U.S. patent application number 09/052033 was filed with the patent office on 2001-06-14 for magnetic recording medium.
This patent application is currently assigned to MASASHI AONUMA, et al. Invention is credited to AONUMA, MASASHI, NOGUCHI, HITOSHI, SAITO, SHINJI, YAMAZAKI, NOBUO.
Application Number | 20010003630 09/052033 |
Document ID | / |
Family ID | 27466463 |
Filed Date | 2001-06-14 |
United States Patent
Application |
20010003630 |
Kind Code |
A1 |
AONUMA, MASASHI ; et
al. |
June 14, 2001 |
MAGNETIC RECORDING MEDIUM
Abstract
A magnetic recording medium is described, which comprises a
support having thereon a magnetic layer comprising a ferromagnetic
metal powder dispersed in a binder, which is a magnetic recording
medium for recording signals of from 0.17 to 2 G bit/inch.sup.2 of
areal recording density, wherein the coercive force of said
magnetic layer is 1,800 Oe or more, said ferromagnetic metal powder
comprises Fe and Co, and the atomic ratio of Al/(Fe+Co) is from 3.0
to 15.4%.
Inventors: |
AONUMA, MASASHI; (KANAGAWA,
JP) ; SAITO, SHINJI; (KANAGAWA, JP) ; NOGUCHI,
HITOSHI; (KANAGAWA, JP) ; YAMAZAKI, NOBUO;
(KANAGAWA, JP) |
Correspondence
Address: |
STROOCK & STROOCK & LAVAN
180 MAIDEN LANE
NEW YORK
NY
10038
|
Assignee: |
MASASHI AONUMA, et al
|
Family ID: |
27466463 |
Appl. No.: |
09/052033 |
Filed: |
March 30, 1998 |
Current U.S.
Class: |
428/843.3 ;
G9B/5.243; G9B/5.256 |
Current CPC
Class: |
Y10S 428/90 20130101;
G11B 5/70621 20130101; Y10T 428/257 20150115; Y10T 428/265
20150115; Y10T 428/24355 20150115; Y10T 428/25 20150115; G11B 5/70
20130101; Y10T 428/256 20150115 |
Class at
Publication: |
428/694.0BA |
International
Class: |
G11B 005/706 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 1997 |
JP |
P.HEI.9-80669 |
Mar 31, 1997 |
JP |
P.HEI.9-80670 |
Jul 28, 1997 |
JP |
P.HEI.9-201659 |
Aug 22, 1997 |
JP |
P.HEI.9-226698 |
Claims
What is claimed is:
1. A magnetic recording medium which comprises a support having
thereon a magnetic layer comprising a ferromagnetic metal powder
dispersed in a binder, which is a magnetic recording medium for
recording signals of from 0.17 to 2 G bit/inch.sup.2 of areal
recording density, wherein the coercive force of said magnetic
layer is 1,800 Oe or more, said ferromagnetic metal powder
comprises Fe and Co, and the atomic ratio of Al/(Fe+Co) is from 3.0
to 15.4%.
2. A magnetic recording medium which comprises a support having
thereon a magnetic layer comprising a ferromagnetic metal powder
dispersed in a binder, which is a magnetic recording medium for
recording signals of from 0.17 to 2 G bit/inch.sup.2 of areal
recording density, wherein the coercive force of said magnetic
layer is 1,800 Oe or more, said ferromagnetic metal powder
comprises Fe and Co, and the atomic ratio of the sum total of rare
earth elements/(Fe+ Co) is from 0.5 to 9.0%.
3. The magnetic recording medium as claimed in claim 1, wherein a
substantially nonmagnetic lower layer is provided between said
support and said magnetic layer.
4. The magnetic recording medium as claimed in claim 1, wherein
said magnetic recording medium is a disc-like magnetic recording
medium.
5. The magnetic recording medium as claimed in claim 1, wherein
said magnetic layer has a dry thickness of from 0.05 to 0.30 .mu.m
and .phi.m of from 10.0.times.10.sup.-3 to 1.0.times.10.sup.-3
emu/cm.sup.2.
6. The magnetic recording medium as claimed in claim 1, wherein
said ferromagnetic metal powder comprises Fe and Co, and the atomic
ratio of the sum total of rare earth elements/Al is from 0.05 to
1.20.
7. The magnetic recording medium as claimed in claim 1, wherein
said ferromagnetic metal powder comprises Fe and Co, the atomic
ratio of the sum total of rare earth elements/(Fe+ Co) is from 1.0
to 6.0%, and the atomic ratio of the sum total of rare earth
elements/Al is from 0.1 to 0.6.
8. The magnetic recording medium as claimed in claim 1, wherein
said ferromagnetic metal powder has the atomic ratio of the sum
total of rare earth elements/(Fe+Co) of from 1 to 8%, and the
atomic ratio of Mg/(Fe+Co) of from 0.05 to 3.0%.
9. The magnetic recording medium as claimed in claim 1, wherein
said ferromagnetic metal powder has an acicular ratio of from 3.0
to 9.0.
10. The magnetic recording medium as claimed in claim 1, wherein
said ferromagnetic metal powder has a long axis length of from 0.04
to 0.12 .mu.m and a crystallite size of from 80 to 180 .ANG..
11. The magnetic recording medium as claimed in claim 2, wherein
said rare earth element is Y or Nd.
12. The magnetic recording medium as claimed in claim 1, wherein
the content of an abrasive having a Mohs' hardness of 6 or more
contained in said magnetic layer is 15.0 wt % or less based on the
weight of said ferromagnetic metal powder.
13. The magnetic recording medium as claimed in claim 1, wherein
said magnetic layer contains at least a diamond powder having an
average particle size of 2.0 .mu.m or less.
14. The magnetic recording medium as claimed in claim 1, wherein
said magnetic layer contains two kinds of abrasives having a Mohs'
hardness of 9 or more.
15. The magnetic recording medium as claimed in claim 1, wherein
said magnetic layer contains an .alpha.-alumina and a diamond
powder.
16. The magnetic recording medium as claimed in claim 1, wherein
said magnetic layer contains at least a saturated fatty acid and
further at least a saturated fatty acid ester or an unsaturated
fatty acid ester.
17. The magnetic recording medium as claimed in claim 1, wherein
said magnetic layer contains fatty acid monoester and fatty acid
diester.
18. The magnetic recording medium as claimed in claim 1, wherein
said magnetic layer has a central plane average surface roughness
of 4.0 nm or less measured by 3D-MIRAU method.
19. The magnetic recording medium as claimed in claim 1, wherein
said magnetic layer has a dry thickness of from 0.05 to 0.20 .mu.m
and said magnetic layer contains an abrasive having an average
particle size of from 0.02 to 0.3 .mu.m.
20. The magnetic recording medium as claimed in claim 1, wherein
the coercive force of said magnetic layer is 2,000 Oe or more.
21. The magnetic recording medium as claimed in claim 1, wherein
said magnetic layer has a dry thickness of from 0.05 to 0.25 .mu.m
and .phi.m of from 8.0.times.10.sup.-3 to 1.0.times.10.sup.-3
emu/cm.sup.2.
22. The magnetic recording medium as claimed in claim 1, wherein
said magnetic recording medium is a magnetic recording medium for a
system of high transfer rate of 1.0 MB/sec or more.
23. The magnetic recording medium as claimed in claim 1, wherein
said magnetic recording medium is a magnetic recording medium for a
high capacity floppy disc system of disc rotation speed of 1,800
rpm or more.
24. The magnetic recording medium as claimed in claim 1, wherein
said support has a central plane average surface roughness of 5.0
nm or less measured by 3D-MIRAU method.
25. The magnetic recording medium as claimed in claim 1, wherein
said support has a thermal shrinkage factor of 0.5% or less at
100.degree. C. for 30 minutes and 0.2% or less at 80.degree. C. for
30 minutes in every direction of in-plane of said support.
26. The magnetic recording medium as claimed in claim 1, wherein
said lower layer contains a carbon black having an average particle
size of from 5 to 80 nm and said magnetic layer contains a carbon
black having an average particle size of from 5 to 300 nm.
27. The magnetic recording medium as claimed in claim 1, wherein
said lower layer contains an acicular inorganic powder having an
average long axis length of 0.20 .mu.m or less and an acicular
ratio of from 4.0 to 9.0.
28. The magnetic recording medium as claimed in claim 1, wherein
said lower layer contains an acicular inorganic powder and said
magnetic layer contains an acicular ferromagnetic metal powder and
the average long axis length of said acicular inorganic powder is
from 1.1 to 3.0 times of the average long axis length of said
acicular ferromagnetic metal powder.
29. The magnetic recording medium as claimed in claim 1, wherein
said magnetic recording medium is a magnetic recording medium for a
high capacity floppy disc system of disc rotation speed of 3,000
rpm or more.
30. The magnetic recording medium as claimed in claim 1, wherein
said magnetic recording medium is a magnetic recording medium for a
system of high transfer rate of 2.0 MB/sec or more.
31. The magnetic recording medium as claimed in claim 1, wherein
said magnetic recording medium is a magnetic recording medium for a
high capacity floppy disc system which has realized subordination
transposition capable of recording/reproduction with conventional
3.5 inch type floppy discs.
32. The magnetic recording medium as claimed in claim 1, wherein
said magnetic recording medium is a magnetic recording medium for a
high capacity floppy disc system adopting a dual discrete gap head
having both a narrow gap for high density recording and a broad gap
for conventional 3.5 inch type floppy discs.
33. The magnetic recording medium as claimed in claim 1, wherein
said magnetic recording medium is a magnetic recording medium for a
high capacity floppy disc system adopting a head which floats by
disc rotation.
34. The magnetic recording medium as claimed in claim 1, wherein
said magnetic recording medium is a magnetic recording medium for a
high capacity floppy disc system adopting a head which floats by
disc rotation and using a linear type voice coil motor as a driving
motor of the head.
35. A magnetic recording medium which comprises a support having
thereon a magnetic layer comprising a ferromagnetic metal powder
dispersed in a binder, which is a magnetic recording medium for
recording signals of from 0.17 to 2 G bit/inch.sup.2 of areal
recording density, wherein said ferromagnetic metal powder comprise
at lest Fe and Co and the magnetic layer after storage has a
fluctuation of Hc (.DELTA.Hc) of from -5.0% to +10.0%.
36. A magnetic recording medium which comprises a support having
thereon a magnetic layer comprising a ferromagnetic metal powder
dispersed in a binder, which is a magnetic recording medium for
recording signals of from 0.17 to 2 G bit/inch.sup.2 of areal
recording density, wherein said ferromagnetic metal powder
comprises at least Fe and Co and the magnetic layer after storage
has a decrease of .phi.m (.DELTA..phi.m) of within 10%.
37. A magnetic recording medium, comprising: a support; at least
one lower layer disposed over the support; at least one magnetic
layer formed over the lower layer, said magnetic layer comprising a
ferromagnetic metal powder dispersed in a binder, said magnetic
layer having an areal recording density of from 0.17 to 2 G
bit/inch.sup.2 and a coercive force of 1,800 Oe or more, and said
ferromagnetic metal powder comprising Fe, Co, Al, Mg or a rare
earth element.
38. The magnetic recording medium as claimed in claim 38, wherein
the atomic ratio of Al/(Fe+Co) is from 3.0 to 15.4%.
39. The magnetic recording medium as claimed in claim 37, wherein
said rare earth element is selected from the group consisting of
Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and
Lu.
40. The magnetic recording medium comprising: a support; at least
one magnetic layer supported by said support, said magnetic layer
comprising a diamond powder.
41. The magnetic recording medium as claimed in claim 40, wherein
the diamond powder has a particle size of 2 .mu.m, more preferably
from 0.05 to 0.8 .mu.m and most preferably from 0.05 to 0.3
.mu.m.
42. The magnetic recording medium as claimed in claim 40, and
including at least one lower layer disposed between said support
and said at least one magnetic layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a coating type high
capacity magnetic recording medium capable of high density
recording. More specifically, the present invention relates to a
coating type high capacity magnetic recording medium for high
density recording which comprises a magnetic layer on a
substantially nonmagnetic lower layer wherein the uppermost
magnetic layer contains a ferromagnetic metal powder.
BACKGROUND OF THE INVENTION
[0002] In the field of a magnetic disc, a 2 MB MF-2HD floppy disc
using Co-modified iron oxide has been generally loaded in a
personal computer. However, along with the increase in the amount
of data to be dealt with, the capacity thereof has become
insufficient and the increase of the capacity of the floppy disc
has been demanded.
[0003] In the field of a magnetic tape, with the prevalence of an
office computer, such as a minicomputer, a personal computer and a
work station, a magnetic tape for recording computer data as
external storage medium (a so-called backup tape) has been
vigorously studied. For the realization of the magnetic tape for
such a use, the improvement of recording capacity has been strongly
demanded conjointly with the miniaturization of a computer and the
increase of information processing ability (e.g., information
throughput).
[0004] Magnetic layers comprising an iron oxide, a Co-modified iron
oxide, CrO.sub.2, a ferromagnetic metal powder, or a hexagonal
ferrite powder dispersed in a binder, which are coated on a
nonmagnetic support, have been conventionally widely used in
magnetic recording media. Ferromagnetic metal powders and hexagonal
ferrite powders among these have been known to have excellent high
density recording characteristics.
[0005] In the case of a disc, as high capacity discs using
ferromagnetic metal powders which are excellent in high density
recording characteristics, there are 10 MB MF-2TD and 21 MB MF-2SD,
and as high capacity discs using hexagonal ferrite, there are 4 MB
MF-2ED and 21 MB Floptical, however, any of these are not
satisfactory with respect to capacities and properties. As is the
circumstance, various attempts have been made to improve high
density recording characteristics. Examples thereof are described
below.
[0006] For improving characteristics of a disc-like magnetic
recording medium, JP-A-64-84418 (the term "JP-A" as used herein
means an "unexamined published Japanese patent application")
proposes the use of a vinyl chloride resin having an acidic group,
an epoxy group and a hydroxyl group, JP-B-3-12374 (the term "JP-B"
as used herein means an "examined Japanese patent publication")
proposes the use of a metal powder having a coercive force Hc of
1,000 Oe or more and a specific surface area of from 25 to 70
m.sup.2/g, and JP-B-6-28106 proposes to regulate the specific
surface area and magnetic susceptibility of magnetic powders and
contain an abrasive.
[0007] For improving the durability of a disc-like magnetic
recording medium, JP-B-7-85304 proposes use of a fatty acid ester
having an unsaturated fatty acid ester and an ether bond,
JP-B-7-70045 proposes the use of a fatty acid ester having a
branched fatty acid ester and an ether bond, JP-A-54-124716
proposes the use of a-nonmagnetic powder having a Mohs' hardness of
6 or more and a higher fatty acid ester, JP-B-7-89407 proposes to
regulate the volume and the surface roughness of voids containing a
lubricant to from 0.005 to 0.025 .mu.m, JP-A-61-294637 proposes the
use of a fatty acid ester having a low melting point and a fatty
acid ester having a high melting point, JP-B-7-36216 proposes use
an abrasive having a particle size of from 1/4to 3/4of the magnetic
layer thickness and a fatty acid ester having a low melting point,
and JP-A-3-203018 proposes use of a metallic magnetic powder
containing Al and a chromium oxide.
[0008] As the constitution of a disc-like magnetic recording medium
having a nonmagnetic lower layer and an intermediate layer,
JP-A-3-120613 proposes the constitution comprising an electrically
conductive layer and a magnetic layer containing a metal powder,
JP-A-6-290446 proposes the constitution comprising a magnetic layer
having a thickness of 1 .mu.m or less and a nonmagnetic layer,
JP-A-62-159337 proposes the constitution comprising an intermediate
layer comprising a carbon and a magnetic layer containing a
lubricant, and JP-A-5-290358 proposes the constitution comprising a
nonmagnetic layer in which the carbon size is regulated.
[0009] On the other hand, a disc-like magnetic recording medium
comprising a thin magnetic layer and a functional nonmagnetic layer
has been developed in recent years and floppy discs of the class
with the capacity of 100 MB are now on the market. As floppy
discs-showing these characteristics, JP-A-5-109061 proposes the
constitution comprising a magnetic layer having Hc of 1,400 Oe or
more and a thickness of 0.5 .mu.m or less and a nonmagnetic layer
containing electrically conductive particles, JP-A-5-197946
proposes the constitution comprising abrasives having particle
sizes larger than the thickness of the magnetic layer,
JP-A-5-290354 proposes the constitution comprising a magnetic layer
having the thickness of 0.5 .mu.m or less and the fluctuation of
the thickness of within .+-.15%, in which the surface electric
resistance is regulated, and JP-A-6-68453 proposes the constitution
in which two kinds of abrasives having different particle sizes are
contained and the amount of the abrasives on the surface is
regulated.
[0010] Further, in the field of a tape-like magnetic recording
medium, with the prevalence of an office computer, such as a
minicomputer and a personal computer, a magnetic tape for recording
computer data as external storage medium (a so-called backup tape)
has been vigorously studied. For the realization of the magnetic
tape for such use, the improvement of recording capacity has been
strongly demanded conjointly with the miniaturization of a computer
and the increase of information processing capability. In addition,
the use in various environmental conditions due to widening of use
environments of magnetic tapes (in particular, under fluctuating
temperature/humidity conditions), reliability on data storage, and
reliability on performance, such as stable recording/readout of
data in multiple running due to repeating use at high speed, have
been increasingly demanded.
[0011] Magnetic tapes which are used in digital signal recording
systems vary according to each system, for example, magnetic tapes
corresponding to a so-called DLT type, 3480, 3490, 3590, QIC, a D8
type and a DDS type are known. In every system, the magnetic tape
comprises, on one surface side of a nonmagnetic support, a magnetic
layer of a single layer structure having a comparatively thick
layer thickness, e.g., from 2.0 to 3.0 .mu.m, containing a
ferromagnetic powder, a binder and an abrasive, and a back coating
layer provided on the surface side of the support opposite to the
side having the magnetic layer for purposes of preventing winding
disarrangement and maintaining good running durability. However, in
general, in a magnetic layer of a single layer structure having a
comparatively thick layer thickness as described above, there is a
problem of thickness loss which generates the reduction of
output.
[0012] For the improvement of the reduction of reproduction output
due to thickness loss, thinning of a magnetic layer has been known.
For example, JP-A-5-182178 discloses a magnetic recording medium
comprising a nonmagnetic support having thereon a lower nonmagnetic
layer containing an inorganic powder dispersed in a binder and an
upper magnetic layer containing a ferromagnetic powder dispersed in
a binder and having a thickness of 1.0 .mu.m or less, which is
coated on the lower nonmagnetic layer while the nonmagnetic layer
is still wet.
[0013] However, with the rapid trend of the increase of the
capacity and density of disc-like and tape-like magnetic recording
media, it has become difficult to obtain satisfactory
characteristics even with these techniques. It has also become
difficult to make the increase of the capacity and density
compatible with durability. The present inventors have proposed in
JP-A-63-103423 a magnetic recording medium comprising a magnetic
layer containing a ferromagnetic metal powder containing from 1 to
6 wt % of aluminum in terms of the weight of metals, and a binder
containing a resin having a polar group, wherein the ferromagnetic
metal powder is invested with higher hardness than the hardness of
conventional similar ferromagnetic metal powders without reducing
magnetic characteristics when the ferromagnetic metal powder
contains an aluminum component in a specific range of amount.
Further, the present inventors have proposed that a magnetic layer
comprising a ferromagnetic metal powder and an ordinary abrasive
dispersed in a binder containing a resin having a polar group
possesses both high running durability and electromagnetic
characteristics. The magnetic medium has certainly exhibited
excellent durability but when it is applied to a magnetic disc of
high capacity and high density at high speed rotation of 1,800 rpm
or more, there is still room for improvement of durability and
electromagnetic characteristics.
SUMMARY OF THE INVENTION
[0014] An object of the present invention is to provide a magnetic
recording medium which is markedly improved in electromagnetic
characteristics, in particular, high density recording
characteristics, and which unites high durability, in particular,
an error rate in high density recording region is conspicuously
improved. Specifically, an object of the present invention is to
provide a high capacity magnetic recording medium, in particular, a
disc-like magnetic recording medium having a high recording
capacity of from 0.17 to 2 G bit, preferably from 0.2 to 2 G bit,
and particularly preferably from 0.35 to 2 G bit.
[0015] As a result of earnest studies to provide a magnetic
recording medium which is excellent in electromagnetic
characteristics and durability, in particular, markedly improved in
an error rate in a high density recording region, the present
inventors have found that high capacity, high density recording
characteristics and excellent durability of the object of the
present invention can be obtained by the magnetic recording medium
having the constitution described below, thus the present invention
has been attained.
[0016] That is, the present invention can be attained by a magnetic
recording medium which comprises a support having thereon a
magnetic layer comprising a ferromagnetic metal powder dispersed in
a binder, which is a magnetic recording medium for recording
signals of from 0.17 to 2 G bit/inch.sup.2 of areal recording
density, wherein the coercive force of the magnetic layer is 1,800
Oe or more, the ferromagnetic metal powder comprises Fe and Co, and
the atomic ratio of Al/(Fe+ Co) is from 3.0 to 15.4%, or the
present invention can be attained by a magnetic recording medium
which comprises a support having thereon a magnetic layer
comprising a ferromagnetic metal powder dispersed in a binder,
which is a magnetic recording medium for recording signals of from
0.17 to 2 G bit/inch.sup.2 of areal recording density, wherein the
coercive force of the magnetic layer is 1,800 Oe or more, the
ferromagnetic metal powder comprises Fe and Co, and the atomic
ratio of the sum total of rare earth elements/(Fe+ Co) is from 0.5
to 9.0%.
[0017] Preferred embodiments of the present invention are described
below.
[0018] (1) In the above magnetic recording medium, a substantially
nonmagnetic lower layer is provided between the support and the
magnetic layer.
[0019] (2) The above magnetic recording medium is a disc-like
magnetic recording medium.
[0020] (3) In the above magnetic recording medium, the magnetic
layer has a dry thickness of from 0.05 to 0.30 .mu.m and .phi.m of
from 10.0.times.10.sup.-3 to 1.0.times.10.sup.-3 emu/cm.sup.2.
[0021] (4) In the above magnetic recording medium, the
ferromagnetic metal powder comprises Fe and Co, and the atomic
ratio of the sum total of rare earth elements/Al is from 0.05 to
1.2.
[0022] (5) In the above magnetic recording medium, the
ferromagnetic metal powder comprises Fe and Co, and the atomic
ratio of the sum total of rare earth elements/Al is from 0.1 to
0.6.
[0023] (6) In the above magnetic recording medium, the
ferromagnetic metal powder comprises Fe and Co, the atomic ratio of
the sum total of rare earth elements/(Fe+Co) is from 1.0 to 6.0%,
and the atomic ratio of the sum total of rare earth elements/Al is
from 0.1 to 0.6.
[0024] (7) In the above magnetic recording medium, the
ferromagnetic metal powder has the atomic ratio of the sum total of
rare earth elements/(Fe+Co) of from 1.0 to 8.0%, and the atomic
ratio of Mg/(Fe+Co) of from 0.05 to 3.0%.
[0025] (8) In the above magnetic recording medium, the
ferromagnetic metal powder has an acicular ratio of from 3.0 to
9.0.
[0026] (9) In the above magnetic recording medium, the
ferromagnetic metal powder has a long axis length of from 0.04 to
0.12 .mu.m and a crystallite size of from 80 to 180 .ANG..
[0027] (10) In the above magnetic recording medium, the rare earth
element is Y or Nd.
[0028] The object of the present invention can be achieved by the
above-described construction. Preferably the magnetic layer has a
dry thickness of from 0.05 to 0.25 .mu.m, .phi.m of from
8.0.times.10.sup.-3 to 1.0.times.10.sup.-3 emu/cm.sup.2, and the
magnetic recording medium of the present invention is a magnetic
recording medium for recording signals of from 0.17 to 2 G
bit/inch.sup.2 of areal recording density. The present inventors
have found that the magnetic recording medium having high capacity,
excellent high density characteristics and excellent durability, in
which, in particular, the error rate in high density recording
region has been markedly improved, which could not be obtained by
conventional techniques, could be obtained by adopting the
constitution of the present invention.
[0029] In the magnetic recording medium of the present invention,
the ferromagnetic metal powder comprises at least Fe and Co.
[0030] The magnetic layer after storage has a fluctuation of Hc
(.DELTA.Hc) of preferably from -5.0% to +10.0%, more preferably
from -3.0% to +8.0%, most preferably from -1.0% to +6.0%. Further,
the magnetic layer after storage has decrease of .phi.m (i.e.,
.DELTA..phi.m) of preferably within 10%, more preferably within
6%.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The term "a substantially nonmagnetic lower layer" means a
lower layer which may have a magnetic property but not too much for
participating in recording and hereinafter referred to simply as "a
lower layer" or "a nonmagnetic layer". When a magnetic powder is
contained in the lower layer, the content is preferably less than
1/2of the content of the inorganic powder.
[0032] Areal recording density is a value obtained by multiplying
linear recording density by track density.
[0033] .phi.m is the amount of magnetic moment (emu/cm.sup.2) which
can be directly measured from the magnetic layer per unit area of
one side using a vibrating sample magnetometer (VSM, a product of
Toei Kogyo Co., Ltd.) at Hm 10 KOe, which is equal to the value
obtained by multiplying magnetic flux density (Bm) (unit G=4
.pi.emu/cm.sup.3) by the thickness (cm). Accordingly, the unit of
.phi.m is represented by emu/cm.sup.2 or G.multidot.cm.
[0034] Linear recording density means a bit number of signals
recorded per 1 inch in the recording direction.
[0035] These linear recording density, track density and areal
recording density are values determined by each system.
[0036] That is, the present inventors have elaborated some means in
an attempt to improve the magnetic layer thickness, the coercive
force (Hc) and the central plane average surface roughness as to
the linear recording density, and the optimization of the .phi.m as
to the track density for the improvement of the areal recording
density.
[0037] More preferred embodiments of the present invention are
described below.
[0038] As for the magnetic recording medium as a whole, (1) the
magnetic layer has a central plane average surface roughness of 5.0
nm or less, preferably 4.0 nm or less, and more preferably 3.0 nm
or less, measured by 3D-MIRAU method; (2) the magnetic layer has a
coercive force of preferably 1,800 Oe or more, more preferably
2,000 Oe or more, and most preferably 2,100 Oe or more and the
ferromagnetic metal powder has preferably a long axis length of
0.12 .mu.m or less, more preferably 0.10 .mu.m or less
(particularly lower limit is 0.04 .mu.m); (3) the magnetic
recording medium is a magnetic recording medium for recording
signals of from 0.17 to 2 G bit/inch.sup.2, preferably from 0.20 to
2 G bit/inch.sup.2, and more preferably from 0.35 to 2 G
bit/inch.sup.2, of areal recording density: (4) the magnetic
recording medium is a magnetic recording medium for the system of a
high transfer rate of 1.0 MB/sec. or more, more preferably 2.0
MB/sec. or more; (5) the magnetic recording medium is a magnetic
recording medium for the high capacity floppy disc system of disc
rotation speed of preferably 1,800 rpm or more, more preferably
2,000 rpm or more, and most preferably 3,000 rpm or more; (6) the
magnetic recording medium is a magnetic recording medium which has
realized subordination transposition capable of
recording/reproduction with the conventional 3.5 inch type floppy
discs; (7) the magnetic recording medium is a magnetic recording
medium for a high capacity floppy disc system adopting a dual
discrete gap head having both a narrow gap for high density
recording and a broad gap for the conventional 3.5 inch type floppy
discs; (8) the magnetic recording medium is a magnetic recording
medium for the high capacity floppy disc system adopting a head
which floats by disc rotation; and (9) the magnetic recording
medium is a magnetic recording medium for the high capacity floppy
disc system adopting a head which floats by disc rotation and, at
the same time, a linear type voice coil motor as a driving motor of
the head. As for the improvement of the magnetic powder, (1) the
ferromagnetic metal powder comprises Fe as a main component, has an
average long axis length of from 0.12 .mu.m or less, and an
acicular ratio of from 4.0 to 9.0; and (2) the ferromagnetic metal
powder comprises Fe as a main component, has an average long axis
length of from 0.10 .mu.m or less, and a crystallite size of from
80 to 180 .ANG.. As for the improvement of the support, (1) the
support has a central plane average surface roughness of preferably
5 nm or less, more preferably 4.0 nm or less; (2) the support has a
thermal shrinkage factor of 0.5% or less both at 100.degree. C. for
30 minutes and at 80.degree. C. for 30 minutes in every direction
of in-plane of the support; and (3) the support has a temperature
expansion coefficient of from 10.sup.-4 to 10.sup.-8/.degree. C. in
every direction of in-plane of the support. As for the improvement
of the lubricant, (1) the lower layer and/or the magnetic layer
contain(s) at least three kinds in total of a fatty acid and/or a
fatty acid ester; (2) the fatty acid and the fatty acid ester have
the same fatty acid residues with each other; (3) the fatty acid
contains at least a saturated fatty acid and the fatty acid ester
contains at least a saturated fatty acid ester or an unsaturated
fatty acid ester; (4) the fatty acid ester contains a monoester and
a diester; (5) the fatty acid ester contains a saturated fatty acid
ester and an unsaturated fatty acid ester; and (6) the surface of
the magnetic layer has a C/Fe peak ratio of from 5 to 100 when the
surface is measured by the Auger electron spectroscopy. As for the
improvement of the nonmagnetic powder for the lower layer, (1) the
lower layer contains a carbon black having a particle size of from
5 nm to 80 nm and the magnetic layer contains a carbon black having
a particle size of from 5 nm to 300 nm; (2) the lower layer
contains a carbon black having an average particle size of from 5
nm to 80 nm a carbon black having an average particle size of 80 nm
or more; (3) the lower layer and the magnetic layer each contains a
carbon black having an average particle size of from 5 nm to 80 nm;
(4) the lower layer contains an acicular inorganic powder having an
average long axis length of preferably 0.20 .mu.m or less, more
preferably 0.10 .mu.m or less, and an acicular ratio of preferably
from 3.0 to 9.0, more preferably from 4.0 or 9.0; (5) the lower
layer contains an acicular inorganic powder and the magnetic layer
contains an acicular ferromagnetic metal powder, and the average
long axis length of the acicular inorganic powder is from 1.1 to
3.0 times of the average long axis length of the acicular
ferromagnetic metal powder; and (6) the lower layer and/or the
magnetic layer contain(s) a phosphorus compound and the lower layer
contains an acicular or spherical inorganic powder. As for the
improvement of the abrasive for the magnetic layer, (1) the
magnetic layer contains at least an abrasive having an average
particle size of from 0.01 to 0.30 .mu.m; (2) the magnetic layer
contains at least a diamond powder having an average particle size
of 0.2 .mu.m or less, preferably from 0.01 to 1.0 .mu.m; (3) the
magnetic layer contains two kinds of abrasives having a Mohs'
hardness of 9 or more; (4) the magnetic layer preferably contains
an .alpha. -alumina and a diamond powder; and (5) the magnetic
layer preferably contains at least an abrasive in an amount of 15.0
wt % or less based on the weight of the magnetic metal powder. As
for the improvement of the binder, (1) the lower layer and/or the
magnetic layer contain(s) at least a polyurethane having a glass
transition temperature of preferably from 0.degree. C. to
100.degree. C., more preferably from 30.degree. C. to 100.degree.
C.; and (2) the lower layer and/or the magnetic layer contains at
least a polyurethane having a breaking stress of from 0.05 to 10
kg/mm.sup.2.
[0039] The present inventors have found that a magnetic recording
medium, in particular, a disc-like magnetic recording medium, in
the recording capacity system of areal recording density of from
0.17 to 2 G bit/inch.sup.2, having excellent high density
characteristics and excellent durability, in particular, markedly
improved error rate in high density recording region, which could
not be obtained by conventional techniques, could be obtained by
adopting the above constitution of the present invention.
[0040] A magnetic recording medium, in particular, a disc-like
magnetic recording medium, having high density characteristics and
high durability in the recording capacity system of areal recording
density of from 0.17 to 2 G bit/inch.sup.2, preferably from 0.2 to
2 G bit/inch.sup.2, more preferably from 0.35 to 2 G
bit/inch.sup.2, which could never be achieved by any coating type
magnetic recording media known in the world, can be obtained as a
result of organically combining and synthesizing these points as
shown below.
[0041] The points aimed at in the present invention include (1)
high Hc and hyper-smoothing, (2) ensuring of durability by the
improvement of a composite lubricant, a binder with high durability
and a ferromagnetic powder, and the use of an abrasive with high
hardness (3) ultra-thinning of the magnetic layer and the reduction
of fluctuation in the interface between the magnetic layer and the
lower layer, (4) the increase of packing density of powders (a
ferromagnetic powder and a nonmagnetic powder), (5) ultra-fine
granulation of powders (a ferromagnetic powder and a nonmagnetic
powder), (6) stabilization of head touch, (7) dimensional stability
and servomechanism, (8) improvement of thermal shrinkage factors of
the magnetic layer and the support, and (9) the functions of a
lubricant at high temperature and low temperature, and the present
invention has been achieved by combining and synthesizing these
points.
[0042] High Hc and hyper-smoothing are described in the first
place. Hc of the magnetic layer can be increased to 1,800 Oe or
more, preferably 2,000 Oe or more, and more preferably 2,100 Oe or
more by using a ferromagnetic powder with high Hc, thereby high
capacity and high density can be obtained. With respect to
hyper-smoothing, a smooth magnetic layer can be obtained by making
the central plane average surface roughness 5.0 nm or less,
preferably 4.0 nm or less, and employing ATOMM.RTM. structure. High
capacity and high density can be attained by the central plane
average surface roughness of preferably 4.0 nm or less.
Subsequently, the above item (2) ensuring of durability by the
improvement of a composite lubricant, a binder with high durability
and a ferromagnetic powder, and the use of an abrasive with high
hardness is described. With respect to a composite lubricant,
fundamental concepts for the enhancement of lubrication capability
are shown below.
[0043] (1) A plurality of lubricants having different functions and
capabilities are used in combination.
[0044] (2) A plurality of lubricants having similar functions and
capabilities are used in combination.
[0045] Due to the above item (1), a variety of functions and
capabilities can be attained under various conditions. Further, due
to the above item (2), affinity and compatibility of lubricants
with each other can be ensured and good functions of lubricants can
be exhibited.
[0046] Examples of combinations of a plurality of lubricants having
different functions and capabilities as in the above item (1) are
shown below.
[0047] 1) A Lubricant having a fluid lubrication function and a
lubricant having a boundary lubrication function are used in
combination.
[0048] 2) A polar lubricant and a nonpolar lubricant are used in
combination.
[0049] 3) A liquid lubricant and a solid lubricant are used in
combination.
[0050] 4) Lubricants having different polarities, in particular, a
fatty acid and/or a fatty acid ester, are used in combination. For
example, a fatty acid monoester and a fatty acid diester are used
in combination.
[0051] 5) Lubricants having different melting points and different
boiling points, in particular, a fatty acid and/or a fatty acid
ester, are used in combination.
[0052] 6) Lubricants which differ in lengths of carbon atom number,
in particular, a fatty acid and/or a fatty acid ester, are used in
combination.
[0053] 7) A straight chain lubricant and a branched chain
lubricant, in particular, a fatty acid and/or a fatty acid ester,
are used in combination. For example, a straight chain fatty acid
ester and a branched fatty acid ester are used in combination.
[0054] 8) A lubricant having saturated carbon chain and a lubricant
having unsaturated carbon chain, in particular, a fatty acid and/or
a fatty acid ester, are used in combination. For example, a
saturated fatty acid ester and unsaturated fatty acid ester are
used in combination.
[0055] 9) Lubricants having different affinities with a binder are
used in combination.
[0056] 10) Lubricants having different affinities with an inorganic
powder are used in combination.
[0057] By the combined use of lubricants as in the above item (1),
a variety of functions and capabilities can be attained under
various conditions.
[0058] Examples of combinations of a plurality of lubricants having
similar functions and capabilities as in the above item (2) are
shown below.
[0059] 1) Fatty acid residues of a fatty acid and a fatty acid
ester are made the same with each other.
[0060] 2) Fatty acid esters having the same fatty acid residues
with each other and/or having the same alcohol residues with each
other are used in combination.
[0061] 3) Two or more saturated fatty acid are used in
combination.
[0062] 4) Saturated fatty acids are respectively used in the fatty
acid residue parts of a fatty acid and a fatty acid ester.
[0063] 5) Unsaturated fatty acids are respectively used in the
fatty acid residue parts of a fatty acid and a fatty acid
ester.
[0064] 6) Three or more fatty acid esters alone are used in
combination.
[0065] 7) Fatty acid parts of a fatty acid and a fatty acid amide
are made the same with each other.
[0066] By the combined use of lubricants as in the above item (2),
affinity and compatibility of lubricants with each other can be
ensured and good functions of lubricants can be exhibited.
[0067] By the combined use of lubricants as in the above items (1)
and (2), not only a variety of functions and capabilities can be
attained under various conditions but also affinity and
compatibility of lubricants with each other can be ensured and good
functions of lubricants can be exhibited.
[0068] The binder with high durability is described below. By the
incorporation of a resin having a polar group, in particular, a
polyurethane resin, having high dispersibility, high glass
transition temperature and high breaking stress, durability of the
binder can be improved. It is preferred for the polyurethane resin
to have 2 or more OH groups, more preferably 3 or more, and most
preferably 4 or more, at the terminal of the molecule because the
reactivity with polyisocyanate, which is a polyfunctional curing
agent, becomes high and the coated film of three dimensional
network can be formed after curing. With respect to the improvement
of the ferromagnetic powder, durability can be improved by
increasing the Al component which can heighten the hardness of the
ferromagnetic powder. Ensuring of durability by using an abrasive
having high hardness is described. Higher durability can be
obtained by the combined use of a diamond powder having a Mohs'
hardness of 10 with conventionally used abrasives having a Mohs'
hardness of 9 or so, e.g., an .alpha.-alumina. Next, ultra-thinning
of the magnetic layer and the reduction of fluctuation in the
interface between the magnetic layer and the lower layer in the
above item (3) is described. By reducing the thickness of the
magnetic layer to preferably from 0.05 to 0.30 .mu.m, more
preferably from 0.05 to 0.25 .mu.m, and reducing the fluctuation in
the interface between the magnetic layer and the lower layer,
uniform, smooth and thin magnetic layer can be obtained, thereby
higher capacity and higher density of the magnetic recording medium
can be attained. The increase of the packing density of powders (a
ferromagnetic powder and a nonmagnetic powder) in the above item
(4) is described. By high packing density of a fine ferromagnetic
powder, specifically, a fine ferromagnetic metal powder, preferably
having an average particle length of 0.15 .mu.m or less, more
preferably 0.12 .mu.m or less, and most preferably 0.10 .mu.m or
less, high .phi.m can be obtained thereby higher capacity and
higher density of the magnetic recording medium can be attained.
Durability can be improved by high packing density of an inorganic
powder. Ultra-fine granulation of powders (a ferromagnetic powder
and a nonmagnetic powder) in the above item (5) is described. By
the use of a fine ferromagnetic powder, specifically, a fine
ferromagnetic metal powder, having an average long axis length of
preferably 0.15 .mu.m or less, more preferably 0.12 .mu.m or less,
and most preferably 0.10 .mu.m or less, in particular, with the
case of a ferromagnetic metal powder, by the ultra-fine granulation
of the powder such as an average long axis length of 0.10 .mu.m or
less, an acicular ratio of from 4.0 to 9.0, and a crystallite size
of from 80 .ANG. to 180 .ANG., and with the case of a nonmagnetic
powder in the lower layer, if it is an acicular powder, by the
ultra-fine granulation of the powder such as an average long axis
length of preferably 0.20 .mu.m or less, more preferably 0.10 .mu.m
or less, higher packing density and hyper-smoothing of the magnetic
layer can be attained, thereby higher capacity and higher density
of the magnetic recording medium can be attained. Stabilization of
head touch in the above item (6) is described. Stabilization of
head touch can be contrived by an appropriate strength, flexibility
and smoothness of the magnetic recording medium as a whole, thereby
higher capacity and higher density of the magnetic recording medium
can be attained stably even at high speed running and high rotation
rate. Dimensional stability and servomechanism in the above item
(7) is described. For example, when the support has a thermal
shrinkage factor of 0.5% or less both at 100.degree. C. for 30
minutes and at 80.degree. C. for 30 minutes in every direction of
in-plane of the support, and a temperature expansion coefficient of
from 10.sup.-4 to 10.sup.-8/.degree. C. in every direction of
in-plane of the support, dimensional stability of the support can
be obtained, thereby higher capacity and higher density of the
magnetic recording medium can be attained stably even at high speed
running and high rotation rate, and improvement of thermal
shrinkage factors of the magnetic layer and the support in the
above item (8) can also achieved. With respect to the functions of
a lubricant at high temperature and low temperature in the above
item (9), desired lubricating functions at both high temperature
and low temperature can be obtained by selecting and combining
various lubricants described above based on specific concepts.
[0069] In the field of personal computers where the tendency of
multimedia has been increasingly progressed, high capacity
recording media have attracted public attentions in place of
conventional floppy discs and, for example, ZIP disc, has been on
sale from IOMEGA CORP., U.S.A. This is a recording medium
comprising a lower layer and a magnetic thin layer developed by the
present inventors using ATOMM.RTM. (Advanced Super Thin Layer &
High Output Metal Media Technology), and products of 3.7 inches
with the recording capacity of 100 MB or more are on the market.
The capacity of from 100 to 120 MB discs is almost equal to the
capacity of MO (3.5 inches), i.e., one disc has the capacity of
recording newspaper articles of from seven to eight month period. A
transfer rate indicating write/readout time of data is 2 MB or more
per a second, which is equal to a hard disc, and the working speed
is 20 times of conventional floppy discs and more than 2 times of
the MO, therefore, extremely advantageous. In addition, as this
recording medium comprising a lower layer and a magnetic thin layer
is the same coating type medium as floppy discs used at present,
mass production is feasible, accordingly inexpensive as compared
with hard discs and the MO.
[0070] As a result of eager investigations based on the knowledge
on these media, the present inventors have achieved the present
invention of a magnetic recording medium, in particular, a
disc-like magnetic recording medium, in the recording capacity
system of areal recording density of from 0.17 to 2 G
bit/inch.sup.2, preferably from 0.2 to 2 G bit/inch.sup.2, more
preferably from 0.35 to 2 G bit/inch.sup.2, and .phi.m of
preferably from 10.0.times.10.sup.-3 to 1.0.times.10.sup.-3
emu/cm.sup.2, particularly preferably from 8.0.times.10.sup.-3 to
1.0.times.10.sup.-3 emu/cm.sup.2, which has markedly big recording
capacity as compared with the above ZIP disc and the MO (3.5
inches). This recording medium also has high capacity, high density
characteristics and excellent durability which could never be
achieved by any products known in the world and, in particular, the
error rate in high density recording region is noticeably improved,
and this is the invention applicable to a magnetic tape, e.g., a
computer tape.
[0071] The magnetic recording medium of the present invention
comprises an ultra-thin magnetic layer containing a magnetic powder
of ultra-fine particles excellent in high output and high
dispersibility, and a lower layer containing a spherical or
acicular inorganic powder, and by thus reducing the thickness of
the magnetic layer, a magnetic force offset in the magnetic layer
can be reduced, the output in a high frequency region can be
markedly increased, further, overwriting characteristics can be
improved. By the improvement of a magnetic head, the effect of the
ultra-thin magnetic layer can be further exhibited by the combined
use with a narrow gap head and digital recording characteristics
can be improved.
[0072] The upper magnetic layer is a thin layer having a thickness
of preferably from 0.05 to 0.30 .mu.m, more preferably from 0.05 to
0.25 .mu.m, so as to match the performance required from the
magnetic recording system and the magnetic head of high density
recording. Such a uniform and ultra-thin magnetic layer is attained
by high dispersion and high packing density realized by the
combined use of a fine magnetic powder and nonmagnetic powder with
a dispersant and a high dispersible binder. The magnetic powders
used are preferably magnetic powders capable of achieving high
output, excellent in high dispersibility and high randomizing
property for inducing suitabilities of high capacity floppy discs
and computer tapes as far as possible. That is, high output and
high durability can be attained by the use of ferromagnetic metal
powders of extremely fine particles which are capable of achieving
high output, in particular, having an average long axis length of
preferably 0.15 .mu.m or less, more preferably 0.12 .mu.m or less,
and particularly preferably 0.10 .mu.m or less, a crystallite size
of from 80 to 180 .ANG., and containing a large amount of Co, and
further Al, Si, Y and Nd components. For the realization of a high
transfer rate, running stability and durability during high speed
rotation can be ensured making use of a three dimensional network
binder system suitable for an ultra-thin magnetic layer. A
composite lubricant capable of maintaining the effect thereof
during use under various temperature and humidity conditions and in
high rotation use can be incorporated into upper and lower layers
and, further, with making the lower layer have a role of the tank
of the lubricant so as to be able to always supply an appropriate
amount of the lubricant to the upper magnetic layer to thereby
heighten the durability of the upper magnetic layer to improve the
reliance. Cushioning effect of the lower layer can bring about good
head touch and stable running property.
[0073] A high transfer rate is required in a high capacity
recording system, e.g., a transfer rate of 1.4 MB/sec. in Zip and a
maximum transfer rate of 3.6 MB/sec. in HiFD.RTM.. For that sake,
it is necessary that the rotation speed of a magnetic disc for a
high capacity recording system should be taken up one or more
places as compared with conventional FD systems. Specifically, the
rotation speed of a magnetic disc is preferably 1,800 rpm or more,
more preferably 2,000 rpm or more, and most preferably 3,000 rpm or
more. For example, the rotation speed of a magnetic disc is 2,968
rpm in Zip and 3,600 rpm in HiFD.RTM.. In the other systems, it is
estimated that the rotation speed of a magnetic disc is 5,400 rpm
and the transfer rate is 7.5 MB/sec when a recording capacity is
650 MB (0.65 GB). Recording track density is improved with the
increase of capacity/density of magnetic recording. In general, a
servo recording area is provided on a medium to ensure traceability
of a magnetic head against a recording track. In the magnetic
recording medium according to the present invention, a base whose
dimensional stability is isotropically heightened is preferably
used as the support base, thereby further stabilization of the
traceability is devised. The smoothness of the magnetic layer can
be further improved by using a hyper-smooth base.
[0074] The increment of density of magnetic recording of a
disc-like magnetic recording medium requires the improvement of
linear recording density and track density. Characteristics of a
support are important factors for the improvement of track density.
The dimensional stability of a support base, in particular,
isotropy, is considered in the recording medium according to the
present invention. Servo recording is an indispensable technique in
recording/reproduction of high track density, but the improvement
can be contrived from the medium side by making the support base
isotropic as far as possible.
[0075] Advantages of changing the magnetic layer of the present
invention from a monolayer (i.e., a single layer) to the ATOMM.RTM.
structure are thought to be as follows.
[0076] (1) Improvement of electromagnetic characteristics by the
thin layer structure of the magnetic layer;
[0077] (2) Improvement of durability by stable supply of
lubricants;
[0078] (3) High output by smoothing the upper magnetic layer;
and
[0079] (4) Easiness of imparting required functions by functional
separation of the magnetic layer.
[0080] These functions cannot be sufficiently attained only by
making the magnetic layer a multilayer structure. For constituting
a multilayer structure, a successive multilayer system comprising
successively constituting the layers is generally used. In this
system, the lower layer is coated, cured or dried, then the upper
magnetic layer is coated in the same way, cured, and
surface-treated. In the case of a floppy disc (FD), as different
from a magnetic tape, the same treatments are conducted on both
surface sides. After a coating step, a disc undergoes a slitting
step, a punching step, a shell incorporation step, and a certifying
step, thus a final product is completed. Simultaneous coating or
successive coating of coating the upper magnetic layer while the
lower layer is still wet is preferred in view of the
productivity.
[0081] Electromagnetic characteristics can be widely improved by
the thin layer structure of the magnetic layer as follows.
[0082] (1) Improvement of the output in a high frequency region by
the improvement of characteristics of recording
demagnetization;
[0083] (2) Improvement of overwriting characteristics; and
[0084] (3) Security of window margin.
[0085] Durability is an important factor for a magnetic recording
disc. In particular, for realizing a high transfer rate, it is
necessary that the rotation speed of a magnetic disc should be
taken up one or more places as compared with conventional FD
systems, and security of the durability of a magnetic disc is an
important problem when the magnetic disc is sliding with a magnetic
head and parts in a cartridge at a high speed. For improving
durability of a disc, there are means such as a binder formulation
to increase the film strength of a disc per se, and a lubricant
formulation to maintain a sliding property of a disc with a
magnetic head. In the magnetic recording medium according to the
present invention, a three dimensional network binder system which
has shown actual results in conventional FD systems is used in the
binder formulation by being modified.
[0086] In the present invention, lubricants are used in combination
of a plurality of kinds respectively exhibiting superior effects in
various temperature and humidity conditions under which they are
used, and each lubricant exhibits its function in different
temperature (low temperature, room temperature, high temperature)
and humidity (low humidity, high humidity) atmospheres, thereby
totally stable lubrication effect can be maintained.
[0087] By using two layer structure, the durability of the upper
magnetic layer can be heightened with making the lower layer have a
role of the tank of a lubricant capable of always supplying an
appropriate amount of a lubricant to the upper magnetic layer.
There is a limit on the amount of a lubricant which can be
contained in the ultra-thin magnetic layer. Simple reduction of the
thickness of the magnetic layer causes the reduction of the
absolute amount of a lubricant, and it follows that running
durability is deteriorated. In this case, it was difficult to well
balance the thickness of the magnetic layer with the amount of the
lubricant. The improvement of electromagnetic characteristics could
be compatible with the improvement of durability by imparting
different functions to the upper layer and the lower layer and
making up for each other. This functional separation was
particularly effective in a system where a medium was slid on a
magnetic head at a high speed.
[0088] In addition to the maintaining function of a lubricant, a
controlling function of surface electrical resistance can be
imparted to the lower layer. For controlling electrical resistance,
in general, a solid electrically conductive material such as a
carbon black is added to a magnetic layer in many cases. Such a
material not only restricts the increase of the packing density of
magnetic powders but also influences the surface roughness of the
magnetic layer as the thickness of the magnetic layer becomes
thinner. Incorporation of electrically conductive materials in the
lower layer can eliminate these defects.
[0089] With the progress of multimedia in society, needs for image
recording have been increased more and more not only in the
industry but also in general homes. The high capacity magnetic
recording medium according to the present invention has
capabilities capable of sufficiently responding to demands such as
function/cost as a medium for recording images, as well as data
such as letters and figures. The high capacity magnetic recording
medium according to the present invention is based on the coating
type magnetic recording medium which has shown actual results and
ensures reliability for a long period of time and is excellent in
cost performance.
[0090] The present invention has been attained for the first time
by heaping up the above various factors, and making them worked
synergistically and organically. The thus-obtained magnetic
recording medium by adopting or rejecting and combining every
technique has capability applicable to, e.g., HiFD.RTM., which has
been developed by joint development by Fuji Photo Film Co., Ltd.
with Sony Corp. HiFD.RTM. has been developed to meet the demand for
a new data recording system of high performance having high
capacity and high data transfer rate with the rapid development of
information processing capability of personal computers in recent
years and sharp increase of throughput to be dealt with. On the
other hand, 3.5 inch type floppy discs of the present have
prevailed worldwide as easily usable recording media. HiFD.RTM. has
been developed as a new system which can read out and reuse
accumulated massive data using these discs even after this.
HiFD.RTM. for 3.5 inch type floppy disc is a high capacity floppy
disc system of the next generation which has high capacity of 200
MB, high transfer rate of 3.6 MB/sec, and has realized
subordination transposition capable of recording/reproduction with
3.5 inch type floppy discs of the present. High capacity of 200 MB
of HiFD.RTM. has been realized by an ultra-thin layer coating type
metal disc newly developed and by the adoption of a dual discrete
gap head having both a narrow gap for high density recording and a
broad gap for a 3.5 inch type floppy disc of the present, which can
easily process data file of huge volume such as an image and a
sound. HiFD.RTM. has realized a high transfer rate of a maximum of
3.6 MB/sec. as compared with a transfer rate of 0.06 MB/sec. of
conventional 3.5 inch type floppy discs (2HD) due to high linear
recording density and high speed disc rotation such as 3,600 rpm,
which is high speed processing of about 60 times as compared with
conventional systems. Further, by the adoption of a floatation type
dual discrete gap head and, at the same time, by the use of a
linear type voice coil motor as a driving motor of the head,
HiFD.RTM. has achieved high speed random access of about 3 to 4
times as compared with conventional 3.5 inch type floppy disc
drives. The floatation type dual discrete gap head, similar to a
hard disc, floats by disc rotation and does not contact with the
disc during recording/reproduction leading to long lifetime and
high reliability. In addition, by the dual discrete gap head,
subordination transposition capable of recording/reproduction with
3.5 inch type floppy discs of the present has been realized.
Further, abrasion of a disc can be reduced by the integration of a
new mechanism capable of soft head loading, and high reliability
can be attained by the loading of an error correcting function. The
magnetic recording medium according to the present invention has
been developed to be applicable to such a high capacity floppy disc
system of the next generation which has high capacity of 200 MB,
high transfer rate of 3.6 MB/sec, and has realized subordination
transposition capable of recording/reproduction with 3.5 inch type
floppy discs of the present.
[0091] Magnetic Layer
[0092] The lower layer and the ultrathin magnetic layer of the
magnetic recording medium according to the present invention may be
provided on either one side of the support or may be provided on
both sides. The upper layer may be coated while the lower layer
coated is still wet (W/W coating) or may be coated after the lower
layer coated is dried (W/D coating). Simultaneous or successive wet
on wet coating is preferred in view of the productivity but in the
case of a disc-like medium, wet on dry coating can be sufficiently
used. In the multilayer construction according to the present
invention, as the upper layer and the lower layer can be formed
simultaneously or successively (with W/W coating), a surface
treatment step, e.g., a calendering step, can be utilized
effectively and surface roughness of the upper magnetic layer can
be improved even the layer is an ultrathin layer. The coercive
force (Hc) of the magnetic layer is essential to be 1,800 Oe or
more, and the maximum magnetic flux density (Bm) of the magnetic
metal powder is preferably from 2,000 to 5,000 G.
[0093] Ferromagnetic Powder
[0094] Ferromagnetic powders which can be used in the present
invention are preferably ferromagnetic alloy powders containing
.alpha.-Fe as a main component.
[0095] Ferromagnetic metal powders which can be used in the present
invention preferably comprise Fe and Co as main components and also
contain Al, Mg and a rare earth element.
[0096] The ferromagnetic metal powder of the present invention is
composed of Fe and Co as main components, and the atomic ratio of
Al/(Fe+Co) is from 3.0 to 15.4%, preferably from 4.5 to 15.0%, and
more preferably from 6.0 to 12.0%. The atomic ratio of the sum
total of rare earth elements/(Fe+Co) is from 0.5 to 9.0%,
preferably from 1.0 to 8.0%, and more preferably from 1.0 to 6.0%.
The atomic ratio of Mg/(Fe+ Co) is preferably from 0.05 to 3.0%,
more preferably from 0.1 to 2.5%, and particularly preferably from
0.1 to 2.0%.
[0097] The proportion of Co to Fe is necessary to be from 3 to 50
atomic %, preferably from 5 to 45 atomic %, more preferably from 10
to 45 atomic %, and particularly preferably from 20 to 35 atomic %.
The ferromagnetic metal powder comprises Fe and Co, and the atomic
ratio of the sum total of rare earth elements/Al is preferably from
0.05 to 1.20, more preferably from 0.10 to 1.0, and particularly
preferably from 0.1 to 0.6. The elements largely exist on surface
of the powder.
[0098] When the ferromagnetic metal powder used in the magnetic
layer of the present invention contains Fe, Co, Al, Mg and rare
earth elements within the above ranged, excellent high density
recording characteristics, excellent running durability and
excellent weather resistance can be obtained. By virtue of the
incorporation of the above essential components in the
above-described respective ranges, the ferromagnetic metal powder
is imparted with narrow and uniform particle size distribution,
appropriate hardness and excellent dispersibility, although
particle sizes are fine, as a result, excellent running durability
and excellent weather resistance can be obtained. That is, if the
atomic ratio of Al/(Fe+Co) is less than 4.5%, running durability is
deteriorated, while when it is greater than 15.4%, high density
recording characteristics are deteriorated. If the atomic ratio of
the sum total of rare earth elements/(Fe+ Co) is less than 0.5%, it
is difficult to obtain excellent high density recording
characteristics, and if it is greater than 9.0%, excellent running
durability can hardly be obtained. When the atomic ratio of
Mg/(Fe+Co) is less than 0.05%, the compatibility of excellent high
density recording characteristics and excellent running durability
becomes difficult, and when it is greater than 3.0%, excellent high
density recording characteristics, excellent running durability and
excellent weather resistance are liable to be incompatible.
[0099] Examples of the rare earth elements which can be used in the
ferromagnetic metal fine powder of the present invention include
Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and
Lu. Among these, Y is preferred.
[0100] The content of Y is preferably from 40 to 100 atomic %, more
preferably from 55 to 100 atomic %, based on the sum total of rare
earth elements.
[0101] The preparation method of the ferromagnetic metal fine
powder is not particularly limited but the method as described in
JP-A-8-279137 is exemplified.
[0102] Concretely, goethite is first formed from an aqueous
solution of Fe salt, or an aqueous solution of Fe salt and Co salt,
and then to the obtained suspension of goethite are added and mixed
an aqueous solution of a Co-containing compound, a rare earth
element compound, an Al-containing compound, an Mg-containing
compound or a compound of elements described below to prepare the
goethite suspension.
[0103] The thus-obtained goethite suspension is granulated, dried,
reduced and then gradually oxidized to obtain the ferromagnetic
metal fine powder of the present invention.
[0104] Further, the following methods other than the above method
for preparing the ferromagnetic metal fine powder is
exemplified.
[0105] For example, the monodispersed hematite particle, or the
goethited hematite particle, if necessary, is treated with a
Co-containing compound, a rare earth element compound, an
Al-containing compound or an Mg-containing compound, and then
reduced to prepare the ferromagnetic metal fine powder of the
present invention.
[0106] Now, in the step of forming the goethite, a part of Al
compound may be added.
[0107] The ferromagnetic metal fine powder of the present invention
can be prepared by well-known processes, such as a method
comprising reducing a composite organic acid salt (mainly an
oxalate) with a reducing gas (e.g., hydrogen); a method comprising
pyrolysis of a metal carbonyl compound; a method comprising adding
to an aqueous solution of a ferromagnetic metal a reducing agent
(e.g., sodium boronhydride, hypophosphite or hydrazine) to conduct
reduction; and a method comprising evaporating a metal in a low
pressure inert gas to obtain a fine powder.
[0108] Now, in order to satisfy the composition of the
ferromagnetic metal fine powder and further the desired
electromagnetic characteristics according to the present invention,
the kind and amount of the compound added, the dehydration
condition, the reducing condition and the gradual oxidation
condition may be properly changed. Also, in order to improve the
electromagnetic characteristics, the ferromagnetic metal fine
powder may be reduced again.
[0109] Examples of the gradual oxidation treatments include a
method comprising immersing powders in an organic solvent and then
drying; a method comprising immersing powders in an organic solvent
and then charging an oxygen-containing gas to form oxide films on
the surfaces thereof and drying; and a method comprising forming
oxide films on the surfaces of powders by regulating partial
pressure of an oxygen gas and an inert gas without using an organic
solvent.
[0110] Among these treatments, the method comprising forming oxide
films on the surfaces of powders by regulating partial pressure of
an oxygen gas and an inert gas without using an organic solvent is
preferred for the present invention.
[0111] These ferromagnetic powders may contain, in addition to the
prescribed atoms, the following atoms, e.g., Si, S, Ca, Ti, V, Cr,
Cu, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, P,
Mn, Zn, Ni, Sr and B. These elements (i.e., atoms) are usually used
in an amount of from 5.times.10.sup.-4 to 1.times.10.sup.-1 atomic
% based on Fe.
[0112] The concrete examples in which Co, Al and rare earth
elements are added to the ferromagnetic metal fine powders in
addition to Fe are described in JP-A-6-215360, JP-A-7-210856,
JP-A-8-185624 and JP-A-8-279142. These ferromagnetic powders may be
previously treated with the later described dispersant, lubricant,
surfactant, and antistatic agent before dispersion. Specific
examples thereof are disclosed in 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. Specific
examples of ferromagnetic metal powders containing Mg are disclosed
in JP-B-1-51042, JP-B-8-31366, JP-A-63-222404 and JP-A-5-54371.
Specific examples of ferromagnetic metal powders containing Fe, Co,
Al, Mg and rare earth elements are disclosed in JP-A-9-27117 and
JP-A-9-35247.
[0113] Ferromagnetic powders which can be preferably used in the
magnetic layer according to the present invention have a specific
surface area (S.sub.BET) as measured by the BET method of from 40
to 80 m.sup.2/g, preferably from 45 to 70 m.sup.2/g. When S.sub.BET
is less than 40 m.sup.2/g, noise increases and when more than 80
m.sup.2/g, good surface property is obtained with difficulty, which
is not preferred. Ferromagnetic powders which can be preferably
used in the magnetic layer according to the present invention have
a crystallite size of generally from 80 to 180 .ANG., preferably
from 100 to 180 .ANG., and more preferably from 110 to 175 .ANG..
The length of the long axis of ferromagnetic powders is generally
from 0.01 to 0.25 .mu.m, preferably from 0.03 to 0.15 .mu.m, and
more preferably from 0.03 to 0.12 .mu.m. Ferromagnetic powders
preferably have an acicular ratio of from 3.0 to 15.0, more
preferably from 3.0 to 12.0, and most preferably from 3.0 to 9.0.
Ferromagnetic metal powders have a saturation magnetization
(.sigma..sub.s) of generally from 100 to 180 emu/g, preferably from
110 to 170 emu/g, and more preferably from 125 to 160 emu/g.
Ferromagnetic metal powders have a coercive force (Hc) of
preferably from 1,700 to 3,500 Oe, and more preferably from 1,800
to 3,000 Oe. Surfaces of ferromagnetic metal powders are preferably
covered with a dense oxide film. Ferromagnetic metal powders have a
coercive force of preferably from 2,000 to 4,000 Oe, and more
preferably from 2,100 to 3,500 Oe.
[0114] Ferromagnetic metal powders preferably have a water content
of from 0.01 to 2%. The water content of ferromagnetic metal
powders is preferably optimized by selecting the kinds of binders.
The pH of ferromagnetic metal powders is preferably optimized by
the combination with the binder to be used. The pH range is from 4
to 12, preferably from 6 to 10. Ferromagnetic metal powders may be
surface-treated with Al, Si, P or oxides thereof, if necessary. The
amount thereof is from 0.1 to 20%, preferably from 0.1 to 10%,
based on the ferromagnetic metal powders. Adsorption of a
lubricant, e.g., fatty acid, becomes 100 mg/m.sup.2 or less by
conducting a surface treatment, which is, therefore, preferred.
Soluble inorganic ions (e.g., Na, Ca, Fe, Ni, Sr, etc.) are
sometimes contained in ferromagnetic metal powders. It is preferred
substantially not to contain such soluble inorganic ions but the
properties of ferromagnetic metal powders are not particularly
affected if the content is 200 ppm or less. Ferromagnetic metal
powders for use in the present invention preferably have less voids
and the value thereof is 20% by volume or less, more preferably 5%
by volume or less. The shape of ferromagnetic metal powders is not
particularly limited, and any shape such as an acicular shape, an
ellipsoidal shape or a spindle shape may be used so long as it
satisfies the above-described properties as to particle sizes.
Switching Field Distribution (SFD) of a ferromagnetic metal powder
itself is preferably small, preferably 1.0 or less. It is necessary
to make Hc distribution of ferromagnetic metal powders narrow. When
the SFD is 1.0 or less, electromagnetic characteristics are
excellent, high output can be obtained, reversal of magnetization
becomes sharp and peak shift is less, therefore, suitable for high
density digital magnetic recording. For achieving small Hc
distribution, making particle size distribution of goethite in
ferromagnetic metal powders good and preventing sintering are
effective methods.
[0115] Nonmagnetic Layer
[0116] The lower layer is described in detail below. Inorganic
powders contained in the lower layer of the present invention are
nonmagnetic powders. They can be selected from the following
inorganic compounds such as metal oxide, metal carbonate, metal
sulfate, metal nitride, metal carbide, metal sulfide, etc. Examples
of inorganic compounds are selected from the following compounds
and they can be used alone or in combination, e.g., .alpha.-alumina
having an alpha-conversion rate of 90% or more, .beta.-alumina,
.gamma.-alumina, .theta.-alumina, silicon carbide, chromium oxide,
cerium oxide, .alpha.-iron oxide, hematite, goethite, corundum,
silicon nitride, titanium carbide, titanium oxide, silicon dioxide,
stannic oxide, magnesium oxide, tungsten oxide, zirconium oxide,
boron nitride, zinc oxide, calcium carbonate, calcium sulfate,
barium sulfate, and molybdenum disulfide. Of these compounds,
particularly preferred are titanium dioxide, zinc oxide, iron oxide
and barium sulfate because they have small particle size
distribution and various means for imparting functions, and more
preferred are titanium dioxide and .alpha.-iron oxide. These
nonmagnetic powders preferably have an average particle size of
from 0.005 to 2 .mu.m. If desired, a plurality of nonmagnetic
powders each having a different average particle size may be
combined, or a single nonmagnetic powder having a broad particle
size distribution may be employed so as to attain the same effect
as such a combination. A particularly preferred particle size of
nonmagnetic powders is from 0.01 to 0.2 .mu.m. In particular, when
the nonmagnetic powder is a granular metal oxide, the average
particle size thereof is preferably 0.08 .mu.m or less, and when it
is an acicular metal oxide, the average long axis length thereof is
preferably 0.3 .mu.m or less, more preferably 0.2 .mu.m or less,
and particularly preferably 0.10 .mu.m or less. Nonmagnetic powders
for use in the present invention have an acicular ratio of
preferably from 3 to 12, more preferably from 4 to 9; a tap density
of from 0.05 to 2 g/ml, preferably from 0.2 to 1.5 g/ml; a water
content of from 0.1 to 5 wt %, preferably from 0.2 to 3 wt %, and
more preferably from 0.3 to 1.5 wt %; a pH value of from 2 to 11,
particularly preferably between 5.5 and 10; a specific surface area
(S.sub.BET) of from 1 to 100 m.sup.2/g, preferably from 5 to 80
m.sup.2/g, and more preferably from 10 to 70 m.sup.2/g; a
crystallite size of from 0.004 to 1 .mu.m, and more preferably from
0.04 to 0.1 .mu.m; an oil absorption amount using DBP (dibutyl
phthalate) of from 5 to 100 ml/100 g, preferably from 10 to 80
ml/100 g, and more preferably from 20 to 60 ml/100 g; and a
specific gravity of from 1 to 12, preferably from 3 to 6. The shape
of nonmagnetic powders may be any of acicular, spherical,
polyhedral, or tabular shape. Nonmagnetic powders preferably have a
Mohs' hardness of from 4 to 10. The SA (stearic acid) absorption
amount of nonmagnetic powders is from 1 to 20 .mu.mol/m.sup.2,
preferably from 2 to 15 .mu.mol/m.sup.2, and more preferably from 3
to 8 .mu.mol/m.sup.2. The pH thereof is preferably between 3 and 6.
The surfaces of these nonmagnetic powders are preferably covered
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. Preferred in the point of
dispersibility are Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 and
ZrO.sub.2, and more preferred are Al.sub.2O.sub.3, SiO.sub.2 and
ZrO.sub.2. They can be used in combination or alone. A method in
which the surface treatment may be performed by coprecipitation,
alternatively, surface treatment of particles may be previously
performed to be covered with alumina in the first place, then the
alumina-covered surface is covered with silica, or vice versa,
according to purposes. The surface-covering layer may be porous
layer, if necessary, but a homogeneous and dense surface is
generally preferred.
[0117] Specific examples of nonmagnetic powders for use in the
lower layer according to the present invention include HIT-100
(average particle size: 0.11 .mu.m), and ZA-G1 (manufactured by
Sumitomo Chemical Co., Ltd.) as alumina, Nanotite (average particle
size: 0.06 .mu.m) (manufactured by Showa Denko Co., Ltd.),
.alpha.-hematite DPN-250, DPN-250BX (average long axis length: 0.16
.mu.m, average short axis length: 0.02 .mu.m, axis ratio: 7.45),
DPN-245, DPN-270BX, DPN-550BX, DPN-550RX (average long axis length:
0.15 .mu.m, average short axis length: 0.02 .mu.m, axis ratio:
7.5), and DPN-650RX (manufactured by Toda Kogyo Co., Ltd.),
.alpha.-hematite .alpha.-40 (manufactured by Titan Kogyo Co.,
Ltd.), .alpha.-hematite E270, E271, E300 and E303 (manufactured by
Ishihara Sangyo Kaisha Ltd.) as iron oxide, TTO-51B (average
particle size: from 0.01 to 0.03 .mu.m), TTO-55A (average particle
size: from 0.03 to 0.05 .mu.m), TTO-55B (average particle size:
from 0.03 to 0.05 .mu.m), TTO-55C (average particle size: from 0.03
to 0.05 .mu.m), TTO-55S (average particle size: from 0.03 to 0.05
.mu.m), TTO-55D (average particle size: from 0.03 to 0.05 .mu.m),
and SN-100 (manufactured by Ishihara Sangyo Kaisha Ltd.), titanium
oxide STT-4D (average particle size: 0.013 .mu.m), STT-30D (average
particle size: 0.09 .mu.m), STT-30 (average particle size: 0.12
.mu.m), STT-65C (average particle size: 0.12 .mu.m) (manufactured
by Titan Kogyo Co., Ltd.), titanium oxide MT-100S (average particle
size: 0.015 .mu.m, MT-100T (average particle size: 0.015 .mu.m),
MT-150W (average particle size: 0.015 .mu.m), MT-500B (average
particle size: 0.035 .mu.m), MT-600B (average particle size: 0.050
.mu.m), MT-100F, and MT-500HD (manufactured by Teika Co., Ltd.) as
titanium oxide, FINEX-25 (average particle size: 0.5 .mu.m)
(manufactured by Sakai Chemical Industry Co., Ltd.) as zinc oxide,
BF-1 (average particle size: 0.05 .mu.m, BF-10 (average particle
size: 0.06 .mu.m), BF-20 (average particle size: 0.03 .mu.m), and
ST-M (manufactured by Sakai Chemical Industry Co., Ltd.) as barium
sulfate, DEFIC-Y and DEFIC-R (manufactured by Dowa Mining Co.,
Ltd.), AS2BM and TiO.sub.2 P25 (manufactured by Nippon Aerosil Co.,
Ltd.), and 100A, 500A and calcined products thereof (manufactured
by Ube Industries, Ltd.). Particularly preferred nonmagnetic
powders are titanium dioxide and .alpha.-iron oxide.
[0118] Preparation of .alpha.-iron oxide (hematite) is performed as
follows. .alpha.-Fe.sub.2O.sub.3 powders are obtained from acicular
goethite particles as precursor particles. Acicular goethite
particles can be produced by any of the following methods.
[0119] (1) A method in which an aqueous alkali hydroxide solution
is added to an aqueous ferrous salt solution in equivalent or more
amount to thereby prepare a suspension having pH of 11 or more
containing ferrous hydroxide colloid, then an oxygen-containing gas
is introduced to the suspension obtained at 80.degree. C. or less
to form acicular goethite particles by the oxidation reaction of
ferrous ions;
[0120] (2) A method in which an aqueous ferrous salt solution is
reacted with an aqueous alkali carbonate solution to thereby
prepare a suspension containing FeCO.sub.3, then an
oxygen-containing gas is introduced to the suspension obtained to
form spindle-like goethite particles by the oxidation reaction of
ferrous ions;
[0121] (3) A method in which an aqueous alkali hydroxide solution
or an aqueous alkali carbonate solution is added to an aqueous
ferrous salt solution in the amount of less than equivalent,
thereby an aqueous ferrous salt solution containing ferrous
hydroxide colloid is prepared, then an oxygen-containing gas is
introduced to the aqueous ferrous salt solution obtained to form
acicular goethite nucleus particles by the oxidation reaction of
ferrous ions, thereafter an aqueous alkali hydroxide solution is
added to the aqueous ferrous salt solution containing the acicular
goethite nucleus particles in the amount of equivalent or more
based on Fe.sup.2+in the aqueous ferrous salt solution, then again
an oxygen-containing gas is introduced to the aqueous ferrous salt
solution to grow the acicular goethite nucleus particles; and
[0122] (4) A method in which an aqueous alkali hydroxide solution
or an aqueous alkali carbonate solution is added to an aqueous
ferrous salt solution in the amount of less than equivalent,
thereby an aqueous ferrous salt solution containing ferrous
hydroxide colloid is prepared, then an oxygen-containing gas is
introduced to the aqueous ferrous salt solution obtained to form
acicular goethite nucleus particles by the oxidation reaction of
ferrous ions, thereafter the acicular goethite nucleus particles
are grown in an acidic or neutral region.
[0123] Further, different kinds of elements such as Ni, Zn, P or
Si, which are generally added to the reaction solution during the
goethite particle-forming reaction for improving the properties of
the powder, may be added. Acicular .alpha.-Fe.sub.2O.sub.3
particles can be obtained by dehydrating acicular goethite
particles, which are precursor particles, in the range of 200 to
500.degree. C. and further, if necessary, annealing the particles
by heat treatment at 350 to 800.degree. C. A sintering inhibitor
such as P, Si, B, Zr or Sb may be adhered to the surface of
acicular goethite particles to be dehydrated or annealed. The
reason why annealing by heat treatment at 350 to 800.degree. C. is
conducted is because it is preferred to fill the voids which have
occurred on the surface of acicular .alpha.-Fe.sub.2O.sub.3
particles obtained by the dehydration by melting the extreme
surface of particles to obtain smooth surfaces.
[0124] The .alpha.-Fe.sub.2O.sub.3 powder for use in the present
invention can be obtained by dispersing acicular
.alpha.-Fe.sub.2O.sub.3 particles obtained by dehydration or
annealing in an aqueous solution to make a suspension, adding Al
compounds to the suspension obtained and adjusting the pH, covering
the surface of acicular .alpha.-Fe.sub.2O.sub.3 particles with the
above Al compounds, filtering, washing, drying, pulverizing and, if
necessary, performing other treatments such as deaeration,
compaction and the like. Aluminum salt such as aluminum acetate,
aluminum sulfate, aluminum chloride, and aluminum nitride, and
aluminic acid alkali salt such as sodium aluminate can be used as
the aluminum compound to be used. In this case, the addition amount
of the Al compound is from 0.01 to 50% by weight in terms of Al
based on the .alpha.-Fe.sub.2O.sub.3 powder. If the content is less
than 0.01% by weight, dispersion in the binder resin is
insufficient and if it exceeds 50% by weight, Al compounds
suspending around surfaces of particles unfavorably interact with
each other. The nonmagnetic powder for use in the lower layer
according to the present invention may be covered with one or two
or more selected from the group consisting of P, Ti, Mn, Ni, Zn,
Zr, Sn and Sb, as well as Si compounds, together with Al compounds.
The content of these compounds used together with Al compounds is
each from 0.01 to 50% by weight based on the
.alpha.-Fe.sub.2O.sub.3 powder. If the content is less than 0.01%
by weight, the improvement of dispersibility by the addition can
hardly be obtained, and if it exceeds 50% by weight, Al compounds
suspending around surfaces of particles unfavorably interact with
each other.
[0125] The producing method of titanium dioxide is as follows. The
producing method of titanium dioxide mainly comprises a sulfuric
acid process and a chlorine process. A sulfuric acid process
comprises digesting raw ores of ilmenite with sulfuric acid and
extracting Ti and Fe as sulfate. Iron sulfate is removed by
crystallization-separation, the resulting titanyl sulfate solution
is purified by filtration, water-containing titanium oxide is
precipitated by thermal hydrolysis, the precipitated product is
filtrated and washed, impurities are removed by washing, then a
particle size-adjusting agent is added and calcined at 80 to
1,000.degree. C., thereby crude titanium oxide is obtained. A
rutile type and an anatase type are separated by the kind of
nucleating agent added at hydrolysis. This crude titanium oxide is
pulverized, graded, and surface treated. In a chlorine process,
natural rutile and synthetic rutile are used as raw ores. Ores are
chlorinated in a high temperature reduction state, Ti becomes
TiCl.sub.4 and Fe becomes FeCl.sub.2, and the iron oxide solidified
by cooling is separated from the liquid TiCl.sub.4. The crude
TiCl.sub.4 obtained is purified by fraction, then a nucleating
agent is added thereto and reacted with oxygen instantaneously at
1,000.degree. C. or more, thereby crude titanium oxide is obtained.
The finishing method for imparting to the crude titanium oxide
formed in the oxidation decomposition process the property of
pigment is the same as in the sulfuric acid process.
[0126] After the above titanium oxide material is dry ground, water
and a dispersant are added thereto, grains are wet ground, and
coarse grains are classified by means of a centrifugal separator.
Subsequently, a fine grain slurry is put in a surface treatment
bath and surface covering with metal hydroxide is conducted here.
In the first place, a predetermined amount of an aqueous solution
of salts such as Al, Si, Ti, Zr, Sb, Sn, Zn is added to the tank,
acid or alkali is added to neutralize the solution, and surfaces of
titanium oxide particles are covered with the hydroxide produced.
The water-soluble salts by-produced are removed by decantation,
filtration and washing, the pH of the slurry is adjusted finally
and filtrated, and washed with pure water. The washed cake is dried
using a spray drier or a band drier. The dried cake is finally
ground by jet milling, thereby the final product is obtained.
[0127] Besides the water system, it is also possible to perform
surface treatment by introducing AlCl.sub.3 and SiCl.sub.4 vapor to
the titanium oxide powder, then water vapor is flowed to conduct
surface treatment with Al and Si.
[0128] By the incorporation of carbon blacks into the lower layer,
a desired micro Vickers' hardness can be obtained in addition to
the well-known effects of reducing surface electrical resistance
(Rs) and light transmittance. Further, it is also possible to
obtain the effect of stocking a lubricant by the incorporation of
carbon blacks into the lower layer. Furnace blacks for rubbers,
thermal blacks for rubbers, carbon blacks for coloring, acetylene
blacks, etc. can be used therefor. Carbon blacks used in the lower
layer should optimize the following characteristics by the desired
effects and sometimes more effects can be obtained by the combined
use.
[0129] Carbon blacks for use in the lower layer according to the
present invention have a specific surface area (S.sub.BET) of from
100 to 500 m.sup.2/g, preferably from 150 to 400 m.sup.2/g, a DBP
oil absorption of from 20 to 400 ml/100 g, preferably from 30 to
400 ml/100 g, an average particle size of from 5 to 80 nm,
preferably from 10 to 50 nm, and more preferably from 10 to 40 nm,
and a small amount of carbon blacks having an average particle size
of larger than 80 nm may be contained in the lower layer. Carbon
blacks for use in the lower layer have pH of from 2 to 10, a water
content of from 0.1 to 10%, and a tap density of from 0.1 to 1
g/ml. Specific examples of carbon blacks for use in the present
invention include BLACKPEARLES 2000 (average particle size: 15 nm),
1400 (average particle size: 13 nm), 1300 (average particle size:
13 nm), 1100 (average particle size: 14 nm), 1000, 900 (average
particle size: 15 nm), 800, 880 and 700, L (average particle size:
24 nm), VULCAN XC-72 (average particle size: 30 nm), and P (average
particle size: 19 nm) (manufactured by Cabot Co., Ltd.), #3050B,
#3150B, #3250B (average particle size: 30 nm), #3750B, #3950B
(average particle size: 16 nm), #950 (average particle size: 16
nm), #650B, #970B, #850B (average particle size: 18 nm), MA-600
(average particle size: 18 nm), MA-230, #4000 and #4010
(manufactured by Mitsubishi Kasei Corp.), CONDUCTEX SC (average
particle size: 17 nm), SC-U (average particle size: 20 nm), 975
(average particle size: 20 nm), RAVEN 8800 (average particle size:
13 nm), 8000 (average particle size: 13 nm), 7000 (average particle
size: 14 nm), 5750 (average particle size: 17 nm), 5250 (average
particle size: 19 nm), 5000 (average particle size: 12 nm), 3500
(average particle size: 16 nm), 2100 (average particle size: 17
nm), 2000 (average particle size: 18 nm), 1800 (average particle
size: 18 nm), 1500 (average particle size: 18 nm), 1255 (average
particle size: 23 nm), 1250 (average particle size: 21 nm), and
1035 (average particle size: 27 nm) (manufactured by Columbia
Carbon Co., Ltd.), Ketjen Black EC (average particle size: 30 nm)
(manufactured by Akzo Co., Ltd.), and #80 (average particle size:
20 nm), #70 (average particle size: 27 nm), #60 (average particle
size: 49 nm), #55 (average particle size: 68 nm), and Asahi Thermal
(average particle size: 72 nm) (manufactured by Asahi Carbon Co.,
Ltd.). Carbon blacks having an average particle size of larger than
80 nm which may be used in the lower layer include #50 (average
particle size: 94 nm) and #35 (average particle size: 82 nm)
(manufactured by Asahi Carbon Co., Ltd.). Carbon blacks for use in
the present invention may previously be surface-treated with a
dispersant, may be grafted with a resin, or a part of the surface
thereof may be graphitized before use. Carbon blacks may be
previously dispersed in a binder before addition to the coating
solution. Carbon blacks can be used within the range not exceeding
50% by weight based on the above inorganic powders and not
exceeding 40% by weight based on the total weight of the
nonmagnetic layer. These carbon blacks can be used alone or in
combination. Regarding carbon blacks for use in the present
invention, for example, the disclosure in Handbook of Carbon Blacks
(edited by Carbon Black Association of Japan) may be referred
to.
[0130] Organic powders can be used in the lower layer according to
the purpose. Examples of such organic powders include an acryl
styrene resin powder, a benzoguanamine resin powder, a melamine
resin powder, and a phthalocyanine pigment. In addition, at least
one of a polyolefin resin powder, a polyester resin powder, a
polyamide resin powder, a polyimide resin powder, and a
polyethylene fluoride resin powder can also be used. The producing
methods thereof are disclosed in JP-A-62-18564 and
JP-A-60-255827.
[0131] Binder
[0132] Binder resins, lubricants, dispersants, additives, solvents,
dispersing methods, etc., used for the magnetic layer described
below can be used in the lower layer and the backing layer. In
particular, with respect to the amounts and the kinds of binder
resins, and the amounts and the kinds of additives and dispersants,
well-known prior art techniques regarding the magnetic layer can be
applied in the lower layer according to the present invention.
[0133] Conventionally well-known thermoplastic resins,
thermosetting resins, reactive resins and mixtures of these resins
are used as a binder in the present invention. Thermoplastic resins
having a glass transition temperature of from -100 to 150.degree.
C., a number average molecular weight of from 1,000 to 200,000,
preferably from 10,000 to 100,000, and a polymerization degree of
about 50 to 1,000 can be used in the present invention.
[0134] Examples thereof include polymers or copolymers containing
as a constituting unit the following compounds, such as vinyl
chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid,
acrylate, vinylidene chloride, acrylonitrile, methacrylic acid,
methacrylate, styrene, butadiene, ethylene, vinyl butyral, vinyl
acetal and vinyl ether; polyurethane resins and various rubber
resins. Examples of thermosetting resins and reactive resins usable
in the present invention include phenol resins, epoxy resins,
curable type polyurethane resins, urea resins, melamine resins,
alkyd resins, acrylic reactive resins, formaldehyde resins,
silicone resins, epoxy-polyamide resins, mixtures of polyester
resins and isocyanate prepolymers, mixtures of polyesterpolyol and
polyisocyanate, and mixtures of polyurethane and polyisocyanate.
Details on these resins are described in Plastic Handbook,
published by Asakura Shoten. It is also possible to use well-known
electron beam curable type resins in each layer. Examples of these
resins and producing methods are disclosed in detail in
JP-A-62-256219. These resins can be used alone or in combination.
Examples of preferred combinations include at least one selected
from vinyl chloride resins, vinyl chloride-vinyl acetate
copolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers,
and vinyl chloride-vinyl acetate-maleic anhydride copolymers with
polyurethane resins, or combinations of these resins with
polyisocyanate.
[0135] As polyurethane resins, those having well-known structures
can be used, e.g., polyester polyurethane, polyether polyurethane,
polyether polyester polyurethane, polycarbonate polyurethane,
polyester polycarbonate polyurethane, polycaprolactone
polyurethane, etc. Preferably, at least one polar group selected
from the following groups is introduced into the above binders by
copolymerization or addition reaction for the purpose of further
improving the dispersibility and the durability, e.g., --COOM,
--SO.sub.3M, --OSO.sub.3M, --P.dbd.O(OM).sub.2,
--O--P.dbd.O(OM).sub.2 (wherein M represents a hydrogen atom, or an
alkali metal salt group), --NR.sup.2, --N.sup.+R.sub.3 (R
represents a hydrocarbon group), an epoxy group, --SH, or --CN. The
content of the polar group is from 10.sup.-1 to 10.sup.-8 mol/g,
preferably from 10.sup.-2 to 10.sup.-6 mol/g. It is preferred that
polyurethane resins have at least one OH group at each terminal of
polyurethane molecule, i.e., two or more in total, in addition to
the above polar groups. As OH groups form three dimensional network
structure by crosslinking with the polyisocyanate curing agent,
they are preferably contained in the molecule as many as possible.
In particular, as the reactivity with the curing agent is high, OH
groups are preferably present at terminals of the molecule. It is
preferred for polyurethane to have 3 or more OH groups,
particularly preferably 4 or more OH groups, at terminals of the
molecule. When polyurethane is used in the present invention, the
polyurethane has a glass transition temperature of from -50 to
150.degree. C., preferably from 0 to 100.degree. C., breaking
extension of from 100 to 2,000%, breaking stress of from 0.05 to 10
kg/mm.sup.2, and a yielding point of from 0.05 to 10 kg/mm.sup.2.
Due to these physical properties, coated film exhibiting good
mechanical properties at high rotation rate of preferably 1,800 rpm
or more, more preferably 3,000 rpm or more, can be obtained.
[0136] Specific examples of binders for use in the present
invention include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC,
VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, and PKFE (manufactured by Union
Carbide Co., Ltd.), MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF,
MPR-TS, MPR-TM, and MPR-TAO (manufactured by Nisshin Chemical
Industry Co., Ltd.), 1000W, DX80, DX81, DX82, DX83, and 100FD
(manufactured by Electro Chemical Industry Co., Ltd.), MR-104,
MR-105, MR-110, MR-100, MR-555, and 400X-110A (manufactured by
Nippon Zeon Co., Ltd.) as vinyl chloride copolymers; Nippollan
N2301, N2302, and N2304 (manufactured by Nippon Polyurethane Co.,
Ltd.), Pandex T-5105, T-R3080, T-5201, Burnock D-400, D-210-80,
Crisvon 6109 and 7209 (manufactured by Dainippon Chemicals and
Ink.), Vylon UR8200, UR8300, UR8700, RV530, and RV280 (manufactured
by Toyobo Co., Ltd.), polycarbonate polyurethane, Daipheramine
4020, 5020, 5100, 5300, 9020, 9022, and 7020 (manufactured by
Dainichi Seika K.K.), polyurethane, MX5004 (manufactured by
Mitsubishi Kasei Corp.), polyurethane, Sunprene SP-150
(manufactured by Sanyo Chemical Industries Co. Ltd.), polyurethane,
Salan F310 and F210 (manufactured by Asahi Chemical Industry Co.,
Ltd.) as polyurethane resins, etc.
[0137] The amount of the binder for use in the nonmagnetic layer
and the magnetic layer according to the present invention is from 5
to 50 wt %, preferably from 10 to 30 wt %, based on the amount of
the nonmagnetic powder or the magnetic powder. When vinyl chloride
resins are used, the amount thereof is from 5 to 30 wt %, when
polyurethane resins are used, the amount of the polyurethane resin
is from 2 to 20 wt %, and it is preferred polyisocyanate is used in
an amount of from 2 to 20 wt % in combination with these resins.
However, for instance, when head corrosion is caused by a slight
amount of chlorine due to dechlorination, it is possible to use
polyurethane alone or a combination of polyurethane and isocyanate
alone.
[0138] The magnetic recording medium according to the present
invention may comprise two or more layers. Accordingly, the amount
of the binder, the amounts of vinyl chloride resins, polyurethane
resins, polyisocyanate or other resins contained in the binder, the
molecular weight of each resin constituting the magnetic layer, the
amount of polar groups, or the above-described physical properties
of resins can of course be varied in the nonmagnetic layer and the
magnetic layer, according to necessity. These factors should be
rather optimized in respective layers. Well-known techniques with
respect to multilayer magnetic layers can be used in the present
invention. For example, when the amount of the binder is varied in
each layer, it is effective to increase the amount of the binder
contained in the magnetic layer to reduce scratches on the surface
of the magnetic layer. For improving the head touch against the
head, it is effective to increase the amount of the binder in the
nonmagnetic layer to impart flexibility.
[0139] Examples of the polyisocyanates which can be used in the
present invention include isocyanates, e.g., tolylenediisocyanate,
4,4'-diphenylmethanediisocyanate, hexamethylenediisocyanate,
xylylenediisocyanate, naphthylene-1,5-diisocyanate,
o-toluidinediisocyanate, isophoronediisocyanate, and
triphenylmethanetriisocyanate; reaction products of these
isocyanates with polyalcohols; and polyisocyanates formed by
condensation reaction of isocyanates. These polyisocyanates are
commercially available under the trade names of Coronate L,
Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, and
Millionate MTL (manufactured by Nippon Polyurethane Co., Ltd.),
Takenate D-102, Takenate D-110N, Takenate D-200, and Takenate D-202
(manufactured by Takeda Chemical Industries, Ltd.), and Desmodur L,
Desmodur IL, Desmodur N, and Desmodur HL (manufactured by Sumitomo
Bayer Co., Ltd.). These may be used alone or in combinations of two
or more thereof, taking advantage of a difference in curing
reactivity in each layer.
[0140] Carbon Black, Abrasive
[0141] Examples of carbon blacks for use in the magnetic layer
according to the present invention include furnace blacks for
rubbers, thermal blacks for rubbers, carbon blacks for coloring,
acetylene blacks, etc. Carbon blacks for use in the magnetic layer
of the present invention preferably have a specific surface area
(S.sub.BET) of from 5 to 500 m.sup.2/g, a DBP oil absorption of
from 10 to 400 ml/100 g, an average particle size of from 5 to 300
nm, pH of from 2 to 10, a water content of from 0.1 to 10%, and a
tap density of from 0.1 to 1 g/ml. Specific examples of carbon
blacks for use in the magnetic layer of the present invention
include BLACKPEARLES 2000 (average particle size: 15 nm), 1300
(average particle size: 13 nm), 1000 (average particle size: 16
nm), 900 (average particle size: 15 nm), 905, 800 (average particle
size: 17 nm), and 700 (average particle size: 18 nm), VULCAN XC-72
(average particle size: 30 nm), and STERLING FT (average particle
size: 180 nm) (manufactured by Cabot Co., Ltd.), #80 (average
particle size: 20 nm), #60 (average particle size: 49 nm), #55
(average particle size: 68 nm), #50 (average particle size: 94 nm),
and #35 (average particle size: 94 nm) (manufactured by Asahi
Carbon Co., Ltd.), #2400B (average particle size: 15 nm), #2300
(average particle size: 15 nm), #900 (average particle size: 16
nm), #1000 (average particle size: 18 nm), #30 (average particle
size: 30 nm), #40 (average particle size: 20 nm), and #10B (average
particle size: 84 nm) (manufactured by Mitsubishi Kasei Corp.),
CONDUCTEX SC (average particle size: 17 nm), RAVEN 150 (average
particle size: 18 nm), 50 (average particle size: 21 nm), 40
(average particle size: 24 nm), and 15 (average particle size: 27
nm), RAVEN-MT-P (average particle size: 275 nm) and RAVEN-MT-P
beads (average particle size: 330 nm) (manufactured by Columbia
Carbon Co., Ltd.), Ketjen Black EC40 (average particle size: 30 nm)
(manufactured by Akzo Co., Ltd.), and Thermal Black (average
particle size: 270 nm) (manufactured by Cancarb Co., Ltd.). Carbon
blacks for use in the present invention may previously be
surface-treated with a dispersant, may be grafted with a resin, or
a part of the surface thereof may be graphitized before use. Carbon
blacks may be previously dispersed in a binder before addition to
the magnetic coating solution. These carbon blacks may be used
alone or in combination. Carbon blacks are preferably used in an
amount of from 0.1 to 30 wt % based on the amount of the
ferromagnetic powder. Carbon blacks can serve various functions
such as preventing static charges, reducing a friction coefficient,
imparting a light-shielding property and improving a film strength.
Such functions vary depending upon the kind of carbon blacks to be
used. Accordingly, it is of course possible in the present
invention to select and determine the kinds, the amounts and the
combinations of the carbon blacks to be added to the upper magnetic
layer and the lower nonmagnetic layer, on the basis of the above
mentioned various properties such as the particle size, the oil
absorption amount, the electroconductivity and the pH value, or
these should be rather optimized in respective layers. Regarding
carbon blacks for use in the magnetic layer of the present
invention, for example, the disclosure in Handbook of Carbon Blacks
(edited by Carbon Black Association of Japan) can be referred
to.
[0142] As the abrasive usable in the present invention, well-known
materials essentially having a Mohs' hardness of 6 or more may be
used alone or in combination. Examples of such abrasives include
.alpha.-alumina having an alpha-conversion rate of 90% or more,
.beta.-alumina, silicon carbide, chromium oxide, cerium oxide,
.alpha.-iron oxide, corundum, artificial diamond, silicon nitride,
silicon carbide, titanium carbide, titanium oxide, silicon dioxide,
and boron nitride. Composites composed of these abrasives
(abrasives obtained by surface-treating with other abrasives) may
also be used. Compounds or elements other than the main component
are often contained in these abrasives, but the intended effect can
be attained so far as the content of the main component is 90% or
more. Abrasives essentially have a particle size of from 0.01 to 2
.mu.m, preferably from 0.01 to 1 .mu.m, more preferably from 0.01
to 0.5 .mu.m, and particularly preferably from 0.01 to 0.3 .mu.m,
and, in particular, for improving electromagnetic characteristics,
abrasives having narrow particle size distribution are preferred.
For improving durability, a plurality of abrasives each having a
different particle size may be combined according to necessity, or
a single abrasive having a broad particle size distribution may be
employed so as to attain the same effect as such a combination.
Preferably, abrasives for use in the present invention have a tap
density of from 0.3 to 2 g/ml, a water content of from 0.1 to 5%, a
pH value of from 2 to 11 and a specific surface area (S.sub.BET) of
from 1 to 30 m.sup.2/g. The shape of the abrasives to be used in
the present invention may be any of acicular, spherical and
die-like shapes. Preferably, the abrasive has a shape partly with
edges, because a high abrasive property is given. Specific examples
of abrasives for use in the present invention include as examples
of .alpha.-iron oxide, AKP-12 (average particle size: 0.50 .mu.m),
AKP-15 (average particle size: 0.45 .mu.m), AKP-20 (average
particle size: 0.39 .mu.m), AKP-30 (average particle size: 0.23
.mu.m), AKP-50 (average particle size: 0.16 .mu.m), HIT-20, HIT-30,
HIT-55 (average particle size: 0.20 .mu.m), HIT-60, HIT-70 (average
particle size: 0.15 .mu.m), HIT-80, and HIT-100 (average particle
size: 0.11 .mu.m) (manufactured by Sumitomo Chemical Co., Ltd.),
ERC-DBM (average particle size: 0.22 .mu.m), HP-DBM (average
particle size: 0.22 .mu.m), and HPS-DBM (average particle size:
0.19 .mu.m) (manufactured by Reynolds International Inc.), WA10000
(average particle size: 0.29 .mu.m) (manufactured by Fujimi Kenma
K.K.), UB20 (average particle size: 0.13 .mu.m) (manufactured by
Uemura Kogyo K. K.), G-5, as examples of chromium oxide, G-5
(average particle size: 0.32 .mu.m), Kromex U2 (average particle
size: 0.18 .mu.m), and Kromex U1 (average particle size: 0.17
.mu.m) (manufactured by Nippon Chemical Industrial Co., Ltd.), as
examples of .alpha.-iron oxide, TF100 (average particle size: 0.14
.mu.m) and TF140 (average particle size: 0.17 .mu.m) (manufactured
by Toda Kogyo Co., Ltd.), as examples of silicon carbide,
.beta.-Random Ultrafine (average particle size: 0.16 .mu.m)
(manufactured by Ibiden Co., Ltd.), and as examples of silicon
dioxide, B-3 (average particle size: 0.17 .mu.m) (manufactured by
Showa Mining Co., Ltd.). These abrasives may also be added to a
nonmagnetic layer, if necessary. By incorporating abrasives into a
nonmagnetic layer, it is possible to control the surface shape or
prevent abrasives from protruding. Particle sizes and amounts of
abrasives to be added to a magnetic layer and a nonmagnetic layer
should be selected independently at optimal values.
[0143] In the case of high capacity floppy discs of rotation rate
of 1,800 rpm or more, in particular 3,000 rpm or more, it is
preferred to use a diamond powder as an abrasive.
[0144] Diamond powders which can be used in the present invention
are essential to have the average particle size of 2 .mu.m or less,
preferably from 0.01 to 1.0 .mu.m, more preferably from 0.05 to 0.8
.mu.m, and most preferably from 0.05 to 0.3 .mu.m. When the average
particle size is less than 0.01 .mu.m, the effect of improving
durability is liable to lower as compared to the addition amount,
while when it is larger than 2.0 .mu.m, noise is liable to increase
even though durability is improved, which is not suitable for
achieving the object of the present invention.
[0145] In the present invention, the maximum size of each diamond
particle is taken as a particle size, and the average value of
determined values of 500 particles by random sampling by means of
an electron microscope is taken as an average particle size.
[0146] The addition amount of a diamond powder in the present
invention is from 0.01 to 5 wt %, preferably from 0.03 to 3.00 wt
%, based on the weight of the ferromagnetic powder. If the addition
amount is less than 0.01 wt %, durability is obtained with
difficulty and if it exceeds 5 wt %, the effect of noise reduction
by means of the addition of a diamond powder is reduced.
[0147] The addition amount and the average particle size of a
diamond powder are regulated within the above ranges from the
viewpoint of noise and durability, but the addition amount thereof
is preferably as small as possible in view of noise. It is
preferred in the magnetic recording medium of the present invention
to appropriately select the amount and the average particle size
suitable for magnetic recording devices from the above ranges.
[0148] Further, with respect to the particle size distribution of a
diamond powder, it is preferred that the number of particles having
the particle size of 200% or more of the average particle size
accounts for 5% or less of the entire number of diamond particles,
and the number of particles having the particle size of 50% or less
of the average particle size accounts for 20% or less of the entire
number of diamond particles. The maximum value of the particle size
of the diamond powder for use in the present invention is about
3.00 .mu.m, preferably about 2.00 .mu.m, and the minimum value is
about 0.01 .mu.m, preferably about 0.02 .mu.m.
[0149] Particle size distribution is found by counting numbers of
respective sizes based on the average particle size at the time of
particle size determination.
[0150] Particle size distribution of a diamond powder also
influences durability and noise of the magnetic medium. If the
particle size distribution is broader than the above-described
range, the effect corresponding to the average particle size set up
in the present invention deviates as described above, i.e., if many
particles have too large particle sizes, noise is increased and the
head is scratched. While when there exist many particles having too
small particle sizes, abrasive effect is insufficient. Further, a
diamond powder having extremely narrow particle size distribution
is expensive, therefore, the above-described range is economically
advantageous as well.
[0151] Diamond powders can be used in combination with
conventionally used abrasives, e.g., an alumina abrasive, in the
present invention. Better effects on durability and SN ratio are
obtained when a small amount of a diamond powder alone is used but,
for economical reason, etc., an alumina abrasive can be used in
combination with a diamond powder in an amount of preferably from 1
to 30 wt %, more preferably from 3 to 25 wt %, based on the
magnetic powder. In this case, addition amount of abrasives can be
considerably reduced due to the addition of a diamond powder as
compared with the amount necessary to ensure durability with
alumina alone, which is preferred in view of the security of
durability and the reduction of noise.
[0152] Producing methods of a diamond powder of a micrometer size
include (1) a static high pressure method, (2) an explosion method,
and (3) a vapor phase method. In a static high pressure method (1),
a crystal having a particle size of several 10 .mu.m or more is
prepared in the first place, and the resulting crystal is
pulverized to obtain a diamond powder of a sub-micrometer size. In
an explosion method (2), extra high pressure is generated by shock
wave by means of the explosion of an explosive, and a black smoke
is converted to a diamond by making use of the generated extra high
pressure. The diamond produced by this method is a polycrystalline
diamond which is said to have a primary particle of something
between 20 .ANG. and 50 .ANG.. In a vapor phase method (3), a
gaseous compound containing a carbon such as a hydrocarbon is
charged into a closed container under the condition of normal
pressure or less with a hydrogen gas, a high temperature zone is
formed therein by plasma, etc., and the starting compound is
decomposed to precipitate a diamond on a substrate, e.g., Si or
Mo.
[0153] Specific examples of diamond powders include LS600F, LS600T,
LS600F coated products (coated products coated with 30% or 56%
nickel), LS-NPM and BN2600 (manufactured by LANDS SUPERABRASIVES,
CO.), which are preferred as diamond powders with arbitrary
particle sizes of from 0 to 100 .mu.m are available. Besides the
above, IRM 0-1/4 (average particle size: 0.12 .mu.m), and IRM 0-1
(average particle size: 0.60 .mu.m) (manufactured by Tomei Diamond
Industrial Co., Ltd.) can be used.
[0154] Additive
[0155] As additives which can be used in the magnetic layer and the
nonmagnetic layer of the present invention, those having a
lubrication effect, an antistatic effect, a dispersing effect and a
plasticizing effect may be used, and by the combined use of
additives, comprehensive improvement of capacities can be
contrived. As additives having a lubricating effect, lubricants
giving remarkable action on adhesion caused by the friction of
surfaces of materials with each other are used. Lubricants are
classified into two types. Lubricants used for a magnetic recording
medium cannot be judged completely whether they show fluid
lubrication or boundary lubrication, but according to general
concepts they are classified into higher fatty acid esters, liquid
paraffins and silicon derivatives showing fluid lubrication and
long chain fatty acids, fluorine surfactants and
fluorine-containing high polymers showing boundary lubrication. In
a coating type magnetic recording medium, lubricants exist in a
state dispersed in a binder or in a state partly adsorbed onto the
surface of a ferromagnetic powder, and they migrate to the surface
of a magnetic layer. The speed of migration depends on whether the
compatibility of the binder and the lubricant is good or bad. The
speed of migration is slow when the compatibility of the binder and
the lubricant is good and the migration speed is fast when the
compatibility is bad. As one idea as to good or bad of the
compatibility, there is a means of comparison of dissolution
parameters of both. A nonpolar lubricant is effective for fluid
lubrication and a polar lubricant is effective for boundary
lubrication. In the present invention, at least three in total of
these higher fatty acid ester showing fluid lubrication and long
chain fatty acid showing boundary lubrication having respectively
different characteristics are preferably used in combination to
obtain high capacity, high density and high durability. Solid
lubricants can also be used in combination with these.
[0156] Examples of solid lubricants which can be used in the
present invention include molybdenum disulfide, tungsten graphite
disulfide, boron nitride, and graphite fluoride. Examples of long
chain fatty acids showing boundary lubrication include monobasic
fatty acids having from 10 to 24 carbon atoms (which may contain an
unsaturated bond or which may be branched) and metal salts thereof
(e.g., with Li, Na, K or Cu). Examples of fluorine surfactants and
fluorine-containing high polymers include fluorine-containing
silicons, fluorine-containing alcohols, fluorine-containing esters,
fluorine-containing alkyl sulfates and alkali metal salts thereof.
Examples of higher fatty acid esters showing fluid lubrication
include mono-fatty acid esters, di-fatty acid esters or tri-fatty
acid esters composed of a monobasic fatty acid having from 10 to 24
carbon atoms (which may contain an unsaturated bond or may be
branched) and any one of mono-, di-, tri-, tetra-, penta- and
hexa-alcohols having from 2 to 12 carbon atoms (which may contain
an unsaturated bond or may be branched), and fatty acid esters of
monoalkyl ethers of alkylene oxide polymers. In addition to the
above, examples further include liquid paraffins, and as silicon
derivatives, silicone oils such as dialkylpolysiloxane (the alkyl
has from 1 to 5 carbon atoms), dialkoxypolysiloxane (the alkoxy has
from 1 to 4 carbon atoms), monoalkyl-monoalkoxypolysiloxane (the
alkyl has from 1 to 5 carbon atoms and the alkoxy has from 1 to 4
carbon atoms), phenylpolysiloxane, and fluoroalkylpolysiloxane (the
alkyl has from 1 to 5 carbon atoms), silicons having a polar group,
fatty acid-modified silicons and fluorine-containing silicons.
[0157] Examples of other lubricants which can be used in the
present invention include alcohols such as mono-, di-, tri-,
tetra-, penta- or hexa-alcohols having from 12 to 22 carbon atoms
(which may contain an unsaturated bond or may be branched), alkoxy
alcohols having from 12 to 22 carbon atoms, and fluorine-containing
alcohols, polyethylene waxes, polyolefins such as polypropylenes,
ethylene glycols, polyglycols such as polyethylene oxide waxes,
alkyl phosphates and alkali metal salts thereof, alkyl sulfates and
alkali metal salts thereof, polyphenyl ethers, fatty acid amides
having from 8 to 22 carbon atoms, and aliphatic amines having from
8 to 22 carbon atoms.
[0158] Examples of additives having an antistatic effect, a
dispersing effect and a plasticizing effect include
phenylphosphonic acids, specifically PPA (manufactured by Nissan
Chemical Industries, Ltd.), etc., .alpha.-naphthylphosphoric acids,
phenylphosphoric acids, diphenylphosphoric acids,
p-ethylbenzenephosphonic acids, phenylphosphinic acids,
aminoquinones, various kinds of silane coupling agents, titanium
coupling agents, fluorine-containing alkyl sulfates and alkali
metal salts thereof.
[0159] A lubricant particularly preferably used in the present
invention are fatty acids and fatty acid esters, in addition, other
different lubricants and additives can be used in combination with
them as well. Specific examples thereof are exemplified below. As
fatty acid, examples of saturated fatty acids include caprylic acid
(C.sub.7H.sub.15COOH, melting point: 16.degree. C.), pelargonic
acid (C.sub.8H.sub.17COOH, melting point: 15.degree. C.), capric
acid (C.sub.9H.sub.19COOH, melting point: 31.5.degree. C.),
undecylic acid (C.sub.10H.sub.21COOH, melting point: 28.6.degree.
C.), lauric acid (C.sub.11H.sub.23COOH, melting point: 44.degree.
C.), specifically NAA-122 (manufactured by Nippon Oils and Fats
Co., Ltd.), tridecylic acid (C.sub.12H.sub.25COOH, melting point:
45.5.degree. C.), myristic acid (C.sub.13H.sub.27COOH, melting
point: 58.degree. C.), specifically NAA-142 (manufactured by Nippon
Oils and Fats Co., Ltd.), pentadecylic acid (C.sub.14H.sub.29COOH,
melting point: 53 to 54.degree. C.), palmitic acid
(C.sub.15H.sub.31COOH, melting point: 63 to 64.degree. C.),
specifically NAA-160 (manufactured by Nippon Oils and Fats Co.,
Ltd.), heptadecylic acid (C.sub.16H.sub.33COOH, melting point: 60
to 61.degree. C.), stearic acid (C.sub.17H.sub.35COOH, melting
point: 71.5 to 72.degree. C.), specifically NAA-173K (manufactured
by Nippon Oils and Fats Co., Ltd.), nonadecanoic acid
(C.sub.18H.sub.37COOH, melting point: 68.7.degree. C.), arachic
acid (C.sub.19H.sub.39COOH, melting point: 77.degree. C.), and
behenic acid (C.sub.21H.sub.43COOH, melting point: 81 to 82.degree.
C.). Examples of unsaturated fatty acids include oleic acid
(C.sub.17H.sub.33COOH(cis), melting point: 16.degree. C.),
specifically oleic acid manufactured by Kanto Kagaku Co., Ltd.,
elaidic acid (C.sub.17H.sub.33COOH(trans), melting point: 44 to
45.degree. C.), specifically elaidic acid manufactured by Wako Pure
Chemical Industries Ltd., cetoleic acid (C.sub.21H.sub.41COOH,
melting point: 33.7.degree. C.), erucic acid (C.sub.21H.sub.41COOH,
melting point: 33.4 to 34.degree. C.), specifically erucic acid
manufactured by Nippon Oils and Fats Co., Ltd., brassidic acid
(C.sub.21H.sub.41COOH(trans), melting point: 61.5.degree. C.),
linoleic acid (C.sub.17H.sub.31COOH, boiling point: 228.degree. C.
(14 mm)), and linolenic acid (C.sub.17H.sub.29COOH, boiling point:
197.degree. C. (4 mm)). Examples of branched saturated fatty acids
include isostearic acid (CH.sub.3CH(CH.sub.3)(CH.sub.2).sub.14COOH,
melting point: 67.6 to 68.1.degree. C.).
[0160] Examples of esters are described below. Examples of laurates
include isocetyl laurate
(C.sub.11H.sub.23COOCH.sub.2CH(C.sub.6H.sub.13)C- .sub.8H.sub.17),
oleyl laurate (C.sub.11H.sub.23COOC.sub.18H.sub.35), and stearyl
laurate (C.sub.11H.sub.23COOC.sub.18H.sub.37); examples of
myristates include isopropyl myristate
(C.sub.13H.sub.27COOCH(CH.sub.3).s- ub.2), specifically Enujerubu
IPM (manufactured by Shin-Nihon Rika Co., Ltd.), butyl myristate
(C.sub.13H.sub.27COOC.sub.4H.sub.9), isobutyl myristate
(C.sub.13H.sub.27COO-iso-C.sub.4H.sub.9), specifically Enujerubu
IBM (manufactured by Shin-Nihon Rika Co., Ltd.), heptyl myristate
(C.sub.13H.sub.27COOC.sub.7H.sub.15), octyl myristate
(C.sub.13H.sub.27COOC.sub.8H.sub.17), isooctyl myristate
(C.sub.13H.sub.27COOCH.sub.2CH(C.sub.2H.sub.5)C.sub.4H.sub.9), and
isocetyl myristate
(C.sub.13H.sub.27COOCH.sub.2CH(C.sub.6H.sub.13)C.sub.8-
H.sub.17).
[0161] Examples of palmitates include octyl palmitate
(C.sub.15H.sub.31COOC.sub.8H.sub.17), decyl palmitate
(C.sub.15H.sub.31COOC.sub.10H.sub.21), isooctyl palmitate
(C.sub.15H.sub.31COOCH.sub.2CH(C.sub.2H.sub.5)C.sub.4H.sub.9),
isocetyl palmitate
(C.sub.15H.sub.31COOCH.sub.2CH(C.sub.6H.sub.13)C.sub.8H.sub.17)- ,
2-octyldodecyl palmitate
(C.sub.15H.sub.31COOCH.sub.2CH(C.sub.8H.sub.17)- C.sub.12H.sub.25),
2-hexyldodecyl palmitate (C.sub.15H.sub.31COOCH.sub.2CH-
(C.sub.6H.sub.13)C.sub.12H.sub.25), and oleyl palmitate
(C.sub.15H.sub.31COOC.sub.18H.sub.35).
[0162] Examples of stearates include propyl stearate
(C.sub.17H.sub.35COOC.sub.3H.sub.7), isopropyl stearate
(C.sub.17H.sub.35COOCH(CH.sub.3).sub.2), butyl stearate
(C.sub.17H.sub.35COOC.sub.4H.sub.9), specifically butyl stearate
manufactured by Nippon Oils and Fats Co., Ltd., sec-butyl stearate
(C.sub.17H.sub.35COOCH(CH.sub.3)C.sub.2H.sub.5), tert-butyl
stearate (C.sub.17H.sub.35COOC(CH.sub.3).sub.3), amyl stearate
(C.sub.17H.sub.35COOC.sub.5H.sub.11), isoamyl stearate
(C.sub.17H.sub.35COOCH.sub.2CH.sub.2CH(CH.sub.3).sub.2), hexyl
stearate (C.sub.17H.sub.35COOC.sub.6H.sub.13), heptyl stearate
(C.sub.17H.sub.35COOC.sub.7H.sub.15), specifically MYB-185
(manufactured by Matsumoto Yushi Co., Ltd.), octyl stearate
(C.sub.17H.sub.35COOC.sub.8- H.sub.17), specifically N-octyl
stearate manufactured by Nippon Oils and Fats Co., Ltd., isooctyl
stearate (C.sub.17H.sub.35COO-iso-C.sub.8H.sub.1- 7), specifically
FAL-123 (manufactured by Takemoto Yushi Co., Ltd.), decyl stearate
(C.sub.17H.sub.35COOC.sub.10H.sub.21), isodecyl stearate
(C.sub.17H.sub.35COO-iso-C.sub.10H.sub.21), dodecyl stearate
(C.sub.17H.sub.35COOC.sub.12H.sub.25), isotridecyl stearate
(C.sub.17H.sub.35COO-iso-C.sub.13H.sub.27), 2-ethylhexyl stearate
(C.sub.17H.sub.35COOCH.sub.2CH--(C.sub.2H.sub.5)C.sub.4H.sub.9),
isohexadecyl stearate or isocetyl stearate
(C.sub.17H.sub.35COO-iso-C.sub- .16H.sub.33), specifically
Enujerubu HDS (manufactured by Shin-Nihon Rika Co., Ltd.),
isostearyl stearate (C.sub.17H.sub.35COO-iso-C.sub.18H.sub.37- ),
and oleyl stearate (C.sub.17H.sub.35COOC.sub.18H.sub.37).
[0163] Examples of behenates include isotetracosyl behenate
(C.sub.21H.sub.43COOCH.sub.2CH (C.sub.6H.sub.13)C.sub.12H.sub.25),
specifically Enujerubu DTB (manufactured by Shin-Nihon Rika Co.,
Ltd.).
[0164] Examples of glycol type esters include those disclosed in
JP-A-59-227030 and JP-A-59-65931, e.g., butoxyethyl stearate
(C.sub.17H.sub.35COOCH.sub.2CH.sub.2OC.sub.4H.sub.9), butoxyethyl
oleate (C.sub.17H.sub.33COOCH.sub.2CH.sub.2OC.sub.4H.sub.9),
diethylene glycol monobutyl ether stearate or butoxyethoxyethyl
stearate (C.sub.17H.sub.35COO(CH.sub.2CH.sub.2O).sub.2--
C.sub.4H.sub.9), tetraethylene glycol monobutyl ether stearate
(C.sub.17H.sub.35COO(CH.sub- .2CH.sub.2O).sub.4C.sub.4H.sub.9),
diethylene glycol monophenyl ether stearate
(C.sub.17H.sub.35COO(CH.sub.2CH.sub.2O).sub.2C.sub.6H.sub.6), and
diethylene glycol mono-2-ethylhexyl ether stearate
(C.sub.17H.sub.35COO(CH.sub.2CH.sub.2O).sub.2CH.sub.2CH(C.sub.2H.sub.5)C.-
sub.4H.sub.9).
[0165] Examples of isostearates include isocetyl isostearate
(iso-C.sub.17H.sub.35COOCH.sub.2CH(C.sub.6H.sub.13)C.sub.8H.sub.17),
specifically I.C.I.S. (manufactured by Higher Alcohol Co., Ltd.),
oleyl isostearate (iso-C.sub. 17H.sub.35COOC.sub.18H.sub.35),
stearyl isostearate (iso-C.sub.17H.sub.35COOC.sub.18H.sub.37),
isostearyl isostearate
(iso-C.sub.17H.sub.35COO-iso-C.sub.18H.sub.37), and eicosenyl
isostearate (iso-C.sub.17H.sub.35COOC.sub.22H.sub.43).
[0166] Examples of oleates include butyl oleate
(C.sub.17H.sub.33COOC.sub.- 4H.sub.9), specifically Enujerubu BO
(manufactured by Shin-Nihon Rika Co., Ltd.), oleyl oleate
(C.sub.17H.sub.33COOC.sub.18H.sub.35), and ethylene glycol dioleyl
(C.sub.17H.sub.33COOCH.sub.2CH.sub.2OCOC.sub.17H.sub.33).
[0167] Examples of erucates include oleyl erucic acid
(C.sub.21H.sub.41COOC.sub.18H.sub.35).
[0168] Examples of diesters include dioleyl maleate
(C.sub.18H.sub.35OCOCH.dbd.CHCOOC.sub.18H.sub.35), neopentyl glycol
didecanoate
(C.sub.10H.sub.21COOCH.sub.2C(CH.sub.3).sub.2CH.sub.2OCOC.sub-
.10H.sub.21), ethylene glycol dilaurate
(C.sub.11H.sub.23COOCH.sub.2CH.sub- .2OCOC.sub.11H.sub.23),
ethylene glycol dioleyl (C.sub.17H.sub.33COOCH.sub-
.2CH.sub.2OCOC.sub.17H.sub.33), 1,4-butanediol distearate
(C.sub.17H.sub.35COO(CH.sub.2).sub.4OCOC.sub.17H.sub.35),
1,4-butanediol dibehenate
(C.sub.21H.sub.43COO(CH.sub.2).sub.4OCOC.sub.21H.sub.43),
1,10-decanediol dioleyl
(C.sub.17H.sub.33COO(CH.sub.2).sub.10OCOC.sub.17H- .sub.33), and
2-butene-1,4-diol cetoleyl (C.sub.21H.sub.41COOCH.sub.2CH.db-
d.CHCH.sub.2OCOC.sub.21H.sub.41).
[0169] Examples of triesters include caprylic acid triglyceride
(C.sub.7H.sub.15COOCH.sub.2CH(OCOC.sub.7H.sub.15)CH.sub.2OCOC.sub.7H.sub.-
15.
[0170] In addition to the above-described fatty acid esters and
fatty acids, examples of additives which can be used include
alcohols such as oleyl alcohol (C.sub.18H.sub.35OH), stearyl
alcohol (C.sub.18H.sub.37OH), and lauryl alcohol
(C.sub.12H.sub.25OH).
[0171] Examples of fatty acid amides include lauric acid amide
(C.sub.11H.sub.23CONH.sub.2), specifically lauric acid amide
manufactured by Tokyo Kasei Co., Ltd., myristic acid amide
(C.sub.13H.sub.27CONH.sub.2- ), palmitic acid amide
(C.sub.15H.sub.31CONH.sub.2), oleic acid amide
(cis-C.sub.8H.sub.17CH.dbd.CH(CH.sub.2).sub.7CONH.sub.2),
specifically Armoslip CP-P (manufactured by Lion Akzo Co., Ltd.),
erucic acid amide
(cis-C.sub.8H.sub.17CH.dbd.CH(CH.sub.2).sub.11CONH.sub.2),
specifically Armoslip E (manufactured by Lion Akzo Co., Ltd.), and
stearic acid amide (C.sub.17H.sub.35CONH.sub.2), specifically
Armide HT (manufactured by Lion Akzo Co., Ltd.).
[0172] Examples of silicone compounds include TAV-3630, TA-3, and
KF-69 (manufactured by Shin-Etsu Chemical Co., Ltd.).
[0173] Additionally, examples of other additives which may be used
include nonionic surfactants such as alkylene oxides, glycerols,
glycidols and alkylphenol-ethylene oxide adducts; cationic
surfactants such as cyclic amines, ester amides, quaternary
ammonium salts, hydantoin derivatives, heterocyclic compounds,
phosphoniums and sulfoniums; anionic surfactants containing an
acidic group such as carboxylic acid, sulfonic acid, phosphoric
acid, sulfate groups or phosphate groups; and amphoteric
surfactants such as amino acids, aminosulfonic acids, sulfates or
phosphates of amino alcohols and alkylbetains. The details of these
surfactants are described in Handbook of Surfactants (published by
Sangyo Tosho Co., Ltd.). These lubricants and antistatic agents may
not always be 100% pure and may contain impurities such as isomers,
non-reacted materials, byproducts, decomposed products and oxides,
in addition to the main component. However, the content of such
impurities is preferably 30% or less, more preferably 10% or
less.
[0174] Lubricants and surfactants for use in the present invention
respectively have different physical functions. The kinds, amounts
and proportions of combination generating synergistic effect of
these lubricants should be determined optimally in accordance with
the purpose. The nonmagnetic layer and the magnetic layer can
separately contain different fatty acids each having a different
melting point so as to prevent bleeding out of the fatty acids to
the surface, or different esters each having a different boiling
point, a different melting point or a different polarity so as to
prevent bleeding out of the esters to the surface. Also, the
amounts of surfactants are controlled so as to improve the coating
stability, or the amount of the lubricant in the lower layer is
made larger than the amount in the magnetic layer so as to improve
the lubricating effect of the surface thereof. Examples are by no
means limited thereto. In general, the total amount of the
lubricants is from 0.1 to 50%, preferably from 2 to 25%, based on
the amount of the magnetic powder or the nonmagnetic powder.
[0175] All or a part of the additives to be used in the present
invention may be added to the magnetic coating solution or the
nonmagnetic coating solution in any step of the preparation. For
example, additives may be blended with a magnetic powder before the
kneading step, may be added during the step of kneading a magnetic
powder, a binder and a solvent, may be added during the dispersing
step, may be added after the dispersing step, or may be added
immediately before coating. According to the purpose, there is a
case of capable of attaining the object by coating all or a part of
the additives simultaneously with or successively after the coating
of the magnetic layer. According to the purpose, lubricants may be
coated on the surface of the magnetic layer after the calendering
treatment or after the completion of slitting.
[0176] Layer Construction
[0177] The thickness of the support in the magnetic recording
medium of the present invention is, for example, from 2 to 100
.mu.m, preferably from 2 to 80 .mu.m. Particularly, the thickness
of the support for a computer tape is from 3.0 to 6.5 .mu.m,
preferably from 3.0 to 6.0 .mu.m, more preferably from 4.0 to 5.5
.mu.m.
[0178] An undercoating layer (or a subbing layer) may be provided
between the support and the nonmagnetic or magnetic layer for
adhesion improvement. The thickness of this undercoating layer is
from 0.01 to 0.5 .mu.m, preferably from 0.02 to 0.5 .mu.m. The
nonmagnetic layer and the magnetic layer of the magnetic recording
medium according to the present invention may be provided on both
surface sides of the support or may be provided on either one
surface side. When the nonmagnetic layer and the magnetic layer are
provided on only one surface side of the support, a back coating
layer may be provided on the surface side of the support opposite
to the side having the nonmagnetic layer and magnetic layer for the
purpose of static charge prevention and curling correction. The
thickness of this back coating layer is from 0.1 to 4 .mu.m,
preferably from 0.3 to 2.0 .mu.m. Well-known undercoating layers
and back coating layers can be used for this purpose.
[0179] The thickness of the magnetic layer of the magnetic
recording medium of the present invention can be optimally selected
according to the saturation magnetization amount of the head used,
the head gap length, and the recording signal zone, and is
generally from 0.05 to 0.30 .mu.m, preferably from 0.05 to 0.25
.mu.m, and more preferably from 0.05 to 0.20 .mu.m. The magnetic
layer may comprise two or more layers each having different
magnetic characteristics and well-known multilayer magnetic layer
structures can be applied to the present invention.
[0180] The thickness of the lower nonmagnetic layer of the medium
according to the present invention is generally from 0.2 to 5.0
.mu.m, preferably from 0.3 to 3.0 .mu.m, and more preferably from
1.0 to 2.5 .mu.m. The lower layer of the recording medium of the
present invention exhibits the effect of the present invention so
long as it is substantially a nonmagnetic layer even if, or
intendedly, it contains a small amount of a magnetic powder as an
impurity, which is as a matter of course regarded as essentially
the same construction as in the present invention. The term
"substantially a nonmagnetic layer" means that the residual
magnetic flux density of the lower layer is 100 G or less and the
coercive force of the lower layer is 100 Oe or less, preferably the
residual magnetic flux density and the coercive force are zero. If
the lower layer contains a magnetic powder, the content of the
magnetic layer is less than 1/2of the entire inorganic powder
contained in the lower layer.
[0181] Back Coating Layer
[0182] In general, a magnetic tape for a computer data recording is
decidedly required to have an excellent repeating-running property
as compared with a video tape and an audio tape. For maintaining
such a high running durability, it is preferred for the back
coating layer to contain a carbon black and an inorganic
powder.
[0183] Two kinds of carbon blacks respectively having different
average particle sizes are preferably used in combination. In this
case, a combined use of a fine carbon black having the average
particle size of from 10 to 20 nm and a coarse carbon black having
the average particle size of from 230 to 300 nm is preferred. In
general, by the incorporation of a fine carbon black as above, the
surface electrical resistance of the back coating layer and light
transmittance can be set up at low values. There are many kinds of
magnetic recording apparatuses making use of light transmittance of
a tape and making it as signals of operation, therefore, the
addition of fine carbon blacks are particularly effective in such a
case. In addition, a fine carbon black is in general excellent in
retention of a liquid lubricant and contributes to the reduction of
a friction coefficient when a lubricant is used in combination. On
the other hand, a coarse carbon black having a particle size of
from 230 to 300 nm has a function as a solid lubricant and forms
minute protrusions on the surface of a back coating layer to reduce
contact area and contributes to the reduction of a friction
coefficient. However, a coarse carbon black has a drawback such
that particles are liable to drop out from the back coating layer
due to the tape sliding during severe running leading to the
increase of the error rate.
[0184] Specific examples of fine carbon blacks commercially
available include RAVEN 2000B (average particle size: 18 nm) and
RAVEN 1500B (average particle size: 17 nm) (manufactured by
Columbia Carbon Co., Ltd.), BP800 (average particle size: 17 nm)
(manufactured by Cabot Co., Ltd.), PRINTEX90 (average particle
size: 14 nm), PRINTEX95 (average particle size: 15 nm), PRINTEX85
(average particle size: 16 nm), PRINTEX75 (average particle size:
17 nm) (manufactured by Degussa Co., Ltd.), and #3950 (average
particle size: 16 nm) (manufactured by Mitsubishi Kasei Corp.).
[0185] Specific examples of coarse carbon blacks commercially
available include THERMAL BLACK (average particle size: 270 nm)
(manufactured by Cancarb Co., Ltd.) and RAVEN MTP (average particle
size: 275 nm) (manufactured by Columbia Carbon Co., Ltd.).
[0186] When two kinds of carbon blacks respectively having
different average particle sizes are used in combination in a back
coating layer, the proportion of the contents (by weight) of a fine
carbon black having a particle size of from 10 to 20 nm and a
coarse carbon black having a particle size of from 230 to 300 nm is
preferably the former/the latter of from 98/2 to 75/25, more
preferably from 95/5 to 85/15.
[0187] The content of the carbon black in a back coating layer (the
total amount when two kinds are used) is generally from 30 to 80
weight parts, preferably from 45 to 65 weight parts, based on 100
weight parts of the binder.
[0188] It is preferred to use two kinds of inorganic powders
respectively having different hardness.
[0189] Specifically, a soft inorganic powder having a Mohs'
hardness of from 3 to 4.5 and a hard inorganic powder having a
Mohs' hardness of from 5 to 9 are preferably used in
combination.
[0190] By the addition of a soft inorganic powder having a Mohs'
hardness of from 3 to 4.5, a friction coefficient can be stabilized
against repeating-running. Moreover, a sliding guide pole is not
scratched off in hardness within this range. The average particle
size of such a soft inorganic powder is preferably from 30 to 50
nm.
[0191] Examples of soft inorganic powders having a Mohs' hardness
of from 3 to 4.5 include, e.g., calcium sulfate, calcium carbonate,
calcium silicate, barium sulfate, magnesium carbonate, zinc
carbonate and zinc oxide. They can be used alone or in combination
of two or more. Of these, calcium carbonate is particularly
preferred.
[0192] The content of the soft inorganic powder in a back coating
layer is preferably from 10 to 140 weight parts, more preferably
from 35 to 100 weight parts, based on 100 weight parts of the
carbon black.
[0193] By the addition of a hard inorganic powder having a Mohs'
hardness of from 5 to 9, the strength of the back coating layer is
increased and running durability is improved. When such hard
inorganic powders are used together with carbon blacks and the
above-described soft inorganic powders, deterioration due to
repeating sliding is reduced and strong back coating layer can be
obtained. Appropriate abrasive capability is imparted to the back
coating layer by the addition of the hard inorganic powder and the
adhesion of scratched powders to a tape guide pole is reduced. In
particular, when the hard inorganic powder is used in combination
with a soft inorganic powder (in particular, calcium carbonate),
sliding characteristics against a guide pole having a rough surface
is improved and the stabilization of a friction coefficient of the
back coating layer can also be brought about.
[0194] The average particle size of hard inorganic powders is
preferably from 80 to 250 nm, more preferably from 100 to 210
nm.
[0195] Examples of hard inorganic powders having a Mohs' hardness
of from 5 to 9 include, e.g., .alpha.-iron oxide, .alpha. -alumina,
and chromium oxide (Cr.sub.2O.sub.3). These powders may be used
alone or in combination. Of the above, .alpha.-iron oxide and
.alpha.-alumina are preferred. The content of hard inorganic
powders in the back coating layer is generally from 3 to 30 weight
parts, preferably from 3 to 20 weight parts, based on 100 weight
parts of the carbon black.
[0196] When the above soft inorganic powder and hard inorganic
powder are used in combination in the back coating layer, it is
preferred to use them selectively such that the difference of
hardness between soft and hard inorganic powders is 2 or more, more
preferably 2.5 or more, and particularly preferably 3 or more.
[0197] It is preferred that the above-described two kinds of
inorganic powders respectively having different hardness and
specific average particle sizes and the above-described two kinds
of carbon blacks respectively having different specific average
particle sizes are contained in the back coating layer. In
particular, in this combination, calcium carbonate is preferably
contained as a soft inorganic powder.
[0198] Lubricants may be contained in the back coating layer.
Lubricants can be arbitrarily selected from among those which can
be used in a magnetic layer or a nonmagnetic layer as described
above. The content of lubricants added to the back coating layer is
generally from 1 to 5 weight parts based on 100 weight parts of the
binder.
[0199] Support
[0200] The support for use in the present invention preferably has
a thermal shrinkage factor of 0.5% or less both at 100.degree. C.
for 30 minutes and at 80.degree. C. for 30 minutes in every
direction of in-plane of the support, more preferably 0.2% or less.
Moreover, the above-described thermal shrinkage factors of the
support at 100.degree. C. for 30 minutes and at 80.degree. C. for
30 minutes are preferably almost equal in every direction of
in-plane of the support with difference of not more than 10%. The
support is preferably a nonmagnetic support. As a nonmagnetic
support for use in the present invention, well-known films such as
polyesters (e.g., polyethylene terephthalate or polyethylene
naphthalate), polyolefins, cellulose triacetate, polycarbonate,
polyamide, polyimide, polyamideimide, polysulfone, polyaramide,
aromatic polyamide, or polybenzoxazole can be used. Highly strong
supports such as polyethylene naphthalate or polyamide are
preferably used. If necessary, a lamination type support as
disclosed in JP-A-3-224127 can be used to vary the surface
roughnesses of the magnetic layer surface and the base surface. The
support may be previously subjected to surface treatments, such as
a corona discharge treatment, a plasma treatment, an adhesion
assisting treatment, a heat treatment, and a dust removing
treatment. Aluminum or glass substrate can also be used as a
support in the present invention.
[0201] For attaining the object of the present invention, it is
preferred to use the support having a central plane average surface
roughness (SRa) of 5.0 nm or less, preferably 4.0 nm or less, more
preferably 2.0 nm or less, measured by a surface roughness meter
"TOPO-3D" (a product of WYKO Co., Ltd., U.S.A.) by MIRAU method. It
is preferred that the support not only has a small central plane
average surface roughness but also is free from coarse protrusions
(having a height) of 0.5 .mu.m or more. Surface roughness
configuration is freely controlled by the size and the amount of
fillers added to the support. Examples of such fillers include
acryl-based organic powders, as well as oxides or carbonates of Ca,
Si and Ti. The support for use in the present invention preferably
has the maximum height (SRmax) of 1 .mu.m or less, ten point
average roughness (SRz) of 0.5 .mu.m or less, central plane peak
height (SRp) of 0.5 .mu.m or less, central plane valley depth (SRv)
of 0.5 .mu.m or less, central plane area factor (SSr) of from 10%
to 90%, and average wavelength (S.lambda.a) of from 5 .mu.m to 300
.mu.m. For obtaining desired electromagnetic characteristics and
durability, surface protrusion distribution of the support can be
controlled arbitrarily by fillers, e.g., the number of protrusions
having sizes of from 0.01 .mu.m to 1 .mu.m can be controlled each
within the range of from 0 to 2,000 per 0.1 mm.sup.2.
[0202] The F-5 value of the nonmagnetic support for use in the
present invention is preferably from 5 to 50 kg/mm.sup.2, a thermal
shrinkage factor of the support at 100.degree. C. for 30 minutes is
preferably 3% or less, more preferably 1.5% or less, and
particularly preferably 0.5% or less, and a thermal shrinkage
factor at 80.degree. C. for 30 minutes is preferably 1% or less,
more preferably 0.5% or less, and particularly preferably 0.2% or
less. The support has a breaking strength of from 5 to 100
kg/mm.sup.2 , an elastic modulus of from 100 to 2,000 kg/mm.sup.2,
a temperature expansion coefficient of from 10.sup.-4 to
10.sup.-8/.degree. C., preferably from 10.sup.-5 to
10.sup.-6/.degree. C., and a humidity expansion coefficient of
10.sup.-4/RH % or less, preferably 10.sup.-5/RH % or less. These
thermal characteristics, dimensional characteristics and mechanical
strength characteristics are preferably almost equal in every
direction of in-plane of the support with difference of not more
than 10%.
[0203] Producing Method
[0204] Processes of preparing the magnetic coating solution for use
in the magnetic recording medium of the present invention comprises
at least a kneading step, a dispersing step and, optionally,
blending steps to be carried out before and/or after the kneading
and dispersing steps. Any of these respective steps may be composed
of two or more separate stages. Materials such as a magnetic
powder, a nonmagnetic powder, a binder, a carbon black, an
abrasive, an antistatic agent, a lubricant, a solvent, and the like
for use in the present invention may be added at any step at any
time. Each material may be added at two or more steps dividedly.
For example, polyurethane can be added dividedly at a kneading
step, a dispersing step, or a blending step for adjusting viscosity
after dispersion. For achieving the object of the present
invention, the above steps can be performed partly with
conventionally well-known techniques. Powerful kneading machines
such as an open kneader, a continuous kneader, a pressure kneader
or an extruder are preferably used in a kneading step. When a
kneader is used, all or a part of a binder (preferably 30% or more
of the total binders) are kneading-treated in the range of from 15
parts to 500 parts per 100 parts of the magnetic powder or
nonmagnetic powder together with a magnetic powder or a nonmagnetic
powder. Details of these kneading are disclosed in JP-A-1-106338
and JP-A-1-79274. When dispersing a magnetic layer solution and a
nonmagnetic layer solution, glass beads can be used but dispersing
media having a high specific gravity is preferably used and
zirconia beads, titania beads and steel beads are suitable for this
purpose. Optimal particle size and packing density of these
dispersing media should be selected. Well-known dispersing
apparatuses can be used in the present invention.
[0205] The following methods are preferably used for coating the
magnetic recording medium having a multilayer construction of the
present invention. As the first method, the lower layer is coated
by any of gravure coating, roll coating, blade coating, and
extrusion coating apparatuses, which are ordinarily used in the
coating of a magnetic coating solution, and the upper layer is
coated while the lower layer is still wet by means of the support
pressing type extrusion coating apparatus disclosed in
JP-B-1-46186, JP-A-60-238179 and JP-A-2-265672. As the second
method, the upper layer and the lower layer are coated almost
simultaneously using the coating head equipped with two slits for
feeding coating solution as disclosed in JP-A-63-88080,
JP-A-2-17971 and JP-A-2-265672. And as the third method, the upper
layer and the lower layer are coated almost simultaneously using
the extrusion coating apparatus equipped with a backup roll as
disclosed in JP-A-2-174965. For preventing the deterioration of the
electromagnetic characteristics of the magnetic recording medium
due to agglomeration of magnetic powders, it is preferred to impart
shear to the coating solution in the coating head by the methods as
described in JP-A-62-95174 and JP-A-1-236968. With respect to the
viscosity of the coating solution, the range of the numeric values
disclosed in JP-A-3-8471 is necessary to be satisfied. For
realizing the constitution of the present invention, successive
multilayer coating method in which the magnetic layer is coated on
the lower layer after the lower layer has been coated and dried can
of course be used without impairing the effect of the present
invention. However, for reducing coating defects and improving
quality, e.g., dropout, it is preferred to use the above
simultaneous multilayer coating method.
[0206] In the case of a magnetic disc, isotropic orienting property
can be sufficiently obtained in some cases without conducting
orientation using orientating apparatus, but it is preferred to use
well-known random orientation apparatuses, such as disposing cobalt
magnets diagonally and alternately or applying an alternating
current magnetic field using a solenoid. Isotropic orientation in a
ferromagnetic metal fine powder is in general preferably in-plane
two dimensional random orientation, but it may be three dimensional
random orientation having vertical components. Further, it is
possible to impart isotropic magnetic characteristics in the
circumferential direction by vertical orientation using well-known
methods, e.g., using different pole and counter position magnets.
In particular, vertical orientation is preferred when the disc is
used for high density recording. Circumferential orientation can be
conducted using spin coating.
[0207] In the case of a magnetic tape, orientation is conducted in
the machine direction using a cobalt magnet and a solenoid. In
orientation, it is preferred that the drying position of the coated
film can be controlled by controlling the temperature and the
amount of drying air and coating rate. Coating rate is preferably
from 20 to 1,000 m/min. and the temperature of drying air is
preferably 60.degree. C. or more. Preliminary drying can be
performed appropriately before entering the magnet zone.
[0208] Use of heat resisting plastic rolls such as epoxy,
polyimide, polyamide and polyimideamide, or metal rolls is
effective for calendering treatment. Metal rolls are usable for the
treatment particularly when magnetic layers are coated on both
surface sides. Treatment temperature is preferably 50.degree. C. or
more, more preferably 100.degree. C. or more. Line pressure is
preferably 200 kg/cm or more, more preferably 300 kg/cm or
more.
[0209] Physical Properties
[0210] Saturation magnetic flux density of the magnetic layer of
the magnetic recording medium according to the present invention is
from 2,000 to 5,000 G when a ferromagnetic metal powder is used.
Coercive force (Hc) and (Hr) are from 1,800 to 5,000 Oe, preferably
from 1,800 to 3,000 Oe. Coercive force distribution is preferably
narrow, and SFD and SFDr are preferably 0.6 or less. Squareness
ratio is from 0.55 to 0.67, preferably from 0.58 to 0.64, in the
case of two dimensional random orientation, from 0.45 to 0.55 in
the case of three dimensional random orientation, and in the case
of vertical orientation, 0.6 or more, preferably 0.7 or more in the
vertical direction, and when diamagnetical correction is conducted,
0.7 or more, preferably 0.8 or more. Orientation ratio of two
dimensional random orientation and three dimensional random
orientation is preferably 0.8 or more. In the case of two
dimensional random orientation, squareness ratio, Br, Hc and Hr in
the vertical direction are preferably from 0.1 to 0.5 times of
those in the in-plane direction.
[0211] In the case of a magnetic tape, squareness ratio is 0.7 or
more, preferably 0.8 or more.
[0212] The friction coefficient of the magnetic recording medium
according to the present invention against a head at temperature of
-10.degree. C. to 40.degree. C. and humidity of 0% to 95% is 0.5 or
less, preferably 0.3 or less, the surface inherent resistivity of
the magnetic surface is preferably from 10.sup.4 to 10.sup.12
.OMEGA./sq, the charge potential is preferably from -500 V to +500
V, the elastic modulus at 0.5% elongation of the magnetic layer is
preferably from 100 to 2,000 kg/mm.sup.2 in every direction of
in-plane, the breaking strength is preferably from 10 to 70
kg/mm.sup.2, the elastic modulus of the magnetic recording medium
is preferably from 100 to 1,500 kg/mm.sup. 2 in every direction of
in-plane, the residual elongation is preferably 0.5% or less, and
the thermal shrinkage factor at every temperature of 100.degree. C.
or less is preferably 1% or less, more preferably 0.5% or less, and
most preferably 0.1% or less. The glass transition temperature of
the magnetic layer (the maximum of elastic modulus loss by dynamic
viscoelasticity measurement at 110 Hz) is preferably from
50.degree. C. to 120.degree. C., and that of the lower nonmagnetic
layer is preferably from 0.degree. C. to 100.degree. C. The elastic
modulus loss is preferably within the range of from
1.times.10.sup.6 to 8.times.10.sup.9 dyne/cm.sup.2, and loss
tangent is preferably 0.2 or less. If loss tangent is too large,
adhesion failure is liable to occur. These thermal and mechanical
characteristics are preferably almost equal in every direction of
in-plane of the medium with difference of not more than 10%. The
residual amount of the solvent in the magnetic layer is preferably
100 mg/m.sup.2 or less, more preferably 10 mg/m.sup.2 or less. The
void ratio is preferably 30% by volume or less, more preferably 20%
by volume or less, with both of the nonmagnetic layer and the
magnetic layer. The void ratio is preferably smaller for obtaining
high output but in some cases a specific value should be preferably
secured depending on purposes. For example, in a disc-like medium
which is repeatedly used, large void ratio contributes to good
running durability in many cases.
[0213] The magnetic layer has a central plane surface roughness
(Ra) of 5.0 nm or less, preferably 4.0 nm or less, more preferably
3.5 nm or less, and particularly preferably 3.3 nm or less,
measured by a surface roughness meter "TOPO-3D" (a product of WYKO
Co., Ltd., U.S.A.) by MIRAU method. The magnetic layer for use in
the present invention preferably has the maximum height (SRmax) of
0.5 .mu.m or less, ten point average roughness (SRz) of 0.3 .mu.m
or less, central plane peak height (SRp) of 0.3 .mu.m or less,
central plane valley depth (SRv) of 0.3 .mu.m or less, central
plane area factor (SSr) of from 20% to 80%, and average wavelength
(S.lambda.a) of from 5 .mu.m to 300 .mu.m. For obtaining desired
electromagnetic characteristics and a friction coefficient, a
number of surface protrusion of the magnetic layer having sizes
(i.e., height) of from 0.01 .mu.m to 1 .mu.m can be controlled
arbitrarily within the range of from 0 to 2,000 by controlling the
surface property due to fillers in the support, the particle size
and the amount of the magnetic powders added to the magnetic layer,
or by the surface shape of rolls of calender treatment. The range
of curling is preferably not within .+-.3 mm.
[0214] When the magnetic recording medium according to the present
invention comprises a nonmagnetic layer and a magnetic layer, these
physical properties in the nonmagnetic layer and the magnetic layer
can be varied according to purposes. For example, the elastic
modulus of the magnetic layer is made higher to improve running
durability and at the same time the elastic modulus of the
nonmagnetic layer is made lower than that of the magnetic layer to
improve the head touching of the magnetic recording medium.
EXAMPLE
[0215]
1 Preparation of Coating Solution Magnetic Coating Solution: 1ML-1
Ferromagnetic metal powder: 1M-1 100 parts Composition: Fe 100%, Co
30% (atomic ratio) Hc: 2,550 Oe Specific surface area: 55 m.sup.2/g
.sigma..sub.s: 140 emu/g Crystallite size: 120 .ANG. Long axis
length: 0.048 .mu.m Acicular ratio: 4 Al [Al/Fe, atomic ratio:
8.8%, Al/(Fe + Co) = 6.2, atomic %] Y (Y/Fe, atomic ratio: 4.6%)
Vinyl chloride copolymer 12 parts MR110 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 3 parts UR 8200 (manufactured by
Toyobo Co., Ltd.) .alpha.-Alumina 10 parts HIT55 (manufactured by
Sumitomo Chemical Co., Ltd.) Carbon black 5 parts #50 (manufactured
by Asahi Carbon Co., Ltd.) Phenylphosphonic acid 3 parts Butyl
stearate 10 parts Butoxyethyl stearate 5 parts Isohexadecyl
stearate 3 parts Stearic acid 2 parts Methyl ethyl ketone 180 parts
Cyclohexanone 180 parts Magnetic Coating Solution: 1ML-2
Ferromagnetic metal powder: 1M-2 100 parts Composition: Fe 100%, Co
30% (atomic ratio) Hc: 2,360 Oe Specific surface area: 49 m.sup.2/g
.sigma..sub.s: 146 emu/g Crystallite size: 170 .ANG. Long axis
length: 0.100 .mu.m Acicular ratio: 6 SFD: 0.950 Al [Al/Fe, atomic
ratio: 11.1%, Al/(Fe + Co) = 8.5 atomic %] Y (Y/Fe, atomic ratio:
6.7%) pH: 9.4 Vinyl chloride copolymer 10 parts MR110 (manufactured
by Nippon Zeon Co., Ltd.) Polyurethane resin 4 parts UR 5500
(manufactured by Toyobo Co., Ltd.) .alpha.-Alumina 10 parts HIT70
(manufactured by Sumitomo Chemical Co., Ltd.) Carbon black 1 part
#50 (manufactured by Asahi Carbon Co., Ltd.) Phenylphosphonic acid
3 parts Oleic acid 1 part Stearic acid 0.6 part Ethylene glycol
dioleyl 12 parts Methyl ethyl ketone 180 parts Cyclohexanone 180
parts Magnetic Coating Solution: 1ML-3 (acicular magnetic powder
was used) Ferromagnetic metal powder: 1M-3 100 parts Composition:
Fe/Ni = 96/4 Hc: 1,600 Oe Specific surface area: 45 m.sup.2/g
Crystallite size: 220 .ANG. .sigma..sub.s: 135 emu/g Average long
axis length: 0.20 .mu.m Acicular ratio: 9 Vinyl chloride copolymer
12 parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane
resin 5 parts UR 8600 (manufactured by Toyobo Co., Ltd.)
.alpha.-Alumina (particle size: 0.65 .mu.m) 2 parts Chromium oxide
(particle size: 0.35 .mu.m) 15 parts Carbon black (particle size:
0.03 .mu.m) 2 parts Carbon black (particle size: 0.3 .mu.m) 9 parts
Isohexadecyl stearate 4 parts n-Butyl stearate 4 parts Butoxyethyl
stearate 4 parts Oleic acid 1 part Stearic acid 1 part Methyl ethyl
ketone 300 parts Nonmagnetic Coating Solution: 1NU-1 (spherical
inorganic powder was used) Nonmagnetic powder, TiO.sub.2, crystal
system 80 parts rutile Average primary particle size: 0.035 .mu.m
Specific surface area (S.sub.BET): 40 m.sup.2/g pH: 7
TiO.sub.2content: 90% or more DBP oil absorption: 27 to 38 ml/100 g
Surface-covering compound: Al.sub.2O.sub.3, 8 wt % based on total
particles Carbon black 20 parts CONDUCTEX SC-U (manufactured by
Columbia Carbon Co., Ltd.) Vinyl chloride copolymer 12 parts MR110
(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 5 parts
UR 8200 (manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4
parts Butyl stearate 10 parts Butoxyethyl stearate 5 parts
Isohexadecyl stearate 2 parts Stearic acid 3 parts Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent) Nonmagnetic
Coating Solution: 1NU-2 (spherical inorganic powder was used)
Nonmagnetic powder, TiO.sub.2, crystal system 100 parts rutile
Average primary particle size: 0.035 .mu.m Specific surface area
(S.sub.BET): 40 m.sup.2/g pH: 7 TiO.sub.2content: 90% or more DBP
oil absorption: 27 to 38 ml/100 g Surface-covering compound:
Al.sub.2O.sub.3and SiO.sub.2 Ketjen Black EC 13 parts (manufactured
by Akzo Nobel Co., Ltd.) Average primary particle size: 30 nm DBP
oil absorption: 350 ml/100 g pH: 9.5 Specific surface area
(S.sub.BET): 950 m.sup.2/g Volatile content: 1.0% Vinyl chloride
copolymer 16 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)
Polyurethane resin 6 parts UR 8200 (manufactured by Toyobo Co.,
Ltd.) Phenylphosphonic acid 4 parts Ethylene glycol dioleyl 16
parts Oleic acid 1 part Stearic acid 0.8 part Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent) Nonmagnetic
Coating Solution 1NU-3 (acicular inorganic powder was used)
Nonmagnetic powder, .alpha.-Fe.sub.2O.sub.3, hematite 80 parts Long
axis length: 0.15 .mu.m Specific surface area (S.sub.BET): 50
m.sup.2/g Acicular ratio: 6 pH: 9 Surface-covering compound:
Al.sub.2O.sub.3, 8 wt % based on total particles Carbon black 20
parts CONDUCTEX SC-U (manufactured by Columbia Carbon Co., Ltd.)
Vinyl chloride copolymer 12 parts MR110 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 5 parts UR 8200 (manufactured by
Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts Butyl stearate 10
parts Butoxyethyl stearate 5 parts Isohexadecyl stearate 2 parts
Stearic acid 3 parts Methyl ethyl ketone/cyclohexanone 250 parts
(8/2 mixed solvent) Nonmagnetic Coating Solution 1NU-4 (acicular
inorganic powder was used) Nonmagnetic powder,
.alpha.-Fe.sub.2O.sub.3, hematite 100 parts Long axis length: 0.15
.mu.m Specific surface area (S.sub.BET): 50 m.sup.2/g Acicular
ratio: 6 pH: 9 Surface-covering compound: Al.sub.2O.sub.3, 8 wt %
based on total particles Carbon black 18 parts #3250B (manufactured
by Mitsubishi Kasei Corp.) Vinyl chloride copolymer 15 parts MR104
(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 7 parts
UR 5500 (manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4
parts Ethylene glycol dioleyl 16 parts Oleic acid 1.3 parts Stearic
acid 0.8 part Methyl ethyl ketone/cyclohexanone 250 parts (8/2
mixed solvent) Preparation Method 1-1 (W/W)
[0216] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0217] These coating solutions were simultaneously
multilayer-coated on a polyethylene terephthalate support having a
thickness of 62 .mu.m and a central plane average surface roughness
of 3 nm of the surface side on which a magnetic layer was to be
coated. The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m, immediately thereafter the magnetic layer
coating solution was coated on the nonmagnetic layer so as to give
the magnetic layer having the thickness of 0.15 .mu.m. The coated
layers were subjected to random orientation while the magnetic
layer and the nonmagnetic layer were still wet by passing through
an alternating current magnetic field generator having two magnetic
field intensities of frequency of 50 Hz, magnetic field intensity
of 250 Gauss and frequency of 50 Hz, magnetic field intensity of
120 Gauss. After drying, the coated layer was subjected to
calendering treatment with calenders of 7 stages at 90.degree. C.
at line pressure of 300 kg/cm. The obtained web was punched to a
disc of 3.7 inches, the disc was subjected to a surface treatment
by abrasives, encased in 3.7 inch cartridge having a liner inside
(a zip-disc cartridge manufactured by Iomega Co., Ltd., U.S.A.),
and equipped the cartridge with prescribed mechanism parts to
obtain a 3.7 inch floppy disc. A part of samples was subjected to
machine direction orientation using Co magnets with the same pole
and counter positions of 4,000 G before random orientation
treatment.
[0218] In this case, it is preferred to increase the frequency and
magnetic field intensity of the alternating current magnetic field
generator so as to achieve finally sufficient random orientation,
thereby 98% or more of orientation ratio can be obtained.
[0219] Preparation Method 1-2 (W/D)
[0220] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0221] The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m on a polyethylene terephthalate support
having a thickness of 62 .mu.m and a central plane average surface
roughness of 3 nm of the surface side on which the magnetic layer
was to be coated, dried, and subjected to calendering treatment.
The magnetic layer coating solution was coated by blade coating on
the nonmagnetic layer so as to give the magnetic layer having the
thickness of 0.15 .mu.m. The coated layers were subjected to random
orientation by passing through an alternating current magnetic
field generator having two magnetic field intensities of frequency
of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50
Hz, magnetic field intensity of 120 Gauss. The procedure was
carried out in the same manner as in Preparation Method 1-1
hereafter. Calendering of the nonmagnetic layer may be omitted.
[0222] Preparation Method 1-3 (Spin Coating)
[0223] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0224] The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m by spin coating on a polyethylene
terephthalate support having a thickness of 62 .mu.m and a central
plane average surface roughness of 3 nm of the surface side on
which a magnetic layer was to be coated and dried. The magnetic
layer coating solution was coated by spin coating on the
nonmagnetic layer so as to give the magnetic layer having the
thickness of 0.15 .mu.m. The coated layers were oriented using a Co
magnets with the same pole and counter positions of 6,000 G in the
circumferential direction and the surface of the magnetic layer was
smoothed by batch system rolling treatment by which the same
pressure as in Preparation Method 1-1 can be applied. The procedure
was carried out in the same manner as in Preparation Method 1-1
hereafter. Also, the magnetic layer may be coated by spin coating
on the nonmagnetic layer while the nonmagnetic layer coated by spin
coating is still wet. By using the spin coating process, not only
the amount of residual magnetization in the recording direction can
be made large but also vertical magnetization components of
ferromagnetic metal powders of short acicular ratio can be reduced
and symmetric property of reproduced wave form can be improved.
[0225] Preparation Method 1-4 (Monolayer)
[0226] The coating solution for the magnetic layer prepared in
Preparation Method 1-1 was coated on a polyethylene terephthalate
support so as to reach the thickness of 0.15 .mu.m. The procedure
was carried out in the same manner as in Preparation Method 1-1
hereafter.
2 Support 1B-1: Polyethylene Terephthalate Thickness: 62 .mu.m F-5
value: MD: 114 MPa, TD: 107 MPa Breaking strength: MD: 276 MPa, TD:
281 MPa Breaking extension: MD: 174 MPa, TD: 139 MPa Thermal
shrinkage factor (80.degree. C., 30 minutes): MD: 0.04%, TD: 0.05%
Thermal shrinkage factor (100.degree. C., 30 minutes): MD: 0.2%,
TD: 0.3% Temperature expansion coefficient: Long axis: 15 .times.
10.sup.-5/.degree. C. Short axis: 18 .times. 10.sup.-5/.degree. C.
Central plane average surface roughness: 3 nm Support 1B-2:
Polyethylene Naphthalate Thickness: 55 .mu.m Central plane average
surface roughness: 1.8 nm Thermal shrinkage factor (80.degree. C.,
30 minutes): MD: 0.007%, TD: 0.007% Thermal shrinkage factor
(100.degree. C., 30 minutes): MD: 0.02%, TD: 0.02% Temperature
expansion coefficient: Long axis: 10 .times. 10.sup.-5/.degree. C.
Short axis: 11 .times. 10.sup.-5/.degree. C. Orientation 1O-1:
Random orientation 1O-2: Orientation in the machine direction using
a Co magnet first, then random orientation 1O-3: Orientation in the
machine direction using a Co magnet first, then in the machine
direction using a solenoid
EXAMPLES 1-1 TO 1-16, COMPARATIVE EXAMPLE 1-1, AND REFERENCE
EXAMPLE 1-1
[0227] Various samples were prepared by combining the
above-described each method arbitrarily as shown in Table 1-1. The
results of evaluations are shown in Table 1-2.
EXAMPLES 1-17 TO 1-32 AND COMPARATIVE EXAMPLES 1-2 TO 1-4
[0228] Various samples of magnetic discs were prepared in the same
manner as in Example 1-11, except that the components of the
magnetic coating solution 1ML-2 (a ferromagnetic metal powder and
an .alpha.-alumina (an abrasive)) were changed as shown in Tables
1-3 and 1-4. The results of evaluations are shown in Tables 1-5 and
1-6.
[0229] Methods of evaluations of each sample are as follows.
[0230] (1) Magnetic characteristics (Hc):
[0231] Magnetic characteristics were measured using a vibrating
sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10
KOe.
[0232] (2) Central plane average surface roughness (Ra):
[0233] Surface roughness (Ra) of the area of about 250
.mu.m.times.250 .mu.m, Rrms, peak-valley values were measured using
"TOPO3D" (a product of WYKO, U.S.A.) by 3D-MIRAU method. The
wavelength of measurement was about 650 nm and spherical
compensation and cylindrical compensation were applied. Measurement
was performed using a light interference type non-contact surface
roughness meter.
[0234] (3) Areal recording density:
[0235] Areal recording density means a value obtained by
multiplying linear recording density by track density.
[0236] (4) Linear recording density:
[0237] Linear recording density means a bit number of signals
recorded per 1 inch in the recording direction.
[0238] (5) Track density:
[0239] Track density means a track number per 1 inch.
[0240] (6) .phi.m:
[0241] .phi.m is the amount of magnetization per unit area of a
magnetic recording medium, which is represented by Bm (Gauss)
multiplying thickness. This is the value obtained by using a
vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.)
at Hm 10 KOe, which can be directly measured.
[0242] (7) Error rate of disc:
[0243] The above signals of linear recording density were recorded
on the disc by (2,7) RLL modulation system and error rate was
measured.
[0244] (8) Thickness of magnetic layer:
[0245] The sample having a thickness of about 0.1 .mu.m was cut out
with a diamond cutter in the machine direction of the magnetic
medium, observed with a transmission type electron microscope of
from 10,000 to 100,000, preferably from 20,000 to 50,000
magnifications and photographed. The print size of the photograph
was from A4 (i.e., 297.times.210 mm) to A5 (i.e., 210.times.148 mm)
sizes. The present inventors paid attentions to the difference of
the shapes of the ferromagnetic powders and the nonmagnetic powders
of the magnetic layer and the nonmagnetic layer and rimmed the
interface and the surface of the magnetic layer with black color by
visual judgement. Thereafter, the distance of the rimmed lines was
measured by the image processing apparatus "IBAS2" (manufactured by
Zeiss Corp.). Measurement was conducted from 85 to 300 times when
the length of the sample photograph was 21 cm. The average measured
value at that time was taken as d, and the standard deviation of
the measured value was taken as .sigma.. d depended on the
description in JP-A-5-298653 and .sigma. was obtained by equation
(2) in JP-A-5-298653. di means each measured value and n is from 85
to 300.
[0246] (9) Running durability:
[0247] A floppy disc drive ("ZIP100", a product of IOMEGA CORP.,
U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at
the position of radius of 38 mm. Recording was conducted at
recording density of 34 kfci, then reproduced the signals recorded
and this was taken as 100%. The disc was run for 25 days (600
hours) under the following thermo-cycle condition, which being
taken as one cycle. Output was monitored every 24 hours of running
and the point when the initial reproduction output became 70% or
less was taken as NG.
[0248] Thermo-Cycle Flow
[0249] 25.degree. C., 50% RH, 1 hr.fwdarw.(temperature up, 2
hr).fwdarw.60.degree. C., 20% RH, 7 hr.fwdarw.(temperature down, 2
hr).fwdarw.25.degree. C., 50% RH, 1 hr.fwdarw.(temperature down, 2
hr).fwdarw.5.degree. C., 50% RH, 7 hr.fwdarw.(temperature up, 2
hr).fwdarw.(this cycle was repeated).
[0250] (10) Liner wear:
[0251] A sample was run for 600 hours under the same condition as
in the evaluation of running durability with the head being off,
and after completion of running, cartridge case of the sample was
opened and the surface of the magnetic layer was visually evaluated
by the following criteria.
[0252] o: No defect was observed on the surface of the magnetic
layer.
[0253] .DELTA.: Fine scratches were generated on a part of the
surface of the magnetic layer.
[0254] x: Fine scratches were generated on the entire surface of
the magnetic layer.
[0255] (11) Characteristics of ferromagnetic metal powder:
[0256] Evaluation of the characteristics of the ferromagnetic metal
powder was conducted according to the method disclosed in
JP-A-8-279137.
[0257] (12) Weather resistance of magnetic layer:
[0258] The weather resistance of the magnetic layer was measured by
the following measurement method after the magnetic recording
medium was stored under the circumstance of 60.degree. C. and 90%
RH for 7 days.
[0259] The fluctuation of Hc (.DELTA.Hc) of the magnetic layer
after storage was calculated by the following equation:
.DELTA.Hc(%)=100.times.(Hc after storage-Hc before storage)/Hc
before storage
[0260] The fluctuation of Hc (.DELTA.Hc) after storage is
preferably from -5.0% to +10.0%, more preferably from -3.0% to
+8.0%, most preferably from -1.0% to +6.0%.
[0261] Also, decrease of .phi.m (i.e., .DELTA..phi.m) was
calculated by the following equation:
.DELTA..phi.m (%)=100.times.(.phi.m before storage-.phi.m after
storage)/.phi.m before storage
[0262] The .phi.m's before storage and after storage were measured
using a vibrating sample magnetometer ("VSM-5" produced by Toei
Kogyo Co., Ltd.) under the condition of a time constant of 0.1
sec., a sweeping speed of 3 min./10 KOe and a magnetic field
measured of 10 KOe.
[0263] The decrease of .phi.m (i.e., .DELTA..phi.m) is preferably
within 10%, more preferably within 6%.
3TABLE 1-1 Disc Magnetic Layer Thick- Surface Prepara- Prescrip-
ness Hc Roughness .phi.m Lower tion Orien- Sample No. tion (.mu.m)
(Oe) (nm) (emu/cm.sup.2) Layer Support Method tation Example 1-1
1ML-2 0.15 2,360 3.5 4.8 .times. 10.sup.-3 1NU-1 1B-1 Method 1-1
10-1 Example 1-2 1ML-2 0.15 2,360 2.3 4.8 .times. 10.sup.-3 1NU-2
1B-1 Method 1-1 10-1 Example 1-3 1ML-2 0.15 2,360 1.9 4.8 .times.
10.sup.-3 1NU-3 1B-1 Method 1-1 10-1 Example 1-4 1ML-2 0.15 2,360
1.7 4.8 .times. 10.sup.-3 1NU-4 1B-1 Method 1-1 10-1 Example 1-5
1ML-2 0.05 2,400 1.4 1.6 .times. 10.sup.-3 1NU-4 1B-1 Method 1-1
10-1 Example 1-6 1ML-2 0.10 2,380 1.6 3.2 .times. 10.sup.-3 1NU-4
1B-1 Method 1-1 10-1 Example 1-7 1ML-2 0.20 2,330 1.9 6.4 .times.
10.sup.-3 1NU-4 1B-1 Method 1-1 10-1 Example 1-8 1ML-2 0.15 2,360
1.5 4.8 .times. 10.sup.-3 1NU-4 1B-2 Method 1-1 10-1 Example 1-9
1ML-1 0.15 2,550 2.5 4.2 .times. 10.sup.-3 1NU-4 1B-1 Method 1-1
10-1 Example 1-10 1ML-2 0.15 2,360 2.5 4.8 .times. 10.sup.-3 1NU-4
1B-1 Method 1-2 10-1 Example 1-11 1ML-2 0.15 2,360 1.7 4.8 .times.
10.sup.-3 1NU-4 1B-1 Method 1-1 10-2 Example 1-12 1ML-2 0.15 2,660
1.6 4.8 .times. 10.sup.-3 1NU-4 1B-1 Method 1-3 10-3 Example 1-13
1ML-2 0.15 2,370 3.4 4.6 .times. 10.sup.-3 None 1B-1 Method 1-4
10-1 ReferenceExample 1-1 1ML-2 0.32 2,300 2.5 10.2 .times.
10.sup.-3 1NU-4 1B-1 Method 1-1 10-1 Comparative Example 1ML-3 0.15
1,600 3.1 4.8 .times. 10.sup.-3 1NU-4 1B-1 Method 1-1 10-1 1-1
[0264]
4 TABLE 1-2 Linear Areal Re- Recording Track cording Density Error
Magnetic Layer Density Density (G bit/ Rate .DELTA.Hc .DELTA..phi.m
Sample No. (tpi) (kbpi) inch.sup.2) (10.sup.-5) (%) (%) Example 1-1
5,200 144 0.75 0.2 +2.6 +5.0 Example 1-2 5,200 144 0.75 0.08 +2.5
+5.0 Example 1-3 5,200 144 0.75 0.03 +2.5 +4.8 Example 1-4 5,200
144 0.75 0.01 +2.5 +4.8 Example 1-5 5,200 144 0.75 0.06 +3.3 +5.1
Example 1-6 5,200 144 0.75 0.01 +3.0 +5.1 Example 1-7 5,200 144
0.75 0.2 +2.3 +5.0 Example 1-8 5,200 144 0.75 0.008 +2.5 +4.9
Example 1-9 5,200 144 0.75 0.004 +4.0 +6.0 Example 1-10 5,200 144
0.75 0.1 +2.5 +5.0 Example 1-11 5,200 144 0.75 0.001 +2.5 +5.0
Example 1-12 5,200 144 0.75 0.0002 +2.5 +5.0 Example 1-13 5,200 144
0.75 0.2 +2.0 +4.8 Reference 5,200 144 0.75 3.5 +2.1 +4.9 Example
1-1 Comparative 5,200 144 0.75 40 -1.0 +8.2 Example 1-1 Example
1-14 7,500 200 1.5 0.8 +2.5 +5.0 Example 1-15 6,000 166 1.0 0.08
+2.5 +5.0 Example 1-16 3,000 120 0.36 0.007 +2.5 +5.0
[0265] In each of Examples 1-14 to 1-16, the disc in Example 1-11
was used and error rate was determined with varying linear
recording density and track density.
[0266] From the results in Table 1-2, it can be seen that the error
rate of the magnetic disc according to the present invention, in
particular, in high density recording region is 1.times.10.sup.-5
or less, which is conspicuously excellent.
5TABLE 1-3 Ferromagnetic Metal Powder Particle Size Ferro- Long
Crystal- magnetic Composition (atomic %) Axis lite Powder Rare
Earth Length Size No. Co/Fe Al/Fe + Co Element/Fe + Co (.mu.m)
(.ANG.) 1M-4 10 14.8 Y: 4.0 0.12 175 1M-5 20 11.2 Y: 4.8 0.11 160
1M-6 30 8.5 Sm: 1.5 0.09 120 1M-7 35 5.9 Y: 3.3 0.09 145 1M-8 30
6.3 Y: 3.4 0.07 135 1M-9 30 10.5 Y: 4.6 0.08 145 1M-10 30 8.5 Nd:
1.3 0.10 160 Y: 2.0 1M-11 5 14.0 Y: 1.3 0.11 180 1M-12 10 15.6 Y:
6.2 0.11 175 1M-13 30 8.6 Y: 5.5 0.08 175 1M-14 30 4.3 Y: 3.5 0.08
175 1M-15 7 1.9 Y: 0.3 0.08 175
[0267]
6TABLE 1-4 Abrasive Average Particle Kind of Size Trade Abrasive
(.mu.m) Manufacturer Name P-1: .alpha.-Alumina 0.23 Sumitomo
Chemical Co., Ltd. AKP P-2: .alpha.-Alumina 0.16 Sumitomo Chemical
Co., Ltd. HIT P-3: .alpha.-Alumina 0.11 Sumitomo Chemical Co., Ltd.
HIT P-4: Cr.sub.2O.sub.3 0.29 Nippon Chemical Industrial Co.,
Kromex Ltd. P-5: SiC 0.16 Ibiden Co., Ltd. .beta.-Random
[0268]
7 TABLE 1-5 Magnetic Layer Magnetic Surface Powder Abrasive Thick-
Rough- Error .sigma..sub.s Amount ness Hc ness .phi.m Rate Sample
No. Kind (emu/g) Kind (parts) (.mu.m) (Oe) (nm) (emu/cm.sup.2)
(10.sup.-5) Example 1-17 1M-4 136 P-2 8 0.15 1,850 2.5 4.8 .times.
10.sup.-3 0.2 Example 1-18 1M-5 140 P-2 8 0.15 2,100 2.2 5.0
.times. 10.sup.-3 0.08 Example 1-19 1M-6 145 P-2 8 0.15 2,290 2.0
5.2 .times. 10.sup.-3 0.02 Example 1-20 1M-7 148 P-2 8 0.15 2,400
1.9 5.4 .times. 10.sup.-3 0.007 Example 1-21 1M-8 152 P-2 8 0.15
2,360 2.2 6.4 .times. 10.sup.-3 0.01 Example 1-22 1M-9 148 P-2 8
0.15 2,360 1.8 5.3 .times. 10.sup.-3 0.004 Example 1-23 1M-10 143
P-2 8 0.15 2,200 2.2 5.2 .times. 10.sup.-3 0.08 Example 1-24 1M-9
148 P-2 15 0.15 2,390 2.2 4.5 .times. 10.sup.-3 0.1 Example 1-25
1M-9 148 P-2 5 0.15 2,330 1.6 6.0 .times. 10.sup.-3 0.001 Example
1-26 1M-9 148 P-2 2 0.15 2,310 1.4 7.0 .times. 10.sup.-3 0.0006
Example 1-27 1M-4 136 P-2 0.5 0.15 1,810 1.0 5.5 .times. 10.sup.-3
0.08 Example 1-28 1M-5 140 P-2 2 0.15 2,040 1.4 6.0 .times.
10.sup.-3 0.007 Example 1-29 1M-9 148 P-1 8 0.15 2,330 2.5 5.6
.times. 10.sup.-3 0.08 Example 1-30 1M-9 148 P-3 8 0.15 2,360 1.6
5.2 .times. 10.sup.-3 0.001 Example 1-31 1M-9 148 P-4 8 0.15 2,350
2.2 5.0 .times. 10.sup.-3 0.03 Example 1-32 1M-9 148 P-5 8 0.15
2,340 2.2 5.0 .times. 10.sup.-3 0.04 Comparative 1M-11 134 P-2 8
0.15 1,750 3.2 4.5 .times. 10.sup.-3 10 Example 1-2 Comparative
1M-12 132 P-2 8 0.15 1,760 2.8 4.4 .times. 10.sup.-3 5 Example 1-3
Comparative 1M-15 145 P-2 8 0.15 1,900 3.0 4.9 .times. 10.sup.-3 5
Example 1-4
[0269]
8TABLE 1-6 Running Magnetic Layer Durability Liner .DELTA.Hc
.DELTA..phi.m Sample No. (hour) Wear (%) (%) Example 1-17 600
.smallcircle. .+-.0.0 +3.0 Example 1-18 600 .smallcircle. +1.5 +2.4
Example 1-19 600 .smallcircle. +2.8 +5.3 Example 1-20 600
.smallcircle. +3.3 +5.0 Example 1-21 600 .smallcircle. +3.0 +4.8
Example 1-22 600 .smallcircle. +2.5 +5.0 Example 1-23 600
.smallcircle. +2.6 +5.1 Example 1-24 600 .smallcircle. +2.3 +4.8
Example 1-25 600 .smallcircle. +2.6 +5.0 Example 1-26 600
.smallcircle. +2.7 +5.2 Example 1-27 600 .smallcircle. +0.1 +2.1
Example 1-28 600 .smallcircle. +1.6 +2.5 Example 1-29 600
.smallcircle. +2.5 +4.8 Example 1-30 600 .smallcircle. +2.5 +4.9
Example 1-31 600 .smallcircle. +2.4 +4.9 Example 1-32 600
.smallcircle. +2.4 +4.8 Comparative 600 .smallcircle. -2.4 +8.9
Example 1-2 Comparative 600 .smallcircle. -0.1 +3.5 Example 1-3
Comparative 240 x -1.1 +6.5 Example 1-4
[0270] In running durability of Table 1-6, 600 means 600 hours.
[0271] As is apparent from the results in Table 1-6, the magnetic
disc according to the present invention is excellent in high
density characteristics and also has excellent running
durability.
[0272] The magnetic recording medium according to the present
invention comprises a support having thereon a magnetic layer
comprising a ferromagnetic metal powder dispersed in a binder,
which is a magnetic disc for recording signals of from 0.17 to 2 G
bit/inch.sup.2 of areal recording density, wherein the coercive
force of the magnetic layer is 1,800 Oe or more, the ferromagnetic
metal powder is composed of Fe and Co as main components, and the
atomic ratio of Al/(Fe+Co) is from 4.5 to 15.0%, the magnetic layer
has a dry thickness of from 0.05 to 0.25 .mu.m and .phi.m of
preferably from 8.0.times.10.sup.-3 to 1.0.times.10.sup.-3
emu/cm.sup.2. It can be seen that the magnetic recording medium of
the present invention having high capacity, excellent high density
characteristics, in particular, markedly improved running
durability, has been realized due to the above constitution of the
present invention, which could never be obtained by conventional
techniques.
9 Preparation of Coating Solution Magnetic Coating Solution: 2ML-1
Ferromagnetic metal powder: 2M-1 100 parts Composition: Fe 100%, Co
30% (atomic ratio) Hc: 2,550 Oe Specific surface area: 55 m.sup.2/g
.sigma..sub.s: 140 emu/g Crystallite size: 120 .ANG. Long axis
length: 0.048 .mu.m Acicular ratio: 4 Al (Al/Fe, atomic ratio:
8.8%) Y (Y/Fe, atomic ratio: 4.6%) Atomic ratio A [Rare Earth
Element/(Fe + Co)]: 3.2% Atomic ratio B [Rare Earth Element/Al]:
0.52% Vinyl chloride copolymer 12 parts MR110 (manufactured by
Nippon Zeon Co., Ltd.) Polyurethane resin 3 parts UR 8200
(manufactured by Toyobo Co., Ltd.) .alpha.-Alumina 10 parts HIT55
(manufactured by Sumitomo Chemical Co., Ltd.) Carbon black 5 parts
#50 (manufactured by Asahi Carbon Co., Ltd.) Phenylphosphonic acid
3 parts Butyl stearate 10 parts Butoxyethyl stearate 5 parts
Isohexadecyl stearate 3 parts Stearic acid 2 parts Methyl ethyl
ketone 180 parts Cyclohexanone 180 parts Magnetic Coating Solution:
2ML-2 Ferromagnetic metal powder: 2M-2 100 parts Composition: Fe
100%, Co 30% (atomic ratio) Hc: 2,360 Oe Specific surface area: 49
m.sup.2/g .sigma..sub.s: 146 emu/g Crystallite size: 170 .ANG. Long
axis length: 0.100 .mu.m Acicular ratio: 6 SFD: 0.950 Al (Al/Fe,
atomic ratio: 11.4%) Y (Y/Fe, atomic ratio: 6.7%) Atomic ratio A:
5.2% Atomic ratio B: 0.59% pH: 9.4 Vinyl chloride copolymer 10
parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane
resin 4 parts UR 5500 (manufactured by Toyobo Co., Ltd.)
.alpha.-Alumina 10 parts HIT70 (manufactured by Sumitomo Chemical
Co., Ltd.) Carbon black 1 part #50 (manufactured by Asahi Carbon
Co., Ltd.) Phenylphosphonic acid 3 parts Oleic acid 1 part Stearic
acid 0.6 part Ethylene glycol dioleyl 12 parts Methyl ethyl ketone
180 parts Cyclohexanone 180 parts Magnetic Coating Solution: 2ML-3
(acicular magnetic powder was used, comparative example)
Ferromagnetic metal powder: 2M-3 100 parts Composition: Fe/Ni =
96/4 Hc: 1,600 Oe Specific surface area: 45 m.sup.2/g Crystallite
size: 220 .ANG. .sigma..sub.s: 135 emu/g Average long axis length:
0.20 .mu.m Acicular ratio: 9 Vinyl chloride copolymer 12 parts
MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 5
parts UR 8600 (manufactured by Toyobo Co., Ltd.) .alpha.-Alumina
(particle size: 0.65 .mu.m) 2 parts Chromium oxide (particle size:
0.35 .mu.m) 15 parts Carbon black (particle size: 0.03 .mu.m) 2
parts Carbon black (particle size: 0.3 .mu.m) 9 parts Isohexadecyl
stearate 4 parts n-Butyl stearate 4 parts Butoxyethyl stearate 4
parts Oleic acid 1 part Stearic acid 1 part Methyl ethyl ketone 300
parts Nonmagnetic Coating Solution: 2NU-1 (spherical inorganic
powder was used) Nonmagnetic powder, TiO.sub.2, crystal system 80
parts rutile Average primary particle size: 0.035 .mu.m Specific
surface area (S.sub.BET): 40 m.sup.2/g pH: 7 TiO.sub.2content: 90%
or more DBP oil absorption: 27 to 38 ml/100 g Surface-covering
compound: Al.sub.2O.sub.3, 8 wt % based on total particles Carbon
black 20 parts CONDUCTEX SC-U (manufactured by Columbia Carbon Co.,
Ltd.) Vinyl chloride copolymer 12 parts MR110 (manufactured by
Nippon Zeon Co., Ltd.) Polyurethane resin 5 parts UR 8200
(manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts
Butyl stearate 10 parts Butoxyethyl stearate 5 parts Isohexadecyl
stearate 2 parts Stearic acid 3 parts Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent) Nonmagnetic
Coating Solution: 2NU-2 (spherical inorganic powder was used)
Nonmagnetic powder, TiO.sub.2, crystal system 100 parts rutile
Average primary particle size: 0.035 .mu.m Specific surface area
(S.sub.BET): 40 m.sup.2/g pH: 7 TiO.sub.2content: 90% or more DBP
oil absorption: 27 to 38 ml/100 g Surface-covering compound:
Al.sub.2O.sub.3and SiO.sub.2 Ketjen Black EC 13 parts (manufactured
by Akzo Nobel Co., Ltd.) Average primary particle size: 30 nm DBP
oil absorption: 350 ml/100 g pH: 9.5 Specific surface area
(S.sub.BET): 950 m.sup.2/g Volatile content: 1.0% Vinyl chloride
copolymer 16 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)
Polyurethane resin 6 parts UR 8200 (manufactured by Toyobo Co.,
Ltd.) Phenylphosphonic acid 4 parts Ethylene glycol dioleyl 16
parts Oleic acid 1 part Stearic acid 0.8 part Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent) Nonmagnetic
Coating Solution 2NU-3 (acicular inorganic powder was used)
Nonmagnetic powder, .alpha.-Fe.sub.2O.sub.3, hematite 80 parts Long
axis length: 0.15 .mu.m Specific surface area (S.sub.BET): 50
m.sup.2/g pH: 9 Surface-covering compound: Al.sub.2O.sub.3, 8 wt %
based on total particles Carbon black 20 parts CONDUCTEX SC-U
(manufactured by Columbia Carbon Co., Ltd.) Vinyl chloride
copolymer 12 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)
Polyurethane resin 5 parts UR 8200 (manufactured by Toyobo Co.,
Ltd.) Phenylphosphonic acid 4 parts Butyl stearate 10 parts
Butoxyethyl stearate 5 parts Isohexadecyl stearate 2 parts Stearic
acid 3 parts Methyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed
solvent) Nonmagnetic Coating Solution 2NU-4 (acicular inorganic
powder was used) Nonmagnetic powder, .alpha.-Fe.sub.2O.sub.3,
hematite 100 parts Long axis length: 0.15 .mu.m Specific surface
area (S.sub.BET): 50 m.sup.2/g pH: 9 Surface-covering compound:
Al.sub.2O.sub.3, 8 wt % based on total particles Carbon black 18
parts #3250B (manufactured by Mitsubishi Kasei Corp.) Vinyl
chloride copolymer 15 parts MR104 (manufactured by Nippon Zeon Co.,
Ltd.) Polyurethane resin 7 parts UR 5500 (manufactured by Toyobo
Co., Ltd.) Phenylphosphonic acid 4 parts Ethylene glycol dioleyl 16
parts Oleic acid 1.3 parts Stearic acid 0.8 part Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent)
[0273] Preparation Method 2-1 (W/W)
[0274] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0275] These coating solutions were simultaneously
multilayer-coated on a polyethylene terephthalate support having a
thickness of 62 .mu.m and a central plane average surface roughness
of 3 nm of the surface side on which a magnetic layer was to be
coated. The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m, immediately thereafter the magnetic layer
coating solution was coated on the nonmagnetic layer so as to give
the magnetic layer having the thickness of 0.15 .mu.m. The coated
layers were subjected to random orientation while the magnetic
layer and the nonmagnetic layer were still wet by passing through
an alternating current magnetic field generator having two magnetic
field intensities of frequency of 50 Hz, magnetic field intensity
of 250 Gauss and frequency of 50 Hz, magnetic field intensity of
120 Gauss. After drying, the coated layer was subjected to
calendering treatment with calenders of 7 stages at 90.degree. C.
at line pressure of 300 kg/cm. The obtained web was punched to a
disc of 3.7 inches, the disc was subjected to a surface treatment
by abrasives, encased in 3.7 inch cartridge having a liner inside
(a zip-disc cartridge manufactured by Iomega Co., Ltd., U.S.A.),
and equipped the cartridge with prescribed mechanism parts to
obtain a 3.7 inch floppy disc. A part of samples was subjected to
machine direction orientation using Co magnets with the same pole
and counter positions of 4,000 G before random orientation
treatment.
[0276] In this case, it is preferred to increase the frequency and
magnetic field intensity of the alternating current magnetic field
generator so as to achieve finally sufficient random orientation,
thereby 98% or more of orientation ratio can be obtained.
[0277] Preparation Method 2-2 (W/D)
[0278] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0279] The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m on a polyethylene terephthalate support
having a thickness of 62 .mu.m and a central plane average surface
roughness of 3 nm of the surface side on which the magnetic layer
was to be coated, dried, and subjected to calendering treatment.
The magnetic layer coating solution was coated by blade coating on
the nonmagnetic layer so as to give the magnetic layer having the
thickness of 0.15 .mu.m. The coated layers were subjected to random
orientation by passing through an alternating current magnetic
field generator having two magnetic field intensities of frequency
of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50
Hz, magnetic field intensity of 120 Gauss. The procedure was
carried out in the same manner as in Preparation Method 2-1
hereafter. Calendering of the nonmagnetic layer may be omitted.
[0280] Preparation Method 2-3 (Spin Coating)
[0281] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0282] The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m by spin coating on a polyethylene
terephthalate support having a thickness of 62 .mu.m and a central
plane average surface roughness of 3 nm of the surface side on
which a magnetic layer was to be coated and dried. The magnetic
layer coating solution was coated by spin coating on the
nonmagnetic layer so as to give the magnetic layer having the
thickness of 0.15 .mu.m. The coated layers were oriented using a Co
magnets with the same pole and counter positions of 6,000 G in the
circumferential direction and the surface of the magnetic layer was
smoothed by batch system rolling treatment by which the same
pressure as in Preparation Method 2-1 can be applied. The procedure
was carried out in the same manner as in Preparation Method 2-1
hereafter. Also, the magnetic layer may be coated by spin coating
on the nonmagnetic layer while the nonmagnetic layer coated by spin
coating is still wet. By using the spin coating process, not only
the amount of residual magnetization in the recording direction can
be made large but also vertical magnetization components of
ferromagnetic metal powders of short acicular ratio can be reduced
and symmetric property of reproduced wave form can be improved.
[0283] Preparation Method 2-4 (Monolayer)
[0284] The coating solution for the magnetic layer prepared in
Preparation Method 2-1 was coated on a polyethylene terephthalate
support so as to reach the thickness of 0.15 .mu.m. The procedure
was carried out in the same manner as in Preparation Method 2-1
hereafter.
10 Support 2B-1: Polyethylene Terephthalate Thickness: 62 .mu.m F-5
value: MD: 114 MPa, TD: 107 MPa Breaking strength: MD: 276 MPa, TD:
281 MPa Breaking extension: MD: 174 MPa, TD: 139 MPa Thermal
shrinkage factor (80.degree. C., 30 minutes): MD: 0.04%, TD: 0.05%
Thermal shrinkage factor (100.degree. C., 30 minutes): MD: 0.2%,
TD: 0.3% Temperature expansion coefficient: Long axis: 15 .times.
10.sup.-5/.degree. C. Short axis: 18 .times. 10.sup.-5/.degree. C.
Central plane average surface roughness: 3 nm Support 2B-2:
Polyethylene Naphthalate Thickness: 55 .mu.m Central plane average
surface roughness: 1.8 nm Thermal shrinkage factor (80.degree. C.,
30 minutes): MD: 0.007%, TD: 0.007% Thermal shrinkage factor
(100.degree. C., 30 minutes): MD: 0.02%, TD: 0.02% Temperature
expansion coefficient: Long axis: 10 .times. 10.sup.-5/.degree. C.
Short axis: 11 .times. 10.sup.-5/.degree. C. Orientation 2O-1:
Random orientation 2O-2: Orientation in the machine direction using
a Co magnet first, then random orientation 2O-3: Orientation in the
machine direction using a Co magnet first, then in the machine
direction using a solenoid
EXAMPLES 2-1 TO 2-16, COMPARATIVE EXAMPLE 2-1, AND REFERENCE
EXAMPLE 2-1
[0285] Various samples were prepared by combining the
above-described each method arbitrarily as shown in Table 2-1. The
results of evaluations are shown in Table 2-2.
EXAMPLES 2-17 TO 2-22 AND COMPARATIVE EXAMPLES 2-2 TO 2-4
[0286] Various samples of magnetic discs were prepared in the same
manner as in Example 2-11, except that the components of the
magnetic coating solution 2ML-2 (a ferromagnetic metal powder) were
changed as shown in Table 2-3 and the addition amount of
.alpha.-alumina was changed to 8 parts. Further, in Table 2-3, the
atomic ratio A of ferromagnetic metal powder 2M-9 shows atomic
ratio of each component of Nd and Y to (Fe+Co), the atomic ratio A
of 2M-9 is, therefore, 3.3%. The sum total of rare earth elements
to calculate the atomic ratio A of other ferromagnetic metal
powders is considered substantially a component of Y or Nd. The
results of evaluations are shown in Tables 2-4 and 2-5.
[0287] Methods of evaluations of each sample are as follows.
[0288] (1) Magnetic characteristics (Hc):
[0289] Magnetic characteristics were measured using a vibrating
sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10
KOe.
[0290] (2) Central plane average surface roughness (Ra):
[0291]
[0292] Surface roughness (Ra) of the area of about 250
.mu.m.times.250 .mu.m, Rrms, peak-valley values were measured using
"TOPO3D" (a product of WYKO, U.S.A.) by 3D-MIRAU method. The
wavelength of measurement was about 650 nm and spherical
compensation and cylindrical compensation were applied. Measurement
was performed using a light interference type non-contact surface
roughness meter.
[0293] (3) Areal recording density:
[0294] Areal recording density means a value obtained by
multiplying linear recording density by track density.
[0295] (4) Linear recording density:
[0296] Linear recording density means a bit number of signals
recorded per 1 inch in the recording direction.
[0297] (5) Track density:
[0298] Track density means a track number per 1 inch.
[0299] (6) .phi.m:
[0300] .phi.m is the amount of magnetization per unit area of a
magnetic recording medium, which is represented by Bm (Gauss)
multiplying thickness. This is the value obtained by using a
vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.)
at Hm 10 KOe, which can be directly measured.
[0301] (7) Error rate of disc:
[0302] The above signals of linear recording density were recorded
on the disc by (2,7) RLL modulation system and error rate was
measured.
[0303] (8) Thickness of magnetic layer:
[0304] The sample having the thickness of about 0.1 .mu.m was cut
out with a diamond cutter in the machine direction of the magnetic
medium, observed with a transmission type electron microscope of
from 10,000 to 100,000, preferably from 20,000 to 50,000
magnifications and photographed. The print size of the photograph
was from A4 (i.e., 297.times.210 mm) to A5 (i.e., 210.times.148 mm)
sizes. The present inventors paid attentions to the difference of
the shapes of the ferromagnetic powders and the nonmagnetic powders
of the magnetic layer and the nonmagnetic layer and rimmed the
interface and the surface of the magnetic layer with black color by
visual judgement. Thereafter, the distance of the rimmed lines was
measured by the image processing apparatus "IBAS2" (manufactured by
Zeiss Corp.). Measurement was conducted from 85 to 300 times when
the length of the sample photograph was 21 cm. The average measured
value at that time was taken as d, and the standard deviation of
the measured value was taken as .sigma.. d depended on the
description in JP-A-5-298653 and a was obtained by equation (2) in
JP-A-5-298653. di means each measured value and n is from 85 to
300.
[0305] (9) Running durability:
[0306] A floppy disc drive ("ZIP100", a product of IOMEGA CORP.,
U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at
the position of radius of 38 mm. Recording was conducted at
recording density of 34 kfci, then reproduced the signals recorded
and this was taken as 100%. The disc was run for 25 days (600
hours) under the following thermo-cycle condition, which being
taken as one cycle. Output was monitored every 24 hours of running
and the point when the initial reproduction output became 70% or
less was taken as NG.
[0307] Thermo-Cycle Flow
[0308] 25.degree. C., 50% RH, 1 hr.fwdarw.(temperature up, 2
hr).fwdarw.60.degree. C., 20% RH, 7 hr.fwdarw.(temperature down, 2
hr).fwdarw.25.degree. C., 50% RH, 1 hr.fwdarw.(temperature down, 2
hr).fwdarw.5.degree. C., 50% RH, 7 hr.fwdarw.(temperature up, 2
hr).fwdarw.(this cycle was repeated).
[0309] (10) Liner wear:
[0310] A sample was run for 600 hours under the same condition as
in the evaluation of running durability with the head being off,
and after completion of running, cartridge case of the sample was
opened and the surface of the magnetic layer was visually evaluated
by the following criteria.
[0311] o: No defect was observed on the surface of the magnetic
layer.
[0312] .DELTA.: Fine scratches were generated on a part of the
surface of the magnetic layer.
[0313] x: Fine scratches were generated on the entire surface of
the magnetic layer.
[0314] (11) Characteristics of ferromagnetic metal powder:
[0315] Evaluation of the characteristics of the ferromagnetic metal
powder was conducted according to the method disclosed in
JP-A-8-279137.
11 TABLE 2-1 Magnetic Layer Thick- Surface Prepara- Prescrip- ness
Hc Roughness .phi.m Lower tion Orien- Sample No. tion (.mu.m) (Oe)
(nm) (emu/cm.sup.2) Layer Support Method tation Example 2-1 2ML-2
0.15 2,360 3.5 4.8 .times. 10.sup.-3 2NU-1 2B-1 Method 2-1 20-1
Example 2-2 2ML-2 0.15 2,360 2.3 4.8 .times. 10.sup.-3 2NU-2 2B-1
Method 2-1 20-1 Example 2-3 2ML-2 0.15 2,360 1.9 4.8 .times.
10.sup.-3 2NU-3 2B-1 Method 2-1 20-1 Example 2-4 2ML-2 0.15 2,360
1.7 4.8 .times. 10.sup.-3 2NU-4 2B-1 Method 2-1 20-1 Example 2-5
2ML-2 0.05 2,400 1.4 1.6 .times. 10.sup.-3 2NU-4 2B-1 Method 2-1
20-1 Example 2-6 2ML-2 0.10 2,380 1.6 3.2 .times. 10.sup.-3 2NU-4
2B-1 Method 2-1 20-1 Example 2-7 2ML-2 0.20 2,330 1.9 6.4 .times.
10.sup.-3 2NU-4 2B-1 Method 2-1 20-1 Example 2-8 2ML-2 0.15 2,360
1.5 4.8 .times. 10.sup.-3 2NU-4 2B-2 Method 2-1 20-1 Example 2-9
2ML-1 0.15 2,550 2.5 4.2 .times. 10.sup.-3 2NU-4 2B-1 Method 2-1
20-1 Example 2-10 2ML-2 0.15 2,360 2.5 4.8 .times. 10.sup.-3 2NU-4
2B-1 Method 2-2 20-1 Example 2-11 2ML-2 0.15 2,360 1.7 4.8 .times.
10.sup.-3 2NU-4 2B-1 Method 2-1 20-2 Example 2-12 2ML-2 0.15 2,660
1.6 4.8 .times. 10.sup.-3 2NU-4 2B-1 Method 2-3 20-3 Example 2-13
2ML-2 0.15 2,370 3.4 4.6 .times. 10.sup.-3 none 2B-1 Method 2-4
20-1 Reference Example 2ML-2 0.32 2,300 2.5 10.2 .times. 10.sup.-3
2NU-4 2B-1 Method 2-1 20-1 2-1 Comparative Example 2ML-3 0.15 1,600
3.1 4.8 .times. 10.sup.-3 2NU-4 2B-1 Method 2-1 20-1 2-1
[0316]
12TABLE 2-2 Areal Linear Recording Magnetic Track Recording Density
Error Layer Density Density (G bit/ Rate .DELTA.Hc .DELTA..phi.m
Sample No. (tpi) (kbpi) inch.sup.2) (10.sup.-5) (%) (%) Example 2-1
5,200 144 0.75 0.2 +2.6 +5.0 Example 2-2 5,200 144 0.75 0.08 +2.5
+5.0 Example 2-3 5,200 144 0.75 0.03 +2.5 +4.8 Example 2-4 5,200
144 0.75 0.01 +2.5 +4.8 Example 2-5 5,200 144 0.75 0.06 +3.3 +5.1
Example 2-6 5,200 144 0.75 0.01 +3.0 +5.1 Example 2-7 5,200 144
0.75 0.2 +2.3 +5.0 Example 2-8 5,200 144 0.75 0.008 +2.5 +4.9
Example 2-9 5,200 144 0.75 0.004 +4.0 +6.0 Example 2-10 5,200 144
0.75 0.1 +2.5 +5.0 Example 2-11 5,200 144 0.75 0.001 +2.5 +5.0
Example 2-12 5,200 144 0.75 0.0002 +2.5 +5.0 Example 2-13 5,200 144
0.75 0.2 +2.0 +4.8 Reference 5,200 144 0.75 3.5 +2.1 +4.9 Example
2-1 Comparative 5,200 144 0.75 40 -1.0 +8.2 Example 2-1 Example
2-14 7,500 200 1.5 0.8 +2.5 +5.0 Example 2-15 6,000 166 1.0 0.08
+2.5 +5.0 Example 2-16 3,000 120 0.36 0.007 +2.5 +5.0
[0317] In each of Examples 2-14 to 2-16, the disc in Example 2-11
was used and error rate was determined with varying linear
recording density and track density.
[0318] From the results in Table 2-2, it can be seen that the error
rate of the magnetic disc according to the present invention, in
particular, in high density recording region is 1.times.10.sup.-5
or less, which is conspicuously excellent.
13TABLE 2-3 Ferromagnetic Metal Powder Composition (atomic %)
Particle Size Ferro- Rare Rare Long Crystal- magnetic Al/ Earth
Earth Axis lite Powder Fe + Element/ Element/ Length Size No. Co/Fe
Co Fe + Co Al (.mu.m) (.ANG.) 2M-4 10 14.8 Y: 4.0 0.27 0.12 175
2M-5 20 11.2 Y: 4.8 0.43 0.11 160 2M-6 35 5.9 Y: 3.3 0.56 0.09 145
2M-7 30 6.3 Y: 3.4 0.54 0.07 135 2M-8 30 10.5 Y: 4.6 0.44 0.08 145
2M-9 30 8.5 Nd: 1.3 0.39 0.10 160 Y: 2.0 2M-10 5 14.0 Y: 1.3 0.09
0.11 180 2M-11 10 15.6 Y: 6.2 0.40 0.11 175 2H-12 30 8.6 Y: 5.5
0.64 0.08 175 2M-13 30 4.3 Y: 3.5 0.81 0.08 175 2M-14 5 4.0 Nd: 0.8
0.20 0.10 160 2M-15 8 2.5 Y: 0.2 0.08 0.08 175
[0319]
14 TABLE 2-4 Magnetic Magnetic Layer Powder Surface Error
.sigma..sub.s Thickness Hc Roughness .phi.m Rate Sample No. Kind
(emu/g) (.mu.m) (Oe) (nm) (emu/cm.sup.2) (10.sup.-5) Example 2-17
2M-4 136 0.15 1,850 2.5 4.8 .times. 10.sup.-3 0.2 Example 2-18 2M-5
140 0.15 2,100 2.2 5.0 .times. 10.sup.-3 0.08 Example 2-19 2M-6 148
0.15 2,400 1.9 5.4 .times. 10.sup.-3 0.007 Example 2-20 2M-7 152
0.15 2,360 2.2 6.4 .times. 10.sup.-3 0.01 Example 2-21 2M-8 148
0.15 2,360 1.8 5.3 .times. 10.sup.-3 0.004 Example 2-22 2M-9 143
0.15 2,200 2.2 5.2 .times. 10.sup.-3 0.08 Comparative 2M-10 134
0.15 1,750 3.2 4.5 .times. 10.sup.-3 10 Example 2-2 Comparative
2M-11 132 0.15 1,760 2.8 4.4 .times. 10.sup.-3 5 Example 2-3
Comparative 2M-15 145 0.15 1,850 2.0 5.0 .times. 10.sup.-3 2
Example 2-4
[0320]
15 TABLE 2-5 Running Magnetic Layer Durability Liner .DELTA.Hc
.DELTA..phi.m Sample No. (hour) Wear (%) (%) Example 2-17 600
.smallcircle. .+-.0.0 +3.0 Example 2-18 600 .smallcircle. +1.5 +2.4
Example 2-19 600 .smallcircle. +3.3 +5.0 Example 2-20 600
.smallcircle. +3.0 +4.8 Example 2-21 600 .smallcircle. +2.5 +5.0
Example 2-22 600 .smallcircle. +2.6 +5.1 Comparative 600
.smallcircle. -2.4 +8.9 Example 2-2 Comparative 600 .smallcircle.
-0.1 +3.5 Example 2-3 Comparative 300 x -1.2 +6.9 Example 2-4
[0321] In running durability of Table 2-5, 600 means 600 hours.
[0322] As is apparent from the results in Table 2-5, the magnetic
disc according to the present invention is excellent in high
density characteristics and also has excellent running
durability.
[0323] The magnetic recording medium according to the present
invention comprises a support having thereon a magnetic layer
comprising a ferromagnetic metal powder dispersed in a binder,
which is a magnetic disc for recording signals of from 0.17 to 2 G
bit/inch.sup.2, preferably from 0.2 to 2 G bit/inch.sup.2, of areal
recording density, wherein the magnetic layer has a dry thickness
of from 0.05 to 0.25 .mu.m, .phi.m of preferably from
8.0.times.10.sup.-3 to 1.0.times.10.sup.-3 emu/cm.sup.2, and the
coercive force of 1,800 Oe or more, and the ferromagnetic metal
powder is composed of Fe and Co as main components, the atomic
ratio of the sum total of rare earth elements/(Fe+ Co) is
preferably from 1.0 to 6.0%, and the atomic ratio of the sum total
of rare earth elements/Al is from 0.10 to 0.60. The magnetic
recording medium of the present invention having excellent high
density characteristics and, in particular, markedly improved
running durability has been realized due to the above constitution
of the present invention, which could never be obtained by
conventional techniques.
16 Preparation of Coating Solution Magnetic Coating Solution: 3ML-1
Ferromagnetic metal powder: 3M-l 100 parts Composition: Fe 100%, Co
36% (atomic ratio) Hc: 2,550 Oe Specific surface area: 55 m.sup.2/g
.sigma..sub.s: 140 emu/g Crystallite size: 110 .ANG. Long axis
length: 0.048 .mu.m Acicular ratio: 4 Atomic ratio A [Al/(Fe +
Co)]: 6.2% Atomic ratio B [Y/(Fe + Co)]: 3.2% Atomic ratio C
[Mg/(Fe + CO)]: 1.2% Vinyl chloride copolymer 12 parts MR110
(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 3 parts
UR 8200 (manufactured by Toyobo Co., Ltd.) .alpha.-Alumina 10 parts
HIT55 (manufactured by Sumitomo Chemical Co., Ltd.) Carbon black 5
parts #50 (manufactured by Asahi Carbon Co., Ltd.) Phenylphosphonic
acid 3 parts Butyl stearate 10 parts Butoxyethyl stearate 5 parts
Isohexadecyl stearate 3 parts Stearic acid 2 parts Methyl ethyl
ketone 180 parts Cyclohexanone 180 parts Magnetic Coating Solution:
3ML-2 Ferromagnetic metal powder: 3M-2 100 parts Composition: Fe
100%, Co 30% (atomic ratio) Hc: 2,360 Oe Specific surface area: 49
m.sup.2/g .sigma..sub.s: 144 emu/g Crystallite size: 140 .ANG. Long
axis length: 0.085 .mu.m Acicular ratio: 5.6 SFD: 0.920 Atomic
ratio A [Al/(Fe + Co)]: 8.8% Atomic ratio B [Y/(Fe + Co)]: 5.2%
Atomic ratio C [Mg/(Fe + Co)]: 0.8% pH: 9.4 Vinyl chloride
copolymer 10 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)
Polyurethane resin 4 parts UR 5500 (manufactured by Toyobo Co.,
Ltd.) .alpha.-Alumina 10 parts HIT70 (manufactured by Sumitomo
Chemical Co., Ltd.) Carbon black 1 part #50 (manufactured by Asahi
Carbon Co., Ltd.) Phenylphosphonic acid 3 parts Oleic acid 1 part
Stearic acid 0.6 part Ethylene glycol dioleyl 12 parts Methyl ethyl
ketone 180 parts Cyclohexanone 180 parts Magnetic Coating Solution:
3ML-3 (comparative example) Ferromagnetic metal powder: 3M-3 100
parts Composition: Fe/Ni = 96/4 Hc: 1,600 Oe Specific surface area:
45 m.sup.2/g Crystallite size: 220 .ANG. .sigma..sub.s: 135 emu/g
Average long axis length: 0.20 .mu.m Acicular ratio: 9 Vinyl
chloride copolymer 12 parts MR110 (manufactured by Nippon Zeon Co.,
Ltd.) Polyurethane resin 5 parts UR 8600 (manufactured by Toyobo
Co., Ltd.) .alpha.-Alumina (particle size: 0.65 .mu.m) 2 parts
Chromium oxide (particle size: 0.35 .mu.m) 15 parts Carbon black
(particle size: 0.03 .mu.m) 2 parts Carbon black (particle size:
0.3 .mu.m) 9 parts Isohexadecyl stearate 4 parts n-Butyl stearate 4
parts Butoxyethyl stearate 4 parts Oleic acid 1 part Stearic acid 1
part Methyl ethyl ketone 300 parts Nonmagnetic Coating Solution:
3NU-1 (spherical inorganic powder was used) Nonmagnetic powder,
TiO.sub.2, crystal system 80 parts rutile Average primary particle
size: 0.035 .mu.m Specific surface area (S.sub.BET): 40 m.sup.2/g
pH: 7 TiO.sub.2 content: 90% or more DBP oil absorption: 27 to 38
ml/100 g Surface-covering compound: Al.sub.2O.sub.3, 8 wt % based
on total particles Carbon black 20 parts CONDUCTEX SC-U
(manufactured by Columbia Carbon Co., Ltd.) Vinyl chloride
copolymer 12 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)
Polyurethane resin 5 parts UR 8200 (manufactured by Toyobo Co.,
Ltd.) Phenylphosphonic acid 4 parts Butyl stearate 10 parts
Butoxyethyl stearate 5 parts Isohexadecyl stearate 2 parts Stearic
acid 3 parts Methyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed
solvent) Nonmagnetic Coating Solution: 3NU-2 (spherical inorganic
powder was used) Nonmagnetic powder, TiO.sub.2, crystal system 100
parts rutile Average primary particle size: 0.035 .mu.m Specific
surface area (S.sub.BET): 40 m.sup.2/g pH: 7 TiO.sub.2 content: 90%
or more DBP oil absorption: 27 to 38 ml/100 g Surface-covering
compound: Al.sub.2O.sub.3 and SiO.sub.2 Ketjen Black EC 13 parts
(manufactured by Akzo Nobel Co., Ltd.) Average primary particle
size: 30 nm DBP oil absorption: 350 ml/100 g pH: 9.5 Specific
surface area (S.sub.BET): 950 m.sup.2/g Volatile content: 1.0%
Vinyl chloride copolymer 16 parts MR110 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 6 parts UR 8200 (manufactured by
Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts Ethylene glycol
dioleyl 16 parts Oleic acid 1 part Stearic acid 0.8 part Methyl
ethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)
Nonmagnetic Coating Solution 3NU-3 (acicular inorganic powder was
used) Nonmagnetic powder, .alpha.-Fe.sub.2O.sub.3, hematite 80
parts Long axis length: 0.15 .mu.m Specific surface area
(S.sub.BET): 50 m.sup.2/g Acicular ratio: 6 pH: 9 Surface-covering
compound: Al.sub.2O.sub.3, 8 wt % based on total particles Carbon
black 20 parts CONDUCTEX SC-U (manufactured by Columbia Carbon Co.,
Ltd.) Vinyl chloride copolymer 12 parts MR110 (manufactured by
Nippon Zeon Co., Ltd.) Polyurethane resin 5 parts UR 8200
(manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts
Butyl stearate 10 parts Butoxyethyl stearate 5 parts Isohexadecyl
stearate 2 parts Stearic acid 3 parts Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent) Nonmagnetic
Coating Solution 3NU-4 (acicular inorganic powder was used)
Nonmagnetic powder, .alpha.-Fe.sub.2O.sub.3, hematite 100 parts
Long axis length: 0.15 .mu.m Specific surface area (S.sub.BET): 50
m.sup.2/g Acicular ratio: 6 pH: 9 Surface-covering compound:
Al.sub.2O.sub.3, 8 wt % based on total particles Carbon black 18
parts #3250B (manufactured by Mitsubishi Kasei Corp.) Vinyl
chloride copolymer 15 parts MR104 (manufactured by Nippon Zeon Co.,
Ltd.) Polyurethane resin 7 parts UR 5500 (manufactured by Toyobo
Co., Ltd.) Phenylphosphonic acid 4 parts Ethylene glycol dioleyl 16
parts Oleic acid 1.3 parts Stearic acid 0.8 part Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent)
[0324] Preparation Method 3-1 (W/W)
[0325] Each of the above compositions of the coating solutions for
the magnetic layer and the lower nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the lower nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0326] These coating solutions were simultaneously
multilayer-coated on a polyethylene terephthalate support having a
thickness of 62 .mu.m and a central plane average surface roughness
of 3 nm of the surface side on which a magnetic layer was to be
coated. The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m, immediately thereafter the magnetic layer
coating solution was coated on the nonmagnetic layer so as to give
the magnetic layer having the thickness of 0.15 .mu.m. The coated
layers were subjected to random orientation while the magnetic
layer and the nonmagnetic layer were still wet by passing through
an alternating current magnetic field generator having two magnetic
field intensities of frequency of 50 Hz, magnetic field intensity
of 250 Gauss and frequency of 50 Hz, magnetic field intensity of
120 Gauss. After drying, the coated layer was subjected to
calendering treatment with calenders of 7 stages at 90.degree. C.
at line pressure of 300 kg/cm. The obtained web was punched to a
disc of 3.7 inches, the disc was subjected to a surface treatment
by abrasives, encased in 3.7 inch cartridge having a liner inside
(a zip-disc cartridge manufactured by Iomega Co., Ltd., U.S.A.),
and equipped the cartridge with prescribed mechanism parts to
obtain a 3.7 inch floppy disc. A part of samples was subjected to
machine direction orientation using Co magnets with the same pole
and counter positions of 4,000 G before random orientation
treatment.
[0327] In this case, it is preferred to increase the frequency and
magnetic field intensity of the alternating current magnetic field
generator so as to achieve finally sufficient random orientation,
thereby 98% or more of orientation ratio can be obtained.
[0328] Preparation Method 3-2 (W/D)
[0329] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0330] The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m on a polyethylene terephthalate support
having a thickness of 62 .mu.m and a central plane average surface
roughness of 3 nm of the surface side on which the magnetic layer
was to be coated, dried, and subjected to calendering treatment.
The magnetic layer coating solution was coated by blade coating on
the nonmagnetic layer so as to give the magnetic layer having the
thickness of 0.15 .mu.m. The coated layers were subjected to random
orientation by passing through an alternating current magnetic
field generator having two magnetic field intensities of frequency
of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50
Hz, magnetic field intensity of 120 Gauss. The procedure was
carried out in the same manner as in Preparation Method 3-1
hereafter. Calendering of the nonmagnetic layer may be omitted.
[0331] Preparation Method 3-3 (Spin Coating)
[0332] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0333] The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m by spin coating on a polyethylene
terephthalate support having a thickness of 62 .mu.m and a central
plane average surface roughness of 3 nm of the surface side on
which a magnetic layer was to be coated and dried. The magnetic
layer coating solution was coated by spin coating on the
nonmagnetic layer so as to give the magnetic layer having the
thickness of 0.15 .mu.m. The layers coated were oriented using a Co
magnets with the same pole and counter positions of 6,000 G in the
circumferential direction and the surface of the magnetic layer was
smoothed by batch system rolling treatment by which the same
pressure as in Preparation Method 3-1 can be applied. The procedure
was carried out in the same manner as in Preparation Method 3-1
hereafter. Also, the magnetic layer may be coated by spin coating
on the nonmagnetic layer while the nonmagnetic layer coated by spin
coating is still wet. By using the spin coating process, not only
the amount of residual magnetization in the recording direction can
be made large but also vertical magnetization components of
ferromagnetic metal powders of short acicular ratio can be reduced
and symmetric property of reproduced wave form can be improved.
[0334] Preparation Method 3-4 (Monolayer)
[0335] The coating solution for the magnetic layer prepared in
Preparation Method 3-1 was coated on a polyethylene terephthalate
support so as to reach the thickness of 0.15 .mu.m. The procedure
was carried out in the same manner as in Preparation Method 3-1
hereafter.
17 Support 3B-1: Polyethylene Terephthalate Thickness: 62 .mu.m F-5
value: MD: 114 MPa, TD: 107 MPa Breaking strength: MD: 276 MPa, TD:
281 MPa Breaking extension: MD: 174 MPa, TD: 139 MPa Thermal
shrinkage factor (80.degree. C., 30 minutes): MD: 0.04%, TD: 0.05%
Thermal shrinkage factor (100.degree. C., 30 minutes): MD: 0.2%,
TD: 0.3% Temperature expansion coefficient: Long axis: 15 .times.
10.sup.-5/.degree. C. Short axis: 18 .times. 10.sup.-5/.degree. C.
Central plane average surface roughness: 3 nm Support 3B-2:
Polyethylene Naphthalate Thickness: 55 .mu.m Central plane average
surface roughness: 1.8 nm Thermal shrinkage factor (80.degree. C.,
30 minutes): MD: 0.007%, TD: 0.007% Thermal shrinkage factor
(100.degree. C., 30 minutes): MD: 0.02%, TD: 0.02% Temperature
expansion coefficient: Long axis: 10 .times. 10.sup.-5 /.degree. C.
Short axis: 11 .times. 10.sup.-5/.degree. C. Orientation 3O-1:
Random orientation 3O-2: Orientation in the machine direction using
a Co magnet first, then random orientation 3O-3: Orientation in the
machine direction using a Co magnet first, then in the machine
direction using a solenoid
EXAMPLES 3-1 TO 3-16, COMPARATIVE EXAMPLE 3-1, AND REFERENCE
EXAMPLE 3-1
[0336] Various samples were prepared by combining the
above-described each method arbitrarily as shown in Table 3-1. The
results of evaluations are shown in Table 3-2.
EXAMPLES 3-17 TO 3-23 AND COMPARATIVE EXAMPLE 3-2
[0337] Various samples of magnetic recording media were prepared in
the same manner as in Example 3-11, except that the components of
the magnetic coating solution 3ML-2 (a ferromagnetic metal powder)
were changed as shown in Table 3-3 and the addition amount of
.alpha.-alumina was changed to 8 parts. Further, in Table 3-3, the
atomic ratio B of ferromagnetic metal powder 3M-10 shows atomic
ratio of each component of Nd and Y to (Fe+Co), the atomic ratio A
of 3M-10 is, therefore, 6.3%. The sum total of rare earth elements
to calculate the atomic ratio B of other ferromagnetic metal
powders is considered substantially a component of Y or Sm. The
results of evaluations are shown in Tables 3-4 and 3-5.
[0338] Methods of evaluations of each sample are as follows.
[0339] (1) Magnetic characteristics (Hc):
[0340] Magnetic characteristics were measured using a vibrating
sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10
KOe.
[0341] (2) Central plane average surface roughness (Ra):
[0342] Surface roughness (Ra) of the area of about 250
.mu.m.times.250 .mu.m was measured using "TOPO3D" (a product of
WYKO, U.S.A.) by 3D-MIRAU method. The wavelength of measurement was
about 650 nm and spherical compensation and cylindrical
compensation were applied. Measurement was performed using a light
interference type non-contact surface roughness meter.
[0343] (3) Areal recording density:
[0344] Areal recording density means a value obtained by
multiplying linear recording density by track density.
[0345] (4) Linear recording density:
[0346] Linear recording density means a bit number of signals
recorded per 1 inch in the recording direction.
[0347] (5) Track density:
[0348] Track density means a track number per 1 inch.
[0349] (6) .phi.m:
[0350] .phi.m is the amount of magnetization per unit area of a
magnetic recording medium, which is represented by Bm (Gauss)
multiplying thickness. This is the value obtained by using a
vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.)
at Hm 10 KOe, which can be directly measured.
[0351] (7) Error rate of disc:
[0352] The above signals of linear recording density were recorded
on the disc by (2,7) RLL modulation system and error rate was
measured.
[0353] (8) Thickness of magnetic layer:
[0354] The sample having the thickness of about 0.1 .mu.m was cut
out with a diamond cutter in the machine direction of the magnetic
medium, observed with a transmission type electron microscope of
from 10,000 to 100,000, preferably from 20,000 to 50,000
magnifications and photographed. The print size of the photograph
was from A4 (i.e., 297.times.210 mm) to A5 (i.e., 210.times.148 mm)
sizes. The present inventors paid attentions to the difference of
the shapes of the ferromagnetic powders and the nonmagnetic powders
of the magnetic layer and the nonmagnetic layer and rimmed the
interface and the surface of the magnetic layer with black color by
visual judgement. Thereafter, the distance of the rimmed lines was
measured by the image processing apparatus "IBAS2" (manufactured by
Zeiss Corp.). Measurement was conducted from 85 to 300 times when
the length of the sample photograph was 21 cm. The average measured
value at that time was taken as d, and the standard deviation of
the measured value was taken as .sigma.. d depended on the
description in JP-A-5-298653 and .sigma. was obtained by equation
(2) in JP-A-5-298653. di means each measured value and n is from 85
to 300.
[0355] (9) Running durability:
[0356] A floppy disc drive ("ZIP100", a product of IOMEGA CORP.,
U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at
the position of radius of 38 mm. Recording was conducted at
recording density of 34 kfci, then reproduced the signals recorded
and this was taken as 100%. The disc was run for 25 days (600
hours) under the following thermo-cycle condition, which being
taken as one cycle. Output was monitored every 24 hours of running
and the point when the initial reproduction output became 70% or
less was taken as NG.
[0357] Thermo-Cycle Flow
[0358] 25.degree. C., 50% RH, 1 hr.fwdarw.(temperature up, 2
hr).fwdarw.60.degree. C., 20% RH, 7 hr.fwdarw.(temperature down, 2
hr).fwdarw.25.degree. C., 50% RH, 1 hr.fwdarw.(temperature down, 2
hr).fwdarw.5.degree. C., 50% RH, 7 hr.fwdarw.(temperature up, 2
hr).fwdarw.(this cycle was repeated).
[0359] (10) Liner wear:
[0360] A sample was run for 600 hours under the same condition as
in the evaluation of running durability with the head being off,
and after completion of running, cartridge case of the sample was
opened and the surface of the magnetic layer was visually evaluated
by the following criteria.
[0361] o: No defect was observed on the surface of the magnetic
layer.
[0362] .DELTA.: Fine scratches were generated on a part of the
surface of the magnetic layer.
[0363] x: Fine scratches were generated on the entire surface of
the magnetic layer.
[0364] (11) Characteristics of ferromagnetic metal powder:
[0365] Evaluation of the characteristics of the ferromagnetic metal
powder was conducted according to the method disclosed in
JP-A-8-279137.
[0366] (12) Starting torque:
[0367] Starting torque at the time of head-on in 3.5 inch floppy
disc drive was determined using torque gauge model 300 ATG (a
product of Tonichi Seisakusho Co., Ltd.). (1) The sample was stored
at 23.degree. C., 50% RH as it was (before storage), (2) the sample
was stored at 60.degree. C., 90% RH for 10 days, then allowed to
stand at room temperature for 24 hours, and determined at
23.degree. C., 50% RH (after storage), and starting torque was
compared before and after storage.
[0368] o: No difference was observed in starting torque before and
after storage
[0369] .DELTA.: A little increase was observed in starting torque
after storage
[0370] x: Considerable increase was observed in starting torque
after storage
18TABLE 3-1 Disc Magnetic Layer Thick- Surface Prepara- Prescrip-
ness Hc Roughness .phi.m Lower tion Orien- Sample No. tion (.mu.m)
(Oe) (nm) (emu/cm.sup.2) Layer Support Method tation Example 3-1
3ML-2 0.15 2,360 3.4 4.8 .times. 10.sup.-3 3NU-1 3B-1 Method 3-1
30-1 Example 3-2 3ML-2 0.15 2,360 2.2 4.8 .times. 10.sup.-3 3NU-2
3B-1 Method 3-1 30-1 Example 3-3 3ML-2 0.15 2,360 1.9 4.8 .times.
10.sup.-3 3NU-3 3B-1 Method 3-1 30-1 Example 3-4 3ML-2 0.15 2,360
1.6 4.8 .times. 10.sup.-3 3NU-4 3B-1 Method 3-1 30-1 Example 3-5
3ML-2 0.05 2,400 1.4 1.6 .times. 10.sup.-3 3NU-4 3B-1 Method 3-1
30-1 Example 3-6 3ML-2 0.10 2,380 1.5 3.2 .times. 10.sup.-3 3NU-4
3B-1 Method 3-1 30-1 Example 3-7 3ML-2 0.20 2,330 1.8 6.4 .times.
10.sup.-3 3NU-4 3B-1 Method 3-1 30-1 Example 3-8 3ML-2 0.15 2,360
1.5 4.8 .times. 10.sup.-3 3NU-4 3B-2 Method 3-1 30-1 Example 3-9
3ML-1 0.15 2,550 2.4 4.2 .times. 10.sup.-3 3NU-4 3B-1 Method 3-1
30-1 Example 3-10 3ML-2 0.15 2,360 2.3 4.8 .times. 10.sup.-3 3NU-4
3B-1 Method 3-2 30-1 Example 3-11 3ML-2 0.15 2,360 1.7 4.8 .times.
10.sup.-3 3NU-4 3B-1 Method 3-1 30-2 Example 3-12 3ML-2 0.15 2,660
1.6 4.8 .times. 10.sup.-3 3NU-4 3B-1 Method 3-3 30-3 Example 3-13
3ML-2 0.15 2,370 3.2 4.6 .times. 10.sup.-3 None 3B-1 Method 3-4
30-1 Reference Example 3ML-2 0.32 2,300 2.5 10.2 .times. 10.sup.-3
3NU-4 3B-1 Method 3-1 30-1 3-1 Comparative Example 3ML-3 0.15 1,600
3.1 4.8 .times. 10.sup.-3 3NU-4 3B-1 Method 3-1 30-1 3-1
[0371]
19TABLE 3-2 Areal Linear Recording Magnetic Track Recording Density
Error Layer Density Density (G bit/ Rate .DELTA.Hc .DELTA..phi.m
Sample No. (tpi) (kbpi) inch.sup.2) (10.sup.-5) (%) (%) Example 3-1
5,200 144 0.75 0.1 +2.2 +4.4 Example 3-2 5,200 144 0.75 0.07 +2.2
+4.4 Example 3-3 5,200 144 0.75 0.03 +2.2 +4.3 Example 3-4 5,200
144 0.75 0.01 +2.2 +4.3 Example 3-5 5,200 144 0.75 0.05 +2.5 +4.8
Example 3-6 5,200 144 0.75 0.01 +2.3 +4.6 Example 3-7 5,200 144
0.75 0.1 +2.0 +4.3 Example 3-8 5,200 144 0.75 0.008 +2.2 +4.3
Example 3-9 5,200 144 0.75 0.002 +4.0 +5.8 Example 3-10 5,200 144
0.75 0.08 +2.2 +4.4 Example 3-11 5,200 144 0.75 0.001 +2.2 +4.4
Example 3-12 5,200 144 0.75 0.0002 +2.2 +4.4 Example 3-13 5,200 144
0.75 0.1 +2.1 +4.2 Reference 5,200 144 0.75 3.5 +2.2 +4.3 Example
3-1 Comparative 5,200 144 0.75 40 -1.2 +8.2 Example 3-1 Example
3-14 7,500 200 1.5 0.8 +2.2 +4.4 Example 3-15 6,000 166 1.0 0.07
+2.2 +4.4 Example 3-16 3,000 120 0.36 0.006 +2.2 +4.4
[0372] In each of Examples 3-14 to 3-16, the disc in Example 3-11
was used and error rate was determined with varying linear
recording density and track density.
[0373] From the results in Table 3-2, it can be seen that the error
rate of the magnetic medium according to the present invention, in
particular, in high density recording region is 1.times.10.sup.-5
or less, which is conspicuously excellent.
20TABLE 3-3 Ferromagnetic Metal Powder Composition (atomic Z)
Particle Size Ferro- Rare Long Crystal- magnetic Mg/ Earth Axis
lite Powder Fe + Element/ Length Size No. Co/Fe Co Al/Fe + Co Fe +
Co (.mu.m) (.ANG.) 3M-4 20 0.5 14.8 Y: 4.0 0.10 175 3M-5 20 1.8
11.2 Y: 4.8 0.11 160 3M-6 30 0.2 8.5 Y: 1.5 0.07 150 3M-7 35 1.0
5.9 Sm: 3.3 0.07 120 3M-8 30 1.2 6.3 Y: 3.4 0.06 135 3M-9 30 1.9
10.5 Y: 4.6 0.07 145 3M-10 30 2.4 8.5 Nd: 2.3 0.07 160 Y: 4.0 3M-11
20 0 9.5 Y: 6.6 0.11 180 3M-12 20 3.2 10.6 Y: 6.2 0.11 175 3M-13 30
1.0 15.6 Y: 5.5 0.08 155 3M-14 30 0.8 3.5 Y: 5.5 0.08 155 3M-15 30
0.4 8.5 Y: 0.5 0.08 155 3M-16 30 0.7 10.5 Y: 9.2 0.08 155 3M-17 8 0
1.5 Y: 0.2 0.08 155
[0374]
21 TABLE 3-4 Magnetic Magnetic Layer Powder Surface Error
.sigma..sub.s Thickness Hc Roughness .phi.m Rate Sample No. Kind
(emu/g) (.mu.m) (Oe) (nm) (emu/cm.sup.2) (10.sup.-5) Example 3-17
3M-4 136 0.15 2,200 2.3 4.0 .times. 10.sup.-3 0.09 Example 3-18
3M-5 140 0.15 2,220 2.0 4.2 .times. 10.sup.-3 0.06 Example 3-19
3M-6 152 0.15 2,310 2.5 5.8 .times. 10.sup.-3 0.01 Example 3-20
3M-7 145 0.15 2,480 1.6 5.2 .times. 10.sup.-3 0.005 Example 3-21
3M-8 142 0.15 2,400 2.0 4.6 .times. 10.sup.-3 0.008 Example 3-22
3M-9 138 0.15 2,400 1.5 4.2 .times. 10.sup.-3 0.004 Example 3-23
3M-10 128 0.15 2,220 2.2 3.4 .times. 10.sup.-3 0.07 Comparative
3M-17 134 0.15 1,900 3.2 4.0 .times. 10.sup.-3 5 Example 3-2
[0375]
22TABLE 3-5 Running Magnetic Layer Durability Liner Starting
.DELTA.Hc .DELTA..phi.m Sample No. (hour) Wear Torque (%) (%)
Example 3-17 600 .smallcircle. .smallcircle. +1.4 +2.4 Example 3-18
600 .smallcircle. .smallcircle. +1.3 +2.3 Example 3-19 600
.smallcircle. .smallcircle. +2.0 +2.6 Example 3-20 600
.smallcircle. .smallcircle. +3.2 +4.4 Example 3-21 600
.smallcircle. .smallcircle. +3.1 +4.6 Example 3-22 600
.smallcircle. .smallcircle. +2.6 +4.7 Example 3-23 600
.smallcircle. .smallcircle. +2.3 +4.9 Comparative 200 x x -1.5 +7.7
Example 3-2
[0376] In running durability of Table 3-5, 600 means 600 hours.
[0377] As is apparent from the results in Table 3-5, the magnetic
disc according to the present invention is excellent in high
density characteristics and also has excellent running
durability.
[0378] The magnetic recording medium according to the present
invention comprises a support having thereon a magnetic layer
comprising a ferromagnetic metal powder dispersed in a binder,
which is a magnetic recording medium for recording signals of from
0.17 to 2 G bit/inch.sup.2 of areal recording density, wherein the
ferromagnetic metal powder is composed of Fe and Co as main
components, the atomic ratio A of Al/(Fe+Co) is from 4.5 to 15.0%,
the magnetic layer has a dry thickness of preferably from 0.05 to
0.25 .mu.m, and .phi.m of preferably from 1.0.times.10.sup.-3 to
8.0.times.10.sup.-3 emu/cm.sup.2, and the atomic ratio B of the sum
total of rare earth elements/(Fe+ Co) is preferably from 1 to 8%,
and the atomic ratio C of Mg/(Fe+Co) is preferably from 0.05 to
3.0%. The magnetic recording medium of the present invention having
excellent high density characteristics and, in particular, markedly
improved running durability has been realized due to the above
constitution of the present invention, which could never be
obtained by conventional techniques.
23 Preparation of Coating Solution Magnetic Coating Solution: 4ML-1
(acicular magnetic powder was used) Ferromagnetic metal powder:
4M-1 100 parts Composition: Co/Fe (atomic ratio), 30% Hc: 2,550 Oe
Specific surface area: 55 m.sup.2/g .sigma..sub.s: 140 emu/g
Crystallite size: 120 .ANG. Long axis length: 0.048 .mu.m Acicular
ratio: 4 Sintering inhibitor: Al compound (Al/Fe, atomic ratio: 8%)
Y compound (Y/Fe, atomic ratio: 6%) Vinyl chloride copolymer 12
parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane
resin 3 parts UR 8200 (manufactured by Toyobo Co., Ltd.)
.alpha.-Alumina 10 parts HIT55 (manufactured by Sumitomo Chemical
Co., Ltd.) Average particle size: 0.20 .mu.m Specific surface area:
8.0 to 9.0 m.sup.2/g Mohs' hardness: 9 pH: 7.7 to 9.0 Carbon black
5 parts #50 (manufactured by Asahi Carbon Co., Ltd.) Average
particle size: 94 nm Specific surface area: 28 m.sup.2/g DBP oil
absorption: 61 ml/100 g pH: 7.5 Volatile content: 1.0 wt %
Phenylphosphonic acid 3 parts Butyl stearate 10 parts Butoxyethyl
stearate 5 parts Isohexadecyl stearate 3 parts Stearic acid 2 parts
Methyl ethyl ketone 180 parts Cyclohexanone 180 parts Magnetic
Coating Solution: 4ML-2 (acicular magnetic powder was used)
Ferromagnetic metal powder: 4M-2 100 parts Composition: Co/Fe
(atomic ratio), 30% Hc: 2,360 Oe Specific surface area: 49
m.sup.2/g .sigma..sub.s: 146 emu/g Crystallite size: 170 .ANG.
Average long axis length: 0.100 .mu.m Acicular ratio: 6 SFD: 0.950
Sintering inhibitor: Al compound (Al/Fe, atomic ratio: 5%) Y
compound (Y/Fe, atomic ratio: 5%) pH: 9.4 Vinyl chloride copolymer
10 parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane
resin 4 parts UR 5500 (manufactured by Toyobo Co., Ltd.)
.alpha.-Alumina 10 parts HIT7O (manufactured by Sumitomo Chemical
Co., Ltd.) Average particle size: 0.15 .mu.m Specific surface area:
17 m.sup.2/g Mohs' hardness: 9 pH: 7.7 to 9.0 Carbon black 1 part
#50 (manufactured by Asahi Carbon Co., Ltd.) Average particle size:
94 nm Specific surface area: 28 m.sup.2/g DBP oil absorption: 61
ml/100 g pH: 7.5 Volatile content: 1.0 wt % Phenylphosphonic acid 3
parts Oleic acid 1 part Stearic acid 0.6 part Ethylene glycol
dioleyl 12 parts Methyl ethyl ketone 180 parts Cyclohexanone 180
parts Magnetic Coating Solution: 4ML-3 (acicular magnetic powder
was used, comparative example) Ferromagnetic metal powder: 4M-3 100
parts Composition: Fe/Ni = 96/4 Hc: 1,600 Oe Specific surface area:
45 m.sup.2/g Crystallite size: 220 .ANG. .sigma..sub.s: 135 emu/g
Average long axis length: 0.20 .mu.m Acicular ratio: 9 Vinyl
chloride copolymer 12 parts MR110 (manufactured by Nippon Zeon Co.,
Ltd.) Polyurethane resin 5 parts UR 8600 (manufactured by Toyobo
Co., Ltd.) .alpha.-Alumina (particle size: 0.65 .mu.m) 2 parts
Chromium oxide (particle size: 0.35 .mu.m) 15 parts Carbon black
(particle size: 0.03 .mu.m) 2 parts Carbon black (particle size:
0.3 .mu.m) 9 parts Isohexadecyl stearate 4 parts n-Butyl stearate 4
parts Butoxyethyl stearate 4 parts Oleic acid 1 part Stearic acid 1
part Methyl ethyl ketone 300 parts Magnetic Coating Solution: 4ML-6
(acicular magnetic powder was used) Ferromagnetic metal powder:
4M-2 100 parts Composition: Co/Fe (atomic ratio), 30% Hc: 2,360 Oe
Specific surface area: 49 m.sup.2/g .sigma..sub.s: 146 emu/g
Crystallite size: 170 .ANG. Average long axis length: 0.100 .mu.m
Acicular ratio: 6 SFD: 0.950 Sintering inhibitor: Al compound
(Al/Fe, atomic ratio: 5%) Y compound (Y/Fe, atomic ratio: 5%) pH:
9.4 Vinyl chloride copolymer 10 parts MR110 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 4 parts UR 5500 (manufactured by
Toyobo Co., Ltd.) .alpha.-Alumina 10 parts HIT7O (manufactured by
Sumitomo Chemical Co., Ltd.) Average particle size: 0.15 .mu.m
Specific surface area: 17 m.sup.2/g Mohs' hardness: 9 pH: 7.7 to
9.0 Carbon black 1 part #50 (manufactured by Asahi Carbon Co.,
Ltd.) Average particle size: 94 nm Specific surface area: 28
m.sup.2/g DBP oil absorption: 61 ml/100 g pH: 7.5 Volatile content:
1.0 wt % Phenylphosphonic acid 3 parts Myristic acid 1 part Stearic
acid 0.6 part Butyl stearate 4 parts Cetyl palmitate 4 parts Oleyl
oleate 4 parts Methyl ethyl ketone 180 parts Cyclohexanone 180
parts Magnetic Coating Solution: 4ML-7 (acicular magnetic powder
was used) Ferromagnetic metal powder: 4M-2 100 parts Composition:
Co/Fe (atomic ratio), 30% Hc: 2,360 Oe Specific surface area: 49
m.sup.2/g .sigma.: 146 emu/g Crystallite size: 170 .ANG. Average
long axis length: 0.100 .mu.m Acicular ratio: 6 SFD: 0.950
Sintering inhibitor: Al compound (Al/Fe, atomic ratio: 5%) Y
compound (Y/Fe, atomic ratio: 5%) pH: 9.4 Vinyl chloride copolymer
10 parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane
resin 4 parts UR 5500 (manufactured by Toyobo Co., Ltd.)
.alpha.-Alumina 10 parts HIT70 (manufactured by Sumitomo Chemical
Co., Ltd.) Average particle size: 0.15 .mu.m Specific surface area:
17 m.sup.2/g Mohs' hardness: 9 pH: 7.7 to 9.0 Carbon black 1 part
#50 (manufactured by Asahi Carbon Co., Ltd.) Average particle size:
94 .mu.m Specific surface area: 28 m.sup.2/g DBP oil absorption: 61
ml/100 g pH: 7.5 Volatile content: 1.0 wt % Phenylphosphonic acid 3
parts Amyl stearate 4 parts Butoxyethyl stearate 6 parts Oleyl
oleate 4 parts Methyl ethyl ketone 180 parts Cyclohexanone 180
parts Magnetic Coating Solution: 4ML-8 (acicular magnetic powder
was used) Ferromagnetic metal powder: 4M-2 100 parts Composition:
Co/Fe (atomic ratio), 30% Hc: 2,360 Oe Specific surface area: 46
m.sup.2/g .sigma..sub.s: 153 emu/g Crystallite size: 160 .ANG.
Average long axis length: 0.100 .mu.m Acicular ratio: 6 SFD: 0.950
pH: 9.4 Sintering inhibitor: Al compound (Al/Fe, atomic ratio: 11%)
Y compound (Y/Fe, atomic ratio: 7%) Mg compound (Mg/Fe, atomic
ratio: 1%) Vinyl chloride copolymer 10 parts MR110 (manufactured by
Nippon Zeon Co., Ltd.) Polyurethane resin 4 parts UR 5500
(manufactured by Toyobo Co., Ltd.) .alpha.-Alumina 10 parts HIT55
(dispersion product of 5 parts/ 1 part/4 parts of HIT55/MR110/MEK,
which were previously dispersed manufactured by Sumitomo Chemical
Co., Ltd.) Average particle size: 0.20 .mu.m Specific surface area:
8.0 to 9.0 m.sup.2/g Mohs' hardness: 9 pH: 7.7 to 9.0 Diamond 1
part LS600F (manufactured by LANDS SUPERABRASIVES, CO.) Average
particle size: 0.27 .mu.m Carbon black 1 part #50 (manufactured by
Asahi Carbon Co., Ltd.) Average particle size: 94 .mu.m Specific
surface area: 28 m.sup.2/g DBP oil absorption: 61 ml/100 g pH: 7.5
Volatile content: 1.0 wt % Phenylphosphonic acid 3 parts Stearic
acid 1 part Oleic acid 1 part Butyl stearate 4 parts Butoxyethyl
stearate 4 parts Neopentyl glycol didecanoate 2 parts Ethylene
glycol dioleyl 2 parts Methyl ethyl ketone 180 parts Cyclohexanone
180 parts Nonmagnetic Coating Solution: 4NU-1 (spherical inorganic
powder was used) Nonmagnetic powder, TiO.sub.2, crystal system 80
parts rutile Average primary particle size: 0.035 .mu.m Specific
surface area (S.sub.BET): 40 m.sup.2/g pH: 7 TiO.sub.2 content: 90%
or more DBP oil absorption: 27 to 38 ml/100 g Surface-covering
compound: Al.sub.2O.sub.3, 8 wt % based on total particles Carbon
black 20 parts CONDUCTEX SC-U (manufactured by Columbia Carbon Co.,
Ltd.) Average primary particle size: 20 nm DBP oil absorption: 115
ml/100 g pH: 7.0 Specific surface area (S.sub.BET): 220 m.sup.2/g
Volatile content: 1.5% Vinyl chloride copolymer 12 parts MR110
(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 5 parts
UR 8200 (manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4
parts Butyl stearate 10 parts Butoxyethyl stearate 5 parts
Isohexadecyl stearate 2 parts Stearic acid 3 parts Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent) Nonmagnetic
Coating Solution: 4NU-2 (spherical inorganic powder was used)
Nonmagnetic powder, TiO.sub.2, crystal system 100 parts rutile
Average primary particle size: 0.035 .mu.m Specific surface area
(S.sub.BET): 40 m.sup.2/g pH: 7 TiO.sub.2 content: 90% or more DBP
oil absorption: 27 to 38 ml/l00 g Surface-covering compound:
Al.sub.2O.sub.3 and SiO.sub.2 Ketjen Black EC 13 parts
(manufactured by Akzo Nobel Co., Ltd.) Average primary particle
size: 30 nm DBP oil absorption: 350 ml/100 g pH: 9.5 Specific
surface area (S.sub.BET): 950 m.sup.2/g Volatile content: 1.0%
Vinyl chloride copolymer 16 parts MR110 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 6 parts UR 8200 (manufactured by
Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts Ethylene glycol
dioleyl 16 parts Oleic acid 1 part Stearic acid 0.8 part Methyl
ethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)
Nonmagnetic Coating Solution 4NU-3 (spherical inorganic powder was
used, comparative example) Nonmagnetic powder, TiO.sub.2, crystal
system 75 parts rutile Average primary particle size: 0.035 .mu.m
Specific surface area: 40 m.sup.2/g pH: 7 TiO.sub.2 content: 90% or
more DBP oil absorption: 27 to 38 ml/100 g Surface-covering
compound: Al.sub.2O.sub.3 and SiO.sub.2 Carbon black 10 parts
Ketjen Black EC (manufactured by Akzo Nobel Co., Ltd.) Average
primary particle size: 30 nm DBP oil absorption: 350 ml/100 g pH:
9.5 Specific surface area (S.sub.BET): 950 m.sup.2/g Volatile
content: 1.0% .alpha.-Alumina 15 parts AKP-15 (manufactured by
Sumitomo Chemical Co., Ltd.) Average particle size: 0.65 .mu.m
Vinyl chloride copolymer 12 parts MR110 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 5 parts UR 8200 (manufactured by
Toyobo Co., Ltd.) Isohexadecyl stearate 4 parts n-Butyl stearate 4
parts Butoxyethyl stearate 4 parts Oleic acid 1 part Stearic acid 1
part Methyl ethyl ketone 300 parts Nonmagnetic Coating Solution
4NU-4 (acicular inorganic powder was used) Nonmagnetic powder,
.alpha.-Fe.sub.2O.sub.3, hematite 80 parts Average long axis
length: 0.15 .mu.m Specific surface area (S.sub.BET): 50 m.sup.2/g
Acicular ratio: 6 pH: 9 Surface-covering compound: Al.sub.2O.sub.3,
8 wt % based on total particles Carbon black 20 parts CONDUCTEX
SC-U (manufactured by Columbia Carbon Co., Ltd.) Average primary
particle size: 20 nm DBP oil absorption: 115 ml/100 g pH: 7.0
Specific surface area (S.sub.BET): 220 m.sup.2/g Volatile content:
1.5% Vinyl chloride copolymer 12 parts MR110 (manufactured by
Nippon Zeon Co., Ltd.) Polyurethane resin 5 parts UR 8200
(manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts
Butyl stearate 10 parts Butoxyethyl stearate 5 parts Isohexadecyl
stearate 2 parts Stearic acid 3 parts Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent) Nonmagnetic
Coating Solution 4NU-5 (acicular inorganic powder was used)
Nonmagnetic powder, .alpha.-Fe.sub.2O.sub.3, hematite 100 parts
Average long axis length: 0.15 .mu.m Specific surface area
(S.sub.BET): 50 m.sup.2/g Acicular ratio: 6 pH: 9 Surface-covering
compound: Al.sub.2O.sub.3, 8 wt % based on total particles Carbon
black 18 parts #3250B (manufactured by Mitsubishi Kasei Corp.)
Average particle size: 30 nm Specific surface area: 245 m.sup.2/g
DBP oil absorption: 155 ml/100 g pH: 6.0 Volatile content: 1.5 wt %
Vinyl chloride copolymer 15 parts MR104 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 7 parts UR 5500 (manufactured by
Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts Ethylene glycol
dioleyl 16 parts Oleic acid 1.3 parts Stearic acid 0.8 part Methyl
ethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)
Nonmagnetic Coating Solution 4NU-6 (acicular inorganic powder was
used) Nonmagnetic powder, .alpha.-Fe.sub.2O.sub.3, hematite 100
parts Average long axis length: 0.15 .mu.m Specific surface area
(S.sub.BET): 50 m.sup.2/g Acicular ratio: 6 pH: 9 Surface-covering
compound: Al.sub.2O.sub.3, 8 wt % based on total particles Carbon
black 18 parts #3250B (manufactured by Mitsubishi Kasei Corp.)
Average particle size: 30 nm Specific surface area: 245 m.sup.2/g
DBP oil absorption: 155 ml/100 g pH: 6.0 Volatile content: 1.5 wt %
Vinyl chloride copolymer 15 parts MR104 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 7 parts UR 5500 (manufactured by
Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts Myristic acid 1
part Stearic acid 0.6 part Butyl stearate 4 parts Cetyl palmitate 4
parts Oleyl oleate 4 parts Methyl ethyl ketone/cyclohexanone 250
parts (8/2 mixed solvent) Nonmagnetic Coating Solution 4NU-7
(acicular inorganic powder was used) Nonmagnetic powder,
.alpha.-Fe.sub.2O.sub.3, hematite 100 parts Average long axis
length: 0.15 .mu.m Specific surface area (S.sub.BET): 50 m.sup.2/g
Acicular ratio: 6 pH: 9 Surface-covering compound: Al.sub.2O.sub.3,
8 wt % based on total particles Carbon black 10 parts CONDUCTEX
SC-U (manufactured by Columbia Carbon Co., Ltd.) Average primary
particle size: 20 nm DBP oil absorption: 115 ml/100 g pH: 7.0
Specific surface area (S.sub.BET): 220 m.sup.2/g Volatile content:
1.5% Carbon black 10 parts #50 (manufactured by Asahi Carbon Co.,
Ltd.) Average particle size: 94 nm Specific surface area: 28
m.sup.2/g DBP oil absorption: 61 ml/100 g pH: 7.5 Volatile content:
1.0 wt % Vinyl chloride copolymer 15 parts MR104 (manufactured by
Nippon Zeon Co., Ltd.) Polyurethane resin 7 parts UR 5500
(manufactured by Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts
Amyl stearate 4 parts Butoxyethyl stearate 6 parts Oleyl oleate 4
parts Methyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed
solvent) Nonmagnetic Coating Solution 4NU-8 (acicular inorganic
powder was used) Nonmagnetic powder, .alpha.-Fe.sub.2O.sub.3,
hematite 100 parts Average long axis length: 0.08 .mu.m Specific
surface area (S.sub.BET): 50 m.sup.2/g Acicular ratio: 6.5 pH: 9
Surface-covering compound: Al.sub.2O.sub.3, 8 wt % based on total
particles Carbon black 25 parts CONDUCTEX SC-U (manufactured by
Columbia Carbon Co., Ltd.) Average primary particle size: 20 nm DBP
oil absorption: 115 ml/100 g pH: 7.0 Specific surface area
(S.sub.BET): 220 m.sup.2/g Volatile content: 1.5% Vinyl chloride
copolymer 16 parts MR104 (manufactured by Nippon Zeon Co., Ltd.)
Polyurethane resin 7 parts UR 5500 (manufactured by Toyobo Co.,
Ltd.) Phenylphosphonic acid 4 parts Stearic acid 1 part Oleic acid
1 part Butyl stearate 4 parts Butoxyethyl stearate 4 parts
Neopentyl glycol dioleyl 2 parts Ethylene glycol dioleyl 2 parts
Methyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed solvent)
[0379] Preparation Method 4-1 (Disc: W/W)
[0380] Each of the above ten compositions of the coating solutions
for the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0381] These coating solutions were simultaneously
multilayer-coated on a polyethylene terephthalate support having a
thickness of 62 .mu.m and a central plane average surface roughness
of 3 nm of the surface side on which a magnetic layer was to be
coated. The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m, immediately thereafter the magnetic layer
coating solution was coated on the nonmagnetic layer so as to give
the magnetic layer having the thickness of 0.15 .mu.m. The coated
layers were subjected to random orientation while the magnetic
layer and the nonmagnetic layer were still wet by passing through
an alternating current magnetic field generator having two magnetic
field intensities of frequency of 50 Hz, magnetic field intensity
of 250 Gauss and frequency of 50 Hz, magnetic field intensity of
120 Gauss. After drying, the coated layer was subjected to
calendering treatment with calenders of 7 stages at 90.degree. C.
at line pressure of 300 kg/cm. The obtained web was punched to a
disc of 3.7 inches, the disc was subjected to a surface treatment
by abrasives, encased in 3.7 inch cartridge having a liner inside
(a zip-disc cartridge manufactured by Iomega Co., Ltd., U.S.A.),
and equipped the cartridge with prescribed mechanism parts to
obtain a 3.7 inch floppy disc. A part of samples was subjected to
machine direction orientation using Co magnets with the same pole
and counter positions of 4,000 G before random orientation
treatment.
[0382] In this case, it is preferred to increase the frequency and
magnetic field intensity of the alternating current magnetic field
generator so as to achieve finally sufficient random orientation,
thereby 98% or more of orientation ratio can be obtained.
[0383] When barium ferrite magnetic powder is used, vertical
orientation can be performed besides the above-described
orientation. Further, if necessary, discs after being punched may
be subjected to post treatments, e.g., a thermal treatment at high
temperature (generally from 50 to 90.degree. C.) to accelerate
curing of coated layers, or a burnishing treatment with an abrasive
tape to scrape surface protrusions.
[0384] Preparation Method 4-2 (Computer Tape: W/W)
[0385] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer were blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 2.5 parts to
the nonmagnetic layer coating solution, and 3 parts to the magnetic
layer coating solution. Further, 40 parts of cyclohexanone was
added to each solution. Each solution was filtered through a filter
having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0386] These coating solutions were simultaneously
multilayer-coated on an aramide support (trade name: Mictron)
having a thickness of 4.4 .mu.m and a central plane average surface
roughness of 2 nm of the surface side on which a magnetic layer was
to be coated. The nonmagnetic layer coating solution was coated in
a dry thickness of 1.7 .mu.m, immediately thereafter the magnetic
layer coating solution was coated on the nonmagnetic layer so as to
give the magnetic layer having a thickness of 0.15 .mu.m. The
coated layers were oriented with a cobalt magnet having a magnetic
force of 6,000 G and a solenoid having a magnetic force of 6,000 G
while both layers were still wet. After drying, the coated layer
was subjected to calendering treatment with calenders of 7 stages
comprising metal rolls at 85.degree. C. at a rate of 200 m/min.
Subsequently, a backing layer (100 parts of a carbon black having
an average particle size of 17 nm, 80 parts of calcium carbonate
having an average particle size of 40 nm, and 5 parts of
.alpha.-alumina having an average particle size of 0.2 .mu.m were
dispersed in a nitrocellulose resin, a polyurethane resin and a
polyisocyanate) having a thickness of 0.5 .mu.m was coated. The
obtained web was slit to a width of 3.8 mm. The magnetic layer
surface of the thus-produced tape was cleaned with a tape cleaning
apparatus of a nonwoven fabric and a razor blade pressed against
the surface of the tape, which was attached to a machine having
delivery and winding-up movement of a slit product. The
thus-obtained magnetic tape was incorporated in a cartridge for
DDS.
[0387] Preparation Method 4-3 (Disc: W/D)
[0388] Each of the above ten compositions of the coating solutions
for the magnetic layer and the nonmagnetic layer was blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0389] The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m on a polyethylene terephthalate support
having a thickness of 62 .mu.m and a central plane average surface
roughness of 3 nm of the surface side on which the magnetic layer
was to be coated, dried, and subjected to calendering treatment.
The magnetic layer coating solution was coated by blade coating on
the nonmagnetic layer so as to give the magnetic layer having the
thickness of 0.15 .mu.m. The coated layers were subjected to random
orientation by passing through an alternating current magnetic
field generator having two magnetic field intensities of frequency
of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50
Hz, magnetic field intensity of 120 Gauss. The procedure was
carried out in the same manner as in Preparation Method 4-1
hereafter. Calendering of the nonmagnetic layer may be omitted.
[0390] Preparation Method 4-4 (Computer Tape: W/D)
[0391] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer were blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 2.5 parts to
the nonmagnetic layer coating solution, and 3 parts to the magnetic
layer coating solution. Further, 40 parts of cyclohexanone was
added to each solution. Each solution was filtered through a filter
having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0392] The nonmagnetic layer coating solution was coated in a dry
thickness of 1.7 .mu.m on an aramide support (trade name: Mictron)
having a thickness of 4.4 .mu.m and a central plane average surface
roughness of 2 nm of the surface side on which a magnetic layer was
to be coated, dried, and subjected to calendering treatment. The
magnetic layer coating solution was coated by blade coating on the
nonmagnetic layer so as to give the magnetic layer having the
thickness of 0.15 .mu.m. The coated layers were oriented with a
cobalt magnet having a magnetic force of 6,000 G and a solenoid
having a magnetic force of 6,000 G. The procedure was carried out
in the same manner as in Preparation Method 4-2 hereafter.
Calendering of the nonmagnetic layer may be omitted.
[0393] Preparation Method 4-5 (Disc: Spin Coating)
[0394] Each of the above ten compositions of the coating solutions
for the magnetic layer and the nonmagnetic layer were blended in a
kneader, then dispersed with a sand mill. Polyisocyanate was added
to each resulting dispersion solution, in an amount of 10 parts to
the nonmagnetic layer coating solution, and 10 parts to the
magnetic layer coating solution. Further, 40 parts of cyclohexanone
was added to each solution. Each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0395] The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m by spin coating on a polyethylene
terephthalate support having a thickness of 62 .mu.m and a central
plane average surface roughness of 3 nm of the surface side on
which a magnetic layer was to be coated and dried. The magnetic
layer coating solution was coated by spin coating on the
nonmagnetic layer so as to give the magnetic layer having the
thickness of 0.15 .mu.m. The coated layers were oriented using a Co
magnets with the same pole and counter positions of 6,000 G in the
circumferential direction and the surface of the layer was smoothed
by batch system rolling treatment by which the same pressure as in
Preparation Method 4-1 can be applied. The procedure was carried
out in the same manner as in Preparation Method 4-1 hereafter.
Also, the magnetic layer may be coated by spin coating on the
nonmagnetic layer while the nonmagnetic layer coated by spin
coating is still wet. By using the spin coating process, not only
the amount of residual magnetization in the recording direction can
be made large but also vertical magnetization components of
ferromagnetic metal powders of short acicular ratio can be reduced
and symmetric property of reproduced wave form can be improved.
24 Support 4B-1: Polyethylene Terephthalate Thickness: 62 .mu.m F-5
value: MD: 114 MPa, TD: 107 MPa Breaking strength: MD: 276 MPa, TD:
281 MPa Breaking extension: MD: 174 MPa, TD: 139 MPa Thermal
shrinkage factor (80.degree. C., 30 minutes): MD: 0.04%, TD: 0.05%
Thermal shrinkage factor (100.degree. C., 30 minutes): MD: 0.2%,
TD: 0.3% Temperature expansion coefficient: Long axis: 15 .times.
10.sup.-5/.degree. C. Short axis: 18 .times. 10.sup.-5/.degree. C.
Central plane average surface roughness: 3 nm Support 4B-2:
Polyethylene Naphthalate Thickness: 55 .mu.m Central plane average
surface roughness: 1.8 nm Thermal shrinkage factor (80.degree. C.,
30 minutes): MD: 0.007%, TD: 0.007% Thermal shrinkage factor
(100.degree. C., 30 minutes): MD: 0.02%, TD: 0.02% Temperature
expansion coefficient: Long axis: 10 .times. 10.sup.-5/.degree. C.
Short axis: 11 .times. 10.sup.-5/.degree. C. Support 4B-3:
Polyethylene Terephthalate Thickness: 62 .mu.m Central plane
average surface roughness: 9 nm Support 4B-4: Aramide Thickness:
4.4 .mu.m Central plane average surface roughness: 2 nm Orientation
4O-1: Random orientation 4O-2: Orientation in the machine direction
using a Co magnet first, then random orientation 4O-3: Orientation
in the machine direction using a Co magnet first, then in the
machine direction using a solenoid 4O-4: Orientation in the
vertical direction using a Co magnet 4O-5: Orientation in the
circumferential direction using a Co magnet Backing Layer Coating
Solution: BL-1 Fine carbon black powder 100 parts BP-800 (average
particle size: 17 nm, manufactured by Cabot Co., Ltd.) Coarse
carbon black powder 10 parts Thermal Black (average particle size:
270 nm, manufactured by Cancarb Co., Ltd.) Calcium carbonate (soft
inorganic powder) 80 parts Hakuenka O (average particle size: 40
nm, Mohs' hardness: 3, manufactured by Shiraishi Kogyo Co., Ltd.)
.alpha.-Alumina (hard inorganic powder) 5 parts (average particle
size: 200 nm, Mohs' hardness: 9) Nitrocellulose resin 140 parts
Polyurethane resin 15 parts Polyisocyanate 40 parts Polyester resin
5 parts Dispersant: Copper oleate 5 parts Copper phthalocyanine 5
parts Barium sulfate 5 parts Methyl ethyl ketone 2,200 parts Butyl
acetate 300 parts Toluene 600 parts
[0396] The above compositions of the coating solution for the
backing layer were blended in a continuous kneader, then dispersed
with a sand mill. The resulting dispersion solution was filtered
through a filter having an average pore diameter of 1 .mu.m to
obtain a coating solution for forming the backing layer.
[0397] With respect to samples obtained by combining the
above-described each method arbitrarily as shown in Table 4-1,
magnetic characteristics, central plane average surface roughness,
areal recording density, etc., were determined.
[0398] (1) Magnetic characteristics (Hc):
[0399] Magnetic characteristics were measured using a vibrating
sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10
KOe.
[0400] (2) Central plane average surface roughness (Ra):
[0401] Surface roughness (Ra) of the area of about 250
.mu.m.times.250 .mu.m, Rrms, peak-valley values were measured using
"TOPO3D" (a product of WYKO, U.S.A.) by 3D-MIRAU method. The
wavelength of measurement was about 650 nm and spherical
compensation and cylindrical compensation were applied. Measurement
was performed using a light interference type non-contact surface
roughness meter.
[0402] (3) Areal recording density:
[0403] Areal recording density means a value obtained by
multiplying linear recording density by track density.
[0404] (4) Linear recording density:
[0405] Linear recording density means a bit number of signals
recorded per 1 inch in the recording direction.
[0406] (5) Track density:
[0407] Track density means a track number per 1 inch.
[0408] (6) .phi.m:
[0409] .phi.m is the amount of magnetization per unit area of a
magnetic recording medium, which is represented by Bm (Gauss)
multiplying thickness. This is the value obtained by using a
vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.)
at Hm 10 KOe, which can be directly measured.
[0410] (7) Error rate of tape:
[0411] The above signals of linear recording density were recorded
on the tape by 8-10 conversion PR1 equalization system and error
rate of the tape was measured using a DDS drive.
[0412] (8) Error rate of disc:
[0413] The above signals of linear recording density were recorded
on the disc by (2,7) RLL modulation system and error rate was
measured.
[0414] (9) Thickness of magnetic layer:
[0415] The sample having the thickness of about 0.1 .mu.m was cut
out with a diamond cutter in the machine direction of the magnetic
medium, observed with a transmission type electron microscope of
from 10,000 to 100,000, preferably from 20,000 to 50,000
magnifications and photographed. The print size of the photograph
was from A4 (i.e., 297.times.210 mm) to A5 (i.e., 210.times.148 mm)
sizes. The present inventors paid attentions to the difference of
the shapes of the ferromagnetic powders and the nonmagnetic powders
of the magnetic layer and the nonmagnetic layer and rimmed the
interface and the surface of the magnetic layer with black color by
visual judgement. Thereafter, the distance of the rimmed lines was
measured by the image processing apparatus "IBAS2" (manufactured by
Zeiss Corp.). Measurement was conducted from 85 to 300 times when
the length of the sample photograph was 21 cm. The average measured
value at that time was taken as d, and the standard deviation of
the measured value was taken as .sigma.. d depended on the
description in JP-A-5-298653 and a was obtained by equation (2) in
JP-A-5-298653. di means each measured value and n is from 85 to
300.
[0416] (10) Running durability:
[0417] A floppy disc drive ("ZIP100", a product of IOMEGA CORP.,
U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at
the position of radius of 38 mm. Recording was conducted at
recording density of 34 kfci, then reproduced the signals recorded
and this was taken as 100%. The disc was run for 1,500 hours under
the following thermo-cycle condition, which being taken as one
cycle. Output was monitored every 24 hours of running and the point
when the initial reproduction output became 70% or less was taken
as NG.
[0418] Thermo-Cycle Flow
[0419] 25.degree. C., 50% RH, 1 hr.fwdarw.(temperature up, 2
hr).fwdarw.60.degree. C., 20% RH, 7 hr.fwdarw.(temperature down, 2
hr).fwdarw.25.degree. C., 50% RH, 1 hr.fwdarw.(temperature down, 2
hr).fwdarw.5.degree. C., 50% RH, 7 hr.fwdarw.(temperature up, 2
hr).fwdarw.(this cycle was repeated).
[0420] (11) Liner wear:
[0421] A sample was run for 1,000 hours under the same condition as
in the evaluation of running durability with the head being off,
and after completion of running, cartridge case of the sample was
opened and the surface of the magnetic layer was visually evaluated
by the following criteria.
[0422] o: No defect was observed on the surface of the magnetic
layer.
[0423] .DELTA.: Fine scratches were generated on a part of the
surface of the magnetic layer.
[0424] x: Fine scratches were generated on the entire surface of
the magnetic layer.
[0425] (12) Liner adhesion:
[0426] A sample was run for 1,000 hours under the same condition as
in the evaluation of running durability with the head being off,
and after completion of running, cartridge case of the sample was
opened and the surface of the magnetic layer was visually evaluated
by the following criteria.
[0427] o: Liner was not adhered on the surface of the magnetic
layer.
[0428] .DELTA.: Liner was adhered on a part of the surface of the
magnetic layer.
[0429] x: Liner was adhered on the entire surface of the magnetic
layer.
[0430] (13) Starting torque:
[0431] Starting torque at the time of head-on in LS-102 drive (a
product of Imation Co., Ltd.) was determined using torque gauge
model 300 ATG (a product of Tonichi Seisakusho Co., Ltd.) (unit:
g.multidot.cm).
[0432] (14) Measurement of C/Fe
[0433] C/Fe value was determined using Auger electron spectrometer
PHI-660 type manufactured by .PHI. Co. Conditions of measurement
were as follows.
[0434] Accelerating voltage of primary electron beam: 3 KV
[0435] Electric current of sample: 130 nA
[0436] Magnification: 250-fold
[0437] Inclination angle: 30.degree.
[0438] The C/Fe ratio is obtained as the C/Fe peak by integrating
the values obtained by the above conditions in the region of
kinetic energy of 130 eV to 730 eV three times and finding the
strengths of KLL peak of the carbon and LMM peak of the iron as
differentials.
25TABLE 4-1 Disc Magnetic Layer Thick- Surface Prepara- Prescrip-
ness Hc Roughness .phi.m Lower tion Orien- Sample No. tion (.mu.m)
(Oe) (nm) (emu/cm.sup.2) Layer Support Method tation Example 4-1
4ML-2 0.15 2,360 3.5 4.8 .times. 10.sup.-3 4NU-1 4B-1 Method 4-1
40-1 Example 4-2 4ML-2 0.15 2,360 2.3 4.8 .times. 10.sup.-3 4NU-2
4B-1 Method 4-1 40-1 Example 4-3 4ML-2 0.15 2,360 1.9 4.8 .times.
10.sup.-3 4NU-4 4B-1 Method 4-1 40-1 Example 4-4 4ML-2 0.15 2,360
1.7 4.8 .times. 10.sup.-3 4NU-5 4B-1 Method 4-1 40-1 Example 4-5
4ML-2 0.05 2,400 1.4 1.6 .times. 10.sup.-3 4NU-5 4B-1 Method 4-1
40-1 Example 4-6 4ML-2 0.10 2,380 1.6 3.2 .times. 10.sup.-3 4NU-5
4B-1 Method 4-1 40-1 Example 4-7 4ML-2 0.20 2,330 1.9 6.4 .times.
10.sup.-3 4NU-5 4B-1 Method 4-1 40-1 Example 4-8 4ML-2 0.15 2,360
1.5 4.8 .times. 10.sup.-3 4NU-5 4B-2 Method 4-1 40-1 Example 4-9
4ML-1 0.15 2,550 2.5 4.2 .times. 10.sup.-3 4NU-5 4B-1 Method 4-1
40-1 Comparative 4ML-3 0.15 1,600 3.1 4.8 .times. 10.sup.-3 4NU-5
4B-1 Method 4-1 40-1 Example 4-1 Example 4-12 4ML-2 0.15 2,360 2.5
4.8 .times. 10.sup.-3 4NU-5 4B-1 Method 4-3 40-1 Example 4-13 4ML-2
0.15 2,360 1.7 4.8 .times. 10.sup.-3 4NU-5 4B-1 Method 4-1 40-2
Example 4-16 4ML-2 0.15 2,660 1.6 4.8 .times. 10.sup.-3 4NU-5 4B-1
Method 4-5 40-5 Example 4-21 4ML-6 0.15 2,360 1.7 4.8 .times.
10.sup.-3 4NU-6 4B-1 Method 4-1 40-1 Example 4-22 4ML-7 0.15 2,360
1.7 4.8 .times. 10.sup.-3 4NU-7 4B-1 Method 4-1 40-1 Example 4-23
4ML-8 0.15 2,360 1.7 4.8 .times. 10.sup.-3 4NU-8 4B-2 Method 4-1
40-1
[0439]
26 TABLE 4-2 Linear Areal Track Recording Recording Error Magnetic
Layer Density Density Density Rate .DELTA.Hc .DELTA..phi.m Sample
No. (tpi) (kbpi) (G bit/inch.sup.2) (10.sup.-5) C/Fe (%) (%)
Example 4-1 5,200 144 0.75 0.2 40 +2.6 +5.0 Example 4-2 5,200 144
0.75 0.08 10 +2.5 +5.0 Example 4-3 5,200 144 0.75 0.03 70 +2.5 +4.8
Example 4-4 5,200 144 0.75 0.01 25 +2.5 +4.8 Example 4-5 5,200 144
0.75 0.06 25 +3.3 +5.1 Example 4-6 5,200 144 0.75 0.01 25 +3.0 +5.1
Example 4-7 5,200 144 0.75 0.2 25 +2.3 +5.0 Example 4-8 5,200 144
0.75 0.008 25 +2.5 +4.9 Example 4-9 5,200 144 0.75 0.004 30 +4.0
+6.0 Comparative 5,200 144 0.75 40 30 -1.0 +8.2 Example 4-1 Example
4-12 5,200 144 0.75 0.1 25 +2.5 +5.0 Example 4-13 5,200 144 0.75
0.001 25 +2.5 +5.0 Example 4-16 5,200 144 0.75 0.0002 25 +2.5 +5.0
Example 4-18 7,500 200 1.5 0.8 25 +2.5 +5.0 Example 4-19 6,000 166
1.0 0.08 25 +2.5 +5.0 Example 4-20 3,000 120 0.36 0.007 25 +2.5
+5.0 Example 4-21 5,200 144 0.75 0.01 45 +2.4 +4.9 Example 4-22
5,200 144 0.75 0.01 60 +2.5 +5.0 Example 4-23 5,200 144 0.75 0.01
75 +2.6 +5.0 Reference 2,000 50 0.1 0.5 25 +2.5 +5.0 Example
4-1
[0440] In each of Examples 4-18 to 4-20 and Reference Example 4-1,
the disc in Example 4-13 was used and error rate was determined
with varying linear recording density and track density.
27TABLE 4-3 Computer Tape Magnetic Layer Thick- Surface Prepara-
Prescrip- ness Hc Roughness .phi.m Lower tion Orien- Sample No.
tion (.mu.m) (Oe) (nm) (emu/cm.sup.2) Layer Support Method tation
Example 4-24 4ML-2 0.15 2,460 3.7 4.8 .times.10.sup.-3 4NU-4 4B-4
Method 4-2 40-3 Example 4-25 4ML-2 0.15 2,460 2.4 4.8
.times.10.sup.-3 4NU-2 4B-4 Method 4-2 40-3 Example 4-26 4ML-2 0.15
2,460 2.1 4.8 .times.10.sup.-3 4NU-4 4B-4 Method 4-2 40-3 Example
4-27 4ML-2 0.15 2,460 1.8 4.8 .times.10.sup.-3 4NU-5 4B-4 Method
4-2 40-3 Example 4-28 4ML-2 0.05 2,500 1.7 1.6 .times.10.sup.-3
4NU-5 4B-4 Method 4-2 40-3 Example 4-29 4ML-2 0.10 2,480 1.7 3.2
.times.10.sup.-3 4NU-5 4B-4 Method 4-2 40-3 Example 4-30 4ML-2 0.20
2,430 2.0 6.4 .times.10.sup.-3 4NU-5 4B-4 Method 4-2 40-3 Example
4-31 4ML-1 0.15 2,650 2.6 4.2 .times.10.sup.-3 4NU-5 4B-4 Method
4-2 40-3 Comparative 4ML-3 0.15 1,700 3.3 4.8 .times.10.sup.-3
4NU-5 4B-4 Method 4-2 40-3 Example 4-2 Example 4-32 4ML-2 0.15
2,460 2.7 4.8 .times.10.sup.-3 4NU-5 4B-4 Method 4-4 40-3
[0441]
28 TABLE 4-4 Linear Areal Track Recording Recording Error Magnetic
Layer Density Density Density Rate .DELTA.Hc .DELTA..phi.m Sample
No. (tpi) (kbpi) (G bit/inch.sup.2) (10.sup.-5) C/Fe (%) (%)
Example 4-24 3,000 122 0.366 0.09 40 +2.5 +5.0 Example 4-25 3,000
122 0.366 0.02 10 +2.5 +5.0 Example 4-26 3,000 122 0.366 0.003 70
+2.5 +4.9 Example 4-27 3,000 122 0.366 0.001 25 +2.5 +4.8 Example
4-28 3,000 122 0.366 0.01 25 +3.3 +5.1 Example 4-29 3,000 122 0.366
0.002 25 +2.3 +5.0 Example 4-30 3,000 122 0.366 0.01 25 +2.5 +4.9
Example 4-31 3,000 122 0.366 0.0005 30 +4.0 +6.0 Comparative 3,000
122 0.366 11 30 -1.1 +8.3 Example 4-2 Example 4-32 3,000 122 0.366
0.02 25 +2.2 +4.6 Example 4-33 4,000 150 0.6 0.02 40 +2.2 +4.6
Example 4-34 5,000 170 0.85 0.5 40 +2.2 +4.6 Reference 3,000 50
0.15 0.1 40 +2.2 +4.6 Example 4-2
[0442] As described above, the above signals of linear recording
density were recorded on the tape by 8-10 conversion PR1
equalization system and error rate of the tape was measured using a
DDS drive. In each of Examples 4-33, 4-34 and Reference Example
4-2, the tape in Example 4-24 was used and error rate was
determined with varying linear recording density and track
density.
[0443] From the results in Tables 4-2 and 4-4, it can be seen that
the error rates of the magnetic recording media (i.e., disc and
computer tape) according to the present invention, in particular,
in high density recording region, are 1.times.10.sup.-5 or less,
which are conspicuously excellent as compared with conventional
disc-like media.
[0444] The magnetic recording medium according to the present
invention comprises a support having provided thereon in the order
from the support a substantially nonmagnetic lower layer and a
magnetic layer comprising a ferromagnetic metal powder dispersed in
a binder, which is a magnetic recording medium for recording
signals of from 0.17 to 2 G bit/inch.sup.2, preferably from 0.20 to
2 G bit/inch.sup.2, of areal recording density, wherein the
coercive force of the magnetic layer is 1,800 Oe or more, the
ferromagnetic metal powder has the average particle length of 0.12
.mu.m or less, and the magnetic layer has a dry thickness of from
0.05 to 0.30 .mu.m, preferably from 0.05 to 0.25 .mu.m, and .phi.m
of from 10.0.times.10.sup.-3 to 1.0.times.10.sup.-3 emu/cm.sup.2,
preferably 8.0.times.10.sup.-3 to 1.0.times.10.sup.-3 emu/cm.sup.2.
The magnetic recording medium of the present invention having high
capacity, excellent high density characteristics, high durability,
and markedly improved error rate in high density recording region
has been realized due to the above constitution of the present
invention, which could never be obtained by conventional
techniques.
EXAMPLE 5
[0445] Magnetic characteristics of the magnetic powders used in
Example 5 are shown in Table 5-1.
29TABLE 5-1 Ferromagnetic Metal Powder Average Element Long
Contained Axis (mol per Magnetic Length Acicular Hc .alpha..sub.s
100 mol Powder (nm) Ratio (Oe) (emu/g) of Fe) MP (1) 80 5 2,340 160
Co: 30 Al: 7 Y: 6 MP (2) 50 6 2,320 140 Co: 30 Al: 10 Y: 5 MP (3)
80 5 1,890 140 Co: 20 Al: 7 Y: 5 HP (4) 80 5 1,700 140 Co: 20 Al: 6
Y: 6
[0446] Magnetic discs and magnetic tapes were produced using the
magnetic powders shown in Table 5-1 below.
30 Preparation of Coating Solution Magnetic Coating Solution A
(ferromagnetic metal powder was used, disc) Ferromagnetic metal
powder: MP (1) to MP (4) 100 parts Vinyl chloride copolymer 12
parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane
resin 3 parts UR 8200 (manufactured by Toyobo Co., Ltd.) Carbon
black 5 parts #50 (manufactured by Asahi Carbon Co., Ltd.)
Phenylphosphonic acid 3 parts Butyl stearate 10 parts Butoxyethyl
stearate 5 parts Isohexadecyl stearate 3 parts Stearic acid 2 parts
Methyl ethyl ketone 180 parts Cyclohexanone 180 parts Magnetic
Coating Solution B (ferromagnetic metal powder was used, tape)
Ferromagnetic metal powder: MP (1) and MP (2) 100 parts Vinyl
chloride copolymer 12 parts MR110 (manufactured by Nippon Zeon Co.,
Ltd.) Polyurethane resin 3 parts UR 8200 (manufactured by Toyobo
Co., Ltd.) Carbon black 1 part #50 (manufactured by Asahi Carbon
Co., Ltd.) Butyl stearate 1 part Stearic acid 5 parts Methyl ethyl
ketone 100 parts Cyclohexanone 20 parts Toluene 60 parts
Nonmagnetic Coating Solution a (lower layer, disc) Nonmagnetic
powder, TiO.sub.2, crystal system 80 parts rutile Average primary
particle size: 0.035 .mu.m Specific surface area (S.sub.BET): 40
m.sup.2/g pH: 7 TiO.sub.2content: 90% or more DBP oil absorption:
27 to 38 ml/100 g Surface-covering compound: Al.sub.2O.sub.3, 8 wt
% based on total particles Carbon black 20 parts CONDUCTEX SC-U
(manufactured by Columbia Carbon Co., Ltd.) Vinyl chloride
copolymer 12 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)
Polyurethane resin 5 parts UR 8200 (manufactured by Toyobo Co.,
Ltd.) Phenylphosphonic acid 4 parts Butyl stearate 10 parts
Butoxyethyl stearate 5 parts Isohexadecyl stearate 2 parts Stearic
acid 3 parts Methyl ethyl ketone/cyclohexanone 250 parts (8/2 mixed
solvent) Nonmagnetic Coating Solution b (lower layer, tape)
Nonmagnetic powder, TiO.sub.2, crystal system 80 parts rutile
Average primary particle size: 0.035 .mu.m Specific surface area
(S.sub.BET): 40 m.sup.2/g pH: 7 TiO.sub.2content: 90% or more DBP
oil absorption: 27 to 38 ml/100 g Surface-covering compound:
Al.sub.2O.sub.3, 8 wt % based on total particles Carbon black 20
parts CONDUCTEX SC-U (manufactured by Columbia Carbon Co., Ltd.)
Vinyl chloride copolymer 12 parts MR110 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 5 parts UR 8200 (manufactured by
Toyobo Co., Ltd.) Phenylphosphonic acid 4 parts Butyl stearate 1
part Stearic acid 3 parts Methyl ethyl ketone/cyclohexanone 250
parts (8/2 mixed solvent)
[0447] Preparation Method 5-1a (Disc)
[0448] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then the diamond powder shown in Table 5-2 was added to
each solution (or not added) and dispersed with a sand mill.
Subsequently, the prescribed amount of dispersed .alpha.-alumina
HIT55 (SLH55, manufactured by Sumitomo Chemical Co., Ltd) shown in
Table 5-2 was added (or not added), further, polyisocyanate was
added to each resulting dispersion solution, in an amount of 10
parts to the nonmagnetic layer coating solution, and 10 parts to
the magnetic layer coating solution. Further, 40 parts of
cyclohexanone was added to each solution. Each solution was
filtered through a filter having an average pore diameter of 1
.mu.m to obtain coating solutions for forming the nonmagnetic layer
and the magnetic layer.
[0449] These coating solutions were simultaneously
multilayer-coated on a polyethylene terephthalate support having a
thickness of 62 .mu.m and a central plane average surface roughness
of 3 nm of the surface side on which a magnetic layer was to be
coated. The nonmagnetic layer coating solution was coated in a dry
thickness of 1.5 .mu.m, immediately thereafter the magnetic layer
coating solution was coated on the nonmagnetic layer so as to give
the magnetic layer having the thickness of 0.2 .mu.m. The layer
containing the ferromagnetic metal powder was subjected to random
orientation while the magnetic layer and the nonmagnetic layer were
still wet by passing through an alternating current magnetic field
generator having two magnetic field intensities of frequency of 50
Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz,
magnetic field intensity of 120 Gauss. After drying, the coated
layer was subjected to calendering treatment with calenders of 7
stages at 90.degree. C. at line pressure of 300 kg/cm. The obtained
web was punched to a disc of 3.7 inches, the disc was subjected to
a surface treatment by abrasives, thereby a magnetic recording disc
was obtained.
[0450] Preparation Method 5-1b (Disc)
[0451] Magnetic recording discs were prepared in the same manner as
in Preparation Method 5-1a, except that the polyethylene
terephthalate support having a central plane average surface
roughness of 7 nm was used.
[0452] Preparation Method 5-2 (Computer Tape)
[0453] Each of the above compositions of the coating solutions for
the magnetic layer and the nonmagnetic layer was blended in a
kneader, then the diamond powder shown in Table 5-4 was added to
each solution (or not added), further, the prescribed amount of
dispersed .alpha.-alumina HIT55 (SLH55, manufactured by Sumitomo
Chemical Co., Ltd) shown in Table 5-4 was added (or not added) and
dispersed with a sand mill. Subsequently, polyisocyanate was added
to each resulting dispersion solution, in an amount of 2.5 parts to
the nonmagnetic layer coating solution, and 3 parts to the magnetic
layer coating solution. Further, 40 parts of cyclohexanone was
added to each solution and each solution was filtered through a
filter having an average pore diameter of 1 .mu.m to obtain coating
solutions for forming the nonmagnetic layer and the magnetic
layer.
[0454] These coating solutions obtained were simultaneously
multilayer-coated on an aramide support (trade name: Mictron)
having a thickness of 4.4 .mu.m and a central plane average surface
roughness of 2 nm of the surface side on which a magnetic layer was
to be coated. The nonmagnetic layer coating solution was coated in
a dry thickness of 1.7 .mu.m, immediately thereafter the magnetic
layer coating solution was coated on the nonmagnetic layer so as to
give the magnetic layer having a thickness of 0.15 .mu.m. The
coated layers were oriented with a cobalt magnet having a magnetic
force of 6,000 G and a solenoid having a magnetic force of 6,000 G
while both layers were still wet. After drying, the coated layer
was subjected to calendering treatment with calenders of 7 stages
comprising metal rolls at 85.degree. C. at a rate of 200 m/min.
Subsequently, a backing layer (100 parts of a carbon black having
an average particle size of 17 nm, 80 parts of calcium carbonate
having an average particle size of 40 nm, and 5 parts of
.alpha.-alumina having an average particle size of 200 nm were
dispersed in a nitrocellulose resin, a polyurethane resin and a
polyisocyanate) having a thickness of 0.5 .mu.m was coated. The
obtained web was slit to a width of 3.8 mm. The magnetic layer
surface of the thus-produced tape was cleaned with a tape cleaning
apparatus of a nonwoven fabric and a razor blade pressed against
the surface of the tape, which was attached to a machine having
delivery and winding-up movement of a slit product. Thus, tape
samples were obtained.
[0455] The above-obtained magnetic powders, magnetic discs and
computer tapes were respectively evaluated as follows.
[0456] Measuring Method
[0457] (1) Magnetic characteristics (Hc, .sigma..sub.2):
[0458] Magnetic characteristics were measured using a vibrating
sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10
KOe.
[0459] (2) Diamond powder (particle size distribution):
[0460] An appropriate amount of a diamond powder was photographed
by an electron microscope. From the electron microphotographs, 500
particles were randomly sampled, and an average particle size
.phi., the proportion of the number of particles having the
particle size of 200% or more of the average particle size .phi.
accounting for in the entire number of diamond particles
(.DELTA.N200) (%), and the proportion of the number of particles
having the particle size of 50% or less of the average particle
size .phi. accounting for in the entire number of diamond particles
(.DELTA.N50) (%) were obtained by the above-described methods.
[0461] (3) Central plane average surface roughness (Ra):
[0462] Surface roughness (Ra) of the area of about 250
.mu.m.times.250 .mu.m was measured using "TOPO3D" (a product of
WYKO, U.S.A.) by 3D-MIRAU method. The wavelength of measurement was
about 650 nm and spherical compensation and cylindrical
compensation were applied. Measurement was performed using a light
interference type non-contact surface roughness meter.
[0463] (4) Electromagnetic characteristics of a disc:
[0464] Output/reproduction was measured using a disc tester
manufactured by Kokusai Denshi Kogyo Co., Ltd. (the late Tokyo
Engineering Co., Ltd.) and a SK606B type disc evaluation apparatus
by a metal-in-gap head having a gap length of 0.3 .mu.m. Recording
was conducted at the position of radius of 24.6 mm by a recording
wavelength of 90 KFCI, then reproduction output of a head amplifier
was determined by an oscilloscope 633 type manufactured by
Techtronics Co., Ltd.
[0465] S/N ratio: After DC erasure of the disc used for determining
reproduction output, reproduction output (noise level) was measured
by a TR4171 type spectrum analyzer (manufactured by Advantes Co.,
Ltd.).
[0466] -20log (noise/reproduction output) was taken as S/N ratio
and represented as a relative value taking the S/N value of Sample
No. 5-1 as 0 dB.
[0467] (5) Electromagnetic characteristics of a tape:
[0468] C/N ratio (tape): digital signals were recorded and
reproduced with a recording head (MIG, gap length: 0.15 .mu.m, 1.8
T) being attached to a drum tester. Relative speed of head-medium
was 3 m/sec, recording wavelength was 0.35 .mu.m, and modulated
noise was determined.
[0469] (6) Durability:
[0470] (1) Durability of a magnetic disc:
[0471] A floppy disc drive ("ZIP100", a product of IOMEGA CORP.,
U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at
the position of radius of 38 mm. The disc was run under the
following thermo-cycle condition, which being taken as one cycle.
The point when scratches were visually observed on the surface of
the sample was taken as NG. The time of durability of Sample No.
5-1 was taken as 100%.
[0472] Thermo-Cycle Flow
[0473] 25.degree. C., 50% RH, 1 hr.fwdarw.(temperature up, 2
hr).fwdarw.60.degree. C., 20% RH, 7 hr.fwdarw.(temperature down, 2
hr).fwdarw.25.degree. C., 50% RH, 1 hr.fwdarw.(temperature down, 2
hr).fwdarw.5.degree. C., 10% RH, 7 hr.fwdarw.(temperature up, 2
hr).fwdarw.(this cycle was repeated).
[0474] (1) Durability of a computer tape:
[0475] Predetermined signals were recorded using a DDS drive. The
disc was run at 50.degree. C., 10% RH while monitoring reproduced
signals, and the point when the initial reproduction output reached
70% was taken as NG. The durability of Sample No. 5-13 was taken as
100%.
[0476] The results of evaluations of magnetic discs and computer
tapes are shown in Tables 5-3 and 5-5, respectively.
31TABLE 5-2 Disc in which ferromagnetic metal powder is used
Average Long Magnetic Axis Layer Diamond Powder Sample Magnetic
Length Hc Ra .phi. .DELTA.N.sub.200 .DELTA.N.sub.50 Alumina No.
Remarks Powder (nm) (Oe) (nm) (.mu.m) (%) (%) A B 5-1 Ref. Ex. MP
(1) 80 2,300 4.1 -- -- -- 0 20 5-2 Ref. Ex. MP (1) 80 2,300 3.5 --
-- -- 0 19 5-3 Ex. MP (1) 80 2,310 3.1 0.2 2 6 1.5 0 5-4 Ex. MP (1)
80 2,310 3.3 0.3 0 5 1.5 0 5-5 Ex. MP (1) 80 2,310 3.0 0.3 0 5 0.3
7.5 5-6 Ex. MP (2) 50 2,300 3.0 0.3 0 5 0.3 7.5 5-7 Ex. MP (1) 80
2,360 1.9 0.8 2 10 1.0 0 5-8 Ex. MP (1) 80 2,350 2.7 0.4 3 7 0.2 10
5-9 Ex. MP (1) 80 2,360 3.0 0.4 7 3 0.2 10 5-10 Ex. MP (1) 80 2,350
3.0 0.4 0 23 0.2 10 5-11 Ex. MP (3) 80 1,850 3.0 0.4 3 7 0.2 10
5-12 Ref. Ex. MP (4) 80 1,720 3.1 0.4 2 7 0.2 10 .phi.: Average
particle size of a diamond powder; A: The addition amount of
diamond powder in % by weight based on magnetic powder; B: The
addition amount of alumina in % by weight based on magnetic
power.
[0477]
32TABLE 5-3 Evaluation of disc in which ferromagnetic metal powder
is used Linear Areal Track Recording Recording S/N Magnetic Layer
Sample Density Density Density Ratio Durability .DELTA.Hc
.DELTA..phi.m No. Remarks (tpi) (kbpi) (G bit/inch.sup.2) (dB) (%)
(%) (%) 5-1 Reference 5,200 144 0.75 0 100 +2.4 +5.1 Example 5-2
Reference 5,200 144 0.75 2.1 20 +2.4 +5.1 Example 5-3 Example 5,200
144 0.75 3.5 100 +2.3 +5.0 5-4 Example 5,200 144 0.75 3.5 100 +2.3
+4.9 5-5 Example 5,200 144 0.75 3.6 100 +2.3 +4.9 5-6 Example 5,200
144 0.75 4.0 100 +2.5 +5.2 5-7 Example 5,200 144 0.75 3.0 100 +2.3
+4.8 5-8 Example 5,200 144 0.75 3.5 100 +2.3 +4.8 5-9 Example 5,200
144 0.75 1.8 100 +2.3 +4.8 5-10 Example 5,200 144 0.75 3.3 100 +2.3
+4.8 5-11 Example 5,200 144 0.75 2.0 100 +1.2 +6.3 5-12 Reference
5,200 144 0.75 0.2 100 +1.1 +6.2 Example
[0478]
33TABLE 5-4 Tape in which ferromagnetic metal powder is used
Average Long Magnetic Axis Layer Diamond Powder Sample Magnetic
Length Hc Ra .phi. .DELTA.N.sub.200 .DELTA.N.sub.50 Alumina No.
Remarks Powder (nm) (Oe) (nm) (.mu.m) (%) (%) A B 5-13 Reference MP
(1) 80 2,340 4.3 -- -- -- 0 12 Example 5-14 Reference MP (1) 80
2,340 3.3 -- -- -- 0 6 Example 5-15 Example MP (1) 80 2,350 2.9 0.2
2 6 1.0 0 5-16 Example MP (1) 80 2,350 3.0 0.3 0 5 0.2 6 5-17
Example MP (2) 50 2,330 3.2 0.3 0 5 0.2 6 5-18 Example MP (2) 50
2,390 2.8 0.4 3 7 0.2 10 .phi.: Average particle size of a diamond
powder; A: The addition amount of diamond powder in % by weight
based on magnetic powder; B: The addition amount of alumina in % by
weight based on magnetic powder.
[0479]
34TABLE 5-5 Evaluation of tape in which ferromagnetic metal powder
is used Linear Areal Track Recording Recording S/N Magnetic Layer
Sample Density Density Density Ratio Durability .DELTA.Hc
.DELTA..phi.m No. Remarks (tpi) (kbpi) (G bit/inch.sup.2) (dB) (%)
(%) (%) 5-13 Reference 3,000 122 0.366 0 100 +2.3 +5.0 Example 5-14
Reference 3,000 122 0.366 1.5 20 +2.3 +5.1 Example 5-15 Example
3,000 122 0.366 1.8 100 +2.3 +4.9 5-16 Example 3,000 122 0.366 2.0
100 +2.3 +5.0 5-17 Example 3,000 122 0.366 1.5 100 +2.5 +5.2 5-18
Example 3,000 122 0.366 1.3 100 +2.5 +5.1
[0480] As is apparent from the results in Example 5, noise in
electromagnetic characteristics can be improved while retaining
durability by adding to the magnetic layer a diamond powder
preferably having an average particle size of from 0.01 to 1.0
.mu.m in an amount of preferably from 0.01 to 10% by weight based
on the magnetic powder.
EFFECT OF THE INVENTION
[0481] The magnetic recording medium according to the present
invention comprises a support having thereon a magnetic layer
comprising a ferromagnetic metal powder dispersed in a binder,
which is a magnetic recording medium for recording signals of from
0.17 to 2 G bit/inch.sup.2 of areal recording density, wherein the
magnetic layer has a coercive force of 1,800 Oe or more, and the
ferromagnetic metal powder is composed of Fe and Co as main
components, the atomic ratio of Al/(Fe+Co) is from 3.5 to 15.4%; or
the magnetic recording medium according to the present invention
comprises a support having thereon a magnetic layer comprising a
ferromagnetic metal powder dispersed in a binder, which is a
magnetic recording medium for recording signals of from 0.17 to 2 G
bit/inch.sup.2 of areal recording density, wherein the magnetic
layer has a coercive force of 1,800 Oe or more, the ferromagnetic
metal powder is composed of Fe and Co as main components, the
atomic ratio of the sum total of rare earth elements/(Fe+Co) is
preferably from 0.5 to 9.0%, and more preferably the magnetic
recording medium of the present invention is a magnetic recording
medium for recording signals of from 0.20 to 2 G bit/inch.sup.2 of
areal recording density, wherein the magnetic recording medium has
a dry thickness of preferably from 0.05 to 0.30 .mu.m. The magnetic
recording medium of the present invention having high capacity,
excellent high density characteristics, high durability and
markedly improved error rate in high density recording region has
been realized due to the above constitution of the present
invention, which could never be obtained by conventional techniques
of coating type magnetic recording medium.
[0482] While the invention has been described in detail and with
reference to specific examples thereof, it will be apparent to one
skilled in the art that various changes and modifications can be
made therein without departing from the spirit and scope
thereof.
* * * * *