Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2001-08-03
2002-06-04
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S456000, C423S633000
Reexamination Certificate
active
06398863
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to spindle-shaped goethite particles, spindle-shaped hematite particles and magnetic spindle-shaped metal particles containing iron as a main component. More particularly, the present invention relates to spindle-shaped goethite particles which are fine particles and exhibit a good particle size distribution (standard deviation/average major axial diameter); spindle-shaped hematite particles which can be prevented as highly as possible from causing destruction of particle shape when subjected to a heat-reduction step for producing magnetic metal particles, and which are suitable as a starting material for the production of spindle-shape magnetic metal particles containing iron as a main component exhibiting a high coercive force, a large saturation magnetization, an excellent oxidation stability and an excellent coercive force distribution (switching field distribution) when incorporated into a magnetic coating film (hereinafter sometimes referred to merely as “SFD” or “sheet SFD”); and the magnetic spindle-shaped metal particles containing iron as a main component which are produced from the spindle-shaped goethite particles or the spindle-shaped hematite particles as a starting material, which exhibit a high coercive force, an excellent particle coercive force distribution (switching field distribution) (hereinafter referred to as merely “SFDr” or “particle SFDr”), a large saturation magnetization and an excellent oxidation stability, and which are excellent in a squareness (Br/Bm) of the sheet due to a good dispersibility in a binder resin.
In recent years, miniaturization, lightening, recording-time prolongation, high density recording and high storage capacity of recording and reproducing apparatuses for audio, video or computers, have proceeded more remarkably. With this progress, magnetic recording media such as magnetic tapes and magnetic discs have been increasingly required to have a high performance and a high recording density.
Magnetic recording media have been required to show a high image quality, high output characteristics, and especially improved frequency characteristics. For this reason, it has been demanded to enhance a residual magnetic flux density (Br) and a coercive force of the magnetic recording media.
These characteristics of the magnetic recording media have a close relation to the magnetic particles used therefor. In recent years, magnetic metal particles containing iron as a main component have attracted attention because such particles can show a higher coercive force and a larger saturation magnetization as compared to those of conventional magnetic iron oxide particles, and have been put into practice and applied to magnetic recording media such as digital audio tapes (DAT), 8-mm video tapes, Hi-8 tapes, video floppies or W-VHS tapes for Hi-vision. Further, the magnetic metal particles containing iron as a main component have been adopted in DVC system for digital recording, Zip or super-discs for computers, and recently, large-capacity Hi-FD which are being now industrially put into practice.
In consequence, it has also been strongly demanded to further improve properties of these magnetic metal particles containing iron as a main component.
As to the relationship between various characteristics of the magnetic recording media and properties of the magnetic particles used therefor, in order to achieve high density recording, it is generally required that the magnetic particles are fine particles and have a good particle size distribution.
In order to obtain a high image quality, the magnetic recording media for video are required to have a high coercive force (Hc) and a large residual magnetic flux density (Br). In order to impart such a high coercive force (Hc) and a large residual magnetic flux density (Br) to the magnetic recording media, the magnetic particles used therefor are also required to have a coercive force (Hc) as high as possible, an excellent particle coercive force distribution (SFDr) and a large saturation magnetization.
For example, in Japanese Patent Application Laid-Open (KOKAI) No. 63-26821(1988), it is described that “
FIG. 1
shows a relationship between the SFD measured on the magnetic disc and the reproduction output thereof. . . As is apparent from
FIG. 1
, the characteristic curve representing the relationship between the SFD and the reproduction output becomes linear. Therefore, it is recognized that the reproduction output of the magnetic disc can be increased by using ferromagnetic particles having a small SFD. Namely, in order to obtain a high reproduction output, it is preferred that the SFD is small, and for example, when it is intended to obtain a more reproduction output than ordinary one, the SFD is required to be not more than 0.6.” Thus, in order to enhance the reproduction output of magnetic recording media, it is necessary that the SFD (Switching Field Distribution) of the magnetic recording media is small, i.e., the sheet coercive force distribution of the magnetic recording media is narrow. Further, for this purpose, it is required that the magnetic particles used therefor has a good particle size distribution and contain no dendritic particles therein.
As to the magnetic metal particles containing iron as a main component, the finer the particle size thereof becomes, the larger the surface activity thereof becomes, so that the magnetic properties is considerably deteriorated even in air, because such fine particles readily undergo the oxidation reaction by oxygen therein. As a result, it is not possible to produce magnetic metal particles containing iron as a main component, which can show the aimed high coercive force and large saturation magnetization.
In consequence, it has been required to provide magnetic metal particles containing iron as a main component which are excellent in oxidation stability.
As described above, at present, there has been a strongest demand for providing magnetic metal particles containing iron as a main particles which are fine particles, contain no dendritic particles, and have a good particle size distribution, a high coercive force, an excellent particle coercive force distribution (SFDr), a large saturation magnetization and an excellent oxidation stability.
On the other hand, in the production of magnetic recording media, when the magnetic metal particles containing iron as a main component becomes finer or have a larger saturation magnetization, there tends to be caused such a problem that the particles show a poor dispersibility due to the increase in attraction force between particles or magnetic cohesive force when kneaded and dispersed in a binder resin in an organic solvent. As a result, the magnetic recording media produced therefrom tend to be deteriorated in magnetic characteristics, especially squareness (Br/Bm). Therefore, it have been required that the magnetic metal particles are further improved in magnetic properties.
In general, the magnetic metal particles containing iron as a main component can be produced by using as starting particles, goethite particles, hematite particles obtained by heat-dehydrating the goethite particles, or particles obtained by incorporating different kind of metals other than iron into these particles; heat-treating the starting particles, if necessary, in a non-reducing atmosphere; and heat-reducing the thus-treated particles in a reducing gas atmosphere. It is known that the obtained magnetic metal particles containing iron as a main component have a similar shape to that of goethite particles as the starting particles. Therefore, in order to obtain magnetic metal particles containing iron as a main component which satisfy the above various properties, it is necessary to use goethite particles which are fine particles, have a good particle size distribution and an appropriate particle shape, and contain no dendritic particles. Further, it is required to retain the appropriate particle shape and the good particle size distribution of the goethite particles durin
Maekawa Masaaki
Okinaka Kenji
Koslow C. Melissa
Nixon & Vanderhye
Toda Kogyo Corporation
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