WO2025230015A1 - Aluminum atom-containing hollow silica sol and method for producing same - Google Patents

Abstract

[Problem] To provide hollow silica particles that have high dispersion stability in a medium and are free from concerns such as outflow of alkali metals. [Solution] Provided are an aluminum atom-containing hollow silica sol, in which the amount of sulfuric acid contained in the hollow silica sol is 1 ppm to 150 ppm, and a method for producing the same.

Description

Aluminum atom-containing hollow silica sol and method for producing the same

The present invention relates to a sol in which hollow silica particles containing aluminum atoms are dispersed (aluminum atom-containing hollow silica sol), a method for producing the same, and a coating-forming composition.

Silica sol is used in a variety of fields as an abrasive, functional inorganic filler, and more. Although silica sol is stable and does not gel in alkaline conditions, when alkaline silica sol is blended into a composition that uses an organosilicon compound or resin as a binder, the alkaline condition can easily cause cloudiness and thickening, and there are also issues with gelling when mixed with alcohol or other substances when preparing organosilica sol through solvent substitution. On the other hand, in acidic conditions, the zeta potential of silica particles is small, resulting in little electrical repulsion. Although silica sol is unstable and prone to gelling, it is often required to use silica sol in acidic conditions, such as in acidic abrasives, raw materials for ceramic fibers, and chromium-based surface treatment agents.

One known method for improving the stability of silica sol in the acidic range is to modify the surface of silica particles with an aluminum compound. In this method, aluminosilicate sites are formed on the surface of silica particles by reaction between aluminate ions derived from the aluminum compound and silanol groups on the surface of the silica particles. The aluminosilicate sites impart a negative charge to the silica particles, i.e., increase the negative zeta potential of the silica particles, thereby improving the dispersion stability of the silica particles in the dispersion medium. This improves the compatibility of silica particles with highly polar organic solvents and charged resins in particular. For example, a method for producing an acidic silica sol has been disclosed (see Patent Document ₁ ), in which an aqueous alkali aluminate solution is added to a dispersion of solid silica particles so that the _Al2O3 / _SiO2 molar ratio is greater than 0.0006 and less than 0.004, and the resulting silica sol is heated at 80 to 250°C, followed by cation exchange.

Furthermore, hollow silica particles have a silica outer shell and a space inside the shell, and due to these characteristics, they have properties such as a low refractive index, low thermal conductivity (thermal insulation), and electrical insulation.
Hollow silica particles consist of a core corresponding to the hollow portion and an outer shell that forms the outside of the core. An aqueous dispersion of hollow silica particles can be obtained by forming a silica layer on the outside of a template particle in an aqueous medium and then removing the template particle.
For example, a method has been disclosed in which a core-shell particle having an aluminosilicate shell is produced by reacting a silane compound and an aluminum precursor with a Si/Al molar ratio in the range of 7 to 15 on a template core made of an organic polymer in a micellar or reverse micellar form, and then reacting this with a basic or acidic aqueous solution to simultaneously form pores in the shell (outer shell) and remove the core, followed by a hydrothermal reaction by heating at 160 to 200°C to produce a hollow silica sol with a densified shell (see Patent Document 2).

Japanese Unexamined Patent Publication No. 6-199515 Korean Patent No. 10-1659709

In order to stabilize silica sol containing hollow silica particles with cavities inside their shells, the particles are modified from the outside with an aluminum compound (i.e., impregnated with the aluminum compound). The aluminum compound penetrates the shells, and the aluminum compound remaining on the outside of the shells (impregnated) and the aluminum compound reaching the inside of the shells (impregnated) each form aluminosilicate sites. The alkali metals present in the silica particles and derived from the alkali metal silicate, which is the raw material for the silica sol, are captured by the respective aluminosilicate sites.
However, depending on the manufacturing conditions, use conditions, storage conditions, etc. of the silica particles, the alkali metal encapsulated within the silica particles may be released outside the system over time, resulting in an increase in the pH in the system and causing the above-mentioned instability. In the case of hollow silica particles, the alkali metal present in the aluminosilicate sites outside the shell can be removed during manufacturing by cation exchange or the like, but it is difficult to remove the alkali metal present in the aluminosilicate sites inside the shell during manufacturing. In addition, since aluminosilicate may be generated even inside the silica particles, which is not originally involved in the dispersion stabilization of the silica particles, the amount of aluminum present in the aluminosilicate per silica particle increases, and as a result, the amount of alkali metal present in the aluminosilicate sites also increases. Moreover, the alkali metal may leak into the dispersion medium through the pores of the shell over time, which may cause an increase in the pH in the system and may impair the dimensional stability of the hollow silica particles and the storage stability of the silica sol.

In light of the above circumstances, the inventors conducted extensive research and discovered that by specifying the amount of sulfuric acid added to the system to adjust the pH, hollow silica particles can be obtained that are highly stable in dispersion in the medium and free from concerns about alkali metal leakage, etc.

That is, in a first aspect, the present invention relates to an aluminum atom-containing hollow silica sol containing aluminum atom-containing hollow silica particles and sulfuric acid, wherein the amount of sulfuric acid contained in the hollow silica sol is 1 ppm to 150 ppm.
As a second aspect, the present invention relates to the hollow silica sol according to the first aspect, in which an amount of sulfuric acid contained in the hollow silica sol is 1 ppm to 5000 ppm/ _SiO2 relative to the mass of the hollow silica particles contained in the hollow silica sol.
As a third aspect, the hollow silica sol relates to the hollow silica sol according to the first aspect, in which, in ^27Al -NMR measurement, the ratio [(α ₀ )/{(α 0 )+(β ₀ )}] of the total integral value (α ₀ ) of peaks representing tetracoordinated aluminum atoms to the sum of the total integral value (α ₀ ) of peaks representing tetracoordinated aluminum atoms and the total integral value (β ₀ ) of _peaks representing aluminum atoms other than tetracoordinated aluminum atoms is 0.4 to 1.0.
As a fourth aspect, the present invention relates to the hollow silica sol according to the first aspect, in which the amount of aluminum atoms present in all of the hollow silica particles in the hollow silica _sol is 120 to 50,000 ppm/ _SiO2 in terms of _Al2O3 relative to the mass of the hollow silica particles.
As a fifth aspect, the present invention relates to the hollow silica sol according to the first aspect, in which a surface charge amount calculated per 1 g of hollow silica particles in the hollow silica sol is 5 to 250 μeq/g.
As a sixth aspect, the hollow silica particles in the hollow silica sol are at least partially coated with a silane compound, and the silane compound is a compound represented by the formula (1) and the formula (2):
(In formula (1),
^R1 's are groups bonded to silicon atoms, and each R1 independently represents an alkyl group, a halogenated alkyl group, an alkenyl group, or an aryl group, or an organic group having an epoxy group, a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, a polyether group, a carboxy group, a protected carboxy group, a carboxy group-generating group, an imide group, or a cyano group, and which are bonded to silicon atoms via a Si-C bond, or a combination of these groups;
^R2 is a group or atom bonded to the silicon atom, and independently represents an alkoxy group, an acyloxy group, a hydroxy group, or a halogen atom, or a combination of these groups or atoms;
a represents an integer of 1 to 3;
In formula (2),
^R3 is a group bonded to a silicon atom, and each R3 independently represents an alkyl group, a halogenated alkyl group, an alkenyl group, or an aryl group, or an organic group having an epoxy group, a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, a polyether group, a carboxy group, a protected carboxy group, a carboxy group-generating group, an imide group, or a cyano group, and which is bonded to a silicon atom via a Si-C bond, or a combination of these groups;
^R4 is a group or atom bonded to the silicon atom, and independently represents an alkoxy group, an acyloxy group, a hydroxy group, or a halogen atom, or a combination of these groups or atoms;
b represents an integer of 1 to 3, c represents an integer of 0 or 1,
Y is a group or atom bonded to the silicon atom, and represents an alkylene group, an NH group, or an oxygen atom.
The hollow silica sol according to the first aspect of the present invention relates to at least one silane compound selected from the group consisting of compounds represented by the following formula (1):
As a seventh aspect, the present invention relates to the hollow silica sol according to the first aspect, in which the hollow silica particles in the hollow silica sol have an average particle size of 20 to 150 nm as measured by a dynamic light scattering method.
As an eighth aspect, the present invention relates to the hollow silica sol according to the first aspect, in which the hollow silica sol contains an organic solvent selected from the group consisting of alcohols having 1 to 10 carbon atoms, ketones having 1 to 10 carbon atoms, ethers having 1 to 10 carbon atoms, esters having 1 to 10 carbon atoms, and amides.
According to a ninth aspect, there is provided a film-forming composition comprising the aluminum atom-containing hollow silica sol according to any one of the first to eighth aspects and an organic resin.
According to a tenth aspect, there is provided a method for producing an aluminum atom-containing hollow silica sol according to any one of the first to eighth aspects, comprising:
The following steps (I) and (II):
Step (I): preparing a hollow silica aqueous sol having a sulfuric acid content of 1 ppm to 5000 ppm/ _SiO2 relative to the mass of aluminum atom-containing hollow silica particles;
Step (II): adjusting the amount of sulfuric acid contained in the hollow silica aqueous sol prepared in step (I) to 1 ppm to 150 ppm;
The present invention relates to a method for producing an aluminum atom-containing hollow silica sol, comprising the steps of:
According to an eleventh aspect, there is provided a method for producing an aluminum atom-containing hollow silica sol according to any one of the first to eighth aspects, comprising:
The following steps (III), (IV), (V), and (VI):
Step (III): preparing a hollow silica aqueous sol containing hollow silica particles;
Step (IV): adding an aluminum compound to the hollow silica aqueous sol prepared in Step (III) in a proportion of 0.0001 to 0.5 g, calculated as Al ₂ O ₃ , per 1 g of hollow silica particles, and maintaining the mixture at 40 to 260°C for 0.1 to 48 hours to obtain an aluminum atom-containing hollow silica aqueous sol;
Step (V): adding sulfuric acid to the aluminum atom-containing hollow silica aqueous sol obtained in Step (IV) in a ratio of 1 ppm to 5000 ppm/ _SiO2 relative to the mass of the hollow silica particles in the sol, and maintaining the mixture at 5 to 100°C for 0.1 to 48 hours;
Step (VI): contacting the aluminum atom-containing hollow silica aqueous sol obtained in step (V) with a cation exchange resin;
The present invention relates to a method for producing an aluminum atom-containing hollow silica sol, comprising the steps of:
According to a twelfth aspect, the present invention further comprises the following step (VII):
Step (VII): A step of replacing the dispersion medium in the aluminum atom-containing hollow silica aqueous sol obtained in the step (VI) from water to an organic solvent by heating replacement under reduced pressure, heating replacement under normal pressure, or ultrafiltration,
The present invention relates to a method for producing an aluminum atom-containing hollow silica sol according to an eleventh aspect.
As a thirteenth aspect, the method further comprises a step of replacing the dispersion medium in the aluminum atom-containing hollow silica aqueous sol obtained in the step (II) from water to an organic solvent by heating replacement under reduced pressure, heating replacement under normal pressure, or ultrafiltration.
The present invention relates to a method for producing an aluminum atom-containing hollow silica sol according to the tenth aspect.

According to the present invention, it is possible to provide aluminum atom-containing hollow silica sol that exhibits good storage stability, with the DLS average particle diameter changing by no more than 5% even after storage at 50°C for one week.

FIG. 1 is a schematic diagram of the ²⁷ Al-NMR spectrum showing the peak positions of tetrahedral aluminum atoms and non-tetrahedral aluminum atoms. FIG. 2 shows the results of ²⁷ Al-NMR measurement spectrum of the heat-treated water-dispersed sol (before adding sulfuric acid) prepared in Synthesis Example 1 (FIG. 2(A)), and the results of ²⁷ Al-NMR measurement spectrum of the water-dispersed sol (A1) of aluminum atom-containing hollow silica particles (FIG. 2(B)).

The present invention relates to an aluminum atom-containing hollow silica sol characterized in that the amount of sulfuric acid contained in the hollow silica sol is controlled to between 1 ppm and 150 ppm.

The aluminum atom-containing hollow silica sol (hereinafter also referred to simply as "hollow silica sol" or "silica sol") according to the present invention is a dispersion system containing aluminum atom-containing hollow silica particles (hereinafter also referred to simply as "hollow silica particles,""hollowsilica," or "silica particles") and a solvent, in which the hollow silica particles are dispersed as dispersoids in the solvent. The hollow silica particles have a silica ( _SiO2 )-containing outer shell and have a space inside the shell.

In the aluminum atom-containing hollow silica sol of the present invention, the hollow silica particles are considered to have the form of aluminum atom-containing hollow particles due to the aluminum atoms forming aluminosilicate sites at least on their surfaces. The aluminosilicate sites formed on the surfaces of the hollow silica particles are partially converted to aluminum sulfate upon contact with sulfuric acid, which is added to adjust the pH of the silica sol to remove impurities. Aluminum sulfate is a compound with such high coagulation power that it is used as a flocculant. When aluminum sulfate reacts with an alkali metal in water to produce positively charged aluminum hydroxide, it neutralizes the negative charge on the silica particle surface, causing aggregation. The aforementioned aluminum sulfate conversion essentially means the release of aluminum from the aluminosilicate sites on the silica particle surface, which can promote the outflow of the alkali metal encapsulated by the aluminosilicate sites, potentially resulting in an increase in the pH of the silica sol and a tendency toward alkalinity.
In the present invention, it has been found that by controlling the amount of sulfuric acid in the hollow silica sol, it is possible to suppress the conversion of aluminosilicate sites formed on the particle surfaces to aluminum sulfate, thereby obtaining a sol with excellent storage stability in which the change in particle size of hollow silica particles before and after long-term storage is small and aggregation of the particles is suppressed. The amount of sulfuric acid includes both sulfuric acid itself present in the system and sulfuric acid in the form of aluminum sulfate. Furthermore, control of the amount of sulfuric acid can be suitably carried out in an aqueous hollow silica sol (aqueous dispersion sol), and storage stability can also be ensured in an organic solvent sol obtained by solvent substitution of this.

Furthermore, in the aluminum atom-containing hollow silica sol according to the present invention, the amount of sulfuric acid contained in the hollow silica sol can be, for example, 1 ppm to 150 ppm, 1 ppm to 140 ppm, 10 ppm to 150 ppm, 30 ppm to 150 ppm, or 50 ppm to 140 ppm. By setting the amount of sulfuric acid contained in the hollow silica sol to 1 ppm or more, the thickness of the electric double layer of the hollow silica particles contained in the hollow silica sol becomes thinner, thereby reducing the viscosity of the hollow silica sol and stabilizing the pH in the system, thereby improving the storage stability of the silica sol. By setting the amount of sulfuric acid contained in the hollow silica sol to 150 ppm or less, aluminum sulfate reacts with alkali metal in water to produce positively charged aluminum hydroxide, which neutralizes the negative charge on the surface of the silica particles and suppresses aggregation.

In the aluminum atom-containing hollow silica sol according to the present invention, the amount of sulfuric acid relative to the mass (g) of the hollow silica particles contained in the sol can be, for example, 1 ppm to 5000 ppm/SiO ₂ , 1 ppm to 3000 ppm/SiO ₂ , 1 ppm to 1500 ppm/SiO ₂ , 1 ppm to 1100 ppm/SiO ₂ , 1 ppm to 1000 ppm/SiO ₂ , 10 ppm to 1000 ppm/SiO ₂ , 100 ppm to 1000 ppm/SiO ₂ , 100 ppm to 3000 ppm/SiO ₂ , 200 ppm to 1000 ppm/SiO ₂ , or 400 ppm to 1000 ppm/SiO ₂ . By setting the amount of sulfuric acid relative to the mass (g) of the hollow silica particles contained in the hollow silica sol to 1 ppm/SiO2 _{or more} , the thickness of the electric double layer of the hollow silica particles contained in the hollow silica sol becomes thin, the viscosity of the hollow silica sol can be reduced, the pH in the system can be stabilized, and the storage stability of the silica sol can be improved. By setting the amount of sulfuric acid relative to the mass (g) of the hollow silica particles contained in the hollow silica sol to 5000 ppm/SiO2 or _less , aluminum sulfate reacts with the alkali metal in water to produce positively charged aluminum hydroxide, which neutralizes the negative charge on the surface of the silica particles and suppresses aggregation.
In this specification, the unit of the amount of sulfuric acid in the silica sol may be distinguished as "ppm" or "ppm/sol" when indicating the amount of sulfuric acid relative to the silica sol, and as "ppm/SiO ₂ (silica particles)" when indicating the amount of sulfuric acid relative to the mass of hollow silica particles in the silica sol.

The hollow silica sol according to the present invention can be determined, from the peak position of the spectrum obtained by measuring the hollow silica sol by ^27Al -NMR, whether the aluminum in the system is in the form of aluminosilicate sites (tetracoordinated aluminum atoms) formed on the particle surfaces or in the form of aluminum atoms dissolved in the system and cationized (aluminum atoms other than tetracoordinated (e.g., tricoordinated or hexacoordinated)).
The peaks representing tetracoordinated aluminum atoms obtained by ^27Al -NMR measurement are observed around 54 ppm (50 to 65 ppm), and the peaks representing non-tetracoordinated aluminum atoms are observed around 0 ppm (-5 to 10 ppm) (see Figure 1). By comparing the integral values of these peaks, the proportion of tetracoordinated aluminum atoms and non-tetracoordinated aluminum atoms can be evaluated.
In the present invention, the amount of sulfuric acid in the silica sol system is controlled to suppress the conversion of aluminosilicate sites to aluminum sulfate, and the sol has a low amount of aluminum in the aluminum sulfated (cationized) system, which leads to good stability of the silica sol after long-term storage.

For example, in the hollow silica sol according to the present invention, in ^27Al -NMR measurement of the hollow silica sol, the ratio [(α0)/{( _α0 )+( _β0 )}] of the total integral value (α) of peaks representing tetracoordinated aluminum atoms to the _sum of the total integral value ( _α0 ) of peaks representing tetracoordinated aluminum atoms and the total integral value ( _β0 ) of peaks representing aluminum atoms other than tetracoordinated aluminum atoms can be 0.4 to 1.0, and can also be 0.45 to 1.0, or 0.50 to 1.0. By making the ratio [( _α0 )/{( _α0 )+( _β0 )}] of the total integral value (α) of peaks representing tetracoordinated aluminum atoms of the hollow silica sol 0.4 or more, aluminum sulfate reacts with alkali metals in water to produce positively charged aluminum hydroxide, which neutralizes the negative charge on the silica particle surface and suppresses aggregation.

Furthermore, the hollow silica sol may have a tetrahedral Al ratio of 0.4 to 1.0, for example, 0.45 to 1.0, or alternatively, 0.50 to 1.0, calculated from the results of ^27Al -NMR measurement of the hollow silica sol using the following formula (1):
4-coordinate Al ratio = (α) / {(α) + (β)} ...Formula (1)
(α): The integral sum of the peaks observed at 50 to 65 ppm in the ^27Al -NMR measurement. (β): The integral sum of the peaks observed at −5 to 10 ppm in the ^27Al -NMR measurement. The peaks observed at 50 to 65 ppm in the ^27Al -NMR measurement are derived from tetracoordinated aluminum atoms, and the peaks observed at −5 to 10 ppm are derived from aluminum atoms other than tetracoordinated aluminum atoms.
The above-mentioned ^27Al -NMR measurement can be carried out on both an aqueous hollow silica sol (water-dispersed sol) and an organic solvent sol obtained by solvent substitution of the aqueous hollow silica sol. For example, data obtained from the water-dispersed sol can be selected as peak data to be used in calculating the tetrahedral Al ratio.

In the hollow silica particles of the present invention, aluminum atoms may be present as aluminosilicate, and aluminosilicate may be formed at least on the surface of the particle, and aluminosilicate may be formed not only on the particle surface but also inside the hollow silica particles.
The aluminum atom-containing hollow silica sol according to the present invention can be dissolved using an aqueous hydrofluoric acid solution (also referred to as an aqueous hydrofluoric acid solution) to show the amount of aluminum atoms present in the entire hollow silica particles (i.e., the entire particles including the particle surfaces and interiors) in the silica sol (i.e., the aluminum atom content in the hollow silica particles) converted into Al ₂ O _3. Specifically, the hollow silica particles are dissolved in an aqueous hydrofluoric acid solution, and the resulting solution is measured and analyzed using an ICP emission spectrometer, whereby the amount of aluminum atoms present in the entire hollow silica particles can be shown converted into Al ₂ O ₃ .
More specifically, the silica sol is first dried to remove the dispersant, yielding hollow silica particles. Then, 250 mg of the particles are dissolved in a hydrofluoric acid solution (e.g., a mixture of 2.5 ml of nitric acid and _2.5 ml of 38% hydrofluoric acid) to obtain an aqueous solution. The amount of aluminum atoms in the aqueous solution is measured using an ICP emission spectrometer to obtain the aluminum atom content (ppm) converted to _Al2O3 . This is then divided by the mass of the silica particles to determine the amount of aluminum atoms present in the entire hollow silica particle ( _{Al2O3 (} ppm)/ _SiO2 ).
For example, the aluminum atom-containing hollow silica sol according to the present invention preferably has an amount of aluminum atoms present in all hollow silica particles in the sol, calculated as Al ₂ O ₃ relative to the mass of the hollow silica particles, of 120 to 50,000 ppm/SiO ₂ (silica particles) when measured by a dissolution method using an aqueous hydrofluoric acid solution, and can be, for example, 300 to 20,000 ppm/SiO ₂ , or 500 to 20,000 ppm/SiO ₂ , or 500 to 10,000 ppm/SiO ₂ , or 500 to 5,000 ppm/SiO ₂ , or 500 to 1,000 ppm/SiO ₂ .
By _adjusting the amount of aluminum atoms present throughout the hollow silica particles to 120 to 50,000 ppm/ _SiO2 in terms of _Al2O3 relative to the mass of the hollow silica particles, the negative charge on the surface of the silica particles increases, and charge repulsion occurs to an extent that the silica particles do not come into contact with each other, allowing the dispersion state to be maintained, thereby improving the storage stability of the hollow silica sol.
In this specification, the amount of aluminum atoms present in the entire hollow silica particle is expressed as "ppm/SiO ₂ " as a unit showing the amount relative to the mass (g) of the hollow silica particle.

The hollow silica particles in the hollow silica sol according to the present invention preferably have a surface charge amount (negative charge amount) calculated per 1 g of hollow silica particles of, for example, 5 to 250 μeq/g, or alternatively, can be in the range of 5 to 150 μeq/g, or 10 to 150 μeq/g, or 20 to 150 μeq/g, or 10 to 100 μeq/g, or 15 to 100 μeq/g, or 20 to 50 μeq/g, or 20 to 40 μeq/g.
By making the surface charge amount (negative charge amount) calculated per 1 g of the hollow silica particles 5 μeq/g or more, the charge repulsion force between the silica particles increases, thereby improving the dispersion stability of the silica particles in the dispersion medium.
Furthermore, by making the surface charge amount (negative charge amount) calculated per 1 g of the hollow silica particles 250 μeq/g or less, the amount of aluminum eluted from the hollow silica particles can be reduced, the amount of aluminum sulfate produced can be reduced, and the aggregation of particles can be suppressed.Furthermore, by making the surface charge amount (negative charge amount) 250 μeq/g or less, the outflow of alkali metals present in the aluminosilicate into the dispersion medium through the pores of the silica particle outer shell over time can be reduced, and the increase in pH in the system can be suppressed, thereby improving the dimensional stability of the hollow silica particles and the storage stability of the silica sol.

In addition, the high absolute value of the zeta potential of hollow silica particles in the acidic region causes electrical repulsion, which is desirable from the perspective of dispersibility, and the amount of surface charge of hollow silica particles varies, in part, depending on the amount of aluminum atoms (aluminosilicate) present in the hollow silica particles.
For example, if the amount of aluminum atoms present in the entire silica particle (calculated as Al ₂ O ₃ relative to SiO ₂ ) is less than 120 ppm/SiO ₂ , the stability of the hollow silica particles tends to decrease.
On the other hand, when the amount of aluminum atoms (in terms of Al ₂ O ₃ , relative to SiO ₂ ) present in the entire silica particles is 50,000 ppm/SiO ₂ or more, the particle size after doping tends to increase compared to the particle size measured by dynamic light scattering (DLS) before doping with aluminum atoms in the aqueous sol stage.

In the hollow silica sol according to the present invention, the hollow silica particles preferably have an average particle size (DLS average particle size: Z-average particle size, harmonic mean particle size) measured by dynamic light scattering (DLS) of 20 to 150 nm, and can be, for example, in the range of 30 to 150 nm, or 40 to 150 nm, or 50 to 150 nm, or 50 to 120 nm, or 50 to 100 nm.
The DLS average particle size represents the average value of the secondary particle size (dispersed particle size), and it can be determined that the larger the DLS average particle size, the more the silica particles in the medium are in an agglomerated state.

Furthermore, in the hollow silica sol according to the present invention, the average primary particle diameter of the hollow silica particles as determined by transmission electron microscope (TEM) observation can be, for example, in the range of 20 to 150 nm, 30 to 150 nm, 40 to 150 nm, 50 to 150 nm, 50 to 120 nm, or 50 to 100 nm.

Furthermore, the hollow silica particles according to the present invention may have a specific surface area measured by the BET method (nitrogen gas adsorption method) of, for example, 18 to 200 m ² /g, or 50 to 160 m ² /g, or 60 to 160 m ² /g, or 70 to 160 m ² /g, or 80 to 150 m ² /g.

The outer shell of the hollow silica particles can be observed using a transmission electron microscope (TEM).
In the hollow silica particles according to the present invention, the thickness of the outer shell as determined by observation with a transmission electron microscope can be, for example, in the range of 3.0 to 15.0 nm, or 4.0 to 12.0 nm, or 5.0 to 10.0 nm.

Furthermore, the number density of silanol groups on the surface of the hollow silica particles can be, for example, 0.2 to 6.0 groups/nm ² , or 0.5 to 5.0 groups/nm ² , 0.5 to 3.0 groups/nm ² , 0.5 to 2.0 groups/nm ² , 0.7 to 2.0 groups/nm ² , or 1.1 to 2.0 groups/nm ² .
By setting the number density of silanol groups on the silica particle surface to 0.2/nm² ^{or more} , the negative charge of the silica particles increases, improving the dispersion stability of the silica particles in the dispersion medium. Furthermore, by setting the number density of silanol groups on the silica particle surface to 6.0/nm² ^or less, aggregation due to dehydration condensation between silanol groups between silica particles is suppressed, improving dispersion stability.
The number density of silanol groups on the surface of silica particles can be measured, for example, by the Sears method described in “Determination of Specific Surface Area of Colloidal Silica by Titration with Sodium Hydroxide” (G. W. Sears, Jr., Analytical Chemistry, 28(12), 1981 (1956)).

The refractive index of the hollow silica particles according to the present invention can be set to, for example, in the range of 1.20 to 1.45, or 1.20 to 1.40, or 1.20 to 1.30.
Furthermore, the hollow silica particles according to the present invention can have a carbon content measured by elemental analysis in the range of, for example, 0.1% to 10.0% by mass. Elemental analysis involves first selecting a poor solvent and a good solvent for a hollow silica sol in which the hollow silica particles to be measured are dispersed, separating the hollow silica particles from the organic components not bonded to the hollow silica particles using a centrifuge or the like, and then drying the resulting mixture to remove even the adsorbed water, thereby preparing a measurement sample. The carbon content (%) of the obtained silica particle measurement sample can be determined by measuring the obtained silica particle measurement sample using an elemental analyzer.

In the present invention, the hollow silica particles in the aluminum atom-containing hollow silica sol may be at least partially coated with a silane compound.
In the present invention, "coated with a silane compound" refers to an embodiment in which the surface of a silica particle is coated with a silane compound, and also includes an embodiment in which a silane compound is bonded to the surface of a silica particle.
The "embodiment in which the surface of the silica particles is coated with a silane compound" may refer to an embodiment in which at least a portion of the surface of the silica particles is coated with a silane compound, i.e., it includes an embodiment in which the silane compound covers a portion of the surface of the silica particles and an embodiment in which the silane compound covers the entire surface of the silica particles. This embodiment does not require bonding between the silane compound and the surface of the silica particles.
Furthermore, "an embodiment in which a silane compound is bonded to the surface of a silica particle" means an embodiment in which a silane compound is bonded to at least a portion of the surface of a silica particle, i.e., an embodiment in which the silane compound is bonded to a portion of the surface of a silica particle, an embodiment in which the silane compound is bonded to a portion of the surface of a silica particle and covers at least a portion of the surface, and even an embodiment in which the silane compound is bonded to the entire surface of a silica particle and covers the entire surface.

The silane compound may be at least one silane compound selected from the group consisting of compounds represented by the following formulas (1) and (2).

In the above formula (1), R ^{1 is a group bonded to a silicon atom, and each R 1} independently represents an alkyl group, a halogenated alkyl group, an alkenyl group, or an aryl group, or an organic group having an epoxy group, a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, a polyether group, a carboxy group, a protected carboxy group, a carboxy group-generating group, an imide group, or a cyano group, and which is bonded to a silicon atom via a Si-C bond, or a combination of these groups;
^R2 is a group or atom bonded to the silicon atom, and independently represents an alkoxy group, an acyloxy group, a hydroxy group, or a halogen atom, or a combination of these groups or atoms;
a represents an integer of 1 to 3.
In the definition of the groups of the silane compound in this specification, the term "independently from each other" means that multiple groups can each independently represent the groups defined as options. That is, for example, in formula (1), when there are two or more R ^1s (a is 2 to 3), the multiple R ^1s may be the same group (e.g., all methyl groups) or a combination of different groups (when a is 2, for example, a methyl group and a phenyl group, or a methyl group and a (meth)acryloylpropyl group).

In the above formula (2), R ^{3 is a group bonded to a silicon atom, and each R 3} independently represents an alkyl group, a halogenated alkyl group, an alkenyl group, or an aryl group, or an organic group having an epoxy group, a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, a polyether group, a carboxy group, a protected carboxy group, a carboxy group-generating group, an imide group, or a cyano group, and which is bonded to a silicon atom via a Si-C bond, or a combination of these groups;
^R4 is a group or atom bonded to the silicon atom, and independently represents an alkoxy group, an acyloxy group, a hydroxy group, or a halogen atom, or a combination of these groups or atoms;
Y is a group or atom bonded to the silicon atom and represents an alkylene group, an NH group, or an oxygen atom;
b represents an integer of 1 to 3, and c represents an integer of 0 or 1.

In the above formula, examples of alkyl groups include linear or branched alkyl groups having 1 to 18 carbon atoms and cyclic alkyl groups having 3 to 10 carbon atoms. Examples include, but are not limited to, methyl groups, ethyl groups, linear, branched, or cyclic propyl groups, butyl groups, pentyl groups, hexyl groups, heptyl groups, octyl groups, nonyl groups, and decyl groups.

The halogenated alkyl group is an alkyl group substituted with one or more halogen atoms, and specific examples of such alkyl groups are the same as those mentioned above, i.e., linear or branched alkyl groups having 1 to 18 carbon atoms and cyclic alkyl groups having 3 to 10 carbon atoms.
Examples of the halogen atom include a fluorine atom (for example, as a trifluoropropyl group), a chlorine atom, a bromine atom, and an iodine atom.

The above-mentioned alkenyl group may be, for example, an alkenyl group having 2 to 10 carbon atoms, and may be linear, branched, or cyclic. Furthermore, the position of the double bond contained in the alkenyl group is not particularly limited. Examples include, but are not limited to, ethenyl groups (vinyl groups), linear, branched, or cyclic propenyl groups, butenyl groups, pentenyl groups, and hexenyl groups.

The aryl group may be, for example, an aryl group having 6 to 30 carbon atoms, such as a phenyl group, a 1-naphthyl group, a 2-naphthyl group, a 1-anthryl group, a 2-anthryl group, a 9-anthryl group, a 1-pyrenyl group, or a 2-pyrenyl group.

Examples of the organic group having an epoxy group include a glycidoxymethyl group, a glycidoxyethyl group, a glycidoxypropyl group, a glycidoxybutyl group, and a 2-(3,4-epoxycyclohexyl)ethyl group.
The (meth)acryloyl group refers to both an acryloyl group and a methacryloyl group. Examples of organic groups having a (meth)acryloyl group include a methacryloyloxymethyl group, an acryloyloxymethyl group, a methacryloyloxyethyl group, an acryloyloxyethyl group, a 3-methacryloyloxypropyl group, and a 3-acryloyloxypropyl group. The methacryloyloxy group and the acryloyloxy group are also referred to as a methacryloxy group and an acryloxy group.
Examples of the organic group having a mercapto group include an ethyl mercapto group, a 3-mercaptopropyl group, a butyl mercapto group, a hexyl mercapto group, an octyl mercapto group, and a mercaptophenyl group.
Examples of the organic group having an amino group include an aminomethyl group, a 2-aminoethyl group, a 3-aminopropyl group, an N-2-(aminoethyl)-3-aminopropyl group, an N-(1,3-dimethyl-butylidene)aminopropyl group, an N-phenyl-3-aminopropyl group, an N-(vinylbenzyl)-2-aminoethyl-3-aminopropyl group, a dimethylaminoethyl group, and a dimethylaminopropyl group.
Examples of the organic group having a carboxy group include a carboxymethyl group, a carboxyethyl group, a carboxypropyl group, and a carboxybutyl group.
The term "protected carboxy group" refers to a carboxy group protected by a protecting group used in ordinary organic synthesis reactions. The term "carboxy group generating group" refers to a group in which a carboxy group is esterified or amidated with alcohols, amines, or the like. A specific example of a silane compound containing a protected carboxy group and an organic group having a carboxy group generating group is a silane coupling agent having a carboxylic acid ester structure. In the silane coupling agent, the carboxylic acid ester moiety and the alkoxysilyl group may be linked by an alkylene group or an alkylene group containing a heteroatom (nitrogen atom, oxygen atom). The carboxylic acid ester moiety of the silane coupling agent is hydrolyzed to a carboxylic acid, and if the silane coupling agent contains a nitrogen atom (heteroatom), it is hydrolyzed to an amino acid due to the presence of a carboxy group and an amino group. Therefore, the silane coupling agent can be used as an amino acid generator. For example, the product name X-88-475, manufactured by Shin-Etsu Chemical Co., Ltd., represented by formula (1-1), can be used.

The above alkoxy groups include alkoxy groups having 1 to 10 carbon atoms, such as, but not limited to, methoxy, ethoxy, propoxy, and isopropoxy groups.

The above-mentioned acyloxy group is a group derived by removing a hydrogen atom from the carboxy group (-COOH) of a carboxylic acid compound, and a specific example is an acyloxy group having 2 to 10 carbon atoms.

The above-mentioned halogen atoms include fluorine atoms, chlorine atoms, bromine atoms, iodine atoms, etc.

Furthermore, examples of the alkylene group include alkylene groups derived from the alkyl groups described above. Specific examples include, but are not limited to, linear, branched, or cyclic alkylene groups having 1 to 10 carbon atoms, such as methylene, ethylene, trimethylene, tetramethylene, pentamethylene, hexamethylene, heptamethylene, octamethylene, nonamethylene, and decamethylene.

Specific preferred examples of the silane compounds represented by the above formulas (1) and (2) include compounds that can form trimethylsilyl groups on the surfaces of silica particles.
Examples of such compounds include compounds represented by the following formulas (1-2), (2-1) and (2-2).
In the formula (1-2), R ¹² is an alkoxy group, such as a methoxy group or an ethoxy group. Silane compounds represented by the formulas (1-2), (2-1), and (2-2) can be silane compounds manufactured by Shin-Etsu Chemical Co., Ltd.

The hollow silica particles at least partially coated with a silane compound can be obtained, for example, by adding a silane compound to a hollow silica sol and then heat-treating the mixture for about 0.1 to 20 hours at 10 to 100° C. In this case, the amount of silane compound added relative to the hollow silica particles in the hollow silica sol (aqueous sol or organic solvent dispersion sol) can be set to, for example, a mass ratio of silane compound/hollow silica particles of 0.1 to 10.0.
The amount of the silane compound coated on the surface of the hollow silica particles (amount of surface treatment) can be set to, for example, about 0.1 to 12, or about 0.1 to 6 silicon atoms per ¹ nm2 of the surface of the hollow silica particles.

The reaction between silica particles and silane compounds proceeds through a reaction between silanol groups generated by hydrolysis of the silane compounds and hydroxyl groups (silanol groups) on the surface of the silica particles. The presence of water is required for this hydrolysis, but if the silica sol is an aqueous solvent sol, the aqueous solvent can fulfill this role. Alternatively, if the silica sol is an organic solvent sol in which the aqueous medium has been replaced with an organic solvent, the water remaining in the organic solvent can fulfill this role. For example, water present in the organic solvent at 0.01 to 1% by mass can be used for the hydrolysis.

The hydrolysis can be carried out with or without a catalyst.
If the silica particle surface is on the acidic side (pH less than 7), hydrolysis can be carried out without a catalyst.
When a catalyst is used, examples of the catalyst include metal chelate compounds, organic acids (acetic acid, oxalic acid, lactic acid, etc.), inorganic acids (hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, etc.), organic bases (heterocyclic amines, quaternary ammonium salts, sodium methoxide, sodium ethoxide, potassium methoxide, potassium ethoxide, etc.), and inorganic bases (ammonia, sodium hydroxide, potassium hydroxide, etc.).

The hollow silica sol according to the present invention can contain, as its dispersion medium, an organic solvent selected from the group consisting of alcohols having 1 to 10 carbon atoms, ketones having 1 to 10 carbon atoms, ethers having 1 to 10 carbon atoms, esters having 1 to 10 carbon atoms, and amides. The number of carbon atoms means the total number of carbon atoms contained in the compounds such as the alcohols.
The hollow silica sol according to the present invention also includes an aqueous sol (water-dispersed sol). The aqueous sol can be solvent-substituted with an organic solvent selected from the group consisting of the above-mentioned alcohols, ketones, ethers, esters, and amides. After the hollow silica particles are coated with the above-mentioned silane compound using the aqueous sol, the solvent can be solvent-substituted with an organic solvent such as the above-mentioned alcohol.

The alcohol having 1 to 10 carbon atoms includes an aliphatic alcohol, which may be a primary alcohol, a secondary alcohol, or a tertiary alcohol. Furthermore, it is also possible to use polyhydric alcohols such as dihydric alcohols and trihydric alcohols as the alcohol.
Examples of the monohydric primary alcohol include methanol, ethanol, 1-propanol, 1-butanol, and 1-hexanol.
Examples of the monohydric secondary alcohol include 2-propanol, 2-butanol, cyclohexanol, propylene glycol monomethyl ether, and propylene glycol monoethyl ether.
The monohydric tertiary alcohol may, for example, be tert-butyl alcohol.
Examples of the dihydric alcohol (glycol) include methanediol, ethylene glycol, and propylene glycol.
The trihydric alcohol includes glycerin.

Aliphatic ketones are preferably used as the ketones having 1 to 10 carbon atoms. Examples include acetone, methyl ethyl ketone, diethyl ketone, methyl propyl ketone, methyl isobutyl ketone, methyl amyl ketone, cyclohexanone, cyclopentanone, and methylcyclopentanone.

Aliphatic ethers are preferably used as the ethers having 1 to 10 carbon atoms. Examples include dimethyl ether, ethyl methyl ether, diethyl ether, tetrahydrofuran, and 1,4-dioxane.

Aliphatic esters can be preferably used as the esters having 1 to 10 carbon atoms. Examples include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, ethyl lactate, methyl propionate, ethyl propionate, propyl propionate, methyl acrylate, ethyl acrylate, propyl acrylate, dimethyl maleate, diethyl maleate, dipropyl maleate, dimethyl adipate, diethyl adipate, dipropyl adipate, methyl 2-hydroxyisobutyrate, and propylene glycol monomethyl ether acetate.

Further examples of the amide include N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, and tetramethylurea.

In the hollow silica sol according to the present invention, the concentration of the hollow silica particles can be, for example, 1 to 50% by mass, or 3 to 40% by mass, or 5 to 40% by mass, and typically 10 to 30% by mass.
In this specification, the hollow silica particle concentration in a silica sol is a value determined by a calcination method, specifically, by calcining the target silica sol at, for example, 1000° C. for 30 minutes or more, and dividing the mass of the resulting calcination residue by the mass of the target silica sol. The mass of the calcination residue is also sometimes referred to as the “silica solid content.”

The pH of the hollow silica sol of the present invention can be adjusted from acidic to alkaline. Adjustment to acidic is achieved by adding an inorganic or organic acid. Adjustment to alkaline is achieved by adding an inorganic or organic base, and amines can be added as organic bases for the purposes of pH adjustment and adjusting the surface charge amount, as described below. The pH can be set to between 1 and less than 7 on the acidic side, and between 7 and 13 on the alkaline side.

If the hollow silica sol is an aqueous sol, before adding the amine, the pH can be set to, for example, a pH range of 2.0 to 6.0 or 2.0 to 4.5 on the acidic side, and by adding the amine, the pH can be adjusted to, for example, a pH range of 3.0 to 10.0 or 3.0 to 9.0. On the alkaline side, the pH of the aqueous sol can be set to 7 to 10.

When the hollow silica sol is an organic solvent sol, the pH can be the pH when the organic solvent sol and the same mass of pure water are mixed at a ratio of 1: 1. The pH is measured in the form of a sol of an organic solvent miscible with water, and when the solvent is subsequently replaced with a hydrophobic organic solvent, the pH is measured in advance at the stage of a sol of a hydrophilic organic solvent such as methanol, or the pH is measured after adding a hydrophilic organic solvent to the sol of the hydrophobic solvent.
For example, when the dispersion medium, such as methanol sol or propylene glycol monomethyl ether sol, is a hydrophilic organic solvent, the pH can be measured using a solution prepared by mixing pure water and the sol in a mass ratio of 1: 1. When the dispersion medium, such as methyl ethyl ketone sol, cyclopentanone sol, or cyclohexanone sol, is a hydrophobic organic solvent, the pH can be measured using a solution prepared by mixing pure water, methanol, and the sol in a mass ratio of 1: 1: 1.

The conversion from an aqueous sol to a hydrophobic organic solvent sol can be achieved by solvent-substituting the aqueous medium with a hydrophilic organic solvent (such as an alcohol), followed by further solvent-substituting with a hydrophobic organic solvent, and moisture may remain during this process.
For example, the residual moisture content in an alcohol sol of hollow silica particles can be about 0.1 to 3.0% by mass, or about 0.1 to 1.0% by mass, and the residual moisture content in an organic solvent sol of hollow silica particles (wherein the dispersion medium is an organic solvent other than alcohol) can be about 0.01 to 0.5% by mass.

Furthermore, the viscosity (25°C) of the hollow silica organic solvent sol can be set in the range of 0.5 to 10.0 mPa·s, or alternatively, in the range of 1.0 to 10.0 mPa·s.

The hollow silica sol (aqueous sol, organic solvent sol, etc.) according to the present invention may contain an amine, or an amine and ammonia. The amine may be added and contained in an amount of 0.001 to 10% by mass, 0.01 to 10% by mass, or 0.1 to 10% by mass relative to the mass of the hollow silica particles.
The amine, or the amine and ammonia, can be contained in such an amount that the total nitrogen amount in the sol containing these basic components and hollow silica particles is in the range of, for example, 0.001 to 10 mass%, or 0.01 to 1 mass%, or 0.01 to 0.3 mass%, or 0.01 to 0.2 mass%, typically 0.02 to 0.2 mass%.

The amine may be, for example, an aliphatic amine or an aromatic amine, with aliphatic amines being preferred. At least one amine selected from the group consisting of primary, secondary, and tertiary amines having 1 to 10 carbon atoms can be used. These amines are water-soluble and are at least one amine selected from the group consisting of primary, secondary, and tertiary amines having 1 to 10 carbon atoms.

Examples of primary amines include monomethylamine, monoethylamine, monopropylamine, monoisopropylamine, monobutylamine, monoisobutylamine, monosec-butylamine, mono-tert-butylamine, monomethanolamine, monoethanolamine, monopropanolamine, monoisopropanolamine, monobutanolamine, monoisobutanolamine, monosec-butanolamine, and mono-tert-butanolamine.
Examples of secondary amines include dimethylamine, diethylamine, dipropylamine, diisopropylamine, N-methylethylamine, N-ethylisobutylamine, dimethanolamine, diethanolamine, dipropanolamine, diisopropanolamine, N-methanolethylamine, N-methylethanolamine, N-ethanolisobutylamine, and N-ethylisobutanolamine.
Examples of tertiary amines include trimethylamine, triethylamine, tripropylamine, triisopropylamine, diisopropylethylamine, tributylamine, triisobutylamine, trisecbutylamine, tritertbutylamine, trimethanolamine, triethanolamine, tripropanolamine, triisopropanolamine, tributanolamine, triisobutanolamine, trisecbutanolamine, tritertbutanolamine, tripentylamine, 3-(dimethylamino)ethyl acrylate, 2-(dimethylamino)ethyl acrylate, 2-(dimethylamino)ethyl methacrylate, 2-(diethylamino)ethyl acrylate, and 2-(diethylamino)ethyl methacrylate.
The water solubility of the amine is preferably 80 g/L or more, or 100 g/L or more. Among these, primary amines and secondary amines are preferred, with secondary amines being more preferred due to their low volatility and high solubility, such as diisopropylamine and diethanolamine. Diisopropylethylamine, which is a tertiary amine, can also be preferably used.

<Method for producing aluminum atom-containing hollow silica sol>
The aluminum atom-containing hollow silica sol of the present invention can be produced by a production method including the following steps (I) and (II).
Step (I): preparing a hollow silica aqueous sol having an amount of sulfuric acid of 1 ppm to 5000 ppm/ _SiO2 relative to the mass of aluminum atom-containing hollow silica particles;
Step (II): A step of adjusting the amount of sulfuric acid contained in the hollow silica aqueous sol prepared in step (I) to 1 ppm to 150 ppm.

More specifically, the aluminum atom-containing hollow silica sol of the present invention can be produced by a production method including the following steps (III), (IV), (V), and (VI): In this specification, steps (III) to (VI) and steps (I) to (II) exist independently, and step (III) is not numbered with the intention of being carried out after step (II).
Step (III): preparing a hollow silica aqueous sol;
Step (IV): adding an aluminum compound to the hollow silica aqueous sol prepared in step (III) in a proportion of 0.0001 to 0.5 g, calculated as Al ₂ O ₃ , per 1 g of hollow silica particles, and maintaining the mixture at 40 to 260°C for 0.1 to 24 hours to obtain an aluminum atom-containing hollow silica aqueous sol;
Step (V): A step of adding sulfuric acid to the aluminum atom-containing hollow silica aqueous sol obtained in the step (IV) in a ratio of 1 ppm to 5000 ppm/ _SiO2 relative to the mass of the hollow silica particles in the sol, and maintaining the mixture at 5 to 100°C for 0.1 to 48 hours.
Step (VI): A step of contacting the aluminum atom-containing hollow silica aqueous sol obtained in the step (V) with a cation exchange resin.
By the above-mentioned method for producing hollow silica sol, the amount of sulfuric acid contained in the sol can be adjusted to a predetermined amount.

[(III) Process]
The hollow silica particles constituting the hollow silica sol prepared in step (III) have an outer shell containing silica and have a space inside the outer shell. Note that the "hollow silica sol" and "hollow silica particles" prepared in step (III) are distinguished from the abbreviations for the aluminum atom-containing hollow silica sol and aluminum atom-containing hollow silica particles mentioned above, and are, so to speak, the raw materials for the silica sol and silica particles according to the present invention. That is, the "hollow silica particles" in this step are hollow silica particles that do not contain aluminum atoms (however, the inclusion of aluminum atoms as impurities is permitted).
The raw material hollow silica particles are obtained by forming a silica-based shell on the surface of a core portion, called a template, in an aqueous dispersion medium, and then removing the core portion (template). The template can be made of either an organic material (e.g., hydrophilic organic resin particles such as polyethylene glycol, polystyrene, or polyester) or an inorganic material (e.g., hydrophilic inorganic compound particles such as calcium carbonate or sodium aluminate).

The raw material hollow silica aqueous sol prepared in step (III) can be any of a non-hydrothermally treated hollow silica aqueous sol, a hydrothermally treated hollow silica aqueous sol, or a mixture thereof.
The non-hydrothermally treated hollow aqueous silica sol is an aqueous sol of silica particles treated in an aqueous medium via a heating temperature of less than 100°C, for example, from 20°C to less than 100°C, or from 40°C to less than 100°C, or from 50°C to less than 100°C. The hydrothermally treated silica aqueous sol is an aqueous sol of silica particles treated in an aqueous medium via a heating temperature of from 100°C to 240°C, or from 110°C to 240°C.

The raw material hollow silica aqueous sol (non-hydrothermally treated hollow silica aqueous sol, hydrothermally treated hollow silica aqueous sol, or a mixture thereof) can be in a form in which the hollow silica particles as the raw material for the aqueous sol contain aluminum atoms in step (IV) described below, specifically in a form in which aluminosilicate sites are formed on the outer shells of the hollow silica particles. Because aluminosilicate sites can retain alkali metals, the raw material hollow silica sol can be selected so that aluminum atoms are present on the surface of the hollow silica particles in a ratio of 100 to _20,000 ppm/ _SiO2 (based on the mass of the hollow silica particles) calculated as _Al2O3 , as measured by the so-called leaching method using a mineral acid.

[(IV) Process]
In step (IV), an aluminum compound is added to the raw material hollow silica aqueous sol prepared in step (III) and heated to obtain an aluminum atom-containing hollow silica aqueous sol. This step allows the aluminum compound to act on the raw material hollow silica particles from the outside (i.e., impregnate them), causing aluminum atoms to be present on the surfaces of the hollow silica particles, i.e., forming aluminosilicate sites at least on the surfaces of the particles.

When an aluminum compound is externally applied (impregnated) to hollow silica particles (raw material) after their formation, there are two methods: one is to subject hollow silica particles before impregnation to a hydrothermal treatment in advance, thereby increasing the density of the outer shell, and then impregnate the hollow silica particles with an aluminum compound by heat treatment; and the other is to impregnate hollow silica particles that have not been subjected to a hydrothermal treatment in advance with an aluminum compound by heat treatment.
In either the former method or the latter method, it is preferable to impregnate the hollow silica particles with the aluminum compound so that the amount of aluminum atoms (in terms of Al ₂ O ₃ ) present throughout the hollow silica particles is in the above-mentioned specific ratio, as measured by the dissolution method using an aqueous hydrofluoric acid solution.

The aluminum compound used in step (IV) can be added in an amount of 0.0001 to 0.5 g, or 0.001 to 0.1 g, or 0.001 to 0.05 g, calculated as Al ₂ O _3, per gram of hollow silica particles in the hollow silica aqueous sol.
The heating temperature in step (IV) is 40 to 260° C., or 50 to 260° C., or 60 to 240° C. The heating temperature can be 40 to less than 100° C., or 50 to less than 100° C., or 60 to less than 100° C. in the case of a non-hydrothermal treatment, and 100 to 260° C., or 150 to 240° C. in the case of a hydrothermal treatment.
The heating time in step (IV) can be in the range of 0.1 to 48 hours, or 0.1 to 24 hours, or 0.1 to 10 hours, or 1 to 10 hours.
Incidentally, whether the aluminum compound is impregnated into the hollow silica particles to form an aluminosilicate and whether the amount of aluminum atoms present is the desired amount depends on the treatment temperature in step (IV), and it is important to carry out the heat treatment within the above temperature range.

Specific examples of the aluminum compound include at least one aluminum compound selected from the group consisting of aluminates, aluminum alkoxides, and hydrolysates thereof.
Examples of the aluminates include sodium aluminate, potassium aluminate, calcium aluminate, magnesium aluminate, ammonium aluminate, and amine aluminate. Examples of the aluminum alkoxides include aluminum isopropoxide and aluminum butoxide. Among these, aluminates such as sodium aluminate are preferably used.

The aluminum compound can be added to the hollow silica aqueous sol in the form of a solid or an aqueous solution, and is preferably added in the form of an aqueous solution. When the aluminum compound is added to the hollow silica aqueous sol in the form of an aqueous solution, the concentration of the aluminum compound in the aqueous solution can be in the range of 0.01 to 20% by mass, or 0.1 to 10% by mass, or 0.5 to 5% by mass.
The aluminum compound can be added while stirring the hollow silica aqueous sol. The addition may be completed before the heating, may be started before the heating and completed during the heating, or may be continued throughout the entire heating period.

The step (IV) may further include a step (IV-i) of adding an amine. The amine may be any of the above-mentioned amines, and may be contained in the hollow silica sol in the above-mentioned range.
Step (IV) may also include step (IV-ii) of incorporating a neutral salt composed of a combination of at least one cation selected from sodium ions, potassium ions, and ammonium ions with an inorganic or organic anion in an amount of 0.1 to 10 mass % relative to the mass of the hollow silica particles. Examples of the inorganic anion used in step (IV-ii) include sulfate ions, chloride ions, or phosphate ions, and examples of the organic anion include carboxylate ions, oxycarboxylate ions, and amino acids. Examples of preferred neutral salts include sodium sulfate, potassium sulfate, and ammonium sulfate.

[(V) Process]
Step (V) is a step of adding sulfuric acid to the aluminum atom-containing hollow silica aqueous sol obtained in step (IV) above at a ratio of 1 ppm to 5000 ppm/ _SiO2 relative to the hollow silica particles in the sol, and maintaining the mixture at a predetermined temperature. This step is a so-called leaching step in which aluminum atom-containing components that were not doped onto the surface or interior of the hollow silica particles in step (IV) above, metal impurities contained in the particles, and impurity basic components such as counter ion components that are eluted by the added sulfuric acid are eluted into a liquid.
In this step, after adding sulfuric acid, the mixture is maintained at 5 to 100° C. for 0.1 to 48 hours.
By passing through this step, the above-mentioned step (I): a step of preparing a hollow silica aqueous sol in which the amount of sulfuric acid relative to the mass of aluminum atom-containing hollow silica particles is 1 ppm to 5000 ppm/SiO ₂ is realized.

[(VI) Process]
Step (VI) is a step in which the aluminum atom-containing hollow silica aqueous sol obtained in step (V) is contacted with a cation exchange resin. This step makes it possible to remove metal-containing components, such as aluminum sulfate, or impurity base components that have been eluted into the solution by the addition of sulfuric acid. In other words, this step achieves the aforementioned step (II): a step in which the amount of sulfuric acid contained in the hollow silica aqueous sol prepared in step (I) is adjusted to 1 ppm to 150 ppm. Furthermore, because this step makes it possible to remove impurity base components that affect the stability of the sol, improved storage stability of the sol can be expected.
The operation in step (VI) may be carried out before the holding at the predetermined temperature in step (V), i.e., before or after the holding at the predetermined temperature in step (V), and the above operation may be carried out multiple times, for example, 2 to 10 times, 2 to 8 times, 2 to 5 times, or 3 to 5 times, as necessary.

Furthermore, after step (VI), an ultrafiltration (UF) step may be included, if desired. This step involves ultrafiltration (UF) of the silica sol obtained in step (VI), with the aim of adjusting the amount of sulfuric acid in the system to the desired value.

[(VII) Process]
Furthermore, after completion of the above step (II) or the above step (VI) or any subsequent UF step, a step (VII) can be included in which the dispersion medium (water) in the aluminum atom-containing hollow silica aqueous sol obtained in the above steps is replaced with an organic solvent by vacuum replacement (heat replacement under reduced pressure), heat replacement under normal pressure, or ultrafiltration (UF).
Examples of the organic solvent to be substituted include alcohols having 1 to 10 carbon atoms, ketones having 1 to 10 carbon atoms, ethers having 1 to 10 carbon atoms, and esters having 1 to 10 carbon atoms. Specific examples of these compounds include the organic solvents listed as examples of the dispersion medium for the hollow silica sol according to the present invention.
The pressure reduction conditions may be about 10 to 600 Torr, and heating at about 30 to 200° C. may be performed in combination with the pressure reduction.

Furthermore, after the step (VI), a step (surface modification step) of coating the surface of the silica particles with the aforementioned silane compound [at least one silane compound selected from the group consisting of compounds represented by formula (1) and formula (2)] may be included.
This step can be carried out by adding the above-mentioned silane compound after the step (VI) and then heat treating at 10 to 100° C. for about 0.1 to 20 hours.
As described above, the surface modification step is a reaction that proceeds by hydrolysis of the silane compound. Therefore, it is preferable to carry out the surface modification step in the presence of water, for example, in the form of an aqueous sol, that is, before the solvent substitution step (VII) (if carried out) (or without carrying out the solvent substitution step).
Alternatively, the solvent substitution step (VII) may be carried out after the step (VI), and then the surface modification step may be carried out, followed by the solvent substitution step (VII).

<Film-forming composition>
The present invention also covers a film-forming composition containing the aluminum atom-containing hollow silica sol and an organic resin. Furthermore, the film-forming composition according to the present invention can be prepared by removing the dispersion medium (water, organic solvent) from the aluminum atom-containing hollow silica sol to form aluminum atom-containing hollow silica particles, and then adding the aluminum atom-containing hollow silica particles to the organic resin.
In the film-forming composition, the aluminum atom-containing hollow silica particles can account for, for example, 1% to 90% by mass of the total solid content (total 100% by mass), and the organic resin can account for, for example, 10% to 99% by mass. The solid content in the film-forming composition refers to all components other than the solvent, and can be the value calculated from the residue obtained by heating the film-forming composition at a temperature of about 200 to 300°C to remove the solvent.

The organic resin may be, for example, a thermosetting or photocurable resin (curable resin). Examples include, but are not limited to, styrene-based resins, epoxy-based resins, thioepoxy resins, novolac-based resins, cyanate-based resins, phenol-based resins, acrylic-based resins, maleimide-based resins, polyester-based resins, urethane-based resins, polyurea resins, polyimide-based resins, polyamide-based resins, polyamic acid resins, polyhydroxyimide resins, polybenzoxazole resins, polybenzimidazole resins, polybenzothiazole resins, polyhydroxyamide resins, polyhydroxyazomethine resins, polyether-based resins, polybenzoxazine resins, polytetrafluoroethylene-based resins, cycloolefin polymer-based resins, unsaturated polyester-based resins, vinyl triazine-based resins, polyphenylene sulfide-based resins, crosslinkable polyphenylene oxide-based resins, curable polyphenylene ether-based resins, and condensation-based resins.

Furthermore, the film-forming composition of the present invention may contain various curing agents as needed, such as amine-based curing agents, acid anhydride-based curing agents, radical generator-based curing agents (thermal radical generators, photoradical generators), acid generator-based curing agents (thermal acid generators or photoacid generators), and base generators (thermal base generators, photobase generators), as well as curing aids (organic phosphorus compounds, quaternary phosphonium salts, quaternary ammonium salts).
Furthermore, the film-forming composition of the present invention may contain conventional additives as needed. Examples of such additives include surfactants (leveling agents), pigments, colorants, thickeners, adhesion promoters, sensitizers, antifoaming agents, coatability improvers, lubricants, stabilizers (antioxidants, heat stabilizers, light resistance stabilizers, etc.), plasticizers, dissolution promoters, fillers, antistatic agents, development inhibitors (diazonaphthoquinone, etc.), etc. These additives may be used alone or in combination of two or more.

The film-forming composition can be applied to or filled on a substrate and then heated, irradiated with light, or a combination thereof to form a cured product.
For example, a photocurable film-forming composition can be applied to a substrate to form a coating film, which can then be cured by irradiating the coating film with light to obtain a coating (cured product). Heating can also be performed before or after light irradiation.
Examples of methods for applying the film-forming composition include flow coating, spin coating, spray coating, screen printing, casting, bar coating, curtain coating, roll coating, gravure coating, dipping, and slitting.
The thickness of the coating film can be selected from the range of about 0.01 μm to 10 mm depending on the application of the cured product. For example, when used as a photoresist, it can be about 0.05 to 10 μm (particularly 0.1 to 5 μm), when used as a printed wiring board, it can be about 5 μm to 5 mm (particularly 100 to 1 mm), and when used as an optical thin film, it can be about 0.1 to 100 μm (particularly 0.3 to 50 μm).
When a transparent coating is to be obtained, it is desirable that the visible light transmittance of the coating be 80% or more, or 90% or more, typically 90% to 96%.
When the film-forming composition of the present invention is a thermosetting film-forming composition, it can be obtained in the form of a thermosetting varnish by mixing the thermosetting resin (curable resin), the curing agent (such as a thermal acid generator), and, if desired, the curing aid. Mixing can be carried out in a reaction vessel using a stirring blade or a kneader.

Thermosetting materials and photocurable materials using the coating-forming composition of the present invention have characteristics such as rapid curing, transparency, and small cure shrinkage, and can be used for coating or bonding electronic components, optical components (anti-reflection coatings), and precision mechanical components.
For example, it can be used to bond mobile phone and camera lenses, optical elements such as light-emitting diodes (LEDs) and semiconductor lasers (LDs), liquid crystal panels, biochips, camera lenses and prisms, magnetic parts in hard disks of computers and the like, pickups in CD and DVD players (the part that captures the optical information reflected from the disc), speaker cones and coils, motor magnets, circuit boards, electronic parts, and parts inside automobile engines.
The present invention can also be used as a hard coating material (coating material) for surface protection of automobile bodies, lamps, electrical appliances, building materials, plastics, etc., and can be applied to, for example, automobile and motorcycle bodies, headlight lenses and mirrors, plastic lenses for eyeglasses, mobile phones, game consoles, optical films, ID cards, etc.
Further applications include application to complex three-dimensional cured materials in combination with three-dimensional CAD, application to photolithography such as model production of industrial products, application to coating of optical fibers, adhesion, optical waveguides, thick film resists, etc.

The film-forming composition of the present invention can also be suitably used as an insulating resin for electronic materials such as anti-reflection films, semiconductor encapsulation materials, adhesives for electronic materials, printed wiring board materials, interlayer insulating film materials, and encapsulants for power modules, as well as an insulating resin for high-voltage equipment such as generator coils, transformer coils, and gas-insulated switchgear.

The present invention will be explained in detail using the following examples, but the present invention is not limited to these examples.

The hollow silica sol, silane compound, pH adjuster, and solvent used were as follows:
(Raw material: hollow silica aqueous sol)
Product name HKT-A20-40D: water-dispersed silica sol of hollow silica particles (manufactured by Ningbo Dilato Co., Ltd.), average particle size by DLS method: 47 nm, average primary particle size by TEM: 40 nm, specific surface area by BET method: 125 m ² /g, shell thickness: 5 nm, refractive index: 1.3, silanol group density: 1.6/nm ² , pH: 10.4, viscosity: 9.6 mPa·s, silica particle concentration: 20.2 mass%
(Silane compound, manufactured by Shin-Etsu Chemical Co., Ltd.)
AcPS: 3-acryloxypropyltrimethoxysilane, MPS: 3-methacryloxypropyltrimethoxysilane, PTMS: phenyltrimethoxysilane, MPMDMS: 3-methacryloxypropylmethyldimethoxysilane, DMDMS: dimethyldimethoxysilane, HMDS: hexamethyldisiloxane, TMPSA: 3-trimethoxysilylpropylsuccinic anhydride, AAPS: 2-(allyloxymethyl)acrylate (trimethoxysilyl)propyl, MTMS: methyltrimethoxysilane, DMEVS: dimethylethoxyvinylsilane, DMPPS: dimethoxymethylphenylsilane (pH adjuster: base)
DiPA: diisopropylamine DiPEA: diisopropylethylamine (organic solvent)
MeOH: Methanol, MEK: Methyl ethyl ketone, PGME: Propylene glycol monomethyl ether, CPN: Cyclopentanone, PGMEA: Propylene glycol monomethyl ether acetate, MIBK: Methyl isobutyl ketone, EL: Ethyl lactate, HBM: Methyl 2-hydroxyisobutyrate

The methods and procedures for measuring the physical properties of each hollow silica sol are as follows.
(Measurement of silica particle concentration in silica sol)
1 g of each hollow silica sol was placed in a crucible, and the sol was heated and dried on a hot plate at a temperature 10°C higher than the boiling point of the dispersion medium (water or methanol (MeOH)) to remove the solvent, and then baked at 1000°C for 30 minutes. The baked residue was weighed and the silica particle concentration (% by mass) was calculated. As will be described later, the sol prepared in each synthesis example contains sulfuric acid, an amine for adjusting the pH, and the like. However, after the above-mentioned baking, the organic components such as the amine are almost completely eliminated by volatilization/thermal decomposition, and the amounts of these components, including sulfuric acid, added are very small, so the concentration calculated by the above-mentioned method can be treated as substantially the hollow silica particle concentration in the silica sol.

(pH measurement of silica sol)
The pH of the water-dispersed hollow silica sol was measured at 23° C. using a pH meter (manufactured by DKK-TOA Corporation, product name: MM-43X).
The pH of the organic solvent-dispersed hollow silica sol was measured at 20°C using a pH meter (MM-43X, manufactured by DKK-TOA Corporation) using a solution prepared by mixing pure water and the sol at a mass ratio of 1:1 for sols of organic solvents that can be arbitrarily mixed with water, such as MeOH dispersion sol and PGME dispersion sol, or a solution prepared by mixing pure water, methanol, and the organic solvent sol at a mass ratio of 1:1:1 for sols of organic solvents that have low solubility in water, such as MEK dispersion sol and CPN dispersion sol.

(Viscosity Measurement)
Measurement was carried out at 25°C using an Ostwald viscometer (manufactured by Shibata Scientific Co., Ltd.).

(Measurement of moisture content)
The water content of the organic solvent dispersion sol was measured by Karl Fischer titration.

(Measurement of MeOH amount)
The amount of methanol (MeOH) contained in the target sol was measured using gas chromatography (manufactured by Shimadzu Corporation, product name: GC-2014s) under the conditions described below.
Column: 3 mm x 1 m glass column Packing material: Polapack Q
Column temperature: 130 to 230°C (heating rate: 8°C/min)
Carrier: _N2 40mL/min
Detector: FID
Injection volume: 1μL
Internal standard: Acetonitrile.

(Measurement of specific surface area ( _SN2 ) by nitrogen adsorption method)
The nitrogen adsorption specific surface area (S _N2 ) of the hollow silica particles in the water-dispersed hollow silica sol was measured by removing water-soluble cations in the water-dispersed hollow silica sol with an H-type cation exchange resin (manufactured by The Dow Chemical Company, trade name: Amberlite IR-120B), drying the silica sol at 290°C, pulverizing the dried product in a mortar, and further heating it at 250°C for 2 hours to prepare a measurement sample, which was then measured by the BET single-point method using a Monosorb nitrogen adsorption specific surface area measuring device (manufactured by Quantachrome Instruments Japan LLC) with a mixed gas of 30% N ₂ (nitrogen) and 70% He (helium) as the carrier gas.
The nitrogen adsorption specific surface area (S _N2 ) of the hollow silica particles in the organic solvent-dispersed hollow silica sol was measured by the BET single-point method using a measurement sample obtained by the following procedure. 4 ml of the organic solvent-dispersed hollow silica sol was added to a 42 ml pear-shaped settling tube (manufactured by Thermo Fisher Scientifics, trade name: Nalgene Oak Ridge), and 4 ml of MEK and 20 ml of hexane were added, followed by standing for 5 minutes to cause cloudiness due to aggregation, separation, or precipitation. The mixture was then centrifuged (temperature: 5°C, rotation speed: 20,000 rpm, time: 30 minutes) using a centrifuge (manufactured by Tomy Seiko Co., Ltd., trade name: High-Speed Refrigerated Centrifuge Suprema 21), and the supernatant was removed. 4 ml of acetone was then added, and the precipitate formed by centrifugation was redissolved using a test tube mixer (manufactured by AS ONE Corporation, trade name: MVM-10), after which 20 mL of hexane was added. The mixture was then centrifuged and the supernatant was removed. 4 ml of acetone was then added, and the precipitate formed by centrifugation was redissolved in a test tube mixer, after which 20 ml of hexane was added. The mixture was then centrifuged and the supernatant was removed. The resulting precipitate (mixture) was vacuum dried (temperature: 60°C, pressure: -0.1 MPa), and the resulting powder was ground in a mortar for 10 minutes to obtain silica particles (powder). The resulting silica particles were heated at 150°C to remove adsorbed water, and a measurement sample was prepared.

(Particle size measured by dynamic light scattering (average particle size (nm) measured by DLS method)
The average particle size (Z-average particle size) by the DLS method was measured using a dynamic light scattering particle size analyzer (manufactured by Malvern Panalytical, product name: Zetasizer Nano). The dilution solvent used when measuring the average particle size by the DLS method was the same as the dispersion medium of each sol. Specifically, 0.1 g of the target silica sol was dispensed into a glass cell with an optical path length of 10 mm, and the same solvent as the dispersion medium of the silica sol was added to obtain a silica sol with a silica particle concentration adjusted so that the count rate at an attenuator setting of 7 was 200 to 400 kcps. The prepared silica sol was placed in the cell so that the height of the liquid surface from the bottom of the cell was approximately 1 cm, and the dynamic light scattering particle size of the silica sol was measured using an attenuator setting of 7.

(Measurement of average primary particle size using TEM (transmission electron microscope))
The particles in the hollow silica sol were photographed using a transmission electron microscope (manufactured by JEOL Ltd., trade name: JEM-F200), and 300 arbitrarily selected particles were binarized using an automatic image processing analyzer (manufactured by Nireco Corporation, trade name: LUZEX' AP), and the diameter of the projected area converted into a circle was measured as the average primary particle diameter (HEYWOOD diameter).

(Shell thickness measurement)
The particles in the hollow silica sol were photographed using a transmission electron microscope (manufactured by JEOL Ltd., trade name: JEM-F200), and 300 arbitrarily selected particles were binarized using an automatic image processing analyzer (manufactured by Nireco Corporation, trade name: LUZEX' AP). The inner diameters obtained by converting the projected areas into circles were averaged, and the difference from the primary particle diameter measured by TEM was measured, and the obtained results were then averaged.

(Measurement of tetrahedral Al content by ^27Al -NMR)
1.5 mL of the water-dispersed hollow silica sol was mixed with 1.5 mL of heavy water and added to a 10 mm diameter Teflon (registered trademark) test tube, followed by measurement using a nuclear magnetic resonance (NMR) spectrometer (trade name: ECA500, manufactured by JEOL Ltd.) under the following conditions.
The area of the signal obtained from the measurement at 50 to 65 ppm was defined as (α ₀ ), and the area of the signal obtained from -5 to 10 ppm was defined as (β ₀ ). When multiple signals were observed, the areas of all the signals were taken as the sum. From the above (α ₀ ) and (β ₀ ), the tetracoordinated Al ratio (maximum value: 1) was calculated using the following formula:
4-coordinate Al ratio=(α ₀ )/{(α ₀ )+(β ₀ )}]
Probe: 10 mm silicon BG-free probe Nuclide: ²⁷ Al (with 1H decoupling, NOE off)
・Observation frequency: 500.15992MHz
・Measurement temperature: 23℃)
90 degree pulse width: 15 μs
Pulse repetition time: 2.5 s
・Waiting time: 2 seconds
・Number of accumulations: 10,000
・Number of points: 32,768
・Rotation: off
Analysis software: Delta (V5.0.6) (manufactured by JEOL Ltd.)
・Window function: sexp
・BF: 5.0Hz

(Measurement of the amount of sulfuric acid in the water-dispersed sol (hereinafter referred to as the amount of sulfuric acid in the system (ppm/sol)))
The obtained water-dispersed hollow silica sol was diluted 10 times with pure water, and then the amount of sulfuric acid in the system (ppm/sol) was measured by ion chromatography using an anion analyzer (trade name Dionex ICS-2100, manufactured by Thermo Scientific).

(Measurement of the amount of sulfuric acid in hollow silica particles (hereinafter referred to as the amount of sulfuric acid in the system (ppm/SiO ₂ )))
The amount of sulfuric acid (ppm) in the system was divided by the concentration of silica particles (mass %) to calculate a value, and the resulting value was taken as a constant (ppm/SiO ₂ ).

(Measurement of the amount of aluminum present throughout the hollow silica particle / Dissolution method using aqueous hydrofluoric acid solution)
The precisely weighed hollow silica sol was dried, and 250 mg of the obtained particles were dissolved in 2.5 ml of nitric acid (manufactured by Kanto Chemical Co., Inc., trade name: Nitric Acid 1.38, purity 60.0%) and 2.5 ml of 38% hydrofluoric acid (manufactured by Tama Chemicals Co., Ltd., trade name: Hydrofluoric Acid) to obtain an aqueous solution. The amount of aluminum in the obtained aqueous solution was measured using an ICP-OES analyzer (manufactured by Rigaku Corporation, trade name: CIROS120 EOP), and the amount of aluminum present in the entire hollow silica particles was _calculated in terms of _Al2O3 (per mass of hollow _silica particles) ( _Al2O3 (ppm)/ _SiO2 ).

(Measurement of surface charge amount of hollow silica particles)
The hollow silica sol was added to 10 mL of methanol and diluted to a silica particle concentration of 0.5% by mass to prepare a measurement sample. Using a particle charge meter (trade name PCD-06, manufactured by Voith Turbo K.K.), a 0.001 mol/L (N/1000) DADMAC solution (manufactured by Voith Turbo K.K.) was used as a standard cation titrant to measure the titration value until the streaming potential of the measurement sample reached zero. The obtained titration value was divided by the amount of silica particles contained in the measurement sample, and the value converted to the surface charge amount (μeq/g) per 1 g of hollow silica particles was determined. DADMAC stands for diallyldimethylammonium chloride.

(Synthesis Example 1) Preparation of Water-Dispersed Sol (A1) of Aluminum Atom-Containing Hollow Silica Particles 1856 g of water-dispersed hollow silica sol HKT-A20-40D (trade name, manufactured by Ningbo Dilato Co., Ltd.) was placed in a 3 L plastic container, and 32.2 g of a sodium aluminate aqueous solution diluted to a concentration of 1.0 mass% (calculated as Al ₂ O _{3 )} was added dropwise over 1 minute while stirring at a rotation speed of 650 rpm using a mechanical stirrer equipped with a glass stirring blade. The mixture was then stirred at the same rotation speed for 30 minutes. 643.6 g of pure water was then added, and the mixture was stirred for an additional 20 minutes to obtain a mixture. Next, 2502 g of this mixture was placed in a 3 L SUS autoclave container, heated at 150 ° C for 5 hours while stirring at 80 rpm, and then cooled to below 50 ° C.
The above series of operations was carried out twice, and the results were combined to obtain 4605 g of a heat-treated water-dispersed sol. Then, 5.6 g of an 8.2% aqueous sulfuric acid solution was added dropwise to 3070 g of the heat-treated water-dispersed sol, and the mixture was stirred at 25° C. for 1 hour to obtain a sulfuric acid-added heat-treated water-dispersed sol.
Next, the obtained sulfuric acid-added heat-treated water-dispersed sol was passed through a 400 mL column-packed cation exchange resin (H-type Amberlite (trade name) IR-120B, harmonic mean diameter 0.6 to 0.8 mm, Organo Corporation) at a space velocity (SV) of 5/hour to obtain a water-dispersed sol of aluminum atom-containing hollow silica particles.
Thereafter, the obtained water-dispersed sol was subjected to a heat treatment at 80°C for 10 hours, and then cooled to 30°C or lower. After that, the liquid was passed through a cation exchange resin (H-type Amberlite (trade name) IR-120B) packed in a column again at a space velocity (SV) of 5/hour to obtain a water-dispersed sol (A1) of aluminum atom-containing hollow silica particles.
The pH of the obtained water-dispersed sol (A1) was 2.5, and the amount of sulfuric acid in the system was 136 ppm/sol. It also had a BET specific surface area of 125 ^m /g, a silica particle concentration of 14.8 mass%, a viscosity of 1.4 mPa·s, an average particle size of 50 nm by DLS, a tetrahedral Al ratio of 0.59, an amount of aluminum atoms present in the whole hollow silica particles _converted into _Al2O3 of 688 ppm/ _SiO2 , a surface charge amount converted into per gram of hollow silica particles of 29 μeq/g, and a shell thickness of 5 nm.
FIG. 2 shows the ²⁷ Al-NMR spectrum of the heat-treated water-dispersed sol (before adding sulfuric acid) (FIG. 2(A)), and the ²⁷ Al-NMR spectrum of the water-dispersed sol (A1) of aluminum atom-containing hollow silica particles (after adding sulfuric acid) (FIG. 2(B)).

Synthesis Example 2 Preparation of Water-Dispersed Sol (A2) of Aluminum-Atom-Containing Hollow Silica Particles A water-dispersed sol (A2) of aluminum-atom-containing hollow silica particles was obtained in the same manner as in Synthesis Example 1, except that the amount of 8.2% aqueous sulfuric acid solution added was 1.4 g per 1,535 g of the heat-treated water-dispersed sol.
The pH of the obtained water-dispersed sol (A2) was 2.7, and the amount of sulfuric acid in the system was 67 ppm/sol. It also had a BET specific surface area of 125 ^m /g, a silica particle concentration of 14.8 mass%, a viscosity of 1.6 mPa·s, an average particle size of 51 nm by DLS, a tetrahedral Al ratio of 0.72, an amount of aluminum atoms present in the whole hollow silica particles _converted into _Al2O3 of 701 ppm/ _SiO2 , a surface charge amount converted into per gram of hollow silica particles of 26 μeq/g, and a shell thickness of 5 nm.

Synthesis Example 3 Preparation of Water-Dispersed Sol (A3) of Aluminum-Atom-Containing Hollow Silica Particles 1,400 g of the water-dispersed sol (A1) of aluminum-atom-containing hollow silica particles obtained in Synthesis Example 1 and 186 g of pure water were placed in a stirring type Ultraholder UHP-150K (manufactured by Advantec Co., Ltd.) equipped with a 150 mm diameter, 200,000 fractionation ultrafilter (manufactured by Advantec Co., Ltd.), and then filtered while applying a nitrogen pressure of 0.2 MPa. Thereafter, 365 g of pure water was added to 905 g of the filtered water-dispersed sol to obtain 1,270 g of water-dispersed sol (A3) of aluminum-atom-containing hollow silica particles.
The pH of the obtained water-dispersed sol (A3) was 2.7, and the amount of sulfuric acid in the system was 90 ppm/sol. It also had a BET specific surface area of 125 ^m2 /g, a silica particle concentration of 14.8 mass%, a viscosity of 1.5 mPa·s, an average particle size of 50 nm by DLS, a tetrahedral Al ratio of 0.65, an amount of aluminum atoms present in the whole hollow silica particles _converted into _Al2O3 of 688 ppm/ _SiO2 , a surface charge amount converted into per gram of hollow silica particles of 29 μeq/g, and a shell thickness of 5 nm.

Synthesis Example 4 Preparation of Water-Dispersed Sol (A4) of Aluminum-Atom-Containing Hollow Silica Particles 2500 g of water-dispersed hollow silica sol HKT-A20-40D (trade name, manufactured by Ningbo Dilato Co., Ltd.) was placed in a 3 L plastic container, and 42.5 g of a sodium aluminate aqueous solution diluted to a concentration of 1.0 mass% in terms of Al ₂ O ₃ was added dropwise over 1 minute while stirring at a rotation speed of 600 rpm with a mechanical stirrer equipped with a glass stirring blade. Stirring was continued for an additional 30 minutes at the same rotation speed. Next, 2442 g of this mixture was placed in a 3 L SUS autoclave container, heated at 150°C for 5 hours while stirring at 80 rpm, and then cooled to below 50°C.
Thereafter, 5.31 g of an 8% aqueous sulfuric acid solution was added dropwise to 2,178 g of the obtained silica sol, and the mixture was stirred at 25°C for 2 hours to obtain a sulfuric acid-added, heat-treated water-dispersed sol. Thereafter, the same procedures as in Synthesis Example 1 were carried out to obtain a water-dispersed sol (A4) of aluminum atom-containing hollow silica particles.
The pH of the obtained water-dispersed sol (A4) was 2.2, and the amount of sulfuric acid in the system was 201 ppm/sol. The silica particle concentration was 17.7% by mass, the average particle size measured by DLS was 60 nm, the specific surface area measured by BET was 125 ^m /g, the tetrahedral Al ratio was 0.42, the amount of aluminum atoms present in the _entire hollow silica particles was 620 ppm/ _SiO2 in terms of _Al2O3 , the surface charge amount measured per 1 g of hollow silica particles was 24 μeq/g, the particle refractive index was 1.3, and the shell thickness was 5 nm.

(Synthesis Example 5) Preparation of Water-Dispersed Sol of Aluminum Atom-Containing Hollow Silica Particles (A5) 1856 g of water-dispersed hollow silica sol HKT-A20-40D (Ningbo Dilato Co., Ltd., trade name) was placed in a 3 L plastic container, and stirred at a rotation speed of 650 rpm with a mechanical stirrer equipped with a glass stirring blade. 32.2 g of sodium aluminate aqueous solution diluted to a concentration of 1.0 mass% in terms of Al ₂ O ₃ was added dropwise over 1 minute, and further stirred at the same rotation speed for 30 minutes. Next, 1888 g of this mixture was placed in a 3 L SUS autoclave container, and subjected to heat treatment at 150 ° C. for 5 hours while stirring at 80 rpm, and then cooled to 50 ° C. or below. Thereafter, 4.4 g of an 8.2% aqueous sulfuric acid solution was added dropwise to the resulting 1888 g of heat-treated water-dispersed sol, and the mixture was stirred at 25 ° C. for 1 hour to obtain a sulfuric acid-added heat-treated water-dispersed sol. Thereafter, the obtained sulfuric acid-added heat-treated water-dispersed sol was passed through a 200 mL column-packed cation exchange resin (H-type Amberlite (trade name) IR-120B, harmonic mean diameter 0.6 to 0.8 mm, Organo Corporation) at a space velocity (SV) of 5/hour to obtain a water-dispersed sol of aluminum atom-containing hollow silica particles.
Thereafter, the obtained water-dispersed sol was subjected to a heat treatment at 100°C for 5 hours, and then cooled to 30°C or lower. After that, the liquid was passed through a cation exchange resin (H-type Amberlite (trade name) IR-120B) packed in a column again at a space velocity (SV) of 5/hour to obtain a water-dispersed sol of aluminum atom-containing hollow silica particles.
The resulting water-dispersed sol had a pH of 2.4, a silica particle concentration of 13.1% by mass, and an average particle size measured by DLS of 55 nm.
The obtained water-dispersed sol was then placed in a stirring type ultra holder UHP-150K (manufactured by Advantec Co., Ltd.) equipped with a 150 mm diameter, 200,000 fractionation ultra filter (manufactured by Advantec Co., Ltd.), and then filtered while applying a nitrogen pressure of 0.2 MPa until the silica particle concentration reached 20 mass %, thereby obtaining a water-dispersed sol (A5) of hollow silica particles.
The pH of the obtained water-dispersed sol (A5) was 2.4, the amount of sulfuric acid in the system was 115 _{ppm/sol, the silica particle concentration was 20.4 mass%, the average particle size by DLS was 55 nm, the specific surface area by BET was 125 m2/g, the amount of aluminum atoms present in the whole hollow silica particles was 611 ppm/SiO2 in terms of Al2O3} ^, _{the surface} charge amount converted to 1 g of hollow silica particles was 24 μeq/g, the particle refractive index was 1.3, and the shell thickness was 5 nm.

(Synthesis Example 6) Preparation of water-dispersed sol (LA1) of aluminum-atom-containing hollow silica particles by long-term storage of water-dispersed sol (A1) of aluminum-atom-containing hollow silica particles The water-dispersed sol (A1) of aluminum-atom-containing hollow silica particles obtained in Synthesis Example 1 was stored at 23 ° C for 2 months to obtain water-dispersed sol (LA1) of aluminum-atom-containing hollow silica particles that had been stored for a long period of time. The tetrahedral Al ratio was 0.46.

(Synthesis Example 7) Preparation of water-dispersed sol (LA2) of aluminum-atom-containing hollow silica particles by long-term storage of water-dispersed sol (A2) of aluminum-atom-containing hollow silica particles The water-dispersed sol (A2) of aluminum-atom-containing hollow silica particles obtained in Synthesis Example 2 was stored at 23°C for 2 months to obtain water-dispersed sol (LA2) of aluminum-atom-containing hollow silica particles that had been stored for a long period of time. The tetrahedral Al ratio was 0.67.

(Synthesis Example 8) Preparation of water-dispersed sol (LA3) of aluminum-atom-containing hollow silica particles by long-term storage of water-dispersed sol (A3) of aluminum-atom-containing hollow silica particles The water-dispersed sol (A3) of aluminum-atom-containing hollow silica particles obtained in Synthesis Example 3 was stored at 23°C for 2 months to obtain water-dispersed sol (LA3) of aluminum-atom-containing hollow silica particles that had been stored for a long period of time. The tetrahedral Al ratio was 0.62.

(Synthesis Example 9) Preparation of water-dispersed sol (LA4) of aluminum-atom-containing hollow silica particles by long-term storage of water-dispersed sol (A4) of aluminum-atom-containing hollow silica particles The water-dispersed sol (A4) of aluminum-atom-containing hollow silica particles obtained in Synthesis Example 4 was stored at 23°C for 3 months to obtain water-dispersed sol (LA4) of aluminum-atom-containing hollow silica particles that had been stored for a long period of time. The tetrahedral Al ratio was 0.47.

(Synthesis Example 10) Preparation of water-dispersed sol (LA5) of aluminum-atom-containing hollow silica particles by long-term storage of water-dispersed sol (A5) of aluminum-atom-containing hollow silica particles The water-dispersed sol (A5) of aluminum-atom-containing hollow silica particles obtained in Synthesis Example 5 was stored at 23°C for 5 months to obtain water-dispersed sol (LA5) of aluminum-atom-containing hollow silica particles that had been stored for a long period of time. The tetrahedral Al ratio was 0.56.

Synthesis Example 11 Preparation of MeOH Dispersion Sol (Me1) of Aluminum Atom-Containing Hollow Silica Particles 80 g of the aqueous dispersion sol (A1) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 1 was placed in a 200 mL recovery flask. The pressure was reduced to 580 Torr using a rotary evaporator, and the solvent was replaced with methanol (MeOH) while the mixture was heated to 120°C. Methanol was then added to adjust the concentration, yielding a 20 mass% MeOH dispersion sol (Me1) of aluminum atom-containing hollow silica particles.
The physical properties of the obtained sol were a BET specific surface area of 125 m ² /g, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 3.1, an average particle diameter of 72 nm by DLS, and a water content of 1.2 mass %.

(Synthesis Example 12) Preparation of MeOH Dispersion Sol (Me2) of Aluminum Atom-Containing Hollow Silica Particles [0123] The same method as in Synthesis Example 11 was carried out, except that the water-dispersion sol (A2) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 2 was used as the target of solvent substitution, to obtain a 20 mass% MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles.
The physical properties of the obtained sol were a BET specific surface area of 125 m ² /g, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 3.3, an average particle diameter of 74 nm by DLS, and a water content of 1.3 mass %.

(Synthesis Example 13) Preparation of MeOH dispersion sol (Me3) of aluminum atom-containing hollow silica particles [0123] The same method as in Synthesis Example 11 was carried out, except that the water dispersion sol (A3) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 3 was used as the target of solvent substitution, to obtain a 20 mass% MeOH dispersion sol (Me3) of aluminum atom-containing hollow silica particles.
The physical properties of the obtained sol were a BET specific surface area of 125 m ² /g, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 3.2, an average particle diameter of 71 nm by DLS, and a water content of 1.0 mass %.

(Synthesis Example 14) Preparation of MeOH Dispersion Sol (Me4) of Aluminum Atom-Containing Hollow Silica Particles Into a 200 mL recovery flask, 118.3 g of the aqueous dispersion sol (A4) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 4 was placed, and 15.0 g of methanol was added. The pressure was reduced to 580 Torr using a rotary evaporator, and the solvent was replaced with methanol while heating to 120°C. Methanol was then added to adjust the concentration, and a 20.2 mass% MeOH dispersion sol (Me4) of aluminum atom-containing hollow silica particles was obtained.
The physical properties of the obtained sol were a BET specific surface area of 125 m ² /g, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 3.6, an average particle diameter of 70 nm by DLS, and a water content of 1.0 mass %.

(Synthesis Example 15) Preparation of MeOH dispersion sol (LMe1) of aluminum atom-containing hollow silica particles A 20 mass% MeOH dispersion sol (LMe1) of aluminum atom-containing hollow silica particles was obtained by the same method as in Synthesis Example 11, except that the water dispersion sol (LA1) of aluminum atom-containing hollow silica particles that had been stored for a long period of time obtained in Synthesis Example 6 was used as the target of solvent substitution.
The physical properties of the obtained sol were a BET specific surface area of 125 m ² /g, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 3.4, an average particle diameter of 71 nm by DLS, and a water content of 1.1 mass %.

(Synthesis Example 16) Preparation of MeOH dispersion sol (LMe2) of aluminum atom-containing hollow silica particles A 20 mass% MeOH dispersion sol (LMe2) of aluminum atom-containing hollow silica particles was obtained by the same method as in Synthesis Example 11, except that the water dispersion sol (LA2) of aluminum atom-containing hollow silica particles that had been stored for a long period of time obtained in Synthesis Example 7 was used as the target of solvent substitution.
The physical properties of the obtained sol were a specific surface area of 125 m ² /g by BET method, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 3.7, an average particle diameter of 76 nm by DLS method, and a water content of 1.2 mass %.

(Synthesis Example 17) Preparation of MeOH dispersion sol (LMe3) of aluminum atom-containing hollow silica particles A 20 mass% MeOH dispersion sol (LMe3) of aluminum atom-containing hollow silica particles was obtained by the same method as in Synthesis Example 11, except that the water dispersion sol (LA3) of aluminum atom-containing hollow silica particles that had been stored for a long period of time obtained in Synthesis Example 8 was used as the target of solvent substitution.
The physical properties of the obtained sol were a BET specific surface area of 125 m ² /g, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 3.4, an average particle diameter of 74 nm by DLS, and a water content of 1.0 mass %.

(Synthesis Example 18) Preparation of MeOH dispersion sol (LMe4) of aluminum atom-containing hollow silica particles A 20 mass% MeOH dispersion sol (LMe4) of aluminum atom-containing hollow silica particles was obtained by the same method as in Synthesis Example 14, except that the water dispersion sol (LA4) of aluminum atom-containing hollow silica particles that had been stored for a long period of time obtained in Synthesis Example 9 was used as the target of solvent substitution.
The physical properties of the obtained sol were a BET specific surface area of 125 m ² /g, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 3.7, an average particle diameter of 64 nm by DLS, and a water content of 1.0 mass %.

(Synthesis Example 19) Preparation of MeOH dispersion sol (LMe5) of aluminum atom-containing hollow silica particles A 20 mass% MeOH dispersion sol (LMe5) of aluminum atom-containing hollow silica particles was obtained by the same method as in Synthesis Example 11, except that the water dispersion sol (LA5) of aluminum atom-containing hollow silica particles that had been stored for a long period of time obtained in Synthesis Example 10 was used as the target of solvent substitution.
The physical properties of the obtained sol were a BET specific surface area of 125 m ² /g, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 3.4, an average particle diameter of 74 nm by DLS, and a water content of 0.5% by mass.

(Synthesis Example 20) Preparation of PGME-Dispersed Sol (P1) of Aluminum-Atom-Containing Hollow Silica Particles 50 g of the aqueous dispersion sol (A2) of hollow silica particles obtained in Synthesis Example 2 was placed in a 100 mL eggplant-shaped flask and set in a rotary evaporator. Distillation was carried out while supplying PGME at a bath temperature of 85°C under a reduced pressure of 350 to 100 Torr, and the dispersion medium was replaced with PGME to obtain a PGME-Dispersed Sol (P1) of aluminum-atom-containing hollow silica particles.
The physical properties of the obtained sol were an average particle size of 57 nm by DLS, pH 3.8, viscosity 3.0 mPa s, silica particle concentration 19.7 mass%, and water content 0.4 mass%. The obtained sol was free of sediment and showed good dispersibility (a state in which the sol was dispersed in the dispersion medium without cloudiness or aggregation; the same applies hereinafter).

(Synthesis Example 21) Preparation of PGME dispersion sol (O1) of hollow silica particles obtained by coating PGME dispersion sol (P1) of aluminum atom-containing hollow silica particles with a silane compound PGME dispersion sol (P1) 50 g of hollow silica particles obtained in Synthesis Example 20 was placed in a 100 mL eggplant-shaped flask, and while stirring with a magnetic stirrer, 0.7 g of pure water and 15.0 g of PGME were added, and the concentration was adjusted to 15.0 mass%. Then, 0.47 g of MPS was added, and the mixture was heated to 70 ° C. and maintained for 5 hours, thereby obtaining a PGME dispersion sol (O1) of aluminum atom-containing hollow silica particles whose surface was coated with a silane compound (MPS).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 58 nm by DLS, pH 4.4, viscosity 3.0 mPa s, silica particle concentration 14.9 mass %, and water content 1.2 mass %. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 22) MeOH dispersion sol (Me2) of hollow silica particles containing aluminum atoms prepared by coating the hollow silica particles with a silane compound (O2) MeOH dispersion sol (Me2) of aluminum atoms containing hollow silica particles obtained in Synthesis Example 12 was placed in a 100 mL eggplant-shaped flask, and while stirring with a magnetic stirrer, MeOH was added, and the concentration was adjusted to 15.5% by mass. Then, 0.46 g of AcPS was added, and the mixture was heated to 60 ° C. and maintained for 5 hours, thereby obtaining a MeOH dispersion sol (O2) of aluminum atoms containing hollow silica particles whose surface was coated with a silane compound (AcPS).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 78 nm by DLS, pH 4.0, viscosity 0.9 mPa s, silica particle concentration 15.8 mass %, water content 1.0 mass %, and silanol group density 1.2/ ^nm² . The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 23) Preparation of MeOH dispersion sol (O3) of hollow silica particles obtained by coating the aluminum atom-containing hollow silica particles with a silane compound in MeOH dispersion sol (Me2) 30 g of the silane compound-coated hollow silica particles in MeOH dispersion sol (O2) obtained in Synthesis Example 22 was placed in a 100 mL eggplant-shaped flask and placed in a rotary evaporator. Thereafter, the pressure was reduced to 550 to 350 Torr, and the solvent methanol (MeOH) was distilled off under a heated condition at 70 ° C., and the silica particle concentration was concentrated to 30 mass %, resulting in a silane compound (AcPS)-coated aluminum atom-containing hollow silica particle MeOH dispersion sol (O3).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 74 nm by DLS, pH 3.3, viscosity 2.4 mPa s, silica particle concentration 31.3 mass %, water content 1.8 mass %, and silanol group density 1.2/ ^nm² . The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 24) MeOH dispersion sol of aluminum atom-containing hollow silica particles (Me2) coated with a silane compound to prepare MeOH dispersion sol (O4) of hollow silica particles. 50 g of MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 12 was added to a 500 mL eggplant-shaped flask, and while stirring with a magnetic stirrer, 1.0 g of pure water, 7.5 g of MEK, and 3.7 g of DMDMS were added, and the mixture was heated to 60 ° C. and maintained for 3 hours. Next, 3.8 g of HMDS was added, and the mixture was heated to 60 ° C. and maintained for 3 hours. Thereafter, DiPA was added so that the pH was 8.0 to 10.0, and the mixture was heated to 60 ° C. and maintained for 1 hour to obtain a MeOH dispersion sol (O4) of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (DMDMS + HMDS).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 73 nm by DLS, a pH of 8.9, a viscosity of 1.0 mPa s, a silica particle concentration of 16.9% by mass, and a water content of 2.7% by mass. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 25) Preparation of CPN dispersion sol (O5) of hollow silica particles obtained by coating the MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles with a silane compound 89.4 g of the MeOH sol (O4) of hollow silica particles coated with a silane compound obtained in Synthesis Example 24 was placed in a 200 mL eggplant-shaped flask and set in a rotary evaporator. Distillation was carried out at a bath temperature of 80°C and a reduced pressure of 30 to 100 Torr while supplying CPN, replacing the dispersion medium with CPN, thereby obtaining a CPN dispersion sol (O5) of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (DMDMS+HMDS).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 56 nm by DLS, pH 5.1, silica particle concentration of 30.6 mass%, water content 1.2 mass%, and MeOH content less than 0.1 mass%. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 26) Preparation of MEK dispersion sol (O6) of hollow silica particles obtained by coating the MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles with a silane compound Step A: 50 g of the MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 12 was placed in a 1 L eggplant flask. Then, using a rotary evaporator, the pressure was reduced to 580 Torr and the solvent methanol (MeOH) was distilled off while heating to 120 ° C., and the silica particle concentration was concentrated to 30 wt%. The physical properties of the obtained sol were an average particle size of 72 nm by DLS method, pH 3.1, viscosity 1.8 mPa s, water content 1.7 wt%, and silica particle concentration 30.3 wt%. The obtained sol showed no sediment and showed good dispersibility.
Step B: 30 g of the MeOH sol with a silica particle concentration of 30% by mass obtained in step A was added to a 100 mL eggplant-shaped flask, and while stirring with a magnetic stirrer, 0.47 g of pure water, 4.5 g of MEK, and 0.43 g of MPMDMS were added, and the mixture was heated to 60 ° C. and maintained for 3 hours. Next, 0.5 g of HMDS was added, and the mixture was heated to 60 ° C. and maintained for 3 hours. Thereafter, DiPA was added so that the pH was 8.0 to 10.0, and the mixture was heated to 60 ° C. and maintained for 1 hour to obtain a MeOH sol of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (MPMDMS + HMDS). Thereafter, the mixture was set in a rotary evaporator, and distillation was carried out while supplying MEK at a bath temperature of 80°C under a reduced pressure of 550 to 400 Torr, and the dispersion medium was replaced with MEK, thereby obtaining an MEK dispersion sol (O6) of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (MPMDMS+HMDS).
The physical properties of the obtained sol were an average primary particle size of 40 nm by TEM, an average particle size of 64 nm by DLS, pH 6.7, viscosity 2.9 mPa s, silica particle concentration 31.2 mass%, water content 0.2 mass%, and MeOH content 0.2 mass%. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 27) Preparation of MEK dispersion sol (O7) of hollow silica particles obtained by coating the MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles with a silane compound 30 g of 30 mass% MeOH sol of aluminum atom-containing hollow silica particles obtained in Step A of Synthesis Example 26 was added to a 100 mL recovery flask, and while stirring with a magnetic stirrer, 0.50 g of pure water and 0.48 g of PTMS were added, and the mixture was heated to 60 ° C and maintained for 2 hours. Thereafter, DiPEA was added so that the pH was 7.5 to 8.5, and the mixture was heated to 60 ° C and maintained for 2 hours. Thereafter, 0.25 g of PTMS was added, and the mixture was heated to 60 ° C and maintained for 2 hours.
The eggplant-shaped flask containing the obtained sol was set in a rotary evaporator, and distillation was carried out while supplying MEK at a bath temperature of 80°C under a reduced pressure of 550 to 350 Torr, and the dispersion medium was replaced with MEK, thereby obtaining an MEK-dispersed sol (O7) of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (PTMS).
The physical properties of the obtained sol were as follows: average primary particle diameter by TEM: 40 nm, average particle diameter by DLS method: 70 nm, pH 5.4, viscosity 1.8 mPa s, silica particle concentration 30.9 mass%, water content less than 0.1 mass%, MeOH content less than 0.1 mass%. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 28) MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles coated with a silane compound MeOH dispersion sol (O8) of hollow silica particles 30 mass% MeOH sol of aluminum atom-containing hollow silica particles obtained in step A of Synthesis Example 26 was added to a 100 mL eggplant flask, and while stirring with a magnetic stirrer, 0.37 g of TMPSA was further added, heated to 60 ° C. and held for 3 hours. Thereafter, in a rotary evaporator, the pressure was reduced to 580 Torr, and the solvent methanol (MeOH) was distilled off under a heated condition at 120 ° C., and the silica particle concentration was concentrated to 30 mass%, thereby obtaining a MeOH dispersion sol (O8) of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (TMPSA).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 77 nm by DLS, pH 3.2, viscosity 3.4 mPa s, silica particle concentration 31.6 mass %, water content 3.2 mass %, and silanol group density 2.0/ ^nm² . The obtained sol was free of sediment and showed good dispersibility.

Synthesis Example 29 Preparation of MEK Dispersion Sol (O9) of Hollow Silica Particles in Which the MeOH Dispersion Sol (Me2) of Aluminum-Atom-Containing Hollow Silica Particles is Coated with a Silane Compound 30 g (460 g) of the 30% by mass MeOH sol of aluminum-atom-containing hollow silica particles obtained in Step A of Synthesis Example 26 Was added to a 1 L recovery ?ask, and While stirring With a magnetic stirrer, 0.5 g of pure Water, 4.5 g of MEK, and 0.99 g of HMDS Were added, and the mixture Was heated to 60° C. and maintained for 3 hours. The recovery ?ask containing the obtained sol Was set in a rotary evaporator, and distillation Was performed While feeding MEK at a bath temperature of 80° C. under a reduced pressure of 550 to 350 Torr, and the dispersing medium Was replaced With MEK to obtain an MEK dispersion sol (O9) of aluminum-atom-containing hollow silica particles having their surfaces coated With a silane compound (HMDS).
The physical properties of the obtained sol were as follows: average particle size by DLS method: 71 nm, pH: 3.5, viscosity: 1.7 mPa·s, silica particle concentration: 31.7 mass%, silanol group density: 1.3/ ^nm2 , water content: 0.1 mass%, MeOH content: 0.2 mass%. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 30) MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles coated with a silane compound MeOH dispersion sol (O10) of hollow silica particles 30 mass% MeOH sol of aluminum atom-containing hollow silica particles obtained in step A of Synthesis Example 26 was added to a 100 mL eggplant-shaped flask, and while stirring with a magnetic stirrer, 0.37 g of AAPS was further added, heated to 60 ° C. and held for 3 hours. Thereafter, in a rotary evaporator, the pressure was reduced to 580 Torr, and the solvent methanol (MeOH) was distilled off under a heated condition at 120 ° C., and the silica particle concentration was concentrated to 30 mass%, thereby obtaining a MeOH dispersion sol (O10) of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (AAPS).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 76 nm by DLS, pH 3.4, viscosity 2.5 mPa s, silica particle concentration 31.2 mass %, and water content 1.3 mass %. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 31) MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles coated with a silane compound to prepare a MeOH dispersion sol (O11) of hollow silica particles 30 mass% MeOH sol of aluminum atom-containing hollow silica particles obtained in step A of Synthesis Example 26 was added to a 500 mL eggplant-shaped flask, and while stirring with a magnetic stirrer, 0.17 g of MTMS was further added, and the mixture was heated to 60 ° C. and held for 3 hours. Thereafter, the pressure was reduced to 580 Torr in a rotary evaporator, and the solvent MeOH was distilled off under a heated condition at 120 ° C., and the silica particle concentration was concentrated to 30 mass%, thereby obtaining a MeOH dispersion sol (O11) of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (MTMS).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 74 nm by DLS, pH 3.4, viscosity 2.1 mPa s, silica particle concentration 31.4 mass %, and water content 1.3 mass %. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 32) Preparation of PGME dispersion sol (O12) of hollow silica particles obtained by coating PGME dispersion sol (P1) of aluminum atom-containing hollow silica particles with a silane compound PGME dispersion sol (P1) 30g of hollow silica particles obtained in Synthesis Example 20 was placed in a 100mL eggplant-shaped flask, and while stirring with a magnetic stirrer, 0.4g of pure water and 9g of PGME were added, and the concentration was adjusted to 15.0% by mass. Then, 0.27g of AcPS was added, and the mixture was heated to 70 ℃ and maintained for 5 hours, thereby obtaining PGME dispersion sol (O12) of aluminum atom-containing hollow silica particles whose surface was coated with a silane compound (AcPS).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 59 nm by DLS, a pH of 3.6, a silica particle concentration of 15.0 mass%, and a water content of 1.1 mass%. The obtained sol was free of sediment and showed good dispersibility.

Synthesis Example 33 Preparation of Water-Dispersed Sol (A6) of Aluminum-Atom-Containing Hollow Silica Particles 1745 g of water-dispersed hollow silica sol HKT-A20-40D (trade name, manufactured by Ningbo Dilato Co., Ltd.) was placed in a 3 L plastic container, and 30 g of a sodium aluminate aqueous solution diluted to a concentration of 1.0 mass% (calculated as Al ₂ O _{3 )} was added dropwise over 1 minute while stirring at a rotation speed of 650 rpm using a mechanical stirrer equipped with a glass stirring blade. 30 g of the diluted sodium aluminate aqueous solution was added dropwise over 1 minute, and the mixture was stirred at the same rotation speed for 30 minutes. 725 g of pure water was then added, and the mixture was stirred for an additional 20 minutes to obtain a mixture. Next, 2320 g of this mixture was placed in a 3 L SUS autoclave container, heated at 150 ° C for 5 hours while stirring at 80 rpm, and then cooled to below 50 ° C. To 500 g of the heat-treated water-dispersed sol, 0.03 g of an 8.2% aqueous sulfuric acid solution was added dropwise, and the mixture was stirred at 25° C. for 1 hour to obtain a water-dispersed sol (A6) of aluminum atom-containing hollow silica particles.
The pH of the obtained water-dispersed sol (A6) was 9.9, and the amount of sulfuric acid in the system was 2 ppm/sol. It also had a BET specific surface area of 125 m ² /g, a silica particle concentration of 13.8 mass%, a viscosity of 1.7 mPa·s, an average particle size of 50 nm by DLS, a tetrahedral Al ratio of 1.0, an amount of aluminum atoms present in the whole hollow silica particles converted into Al ₂ O ₃ of 688 ppm/SiO ₂ , a surface charge amount converted into per gram of hollow silica particles of 142 μeq/g, and a shell thickness of 5 nm.

Synthesis Example 34 Preparation of Water-Dispersed Sol (A7) of Aluminum-Atom-Containing Hollow Silica Particles A water-dispersed sol (A7) of aluminum-atom-containing hollow silica particles was obtained in the same manner as in Synthesis Example 33, except that the amount of 8.2% aqueous sulfuric acid solution added was 0.2 g per 33 g of the heat-treated water-dispersed sol, and then 117 g of pure water was added.
The pH of the obtained water-dispersed sol (A7) was 9.4, and the amount of sulfuric acid in the system was 138 ppm/sol. It also had a BET specific surface area of 125 ^m /g, a silica particle concentration of 3.1 mass%, a viscosity of 1.0 mPa·s, an average particle size of 50 nm by DLS, a tetrahedral Al ratio of _1.0 , an amount of aluminum atoms present in the whole hollow silica particles converted into _Al2O3 of 688 ppm/ _SiO2 , a surface charge amount converted into per gram of hollow silica particles of 126 μeq/g, and a shell thickness of 5 nm.

Synthesis Example 35 Preparation of MeOH Dispersion Sol (Me6) of Aluminum Atom-Containing Hollow Silica Particles Into a 200 mL recovery flask, 350 g of the aqueous dispersion sol (A6) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 33 was placed. Using a rotary evaporator, the pressure was reduced to 580 Torr, and the solvent was replaced with methanol (MeOH) while heating to 120°C. Methanol was then added to adjust the concentration, and a 15 mass% MeOH dispersion sol (Me6) of aluminum atom-containing hollow silica particles was obtained.
The physical properties of the obtained sol were a BET specific surface area of 125 m ² /g, a particle refractive index of 1.3, a shell thickness of 5 nm, a pH of 7.1, an average particle diameter of 77 nm by DLS, and a water content of 0.9 mass %.

(Synthesis Example 36) MeOH dispersion sol of aluminum atom-containing hollow silica particles (Me6) coated with a silane compound to prepare a MeOH dispersion sol (O13) of hollow silica particles. 120 g of the MeOH dispersion sol (Me6) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 35 was added to a 500 mL eggplant-shaped flask, and while stirring with a magnetic stirrer, 1.5 g of pure water and 1.0 g of DMMPS were added, and the mixture was heated to 60 ° C. and maintained for 3 hours. Next, 1.2 g of DMEVS was added, and the mixture was heated to 60 ° C. and maintained for 3 hours. Thereafter, DiEPA was added so that the pH was 8.0 to 10.0, and the mixture was heated to 60 ° C. and maintained for 1 hour to obtain a MeOH dispersion sol (O13) of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (DMMPS + DMEVS).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 72 nm by DLS, a pH of 10.0, a viscosity of 1.0 mPa s, a silica particle concentration of 14.3 mass%, and a water content of 2.1 mass%. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 37) Preparation of PGMEA-Dispersion Sol (O14) of Hollow Silica Particles Prepared by Coating the MeOH-Dispersion Sol (Me6) of Aluminum-Atom-Containing Hollow Silica Particles with a Silane Compound 20 g of the MeOH sol (O13) of hollow silica particles coated with a silane compound obtained in Synthesis Example 36 was placed in a 200 mL eggplant-shaped flask and set in a rotary evaporator. Distillation was carried out at a bath temperature of 80°C and a reduced pressure of 100 Torr while supplying PGMEA, replacing the dispersion medium with PGMEA, thereby obtaining a PGMEA-dispersion sol (O14) of aluminum-atom-containing hollow silica particles whose surfaces were coated with silane compounds (DMMPS+DMEVS).
The physical properties of the obtained sol were an average primary particle size of 40 nm by TEM, an average particle size of 107 nm by DLS, pH 7.9, silica particle concentration of 17.8 mass%, water content of 1.0 mass%, and MeOH content of 3.3 mass%. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 38) Preparation of MIBK dispersion sol (O15) of hollow silica particles obtained by coating the MeOH dispersion sol (Me6) of aluminum atom-containing hollow silica particles with a silane compound 20 g of the MeOH sol (O13) of hollow silica particles coated with a silane compound obtained in Synthesis Example 36 was placed in a 200 mL eggplant-shaped flask and set in a rotary evaporator. Distillation was carried out while supplying MIBK at a bath temperature of 80°C under a reduced pressure of 100 Torr, and the dispersion medium was replaced with MIBK to obtain an MIBK dispersion sol (O15) of aluminum atom-containing hollow silica particles whose surfaces were coated with silane compounds (DMMPS+DMEVS).
The physical properties of the obtained sol were an average primary particle diameter of 40 nm by TEM, an average particle diameter of 73 nm by DLS, pH 8.4, silica particle concentration of 16 mass%, water content 1.0 mass%, and MeOH content less than 0.1 mass%. The obtained sol was free of sediment and showed good dispersibility.

Synthesis Example 39 Preparation of MeOH Dispersion Sol (O16) of Aluminum Atom-Containing Hollow Silica Particles 80 g of the aqueous dispersion sol (A7) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 34 was placed in a 200 mL recovery flask. The pressure was reduced to 580 Torr using a rotary evaporator, and the solvent was replaced with methanol (MeOH) while the mixture was heated to 120°C. Methanol was then added to adjust the concentration, yielding a 20 mass% MeOH dispersion sol (O16) of aluminum atom-containing hollow silica particles.
The physical properties of the obtained sol were an average primary particle size of 40 nm by TEM, an average particle size of 70 nm by DLS, a pH of 3.7, a viscosity of 0.6 mPa s, a silica particle concentration of 2.9 mass%, and a water content of 1.0 mass%. The obtained sol was free of sediment and showed good dispersibility.

(Synthesis Example 40) Preparation of EL-dispersion sol (O17) of hollow silica particles obtained by coating the MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles with a silane compound 200 g of the MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles obtained in Synthesis Example 12 was added to a 500 mL eggplant-shaped flask, and while stirring with a magnetic stirrer, 0.78 g of MPS was further added, and the mixture was heated to 60°C and maintained at this temperature for 5 hours.
Thereafter, a 100 mL eggplant-shaped flask containing 60 g of the obtained MeOH dispersion sol of hollow silica particles coated with the silane compound was set in a rotary evaporator, and distillation was carried out while supplying EL at a bath temperature of 90°C under a reduced pressure of 500 to 80 Torr, and the dispersion medium was replaced with EL, thereby obtaining an EL dispersion sol (O17) of aluminum atom-containing hollow silica particles whose surfaces were coated with the silane compound (MPS).
The physical properties of the obtained EL dispersion sol were as follows: average primary particle diameter by TEM: 40 nm, average particle diameter by DLS method: 60 nm, pH: 2.7, viscosity: 5.3 mPa s, silica particle concentration: 14.8 mass%, water content: 0.1 mass%, and MeOH content: less than 0.1 mass%.

(Synthesis Example 41) Preparation of EL dispersion sol (O18) of hollow silica particles obtained by coating MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles with a silane compound An EL dispersion sol (O18) of aluminum atom-containing hollow silica particles surface-coated with a silane compound (TMPSA) was obtained in the same manner as in Synthesis Example 40, except that 1.39 g of TMPSA was added instead of 0.78 g of MPS added in Synthesis Example 40.
The physical properties of the obtained EL dispersion sol were as follows: average primary particle diameter by TEM: 40 nm, average particle diameter by DLS method: 60 nm, pH: 2.7, viscosity: 5.5 mPa s, silica particle concentration: 15.6 mass%, water content: 0.1 mass%, and MeOH content: less than 0.1 mass%.

(Synthesis Example 42) Preparation of HBM dispersion sol (O19) of hollow silica particles obtained by coating MeOH dispersion sol (Me2) of aluminum atom-containing hollow silica particles with a silane compound A 100 mL recovery flask containing 30 g of the MEK dispersion sol (O6) obtained in Synthesis Example 26 was placed in a rotary evaporator, and distillation was carried out while supplying HBM at a bath temperature of 90°C under reduced pressure of 400 to 100 Torr, replacing the dispersion medium with HBM to obtain an HBM dispersion sol (O19) of aluminum atom-containing hollow silica particles whose surfaces were coated with a silane compound (MPMDMS+HMDS).
The physical properties of the obtained HBM dispersion sol were an average primary particle diameter of 40 nm measured by TEM, an average particle diameter of 73 nm measured by DLS, a pH of 6.7, a silica particle concentration of 36.8% by mass, a water content of less than 0.1% by mass, and a MeOH content of less than 0.1% by mass.

Examples 1 to 7 and Comparative Examples 1 and 2: Storage stability results of MeOH-dispersed sols Each of the MeOH-dispersed sols prepared in Synthesis Examples 11 to 19 was stored at 50°C for 1 week and 4 weeks, and the change in average particle size (%, absolute value) was calculated using the following formula from the results of measuring the average particle size by DLS before and after storage. The results are also shown in Table 1.
DLS change rate (%, absolute value) = 100 × [(DLS average particle size after storage at 50°C - DLS average particle size before storage at 50°C) / DLS average particle size before storage at 50°C]
The storage stability was evaluated based on the obtained DLS change rate according to the following criteria. A indicates the best storage stability, B indicates a slightly better result, and C indicates an unfavorable result. The results are also shown in Table 1.
<Storage stability evaluation>
A: DLS change rate: 0% or more and less than 6% as an absolute value B: DLS change rate: 6% or more and less than 30% as an absolute value C: DLS change rate: 30% or more as an absolute value

As shown in Table 1, the water-dispersed sols in which the amount of sulfuric acid in the system (amount of sulfuric acid in the sol) was 150 ppm/sol or less and the tetracoordinated Al ratio was 0.45 or more showed a DLS change rate of 5% or less before and after storage at 50°C for 1 week and 4 weeks in the organic solvent-dispersed sols (MeOH-dispersed sols), indicating good storage stability.
On the other hand, in the case of a water-dispersed sol in which the amount of sulfuric acid in the system (amount of sulfuric acid in the sol) was 200 ppm/sol or more, even though the tetracoordinated Al ratio was a low value of around 0.45, the DLS change rate in the organic solvent-dispersed sol exceeded 100% after storage at 50°C for 1 week, resulting in a lack of storage stability.

Examples 8 to 25: Storage Stability Results of Organic Sol Dispersion Sols The organic solvent dispersion sols prepared in Synthesis Examples 21 to 32 and Synthesis Examples 37 to 42 were stored at 50°C for one week. From the results of measuring the average particle size by the DLS method before and after storage, the rate of change in average particle size (%, absolute value) was calculated using the formula for determining the [DLS Change Rate] shown in the [Storage Stability Results of MeOH Dispersion Sols] above. In addition, the storage stability was evaluated according to the above <Storage Stability Evaluation> from the obtained DLS change rate value. The obtained results are shown in Table 2 (Table 2-1, Table 2-2).
As shown in Table 2 (Table 2-1, Table 2-2), all of the organic solvent-dispersed sols obtained in Synthesis Examples 21 to 32 and Synthesis Examples 37 to 42 had DLS change rates of 0% or more and less than 6% as absolute values, and exhibited good dispersion stability.

 

Claims (13)

An aluminum atom-containing hollow silica sol containing aluminum atom-containing hollow silica particles and sulfuric acid,
the amount of sulfuric acid contained in the hollow silica sol is 1 ppm to 150 ppm;
Hollow silica sol. 2. The hollow silica sol according to claim 1, wherein the amount of sulfuric acid contained in the hollow silica sol is 1 ppm to 5000 ppm/SiO ₂ relative to the mass of the hollow silica particles contained in the hollow silica sol. The hollow silica sol is
In ^27Al -NMR measurement, the ratio [(α ₀ )/{(α 0 )+(β ₀ )}] of the total integral value (α ₀ ) of peaks representing tetracoordinated aluminum atoms to the sum of the total integral value (α ₀ ) of peaks representing tetracoordinated aluminum atoms and the total integral value (β ₀ ) of peaks representing aluminum atoms other than tetracoordinated _aluminum atoms is 0.4 to 1.0.
The hollow silica sol according to claim 1. The amount of aluminum atoms present in the entire hollow silica particles in the hollow silica sol is
The content of Al ₂ O ₃ is 120 to 50,000 ppm/SiO ₂ relative to the mass of the hollow silica particles,
The hollow silica sol according to claim 1. the surface charge amount calculated per 1 g of hollow silica particles in the hollow silica sol is 5 to 250 μeq/g;
The hollow silica sol according to claim 1. At least a portion of the hollow silica particles in the hollow silica sol is coated with a silane compound,
The silane compound is represented by formula (1) and formula (2):
(In formula (1),
^R1 's are groups bonded to silicon atoms, and each R1 independently represents an alkyl group, a halogenated alkyl group, an alkenyl group, or an aryl group, or an organic group having an epoxy group, a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, a polyether group, a carboxy group, a protected carboxy group, a carboxy group-generating group, an imide group, or a cyano group, and which are bonded to silicon atoms via a Si-C bond, or a combination of these groups;
^R2 is a group or atom bonded to the silicon atom, and independently represents an alkoxy group, an acyloxy group, a hydroxy group, or a halogen atom, or a combination of these groups or atoms;
a represents an integer of 1 to 3;
In formula (2),
^R3 is a group bonded to a silicon atom, and each R3 independently represents an alkyl group, a halogenated alkyl group, an alkenyl group, or an aryl group, or an organic group having an epoxy group, a (meth)acryloyl group, a mercapto group, an amino group, a ureido group, a polyether group, a carboxy group, a protected carboxy group, a carboxy group-generating group, an imide group, or a cyano group, and which is bonded to a silicon atom via a Si-C bond, or a combination of these groups;
^R4 is a group or atom bonded to the silicon atom, and independently represents an alkoxy group, an acyloxy group, a hydroxy group, or a halogen atom, or a combination of these groups or atoms;
b represents an integer of 1 to 3, c represents an integer of 0 or 1,
Y is a group or atom bonded to the silicon atom, and represents an alkylene group, an NH group, or an oxygen atom.
At least one silane compound selected from the group consisting of compounds represented by
The hollow silica sol according to claim 1. the hollow silica particles in the hollow silica sol have an average particle size of 20 to 150 nm as measured by a dynamic light scattering method;
The hollow silica sol according to claim 1. the hollow silica sol contains an organic solvent selected from the group consisting of alcohols having 1 to 10 carbon atoms, ketones having 1 to 10 carbon atoms, ethers having 1 to 10 carbon atoms, esters having 1 to 10 carbon atoms, and amides;
The hollow silica sol according to claim 1. A film-forming composition comprising the aluminum atom-containing hollow silica sol according to any one of claims 1 to 8 and an organic resin. A method for producing the aluminum atom-containing hollow silica sol according to any one of claims 1 to 8, comprising:
The following steps (I) and (II):
Step (I): preparing a hollow silica aqueous sol having a sulfuric acid content of 1 ppm to 5000 ppm/ _SiO2 relative to the mass of aluminum atom-containing hollow silica particles;
Step (II): adjusting the amount of sulfuric acid contained in the hollow silica aqueous sol prepared in step (I) to 1 ppm to 150 ppm;
A method for producing an aluminum atom-containing hollow silica sol, comprising: A method for producing the aluminum atom-containing hollow silica sol according to any one of claims 1 to 8, comprising:
The following steps (III), (IV), (V), and (VI):
Step (III): preparing a hollow silica aqueous sol containing hollow silica particles;
Step (IV): adding an aluminum compound to the hollow silica aqueous sol prepared in Step (III) in a proportion of 0.0001 to 0.5 g, calculated as Al ₂ O ₃ , per 1 g of hollow silica particles, and maintaining the mixture at 40 to 260°C for 0.1 to 48 hours to obtain an aluminum atom-containing hollow silica aqueous sol;
Step (V): adding sulfuric acid to the aluminum atom-containing hollow silica aqueous sol obtained in Step (IV) in a ratio of 1 ppm to 5000 ppm/ _SiO2 relative to the mass of the hollow silica particles in the sol, and maintaining the mixture at 5 to 100°C for 0.1 to 48 hours;
Step (VI): contacting the aluminum atom-containing hollow silica aqueous sol obtained in step (V) with a cation exchange resin;
Including,
A method for producing hollow silica sol containing aluminum atoms. Furthermore, the following step (VII):
Step (VII): A step of replacing the dispersion medium in the aluminum atom-containing hollow silica aqueous sol obtained in the step (VI) from water to an organic solvent by heating replacement under reduced pressure, heating replacement under normal pressure, or ultrafiltration,
The method for producing the aluminum atom-containing hollow silica sol according to claim 11. Furthermore,
a step of replacing the dispersion medium in the aluminum atom-containing hollow silica aqueous sol obtained in the step (II) from water to an organic solvent by heating under reduced pressure, heating under normal pressure, or ultrafiltration,
The method for producing the aluminum atom-containing hollow silica sol according to claim 10.