U.S. patent application number 11/253604 was filed with the patent office on 2006-03-09 for hexagonal ferrite magnetic powder, method for producing the same and magnetic recording medium.
This patent application is currently assigned to FUJI PHOTO FILM CO., LTD.. Invention is credited to Akira Manabe, Hiroyuki Suzuki, Masatoshi Takahashi, Nobuo Yamazaki.
Application Number | 20060051624 11/253604 |
Document ID | / |
Family ID | 35480958 |
Filed Date | 2006-03-09 |
United States Patent
Application |
20060051624 |
Kind Code |
A1 |
Yamazaki; Nobuo ; et
al. |
March 9, 2006 |
Hexagonal ferrite magnetic powder, method for producing the same
and magnetic recording medium
Abstract
A hexagonal ferrite magnetic powder having an average tabular
diameter of from 15 to 30 nm, a coercive force (Hc) of from 2,000
to 5,000 Oe (from 160 to 400 kA/m) and a saturated magnetization
(.sigma.s) of equal to or more than [the average tabular diameter
(nm).times.0.37+45] Am.sup.2/kg. This magnetic powder is obtained
by melting a starting material containing a material which has a
composition within the hatched region (1) in the triangular phase
diagram shown in FIG. 1 and quenching the molten product to obtain
an amorphous product, subjecting the amorphous product to a thermal
treatment, acid treatment, and washing. Also, a magnetic recording
medium is obtained by adding this magnetic powder to the magnetic
layer and coating it on the support.
Inventors: |
Yamazaki; Nobuo; (Kanagawa,
JP) ; Takahashi; Masatoshi; (Kanagawa, JP) ;
Manabe; Akira; (Shizuoka, JP) ; Suzuki; Hiroyuki;
(Shizuoka, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
FUJI PHOTO FILM CO., LTD.
ASAHI GLASS CO., LTD.
|
Family ID: |
35480958 |
Appl. No.: |
11/253604 |
Filed: |
October 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11156527 |
Jun 21, 2005 |
|
|
|
11253604 |
Oct 20, 2005 |
|
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|
Current U.S.
Class: |
428/842.8 ;
252/62.58; 252/62.63; G9B/5.267 |
Current CPC
Class: |
Y10T 428/2982 20150115;
G11B 5/70678 20130101; G11B 5/714 20130101 |
Class at
Publication: |
428/842.8 ;
252/062.63; 252/062.58 |
International
Class: |
G11B 5/708 20060101
G11B005/708; H01F 1/00 20060101 H01F001/00; C04B 35/26 20060101
C04B035/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 21, 2004 |
JP |
P.2004-182631 |
Jun 20, 2005 |
JP |
P.2005-179481 |
Claims
1. (canceled)
2. A method for producing a hexagonal ferrite magnetic powder
having an average tabular diameter of from 15 to 30 nm, a coercive
force (Hc) of from 2,000 to 5,000 Oe (from 160 to 400 kA/m) and a
saturated magnetization (.sigma.s) of equal to or more than [the
average tabular diameter (nm).times.0.37+45] Am.sup.2/kg, and the
method comprising: melting a starting material containing a
material that has a composition within a composition region
surrounded by four points of a, b, c and d in a triangular phase
diagram wherein each of AO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3
constitutes an apex, and wherein A represents at least one selected
from Ba, Sr, Ca and Pb, and a, b, c and d each represents: (a)
B.sub.2O.sub.3=50, AO=40, Fe.sub.2O.sub.3=10 mol %; (b)
B.sub.2O.sub.3=45, AO=45, Fe.sub.2O.sub.3=10 mol %; (c)
B.sub.2O.sub.3=25, AO=25, Fe.sub.2O.sub.3=50 mol %; and (d)
B.sub.2O.sub.3=30, AO=20, Fe.sub.2O.sub.3=50 mol %, so as to form a
melted starting material, and quenching the melted starting
material to obtain an amorphous product; and subjecting the
amorphous product to a thermal treatment to precipitate a hexagonal
ferrite.
3. (canceled)
4. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a hexagonal ferrite
magnetic powder, a method for producing the same and a magnetic
recording medium. More particularly, it relates to a hexagonal
ferrite magnetic powder which enables one to reduce noise without
reducing .sigma.s and which is adapted for a magnetic recording
medium for high-density recording reproducible by a highly
sensitive head such as an MR head or a GMR head. Also, the
invention relates to a magnetic recording medium containing the
hexagonal ferrite magnetic powder in the magnetic layer.
[0003] 2. Description of the Related Art
[0004] In the field of magnetic recording, a magnetic head based on
the principle of electromagnetic induction (induction type magnetic
head) has been used and spread. However, the induction type
magnetic head is becoming insufficient for use in the presently
required reproduction of high-density records. That is, when the
number of coil turns of a reproduction head is increased in order
to obtain a larger reproduction output, there results an increased
inductance and an increase in resistance in high frequency region,
leading to a problem of reduction in reproduction output. Thus, in
recent years, a reproduction head based on the principle of MR
(magnetic reluctance) has been proposed and used for a hard disc or
the like. The MR head yields a reproduction output several times as
much as that of the induction type magnetic head and, since no
induction coils are used, device noises such as impedance noise are
markedly reduced. Therefore, a large SN ratio can be obtained.
[0005] On the other hand, the improvement of high-density recording
characteristics can also be made by reducing magnetic recording
medium noise having conventionally been hidden behind the device
noises.
[0006] In order to attain such object, there has been proposed, for
example, a magnetic recording medium comprising a non-magnetic
support having provided thereon a magnetic layer containing a
hexagonal ferrite magnetic powder dispersed in a binder (see, for
example, JP-A-10-312525).
[0007] Also, improvement of the hexagonal ferrite magnetic powder
is disclosed in JP-A-56-169128, JP-A-58-169902 and
JP-A-10-92620.
[0008] However, the above-mentioned related art fails to achieve
noise reduction without reducing .sigma.s and fails to provide a
magnetic recording medium for high-density recording currently
required.
[0009] It can be considered to reduce particle size of the magnetic
powder for the purpose of reducing noise of the magnetic recording
medium. In general, however, reduction of the particle size of the
magnetic powder leads to reduction of .sigma.s. With hexagonal
ferrite, too, reduction of the particle size of the magnetic powder
causes reduction of .sigma.s. Also, in order to obtain a high SN
ratio, low noise and high output are necessary. However, reduction
of .sigma.s of the magnetic powder causes reduction of output. On
the other hand, with a magnetic recording medium for use in
high-density recording, it is necessary to reduce the thickness of
the magnetic layer in order to reduce thickness loss and reduce
PW50.
[0010] However, since the thickness of the magnetic layer is
rendered small and .sigma.s of the magnetic powder is reduced, the
output becomes smaller, thus sufficient performance not being
obtained. In addition, low noise and high output are required for a
magnetic recording medium upon conducting high-density
recording.
SUMMARY OF THE INVENTION
[0011] An object of the invention is to provide a hexagonal ferrite
magnetic powder adapted for a magnetic recording medium for
high-density recording which permits reduction of noise without
reducing .sigma.s and which is adapted for a magnetic recording
medium for high-density recording reproducible by a highly
sensitive head such as an MR head or a GMR head, a method for
producing the same and a magnetic recording medium.
[0012] The invention is as follows.
[0013] (1) A hexagonal ferrite magnetic powder having an average
tabular diameter of from 15 to 30 nm, a coercive force (Hc) of from
2,000 to 5,000 Oe (from 160 to 400 kA/m) and a saturated
magnetization (.sigma.s) of equal to or more than [the average
tabular diameter (nm).times.0.37+45] Am.sup.2/kg.
[0014] (2) A method for producing a hexagonal ferrite magnetic
powder as described in (1) above, the method comprising: [0015]
melting a starting material containing a material that has a
composition within a composition region surrounded by four points
of a, b, c and d in a triangular phase diagram wherein each of AO,
B.sub.2O.sub.3 and Fe.sub.2O.sub.3 constitutes an apex, and wherein
A represents at least one selected from Ba, Sr, Ca and Pb, and a,
b, c and d each represents: [0016] (a) B.sub.2O.sub.3=50, AO=40,
Fe.sub.2O.sub.3=10 mol %; [0017] (b) B.sub.2O.sub.3=45, AO=45,
Fe.sub.2O.sub.3=10 mol %; [0018] (c) B.sub.2O.sub.3=25, AO=25,
Fe.sub.2O.sub.3=50 mol %; and [0019] (d) B.sub.2O.sub.3=30, AO=20,
Fe.sub.2O.sub.3=50 mol %, so as to form a melted starting material,
and quenching the melted starting material to obtain an amorphous
product; and [0020] subjecting the amorphous product to a thermal
treatment to precipitate a hexagonal ferrite.
[0021] (3) A magnetic recording medium comprising: [0022] a
non-magnetic support; and [0023] a magnetic layer containing a
hexagonal ferrite magnetic powder dispersed in a binder, [0024]
wherein the hexagonal ferrite magnetic powder is a hexagonal
ferrite magnetic powder as described in (1) above.
[0025] (4) The magnetic recording medium as described in (3) above,
which further comprises a non-magnetic layer containing a
non-magnetic powder dispersed in a binder between the non-magnetic
support and the magnetic layer.
BRIEF DESCRIPTION OF THE DRAWING
[0026] FIG. 1 shows a triangular phase diagram in accordance with
the invention, wherein AO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3
constitute apexes; and
[0027] FIG. 2 shows a plot data of Table 1 together with the
relation line [the average tabular diameter (nm).times.0.37+45]
Am.sup.2/kg.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The invention is described in more detail below.
[0029] The hexagonal ferrite magnetic powder of the invention is
characterized in that it has an average tabular diameter of from 15
to 30 nm, a coercive force (Hc) of from 2,000 to 5,000 Oe (from 160
to 400 kA/m) and a saturated magnetization (.sigma.s) of [average
tabular diameter (nm).times.0.37+45]Am.sup.2/kg or more. In case
when the average tabular diameter is less than 15 nm, there results
insufficient magnetic characteristics whereas, in case when the
average tabular diameter exceeds 30 nm, there results such a
serious noise that a necessary SN ratio for a magnetic recording
medium for high-density recording can not be ensured. In case when
Hc is less than 2,000 Oe, there results an insufficient linear
recording density. Also, in the range of from 15 to 30 nm in the
average tabular diameter, it is difficult to produce a magnetic
powder having an Hc exceeding 5,000 Oe. Further, .sigma.s of the
hexagonal ferrite magnetic powder of the invention is equal to
[average tabular diameter (nm).times.0.37+45] Am.sup.2/kg or more
than that. Such a high .sigma.s serves to provide a hexagonal
ferrite magnetic powder adapted for high-density recording which is
reproducible by means of a highly sensitive head such as an MR head
or a GMR head.
[0030] In the present invention, the basis for finding out the
hexagonal ferrite magnetic powder which satisfies the relation of
above described average tabular diameter, coercive force (Hc) and
saturated magnetization (.sigma.s) is as described below.
[0031] As mentioned above, low noise and high output are required
upon conducting high-density recording.
[0032] In order to lower noise, it is necessary to reduce the
particle size of the magnetic material to be used as above
described average tabular diameter.
[0033] When the particle size of the magnetic material is reduced,
the saturated magnetization .sigma.s is known to be lowered. This
reason is based on the increase of the specific surface area of the
particle along with the decrease of the particle size, and the
particle surface of ferromagnetic material is nonmagnetic or small
in magnetization amount.
[0034] When as is low, it is hard to obtain high output, thus it is
difficult to exert the full effect of low noise by making into fine
particles, however by defining the simultaneous pursuit of the
reduction of the average tabular diameter and high .sigma.s as
above, conventionally unknown high-density recording is
achieved.
[0035] Conventionally, Ba ferrite where Co--Zn--Nb is added is
known as a hexagonal ferrite excellent in high-density recording
(JP-A-10-92620). The relation of Hc, .sigma.s and average tabular
diameter of Ba ferrite sample 1 to 16 obtained by this method
(provided that sample 14 is the data of Example in JP-A-10-92620)
together with the data of Examples in JP-A-10-92620 are shown in
Table 1 and plotted in FIG. 2. [the average tabular diameter
(nm).times.0.37+45] Am.sup.2/kg as the relation line that connects
the highest .sigma.s is obtained.
[0036] The present application discloses the development of
magnetic material capable of high-density recording by particularly
the simultaneous pursuit of the average tabular diameter and
.sigma.s in high level. TABLE-US-00001 TABLE 1 Average Tabular Hc
.sigma.s Diameter Oe A m.sup.2/kg nm Sample 1 2000 52 22 Sample 2
2500 53 23 Sample 3 2500 54 25 Sample 4 2500 55 30 Sample 5 3000 54
30 Sample 6 2900 52 27 Sample 7 2300 54 29 Sample 8 2300 53 28
Sample 9 2000 49 18 Sample 10 4000 54 29 Sample 11 3500 50 22
Sample 12 2500 51 18 Sample 13 3000 52 20 Sample 14 2800 48 23
Sample 15 2000 48 16 Sample 16 2200 50 17 JP-A-10-92620 1590 55 30
JP-A-10-92620 1810 60 40 JP-A-10-92620 1460 48 22 JP-A-10-92620
1900 62 60 JP-A-10-92620 1750 57 50
[0037] The hexagonal ferrite magnetic powder can be obtained by the
following production method.
[0038] That is, the hexagonal ferrite magnetic powder of the
invention can be obtained by melting a starting material containing
at least a material which has a composition within the composition
region (hatched area (1)) shown in FIG. 1 surrounded by the
following four points of a, b, c and d in the triangular phase
diagram wherein AO (wherein A represents at least one member
selected from among Ba, Sr, Ca and Pb), B.sub.2O.sub.3 and
Fe.sub.2O.sub.3 constitute apexes): [0039] (a) B.sub.2O.sub.3=50,
AO=40, Fe.sub.2O.sub.3=10 mol % [0040] (b) B.sub.2O.sub.3=45,
AO=45, Fe.sub.2O.sub.3=10 mol % [0041] (c) B.sub.2O.sub.3=25,
AO=25, Fe.sub.2O.sub.3=50 mol % [0042] (d) B.sub.2O.sub.3=30,
AO=20, Fe.sub.2O.sub.3=50 mol % and quenching the molten product to
obtain an amorphous product, then subjecting the amorphous product
to a thermal treatment to precipitate hexagonal ferrite.
[0043] A hexagonal ferrite magnetic powder having an Hc of from
2,000 to 5,000 and a .sigma.s value equal to [average tabular
diameter (nm).times.0.37+45] Am.sup.2/kg or more than that can be
obtained by using a starting material having an
AO--B.sub.2O.sub.3--Fe.sub.2O.sub.3 composition within the region
surrounded by the four points of a, b, c and d shown in FIG. 1.
Additionally, for reference, particular composition regions in the
triangular phase diagram wherein AO--B.sub.2O.sub.3 and
Fe.sub.2O.sub.3 constitute apexes, disclosed in the aforesaid
JP-A-56-169128 and JP-A-58-169902 are also shown as hatched areas
(2) and (3).
[0044] Also, the hexagonal ferrite in the invention may partly be
substituted by metal elements. Examples of such substituting
element include Co--Zn--Nb, Co--Ti, Co--Ti--Sn, Co--Sn--Nb,
Co--Zn--Sn--Nb, Co--Zn--Zr--Nb and Co--Zn--Mn--Nb. Additionally,
selection, compounding ratio and introducing amount of the metal
element may properly be determined depending upon necessary Hc and
.sigma.s.
[0045] The step of melting the starting materials is conducted at a
temperature of, for example, 1,250 to 1,450.degree. C., preferably
1,300 to 1,400.degree. C. The quenching step may be conducted in a
known manner, for example, by pouring a molten material onto a
water-cooled twin roller rotating at a high speed to thereby mill
and quench. Conditions for the subsequent step of thermally
treating the thus-obtained amorphous product are, for example, 600
to 700.degree. C., preferably 620 to 680.degree. C., in
temperature, and 2 to 12 hours, preferably 3 to 6 hours, in
retention time. Subsequently, the product is acid-treated under
heating to remove excess glass components, followed by washing with
water and drying to obtain the hexagonal ferrite magnetic powder of
the invention. Additionally, although it is described in the
aforementioned JP-A-56-169128 that, in the case of obtaining a
magnetic powder by using a starting material falling within the
composition region surrounded by the four points of a, b, c and d
in the triangular phase diagram shown by FIG. 1, there results a
product mingled with .alpha.-hematite, it can be avoided by
substituting part of Fe by a metal element or by properly setting
the thermally treating temperature (for example, at a comparatively
low temperature). Though it is not clear why the hexagonal ferrite
magnetic powder of the invention shows a high .sigma.s value, it
may be surmised that, since the content of AO with respect to the
content of Fe.sub.2O.sub.3 is less than that described in, for
example, the JP-A-56-169128, lack of A possibly exiting in part of
AO.6Fe.sub.2O.sub.3 crystal lattice causes lattice distortion which
produces magnetic distortion, resulting in change of .sigma.s.
[0046] The specific surface area of the hexagonal ferrite magnetic
powder of the invention is desirably in the range of from 45 to 80
m.sup.2/g obtained by the BET method In consideration of a packing
property and an orientation property in the medium, the tabular
ratio of the magnetic powder is desirably in the range of from 2.0
to 4.9, more preferably from 2.5 to 4.2. As the tabular ratio
increases, the orientation property is more improved, and the
switching field distribution (SFD) becomes sharper, but the packing
property is deteriorated. In order to increase reproduction output,
it is important to strike a balance among the improvement of
packing property, the improvement of the orientation property and
sharpening of SFD and, as a result of such consideration, the
above-mentioned regions having been found to be preferred.
[0047] The hexagonal ferrite magnetic powder of the invention may
be subjected, as needed, to surface treatment with Al, Si, P, or
oxide or hydroxide thereof. The amount thereof is preferably from
0.1 to 10% by weight based on the magnetic powder.
[0048] The invention also provides a magnetic recording medium
comprising a non-magnetic support having provided thereon a
magnetic layer containing the hexagonal ferrite magnetic powder of
the invention dispersed in a binder. The magnetic recording medium
of the invention is described hereinafter.
Non-Magnetic Support
[0049] As a support to be used in the invention, a flexible support
is preferred, and known films of, for example, polyesters such as
polyethylene terephthalate (PET) and polyethylene naphthalate
(PEN); polyolefins; cellulose triacetate; polycarbonates;
polyamides; polyimides; polyamideimides; polysulfone; and aromatic
polyamides such as aramide may be used. These supports may
previously be subjected to corona discharge treatment, plasma
treatment, adhesion-enhancing treatment, thermal treatment or
dust-removing treatment. For effectively achieving the objects of
the invention, it is preferred to use a support having a
center-line average surface roughness of 0.03 .mu.m or less,
preferably 0.02 .mu.m or less, more preferably 0.01 .mu.m or less.
Besides being small in center-line average surface roughness, the
support is preferably free of coarse projections measuring 1 .mu.m
or above in height. Further, the surface roughness dimensions can
be adjusted freely by selecting sizes and amounts of fillers added
to the support as needed. Examples of such fillers include oxides
and carbonates of Ca, Si and Ti and organic fine powders of, for
example, acrylic resins.
Magnetic Layer
[0050] The binder to be used in the magnetic layer of the invention
is a conventionally known thermoplastic resin, thermosetting resin,
reactive resin or a mixture thereof. Examples of the thermoplastic
resin include polymers and copolymers comprising structural units
such as vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid,
acrylic acid, acrylic acid ester, vinylidene chloride,
acrylonitrile, methacrylic acid, methacrylic acid ester, styrene,
butadiene, ethylene, vinyl butyral, vinyl acetal and vinyl ether;
polyurethane resins; and various rubber resins.
[0051] Also, examples of the thermosetting resins and the reactive
resins include phenol resins, epoxy resins, polyurethane curable
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 polyester polyols and polyisocyanates, and
mixtures of polyurethane and polyisocyanates. The thermoplastic
resins, the thermosetting resins and the reactive resins are
described in detail in Plastic Handbook published by Asakura
Shoten.
[0052] Further, when an electron beam-curable resin is used in the
magnetic layer, not only coating strength can be improved to
improve durability, but also the surface is rendered smooth to
enhance electromagnetic transducing characteristics. Examples
thereof and methods for their production are described in detail in
JP-A-62-256219.
[0053] These resins may be used independently or in combination
thereof. Of these, use of polyurethane resin is preferred. Further,
use of the following polyurethane resins is further preferred; a
polyurethane resin prepared by reacting a cyclic compound such as
hydrogenated bisphenol A or a polypropylene oxide adduct of
hydrogenated bisphenol A, a polyol with a molecular weight of 500
to 5,000 having an alkylene oxide chain, a chain-extending agent of
a polyol with a molecular weight of 200 to 500 having a cyclic
structure, and an organic diisocyanate, as well as introducing a
polar group; a polyurethane resin prepared by reacting an aliphatic
dibasic acid such as succinic acid, adipic acid or sebacic acid, a
polyester polyol comprising an aliphatic diol not having a cyclic
structure having an alkyl branched side chain such as
2,2-dimethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol or
2,2-diethyl-1,3-propanediol, a chain-extending agent such as an
aliphatic diol having a branched alkyl side chain containing three
or more carbon atoms, such as 2-ethyl-2-butyl-1,3-propanediol or
2,2-diethyl-1,3-propanediol, and an organic diisocyanate, as well
as introducing a polar group; and a polyurethane resin prepared by
reacting a cyclic structure such as a dimmer diol, a polyol
compound having a long alkyl chain, and an organic diisocyanate, as
well as introducing a polar group.
[0054] The average molecular weight of the polyurethane resin
containing the polar group which is employed in the invention is
preferably from 5,000 to 100,000, more preferably from 10,000 to
50,000. The average molecular weight is preferably 5,000 or more in
that it yields a magnetic coating that does not undergo a decrease
in physical strength such as becoming brittle, and that does not
affect the durability of the magnetic recording medium. Also, the
molecular weight of 100,000 or less than that does not reduce
solubility in solvent and thus affords a good dispersibility.
Further, since the coating composition viscosity does not become
high at a given concentration, the coating composition provides
good manufacturing properties and facilitates its handling.
[0055] Examples of the polar group contained in the above-described
polyurethane resins include --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 base), --OH, --NR.sub.2,
--N.sup.+R.sub.3 (wherein R represents a hydrocarbon group), an
epoxy group, --SH and --CN. At least one of these polar groups may
be incorporated by copolymerization or an addition reaction for
use. When the polar group-containing polyurethane resin has an OH
group, a branched OH group is preferred from the viewpoint of
curing properties and durability. The number of the branched OH
group is preferably from 2 to 40, more preferably from 3 to 20, per
molecule. The quantity of such polar groups ranges from 10.sup.-1
to 10.sup.-8 mol/g, preferably from 10.sup.-2 to 10.sup.-6
mol/g.
[0056] Specific examples of these binders include VAGH, VYHH, VMCH,
VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC
and PKFE [manufactured by Union Carbide Corporation]; MPR-TA,
MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM and MPR-TAO
[manufactured by Nissin Kagaku Kogyo K.K.]; 1000W, DX80, DX81,
DX82, DX83 and 100FD [manufactured by Denki Kagaku Kogyo K.K.];
MR-104, MR-105, MR110, MR100, MR555 and 400X-110A [manufactured by
Nippon Zeon Co., Ltd.], 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 Ink & Chemicals Incorporated]; Vylon
UR8200, UR8300, UR-8700, RV530 and RV280 [manufactured by Toyobo
Co., Ltd.]; Daipheramine 4020, 5020, 5100, 5300, 9020, 9022 and
7020 [manufactured by Dainichiseika Color & Chemicals Mfg. Co.,
Ltd.]; MX5004 [manufactured by Mitsubishi Chemical Corporation];
Sanprene SP-150 [manufactured by Sanyo Chemical Industries, Ltd.];
and Saran F310 and F210 [manufactured by Asahi Chemical. Industry.
Col. Ltd.].
[0057] The addition amount of the binder to be used in the magnetic
layer of the invention ranges from 5 to 50 parts by weight,
preferably from 10 to 30 parts by weight, based on the weight of
the magnetic powder. In the case of using the polyurethane resin,
it is preferably employed in a quantity of from 2 to 20% by weight
and, in the case of using the polyisocyanate, it is preferably
employed in a quantity of from 2 to 20% by weight. It is preferable
to employ them together. However, for example, when head corrosion
occurs due to the release of a trance amount of chlorine, it is
possible to employ only polyurethane or polyurethane and
isocyanate. In the case of using a vinyl chloride-based resin as
other resin, the preferred range is 5 to 30% by weight. When using
polyurethane in the invention, the glass transition temperature
ranges from -50 to 150.degree. C., preferably from 0 to 100.degree.
C., the elongation at break preferably ranges from 100 to 2,000%,
the stress at break preferably ranges from 0.49 to 98 MPa (from
0.05 to 10 kg/mm.sup.2), and the yield point preferably ranges from
0.49 to 98 MPa (from 0.05 to 10 kg/mm.sup.2).
[0058] The magnetic recording medium to be used in the invention in
the form of, for example, a floppy disk can be constituted by two
or more layers provided on each side of a support. Accordingly, the
quantity of binder, the proportion of vinyl chloride resin,
polyurethane resin, polyisocyanate or some other resin in the
binder, the molecular weight and quantity of polar groups in the
various resins forming the magnetic layer, and the physical
characteristics of the aforesaid resins may be varied as needed in
the non-magnetic layer and the individual magnetic layers or,
rather, should be optimized with each layer. Known techniques for a
multi-layered magnetic layer may be applied. For example, when
varying the quantity of binder in each layer, the quantity of
binder in the magnetic layer may be increased to effectively reduce
rubbing damage on the surface of the magnetic layer, and the
quantity of binder in the non-magnetic layer may be increased to
impart flexibility for good head touch.
[0059] Examples of the polyisocyanate to be used in the invention
include isocyanates such as tolylenediisocyanate,
4,4'-diphenylmethanediisocyanate, hexamethylenediisocyanate,
xylylenediisocyanate, naphthylene-1,5-diisocyanate,
o-toluidinediisocyanate, isophoronediisocyanate and
triphenylmethanetriisocyanate; adducts between these isocyanates
and polyalcohols; and polyisocyanates produced by condensation of
isocyanates Commercially available products of these isocyanates
includes Colonate L, Colonate HL, Colonate 2030, Colonate 2031,
Millionate MR and Millionate MTL [manufactured by Nippon
Polyurethane Industry 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 Urethane Co., Ltd.].
These may be used independently or as a combination of two or more
thereof utilizing difference in curing activity in the respective
layers.
[0060] As needed, additives can be added to the magnetic layer in
the invention. Examples of the additives include abrasives,
lubricants, dispersants, dispersing aids, fungicides, antistatic
agents, anatioxidants, solvents and carbon black. As the additives,
there may be used, for example, molybdenum disulfide; tungsten
disulfide; graphite; boron nitride; graphite fluoride; silicone
oils; silicones having a polar group; fatty acid-modified
silicones; fluorine-containing silicones; fluorine-containing
alcohols; fluorine-containing esters; polyolefins; polyglycols;
polyphenyl ethers; aromatic ring-containing organic phosphonic
acids such as phenylphosphonic acid, benzylphosphonic acid,
phenethylphosphonic acid, .alpha.-methylbenzylphosphonic acid,
1-methyl-1-phenethylphosphonic diphenyulmethylphosphonic acid,
biphenylphosphonic acid, benzylphenylphosphonic acid,
.alpha.-cumylphosphonic acid, toluylphosphonic acid,
xylylphosphonic acid, ethylphenylphosphonic acid, cumenylphosphonic
acid, propylphenyllphosphonic acid, butylphenylphosphonic acid,
heptylphenylphosphonic acid, octylphenylphosphonic acid and
nonylphenylphosphonic acid, and the alkali metal salts thereof;
alkylphosphonic acids such as octylphosphonic acid,
2-ethylhexylphosphonic acid, iso-octyl phosphonic acid,
iso-nonylphosphonic acid, iso-decylphosphonic acid,
isoundecylphosphonic acid, iso-dodecylphosphonic acid,
iso-hexadecylphosphonic acid, iso-octadecylphosphonic acid and
iso-eicosylphosphonic acid, and the alkali metal salts thereof;
phosphoric acid aromatic esters such as phenyl phosphate, benzyl
phosphate, phenethyl phosphate, .alpha.-methylbenzyl phosphate,
1-methyl-1-phenethyl phosphate, diphenylmethyl phosphate, biphenyl
phosphate, benzylphenyl phosphate, .alpha.-cumyl phosphate, toluyl
phosphate, xylyl phosphate, ethylphenyl phosphate, cumenyl
phosphate, propylphenyl phosphate, butylphenyl phosphate,
heptylphenyhl phosphate, octylphenyl phosphate and nonylphenyl
phosphate, and the alkali metal salts thereof; alkyl phosphates
such as octyl phosphate, 2-ethylhexyl phosphate, iso-octyl,
phosphate, iso-decyl, phosphate, iso-undecyl phosphate, iso-dodecyl
phosphate, iso-hexadecyl phosphate, iso-octadecyl phosphate and
iso-eicosyl phosphate, and the alkali metal salts thereof;
alkylsulfonic acid esters and the alkali metal salts thereof;
fluorine-containing alkylsulfuric acid esters and the alkali metal
salts thereof; monobasic fatty acids with 10 to 24 carbon atoms
(which may contain an unsaturated bond or may be branched) such as
lauric acid, myristic acid, palmitic acid, stearic acid, behenic
acid, butyl stearate, oleic acid, linoleic acid, linolenic acid,
elaidic acid and erucic acid, and the alkali metal salts thereof;
monofatty acid esters, difatty acid esters or polyfatty acid esters
comprising a monobasic fatty acid with 2 to 22 carbon atoms (which
may have an unsaturated bond or may be branched) and at least one
of alcohols having 1 to 6 hydroxyl groups with 12 to 22 carbon
atoms (which may have an unsaturated bond or may be branched),
alkoxyalcohols with 12 to 22 carbon atoms (which may have an
unsaturated bond or may be branched) and monoalkyl ethers of
alkylene oxide polymers, such as butyl stearate, octyl stearate,
amyl stearate, iso-octyl stearate, octyl myristate, butyl laurate,
butoxyethyl stearate, anhydrosorbitan monostearate and
anhydrosorbitan tristearate; aliphatic acid amides containing 2 to
22 carbon atoms; and aliphatic amines containing 8 to 22 carbon
atoms. Compounds having an alkyl group, an aryl group or an aralkyl
group substituted by a group other than the above-mentioned
hydrocarbon groups, such as a nitro group, F, Cl, Br or a
halogen-containing hydrocarbon group, may also be employed.
[0061] Further, nonionic surfactants such as alkylene oxide-based
surfactants, glycerin-based surfactants, glycidol-based surfactants
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 having an acidic group such as
carboxylic acid, sulfonic acid or sulfuric acid ester group; and
amphoteric surfactants such as amino acids, aminosulfonic acids,
sulfuric and phosphoric acid esters of aminoalcohols and
alkylbetaines may also be used. These surfactants are described in
detail in Kaimen Kasseizai Binran published by Sangyo Tosho
K.K.
[0062] These lubricants, antistatic agents, etc. need not
necessarily be pure, and may contain impurities such as isomers,
unreacted materials, by-products, decomposition products and
oxides. The content of the impurities be preferably 30% by weight
or less, more preferably 10% by weight or less.
[0063] Specific examples of these additives include NAA-102,
hydrogenated castor oil fatty acid, NAA-42, Cation SA, Nymeen
L-201, Nonion E-208, Anon BF and Anon LG (manufactured by Nippon
Yushi K.K.); FAL-205 and FAL-123 (manufactured by Takemoto Oil
& Fat Co., Ltd.); NJLUB OL (manufactured by New Japan Chemical
Co., Ltd.); TA-3 (manufactured by Shin-Etsu Chemical Co., Ltd.);
Armide P (manufactured by Lion Armour Co., Ltd.); Duomine TDO
(manufactured by Lion Corporation); BA-41G (manufactured by Nisshin
Oil Mills, Ltd.); and Profan 2012E, Newpole PE61 and Ionet MS-400
(manufactured by Sanyo Chemical Industries, Ltd.).
[0064] Further, to the magnetic layer in the invention may be added
carbon black as needed. Examples of the carbon black that can be
used in the magnetic layer include furnace black for rubber,
thermal black for rubber, carbon black for color and acetylene
black. The preferred carbon black has a specific surface area of
from 5 to 500 m.sup.2/g, a DBP absorptive capacity of from 10 to
400 ml/100 g, an average particle size of from 5 to 300 mu, a 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 black to be used
in the invention include BLACKPEARLS 2000, 1300, 1000, 900, 905,
800, 700, VULCAN XC-72 [products of Cabot Co.]; #80, #60, #55, #50
and #35 [products of Asahi Carbon Co., Ltd.], #2400B, #2300, #900,
#1000, #30, #40 and #10B [products of Mitsubishi Chemical
Corporation]; CONDUCTEX SC, RAVEN150, 50, 40, 15 and RAVEN-MT-P
[products of Colombian Carbon Co.]; and KEETJENBLACK EC [product of
Nippon EC]. Carbon black may be surface-treated with a dispersing
agent or may be grafted with a resin, or carbon black surface may
be partly converted to graphite before use. Also, in advance of its
addition to a magnetic coating composition, carbon black may be
dispersed into a binder. These carbon black products can be used
independently or in combination thereof. In using carbon black, it
is preferably used in an amount of from 0.1 to 30% by weight based
on the weight of the magnetic powder. In the magnetic layer, carbon
black functions to prevent static, reduce the coefficient of
friction, impart light-blocking properties and enhance film
strength, which varies depending upon kind of the carbon black
employed. Accordingly, the kind, quantity, and combination of the
carbon blacks may be determined separately for the magnetic layer
and the non-magnetic layer based on the various characteristics
described above, such as particle size, oil absorption capacity,
electrical conductivity, and pH depending upon the object, or
rather be optimized for each layer. As to carbon blacks which can
be used in the magnetic layer of the invention, reference can be
made to, for example, Carbon Black Binran compiled by the Carbon
Black Assosiation.
[0065] As organic solvents to be used in the invention, known ones
can be used. The organic solvent employed in the invention may be
used in any ratio. Examples thereof include ketones such as
acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl
ketone, cyclohexanone, isophorone and tetrarydrofuran; alcohols
such as methanol, ethanol, propanol, butanol, isobutyl alcohol,
isopropyl alcohol and methylcyclohexanol; esters such as methyl
acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl
lactate and glycol acetate; glycol ethers such as glycol dimethyl
ether, glycol monoethyl ether and dioxane; aromatic hydrocarbons
such as benzene, toluene, xylene, cresol and chlorobenzene;
chlorinated hydrocarbons such as methylene chloride, ethylene
chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin
and dichlorobenzene; N,N-dimethylformamide; and hexane.
[0066] These organic solvents need not be 100% pure and may contain
impurities such as isomers unreacted materials by-products,
decomposition products, oxides and moisture in addition to the
major components. The content of these impurities is preferably 30%
or less, more preferably 10% or less. Preferably the same type of
organic solvents be used as the organic solvents in the invention
in both the magnetic layer and the non-magnetic layer. However, the
addition amount may be varied. The stability of coating is
increased by using a solvent with a high surface tension (such as
cyclohexanone or dioxane) in the non-magnetic layer. Specifically,
it is important that the arithmetic mean value of the upper layer
solvent composition be not less than the arithmetic mean value of
the non-magnetic layer solvent composition. To improve dispersion
properties, a solvent having a somewhat strong polarity is
preferred. Of the solvent compositions, a solvent composition
containing a solvent having a dielectric constant of 15 or more in
a content of 50% or more is preferred. Further, the dissolution
parameter is preferably from 8 to 11.
[0067] Different types and quantities of these dispersants,
lubricants and surfactants to be used in the invention may be used
as needed in the magnetic layer and the non-magnetic layer.
Needless to say, the examples given here are not to be construed as
limitative. The dispersant exhibits adsorptive or bonding
properties through the polar groups, adsorbing or binding by means
of the polar groups chiefly to the surface of the hexagonal ferrite
magnetic powder in the magnetic layer and chiefly to the surface of
the non-magnetic powder in the non-magnetic layer. It is surmised
that the organic phosphorus compound once having adsorbed
difficultly desorbs from the surface of a metal or metallic
compound. Accordingly, since the surface of the hexagonal ferrite
magnetic powder of the invention or the surface of the non-magnetic
powder is coated with alkyl groups or aromatic groups, affinity of
the hexagonal ferrite magnetic powder or the non-magnetic powder
for the binder resin component increases, and the dispersion
stability of the hexagonal ferrite magnetic powder or the
non-magnetic powder improves. Further, since lubricants are present
in a free state, it is conceivable to employ fatty acids having
different melting points in the non-magnetic layer and magnetic
layer to control seepage out onto the surface, employ esters having
different melting points and polarities to control seepage out onto
the surface, adjust the quantity of surfactant to improve the
stability of the coating, and increase the quantity of lubricant in
the non-magnetic layer to improve the lubricating effect. Also, all
or a portion of the additive employed in the invention may be added
during any step in the manufacturing of the coating liquid for the
magnetic layer or the non-magnetic layer. For example, there are
cases where additives are admixed with the ferromagnetic powder
prior to the kneading step, cases where they are added during the
step of kneading the ferromagnetic powder, binder and solvent,
cases where they are added during the dispersion step, cases where
they are added following dispersion, and cases where they are added
immediately before coating.
Non-Magnetic Layer
[0068] Next, the non-magnetic layer is described in detail. The
magnetic recording medium of the invention can have a non-magnetic
layer, formed on a support, which contains a binder and a
non-magnetic powder. The non-magnetic powder to be used in the
non-magnetic layer may be of an inorganic or organic substance.
Carbon black may also be employed. Examples of the inorganic
substance include metals, metal oxides, metal carbonates, metal
sulfates, metal nitrides, metal carbides and metal sulfides.
[0069] Specifically, titanium oxides such as titanium dioxide,
cerium oxide, tin oxide, tungsten oxide, ZnO, ZArO.sub.2,
SiO.sub.2, Cr.sub.2O.sub.3, .alpha.-alumina with an
.alpha.-conversion ratio of 90 to 100%, .beta.-alumina,
.gamma.-alumina, .alpha.-iron oxide, goethite, corundum, silicon
nitride, titanium carbide, magnesium oxide, boron nitride,
molybdenum disulfite, copper oxide, MgCO.sub.3, CaCO.sub.3,
BaCO.sub.3, SrCO.sub.3, BasO.sub.4, silicon carbide and titanium
carbide can be used independently or in combination of two or more
thereof. Of these, .alpha.-iron oxide and titanium oxide are
preferred.
[0070] As to the shape of the non-magnetic powder, it may be
acicular, spherical, polyhedral or tabular. The crystallite size of
the non-magnetic powder is preferably from 4 nm to 1 .mu.m, more
preferably from 40 to 100 nm. A crystallite size falling within the
range of 4 nm to 1 .mu.m is preferred in that it facilitates
dispersion and imparts a suitable surface roughness. The preferred
average particle size of the non-magnetic powder ranges from 5 nm
to 2 .mu.m. It is also possible to combine, as needed, non-magnetic
powders different in the average particle size or, in the case of
using a single non-magnetic powder, to broaden the particle size
distribution to thereby obtain the same effect. A particularly
preferred average particle size of the non-magnetic powder ranges
from 10 to 200 nm. Within a range of 5 nm to 2 .mu.m, dispersion is
good and good surface roughness is achieved, thus such average
particle size being preferred.
[0071] The specific surface area of the non-magnetic powder is
preferably from 1 to 100 m.sup.2/g, more preferably from 5 to 70
m.sup.2/g, still more preferably from 10 to 65 m.sup.2/g. Within
the specific surface area ranging from 1 to 100 m.sup.2/g, suitable
surface roughness is achieved and dispersion is possible with the
desired quantity of binder, thus such specific surface area being
preferred. The oil absorption capacity measured by using dibutyl
phthalate (DBP) ranges from 5 to 100 ml/100 g, preferably from 10
to 80 ml/100 g, more preferably from 20 to 60 ml/100 g. The
specific gravity ranges from 1 to 12, preferably from 3 to 6. The
tap density ranges from 0.05 to 2 g/ml, preferably from 0.2 to 1.5
g/ml. A tap density falling within a range of 0.05 to 2 g/ml
reduces the amount of scattering particles, thereby facilitating
handling, and tends to prevent deposition of the particles onto the
device. The pH of the non-magnetic powder is preferably from 2 to
11, particularly preferably from 6 to 9. When the pH falls within a
range of 2 to 11, the coefficient of friction does not become high
at a high temperature or under a high humidity or due to liberation
of fatty acids. The moisture content of the non-magnetic powder
ranges from 0.1 to 5% by weight, preferably from 0.2 to 3% by
weight, still more preferably from 0.3 to 1.5% by weight. A
moisture content falling within a range of 0.1 to 5% by weight is
preferred because it produces good dispersion and yields a stable
coating viscosity following dispersion. An ignition loss of 20% by
weight or less is preferred, with non-magnetic powders having a low
ignition loss being preferred.
[0072] When the non-magnetic powder is an inorganic powder, the
Mohs' hardness thereof is preferably 4 to 10. Durability can be
ensured if the Mohs' hardness ranges from 4 to 10. The stearic acid
adsorption capacity of the non-magnetic powder ranges preferably
from 1 to 20 .mu.mol/m.sup.2, more preferably from 2 to 15
.mu.m/m.sup.2. The heat of wetting in 25.degree. C. water of the
non-magnetic powder is preferably within the range of from 200 to
600 erg/cm.sup.2 (200 to 600 mJ/m.sup.2). A solvent with a heat of
wetting in this range may be used. The quantity of water molecules
on the surface at 100 to 400.degree. C. suitably ranges from 1 to
10 molecules per 100 .ANG.. The pH of the isoelectric point in
water preferably ranges from 3 to 9. The surface of these
non-magnetic powders is preferably subjected to surface treatment
for Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, SnO.sub.2,
Sb.sub.2O.sub.3 or ZnO to exist on the surface. Of these,
Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 and ZrO.sub.2 are preferred
in view of dispersibility, with Al.sub.2O.sub.3, SiO.sub.2 and
ZrO.sub.2 being more preferred. These may be used in combination or
independently. Depending on the object, a surface treatment coating
layer formed by co-precipitation may also be employed. It is also
employable to first treat with alumina and then treat the surface
layer with silica or to treat in the reverse order. Depending on
the object, the surface treatment coating layer may be a porous
layer, with a homogeneous and dense layer being generally
preferred.
[0073] Specific examples of the non-magnetic powder to be used in
the non-magnetic layer of the invention include Nanotite
manufactured by Showa Denko K.K.; DPN-250, DPN-250BX, DPN-245,
DPN-270BX, DPB-550BX and DPN-550RX manufactured by Toda Kogyo
Corp.; titanium oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S,
TTO-55D, SN-100, MJ-7, .alpha.-iron oxide E270, E271 and E300
manufactured by Ishihara Sangyo Co., Ltd.; STT-4D, STT-30D, STT-30
and STT-65C manufactured by Titan Kogyo K.K.; and MT-100S, MT-100T,
MT-150W, MT-500B, T-600B, T-100F and T-500HD manufactured by Tayca
Corporation. Further, there are illustrated FINEX-25, BF-1, BF-10,
BF-20 and ST-M manufactured by Sakai Chemical Industry Co., Ltd.;
DEFIC-Y and DEFIC-R manufactured by Dowa Mining Co., Ltd.; 100A and
500A manufactured by Ube Industries, Ltd.; Y-LOP manufactured by
Titan Kogyo K.K.; and sintered products thereof. Particularly
preferred non-magnetic powders are titanium dioxide and
.alpha.-iron oxide.
[0074] In the non-magnetic layer, carbon black may be mixed with
the non-magnetic powder to decrease surface resistivity and
transmittance of light and achieve the desired micro Vicker's
hardness. The micro Vicker's hardness normally ranges from 25 to 60
kg/mm.sup.2 (from 245 to 588 MPa), preferably from 30 to 50
kg/mm.sup.2 (from 294 to 490 MPa) to adjust head touch. It can be
measured by means of a thin-film hardness meter (HMA-400;
manufactured by NEC Corporation) using a triangular diamond
indenter tip with a front end radius of 0.1 .mu.m and an edge angle
of 80.degree.. The transmittance of light is generally standardized
to be 3% or less in terms of absorption of infrared rays having a
wavelength of about 900 nm, for example, 0.8% or less for a VHS
magnetic tape. For such purposes, furnace black for rubber, thermal
black for rubber, black for coloring and acetylene black may be
used.
[0075] The specific surface area of carbon black to be used in the
non-magnetic layer of the invention ranges from 100 to 500
m.sup.2/g, preferably from 150 to 400 m.sup.2/g, and the DBP oil
absorption capacity ranges from 20 to 400 ml/100 g, preferably from
30 to 200 ml/g. The particle size of the carbon black ranges from 5
to 80 nm, preferably from 10 to 50 nm, more preferably from 10 to
40 nm. The pH of the carbon black preferably ranges from 2 to 10,
the moisture content preferably ranges from 0.1 to 10%, and the tap
density preferably ranges from 0.1 to 1 g/ml.
[0076] Specific examples of carbon black to be used in the
non-magnetic layer of the invention include BLACKPEARLS 2000, 1300,
1000, 900, 800, 880, 700 and VULCAN C-72 manufactured by Cabot
Corporation; #3050B, #3150B, #3250B, #3950B, #950, #650B, #970B,
#850B and MA-600 manufactured by Mitsubishi Chemical Corporation;
CONDUCTEX SC, RAVEN8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000,
1800, 1500, 1255 and 1250 manufactured by Columbia Carbon Co.,
Ltd.; and Ketjen Black EC manufatucured by Akzo Co.
[0077] Also, carbon black may be surface-treated with a dispersing
agent or may be grafted with a resin, or carbon black surface may
be partly converted to graphite before use. Further, in advance of
its addition to a coating composition, carbon black may be
dispersed into a binder. These carbon black products may be used in
an amount not exceeding 50% by weight based on the weight of the
inorganic powder and not exceeding 40% by weight based on the whole
weight of the non-magnetic layer. These carbon black products can
be used independently or in combination thereof. As to carbon
blacks which can be used in the non-magnetic layer of the
invention, reference can be made to, for example, Carbon Black
Binran compiled by the Carbon Black Assosiation.
[0078] To the non-magnetic layer, organic powder can also be added,
as needed depending upon the object. Examples of such organic
powder include acrylic-styrene resin powder, benzoquanamine resin
powder, melamine resin powder and phthalocyanine-based pigment.
Further, polyolefin resin powder, polyester resin powder, polyamide
resin powder, polyimide resin powder and polyfluoroethylene resin
may be used. As to methods for their production, those which are
described in JP-A-62-18564 and JP-A-60-255827 may be employed.
[0079] As binder resins, lubricants, dispersants, additives and
dispersing methods, those for the magnetic layer can be applied. In
particular, as to the amount and kind of binder resins and the
amount and kind of additives and dispersants, known techniques
relating to the magnetic layer can be applied.
[0080] The magnetic recording medium of the invention may have an
undercoat layer. The undercoat layer serves to improve the adhesion
force between the support and the magnetic layer or the
non-magnetic layer. As the undercoat layer, a polyester resin
soluble in the solvent is used.
Layer Structure
[0081] In the magnetic recording medium of the invention, the
thickness of the support is preferably from 3 to 80 .mu.m. Further,
when providing an undercoat layer between the support and the
non-magnetic layer or the magnetic layer, the thickness of the
undercoat layer is from 0.01 to 0.8 .mu.m, preferably from 0.02 to
0.6 .mu.m.
[0082] The thickness of the magnetic layer is optimized based on
the saturated magnetization level and head gap length of the
magnetic head employed and the recording signal band, but is
generally from 10 to 150 nm, preferably from 20 to 80 nm, more
preferably from 30 to 80 nm. Further, the thickness fluctuation
ratio of the magnetic layer is preferably within .+-.50%, more
preferably within .+-.40%. The magnetic layer comprises at least
one layer, but may be separated into two or more layers having
different magnetic characteristics. Known multi-layered magnetic
layer configurations may be employed.
[0083] The thickness of the non-magnetic layer is from 0.5 to 2.0
.mu.m, preferably from 0.8 to 1.5 .mu.m, more preferably from 0.8
to 1.2 .mu.m. Additionally, the non-magnetic layer of the magnetic
recording medium of the invention can exhibit its effect so long as
it is essentially non-magnetic. For example, even when an impurity
or an intentional trace amount of magnetic material is contained,
the effect of the invention is exhibited and the configuration can
be seen as being essentially identical to that of the magnetic
recording medium of the invention. Additionally, the term
"essentially identical" as used herein means that the residual
magnetic flux density of the non-magnetic layer is 10 mT or less or
the coercive force is 7.96 kA/m (100 Oe) or less, with the absence
of residual magnetic flux density and coercive force being
preferred.
Manufacturing Method
[0084] The process of manufacturing the coating composition to be
used in the invention for forming the magnetic layer of the
magnetic recording medium comprises at least a kneading step,
dispersion step, and mixing steps provided as needed before and
after these steps. Each of the steps may be divided into two or
more stages. All of the starting materials employed in the
invention, including the hexagonal ferrite magnetic powder, the
non-magnetic powder, the binder, carbon black, the abrasive, the
antistatic agent, the lubricant and the solvent may be added at the
beginning or during any step. Further, each of the starting
materials may be divided and added during two or more steps. For
example, polyurethane may be divided and added in the kneading
step, dispersing step and mixing step for viscosity adjustment
after dispersion. In order to achieve the object of the invention,
conventionally known manufacturing techniques may be employed for
some of the steps. A kneading device of high kneading strength,
such as an open kneader, continuous kneader, pressure kneader or
extruder, is preferably used in the kneading step. Details of these
kneading treatments are described in JP-A-1-106338 and
JP-A-1-79274. Further, glass beads may be employed to disperse the
magnetic layer-forming coating composition and the non-magnetic
layer-forming coating composition. A dispersion medium having a
high specific gravity such as zirconia beads, titania beads or
steel beads is suitable for use as the glass beads. The particle
size and the packing ratio of the dispersion medium are optimized
for use. As a dispersing machine, any known dispersing machine may
be used.
[0085] In the method of manufacturing the magnetic recording medium
of the invention, the magnetic layer is formed by coating a
magnetic layer-forming coating composition on the surface of a
running support in a predetermined thickness. Here, a plurality of
magnetic layer-forming coating compositions may be sequentially or
simultaneously multi-layer coated, and the non-magnetic
layer-forming coating composition and the magnetic layer-forming
coating composition may be sequentially or simultaneously
multi-layer coated. As coating machines suited for use in coating
the magnetic layer-forming coating composition and the non-magnetic
layer forming coating composition, an air doctor coater, a blade
coater, a rod coater, an extrusion coater, an air knife coater, a
squeeze coater, an immersion coater, a reverse roll coater, a
transfer roll coater, a gravure coater, a kiss coater, a cast
coater, a spray coater and a spin coater can be used. As to these
coating machines, reference can be made to, for example, Saishin
Coating Gijutsu (May 31, 1983), issued by K.K. Sogo Gijutsu
Center.
[0086] With a magnetic tape, the layer formed by coating the
magnetic layer-forming coating composition is subjected to the
orientation treatment in the longitudinal direction by applying a
cobalt magnet or solenoid to the hexagonal ferrite magnetic powder
contained in the magnetic layer-forming coating composition. With a
disk, although isotropic orientation can be sufficiently achieved
in some cases without orientation using an orientation device, the
positioning of cobalt magnets at mutually oblique angles or the use
of a known random orientation device such as the application of an
alternating current magnetic field with solenoids is preferably
employed with a hexagonal ferrite magnetic powder, the term
"isotropic orientation" generally preferably means two-dimensional
in-plane randomness, but can also mean three-dimensional randomness
when a vertical component is imparted. With the hexagonal ferrite,
three-dimensional randomness in the in-plane and vertical
directions is generally readily achieved, but two-dimensional
in-place randomness is also possible. Also, a known technique such
as opposed magnets with different poles may be employed to impart
isotropic magnetic characteristics in a circumferential direction
using a vertical orientation. Vertical orientation is particularly
preferred in the case of conducting high-density recording.
Further, spin coating may be employed to achieve circumferential
orientation.
[0087] It is preferred to control the drying position of the coated
film by controlling the temperature and flow rate of drying air and
the coating rate. The coating rate is preferably from 20 m/min to
1,000 m/min, and the temperature of the drying air is preferably
60.degree. C. or higher. Also, a moderate preliminary drying may be
conducted before entering into a magnet zone.
[0088] Following drying, a surface smoothening treatment is applied
to the coated layer. In the surfaced smoothening treatment, there
may be utilized, for example, supercalender rolls. The surface
smoothening treatment eliminates voids produced by the removal of
solvent during drying and improves the packing ratio of the
hexagonal ferrite magnetic powder in the magnetic layer, making it
possible to obtain a magnetic recording medium having high
electromagnetically transducing characteristics. As
calender-processing rolls, rolls of heat-resistant plastics such as
epoxy, polyimide, polyamide and polyamidimide are used. It is also
possible to process with metal rolls.
[0089] The surface of the magnetic recording medium of the
invention preferably has an extremely excellent smoothness of 0.1
to 4 nm, preferably 1 to 3 nm, in terms of center-line average
surface roughness (cut-off value: 0.25 mm). Such surface roughness
is obtained by the method of, for example, subjecting the magnetic
layer formed by selecting the specific hexagonal ferrite magnetic
powder and the binder as described hereinbefore to the
above-mentioned calender processing. Calender processing conditions
are: 60 to 100.degree. C., preferably 70 to 100.degree. C.,
particularly preferably 80 to 100.degree. C., in temperature; and
100 to 500 kg/cm (98 to 490 kN/m), preferably 200 to 450 kg/cm (196
to 441 kN/m), particularly preferably 300 to 400 kg/cm (294 to 392
kN/m), in pressure.
[0090] In the case where the magnetic recording medium of the
invention is a magnetic tape, Hc (in the longitudinal direction) is
preferably 167 to 350 kA/m (more preferably 180 to 340 kA/m,
particularly preferably 200 to 320 kA/m), SQ (squareness ratio) is
preferably 0.50 to 0.80 (more preferably 0.60 to 0.80, particularly
preferably 0.65 to 0.80), and Bm (maximum magnetic flux density) is
preferably 1,000 to 2,000 mT (more preferably 1,200 to 2,000 mT,
particularly preferably 1,500 to 2,000).
[0091] In the case where the magnetic recording medium of the
invention is a magnetic disk, Hc (in-plane) is preferably 160 to
350 kA/m (more preferably 180 to 340 kA/m, particularly preferably
200 to 320 kA/m), SQ (squareness ratio) is preferably 0.40 to 0.60
(more preferably 0.45 to 0.60, particularly preferably 0.50 to
0.60), and Bm (maximum magnetic flux density) is preferably 1,000
to 2,000 mT (more preferably 1,200 to 2,000 mT, particularly
preferably 1,500 to 2,000).
[0092] The resulting magnetic recording medium can be cut into a
desired size for use by using a cutting machine. Such cutting
machine is not particularly limited, but those wherein plural sets
of a rotating upper blade (male blade) and a lower blade (female
blade) are provided are preferred. The slitting speed, depth of
engagement of the blades, ratio of the peripheral speed of the
upper blade (male blade) to the peripheral speed of the lower blade
(female blade) (upper blade peripheral speed/lower blade peripheral
speed) and time of continuously using the slitting blades are
properly selected.
Physical Properties
[0093] The coefficient of friction of the magnetic recording medium
of the invention with the head is preferably 0.5 or less,
preferably 0.3 or less, over a temperature range of -10 to
40.degree. C. and a humidity range of 0 to 95%. The intrinsic
surface resistivity is preferably from 10.sup.4 to 10.sup.12
.OMEGA./sq on the magnetic surface, and the charge potential is
preferably within the range of -500 V to +500 V. The modulus of
elasticity of the magnetic layer at an elongation of 0.5% is
preferably from 0.98 to 19.6 GPa (from 100 to 2000 kg/mm.sup.2) in
all in-plane directions, and the breaking strength is preferably
from 98 to 686 MPa (from 10 to 70 kg/mm.sup.2). The modulus of
elasticity of the magnetic recording medium is preferably from 0.98
to 14.7 GPa (from 100 to 1,500 kg/mm.sup.2), the residual
elongation is preferably 0.5% or less, the thermal shrinkage rate
at any temperature equal to or less than 100.degree. C. is
preferably 0.1% or less, more preferably 0.5% or less, most
preferably 0.1% or less.
[0094] The glass transition temperature of the magnetic layer (the
peak loss elastic modulus of dynamic viscoelasticity measured at
110 Hz) is preferably from 50 to 180.degree. C., and that of the
non-magnetic layer is preferably from 0 to 180.degree. C. The loss
elastic modulus falls within the range of 1.times.10.sup.7 to
8.times.10.sup.8 Pa (1.times.10 to 8.times.10.sup.9 dyne/cm.sup.2)
and the loss tangent is preferably 0.2 or less. A too large loss
tangent tends to cause an adhesion trouble. These thermal and
mechanical characteristics are preferably identical within 10% in
all in-plane directions of the medium.
[0095] The residual solvent contained 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 of the coated layer is preferably 30% by
volume or less, more preferably 20% by volume or less in both the
non-magnetic and magnetic layers. A smaller void ratio is preferred
to achieve a high output, but there are cases where ensuring a
certain value is good. For example, with a disk medium in which
repeated uses are important, a high void ratio is often preferred
for running durability.
[0096] The maximum height SR.sub.max of the magnetic layer is
preferably 0.5 .mu.m or less, the ten-point average roughness SRz
is preferably 0.3 .mu.m or less, the center surface peak SRp is
preferably 0.3 .mu.m or less, the center surface valley depth SRv
is preferably 0.3 .mu.m or less, the center surface surface area
ratio SSr is preferably from 20 to 80%, and the average wavelength
S.lamda.a is preferably from 5 to 300 .mu.m. These can readily be
controlled by controlling the surface properties by means of
fillers employed in the support and the surface shape of the rolls
employed in calendering. Curling is preferably within .+-.3 mm.
[0097] When the magnetic recording medium of the invention is
constituted by the non-magnetic layer and the magnetic layer, it is
possible to vary the physical characteristics between the
non-magnetic layer and the magnetic layer depending upon the
object. For example, while increasing the modulus of elasticity of
the magnetic layer to improve running durability, it is possible to
make the modulus of elasticity of the non-magnetic layer lower than
that of the magnetic layer to improve contact between the magnetic
recording medium and the head.
EXAMPLES
[0098] The invention is described in more detail below by reference
to Examples and Comparative Examples which, however, do not limit
the invention.
Examples 1 to 11 and Comparative Examples 1 to 8
Preparation of Hexagonal Ferrite Magnetic Powder
[0099] A glass mother phase component comprising
BaO--B.sub.2O.sub.3 in the proportion shown in Table 2 and a Ba
ferrite component represented by the composition formula of
Bao.Fe.sub.12-3(x+y)/2Co.sub.xZn.sub.yNb.sub.(x+y)/2O.sub.18 were
weighed and well mixed. The resulting mixture was put in a platinum
crucible and molten at a temperature of 1350.degree. C. by means of
a high-frequency heating furnace. After melting all of the starting
materials, the molten mixture was stirred for 1 hour to homogenize.
The thus-homogenized molten product was poured onto a water-cooled
twin rollers rotating at a high speed to thereby mill and quench,
thus an amorphous product being obtained. The resultant amorphous
product was maintained at a crystallization temperature shown in
Table 2 for 5 hours to crystallize. Subsequently, the crystallized
product was pulverized, then acid-treated in a 10% acetic acid
solution for 4 hours under stirring while controlling the solution
temperature at 80.degree. C. or above. The excess glass component
dissolved out of the amorphous product with the acid was removed by
repeatedly conducting washing with water. Finally, the slurry was
dried to obtain a magnetic powder. Characteristic properties of the
thus-obtained magnetic powder are shown in Table 2. Additionally,
as to the tabular diameter, a powder sample was photographed at an
arbitrarily selected position under a transmission electron
microscope (400,000.times.), and 300 particles whose side was
photographed were measured, followed by determining the average
value. The magnetic characteristics (Hc, .sigma.s) were measured by
means of a vibrating sample magnetometer (manufactured by Toei
Kogyo K.K.) under the conditions of 23.degree. C. in temperature
and 10 KOe in applied magnetic field. TABLE-US-00002 TABLE 2(A)
Crystallization B.sub.2O.sub.3 BaO Fe.sub.2O.sub.3 Temp.
Composition of magnetic powder mol % mol % mol % .degree. C.
(atomic ratio) Example 1 34 34 32 660.degree. C.
BaFe.sub.11.14Co.sub.0.12Zn.sub.0.45Nb.sub.0.29O.sub.19 Example 2
34 34 32 660.degree. C.
BaFe.sub.11.37Co.sub.0.12Zn.sub.0.30Nb.sub.0.21O.sub.19 Example 3
36 33 31 660.degree. C.
BaFe.sub.11.37Co.sub.0.12Zn.sub.0.30Nb.sub.0.21O.sub.19 Example 4
37 32 31 660.degree. C.
BaFe.sub.11.37Co.sub.0.12Zn.sub.0.30Nb.sub.0.21O.sub.19 Example 5
47 38 15 660.degree. C.
BaFe.sub.11.29Co.sub.0.12Zn.sub.0.35Nb.sub.0.24O.sub.19 Example 6
43 42 15 660.degree. C.
BaFe.sub.11.29Co.sub.0.12Zn.sub.0.35Nb.sub.0.24O.sub.19 Example 7
32 23 45 740.degree. C.
BaFe.sub.11.29Co.sub.0.12Zn.sub.0.35Nb.sub.0.24O.sub.19 Example 8
28 27 45 740.degree. C.
BaFe.sub.11.29Co.sub.0.12Zn.sub.0.35Nb.sub.0.24O.sub.19 Example 9
34 34 32 610.degree. C.
BaFe.sub.11.52Co.sub.0.12Zn.sub.0.20Nb.sub.0.15O.sub.19 Example 10
35 33 32 700.degree. C.
BaFe.sub.11.55Co.sub.0.12Zn.sub.0.18Nb.sub.0.15O.sub.19 Example 11
35 33 32 700.degree. C.
BaFe.sub.11.67Co.sub.0.12Zn.sub.0.10Nb.sub.0.11O.sub.19 Comparative
31 37 32 660.degree. C.
BaFe.sub.11.37Co.sub.0.12Zn.sub.0.30Nb.sub.0.21O.sub.19 Example 1
Comparative 31 37 32 720.degree. C.
BaFe.sub.10.8Co.sub.0.1Zn.sub.0.7Nb.sub.0.4O.sub.19 Example 2
Comparative 28 40 32 640.degree. C.
BaFe.sub.11.44Co.sub.0.12Zn.sub.0.25Nb.sub.0.19O.sub.19 Example 3
Comparative 31 37 32 660.degree. C.
BaFe.sub.11.67Co.sub.0.12Zn.sub.0.10Nb.sub.0.11O.sub.19 Example 4
Comparative 36 33 31 680.degree. C.
BaFe.sub.11.37Co.sub.0.12Zn.sub.0.30Nb.sub.0.21O.sub.19 Example 5
Comparative 41 44 15 660.degree. C.
BaFe.sub.11.29Co.sub.0.12Zn.sub.0.35Nb.sub.0.24O.sub.19 Example 6
Comparative 25 30 45 740.degree. C.
BaFe.sub.11.29Co.sub.0.12Zn.sub.0.35Nb.sub.0.24O.sub.19 Example 7
Comparative 34 34 32 590.degree. C.
BaFe.sub.11.52Co.sub.0.12Zn.sub.0.20Nb.sub.0.16O.sub.19 Example
8
[0100] TABLE-US-00003 TABLE 2(B) Average Tabular Hc .sigma.s
Diameter Oe kA/m A m.sup.2/kg nm Example 1 2000 160 57 22 Example 2
2500 200 57 23 Example 3 2500 200 58 25 Example 4 2500 200 59 30
Example 5 3000 240 57 30 Example 6 2900 232 56 27 Example 7 2300
184 58 29 Example 8 2300 184 57 28 Example 9 2000 160 52 18 Example
10 3500 280 59 28 Example 11 4000 320 58 29 Comparative Example 1
2500 200 48 22 Comparative Example 2 1400 112 48 23 Comparative
Example 3 2000 160 44 20 Comparative Example 4 3000 240 47 27
Comparative Example 5 2700 216 59 32 Comparative Example 6 2650 212
50 28 Comparative Example 7 2300 184 49 26 Comparative Example 8
1200 96 39 14
[0101] TABLE-US-00004 Preparation of a coating composition for tape
Coating composition for forming a magnetic layer Barium ferrite
magnetic powder 100 parts Polyurethane resin 12 parts
weight-average molecular weight: 10,000 sulfonic acid functional
group: 0.5 meq/g .alpha.-Alumina 8 parts HIT60 (manufactured by
Sumitomo Chemical) Carbon black (particle size: 0.015 .mu.m) 0.5
part #55 (manufactured by Asahi Carbon) Stearic acid 0.5 part Butyl
stearate 2 parts Methyl ethyl ketone 180 parts Cyclohexanone 100
parts Coating composition for forming a non-magnetic layer
Non-magnetic powder, .alpha.-iron oxide 100 parts Average major
axis length: 0.09 .mu.m; Specific surface area by the BET method:
50 m.sup.2/g pH: 7 DBP oil absorption capacity: 27-38 ml/100 g
Surface treatment layer Al.sub.2O.sub.3: 8% by weight Carbon black
25 parts Conductex SC-U (manufactured by Colombian Carbon Co.)
Vinyl chloride copolymer 13 parts MR104 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 5 parts UR8200 (manufactured by
Toyobo Co., Ltd.) Phenylphosphonic acid 3.5 parts Butyl stearate 1
part Stearic acid 2 parts Methyl ethyl ketone 205 parts
Cyclohexanone 135 parts
Method for Preparing a Magnetic Tape
[0102] Individual components for each coating composition were
kneaded in a kneader. Each of the resulting coating compositions
was introduced into a horizontal sand mill retaining 1.0-mm.phi.
zirconia beads in an amount of 65% based on the volume of the
dispersing zone thereof using a pump, and dispersed for 120 minutes
(time during which the composition substantially stayed in the
dispersing zone) at 2,000 rpm. To the resulting dispersion of the
non-magnetic layer-forming coating composition was added 5.0 parts
of polyisocyanate, and to the resulting dispersion of the magnetic
layer-forming coating composition was added 2.5 parts of the
polyisocyanate. Further, 3 parts of methyl ethyl ketone was added
to each coating composition, and each of the resulting coating
compositions was filtered through a filter of 1 .mu.m in average
pore size. Thus, there were prepared a coating composition for
forming a non-magnetic layer and a coating composition for forming
a magnetic layer.
[0103] The thus-obtained coating composition for forming the
non-magnetic layer was coated on a 4-.mu.m thick polyethylene
terephthalate base in a dry thickness of 1.5 .mu.m and, after
drying, the coating composition for forming the magnetic layer was
successively multi-layer coated in a thickness of the magnetic
layer of 70 nm and, while the magnetic layer was still in a wet
state, the orientation was performed by means of a cobalt magnet
having a magnetic force of 6,000 G (600 mT) and a solenoid having a
magnetic force of 6,000 G, followed by drying. Subsequently,
calendering was conducted using a 7-stage calender at a temperature
of 90.degree. C. and a linear pressure of 300 kg/cm (294 kN/m).
Then, a 0.5-.mu.m thick backcoat layer (prepared by dispersing 100
parts of carbon black of 17 nm in average particle size, 80 parts
of calcium carbonate of 40 nm in average particle size and 5 parts
of .alpha.-alumina of 200 nm in average particle size were
dispersed in nitrocellulose resin, polyurethane resin and
polyisocyanate) was coated. The resulting web-shaped magnetic
recording medium was slit into 3.8 mm-wide product. The slit
product was then subjected to surface-cleaning treatment by means
of a tape-cleaning device provided in an apparatus having a tape
delivery mechanism and a tape wind-up mechanism, wherein non-woven
fabric and a razor blade are provided so as to be pressed against
the magnetic surface. Thus, a magnetic tape medium was
obtained.
[0104] Various magnetic characteristics of the thus-obtained
magnetic tapes were examined as described above. Also, output and
noise were checked. These were measured by fitting a recording head
(MIG; gap: 0.15 .mu.m; 1.8 T) and an AMR head for reproduction to a
drum tester. The head-medium relative speed was adjusted to 15
m/sec, and noise was measured in terms of modulated noise. SN was
given with SN in Comparative Example 2 as 0 dB.
[0105] The results are shown in Table 3. Additionally, in Table 3,
numbers of Examples and Comparative Examples correspond to the
numbers of Examples and Comparative Examples of magnetic powders
shown in Table 2. TABLE-US-00005 TABLE 3 Hc Bm Output Noise S/N
Example Oe kA/m SQ mT dB dB dB Example 1 2140 171 0.62 165 2.4 0.0
2.4 Example 2 2670 214 0.64 165 2.9 0.3 2.6 Example 3 2640 211 0.68
167 3.1 0.7 2.4 Example 4 2650 212 0.78 171 3.3 1.7 1.6 Example 5
3170 254 0.81 165 3.4 1.7 1.7 Example 6 3070 246 0.77 162 3.1 1.1
2.0 Example 7 2450 196 0.76 169 2.9 1.5 1.4 Example 8 2460 197 0.74
164 2.7 1.3 1.4 Example 9 2140 171 0.54 150 0.5 -0.9 1.4 Example 10
3690 295 0.83 169 4.3 1.4 2.9 Example 11 4200 336 0.85 167 4.6 1.7
2.9 Comparative 2670 214 0.62 139 0.6 0.1 0.5 Example 1 Comparative
2520 202 0.64 138 0.0 0.0 0.0 Example 2 Comparative 2140 171 0.58
127 -0.2 -0.6 0.4 Example 3 Comparative 3170 254 0.79 136 1.4 0.9
0.5 Example 4 Comparative 2860 229 0.82 170 3.5 3.0 0.5 Example 5
Comparative 2810 225 0.74 143 1.7 1.2 0.5 Example 6 Comparative
2450 196 0.70 141 1.1 0.7 0.4 Example 7 Comparative 1320 106 0.46
111 -2.0 -2.0 0.0 Example 8
Results of Evaluating the Magnetic Tape Media
[0106] It is seen from the results shown in Tables 2 and 3 that, it
is seen that the magnetic powders produced by using the starting
materials having a composition within the composition range
surrounded by the four points of a, b, c and d in the triangular
phase diagram shown in FIG. 1 show a high .sigma.s value even when
their tabular sizes are small. It is also seen that the magnetic
powder produced by using starting materials outside the aforesaid
composition range show a low .sigma.s value. Further, magnetic tape
media containing in the magnetic layer the magnetic powders
produced by using the starting materials having a composition
within the composition range surrounded by the four points of a, b,
c and d in the triangular phase diagram shown in FIG. 1 show a high
SN ratio even when the thickness of the magnetic layer is small
whereas, when magnetic powders outside the range are used, there
results a low SN ratio.
[0107] Next, magnetic disc media containing hexagonal ferrite
magnetic powder of the invention in the magnetic layer were
prepared. TABLE-US-00006 Preparation of a coating composition for
disc Coating composition for forming a magnetic layer Barium
ferrite magnetic powder 100 parts Polyurethane resin 12 parts
weight-average molecular weight: 10,000 sulfonic acid functional
group: 0.5 meq/g Diamond fine particles 2 parts Average particle
size: 0.10 .mu.m Carbon black (particle size: 0.015 .mu.m) 0.5 part
#55 (manufactured by Asahi Carbon) Stearic acid 0.5 part Butyl
stearate 2 parts Methyl ethyl ketone 230 parts Cyclohexanone 150
parts Coating composition for forming a non-magnetic layer
Non-magnetic powder, .alpha.-iron oxide 100 parts Average major
axis length: 0.09 .mu.m; Specific surface area by the BET method:
50 m.sup.2/g pH: 7 DBP oil absorption capacity: 27-38 ml/100 g
Surface treatment layer Al.sub.2O.sub.3: 8% by weight Carbon black
25 parts Conductex SC-U (manufactured by Colombian Carbon Co.)
Vinyl chloride copolymer 13 parts MR104 (manufactured by Nippon
Zeon Co., Ltd.) Polyurethane resin 5 parts UR8200 (manufactured by
Toyobo Co., Ltd.) Phenylphosphonic acid 3.5 parts Butyl stearate 1
part Stearic acid 2 parts Methyl ethyl ketone 205 parts
Cyclohexanone 135 parts
Method for Preparing a Magnetic Disc
[0108] Individual components for each coating composition were
kneaded in a kneader. Each of the resulting coating compositions
was introduced into a horizontal sand mill retaining 1.0-mm.phi.
zirconia beads in an amount of 65% based on the volume of the
dispersing zone thereof by means of a pump, and dispersed for 120
minutes (time during which the composition substantially stayed in
the dispersing zone) at 2,000 rpm. To the resulting dispersion of
the non-magnetic layer-forming coating composition was added 6.5
parts of polyisocyanate, and to the resulting dispersion of the
magnetic layer-forming coating composition was added 2.5 parts of
the polyisocyanate. Further, 7 parts of methyl ethyl ketone was
added to each coating composition, and each of the resulting
coating compositions was filtered through a filter of 1 .mu.m in
average pore size. Thus, there were prepared a coating composition
for forming a non-magnetic layer and a coating composition for
forming a magnetic layer.
[0109] The thus-obtained coating composition for forming the
non-magnetic layer was coated on a 62-.mu.m thick polyethylene
terephthalate base in a dry thickness of 1.5 .mu.m and, after
drying, the coating composition for forming the magnetic layer was
successively multi-layer coated in a thickness of the magnetic
layer of 0.10 .mu.m. After drying, calendering was conducted using
a 7-stage calender at a temperature of 90.degree. C. and a linear
pressure of 300 kg/cm. These procedures were conducted with both
sides of the non-magnetic support. The thus obtained magnetic
material was stamped into a disc measuring 3.5 inches, and the disc
was subjected to surface abrasion treatment to obtain a magnetic
disc medium.
[0110] With the thus obtained magnetic disc media, magnetic
characteristics and noise were measured as is the same with the
magnetic tape media. Additionally, output and noise were measured
by fitting a recording head (MIG; gap: 0.15 .mu.m; 1.8 T) and an
AMR head for reproduction to a spin stand. The medium rotation
number was 4,000 rpm, recording wavelength was 0.20 .mu.m, and
noise was measured in terms of modulated noise. SN was given with
SN in Comparative Example 2 as 0 dB.
[0111] The results are shown in Table 4. Additionally, in Table 4,
numbers of Examples and Comparative Examples correspond to the
numbers of Examples and Comparative Examples of magnetic powders
shown in Table 2. TABLE-US-00007 TABLE 4 Hc Bm Output Noise S/N
Example Oe kA/m SQ mT dB dB dB Example 1 2000 160 0.50 161 1.2 -0.1
1.3 Example 2 2450 196 0.52 161 1.6 0.1 1.5 Example 3 2450 196 0.52
164 1.7 0.5 1.2 Example 4 2450 196 0.52 167 2.3 1.2 1.1 Example 5
2900 232 0.54 161 2.6 1.3 1.3 Example 6 2810 225 0.54 158 1.9 0.9
1.0 Example 7 2270 182 0.51 164 1.9 0.7 1.2 Example 8 2270 182 0.51
161 1.8 0.6 1.2 Example 9 2000 160 0.50 147 0.4 -0.8 1.2 Example 10
3350 268 0.55 167 2.7 1.1 1.6 Example 11 3800 304 0.56 164 3.0 1.3
1.7 Comparative 2450 196 0.52 136 0.3 0.0 0.3 Example 1 Comparative
1460 117 0.44 135 0.0 0.0 0.0 Example 2 Comparative 2000 160 0.50
124 0.0 -0.3 0.3 Example 3 Comparative 2900 232 0.54 133 1.2 0.8
0.4 Example 4 Comparative 2630 210 0.53 167 1.9 1.8 0.1 Example 5
Comparative 2585 207 0.53 141 1.3 1.0 0.3 Example 6 Comparative
2270 182 0.51 137 0.9 0.6 0.3 Example 7 Comparative 1280 102 0.42
110 -2.4 -1.8 -0.6 Example 8
Results of Evaluating the Magnetic Disc Media
[0112] It is seen from the results shown in Table 4 that the
magnetic disc media each containing in the magnetic layer the
magnetic powder produced by using the starting materials within the
composition region surrounded by the four points of a, b, c and d
in the triangular phase diagram shown in FIG. 1 have good output
and noise characteristics.
[0113] According to the invention, there are provided a hexagonal
ferrite magnetic powder adapted for a magnetic recording medium for
high-density recording which permits reduction of noise without
reducing .sigma.s and which is adapted for a magnetic recording
medium for high-density recording reproducible by a highly
sensitive head such as an MR head or a GMR head, a method for
producing same and a magnetic recording medium.
[0114] Additionally, the aforesaid JP-A-56-169128 and
JP-A-58-0169902 disclose a composition region surrounded by 4
particular points of a, b, c and d in the triangular phase diagram
wherein AO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 constitute apexes.
However, the composition is different from that of the material
used in the invention.
[0115] The entire disclosure of each and every foreign patent
application from which the benefit of foreign priority has been
claimed in the present application is incorporated herein by
reference, as if fully set forth.
* * * * *