Plate-shaped alumina powder, method for producing same, and paint or cosmetic

Abstract

Provided are: a plate-shaped alumina powder which is suppressed in sparkling (sparkling sensation) having a star point feeling even if the particle diameter is the same as that of conventional plate-shaped alumina powder; and a method for producing the plate-shaped alumina powder. The plate-shaped alumina powder is composed of a plurality of plate-shaped alpha alumina particles, and has an average particle diameter of 2-100 [mu] m as determined by SEM observation, and an average thickness of 0.2-3.0 [mu] m as determined by SEM observation. Wherein the plate-shaped alpha alumina particles are polycrystalline, and the average value of the ratio (L2 / L1) of the total length (L2) of grain boundaries to the circumference (L1) of the plate-shaped alpha alumina particles on the plate surface of the plate-shaped alpha alumina particles is 0.30-2.00 (inclusive).

Description

Technical Field

[0001] This invention relates to plate-shaped alumina powder and its manufacturing method, as well as coatings or cosmetics. Background Technology

[0002] Alpha-alumina (Al2O3) is chemically stable, exhibiting excellent heat resistance, corrosion resistance, wear resistance, and insulation properties, as well as high strength and hardness. Utilizing these characteristics, alpha-alumina powder is widely used in various applications, including structural components, tools, abrasives, fillers, spark plugs, insulators, electronic substrates, and refractory materials.

[0003] In particular, plate-shaped alumina powder has high light reflectivity and high brightness on its particle surface. Therefore, it is used as a pigment added to coatings and cosmetics, taking advantage of this property. Plate-shaped alumina powder is a powder composed of multiple plate-shaped alumina particles, and is also known as flake-shaped alumina powder, thin-film alumina powder, flat alumina powder, or alumina flakes.

[0004] Patent Document 1 discloses the application of plate-shaped alumina powder as a pigment, and discloses an Al2O3 sheet having a thickness of more than 500 nm and a density of 15-30 μm. 50 Value and D 30-45μm 90 The Al2O3 sheet possesses high chemical stability, a smooth surface, and high whiteness, and is used as a pigment substrate (claims 1, paragraphs

[0010] and

[0015] of Patent Document 1). Furthermore, Patent Document 1 also describes the Al2O3 sheet being coated with a high-refractive-index layer such as TiO2 or a low-refractive-index layer such as SiO2, thereby imparting higher gloss, interference color, or color-changing effects (paragraphs

[0040] to

[0046] of Patent Document 1).

[0005] Patent Document 2 discloses a coating composition containing a glossy pigment, which includes alumina flakes as the glossy pigment, and describes its suitability for use as an automotive topcoat, which forms an unprecedentedly strong gloss and a novel, aesthetically pleasing composite coating film without compromising the final appearance (claims 1,

[0001] and

[0108] of Patent Document 2). Furthermore, Patent Document 2 also describes the alumina flakes as being formed by coating alumina (Al2O3) with a metal oxide such as titanium dioxide, having a particle size of 10–30 μm and a thickness of 0.3–0.4 μm (paragraph

[0007] of Patent Document 2).

[0006] Patent Document 3 discloses a pearlescent pigment containing flake-shaped alumina crystals. These flake-shaped alumina crystals contain alumina and zinc oxide as main components in a mass ratio of 100:0.1 to 5, and are coated with metal or metal precursor particles (claim 1 of Patent Document 3). Furthermore, Patent Document 3 also describes that the average particle thickness of the crystals is less than 0.5 μm, the average particle size is more than 15 μm, and the aspect ratio is more than 50, thus exhibiting excellent gloss (paragraph

[0001] of Patent Document 3).

[0007] Patent Document 4 discloses a method for manufacturing plate-shaped alumina-based powder by calcining plate-shaped boehmite at 400–1500°C. This plate-shaped alumina-based powder is characterized by having only α-, β-, or γ-alumina crystal structures, or having two or more crystal structures (claim 2 of Patent Document 4). Furthermore, Patent Document 4 also describes coating plate-shaped boehmite or plate-shaped alumina powder with a hydrophobic compound such as polysiloxane, and then incorporating the coated powder into cosmetics, thereby obtaining cosmetics with a pleasant user experience (paragraph

[0013] of Patent Document 4).

[0008] Patent Document 5 discloses a hexagonal plate-shaped alumina obtained by firing hexagonal plate-shaped boehmite at a temperature of 450 to 1500°C. The hexagonal plate-shaped alumina is characterized by being approximately hexagonal in shape, having a major-to-minor axis ratio of 1 to 1.3, and an aspect ratio of 40 to 100. It is also described as having high orientation, reduced diffuse reflection, and further increased gloss. It is also suitable for use as a filler for gloss purposes in coatings and cosmetics (claims 4 and

[0045] of Patent Document 5).

[0009] Patent Document 6 discloses a plate-shaped alumina particle characterized by a thickness of 0.01–5 μm, an average particle size of 0.1–500 μm, a particle size-to-thickness ratio (length-to-width ratio) of 2–500, a polygonal plate shape, and the presence of molybdenum within the particle (claim 1 of Patent Document 6). Furthermore, Patent Document 6 also describes the applicability of this plate-shaped alumina particle to thermally conductive fillers, cosmetics, abrasives, high-gloss pigments, lubricants, the matrix of conductive powders, ceramic materials, etc. (paragraph

[0101] of Patent Document 6). Existing technical documents Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2014-218424 Patent Document 2: Japanese Patent Application Publication No. 10-298458 Patent Document 3: Japanese Patent Publication No. 2010-502774 Patent Document 4: Japanese Patent Application Publication No. 2012-071996 Patent Document 5: Japanese Patent Application Publication No. 2003-002642 Patent Document 6: Japanese Patent Application Publication No. 2019-123664 Summary of the Invention The technical problem that the invention aims to solve

[0011] Therefore, although the improvement of the glossiness of plate-shaped alumina powder has been under investigation, there is still room for improvement in existing plate-shaped alumina powders. That is, increasing the particle size of the powder is effective in utilizing the high reflectivity of plate-shaped alumina powder.

[0012] However, if the particle size is too large, the shimmering (sparkling) effect becomes stronger. Here, "sparkling" refers to the localized presence of bright areas (highlights) within the powder. For example, in cases where the brightness produced by the particles constituting plate-like alumina powder varies greatly depending on the particle orientation (the angle between the particle and the viewing direction), the brightness produced by specific areas increases. This shimmering (sparkling) effect is sometimes undesirable depending on the application. Especially in cosmetic applications, pigments with a shimmering effect tend to be avoided.

[0013] In view of the above-mentioned problems, the inventors conducted in-depth research. As a result, it was found that, compared with existing plate-shaped alumina powders, plate-shaped alumina powders with specific average particle size and average thickness and specific polycrystalline state, even with the same particle size, have suppressed the scintillation (flickering) effect with a star-like appearance.

[0014] This invention is based on the insight that the technical problem it aims to solve is to provide a plate-shaped alumina powder and its manufacturing method, which, compared with existing plate-shaped alumina powders, suppresses the star-like shimmering (sparkling) effect even with the same particle size. means of solving technical problems

[0015] The present invention includes the forms described in (1) to (8) below. Furthermore, in this specification, the expression “~” includes the numerical values ​​at both ends. That is, “X~Y” is synonymous with “X or more and Y or less”.

[0016] (1) A plate-shaped alumina powder, comprising multiple plate-shaped α-alumina particles, wherein the average particle size, as determined by SEM observation, is greater than 2 μm and less than 100 μm, and the average thickness, as determined by SEM observation, is greater than 0.2 μm and less than 3.0 μm, wherein... The plate-shaped α-alumina particles are polycrystalline. On the plate surface of the plate-shaped α-alumina particles, the average value of the ratio (L2 / L1) of the total length of the grain boundaries (L2) to the perimeter (L1) of the plate-shaped α-alumina particles is 0.30 or more and 2.00 or less.

[0017] (2) The plate-shaped alumina powder as described in (1) above, wherein the average value of L2 / L1 is 0.50 or more and 1.50 or less.

[0018] (3) The plate-shaped alumina powder as described in (1) or (2) above, wherein the average particle size is 5 μm or more and 50 μm or less, and the average thickness is 0.3 μm or more and 1.0 μm or less.

[0019] (4) A method for manufacturing plate-shaped alumina powder, which is any one of the plate-shaped alumina powder manufacturing methods of (1) to (3) above, comprising: The process for preparing a raw material mixture containing aluminum hydroxide powder and additives, and The process of firing the raw material mixture at a temperature between 1000°C and 1300°C for less than 20 hours. The raw material mixture comprises, in the following amounts: alkali metal (AM) equivalent to 0.01% to 0.5% by mass (AM2O), silicon (Si) equivalent to 0.1% to 0.3% by mass (SiO2), and fluorine (F) equivalent to 0.1% to 5.0% by mass (F). The heating rate during the firing process of the raw material mixture is 50°C / hour or more and 150°C / hour or less.

[0020] (5) The method as described in (4) above, wherein the additive comprises alkali metal silicon fluoride (AM2SiF6), or aluminum fluoride (AlF3) and silicon oxide (SiO2).

[0021] (6) The method as described in (5) above, wherein the additive further comprises one or both of an oxide of an alkali metal (AM) (AM2O) and a carbonate (AM2CO3).

[0022] (7) Any of the methods described in (4) to (6) above, wherein the alkali metal (AM) is one or both of sodium (Na) and potassium (K).

[0023] (8) A coating or cosmetic comprising any one of the plate-shaped alumina powders described in (1) to (3) above. Invention Effects

[0024] According to the present invention, a plate-shaped alumina powder and a method for manufacturing the same are provided. Compared with existing plate-shaped alumina powders, the plate-shaped alumina powder has suppressed the shimmering (flickering) effect of star-like particles, even with the same particle size. Attached Figure Description

[0025] Figure 1 The mechanism by which the glossiness of plate-shaped alumina powder is manifested is shown. Figure 2 The diagram illustrates light scattering through a polycrystalline material. Figure 3 The surface of the plate-like particles is shown schematically. Detailed Implementation

[0026] The specific embodiments of the present invention (hereinafter referred to as "this embodiment") will be described below. However, the present invention is not limited to the following embodiments, and various modifications can be made without departing from the spirit of the present invention. In addition, in this specification, any combination of preferred methods can be adopted as long as technical integration can be achieved. For example, one of the appropriate numerical ranges can be arbitrarily combined with another.

[0027] <<1. Plate-shaped alumina powder>> The plate-shaped alumina powder (hereinafter sometimes simply referred to as "alumina powder") of this embodiment is composed of multiple plate-shaped α-alumina (Al2O3) particles (hereinafter sometimes simply referred to as "alumina particles," "plate-shaped particles," or "particles"). That is, it contains plate-shaped α-alumina particles as the main component. Here, the plate-shaped α-alumina particles are particles composed of α-alumina (corundum). α-alumina has trigonal corundum crystals, which have excellent chemical stability, heat resistance, corrosion resistance, wear resistance, and insulation properties, as well as high strength and hardness. In addition, it has high light reflectivity, high brightness, and high whiteness. Since the alumina powder of this embodiment contains plate-shaped α-alumina particles as the main component, these properties can be fully utilized.

[0028] In addition, in this specification, "powder" refers to an aggregate of a large number of independent particles. That is, a large number of particles aggregate together to form powder. Since each particle constituting powder is independent, the powder as a whole exhibits fluidity. Most of the particles constituting powder need to be independent. As long as the powder as a whole exhibits fluidity, some particles can also combine to form aggregates. Furthermore, an aggregate of a large number of particles dispersed in a medium such as a liquid or resin is also called powder.

[0029] The alumina particles constituting the plate-shaped alumina powder of this embodiment are plate-shaped (flaky, sheet-like, flat, or lamellar). That is, each particle has a large plate surface and a small plate thickness. Specifically, the average particle size of the plate-shaped alumina powder is 2 μm or more and 100 μm or less, and the average thickness is 0.2 μm or more and 3.0 μm or less. Here, the average particle size and average thickness are averages of the number of particles constituting the alumina powder, based on the particle size and thickness. Furthermore, particle size refers to the major axis of the particle. That is, the major axis diameter on the plate surface of the particle is the particle size. Furthermore, thickness refers to the plate thickness. The particle size and thickness can be determined by observing the particles constituting the alumina powder using a scanning electron microscope (SEM). That is, the average particle size and average thickness are values ​​determined by SEM observation. In addition, the average particle size is usually greater than the average thickness.

[0030] In this way, by using particles with large surface areas and small thicknesses, the gloss of the alumina powder can be improved. This is used for... Figure 1 To illustrate. For example Figure 1 As shown, when powder containing particles with large surface areas and small thicknesses is formed onto a substrate by methods such as coating, the large particle surfaces are easily aligned parallel to the substrate surface. Therefore, the larger the particle size, the larger the surface area of ​​the particle surfaces aligned parallel to the substrate surface. Furthermore, light incident from the outside is reflected off the particle surfaces and emitted as reflected light. Thus, the larger the particle size, the easier it is for incident light to be reflected off the particle surfaces, resulting in an increased visual brightness.

[0031] If the average particle size is less than 2 μm or the average thickness is greater than 3.0 μm, the powder may lack luster. On the other hand, powders with an average particle size greater than 100 μm or an average thickness less than 0.2 μm are difficult to manufacture. Moreover, they have low strength, making handling difficult. Preferably, the plate-shaped alumina powder has an average particle size of 5 μm or more and 50 μm or less, and an average thickness of 0.3 μm or more and 1.0 μm or less.

[0032] The alumina powder is preferably selected with an average aspect ratio of 20 or higher and 50 or lower. Here, the average aspect ratio is the ratio of average particle size to average thickness (average particle size / average thickness). By setting the average aspect ratio to 20 or higher, the gloss of the powder can be further improved. In addition, by setting the average aspect ratio to 50 or lower, the strength of the alumina powder is improved, and the handling becomes easier.

[0033] Furthermore, the particles constituting the alumina powder do not necessarily all have to be plate-shaped α-alumina particles. As long as the alumina powder as a whole meets the aforementioned necessary conditions for average particle size and average thickness, it may also contain particles other than α-alumina particles. However, in order to take advantage of the superior effects brought by plate-shaped α-alumina particles, a high content of plate-shaped α-alumina particles is preferred. The content of plate-shaped α-alumina particles is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.

[0034] The plate-shaped α-alumina particles constituting the alumina powder of this embodiment are polycrystalline. That is, each particle is not composed of a single crystal, but rather of multiple grains (or crystalline particles). In each particle, adjacent grains are connected by grain boundaries. Furthermore, on the plate surface of the plate-shaped α-alumina particles, the average value of the ratio (L2 / L1) of the total length (L2) of the grain boundaries to the perimeter (L1) of the plate-shaped α-alumina particles is 0.30 or more and 2.00 or less. In this way, the alumina particles are composed of polycrystalline particles, and the ratio of their grain boundary length to perimeter (L2 / L1) is limited to a specified range, thereby suppressing the scintillation (flickering) effect that gives a star-like appearance.

[0035] This is illustrated using a schematic diagram of light scattering through a polycrystalline material. Figure 2 Let me explain. When alumina particles are polycrystalline, the crystal structure within each grain is uniform, and the refractive index is consistent. Therefore, light passing through the grains travels straight without scattering. On the other hand, at grain boundaries, the crystal structure is disordered, sometimes becoming amorphous. Therefore, the refractive index changes at grain boundaries. As a result, light does not travel straight at grain boundaries but is scattered in all directions. Consequently, reflected light is reduced, suppressing flicker. Conversely, when alumina particles are monocrystalline, scattered light is suppressed while reflected light is enhanced, resulting in excessively strong flicker.

[0036] The shimmering (flickering) effect, reminiscent of starlight, can be controlled by adjusting the proportion of grain boundaries in each particle. This proportion can be estimated using the ratio (L2 / L1) of the total length of the grain boundaries (L2) to the perimeter of the particle (L1). Here, as... Figure 3 As shown, the perimeter of the particle (L1) is the perimeter of the plate-like particle's surface. The total grain boundary length (L2) is the total length of the grain boundaries exposed on the plate-like particle's surface. Furthermore, Figure 3 The plate-like surface of the plate-like particles is schematically shown. Figure 3 In the diagram, L1 is represented by a solid line, and L2 by a dashed line. Furthermore, L2 / L1 is the ratio of the total length (L2) of the grain boundaries of all particles to their perimeter (L1), and the average value of L2 / L1 refers to the average value of the ratio (L2 / L1) of multiple particles.

[0037] When the average value of L2 / L1 is 0.30 or higher and 2.00 or lower, the proportion of grain boundaries becomes suitable, achieving both high gloss and suppressed flicker. Conversely, if the average value of L2 / L1 is less than 0.30, the proportion of grain boundaries is insufficient, thus the effect of reducing flicker caused by light scattering becomes inadequate. On the other hand, if the average value of L2 / L1 is greater than 2.00, the proportion of grain boundaries becomes too large, resulting in increased scattered light, reduced reflected light, and loss of gloss. From the viewpoint of further achieving both high gloss and suppressed flicker simultaneously, the average value of L2 / L1 is preferably 0.40 or higher and 1.80 or lower, more preferably 0.50 or higher and 1.50 or lower.

[0038] Furthermore, whether plate-shaped α-alumina particles are polycrystalline can be investigated by observing the particles using SEM. The average L2 / L1 ratio can also be determined by observing the plate-shaped alumina powder using SEM. Specifically, when observing the particle surface or cross-section using SEM, grain boundaries are observed, thus confirming that the plate-shaped α-alumina particles are polycrystalline. The ratio of L1 to L2 can be used to determine the ratio of the grain boundary length on the particle surface to the particle's perimeter, which is then used to evaluate polycrystallineity.

[0039] In this embodiment, the plate-shaped alumina powder preferably has an intensity ratio of diffraction peaks based on α-alumina in the powder X-ray diffraction (XRD) pattern obtained in the oriented state within a specified range. Specifically, the intensity ratio of the diffraction peaks based on the α-alumina (006) plane is I. A The intensity I of the diffraction peak based on the α-alumina (113) plane B The ratio (I) A / I B The preferred value is 0.3 or higher and 10 or lower. The intensity I based on the diffraction peak of the α-alumina (104) plane... C The intensity I of the diffraction peak based on the α-alumina (113) plane B The ratio (I) C / I B The preferred value is 4.0 or higher and 10 or lower. Furthermore, the intensity I based on the diffraction peak of the α-alumina (116) plane... D The intensity I of the diffraction peak based on the α-alumina (113) plane B The ratio (I) D / I B The preferred value is 3.0 or higher and 7.5 or lower. The intensity I based on the diffraction peak of the α-alumina (018) plane... E The intensity I of the diffraction peak based on the α-alumina (113) plane B The ratio (I) E / I B The preferred value is 1.0 or higher and 7.5 or lower. The intensity I based on the diffraction peak of the α-alumina (1010) plane...F The intensity I of the diffraction peak based on the α-alumina (113) plane B The ratio (I) F / I B The preferred value is 5.0 or higher and 20.0 or lower.

[0040] The (006) plane of α-alumina corresponds to the crystal plane (c-plane) perpendicular to the c-axis of the α-alumina crystal. Furthermore, the (104), (116), (018), and (1010) planes of α-alumina are all crystal planes with small angles to the c-plane, i.e., similar to the c-plane. The plate-shaped alumina powder of this embodiment is composed of plate-shaped α-alumina particles with large plate surfaces and small plate thicknesses. Therefore, when the plate-shaped alumina powder is filled into the measuring support to obtain a powder XRD pattern, the particles are oriented so that the plate surfaces of the plate-shaped α-alumina particles align with the measuring face. Therefore, in the XRD pattern of the plate-shaped alumina powder, diffraction peak intensities (I0) based on the c-plane (plate surface) or similar planes, i.e., the (006), (104), (116), (018), and / or (1010) planes, are clearly observed. A I C I D I E and I F ).

[0041] Furthermore, the XRD pattern in the oriented state can be obtained as follows. The sample is placed on a glass sample plate, pressed to flatten the sample surface, and then measured using an XRD measuring device. Since the sample is a plate-shaped particle, when filling the powder sample, the surface relative to the sample plate is easily aligned with a specific crystal axis, resulting in a selective orientation effect. If the plate-shaped alumina powder is pulverized when preparing the sample for measurement, the shape of the powder will change, which may impair the selective orientation effect. Therefore, the particle size is not adjusted for measurement. In addition, in this specification, peak intensity refers to peak area (integral intensity). Furthermore, when using CuKα as the X-ray source, the diffraction peak based on the α-alumina (006) plane appears at 2θ = 41.7 ± 0.5°. Similarly, the diffraction peak based on the (113) plane appears at 2θ = 43.3 ± 0.5°. The diffraction peak based on the (104) plane appears at 2θ = 35.1 ± 0.5°. The diffraction peak based on the (116) plane appears at 2θ = 57.5 ± 0.5°. The diffraction peak based on the (018) plane appears at 2θ = 61.3 ± 0.5°. The diffraction peak based on the (1010) plane appears at 2θ = 76.9 ± 0.5°.

[0042] The alumina powder of this embodiment preferably contains an alkali metal (AM) in an amount of 0.01% to 0.5% by mass (equivalent to AM2O) and a silicon (Si) in an amount of 0.01% to 0.3% by mass (equivalent to SiO2). The alkali metal (AM) content is more preferably 0.01% to 0.3% by mass (equivalent to AM2O). The silicon (Si) content is more preferably 0.01% to 0.2% by mass (equivalent to SiO2). It is further preferred that the alkali metal (AM) is one or both of sodium (Na) and potassium (K). By including these components, the glossiness of the alumina powder can be further improved. It is particularly preferred that it contains sodium (Na) in an amount of 0.01% to 0.5% by weight (equivalent to Na2O). This allows for more effective suppression of the shimmering effect of the alumina powder.

[0043] The alumina powder of this embodiment preferably contains at least aluminum (Al), alkali metal (AM), silicon (Si), and oxygen (O). However, other components (elements other than Al, AM, Si, and O) may be included as long as the above requirements are met. Furthermore, the presence of impurities inevitably introduced during the manufacturing process is permissible. Fluorine (F) is an example of such another component. Fluorine (F) is a component derived from additives added during the manufacture of alumina powder. That is, as described later, the additives added during the manufacture of alumina powder contain fluorine (F). During the firing process, most of the fluorine volatilizes, but sometimes a portion remains. The amount of fluorine in the alumina powder is typically 0.5% by mass or less, 0.3% by mass or less, or 0.1% by mass or less.

[0044] If alumina powder contains a large amount of other components (elements other than Al, AM, Si, and O), its glossiness may be compromised. Therefore, the content of these other components is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 1% by mass or less, particularly preferably 0.5% by mass or less, and most preferably 0.1% by mass or less. Iron (Fe), in particular, has a coloring effect on alumina powder, so its content should be kept low. Preferably, the iron (Fe) content, converted to iron oxide (Fe₂O₃), is 0.05% by mass or less.

[0045] The specific surface area (S) of the alumina powder in this embodiment BET The preferred value is 0.1m. 2 / g or more and 5.0m 2 / g or less, more preferably 0.3m 2 / g or more and 2.0m 2 Below / g. If the specific surface area is too large, the particle size of the alumina powder will become smaller, which may reduce the gloss. On the other hand, alumina powder with a small specific surface area is difficult to manufacture.

[0046] The alumina powder of this embodiment exhibits high chemical stability due to its primary component being α-alumina. Furthermore, it possesses a large average particle size and excellent dispersibility. Alumina powder with these characteristics is suitable for use in coatings and cosmetics. However, the alumina powder of this embodiment is not limited to coatings and cosmetics. It is also suitable for use as a reinforcing or barrier material in plastic or resin films, or in known applications such as structural components, tools, abrasives, fillers, spark plugs, insulators, electronic substrates, and refractories.

[0047] <2. Method for manufacturing plate-shaped alumina powder> The manufacturing method of the plate-shaped alumina powder of this embodiment is not limited as long as the above-mentioned requirements are met. However, it is preferred to manufacture it in the following order. A suitable manufacturing method includes: a step of preparing a raw material mixture containing aluminum hydroxide powder and additives (raw material mixing step), and a step of firing the raw material mixture at a temperature in the range of 1000°C to 1300°C for less than 20 hours (firing step). The raw material mixture contains, in the following amounts: alkali metal (AM) equivalent to 0.01% to 0.5% by mass (calculated as AM2O), silicon (Si) equivalent to 0.1% to 0.3% by mass (calculated as SiO2), and fluorine (F) equivalent to 0.1% to 5.0% by mass (calculated as F). The heating rate in the firing step of the raw material mixture is 50°C / hour or more and 150°C / hour or less. In addition, a post-processing step may be provided after the firing step if necessary.

[0048] The manufacturing method of this embodiment is particularly characterized by controlling the composition of the raw material mixture and the firing conditions within a specific range while using aluminum hydroxide powder as a raw material. Based on this characteristic, it is possible to obtain aluminum oxide powder with desired size and polycrystalline state. As a result, it is possible to obtain aluminum oxide powder that maintains high gloss while suppressing the shimmering (sparkling) effect. Each step is described in detail below.

[0049] <Raw Material Mixing Process> In the raw material mixing process, a raw material mixture containing aluminum hydroxide powder and additives is adjusted. Aluminum hydroxide is a compound represented by the chemical formula Al(OH)3, which is transformed into α-alumina (α-alumina) through heating and dehydration. Two types of aluminum hydroxide are known: gibbsite (γ-aluminum hydroxide) and boehmite (α-aluminum hydroxide). In this embodiment, either gibbsite or boehmite can be used. However, powder composed of thermodynamically stable gibbsite is preferred.

[0050] The size of the aluminum hydroxide powder used as a raw material is not particularly limited, as long as the alumina powder of this embodiment can be obtained. However, a volume average particle size (D) is preferred.50 The specific surface area is greater than 0.5 μm and less than 15 μm. Alternatively, the specific surface area (S) BET ) is 0.5m 2 / g or more and 20m 2 / g or less. Additionally, D 50 It is the cumulative 50% diameter in the volume-based particle size distribution determined by a laser diffraction and scattering particle size distribution measuring device. Additionally, S BET The value was obtained using the N2 gas adsorption method.

[0051] The additives are components that become liquid in the subsequent calcination process and act as mineralizers. The additives contain at least silicon (Si) and fluorine (F), and may also contain alkali metals (AM) if desired. As described later, when the composition of the liquid phase is properly controlled, the crystallization and grain growth of alumina particles proceed appropriately, resulting in alumina powder with the desired size and polycrystalline state.

[0052] Therefore, to obtain the desired alumina powder, it is important to properly control the composition of the raw material mixture containing additives. Specifically, the composition of the additives is adjusted so that the raw material mixture contains the following amounts: alkali metal (AM) equivalent to 0.01% to 0.5% by mass (calculated as AM2O), silicon (Si) equivalent to 0.1% to 0.3% by mass (calculated as SiO2), and fluorine (F) equivalent to 0.1% to 5.0% by mass (calculated as F). When the composition of the raw material mixture is within the above range, alumina powder with the desired size and polycrystalline state can be obtained, resulting in alumina powder that maintains high gloss while suppressing flicker. Conversely, an inappropriate composition of the raw material mixture may adversely affect the particle size and crystallinity.

[0053] The alkali metal (AM) content of the raw material mixture is more preferably 0.01% by mass and 0.5% by mass (calculated as AM2O). The alkali metal is also preferably one or both of sodium (Na) and potassium (K), with sodium (Na) being particularly preferred. The silicon (Si) content of the raw material mixture is more preferably 0.10% by mass and 0.20% by mass (calculated as SiO2). The fluorine (F) content of the raw material mixture is more preferably 0.1% by mass and 1.0% by mass (calculated as F). By limiting the composition of the raw material mixture within the above ranges, the polycrystalline state of the alumina powder can be controlled within a more preferred range.

[0054] The additive contains at least silicon (Si) and fluorine (F). Additionally, the additive may or may not contain alkali metals (AM). Aluminum hydroxide powder, used as a raw material, sometimes contains alkali metals (AM) as impurities. If the amount of alkali metal in the aluminum hydroxide powder is sufficient, the additive may be alkali metal-free. On the other hand, if the amount of alkali metal in the aluminum hydroxide powder is insufficient, it is preferable to add an alkali metal as an additive. Furthermore, the additive may contain aluminum (Al). If the additive contains aluminum, this aluminum will be incorporated into the alumina powder during the firing process.

[0055] As long as alumina powder with the desired size and polycrystalline state can be obtained, the additive may also contain components other than alkali metals (AM), silicon (Si), fluorine (F), aluminum (Al), and oxygen (O). However, excessive amounts of other components may adversely affect particle size and polycrystalline state. The content of other components is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and particularly preferably 1% by mass or less. The additive may be free of components other than alkali metals (AM), silicon (Si), fluorine (F), aluminum (Al), and oxygen (O) in amounts exceeding impurity levels.

[0056] According to a preferred embodiment, the additive comprises aluminum fluoride (AlF3) and silicon oxide (SiO2). Furthermore, when the amount of alkali metal in the aluminum hydroxide powder is insufficient, the additive preferably further comprises one or both of an oxide (AM) of an alkali metal (AM) (AM2O) and a carbonate (AM2CO3). According to another more preferred embodiment, the additive comprises an alkali metal silicon fluoride (AM2SiF6). By using appropriate amounts of these compounds (AlF3, SiO2, AM2O, AM2CO3, AM2SiF6) as additives, alumina powder with the desired size and polycrystalline state can be reliably produced. Moreover, these compounds are inexpensive and readily available. Therefore, plate-shaped alumina powder can be obtained more cost-effectively.

[0057] The preparation of the raw material mixture can be carried out by mixing aluminum hydroxide powder with additives. Mixing can be performed using known methods. It can be done dry or wet. Dry mixing can be performed using, for example, air mixers, V-type mixers, oscillating mixers, Henschel mixers, cone mixers, etc. In the case of wet mixing, a solvent such as water is added to the aluminum hydroxide powder and additives to form a slurry, and the resulting slurry is then mixed using a wet mixer such as a ball mill, atomizer, or bead mill.

[0058] <Firing Process> In the firing process, the obtained raw material mixture is fired at a temperature between 1000°C and 1300°C for less than 20 hours. This yields the fired product. During firing, aluminum hydroxide (Al(OH)3) dehydrates and transforms into α-alumina (Al2O3). Furthermore, the alkali metals (AM), silicon (Si), and fluorine (F) in the raw material mixture become liquid phases in oxide form, acting as mineralizing agents that promote the crystallization of α-alumina. That is, the crystal structure changes during the transformation from Al(OH)3 to α-alumina. By adding mineralizing agents, the crystallization of α-alumina (corundum) is promoted, thus enabling the low-temperature transfer process.

[0059] During firing, the mineralizing agent components (AM, Si, F) that become a liquid phase cover the surface of alumina particles, and some penetrate into the particle interior. This liquid phase component promotes particle diffusion and growth. Specifically, some aluminum atoms (Al) dissolve from the surface of the alumina particles into the liquid phase, and the dissolved aluminum precipitates in other parts of the particle surface. This causes the alumina particles to grow. At this time, depending on the composition of the liquid phase, the growth direction of the alumina particles varies, and the growth rate also changes. Therefore, if the composition of the liquid phase is appropriately adjusted, the plate-like growth of aluminum particles can be controlled, resulting in particles with high plate-like properties and the desired polycrystalline state. Furthermore, the liquid phase component also has the effect of smoothing the plate-like surface of the alumina particles. If the surface of the alumina particles is uneven, these uneven parts preferentially diffuse into the liquid phase, thus eliminating the unevenness. Therefore, if the composition of the liquid phase is appropriately controlled, alumina powder with the desired size and polycrystalline state and a smooth surface can be obtained.

[0060] If the firing temperature is below 1000°C, the transformation to α-alumina and grain growth may be insufficient. Therefore, it may be difficult to obtain alumina powder with excellent dispersibility. On the other hand, if the firing temperature exceeds 1300°C, the particles may sometimes sinter together and form strong sintered blocks. Furthermore, excessive energy consumption for firing may lead to increased manufacturing costs. The firing temperature is preferably 1000°C or higher and 1200°C or lower, more preferably 1100°C or higher and 1200°C or lower. The firing time is preferably 10 hours or more and less than 20 hours. This ensures reliable transformation to α-alumina and grain growth while preventing over-sintering of particles. The firing furnace is not limited as long as the desired alumina powder can be obtained. However, from the viewpoint of effectively utilizing the additives during firing, a stationary furnace using a closed firing vessel is preferred.

[0061] In the manufacturing method of this embodiment, the heating rate during the firing process is 50°C / hour or more and 150°C / hour or less. In controlling the polycrystalline state of alumina powder, not only the composition of the liquid phase and the firing temperature are important, but the heating rate during firing is also crucial. A higher (faster) heating rate results in a longer grain boundary length. Therefore, the ratio (L2 / L1) of the total grain boundary length (L2) to the perimeter (L1) of the alumina powder increases. Conversely, a lower (slower) heating rate results in a smaller ratio (L2 / L1). By adjusting the composition of the liquid phase and the firing temperature to a predetermined range, and by adjusting the heating rate to a predetermined range, the ratio (L2 / L1) can be controlled within a desired range. As a result, alumina powder with suppressed scintillation (flashing) and a star-like appearance can be obtained. Furthermore, the aforementioned heating rate is a heating rate within the temperature range of at least 25°C (room temperature) and below 1000°C.

[0062] Post-processing As needed, the calcined material obtained after the firing process can undergo post-treatment such as dealkali treatment, crushing, and / or grading. In the dealkali treatment, excess alkali metal components adhering to the surface of the calcined material are removed. This allows adjustment of the alkali metal content in the final alumina powder. Alkali metal components on the particle surface can be removed, for example, by washing and filtering the calcined material. In the crushing treatment, slight mechanical energy is applied to the calcined material to break the adhesion between the aggregated particles formed during firing. Crushing can be carried out using dry or wet crushers such as pot mills, pin mills, and / or jaw crushers. In the grading treatment, particles are screened according to size to obtain alumina powder of the desired particle size. Grading can be carried out by methods such as sieving, air classification, panning, and / or centrifugation. Post-treatment can be performed as needed. If the desired alumina powder can be obtained immediately after firing, post-treatment can be omitted.

[0063] Thus, the plate-shaped alumina powder of this embodiment can be produced. The resulting alumina powder has the desired size and polycrystalline state, and a smooth surface. Therefore, while maintaining a high gloss, the shimmering (sparkling) effect is also suppressed. Such alumina powder is suitable for various applications, such as coatings and cosmetics.

[0064] <<3. Coatings and Cosmetics>> The coating or cosmetic of this embodiment comprises the above-described plate-shaped alumina powder. The coating or cosmetic may comprise alumina powder in its pure form, or it may comprise alumina powder in a surface-treated form. Surface treatment can be performed by applying surface treatment agents such as silicon-based compounds, alkylsilane-based compounds, and / or fluorine compounds to the surface of the plate-shaped alumina powder particles. Furthermore, in addition to plate-shaped alumina powder, the coating and cosmetic may also contain solvents and resins. The solvent may be aqueous or non-aqueous. Additionally, the coating and cosmetic may also contain known additives other than oils, pigments, fillers, surfactants, viscosity modifiers, preservatives, fragrances, humectants, physiologically active ingredients, salts, chelating agents, neutralizing agents, and / or pH adjusters. Example

[0065] The invention is illustrated in more detail using the following examples and comparative examples. However, the invention is not limited to the following examples.

[0066] (1) Preparation of plate-shaped alumina powder [Examples 1-5 and Comparative Example 1] As the alumina source for Examples 1-3 and Comparative Example 1, aluminum hydroxide (gibbsite) powder (Nippon Light Metals Co., Ltd.) was prepared. Additionally, as the alumina source for Examples 4 and 5, another aluminum hydroxide (gibbsite) powder (Nippon Light Metals Co., Ltd.) was prepared. The characteristics of the prepared aluminum hydroxide powders are shown in Table 1 below. In Table 1, Na₂O represents the combined value of the dissolved sodium content on and near the crystalline surface of aluminum hydroxide and the insoluble sodium content introduced into the crystalline lattice; f-Na₂O represents only the dissolved sodium content on and near the crystalline surface of aluminum hydroxide.

[0067] In addition, aluminum fluoride (DO-FLUORIDECHEMICALS CO., LTD.; AlF3), silicon dioxide (Marukamado Ceramics Co., Ltd., Snow Brand Silica SP-3; SiO2), sodium carbonate (Kanto Chemical Co., Ltd.; Na2CO3) and sodium fluoride (Kanto Chemical Co., Ltd., Sodium Hexafluorosilicate; Na2SiF6) were prepared as additives.

[0068] Then, the prepared aluminum hydroxide powder was mixed with the additives to prepare a raw material mixture. The amount of additives was adjusted so that the content ratio of each additive in the raw material mixture was as shown in Table 1 below. Mixing was carried out by placing the raw materials in a bag and shaking them (manual method; Examples 1, 2, 4 and 5) or by using a oscillating mixer (mechanical method; Examples 3 and Comparative Example 1).

[0069] <Firing Process> The obtained raw material mixture is put into a firing container, and the firing container is covered with a special lid. The covered firing container is set in a firing furnace and fired in the atmosphere. The firing is carried out using an electric furnace or a shuttle kiln. During firing, the temperature is raised at a rate of 100 °C / hour to the firing temperature, and then held at this temperature for 10 hours (Examples 1 to 5) or 20 hours (Comparative Example 1), and then naturally cooled. The firing temperature is 1100 °C (Examples 1 to 5) or 1050 °C (Comparative Example 1). After the temperature in the furnace drops, the firing container is taken out of the furnace, and the fired product (aluminum oxide) is recovered.

[0070] <Post-treatment> The obtained fired product is post-treated. The crushing strength is adjusted so that the plate shape is not damaged, wet crushing is carried out, and a filtration treatment is implemented to obtain a crushed product (aluminum oxide). Subsequently, using a dryer, the obtained crushed product is dried at 105 °C for one night. Then, coarse and fine particles are removed after the crushing treatment.

[0071] Thus, plate-shaped aluminum oxide powders of Examples 1 to 5 and Comparative Example 1 were produced. Among them, the manufacturing conditions of the plate-shaped aluminum oxide powders are summarized in Table 1 below.

[0072] [Comparative Example 2] In Comparative Example 2, a commercially available plate-shaped aluminum oxide powder (Kinsai Mateck Co., Ltd., YFA10030) was purchased and its properties were evaluated.

[0073] Table 1

[0074] (2) Evaluation The plate-shaped aluminum oxide powders obtained in Examples 1 to 5, Comparative Examples 1 and 2 were used as samples, and various properties were evaluated as follows.

[0075] <Particle size distribution> The particle size distribution of the aluminum oxide powder was measured using a laser diffraction / scattering particle size distribution measuring device (Nikkiso Co., Ltd., Microtrack MT3300). Specifically, the aluminum oxide powder was directly put into the inlet of the measuring device, and after being dispersed for 1 minute using the dispersion function equipped in the device, the particle size distribution was measured. The average particle diameter (D 50 ) was obtained as the particle diameter corresponding to a cumulative particle size distribution rate of 50 vol%.

[0076] <SEM observation> Alumina powder was evaluated using a scanning electron microscope (JEOL Ltd., JSM-F100; SEM) and image analysis software (ImageJ). Specifically, SEM images of the alumina powder were captured, and ImageJ was used to evaluate the particles (plate-like α-alumina particles) in the images, measuring particle size and thickness, and calculating the average particle size and average thickness. The average particle size was calculated as the average of 80 particles, and the average thickness as the average of 30 particles. Then, the ratio of average particle size to average thickness (average particle size / average thickness) was calculated as the average aspect ratio.

[0077] Furthermore, electron backscatter diffraction (EBSD) was used to analyze the crystalline state of the alumina powder particles (plate-shaped α-alumina particles). Specifically, EBSD measurements were performed using a scanning electron microscope (JEOL Ltd., JSM-6490A; SEM) equipped with an electron microscope crystal orientation measurement device and analysis tool (TSL Solutions Ltd., OIM EBSD system) to observe the crystalline morphology of the alumina. The plate-like surfaces of the particles were observed, and the perimeter (L1), the total length of the grain boundaries (L2), and the major axis (L3) of the particles on the plate-like surfaces were measured. L2 / L1 and L2 / L3 were then calculated. The same operation was performed on 30 particles, and the average values ​​of L2 / L1 and L2 / L3 were obtained.

[0078] <Crystal phase> The crystal phase of alumina powder was investigated using an X-ray diffraction apparatus (RIGHU Corporation, RINT Ultima III). Specifically, the obtained alumina powder (sample) was first placed on a glass sample plate and pressed to flatten the sample surface and selectively orient it. Then, XRD analysis was performed using an X-ray diffraction apparatus. CuKα was used as the X-ray source. Next, the presence or absence of diffraction peaks based on α-alumina (corundum phase) was investigated in the obtained X-ray diffraction pattern.

[0079] Furthermore, for samples where diffraction peaks based on α-alumina were detected, the intensity (height) of the diffraction peaks based on the (006), (113), (104), (116), (018), and (1010) planes of α-alumina was calculated, which is I. A I B I C I D I E and I F Calculate the peak intensity ratio I A / I B I C / I B I D / IB I E / I B and I F / I B .

[0080] <Component Analysis> Compositional analysis was performed using a scanning X-ray fluorescence analyzer (Rigaku Corporation, ZSX Primus IV). Specifically, alumina powder was placed in a platinum crucible and then placed in a high-frequency melting apparatus. After preheating at 700°C for 60 seconds, the sample was shaken at 1200°C for 120 seconds to prepare glass beaded samples. The glass bead samples were then evaluated using a scanning X-ray analyzer. The obtained data were compared with the calibration curve using data from standard samples to obtain the evaluation results.

[0081] Specific surface area Specific surface area (S) BET The evaluation was performed using a manual surface area measuring device (Micromeritics Instrument Corp., FlowSorbIII 2305) via N2 adsorption.

[0082] <A twinkling effect (a sparkling, star-like quality)> The shimmering effect of the plate-shaped alumina powder was evaluated. Specifically, five healthy adults (three men and two women) were used as evaluators. A suitable amount (approximately 1 g) of plate-shaped alumina powder was applied to the back of their hands, and the powder was visually observed after application. Then, scores were assigned according to the evaluation criteria described below.

[0083] 3 points: The shimmering starlight effect (twinkling) is suppressed. 2 points: The sparkling star effect is slightly strong. 1 point: Strong sense of sparkling stars

[0084] Then, for each sample, the average of the scores from the above 5 evaluators was taken, and the average value (rounded to 2 decimal places) was used to evaluate the starlight (flickering sensation).

[0085] (3) Evaluation Results The evaluation results obtained for Examples 1-5, Comparative Examples 1 and 2 are summarized in Table 2 below.

[0086] In Examples 1-5, the average particle size and average thickness were both within the ranges specified in this embodiment (average particle size 2-100 μm, average thickness 0.2-3.0 μm). Furthermore, the particles constituting the powder were polycrystalline, and the average value of the ratio (L2 / L1) of the total length of the grain boundaries (L2) to the perimeter of the particles (L1) was within the range specified in this embodiment (0.30-2.0). Among these, Examples 1-3 showed higher evaluation results for the star-like appearance, ranging from 2.4 to 3.0.

[0087] In contrast, the average value of the ratio (L2 / L1) of Comparative Example 1 is small and does not meet the range (0.30 to 2.0) specified in this embodiment. Therefore, the evaluation result of star-like perception is as low as 1.2. Moreover, Comparative Example 2 is composed of single crystal particles.

[0088] Table 2

[0089] Based on the above results, it can be understood that this embodiment provides a plate-shaped alumina powder and its manufacturing method. Compared with existing plate-shaped alumina powders, even with the same particle size, the shimmering (sparkling) effect of the plate-shaped alumina powder is suppressed.

Claims

1. A plate-shaped alumina powder, comprising multiple plate-shaped α-alumina particles, wherein the average particle size, as determined by SEM observation, is greater than 2 μm and less than 100 μm, and the average thickness, as determined by SEM observation, is greater than 0.2 μm and less than 3.0 μm, wherein... The plate-shaped α-alumina particles are polycrystalline. On the plate surface of the plate-shaped α-alumina particles, the average value of the ratio (L2 / L1) of the total length of the grain boundaries (L2) to the perimeter (L1) of the plate-shaped α-alumina particles is 0.30 or more and 2.00 or less.

2. The plate-shaped alumina powder as described in claim 1, wherein, The average value of L2 / L1 is above 0.50 and below 1.

50.

3. The plate-shaped alumina powder as described in claim 1 or 2, wherein, The average particle size is 5 μm or more and 50 μm or less, and the average thickness is 0.3 μm or more and 1.0 μm or less.

4. A method for manufacturing plate-shaped alumina powder, which is the method for manufacturing plate-shaped alumina powder according to claim 1 or 2, comprising: The process for preparing a raw material mixture containing aluminum hydroxide powder and additives, and The process of firing the raw material mixture at a temperature between 1000°C and 1300°C for less than 20 hours. The raw material mixture comprises, in the following amounts: alkali metal (AM) equivalent to 0.01% to 0.5% by mass (AM2O), silicon (Si) equivalent to 0.1% to 0.3% by mass (SiO2), and fluorine (F) equivalent to 0.1% to 5.0% by mass (F). The heating rate during the firing process of the raw material mixture is 50°C / hour or more and 150°C / hour or less.

5. The method of claim 4, wherein, The additives include alkali metal silicon fluoride (AM2SiF6), or aluminum fluoride (AlF3) and silicon oxide (SiO2).

6. The method of claim 5, wherein, The additive also contains one or both of alkali metal (AM) oxides (AM2O) and carbonates (AM2CO3).

7. The method of claim 4, wherein, The alkali metal (AM) is one or both of sodium (Na) and potassium (K).

8. A coating or cosmetic comprising the plate-shaped alumina powder as described in claim 1 or 2.

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