Method for producing silica particles and method for producing silica sol

Abstract

To provide a method for producing silica particles having a low refractive index.SOLUTION: A method for producing silica particles comprises subjecting tetra-alkoxysilane to a hydrolysis reaction and a condensation reaction, the hydrolysis reaction and the condensation reaction have a reaction temperature of 28°C or lower from a start of a reaction to the end of the reaction, the hydrolysis reaction and the condensation reaction have a concentration of the alkali catalyst of 0.95 mass% or less in the reaction solution from the start of the reaction to the end of the reaction, and the method for producing silica sol comprises the method for producing the silica particles.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Silica films possess properties such as low refractive index, transparency, and electrical insulation, and are widely used in various industries. In particular, applications utilizing the low refractive index include anti-reflective coatings, optical waveguides, and lenses, and their practical use is being considered for electronic equipment displays, automotive panels, lighting fixtures, solar energy devices, and camera lenses. The refractive index, one of the properties of silica films, largely depends on the refractive index of the silica particles used to form it, making its control extremely important.

[0003] Many studies have been conducted on methods for producing silica particles. Known methods include the thermal decomposition of silicon tetrachloride (fumed silica, etc.), deionization of alkali silicates such as water glass, and hydrolysis and condensation reactions of alkoxysilanes (generally referred to as the "sol-gel method"). For example, Patent Documents 1 and 2 disclose a method for producing silica particles by hydrolysis and condensation reactions of alkoxysilanes. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2005-060217 [Patent Document 2] Japanese Patent Publication No. 2021-123527 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, Patent Documents 1 and 2 do not disclose anything regarding the refractive index of silica particles or methods for controlling it, and effective means for controlling the refractive index of silica particles were not clear.

[0006] This invention has been made in view of these problems, and the object of this invention is to provide a method for producing silica particles having a low refractive index. [Means for solving the problem]

[0007] Conventionally, suitable manufacturing conditions for obtaining silica particles with a low refractive index have not been disclosed. However, after diligent research, the inventors discovered that silica particles with a low refractive index can be obtained by optimizing the reaction temperature of the hydrolysis and condensation reactions and the concentration of the alkaline catalyst in the reaction solution, thus completing the present invention.

[0008] The gist of this invention is as follows: [1] A method for producing silica particles by hydrolyzing and condensing tetraalkoxysilane, wherein the reaction temperature from the start to the end of the hydrolysis and condensation reactions is 28°C or lower, and the concentration of the alkaline catalyst in the reaction solution from the start to the end of the hydrolysis and condensation reactions is 0.95% by mass or lower. [2] The method for producing silica particles according to [1], wherein the hydrolysis and condensation reactions are carried out by adding a liquid (B) containing tetraalkoxysilane and a liquid (C) containing an alkaline catalyst to a liquid (A) containing an alkaline catalyst, and causing the tetraalkoxysilane to undergo hydrolysis and condensation reactions. [3] A method for producing silica particles according to [1] or [2], wherein the concentration of the alkaline catalyst in the reaction solution from the start of the hydrolysis reaction and the condensation reaction to the end of the reaction is 0.88% by mass or less. [4] A method for producing silica particles according to any one of [1] to [3], wherein the time from the start of the hydrolysis reaction and the condensation reaction to the end of the reaction is 200 minutes or more. [5] A method for producing silica particles according to any one of [1] to [4], wherein the average primary particle diameter of the silica particles is 30 nm to 40 nm. [6] The method for producing silica particles according to any one of [1] to [5], further comprising the following step (1). Step (1): a step of concentrating the reaction solution after the hydrolysis reaction and condensation reaction, and adding a dispersion medium to obtain a dispersion of the silica particles [7] The method for producing silica particles according to [6], further comprising the following step (2). Step (2): a step of subjecting the dispersion of silica particles obtained in step (1) to pressure and heat treatment [8] A method for producing silica sol, comprising the method for producing silica particles according to any one of [1] to [7]. [9] The method for producing silica sol according to [8], wherein the concentration of the silica particles in the silica sol is 3% by mass to 50% by mass based on 100% by mass of the total amount of the silica sol.

Effect of the Invention

[0009] Silica particles having a low refractive index can be obtained by the method for producing silica particles of the present invention.

Mode for Carrying Out the Invention

[0010] The present invention is described in detail below, but the present invention is not limited to the following embodiments, and can be implemented with various modifications within the scope of the gist thereof. In addition, when the expression "~" is used in the present specification, it is used as an expression including the numerical values or physical property values before and after it.

[0011] (Method for Producing Silica Particles) The method for producing silica particles of the present invention is a method for producing silica particles in which tetraalkoxysilane is subjected to hydrolysis reaction and condensation reaction, wherein the reaction temperature from the start of the hydrolysis reaction and condensation reaction to the end of the reaction (hereinafter, also simply referred to as "from the start of the reaction to the end of the reaction") is 28°C or lower, and the concentration of the alkali catalyst in the reaction solution from the start of the hydrolysis reaction and condensation reaction to the end of the reaction is 0.95% by mass or less. It is preferable that the concentration of the alkali catalyst in the reaction solution is constant from the start of the reaction to the completion of the reaction, but the concentration may vary on the condition that the concentration satisfies 0.95% by mass or less.

[0012] The reaction solution refers to a solution in which substances in the reaction system for the hydrolysis reaction and condensation reaction are dissolved in a liquid. The reaction temperature refers to the temperature of the reaction solution from the start to the completion of the hydrolysis reaction and condensation reaction. The concentration of the alkali catalyst in the reaction solution refers to the ratio of the total amount of the alkali catalyst to the total amount of the reaction solution, that is, the total amount of the liquid and the substances dissolved in the liquid in the reaction system for the hydrolysis reaction and condensation reaction, expressed in mass%. At the start of the reaction, the total amount of the liquid and the substances dissolved in the liquid in the reaction system is only the total amount of the solution (A) described later. During the reaction, it is the amount obtained by subtracting the amounts of tetraalkoxysilane and water consumed in the reaction from the sum of the total amount of the solution (A) described later, the amount of the solution (B) described later that has been supplied, the amount of the solution (C) described later that has been supplied, and the amount of alcohol produced in the reaction. It should be noted that silica particles dispersed in the liquid are not included in the liquid and the substances dissolved in the liquid in the reaction system.

[0013] By setting the reaction temperature from the start to the completion of the reaction to 28°C or lower and the concentration of the alkali catalyst in the reaction solution to 0.95% by mass or less, silica particles having a low refractive index can be obtained.

[0014] Examples of the alkali catalyst include ethylenediamine, diethylenetriamine, triethylenetetramine, ammonia, urea, ethanolamine, tetramethylammonium hydroxide, and the like. One of these alkali catalysts may be used alone, or two or more of them may be used in combination. Among these alkali catalysts, ammonia is preferable because it has excellent catalytic action, facilitates control of particle shape, can suppress contamination of metal impurities, has high volatility, and exhibits excellent removability after the hydrolysis reaction and condensation reaction.

[0015] In the method for producing silica particles of the present invention, it is preferable that the hydrolysis reaction and condensation reaction are steps of adding a solution (B) containing tetraalkoxysilane and a solution (C) containing an alkaline catalyst to a solution (A) containing an alkaline catalyst, and causing the tetraalkoxysilane to undergo hydrolysis and condensation reactions.

[0016] Examples of alkaline catalysts in solution (A) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. These alkaline catalysts may be used individually or in combination of two or more. Among these alkaline catalysts, ammonia is preferred because it exhibits excellent catalytic activity, allows for easy control of particle shape, suppresses the inclusion of metal impurities, and has high volatility, resulting in excellent removal after hydrolysis and condensation reactions.

[0017] The concentration of the alkaline catalyst in solution (A) is 0.95% by mass or less, preferably 0.85% by mass or less, and more preferably 0.75% by mass or less, based on 100% by mass of solution (A). The concentration of the alkaline catalyst in solution (A) is preferably 0.50% by mass or more, more preferably 0.60% by mass or more, and even more preferably 0.70% by mass or more. It is preferable to keep the concentration of the alkaline catalyst in the reaction solution constant from the start to the end of the reaction, at the same concentration as the alkaline catalyst in solution (A), by adding solution (C) as described later.

[0018] Solution (A) preferably contains water because it allows the hydrolysis and condensation reactions of tetraalkoxysilane to proceed.

[0019] Solution (A) is preferable to contain a solvent other than water because it exhibits excellent dispersibility in the reaction solution of tetraalkoxysilane. Examples of solvents other than water in solution (A) include methanol, ethanol, propanol, isopropanol, and ethylene glycol. These non-water solvents may be used individually or in combination of two or more. Among these non-water solvents, alcohols are preferred, methanol and ethanol are more preferred, and methanol is even more preferred, because the by-products used in the hydrolysis and condensation reactions are the same, and they offer excellent manufacturing convenience.

[0020] The concentration of water in solution (A) is preferably 3% to 25% by mass, and more preferably 7% to 20% by mass, in 100% by mass of solution (A), as this makes it easier to obtain silica particles having an average primary particle size within the preferred range described later.

[0021] Examples of tetraalkoxysilanes in solution (B) include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetraisopropoxysilane. These tetraalkoxysilanes may be used individually or in combination of two or more. Among these tetraalkoxysilanes, tetramethoxysilane and tetraethoxysilane are preferred, and tetramethoxysilane is more preferred, because they undergo rapid hydrolysis, leave little unreacted residue, have excellent productivity, and allow for easy acquisition of a stable silica sol.

[0022] While raw materials other than tetraalkoxysilane, such as low-level condensates of tetraalkoxysilane, may be used as raw materials for silica particles, it is preferable that, due to their excellent reactivity, the silica particles consist of 100% by mass of tetraalkoxysilane at 50% or more by mass and raw materials other than tetraalkoxysilane at 50% or less by mass, and more preferably 90% or more by mass of tetraalkoxysilane and raw materials other than tetraalkoxysilane at 10% or less by mass.

[0023] Solution (B) may contain only tetraalkoxysilane without a solvent, but it is preferable to include a solvent because it provides excellent dispersibility of the tetraalkoxysilane in the reaction solution. Examples of solvents in solution (B) include methanol, ethanol, propanol, isopropanol, and ethylene glycol. These solvents may be used individually or in combination of two or more. Among these solvents, alcohols are preferred, methanol and ethanol are more preferred, and methanol is even more preferred, because the by-products used in the hydrolysis and condensation reactions are the same, and they offer excellent manufacturing convenience.

[0024] The concentration of tetraalkoxysilane in solution (B) is preferably 60% to 95% by mass, and more preferably 70% to 90% by mass, based on 100% by mass of solution (B). When the concentration of tetraalkoxysilane in solution (B) is 60% by mass or higher, the reaction solution tends to become more homogeneous. Furthermore, when the concentration of tetraalkoxysilane in solution (B) is 95% by mass or lower, the formation of gel-like substances can be suppressed.

[0025] The concentration of the solvent in solution (B) is preferably 5% to 40% by mass, and more preferably 10% to 30% by mass, based on 100% by mass of solution (B). If the concentration of the solvent in solution (B) is 5% by mass or higher, the formation of gel-like substances can be suppressed. Also, if the concentration of the solvent in solution (B) is 40% by mass or lower, the reaction solution tends to become more homogeneous.

[0026] The rate at which solution (B) is added per unit time relative to the volume of solution (A) is preferably 0.05 kg / hour / L to 1.3 kg / hour / L, and more preferably 0.1 kg / hour / L to 0.8 kg / hour / L. When the rate at which solution (B) is added is 0.05 kg / hour / L or higher, the productivity of silica particles is excellent. Furthermore, when the rate at which solution (B) is added is 1.3 kg / hour / L or lower, the formation of gel-like substances can be suppressed.

[0027] Examples of alkaline catalysts in solution (C) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, and tetramethylammonium hydroxide. These alkaline catalysts may be used individually or in combination of two or more. Among these alkaline catalysts, ammonia is preferred because it exhibits excellent catalytic activity, allows for easy control of particle shape, suppresses the inclusion of metal impurities, and has high volatility, resulting in excellent removal after hydrolysis and condensation reactions.

[0028] Solution (C) preferably contains a solvent because it can reduce fluctuations in the concentration of the alkaline catalyst in the reaction mixture. Examples of solvents in solution (C) include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These solvents may be used individually or in combination of two or more. Among these solvents, water and alcohol are preferred, and water is more preferred, because the by-products used in the hydrolysis and condensation reactions are the same, and they offer excellent manufacturing convenience.

[0029] The concentration of the alkaline catalyst in solution (C) is preferably 0.5% to 10% by mass, and more preferably 1% to 6% by mass, based on 100% by mass of solution (C). When the concentration of the alkaline catalyst in solution (C) is 0.5% by mass or higher, it is easy to adjust the concentration of the alkaline catalyst in the reaction solution from the start to the end of the reaction. Furthermore, when the concentration of the alkaline catalyst in solution (C) is 10% by mass or lower, fluctuations in the concentration of the alkaline catalyst in the reaction solution can be reduced.

[0030] The concentration of the solvent in solution (C) is preferably 90% to 99.5% by mass, and more preferably 94% to 99% by mass, based on 100% by mass of solution (C). When the concentration of the solvent in solution (C) is 90% by mass or higher, fluctuations in the concentration of the alkaline catalyst in the reaction solution can be minimized. Furthermore, when the concentration of the solvent in solution (C) is 99.5% by mass or lower, it is easier to adjust the concentration of the alkaline catalyst in the reaction solution from the start to the end of the reaction.

[0031] The rate at which solution (C) is added per unit time relative to the volume of solution (A) is preferably 0.02 kg / hour / L to 0.5 kg / hour / L, and more preferably 0.03 kg / hour / L to 0.3 kg / hour / L. When the rate at which solution (C) is added is 0.02 kg / hour / L or higher, the productivity of silica particles is excellent. Furthermore, when the rate at which solution (C) is added is 0.5 kg / hour / L or lower, the formation of gel-like substances can be suppressed.

[0032] The water concentration in the reaction system for hydrolysis and condensation reactions is preferably maintained at 3% to 25% by mass, and more preferably at 7% to 20% by mass, of the total amount in the reaction system (100% by mass), in order to easily obtain silica particles having an average primary particle size within the preferred range described later.

[0033] The reaction temperature from the start to the end of the reaction is preferably 28°C or lower, more preferably 25°C or lower, and even more preferably 22°C or lower. Furthermore, the reaction temperature from the start to the end of the reaction is preferably 5°C or higher, more preferably 7°C or higher, and even more preferably 10°C or higher. By keeping the reaction temperature from the start to the end of the reaction within the above range, silica particles with a low refractive index and excellent reaction control are obtained, which is preferable.

[0034] The concentration of the alkaline catalyst in the reaction solution from the start to the end of the reaction is preferably 0.88% by mass or less, more preferably 0.85% by mass or less, and even more preferably 0.75% by mass or less. Furthermore, the concentration of the alkaline catalyst in the reaction solution from the start to the end of the reaction is preferably 0.50% by mass or more, more preferably 0.60% by mass or more, and even more preferably 0.70% by mass or more. By keeping the concentration of the alkaline catalyst in the reaction solution within the above range from the start to the end of the reaction, silica particles with a low refractive index and excellent reaction control are obtained, which is preferable. The concentration of the alkaline catalyst in the reaction solution from the start to the end of the reaction can be confirmed, for example, by analyzing the reaction solution each time using gas chromatography.

[0035] The time from the start of the reaction to the end of the reaction (hereinafter also simply referred to as "reaction time") is preferably 200 minutes or more, more preferably 230 minutes or more, and even more preferably 250 minutes or more. Furthermore, the time from the start of the reaction to the end of the reaction is preferably 300 minutes or less, more preferably 280 minutes or less, and even more preferably 270 minutes or less. By setting the time from the start to the end of the reaction within the above range, silica particles with a low refractive index and excellent reaction control are obtained, which is preferable.

[0036] The method for producing silica particles of the present invention is preferable to further include the following step (1), since it is possible to remove unnecessary components and add necessary components. Step (1): A step to obtain a dispersion of silica particles by concentrating the reaction solution after the hydrolysis and condensation reactions and adding a dispersion medium.

[0037] Examples of dispersion media include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These dispersion media may be used individually or in combination of two or more. Among these dispersion media, water and alcohol are preferred, with water being more preferred, due to their excellent affinity for silica particles.

[0038] Since the method for producing silica particles of the present invention can increase the degree of condensation of the silica particles, it is preferable to further include the following step (2). Step (2): A process of pressurizing and heating the dispersion of silica particles obtained in step (1).

[0039] The pressure for the pressurized heat treatment is preferably 0.10 MPa to 2.3 MPa, and more preferably 0.14 MPa to 1.0 MPa. When the pressurized heat treatment pressure is 0.10 MPa or higher, the degree of condensation of silica particles can be increased. Furthermore, when the pressurized heat treatment pressure is 2.3 MPa or lower, silica particles can be produced without significantly changing the average primary particle diameter, average secondary particle diameter, association ratio, or refractive index, and the dispersion stability of the silica sol is excellent. Pressurization can be achieved by heating the silica particle dispersion to above the boiling point of the dispersion medium while it is sealed. When the silica particle aqueous dispersion is heated to above 100°C while sealed, the pressure will be equal to the saturated water vapor pressure at that temperature.

[0040] The temperature for the pressurized heat treatment is preferably 100°C to 220°C, and more preferably 110°C to 180°C. If the pressurized heat treatment temperature is 100°C or higher, the degree of condensation of silica particles can be increased. If the pressurized heat treatment temperature is 220°C or lower, silica particles can be produced without significantly changing the average primary particle diameter, average secondary particle diameter, association ratio, or refractive index, and the dispersion stability of the silica sol is excellent.

[0041] The pressurized heat treatment time is preferably 0.25 to 10 hours, and more preferably 0.5 to 8 hours. If the pressurized heat treatment time is 0.25 hours or longer, the degree of condensation of silica particles can be increased. If the pressurized heat treatment time is 10 hours or less, silica particles can be produced without significantly changing the average primary particle diameter, average secondary particle diameter, association ratio, and refractive index, and the dispersion stability of the silica sol is excellent.

[0042] Pressurized heat treatment is more preferably performed in an aqueous dispersion because it can increase the degree of condensation of silica particles without significantly changing the average primary particle diameter, average secondary particle diameter, association ratio, or refractive index.

[0043] The pH when performing the pressurized heat treatment in an aqueous dispersion is preferably 6.0 to 8.0, and more preferably 6.5 to 7.8. When the pH when performing the pressurized heat treatment in an aqueous dispersion is 6.0 or higher, gelation of the silica sol can be suppressed. Furthermore, when the pH when performing the pressurized heat treatment in an aqueous dispersion is 8.0 or lower, the degree of condensation of silica particles can be increased without significantly changing the average primary particle diameter, average secondary particle diameter, association ratio, or refractive index.

[0044] Furthermore, steps (1) and (2) described above are steps that occur after the hydrolysis reaction and the condensation reaction, and are not included in the hydrolysis reaction and the condensation reaction.

[0045] (Physical properties of silica particles) Silica particles preferably have a low refractive index; specifically, a refractive index of 1.398 or less is preferred, 1.395 or less is more preferred, and 1.390 or less is even more preferred. A low refractive index in silica particles is preferable because it reduces reflected light and improves light transmittance. There is no particular lower limit to the refractive index of the silica particles, but for example, 1.360 or higher is preferred, 1.370 or higher is more preferred, and 1.380 or higher is even more preferred.

[0046] The refractive index of silica particles is determined by adding special grade 2-propanol and special grade toluene in varying ratios to a container containing silica particles. The supernatant liquid, when the silica particles in the container become clear, is measured using an Abbe refractometer, and the refractive index at that time is defined as the refractive index of the silica particles.

[0047] The average primary particle diameter of silica particles is preferably 30 nm to 40 nm, and more preferably 32 nm to 38 nm. When the average primary particle diameter of silica particles is 30 nm or more, the dispersibility is excellent. Furthermore, when the average primary particle diameter of silica particles is 40 nm or less, the sedimentation of silica particles is suppressed.

[0048] The average primary particle diameter of silica particles is measured by the BET method. Specifically, the specific surface area of ​​silica particles is measured using an automated specific surface area measuring device, and the average primary particle diameter is calculated using the following formula (1). Average primary particle diameter (nm) = 6000 / (specific surface area (m 2 / g) x density (g / cm 3 )) ··· (1)

[0049] The average primary particle size of silica particles can be set according to the conditions described above.

[0050] The average secondary particle diameter of silica particles is preferably 50 nm to 90 nm, and more preferably 55 nm to 85 nm. When the average secondary particle diameter of silica particles is 50 nm or more, dispersibility is excellent. Furthermore, when the average secondary particle diameter of silica particles is 90 nm or less, sedimentation of silica particles is suppressed.

[0051] The average secondary particle size of silica particles is measured by the DLS method. Specifically, it is measured using a dynamic light scattering particle size analyzer.

[0052] The average secondary particle size of silica particles can be set according to the conditions described above.

[0053] The CV value of the silica particles is preferably 15 or higher, more preferably 20 or higher, even more preferably 25 or higher, and also preferably 50 or lower, more preferably 40 or lower, and even more preferably 35 or lower. When the CV value of the silica particles is 15 or higher, the film exhibits excellent uniformity. Furthermore, when the CV value of the silica particles is 50 or lower, the film exhibits excellent smoothness.

[0054] The cv value of silica particles is calculated using the following formula (2) after measuring the average secondary particle diameter of silica particles with a dynamic light scattering particle diameter analyzer, and is an indicator of uniform particle diameter. cv value = (standard deviation (nm) / mean secondary particle diameter (nm)) × 100 ... (2)

[0055] The association ratio of silica particles is preferably 1.0 to 4.0, and more preferably 1.1 to 3.0. When the association ratio of silica particles is 1.0 or higher, the uniformity of the film is excellent. Furthermore, when the association ratio of silica particles is 4.0 or lower, the smoothness of the film is excellent.

[0056] The association ratio of silica particles is calculated using the following formula (3) from the average primary particle diameter and the average secondary particle diameter measured using the aforementioned measurement method. Association ratio = average secondary particle diameter / average primary particle diameter (3)

[0057] The surface silanol group density of silica particles is 0.1 groups / nm. 2 ~10 pieces / nm 2 Preferably, 0.5 particles / nm 2 ~7.5 pieces / nm 2 More preferably, 2.0 pieces / nm 2 ~7.0 pieces / nm 2is more preferable. When the surface silanol group density of the silica particles is 0.1 groups / nm 2 or more, the silica particles have appropriate surface repulsion, and the silica sol is excellent in dispersion stability. In addition, when the surface silanol group density of the silica particles is 10 groups / nm 2 or less, the silica particles have appropriate surface repulsion, and aggregation of the silica particles can be suppressed.

[0058] The surface silanol group density of silica particles is measured by the Sears method. Specifically, measurement and calculation are performed under the conditions shown below. A silica sol equivalent to 1.5 g of silica particles is collected, and pure water is added to adjust the liquid volume to 90 mL. Under an environment of 25°C, a 0.1 mol / L aqueous hydrochloric acid solution is added until the pH reaches 3.6, 30 g of sodium chloride is added, sodium chloride is completely dissolved while gradually adding pure water, and finally pure water is added until the total volume of the test solution reaches 150 mL to obtain a test solution. The obtained test solution is placed in an automatic titrator, a 0.1 mol / L aqueous sodium hydroxide solution is added dropwise, and the titration volume A (mL) of the 0.1 mol / L aqueous sodium hydroxide solution required for the pH to change from 4.0 to 9.0 is measured. Using the following formula (4), the consumption V (mL) of the 0.1 mol / L aqueous sodium hydroxide solution required for the pH to change from 4.0 to 9.0 per 1.5 g of silica particles is calculated, and using the following formula (5), the surface silanol group density ρ (groups / nm 2 ) is calculated. V=(A×f×100×1.5) / (W×C) ··· (4) A: the titration volume (mL) of the 0.1 mol / L aqueous sodium hydroxide solution required for the pH to change from 4.0 to 9.0 per 1.5 g of silica particles f: the titer of the 0.1 mol / L aqueous sodium hydroxide solution used C: the concentration of silica particles in the silica sol (mass%) W: the collected amount of the silica sol (g) ρ=(B×N A ) / (10 18 ×M×S BET ) ··· (5) The amount of sodium hydroxide (mol) required to change the pH of 1.5g of silica particles from 4.0 to 9.0, calculated from B:V. N A Avogadro's number (particles / mol) M: Silica particle amount (1.5g) S BET :Specific surface area (m²) of silica particles measured when calculating the average primary particle diameter 2 / g)

[0059] Furthermore, the methods for measuring and calculating the surface silanol group density of the silica particles were based on the following references: "GWSears, Jr., Analytical Chemistry, Vol.28, No.12, pp.1981-1983 (1956)," "Shinichi Haba, Development of Polishing Agents for Semiconductor Integrated Circuit Processes, Doctoral Dissertation, Kochi University of Technology, pp.39-45, March 2004," "Patent Publication No. 5967118," and "Patent Publication No. 6047395."

[0060] The surface silanol group density of silica particles can be set to a desired range by adjusting the conditions of the hydrolysis and condensation reactions of alkoxysilanes.

[0061] The metal impurity content of the silica particles is preferably 5 ppm or less, and more preferably 2 ppm or less.

[0062] The presence of metallic impurities in a silica film significantly reduces its light transmittance. Furthermore, the presence of metal impurities in silica particles can lead to coordination interactions between the acidic surface silanol groups and the metal impurities. This can alter the chemical properties of the surface silanol groups (such as acidity), change the steric environment of the silica particle surface (such as the ease with which silica particles aggregate), and affect film formation.

[0063] The metal impurity content of silica particles is measured by inductively coupled plasma mass spectrometry (ICP-MS). Specifically, a silica sol containing 0.4 g of silica particles is accurately weighed, sulfuric acid and hydrofluoric acid are added, the mixture is heated, dissolved, and evaporated, and pure water is added to the remaining sulfuric acid droplets to create a test solution with a total volume of exactly 10 g. This test solution is then measured using an inductively coupled plasma mass spectrometer. The target metals are sodium, potassium, iron, aluminum, calcium, magnesium, zinc, cobalt, chromium, copper, manganese, lead, titanium, silver, and nickel, and the sum of the content of these metals is considered the metal impurity content.

[0064] The metal impurity content of silica particles can be reduced to 5 ppm or less by performing hydrolysis and condensation reactions using alkoxysilane as the main raw material to obtain silica particles. In methods involving the deionization of alkali silicates such as water glass, residual sodium and other substances from the raw materials remain, making it extremely difficult to reduce the metal impurity content of silica particles to 5 ppm or less.

[0065] Silica particles can take various shapes, such as spherical, chain-like, or cocoon-like (also referred to as knob-like or peanut-like). Examples of silica particle shapes include irregular shapes (e.g., warty, bent, branched, etc.). Among these shapes of silica particles, a cocoon-like shape is preferred for film uniformity and smoothness.

[0066] (Method for producing silica sol) The present invention includes a method for producing silica sol, which includes a method for producing silica particles.

[0067] The silica sol may be produced by using the silica particle dispersion obtained by the silica particle production method of the present invention as is, or by removing unnecessary components and adding necessary components from the obtained silica particle dispersion.

[0068] The silica sol preferably contains silica particles and a dispersion medium. Examples of dispersion media in silica sols include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These dispersion media in silica sols may be used individually or in combination of two or more. Among these dispersion media in silica sols, water and alcohol are preferred, with water being more preferred, due to their excellent affinity for silica particles.

[0069] The silica particle content in the silica sol is preferably 3% to 50% by mass, more preferably 4% to 40% by mass, and even more preferably 5% to 30% by mass, based on 100% by mass of the total silica sol. When the silica particle content in the silica sol is 3% by mass or more, the film production is excellent. Furthermore, when the silica particle content in the silica sol is 50% by mass or less, the aggregation of silica particles in the silica sol can be suppressed, resulting in excellent storage stability of the silica sol.

[0070] The content of the dispersion medium in the silica sol is preferably 50% to 97% by mass, more preferably 60% to 96% by mass, and even more preferably 70% to 95% by mass, based on 100% by mass of the total silica sol. When the content of the dispersion medium in the silica sol is 50% by mass or more, aggregation of silica particles in the silica sol can be suppressed, resulting in excellent storage stability of the silica sol. Furthermore, when the content of the dispersion medium in the silica sol is 97% by mass or less, excellent film production is achieved.

[0071] The content of silica particles and dispersion medium in the silica sol can be set to a desired range by removing unnecessary components and adding necessary components from the components of the resulting silica particle dispersion.

[0072] In addition to silica particles and dispersion medium, silica sol may contain other components as needed, such as oxidizing agents, preservatives, fungicides, pH adjusters, pH buffers, surfactants, chelating agents, and antimicrobial biocides, to the extent that it does not impair its performance. In particular, since silica sol has excellent storage stability, it is preferable to include an antimicrobial biocide in the silica sol.

[0073] Examples of antimicrobial biocides include hydrogen peroxide, ammonia, quaternary ammonium hydroxide, quaternary ammonium salt, ethylenediamine, glutaraldehyde, methyl p-hydroxybenzoate, and sodium chlorite. These antimicrobial biocides may be used individually or in combination of two or more. Among these antimicrobial biocides, hydrogen peroxide is preferred due to its excellent affinity for silica sol. Biocides include what are commonly known as fungicides.

[0074] The content of the antimicrobial biocide in the silica sol is preferably 0.0001% to 10% by mass, and more preferably 0.001% to 1% by mass, based on 100% by mass of the total silica sol. When the content of the antimicrobial biocide in the silica sol is 0.0001% by mass or more, the silica sol exhibits excellent storage stability. When the content of the antimicrobial biocide in the silica sol is 10% by mass or less, the original performance of the silica sol is not impaired.

[0075] The pH of the silica sol is preferably 6.0 to 8.0, and more preferably 6.5 to 7.8. A pH of 6.0 or higher provides excellent dispersion stability and suppresses the aggregation of silica particles. Furthermore, a pH of 8.0 or lower prevents the dissolution of silica particles and provides excellent long-term storage stability. The pH of silica sol can be set to a desired range by adding a pH adjusting agent.

[0076] (Application) The silica particles obtained by the silica particle manufacturing method of the present invention, and the silica sol obtained by the silica sol manufacturing method of the present invention, have properties such as low refractive index, transparency, and electrical insulation, and in particular the low refractive index, so they can be suitably used in applications such as anti-reflective coatings, optical waveguides, and lenses. For example, practical application in electronic equipment displays, automobile panels, lighting fixtures, solar energy utilization devices, camera lenses, etc., is being considered. [Examples]

[0077] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples without departing from its essence.

[0078] (Measurement of average primary particle diameter) The silica particle dispersions obtained in the examples and comparative examples were dried at 150°C, and the specific surface area of ​​the silica particles was measured using an automatic specific surface area analyzer "BELSORP-MR1" (model name, Microtrac-Bel Corporation). The density was then calculated to be 2.2 g / cm³ using the following formula (1). 3 The average primary particle size was then calculated. Average primary particle diameter (nm) = 6000 / (specific surface area (m 2 / g) x density (g / cm 3 )) ··· (1)

[0079] (Measurement of average secondary particle size and cv value) The average secondary particle diameter of the silica particles in the dispersions obtained in the examples and comparative examples was measured using a dynamic light scattering particle size analyzer "ZetaSizer Nano ZS" (model name, manufactured by Malvern), and the cv value was calculated using the following formula (2). cv value = (standard deviation (nm) / mean secondary particle diameter (nm)) × 100 ... (2)

[0080] (Calculation of meeting ratio) The association ratio was calculated using the following formula (3) from the measured average primary particle diameter and average secondary particle diameter. Association ratio = average secondary particle diameter / average primary particle diameter (3)

[0081] (Measurement of refractive index) From the dispersions of silica particles obtained in the examples and comparative examples, dried silica particle powder was obtained. 0.1 g of the dried silica particle powder was placed in a 10 mL glass bottle, and special grade 2-propanol and special grade toluene were added in varying ratios. The supernatant when the powder in the glass bottle became clear was measured with an Abbe refractometer to determine the refractive index of the silica particles.

[0082] [Example 1] Solution (B) was prepared by mixing tetramethoxysilane and methanol in a 4.4:1 (volume ratio), and solution (C) was prepared by mixing a 2.5% by mass aqueous ammonia solution. Solution (A), which had been pre-mixed with methanol, pure water, and ammonia, was charged into a reaction vessel equipped with a thermometer, stirrer, feed pipe, and distillation line. The concentration of water in solution (A) was set to 17.5% by mass, and the concentration of ammonia in solution (A) was set to 0.70% by mass. While maintaining the reaction solution temperature at 20°C, 100% by volume of solution (B) and 27% by volume of solution (C) were added dropwise to 167% by volume of solution (A) at a constant rate over 255 minutes to obtain a dispersion of silica particles. The concentration of the alkaline catalyst in the reaction solution was maintained at 0.70% by mass from the start to the end of the reaction by adding solution (C). Table 1 shows the reaction temperature from the start to the end of the hydrolysis and condensation reactions, the reaction time from the start to the end of the hydrolysis and condensation reactions, and the alkali catalyst concentration from the start to the end of the hydrolysis and condensation reactions in Example 1, as well as in Examples 2 to 4 and Comparative Examples 1 to 4 described later.

[0083] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass. Table 2 shows the evaluation results of silica particles obtained in Example 1, as well as in Examples 2-4 and Comparative Examples 1-4, which will be described later.

[0084] [Example 2] Solution (B) was prepared by mixing tetramethoxysilane and methanol in a 4.4:1 (volume ratio), and solution (C) was prepared by mixing a 2.6% by mass aqueous ammonia solution. Solution (A), which had been pre-mixed with methanol, pure water, and ammonia, was charged into a reaction vessel equipped with a thermometer, stirrer, feed pipe, and distillation line. The concentration of water in solution (A) was set to 17.5% by mass, and the concentration of ammonia in solution (A) was set to 0.75% by mass. While maintaining the reaction solution temperature at 21°C, 100% by volume of solution (B) and 27% by volume of solution (C) were added dropwise to 166% by volume of solution (A) at a constant rate over 255 minutes to obtain a dispersion of silica particles. The concentration of the alkaline catalyst in the reaction solution was maintained at 0.75% by mass from the start to the end of the reaction by adding solution (C).

[0085] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass.

[0086] [Example 3] Solution (B) was prepared by mixing tetramethoxysilane and methanol in a 4.4:1 (volume ratio), and solution (C) was prepared by mixing a 2.6% by mass aqueous ammonia solution. Solution (A), which had been pre-mixed with methanol, pure water, and ammonia, was charged into a reaction vessel equipped with a thermometer, stirrer, feed pipe, and distillation line. The concentration of water in solution (A) was set to 11.5% by mass, and the concentration of ammonia in solution (A) was set to 0.85% by mass. While maintaining the reaction solution temperature at 23°C, 100% by volume of solution (B) and 32% by volume of solution (C) were added dropwise to 162% by volume of solution (A) at a constant rate over 266 minutes to obtain a dispersion of silica particles. The concentration of the alkaline catalyst in the reaction solution was maintained at 0.85% by mass from the start to the end of the reaction by adding solution (C).

[0087] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass.

[0088] [Example 4] Solution (B) was prepared by mixing tetramethoxysilane and methanol in a volume ratio of 4.4:1, and solution (C) was prepared by mixing a 2.1% by mass aqueous solution of ammonia. Solution (A), which had been pre-mixed with methanol, pure water, and ammonia, was charged into a reaction vessel equipped with a thermometer, stirrer, feed pipe, and distillation line. The concentration of water in solution (A) was set to 11.5% by mass, and the concentration of ammonia in solution (A) was set to 0.70% by mass. While maintaining the reaction solution temperature at 18°C, 100% by volume of solution (B) and 32% by volume of solution (C) were added dropwise to 163% by volume of solution (A) at a constant rate over 266 minutes to obtain a dispersion of silica particles. The concentration of the alkaline catalyst in the reaction solution was maintained at 0.70% by mass from the start to the end of the reaction by adding solution (C).

[0089] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass.

[0090] [Comparative Example 1] Solution (B) was prepared by mixing tetramethoxysilane and methanol in a 4.4:1 (volume ratio), and solution (C) was prepared by mixing a 6.6% by mass aqueous ammonia solution. Solution (A), which had been pre-mixed with methanol, pure water, and ammonia, was charged into a reaction vessel equipped with a thermometer, stirrer, feed pipe, and distillation line. The concentration of water in solution (A) was set to 15.0% by mass, and the concentration of ammonia in solution (A) was set to 2.40% by mass. While maintaining the reaction solution temperature at 50°C, 100% by volume of solution (B) and 39% by volume of solution (C) were added dropwise to 153% by volume of solution (A) at a constant rate over 153 minutes to obtain a dispersion of silica particles. The concentration of the alkaline catalyst in the reaction solution was maintained at 2.40% by mass from the start to the end of the reaction by adding solution (C).

[0091] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass.

[0092] [Comparative Example 2] Solution (B) was prepared by mixing tetramethoxysilane and methanol in a 4.4:1 (volume ratio), and solution (C) was prepared by mixing a 3.4% by mass aqueous ammonia solution. Solution (A), which had been pre-mixed with methanol, pure water, and ammonia, was charged into a reaction vessel equipped with a thermometer, stirrer, feed pipe, and distillation line. The concentration of water in solution (A) was set to 15.0% by mass, and the concentration of ammonia in solution (A) was set to 1.20% by mass. While maintaining the reaction solution temperature at 36°C, 100% by volume of solution (B) and 37% by volume of solution (C) were added dropwise to 155% by volume of solution (A) at a constant rate over 153 minutes to obtain a dispersion of silica particles. The concentration of the alkaline catalyst in the reaction solution was maintained at 1.20% by mass from the start to the end of the reaction by adding solution (C).

[0093] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass.

[0094] [Comparative Example 3] Solution (B) was prepared by mixing tetramethoxysilane and methanol in a 4.4:1 (volume ratio), and solution (C) was prepared by mixing a 4.7% by mass aqueous solution of ammonia. Solution (A), which had been pre-mixed with methanol, pure water, and ammonia, was charged into a reaction vessel equipped with a thermometer, stirrer, feed pipe, and distillation line. The concentration of water in solution (A) was set to 15.0% by mass, and the concentration of ammonia in solution (A) was set to 1.20% by mass. While maintaining the reaction solution temperature at 36°C, 100% by volume of solution (B) and 23% by volume of solution (C) were added dropwise to 172% by volume of solution (A) at a constant rate over 138 minutes to obtain a dispersion of silica particles. The concentration of the alkaline catalyst in the reaction solution was maintained at 1.20% by mass from the start to the end of the reaction by adding solution (C).

[0095] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass.

[0096] [Comparative Example 4] Solution (B) was prepared by mixing tetramethoxysilane and methanol in a 4.4:1 (volume ratio), and solution (C) was prepared by mixing a 4.0% by mass aqueous ammonia solution. Solution (A), which had been pre-mixed with methanol, pure water, and ammonia, was charged into a reaction vessel equipped with a thermometer, stirrer, feed pipe, and distillation line. The concentration of water in solution (A) was set to 7.5% by mass, and the concentration of ammonia in solution (A) was set to 1.20% by mass. While maintaining the reaction solution temperature at 28°C, 100% by volume of solution (B) and 28% by volume of solution (C) were added dropwise to 169% by volume of solution (A) at a constant rate over 144 minutes to obtain a dispersion of silica particles. The concentration of the alkaline catalyst in the reaction solution was maintained at 1.20% by mass from the start to the end of the reaction by adding solution (C).

[0097] The resulting silica particle dispersion was then heated to remove methanol and ammonia while adjusting the volume by adding pure water so that the silica particle content was approximately 20% by mass, thereby obtaining a silica particle dispersion with a silica particle content of approximately 20% by mass.

[0098] [Table 1]

[0099] [Table 2]

[0100] As can be seen from Table 2, silica particles produced under conditions such as those in Examples 1 to 4, where the reaction temperature from the start to the end of the hydrolysis and condensation reactions was 28°C or lower, and the concentration of the alkaline catalyst in the reaction solution from the start to the end of the hydrolysis and condensation reactions was 0.95% by mass or lower, exhibited a low refractive index. On the other hand, silica particles produced under conditions that did not satisfy the requirements of keeping the reaction temperature from the start to the end of the hydrolysis and condensation reactions at 28°C or lower, and keeping the concentration of the alkaline catalyst in the reaction solution at 0.95% by mass or lower from the start to the end of the hydrolysis and condensation reactions, as in Comparative Examples 1 to 4, showed a higher refractive index compared to the silica particles of Examples 1 to 4. [Industrial applicability]

[0101] The silica particles obtained by the silica particle production method of the present invention, and the silica sol obtained by the silica sol production method of the present invention, can be suitably used in applications such as anti-reflective coatings, optical waveguides, and lenses.

Claims

1. A method for producing silica particles by hydrolyzing and condensing a tetraalkoxysilane, wherein the reaction temperature from the start to the end of the hydrolysis and condensation reactions is 28°C or lower, and the concentration of the alkaline catalyst in the reaction solution from the start to the end of the hydrolysis and condensation reactions is 0.70% by mass or more and 0.88% by mass or less.

2. The method for producing silica particles according to claim 1, wherein the hydrolysis reaction and condensation reaction are steps of adding a liquid (B) containing tetraalkoxysilane and a liquid (C) containing an alkaline catalyst to a liquid (A) containing an alkaline catalyst, and causing the tetraalkoxysilane to undergo hydrolysis and condensation reactions.

3. A method for producing silica particles according to claim 1 or 2, wherein the time from the start of the hydrolysis reaction and the condensation reaction to the end of the reaction is 200 minutes or more.

4. A method for producing silica particles according to any one of claims 1 to 3, wherein the average primary particle diameter of the silica particles is 30 nm to 40 nm.

5. Furthermore, a method for producing silica particles according to any one of claims 1 to 4, comprising the following step (1). Step (1): A step of concentrating the reaction solution after the hydrolysis and condensation reactions and adding a dispersion medium to obtain a dispersion of silica particles.

6. Furthermore, the method for producing silica particles according to claim 5, further comprising the following step (2). Step (2): A step of pressurizing and heating the dispersion of silica particles obtained in step (1).

7. A method for producing silica sol, comprising the method for producing silica particles according to any one of claims 1 to 6.

8. The method for producing silica sol according to claim 7, wherein the concentration of silica particles in the silica sol is 3% to 50% by mass of the total amount of silica sol (100% by mass).

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