Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2000-05-19
2002-05-21
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S403000, C428S690000, C428S900000, C420S080000, C420S083000, C420S103000, C420S536000, C420S062000, C420S119000, C420S536000, C420S536000, C420S536000
Reexamination Certificate
active
06391450
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to spindle-shaped magnetic goethite particles and spindle-shaped hematite particles, which have a uniform particle size, a larger minor axis diameter and a suitable aspect ratio and are very excellent in sintering preventing performance, and to spindle-shaped magnetic iron-based alloy particles which are obtained from said spindle-shaped hematite particles as a starting material, are excellent in dispersibility (high squareness and high orientation), excellent both in weatherability and in coercive force distribution and are preferably usable as household DAT, 8 mm, Hi-8 tapes, VTR tapes for business purposes, computer tapes or floppy disks.
2. Description of the Prior Art
Miniaturized and lightweight magnetic recording and reproducing devices for long-time recording in audios, videos and computers such as household DAT, 8-mm video tapes, Hi-8 tapes, VTR tapes for business purposes, computer tapes or floppy disks have recently shown a remarkable progress. Especially, video tape recorders (VTR) have rapidly spread widely and the development of miniaturized and lighter-weight VTR for longer-time recording, particularly attempted shifting from analogue to digital systems, has been rapid. With this development, magnetic recording media such as a magnetic tape have been strongly required to have a higher performance and a higher recording density and to improve reliability in recording.
Magnetic recording media are required to improve higher picture qualities, higher output characteristics and especially frequency characteristics, and for this purpose, it is necessary to improve the residual flux density (Br), the coercive force, the dispersibility, the packing property, the surface smoothness of a magnetic tape and the S/N ratio.
These properties of magnetic recording media have a close relationship with the magnetic particles used for the magnetic recording media. In recent years, magnetic iron-based alloy particles have attracted attention due to their coercive force and saturation magnetization which are superior to those of conventional iron oxide magnetic particles, and have been put to practical use as magnetic recording media such as digital audio tapes (DAT), 8-mm video tapes, Hi-8 tapes, VTR tapes for business purposes, computer tapes and floppy disks. Such magnetic iron-based alloy particles, however, are also strongly demanded to improve the properties.
The properties of magnetic recording media are described in detail as follows: In order to achieve high picture qualities as video magnetic recording media, it is required to improve SIN ratio and video frequency characteristics. For this improvement, it is important to improve the dispersibility of magnetic particles in a coating material, orientation in a coating film and the packing property and to improve the surface smoothness of the magnetic recording media. In order to improve the video frequency characteristics, it is necessary that the coercive force of the magnetic recording media is not only high, but the residual flux density is large, and further that the S.F.D. (Switching Field Distribution) of the magnetic recording media, namely, the coercive force distribution is small. In addition, the repeated running ability, still characteristics or reliability of the magnetic recording media in recording for use in a severe environment should also be secured, in other words, the improvement of durability is also important.
It is considered that such metal magnetic particles preferably have a larger particle size for the dispersibility and weatherability thereof, and preferably have a larger aspect ratio for the squareness and orientation thereof in the form of a coating film. On the other hand, those having a smaller particle size are preferable from the viewpoint of surface smoothness and noises, but as the particle size is decreased, the particles are hardly dispersed and their weatherability is deteriorated. From the viewpoint of saturation magnetization, those having a larger particle size with an excellent distribution of particle sizes are preferable, but if the particle size is larger than necessary, the coercive force tends to be reduced, and it is thus necessary to maintain the coercive force by increasing the aspect ratio of the metal magnetic particles.
Generally, for preparation of the metal magnetic particles, goethite particles as the starting material, hematite particles obtained by heat dehydrating the goethite particles, or particles prepared by adding heterogeneous non-iron metals to these particles are heat-treated, if necessary, in a non-reducing atmosphere and then heat-reduced in a reducing atmosphere to form metal magnetic particles. During this process, it is necessary to suitably regulate the shape and size of goethite particles as the starting material and to prevent the fusion of particles upon heat treatment such as heating and reduction or the deformation and destruction of a single particle, whereby the shape and size of goethite particles as the starting material are maintained and inherited by the resulting metal magnetic particles.
Morphologically, there are two kinds of goethite particles as the starting material, that is, needle-shaped goethite particles obtained from an alkali hydroxide as a base and spindle-shaped goethite particles obtained from an alkali carbonate as a base. Although the needle-shape goethite particles are characterized in that generally those having a larger aspect ratio can be easily obtained, there is the problem that they are inferior to spindle-shaped particles in particle size distribution and those having a small particle size are hardly obtained. This particle size distribution is an indicator of the uniformity of primary particles and is thus also closely related to the coercive force distribution of the metal magnetic particles or to their weatherability, that is, oxidation stability. Japanese Patent Application Laid-Open (KOKAI) No. 5-98321 discloses the technique of obtaining fine needle-shaped particles having a relatively large aspect ratio, a high coercive force and a large saturation magnetization, but the oxidation stability and heat resistance of the metal magnetic particles or the weatherability of magnetic coating film therefrom have not sufficiently been examined.
On the other hand, the spindle-shaped goethite particles are characterized by being generally superior in particle size distribution of, but those having a larger aspect ratio are hardly obtained, and if the particle size is increased, the coercive force of the metal magnetic particles is low as compared with the needle-shaped particles, and thus the coercive force is maintained usually by decreasing the particle size. As a result, since the particle size is relatively small, there is the problem that the dispersibility thereof in a coating material is not good, and further due to the low aspect ratio, the squareness and orientation of the coating film are low, and the weatherability of the coating film is not satisfactory due to the small particle size in spite of good particle size distribution. Japanese Patent Application Laid-Open (KOKAI) No. 5-62166 discloses the technique of securing the coercive force and improving the dispersibility on the basis of the idea of enlarging the aspect ratio of spindle-shaped metal magnetic particles, but the oxidation stability of the metal magnetic particles and the weatherability of magnetic coating film therefrom are not taken into consideration.
For the reason described above, metal magnetic particles used in media for magnetic recording and reproducing devices in audios, videos and computers such as household DAT, 8-mm video tapes, Hi-8 tapes, VTR tapes for business purposed, computer tapes or floppy disks generally make use of needle-shaped metal magnetic particles with a coercive force of 103.5 to 143.2 KA/m (1300 to 1800 Oe) having a large particle size, a large aspect ratio, and a high squareness and orientation thereof in a magnetic coating film. How
Okinaka Kenji
Uegami Masayuki
Armstrong Westerman & Hattori, LLP
Kiliman Leszek
Toda Kogyo Corporation
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