Flaky alumina powder, production method for same, and paint and cosmetic containing same

Abstract

The present invention provides: a flaky alumina powder in which there are few aggregated particles or aggregated particles can be easily separated; a production method for the same; and a paint and a cosmetic containing the same. Provided is a flaky alumina powder composed of a plurality of flaky α-alumina particles, wherein the average thickness is 0.2-3.0 μm, the average particle size (Dp50) after ultrasonic dispersion at an ultrasonic intensity of 600W is 2-100 μm, and the proportion of particles that are at least three times the average particle size Dp50 (3×Dp50) is 1.5% or less. Also provided are a production method for said flaky alumina powder, a production method suitable therefor, and a paint and a cosmetic containing said flaky alumina powder.

Description

Plate-like alumina powder, its manufacturing method, and paints and cosmetics containing the same

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

[0002] Conventionally, plate-like alumina powder has a high light reflectance at its particle surface and high brightness. Therefore, by taking advantage of this characteristic, it has been used as a pearl pigment added to paints, cosmetics, and the like. When plate-like alumina powder is used in cosmetics, for example, the flat surfaces of the plate-like alumina powder reflect light, giving the skin a radiant glow after makeup is applied. Plate-like alumina powder is a powder composed of a plurality of plate-like alumina particles, and is also called scaly alumina powder, thin flaky alumina powder, flat alumina powder, or alumina flakes.

[0003] 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 Al having a value ２ O ３ The Al flakes are disclosed. ２ O ３ It is described that the flakes have high chemical stability, a smooth surface, and high whiteness at the same time, and are used as a pigment substrate (Patent Document 1, claims 1,

[0010] and

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

[0040] to

[0046] ).

[0004] Patent Document 2 discloses a luster pigment-containing paint composition that contains alumina flakes as a luster pigment, and describes that it can be suitably used as an automotive top coat paint, and that it forms a composite coating film with an unprecedented high luster and novel high designability without deteriorating the finished appearance, etc. (Claim 1,

[0001] and

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

[0007] ).

[0005] Patent Document 3 discloses a pearlescent pigment containing flaky alumina crystals that contain aluminum oxide and zinc oxide as main components in a mass ratio of 100:0.1 to 5 and are coated with metal and / or metal precursor particles (claim 1 of Patent Document 3). Patent Document 3 also describes that the crystals have an average particle thickness of 0.5 μm or less, an average particle size of 15 μm or more, and an aspect ratio of 50 or more, and therefore have excellent gloss (paragraph

[0001] of Patent Document 3).

[0006] Patent Document 4 discloses a method for producing a plate-like alumina powder characterized by having a crystal structure of only α-, β-, or γ-alumina, or two or more types of crystal structures, by firing plate-like boehmite at 400°C to 1500°C (claim 2 of Patent Document 4). Patent Document 4 also describes that a cosmetic product with a good feel when used can be obtained by surface-coating plate-like boehmite or plate-like alumina powder with a hydrophobic compound such as polysiloxane and incorporating the coated powder into a cosmetic product (paragraph

[0013] of Patent Document 4).

[0007] Patent Document 5 discloses hexagonal plate-like alumina obtained by firing hexagonal plate-like boehmite at a temperature of 450 to 1500°C, which has a substantially hexagonal plate shape, a ratio of the major axis to the minor axis of 1 to 1.3, and an aspect ratio of 40 to 100. It also describes that the hexagonal plate-like alumina has high orientation, reduces diffused reflection, and enhances brilliance, and can be suitably used as a filler for the purpose of increasing the brilliance of paints and cosmetics (claims 4 and

[0045] of Patent Document 5).

[0008] Patent Document 6 discloses plate-like alumina particles characterized by having a thickness of 0.01 to 5 μm, an average particle diameter of 0.1 to 500 μm, an aspect ratio (the ratio of particle diameter to thickness) of 2 to 500, a polygonal plate shape, and containing molybdenum within the particles (Claim 1 of Patent Document 6). Patent Document 6 also describes that the plate-like alumina particles can be suitably used for thermally conductive fillers, cosmetics, abrasives, high luster pigments, lubricants, substrates for conductive powders, ceramic materials, etc. (Refer to

[0101] of Patent Document 6).

[0009] Japanese Patent Laid-Open No. 2014-218424 Japanese Patent Laid-Open No. 10-298458 Japanese Patent Laid-Open No. 2010-502774 Japanese Patent Laid-Open No. 2012-071996 Japanese Patent Laid-Open No. 2003-002642 Japanese Patent Laid-Open No. 2019-123664

[0010] Thus, although research has been conducted to improve the glitter of plate-like alumina powders, there is still room for improvement in conventional plate-like alumina powders. Specifically, plate-like alumina powders can sometimes form agglomerates during production. Agglomerates of plate-like alumina powders, when used directly in cosmetics, can feel unpleasant on the skin, so they must be crushed and broken down into individual pieces. However, chipping is likely to occur during the crushing process, and there is a limit to how easily these chipped pieces can be removed. When a large amount of chipped pieces are mixed into plate-like alumina powder, the glitter of the powder as a pearlescent pigment is reduced, and the powdery texture impairs the application of makeup, making it difficult to use as a pearlescent pigment.

[0011] Therefore, an object of the present invention is to provide a plate-like alumina powder having few agglomerates or in which agglomerates are easily disaggregated, a method for producing the same, and paints and cosmetics containing the same.

[0012] The present invention encompasses the following aspects <1> to <6>. In this specification, the expression "to" includes the numerical values ​​at both ends. In other words, "X to Y" is synonymous with "at least X and at most Y."

[0013] <1> A powder comprising a plurality of plate-like α-alumina particles, having an average thickness of 0.2 μm or more and 3.0 μm or less, and having an average particle diameter (Dp ５０ ) is 2 μm or more and 100 μm or less, and the average particle diameter Dp ５０ 3 times (3 x Dp ５０ ) or larger particles account for 1.5% or less.

[0014] <2> A method for producing the plate alumina powder according to <1>, comprising: a raw material mixing step of preparing a raw material mixture containing at least aluminum hydroxide powder and an additive; and a firing step of firing the raw material mixture at a firing temperature in the range of 1000°C to 1300°C for 20 hours or less, wherein the raw material mixture contains sodium (Na) as Na 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 to 5.0 mass% in terms of F, and in the calcination step, a heating rate for raising the temperature to the calcination temperature is within a range of 50°C / hour to 150°C / hour.

[0015] <3> The additive is sodium silicofluoride (Na ２ SiF ６ ), and / or aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ <2> The method for producing a plate-like alumina powder according to <2>, comprising:

[0016] <4> As the additive, an oxide of sodium (Na) (Na ２ O) and carbonate (Na ２ CO ３<3> The method for producing a plate-like alumina powder according to <3>, further comprising either one or both of the following:

[0017] <5> Volume average particle diameter (Dp ５０ ) is 0.5 μm or more and 5 μm or less, and the 45 μm sieve amount (+45 μm) is 0.01 mass% or less, and the BET specific surface area (N ２ The BET equivalent diameter (D ＢＥＴ ) to the volume average particle diameter (Dp ５０ ) ratio (Dp ５０ / D ＢＥＴ a raw material mixing step of adding an additive to aluminum hydroxide powder having a stoichiometric ratio (Sr) of 5.0 or less, and mixing the resulting mixture to obtain a raw material mixture; and a firing step of filling the raw material mixture into a sealed firing vessel and firing the mixture at a temperature of 1000°C or more for 20 hours or less.

[0018] <6> The raw material mixture obtained in the raw material mixing step contains sodium (Na) ２ O equivalent of 0.01 mass % or more and 0.5 mass % or less, silicon (Si) is SiO ２ The method according to <5>, wherein the content of fluorine (F) is 0.1 mass% or more and 0.3 mass% or less, calculated as F, and the content of fluorine (F) is 0.1 mass% or more and 5.0 mass% or less, calculated as F.

[0019] <7> A coating material containing the platelet alumina powder according to <1>.

[0020] <8> A cosmetic product comprising the plate-like alumina powder according to <1>.

[0021] According to the present invention, it is possible to provide a plate-like alumina powder having few agglomerated particles or in which the agglomerated particles are easily disaggregated, a method for producing such a plate-like alumina powder, and a paint and a cosmetic containing the same.

[0022]

[0023] Fig. 1 is a schematic diagram showing an embodiment in which a plurality of plate-like alumina particles are bonded together, showing a state in which the plate-like alumina particles are bonded together through point contact. Fig. 2 is a schematic diagram showing an embodiment in which a plurality of plate-like alumina particles are bonded together, showing a state in which one plate-like alumina particle is penetrated by another plate-like alumina particle and bonded together. Fig. 3 is a schematic diagram showing an embodiment in which a plurality of plate-like alumina particles are bonded together, showing a state in which the plurality of plate-like alumina particles are bonded together in a complex entangled manner, penetrating and being penetrated by each other.

[0023] 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.

[0024] First, the plate-like alumina powder according to this embodiment will be described, and then the method for producing the plate-like alumina powder according to this embodiment will be described in detail.

[0025] [Plate-like alumina powder] The plate-like alumina powder according to this embodiment (hereinafter, sometimes simply referred to as "alumina powder") is a powder containing a plurality of plate-like α-alumina (Al ２ O ３ ) particles (hereinafter, sometimes simply referred to as "alumina particles," "plate-like particles," or "particles"). That is, it contains plate-like α-alumina particles as a main component. Here, the plate-like α-alumina particles are particles made of α-alumina (corundum). α-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. The alumina powder according to this embodiment is able to fully utilize these characteristics because it contains plate-like α-alumina particles as a main component.

[0026] In this specification, powder refers to an aggregate of many independent particles. In other words, a large number of particles aggregate to form a powder. Because each particle constituting a powder is independent, the powder exhibits overall fluidity when in its pure form. It is sufficient for the majority of the particles constituting a powder to be independent. As long as the powder as a whole exhibits fluidity, a small portion of the particles may bond together to form aggregated particles, but in the present invention, the content of these aggregated particles can be reduced. Incidentally, an aggregate of many particles dispersed in a medium such as a liquid or resin is also called a powder.

[0027] The alumina particles constituting the plate-like alumina powder according to this embodiment are plate-like (scale-like, thin-plate-like, flat-like, flake-like). That is, each particle has a large plate surface and a small plate thickness. Specifically, the average particle diameter (Dp ５０ ) is 2 μm or more and 100 μm or less, the average thickness is 0.2 μm or more and 3.0 μm or less, and the average aspect ratio is preferably 10 or more and 200 or less. ５０ ) is the volume-based average particle diameter of each particle (primary particle) constituting the alumina powder after ultrasonic dispersion at an ultrasonic intensity of 600 W. The average thickness is the number-based average thickness of each particle constituting the alumina powder. The particle diameter is the long diameter of the particle. That is, the particle diameter is the long axis diameter of the particle plate surface. The thickness is the plate thickness of the plate surface. The average aspect ratio is the ratio of the average particle diameter to the average thickness (average particle diameter / average thickness). The particle diameter can be determined by measuring the particles constituting the alumina powder with a laser diffraction / scattering particle size distribution analyzer, and the thickness and aspect ratio can be determined by observing the particles constituting the alumina powder with a scanning electron microscope (SEM).

[0028] The average particle diameter Dp of the plate-like alumina powder according to this embodiment ５０The average thickness of the tabular alumina powder is preferably 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 40 μm or less. The average thickness of the tabular alumina powder is preferably 0.3 μm or more and 1.0 μm or less, and more preferably 0.5 μm or more and 1.0 μm or less. The average aspect ratio of the tabular alumina powder is more preferably 5 or more and 60 or less, and even more preferably 10 or more and 40 or less.

[0029] By adjusting the average particle size, average thickness and average aspect ratio of the plate-like alumina powder to appropriate values, the plate-like alumina powder can be given high brilliance and excellent texture when used in cosmetics.

[0030] The plate-like alumina powder according to this embodiment has an average particle diameter Dp after ultrasonic dispersion at an ultrasonic intensity of 600 W. ５０ 3 times (3 x Dp ５０ ) (hereinafter referred to as "standard agglomerated particles") or larger (particles of this size are referred to as "agglomerated particles (≧3×Dp ５０ The plate-like alumina powder according to this embodiment has a ratio of agglomerated particles (≧3×Dp ５０ ), and therefore, when used in cosmetics, for example, the feel on the skin is excellent. Furthermore, the plate-like alumina powder according to this embodiment is easily disintegrated even if it contains agglomerated particles, so chipping is unlikely to occur during disintegration, the high brilliance of the pearl pigment is maintained, and when used in cosmetics, the makeup spreads well, and it is easy to handle as a pearl pigment. The plate-like alumina powder according to this embodiment has a low agglomerated particle count (≧3×Dp) after ultrasonic dispersion at an ultrasonic intensity of 600 W. ５０ The proportion of ) is preferably 1.0% or less, and more preferably 0.5% or less.

[0031] The method for ultrasonically dispersing plate-like alumina powder involves dispersing the plate-like alumina powder at a concentration of approximately 0.5 to 1.0 g / L in an aqueous solution of 0.0005 mol / L sodium hexametaphosphate, followed by standard dispersion using an ultrasonic disperser with an ultrasonic intensity of 600 W. The amount of water in the dispersion can be adjusted to suit the specifications of the ultrasonic disperser used, but is typically selected from a range of approximately 150 to 200 mL. Trace amounts of various chelating agents may also be added to the dispersion, if necessary. The ultrasonic dispersion time is preferably 40 seconds or longer, and more preferably approximately 50 to 70 seconds. As described below, in the examples, ultrasonic dispersion was performed for 1 minute.

[0032] As the ultrasonic disperser, a general device commercially available as an ultrasonic disperser or ultrasonic homogenizer may be used. If the ultrasonic disperser has a variable ultrasonic intensity (high frequency output), it is set to 600 W.

[0033] The particle size distribution of the dispersion after ultrasonic dispersion was measured using a laser diffraction / scattering particle size distribution measuring device to determine the average particle diameter Dp of the primary particles of the plate-like alumina powder. ５０ (volume basis) and agglomerated particles (≧3×Dp ５０ The volume ratio of the average particle diameter Dp ５０ Three times the value of (3 × Dp ５０ ) agglomerated particles equal to or larger than the standard agglomerated particles (≧3×Dp ５０ The reason why it is preferable to reduce the amount of ) will be explained below.

[0034] Figures 1 to 3 are schematic diagrams showing three modes in which a plurality of plate-like alumina particles are bonded together. Specifically, Figure 1 shows a state in which plate-like alumina particles are bonded together through point contact, Figure 2 shows a state in which a plate-like alumina particle is bonded to another plate-like alumina particle by penetrating it, and Figure 3 shows a state in which a plurality of plate-like alumina particles are bonded together in a complex entanglement through point contact or penetration with one another.

[0035] 1 and 2 can be said to be agglomerated particles that are relatively easy to disaggregate, but the "agglomerated particles that are difficult to disaggregate" that are problematic in the present invention correspond to agglomerated particles with a complex shape such as the state shown in Fig. 3. Agglomerated particles such as those shown in Fig. 3 are characterized in that each of the constituent plate-like alumina particles is in point contact with or penetrates two or more other plate-like alumina particles. In other words, one agglomerated particle contains three or more plate-like alumina particles that are bonded to each other by point contact, penetration, or the like. Therefore, in this embodiment, the size of the "agglomerated particles that are difficult to disaggregate" is determined based on the average particle diameter Dp of the primary particles of the plate-like alumina powder. ５０ It is said that the particle size is more than three times the standard agglomerated particle size.

[0036] Therefore, in the measured particle size distribution, the standard agglomerate particles (3 × Dp ５０ The percentage (%) of "aggregated particles that do not easily disintegrate" can be calculated by calculating the volume percentage of particles with a particle diameter of at least 100 μm. ５０ and agglomerated particles (≧3×Dp ５０ The method for measuring the volume ratio of (a) will be described in detail in the Examples.

[0037] As described above, according to the plate-like alumina powder of this embodiment, the agglomerated particles (≧3×Dp ５０ ) is extremely low, it has high brilliance when used in paints and cosmetics, and has an excellent texture when used in cosmetics.

[0038] [Method for Producing Plate-Like Alumina Powder] The method for producing plate-like alumina powder according to this embodiment includes a raw material mixing step and a firing step. There are two methods for producing plate-like alumina powder according to this embodiment: a first method shown in (1) below and a second method shown in (2) below.

[0039] (1) A method for producing a plate-like alumina powder according to the embodiment described above, comprising: a raw material mixing step of preparing a raw material mixture containing at least aluminum hydroxide powder and an additive; and a firing step of firing the raw material mixture at a firing temperature in the range of 1000°C to 1300°C for 20 hours or less, wherein the raw material mixture contains sodium (Na) as Na 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 to 5.0 mass% in terms of F, and in the calcination step, a heating rate for raising the temperature to the calcination temperature is within a range of 50°C / hour to 150°C / hour.

[0040] (2) Volume average particle diameter (Dp ５０ ) is 0.5 μm or more and 5 μm or less, and the 45 μm sieve amount (+45 μm) is 0.01 mass% or less, and the BET specific surface area (N ２ The BET equivalent diameter (D ＢＥＴ ) to the volume average particle diameter (Dp ５０ ) ratio (Dp ５０ / D ＢＥＴ a raw material mixing step of adding an additive to aluminum hydroxide powder having a stoichiometric ratio (Sr) of 5.0 or less, and mixing the resulting mixture to obtain a raw material mixture; and a firing step of filling the raw material mixture into a sealed firing vessel and firing the mixture at a temperature of 1000°C or more for 20 hours or less.

[0041] The first method shown in (1) above is a method for producing the plate-like alumina powder according to the embodiment described above, but the second method shown in (2) above is not limited to producing the plate-like alumina powder according to the embodiment described above. However, the second method shown in (2) above is suitable for producing the plate-like alumina powder according to the embodiment described above. Below, the method for producing the plate-like alumina powder according to this embodiment will be divided into the first method shown in (1) above and the second method shown in (2) above, and each step will be described.

[0042] (1) First Method <Raw Material Mixing Step> In the first method, the raw material mixing step is a step of preparing a raw material mixture containing at least aluminum hydroxide powder and additives. Aluminum hydroxide has the chemical formula Al(OH) ３ It is a compound represented by the formula: 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 the first method, either gibbsite or bayerite may be used. However, powder made of thermodynamically stable gibbsite is preferred.

[0043] In the first method, the aluminum hydroxide powder may be, for example, that obtained by the Bayer process. The Bayer process is a production method in which bauxite is dissolved in a high-temperature sodium hydroxide solution, impurities are removed by filtration, and then the solution is cooled to precipitate aluminum hydroxide. The aluminum hydroxide obtained by the Bayer process inevitably contains sodium, which is a component that acts advantageously to form the alumina into a plate shape.

[0044] In the first method, the volume average particle diameter (Dp ５０ The particle size of the aluminum hydroxide powder as a raw material affects the particle size of the plate-like alumina powder to be produced, so the aluminum hydroxide powder should be adjusted to an appropriate volume average particle size (Dp ５０ It is desirable that the volume average particle diameter (Dp ５０ ) is the cumulative 50% diameter in the particle size distribution on a volume basis determined using a laser diffraction / scattering particle size distribution analyzer.

[0045] Furthermore, the 45 μm sieve amount (+45 μm) of the aluminum hydroxide powder used is preferably 0.01 mass% or less, more preferably 0.007 mass% or less, and even more preferably 0.005 mass% or less. Reducing the 45 μm sieve amount (+45 μm), i.e., reducing the content of large particles with a particle size of 45 μm or more, leads to a reduction in the content of secondary particles, and can suppress the formation of agglomerated particles in the produced plate-like alumina powder. The 45 μm sieve amount (+45 μm) of the aluminum hydroxide powder can be measured using a standard sieve mesh with a mesh size of 45 μm (Japanese Industrial Standard JIS Z8801).

[0046] In addition, the aluminum hydroxide powder used has a BET specific surface area (N ２ The BET equivalent diameter (D ＢＥＴ ) to the volume average particle diameter (Dp ５０ ) ratio (Dp ５０ / D ＢＥＴ ) is preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less. ５０ / D ＢＥＴ The ratio (Dp) is an index that represents the shape of the particles, and the larger this value, the more the crushing or pulverization has progressed. ５０ / D ＢＥＴ By using an aluminum hydroxide powder having a sufficiently small particle size, it is possible to suppress the formation of agglomerated particles in the produced plate-like alumina powder.

[0047] By using aluminum hydroxide powder having the above-described shape, it is possible to suppress the generation of agglomerated particles in the produced plate-like alumina powder, thereby producing the plate-like alumina powder according to the embodiment. Also, by using aluminum hydroxide powder having the above-described shape, it is possible to suppress the generation of chipping fragments.

[0048] The additives added to the aluminum hydroxide powder are primarily components that become liquid in the subsequent calcination process and act as mineralizers. The additives preferably contain at least one element selected from the group consisting of silicon (Si), phosphorus (P), sulfur (S), and zirconium (Zr), as well as fluorine (F). The additives may further contain an alkali metal (AM) or an alkaline earth metal (AEM) as necessary. When the composition of the liquid phase components is appropriately controlled, the crystallization and grain growth of the alumina particles proceeds appropriately, resulting in an alumina powder having a desired size and a low content of agglomerated particles.

[0049] Therefore, in order to obtain a desired alumina powder, it is preferable to appropriately control the composition of the raw material mixture containing additives. Specifically, the raw material mixture obtained in this step contains an alkali metal (AM), particularly preferably sodium (Na), as AM. ２ O conversion (especially Na ２ It is preferable to control the silicon (Si) content in the raw material mixture obtained in this step to 0.01 mass % or more and 0.5 mass % or less (calculated as O). ２ It is preferable to control the fluorine (F) content in the raw material mixture obtained in this step to 0.1 mass % or more and 0.3 mass % or less in terms of F. Furthermore, it is preferable to adjust the composition of the additives so that the raw material mixture obtained in this step contains fluorine (F) in an amount of 0.1 mass % or more and 5.0 mass % or less in terms of F.

[0050] It is also preferable that the raw material mixture obtained in this step contains at least one element selected from the group consisting of phosphorus (P), sulfur (S) and zirconium (Zr) instead of or together with silicon (Si). In this case, the amount of phosphorus (P) in the raw material mixture is P ２ O ５ The amount of sulfur (S) in the raw material mixture is preferably 1.0 mass % or more and 5.0 mass % or less in terms of SO ２ The amount of zirconium (Zr) is preferably 0.2 mass % or more and 5.0 mass % or less in terms of ZrO ２The content of the alumina powder in the raw material mixture is preferably 0.1% by mass or more and 3.0% by mass or less, calculated as a percentage. When the composition of the raw material mixture is within the above-mentioned range, it is possible to obtain an alumina powder having a desired size and a low content of agglomerated particles and chippings. On the other hand, if the composition of the raw material mixture is inappropriate, it may have an adverse effect on the particle size and the generation of agglomerated particles.

[0051] The amount of alkali metal (AM) and alkaline earth metal (AEM) in the raw material mixture is ２ O conversion and AEM ２ The content is more preferably 0.01% by mass or more and 0.5% by mass or less in terms of O. The alkali metal (AM) is preferably one or both of sodium (Na) and potassium (K), and particularly preferably sodium (Na). The alkaline earth metal (AEM) can include magnesium (Mg) and calcium (Ca).

[0052] The amount of sodium (Na) in the raw material mixture is Na ２ The amount of silicon (Si) in the raw material mixture is more preferably 0.01% by mass or more and 0.5% by mass or less in terms of O. ２ The amount of phosphorus (P) in the raw material mixture is more preferably 0.1 mass % or more and 0.2 mass % or less in terms of P. ２ O ５ The amount of sulfur (S) in the raw material mixture is more preferably 1.0 mass % or more and 3.0 mass % or less in terms of sulfur content. ２ The amount of zirconium (Zr) in the raw material mixture is more preferably 0.2 mass % or more and 3.0 mass % or less in terms of ZrO ２ The amount of fluorine (F) in the raw material mixture is more preferably 0.5% by mass or more and 2.0% by mass or less in terms of F. By limiting the composition of the raw material mixture to within the above range, it is possible to obtain an alumina powder with a low content of agglomerated particles and chipping fragments.

[0053] The additive added to the aluminum hydroxide powder preferably contains at least one selected from the group consisting of silicon (Si), phosphorus (P), sulfur (S), and zirconium (Zr), as well as fluorine (F). The additive may or may not contain an alkali metal (AM) or alkaline earth metal (AEM). The raw aluminum hydroxide powder may contain alkali metals (AM) or alkaline earth metals (AEM) as impurities. If the amount of alkali metal or alkaline earth metal in the aluminum hydroxide powder is sufficient, the additive may not contain an alkali metal or alkaline earth metal. On the other hand, if the amount of alkali metal or alkaline earth metal in the aluminum hydroxide powder is insufficient, it is preferable to add an alkali metal or alkaline earth metal as an additive. The additive may further contain aluminum (Al). If the additive contains aluminum, this aluminum is incorporated into the alumina powder during the firing process.

[0054] The additives added to the aluminum hydroxide powder may contain a magnesium compound and / or a calcium compound as alkaline earth metals (AEM). By including a magnesium compound or a calcium compound in the additive, it is possible to obtain plate-like alumina with a smaller particle size. This is due to the grain growth suppression effect of magnesium and calcium. Examples of magnesium compounds that can be added to the additive include magnesium nitrate (Mg(NO ３ ) ２ ), magnesium aluminate (MgAl ２ O ４ ), magnesium chloride (MgCl ２ ), magnesium acetate ((CH ３ COO) ２ Examples of calcium compounds that can be added to the additive include calcium hydroxide (Ca(OH) ２ ), calcium carbonate (CaCO ３ ), calcium nitrate (Ca(NO ３ ) 2) etc. can be mentioned.

[0055] The amount of magnesium compound in the raw material mixture is preferably in the range of 0.01% by mass to 0.5% by mass, and more preferably 0.05% by mass to 0.3% by mass, in terms of magnesium (Mg) content, calculated as MgO. The amount of calcium compound in the raw material mixture is preferably in the range of 0.01% by mass to 1.0% by mass, and more preferably 0.1% by mass to 0.5% by mass, in terms of calcium (Ca) content, calculated as CaO. If the amount of magnesium compound and / or calcium compound is too large, the grain growth suppression effect increases, and there is a risk that particles will not grow sufficiently to form plate-like alumina.

[0056] As long as an alumina powder having the desired size, shape, and polycrystalline state is obtained, the additive may contain components other than alkali metals (AM), silicon (Si), phosphorus (P), sulfur (S), zirconium (Zr), fluorine (F), magnesium (Mg), calcium (Ca), aluminum (Al), and oxygen (O). However, excessively large amounts of other components may adversely affect particle size, shape, and agglomerate formation. 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), phosphorus (P), sulfur (S), zirconium (Zr), fluorine (F), magnesium (Mg), calcium (Ca), aluminum (Al), and oxygen (O) in amounts exceeding the impurity level.

[0057] According to a preferred embodiment, the additive is aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ According to another preferred embodiment, the additive includes an alkali silicofluoride (AM ２ SiF ６ Among alkali silicofluorides, sodium silicofluoride (Na ) is used, in which the alkali metal (AM) is sodium. ２ SiF ６ ) is particularly preferred. Therefore, in the first method, the additive is sodium silicofluoride (Na ２ SiF ６), and / or aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ ) is preferably included.

[0058] In addition, when the amount of alkali metal in the aluminum hydroxide powder is insufficient, the additive may be an oxide of alkali metal (AM) (AM ２ O) and carbonate (AM ２ CO ３ Among these, it is preferable that the alkali metal (AM) further contains one or both of the oxides of sodium (Na ２ O) and carbonate (Na ２ CO ３ ) Either one or both of these is particularly preferred.

[0059] The above compounds (AlF ３ , SiO ２ , A.M. ２ O, A.M. ２ CO ３ , A.M. ２ SiF ６ By using an appropriate amount of these compounds as additives, it is possible to reliably produce alumina powder with the desired size and a low content of agglomerated particles. Furthermore, these compounds are inexpensive and easily available. Therefore, plate-like alumina powder can be obtained more inexpensively.

[0060] 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 mixer, Henschel mixer, or Nauta mixer. In the case of wet mixing, a solvent such as water can be 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. In both dry mixing and wet mixing, the raw materials can be mixed manually by putting them in a bag and shaking them, or by putting them in a container and stirring them using an appropriate stirrer.

[0061] <Firing step> In the firing step, the raw material mixture obtained in the raw material mixing step is filled into a sealed firing container and fired at a firing temperature in the range of 1000°C to 1300°C for 20 hours or less. This produces a fired product. During firing, aluminum hydroxide (Al(OH) ３ ) is dehydrated to form α-alumina (Al ２ O ３ In addition, alkali metals (AM), silicon (Si), phosphorus (P), sulfur (S), zirconium (Zr) and fluorine (F) in the raw material mixture become liquid oxides and act as mineralizers that promote the crystallization of alpha alumina. That is, Al(OH) ３ The crystal structure changes when the transition from Al(OH) to α-alumina occurs. By adding a mineralizer, the crystallization of α-alumina (corundum) is promoted, making it possible to lower the temperature of the transition. ３ If magnesium (Mg) and / or calcium (Ca) is added during the transformation from α-alumina to α-alumina, the grain growth suppression effect of Mg and Ca can be used to obtain plate-like alumina with a smaller particle size.

[0062] If the firing temperature is too low, the transition to α-alumina and grain growth may be insufficient. Therefore, it may be difficult to obtain an alumina powder with a low content of agglomerated particles and chipping fragments. The lower limit of the firing temperature is 1000°C or higher, but preferably 1100°C or higher. On the other hand, the upper limit of the firing temperature is 1300°C or lower, but preferably 1200°C or lower. If the firing temperature is too high, particles may sinter together, forming a strong sintered mass. Therefore, a large force must be applied to break down the agglomerated particles, which may lead to excessive chipping fragments. Furthermore, excessive energy consumption for firing may result in increased production costs.

[0063] The rate of temperature increase to the above-mentioned predetermined firing temperature is within the range of 50°C / hour to 150°C / hour. By taking an appropriate time for temperature increase, plate-like alumina particles with sufficient particle growth can be obtained. However, taking too long for temperature increase may result in excessive energy consumption for firing, which may lead to increased production costs.

[0064] The calcination time is preferably 10 hours or more and 20 hours or less. Here, "calcination time" refers to the time for which the calcination temperature is maintained after being raised to a predetermined calcination temperature. By setting the calcination time appropriately, it is possible to reliably ensure the transition to α-alumina and grain growth while preventing excessive sintering of particles. The calcination furnace is not limited as long as the desired alumina powder is obtained. However, a stationary furnace capable of calcining using a sealed calcination container is preferred. In the calcination step, the raw material mixture is filled into a sealed calcination container and then calcined, thereby allowing the additives to exert their effects more effectively.

[0065] When the raw material mixture is filled into a sealed firing vessel, it is preferable to fill the raw material mixture at a filling bulk density ratio of 60% to 75% and more preferably 60% to 70%. Here, the filling bulk density ratio is the ratio of the bulk density BSG when filled into the firing vessel to the packed bulk density UTD specified in Japanese Industrial Standards JIS R 9301-2-3 (BSG / UTD x 100%). By filling the raw material mixture into a sealed firing vessel at an appropriate filling bulk density ratio, abnormal particle growth can be suppressed, particle shape can be made more uniform, and the generation of aggregated particles can be suppressed.

[0066] <Post-treatment process> 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 process, excess alkali metal components adhering to the surface of the fired product are removed. This allows the amount of alkali metal in 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 process, mild mechanical energy is applied to the fired product to break up the bonds between agglomerated particles formed during firing.

[0067] The crushing treatment can be carried out by a dry method or a wet method using a crusher such as a pot mill, a pin mill, and / or a jaw crusher. Of course, according to the production method according to the first method, since the content of agglomerated particles is small, there is no need to carry out a crushing treatment or to carry out a strong crushing treatment.

[0068] In the classification process, particles are separated according to size to obtain alumina powder of the desired particle size. The classification process can be performed by techniques such as sieving, air classification, elutriation, and / or centrifugation. Post-processing can be performed as needed. If the desired alumina powder can be obtained after firing, post-processing may be omitted.

[0069] (2) Second Method <Raw Material Mixing Step> In the second method, the raw material mixing step is a step of preparing a raw material mixture by adding at least an additive to aluminum hydroxide powder and mixing them. In the second method, the aluminum hydroxide or aluminum hydroxide powder is as described in the first method, and either gibbsite or bayerite may be used, with powder made of gibbsite being preferred and one obtained by the Bayer process also being usable, as in the first method.

[0070] In the second method, the volume average particle diameter (Dp ５０ The particle size of the aluminum hydroxide powder as a raw material affects the particle size of the plate-like alumina powder to be produced, so the aluminum hydroxide powder should be adjusted to an appropriate volume average particle size (Dp ５０ It is desirable that the volume average particle diameter (Dp ５０ ) is as explained in the first method.

[0071] In the second method, the 45 μm sieve content (+45 μm) of the aluminum hydroxide powder used is 0.01 mass% or less, preferably 0.007 mass% or less, and more preferably 0.005 mass% or less. Reducing the 45 μm sieve content (+45 μm), i.e., reducing the content of large particles with a particle size of 45 μm or more, leads to a reduction in the content of secondary particles, and can suppress the formation of agglomerates in the produced plate-like alumina powder. The 45 μm sieve content (+45 μm) of the aluminum hydroxide powder can be measured by the method described in the section on the first method.

[0072] In addition, in the aluminum hydroxide powder used in the second method, the BET specific surface area (N ２ The BET equivalent diameter (D ＢＥＴ ) to the volume average particle diameter (Dp ５０ ) ratio (Dp ５０ / D ＢＥＴ ) is 5.0 or less, preferably 4.0 or less, and more preferably 3.0 or less. ５０ / D ＢＥＴ The ratio (Dp) is an index that represents the shape of the particles, and the larger this value, the more the crushing or pulverization has progressed. ５０ / D ＢＥＴ By using an aluminum hydroxide powder having a sufficiently small particle size, it is possible to suppress the formation of agglomerates in the produced plate-like alumina powder.

[0073] By using aluminum hydroxide powder having the above-mentioned shape, it is possible to suppress the formation of agglomerates in the produced plate-like alumina powder, and in turn, it is possible to suppress the formation of chipping fragments.

[0074] The functions and components of the additives added to the aluminum hydroxide powder in the second method are also similar to those in the first method. When the composition of the liquid phase component is appropriately controlled, the crystallization and grain growth of the alumina particles proceeds appropriately, and as a result, it becomes possible to obtain an alumina powder having a desired size and a low content of agglomerates.

[0075] In the second method, it is preferable to appropriately control the composition of the raw material mixture containing additives in order to obtain the desired alumina powder. Specifically, the raw material mixture obtained in this step contains an alkali metal (AM), particularly preferably sodium (Na), as AM. ２ It is preferable to control the silicon (Si) content in terms of O to 0.01 mass % or more and 0.5 mass % or less. ２ It is preferable to control the fluorine (F) content in the raw material mixture obtained in this step to 0.1 mass % or more and 0.3 mass % or less in terms of F. Furthermore, it is preferable to adjust the composition of the additives so that the raw material mixture obtained in this step contains fluorine (F) in an amount of 0.1 mass % or more and 5.0 mass % or less in terms of F.

[0076] It is also preferable that the raw material mixture obtained in this step contains at least one element selected from the group consisting of phosphorus (P), sulfur (S) and zirconium (Zr) instead of or together with silicon (Si). In this case, the amount of phosphorus (P) in the raw material mixture is P ２ O ５ The amount of sulfur (S) in the raw material mixture is preferably 1.0 mass % or more and 5.0 mass % or less in terms of SO ２ The amount of zirconium (Zr) is preferably 0.2 mass % or more and 5.0 mass % or less in terms of ZrO ２ The content of the alumina powder in the raw material mixture is preferably 0.1% by mass or more and 3.0% by mass or less, calculated as a percentage. When the composition of the raw material mixture is within the above-mentioned range, an alumina powder having a desired size and a low content of agglomerates and chippings can be obtained. On the other hand, if the composition of the raw material mixture is inappropriate, it may have a negative effect on the particle size and agglomerate formation.

[0077] The amount of alkali metal (AM) in the raw material mixture is AM ２ The amount of silicon (Si) in the raw material mixture is more preferably 0.01% by mass or more and 0.5% by mass or less in terms of O. The alkali metal is preferably one or both of sodium (Na) and potassium (K), and particularly preferably sodium (Na). ２ The amount of phosphorus (P) in the raw material mixture is more preferably 0.1 mass % or more and 0.2 mass % or less in terms of P. ２ O ５ The amount of sulfur (S) in the raw material mixture is more preferably 1.0 mass % or more and 3.0 mass % or less in terms of sulfur content. ２ The amount of zirconium (Zr) in the raw material mixture is more preferably 0.2 mass % or more and 3.0 mass % or less in terms of ZrO ２ The amount of fluorine (F) in the raw material mixture is more preferably 0.5% by mass or more and 2.0% by mass or less in terms of F. 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 range, it is possible to obtain an alumina powder with a low content of aggregates and chipping fragments.

[0078] In the second method, the additives added to the aluminum hydroxide powder preferably contain at least one selected from the group consisting of silicon (Si), phosphorus (P), sulfur (S), and zirconium (Zr), as well as fluorine (F), and may or may not contain an alkali metal (AM) or an alkaline earth metal (AEM), similar to the first method. Furthermore, in the second method, the additives do not need to contain alkali metals when the aluminum hydroxide powder as a raw material contains a sufficient amount of alkali metals (AM) as impurities, and when the amount of alkali metal in the aluminum hydroxide powder is insufficient, it is preferable to add an alkali metal as an additive, and the additives may further contain aluminum (Al), similar to the first method.

[0079] In the second method, the additives added to the aluminum hydroxide powder may contain a magnesium compound and / or a calcium compound as alkaline earth metals (AEM), and the significance thereof, the magnesium compounds and calcium compounds that can be added, the preferred amounts of magnesium compounds and calcium compounds in the raw material mixture, and the adverse effects of an excessive amount of magnesium compound and calcium compound are the same as those in the first method.

[0080] In the second method, the additive may contain other components than alkali metals (AM), silicon (Si), phosphorus (P), sulfur (S), zirconium (Zr), fluorine (F), alkaline earth metals (AEM), aluminum (Al) and oxygen (O), and the amount of the other components may be the same as in the first method, as long as an alumina powder having the desired size, shape and polycrystalline state is obtained.

[0081] In the second method, according to a preferred embodiment, the additive is aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ In addition, when the amount of alkali metal in the aluminum hydroxide powder is insufficient, the additive contains an oxide of alkali metal (AM) (AM ２ O) and carbonate (AM ２ CO ３In another preferred embodiment, the additive further comprises one or both of an alkali silicofluoride (AM ２ SiF ６ These compounds (AlF ３ , SiO ２ , A.M. ２ O, A.M. ２ CO ３ , A.M. ２ SiF ６ By using an appropriate amount of these compounds as additives, it is possible to reliably produce alumina powder with the desired size and a low content of agglomerates. Furthermore, these compounds are inexpensive and easily available. Therefore, plate-like alumina powder can be obtained more inexpensively.

[0082] In the second method, the raw material mixture is prepared by mixing aluminum hydroxide powder and additives, similarly to the first method. The second method also has the same mixing technique and mode (dry or wet), usable dry mixers and wet mixers, and the possibility of manual mixing.

[0083] <Firing step> In the second method, in the firing step, the obtained raw material mixture is filled into a sealed firing vessel and fired at a temperature of 1000°C or higher for 20 hours or less. This produces a fired product. During firing, aluminum hydroxide (Al(OH) ３ ) is dehydrated to form α-alumina (Al ２ O ３ In addition, alkali metals (AM), silicon (Si), phosphorus (P), sulfur (S), zirconium (Zr) and fluorine (F) in the raw material mixture become liquid oxides and act as mineralizers that promote the crystallization of alpha alumina. That is, Al(OH) ３ The crystal structure changes when the transition from Al(OH) to α-alumina occurs. By adding a mineralizer, the crystallization of α-alumina (corundum) is promoted, making it possible to lower the temperature of the transition. ３ If magnesium (Mg) and / or calcium (Ca) is added during the transformation from α-alumina to α-alumina, the grain growth suppression effect of Mg and Ca can be used to obtain plate-like alumina with a smaller particle size.

[0084] If the firing temperature is too low, the transition to α-alumina and grain growth may be insufficient. Therefore, it may be difficult to obtain an alumina powder with a low content of agglomerates and chipping fragments. The lower limit of the firing temperature is preferably 1100°C or higher. On the other hand, the upper limit of the firing temperature is preferably 1300°C or lower, more preferably 1200°C or lower. If the firing temperature is too high, particles may sinter together, forming a strong sintered mass. Therefore, a large force must be applied to break down the agglomerates, which may lead to excessive chipping fragments. Furthermore, excessive energy consumption for firing may result in increased production costs.

[0085] In the second method, the preferred temperature rise rate, the preferred firing time, the significance thereof, and the firing furnace that can be used are as explained in the section for the first method. In the second method, the raw material mixture is filled into a sealed firing vessel and then fired in the firing step, which allows the additives to exert their effects effectively.

[0086] The preferred packed bulk density ratio of the raw material mixture when packed into a closed firing vessel and its significance are the same as those in Method 1. The significance of the packed bulk density ratio and the measuring device are the same as those explained in the section for Method 1.

[0087] <Post-treatment step> In the second method, the fired product obtained through the firing step may be subjected to post-treatments such as soda removal, crushing, and / or classification, as necessary, as in the first method. The types and details of these post-treatments in the post-treatment step are as described in the section for the first method.

[0088] <Uses of the Produced Plate Alumina Powder> The plate alumina powder produced by the production methods according to the present embodiment (the first and second methods) can be used in a variety of applications, including paints and cosmetics. Paints and cosmetics using the produced plate alumina powder may contain the plate alumina powder alone or in a surface-treated form. The surface treatment may be achieved by providing a surface treatment agent, such as a silicone compound, an alkylsilane compound, and / or a fluorine-based compound, on the particle surfaces of the plate alumina powder. Furthermore, the paints and cosmetics may contain solvents and resins in addition to the plate alumina powder. The solvent may be aqueous or non-aqueous. Furthermore, the paints and cosmetics may contain known additives, such as oils, pigments other than the plate alumina powder, fillers, surfactants, viscosity modifiers, preservatives, fragrances, moisturizers, physiologically active ingredients, salts, chelating agents, neutralizing agents, and / or pH adjusters.

[0089] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0090] <Raw Material Mixing Step> (Preparation of Aluminum Hydroxide Powder) As the alumina source used in Examples 1 to 11 and Comparative Examples 1 to 4, aluminum hydroxide (gibbsite) powders A to O having the compositions and particle sizes and shapes shown in Tables 1 and 2 below were prepared by adjusting various conditions using the Bayer process.

[0091] In addition, Na shown in Tables 1 and 2 below ２ O is the total value of the dissolved sodium content on and near the crystal surface of aluminum hydroxide and the undissolved sodium content incorporated within the crystal lattice, and f-Na ２ O represents only the sodium content eluted from the surface of the aluminum hydroxide crystal and its vicinity. １０ , Dp ５０ and Dp ９０ are the cumulative 10% diameter, cumulative 50% diameter, and cumulative 90% diameter, respectively, in the particle size distribution on a volume basis determined using a laser diffraction / scattering particle size distribution analyzer (Microtrac 9320HRA (X100) manufactured by Nikkiso Co., Ltd.). ＢＥＴThat is, N ２ This is the BET specific surface area determined by gas adsorption method.

[0092]

[0093]

[0094] (Preparation of Additives) Aluminum fluoride (DO-FLUORIDE CHEMICALS CO., LTD.; AlF ３ ), silicon oxide (Marukama Kamado Toryo Co., Ltd., Snow Brand Silica SP-3; SiO ２ ), aluminum phosphate (Kanto Chemical Co., Ltd., Grade 1; AlPO ４ ), sulfuric acid (Kanto Chemical Co., Ltd., special grade; H ２ SO ４ ), zirconium nitrate (Nippon Light Metal Co., Ltd.; Zr(NO ３ ) ４ ), aluminum sulfate 14-18 water (Kanto Chemical Co., Ltd., special grade; Al ２ (SO ４ ) ３ ・14 to 18 hours ２ O), potassium carbonate (Kanto Chemical Co., Ltd., special grade; K ２ CO ３ ), magnesium nitrate hexahydrate (Fujifilm Wako Pure Chemical Industries, Ltd.; Mg(NO ３ ) ２ ・6H ２ O), magnesium aluminate (Kojundo Chemical Laboratory Co., Ltd.; MgAl ２ O ４ ), and magnesium chloride hexahydrate (Kanto Chemical Co., Ltd., Grade 1; MgCl ２ ・6H ２ O) was prepared.

[0095] (Preparation of raw material mixture) Next, the prepared aluminum hydroxide powders A to O were mixed with additives to prepare raw material mixtures. The amounts of additives were adjusted so that the content ratio of each additive in the raw material mixture was the value shown in Table 1 above. Mixing was performed using a rocking mixer (mechanical method) only in Example 3, while the other examples were mixed by putting the raw materials in a bag and shaking (manual method). In this way, the raw material mixtures used in Examples 1 to 11 and Comparative Examples 1 to 4 were prepared.

[0096] <Firing step> Each raw material mixture prepared as described above was fired. The various conditions for the firing step and the firing vessels, firing furnaces, etc. used in Examples 1 to 11 and Comparative Examples 1 to 4 are summarized in Tables 3 and 4 below.

[0097]

[0098]

[0099] The obtained raw material mixture was filled into a sealed firing container, such as a sagger or crucible, and sealed with a dedicated lid. At this time, the packed bulk specific gravity ratio of each filled raw material mixture was measured (see Tables 3 and 4 above). The sealed firing container was placed in a firing furnace and fired. The firing furnace was an electric furnace or a gas furnace, as shown in Tables 3 and 4 above. During firing, the temperature was raised to a firing temperature of 1,100°C at a heating rate of 100°C / hour, and then maintained at that temperature for 10 hours, followed by natural cooling. After the temperature inside the furnace had completely dropped, the firing container was removed from the furnace, and each fired product (alumina) was recovered.

[0100] <Post-treatment> Each of the obtained fired products was dry sieved using a sieve with a mesh size of 500 μm and a wire diameter of 315 μm to produce plate-like alumina powders of Examples 1 to 11 and Comparative Examples 1 to 4.

[0101] <Evaluation Test> (Particle Size Distribution) The particle size distribution of each of the obtained plate-like alumina powders of Examples 1 to 11 and Comparative Examples 1 to 4 was measured using a laser diffraction / scattering particle size distribution measuring device (Microtrac SYNC, Microtrac Bell Co., Ltd.). Specifically, each plate-like alumina powder was directly placed into the inlet of the measuring device, and the particle size distribution was measured. The average particle size was determined as the particle diameter Dp corresponding to an integrated particle size distribution rate of 50% by volume. ５０ The results are summarized in Tables 5 and 6 below.

[0102] (SEM Observation) Using a scanning electron microscope (JEOL Ltd., JSM-F100; SEM) and image analysis software (ImageJ), the plate-like alumina powders of Examples 1 to 6 and Comparative Examples 1 to 3 were evaluated. Specifically, SEM images of the obtained plate-like alumina powders were taken, and the particles (plate-like α-alumina particles) shown in the images were evaluated using ImageJ to measure the particle thickness and calculate the average thickness. The average thickness was taken as the average value of 30 particles. The ratio of the average particle diameter determined using a laser diffraction / scattering particle size distribution analyzer to the average thickness (average particle diameter / average thickness) was then calculated as the average aspect ratio. The results are summarized in Tables 5 and 6 below.

[0103] (Crystalline Phase) The crystalline phase of each of the plate-like alumina powders of Examples 1 to 11 and Comparative Examples 1 to 4 was examined using an X-ray diffractometer (Rigaku Corporation, RINT Ultima III). Specifically, the obtained alumina powder was placed on a glass sample plate, pressed so that the sample surface was flat to allow for preferential orientation, and then measured using an XRD measurement device. CuKα was used as the X-ray source. It was then examined whether or not a diffraction peak based on α-alumina (corundum phase) was detected in the obtained X-ray diffraction patterns. As a result, a diffraction peak based on the corundum phase and a diffraction peak based on the (006) plane were detected in all of the plate-like alumina powders of Examples 1 to 11 and Comparative Examples 1 to 4.

[0104] In the above-mentioned method of measuring a sample by selectively orienting it, the orientation of the sample is affected, so the sample is always rotated to make it non-oriented, and measurement is performed by the capillary transmission method, which is not affected by orientation. As a result, a diffraction peak due to the corundum phase was detected, but a diffraction peak due to the (006) plane was not detected.

[0105] <Component Analysis> Component analysis was performed using a scanning X-ray fluorescence analyzer (Rigaku Corporation, ZSX Primus IV). Specifically, alumina powder was placed in a platinum crucible and set in a high-frequency melting apparatus. The crucible was then preheated at 700°C for 60 seconds and then rocked at 1200°C for 120 seconds to produce a glass beaded sample. The glass beaded sample was then evaluated using the scanning X-ray fluorescence analyzer. The obtained data was compared with the data of a standard sample for a calibration curve to obtain evaluation results.

[0106] (Particle size distribution) (Measurement of the proportion of aggregated particles) Each of the plate-like alumina powders in Examples 1 to 11 and Comparative Examples 1 to 4 was dissolved in 0.0005 mol / L of sodium hexametaphosphate (Kanto Chemical Co., Ltd., special grade; (NaPO ３ ) ６ ) and ultrasonically dispersed for 1 minute at 600 W using a homogenizer (Nippon Seiki Seisakusho, US-600AT type).

[0107] The particle size distribution of each plate-like alumina powder was measured for the dispersion after ultrasonic dispersion using a laser diffraction / scattering particle size distribution analyzer (Microtrac SYNC, Microtrac Bell Co., Ltd.). Specifically, each plate-like alumina powder was directly placed into the inlet of the analyzer, and the particle size distribution on a volume basis was measured. The particle diameter Dp corresponding to an integrated particle size distribution rate of 50% by volume was ５０ was calculated and used as the average particle size.

[0108] In addition, the calculated average particle diameter Dp ５０ The value of the standard agglomerated particle is calculated by multiplying the standard agglomerated particle size by 3×Dp ５０ ) or more, and this is defined as the volume ratio of agglomerated particles (≧3×Dp ５０The results are shown in Tables 5 and 6 below.

[0109]

[0110]

[0111] As shown in Tables 5 and 6, the plate-like alumina powders of the examples were agglomerated particles (≧3×Dp ５０ ) is extremely low at 1.5% or less. Therefore, when used in paints or cosmetics, it has high brilliance and when used in cosmetics, it has an excellent texture. On the other hand, the plate-like alumina powders of Comparative Examples 1 and 2 have a high proportion of agglomerated particles (≧3×Dp ５０ Since the proportion of ) is much more than 1.5%, the brilliance is poor when used in paints or cosmetics. In Comparative Examples 3 and 4, the alumina powder obtained was not plate-shaped, and therefore the brilliance was significantly poor.

Claims

1. Composed of a plurality of plate-shaped alpha alumina particles, the average thickness is 0.2 μm or more and 3.0 μm or less, and the average particle diameter (Dp) after ultrasonic dispersion at an ultrasonic intensity of 600 W is ５０ ) is 2 μm or more and 100 μm or less, and the average particle diameter Dp ５０ 3 times (3 x Dp ５０ ) or larger particles account for 1.5% or less.

2. A method for producing the plate alumina powder according to claim 1, comprising: a raw material mixing step of preparing a raw material mixture containing at least aluminum hydroxide powder and an additive; and a firing step of firing the raw material mixture by holding it at a firing temperature in the range of 1000°C to 1300°C for 20 hours or less, wherein the raw material mixture contains sodium (Na) as a raw material. 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 to 5.0 mass% in terms of F, and in the calcination step, a heating rate for raising the temperature to the calcination temperature is within a range of 50°C / hour to 150°C / hour.

3. The additive is sodium silicofluoride (Na ２ SiF ６ ), and / or aluminum fluoride (AlF ３ ) and silicon oxide (SiO ２ 3. The method for producing a plate-like alumina powder according to claim 2, comprising:

4. As the additive, sodium (Na) oxide (Na ２ O) and carbonate (Na ２ CO ３ 4. The method for producing a plate-like alumina powder according to claim 3, further comprising either one or both of the following:

5. Volume average particle diameter (Dp ５０ ) is 0.5 μm or more and 5 μm or less, and the 45 μm sieve amount (+45 μm) is 0.01 mass% or less, and the BET specific surface area (N ２ The BET equivalent diameter (D ＢＥＴ ) to the volume average particle diameter (Dp ５０ ) ratio (Dp ５０ / D ＢＥＴ a raw material mixing step of adding an additive to aluminum hydroxide powder having a stoichiometric ratio (Sr) of 5.0 or less, and mixing the resulting mixture to obtain a raw material mixture; and a firing step of filling the raw material mixture into a sealed firing vessel and firing the mixture at a temperature of 1000°C or more for 20 hours or less.

6. The raw material mixture obtained in the raw material mixing step is added with sodium (Na) ２ O equivalent of 0.01 mass % or more and 0.5 mass % or less, silicon (Si) is SiO ２ 6. The method according to claim 5, wherein the carbon black contains 0.1% by mass or more and 0.3% by mass or less in terms of carbon dioxide equivalent, and fluorine (F) in an amount of 0.1% by mass or more and 5.0% by mass or less in terms of F.

7. A paint comprising the platy alumina powder according to claim 1.

8. A cosmetic product comprising the platy alumina powder according to claim 1.

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