Highly luminescent scale-like α-alumina powder and method for producing the same

Abstract

To provide scale-like α-alumina powder having a large average particle diameter and aspect ratio and excellent luminosity, and a method for producing the scale-like α-alumina powder by a simple method.SOLUTION: A highly luminescent scale-like α-alumina powder has an average particle diameter 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, an average aspect ratio of 50 or more and 600 or less, and a luminosity intensity Si value of 1.00 or more and 15.00 or less at measurement angles θ of 15° and 45° in luminosity measurement.SELECTED DRAWING: Figure 5

Description

Technical Field

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

Background Art

[0002] α-alumina (Al 2 O 3 ) is chemically stable, excellent in heat resistance, corrosion resistance, abrasion resistance, and insulation, and further has high strength and hardness. Taking advantage of this feature, α-alumina powder is widely used in various applications such as structural members, tools, abrasives, fillers, spark plugs, insulators, electronic substrates, and refractories.

[0003] In particular, flaky α-alumina powder has a high light reflectance on the particle surface and high brightness. Therefore, taking advantage of this feature, it is used as a high-brightness pigment added to paints and cosmetics. Also, in pigment applications, an inorganic coating composed of TiO 2 etc. is applied to the particle surface of flaky α-alumina powder. Since light interference occurs due to the coating, it is said that the glossiness is increased by utilizing this interference.

[0004] Furthermore, flaky α-alumina powder has high strength and excellent gas barrier properties. Therefore, it is used as a reinforcing material or a gas barrier material added to plastics and resin films. Note that 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 flake.

[0005] Patent Document 1 discloses Al 50 flake having a thickness of 500 nm or more, a D 90 value of 15 to 30 μm, and a D 2 O 3 value of 30 to 45 μm, and the Al 2 O 3Flakes are described as having high chemical stability, a smooth surface, and high whiteness simultaneously and being used as a pigment substrate (Claims 1,

[0010] , and

[0015] of Patent Document 1). Patent Document 1 also mentions that Al 2 O 3 Flakes are coated with a high refractive index layer such as TiO 2 or a low refractive index layer such as SiO 2 Thereby, an increased luster, interference color, or color flop effect is imparted (

[0040] -

[0046] of Patent Document 1).

[0006] Patent Document 2 discloses a bright pigment-containing paint composition containing alumina flakes as a bright pigment, which can be suitably used as an automotive topcoat paint, and forms a novel composite coating film with an unprecedented strong brightness and high design property without degrading the finish appearance, etc. (Claims 1,

[0001] , and

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

[0007] of Patent Document 2).

[0007] )

[0007] Patent Document 3 discloses a nacreous luster pigment containing flaky alumina crystals mainly composed of aluminum oxide and zinc oxide in a mass ratio of 100:0.1 - 5 and 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 diameter of 15 μm or more, and an aspect ratio of 50 or more, so they have excellent luster (

[0001] of Patent Document 3).

[0008] Patent Document 4 discloses a method for producing plate-shaped alumina-based powder, which is characterized in that plate-shaped boehmite is fired at 400°C to 1500°C to have a crystal structure of α, β, γ-alumina alone or two or more crystal structures (Claim 2 of Patent Document 4). Patent Document 4 also describes that by surface coating plate-shaped boehmite or plate-shaped alumina powder with a compound exhibiting hydrophobicity such as polysiloxane and blending this coated powder into cosmetics, cosmetics with good usability can be obtained (

[0013] of Patent Document 4).

[0009] Patent Document 5 discloses hexagonal plate-shaped alumina obtained by firing hexagonal plate-shaped boehmite at a temperature of 450 to 1500°C, having a substantially hexagonal plate shape, a ratio of major axis to minor axis of 1 to 1.3, and an aspect ratio of 40 to 100. It is described that the orientation is high, the diffuse reflection is small, the brilliance is increased, and it can also be suitably used as a filler for enhancing the brilliance of paints and cosmetics (Claims 4 and

[0045] of Patent Document 5).

[0010] Patent Document 6 discloses plate-shaped alumina particles characterized by 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, being polygonal plate-shaped, and containing molybdenum in the particles (Claim 1 of Patent Document 6). Patent Document 6 also describes that the plate-shaped alumina particles can be suitably used for thermal conductivity fillers, cosmetics, abrasives, high-brilliance pigments, lubricants, substrates for conductive powders, ceramic materials, etc. (

[0101] of Patent Document 6).

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0012] Thus, in fields such as paints and cosmetics, it has been conventionally proposed to use flaky α-alumina powder as a high-brightness pigment. However, as a result of investigations by the present inventors, it has been found that there is room for improvement in the conventional technology. That is, it is desirable to enhance the brilliance of alumina powder when used as a high-brightness pigment. However, conventional flaky α-alumina powder has limitations in enhancing the reflectance of the particle surface.

[0013] In particular, it has been difficult to increase the average particle diameter and aspect ratio of conventional flaky alumina powder. That is, the larger the average particle diameter and aspect ratio, the larger the area of the plate surface of the flat particles. Therefore, even if the surface reflectance is the same, the larger the area of the plate surface, the more likely it is to obtain alumina powder with excellent brilliance. Conventional alumina powder has an insufficiently large average particle diameter and aspect ratio, and there are limitations in enhancing the brilliance.

[0014] Furthermore, as proposed in Patent Documents 1 to 3, although a technique for improving the glossiness by utilizing the interference effect of light by an inorganic coating has been proposed, such a technique cannot obtain flaky α-alumina powder at low cost. That is, after producing flaky α-alumina powder, it is necessary to separately form an inorganic coating such as TiO 2 on its surface, which may increase the manufacturing cost.

[0015] In view of such conventional problems, the present inventors have conducted intensive studies. As a result, it has been found that scaly α-alumina powder can be obtained by a simple method of firing a raw material mixture containing scaly boehmite powder and an additive having a predetermined composition, and the obtained scaly α-alumina powder has a large average particle diameter and aspect ratio and excellent brilliance.

[0016] The present invention has been completed based on such findings, and an object thereof is to provide a scaly α-alumina powder having a large average particle diameter and aspect ratio and excellent brilliance, and a production method capable of obtaining the scaly α-alumina powder by a simple method.

Means for Solving the Problems

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

[0018] (1) The average particle diameter is 20 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, In the brilliance measurement, the brilliance intensity Si values at the measurement angles θ of 15° and 45° are in the range of 1.00 or more and 15.00 or less, and the high-brilliance scaly α-alumina powder.

[0019] (2) The high-brilliance scaly α-alumina powder according to the above (1), wherein the average particle diameter 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) is contained in an amount of 0.2% by mass or more and 5.0% by mass or less in terms of AM 2 O conversion, and silicon (Si) is contained in an amount of SiO 2The high-brightness flaky α-alumina powder of (1) or (2) above, contained in an amount of 0.3% by mass or more and 10.0% by mass or less in terms of conversion.

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

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

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

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

[0025] (8) The method of (7) above, wherein the flaky boehmite powder has an average particle diameter 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 method of (7) or (8) above, wherein the additive contains aluminum fluoride (AlF 3 ) and silicon oxide (SiO 2 ).

[0027] (10) The additive is an oxide of an alkali metal (AM) (AM2 O) and / or carbonate (AM 2 CO 3 ) further comprising the method of (9) above.

[0028] (11) The method of (7) or (8) above, wherein the additive comprises an alkali metal fluoride (AM 2 SiF 6 ) is included.

[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 comprising the high-brightness flaky α-alumina powder according to any one of (1) to (6) above.

Advantages of the Invention

[0031] According to the present invention, there are provided a flaky α-alumina powder having a large average particle diameter and aspect ratio and excellent in luster, and a production method capable of obtaining this flaky α-alumina powder by a simple method.

Brief Description of the Drawings

[0032]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0033] A specific embodiment of the present invention (hereinafter referred to as "this embodiment") will be described. Note that the present invention is not limited to the following embodiments, and various modifications are possible without changing the gist of the present invention.

[0034] <<1. High-brightness flaky α-alumina powder>> The high-brightness flaky α-alumina powder of this embodiment (hereinafter sometimes simply referred to as "α-alumina powder" or "alumina powder") is a powder containing α-alumina (Al 2 O 3 ) as the main component. α-alumina has a trigonal corundum crystal, is excellent in chemical stability, heat resistance, corrosion resistance, abrasion resistance, and insulation, and has high strength and hardness. Furthermore, it has a high light reflectance, high brightness, and high whiteness. In the alumina powder of this embodiment, such characteristics can be fully utilized. In this specification, the powder means an aggregate of a large number of independent particles. That is, a large number of particles aggregate to form a powder. Since each of the particles constituting the powder is independent, the powder exhibits fluidity as a whole. As long as the powder exhibits fluidity as a whole, some of the particles may be combined to form aggregates.

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

[0036] The alumina powder of this embodiment has particles that are scaly (flaky, flat, plate-shaped). That is, each particle has a large plate surface area and a small plate thickness. Specifically, the alumina powder has an average particle diameter 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. Here, the average particle diameter and the average thickness are the number-based average values of the particle diameters and thicknesses of the respective particles that make up the alumina powder. The particle diameter is the major axis of the particle. That is, the longest diameter of the particle plate surface is taken as the particle diameter. The thickness is the plate thickness of the plate surface. Further, the average aspect ratio is 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 make up the alumina powder with a scanning electron microscope (SEM).

[0037] By increasing the particle diameter (major axis) and the aspect ratio, it becomes possible to enhance the brilliance of the alumina powder. This will be described with reference to FIG. 1. As shown in FIG. 1, when a powder with a high aspect ratio is formed on a substrate by a method such as coating, the large particle plate surfaces are likely to be oriented parallel to the substrate surface. And the larger the particle diameter, the larger the surface area of the particle plate surfaces that are aligned parallel to the substrate surface. Also, the light incident from the outside is reflected by the particle plate surfaces and radiated to the outside as reflected light. Therefore, the larger the particle diameter and the aspect ratio, the easier it is for the incident light to be reflected by the particle plate surfaces, and as a result, the brilliance in visual recognition becomes higher.

[0038] The larger the particle diameter and aspect ratio are, the higher the brilliance of the alumina powder becomes. If the average particle diameter 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, alumina powder with an average particle diameter exceeding 200 μm, an average thickness less than 0.20 μm, or an average aspect ratio exceeding 600 is difficult to manufacture. Also, its strength is low and handling is difficult. The average particle diameter 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, more preferably 0.20 μm or more and 0.30 μm or less. Preferably, the average particle diameter 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, more preferably 250 or more and 600 or less.

[0039] Note that not all particles constituting the alumina powder need to be scaly. As long as the alumina powder as a whole satisfies the requirements for the average particle diameter, average thickness, and average aspect ratio described above, the alumina powder may contain particles having a shape other than scaly.

[0040] In the measurement of brilliance of the high-brilliance scaly α-alumina powder of the present embodiment, the brilliance intensity Si value at the measurement angles θ of 15° and 45° is in the range of 1.00 or more and 15.00 or less. Here, the brilliance intensity Si is a value obtained during the brilliance evaluation described later and is an index indicating the intensity of the luminance based on the bright spot of the sample (alumina powder). The larger the brilliance intensity Si value is, 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. Also, alumina powder with a brilliance intensity Si value exceeding 15.00 is difficult to manufacture. The brilliance intensity Si value is preferably 5.00 or more and 15.00 or less, more preferably 7.00 or more and 15.00 or less.

[0041] Preferably, the Sa value of the brightening area at the measurement angles θ of 15° and 45° is 1.00 or more and 30.00 or less. The brightening area Sa is a value obtained during the brightening sensation evaluation and is an index indicating the size of each bright spot of the sample (alumina powder). The Sa value of the brightening area 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 brightening sensation of the alumina powder can be evaluated using a multi-angle colorimetric, brightening sensation, and particle size measuring instrument (BYK, BYK-mac i). The evaluation method will be described with reference to FIGS. 2 and 3. During the measurement, first, a sample holder attached to the apparatus is filled with a sample (alumina powder) that is ground to the maximum capacity. Then, the sample holder containing the sample is set in the apparatus for measurement. In the apparatus, a CCD chip is provided on the extension of the vertical axis with respect to the measurement surface of the sample holder. Also, a light source is provided at a position separated from the vertical axis by a predetermined measurement angle θ. As shown in FIG. 2, during the measurement, light is incident from the light source toward the sample, is reflected and scattered on the measurement surface, and then enters 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 illustrated in FIG. 3, in the image data, a large number of bright spots based on each particle constituting the sample (alumina powder) are observed. By analyzing and quantifying the obtained image data, the brightening intensity Si value and the brightening area Sa value can be determined.

[0043] The high-brightness flaky α-alumina powder of the present embodiment preferably contains an alkali metal (AM) in an amount of 0.2% by mass or more and 5.0% by mass or less in terms of AM 2 O, and contains silicon (Si) in an amount of 0.3% by mass or more and 10.0% by mass or less in terms of SiO 2 The content of the alkali metal (AM) is more preferably 0.2% by mass or more and 2.0% by mass or less in terms of AM 2 O, and even more preferably 0.2% by mass or more and 1.0% by mass or less. The content of silicon (Si) is in terms of SiO 2More preferably, the amount is 0.5% by mass or more and 3.0% by mass or less in terms of the total mass of the alumina powder. In addition, the alkali metal (AM) is preferably at least one selected from the group consisting of sodium (Na) and potassium (K), that is, sodium (Na) and potassium (K). By including these components, it is possible to further enhance the brilliance of the alumina powder.

[0044] To explain this point, as 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, the crystallization, grain growth, and surface smoothing of the alumina particles are promoted. This makes it possible to obtain alumina powder with a large average particle size and aspect ratio, a smooth particle surface, 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. In other words, the liquid phase components generated during the firing process when manufacturing the alumina powder are segregated as a different phase such as glass on the particle surface and / or inside the manufactured alumina powder. The different phase (glass phase, etc.) has a different refractive index from alumina. Therefore, it is speculated that the presence of such a different phase strengthens 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 amount 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 amount of alkali metal and silicon are inappropriate, the brilliance may decrease.

[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 satisfied, other components (elements) other than the essential components (Al, AM, Si, O) may be included. Also, the presence of impurities inevitably mixed in during the manufacturing process is allowed. Examples of such other components include fluorine (F). Among these, fluorine (F) is a component derived from the additive added during the production of alumina powder. That is, as will be described later, the additive added during the production of alumina powder contains fluorine (F). Although most of the fluorine volatilizes during the firing process, some may remain. The amount of fluorine in the alumina powder is typically 0.5% by mass or less, 0.3% by mass or less, or 0.1% by mass or less.

[0048] Also, if a large amount of other components is contained, 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, still 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 suppress its content. Preferably, the content of iron (Fe) is 0.05% by mass or less in terms of iron oxide (Fe 2 O 3 ).

[0049] The specific surface area (S BET ) of the high-brightness flaky α-alumina powder of this embodiment is preferably 0.1 m 2 / g or more and 5.0 m 2 / g or less, and more preferably 0.5 m 2 / g or more and 2.0 m 2 / g or less. If the specific surface area is excessively large, the particle size of the alumina powder becomes small, so there is a risk that the brilliance will decrease. On the other hand, it is difficult to produce 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 polycrystals. That is, each particle constituting the alumina powder is in a polycrystalline state. By being composed of polycrystals, it becomes possible to further enhance the brilliance of the alumina powder. Although the details of the reason are unclear, it is presumed that silicate (AM 2 SiO 3 ) is generated by the essential components (AM, Si, O), and this promotes the fusion and adhesion of alumina particles to cause polycrystallization.

[0051] The high-brightness flaky α-alumina powder of this embodiment has high chemical stability because α-alumina is the main component. It also has the characteristics of a large average particle size and aspect ratio, and excellent brilliance. The α-alumina powder having such characteristics is preferably used in the fields of paints and cosmetics. However, the α-alumina powder of this embodiment is not limited to those used in paints and cosmetics. It can be applied to known uses such as reinforcing materials and gas barrier materials mixed into plastics and resin films, or structural members, tools, abrasives, fillers, spark plugs, insulators, electronic substrates, and refractories.

[0052] <<2. Manufacturing method of high-brightness flaky α-alumina powder>> The manufacturing method of the high-brightness flaky 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 preferred manufacturing method includes a step of adding an additive to flaky boehmite powder and then mixing to prepare a raw material mixture (raw material mixing step), and a step of firing this raw material mixture at a temperature within the range of 1000°C or higher and 1300°C or lower (firing step). Also, if necessary, a post-treatment step may be provided after the firing step. The raw material mixture contains an alkali metal (AM) in an amount of 0.5 mass% or more and 10.0 mass% or less in terms of AM 2 O conversion, silicon (Si) in an amount of 0.1 mass% or more and 10.0 mass% or less in terms of SiO 2 conversion, and fluorine (F) in an amount of 0.1 mass% or more and 5.0 mass% or less. Each step will be described in detail below.

[0053] <Raw Material Mixing Step> In the raw material mixing step, an additive is added to the flaky boehmite powder (hereinafter sometimes simply referred to as "boehmite powder") and then mixed to prepare a raw material mixture. The flaky boehmite powder is a flaky powder mainly composed of boehmite (AlOOH) which is aluminum monohydrate, and is an aluminum source of the alumina powder. By using the flaky boehmite powder as a raw material, an alumina powder having a large average particle diameter and aspect ratio can be obtained. That is, in the subsequent firing step, boehmite is transferred to α-alumina via intermediate alumina. When the flaky boehmite powder is used as a raw material, since its particle shape is inherited, a flaky α-alumina powder can be obtained.

[0054] The size of the boehmite powder is not particularly limited as long as the alumina powder of the present 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. By using the boehmite powder having such a size, it becomes possible to easily obtain the alumina powder of the present embodiment. Further, the boehmite powder having such a size has a known production method and is easily available. For example, Patent Document 5 discloses a method for producing hexagonal plate-like boehmite in which aluminum hydroxide and boric acid are hydrothermally treated by adding a pH adjuster such as sodium hydroxide, and the obtained hexagonal plate-like boehmite has an outer diameter size of 0.7 to 15.0 μm and an aspect ratio of 40 to 100 (Claim 2,

[0017] and

[0018] of Patent Document 5).

[0055] The additive is a component that becomes a liquid phase in the subsequent firing process and acts as a mineralizer. The additive contains at least silicon (Si) and fluorine (F), and may further contain an alkali metal (AM) as required. As described later, when the composition of the liquid phase component is appropriately controlled, the crystallization, grain growth, and surface smoothing of alumina particles are promoted. As a result, it becomes possible to obtain alumina powder having a large average particle size, a large aspect ratio, a smooth particle surface, and excellent luster. In addition, the liquid phase component exists as a foreign phase such as glass in the alumina powder after production, and has an effect of further enhancing the luster.

[0056] Therefore, in order to obtain alumina powder having a large average particle size, a large aspect ratio, and excellent luster, it is important to appropriately control the composition of the raw material mixture containing the additive. Specifically, the raw material mixture contains an alkali metal (AM) in an amount of 0.5% by mass or more and 10.0% by mass or less in terms of AM 2 in terms of O conversion, silicon (Si) in an amount of 0.1% by mass or more and 10.0% by mass or less in terms of SiO 2 conversion, and fluorine (F) in an amount of 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-described range, alumina powder having a large average particle size, a large aspect ratio, and excellent luster can be obtained. On the other hand, if the composition of the raw material mixture is inappropriate, the luster of the alumina powder may decrease.

[0057] The amount of the alkali metal (AM) in the raw material mixture is more preferably 0.5% by mass or more and 5.0% by mass in terms of AM 2 in terms of O conversion, and even more preferably 0.5% by mass or more and 3.0% by mass or less. 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 amount of silicon (Si) in the raw material mixture is in terms of SiO 2It is more preferably 0.5% by mass or more and 10.0% by mass or less, and even more preferably 1.0% by mass or more and 5.0% by mass or less in terms of conversion. The amount of fluorine (F) 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 within the above-described range, it becomes possible to further enhance the brilliance of the obtained 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 material boehmite powder may contain an alkali metal (AM) as an impurity. When the amount of the alkali metal in the boehmite powder is sufficient, the additive may not contain an alkali metal. However, when the amount of the alkali metal in the boehmite powder is insufficient, it is preferable to add an alkali metal as an additive. Further, the additive may contain aluminum (Al). When the additive contains aluminum, this aluminum is incorporated into the alumina powder in the firing step.

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

[0060] According to a preferred embodiment, the additive contains aluminum fluoride (AlF 3 ) and silicon oxide (SiO 2 ). When the amount of the alkali metal in the boehmite powder is insufficient, the additive contains an oxide (AM 2 O) and / or a carbonate (AM2 CO 3 )), that is, an oxide (AM 2 O) and a carbonate (AM 2 CO 3 ). It is preferably further included at least one selected from the group consisting of. According to another preferred embodiment, the additive is an alkali silicofluoride (AM 2 SiF 6 ). By using these compounds (AlF 3 , SiO 2 , AM 2 O, AM 2 CO 3 , AM 2 SiF 6 ) as an additive in an appropriate amount, it is possible to surely produce alumina powder having a large average particle diameter and aspect ratio and excellent luster. Also, these compounds are inexpensive and easily available. Therefore, scaly alumina powder can be obtained at a lower cost.

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

[0062] <Firing step> In the firing step, the raw material mixture containing scaly boehmite powder and the additive is fired at a temperature in the range of 1000 °C or higher and 1300 °C or lower to obtain a fired product. During firing, boehmite (AlOOH) passes through intermediate alumina such as metastable γ-Al 2 O 3 and transforms into high-temperature stable α-alumina (Al 2 O 3 ). Since the particle shape is maintained during the transition, scaly α-alumina (Al 2 O 3) is obtained. Also, during firing, the alkali metal (AM), silicon (Si), and fluorine (F) in the raw material mixture become a liquid phase in the oxide state and act as a mineralizer that promotes the crystallization of α-alumina. That is, when transitioning from intermediate alumina to α-Al 2 O 3 , the crystal structure changes from a cubic close-packed structure to a hexagonal close-packed structure, and accordingly, the oxygen in the crystal rearranges. Therefore, a certain amount of thermal energy needs to be added for the transition to α-Al 2 O 3 . By adding a mineralizer, the crystallization of the α-Al 2 O 3 crystal structure (corundum structure) is promoted, so the transition temperature can be lowered.

[0063] Also, the mineralizer components (AM, Si, F) that become a liquid phase during firing cover the surface of the alumina particles and partially penetrate into the particles. This liquid phase component has the function of promoting the diffusion and growth of the particles. That is, a part of the aluminum atoms (Al) dissolves into the liquid phase from the surface of the alumina particles, and the dissolved aluminum precipitates at another location on the particle surface. As a result, the growth of the alumina particles occurs. At this time, depending on the composition of the liquid phase component, differences occur in the growth direction of the alumina particles and the growth rate changes. Therefore, when the composition of the liquid phase component is appropriately controlled, the growth of the alumina particles in the plate surface direction proceeds significantly. Also, the liquid phase component has the effect of smoothing the plate surface of the alumina particles. If there are irregularities on the surface of the alumina particles, these irregularities preferentially diffuse into the liquid phase, so the irregularities disappear. Therefore, when the composition of the liquid phase component is appropriately controlled, it becomes possible to obtain alumina powder with a large average particle diameter, a large aspect ratio, a smooth particle surface, and excellent luster.

[0064] If the firing temperature is less than 1000°C, the transition to α-alumina and grain growth may be insufficient. Therefore, it may be difficult to obtain α-alumina powder with excellent luster. On the other hand, if the firing temperature is over 1300°C, particles may sinter together to form a strong sintered mass. Also, the energy consumption for firing may become excessive, leading to an increase in manufacturing costs. The firing temperature is preferably 1000°C or more and 1200°C or less, more preferably 1000°C or more and 1100°C or less. Also, the firing time is preferably 10 hours or more and 20 hours or less. This can ensure the transition to α-alumina and grain growth while preventing excessive sintering of the particles. The firing furnace is not limited as long as the desired alumina powder can be obtained. However, from the viewpoint of effectively exerting the action of the additive during firing, a stationary furnace that can be fired using a sealed firing container is preferred.

[0065] <Post-treatment> If necessary, the fired product obtained through the firing process may be subjected to post-treatment such as desoda treatment, crushing treatment, and / or classification treatment. In the desoda treatment, excessive alkali metal components adhering to the surface of the fired product are removed. This can adjust the amount of alkali metal in the finally obtained α-alumina powder. The alkali metal components on the particle surface can be removed, for example, by washing and filtering the fired product with water. In the crushing treatment, mild mechanical energy is applied to the fired product to break the bonding of the aggregated particles formed during firing. The crushing treatment can be performed dry or wet using a crusher such as a pot mill, pin mill, and / or jaw crusher. In the classification treatment, particles are sorted according to size to obtain alumina powder with a desired particle size. The classification treatment can be performed by methods such as sieving, air classification, elutriation, and / or centrifugal separation. The post-treatment may be performed as necessary. If the desired alumina powder can be obtained after firing, the post-treatment may be omitted.

[0066] In this way, the flaky α-alumina powder of the present embodiment can be produced. The obtained alumina powder has a high average particle diameter and aspect ratio and excellent luster. Therefore, it can be suitably used for various applications including paints and cosmetics.

[0067] <<3. Paint and Cosmetics>> The paint or cosmetic of this embodiment contains the above-described high-brightness flaky α-alumina powder. The paint or cosmetic may contain the alumina powder alone or in a surface-treated form. For the surface treatment, a surface treatment agent such as a silicon-based compound, an alkylsilane-based compound, and / or a fluorine-based compound may be provided on the particle surface of the flaky α-alumina powder. Further, the paint and cosmetic may contain a solvent and a resin in addition to the flaky α-alumina powder. The solvent may be aqueous or non-aqueous. Furthermore, the paint and cosmetic may contain known additive components such as an oil agent, a pigment other than the flaky α-alumina powder, a filler, a surfactant, a viscosity modifier, a preservative, a fragrance, a humectant, a physiologically active ingredient, salts, a chelating agent, a neutralizing agent, and / or a pH adjuster.

Example

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

[0069] (1) Preparation of Alumina Powder [Example 1] <Raw Material Mixing Step> Flaky boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160) was prepared as an alumina source. Also, aluminum fluoride (DO-FLUORIDE CHEMICALS CO., LTD.; AlF 3 ) and silicon oxide (Maruka Kamaoto Ceramics Co., Ltd. Snowprint Silica SP-3; SiO 2 ) were prepared. The characteristics of the prepared flaky boehmite powder are shown in Table 1 below.

[0070] Next, 50 g of the prepared flaky boehmite powder, 1.2 g of aluminum fluoride (AlF 3 ), and 0.4 g of silicon oxide (SiO 2 ) were mixed to obtain a raw material mixture. The mixing was performed by putting the raw materials in a bag and shaking them. The obtained raw material mixture contained sodium (Na) as Na2 0.7 mass% in terms of oxygen, containing 0.8 mass% of silicon (Si) in terms of SiO 2 and 1.6 mass% of fluorine (F).

[0071] <Firing Process> The obtained raw material mixture was put into a crucible (Nikkato Corporation, SSA-H 500 mL), and the crucible was closed with a dedicated lid. This crucible had a composition of Al 2 O 3 : 95%, SiO 2 : 3%. Subsequently, the covered crucible was placed in a firing furnace (Silconit Corporation, box-type Silconit electric furnace, BSH-3570) and fired in the air. 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 cooled naturally. After the temperature in the furnace had dropped completely, the crucible was taken out of the furnace, and the fired product (alumina) was recovered.

[0072] <Desoda Process> The obtained fired product (alumina) was subjected to desoda treatment. Specifically, 340 g of the fired product was put into 7 L of pure water and then mixed to form a slurry. Subsequently, the obtained slurry was filtered to obtain an alumina residue. This removed the sodium content adhering to the fired product.

[0073] <Crushing Process> The obtained alumina residue was subjected to crushing treatment. Specifically, pure water was added again to the alumina residue and then stirred to form a slurry. The obtained slurry was put into a pot mill container with an internal volume of 10 L together with 15 kg of 1 mm Φ zirconia beads, and the pot mill container was rotated at a rotational speed of 60 rpm for 3 hours. Subsequently, the slurry was taken out of the pot mill container and subjected to filtration treatment to obtain a crushed product (alumina). Then, using a dryer, the obtained crushed product was dried at 105 °C overnight.

[0074] <Classification Process> 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 gyro sifter (Tokuju Kousakusho Co., Ltd., GS-A1H) with an aperture of 25 μm. The sieving was performed under the condition of a rotation speed of 60 rpm to remove coarse particles with a size exceeding 25 μm. Thereafter, the material passing through the sieve from which the coarse particles had been removed was filtered using a SUS mesh with an aperture of 12 μm to remove fine particles with a size less than 12 μm. In this way, alumina powder was produced.

[0075] [Example 2] In the raw material mixing step, 50 g of flaky boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160), 1.2 g of aluminum fluoride (AlF 3 ), and 0.6 g of silicon oxide (SiO 2 ) were mixed to obtain a raw material mixture. The obtained raw material mixture contained 0.7% by mass of sodium (Na) in terms of Na 2 O, 1.2% by mass of silicon (Si) in terms of SiO 2 O, and 1.6% by mass of fluorine (F). Also, without performing the desoda treatment and the crushing treatment, the classification process was carried out in the following procedure. Otherwise, alumina powder was produced in the same manner as in Example 1.

[0076] <Classification step> For the fired product (alumina) obtained in the firing step, elutriation classification using a winnowing tube and particle size adjustment using a SUS mesh were performed to remove extremely thick particles and fine particles. Specifically, after adding pure water to 300 g of the fired product and stirring, a slurry with a concentration of 100 g / L was obtained. The obtained slurry was put into a classification device (winnowing tube), and pure water was continuously introduced thereon at a flow rate of 1 L / min for 3 hours. Thereafter, the slurry containing the precipitate was recovered from the classification device. Next, manual sieving was performed successively using a SUS mesh with an aperture of 63 μm and a SUS mesh with an aperture of 45 μm to remove fine particles with a size less than 45 μm from the recovered slurry. The residue obtained after the manual sieving was dried overnight using a dryer.

[0077] [Example 3] In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160), 1.2 g of aluminum fluoride (AlF 3 ), and 0.8 g of silicon oxide (SiO 2 ) were mixed to obtain a raw material mixture. The obtained raw material mixture contained 0.7% by mass of sodium (Na) in terms of Na 2 O, 1.6% by mass of silicon (Si) in terms of SiO 2 , and 1.6% by mass of fluorine (F). Also, without performing the desoda treatment and the crushing treatment, the classification treatment was carried out according to the following procedure. Otherwise, alumina powder was produced in the same manner as in Example 1.

[0078] <Classification step> For the fired product (alumina) obtained in the firing step, elutriation classification using a winnowing tube and particle size adjustment using a SUS screen were performed to remove extremely thick particles and fine particles. Specifically, after adding pure water to 300 g of the fired product and stirring to obtain a slurry with a concentration of 100 g / L. The obtained slurry was put into a classification device (winnowing tube), and pure water was continuously poured onto it at a flow rate of 1 L / min for 3 hours. Then, the slurry containing the precipitate was recovered from the classification device. Next, hand sieving was performed using a SUS screen with an opening of 45 μm to remove fine particles with a size of less than 45 μm from the recovered slurry. The residue obtained after hand sieving was dried overnight using a dryer.

[0079] [Example 4] In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160), 1.2 g of aluminum fluoride (AlF 3 ), 0.8 g of silicon oxide (SiO 2 ), and 0.08 g of sodium carbonate (Na 2 CO 3 ) were mixed to obtain a raw material mixture. The obtained raw material mixture contained 0.6% by mass of sodium (Na) in terms of Na 2 O, 1.6% by mass of silicon (Si) in terms of SiO 2 , and 1.6% by mass of fluorine (F). Also, without performing the desoda treatment and the crushing treatment, the classification treatment was carried out according to the following procedure. Otherwise, alumina powder was produced in the same manner as in Example 1.

[0080] <Classification process> For the fired product (alumina) obtained in the firing process, elutriation classification using a sifting tube and particle size adjustment using a SUS screen were performed to remove extremely thick particles and fine particles. Specifically, after adding pure water to 300 g of the fired product and stirring, a slurry with a concentration of 100 g / L was obtained. The obtained slurry was put into a classification device (sifting tube), and pure water was continuously introduced onto it at a flow rate of 1 L / min for 3 hours. Then, the slurry containing the precipitate was recovered from the classification device. Next, hand sieving was performed using a SUS screen with an opening size of 63 μm to remove fine particles with a size of less than 63 μm from the recovered slurry. The residue obtained after hand sieving was dried overnight using a dryer.

[0081] [Example 5] In the raw material mixing process, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160) and 2.5 g of sodium hexafluorosilicate (Na 2 SiF 6 ): were mixed to obtain a raw material mixture. The obtained raw material mixture contained 0.7% by mass of sodium (Na) in terms of Na 2 O, 1.6% by mass of silicon (Si) in terms of SiO 2 O, and 2.8% by mass of fluorine (F). Also, no desoda treatment, crushing treatment, and classification treatment were performed. Otherwise, alumina powder was produced in the same manner as in Example 1.

[0082] [Example 6] Scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-920) was used as the alumina source. The obtained raw material mixture contained 0.6% by mass of sodium (Na) in terms of Na 2 O, 1.6% by mass of silicon (Si) in terms of SiO 2 O, and 1.6% by mass of fluorine (F). Also, no desoda treatment, crushing treatment, and classification treatment were performed. Otherwise, alumina powder was produced in the same manner as in Example 3.

[0083] [Example 7] In the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160), 1.2 g of aluminum fluoride (AlF 3 ), 2.4 g of silicon oxide (SiO 2 ), and 1.8 g of sodium carbonate (Na 2 CO 3 ) were mixed to obtain a raw material mixture. The obtained raw material mixture contained 0.7% by mass of sodium (Na) in terms of Na 2 O, 4.4% by mass of silicon (Si) in terms of SiO 2 , and 1.5% by mass of fluorine (F). Also, desoda treatment, crushing treatment, and classification treatment were not performed. Otherwise, alumina powder was produced in the same manner as in Example 1.

[0084] [Example 8] Scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-520) was used as the alumina source. Also, in the raw material mixing step, 50 g of scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-520), 1.2 g of aluminum fluoride (AlF 3 ), 4.8 g of silicon oxide (SiO 2 ), and 1.2 g of sodium carbonate (Na 2 CO 3 ) were mixed to obtain a raw material mixture. The obtained raw material mixture contained 0.6% by mass of sodium (Na) in terms of Na 2 O, 8.4% by mass of silicon (Si) in terms of SiO 2 , and 1.4% by mass of fluorine (F). Furthermore, desoda treatment, crushing treatment, and classification treatment were not performed. Otherwise, alumina powder was produced in the same manner as in Example 1.

[0085] [Example 9] [Raw Material Mixing Step] Scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-520) was used as the alumina source. Also, 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 to obtain Na 2 SiF 6A raw material mixture containing 3.8% by mass was obtained.

[0086] <Firing process> The obtained raw material mixture was put into a crucible (Acceramic Co., Ltd., KR-4A, 300 mm × 300 mm × 100 mH), closed with a special lid and sealed. Then, the sealed crucible was placed in a firing furnace (Siliconit Co., Ltd., box-type siliconit electric furnace, BSH-3570) and fired in the air. 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 in the furnace had dropped completely, the crucible was taken out of the furnace and the fired product (alumina) was recovered.

[0087] The recovered fired product (alumina) was used as alumina powder. At this time, desoda treatment, crushing treatment, and classification treatment were not performed.

[0088] [Example 10] <Raw material mixing process> Scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-920) was used as the alumina source. Also in the raw material mixing process, scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-920) and sodium hexafluorosilicate (Na 2 SiF 6 ) were mixed to obtain a raw material mixture containing 1.3% by mass of Na 2 SiF 6 .

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

[0090] <Crushing process> The obtained fired product (alumina) was subjected to a crushing process. 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 mmΦ zirconia beads, and the pot mill container was rotated at a rotational speed of 60 rpm for 3 hours. Subsequently, the slurry was taken out from the pot mill container to obtain a crushed product (alumina).

[0091] <Classification step> For the crushed product (alumina) obtained in the crushing step, water elutriation classification using a sifting tube and particle size adjustment using a SUS screen were performed to remove extremely thick particles and fine particles. Specifically, the slurry containing the crushed product was charged into a classification device (sifting tube), and pure water was continuously charged thereon at a flow rate of 1 L / min for 3 hours. Thereafter, the slurry containing the precipitate was recovered from the classification device. Subsequently, hand sieving was performed sequentially using a SUS screen with an opening of 108 μm and a SUS screen with an opening of 45 μm, and fine particles with a size of less than 45 μm were removed from the recovered slurry. The residue obtained after hand sieving was dried overnight using a dryer.

[0092] [Example 11] <Raw material mixing step> Scaly boehmite powder (Kawai Lime Industry Co., Ltd., BMF-920) was used as the alumina source. Also, 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 obtain a raw material mixture containing 2.7% of AlF 3 , 1.0 mass% of SiO 2 , and 1.7 mass% of Na 2 CO 3 .

[0093] <Firing step> The obtained raw material mixture was fired to recover a fired product (alumina). The firing was performed 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. At this time, desoda treatment, crushing treatment, and classification treatment were not performed.

[0095] [Example 12] The raw materials were mixed so as to obtain a raw material mixture containing 3.1% by mass of sodium silicofluoride (Na 2 SiF 6 ). Also, the holding time during firing was set to 20 hours. Alumina powder was produced in the same manner as in Example 10 except for this.

[0096] [Comparative Example 1] <Raw material mixing step> As the alumina source, alumina powder having a BET specific surface area of 0.8 to 1.2 m 2 / g produced by rotary kiln firing was prepared. The properties of the prepared alumina powder are shown in Table 1 below.

[0097] Next, alumina powder and silicon oxide (SiO 2 ) were mixed to obtain a raw material mixture. Silicon oxide was added so as to be 0.3% by mass with respect to the alumina powder. Silicon oxide changes sodium (Na) in the alumina powder into the form of sodalite in the melting step described later. Since sodalite can be removed in the pickling step, it is possible to lower the amount of sodium component in the alumina by changing it into the form of sodalite.

[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 molten metal temperature was over 2000°C. After melting, the molten metal was cooled to obtain a molten mass.

[0099] <Crushing and grinding> The obtained molten mass was broken into the size of a human head using a power man, and then sieved using a trommel (25 mm × 25 mm sieve). Then, the pieces larger than 25 mm remaining on the sieve mesh were recovered and ground until grains having a size of several tens of μm to several mm were obtained.

[0100] <Pickling> The obtained crushed material was washed with a 4% sulfuric acid solution to wash the sodalite formed in the fused magnet. The processed material after washing was filtered using a horizontal filter, dried using a kiln, and then sieved to obtain fused alumina with an average particle size of 250 to 350 μm.

[0101] <Crushing> The obtained fused alumina was crushed using a continuous vibration mill to obtain alumina powder. The average particle size of the obtained alumina powder was 18 μm.

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

[0103] <Firing> The prepared aluminum hydroxide A was quantitatively charged into the raw material charging section of the rotary kiln firing equipment. The attached moisture of aluminum hydroxide A was removed in the pre-drying section of the firing equipment, and then aluminum fluoride (AlF 3 ) was quantitatively added. Thereafter, the aluminum hydroxide A added with aluminum fluoride was fired with the fired product. The firing was carried out under the conditions of 1280 to 1380 °C × 2 hours. Thereby, alumina powder was obtained.

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

[0105] [Comparative Example 4] In the raw material mixing step, 50 g of flaky boehmite powder (Kawai Lime Industry Co., Ltd., BMF-1160) and 1.2 g of aluminum fluoride (AlF 3 ) were mixed, but silicon oxide (SiO 2 ) was not added. The obtained raw material mixture contained sodium (Na) as Na2 0.7 mass% in terms of oxygen, containing silicon (Si) at 0.02 mass% in terms of SiO 2 and fluorine (F) at 1.6 mass%. Also, the desoda treatment, crushing treatment, and classification treatment were not performed. Otherwise, alumina powder was produced in the same manner as in Example 1.

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

[0107] [Table 1]

[0108] [Table 2]

[0109] [Table 3]

[0110] (2) Evaluation For the alumina powders obtained in Examples 1 to 12 and Comparative Examples 1 to 4, the evaluation of various properties was carried out as follows.

[0111] <Average particle diameter and thickness> Evaluation was carried out using a scanning electron microscope (JEOL Ltd., JSM-F100; SEM) and image analysis software (ImageJ). Specifically, the SEM image of the obtained alumina powder was taken, and the particle diameter and thickness were measured by evaluating the particles shown in the image using ImageJ. The average particle diameter was the average value of 80 particles, and the average thickness was the average value of 30 particles.

[0112] <Crystal phase> The crystalline phase was evaluated using an X-ray diffractometer (Rigaku Corporation, RINT UltimaIII). Specifically, first, the obtained alumina powder was 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 evaluated using a scanning fluorescence X-ray analyzer (Rigaku Corporation, ZSX PrimusIV). Specifically, first, the obtained alumina powder was placed in a platinum crucible and set in a high-frequency melting apparatus. After preheating at 700 °C for 60 seconds, a rocking treatment was performed at 1200 °C for 120 seconds to prepare a sample vitrified into glass beads. Next, the glass bead sample was evaluated using the scanning X-ray analyzer. The obtained data was compared with the data of the calibration standard sample to obtain the evaluation result.

[0114] <Specific Surface Area> The specific surface area was evaluated by the N 2 gas adsorption method using a specific surface area manual measurement device (Micromeritics Instrument Corp., FlowSorbIII 2305 type).

[0115] <Gloss Evaluation> The gloss of the alumina powder was evaluated using a multi-angle colorimeter (BYK-Gardner, BYK-mac i, measurement aperture: 12 mm). Specifically, a measurement sample was prepared by filling the opening of the sample holder (35 mm Φ × 5 mm H) with the ground alumina powder, and this measurement sample was set in the colorimeter for measurement. During the measurement, a CCD chip was placed on the axis perpendicular to the measurement surface of the sample holder, and light was irradiated from directions at angles of 15°, 45°, or 75° from the perpendicular axis, and an image was taken with the CCD chip. Then, the obtained image was analyzed using an image analysis algorithm to obtain the glossiness Si value and the gloss area Sa value at each measurement angle (15°, 45°, 75°).

[0116] (3) Evaluation Results Table 4 summarizes the evaluation results obtained for the alumina powders obtained in Examples 1 to 12 and Comparative Examples 1 to 4. The SEM images of the alumina powder obtained in Example 1 are shown in Fig. 4, and the SEM images of the alumina powder obtained in Example 3 are shown in Figs. 5 and 6.

[0117] As shown in Table 4, the alumina powders of Examples 1 to 12 prepared using scaly boehmite powder and an additive with a predetermined composition as raw materials had their average particle diameter, average thickness, and average aspect ratio within a predetermined range. Also, the amount of sodium oxide (Na 2 O) and the amount of silicon oxide (SiO 2 ) satisfied the ranges 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 larger the average particle diameter, the higher the brilliance intensity Si value.

[0118] Also, from the SEM images of Figs. 4 and 5, it was found that the alumina powders of Examples 1 and 3 had a scaly shape with a large plate surface and a small plate thickness, and the surface of the plate surface was smooth. From the cross-sectional SEM image and the SEM enlarged image of Fig. 6, it was found that the alumina powder of Example 3 had a polycrystalline structure.

[0119] On the other hand, in Comparative Examples 1 to 3 prepared using alumina or aluminum hydroxide as raw materials, flat alumina powder could not be obtained. Also, the alumina powders of Comparative Examples 1 to 3 had small brilliance intensity Si values. Furthermore, for the alumina powder of Comparative Example 4 which used scaly boehmite powder as a raw material but did not add silicon oxide (SiO 2 ), the average particle diameter and aspect ratio were small, and the brilliance intensity Si value was also small.

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

[0121]

Table 4

Claims

1. The average particle size is 20 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. 2. The high-brightness flaky α-alumina powder according to claim 1, wherein the average particle size is from 20 μm to 120 μm, and the average thickness is from 0.20 μm to 0.50 μm.

3. 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.

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

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

6. 5. 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.

7. A method for producing the high-brightness flaky α-alumina powder according to any one of claims 1 to 4, 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.

8. The method according to claim 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.

9. The additive is aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ 8. The method of claim 7, comprising:

10. The additive is an oxide of an alkali metal (AM). ２ O) and / or carbonate (AM ２ CO ３ 10. The method of claim 9, further comprising:

11. The additive is an alkali silicofluoride (AM ２ SiF ６ 8. The method of claim 7, comprising:

12. 8. The method according to claim 7, wherein the alkali metal (AM) is sodium (Na) and / or potassium (K).

13. A paint or cosmetic comprising the high-brightness scaly α-alumina powder according to any one of claims 1 to 4.

Images