High-luminance scaly α-alumina powder and method for producing same

Abstract

Provided is a scaly α-alumina powder having a large average particle diameter and aspect ratio as well as excellent perceived luminosity, and a production method capable of obtaining the scaly alumina powder by a simple technique. This high-luminance scaly α-alumina powder has an average particle diameter of 20 μm to 200 μm, an average thickness of 0.20 μm to 2.00 μm, and an average aspect ratio of 50 to 600. Also, a perceived luminosity measurement reveals that the luminosity intensity Si at a measurement angle θ of 15° and 45° falls in the range from 1.00 to 15.00.

Description

High-brightness flaky α-alumina powder and method for producing the same

[0001] The present invention relates to a high-brightness flaky α-alumina powder and a method for producing the same.

[0002] α-alumina (Al ２ O ３ ) is chemically stable and has excellent heat resistance, corrosion resistance, wear resistance, and insulating properties, as well as high strength and hardness. Taking advantage of these characteristics, α-alumina powder is widely used in a variety of applications, such as structural members, tools, abrasives, fillers, spark plugs, insulators, electronic substrates, and refractories.

[0003] In particular, scaly α-alumina powder has a high light reflectance on its particle surface, resulting in high brightness. Therefore, by taking advantage of this characteristic, it is used as a high-brightness pigment added to paints and cosmetics. In addition, in pigment applications, the scaly α-alumina powder has been coated with TiO on its particle surface. ２ The coating causes light interference, and it is said that this interference can be used to increase gloss.

[0004] Furthermore, flaky α-alumina powder has high strength and excellent gas barrier properties. For this reason, it is used as a reinforcing material or gas barrier material added to plastics and resin films. Flaky α-alumina powder is a powder composed of flaky α-alumina particles, and is also called flaky alumina powder, flat alumina powder, plate-like alumina powder, or alumina flakes.

[0005] Patent Document 1 describes a method for forming a thin film having a thickness of 500 nm or more and a D of 15 to 30 μm. ５０ value and D of 30 to 45 μm ９０ Al having a value ２ O ３ The Al flakes are disclosed. ２ O ３ It is described that the flakes have high chemical stability, a smooth surface, and high whiteness at the same time, and are used as a pigment substrate (Patent Document 1, claims 1,

[0010] and

[0015] ). ２ O ３ The flakes are TiO ２High refractive index layers such as SiO ２ It is described that the glass is coated with a low refractive index layer such as a fluorine-containing film, thereby imparting increased gloss, interference color, or color flop effect (Patent Document 1, paragraphs

[0040] to

[0046] ).

[0006] Patent Document 2 discloses a luster pigment-containing paint composition that contains alumina flakes as a luster pigment, and describes that it can be suitably used as an automotive top coat paint, and that it forms a composite coating film with an unprecedented high luster and novel high designability without deteriorating the finished appearance, etc. (Claim 1,

[0001] and

[0108] of Patent Document 2). Patent Document 2 also describes that the alumina flakes are aluminum oxide (Al ２ O ３ ) coated with a metal oxide such as titanium dioxide, and has a particle size of 10 to 30 μm and a thickness of 0.3 to 0.4 μm (Patent Document 2,

[0007] ).

[0007] Patent Document 3 discloses a pearlescent pigment containing flaky alumina crystals that contain aluminum oxide 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). Patent Document 3 also describes that the crystals have an average particle thickness of 0.5 μm or less, an average particle size of 15 μm or more, and an aspect ratio of 50 or more, and therefore have excellent gloss (paragraph

[0001] of Patent Document 3).

[0008] Patent Document 4 discloses a method for producing a plate-like alumina powder characterized by having a crystal structure of only α-, β-, or γ-alumina, or two or more types of crystal structures, by firing plate-like boehmite at 400°C to 1500°C (claim 2 of Patent Document 4). Patent Document 4 also describes that a cosmetic product with a good feel when used can be obtained by surface-coating plate-like boehmite or plate-like alumina powder with a hydrophobic compound such as polysiloxane and incorporating the coated powder into a cosmetic product (paragraph

[0013] of Patent Document 4).

[0009] Patent Document 5 discloses hexagonal plate-like alumina obtained by firing hexagonal plate-like boehmite at a temperature of 450 to 1500°C, which has a substantially hexagonal plate shape, a ratio of the major axis to the minor axis of 1 to 1.3, and an aspect ratio of 40 to 100. It also describes that the hexagonal plate-like alumina has high orientation, reduces diffused reflection, and enhances brilliance, and can be suitably used as a filler for the purpose of increasing the brilliance of paints and cosmetics (claims 4 and

[0045] of Patent Document 5).

[0010] Patent Document 6 discloses plate-like alumina particles characterized by having a thickness of 0.01 to 5 μm, an average particle diameter of 0.1 to 500 μm, an aspect ratio (the ratio of particle diameter to thickness) of 2 to 500, a polygonal plate shape, and containing molybdenum within the particles (Claim 1 of Patent Document 6). Patent Document 6 also describes that the plate-like alumina particles can be suitably used for thermally conductive fillers, cosmetics, abrasives, high luster pigments, lubricants, substrates for conductive powders, ceramic materials, etc. (Refer to

[0101] of Patent Document 6).

[0011] Japanese Patent Application Laid-Open No. 2014-218424 Japanese Patent Application Laid-Open No. 10-298458 Japanese Patent Application Laid-Open No. 2010-502774 Japanese Patent Application Laid-Open No. 2012-071996 Japanese Patent Application Laid-Open No. 2003-002642 Japanese Patent Application Laid-Open No. 2019-123664

[0012] Thus, the use of flaky α-alumina powder as a high-brightness pigment has been proposed in the fields of paints, cosmetics, and the like. However, the inventors' investigations have revealed that there is room for improvement in the conventional techniques. That is, it is desirable to enhance the brilliance of alumina powder when used as a high-brightness pigment. However, conventional flaky α-alumina powders have limitations in terms of increasing the reflectance of the particle surface.

[0013] In particular, it has been difficult to increase the average particle size and aspect ratio of conventional scaly alumina powder. That is, the larger the average particle size and aspect ratio, the larger the plate surface area of ​​the flat particles. Therefore, even if the surface reflectance is the same, the larger the plate surface area, the more excellent the brilliance of the alumina powder that can be obtained. However, the average particle size and aspect ratio of conventional alumina powder are not sufficiently large, and there are limitations in increasing the brilliance.

[0014] Furthermore, as proposed in Patent Documents 1 to 3, techniques have been proposed to improve glossiness by utilizing the light interference effect of inorganic coating, but these techniques do not allow for inexpensive production of flaky α-alumina powder. ２ In this case, it is necessary to separately form an inorganic coating such as a fluorine-containing compound, which may increase the manufacturing cost.

[0015] In view of these conventional problems, the present inventors have conducted extensive research and have found that scaly α-alumina powder can be obtained by a simple method of firing a raw material mixture containing scaly boehmite powder and a predetermined composition of additives, and that the obtained scaly α-alumina powder has a large average particle size and aspect ratio and is excellent in brilliance.

[0016] The present invention was completed based on these findings, and an object of the present invention is to provide a flaky α-alumina powder that has a large average particle size and aspect ratio and is excellent in brilliance, and a production method by which this flaky α-alumina powder can be obtained by a simple method.

[0017] The present invention encompasses the following aspects (1) to (13). In this specification, the expression "to" includes both the numerical values ​​at both ends. In other words, "X to Y" is synonymous with "X or more and Y or less." In addition, in this specification, any combination of suitable aspects can be adopted as long as technical consistency can be achieved. For example, one of the suitable numerical ranges can be combined with the other.

[0018] (1) A high-brightness flaky α-alumina powder having an average particle size of 20 μm or more and 200 μm or less, an average thickness of 0.20 μm or more and 2.00 μm or less, and an average aspect ratio of 50 or more and 600 or less, and in a brilliance measurement, the brilliance intensity Si value at measurement angles θ of 15° and 45° is in the range of 1.00 or more and 15.00 or less.

[0019] (2) The high-brightness flaky α-alumina powder according to (1) above, wherein the average particle size is 20 μm or more and 120 μm or less, and the average thickness is 0.20 μm or more and 0.50 μm or less.

[0020] (3) Alkali metal (AM) ２ O in an amount of 0.2 mass % or more and 5.0 mass % or less, and silicon (Si) is contained in the amount of SiO ２ The high-brightness flaky α-alumina powder according to (1) or (2) above, containing 0.3% by mass or more and 10.0% by mass or less in terms of the amount of α-alumina converted into α-alumina.

[0021] (4) The high-brightness flaky α-alumina powder according to (3) above, wherein the alkali metal (AM) is sodium (Na) and / or potassium (K).

[0022] (5) The high-brightness flaky α-alumina powder according to any one of (1) to (4) above, wherein the average particle size is 50 μm or more and 120 μm or less.

[0023] (6) The high-brightness flaky α-alumina powder according to any one of (1) to (5) above, wherein the brilliance intensity Si value is in the range of 5.00 to 15.00.

[0024] (7) A method for producing a high-brightness flaky α-alumina powder according to any one of (1) to (6) above, comprising: a step of adding an additive to flaky boehmite powder and then mixing them to prepare a raw material mixture; and a step of firing the raw material mixture at a temperature in the range of 1000°C to 1300°C, wherein the raw material mixture contains an alkali metal (AM) as the AM. ２ 0.5 mass % or more and 10.0 mass % or less in terms of O, silicon (Si) is SiO ２ and fluorine (F) in an amount of 0.1 mass% or more and 10.0 mass% or less, calculated as a carbon black equivalent, and 0.1 mass% or more and 5.0 mass% or less.

[0025] (8) The method according to (7), wherein the scaly boehmite powder has an average particle size of 2 μm or more and 15 μm or less and an average aspect ratio of 20 or more and 60 or less.

[0026] (9) The additive is aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ The method according to (7) or (8), comprising:

[0027] (10) The additive is an oxide of an alkali metal (AM) ２ O) and / or carbonate (AM ２ CO ３ The method according to (9), further comprising:

[0028] (11) The additive is an alkali silicofluoride (AM ２ SiF ６ The method according to (7) or (8), comprising:

[0029] (12) The method according to any one of (7) to (11) above, wherein the alkali metal (AM) is sodium (Na) and / or potassium (K).

[0030] (13) A paint or cosmetic product containing the high-brightness flaky α-alumina powder according to any one of (1) to (6) above.

[0031] According to the present invention, there are provided a flaky α-alumina powder having a large average particle size and aspect ratio and excellent brilliance, and a method for producing this flaky α-alumina powder by a simple technique.

[0032] FIG. 1 shows the mechanism by which a brilliance feeling is expressed by a scaly α-alumina powder. FIG. 2 is a drawing provided for explaining a method for evaluating a brilliance feeling. FIG. 3 is a drawing provided for explaining a method for evaluating a brilliance feeling. FIG. 4 shows an SEM image of a scaly α-alumina powder (Example 1). FIG. 5 shows an SEM image of a scaly α-alumina powder (Example 3). FIG. 6 shows an SEM image of a scaly α-alumina powder (Example 3).

[0033] A specific embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described. Note that the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the present invention.

[0034] <<1. High-brightness flaky α-alumina powder>> The high-brightness flaky α-alumina powder of this embodiment (hereinafter, may be simply referred to as “α-alumina powder” or “alumina powder”) is an α-alumina (Al ２ O ３ ) as a main component. α-alumina has trigonal corundum crystals and is excellent in chemical stability, heat resistance, corrosion resistance, wear resistance, and insulation, as well as in strength and hardness. It also has high light reflectance, brightness, and whiteness. The alumina powder of this embodiment can fully utilize these characteristics. In this specification, powder refers to an aggregate of many independent particles. In other words, many particles aggregate to form the powder. Since each particle constituting the powder is independent, the powder as a whole exhibits fluidity. It is sufficient that the majority of the particles constituting the powder are independent. As long as the powder as a whole exhibits fluidity, some particles may be bonded to form aggregates.

[0035] Furthermore, the alumina powder of this embodiment may contain other components as long as it contains α-alumina as the main component. However, to maximize the effects of α-alumina, a higher α-alumina content is preferable. The α-alumina content is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. The α-alumina content can be evaluated by determining the amount of components in the alumina powder by X-ray fluorescence analysis.

[0036] The alumina powder of this embodiment is composed of particles that have a flaky (thin, flat, or plate-like) shape. That is, each particle has a large plate surface and a small plate thickness. Specifically, the alumina powder has an average particle diameter of 20 μm to 200 μm, an average thickness of 0.20 μm to 2.00 μm, and an average aspect ratio of 50 to 600. Here, the average particle diameter and average thickness are the number-based average values ​​of the particle diameter and thickness of each particle that constitutes the alumina powder. The particle diameter refers to the longest diameter of the particle. The thickness refers to the plate thickness of the particle surface. The average aspect ratio refers to the ratio of the average particle diameter to the average thickness (average particle diameter / average thickness). The particle diameter, thickness, and aspect ratio can be determined by observing the particles that constitute the alumina powder with a scanning electron microscope (SEM).

[0037] Increasing the particle diameter (long diameter) and aspect ratio makes it possible to enhance the brilliance of alumina powder. This point will be explained using Figure 1. As shown in Figure 1, when powder with a high aspect ratio is formed on a substrate by a method such as coating, the large-area particle plate surfaces are likely to be oriented so that they are parallel to the substrate surface. The larger the particle diameter, the greater the surface area of ​​the particle plate surfaces that are parallel to the substrate surface. Furthermore, light incident from the outside is reflected by the particle plate surfaces and emitted to the outside as reflected light. Therefore, the larger the particle diameter and aspect ratio, the more easily incident light is reflected by the particle plate surfaces, resulting in a higher visual brilliance.

[0038] The larger the particle size and aspect ratio, the higher the brilliance of the alumina powder. If the average particle size is less than 20 μm, the average thickness is more than 2.00 μm, or the average aspect ratio is less than 50, the brilliance of the alumina powder is insufficient. On the other hand, if the average particle size is more than 200 μm, the average thickness is less than 0.20 μm, or the average aspect ratio is more than 600, it is difficult to produce. Furthermore, the strength is low, making handling difficult. The average particle size is preferably 50 μm or more and 120 μm or less, more preferably 70 μm or more and 120 μm or less, and even more preferably 90 μm or more and 120 μm or less. The average thickness is preferably 0.20 μm or more and 0.40 μm or less, and more preferably 0.20 μm or more and 0.30 μm or less. Preferably, the average particle size is 20 μm or more and 120 μm or less, and the average thickness is 0.20 μm or more and 0.50 μm or less. The average aspect ratio is preferably 150 or more and 600 or less, and more preferably 250 or more and 600 or less.

[0039] It is not necessary for all particles constituting the alumina powder to be flaky, and the alumina powder may contain particles having shapes other than flaky, as long as the alumina powder as a whole satisfies the above-mentioned requirements for the average particle size, average thickness, and average aspect ratio.

[0040] The high-brightness scaly α-alumina powder of this embodiment has a brilliance intensity Si value in the range of 1.00 to 15.00 in the brilliance measurement at measurement angles θ of 15° and 45°. Here, the brilliance intensity Si is a value determined in the brilliance evaluation described below, and is an index showing the intensity of brilliance based on the bright spots of the sample (alumina powder). The higher the brilliance intensity Si value, the higher the brilliance of the alumina powder. If the brilliance intensity Si value is less than 1.00, the brilliance of the alumina powder is insufficient. Furthermore, alumina powder with a brilliance intensity Si value exceeding 15.00 is difficult to produce. The brilliance intensity Si value is preferably 5.00 to 15.00, and more preferably 7.00 to 15.00.

[0041] Preferably, the luminous area Sa value at measurement angles θ of 15° and 45° is 1.00 or more and 30.00 or less. The luminous area Sa is a value determined when evaluating luminous feeling, and is an index showing the size of each bright spot of the sample (alumina powder). The luminous area Sa value is more preferably 10.00 or more and 30.00 or less, and even more preferably 15.00 or more and 30.00 or less.

[0042] The brilliance of alumina powder can be evaluated using a multi-angle colorimeter, brilliance, and particle size analyzer (BYK, BYK-mac i). The evaluation method is explained using Figures 2 and 3. To perform the measurement, first, a sample (alumina powder) is placed level in the sample holder attached to the instrument. Then, the sample holder containing the sample is placed in the instrument and the measurement is performed. The instrument is equipped with a CCD chip on an extension of the vertical axis relative to the measurement surface of the sample holder. A light source is also installed at a predetermined measurement angle θ from the vertical axis. As shown in Figure 2, during measurement, light is incident from the light source toward the sample, reflected and scattered by the measurement surface, and then incident on the CCD chip. The light intensity of the reflected and scattered light incident on the CCD chip is measured and analyzed to obtain image data. As shown in Figure 3, numerous bright spots corresponding to the individual particles constituting the sample (alumina powder) are observed in the image data. The obtained image data can be analyzed and quantified to determine the brilliance intensity Si and brilliance area Sa.

[0043] The high-brightness flaky α-alumina powder of this embodiment preferably contains an alkali metal (AM). ２ O in an amount of 0.2 mass % or more and 5.0 mass % or less, and silicon (Si) is contained in the amount of SiO ２ The alkali metal (AM) content is 0.3 mass % or more and 10.0 mass % or less in terms of AM. ２ The silicon (Si) content is more preferably 0.2 mass % or more and 2.0 mass % or less, and even more preferably 0.2 mass % or more and 1.0 mass % or less, calculated as O. ２The alkali metal (AM) is preferably at least 0.5% by mass or more and at most 3.0% by mass or less in terms of the total mass of the alumina powder. The alkali metal (AM) is preferably at least one selected from the group consisting of sodium (Na) and potassium (K). The inclusion of these components further enhances the brilliance of the alumina powder.

[0044] To explain this point, as will be described later, alkali metals (AM) and silicon (Si) are components contained in the raw material mixture during the production of alumina powder, and become liquid during firing. When the composition of the liquid phase components is appropriately controlled, crystallization, grain growth, and surface smoothing of alumina particles are promoted. This makes it possible to obtain alumina powder with a large average particle size and aspect ratio, smooth particle surfaces, and excellent brilliance.

[0045] In addition, the alkali metal oxides (AM) contained in the alumina powder ２ O) and silicon oxide (SiO ２ ) itself has the effect of enhancing the brilliance of the alumina powder. That is, the liquid phase components generated during the firing process during the production of alumina powder segregate as a heterogeneous phase such as glass on the particle surface and / or inside the alumina powder after production. The heterogeneous phase (glass phase, etc.) has a different refractive index from alumina. Therefore, it is speculated that the presence of such a heterogeneous phase intensifies the reflection and scattering of light incident from the outside, resulting in an enhanced brilliance.

[0046] In this way, by controlling the average particle size and aspect ratio and appropriately adjusting the amounts of alkali metal (AM) and silicon (Si), it is possible to further enhance the brilliance of the alumina powder. On the other hand, if the amounts of alkali metal and silicon are inappropriate, the brilliance may be reduced.

[0047] The high-brightness flaky α-alumina powder of this embodiment contains at least aluminum (Al), alkali metal (AM), silicon (Si), and oxygen (O) as essential components (elements). However, as long as the above-mentioned requirements are met, other components (elements) besides the essential components (Al, AM, Si, O) may be contained. Furthermore, the presence of impurities inevitably mixed in during the manufacturing process is permitted. One such other component is fluorine (F). Among these, fluorine (F) is a component derived from additives added during the manufacturing of the alumina powder. That is, as described below, the additives added during the manufacturing of the alumina powder contain fluorine (F). Most of the fluorine volatilizes during the firing process, but some may remain. The fluorine content 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.

[0048] Furthermore, if other components are contained in large amounts, the brilliance of the alumina powder may be impaired. Therefore, the content of components other than the essential components (Al, AM, Si, O) 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. In particular, iron (Fe) has the effect of coloring the alumina powder, so it is preferable to reduce its content. Preferably, the content of iron (Fe) is less than that of iron oxide (Fe ２ O ３ ) is 0.05 mass % or less.

[0049] The specific surface area (S ＢＥＴ ) is 0.1 m ２ / g or more 5.0m ２ / g or less is preferable, and 0.5m ２ / g or more 2.0m ２ / g or less is more preferable. If the specific surface area is too large, the particle size of the alumina powder will be small, which may result in a decrease in the sense of brilliance. On the other hand, it is difficult to produce a flaky α-alumina powder with a small specific surface area.

[0050] The high-brightness flaky α-alumina powder of this embodiment is preferably composed of a polycrystalline body. That is, each particle constituting the alumina powder is in a polycrystalline state. By constituting it as a polycrystalline body, it is possible to further enhance the brilliance of the alumina powder. Although the details of the reason for this are unknown, it is thought that the essential components (AM, Si, O) combine to form silicate (AM). ２ SiO ３ ) is generated, which promotes fusion and adhesion between alumina particles, resulting in polycrystallization.

[0051] The high-brightness flaky α-alumina powder of this embodiment has high chemical stability because it is primarily composed of α-alumina. It also has a large average particle size and aspect ratio, resulting in excellent brilliance. α-alumina powder with these characteristics is suitable for use in the fields of paints and cosmetics. However, the α-alumina powder of this embodiment is not limited to use in paints and cosmetics. It can be used in known applications such as reinforcing materials and gas barrier materials to be mixed into plastics and resin films, structural members, tools, abrasives, fillers, spark plugs, insulators, electronic substrates, and refractories.

[0052] <<2. Manufacturing Method of High-Brightness Scaly α-Alumina Powder>> The manufacturing method of the high-brightness scaly α-alumina powder of this embodiment is not limited as long as it satisfies the above-mentioned requirements. However, it is preferably manufactured by the following procedure. A suitable manufacturing method includes a step of adding additives to scaly boehmite powder and then mixing them to prepare a raw material mixture (raw material mixing step), and a step of firing this raw material mixture at a temperature in the range of 1000°C to 1300°C (firing step). If necessary, a post-treatment step may be provided after the firing step. The raw material mixture is prepared by adding an alkali metal (AM) to the AM. ２ 0.5 mass % or more and 10.0 mass % or less in terms of O, silicon (Si) is SiO ２ The amount of fluorine (F) is 0.1 mass % or more and 10.0 mass % or less in terms of the total mass of the carbon black, and 0.1 mass % or more and 5.0 mass % or less in terms of the total mass of the carbon black. Each step will be described in detail below.

[0053] <Raw Material Mixing Step> In the raw material mixing step, additives are added to scaly boehmite powder (hereinafter sometimes simply referred to as "boehmite powder") and then mixed to prepare a raw material mixture. The scaly boehmite powder is a scaly powder whose main component is boehmite (AlOOH), which is alumina monohydrate, and is an aluminum source for alumina powder. By using the scaly boehmite powder as a raw material, alumina powder with a large average particle size and aspect ratio can be obtained. That is, in the subsequent firing step, the boehmite transforms into α-alumina via intermediate alumina. When the scaly boehmite powder is used as a raw material, the particle shape of the scaly boehmite powder is retained, and therefore scaly α-alumina powder can be obtained.

[0054] The size of the boehmite powder is not particularly limited as long as the alumina powder of this embodiment can be obtained. However, preferably, the average particle diameter is 2 μm or more and 15 μm or less, and the average aspect ratio is 20 or more and 60 or less. The alumina powder of this embodiment can be easily obtained by using boehmite powder having such a size. Furthermore, methods for producing boehmite powder having such a size are publicly known and readily available. For example, Patent Document 5 discloses a method for producing hexagonal plate-shaped boehmite by hydrothermally treating aluminum hydroxide and boric acid with the addition of a pH adjuster such as sodium hydroxide. The obtained hexagonal plate-shaped boehmite is described as having an outer diameter of 0.7 to 15.0 μm and an aspect ratio of 40 to 100 (see claims 2,

[0017] and

[0018] of Patent Document 5).

[0055] The additives are components that become liquid in the subsequent firing process and act as mineralizers. The additives contain at least silicon (Si) and fluorine (F), and may further contain an alkali metal (AM) as needed. As described below, when the composition of the liquid phase components is appropriately controlled, crystallization, grain growth, and surface smoothing of the alumina particles are promoted, resulting in an alumina powder with a large average particle size and aspect ratio, a smooth particle surface, and excellent brilliance. Furthermore, the liquid phase components exist as a different phase, such as glass, in the alumina powder after production, further enhancing the brilliance.

[0056] Therefore, in order to obtain an alumina powder having a large average particle size and aspect ratio and excellent brilliance, it is important to appropriately control the composition of the raw material mixture containing additives. ２ 0.5 mass % or more and 10.0 mass % or less in terms of O, silicon (Si) is SiO ２ The composition of the additives is adjusted so that the total amount of fluorine (F) is 0.1% by mass or more and 10.0% by mass or less, calculated as a percentage of fluorine (F), and 0.1% by mass or more and 5.0% by mass or less. When the composition of the raw material mixture is within the above-mentioned ranges, an alumina powder with a large average particle size and aspect ratio and excellent brilliance can be obtained. On the other hand, if the composition of the raw material mixture is inappropriate, the brilliance of the alumina powder may be reduced.

[0057] The amount of alkali metal (AM) in the raw material mixture is AM ２ The content of silicon (Si) in the raw material mixture is preferably 0.5 mass % or more and 5.0 mass % or less, and more preferably 0.5 mass % or more and 3.0 mass % or less, calculated as O. The alkali metal is preferably sodium (Na) and / or potassium (K), that is, at least one selected from the group consisting of sodium (Na) and potassium (K). The silicon (Si) content of the raw material mixture is preferably 0.5 mass % or more and 5.0 mass % or less, and even more preferably 0.5 mass % or more and 3.0 mass % or less, calculated as O. ２ The fluorine (F) content in the raw material mixture is preferably 0.1% by mass or more and 5.0% by mass or less, and more preferably 1.0% by mass or more and 3.0% by mass or less, calculated as a percentage of the total mass of the alumina powder. The fluorine (F) content in the raw material mixture is more preferably 0.1% by mass or more and 5.0% by mass or less, and even more preferably 1.0% by mass or more and 3.0% by mass or less. By limiting the composition of the raw material mixture to the above-mentioned range, it is possible to further enhance the brilliance of the resulting alumina powder.

[0058] The additive contains at least silicon (Si) and fluorine (F). The additive may or may not contain an alkali metal (AM). The raw boehmite powder may contain an alkali metal (AM) as an impurity. If the amount of alkali metal in the boehmite powder is sufficient, the additive may not contain an alkali metal. However, if the amount of alkali metal in the boehmite 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 is incorporated into the alumina powder during the firing process.

[0059] As long as an alumina powder with a high aspect ratio and brilliance is obtained, the additive may contain components other than alkali metals (AM), silicon (Si), fluorine (F), aluminum (Al), and oxygen (O). However, if the amount of other components is excessively large, it may be difficult to obtain an alumina powder with a high brilliance. Therefore, 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 does not need to contain components other than alkali metals (AM), silicon (Si), fluorine (F), aluminum (Al), and oxygen (O) in amounts exceeding the impurity amount.

[0060] According to a preferred embodiment, the additive is aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ In addition, when the amount of alkali metal in the boehmite powder is insufficient, the additive contains an oxide of alkali metal (AM) (AM ２ O) and / or carbonate (AM ２ CO ３ ), that is, oxide (AM ２ O) and carbonate (AM ２ CO ３ In another preferred embodiment, the additive further comprises at least one selected from the group consisting of alkali silicofluorides (AM ２ SiF ６ These compounds (AlF ３ , SiO ２, A.M. ２ O, A.M. ２ CO ３ , A.M. ２ SiF ６ By using an appropriate amount of flaky alumina powder as an additive, it is possible to reliably produce alumina powder with a large average particle size and aspect ratio and excellent brilliance. Furthermore, these compounds are inexpensive and easily available. Therefore, flaky alumina powder can be obtained more inexpensively.

[0061] The boehmite powder and the additives may be mixed by a known method. The mixing may be performed in a dry or wet manner. Dry mixing may be performed using a dry mixer such as an air blender, a V-type blender, a rocking blender, a Henschel mixer, or a Nauta mixer. In the case of wet mixing, a solvent such as water is added to the boehmite powder and the additives to form a slurry, and the resulting slurry may be mixed using a wet mixer such as a ball mill, an attritor, or a bead mill.

[0062] <Firing step> In the firing step, a raw material mixture containing scaly boehmite powder and additives is fired at a temperature in the range of 1000°C to 1300°C to obtain a fired product. During firing, boehmite (AlOOH) is converted into a metastable phase of γ-Al ２ O ３ The high-temperature stable phase α-alumina (Al ２ O ３ During the transformation, the particle shape is maintained, so it is possible to obtain scaly α-alumina (Al ２ O ３ During the firing, the alkali metals (AM), silicon (Si) and fluorine (F) in the raw material mixture become liquid oxides and act as mineralizers that promote the crystallization of α-alumina. ２ O ３ During the transition from α-Al to α-Al, the crystal structure changes from a cubic close-packed structure to a hexagonal close-packed structure, and oxygen atoms in the crystal are rearranged. ２ O ３ The transformation to α-Al requires the addition of a certain amount of thermal energy. ２ O ３Since the crystallization of the crystalline structure (corundum structure) is promoted, the transition temperature can be lowered.

[0063] Furthermore, the mineralizer components (AM, Si, F) that become liquid during firing cover the alumina particle surface and partially penetrate into the particle interior. This liquid-phase component promotes particle diffusion and growth. Specifically, some aluminum atoms (Al) from the alumina particle surface dissolve into the liquid phase, and the dissolved aluminum precipitates elsewhere on the particle surface. This causes alumina particle growth. Depending on the composition of the liquid-phase component, the growth direction of the alumina particles varies, as does the growth rate. Therefore, if the composition of the liquid-phase component is properly controlled, the growth of alumina particles toward the plate surface progresses significantly. The liquid-phase component also has the effect of smoothing the plate surface of the alumina particles. If the alumina particle surface is uneven, these uneven portions preferentially diffuse into the liquid phase, eliminating the uneven portions. Therefore, if the composition of the liquid-phase component is properly controlled, it is possible to obtain alumina powder with a large average particle size and aspect ratio, smooth particle surfaces, and excellent brilliance.

[0064] If the firing temperature is below 1000°C, the transition to α-alumina and grain growth may be insufficient. This may make it difficult to obtain α-alumina powder with excellent brilliance. On the other hand, if the firing temperature exceeds 1300°C, particles may sinter together, forming a strong sintered mass. Furthermore, excessive energy consumption for firing may result, leading to increased production costs. The firing temperature is preferably 1000°C or higher and 1200°C or lower, and more preferably 1000°C or higher and 1100°C or lower. Furthermore, the firing time is preferably 10 hours or higher and 20 hours or lower. This ensures the transition to α-alumina and grain growth while preventing excessive particle sintering. The firing furnace is not limited as long as the desired alumina powder is obtained. However, from the viewpoint of effectively utilizing the effects of additives during firing, a stationary furnace capable of firing using a sealed firing container is preferred.

[0065] <Post-treatment> If necessary, the fired product obtained through the firing step may be subjected to post-treatments such as soda removal, crushing, and / or classification. In the soda removal treatment, excess alkali metal components adhering to the surface of the fired product are removed. This allows the alkali metal content of the final α-alumina powder to be adjusted. The alkali metal components on the particle surfaces can be removed, for example, by washing and filtering the fired product. In the crushing treatment, mild mechanical energy is applied to the fired product to break up the bonds between agglomerated particles formed during firing. The crushing treatment can be carried out in a dry or wet manner using a crusher such as a pot mill, pin mill, and / or jaw crusher. In the classification treatment, particles are separated according to size to obtain alumina powder of the desired particle size. The classification treatment can be carried out by techniques such as sieving, air classification, elutriation, and / or centrifugation. Post-treatment can be carried out as needed. If the desired alumina powder can be obtained after firing, post-treatment may be omitted.

[0066] In this manner, the scaly α-alumina powder of the present embodiment can be produced. The resulting alumina powder has a large average particle size and aspect ratio, and is excellent in brilliance. Therefore, it can be suitably used in a variety of applications, including paints and cosmetics.

[0067] <<3. Paints and Cosmetics>> The paints or cosmetics of this embodiment contain the high-brightness scaly α-alumina powder described above. The paints or cosmetics may contain the alumina powder alone or in a surface-treated form. The surface treatment may be achieved by providing a surface treatment agent such as a silicon-based compound, an alkylsilane-based compound, and / or a fluorine-based compound on the particle surfaces of the scaly α-alumina powder. The paints and cosmetics may also contain a solvent or resin in addition to the scaly α-alumina powder. The solvent may be aqueous or non-aqueous. The paints and cosmetics may also contain known additives such as oils, pigments other than the scaly α-alumina powder, fillers, surfactants, viscosity modifiers, preservatives, fragrances, moisturizers, physiologically active ingredients, salts, chelating agents, neutralizing agents, and / or pH adjusters.

[0068] The present invention will be described in more detail using the following examples, but the present invention is not limited to the following examples.

[0069] (1) Preparation of Alumina Powder [Example 1] <Raw Material Mixing Step> Scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160) was prepared as an alumina source. Aluminum fluoride (DO-FLUORIDE CHEMICALS CO., LTD.; AlF ３ ) and silicon oxide (Marukama Kamado Toryo Co., Ltd. Snow Brand Silica SP-3; SiO ２ The properties of the prepared scaly boehmite powder are shown in Table 1 below.

[0070] Next, 50 g of the prepared scaly boehmite powder and aluminum fluoride (AlF ３ ): 1.2 g, and silicon oxide (SiO ２ The raw material mixture was obtained by mixing 0.4 g of sodium (Na) in a bag and shaking the bag. ２ 0.7 mass% in terms of O, silicon (Si) is SiO ２ The carbon content was 0.8 mass % in terms of carbon dioxide and 1.6 mass % of fluorine (F).

[0071] <Firing step> The obtained raw material mixture was placed in a crucible (Nikkato Corporation, SSA-H 500 mL), and the crucible was closed with a dedicated lid. ２ O ３ : 95%, SiO ２ The composition was 3%. The covered crucible was then placed in a firing furnace (Siliconite Corporation, box-type Siliconite electric furnace, BSH-3570) and fired in the atmosphere. During firing, the temperature was raised to 1100°C at a rate of 100°C / hour, and then held at 1100°C for 10 hours, followed by natural cooling. After the temperature inside the furnace had completely dropped, the crucible was removed from the furnace and the fired product (alumina) was recovered.

[0072] <Soda removal step> The obtained calcined product (alumina) was subjected to a soda removal process. Specifically, 340 g of the calcined product was added to 7 L of pure water and mixed to form a slurry. The obtained slurry was then filtered to obtain an alumina residue. This removed the sodium content adhering to the calcined product.

[0073] <Crushing Step> The obtained alumina residue was subjected to a crushing treatment. Specifically, pure water was added again to the alumina residue, and the mixture was stirred to form a slurry. The obtained slurry was placed in a pot mill container with an internal volume of 10 L together with 15 kg of 1 mm diameter zirconia beads, and the pot mill container was rotated at a rotation speed of 60 rpm for 3 hours. Next, the slurry was removed from the pot mill container and subjected to a filtration treatment to obtain a crushed product (alumina). The obtained crushed product was then dried overnight at 105°C using a dryer.

[0074] <Classification step> The obtained crushed material was subjected to a classification process. Specifically, pure water was added to the crushed material and then stirred to form a slurry. Next, the obtained slurry was sieved using a resin mesh gyrosifter (Tokuju Kosakusho Co., Ltd., GS-A1H) with a mesh size of 25 μm. The sieving was carried out at a rotation speed of 60 rpm, and coarse particles with a size of more than 25 μm were removed. Thereafter, the sieved material from which the coarse particles had been removed was filtered using a SUS mesh with a mesh size of 12 μm, and fine particles with a size of less than 12 μm were removed. In this way, an alumina powder was produced.

[0075] [Example 2] In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160), 50 g of aluminum fluoride (AlF ３ ): 1.2 g, and silicon oxide (SiO ２ ): 0.6 g were mixed to obtain a raw material mixture. The obtained raw material mixture contained sodium (Na) in the form of Na ２ 0.7 mass% in terms of O, silicon (Si) is SiO ２ The alumina powder contained 1.2 mass% of fluorine (F) in terms of carbon dioxide equivalent and 1.6 mass% of fluorine (F). Furthermore, the alumina powder was produced in the same manner as in Example 1, without being subjected to a soda removal treatment or a crushing treatment, and instead being classified by the following procedure.

[0076] <Classification Step> The calcined product (alumina) obtained in the calcination step was subjected to elutriation using a sorting tube and particle size adjustment using a SUS mesh to remove extremely thick particles and fine particles. Specifically, pure water was added to 300 g of the calcined product and stirred to obtain a slurry with a concentration of 100 g / L. The resulting slurry was placed in a classification device (sorting tube), and pure water was added thereto at a flow rate of 1 L / min for 3 hours. The slurry containing the precipitate was then recovered from the classification device. Next, the slurry was manually sieved sequentially using a SUS mesh with a mesh size of 63 μm and a SUS mesh with a mesh size of 45 μm, and fine particles with a size less than 45 μm were removed from the recovered slurry. The residue obtained after manual sieving was dried overnight in a dryer.

[0077] [Example 3] In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160), 50 g of aluminum fluoride (AlF ３ ): 1.2 g, and silicon oxide (SiO ２ ): 0.8 g were mixed to obtain a raw material mixture. The obtained raw material mixture contained sodium (Na) in the form of Na ２ 0.7 mass% in terms of O, silicon (Si) is SiO ２ The alumina powder contained 1.6 mass% of fluorine (F) in terms of carbon dioxide equivalent, and 1.6 mass% of fluorine (F). Furthermore, the alumina powder was produced in the same manner as in Example 1, without being subjected to a soda removal treatment or a crushing treatment, and instead being classified by the following procedure.

[0078] <Classification step> The calcined product (alumina) obtained in the calcination step was subjected to elutriation using a sorting tube and particle size adjustment using a SUS mesh to remove extremely thick particles and fine particles. Specifically, pure water was added to 300 g of the calcined product and stirred to obtain a slurry with a concentration of 100 g / L. The resulting slurry was placed in a classification device (sorting tube), and pure water was added thereto at a flow rate of 1 L / min for 3 hours. The slurry containing the precipitate was then recovered from the classification device. Next, the slurry was manually sieved using a SUS mesh with a mesh size of 45 μm, and fine particles less than 45 μm in size were removed from the recovered slurry. The residue obtained after manual sieving was dried overnight in a dryer.

[0079] [Example 4] In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160), 50 g of aluminum fluoride (AlF ３ ): 1.2 g, silicon oxide (SiO ２ ): 0.8 g, and sodium carbonate (Na ２ CO ３ ): 0.08 g were mixed to obtain a raw material mixture. The obtained raw material mixture contained sodium (Na) in the form of Na ２ 0.6 mass% in terms of O, silicon (Si) is SiO ２ The alumina powder contained 1.6 mass% of fluorine (F) in terms of carbon dioxide equivalent, and 1.6 mass% of fluorine (F). Furthermore, the alumina powder was produced in the same manner as in Example 1, without being subjected to a soda removal treatment or a crushing treatment, and instead being classified by the following procedure.

[0080] <Classification Step> The calcined product (alumina) obtained in the calcination step was subjected to elutriation using a sorting tube and particle size adjustment using a SUS mesh to remove extremely thick particles and fine particles. Specifically, pure water was added to 300 g of the calcined product and stirred to obtain a slurry with a concentration of 100 g / L. The resulting slurry was placed in a classification device (sorting tube), and pure water was added thereto at a flow rate of 1 L / min for 3 hours. The slurry containing the precipitate was then recovered from the classification device. Next, the slurry was manually sieved using a SUS mesh with a mesh size of 63 μm, and fine particles with a size less than 63 μm were removed from the recovered slurry. The residue obtained after manual sieving was dried overnight in a dryer.

[0081] [Example 5] In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160) and sodium silicofluoride (Na ２ SiF ６ ): 2.5 g were mixed to obtain a raw material mixture. The obtained raw material mixture contained sodium (Na) in the form of Na ２ 0.7 mass% in terms of O, silicon (Si) is SiO ２ The alumina powder contained 1.6 mass% of carbon dioxide (CO₂) and 2.8 mass% of fluorine (F) in terms of carbon dioxide equivalent. Furthermore, the alumina powder was prepared in the same manner as in Example 1, without being subjected to soda removal treatment, crushing treatment, or classification treatment.

[0082] [Example 6] Scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-920) was used as the alumina source. ２ 0.6 mass% in terms of O, silicon (Si) is SiO ２ The alumina powder was produced in the same manner as in Example 3 except that it contained 1.6 mass% of fluorine (F) and 1.6 mass% of sodium hydroxide.

[0083] [Example 7] In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160), 50 g of aluminum fluoride (AlF ３ ): 1.2 g, silicon oxide (SiO ２ ): 2.4 g, and sodium carbonate (Na ２ CO ３ ): 1.8 g were mixed to obtain a raw material mixture. The obtained raw material mixture contained sodium (Na) in the form of Na ２ 0.7 mass% in terms of O, silicon (Si) is SiO ２ The alumina powder contained 4.4 mass% of carbon dioxide (CO₂) and 1.5 mass% of fluorine (F) in terms of carbon dioxide equivalent. Furthermore, the alumina powder was produced in the same manner as in Example 1, without being subjected to soda removal treatment, crushing treatment, or classification treatment.

[0084] Example 8 As an alumina source, scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-520) was used. In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-520), 100 g of aluminum fluoride (AlF ３ ): 1.2 g, silicon oxide (SiO ２ ): 4.8 g, and sodium carbonate (Na ２ CO ３ ): 1.2 g were mixed to obtain a raw material mixture. The obtained raw material mixture contained sodium (Na) in the form of Na ２ 0.6 mass% in terms of O, silicon (Si) is SiO ２ The alumina powder contained 8.4 mass% of carbon dioxide (CO₂) and 1.4 mass% of fluorine (F) in terms of carbon dioxide equivalent. Furthermore, the alumina powder was prepared in the same manner as in Example 1, without further soda removal treatment, crushing treatment, or classification treatment.

[0085] [Example 9] <Raw Material Mixing Step> As an alumina source, scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-520) was used. In the raw material mixing step, scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-520) and sodium silicofluoride (Na ２ SiF ６ ) were mixed and Na ２ SiF ６ A raw material mixture containing 3.8 mass % of

[0086] <Firing step> The obtained raw material mixture was placed in a sagger (KR-4A, 300 mm x 300 mm x 100 mH, manufactured by Accorcera Corporation) and sealed with a dedicated lid. The sealed sagger was then placed in a firing furnace (Bulk Siliconit Electric Furnace, BSH-3570, manufactured by Siliconit Corporation) and fired in the atmosphere. During firing, the temperature was raised to 1100°C at a rate of 100°C / hour, then held at 1100°C for 10 hours, and then naturally cooled. After the temperature inside the furnace had completely dropped, the sagger was removed from the furnace and the fired product (alumina) was recovered.

[0087] The recovered fired product (alumina) was used as alumina powder without undergoing soda removal, crushing, or classification.

[0088] Example 10 Raw Material Mixing Step: As an alumina source, scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-920) was used. In the raw material mixing step, the scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-920) and sodium silicofluoride (Na ２ SiF ６ ) were mixed and Na ２ SiF ６ A raw material mixture containing 1.3 mass % of

[0089] <Firing process> A shuttle kiln (Takasago Kogyo Co., Ltd., 0.6 m ３ A shuttle kiln (under-upper type) was used. Except for this, firing was carried out under the same conditions as in Example 9 to obtain a fired product (alumina).

[0090] <Crushing step> The obtained fired product (alumina) was subjected to a crushing treatment. Specifically, pure water was added to the fired product and then stirred to form a slurry. The obtained slurry was placed in a pot mill container with an internal volume of 10 L together with 15 kg of 1 mm diameter zirconia beads, and the pot mill container was rotated at a rotation speed of 60 rpm for 3 hours. Next, the slurry was removed from the pot mill container to obtain a crushed product (alumina).

[0091] <Classification step> The crushed material (alumina) obtained in the crushing step was subjected to elutriation using a sorting tube and particle size adjustment using a SUS mesh to remove extremely thick particles and fine particles. Specifically, the slurry containing the crushed material was placed in a classification device (sorting tube), and pure water was added thereto at a flow rate of 1 L / min for 3 hours. The slurry containing the precipitate was then recovered from the classification device. Next, the slurry was manually sieved using a SUS mesh with a mesh size of 108 μm and a SUS mesh with a mesh size of 45 μm, successively, to remove fine particles with a size of less than 45 μm from the recovered slurry. The residue obtained after manual sieving was dried overnight using a dryer.

[0092] Example 11 Raw Material Mixing Step: As an alumina source, scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-920) was used. In the raw material mixing step, scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-920), aluminum fluoride (AlF ３ ), silicon oxide (SiO ２ ), and sodium carbonate (Na ２ CO ３ ) were mixed to form AlF ３ 2.7%, SiO ２ 1.0 mass% and Na ２ CO ３ A raw material mixture containing 1.7 mass % of

[0093] <Caking Step> The resulting raw material mixture was calcined, and the calcined product (alumina) was collected. The calcination was carried out under the same conditions as in Example 10, except that the holding time was 20 hours.

[0094] The recovered fired product (alumina) was used as alumina powder without undergoing soda removal, crushing, or classification.

[0095] [Example 12] Sodium silicofluoride (Na ２ SiF ６ The raw materials were mixed so as to obtain a raw material mixture containing 3.1 mass % of ammonium nitrate. The holding time during firing was set to 20 hours. Except for this, an alumina powder was prepared in the same manner as in Example 10.

[0096] [Comparative Example 1] <Raw Material Mixing Step> As an alumina source, alumina having a BET specific surface area of ​​0.8 to 1.2 m2 produced by rotary kiln firing was used. ２ The properties of the prepared alumina powder are shown in Table 1 below.

[0097] Then, alumina powder and silicon oxide (SiO ２ ) were mixed to obtain a raw material mixture. Silicon oxide was added so that the silicon oxide content was 0.3 mass % relative to the alumina powder. Silicon oxide converts sodium (Na) in the alumina powder into sodalite in the melting process described below. Since sodalite can be removed in the pickling process, converting it into sodalite makes it possible to reduce the amount of sodium in the alumina.

[0098] <Melting step> The obtained raw material mixture was subjected to arc discharge treatment in an electric arc furnace for 8 to 9 hours. During the treatment, the raw material mixture melted into a molten metal. The temperature of the molten metal exceeded 2000°C. After melting, the molten metal was cooled to obtain a molten mass.

[0099] <Crushing and Pulverization> The obtained molten mass was broken into pieces the size of a human head using a Powerman, and then sieved using a trommel (25 mm x 25 mm sieve). Then, masses larger than 25 mm remaining on the sieve were collected and pulverized until particles of several tens of μm to several mm in size were obtained.

[0100] <Pickling> The resulting pulverized material was washed with a 4% sulfuric acid solution to remove sodalite formed during fusion. The washed product was filtered using a horizontal filter, dried in a kiln, and then sieved to obtain fused alumina with an average particle size of 250 to 350 μm.

[0101] <Pulverization> The obtained electrofused alumina was pulverized using a continuous vibration mill to obtain an alumina powder having an average particle size of 18 μm.

[0102] [Comparative Example 2] Aluminum hydroxide A (Al(OH)) produced by the Bayer process was used as an alumina source. ３ The properties of the prepared aluminum hydroxide A are shown in Table 1 below.

[0103] <Firing> A certain amount of the prepared aluminum hydroxide A was charged into the raw material charging section of a rotary kiln firing equipment. The moisture adhering to the aluminum hydroxide A was removed in the pre-drying section of the firing equipment, and then aluminum fluoride (AlF ３ ) was added in a fixed amount. Then, aluminum hydroxide A with added aluminum fluoride was calcined with the calcined product. The calcination was carried out under the conditions of 1280 to 1380°C x 2 hours. As a result, alumina powder was obtained.

[0104] [Comparative Example 3] As an alumina source, imported aluminum hydroxide B (Al(OH) ３ The properties of the aluminum hydroxide B used are shown in Table 1 below. Except for these, an alumina powder was prepared in the same manner as in Comparative Example 2.

[0105] [Comparative Example 4] In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160) and 100 g of aluminum fluoride (AlF ３ ): 1.2 g was mixed, but silicon oxide (SiO ２ The resulting raw material mixture contained sodium (Na) in the amount of Na ２ 0.7 mass% in terms of O, silicon (Si) is SiO ２ The alumina powder contained 0.02 mass% of carbon dioxide (CO₂) and 1.6 mass% of fluorine (F) in terms of carbon dioxide equivalent. Furthermore, the alumina powder was produced in the same manner as in Example 1, without being subjected to soda removal treatment, crushing treatment, or classification treatment.

[0106] The manufacturing conditions for the alumina powders of Examples 1 to 12 and Comparative Examples 1 to 4 are summarized in Tables 2 and 3 below.

[0107]

[0108]

[0109]

[0110] (2) Evaluation The alumina powders obtained in Examples 1 to 12 and Comparative Examples 1 to 4 were evaluated for various properties as follows.

[0111] <Average particle size and thickness> Evaluation was performed using a scanning electron microscope (JEOL Ltd., JSM-F100; SEM) and image analysis software (ImageJ). Specifically, an SEM image of the obtained alumina powder was taken, and the particles shown in the image were evaluated using ImageJ to measure the particle size and thickness. The average particle size was determined as the average value of 80 particles, and the average thickness was determined as the average value of 30 particles.

[0112] <Crystalline Phase> The crystalline phase was evaluated using an X-ray diffractometer (Rigaku Corporation, RINT Ultima III). Specifically, the obtained alumina powder was first placed on a dedicated sample plate and gently spread to a size of 20 mm × 20 mm × 0.5 mm to prepare a measurement sample. Next, the X-ray diffraction pattern of the measurement sample was evaluated using the X-ray diffractometer.

[0113] <Component Analysis> Component analysis was performed using a scanning X-ray fluorescence analyzer (Rigaku Corporation, ZSX Primus IV). Specifically, the obtained alumina powder was first placed in a platinum crucible and set in a high-frequency melting apparatus. The crucible was then preheated at 700°C for 60 seconds and then rocked at 1200°C for 120 seconds to produce a glass bead sample. The glass bead sample was then evaluated using the scanning X-ray analyzer. The obtained data was compared with the data of a standard sample for a calibration curve to obtain evaluation results.

[0114] <Specific Surface Area> The specific surface area was measured using a manual specific surface area measuring device (Micromeritics Instrument Corp., FlowSorbIII 2305 type) ２ The evaluation was carried out by gas adsorption method.

[0115] <Brilliance Evaluation> The brilliance of the alumina powder was evaluated using a multi-angle colorimeter (BYK-Gardner, BYK-mac i, measurement opening diameter: 12 mm). Specifically, a measurement sample was prepared by filling the opening (35 mmΦ x 5 mmH) of a sample holder with alumina powder to the brim, and this measurement sample was set in the colorimeter and measured. During measurement, a CCD chip was placed on an axis perpendicular to the measurement surface of the sample holder, and light was irradiated from a direction at an angle of 15°, 45°, or 75° from the perpendicular axis, and an image was taken with the CCD chip. The obtained images were then analyzed using an image analysis algorithm to determine the light luminance Si value and the brilliance area Sa value at each measurement angle (15°, 45°, 75°).

[0116] (3) Evaluation Results The evaluation results obtained for the alumina powders obtained in Examples 1 to 12 and Comparative Examples 1 to 4 are summarized in Table 4. SEM images of the alumina powder obtained in Example 1 are shown in Fig. 4, and SEM images of the alumina powder obtained in Example 3 are shown in Figs.

[0117] As shown in Table 4, the alumina powders of Examples 1 to 12, which were prepared using scaly boehmite powder and additives of predetermined compositions as raw materials, had average particle diameters, average thicknesses, and average aspect ratios that fell within the predetermined ranges. ２ O) amount, and silicon oxide (SiO ２ ) amount satisfied the range specified in this embodiment, and the brilliance intensity Si value was high. In particular, as can be seen from the results of the samples of Examples 1 to 3, the brilliance intensity Si value increased as the average particle diameter increased.

[0118] 4 and 5, it was found that the alumina powders of Examples 1 and 3 had a scaly shape with large plate surfaces and small plate thicknesses, and that the surfaces of the plate surfaces were smooth. The cross-sectional SEM image and enlarged SEM image of Fig. 6 revealed that the alumina powder of Example 3 had a polycrystalline structure.

[0119] On the other hand, in Comparative Examples 1 to 3, which were produced using alumina or aluminum hydroxide as raw materials, flat alumina powder could not be obtained. In addition, the alumina powders of Comparative Examples 1 to 3 had low brilliance intensity Si values. Furthermore, although scaly boehmite powder was used as a raw material, silicon oxide (SiO ２ The alumina powder of Comparative Example 4, to which no fluorine-containing fluoride was added, had a small average particle size and aspect ratio, and also had a small brilliance intensity Si value.

[0120] From the above results, it was found that the scaly α-alumina powder of this embodiment has a large average particle size and aspect ratio, and is excellent in brilliance.

[0121]

Claims

1. The average particle size is 50 μm or more and 200 μm or less, the average thickness is 0.20 μm or more and 2.00 μm or less, and the average aspect ratio is 50 or more and 600 or less, A high-brightness flaky α-alumina powder having a brilliance intensity Si value in a brilliance measurement at measurement angles θ of 15° and 45° in the range of 1.00 or more and 15.00 or less.

2. Alkali metal (AM) ２ The silicon (Si) content is 0.2 mass% or more and 5.0 mass% or less in terms of O, and the silicon (Si) content is SiO ２ The high-brightness flaky α-alumina powder according to claim 1, wherein the α-alumina powder contains 0.3% by mass or more and 10.0% by mass or less in terms of the amount of α-alumina converted into α-alumina.

3. 3. The high-brightness flaky α-alumina powder according to claim 2, wherein the alkali metal (AM) is sodium (Na) and / or potassium (K).

4. 4. The high-brightness flaky α-alumina powder according to claim 1, wherein the luminance intensity Si value is in the range of 5.00 or more and 15.00 or less.

5. A method for producing the high-brightness flaky α-alumina powder according to any one of claims 1 to 3, comprising the steps of: Adding additives to the scaly boehmite powder and then mixing to prepare a raw material mixture; Firing the raw material mixture at a temperature in the range of 1000° C. to 1300° C.; Equipped with The raw material mixture is an alkali metal (AM) ２ O equivalent of 0.5 mass% or more and 10.0 mass% or less, silicon (Si) is SiO ２ and fluorine (F) in an amount of 0.1 mass% or more and 10.0 mass% or less, calculated as carbon black or silver halide.

6. The method according to claim 5 , wherein the scaly boehmite powder has an average particle size of 2 μm or more and 15 μm or less and an average aspect ratio of 20 or more and 60 or less.

7. The additive is aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ 6. The method of claim 5, comprising:

8. The additive is an oxide of an alkali metal (AM). ２ O) and / or carbonate (AM ２ CO ３ 8. The method of claim 7, further comprising:

9. The additive is an alkali silicofluoride (AM ２ SiF ６ 6. The method of claim 5, comprising:

10. 6. The method according to claim 5, wherein the alkali metal (AM) is sodium (Na) and / or potassium (K).

11. A paint or cosmetic comprising the high-brightness flaky α-alumina powder according to any one of claims 1 to 3.