JP7537735B2 - Dispersion of inorganic fine particles, its manufacturing method, and decorative article using the same - Google Patents

Description

(1) Published in the preliminary paper of the Nano/Particle Forum at KISTEC Innovation Hub 2019 in EBINA on the Kanagawa Institute of Industrial Science and Technology website on October 4, 2019, and presented at the same forum on October 31, 2019. (2) Exhibited at the Industrial Exchange Exhibition 2019 hosted by the Tokyo Metropolitan Government and others from November 13 to 15, 2019. (3) Published in KISTEC NEWS Vol. 1, published by the Kanagawa Institute of Industrial Science and Technology on December 24, 2019. (4) Exhibited at Technical Show Yokohama 2020, sponsored by the Kanagawa Industrial Promotion Center and others, from February 5 to 7, 2020. (5) Published in the 2020 Annual Meeting Proceedings posted on the Ceramic Society of Japan website on March 2, 2020, and in the Proceedings of the same meeting published on March 2, 2020. (6) Published in the Journal of Asian Ceramic Society Vol. posted on the Taylor & Francis website on May 31, 2020. (7) Published in the presentation materials for the New Manufacturing Technology Briefing on the website of the Japan Science and Technology Agency on June 2, 2020 (8) Published in the press release materials on the website of the Kanagawa Prefectural Institute of Industrial Science and Technology on June 8, 2020, and announced at the Kanagawa Prefectural Press Club on June 8, 2020 (9) Published in Convertech 2020 Vol. 568 No. 48, published by the Processing Technology Research Association on July 15, 2020

The present invention relates to a dispersion of inorganic fine particles used for structural color decoration, a method for producing the same, and a decorated product exhibiting structural color using the dispersion. More specifically, the present invention relates to a dispersion in which inorganic fine particles are dispersed in a liquid phase containing unreacted alkoxide components, which, when applied to any substrate, forms a periodic structure in which the inorganic fine particles are regularly arranged, and can uniformly express the high brightness coloring and angle-dependent coloring characteristic of structural colors over any wide area.

The present invention also relates to a manufacturing method for the dispersion, which requires no capital investment, has low raw material costs, and places little strain on the environment, and can be obtained by simply stirring general-purpose raw materials at room temperature. The present invention also relates to a decorative article, which is made using the dispersion and has a structural color that is beautifully expressed continuously in any desired area.

Light is a type of electromagnetic wave, and the wavelength of light that is visible to humans is said to be approximately 380 to 780 nm. It is known that when a periodic structure is formed on the order of several hundred nanometers, which is the same scale as the wavelength of visible light, the phases of light at specific wavelengths in the visible range overlap and are reflected in a reinforced way due to the reflection and refraction of light by the periodic structure, resulting in the appearance of chromatic colors. Colors that appear due to such microstructures are called structural colors or play of colors, and can be seen in natural gemstones such as opals, and in living things such as morpho butterflies and peacocks.

It is also possible to artificially obtain materials that exhibit structural colors. For example, if spherical particles with diameters of several hundred nanometers are created and then densely packed and arranged in a regular pattern, the particle aggregate will form a periodic structure and exhibit structural colors. Such particle aggregates have a structure and color similar to that of the natural gemstone opal, and are therefore called artificial opals, including those made from materials other than silicon oxide, the main component of opal. They can produce high-brightness colors due to multiple reflections of light and angle-dependent colors that change color depending on the viewing angle, resulting in beautiful gemstone-like colors.

In the research and development of artificial opals, polymer particles such as latex, which have a uniform particle size and are easy to form periodic structures with, are often created and used, but polymer particles, which are organic, have drawbacks such as low heat resistance, weak mechanical strength, susceptibility to wear, and deterioration over time. For applications where these drawbacks have a negative effect on products based on inorganic materials such as pottery, ceramics, and metals, it is appropriate to use artificial opals made from inorganic particles such as silicon oxide, which have excellent heat resistance, mechanical strength, wear resistance, light resistance, weather resistance, and chemical stability.

For example, bone china is known in which artificial opal particles (bulk body) made of pre-prepared silicon oxide particles are dispersed in a glaze layer, i.e., a glass layer, and attached to the base (Patent Document 1). By using this technology, it is possible to fix gem-like artificial opals to the base by making the surface glass layer transparent, resulting in beautiful ceramics that show the structural color unique to artificial opals.

Comparing the coloring mechanisms of conventional pigments and dyes with those of artificial opals and other structural colors, pigments and dyes absorb visible light of specific wavelengths and reflect other light, resulting in their color. On the other hand, the structural colors of artificial opals and other materials are not produced by light absorption, but by reflecting visible light of specific wavelengths in such a way that the phases of the light overlap and reinforce each other. When the periodic surfaces that produce structural colors are stacked, the number of light beams whose phases overlap increases, resulting in a bright and brilliant color.

On the other hand, if the number of layers is too high, the contribution of diffuse reflection increases, resulting in a whitish appearance. For this reason, it is necessary to adjust the coating thickness so that the number of layers is not excessive; however, when using a substrate with high brightness, such as paper or ceramics, the light that reaches the substrate is strongly reflected by the substrate and overlaps with the reflected light from the structural color. In other words, the problem with structural color is that when a substrate with high brightness is used, it is difficult to create a contrast between the structural color and the background color of the substrate, making it difficult to see the structural color.

As a means of solving the above problems, it has been reported that vivid structural colors can be produced by mixing black particles with spherical artificial opal inorganic microparticles (Non-Patent Document 1). This document shows that when 0.2 to 1.7 wt% of carbon black particles were added to and mixed with two types of silicon oxide particles with average particle sizes of 280 nm and 360 nm, respectively, vivid green and red colors were produced.

JP 2011-73948 A

Cosmetology Research Report, KOSÉ Cosmetology Research Foundation, 2015, vol.23, p48-54.

However, although artificial opal, an inorganic material, exhibits beautiful structural colors similar to those of the natural gemstone opal and has excellent properties such as heat resistance, mechanical strength, and abrasion resistance, it cannot necessarily be said that it has been actively used in fields other than jewelry. The reason for this is that it is difficult to control the particle size and microstructure of inorganic particles. In order to produce structural colors, it is necessary to produce fine particles on the order of several hundred nanometers with uniform particle sizes and arrange them in a highly regular pattern to form a periodic structure.

For polymer particles such as latex produced by emulsion polymerization, it is easy to control both the nucleation and particle growth, and there is established technology to make the particle size uniform, even for fine particles. On the other hand, for inorganic materials, it is not easy, even for silicon oxide fine particles, for which technology to control the particle size is relatively well established. Even with the liquid phase method commonly used to synthesize inorganic fine particles, it is not easy to create a dispersion of inorganic fine particles with a uniform particle size in the size range of the order of several hundred nanometers. In particular, if the raw material concentration is increased in an attempt to produce particles with a large particle size, gelation occurs easily and a dispersion cannot be obtained, so it is even more difficult to create a dispersion of inorganic fine particles with a uniform particle size of 350 nm to 400 nm or more and use it to express structural color.

In addition, in order to use a dispersion of inorganic fine particles with uniform particle size that can produce structural color for decoration, it is necessary to apply the dispersion and form a periodic structure through the regular arrangement of the particles as the solvent evaporates, thereby producing structural color, and to fix the particles to the applied surface so that they do not peel off due to abrasion, etc.

The bone china of Patent Document 1 uses artificial opal particles (bulk bodies), and since the artificial opals are scattered, it is not possible to form a continuous colored area over a wide area. The document states that the preferable diameter of the bulk bodies is up to 0.5 mm, and the maximum area of each scattered area is less than 0.2 ^mm2 . In addition, there is a problem in that the expression techniques using structural colors are extremely limited, such as not being able to express gradations of structural colors and not being able to express different structural colors by overlapping them in the same area. Furthermore, there is a problem in that a large amount of glaze and high-temperature firing are required to distribute and hold the thick bulk bodies in the glaze layer, which results in high manufacturing costs and environmental load.

The inventors have been researching a technology for continuously forming a periodic structure in which inorganic particles are regularly arranged over a wide area in a drying time of several tens of seconds by preparing a dispersion liquid containing roughly spherical inorganic particles and a component that will become a glass phase for bonding the particles, and directly applying this to an inorganic substrate. Using this technology, inorganic particles on the order of several hundred nanometers can be fixed to a substrate to form a multilayer periodic structure, and the structural color that is unique to artificial opals and has excellent aesthetics can be continuously expressed over a wide area. It is also possible to express gradations of structural colors and apply multiple layers of color, making it possible to obtain inorganic products with excellent aesthetics that are not available with existing publicly known technologies.

In addition, to address the issue of structural colors becoming difficult to see when a highly bright base material is used, adding and mixing other substances such as black particles with artificial opal inorganic microparticles impairs the high brightness and angle-dependent coloring that are characteristic of structural colors. The reason for this is that the added substances disrupt the regular arrangement of the inorganic microparticles, and particles that are only attached to each other by weak forces such as van der Waals forces are rearranged by external physical forces, etc., preventing a highly regular arrangement from being achieved and maintained. This issue is not limited to black particles added to darken the background color, but also occurs when other substances are added.

When such additives disrupt the particle arrangement, only a partial regular arrangement is formed. In this case, a single color is visible that corresponds to the periodic distance √6R/3 (R is the diameter of the sphere) that is most likely to be formed in the close-packed structure of spheres. In Non-Patent Document 1, such a partial regular arrangement structure is expressed as an "amorphous aggregate" and it is clearly stated that the color is not angle-dependent. In order to enable the high brightness coloring and angle-dependent coloring that are unique to structural colors even when additives are added, it is necessary to prepare fine particles of the order of several hundred nanometers with uniform particle size, arrange them in a highly regular manner to form a periodic structure, fix the particles by means of bonding the particles together, etc., and maintain the periodic structure so that the particles do not rearrange.

In addition, when inorganic fine particles with large particle sizes are produced and packed closely together, a periodic structure is formed in which the periodic distance is an integer multiple of the wavelength of light, making it possible to produce a wider variety of colors than periodic structures made of small particle sizes. However, when attempting to produce inorganic fine particles with large particle sizes, the particles tend to aggregate and gel during synthesis. In the large particle size range, it is difficult to obtain a dispersion with particle sizes uniform enough to produce structural colors, and more stringent manufacturing conditions are required.

The present invention has been made in consideration of the above-mentioned background art and the problems associated with it, and aims to provide a dispersion liquid that realizes a highly ordered structure and maintains the ordered structure through interparticle bonding in inorganic fine particles that have excellent properties such as heat resistance, mechanical strength, abrasion resistance, light resistance, weather resistance, and chemical stability, and that can continuously express the high brightness coloring and angle-dependent coloring characteristic of structural colors over a wide area.

Another objective is to provide a dispersion liquid that allows for a variety of colors to be produced from a regular array structure of large-diameter particles, and that allows for high-brightness coloring and angle-dependent coloring that are characteristic of structural colors, even when applied to a highly luminous substrate.A further objective is to provide a low-cost, safe, and environmentally friendly manufacturing method that can produce these dispersion liquids using only general-purpose raw materials and small-scale equipment.Another objective is to provide a decorative product made using these dispersion liquids that continuously expresses aesthetically pleasing structural colors over any wide area.

The inventors conducted extensive research to solve the above problems and discovered that it is optimal to bond roughly spherical inorganic fine particles with a component in which part of the raw material, the alkoxide, is unreacted and exists in a liquid state. They also discovered that optimizing the synthesis conditions makes it possible to obtain a monodisperse dispersion without gelling, even in the large particle size range of 375 to 550 nm in number average diameter, and thus realize a variety of colors that change color depending on the angle of light.

Furthermore, we have discovered that even when dark color particles are added and a bright base material is used, it is possible to achieve high brightness and angle-dependent coloring that are characteristic of structural color, and that when plate-shaped dark color particles are used, it is possible to express the characteristics of structural coloring more prominently. As a result of further research into the structure and manufacturing method, we have completed this invention.

That is, the present invention is a dispersion liquid containing approximately spherical inorganic fine particles and unreacted components in which a portion of the alkoxide, which is the raw material of the inorganic fine particles, is unreacted and exists in a liquid state, and the dispersion liquid is characterized in that the aggregates formed by the inorganic fine particles being arranged in an approximately regular pattern exhibit structural colors due to the number average diameter of the primary particles of the inorganic fine particles, the particle size distribution, and the content ratio of the inorganic fine particles.

The dispersion of the present invention is composed of roughly spherical inorganic fine particles and an alkoxide component, and by applying this to a substrate, it is possible to obtain excellent heat resistance, mechanical strength, hardness, abrasion resistance, chemical stability, corrosion resistance, light resistance, and weather resistance, and to achieve high brightness and angle-dependent coloring characteristic of structural colors, which are derived from a periodic structure consisting of a regular arrangement of particles across multiple layers. The alkoxide component is interposed between the inorganic fine particles to bond the particles, making it possible to maintain the regular arrangement structure of the particles, and further improving adhesion to the substrate and making them less likely to peel off.

When the coating is heated to a temperature of 200°C or higher, the alkoxide component forms a metal-oxygen bond, and when applied to an inorganic substrate, it can form a strong chemical bond even at the interface between the substrate and the coating. It is also possible to achieve a glossy, or lustrous, color by adjusting the content of the alkoxide component to sufficiently fill the gaps between the inorganic particles until the surface of the coating is smooth.

The structural color in the present invention is produced due to the periodic structure formed by inorganic fine particles that are stacked and regularly arranged on the substrate. The periodic distance of the arranged particles is not affected by slight unevenness of the substrate or the coating thickness. Therefore, unlike the interference colors of thin films that have been used traditionally, the intended color can be produced uniformly over a wide area. In addition, because it is a structural color, it is not necessary to contain heavy metals used in conventional inorganic pigments as light absorbing elements, and materials can be selected without being restricted by chemical composition. For example, colors can be produced using silicon oxide, which is the most abundant solid material in the earth's crust, and it is possible to select materials that are abundant in resources, have low material costs, are less harmful to the human body, and have a low environmental impact.

The present invention also relates to the dispersion liquid in which the number-average diameter of the primary particles of the inorganic fine particles is 375 to 550 nm. In a periodic structure consisting of a regular arrangement of inorganic fine particles with large particle sizes, periodic surfaces corresponding to an integer multiple of the wavelength of visible light are formed, and therefore, compared to a periodic structure consisting of inorganic fine particles with small particle sizes, more periodic surfaces are formed, making it possible to produce a wide variety of colors.

The present invention further relates to the dispersion liquid containing dark color particles. Even when the dispersion liquid is applied to a substrate with high brightness, the dark color of the added particles acts as a background color, creating color contrast, enabling vivid structural coloring. By realizing a sharp particle size distribution of inorganic fine particles under appropriate synthesis conditions, interparticle bonding by the alkoxide component, and the associated maintenance of a regular array structure, high brightness coloring and angle-dependent coloring characteristic of structural colors are possible even in a mixed system to which a dark color is added.

The present invention also relates to the dispersion liquid in which the dark color particles are plate-shaped. As described above, structural coloring using the dispersion liquid of the present invention can produce uniform color over a wide area. In particular, when the dispersion liquid is applied to plate-shaped dark color particles, particles with an appearance similar to the glitter used in cosmetics and the like can be obtained. By adjusting the particle size of the inorganic fine particles, the intended color such as purple, blue, green, yellow-green, yellow, orange, red, etc. can be produced uniformly without being affected by the unevenness of the base material or the coating thickness. Naturally, high brightness coloring and angle-dependent coloring unique to structural colors are also possible.

The present invention is also a method for producing the dispersion, which includes the steps of dissolving an alkoxide raw material in a water-containing liquid in advance, precipitating inorganic fine particles by adding a reaction accelerator thereto, and preparing a dispersion containing the inorganic fine particles and unreacted components in which a portion of the alkoxide raw material is unreacted and in a liquid state. With this production method, it is possible to stably obtain a dispersion of inorganic fine particles that realizes high brightness coloring and angle-dependent coloring characteristic of structural colors, which balances highly regular arrangement of inorganic fine particles, maintenance of the arrangement structure, and bonding to a substrate. As a result, it is expected that this method will be used as a new and unprecedented decorative technique in applications requiring high decorativeness and design, in addition to art and crafts and decorative items.

The manufacturing method of the present invention described above can be used to produce the dispersion of the present invention by simply stirring general-purpose liquid raw materials, specifically metal alkoxides, alcohol, water, and basic aqueous solutions, at room temperature, with only a small investment in equipment. Because the process is simple and requires only stirring, and the reaction is carried out at room temperature, the manufacturing process is less hazardous and easy to manage.

The present invention also relates to a method for producing the dispersion liquid, which further includes a step of preparing coated particles by applying the dispersion liquid to the surface of the base particles, and a step of dispersing the coated particles in a liquid. The unreacted raw material components act as bonds between the inorganic fine particles and between the inorganic fine particles and the base particles, and it is possible to realize high-brightness coloring and angle-dependent coloring that are characteristic of structural colors on the surface of the base particles. In particular, when plate-shaped base particles are used, the coloring that is characteristic of these structural colors can be manifested significantly.

The present invention further provides a decorative article comprising an aggregate in which inorganic fine particles formed from any of the above dispersions are arranged in a substantially regular pattern and a bonding phase consisting of unreacted components, and a base material serving as a substrate, in which the inorganic fine particles are arranged in a substantially regular pattern on the surface of the base material to form an aggregate, a part or all of the inorganic fine particles forming the aggregate are bonded by the bonding phase, and the decorative article has an area in which a structural color derived from the particle size and regular arrangement of the inorganic fine particles is expressed.

The decorative product of the present invention forms a periodic structure using the dispersion of the present invention, and therefore the high brightness coloring and angle-dependent coloring characteristic of structural coloring can be continuously expressed over a wide area in any region, and the decorative layer is made of an inorganic material with high bonding strength and has excellent heat resistance. It is also possible to achieve a glossy, or lustrous, coloring, and vivid structural coloring is possible even when a highly luminous base material is used.

The dispersion of the present invention exhibits structural color by forming a periodic structure in which inorganic fine particles are regularly arranged so as to be closely packed as the solvent evaporates. Inorganic fine particles have excellent properties such as heat resistance, mechanical strength, abrasion resistance, light resistance, weather resistance, and chemical stability, and can continuously and uniformly develop high-brightness coloring resulting from multiple reflections of light, angle-dependent coloring resulting from a highly regular arrangement, and diverse coloring resulting from coarse particle sizes over a wide area. Even when dark color particles are mixed, the highly regular arrangement of inorganic fine particles and the maintenance of that arrangement structure can be achieved, and the high-brightness coloring and angle-dependent coloring characteristic of structural colors, as well as diverse coloring, can be developed.

The method for producing the dispersion of the present invention can also be used to produce the product by simply stirring general-purpose liquid raw materials at room temperature, and the product can be produced safely using low-cost raw materials and small-scale equipment. Furthermore, decorations using the dispersion of the present invention can continuously express the high brightness coloring and angle-dependent coloring characteristic of structural colors in any desired area, and have an aesthetic appearance that has never been seen before.

1A is an external view of Sample 1 prepared in Example 1, and FIG. 1B is a magnified image of the surface of Sample 1 observed with a scanning electron microscope at a magnification of 50,000 times. 1A is an external view of Sample 2 prepared in Example 1, and FIG. 1B is a magnified image of the surface of Sample 2 observed with a scanning electron microscope at a magnification of 50,000 times. 1A is an external view of the sample prepared in Comparative Example 1, and FIG. 1B is a magnified image of the surface of the sample observed with a scanning electron microscope at a magnification of 50,000 times. 1 shows external images of black ceramic obtained by applying the 12 types of dispersion liquid prepared in Example 2 to the ceramic. 1 is a magnified image of a surface of a test piece coated with the 12 types of dispersion liquid prepared in Example 2, observed at a magnification of 50,000 times using a scanning electron microscope. FIG. 11 is an explanatory diagram showing the relationship between color and particle size observed in Example 2.

1 is an external image of white ceramic obtained by applying the dispersion liquid prepared in Example 3 to the white ceramic. 1 shows an external image of ceramic ware to which a dispersion liquid was applied in Example 4. 1 shows an external image of ceramic to which the dispersion liquid was applied in Example 5. 1A and 1B are magnified images of the dispersion used in Example 5 observed under a scanning electron microscope at a magnification of 2,000 times and a magnification of 20,000 times, respectively. 1 is an external image of paper coated with the dispersion in Example 6. 1 is an external image of paper coated with the dispersion in Example 7.

The dispersion of the present invention, its manufacturing method, and decorative articles using the same are described in detail below. Note that structures, characteristics, compositions, manufacturing methods, etc. that are not described here may be the same or substantially the same as those known to those skilled in the art.

In addition, in the present invention, "approximately" is not limited to being strictly identical, but is a concept that includes errors and deformations that do not lose their identity. For example, "approximately spherical" is not limited to being strictly spherical, but includes cases that can be regarded as being the same as a sphere.

The chemical composition of the inorganic fine particles of the present invention includes silicon oxide, aluminum oxide, titanium oxide, iron oxide, magnesium oxide, calcium oxide, manganese oxide, cobalt oxide, copper oxide, zinc oxide, yttrium oxide, zirconium oxide, palladium oxide, silver oxide, and composite oxides thereof, silicon nitride, aluminum nitride, titanium nitride, silicon carbide, sialon ceramics, calcium carbonate, calcium phosphate, apatite compounds, carbon, feldspars such as alkali feldspar and plagioclase, clays such as kaolinite, montmorillonite, smectite, allophane, zeolite, and layered double hydroxides, and metals such as platinum, gold, silver, copper, iron, titanium, steel, and various alloys.

Among these, oxides or metals whose surfaces are covered with oxides are preferred from the viewpoint of stability in the atmosphere, and silicon oxide, aluminum oxide, aluminum silicate, or chemical compositions containing small amounts of alkali or alkaline earth metals are more preferred from the viewpoint of cost and environmental friendliness. From the viewpoint of particle size controllability and refractive index uniformity, silicon oxide and aluminum oxide, which are oxides of a single composition, are even more preferred. The use of silicon oxide, which is the same material as opals, is the most preferred because it can be described as a feature that it is the same material as natural gemstones and is an abundant resource.

Alkoxides used as raw materials for the inorganic fine particles of the present invention include metal alkoxides containing elements such as silicon, titanium, aluminum, magnesium, and zirconium. Among these, tetramethyl orthosilicate and tetraethyl orthosilicate are preferred because the reaction rate is easily controlled and the raw materials are inexpensive.

By containing these alkoxides in a partially unreacted and liquid state, it is possible to strengthen the bonds between inorganic fine particles and between the inorganic fine particles and the substrate, and to impart gloss to the coating film (decorative layer). By bonding the inorganic fine particles to each other, it is possible to prevent the inorganic fine particles from moving and rearranging due to external forces such as rubbing. This maintains the close-packed structure that occurs when the solvent in the coating film evaporates, which is the most likely place for the inorganic fine particles to be arranged regularly, and this highly ordered arrangement makes it possible to produce high-brightness coloring due to the multiple reflections of light that are unique to structural colors, as well as coloring that is angle-dependent.

When the dispersion is applied to the surface of the substrate (object to be decorated), the inorganic fine particles approach each other as the solvent evaporates, arranging in a generally regular pattern on the substrate surface to form a close-packed structure, forming an aggregate of inorganic fine particles with a periodic structure. At this time, the alkoxide component, which has a low vapor pressure, remains in the aggregate and is interposed between the inorganic fine particles. When the volume ratio of the alkoxide component to the inorganic fine particles (alkoxide component/inorganic fine particles) is 1-5%, the inorganic fine particles are generally bonded together at points, when it is 26% or more, most of the gaps between the inorganic fine particles are filled, and when it is 6-25%, either of the two forms can be taken depending on the coating area.

The higher the ratio, the stronger the bond between the inorganic fine particles and between the inorganic fine particles and the substrate after application, and the gloss of the coating film will increase. On the other hand, the lower the ratio, the clearer the structural color of the coating film will be. Therefore, the volume ratio of the alkoxide component contained in the dispersion to the inorganic fine particles can be selected depending on the application, and if emphasis is placed on structural color, 1 to 25% is preferred, and 2 to 20% is more preferred. If emphasis is placed on bonding strength, 10 to 30% is preferred, and 15 to 28% is more preferred.

Here, the wavelength of light of the structural color that appears due to the periodic structure formed by the regular arrangement of inorganic fine particles can be predicted using the following Bragg-Snell equation.
λ=2(d/m)(n ² -sin ² θ) ^0.5
When the above Bragg-Snell equation is applied to the dispersion of the present invention, λ is the wavelength (nm) of light reflected by the periodic structure formed by the inorganic fine particles due to constructive interaction with the light, d is the periodic distance (nm) of the periodic structure determined by the diameter R of the inorganic fine particles, m is an integer, n is the refractive index of the coated surface layer containing the inorganic fine particles, and θ is the angle from the normal to the coated surface layer. In other words, the structural color observed after coating the dispersion depends on the particle size of the inorganic fine particles, the refractive index of the coated surface layer, and the observation angle.

In this specification, the term "particle size" refers to the diameter of a primary particle when it is approximated as a sphere. Spherical particles are densely packed to form a periodic structure characterized by a short periodic distance, so there is a high possibility that there are multiple wavelengths of visible light that satisfy the Bragg-Snell equation. In other words, in a periodic structure formed by densely packed spherical particles, different colors can be observed depending on the observation angle θ. The characteristic of observing different colors depending on the observation angle is a unique characteristic of structural color that is not seen in general pigments and dyes that are based on the principle of light absorption, and for this reason it is preferable to use spherical particles.

Even if the particle surface has ultra-fine irregularities, if the appearance in a magnified image observed under an electron microscope can be approximated to a sphere, it is considered to be approximately spherical and is within the scope of the present invention. On the other hand, since it is difficult to develop structural color derived from regular arrangement and therefore periodic structure in oblong particles, they are not within the scope of the present invention. In the present invention, the boundary between a sphere and an oblong spheroid is the value of the major axis divided by the minor axis of 1.3, and particles with this value of 1.3 or less are considered to be approximately spherical.

The inorganic particles may be solid, hollow, or porous, as they can exhibit structural color depending on their refractive index. When the coating is not covered with other substances and is exposed to the surface, if the inorganic particles are solid or hollow, liquid such as water will penetrate into the gaps between the inorganic particles, changing the effects of light refraction, reflection, diffraction, scattering, etc., and the structural color will change or disappear depending on the difference in refractive index between the inorganic particles and the liquid, resulting in a unique characteristic. In addition, if the inorganic particles are porous, the liquid will penetrate into the gaps between the inorganic particles and into the pores of the inorganic particles at the same time, resulting in the structural color disappearing regardless of the refractive index of the liquid used. The color change due to liquid penetration will return to the original color when the liquid is removed by evaporation, etc. This characteristic is not seen in general pigments and dyes that use light absorption as a principle, and is unique to structural colors.

When a dispersion of roughly spherical inorganic particles is applied and the solvent evaporates, the particles approach each other as the solvent evaporates, forming a densely packed structure. In a dispersion containing only spherical particles of uniform diameter, a close-packed structure is usually formed. In a close-packed structure consisting of spheres of diameter R, a periodic surface with a periodic distance of √6R/3 is formed in the direction in which the particles are stacked, and a periodic surface with a periodic distance of R/2 is formed in the perpendicular direction. The wavelength of light calculated by applying these periodic distances to the Bragg-Snell formula mentioned above matches well with the wavelength of the color actually observed. Periodic surfaces are also formed in oblique directions, which means that the color seen changes depending on the viewing angle and the angle of the light.

The structural color that appears due to the periodic structure formed by inorganic fine particles is purple in the √6R/3 periodic plane in the case of silicon oxide fine particles with a diameter of about 150 nm, and as the particle size increases, it shifts to longer wavelengths of blue-purple, blue, blue-green, green, yellow-green, yellow, orange, and red. For materials with the same refractive index, the wavelength of light that is reflected constructively is directly proportional to the diameter of the particles that form the close-packed structure, as can be seen from the Bragg-Snell equation. When the particle size becomes larger than this, the constructive color under the condition of m = 1 and the constructive color under the condition of m = 2 mix, resulting in a reddish purple or even a blue-purple color. When the particle size becomes even larger, the constructive color under the condition of m = 1 enters the near-infrared region and cannot be detected by human photoreceptor cells, and the constructive purple or blue color under the condition of m = 2 is displayed, followed by a cycle of blue-green and green.

As can be seen from the Bragg-Snell formula, constructive colors under the condition of m=2 are colors with wavelengths where the optical path difference, calculated by converting the effects of refraction into length, is equal to twice the wavelength of light, resulting in constructive colors. Similarly, m=3, m=4, and so on, but when the coating thickness is the same, the larger the value of m, the less reflected light with overlapping phases, resulting in a blurred color with low brightness. For this reason, it is preferable that the diameter of the inorganic fine particles is 600 nm or less. Also, if the particle size is large, the appearance tends to be whitish, so to avoid this, it is more preferable that the diameter is 550 nm or less.

In the case of a periodic surface with a periodic distance of R/2 in the vertical direction, silicon oxide particles with a diameter of about 240 nm are purple, and as in the case of a √6R/3 periodic surface, the color of the longer wavelength side appears as the particle size increases. In the case of an R/2 periodic surface, light is constructive even under conditions where m = 2, m = 3, m = 4, where m is an integer. In other words, the larger the particle size of the inorganic particles, the easier it is to form a periodic surface at various angles, and multiple reflections of light corresponding to integer multiples of the wavelength m = 2, 3, 4, etc. tend to occur, resulting in the expression of a color characteristic of structural colors, in which the color changes depending on the angle. In particular, when the wavelength of light that is constructively reflected by a √6R/3 periodic surface is close to the wavelength of light that is constructively reflected by an R/2 periodic surface, the color changes with a slight difference in the angle of light, making it appear colorful. For this reason, it is preferable that the diameter of the inorganic particles is 375 nm or more.

In order to produce the desired structural color using the dispersion of the present invention, it is preferable to prepare inorganic fine particles with a sharp particle size distribution and uniform particle size. By making the particle size of the inorganic fine particles uniform and making the particle size distribution sharp, the inorganic fine particles are regularly arranged based on the action of self-organization accompanying evaporation of the solvent, forming a multilayer periodic structure. When the variation in particle size is expressed as a standard deviation, it is preferable that the value of the relative standard deviation obtained by dividing the standard deviation by the average particle size is within 10%. It is also possible to intentionally use multiple inorganic fine particles that show different levels of particle size distribution.

In this study, both inorganic and organic pigments can be used as dark color particles, but inorganic pigments are preferable for applications requiring heat resistance, mechanical strength, hardness, abrasion resistance, chemical stability, corrosion resistance, light resistance, and weather resistance, specifically applications involving exposure outdoors or to liquids, applications in which the surface is rubbed or cleaning liquids other than water are used for cleaning purposes, or applications involving a heating process after application. The term "dark color" as used here refers to a color classified into 0 to 4 according to the method of expressing lightness defined in JIS standard Z8721-1993, where ideal black is 0 and ideal white is 10, with the difference in lightness perception being divided into values between 0 and 4 at approximately equal rates, and it may be either achromatic or chromatic.

The shape of the dark color particles may be any of spherical, ellipsoidal, needle-like, wire-like, tubular, plate-like, polyhedral, and amorphous, but it is preferable to use plate-like particles in which inorganic fine particles are easily stacked to form periodic surfaces with a regular arrangement. Plate-like particles obtained chemically by gas-phase synthesis or liquid-phase synthesis may be used, but considering the cost, it is preferable to use plate-like clay minerals, which are natural resources. Specific examples include mica, biotite, charcoal, vermiculite, montmorillonite, kaolinite, bentonite, and smectite. The term "plate-like" as used here includes flat, plate-like, flaky, scaly, etc., and refers to particles whose thickness in the depth direction is sufficiently thin compared to the long and short sides in a planar view. Specifically, particles with a ratio of the long side to the thickness (long side/thickness) of 5 or more are considered to be plate-like.

The size of the dark color particles may be larger than the inorganic fine particles so that the inorganic fine particles are stacked, or may be smaller than the inorganic fine particles so that they enter the gaps between the inorganic fine particles and absorb light as a background color, so the appropriate size range varies depending on the purpose. When using the plate-like particles mentioned above, it is preferable to use particles with a long side length of 1 μm or more. Depending on the surface charge of the dark color particles, the larger the dark color particles, the more likely they are to settle in the dispersion liquid, but this is not a problem as long as they can be applied. By applying the dispersion liquid of the present invention in advance to dark color particles with a long side length of 1 mm or more, crushing them and dispersing them in the liquid, it is possible to obtain a dispersion liquid in which composite particles that significantly express the high brightness coloring and angle-dependent coloring unique to structural colors are dispersed.

Depending on the blending ratio of the dark color particles added, the color of the coating film when applied to a substrate with high brightness can be adjusted. For example, when the dispersion of the present invention is applied to a white substrate without adding dark color particles, the coating will have a color that looks like a sparkling jewel. It is possible to produce only a specific color according to the particle size and refractive index of the inorganic fine particles, or to produce a color that is angle-dependent. In a substrate with high brightness, the light reflected from the substrate is strong, so the contrast of the multiple reflections of the dispersion coating layer is low, making the structural color of the coating film difficult to see. As the blending ratio of the dark color particles is increased, the added dark color particles absorb light and suppress light reflection by the substrate, making it easier to see the structural color due to the multiple reflections of the coating film.

In other words, by adjusting the blending ratio of the dark color particles, it is possible to achieve both sparkling, jewel-like coloring and clearly visible structural coloring, and it is possible to use either depending on the intended design. The content of the dark color particles to improve the visibility of the structural color varies depending on the brightness and color of the base material and the dark color particles, but is preferably in the range of 0.5 to 15 wt%, and more preferably in the range of 1.5 to 5 wt%.

In addition to the dark colored particles, the dispersion may contain an appropriate amount of additives for the purpose of strengthening the adhesion during or after application, adjusting the viscosity, wettability, and evaporation rate of the liquid, and adjusting the color and thickness of the coating film, except in cases where this significantly impairs the dispersibility of the inorganic fine particles.

Additives used to strengthen bonds and adjust viscosity include carboxymethylcellulose, methylcellulose and their salts, latex, natural rubber, glue, dextrin, etc. These may also be used in combination.

Additives used to adjust wettability and evaporation rate include acetone, methyl ethyl ketone, toluene, xylene, benzene, phenol, n-hexane, formic acid, acetic acid, methyl acetate, ethyl acetate, dibutyl phthalate, acetonitrile, and dimethylformamide, and these may be added in combination. A surfactant may also be added to improve the dispersion stability of the inorganic fine particles. The surfactant used may be any of anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants. Multiple ionic surfactants may be combined, or an ionic surfactant and a nonionic surfactant may be combined.

As described above, when the dispersion of the present invention is applied to a base material, the inorganic fine particles are arranged in a generally regular manner as the solvent evaporates, forming an aggregate of inorganic fine particles having a periodic structure. This inorganic fine particle aggregate may be an aggregate in which a plurality of inorganic fine particles are arranged in a generally regular manner and maintain a certain shape, and is an aggregate in which inorganic fine particles are assembled by physicochemical forces. In the present invention, even if partial cracks, defects, or fallen particles are found in the inorganic fine particle aggregate under magnification observation, as long as the aggregate can continuously express the desired structural color when observed with the naked eye, it is considered to be arranged in a generally regular manner and can be regarded as being the same as an aggregate that is arranged in a precise regular manner.

The inorganic fine particle aggregates that exhibit structural color can be formed in any region of the substrate by adjusting the shape and area to which the dispersion is applied. In the present invention, the term "any region" refers to any region that is within a part or entire range of the substrate and includes all shapes in a planar view direction that can be formed by coating the dispersion at the discretion of the creator. The region in which the inorganic fine particle aggregates are formed may be one or more locations. In order to continuously exhibit structural color over a wide area, the area of each region in which the inorganic fine particle aggregates are formed is preferably 0.2 ^mm2 or more.

The thickness of the inorganic fine particle aggregate can be formed to any thickness by adjusting the coating method, liquid amount, and number of times. In order to form a multi-layered periodic structure and to express a structural color that is clearly visible to the naked eye, the thickness after firing is preferably 700 nm or more, and more preferably 1000 nm or more. In terms of the number of layers, 2 to 3 layers or more are preferable, and 4 to 5 layers or more are more preferable. The thicker the layer, the clearer the structural color will be, and a thinner layer is advantageous in terms of recoating and cost. On the other hand, if you want to express a faint color and a faint hue, 1 to 2 layers are preferable. In addition to the design to be expressed by the decoration, the brightness, abrasion resistance, peeling resistance, etc. of the base material are also taken into consideration when setting the thickness.

In inorganic fine particle aggregates, some or all of the particles are bonded together and fixed or bonded to any region of the substrate surface by a bonding phase formed by hydrolysis and dehydration condensation of unreacted components of the alkoxide raw material present in the gaps. When the dispersion is applied, dried, and then heated at a temperature of 200°C or higher, the alkoxide component forms a metal-oxygen bond, forming a strong chemical bond. In particular, when the dispersion is applied to an inorganic substrate, a strong chemical bond is also formed at the interface between the inorganic fine particle aggregate and the inorganic substrate. Furthermore, when the dispersion is applied to an inorganic substrate such as ceramics, dried, and then fired at a high temperature of 900°C or higher, the inorganic fine particles and the alkoxide component partially melt, forming an even stronger bond.

The inorganic fine particles and the bonding phase formed from the unreacted components are formed from the same starting material. When the alkoxide component forms a metal-oxygen bond by heating and sintering, the inorganic fine particles and the bonding phase will have the same chemical composition. Having the same chemical composition means that the types of constituent elements of the inorganic compounds that form the inorganic fine particles and the bonding phase are the same.

The dispersion of the present invention can be applied to a substrate of any color in any region like paint and allowed to dry naturally, thereby producing a structural color on the substrate surface. Therefore, the substrate to which the dispersion of the present invention is applied is not limited by the manufacturing process such as heating, and can be selected from a wide range of substrates, such as organic substrates, inorganic substrates, and natural materials, depending on the application. The form of the substrate that serves as the base is not particularly limited as long as it can produce the desired structural color. It can be in any form, such as a molded product made of organic or inorganic material, paper or woven fabric made of fiber, or a component made of processed natural material.

As the organic substrate, a resin with high hydrophilicity is preferable in consideration of the wettability of the aqueous dispersion to the substrate surface, and examples thereof include acrylic, methacrylic, urethane, amide, and polyester resins, copolymers thereof, and cellulose resins. If necessary, the surface may be subjected to a chemical or physical hydrophilic treatment such as ozone treatment, or may be coated with a hydrophilic layer. Alternatively, the wettability of the dispersion may be improved by a method such as adjusting the amount of unreacted components of the alkoxide or adding a solvent or surfactant. By performing these surface treatments and component adjustments, it may be possible to apply the dispersion to resins with low hydrophilicity.

Inorganic substrates include ceramics such as porcelain, various tiles, glass, carbon, and silicon, natural stones such as marble, Oya stone, and granite, and metals such as iron, steel, aluminum, titanium, gold, silver, copper, platinum, and various alloys. Composite materials combining a plurality of these inorganic materials may also be used. When drying and firing after coating, carbon that disappears when fired in air, silicon and metals that oxidize, etc. are fired in an inert atmosphere, vacuum atmosphere, or reducing atmosphere. If the surface of the inorganic substrate is porous and the applied dispersion liquid penetrates into the interior, a base layer may be provided on the substrate surface. For example, a glassy base layer may be provided by coating and firing a glaze. The structural color may be emphasized by making the base layer a dark color as the background color.

Examples of natural materials and processed products thereof include paper such as Japanese paper and Western paper, wood such as solid wood, laminated wood and plywood, fibers such as cotton and hemp, leather of cows and pigs, etc. Similarly, if the surface of the substrate is porous and the dispersion liquid penetrates into the interior, a base layer may be provided.

The means for applying the dispersion of the present invention to a substrate is not limited to coating the surface of a molded product or a finished product. For example, it is possible to process the dispersion into a powder state by adding a binder component as an adhesive to the dispersion and drying it, or by mixing a binder with the powder obtained by drying the dispersion, and then using this to form an aggregate of inorganic fine particles having a periodic structure on or near the surface of the substrate, thereby expressing a structural color.

The dispersion of the present invention is capable of producing highly luminous colors due to multiple reflections of light, and color that changes depending on the viewing angle, resulting in excellent aesthetics. Not only can it be used for products such as fine arts and crafts and decorative items that are objects of appreciation, but it can also be used as a dispersion that allows for new designs through new coloring, in the creation of art such as paintings, prints, dyeing and ceramics, as well as for everyday items, components, structural materials and industrial products.

Specific examples include cosmetics, tableware, vases, cooking utensils, writing implements, tools, paper, woodwork, plastic products, rubber products, clothes, bags, shoes, hats, pouches, household appliances and other daily necessities and their parts, jewelry, accessories, watches and other decorative items and their parts, mobile phones, smartphones and other portable information and communication devices, display panels, housings and parts of electrical appliances, furniture components such as tables, desks and shelves, building materials and fixtures such as roofs, walls, floors, windows and doors, components for housing facilities such as baths, toilets, kitchens and washbasins, frames, bodies and interior and exterior parts of transportation equipment such as bicycles, motorcycles, automobiles, trains and airplanes, stone products such as monuments, lanterns and gravestones, ceramics and crafts, etc.

In addition, when applied to a bright substrate without mixing in dark-colored particles, it has the characteristic of exhibiting angle-dependent color development at specific observation angles, and because it is a unique dispersion that has never been seen before, it can be used to prevent counterfeiting of banknotes, coupons, securities, identification cards, insurance cards, tickets, certificates, contracts, etc.

Next, the manufacturing method of the present invention adjusts the reaction so that a portion of the alkoxide used as the raw material for inorganic fine particles is intentionally left unreacted, and produces a dispersion containing inorganic fine particles and unreacted alkoxide components in a liquid state. The alkoxide is hydrolyzed, and then a dehydration condensation reaction generates baby nuclei and nuclei of inorganic fine particles, and the nuclei grow to produce inorganic fine particles. The color development, such as color and gloss, can be adjusted by the balance between the generation of baby nuclei and nuclei, which determines the number of particles, and the particle growth reaction, which grows from the baby nuclei and nuclei to particles and determines the size of the particles, and by the balance of the ratio of the alkoxide used in the reaction to the alkoxide that remains unreacted.

In the present invention, inorganic fine particles are produced from alkoxide raw materials by liquid phase synthesis to produce a dispersion liquid. For example, particles of uniform size can also be obtained by gas phase synthesis, but the particles recovered by gas phase synthesis are somewhat aggregated, and it is difficult to completely disperse the particles in the liquid to obtain a monodispersed particle, or to control the degree of aggregation, so liquid phase synthesis is adopted in the present invention. Among liquid phase syntheses, the alkoxide method can produce inorganic fine particles while controlling the particle size by adjusting the balance of the reaction rate, and the reaction residue of the alkoxide can be used to strengthen the bonding of the coating layer, maintain the regular arrangement of the particles, and impart gloss to the coating film, so the alkoxide method is adopted in the present invention.

To obtain a dispersion of inorganic fine particles with uniform particle size, it is necessary to generate inorganic fine particle nuclei and nuclei quickly and uniformly. Therefore, the faster the stirring speed and catalyst addition speed during liquid phase synthesis, the better. In the nucleation and particle growth processes of liquid phase synthesis, the synthesis reaction may be promoted by applying external energy using ultrasound or a vibrator, and the nucleation rate and particle growth rate may be controlled by heating or cooling the liquid from which the particles are precipitated.

When using an alkoxide with a low hydrolysis rate as the raw material, a reaction accelerator is added to produce inorganic fine particles. In order to uniformly generate inorganic fine particle nuclei, it is preferable to dissolve the alkoxide raw material in water in advance and react it to generate immature nuclei, and then add the reaction accelerator. This allows the nuclei of inorganic fine particles to be generated all at once, resulting in the effect of making the particle size of the inorganic fine particles uniform.

The reaction time for growing inorganic microparticles is adjusted appropriately within a range of several minutes to several days so as to obtain inorganic microparticles of the desired particle size. It varies depending on conditions such as the type of alkoxide raw material used, the compounding ratio of the alkoxide raw material, the compounding ratio of the reaction accelerator, and the reaction temperature, but for example, under mild conditions in which a reaction is carried out at room temperature with stirring only using a general-purpose liquid alkoxide raw material, alcohol, water, and a basic aqueous solution, it is adjusted appropriately within a range of several hours to three days.

In the manufacturing method of the present invention, the reaction of the alkoxide raw material is adjusted so that the alkoxide raw material is not completely reacted, and some raw material components are intentionally left unreacted, to prepare a dispersion. The reaction rate in the synthesis is controlled by adjusting the rate of nucleation and particle growth of the inorganic fine particles, for example, by adjusting the mixing ratio of the alkoxide raw material, the mixing ratio of the reaction accelerator, the mixing ratio of water added for the hydrolysis reaction, the temperature, pressure, stirring speed, solubility parameter, and reaction time.

Methods for applying the dispersion include dip coating, in which the substrate is immersed in the dispersion, application with a brush or writing brush, spray coating by spraying, and, in addition, when the substrate has a smooth shape, bar coating or spin coating can be used. In the present invention, the dispersion can be applied directly to the substrate to form a periodic structure, so that structural color can be continuously expressed over any large area. In addition, even if the inorganic substrate has a curved surface, it can be applied uniformly, and it is possible to apply the dispersion to the entire surface of the substrate, or to apply the dispersion by spraying to scatter structural color coloring regions. An application method suitable for the intended color expression can be selected from the above methods, and multiple application methods may be combined.

It is preferable to use the dispersion liquid by applying it to a substrate in a monodispersed state in which inorganic fine particle nuclei are generated from the alkoxide raw material and the particles are grown. Once the solvent of the dispersion liquid evaporates and the inorganic fine particles aggregate, it is difficult to disperse them again to a monodispersed state. However, a manufacturing method in which the dispersion liquid is applied to large-sized mother particles to form a regular array structure of inorganic fine particles on the mother particle surface and the coated particles coated with inorganic fine particles are dispersed in a liquid is preferable because it can fully utilize the structural coloring due to the regular array structure. When plate-shaped mother particles are used, it is particularly preferable because a dispersion liquid in which coated particles with excellent glitter-like aesthetics are dispersed can be obtained. Alternatively, when a faint coloring is intended to be designed, the dispersion liquid may be used in a state in which the solvent is evaporated and the particles are aggregated.

The shape of the base particles is similar to that of the dark colored particles, which are larger than the inorganic fine particles described above. Specific examples include mica, biotite, carbon, vermiculite, montmorillonite, kaolinite, bentonite, smectite, talc, calcium carbonate, titanium oxide, silicon oxide, aluminum oxide, chromium oxide, zinc oxide, iron oxide, lead oxide, lead chromate, metal powder, barium sulfate, iron sulfate, molybdate, phthalocyanine pigment, azo pigment, lake pigment, and fluorescent pigment. Also, coated particles may be produced using dark colored particles as base particles.

Conventionally, glitter used in cosmetics and the like has been used by forming a thin film on plate-like particles to produce interference colors, but even slight differences in film thickness can result in unintended colors being produced, making color control difficult. On the other hand, in a dispersion containing coated particles obtained by the manufacturing method of the present invention, the color is determined by the periodic distance of the periodic structure formed by the inorganic fine particles stacked on the plate-like particles, so the intended specific color can be produced uniformly without being affected by film thickness.

The dispersion liquid of the present invention and its manufacturing method will be specifically described below with reference to examples and comparative examples. In addition, in order to supplement the explanation of the color and color development, color photographs corresponding to Figures 1(a), 2(a), 3(a), 4, 6, 7, 8, 9, 11, and 12 are attached to the matter submission document dated the same day as this application. Note that the present invention is not limited to these examples, etc., and various modifications are possible within the scope of the technical idea of the present invention.

[Example 1]
20g of ethanol, 5g of water, and 7.0g of tetraethyl orthosilicate (TEOS) were mixed, and 1 mol/L of ammonia water was added while stirring with a stirrer, and the mixture was stirred at room temperature to precipitate silicon oxide fine particles. The amount of ammonia water added was set to two conditions of 6.0mL and 7.0mL, and stirring was continued for 24 hours. Each of the two types of dispersion liquids prepared was applied with a brush to biotite (Tokyo Science Sales, experimental mineral biotite product No. MD011) with a size of approximately 20mm x 30mm square. Partial coating was performed multiple times to adjust the thickness.

Figures 1(a) and 2(a) show the external appearance images of two types of samples 1 and 2 obtained by applying the dispersion liquid to biotite, taken with a digital camera. Color development was confirmed over a wide area of the order of several ^cm2 . The color differed depending on the amount of ammonia water added, with the 6 mL sample (sample 1) exhibiting green and the 7 mL sample (sample 2) exhibiting red. In particular, glossy, highly luminous color development was confirmed in the 6 mL sample.

Images of the two obtained samples, Sample 1 and Sample 2, magnified at 50,000 times by a scanning electron microscope are shown in Figures 1(b) and 2(b). Both samples were confirmed to contain spherical particles with uniform sizes of several hundred nanometers. In the 6 mL sample, the spaces between the particles were found to be filled, forming a so-called sea-island structure. On the other hand, in the 7 mL sample, the particles were found to have a structure in which they were partially connected and joined by faces or points. In the 6 mL sample, the gaps between the particles were filled, making the top surface of the coating smooth, which is thought to make it easier for light to be reflected specularly and thus increasing the gloss.

[Comparative Example 1]
A dispersion and a sample coated with the same on biotite were obtained under the same conditions as in Example 1, except that the amount of ammonia water added was 8 mL and the TEOS raw material was allowed to react completely. The appearance image is shown in Figure 3(a). A blue color was observed, but compared to the sample in Example 1, the gloss was poor and the coating film was easily peeled off even with a weak force.

Figure 3(b) shows a magnified image of the sample of Comparative Example 1 observed with a scanning electron microscope at a magnification of 50,000 times. As with Example 1, spherical particles of uniform size were confirmed, but unlike the two samples of Example 1, it was found that the particles were not attached to each other and had a microstructure in which they existed independently. It is believed that the coating peeled off more easily compared to Example 1 because the particles were not bonded to each other.

[Example 2]
Twelve types of dispersions were prepared under the same conditions as in Example 1, except that the amounts of the raw reagents were changed, and were applied to a black bowl-shaped ceramic (Tokyu Hands, black matte tea bowl, φ11 cm, product no. 2401026303928) at room temperature. The appearance of the ceramic is shown in FIG. 4, and a scanning electron microscope image is shown in FIG. 5. The amounts of the raw reagents and the number-average diameter of the inorganic fine particles contained in the obtained dispersions are summarized in Table 1, and the colors observed when the ceramic was viewed from above are summarized in Table 2.

The samples were numbered with the amount of TEOS raw material as the first reference value and the amount of ammonia water as the second reference value, with smaller numbers being assigned to samples with smaller amounts. The number-average diameter of the inorganic fine particles was calculated as an average value from the images of 10 particles with relatively clear boundaries in scanning electron microscope images observed at 50,000x magnification. For Nos. 2, 8, and 9, where inorganic fine particles consisting of two levels of particle size were confirmed in microstructural observation, the number-average diameters were measured for both levels. The observed colors shown in Table 2 were observed with the naked eye. From the scanning electron microscope images in Figure 5, it was possible to confirm that the particles were partially joined at faces or points for all 12 types.

In general, within the range of raw material blend amounts in Table 1, the greater the blend amount of TEOS and aqueous ammonia, the larger the number average diameter of the precipitated inorganic fine particles tended to be. However, as in No. 12, when the blend amount of both raw materials exceeded a certain boundary value, the diameter of the precipitated particles suddenly became smaller and reached a maximum value. When TEOS, the silicon source, was present at a high concentration exceeding the boundary value, many particle nuclei were generated the moment aqueous ammonia was added, and as a result, less TEOS raw material was used for grain growth, which is thought to have resulted in the small diameter of the inorganic fine particles in No. 12. The tendency for the particle size of the inorganic fine particles produced when the TEOS raw material concentration was excessive was also confirmed in a comparison of Nos. 8, 9, and 10, in which the aqueous solution concentration was changed by the amount of water added.

When the aqueous solution concentration was changed, the electron microscope images in Figure 5 showed that there were two levels of particle size in samples No. 8 and No. 9, which had a reduced amount of water added. It is believed that adding water before adding ammonia water produces young nuclei, and when the amount of water is low, ammonia water is added, which rapidly promotes the reaction, before young nuclei have been produced sufficiently. As a result, in samples No. 8 and No. 9, the nuclei split into two, those that grew immediately after the addition of ammonia water and those that grew later, resulting in two levels of particle size.

When the ceramics were viewed from directly above, a highly luminous color was observed at the bottom of the ceramics due to regular reflection light. Observation from directly above means that the view is taken from a direction perpendicular to the layered surface of the close-packed structure created by the spherical particles. Because there are multiple periodic surfaces due to the layering of inorganic fine particles, it is thought that the light is reflected multiple times by these surfaces and the phases of multiple lights overlap, resulting in a highly luminous color. The color was whitish-blue for sample No. 1 with a particle diameter of 357 nm, and as the particle diameter increased, the color shifted to longer wavelengths, from green, to yellow-green, to yellow, and was red at 466 nm (No. 7). At larger wavelengths of 481 nm (No. 6) and 508 nm (No. 11), the color was reddish purple, and at 561 nm (No. 10), the color was green.

As the particle size increases, the periodic distance created by the close-packed structure of the particles becomes longer, which causes the phase to align and reinforce each other, resulting in longer wavelengths of reflected light. As a result, the observed colors shift to longer wavelengths along with the particle size. These high-brightness colors caused by specularly reflected light are angle-dependent colors that change color depending on the viewing angle (angle of light). For example, the red color seen when observed from directly above changes to orange or yellow-green when viewed at an angle.

Because ceramics are bowl-shaped and curved, when observed from directly above, structural colors are simultaneously detected at different angles of light irradiation depending on the curve of the inside surface of the ceramic. With a particle size of 357 nm (No. 1), only blue was visible on the inside surface, but with a particle size of 378 nm (No. 2), green was seen in addition to blue. With an even larger particle size of 398 nm (No. 3), blue, green, orange and red were seen, producing colors that could truly be described as rainbow colors. It is believed that when the particle size is large enough, the periodic distance of the close-packed structure corresponds to an integer multiple of the wavelength of light, satisfying the conditions for structural coloring, which is why the diverse colors were observed.

Conventionally, rainbow coloring has been known to utilize the interference colors of thin-film coatings, but with thin films, even slight unevenness in the substrate or slight differences in film thickness can change the color, making it difficult to control the color, and conversely, rainbow colors can appear unintentionally. On the other hand, the structural color of this invention is derived from the mechanism by which the close-packed structure made of inorganic fine particles piled up on a ceramic substrate reflects light of a specific color in a reinforced manner, and the periodic distance of the close-packed structure made by the particles is constant and not affected by the unevenness of the substrate or the film thickness.

This makes it easy to control and produce the intended color. Specifically, the iridescent coloring of the ceramic shown in Figure 4 depends only on the difference in the angle of light according to the curve of the ceramic, and is not affected by the unevenness of the base material or the thickness of the film, making it possible to create a rainbow design that appears as concentric circles when observed from directly above. The coloring of this rainbow design has an angle dependency unique to structural colors, and when observed from a different angle, the areas where the rainbow colors are visible change.

In addition to the rainbow colors, purple, blue, and green were also observed on the inside of the ceramics. It is thought that these colors on the lower wavelength side were observed because the shorter the wavelength of light, the more easily it is scattered. The colors that appear due to diffuse reflected light are not angle-dependent, and appear the same color when viewed from any angle.

The above relationship between particle size and color is summarized in Figure 6 based on the experimental results. The six straight lines inserted in Figure 6 are lines that represent the relationship between particle size and the wavelength of light that is reflected constructively by the stacking direction of the close-packed structure and the periodic planes in the perpendicular direction, and the slope was determined using the calculated value of the interplanar spacing of the stacking planes of the close-packed structure. For example, since the particle size of sample No. 1 is 357 nm, looking at 357 nm in Figure 6, it can be seen that it intersects with two lines, one for the bottom plane and one for the side plane. In other words, from Figure 6, it is expected that blue will be seen on the bottom plane and blue-green on the side perpendicular to it. In fact, the observation of Figure 4 confirmed only blue, which is thought to be because blue has a shorter wavelength than green and is more easily scattered and expressed.

Similarly, looking at the 378 nm particle size of sample No. 2 in Figure 6, two cycles on the bottom surface are well within the green region, which is thought to explain the blue to green gradation seen in Figure 4. For the even larger particle size of 398 nm (No. 3), it can be seen in Figure 6 that the straight line for one cycle on the side surface is in the red wavelength region, and the straight line for two cycles on the bottom surface is in the green wavelength region. From this, in addition to the blue color of low-wavelength light that is easily scattered, it is thought that the color changes from the bottom to the top of the bowl, from green to yellow, orange, and red, which could explain the phenomenon of the rainbow design seen in Figure 4.

On the inner surface of the No. 8 sample with a larger particle size of 501 nm, only blue and purple were observed, rather than the colorful colors that could be called a rainbow. It is noteworthy that the lower part near the bottom is blue and the upper part is purple, indicating that the colors shift to lower wavelengths from the bottom to the top. This color order is the opposite of the direction of the rainbow design, which shifts to longer wavelengths from the bottom to the top. In the 501 nm particle size region in Figure 6, the straight lines of the first cycle on the side and the second cycle on the bottom both fall into the red region, causing the reddish rainbow colors to disappear, and the third cycle on the bottom falls into the blue wavelength region and the second cycle on the side falls into the purple wavelength region, which explains why a gradation coloring with a reversed color order was observed.

In samples No. 9 and 10, which have a larger particle size of over 550 nm, rainbow coloring was observed, but some parts appeared whitish and the color was uneven. This is thought to be due to partial aggregation of inorganic fine particles in the dispersion liquid.

In the past, small particle diameters have been preferred for structural coloring technology that utilizes the close-packed structure of spherical particles. In particular, in the case of inorganic fine particles, if the concentration of the alkoxide raw material is increased to increase the particle diameter, the particles aggregate and gel, making it impossible to obtain a dispersion. Another reason for the use of small particle diameters is that the condition in which the periodic distance corresponds to one period of the wavelength of light when observed in the vertical direction makes it easiest to evaluate the color and produces a vivid color. The structural coloring targeted by these past technologies corresponds to the straight line of "one period of the bottom surface" in Figure 6.

This is also the range that the inventor, who has been conducting research for some time, initially targeted. However, even when the particle size of the inorganic fine particles is large, by making the particle size highly uniform and by bonding the particles together and fixing the microstructure so that the close-packed structure is not rearranged by external forces as shown in Figure 5, it was found that it is possible to clearly express structural colors resulting from light reflection equivalent to two periods of the wavelength, as well as colors observed from an oblique direction. It can be said that this made it possible to express the gradational design of samples No. 2 to No. 11 shown in Figure 4, and even the rainbow color design.

[Example 3]
Four types of dispersions A to D prepared under the conditions of the raw material composition shown in Table 3 were applied to white ceramics (Tokyu Hands sales, tea bowl, white matte, φ11 cm, product number 2401026303911) with a brush. Dispersion A was applied to the bottom of the ceramics in a circular manner, and dispersions B to D were applied to the inside surface of the ceramics in a concentric line manner. The appearance of the ceramics is shown in Figures 7 (a) and (b). The reddish purple color due to the coating of dispersion A and the green color due to the coating of dispersion B were confirmed on the bottom of the ceramics, but the other colors were so faint that they could not be seen with the naked eye. It was found that it was possible to create a design that gives a glittering impression like a jewel based on white. It is thought that the white ceramic used as the base material strongly diffused light in the visible wavelength range on the base, resulting in a sparkling appearance.

[Example 4]
To each of the four dispersions used in Example 3 (2 g), 0.05 g of black paint (Sakura Matte Watercolor Multi Black #49, manufactured by Sakura Color Products Co., Ltd.) and 0.17 g of water were added to prepare dispersions A' to D'. Dispersions A' to D' were applied to white ceramics in the same manner as in Example 3. The appearance of the ceramics is shown in FIG. 8. In the areas where dispersions A' and B' were applied, a more vivid color was visually observed than in Example 3. Although the appearance was different from the brilliant color of Example 3, the color was highly luminous and different from that of conventional pigments that use light absorption as a coloring principle.

The black particles added to the dispersion absorbed visible light over a wide wavelength range as a background color, which resulted in an emphasis on the contrast between the structural color due to the periodic structure created by the inorganic microparticles and the color of the base, making the structural color more clearly visible. Dispersion D' had a weak, dark structural color, but dispersion C' also produced a vivid blue color due to a mechanism similar to that of dispersions A' and B'. It was also found that the blue color of dispersion C' appeared as a reddish purple color in the region of specular reflection.

[Example 5]
Only the dispersion A' prepared in Example 4 was applied to the bottom of a white ceramic ware in a wide circular pattern. The external appearance of the ceramic ware is shown in Figure 9. When the ceramic ware was viewed from directly above, a high intensity reddish purple color was observed, and when viewed from an oblique direction, a high intensity orange or yellow-green color was observed, confirming that this is an angle-dependent coloring characteristic of structural colors. Magnified images of the powder obtained by evaporating the solvent of the dispersion A', observed with a scanning electron microscope at 2,000 and 20,000 magnifications, are shown in Figures 10(a) and (b).

It was found that the microstructure is made up of inorganic particles of several hundred nanometers that are bonded together at points or surfaces to form aggregates. By bonding together inorganic particles of highly uniform particle size, the particles maintain their periodic structure without being rearranged by external forces, making it possible to create and maintain a highly ordered structure, which is thought to enable high brightness coloring due to the multiple reflections of light that are characteristic of structural colors, and angle-dependent coloring due to the formation of a periodic structure in an oblique direction.

[Example 6]
Four types of dispersions E to H prepared under the conditions of the raw material blending amounts shown in Table 4 were applied to white paper with a brush so as to draw a straight line. The appearance after application is shown in FIG. 11. Although the color was faint, a sparkling color was observed at a specific observation angle that satisfied the condition of light reinforcement, as in the case of application to the white ceramic in Example 3. When observed from directly above in the perpendicular direction to the applied surface, the color was blue for dispersion E, blue-green for F, green for G, and yellow-green for H. When the coating film of dispersion E was observed obliquely, it appeared red, and when observed from an even lower angle, it appeared yellow. Dispersion F also changed from blue-green to purple and then to red at a low angle, and dispersion G changed from green to blue and then to purple at a low angle. In this way, angle-dependent color development was confirmed.

0.3 g of black paint (Sakura Matte Watercolor Multi Black #49, manufactured by Sakura Color Products Co., Ltd.) was added to 2 g of each of the four dispersions used in Example 6 to prepare dispersions E' to H'. Dispersions E' to F' were each applied to white paper in the same manner as in Example 6. Figure 12 shows the appearance after application. In the region of regular reflection, the color was highly luminous, like that of a jewel, and was different from that of conventional pigments whose color development principle is light absorption. When observed from a vertical direction, dispersion E was blue, F was blue-green, G was green, and H was yellow-green, and when observed from an oblique angle, the color changed in the same way as in Example 6.

When observed from a low angle, dispersion H' was also confirmed to have a highly brilliant purple color. As in Example 4, where the dispersion was applied to ceramics, the black particles added to the dispersion absorbed visible light over a wide wavelength range as a background color, and as a result, the contrast between the structural color due to the periodic structure created by the inorganic fine particles and the color of the base was emphasized, which is thought to have made the structural color clearer than in Example 6.

The dispersion of the present invention is composed of inorganic fine particles and unreacted alkoxide components, and can form a coating film that exhibits structural colors that are excellent in heat resistance, mechanical strength, hardness, abrasion resistance, chemical stability, corrosion resistance, and weather resistance. The coloring is high brightness and angle-dependent, which are characteristic of structural colors, and can be achieved even when additives such as black particles are added. In particular, by using inorganic fine particles with a particle size of 375 nm or more, it is possible to achieve a variety of colors that change color depending on the slight difference in the angle of light, and it is also possible to apply a concentric rainbow design.

By adding dark colored particles as a background color, such as black particles, the structural color can be clearly expressed even when applied to a bright substrate. In particular, when inorganic fine particles are applied to plate-shaped particles, the color characteristic of structural color can be prominently expressed. When applied to a bright substrate without adding dark colored particles, the appearance is like a sparkling jewel. It is also possible to achieve these effects by using inorganic fine particles such as silicon oxide and aluminum oxide, which are rare metal-free, inexpensive, hazardous to the human body, and have a low environmental impact.

The dispersion of the present invention can be produced simply by mixing general-purpose liquid raw materials at room temperature, requiring only a small initial investment in production equipment, and the production costs, including raw materials, are low, making it safe and environmentally friendly. Furthermore, the decorative product of the present invention can continuously express the high brightness coloring and angle-dependent coloring characteristic of structural colors in any desired area, providing an aesthetic appearance not previously seen.

That is, the dispersion of the present invention, its manufacturing method, and decorative articles using the same can be used in cosmetics, tableware, vases, cooking utensils, writing implements, tools, paper, woodwork products, plastic products, rubber products, clothes, bags, shoes, hats, bags, household appliances, and other daily necessities and parts thereof, jewelry, accessories, watches, and other decorative articles and parts thereof, mobile phones, smartphones, and other portable information and communication devices, and display panels, housings and parts, furniture components such as tables, desks, and shelves for electrical appliances, building materials and fixtures such as roofs, walls, floors, windows, and doors, components for housing facilities such as baths, toilets, kitchens, and washbasins, frames, bodies, and interior and exterior parts of transportation equipment such as bicycles, motorcycles, automobiles, trains, and airplanes, stone processed products such as monuments, lanterns, and gravestones, ceramics and crafts, and are expected to contribute to the development of industry in various industrial fields.

Claims (8)

A dispersion liquid containing substantially spherical inorganic fine particles of silicon oxide and an unreacted component in which a part of a silicon alkoxide, which is a raw material of the inorganic fine particles, is unreacted and exists in a liquid state,
The number average particle diameter of the primary particles of the inorganic fine particles is in the range of 375 to 550 nm,
a dispersion in which aggregates formed by the inorganic fine particles being arranged in a substantially regular pattern exhibit a structural color due to the number average diameter of primary particles of the inorganic fine particles, particle size distribution, and a volume ratio of the unreacted components to the inorganic fine particles. 2. The dispersion according to claim 1 , wherein the volume ratio of the unreacted components to the inorganic fine particles is in the range of 6 to 30%. The dispersion according to claim 1 or 2, further comprising dark color particles. The dispersion according to claim 3, wherein the dark particles are plate-shaped. 5. A method for producing a dispersion liquid according to claim 1, comprising the steps of: dissolving a silicon alkoxide raw material in advance in a liquid containing water; and adding a reaction accelerator thereto to precipitate substantially spherical inorganic fine particles of silicon oxide ; and preparing a dispersion liquid in which the inorganic fine particles and a portion of the alkoxide raw material are unreacted and contain unreacted components present in a liquid state, and the number average diameter of primary particles of the inorganic fine particles is in the range of 375 to 550 nm . The method for producing the dispersion liquid according to claim 5 further includes a step of preparing coated particles by applying a dispersion liquid containing inorganic fine particles and unreacted components to the surface of the base particles, and a step of dispersing the coated particles in the liquid. A composite material comprising: an aggregate of inorganic fine particles formed from the dispersion liquid according to any one of claims 1 to 4 and a bonding phase comprising an unreacted component; and a substrate serving as a base;
The inorganic fine particles are arranged in a generally regular pattern on the surface of the substrate to form an aggregate, and some or all of the inorganic fine particles forming the aggregate are bonded by the bonding phase, and the decorative article has an area that exhibits a structural color derived from the particle size and regular arrangement of the inorganic fine particles. 5. A method for producing a decorative article having an area exhibiting a structural color resulting from the particle size and regular arrangement of inorganic microparticles, the method comprising the steps of coating a part or all of a surface of a substrate with the dispersion liquid according to claim 1 and drying the coating to form an aggregate on the surface of the substrate in which inorganic microparticles are arranged in a generally regular pattern.