Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
2000-09-20
2003-10-21
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S336000, C428S480000, C428S690000
Reexamination Certificate
active
06635338
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording tape that is excellent in durability, particularly in a cyclic environment, and a method for manufacturing the same, and more particularly it relates to a magnetic recording tape that has suppressed ununiform elongation of the tape and thus can record and reproduce data with high reliability even after repeated use under a cyclic environment, and a method for manufacturing the same.
BACKGROUND OF THE INVENTION
As means for increasing a capacity of a magnetic recording medium, such as a tape, there has been a method of decreasing a thickness of a substrate. When the thickness of a substrate is decreased, however, there are cases where such a phenomenon occurs in that the medium is ununiformly elongated in the longitudinal direction due to a driving tension on recording and reproducing to cause turbulent winding or decrease in running stability, whereby accurate recording and reproduction of data cannot be achieved. The phenomenon becomes conspicuous upon storage and use under high temperatures, under which thermal contraction is liable to occur. The phenomenon brings about a serious problem in a field of a recording medium for backing up data, in which the reliability of data recording and reproduction is extremely important. There is thus demanded to develop a highly reliable magnetic recording medium having high durability that does not cause ununiform elongation of the medium even under severe environments.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a magnetic recording tape having improved durability capable of recording and reproducing data with high reliability.
Another object of the invention is to provide a method for manufacturing the magnetic recording tape.
As a result of various investigations for shape stability of magnetic recording media conducted by the inventors, a magnetic recording medium that attains the object can be obtained by such a manner that a creep deformation ratio of the medium measured under a specific condition is set less than a specific value.
The invention has been made based on the above finding and relates to a magnetic recording tape which comprises a flexible substrate having a thickness equal to or less than 7 &mgr;m and at least one magnetic layer provided on one surface of the substrate, the magnetic recording tape having a longitudinal creep deformation ratio in a range of less than 0.04% when the magnetic recording tape is subjected to a longitudinal tensile stress of 19.1 MPa for 25 minutes at 50° C.
The invention also relates to a method for manufacturing a magnetic recording tape comprising a flexible substrate having a thickness equal to or less than 7 &mgr;m and at least one magnetic layer provided on one surface of the substrate, the method comprising subjecting said magnetic recording tape to a heating treatment at such a temperature (T) below the equilibrium glass transition temperature (ETg) of the substrate that (ETg-T) is from 1 to 40° C. for 0.5 to 240 hours, thereby obtaining the magnetic recording tape having a longitudinal creep deformation ratio in a range of less than 0.04% when the magnetic recording tape is subjected to a longitudinal tensile stress of 19.1 MPa for 25 minutes at 50° C.
DETAILED DESCRIPTION OF THE INVENTION
The magnetic recording tape of the invention will be described in detail with reference to preferred embodiments thereof below.
In one embodiment of the magnetic recording tape of the invention, a magnetic or non-magnetic lower layer is provided on one surface of a substrate, and a magnetic layer as an uppermost layer (hereinafter simply referred to as an upper layer) is provided on the lower layer. A back layer is provided on another surface of the substrate.
In the magnetic recording tape of the invention, a substrate having a thickness equal to or less than 7 &mgr;m, preferably a thickness of from 2 to 7 &mgr;m, and more preferably from 2 to 4.5 &mgr;m, is used. In other words, the magnetic recording tape of the invention is one of an extremely thin type. When a thin magnetic recording tape is subjected to recording and reproduction on a drive, the tape causes ununiform elongation due to a driving tension to deteriorate the running stability. However, the magnetic recording tape of the invention, owing to the creep deformation ratio of less than the specific value measured under the specified condition, causes no ununiform elongation even when the medium is stored or used under severe environments, so as to improve the durability, particularly cyclic durability, of the medium. As a result, recording and reproduction of data can be achieved with high reliability.
When the creep deformation ratio is less than 0.04%, and preferably less than 0.038%, ununiform elongation of the medium is effectively prevented. It is more preferred as the creep deformation ratio is smaller and is ideally zero, but the lower limit thereof that can be attained by the current level of technology is about 0.012%.
While the standard relating to the creep deformation ratio of a magnetic recording tape is defined in Japanese Industrial Standard(JIS) as remaining elongation (JIS X6141), the remaining elongation defined in JIS is the one measured under considerably mild conditions but not under the severe conditions as in the invention. Therefore, even a tape satisfying the JIS standard cannot always attain the object of the invention. The invention is characterized by the fact that when the creep deformation ratio measured under the specified conditions becomes less than the specified value, no ununiform elongation occurs when the tape is stored or used under severe conditions, independent as to whether or not the tape satisfies the JIS standard. When the creep deformation ratio of the invention is measured according to the JIS standard, no result of significant difference can be obtained. Therefore, the creep deformation ratio of the invention is measured by the method described in detail in Examples described later.
While a method for obtaining the creep deformation ratio in the specified range is not particularly limited, it has been found effective to subject the tape to a heating treatment at a prescribed temperature for a prescribed period of time.
The temperature of the heating treatment is preferably lower than the equilibrium glass transition temperature (hereinafter referred to as ETg) of a material constituting the substrate of the magnetic recording tape by from 1 to 40° C., more preferably from 1 to 35° C., further preferably from 1 to 25° C., and particularly preferably from 1 to 20° C. When the heating treatment temperature is lower than the ETg of the material constituting the substrate by more than 40° C., the heating treatment time becomes too long. When the heating treatment temperature is higher than the ETg, there are cases where the creep deformation ratio cannot be less than the specified value. The ETg is herein defined as follows. A peak temperature of loss modulus of the substrate measured in a frequency range of from 0.0016 to 16 Hz is plotted against reciprocals of the frequency and the temperature to form an Arrhenius plot that is obtained in such a manner that the support is subjected to a suitable heating treatment to make the Arrhenius plot being the Williams-Landel-Ferry(WLF) type (M. L. Williams, et al., J. Am. Chem. Soc., vol. 77, p. 3710 (1955)), and the Arrhenius plot is extrapolated to the lower frequency side to obtain a limiting temperature, which is designated as an ETg.
The specific temperature of the heating treatment varies depending on the material constituting the substrate, and for example, in the case of polyethylene terephthalate (hereinafter referred to as PET) having an ETg of 95° C., the heating treatment temperature is preferably from 55 to 94° C., more preferably from 70 to 94° C., and most preferably from 75 to 94° C.
The period of time for the heating treatment is, while depending on the heating temperature, preferably from a several minutes to 240 hours, mor
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