Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Patent
1984-03-01
1986-09-23
Robinson, Ellis P.
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
252 6254, 360134, 360135, 360136, 427128, 428329, 4284259, 428694, 428900, 428522, 428480, G11B 570
Patent
active
046135456
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a magnetic recording medium and, more particularly, to a short-wave recording magnetic recording medium in which a magnetic layer having a high coercive force and an excellent dispersibility is formed on a nonmagnetic support.
BACKGROUND ART
A tendency toward short-wave recording is significant for high-density recording in magnetic recording media, especially, magnetic recording media for VTRs. In order to obtain a high reproducing output in short-wave recording, it is indispensable that a magnetic layer of a magnetic recording medium has a high coercive force and a high residual flux density. However, such satisfactory magnetic characteristics cannot be obtained in a current magnetic recording medium which uses as a magnetic powder iron oxide magnetic powders such as .gamma.--Fe.sub.2 O.sub.3 or a chromium dioxide magnetic powder. The central recording wavelength attainable in practice is limited to about 1 .mu.m.
In contrast to this, in a magnetic recording medium using a ferromagnetic powder as a magnetic powder, a coercive force Hc and a residual flux density Br of a magnetic layer formed thereon are both high, so that high reproducing output can also be obtained in a short-wave band. Since such a medium permits high-density recording, it is considered to be most promising and various associated developments are being made.
In Japanese Patent Application Nos. 56-20388 and 56-40202, the present applicant has previously proposed a magnetic recording medium which uses such a ferromagnetic metal powder and which is therefore suitable for short-wave recording, especially, short-wave recording having a central recording wavelength of 1 .mu.m or less. However, in such a magnetic recording medium, when a fine ferromagnetic metal powder having a great specific surface area as used to improve, for example, the S/N ratio, various problems such is degradation in dispersibility of the ferromagnetic metal powder in a binder and an increase in the viscosity of a magnetic coating are raised. These problems adversely affect the surface properties, durability, magnetic characteristics and so on of a magnetic layer, which prevents the manufacture of a high-performance magnetic recording medium. This also impairs the workability in the manufacture of a magnetic recording medium. In order to improve the dispersibility of the magnetic powder, methods have been proposed wherein a surfactant is used as a dispersant, and the content of a hydrophillic group such as an OH group in the binder is increased. However, in either case, a satisfactory dispersibility can not be obtained. Moreover, it has been found that physical properties and mechanical properties and abrasion resistance, such as especially rubbing-off, of a magnetic layer are largely degraded.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide a magnetic recording medium for short-wave recording which uses a ferromagnetic metal powder as a magnetic powder, which has an improved dispersibility of the magnetic powder, a low noise level, and excellent magnetic characteristics such as squareness ratio.
For this reason, the present invention relates to a magnetic recording medium having, on a nonmagnetic support, a magnetic layer consisting mainly of an acicular ferromagnetic metal powder and a binder, characterized in that the metal powder has a coercive force of not less than 1,000 Oe and a specific surface area determined by the BET method is not less than 45 m.sup.2/ g, said binder containing not less than 50% by weight of a resin having at least one polar group selected from the group consisting of polar groups having the general formulas: ##STR2## (wherein M is hydrogen, lithium, sodium, or potassium; and M.sub.1 and M.sub.2 are hydrogen, lithium, sodium, potassium or alkyl groups).
According to the present invention, since a fine ferromagnetic metal powder having a high coercive force is used in combination with a binder having a polar group, a short-wave recording magnetic recording m
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Chubachi Ryoji
Nihei Nobuyuki
Robinson Ellis P.
Sony Corporation
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