:2 to 1:10, preferably 1:3 to 1:8, and more preferably 1:4 to 1:7. If the molar ratio of water to tetraalkoxy is below the upper limit, the time required for gelation can be shortened, and even after gelation, the time required for the wet gel to reach a hardness suitable for the grinding process can be shortened, and the process of evaporating excess water becomes unnecessary, which is preferable in terms of production efficiency. Also, if the molar ratio of water to tetraalkoxy is above the lower limit, hydrolysis proceeds sufficiently, and therefore gelation is carried out sufficiently. Since the hydrolysis reaction has almost completely proceeded once a homogeneous solution of tetraalkoxysilane and water has been formed, it is then sufficient to leave the solution to stand until it gels and becomes integrated.

[0027] The conditions for hydrolysis and gelation vary depending on the raw materials used, but are generally preferably carried out at a temperature of 20 to 100°C, more preferably at 25 to 75°C, and even more preferably at 35 to 55°C. Excellent gel strength is achieved when the hydrolysis and gelation temperatures are below the upper limit. Conversely, excellent productivity is achieved when the hydrolysis and gelation temperatures are above the lower limit.

[0028] Furthermore, while the hydrolysis reaction and gelation are not particularly limited, they are preferably carried out under pressure conditions of atmospheric pressure to 0.2 MPa. The time required for the hydrolysis reaction and gelation is not particularly limited, but it is preferably about 20 minutes to 10 hours.

[0029] The gelation of hydrolysates proceeds within a few hours even at room temperature (e.g., 23°C) and can be accelerated by heating; therefore, the gelation time can be adjusted by adjusting the various conditions mentioned above. These conditions can be adjusted, for example, by passing hot water through the reactor jacket or by creating a vacuum inside the reactor.

[0030] Furthermore, to facilitate handling during the grinding process, a coarse grinding treatment may be performed after the hydrolysis reaction, in which the wet gel is coarsely ground to a size of several centimeters. The method of coarse grinding is not limited, but for example, it can be done by placing the wet gel in the hydrolysis container under reduced pressure after the hydrolysis reaction to create cracks in the gel, and then grinding the gel by rotating or oscillating the hydrolysis container containing the wet gel.

[0031] (Grinding Process S2) In grinding process S2, the wet gel is ground at this stage to adjust the particle size of the final product. Here, the ground wet gel is called "ground wet gel". The wet gel obtained in hydrolysis process S1 is transferred from the hopper to the grinder. At this time, the flow rate of the wet gel is controlled by a motor-operated flow control device and supplied quantitatively. The wet gel transferred to the grinder is then ground and transferred back to the hopper as ground wet gel.

[0032] During pulverization, since wet gel is brittle and prone to generating fine powder, it is preferable to shorten the residence time in the pulverizer and use a one-pass continuous pulverizer in order to obtain silica particles within a desirable median diameter range in good yield. The residence time is preferably within 1 minute. Examples of pulverizer types include mesh pulverizers that press the wet gel against a mesh made of synthetic resin or the like and pass it through the mesh; jet mills that use a high-speed airflow to collide particles and pulverize them; bead mills that use small beads to forcibly agitate and pulverize particles; and high-speed rotary mills that rotate hammers, blades, pins, etc. at high speed and pulverize by impact and shear (especially screen mills that have a screen (perforated plate) built into the pulverizer for classifying the pulverized material). These can be used individually or in combination of two or more types. To reduce the median diameter of silica gel to 5 μm or less, it is preferable to pulverize it at least in a jet mill and a bead mill, and more preferably in a jet mill.

[0033] (Drying process S3) In drying process S3, the pulverized wet gel is dried to remove organic solvents and water contained in the gel. Drying of the pulverized wet gel is usually done in batches. Here, the dried wet gel is called a "dry gel".

[0034] The pulverized wet gel obtained in the pulverization process S2 is stored in a hopper, and when the amount reaches the batch size, it is transferred to a dryer. For example, a conical rotary dryer is used as the dryer. The drying conditions are adjusted by supplying steam introduced from a steam introduction mechanism to the dryer's jacket to control the temperature, and by evacuating the inside of the dryer using a vacuum mechanism to control the pressure. For drying, for example, heating is performed under reduced pressure, usually at 90 to 300°C for 60 to 600 minutes, more preferably at 100 to 280°C, and even more preferably at 150 to 250°C for 120 to 360 minutes.

[0035] Furthermore, to facilitate handling of the gel during the firing process, the dry gel may be pre-classified using a classifier after drying, if necessary. This also aims to appropriately adjust the final particle size distribution range. A vibrating sieve classifier is used as the classifier.

[0036] <Introduction of metals into silica gel> This manufacturing method includes a step of adding metals with a melting point of 1650°C or higher to silica gel in an amount of 5 ppm by mass or more in terms of metallic elements.

[0037] The metals with a melting point of 1650°C or higher (hereinafter sometimes referred to as "high-melting-point metals") are not particularly limited as long as they are high-melting-point metals and are components that are ultimately incorporated into silica particles. Here, "metals" means metals, alloys, metal oxides, metal hydroxides, and metal salts. Among these, from the viewpoint of effectively preventing the fusion of silica particles, metals, metal oxides, and metal salts are preferred, and metal oxides are more preferred. These can be used individually or in combination of two or more. Metals tend to form a protective oxide film on their surface by firing or other processes, and it is presumed that this protective film protects the particles from fusing together, thereby improving resistance to fusion.

[0038] Furthermore, while the type of metal used for the high-melting-point metals is not particularly limited as long as it is a high-melting-point metal, it is preferable to include at least one metal selected from the group consisting of Group 4 elements (Ti, Zr), Group 6 elements (W), and aluminum (Al) of the periodic table, because it forms an oxide film and has excellent corrosion resistance. More preferably, it is at least one metal selected from the group consisting of tungsten (W), titanium (Ti), zirconium (Zr), and aluminum (Al), and even more preferably zirconium (Zr) and / or aluminum (Al). Zirconium (Zr) and / or aluminum (Al) offer excellent handling and economic advantages for industrial use when introduced into silica gel.

[0039] As raw materials for high melting point metals, for example, as Al raw materials, aluminum acetate, aluminum lactate, etc. can be mentioned. As Zr raw materials, zirconium acetylacetonate, etc. can be mentioned. As Ti raw materials, titanium oxide, etc. can be mentioned. As W raw materials, tungsten oxide, sodium tungstate, lithium tungstate, etc. can be mentioned.

[0040] As a method for introducing high melting point metals into silica gel, for example, a method of mixing high melting point metals in the form of powder, aqueous solution, or organic solvent solution with silica gel can be mentioned. Among them, a method of mixing high melting point metals in the form of aqueous solution and / or organic solvent solution with silica gel is preferable from the viewpoint of homogeneity.

[0041] As a method for introducing high melting point metals into silica gel in the form of aqueous solution and / or organic solvent solution (metal-containing solution), specifically, by adjusting the metal concentration of the metal-containing solution and the mixing ratio of the metal-containing solution and silica gel, the content of high melting point metals in silica gel can be adjusted.

[0042] The metal concentration of the metal-containing solution is preferably 1 to 5000 mass ppm, more preferably 3 to 3000 mass ppm, still more preferably 5 to 2000 mass ppm, particularly preferably 10 to 1000 mass ppm, and most preferably 15 to 900 mass ppm. When it is above the lower limit value, there is a tendency to have an excellent effect of preventing the fusion of silica particles. When it is below the upper limit value, there is a tendency to be excellent due to the reduction of silanol groups. Although the mechanism by which the silanol groups are easily reduced when it is less than the upper limit value is not clear, it is considered that water molecules are attracted and adhered to the metal species showing Lewis acidity present on the silica surface and changed to silanol groups after firing. In addition, in the case of metals forming an oxide film, oxygen vacancies occur during firing, and these vacancies function as adsorption sites to attract water molecules and change to silanol groups after firing.

[0043] The mixing ratio of silica gel to the metal-containing solution (silica gel / metal-containing solution) is preferably 20 / 80 to 80 / 20, more preferably 30 / 70 to 70 / 30, and even more preferably 40 / 60 to 60 / 40. By adjusting the metal concentration and mixing ratio of the metal-containing solution as described above, a specific amount of high-melting-point metals can be uniformly dispersed in the silica gel.

[0044] A slurry can be obtained by mixing silica gel and the metal-containing solution, and by drying such a slurry, silica gel powder (dry gel) containing high-melting-point metals can be obtained.

[0045] The drying of the slurry can be applied by analogy to the drying step S3. For example, under reduced pressure, it is usually heated at 90 to 300 °C for 30 to 600 minutes, but more preferably at 100 to 150 °C for 60 to 300 minutes to obtain a dry gel.

[0046] As a method of introducing high-melting-point metals into silica gel in a powder state, in order to efficiently concentrate high-melting-point metals on the silica gel surface, it is desirable to use particles with a particle size of 1 / 10 or less of the median particle size of the silica gel. Also, as a method of mixing fine high-melting-point metals into silica gel, at the stage of pulverizing the raw silica gel, a method can be used in which wear powder of high-melting-point metals is generated using high-melting-point metals as a pulverizing medium such as balls in ball mill pulverization, and the high-melting-point metals are added.

[0047] <Silica gel before firing> The silica gel used in this manufacturing method can be prepared by the above process. The content of high-melting-point metals in the prepared silica gel is 5 ppm by mass or more, preferably 8 ppm by mass or more, and more preferably 10 ppm by mass or more, in terms of metal elements. Furthermore, the upper limit is preferably 3000 ppm by mass or less, more preferably 2000 ppm by mass or less, even more preferably 1500 ppm by mass or less, particularly preferably 1000 ppm by mass or less, especially preferably 500 ppm by mass or less, and most preferably 200 ppm by mass or less. Examples of the content range include 5 to 3000 ppm by mass, 8 to 2000 ppm by mass, etc. If the content of high-melting-point metals in silica gel is above the lower limit, the silica particles are less likely to sinter or fuse with each other, and firing can be performed under higher temperature conditions. On the other hand, if the content of high-melting-point metals in silica gel is below the upper limit, the properties of amorphous silica can be maintained.

[0048] The mechanism by which adding high-melting-point metals to silica gel makes it less likely for silica particles to sinter or fuse together is not entirely clear, but it is thought to be due to the fact that the melting point of silica increases when the metal is dissolved in the silica compared to silica alone.

[0049] The content of high-melting-point metals in silica gel can be evaluated, for example, by dissolving the sample in acid and performing ICP emission spectrometry.

[0050] The prepared silica gel has a median diameter D50 of 10 μm or less, preferably 8 μm or less, more preferably 7 μm or less, and even more preferably 5 μm or less. When the median diameter of the silica gel is below the upper limit, the effect of reducing silanol groups by calcination becomes excellent. The lower limit of the median diameter is preferably 0.3 μm or more, more preferably 0.5 μm or more, even more preferably 1 μm or more, and particularly preferably 2 μm or more. When the median diameter is above the lower limit, silanol can be reduced efficiently. Examples of median diameter ranges include 0.3 to 10 μm and 0.5 to 8 μm.

[0051] The median diameter of silica gel can be measured using known measurement methods such as laser diffraction scattering and dynamic light scattering (DLS).

[0052] <<Silica Gel Calcination Process>> This manufacturing method includes a process of calcining the prepared silica gel. The calcination method is not particularly limited, but one example is the use of an electric furnace. For example, the dry gel before calcination, which has been stored in a hopper or the like, is transferred to a quartz glass crucible via a classifier as needed. Then, the silica gel is heated in an electric furnace to produce silica particles. At this point, if necessary, dry air or helium gas is circulated into the crucible through a quartz glass tube from a gas introduction mechanism.

[0053] The temperature of the firing process is preferably 1100 to 1280°C, more preferably 1150 to 1250°C, and even more preferably 1200 to 1230°C. If the firing temperature is below the upper limit, crystallization of the resulting silica particles can be prevented. On the other hand, if the firing temperature is above the lower limit, silanol groups can be efficiently reduced, resulting in advantages in terms of time and cost. The firing time is generally about 10 to 100 hours.

[0054] Dry gels before firing are generally porous and therefore unsuitable as raw materials for forming glass layers such as optical fibers. For this reason, it is preferable to heat and fire the obtained dry gel in the firing process to densify it and convert it into non-porous silica particles.

[0055] After the firing process, a magnetic separation process may be included, if necessary, in which the silica particles are passed through a magnetic separator. The magnetic separation process can remove foreign particles from the silica particles. Furthermore, a classification process may be included.

[0056] <Silica particles after firing> Silica particles obtained by this manufacturing method tend to have little variation in the content of high-melting-point metals due to firing. The silica particles obtained by this manufacturing method and the silica particles according to one embodiment of the present invention may collectively be referred to as "these silica particles" below.

[0057] The silica particles contain high-melting-point metals in terms of metallic elements of 5 ppm or more by mass, preferably 8 ppm or more, and more preferably 10 ppm or more. The upper limit is preferably 3000 ppm or less by mass, more preferably 2000 ppm or less by mass, even more preferably 1500 ppm or less by mass, particularly preferably 1000 ppm or less by mass, especially preferably 500 ppm or less by mass, and most preferably 200 ppm or less by mass. Examples of the content range include 5 to 3000 ppm by mass, 8 to 2000 ppm by mass, etc. If the content of high-melting-point metals in the silica particles is above the lower limit, the silica particles are less likely to sinter or fuse with each other, and can be fired under higher temperature conditions. On the other hand, if the content of high-melting-point metals in the silica particles is below the upper limit, the properties of amorphous silica can be maintained.

[0058] The content of high-melting-point metals in silica particles can be evaluated, for example, by dissolving the sample in acid and performing ICP emission spectrometry.

[0059] The silica particles have a median diameter D50 of 10 μm or less, preferably 8 μm or less, more preferably 7 μm or less, and even more preferably 5 μm or less. When the median diameter of the silica particles is below the upper limit, the effect of reducing silanol groups by calcination becomes excellent. The lower limit of the median diameter is preferably 0.3 μm or more, more preferably 0.5 μm or more, even more preferably 1 μm or more, and particularly preferably 2 μm or more, and when the median diameter is above the lower limit, silanol is sufficiently reduced. Examples of median diameter ranges include 0.3 to 10 μm, 0.5 to 8 μm, etc.

[0060] The median diameter can be measured using known measurement methods such as laser diffraction scattering and dynamic light scattering (DLS).

[0061] The ratio of the median diameter of silica particles after firing to the median diameter of silica gel (dry gel) before firing (after firing / before firing) is preferably 1 or less, more preferably 0.99 or less, and even more preferably 0.98 or less. The ratio of the median diameter of silica particles after firing to the median diameter of silica gel (dry gel) before firing (after firing / before firing) is preferably 0.80 or more, more preferably 0.90 or more, even more preferably 0.94 or more, and especially preferably 0.95 or more. Silica gel usually tends to have a smaller median diameter after firing, but if there is fusion between silica particles, the median diameter will increase. Therefore, the above ratio of median diameter after firing / before firing can be used as an indicator that there is no fusion or little fusion when it is below the upper limit. Furthermore, if the ratio of the median diameter after firing to the diameter before firing is above the lower limit, silica particles with uniform performance can be obtained without significant changes in particle size or particle size distribution from the silica gel (dry gel) before firing, and without local density differences or strain.

[0062] The silica particles preferably have an amorphous (non-crystalline) structure. In particular, amorphous silica generally has fewer impurities and a lower coefficient of thermal expansion compared to crystalline silica, making it suitable for use in electronic materials. The amorphous structure of silica particles can be evaluated, for example, by X-ray diffraction.

[0063] These silica particles achieve a reduction in silanol group content through calcination. Specifically, the amount of silanol groups in the silica particles is preferably 150 ppm by mass or less, more preferably 100 ppm by mass or less, and even more preferably 50 ppm by mass or less. As a lower limit, the amount of silanol groups is preferably 5 ppm by mass or more, more preferably 10 ppm by mass or more, and even more preferably 15 ppm by mass or more. The range of silanol group content is, for example, 5 to 150 ppm by mass, 10 to 100 ppm by mass, etc. When the amount of silanol groups in the silica particles is below the upper limit, the amount of foam generated when the silica is melted can be suppressed. On the other hand, when the amount of silanol groups in the silica particles is above the lower limit, the time and energy required for calcination can be reduced, resulting in excellent ease of manufacture.

[0064] The amount of silanol groups in the silica particles can be evaluated, for example, by an infrared spectrometer (FTIR).

[0065] These silica particles tend to reduce the specific surface area by BET method (hereinafter referred to as "BET specific surface area") upon firing. The BET specific surface area of these silica particles is preferably less than 3 m ２ / g, more preferably 2.5 m ２ / g or less, and even more preferably 2.0 m ２ / g or less. The lower limit is usually 0.5 m ２ / g. When the BET specific surface area is less than the above upper limit value, the smoothness of the particle surface is further improved, and high filling, thinning, and high fluidity requirements in the molding resin composition can be met.

[0066] The BET specific surface area is measured by the BET method. The BET method is a method for calculating the surface area of a solid from the monolayer adsorption amount and the molecular cross-sectional area of the adsorbate using an adsorption isotherm (BET equation) derived based on multilayer adsorption, and is a known method. The BET specific surface area can be measured, for example, using a specific surface area measuring device by the nitrogen gas adsorption method.

[0067] The content of alkali metals (for example, Na, K, Li, etc.) contained in these silica particles is preferably 100 mass ppm or less, more preferably 50 mass ppm or less, and even more preferably 10 mass ppm or less. If the alkali metal content is below these upper limit values, even when highly filled in the encapsulating resin, elution of ionic impurities that cause corrosion can be suppressed without impairing electrical reliability. The lower limit value of the alkali metal content is not particularly limited, but is usually mass ppm or more. Alkali metals are factors that lower the viscosity of silica glass during melting and also deteriorate electrical insulation, so a low concentration is preferred. Since alkali metals have a low melting point, they are usually excluded from the "metals with a melting point of 1650 °C or higher" used in this embodiment. The alkali metal content can be measured by a known method such as the ICP-MS method (inductively coupled plasma mass spectrometry).

[0068] <Applications of Silica Particles> These silica particles are micrometer-sized, non-fusing, and have a low silanol group content. Therefore, they can be used as semiconductor encapsulants in automobiles, portable electronic devices, personal computers, and home appliances, as well as in laminates on which semiconductors are mounted, and as fillers in putties, sealants, various types of rubber, and various engineering plastics. In particular, these silica particles are also useful as electronic materials such as insulating materials for high-frequency printed circuit boards, 5G antenna substrate materials, and materials for electronic components for high-speed transmission.

[0069] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples without departing from its essence.

[0070] <Measurement Method> (Method for measuring silanol group content) The amount of silanol group content was measured using an infrared spectrometer "Nicolet iN10" (manufactured by Thermo Fisher Scientific) at 3660 cm⁻¹. －１ The concentration was calculated by comparing it to the standard sample based on the height of the nearby absorption peaks.

[0071] (Method for measuring particle size distribution) The particle size distribution of silica gel and silica particles was measured using a particle size distribution analyzer "MT3000II" (manufactured by MicrotrackBEL). One microspatulaful of sample and approximately 3cc of desalinated water were added to a 9cc screw tube and the sample was prepared by ultrasonic dispersion. Four drops of the obtained slurry were added to the sample cell using a syringe and further ultrasonic dispersion was performed before measurement.

[0072] (Confirmation of particle fusion) The presence or absence of particle fusion was determined by observing SEM images using a "JSM-6060" (manufactured by JEOL Ltd.). The SEM image acquisition conditions were an acceleration voltage of 5 kV and a working distance of 17 mm, with silica particles fixed on carbon tape attached to a brass sample stage. To suppress surface charging, the particles were sputtered with platinum.

[0073] (Metal Element Content) The metal element content of silica gel and silica particles was measured by ICP emission spectrometry. For the measurement, 300 mg of silica particles was accurately weighed out, hydrofluoric acid and a small amount of sulfuric acid were added, and the mixture was heated and dissolved. Further heating was performed until sulfuric acid droplets remained. Nitric acid was added to the remaining sulfuric acid droplets, and then pure water was added to dissolve them. After collection, the mixture was diluted as appropriate and measured using ICP-MS "ELEMENT2" (manufactured by Thermofisher Scientific).

[0074] (BET specific surface area of ​​silica particles) The BET specific surface area of ​​silica particles is determined by drying the obtained dispersion of silica particles at 150°C and measuring the specific surface area of ​​the silica particles using the automatic specific surface area measuring device "BELSORP-MR1" (manufactured by Microtrac-Bel).

[0075] (Alkali Metal Content) The alkali metal content in silica particles was measured using ICP-MS (Inductively Coupled Plasma Mass Spectrometry) on the sample powder. The analysis was performed according to known conditions, and the alkali metal content (total content of Na, K, and Li) was determined in ppm by mass.

[0076] [Example 1] 300 g of tetramethoxysilane was placed in a 1 L glass jacketed separable flask equipped with a stirrer. After heating to an internal temperature of 45°C, 210 g of ultrapure water (6 times the molar ratio to tetramethoxysilane) was added to carry out the hydrolysis reaction of tetramethoxysilane. The resulting agar-like wet gel was removed and pulverized by pressing the gel against a sieve with a mesh size of 0.8 mm. Next, the pulverized gel was placed in a vacuum dryer and heated to 200°C under reduced pressure to evaporate the water and methanol from the gel, obtaining 160 g of silica gel powder. The obtained silica gel powder was pulverized in a jet mill to a median diameter D50 < 10 μm. The Zr concentration of the zirconium acetylacetonate aqueous solution was adjusted to 1300 ppm by mass. 160 g of the pulverized silica gel was mixed with 160 g of the adjusted aqueous solution to prepare a slurry. Subsequently, the slurry was vacuum dried at 130°C to obtain silica gel powder containing Zr. This powder was used as a raw material and placed in a quartz glass crucible, then heated at 1200°C for 5 hours in an electric furnace with dry air circulating through it. The dew point of the dry air was set to -50°C. The silica particles obtained after calcination had a silanol group content of 116 ppm by mass and a Zr content of 1260 ppm by mass. No fusion between particles was observed from the SEM images.

[0077] [Example 2] Silica particles were obtained under the same conditions as in Example 1, except that the Zr concentration of the zirconium acetylacetonate aqueous solution was 130 ppm by mass. After calcination, the silanol group content of the silica particles was 84 ppm by mass, and the Zr content was 150 ppm by mass. No fusion between particles was observed from the SEM images.

[0078] [Example 3] Silica particles were obtained under the same conditions as in Example 1, except that the Zr concentration of the zirconium acetylacetonate aqueous solution was 13 ppm by mass. After calcination, the silanol group content of the silica particles was 74 ppm by mass, and the Zr content was 14 ppm by mass. No fusion between particles was observed from the SEM images. Figure 2 shows the SEM images of the silica particles obtained in Example 3.

[0079] [Example 4] Silica particles were obtained under the same conditions as in Example 1, except that an aqueous aluminum acetate solution with an Al concentration of 970 ppm by mass was used instead of an aqueous zirconium acetylacetonate solution. After calcination, the silica particles had a silanol group content of 121 ppm by mass and an Al content of 1280 ppm by mass. No fusion between particles was observed from the SEM images.

[0080] [Example 5] Silica particles were obtained under the same conditions as in Example 4, except that the Al concentration of the aluminum acetate aqueous solution was 88 ppm by mass. After calcination, the silanol group content of the silica particles was 113 ppm by mass, and the Al content was 106 ppm by mass. No fusion between particles was observed from the SEM images.

[0081] [Example 6] Silica particles were obtained under the same conditions as in Example 4, except that the Al concentration of the aluminum acetate aqueous solution was 11 ppm by mass. After calcination, the silanol group content of the silica particles was 95 ppm by mass, and the Al content was 14 ppm by mass. No fusion between particles was observed from the SEM images.

[0082] [Example 7] Silica particles were obtained under the same conditions as in Example 4, except that the Al concentration of the aluminum acetate aqueous solution was 335 ppm by mass. After calcination, the silanol group content of the silica particles was 120 ppm by mass, and the Al content was 400 ppm by mass. No fusion between particles was observed from the SEM images.

[0083] [Comparative Example 1] Silica particles were obtained under the same conditions as in Example 1, except that the Zr concentration in the zirconium acetylacetonate aqueous solution was 1 ppm by mass. After calcination, the amount of silanol groups in the silica particles was 84 ppm by mass, and the Zr content was 1.8 ppm by mass. Fusion between particles was observed from the SEM image. Figure 3 shows the SEM image of the silica particles obtained in the comparative example.

[0084] [Comparative Example 2] Silica particles were obtained under the same conditions as in Example 1, except that the Zr concentration of the zirconium acetylacetonate aqueous solution was 0.1 ppm by mass. After calcination, the amount of silanol groups in the silica particles was 82 ppm by mass, and the Zr content was 0.14 ppm by mass. Fusion between particles was observed from the SEM image.

[0085] [Comparative Example 3] The procedure was the same as in Example 4, except that the Al concentration of the aluminum acetate aqueous solution was set to 1 ppm by mass. After calcination, the amount of silanol groups in the silica particles was 75 ppm by mass, and the Al content was 1.5 ppm by mass. Fusion between particles was observed from the SEM image.

[0086] Table 1 shows the evaluation results of the obtained silica particles. It was confirmed using an X-ray diffractometer "X'Pert PRO" (manufactured by PANICAL) that the silica particles in both the examples and comparative examples had an amorphous (non-crystalline) structure.

[0087]

[0088] As can be seen from Table 1, in Comparative Examples 1 to 3, Zr and Al are all 5 ppm by mass or less in terms of metallic elements, and although the amount of silanol groups is small, particle fusion occurs. On the other hand, in Examples 1 to 7, Zr and Al are all 5 ppm by mass or more in terms of metallic elements, and particle fusion is prevented in addition to the small amount of silanol groups.

[0089] While the above embodiments illustrate specific forms of the present invention, these embodiments are merely illustrative and should not be interpreted restrictively. Various modifications that are obvious to those skilled in the art are intended to fall within the scope of the present invention.

[0090] The silica particles and the method for producing the same of the present invention can be used to obtain silica particles suitable as additives to silica molded articles and resin compositions.

Claims

1. A method for producing silica particles, comprising the steps of: preparing silica gel containing 5 ppm or more by mass of metals with a melting point of 1650°C or higher in terms of metallic elements, and having a median diameter of 10 μm or less; and calcining the silica gel.

2. The method for producing silica particles according to claim 1, wherein the temperature of the firing step is 1100 to 1280°C.

3. The method for producing silica particles according to claim 1 or 2, wherein the median diameter of the silica particles after firing is 10 μm or less.

4. The method for producing silica particles according to claim 1 or 2, wherein the ratio of the median diameter of the silica particles after firing to the median diameter of the silica gel before firing (after firing / before firing) is 1 or less.

5. The BET specific surface area of ​​the silica particles is 3 m². ２ A method for producing silica particles according to claim 1 or 2, wherein the amount is less than / g.

6. The method for producing silica particles according to claim 1 or 2, wherein the alkali metal content of the silica particles is 100 ppm by mass or less.

7. Silica particles containing 5 ppm or more by mass of metals with a melting point of 1650°C or higher (calculated as metallic elements), having a median diameter of 10 μm or less, and a silanol group content of 150 ppm or less by mass.

8. The silica particles according to claim 7, wherein the silica particles have an amorphous structure.

9. The silica particles according to claim 7 or 8, wherein the metals are at least one metal selected from the group consisting of metals, metal oxides, and metal salts.

10. The silica particles according to claim 7 or 8, wherein the metal species of the metals is at least one metal selected from the group consisting of Group 4 elements, Group 6 elements of the periodic table, and aluminum.

11. BET specific surface area is 3 m² ２ Silica particles according to claim 7 or 8, wherein the amount is less than / g.

12. Silica particles according to claim 7 or 8, wherein the alkali metal content is 100 ppm by mass or less.

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