Ykk Corporation

Kinsei Matec Co., Ltd.

Ryuichi Shido

Takeshi Fukuda

Flake-like &#x3b1;-alumina particles having an average major diameter of 0.5 to 25 &#x3bc;m and an aspect ratio, expressed by particle major diameter/average thickness, of greater than 50 to 2000 and having a thin flat form. The flake-like &#x3b1;-alumina particles are produced by a hydrothermal synthesis process of an aqueous slurry in which the slurry comprises an alumina hydrate and/or an alumina gel, having a particle size of not more than 2 &#x3bc;m and a maximum size of not more than 5.0 &#x3bc;m and phosphoric acid ions in an amount of 1.0&#xd7;10to 1.0&#xd7;10mol per mol of the alumina hydrate and/or alumina gel. The flake-like &#x3b1;-alumina particles exhibit good dispersibility when being kneaded as fillers or pigments in rubbers or plastics or as coating agents with a resin and also can be easily dispersed as primary particles in aqueous solvent with high dispersion stability when added to an aqueous slurry of precision abrasives or cosmetics. The particles are desirable in providing cosmetics with good smoothness, tackiness to the skin and spreadability.

Flaky &#x3b1;-alumina particles and method for producing the same

Alumina particles having high dispersibility and plasticity, at least on the surface of which a phosphoric acid or phosphate is present and suitable for use as a material for pigments for paints, precision abrasives or ceramics. The alumina particles are produced by adding a crystallization inhibitor containing at least phosphate ion to aluminum hydroxide or alumina hydrate and then conducting a hydrothermal synthesis treatment. The amount of the phosphate ion to be added is in the range of 3.0&times;10,to 2.5&times;10,mol per mol of aluminum hydroxide or alumina hydrate. The particle size of aluminum hydroxide or alumina hydrate is 0.1 to 5.0 &mgr;m. The hydrothermal synthesis is preferably conducted at 350&deg; C. or above under a pressure of 50 to 200 atm and at a temperature elevation rate of 5&deg; C./min to 0.3&deg; C./min. The thus obtained alumina is &agr;-alumina in the form of hexagonal plate single crystal having a particle size of 0.2 to 15 &mgr;m, an aspect ratio of 15 to 50 and an isoelectric point at which the zeta-potential is 0 of pH 4 to 8.

Process for producing alumina particles having high dispersibility and plasticity

Alumina particles having high dispersibility and plasticity, at least on the surface of which a phosphoric acid or phosphate is present and suitable for use as a material for pigments for paints, precision abrasives or ceramics. The alumina particles are produced by adding a crystallization inhibitor containing at least phosphate ion to aluminum hydroxide or alumina hydrate and then conducting a hydrothermal synthesis treatment. The amount of the phosphate ion to be added is in the range of 3.0.times.10.sup.-3 to 2.5.times.10.sup.-2 mol per mol of aluminum hydroxide or alumina hydrate. The particle size of aluminum hydroxide or alumina hydrate is 0.1 to 5.0 .mu.m. The hydrothermal synthesis is preferably conducted at 350.degree. C. or above under a pressure of 50 to 200 atm and at a temperature elevation rate of 50.degree. C./min to 0.3.degree. C./min. The thus obtained alumina is .alpha.-alumina in the form of hexagonal plate single crystal having a particle size of 0.2 to 15 .mu.m, an aspect ratio of 15 to 50 and an isoelectric point at which the zeta-potential is 0 of pH 4 to 8.

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Kurobe, Japan

Ryuichi Shido