Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
1999-09-24
2001-08-14
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S336000, C428S690000, C428S690000, C428S900000, C428S065800, C360S133000, C360S135000
Reexamination Certificate
active
06274227
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a magnetic recording medium, and more specifically, to a magnetic recording medium comprising a nonmagnetic substrate, a liquid lubricant-containing coating layer formed on the substrate, and a magnetic layer formed directly on the coating layer.
DESCRIPTION OF THE RELATED ART
There is a seemingly never ending demand for increasing the storage capacity, and thus usually, the storage density, of removable storage media. One approach for accomplishing this is to reduce the thickness of the magnetic recording layer. However, when the thickness of the magnetic layer is reduced to less than 1 &mgr;m, some practical problems such as decrease of durability of the magnetic layer arise. This is because, since the thickness of the magnetic layer which directly contacts a magnetic head is decreased, the magnetic layer cannot have a volume of pores sufficient to create a lubricant reservoir capable of improving the durability of the magnetic layer by the improvement of tribologic properties such as the decrease of a coefficient of friction in the case of contact of the magnetic layer to the magnetic head and increase of wear resistance.
One type of magnetic recording media for solving such problems comprises a nonmagnetic support or substrate, a coating layer formed on the substrate, and a magnetic layer formed directly on the coating layer. The coating layer can contain an amount of a lubricant necessary for the magnetic layer. In this medium, the coating layer may be a magnetic layer which comprises a magnetic material and a binder, and which is formed independently from the above magnetic layer, or a nonmagnetic layer comprising a nonmagnetic powder and a binder.
Examples of such magnetic recording media follow.
(1) Japanese Patent Kokai Publication No. 175922/1986 is one of the earliest patent applications which tried to solve the above problems by differentiating a porosity between the coating layer and the magnetic layer formed directly on the coating layer.
This Kokai publication discloses a magnetic recording medium comprising a nonmagnetic support, a nonmagnetic undercoat layer as a coating layer formed on the nonmagnetic support, and a magnetic layer formed directly on the undercoat layer, characterized in that a porosity (M) of the magnetic layer is smaller than a porosity (U) of the undercoat layer.
In the working example of this Kokai publication, there is disclosed a magnetic recording medium in which the porosity of the magnetic layer is 20 vol. % and that of the undercoat layer is 45 vol. %.
Magnetic recording media which intend to achieve the same object as Japanese Kokai Publication No. 175922/1986 by making the porosity of the coating layer larger than that of the magnetic layer on the coating layer are disclosed also in Japanese Kokai Publication Nos. 260124/1990 and 182183/1993.
A magnetic recording medium disclosed in Japanese Kokai Publication No. 260124/1990 has a first magnetic layer as a coating layer and a second magnetic layer formed directly on the first magnetic layer and is characterized by a porosity of the first magnetic layer of from 0.25 to 0.55 and a porosity of the second magnetic layer of from 0.10 to less than 0.25.
A magnetic recording medium disclosed in Japanese Patent Kokai Publication No. 182183/1993 is characterized by a porosity of the first magnetic layer of from 30 to 50% and a porosity for the second magnetic layer of from 10 to 30% by weight.
(2) In contrast to the above recording media, Japanese Kokai Publication No. 293718/1988 discloses a magnetic recording medium which achieves the above object by making the porosity of the coating layer smaller than that of the magnetic layer formed on the coating layer. The magnetic recording medium comprises a first magnetic layer as a coating layer and a second magnetic layer formed directly on the first magnetic layer, and is characterized by a ratio of porosity (M
2
) of the second magnetic layer to a porosity (M
1
) of the first magnetic layer (M
2
/M
1
) of from 1.1 to 2.
(3) Japanese Kokai Publication No. 168439/1994 discloses a magnetic recording medium which achieves the same object as in the media of (1) by differentiating contents of a fatty acid and a fatty acid ester as a lubricant between the coating layer and the magnetic layer on the coating layer.
The magnetic recording medium disclosed in this Kokai has a nonmagnetic layer as a coating layer and a magnetic layer formed directly on the nonmagnetic layer and is characterized by a porosity of the nonmagnetic layer of at least 15%, a porosity of the magnetic layer of 15% or less, and a content of the aliphatic acid in the magnetic layer larger than that in the nonmagnetic layer, while a content of the fatty acid ester in the non-magnetic layer is larger than that in the magnetic layer.
(4) Japanese Kokai Publication No. 4854/1994 discloses a magnetic recording medium which achieves the same object as those in (1) by separating a coating layer into two sublayers. That is, the disclosed magnetic recording medium has a nonmagnetic layer as a coating layer and a magnetic layer formed on the nonmagnetic layer and is characterized in that the nonmagnetic layer is divided in two sublayers, and the porosity of a lower nonmagnetic sublayer on the nonmagnetic support side is larger than that of the upper nonmagnetic sublayer.
SUMMARY OF THE INVENTION
However, the durability of the magnetic layer which directly contacts the magnetic head may not be sufficiently improved only by controlling the porosities of the magnetic layer and the coating layer in the specific ranges as disclosed in the prior art. In particular, with a high density magnetic recording medium, a magnetic layer of which should have a decreased thickness, for example, 0.5 &mgr;m or less, it is very difficult to improve the durability of the magnetic layer even by the above prior art. An object of the present invention is to provide a magnetic recording medium which improves both the durability and carrier-to-noise ratio characteristics of the magnetic layer.
To solve the above problem, the present invention provides a magnetic recording medium comprising a nonmagnetic support, a liquid lubricant-containing coating layer formed on the nonmagnetic support, and a magnetic layer formed directly on the coating layer. The magnetic layer has a porosity (M) and the coating layer has a porosity (U). The ratio of the porosity (M) of the magnetic layer to the porosity (U) of said coating layer is from 0.75 to 1.25, i.e., 0.75 ≦M/U≦1.25. The volume percentage of the liquid lubricant contained in the pores of the coating layer based on the whole pore volume of the coating layer is from 35 to 85%. This improves the durability of the magnetic layer.
DETAILED DESCRIPTION
When the ratio of the porosities (M/U) is less than 0.75, since a sufficient amount of the liquid lubricant is not supplied from the coating layer to the magnetic layer, the durability of the magnetic layer is deteriorated. When the ratio of the porosities (M/U) exceeds 1.25, since an excessive amount of the liquid lubricant is present on the surface of the magnetic layer, a coefficient of friction between the magnetic head and the magnetic layer (namely, a torque when the magnetic head runs over the magnetic layer) is increased, so that the durability of the magnetic layer is deteriorated. In addition the increase of the coefficient of friction leads to failure of the magnetic head, namely unstable recording and reproducing. From this view point, the ratio of porosities M is preferably from 0.9 to 1.2.
When the percentage of the volume of the liquid lubricant contained in the pores of the coating layer based on the whole pore volume in the coating layer is less than 35%, since a sufficient amount of the liquid lubricant is not supplied from the coating layer to the magnetic layer, the durability of the magnetic layer is deteriorated. When this percentage exceeds 85%, since an excessive amount of the liquid lubricant is present on the surface of the magnetic
Oiri Shigeto
Tsukamoto Yoji
Yasui Hideaki
Buharin Amelia A.
Imation Corp.
Levinson Eric D.
Resan Stevan A.
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