Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-07-05
2004-09-28
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S328000, C428S332000, C428S336000, C428S690000, C428S690000
Reexamination Certificate
active
06797373
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a particulate magnetic recording medium. The present invention particularly relates to a magnetic recording medium suited to high-density recording comprising a thin magnetic layer and a nonmagnetic layer.
BACKGROUND OF THE INVENTION
In recent years, recording wavelengths have tended to shorten as recording densities have increased. The problems of self-magnetization loss during recording and thickness loss during reproduction where output drops due to thick magnetic layer have become significant. Thus, the magnetic layer has been reduced in thickness. However, when a magnetic layer of 2 &mgr;m or less is coated directly onto a support, the nonmagnetic support tends to affect the magnetic layer surface, and deterioration of electromagnetic characteristics and dropout tend to result.
Under such conditions, recording and reproduction systems in which the track width is further narrowed have been recently developed to further increase recording density. By contrast, reproduction systems employing high-sensitivity magnetoresistive heads (MR heads) have been proposed and implemented in hard disks and the like.
However, in existing magnetic recording media, a high level of noise is encountered even when the output is adequately high. When MR heads are employed, a good S/N is not necessarily achieved. Examples thereof are given below.
For example, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 3-214422 discloses a magnetic recording medium affording high reproduction output and low noise obtained by providing a dummy layer comprised principally of polymer resin and nonmagnetic particles on a nonmagnetic support, smoothing the surface of the dummy layer by calendering, and forming a magnetic layer over the dummy layer to achieve good magnetic layer surface properties. However, since the magnetic recording medium obtained by this method employs &ggr;-iron oxides (&ggr;-Fe
2
O
3
and the like) as magnetic powder and exhibits a high magnetic layer surface roughness of 0.01 to 0.015 &mgr;m, it is impossible to ensure the S/N required for high density recording. Further, since this magnetic recording medium has a structure tending to collect static electricity, it presents a problem in that electrostatic damage tends to occur during recording and reproduction with MR heads.
Further, Japanese Unexamined Patent Publication (KOKAI) No. 2000-11354 discloses a magnetic recording medium in which a lower nonmagnetic layer is applied and dried, after which an upper magnetic layer is applied in a wet-on-dry coating method. However, this magnetic recording medium has a high lower layer surface roughness of 3.0 nm, precluding the obtaining of an adequate S/N particularly when employing MR heads. There is a further problem in that MR heads tend to be electrostatically damaged during recording and reproduction with this magnetic recording medium.
Accordingly, it is an object of the present invention is to provide a magnetic recording medium ensuring an adequate SIN even in high density magnetic recording, and tending not to electrostatically damage MR heads during recording and reproduction employing MR heads.
SUMMARY OF THE INVENTION
The present inventors conducted extensive research into improving the surface conditions on the magnetic layer in particulate magnetic recording media, resulting in the successful development of a magnetic recording medium capable of ensuring a high S/N and in which there is no electrostatic damage during recording and reproduction with MR heads; the present invention was devised on that basis.
That is, the present invention relates to a magnetic recording medium comprising a nonmagnetic layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic metal powder or a ferromagnetic hexagonal ferrite powder and a binder in this order on at least one side of a nonmagnetic support, characterized in that
said nonmagnetic layer comprises 10 to 50 mass parts of carbon black with a mean particle diameter of 10 to 30 nm per 100 mass parts of said nonmagnetic powder,
said magnetic layer has a thickness equal to or less than 0.2 &mgr;m,
the standard deviation b of the average intensity a of elements due to said ferromagnetic powder as determined by electron-beam microanalysis is 0.03≦b/a≦0.4, and
said magnetic layer has a center surface average roughness Ra equal to or less than 5 nm and a ten-point average roughness Rz equal to or less than 50 nm.
The preferred modes of the present invention are as follows:
(1) The magnetic recording medium in which the nonmagnetic layer is formed by coating and drying on the nonmagnetic support the nonmagnetic layer coating material comprising a nonmagnetic powder and a binder, after which the magnetic layer is formed;
(2) The magnetic recording medium in which the nonmagnetic layer is formed and then calendered; and
(3) The magnetic recording medium for MR head reproduction employing MR heads during recording and reproduction.
The magnetic recording medium of the present invention is described in detail below.
The first characteristic of the magnetic recording medium of the present invention is that the nonmagnetic layer (also referred to as the “lower layer” hereinafter) comprises 10 to 50 mass parts of carbon black with a mean particle diameter of 10 to 30 nm per 100 mass parts of the nonmagnetic powder.
The mean particle diameter of the carbon black employed in the nonmagnetic layer of the present invention is 10 to 30 nm, preferably 15 to 25 nm, and more preferably 18 to 22 nm.
Further, the carbon black content in the lower layer is set to 10 to 50 mass parts, preferably 20 to 40 mass parts, and more preferably, 20 to 30 mass parts, per 100 mass parts of the nonmagnetic powder.
Restricting the mean particle diameter of the carbon black to within a range of 10 to 30 nm and the content of carbon black relative to the nonmagnetic powder to within a range of 10 to 50 mass parts reduces the surface electrical resistivity of the magnetic layer, results in a structure tending not to accumulate static electricity, and prevents electrostatic damage to MR heads.
Additionally, the incorporation of carbon black into the lower layer reduces light transmittance and achieves known effects such as the desired micro-Vickers hardness. Further, the incorporation of carbon black into the lower layer also achieves the effect of lubricant stockpiling.
The specific surface area of carbon black employed in the lower layer normally ranges from 100 to 500 m
2
/g, preferably from 150 to 400 m
2
/g. The DBP oil absorption capacity ranges from 20 to 400 ml/100 g, preferably from 30 to 400 ml/100 g. Carbon black preferably has a pH ranging from 2 to 10, a moisture content ranging from 0.1 to 10 percent, and a tap density ranging from 0.1 to 1 g/ml.
Specific examples of carbon black suitable for use in the lower layer are those described in WO98/35345. Carbon black can be used singly or in combination. The
Carbon Black Handbook
compiled by the Carbon Black Association may be consulted for types of carbon black suitable for use in the present invention.
The second characteristic of the magnetic recording medium of the present invention is a magnetic layer equal to or less than 0.2 &mgr;m in thickness and a standard deviation b of the average intensity a of elements due to the ferromagnetic powder as determined by electron-beam microanalysis of 0.03≦b/a≦0.4.
The thickness of the magnetic layer is equal to or less than 0.2 &mgr;m, preferably 0.01 to 0.15 &mgr;m, and more preferably 0.01 to 0.1 &mgr;m. Restricting the thickness of the magnetic layer to equal to or less than 0.2 &mgr;m reduces self-magnetization loss and increases output, yields good overwrite characteristics (referred to hereinafter as “PW50”), and is advantageous to high-density recording. Further, when the thickness of the magnetic layer is equal to or less than 0.2 &mgr;m, surface electrical resistivity is high and MR heads are not damaged by static electricity.
The standard deviation b of t
Noguchi Hitoshi
Saito Shinji
Yamazaki Nobuo
Fuji Photo Film Co. , Ltd.
Sughrue & Mion, PLLC
Thibodeau Paul
Uhlir Nikolas J.
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