Colloidal silica and method for producing the same

Abstract

To provide colloidal silica containing silica particles with excellent denseness and excellent maintenability of an uneven surface shape under basic conditions, and a method for producing the colloidal silica.SOLUTION: Colloidal silica that contains silica particles having an uneven surface shape, is characterized in that (1) the silica particles have an alkoxy group content of 1000 ppm or more, and (2) the silica particles have a specific surface area reduction rate of 15.0% or less when heat-treated under basic conditions.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to colloidal silica and a method for producing the same, and more particularly to colloidal silica containing silica particles having an irregular surface and a method for producing the same. [Background technology]

[0002] Colloidal silica is a dispersion of silica fine particles in a medium such as water, and is used as a property improver in fields such as paper, textiles, and steel, as well as an abrasive for electronic materials such as semiconductor wafers. The silica particles dispersed in colloidal silica used for such purposes must be highly pure and dense.

[0003] As a method for producing colloidal silica that can meet the above requirements, for example, a method for producing an aqueous silica sol in which an alkyl silicate is added to a reaction medium having an alkali concentration within a specific range has been disclosed (see, for example, Patent Document 1).

[0004] However, according to the manufacturing method described in Patent Document 1, spherical particles are produced, and the shape of the silica particles is not considered.

[0005] A method for producing colloidal silica containing silica particles having small protrusions on the particle surface using a quaternary ammonium salt or the like as a hydrolysis catalyst has been disclosed (see, for example, Patent Document 2). Colloidal silica can exhibit higher polishing properties as an abrasive when the silica particles are deformed, such as when they have protrusions on the surface. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 6-316407 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-153732 Summary of the Invention [Problem to be solved by the invention]

[0007] The present inventors have found that colloidal silica produced by the production method described in Patent Document 2 has the problem that it is unable to maintain its surface irregularities under basic conditions.

[0008] As a result of extensive research, the present inventors have succeeded in producing colloidal silica containing silica particles that are excellent in maintaining the surface irregularities even under basic conditions. They have then come to the realization that such colloidal silica can be suitably used as an abrasive and can effectively solve the above-mentioned problems, thereby completing the present invention.

[0009] An object of the present invention is to provide colloidal silica containing silica particles that are highly dense and have excellent ability to maintain the surface irregularity even under basic conditions, and a production method by which the colloidal silica can be produced. [Means for solving the problem]

[0010] As a result of extensive research into achieving the above-mentioned object, the present inventors have found that the above-mentioned object can be achieved by using colloidal silica containing silica particles having an uneven surface, the silica particles having an alkoxy group content within a specific range, and the silica particles exhibiting a specific range of reduction rate in specific surface area when heat-treated under basic conditions, and have completed the present invention.

[0011] That is, the present invention relates to the following colloidal silica and a method for producing the same. 1. Colloidal silica containing silica particles having an irregular surface, (1) The silica particles have an alkoxy group content of 1000 ppm or more, (2) The silica particles have a specific surface area reduction rate of 15.0% or less when heat-treated under basic conditions. Colloidal silica characterized by: 2. The colloidal silica according to Item 1, wherein the true specific gravity of the silica particles is 1.95 or more. 3. The colloidal silica according to Item 1 or 2, wherein the silica particles contain 5 μmol or more of at least one amine selected from the group consisting of primary amines, secondary amines, and tertiary amines (excluding hydroxyl groups as substituents) per 1 g of silica particles. 4. A method for producing colloidal silica containing silica particles having an irregular surface, comprising: (1) a step 1 of preparing a mother liquor containing an alkali catalyst and water; (2) Step 2 of adding an alkoxysilane to the mother liquid to prepare a seed particle dispersion; and (3) a step 3 of adding water, an alkali catalyst, and an alkoxysilane to the seed particle dispersion in this order; the alkali catalyst is at least one amine selected from the group consisting of primary amines, secondary amines, and tertiary amines (excluding hydroxyl groups as substituents); In the step 3, the molar ratio (s3 / c3) of the amount s3 (mol) of the alkoxysilane added to the amount c3 (mol) of the alkali catalyst added is greater than 185 and equal to or less than 400. A method for producing colloidal silica. [Effects of the Invention]

[0012] The colloidal silica of the present invention contains silica particles that are highly dense and have excellent surface irregularity retention under basic conditions. The colloidal silica production method of the present invention can produce the colloidal silica. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is an SEM photograph of silica particles of colloidal silica produced in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0014] The colloidal silica of the present invention and its production method will be described in detail below.

[0015] The colloidal silica of the present invention contains silica particles having an irregular surface shape, and therefore exhibits high polishing properties.Furthermore, the colloidal silica of the present invention has an alkoxy group content of 1000 ppm or more in the silica particles, so that the amount of alkoxy groups per unit weight of the silica particles is high, and defects on the surface of the substrate or the like to be polished can be suppressed.Furthermore, the colloidal silica of the present invention has a specific surface area reduction rate of 15.0% or less when heat-treated under basic conditions, so that the colloidal silica is excellent in maintaining the irregular surface shape under basic conditions, and can maintain high polishing properties even under basic conditions. Furthermore, the method for producing colloidal silica of the present invention uses, as the alkali catalyst, at least one amine selected from the group consisting of primary amines, secondary amines, and tertiary amines (however, hydroxy groups are excluded as substituents), and performs the sol-gel reaction in step 3 with a molar ratio (s3 / c3) of the amount of alkoxysilane added s3 (mol) to the amount of alkali catalyst added c3 (mol) within a specific range, thereby making it possible to produce colloidal silica that contains few metal impurities, is excellent in maintaining the surface irregularity shape under basic conditions, and can maintain high polishing properties even under basic conditions.

[0016] 1. Colloidal silica The colloidal silica of the present invention is a colloidal silica containing silica particles having an irregular surface. The silica is characterized in that (1) the silica particles have an alkoxy group content of 1000 ppm or more, and (2) the silica particles have a specific surface area reduction rate of 15.0% or less when heat-treated under basic conditions.

[0017] In this specification, the term "surface irregularity of silica particles" refers to a shape in which the surface of the silica particles has minute protrusions, and the silica particles have a shape similar to that of confetti. Such surface irregularity can be defined by the range of surface roughness (B1 / S1), calculated by dividing the BET specific surface area (B1) by the specific surface area (S1) calculated from the SEM minor axis. The specific surface area (S1) can be calculated by converting the value of 2727 / SEM minor axis (nm) assuming that the true specific gravity of silica is 2.2. The surface roughness (B1 / S1) is preferably 1.1 or more, more preferably 1.4 or more, and is preferably 2.0 or less, more preferably 1.8 or less.

[0018] The silica particles have an alkoxy group content of 1000 ppm or more. If the alkoxy group content is less than 1000 ppm, the polishing ability of the colloidal silica of the present invention will decrease, and defects on the surface of the object to be polished will not be suppressed. The alkoxy group content is preferably 4000 ppm or more, more preferably 5000 ppm or more. Furthermore, the alkoxy group content is preferably 45000 ppm or less, more preferably 40000 ppm or less. When the upper limit of the alkoxy group content is within the above range, the polishing ability of the colloidal silica of the present invention will be further improved.

[0019] The alkoxy group content can be measured by the following method.

[0020] (Alkoxy group content (ppm)) The colloidal silica was centrifuged at 215,000 G for 90 minutes, the supernatant was discarded, and the solids were vacuum dried at 60°C for 90 minutes. 0.50 g of the resulting dried silica was weighed and placed in 50 ml of 1 M aqueous sodium hydroxide solution. The silica was dissolved by heating at 50°C for 24 hours with stirring. The silica solution was analyzed by gas chromatography to determine the alcohol content and calculate the amount of alkoxy per gram of silica. A flame ionization detector (FID) was used as the detector for the gas chromatography. Gas chromatography analysis was performed in accordance with JIS K0114.

[0021] (BET specific surface area (m 2 / g)) The colloidal silica was pre-dried on a hot plate and then heat-treated for 1 hour at 800°C to prepare a measurement sample. The prepared measurement sample was measured by the nitrogen gas adsorption method (BET method).

[0022] (Average primary particle diameter (nm)) Assuming that the true specific gravity of silica is 2.2, the above BET specific surface area measurement is calculated as 2727 / BET specific surface area (m 2 / g) is converted to the average primary particle diameter (nm) of silica particles in the colloidal silica.

[0023] The silica particles have a specific surface area reduction rate of 15.0% or less when heat-treated under basic conditions. If the specific surface area reduction rate exceeds 15.0%, the base resistance of the protrusions decreases, the surface irregularity of the silica particles is less able to be maintained under basic conditions, and polishing properties under basic conditions cannot be maintained. The specific surface area reduction rate is preferably 14.5% or less, more preferably 14.3% or less. There is no particular lower limit to the specific surface area reduction rate, and it is sufficient if it is about 0.1%.

[0024] The reduction rate of the specific surface area is measured by the following method. (Reduction rate of specific surface area) 3-Ethoxypropylamine is added to 800 g of colloidal silica to adjust the pH to 9.9 to 10.3. The colloidal silica is placed in a flask equipped with a reflux condenser and heated, and the base treatment is carried out by maintaining the reflux state for 3 hours. The pH of the base-treated colloidal silica is adjusted to 7.6 to 7.8, and the BET specific surface area is measured according to the method for measuring BET specific surface area described above. The reduction rate of the specific surface area is calculated using the following formula based on the BET specific surface areas before and after the base treatment. Reduction rate of specific surface area (%) = (BET specific surface area before base treatment - BET specific surface area after base treatment) / BET specific surface area before base treatment x 100

[0025] The silica particles preferably have a true specific gravity of 1.95 or more. A true specific gravity of 1.95 or more further improves the hardness of the silica particles and the abrasiveness of the colloidal silica. The true specific gravity of the silica particles is more preferably 2.00 or more, and even more preferably 2.10 or more. The true specific gravity is preferably 2.20 or less, and more preferably 2.16 or less.

[0026] The true specific gravity of the silica particles can be measured by a measurement method in which colloidal silica is dried on a hot plate at 150°C, then held in a furnace at 300°C for 1 hour, and then measured by a liquid phase substitution method using ethanol.

[0027] The silica particles preferably contain at least one amine selected from the group consisting of primary amines, secondary amines, and tertiary amines. The amine is not particularly limited and is represented by the following general formula (X): NR a R b R c (X) (In the formula, R a , R b , R c represents an optionally substituted alkyl group having 1 to 12 carbon atoms, or hydrogen. a , R b , R c is hydrogen, i.e., ammonia is excluded.) R a , R b , R c may be the same or different. a , R b , R c may be linear, branched or cyclic.

[0028] The number of carbon atoms in the linear or branched alkyl group may be 1 to 12, preferably 1 to 8, and more preferably 1 to 6. Examples of the linear alkyl group include a methyl group, an ethylene group, and the like. Examples of branched alkyl groups include isopropyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 1-methyl-1-ethylpropyl, 2-methyl-2-ethylpropyl, 1-ethylbutyl, 2-ethylbutyl, 1-ethylhexyl, 2-ethylhexyl, 3-ethylhexyl, 4-ethylhexyl, and 5-ethylhexyl groups. Preferred linear or branched alkyl groups include an n-propyl group, an n-hexyl group, a 2-ethylhexyl group, and an n-octyl group.

[0029] The number of carbon atoms in the cyclic alkyl group may be, for example, 3 to 12, and preferably 3 to 6. Examples of the cyclic alkyl group include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, and a cyclooctyl group. A preferred cyclic alkyl group is a cyclohexyl group.

[0030] R in the above general formula (X) a , R b , R c In the formula, the alkyl group may be substituted. The number of substituents may be, for example, 0, 1, 2, 3, 4, etc., preferably 0, 1 or 2, more preferably 0 or 1. In addition, when the number of substituents is 0, The alkyl group in this case refers to an unsubstituted alkyl group. Examples of the substituent include an alkoxy group having 1 to 3 carbon atoms (e.g., a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group), an amino group, a primary amino group substituted with a linear alkyl group having 1 to 4 carbon atoms, an amino group di-substituted with a linear alkyl group having 1 to 4 carbon atoms (e.g., a dimethylamino group, a di-n-butylamino group, etc.), and an unsubstituted amino group. However, a hydroxyl group is excluded as a substituent. In an alkyl group having multiple substituents, the substituents may be the same or different.

[0031] R in the above general formula (X) a , R b , R c is a linear or branched alkyl group having 1 to 8 carbon atoms (preferably 1 to 6 carbon atoms) that may be substituted. a , R b , R c teeth and a linear or branched alkyl group having 1 to 8 carbon atoms (preferably 1 to 6 carbon atoms) which may be substituted with an alkoxy group having 1 to 3 carbon atoms.

[0032] Also, R a , R b , R c may be unsubstituted. Preferably, R a , R b , R c is an unsubstituted linear or branched alkyl group having 1 to 12 carbon atoms, or a linear or branched alkyl group having 1 to 12 carbon atoms substituted with an alkoxy group.

[0033] Examples of the amine include at least one amine selected from the group consisting of aliphatic etheramines such as 3-ethoxypropylamine, 2-methoxyethylamine, 2-2-ethoxyethylamine, 3-methoxypropylamine, 3-propoxypropylamine, 3-isopropoxypropylamine, 3-butoxypropylamine, 3-isobutoxypropylamine, 3-(2-ethylhexyloxy)propylamine, and 3-(2-methoxyethoxy)propylamine, and aliphatic amines such as pentylamine, hexylamine, dipropylamine, and triethylamine. Among these, aliphatic etheramines are preferred, and 3-ethoxypropylamine is more preferred, in that they can increase the content of silica particles that are more excellent in maintaining the surface irregularity shape under basic conditions.

[0034] The above amines may be used alone or in combination of two or more.

[0035] The content of at least one amine selected from the group consisting of primary amines, secondary amines, and tertiary amines (excluding hydroxyl groups as substituents) in the silica particles is preferably 5 μmol or more per 1 g of silica particles, more preferably 10 μmol or more per 1 g of silica particles. By setting the lower limit of the amine content within the above range, the content of silica particles in the colloidal silica that are excellent in maintaining the surface irregularity under basic conditions is further increased, and the colloidal silica exhibits even more sufficient polishing properties. Furthermore, the amine content is preferably 100 μmol or less per 1 g of silica particles, more preferably 90 μmol or less per 1 g of silica particles. By setting the upper limit of the amine content within the above range, it is even easier to produce silica particles with an irregular surface.

[0036] The amine content can be measured by the following method. Specifically, the colloidal silica is centrifuged at 215,000 G for 90 minutes, the supernatant is discarded, and the solid is vacuum-dried at 60°C for 90 minutes. 0.50 g of the resulting dried silica is weighed and placed in 50 ml of 1 M aqueous sodium hydroxide solution. The mixture is heated at 50°C for 24 hours with stirring to dissolve the silica. The silica solution is analyzed by ion chromatography to determine the amine content. The ion chromatography analysis is performed in accordance with JIS K0127.

[0037] The boiling point of the amine is preferably 85°C or higher, more preferably 90°C or higher. By setting the lower limit of the boiling point within the above range, vaporization during the reaction is further suppressed, making it suitable for use as a catalyst. The boiling point is preferably 500°C or lower, more preferably 300°C or lower.

[0038] The colloidal silica of the present invention preferably contains 20% or more, and more preferably 30% or more, of silica particles having an irregular surface, based on the number of particles in an arbitrary field of view at 200,000 magnifications observed under a scanning electron microscope. When the lower limit of the content of the silica particles is within the above range, the abrasiveness of the colloidal silica is further improved. The upper limit of the content is not particularly limited and may be 100% or 70%.

[0039] In this specification, the content of silica particles having the above-mentioned irregular surface shape can be measured by the following measurement method: That is, particles having an irregular surface shape are counted from the number of particles in an arbitrary field of view observed under a scanning electron microscope (SEM) at 200,000 magnifications, and the proportion of these particles is taken as the content (%).

[0040] The SEM minor axis of the silica particles in the colloidal silica is preferably 8 nm or more, and more preferably 15 nm or more. When the lower limit of the SEM minor axis of the silica particles is within the above range, the polishing properties of the colloidal silica of the present invention are further improved. Furthermore, the SEM minor axis of the silica particles is preferably 100 nm or less, and more preferably 80 nm or less. When the upper limit of the SEM minor axis of the silica particles is within the above range, the occurrence of scratches on the polished object is further reduced.

[0041] The SEM minor diameter can be measured by the following method: An image of silica particles taken with a scanning electron microscope is analyzed using image analysis software ("WinRoof2015" manufactured by Mitani Corporation). ) 1000 particles were approximated as an ellipse and the minor axis of the ellipse was measured. The number frequency distribution of the minor axis of the ellipse was calculated, and the minor axis of the ellipse with a number frequency of 50% was taken as the SEM minor diameter (nm).

[0042] The average secondary particle diameter of the silica particles in the colloidal silica is preferably 8 nm or more, more preferably 15 nm or more. When the lower limit of the average secondary particle diameter of the silica particles is within the above range, the polishing properties of the colloidal silica of the present invention are further improved. Furthermore, the average secondary particle diameter of the silica particles is preferably 400 nm or less, more preferably 300 nm or less. When the upper limit of the average secondary particle diameter of the silica particles is within the above range, the occurrence of scratches on the polished object is further reduced.

[0043] In this specification, the average secondary particle size of the silica particles in the colloidal silica can be measured by the following measurement method. Specifically, a measurement sample for dynamic light scattering is prepared by adding colloidal silica to a 0.3 wt % aqueous citric acid solution and homogenizing it. The secondary particle size of this measurement sample is measured by dynamic light scattering (using an ELSZ-2000S, manufactured by Otsuka Electronics Co., Ltd.).

[0044] The aspect ratio of the silica particles in the colloidal silica is preferably 1.0 or more, more preferably 1.1 or more. When the lower limit of the aspect ratio is within the above range, polishing properties are further improved. Furthermore, the aspect ratio of the silica particles is preferably 4.0 or less, more preferably 3.0 or less. When the lower limit of the aspect ratio is within the above range, scratches on the polished object are further suppressed.

[0045] In this specification, the aspect ratio of the silica particles in the colloidal silica can be measured by the following method. That is, an image of the silica particles taken with a scanning electron microscope is analyzed by image analysis software ("WinRoof2015" manufactured by Mitani Shoji Co., Ltd.) for 1000 particles. Each particle is approximated as an ellipse, and the major and minor axes of the ellipse are measured. The ratio of the major and minor axes of the ellipse (major axis of the ellipse / minor axis of the ellipse) of each particle is calculated, and the average value is taken as the aspect ratio.

[0046] The colloidal silica of the present invention contains sodium, potassium, iron, aluminum, calcium, magnesium, titanium, nickel, chromium, copper, zinc, lead, silver, manganese, cobalt, etc. The content of metal impurities is preferably 1 ppm or less. When the content of metal impurities is 1 ppm or less, the polishing powder can be suitably used for polishing electronic materials and the like.

[0047] In this specification, the content of the metal impurities is a value measured using an atomic absorption spectrometer.

[0048] The association ratio of silica particles in the colloidal silica is preferably 1.5 or more, more preferably 1.7 or more. When the lower limit of the association ratio of silica particles is within the above range, the polishing properties of the colloidal silica of the present invention are further improved. Furthermore, the association ratio of silica particles is preferably 5.5 or less, more preferably 5.0 or less. When the upper limit of the association ratio of silica particles is within the above range, the occurrence of scratches on the polished object is further reduced.

[0049] In this specification, the association ratio of silica particles in the colloidal silica is a value obtained by calculating the average secondary particle size / average primary particle size of silica particles in the colloidal silica.

[0050] The silanol group density of silica particles in colloidal silica is 1.5 / nm 2 More than 1.6 particles / nm is preferable. 2 The above is more preferable. When the lower limit of the silanol group density is within the above range, the occurrence of scratches on the object to be polished is further reduced. In addition, the silanol density of the silica particles is 5.0 particles / nm 2 Preferably 4.0 or less / nm 2 The following is more preferable: When the upper limit of the silanol group density is within the above range, the polishing properties of the colloidal silica of the present invention are further improved.

[0051] The silanol group density of silica particles in colloidal silica can be determined by the Sears method. The Sears method is described in GW Sears, Jr., "Determination of Specific Surface Area of Colloidal Silica" The measurement was carried out with reference to the description in "Silica by Titration with Sodium Hydroxide", Analytical Chemistry, 28(12), 1981 (1956). A 1 wt% silica dispersion was used for the measurement, and titration was carried out with a 0.1 mol / L aqueous sodium hydroxide solution, and the silanol group density was calculated based on the following formula. ρ = (a × f × 6022) ÷ (c × S) In the above formula, ρ: Silanol group density (number / nm 2 ), a: Drop amount (mL) of 0.1 mol / L sodium hydroxide aqueous solution with pH 4-9, f: 0.1 mol / L sodium hydroxide aqueous solution Solution factor, c: mass of silica particles (g), S: BET specific surface area (m 2 / g).

[0052] 2. Colloidal Silica Manufacturing Method The method for producing colloidal silica of the present invention is a method for producing colloidal silica containing silica particles having an uneven surface, comprising the steps of: (1) Step 1: preparing a mother liquor containing an alkali catalyst and water; (2) Step 2 of adding an alkoxysilane to the mother liquid to prepare a seed particle dispersion; and (3) a step 3 of adding water, an alkali catalyst, and an alkoxysilane to the seed particle dispersion in this order; the alkali catalyst is at least one amine selected from the group consisting of primary amines, secondary amines, and tertiary amines (excluding hydroxyl groups as substituents); In the production method, the molar ratio (s3 / c3) of the amount s3 (mol) of the alkoxysilane added to the amount c3 (mol) of the alkali catalyst added in step 3 is greater than 185 and 400 or less.

[0053] (Process 1) Step 1 is a step of preparing a mother liquor containing an alkali catalyst and water.

[0054] The alkali catalyst is at least one amine selected from the group consisting of primary amines, secondary amines, and tertiary amines (excluding hydroxyl groups as substituents). The amine may be the same as the amine described above for the colloidal silica.

[0055] The amine content in the mother liquor is preferably 0.30 mmol / kg or more, more preferably 0.50 mmol / kg or more. When the lower limit of the amine content is within the above range, it becomes easier to control the particle size. Furthermore, the amine content in the mother liquor is preferably 20.0 mmol / kg or less, more preferably 15.0 mmol / kg or less. When the amine content is within the above range, gelation during the reaction is less likely to occur.

[0056] The method for preparing the mother liquor is not particularly limited, and it is sufficient to add an alkali catalyst to water by a conventionally known method and stir the mixture.

[0057] The pH of the mother liquor is not particularly limited, but is preferably 9.5 or higher, more preferably 10.0 or higher. When the lower limit of the pH of the mother liquor is within the above range, it becomes easier to control the particle size. Furthermore, the pH of the mother liquor is preferably 12.0 or lower, more preferably 11.5 or lower. When the upper limit of the pH of the mother liquor is within the above range, it becomes easier to control the average secondary particle size of the silica particles having an uneven surface, and aggregation of the seed particles in the seed particle dispersion obtained in step 2 described below is suppressed, thereby further improving the storage stability of the colloidal silica.

[0058] (Process 2) Step 2 is a step of adding an alkoxysilane to the mother liquid to prepare a seed particle dispersion.

[0059] The alkoxysilane is not particularly limited, and may be a compound represented by the following general formula (2): Si(OR 1 )4(2) (In the formula, R 1 represents an alkyl group. Examples of the alkoxysilane include those represented by the following formula:

[0060] In the above general formula (2), R 1 represents an alkyl group. 1 is not particularly limited as long as it is an alkyl group, and is preferably a lower alkyl group having 1 to 8 carbon atoms, and more preferably a lower alkyl group having 1 to 4 carbon atoms. Specific examples of the alkyl group include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. The alkoxysilane represented by the general formula (2) above includes a group in which R 1 Tetramethoxysilane (tetramethyl orthosilicate) where R is a methyl group 1 Tetraethoxysilane (tetraethyl orthosilicate) where R is an ethyl group 1 is an isopropyl group, and tetraisopropoxysilane is preferred. 1 tetramethoxysilane, where R is a methyl group; 1 Tetraethoxysilane, in which R is an ethyl group, is more preferred, and tetramethoxysilane is even more preferred.

[0061] The alkoxysilane represented by the general formula (2) may be a derivative, such as a low condensate obtained by partially hydrolyzing the alkoxysilane represented by the general formula (2).

[0062] The alkoxysilanes represented by the general formula (2) may be used alone or in combination of two or more.

[0063] The amount of alkoxysilane represented by the general formula (2) added in the seed particle dispersion is not particularly limited, and the molar ratio (s2 / c1) of the amount of alkoxysilane added in step 2, s2 (mol), to the amount of alkali catalyst, c1 (mol), in the mother liquor is preferably 10 or more, more preferably 100 or more, and even more preferably 150 or more. When the lower limit of s2 / c1 is within the above range, the content of silica particles in the colloidal silica can be further increased. Furthermore, s2 / c1 is preferably 8500 or less, more preferably 8000 or less. When the upper limit of s2 / c1 is within the above range, gelation during the reaction is less likely to occur.

[0064] The addition time of the alkoxysilane in step 2 is preferably 5 minutes or more, more preferably 10 minutes or more. When the lower limit of the addition time is within the above range, gelation is less likely to occur during the reaction. Furthermore, the addition time of the alkoxysilane is preferably 1000 minutes or less, more preferably 600 minutes or less. When the upper limit of the addition time is within the above range, productivity is further improved and production costs can be further reduced.

[0065] The pH of the seed particle dispersion is preferably 8.5 or less, more preferably 8.0 or less. When the upper limit of the pH of the seed particle dispersion is within the above range, it becomes easier to form silica particles having an uneven surface. Furthermore, the pH of the seed particle dispersion is preferably 4.5 or more, more preferably 4.9 or more. When the lower limit of the pH of the seed particle dispersion is within the above range, gelation is further suppressed.

[0066] The temperature of the seed particle dispersion in step 2 is preferably 70°C or higher, more preferably 75°C or higher. When the lower limit of the temperature of the seed particle dispersion is within the above range, gelation during the reaction is further suppressed. Furthermore, the temperature of the seed particle dispersion is preferably 95°C or lower, more preferably 90°C or lower. When the upper limit of the temperature of the seed particle dispersion is within the above range, vaporization of the alkoxysilane is further suppressed.

[0067] (Step 3) Step 3 is a step of adding water, an alkali catalyst, and an alkoxysilane to the seed particle dispersion.

[0068] The alkali catalyst is at least one amine selected from the group consisting of primary amines, secondary amines, and tertiary amines (excluding hydroxyl groups as substituents). The amine may be the same as the amine described above for the colloidal silica. The alkali catalyst used in step 3 may be the same as or different from the alkali catalyst used in step 1.

[0069] In step 3, silica particles having an irregular surface are formed in the colloidal silica. The mechanism of this action is unclear, but is presumed to be as follows. Specifically, in step 3, the pH of the seed particle dispersion is lowered by the addition of alkoxysilane. In the reaction for forming silica particles in step 3, under basic conditions with a relatively high pH, new seed particles are not generated, and the alkoxysilane is consumed so that the silica particles simply grow, preventing the generation of silica particles having an irregular surface. In contrast, as the pH gradually decreases and the seed particle dispersion becomes weakly basic, the condensation rate of the alkoxysilane hydrolysate increases, and dissolution of the embryos, which are the precursors of the seed particles, ceases. This is expected to result in the generation of new seed particles. Furthermore, as the pH decreases to near neutral, the seed particles generated under weakly basic conditions bond with the original seed particles generated in step 2, forming irregularities on the particle surfaces. Therefore, it is believed that colloidal silica having an irregular surface can be produced by the production method of the present invention.

[0070] From the above-mentioned expected mechanism of action, in step 3, the pH of the seed particle dispersion is changed from a strong basic to a moderate basic. It is preferable that the pH be decreased while being controlled to an appropriate pH level to near the pH limit. For this reason, an alkali catalyst that maintains a high pH or an alkali catalyst that causes a sudden decrease in pH is not suitable for use in step 3, and an alkali catalyst that has buffering capacity and gradually decreases the pH while controlling it to an appropriate pH range is preferably used.

[0071] Due to the above-described mechanism of action, the pKa value, which indicates the physical properties of an acid or base, is the central value in the buffer region, and therefore serves as a criterion for determining whether a substance has a buffering capacity that gradually decreases while controlling the pH within an appropriate range in step 3. That is, the alkali catalyst used in step 3 is preferably an amine represented by the above general formula (X) having a pKa value of 8.5 or more and less than 11, and more preferably an amine represented by the above general formula (X) having a pKa value of 9 or more and less than 10.

[0072] The amines represented by the general formula (X) and their pKa values are as follows: aliphatic etheramines: 3-ethoxypropylamine (9.79), 2-methoxyethylamine (9.89), 3-methoxypropylamine (9.73), 3-propoxypropylamine (9.78), 3-isopropoxypropylamine (9.82), and 3-butoxypropylamine (9.77).Also, aliphatic amines: pentylamine (10.63), hexylamine (10.56), dipropylamine (10.91), and triethylamine (10.75).

[0073] The alkoxysilane used in step 3 is not particularly limited, and the same alkoxysilane as described in step 2 can be used. The alkoxysilane used in step 3 may be the same as or different from the alkoxysilane used in step 2, but it is preferable to use the same alkoxysilane as the alkoxysilane used in step 2.

[0074] In step 3, the molar ratio (s3 / c3) of the amount s3 (mol) of alkoxysilane added to the amount c3 (mol) of alkali catalyst added exceeds 185. When the lower limit of s3 / c3 exceeds 185, it becomes easier to form an uneven surface. s3 / c3 is preferably 200 or more, and more preferably 220 or more. Furthermore, s3 / c3 is 400 or less. When s3 / c3 is 400 or less, gelation of colloidal silica is further suppressed. s3 / c3 is preferably 380 or less, and more preferably 350 or less.

[0075] In step 3, in addition to the water, alkali catalyst, and alkoxysilane, an alcohol may be added to the seed particle dispersion.

[0076] The alcohol is not particularly limited as long as it is soluble in water, and is preferably the same alcohol as the alcohol that is a by-product when the alkoxysilane used is hydrolyzed. For example, when the alkoxysilane is tetramethyl orthosilicate, it is preferable to use methanol, and when the alkoxysilane is tetraethyl orthosilicate, it is preferable to use ethanol.

[0077] In step 3, the alcohol content relative to 100% by mass of the mixture obtained by mixing the seed particle dispersion, water, alkali catalyst, and alcohol is preferably 25% by mass or less, and more preferably 20% by mass or less. When the upper limit of the alcohol content is within the above range, it becomes easier to increase the temperature of the mixture in step 3. In addition, the lower limit of the alcohol content is not particularly limited, and may be 0% by mass or 2% by mass.

[0078] The amount of alkoxysilane added in step 3 is not particularly limited, and the molar ratio (s3 / sp3) of the amount of alkoxysilane added in step 3, s3 (mol), to the amount of seed particles, sp3 (mol), in the mixture of the seed particle dispersion, water, alkali catalyst, and alcohol is 0 or more and 30 or less. When the upper limit of s3 / sp3 is within the above range, new core particles are less likely to be generated during the reaction, and the growth of main particles is further promoted. Note that the above molar ratio is a value defined assuming that the molecular weight of the seed particles is 60.08 g / mol.

[0079] The temperature of the mixed liquid in step 3 is preferably 70°C or higher, more preferably 75°C or higher. When the lower limit of the temperature of the mixed liquid is within the above range, gelation during the reaction is further suppressed. Furthermore, the temperature of the mixed liquid is preferably 95°C or lower, more preferably 90°C or lower. When the upper limit of the temperature of the seed particle dispersion is within the above range, vaporization of the alkoxysilane is further suppressed.

[0080] The addition time of the alkoxysilane in step 3 is preferably 5 minutes or more, more preferably 10 minutes or more. When the lower limit of the addition time is within the above range, gelation during the reaction is unlikely. Furthermore, the addition time of the alkoxysilane is preferably 1000 minutes or less, more preferably 600 minutes or less. When the upper limit of the addition time is within the above range, productivity is further improved and production costs can be further reduced.

[0081] The colloidal silica of the present invention can be produced by the production method described above.

[0082] The pH of the colloidal silica is preferably 11.0 or less, more preferably 10.0 or less. When the upper limit of the pH of the colloidal silica is within the above range, dissolution of the silica particles is further suppressed. Furthermore, the pH of the colloidal silica is preferably 5.8 or more, more preferably 6.0 or more. When the lower limit of the pH of the colloidal silica is within the above range, gelation is further suppressed.

[0083] The method for producing colloidal silica of the present invention may further include a step of concentrating the colloidal silica after the above step 3. The concentration method is not particularly limited, and the colloidal silica can be concentrated by a conventionally known method. Examples of such concentration methods include a method of heating and concentrating the colloidal silica at a temperature of about 65 to 100°C.

[0084] The concentration of silica particles in the concentrated colloidal silica is not particularly limited, but is preferably about 1 to 50% by mass, with the colloidal silica being 100% by mass.

[0085] The method for producing colloidal silica of the present invention may further include, after step 3, a step of distilling off methanol generated as a by-product during the reaction. The method for distilling off methanol is not particularly limited, and examples thereof include a method in which pure water is added dropwise to the colloidal silica while heating it, and the volume is kept constant, thereby replacing the dispersion medium with pure water. Another example of a method is to separate the colloidal silica from the solvent by precipitation, separation, centrifugation, or the like, and then redisperse the colloidal silica in water.

[0086] The colloidal silica of the present invention and colloidal silica produced by the production method of the present invention can be used in various applications such as abrasives and paper coating agents. Abrasives containing the above-mentioned colloidal silica also constitute the present invention. The colloidal silica of the present invention can be highly purified to a content of metal impurities such as sodium of 1 ppm or less, and therefore can be suitably used, particularly as an abrasive for chemical mechanical polishing of semiconductor wafers. [Example]

[0087] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0088] Example 1 (Step 1) 6,767 g of pure water as a solvent and 6.98 g of 3-ethoxypropylamine (3-EOPA) as an alkaline catalyst were placed in a flask to prepare a mother liquor. The pH of the mother liquor was 11.0. (Step 2) After heating the mother liquor to an internal temperature of 80°C, tetramethyl orthosilicate was added to the mother liquor. While controlling the temperature so as not to fluctuate the internal temperature, 2472 g of the solution was added dropwise at a constant rate over 210 minutes to prepare a seed particle dispersion. (Step 3) 5704 g of pure water as a solvent, 6.50 g of 3-ethoxypropylamine (3-EOPA) as an alkali catalyst, and 1075 g of the seed particle dispersion liquid prepared in step 2 were placed in a flask. Next, after heating to an internal temperature of 80°C, 2397 g of tetramethyl orthosilicate was added dropwise at a constant rate over 180 minutes while controlling the temperature so as not to fluctuate the internal temperature. After the completion of the addition, stirring was continued for 15 minutes to prepare a colloidal silica dispersion liquid. Next, the colloidal silica The silica dispersion was heated and concentrated under normal pressure to a base volume of 800 mL until the silica concentration reached 20 wt%. Next, in order to distill off the methanol by-product during the reaction, the dispersion medium was replaced with 500 mL of pure water while keeping the volume constant, to prepare colloidal silica.

[0089] In Example 1, the molar ratio (s3 / c3) of the amount s3 (mol) of alkoxysilane (tetramethyl orthosilicate) added in step 3 to the amount c3 (mol) of alkali catalyst (3-ethoxypropylamine) added was 250.

[0090] Example 2 (Step 1) 6767 g of pure water as a solvent and 10.47 g of 3-ethoxypropylamine (3-EOPA) as an alkaline catalyst were placed in a flask to prepare a mother liquor. The pH of the mother liquor was 11.3. (Step 2) After heating the mother liquor to an internal temperature of 85°C, tetramethyl orthosilicate was added to the mother liquor. While controlling the temperature so as not to fluctuate the internal temperature, 2472 g of the solution was added dropwise at a constant rate over 210 minutes to prepare a seed particle dispersion. (Step 3) 5704 g of pure water as a solvent, 6.50 g of 3-ethoxypropylamine (3-EOPA) as an alkali catalyst, 242 g of methanol, and 667 g of the seed particle dispersion liquid prepared in step 2 were placed in a flask. Next, the flask was heated to an internal temperature of 80°C, and then 2397 g of tetramethyl orthosilicate was added dropwise at a constant rate over 180 minutes while controlling the temperature so as not to fluctuate the internal temperature. After the completion of the addition, stirring was continued for 15 minutes to prepare a colloidal silica dispersion liquid. Next, The colloidal silica dispersion was then heated and concentrated under normal pressure to a base volume of 9000 mL until the silica concentration reached 20 wt%. Next, in order to distill off the methanol by-product during the reaction, the dispersion medium was replaced with 5680 mL of pure water while maintaining the volume constant, to prepare colloidal silica.

[0091] In Example 2, the molar ratio (s3 / c3) of the amount s3 (mol) of alkoxysilane (tetramethyl orthosilicate) added in step 3 to the amount c3 (mol) of alkali catalyst (3-ethoxypropylamine) added was 250.

[0092] Comparative Example 1 (Step 1) 7,500 g of pure water as a solvent and 1.35 g of 3-ethoxypropylamine (3-EOPA) as an alkaline catalyst were placed in a flask to prepare a mother liquor. The pH of the mother liquor was 10.3. (Step 2) After heating the mother liquor to an internal temperature of 85°C, tetramethyl orthosilicate was added to the mother liquor. 2740 g of acetone was added dropwise at a constant rate over 60 minutes while controlling the temperature so as not to fluctuate the internal temperature, and the mixture was stirred for 15 minutes to prepare a seed particle dispersion. (Step 3) 50 g of 3-ethoxypropylamine (3-EOPA) was added as an alkaline catalyst to the seed particle dispersion to prepare a mixed solution. 5,379 g of pure water was placed in a separate flask as a solvent, and 2,382 g of the mixed solution of 3-ethoxypropylamine and the seed particle dispersion was added. Next, the internal temperature was heated to 80°C, and 1712.5 g of tetramethyl orthosilicate was added dropwise at a constant rate over 180 minutes while controlling the temperature so as not to fluctuate the internal temperature. Stirring was continued for a period of time to prepare a colloidal silica dispersion. The colloidal silica dispersion was then heated and concentrated under normal pressure to a base volume of 800 mL until the silica concentration reached 20 wt%. Next, in order to distill off the methanol by-product during the reaction, the dispersion medium was replaced with 500 mL of pure water while maintaining the volume constant, thereby preparing colloidal silica. The resulting particles did not have any surface irregularities.

[0093] In Comparative Example 1, the molar ratio (s3 / c3) of the amount s3 (mol) of alkoxysilane (tetramethyl orthosilicate) added in step 3 to the amount c3 (mol) of alkali catalyst (3-ethoxypropylamine) added was 100.

[0094] Comparative Example 2 (Step 1) 9,492 g of pure water as a solvent and 3.28 g of triethanolamine (TEA) as an alkaline catalyst were placed in a flask to prepare a mother liquor. The pH of the mother liquor was 9.4. (Step 2) After heating the mother liquid to an internal temperature of 80°C, 1704 g of tetramethyl orthosilicate was added dropwise to the mother liquid at a constant rate over 180 minutes while controlling the temperature to prevent fluctuations in the internal temperature. After the supply of tetramethyl silicate to the reaction vessel was completed, the reaction liquid in the reaction vessel was heated, and while distilling methanol through a distillation tube with a condenser, the reaction liquid prepared under the same conditions was fed into the reaction vessel to concentrate it, thereby preparing a seed particle dispersion liquid with a silica concentration of 12.2 wt%. (Step 3) A flask was charged with 5582 g of pure water as a solvent, 9.43 g of triethanolamine (TEA) as an alkaline catalyst, and 857 g of the seed particle dispersion liquid prepared in step 2. Next, the internal temperature was heated to 80°C, and 3878 g of tetramethyl orthosilicate was added dropwise to the mother liquid at a constant rate over 180 minutes while controlling the temperature to prevent fluctuations in the internal temperature. After the completion of the addition, the mixture was stirred for 15 minutes. Stirring was continued to prepare a colloidal silica dispersion. Next, the colloidal silica dispersion was adjusted to a base volume of 4500 mL under normal pressure, and heated and concentrated until the silica concentration reached 20 wt%. Next, in order to distill off methanol, which was by-produced during the reaction, the dispersion medium was replaced with 5680 mL of pure water while maintaining the volume constant, to prepare colloidal silica.

[0095] In Comparative Example 2, the molar ratio (s3 / c3) of the amount s3 (mol) of alkoxysilane (tetramethyl orthosilicate) added in step 3 to the amount c3 (mol) of alkali catalyst (triethanolamine) added was 403.

[0096] The properties of the colloidal silica of the Examples and Comparative Examples obtained as described above were evaluated by the following methods.

[0097] (Alkoxy group content (ppm)) The colloidal silica was centrifuged at 215,000 G for 90 minutes, the supernatant was discarded, and the solid was vacuum dried at 60°C for 90 minutes. 0.50 g of the resulting dried silica was weighed and added to 50 ml of 1 M aqueous sodium hydroxide solution, and heated at 50°C for 24 hours with stirring to dissolve the silica. The silica solution was analyzed by gas chromatography to determine the alcohol content, which was taken as the alkoxy content. A flame ionization detector (FID) was used as the detector for the gas chromatography. The gas chromatography analysis was performed in accordance with JIS K0114.

[0098] (BET specific surface area (m 2 / g)) The colloidal silica was pre-dried on a hot plate and then heat-treated at 800°C for 1 hour to prepare a measurement sample. The BET specific surface area of the prepared measurement sample was measured by the nitrogen gas adsorption method (BET method) described below. Nitrogen gas adsorption method Pretreatment device: BELPREP-vacII (Microtrac BEL Co., Ltd.) Pretreatment method: Vacuum degassing was carried out at 120°C for 8 hours. Measurement device: BELSORP-miniII (Microtrack BEL Co., Ltd.) Measurement method: Adsorption isotherms with nitrogen were measured using a constant volume method. Measurement conditions: Adsorption temperature 77K; Adsorbate nitrogen; Saturated vapor pressure measured; Adsorbate cross section 0.162nm 2 Equilibrium waiting time (waiting time after reaching adsorption equilibrium state (state where pressure change during adsorption / desorption is below a predetermined value)) 500 seconds From the measurement results, the specific surface area was calculated by the BET method.

[0099] (Average primary particle diameter (nm)) Assuming that the true specific gravity of silica is 2.2, the above BET specific surface area measurement is calculated as 2727 / BET specific surface area (m 2 / g) was converted to the average primary particle diameter (nm) of silica particles in the colloidal silica.

[0100] (Average secondary particle size) A sample for dynamic light scattering measurement was prepared by adding colloidal silica to a 0.3 wt% aqueous citric acid solution and homogenizing it. The average secondary particle diameter of the sample was measured by dynamic light scattering ("ELSZ-2000S" manufactured by Otsuka Electronics Co., Ltd.).

[0101] (Reduction rate of specific surface area) 3-Ethoxypropylamine was added to 800 g of colloidal silica to adjust the pH to 9.9 to 10.3. The colloidal silica was placed in a flask equipped with a reflux condenser and heated, and the reflux state was maintained for 3 hours to perform base treatment. The pH of the base-treated colloidal silica was adjusted to 7.6 to 7.8, and the BET specific surface area was measured according to the method for measuring BET specific surface area described above. The reduction rate of the specific surface area was calculated using the BET specific surface areas before and after the base treatment according to the following formula: Reduction rate of specific surface area (%) = (BET specific surface area before base treatment - BET specific surface area after base treatment) / BET specific surface area before base treatment x 100

[0102] (SEM short axis) The images of the silica particles taken with a scanning electron microscope were analyzed using image analysis software ("WinRoof2015" manufactured by Mitani Shoji Co., Ltd.), and 1000 particles were each approximated as an ellipse, and the minor axis of the ellipse was measured. The number frequency distribution of the minor axes of the circles was calculated, and the minor axis of the ellipse with a number frequency of 50% was taken as the SEM minor axis (nm).

[0103] (aspect ratio) The images of silica particles taken with a scanning electron microscope were analyzed using image analysis software ("WinRoof2015" manufactured by Mitani Shoji Co., Ltd.) to approximate each of 1,000 particles into an ellipse, and the major and minor axes of the ellipse were calculated. The ratio of the major axis of the ellipse to the minor axis of the ellipse (major axis of the ellipse / minor axis of the ellipse) of each particle was calculated, and the average value was taken as the aspect ratio.

[0104] (Surface roughness) The surface roughness was calculated by dividing the BET specific surface area (B1) by the specific surface area (S1) calculated from the SEM minor axis (B1 / S1). The specific surface area (S1) was calculated by converting the value of 2727 / SEM minor axis (nm) assuming the true specific gravity of silica to be 2.2.

[0105] (true specific gravity) The colloidal silica was dried on a hot plate at 150°C, and then held in a furnace at 300°C for 1 hour, after which the true specific gravity was measured by a liquid phase displacement method using ethanol.

[0106] (amine content) The colloidal silica was centrifuged at 215,000 G for 90 minutes, the supernatant was discarded, and the solid was vacuum dried at 60°C for 90 minutes. 0.50 g of the resulting dried silica was weighed and added to 50 ml of 1 M aqueous sodium hydroxide solution. The silica was dissolved by heating at 50°C for 24 hours with stirring. The silica solution was analyzed by ion chromatography to determine the amine content. The ion chromatography analysis was performed in accordance with JIS K0127.

[0107] (Silanol group density) The silanol group density of silica particles was determined by the Sears method. The Sears method was performed with reference to the description in GW Sears, Jr., "Determination of Specific Surface Area of Colloidal Silica by Titration with Sodium Hydroxide," Analytical Chemistry, 28(12), 1981 (1956). A 1 wt% silica dispersion was used for measurement, and titration was performed with a 0.1 mol / L aqueous sodium hydroxide solution. The silanol group density was calculated using the following formula: ρ = (a × f × 6022) ÷ (c × S) In the above formula, ρ: Silanol group density (number / nm 2 ), a: Drop amount (mL) of 0.1 mol / L sodium hydroxide aqueous solution with pH 4-9, f: 0.1 mol / L sodium hydroxide aqueous solution Solution factor, c: mass of silica particles (g), S: BET specific surface area (m 2 / g).

[0108] (Metal impurity content) The content of metal impurities was measured using an atomic absorption spectrometer. The total content of sodium, potassium, iron, aluminum, calcium, magnesium, titanium, nickel, chromium, copper, zinc, lead, silver, manganese, and cobalt in the colloidal silica was defined as the content of metal impurities.

[0109] [Table 1]

[0110] *1: In Comparative Example 2, no amine content was detected because no amine selected from the group consisting of primary amines, secondary amines, and tertiary amines (however, hydroxyl groups are excluded as substituents) was used.

Claims

1. A colloidal silica containing silica particles, (1) The silica particles have a BET specific surface area reduction rate of 15.0% or less when heat-treated under basic conditions; (2) The silica particles have a surface roughness (B1 / S1) of 1.1 or more and 1.8 or less, calculated by dividing a BET specific surface area (B1) measured by the BET specific surface area measurement method described below by a specific surface area (S1) calculated from a SEM minor axis; The reduction rate of the BET specific surface area, the BET specific surface area (B1), and the specific surface area (S1) calculated from the SEM minor axis are measured by the following measurement method: Colloidal silica characterized by: [Method for measuring the reduction rate of BET specific surface area] 3-Ethoxypropylamine is added to 800 g of colloidal silica to adjust the pH to 9.9 to 10.

3. The colloidal silica is placed in a flask equipped with a reflux condenser and heated, and the reflux state is maintained for 3 hours to perform a base treatment. The pH of the base-treated colloidal silica is adjusted to 7.6 to 7.8, and the BET specific surface area is measured according to the BET specific surface area measurement method described below. The reduction rate of the BET specific surface area is calculated based on the BET specific surface area before and after the base treatment according to the following formula. Reduction rate of BET specific surface area (%) = (BET specific surface area before base treatment - BET specific surface area after base treatment) / base BET specific surface area before treatment x 100 [Method for measuring BET specific surface area] Colloidal silica was pre-dried on a hot plate and then heat-treated at 800°C for 1 hour. The prepared measurement sample is analyzed by nitrogen gas adsorption method (BET method). The BET specific surface area is measured. [Method for measuring specific surface area (S1) calculated from SEM minor axis] An image of silica particles taken with a scanning electron microscope is used to approximate 1,000 particles into an ellipse and measure the minor axis of the ellipse using image analysis software ("WinRoof2015" manufactured by Mitani Shoji Co., Ltd.). The number frequency distribution of the minor axis of the ellipse is calculated, and the minor axis of the ellipse with a number frequency of 50% is defined as the SEM minor axis (nm). The true specific gravity of silica is assumed to be 2.2, and the value of 2727 / SEM minor axis (nm) is calculated. The specific surface area (S1) is calculated from the SEM minor axis.

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