Abstract: A process for producing roundish alumina particles includes the steps of granulating a composition containing at least one pulverized product of electrofused alumina and sintered alumina having a mean particle size of 5 to 35 ?m and at least one species selected from the group consisting of a halogen compound, a boron compound and an alumina hydrate to obtain a granulated product, heating the granulated product at 1,000 to 1,600° C., and crushing the heated product.

Abstract: Particulate alumina has a mean particle size corresponding to a volume-cumulative 50% mean particle size (D50) falling within a range of 3 to 6 ?m, has a ratio of D90 to D10 that is 2.5 or less, contains particles that have a particle size of at least 12 ?m in an amount of 0.5 mass % or less, particles that have a particle size of 20 ?m or more in an amount of 0.01 mass % or less and particles that have a particle size of 1.5 ?m or less in an amount of 0.2 mass % or less, and contains an ?-phase as a predominant phase. In addition, the particulate alumina has a ratio of longer diameter (DL) to shorter diameter (DS) that is 2 or less and a ratio of D50 to mean primary particle size (DP) that is 3 or less. With these features, the particulate alumina has a narrow particle size distribution profile, causes little wear and exhibits excellent flow characteristics.

Abstract: A process for producing roundish alumina particles includes the steps of granulating a composition containing at least one pulverized product of electrofused alumina and sintered alumina having a mean particle size of 5 to 35 ?m and at least one species selected from the group consisting of a halogen compound, a boron compound and an alumina hydrate to obtain a granulated product, heating the granulated product at 1,000 to 1,600° C., and crushing the heated product.

Abstract: A rubber composition for a tire tread which comprises (A) a rubber component selected from the group consisting of natural rubber and synthetic diene-based rubbers, (B) 5 to 50 parts by weight per 100 parts by weight of the rubber component of (b1) aluminum hydroxide particles which are treated with a surface treating agent on the surface and have an average diameter of secondary particles D2 of 10 &mgr;m or smaller or (b2) aluminum hydroxide particles which have an average diameter of secondary particles D2 of 0.8 &mgr;m or smaller and a ratio (D2/D1) of the average diameter of secondary particles D2 to an average diameter of primary particles D1 of 1.7 or smaller and (C) 5 to 80 parts by weight per 100 parts by weight of the rubber component of at least one filler selected from carbon black and silica.

Abstract: A rubber composition for a tire tread which comprises (A) a rubber component selected from the group consisting of natural rubber and synthetic diene-based rubbers, (B) 5 to 50 parts by weight per 100 parts by weight of the rubber component of (b1) aluminum hydroxide particles which are treated with a surface treating agent on the surface and have an average diameter of secondary particles D2 of 10 &mgr;m or smaller or (b2) aluminum hydroxide particles which have an average diameter of secondary particles D2 of 0.8 &mgr;m or smaller and a ratio (D2/D1) of the average diameter of secondary particles D2 to an average diameter of primary particles D1 of 1.7 or smaller and (C) 5 to 80 parts by weight per 100 parts by weight of the rubber component of at least one filler selected from carbon black and silica.

Abstract: The present invention provides particulate alumina having a mean particle size corresponding to a volume-cumulative (50%) mean particle size (D50) of 1.5 to 4 &mgr;m and a ratio (D90/D1O) of D90 to D10 of 2.5 or less. The alumina contains secondary particles having a particle size of at least 10 &mgr;m in an amount of 0.1 mass % or less; secondary particles having a particle size of 0.5 &mgr;m or less in an amount of 5 mass % or less; and an &agr;-phase as a predominant phase.fluoride. The present invention also provides a method for producing the particulate alumina.