Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
2003-06-17
2004-11-23
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, C428S690000
Reexamination Certificate
active
06821603
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a high-density magnetic recording tape excellent in electromagnetic characteristics and running durability.
BACKGROUND OF THE INVENTION
Magnetic recording media are widely used for recording all kinds of information including sound data, image data, and literal data. The recent innovation of technology has increased data storage capacity and data transfer rate and thus boosted the need for high-density recording. With this trend, magnetic recording media having high electromagnetic characteristics have been demanded. High reliability in repeated use of data and for data storage is also demanded. Accordingly, magnetic recording media are required to have satisfactory running durability as well as excellent electromagnetic characteristics. It is known that running durability of a magnetic tape media can be improved by properly designing a back coating layer. Included among such attempts are roughening the surface of a back coating layer by providing projections or adding coarse carbon particles of 0.1 &mgr;m or greater. However, when a magnetic recording tape having a back coating layer with such surface roughness is stored or handled for processing in form of a tape pack wound on a hub, the roughness can imprint itself in the magnetic layer under tape pack stresses, which can result in deteriorated electromagnetic characteristics. To overcome the roughness imprint problem, it has been attempted to smoothen the back coating layer surface, but back side smoothening often causes poor tape pack wind quality. That is, entrapped air hardly escapes on tape winding, which can result in irregularities of a tape pack, such as popped strands of tape protruding from the edge of a pack.
The recent computer storage tape media are required to reduce in thickness for increasing recording capacity. Thickness reduction is achieved by, for example, reducing the thickness of a flexible support or the thickness of a non-magnetic layer provided in a particulate magnetic recording tape between a flexible support and a magnetic layer. When a pack of tape with an extremely thin flexible support has a poor wind quality with popped strands, the strands that stick out of the pack edge can be bent during storage, or the tape pack undergoes winding defects, such as cinching and spoking, which will lead to increased errors.
As an approach to improve electromagnetic characteristic by smoothening a back coating layer to prevent roughness imprints, JP-A-11-259851 discloses a magnetic tape medium having a back coating layer mainly comprising particulate titanium oxide and carbon black. Whereas the technique brings about improved back side smoothness and reduction of roughness imprints, the tape has a poor wind quality, and an increased error rate results after storage.
Hence, the related techniques have not provided a magnetic recording tape satisfactory in both electromagnetic characteristics and wind quality. Considering the increasing demand for further reduction in tape thickness, the state-of-the-art magnetic recording tapes are still unsatisfactory in electromagnetic characteristics and wind quality.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic recording medium for high density recording which is substantially free from the roughness imprint problem and has excellent electromagnetic characteristics and a good wind quality.
As a result of extensive investigations, the inventors have found that a magnetic recording tape having a controlled surface roughness on its back coating layer side and showing moderate curvature satisfies both electromagnetic characteristics and tape wind quality.
The present invention provides a magnetic recording tape comprising a flexible support, a magnetic layer provided on one side of the support, and a back coating layer on the other side of the support, wherein the back coating layer has on its surface 10 to 200 projections having a height of 50 to 100 nm per 90 &mgr;m side square as measured with an atomic force microscope (AFM) and 10 or fewer projections having a height greater than 100 nm and not greater than 500 nm per 90 &mgr;m side square as measured with an AMF and the tape has a curvature of 3 to 30% of the tape width per meter.
DETAILED DESCRIPTION OF THE INVENTION
The surface roughness of the back coating layer can be controlled by properly selecting the physical properties, shape, size, and amount of particles, the kind of the binder, and other factors as will be discussed in detail.
The terminology “curvature” of a magnetic recording tape has the following meaning. Reference is made to
FIG. 1
, a plan view of a tape placed on a flat surface. A length of 1 m is cut out of a magnetic recording tape product in a cassette, etc. The cut piece
1
is placed on a flat surface with its magnetic layer side or back coating layer side down and with no tension applied. When the tape has a curvature, the cut piece
1
depicts an arc when seen from above. In the present invention, the curvature is such that the maximum distance L between the line
4
connecting the edges
2
and
3
of the specimen 1 and the inner edge
5
of the specimen 1 is 3 to 30%, preferably 4 to 20%, of the tape width W. In the measurement of a curvature, the specimen is taken from the last turn of the tape pack of a reel immediately after winding (before heating and before storage).
The curvature can be controlled through various methods. Among them are a method in which a coating thickness is varied in the tape width direction and a method in which a tape is wound on to a tapered hub into a tape pack, and the pack is subjected to heat treatment. The latter method is preferred. According to the latter method, the degree of curvature can be controlled by adjusting the tapering angle, heat treating conditions, and the like. For example, a curvature is imparted to a tape by heat treating a tape pack on a tapered hub at 40 to 80° C., preferably 50 to 70° C., for 6 to 72 hours, preferably 12 to 48 hours. The heat treatment for imparting curvature is preferably carried out after slitting into widths, and the heat-treated tape is wound onto a hub of a cassette, etc.
With the tape curvature falling within the above-specified range, the tension imposed to the tape varies between the two edges. As a result, it is considered that entrapped air is allowed to escape quickly during winding so as to give even packing. If the curvature is out of the range, the wind quality is not improved, resulting in adverse influences on the electromagnetic characteristics. Excessive curvature results in off-track, which leads to further deterioration of electromagnetic characteristics.
The magnetic recording tape of the present invention embraces a wide range of products comprising a flexible support, a magnetic layer on one side of the support, and a back coating layer on the other side of the support. The magnetic recording tape of the invention may comprise additional layers, such as a non-magnetic layer containing non-magnetic powder, a soft magnetic layer containing soft magnetic powder, a second magnetic layer, a cushioning layer, an overcoating layer, an adhesive layer, and a protective layer. These optional layers are provided at an appropriate position in the tape thickness direction where their function is effectively displayed.
A preferred layer structure of the magnetic recording tape of the invention has a non-magnetic layer containing a non-magnetic inorganic powder and a binder between the flexible support and the magnetic layer. In this configuration, a preferred thickness of the magnetic layer is usually 0.01 to 1 &mgr;m, preferably 0.03 to 0.5 &mgr;m, still preferably 0.03 to 0.2 &mgr;m, and a preferred thickness of the non-magnetic layer is usually 0.5 to 3 &mgr;m, preferably 0.8 to 3 &mgr;m. It is preferred for the non-magnetic layer be thicker than the magnetic layer.
A layer structure having two magnetic layers is also preferred. The upper magnetic layer usually has a thickness of 0.2 to 2 &mgr;m, preferably 0.2 to 1.5 &m
Ejiri Kiyomi
Inoue Noriko
Fuji Photo Film Co. , Ltd.
Resan Stevan A.
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