Note: Descriptions are shown in the official language in which they were submitted.
~:045806
1 BACKGROUND OF THE INVENTION
1 E`ield of the Invention
This invention relates to a method of producing a
magnetic material for use in a magnetic recording medium and,
in particular, to a new metal powder magnetic material and a
method for the production thereof.
2. Description of the Prior Art
Conventionally, powdered ferromagnetic materials such
as y-Fe203, Fe304, Co containing y-Fe203, Co containing Fe304,
CrO2, etc., have been used for magnetic recording media. Such
materials, however, are not completely satisfactory for recording
signals with short wave-lengths (about 10 microns or less). In
other words, their magnetic properties of coercive force (Hc)
and maximum residual magnetic flux density (Br) are insufficient.
Recently, various ferromagnetic powder materials suitable for
high density recording have been extensively developed. One
; group of such materials is a finely divided ferromagnetic metal,
comprising pure metal, a metal alloy or an intermetallic compound.
Suitable metals include Fe, Co, and Ni, to which, depending
on the requirements, Cr, Mn, rare earth metals, Zn, etc., are
added.
The following methods of producing ferromagnetic
materials are known.
(I) Hydrolysis of an organic acid salt of a ferromagnetic
metal, followed by reduction with a reducing gas, as is
described in, for example, ~apanese Patent Publication Nos.
11412/1961 and 38417/72.
~ II) Reduction of needle-shaped iron oxyhydroxide, needle-
shaped iron oxyhydroxide containing a foreign metal, or needle-
i~ '
. - . :.
1045806
S shaped iron oxide obtained from these oxyhydroxides, as is
disclosed in Japanese Patent Publication Nos. 3862/1960 and
39477/1972, or in British Patent 1,192,167.
(III) Vaporization of a ferromagnetic material in an atmos-
phere of an inert gas, as is described in Japanese Patent
Publication No.27718jl972, or "Ohyo sutsuri" (Applied Physics)
Vo. 40, No. 1, p. 110 (19671).
~ IV) Decomposition of a metal carbonyl compound, as is
described in U.S. Patents 2,983,997 and 3,228,882.
~V) Electrodeposition of a ferromagnetic metal onto a
mercury cathode, followed by separation of the metal from the
mercury, as is disclosed in Japanese Patent Publication Nos.
15525/1964 and 8123/1965.
~ VI) - Reduction of a salt of a ferromagnetic metal in
solution, as is described in Japanese Patent Publication Nos.
- 20520/1963, 26555/1963, 20116/1968 and 41718/1972, U.S. Patent
Nos. 3,663,318 and 3,661,556, and German Patent OLS 2,132,430.
The present invention relates to method (VI) above
and especially to a method which employs a borohydride compound
or a derivative thereof as a reducing agent.
Conventional methods of reducing a ferromagnetic
metal salt in solution with a borohydride compound or a
derivative thereof,have the following drawbacks. Firstly, the
metal powder thus prepared, especially when the powder contains
Fe, is sensitive to moisture so that it is gradually oxidized
even when kept at room temperature conditions. In an extreme
case, it has been proved that the ferromagnetic powder is finally -
converted into a non-magnetic material. Secondly, because of
the high surface activity of the powder prepared by this method,
the treatment of the powder during production has been quite
difficult from an industrial standpoint. That is, a danger of fire
- . . ... .... . - ., . ..
10458Q6
1 exists when such powder is treated in the air; moreover, this
active powder tends to decompose or degrade a binder material
when blended therewith, thus causing poor dispersion or diffi-
culties in coating.
SUMMARY OF THE INVENTION
A principal object of the present invention is to
overcome these drawbacks of the prior art.
An object of the present invention is to provide
finely divided magnetic materials having improved magnetic pro-
perties.
Another object of the present invention is to provide
a new method of producing finely divided magnetic materials
which are highly resistant to humidity and also withstand
oxidation.
Still a further object of the present invention is to
provide a magnetic material which is stable during production
and is thus easy to manufacture.
A method of producing a weather-resistant, stable
ferromagnetic metal powder has been found involving dissolving
a borohydride compound or a derivative thereof in a solution
containing an alkali hydroxide and using the resulting solution
as a reducing agent.
Thus the present invention provides a method of
producing a ferromagnetic material comprising reducing a
solution of a salt of a metal capable of forming a ferromagnetic
material using as a reducing agent an alkaline solution of a
borohydride or a derivative thereof containing hydroxide ion in
a concentration of at least about O.OOlN and not higher than -
about 0.6N.
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104S8Q6
1 DETAILED DESCRIPTION OF THE INVENTION
Suitable reducing agents for the present invention
include borohydrides and derivatives thereof such as borane,
borazane,borohydride, sodium borohydride, potassium borohydride,
dimethylaminoborane,diethylaminoborane, etc. These reducing
agents can be used individually or in combination. These
compounds are compounds which contain boron and are water-soluble.
Upon reaction hydrogen and metal powder are produced.
In the past, when a ferromagnetic metal powder was
0 prepared through a reducing reaction with a borohydride compound,
the borohydride reducing agent was supplied to the reaction
in the form of a powder or of an aqueous solution. It has been
found,however, that the resulting ferromagnetic powder has
improved weather resistance if such a reducing agent is fed to
the reaction after dissolution in hydroxide ion containing
solution.
The alkaline materials which can be suitably used in
the present invention include those compounds which are water-
- soluble, produce hydroxide ions, and generate a pH of greater
than about 8 when dissolved in water. Suitable specific examples
include inorganic alkaline materials, e.g., alkali metal
hydroxides such as sodium hydroxide,potassium hydroxide, or
lithium hydroxide, alkaline earth metal hydroxides such as calcium
hydroxide, or barium hydroxide, ammonium hydroxide, and mixtures
thereof and organic alkaline materials such as n-butylamine,
; isopropylamine, hydrazine, etc. These alkaline materials are
dissolved in water, or mixtures containing water-miscible organic
solvents in an amount of less than about 50% by weight, preferably
less than 25% by weight. Suitable water-miscible organic sol-
vents are alcohols such as methanol, ethanol, propanol, butanol;
' ~
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1 ketones such as acetone, methyl ethyl ketone, dimethyl sulfoxide,
tetrahydrofuran, etc. The hydroxide ion containing solution should
have a concentration not lower than O.OOlN. At concentrations
lower than this the advantageous effect of the present invention
is not obtained satisfactorily. On the other hand, a concentration
above 0.6N is not preferred since with such an extremely strongly
alkaline solution the hydroxide or oxyhydroxide tends to
contaminate the resulting ferromagnetic powder.
The mechanism is not clear on why the ferromagnetic
powder has a decreased surface activity and thus improved
weather resistance by the use of the reducing agent in an alkaline
hydroxide solution. However, the property change of the
resulting powder surface suggests the formation of a thin oxide
or hydroxide layer on the ferromagnetic metal powder surface,
and this layer is believed to play the role of a protective
coating.
The "metal salt capable of providing a ferromagnetic
material" of the present invention includes those materials
which contain at least one of Fe, Co, Fe-Co, Fe-Ni, Fe-Co-Ni,
or Co-Ni, which can, for the purpose of improvement of the
magnetic properties as well as oxidation resistance, contain a
suitable amount of a rare earth metal element such as La, Ce,
Nd, Sm, etc., or Sn, Al, W, Cr, Mn, Cu, Zn, Ag, Pd, Ti etc. More
precisely, the metal salt is a water-soluble salt producing in
solution a metal ion or a-hydrated metal ion such as the chloride,
sulfate, nitrate,formate, acetate, pyrophosphate,sulfamate, etc.
of such a metal element. -~
In addition to the above-described ingredients, a
chelating agent which is water-soluble and forms a complex ion
with one of the above-described metal ions, a pH buffering agent,
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a pH controlling agent,etc., can be added to the metal salt
solution, e.g., in an amount of less than about 20% by weight,
preferably less than 12% by weight, as required. pH buffering
agents which also act as chelating agents include monocarboxylic
acids such as formic acid, acetic acid, propionic acid, butyric
acid, n-valeric acid, acrylic acid, trimethylacetic acid,
benzoic acid, chloroacetic acid, etc. and their salts; chelating
a~ents include dicarboxylic acids such as oxalic, succinic,
malonic, maleic, itaconic, p-phthalic, etc., and their salts,
and oxycarboxylic acids such as glycolic, lactic, salicylic,
tartaric, citric, etc. and their salts; pH controlling agents
which can also act as pH buffering agents include boric acid,
carbonic acid, sulfinic acid, etc., and pH controlling agents
include other inorganic and organic acids, ammonia, an alkaline
`~ hydroxide, etc~ It should be noted that each compound described-
above not only functions individually but gives rise to
~ interactions. Certain compounds, for example, act as chelating
; agents as well as pH buffering agents. Thus, the functions
of these Compounds need not be limited to those described
20 above. ~ -
Further, to the reaction solution there can be added
as desired a soluble protein together with a proteolytic enzyme,
a carbohydrate together with a carbohydrate decomposer, and an
organic solvent, e.g., in an amount less than about 10% by
weight, preferably less than about 6% by weight.
Suitable soluble proteins include simple proteins which
decompose into chiefly amino acids upon hydrolysis, conjugated
proteins comprising polypeptide chains connected to other
compounds, derived proteins which have been modified by various
physical, thermal, photochemical or chemical proceduFes, etc.
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,
16)458Q6
1 They are all composed of macromolecules of polypeptides, com-
prising amino acids, having a molecular weight of from about 10,000
to about 5,000,000.
Simple proteins suited for the present invention include
albumins such as egg albumin, blood serum albumin, lactoalbumin,
etc., globulins such as serum globulin, lactoglobulin, myosin,
edestin, ammandin, legumin, etc., glutelins such as glutenin,
hordenin, oryzenin, etc., prolamins such as gliadin, hordein,
zeinj etc., albuminoids such as collagen, elastin, keratin,
0 fibroin, etc., histones such as thymus-histone, liver-histone,
scombrone, etc., and protamines such as salmine, clupeine,
strurine, iridine, etc.
Conjugated proteins include chromoproteins, phospho-
proteins, metalloproteins, lipoproteins, glycoproteins,
nucleoproteins, etc. Examples of chromoproteins are hemoproteins
such as hemoglobin, myoglobin, cytochrome, catalase, peroxydaze,
etc., chlorophyll proteins such as chlorophylin; carotinoid
proteins such as rhodopsin, etc., flavoproteins such as flavin
mononucleotide, flavin adeninedinucleotide, etc. Phosphoproteins
include casein, hyderin, apoferritin, etc. Metalloproteins
include iron proteins such as feritin, apoferritin, etc.,
copper proteins such as hemocyanine, etc., zinc proteins; man-
ganese proteins, etc. Lipoproteins include thrombplastein,
al-lipoprotein, a2-lipoprotein, lipobilitein, lipotenilin,
etc., glucoproteins include mucin, celluloplasmin, siderophilin,
fibrin, prothrombin collagen, enterokinase, haptoglobin, mucoid,
etc., and nucleoproteins include nucleic acids, deoxypentose-
nucleoprotein, pentose-nucleoprotein, viruses, bacteriophages,
etc.
Derived proteins include gelatin modified protein,
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11;)458Q6
proteans, proteoses, peptones, polypeptides, metap~oteins, etc.
These proteins can be used individually or in combination.
To decompose any of these proteins, proteases belonging
to the hydrolase type can be employed, including pepsin, trypsin,
chymotrypsin, cathepsin, rennin, papain, promelin, ficin,
thrombin, enteropeptidase, plasmin, mold protease, yeast pro-
tease, bacterium protease, etc. Such proteolytic enzymes can be
used individually or in combination.
Carbohydrates comprise monosaccharides, oligosaccharides
such as di-, tri- or tetrasaccharides and polysaccharides.
Monosaccharides include polyhydric alcohols containing - -
aldehyde groups (aldoses) and those containing keto groups
~ketoses).
Oligosaccharides comprise dimers, trimers or tetramers
of monosaccharides, each of which may be common or different,
through glycoside linkages. Polysaccharides are polymers of
mono- or oligosaccharides. -
Suitable monosaccharides include glycol aldehydes which
is the aldose of a diose (C2H402), a triose (C3H603) such as
glyceraldehyde ~aldose) or dihydroxyacetone (ketose), etc.,a
tetrose (C4H804) such as erythrose (aldose), erythrulose (ketose),
etc., a pentose (C5H1005) such as ribose, xylose, arabinose
(aldose),arabinulose (ketose), xylulose (ketose), etc., a
hexose (C6H1206) such as D-glucose,mannose, galactose, (aldose),
D-fructose, sorbose (ketose), and a heptose (C7H1407) such as
mannoheptose, which is an aldose, or heptulose which is a
ketose.
Oligosaccharides, include disaccharides such as
lactose (comprising D-glucose and D-galactose), maltose (comprising
two moles of D-glucose), sucrose (comprising D-glucose and D-
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1045806
1 fructose), trehalose (comprising D-glucose); trisaccharides such
as raffinose (comprising D-glucose, D-fructose and D-galactose),
gentianose (comprising D-fructose and two molecules of D-
glucose), mannotriose (the trimer of mannose), etc; and tetra-
saccharides such as stachyose (comprising two molecules of D-
galactose, D-glucose and D-fructose), etc.
Polysaccharides which can be used for the present
invention include dextrin (D-glucose), starch (D-glucose),
glycogen ~D-glucose), cellulose (D-glucose), inulin (fructose),
mannan (mannose~, araban (arabinose), xylan (xylose), dextran
(D-glucose), galactan (galactose), gum arabic (comprising
galactose, glucuronic acid, hexose and methylpentose), etc.
As is clear from the above explanations, suitable
carbohydrates for the present invention are monosaccharides and
polysaccharides comprising a multiplicity of monosaccharides
connected by glycoside linkages. Further, as the enzyme to
decompose such a carbohydrate, a hydrolase such as a-amylase
(diastase), ~-amylase (diastase), cellulose, lichenase,
laminarinase, inulase, a-glucosidase, ~-glucosidase, ~-galacto-
sidase, ~-mannosidase, etc., all of which are glycosidases, can
be used.
An inorganic acid such as hydrogen chloride, sulfuric
acid, nitric acid, etc. can also be used to decompose carbo-
hydrates.
Ultrasonic waves can be effectively used during the
reducing reaction to improve the properties of the resulting
material.
The application of a magnetic field to the reaction
solution during the reducing reaction results in a remarkable
improvement of coercive force and squareness ratio. A higher
field strength is preferred, especially of from about 500 to
_ g _ :,
1~45806
3,000 Oe, although a field stronger than several tens of Oe
can be effectively used. A reaction temperature not higher
than about 65C is desirable for a reducing reaction utilizing
a borohydride compound. A suitable reaction temperature can
range from about -10 to about 65C. At a temperature above
65C, the characteristics of the magnetic powder produced
deteriorate and at temperatures less than about -10C, the
reactivity is reduced and the solution tends to freeze.
A preferred metal ion concentration range between about
0.002 and 2 mol/Q, and more preferably between 0.01 and 0.5
mol/Q.
The concentration of the borohydride compound or a
derivative thereof reducing agent ranges between about 0.0001
to 15 mol/Q, preferably 0.0002 and 10 mol/Q. In addition,
a desirable result is obtained when the reaction is carried out
with a mol ratio of the reducing agent to the metal ion of from
about 0.1:1 to 5:1, preferably 0.25:1 to 4:1.
The ferromagnetic powder obtained by the present
invention exists in a particulate form of a size of from about
50 to 1,000 A with the tendency to form filaments, rods, sticks
or a necklace-like structure comprising several to several tens
of particles.
The ferromagnetic powder of the present invention
exhibits still further improved magnetic properties when heat
treated for about 15 seconds to about 120 hours, preferably
3 minutes ~ 24 hours, at a temperature of about 120C to 450C
preferably 150C to 370C under a non-oxidizing atmosphere or
in the presence of a trace of water or of 2 Suitable
non-oxidizing atmospheres include inert gases such as helium,
neon, argon, krypton, xenon, etc., and gases such as nitrogen,
-- 10 - '~
1~)45806
1 carbon monoxide, carbon dioxide, etc. Where oxygen is present
a suitable oxygen concentration is less than about 50 mmHg,
preferably is less than 20 mmHg, and a suitable amount of water
is a relative humidity of less than about 10%.
The ferromagnetic material obtained by the present
invention contains a trace of boron, which may originate from
the boron hydride compound used as the raw material. Chemical
analysis establishes that the boron content ranges from about
2.0 to 10.5%.
The ferromagnetic material of the present invention
can be combined with various materials to prepare recording
media.
Binder materials which can be used in combination with
the ferromagnetic material of the present invention include
thermoplastic or thermosetting resins known in the art.
Thermoplastic resins are those with a softening
temperature not higher than about 150C, an average molecular
weight of from about 10,000 to 200,000 and with a degree of
polymerization of from about 300 to 1,000, including, e.g.,
vinyl chloride/vinyl acetate copolymers, vinyl chloride/
vinylidene chloride copolymers, vinyl chloride/acrylonitrile
copolymers, acrylic acid ester/acrylonitrile copolymers,
acrylic acid ester/vinylidene chloride copolymers, acrylic acid
ester/styrene copolymers, methacrylic acid ester/acrylonitrile
copolymers, methacrylic acid ester/vinylidene chloride copolymers,
methacrylic acid ester/styrene copolymers urethane elastomers,
polyvinylidene fluoride, vinylidene chloride/acrylonitrile copo-
lymers, butadiene/acrylonitrile copolymers, polyamides,
poly(vinylbutyral), cellulose derivatives such as cellulose
acetate butyrate, cellulose diacetate, cellulose triacetate,
1~)45806
1 cellulose propionate, nitrocellulose, etc., styrene/butadiene
copolymers, polyester resins, chlorovinylether/acrylic acid ester
copolymers, amino resins, various synthetic rubber based
thermoplastic resins, and mixtures thereof.
Use of these resins is described in the following
Japanese Patent Publication Nos. 6877/1962, 12528/1964,
19282/1964, 5349/1965, 20907/1965, 9463/1966, 14059/1966,
16985/1966, 6428/1967, 11621/1967, 4623/1968, 15206/1968,
2889/1969, 17947/1969, 18232/1969, 14020/1970, 14500/1970,
18573/1972, 22063/1972, 22064/1972, 22068/1972, 22069/1972,
22070/1972 and 27886/1972, and U.S. Patent Nos. 3,144,352,
: 3,419,420, 3,499,789 and 3,713,887, etc~
Suitable thermosetting resins which can be used are
those which have a molecular weight not greater than about
200,000 when present in the coating mixture, and which form
a network with an infinite molecular weight through a condensation
or addition reaction. Further those which do not soften or
melt until thermal decomposition are preferred. Specific
examples are phenol resins, epoxy resins, curable polyurethane
resins, melamine resins, urea resins, alkyd resins, silicone
resins, reactive acrylic resins, epoxy-polyamide resins,
nitro-cellulose-melamine resins, mixtures of a high molecular
weight polyester resin with an isocyanate prepolymer, mixtures
of methacrylic acid ester copolymers with diisocyanate pre-
polymers, mixtures of polyester polyols with polyisocyanates,
urea-formaldehyde resins, low molecular weight glycoljhigh
molecular weight diol-triphenylmethane triisocyanate mixtures,
polyamide resins, and mixtures thereof.
Use of these resins is described in the following
Japanese Patent Publication Nos. 8103/1964, 9779/1965, 7192/1966,
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' ' . ," ' ' ' ' ' '
1~4S806
1 8016/1966, 14275/1966, 18179/1967, 12081/1968, 28023/1969,
14501/1970, 24902/1970, 13103/1971, 2206S/1972, 22066/1972,
22067/1972, 22072/1972, 22073/1972, 28045/1972, 28048/1972, and
28922/1972, and U.S. Patent Nos. 3,144,353, 3,320,090, 3t437,510,
3,597,273, 3,781,210 and 3,781,211, etc.
These resin binders can be utilized individually or
in combination, and further additives can be employed. The
mixing ratio between the ferromagnetic powder and the binder
is from about 100:10 to 100:200 by weight, preferably 100:25
to 100:120 by weight.
Typical additives are a dispersing agent, a lubricating
agent, and an abrasive. Dispersing agents include caprylic
acid, caproic acid, lauric acid, myristic acid, palmitic acid,
stearic acid, oleic acid, elaidic acid, linoleic acid, linolenic
acid, stearolic acid, all of which have from about 12 to 18
carbon atoms with the general formula RlCOOH (in which Rl
represents an alkyl group having about 11 to 17 carbon atoms);
a fatty acid ester with the general formula R2COOR3 wherein
R2 and R3 each represents an alkyl group of having 1 to about 12
carbon atoms including ethyl acetate, butyl acetate, ethyl
propionate, methyl butyrate, ethyl caprylate, propyl laurate,
etc.; a metal soap comprising an alkali metal (e.g., sodium,
potassium, etc.) or an alkaline earth metal (e.g., magnesium,
calcium, etc.) salt of the above-cited fatty acid; lecithin,
etc. Further, a higher alcohol having about 12 to 24 carbon
atoms such as lauryl alcohoI, pentadecyl alcohol, cetyl alcohol,
stearyl alcohol, nonadecyl alcohol, etc. and the sulfuric
acid ester thereof can be employed. Su~h a dispersing agent can
be incorporated in an amount of from about 1 to 20 parts by
weight per 100 parts by weight of the binder.
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~)45806
1 Suitable lubricants include silicone oil, graphite,
molybdenum disulfide, tungsten disulfide, a fatty acid ester
comprising a monocarboxylic fatty acid having about 12 to 16
carbon atoms and a monohydric alcohol having about 3 to 12
carbon atoms, a fatty acid ester comprising a monocarboxylic
acid with more than about 17 carbon atoms and a monohydric
alcohol which will form an ester having about 15 to 28 total
carbon atoms. Suitable fatty acid esters include ethyl caprylate,
ethyl laurate, propyl myristate, methyl palmitate, ethyl
stearate, amyl stearate, ethyl behenate, ethyl oleate, propyl
linolate, methyl linolenate, etc. Such a lubricant can be
used at from about 0.2 to 20 parts by weight per 100 parts by
weight of the binder. Descriptions as to the lubricant can be
found in Japanese Patent Publication No. 23889/1968, and
Japanese Patent Publication 24041/1973 and ~o. 18482/1973,
U.S. Patent Nos. 3,470,021, 3,492,235, 3,497,411, 3,523,086,
3,625,760, 3,630,772, 3,634,253, 3,647,539 and 3,687,725,
IBM Technical Disclosure Bulletin Vol. 9, No. 7,Page 779,
.. :
December 1966, and ELEKTRONIK 1961, No. 12, Page 380, etc.
Asb~ the abrasive, those conventionally used abrasives
can be used including alumina, fused alumina, silicon carbide, chro-
mium sesquioxide~ corundum, synthetic corundum, diamond, synthetic
diamond, garnet, emery ~main ingredient; corundum and magnetite),
etc. A preferable size of such an abrasive ranges from about
0.1 to 2 microns in average. The abrasive can be present in an -
amount of from about 1 to 20 parts by weight per 100 parts
by weight of the binder. Descriptions of abrasives can be found
in Japanese Patent Application (O.P.I.) No. 115510/1974, U.S. Patent
Nos. 3,007,807, 3,041,196, 3,293,066, 3,630,910 and 3,687,725,
30 British Patent No. 1,145,349 and German Patent No. 853,211.
.
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1 A magnetic recording layer can be formed by dissolving
the above described components in a suitable organic solvent
system and coating the resulting mlxture on a substrate.
The substrate can have a thickness of about 5 to 50
microns, more preferably from 10 to 40 microns, and examples
include a polyester such as poly~ethylene terephthalate), a
polyolefin such as polypropylene, a cellulose derivative such
as cellulose triacetate, cellulose diacetate etc., a poly-
carbonate, etc.
Suitable methods of coating a magnetic recording layer
on a substrate include various coating techniques such as air
doctor coating, blade coating, air knife coating, squeeze
coating, impregnation coating, reverse roll coating, transfer
roll coating, gravure coating, kiss coating, cast coating or
spray coating. Other methods can be used and all of these
methods are explained in detail in Coating Enq-neeringi p. 253- -
277, Asakura Shoten, March 20, 1971.
Organic solvents which can be used for coating include
ketones such as acetone, methyl ethyl ketone, methyl isobutyl
ketone, cyclohexanone; alcohols such as methanol, ethanol,
propanol, butanol, etc.; esters such as methyl acetate, ethyl
acetate, butyl acetate ethyl lactate, glycol acetate
monoethylether, etc.; ethers and glycol ethers such as diethyl
ether, glycol dimethylether, glycol monoethylether, dioxane,
tetrahydrofuran,etc.; aromatic hydrocarbons such as benzene,
toluene, xylene, etc.; chlorinated hydrocarbons such as methylene
chloride, ethylene chloride, carbon tetrachloride, ethylene ;
chlorohydrin, chloroform, dichlorobenzene, etc., dimethyl
sulfoxide, and dimethylformamide. A suitable concentration of
the ferromagnetic powder in the coating composition can range
from about 150 to 500 g, preferably 200 to 450 g per kg of solvent.
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. ~ ,.
1~)45806
1 The present invention will be further illustrated
in greater detail m the following examples and comparative
examples. It will be self-evident to those skilled in the art.
that the ratios, ingredients in the following formulations, and
the order of operations can be modified within the scope of the
present invention. Therefore, the present invention is not to
be interpreted as being limited to the following examples.
All parts, percents and the like are by weight, unless otherwise
indicated.
EXAMPLE I
FeSO4.7H2O ~5.6 kg), CoC12.6H2O (1.2 kg), and
concentrated hydrochloric acid ~50 ml) were dissolved in water
to form a 100Q solution, which was subjected to a DC magnetic
field of 500 Oe and kept at 20C. On the other hand, a reducing -
agent solution(20Q), which was prepared by dissolving sodium
borohydride (1.9 kg), and sodium hydroxide (80 g) in water, was
added to the metal salt solutîon at a rate of 1.2Q/sec.
Immediately a vigorous reaction took place with the generation
of a violet foam.
After the reaction ceased, the black precipitate
obtained was thoroughly washed with water and then with acetone.
The precipitate was dried in an atmosphere with a humidity
~below 30% at 25C. The thus obtained ferromagnetic metal
powder exhibited a Bs value of 8600G. Using the powder
(designated #P-l),a magnetic coating mixture having the
following formulation was prepared.
Parts
Ferromagnetic Powder #P-l 300
Polyester Polyol * 40
Cellulose Propionate 20
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1~45~06
Parts
Polyisocyanate ** 20
Chromium Sesquioxide*** 5
Lecithin 5
Methyl Ethyl Ketone 450
Butyl Acetate 450
* A condensate of adipic acid and butanediol, havingterminal hydroxy groups and a molecular weight of
about 1800.
** An adduct of 3 moles of toluene diisocyanate and
l mole of trimethylolpropane.
*** Average particle size of about 0.1 ~ microns in
length and 0.04 microns in width.
This mixture was coated on a 25 micron thick poly-
(ethylene terephthalate) film to give a 5 micron dry thickness
under the application of magnetic field,and then dried by
heating. The magnetic recording web thus obtained was
supercalendered, and then slit to a l/2 inch width to give a
video tape, designated #T-l.
EXAMPLE I I
Another ferromagnetic powder (#P-2) and another video
tape (#T-2) therefrom were obtained following the same
procedures as in Example I except that the sodium hydroxide
(80 g) was replaced by potassium hydroxide (llO g).
EXAMPLE I I I
Example I was modified only by replacing the sodium
borohydride ~l.9 kg) with potassium borohydride (0.3 kg) and
sodium borohydride 1.7 kg. The resulting powder (#P-3) was
used to prepare a video tape, designated #T-3. 7"'" '
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1~45806
1 Comparative Example
Sodium hydroxide ~80 g) was omitted from the composition
used in Example I. Except for this elimination, all of the
procedures of the example were exactly followed to give a
ferromagnetic powder and a video tape.
It was observed that the reducing solution of the
comparative example had a noticeable tendency to generate foam
during storage compared with those employed in the foregoing
examples, containing the alkaline hydroxide. Actually, where
the reducing solution free from alkaline hydroxide was used for
the reduction reaction after prolonged storage, a magnetic
powder with poor magnetic properties resulted.
The various characteristic values of these magnetic
powders are shown inTable I.
TABLE I ~gauss)
Sample No. 8s(l)* BS(2)**
. .
#P-1 8600 7200
#P-2 8200 7350
#P 3 8540 7620
Comparative
example 8800 4800
* Bs(l) is the value of saturation magnetization before
weather-resistance testing.
** Bs(2) is the corresponding value after one week of
storage at 45 C, 85% R.H.
This result shows that the magnetic powders prepared in accordance
with the present invention have improved weather-resistance.
The low initial, Bs value of the comparative example may be
attributed to an oxidation which took place during the washing
with water and the drying.
- 18 -
~)45806
1 The various properties of the video tapes produced
are shown in Table II. The measurements were made at a
magnetic field of 2000 Oe.
TABLE II
Sample No. Hc B /BVideo Recording*
r sOutput
(dB)
#T-l 1080 0.80 +1.2
#T-2 1050 0.81 +0.4
#T-3 1080 0.78 +0.6 ~-~
Comparative
example 1100 0.73 0.0
* Values at biasless recording at 5MHz, expressed
relatively to the output of the comparative example
as the control.
The figure shows the characteristics of the magnetic ~-
powder of the present invention, in which the abscissa
corresponds to the concentration ~log) of the alkaline hydroxide
in the solution containing a borohydride compound, and on the
ordinate the values of Bs(2)/Bs(l) x 100~ shown in Table I
20 are plotted, thus showing the change of Bs.
With an alkaline hydroxide concentration below 0.001N
or above 0.6N, the desirable effects on weather-resistance
disappeared and moreover an adverse effect resulted.
f The result shows that the control has a lower sen-
.. ... . .
; sitivity than the tapes of the present invention in spite of
its slightly higher coercive force. This may be due to the low
squareness ratio and also to a partial oxidation of the
ferromagnetic material during tape production.
While the invention has been described in detail and
with reference to specific embodiments thereof, it will be appa-
rent to one skilled in the art that various changes and modifi-
cations can be made therein without departing from the spirit
and scope thereof.
19
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