Coated particle, ultraviolet shielding agent, cosmetic, paint, and production method of coated particle

Abstract

To coat a ZnO surface with a safe and stable oxide.SOLUTION: A coated particle of the present invention includes ZnO of which at least a part is covered with ZnMOx (M is a metal element capable of constituting a composite metal oxide together with Zn). The coated particle is obtained by hydrothermal treatment of ZnO in the presence of a hydroxide of a metal capable of constituting a composite metal oxide together with Zn. The coated particle is suitable for an ultraviolet shielding agent, a cosmetic containing the same, and a paint having an ultraviolet shielding function, a gas permeation suppression function, or the like.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to coated particles, ultraviolet screening agents, cosmetics, paints, and methods for producing coated particles. [Background technology]

[0002] TiO2 is used as a material for blocking ultraviolet rays. It is also possible to ensure transparency by miniaturizing the particle size. However, there are concerns that the photocatalytic activity of TiO2 may have adverse effects on the skin.

[0003] ZnO is also expected to block ultraviolet rays, but concerns remain about its elution into water and adverse effects on the skin (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-207039 Summary of the Invention [Problem to be solved by the invention]

[0005] If the ZnO surface can be covered with a safe and stable oxide, it is expected that Zn elution can be suppressed. Furthermore, if the surface can be highly organically modified, it is expected that Zn elution from ZnO can be suppressed.

[0006] The present invention has been made in view of these problems, and aims to coat the ZnO surface with a safe and stable oxide. [Means for solving the problem]

[0007] As a result of extensive research, the present inventors have discovered that by subjecting ZnO to hydrothermal treatment in the presence of a hydroxide of a metal capable of forming a composite metal oxide with Zn, at least a portion of ZnO can be converted to ZnMO. xThe present inventors have found that ZnO can be coated with ZnO and that the amount of Zn dissolved in water can be reduced to one-tenth or less of that in the case of untreated ZnO, and have completed the present invention. Specifically, the present invention provides the following.

[0008] The present invention provides a method for producing a composite metal oxide of ZnO, in which at least a part of the ZnO is converted into ZnMO, which comprises a step of hydrothermally treating ZnO in the presence of a hydroxide of a metal capable of forming a composite metal oxide with Zn. x (M is a metal element capable of forming a composite metal oxide together with Zn) is a method for producing coated particles.

[0009] In this method, the pH during the hydrothermal treatment is preferably 8 or more and 11 or less.

[0010] ZnO has such high solubility in water that it dissolves even at room temperature, and dissolution progresses even more in high-temperature hydrothermal fields. Dissolution also progresses more at low pH. Dissolution is suppressed at high pH, ​​but the solubility of hydroxides increases with temperature.

[0011] At high pH conditions, metal elements M to MO, which are different from Zn, x However, under high pH conditions, the raw material M is M(OH)x, i.e., hydroxide. At high temperatures, dissolution also proceeds.

[0012] Under these conditions, ZnO and MO precipitated, and ZnMO x The substance with the lowest solubility among them will precipitate.

[0013] This principle has already been reported by Adshiri et al. in their previous research (YAG synthesis) (JP Patent Publication No. 2008-162864). At that time, YOOH was produced as a by-product at high pH, ​​and AlOOH was produced at low pH. Similarly, by optimizing the pH, the precipitation of Zn and M was suppressed, and ZnMO was obtained. x It is possible to selectively generate

[0014] The solubility of metal oxides in supercritical and subcritical water can be predicted using the improved HKF model (previous research by Adjiri et al.). Tadafumi Adschiri, Yukiya Hakuta, Kiwamu Sue and Kunio Arai, Hydrothermal synthesis of metal oxide nanoparticles at supercritical conditions, Journal of Nanoparticle Research 3: 227-235, 2001. Kiwamu Sue, Yukiya Hakuta, Richard L. Smith, Tadafumi Adschiri, and Kunio Arai, Solubility of Lead (II) Oxide and Copper (II) Oxide in Subcritical and Supercritical Water, Journal of Chemical and Engineering Data, 44, 6, 1422-1426, 1999.

[0015] In this method, the concentration of the raw material ZnO is preferably 0.5 mol / L or more.

[0016] ZnMo x The precipitation of occurs in parallel with the generation of homogeneous nuclei in the liquid phase and precipitation onto particles. However, the former is proportional to the solution space V, and the latter is proportional to the particle surface area S. Therefore, when the particle concentration is high, precipitation onto particles becomes dominant.

[0017] Under conditions where the former predominates (low particle concentration and / or high concentration of precipitation raw material), homogeneous nucleation occurs. Therefore, even if it appears that precipitation has occurred on the ZnO, it is actually the accumulation of fine particles generated by homogeneous nucleation, resulting in a powdery deposition on the ZnO particles. In some cases, a porous body is formed, and in this case, although a coating structure appears to have been formed, ZnO dissolves in water or solution and diffuses into the voids of the porous body, resulting in the risk of leaching.

[0018] Under the latter condition (high particle concentration and low concentration of the precipitation raw material), precipitation on the target ZnO particles occurs.

[0019] In the present method, M is preferably one or more selected from B, Al, Si, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Se, Nb, Mo, Cd, Sn, Sb, Ta, W, Pt, and Bi.

[0020] It is not easy to create a sound shell structure with different materials due to the mismatch in lattice constants. In this case, two-dimensional nucleation occurs on the surface of ZnO, and the islands grow. If the mismatch is large, wettability is poor, and in extreme cases, droplet (spherical) precipitation occurs on the particles, resulting in a product that at first glance appears as a powdery precipitation or aggregated particle state. If the affinity is high, the precipitation occurs as if it is wetting the surface, and large islands grow, eventually forming polycrystals on the particle surface.

[0021] ZnMo x Any M may be used, but it is preferable that the lattice constant of M is close to that of ZnO. It is also preferable that the ionic radius of M does not differ too much from that of Zn.

[0022] In addition, ZnMO x More than MO x In some cases, precipitation of TiO2 or ZrO2 is dominant. For example, TiO2 and ZrO2 have low solubility even under high-temperature, high-pressure hydrothermal conditions. These do not precipitate on ZnO particles, but tend to form homogeneous nuclei. Therefore, unless the raw material concentration is set very low, the precipitation will be powdery, which is undesirable.

[0023] The present method preferably includes a cooling step of cooling the coated particles obtained by the hydrothermal treatment.

[0024] In both batch reactions and flow-through reactors, cooling can cause the precipitation of dissolved hydroxides of Zn and M. Rapid cooling results in a homogeneous phase precipitation of fine nanoparticles, while slow cooling results in a film-like precipitation on the particles. These are generally unstable, highly soluble precipitates, which can be removed relatively easily by washing after product recovery. Considering washability, rapid cooling is preferred because it results in more unstable, finer precipitates.

[0025] The present method preferably includes a step of washing the coated particles obtained by the hydrothermal treatment, and then organically modifying the coated particles with a basic region.

[0026] In general, in supercritical hydrothermal synthesis, the following occurs between metal oxides and organic molecules: M(OH) + RCOOH = MOOR + HO This is a dehydration reaction that occurs on the acid side.

[0027] Therefore, even if an organic modification group is present in the reaction field under the above-mentioned high pH conditions, even if it is on ZnO, the ZnMO x Even if the above is the case, good organic modification cannot be expected.

[0028] Once, ZnMO x After thoroughly washing the core-shell ZnO on the surface, the ZnMO was obtained by organic modification in the neutral to weakly basic range. x Organic modification can then proceed on top of this.

[0029] In this method, the organic modification is preferably carried out using a flow reactor, and the reaction time for the organic modification is preferably 0.5 seconds to 30 minutes.

[0030] The organic functional groups form complexes with Zn and M, and dissolution and reprecipitation through these complexes occurs at high speed. x This leads to the promotion of re-dissolution of ZnO.

[0031] The time constant of the organic modification reaction is on the order of seconds to minutes. On the other hand, the dissolution reaction takes more than minutes. To prevent re-dissolution and promote organic modification, it is desirable to use a flow reactor and perform the modification on the order of seconds to minutes. [Effects of the Invention]

[0032] According to the present invention, a shell structure can be formed on the ZnO surface using a safe and stable oxide. [Brief explanation of the drawings]

[0033] [Figure 1] FIG. 1 shows FT-IR spectra of ZnO used in Test Example 1 and Samples A to D obtained from Example 1 and Comparative Examples 1-1 to 1-3. [Figure 2] FIG. 2 shows XRD patterns of ZnO used in Test Example 1 and Samples A to D obtained from Example 1 and Comparative Examples 1-1 to 1-3. [Figure 3] FIG. 3 shows an image of Sample A obtained in Example 1 observed with a scanning electron microscope (SEM) and the mapping results of energy dispersive X-ray analysis (EDX analysis). [Figure 4] FIG. 4 shows an image of Sample B obtained in Example 1-1 by SEM observation and the mapping results of EDX analysis. [Figure 5] FIG. 5 shows a scanning transmission electron microscope image and mapping results of energy dispersive X-ray analysis of the surface-modified ZnO (sample a) obtained in Example 1. [Figure 6] FIG. 6 shows the diffuse reflectance spectra in the ultraviolet-visible region of ZnO used in Test Example 1 and Samples A to D obtained from Example 1 and Comparative Examples 1-1 to 1-3. DETAILED DESCRIPTION OF THE INVENTION

[0034] Specific embodiments of the present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be implemented with appropriate modifications within the scope of the object of the present invention.

[0035] <Method of manufacturing coated particles> In the coated particles of the present invention, at least a part of the ZnO is ZnMO. x (M is a metal element that can form a complex metal oxide with Zn.) The coated particles are obtained by hydrothermal treatment of ZnO in the presence of a hydroxide of a metal that can form a complex metal oxide with Zn.

[0036] [Hydrothermal treatment process] [Raw materials] The raw materials are ZnO and a metal M that can form a composite metal oxide with Zn.

[0037] (ZnO) The ZnO concentration during the hydrothermal reaction treatment is preferably 0.5 mol / L or more, and more preferably 1 mol / L or more.

[0038] ZnMo x The precipitation of occurs in parallel with the generation of homogeneous nuclei in the liquid phase and precipitation onto particles. However, the former is proportional to the solution space V, and the latter is proportional to the particle surface area S. Therefore, when the particle concentration is high, precipitation onto particles becomes dominant.

[0039] Under conditions where the former is dominant (low particle concentration and / or high raw material concentration), the raw material to be coated undergoes uniform nucleation. As a result, the ZnO particles are precipitated in a powdery state. In some cases, a porous body is formed, and although a coating structure appears to have been formed, there is a risk of leaching from the ZnO.

[0040] Under the latter condition (high particle concentration and low raw material concentration), the desired precipitation occurs on ZnO particles.

[0041] As long as the coating raw material concentration is constant, there is no particular upper limit to the ZnO concentration C. However, care must be taken when precipitating while maintaining a constant coating thickness. Increasing the particle concentration increases the surface area (and the precipitation rate), but the feed M concentration also increases by the amount of the total ZnOx coating, which increases the homogeneous nucleation rate. Under conditions where the precipitation raw material concentration M is relatively low, the increases in precipitation rate and homogeneous nucleation rate compensate for each other, and the voids V decrease with an increase in S, so there is no problem. However, care must be taken when M is high, as homogeneous nucleation may increase rapidly. In this case, to prevent excess ZnO from remaining, the ZnO concentration is preferably 10 mol / L or less, and more preferably 5 mol / L or less.

[0042] (Metal M that can form a complex metal oxide with Zn) Any M may be used, but it is preferable that the lattice constant of M is close to that of ZnO. It is also preferable that the ionic radius of M does not differ too much from that of Zn.

[0043] Specific examples of M include one or more selected from B, Al, Si, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Se, Nb, Mo, Cd, Sn, Sb, Ta, W, Pt, and Bi.

[0044] Creating a sound shell structure with different materials is not easy due to the mismatch in lattice constants. In this case, two-dimensional nucleation occurs on the surface of the ZnO, and the islands grow. If the mismatch is large, wettability is poor, and in extreme cases, droplets (spheres) form on the particles, resulting in a powdery precipitate or aggregate-like state. If the affinity is high, the precipitate wets the surface, resulting in the growth of large islands, and ultimately forming polycrystals on the particle surface.

[0045] In addition, ZnMO x More than MO xIn some cases, precipitation of TiO2 or ZrO2 is dominant. For example, TiO2 and ZrO2 have low solubility even under high-temperature, high-pressure hydrothermal conditions. These do not precipitate on ZnO particles, but tend to form homogeneous nuclei, resulting in a powdery precipitation, which is undesirable.

[0046] [Water-based materials] The water-based material refers to water, a polar organic solvent, or a mixed solvent of water and a polar organic solvent, such as water, alcohols, carboxylic acids, ketones, ethers, esters, amides, amines, sulfur compounds, and mixtures thereof.

[0047] Examples of alcohols include methanol, ethanol, isopropyl alcohol, t-butyl alcohol, propylene glycol, and phenol.

[0048] Examples of the carboxylic acids include lower carboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, and caproic acid.

[0049] Examples of the ketones include acetone, methyl ethyl ketone, and methyl isobutyl ketone.

[0050] Examples of the ethers include ethylene glycol monobutyl ether, ethylene glycol monoethyl ether, tetrahydrofuran, dioxane, and methyl cellosolve.

[0051] Examples of the esters include ethyl acetate and butyl acetate.

[0052] Examples of amides include formamide, dimethylformamide, acetamide, dimethylacetamide, nitromethane, and acetonitrile.

[0053] Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine.

[0054] Examples of sulfur compounds include dimethyl sulfoxide.

[0055] Among these, the aqueous material preferably contains one or more selected from water, alcohols, and carboxylic acids, and more preferably water, because it is easy to handle.

[0056] In addition, to control the reaction field, a pH adjuster or an oxidizing or reducing agent can be added to the aqueous material.

[0057] Examples of pH adjusters include acids such as hydrochloric acid, nitric acid, acetic acid, sulfuric acid, carbonic acid, and ammonium salts thereof, and alkalis such as potassium hydroxide, sodium hydroxide, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, and ammonia.

[0058] Examples of oxidizing and reducing agents include hydrogen peroxide, oxygen, nitric acid, formic acid, hydrazine, hydrogen, ammonia, ethanol, and formaldehyde.

[0059] The raw materials are charged into the hydrothermal reactor by dispersing them in an aqueous material. The raw materials can be charged into the hydrothermal reactor as powders or as a fluid. There are no particular limitations on the fluidity of the material, and examples include an aqueous solution, slurry, paste, or suspension containing the raw material components.

[0060] If it is difficult to prepare a water slurry, the raw material may be dispersed in an aqueous material such as ethanol to form a slurry.

[0061] The pH of the raw material liquid charged into the hydrothermal reaction apparatus is preferably 8 or more and 11 or less.

[0062] ZnO has such high solubility in water that it dissolves even at room temperature, and dissolution progresses even more in high-temperature hydrothermal fields. Dissolution also progresses more at low pH. Dissolution is suppressed at high pH, ​​but the solubility of hydroxides increases with temperature.

[0063] At high pH conditions, metal elements M to MO, which are different from Zn, x However, under high pH conditions, the raw material M is M(OH) x Therefore, under high pH conditions, the reaction site is in the presence of a hydroxide of metal M, which can form a composite metal oxide with Zn. Also, at high temperatures, dissolution proceeds in the same way.

[0064] Under these conditions, ZnO and MO precipitated, and ZnMO x The substance with the lowest solubility among them will precipitate.

[0065] This principle has already been reported by Adshiri et al. in their previous research (YAG synthesis) (JP Patent Publication No. 2008-162864). At that time, YOOH was produced as a by-product at high pH, ​​and AlOOH was produced at low pH. Similarly, by optimizing the pH, the precipitation of Zn and M was suppressed, and ZnMO was obtained. x It is possible to selectively generate

[0066] The solubility of metal oxides in supercritical and subcritical water can be predicted using the improved HKF model (previous research by Adjiri et al.). Tadafumi Adschiri, Yukiya Hakuta, Kiwamu Sue and Kunio Arai, Hydrothermal synthesis of metal oxide nanoparticles at supercritical conditions, Journal of Nanoparticle Research 3: 227-235, 2001. Kiwamu Sue, Yukiya Hakuta, Richard L. Smith, Tadafumi Adschiri, and Kunio Arai, Solubility of Lead (II) Oxide and Copper (II) Oxide in Subcritical and Supercritical Water, Journal of Chemical and Engineering Data, 44, 6, 1422-1426, 1999.

[0067] The timing of bringing the raw material liquid into the presence of a base is not particularly limited, and it is sufficient that the raw material liquid is brought into the presence of a base before the raw material liquid and the aqueous material come into contact in the hydrothermal reaction apparatus. However, in order to simplify the apparatus configuration, it is preferable to supply an acid or a base at the stage of preparing the raw material to bring the raw material liquid into a basic state.

[0068] [Water-based reaction equipment] The hydrothermal reaction apparatus is not particularly limited as long as it is an apparatus that can achieve high temperature and high pressure conditions, and can be selected from apparatuses that are widely known to those skilled in the art in this field. For example, either a batch type apparatus or a flow type apparatus can be used.

[0069] The water used in the reaction of the present invention may be supercritical water (SCW) or subcritical water. Subcritical water includes water in the gas phase or in a state known as water vapor (or steam). Subcritical water also includes water in a state known as subcritical water. In the case of subcritical water, it is preferable that the water is in a liquid state (liquid phase) or contains a liquid phase as the main phase. Under such hydrothermal conditions, it has the ability to form a single phase with relatively heavy hydrocarbons, and near the critical point, the solvent effect (dielectric constant, effects on reaction equilibrium and rate due to the formation of hydration structures) can be significantly controlled by temperature and pressure.

[0070] The "hydrothermal conditions" according to the present invention are defined as conditions in which water coexists at the following reaction temperatures: As described above, the "hydrothermal conditions" herein include conditions in which gas-phase water or water in a state called water vapor (or steam) coexists.

[0071] Although not particularly limited, the lower limit of the reaction temperature is preferably 150°C or higher, more preferably 200°C or higher, even more preferably 250°C or higher, and particularly preferably 300°C or higher.

[0072] The upper limit of the reaction temperature is preferably 1000°C or lower, more preferably 600°C or lower, further preferably 500°C or lower, and particularly preferably 450°C or lower.

[0073] The reaction pressure of the present invention is desirable under subcritical conditions, since a precipitation reaction occurs in the solution if it is above the saturated vapor pressure. A pressure several atmospheres higher than the saturated vapor pressure at that temperature is desirable to prevent gas-liquid two-phase separation. Under supercritical conditions, a similar reaction can be expected at pressures near or above the critical pressure, so the pressure is not particularly limited, but may be 50 MPa or less, or 40 MPa or less. Due to the pressure resistance of the high-pressure vessel, a low pressure is preferable, such as 30 MPa or less, and more preferably 25 MPa or less. However, if the pressure is below the critical pressure of 22.1 MPa, the density drops sharply, resulting in a gas phase, which causes high-speed precipitation from the gas phase and makes it difficult to form a uniform film. A pressure of 5 MPa or more is desirable.

[0074] Furthermore, the reaction time of the present invention is not particularly limited, but may be, for example, 1 minute to 48 hours, more generally 5 minutes to 24 hours, and more typically 10 minutes to 12 hours.

[0075] The type of heating device for heating the water-based material is not particularly limited. Examples of the heating device include a heating device that irradiates the water-based material with microwaves, and a heating device that heats the water-based material by heat conduction from a heating element such as a heater.

[0076] High-temperature steam may also be available. This steam can be used in a heat exchanger or in combination with the heating device described above to produce pressurized water. If clean, impurity-free heated steam at a temperature higher than the processing temperature is available, it can be introduced directly. By controlling the pressure with a pressure control valve, the steam can be converted into a liquid phase and used as heated water for extraction.

[0077] [Cooling process] Although not essential, it is preferable to include a cooling step of cooling the coated particles obtained by the hydrothermal treatment after the hydrothermal treatment step.

[0078] In both batch reactions and flow-through reactors, cooling can cause the precipitation of dissolved hydroxides of Zn and M. Rapid cooling results in a homogeneous phase precipitation of fine nanoparticles, while slow cooling results in a film-like precipitation on the particles. These are generally unstable, highly soluble precipitates, which can be removed relatively easily by washing after product recovery. Considering washability, rapid cooling is preferred because it results in more unstable, finer precipitates.

[0079] Cooling is achieved by mixing a high-temperature, high-pressure fluid supplied from the hydrothermal reactor with a low-temperature, high-pressure aqueous material pressurized by a pressure pump or the like. By mixing the high-temperature, high-pressure fluid with the low-temperature, high-pressure fluid, the heat associated with the change in state of the fluid, i.e., the heat equivalent to the latent heat of vaporization, can be quickly removed, enabling safe and stable operation. Furthermore, if the high-temperature, high-pressure fluid is cooled below its critical temperature by this mixing, the high-temperature, high-pressure fluid is rapidly cooled, and the reaction that produces particles can be stopped almost instantly. Therefore, the product particles can be made to have a nearly uniform particle size.

[0080] Such a cooling method is sufficient when operating at low temperatures, but when operating at relatively high temperatures, heat recovery may be required from the perspective of energy utilization. In such cases, a cooling pipe is installed to perform indirect cooling, i.e., heat exchange, and the recovered heat is used to preheat the raw materials or circulating water.

[0081] Regarding particle recovery, if the powder is placed in a container, the particles are recovered as is. If the particles are recovered as a slurry, the fluid containing the product particles is passed through a filter to separate the product particles from the fluid. The type of filter is not particularly limited, but an in-line filter is an example.

[0082] [Organic modification process] Although not essential, the method preferably includes a step of washing the coated particles obtained by the hydrothermal treatment and then organically modifying the coated particles with a basic region.

[0083] [Organic modifiers] The organic modifier is not particularly limited as long as it can strongly bond hydrocarbons to the surface of the fine particles, and can be selected from organic substances widely known in fields where fine particle applications are expected, including organic chemistry, inorganic materials, and polymer chemistry. Examples of the organic modifier include those that allow the formation of strong bonds such as ether bonds, ester bonds, bonds via N atoms, bonds via S atoms, metal-C- bonds, metal-C= bonds, and metal-(C=O)- bonds. The number of carbon atoms in the hydrocarbon is not particularly limited, but it is preferably 18 or less.

[0084] In particular, taking into consideration that the aromatic group is used as a functional group and that the number of linked hydrocarbons is about 2, it is more preferable that the number of carbon atoms constituting the hydrophobic group is 9 or less.

[0085] The organic modifier may be linear, branched, or cyclic. The organic modifier may be substituted or unsubstituted. The substituent may be selected from functional groups widely known in the fields of organic chemistry, inorganic materials, polymer chemistry, etc. One or more substituents may be present, and when there are multiple substituents, they may be the same or different.

[0086] Examples of organic modifying agents include alcohols, aldehydes, ketones, carboxylic acids, esters, amines, thiols, amides, oximes, phosgene, enamines, amino acids, peptides, and sugars.

[0087] Representative modifying agents include, for example, pentanol, pentanal, pentanoic acid, pentanamide, pentanethiol, hexanol, hexanal, hexanoic acid, hexanamide, hexanethiol, heptanol, heptanal, heptanoic acid, heptanamide, heptanethiol, octanol, octanal, octanoic acid, octanamide, octanethiol, decanol, decanal, decanoic acid, decanamide, and decanethiol.

[0088] Examples of the hydrocarbon group include an optionally substituted linear or branched alkyl group, an optionally substituted cyclic alkyl group, an optionally substituted aryl group, an optionally substituted aralkyl group, an optionally substituted saturated or unsaturated heterocyclic group, etc. Examples of the substituent include a carboxy group, a cyano group, a nitro group, a halogen atom, an ester group, an amide group, a ketone group, a formyl group, an ether group, a hydroxyl group, an amino group, a sulfonyl group, -O-, -NH-, -S-, etc.

[0089] [Hydrothermal organic modification] The conditions for hydrothermal organic modification can basically be the same as those for the hydrothermal treatment step described above, but the optimum pH for the hydrothermal treatment / coating reaction and the optimum pH for organic modification generally differ.

[0090] In general, in supercritical hydrothermal synthesis, the following occurs between metal oxides and organic molecules: M(OH) + RCOOH = MOOR + HO Generally, dehydration reactions tend to occur on the acid side. Generally, the surface of a metal oxide has a zeta potential in solution. At low pH, it is positively charged, and at high pH, ​​it is negatively charged. The pH at which it is neutral is called the isoelectric point. In the case of ZnO, the isoelectric point is high, at 9.3 to 10.3, and below this pH it is positively charged.

[0091] On the other hand, the dissociation constant pKa of COOH is around 4. If it is above this pKa, COOH will dissociate. - In other words, if the pH is between 4 and 10, electrostatic interactions make it easy for organic molecules to bond on the ZnO solid surface.

[0092] Considering that ZnO and the like dissolve on the acid side, it is preferable to carry out the dehydration reaction (organic modification) on the base side.

[0093] Once, ZnMO x After thoroughly washing the core-shell ZnO on the surface, organic modification in the neutral to basic range was performed to form ZnO and ZnMO. x Organic modification can then proceed on top of this.

[0094] The lower limit of the pH is preferably 6 or higher, more preferably 7 or higher, and even more preferably 8 or higher.

[0095] The upper limit of the pH is preferably 13 or less, more preferably 12 or less, and even more preferably 10 or less.

[0096] The organic modification is preferably carried out using a flow reactor. In this case, the lower limit of the reaction time for the organic modification is preferably 0.5 seconds or more, more preferably 1 second or more, and even more preferably 3 seconds or more. On the other hand, the upper limit of the reaction time is preferably 30 minutes or less, more preferably 20 minutes or less, and even more preferably 15 minutes or less.

[0097] The organic functional groups form complexes with Zn and M, and dissolution and reprecipitation through these complexes occurs at high speed.x This leads to the promotion of re-dissolution of ZnO.

[0098] The time constant of the organic modification reaction is on the order of seconds to minutes. On the other hand, the dissolution reaction takes more than minutes. To prevent re-dissolution and promote organic modification, it is desirable to use a flow reactor and perform the modification on the order of seconds to minutes.

[0099] <Coated particles> The coated particles obtained as a product by this method have a structure in which at least a part of the ZnO is ZnMO. x (M is a metal element capable of forming a composite metal oxide together with Zn). The coated particles are preferably surface-modified with an organic compound.

[0100] The coated particles obtained as a product can be placed in 40 mL of water at 25°C and ultrasonically dispersed for 10 minutes. The amount of Zn contained in the water can be reduced to less than one-tenth of that in the case of untreated ZnO.

[0101] The coated particles can be used in ultraviolet screening agents and cosmetics containing the same, as well as in paints and the like having ultraviolet screening and gas permeation inhibiting functions.

[0102] The average particle size of the coated particles is not particularly limited as long as it is within a range preferred for use as an ultraviolet screening agent. In consideration of workability, the average particle size is preferably 10 nm or more, more preferably 100 nm or more, and even more preferably 500 nm or more. In consideration of the ultraviolet screening effect, the average particle size is preferably 20 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less. [Example]

[0103] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples. There is no.

[0104] <Test Example 1> [Sample preparation] [Example 1] (Fabrication of ZnO-ZnAl2O4 core-shell bodies under basic conditions) The experiment was conducted using a batch-type test machine (device name: TAS-H02HC type reactor, manufactured by Taiatsu Glass Industry Co., Ltd.). 100 ml of 1.5 mol / L NaOH aqueous solution and 18.6 g of Al(NO3)3·9H2O were placed in a 200 ml Inconel container and mixed well. 10 g of ZnO was then added and ultrasonically dispersed to obtain a raw material slurry with a pH of 10. This was heated in the batch-type test machine at 300°C for 10 minutes (pressure: 10 MPa). After cooling, the mixture was thoroughly washed with water to obtain a ZnO-ZnAl2O4 core-shell body (sample a).

[0105] (Preparation of oleic acid-modified ZnO-ZnAl2O4 core-shell bodies) Five grams of ZnO-ZnAl2O4 core-shell material (sample a) was placed in a 200 ml Inconel container together with 10 g of sodium oleate, and 100 ml of H2O was added and mixed well. The mixture was then heated at 340 °C for 10 minutes in a batch tester (pressure: 15 MPa). The pH was 12. After cooling, the mixture was thoroughly washed with water and ethanol to obtain an oleic acid-modified ZnO-ZnAl2O4 core-shell material (sample A).

[0106] [Comparative Example 1-1] (Fabrication of ZnO-ZnAl2O4 core-shell bodies under neutral conditions) 100 ml of H2O and 3.9 g of Al(OH)3 were placed in a 200 ml Inconel container and mixed thoroughly. 10 g of ZnO was then added and ultrasonically dispersed to obtain a raw material slurry with a pH of 7.5. This was heated at 300 °C for 10 minutes in a batch tester (pressure: 10 MPa). After cooling, the mixture was thoroughly washed with water to obtain a ZnO-ZnAl2O4 core-shell body (sample b).

[0107] (Preparation of oleic acid-modified ZnO-ZnAl2O4 core-shell bodies) An oleic acid modified product (sample B) was recovered in the same manner as in Example 1, except that sample b was used.

[0108] [Comparative Example 1-2] (Fabrication of ZnO-ZnAl2O4 core-shell bodies under strongly basic conditions) A ZnO-ZnAl2O4 core-shell body (sample c) was obtained in the same manner as in Example 1, except that a slurry of pH 12 was prepared using a 1.875 mol / L NaOH aqueous solution.

[0109] (Preparation of oleic acid-modified ZnO-ZnAl2O4 core-shell bodies) An oleic acid modified product (sample C) was recovered in the same manner as in Example 1, except that sample c was used.

[0110] [Comparative Example 1-3] (Production of oleic acid-modified ZnO) An oleic acid modified product (sample D) was recovered in the same manner as in Example 1, except that the ZnO surface was not covered with ZnAl2O4 and was used as is.

[0111] 〔evaluation〕 1 shows the FT-IR spectra of the ZnO used and samples A to D obtained from Example 1 and Comparative Examples 1-1 to 1-3. -1 The peak due to CH stretching around 700 cm suggests that oleic acid is modified. -1 The peak due to Al-O stretching appears around 3400 cm, suggesting that the surface is treated with Al. However, the peak is small, especially in sample C, and the peak at 3400 cm is small. -1 The strong peak due to OH stretching in the vicinity suggests the presence of hydroxyl groups on the surface, i.e., the surface is not well organically modified.

[0112] Figure 2 shows the X-ray diffraction patterns (XRD patterns) of ZnO and samples A to C. Samples A to C all show peaks derived from ZnAl2O4 at the positions indicated by the arrows, suggesting the formation of ZnAl2O4 on the ZnO surface. However, the peak in sample C is relatively small, suggesting that there is little ZnAl2O4.

[0113] Figure 3 shows an image of sample A observed with a scanning electron microscope (SEM) and the mapping results of energy dispersive X-ray analysis (EDX analysis). The SEM image confirmed that there was no significant change in the shape of the sample before and after treatment. The EDX analysis confirmed that Al was present throughout the sample, suggesting that it was on the surface of the ZnO.

[0114] Figure 4 shows an image of sample B taken by SEM observation and the mapping results of EDX analysis. The SEM image confirmed the presence of an octahedral structure. Furthermore, EDX analysis confirmed that a large amount of Al was contained in the octahedral structure, and that ZnAl2O4, which has an octahedral structure, had precipitated as a single substance.

[0115] Figure 5 shows an image of sample a taken with a scanning transmission electron microscope (STEM) and the mapping results of EDX analysis. The presence of Al on the ZnO surface can be confirmed, but it does not completely cover the surface, with the coverage rate estimated from microscope observation being around 60%.

[0116] In general, when creating a core-shell structure, it is easier to create a uniform shell by using an aluminate with a lattice constant close to that of ZnO.

[0117] However, if precipitation does occur on the surface and the islands become aluminate, subsequent aluminate deposition tends to occur in locations with higher affinity, i.e., on or near the precipitated aluminate. As a result, it is not easy to find the optimal conditions for creating a uniform shell structure on ZnO, and further deposition in an attempt to achieve a complete coating inevitably results in a thicker shell.

[0118] In addition, precipitation of impurities such as ZnO and AlOOH may occur.

[0119] However, by setting conditions that facilitate repeated dissolution and deposition, the above-mentioned heterogeneous precipitation can be suppressed. Whether aluminate precipitates on ZnO or aluminate is determined by equilibrium affinity, but the difference can be reduced by setting a high deposition rate. Adschiri et al. reported that the coexistence of organic modifiers virtually increases solubility, resulting in high dissolution and deposition rates (Tatsuya Fujii, Shin-ichiro Kawasaki, and Tadafumi Adschiri, Kinetic study of octanoic acid enhanced crystal growth of boehmite under sub- and supercritical hydrothermal conditions, The Journal of Supercritical Fluids, 118, 148-152, 2016). This facilitates the formation of a thin shell layer that cannot be observed by SEM, and in particular, organic modification can sometimes exert a dissolution-suppressing effect. This suggests that significant suppression of dissolution can be achieved, even if the surface does not appear to be completely coated at the SEM level.

[0120] Figure 6 shows the diffuse reflectance spectra in the ultraviolet-visible region of samples A to D and the original ZnO. Compared to the original ZnO, there is no particular change in absorption in the UV region below 400 nm, especially in the UVA region from 320 to 400 nm, for samples A to D, confirming that the UV-blocking ability is maintained even after surface modification.

[0121] Table 1 shows the results of the water elution test. [Table 1]

[0122] The water elution test was carried out by placing 200 mg of each of Samples A to D and the original ZnO in 40 ml of pure water, ultrasonically dispersing for 10 minutes, centrifuging the dispersion, and then aliquoting 10 ml of the resulting supernatant and filtering the filtrate, which was then diluted two-fold and measuring the zinc concentration in the solution by atomic absorption spectrometry (detection wavelength: 213.8 nm).The zinc oxide obtained according to the present invention showed approximately one-tenth the amount of Zn elution compared to the comparative sample, demonstrating its excellent water elution properties.

[0123] <Test Example 2> [Example 2-1] Oleic acid modified ZnO-ZnAl2O4 core-shell structure with increased Al content The oleic acid modified product (sample E) was recovered in the same manner as in Example 1, except that the amount of Al(NO3)3·9H2O added was increased to 37.2 g and the pH of the slurry was adjusted to 10 by adding NaOH.

[0124] Evaluation by STEM observation showed that the Al coverage of the ZnO surface had improved to approximately 90%. Furthermore, elution tests showed that Zn elution was suppressed to approximately one-twentieth of that of the comparative sample, demonstrating superior water elution properties.

[0125] [Example 2-2] Oleic acid-modified ZnO-ZnAl2O4 core-shell structure with increased amount of organic modification An oleic acid modified product (sample F) was recovered in the same manner as in Example 1, except that the amount of sodium oleate added was changed to 20 g.

[0126] Evaluation by FT-IR spectroscopy and thermogravimetric analysis showed that the amount of organic modification on the ZnO-ZnAl2O4 surface was improved to approximately 1.5 times that of Example 1. Furthermore, the results of the elution test showed that the elution of Zn was suppressed to about one-twentieth of that of the sample in the comparative example, demonstrating superior water elution properties.

Claims

1. At least a part of ZnO is ZnMO ｘ (M is a metal element capable of forming a composite metal oxide together with Zn), Coated particles, in which 200 mg of the coated particles are placed in 40 mL of water at a temperature of 25°C and ultrasonically dispersed for 10 minutes, and the amount of Zn contained in the water is one-tenth or less of that in the case of untreated ZnO.

2. 2. The coated particle according to claim 1, wherein M is one or more selected from B, Al, Si, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Se, Nb, Mo, Cd, Sn, Sb, Ta, W, Pt, and Bi.

3. The coated particle according to claim 1 , wherein the surface is organically modified.

4. An ultraviolet screening agent comprising the coated particles according to any one of claims 1 to 3.

5. A cosmetic comprising the ultraviolet screening agent according to claim 4.

6. A paint containing the coated particles according to any one of claims 1 to 3.

7. The method for producing coated particles according to claim 1 , comprising a step of hydrothermally treating ZnO in the presence of a hydroxide of a metal capable of forming a composite metal oxide with Zn.

8. The method according to claim 6, wherein the pH during the hydrothermal treatment is 8 or more and 11 or less.

9. The method according to claim 6, wherein the concentration of the raw material ZnO is 0.5 mol / L or more.

10. The method according to claim 6, further comprising a cooling step of cooling the coated particles obtained by the hydrothermal treatment.

11. The method according to claim 6, further comprising the step of washing the coated particles obtained by the hydrothermal treatment and then organically modifying the coated particles with a basic region.

12. The method according to claim 10, wherein the organic modification is carried out using a flow reactor, and the reaction time of the organic modification is from 0.5 seconds to 30 minutes.

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