Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2001-08-23
2003-03-18
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S690000, C428S900000
Reexamination Certificate
active
06534168
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium exhibiting high output and a good C/N ratio in high-density recording.
RELATED ART
In recent years, recording wavelengths have tended to shorten as recording densities have increased. However, the problem of so-called self-magnetization loss where output drops when recording with a short wavelength on a thick magnetic layer has become significant. Thus, the magnetic layer has been reduced in thickness. However, in particulate magnetic recording media, when a magnetic layer equal to or less than 2 &mgr;m in thickness is directly coated onto a support, the nonmagnetic support tends to affect the surface of the magnetic layer. As a result, deterioration of electromagnetic characteristics and dropout tend to appear. This problem is solved by a method employing a simultaneous multilayer coating method in which a nonmagnetic layer is provided as a lower layer and magnetic coating liquid with high concentration is thinly applied (for example, see Japanese Unexamined Patent Publication (KOKAI) Showa Nos. 63-191315 and 63-187418). Such inventions have made it possible to achieve particulate magnetic recording media having good electromagnetic characteristics with dramatically improved yields.
In recent popular digital VCR systems and the like, the need to further reduce medium noise in particulate magnetic recording media has emerged. In popular digital VCR systems, surface roughness has been found to affect medium noise at a wavelength pitch of about 4 &mgr;m. Further, high output and suitability to overwriting (O/W) is being demanded of the tapes employed in popular digital VCR systems, resulting in further reduction in thickness of the magnetic layer to about 0.1 &mgr;m. When the thickness of the magnetic layer is reduced in this manner, for example, the effects on the surface properties of the magnetic layer of factors resulting from the magnetic layer such as aggregation of the magnetic material and orientational turbulence are reduced, with the surface properties of the lower nonmagnetic layer determining the surface properties of the magnetic layer.
Known methods of smoothing the surface of the lower nonmagnetic layer include, for example, the method of employing acicular particles in the lower layer nonmagnetic powder described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 4-325915, and the method of employing scale-like particles or plate-shaped particles in the lower layer nonmagnetic powder described in Japanese Patent No. 2,698,770. However, in these methods, it is difficult to reduce roughness at a wavelength pitch of from several &mgr;m to several tens of &mgr;m. Further, when the thickness of magnetic recording tapes is reduced, that is, when the tape length is increased, it is possible to increase the volume density and achieve high density. However, it has been necessary to reduce the thickness of the support relative to metal evaporation (ME) tapes in magnetic recording tapes having a lower nonmagnetic layer such as the present inventions. As the support has been thinned, high-strength supports extended in the direction of width (TD) have come to be employed to achieve good head contact and ensure running durability. However, when such supports develop minute scratches, they tend to sever during coating, calendering, and slitting steps, so that the yield deteriorates, and there are problems with production properties.
From these perspectives, in magnetic recording tapes having a lower nonmagnetic layer, the demand for further thinning of the lower layer is increasing. However, when the lower layer is thinned in the layer configurations described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 4-325915 and Japanese Patent No. 1698770, calendering molding properties decrease, the effects of protrusions appear on the support surface, aggregates form in the upper and lower layers, additives such as abrasives are exposed, the surface of the magnetic layer is roughened, output drops, and noise increases. In particular, when the lower layer is thinned to 0.1 to 0.5 &mgr;m, these effects become marked.
Accordingly, the object of the present invention is to provide a magnetic recording medium exhibiting a high C/N ratio (low noise) in high-density magnetic recording.
SUMMARY OF THE INVENTION
The present inventors conducted extensive research into achieving the above-stated object in a magnetic recording medium which comprises a nonmagnetic lower layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic powder and a binder provided in this order on a flexible nonmagnetic support, resulting in the discovery that a magnetic recording medium exhibiting a high C/N ratio (lower noise) was obtained by incorporating flat acicular &agr;-iron oxide powder and granular nonmagnetic particles into the nonmagnetic lower layer wherein said flat acicular &agr;-iron oxide powder has a major axis length ranging from 0.05 to 0.5 &mgr;m and a ratio of the major width m to the minor width k of the minor axis cross-section when sectioned at an angle perpendicular to the major axis (m/k) is higher than 1, and said granular nonmagnetic particles have a mean particle diameter equal to or less than 0.04 &mgr;m; the present invention was devised on that basis.
In the magnetic recording medium of the present invention, the use of flat acicular &agr;-iron oxide powder such as set forth above permits inhibition of surface roughness due to vortex flows by increasing the thixotropic properties of the liquid to a greater level than in conventional acicular &agr;-iron oxide powder. Further, the orientation of the nonmagnetic powder can be increased, permitting the production of a smooth surface during coating film formation. However, the increase in orientation of the nonmagnetic powder decreases voids in the coating film, thereby decreasing calender forming properties, resulting in the problem that the surface property cannot be rendered smooth by calendering.
In the present invention, the combined use of the granular nonmagnetic particles, as described above, ensures the presence of voids in the coating film while inhibiting orientation turbulence of the flat acicular &agr;-iron oxide powder. Therefore, a magnetic recording medium is obtained with a smooth magnetic layer surface property during coating film formation and with improved calendering forming properties.
Further, from the perspective of ensuring voids and increasing the thixotropic property of the liquid, the above-described granular nonmagnetic particles are preferably particles that easily form structures. From this perspective, the granular nonmagnetic particles are most preferably carbon black. However, so long as the particles easily form a structure, they may be ceramic oxides such as titanium dioxide or zirconia oxide, or carbon black may be employed in combination with ceramic oxides. Further, from the perspective of ensuring voids and increasing the thixotropic property of the liquid, the above-mentioned flat acicular particles are preferably particles capable of being dispersed in a small quantity of binder, that is, particles having a low specific surface area in spite of small particle size. From this perspective, the flat acicular particles are preferably &agr;-iron oxide rather than iron oxyhydroxide.
One characteristic of the magnetic recording medium of the present invention is that the nonmagnetic lower layer comprises flat acicular &agr;-iron oxide powder. In the present invention, the flat acicular &agr;-iron oxide powder consists of particles with a major axis length ranging from 0.05 to 0.5 &mgr;m, and having a ratio of major width to minor width (major width/minor width) of the minor axis cross-section when sectioned at an angle perpendicular to the major axis of greater than 1. When the major axis length of the flat acicular &agr;-iron oxide powder is less than 0.05 &mgr;m, the improvement in surface smoothness decreases. Further, flat acicular &agr;-iron oxide powder with
Ejiri Kiyomi
Naoe Koji
Fuji Photo Film Co. , Ltd.
Le H. Thi
Stroock & Stroock & Lavan LLP
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