Method for producing hollow silica particles

Abstract

To provide a method for producing hollow silica particles that enables pulverization to a particle size below a specified level while preventing cracking of the hollow silica particles.SOLUTION: A method for producing hollow silica particles comprises a step of pulverizing a hollow silica particle aggregate using a pulverizer having a pulverization part and a classification part.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing hollow silica particles. [Background technology]

[0002] The use of high frequencies of several tens of GHz is being considered for high-speed communication technologies such as 5G and radars used in autonomous driving. In high-frequency circuits that handle such high-frequency radio waves, insulating materials with excellent dielectric properties such as low dielectric constant and low dielectric tangent are required to reduce transmission loss and transmission delay, and similar performance is required for silica particles that are mixed into insulating materials to improve their dielectric properties. In response to these requirements, the use of hollow silica particles has been considered to improve the dielectric properties of silica particles. In addition, miniaturization of high-frequency circuits is desired, and the particle size of silica particles mixed into insulating materials is also required to be reduced.

[0003] Patent Document 1 describes a hollow silica particle having a shell layer containing silica and having a space inside the shell layer, which has a wavelength of 3746 cm by infrared spectroscopy. -1 The hollow silica particles disclosed herein have a SiOH-derived peak in the vicinity of 0.60 or less, a relative dielectric constant at 1 GHz of 1.3 to 5.0, and a dielectric loss tangent at 1 GHz of 0.0001 to 0.05. Patent Document 2 discloses hollow silica particles having an outer shell that forms an internal space, the outer shell being composed of a component containing silica, and the hollow silica particles are obtained by calcining a hollow silica particle precursor at 1100°C, pulverizing the resulting hollow silica particles in a jet mill, and classifying the resulting hollow silica particles in a classifier to adjust the average particle size and particle size distribution. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 172294 [Patent Document 2] JP 2020-83736 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the manufacturing process of the hollow silica particles of Patent Document 1, the silanols on the particle surface condense with each other during firing, causing the particles to strongly aggregate, resulting in a large particle size, which makes it impossible to sufficiently incorporate the hollow silica particles into insulating materials as they are. Furthermore, if the hollow silica fired particles are crushed by a normal method as required, the hollow silica particles break, causing an increase in the relative dielectric constant and dielectric loss tangent, making them unsuitable for insulating materials. In addition, in the manufacturing process of Patent Document 2, the obtained hollow silica particles (aggregates) are crushed by a crusher and then classified by a classifier. However, in the crusher, when the crushing pressure is increased to crush the hollow silica particles to a predetermined particle size, the occurrence of cracks in the hollow silica particles also increases, so that even if the crushed hollow silica particles are blended into an insulating material, the relative dielectric constant and dielectric loss tangent are not sufficiently low. In addition, if the crushing pressure is adjusted to be low in order to suppress the occurrence of cracks in the hollow silica particles, the hollow silica particles are not sufficiently crushed and remain as aggregates, and there are cases in which the hollow silica particles cannot be blended uniformly into the insulating material. The present invention relates to a method for producing hollow silica particles, which involves crushing hollow silica particles to a particle size equal to or smaller than a predetermined size while suppressing cracking of the hollow silica particles. [Means for solving the problem]

[0006] The present inventors have found that the above-mentioned problems can be solved by using a disintegrator having a disintegrating section and a classifying section when disintegrating hollow silica particle agglomerates. The present invention relates to the following [1]. [1] A method for producing hollow silica particles, comprising a step of disintegrating hollow silica particle agglomerates using a disintegrator having a disintegrator section and a classifier section. Effect of the Invention

[0007] The present invention provides an efficient method for producing hollow silica particles, which can crush the hollow silica particles to a predetermined particle size or less while suppressing cracking of the hollow silica particles. In addition, since the hollow silica particles obtained by the present invention are suppressed from cracking, the relative dielectric constant and dielectric loss tangent can be kept low. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] [Method of manufacturing hollow silica particles] The present invention provides a method for producing hollow silica particles, comprising a step of disintegrating hollow silica particle agglomerates using a disintegrator having a disintegrator section and a classifier section. According to the present invention, the hollow silica particles can be crushed to a predetermined particle size or less by suppressing the cracking of the hollow silica particles. In addition, since the hollow silica particles obtained by the present invention are suppressed from cracking, the relative dielectric constant and the dielectric loss tangent can be kept low.

[0009] Although the details of the mechanism by which the effects of the present invention are manifested are not clear, it is presumed as follows. Usually, in order to obtain hollow silica particles with low relative dielectric constant and dielectric loss tangent, it is effective to carry out a process of baking the precursor of hollow silica particles at a high temperature of 1000°C or more for 1 hour or more. However, when baking at a high temperature of 1000°C or more, the obtained hollow silica particles will aggregate, resulting in a state of so-called hollow silica particle aggregates. Since the hollow silica particle aggregates have a large particle diameter, it is difficult to directly mix them with resins used for insulating materials, etc., so it is necessary to crush the hollow silica particle aggregates to obtain hollow silica particles with a particle diameter of a predetermined particle diameter or less. In conventional crushing using a hammer mill or a bead mill, hollow silica particle aggregates having cavities are crushed with a hard hammer or bead, so the resulting hollow silica particles are easily broken. If the proportion of broken hollow silica particles increases, the porosity of the resulting silica particles decreases, so the relative dielectric constant increases. In addition, silanol groups are generated on the cross sections of the broken hollow silica particles, so the dielectric tangent also increases. Therefore, the hollow silica particles obtained by the conventional manufacturing method have a high relative dielectric constant and dielectric tangent, so they are not suitable for incorporation into insulating materials, etc. On the other hand, in the method for producing hollow silica particles of the present invention, in a disintegrator having a disintegrator and a classifier, hollow silica particle agglomerates are disintegrated and then classified by the classifier, so that hollow silica particles that have become smaller than a predetermined particle size do not remain in the disintegrator more than necessary, and therefore the cracking of hollow silica particles can be suppressed. In addition, hollow silica particle agglomerates that are larger than a predetermined particle size and are not discharged outside the disintegrator in the classifier (i.e., hollow silica particle agglomerates that have not been sufficiently disintegrated) are again fed into the disintegrator and disintegrated. Therefore, hollow silica particles that have been disintegrated to a predetermined particle size or less can be efficiently obtained. Since the hollow silica particles obtained in the present invention are prevented from cracking, the relative dielectric constant and the dielectric loss tangent can be kept low.

[0010] <Crusher having a crushing section and a classifying section> In the present invention, the crusher has a crushing section and a classifying section.

[0011] [Crushing section] In the present invention, the disintegrating section of the disintegrator preferably has a mechanism for disintegrating hollow silica particle aggregates by causing them to collide with each other using a high-speed jet stream ejected from a nozzle. By this mechanism, the hollow silica particle agglomerates are disintegrated by collisions and friction between the hollow silica particle agglomerates in the airflow. This mechanism does not involve collisions with objects such as the hammer or beads, which have a much larger mass than the hollow silica particle agglomerates, but rather collisions between hollow silica particle agglomerates of the same mass, thereby making it possible to obtain hollow silica particles with fewer cracks. As a structure for colliding hollow silica particle aggregates with each other by an airflow from a nozzle that emits a high-speed jet airflow, the arrangement of the nozzle that generates the high-speed jet airflow is preferably known to be a swirling airflow type, an opposed nozzle type, etc., but in the present invention, the opposed nozzle type is preferred. That is, in the present invention, it is more preferred that the nozzles that emit the high-speed jet airflow are arranged at positions opposite each other. In the present invention, a configuration in which nozzles that emit high-speed jet air streams are arranged in positions facing each other means that the nozzles are arranged in a positional relationship such that the high-speed jet air stream emitted from one nozzle and the high-speed jet air stream emitted from the other nozzle collide at positions equidistant from each other.

[0012] [Classification Department] The classification section of the crusher preferably has a mechanism for discharging hollow silica particles having a particle size equal to or smaller than a predetermined size outside the crusher, and extracting hollow silica particles and hollow silica particle aggregates having a particle size larger than the predetermined size and reintroducing them into the crushing section. In addition, the classification section is preferably independent of the crushing section. The above-mentioned mechanism can disintegrate the hollow silica particle aggregates larger than the predetermined particle size again, so that the hollow silica particles disintegrated to the predetermined particle size or less can be efficiently obtained. Also, the above-mentioned mechanism can prevent the hollow silica particles smaller than the predetermined particle size from remaining in the disintegration section more than necessary, so that the cracking of the obtained hollow silica particles can be easily prevented.

[0013] It is more preferable that the mechanism is a mechanism in which the hollow silica particles and hollow silica particle agglomerates disintegrated in the crushing section are classified into hollow silica particles having a predetermined particle size or less and hollow silica particles and hollow silica particle agglomerates having a larger particle size than the predetermined particle size, the hollow silica particles having the predetermined particle size or less are discharged outside the crusher, and the hollow silica particles and hollow silica particle agglomerates having a larger particle size than the predetermined particle size are extracted, mixed again with newly introduced hollow silica particle agglomerates in a predetermined ratio, and then reintroduced into the crushing section. The method for withdrawing the hollow silica particles and hollow silica particle agglomerates having a particle size larger than the predetermined size is preferably one selected from a continuous withdrawal method and an intermittent withdrawal method, and more preferably the continuous withdrawal method.

[0014] In the present invention, the predetermined particle size is a particle size that is a dividing line when the classification section separates particles to be discharged outside the crusher from particles to be reintroduced into the crushing section. In the present invention, the predetermined particle size is preferably 10 μm, more preferably 7 μm, and even more preferably 5 μm. That is, in the present invention, the classification section preferably has a mechanism for discharging particles of 10 μm or less outside the crusher and reintroducing particles larger than 10 μm into the crushing section, more preferably has a mechanism for discharging particles of 7 μm or less outside the crusher and reintroducing particles larger than 7 μm into the crushing section, and even more preferably has a mechanism for discharging particles of 5 μm or less outside the crusher and reintroducing particles larger than 5 μm into the crushing section.

[0015] In the present invention, the crusher is preferably a jet mill with a classifier. Examples of the jet mill with a classifier include the Super Jet Mill (nozzles for ejecting high-speed jet streams are arranged on the tangent line of the circumference: swirling airflow type) manufactured by Nisshin Engineering Co., Ltd., the Counter Jet Mill AFG series and AFG-CRS series (nozzles for ejecting high-speed jet streams are arranged in positions facing each other: opposed nozzle type) manufactured by Hosokawa Micron Corporation, the fluidized bed type jet mill CGS series (nozzles for ejecting high-speed jet streams are arranged in positions facing each other: opposed nozzle type) manufactured by Toyo Hightec Co., Ltd., and the Cross Jet Mill (nozzles for ejecting high-speed jet streams are arranged in positions facing each other: opposed nozzle type) manufactured by Kurimoto Iron Works Co., Ltd.

[0016] <Method of manufacturing hollow silica particles> In the present invention, the hollow silica particles are preferably produced by a production method including the following steps 1 to 4. Step 1: preparing an aqueous emulsion A of a hydrophobic liquid using a cationic surfactant A. Step 2: A step of adding a silanol precursor, a cationic surfactant B, and an alkaline substance to the obtained aqueous emulsion A, and subjecting the silanol obtained by hydrolyzing the silanol precursor to a condensation reaction to produce hollow silica particle precursors. Step 3: A step of calcining the obtained hollow silica particle precursor at 1000° C. or higher to obtain silica particle aggregates. Step 4: A step of disintegrating and classifying the obtained hollow silica particle agglomerates using a disintegrator having a disintegrating section and a classifying section to obtain hollow silica particles.

[0017] [Process 1] In step 1, an aqueous medium, a cationic surfactant A, and a hydrophobic liquid are mixed and stirred to obtain an aqueous emulsion A of the hydrophobic liquid in which the hydrophobic liquid is emulsified in the aqueous medium. The aqueous emulsion A of the hydrophobic liquid can be prepared by a general method.

[0018] The aqueous medium preferably contains water. Examples of the water contained in the aqueous medium include distilled water, ion-exchanged water, and ultrapure water. In addition, the aqueous medium may contain an organic solvent compatible with water from the viewpoint of more uniform and stable emulsion of the hydrophobic liquid. Examples of the organic solvent compatible with water include lower alcohols such as methanol, ethanol, and isopropyl alcohol, and acetone. From the viewpoint of instantly reducing the solubility of the hydrophobic liquid in the aqueous medium, the water content in the aqueous medium is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 98% by mass or more, and even more preferably 100% by mass.

[0019] [Cationic surfactant A] From the viewpoint of facilitating the formation of a complex with the condensed silanol in step 2 described below, and from the viewpoint of decomposition and volatilization in step 3 described below, the cationic surfactant A is preferably a quaternary ammonium salt, more preferably one or more types selected from the group consisting of quaternary ammonium salts represented by the following general formula (1) or general formula (2), and even more preferably one or more types selected from alkyltrimethylammonium salts and dialkyldimethylammonium salts. [R 1 R 3 3N] + X ― (1) [R 1 R 2 R 3 2N] + X ― (2)

[0020] In the general formula (1) and the general formula (2), R 1 and R 2 each independently represents a linear or branched alkyl group having 4 to 24 carbon atoms; R 3 represents an alkyl group having 1 to 3 carbon atoms, and multiple R 3 may each be a different group, and X ― indicates a monovalent anion. Examples of the alkyl group having 4 to 24 carbon atoms include various butyl groups, various pentyl groups, various hexyl groups, various heptyl groups, various octyl groups, various nonyl groups, various decyl groups, various dodecyl groups, various tetradecyl groups, various hexadecyl groups, various octadecyl groups, various eicosyl groups, various docosyl groups, and various tetracosyl groups. Examples of the alkyl group having 1 to 3 carbon atoms include a methyl group, an ethyl group, an n-propyl group, and an isopropyl group. 3 is preferably a methyl group.

[0021] X in general formulas (1) and (2) ― From the viewpoint of being easily decomposed and volatilized during firing, X is preferably at least one type selected from monovalent anions such as halogen ions, hydroxide ions, and nitrate ions. ―More preferably, the cation is a halide ion, and even more preferably, a chloride ion.

[0022] Examples of the alkyl trimethyl ammonium salt represented by the general formula (1) include butyl trimethyl ammonium chloride, hexyl trimethyl ammonium chloride, octyl trimethyl ammonium chloride, decyl trimethyl ammonium chloride, lauryl trimethyl ammonium chloride (dodecyl trimethyl ammonium chloride), tetradecyl trimethyl ammonium chloride, hexadecyl trimethyl ammonium chloride, stearyl trimethyl ammonium chloride, behenyl trimethyl ammonium chloride, butyl trimethyl ammonium bromide, hexyl trimethyl ammonium bromide, octyl trimethyl ammonium bromide, decyl trimethyl ammonium bromide, lauryl trimethyl ammonium bromide, tetradecyl trimethyl ammonium bromide, hexadecyl trimethyl ammonium bromide, stearyl trimethyl ammonium bromide, and behenyl trimethyl ammonium bromide.

[0023] From the viewpoint of facilitating the formation of a complex with the condensed silanol in step 2, and facilitating decomposition and volatilization in step 3, the quaternary ammonium salt is preferably one or more selected from lauryl trimethyl ammonium chloride, stearyl trimethyl ammonium chloride, and behenyl trimethyl ammonium chloride, more preferably one or more selected from lauryl trimethyl ammonium chloride and behenyl trimethyl ammonium chloride, and even more preferably behenyl trimethyl ammonium chloride.

[0024] (hydrophobic liquid) The hydrophobic liquid is preferably capable of forming emulsified droplets (emulsified oil droplets) in an aqueous medium. In view of using the aqueous medium as a dispersion medium and improving the utilization efficiency of the hydrophobic liquid, the temperature range in which the hydrophobic liquid is in a liquid state is preferably 0 to 100°C, and more preferably 20 to 90°C. Specific examples of hydrophobic liquids include those described in paragraphs

[0015] to

[0023] of JP2016-121060A. Among these, hydrocarbons having 6 to 18 carbon atoms are preferred, hydrocarbons having 8 to 14 carbon atoms are more preferred, and dodecane is more preferred.

[0025] In step 1, the mass ratio of the hydrophobic liquid to the aqueous medium [hydrophobic liquid / aqueous medium] is, from the viewpoint of keeping the volume average particle size of the hydrophobic liquid particles in the resulting aqueous emulsion of the hydrophobic liquid within an appropriate range, preferably 0.2 or more, more preferably 0.25 or more, even more preferably 0.3 or more, and is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.7 or less. Furthermore, when the aqueous medium consists of only water, in step 1, the mass ratio of the hydrophobic liquid to water [hydrophobic liquid / water] is, from the viewpoint of keeping the particle size of the resulting droplets containing the hydrophobic liquid within an appropriate range, preferably 0.3 or more, more preferably 0.35 or more, even more preferably 0.4 or more, and is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.7 or less.

[0026] In step 1, the mass ratio of cationic surfactant A to the hydrophobic liquid [cationic surfactant A / hydrophobic liquid] is, from the viewpoint of emulsifying the hydrophobic liquid in an aqueous medium, preferably 0.001 or more, more preferably 0.005 or more, even more preferably 0.01 or more, and is preferably 0.05 or less, more preferably 0.04 or less, even more preferably 0.035 or less.

[0027] In step 1, the particle size of the resulting droplets containing the hydrophobic liquid can be adjusted to an appropriate range by appropriately adjusting the stirring speed, temperature, etc. Step 1 is preferably carried out at a temperature of 15°C to 80°C. The volume average particle diameter of the droplets containing the hydrophobic liquid is preferably 0.1 μm or more, more preferably 0.3 μm or more, even more preferably 0.4 μm or more, and is preferably 2.5 μm or less, more preferably 2.0 μm or less, even more preferably 1.5 μm or less, from the viewpoint of setting the average particle diameter of the hollow silica particles in the range described below. The volume average particle size of droplets containing a hydrophobic liquid can be determined by the method described in the Examples.

[0028] [Process 2] In step 2, a silanol precursor and a cationic surfactant B are added to the aqueous emulsion A obtained in step 1, and the silanol precursor present on the surface of the droplets containing the hydrophobic liquid is hydrolyzed in the presence of an alkaline substance to obtain silanols. The obtained silanols are condensed in the presence of the alkaline substance to form hollow silica particle precursors having an outer shell containing silica and the cationic surfactant B on the surface of the droplets containing the hydrophobic liquid and containing the hydrophobic liquid inside. The addition of the silanol precursor and the cationic surfactant B to the aqueous emulsion A may be performed by adding the silanol precursor and the cationic surfactant B to the aqueous emulsion A simultaneously or separately, or by adding the aqueous emulsion A to either the silanol precursor or the cationic surfactant B and then adding the remaining one.

[0029] (Silanol precursor) The silanol precursor is a compound that generates a silanol compound by hydrolysis of alkoxysilane or the like, and is preferably one or more selected from orthosilicate alkyl esters and pyrosilicate alkyl esters. Specific examples include compounds represented by the following general formulas (3) to (7), or combinations thereof. SiY4(3) R 4 SiY3(4) R 4 2SiY2(5) R 4 3SiY (6) Y3Si-O-SiY3(7)

[0030] In general formulas (3) to (7), R 4 each independently represents a hydrocarbon group in which a carbon atom is directly bonded to a silicon atom, and Y represents a monovalent hydrolyzable group that becomes a hydroxyl group upon hydrolysis.

[0031] In general formulas (4) to (6), R 4 are each independently preferably a hydrocarbon group having 1 to 22 carbon atoms in which some of the hydrogen atoms may be substituted with fluorine atoms, and from the viewpoint of improving the utilization efficiency of the hydrophobic organic substance, are preferably an alkyl group, a phenyl group, or a benzyl group having 1 to 22 carbon atoms, more preferably 4 to 18 carbon atoms, and even more preferably 8 to 16 carbon atoms. In the general formulas (3) to (7), Y is preferably an alkoxy group having 1 to 8 carbon atoms or a halogen group other than fluorine, and more preferably an alkoxy group having 2 to 4 carbon atoms. When Y is an alkoxy group having 1 carbon atom or a halogen group other than fluorine, the hydrolysis reaction rate is too fast, so that the outer shell of the hollow silica precursor is difficult to become dense, and shrinkage during firing increases, so that the relative dielectric constant and dielectric loss tangent of the hollow silica particles tend to be high. Conversely, an alkoxy group having 5 or more carbon atoms slows down the hydrolysis rate.

[0032] The silanol precursor is preferably one or more selected from the compounds represented by general formula (3) and general formula (7). Among them, from the viewpoint of suppressing the generation of metal-corrosive acid and from the viewpoint of hydrolysis reactivity, the silanol precursor is preferably one or more selected from the compounds represented by general formula (3) and general formula (7) in which Y is an alkoxy group having 2 to 4 carbon atoms, and more preferably one or more selected from the compounds represented by general formula (3) and general formula (7) in which Y is an ethoxy group. The silanol precursor can be used alone or in combination of two or more.

[0033] The mass ratio of the silanol precursor to the hydrophobic liquid [silanol precursor / hydrophobic liquid] is preferably 10 or more, more preferably 20 or more, even more preferably 25 or more, and is preferably 90 or less, more preferably 80 or less, even more preferably 75 or less, from the viewpoint of keeping the porosity of the hollow silica particles in an appropriate range.

[0034] (Cationic Surfactant B) As the cationic surfactant B, the same cationic surfactant B as the cationic surfactant A shown in step 1 can be used. As the cationic surfactant B, from the viewpoint of facilitating the formation of a complex with the condensed silanol and facilitating decomposition and volatilization in step 3, it is preferably a quaternary ammonium salt, more preferably one or more selected from lauryl trimethyl ammonium chloride (dodecyl trimethyl ammonium chloride), stearyl trimethyl ammonium chloride, and behenyl trimethyl ammonium chloride, and further preferably lauryl trimethyl ammonium chloride. The cationic surfactant B used in this step may be the same as or different from the cationic surfactant A used in step 1. In addition, the cationic surfactant B may be used alone or in combination of two or more kinds.

[0035] The mass ratio of the silanol precursor to the cationic surfactant B [silanol precursor / cationic surfactant B] is, from the viewpoint of dispersibility of the hollow silica particle precursor, preferably 3 or more, more preferably 5 or more, even more preferably 6 or more, and is preferably 25 or less, more preferably 20 or less, even more preferably 18 or less.

[0036] (Alkaline substances) The silanol precursor is hydrolyzed by an alkaline substance to form silanol, which is then dehydrated and condensed to form silica. Specific examples of the alkaline substance include those described in paragraph

[0014] of JP 2016-121060 A. Among these, hydroxide salts of quaternary ammonium are preferred. Specific examples of hydroxide salts of quaternary ammonium include tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, tributylmethylammonium hydroxide, trimethylhydroxyethylammonium hydroxide (choline), tetraethanolammonium hydroxide, methyltriethanolammonium hydroxide, and dimethylbis(2-hydroxyethyl)ammonium hydroxide. From the viewpoint of making the outer shell of the hollow silica particle precursor dense, it is preferably one or more selected from tetramethylammonium hydroxide, tetraethylammonium hydroxide, trimethylhydroxyethylammonium hydroxide, methyltriethanolammonium hydroxide, and dimethylbis(2-hydroxyethyl)ammonium hydroxide, more preferably one or more selected from tetramethylammonium hydroxide and dimethylbis(2-hydroxyethyl)ammonium hydroxide, and even more preferably dimethylbis(2-hydroxyethyl)ammonium hydroxide.

[0037] The mass ratio of the silanol precursor to the alkaline substance [silanol precursor / alkaline substance] is preferably 5 or more, more preferably 10 or more, and even more preferably 20 or more, from the viewpoint of densely forming the outer shell of the hollow silica particle precursor, and is preferably 100 or less, more preferably 80 or less, and even more preferably 70 or less, from the viewpoint of efficiently carrying out the condensation reaction of the silanol precursor.

[0038] The alkaline substance may contain, in addition to the above-mentioned quaternary ammonium hydroxide salt, for example, an alkali metal salt, an alkaline earth metal salt, etc., but in order to reduce the content of alkali metals and alkaline earth metals in the obtained hollow silica particles, the total content of alkali metals and alkaline earth metals in the silanol precursor is preferably 50 ppm by mass or less, more preferably 30 ppm by mass or less, even more preferably 10 ppm by mass or less, and even more preferably 0 ppm by mass, relative to the mass of the obtained hollow silica particles.

[0039] By mixing the alkaline substance with the cationic surfactant B and contacting it with the silanol precursor, hollow silica particles having a small maximum particle size and an appropriate particle size can be obtained. The contact between the mixture of the alkaline substance and the cationic surfactant B and the silanol precursor may be performed by adding the mixture of the alkaline substance and the cationic surfactant B to a reaction system containing the silanol precursor, or the silanol precursor may be added to a reaction system containing the mixture of the alkaline substance and the cationic surfactant B. However, from the viewpoint of increasing the porosity and increasing the synthesis concentration to increase productivity, it is preferable to add the mixture of the alkaline substance and the cationic surfactant B to a reaction system containing the silanol precursor.

[0040] The temperature at which step 2 is carried out can be appropriately adjusted depending on the type and amount of the silanol precursor and alkaline substance used, and is preferably from 0° C. to 100° C. in terms of making the outer shell of the hollow silica particle precursor dense. For example, when orthosilicate tetraethyl ester or pyrosilicate hexaethyl ester is used as the silanol precursor, the temperature is preferably from 20° C. to 45° C., and when orthosilicate tetramethyl ester or pyrosilicate hexamethyl ester is used, the temperature is preferably from 0° C. to 20° C.

[0041] The time for carrying out step 2 is preferably 30 minutes or more, more preferably 1 hour or more, and even more preferably 2 hours or more from the viewpoint of densifying the outer shell of the hollow silica particle precursor, and is preferably 24 hours or less, more preferably 20 hours or less, and even more preferably 16 hours or less from the viewpoint of production efficiency.

[0042] Step 2 may include, after the formation of the hollow silica particle precursor and before step 3, a step of aggregating the hollow silica particle precursor, a step of isolating the hollow silica particle precursor, and a step of drying the hollow silica particle precursor, as necessary.

[0043] (Step of aggregating hollow silica particle precursor) The step of aggregating the hollow silica particle precursor is a step of aggregating the hollow silica particle precursor in a liquid phase after forming the hollow silica particle precursor in step 2. It is considered that the inclusion of this step can cause weak aggregation (flocculation) that maintains a certain distance between the particles of the hollow silica particle precursor. As a result, in step 3, the distance between the particles in the obtained hollow silica particle aggregate can be kept constant, and the hollow silica particles can be easily disintegrated even with a weak disintegration force that does not easily break the hollow silica particles, and the breakage of the hollow silica particles can be easily suppressed. Furthermore, since the obtained hollow silica particle precursor becomes an aggregate and has a certain particle diameter, it is easy to efficiently recover the hollow silica particle precursor by an operation such as filtration in the step of isolating the hollow silica particle precursor described later. The step of aggregating the hollow silica particle precursor is preferably a step of adding an aggregating agent to the liquid phase containing the hollow silica particle precursor obtained in step 2.

[0044] <Flocculant> The flocculant is preferably an anionic polymer, and an anionic polymer that can cause particle flocculation by adding it to the hollow silica particle precursor in a liquid phase can be suitably selected. As the anionic polymer, from the viewpoint of obtaining hollow silica particles that can easily cause weak aggregation between particles of the hollow silica particle precursor, can suppress cracking even when crushed, and can maintain low relative dielectric constant and dielectric tangent, polycarboxylates and polysulfonates are preferable. The polycarboxylate is preferably a salt of polyacrylic acid and a salt of an acrylic acid-maleic acid copolymer, and is preferably a metal salt (salt of an alkali metal such as sodium or potassium), an amine salt, or an ammonium salt of polyacrylic acid and an acrylic acid-maleic acid copolymer. Among these, from the viewpoint of obtaining hollow silica particles that can be prevented from cracking even when crushed and can maintain a low relative dielectric constant and dielectric loss tangent, one or more selected from ammonium polyacrylate and ammonium acrylic acid-maleic acid copolymer are preferred. The polysulfonate is preferably a salt of a formalin condensate of an aromatic sulfonic acid and a salt of a formalin condensate of a lignin sulfonic acid. The salt of the formalin condensate of aromatic sulfonic acid may be a salt of condensed naphthalene sulfonic acid. The salt of condensed naphthalene sulfonic acid is preferably a metal salt (sodium, potassium, or other alkali metal salt), an amine salt, or an ammonium salt of condensed naphthalene sulfonic acid. Among these, the ammonium salt of condensed naphthalene sulfonic acid is preferred from the viewpoint of obtaining hollow silica particles that can suppress cracking even when crushed and can maintain low relative dielectric constant and dielectric loss tangent. Among the above, from the viewpoint of obtaining hollow silica particles that can be prevented from cracking even when crushed and can maintain a low relative dielectric constant and dielectric loss tangent, the anionic polymer is more preferably one or more selected from ammonium polyacrylate, ammonium acrylic acid-maleic acid copolymer, and ammonium salt of condensed naphthalenesulfonic acid, and even more preferably ammonium polyacrylate. Furthermore, using the above ammonium salt as the anionic polymer is also preferable from the viewpoint of use in insulating materials, etc., and from the viewpoint of reducing the content of metal ions in the obtained hollow silica particles.

[0045] The temperature at which the step of aggregating the hollow silica particle precursor is carried out is preferably 0°C or higher and 100°C or lower, more preferably 10°C or higher and 80°C or lower, even more preferably 15°C or higher and 60°C or lower, and still more preferably 20°C or higher and 45°C or lower, from the viewpoint of adjusting the agglomeration strength of the hollow silica particles, suppressing cracking even when crushed, and obtaining hollow silica particles that can maintain a low relative dielectric constant and dielectric tangent. In addition, the process of aggregating the hollow silica particle precursor is preferably performed by stirring after adding the aggregating agent. In the process of aggregating the hollow silica particle precursor, the stirring time is preferably 1 minute or more, more preferably 3 minutes or more, even more preferably 5 minutes or more, even more preferably 8 minutes or more, and is preferably 60 minutes or less, more preferably 45 minutes or less, even more preferably 30 minutes or less, even more preferably 15 minutes or less, from the viewpoint of adjusting the aggregation strength of the hollow silica particle precursor, suppressing cracking even when crushed, and obtaining hollow silica particles that can maintain low relative dielectric constant and dielectric loss tangent.

[0046] The hollow silica particle precursor can be isolated, for example, by filtration. If the boiling point of the hydrophobic liquid containing the hollow silica particle precursor is higher than 100°C, the hollow silica particle precursor can be dried, for example, by heating to a temperature of 100°C or higher and lower than the boiling point of the hydrophobic liquid. If the boiling point of the hydrophobic liquid contained in the hollow silica particle precursor is 100°C or lower, the hollow silica particle precursor can be dried, for example, by freeze-drying.

[0047] (Hollow silica particle precursor) The hollow silica particle precursor is a composite silica particle having an outer shell containing silica and a hydrophobic liquid inside the outer shell, in which a cationic surfactant is radially oriented toward the center of the particle.

[0048] [Step 3] In step 3, the hollow silica particle precursor obtained in step 2 is calcined at 1000°C or higher to decompose and volatilize the cationic surfactant present in the outer shell of the hollow silica particle precursor and volatilize the internal hydrophobic liquid, and then the pores present in the outer shell are closed by calcination to obtain hollow silica particle aggregates having uniform outer shells.

[0049] The calcination temperature in step 3 is, from the viewpoint of reducing silanol groups on the surface of the hollow silica particles, 1000°C or higher, preferably 1010°C or higher, more preferably 1030°C or higher, and even more preferably 1050°C or higher, and, from the viewpoint of suppressing aggregation of the hollow silica particles, is preferably 1200°C or lower, more preferably 1190°C or lower, even more preferably 1180°C or lower, and even more preferably 1160°C or lower.

[0050] The calcination time in step 3 is preferably 15 minutes or more, more preferably 30 minutes or more, and even more preferably 45 minutes or more, from the viewpoint of reducing silanol groups on the surface of the hollow silica particles, and is preferably 3 hours or less, more preferably 2 hours or less, and even more preferably 1.5 hours or less, from the viewpoint of suppressing aggregation of the hollow silica particles.

[0051] [Step 4] In step 4, the hollow silica particle agglomerates obtained in step 3 are disintegrated and classified by a disintegrator having a disintegrator section and a classifier section to obtain hollow silica particles. In step 4, by performing crushing and classification using a crusher having a crushing section and a classification section, it is possible to suppress the cracking of the hollow silica particles, and to crush the resulting hollow silica particles to a particle size of a predetermined particle size or less, thereby enabling hollow silica particles to be obtained efficiently. The crusher having a crushing section and a classification section used in step 4 is as described above.

[0052] [Physical Properties of Hollow Silica Particles Obtained by the Present Invention] The hollow silica particles obtained by the present invention preferably have a relative dielectric constant of 2.5 or less at a measurement frequency of 10 GHz and a dielectric loss tangent of 0.0050 or less at a measurement frequency of 10 GHz. The dielectric constant at a measurement frequency of 10 GHz is preferably 2.5 or less, more preferably 2.2 or less, even more preferably 2.0 or less, and even more preferably 1.8 or less, from the viewpoint of sufficiently low dielectric constant when used in insulating materials, etc., and is preferably 1.1 or more, more preferably 1.2 or more, and even more preferably 1.3 or more, from the viewpoint of sufficient strength of the hollow silica particles after crushing. In addition, the dielectric tangent at a measurement frequency of 10 GHz is preferably 0.0050 or less, more preferably 0.0048 or less, and even more preferably 0.0046 or less, from the viewpoint of sufficiently lowering the dielectric tangent when used in an insulating material or the like, and is preferably 0.0001 or more, more preferably 0.0005 or more, and even more preferably 0.0010 or more, from the viewpoint of maintaining the strength of the hollow silica particles and suppressing cracking even when crushed.

[0053] The volume-based average particle size of the hollow silica particles obtained by the present invention is, from the viewpoints of making it easy to incorporate into insulating materials and maintaining processability, preferably 0.1 μm or more, more preferably 0.5 μm or more, even more preferably 0.7 μm or more, still more preferably 0.9 μm or more, and is preferably 3.0 μm or less, more preferably 2.6 μm or less, even more preferably 2.4 μm or less, and still more preferably 2.2 μm or less. In the present invention, the volume-based average particle size of the hollow silica particles means the volume-based average particle size calculated by the Coulter counter method (the sum of (each particle size calculated by measurement using the Coulter counter method) x (the volume ratio of each particle size)). The volume-based average particle size is measured by the method described in the Examples.

[0054] The maximum particle size on a volume basis of the hollow silica particles obtained by the present invention is, from the viewpoints of facilitating incorporation into insulating materials and maintaining processability, preferably 0.8 μm or more, more preferably 1.5 μm or more, even more preferably 2.0 μm or more, still more preferably 2.5 μm or more, and is preferably 5.0 μm or less, more preferably 4.6 μm or less, even more preferably 4.4 μm or less, and still more preferably 4.2 μm or less. The maximum volumetric particle size of the hollow silica particles means the particle size at 99% (D99) of the cumulative frequency distribution on a volumetric basis, and can be determined by the method described in the Examples.

[0055] The porosity of the hollow silica particles obtained by the present invention is preferably 40% or more, more preferably 45% or more, and even more preferably 50% or more, from the viewpoint of reducing the relative dielectric constant of the hollow silica particles after crushing, and is preferably 80% or less, more preferably 77% or less, and even more preferably 74% or less, from the viewpoint of ensuring that the hollow silica particles after crushing have sufficient strength.

[0056] In addition, from the same viewpoints as above, the porosity of the hollow silica particle agglomerate in the present invention (hollow silica particle agglomerate after firing and before being disintegrated) is preferably 40% or more, more preferably 45% or more, even more preferably 50% or more, and is preferably 80% or less, more preferably 77% or less, even more preferably 74% or less. The porosity of the hollow silica particles obtained by the present invention and the hollow silica particle agglomerates in the present invention can be determined by the method described in the Examples.

[0057] The ratio of the broken hollow silica particles to the hollow silica particles obtained by the present invention is preferably 20% or less, more preferably 15% or less, even more preferably 10% or less, and still more preferably 5% or less, from the viewpoint of maintaining a low relative dielectric constant and dielectric tangent, and from the viewpoint of making the particles easily blendable into insulating materials and maintaining processability of the insulating materials. The ratio of broken hollow silica particles to the hollow silica particles obtained by the present invention can be determined from the porosity of the hollow silica particles before and after crushing, and can be determined by the method described in the examples.

[0058] The number ratio of the hollow silica particles having a particle diameter of 5 μm or more obtained by the present invention is preferably 50 ppm or less, more preferably 40 ppm or less, even more preferably 30 ppm or less, still more preferably 20 ppm or less, and even more preferably 10 ppm or less, from the viewpoint of making it easy to blend with insulating materials and maintaining processability. The number ratio of the hollow silica particles having a particle diameter of 5 μm or more obtained by the present invention can be determined by the method described in the Examples.

[0059] Since the hollow silica particles obtained by the present invention have the above-mentioned characteristics, the relative dielectric constant and dielectric loss tangent can be kept low, and further, the particles can be easily blended into insulating materials and the like, and the processability of the insulating materials can be maintained. EXAMPLES

[0060] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. Each property value was measured and evaluated by the following methods.

[0061] [Measurement method] <Measurement of the volume average particle size of droplets containing hydrophobic liquid in water-based emulsion A> The volume average particle size of the particles in the aqueous emulsion A was measured using a particle size measuring device "Zetasizer Nano ZS" (manufactured by Malvern Panalytical) by dynamic light scattering method, using a square cell with an optical path length of 10 mm.

[0062] <Measurement of the average particle size, maximum particle size, and number ratio of particles 5 μm or larger of hollow silica particles> The average particle size, maximum particle size, and number ratio of particles 5 μm or larger of the hollow silica particles were measured using a particle size measuring device (Beckman Coulter, Inc., Multisizer 3 (using a 20 μm aperture tube)) based on the Coulter counter method. The average particle size of the hollow silica particles was determined to be the volume-based average particle size calculated by the Coulter counter method (the sum of (each particle size calculated by measurement using the Coulter counter method) x (the volume ratio of each particle size)). The maximum particle size of the hollow silica particles was determined as the particle size at 99% (D99) of the cumulative frequency distribution on a volume basis. The percentage of hollow silica particles having a size of 5 μm or more was determined by counting the number of particles having a size of 5 μm or more on a volume basis using the above-mentioned device and dividing the number by the total number of hollow silica particles measured using the same device.

[0063] <Measurement of porosity of hollow silica particle aggregates and hollow silica particles> The porosity of the hollow silica particle aggregates and hollow silica particles was calculated from the density measured using a true density measuring device (ULTRAPYCNMETER1200e manufactured by Quantachrome) with nitrogen as the measurement gas according to the following formula. The true density of the silica particles was 2.2 g / cm 3 It was decided. Porosity (%) = [1-(measured sample density / true density of silica particles)] x 100

[0064] <Measurement of the percentage of hollow silica particles broken by crushing> The porosity of the hollow silica particle agglomerates before disintegration and the porosity of the hollow silica particles obtained by the present invention were each measured, and the porosity was calculated according to the following formula, assuming that the porosity of the hollow silica particle agglomerates before disintegration is η and the porosity of the hollow silica particles is η'. Percentage of broken hollow silica particles (%) = [1-(η' / η)] x 100

[0065] <Measurement of the dielectric constant and dielectric loss tangent of hollow silica particles> The dielectric constant and dielectric loss tangent of the hollow silica particles were measured at a temperature of 25°C and a frequency of 10 GHz using a network analyzer (Agilent Technologies, product name: N5221A) connected to a perturbation method cavity resonator (CP-580) manufactured by Kanto Electronics Application Development Co., Ltd., using the cavity resonator perturbation method (CP-MA dielectric constant measurement software, manufactured by Kanto Electronics Application Development Co., Ltd.). The measurement sample was prepared by filling hollow silica particles into a Teflon (registered trademark) tube (manufactured by Chukoh Chemical Industry Co., Ltd.: PTFE tube, inner diameter 1.5 mm, outer diameter 2.5 mm) so that all the filled hollow silica particles were within the measurement range (between 6.75 mm and 36.35 mm from the bottom). The amount of silica filled was calculated by measuring the weight before and after filling the Teflon tube with silica, and the volume of silica filled in the Teflon tube was calculated from the filling weight and specific gravity of the silica. The relative dielectric constant and dielectric loss tangent were determined by taking an empty Teflon tube not filled with silica as a blank and calculating the difference between them and a Teflon tube filled with silica.

[0066] [Production of hollow silica particles] <Example 1> (Process 1) 4.64 kg of ion-exchanged water, 3.20 kg of dodecane (Kishida Chemical Co., Ltd.: primary n-dodecane), and 0.17 kg of Coatamin 2285E (Kao Corporation: containing 58% by mass of behenyl trimethyl ammonium chloride) as surfactant A were mixed and stirred to obtain water-based emulsion A. The volume average particle size of droplets containing the hydrophobic liquid in the obtained water-based emulsion A was 0.9 μm. (Process 2) A reaction tank was charged with 755.21 kg of ion-exchanged water, 7.66 kg of aqueous emulsion A, 7.67 kg of Coatamin 24P (manufactured by Kao Corporation: containing 27.5% by mass of lauryl trimethylammonium chloride) as surfactant B, and 173.20 kg of orthosilicate ethyl ester (manufactured by Wacker Asahi Kasei Silicones Corporation: SEMICOSIL TEOS999-LB) as a silanol precursor, and the mixture was heated to 40°C with stirring and then stirred for 10 minutes to obtain preparation B. Next, 12.82 kg of AH212-CS (manufactured by Yokkaichi Chemical Co., Ltd.: containing 50% by mass of dimethylbis(2-hydroxyethyl)ammonium hydroxide) as an alkaline substance and 43.44 kg of Courtamin 24P as surfactant B were uniformly mixed to obtain preparation C. Preparation liquid C was added to preparation liquid B, and then the mixture was stirred at 40° C. for 3 hours to obtain a cloudy white liquid D. Next, the obtained cloudy liquid D was filtered using filter paper No. 5C (manufactured by Advantec Toyo Kaisha, Ltd.) and then dried at 110° C. for 15 hours to obtain an aggregate of hollow silica particle precursors. (Step 3) The obtained dried aggregate of hollow silica particle precursor was fired at 1100° C. for 1 hour to obtain hollow silica particle aggregate. (Step 4) The obtained hollow silica particle aggregates were crushed in a counter jet mill (Hosokawa Micron Corporation: 200AFG) using three φ3.0 nozzles at a crushing air pressure of 0.30 MPa and a crushing air flow rate of 1.0 Nm 3Disintegration was performed at a speed of 1000 / min, a classifier rotation speed of 11,500 rpm, and a sample supply rate of 100 g / min, and classification was performed in the classification section to obtain hollow silica particles. In the classification section, hollow silica particle aggregates with a particle size of more than 5 μm were continuously reintroduced into the disintegration section. The disintegrated hollow silica particles were collected using a pulse jet collector (KP-1-700, manufactured by Hosokawa Micron Corporation). The physical properties of the obtained hollow silica particles are shown in Table 1.

[0067] <Example 2> Hollow silica particles were obtained in the same manner as in Example 1, except that in step 3, the obtained hollow silica particle precursor was calcined at 1,150° C. for 1 hour. The physical properties of the obtained hollow silica particles are shown in Table 1.

[0068] <Example 3> (Process 1) In the same manner as in Example 1, a water-based emulsion A was obtained. (Process 2) Into a reaction vessel, 210.00 kg of ion-exchanged water, 1.73 kg of aqueous emulsion A, 4.58 kg of Catiogen TML (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.: containing 30% by mass of dodecyltrimethylammonium chloride) as surfactant B, and 1.35 kg of tetramethylammonium hydroxide (manufactured by Showa Denko K.K.: industrial TMAH, TMAH content 25% by mass) as an alkaline substance were added, and stirred for 10 minutes at 15° C. Then, 21.9 kg of tetraethyl orthosilicate (manufactured by Asahi Kasei Wacker Silicone Co., Ltd.: SEMICOSIL TEOS999-LB) was added as a silanol precursor, and stirred at 25° C. for 15 hours to obtain a cloudy liquid D'. Next, the resulting cloudy liquid D' was filtered using 5C filter paper (manufactured by Advantec Toyo Kaisha, Ltd.) and then dried at 110°C for 15 hours to obtain an aggregate of white hollow silica particle precursors. (Step 3) The obtained aggregates of hollow silica particle precursors were fired at 1100° C. for 1 hour to obtain hollow silica particle aggregates. (Step 4) The same procedure as in step 4 of Example 1 was carried out to obtain hollow silica particles. The physical properties of the obtained hollow silica particles are shown in Table 1.

[0069] <Example 4> Hollow silica particles were obtained in the same manner as in Example 3, except that in step 3, the obtained hollow silica particle precursor was calcined at 1,150° C. for 1 hour. The physical properties of the obtained hollow silica particles are shown in Table 1.

[0070] <Comparative Example 1> After obtaining hollow silica particle agglomerates in the same manner as in Example 1 in steps 1 to 3, the obtained hollow silica particle agglomerates were disintegrated in step 4 by the following method to obtain hollow silica particles. The physical properties of the obtained hollow silica particles are shown in Table 1. (Step 4) The obtained hollow silica particle aggregates were crushed in a swirling airflow type jet mill (Seishin Enterprise Co., Ltd.: CO-JET SYSTEM α MARK III) at P NOZZLE pressure: 0.30 MPa, G NOZZLE pressure: 0.30 MPa, and sample supply rate: 10 g / min to obtain hollow silica particles. The crushed hollow silica particles were collected in a cyclone and a bag filter, and then mixed.

[0071] <Comparative Example 2> After obtaining hollow silica particle agglomerates in the same manner as in Example 1 in steps 1 to 3, the obtained hollow silica particle agglomerates were disintegrated in step 4 by the following method to obtain hollow silica particles. The physical properties of the obtained hollow silica particles are shown in Table 1. (Step 4) The obtained hollow silica particle aggregates were crushed in a hammer mill (Dalton Co., Ltd.: Labo Mill LM-05) at a hammer rotation speed of 16,000 rpm, a screen opening of 1 mm, and a sample supply rate of 10 g / min to obtain hollow silica particles. The crushed hollow silica particles were collected in a bag filter.

[0072] <Comparative Example 3> After obtaining hollow silica particle agglomerates in the same manner as in Example 1 in steps 1 to 3, the obtained hollow silica particle agglomerates were disintegrated in step 4 by the following method to obtain hollow silica particles. The physical properties of the obtained hollow silica particles are shown in Table 1. (Step 4) The obtained hollow silica particle aggregates were mixed in ethanol at a concentration of 10% by mass to prepare a slurry. In a wet bead mill (Star Mill LMZ-015 manufactured by Ashizawa Finetech Co., Ltd.), φ0.3 mm zirconia beads were filled in the grinding chamber at a filling rate of 80%, and the rotor was rotated at a peripheral speed of 12 m / s. The slurry was passed through the rotor for one pass to disintegrate the particles, obtaining a disintegrated slurry. The obtained disintegrated slurry was dried for 12 hours in a dryer adjusted to a temperature of 130°C to obtain hollow silica particles.

[0073] [Table 1]

[0074] From Table 1, it is confirmed that the hollow silica particles of Examples 1 to 4 obtained by the present invention are prevented from cracking and are crushed to a predetermined particle size or less, compared with Comparative Examples 1 to 3. Furthermore, since the hollow silica particles of Examples 1 to 4 are prevented from cracking, the relative dielectric constant and dielectric loss tangent can be kept low.

Claims

1. A method for producing hollow silica particles, comprising the step of crushing hollow silica particle aggregates using a crusher having a crushing section and a classification section.

2. The method for producing hollow silica particles according to claim 1, wherein the classification section of the crusher has a mechanism for discharging hollow silica particles of a predetermined particle size or smaller to the outside of the crusher, and for extracting hollow silica particles larger than the predetermined particle size and hollow silica particle aggregates and reintroducing them into the crushing section.

3. A method for producing hollow silica particles according to claim 1 or 2, wherein the crushing section of the crushing machine has a mechanism for crushing hollow silica particle aggregates by an airflow from a nozzle that ejects a high-speed jet stream.

4. The method for producing hollow silica particles according to claim 3, wherein the nozzles for ejecting the high-speed jet stream are arranged in positions facing each other.

5. A method for producing hollow silica particles according to claim 1 or 2, wherein the relative permittivity of the hollow silica particles at 10 GHz is 2.5 or less, and the dielectric loss tangent at 10 GHz is 0.0050 or less.

6. A method for producing hollow silica particles according to claim 1 or 2, wherein the number ratio of hollow silica particles with a particle diameter of 5 μm or more is 50 ppm or less.