Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2001-11-21

2004-02-03

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, C428S329000, C428S336000, C428S690000, C428S690000, C428S690000, C428S690000

Reexamination Certificate

active

06686022

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium provided a magnetic layer in which a ferromagnetic powder and a binder are dispersed on a nonmagnetic support, especially, having excellent electromagnetic characteristics and running durability.
BACKGROUND OF THE INVENTION
Particulate magnetic recording media in which a magnetic layer comprised of ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO
2
, ferromagnetic alloy powders, or the like dispersed in a binder is particulate on a nonmagnetic support have been widely employed as magnetic recording media such as video tapes, audio tapes, and magnetic tapes. Further, in tape-like particulate magnetic recording media, a backcoat layer is provided as needed on the reverse surface of the support from the magnetic layer.
In such magnetic recording media, a higher degree of original sound reproduction capability is demanded of audio tapes used to record and reproduce music. In video tapes, good original image reproduction capability is demanded. In computer backup tapes and disks, good durability and no data loss are demanded.
With the trends toward high density with short recording wavelengths in recent years, a thick magnetic layer has resulted in problems in the form of self-magnetization loss during recording and thickness loss during reproduction, leading to decreased output. Thus, the thickness of the magnetic layer has been reduced. However, when the thickness of the magnetic layer is reduced to about 2 &mgr;m or less, the nonmagnetic support tends to affect the surface of the magnetic layer, and electromagnetic characteristics and dropout tend to deteriorate.
A method employing a simultaneous multilayer coating technique by which a nonmagnetic coated layer is provided as a lower layer for the magnetic layer and a high-density magnetic coating solution is thinly coated is one method of solving this problem (Japanese Unexamined Patent Publication (KOKAI) Showa Nos. 63-191315 and 63-187418). Based on these inventions, it is possible to manufacture a particulate magnetic recording medium with a thin magnetic layer and to obtain a magnetic recording medium with better electromagnetic characteristics. However, in recent years, the demand for high-density magnetic recording media has increased markedly.
Conventionally, to reduce magnetic tape wear and tape damage during VTR running, solid lubricants such as carbon black and liquid lubricants such as fatty acid esters have been added to the magnetic layer. The addition of abrasives such as alumina to the magnetic layer is also generally practiced. However, when the quantity of solid lubricants and abrasives added is increased, the fill ratio of the magnetic powder in the magnetic layer and the surface smoothness of the magnetic layer decrease, causing a drop in reproduction output. Since the presence of liquid lubricants on the magnetic layer surface and in the magnetic layer is affected, there are problems in the form of dependency on the environment during use and over time. The addition of a large quantity of abrasives causes wear of the magnetic head, minute changes in the contact state in proximity to the gap, and changes in the reproduction envelope, thereby shortening the service life of the VTR.
It is desirable for such magnetic tapes themselves not to be worn down or damaged by the magnetic head or running contact parts of the VTR or the like, and for the magnetic tapes not to cause wear of the magnetic head as small as possible.
To satisfy the above-stated requirements, various investigation has been conducted to-date:
1) Japanese Unexamined Patent Publication (KOKAI) Heisei No. 6-52541: Describes limiting the average protrusion height of abrasive particles present in the magnetic layer surface to equal to or less than 15 nm, thereby affording improvement in head wear and head fouling.
2) Japanese Unexamined Patent Publication (KOKAI) Heisei No. 6-309650: Discloses specifying the content of lubricants and the protrusion height and their present concentration on the magnetic layer surface to improve running properties.
3) Japanese Unexamined Patent Publication (KOKAI) Heisei Nos. 6-12651 and 6-12652: Disclose specifying the surface roughness of the magnetic layer and the protrusion height and present concentration on the magnetic layer surface to balance electromagnetic characteristics, running properties, and durability.
In recent years, with high-density recording and high recording rates, shortening of the wavelength of the recording frequency and narrowing of magnetic recording head tracks have progressed as means of increasing the recording capacity per unit area. In particular, there is a demand for magnetic recording media recording and reproducing at high density with a minimum recording wavelength equal to or less than 0.7 &mgr;m and a track pitch equal to or less than 25 &mgr;m. Further, the trend is toward increasing the relative head/tape velocity by increasing the rotational speed of the cylinder. Thus, in the above-cited prior art, it is no longer possible to balance electromagnetic characteristics and running durability.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a particulate magnetic recording medium with good output at high frequency, yielding a low coefficient of friction, and having good running properties. A further object of the present invention is to stably provide a magnetic recording medium performing high-density recording and reproduction at a minimum recording wavelength equal to or less than 0.7 &mgr;m and a track pitch equal to or less than 25 &mgr;m affording good electromagnetic characteristics and good repeat durability.
The present inventors have extensively researched as for structures and manufacture method of a magnetic layer and a nonmagnetic layer in order to achieve the above-mentioned problems. As a result, they discovered that the above-mentioned problems can be solved by a magnetic recording medium provided a lower layer comprising a nonmagnetic powder and a binder and an upper magnetic layer comprising a ferromagnetic powder and a binder, as well as further comprising at least one abrasives in this order on a nonmagnetic support, wherein the number of abrasive particles present on said magnetic layer surface satisfied the relation 0.2≦X≦1.2 (particles/&mgr;m
2
), and when the number of pairs of abrasive particles satisfying the relation L≦2K (where K denotes the mean particle diameter of the abrasive and L denotes the distance from the center of gravity of one of the abrasive particles to the center of gravity of the other abrasive particle) is denoted as Y (particles/&mgr;m
2
), Y/X is equal to or less than 2.
To achieve good sliding characteristics of the head and tape while maintaining smoothness of the magnetic layer surface without greatly decreasing the fill rate, it is considered necessary for the abrasive contained in the magnetic layer to be in the form of microparticles (for example, with a mean particle diameter of 0.1 to 0.6 &mgr;m). However, when the abrasive is in the form of microparticles, severe aggregation of abrasive particles is known to occur; in particular, when the magnetic layer is thin, as the particle diameter of the abrasive approaches the thickness of the magnetic layer, aggregates are often observed. This is attributed to aggregates of abrasive that were concealed in the direction of thickness of the magnetic layer becoming prominent with the decrease in layer thickness.
Such abrasive aggregates are known to cause increased noise and head abrasion in the same manner as when coarse abrasives are employed, irrespective of whether or not microgranular abrasives have been employed. A portion or all of the abrasive particles drop out from the aggregate, causing abrasive wear between the tape surface and the cylinder and head, as well as head fouling and cylinder fouling.
In response to the above problems, the present inventors discovered that by inhibiting such abrasive aggregation (dispersing microgranular abrasi

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