Magnetic recording medium comprising a magnetic layer containing magnetic powder, an organic dye and a binder having an amino group or ammonium salt group

A magnetic layer containing a magnetic powder whose surface is composed mainly of carbon and/or iron carbide and a binder is provided on a non-magnetic substrate, the magnetic layer further contains an organic dye compound having at least one polar group selected from the group consisting of a hydroxyl group, a carboxylic acid group, a sulfonic acid group and a salt thereof, and the binder contains an amino group and/or an ammonium salt group. Due to the improved dispersibility of a magnetic paint, the magnetic recording medium of the invention is improved in surface morphology, magnetic properties, and electromagnetic properties.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to magnetic recording media of the coating type 
including a variety of magnetic tapes. 
2. Prior Art 
Needle iron oxide is typical of the magnetic powders used in magnetic 
recording media. However, since the needle iron oxide as such cannot 
accommodate a demand for increased magnetic recording density, a variety 
of magnetic materials having high coercivity and high saturation 
magnetization have also been developed. 
For example, iron carbide system magnetic powders in which at least the 
surface is of iron carbide are proposed in Japanese Patent Application 
Kokai (JP-A) Nos. 71509/1985, 124023/1985, 184576/1985, 211625/1985, 
212821/1985, 269225/1986, 85403/1987, 86537/1987, 86531/1987, etc. Also 
the inventors proposed a magnetic powder whose surface is composed mainly 
of carbon in Japanese Patent Application No. 272057/1991. These powders 
are characterized by high coercivity, high saturation magnetization, good 
electrical conductivity, and effective light shielding. 
These prior art proposals, however, are insufficient in packing density and 
orientation of the magnetic layer because the dispersibility of magnetic 
powder in a binder is not taken into account. For example, some of the 
above-referred publications disclose to add fatty acids such as lauric 
acid and stearic acid to a magnetic paint as a dispersant, but such means 
alone is still insufficient in packing density and orientation. 
Then the inventors proposed in Japanese Patent Application No.338016/1992 
to increase the dispersibility of a magnetic powder whose surface is 
composed mainly of carbon or iron carbide by subjecting the magnetic 
powder to pre-treatment by kneading and dispersing the magnetic powder 
optionally with an anionic or ampholytic surfactant and a fatty acid, and 
using a resin containing an amino or ammonium salt group as a binder. 
However, gloss and surface roughness are still insufficient. There often 
occur coating defects such as longitudinal streaks. A further improvement 
in electromagnetic properties is also desired. 
DISCLOSURE OF THE INVENTION 
A primary object of the present invention is to provide a magnetic 
recording medium which is improved in surface morphology, coating 
properties, magnetic properties, and electromagnetic properties by 
increasing the dispersibility in a magnetic paint of a magnetic powder 
whose surface is composed mainly of carbon or iron carbide. 
This and other objects are attained by the present invention which is 
defined below as (1) to (6). 
(1) A magnetic recording medium comprising on a non-magnetic substrate a 
magnetic layer containing a magnetic powder whose surface is composed 
mainly of carbon and/or iron carbide and a binder, 
said magnetic layer containing an organic dye compound having a polar 
group. 
(2) The magnetic recording medium of (1) wherein said polar group is at 
least one selected from the group consisting of a hydroxyl group, a 
carboxylic acid group, a sulfonic acid group and a salt thereof. 
(3) The magnetic recording medium of (1) wherein the content of said 
organic dye compound is 0.5 to 10 parts by weight per 100 parts by weight 
of said magnetic powder. 
(4) The magnetic recording medium of (1) wherein said binder contains an 
amino group and/or an ammonium salt group. 
(5) The magnetic recording medium of (1) wherein said magnetic powder is 
one obtained by previously kneading and dispersing a magnetic powder whose 
surface is composed mainly of carbon and/or iron carbide in a solvent. 
(6) The magnetic recording medium of (1) wherein a magnetic paint 
containing the magnetic powder whose surface is composed mainly of carbon 
and/or iron carbide, the binder, and the organic dye compound having a 
polar group is coated onto the non-magnetic substrate for improving 
coating defects at the surface of the magnetic layer. 
OPERATION AND BENEFITS 
The magnetic recording medium of the invention includes a magnetic layer 
containing a magnetic powder whose surface is composed mainly of carbon 
and/or iron carbide and a binder on a non-magnetic substrate. The magnetic 
layer further contains an organic dye compound having a polar group or a 
salt thereof. Preferably the binder used is one containing an amino group 
and/or an ammonium salt group. 
In preparing a magnetic coating composition or paint containing these 
components, the magnetic powder is preferably subject to a pre-treatment 
by kneading and dispersing it in a solvent. Preferably, the organic 
compound is also mixed and dispersed during this pre-treatment, obtaining 
a magnetic paint containing the binder. 
The thus prepared magnetic paint is enhanced in the dispersibility of the 
magnetic powder and for unknown reasons, is changed in viscosity and 
reduced in yield value. As a result, the magnetic layer is increased in 
surface gloss and reduced in surface roughness, that is, improved in 
surface morphology. Also the frequency of unacceptable appearance 
resulting from coating defects at the magnetic layer surface such as 
longitudinal streaks which are observable under a microscope is reduced, 
indicating improved coating properties. These lead to significant benefits 
including improved magnetic field orienting effect, an increased 
squareness ratio, and improved electromagnetic properties such as RF 
output. 
A lowering of yield value is recognized significant particularly when the 
magnetic powder whose surface is composed mainly of carbon or iron carbide 
is combined with the organic dye compound. These significant benefits 
resulting from such improved dispersibility and a reduced yield value are 
not obtained from those magnetic paints which are prepared by using 
commonly used magnetic powders, for example, fine powders of Fe, Co, Ni 
and alloys thereof, and oxide fine powders such as .gamma.-Fe.sub.2 
O.sub.3, cobalt-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, 
cobalt-doped Fe.sub.3 O.sub.4, CrO.sub.2, barium ferrite and strontium 
ferrite and adding the organic dye compound thereto. 
It is disclosed in JP-A 98525/1985, 261817/1986 and 3430/1987 to improve 
the dispersibility of magnetic powder in the magnetic layer by adding 
thereto an organic dye compound as mentioned above. These publications, 
however, disclose only those examples of employing commonly used magnetic 
powders such as metal or alloy magnetic powders and oxide fine powders as 
mentioned above. The dispersibility and dispersion stability are increased 
and electromagnetic properties are improved with satisfactory gloss 
values. Note that no reference is made to an improvement in coating 
properties. 
ILLUSTRATIVE CONSTRUCTION 
Now the illustrative construction of the present invention is described in 
more particularity. 
The magnetic recording medium of the invention includes a magnetic layer 
which contains a magnetic powder comprised of magnetic particles whose 
surface is composed mainly of carbon and/or iron carbide, an organic dye 
compound, and a binder. 
The magnetic powder whose surface is composed of iron carbide may be 
prepared by mixing an iron cyanide with a sulfate, sulfite or sulfide, 
placing the mixture in an iron-made reactor, and heat reducing the mixture 
while introducing CO into the reactor, followed by cooling. It may also be 
prepared by reducing an iron oxide, for example, iron oxyhydroxides such 
as .alpha.-FeOOH (goethite), .beta.-FeOOH (akaganite) and .gamma.-FeOOH 
(lepidocrocite), etc. or iron oxides such as .alpha.-Fe.sub.2 O.sub.3, 
.gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, .gamma.-Fe.sub.2 O.sub.3 
-Fe.sub.3 O.sub.4 (solid solution), etc., with carbon monoxide or a gas 
mixture of hydrogen and carbon monoxide as disclosed in JP-A 71509/1985 
and 124023/1985. 
An alternative preparation method is by reducing a slurry mixture of any of 
these iron oxides in an aqueous colloidal carbon black particle suspension 
with hydrogen, carbon monoxide or a mixture of hydrogen and carbon 
monoxide. Examples of the iron cyanides which can be used herein include 
hexacyano iron salts such as Turnbull's blue, Berlin white, etc., and 
ferro- and ferricyanides such as potassium ferrocyanide, sodium 
ferrocyanide, potassium ferricyanide, sodium ferricyanide, etc. Examples 
of the additives which can be used herein include sulfates such as 
potassium sulfate, sodium sulfate, ammonium sulfate, iron sulfate, sodium 
hydrogen sulfate, and potassium hydrogen sulfate; sulfites such as 
potassium sulfite, sodium sulfite, ammonium sulfite, and potassium 
hydrogen sulfite; and sulfides such as sodium thiosulfate, potassium 
thiosulfate, sodium sulfide, potassium sulfide, iron sulfide, sodium 
rhodanide, potassium rhodanide, sodium isothiocyanate, and potassium 
isothiocyanate. The gas used in the heat reducing atmosphere is not 
limited to CO, and carbon-bearing reducing gases such as CH.sub.4, water 
gas, and propane may also be used. Alternatively, pure iron particles are 
formed and subjected to any of the foregoing heat reducing treatments. For 
reduction purpose, a heating temperature of about 300.degree. to 
700.degree. C. and a heating time of about 30 minutes to about 10 hours 
may be employed. 
There is thus produced a magnetic powder represented by the formula 
Fe.sub.n C wherein n.gtoreq.2, especially n is from 2 to 3. Although it is 
not necessary that n be an integer or the material have a stoichiometric 
composition, there are often formed Fe.sub.2 C, Fe.sub.5 C.sub.2, and 
Fe.sub.3 C. Particles may have a graded concentration, and iron carbide 
need not necessarily be present throughout particles insofar as iron 
carbide is present at the surface. 
The magnetic powder whose surface is composed mainly of carbon may be 
prepared by heat treating the aforementioned iron carbide powder in a 
non-oxidizing atmosphere, especially in a non-oxidizing or inert gas 
atmosphere such as nitrogen, at 300.degree. to 400.degree. C. for 12 to 48 
hours. The thus obtained magnetic powder is a black powder having a carbon 
base surface and an iron base internal core. The iron is present 
essentially as .alpha.-iron in the particle core. This magnetic powder 
exhibits significantly high .sigma.s and good retention of magnetic 
properties with time as compared with conventional metallic magnetic 
powders obtained by reducing iron oxide. 
The presence of carbon at the magnetic powder surface can be judged by 
effecting secondary ion mass spectroscopy (SIMS) analysis to detect 
whether or not a C--C bond is present. The presence of .alpha.-iron can be 
confirmed by X-ray diffractometry (XRD). Preferably, the magnetic powder 
contains up to 20%, especially about 5 to 15% by weight of carbon with the 
balance of essentially .alpha.-iron because this composition ensures very 
high .sigma.s. Carbon contents insure retention of magnetic properties 
with time whereas .sigma.s becomes low with too high carbon contents. 
The above-mentioned magnetic powder whose surface is composed mainly of 
carbon or iron carbide is in needle or granular form and may be suitably 
selected in accordance with the intended application of the magnetic 
recording medium although particles having a major diameter or length of 
0.1 to 1 .mu.m and an aspect or length-to-breadth ratio of from 1 to 20 
are generally used. Needle form particles destined for video and audio 
tapes preferably have a length of 0.1 to 0.5 .mu.m and a needle ratio of 
from 4 to 15. 
Also, especially the magnetic powder having a carbon base surface should 
preferably have a specific surface area of about 20 to 70 m.sup.2 /g as 
measured by BET based on nitrogen adsorption. It has a coercivity Hc of 
1,000 to 1,800 Oe, especially 1,200 to 1,600 Oe and a saturation 
magnetization .sigma.s of at least 140 emu/g, especially 150 to 170 emu/g. 
It will be understood that a mixture of carbon and iron carbide may be 
present at the surface of magnetic powder according to the present 
invention. 
In addition to the magnetic powder whose surface is composed mainly of 
carbon or iron carbide, the magnetic powder used in herein may contain any 
of well-known magnetic powders, for example, oxide fine powders such as 
.gamma.-Fe.sub.2 O.sub.3, cobalt-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 
O.sub.4, cobalt-doped Fe.sub.3 O.sub.4, CrO.sub.2, barium ferrite and 
strontium ferrite and fine powders of Fe, Co, Ni and alloys thereof, in an 
amount of up to 50% by weight of the entire magnetic powder. 
Preferably the content of magnetic powder in the magnetic layer is 70 to 
95% by weight, more preferably 80 to 90% by weight. Higher contents would 
make it difficult to improve surface smoothness by calendering whereas 
with lower contents, the magnetic layer tends to lower its magnetic 
properties. 
The organic dye compound used herein has a polar group which is preferably 
a hydroxyl group, a carboxylic acid group, a sulfonic acid group or the 
like. The organic dye compounds has such a polar group include azo 
compounds, for example, .beta.-naphthol systems, .beta.-oxynaphthoic acid 
systems, .beta.-oxynaphthoic anilide systems, acetoacetic anilide systems, 
acetoacetic aminobenzimidazolone systems, pyrazolone systems, and fused 
azo systems, fused polycyclic compounds such as phthalocyanine systems, 
quinacridone systems, anthraquinone systems, perylene systems, perinone 
systems, thioindigo systems, dioxazine systems, isoindolenone systems, 
quinophthalone systems, and triphenylmethane systems, and metal complex 
compounds, such as azo, nitroso and azomethine systems. These compounds 
may take the form of a ligand without forming a complex with a metal. Also 
included are dyeing lake compounds, for example, acidic dye lake systems 
and basic dye lake systems. The polar group may be attached to the organic 
dye compound directly or through a saturated or unsaturated alkylene or 
arylene group having 1 to 20 carbon atoms which may be substituted 
directly or through a carboxylic amide or sulfonic amide. 
The polar group may form a salt with a mono- or di-valent metal cation or 
organic ammonium. Exemplary metals that form the metal salt are Na, K, Mg, 
Ca, Zn, Ba, Sr, Mn, Fe, Ni, Co, etc. The ammonium salt may be ammonium or 
any of aliphatic primary to quaternary ammonium salts. 
Preferably one to five polar groups, especially one to two polar groups are 
contained in the organic dye compound per molecule. Less polar groups 
would detract from dispersibility whereas too much polar groups would 
obstruct the bond between the magnetic powder surface and the binder, also 
detracting from dispersibility. 
In addition to the polar group, the organic dye compound used in the 
magnetic layer of the magnetic recording medium of the invention may have 
a substituent, for example, a saturated or unsaturated alkyl group having 
1 to 20 carbon atoms, aryl group, halogen atoms, --OR.sub.1, --NR.sub.1 
R.sub.2, --CONR.sub.1 R.sub.2, --SO.sub.2 NR.sub.1 R.sub.2, --SR.sub.1, 
--CN, --NO.sub.2, --NR.sub.1 COR.sub.2, --NR.sub.1 SO.sub.2 R.sub.2, 
--COOR.sub.3, --OCOR.sub.2 wherein each of R.sub.1 and R.sub.2 is H, a 
saturated or unsaturated alkyl group having 1 to 20 carbon atoms or an 
aryl group, and R.sub.3 is a saturated or unsaturated alkyl group having 1 
to 20 carbon atoms. 
Containment of one or more of these compounds and a magnetic powder whose 
surface is composed mainly of carbon or iron carbide not only improves the 
dispersion of the magnetic powder in the magnetic paint, but also offers 
the significant benefits that the magnetic paint is changed in viscosity 
and reduced in yield value. 
Preferably the content of the organic dye compound is 0.5 to 10 parts by 
weight, more preferably 3 to 7 parts by weight per 100 parts by weight of 
the magnetic powder. Higher contents would allow the organic dye compound 
to precipitate out of the magnetic layer, inviting clogging of the 
magnetic head. Lower contents would be less effective for improving the 
dispersion of magnetic powder in the magnetic paint and lowering the yield 
value. The organic dye compound is present dispersed in the magnetic layer 
while its diameter is of the order of 5 nm to 20 nm. 
In the practice of the invention, the magnetic powder whose surface is 
composed mainly of carbon or iron carbide is preferably pretreated for 
precluding aggregation of the powder and causing the powder to bear the 
organic dye compound thereon for providing improved dispersion with the 
binder as will be described later. More particularly, the magnetic powder 
and an organic solvent may be admitted into a ball mill, optionally 
together with one or more organic dye compounds, and agitated therein for 
milling and dispersion, or they may be kneaded and dispersed in a kneader. 
The organic solvent used herein is not particularly limited and may be any 
of solvents commonly used in magnetic coating compositions, for example, 
one or more of the solvents which will be described later. Most effective 
are ketone and aromatic solvents, especially cyclohexanone, methyl ethyl 
ketone, methyl isobutyl ketone, and toluene. More than one of these ketone 
and aromatic solvents may be used. Alternatively, various organic solvents 
which will be described later may be mixed into a mixture wherein ketone 
and aromatic solvents occupy more than 50% by weight. 
Preferably the amount of solvent mixed with the magnetic powder in the 
pre-treatment step is 20 to 50% by weight based on 100 parts by weight of 
the magnetic powder. Excess amounts could not fully eliminate aggregation 
whereas lesser amounts would cause fracture of particles. 
In the pretreatment step, the kneading time is about 15 minutes to 12 
hours. Then the organic solvent is borne on at least a portion of the 
magnetic powder surface. In this regard, the solvent not only covers the 
surface, but can also penetrate into the magnetic powder. The organic 
solvent can undergo air oxidation or be modified by magnetic particles 
whereupon the modified solvent covers the magnetic powder surface. 
The magnetic powder which has been pre-treated in this way so as to carry 
the solvent and the organic dye compound thereon is further kneaded with 
the organic solvent along with the binder, inorganic fine particles, and 
optionally, a dispersant other than the organic dye compound, a lubricant, 
and other additives, obtaining a magnetic paint or coating composition. 
The preferred binder used herein is a binder containing an amino group or 
an ammonium salt group. The binder used herein may be obtained by 
copolymerizing a monomer containing an amino group or an ammonium salt 
group or by reacting a resin with amine or the like for amino or ammonium 
modification. Preferably the binder resin contains about 300 to 1,000 ppm 
of N from the amino group or ammonium salt group in its molecule. Higher 
contents would allow for agglomeration between binder molecules, 
detracting from dispersibility. Lower contents would also fail to achieve 
satisfactory dispersion. 
The resin containing an amino group and/or ammonium salt group as a 
functional group preferably has a number average molecular weight of about 
10,000 to 200,000. Exemplary resin skeletons include vinyl 
chloride-(meth)acrylate copolymers (which may contain an epoxy group), 
vinyl chloride-vinyl acetate copolymers (which may contain carboxylic 
acids), vinyl chloride-vinyl alcohol-vinyl acetate copolymers (which may 
contain carboxylic acids), phenolic resins, epoxy resins, urethane resins, 
vinyl chloride-vinylidene chloride copolymers, urea resins, butyral 
resins, formal resins, melamine resins, alkyd resins, etc. An amino or 
ammonium group is introduced into these resin skeletons, often at their 
side chain. 
The resin containing an amino group or ammonium salt group should occupy at 
least 50% by weight of the entire binder. The binder used herein may be an 
electron beam curable, thermoplastic, thermosetting or reactive resin or a 
mixture thereof although the thermosetting and electron beam curable 
resins are preferred for film strength and other reasons. Where another 
resin free of an amino group or ammonium group is additionally used, it 
may be selected from conventional well-known resins. The binder content of 
the magnetic layer is not particularly limited although the binder content 
is preferably 15 to 25 parts by weight per 100 parts by weight of the 
magnetic powder. 
Preferred examples of the thermosetting resins include mixtures of a 
crosslinking agent and a vinyl copolymeric resin such as vinyl 
chloride-(meth)acrylate copolymers (which may contain an epoxy group), 
vinyl chloride-vinyl acetate copolymers (which may contain carboxylic 
acid), vinyl chloride-vinyl alcohol-vinyl acetate copolymers (which may 
contain carboxylic acid), vinyl chloride-vinylidene chloride copolymers, 
chlorinated polyvinyl chloride, vinyl chloride-acrylonitrile copolymers, 
vinyl butyral copolymers, vinyl formal copolymers, etc.; mixtures of a 
crosslinking agent and a cellulosic resin such as nitrocellulose, 
cellulose acetobutyrate, etc.; mixtures of a crosslinking agent and a 
synthetic rubber such as butadiene-acrylonitrile, etc.; resins of 
condensation polymerization type such as phenol resins, epoxy resins, 
polyurethane curable resins, urea resins, butyral resins, formal resins, 
melamine resins, alkyd resins, silicone resins, acrylic reactive resins, 
polyamide resins, epoxypolyamide resins, saturated polyester resins, and 
ureaformaldehyde resins; mixtures of a high molecular weight polyester 
resin and an isocyanate prepolymer, mixtures of a methacrylate copolymer 
and a diisocyanate prepolymer, mixtures of a polyester polyol and a 
polyisocyanate, mixtures of low molecular weight glycol/high molecular 
weight diol/triphenylmethane triisocyanate, etc.; and mixtures of any one 
of the foregoing condensation polymerization resins and a crosslinking 
agent such as isocyanates, wherein these resins contain an amino group or 
ammonium salt group. 
The crosslinking agents which can be used to cure these binder resins 
include various polyisocyanates, preferably diisocyanates such as tolylene 
diisocyanate, hexamethylene diisocyanate and methylene diisocyanate. These 
crosslinking agents are reactive with hydroxyl, amino or ammonium salt 
groups in the binder resins, thereby causing crosslinking of the binder 
resins. Usually 10 to 30 parts by weight of the crosslinking agent is used 
per 100 parts by weight of the resin. These thermosetting resins are 
generally cured by heating in an oven at 50.degree. to 70.degree. C. for 
12 to 48 hours. 
Among the preferred binders are electron beam-curable resins, that is, 
resins obtained by curing electron beam-curable compounds. Illustrative 
electron beam-curable resins are thermoplastic resins having contained or 
incorporated in their molecule groups capable of crosslinking or 
polymerizing upon exposure to electron beams, for example, acrylic double 
bonds as given by acrylic and methacrylic acids having an unsaturated 
double bond capable of radical polymerization and esters thereof, allyl 
double bonds as given by diallyl phthalate, and unsaturated bonds as given 
by amine-modified maleic acid and maleic derivatives. Other compounds 
having unsaturated double bonds capable of crosslinking or polymerizing 
upon exposure to electron beams may also be used. 
The thermoplastic resins which can be modified into electron beam-curable 
resins include, for example, vinyl chloride copolymers, vinyl 
chloride-vinyl acetate-vinyl alcohol copolymers, vinyl chloride-acrylic 
copolymers, epoxy resins of saturated polyesters, urethane resins, phenoxy 
resins, and cellulosic resins. They may be acryl modified in a 
conventional manner. Thereafter, amino or ammonium salt groups are 
incorporated therein. 
Preferably, these resins have a number average molecular weight of 5,000 to 
50,000. 
Inorganic fine particles such as .alpha.-Al.sub.2 O.sub.3, Cr.sub.2 
O.sub.3, TiO.sub.2, SiC and .alpha.-Fe.sub.2 O.sub.3 may be added to the 
magnetic paint for enhancing the mechanical strength of a magnetic layer 
formed therefrom. If desired, the magnetic paint may further contain 
lubricants such as carbon black and silicone oil and various other 
additives. 
Further, another dispersant such as a surfactant may be used in combination 
with the organic dye compound. The surfactants used herein are preferably 
anionic and ampholytic surfactants. The preferred anionic surfactants used 
herein are those having a hydrophilic group in the form of a carboxylate 
salt, sulfonate salt, sulfate ester, phosphate ester, taurinate salt or 
phosphonate salt. The ampholytic surfactants are preferably those having 
betaine, phosphocholine, amic acid, aminosulfate, sulfobetaine, etc. as a 
hydrophilic group. These dispersants may be added during the 
above-mentioned pretreatment as is the organic dye compound. 
The solvent used in the magnetic paint is not particularly limited, but is 
preferably selected from ketones such as cyclohexanone, methyl ethyl 
ketone and methyl isobutyl ketone, esters such as ethyl acetate and butyl 
acetate, aromatics such as toluene and xylene, tetrahydrofuran, and 
dimethylformamide alone or mixtures thereof. The amount of the solvent 
used in the magnetic paint is not particularly limited although about 150 
to 1,000 parts by weight of the solvent is preferably mixed with 100 parts 
by weight of the ferromagnetic powder. 
The magnetic paint having a composition as mentioned above is coated onto a 
non-magnetic substrate to form a magnetic layer which is subject to 
orientation, drying, calendering, curing and the like before a magnetic 
recording medium is obtained. 
The material of which the non-magnetic substrate is made is not 
particularly limited and may be selected for a particular purpose from 
various flexible materials and various rigid materials and configured to a 
predetermined shape (such as tape) and size meeting any desired one of the 
standards. Exemplary flexible materials are various resins including 
polyesters such as polyethylene terephthalate and polyethylene naphthalate 
and polyamides. 
Although the thickness of the magnetic layer varies with a particular 
application and purpose, it is generally about 0.1 to 4 .mu.m thick. If 
desired, the magnetic layer may have a multi-layer structure, an undercoat 
layer may be formed between the non-magnetic substrate and the magnetic 
layer, and a backcoat layer may be provided.

EXAMPLE 
Examples of the present invention are given below by way of illustration. 
Example 1 
An iron carbide powder consisting essentially of Fe.sub.5 C.sub.2 was heat 
treated in H.sub.2, obtaining a magnetic powder having a coercivity Hc of 
1,550 Oe, a saturation magnetization .sigma.s of 150 emu/g, and a specific 
surface area of 60 m.sub.2 /g as measured by a BET method. On SIMS 
analysis of this powder, the presence of C--C bond and predominance of 
carbon on the surface were ascertained. On X-ray diffractometry, the peak 
of Fe.sub.5 C.sub.2 almost disappeared and the peak of .alpha.-Fe newly 
appeared. The carbon content was about 10% by weight. 
This magnetic powder, 15 grams, was admitted into a vibratory ball mill 
having an internal volume of 100 ml charged with 230 grams of steel balls 
with a diameter of 2.5 mm. To the magnetic powder was added 4.5 grams of 
methyl ethyl ketone as a solvent. The ingredients were kneaded and 
dispersed at room temperature for one hour. Thereafter, 0.45 gram of an 
organic dye compound I shown below was added and the ingredients were 
further kneaded and dispersed for 1/2 hour. 
##STR1## 
Using the magnetic powder having the solvent and organic dye compound 
carried thereon, the following composition was prepared. 
______________________________________ 
Composition 
Magnetic powder 100 pbw 
Dispersant (organic dye compound I) 
3 pbw 
Vinyl chloride resin 14 pbw 
polar group: ammonium salt group 
(average ammonium salt group 
per molecule: 500 ppm of N) 
degree of polymerization: .about.310 
Polyurethane resin 6 pbw 
acid: t-PhA/i-PhA/AA glycol: 
NPG/HG/EG/MPD 
chain extender: NPG isocyanate: MDI 
polar group: sulfonic acid group, ammonium salt 
group number average molecular weight: .about.19,000 
.alpha.-Al.sub.2 O.sub.3 10 pbw 
Stearic acid 1 pbw 
Butyl stearate 0.5 pbw 
Methyl ethyl ketone 105 pbw 
Toluene 70 pbw 
Cyclohexanone 105 pbw 
______________________________________ 
The composition was dispersed in a vibratory ball mill. Then 3 parts by 
weight of tolylene diisocyanate was added to the composition, which was 
coated onto a polyester film of 22 .mu.m thick and dried in an orienting 
magnetic field of 2,000 G applied. The coating was then subject to 
calendering under a gage pressure of 21 kg/cm.sup.2 (linear pressure 200 
kg/cm) at 60.degree. C. and curing reaction for 24 hours, obtaining a 
sample No. 1. The magnetic layer had a final thickness of 3.0 .mu.m. 
Sample No. 1 was cut to a width of 1/2 inch, obtaining a video tape 
sample. 
For evaluating the dispersibility of the resulting sample No. 1, it was 
measured for 60.degree. gloss by means of a gloss meter, squareness ratio 
Br/Bm by means of a VSM, and surface roughness by means of a contact type 
surface roughness meter. For the evaluation of coating properties, the 
yield value of the magnetic paint prior to the addition of the curing 
agent was calculated from a Casson plot determined by means of a cone 
plate viscometer, and streaks at the coating surface immediately after 
drying were observed under an optical microscope with a magnifying power 
of 50.times. to count the number of streaks per millimeter transversely of 
the tape. Further, using a VCR deck for RF output measurement, the VHS 
video tape form of sample No. 1 was measured for reproduction output at 7 
MHz. The results are collectively shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Dispersant Squareness 
Yield Surface 
Surface 
RF 
Sample Amount 
Gloss 
ratio value roughness 
streaks 
output 
No. Compound 
(pbw) 
(%) Br/Bm (dyn .multidot. cm) 
(nm) (count) 
(dB) 
__________________________________________________________________________ 
1 I 3 169 0.870 402 3.9 1 +1.3 
2 I 1 165 0.856 421 4.3 3 +1.0 
3 II 3 170 0.868 405 3.6 1 +1.5 
4 III 3 158 0.867 410 3.9 1 +1.2 
5 IV 3 167 0.866 410 3.9 1 +1.1 
6 V 3 168 0.868 406 3.8 0 +1.4 
7 VI 3 171 0.870 401 3.7 0 +1.6 
8 VII 3 169 0.868 403 3.7 1 +1.5 
9 phosphate 
3 151 0.831 485 6.8 11 +0.6 
(comparison) 
10 -- 0 130 0.788 510 8.2 18 0.0 
(comparison) 
__________________________________________________________________________ 
Example 2 
Sample No. 2 was obtained by the same procedure as Example 1 except that 
the amount of organic dye compound I added was changed to 1 part by 
weight. The resulting sample was evaluated as in Example 1. The results 
are collectively shown in Table 1. 
Example 3 
Sample No. 3 was obtained by the same procedure as Example 1 except that 
the following organic dye compound II was used instead of organic dye 
compound I. The resulting sample was evaluated as in Example 1. The 
results are collectively shown in Table 1. 
##STR2## 
Example 4 
Sample No. 4 was obtained by the same procedure as Example 1 except that 
the following organic dye compound III was used instead of organic dye 
compound I. The resulting sample was evaluated as in Example 1. The 
results are collectively shown in Table 1. 
##STR3## 
Example 6 
Sample No. 5 was obtained by the same procedure as Example 1 except that 
the following organic dye compound IV was used instead of organic dye 
compound I. The resulting sample was evaluated as in Example 1. The 
results are collectively shown in Table 1. 
##STR4## 
Example 6 
Sample No. 6 was obtained by the same procedure as Example 1 except that 
the following organic dye compound V was used instead of organic dye 
compound I. The resulting sample was evaluated as in Example 1. The 
results are collectively shown in Table 1. 
##STR5## 
Example 7 
Sample No. 7 was obtained by the same procedure as Example 1 except that 
the following organic dye compound VI was used instead of organic dye 
compound I. The resulting sample was evaluated as in Example 1. The 
results are collectively shown in Table 1. 
##STR6## 
Example 8 
Sample No. 8 was obtained by the same procedure as Example 1 except that 
the following organic dye compound VII was used instead of organic dye 
compound I. The resulting sample was evaluated as in Example 1. The 
results are collectively shown in Table 1. 
##STR7## 
Comparative Example 1 
Sample No. 9 was obtained by the same procedure as Example 1 except that a 
phosphate ester dispersant (RE-610, manufactured by Toho Chemical K.K. ) 
was used instead of organic dye compound I. The resulting sample was 
evaluated as in Example 1. The results are collectively shown in Table 1. 
Comparative Example 2 
Sample No. 10 was obtained by the same procedure as Example 1 except that 
organic dye compound I was omitted. The resulting sample was evaluated as 
in Example 1. The results are collectively shown in Table 1. 
Example 9 
There was furnished a magnetic powder having a coercivity Hc of 950 Oe, a 
saturation magnetization .sigma.s of 96 emu/g, and a specific surface area 
of 48 m.sup.2 /g as measured by a BET method. On X-ray diffractometry and 
SIMS analysis of this powder, the presence of iron carbide (Fe.sub.5 
C.sub.2) at the surface was ascertained. Sample No. 11 was obtained by the 
same procedure as Example 1 except that this magnetic powder was used. The 
resulting sample was evaluated as in Example 1. Note that the RF output 
was measured with the measurement frequency changed from 7 MHz to 5 MHz. 
The results are collectively shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Dispersant Squareness 
Yield Surface 
Surface 
RF 
Sample Amount 
Gloss 
ratio value roughness 
streaks 
output 
No. Compound 
(pbw) 
(%) Br/Bm (dyn .multidot. cm) 
(nm) (count) 
(dB) 
__________________________________________________________________________ 
11 I 3 171 0.885 364 3.9 1 +1.9 
12 II 3 170 0.887 362 3.8 0 +1.8 
13 III 3 169 0.886 360 3.9 0 +1.8 
14 IV 3 168 0.883 363 4.2 2 +1.5 
15 V 3 169 0.885 361 4.0 1 +1.7 
16 VI 3 172 0.888 359 3.8 0 +2.0 
17 VII 3 171 0.888 362 3.7 0 +2.1 
18 phosphate 
3 155 0.845 432 6.6 10 +0.8 
(comparison) 
19 -- 0 132 0.795 471 7.9 15 0.0 
(comparison) 
__________________________________________________________________________ 
Example 10 
Sample No. 12 was obtained by the same procedure as Example 1 except that 
there were used the magnetic powder used in Example 9 and organic dye 
compound II used in Example 3. The resulting sample was evaluated as in 
Example 9. The results are collectively shown in Table 2. 
Example 11 
Sample No. 13 was obtained by the same procedure as Example 1 except that 
there were used the magnetic powder used in Example 9 and organic dye 
compound III used in Example 4. The resulting sample was evaluated as in 
Example 9. The results are collectively shown in Table 2. 
Example 12 
Sample No. 14 was obtained by the same procedure as Example 1 except that 
there were used the magnetic powder used in Example 9 and organic dye 
compound IV used in Example 5. The resulting sample was evaluated as in 
Example 9. The results are collectively shown in Table 2. 
Example 13 
Sample No. 15 was obtained by the same procedure as Example 1 except that 
there were used the magnetic powder used in Example 9 and organic dye 
compound V used in Example 6. The resulting sample was evaluated as in 
Example 9. The results are collectively shown in Table 2. 
Example 14 
Sample No. 16 was obtained by the same procedure as Example 1 except that 
there were used the magnetic powder used in Example 9 and organic dye 
compound VI used in Example 7. The resulting sample was evaluated as in 
Example 9. The results are collectively shown in Table 2. 
Example 15 
Sample No. 17 was obtained by the same procedure as Example 1 except that 
there were used the magnetic powder used in Example 9 and organic dye 
compound VII used in Example 8. The resulting sample was evaluated as in 
Example 9. The results are collectively shown in Table 2. 
Comparative Example 3 
Sample No. 18 was obtained by the same procedure as Comparative Example 1 
except that there was used the magnetic powder used in Example 9. The 
resulting sample was evaluated as in Example 9. The results are 
collectively shown in Table 2. 
Comparative Example 4 
Sample No. 19 was obtained by the same procedure as Comparative Example 2 
except that there was used the magnetic powder used in Example 9. The 
resulting sample was evaluated as in Example 9. The results are 
collectively shown in Table 2. 
As understood from the results of Table 1 using a magnetic powder whose 
surface was composed mainly of carbon and the results of Table 2 using a 
magnetic powder whose surface was composed mainly of iron carbide, those 
samples using organic dye compounds according to the invention as a 
dispersant exhibited good results with respect to all of gloss, squareness 
ratio, yield value, surface roughness, the number of streaks extending 
longitudinally of tape, and RF output.