Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2000-05-25
2003-03-11
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S402000, C428S403000, C428S404000, C428S405000, C428S407000, C428S690000, C106S287120, C106S287130, C252S062560
Reexamination Certificate
active
06531211
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to black plate-shaped ferrite composite particles with magnetoplumbite structure and a magnetic recording medium using the black plate-shaped ferrite composite particles with magnetoplumbite structure. More particularly, the present invention relates to black plate-shaped ferrite composite particles with magnetoplumbite structure which are excellent in dispersibility in a vehicle due to less amount of carbon black fallen-off from the surface of each black plate-shaped ferrite composite particles with magnetoplumbite structure, and have a high blackness and a low volume resistivity, and a magnetic recording medium capable of not only showing a low light transmittance and a low surface resistivity even when the amount of carbon black fine particles added to a magnetic recording layer thereof is as small as possible, but also having a smooth surface, by using the black plate-shaped ferrite composite particles with magnetoplumbite structure.
With a development of miniaturized, lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly desired to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
Especially, video tapes have recently been desired more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
In order to enhance output characteristics of magnetic recording media, especially an S/N ratio thereof with respect to signals having a short wavelength, there have been demanded fineness of magnetic particles, reduction in thickness of a magnetic recording layer, high dispersibility of magnetic particles and surface smoothness of a magnetic coating film.
In general, as magnetic particles having a high coercive force, there are known magnetic metal particles containing iron as a main component, plate-shaped ferrite particles with magnetoplumbite structure, or the like.
Since the above-mentioned plate-shaped ferrite particles with magnetoplumbite structure are a stable oxide compound, there have been a large demand thereof especially in the field of data tapes which are required to retain recorded information for a long period of time.
However, it is also known that due to the fact that the plate-shaped ferrite particles with magnetoplumbite structure are a stable oxide compound, electrons therewithin are less movable, thereby exhibiting a volume resistivity as high as not less than 1.0×10
8
&OHgr;·cm. When such plate-shaped ferrite particles with magnetoplumbite structure are used as magnetic particles for magnetic recording media, there arises such a disadvantage that the surface resistivity of the obtained magnetic recording media becomes too high, especially more than 1.0×10
12
&OHgr;/sq.
The magnetic recording media having such a high surface resistivity value cause problems such as attachment of cut chips, dusts or the like onto the surface thereof during the production process or upon use due to the increase in electrostatic charge amount, resulting in frequently generating drop-out. Therefore, in order to produce magnetic recording media whose surface resistivity value is reduced to not more than 10
10
&OHgr;/sq, it has been strongly demanded to provide plate-shaped ferrite particles with magnetoplumbite structure which have a volume resistivity value as low as possible.
On the other hand, at the present time, the end position of a magnetic recording medium such as magnetic tapes has been detected by sensing a high light transmittance portion of the magnetic recording medium by means of a video deck. In the case where the particle size of magnetic particles dispersed in the magnetic recording layer become finer and the thickness of the magnetic recording medium is reduced in order to meet the requirement for high performance of the magnetic recording medium as described hereinbefore, the magnetic recording medium shows a high light transmittance as a whole, so that it has been difficult to detect the end position thereof by means of the video deck. In order to solve this problem, carbon black fine particles have been added to the magnetic recording layer in an amount of usually about 6 to 12 parts by weight based on 100 parts by weight of the magnetic particles, thereby reducing the light transmittance of the magnetic recording medium. For this reason, in current videotapes, it is indispensable to add carbon black fine particles, etc., to the magnetic recording layer thereof.
When a large amount of carbon black fine particles are added to the magnetic recording layer, the obtained magnetic recording media can show not only a low light transmittance but also a low surface resistivity value, because the carbon black fine particles are conductive particles. However, since the carbon black fine particles are fine particles having an average particle size as small as about 0.002 to about 0.05 &mgr;m, and have a large BET specific surface area and a low solvent-wettability, it is difficult to disperse these particles in vehicles. For this reason, it also becomes difficult to produce magnetic recording media having a smooth surface. In addition, since the addition of the large amount of such carbon black fine particles becomes to increase in amount of non-magnetic components in the magnetic recording layer, the obtained magnetic recording media is deteriorated in signal recording properties, thereby obstructing not only high-density recording but also reduction in thickness of the magnetic recording layer.
Consequently, it has been required to provide plate-shaped ferrite particles with magnetoplumbite structure having an excellent blackness and a low volume resistivity value in order to obtain magnetic recording media which can exhibit a sufficiently low surface resistivity value and a sufficiently low light transmittance even when the amount of carbon black fine particles added to the magnetic recording layer is reduced to a level as low as possible, especially less than 6 parts by weight based on 100 parts by weight of magnetic particles.
Hitherto, as methods of reducing the surface resistivity value of magnetic recording media using the plate-shaped ferrite particles with magnetoplumbite structure, there are known a method of adding a large amount of carbon fine particles to the magnetic recording layer, and a method of lowering the volume resistivity value of the magnetic particles per se, as described hereinabove. As the method of lowering the volume resistivity value, there are known a method of incorporating Fe
2+
into plate-shaped hexagonal ferrite particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 62-154228(1987) and 2-208821(1990), etc.), a method of depositing 2 to 20% by weight of carbon onto the surfaces of particles by heat-treating plate-shaped hexagonal ferrite particles at a temperature of 100 to 450° C. by a hydrogen gas stream and then treating the reduced particles under a carbon dioxide gas stream (Japanese Patent Application Laid-Open (KOKAI) No. 4-157615(1992), etc.), or the like.
At the present time, it has been most strongly demanded to provide plate-shaped ferrite particles with magnetoplumbite structure, which show an excellent blackness and a low volume resistivity value. However, such plate-shaped ferrite particles with magnetoplumbite structure, which can satisfy these properties have not been obtained yet.
That is, the above-mentioned Fe
2+
-containing plate-shaped hexagonal ferrite particles show a low volume resistivity value. However, as shown in Comparative Examples hereinafter, since the plate-s
Hayashi Kazuyuki
Iwasaki Keisuke
Kamigaki Mamoru
Morii Hiroko
Tanaka Yasuyuki
Bernatz Kevin M.
Chen Vivian
Toda Kogyo Corporation
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