JP7315354B2 - Coated zinc oxide particles and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to zinc oxide particles and a method for producing the same.

Various techniques for shielding ultraviolet rays and infrared rays using zinc oxide are known. For example, Patent Literature 1 describes a technique for improving the infrared shielding property of zinc oxide particles that contain Zn and a trivalent and/or tetravalent metal element as metal components and exhibit zinc oxide crystallinity in terms of X-ray diffraction, by setting the content of an impurity element such as chlorine to a specific value or less. In addition, the present applicant has previously proposed a zinc oxide powder in which the photocatalytic activity of zinc oxide is suppressed without impairing the ultraviolet absorption efficiency by subjecting the surface of the zinc oxide powder to a sulfurizing treatment (see Patent Document 2).

Patent Document 3 describes a composite powder obtained by coating the surface of zinc oxide with an organic compound having a low solubility in a medium and having an ultraviolet absorption ability using an organic solvent with high solubility.

JP-A-10-338521 JP-A-2000-109601 JP 2012-121810 A

As described above, although there are various techniques for shielding ultraviolet rays and infrared rays using zinc oxide, in recent years, there has been an increasing demand for materials capable of shielding long-wave ultraviolet rays close to the wavelength region of visible light.

Accordingly, an object of the present invention is to improve the technology for shielding ultraviolet rays using zinc oxide, and more specifically, to provide zinc oxide particles capable of shielding ultraviolet rays of long wavelengths.

As a result of intensive studies aimed at solving the above-mentioned problems, the inventors of the present invention have found that the change in bandgap caused by doping zinc oxide with a specific dissimilar element is related to the degree of ultraviolet shielding, and that the change in unpaired electrons that occurs at that time can be detected by the electron spin resonance method. The present invention was made based on this finding,
At 84 K, the absorption intensity of a 2.0 mmol/L toluene solution of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl obtained by an electron spin resonance method is defined as I0,
At 84 K, when the spectral absorption intensity per 1 g of zinc oxide particles obtained by the electron spin resonance method is I,
The above problem has been solved by providing zinc oxide particles having a relative spectral absorption intensity represented by I/I0 of 0.001 or more.

In addition, the present invention provides a suitable method for producing the zinc oxide particles,
mixing an aqueous solution containing zinc and a halogen with a basic substance to form a precipitate containing zinc;
washing the precipitate with water until the conductivity of the washing water is 2 μS/cm or more and 10000 μS/cm or less;
The present invention provides a method for producing zinc oxide particles, comprising firing the precipitate after washing with water at 350° C. or higher and 750° C. or lower in an air atmosphere.

INDUSTRIAL APPLICABILITY According to the present invention, zinc oxide particles capable of shielding ultraviolet rays having long wavelengths close to the wavelength region of visible light are provided.

The present invention will be described below based on its preferred embodiments. One of the characteristics of the zinc oxide (ZnO) particles of the present invention is that they can block long-wave ultraviolet rays close to the wavelength region of visible light. That is, one of the characteristics is that it has a high ability to absorb ultraviolet rays of long wavelengths close to the wavelength region of visible light. As a result of intensive studies on the UV shielding ability of zinc oxide, the present inventors have found that the change in bandgap caused by doping zinc oxide with a specific dissimilar element is related to the degree of UV shielding. Further, the inventors have found that a change in unpaired electrons occurs due to a change in bandgap, and that the change in unpaired electrons can be detected by an electron spin resonance method (hereinafter also referred to as "ESR"). In general, the electronic state of a substance is reflected in the g value and intensity value measured by ESR. That is, the zinc oxide particles of the present invention are ultraviolet absorbing materials characterized by specific physical property values obtained by ESR measurement. In this specification, long-wavelength ultraviolet rays close to the wavelength region of visible light are ultraviolet rays having a wavelength region of 360 nm or more and 400 nm or less.

In the present invention, I0 is the absorption intensity of 100 μL of a 2.0 mmol/L toluene solution of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (hereinafter also referred to as “TEMPOL”) obtained by ESR measurement at 84K. TEMPOL is used as a standard substance for ESR measurement in the present invention. The reason why TEMPOL was used as the standard substance is that the radicals generated from this substance are stable. The reason why the ESR measurement temperature is set to 84 K is that the number of unpaired electrons involved in microwave absorption in ESR measurement increases as the temperature decreases, and as a result, the sensitivity increases as the temperature decreases.

In the present invention, I is the spectral absorption intensity per 1 g of zinc oxide particles obtained by ESR measurement at 84K. When the relative spectral absorption intensity is defined as I/I0, zinc oxide particles having a relative spectral absorption intensity of 0.001 or more have a high ability to shield long-wave ultraviolet rays. The larger the value of the relative spectral absorption intensity, the better. Specifically, it is preferably 0.005 or more, and more preferably 0.008 or more. The upper limit of the relative spectral absorption intensity is not particularly limited, but the upper limit that can be reached at present is about 0.1.

ESR measurements provide information on g-values as well as the relative spectral absorption intensities described above. The g-value is a material-specific value determined by the electron spin environment. The zinc oxide particles of the present invention preferably have a g value of 1.957 or more and 1.964 or less, more preferably 1.958 or more and 1.963 or less, and even more preferably 1.959 or more and 1.962 or less. When the g value is within this range, the zinc oxide particles of the present invention are more capable of shielding long-wave ultraviolet rays close to the wavelength region of visible light.

The zinc oxide particles of the present invention are also characterized by their bandgap. The bandgap is the energy difference between the valence band top and the conduction band bottom. The zinc oxide particles of the present invention preferably have a bandgap of 2.60 eV or more and 3.35 eV or less, more preferably 2.65 eV or more and 3.30 eV or less, and even more preferably 2.70 eV or more and 3.25 eV or less.

The zinc oxide particles of the present invention are also characterized by their tint. The zinc oxide particles of the present invention are yellowish, in contrast to the fact that ordinary zinc oxide particles are white. The present inventor believes that this also contributes to the long-wavelength ultraviolet shielding ability. Specifically, when represented by the L*a*b* color system (CIE1976 L*a*b* color space), the zinc oxide particles of the present invention preferably have an L* value of 90 or more and 99.5 or less, more preferably 93 or more and 99 or less, and even more preferably 97 or more and 98.5 or less. The a* value is preferably -15 or more and 1 or less, more preferably -12 or more and 0 or less, and even more preferably -10 or more and 0 or less. Furthermore, the b* value is preferably 0 or more and 60 or less, more preferably 1 or more and 50 or less, and even more preferably 10 or more and 50 or less. These values can be measured using a spectral color difference meter SE600 manufactured by Nippon Denshoku Industries Co., Ltd.

As a result of investigations by the present inventors, it has been found that it is advantageous to allow zinc oxide to contain a trace amount of one or more halogens such as chlorine in the method for producing zinc oxide particles described later in order for the zinc oxide particles of the present invention to have the various physical properties described above. In the present invention, halogen means fluorine, chlorine, bromine and iodine among Group 17 elements. Astatine and tennessine are not included in the halogens referred to in the present invention. In the method for producing zinc oxide particles, it is particularly preferable to use one or more elements selected from the group consisting of chlorine, iodine and bromine as the halogen, since the ability to shield long-wavelength ultraviolet rays is even better. The halogen content in the zinc oxide particles is preferably 0.02% by mass or more and 1.0% by mass or less, more preferably 0.05% by mass or more and 0.8% by mass or less, and even more preferably 0.08% by mass or more and 0.7% by mass or less for each halogen. In particular, the total content of all halogens is preferably within this range. By containing a halogen within this range, the zinc oxide particles of the present invention become even more excellent in long-wavelength ultraviolet shielding ability.

The amount of halogen contained in the zinc oxide particles is measured by an ion chromatograph ICS-1000 manufactured by Dionex Corporation after pretreatment with an automatic sample combustion apparatus AQF-100 manufactured by Mitsubishi Chemical Analytech.

In contrast to the fact that the zinc oxide particles of the present invention preferably contain a halogen such as chlorine, the zinc oxide particles are preferably free of elements that can be trivalent or tetravalent. Examples of trivalent elements that can be trivalent or tetravalent include Al, Cr, Fe, B, In, and Ga. On the other hand, examples of tetravalent elements include Zr, Ti, and Hf. Since these elements are not advantageous in that they contribute to a reduction in visible light transmittance, it is desirable not to include them in the zinc oxide particles. From this point of view, it is preferable that the zinc oxide particles of the present invention do not particularly contain Al, Ga, and In as elements that can be trivalent or tetravalent. The total amount of elements that can be trivalent or tetravalent is preferably less than 0.2 atomic weight %, more preferably less than 0.1 atomic weight %, relative to the zinc atoms in the zinc oxide particles of the present invention. The contents of these elements in the zinc oxide particles of the present invention are measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES) SPS5100 manufactured by Hitachi High-Tech Sunence.

The long-wavelength UV shielding ability of the zinc oxide particles of the present invention is preferably 20% or less, more preferably 15% or less, and even more preferably 10% or less, expressed as a transmittance of UV light with a wavelength of 380 nm in the form of a film. The ultraviolet transmittance is measured, for example, by a UV-visible-near-infrared spectrophotometer V-670 (trade name) manufactured by JASCO Corporation.

In contrast to their high ultraviolet shielding ability, the zinc oxide particles of the present invention are excellent in their ability to transmit infrared rays. The infrared transmittance of the film is preferably 80% or more, more preferably 84% or more, and even more preferably 87% or more, in terms of infrared transmittance at a wavelength of 1500 nm. The infrared transmittance is measured, for example, by a UV-visible-near-infrared spectrophotometer V-670 (trade name) manufactured by JASCO Corporation.

In relation to the above-described ultraviolet transmittance and infrared transmittance, the zinc oxide particles of the present invention preferably show a low haze value of 10% or less in a film state. From this point of view, the zinc oxide particles of the present invention are advantageous as a transparent material with high ultraviolet shielding ability. From the viewpoint of making this advantage more remarkable, the haze is more preferably 7% or less, and even more preferably 5% or less. Haze is measured with a haze meter (NDH-4000, manufactured by Nippon Denshoku Industries Co., Ltd.).

The above-described ultraviolet transmittance, infrared transmittance and haze are measured with the zinc oxide particles of the present invention formed into a film. Specifically, the zinc oxide particles of the present invention, an acrylic resin (LR-167 (trade name) manufactured by Mitsubishi Chemical Corporation, solid content 46% by mass), and methyl ethyl ketone are placed in a plastic container and mixed to prepare a dispersion. The concentration of zinc oxide particles in the dispersion is set at 29% by weight, and the solid content concentration of the dispersion is set at 40% by weight. A paint shaker is used to prepare the dispersion. Dispersion time is 3 hours. The dispersion thus obtained is applied to a polyethylene terephthalate film (thickness: 100 μm, T-60 (trade name) manufactured by Toray Industries, Inc.). A bar coater (#7) is used for coating. The coating amount is 3 g/m ² . The coating film thus obtained is dried at 80° C. in the air. The drying time is 15 minutes. This forms the desired film.

The UV shielding ability and visible light transparency of the zinc oxide particles of the present invention are also related to the size of the particles. From the viewpoint of further increasing the ultraviolet shielding ability and transparency in visible light, the particle diameter of the zinc oxide particles of the present invention is preferably 0.1 μm or more and 10 μm or less, more preferably 0.2 μm or more and 8 μm or less, and still more preferably 0.3 μm or more and 5 μm or less, when represented by the volume cumulative particle diameter _D50 at a cumulative volume of 50% by volume measured by laser diffraction scattering particle size distribution measurement. The particle size can be controlled by adjusting the grinding conditions and the like when zinc oxide particles are produced according to the production method described below.

In relation to the particle size, the ultraviolet shielding ability of the zinc oxide particles of the present invention is also related to the crystallite size of the zinc oxide. The crystallite size of the zinc oxide particles of the present invention is preferably 5 nm or more and 100 nm or less, more preferably 10 nm or more and 80 nm or less, and still more preferably 15 nm or more and 60 nm or less, from the viewpoint of further increasing the ultraviolet shielding ability. The crystallite size can be controlled by adjusting the conditions when zinc oxide particles are produced according to the production method described below, such as calcination conditions.

The crystallite size of zinc oxide is measured by the following method. XRD measurement is performed using an X-ray diffractometer Ultima IV manufactured by Rigaku Corporation. The conditions are X-ray: CuKα, 40 kV, 50 mA, measurement range 20° or more and 100° or less, radiation source: CuKα, scanning axis: 2θ/θ, measurement method: FT, coefficient unit: Counts, step width: 0.01°, coefficient time: 3 seconds, divergence slit: 2/3°, divergence longitudinal limiting slit: 0.8 mm. Subsequently, the measurement data is read using Rigaku's analysis software PDXL, and the crystallite size is calculated by the Scherrer method.

In relation to the crystallite size, the primary particle size of the zinc oxide particles is preferably 5 nm or more and 100 nm or less, more preferably 10 nm or more and 90 nm or less, and even more preferably 20 nm or more and 80 nm or less. A primary particle is an object recognized as a minimum unit as a particle judging from the apparent geometric form when the zinc oxide particle is observed with an electron microscope at a magnification of 50000 times. When the primary particle size of the zinc oxide particles is within this range, it is advantageous from the viewpoint of particle transparency. The primary particle size can be controlled by adjusting the conditions when zinc oxide particles are produced according to the production method described below, such as the firing conditions.

The primary particle size of zinc oxide particles is measured by the following method. Using a transmission electron microscope JEM-2100 manufactured by JEOL Ltd., a transmission electron microscope image of zinc oxide particles magnified 50,000 times is obtained. 30 particles observed in the imaging field are arbitrarily selected, and the Feret diameter is measured. An arithmetic mean value of the Feret diameters is obtained, and the obtained value is defined as the primary particle diameter of the zinc oxide particles.

For the purpose of further enhancing the UV shielding ability, the zinc oxide particles of the present invention may have, on their surface, at least one agent selected from the group consisting of an agent that enhances compatibility with a resin, an agent that has UV shielding properties, and an agent that enhances the dispersibility of the particles in the resin. Agents for enhancing compatibility with resins include, for example, polyoxyethylene-based compounds, silane coupling agents, polymeric unsaturated carboxylic acid-based compounds, cationic group-containing acrylic polymers, and the like. Examples of ultraviolet absorbers include benzotriazole-based compounds, benzophenone-based compounds, triazine-based compounds, cyanoacrylate-based compounds, oxalinide-based compounds, salicylate-based compounds, and formamidine-based compounds. Agents for enhancing dispersibility in resins include, for example, silane coupling agents, silicone oils, fatty acids, fatty acid esters, fatty acid soaps and organic titanate compounds.

Specifically, silane coupling agents include hexyltrimethoxysilane, hexyltriethoxysilane, methyltriethoxysilane, phenyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-acryloxypropyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(β-methoxyethoxy)silane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, and 3-glycidoxypropylmethyl. diethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane and the like.
Examples of silicone oil include dimethyl silicone oil, methylphenyl silicone oil, cyclic silicone oil, polyether silicone oil, modified silicone oil, and methyl hydrogen silicone oil.
Fatty acids include saturated fatty acids such as n-decanoic acid, caprylic acid, myristic acid, rosin acid, lauric acid, stearic acid, behenic acid and palmitic acid, and unsaturated fatty acids such as linoleic acid, linoleic acid and oleic acid.
Fatty acid esters include dextrin fatty acid esters, cholesterol fatty acid esters, sucrose fatty acid esters and starch fatty acid esters.
Fatty acid soaps include aluminum stearate, calcium stearate, aluminum 12-hydroxystearate.
Organic titanate compounds include isopropyl triisostearoyl titanate, isopropyl dimethacrylisostearoyl titanate, isopropyl tri(dodecyl)benzenesulfonyl titanate, neopentyl(diallyl)oxytri(dioctyl)phosphate titanate.
Polyoxyethylene-based compounds include polyoxyethylene stearyl ether phosphate, polyoxyethylene lauryl ether phosphate, polyoxyethylene alkyl ether phosphate, and polyoxyethylene oleyl ether phosphate.
In addition to these, hydroxyphenylbenzotriazole, 2-hydroxy-4-methoxybenzophenone, 2,2'-dihydroxy-4-methoxybenzophenone, 2,2',4,4'-tetrahydroxybenzophenone, 4-t-butylphenyl salicylate, 2,4-dibenzoylresorcinol, and the like can be used.

The zinc oxide particles of the present invention are preferably produced, for example, by the method described below. First, a zinc halide such as zinc chloride (ZnCl2) is prepared as a starting material and dissolved in water to obtain an aqueous solution containing zinc and halogen. It is preferable to use a high-purity zinc halide that does not contain an element that can be trivalent or tetravalent. For example, it is preferable to use one with a purity of 99% by mass or more. From the viewpoint of particle size control, the concentration of zinc in the aqueous solution is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 70% by mass or less, and even more preferably 30% by mass or more and 60% by mass or less.

Instead of preparing the aqueous solution containing zinc and halogen by dissolving zinc halide in water, it may be prepared by dissolving a poorly soluble zinc compound such as metallic zinc, zinc oxide, zinc hydroxide, and zinc carbonate in various hydrohalic acids such as hydrochloric acid, hydrobromic acid, and hydroiodic acid, or by dissolving a water-soluble zinc salt such as zinc sulfate and an alkali metal halide such as sodium chloride in water.

The zinc halide aqueous solution thus prepared is mixed with a basic substance to form a zinc-containing precipitate in the liquid. The basic substance may be mixed with the zinc halide aqueous solution as it is, or may be mixed with the zinc halide aqueous solution in the form of an aqueous solution. In any case, the addition amount of the basic substance is preferably 1.2 times or more and 2.8 times or less of the metal ion neutralization equivalent on a molar basis. At that time, the concentration of the aqueous metal salt solution is preferably 1% by mass or more and 40% by mass or less, and the concentration of the aqueous solution of the basic substance is preferably 5% by mass or more and 50% by mass or less.

The zinc chloride aqueous solution and the basic substance may be mixed at room temperature in an unheated state or in a heated state. When mixing in a heated state, the temperature of the mixed liquid is preferably set to 30° C. or higher and 90° C. or lower, more preferably 35° C. or higher and 80° C. or lower, and more preferably set to 40° C. or higher and 70° C. or lower. By mixing the zinc chloride aqueous solution and the basic substance at a temperature within this range, the aforementioned precipitate can be successfully formed.

The zinc chloride aqueous solution and the basic substance can be mixed, for example, by adding the basic substance to the zinc chloride aqueous solution. In this case, the basic substance may be added all at once or sequentially. When the basic substance is added successively, it is preferable to continuously add the basic substance in the form of an aqueous solution over a predetermined period of time in order to successfully form the precipitate.

Examples of basic substances include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, ammonia, inorganic carbonates and inorganic hydrogencarbonates. These basic substances can be used singly or in combination of two or more. Among these basic substances, it is preferable to use an alkali metal hydroxide from the viewpoint of successfully forming a precipitate, and it is particularly preferable to use sodium hydroxide from an economical point of view.

When a precipitate is formed in the liquid in this manner, the precipitate is filtered and washed with water. The degree of water washing is related to the amount of chlorine contained in the finally obtained zinc oxide particles. From this point of view, the precipitate is washed with water so that the conductivity (25° C.) of the washing water is preferably 1 μS/cm or more and 10000 μS/cm or less, more preferably 2 μS/cm or more and 5000 μS/cm or less, and still more preferably 5 μS/cm or more and 4000 μS/cm or less. By washing with water under these conditions, the amount of chlorine contained in the finally obtained zinc oxide particles can be within the desired range. Moreover, zinc oxide particles with few impurities can be obtained.

After washing the precipitate with water, the precipitate is calcined to obtain the desired zinc oxide particles. Examples of the firing atmosphere include an oxidizing atmosphere such as air, a reducing atmosphere such as a hydrogen-containing atmosphere, and an inert gas atmosphere such as a nitrogen atmosphere. Considering safety and economy, it is preferable to use an oxidizing atmosphere such as air.

In any of the above-described firing atmospheres, the firing temperature is preferably 350° C. or higher and 750° C. or lower, more preferably 400° C. or higher and 700° C. or lower, and even more preferably 450° C. or higher and 650° C. or lower. By firing at a temperature within this range, zinc oxide particles having the desired UV shielding ability can be successfully obtained. From the same point of view, the firing time is preferably from 0.5 hours to 20 hours, more preferably from 0.5 hours to 10 hours, and even more preferably from 1 hour to 5 hours.

In this way, the desired zinc oxide particles are obtained. The zinc oxide particles are subjected to a post-treatment such as pulverization, if necessary, so as to have a desired particle size. There are no particular restrictions on the pulverizing means, and various media mills can be used.

The zinc oxide particles thus obtained can be applied to various fields by making use of their high ultraviolet shielding ability, particularly ultraviolet shielding ability in the long wavelength range. For example, an ultraviolet shielding film can be formed by mixing zinc oxide particles with various binder components made of a polymer compound to prepare an ultraviolet shielding composition, and applying this composition to various substrates. That is, according to the present invention, an ultraviolet shielding composition containing zinc oxide particles is provided. Examples of binder components include acrylic resins, methacrylic resins, polyvinyl butyral resins, polyvinyl alcohol resins, urethane resins, epoxy resins, polyimide resins, polyamide resins, polyamideimide resins, phenol resins, polyacetal resins, polycarbonate resins, amino resins, fluorine resins, silicone resins, styrene resins, unsaturated polyester resins, nylon resins, and the like. Examples of substrates include glass, resin films, textiles, and pottery.

Moreover, it can also be used in the form of a masterbatch comprising a resin composition in which the zinc oxide particles of the present invention are kneaded into a resin. By molding using this masterbatch, it is possible to obtain a molded article having an ultraviolet shielding ability. Examples of masterbatch resins include polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyvinyl chloride, polyacetal, polyphenylene sulfide, polyether ether ketone, ABS, polycarbonate, polyphenylene ether, polyether sulfone, and PMMA.

EXAMPLES The present invention will be described in more detail below with reference to examples. However, the scope of the invention is not limited to such examples. "%" means "% by mass" unless otherwise specified.

[Example 1]
An aqueous solution was obtained by dissolving 600 g of zinc chloride having a purity of 99% in 12 liters of pure water. Separately from this operation, 1400 g of a 25% sodium hydroxide aqueous solution was prepared. An aqueous sodium hydroxide solution was continuously supplied to an aqueous zinc chloride solution using a pump capable of adjusting the flow rate. The feed rate was set at 40 ml/min. During this time, the mixture was heated to 60° C. and the pH was adjusted to 9-10. In addition, the liquid was continuously stirred at high speed during the supply of the aqueous sodium hydroxide solution. This produced a precipitate containing zinc in the liquid.

The liquid in which the precipitate was generated was filtered, and washing with water was repeated until the electric conductivity (25° C.) of the washing water became 400 μs/cm or less. The washed precipitate was then dried at 150° C. and subsequently calcined at 550° C. for 1.5 hours in an air atmosphere. Zinc oxide particles were thus obtained. The zinc oxide particles were ground in a bead mill to the desired particle size.

[Example 2]
In Example 1, the precipitate was washed with water until the conductivity of the washing water (25° C.) was 7 μs/cm or less. Zinc oxide particles were obtained in the same manner as in Example 1 except for this.

[Example 3]
In Example 1, the precipitate was washed with water until the conductivity of the washing water (25° C.) was 3300 μs/cm or less. Zinc oxide particles were obtained in the same manner as in Example 1 except for this.

[Example 4]
The zinc oxide particles obtained in Example 1 were surface-treated with a benzophenone derivative. 2,2′,4,4′-tetrahydroxybenzophenone manufactured by Kanto Kagaku Co., Ltd. was used as a benzophenone derivative. 100 g of benzophenone derivative was dissolved in 2 liters of toluene and stirred for 5 minutes to prepare a treatment liquid. 1 kg of zinc oxide particles obtained in Example 1 was added to this treatment liquid and stirred for an additional 30 minutes. Then, the zinc oxide particles were collected by filtration and dried at 100° C. to obtain surface-treated zinc oxide particles.

[Example 5]
An aqueous solution was obtained by dissolving 500 g of zinc bromide with a purity of 95% in 6 liters of pure water. Separately from this operation, 700 g of a 25% sodium hydroxide aqueous solution was prepared. The sodium hydroxide aqueous solution was continuously supplied to the zinc bromide aqueous solution using a pump capable of adjusting the flow rate. The feed rate was set at 20 ml/min. During this time, the mixture was heated to 60° C. and the pH was adjusted to 9-10. In addition, the liquid was continuously stirred at high speed during the supply of the aqueous sodium hydroxide solution. This produced a precipitate containing zinc in the liquid. Thereafter, zinc oxide particles were obtained in the same manner as in Example 1.

[Example 6]
An aqueous solution was obtained by dissolving 500 g of zinc iodide having a purity of 95% in 5 liters of pure water. Separately from this operation, 400 g of a 25% sodium hydroxide aqueous solution was prepared. An aqueous sodium hydroxide solution was continuously supplied to an aqueous zinc iodide solution using a pump capable of adjusting the flow rate. The feed rate was set at 20 ml/min. During this time, the mixture was heated to 60° C. and the pH was adjusted to 9-10. In addition, the liquid was continuously stirred at high speed during the supply of the aqueous sodium hydroxide solution. This produced a precipitate containing zinc in the liquid. Thereafter, zinc oxide particles were obtained in the same manner as in Example 1.

[Comparative Example 1]
Instead of the zinc chloride used in Example 1, 1200 g of 98% pure zinc sulfate heptahydrate was used. Zinc oxide particles were obtained in the same manner as in Example 1 except for this.

[Comparative Example 2]
Zinc oxide (product name: FINEX-30) obtained from Sakai Chemical Industry Co., Ltd. was used. Zinc oxide particles were obtained in the same manner as in Example 1 except for this.

〔evaluation〕
The zinc oxide particles obtained in Examples and Comparative Examples were subjected to ESR measurement by the method described below to determine the relative spectral absorption intensity and g value. In addition, the bandgap of zinc oxide was measured by the method described below. Further, the halogen (F, Cl, I and Br) content, zinc oxide crystallite size, primary particle size and hue were measured by the methods described above. Furthermore, the ultraviolet transmittance at 380 nm and 370 nm, the infrared transmittance at 1500 nm, and the haze were measured by the methods described above. These results are shown in Table 1 below.

[ESR measurement]
It was measured using an X-band (9 GHz band) electron spin resonance apparatus (JES-X330) manufactured by JEOL Ltd. A measurement sample was placed in a quartz tube having an outer diameter of 5 mm, and measurement was performed under the following measurement conditions to obtain an ESR spectrum. For zinc oxide particles, the absorption intensity of the obtained ESR spectrum was converted to a value per 1 g of the sample.
Sample mass: 20 mg
Quartz tube: No. manufactured by JEOL Ltd. 193-S (for high accuracy)
Microwave output: 0.25mW
Magnetic field sweep width: 20mT
Modulation magnetic field: 0.3mT
Sensitivity: 50
Time constant: 0.03 seconds Measurement time: 30 seconds Measurement temperature: 84K

[Measurement of g value]
It was measured using a standard sample Mn marker (Mn ²⁺ /MgO) attached to the electron spin resonance apparatus described above. The measurement of the Mn marker is performed simultaneously with the measurement of the target sample.

[Calculation of relative absorption intensity]
For the sample to be measured, the value obtained by integrating twice the ESR spectrum in the magnetic field range where the g value becomes 1.94 to 2.02 when the Mn marker was inserted was divided by the mass of the sample to be measured, and this was taken as the absorption intensity I of the sample.
For the standard substance, the value obtained by integrating twice the ESR spectrum in the magnetic field range where the g value becomes 1.94 to 2.10 when the Mn marker was inserted was divided by the mass of the standard substance, and this was taken as the absorption intensity I0 of the sample.
The absorption intensity I of the measurement sample was divided by the absorption intensity I0 of the standard substance, and the resulting value was taken as the relative absorption intensity.

[Measurement of bandgap of zinc oxide]
The reflectance of the zinc oxide particles was measured using an ultraviolet-visible-near-infrared spectrophotometer V-670 (trade name) manufactured by JASCO Corporation. The measured data were analyzed by the software attached to the spectrophotometer (VWBG-773 type bandgap program). In the analysis using this software, first, "direct permissible transition" was selected from among the options for "calculation method" displayed on the display. Next, a graph was prepared by converting the wavelength on the horizontal axis into energy (unit: eV) and converting the vertical axis into (hνα) ² . Energy is calculated by E=hv=hc/λ.
E: energy (unit: eV)
h: Planck's constant ν: frequency of incident light (unit: Hz)
λ: wavelength of incident light (unit: m)
c: speed of light (unit: m/s)
α: extinction coefficient Next, the baseline was determined by selecting “baseline” from the “calculation area” options displayed on the display, and setting the lower limit of the baseline to 2.5 eV and the upper limit to 3.0 eV.
Next, select "Slope Line" from the "Calculation Area" options displayed on the display, and select the energy range for which the slope line is to be calculated. The selection was determined by temporarily selecting an energy range in which the graph rises, and then adjusting the upper and lower limits of the energy range so that the "correlation coefficient" displayed on the display approaches 1 as much as possible. Determining the upper and lower limits yields a slope line.
After the baseline and the slant line are determined, the energy (unit: eV) at the intersection of the baseline and the slant line is displayed as the value of the bandgap on the display.

As is clear from the results shown in Table 1, the coating film formed using the zinc oxide particles obtained in each example has a lower UV transmittance of 380 nm, which is a long-wavelength UV ray, than the comparative example, indicating excellent UV shielding ability. On the other hand, it can be seen that the transmission of infrared rays is approximately the same between the example and the comparative example. The table does not show the content of fluorine as the content of halogen, but the reason for this is that fluorine was not detected.

Claims (11)

At 84 K, the absorption intensity of a 2.0 mmol/L toluene solution of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl obtained by an electron spin resonance method is defined as I0,
At 84 K, when the spectral absorption intensity per 1 g of zinc oxide particles obtained by the electron spin resonance method is I,
Zinc oxide particles having a relative spectral absorption intensity represented by I/I0 of 0.001 or more ;
an ultraviolet absorber present on the surface of the zinc oxide particles;
Coated zinc oxide particles comprising : 2. The coated zinc oxide particles according to claim 1, having a bandgap of 2.60 eV or more and 3.35 eV or less. 3. The coated zinc oxide particles according to claim 1, which contain halogen and the content thereof is 0.015% by mass or more and 1.0% by mass or less. 4. Coated zinc oxide particles according to claim 3, wherein the halogen is chlorine. 4. Coated zinc oxide particles according to claim 3, wherein the halogen is iodine. 4. Coated zinc oxide particles according to claim 3, wherein the halogen is bromine. 7. The coated zinc oxide particles according to any one of claims 1 to 6 , having a crystallite size of 5 nm or more and 100 nm or less. The coated zinc oxide particles according to any one of claims 1 to 7 , having a primary particle size of 5 nm or more and 100 nm or less. 9. The coated zinc oxide particles according to any one of claims 1 to 8 , which do not contain an element that can be trivalent or tetravalent. A UV screening composition comprising coated zinc oxide particles according to any one of claims 1 to 9 . mixing an aqueous solution containing zinc and a halogen with a basic substance to form a precipitate containing zinc;
washing the precipitate with water until the conductivity of the washing water is 2 μS/cm or more and 10000 μS/cm or less;
The precipitate washed with water is calcined at 350° C. or higher and 750° C. or lower in an air atmosphere to obtain zinc oxide particles,
A method for producing coated zinc oxide particles, comprising attaching an ultraviolet absorber to the zinc oxide particles.