Plate-shaped alumina powder and production method thereof, and paint or cosmetic

Abstract

To provide: plate-shaped alumina powder having excellent dispersibility and a production method thereof; and a paint or cosmetic containing the plate-shaped alumina powder.SOLUTION: This plate-shaped alumina powder comprises a plurality of plate-shaped α-alumina particles and has an average particle diameter of 2 μm or more and 100 μm or less and an average thickness of 0.2 μm or more and 3.0 μm or less. The plate-shaped alumina powder has a number percentage (high circularity particle percentage) of particles (high circularity alumina particles) having a circularity of 0.953 or more of 25% or more when analyzed by a dry particle image analyzing device.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a platelet alumina powder, a method for producing the same, and a paint or cosmetic. [Background technology]

[0002] Alumina (Al2O3) 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, alumina powder is widely used in a variety of applications, including structural members, tools, abrasives, fillers, spark plugs, insulators, electronic substrates, and refractories.

[0003] In particular, plate-like alumina powder has high light reflectance and brightness on its particle surface. Taking advantage of these characteristics, it is used as a pigment in paints and cosmetics. Plate-like alumina powder is a powder composed of plate-like alumina particles, and is also called scaly alumina powder, thin flaky alumina powder, flat alumina powder, or alumina flakes.

[0004] As a document disclosing the use of plate-like alumina powder as a pigment, Patent Document 1 discloses a plate-like alumina powder 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 The document discloses Al2O3 flakes having a high refractive index, and describes that the Al2O3 flakes simultaneously have high chemical stability, a smooth surface, and high whiteness, and are used as a pigment substrate (claim 1,

[0010] and

[0015] of Patent Document 1). Patent Document 1 also describes that the Al2O3 flakes are coated with a high refractive index layer such as TiO2 or a low refractive index layer such as SiO2, thereby imparting increased gloss, interference color, or color flop effects (

[0040] to

[0046] of Patent Document 1).

[0005] 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 and that it forms a composite coating film with an unprecedented high luster and novel designability without compromising the finished appearance (claim 1,

[0001] and

[0108] of Patent Document 2). Patent Document 2 also describes that the alumina flakes are aluminum oxide (Al2O3) coated with a metal oxide such as titanium dioxide, with a particle size of 10 to 30 μm and a thickness of 0.3 to 0.4 μm (

[0007] of Patent Document 2).

[0006] Patent Document 3 discloses a pearlescent pigment containing aluminum oxide and zinc oxide as main components in a mass ratio of 100:0.1 to 5, and containing flaky alumina crystals 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 (

[0001] of Patent Document 3).

[0007] 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 by coating the surface of plate-like boehmite or plate-like alumina powder with a hydrophobic compound such as polysiloxane and incorporating this coated powder into cosmetics, cosmetics that feel good when used can be obtained (paragraph

[0013] of Patent Document 4).

[0008] Patent Document 5 discloses hexagonal plate-shaped alumina obtained by firing hexagonal plate-shaped boehmite at temperatures of 450 to 1500°C, which has a roughly 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-shaped alumina has high orientation, reduces diffuse reflection, and increases brilliance, and is suitable for use as a filler for the purpose of increasing the brilliance of paints and cosmetics (claims 4 and

[0045] of Patent Document 5).

[0009] 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 in thermally conductive fillers, cosmetics, abrasives, high luster pigments, lubricants, substrates for conductive powders, ceramic materials, etc. (

[0101] of Patent Document 6). [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-218424 [Patent Document 2] Japanese Patent Application Publication No. 10-298458 [Patent Document 3] Special Publication No. 2010-502774 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-071996 [Patent Document 5] Japanese Patent Application Laid-Open No. 2003-002642 [Patent Document 6] Japanese Patent Application Publication No. 2019-123664 Summary of the Invention [Problem to be solved by the invention]

[0011] As described above, the use of plate-like alumina powder as a pigment in paints and cosmetics has been proposed. However, there is room for improvement in conventional plate-like alumina powders. That is, pigments are used in a state dispersed in a vehicle containing a solvent or resin. Pigments used in such applications are desired to have transparent crystals, excellent plate-like properties, and excellent dispersibility. High dispersibility is particularly desirable for pigments used in cosmetics. This is because the higher the dispersibility of a pigment, the more transparent the cosmetics can be.

[0012] However, conventional plate-like alumina powders have been insufficient in meeting the above-mentioned demands. In particular, conventional plate-like alumina powders have limitations in terms of improving dispersibility, making it difficult to further improve transparency when used in applications such as cosmetics.

[0013] In view of these problems, the present inventors have conducted extensive research and have found that a plate-like alumina powder having a specific average particle size and average thickness and containing a large number of plate-like particles with high circularity has excellent dispersibility.

[0014] The present invention was completed based on these findings, and aims to provide a plate-like alumina powder with excellent dispersibility and a method for producing the same, as well as a paint or cosmetic containing the plate-like alumina powder. [Means for solving the problem]

[0015] The present invention includes the following aspects (1) to (9). In this specification, the expression "to" includes the numerical values ​​on both ends. In other words, "X to Y" is synonymous with "at least X and at most Y."

[0016] (1) A plate-like alumina powder composed of a plurality of plate-like alpha alumina particles, having an average particle diameter of 2 μm or more and 100 μm or less and an average thickness of 0.2 μm or more and 3.0 μm or less, When the plate-like alumina powder is analyzed using a dry particle image analyzer, the number ratio (high circularity particle ratio) of particles having a circularity of 0.953 or more (high circularity alumina particles) is 25% or more.

[0017] (2) The plate-like alumina powder according to (1) above, wherein the average particle diameter is 5 μm or more and 50 μm or less, the average thickness is 0.3 μm or more and 1.0 μm or less, and the proportion of highly circular particles is 40% or more.

[0018] (3) The plate-like alumina powder of (1) or (2) above, containing an alkali metal (AM) in an amount of 0.01% by mass or more and 0.5% by mass or less, calculated as AM2O, and silicon (Si) in an amount of 0.01% by mass or more and 0.3% by mass or less, calculated as SiO2.

[0019] (4) The plate-like alumina powder according to (3) above, wherein the alkali metal (AM) is one or both of sodium (Na) and potassium (K).

[0020] (5) A method for producing a plate-like alumina powder according to any one of (1) to (4) above, comprising the following steps: preparing a raw material mixture containing aluminum hydroxide powder and an additive; and a step of firing the raw material mixture at a temperature of 900°C or higher and 1300°C or lower; Equipped with The raw material mixture contains alkali metal (AM) in an amount of 0.01% by mass or more and 0.5% by mass or less in terms of AM2O, silicon (Si) in an amount of 0.01% by mass or more and 0.3% by mass or less in terms of SiO2, and fluorine (F) in an amount of 0.1% by mass or more and 5.0% by mass or less in terms of F.

[0021] (6) The method according to (5) above, wherein the additive comprises an alkali silicofluoride (AM2SiF6), or aluminum fluoride (AlF3) and silicon oxide (SiO2).

[0022] (7) The method according to (5) or (6) above, wherein the additive further comprises one or both of an oxide (AM2O) and a carbonate (AM2CO3) of an alkali metal (AM).

[0023] (8) The method according to any one of (5) to (7) above, wherein the alkali metal (AM) is one or both of sodium (Na) and potassium (K).

[0024] (9) A paint or cosmetic containing the platelet alumina powder according to any one of (1) to (4) above. [Effects of the Invention]

[0025] According to the present invention, there are provided a plate-like alumina powder having excellent dispersibility, a method for producing the same, and a paint or cosmetic containing the plate-like alumina powder. [Brief explanation of the drawings]

[0026] [Figure 1] The mechanism by which plate-like alumina powder produces a shiny appearance is shown. [Figure 2] 1 is a diagram illustrating circularity. [Figure 3] 4 shows the results of measuring the zeta potential of the plate-like alumina powder. DETAILED DESCRIPTION OF THE INVENTION

[0027] Specific embodiments of the present invention (hereinafter referred to as "present embodiments") are described below. However, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention. Furthermore, in this specification, any combination of preferred aspects can be adopted as long as technical consistency can be achieved. For example, one preferred numerical range and another preferred numerical range can be combined arbitrarily.

[0028] <<1. Plate-shaped alumina powder>> The plate-like alumina powder of this embodiment is composed of a plurality of plate-like α-alumina particles, and has an average particle diameter of 2 μm to 100 μm and an average thickness of 0.2 μm to 3.0 μm. When this plate-like alumina powder is analyzed using a dry particle image analyzer, the number ratio (high circularity particle ratio) of particles having a circularity of 0.953 or more (high circularity alumina particles) is 25% or more.

[0029] The plate-like alumina powder of this embodiment (hereinafter sometimes simply referred to as "alumina powder") is composed of a plurality of plate-like alpha-alumina (Al2O3) particles (hereinafter sometimes simply referred to as "alumina particles," "plate-like particles," or "particles"). That is, it contains plate-like alpha-alumina particles as a main component. Here, the plate-like alpha-alumina particles are particles made of alpha-alumina (corundum). Alpha-alumina has a trigonal corundum crystal system and is excellent in chemical stability, heat resistance, corrosion resistance, wear resistance, and insulation, as well as high strength and hardness. Furthermore, it has high light reflectance, brightness, and whiteness. Because the alumina powder of this embodiment contains plate-like alpha-alumina particles as a main component, it is possible to fully utilize these characteristics.

[0030] In this specification, powder refers to an aggregate of many independent particles. In other words, many particles aggregate to form a powder. Because each particle that makes up a powder is independent, the powder exhibits fluidity as a whole when in its own state. It is sufficient for most of the particles that make up a powder to be independent. As long as the powder as a whole exhibits fluidity, some particles may be bonded to form aggregates. In addition, an aggregate of many particles dispersed in a medium such as a liquid or resin is also called a powder.

[0031] The alumina particles constituting the plate-like alumina powder of this embodiment are plate-like (scale-like, thin-plate-like, flat, flake-like). That is, each particle has a large plate surface and a small plate thickness. Specifically, the average particle diameter of the plate-like 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 diameter and average thickness are the number-based average values ​​of the particle diameter and thickness of each particle constituting the alumina powder. The particle diameter refers to the long diameter of the particle. That is, the particle diameter is the long axis diameter of the particle plate surface. The thickness refers to the plate thickness of the plate surface. The particle diameter and thickness can be determined by observing the particles constituting the alumina powder with a scanning electron microscope (SEM). That is, the average particle diameter and average thickness are values ​​determined by SEM observation. In addition, the average particle diameter is usually larger than the average thickness.

[0032] In this way, by using particles with large platelets and small platelets, the brilliance of the alumina powder can be enhanced. This point will be explained using Figure 1. As shown in Figure 1, when a powder containing particles with large platelets and small platelets is formed on a substrate by a coating method or other method, the large-area platelets of the particles tend to be oriented so that they are parallel to the substrate surface. The larger the particle diameter, the greater the surface area of ​​the platelets that are parallel to the substrate surface. Furthermore, light incident from outside is reflected by the platelets of the particles and emitted to the outside as reflected light. Therefore, the larger the particle diameter, the more easily incident light is reflected by the platelets of the particles, resulting in a higher visual brilliance.

[0033] If the average particle size is less than 2 μm or the average thickness is more than 3.0 μm, the powder may not have sufficient brilliance. On the other hand, if the average particle size is more than 100 μm or the average thickness is less than 0.2 μm, it is difficult to produce. In addition, the strength is low and handling is difficult. The average particle size of the plate-like alumina powder is preferably 5 μm or more and 50 μm or less, and the average thickness is preferably 0.3 μm or more and 1.0 μm or less.

[0034] Preferably, the average aspect ratio of the alumina powder is 20 or more and 50 or less. Here, the average aspect ratio is the ratio of the average particle diameter to the average thickness (average particle diameter / average thickness). By making the average aspect ratio 25 or more, it is possible to further enhance the brilliance of the powder. Furthermore, by making the average aspect ratio 50 or less, the strength of the alumina powder is increased and handling becomes easier.

[0035] It is not necessary for all particles constituting the alumina powder to be plate-like α-alumina particles. As long as the alumina powder as a whole satisfies the above-mentioned requirements for average particle size and average thickness, it may contain particles other than plate-like α-alumina particles. However, in order to take advantage of the excellent effects of the plate-like α-alumina particles, a high content of plate-like α-alumina particles is preferable. The content of plate-like 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.

[0036] When the alumina powder of this embodiment is analyzed using a dry particle image analyzer, the number ratio of high-circularity alumina particles (high-circularity particle ratio) is 25% or more. Here, high-circularity alumina particles are plate-like α-alumina particles with a circularity of 0.953 or more. Circularity is a numerical representation of how close an object is to a perfect circle, and in principle, is 1.000 or less. As shown in Figure 2, the circularity of a perfect circle is 1, and the further away from a perfect circle the smaller the circularity. For example, the circularity of a regular hexagon is 0.9523, and the circularity of a square is 0.89.

[0037] The alumina powder of this embodiment contains a large number (25% or more) of particles with a plate surface shape close to a circle (high circularity alumina particles). As a result, it has various advantageous effects. First, this alumina powder has the effect of excellent dispersibility. This is because the high circularity particles have a small contact area between particles, which reduces physical contact and resulting interference between particles, making problems such as aggregation less likely to occur. In addition, the alumina powder of this embodiment has the effect of imparting excellent transparency when used as an inorganic pigment. This is because the high circularity particles do not have sharp angles, which makes it less likely to cause diffuse reflection that reduces transparency. Furthermore, the alumina powder of this embodiment has the effect of being less likely to damage other components. This is because the high circularity particles do not have sharp angles, so there is little risk of them causing damage (scratching) when they come into contact with other components.

[0038] In contrast, if the proportion of highly circular particles is low (less than 25%), dispersibility is poor and problems such as transparency may arise. For example, particles with low circularity, such as polygonal plate-like particles, are prone to physical interference with other particles, resulting in poor dispersibility. Furthermore, particle corners are prone to chipping during post-processing. Chipped particle corners are prone to diffuse reflection, degrading transparency. Furthermore, there is also the problem that particle corners are prone to scratching other components, such as processing equipment.

[0039] From the viewpoint of utilizing the excellent effects of the high circularity particles, it is desirable that the proportion of high circularity particles (proportion of the number of high circularity alumina particles) is high. The proportion of high circularity particles is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more. There is no upper limit to the proportion of high circularity particles, and it is sufficient that the proportion of high circularity particles is 100% or less. The proportion of high circularity particles may be 90% or less, 80% or less, 70% or less, or 60% or less.

[0040] The proportion of high-circularity particles is determined as follows. First, the alumina powder is analyzed using a dry particle image analyzer. Examples of dry particle image analyzers include a fully automated image-based particle size distribution analyzer (Spectris Corporation, Morphologi 4) or a similar device. During the analysis, the particle morphology is evaluated based on a two-dimensional projection image of each particle constituting the alumina powder. Specifically, the area (s) and perimeter (l) of each particle are measured in the two-dimensional projection image. The perimeter (l) of the equivalent area circle of each particle is then calculated. Here, the equivalent area circle is a virtual circle with an area equal to the area (s) of the particle. The perimeter (l) of the equivalent area circle is calculated from the particle area (s) according to the following formula (1). The circularity of each particle is then calculated from the particle perimeter (l) and the equivalent area circle perimeter (l) according to the following formula (2).

[0041]

number

[0042] Then, the number of analyzed particles and the number of particles with a circularity of 0.953 or more (high circularity alumina particles) are counted, and the ratio of these numbers (number of high circularity alumina particles / number of analyzed alumina particles) is calculated as the ratio of high circularity particles.

[0043] The alumina powder of this embodiment preferably contains an alkali metal (AM) in an amount of 0.01% by mass or more and 0.5% by mass or less, calculated as AM2O, and silicon (Si) in an amount of 0.01% by mass or more and 0.3% by mass or less, calculated as SiO2. The content of the alkali metal (AM) is more preferably 0.01% by mass or more and 0.3% by mass or less, calculated as AM2O. The content of silicon (Si) is more preferably 0.01% by mass or more and 0.2% by mass or less, calculated as SiO2. The alkali metal (AM) is preferably one or both of sodium (Na) and potassium (K). By including these components, it is possible to further enhance the brilliance of the alumina powder.

[0044] 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 contained as long as the above-mentioned requirements are met. Furthermore, the presence of impurities inevitably mixed in during the manufacturing process is permitted. One such other component is fluorine (F). 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.

[0045] If other components (elements other than Al, AM, Si, and O) are contained in large amounts, the brilliance of the alumina powder may be impaired. Therefore, the content of 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. 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 0.05% by mass or less in terms of iron oxide (Fe2O3).

[0046] The specific surface area (S BET ) is 0.1m 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 reduce the brilliance. On the other hand, alumina powder with a small specific surface area is difficult to produce.

[0047] The alumina powder of this embodiment has high chemical stability because it is primarily composed of α-alumina. It also has the characteristics of a large average particle size and excellent dispersibility. Alumina powder with these characteristics is suitable for use in the fields of paints and cosmetics. However, the alumina powder of this embodiment is not limited to use in paints and cosmetics. It can also be used in known applications such as reinforcing materials and gas barrier materials mixed into plastics and resin films, structural members, tools, abrasives, fillers, spark plugs, insulators, electronic substrates, and refractories.

[0048] <<2. Manufacturing method of plate-shaped alumina powder>> The plate-like alumina powder of this embodiment can be produced by any method as long as it satisfies the above-mentioned requirements. However, it is preferably produced by the following procedure. A preferred production 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 this raw material mixture at a temperature of 900°C to 1300°C (firing step). The raw material mixture contains an alkali metal (AM) in an amount of 0.01% by mass to 0.5% by mass calculated as AM2O, silicon (Si) in an amount of 0.01% by mass to 0.3% by mass calculated as SiO2, and fluorine (F) in an amount of 0.1% by mass to 5.0% by mass. If necessary, a post-treatment step may be performed after the firing step.

[0049] The manufacturing method of this embodiment is particularly characterized in that aluminum hydroxide powder is used as a raw material and the composition of the raw material mixture is controlled within a specific range, which makes it possible to obtain an alumina powder that has a desired size and contains a large amount of highly circular alumina particles, resulting in excellent dispersibility. Each step will be described in detail below.

[0050] <Raw material mixing process> In the raw material mixing step, a raw material mixture containing aluminum hydroxide powder and additives is prepared. Aluminum hydroxide is a compound represented by the chemical formula Al(OH)3, and when heated, it is dehydrated and converted into α-type aluminum oxide (α-alumina). Two types of aluminum hydroxide are known: gibbsite (γ-type aluminum hydroxide) and bayerite (α-type aluminum hydroxide). In this embodiment, either gibbsite or bayerite may be used. However, a powder made of thermodynamically stable gibbsite is preferred.

[0051] The size of the aluminum hydroxide powder as the raw material is not particularly limited as long as the alumina powder of this embodiment can be obtained. However, the volume average particle size (D50) is preferably 0.5 μm or more and 15 μm or less. Alternatively, the specific surface area (S BET ) is 0.5m 2 / g or more 20m 2 / g or less. D50 is the cumulative 50% diameter in the particle size distribution on a volume basis determined by a laser diffraction / scattering particle size distribution analyzer. BET is a value obtained by measurement using the N2 gas adsorption method.

[0052] The additives are components that become liquid in the subsequent calcination step 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 will be described later, when the composition of the liquid phase components is appropriately controlled, the crystallization and grain growth of alumina particles proceeds appropriately, resulting in the production of alumina powder that has a desired size and contains a large amount of highly circular alumina particles.

[0053] Therefore, to obtain alumina powder with excellent dispersibility, it is important to appropriately 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 alkali metal (AM) in an amount of 0.01 to 0.5 mass% in terms of AM2O, silicon (Si) in an amount of 0.01 to 0.3 mass% in terms of SiO2, and fluorine (F) in an amount of 0.1 to 5.0 mass% in terms of F. When the composition of the raw material mixture is within the above ranges, alumina powder with excellent dispersibility, containing a large average particle size and a large amount of highly circular alumina particles, can be obtained. On the other hand, an inappropriate raw material mixture composition may adversely affect the particle size and the content of highly circular alumina particles.

[0054] The amount of alkali metal (AM) in the raw material mixture is more preferably 0.01% by mass or more and 0.5% by mass or less in terms of AM2O. The alkali metal is preferably one or both of sodium (Na) and potassium (K). The amount of silicon (Si) in the raw material mixture is more preferably 0.05% by mass or more and 0.15% by mass or less in terms of SiO2. The amount of fluorine (F) in the raw material mixture is more preferably 0.1% by mass or more and 1.0% by mass or less in terms of F. By limiting the composition of the raw material mixture to within the above-mentioned ranges, the content of high-circularity alumina particles can be further increased, and as a result, it is expected that an alumina powder with even better dispersibility can be obtained.

[0055] The additive contains at least silicon (Si) and fluorine (F). The additive may or may not contain an alkali metal (AM). The raw material aluminum hydroxide powder may contain an alkali metal (AM) as an impurity. If the amount of alkali metal in the aluminum hydroxide powder is sufficient, the additive may not contain an alkali metal. 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 is incorporated into the alumina powder during the firing process.

[0056] The additive may contain components other than alkali metals (AM), silicon (Si), fluorine (F), aluminum (Al), and oxygen (O), as long as an alumina powder having the desired size and containing a large amount of highly circular alumina particles is obtained. However, excessively large amounts of other components may adversely affect the particle size and the content of highly circular alumina particles. 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.

[0057] According to a preferred embodiment, the additive contains aluminum fluoride (AlF) and silicon oxide (SiO). Furthermore, when the amount of alkali metal in the aluminum hydroxide powder is insufficient, it is preferable that the additive further contains one or both of an oxide (AM) and a carbonate (AMCO). According to another preferred embodiment, the additive contains an alkali silicon fluoride (AMSiF). By using an appropriate amount of these compounds (AlF, SiO, AMO, AMCO, AMSiF) as additives, it is possible to reliably produce alumina powder having a desired size and containing a large amount of highly circular alumina particles. Furthermore, these compounds are inexpensive and easily available. Therefore, plate-like alumina powder can be obtained more inexpensively.

[0058] The raw material mixture can be prepared by mixing aluminum hydroxide powder and additives. Mixing can be performed by a known method. It can be performed dry or wet. Dry mixing can be performed using a dry mixer such as an air blender, V-type blender, rocking blender, Henschel mixer, or Nauta mixer. 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 can be mixed using a wet mixer such as a ball mill, attritor, or bead mill.

[0059] <Firing process> In the firing process, the resulting raw material mixture is fired at a temperature between 900°C and 1300°C to obtain a fired product. During firing, aluminum hydroxide (Al(OH)3) dehydrates and transforms into alpha alumina (Al2O3). Additionally, 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 alpha alumina. In other words, the crystal structure changes during the transformation from Al(OH)3 to alpha alumina. Adding mineralizers promotes the crystallization of alpha alumina (corundum), making it possible to lower the temperature of the transformation.

[0060] The mineralizer components (AM, Si, F) that become liquid during calcination coat 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. The growth direction and growth rate of alumina particles vary depending on the composition of the liquid-phase component. Therefore, if the composition of the liquid-phase component is properly adjusted, the growth of the aluminum particle plate surface is controlled, resulting in highly circular particles. The liquid-phase component also smooths the alumina particle plate surface. If the alumina particle surface has irregularities, these irregularities will preferentially diffuse into the liquid phase, eliminating the irregularities. Therefore, if the composition of the liquid-phase component is properly controlled, it is possible to obtain a highly dispersible alumina powder containing a large number of alumina particles with the desired size, smooth surfaces, and high circularity.

[0061] If the firing temperature is below 900°C, the transition to α-alumina and particle growth may be insufficient. This may make it difficult to obtain an alumina powder with excellent dispersibility. On the other hand, if the firing temperature exceeds 1300°C, particles may sinter together, forming a strong sintered mass. Furthermore, excessive energy consumption during firing may result, leading to increased production costs. The firing temperature is preferably 1000°C or higher and 1200°C or lower, 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 particle 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.

[0062] <Post-processing> If necessary, the fired product obtained through the firing process 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 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. Crushing can be performed 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. Classification can be performed by techniques such as sieving, air classification, elutriation, and / or centrifugation. Post-treatment can be performed as needed. If the desired alumina powder can be obtained after firing, post-treatment may be omitted.

[0063] In this manner, the plate-like alumina powder of this embodiment can be produced. The obtained alumina powder contains many alumina particles having the desired size, a smooth surface, and high circularity. Therefore, it has excellent dispersibility. Such alumina powder can be suitably used in various applications, including paints and cosmetics.

[0064] <<3. Paints and Cosmetics>> The paint or cosmetic of this embodiment contains the above-mentioned plate-like alumina powder. The paint or cosmetic 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 surface of the plate-like alumina powder. The paint or cosmetic may also contain a solvent or resin in addition to the plate-like alumina powder. The solvent may be aqueous or non-aqueous. The paint or cosmetic may also contain known additives, such as oils, pigments other than the plate-like alumina powder, fillers, surfactants, viscosity modifiers, preservatives, fragrances, moisturizers, physiologically active ingredients, salts, chelating agents, neutralizing agents, and / or pH adjusters. [Example]

[0065] The present invention will be described in more detail using the following examples, but the present invention is not limited to the following examples.

[0066] (1) Preparation of plate-shaped alumina powder [Examples 1 to 6 and Comparative Examples 1 to 7] <Raw material mixing process> Aluminum hydroxide (gibbsite) powder (Nippon Light Metal Co., Ltd., BF013) was prepared as the alumina source for Examples 1 to 5 and Comparative Examples 1 to 7. Aluminum hydroxide (gibbsite) powder (Nippon Light Metal Co., Ltd., BE043) was prepared as the alumina source for Example 6. The properties of the prepared aluminum hydroxide powders are shown in Table 1 below. Note that Na2O shown in Table 1 is the combined value of the eluted sodium content on and near the crystal surface of the aluminum hydroxide and the uneluted sodium content incorporated within the crystal lattice, while f-Na2O is only the eluted sodium content on and near the crystal surface of the aluminum hydroxide.

[0067] Separately, aluminum fluoride (DO-FLUORIDE CHEMICALS CO., LTD.; AlF3), silicon oxide (Marukama Kamado Toryo Co., Ltd., Snow Brand Silica SP-3; SiO2), sodium carbonate (Kanto Chemical Co., Ltd.; Na2CO3), and sodium silicofluoride (Kanto Chemical Co., Ltd., sodium hexafluorosilicate; Na2SiF6) were prepared as additives.

[0068] Next, the prepared aluminum hydroxide powder and additives were mixed 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 would be the value shown in Table 1 below. Mixing was performed by placing the raw materials in a bag and shaking them (manual method; Examples 1, 2, 4, 5 and Comparative Examples 2 to 7) or by using a rocking mixer (mechanical method; Examples 3, 6 and Comparative Example 1).

[0069] <Firing process> The obtained raw material mixture was placed in a firing container, and the firing container was closed with a dedicated lid. The lidded firing container was placed in a firing furnace and fired in the atmosphere. The firing was carried out using an electric furnace or a shuttle kiln. During firing, the temperature was raised to the firing temperature at a rate of 100°C / hour, and then maintained at that temperature for 10 hours, followed by natural cooling. The firing temperature was 1000°C (Comparative Example 7) or 1100°C (Examples 1 to 6 and Comparative Examples 1 to 6). After the temperature inside the furnace had completely dropped, the firing container was removed from the furnace, and the fired product (alumina) was recovered.

[0070] <Post-treatment>

[0071]

[0072]

Table 1

[0073] (2) Evaluation

[0074] <Particle size distribution>

[0075] <SEM observation> The alumina powder was evaluated using a scanning electron microscope (JEOL Ltd., JSM-F100; SEM) and image analysis software (ImageJ). Specifically, SEM images of the obtained alumina powder were taken, and the particles captured in the images were evaluated using ImageJ to measure particle diameter and thickness, and the average particle diameter and thickness were calculated. The average particle diameter was calculated as the average value of 80 particles, and the average thickness was calculated as the average value of 30 particles. The ratio of the average particle diameter to the average thickness (average particle diameter / average thickness) was then calculated as the average aspect ratio.

[0076] <Crystal phase> The crystalline phase of the alumina powder was investigated using an X-ray diffractometer (Rigaku Corporation, RINT Ultima III). Specifically, the obtained alumina powder was placed on a dedicated sample plate and gently spread to a size of 20 mm x 20 mm x 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. Then, it was investigated whether diffraction peaks based on α-alumina (corundum phase) were detected in the obtained X-ray diffraction pattern.

[0077] <Component analysis> The component analysis was performed using a scanning X-ray fluorescence analyzer (Rigaku Corporation, ZSX PrimusIV). Specifically, alumina powder was placed in a platinum crucible and set in a high-frequency melting device. The crucible was then preheated at 700°C for 60 seconds, followed by a rocking treatment 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 the calibration curve to obtain the evaluation results.

[0078] <Specific surface area> Specific surface area (S BET ) was evaluated by N2 gas adsorption method using a manual specific surface area measuring device (Micromeritics Instrument Corp., FlowSorbIII 2305 model).

[0079] <Image analysis of particles> Image analysis of the alumina particles contained in the alumina powder was performed to determine the proportion of highly circular particles. Specifically, a fully automated image-based particle size distribution analyzer (Spectris Corporation, Morphologi 4) was used to evaluate the particle morphology based on the two-dimensional projection image of each particle constituting the alumina powder. More specifically, the area (s) and perimeter (l) of each particle were measured in the two-dimensional projection image. Next, the perimeter (l) of the equivalent area circle of this particle was calculated according to the following equation (1). The circularity of each particle was then calculated from the particle perimeter (l) and the equivalent area circle perimeter (l) according to the following equation (2).

[0080]

number

[0081] The number of analyzed particles and the number of particles with a circularity of 0.953 or more (high circularity alumina particles) were counted, and the ratio of these numbers (number of high circularity alumina particles / number of analyzed alumina particles) was calculated as the ratio of high circularity particles.

[0082] <Zeta potential> The zeta potential of the alumina powder was measured. Specifically, an alumina powder dispersion (solvent: distilled water) was prepared by dispersing the alumina powder using a homogenizer (Nippon Seiki Seisakusho Co., Ltd., US-300T) at 300 W, 20 kHz, and for 1 minute. The dispersion was then placed in a cell with electrodes and attached to a zeta potential measurement device (Spectris, Zetasizer Nano ZS) in a designated position. A voltage was applied to the dispersion, causing the alumina particles to electrophoretically migrate. A laser beam was irradiated onto the cell, and the scattered light from the alumina particles was detected.

[0083] (3) Evaluation results The evaluation results obtained for Examples 1 to 6 and Comparative Examples 1 to 7 are summarized in Table 2 below.

[0084] The alumina powders of Examples 1 to 6, which had appropriate compositions and amounts of additives, had a high percentage of high circularity particles of 28% or more. In contrast, the alumina powders of Comparative Examples 1 to 7 had a low percentage of high circularity particles of 14% or less. A more detailed study revealed the following.

[0085] Comparing the results of Examples 3 and 4 and Comparative Examples 1 and 2, it was confirmed that when the amount of SiO2 added was 0.05 mass% or less or 0.20 mass% or more, which was outside the range of the appropriate amount, the particle shape was not disk-like but jagged, and the circularity was lower than that of hexagonal plate-like. This shows that by optimizing the amount of SiO2 added, disk-shaped alumina particles with high circularity can be obtained.

[0086] Comparing the results of Examples 3 and 5 and Comparative Example 1, it was found that disc-shaped alumina particles could be obtained by optimizing the amount of AlF3 added. However, if the amount of AlF3 added was too large, the particle size became small.

[0087] From the results of Comparative Example 3, it was confirmed that when the amount of SiO2 added to the amount of AlF3 added exceeded a certain ratio, α-alumina was no longer detected in the alumina powder. It is believed that the phase transition to α-alumina did not proceed, and the alumina remained as transition alumina.

[0088] From the results of Comparative Example 4, it was confirmed that when only AlF3 was added and fired, an alumina powder containing a mixture of spherical particles in addition to plate-like particles with small particle diameters was obtained.

[0089] From the results of Comparative Examples 5 and 6, it was confirmed that when only SiO2 was added and fired, α-alumina was not detected because there was no AlF3 to promote the phase transition to α-alumina. The same was true when no additive was added, and α-alumina was not detected.

[0090] Comparison of the results of Example 4 and Comparative Example 7 revealed that when the firing temperature was changed from 1100°C to 1000°C, the amount of heat applied to the raw material mixture during firing was insufficient, making it difficult for particle growth to proceed, and disc-shaped particles could not be obtained.

[0091] [Table 2]

[0092] A graph of the zeta potential measurement results obtained for the alumina powder of Example 6 is shown in Figure 3. Figure 3 also shows the zeta potential measurement results for commercially available plate-like polygonal alumina powders A and B. The graph shown in Figure 3 shows the relationship between zeta potential and total scattering intensity. Here, total scattering intensity is a quantity that represents the amount of light or brightness of scattered light from particles. The properties of the alumina powder of Example 6 and the plate-like polygonal alumina powders A and B are shown in Table 3 below.

[0093] As shown in Figure 3, the alumina powder of Example 6 had a higher total scattering intensity (quantity and brightness of scattered light) than the platelet polygonal alumina powders A and B. The alumina powder of Example 6 had almost the same particle size as the platelet polygonal alumina powders A and B. Nevertheless, the result that the quantity and brightness of scattered light were high indicates that the alumina powder of Example 6 had excellent dispersibility.

[0094] [Table 3]

[0095] From the above results, it can be seen that the present embodiment provides a plate-like alumina powder with excellent dispersibility and a method for producing the same.

Claims

1. A plate-like alumina powder comprising a plurality of plate-like α-alumina particles, the plate-like alumina powder having an average particle diameter of 2 μm or more and 100 μm or less and an average thickness of 0.2 μm or more and 3.0 μm or less, When the plate-like alumina powder is analyzed using a dry particle image analyzer, the number ratio (high circularity particle ratio) of particles having a circularity of 0.953 or more (high circularity alumina particles) is 25% or more.

2. 2. The plate-like alumina powder according to claim 1, wherein the average particle diameter is 5 μm or more and 50 μm or less, the average thickness is 0.3 μm or more and 1.0 μm or less, and the proportion of high circularity particles is 40% or more.

3. Alkali metal (AM) ２ The silicon (Si) content is 0.01 mass % or more and 0.5 mass % or less in terms of O, and the silicon (Si) content is SiO ２ The plate-like alumina powder according to claim 1, wherein the plate-like alumina powder contains 0.01 mass % or more and 0.3 mass % or less in terms of the amount of SiO 2 converted into SiO 2 .

4. The plate-like alumina powder according to claim 3, wherein the alkali metal (AM) is one or both of sodium (Na) and potassium (K).

5. A method for producing the plate-like alumina powder according to any one of claims 1 to 4, comprising the following steps: preparing a raw material mixture containing aluminum hydroxide powder and an additive; and a step of firing the raw material mixture at a temperature of 900°C or higher and 1300°C or lower; Equipped with The raw material mixture contains an alkali metal (AM) ２ O equivalent of 0.01 mass % or more and 0.5 mass % or less, silicon (Si) is SiO ２ and fluorine (F) in an amount of 0.1% by mass to 5.0% by mass, calculated as F.

6. The additive is an alkali silicofluoride (AM ２ SiF ６ ), or aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ 6. The method of claim 5, comprising:

7. The additive is an oxide of an alkali metal (AM) ２ O) and carbonate (AM ２ CO ３ 7. The method of claim 6, further comprising one or both of:

8. 6. The method of claim 5, wherein the alkali metal (AM) is one or both of sodium (Na) and potassium (K).

9. A paint or cosmetic product comprising the platelet alumina powder according to any one of claims 1 to 4.

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