JP7832781B2 - Method for producing crystalline inorganic oxide particles - Google Patents

Description

This invention relates to a method for producing crystalline inorganic oxide particles.

Inorganic oxide particles such as titanium dioxide, zirconium oxide, yttrium oxide, tin oxide, silica, and alumina are widely used as materials for paints, inks, and coatings, catalysts, UV protection agents, and pigments. For example, Patent Document 1 discloses a method for producing a porous titanium dioxide structure by preparing a titanium dioxide paste containing titanium dioxide particles, porosity-forming resin particles, and an organic solvent, coating it, drying it, and then firing the resulting paste.

Japanese Patent Publication No. 2012-82124

Incidentally, when producing crystalline inorganic oxide particles by calcining inorganic oxide particles, if the inorganic oxide particles bond together during calcination, the particle size of the resulting crystalline inorganic oxide particles increases, which presents a problem in that the desired properties cannot be obtained.

The objective of this invention is to suppress the bonding between inorganic oxide particles during firing.

This invention relates to a method for producing crystalline inorganic oxide particles from inorganic oxide particles, comprising preparing a mixture containing the inorganic oxide particles and spacer particles, and heating the mixture to calcine the inorganic oxide particles.

According to the present invention, by heating a mixture containing inorganic oxide particles and spacer particles to calcine the inorganic oxide particles, bonding between inorganic oxide particles during calcination can be suppressed.

This is a scanning electron microscope image of the crystalline thin-film titanium oxide particles obtained in Example 1. This is a scanning electron microscope image of the crystalline thin-film titanium oxide particles obtained in Example 2. This is a scanning electron microscope image of the crystalline thin-film titanium oxide particles obtained in Comparative Example 1. This is a scanning electron microscope image of the crystalline thin-film titanium oxide particles obtained in Example 3. This is a scanning electron microscope image of the crystalline thin-film titanium oxide particles obtained in Example 4. This is a scanning electron microscope image of the crystalline thin-film titanium oxide particles obtained in Comparative Example 2. This figure shows the number distribution of film thickness for thin-film titanium oxide particles A, and for crystalline thin-film titanium oxide particles obtained in Examples 1 and 2 and Comparative Example 1. This figure shows the number distribution of film thickness for thin-film titanium oxide particles B, and for crystalline thin-film titanium oxide particles obtained in Examples 3 and 4 and Comparative Example 2. This figure shows the X-ray diffraction patterns of thin-film titanium oxide particles A and B, Examples 1, 2, 3, and 4, and crystalline thin-film titanium oxide obtained in Comparative Examples 1 and 2.

The embodiments will be described in detail below.

In the method for producing crystalline inorganic oxide particles according to this embodiment, crystalline inorganic oxide particles are produced from inorganic oxide particles. At that time, a mixture containing inorganic oxide particles and spacer particles is prepared, and this mixture is heated to calcine the inorganic oxide particles.

According to the method for producing crystalline inorganic oxide particles according to this embodiment, by heating a mixture containing inorganic oxide particles and spacer particles to calcine the inorganic oxide particles, bonding between inorganic oxide particles during calcination can be suppressed. This is thought to be because the spacer particles interpose between the inorganic oxide particles, physically inhibiting contact between them, and consequently preventing bonding between them. In other words, in this invention, spacer particles are particles that have the function of suppressing bonding between inorganic oxides during the calcination of the inorganic oxide particles.

Here, examples of inorganic oxides constituting inorganic oxide particles include titanium oxide, zirconium oxide, yttrium oxide, tin oxide, silica, and alumina. It is preferable that the inorganic oxide particles are composed of one or more of these inorganic oxides. The inorganic oxide particles may also be composed of a composite of two or more of these inorganic oxides. The inorganic oxide particles may also be derived from an inorganic substance having hydrolyzable functional groups through a sol-gel transition involving hydrolysis and polycondensation reactions. A method for producing inorganic oxide particles using this sol-gel transition is disclosed in detail in Japanese Patent Application Publication No. 2021-54696. The crystalline form of the inorganic oxide particles is preferably amorphous.

The average particle size of inorganic oxide particles is preferably 1 nm or larger, more preferably 0.1 μm or larger, and even more preferably 1 μm or larger, from the viewpoint of suppressing bonding between inorganic oxide particles during firing. Similarly, from the same viewpoint, it is preferably 1 mm or smaller, more preferably 500 μm or smaller, even more preferably 100 μm or smaller, and even more preferably 50 μm or smaller. This average particle size is the volume-based average particle size measured by laser diffraction scattering for dried inorganic oxide particles.

The shape of the inorganic oxide particles can be, for example, thin film or granular. When the inorganic oxide particles are thin film, the average film thickness is preferably 1 nm or more, more preferably 10 nm or more, and even more preferably 20 nm or more, from the viewpoint of suppressing bonding between inorganic oxide particles during firing. Similarly, from the same viewpoint, it is preferably 100 μm or less, more preferably 10 μm or less, even more preferably 1 μm or less, and even more preferably 500 nm or less. This average film thickness is the number average of the film thicknesses of 100 inorganic oxide particles arbitrarily selected from images obtained by scanning electron microscopy observation of dried inorganic oxide particles. The method for measuring the average particle diameter is the method described in the examples.

The ratio of the average particle diameter of thin-film inorganic oxide particles to the average film thickness (average particle diameter / average film thickness) is preferably 3 or more, more preferably 5 or more, and even more preferably 10 or more, from the viewpoint of suppressing bonding between inorganic oxide particles during firing. Similarly, from the same viewpoint, it is preferably 2000 or less, more preferably 1500 or less, even more preferably 1000 or less, even more preferably 300 or less, and even more preferably 150 or less.

The material used to form the spacer particles may be either an inorganic or organic substance. Examples of inorganic substances include silica. Examples of organic substances include thermosetting resins and thermoplastic resins. Examples of thermosetting resins include melamine resin, phenolic resin, and epoxy resin. Examples of thermoplastic resins include polymethyl methacrylate resin and polyamide resin.

From the viewpoint of suppressing bonding between inorganic oxide particles during firing, it is preferable that the spacer particles be made of a material with high heat resistance. On the other hand, from the viewpoint of suppressing the residue of impurities, it is preferable that the spacer particles burn off when the mixture is heated. For these reasons, it is preferable that the spacer particles be made of an organic substance with high heat resistance, resin particles are preferable, and thermosetting resin particles are more preferable. From the viewpoint of suppressing bonding between inorganic oxide particles during firing, the temperature at which the spacer particles burn off is preferably 200°C or higher, more preferably 220°C or higher, and even more preferably 250°C or higher, and from the viewpoint of ensuring complete burn-off when the mixture is heated, it is preferably 900°C or lower, more preferably 600°C or lower. Furthermore, the spacer particles may be hollow particles or porous particles.

The average particle diameter of the spacer particles is preferably 1 nm or larger, more preferably 10 nm or larger, and even more preferably 20 nm or larger, from the viewpoint of suppressing bonding between inorganic oxide particles during firing. Similarly, from the same viewpoint, it is preferably 1 mm or smaller, more preferably 500 μm or smaller, even more preferably 100 μm or smaller, and even more preferably 20 μm or smaller. This average particle diameter is the volume-based average particle size measured by the laser diffraction scattering method described in the examples.

The ratio of the average particle diameter of inorganic oxide particles to the average particle diameter of spacer particles (average particle diameter of inorganic oxide particles / average particle diameter of spacer particles) is preferably 0.001 or higher, more preferably 0.01 or higher, even more preferably 0.1 or higher, and even more preferably 1 or higher, from the viewpoint of suppressing bonding between inorganic oxide particles during firing. Similarly, it is preferably 1000 or lower, more preferably 500 or lower, even more preferably 100 or lower, and even more preferably 50 or lower.

In a mixture of inorganic oxide particles and spacer particles, the mass ratio of the inorganic oxide particle content to the spacer particle content (inorganic oxide particle content / spacer particle content) is preferably 0.01 or higher, more preferably 0.05 or higher, and even more preferably 0.1 or higher, from the viewpoint of suppressing bonding between inorganic oxide particles during firing. Similarly, from the same viewpoint, it is preferably 1000 or lower, more preferably 100 or lower, even more preferably 50 or lower, and even more preferably 10 or lower.

The preparation of the mixture is preferably carried out by mixing inorganic oxide particles and spacer particles via a solvent, from the viewpoint of suppressing bonding between inorganic oxide particles during firing. Examples of solvents used include organic solvents such as water and ethanol. The mass ratio of the inorganic oxide particle content to the solvent content in the mixture (inorganic oxide particle content / solvent content) is preferably 0.001 or higher, more preferably 0.005 or higher, even more preferably 0.01 or higher, and from the same viewpoint, preferably 10 or lower, more preferably 5 or lower, even more preferably 1 or lower, and even more preferably 0.2 or lower.

The mixture may contain, in addition to inorganic oxide particles and spacer particles, other materials such as crystalline inorganic oxides.

From the viewpoint of suppressing bonding between inorganic oxide particles during firing, the heating temperature of the mixture is preferably 200°C or higher, more preferably 220°C or higher, and even more preferably 250°C or higher. Similarly, from the same viewpoint, it is preferably 1200°C or lower, more preferably 1100°C or lower, and even more preferably 1000°C or lower.

The average particle size of crystalline inorganic oxide particles obtained by calcining inorganic oxide particles is often smaller than the average particle size of the inorganic oxide particles themselves. The average particle size of crystalline inorganic oxide particles is preferably 1 nm or larger, more preferably 0.1 μm or larger, even more preferably 1 μm or larger, and also preferably 1 mm or smaller, more preferably 500 μm or smaller, even more preferably 100 μm or smaller, and even more preferably 50 μm or smaller. This average particle size is the volume-based average particle size measured by laser diffraction scattering.

Thin-film crystalline inorganic oxide particles can be obtained from thin-film inorganic oxide particles. In this case, the average film thickness of the crystalline inorganic oxide particles is often smaller than the average film thickness of the inorganic oxide particles. The average film thickness of the crystalline inorganic oxide particles is preferably 1 nm or more, more preferably 10 nm or more, even more preferably 20 nm or more, and also preferably 100 μm or less, more preferably 10 μm or less, even more preferably 1 μm or less, and even more preferably 500 nm or less. This average film thickness is the number average of the film thicknesses of 100 crystalline inorganic oxide particles arbitrarily selected from images obtained by scanning electron microscopy.

The ratio of the average particle diameter to the average film thickness of the resulting thin-film crystalline inorganic oxide particles (average particle diameter / average film thickness) is preferably 3 or more, more preferably 5 or more, even more preferably 10 or more, and also preferably 2000 or less, more preferably 1500 or less, even more preferably 1000 or less, even more preferably 300 or less, and even more preferably 150 or less.

Examples of crystalline inorganic oxide particles include rutile, anatase, and brookite forms for titanium oxide. Of these, the rutile form is preferred. This crystal form is analyzed by X-ray crystal structure analysis.

Crystalline inorganic oxide particles can be used, for example, as materials for paints, inks, and coatings, as catalysts, UV protection agents, pigments, and the like.

As described in Examples 1 to 4 and Comparative Examples 1 to 2 below, experiments were conducted to produce crystalline thin-film titanium oxide particles from thin-film titanium oxide particles. The average particle size and average film thickness of both the thin-film titanium oxide particles and the crystalline thin-film titanium oxide particles were measured, and the crystal type of the crystalline thin-film titanium oxide particles was analyzed. The details are also shown in Tables 1 and 2.

Here, the average particle size of thin-film titanium oxide particles and crystalline thin-film titanium oxide particles was measured using a laser diffraction/scattering particle size distribution analyzer (LA-960, Horiba, Ltd.) by laser diffraction/scattering method to determine the volume-based average particle size. Measurements were performed using a flow cell with water as the medium and the refractive index set to relative refractive index: 1.12 when measuring thin-film titanium oxide before firing, and 2.0 when measuring crystalline thin-film titanium oxide. The aqueous dispersion containing the titanium particles was added to the flow cell, and measurements were performed at a concentration showing a transmittance of approximately 95%, and the volume-based average particle size was determined.

For thin-film titanium oxide particles, the average film thickness was calculated by taking 100 randomly selected thin-film titanium oxide particles dried at 105°C and observing their images using a scanning electron microscope. For crystalline thin-film titanium oxide particles, the average film thickness was calculated by taking 100 randomly selected crystalline thin-film titanium oxide particles and observing their images using a scanning electron microscope.

The crystalline structure of thin-film titanium oxide particles was analyzed by X-ray crystallography after drying at room temperature. The crystalline structure of crystalline thin-film titanium oxide particles was analyzed by X-ray crystallography of the powder after calcination. A desktop X-ray diffractometer (MiniFlex 600, Rigaku) was used for the X-ray crystallography. The measurement conditions were: X-ray source: Cu/Kα-radiation, tube voltage: 40 kV, tube current: 15 mA, measurement range: diffraction angle 2θ = 20–60°, X-ray scan speed: 10°/min. In X-ray diffraction, particles showing a diffraction peak at a diffraction angle (2θ) of 25.4 ± 0.3° were considered anatase type, and particles showing a diffraction peak at a diffraction angle (2θ) of 27.5 ± 0.3° were considered rutile type.

(Manufacturing of crystalline thin-film titanium oxide particles)
<Example 1>
Thin-film titanium oxide particles were prepared based on the method disclosed in Japanese Patent Publication No. 2021-54696. The thin-film titanium oxide particles had an average particle diameter of 14 μm and an average film thickness of 282 nm. The crystalline form of the thin-film titanium oxide particles was amorphous. Hereinafter, these thin-film titanium oxide particles will be referred to as "thin-film titanium oxide particles A".

A dispersion containing 5% by mass of thin-film titanium oxide particles A was prepared by dispersing thin-film titanium oxide particles A in water. To this dispersion, 5 parts by mass of melamine-formaldehyde resin particles (Epostor S6, manufactured by Nippon Shokubai Co., Ltd., average particle size: 0.42 μm, burning temperature 300°C) were added per 100 parts by mass of the dispersion to prepare a mixture.

The particle size of the spacer particles was measured using a laser diffraction/scattering particle size distribution analyzer (LA-960, Horiba, Ltd.) and the volume-based average particle size was determined by the laser diffraction/scattering method. A batch cell was used for the measurement, with ethanol as the solvent and the refractive index set to 1.22 (relative refractive index). The spacer particles were added to the batch cell, and measurements were performed at a concentration where the transmittance was approximately 90%. The volume-based average particle size was then determined.

The mass ratio of the content of thin-film titanium oxide particles A to the content of spacer particles in the mixture is 1. The mass ratio of the content of thin-film titanium oxide particles A to the content of water in the mixture is 0.05.

The mixture was subjected to ultrasonic cleaning using an ultrasonic cleaner (VS-100III, manufactured by AS ONE Corporation), irradiated with ultrasound at a frequency of 28 kHz for 1 minute, to disperse thin-film titanium oxide particles A and spacer particles.

Then, under an environment where nitrogen was flowed at a flow rate of 5 L/min, the mixture was heated at 850°C for 60 minutes to calcine the thin-film titanium oxide particles A, thereby obtaining crystalline thin-film titanium oxide particles and burning off the spacer particles.

Figure 1 shows the obtained crystalline thin-film titanium oxide particles. The crystalline thin-film titanium oxide particles obtained in Example 1 had an average particle diameter of 9.5 μm and an average film thickness of 238 nm. The crystalline form of the crystalline thin-film titanium oxide particles was a mixture of rutile and anatase forms.

<Example 2>
Crystalline thin-film titanium oxide particles were obtained in the same manner as in Example 1, except that polymethyl methacrylate resin particles (NMB-0220C, manufactured by ENEOS Liquid Crystal Corporation, average particle size: 2.6 μm, burn-out temperature 340°C) were used as thin-film titanium oxide particles A and spacer particles.

The particle size of the spacer particles was measured using a laser diffraction/scattering particle size distribution analyzer (LA-960V2, Horiba, Ltd.) by laser diffraction/scattering method to determine the volume-based average particle size. A batch cell was used for the measurement, with water as the solvent, a solvent refractive index of 1.333, and a solute refractive index of 1.49. The spacer particles were added to the batch cell, and measurements were performed at a concentration where the transmittance was approximately 90%. The volume-based average particle size was then determined.

Figure 2 shows the obtained crystalline thin-film titanium oxide particles. The crystalline thin-film titanium oxide particles obtained in Example 2 had an average particle diameter of 12.5 μm and an average film thickness of 262 nm. The crystal type of the crystalline thin-film titanium oxide particles was rutile.

<Comparative Example 1>
Crystalline thin-film titanium oxide particles were obtained in the same manner as in Example 1, except that thin-film titanium oxide particles A were used and spacer particles were not used.

Figure 3 shows the obtained crystalline thin-film titanium oxide particles. The crystalline thin-film titanium oxide particles obtained in Comparative Example 1 had an average particle diameter of 13.8 μm and an average film thickness of 334 nm. The crystal type of the crystalline thin-film titanium oxide particles was rutile.

<Example 3>
Thin-film titanium oxide particles were prepared based on the method disclosed in Japanese Patent Publication No. 2021-54696. The thin-film titanium oxide particles had an average particle diameter of 16 μm and an average film thickness of 163 nm. The crystalline form of the thin-film titanium oxide particles was amorphous. Hereinafter, these thin-film titanium oxide particles will be referred to as "thin-film titanium oxide particles B".

A dispersion containing 4% by mass of thin-film titanium oxide particles B was prepared by dispersing thin-film titanium oxide particles B in water. To this dispersion, 4 parts by mass of melamine-formaldehyde resin particles (spacer particles) were added per 100 parts by mass of the dispersion to prepare a mixture.

The mass ratio of the content of thin-film titanium oxide particles B in the mixture to the content of spacer particles is 1. The mass ratio of the content of thin-film titanium oxide particles B in the mixture to the content of water is 0.04.

The mixture was subjected to ultrasonic cleaning using an ultrasonic cleaner, irradiating it with ultrasound at a frequency of 28 kHz for 1 minute, to disperse thin-film titanium oxide particles B and spacer particles.

Then, under an environment where nitrogen was flowed at a flow rate of 5 L/min, the mixture was heated at 850°C for 300 minutes to calcine the thin-film titanium oxide particles B, thereby obtaining crystalline thin-film titanium oxide particles and burning off the spacer particles.

Figure 4 shows the obtained crystalline thin-film titanium oxide particles. The crystalline thin-film titanium oxide particles obtained in Example 3 had an average particle diameter of 13.3 μm and an average film thickness of 218 nm. The crystal type of the crystalline thin-film titanium oxide particles was rutile.

<Example 4>
Crystalline thin-film titanium oxide particles were obtained in the same manner as in Example 3, except that thin-film titanium oxide particles B were used, and the amount of melamine-formaldehyde resin particles added as spacer particles was 19 parts by mass per 100 parts by mass of the dispersion. The mass ratio of the content of thin-film titanium oxide particles B in the mixture to the content of spacer particles was 0.21.

Figure 5 shows the obtained crystalline thin-film titanium oxide particles. The crystalline thin-film titanium oxide particles obtained in Example 4 had an average particle diameter of 10.7 μm and an average film thickness of 164 nm. The crystalline form of the crystalline thin-film titanium oxide particles was a mixture of rutile and anatase forms.

<Comparative Example 2>
Crystalline thin-film titanium oxide particles were obtained in the same manner as in Example 3, except that thin-film titanium oxide particles B were used and spacer particles were not used.

Figure 6 shows the obtained crystalline thin-film titanium oxide particles. The crystalline thin-film titanium oxide particles obtained in Comparative Example 2 had an average particle diameter of 6.9 μm and an average film thickness of 321 nm. The crystal type of the crystalline thin-film titanium oxide particles was rutile.

(Experimental results)
Figure 7 shows the number distribution of film thickness for thin-film titanium oxide particles A, and the crystalline thin-film titanium oxide particles obtained in Examples 1, 2, and Comparative Example 1. Figure 8 shows the number distribution of film thickness for thin-film titanium oxide particles B, and the crystalline thin-film titanium oxide particles obtained in Examples 3, 4, and Comparative Example 2. Figure 9 shows the X-ray diffraction patterns of crystalline thin-film titanium oxide obtained in thin-film titanium oxide particles A and B, Examples 1, 2, 3, and 4, and Comparative Examples 1 and 2.

According to Figures 7 and 8, in Examples 1 to 4 using spacer particles, the film thickness of the thin-film titanium oxide particles A and B before firing and the film thickness of the crystalline thin-film titanium oxide particles after firing have almost the same particle number distribution. On the other hand, in Comparative Examples 1 and 2, which do not use spacer particles, the film thickness of the crystalline thin-film titanium oxide particles after firing shows that the particle number distribution is wider towards the thicker side than that of the thin-film titanium oxide particles A and B before firing.

This invention is useful in the technical field of methods for producing crystalline inorganic oxide particles.

Claims (9)

A method for producing crystalline inorganic oxide particles from inorganic oxide particles,
The inorganic oxide particles are thin-film titanium oxide particles, and the crystalline inorganic oxide particles are crystalline thin-film titanium oxide particles.
A method for producing crystalline inorganic oxide particles , comprising mixing the inorganic oxide particles and resin spacer particles through a solvent to prepare a mixture, and heating the mixture to calcine the inorganic oxide particles. The method for producing crystalline inorganic oxide particles according to claim 1, wherein the crystalline form of the inorganic oxide particles is amorphous. A method for producing crystalline inorganic oxide particles according to claim 1 or 2 , wherein the spacer particles are burned off when the mixture is heated. A method for producing crystalline inorganic oxide particles according to any one of claims 1 to 3 , wherein the temperature at which the spacer particles burn out is 200°C or higher. A method for producing crystalline inorganic oxide particles according to any one of claims 1 to 4 , wherein the inorganic oxide particles are induced from an inorganic substance having hydrolyzable functional groups by a sol-gel transition involving a hydrolysis reaction and a polycondensation reaction. A method for producing crystalline inorganic oxide particles according to any one of claims 1 to 5 , wherein the average particle diameter of the inorganic oxide particles is 1 nm or more and 1 mm or less. A method for producing crystalline inorganic oxide particles according to any one of claims 1 to 6 , wherein the average particle diameter of the spacer particles is 1 nm or more and 1 mm or less. A method for producing crystalline inorganic oxide particles according to any one of claims 1 to 7 , wherein the mass ratio of the content of the inorganic oxide particles in the mixture to the content of the solvent is 0.001 or more and 10 or less. A method for producing crystalline inorganic oxide particles according to any one of claims 1 to 8 , wherein the mass ratio of the content of the inorganic oxide particles in the mixture to the content of the spacer particles is 0.01 or more and 1000 or less.