Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
2000-04-24
2002-05-07
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S216000, C428S323000, C428S328000, C428S690000, C428S690000, C428S690000, C428S900000
Reexamination Certificate
active
06383605
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnetic recording medium exhibiting a high output and a good C/N ratio under high-density recording.
2. Description of Related Art
In recent years, a recording wavelength tends to be shorter along with high densification, and if the magnetic layer is thick, the output tends to be lowered, thereby raising problems in a self-demagnetization loss during recording and a thickness loss during reproducing. Therefore, a magnetic layer is tried to be thinner, but the is influence of a nonmagnetic support may easily appear on the surface of the magnetic layer when the magnetic layer of 2 &mgr;m. or less is directly applied to the support, and a deteriorating tendency in electromagnetic characteristics or dropouts is seen.
As a means to solve such problems, there is a method for forming, by using a simultaneous multiplayer coating technique, on a support, a nonmagnetic layer and a magnetic layer in this order, thereby coating high concentration magnetic coating liquid with a thin thickness as disclosed in Japanese Unexamined Patent Publication (KOKAI) Showa No. 63-191,315 and U.S. Pat. No. 4,863,793. With these inventions, production yields were remarkably improved, and good electromagnetic characteristics were gained. However, a recording and reproducing system with a further narrowed track width has been developed to further improve a recording density.
In response to this, the inventors propose, for the purpose of improving a medium output, that a lower layer thickness be less than 0.5 &mgr;m to enhance Hc of the respective longitudinal, width and normal direction of the magnetic layer (See Japanese Uhexamined Patent (KOKAI) Heisei No. 8-339,526; hereinafter referred to as Patent Publication (1)). Along with digitalization of recording signals, however, it has been strongly requested to reduce noises caused by not only an output but also surface roughness or distribution condition of magnetic powder.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a magnetic recording medium exhibiting a high output and a high C/N ratio (low noises) during high-density magnetic recording.
The inventors have diligently researched structures of magnetic layers and nonmagnetic layers, and the surface roughness, the Young's modulus or the like of the support to accomplish the above object. Consequently, they have found out that the above object could be accomplished by a magnetic recording medium comprising a flexible nonmagnetic support, a nonmagnetic layer containing nonmagnetic powder and a binder formed on the flexible nonmagnetic support and a magnetic layer containing ferromagnetic powder and a binder formed on the nonmagnetic layer, wherein the magnetic layer has an average thickness in the range of from 0.01 &mgr;m. to 0.3 &mgr;m., the nonmagnetic layer has an average thickness equal to or less than 0.5 &mgr;m., and, according to a surface roughness spectrum measured with an atomic force microscope, the side of the flexible nonmagnetic support to which the nonmagnetic layer and the magnetic layer are formed has a power spectrum density of roughness (abbreviated to “PSD” hereinafter) equal to or less than 0.5 nm
2
in the wavelength of from 1 to 5 &mgr;m. and a PSD of 0.02 to 0.5 nm
2
in the wavelength of from 0.5 &mgr;m. or higher to less than 1 .m, and the flexible nonmagnetic support has a ratio, MD/TD, of Young's modulus in the direction of MD to the direction of TD in the range of from 1/5 to 1/1.
As for the magnetic recording medium of the invention, it is preferable that the nonmagnetic powder contained in the nonmagnetic layer is an acicular nonmagnetic powder having a mean length of the major axis equal to or less than 0.2 &mgr;m.
It is desirable in the magnetic recording medium of the invention that the nonmagnetic layer contains carbon black of an average primary particle size equal to or less than 30 nm and an oil absorption amount equal to or less than 200 ml/100 g, and that the carbon black is contained in a ratio with respect to the nonmagnetic powder in the range of from 5:95 to 30:70.
It is also preferable in the magnetic recording medium of the invention that a binder contained in the nonmagnetic layer has a molecular weight ranging from 20,000 to 50,000 and one or more polar groups selected from groups constituted of —SO
3
M, —COOM, —OSO
3
M, —PO(OM)
2
and —OPO(OM)
2
, and the content of the binder in the nonmagnetic layer is 12 to 30 parts with respect to the 100-part-total amount of the nonmagnetic powder and carbon black, and a content of the binder in the nonmagnetic layer is larger than that of the binder in the magnetic layer.
DESCRIPTION OF PREFERRED EMBODIMENTS
In the magnetic recording medium according to the invention, one of features is that the average thickness of the magnetic layer ranges from 0.01 m. to 0.3 &mgr;m, and the average thickness of the nonmagnetic layer is equal to or less than 0.5 &mgr;m.
Required magnetism for recording is secured by making an average value of the thickness of the magnetic layer equal to or more than 0.01 &mgr;m., and resolution property and overwrite can be improved by making the average thickness equal to or less than 0.3 &mgr;m. In particular, the average value of the thickness of the magnetic layer is preferably in the range of from 0.01 to 0.2 &mgr;m, more preferably from 0.01 to 0.1 &mgr;m from the viewpoint that the above characteristics can be attained in a good balance. In addition, not only the effect improving Hc but also the effect improving magnetic-material filling-degree in the magnetic layer can be obtained by making the average thickness of the nonmagnetic layer equal to or less than 0.5 &mgr;m. The average thickness of the nonmagnetic layer is preferably in the range of from 0.2 to 0.5 &mgr;m. from a viewpoint to reflect the smooth surface of the support on the magnetic surface.
It has been well-known to make the average thickness of the nonmagnetic layer 0.5 &mgr;m. or less. However, it is found in the invention that the magnetic-material filling degree in the magnetic layer is enhanced in addition to the Hc improving effect as indicated in the invention disclosed in the Patent Publication (1). This effect can be obtained by making the average thickness of the magnetic layer 0.01 to 0.3 &mgr;m. According to the invention of the Patent Publication (1) where it is disclosed that a thickness of the magnetic layer is 1 &mgr;m. or less and the lower layer thickness is 0.5 &mgr;m., in this invention, however, the magnetic layer is made to be further thinner, in response to the request for high densification. The Hc improving effect and the magnetic-material filling-degree improving effect in the magnetic layer can be obtained by making the nonmagnetic layer thin. Those effects are provided due to decreasing in the magnetic upper layer filling degree by a binder provided from the nonmagnetic layer during application and drying.
As another feature of the magnetic recording medium of the invention, the side of the flexible nonmagnetic support to which the nonmagnetic layer and the magnetic layer formed has a PSD equal to or less than 0.5 nm
2
in the wavelength of from 1 to 5 &mgr;m and a PSD of 0.02 to 0.5 nm
2
in the wavelength of from 0.5 &mgr;m. or higher to less than 1 &mgr;m. according to a surface roughness spectrum measured with an atomic force microscope, and the flexible nonmagnetic support has a ratio, MD/TD, of Young's modulus in the direction of MD to the direction of TD in the range of from 1/5 to 1/1.
When the surface roughness of the support is measured with an atomic force microscope, the wavelength and strength of the roughness can be obtained. A roughness spectrum indicates changes in strength in the wavelength region of a specified roughness. In the magnetic recording medium of the invention, the surface of the side of the flexible nonmagnetic support to which the nonmagnetic layer and the magnetic layer formed has a PSD equal to or less than 0.5 nm
2
in the wavelength of fr
Fuji Photo Film Co. , Ltd.
Resan Stevan A.
Sughrue & Mion, PLLC
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