JP2001048520A - Elongated silica sol and method for producing the same - Google Patents

Abstract

[57] [Abstract] [Problem] An ink-absorbing filler for printing paper, an agent for improving the spreadability of paint, a hydrophilic coating material for various material surfaces, a high-strength binder, and a raw material for high-purity silica gel and high-purity ceramics And a silica sol useful as a catalyst binder, an abrasive for electronic materials, and the like. SOLUTION: Elongated amorphous silica particles having an average diameter in a thickness direction of 5 to 100 nm and a length of 1.5 to 50 times the length thereof observed by an electron microscope are dispersed in a liquid dispersion. A silica sol characterized by the above-mentioned. A method for producing the silica sol is also disclosed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a silica sol and a method for producing the same. More specifically, the present invention relates to a silica sol in which elongated amorphous silica particles are dispersed in a liquid dispersion, and a method for producing the same. The silica sol obtained by the production method of the present invention is applied to an ink-absorbing filler for printing paper, a spreadability improving agent for paint, a hydrophilic coating material on various material surfaces, a high-strength binder, and the like due to its shape. it can. Further, silica sol containing no impurities is useful for high-purity silica gel, raw materials for high-purity ceramics, binders for catalysts, abrasives for electronic materials, and the like.

[0002]

2. Description of the Related Art A commercially available silica sol is produced from water glass as a raw material by a method such as neutralization with an acid or ion exchange. The silicic acid solution from which the alkali component has been removed by ion-exchanging the diluted water glass is called "activated silicic acid solution". When this activated silicic acid solution is heated in an alkaline range, it becomes a normal silica sol of spherical silica particles. There are several methods for producing silica sols of elongated particles, for example, U.S. Pat. No. 2,807,721 states that non-spherical silica sols can be formed by a specific hydrothermal treatment at 160-300.degree.
JP-A-1-317115 describes a method in which an alkali is added to an activated silicic acid solution to which a calcium salt or the like has been added, and then hydrothermal treatment at 60 to 250 ° C. is performed to produce silica sol of elongated particles. JP-A-4-187512 describes a method of preparing a silica sol having elongated particles by adding an active silicate solution containing an aluminum salt or the like to an alkali silicate.

[0003] A high-purity silica sol obtained by thermally decomposing silicon tetrachloride, which is obtained by dispersing high-purity silica fine powder containing almost no impurities such as metals in water, is used for polishing and the like. High-purity silica sol produced by a method of hydrolyzing an alkyl silicate in an alcohol solution containing ammonia is also used for polishing and the like. A method for obtaining the silica particles is described by Stever et al. In J. Colloid and Interface Sci. Vol. 26 (196
(8 years) P. 62-69, 0.28 mol / L of tetraethyl silicate was added to an alcohol solution containing several mol / L of ammonia and several mol / L to 15 mol / L of water. It is reported that silica particles having a particle size of 50 to 900 nm can be obtained by hydrolysis.

Japanese Patent Application Laid-Open No. 62-275005 discloses a water-alcohol dispersion containing seed particles (alcohols 35 to 97).
%) Is obtained by adding an alkyl silicate and hydrolyzing the mixture while maintaining alkalinity with ammonia or the like to obtain silica particles having a particle size of 0.1 to 10 μm. JP-A-63-74911 discloses that in an alcohol solution containing an alkaline catalyst such as ammonia in a molar ratio of 0.5 to 10 with respect to an alkyl silicate and water in a concentration of 5 to 20 mol / l, 30 alkyl silicates
By hydrolysis at a temperature of at least
A method for obtaining silica particles of m or less is described. The publication discloses that the higher the reaction temperature, the smaller the particle size of the formed silica.

Japanese Patent Application Laid-Open No. 61-209910 discloses that 0.5 to 1
0% quaternary ammonium hydroxide and 0.1% as dispersant.
It describes a method of obtaining a silica sol having a particle diameter of about 10 nm by a method of hydrolyzing a silicate ester in the presence of a surfactant of 01 to 0.001% and then concentrating at 60 to 70 ° C. JP-A-6-316407 discloses a reaction medium,
While maintaining the alkali concentration of 0.002 to 0.1 mol per liter and the water concentration of 30 mol or more per liter, the reaction medium contains 7 to 7 moles of Si atoms per mole of the alkali.
80 mol of alkyl silicate was added,
The alkyl silicate is hydrolyzed at a temperature equal to or higher than the boiling point of the reaction medium, and the polymerization of silicic acid generated by the hydrolysis is advanced to obtain a particle size of 3 to 100 n.
A method for producing colloidal silica having a uniform particle size of m is disclosed.

[0006]

In a method for obtaining silica sol by neutralizing or ion-exchanging water glass as a raw material,
In addition to the problem of impurities in water glass, there are complicated ion exchange steps and generation of large amounts of wastewater.
In the methods described in JP-A No. 15 and JP-A-7-118008, addition of a metal oxide is required for non-spheroidization, and a high-purity silica sol cannot be obtained. Silica fine powder obtained by the pyrolysis method of silicon tetrachloride is spherical particles, and can be fused and non-spheroidized by heat treatment. However, dispersibility in water is basically poor, and a monodispersed sol is obtained. I can't do that.

In the method of hydrolyzing an alkyl silicate in an alcohol, the formed silica particles have a particle size of 50 nm or more.
It has a size of several μm, is spherical or almost spherical, and does not have an elongated shape. When the particle size is 100 nm or less, the monodispersibility is not good. The above JP
In the method of 63-74911 or the method of JP-A-61-209910, the control of the produced silica particle diameter can be improved, but the particle shape cannot be elongated. The method disclosed in JP-A-6-31640 is a method of hydrolyzing an alkyl silicate in water. However, the particle diameter of the produced silica can be controlled, but the particle shape cannot be elongated. A solution obtained by hydrolyzing an alkyl silicate with an acid catalyst may be referred to as silica sol, but this silica does not have a particle shape and is polymerized with time to form an agar-like silica gel. The present invention provides a silica sol obtained from an alkyl silicate, which has a long and narrow shape rather than a conventional true spherical shape, and provides a high-purity and stable silica sol capable of responding to requests in various industrial fields and a method for producing the same. What you want to do.

[0008]

According to the present invention, particles are grown by hydrolyzing an alkyl silicate with an acid catalyst, adding an alkali catalyst so that the pH becomes 7 or more, and heating to promote polymerization of silicic acid. A silica sol particle having an average diameter in a thickness direction of 5 to 100 nm as observed by an electron microscope and a length of 1.5 to 50 times the length of the elongated amorphous silica particle in a liquid dispersion. Pertains to silica sol dispersed in Also, the present invention provides a silica concentration of 1 to 1.
8 mol / l, acid concentration 0.0018 to 0.18 mol / l, water concentration 2 to 30 mol / l, alkyl silicate hydrolyzed with an acid catalyst without using a solvent, silica concentration Is 0.2 to 1.5 mol /
Diluted with water so as to be in the range of 1 liter, and then an alkali catalyst was added and heated so that the pH became 7 or more to promote the polymerization of silicic acid, and the average diameter in the thickness direction by electron microscope observation was 5 to 100 nm. And the length is 1.5-5
The present invention relates to a method for producing a silica sol in which elongated amorphous silica particles having a length of 0 times are dispersed in a liquid dispersion.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The silica sol of the present invention is a silica sol obtained from an alkyl silicate and is characterized by its shape. Such a shape in the silica sol particles has an average diameter in the thickness direction of 5 to 100 nm by electron microscope observation.
And has an elongated shape with a length of 1.5 to 50 times the length. Its shape is a transmission electron microscope (T
EM) can be confirmed by observation. The method for producing an elongated silica sol according to the present invention is characterized in that the silica concentration is 1 to 8 mol / mol.
Liter, with an acid concentration of 0.0018 to 0.18 mol / l and a water concentration of 2 to 30 mol / l, alkyl silicate is hydrolyzed with an acid catalyst without using a solvent to produce silicic acid, Silica concentration of 0.2 to 1.5
The mixture is diluted with water so as to have a mole / liter range, and then an alkali catalyst is added so that the pH becomes 7 or more, and the mixture is heated to promote the polymerization of silicic acid to have a diameter in the thickness direction of 3 to 100 n.
In this method, silica sol is obtained by growing particles so as to obtain m.

The alkyl silicate used in the present invention is a silicic acid monomer or an alkyl ester of a silicic acid oligomer having a polymerization degree of 2 to 10. The alkyl group preferably has 1 to 3 carbon atoms. Further, mixed esters having different alkyl groups in the molecule, esters partially having an alkyl group, and mixtures of these alkyl silicates can also be used. Preferred examples of the alkyl silicate include tetramethyl silicate, tetraethyl silicate and the like. Degree of polymerization 2-10
Alkyl esters of silicic acid oligomers are more preferred because they generate less alcohol.

The acid catalyst used in the present invention is an inorganic acid such as hydrochloric acid or nitric acid, or an organic acid such as formic acid or acetic acid, and can be appropriately selected according to the use of the final product. Although a method of hydrolyzing an alkyl silicate with an acid catalyst is already known, it is an important feature of the present invention that a solvent such as alcohol is not used, and the composition of the hydrolysis system has a silica concentration of 1 to 1.
8 mol / l, acid concentration 0.0018-0.18 mol / l and water concentration 2-30 mol / l. Hydrolysis is an exothermic reaction, and increasing the temperature of the liquid promotes the reaction and ensures completion of the reaction. For this purpose, the composition is appropriately adjusted so that the hydrolysis is completed in about 30 minutes to 2 hours. When the hydrolysis is not completed only by the heat of reaction, heating is performed at about 40 to 60 ° C.

When the concentration of the alkyl silicate is 1 mol / l or less, the hydrolysis is difficult to be terminated, and when the concentration is 8 mol / l or more, the hydrolyzed liquid tends to gel and the handling is poor. When the acid concentration is 0.0018 mol / L or less, the catalytic effect is small and the hydrolysis requires a long time.
If it is 0.18 mol / L or more, the reaction is too rapid, and the solution is apt to gel, and the handling is not good. Water concentration is 2
When the amount is too small, the hydrolysis is difficult to be terminated. When the amount is too large, the temperature rise due to the heat of reaction is small and the hydrolysis is not easily terminated. Next, water is added to the hydrolyzate to dilute it. The silica in the solution after the hydrolysis is in the form of low-polymerized silicic acid, the degree of polymerization increases with time, and eventually silica gel is formed. To delay the polymerization, the hydrolyzate is diluted by adding water. This dilution also serves to lower the liquid temperature.

The concentration of silica in the diluted liquid is 0.2 to 1.
A range of 5 mol / l is preferred. If it is lower than this, an excessive load is applied to heating and concentration in a subsequent step, and if it is higher than this, gelation of the liquid is quick and lacks handling properties. Such a diluted hydrolyzate is hereinafter referred to as "active silicic acid solution". The activated silicic acid solution contains a small amount of a hydrolyzing acid catalyst, which is removed if necessary. An anion exchange resin can be suitably used for the removal. The acid catalyst, which is an anion component, can be removed by passing the activated silicic acid solution through a column of a resin regenerated into OH type with ammonia water or the like.

In the production of the present invention, the activated silicate solution is then
An alkali catalyst is added and heated to polymerize the silicic acid. Such a step of adding an alkali catalyst is usually performed also in a method for producing a silica sol using water glass as a raw material.For example, the method described in US Pat. Simultaneously with a specific addition speed, the method of JP-A-58-15022 is a method of adding an active silicic acid solution in an alkaline aqueous solution at a specific addition speed, JP-A-63-123807. Is a method of adding an active silicic acid solution to an aqueous alkali solution in the presence of an aluminum compound to form a strongly anionic silica sol, and the like, and any of these water glass production methods can be applied.

In the present invention, the alkalis specifically used are hydroxides of alkali metals such as ammonia, Na, K, and Li, water-soluble amines, and quaternary ammonium hydroxides. Hydroxides and quaternary ammonium hydroxides have high reactivity and are preferably used. Also, an aqueous solution of alkali silicate such as quaternary ammonium silicate such as sodium silicate and choline can be used. When it is desired to obtain a silica sol containing no alkali metal, a nitrogen-containing basic compound such as a quaternary ammonium hydroxide or a quaternary ammonium silicate as described above is used. As described above, there are many methods for mixing active silicate liquid with an alkali agent and heating to polymerize silicic acid to grow particles.Therefore, depending on which method is applied, the type and amount of the alkali agent need not be selected. must not. Therefore, the basic conditions are pH
The addition of an alkali catalyst and heating to promote the polymerization of silicic acid so as to be 7 or more is a component of the present invention. More preferably, the pH is 8 or more. The amount of the alkali agent when the acid catalyst as an anionic component is removed is 0.005 to 0.1 mol per 1 mol of silica. When the activated silicic acid solution is made alkaline and heated, the polymerization reaction of silicic acid can proceed. Then, silica core particles are generated in the reaction medium by the progress of the polymerization reaction of the silicic acid, and when the active silicic acid liquid is added while maintaining the alkalinity of the liquid by a method such as adding an alkali, the particles become thicker, The silica sol has a diameter in the thickness direction of 5 to 100 nm, preferably 10 to 50 nm. A method in which the entire amount of the alkali agent is put in the reaction medium in advance, and the active silicic acid solution is added thereto. The reverse is not possible. At this time, the amount of the alkali agent is 0.005 to 0.1 mol per 1 mol of silica.

It is preferable to use the silica sol prepared according to the present invention as seed particles and to use an activated silicate solution prepared from water glass by an ion exchange method as a silica source for thickening, because the raw material cost is reduced. This polymerization particle growth reaction can be performed under any of reduced pressure, normal pressure, and pressurized pressure. However, it is performed under sufficient stirring at a temperature of 60 ° C. or higher, preferably 80 ° C. to the boiling point of the reaction mixture. Simple and good. After the addition of the active silicic acid solution is completed, the temperature is further increased to 70 ° C or higher, preferably 80 to 200 ° C.
It is preferable to perform the heat aging for 30 minutes or more, preferably for about 1 to 6 hours. During or after this heat aging, an aluminum compound or the like may be added and deposited on the surface of the silica particles to change the properties of the particles.

The silica sol obtained by the method of the present invention can be concentrated by a usual method such as an evaporation method and an ultrafiltration method. This concentration results in a stable silica sol of up to about 40%. If necessary, by-product alcohol
It can be distilled off and washed out during the process of evaporation and ultrafiltration. The acid catalyst can also be washed out during the ultrafiltration process. Further, the acid catalyst and the alkali agent can be removed by an ion exchange method, and a silica sol substantially containing no components other than silica can be obtained. The silica sol from which the alkaline agent has been removed by the ion exchange method is subjected to the evaporation and ultrafiltration steps while adding an organic solvent, whereby the water is distilled off and washed out, and the silica sol dispersed in the organic solvent, that is, an organosol It can be.

The silica sol obtained by the production method of the present invention is based on its shape, and is made of an ink-absorbing filler for printing paper, a spreadability improving agent for paint, a hydrophilic coating material on various material surfaces, a high-strength binder. And so on. Furthermore, as a use of silica sol containing no impurities,
It is useful as a raw material for high-purity silica gel, high-purity ceramics, a binder for catalysts, an abrasive for electronic materials, and the like.

[0019]

EXAMPLES The present invention will be described more specifically with reference to the following examples. Example 1 100 g of pure water and 1 g of 35% hydrochloric acid were added and mixed to prepare a diluted hydrochloric acid solution. 400 g of a commercially available ethyl silicate oligomer ("Ethyl silicate 40" manufactured by Colcoat Co., silica concentration: 40% by weight) was charged into a 2 liter glass reaction vessel equipped with a stirrer, and the diluted hydrochloric acid solution was added with stirring. Initially, the two liquids were turbid without mixing, but were transparently mixed after the liquid temperature rose to 52 ° C. due to the heat of hydrolysis reaction. Fifty minutes after the start of hydrolysis, 500 g of pure water was added to dilute the mixture, which was then transferred to another container, diluted with 3500 g of pure water, and diluted with 4500 g of an active silicic acid solution (silica concentration = 0.6
Mol / l). The pH of this activated silicic acid solution is 2.
It was 6. The activated silicic acid solution was passed at a constant rate for 1 hour through a column filled with 200 ml of an anion exchange resin ("Amberlite IRA400" manufactured by Organo Co., Ltd.) previously regenerated to an OH type with ammonia water, and the column was cooled to 50%.
An activated silicate solution from which 00 g of chlorine ions had been removed was obtained. The pH of this activated silicic acid solution was 4.4. 5 with another stirrer
1000 g of the activated silicic acid solution was transferred to a liter container, and 30 g of a 10% aqueous choline solution was added with stirring to adjust the pH to 8.0.
And heated at 95 ° C. for 1 hour. Further, while maintaining the temperature at 95 ° C. and the pH at 9.8, 70 g of 10
% Choline aqueous solution and 4000 g of activated silicic acid solution were added simultaneously over 5 hours. After completion of the addition, the mixture was heated and aged at 95 ° C. for 1 hour. This liquid was a translucent liquid having a bluish color unique to silica sol. After cooling, the mixture was concentrated to a silica concentration of 20% by ultrafiltration to obtain about 0.8 kg of a silica sol dispersed in alcohol and water substantially containing no components other than silica and choline. The silica particles of this sol had an average diameter in the thickness direction of 8 nm as observed by electron microscopy, and had an elongated shape having a length five times as long.

FIG. 1 is an electron micrograph (magnification: 200,000, length from side to side of the photograph is 604 nm) showing the silica particles of the sol, and FIG. FIG. 3 is an electron micrograph (magnification: 400,000, magnification: 302 nm in lateral direction of the photograph) showing the particle structure of the particles. Further, this silica sol did not change in properties such as an increase in viscosity even after standing at room temperature for one year, and maintained a stable sol state.

Example 2 100 g of pure water and 2.9 g of 20% nitric acid were added and mixed to prepare a diluted nitric acid solution. 400 g of commercially available ethyl silicate oligomer ("Ethyl silicate 40" manufactured by Colcoat Co., Ltd., silica concentration: 40% by weight) was charged into a 1-liter glass reaction vessel equipped with a stirrer, and the diluted nitric acid solution was added with stirring. Initially, the two liquids were turbid without mixing, but were transparently mixed after the liquid temperature rose to 55 ° C. due to the heat of hydrolysis reaction. 60 minutes after the start of hydrolysis, 500 g of pure water was added to dilute the mixture, and the mixture was further stirred for 30 minutes. Then, the mixture was transferred to another container and diluted by adding 3000 g of pure water, and 4000 g of an active silicic acid solution (silica concentration = 0) was added. 0.7 mol / l). The pH of this activated silicic acid solution was 2.6. 500 g of the activated silicic acid solution was transferred to another 5 liter container equipped with a stirrer, and 9 g of a 20% aqueous solution of tetramethylammonium hydroxide was added thereto with stirring to adjust the pH to 8.0, followed by heating at 95 ° C. for 1 hour. Raise the temperature to 98 ° C and keep this temperature at 30.2
g of a 20% aqueous solution of tetramethylammonium hydroxide and 3
500 g of activated silicic acid solution was added simultaneously over 6 hours. After completion of the addition, the mixture was heated and aged at 98 ° C. for 1 hour. This liquid was a translucent liquid having a bluish color unique to silica sol. After cooling, the mixture was concentrated to a silica concentration of 10% by ultrafiltration to obtain about 1.6 kg of a silica sol dispersed in alcohol and water.

This sol has a pH of 9.2 and a pH of substantially 1
It contained no components other than 40 ppm HNO ₃ , 0.2% tetramethylammonium hydroxide and silica.
The silica particles of this sol had an average diameter of 18 nm in the thickness direction observed by an electron microscope and had an elongated shape having an average length three times as long. FIG. 3 is an electron micrograph (magnification: 200,000 ×, length from side to side of the photograph is 604 nm) showing the silica particles of the sol, and FIG. 4 is a particle of silica particles at different magnifications. Electron micrograph showing the structure (magnification: 400,000, magnification: lateral length of the photograph is 3
02 nm). Further, this silica sol did not change in properties such as an increase in viscosity even after standing at room temperature for one year, and maintained a stable sol state.

Example 3 200 g of pure water and 2 g of 17% hydrochloric acid were added and mixed to prepare a diluted hydrochloric acid solution. In a 2 liter glass reaction vessel equipped with a stirrer, 571 g of commercially available tetraethyl silicate ("Ethyl silicate 28" manufactured by Colcoat Co., Ltd., silica concentration: 2)
8% by weight), and the diluted hydrochloric acid solution was added thereto with stirring.
Initially, the two liquids were turbid without mixing, but were transparently mixed after the liquid temperature rose to 60 ° C. due to the heat of hydrolysis reaction. Forty minutes after the start of hydrolysis, 500 g of pure water was added to dilute the solution, transferred to another 5-liter container equipped with a stirrer, further diluted with 3500 g of pure water, and further diluted with about 4770 g of an active silicic acid solution (silica concentration = 0.56 mol). / Liter). The pH of this activated silicic acid solution was 2.6. While stirring, 36.4 g of a 10% aqueous solution of sodium hydroxide was added to adjust the pH to 8.0.
And heated at 95 ° C. for 1 hour. This liquid was a translucent liquid having a bluish color unique to silica sol. After cooling, the mixture was concentrated to a silica concentration of 15.8% by ultrafiltration to obtain a silica sol dispersed in about 1 kg of alcohol and water.

This sol has a pH of 9.2 and is substantially 7
It contained no components other than 0 ppm HCl, 0.08% sodium hydroxide and silica. The silica particles have an average diameter of 20 nm in the thickness direction observed by an electron microscope,
It was an elongated shape having a length of 20 times or 20 times or more thereof. FIG. 5 is an electron micrograph (magnification: 200,000 ×, length from side to side of the photograph is 604 nm) showing the silica particles of the sol, and FIG. 6 is a particle of silica particles at different magnifications. It is an electron micrograph (400,000 times magnification, the length of the photograph from side to side is 302 nm) showing the structure. Further, this silica sol did not change in properties such as an increase in viscosity even after standing at room temperature for one year, and maintained a stable sol state.

[0025]

Since the present invention is constructed as described above, the average diameter in the thickness direction measured by electron microscopy is 5 to 100 n.
m, and a silica sol characterized in that elongated amorphous silica particles having a length of 1.5 to 50 times the length thereof are dispersed in a liquid dispersion, can be efficiently produced.
Further, the obtained silica sol can be applied to an ink-absorbing filler for printing paper, a spreadability improving agent for paint, a hydrophilic coating material on the surface of various materials, a high-strength binder, etc. due to its shape. Further, silica sol containing no impurities is useful for high-purity silica gel, raw materials for high-purity ceramics, binders for catalysts, abrasives for electronic materials, and the like.

[Brief description of the drawings]

FIG. 1 is an electron micrograph (magnification: 200,000 ×, length from side to side in the horizontal direction of the photograph: 604 nm) showing silica particles of a sol obtained in Example 1.

FIG. 2 is an electron micrograph (magnification: 400,000 times, length from side to side of the photograph is 302 nm) of the sol obtained in Example 1, in which the magnification of the silica particles is changed.

FIG. 3 is an electron micrograph (magnification: 200,000 ×, length from side to side in the horizontal direction of the photograph: 604 nm) showing the silica particles of the sol obtained in Example 2.

FIG. 4 is an electron micrograph (magnification: 400,000, magnification from side to side of the photograph being 302 nm) of the sol obtained in Example 2 in which the magnification of the silica particles is changed.

FIG. 5 is an electron micrograph (magnification: 200,000, length from side to side in the horizontal direction of the photograph: 604 nm) showing the silica particles of the sol obtained in Example 3.

FIG. 6 is an electron micrograph (magnification: 400,000, magnification of the silica particle of the sol of the sol obtained in Example 3; length from side to side in the horizontal direction of the photograph is 302 nm).

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Claims (2)

[Claims] 1. Silica sol particles obtained by hydrolyzing an alkyl silicate with an acid catalyst, adding an alkali catalyst so as to have a pH of 7 or more and heating to promote the polymerization of silicic acid to grow the particles. The average diameter in the thickness direction by observation is 5 to 100 nm, and the length is 1.5 to 50 nm.
A silica sol in which amorphous silica particles having an elongated shape twice as long are dispersed in a liquid dispersion. 2. A silica concentration of 1 to 8 mol / l, an acid concentration of 0.0018 to 0.18 mol / l and a water concentration of 2 to 8 mol / l.
After hydrolyzing an alkyl silicate with an acid catalyst without using a solvent in a composition in the range of 30 mol / l, diluting with water so that the silica concentration is in the range of 0.2 to 1.5 mol / l, Then, an alkali catalyst is added so that the pH becomes 7 or more, and the polymerization of the silicic acid is advanced by heating, and the average diameter in the thickness direction by electron microscopic observation is 5 to 100 nm.
And a method for producing a silica sol, wherein elongated amorphous silica particles having a length of 1.5 to 50 times the length thereof are dispersed in a liquid dispersion.