Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2000-06-19

2002-11-12

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Continuous and nonuniform or irregular surface on layer or...

C428S213000, C428S323000, C428S336000, C428S402000, C428S690000, C428S690000, C428S690000, C428S690000

Reexamination Certificate

active

06479131

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. Therefore, a magnetic layer is tried to be thinner, but the influence of a nonmagnetic support may easily appear on the surface of the magnetic layer when the magnetic layer having a thickness equal to or less than 2 &mgr;m is directly applied to the support, and a deteriorating tendency in electromagnetic characteristics and dropouts are seen.
To solve such problems, there has been proposed a method for forming a thin thickness magnetic layer and a nonmagnetic layer under the magnetic layer, on a nonmagnetic support, by using a simultaneous multilayer coating technique as well as a concentrated coating liquid for the magnetic layer as disclosed in Japanese Unexamined Patent Publication (KOKAI) Showa No.63-191,315 and Japanese Unexamined Patent Publication (KOKAI) Showa No.63-187,418, thereby obtaining a magnetic recording medium having an improved electromagnetic characteristics. In order to smooth a surface of a magnetic layer to further improve a recording density, it is known that an acicular nonmagnetic powder is used in a nonmagnetic lower layer. However, in order to satisfy the demands that a track width is narrowed and noises are decreased, the smoothing of the surface and orientation improvement of magnetic particles become necessary.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a magnetic recording medium exhibiting a high C/N ratio in high-density magnetic recording.
The inventors have diligently researched surface property improvements of a magnetic layer and orientation improvements of magnetic materials in order to provide a magnetic recording medium exhibiting high C/N ratio in high-density magnetic recording.
That is, the present invention relates to a magnetic recording medium comprising, on a flexible nonmagnetic support, at least a nonmagnetic layer containing a nonmagnetic powder and a binder (also referred to a lower layer or a nonmagnetic lower layer hereinafter) and a magnetic layer containing a ferromagnetic powder and a binder (also referred to a magnetic upper layer hereinafter) on said nonmagnetic layer, characterized in that said magnetic layer has an average thickness in the range of from 0.01 to 0.3 &mgr;m, said nonmagnetic powder has an acicular shape, and the ratio of a mean length L of the major axis of said nonmagnetic powder to an average thickness D of said nonmagnetic layer satisfies the relationship of {fraction (1/10)}≦L/D≦2.
DESCRIPTION OF PREFERRED EMBODIMENTS
In the magnetic recording medium of the present invention, the following embodiments are preferred;
(1) a magnetic recording medium wherein said nonmagnetic layer has a thickness of less than 0.5 &mgr;m;
(2) a magnetic recording medium wherein said nonmagnetic powder has a mean length of the major axis equal to or less than 0.2 &mgr;m and an acicular ratio in the range of from 2 to 20;
(3) a magnetic recording medium wherein said nonmagnetic layer contains granulated particles which has an average primary particle size equal to or less than 50 nm, and a content ratio of said granulated particle to the acicular nonmagnetic powder is in the range of from.5:95 to 40:60;
(4) a magnetic recording medium wherein said granulated particles are carbon black which has 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;
(5) a magnetic recording medium wherein one of the surfaces of the flexible nonmagnetic support, on which a nonmagnetic layer and a magnetic layer are formed, has a power spectrum density of roughness (abbreviated PSD hereinafter) equal to or less than 0.5 nm
2
in the wavelength of from 1 to 5 &mgr;m according to the surface roughness spectrum measured with the atomic force a microscope (AFM) and PSD in the range of from 0.02 to 0.5 nm
2
in the wavelength of from 0.5 &mgr;m or higher to less than 1 &mgr;m.
MODE FOR CARRYING OUT OF THE INVENTION
[Magnetic Layer]
A magnetic layer is also referred to as a magnetic upper layer in this specification.
In the magnetic recording medium of the invention, the thickness of the magnetic upper layer ranges from 0.01 to 0.3 &mgr;m. By setting the thickness of the magnetic upper layer in such range, C/N ratio :and resolution property of digital recording can be improved. In the case that such a very thin magnetic layer is coated, in particular, by simultaneous multilayer coating technique or wet-on-wet coating method, the orientation state of magnetic powder and the surface roughness of the magnetic layer are largely influenced by the lower layer. It was found that the orientations and the surface properties can be improved by the fact that an acicular nonmagnetic powder is used in the nonmagnetic lower layer as well as the length of the major axis of the acicular nonmagnetic powder is appropriately selected in view of the thickness of the lower layer, that is, the ratio of a mean length L of the major axis of the nonmagnetic powder to an average thickness D of the nonmagnetic layer satisfies the relationship of {fraction (1/10)}≦L/D≦2. This is because when a length of the major axis of the nonmagnetic powder is in an appropriate range in view of the thickness of the lower layer, a degree of freedom in the thickness direction of major axis is lowered; thereby the nonmagnetic powder is flatly aligned in the layer. This phenomenon becomes remarkable as the thickness of the lower layer becomes thin.
When the thickness of the lower layer becomes thin, the acicular particles easily and deeply incline and voids between the acicular particles decrease and a molding effect during calendering tends to be smaller, while the effects mentioned above become remarkable. In such a case, it is preferable that a granulated powder having an average primary particle size equal to or less than 50 nm is mixed with said acicular nonmagnetic powder at the specific ratio. It is because the granulated particles enter between the acicular particles, and thus appropriate voids are formed. Desirable effects are gained by using the granulated particles in the mixing ratio mentioned above. In particular, carbon black having 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 is preferably used as the granulated particles mentioned above. Because this type of carbon black has appropriate softness. Mixing carbon black in the lower layer is disclosed in U.S. Pat. No. 5,612,122. However, the oil absorption amount of the carbon black described in this patent is 300 ml/100 g, which is larger than the range described above as a preferable range in the invention. The carbon black having a preferable oil absorption amount in the invention is preferable because of its better dispersibility.
When the magnetic layer in the magnetic recording medium of the invention is thin, surface properties of the nonmagnetic support are important and the effect mentioned above becomes significant by controlling a roughness in the specific wavelength as mentioned below.
It is appropriate in the magnetic medium of the present invention that the mean value d of the thickness of the magnetic layer is in the range of from 0.01 to 0.3 &mgr;m, preferably from 0.01 to 0.2 &mgr;m, more preferably from 0.01 to 0.1 &mgr;m, further more preferably from 0.02 to 0.09 &mgr;m, the most preferably from 0.03 to 0.07 &mgr;m. The object of the invention is accomplished with either single or multiple magnetic layers.
In the case of multi-magnetic layers, the technique described in U.S. Pat. No. 5,447,782 can be applied. Because such a thin magnetic layer of the invention result

Affiliated with

Also associated with

Rating

Magnetic recording medium does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Magnetic recording medium, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Magnetic recording medium will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2959134