Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
1998-06-24
2001-03-27
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S323000, C428S336000, C428S690000, C428S690000, C428S900000, C428S328000, C428S329000
Reexamination Certificate
active
06207252
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium, and more specifically to a magnetic recording medium suitable for high-density recording.
BACKGROUND OF THE INVENTION
Recently, it has been strongly required that a magnetic recording medium is in conformity with high-density recording. For the requirement, a magnetic recording medium using a magnetic recording thin film as a magnetic layer has been investigated and improvements in a magnetic recording particulate medium which is excellent for such reasons as productivity, durability, corrosion resistance, have been particularly extensively made.
The magnetic recording particulate medium is produced by coating a magnetic coating composition containing a ferromagnetic powder and a binder on a non-magnetic support such as a polyethylene terephthalate film, directly or via other layer, followed by drying to form a magnetic layer.
Hitherto a magnetic recording particulate medium, to improve, in particular, the self-demagnetization loss and the overwrite characteristics in high-frequency recording, it has been conventional to thin a magnetic layer. As the result thereof, a magnetic recording particulate medium having a thickness of the magnetic layer of 0.7 &mgr;m or thinner has been proposed. However, in this case, as the result of thinning the magnetic layer, there occurs a problem that the durability and surface property of the magnetic recording medium are deteriorated.
Accordingly, recently, a magnetic recording medium of a double-layer structure that a non-magnetic underlayer containing a non-magnetic powder and a binder is further formed between the magnetic layer and the non-magnetic support has been proposed. The magnetic recording medium of as such a double-layer structure has a feature that after forming the underlayer, the surface form of the underlayer is directly reflected on the surface of the magnetic recording medium. Also, the phenomenon becomes more remarkable in the case of thinning the magnetic layer. Usually, because the non-magnetic underlayer is also formed by coating as the magnetic layer, the coating condition and the drying condition become one factor for determining the surge form of the magnetic recording medium.
About the surface form of the magnetic recording medium, it is known that in particular, at high-density recording, the roughness of the surface of the magnetic recording medium influences the characteristics of the magnetic recording medium. That is, it has been practiced that by smoothing the surface of the magnetic recording medium, the magnetic recording medium is stably brought into contact with a magnetic head to improve the electromagnetic recording characteristics. In this case, as the indicator of the smoothness of the surface of the magnetic recording medium, a surface roughness Ra is used at present.
However, according to our investigations, it has been clarified that by simply smoothing a surface of the magnetic recording medium, that is, by reducing the Ra only, the torque characteristics of rotating components that move the magnetic recording medium in contact with a magnetic head are deteriorated. If the torque characteristics are bad, there occurs a problem that a head adsorption phenomenon occurs. On the other hand, when the surface roughness Ra is relatively increased to improve the torque characteristics, it is in a dilemma that the electromagnetic recording characteristics are reduced.
SUMMARY OF THE INVENTION
The present invention solves the above-described problems, and an object of the present invention is to provide a magnetic recording medium for high-density recording having excellent electromagnetic recording characteristics. Also, another object of the present invention is to provide a magnetic recording medium excellent in the torque characteristics. Still another object of the present invention is to provide a production method capable of producing the above-described magnetic recording medium.
That is, an aspect of the present invention is to provide a magnetic recording medium having a magnetic layer containing a binder and a ferromagnetic powder and having a thickness of 0.7 &mgr;m or thinner on a non-magnetic support, wherein the surface roughness Ra, according to standard nomenclature, of the surface of the magnetic recording medium is from 0.5 to 7 nm and the skewness R
SK
, according to standard nomenclature, of the surface of the magnetic recording medium is 0.1 or lower.
Furthermore, another aspect of the present invention is to provide a production method of a magnetic recording medium, which comprises coating a coating composition for a nonmagnetic underlayer containing a binder and a non-magnetic powder on a non-magnetic support to form a non-magnetic underlayer and, after drying the coated layer, coating a coating composition for a magnetic layer containing a binder, a ferromagnetic powder, and a solvent, whereby a magnetic recording medium having a surface roughness Ra of the source of the medium is from 0.5 to 7 nm and a skewness R
SK
of the surface of the medium is 0.1 or lower is obtained.
DETAILED DESCRIPTION OF THE INVENTION
One of the features of the present invention is that the magnetic recording medium has a relatively thin magnetic layer of the thickness of 0.7 &mgr;m or thinner, the surface roughness of the surface of the magnetic recording medium is form 0.5 to 7 nm, and the skewness R
SK
of the surface thereof is 0.1 or lower. The skewness as referred to herein means the measure of the symmetry of the profile about the center line. That is, in the case of considering the sectional form of the surface, when the portions which becomes “hills” are large, the R
SK
tends to become a large value and when the portions which become “valleys” are large, the R
SK
tends to become a small value. The R
SK
is usually measured based on the following formula.
R
SK
=
1
n
(
R
q
)
∑
(
Y
i
)
3
wherein n is a data number, R
q
is a root-mean-square roughness, according to standard nomenclature, and Y
i
is the i
th
data value.
In the present invention, by making the surface roughness Ra to a certain range and also making the skewness to 0.1 or lower, both the recording characteristics and the torque characteristics can be improved.
The skewness is 0.1 or lower and preferably 0 or lower. If the skewness is too large, the torque characteristics a or recording characteristics are deteriorated. Also, the lower limit value of the skewness is usually about −1 for practical use. Also the source roughness of the surface of the magnetic recording medium is from 0.5 to 7 nm, preferably from 0.5 to about 6 nm, and more preferably from 0.5 to about 5 nm. The surface roughness is, in general, preferably small from the point of the electromagnetic recording characteristics but is usually 0.5 nm or larger.
The thickness of the magnetic layer is 0.7 &mgr;m or thinner, preferably 0.5 &mgr;m or thinner, and more preferably 0.3 &mgr;m or thinner for improving self-demagnetization loss and the overwrite characteristics in high frequency recording. If the thickness of the magnetic layer is thick, the magnetic recording medium becomes less suitable for high-density recording. Also. From the view point of the production, the thickness of the magnetic layer is usually 0.01 &mgr;m or thicker, a preferably 0.05 &mgr;m or thicker.
The magnetic layer contains a ferromagnetic powder and a binder. As the ferromagnetic powder, acicular metal oxide powders such as an iron oxide powder, a Co-modified iron oxide powder; acicular metal powders of a metal such as Fe, Co, Ni, or an alloy of these metals as main constituents. Hexagonal system ferromagnetic powders of barium ferrite, strontium ferrite, can be used, but to obtain a higher output, it is preferred to use the ferromagnetic acicular metal powder or the hexagonal system ferromagnetic powder.
Regarding ferromagnetic acicular metal powder, it is preferred to use such metal powder having a specific surface areas determined by the conventional BET method (or by an analogous Speci
Fenwick & West LLP
Resan Stevan A.
Verbatim Corporation
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