U.S. patent application number 09/933074 was filed with the patent office on 2002-03-07 for particles for non-magnetic undercoat layer of magnetic recording medium, method thereof and magnetic recording medium.
This patent application is currently assigned to TODA KOGYO CORP.. Invention is credited to Hayashi, Kazuyuki, Iwasaki, Keisuke, Morii, Hiroko.
Application Number | 20020028353 09/933074 |
Document ID | / |
Family ID | 17852271 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020028353 |
Kind Code |
A1 |
Hayashi, Kazuyuki ; et
al. |
March 7, 2002 |
Particles for non-magnetic undercoat layer of magnetic recording
medium, method thereof and magnetic recording medium
Abstract
The present invention provides particles for a non-magnetic
undercoat layer of a magnetic recording medium, which comprises
acicular hematite particles having an average major axial diameter
of not more than 0.3 .mu.m, a geometrical standard deviation in the
major axial diameter of not more than 1.50 and a BET specific
surface area of not less than 40 m.sup.2/g, and containing a total
amount of sodium of not more than 50 ppm calculated as Na. The
acicular hematite particles have an excellent dispersibility in a
vehicle so that a non-magnetic undercoat layer containing the
particles is excellent in surface smoothness and strength. A
magnetic recording medium using the non-magnetic undercoat layer is
excellent not only in electromagnetic performance, but in storage
stability.
Inventors: |
Hayashi, Kazuyuki;
(Hiroshima-ken, JP) ; Iwasaki, Keisuke;
(Hiroshima-ken, JP) ; Morii, Hiroko;
(Hiroshima-ken, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN, HATTORI,
MCLELAND & NAUGHTON, LLP
1725 K STREET, NW, SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
TODA KOGYO CORP.
Hiroshima-shi
JP
|
Family ID: |
17852271 |
Appl. No.: |
09/933074 |
Filed: |
August 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09933074 |
Aug 21, 2001 |
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09420008 |
Oct 18, 1999 |
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6299973 |
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Current U.S.
Class: |
428/840.2 ;
G9B/5.284; G9B/5.286 |
Current CPC
Class: |
Y10T 428/25 20150115;
C01P 2004/20 20130101; C01P 2004/82 20130101; C01P 2004/62
20130101; Y10T 428/2982 20150115; C01P 2004/10 20130101; C01P
2006/12 20130101; C01P 2004/03 20130101; C01G 49/06 20130101; C01P
2006/42 20130101; C01P 2004/54 20130101; Y10S 428/90 20130101; C09C
1/24 20130101; C01P 2004/64 20130101; C01P 2006/80 20130101; G11B
5/733 20130101; C01P 2004/51 20130101; B82Y 30/00 20130101; C01P
2006/10 20130101 |
Class at
Publication: |
428/694.0BS |
International
Class: |
G11B 005/733 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 20, 1998 |
JP |
10-297876 |
Claims
What is claimed is:
1. Particles for a non-magnetic undercoat layer of a magnetic
recording medium, which comprises acicular hematite particles
having an average major axial diameter of not more than 0.3 .mu.m,
a geometrical standard deviation in the major axial diameter of not
more than 1.50 and a BET specific surface area of not less than 40
m.sup.2/g, and containing a total amount of sodium of not more than
50 ppm calculated as Na.
2. Particles of claim 1, wherein the acicular hematite particles
are coated with at least one selected from the group consisting of
an aluminium hydroxide, an aluminium oxide, a silicon hydroxide and
a silicon oxide.
3. Particles of claim 1 or 2, wherein an S.sub.BET/S.sub.TEM
defined by a ratio of a specific surface area (S.sub.BET) measured
by a BET method and a surface area (S.sub.TEM) calculated from the
major axial diameter and the minor axial diameter, measured from
the particles in an electron microscopic photograph is 0.5 to
2.5.
4. A method for producing particles for a non-magnetic undercoat
layer of a magnetic recording medium, which comprises the steps of:
dehydrating acicular goethite particles with the surfaces coated
with a sintering-preventing agent to form acicular hematite
particles, reducing the acicular hematite particles at a
temperature of 250to 600.degree. C. under a reducing atmosphere to
form acicular magnetite particles, washing with pure water and
drying the acicular magnetite particles, oxidizing the acicular
magnetite particles at a temperature of 650 to 850.degree. C. under
an oxidizing atmosphere, and washing with pure water and drying the
resulting high-density acicular hematite particles.
5. A method for producing particles for a non-magnetic undercoat
layer of a magnetic recording medium, which comprises the steps of:
dehydrating acicular goethite particles to form acicular hematite
particles, coating the surfaces of the acicular hematite particles
with a sintering-preventing agent, reducing the acicular hematite
particles at a temperature of 250 to 600.degree. C. under a
reducing atmosphere to form acicular magnetite particles, washing
with pure water and drying the acicular magnetite particles,
oxidizing the acicular magnetite particles at a temperature of 650
to 850.degree. C. under an oxidizing atmosphere, and washing with
pure water and drying the resulting high-density acicular hematite
particles.
6. A method of claim 4 or 5, wherein the particles are
wet-pulverized prior to washing with pure water.
7. A method of claim 4 or 5, wherein the high-density acicular
hematite particles are coated with at least one selected from the
group consisting of an aluminium hydroxide, an aluminium oxide, a
silicon hydroxide and a silicon oxide by treating the particles
with an aqueous solution containing an aluminium compound, a
silicon compound or the both compounds.
8. A magnetic recording medium comprising a non-magnetic base film,
a non-magnetic undercoat layer formed on the non- magnetic base
film, which comprises non-magnetic particles and a binder resin,
and a magnetic recording layer formed on the non- magnetic
undercoat layer, which comprises magnetic particles and a binder
resin, the improvement wherein the non-magnetic particles comprises
particles for a non-magnetic undercoat layer defined in any one of
claims 1 to 3.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to acicular hematite particles
suitable for a non-magnetic undercoat layer of a magnetic recording
medium, more particularly, to acicular hematite particles
containing the total sodium amount of not more than 50 ppm, method
thereof and a magnetic recording medium using said acicular
hematite particles.
[0003] 2. Description of the Prior Art
[0004] With a recent development of miniaturized and lightweight
video or audio magnetic recording and reproducing apparatuses for
long-time recording, magnetic recording media such as a magnetic
tape and a magnetic disk have been increasingly and strongly
required to have a higher performance, namely, a higher recording
density, a higher output characteristic and a lower noise
level.
[0005] Especially, much higher recording density of audio or video
tapes is always desired and carrier signals tend to move to a
shorter wavelength range.
[0006] Meanwhile, since an influence of self-demagnetization
becomes prominent as the recording wavelength becomes shorter, it
is necessary to reduce an influence of self-demagnetization by
thinning of a magnetic recording layer with a view to higher
recording density. That is supported, for example, on page 312 of
"Development of Magnetic Materials and Technology for High
Dispersion of Magnetic Powder" published by Sogo Gijutsu Center
(1982), " . . . the conditions for high-density recording in a
coated layer type are that high output characteristic and low level
of noise with respect to short wavelength signals are maintained.
To meet such conditions, it is required to have large coercive
force Hc and residual magnetization Br . . . and to have a thinner
thickness of the coated film . . . ".
[0007] In light of the foregoing situation, it is proposed and
practiced to reduce self-demagnetization by decreasing the
thickness of an upper magnetic recording layer and to solve the
problems such as a deterioration in surface smoothness and a
deterioration in electromagnetic performance by forming on a
non-magnetic base film at least one non-magnetic undercoat layer
which comprises dispersing non-magnetic particles such as hematite
Or particles in a binder (Japanese Patent Examined Publication
(Kokoku) No. 6-93297, Japanese Patent Non-examined Publication
(Kokai) Nos. 62-159338, 63-187413, 4-167225, 4-325915, 5-73882,
5-182177, 5-347017, 6-60362, etc.)
[0008] Moreover, with a development of miniaturized and lightweight
video or audio magnetic recording and reproducing apparatuses for
long-time recording, surroundings in which a magnetic recording
medium is used and stored become diversified and it is required to
have storage stability not only in ordinary conditions, but in high
temperature and high humidity conditions.
[0009] As a cause of lowering the electromagnetic performance, the
storage stability of a magnetic recording medium and the dispersion
stability of a coating composition, a water-soluble alkali metal,
in particular, a water-soluble sodium contained in the magnetic
recording medium is pointed out.
[0010] The Japanese Patent Non-examined Publication (Kokai) No.
9-22524, for example, on page 3, column 3, lines 14-20 discloses; "
. . . When a free fatty acid increases and water-soluble Na and Ca
contained in non-magnetic particles abound, Na and Ca salts of the
fatty acid tend to deposit to thus afford an adverse effect to the
electromagnetic performance such as output performance and C/N, but
by reducing those salts to the specific amount or less, an
excellent strage stability and a low friction coefficient are
obtained without a deterioration in electromagnetic performance.
The Japanese Patent Non-examined Publication (Kokai) No. 62-209806
discloses on page 2, left upper column, lines 3-19; " . . . the
residual Na.sup.+has been known to have a great influence on the
quality of a magnetic coated film. As a typical example, a
so-called "salt-depositing phenomenon" is pointed out. That is,
when a polyvinyl chloride-based resin is used as part of a binder,
crystals of NaCl deposit on the surface of a magnetic recording
layer which invites D.O. (dropout) to thus damage the quality of
magnetic tapes. Moreover, there are data showing that the
above-mentioned phenomenon becomes a cause of "blocking" which is a
characteristic of video tapes (Blocking is a phenomenon that when
running of a video tape was stopped by power failure or the like
with the video tape being loaded on a video deck under high
temperature and high humidity conditions, the magnetic recording
layer is held to be attached to an upper cylinder of the video
deck, in consequence, part of the magnetic recording layer peels
off.). Thus, the reduction of the residual Na.sup.+ contained in
the magnetic powders has been long-waited." Furthermore, Japanese
Patent No. 2641662 discloses on page 2, column 3, line 38 to column
4, line 14; " . . . fatty acids react with alkali metals such as
sodium and potassium which are impurities of carbonblack to thereby
form alkali metal salts of the fatty acids. . . . These alkali
metal salts of the fatty acids are insoluble in an organic solvent
. . . powder of these alkali metal salts of the fatty acids deposit
on the surface of the magnetic recording layer to thus become a
cause of dropout.
[0011] . . . The decomposition amount of the organic solvent is
proportional to the amount of the alkali metals such as Na and K. .
. . The decomposition products of the organic solvent lower
adsorptivity of a binder resin to the surface of an inorganic fine
particle filler to result in a decrease in mechanical strength of a
coated film. In addition, storage stability as a coating
composition deteriorates."
[0012] It has been reported that an improvement in storage
stability of a magnetic recording medium is tried by reducing a
water-soluble sodium salt contained in a magnetic recording medium
or in non-magnetic or magnetic particles added to the magnetic
recording medium (Japanese Patent Non-examined Publication (Kokai)
Nos. 62-209726, 62-209806, 9-22524, 9-147350, 9-231546, 9-170003,
10-177714, 10-198948, Japanese Patent Examined Publication (Kokoku)
No. 7-82638, Japanese Patent No. 2641662, etc.).
[0013] As is discussed above, non-magnetic particles for a
non-magnetic undercoat layer have been strongly demanded which are
capable of providing a thin magnetic recording layer having a
smooth surface and uniform thickness when the magnetic recording
layer is formed on the non-magnetic undercoat layer obtained by
dispersing the non-magnetic particles in a vehicle, and further,
capable of providing a magnetic recording medium excellent in
electromagnetic performance and storage stability, but such
non-magnetic particles have not been hitherto obtained.
[0014] That is, in the above-mentioned Japanese Patent Non-examined
Publication (Kokai) No. 9-147350, it is described that the amount
of alkali metals contained in non-magnetic particles of a
non-magnetic layer is less than 1500 ppm. However, as will be
described later as comparative examples, when the non-magnetic
particles contain approximately 1500 ppm of alkali metals,
dispersibility in a vehicle is poor because of high desorption
ratio of resin, and hence, a magnetic recording medium obtained by
employing the non-magnetic particles as ones for a non-magnetic
undercoat layer of a magnetic recording medium is weak in strength
of a coated film and storage stability is not said to be
satisfactory. Moreover, as methods for production of non-magnetic
particles having alkali metals of less than 1500 ppm, a method for
employing as an alkali aqueous solution, for example, an ammonium
aqueous solution not containing alkali metals, and a method for
carrying out sufficient washing after the completion of production
or before the final heat-treatment are described. However,
according to these methods, the total amount of sodium can only be
reduced to approximately 100 ppm as described in the publication
and can not be reduced to 50 ppm or less. Thus, when such
non-magnetic particles are used as ones for a non-magnetic
undercoat layer of a magnetic recording medium, it cannot be said
that the storage stability of the obtained magnetic recording
medium is satisfactory.
[0015] In the above-mentioned Japanese Patent Non-examined
Publication Nos. 9-22524 and 9-170003, it is described that a
soluble sodium contained in non-magnetic particles of a
non-magnetic layer is 0-150 ppm or 300 ppm or less. However, as
will be described later as comparative examples, according to these
methods, a soluble sodim can be reduced to approximately 45 ppm but
a difficultly-soluble sodium is contained in an amount of
approximately 300 ppm. Since the difficultly-soluble sodium
converts into a soluble sodium through moisture contained in air or
a coated film and to come out to deposit, when the non-magnetic
particles are used as ones for a non-magnetic undercoat layre of a
magnetic recording medium, the storage stability of the magnetic
recording medium can not be said to be satisfactory.
SUMMARY OF THE INVENTION
[0016] An object of the present invention is to provide
non-magnetic particles for a non-magnetic undercoat layer of a
magnetic recording medium which can provide a thin magnetic
recording layer having a smooth surface and a uniform thickness
when a magnetic recording layer is formed on the non-magnetic
undercoat layer obtained by dispersing the non-magnetic particles
in a vehicle, and which can provide a magnetic recording medium
excellent in electromagnetic performance and storage stability.
[0017] Another object of the present invention is to provide a
method for producing the non-magnetic particles for a non-magnetic
undercoat layer of a magnetic recording medium.
[0018] Still another object of the present invention is to provide
a magnetic recording medium which is excellent in electromagnetic
performance and storage stability.
[0019] Further objects and advantages of the present invention will
be apparent from the detailed description below.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention is, in a first aspect, to provide
particles for a non-magnetic undercoat layer of a magnetic
recording medium, which comprises acicular hematite particles
having an average major axial diameter of not more than 0.3 .mu.m,
a geometrical standard deviation of the major axial diameter of not
more than 1.50 and a BET specific surface area of not less than 40
m.sup.2/g, and containing a total amount of sodium of not more than
50 ppm calculated as Na.
[0021] As a preferable embodiment, the acicular hematite particles
are coated with at least one selected from the group consisting of
an aluminium hydroxide, an aluminium oxide, a silicon hydroxide and
a silicon oxide.
[0022] As a further preferable embodiment, an S.sub.BET/S.sub.TEM
defined by a ratio of a specific surface area (S.sub.BET) measured
by a BET method and a surface area (S.sub.TEM) calculated from the
major axial diameter and the minor axial diameter, measured from
the particles in an electron microscopic photograph is 0.5 to
2.5.
[0023] The present invention is, in a second aspect, to provide a
method for producing particles for a non-magnetic undercoat layer
of a magnetic recording medium, which comprises the steps of:
[0024] dehydrating acicular goethite particles with the surfaces
coated with a sintering-preventing agent to form acicular hematite
particles,
[0025] reducing the acicular hematite particles at a temperature of
250 to 600.degree. C. under a reducing atmosphere to form acicular
magnetite particles,
[0026] washing with pure water and drying the acicular magnetite
particles,
[0027] oxidizing the acicular magnetite particles at a temperature
of 650 to 850.degree. C. under an oxidizing atmosphere, and
[0028] washing with pure water and drying the resulting
high-density acicular hematite particles.
[0029] The present invention is, in a third aspect, to provide a
method for producing particles for a non-magnetic undercoat layer
of a magnetic recording medium, which comprises the steps of:
[0030] dehydrating acicular goethite particles to form acicular
hematite particles,
[0031] coating the surfaces of the acicular hematite particles with
a sintering-preventing agent,
[0032] reducing the acicular hematite particles at a temperature of
250 to 600.degree. C. under a reducing atmosphere to form acicular
magnetite particles,
[0033] washing with pure water and drying the acicular magnetite
particles,
[0034] oxidizing the acicular magnetite particles at a temperature
of 650 to 850.degree. C. under an oxidizing atmosphere, and
[0035] washing with pure water and drying the resulting
high-density acicular hematite particles.
[0036] As a preferable embodiment in the above-mentioned two
methods, the acicular magnetite particles and high-density acicular
hematite particles are wet-pulverized prior to washing with pure
water.
[0037] As a further preferable embodiment, the high-density
acicular hematite particles are coated with at least one selected
from the group consisting of an aluminium hydroxide, an aluminium
oxide, a silicon hydroxide and a silicon oxide by treating the
particles with an aqueous solution containing an aluminium
compound, a silicone compound or the both compounds.
[0038] The present invention is, in a fourth aspect, to provide a
magnetic recording medium comprising a non-magnetic base film, a
non-magnetic undercoat layer formed on the non-magnetic base film,
comprising non-magnetic particles and a binder resin, and a
magnetic recording layer formed on the non-magnetic undercoat
layer, comprising magnetic particles and a binder resin, the
improvement wherein the non-magnetic particles comprises particles
for a non-magnetic undercoat layer as mentioned above.
[0039] Hereinafter, the present invention will be explained in
detail.
[0040] First, particles for a non-magnetic undercoat layer
according to the present invention will be explained.
[0041] The particles for a non-magnetic undercoat layer according
to the present invention comprises acicular hematite particles
having an average major axial diameter of not more than 0.3 .mu.m,
a geometrical standard deviation of the major axial diamete of not
more than 1.50 and a BET specific surface area of not less than 40
m.sup.2/g, and containing the total amount of sodium of not more
than 50 ppm calculated as Na.
[0042] The word "acicular" of the acicular hematite particles
herein includes not only an acicular shape, literally, but, for
example, a spindle shape and a rice shape.
[0043] The average major axial diameter of the acicular hematite
particles for a non-magnetic undercoat layer is not more than 0.3
.mu.m, preferably 0.005 to 0.3 .mu.m. If it is less than 0.005
.mu.m, the dispersion in a vehicle tends to be difficult upon the
production of a non-magnetic coating composition because of
increased intermolecular force caused by fine particles. If it is
more than 0.3 .mu.m, the surface smoothness of a coated film tends
to be deteriorated because of the increased particle size. The
average major axial diameter is more preferably 0.02 to 0.2 .mu.m
when considering the dispersibility in the vehicle and the surface
smoothness of a coated film.
[0044] The geometrical standard deviation of the major axial
diameter of the acicular hematite particles is not more than 1.50.
If it is more than 1.50, the coarse particles give an adverse
effect to the surface smoothness. It is more preferably not more
than 1.40, still more preferably not more than 1.35, when
considering the surface smoothness of a coated film. Further, if
the industrial productivity is taken into consideration, the lower
limit thereof is approximately 1.01.
[0045] The BET specific surface area of the acicular hematite
particles is not less than 40 m.sup.2/g, preferably 40 to 150
m.sup.2/g. More preferably, it is 45 to 100 m.sup.2/g, still more
preferably 50 to 80 m.sup.2/g for the same reasons as in the upper
and lower limits of the above-mentioned average major axial
diameter.
[0046] The total amount of sodium as calculated as Na contained in
the acicular hematite particles is not more than 50 ppm. If it is
more than 50 ppm, difficultly-soluble sodium salts contained in the
particles are converted into soluble sodium salts through moisture
contained in air and a coated film to thus deposit on the surfaces
of the particles, then the soluble sodium salts react with fatty
acids added to the coated film to thus generate metal salts of the
fatty acids, which gradually lowers electromagnetic performance of
the obtained magnetic recording medium and as a result, the storage
stability becomes worse. In some cases, the dispersibility of the
acicular hematite particles in a vehicle is liable to be
deteriorated and under high humidity surroundings, in particular,
an efflorescence phenomenon sometimes occurs on the surface of the
magnetic recording medium. If the storage stalility of the magnetic
recording medium is taken into consideration, the total amount of
sodium as calculated as Na contained in the acicular hematite
particles is preferably not more than 45 ppm, more preferably not
more than 40 ppm, still more preferably not more than 35 ppm. The
lower limit is approximately 0.01 ppm, if the industrial
productivity is taken into consideration.
[0047] The storage stability of the acicular hematite particles
(the content of the soluble sodium salts calculated as Na contained
in the acicular hematite particles after left to stand for 14 days
under surroundings at a temperature of 60.degree. C. and a relative
humidity of 90%) is preferably not more than 40 ppm. If it exceeds
40 ppm, the fatty acids added to a coated film and the soluble
sodium salts react to generate metal salts of the fatty acids and
they gradually lower the electromagnetic performance of a magnetic
recording medium so that the storage stability is deteriorated.
Under high humidity surroundings, in particular, an efflorescence
phenomenon sometimes occurs on the surface of the magnetic
recording medium. If the storage stability of the magnetic
recording medium obtained is taken into consideration, it is
preferably not more than 35 ppm, more preferably not more than 30
ppm, still more preferably not more than 25 ppm. The lower limit
is, if the industrual productivity is taken into consideration,
approximately 0.01 ppm.
[0048] The average minor axial diameter of the acicular hematite
particles is preferably 0.0025 to 0.15 .mu.m and the aspect ratio
(ratio of average major axial diameter/average minor axial
diameter) is preferably not less than 2.
[0049] The average minor axial diameter is more preferably 0.01 to
0. 10 .mu.m for the same reasons as in the lower and upper limits
of the above-mentioned average major axial diameter.
[0050] The upper limit of the aspect ratio is preferably 20. If it
is more than 20, the particles twine in a vehicle upon the
preparation of a non-magnetic coating composition, which often
lowers dispersibility or increases viscosity. If it is less than 2,
the stiffness of a coated film obtained becomes insufficient. If
the dispersibility in the vehicle and the stiffness of the coated
film are taken into consideration, the aspect ratio is more
preferably 3 to 10.
[0051] The acicular hematite particles have preferably a higher
densification. The degree of densification is defined by the ratio
of S.sub.BET/S.sub.TEM in which the specific surface area S.sub.BET
is measured by a BET method and the surface area S.sub.STEM is
calcularated from the major axial diameter and the minor axial
diameter, which were measured from the particles in an electron
microscopic photograph, and have preferably a value of 0.5 to
2.5.
[0052] If the S.sub.BET/S.sub.TEM value is less than 0.5, the
acicular hematite particles are densified, but the particle size
increases because of sintering of the particles and thus a coated
film excellent in the surface smoothness can not be obtained. If it
is more than 2.5, the densification is not sufficient to allow many
pores to exist in the interiors or on the surfaces of the particles
so that the dispersibility in a vehicle may be insufficient. If the
dispersibility in the vehicle and the surface smoothness of the
coated film are taken into consideration, its value is preferably
0. 7 to 2.0, more preferably 0.8 to 1.6.
[0053] The hematite particles are low in resin desorption ratio and
if it is represented by a value obtained by the method as will be
described later, its value is preferably not more than 30%. If it
is more than 30%, the dispersibility in a vehicle and the
dispersion stability not only lower, but the stiffness of the
coated film lowers. If the dispersibility, dispersion stability in
the vehicle and the stiffness of the coated film are taken into
consideration, it is more preferably not more than 25%, still more
preferably not more than 20%.
[0054] The surfaces of the acicular hematite particles may, if
necessary, be coated with at least one selected from the group
consisting of an aluminium hydroxide, an aluminium oxide, a silicon
hydroxide and a silicon oxide (hereinafter referred to "coating
substance"). The accicular hematite particles coated are improved
in the dispersibility in the vehicle as compared with non-coated
ones.
[0055] The amount of the coating substance is preferably 0.01 to
50% by weight calculated as Al and/or SiO.sub.2 on the basis of the
acicular hematite particles. If it is less than 0.01% by weight,
the dispersibility-improving effect resulting from the coating is
difficult to be obtained, and if it is more than 50% by weight, the
coating effect arrives at the saturated level and thus the coating
more than necessary is meaningless. If the dispersibility in the
vehicle and the industrial productivity are taken into
consideration, it is more preferable 0.05 to 20% by weight.
[0056] The acicular hematite particles coated with the coating
substance have almost the same particle size, aspect ratio, BET
specific surface area, geometrical standard deviation,
S.sub.BET/S.sub.TEM and the total amount of sodium as the
non-coated acicular hematite particles.
[0057] Next, a magnetic recording medium according to the present
invention will be explained.
[0058] The magnetic recording medium according to the present
invention comprises a non-magnetic base film, a non-magnetic
undercoat layer formed on the non-magnetic base film and a magnetic
recording layer formed on the non-magnetic undercoat layer.
[0059] As the non-magnetic base film, synthetic resins such as
polyethylene terephthalate, polyethylene, polypropylene,
polycarbonate, polyethylene naphthalate, polyamide, polyamideimide
and polyimide, foil and plate of a metal such as aluminium and
stainless steel, and various kinds of paper, which are widely used
for the production of a magnetic recording medium can be used. The
thickness of the non-magnetic base film is preferably 1.0 to 300
.mu.m, more preferably 2.0 to 200 .mu.m, though variable depending
on the material employed. In the case of a magnetic disc,
polyethylene terephthalate is ordinarily used as the non-magnetic
base film, and its thickness is ordinarily 50 to 300 .mu.m,
preferably 60 to 200 .mu.m. In the case of a magnetic tape, when
polyethylene therephthalate is used, its thickness is ordinarily 3
to 100 .mu.m, preferably 4 to 20 .mu.m, and when polyethylene
naphthalate is used, its thickness is ordinarily 3 to 50 .mu.m,
preferably 4 to 200 .mu.m, and when the polyamide is used, its
thickness is ordinarily 1 to 100 .mu.m, preferably 3 to 7
.mu.m.
[0060] The non-magnetic undercoat layer of a magnetic recording
medium in the present invention comprises acicular hematite
particles which are particles for a non-magnetic undercoat layer
and a binder resin.
[0061] As the binder resin, various binder resins which are widely
used for the production of a magnetic recording medium can be
used.
[0062] Examples of the binder resin are vinyl chloride-vinyl
acetate copolymer, urethane resin, vinyl chloride-vinyl
acetate-maleic acid terpolymer, urethane elastomer,
butadiene-acrylonitrile copolymer, polyvinyl butyral, cellurose
derivatives such as nitrocellulose, polyester resin, synthetic
rubber resins such as polybutadiene, epoxy resin, polyamide resin,
polyisocyanate, electron beam-curing acryl urethane resin and
mixtures thereof. Each of these binder resins may contain a
functional group such as --OR, --COOH, --SO.sub.3M,
--OPO.sub.2M.sub.2 and --NH.sub.2, wherein M represents H, Na or K.
If the dispersibility of the acicular hematite particles is taken
into consideration, the binder resin containing --COOH or
--SO.sub.3M as the functional group is preferable.
[0063] The blending ratio of the acicular hematite particles and
the binder resin is 5 to 2000 parts by weight, preferably 100 to
1000 parts by weight based on 100 parts by weight of the binder
resin.
[0064] If the acicular hematite particles are less than 5 parts by
weight, the accicular hematite particles contained in a
non-magnetic coating composition is too short to obtain a
continuously dispersed layer of the acicular hematite particles,
and thus the surface smoothness and the stiffness of the
non-magnetic undercoat layer can not be said to be sufficient. If
it is more than 2000 parts by weight, the acicular hematite
particles are too plentiful with respect to the binder resin to
sufficiently disperse in a non-magnetic coating composition and as
a result, the coated film with the sufficient surface smoothness is
difficult to be obtained. Moreover, since the acicular hematite
particles are not bound adequately by the binder resin, the
obtained coated film is apt to be brittle.
[0065] The coated thickness of the non-magnetic undercoat layer is
preferably 0.2 to 10.0 .mu.m. If it is less than 0.2 .mu.m, the
surface roughness of the non-magnetic base film is not improved
sufficiently and the stiffness is also apt to be insufficient. If
the thinning of a magnetic recording medium and the stiffness of
the coated film are taken into consideration, the coated thickness
is more preferably 0.5 to 5.0 .mu.m.
[0066] Meanwlile, it is possible to add to the non-magnetic
undercoat layer, additives such as a lubricant, a polishing agent
and an antistatic agent which are ordinarily used for the
production of a magnetic recording medium.
[0067] The gloss of the non-magnetic undercoat layer using acicular
hematite particles, the surfaces of which are not coated with the
above-mentioned coating substance such as aluminium hydroxide is
190 to 300%, preferably 193 to 300%, more preferably 195 to 300%,
the surface roughness Ra thereof is 0.5 to 8.0 nm, preferably 0.5
to 7.5 nm, more preferably 0.5 to 7.0 nm, and the stiffness
(Young's modulus; relative value) thereof is 125 to 160, preferably
128 to 160.
[0068] The gloss of the non-magnetic undercoat layer using the
acicular hematite particles, which are coated with the
above-mentioned coating substance such as aluminium hydroxide is
192 to 300%, preferably 195 to 300%, more preferably 200 to 300%,
the surface roughness Ra thereof is 0.5 to 7.8 nm, preferably 0.5
to 7.0 nm, more preferably 0.5 to 6.8 nm, and the stiffness of the
coated film (Young's modulus; relative value) is 128 to 160,
preferably 130 to 160.
[0069] The magnetic recording layer in the present invention
comprises magnetic particles and a binder resin.
[0070] As the magnetic particles, Co-coated magnetic iron oxide
particles obtained by coating magnetic iron oxide particles such as
maghemite particles (.gamma.-Fe.sub.20.sub.3) and magnetite
particles (FeOx.multidot.Fe.sub.2O.sub.3, O<x.ltoreq.1) with Co
or Co and Fe, Co-coated magnetic iron oxide particles obtained by
adding the different kinds of elements other than iron, such as Co,
Al, Ni, P, Zn, Si, B and a rare earth element to the
above-mentioned Co-coated magnetic iron oxide particles, acicular
metal magnetic particles mainly containing iron, acicular
iron-based alloy magnetic particles containing elements other than
iron, such as Co, Al, Ni, P, Zn, Si and B, plate-like
magnetoplumbite ferrite particles containing Ba, Sr, Ba-Sr, and
plate-like magnetoplumbite ferrite particles containing one or more
of a coercive force-reducing agent selected from divalent and
tetravalent metals such as Co, Ni, Zn, Mn, Mg, Ti, Sn and Zr are
exemplified, and these may be used singly or in combination of two
or more.
[0071] Meanwhile, under the consideration of high-density recording
of recent magnetic recording medium, among the above megnetic
particles, the acicular metal magnetic particles mainly containing
iron and the acicular iron-based alloy magnetic particles
containing elements other than iron, such as Co, Al, Ni, P, Zn, Si,
B and a rare earth metal element are preferable, The average major
axial diameter of the magnetic particles (average particle size in
the case of a plate-like particle) is 0.01 to 0.50 .mu.m,
preferably 0.03 to 0.30 .mu.m. The shape of the magnetic particles
is acicular or plate-like. The word "acicular" herein means not
only an acicular shape literally, but a spindle shape and a rice
shape.
[0072] When the shape of the magnetic particles is acicular, the
aspect ratio is not less than 3, preferably not less than 5. If the
dispersibility in a vehicle upon the preparation of a magnetic
coating composition is taken into consideration, the upper limit is
approximately 15, preferably approximately 10.
[0073] When the shape of the magnetic particles is plate-like, the
plate ratio (ratio of an average particle size/an average
thickness) is not less than 2, preferably not less than 3. If the
dispersibility in a vehicle upon the preparation of a magnetic
coating composition, the upper limit is approximately 20,
preferably approximately 15.
[0074] As the magnetic properties, the coercive force is 500 to
3200 Oe, preferably 550 to 3200 Oe, the saturation magnetization is
50 to 170 emu/g, preferably 60 to 170 emu/g. If the properties such
as high-density recording of the magnetic recording medium are
taken into consideration, the coercive force is more preferably 900
to 3200 Oe and the saturation magnetization is more poferably 70 to
170 emu/g.
[0075] As the binder resin, the binder resin used for forming the
above-mentioned non-magnetic undercoat layer can be used.
[0076] The blending ratio of the magnetic particles and the binder
resin is 200 to 2000 parts by weight, preferably 300 to 1500 parts
by weight of the magnetic particles based on 100 parts by weight of
the binder resin.
[0077] The coated thickness of the magnetic recording layer formed
on the non-magnetic undercoat layer is 0.01 to 5.0 .mu.m. If it is
less than 0.01 m, the uniform coating is difficult and the uneven
coating tends to occur. If it is more than 5.0 .mu.m, an influence
of the self-demagnetization becomes large and the desired
electromagnetic performance is difficult to be obtained. The coated
thickness is preferably 0.05 to 1.0 .mu.m.
[0078] It is possible to add to the magnetic recording layer,
additives such as a lubricant, a polishing agnet and an antistatic
agent which are normelly used.
[0079] The magnetic recording medium according to the present
invention, when the Co-coated magnetic iron oxide particles are
used as the magnetic particles, has a coercive force of 500 to 1500
Oe, preferably 550 to 1500 Oe, a squareness (residual flux density
Br/saturation flux density Bm) of 0.85 to 0.95, preferably 0.86 to
0.95, a gloss of a coated film of 130 to 200%, preferably 140 to
200%, a surface roughness Ra of a coated film of not more than 12.0
nm, preferably 2.0 to 11.0 nm, more preferably 2.0 to 10.0 nm, a
Young's modulus (relative value to a commercially available video
tape: AV T-120 produced by Victor Company of Japan, Limited) of 125
to 160, preferably 130 to 160. As the electromagnetic performance,
the output at 4 MHz is not less than +0.5 dB as compared with that
of the standard tape obtained by using as the non-magnetic
particles for a non-magnetic undercoat layer, non-magnetic
particles other than those of the present invention and using the
same magnetic particles for a magnetic recording layer as in the
present invention, the output drop at 4 MHz after stored at a
temperature of 60.degree. C. and a relative humidity of 90% for 14
days is not more than 1.0 dB, and the stain on a head after
30-minute running of a magnetic tape is 2, preferably 1, according
to the evaluation method as will be described later.
[0080] The magnetic recording medium according to the present
invention, when the acicular metal magnetic particles mainly
containing iron or acicular iron-based alloy magnetic particles are
used as the magnetic particles, has a coercive force of 800 to 3200
Oe, preferably 900 to 3200 Oe, a squareness (residual flux density
Br/saturation flux density Bm) of 0.85 to 0.95, preferably 0.86 to
0. 95, a gloss of a coated film of 180 to 300%, preferably 190 to
300%, a surface roughness Ra of a coated film of not more than 12.0
nm, preferably 2.0 to 11.0 nm, more preferably 2.0 to 10.0 nm, a
Young's modulus (relative value to a commercially available video
tape: AV T-120 produced by Victor Company of Japan, Limited) of 124
to 160, preferably 128 to 160. As the electromagnetic performance,
the output at 7 MHz is not less than +0.5 dB as compared with that
of the standard tape obtained by using as the non-magnetic
particles for a non-magnetic undercoat layer, the non-magnetic
particles other than those of the present invention and using the
same magnetic particles for a magnetic recording layer as in the
present invention, the output drop at 7 MHz after stored at a
temperature of 60.degree. C. and a relative humidity of 90% for 14
days is not more than 1.0 dB, and the stain on a head after
30-minute running of a magnetic tape is 2, preferably 1, according
to the evaluation method as will be described later.
[0081] The magnetic recording medium according to the present
invention, when the plate-like magnetoplumbite ferrite particles
are used as the magnetic particles, has a coercive force of 800 to
3200 Oe, preferably 900 to 3200 Oe, a squareness (residual flux
density Br/saturation flux density Bm) of 0.85 to 0.95, preferably
0.86 to 0. 95, a gloss of the coated film of 160 to 300%,
preferably 170 to 300%, a surface roughness Ra of a coated film of
not more than 12.0 nm, preferably 2.0 to 11.0 nm, more preferably
2.0 to 10.0 nm, a Young's modulus (relative value to a commercially
available video tape: AV T-120 produced by Victor Company of Japan,
Limited) of 124 to 160, preferably 128 to 160. As the
electromagnetic performance, the output at 7 MHz is not less than
+0.5 dB as compared with that of the standard tape obtained by
using as the non-magnetic particles for a non-magnetic undercoat
layer, non-magnetic particles other than those of the present
invention and using the same magnetic particles for a magnetic
recording layer as in the present invention, the output drop at 7
MHz after stored at a temperature of 60.degree. C. and a relative
humidity of 90% for 14 days is not more than 1.0 dB, and the stain
on a head after 30-minute running of a magnetic tape is 2,
preferably 1, according to the evaluation method as will be
described later.
[0082] Next, the method for producing the acicular hematite
particles for a non-magnetic undercoat layer according to the
present invention will be explained.
[0083] The acicular hematite particles are obtained by dehydrating
by heating acicular goethite particles obtained by an ordinary
method to thus obtain acicular hematite particles, reducing the
obtained acicular hematite particles to thus obtain acicular
magnetite particles, removing by washing sodium deposited on the
surfaces, then oxidizing.
[0084] The goethite particles as the starting material in the
present invention can be obtained by an ordinary method, i.e.,
conducting an oxidation reaction by passing an oxygen-containing
gas such as air into a suspension containing iron-containing
presipitates such as iron hydroxide and iron carbonate obtained by
reacting an aqueous ferrous salt solution with an aqueous alkali
hydroxide solution, an aqueous alkali carbonate solution or an
aqueous alkali hydroxide and an alkali carbonate solution.
[0085] Meanwhile, it is possible to add, during the course of the
synthetic reaction of the acicular goethite particles, the
different elements such as Ni, Zn, P, Si and Al which are added to
enhance the characteristics such as the major axial diameter, minor
axial diameter and aspect ratio.
[0086] Prior to the reduction by heating, it is necessary to
conduct the coating treatment with a sintering-preventing agent.
The coating treatment is performed by adding the
sintering-preventing agent into an aqueous suspension containing
acicular goethite particles or low-density hematite particles
obtained by dehydrating by heating the acicular goethite particles
at a temperature of 250 to 500.degree. C., mixing by stirring,
followed by filtration, washing and drying.
[0087] As the sintering-preventing agent, it is possible to use
sintering-preventing agents which are ordinarily used. Examples are
phosphorus compounds such as sodium hexamethaphosphate,
polyphosphoric acid and orthophosphoric acid, silicon compounds
such as water glass #3, sodium orthosilicate, sodium metasilicate
and colloidal silica, boron compounds such as boric acid, aluminium
compounds such as aluminium acetate, aluminium sulfate, aluminium
chloride, aluminium nitrate and sodium aluminate, and titanium
compounds such as titanyl sulfate. Among these compounds,
orthophosphoric acid, colloidal silica, boric acid and aluminium
acetate are preferable. These are used singly or in combination of
two or more.
[0088] The coated amount of the sintering-preventing agent is
preferably 0.05 to 10% by weight, more preferably 0.1 to 5% by
weight in an amount calculated as P, SiO.sub.2, B, Al or Ti based
on the total weight of the particles.
[0089] The total sodium content calculated as Na of the acicular
goethite particles obtained is 600 to 3000 ppm, the content of
soluble sodium salts calculated as Na is 300 to 1500 ppm, the
average major axial diameter is 0.01 to 0.3 .mu.m, the average
minor axial diameter is 0.001 to 0.15 .mu.m, the aspect ratio is 3
to 25, the geometrical standard deviation of the major axial
diameter is not more than 1.5 and the BET specific surface area is
50 to 250 m.sup.2/g.
[0090] The acicular goethite particles coated with the
sintering-preventing agent is dehydrated by heating at a
temperature of 250 to 500.degree. C. to obtain the low-density
acicular hematite particles.
[0091] The low-density acicular hematite particles are 600 to 3000
ppm in total content of sodium salts calculated as Na, 500 to 2000
ppm in content of soluble sodium salts calculated as Na, 0.005 to
0.3 .mu.m in average major axial diameter, 0.0025 to 0.15 .mu.m in
average minor axial diameter, 3 to 20 in aspect ratio, not more
than 1.5 in geometrical standard deviation of the major axial
diameter, 70 to 350 m.sup.2/g in BET specific surface area, and 2.5
to 6. 0 in degree of densification S.sub.BET/S.sub.TEM.
[0092] If the heating temperature is lower than 250.degree. C., the
dehydration reaction takes a long time and thus it is not
preferable. If it is higher than 500.degree. C., the dehydration
reaction takes place rapidly to thus result in deformation in
particle shape and sintering among the particles, and thus it is
not preferable.
[0093] The acicular hematite particles obtained by the
heat-treatment are low-density particles having a lot of dehydrated
pores, which are dehydrated from the acicular goethite particles,
and the BET specific surface area is approximately 1.2 to 2 times
that of the acicular goethite particles as the starting
particles.
[0094] The low-density hematite particles obtained are then
subjected to the reduction treatment at a temperature of 250 to
600.degree. C. under a reducing atmosphere to thus form low-density
acicular magnetite particles so that sodium compounds contained in
the interiors of particles may be deposited on the surfaces of the
particles.
[0095] If the temperature is lower than 250.degree. C., the
reduction reaction takes a longer time and thus it is not
preferable. If it is higher than 600.degree. C., the reduction
reaction takes place rapidly to thus invite the deformation in
particle shape and sintering among the particles and thus it is not
preferable.
[0096] The low-density magnetite particles obtained are 600 to 3000
ppm in total amount of sodium calculated as Na, 600 to 3000 ppm in
soluble sodium salts calculated as Na, 0.01 to 0.3 .mu.m in average
major axial diameter, 0.005 to 0.15 .mu.m in average minor axial
diameter, 2 to 20 in aspect ratio, not more than 1.5 in geometrical
standard deviation of the major axial diameter, 40 to 250 m.sup.2
/g in BET specific surface area, and 2.5 to 5.0 in degree of
densification S.sub.BET/S.sub.TEM .
[0097] The low-density acicular magnetite particles obtained are
roughly pulverized into coarse particles by a dry method and formed
into a slurry. The slurry is then pulverized by a wet method to
thus remove the coarse particles. The wet method pulverization is
conducted by the use of ball mill, a sand grinder, a Daino mill, a
colloid mill or the like to such an extent that coarse particles of
not less than 44 .mu.m are not more than 10%, preferably not more
than 5%, more preferably 0%. If the coarse particles of not less
than 44 .mu.m remain in an amount of more than 10%, the sufficient
removal effect of the deposited sodium component at the washing
step is not obtained.
[0098] The low-density acicular magnetite particles obtained by
pulverizing the coarse particles by the wet pulverization method
are filtered and washed with pure water by an ordinary method and
the sodium components are removed by washing, then dried.
[0099] As the method for washing with water, any known methods
which are industrially used, such as a decantation method, a
dilution method using a filter thickner and a method of passing
water into a filter press are employed.
[0100] The low-density acicular magnetite particles after washing
with pure water are 50 to 1500 ppm in total sodium content
calculated as Na, 30 to 300 ppm in soluble sodium salts calculated
as Na, 0.01 to 0.3 .mu.m in average major axial diameter, 0.005 to
0.15 .mu.m in average minor axial diameter, 2 to 20 in aspect
ratio, not more than 1.5 in geometrical standard deviation of the
major axial diameter, 40 to 250 m.sup.2/g in BET specific surface
area, and 2.5 to 5. 0 in degree of densification
S.sub.SET/S.sub.TEM.
[0101] Then, the low-density acicular magnetite particles are
subjected to the oxidation reaction at a temperature of 650 to
850.degree. C., under an oxidizing atmosphere to thus obtain
high-density acicular hematite particles by way of acicular
maghemite particles.
[0102] If the temperature is lower than 650.degree. C., since the
acicular maghemite particles are mixed in the acicular hematite
particles, the obtained acicular particles have magnetism.
Moreover, the acicular hematite particles have a large number of
dehydrated pores inside the particles or on the surfaces of the
particles due to the insufficient densification. As a result, the
dispersibility in a vehicle is insufficient and a coated film with
the surface smoothness is difficult to be obtained. If the
temperature is higher than 850.degree. C., though the acicular
hematite particles are sufficiently densified, since sintering
among the particles takes place, the particle size increases and
thus a coated film with the surface smoothness is difficult to be
obtained.
[0103] The high-density acicular hematite particles are 50 to 1500
ppm in total sodium content calculated as Na, 50 to 1500 ppm in
soluble sodium salts calculated as Na, 0.005 to 0.3 .mu.m in
average major axial diameter, 0.0025 to 0.15 .mu.m in average minor
axial diameter, 2 to 20 in aspect ratio, not more than 1.5 in
geometrical standard deviation of the major axial diameter, 40 to
250 m.sup.2 /g in BET specific surface area, and 2.5 to 5.0 in
degree of densification S.sub.SET/S.sub.TEM.
[0104] The obtained high-density acicular hematite particles are,
in the same manner as in the washing step of the low-density
acicular magnetite particles, pulverized by a dry method and formed
into a slurry, thereafter pulverized by a wet method, filtered and
washed with pure water by an ordinary method, whereby the sodium
component deposited on the surfaces of the particles are removed by
washing and dried.
[0105] In order to improve the affinity with the binder resin to
enhance the dispersibility, the so obtained acicular hematite
particles may, if necessary, be coated with at least one selected
from an aluminium hydroxide, an aluminium oxide, a silicon
hydroxide and a silicon oxide.
[0106] The coating treatment is conducted by adding the aluminium
compound or the silicon compound or the both compounds to a
suspension containing the acicular hematite particles obtained by
washing with water after the oxidation reaetion, followed by mixing
and stirring, and further adjusting a pH value, if required,
followed by filtration, washing with water, drying and
pulverization. Deaeration and a compression treatment or the like
may be further conducted, if necessary.
[0107] As the aluminium compound in the present invention,
aluminium salts such as aluminium acetate, aluminium sulfate,
aluminium chloride and aluminium nitrate, aluminium compounds such
as aluminium hydroxide, aluminium oxide and alumina sol are
usable.
[0108] The amount of the aluminium compound calculated as Al is
0.01 to 50% by weight, preferably 0.05 to 20% by weight based on
the weight of the acicular hematite particles. If it is less than
0.01% by weight, the improving effect of dispersing in a vehicle is
not obtained. If it is more than 50% by weight, the
dispersibility-improving effect arrives at the saturation level and
thus addition more than necessary is meaningless.
[0109] As the silicon compound in the present invention, silicates
such as potassium silicate, silicon compounds such as silicon
hydroxide and silicon oxide and collidal silica are usable.
[0110] The amount of the silicon compound added calculated as
SiO.sub.2 is 0.01 to 50% by weight, preferably 0.05 to 20% by
weight based on the weight of the acicular hematite particles. If
it is less than 0.01% by weight, the improving effect of dispersing
in a vehicle is not obtained. If it is more than 50% by weight, the
dispersibility-improving effect arrives at the saturation level and
thus addition more than necessary is meaningless.
[0111] When the aluminium compound and the silicon compound are
mixed, the total amount calculated as Al and SiO.sub.2 is 0.01 to
50% by weight, more preferably 0.05 to 20% by weight.
[0112] Next, the method for producing a non-magnetic substrate for
a magnetic recording medium having a non-magnetic undercoat layer
in the present invention will be explained.
[0113] The non-magnetic substrate for a magnetic recording medium
in the present invention is obtained by coating a non-magnetic
coating composition containing acicular hematite particles, a
binder resin and a solvent, on a non-magnetic base film to thus
form a non-magnetic undercoat lyer thereon, and drying.
[0114] As the solvent, solvents which are ordinarily used for the
production of a magnetic recording medium such as methyl ethyl
ketone, toluene, cyclohexanone, methylisobutyl ketone and
tetranydrofuran are exemplified and these are used singly or in
combination of two or more.
[0115] The amount of the solvent in the non-magnetic coating
composition is preferably 50 to 95 parts by weight based on 100
parts by weight of the non-magnetic coating composition. If it is
less then 50 parts by weight, the viscosity of the non-magnetic
coating composition obtained becomes too high to make the coating
difficult. If it is more than 95 parts by weight, the volatile
amount of the solvent becomes too large and thus it is
disadvantageous industrially.
[0116] The non-magnetic coating composition in the present
invention is excellent in dispersion stability and the change ratio
in gloss showing the dispersion stability of the non-magnetic
coating composition, which is obtained by a measurement method as
will be mentioned later is not more than 5%.
[0117] Next, the method for producing a magnetic recording medium
according to the present invention will be explained.
[0118] The magnetic recording medium of the present invention is
obtained by coating a magnetic coating composition containing
magnetic particles, a binder resin and a solvent on the non-
magnetic substrate having the non-magnetic undercoat layer to thus
form a magnetic recording layer thereon, and drying.
[0119] As the solvent, the above-mentioned solvents as used in the
non-magnetic paint are usable.
[0120] The amount of the solvent is preferably 50 to 95 parts by
weight based on 100 parts by weight of the magnetic coating
composition. If it is less than 50 parts by weight, the viscosity
of the obtained magnetic coating composition becomes too high to
make the coating difficult. If it is more than 95 parts by weight,
the volatile amount of the solvent becomes too large and thus it is
disadvantageous industrially.
[0121] The most important point is that when the high-density
hematite particles having an excellent dispersibility in the
vehicle and containing not more than 50 ppm of the total content of
sodium are used as the non-magnetic particles for a non-magnetic
undercoat layer, it is possible to enhance the surface smoothness
and the strength of the non-magnetic undercoat layer due to the
excellent dispersibility in the binder resin, and that when a
magnetic recording layer is formed on the non-magnetic undercoat
layer, it is possible not only to form a thin layer having the
smooth surface and the uniform thickness, but to obtain a magnetic
recording medium having an excellent electromagnetic performance
and storage stability.
[0122] The reason why the total content of sodium contained in the
acicular hematite particles can be reduced to not more than 50 ppm
is presumably considered that since difficultly-soluble sodium
salts fixed to crystals inside the non-magnetic particles which can
not be removed by an ordinary washing deposit on the surfaces of
the particles with the modification of the crystal form and convert
into soluble sodium salts, it became possible to remove the sodium
by an ordinary washing with water.
[0123] The reason why the surface smoothness and the strength of
the non-magnetic undercoat layer are enhanced is presumably
considered that since it is possible to sufficiently remove by
washing with water the sodium salts which cause the high-density
hematite particles to aggregate by firmly crosslinking, the
aggregates are separated into substantially discrete particles, and
since in the vehicle, the adsorption of the binder resin onto the
surfaces of the acicular hematite particles is made directly
through elements other than the sodium element, the desorption of
the binder resin from the surfaces of the acicular hematite
particles decreases, so that the dispersibility of the acicular
hematite particles is improved.
[0124] The reason why the storage stability as well as the
electromagnetic performance of the magnetic recording medium is
enhanced is presumably considered that since it is possible to
reduce the total content of sodium in the acicular hematite
particles contained in the non-magnetic undercoat layer to not more
than 50 ppm, as a result, to reduce not only the suluble sodium
salts on the surfaces of the particles, but the difficultly-soluble
sodium salts contained in the interiors of the particles which
deposit on the surfaces of the particles as converted to the
soluble sodium salt for some causes such as moisture in air, the
metal salts of fatty acids synthesized by the reaction with fatty
acids added to a coated film can be reduced.
[0125] Next, the typical embodiment of the present invention will
be described.
[0126] The remaining amount of coarse particles on a sieve was
measured by passing through a 325 mesh-sieve (sieve opening:44
.mu.m) a slurry containing 100 g of the particles, the
concentration of which was prelimirarily measured after the
wet-pulverization, and weighing the particles which do not pass
through the sieve.
[0127] The average major axial diameter and the average minor axial
diameter of the particles are represented by an average values of
350 particles in an electron microscopic photograph (.times.30,000)
enlarged 4-fold in the longitudinal and transverse directions.
[0128] The aspect ratio of the particles was calculated from a
ratio of the average major axial diameter to the average minor
axial diameter.
[0129] The particle size distribution of the major axial diameter
of the particles is represented by a geometrical standard deviation
obtained by the following method. That is, the major axial
diameters of 350 particles in the enlarged electron microscopic
photograph were measured. The actual major axial diameters of the
particles and the accumulative number of particles were obtained
from the calculation on the basis of the measured values. In a
logarithmic-normal probability paper, the major axial diameters
were plotted at the same intervals on the abscissa and the
accumulative number of particles passed through the sieve belonging
to each interval of the major axial diameters was plotted by
percentage on the ordinate by a statistical technique. The major
axial diameters corresponding to the number of particles of 50% and
84.13%, respectively, were read from the graph, and the geometrical
standard deviation was determined from the following equation:
[0130] Geometrical standard deviation=(major axial diameter
corresponding to the accumulative number of particles of
84.13%)/(major axial diameter corresponding to the accumulative
number of particles of 50%) (geometrical average diameters)
[0131] As the geometrical standard deviation becomes close to 1,
the particle size distribution of the average axial diameter
becomes excellent.
[0132] The specific surface area was represented by a value
measured by a BET method (nitrogen adsorption method).
[0133] The total content of sodium contained in the particles was
measured by the following method. That is, 1.000 g of the particles
was charged into a 200 ml beaker. Then, 25 ml of a 12 mol/l
hydrochloric acid were added and the particles were dissolved by
heating with a lid of a watch glass put on the beaker. After
cooling, the contents were transferred to a 500 ml measuring flask
and pure water was added accurately to a 500 ml solution, and the
total content of sodium in the solution was measured by the use of
Industively Coupled Plasma Atomic Emission Spectrophotometer
manufactured by Seiko Instruments Inc.
[0134] The content of the soluble salts was measured by the
following method. That is, 5 g of the particles were charged into a
300 ml conical flask. 100 ml of pure water were added, heated and
boiled for 5 minutes, after stoppering, cooled to room temperature.
Then, pure water was added in an amount equivalent to the pure
water lost by boiling, and after stoppering, the contents in the
conical flask were shaked for 1 minute and left to stand for 5
minutes. The supernatant liquid was filtered by the use of a No. 5
C filter paper and the content of Na.sup.+ in the filtrate was
measured by the use of Inductively Coupled Plasma Atomic Emission
Spectrophotometer manufactured by Seiko Instruments Inc.
[0135] The storage stability of the acicular hematite particles was
represented by a content of the soluble salts (calculated as Na)
measured by the same method as mentioned above, after left to stand
at a temperature of 60.degree. C. and a relative humidity of 90%
for 14 days.
[0136] The degree of densification of the particles is represented
by a value of S.sub.BET/S.sub.TEM. The S.sub.BET is a specific
surface area measured by the above-mentioned BET method. The
S.sub.TEM is a value calculated from the following equation on the
assumption that a particle is a rectangular parallelepiped having
the average major axial diameter 1 cm and the average minor axial
diameter w cm which were measured from the particles in an electron
microscopic photograph:
S.sub.TEM
(m.sup.2/g)=[(41w+2w.sup.2)/1w.sup.2.multidot..rho..sub.p].times-
.10.sup.-4
[0137] wherein .rho..sub.p is the density of the hematite particle
and 5.2 g/cm.sup.3 was used.
[0138] The contents of Al, SiO.sub.2 and P were measured by the use
of the fluorescent X-ray analysis apparatus 3063 M-type
manufactured by Rigaku Denki Kogyo Co., Ltd., according to the
rules of fluorescent X-ray analysis of JIS K 0119.
[0139] The resin desorption ratio shows a desorbable degree of a
resin adsorbed onto the acicular hematite particles. As the resin
desorption ratio (%) measured by the following method becomes close
to zero, the resin becomes difficult to be desorbed from the
surfaces of the acicular hematite particles:
[0140] First, 10 g of the acicular hematite particles, 20.5 g of a
resin solution obtained by dissolving 0.5 g of the resin in 20 g of
a mixed solvent (methyl ethyl ketone/toluene/cyclohexanone=5/3/2),
and 100 g of 1 mm.PHI. glass beads were charged into a 140 ml glass
bottle and the contents were mixed and dispersed for 2 hours by the
use of a paint shaker.
[0141] Next, the paint composition obtained was taken and
introduced into a settling tube and centrifuged at 10000 rpm for 15
minutes to separate a supernatant liquid from a solid. The amount
of the resin contained in the supernatant liquid is weighed and the
amount of the resin adsorbed onto the particles Wa (mg/g) is
calcularated by deducting the measured amount from the amount of
the resin charged.
[0142] Next, the solid obtained by the centrifugation is evaporated
to dryness and the dried solid containing 5 g of the particles is
charged into a 140 ml glass bottle. 10 g of the avove-mentioned
mixed solvent and 50 g of 1 mm.PHI. glass beads are added, mixed
and dispersed for 2 hours by the used of the paint shaker.
[0143] The obtained contents are introduced into the settling tube
and centrifuged at 10000 rpm for 15 minutes to separate a
supernatant liquid from a solid. The amount of the resin desorbed
in the supernatant liquid We (mg/g) is weighed and the resin
dersorption ratio is calculated from the following equation:
Resin desorption ratio (%)=(We/Wa).times.100
[0144] The viscosity of the coating composition was measured at
25.degree. C. by the use of an E type Viscometer EMD-R manufactured
by Tokyo Keiki Co., Ltd., at a shear rate of D=1.92 sec.sup.-1.
[0145] The gloss was measured at an angle of incidence of
45.degree. C. by the use of "Glossmeter UGV-5D manufactured by Suga
Shikenki, Co., Ltd.
[0146] The dispersion stability of the non-magnetic coating
composition is represented by a change in gloss (%) of the coated
film measured by the following method. The smaller the change in
gloss, the more excellent the dispersion stability.
[0147] First, the coated film was formed using a non-magnetic
coating composition immediately after being dispesed and the angle
of incidence of 45.degree. gloss (Go) is measured. After the
non-magnetic coating composition is then left to stand for 60
minutes, the coated film is formed in the samd manner, the angle of
incidence of 45.degree. gloss (G) is measured, and the change in
gloss is measured by the following equation:
Change in gloss (%)=[(G.sub.o-G)/G.sub.o].times.100
[0148] The surface roughness Ra is represented by an average value
of the center-line average roughness of the coated film measured by
the use of "Surfcom-575A" manufactured by Tokyo Seimitsu Co.,
Ltd.
[0149] The stiffness of the coated film is obtained by measuring
the Young's modulus of the coated film by the use of "Autograph"
manufactured by Shimadzu Corp. The Young's modulus is represented
by a relative value with that of a commercially available video
tape "AV T-120" manufactured by Victor Company of Japan, Limited.
The higher the relative value, more favorable.
[0150] The magnetic properties were measured under an external
magnetic field of 10 KOe by the use of "Vibration Sample
Magnetometer VSM-15" manufactured by Toei Kogyo Co., Ltd.
[0151] The electromagnetic performance of the magnetic tape was
obtained by the magnetic tape using the coating composition
prepared by the prescription as will be described later, which was
cut to a 1/2 inch width, by the use of "Drumtester-BX-3168"
manufactured by BELDEX Co., Ltd.
[0152] In the case of the magnetic tape using the acicular magnetic
iron oxide particles as the magnetic particles, the electromagnetic
performance was represented by a relative value of the output
performance at a relative speed at 5.8 m/s between the magnetic
tape and a head and the recording frequency of 4 MHz with that of
each of the reference tape of comparative examples as will be
described later.
[0153] In the case of the magnetic tape using the acicular magnetic
particles mainly containing iron or the plate-like magnetoplumbite
ferrite particles as the magnetic particles, the electromagnetic
performance was represented by a relative value of the output
performance at a relative speed of 3.8 m/s between the magnetic
tape and a head and the recording frequency of 7 MHz with that of
each of the reference tapes of comparative examples as will be
described later.
[0154] The storage stability of the magnetic tape was represented
by a change width (drop width) in electromagnetic performance
measured before and after the storage in the same manner, in which
the electromagnetic performance after the storage was measured
after stored at a temperature of 60.degree. C. and a relative
humidity of 90% for 14 days.
[0155] The stain on a head after running of the magnetic tape was
made by four-rank evaluation according to the following criteria of
the visual observation of a stain on the head after the magnetic
tape was caused to run at a relative speed of 16 m/sec with a load
of 200 g for 30 minutes by the use of "Mediadurabilitytester
MDT-3000" manufactured by Steinberg Associates Co., Ltd:
[0156] 1: No stain is observed.
[0157] 2: Stain is slightly observed.
[0158] 3: Stain is observed.
[0159] 4: Stain is noticeably observed.
[0160] The thickness of the non-magnetic base film, the
non-magnetic undercoat layer and the magnetic recording layer
forming the magnetic recording medium were measured in the
following method:
[0161] That is, the film thickness (A) of a non-magnetic base film
is first measured by the use of a digital electromicrometer "K 351
C" manufactured by Anritsu Electric Co., Ltd. Next, film thickness
(B) of a nonmagnetic substrate obtained by forming a non-magnetic
undercoat layer on the non-magnetic base film (total of the
thickness of the non-magnetic base film and the thickness of the
non-magnetic undercoat layer) is measured in the same manner.
Moreover, the film thickness (C) of the magnetic recording medium
obtained by forming a magnetic recording layer on the non-magnetic
undercoat layer (total of the thickness of the non-magnetic base
film, the thickness of the non-magnetic undercoat layer and the
thickness of the magnetic recording layer) is measured in the same
manner.
[0162] Accordingly, the film thickness of the non-magnetic
undercoat layer is calculated from B minus A and the film thickness
of the magnetic recording layer is calculated from C minus B.
< Production of acicular goethite particles and low-density
acicular hematite particles >
[0163] 1500 g of acicular goethite particles obtained by using an
aqueous ferrous sulfate solution and an aqueous sodium carbonate
solution (average major axial diameter:0.213 .mu.m, average minor
axial diameter:0.0246 .mu.m, aspect ratio:8.7, BET specific surface
area(S.sub.BET):113.6 m.sup.2/g, degree of densification
S.sub.BET/S.sub.TEM:3.43, geometrical standard deviation in the
major axial diameter:1.36, total content of sodium calculated as
Na:1864 ppm, soluble sodium salt calculated as Na:301 ppm) were
suspended in water to form a slurry, and the solid concentration
was adjusted to 10 g/liter. 150 liters of the obtained slurry was
heated to 60.degree. C. and the pH value was adjusted to 10.0 by
addition of a 0.1 mol/liter aqueous KOH solution.
[0164] To the alkali slurry, 30 g of phosphoric acid as a
sintering-preventing agent was gradually added and aged for 30
minutes after the termination of addition. Then, a 0.5 mol/liter
aqueous acetic acid solution was added to thus adjust the pH value
to 6.0. Thereafter, the slurry was filtered, washed with water,
dried and pulverized by an ordinary method to thereby obtain
acicular goethite particles coated with the phosphorus compound.
The content of the phosphorus compound calculated as P in the
acicular goethite particles was 0.63% by weight based on the weight
of the acicular goethite particles.
[0165] 1300 g of the acicular goethite particles obtained were
charged into a stainless steel rotary furnace and dehydrated by
heating in air at 320.degree. C. for 30 minutes while rotating the
furnace to thereby obtain low-density acicular hematite
particles.
[0166] The low-density acicular hematite particles obtained had an
average major axial diameter of 0.171 .mu.m, an average minor axial
diameter of 0.0221 .mu.m, an aspect ratio of 7.7, a BET specific
surface area (S.sub.BET) of 141.6 m.sup.2/g, an S.sub.BET/S.sub.TEM
of 3.82, and a geometrical standard deviation in the major axial
diameter of 1.36. The total content of sodium (calculated as Na)
was 1871 ppm, the content of the soluble sodium salts (calculated
as Na) was 568 ppm and the content of the phosphorus compound
(calculated as P) was 0.69% by weight.
< Production of low-density acicular magnetite particles
>
[0167] 1100 g of the obtained low-density acicular hematite
particles were charged into the stainless steel rotary furnace and
reduced by heating in a hydrogen gas atmosphere at 450.degree. C.
for 120 minutes while rotating the furnace to thereby obtain
low-density acicular magnetite particles.
[0168] The low-density acicular magnetite particles obtained had an
average major axial diameter of 0.166 .mu.m, an average minor axial
diameter of 0.0232 .mu.m, an aspect ratio of 7.2, a BET specific
surface area (S.sub.BET) of 54.6 m.sup.2/g, an S.sub.BET/S.sub.TEM
of 1.53, and a geometrical standard deviation in the major axial
diameter of 1.37, The total content of sodium (calculated as Na)
was 1896 ppm, the content of the soluble sodium salts (calculated
as Na) was 1810 ppm and the content of the phosphorus compound
(calculated as P) was 0.70% by weight. < Washing of low-density
acicular magnetite particles with water >
[0169] After 1000 g of the low-density acicular magnetite particles
obtained were roughly pulverized by the use of a Nara pulverizer,
they were added into 10 liters of pure water and were encountered
for 60 minutes by a homomixer manufactured by Tokushu Kika Kogyo
Co., Ltd.
[0170] The obtained slurry of the low-density acicular magnetite
particles was then mixed and dispersed for one hour at an axial
rotation of 2000 rpm while being circulated by a horizontal SGM
(Dispamat SL manufactured by S. C. Adichem, Co., Ltd. The
low-density acicular magnetite particles in the slurry remaining a
325 mesh (sieve opening:44 .mu.m) was zero %. The slurry was washed
with water by a decantation method.
[0171] The washed slurry containing the low-density acicular
magnetite particles was filtered by the use of a filter press and
washed by passing pure water till the electric conductivity of a
filtrate being not more than 5 .mu.S, then dried by an ordinary
method, and pulverized to thereby obtain low-density acicular
magnetite particles.
[0172] The low-density acicular magnetite particles obtained had an
average major axial diameter of 0.163 .mu.m, an average minor axial
diameter of 0.0233 .mu.m, an aspect ratio of 7.0, a BET specific
surface area (S.sub.BET) of 53.2 m.sup.2/g, an S.sub.BET/S.sub.TEM
of 1.50, and a geometrical standard deviation the major axial
diameter of 1.36. The total content of sodium (calculated as Na)
was 140 ppm, the content of the soluble sodium salt (calculated as
Na) was 52 ppm and the content of the phosphorus compound
(calculated as P) was 0.70% by weight. < Production of
high-density acicular hematite particles >
[0173] Next, 800 g of the washed low-density acicular magnetite
particles were charged into a ceramic rotary furnace and oxidized
by heating in the air at 730.degree. C. for 30 minutes while
rotating the furnace to thereby obtain high-density acicular
hematite particles.
[0174] The high-density acicular hematite particles obtained had an
average major axial diameter of 0.159 .mu.m, an average minor axial
diameter of 0.0235 .mu.m, an aspect ratio of 6.8, a BET specific
surface area (S.sub.BET) of 50.0 m.sup.2/g, an S.sub.BET/S.sub.TEM
of 1.42, and a geometrical standard deviation in the major axial
diameter of 1.37. The total content of sodium (calculated as Na)
was 138 ppm, the content of the soluble sodium salts (calculated as
Na) was 126 ppm and the content of the phosphorus compound
(calculated as P) was 0.70% by weight. < Washing of high-density
acicular hematite particles with water >
[0175] After 800 g of the high-density acicular hematite particles
obtained were roughly pulverized by the use of a Nara pulverizer,
they were added into 8 liters of pure water and were encountered
for 60 minutes by a homomixer manufactured by Tokushu Kika Kogyo
Co., Ltd.
[0176] The obtained slurry of the high-density acicular hematite
particles was then mixed and dispersed for one hour at an axial
rotation of 2000 rpm while being circulated by a horizontal SGM
(Dispamat SL manufactured by S. C. Adichem, Co., Ltd. The
high-density acicular hematite particles in the slurry remaining a
325 mesh (sieve opening:44 .mu.m) was zero %. The slurry was washed
with water by a decantation method. To be accurate, the slurry
concentration at this point was measured and confirmed to be 96
g/liter.
[0177] The washed slurry containing the high-density acicular
hematite particles was filtered by the use of a filter press and
washed by passing pure water till the electric conductivity of a
filtrate being not more than 5 .mu.S, then dried by an ordinary
method, and pulverized to thereby obtain high-density acicular
hematite particles.
[0178] The high-density acicular hematite particles obtained had an
average major axial diameter of 0.158 .mu.m, an average minor axial
diameter of 0.0228 .mu.m, an aspect ratio of 6.9, a BET specific
surface area (S.sub.BET) of 50.2 m.sup.2/g, an S.sub.BET/S.sub.TEM
of 1.39, a geometrical standard deviation in the major axial
diameter of 1.37, and a resin desorption ratio of 8.6%. The total
content of sodium (calculated as Na) was 19 ppm, the content of the
soluble sodium salts (calculated as Na) was 8 ppm, the storage
stability under a high temperature and a high relative humidity
(soluble sodium salts calculated as Na) was 9 ppm, and the content
of the phosphorus compound (calculated as P) was 0.69% by
weight.
< Production of a non-magnetic undercoat layer >
[0179] The obtained high-density acicular hematite particles, a
binder resin and a solvent were mixed and kneaded at a solid
concentration of 75% by weight by the use of a plast mill for 30
minutes. Thereafter, a given amount of the kneaded mixture was
taken out and charged into a glass bottle together with glass beads
and solvents, then mixed and despersed for 6 hours by a pain-
conditioner.
[0180] The non-magnetic coating composition obtained was given
below:
1 Acicular hematite particles 100 parts by weight Vinyl
chloride-vinyl acetate 10 parts by weight copolymer resin
containing sodium sulfonate groups Polyurethane resin containing 10
parts by weight sodium sulfonate groups Cyclohexanone 44.6 parts by
weight Methyl ethyl ketone 111.4 parts by weight Toluene 66.9 parts
by weight
[0181] The obtained non-magnetic coating composition was applied by
an applicator to a 14 .mu.m-thick polyethylene terephthalate film
to a thickness of 55 .mu.m and dried to thus form a non-mognetic
undercoat layer.
[0182] The thickness of the non-magnetic undercoat layer was 3.5
.mu.m.
[0183] The gloss of the non-magnetic undercoat layer was 216%, the
surface roughness Ra was 5.6 nm and the Yung's modulus was 135.
[0184] Moreover, after the non-mognet coating composition was left
to stand for 60 minutes, the undercoat layer was formed on a 14
.mu.m-thick polyethylen terephthalate film in the same manner as
above. The gloss of the non-magnet undercoat layer was 213% and the
change in gloss showing the dispersibility of the non-magnetic
coating composition was 1.4%.
< Production of magnetic recording medium >
[0185] Acicular metal magnetic particles mainly containing iron
(average major axial diameter:0.103 .mu.m, average minor axial
diameter:0.0152 .mu.m, aspect ratio:6.8, coercive force:1910 Oe,
saturation magnetization:136 emu/g), a binder resin and a solvent
were mixed and kneaded at a solid concentration of 78% by weight by
the use of a plast mill for 30 minutes. The kneaded mixture was
charged into a glass bottle together with glass beads and solvents,
then mixed and dispersed for 6 hours by a paint conditioner.
[0186] Thereafter, a polishing agent, a lubricant and a hardener
were further added and the mixture was further mixed and dispersed
for 15 minutes. The composition of the magnetic coating composition
obtained was given below:
2 Acicular metal magnetic particles 100 parts by weight mainly
containing iron Vinyl chloride-vinyl acetate 10 parts by weight
copolymer resin containing sodium sulfate groups Polyurethane resin
containing 10 parts by weight sodium sulfate groups Polishing agent
(AKP-30 manufactured 10 parts by weight by Sumitomo Chemical Corp)
Carbonblack (#3250 B manufactured 1 part by weight by Mitsubishi
Chemical Corp.) Lubricant (myristic acid: 3 parts by weight butyl
stearate = 1:2) Hardener (polyisocyanate) 5 parts by weight
Cyclohexanone 64.9 parts by weight Methyl ethyl ketone 162.2 parts
by weight Toluene 97.3 parts by weight
[0187] The magnetic coating composition obtained was applied by an
applicator to the non-magnetic undercoat layer to a thickness of 15
.mu.m and oriented and dried in a magnetic field, then calendered.
The magnetic recording medium was subjected to a curing reaction at
60.degree. C. for 24 hours and the multilayered film was slit into
a width of 0.5 inch to thereby obtain a magnetic tape.
[0188] The thickness of the magnetic recording layer was
1.0.mu.m.
[0189] The coercive force Hc of the obtained magnetic tape was 1980
Oe, the squareness (Br/Bm) was 0.87, the gloss was 236%, the
surface roughness Ra was 5.7 nm and the Young's modulus was 137.
The electromagnetic performance at a relative speed between the
magnetic tape and a head of 3.8 m/s and a recording frequency of 7
MHz was +1.1 dB when the magnetic tape obtained by Comparative
Example 34 as will be described later was used as the reference
tape. The drop width of the electromagnetic performance at a
recording frequency of 7 MHz after stored at a temperature of
60.degree. C. and a relative humidity of 70% for 14 days was 0.1
dB.
[0190] The stain on the head after running of the magnetic tape for
30 minutes was 1.
[0191] Hereinafter, the present invention will be explained in more
detail by way of examples and comparative examples, which in no way
limit the scope of the present invention.
< Kind of acicular goethite particles >
[0192] As the starting material for the production of acicular
hematite particles, the goethite particles described in the
above-mentioned embodiment and acicular goethite particles 1 to 3
set forth in Table 1 were prepared.
3 TABLE I Characteristics of acicular goethite particles Total
Content of Average Average content of soluble major minor
Geometrical sodium sodium salts axial axial Aspect standard
S.sub.BET/ (calculated (calculated Kind of acicular diameter
diameter ratio deviation S.sub.BET S.sub.TEM S.sub.TEM as Na) as
Na) goethite particles (.mu.m) (.mu.m) (-) (-) (m.sup.2/g)
(m.sup.2/g) (-) (ppm) (ppm) Particles described in 0.213 0.0246 8.7
1.36 113.6 33.1 3.43 1,864 301 the embodiment Goethite particles 1
0.153 0.0188 8.1 1.33 171.1 43.4 3.94 1,538 416 Goethite particles
2 0.186 0.0201 9.3 1.35 152.6 40.3 3.78 1,965 513 Goethite
particles 3 0.265 0.0278 9.5 1.32 83.2 29.1 2.86 2,562 458
< Production of low-density acicular hematite particles >
[0193] Particles to be treated (Precursors) 1 to 4, Comparative
Example 1
[0194] Acicular low-density hematite particles to be treated were
obtained in the same manner as in the above-mentioned embodiment
except that the kind of acicular goethite particles, the kind of
sintering-preventing agents and amounts thereof added, heating and
dehydration temperatures and times were veried.
[0195] The main production conditions and the characteristics are
shown in Table 2 and Table 3.
4TABLE 2 Heating and Kind of Sintering-preventing treatment
dehydration Precursor acicular Cal- Amount treatment and goethite
cu- added Temp. Time Comp.Ex. Particles Kind lated (wt. %)
(.degree. C.) (min) Precursor 1 Partictes Phosphoric P 1.51 320 30
described acid in the embo- diment Precursor 2 Goethite Colloidal
SiO.sub.2 2.03 340 30 particles 1 silica Precursor 3 Goethite
Phosphoric P 1.01 310 30 particles 2 acid Precursor 4 Goethite
Colloidal SiO.sub.2 3.06 370 30 particles 3 silica Comp.Ex.1
Particles Colloidal SiO.sub.2 1.03 340 30 described silica in the
embo- diment
[0196]
5 TABLE 3 Characteristics of low-density acicular hematite
particles Average Average Amount major minor Geometrical of
sintering- Total Content Desorption Precursor axial axial standard
Aspect S.sub.BET/ preventing agent content of of soluble ratio of
and diameter diameter deviation ratio S.sub.BET S.sub.TEM S.sub.TEM
Content sodium sodium salts resin Comp. Ex. (.mu.m) (.mu.m) (-) (-)
(m.sup.2/g) (m.sup.2/g) (-) Calculated (wt. %) (ppm) (ppm) (%)
Precursor 1 0.173 0.0229 1.36 7.6 146.8 35.8 4.10 P 1.64 1,888 613
68.3 Precursor 2 0.118 0.0185 1.33 8.0 192.1 44.2 4.35 SiO.sub.2
2.24 1,546 712 71.2 Precursor 3 0.175 0.0196 1.35 8.9 168.3 41.4
4.06 P 1.11 1,983 583 68.6 Precursor 4 0.246 0.0268 1.33 9.2 110.0
30.3 3.63 SiO.sub.2 3.34 2,580 512 69.2 Comp. Ex. 1 0.173 0.0230
1.36 7.5 132.5 35.7 3.71 SiO.sub.2 1.13 1,872 588 69.3
< Production of low-density acicular magnetite particles
>
[0197] Particles to be treated (Precursors) 5 to 8, Comparative
Examples 2 to 4
[0198] Acicular low-density magnetite particles were obtained in
the same manner as in the above-mentioned embodiment except that
the kind of particles to be treated, heating and reduction
temperatures and times were veried.
[0199] The main production conditions and the characteristics are
shown in Table 4 and Table 5.
6 TABLE 4 Heating and reduction Precursor and Kind of Particles to
Temp. Time Com.Ex. be treated Atmosphere (.degree. C.) (min)
Precursor 5 Precursor 1 Hydrogen gas 480 120 Precursor 6 Precursor
2 Hydrogen gas 400 180 Precursor 7 Precursor 3 Hydrogen gas 410 160
Precursor 8 Precursor 4 Hydrogen gas 500 100 Comp.Ex.2 Comp.Ex.1
Hydrogen gas 450 120 Comp.Ex.3 Comp.Ex.1 Hydrogen gas 650 120
Comp.Ex.4 Comp.Ex.1 Hydrogen gas 200 120
[0200]
7 TABLE 5 Characteristics of low-density acicular magnetite
particles Average Average Amount major minor Geometrical of
sintering- Total Content Desorption Precursor axial axial standard
Aspect S.sub.BET/ preventing agent content of of soluble ratio of
and diameter diameter deviation ratio S.sub.BET S.sub.TEM S.sub.TEM
Content sodium sodium salts resin Comp. Ex. (.mu.m) (.mu.m) (-) (-)
(m.sup.2/g) (m.sup.2/g) (-) Calculated (wt. %) (ppm) (ppm) (%)
Precursor 5 0.168 0.0230 1.36 7.3 53.8 35.7 1.51 P 1.67 1,879 1,792
78.8 Precursor 6 0.142 0.0187 1.34 7.6 63.6 43.8 1.45 SiO.sub.2
2.28 1,551 1,474 76.6 Precursor 7 0.170 0.0195 1.35 8.7 52.6 41.7
1.26 P 1.15 1,988 1,927 72.1 Precursor 8 0.211 0.0266 1.33 9.1 43.8
30.5 1.44 SiO.sub.2 3.37 2,590 2,492 76.1 Comp. Ex. 2 0.170 0.0228
1.37 7.5 51.6 36.0 1.43 SiO.sub.2 1.15 1,880 1,821 73.2 Comp. Ex. 3
0.140 0.0503 1.83 2.8 27.6 18.0 1.53 SiO.sub.2 1.15 1,893 1,850
78.6 Comp. Ex. 4 0.170 0.0230 1.36 7.4 116.5 35.7 3.26 SiO.sub.2
1.14 1,886 712 88.9
< Washing of low-density acicular magnetite particles with water
>
[0201] Particles to be treated (Precursors) 9 to 12, Comparative
Examples 5 to 7
[0202] Acicular low-density magnetite particles were obtained in
the same manner as in the above-mentioned embodiment except that
the kind of particles to be treated and presence or absence of a
wet- pulverization were veried.
[0203] The main production conditions and the characteristics are
shown in Table 6 and Table 7.
8 TABLE 6 Wet-pulverization Precursor Remaining amount and Kind of
perticles to Presence on sieve Comp. Ex. be treated or absence (wt.
%) Precursor 9 Precursor 5 Presence 0 Precursor 10 Precursor 6
Presence 0 Precursor 11 Precursor 7 Presence 0 Precursor 12
Precursor 8 Presence 0 Comp.Ex.5 Comp.Ex.2 Presence 0 Comp.Ex.6
Comp.Ex.3 Presence 0 Comp.Ex.7 Comp.Ex.4 Presence 0
[0204]
9 TABLE 7 Characteristics of low-density acicular magnetite
particles after washing with water Average Average Amount major
minor Geometrical of sintering- Total Content Desorption Precursor
axial axial standard Aspect S.sub.BET/ preventing agent content of
of soluble ratio of and diameter diameter deviation ratio S.sub.BET
S.sub.TEM S.sub.TEM Content sodium sodium salts resin Comp. Ex.
(.mu.m) (.mu.m) (-) (-) (m.sup.2/g) (m.sup.2/g) (-) Calculated (wt.
%) (ppm) (ppm) (%) Precursor 9 0.168 0.0230 1.36 7.3 52.9 35.7 1.48
P 1.67 128 43 38.6 Precursor 10 0.113 0.0186 1.34 7.7 63.9 44.0
1.45 SiO.sub.2 2.27 139 65 41.2 Precursor 11 0.170 0.0194 1.35 8.8
53.1 41.9 1.27 P 1.16 99 40 32.6 Precursor 12 0.241 0.0265 1.33 9.1
43.2 30.6 1.41 SiO.sub.2 3.38 180 63 36.8 Comp. Ex. 5 0.170 0.0228
1.36 7.5 51.6 36.0 1.43 SiO.sub.2 1.16 120 63 40.3 Comp. Ex. 6
0.139 0.0504 1.85 2.8 27.3 18.0 1.51 SiO.sub.2 1.15 98 55 45.6
Comp. Ex. 7 0.169 0.0229 1.36 7.4 117.1 35.9 3.26 SiO.sub.2 1.15
1,287 113 76.9
< Production of high-density acicular hematite particles
>
[0205] Particles to be treated (Precursors) 13 to 16, Comparative
Examples 8 to 11
[0206] Acicular high-density hematite particles were obtained in
the same manner as in the above-mentioned embodiment except that
the kind of particles to be treated and heating and oxidation
temperatures and times were veried.
[0207] The main production conditions and the characteristics are
own in Table 8 and Table 9.
10TABLE 8 Precursor Heating and oxidation and Comp. Kind of
particles Temp. Time Ex. to be treated Atmosphere (.degree. C.)
(min) Precursor 13 Precursor 9 Air 750 30 Precursor 14 Precursor 10
Air 700 60 Precursor 15 Precursor 11 Air 730 60 Precursor 16
Precursor 12 Air 780 30 Comp. Ex.8 Comp. Ex.5 Air 750 30 Comp. Ex.9
Comp. Ex.5 Air 880 30 Comp. Ex.10 Comp. Ex.5 Air 500 30 Comp. Ex.11
Comp. Ex.4 Air 700 30
[0208]
11 TABLE 9 Characteristics of high-density acicular hematite
particles Average Average Amount major minor Geometrical of
sintering- Total Content Desorption Precursor axial axial standard
Aspect S.sub.BET/ preventing agent content of of soluble ratio of
and diameter diameter deviation ratio S.sub.BET S.sub.TEM S.sub.TEM
Content sodium sodium salts resin Comp. Ex. (.mu.m) (.mu.m) (-) (-)
(m.sup.2/g) (m.sup.2/g) (-) Calculated (wt. %) (ppm) (ppm) (%)
Precursor 13 0.163 0.0231 1.37 7.1 50.8 35.7 1.42 P 1.67 128 100
51.6 Precursor 14 0.139 0.0187 1.35 7.4 60.6 43.9 1.38 SiO.sub.2
2.28 136 112 48.2 Precursor 15 0.164 0.0196 1.35 8.4 49.8 41.6 1.20
P 1.16 96 93 41.6 Precursor 16 0.234 0.0268 1.34 8.7 42.6 30.3 1.40
SiO.sub.2 3.39 182 139 56.8 Comp. Ex. 8 0.170 0.0229 1.37 7.4 51.8
35.9 1.44 SiO.sub.2 1.16 122 100 48.9 Comp. Ex. 9 0.121 0.0401 1.86
3.0 31.6 22.4 1.41 SiO.sub.2 1.15 101 89 39.6 Comp. Ex. 0.169
0.0230 1.36 7.3 118.0 35.7 3.30 SiO.sub.2 1.15 121 412 71.6 10
Comp. Ex. 0.242 0.0269 1.34 9.0 44.4 30.2 1.47 SiO.sub.2 3.39 1,883
1,471 82.6 11
< Washing of high-density acicular hematite particles with water
>
[0209] Examples 1 to 4, Comparative Example 12
[0210] Acicular high-density hematite particles after washing with
water were obtained in the same manner as in the above-mentioned
embodiment except that the kind of particles to be treated and
presence or absence of a wet-pulverization were veried.
[0211] The main production conditions and the characteristics are
shown in Table 10 and Table 11.
12 TABLE 10 Wet-pulverization Remaining Presence amount Example and
Kind of particles or on sieve Comp. Ex. to be treated absence (wt.
%) Example 1 Precursor 13 Presence 0 Example 2 Precursor 14
Presence 0 Example 3 Precursor 15 Presence 0 Example 4 Precursor 16
Presence 0 Comp. Ex.12 Comp. Ex.11 Presence 0
[0212]
13 TABLE 11 Characteristics of high-density acicular hematite
particles after washing with water Content Content of Average
Average Amount of Total of soluble Desorp- Example major minor
Geometrical sintering- content soluble sodium salts tion and axial
axial standard Aspect S.sub.BET/ preventing agent of sodium after
passage ratio of Comp. diameter diameter deviation ratio S.sub.BET
S.sub.TEM S.sub.TEM Calcu- Content sodium salts of time*1) resin
Ex. (.mu.m) (.mu.m) (-) (-) (m.sup.2/g) (m.sup.2g) (-) lated (wt.
%) (ppm) (ppm) (ppm) (%) Example 1 0.163 0.0230 1.37 7.1 50.3 35.8
1.40 P 1.67 21 6 7 8.2 Example 2 0.139 0.0187 1.34 7.4 61.1 43.9
1.39 SiO.sub.2 2.28 32 13 15 7.6 Example 3 0.163 0.0195 1.35 8.4
48.9 41.8 1.17 P 1.16 8 6 6 6.8 Example 4 0.234 0.0268 1.34 8.7
42.1 30.3 1.39 SiO.sub.2 3.39 44 26 28 12.3 Comp. 0.242 0.0270 1.35
9.0 43.6 30.1 1.45 SiO.sub.2 3.35 365 43 165 36.8 Ex. 12
*.sup.1)Soluble sodium salts (calculated as Na) contained in
particles after left to stand at a temperature of 60.degree. C. and
a relative humidity of 90% for 14 days.
< Coating treatment of high-density acicular hematite particles
>
[0213] Example 5
[0214] After 700 g of the high-density acicular hematite particles
obtained in Example 1 were roughly pulverized by the use of a Nara
pulverizer, they were added into 7 liters of pure water and were
encountered for 60 minutes by a homomixer manufactured by Tokushu
Kika Kogyo Co., Ltd.
[0215] The obtained slurry of the high-density acicular hematite
particles was then mixed and dispersed for 6 hours at an axial
rotation of 2000 rpm while being circulated by a horizontal SGM
(Dispamat SL manufactured by S. C. Adichem, Co., Ltd. The pH value
of the slurry obtained was adjusted to 4.0 by the use of an aqueous
0.1 mol/liter acetic acid solution and pure water was added into
this slurry to adjust the slurry concentration to 96 g/liter. 5
liters of this slurry were heated to 60.degree. C. and 266 ml of an
aqueous 1.0 mol/liter aluminium acetate solution (corresponding to
1. 5% by weight calculated as Al to the high-density acicular
hematite particles) were added into this slurry and maintained for
30 minutes, then the pH value of the slurry was adjusted to 7.0 by
the use of an aqueous 0.1 mol/liter sodium hydroxide solution. The
slurry was maintained for 30 minutes, then filtered by the use of a
filter press and washed by passing pure water till the electric
conductivity of a filtrate being not more than 5 .mu.S in the same
manner as in the above embodiment, then dried and pulverized to
thereby obtain high-density acicular hematite particles, the
surfaces of which were coated with aluminium hydroxide.
[0216] The main production conditions and the characteristics are
shown in Table 12 and Table 13.
[0217] Examples 6 to 8
[0218] Acicular hematite particles were obtained in the same manner
as in Example 5, except that the kind of acicular hematite
particles, the kind and the amount of the coating substances were
varied.
[0219] The main production conditions and the characteristics are
shown in Table 12 and Table 13.
14 TABLE 12 Surface treatment Substances coated Kind of acicular
Amounted added Amount hematite (Al or SiO.sub.2) coated Example
particles Kind (wt. %) Kind.sup.*2) Calculated (wt. %) Example 5
Example 1 Aluminium acetate 1.5 A Al 1.48 Example 6 Example 2
Colloidal silica 1.0 S SiO.sub.2 0.98 Example 7 Example 3 Aluminium
acetate 3.0 A Al 2.91 Example 8 Example 4 Aluminium acetate 2.0 A
Al 1.98 Colloidal silica 0.5 S SiO.sub.2 0.48 .sup.*2)A: aluminium
hydroxide S: silicon oxide
[0220]
15 TABLE 13 Characteristics of high-density acicular hematite
particles washed with water after surface treatment Content Content
of Average Average Amount of Total of soluble Desorp- Example major
minor Geometrical sintering- content soluble sodium salts tion and
axial axial standard Aspect S.sub.BET/ preventing agent of sodium
after passage ratio of Comp. diameter diameter deviation ratio
S.sub.BET S.sub.TEM S.sub.TEM Calcu- Content sodium salts of
time*1) resin Ex. (.mu.m) (.mu.m) (-) (-) (m.sup.2/g) (m.sup.2g)
(-) lated (wt. %) (ppm) (ppm) (ppm) (%) Example 5 0.163 0.0230 1.37
7.1 51.1 35.8 1.43 P 1.65 18 3 3 3.4 Example 6 0.139 0.0188 1.34
7.4 62.1 43.7 1.42 SiO.sub.2 2.26 35 6 6 2.8 Example 7 0.163 0.0191
1.35 8.4 48.1 42.0 1.14 P 1.13 10 8 8 4.1 Example 8 0.234 0.0268
1.34 8.7 41.6 30.3 1.37 SiO.sub.2 3.32 42 21 21 3.0 *.sup.1)Soluble
sodium salts (calculated as Na) contained in particles after left
to stand at a temperature of 60.degree. C. and a relative humidity
of 90% for 14 days.
[0221] Comparative Example 13
[0222] 1500 g of acicular goethite particles (average major axial
diameter:0.248 .mu.m, average minor axial diameter:0.0306 .mu.m,
aspect ratio:8.1, BET specific surface area (S.sub.BET):86.5
m.sup.2 /g, degree of densification S.sub.BET/S.sub.TEM:3.24,
geometrical standard deviation of the major axial diameter: 1.53,
total alkali content (total amount of Na calculated and K
calculated):113 ppm, soluble sodium salt content (Na calculated):21
ppm) obtained by using an aqueous ferrous sulfate solution and an
aqueous ammonia solution were suspended in water to form a slurry.
To the slurry, 15 g of phosphoric acid were added as a
sintering-preventing agent, followed by filtration, washing with
water, drying and pulverization by an ordinary method to thereby
obtain acicular goethite particles with their surfaces coated with
the phosphorus compound. The acicular goethite particles contained
0.32 % by weight of the phosphorus compound calculated as P.
[0223] 1300 g of the acicular goethite particles obtained were then
charged into a ceramic rotary furnace and oxidized by heating in
air at 650.degree. C. for 30 minutes while rotating the
furnace.
[0224] The acicular hematite particles obtained had an average
major axial diameter of 0.2360 .mu.m, an average minor axial
diameter of 0.0311 .mu.m, an aspect ratio of 7.8, a BET specific
surface area (S.sub.BET) of 40.8 m.sup.2 /g, an S.sub.BET/S.sub.TEM
of 1.54, a geometrical standard deviation of the major axial
diameter of 1.37, a total alkali content (total amount of Na
calculated and K calculated) of 102 ppm, a soluble sodium salt
content (calculated as Na)of 21 ppm, a storage stability (soluble
sodium salt calculated as Na) under a high temperature and a high
humidity of 98 ppm and a content of the phosphorus compound
calculated as P of 0.35% by weight.
[0225] Comparative Example 14
[0226] 1500 g of acicular goethite particles (average major axial
diameter:0.236 .mu.m, average minor axial diameter:0.0300 .mu.m,
aspect ratio:7.9, BET specific surface area (S.sub.BET):93.1
m.sup.2/g, degree of densification S.sub.BET/S.sub.TEM:3.4,
geometrical standard deviation of the major axial diameter:1.37,
total sodium content (calculated as Na):1912 ppm, soluble sodium
salt content (calculated as Na):296 ppm) obtained by using an
aqueous ferrous sulfate solution and an aqueous sodium carbonate
solution were suspended in water to form a slurry. To the slurry,
20 g of phosphoric acid were added as a sintering-preventing agent,
and the mixture was filtered by an ordinary method and washed with
water by passing deionized water till the electric conductivity of
a filtrate becomes not more than 1 .mu.S, followed by drying and
pulverization to thereby obtain acicular goethite particles with
their surfaces coated with the phosphorus compound. The acicular
goethite particles contained 0.41% by weight of the phosphorus
compound calculated as P.
[0227] 1300 g of the acicular goethite particles obtained were then
charged into a ceramic rotary furnace and oxidized by heating in
air at 700.degree. C. for 30 minutes while rotating the
furnace.
[0228] The acicular hematite particles obtained had an average
major axial diameter of 0.206 .mu.m, an average minor axial
diameter of 0.0309 2 .mu.m, an aspect ratio of 6.7, a BET specific
surface area (S.sub.BET) of 41.1 m.sup.2/g, an S.sub.BET/S.sub.TEM
of 1.54, a geometrical standard deviation of the major axial
diameter of the major axial diamer of 1. 38, a total sodium content
(Na calculated) of 1480 ppm, a soluble sodium salt content
(calculated as Na)of 116 ppm, a storage stability (soluble sodium
salt calculated as Na) under a high temperature and a high humidity
of 388 ppm and a content of the phosphorus compound calculated as P
of 0.48% by weight.
< Production of a non-magnetic undercoat layer >
[0229] Examples 9 to 16, Comparative Examples 15 to 24
[0230] Non-magnetic undercoat layers were obtained in the same
manner as in the above-mentioned embodiment, except that the kind
of the acicular hematite particles was varied.
[0231] The main production conditions and the characteristics are
shown in Table 14.
16 TABLE 14 Production of non- magnetic composition Weight
Characteristics Characteristics of non-magnetic undercoat layer
Kind of ratio of of non-magnetic Young's acicular particles/
composition Film Change modulus Example and hematite resin
Viscosity thickness Gloss In gloss Ra (relative Comp. Ex. particles
(-) (cP) (.mu.m) (%) (%) (nm) value) Example 9 Example 1 5.0 384
3.5 198 1.4 6.0 135 Example 10 Example 2 5.0 410 3.5 208 3.2 5.8
135 Example 11 Example 3 5.0 384 3.4 201 0.9 6.2 135 Example 12
Example 4 5.0 307 3.4 198 1.5 6.4 138 Example 13 Example 5 5.0 333
3.5 204 2.1 6.0 136 Example 14 Example 6 5.0 410 3.5 211 3.6 5.6
137 Example 15 Example 7 5.0 333 3.4 206 1.6 5.8 138 Example 16
Example 8 5.0 282 3.5 208 2.0 5.8 141 Comp. Ex. 15 Comp. Ex. 1 5.0
23,040 3.8 121 21.4 36.4 110 Comp. Ex. 16 Comp. Ex. 5 5.0 3,072 3.6
68 14.6 56.8 93 Comp. Ex. 17 Comp. Ex. 6 5.0 768 3.5 32 11.0 71.2
70 Comp. Ex. 18 Comp. Ex. 7 5.0 20,480 4.1 72 23.9 46.6 81 Comp.
Ex. 19 Comp. Ex. 8 5.0 640 3.5 173 11.2 14.8 111 Comp. Ex. 20 Comp.
Ex. 9 5.0 410 3.5 101 14.2 18.2 96 Comp. Ex. 21 Comp. Ex. 10 5.0
1,024 3.7 86 22.6 38.2 93 Comp. Ex. 22 Comp. Ex. 12 5.0 384 3.5 189
10.1 8.6 125 Comp. Ex. 23 Comp. Ex. 13 5.0 287 3.5 168 10.6 14.6
121 Comp. Ex. 24 Comp. Ex. 14 5.0 205 3.5 172 12.9 13.8 121
< Production of a magnetic recording medium >
[0232] Examples 17 to 24, Comparative Examples 25 to 37
[0233] Magnetic recording media were produced in the same manner as
in the above-mentioned embodiment, except that the kind of the
non-magnetic undercoat layers and the kind of the magnetic
particles were varied.
[0234] Meanwhile, the characteristics of the magnetic particles M-1
to M-3 are shown in Table 15.
17 TABLE 15 Characteristics of magnetic particles BET Average
Average Geometrical specific major axial minor axial standard
Aspect surface Coercive Saturation Magnetic diameter diameter
deviation ratio area force magnetization particles Kind of magnetic
particles (.mu.m) (.mu.m) (-) (-) (m.sup.2/g) (Oe) (emu/g) M-1
Metal magnetic particles 0.135 0.0191 1.38 7.1 53.5 2,240 138.2 M-2
Ba ferrite particles*.sup.3) 0.053 0.0160 1.21 3.3 58.2 2,510 52.6
M-3 Co-coated magnetite 0.180 0.0252 1.35 7.1 41.6 968 78.6
particles *.sup.3)With respect to Ba ferrite particles, the plate
diameter, the thickness and the plate ratio (plate
diameter/thickness) were regarded as "average major axial
diameter", "average minor axial diameter" and "aspect ratio",
respectively.
[0235] The main production conditions and the characteristics are
shown in Table 16 and Table 17.
[0236] The electromagnetic performance in Examples 17, 18 and
Comparative Examples 25 to 34 were represented by values obtained
by using the magnetic tape of Comparative Example 34 as the
reference tape.
[0237] The electromagnetic performance in Examples 19 and 20 were
represented by values obtained by using the magnetic tape of
Comparative Example 35 as the reference tape.
[0238] The electromagnetic performance in Examples 21 and 22 were
represented by values obtained by using the magnetic tape of
Comparative Example 36 as the reference tape.
[0239] The electromagnetic performance in Examples 23 and 24 were
represented by values obtained by using the magnetic tape of
Comparative Example 37 as the reference tape.
18 TABLE 16 Characteristics of magnetic recording medium Production
of magnetic Film recording medium thick- Kind of Weight ness of
Drop width in non- ratio of magnetic Co- Young's Stain
Electromagnetic electromagnetic magnetic Kind of particles/
recording ercive Br/ modulus on performance performance*.sup.4)
undercoat magnetic resin layer force Bm Gloss Ra (Relative head 4
MHz 7 MHz 4 MHz 7 MHz Example layer particles (-) (.mu.m) (Oe) (-)
(%) (nm) value) (-) (dB) (dB) (dB) (dB) Example Example 9 Particles
5.0 1.1 1,990 0.87 238 5.7 137 1 -- +1.2 -- 0.2 17 in the
embodiment Example Example Particles 5.0 1.0 1,996 0.87 241 5.6 135
2 -- +1.3 -- 0.1 18 10 in the embodiment Example Example M-1 5.0
1.0 2,310 0.88 235 5.2 138 2 -- +2.8 -- 0.3 19 11 Example Example "
5.0 1.0 2,323 0.89 239 5.3 139 1 -- +2.6 -- 0.4 20 12 Example
Example M-2 5.0 1.1 2,566 0.86 266 4.8 136 1 -- +2.1 -- 0.3 21 13
Example Example " 5.0 1.0 2,532 0.86 258 5.0 136 1 -- +2.8 -- 0.5
22 14 Example Example M-3 5.0 1.1 1,042 0.90 183 5.2 143 1 +2.6 --
0.1 -- 23 15 Example Example " 5.0 1.0 1,052 0.91 188 5.6 140 1
+3.1 -- 0.1 -- 24 16 *.sup.1)Drop width in the electromagnetic
performance of the magnetic tape after stored at a temperature of
6o.degree. C. and a relative humidity of 90% for 11 days.
[0240]
19 TABLE 17 Production of Characteristics of magnetic recording
medium magnetic recording medium Film Weight thickness Drop width
in Kind of non- ratio of of Coer- Young's Stain Electromagnetic
electromagnetic magnetic Kind of particles/ magnetic cive modulus
on performance performance*.sup.4) Comp. undercoat magnetic resin
recording force Br/Bm Gloss Ra (Relative head 4 MHz 7 MHz 4 MHz 7
MHz Ex. layer particles (-) layer (.mu.m) (Oe) (-) (%) (nm) value)
(-) (dB) (dB) (dB) (dB) Comp. Comp. Particles 5.0 1.3 1,921 0.78
186 13.8 113 3 -- -1.5 -- 2.6 Ex. 25 Ex. 15 in the embodiment Comp.
Comp. Particles 5.0 1.4 1,930 0.77 132 32.8 98 4 -- -3.8 -- 1.8 Ex.
26 Ex. 16 in the embodiment Comp. Comp. Particles 5.0 1.1 1,968
0.82 116 41.6 81 4 -- -4.1 -- 1 6 Ex. 27 Ex. 17 in the embodiment
Comp. Comp. Particles 5.0 1.3 1,932 0.80 121 38 8 93 4 -- -3.6 --
1.9 Ex. 28 Ex. 18 in the embodiment Comp. Comp. Particles 5.0 1.0
1,978 0.84 204 8.2 115 3 -- -0.8 -- 2.3 Ex. 29 Ex. 19 in the
embodiment Comp. Comp. Particles 5.0 1.1 1,966 0.81 176 16.6 100 4
-- -1.8 -- 2.0 Ex. 30 Ex. 20 in the embodiment Comp. Comp.
Particles 5.0 1.3 1,960 0.79 156 25.8 96 4 -- -2.2 -- 2.2 Ex. 31
Ex. 21 in the embodiment Comp. Comp. Particles 5.0 1.1 1,967 0.84
180 11.2 124 3 -- -1.6 -- 1.3 Ex. 32 Ex. 23 in the embodiment Comp.
Comp. Particles 5.0 1.0 1,970 0.84 184 11.0 124 3 -- -2.0 -- 1.2
Ex. 33 Ex. 24 in the embodiment Comp. Comp. Particles 5.0 1.1 1,976
0.84 215 9.6 128 3 -- (0.0) -- 1.6 Ex. 34 Ex. 22 in the embodiment
Comp. Comp. M-1 5.0 1.0 2,295 0.83 208 11.2 127 3 -- (0.0) -- 1.8
Ex. 35 Ex. 22 Comp. Comp. M-2 5.0 1.1 2,537 0.80 199 16.8 123 3 --
(0.0) -- 1.6 Ex. 36 Ex. 22 Comp. Comp. M-3 5.0 1.0 1,021 0.85 201
8.8 133 3 (0.0) -- 1.2 -- Ex. 37 Ex. 22 *.sup.4)Drop width in the
electromagnetic performance of the magnetic tape after stored at a
temperature of 60.degree. C. and a relative humidity of 90% for 14
days.
[0241] The acicular hematite particles for a non-magnetic undercoat
layer of a magnetic recording medium according to the present
invention have an excellent dispersibility in a vehicle so that it
is possible to enhance the surface smoothness and the strength of
the non-magnetic undercoat layer. When a magnetic recording layer
is formed on the non-magnetic undercoat layer, not only can the
magnetic recording layer be formed into a smooth and uniform thin
film, but also a magnetic recording medium having an excellent
electromagnetic performance and an excellent storage stability can
be obtained.
[0242] The magnetic recording medium according to the present
invention uses as particles for a non-magnetic undercoat layer
acicular hematite particles with the total content of sodium of not
more than 50 ppm which is excellent in storage stability, so that
it is excellent in storage stability as well as electromagnetic
performance.
* * * * *