U.S. patent application number 09/995274 was filed with the patent office on 2002-08-01 for process for producing magnetic disk.
Invention is credited to Nakamikawa, Junichi, Noguchi, Hitoshi, Saito, Shinji.
Application Number | 20020102351 09/995274 |
Document ID | / |
Family ID | 18829756 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020102351 |
Kind Code |
A1 |
Noguchi, Hitoshi ; et
al. |
August 1, 2002 |
Process for producing magnetic disk
Abstract
A process for producing a magnetic disk having a randomly
oriented magnetic powder, comprises: applying a magnetic coating
solution containing at least a magnetic powder to a web that is
being continuously conveyed, so as to prepare a magnetic layer;
applying a first external magnetic field to the magnetic layer
while the magnetic layer is wet; and applying a second external
magnetic field to the magnetic layer while the magnetic layer is
wet, wherein: the first external magnetic field is applied with a
set of a first pair of same-pole-opposed magnets with the web
interposed therebetween and a second pair of same-pole-opposed
magnets with the web interposed therebetween; the first and second
pairs are provided on the same plane of the web and on two equal
sides of an isosceles triangle so that a perpendicular line dropped
from a base of the isosceles triangle forms a line perpendicular to
a conveying direction of the web; and the second external magnetic
field is an alternating magnetic field, and is applied with a pair
of magnets with the web interposed therebetween; the pair of
magnets being provided on the same plane of the web, and in a
direction perpendicular to the conveying direction of the web.
Inventors: |
Noguchi, Hitoshi; (Kanagawa,
JP) ; Nakamikawa, Junichi; (Kanagawa, JP) ;
Saito, Shinji; (Kanagawa, JP) |
Correspondence
Address: |
STROOCK & STROOCK & LAVAN LLP
180 Maiden Lane
New York
NY
10038
US
|
Family ID: |
18829756 |
Appl. No.: |
09/995274 |
Filed: |
November 26, 2001 |
Current U.S.
Class: |
427/128 ;
427/190; 427/599; G9B/5.267; G9B/5.297 |
Current CPC
Class: |
Y10S 428/90 20130101;
G11B 5/845 20130101; G11B 5/70678 20130101 |
Class at
Publication: |
427/128 ;
427/599; 427/190 |
International
Class: |
B05D 005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 24, 2000 |
JP |
P-2000-357743 |
Claims
What is claimed is:
1. A process for producing a magnetic disk having a randomly
oriented magnetic powder, which comprises: applying a magnetic
coating solution containing at least a magnetic powder to a web
that is being continuously conveyed, so as to prepare a magnetic
layer; applying a first external magnetic field to the magnetic
layer while the magnetic layer is wet; and applying a second
external magnetic field to the magnetic layer while the magnetic
layer is wet, wherein: the first external magnetic field is applied
with a set of a first pair of same-pole-opposed magnets with the
web interposed therebetween and a second pair of same-pole-opposed
magnets with the web interposed therebetween; the first and second
pairs are provided on the same plane of the web and on two equal
sides of an isosceles triangle so that a perpendicular line dropped
from a base of the isosceles triangle forms a line perpendicular to
a conveying direction of the web; and the second external magnetic
field is an alternating magnetic field, and is applied with a pair
of magnets with the web interposed therebetween; the pair of
magnets being provided on the same plane of the web, and in a
direction perpendicular to the conveying direction of the web.
2. The process for producing a magnetic disk according to claim 1,
wherein the same-pole-opposed magnets in each of the first and
second pairs are a pair of permanent magnets, and the intensity of
magnetic field in a center of a gap between the opposed permanent
magnets in each of the first and second pairs is from 1/3 to 10
times the coercive force of the magnetic layer.
3. The process for producing a magnetic disk according to claim 1,
wherein the random orientation by the application of an alternating
magnetic field is effected in such a manner that the intensity of
the second external magnetic field is from {fraction (1/40)} to 10
times the intensity of the first external magnetic field.
4. The process for producing a magnetic disk according to claim 1,
wherein the magnetic powder is a ferromagnetic metal powder having
Hc of from 114 to 280 kA/m (from 10,400 t0 3, 500 Oe) and an
average major axis length of from 0.01 .mu.m to 0.18 .mu.m.
5. The process for producing a magnetic disk according to claim 1,
wherein the magnetic powder is a hexagonal ferrite magnetic
material having an average plate diameter of from 0.01 .mu.m to 0.1
.mu.m.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] 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.
[0003] 2. Description of the Related Art
[0004] 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.
[0005] 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.sub.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.
[0006] 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 MB
MF-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.
[0007] It is important that a ferromagnetic powder itself has a
high aciicularity 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).
[0008] 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.sup.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 to 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.
[0009] As techniques for random orientation of magnetic powder in a
magnetic layer there have heretofore been proposed the following
approaches.
[0010] 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 .mu.m and 195 angstrom as calculated
in terms of major axis length and crystalline size,
respectively.
[0011] 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 {fraction (1/10)} to {fraction (1/1)} of Hc of the
ferromagnetic powder and the frequency of the alternating magnetic
field is from {fraction (1/10)} to {fraction (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
.gamma.-Fe.sub.2O.sub.3and Co-containing .gamma.-Fe.sub.2O.sub.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 os value) is
subject to agglomeration. Thus, it is necessary to take a measure
for inhibiting noises.
[0012] 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 .mu.n 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.
[0013] 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
.gamma.-Fe.sub.2O.sub.3, which has a small magnetizability than
magnetic metal powder, sufficient electromagnetic properties cannot
be obtained.
[0014] JP-A-1-105328 discloses a process which comprises subjecting
a magnetic material to crosswise orientation, and then subjecting
the magnetic material to uniform deorientation in a solenoid to
which an alternating magnetic field is applied. In the above cited
patent, it is certain that the in-plane orientation ratio can be
improved. However, there is no definition on the inhibition of
agglomeration of magnetic particles and vertical magnetization.
Thus, this approach leaves something to be desired in attaining a
high S/N ratio.
[0015] JP-B-5-53009 (The term "JF-B" as used herein means an
"examined Japanese patent application") discloses a process which
comprises arranging a plurality of bar-shaped orientation magnets
apart from each other at a certain interval in the direction of
conveyance of the support such that the polarity of magnets differ
from that of adjacent magnets and these magnets are disposed
oblique to the conveying direction and face in the direction
opposed to that of adjacent magnets, where by random orientation is
allowed. In this arrangement, a good modulation can be certainly
obtained, attaining a high orientation ratio. However, since the
magnetic powder used in the examples of the above cited patent is
.gamma.-Fe.sub.2O.sub.3, which has a small magnetizability than
magnetic metal powder, sufficient electromagnetic properties cannot
be obtained. Further, since there is no random orientation
apparatus using an alternating magnetic field, modulation is
deteriorated unless the intensity of magnetic field of the
bar-shaped orientation magnet is predetermined to be not greater
than 50 Oe. A particulate magnetic metal cannot be sufficiently
oriented in such a low magnetic field.
[0016] As mentioned above, there have been disclosed many
techniques for performing so-called random orientation free from
anisotropy as magnetic orientation in the magnetic layer in the
production of magnetic disk. However, no effective means have been
found for realizing desired orientation and reducing noises even by
reducing the particle size and acicularity ratio of magnetic powder
to increase the coercive force thereof for the purpose of
performing high density recording.
SUMMARY OF THE INVENTION
[0017] Therefore, an object of the invention is to provide a
production process suitable for the provision of a large capacity
magnetic disk suitable for digital recording having good
electromagnet properties, S/N ratio and modulation which can
provide a desired orientation and eliminate agglomeration of
magnetic particles to each other to reduce noises even if the
particle size and acicularity ratio of the magnetic powder are
reduced to enhance the coercive force thereof.
[0018] The inventors made extensive studies. As a result, it was
found that the use of the following production process makes it
possible to accomplish the object of the invention, i e., obtain a
desired orientation, eliminate the agglomeration of magnetic
particles to each other and reduce the content of vertically
magnetized components to reduce noises, thus obtaining a large
capacity magnetic disk suitable for digital recording having good
electromagnetic properties, S/N ratio and modulation. Thus, the
present invention has been worked out.
[0019] In other words, the present invention has the following
constitutions.
[0020] (1) A process for producing a magnetic disk having a
randomly oriented magnetic powder, which comprises: applying a
magnetic coating solution containing at least a magnetic powder to
a web that is being continuously conveyed, so as to prepare a
magnetic layer; applying a first external magnetic field to the
magnetic layer while the magnetic layer is wet; and applying a
second external magnetic field to the magnetic layer while the
magnetic layer is wet, wherein: the first external magnetic field
is applied with a set of a first pair of same-pole-opposed magnets
with the web interposed therebetween and a second pair of
same-pole-opposed magnets with the web interposed therebetween; the
first and second pairs are provided on the same plane of the web
and on two equal sides of an isosceles triangle so that a
perpendicular line dropped from a base of the isosceles triangle
forms a line perpendicular to a conveying direction of the web; and
the second external magnetic field is an alternating magnetic
field, and is applied with a pair of magnets with the web
interposed therebetween; the pair of magnets being provided on the
same plane of the web, and in direction perpendicular to the
conveying direction of the web.
[0021] (2) The process for producing a magnetic disk according to
Clause 1, wherein the same-pole-opposed magnets in each of the
first and second pairs are a pair of permanent magnets, and the
intensity of magnetic field in a center of a gap between the
opposed permanent magnets in each of the first and second pairs is
from 1/3 to {fraction (10)} times the coercive force of the
magnetic layer.
[0022] (3) The process for producing a magnetic disk according to
Clause 1, wherein the random orientation by the application of an
alternating magnetic field is effected in such a manner that the
intensity of the second external magnetic field is from {fraction
(1/40)} to 10 times the intensity of the first external magnetic
field.
[0023] (4) The process for producing a magnetic disk according to
Clause 1, wherein the magnetic powder is a ferromagnetic metal
powder having Hc of from 114 to 280 kA/m (from 1,400 to 3,500 Oe)
and an average major axis length of from 0.01 .mu.m to 0.18
.mu.m.
[0024] (5) The process for producing a magnetic disk according to
Clause 1, wherein the magnetic powder is a hexagonal ferrite
magnetic material having an average plate diameter of from 0.01
.mu.m to 0.1 .mu.m.
[0025] In accordance with the foregoing production process, even
when as a magnetic powder there is used one having a small particle
size and acicularity ratio and a great coercive force, a desired
orientation ratio can be obtained, a vertically magnetized
component can be obtained, and the agglomeration of magnetized
particles can be relaxed, making it possible to realize the
reduction of noise.
[0026] The restriction of the orientation ratio (Or) in the plane
of the magnetic layer and the squareness ratio (SQn) in the
direction perpendicular to the plane of the magnetic layer
contributes to the reduction of SN ratio and modulation. The
restriction of the particle size, acicularity ratio and coercive
force (Hc) of the magnetic material is needed to secure
electromagnetic properties, particularly output.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIGS. 1A and 1B are schematic diagrams illustrating how a
set of pairs of magnets, the pairs facing in the opposite
directions are oriented in the magnetization process of the
invention.
[0028] The Reference Numerals and Signs in the drawings are as.
follows.
[0029] M1: One pair of opposing magnets having the same polarity at
the opposing sides
[0030] M2: Other pair of opposing magnet shaving the same polarity
at the opposing sides
[0031] L-L: Central line in the support (indicating the
longitudinal direction)
DETAILED DESCRIPTION OF THE INVENTION
[0032] The production process of the invention is characterized by
applying two kinds of external magnetic fields (first and second
external magnetic fields) while a magnetic layer that is formed by
coating a coating liquid for a magnetic layer containing a magnetic
powder on a support for supporting the magnetic layer (hereinafter
referred to a web) is wet. The first external is applied with a set
of a first pair of same-pole-opposed magnets with the web
interposed therebetween and a second pair of same-pole-opposed
magnets with the web interposed therebetween; the first and second
pairs are provided on the same plane of the web and on two equal
sides of an isosceles triangle so that a perpendicular line dropped
from a base of the isosceles triangle forms a line perpendicular to
a conveying direction of the web. The second external magnetic
field applied after the first external magnetic field is an
alternating magnetic field, and is applied with a pair of magnets
with the web interposed therebetween. The pair of magnets for
applying the second external magnetic field is provided on the same
plane of the web, and in a direction perpendicular to the conveying
direction of the web.
[0033] The embodiment for applying the first external magnetic
field is further explained with drawings FIGS. 1A and 1B are
schematic diagrams illustrating how a set of two pairs of magnets,
and one pair of permanent magnets wherein the same poles are
opposed to each other with the web interposed therebetween (i.e.,
same-pole-opposed magnets) is provided in an oblique direction and
the other pair of same-pole-opposed magnets is provided in a
reverse oblique direction thereof. FIGS. 1A and 1B show a typical
two-step (or multi-step) magnetic field application of the first
external magnetic field. In FIGS. 1A and 1B, the two pairs of
same-pole-opposed magnets for forming one set are provided on each
of two equal sides (not shown in FIGS. 1A and 1B) of an isosceles
triangle. FIG. 1A is a sketch drawing viewed in an oblique
direction (perspective drawing). FIG. 1B is a plane view. In the
present specification, the two-direction applications of the
external magnetic field are referred to as magnetic field
application in an oblique direction and magnetic field application
in a reverse oblique direction, respectively.
[0034] The angle .alpha. between each of the two equal sides of the
isosceles triangle and the longitudinal direction of the web W
(i.e., the angle in the oblique direction and the angle in the
reverse oblique direction) is preferably from 30.degree. to
60.degree., more preferably from 40.degree. to 50.degree. (see FIG.
1B). This means that the angle between the magnetic field in the
oblique direction and the magnetic field in the reverse oblique
direction is more preferably from 80.degree. to 100.degree.. This
also means that the angle between the longitudinal axis of one pair
of permanent magnets in the oblique direction and that of the other
pair of permanent magnets in the reverse oblique direction (the
angle between the centerlines of M1 and M2 in FIG. 1B) is more
preferably from 80.degree. to 100.degree.. The number of a set
consisting of one pair of the same-pole-opposed permanent magnets
for the magnetic field application in the oblique direction (e.g.,
M1 in FIGS. 1A and 1B) and the other pair of the same-pole-opposed
permanent magnets for the magnetic field application in the reverse
oblique direction (e.g., M2 in FIGS. 1A and 1b) is at least one,
preferably one to three.
[0035] In the production process of the invention, the intensity of
magnetic field in the center of gap in the oblique orientation is
preferably from 40 to 560 kA/m (from 500 Oe to 8,000 Oe), more
preferably from 80 to 480 kA/m (from 1,000 Oe to 6,000 Oe).
[0036] In the random orientation by the application of the second
external magnetic field (i.e., alternating magnetic field), the
frequency of the magnetic field is preferably from 20 Hz to 200 Hz,
more preferably from 40 Hz to 100 Hz. The intensity of the
alternating magnetic field in the random orientation is preferably
from 4 to 240 kA/m (from 50 Oe to 3,000 Oe), more preferably from
16 to 80 kA/m (from 200 Oe to 1,000 Oe).
[0037] The shape and size of the magnet to be used in the
production process of the invention are not specifically limited.
In practice, however, a bar magnet having a length great enough to
cover the support in the crosswise direction is preferred. For
example, a bar magnet shielded by a yoke is preferred.
[0038] In the present invention, the ambient temperature during
production can be properly predetermined but normally can be
predetermined to be from 40.degree. C. to 120.degree. C.
[0039] The in-plane orientation ratio (Or) of the magnetic disk
obtained by the production process of the invention is not smaller
than 0.85, preferably not smaller than 0.90, more preferably not
smaller than 0.95, ideally 1 with respect to the surface of the
magnetic layer. When Or falls below 0.85, the resulting modulation
exceeds the tolerance limit, raising the error rate to
disadvantage.
[0040] The squareness ratio (SQn) in the direction perpendicular to
the surface of the magnetic layer in the magnetic disk obtained by
the production process of the invention is not greater than 0.30,
preferably not greater than 0.28, more preferably from 0.26 to
0.15.
[0041] The production process of the invention may relate to the
production of a single-layer magnetic disk having a magnetic layer
formed directly on a non-magnetic support. The production process
of the invention may also relate to a process for the production of
a magnetic disk which comprises providing. a substantially
non-magnetic subbing layer on a support, and then forming a thin
magnetic layer thereon, i.e., the foregoing ATOMM type production
process.
[0042] The term "substantially no-magnetic subbing layer" as used
herein is meant to indicate that the subbing layer may be magnetic
to an extent such that it doesn't take part in recording and will
be hereinafter referred simply to as "underlying layer" or
"non-magnetic layer".
[0043] In the case where ATOMM type process is employed, the
application of the magnetic coating compound and the non-magnetic
coating compound may be accomplished by either wet-on-dry process
(involving the application of the magnetic layer after the drying
of the underlying layer; abbreviated as "W/D") or wet-on-wet
process (application of both the two layers while wet; abbreviated
as "W/W").
[0044] The thickness of the magnetic layer in the magnetic disk
obtained by the production process of the invention is preferably
from 0.02 .mu.m to 0.5 .mu.m. In order to predetermine the
thickness of the magnetic layer to a range of from 0.02 .mu.m to
0.5 .mu.m, the foregoing ATOMM type process is preferably effected.
Further, W/W process is preferably used. The magnetic disk obtained
by the production process of the invention can be used in a
magnetic recording system performing recording at a face recording
density as high as from 0.2 to 2 Gbit/inch.sup.2
[0045] The face recording density is obtained by multiplying the
linear recording density by the track density.
[0046] The linear recording density is the number of bits of signal
to be recorded per inch in the recording direction.
[0047] The linear recording density, track density and face
recording density are determined by the system.
[0048] The magnetic powder to be used in the production process of
the invention preferably is a ferromagnetic metal powder or
hexagonal ferrite having a coercive force (Hc) of from 110 to 280
kA/m (from 1,400 to 3,500 Oe).
[0049] The magnetic powder to be used in the production process of
the invention, if it is a ferromagnetic metal powder, has an
average major axis length of from 0.01 .mu.m to 0.18 .mu.m ,
preferably from 0.04 .mu.m to 0.15 .mu.m, more preferably from 0.06
.mu.m to 0.12 .mu.m. When the average major axis length of the
magnetic powder exceeds 0.18 .mu.m, it is disadvantageous in that
the resulting magnetic disk (hereinafter also referred to as
"media") generates a raised noise. On the contrary, when the
average major axis length of the magnetic powder falls below 0.01
.mu.m, a sufficient dispersion cannot be attained, making it
impossible to exert an effect of reducing noise by the atomization
of magnetic material and increasing the surface roughness of the
magnetic layer to disadvantage.
[0050] The crystalline size of the magnetic powder is from 50 to
180 angstrom, preferably from 80 to 160 angstrom, more preferably
from 100 to 150 angstrom. When the crystalline size of the magnetic
powder exceeds 180 angstrom, it is disadvantageous in that the
resulting media generate a raised noise. On the contrary, when the
crystalline size of the magnetic powder falls below 50angstrom, a
sufficient dispersion cannot be attained, making it impossible to
exert an effect of reducing noise by the atomization of magnetic
material and increasing the surface roughness of the magnetic layer
to disadvantage.
[0051] The acicularity ratio of the magnetic powder is from 2 to 9,
preferably from 4 to 7. When the acicularity ratio of the magnetic
powder exceeds 9, the packing of the magnetic layer is reduced,
lowering the reproduced output. It is further disadvantageous in
that the resulting media generate a raised noise. When the
acicularity ratio of the magnetic powder falls below 2, a
sufficient coercive force cannot be provided, making the resulting
media unsuitable for high density recording. Further, the magnetic
material is subject to three-dimensional random orientation that
increases the vertical magnetized components and hence raises
noise.
[0052] In the case where the magnetic powder to be used is a
hexagonal ferrite magnetic material, its average plate diameter is
from 0.01 .mu.m to 0.1 .mu.m, preferably from 0.02 .mu.m to 0.06
.mu.m, more preferably from 0.03 .mu.m to 0.05 .mu.m. When the
average plate diameter exceeds 0.1 .mu.m, it is disadvantageous in
that the resulting magnetic disk generates a raised noise. On the
contrary, when the average plate diameter falls below 0.01 .mu.m, a
sufficient dispersion cannot be attained, making it impossible to
exert an effect of reducing noise by the atomization of magnetic
material and increasing the surface roughness of the magnetic layer
to disadvantage.
[0053] The tabularity ratio (plate diameter/plate thickness) of the
magnetic powder is from 1 to 7, preferably from 2 to 6, more
preferably from 3 to 5. When the tabularity ratio of the magnetic
powder exceeds 7, the magnetic material undergoes stacking that
raises noise to disadvantage. On the contrary, when the tabularity
ratio of the magnetic powder falls below 1, the magnetic material
is subject to three-dimensional random orientation that increases
the vertical magnetized components and hence raises noise.
[0054] The coercive force Hc of the magnetic layer is from 140 to
280 kA/m (from 1,700 to 3,000 Oe), preferably 150 to 230 kA/m (from
1,800 to 2,700 Oe), more preferably 160 to 200 kA/m (from 2, 000 to
2,500 Oe) When Hc of the magnetic layer falls below 140 kA/m (1,700
Oe), a high linear density recording cannot be conducted, making it
impossible to obtain media having sufficient properties for high
capacity recording. On the contrary, when Hc of the magnetic layer
exceeds 280 kA/m (3, 000 Oe), the state-of-the art recording head
cannot sufficiently record signal on the magnetic layer to
disadvantage.
[0055] In the art of personal computers, which have recently found
themselves engrossed more and more in multimedia system, large
capacity recording media substituting for the conventional floppy
disk have been noted. Such a large capacity recording medium has
been put into the market as ZIP disk (face recording density: 96
Mbit/inch.sup.2)by IOMEGAINC. (U.S.A.). This is an ATOMM type disk
developed by Fuji Photo Film Co., Ltd. The product which has been
put into the market has a diameter of 3.7 inch and a recording
capacity of 100 MB or more. The recording capacity of from 100 MB
to 120 MB is almost the same as that of MO (3.5 inch diameter),
which can store data of from 7 to 8 month newspaper per sheet. The
transfer rate, which indicates the time required to write/read
data, is 2 MB or more per second, which is comparable to that of
hard disc and 20 times that of the conventional FD or twice that of
MO, to great advantage. This magnetic disk, which comprises an
underlying layer and a thin magnetic layer, is of the same type as
the state-of-the-art FD, i.e., particulate type medium and thus can
be mass-produced. Accordingly, this magnetic disk is advantageous
in that it is inexpensive as compared with MO and hard disk.
[0056] The foregoing process for the production of a magnetic disk
by the inventors is a production process for obtaining a magnetic
disk having a drastically higher recording capacity than the
foregoing ZIP disk or MO (3.5 inch diameter) and a face recording
density of from 0.2 to 2 Gbit/inch.sup.2.
[0057] In particular, by incorporating a particulate ferromagnetic
powder having a high output and a high dispersibility in a magnetic
layer having a thickness as extremely small as 0.02 to 0.5 .mu.m,
incorporating a spherical or acicular inorganic powder in a subbing
layer, and reducing the thickness of the magnetic layer,
cancellation of magnetic force in the magnetic layer can be
lessened, the output in a high frequency region can be drastically
enhanced, and the overwriting properties can be improved.
[0058] By improving the magnetic head and combining it with a
narrow gap head, the effect of the ultrathin magnetic layer can be
further exerted, making it possible to improve the digital
recording properties.
[0059] The thickness of the magnetic layer is predetermined to be
as small as 0.02 to 0.5 .mu.m to meet the requirements of high
density magnetic recording system or magnetic head. The packing of
such an extremely thin uniform magnetic layer can be raised by
dispersing particulate magnetic and non-magnetic materials with a
dispersant and a binder having a high dispersibility in
combination. As the magnetic material to be used herein there may
be used a ferromagnetic metal powder which is very particulate, can
attain a high output, contains much Co and contains Al or Y as a
sintering inhibitor to maximize capacity and output. In order to
realize a high transfer rate, a three-dimensional network binder
system suitable for ultrathin magnetic layer can be used to attain
desired running stability and durability during high speed
rotation. A composite lubricant which can maintain its
effectiveness even during use or high speed rotation under wide
humidity and temperature conditions can be incorporated in the two
layers. Further, the underlying layer can act as a lubricant
reservoir from which a proper amount of the lubricant is always
supplied into the upper magnetic layer, making it possible to
enhance the durability of the upper magnetic layer and hence
enhance the reliability of the magnetic disk. The cushioning effect
of the underlying layer can bring about a good head touch and
stable running.
[0060] ATOMM structure, which is a preferred embodiment of the
arrangement of the magnetic disk produced by the process of the
invention, has the following advantages:
[0061] (1) Improvement of electromagnetic properties by the
reduction of the thickness of the magnetic layer
[0062] (a) Enhancement of output in high frequency range by the
improvement of recording demagnetization properties
[0063] (b) Improvement of overwrite properties
[0064] (c) Provision of window margin
[0065] (2) Enhancement of output by the smoothening of the upper
magnetic layer
[0066] (3) Easiness of provision of required functions by the
separation of functions of magnetic layer
[0067] (4) Enhancement of durability by stable supply of
lubricant
[0068] These functions cannot be attained merely by providing a
multi-layer magnetic layer. In order to provide such a multi-layer
structure, coating solutions of underlying layer and upper layer
are applied. The coated support is then normally subjected to
surface treatment such as hardening and calendering. Unlike
magnetic tape, FD is normally subjected to the same treatment on
both sides thereof. After the coating step, the coated supported is
subjected to slit step, punching step, step for insertion into
shell, and certifying step to complete finished product. The
product is then punched into a disk, and then subjected to thermal
treatment at a high temperature (normally 50.degree. C. to
90.degree. C.) to accelerate the curing of the coat layers. The
disk may then be varnished with an abrasive tape to remove surface
rises or otherwise post-treated.
[0069] Durability is an important factor for magnetic disk.
Examples of the means for enhancing the durability of media include
a means for formulating the binder such that the film strength of
the disk itself can be raised, and a means for the adjusting the
formulation of the lubricant to maintain the slipperiness with the
magnetic head.
[0070] A plurality of lubricants which each can exert its excellent
effect under various humidity and temperature conditions used can
be used in combination so that the various lubricants can each
perform its function even under wide temperature (low temperature,
room temperature, high temperature) and humidity conditions (low
humidity, high humidity) to maintain a comprehensively stable
lubricating effect.
[0071] By making the best use of the two-layer structure such that
the underlying layer can exert a tank effect (as a reservoir), the
magnetic layer can be invariably supplied with a proper amount of
lubricant to enhance the durability of the magnetic layer.
[0072] The underlying layer can be rendered capable of retaining a
lubricant as well as adjusting the surface resistivity. In general,
in order to adjust the electrical resistivity, it is often
practiced to incorporate a solid electrolytically-conducting
material such as carbon black in the magnetic layer. This restricts
the rise of the packing density of the magnetic material. Further,
as the thickness of the magnetic layer decreases, this has an
effect on the surface roughness of the magnetic disk. The
incorporation of an electrically-conductive material in the
underlying layer makes it possible to eliminate these defects.
[0073] [Magnetic Layer]
[0074] The magnetic layer formed by the production process of the
invention may be provided on one or both sides of a support. The
magnetic layer may be provided directly on the support or on a
subbing layer. A proper processing may be conducted to provide a
plurality of magnetic layers. The term "thickness of magnetic
layer" as used herein is meant to indicate the total thickness of
the various magnetic layers. In the case of ATOMM type structure,
either W/W or W/D may be employed. From the standpoint of
production efficiency, W/W is preferred, but W/D can be
sufficiently employed. In the case of multi-layer structure formed
by the production process of the invention, the use of W/W makes it
possible to form the upper layer and the underlying layer at the
same time. Thus, a surface treatment step such as calendering step
can be made the effective use of, making it possible to improve
even the surface smoothness of ultrathin magnetic layer. The
coercive force Hc of the magnetic layer is as defined above. Bm of
the magnetic layer is preferably from 2,000 to 5,000 G.
[0075] [Ferromagnetic Powder]
[0076] The ferromagnetic powder to be used in the magnetic layer to
be formed according to the production process of the invention is
not specifically limited but is preferably a ferromagnetic metal
powder comprising a-Fe as a main component. The ferromagnetic
powder may contain besides predetermined atoms Al, Si, S, Sc, Ca,
Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au,
Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, etc. In
particular, at least one of Al, Si, Ca, Y, Ba, La, Nd, Co, Ni and B
is preferably contained in the ferromagnetic powder. More
particularly, at least one of Co, Y and Al is contained in the
ferromagnetic powder. The content of Co is preferably from not
smaller than 0 atm-% (atomic percent) to not greater than 40
atom-%, more preferably from not smaller than 15 atm-% to not
greater than 35 atm-%, even more preferably from not smaller than
1.5 atm-% to not greater than 12 atm-% based on Fe. The content of
Y is preferably from not smaller than 1.5 atm-% to not greater than
12 atm-%, more preferably from not smaller than 3 atm-% to not
greater than 10 atm-%, even more preferably from not smaller than 4
atm-% to not greater than 9 atm-% based on Fe. The content of Al is
preferably from not smaller than 5 atm-% to not greater than 30
atm-%, more preferably from not smaller than 11 atm-% to not
greater than 20 atm-%, even more preferably from not smaller than
12 atm-% to not greater than 18 at=% based on Fe.
[0077] The ferromagnetic powder may be previously subjected to
treatment with a dispersant, lubricant, surface active agent,
antistat or other treatments described later before dispersion. For
the details of this treatment, reference can be made to
JP-B-44-14090, JP-B-45-18372, JP-B-47-22062, JP-B-47-22513,
JP-B-46-28466, JP-B-46-38755, JP-B-47-4286, JP-B-47-12422,
JP-B-47-17284, JP-B-47-18509, JP-B-47-18573, JP-B-39-10307,
JP-B-46-39639, U.S. Pat. Nos. 3,026,215, 3,031,341, 3,100,194,
3,242,005, and 3,389,014.
[0078] The particulate ferromagnetic alloy may contain a small
amount of a hydroxide or oxide. One obtained by any known process
for the preparation of particulate ferromagnetic alloy may be used
As such a preparation process there may be used the following
process. Examples of such a preparation process include a process
which comprises the reduction of a composite organic acid salt
(mainly composed of oxalate) with a reducing gas such as hydrogen,
a process which comprises reducing iron oxide with a reducing gas
such as hydrogen to obtain particulate Fe or Fe-Co, a process which
comprises the thermal decomposition of a metal carbonyl compound, a
process which comprises adding a reducing agent such as sodium
borohydride, hypophosphite and hydrazine to an aqueous solution of
a ferromagnetic metal to effect reduction thereof,, and a process
which comprises allowing a metal to be evaporated in a low pressure
inert gas to obtain a finely divided powder. The ferromagnetic
alloy powder thus obtained may be subjected to any known gradual
oxidation involving a method which comprises dipping the
ferromagnetic alloy powder in an organic solvent, and then drying
the ferromagnetic alloy powder, a method which comprises dipping
the ferromagnetic alloy powder in an organic solvent, supplying an
oxygen-containing gas into the ferromagnetic alloy powder to form
an oxide layer thereon, and then drying the ferromagnetic alloy
powder or a method which comprises adjusting the partial pressure
of oxygen gas and inert gas free from organic solvent to form an
oxide layer on the surface of the ferromagnetic alloy powder.
[0079] The coercive force of the ferromagnetic powder is preferably
from not smaller than 140 kA/m (1,700 Oe) to not greater than 280
kcA/m (3,500 Oe), more preferably from not smaller than 150 kA/m
(1,800 Oe) to not greater than 270 kA/m (3,000 Oe).
[0080] The water content of the ferromagnetic powder is preferably
from 0.01% to 2%. The water content of the ferromagnetic powder is
preferably optimized according to the kind of the binder used.
[0081] The pH value of the ferromagnetic powder is preferably
optimized by the combination with the binder used. The pH value of
the ferromagnetic powder is from 4 to 12, preferably from 6 to 10.
The ferromagnetic powder may be subjected to surface treatment with
Al, Si, P or oxide thereof as necessary. The amount of the surface
treatment to be used is from 0.1% to 10% based on the amount of the
ferromagnetic powder. The surface treatment is preferably effected
because the adsorption of a lubricant such as aliphatic acid is not
greater than 100 mg/m.sup.2. The ferromagnetic powder occasionally
contains a soluble inorganic ion such as Na, Ca, Fe, Ni and Sr
ions. It is essentially preferred that the ferromagnetic powder be
free of these inorganic ions. However, these inorganic ions have
little effect on the properties of the ferromagnetic powder if
their content is not greater than 200 ppm. The ferromagnetic powder
to be used in the production process of the invention preferably
has less voids. The void of the ferromagnetic powder is preferably
not greater than 20% by volume, more preferably not greater than 5%
by volume. The ferromagnetic powder may be in any form such as
needle, grain or spindle so far as the requirements for particle
size as previously defined can be satisfied. SFD (Switching Field
Distribution) of the ferromagnetic powder itself is preferably
small, i.e., not greater than 0.8. The distribution of Hc in the
ferromagnetic powder needs to be small. When SFD of the
ferromagnetic powder is not greater than 0.8, the ferromagnetic
powder exhibits good electromagnetic properties, a high output, a
sharp inversion of magnetization and less peak shifts and thus is
suitable for high density digital magnetic recording. The reduction
of Hc distribution can be by improving the particle size
distribution of goethite in the ferromagnetic metal powder or
preventing sintering of the ferromagnetic metal powder.
[0082] [Non-Magnetic layer]
[0083] The details of the underlying layer will be described
hereinafter. The inorganic powder to be used in the underlying
layer in the production process of the invention is a non-magnetic
powder which may be selected from the group consisting of inorganic
compounds such as metal oxide, metal carbonate, metal sulfate,
metal nitride, metal carbide and metal sulfide. As inorganic
compounds there may be used singly or in combination
.alpha.-alumina having a percent a conversion of not smaller than
90%, .beta.-alumina, .gamma.-alumina, .theta.-alumina, silicon
carbide, chromiumoxide, ceriumoxide, .alpha.-iron oxide, hematite,
goethite, corundum, silicon nitride, titanium carbite, titanium
oxide, silicon dioxide, tin oxide, magnesium oxide, tungsten oxide,
zirconium oxide, boron nitride, zinc oxide, calcium carbonate,
calcium sulfate, barium sulfate and molybdenum disulfide.
Particularly preferred among these inorganic compounds are titanium
dioxide, zinc oxide, iron oxide and barium sulfate because they
provide small particle size distribution and there are many means
of providing function. Even more desirable among these inorganic
compounds are titanium dioxide and a-iron oxide. These non-magnetic
powders preferably have a particle size of from 0.005 .mu.m to 2
.mu.m. If necessary, non-magnetic powders having different particle
sizes may be used in combination or the particle diameter
distribution of a single non-magnetic powder may be increased to
exert the similar effect. In particular, the particle size of the
non-magnetic powder is preferably from 0.01 .mu.m to 0.2 .mu.m. In
particular, in the case where the non-magnetic powder is a granular
metal oxide, the average particle diameter of the non-magnetic
powder is preferably not greater than 0.08 .mu.m. In the case where
the non-magnetic powder is an acicular metal oxide, the major axis
length of the non-magnetic powder is preferably not greater than
0.3 .mu.m, more preferably not greater than 0.2 .mu.m. The tap
density of the non-magnetic powder is from 0.05 to 2 g/ml,
preferably from 0.2 to 1.5 g/ml The water content of the
non-magnetic powder is from 0.1% to 5% by mass, preferably from
0.2% to 3% by mass, more preferably from 0.3% to 1.5% by mass. The
pH value of the non-magnetic powder is preferably from 2 to 11,
particularly from 5.5 to 10. The specific surface area of the
non-magnetic powder is from 1 to 100 m.sup.2/g, preferably from 5
to 80 m.sup.2/g, more preferably from 10 to 70 m.sup.2/g. The
crystalline size of the non-magnetic powder is preferably from
0.004 .mu.m to 1 .mu.m, more preferably from 0.04 .mu.m to 0.1
.mu.m. The DBP (dibutyl phthalate) oil adsorption of the
non-magnetic powder is from 5 to 100 ml/100 g, preferably from 10
to 80 ml/100 g, more particularly from 20 to 60 ml/100 g. The
specific gravity of the non-magnetic powder is from 1 to 12,
preferably from 3 to 6. The non-magnetic powder may be in any form
such as needle, sphere, polyhedron and tablet. The Mohs hardness of
the non-magnetic powder is not smaller than 4, preferably not
smaller than 10 The SR (stearic acid) adsorption of the
non-magnetic powder is from 1 to 20 .mu.mol/m.sup.2, preferably 2
to 15 .mu.mol/m.sup.2, more preferably from 3 to 8 .mu.mol/m.sup.2.
The pR value of the non-magnetic powder is preferably from 3 to 6.
The non-magnetic powder is preferably subjected to surface
treatment with Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2,
SnO.sub.2, Sb.sub.2O.sub.3, ZnO or Y.sub.2O.sub.3. Particularly
preferred among these surface has treatments are Al.sub.2O.sub.3,
SiO.sub.2, TiO.sub.2, and ZrO.sub.2 from the standpoint of
dispersibility. Even more desirable from these surface treatments
are Al.sub.2O.sub.3, SiO.sub.2, and ZrO.sub.2. These surface
treatments may be used singly or in combination. Alternatively,
co-precipitated surface-treated layer may be used depending on the
purpose The non-magnetic powder may be subjected to treatment with
alumina followed by the treatment of the surface layer with silica
or vice versa. The surface-treated layer may be porous depending on
the purpose but normally is preferably homogeneous and dense.
[0084] Specific examples of the non-magnetic powder to be used in
the formation of the underlying layer in the production process of
the invention include Nanotite (produced by Showa Denko K.K.),
HIT-100, ZA-G1 (produced by SUMITOMOCHEMICAL CO., LTD.),
.alpha.-hematite DPN-250, DPN-250BX, DPN-245, DPM-BX, DPN-500BX,
DBN-SA1, DSN-SA3 (produced by TODA KOGYO CORP.), titanium oxide
TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100,
.alpha.-hematite E270, E271, E300, E303 (produced by ISHIHARA
SANGYO KIAISHA, LTD.), titanium oxide STT-4D, STT-30D, STT-30,
STT-65C, .alpha.-hematiteoc-40 (produced by Titan Kogyo K.K.),
MT-100, MT-100T, MT-150W, MT-500B, MT-600B; MT-100F, MT-500HD
(produced by TAYCA CORP.), FINEX-25, BF-1, BF-10, BF-20, ST-M
(produced by SAKAI CHEMICAL INDUSTRY CO., LTD.), DEFIC-Y, DEFIC-R
(produced by DOWA MINING Co., LTD.), AS2BM, TiO2P25 (produced by
Nippon Aerosil Co., Ltd.), 100A, 500A (produced by Ube Industries,
Ltd.), and sintering products thereof. Particularly preferred among
these non-magnetic powders are titanium dioxide and .alpha.-iron
oxide.
[0085] By mixing the underlying layer with carbon black, the
surface electrical resistivity Rs can be lowered as a known effect.
Further, the light transmittance can be reduced, making it possible
to obtain a desired micro Vickers hardness. Moreover, by
incorporating carbon black in the underlying layer, the effect of
storing lubricant can be exerted. As the carbon black there may be
used furnace for rubber, thermal for rubber, black for color,
acetylene black or the like. The underlying layer should be
optimized for the following properties depending on the desired
effect. By optimizing these properties in combination, the desired
effect can be exerted more effectively.
[0086] The carbon black to be incorporated in the underlying layer
exhibits a specific surface area of from 100 to 500 m.sup.2/g,
preferably from 150 to 400 m.sup.2/g, and a DEP oil absorption of
from 30 to 400 ml/100 g. The particle diameter of the carbon black
is from 5 to 80 m.mu., preferably from 10 to 50 m.mu., more
preferably from 10 to 40 .mu.g. The carbon black preferably to
exhibits a pH value of from 2 to 10 (as measured by dipping
process), a water content of from 0.1 to 10%, and a tap density of
from 0.1 to 1 g/ml. Specific examples of the carbon black to be
used in the production process of the invention include BLACKPEALS
2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72 15 (produced by
Cabot Specialty Chemicals Inc.), #3050B, 43150B, 03250B, #3750B,
#3950B, #950, #970B, #850B, MA-600, MA-230, #4000, #4010 (produced
by Mitsubishi Chemical Corporation), CONDUCTEX SC RAVEN 8800, 8000,
7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, 1250
(produced by Columbian Carbon, Ltd.), and KETJENBLACK EC (produced
by Aczo Inc.). The carbon black may be surface-treated with a
dispersant, graphitized with a resin or partially graphitized on
the surface thereof before use. The carbon black may be previously
dispersed in a binder before being added to the coating compound.
The carbon black may be used in an amount of not greater than 50%
by mass based on the foregoing inorganic powder or not greater than
40% by mass based on the total mass of the non-magnetic layer.
These carbon blacks may be used singly or in combination. For the
details of carbon black which can be used in the production process
of the intention, reference can be made to "Kaabon Burakku Binran
(Handbook of Carbon Black)", Association of Carbon Black.
[0087] The underlying layer may also comprise an organic powder
incorporated therein depending on the purpose. Examples of the
organic powder to be incorporated in the underlying layer include
acryl styrene-based resin powder, benzoguanamine resin powder,
melamine-basedresin powder, and phthalocyanine-based pigment. Other
examples of the organic powder employable herein include
polyolefin-based resin powder, polyester-based resin powder,
polyamide-based resin powder, polyimide-based resin powder, and
polyethylene fluoride resin powder. The preparation of these
organic powders can be accomplished by the process disclosed in
JP-A-62-18564 and JP-A-60-255827
[0088] Referring to the binder resin, lubricant, dispersant,
additives and solvent to be used in the underlying layer and other
conditions, those described below can be used. In particular, for
the amount and kind of the binder resin, and the amount and kind of
additives and dispersant, any known technique concerning the
magnetic layer can be employed.
[0089] [Binder]
[0090] As the binder to be in the production process of the
invention there may be used a conventional known thermoplastic
resin, thermosetting resin, reactive resin or mixture thereof.
[0091] As the thermoplastic resin there may be used one having a
glass transition temperature of from -100.degree. C. to 150.degree.
C., a number-average molecular weight of from 1,000 to 200,00,
preferably from 10,000 to 100,000, and a polymerization degree of
from about 50 to 1,000. Examples of such a thermoplastic resin
include polymer or copolymer comprising as a constituent unit vinyl
chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid,
acrylic acid ester, vinylidene chloride, acrylonitrile, methacrylic
acid, methacrylic acid ester, styrene, butadiene, ethylene, vinyl
butyral, vinyl acetal and vinylether, polyethane resin, and various
rubber resins. Examples of the thermosetting resin or reactive
resin include phenol resin, epoxy resin, polyurethane resin, urea
resin, melamine resin, alkyd resin, acrylic reactive resin,
formaldehyde resin, silicone resin, epoxy-polyamide resin, mixture
of polyester resin and isocyanate prepolymer, mixture of
polyesterpolyol and polyisocyanate, and mixture of polyurethane and
polyisocyanate. For the details of these resins, reference can be
made to "Plastic Handbooks", Asakura Shoten. A known electron
radiation-curing resin may be incorporated in various layers. For
the details of such an electron radiation-curing resin and its
preparation process, reference can be made to JP-A-62-256219. These
resins may be used singly or in combination. Preferred examples of
these resins include vinyl chloride resin, combination of at least
one vinyl chloride copolymer selected from the group consisting of
vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl
acetate-vinyl alcohol copolymer and vinyl chloride-vinyl
acetate-maleic anhydride copolymer, and combination thereof with a
polyisocyanate.
[0092] As the polyurethane resin structure there may be used a
known polyurethane structure such as polyester polyurethane,
polyether polyurethane, polyether polyester polyurethane,
polycarbonate polyurethane, polyester polycarbonate polyurethane
and polycaprolactone polyurethane. All these exemplified binders
preferably comprise at least one polar group selected from the
group consisting of CooM, SO.sub.3M, OSO.sub.3M, P=O (OM).sub.2,
O-P=O(OM).sub.2 (in which M is a hydrogen atom or alkaline metal
salt), OH, NR.sub.2, N.sup.+R.sub.3 (in which R is a hydrocarbon
group), epoxy group, SH and CN introduced therein by
copolymerization or addition reaction as necessary to obtain a
better dispersibility and durability. The amount of such a polar
group is from 10.sup.-1 to 10.sub.-9 mols/g, preferably from
10.sup.-2 to .sub.10.sup.-6 mols/g.
[0093] Specific examples of these binders to be used in the
production process of the invention include VAGH, VYHH, VMCH, VAGF,
VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PHHH, PKHJ, PKHC, PKFE
(produced by Union Carbite Inc.), MPR-TA, MPR-TA5, MPR-TAL,
MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, MPR-TAC (produced by Nissin
Chemical Industry Co., Ltd.), 1000W, DX80, DX81 DX83, 100FD
(produced by DENKI KAGAKU KOGYO K.K.), MR-104, MR-105, MR110,
MR100, MR-555, 400X-110A (produced by Nippon Zeon Co., Ltd.),
Nippolan N2301, N2302, N2304 (produced by Nippon Polyurethane
Industry Co., Ltd.), Pandex T-5105, T-R3080, T-5201, BURNOCK D-400,
D-210-80, Crysbon 6109, 7209 (produced by DAINIPPON INK &
CHEMICALS, INC.), Vylon UR200, UR8300, URB7000 RV530, RV280
(produced by TOYOBO CO., LTD.), Daifelamine 4020, 5020, 5100, 5300,
9020, 9022, 7020 (produced by Dainichiseika Color & Chemicals
Mfg. Co., Ltd.), 5004 (produced by Mitsubishi Chemical
Corporation), SANPRENE SP-150 (produced by Sanyo Chemical
Industries, Ltd.), and Saran F310, F210 (produced by Asahi Chemical
Industry Co., Ltd.).
[0094] The binder to be incorporated in the non-magnetic layer and
magnetic layer in the production process of the invention is used
in an amount of from 5 to 50% by weight, preferably from 10 to 30%
by weight based on the weight of the non-magnetic powder or
ferromagnetic powder. The amount of a vinyl chloride-based
copolymer, if any, to be used is from 5 to 30% by weight. The
amount of a polyurethane resin, if any, to be used is from 2 to 20%
by weight. The amount of a polyisocyanate, if any, to be used is
from 2 to 20% by weight. For example, if a slight amount of
chlorine liberated corrodes the head, only the polyurethane resin
may be used optionally in combination with the isocyanate resin.
The polyurethane, if used herein, preferably exhibits a glass
transition temperature of from -50.degree. C. to 100.degree. C.,
preferably 0.degree. C. to 100.degree. C., an elongation at break
of from 100 to 2,000%, a stress at rupture of from 0.05 to 10 5
kg/cm.sup.2 (0.49 to 98.times.10.sup.6 Pa) and a yield point of
from 0.05 to 10 kg/cm.sup.2 (0.49 to 98.times.10.sup.6 Pa)
[0095] In the case where two or more layers are formed on the
non-magnetic support in the process for the production of a
magnetic disk of the invention, it goes without saying that the
amount of the binder, the amount of the vinyl chloride-based
polymer, polyurethane resin, polyisocyanate or other resins to be
incorporated in the binder, the molecular weight of various resins
constituting the magnetic layer, the amount of the polar group, and
the physical properties of these resins, etc. may IS vary from the
non-magnetic layer to the magnetic layer. Optimization should be
conducted in each of these layers. Any known techniques concerning
multi-layer magnetic layer can be applied. For example, if the
amount of the binder varies from layer to layer, it is effective to
increase the amount of the binder in the magnetic layer in order to
lessen scratch on the surface of the magnetic layer. The
improvement of touch of the magnetic recording medium to the
recording head can be accomplished by increasing the amount of the
binder in the non-magnetic layer so that they are rendered
flexible.
[0096] Examples of the polyisocyanate to be used in the production
process of the invention include isocyanates such as tolylene
diisocyanate, 4-4'-diphenylmethanediisocynate, hexamethylene
diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate,
o-toluidine diisocyanate, isophorone diisocyanate and
triphenylmethane triisocyanate, product of reaction of these
isocyanates with polyalcohols, and polyisocyanates produced by
condensation of isocyanates. Examples of trade name of commercially
available isocyanates include Colonate L, Colonate HL, Colonate
2030, Colonate 2031, Millionate MR, Millionate MTL (produced by
Nippon Polyurethane Industry Co.,, Ltd.), Takenate D-102, Takenate
D-11ON, Takenate D-200, Takenate D-202 (produced by Takeda Chemical
Industries Ltd.), and Desmodur L, Desmodur IL, Desmodur N, Desmodur
HL (produced by Sumitomo Bayer Co, Ltd.). These isocyanates may be
used singly in the various layers. Alternatively, two or more of
these isocyanates may be used in both the non-magnetic layer and
magnetic layer in combination to make the best use of difference in
hardening reactivity therebetween.
[0097] [Carbon black, abrasive]
[0098] As the carbon black to be incorporated in the magnetic layer
of the invention there may be used furnace black for rubber,
thermal black for rubber, black for color, acetylene black or the
like. The carbon black to be used here in preferably exhibits a
specific surface area of from 5 to 500m.sup.2/g, DBP oil absorption
of from 10 to 400 ml/100 g, a particle diameter of from 5 m.lambda.
to 300 m.mu., a pH value of from 2 to 10, a water content of from
0.1 to 10% by weight and a tap density of from 0.1 to 1 g/ml.
Specific examples of the carbon black to be used in the production
process of the invention include BLACKPEARLS 2000, 1300, 1000, 900,
905, 800, 700 and VULCANXC-72 (produced by Cabot Corporation), #80,
#60, #55, #50, #35 (produced by Asahi Carbon Co., Ltd.), #2400B,
#2300, #900, #1000, #30, #40, #10B (produced by Mitsubishi Chemical
Corporation), CONDUCTEX SC, RAVEN 150, 50, 40, 15, RAVEN-MT-P
(produced by Colombian Carbon S:D Corporation), and KETJENBLACK EC
(produced by Nihon EC), The carbon black may be subjected to
surface treatment with a dispersant, may be subjected to grafting
with a resin or graphitized partially on the surface thereof before
use. The carbon black may be dispersed with a binder before being
added it to the magnetic coating compound. These carbon black
powders may be used singly or in combination. The amount of the
carbon black, if used, is preferably from 0.1 to 30% by weight
based on the weight of the magnetic powder. The carbon black acts
to inhibit electrostatic charge of the magnetic layer, reduce the
friction coefficient of the magnetic layer, provide the magnetic
layer with light-screening properties or enhance the strength of
the magnetic layer depending on the kind of the carbon black used.
Accordingly, these carbon black powders of the invention can be of
course varied in their kind, amount and combination from the
magnetic layer to the underlying layer and selected properly
depending on the purpose on the basis of the foregoing various
properties such as particle size, oil absorption, electrical
conductivity and pH value. Optimization should be conducted in each
of these layers. For the details of carbon black to be used in the
magnetic layer in the production process of the invention,
reference can be made to "Kaabon Burakku Binran (Handbook of Carbon
Black)", Association of Carbon Black.
[0099] As abrasives to be used in the production process of the
invention there may be mainly used known materials having a Mohs
hardness of not lower than 6 such as .alpha.-alumina having a
percent .alpha. conversion of not lower than 90%, .beta.-alumina,
silicon carbide, chromium oxide, cerium oxide, .alpha.-iron oxide,
corrundum, artificial diamond, silicon nitride, silicon carbide,
titanium carbite, titanium oxide, silicon dioxide and boron
nitride, singly or in combination. Alternatively, a composite of
these abrasives (abrasive surface-treated with another abrasive)
may be used. These abrasives occasionally contain compounds or
elements other than main components. However, these abrasives
remain the same in their effect so far as the content of the main
components is not lower than 90% by weight. The particle size of
these abrasives is preferably from 0.01 to 2 .mu.m. In order to
enhance electromagnetic properties in particular, it is preferred
that these abrasives have a sharp distribution of particle size.
Further, in order to enhance durability, abrasives having different
particle sizes may be used in combination as necessary.
Alternatively, a single abrasive having a wide distribution of
particle diameter may be used to exert the same effect. The
abrasive to be used in the invention preferably exhibits a tap
density of from 0.3 to 2 g/cc, a water content of from 0.1 to 5% by
weights a pH value of from 2 to 11 and a specific surface area of
from 1 to 30 m.sup.2/g. The shape of the abrasive to be used in the
invention may be any of needle, sphere and cube. The abrasive
preferably has a partly cornered shape to have high grinding
properties. Specific examples of the abrasive employable herein
include AKP-12, AKP-15, AKP-20, AKP-30, AKP-50, HIT-20, HIT-55,
HIT-60, HIT-70, HIT-80, HIT-100 (produced by SUMITOMO CHEMICAL CO.,
LTD.), ERC-DBM, HP-DAM, HPS-DBM (produced by Reynolds Inc.), WA1000
(produced by Fujimi Incorporated), UB20 (produced by Uyemura CO.,
LTD.), G5 (produced by Nippon Chemical Industrial Co., Ltd.),
ChromexU2 (produced by CHROMEX INC.), TF-100, TF-140 (produced by
TODA KOGYO CORP.), Beta Random Ultrafine (produced by IBIDEN CO.,
LTD.), and B-3 (produced by SHOWA MINING CO., LTD). These abrasives
may be incorporated in the non-magnetic layer as necessary. By
incorporating these additives in the non-magnetic layer, the
surface shape of the non-magnetic layer or the degree of protrusion
of abrasive can be controlled. Of course, the particle diameter and
amount of the abrasive to be incorporated in the magnetic layer and
non-magnetic layer should be optimized
[0100] [Additive]
[0101] As additives to be used in the formation of the magnetic
layer and non-magnetic layer in the production process of the
invention there may be used those having lubricating effect,
antistatic effect, dispersing effect, plasticizing effect, etc
Examples of these additives employable herein include molybdenum
disulfide, tungsten disulfide, graphite, boron nitride, fluorinated
graphite, silicone oil, silicone having polar group, aliphatic
acid-modified silicone, fluorine-containing silicone,
fluorine-containing alcohol, fluorine-containing ester, polyolefin,
polyglycol, alkylphosphoric acid ester, alkaline metal salt
thereof, alkylsulfuric acid ester, alkaline metal salt thereof,
polyphenylether, phenylphosphonic acid, .alpha.-naphthylphosphoric
acid, phenylphosphoric acid, diphenylphosphoric-acid,
p-ethylbenzenephosphonic acid, phenylphosphinic acid,
aminoquinones, various silane coupling agents, titanium coupling
agents, fluorine-containing alkylsulfuric acid ester, alkaline
metal salt thereof, monobasic aliphatic acid having from 10 to 24
carbon atoms (which may have unsaturated bonds or may be branched),
salt thereof with a metal (e.g., Li, Na, K, Cu), monoaliphatic acid
ester, dialiphatic acid ester or trialiphatic acid ester of any one
of monovalent, divalent, trivalent, tetravalent, pentavalent and
hexavalent alcohols having from 12 to 22 carbon atoms (which may
have unsaturated bonds or may be branched), alkoxyalcohol having
from 12 to 22 carbon atoms, monobasic aliphatic acid having from 10
to 24 carbon atoms (which may have unsaturated bonds or may be
branched) and monovalent, divalent, trivalent, tetravalent,
pentavalent and hexavalent alcohols having from 2 to 12 carbon
atoms (which may have unsaturated bonds or may be branched),
aliphatic acid ester of monoalkylether of alkylene oxide polymer,
aliphatic acid amide having from 8 to 22 carbon atoms, and
aliphatic amine having from 8 to 22 carbon atoms.
[0102] Specific examples of these compounds include aliphatic acids
such as capric acid, caprylic acid, lauric acid, myristic acid,
palmitic acid, stearic acid, behenic acid, oleic acid, elaidic
acid, linoleic acid, linolenic acid and isostearic acid, esters
such as butyl stearate, octyl stearate, amyl stearate, isooctyl
stearate, butyl myristate, octyl myristate, butoxy ethyl stearate,
butoxy diethyl stearate, 2-ethylhexyl stearate, 2-octytldodecyl
palmitate, 2-hexylhexyl stearate, 2-octyldodecyl palmitate,
2-hexyldodecyl palmitate, isohexadecyl stearate, oleyl oleate,
dodecyl stearate, tridecyl stearate, oleyl erucate, neopentyl
glycol didecanoate and ethylene glycoldioleate, and alcohols such
as oleyl alcohol, stearyl alcohol and lauryl alcohol. Other
examples of additives employable herein include nonionic surface
active agent such as alkylene oxide-based surface active agent,
glycerin-based surface active agent, glycidol-based surface active
agent and alkylphenol ethylene oxide adduct, cationic surface
active agent such as cyclic amine, esteramide, quaternary ammonium
salt, hydantoin derivative, heterocyclic group, phosphonium and
sulfonium, anionic surface active agent containing acid group such
as carboxylic acid, sulfonic acid, phosphoric acid, sulfuric acid
ester group and phosphoric acid ester group, and amphoteric surface
active agent such as amino acid, aminosulfonic acid, sulfuric or
phosphoric acid ester of amino alcohol and alkylbetaine. For the
details of these surface active agents, reference can be made to
"Kaimen Kasseizai Binran (handbook of Surface Active Agents",
Sangyo Tosho. These lubricants, antistats or other additives may
not be 100% pure but may contain impurities such as unreacted
matter, by-product, decomposition product and oxide besides main
components. The content of these impurities is preferably not
higher than 30%, more preferably not higher than 10%.
[0103] These lubricants and surface active agents to be used in the
production process of the invention have different physical
actions. The kind and amount of these lubricants and surface active
agents and the proportion of lubricant to be used in combination
should be optimized depending on the purpose For example, the
aliphatic acids to be incorporated in the non-magnetic layer and
the magnetic layer may differ from each other in melting point to
keep the oozing to the surface of the magnetic recording medium
under control. The esters to be used in the non-magnetic layer and
the magnetic layer may differ from each other in boiling point or
polarity to keep the oozing to the surface of the magnetic
recording medium under control. The amount of the surface active
agents to be used in the non-magnetic layer and the magnetic layer
may be properly adjusted to improve the coating stability. The
amount of the lubricant to be incorporated in the underlying layer
may be greater than that in the magnetic layer to enhance the
lubricating effect. Of course, the present invention is not limited
to these proposals. In general, the total amount of lubricant to be
used is predetermined to be from 0.1% to 50%, preferably from 2% to
25% based on the amount of the ferromagnetic powder or non-magnetic
powder.
[0104] The additives to be used in the invention may be entirely or
partly added at any steps during the preparation of the magnetic
coating compound and non-magnetic coating compound. For example,
these additives may be mixed with the magnetic powder before the
kneading step. Alternatively, these additives may be added at the
step of kneading the magnetic powder with the binder and solvent.
These additives may be added at, after or shortly before the
dispersion step. Depending on the purpose, the application of the
magnetic layer may be followed by simultaneous or successive
coating of a part or whole of these additives. Alternatively,
depending on the purpose, the magnetic layer which has been
calendered or slit may be coated with a lubricant.
[0105] As the organic solvent to be used in the production process
of the invention there may be used a known organic solvent. For
example, solvents disclosed in JP-A-6-68453 may be used.
[0106] [Layer arrangement]
[0107] The thickness of the magnetic disk formed by the production
process of the invention is from 2 .mu.m to 100 .mu.m, preferably
from 10 .mu.m to 80 .mu.m. In the case where a single magnetic
layer is provided, the thickness of the magnetic layer is
preferably from 0.02 .mu.m to 2.0 .mu.m, more preferably from 0.02
.mu.m to 0.5 .mu.m
[0108] An undercoating layer may be provided interposed between the
support and the non-magnetic layer or magnetic layer for the
purpose of enhancing the adhesion therebetween. The thickness of
the undercoating layer is from 0.01 .mu.m to 0.5 .mu.m, preferably
from 0.02 .mu.m to 0.5 .mu.m.
[0109] In the case where the magnetic layer is provided on one side
of the support, a back coat layer may be provided on the side of
the support opposite the magnetic layer to exert an effect of
preventing electrostatic charge or correcting curling. The
thickness of the back coat layer is from 0.1 .mu.m to 4 .mu.m,
preferably from 0. 3 .mu.m to 2.0 .mu.m. As the undercoating layer
and back coat layer there may be used any known materials.
[0110] The thickness of the magnetic layer in the magnetic disk
formed by the production process of the invention is optimized
according to the saturated magnetization of the head to be used on
the magnetic disk thus obtained, the length of head gap and the
band width of signal to be recorded. The magnetic layer may be
divided into two or more layers having different magnetic
properties. Any known multi-layer magnetic layer arrangement may be
employed.
[0111] In the case where the magnetic disk formed by the production
process of the invention is of ATOMM type, the thickness of the
non-magnetic layer as underlying layer is normally from not smaller
than 0.2 .mu.m to not greater than 5.0 .mu.m, preferably from not
smaller than 0.3 .mu.m to not greater than 3.0 .mu.n, even more
preferably from not smaller than 1.0 .mu.m to not greater than 2.5
.mu.m. In this case, the underlying layer. in the magnetic disk can
exert its effect so far as it is a substantially non-magnetic
layer. Even when the underlying layer contains a small amount of a
magnetic material as an impurity or intentionally, the desired
effect can be exerted. Thus, it goes without saying that this
structure can be regarded as the same structure as the magnetic
disk obtained by the production process of the invention. The term
"substantially non-magnetic layer" as used herein is meant to
indicate that the underlying layer exhibits a residual magnetic
reflux density of not greater than 30 mT (300 G) or a coercive
force of not greater than 24 kA/m (300 Oe), preferably has no
residual magnetic reflux density and coercive force.
[0112] [Support]
[0113] As the support to be used in the production process of the
invention there maybe used any known film such as polyester (e.g.,
polyethylene terephthalate, polyethylene naphthalate), polyolefin,
cellulose triacetate, polycarbonate, polyamide, polyimide,
polyamidimide, polysulfone, aramide and aromatic polyamide. A high
strength support such as polyethylene naphthalate and polyamide is
preferably used. If necessary, a laminated type support as
disclosed in JP-A-3-224127 may be used to change the surface
roughness of the magnetic layer and base. The support to be used
herein maybe subjected to corona discharge treatment, plasma
treatment, treatment for easy adhesion, heat treatment, dusting,
etc. before use. Alternativelyi as the support to be used in the
production process of the invention there may be used an aluminum
or glass substrate.
[0114] In order to accomplish the basic desired object, a support
having a central area average surface roughness SRa of not greater
than 8.0 nm, preferably not greater than 4.0 nm, more preferably
not greater than 2.0 nm as determined by mirau method of TOPO-3D
produced by WYKO INC. is preferably used. The support preferably
not only has a small central line average roughness but also has no
big protrusions having a size of not smaller than 0.5 .mu.m. If
necessary, the roughness shape of the surface of the support may be
freely controlled by the size and amount of filler to be
incorporated therein. Examples of the filler include oxide and
carbonate of Ca, Si and Ti, and fine powder of organic material
such as acryl. The support preferably has a maximum height SRmax of
not greater than 1 .mu.m, a ten point average roughness SRz of not
greater than 0.5 .mu.m, a central area rise height SRp of not
greater than 0.5 .mu.m, a central area valley depth SRv of not
greater than 0.5 .mu.m, a central area rate SSr of from not smaller
than 10% to not greater than 90%, and an average wavelength Spa of
from not smaller than 0.5 .mu.m to not greater than 300 .mu.m. In
order to obtain desired electromagnetic properties and durability,
the surface rise distribution on the support can be arbitrarily
controlled by using a filler. In some detail, from 0 to 2, 000
filler particles each having a size of from 0.01 .mu.m to 1 .mu.m
may be used per 0.1 mm.sup.2.
[0115] The support to be used in the production process of the
invention preferably exhibits F-5 value of from 5 to 50 Kg/mm.sup.2
(4.9 to 49.times.10.sup.7 Pa) and a thermal shrinkage coefficient
of not greater than 3%, more preferably not greater than 1.5% at
100.degree. C. for 30 minutes, or not greater than 1%, more
preferably not greater than 0.5% at 80.degree. C. for 30 minutes.
The support exhibits a breaking strength of from 5 to 100
Kg/mm.sup.2 (4.9 to 98.times.10.sup.7 Pa) and an elastic modulus of
from 100 to 2, 000 Kg/mm.sup.2 (98 to 1,960.times.10.sub.7 Pa). The
support exhibits a thermal expansion coefficient of from 10.sup.-4
to 10.sup.-8/.degree. C., preferably from 10.sup.-5 to
10.sup.6/.degree. C. The support exhibits a hygroscopic expansion
coefficient of not greater than 10.sup.-4/RH %, preferably
10.sup.-6/RH %. The thermal properties, dimensional properties and
mechanical strength of the support are preferably equal in the
various in-plane directions with a difference of not greater than
10%.
[0116] [Production process]The process for the production of the
magnetic disk of the invention involves at least a kneading step, a
dispersing step, and a mixing step optionally provided before and
after these steps. These steps may each consist of two or more
stages. All the starting materials to be used in the invention such
as ferromagnetic powder, non-magnetic powder, binder, carbon black,
abrasive, antistat, lubricant and solvent may be added at the
beginning of or in the course of any steps. These starting
materials may be each batchwise added at two or more steps. For
example, a polyurethane resin may be added batchwise at a kneading
step, a dispersing step, and a mixing step for adjusting the
viscosity of the coating solution thus dispersed. In order to
accomplish the objects of the invention, a conventional known
production technique may be used as a part of the production
process. At the kneading step, advice having a high kneading force
such as open kneader, continuous kneader, pressure kneader and
extruder can be used. In the case where a kneader is used, the
magnetic powder is kneaded with whole or part of the non-magnetic
powder (preferably not lower than 30% of the total amount of the
binder) in an amount of from 15 to 500 parts by weight based on 100
parts by weight of the magnetic powder. For the details of the
kneading steps reference can be made to Japanese Patent Application
No. 62-264722 and Japanese Patent Application No. 62-236872. In the
case where the coating solution for the non-magnetic layer and
non-magnetic layer are prepared, dispersing media having a high
specific gravity such as zirconia beads, titania beads and steel
beads are preferably used. The particle diameter and packing of
these dispersing media are optimized for use. As the dispersing
machine there may be used any known dispersing machine.
[0117] In the case where a multi-layer magnetic disk is produced by
coating according to the production process of the invention, the
following process is preferably used. Firstly, a lower layer is
applied using a coating device commonly used in the application of
magnetic coating compound such as gravure coating device, roll
coating device, blade coating device and extrusion coating device.
While the lower is wet, an upperlayer is applied using a support
pressure type extrusion coating device disclosed in,JP-B-1-46186,
JP-A-60-238179, and JP-A-2-265672. Secondly, an upper layer and a
lower layer are almost simultaneously applied using one coating
head having two coating solution passing slits as disclosed in
JP-A-63-88080, JP-A-2-17971, and JP-A-2-265672. Thirdly, an upper
layer and a lower layer are almost simultaneously applied using an
extrusion coating device with a backup roll as disclosed in
JP-A-2-174965. In order to prevent the deterioration of
electromagnetic characteristics of the magnetic disk due to
agglomeration of magnetic particles, it is preferred that a method
disclosed in JP-A-62-95174 and JP-1-236968 be used to provide the
coating solution in the coating head with shear. The coating
solution needs to have a viscosity falling within the range defined
in Japanese Patent Application No. 1-312659. In order to realize
the desired arrangement of magnetic disk according to the
production process of the invention, a successive multi-layer
coating process may be, of course, used involving the application
and drying of the underlying layer followed by the provision of the
magnetic layer. Even when this successive multi-layer coating
process is used, the effect of the invention cannot be impaired.
However, in order to lessen coating defectives and hence improve
quality such as dropout resistance, the foregoing simultaneous
multi-layer coating process is preferably used.
[0118] The orientation is preferably conducted by controlling the
temperature and amount of drying air and the coating speed so that
the drying position of coat layer can be controlled. The coating
speed is preferably from 20 to 1,000 m/min. The temperature of the
drying air is preferably not lower than 60.degree. C. A proper
previous drying can be conducted before the coated material enters
in the magnet zone.
[0119] As the calendering roll there may be used a heat-resistant
plastic roll made of epoxy, polyimide, polyamide, polyimidamide or
the like. Alternatively, metallic rolls may be used opposed to each
other if a double-sided magnetic layer is formed. The calendering
temperature is preferably not lower than 50.degree. C., more
preferably not lower than 100.degree. C. The linear pressure is
preferably not lower than 200 kg/cm (1.96.times.10.sup.7 Pa/cm),
more preferably not lower than 300 kg/cm (2.94.times.10.sup.7
Pa/cm).
[0120] [Physical Properties]
[0121] The magnetic disk produced by the production process of the
invention preferably exhibits a surface resistivity of from
10.sup.4 to 10.sup.12 .OMEGA./sq on the magnetic surface thereof
and a charged potential of from -500 V to +500 V. The magnetic
layer preferably has an elastic modulus of from 100 to 2, 000
kg/mm.sup.2 at 0.5% elongation in the various directions in plane
and a break strength of from 10 to 70 kg/cm.sup.2 (9.8 to 58.6
.times.10.sup.7 Pa) The magnetic disk preferably has an elastic
modulus of from 100 to 1,500 kg/mm.sup.2 in the various directions
in plane and a residual elongation of not greater than 0.5%, and a
thermal shrinkage factor of not greater than 1%, more preferably
not greater than 0.5%, most preferably not greater than 0.1% at all
temperatures lower than 100.degree. C. The content of residual
solvent in the magnetic layer is preferably not greater than
100mg/m.sup.2, more preferably not greater than 10 mg/m.sup.2. The
content of residual solvent in the coat layer is preferably not
greater than 30 vol-%, more preferably not greater than 20 vol-%
both in the underlying layer and magnetic layer. The void is
preferably not greater than 30vol-%, more preferably not greater
than 20 vol-% both in the non-magnetic layer and magnetic layer.
The void is preferably small to attain a high output. However, it
is occasionally preferred that the void be greater than a certain
value depending on the purpose. Further, since the repeated use is
thought much of, the coat layer preferably has a great void to
exhibit a good running durability.
[0122] The central area surface roughness Ra of the magnetic layer
as measured by mirau method of TOPO-3D is not greater than 4.0 nm,
preferably not greater than 3.8 nm, more preferably not greater
than 3.5 nm. The magnetic layer preferably exhibits a maximum
height SRmax of not greater than 0.5 .mu.m, a ten point average
roughness SRz of not greater than 0.3 .mu.m, a central area rise
height SRp of not greater than 0.3 .mu.m, a central area valley
depth SRv of not greater than 0.3 .mu.m, a central area rate SSr of
from not smaller than 20% to not greater than 80%, and an average
wavelength Spa of from not smaller than 5 .mu.m to not greater than
300 .mu.m. The surface rise on the magnetic layer can be
arbitrarily predetermined to be from 0 to 2,000 rises each having a
size of from 0.01 .mu.m to 1 .mu.m per 0.1 mm.sup.2. In this
manner, the electromagnetic properties and frictional coefficient
of the magnetic layer is preferably optimized. This can be easily
controlled by controlling the surface properties of the filler to
be incorporated in the support or controlling the particle diameter
and amount of the powder to be incorporated in the magnetic layer
and the surface shape of the calendering roll. The curling of the
magnetic layer is preferably within .+-.3 mm (according to the
sheet sample measuring process defined in IS018910).
[0123] In the case where the magnetic disk comprises a non-magnetic
layer and a magnetic layer, it can be easily presumed that these
physical properties can be varied from the non-magnetic layer to
the magnetic layer depending on the purpose. For example, the
elastic modulus of the magnetic layer can be enhanced to improve
the running durability thereof. At the same time, the elastic
modulus of the non-magnetic layer can be lowered from that of the
magnetic layer to improve the touch of the magnetic disk to the
head.
[0124] The present invention will be further described in the
following examples, but the present invention should not be
construed as being limited thereto.
[0125] The following predetermined coating compound was prepared.
The coating compound was then applied to a support to produce a
magnetic disk.
[0126] <Preparation of Coating Compound>
1 Magnetic coating compound A Ferromagnetic metal powder 100 parts
by mass Formulation: Fe 70%; Co 30% Hc, major axis length: see
FIGS. 1A and 1B Crystalline size: 150 angstrom .sigma.s: 150 emu/g
Sintering inhibitor: Al compound (Al/Fe atomic ratio: 14%) Y
compound (Y/Fe atomic ratio: 7%) Vinyl chloride copolymer 10 parts
by MR110 (produced by Nippon Zeon Co., Ltd.) mass Polyurethane
resin 4 parts by UR8200 (produced by TOYOBO CO., LTD.) mass
.alpha.-Alumina 5 parts by HIT60 (produced by SUMITOMO mass
CHEMICAL CO., LTD.) Carbon black 1 part by #50 (produced by Asahi
Carbon Co., Ltd.) mass Phenylphosphonic acid 3 parts by mass
n-Butyl stearate 3 parts by mass Butoxyethyl stearate 3 parts by
mass Ethylene glycol dioleate 6 parts by mass Stearic acid 1 part
by mass Oleic acid 1 part by mass Methyl ethyl ketone 140 parts by
mass Cyclohexanone 200 parts by mass Magnetic coating compound B
Hexagonal barium ferrite 100 parts by mass Surface treatment:
Al.sub.2O.sub.3 5% by mass; SiO.sub.2: 2% by mass Hc, plate
diameter; see FIGS. 1A and 1B Tabularity ratio: 3 .sigma.s: 56
emu/g Vinyl chloride copolymer 6 parts by MR110 (produced by Nippon
Zeon Co., Ltd.) mass Polyurethane resin 3 parts by UR8200 (produced
by TOYOBO CO., LTD.) mass .alpha.-Alumina 5 parts by HIT60
(produced by SUMITOMO mass CHEMICAL CO., LTD.) Carbon black 1 part
by #50 (produced by Asahi Carbon Co., Ltd.) mass n-Butyl stearate 3
parts by mass Butoxyethyl stearate 3 parts by mass Ethylene glycol
dioleate 6 parts by mass Stearic acid 1 part by mass Oleic acid 1
part by mass Methyl ethyl ketone 80 parts by mass Cyclohexanone 120
parts by mass Non-magnetic coating compound Non-magnetic powder:
TiO.sub.2 crystalline rutile 100 parts by Average primary particle
diameter: 0.035 .mu.m mass Specific surface area: 40 m.sup.2/g pH:
7 TiO.sub.2 content: 90% or more DBP oil absorption: 27-38 g/100 g
Surface treatment: Al.sub.2O.sub.3, SiO.sub.2 Carbon black 13 parts
by KETJENBLACK EC mass Vinyl chloride copolymer 17 parts by MR110
(produced by Nippon Zeon Co., Ltd.) mass Polyurethane resin 6 parts
by UR8600 (produced by TOYOBO CO., LTD.) mass Phenylphosphonic acid
3 parts by mass Ethylene glycol dioleate 8 parts by mass n-Butyl
stearate 4 parts by mass Butoxyethyl stearate 4 parts by mass Oleic
acid 1 part by mass Stearic acid 1 part by mass Methyl ethyl ketone
120 parts by mass Cyclohexanone 180 parts by mass
[0127] Production Process 1
[0128] For each of the coating compounds, the various components
were kneaded by a kneader, and then subjected to dispersion by a
sand mill. To the dispersions thus obtained were then added a
polyisocyanate in an amount of 13 parts by mass for the coating
solution of non-magnetic layer, 4 parts by mass for the coating
solution of magnetic layer A and 5 parts by mass for the coating
solution of magnetic layer B. To these dispersions were then each
added 30 parts by mass of cyclohexanone. These dispersions were
then each filtered through a filter having an average pore diameter
of 1 .mu.m to prepare a non-magnetic layer-forming coating solution
and a magnetic layer-forming coating solution.
[0129] The non-magnetic layer coating solution was applied to a
polyethylene terephthalate support having a thickness of 62 .mu.m
and a central area average surface roughness of 3 nm to a dry
thickness of 1.5 .mu.m. The magnetic layer coating solution was
immediately applied to the polyethylene terephthalate support to a
dry thickness of 0.15 .mu.m in a simultaneous multi-layer coating
manner. While the two layers were wet, the coated material was
subjected to random orientation under the following orientation
conditions 1 to 3. After dried, the coated material was processed
through a 7-stage calender at a temperature of 90.degree. C. and a
linear pressure of 300 kg/cm (2.94.times.10.sup.7 Pa/cm), and then
punched into a 3.7 inch diameter disk. The disk was subjected to
surface polishing, put in a 3.7 inch cartridge (zip-disk cartridge
produced by Iomega Corporation), and then provided with
predetermined mechanism parts to obtain a 3.7 inch floppy disk.
[0130] Production Process 2
[0131] A non-magnetic layer-forming coating solution and a magnetic
layer-forming coating solution were prepared in the same manner as
in the production process 1. The non-magnetic layer coating
solution thus prepared was applied to a polyethylene terephthalate
support having a thickness of 62 .mu.m and a central area average
surface roughness of 3 nm to a dry thickness of 1.5 .mu.m, and then
dried. The magnetic layer coating solution was then applied to the
polyethylene terephthalate support to a dry thickness of 0.15
.mu.m. While the magnetic layer was wet, the coated material was
subjected to random orientation under the following orientation
conditions 1 to 3. After dried, the coated material was processed
through a 7-stage calender at a temperature of 90.degree. C. and a
linear pressure of 300 kg/cm (2.94.times.10.sup.7 Pa/cm), and then
punched into a 3.7 inch diameter disk. The disk was subjected to
surface polishing, put in a 3.7 inch cartridge (zip-disk cartridge
produced by Iomega Corporation), and then provided with
predetermined mechanism parts to obtain a 3.7 inch floppy disk.
[0132] Production Process 3
[0133] A magnetic layer-forming coating solution obtained in the
same manner as in the production process 1 was applied to a
polyethylene terephthalate support having a thickness of 62 .mu.m
and a central area average surface roughness of 3 nm to a dry
thickness of 1.5 .mu.m. While the magnetic layer was wet, the
coated material was subjected to random orientation under the
following orientation conditions 1 to 3. After dried, the coated
material was processed through a 7-stage calender at a temperature
of 90.degree. C. and a linear pressure of 300 kg/cm (2.94
.times.10.sup.7 Pa/cm), and then punched into a 3.7 inch diameter
disk. The disk was subjected to surface polishing, put in a 3.7
inch cartridge (zip-disk cartridge produced by Iomega Corporation),
and then provided with predetermined mechanism parts to obtain a
3.7 inch floppy disk.
[0134] Orientation Condition 1
[0135] The coated material was passed through an alternating
magnetic field generator at a frequency and an intensity of
magnetic field set forth in Table 1.
[0136] Orientation Condition 2
[0137] A pair of same-pole-opposed Co magnets with the support
interposed therebetween was provided so that the longitudinal
direction of the magnet pair was in parallel to the conveying
direction of the support, and a magnetic field was applied in such
an arrangement that the center of the gap has an intensity of
magnetic field set forth in Table 1 so as to subject magnetic
particles to orientation. The coated material was then passed
through the foregoing alternating magnetic field generator.
orientation condition 3
[0138] The magnetic fields in the directions of the oblique
direction and the reverse oblique direction were applied with at
least one set of pairs of same-pole-opposed Co magnets whose
longitudinal direction axis was arranged with the angle with the
longitudinal direction of support set forth in Table 1. The coated
material was then passed through the foregoing alternating magnetic
field generator
[0139] The samples thus obtained were each then evaluated by the
following properties The results are set forth in Table 1
[0140] Electromagnetic properties
[0141] Measurement of S/N ratio
[0142] For the measurement of S/N ratio, a Type RWA1001 disk
evaluating device produced by GUZIK INC. (U.S.A.) and a Type LS-90
spin stand produced by Kyodo Electronics System Co., Ltd. were
used. Using a metal-in gap head having a gap length of 0.3 .mu.m,
there produced output (TAA) at a linear recording density of 90KFCI
and the noise level after DC erase were measured at a radius of
24.6 mm to determine S/N ratio.
[0143] Measurement of modulation
[0144] Using the same conditions and apparatus as in the
measurement of reproduced output, the maximum Vmax and minimum Vmin
in one cycle of reproduced waveform were measured. Modulation was
determined by the following equation:
{(Vmax-Vmin)/(Vmax+Vmin)}.times.100 (%)
[0145] Measurement of Hc and SQn of magnetic layer
[0146] Using a vibrating sample type magnetic flux meter (produced
TOEI INDUSTRY CO., LTD.), measurement was made at Hm of 10 KOe.
2 TABLE 1 Opposing magnets Alternating having the same polarity
magnetic field Average Angle to Angle between Intensity major axis
Intensity longi- Number magnets of Mag- length or Produc- Direction
of tudinal of facing in magnetic Fre- Sample netic Hc average plate
tion of magnetic direction magnets opposite field quency S/N % Mod-
No. solution (Oe) dia. (.mu.m) process orientation field (Oe)
(.degree.) (set) directions (Oe) (Hz) (dB) ulation 1 A 2,300 0.075
1 1 -- -- -- -- 250 55 0.0 100 2 A 2,300 0.075 1 2 1,000 90 -- --
250 55 1.0 85 3 A 2,300 0.075 1 3 1,000 45 1 90 250 55 5.0 100 4 A
2,300 0.075 1 3 700 45 1 90 250 55 2.0 100 5 A 2,300 0.075 1 3
2,000 45 1 90 250 55 5.0 100 6 A 2,300 0.075 1 3 5,000 45 1 90 250
55 4.5 100 7 A 2,300 0.075 1 3 10,000 45 1 90 250 55 4.0 95 8 A
2,300 0.075 1 3 1,000 55 1 70 250 55 4.0 90 9 A 2,300 0.075 1 3
1,000 50 1 80 250 55 5.0 100 10 A 2,300 0.075 1 3 1,000 40 1 100
250 55 5.0 100 11 A 2,300 0.075 1 3 1,000 35 1 110 250 55 4.0 90 12
A 2,300 0.075 1 3 1,000 45 2 90 250 55 5.5 100 13 A 2,300 0.075 1 3
1,000 45 3 90 250 55 60 100 14 A 2,300 0.075 1 3 2,000 45 1 90 OFF
OFF 1.0 85 15 A 2,300 0.075 1 3 2,000 45 1 90 30 55 3.0 90 16 A
2,300 0.075 1 3 2,000 45 1 90 1,000 55 4.0 100 17 A 2,300 0.075 1 3
2,000 45 1 90 2,000 55 5.0 100 18 A 1,200 0.075 1 3 1,000 45 1 90
250 55 1.0 100 19 A 1,400 0.075 1 3 1,000 45 1 90 250 55 3.0 100 20
A 3,500 0.075 1 3 1,000 45 1 90 250 55 6.0 100 21 A 3,800 0.075 1 3
1,000 45 1 90 250 55 1.0 100 22 A 2,300 0.2 1 3 1,000 45 1 90 250
55 1.0 100 23 A 2,300 0.1 1 3 1,000 45 1 90 250 55 3.0 100 24 A
2,300 0.05 1 3 1,000 45 1 90 250 55 4.0 100 25 A 2,300 0.075 2 3
1,000 45 1 90 250 55 3.0 100 26 A 2,300 0.075 3 1 -- -- -- -- 250
55 -2.0 100 27 A 2,300 0.075 3 2 1,000 90 -- -- 250 55 0.0 85 28 A
2,300 0.075 3 3 1,000 45 1 90 250 55 2.0 100 29 B 2,800 0.025 1 1
-- -- -- -- 250 55 0.0 100 30 B 2,800 0.025 1 2 1,000 90 -- -- 250
55 2.0 90 31 B 2,800 0.025 1 3 1,000 45 1 90 250 53 5.0 100
[0147] As can be seen in the results of Table 1, the samples
obtained by the production process involving the two-way
orientation in the oblique and the reverse oblique directions by
the application of the first external magnetic field and the random
orientation by the alternating second external magnetic field
exhibit remarkably improved S/N ratio and modulation as compared
with the comparative samples obtained by other processes.
[0148] In accordance with the process of a magnetic disk of the
invention, even if as a magnetic powder there is one having a small
particle size, the agglomeration of magnetic particles to each
other can be relaxed. Further, the orientation of vertical
component of magnetization can be inhibited. Thus, the drastic
reduction of noise can be attained. Moreover, the magnetic disk
according to the invention has an orientation ratio (Or) close to 1
in the plane of magnetic layer and a good modulation Accordingly,
the use of the production process of the invention makes it
possible to invariably obtain a large capacity magnetic disk having
a good S/N ratio and modulation suitable for digital recording.
[0149] This application is based on Japanese patent application JP
2000-357743, filed Nov. 24, 2000, the entire content of which is
hereby incorporated by reference, the same as if set forth at
length.
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