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JP-2026075334-A - Plate-shaped alumina powder and its manufacturing method, as well as paints or cosmetics

JP2026075334AJP 2026075334 AJP2026075334 AJP 2026075334AJP-2026075334-A

Abstract

[Problem] To provide plate-shaped alumina powder and a method for producing the same, which suppresses the scattered glare (shininess) compared to conventional plate-shaped alumina powder, even with the same particle size. [Solution] A plate-shaped alumina powder composed of a plurality of plate-shaped α-alumina particles, having an average particle diameter of 2 μm or more and 100 μm or less, and an average thickness of 0.2 μm or more and 3.0 μm or less, wherein the plate-shaped α-alumina particles are polycrystalline, and on the plate surface of the plate-shaped α-alumina particles, the average value of the ratio (L2/L1) of the total length of grain boundaries to the perimeter length (L1) of the plate-shaped α-alumina particles to the total length of grain boundaries (L2) is 0.30 or more and 2.00 or less. [Selection Diagram] Figure 2

Inventors

  • 林 大智
  • 吉岡 真良
  • 望月 千歳
  • 井田 猛男

Assignees

  • 日本軽金属株式会社

Dates

Publication Date
20260508
Application Date
20241022

Claims (8)

  1. A plate-shaped alumina powder composed of multiple plate-shaped α-alumina particles, having an average particle diameter of 2 μm or more and 100 μm or less, and an average thickness of 0.2 μm or more and 3.0 μm or less, The aforementioned plate-like α-alumina particles are polycrystalline, Plate-shaped alumina powder, wherein the average value of the ratio (L2/L1) of the total length of grain boundaries to the perimeter length (L1) of the plate-shaped α-alumina particles on the plate surface of the plate-shaped α-alumina particles is 0.30 or more and 2.00 or less.
  2. The plate-shaped alumina powder according to claim 1, wherein the average value of L2/L1 is 0.50 or more and 1.50 or less.
  3. The plate-shaped alumina powder according to claim 1 or 2, wherein the average particle diameter is 5 μm or more and 50 μm or less, and the average thickness is 0.3 μm or more and 1.0 μm or less.
  4. A method for producing plate-shaped alumina powder according to claim 1 or 2, The process comprises the steps of: preparing a raw material mixture containing aluminum hydroxide powder and additives; and firing the raw material mixture by holding it at a temperature in the range of 1000°C to 1300°C for less than 20 hours. The aforementioned raw material mixture contains alkali metals (AM) in an amount of 0.01% to 0.5% by mass (based on AM₂O ), silicon (Si) in an amount of 0.1% to 0.3% by mass (based on SiO₂ ), and fluorine (F) in an amount of 0.1% to 5.0% by mass (based on F). A method wherein the heating rate in the step of calcining the raw material mixture is 50°C/hour or more and 150°C/hour or less.
  5. The method according to claim 4, wherein the additive comprises alkali silica fluoride ( AM₂SiF₆ ), or aluminum fluoride ( AlF₃ ) and silicon dioxide ( SiO₂ ).
  6. The method according to claim 5, wherein the additive further comprises one or both of an alkali metal (AM) oxide ( AM₂O ) and a carbonate ( AM₂CO₃ ).
  7. The method according to claim 4, wherein the alkali metal (AM) is either sodium (Na) or potassium (K), or both.
  8. A paint or cosmetic comprising the plate-shaped alumina powder described in claim 1 or 2.

Description

This invention relates to plate-shaped alumina powder, a method for producing the same, and a paint or cosmetic. Alumina ( Al₂O₃ ) is chemically stable and possesses excellent heat resistance, corrosion resistance, wear resistance, and insulation properties, as well as high strength and hardness. Taking advantage of these characteristics , alumina powder is widely used in various applications such as structural components, tools, abrasives, fillers, spark plugs, insulators, electronic circuit boards, and refractories. In particular, plate-shaped alumina powder has a high light reflectivity at its particle surface, resulting in high brightness. Therefore, this characteristic is utilized as a pigment added to paints and cosmetics. Plate-shaped alumina powder is composed of multiple plate-shaped alumina particles and is also called flake-shaped alumina powder, thin flake-shaped alumina powder, flattened alumina powder, or alumina flakes. Patent Document 1 discloses Al₂O₃ flakes having a thickness of 500 nm or more, a D₅50 value of 15 to 30 μm, and a D₅90 value of 30 to 45 μm, and states that these Al₂O₃ flakes simultaneously possess high chemical stability, a smooth surface, and high whiteness, and are used as a pigment substrate (Claim 1, [0010] and [0015 ] of Patent Document 1). Patent Document 1 also states that Al₂O₃ flakes are coated with a high refractive index layer such as TiO₂ or a low refractive index layer such as SiO₂ , thereby imparting increased gloss, interference color, or color flop effect (Claim 1, [0040] to [0046] of Patent Document 1). Patent Document 2 discloses a paint composition containing alumina flakes as a lustrous pigment, which can be suitably used as an automotive topcoat, and which forms a composite coating film with unprecedentedly strong lustrousness and novel, highly aesthetic design without degrading the finished appearance (Claim 1, [0001] and [0108] of Patent Document 2). Patent Document 2 also states that the alumina flakes are aluminum oxide ( Al₂O₃ ) coated with a metal oxide such as titanium dioxide, with a particle size of 10 to 30 μm and a thickness of 0.3 to 0.4 μm (Claim 2 , [0007] of Patent Document 2). Patent Document 3 discloses a pearlescent pigment containing aluminum oxide and zinc oxide as main components in a mass ratio of 100:0.1 to 5, and containing flaky alumina crystals coated with metal or metal precursor particles (Claim 1 of Patent Document 3). Patent Document 3 also states that the crystals have excellent gloss because they 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 ([0001] of Patent Document 3). Patent Document 4 discloses a method for producing plate-shaped alumina-based powder characterized by having a crystalline structure of α, β, γ-alumina alone, or two or more crystalline structures, by firing plate-shaped boehmite at 400°C to 1500°C (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 hydrophobic compound such as polysiloxane, and incorporating this coated powder into cosmetics, cosmetics with a pleasant feel can be obtained ([0013] of Patent Document 4). Patent Document 5 discloses hexagonal plate-shaped alumina obtained by firing hexagonal plate-shaped boehmite at a temperature of 450 to 1500°C, characterized by being a substantially hexagonal plate shape, having 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 this alumina exhibits high orientation, reduced diffuse reflection, and increased luster, and can be suitably used as a filler for paints and cosmetics where luster is a primary objective (Claim 4 and [0045] of Patent Document 5). Patent Document 6 discloses plate-shaped 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 (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 states that these plate-shaped alumina particles can be suitably used as a thermally conductive filler, cosmetic, abrasive, high-gloss pigment, lubricant, substrate for conductive powders, ceramic material, etc. ([0101] of Patent Document 6). Japanese Patent Publication No. 2014-218424Japanese Patent Application Publication No. 10-298458Special Publication No. 2010-502774Japanese Patent Publication No. 2012-071996Japanese Patent Publication No. 2003-002642Japanese Patent Publication No. 2019-123664 This shows the mechanism by which plate-shaped alumina powder produces a glossy appearance.This diagram schematically illustrates the scattering of light passing through a polycrystalline material.The plate surface of a plate-like particle is schematically shown. Specific embodiments of the present invention (hereinafter referre