Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-07-27
2002-09-10
Resan, Stevan A. (Department: 1773)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C252S062550, C075S348000, C428S690000
Reexamination Certificate
active
06447618
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to magnetic acicular alloy particles containing iron as a main component, and more particularly, to magnetic acicular alloy particles containing iron as a main component which exhibit an excellent dispersibility in a vehicle, especially such a vehicle composed of a binder resin having a polar functional group such as —SO
3
M (wherein M is H, Na or K), —COOH or the like, and in which an orientation and a packing density in coating film are improved; and a magnetic recording medium using the magnetic acicular alloy particles containing iron as a main component.
In recent years, recording-time prolongation, the miniaturization and lightening of magnetic recording and reproducing apparatuses for audio or video have proceeded more rapidly. In particular, VTRs (video tape recorders) are now widespread, so that there have been intensely developed VTRs aiming at prolongation of the recording-time, miniaturization and lightening of the VTRs.
On the other hand, magnetic recording media such as magnetic tapes have been required to have a still higher performance and a still higher recording density.
More specifically, magnetic recording media have been required to show a high image quality, high output characteristics, and especially high frequency characteristics. For this reason, it has been strongly demanded to enhance an S/N ratio of magnetic recording media.
These characteristics of the magnetic recording media have a close relation to the magnetic particles used therefor. In recent years, magnetic acicular alloy particles containing iron as a main component have been used as magnetic particles for magnetic recording media such as digital audio tapes (DAT), 8-mm video tapes, Hi-8 tapes, video floppy or the like, because such particles can show a high coercive force and a large saturation magnetization as compared to those of the conventional magnetic iron oxide particles.
In magnetic recording fields, there has been a continuous demand for the enhancement of these characteristics. In particular, it has been strongly demanded to further improve properties of the magnetic acicular alloy particles containing iron as a main component, which have a considerable influence on the characteristics of magnetic recording media such as S/N ratio or the like.
Namely, it has now been strongly required to provide magnetic acicular alloy particles containing iron as a main component, which show an excellent dispersibility in vehicle, and in which an orientation and a packing density in coating film are improved, and to improve properties of the magnetic acicular alloy particles themselves.
In order to attain the excellent dispersibility in vehicle, and the enhanced orientation and packing density in coating film, there have been widely proposed a method of improving surface conditions of the magnetic acicular alloy particles containing iron as a main component by using various organic or inorganic compounds, or a method of using binder resins having a polar functional group such as —SO
3
M, —COOH or the like.
In order to satisfy the requirements for improving the properties of the magnetic acicular alloy particles themselves, there have been conducted various attempts not only for reducing the size of the magnetic acicular alloy particles containing iron as a main component, but also for enhancing the shape-maintaining property and the aspect ratio (major axial diameter/minor axial diameter) which tend to be deteriorated in association with the size reduction. In addition, it has been attempted to improve magnetic properties of these magnetic acicular alloy particles containing iron as a main component. In order to obtain magnetic acicular alloy particles containing iron as a main component, which have the above-mentioned excellent properties, it is necessary that acicular goethite particles used as a starting material are fine particles having a large aspect ratio and a narrow particle size distribution.
In addition, it is important that in the production of the magnetic acicular alloy particles, the particle shape of the raw acicular goethite particles is retained as similarly as possible. In order to allow the magnetic acicular alloy particles to inherit the particle shape of the raw acicular goethite particles, it has been attempted to coat the surfaces of the acicular goethite particles or acicular hematite particles obtained by heat-dehydrating the acicular goethite particles, with various inorganic or organic compounds in advance of the heat-reduction thereof.
As well known in the arts, the size reduction of the magnetic acicular alloy particles containing iron as a main component adversely affects the orientation and packing density in coating film. In addition, as a matter of course, various inorganic compounds coated on the surfaces of the acicular goethite particles or the acicular hematite particles still remains on the surfaces of the magnetic acicular alloy particles containing iron as a main component, which are obtained by subjecting the acicular goethite particles or the acicular hematite particles to heat-reduction, or are diffused thereinto, thereby deteriorating the surface conditions of the magnetic acicular alloy particles containing iron as a main component. As a result, the conformability or compatibility of the particles with binder resin is considerably deteriorated.
Therefore, the magnetic acicular alloy particles containing iron as a main component have been strongly required to show an enhanced dispersibility in vehicle, high orientation and packing density in coating film, without being adversely influenced by the size reduction or the surface conditions thereof.
Hitherto, in order to improve various properties of the magnetic acicular alloy particles containing iron as a main component, such as dispersibility in vehicle, oxidation stability or the like, there is known such a method of contacting the magnetic acicular alloy particles containing iron as a main component with a basic gas such as ammonia, at various stages of the production process thereof, e.g., at a stage immediately after the heat-reduction and before allowing any oxide layer to be formed on the surfaces of the magnetic acicular alloy particles containing iron as a main component, at a gradual oxidation stage after the heat-reduction, or at a stage where the oxide layer is already formed on the surfaces of the particles by gradual oxidation (Japanese Patent Application Laid-Open (KOKAI) Nos. 49-89899(1974), 49-99004(1974), 51-51796 (1976) and 51-63494 (1976), Japanese Patent Publication (KOKOKU) No. 55-4802 (1980), Japanese Patent Application Laid-Open (KOKAI) Nos. 61-270315 (1986), 62-156202 (1987), 63-88806 (1988) and 3-101103(1991), Japanese Patent Publication (KOKOKU) No. 5-57321 (1993), Japanese Patent Application Laid-Open (KOKAI) No. 6-29112 (1994), etc.).
Thus, at the present time, it has been most strongly demanded to provide magnetic acicular alloy particles containing iron as a main component, which show an excellent dispersibility in vehicle, and in which a high orientation and a high packing density in coating film are improved, without being adversely affected by the size reduction or surface conditions thereof. However, the above-mentioned conventional methods fail to sufficiently satisfy these requirements.
Specifically, in the case of the particles described in Japanese Patent Application Laid-Open (KOKAI) Nos. 49-89899 (1974), 49-99004 (1974), 51-51796 (1976) and 51-63494 (1976), Japanese Patent Publication (KOKOKU) No. 55-4802 (1980), Japanese Patent Application Laid-Open (KOKAI) Nos. 61-270315 (1986), 62-156202 (1987), 63-88806 (1988) and 3-101103 (1991), Japanese Patent Publication (KOKOKU) No. 5-57321 (1993), Japanese Patent Application Laid-Open (KOKAI) No. 6-29112 (1994), etc., the difference between pH values of the water suspensions containing these particles as described hereinafter, is not less than 0. In particular, in the case of the particles described in Japanese Patent Application Laid-Open (KOKAI) No. 63-88806
Mori Kohji
Ota Yasutaka
Bernatz Kevin
Nixon & Vanderhye P.C.
Resan Stevan A.
Toda Kogyo Corporation
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