Citric acid-containing colloidal silica and its manufacturing method

Abstract

The present invention provides a citric acid-containing colloidal silica in which the generation of coarse particles having a size of 0.20 μm or more is suppressed when a shear force is applied. The present invention relates to a citric acid-containing colloidal silica containing silica particles and citric acid, (1) The silanol group density per unit surface area of ​​the silica particles is 1.6 × 10 19 ~14.5×10 19 pieces / m 2 and (2) The content of the citric acid per unit surface area of ​​the silica particles is 1.0 to 10.0 μg / m 2 That is, 2. The citric acid-containing colloidal silica according to claim 1 ,

Description

[Technical field]

[0001] The present invention relates to a citric acid-containing colloidal silica and a method for producing the same. [Background technology]

[0002] Colloidal silica is silica particles dispersed in a medium such as water, and is used as a property improver in the fields of paper, textiles, steel, etc., as well as an abrasive used in polishing semiconductor devices such as semiconductor wafers (CMP).

[0003] In chemical mechanical polishing carried out in the manufacturing process of semiconductor devices, a polishing composition with few coarse particles is required to reduce polishing scratches on the surface of the substrate to be polished. As a raw material for preparing such a polishing composition, colloidal silica with an average particle size of about 10 to 200 nm is used, but due to the need to reduce polishing scratches as described above, the colloidal silica is required to have a small content of coarse particles with a size of 0.20 μm or more.

[0004] When preparing a polishing composition using colloidal silica as an abrasive raw material, a process of stirring a mixture of colloidal silica and other chemical components is usually performed. For example, Patent Document 1 describes a method for preparing a polishing composition in which the silanol group density of silica particles is 1.0 to 3.0 / nm 2 An aqueous dispersion for chemical mechanical polishing has been proposed, and it is disclosed that the content of the polyvalent carboxylic acid is 0.001 to 3.0 mass %. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2010-16344 A Summary of the Invention [Problem to be solved by the invention]

[0006] As a result of intensive research, the present inventors have found that conventional colloidal silica has the following problems. That is, the colloidal silica contained in the aqueous dispersion for chemical mechanical polishing has a high content of polyvalent carboxylic acid per unit surface area of ​​silica particles, and when shear force is applied, the content of coarse particles having a size of 0.20 μm or more increases. Such colloidal silica cannot reduce polishing scratches on the surface of the substrate to be polished in chemical mechanical polishing or the like performed in the manufacturing process of semiconductor devices.

[0007] In view of the above circumstances, an object of the present invention is to provide a polyvalent carboxylic acid-containing colloidal silica in which the generation of coarse particles having a size of 0.20 μm or more is suppressed when a shear force is applied. [Means for solving the problem]

[0008] Means for Solving the Problems The present inventors have conducted intensive research in order to achieve the above-mentioned object, and as a result, have found that, in a colloidal silica containing silica particles and citric acid, the above-mentioned object can be achieved by using a citric acid-containing colloidal silica in which the silanol group density per unit surface area of ​​the silica particles and the content of citric acid per unit surface area of ​​the silica particles are within specific ranges, and have thus completed the present invention.

[0009] That is, the present invention relates to the following citric acid-containing colloidal silica and a production method thereof. 1. A citric acid-containing colloidal silica comprising silica particles and citric acid, (1) The silanol group density per unit surface area of ​​the silica particles is 1.6 × 10 19 ~14.5×10 19 pieces / m 2 and (2) The content of the citric acid per unit surface area of ​​the silica particles is 1.0 to 10.0 μg / m 2 That is, 2. The citric acid-containing colloidal silica according to claim 1 , 2. The citric acid-containing colloidal silica according to item 1, wherein the silica particles have an average secondary particle size of 10 to 200 nm. 3. The BET specific surface area of ​​the silica particles is 20 to 300 m 2 Item 3. The citric acid-containing colloidal silica according to Item 1 or 2, wherein the citric acid-containing colloidal silica has a molecular weight of 1000 or more and a molecular weight of 1000 or more. 4. The amount of silanol groups per unit weight of the silica particles is 1.5×10 21 ~5.0×10 21 Item 4. The citric acid-containing colloidal silica according to any one of Items 1 to 3, wherein the citric acid-containing colloidal silica is present in an amount of 1 to 1 g. 5. The citric acid-containing colloidal silica according to any one of items 1 to 4, wherein the content of the silica particles is 10% by mass or more. 6. A method for producing the citric acid-containing colloidal silica according to any one of items 1 to 5, comprising the steps of: (1) Step 1: preparing a mother liquor containing an alkali catalyst, an alcohol, and water; (2) a step 2 of adding a raw material solution containing an alkoxysilane and an alcohol to the mother liquid to prepare a reaction liquid; and (3) adding citric acid to the reaction solution; The manufacturing method according to claim 1, 7. The method according to item 6, wherein the step 2 is a step of adding the raw material solution to the mother liquor at a constant rate. 8. The method according to item 6 or 7, further comprising, after the step 3, a step 4 of concentrating a silica particle concentration of the citric acid-containing colloidal silica and replacing the solvent with water. Item 9. The method according to item 8, wherein the step 4 is a step of heating the citric acid-containing colloidal silica for a heating time of 15 hours or less. 10. The method according to any one of items 6 to 9, wherein in step 2, the water concentration in the reaction solution is 14.5 mass % or less. 11. The method according to any one of items 6 to 10, wherein in step 2, the reaction solution has a temperature of 15 to 25°C. 12. The method according to any one of items 6 to 11, wherein the amount of citric acid added per 1 g of silica particles is 50 to 600 μg / g. Effect of the Invention

[0010] The citric acid-containing colloidal silica of the present invention is suppressed from generating coarse particles having a size of 0.20 μm or more when a shear force is applied. Moreover, according to the production method of the present invention, the citric acid-containing colloidal silica of the present invention can be produced. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The present invention will be described in detail below. Note that the following description of the components may be based on representative embodiments and specific examples, but the present invention is not limited to such embodiments.

[0012] In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In addition, in the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in an example or a value that can be unambiguously derived from an example. Furthermore, in this specification, a numerical value connected with "~" means a numerical range that includes the numerical values ​​before and after "~" as the lower and upper limits.

[0013] In this specification, the expressions "contain" and "include" include the concepts of "contain", "include", "consist essentially of" and "consist only of".

[0014] The citric acid-containing colloidal silica of the present invention (hereinafter, also simply referred to as "colloidal silica") is a citric acid-containing colloidal silica containing silica particles and citric acid, and (1) the silanol group density per unit surface area of ​​the silica particles is 1.6 × 10 19 ~14.5×10 19 pieces / m 2 (2) the content of the citric acid per unit surface area of ​​the silica particles is 1.0 to 10.0 μg / m 2The colloidal silica of the present invention contains silica particles and citric acid, and has the above-mentioned (1) and (2) configurations, so that even if a shear force is applied under conditions of a high silica particle concentration, the aggregation of particles is unlikely to proceed, and an increase in the content of coarse particles having a size of 0.20 μm or more is suppressed. The colloidal silica can be used very suitably as an abrasive for polishing.

[0015] The production method of the present invention is a method for producing the citric acid-containing colloidal silica of the present invention, which includes (1) step 1 of preparing a mother liquid containing an alkali catalyst, an alcohol, and water, (2) step 2 of adding a raw material solution containing an alkoxysilane and an alcohol to the mother liquid to prepare a reaction liquid, and (3) step 3 of adding citric acid to the reaction liquid. According to such a production method of the present invention, the citric acid-containing colloidal silica of the present invention can be produced.

[0016] The colloidal silica of the present invention and its production method will be described in detail below.

[0017] 1. Colloidal silica containing citric acid The citric acid-containing colloidal silica of the present invention is a citric acid-containing colloidal silica containing silica particles and citric acid, wherein: (1) the silanol group density per unit surface area of ​​the silica particles is 1.6×10 19 ~14.5×10 19 pieces / m 2 (2) the content of the citric acid per unit surface area of ​​the silica particles is 1.0 to 10.0 μg / m 2 It is.

[0018] The silica particles contained in the colloidal silica of the present invention have a silanol group density per unit surface area of ​​1.6×10 19 ~14.5×10 19 pieces / m 2 The silanol group density is 1.6×10 19If the silanol group density is less than 14.5×10, the dispersion stability of the silica particles decreases, and the silica particles aggregate to generate coarse particles when a shear force is applied. 19 pieces / m 2 If the silanol group density exceeds 1.8×10, the dispersion stability of the silica particles decreases, and the silica particles aggregate to generate coarse particles when shear force is applied. 19 ~14.2×10 19 pieces / m 2 is preferred, 1.9×10 19 ~14.0×10 19 pieces / m 2 is more preferred.

[0019] In this specification, the silanol group density per unit surface area of ​​silica particles contained in colloidal silica is measured by the following measurement method.

[0020] (Method for measuring silanol group density per unit surface area of ​​silica particles) [Step 1] The colloidal silica is centrifuged at 77,400 G, 5°C, and for 90 minutes, and the resulting precipitate is dried under reduced pressure at 60°C and a gauge pressure of -0.1 MPa or less for 90 minutes to obtain a dry silica powder.

[0021] [Step 2] The dried silica powder obtained in step 1 is solid 29 Analyzed by Si-DD / MAS-NMR. 29 A Si-NMR spectrum is obtained. In the NMR analysis, the DD-MAS method is used. As an NMR device, an ECZ500R manufactured by JEOL Ltd. or the like can be used. In addition, as a probe for detecting NMR signals, an 8 mm HXMAS probe manufactured by JEOL Ltd. or the like can be used.

[0022] [Step 3] The value obtained in step 2 29The Si-NMR spectrum data is analyzed, and the peak with a chemical shift of about -84 ppm when the signal of the silicon atom of tetramethylsilane is set to 0 ppm is designated as Q1, the peak with a chemical shift of about -92 ppm as Q2, the peak with a chemical shift of about -101 ppm as Q3, and the peak with a chemical shift of about -111 ppm as Q4. The signal areas a1, a2, a3, and a4 of Q1, Q2, Q3, and Q4, respectively, are calculated.

[0023] In analyzing the spectral data, optimization calculations are performed for each peak in the spectrum after Fourier transformation using the nonlinear least squares method with the center position, height, and half-width of the peak shape created by mixing Lorentzian and Gaussian waveforms as variable parameters.

[0024] In addition, Q1 is thought to be derived from a silicon atom with a coordination number of 1 adjacent to an oxygen atom, and the composition formula is SiO 1 / 2 Q2 is believed to be derived from a silicon atom with a coordination number of 2 adjacent to an oxygen atom, and can be expressed as SiO(OH)2 with a formula of 78.10 g / mol. Q3 is believed to be derived from a silicon atom with a coordination number of 3 adjacent to an oxygen atom, and can be expressed as SiO 3 / 2 It can be expressed as (OH) with a formula weight of 69.09g / mol. Q4 is thought to be derived from a silicon atom with a coordination number of 4 adjacent to an oxygen atom, and can be expressed as SiO2 with a formula weight of 60.08g / mol.

[0025] [Step 4]. From the signal areas a1, a2, a3, and a4 and the formula weights of the Q1, Q2, Q3, and Q4 components obtained in step 3, the amount of silanol groups per unit weight of the silica particles is calculated using the following formula. Amount of silanol groups per unit weight of silica particles (×10 21 pieces / g) ={[(a1×3)+(a2×2)+(a3×1)]×N A ×10 -21} ÷{(a1×87.11)+(a2×78.10)+(a3×69.09)+(a4×60.08)} Here, N A is Avogadro's number: 6.022 × 10 23 Represents.

[0026] [Step 5] The silanol group density per unit surface area of ​​the silica particles is calculated using the following formula from the amount of silanol groups per unit weight of the silica particles calculated in step 4 and the BET specific surface area of ​​the silica particles obtained by the measurement method described below. The silanol group density per unit surface area of ​​silica particles (×10 19 pieces / m 2 ) = [silanol group amount per unit weight of silica particles (× 10 21 pieces / g)×10 2 ] ÷BET specific surface area (m 2 / g)

[0027] The BET specific surface area of ​​the silica particles contained in the colloidal silica of the present invention is 20 to 300 m 2 / g is preferable, and 25 to 200m 2 / g is more preferable, and 30 to 150m 2 / g is even more preferable. When the lower limit of the BET specific surface area is within the above range, the flatness of the surface of a substrate to be polished is further improved when the colloidal silica of the present invention is used as an abrasive for polishing. When the upper limit of the BET specific surface area is within the above range, the dispersion stability of the silica particles is further improved.

[0028] In this specification, the BET specific surface area of ​​the silica particles contained in the colloidal silica is measured by the following measurement method.

[0029] (BET specific surface area) Colloidal silica is 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 (m 2 / g) is measured.

[0030] The average primary particle size of the silica particles contained in the colloidal silica of the present invention is preferably 135 nm or less, more preferably 110 nm or less, and even more preferably 90 nm or less. When the upper limit of the average primary particle size is in the above range, the flatness is further improved when polished using the colloidal silica of the present invention. In addition, the average primary particle size of the silica particles is preferably 5 nm or more, more preferably 10 nm or more, and even more preferably 20 nm or more. When the lower limit of the average primary particle size of the silica particles is in the above range, the storage stability of the colloidal silica is further improved. The average primary particle size of the silica particles is measured by the following measurement method.

[0031] (Method of measuring average primary particle size) Colloidal silica is 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 is measured using the prepared measurement sample. The true specific gravity of silica is 2.2, and the formula is 2727 / BET specific surface area (m 2 / g) is converted to the average primary particle size (nm) of the silica particles in the colloidal silica.

[0032] The average secondary particle diameter of the silica particles contained in the colloidal silica of the present invention is 10 to 200 nm. If the average secondary particle diameter is less than 10 nm, the dispersion stability of the silica particles decreases, and coarse particles are likely to be generated when shear force is applied. If the average secondary particle diameter exceeds 200 nm, polishing scratches are likely to be caused on the surface of the substrate to be polished when used as a polishing abrasive. The average secondary particle diameter is preferably 20 to 170 nm, more preferably 30 to 150 nm. The average secondary particle diameter of the silica particles is measured by the following measurement method.

[0033] (Method for measuring average secondary particle size) As a sample for measuring the average secondary particle size, colloidal silica is added to a 0.3% by mass citric acid aqueous solution and homogenized to a silica concentration of 0.8% by mass. The average secondary particle size (nm) is measured using the measurement sample by dynamic light scattering (Otsuka Electronics Co., Ltd. "ELSZ-2000S").

[0034] The association ratio of the silica particles contained in the colloidal silica of the present invention is preferably 3.0 or less, more preferably 2.5 or less, and even more preferably 2.0 or less. When the upper limit of the association ratio is within the above range, the silica particles can be more stably dispersed, and the occurrence of aggregation that causes defects during polishing is further suppressed. In addition, the association ratio of the silica particles is preferably 1.0 or more, more preferably 1.2 or more, and even more preferably 1.4 or more. When the lower limit of the association ratio of the silica particles is within the above range, the polishing performance when the colloidal silica of the present invention is used as an abrasive is further improved.

[0035] (Calculation method of association ratio) The association ratio of the silica particles is calculated using the following formula from the average primary particle size and average secondary particle size of the silica particles obtained by the above-mentioned measurement method. Association ratio = average secondary particle size of silica particles (nm) ÷ average primary particle size of silica particles (nm)

[0036] The colloidal silica of the present invention contains silica particles having a silanol group amount per unit weight of 1.5×10 21 ~5.0×10 21 pcs / g is preferable, 1.8×10 21 ~4.7×10 21 pcs / g is more preferable, 2.0×10 21 ~4.5×10 21 When the amount of silanol groups per unit weight is within the above range, the silica particles are appropriately stabilized by the interaction between the silica particles and citric acid, allowing the silica particles to be more stably dispersed, and the occurrence of aggregation that causes defects during polishing is further suppressed.

[0037] The content of silica particles in the colloidal silica of the present invention is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, based on 100% by mass of colloidal silica. The content of silica in the colloidal silica is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less, based on 100% by mass of colloidal silica. By setting the lower limit of the content of silica particles in the colloidal silica within the above range, the polishing performance when the colloidal silica is used as an abrasive is further improved. By setting the upper limit of the content of silica particles in the colloidal silica within the above range, the dispersion stability of the silica particles is further improved.

[0038] The colloidal silica of the present invention has a citric acid content per unit surface area of ​​the silica particles of 1.0 to 10.0 μg / m 2 The citric acid content is 1.0 μg / m 2 If the content of citric acid is less than 10.0 μg / m, the dispersion stability of the silica particles decreases, and the silica particles aggregate to generate coarse particles when shear force is applied. 2 If the content of citric acid exceeds 1.5 μg / m, the dispersion stability of the silica particles decreases, and the silica particles aggregate to generate coarse particles when shear force is applied. 2 More than 2.0 μg / m is preferable. 2 More preferably, the citric acid content is 9.5 μg / m 2 Less than 9.0 μg / m is preferred 2 The following is more preferred:

[0039] In this specification, the content of citric acid per unit surface area of ​​silica particles is measured by the following measurement method.

[0040] (Citric acid content per unit surface area of ​​silica particles) [Step 1] 5 g of colloidal silica is placed in a crucible, 9 mL of 38% hydrofluoric acid is added, and the mixture is heated on a hot plate at 100° C. for 6 hours to decompose and evaporate the silica.

[0041] [Step 2] Add 6 mL of 0.1 mol / L diammonium hydrogen phosphate aqueous solution to dissolve the residue in the crucible, and recover the entire amount of the aqueous solution. After recovery, add additional 0.1 mol / L diammonium hydrogen phosphate aqueous solution to the aqueous solution to dilute it to a total amount of 10 g, and use it as the diluted solution for measurement.

[0042] [Step 3] The measurement dilution obtained in step 2 is analyzed by liquid chromatography, and the citric acid concentration is measured using the absolute calibration curve method. The liquid chromatography apparatus can be Shimadzu LC-2010CHT, and the chromatography column can be GL Science Inertsil ODS-3 or the like.

[0043] [Step 4] The citric acid concentration of the measurement diluted solution obtained in step 3 is defined as C (μg / g), and the citric acid content per unit surface area of ​​the silica particles is calculated using the following formula from the silica particle content and the BET specific surface area of ​​the silica particles obtained by the above-mentioned measurement method. Citric acid content per unit surface area of ​​silica particles (μg / m 2 ) = {C (μg / g) × 10 ÷ (5 × silica particle content (mass%) ÷ 100)} ÷BET specific surface area of ​​silica particles (m 2 / g)

[0044] The pH of the colloidal silica of the present invention may be appropriately set according to the use of the colloidal silica, and is not particularly limited, but is preferably 2.0 or more, more preferably 3.0 or more. The pH is preferably 11.0 or less, more preferably 10.0 or less. By setting the lower limit of the pH in the above range, the long-term dispersion stability of the silica particles of the colloidal silica is further improved. By setting the upper limit of the pH in the above range, the long-term dispersion stability of the colloidal silica is further improved.

[0045] The colloidal silica of the present invention preferably contains metal impurities such as sodium, potassium, iron, aluminum, calcium, magnesium, titanium, nickel, chromium, copper, zinc, lead, silver, manganese, and cobalt at 1 ppm or less. If the content of metal impurities exceeds 1 ppm, the citric acid contained in the colloidal silica is coordinated to the metal ion, and as a result, the amount of citric acid adsorbed to the silica particle surface decreases, and the dispersion stability of the silica particles decreases. If the content of metal impurities is 1 ppm or less, a sufficient amount of citric acid is adsorbed to the silica particle surface, and the dispersion stability of the silica particles is improved. In addition, if the content of metal impurities is 1 ppm or less, the risk of metal contamination decreases, and therefore the colloidal silica can be suitably used for polishing electronic materials and the like.

[0046] (Method of measuring metal impurity content) The content of metal impurities can be measured using an atomic absorption spectrometer.

[0047] The colloidal silica of the present invention can be used as an abrasive (CMP) for semiconductor devices such as semiconductor wafers. In addition, it can be used as a property improver in the fields of paper, fiber, steel, etc., and can also be used as an additive for fillers, external toner additives, etc. by drying to form a powder.

[0048] 2. Manufacturing method of colloidal silica The method for producing colloidal silica of the present invention is characterized by comprising: (1) step 1 of preparing a mother liquid containing an alkali catalyst, an alcohol, and water; (2) step 2 of adding a raw material solution containing an alkoxysilane and an alcohol to the mother liquid to prepare a reaction liquid; and (3) step 3 of adding citric acid to the reaction liquid. The production method of the present invention has the above-mentioned configuration, and thus can suitably produce the above-mentioned citric acid-containing colloidal silica of the present invention.

[0049] Each step of the production method of the present invention will be described in detail below.

[0050] (Process 1) Step 1 is a step of preparing a mother liquor containing an alkali catalyst, an alcohol, and water.

[0051] The alkali catalyst is not particularly limited, and is preferably at least one amine selected from the group consisting of primary amines, secondary amines, and tertiary amines. As such an amine, an amine represented by the following general formula (X) can be suitably used. 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.

[0052] The amine may be R a , R b , R c In the case where all of the above are hydrogen, that is, ammonia, can be suitably used.

[0053] R a , R b , R c may be the same or different. a , R b , R c may be linear, branched or cyclic.

[0054] 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 linear alkyl groups include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, and an octyl group. 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 n-propyl, n-hexyl, 2-ethylhexyl, and n-octyl groups.

[0055] The number of carbon atoms of the cyclic alkyl group may be, for example, 3 to 12, and is 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.

[0056] R in the above general formula (X) a , R b , R cIn the above, the alkyl group may be substituted. The number of the substituents may be, for example, 0, 1, 2, 3, 4, etc., preferably 0, 1 or 2, more preferably 0 or 1. An alkyl group having 0 substituents is an unsubstituted alkyl group. Examples of the substituents 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 a plurality of substituents, the substituents may be the same or different.

[0057] R in the above general formula (X) a , R b , R c is preferably a linear or branched alkyl group having 1 to 8 carbon atoms (preferably 1 to 6 carbon atoms) which may be substituted. a , R b , R c may be 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.

[0058] 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 an alkoxy-substituted linear or branched alkyl group having 1 to 12 carbon atoms. In one embodiment, the amine may be at least one amine selected from the group consisting of 3-ethoxypropylamine, pentylamine, hexylamine, dipropylamine, and triethylamine.

[0059] The above amines may be used alone or in combination of two or more.

[0060] The content of the alkali catalyst in the mother liquor is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and even more preferably 0.5% by mass or more, based on 100% by mass of the mother liquor. The lower limit of the content of the alkali catalyst is within the above range, which makes it easier to control the particle size of the silica particles. The upper limit of the content of the alkali catalyst in the mother liquor is not particularly limited, and may be 3.0% by mass or less, 2.5% by mass or less, or 2.0% by mass or less.

[0061] The alcohol is not particularly limited, and examples thereof include methanol, ethanol, isopropanol, n-butanol, and isobutanol.

[0062] The content of the alcohol in the mother liquor is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, based on 100% by mass of the mother liquor. The lower limit of the alcohol content is within the above range, which makes it easier to control the particle size of the silica particles. The upper limit of the alcohol content in the mother liquor is not particularly limited, and may be 95% by mass or less, 90% by mass or less, or 87% by mass or less.

[0063] The water content in the mother liquor is preferably 10.0 to 14.5% by mass, more preferably 11.0 to 14.0% by mass, and even more preferably 12.0 to 13.5% by mass. If the water content in the mother liquor is less than the lower limit of the above range, aggregates of silica particles are likely to be formed. If the water content in the mother liquor is more than the upper limit of the above range, the silanol group density per unit surface area of ​​the silica particles is reduced, and the dispersion stability of the silica particles is reduced.

[0064] The method for preparing the mother liquor is not particularly limited, and an alkali catalyst and an alcohol may be added to water and stirred by a conventionally known method.

[0065] The pH of the mother liquor is not particularly limited, but is preferably 9.5 or more, more preferably 10.0 or more. By setting the lower limit of the pH of the mother liquor within the above range, it becomes easier to control the particle size. The upper limit of the pH of the mother liquor is not particularly limited, but is about 11.5 or less, about 11.0 or less.

[0066] By the above-described step 1, a mother liquor containing an alkali catalyst, an alcohol, and water is prepared.

[0067] (Process 2) Step 2 is a step of adding a raw material solution containing an alkoxysilane and an alcohol to the mother liquor to prepare a reaction liquid.

[0068] The alcohol used in step 2 can be the same as the alcohol described in step 1 above.

[0069] The content of alcohol in the raw material solution is not particularly limited as long as colloidal silica can be formed, and can be appropriately adjusted. The content of alcohol is preferably 10% by mass or more, more preferably 15% by mass or more, based on 100% by mass of the raw material solution. The content of alcohol is preferably 40% by mass or less, more preferably 30% by mass or less, based on 100% by mass of the raw material solution.

[0070] The alkoxysilane is not particularly limited, and an alkoxysilane represented by the following general formula (1) can be used. A derivative of the alkoxysilane can also be used.

[0071] Si(OR)4(1) [In the formula, R is an alkyl group, preferably a lower alkyl group having 1 to 8 carbon atoms, and more preferably a lower alkyl group having 1 to 4 carbon atoms.]

[0072] Examples of R include methyl, ethyl, propyl, isopropyl, butyl, pentyl, and hexyl groups, and R is preferably tetramethoxysilane in which R is a methyl group, tetraethoxysilane in which R is an ethyl group, and tetraisopropoxysilane in which R is an isopropyl group. Examples of alkoxysilane derivatives include low condensates obtained by partially hydrolyzing alkoxysilane. In the present invention, it is preferable to use tetramethoxysilane because it is easy to control the hydrolysis rate and there is little residual unreacted material.

[0073] The content of alkoxysilane in the raw material solution is not particularly limited as long as it can form colloidal silica, and can be adjusted appropriately. The content of alkoxysilane is preferably 50% by mass or more, more preferably 70% by mass or more, based on 100% by mass of the raw material solution. The content of alkoxysilane is preferably 95% by mass or less, more preferably 90% by mass or less, based on 100% by mass of the raw material solution.

[0074] The method for preparing the raw material solution is not particularly limited, and it is sufficient to mix an alkoxysilane and an alcohol and stir them by a conventionally known method.

[0075] In step 2, the raw material solution is added to the mother liquor to prepare a reaction liquid.

[0076] The rate at which the raw material solution is added to the mother liquor is not particularly limited, but is preferably a constant rate, preferably 1.0 to 20 g / min / kg of mother liquor, more preferably 2.0 to 15 g / min / kg of mother liquor, and even more preferably 3.0 to 10 g / min / kg of mother liquor.

[0077] In step 2, the temperature of the reaction solution is preferably 15 to 25°C. If the temperature of the reaction solution is below the lower limit of the above range, the silanol group density per unit surface area of ​​the silica particles becomes excessively high, and the dispersion stability of the silica particles decreases. If the temperature of the reaction solution is higher than the upper limit of the above range, the silanol group density per unit surface area of ​​the silica particles becomes low, and the dispersion stability of the silica particles decreases.

[0078] In step 2, the concentration of water in the reaction solution is preferably 14.5% by mass or less, more preferably 14.0% by mass or less. By setting the upper limit of the concentration of water in the reaction solution within the above range, the silanol group density per unit surface area of ​​the silica particles is increased, and the dispersion stability of the silica particles is improved. The lower limit of the concentration of water in the reaction solution is not particularly limited, and may be, for example, 5% by mass.

[0079] In step 2 described above, the raw material solution containing alkoxysilane and alcohol is added to the mother liquor to prepare the reaction liquid.

[0080] (Step 3) Step 3 is a step of adding citric acid to the above reaction solution.

[0081] The method for adding citric acid to the reaction liquid is not particularly limited, and citric acid may be added dropwise to the reaction liquid by a conventionally known method and then stirred.

[0082] In step 3, the amount of citric acid added per 1 g of silica particles when citric acid is added to the reaction solution is preferably 50 to 600 μg / g, more preferably 100 to 595 μg / g. When the lower limit of the amount of citric acid added is within the above range, the dispersion stability of the silica particles is further improved, aggregation of the silica particles is suppressed, and the generation of coarse particles when a shear force is applied can be further suppressed. When the upper limit of the amount of citric acid added is within the above range, the dispersion state is further improved, and the generation of coarse particles when a shear force is applied can be further suppressed.

[0083] Citric acid is added to the reaction liquid in the above-described step 3. The colloidal silica of the present invention can be produced by the above-described step 3.

[0084] (Step 4) The production method of the present invention may further include step 4 of concentrating the silica particle concentration of the citric acid-containing colloidal silica and replacing the solvent with water after step 3. By including step 4 in the production method of the present invention, the solvent containing impurities and the like in the colloidal silica prepared in step 3 is newly replaced with water, the dispersion stability of the silica particles in the colloidal silica is further improved, the aggregation of the silica particles is further suppressed, and the generation of coarse particles when a shear force is applied can be further suppressed.

[0085] The method for concentrating the silica particle concentration of colloidal silica is not particularly limited, and for example, a method in which colloidal silica is heated by a conventionally known method to evaporate the solvent can be mentioned.

[0086] The heating temperature when concentrating the silica particle concentration of the colloidal silica is not particularly limited as long as it is possible to evaporate the solvent of the colloidal silica, and is preferably 50 to 100°C.

[0087] The heating time for concentrating the silica particle concentration of the colloidal silica is preferably 0 to 15 hours. If the heating time exceeds the upper limit of the above range, the silanol group density per unit surface area of ​​the silica particles decreases, and the dispersion stability of the silica particles decreases.

[0088] The method for replacing the solvent of colloidal silica with water is not particularly limited, and examples thereof include a method in which water is added to concentrated colloidal silica and heated by a conventionally known method to replace the solvent with water.

[0089] The heating temperature when replacing the solvent of the colloidal silica with water is preferably 60 to 100° C. If the heating temperature is below the lower limit of the above range, the alcohol contained in the colloidal silica cannot be sufficiently distilled off. If the heating temperature is above the upper limit of the above range, the silanol group density per unit surface area of ​​the silica particles decreases, and the dispersion stability of the silica particles decreases.

[0090] The heating time when replacing the solvent of the colloidal silica with water is preferably 0 to 15 hours. If the heating time exceeds the upper limit of the above range, the silanol group density per unit surface area of ​​the silica particles decreases, and the dispersion stability of the silica particles decreases.

[0091] In step 4, the total heating time during the concentration and the substitution with water, i.e., the heating time of the colloidal silica in step 4, is preferably 15 hours or less, more preferably 13 hours or less. If the heating time in step 4 exceeds the upper limit of the above range, the silanol group density per unit surface area of ​​the silica particles decreases, and the dispersion stability of the silica particles decreases.

[0092] By the above-described step 4, the concentration of silica particles in the colloidal silica is concentrated, and the solvent is replaced with water.

[0093] The above-mentioned citric acid-containing colloidal silica of the present invention can be produced by the production method having the steps described above. EXAMPLES

[0094] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these.

[0095] Example 1 (Production of citric acid-containing colloidal silica) A mother liquid containing 11693 parts by mass of a mixture of 1186 parts by mass of pure water, 452 parts by mass of 26% by mass aqueous ammonia, and 10055 parts by mass of methanol was prepared. Next, 1552 parts by mass of a raw material solution containing 1236 parts by mass of tetramethoxysilane and 316 parts by mass of methanol was injected into the mother liquid at a constant rate over 25 minutes while maintaining the liquid temperature in the reaction system at 23°C, to prepare a silica sol reaction liquid containing water and methanol as a dispersion medium. 7.673 parts by mass of a 2% by mass aqueous citric acid solution was added to the silica sol reaction liquid and stirred and mixed for 30 minutes. 4317 parts by mass of the silica sol after stirring and mixing was concentrated while being heated and distilled under normal pressure. The heating time in the concentration step was 10.0 hours. This concentrated liquid was heated and distilled under normal pressure while adding pure water to keep the volume constant, and the methanol and ammonia in the concentrated liquid were replaced with water, and when the pH became 8 or less, the dropping of pure water and heating were stopped to produce citric acid-containing colloidal silica. The heating time in the water replacement step was 3.0 hours.

[0096] Example 2 A citric acid-containing colloidal silica was produced under the same production conditions as in Example 1, except that the heating time in the concentration step was 7.0 hours and the heating time in the water replacement step was 2.0 hours.

[0097] Example 3 A citric acid-containing colloidal silica was produced under the same production conditions as in Example 2, except that the heating time in the concentration step was 5.0 hours.

[0098] Example 4 Citric acid-containing colloidal silica was produced under the same production conditions as in Example 2, except that the amount of 2 mass % citric acid aqueous solution added to the silica sol reaction liquid was 3.215 parts by mass.

[0099] Example 5 Citric acid-containing colloidal silica was produced under the same production conditions as in Example 2, except that the amount of 2 mass % citric acid aqueous solution added to the silica sol reaction liquid was 11.936 parts by mass.

[0100] Example 6 Citric acid-containing colloidal silica was produced under the same production conditions as in Example 2, except that the liquid temperature in the reaction system was kept at 25°C when the raw material solution was poured into the mother liquor, and the amount of 2 mass% citric acid aqueous solution added to the silica sol reaction liquid was 14.470 parts by mass.

[0101] Example 7 Citric acid-containing colloidal silica was produced under the same production conditions as in Example 2, except that the liquid temperature in the reaction system was kept at 18°C ​​when the raw material solution was poured into the mother liquor, and the amount of 2 mass% citric acid aqueous solution added to the silica sol reaction liquid was 4.409 parts by mass.

[0102] Comparative Example 1 A mother liquid containing 11,693 parts by mass of a mixture of 1,712 parts by mass of pure water, 452 parts by mass of 26% by mass ammonia water, and 9,529 parts by mass of methanol was prepared. Next, 1,552 parts by mass of a raw material solution containing 1,236 parts by mass of tetramethoxysilane and 316 parts by mass of methanol was injected into the mother liquid at a constant rate over 25 minutes while maintaining the liquid temperature in the reaction system at 23°C, to prepare a silica sol reaction liquid containing water and methanol as a dispersion medium. 7.673 parts by mass of a 2% by mass aqueous citric acid solution was added to the silica sol reaction liquid and stirred and mixed for 30 minutes. 4,317 parts by mass of the silica sol after stirring and mixing was concentrated while being heated and distilled under normal pressure. The heating time in the concentration step was 7.0 hours. This concentrated liquid was heated and distilled under normal pressure while adding pure water to keep the volume constant, and the methanol and ammonia in the concentrated liquid were replaced with water, and when the pH became 8 or less, the dropping of pure water and heating were stopped to produce citric acid-containing colloidal silica. The heating time in the water replacement step was 2.0 hours.

[0103] Comparative Example 2 A mother liquid containing 11693 parts by mass of a mixture of 952 parts by mass of pure water, 452 parts by mass of 26% by mass ammonia water, and 10289 parts by mass of methanol was prepared. Next, 1552 parts by mass of a raw material solution containing 1236 parts by mass of tetramethoxysilane and 316 parts by mass of methanol was injected into the mother liquid at a constant rate over 25 minutes while maintaining the liquid temperature in the reaction system at 14°C, to prepare a silica sol reaction liquid containing water and methanol as a dispersion medium. 7.673 parts by mass of a 2% by mass aqueous citric acid solution was added to the silica sol reaction liquid and stirred and mixed for 30 minutes. 4317 parts by mass of the silica sol after stirring and mixing was concentrated while being heated and distilled under normal pressure. The heating time in the concentration step was 7.0 hours. This concentrated liquid was heated and distilled at normal pressure while adding pure water to keep the volume constant, and the methanol and ammonia in the concentrated liquid were replaced with water, and when the pH became 8 or less, the dropping of pure water and heating were stopped to produce citric acid-containing colloidal silica. The heating time in the water replacement step was 2.0 hours.

[0104] Comparative Example 3 Citric acid-containing colloidal silica was produced under the same production conditions as in Example 2, except that the amount of 2 mass % citric acid aqueous solution added to the silica sol reaction liquid was 0.706 parts by mass.

[0105] Comparative Example 4 Citric acid-containing colloidal silica was produced under the same production conditions as in Example 2, except that the amount of 2 mass % citric acid aqueous solution added to the silica sol reaction liquid was 15.347 parts by mass.

[0106] Regarding the production conditions of colloidal silica described in Examples 1 to 7 and Comparative Examples 1 to 4, the injection amount of the raw material solution, the tetramethoxysilane (TMOS) concentration of the raw material solution, the amount of mother liquor, the water concentration of the mother liquor, the ammonia concentration of the mother liquor, the injection rate of the raw material solution into the mother liquor, the reaction temperature, the maximum water concentration during the reaction, the amount of citric acid added to the reaction solution, the heating time in the concentration step, the heating time in the water replacement step, and the total heating time in the concentration step and the water replacement step are shown in Table 1.

[0107] Evaluation method The colloidal silica of the examples and comparative examples obtained as described above were evaluated by the following methods.

[0108] (Silanol group density per unit surface area of ​​silica particles, amount of silanol groups per unit weight of silica particles) The silanol group density per unit surface area of ​​the silica particles was evaluated by the following procedure.

[0109] [Step 1] The colloidal silica was centrifuged at 77,400 G, 5° C., and 90 minutes, and the resulting precipitate was dried under reduced pressure at 60° C. and a gauge pressure of −0.1 MPa or less for 90 minutes to obtain a dried silica powder.

[0110] [Step 2] The dried silica powder obtained in step 1 is solid 29 Analyzed by Si-DD / MAS-NMR. 29 A Si-NMR spectrum was obtained. The NMR analysis was performed using the DD-MAS method. The NMR device used was the ECZ500R manufactured by JEOL Ltd. The probe used for detecting the NMR signal was the 8 mm HXMAS probe manufactured by JEOL Ltd.

[0111] [Step 3] The value obtained in step 2 29 The Si-NMR spectrum data was analyzed, and the peak with a chemical shift of about -84 ppm, when the signal of the silicon atom of tetramethylsilane was set to 0 ppm, was designated as Q1, the peak with a chemical shift of about -92 ppm as Q2, the peak with a chemical shift of about -101 ppm as Q3, and the peak with a chemical shift of about -111 ppm as Q4, and the signal areas a1, a2, a3, and a4 of Q1, Q2, Q3, and Q4, respectively, were calculated.

[0112] In analyzing the spectral data, optimization calculations were performed for each peak in the spectrum after Fourier transformation using the nonlinear least squares method with the center position, height, and half-width of the peak shape created by mixing Lorentzian and Gaussian waveforms as variable parameters.

[0113] [Step 4] From the signal areas a1, a2, a3, and a4 and the formula weights of the Q1, Q2, Q3, and Q4 components obtained in step 3, the amount of silanol groups per unit weight of the silica particles was calculated using the following formula. Amount of silanol groups per unit weight of silica particles (×10 21 pieces / g) ={[(a1×3)+(a2×2)+(a3×1)]×N A ×10 -21} ÷{(a1×87.11)+(a2×78.10)+(a3×69.09)+(a4×60.08)} Here, N A is Avogadro's number: 6.022 × 10 23 Represents.

[0114] [Step 5] The silanol group density per unit surface area of ​​the silica particles was calculated using the following formula from the amount of silanol groups per unit weight of the silica particles calculated in step 4 and the BET specific surface area of ​​the silica particles obtained by the measurement method described below. The silanol group density per unit surface area of ​​silica particles (×10 19 pieces / m 2 ) = [silanol group amount per unit weight of silica particles (× 10 21 pieces / g)×10 2 ] ÷BET specific surface area (m 2 / g)

[0115] (Citric acid content per unit surface area of ​​silica particles) The citric acid content per unit surface area of ​​the silica particles was evaluated by the following procedure.

[0116] [Step 1] 5 g of colloidal silica was placed in a crucible, 9 mL of 38% hydrofluoric acid was added, and the mixture was heated on a hot plate at 100° C. for 6 hours to decompose and evaporate the silica.

[0117] [Step 2] The residue in the crucible was dissolved by adding 6 mL of 0.1 mol / L diammonium hydrogen phosphate aqueous solution, and the entire amount of the aqueous solution was collected. After collection, an additional 0.1 mol / L diammonium hydrogen phosphate aqueous solution was added to the aqueous solution to dilute it to a total amount of 10 g, and this was used as the diluted solution for measurement.

[0118] [Step 3] The diluted solution for measurement obtained in step 2 was analyzed by liquid chromatography, and the citric acid concentration was measured using the absolute calibration curve method. The liquid chromatography apparatus used was Shimadzu Corporation's LC-2010CHT, and the chromatography column was GL Sciences' Inertsil ODS-3.

[0119] [Step 4] The citric acid concentration of the measurement diluted solution obtained in step 3 was designated as C (μg / g), and the citric acid content per unit surface area of ​​the silica particles was calculated using the following formula from the silica particle content and the BET specific surface area of ​​the silica particles obtained by the measurement method described below. Citric acid content per unit surface area of ​​silica particles (μg / m 2 ) = {C (μg / g) × 10 ÷ (5 × silica particle content (mass%) ÷ 100)} ÷BET specific surface area of ​​silica particles (m 2 / g)

[0120] (BET specific surface area) 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 (m 2 / g) was measured.

[0121] (Average primary particle size) 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 was measured using the prepared measurement sample. The true specific gravity of silica was 2.2, and the formula was 2727 / BET specific surface area (m 2 / g) was converted to the average primary particle size (nm) of the silica particles in the colloidal silica.

[0122] (Average secondary particle size) Colloidal silica was added to a 0.3% by mass aqueous solution of citric acid to prepare a homogenized solution with a silica concentration of 0.8% by mass. The average secondary particle diameter (nm) of the measurement sample was measured by dynamic light scattering (Otsuka Electronics Co., Ltd., "ELSZ-2000S").

[0123] (association ratio) The association ratio of the silica particles was calculated using the following formula from the average primary particle size and average secondary particle size of the silica particles obtained by the above-mentioned measurement method. Association ratio = average secondary particle size of silica particles (nm) ÷ average primary particle size of silica particles (nm)

[0124] (Silica particle content) The content of silica particles in the colloidal silica was calculated from the following formula, where 10.0 g of colloidal silica was dried on a hot plate at 150° C. and then heated at 800° C. for 1 hour to remove moisture, and the amount of the resulting solid content was defined as Wg. Content of silica particles in colloidal silica [%] = (W ÷ 10.0) × 100

[0125] (Metal impurity content) The content of metal impurities was measured using an atomic absorption spectrometer. The sum of the contents 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.

[0126] (Increase in LPC when shear force is applied by stirring) The colloidal silica of each of the Examples and Comparative Examples was subjected to stirring treatment by the method described below in order to apply a shear force. The number of large particles (LPC) of 0.2 μm or more was measured before and after the stirring treatment by the method described below, and the LPC increase rate when a shear force was applied by the stirring treatment was calculated using the following formula. Increase in LPC (%) when shear force is applied by stirring = {[LPC after mixing (# / mL) - LPC before mixing (# / mL)] × 100} ÷LPC before mixing (# / mL)

[0127] (Colloidal silica mixing process) In order to apply a shear force to the colloidal silica of each of the Examples and Comparative Examples, a stirring treatment was carried out in the following manner.

[0128] [Step 1] A 500 mL round-bottom flask was charged with 400 mL of colloidal silica.

[0129] [Step 2] Two paddle blades with a blade diameter of 8.6 mm were installed in the round-bottom flask prepared in step 1, and the colloidal silica was stirred at room temperature at a rotation speed of 350 rpm for 24 hours.

[0130] (LPC measurement) The colloidal silica was diluted with ultrapure water to a silica concentration of 1.0 mass%. The diluted solution was used as a measurement sample, and the number of coarse particles of 0.2 μm or more was measured using an Accusizer FX-nano manufactured by Particle Sizing System Inc. The measurement conditions were as follows:

[0131] <System Setup> ·Stirred Vessel Volume:13.22mL Sample Loop Volume: 0.52mL ·Autodilution delay time:3sec. ·Normal Speed ​​Flow Rate:15 mL / min <Sensor Setup Menu> ·FX-Nano HG Minimum Size:0.15μm ·FX-Nano HG Maximum Size:0.27μm ·FX-Nano HG Collection Time:60sec. ·HG Starting Concentration:8000# / mL

[0132] The results are shown in Table 1.

[0133] [Table 1]

Claims

1. A citric acid-containing colloidal silica containing silica particles and citric acid, (1) The silanol group density per unit surface area of ​​the silica particles is 1.6×10 １９ ~14.5 x 10 １９ pieces / m ２ and (2) The content of the citric acid per unit surface area of ​​the silica particles is 1.0 to 10.0 μg / m ２ That is, 2. The citric acid-containing colloidal silica according to claim 1 ,

2. 2. The citric acid-containing colloidal silica according to claim 1, wherein the silica particles have an average secondary particle size of 10 to 200 nm.

3. The BET specific surface area of ​​the silica particles is 20 to 300 m ２ 2. The citric acid-containing colloidal silica according to claim 1, wherein the citric acid-containing colloidal silica has a molecular weight of 1000 or more.

4. The amount of silanol groups per unit weight of the silica particles is 1.5×10 ２１ ~5.0 x 10 ２１ 2. The citric acid-containing colloidal silica according to claim 1, wherein the citric acid-containing colloidal silica has a molecular weight of 1.0 or more and a molecular weight of 1.0 or more.

5. 2. The citric acid-containing colloidal silica according to claim 1, wherein the content of the silica particles is 10 mass % or more.

6. A method for producing the citric acid-containing colloidal silica according to claim 1, comprising the steps of: (1) Step 1 of preparing a mother liquor containing an alkali catalyst, an alcohol and water; (2) a step 2 of adding a raw material solution containing an alkoxysilane and an alcohol to the mother liquid to prepare a reaction liquid; and (3) adding citric acid to the reaction solution; The manufacturing method according to claim 1,

7. The method according to claim 6 , wherein the step 2 is a step of adding the raw material solution to the mother liquor at a constant rate.

8. The method according to claim 6, further comprising, after the step 3, a step 4 of concentrating a silica particle concentration of the citric acid-containing colloidal silica and replacing the solvent with water.

9. The method according to claim 8 , wherein the step 4 is a step of heating the citric acid-containing colloidal silica for a heating time of 15 hours or less.

10. The method according to claim 6 , wherein in the step 2, the water concentration in the reaction solution is 14.5% by mass or less.

11. The method according to claim 6, wherein in the step 2, the temperature of the reaction solution is 15 to 25°C.

12. The method according to claim 6, wherein the amount of citric acid added per 1 g of silica particles is 50 to 600 μg / g.