Hollow Silica Particles

Abstract

The present invention provides novel hollow silica particles in which the shell layer is densified. The hollow silica particles according to the present invention each have a shell layer, that contains silica, and includes a hollow space inside the shell layer. The particle density of the hollow silica particles is found to be 2.00 g / cm3 or greater when density is measured by using a dry-process pycnometer that uses helium gas, and is found to be less than 2.00 g / cm3 when density is measured by using a dry-process pycnometer that uses oxygen gas.

Description

[Technical Field]

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

[0002] Hollow particles have various properties such as low density, low refractive index, and ability to encapsulate substances, and by taking advantage of these properties, they are used in a wide range of fields, including lightweight materials, heat insulating materials, and coloring materials.

[0003] Hollow particles include hollow resin particles and hollow inorganic particles, but in recent years, there has been a movement to replace hollow resin particles with hollow inorganic particles due to growing concerns about marine pollution caused by microplastics.

[0004] Hollow silica particles are a representative example of hollow inorganic particles. These particles have a hollow space inside a shell layer formed of silica. Hollow silica particles are widely used in catalysts, catalyst carriers, cosmetic pigments, resin fillers, adsorbents, desiccants, heat insulating materials, paints, drug delivery systems, optical filters, and other applications due to their diverse particle size, shell layer pore structure, and surface properties. Furthermore, due to their low refractive index resulting from their hollow shape, they are also useful as anti-reflective coating materials.

[0005] Various proposals have been made for such hollow silica particles. For example, Patent Document 1 describes micron-sized spherical silica particles that are synthesized by forming a water-in-oil emulsion and then causing hydrolysis and condensation polymerization reactions between tetraalkoxysilane and water in the solubilized water contained in the water-in-oil emulsion as a reaction field.

[0006] Furthermore, Patent Document 2, for example, describes a method for producing hollow silica particles of nano size, which comprises preparing an organosol in which hollow silica particles are dispersed, adding a silane compound and an alkali catalyst to the organosol at a temperature range of 30°C to 300°C, and reacting the silane compound with the hollow silica particles under conditions where the water content is 0.1 to 50% by weight relative to the silica content. The method also describes a method for producing hollow silica particles of nano size, which have an average particle diameter of 5 to 300 nm and a specific surface area of ​​50 to 1500 m2 as measured by dynamic light scattering. 2 / g, and hollow silica fine particles having cavities formed inside the outer shell, which show a weight loss of 1.0% by weight or more in the temperature range of 200°C to 500°C by thermogravimetry (TG).

[0007] Patent Document 3 describes a method for producing silica-based hollow particles, which includes using polystyrene particles as cores, coating the polystyrene particles with alkoxysilane or the like, and thermally decomposing the polystyrene particles. The method describes silica-based hollow particles in which particles with an aspect ratio of 1.5 or less account for 95% or more of the total particles, the coefficient of variation of particle size is 20 to 60%, and the average particle size is 30 to 150 nm.

[0008] Patent Document 4 describes a method for producing amorphous spherical hollow silica powder, which includes supplying silica raw material powder into a high-temperature flame to spheroidize and hollow the particles. The method describes amorphous spherical hollow silica powder having an average particle size of 0.5 to 8 μm, an average sphericity of 0.85 or more, a 50% collapse pressure of 10 MPa or more, an average hollow fraction of 20 to 70% by volume calculated from the density measured using a pycnometer method, and a maximum particle size of 5 times or less the average particle size. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 11-029318 [Patent Document 2] Japanese Patent Application Publication No. 2013-014506 [Patent Document 3] Japanese Patent Application Publication No. 2017-226567 [Patent Document 4] Japanese Patent No. 4244323 Summary of the Invention [Problem to be solved by the invention]

[0010] When the conventional hollow silica particles described in Patent Documents 1 to 4 are added to a solvent such as water, the solvent penetrates into the interior of the particles, making them unusable for their intended purpose. For example, if water enters the interior of the hollow silica particles when they are added to water, the hollow silica particles may become translucent in water, resulting in a loss of light scattering properties, or may settle and result in a loss of dispersibility. Furthermore, the conventional hollow silica particles produced in a high-temperature flame as described in Patent Document 4 have a thin shell and are prone to cracking due to the negative pressure inside. For example, when hollow silica particles are added to a resin or the like and kneaded, the hollow silica particles crack, causing the void space inside the particles to be lost, and the desired properties of the hollow silica particles cannot be obtained.

[0011] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide new hollow silica particles that have a densified silica shell layer, making it difficult for solvents such as water to penetrate into the interior of the particles, and that allow gases such as helium to pass through the shell, thereby maintaining the interior at normal pressure and reducing cracking and the like during kneading. [Means for solving the problem]

[0012] The present invention relates to the following (1) to (9). (1) Hollow silica particles having a shell layer containing silica and having a space inside the shell layer, wherein the density of the hollow silica particles measured by a dry pycnometer using helium gas is 2.00 g / cm or less. 3 or more, and the particle density measured by a dry pycnometer using oxygen gas is 2.00 g / cm 3 Hollow silica particles that are less than (2) The density of the particles measured by a dry pycnometer using helium gas is 2.00 to 2.40 g / cm 3 The hollow silica particles according to (1) above, (3) The density of the particles measured by a dry pycnometer using oxygen gas is 0.40 to 1.90 g / cm 3 The hollow silica particles according to (1) or (2) above, (4) The hollow silica particles according to any one of (1) to (3) above, which have an average primary particle diameter of 10 nm to 10 μm. (5) BET specific surface area is 5 to 2600 m 2 The hollow silica particles according to any one of (1) to (4) above, wherein the average particle diameter is 1 / g. (6) The hollow silica particles according to any one of (1) to (5) above, which have a sphericity of 0.8 to 1.0. (7) The hollow silica particles according to any one of (1) to (6) above, which have an oil absorption of 30 to 1000 mL / 100 g. (8) The hollow silica particles according to any one of (1) to (7) above, wherein the secondary particle agglomeration diameter (D50) is 0.1 to 50 μm. (9) A method for producing hollow silica particles, comprising forming a shell layer containing silica on the outer periphery of a core to obtain a hollow silica precursor, removing the core from the hollow silica precursor, and heat-treating the resulting mixture at 700°C or higher. [Effects of the Invention]

[0013] According to the present invention, hollow silica particles having a dense shell layer can be provided. Because the hollow silica particles of the present invention are difficult to penetrate by solvents such as water and oil, they can exhibit excellent light scattering properties even in solvents with similar refractive indices. Furthermore, they also have excellent dispersibility in solvents. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 shows a scanning electron microscope (SEM) image of the hollow silica particles obtained in Example 6. [Figure 2] FIG. 2 shows the spectroscopic spectrum of the hollow silica particles obtained in Example 6. [Figure 3] FIG. 3 shows the Raman spectra of the hollow silica particles obtained in Examples 1 and 6. [Figure 4] FIG. 4 shows the solid-state 29Si-NMR spectra of the hollow silica particles obtained in Examples 1 and 6. [Figure 5] FIG. 5 shows the total light transmittance of the films obtained in Examples 30 and 31 before and after the accelerated weathering test. [Figure 6] FIG. 6 shows the parallel light transmittance of the films obtained in Examples 30 and 31 before and after the accelerated weathering test. [Figure 7] FIG. 7 shows the sum of the total light transmittance and the parallel light transmittance of the films obtained in Examples 30 and 31 before and after the accelerated weathering test. [Figure 8] FIG. 8 shows a scanning electron microscope (SEM) image of the hollow silica particles obtained in Example 32. [Figure 9] FIG. 9 shows a scanning electron microscope (SEM) image of the hollow silica particles obtained in Example 8. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described below, but the present invention is not limited to the examples in the following description. In this specification, "mass" is synonymous with "weight."

[0016] (hollow silica particles) The hollow silica particles of the present invention have a shell layer containing silica and have a void space inside the shell layer. The fact that hollow silica particles have a void space inside the shell layer can be confirmed by observation with a transmission electron microscope (TEM) or a scanning electron microscope (SEM). In the case of SEM observation, the hollowness can be confirmed by observing broken particles with partial openings. In this specification, spherical particles having a void space inside the particle that can be confirmed by TEM or SEM observation are defined as "primary particles." Note that, since the primary particles of hollow silica particles are partially bonded to each other during the firing and drying processes, the hollow silica obtained by production is often an aggregate of secondary particles formed by aggregation of primary particles.

[0017] In this specification, the shell layer "contains silica" means that it contains 50% or more by mass of silica (SiO2). The composition of the shell layer can be measured by ICP atomic emission spectrometry, flame atomic absorption spectrometry, or the like. The silica content of the shell layer is preferably 80% or more by mass, more preferably 95% or more by mass. Theoretically, the upper limit is 100% by mass. The silica content of the shell layer is preferably less than 100% by mass, more preferably 99.99% or less by mass. The remainder may include alkali metal oxides, carbon, etc. Furthermore, "having a space inside the shell layer" means that when a cross section of a primary particle is observed, a single space is surrounded by a shell layer, creating a hollow state. In other words, each hollow particle has a large space surrounded by a shell layer.

[0018] The hollow silica particles of the present invention have a particle density of 2.00 g / cm as determined by density measurement with a dry pycnometer using helium gas (hereinafter also referred to as helium pycnometer method). 3 The particle density measured by a dry pycnometer using oxygen gas (also called the oxygen pycnometer method) is 2.00 g / cm 3 When both of these relationships are satisfied, the particles have a hollow shape composed of a shell layer having pores, and the shell layer is dense enough to be difficult for the solvent to penetrate.

[0019] Density measurements using a dry pycnometer with helium gas can determine whether the shell layer of hollow silica particles has pores. The true density of silica is approximately 2.2 g / cm. 3 Since hollow silica particles have spaces inside, the actual particle density is smaller than the true density of silica. However, when helium gas penetrates into the particle, the density approaches the true density of silica. The density of hollow silica particles determined by the helium pycnometer method is 2.00 g / cm 3From the above, it can be seen that the shell layer has pores, since helium gas penetrates the inside of the particle and remains in the internal space. Because the shell layer has pores, gases such as helium, which have small molecular sizes, can pass through the shell. As a result, the inside of the hollow silica particle is at normal pressure, which reduces cracking during kneading. The density of hollow silica particles determined by helium pycnometer method is 2.00-2.40 g / cm 3 Specifically, the lower limit is 2.05 g / cm 3 More preferably, 2.07 g / cm 3 More preferably, 2.09 g / cm 3 More than 2.10 g / cm is particularly preferred. 3 The most preferable value is 2.40 g / cm. 3 Preferably less than 2.30 g / cm 3 The following is more preferred:

[0020] Then, density measurement using a dry pycnometer with oxygen gas determines whether the hollow silica particles are hollow. Since oxygen gas has a larger molecular size than helium gas, if the shell layer is dense, oxygen gas cannot pass through the shell layer, and the actual particle density is measured. If the density of the hollow silica particles determined by the oxygen pycnometer method is 2.00 g / cm 3 If the particle density is less than 1 / 2, the particle density is smaller than the true density of silica, and it can be determined that there is a space inside the particle. The density of silica particles determined by oxygen pycnometer method is 0.40 to 1.90 g / cm 3 Specifically, the upper limit is 1.90 g / cm 3 Preferably less than 1.80 g / cm 3 Less than 1.60 g / cm is more preferable. 3 More preferably, 1.50 g / cm 3 The following is particularly preferred: 1.40 g / cm 3 The most preferable value is 0.40 g / cm or less, and the lower limit is 0.40 g / cm from the viewpoint of the shell strength of the hollow particles. 3 More than 0.50 g / cm is preferable. 3 More preferably, 0.60 g / cm or more3 More preferably, 0.70 g / cm 3 More than 0.80 g / cm is particularly preferred. 3 The above is most preferable.

[0021] When the particle density of hollow silica is greater than that of water, density can also be measured using a pycnometer and water. The sample (hollow silica particles) and water are placed in a pycnometer, which is then placed in a sealed container made of PTFE (polytetrafluoroethylene resin) in a thermostatic chamber at 110°C for 16 hours before measurement. Since water penetration may take time depending on the density of the shell layer of the hollow silica, it is preferable to perform the above pretreatment. If the density measured after pretreatment is 2.00 g / cm, 3 If it is more than this, water has penetrated and the density is 2.00 g / cm 3 If the density is less than this, water will not penetrate and the hollow space will be maintained. The results measured using this method correspond to the results of density measurements using a dry pycnometer using oxygen gas.

[0022] The particle density of the hollow silica primary particles can be adjusted by adjusting the ratio of the shell thickness (shell layer) to the particle diameter of the hollow silica primary particles of the present invention. When the hollow silica primary particles are spherical, the following formula is satisfied. Primary particle volume: 4πr 3 / 3 Volume of space: 4π(rd) 3 / 3 Primary particle weight: 4π(r 3 -(rd) 3 )ρ / 3 Primary particle density:(r 3 -(rd) 3 )ρ / r 3 where r is the primary particle radius, d is the shell thickness, and ρ is the true density of silica.

[0023] The hollow particle fraction is the percentage of completely hollow particles in a sample of hollow silica particles, in which the shell layer is intact and there is space inside the particle. Because the hollow silica particles of the present invention have a dense shell layer, they are impervious to various solvents, argon gas, and gases with a larger kinetic molecular diameter than oxygen molecules. However, if particles with a damaged shell layer (damaged particles) are present, these gases will penetrate into the interior of the particles. Therefore, the particle density of the secondary particles changes depending on the hollow particle fraction. The higher the hollow particle fraction, the lower the particle density of the hollow silica secondary particles; and the lower the hollow particle fraction, the higher the particle density of the hollow silica secondary particles. Using this, assuming a 100% yield, the hollow particle fraction can be calculated from the theoretical density calculated from the amount of raw materials charged and the density measured by oxygen pycnometer. In addition, the primary particle size and shell thickness of particles with undamaged shell layers can be determined from SEM and TEM images, allowing particle density to be estimated. The hollow particle fraction can be calculated from the particle density obtained from the image information and the density measured by the oxygen pycnometer method. The density measured by oxygen pycnometry or with a pycnometer corresponds to the average particle density of the secondary particles.

[0024] Additionally, when manufacturing hollow silica particles, the hollow particle ratio can be determined from the weight change before and after heat treatment using the cake after filtration before the oil core is removed. When the filtered cake is broken up and dried overnight, the oil components inside the broken particles volatilize, while the oil components inside the completely hollow particles are retained. The weight change during heat treatment can be calculated from the amount of raw material charged when all the charged oil components have volatilized (hollow particle ratio 0%) and when all the oil components are retained (hollow particle ratio 100%). Therefore, the hollow particle ratio can be determined from the weight change when a sample that has been filtered and dried overnight is heat treated to 800°C.

[0025] By changing the primary particle density and / or secondary particle density, it is possible to adjust whether the hollow silica particles settle in the solvent, remain dispersed, or float to the top. When hollow silica particles are to be dispersed in a solvent, it is desirable that the solvent density and particle density are close to each other. For example, when hollow silica particles are dispersed in a solvent with a density of 1.0 g / cm 3 If you want to disperse it in water, the particle density should be 0.8 g / cm 3From the above, 1.2g / cm 3 It is recommended to adjust as follows:

[0026] The primary particle size of hollow silica particles can be determined by directly observing their particle diameter using SEM. Specifically, the primary particle size of 100 particles is measured from SEM images, and the primary particle size distribution obtained by aggregating these is estimated to be the overall primary particle size distribution. The average size of the primary particles (average primary particle diameter) is preferably in the range of 10 nm to 10 μm. From the viewpoint of production reproducibility, the lower limit is more preferably 20 nm or more, even more preferably 50 nm or more, particularly preferably 70 nm or more, and most preferably 100 nm or more. From the viewpoint of handleability as a filler, the upper limit is more preferably 7 μm or less, even more preferably 5 μm or less, particularly preferably 3 μm or less, and most preferably 1 μm or less.

[0027] The secondary particle size (aggregation diameter (D50)) of the hollow silica particles is preferably 0.1 to 50 μm. From the viewpoint of handleability, the agglomeration diameter (D50) of the secondary particles is more preferably 0.2 μm or more, even more preferably 0.3 μm or more, particularly preferably 0.4 μm or more, and most preferably 0.5 μm or more. Also, from the viewpoint of dispersibility when mixed with water, oil, fluororesin, etc., the agglomeration diameter (D50) of the secondary particles is more preferably 35 μm or less, even more preferably 30 μm or less, even more preferably 25 μm or less, particularly preferably 15 μm or less, particularly preferably 10 μm or less, and most preferably 6 μm or less. Here, the agglomeration diameter (D50) of secondary particles can be measured, for example, by measuring the median value of particle distribution (diameter) twice using a diffraction / scattering particle distribution measuring device and calculating the average value.

[0028] Hollow silica particles have a BET specific surface area of ​​5 to 2600 m 2 / g is preferred. The denser the shell, the smaller the specific surface area, so the BET specific surface area is 2000m 2 / g or less is more preferable, and 1000m 2 / g or less is more preferable, and 2 / g or less is particularly preferable, and 2 / g or less. The BET specific surface area is preferably 8m 2 / g or more is more preferable, and 10m 2 / g or more is more preferable.

[0029] The specific surface area depends on the diameter of the primary particle of hollow silica and the thickness of the shell layer. When the primary particle diameter of hollow silica is spherical and the surface is smooth, the following formula holds when the radius of the primary particle is r, the thickness of the shell is d, and the true density of the silica is ρ. Primary particle weight: 4π(r 3 -(rd) 3 )ρ / 3 Primary particle surface area: 4πr 2 Specific surface area: 3r 2 / (r 3 -(rd) 3 )ρ Since the shell preferably does not have pores through which oxygen can pass, the specific surface area is preferably close to the theoretical value, and the BET specific surface area is preferably 5 times or less, more preferably 4 times or less, even more preferably 3 times or less, and most preferably 2 times or less of the theoretical value. Here, the BET specific surface area can be measured by a single-point method using a specific surface area measuring device (for example, "Macsorb" manufactured by Mountec Co., Ltd.) and a mixed gas (30% nitrogen as an adsorption gas and 70% helium as a carrier gas).

[0030] The sphericity of the hollow silica particles is preferably 0.8 to 1.0. The sphericity can be expressed as the average value obtained by measuring the circumscribed circle diameter (DL) and the inscribed circle diameter (DS) of each of 100 randomly selected particles in a photographic projection obtained by photographing with a scanning electron microscope (SEM) and calculating the ratio (DS / DL) of the inscribed circle diameter (DS) to the circumscribed circle diameter (DL). From the viewpoints of light scattering properties, tactile feel, etc., the sphericity is more preferably 0.83 or more, even more preferably 0.85 or more, particularly preferably 0.87 or more, and most preferably 0.9 or more.

[0031] The shell thickness of hollow silica particles is preferably 0.01 to 0.3 times the primary particle diameter of 1. If the shell thickness is less than 0.01 times the primary particle diameter of 1, the strength of the hollow silica particles may decrease. If this ratio is greater than 0.3, the hollow space inside the particle becomes too small, and the properties of the hollow shape are not exhibited. The shell thickness is more preferably 0.02 or more, even more preferably 0.03 or more, and more preferably 0.2 or less, even more preferably 0.1 or less, relative to the diameter of the primary particle. Here, the shell thickness is determined by measuring the shell thickness of each particle using a transmission electron microscope (TEM).

[0032] Since hollow silica particles have a hollow space inside, they can encapsulate substances inside the particles. The hollow silica particles of the present invention have a dense shell layer, which makes it difficult for various solvents to penetrate, but if broken particles are present, the solvent will penetrate into the particles. Therefore, the oil absorption capacity changes depending on the proportion of broken particles.

[0033] The oil absorption of the hollow silica particles is preferably 30 to 1000 mL / 100 g. Since too much oil absorption increases viscosity, the oil absorption is more preferably 900 mL / 100 g or less, even more preferably 850 mL / 100 g or less, particularly preferably 830 mL / 100 g or less, and most preferably 800 mL / 100 g or less. Furthermore, since a low oil absorption means that the powder is less likely to be wetted by oil, the oil absorption is more preferably 35 mL / 100 g or more, even more preferably 40 mL / 100 g or more, particularly preferably 45 mL / 100 g or more, and most preferably 50 mL / 100 g or more. In addition, based on the relationship between the proportion of broken particles and the oil absorption as described above, the oil absorption can be adjusted by adjusting the proportion of broken particles. Furthermore, since the spaces between primary particles are also spaces that can hold oil, it is thought that the oil absorption will be high if the aggregate diameter of secondary particles is large, and low if the aggregate diameter of secondary particles is small.

[0034] The oil absorption can be measured in accordance with JIS K 5101-13-1 (established in 2004).

[0035] The fact that the hollow silica particles of the present invention are particles with a dense shell layer can also be confirmed by a water dispersion test. Specifically, 1% by mass of hollow silica particles are added to 5 mL of pure water, dispersed by irradiating with ultrasound for 30 seconds, and the state is confirmed after standing for one week. When the redispersed particles after standing for one week are visually observed in comparison with the sample immediately after dispersion, if the white color is maintained, it can be determined that water has not penetrated into the interior of the particles. When water penetrates into the interior of the hollow silica particles, the particles change from translucent to transparent.

[0036] The fact that the hollow silica particles of the present invention have a dense shell layer can also be confirmed by a redispersion test. Specifically, 1% by mass of hollow silica particles are added to 5 mL of pure water, dispersed by 30 seconds of ultrasonic irradiation, and allowed to stand for one week before confirming their redispersibility. The lid of the sample bottle containing the dispersion is tightly closed and the bottle is slowly inverted twice. If no cake of particles settles to the bottom, it can be determined that water has not penetrated. If water penetrates into the hollow silica particles, the apparent density increases, forming a hard cake when the particles settle, making redispersion difficult.

[0037] Furthermore, whether the hollow silica particles of the present invention have a dense shell layer can also be confirmed by examining the pore size peak measured by a nitrogen adsorption method. For example, the pore size can be determined by measurement using a pore size distribution analyzer manufactured by Micromeritics. If the shell has pores that allow nitrogen gas to pass through, a pore size peak can be observed at 2 to 15 nm. However, if the pore size is too small for nitrogen gas to pass through, no pore size peak can be observed, confirming that the hollow silica particles have a dense shell layer.

[0038] In the present invention, it is preferable that the shell layer of the hollow silica particles contains an alkali metal component. If the alkali metal component is too small, the shell becomes porous, making it difficult to obtain a dense and strong shell. When silicon alkoxide is used as the silica raw material, the alkali metal component is hardly observed. For example, when a sodium silicate aqueous solution is used as the silica raw material, the mass concentration of the Na component in the shell of the obtained hollow silica particles is preferably 200 ppm by mass or more, more preferably 300 ppm by mass or more, even more preferably 500 ppm by mass or more, particularly preferably 800 ppm by mass or more, and most preferably 1000 ppm by mass or more. On the other hand, the mass concentration of the Na component in the shell of general hollow silica particles produced using tetraethyl orthosilicate as the silica raw material is 100 ppm by mass or less. The Ca content is preferably 10 ppm by mass or more, more preferably 30 ppm by mass or more, and even more preferably 50 ppm by mass or more. The Mg content is preferably 5 ppm by mass or more, more preferably 10 ppm by mass or more, and even more preferably 50 ppm by mass or more. However, if a large amount of alkali metals or alkaline earth metals is present, they may leach out when hollow silica is dispersed in water, turning the dispersion alkaline, or may inhibit the function of the surfactant. Therefore, from the perspective of the stability of the blended product, it is preferable that they be contained in a certain amount or less. Furthermore, when hollow silica containing a large amount of alkali metals or alkaline earth metals is used in electronic applications, ion migration may occur. Therefore, the Na content is preferably 1500 ppm by mass or less. The Ca content is preferably 1000 ppm by mass or less, more preferably 800 ppm by mass or less, and even more preferably 500 ppm by mass or less. The Mg content is preferably 500 ppm by mass or less, more preferably 300 ppm by mass or less, and even more preferably 100 ppm by mass or less.

[0039] The amount of alkali metal and alkaline earth metal can be measured by adding perchloric acid and hydrofluoric acid to the obtained hollow silica, igniting the mixture to remove the main component silicon, and then measuring the amount by ICP emission spectrometry. Furthermore, when an alkali metal silicate is used as the silica raw material, the shell layer of the resulting hollow silica particles contains less carbon components derived from the raw material than when a silicon alkoxide is used as the silica raw material.

[0040] Hollow silica particles are expected to be used in a dispersed state in resins, solvents, water, etc., and it is preferable for them to maintain dispersion without settling. Dispersion stability can be evaluated using the following method. Specifically, an aqueous dispersion containing hollow silica particles at a concentration of 250 ppm by mass is prepared, and the particles are dispersed by irradiating them with ultrasound for 30 seconds. The time-dependent change in absorbance at a wavelength of 310 nm is then observed using a spectrophotometer (Shimadzu Corporation, UV-1280). The time immediately after preparation of the aqueous dispersion is set to 0 hours, and the absorbance at 0 hours is set to 1. Dispersion stability is evaluated based on the absorbance ratios after 1 hour, 2 hours, and 15 hours at 25°C. After a predetermined time has passed, water penetrates into the interior of hollow silica particles with open-pore particles or shells that allow water to penetrate. As a result of water penetration, the apparent specific gravity of the particles approaches the true density of the silica, resulting in sedimentation and a corresponding decrease in absorbance. The absorbance ratio after 1 hour is preferably 0.2 or more, more preferably 0.5 or more, even more preferably 0.6 or more, particularly preferably 0.8 or more, and most preferably 0.9 or more. If the dispersion stability is ideal, the absorbance will be constant, and the maximum absorbance ratio will be 1.0 or less. After 2 hours, the absorbance will decrease due to gradual settling or floating depending on the apparent specific gravity of the hollow silica particles. The absorbance ratio after 15 hours is preferably 0.2 or more, more preferably 0.3 or more, even more preferably 0.6 or more, particularly preferably 0.7 or more, and most preferably 0.8 or more.

[0041] Titanium oxide and zinc oxide, inorganic materials used in white pigments, have a reflection peak around 400 nm, which gives them a bluish white appearance. By mixing inorganic particles into a fluororesin coating and applying it to a glass plate as an optical layer, whiteness can be evaluated from reflectance and spectral color. Reflectance in particular can be measured using a spectrophotometer. The ratio of the reflectance at a wavelength of 400 nm to the reflectance at a wavelength of 800 nm of the hollow silica particles of the present invention is preferably 2.00 or less, more preferably 1.50 or less, even more preferably 1.35 or less, particularly preferably 1.20 or less, and most preferably 1.10 or less. If the ratio of the reflectance at a wavelength of 400 nm to the reflectance at a wavelength of 800 nm is too small, the particles will appear reddish, so the reflectance ratio is preferably 0.5 or more, more preferably 0.8 or more, and even more preferably 0.9 or more.

[0042] L for lightness * b for value and chromaticity * The L value of the hollow silica particles of the present invention can be measured using a spectrophotometer. * The value is preferably 30.0 or more, more preferably 40.0 or more, even more preferably 45.0 or more, and particularly preferably 50.0 or more. * = 100, so L * The value is 100 or less. * The value is preferably −5.0 or more, more preferably −4.5 or more, even more preferably −4.0 or more, particularly preferably −3.5 or more, and most preferably −3 or more. * The value is preferably +5.0 or less, more preferably +4.5 or less, even more preferably +4.0 or less, particularly preferably +3.5 or less, and most preferably +3 or less.

[0043] The strength of hollow silica particles can be evaluated from the percentage of particles that break when a predetermined hydrostatic pressure is applied after the hollow silica particles are sealed in a sealer bag. The greater the pressure required to break a certain amount of hollow silica particles, the greater the pressure they can withstand. The pressure at which 10% of particles break due to hydrostatic pressure treatment is preferably 1 MPa or higher, more preferably 5 MPa or higher, even more preferably 10 MPa or higher, particularly preferably 20 MPa or higher, and most preferably 30 MPa or higher. There is no particular upper limit to the pressure, but 1 GPa or lower is preferred.

[0044] The amount of silanol (Si-OH) on the surface of hollow silica particles can be obtained by IR measurement. Specifically, the IR spectrum is measured at 800 cm -1 Normalized to 3800cm -1 After adjusting the baseline, the Si-OH / Si-O-Si peak intensity ratio can be calculated for evaluation. If there is a large amount of surface silanol, the dielectric loss will increase when the material mixed with resin is used for electronic applications. The Si-OH / Si-O-Si peak intensity ratio is preferably 0.50 or less, more preferably 0.30 or less, even more preferably 0.20 or less, particularly preferably 0.15 or less, and most preferably 0.10 or less. Generally, amorphous silica has a large amount of silanol. If the amount of silanol is too small, it is thought to approach crystalline silica, so the Si-OH / Si-O-Si peak intensity ratio is preferably 0.01 or more.

[0045] The water vapor adsorption amount of hollow silica particles is determined using a device that can observe the amount of gas adsorption using water vapor as the gas. A large amount of water vapor adsorption means that there are many areas (adsorption sites) where water vapor can be adsorbed, which is not desirable for electronic component applications. The maximum amount of water vapor adsorption is 5.0 (cm 3 / gSTP) / (m 2 / g) or less is preferable, and 4.0 (cm 3 / gSTP) / (m 2 / g) or less is more preferable, and 3.0 (cm 3 / gSTP) / (m 2 / g) or less is more preferable, and 2.0 (cm 3 / gSTP) / (m 2 / g) or less is particularly preferable, and 1.0 (cm 3 / gSTP) / (m 2 / g) or less is most preferable. Examples of the portion capable of adsorbing water vapor include silanols. If the portion capable of adsorbing water vapor is too small, it is thought that the surface will approach crystalline silica, and therefore the maximum adsorption amount of water vapor is 0.1 (cm 3 / gSTP) / (m 2 / g) or more is preferred.

[0046] Hollow silica particles can scatter light in all directions over a wide wavelength range, from ultraviolet to near-infrared, without absorbing it, depending on the concentration. Furthermore, hollow silica particles do not have the photocatalytic properties found in white pigments such as titanium dioxide, and can be used as a safe white pigment made of amorphous silica. By mixing hollow silica particles with a fluororesin characterized by UV resistance and high UV transmittance, a highly weather-resistant white film can be obtained. Such white films are suitable for membrane structures (natural turf stadiums and agricultural greenhouses) capable of cultivating plants that require UV light for growth. Furthermore, such white films can block direct sunlight while transmitting a moderate amount of UV light as scattered light, which is expected to have a sterilizing effect inside the membrane structure. Furthermore, because such white films reflect or transmit most of the sunlight, the film itself does not absorb energy and the temperature of the film itself is less likely to rise.

[0047] The weather resistance of a fluororesin film containing hollow silica particles can be evaluated by observing the changes in haze, total light transmittance, and parallel light transmittance over time using an accelerated weathering tester. The smaller the change in haze, the better the weather resistance. Therefore, the haze after 1016 hours of weathering test is preferably 1.50 or less, more preferably 1.20 or less, even more preferably 1.10 or less, and particularly preferably 1.05 or less, relative to the initial haze. Since the haze tends to increase as the fluororesin containing hollow silica deteriorates, the lower limit of the haze is 1.00 or more.

[0048] Because the hollow silica particles of the present invention have a dense shell, even when mixed with a resin, the resin does not penetrate into the hollow silica particles, thereby maintaining a void layer. Furthermore, because the hollow silica particles of the present invention form secondary particles, voids also exist between the primary particles. Because air has low thermal conductivity, resins containing air layers formed by adding the hollow silica particles of the present invention are thought to have lower thermal conductivity and improved insulating performance. For example, lightweight resin films and sheets with excellent insulating performance can be produced. Furthermore, by mixing the hollow silica particles with paint, they can be used as insulating paints that can be applied to houses and window glass. When hollow silica particles are added at 20 vol% to a bulk resin that is not porous, the thermal conductivity is preferably at least 5% lower than that of the resin alone, more preferably at least 10%, and most preferably at least 15% lower. Furthermore, since an increase in thermal conductivity due to the addition of hollow silica particles indicates a decrease in thermal insulation performance, it is preferable that the rate of change in the direction of increasing thermal conductivity be 1% or less.

[0049] (Method for producing hollow silica particles) The method for producing hollow silica particles of the present invention includes forming a shell layer containing silica around a core to obtain a hollow silica precursor, removing the core from the hollow silica precursor, and heat-treating the hollow silica precursor at 700°C or higher. Specifically, the method for producing hollow silica particles of the present invention includes using an oil-in-water emulsion containing an aqueous phase, an oil phase, and a surfactant to obtain a hollow silica precursor in which a shell layer containing silica is formed around the core in the emulsion, removing the core from the precursor, and then heat-treating the precursor to obtain hollow silica particles. This oil-in-water emulsion is an emulsion in which an oil phase is dispersed in water, and when a silica raw material is added to this emulsion, the silica raw material adheres to the oil droplets, forming oil-core-silica shell particles.

[0050] The method for producing the hollow silica particles described above includes the steps of adding a first silica raw material to an oil-in-water emulsion containing an aqueous phase, an oil phase, and a surfactant to form a first shell, adding a second silica raw material to the emulsion in which the first shell has been formed to form a second shell to obtain a hollow silica precursor, and obtaining hollow silica particles from the hollow silica precursor. Hereinafter, the oil-in-water emulsion will also be referred to simply as emulsion. The dispersion of oil-core-silica shell particles formed by adding the first silica raw material and before the second silica raw material is added, and the dispersion of oil-core-silica shell particles after the second silica raw material is added, may also be referred to as emulsion. The latter dispersion of oil-core-silica shell particles after the second silica raw material is added may be equivalent to the hollow silica precursor dispersion.

[0051] <First stage shell forming process> First, a first silica raw material is added to an oil-in-water emulsion containing a water phase, an oil phase, and a surfactant to form a first shell.

[0052] The aqueous phase of the emulsion mainly contains water as a solvent. The aqueous phase may further contain additives such as a water-soluble organic liquid or a water-soluble resin. The proportion of water in the aqueous phase is preferably 50 to 100% by mass, more preferably 90 to 100% by mass.

[0053] The oil phase of the emulsion preferably comprises a water-insoluble organic liquid that is incompatible with the aqueous phase components, which forms droplets in the emulsion and form the oil-core portion of the hollow silica precursor.

[0054] Examples of organic liquids include aliphatic hydrocarbons such as n-hexane, isohexane, n-heptane, isoheptane, n-octane, isooctane, n-nonane, isononane, n-pentane, isopentane, n-decane, isodecane, n-dodecane, isododecane, and pentadecane, or mixtures thereof, such as paraffinic base oils; alicyclic hydrocarbons such as cyclopentane, cyclohexane, and cyclohexene, or mixtures thereof, such as naphthenic base oils; benzene, toluene, xylene, ethylbenzene, propylbenzene, cumene, and methacrylic acid; Examples of suitable solvents include aromatic hydrocarbons such as ethylene, tetralin, and styrene; ethers such as propyl ether and isopropyl ether; esters such as ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, butyl lactate, methyl propionate, ethyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, and butyl butyrate; vegetable oils such as palm oil, soybean oil, and rapeseed oil; and fluorine-based solvents such as hydrofluorocarbons, perfluorocarbons, and perfluoropolyethers. Polyoxyalkylene glycols that become hydrophobic liquids at the shell formation reaction temperature can also be used. Examples include polypropylene glycol (molecular weight 1000 or more) and polyoxyethylene-polyoxypropylene block copolymers containing less than 20% by mass of oxyethylene units and having a cloud point (1% by mass aqueous solution) of 40°C or less, preferably 20°C or less. Among these, polyoxypropylene-polyoxyethylene-polyoxypropylene block copolymers are preferred. These may be used alone or in combination of two or more kinds so long as they form a single oil phase.

[0055] The organic liquid is preferably a hydrocarbon having 8 to 16 carbon atoms, particularly 9 to 12 carbon atoms. The organic liquid is selected taking into consideration a comprehensive range of factors, including operability, safety against fire, separability between the hollow silica precursor and the organic liquid, shape characteristics of the hollow silica particles, and solubility of the organic liquid in water. The hydrocarbon having 8 to 16 carbon atoms may be linear, branched, or cyclic, as long as it has good chemical stability, and hydrocarbons with different carbon numbers may be mixed and used. As the hydrocarbon, saturated hydrocarbons are preferred, and linear saturated hydrocarbons are more preferred.

[0056] The flash point of the organic liquid is preferably 20 to 90° C., more preferably 30 to 80° C. When an organic liquid with a flash point of less than 20° C. is used, the flash point is too low, and therefore measures to prevent fire and improve the working environment are necessary.

[0057] The emulsion contains a surfactant to enhance emulsion stability. The surfactant is preferably water-soluble or water-dispersible and is preferably added to the aqueous phase. A nonionic surfactant is preferred. Examples of nonionic surfactants include the following surfactants. Polyoxyethylene-polyoxypropylene copolymer surfactants, Polyoxyethylene sorbitan fatty acid ester surfactants: polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan tristearate, polyoxyethylene sorbitan monooleate, Polyoxyethylene higher alcohol ether surfactants: polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, polyoxyethylene oleyl ether, polyoxyethylene octylphenol ether, polyoxyethylene nonylphenol ether, Polyoxyethylene aliphatic ester surfactants: polyoxyethylene glycol monolaurate, polyoxyethylene glycol monostearate, polyoxyethylene glycol monooleate, Glycerin fatty acid ester surfactants: stearic acid monoglyceride, oleic acid monoglyceride. Furthermore, polyoxyethylene sorbitol fatty acid ester surfactants, sucrose fatty acid ester surfactants, polyglycerin fatty acid ester surfactants, polyoxyethylene hydrogenated castor oil surfactants, etc. may also be used. These may be used alone or in combination of two or more.

[0058] Among the nonionic surfactants described above, polyoxyethylene-polyoxypropylene copolymer surfactants are preferably used. Polyoxyethylene-polyoxypropylene copolymers are block copolymers in which polyoxyethylene blocks (EO) and polyoxypropylene blocks (PO) are bonded. Examples of block copolymers include EO-PO-EO block copolymers and EO-PO block copolymers, with EO-PO-EO block copolymers being preferred. The proportion of oxyethylene units in the EO-PO-EO block copolymer is preferably 20% by mass or more, more preferably 30% by mass or more. The weight average molecular weight of the polyoxyethylene-polyoxypropylene copolymer is preferably 3,000 to 27,000, more preferably 6,000 to 19,000. The total amount of polyoxyethylene blocks is preferably 40 to 90% by mass, and the total amount of polyoxypropylene blocks is preferably 10 to 60% by mass, based on the entire polyoxyethylene-polyoxypropylene copolymer.

[0059] The amount of surfactant used varies depending on conditions such as the type of surfactant, its HLB (Hydrophile-Lipophile Balance), which is an index of the surfactant's hydrophilicity or hydrophobicity, and the particle size of the target silica particles. However, a content of 500 to 20,000 ppm by mass in the aqueous phase is preferred, with 1,000 to 10,000 ppm by mass being more preferred. A content of 500 ppm by mass or more can further stabilize the emulsion. Furthermore, a content of 20,000 ppm by mass or less can reduce the amount of surfactant remaining in the hollow silica particles (the final product).

[0060] The water phase and the oil phase may be mixed at a mass ratio of 200:1 to 5:1, preferably 100:1 to 9:1.

[0061] Methods for preparing oil-in-water emulsions are not limited to the following. The aqueous and oil phases can be prepared in advance, and then the oil phase is added to the aqueous phase and thoroughly mixed or stirred. Other methods that apply strong physical shear forces include ultrasonic emulsification, stirring emulsification, and high-pressure emulsification. Other methods include membrane emulsification, in which a finely divided oil phase is dispersed in an aqueous phase through a membrane with micropores; phase inversion emulsification, in which a surfactant is dissolved in an oil phase and then an aqueous phase is added to emulsify; and phase inversion temperature emulsification, in which a surfactant changes from water-soluble to oil-soluble at a temperature near its cloud point. These emulsification methods can be appropriately selected depending on the desired particle size, particle size distribution, and the like. In order to reduce the particle size of the resulting hollow silica particles and narrow the particle size distribution, it is preferable that the oil phase is sufficiently dispersed and emulsified in the aqueous phase. For example, the mixture can be emulsified using a high-pressure homogenizer at a pressure of 100 bar or more, preferably 400 bar or more.

[0062] In the first shell formation step, a first silica raw material is added to an oil-in-water emulsion. Examples of the first silica raw material include an aqueous solution of water-soluble silica, an aqueous dispersion of solid silica, a mixture of these, and one or more selected from the group consisting of alkali metal silicates, active silicic acid, and silicon alkoxides, or their aqueous solutions or dispersions. Among these, one or more selected from the group consisting of alkali metal silicates, active silicic acid, and silicon alkoxides, or their aqueous solutions or dispersions, are preferred because of their high availability.

[0063] Examples of solid silica include silica sol obtained by hydrolyzing an organosilicon compound and commercially available silica sol. Examples of alkali metals in alkali metal silicates include lithium, sodium, potassium, and rubidium, with sodium being preferred due to its availability and economical advantages. That is, sodium silicate is preferred as the alkali metal silicate. Sodium silicate has a composition expressed as Na2O·nSiO2·mH2O. The ratio of sodium to silicate, expressed as the molar ratio n of Na2O / SiO2, is preferably 1.0 to 4.0, and more preferably 2.0 to 3.5.

[0064] Activated silicic acid is obtained by cation exchange of alkali metal silicate, replacing the alkali metal with hydrogen. An aqueous solution of this activated silicic acid exhibits weak acidity. A hydrogen cation exchange resin can be used for the cation exchange. The alkali metal silicate and active silicic acid are preferably dissolved or dispersed in water before being added to the emulsion. The concentration of the alkali metal silicate and active silicic acid aqueous solution is preferably 3 to 30 mass %, more preferably 5 to 25 mass %, in terms of SiO concentration.

[0065] As the silicon alkoxide, for example, tetraalkylsilanes such as tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane are preferably used. It is also possible to obtain composite particles by mixing other metal oxides, etc. with the silica raw material. Examples of other metal oxides include titanium dioxide, zinc oxide, cerium oxide, copper oxide, iron oxide, and tin oxide.

[0066] The first silica raw material may be any of the above silica raw materials, or a mixture of two or more of them. Among them, it is preferable to use an aqueous solution of alkali metal silicate, particularly an aqueous solution of sodium silicate, as the first silica raw material.

[0067] The first silica raw material is preferably added to the oil-in-water emulsion under acidic conditions, which generates silica particles and creates a network, forming the first coating layer.

[0068] The reaction temperature is preferably 80° C. or lower to maintain emulsion stability, more preferably 70° C. or lower, even more preferably 60° C. or lower, particularly preferably 50° C. or lower, and most preferably 40° C. or lower. From the viewpoint of controlling the rate of network formation of silica fine particles to achieve a uniform coating thickness, the temperature is preferably 4° C. or higher, more preferably 10° C. or higher, even more preferably 15° C. or higher, particularly preferably 20° C. or higher, and most preferably 25° C. or higher.

[0069] The pH of the oil-in-water emulsion during the reaction is preferably 3.0 or less, more preferably 2.4 or less, and more preferably 1.0 or more, from the viewpoint of making the thickness of the coating more uniform and making the silica shell layer of the resulting hollow silica more dense.

[0070] The pH of the oil-in-water emulsion can be made acidic by adding an acid. Examples of the acid include hydrochloric acid, nitric acid, sulfuric acid, acetic acid, perchloric acid, hydrobromic acid, trichloroacetic acid, dichloroacetic acid, methanesulfonic acid, and benzenesulfonic acid.

[0071] When adding the first silica raw material, the amount of the first silica raw material added is preferably such that the SiO2 content in the first silica raw material is 1 to 50 parts by mass, more preferably 3 to 30 parts by mass, per 100 parts by mass of the oil phase contained in the emulsion.

[0072] In the addition of the first silica raw material, after the addition of the first silica raw material, the pH of the emulsion is maintained at an acidic level, preferably for 1 minute or more, more preferably for 5 minutes or more, and even more preferably for 10 minutes or more.

[0073] Next, it is preferable to maintain the pH of the emulsion to which the first silica raw material has been added at 5 or more and 7 or less, thereby making it possible to immobilize the first silica raw material on the surface of the oil droplets. For example, there is a method of adjusting the pH of the emulsion to 5 or higher by adding a base to the emulsion to which the first silica raw material has been added.

[0074] Examples of the base include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, alkaline earth metal hydroxides such as magnesium hydroxide and calcium hydroxide, ammonia, and amines. Alternatively, a method of exchanging anions such as halogen ions for hydroxide ions by anion exchange treatment may be used.

[0075] When adding the base, it is preferable to gradually add the base while stirring the emulsion to which the first silica raw material has been added, thereby gradually increasing the pH of the emulsion. If the stirring is weak or a large amount of base is added all at once, the pH of the emulsion may become uneven, which may result in an uneven thickness of the first coating layer.

[0076] The emulsion is preferably maintained while being stirred. The maintenance time may be 10 minutes or more, preferably 1 hour or more, or may be 4 hours or more. The maintenance temperature is preferably 100°C or less to maintain the stability of the emulsion, more preferably 95°C or less, even more preferably 90°C or less, and particularly preferably 85°C or less. Furthermore, to promote aging, the maintenance temperature is preferably 35°C or more, more preferably 40°C or more, and particularly preferably 45°C or more.

[0077] <Second shell formation process> Next, a second silica raw material is added to the emulsion in the presence of alkali metal ions, thereby obtaining a hollow silica precursor dispersion, in which the hollow silica precursor is an oil core-silica shell particle.

[0078] The second silica source is preferably added to the emulsion under alkaline conditions. When adding the first silica raw material, the emulsion is first acidified and then adjusted to a pH of 5 or higher to ensure more uniform adhesion of the first silica raw material to the oil droplets. The first silica layer obtained by this method is porous and lacks density, resulting in low strength. When adding the second silica raw material, the emulsion is made alkaline, allowing a high-density second silica layer to be formed on top of the first silica layer obtained earlier.

[0079] In order to suppress the generation of new fine particles, the pH of the emulsion when the second silica raw material is added is preferably 8 or higher, more preferably 8.5 or higher, even more preferably 8.7 or higher, particularly preferably 8.9 or higher, and most preferably 9 or higher. Furthermore, since a too high pH increases the solubility of silica, the pH is preferably 13 or lower, more preferably 12.5 or lower, even more preferably 12 or lower, particularly preferably 11.5 or lower, and most preferably 11 or lower.

[0080] The pH of the oil-in-water emulsion can be made alkaline by adding a base, and the same compounds as those mentioned above can be used as the base.

[0081] As the second silica raw material, the same silica raw material as the first silica raw material described above can be used alone or in combination of two or more. Among them, at least one of an aqueous sodium silicate solution and an aqueous activated silicic acid solution is preferably used as the second silica raw material. When adding the second silica raw material to the emulsion under alkaline conditions, a method of adding an alkali metal hydroxide simultaneously with the second silica raw material may be used. Alternatively, a method of using sodium silicate as an alkali metal silicate in the second silica raw material may be used. In this case, the alkaline component, sodium silicate, is added to the weakly acidic emulsion whose pH is adjusted to 5 or higher after the addition of the first silica raw material, so that the pH of the emulsion can be maintained alkaline while the second silica raw material is added. In addition, alkali metal ions become present in the emulsion.

[0082] If the pH becomes too high when using an aqueous solution of sodium silicate as the second silica raw material, an acid may be added to adjust the pH. The acid used here may be the same acid as that used when adding the first silica raw material.

[0083] The second silica raw material is preferably added in the presence of alkali metal ions. The alkali metal ions may be derived from the first silica raw material, the second silica raw material, or a base added for pH adjustment. They can also be incorporated by adding additives to the emulsion. For example, an alkali metal silicate may be used as at least one of the first and second silica raw materials. Alternatively, an alkali metal halide, sulfate, nitrate, fatty acid salt, or the like may be used as an additive to the emulsion.

[0084] The second silica raw material may be added, for example, by adding either an aqueous sodium silicate solution or an aqueous active silicic acid solution to the emulsion after the addition of the first silica raw material, or by adding both. When both are added, the aqueous sodium silicate solution and the aqueous active silicic acid solution may be added all at once or in order.

[0085] For example, the addition of the second silica raw material can be performed by repeating the steps of adding an aqueous sodium silicate solution and adding an aqueous activated silicic acid solution two or more times while adjusting the pH to promote adhesion of the silica raw material onto the first silica layer.

[0086] The second silica raw material is preferably added to the heated emulsion to promote adhesion of the silica raw material to the first silica layer. The heating temperature is preferably 30°C or higher, more preferably 35°C or higher, even more preferably 40°C or higher, particularly preferably 45°C or higher, and most preferably 50°C or higher, to suppress the generation of new fine particles. Since the solubility of silica increases with increasing temperature, the temperature is preferably 100°C or lower, more preferably 95°C or lower, even more preferably 90°C or lower, particularly preferably 85°C or lower, and most preferably 80°C or lower. When using a heated emulsion, after adding the second silica raw material, it is preferable to slowly cool the resulting emulsion to room temperature (23°C).

[0087] In the addition of the second silica raw material, the amount of the second silica raw material added is preferably adjusted so that the amount of SiO2 in the second silica raw material is 20 to 500 parts by mass, and more preferably 40 to 300 parts by mass, per 100 parts by mass of the oil phase. In the addition of the second silica raw material, it is preferable to maintain the emulsion in an alkaline pH state for 10 minutes or more after the addition of the second silica raw material.

[0088] Through the addition of the first silica raw material and the second silica raw material, the total amount of the first silica raw material and the second silica raw material added is preferably adjusted so that the sum of the SiO2 in the first silica raw material and the SiO2 in the second silica raw material is 30 to 500 parts by mass, and more preferably 50 to 300 parts by mass, per 100 parts by mass of the oil phase.

[0089] Although the silica shell layer of the present invention is mainly composed of silica, other metal components such as Ti and Zr may be contained as necessary for refractive index adjustment, etc. The method for containing other metal components is not particularly limited, but for example, a method such as adding a metal sol liquid or an aqueous metal salt solution simultaneously in the step of adding the silica raw material may be used.

[0090] In this manner, a hollow silica precursor dispersion is obtained.

[0091] Methods for obtaining hollow silica precursors from hollow silica precursor dispersions include, for example, filtering the dispersion, heating to remove the aqueous phase, and separating the precursors by sedimentation or centrifugation. One example is a method in which the dispersion is filtered using a filter of about 0.1 μm to 5 μm, and the filtered hollow silica precursor is dried.

[0092] If necessary, the obtained hollow silica precursor may be washed with water, an acid, an alkali, an organic solvent, or the like.

[0093] <Step of obtaining hollow silica particles from hollow silica precursor> The oil core is then removed from the hollow silica precursor (first-stage treatment), and the precursor is further heat-treated at 700°C or higher (second-stage treatment) to obtain hollow silica particles. The first-stage treatment removes the oil core and organic components such as surfactants, and the second-stage treatment densifies the hollow silica particles to densify the shell. Methods for removing the oil core include, for example, a method of calcining a hollow silica precursor to burn and decompose the oil, a method of volatilizing the oil by drying, a method of decomposing the oil by adding an appropriate additive, a method of extracting the oil using an organic solvent, etc. Among these, from the viewpoints of reducing oil residue and operational efficiency when performing heat treatment at 700°C or higher, a method of calcining a hollow silica precursor to decompose the oil core by calcination, followed by heat treatment at 700°C or higher is preferred.

[0094] Hereinafter, a specific description will be given of an example of a method for removing the oil core by calcining a hollow silica precursor.

[0095] In the first-stage treatment, the oil inside the hollow particles must be thermally decomposed, so the calcination temperature is preferably 30° C. or higher, more preferably 100° C. or higher, even more preferably 200° C. or higher, particularly preferably 250° C. or higher, and most preferably 300° C. or higher. If the calcination temperature is too high, the silica shell becomes more densified, making it difficult to remove the organic components inside the particles, so the calcination temperature is preferably 100° C. or higher, more preferably 300° C. or higher, and even more preferably 400° C. or higher than the second-stage heat treatment temperature.

[0096] The baking time for the first stage treatment is preferably 5 minutes or more in order to sufficiently remove the oil core and organic components, more preferably 1 hour or more, and even more preferably 3 hours or more, and from the viewpoint of work efficiency, is preferably 12 hours or less, more preferably 8 hours or less, and even more preferably 6 hours or less.

[0097] In the first stage treatment, treatment may be carried out at a constant baking temperature for a predetermined time, or treatment may be carried out stepwise at multiple temperatures. Even when treatment is carried out at multiple temperatures, it is preferable to carry out treatment for the above-mentioned baking time from the viewpoint of work efficiency.

[0098] Next, in the second stage treatment, heat treatment is carried out at 700° C. or higher. After the first stage treatment, the hollow silica precursor may be returned to room temperature before the second stage treatment, or the temperature may be raised from the firing temperature of the first stage treatment to the heat treatment temperature of the second stage treatment. The heat treatment temperature is more preferably 750° C. or higher, and even more preferably 800° C. or higher. If the temperature is too high, crystallization of amorphous silica occurs, so the heat treatment temperature is preferably 1200° C. or lower, more preferably 1150° C. or lower, even more preferably 1000° C. or lower, particularly preferably 950° C. or lower, and most preferably 900° C. or lower.

[0099] The heat treatment time for the second stage treatment is preferably 1 hour or more, more preferably 3 hours or more, in order to improve the densification of the shell layer, and from the viewpoint of work efficiency, is preferably 12 hours or less, more preferably 8 hours or less, and even more preferably 6 hours or less.

[0100] The hollow silica particles obtained by the above steps may be aggregated due to the drying or calcination steps, and may be crushed to obtain an aggregate size that is easy to handle. Crushing methods include, for example, a method using a mortar, a method using a dry or wet ball mill, a method using a shaking sieve, or a method using a crusher such as a pin mill, a cutter mill, a hammer mill, a knife mill, or a roller mill. The preferred aggregate size of the secondary particles is as described above.

[0101] The hollow silica particles of the present invention have a densified shell layer, and therefore exhibit low permeability to various solvents when added to water, oil, or other solvents. Therefore, they exhibit good dispersibility in various solvents, such as water, oil, and fluororesin, and by making the specific gravity of the hollow silica particles close to that of the solvent used, sedimentation is suppressed. Furthermore, the properties unique to hollow particles can be maintained in the solvent. Therefore, the hollow silica particles of the present invention have the following characteristics. Compared to solid silica particles, they are superior in that their specific gravity can be adjusted to make them non-settling, they exhibit high scattering properties in the ultraviolet and visible light regions, and they have high hiding power. Furthermore, they are less toxic than titanium oxide particles, have no photocatalytic properties, and their specific gravity can be adjusted to make them non-settling. Furthermore, they are superior to resin hollow particles in that they are less environmentally polluting, heat resistant, and safe. They can be used as additives that have the characteristics of being environmentally friendly, gentle on the skin, and easy to disperse and use. Therefore, the hollow silica particles of the present invention can be applied in various fields, and can be used, for example, as cosmetic pigments, low refractive index materials, insulating fillers, heat insulating fillers, low dielectric constant fillers, white pigments, drug carriers, fragrance carriers, pesticide carriers, and UV scattering agents. [Example]

[0102] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these. In the following description, the same components are used. Unless otherwise specified, "%" represents "% by mass." Examples 1 to 3, 32, and 33 are comparative examples, and Examples 4 to 31, 34, and 35 are working examples.

[0103] <Test Example 1> (Examples 1 to 6) "Preparation of emulsion" 7 g of EO-PO-EO block copolymer (ADEKA Pluronic F68) was added to 1250 g of pure water and stirred until dissolved. 42 g of n-dodecane was added to this solution and stirred using an IKA homogenizer until the entire solution became homogenous, producing a crude emulsion. This coarse emulsion was emulsified three times at a pressure of 400 bar using a high-pressure emulsifier (LAB2000 manufactured by SMT Co., Ltd.) to produce a fine emulsion.

[0104] "First stage shell formation" The first and second shell formations were carried out using a glass reaction vessel with a capacity of 2 L. The fine emulsion obtained above was left to stand at room temperature (25°C) for 15 hours before use. To 1,300 g of the fine emulsion, 41 g of a diluted aqueous sodium silicate solution (SiO concentration 10.4 mass%, NaO concentration 3.6 mass%) and 2 M hydrochloric acid were added so as to adjust the pH to 2 (condition (i)), and the mixture was stirred thoroughly while maintaining the temperature at 25°C (condition (iii)). While stirring this liquid well, 1 M aqueous sodium hydroxide solution was slowly added dropwise to adjust the pH to 5 (condition (ii)), to obtain an oil core-silica shell particle dispersion. The obtained oil core-silica shell particle dispersion was retained and aged.

[0105] "Second stage shell formation" The entire oil core-silica shell particle dispersion obtained in the first stage shell formation was heated to 70°C, and 1M NaOH was slowly added with stirring to adjust the pH to 9. Next, 460 g of a diluted aqueous sodium silicate solution (SiO2 concentration 10.4 mass %, Na2O concentration 3.6 mass %) was gradually added together with 0.5 M hydrochloric acid to adjust the pH to 9. This suspension was kept at 70°C for 2 days and then slowly cooled to room temperature to obtain a hollow silica precursor dispersion.

[0106] "Filtering, drying, and baking" The entire hollow silica precursor dispersion was filtered under pressure (pressure 0.28 MPa) using a 0.45 μm hydrophilic PTFE membrane filter. The filtered cake was dried under a nitrogen atmosphere at 60°C for 1 hour and then at 400°C for 4 hours (heating rate 5°C / min) to remove the organic matter and obtain a hollow silica precursor. The obtained precursor was divided into six portions, and each portion was baked for four hours at the baking temperature shown in Table 1 (heating rate: 5°C / min) to bake and tighten the shell, thereby obtaining hollow silica particles.

[0107] "evaluation" The hollow silica particles of Examples 1 to 6 were subjected to the following tests.

[0108] 1. Scanning Electron Microscope Images A scanning electron microscope (SEM) image of the hollow silica particles obtained in Example 6 is shown in Figure 1. The SEM image was observed using a Hitachi High-Tech S4800 at an accelerating voltage of 3 kV. As shown in Figure 1, the hollow particles were evident from the open-pore particles contained in some of the particles. The circumscribed circle diameter (DL) and inscribed circle diameter (DS) of each of 100 particles were measured, and the ratio of the inscribed circle diameter (DS) to the circumscribed circle diameter (DL) (DS / DL) was calculated and the sphericity was calculated from the average value, resulting in a sphericity of 0.91. Furthermore, the presence of particles ranging in size from approximately 100 nm to 1 μm indicated that there was a distribution. The primary particle diameter of 100 particles was measured, and the average value was calculated from the distribution obtained by aggregating these values, resulting in an average primary particle diameter of 370 nm.

[0109] 2.Aggregation diameter The hollow silica particles obtained in Example 6 were measured using a diffraction / scattering particle distribution analyzer (MT3300) manufactured by Microtrac-Bell, and the median value of the particle distribution (diameter) was measured twice and the average value was calculated. As a result, the aggregate diameter (D50) was 12 μm.

[0110] 3.Specific surface area The BET specific surface areas of the hollow silica particles obtained in Examples 1 to 6, measured by the nitrogen adsorption method, are shown in Table 1. The BET specific surface areas were measured by the single-point method using a fully automatic specific surface area measuring device "Macsorb" manufactured by Mountech Co., Ltd., and a mixed gas (30% nitrogen as the adsorption gas and 70% helium as the carrier gas). The higher the calcination temperature, the smaller the BET specific surface area, which is thought to be due to the densification of the shell.

[0111] 4. Check the pore size peak Table 1 shows whether or not the hollow silica particles obtained in Examples 1 to 6 had a pore size peak measured by the nitrogen adsorption method. The pore size peak was measured using a pore size distribution analyzer "3Flex" manufactured by Micromeritics. When a pore size peak was confirmed, the position of the peak was recorded, and when it was not confirmed, "none" was recorded. The higher the calcination temperature, the less a pore size peak was observed by the nitrogen adsorption method, which suggests that the shell became denser.

[0112] 5. Density Measurement Using a Pycnometer The density in water was measured using a 10 mL Gay-Lussac pycnometer. As a pretreatment, the sample and water were placed in the pycnometer, which was then placed in a sealed PTFE container and left to stand in a constant temperature bath at 110°C for 16 hours. After removing it from the bath and cooling it to room temperature, the density was measured. The results are shown in Table 1. Examples 1 to 3 show the true density of silica (approximately 2.2 g / cm 3 ), water has penetrated, and in Examples 4 to 6, the density is 2.00 g / cm 3 Since the thickness is less than 1 / 2 mm, it is thought that water does not penetrate and the hollow part is maintained.

[0113] 6. Density Measurement Using a Dry Pycnometer The density was measured using a dry pycnometer (Micromeritics AccuPycII 1340) under the following measurement conditions. Sample cell: 10cm 3 cell Sample weight: 1.5g Measurement gas: Helium or oxygen Number of purges: 30 Purging process filling pressure: 135kPa Number of cycles: 10 Cycle filling pressure: 135kPa Pressure equilibration rate: 0.05kPa / min The results are shown in Table 1. When helium was used as the measurement gas, the concentration of each sample was approximately 2.2 g / cm 3 The value obtained is equivalent to the true density of silica, indicating that helium gas passes through the shell and penetrates into the interior of the hollow silica. On the other hand, when oxygen gas was used, the density was approximately 1.1 g / cm in Examples 4 to 6. 3It is thought that the particle density obtained did not include the interior of the hollow silica because the oxygen gas passed through the shell slowly. In Examples 1 to 3, the oxygen gas passed through the shell and aggregated or was adsorbed in some of the pores, resulting in a value higher than the true density, or the pressure did not reach equilibrium within the specified time, resulting in the error "Measurement not possible." These results suggest that the pore size of the shell decreases with increasing firing temperature, and in Examples 4 to 6 the pore size becomes equal to or smaller than the diameter of an oxygen molecule.

[0114] [Table 1]

[0115] <Test Example 2> (Examples 7-14) Hollow silica particles were prepared in the same manner as in Example 6, except that the conditions for the first-stage shell formation reaction were changed as shown in Table 2. i) pH after adding sodium silicate solution ii) pH after adding 1M NaOH iii) Reaction temperature during first shell formation

[0116] [Table 2]

[0117] (Examples 15-17) Hollow silica particles were produced in the same manner as in Example 14, except that hydrochloric acid was changed to sulfuric acid, and a glass reaction vessel having a capacity of 2 L was used in Example 15, a glass reaction vessel having a capacity of 5 L in Example 16, and a glass reaction vessel having a capacity of 10 L in Example 17 were used.

[0118] (Examples 18-21) Hollow silica particles were prepared in the same manner as in Example 14, except that the amount of silica added relative to the oil used in the core was changed. Specifically, the ratio of silica added relative to the oil was 1.2 in Example 14, 1.6 in Example 18, 1.8 in Example 19, 2.2 in Example 20, and 0.8 in Example 21.

[0119] (Examples 22-24) Hollow silica particles were prepared in the same manner as in Example 8, except that the step of adding 1 M NaOH to adjust the pH to 9 during the second shell formation was carried out quickly. Specifically, NaOH was added after 1 minute in Example 22, after 5 minutes in Example 23, and after 10 minutes in Example 24.

[0120] (Examples 25-29) Hollow silica particles were prepared in the same manner as in Example 14, except that the standing time and temperature after preparation of the fine emulsion were changed. Specifically, Example 25 was left standing at 25°C for 12 hours, Example 26 was left standing at 25°C for 24 hours, Example 27 was left standing at 25°C for 192 hours, Example 28 was left standing at 25°C for 112 hours, and Example 29 was left standing at 70°C for 15 hours.

[0121] "evaluation" The hollow silica particles of Examples 7 to 29 were subjected to the following tests.

[0122] 1. Sphericity and average primary particle size The sphericity and average primary particle size were determined from scanning electron microscope images in the same manner as in Example 6. The results are shown in Table 3.

[0123] 2. Secondary particle agglomeration diameter The secondary particle agglomeration diameter (D50) was determined in the same manner as in Example 6. The results are shown in Table 3.

[0124] 3.Specific surface area The specific surface area was determined in the same manner as in Examples 1 to 6. The results are shown in Table 3.

[0125] 4. Density Measurement Using a Dry Pycnometer The density was measured using a dry pycnometer in the same manner as in Examples 1 to 6. The results are shown in Table 3.

[0126] 5.Hollow particle rate When the filtered cake, before removing the oil core, is loosened and air-dried overnight, the oil components inside the open particles volatilize, while the oil components inside the completely coated hollow particles are retained. The weight change during heat treatment when all of the charged oil components have volatilized (hollow particle ratio 0%) and when all of the charged oil components are retained (hollow particle ratio 100%) was calculated from the charged amount of raw material, and the hollow particle ratio was estimated from the weight change when the filtered cake was air-dried overnight and heat-treated to 800°C. The results are shown in Table 3.

[0127] 6.Oil absorption amount The oil absorption was determined in accordance with JIS K 5101-13-1 (established in 2004). The results are shown in Table 3. The higher the hollow particle ratio, the lower the oil absorption, which suggests that the internal space of the open-pore particles contributes to the oil absorption.

[0128] 7. Shell thickness The shell thickness was determined by measuring the shell thickness of individual particles using a transmission electron microscope (TEM). TEM images were taken by scattering hollow silica particles on a hydrophilically treated polyvinyl formal film and observing them using a Hitachi HT7700 microscope at an accelerating voltage of 100 kV. The average shell thickness of 50 randomly selected particles was taken as the shell thickness.

[0129] [Table 3]

[0130] The results in Table 3 show that the higher the hollow particle ratio, the lower the oil absorption. This suggests that the internal space of partially open broken particles contributes to the oil absorption, and that the oil absorption can be adjusted by the proportion of broken particles, i.e., the hollow particle ratio.

[0131] <Test Example 3> The particle size distribution of the hollow silica particles of Examples 7 to 14 and 22 to 24 produced in Test Example 2 was measured. The particle size distribution was measured using a laser diffraction particle size distribution analyzer (MT3300 manufactured by Microtrackbell). The particles were dispersed in the device by irradiating them with ultrasonic waves three times for 60 seconds each, and then the measurement was performed. The measurement was performed twice for 120 seconds each, and the average value is shown in Table 4.

[0132] The particle size distribution observed here is not that of a single hollow silica particle, but that of secondary aggregates. Table 4 shows that the size of the secondary aggregates ranges from several hundred nm to several tens of μm.

[0133] [Table 4]

[0134] <Test Example 4> The impurity concentrations of the hollow silica particles of Examples 14 to 17 produced in Test Example 2 were measured. The measurements were carried out by adding perchloric acid and hydrofluoric acid to hollow silica particles, igniting the mixture to remove the main component, silicon, and then quantifying the amount of alkali metals and alkaline earth metals using an ICPE-9000 manufactured by Shimadzu Corporation according to high-frequency inductively coupled plasma atomic emission spectrometry. The hollow silica particles of Examples 14 to 17 were prepared into 14% aqueous dispersions and allowed to stand in a thermostatic chamber at 110°C for 17 hours. This was done to accelerate the penetration of water through the shell into the interior of the hollow silica particles. The aqueous dispersions were then diluted to 0.008% and the absorbance at 310 nm was measured using a spectrophotometer (Shimadzu UV-1280) to evaluate UV scattering and compare water penetration. The results are shown in Table 5.

[0135] It was found that when there were a large amount of alkali metals such as Na and alkaline earth metals such as Ca, the absorbance at a wavelength of 310 nm was high, forming a shell that was difficult for water to penetrate. Alkali metals and alkaline earth metals are known to affect the dissolution and reprecipitation of silica, and it is thought that their presence promotes the densification of the shell during heat treatment.

[0136] [Table 5]

[0137] <Test Example 5> The hollow silica particles of Examples 1 to 6 prepared in Test Example 1 were subjected to measurement of absorbance ratio. A 250 ppm by mass aqueous dispersion was prepared using the hollow silica particles of Examples 1 to 6 and dispersed by ultrasonic irradiation for 30 seconds. A quartz cell (AS ONE Corporation, AZLAB Quartz Cell (two-sided transparent) Q-102, 4.5 mm deep x 12.5 mm wide x 45 mm high, 2 mm optical path length x 10 mm optical path width) was used for absorbance measurement. A spectrophotometer (Shimadzu Corporation, UV-1280) was used to measure absorbance at a wavelength of 310 nm to evaluate UV scattering. The cell was positioned so that the center of the light beam was 15 mm from the bottom. The time immediately after preparation of the aqueous dispersion was set to 0 hours, and the absorbance at 0 hours was set to 1. Dispersion stability was evaluated by the absorbance ratio after leaving the dispersion at room temperature for 1 hour, 2 hours, and 15 hours. The results are shown in Table 6.

[0138] The absorbance ratio after 1 hour is thought to have decreased because water penetrated into the interior of open particles or hollow silica particles with a shell that allows water to penetrate, causing the specific gravity to approach the true density of silica and resulting in sedimentation. After 2 hours, the absorbance decreased because the hollow silica particles gradually settled or floated depending on their specific gravity.

[0139] [Table 6]

[0140] <Test Example 6> The hollow silica particles of Examples 1 to 6 prepared in Test Example 1 were subjected to spectroscopic analysis. 0.1 g of hollow silica particles from Examples 1 to 6 was placed in a powder sample holder for an integrating sphere accessory (Shimadzu Corporation), and the diffuse reflectance was measured in the wavelength range of 300 nm to 800 nm using a spectrophotometer (Shimadzu Corporation, UV-3100PC and MPC-3100). The incident angle of the measurement light was set to 0 degrees. The spectroscopic spectrum of Example 6 at this time is shown in Figure 2.

[0141] 2, no peak was observed at a specific wavelength, which indicated that there was no wavelength dependency. The hollow silica particles of Examples 1 to 5 also exhibited the same spectroscopic spectrum as Example 6.

[0142] <Test Example 7> The optical layers prepared using the hollow silica particles of Examples 8 to 14 in Test Example 2 were measured for reflectance, L * value and b * The values ​​were measured. A fluororesin coating agent (AGC Coatec, Obligard) (18.2 g), hollow silica particles (Examples 8 to 14) (1.8 g), and an antifoaming agent (Teikoku Ink Mfg. Co., Ltd. SM-257) (0.5 g) were added and stirred at 200 rpm for 1 minute using a kneader (Thinky Corporation, Awatori Rentaro). Then, a curing agent (Asahi Kasei Corporation, TPA-B80E) (6 g) was added and further stirred at 200 rpm for 1 minute to obtain a coating solution. The resulting coating liquid was applied to one side of a soda-lime silicate glass plate (AGC KFL, 150 mm long x 75 mm wide, average plate thickness: 3.2 mm) as a substrate layer using a screen printer (Micro-Tec MTVC-320) to a thickness of approximately 30 μm. The coating was then dried and cured in a thermostatic chamber at 130°C for 30 minutes to obtain an optical layer consisting of the glass plate as the substrate layer and the coating layer disposed on the substrate layer.

[0143] The reflectance of the optical layer was measured using a spectrophotometer (V-670 manufactured by JASCO Corporation) in the wavelength range of 200 to 1,500 nm in 5 nm increments at a scanning speed of 1,000 nm / min. The ratio of the reflectance at a wavelength of 400 nm to the reflectance at a wavelength of 800 nm is shown in Table 7.

[0144] The higher this reflectance ratio, the greater the reflection at a wavelength of 400 nm and the bluer the appearance; however, the optical layers using hollow silica particles in Examples 8 to 14 all had a reflectance of 1.4 or less, and therefore did not appear bluish.

[0145] Optical layer L * , b * was measured using a spectrophotometer (SD6000 manufactured by Nippon Denshoku Industries Co., Ltd.). During measurement, a black plate was placed behind the sample, and water was placed between the black plate and the sample. The measurement light was incident from the glass substrate. The measurement results are shown in Table 7.

[0146] The higher the hollow particle ratio of hollow silica particles used, the better the L of the optical layer. * It was confirmed that the value was high and the brightness was high. It is thought that the more particles that maintain a hollow structure in the mixed resin, the more light is scattered and the higher the brightness. * The values ​​were -5 or higher for all samples, and this result also showed that the samples were white and not bluish.

[0147] [Table 7]

[0148] <Test Example 8> The hollow silica particles of Examples 14 and 18 to 20 prepared in Test Example 2 were measured for particle strength. Two grams of hollow silica particles from Examples 14, 18, and 20 were weighed out and placed in a sealer bag (Polyflex Bag, a product of Asahi Kasei Pax Corporation). The bag was sealed using a chamber-type vacuum sealer (FCB-2000, manufactured by Fuji Impulse Corporation), and then subjected to a predetermined hydrostatic pressure for one minute using a CIP device (CPA-50, manufactured by NPA Systems Corporation). The density of the samples before and after hydrostatic pressure treatment was measured using an oxygen pycnometer, and the percentage of particles damaged by hydrostatic pressure treatment was determined. The pressure required for 10% particle damage by hydrostatic pressure treatment is shown in Table 8.

[0149] It was found that the greater the silica loading, the greater the pressure required to break 10% of the particles, resulting in stronger particles.

[0150] [Table 8]

[0151] <Test Example 9> The amount of surface silanol groups in the hollow silica particles of Examples 1 to 6 prepared in Test Example 1 was measured. The hollow silica particles of Examples 1 to 6 were collected on a diamond plate and subjected to IR measurement using a Nic-plan / Nicolet 6700 manufactured by Thermo Fisher Scientific. -1 The Si-OH peak appears at 3800 cm -1 The baseline was adjusted by 1000 kJ / cm 2 , and the intensity ratio of the Si-OH / Si-O-Si peaks was calculated to evaluate the amount of silanol on the particle surface. The results are shown in Table 9.

[0152] The results in Table 9 show that the higher the firing temperature, the smaller the Si-OH / Si-O-Si peak intensity ratio, and the smaller the amount of surface silanol.

[0153] [Table 9]

[0154] <Test Example 10> The hollow silica particles of Examples 1 and 6 prepared in Test Example 1 were analyzed by Raman spectroscopy and solid 29 Si-NMR spectra were obtained.

[0155] Raman spectroscopy was obtained using a LabRAM HR Evolution (Horiba Ltd.). The excitation wavelength was 532 nm, the power was 15 mW per sample, the objective lens was ×100, NA (Numerical Aperture) = 0.8, the confocal aperture was a 200 μm pinhole, the grating was 600, and the center wavelength was 950 cm. -1 The measurements were performed with an exposure time of 10 seconds and an accumulation count of 20. The results of Examples 1 and 6 are shown in Figure 3.

[0156] In Example 1, where the calcination temperature is low, the Si-OH peak intensity is high and the D2 peak (605 cm -1 ) has a high strength, it is thought that there are many Si-OH groups, the degree of cross-linking is low, and there are many unstable planar three-membered ring structures.

[0157] In addition, solid state imaging was performed using AVANCE-III-HD400 manufactured by Bruker Biospin. 29 Si-NMR spectra were obtained using a 7mm CP / MAS probe under the conditions of D1 = 300 sec and NS = 700, 500. The samples of Examples 1 and 6 were packed into 7.0mmφ ZrO2 sample tubes and the measurement results are shown in Figure 4.

[0158] In Example 1, where the firing temperature was low, the Q3 peak detected on the shoulder of the Q4 peak on the low magnetic field side was slightly strong, which suggests that a large amount of Si—OH was present.

[0159] <Test Example 11> The amount of water vapor adsorption was measured for the hollow silica particles of Examples 1 to 6 produced in Test Example 1. Using water vapor as the gas, 3Flex manufactured by Micromeritics was used to measure the amount of water vapor adsorption for the hollow silica particles of Examples 1 to 6. Table 10 shows the maximum adsorption amount for each sample.

[0160] The results in Table 10 show that the higher the firing temperature, the smaller the maximum amount of water vapor adsorption, and therefore the fewer areas (adsorption sites) that can adsorb water vapor.

[0161] [Table 10]

[0162] <Test Example 12> (Example 30) 31 g of the hollow silica particles obtained in Example 14 of Test Example 2 and 1.3 g of phenylmethyl silicone (PMS) were dispersed in 200 g of toluene, and the toluene was then evaporated and removed at 140°C to obtain 32.3 g of hollow particles surface-treated with phenylmethyl silicone. 1500 g of ETFE (AGC, Fluon ETFE88AXB) and 32.3 g of surface-treated hollow particles were dry-blended, and then melt-kneaded and extruded at 300°C using a twin-screw extruder to obtain pellets. The extrusion pressure during pellet molding was 12 MPa. The pellets were extruded at 300°C using a single-screw extruder connected to a T-die at the outlet to obtain a film with a thickness of 102 μm.

[0163] (Example 31) The pellets prepared in Example 30 were further dry-blended with 19 times the amount of ETFE, and then extrusion-molded at 300°C using a uniaxial molding machine to obtain a film having a thickness of 251 µm.

[0164] "evaluation" The optical properties of the films of Examples 30 and 31 were measured.

[0165] The film thickness was measured with a micrometer.

[0166] The haze of the film was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., device name: NDH-5000).

[0167] The total light transmittance and parallel light transmittance of the film were measured using an ultraviolet-visible-near-infrared (UV-VIS-NIR) spectrophotometer (Shimadzu Corporation, device name: UV-3600) according to JIS R 3106:1998.

[0168] The weather resistance test was carried out using an accelerated weather resistance tester (manufactured by Suga Test Instruments Co., Ltd., device name: Eye Super UV Tester). In the weather resistance test, condensation was allowed to occur at 63°C for 2 hours, followed by 1,600 W / m for 10 hours. 2 The cycle was set to irradiate the sample with ultraviolet light.

[0169] The optical properties at the initial stage and after being subjected to an accelerated weathering test are shown in Table 11 and Figures 5, 6, and 7. The figures show the total luminous transmittance, parallel luminous transmittance, and the sum of the total luminous transmittance and parallel luminous transmittance, respectively.

[0170] The films of Examples 30 and 31 maintained stable performance without being affected by ultraviolet rays or humidity in accelerated weathering tests, and were found to disperse light to the transmitting and reflective sides over a wide wavelength range without absorbing energy. Furthermore, no deterioration of the resin (increased haze) due to the photocatalyst was observed.

[0171] [Table 11]

[0172] <Test Example 13> (Example 32) Hollow silica particles were prepared in the same manner as in Example 8, except that the pH during the formation of the second shell was set to 7. The density of the hollow silica particles of Example 32 measured by oxygen pycnometer was 2.77 g / cm 3 It was.

[0173] SEM images were taken of the hollow silica particles of Examples 32 and 8. The SEM images were observed at an accelerating voltage of 3 kV using an S4800 manufactured by Hitachi High-Technologies Corporation. SEM images of Example 32 and Example 8 are shown in Figures 8 and 9, respectively.

[0174] The outside of the shell of the hollow silica particles of Example 32 was found to have large surface irregularities, with fine particles of several tens of nanometers agglomerated, while the outside of the shell of the hollow silica particles of Example 8 was found to have few surface irregularities and was smooth. Furthermore, when the inside of the shell was observed from the open particle, it was found that the surface appeared smooth with few irregularities in both Example 32 and Example 8. This is thought to be a characteristic observed because the process for adding the silica raw material in the method for producing hollow silica particles of the present invention is divided into two or more steps, and the first silica raw material is added to an emulsion containing a surfactant under acidic conditions to form the first silica coating.

[0175] <Test Example 14> (Example 33) A fluororesin coating agent (AGC Coatec's Obligard product) (27.29 g) and an antifoaming agent (Teikoku Ink Mfg. Co., Ltd.'s SM-257 product) (0.78 g) were mixed and stirred at 200 rpm for 1 minute in a kneader (Thinky Corporation's Awatori Rentaro product). Then, a curing agent (Asahi Kasei Corporation's TPA-B80E product) (9.09 g) was added and stirred at 200 rpm for another 1 minute to obtain a mixed solution. The resulting mixture was placed in a polyethylene ointment bottle (made of high-density polyethylene, inner diameter φ50 mm) and dried at room temperature for 3 days to harden, after which the hardened film was collected to obtain a resin sheet.

[0176] (Example 34) A resin sheet was prepared in the same manner as in Example 33, except that 2.91 g of hollow silica particles of Example 14 prepared in Test Example 2 (an amount that would result in 10 wt % and 21 vol % after curing and drying) was further added to prepare a mixed solution.

[0177] (Example 35) A resin sheet was prepared in the same manner as in Example 33, except that 2.91 g of hollow silica particles of Example 29 prepared in Test Example 2 (an amount that would result in 10 wt % and 25 vol % after curing and drying) was further added to prepare a mixed solution.

[0178] "evaluation" The resin sheets of Examples 33 to 35 were subjected to the following tests.

[0179] 1. Thermal conductivity measurement The thermal conductivity was measured at normal pressure and 25° C. using a thermal conductivity measuring device (FOX50 manufactured by Eiko Seiki Co., Ltd.). The results are shown in Table 12.

[0180] 2. Density Measurement The diameter of the prepared resin sheet was measured with a ruler. Furthermore, when the resin sheet was clamped and measured in the thermal conductivity measuring device, the thickness of the resin sheet was also measured, and the volume of the resin sheet was calculated from the diameter and thickness values. The weight of the resin sheet was then measured with a scale, and the density was calculated. The results are shown in Table 12.

[0181] Table 12 shows that Examples 34 and 35, which contained 10% hollow silica particles, had lower densities than Example 33, which contained only fluororesin, allowing for the introduction of an air layer, resulting in a reduction in thermal conductivity of approximately 20%. These results demonstrate that adding the hollow silica particles of the present invention to a resin sheet or coating liquid can improve heat insulation performance.

[0182] [Table 12]

[0183] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on Japanese patent applications filed on February 27, 2020 (Patent Application No. 2020-032046) and September 25, 2020 (Patent Application No. 2020-161378 and Patent Application No. 2020-161379), the contents of which are incorporated herein by reference.

Claims

1. A hollow silica particle having a shell layer containing silica and a space inside the shell layer, The density of the hollow silica particles measured by a dry pycnometer using helium gas is 2.00 g / cm ３ or more, and the particle density determined by density measurement using a dry pycnometer using oxygen gas is 2.00 g / cm ３ is less than The hollow silica particles have a mass concentration of a Na component in the shell layer of 200 ppm by mass or more.

2. The density of the particles measured by a dry pycnometer using helium gas is 2.00 to 2.40 g / cm ３ The hollow silica particle according to claim 1, wherein

3. The density of the particles measured by a dry pycnometer using oxygen gas is 0.40 to 1.90 g / cm ３ The hollow silica particles according to claim 1 or 2,

4. The hollow silica particles according to any one of claims 1 to 3, having an average primary particle diameter of 10 nm to 10 µm.

5. BET specific surface area: 5 to 2600 m ２ The hollow silica particles according to any one of claims 1 to 4, wherein the average molecular weight of the hollow silica particles is 1 / g.

6. The hollow silica particles according to any one of claims 1 to 5, having a sphericity of 0.8 to 1.

0.

7. The hollow silica particles according to any one of claims 1 to 6, having an oil absorption of 30 to 1000 mL / 100 g.

8. The hollow silica particles according to any one of claims 1 to 7, wherein the secondary particle agglomeration diameter (D50) is 0.1 to 50 µm.

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