Metal treatment – Stock – Magnetic
Reexamination Certificate
2002-04-24
2004-08-31
Sheehan, John P. (Department: 1742)
Metal treatment
Stock
Magnetic
C148S301000, C148S311000, C075S348000, C075S352000, C075S365000
Reexamination Certificate
active
06783608
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to secondary agglomerates of magnetic metal particles for magnetic recording and a process for producing the same, and more particularly, to secondary agglomerates of magnetic metal particles containing iron as a main component, which exhibit not only an excellent dispersibility but also a less soluble salt content while maintaining good magnetic properties.
In recent years, miniaturization, lightening, recording-time prolongation and high-density recording of magnetic recording and reproducing apparatuses for audio, video or computer as well as increase in memory capacity thereof have proceeded more rapidly. With such a recent tendency, it has been increasingly required to provide magnetic recording media having a high performance and a high-density recording property, such as magnetic tapes and magnetic disks.
Namely, the magnetic recording media have been required to have high image definition and quality, high output characteristics such as, in particular, good frequency characteristics, an excellent keeping property and a high durability. Therefore, it has been required that the magnetic recording media are reduced in noise due to the magnetic recording media, and exhibit a high coercive force Hc, a narrow coercive force distribution (Switching Field Distribution: SFD) and an excellent weather resistance (&Dgr;Bm).
These properties of the magnetic recording media have a close relation to magnetic particles used therein. In recent years, magnetic metal particles containing iron as a main component have been noticed because the magnetic metal particles can show a higher coercive force and a larger saturation magnetization &sgr;s as compared to those of conventional magnetic iron oxide particles, and have been already used as magnetic particles for magnetic recording media such as digital audio tapes (DAT), 8-mm video tapes, Hi-8 tapes, W-VHS tapes for Hi-Vision, DVC tapes of digital recording type, etc., as well as removable disks for computers such as Zip disks and Super disks. Further, it has been recently attempted to practically apply the magnetic metal particles containing iron as a main component to large-capacity Hi-FD.
Therefore, it has also been strongly required to improve properties of the magnetic metal particles containing iron as a main component.
More specifically, in order to obtain magnetic recording media having a higher coercive force, an excellent coercive force distribution SFD and an excellent weather resistance &Dgr;Bm, the magnetic metal particles containing iron as a main component, which are used as magnetic particles therein, have been strongly required to show a higher coercive force and a larger saturation magnetization as well as a narrower particle size distribution, an excellent dispersibility and an excellent oxidation stability &Dgr;&sgr;s.
In addition, in order to obtain magnetic recording media having a high recording density, a high reliability and an improved durability, the magnetic metal particles have been required to have a less soluble salt content.
The above facts are described in detail below.
In general, the magnetic metal particles containing iron as a main component are produced by using as a starting material, (i) spindle-shaped goethite particles obtained by passing an oxygen-containing gas such as air through a suspension containing a ferrous-containing precipitate obtained by reacting an aqueous ferrous salt solution with an aqueous alkali solution containing sodium hydroxide, potassium hydroxide or sodium carbonate for conducting an oxidation reaction of the suspension, (ii) spindle-shaped hematite particles obtained by heat-dehydrating the spindle-shaped goethite particles, or (iii) spindle-shaped particles obtained by incorporating metal elements other than iron into the above spindle-shaped goethite or hematite particles; and then heat-reducing the starting material in a reducing gas atmosphere.
The thus obtained spindle-shaped magnetic metal particles inevitably contain sodium or calcium owing to the above production process. When the magnetic metal particles containing a soluble sodium salt or a soluble calcium salt are used for producing magnetic recording media, there arises such a problem that compounds derived from the soluble sodium salt or the soluble calcium salt tend to be precipitated on a magnetic coating film thereof. As to this fact, Japanese Patent Application Laid-Open (KOKAI) No. 8-186015(1996) describes that “ . . . in the case where such Fe-containing magnetic metal particles having an increased soluble ion content are used for producing magnetic recording media, although initial properties of the magnetic recording media are excellent, the soluble ion tends to be precipitated in the form of insoluble salts when stored under high-temperature and high-humidity conditions, resulting in problems such as drop-out (DO) and deteriorated output characteristics of the obtained products”.
In order to reduce the amount of soluble salts contained in the magnetic metal particles, there have been adopted (1) the method of non-using of aqueous alkali solutions containing alkali metal salts such as sodium hydroxide, as starting materials, and (2) the method of washing the magnetic metal particles with water to remove the soluble salt therefrom. Meanwhile, the present invention relates to the method (2).
In the case of the above water-washing method, it will be considered that the respective products obtained in each intermediate step of the production process of the magnetic metal particles are washed with water. However, even if the goethite particles or the hematite particles as the starting material are washed with water, only the soluble salts contained inside of these particles are removed, and it is known that when these starting particles are subjected to reduction reaction to produce the magnetic metal particles, insoluble impurities contained therein are transferred onto the surface of the particles and precipitated thereon in the form of soluble salts. As to this fact, Japanese Patent Application Laid-Open (KOKAI) No. 7-22224(1995) describes that “ . . . in order to reduce the content of elements of Group 1a of the Periodic Table to not more than 0.05% by weight, it is necessary to conduct an additional step for removing these element if they are inevitably mixed into the particles in the course of the production process, . . . in particular, as the process proceeds from iron oxide hydroxide to iron oxide and then to magnetic metal particles, these elements are more remarkably precipitated at the surface of the particle . . . ”. On the other hand, when the magnetic metal particles, especially spindle-shaped magnetic metal particles, are washed with water, magnetic properties thereof such as coercive force tend to be deteriorated, and the dispersibility thereof in a magnetic coating composition also tends to be lowered.
As conventional techniques for reducing the contents of impurities such as soluble sodium by washing the magnetic metal particles with water, there are known the methods described in Japanese Patent Application Laid-Open Nos. 56-51029(1981), 7-22224(1995), 8-172005(1996), 8-186015(1996) and 9-305958(1997) or the like.
Meanwhile, as known in the art, the magnetic metal particles have been produced by granulating a starting material such as goethite particles and hematite particles obtained by heat-dehydrating the goethite particles, into an adequate size, and then heat-reducing the granulated product.
In general, upon the production of coating-type magnetic recording media, the granulated product of the magnetic metal particles is directly charged into a kneading apparatus such as kneader, and kneaded therein with various binder resins and organic solvents. The kneaded material is diluted and dispersed in an additional amount of the organic solvent to prepare a magnetic coating composition, and then the obtained magnetic coating composition is coated onto a non-magnetic base film.
As described above, the surface smoothness of a magne
Mori Kohji
Okinaka Kenji
Uegami Masayuki
Sheehan John P.
Toda Kogyo Corporation
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