JP7835565B2 - Spherical zinc oxide particles - Google Patents

Description

This invention relates to spherical zinc oxide particles, and more particularly to spherical zinc oxide particles having a high specific surface area.

Zinc oxide particles possess characteristics such as UV shielding (UV absorption), near-infrared reflection, and photocatalytic effects, making them useful for a variety of applications including cosmetics, photocatalytic materials, UV-blocking materials, antibacterial materials, antifungal materials, and heat-dissipating materials. In particular, shaping zinc oxide particles into spheres improves their dispersibility, fluidity, and packing properties, as well as their handling capabilities, making spherical zinc oxide particles even more useful in the aforementioned applications.

Patent Document 1 discloses a method for producing ultrafine zinc oxide particles from metallic zinc vapor, wherein the obtained ultrafine zinc oxide particles have a specific surface area of 10 to 200 ^m² /g and a tapping density of 4 to 40 mL/g.
Patent Document 2 discloses a method for producing hydroxide and/or oxide fine particles by electrodialysis, with zinc oxide fine particles being an example of the obtained fine particles. The average primary particle size of these fine particles is 0.2 to 15 nm, and the specific surface area when the fine particles are zinc oxide is approximately 22 to 180 ^m² /g.

International Publication No. 2000/046152 International Publication No. 2008/044544

The ultrafine zinc oxide particles actually obtained in Patent Document 1 have a specific surface area of 14 to 34 ^m² /g, a primary particle size of 31 to 76 nm, and an average particle size of 0.47 to 0.55 μm. As such, they have high adhesion and aggregation properties, and there was room for improvement in terms of dispersibility and fluidity.

Similarly, the zinc oxide fine particles obtained in Patent Document 2 also have a specific surface area of 22 to 180 ^m² /g and a primary particle diameter of 0.2 to 15 nm, resulting in high adhesion and aggregation properties, leaving room for improvement in terms of dispersibility and fluidity.

This invention has been made in view of the above circumstances, and aims to provide spherical zinc oxide particles that exhibit low adhesion and aggregation properties, as well as good dispersibility and fluidity.

The inventors have discovered spherical zinc oxide particles that satisfy a specific range of specific surface area and median diameter (D50), exhibiting low adhesion and aggregation properties, as well as good dispersibility and fluidity, and have thus completed the present invention.

In other words, the present invention consists of either (1) or (2) below.
(1) Spherical zinc oxide particles having a median diameter (D50) of 1 to 300 μm and a BET specific surface area of 50 to 300 ^m² /g.
(2) Spherical zinc oxide particles as described in (1), wherein the total pore volume calculated by the BJH method is 0.10 to 1.50 ^cm³ /g.

The spherical zinc oxide particles according to the present invention have a high specific surface area and a median diameter (D50) within a specific range. Therefore, they are expected to exhibit better dispersibility and fluidity than submicron-sized zinc oxide particles. Furthermore, their high specific surface area allows for expected oil absorption, moisture absorption, photocatalytic activity, antibacterial properties, and antifungal properties.

Figure 1 summarizes the SEM images of spherical zinc oxide particles obtained in Examples 1 to 3 and Comparative Example 1.

The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be modified and implemented as appropriate without departing from the spirit of the invention.
In this specification, the "~" indicating a numerical range is used to mean that the numbers before and after it are included as the lower and upper limits, respectively.

<Spherical zinc oxide particles>
The spherical zinc oxide particles according to the present invention have a BET specific surface area of 50 to 300 ^m² /g and are spherical with a median diameter (D50) of 1 to 300 μm.
The term "spherical" for zinc oxide particles refers to a generally rounded shape, and is not limited to being strictly spherical. For example, it can include shapes that are slightly flattened or distorted from a perfect sphere, as well as elliptical shapes.
Furthermore, for applications requiring perfect sphericity, such as cosmetic applications, the spherical zinc oxide particles according to the present invention are preferable as they are as close to perfect spheres as possible. The ratio of the major axis a to the minor axis b (a/b) is calculated for any number of particles, and the closer the arithmetic mean (aspect ratio) is to 1, the closer the particle is to perfect sphere. When even greater sphericity is required, the aspect ratio is preferably 1.15 or less, more preferably 1.10 or less, and preferably as close to 1.00 as possible. Note that the major axis a and minor axis b in the aspect ratio correspond to the longer and shorter sides when a rectangle is drawn circumscribing the particle, respectively.

A spherical median diameter (D50) of 1 μm or more can be expected to provide good dispersibility and fluidity. A median diameter (D50) of 3 μm or more is preferable, and 5 μm or more is more preferable.
On the other hand, when the median diameter (D50) is 300 μm or less, it can exert its effects without hindering the performance of the material when used as a cosmetic, photocatalytic material, UV-blocking material, antibacterial material, antifungal material, heat-dissipating material, etc. The median diameter (D50) is preferably 100 μm or less, and more preferably 20 μm or less.
The median diameter (D50) is determined by a nanoparticle size distribution analyzer and refers to the particle size equivalent to 50% of the cumulative volume.

The BET specific surface area of the spherical zinc oxide particles according to the present invention is 50 ^m² /g or more, which increases the number of active sites and adsorption capacity, and allows for the expectation of good oil absorption properties, moisture absorption properties, photocatalytic activity, antibacterial properties, and antifungal properties. A BET specific surface area of 100 ^m² /g or more is preferable, and 150 ^m² /g or more is more preferable. Furthermore, a higher BET specific surface area is preferable from the viewpoint of exhibiting oil absorption properties.
On the other hand, as the BET specific surface area increases, the density of spherical zinc oxide particles decreases, making the particles more prone to scattering and thus reducing their ease of handling. Therefore, when considering ease of handling, the BET specific surface area is preferably 300 ^m² /g or less, more preferably 280 ^m² /g or less, and even more preferably 250 ^m² /g or less.

The total pore volume of the spherical zinc oxide particles according to the present invention can be calculated by the BJH method from the results of nitrogen adsorption/desorption measurements by the BET method. The total pore volume of the spherical zinc oxide particles according to the present invention, calculated by the BJH method, is preferably 0.10 ^cm³ /g or more from the viewpoint of increasing adsorption capacity, and more preferably 0.15 ^cm³ /g or more. Furthermore, a larger total pore volume is preferable from the viewpoint of increasing adsorption capacity.
On the other hand, as the total pore volume increases, the density of spherical zinc oxide particles decreases, making the particles more prone to scattering and thus reducing their ease of handling. Therefore, when considering ease of handling, the total pore volume is preferably 1.50 ^cm³ /g or less, and more preferably 1.30 ^cm³ /g or less.

The spherical zinc oxide particles according to the present invention preferably have a crystallite size within a certain width range.
Specifically, it is preferable that the full width at half maximum of the zinc oxide (101) plane peak in the X-ray diffraction pattern obtained by X-ray diffraction measurement is 0.6 to 3.5°.
A full width at half maximum (FWHM) of 0.6° or higher prevents the crystallite size constituting the particles from becoming too large, thereby achieving a high specific surface area. Furthermore, a FWHM of 3.5° or lower prevents the crystallites from becoming too small, allowing for a high specific surface area while maintaining the properties of zinc oxide. A FWHM of 1.0° or higher is more preferable, and 3.0° or lower is even more preferable.

The above full width at half maximum can be determined by performing an X-ray diffraction measurement under the following conditions.
(X-ray diffraction measurement conditions)
Radiation source: CuKα, Measurement range (2θ): 10-80°, Step width: 0.02°, Scan speed: 2°/min, Tube voltage: 40kV, Tube current: 30mA, Cathode: Cu, Receiving slit: 0.3mm, Divergence slit: 1°, Scattering slit: 1°
The XRD pattern obtained under the above measurement conditions is subjected to background processing and smoothing, and the width between the two ends at half the intensity value of the zinc oxide (101) plane peak intensity is defined as the width at half maximum.

<Method for producing spherical zinc oxide particles>
The method for producing spherical zinc oxide particles according to the present invention is not particularly limited, but they can be produced, for example, by the following steps (A) to (C).
Step (A): An aqueous solution containing a zinc compound and at least one of ammonium ions and a compound that generates ammonium ions is heated to obtain a precipitate of basic zinc carbonate.
Step (B): The obtained basic zinc carbonate is calcined to obtain zinc oxide particles.
Step (C): After step (A) or step (B) above, the basic zinc carbonate or zinc oxide particles are crushed and spray-dried.

The above process may be carried out in the order of process (A), process (B), process (C), or in the order of process (A), process (C), process (B).
When the process is carried out in the order of steps (A), (B), and (C), spherical zinc oxide particles are obtained in step (C) by crushing and spray-drying the zinc oxide particles obtained in step (B).
On the other hand, when the process is carried out in the order of process (A), process (C), and process (B), in process (C), spherical basic zinc carbonate particles are obtained by crushing and spray-drying the basic zinc carbonate obtained in process (A), and spherical zinc oxide particles are obtained by calcining these particles in process (B).

(Process (A))
The zinc compound in step (A) may contain any zinc salt. Examples of zinc salts include inorganic salts such as zinc nitrate, zinc acetate, zinc sulfate, zinc chloride, zinc formate, zinc oxalate, and zinc citrate, as well as organic salts. The compound may contain one or more of these zinc salts.
Specifically, examples include zinc nitrate hexahydrate, zinc acetate dihydrate, zinc acetate (anhydrous), zinc sulfate heptahydrate, and zinc chloride.

In step (A), an aqueous solution containing ammonium ions can be found in ammonia water. Compounds that generate ammonium ions include urea, hexamethylenetetramine, and ethylenediamine. Among these, urea is preferred.

It is preferable that the aqueous solution in step (A) further contains an organic acid. The inclusion of an organic acid can increase the specific surface area of the resulting spherical zinc oxide particles. This is presumed to be because the addition of the organic acid allows the spherical zinc oxide particles to crystallize appropriately, without excessive or insufficient growth.

Examples of organic acids include carboxylic acids. The mixture may contain one or more organic acids.
Examples of carboxylic acids include citric acid, maleic acid, malic acid, succinic acid, and tartaric acid. Among these, at least one acid selected from the group consisting of citric acid, maleic acid, and malic acid is more preferred because it has a remarkable effect in increasing the specific surface area.

In step (A), a precipitate of basic zinc carbonate is obtained by heating an aqueous solution containing the above-mentioned zinc compound, at least one of ammonium ions and a compound that generates ammonium ions, and preferably an organic acid.

The obtained basic zinc carbonate precipitate can be identified by X-ray diffraction. The precipitate can be isolated by filtration, centrifugation, etc., and, if necessary, washed and dried before proceeding to step (B) or step (C).

(Process (B))
By calcining the basic zinc carbonate obtained in step (A), or the spherical basic zinc carbonate particles obtained by subjecting the basic zinc carbonate obtained in step (A) to step (C) described later, zinc oxide particles or spherical zinc oxide particles can be obtained.
The firing temperature is preferably 150 to 400°C. A firing temperature of 150°C or higher prevents undecomposed basic zinc carbonate from remaining in the resulting particles, while a firing temperature of 400°C or lower allows for a higher specific surface area of the final spherical zinc oxide particles. The firing time is not particularly limited, but 1 to 5 hours is preferred.

(Step (C))
The zinc oxide particles obtained by step (B), which is performed following step (A), or the basic zinc carbonate obtained by step (A), can be made spherical by crushing and spray-drying them.
The pulverization process breaks down the zinc oxide particles or basic zinc carbonate particles, facilitating subsequent spheroidization by spray drying.

The grinding method is not particularly limited; either dry grinding or wet grinding is acceptable, but wet grinding is preferred because it is more effective at breaking down particles.
Specifically, examples include dry bead mills, wet bead mills, ball mills, jet mills, and wet jet mills, but wet bead mills and wet jet mills are preferred.
The dispersant used in wet grinding is not particularly limited, but examples include ammonium polyacrylate and ammonium polycarboxylate.

In spray drying, pulverized zinc oxide particles or basic zinc carbonate particles are sprayed into a gas as a mixture (slurry) with a liquid, and then rapidly dried to form spherical particles. By changing the conditions of the liquid delivery speed and atomizer rotation speed during spray drying, the median diameter (D50) and sphericity of the resulting spherical particles can be adjusted.

Spherical particles can be obtained by drying the material, which has been sprayed in the form of minute droplets, with high-temperature hot air. A drying temperature of 160°C or higher is preferable from the viewpoint of promoting droplet drying. There is no particular upper limit to the drying temperature as long as crystal growth of the obtained particles is not promoted; the upper limit should be the temperature limit of the drying equipment used.

The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these examples.

<Example 1>
(A-1) Zinc nitrate hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd., special grade), urea (manufactured by Wako Pure Chemical Industries, Ltd., special grade), and 0.1% by weight aqueous ammonia (manufactured by Wako Pure Chemical Industries, Ltd., special grade) were added to distilled water according to the composition shown in Table 1. The mixture was then heated at 95°C for 2 hours while stirring with a hot stirrer.
The obtained white precipitate was filtered, washed, and dried in an oven at 110°C for 2 hours. The obtained white precipitate was confirmed to be basic zinc carbonate particles ( _Zn₅ ( _CO₃ ) _₂ (OH) _₆ ) by X-ray diffraction measurement.
(B-1) Subsequently, zinc oxide particles were obtained by calcining the obtained basic zinc carbonate particles at 250°C for 2 hours.
(C-1) The obtained zinc oxide particles were then pulverized using a bead mill, and the resulting slurry was spray-dried to obtain spherical zinc oxide particles. The pulverization conditions using the bead mill and the spray drying conditions are shown in Tables 2 and 3. Furthermore, it was confirmed that the obtained spherical particles were zinc oxide particles by X-ray diffraction measurement.

<Example 2>
(A-2) Zinc nitrate hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd., special grade), urea (manufactured by Wako Pure Chemical Industries, Ltd., special grade), and 0.1% by weight aqueous ammonia (manufactured by Wako Pure Chemical Industries, Ltd., special grade) were added to distilled water according to the composition shown in Table 1. The mixture was then heated at 95°C for 2 hours while stirring with a hot stirrer.
The resulting white precipitate was filtered, washed, and dried in an oven at 110°C for 2 hours. The resulting white precipitate was basic zinc carbonate particles ( _Zn₅ ( _CO₃ ) _₂ (OH) _₆ ).
(C-2) The obtained basic zinc carbonate particles were then pulverized in a bead mill, and the resulting slurry was spray-dried to obtain spherical basic zinc carbonate particles. The pulverization conditions in the bead mill and the spray drying conditions are shown in Tables 2 and 3.
(B-2) Subsequently, spherical zinc oxide particles were obtained by calcining the obtained spherical basic zinc carbonate particles at 250°C for 2 hours.

<Example 3>
(A-3) Zinc nitrate hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd., special grade), urea (manufactured by Wako Pure Chemical Industries, Ltd., special grade), 0.1% by weight aqueous ammonia (manufactured by Wako Pure Chemical Industries, Ltd., special grade), and citric acid monohydrate (manufactured by Wako Pure Chemical Industries, Ltd., food additive) were added to distilled water according to the composition shown in Table 1. The mixture was then heated at 95°C for 2 hours while stirring with a hot stirrer.
The resulting white precipitate was filtered, washed, and dried in an oven at 110°C for 2 hours. The resulting white precipitate was basic zinc carbonate particles ( _Zn₅ ( _CO₃ ) _₂ (OH) _₆ ).
(C-3) The obtained basic zinc carbonate particles were then pulverized in a bead mill, and the resulting slurry was spray-dried to obtain spherical basic zinc carbonate particles. The pulverization conditions in the bead mill and the spray drying conditions are shown in Tables 2 and 3.
(B-3) Subsequently, spherical zinc oxide particles were obtained by calcining the obtained spherical basic zinc carbonate particles at 250°C for 2 hours.

<Comparative Example 1>
(C-4) Spherical zinc oxide particles were obtained by grinding zinc oxide powder (manufactured by Kojun Chemical Laboratory Co., Ltd., 4N) in a bead mill and spray-drying the resulting slurry. The grinding conditions in the bead mill and the spray drying conditions are shown in Tables 2 and 3.

<External observation>
The obtained spherical zinc oxide particles were observed visually using a scanning electron microscope (SEM, JEOL Ltd., JSM-5500LV). Figure 1 summarizes the SEM images of the zinc oxide particles obtained in Examples 1-3 and Comparative Example 1, confirming that all of the zinc oxide particles were spherical. The median diameter (D50) of the obtained spherical zinc oxide particles was calculated using a nanoparticle size distribution analyzer (Shimadzu Corporation, SALD-7100) to determine the particle size equivalent to 50% of the cumulative percentage based on volume. The results are shown in Table 4.

<BET specific surface area, total pore volume>
The BET specific surface area of spherical zinc oxide particles was determined using a specific surface area meter (BELSORP-miniII, manufactured by Microtrac Bell Co., Ltd.), and the total pore volume was calculated using the BJH method. The results are shown in Table 4.

<Full width at half maximum of the zinc oxide (101) surface peak>
The full width at half maximum (FWHM) of the zinc(101) plane peak of spherical zinc oxide particles was determined using an X-ray diffractometer (Shimadzu Corporation, XRD-6000), and the FWHM was calculated. The results are shown in Table 4.

From these results, it was confirmed that spherical zinc oxide particles with a high specific surface area and a median diameter (D50) within a specified range were obtained.

The spherical zinc oxide particles according to the present invention have a high specific surface area and a spherical shape with a median diameter (D50) of 1 μm or more. Therefore, they are expected to exhibit better dispersibility and fluidity than submicron-sized zinc oxide fine particles. Furthermore, their high specific surface area allows for expected oil absorption, moisture absorption, photocatalytic activity, antibacterial properties, and antifungal properties, making them of great technical significance.

Claims (2)

Spherical zinc oxide particles having a median diameter (D50) of 5 to 100 μm and a BET specific surface area of 100 to 300 ^m² /g. Spherical zinc oxide particles according to claim 1, wherein the total pore volume calculated by the BJH method is 0.10 to 1.50 ^cm³ /g.