Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
2002-08-30
2003-12-09
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S336000, C428S690000, C428S690000, C428S690000
Reexamination Certificate
active
06660364
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic recording medium excellent in recording density characteristics and less in wear of head and output deterioration.
Magnetic tapes have various uses such as audiotapes, videotapes and computer tapes. Especially, in the field of tapes for data backup, with increase of capacity of hard disks, the data of which are objects for backup, those of several ten GB in storage capacity per one roll have been put to the market, and for accommodation to further increase of capacity of hard disks, it is essential to enhance the recording density of backup tapes.
In order to accommodate the enhancement of recording density, it has become necessary to produce magnetic tapes by using ferromagnetic powders improved in magnetic characteristics, further improving the dispersibility of ferromagnetic powders or carrying out a processing for reducing the spacing loss between medium and head.
For the improvement in magnetic characteristics of ferromagnetic powders, since the greater residual magnetization of magnetic layer is preferred for increase of output, ferromagnetic iron alloy powders are mainly used as magnetic powders in place of conventional oxide magnetic powders or Co-containing iron oxide magnetic powders, and ferromagnetic iron alloy powders having a coercive force of not less than 135 kA/m (1700 Oe) have been proposed (e.g., JP-A-5-234064, JP-A-6-25702, and JP-A-6-139553).
Furthermore, for improving the dispersibility of ferromagnetic powders, there have been proposed a method of using binders having polar functional groups such as sulfonic acid group, phosphoric acid group and alkali metal salts thereof, a method of using dispersing agents together with the binders, a method of continuously carrying out kneading and dispersing steps of magnetic coating compositions, and a method of adding lubricants after dispersion (e.g., JP-A-62-23226, JP-A-2-101624, JP-A-3-216812, JP-A-3-17827, JP-A-4-47586 and JP-A-8-235566).
For reducing the spacing loss between tape-head, there have been proposed a method of carrying out a smoothening treatment of magnetic layer under the conditions of high temperatures and high pressures at calendering step, and a method of providing a non-magnetic undercoat layer below the magnetic layer to inhibit the effect of the surface property of non-magnetic support on the surface of the magnetic layer in addition to the above-mentioned method of increasing dispersibility of magnetic powders (e.g., JP-B-1-1297, JP-B-7-60504 and JP-A-4-19815).
Furthermore, in addition to the enhancement of performance of magnetic layer as mentioned above, it is recently proposed for making a structure of a magnetic recording medium suitable for short wavelength recording to provide an undercoat layer between a magnetic layer and a non-magnetic support and further to use a thin layer of not more than 1 &mgr;m as the magnetic layer (JP-A-62-154225). In these magnetic recording media, the undercoat layer is provided as a lubricant retaining layer to reduce self-demagnetization loss and reproduction loss caused by using a thin magnetic layer and, besides, to improve deterioration of running properties and endurance caused by allowing the media to run many times. In the case of magnetic recording media having a single thin magnetic layer, the magnetic layer is not compressed at the calendering step, and, hence, improvement in filling properties and smoothness of the magnetic layer cannot be attained. Therefore, from these viewpoints, the undercoat layer must be provided when the magnetic layer is an ultra-thin layer of not more than 1 &mgr;m.
On the other hand, with advancement of digital information recording system, reduction of information recording area has been attempted. The reduction of recording area has been attempted by shortening the recording wavelength and narrowing the recording track width. There are devices of digital data storage system of about 0.5 &mgr;m in the shortest recording wavelength and about 10 &mgr;m in track width. Considering recording and reproduction of information, thickness loss increases and reproduction output does not increase when the ratio of thickness of the magnetic layer to the recording wavelength exceeds a certain value. This thickness loss is caused by self-demagnetization which generates a magnetic field erasing the recording signal inside the magnetic layer, and even when the thickness of the magnetic layer is increased over ½ of the shortest recording wavelength &lgr;, the output usually does not increase. Therefore, when saturation recording is carried out, thickness of the magnetic layer effective for magnetic recording is considered to be ⅓ of the shortest recording wavelength, and in the recording system as mentioned above, thickness of the magnetic layer must be very thin, namely, not more than 0.20 &mgr;m.
In the recording system of large capacity, due to the necessity to increase transfer speed, the relative speed of medium and head tends to be increased. Therefore, the temperature of glass part of magnetic head is apt to rise as compared with conventional recording systems owing to generation of heat caused by sliding of medium-head. For this reason, the phenomenon is often observed that wearing powders of glass or coating film produced by the sliding adhere to the glass part of the head to generate opaque scum, resulting in decrease of reproduction output or scraping powders of the magnetic coating film adhere to the sliding face of the medium above the head to generate incrustation scum, resulting in decrease of output due to increase of the space between medium-head. The stain called head stain can be wiped off with alcohol, but opaque scum or incrustation scum cannot be wiped off with alcohol and can be removed only by abrasion. Therefore, it is supposed that the head stain is caused by exfoliation of the magnetic coating film while the opaque scum or incrustation scum is caused by sticking of matters exfoliating from head materials and magnetic coating film which are burnt with the heat generated by sliding of head-medium.
The present invention has been accomplished to solve the above problems, and the object is to inhibit deterioration of output caused by opaque scum or incrustation scum in magnetic recording media having a magnetic layer of not more than 0.20 &mgr;m in thickness.
SUMMARY OF THE INVENTION
As a result of an intensive research conducted by the inventors, they have provided a magnetic recording medium comprising a non-magnetic support on one side of which at least one non-magnetic undercoat layer and a magnetic layer containing ferromagnetic metal powders are formed in succession and on another side of which is formed a back coat layer, said magnetic layer having a thickness of not more than 0.20 &mgr;m and having a standard deviation of not more than 1.0×10
−5
m in the distribution of height of inequalities on the surface of the magnetic coating film which is obtained by subjecting the surface of the magnetic layer to measurement by a light interference surface roughness meter. This magnetic recording medium has substantially no opaque scum and incrustation scum.
DETAILED DESCRIPTION OF THE INVENTION
In order to inhibit the head stain, the inventors have paid attention to surface properties of the magnetic layer which contacts the magnetic head under sliding. The surface of the magnetic layer is macroscopically highly smooth, but microscopically has unevenness caused by protrusions and dents, and the inventors have noticed that the magnetic recording medium and the magnetic head contact with each other at the protrusions and they have accomplished the present invention. That is, they have found that in order to diminish the head stain in magnetic recording media having a thin magnetic layer, opaque scum or incrustation scum can be diminished by lowering the height of the microscopic protrusions on the surface of the media or reducing the number of the protrusion to inhibit heat generation. Thus, the present invention h
Kaneno Kimihiko
Yoshida Ken-ichiro
Hitachi Maxell Ltd.
Resan Stevan A.
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