Process for producing magnetic disk

Details Process for producing magnetic disk Process for producing magnetic disk

2001-11-26

2002-10-01

Pianalto, Bernard (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Pretreatment of substrate or post-treatment of coated substrate

C427S128000, C427S130000, C427S550000, C427S599000, C428S065100, C428S690000, C428S900000

Reexamination Certificate

active

06458432

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the production of a magnetic recording disk and more particularly to a process for the production of a particulate type magnetic disk, which has a magnetic layer where a ferromagnetic powder is dispersed in a binder, suitable for high density recording involving a specific orientation method.
2. Description of the Related Art
In the art of magnetic disk, the trend is for more 2 MB MF-2HD floppy disks made of Co-modified iron oxide to be normally mounted on personal computers. However, the capacity of such a 2MB MF-2HD floppy disk is not necessarily sufficient under today's circumstances where the amount of data to be treated has shown a sudden increase. It has thus been desired to increase drastically the capacity of floppy disks.
As a magnetic recording medium there has heretofore been widely used one obtained by applying a magnetic layer having an iron oxide, Co-modified iron oxide, CrO
2
, ferromagnetic metal powder and hexagonal ferrite powder dispersed in a binder to a support. Among these materials, ferromagnetic metal powder and hexagonal ferrite powder are known to have excellent high density recording properties.
Examples of magnetic disks include large capacity disks comprising a ferromagnetic metal powder having excellent high density recording properties such as 10 MB MF-2TD and 21 MB MF-2SD, and large capacity disks comprising hexagonal ferrite such as 4 MBMF-2ED and 21 MB floptical. However, these magnetic disks leave something to be desired in capacity and performance. Under these circumstances, many attempts have been made to improve high density recording properties. During the process of development, the following knowledge has been found concerning the orientation of magnetic material.
It is important that a ferromagnetic powder itself has a high acicularity ratio to realize a high coercive force due to its anisotropy in shape. It is important in a tape-like medium that the magnetic layer itself has a raised magnetic orientation in the direction according to the running direction of the head. In a rotary recording medium such as floppy disk, it is more important that the variation of output in the circumferential direction is minimized than the magnitude of output is maximized because digital recording is effected. It is thus important that the magnetic orientation in the magnetic layer is so-called random orientation free of anisotropy (i.e., orientation ratio (anisotropy in magnetic orientation in the plane of magnetic layer normally represented by the ratio of squareness ratio in a predetermined direction to squareness ratio in the direction perpendicular to that direction as a measure) of close to 1).
In order to realize high density recording, it is important to reduce the particle size of the magnetic powder further. However, a problem has arisen during the development of a large capacity floppy disk having a face recording density of greater than 0.2 Gbit/inch
2
that as the particle size of the magnetic material decreases, there occur more noises. In order to inhibit the generation of noises, it is necessary that the agglomeration of magnetic particles to each other be eliminated and the content of vertical magnetization components (vertically magnetized components) be reduced. In order to meet these requirements, orientation becomes a great factor. Further, as the particle size of the magnetic powder decreases, the dispersion of the magnetic powder in the binder during the preparation of the magnetic layer coating compound is made difficult, making it difficult to obtain the desired orientation even after the application of the magnetic layer coating compound to the support.
As techniques for random orientation of magnetic powder in a magnetic layer there have heretofore been proposed the following approaches.
JP-A-6-36261 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a recording medium having a lower non-magnetic layer and a thin magnetic layer obtained by ATOMM (Advanced Super Thin Layer & High Output Metal Media Technology) which comprises performing random orientation under wet conditions, and then performing oblique orientation to attain an in-plane and vertical orientation ratio of not smaller than 0.85 and a vertical squareness ratio of from 0.3 to 0.65. In some detail, a magnetic disk which can give a uniform and high circumferential output and excellent overwrite properties as compared with those obtained by the conventional orientation-free processing and a process for the production thereof are provided. However, the ferromagnetic powder used in the examples is as large as 0.20 &mgr;m and 195 angstrom as calculated in terms of major axis length and crystalline size, respectively.
JP-A-63-148417 discloses a process which comprises applying an alternating magnetic field while the magnetic layer is undried to perform random orientation, wherein the intensity of magnetic field is from 1/10 to 1/1 of Hc of the ferromagnetic powder and the frequency of the alternating magnetic field is from 1/10 to 1/1 of the coating speed. In accordance with this approach, the range of the intensity of magnetic field is predetermined while the relationship between the frequency and the coating speed is predetermined within a predetermined range. In this arrangement, the orientation ratio of 1 can be continuously maintained to enable stable random orientation. Although the approach disclosed in the above cited patent can perform three-dimensional random orientation, a sufficient S/N ratio cannot be secured. Further, the magnetic materials used in the examples of the above cited patent are &ggr;-Fe
2
O
3
and Co-containing &ggr;-Fe
O
3
having Hc of from 240 to 600 Oe. However, a particulate magnetic material having a high coercive force (Hc) required for high density recording (particularly a magnetic metal powder or hexagonal tablet-like hexagonal ferrite having a high as value) is subject to agglomeration. Thus, it is necessary to take a measure for inhibiting noises.
JP-A-1-248321 discloses a process which comprises performing random orientation shortly after vertical orientation. This invention contemplates the combined use of vertical orientation and random orientation that makes it possible to provide a medium having little mechanical orientation, a high orientation ratio and good modulation properties (variation of reproduced output in the circumferential direction on the magnetic disk). However, this approach is disadvantageous in that since vertical orientation has been once effected, the vertical component of magnetization, which factor is the target of control in the present invention, tends to grow. Further, in the examples of the above cited patent, a magnetic powder having a particle size as large as 0.25 &mgr;m as calculated in terms of major axis length and an acicularity ratio as large as 10 is used. Such a magnetic powder having a relatively large particle size and large acicularity ratio can easily be arranged parallel to the surface of the magnetic layer. However, these examples leave something to be desired in attaining a sufficient orientation because a magnetic metal powder having a small major axis length and a small acicularity ratio must be used to secure a high S/N ratio essential for high density recording.
JP-A-63-171428 discloses a process which comprises subjecting a particulate ferromagnetic material to orientation in a magnetic field in a predetermined direction, and then subjecting the ferromagnetic material to orientation in a weak alternating magnetic field in the direction almost perpendicular to that of the former magnetic field for random orientation. However, since the magnetic powder used in the examples of the above cited patent is &ggr;-Fe
2
O
3
, which has a small magnetizability than magnetic metal powder, sufficient electromagnetic properties cannot be obtained.
JP-A-1-105328 discloses a process which comprises subjecting a magnetic material to crosswise

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