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BACKGROUND OF THE INVENTION
The present invention relates to a magnetic recording medium having a
lubricant layer on a magnetic layer or on an inorganic protective layer
formed over a magnetic layer. More particularly, the present invention
relates to a metal thin film type magnetic recording medium which is
excellent in running properties, durability and shelf stability.
Magnetic recording mediums, e.g. magnetic tapes, floppy disks, etc., are
commonly coated at the magnetic layer surface thereof with a lubricant to
improve lubrication between the magnetic recording medium and the magnetic
head and to enhance the running durability of the magnetic recording
medium.
With the achievement of high recording density, the magnetic layer surfaces
of magnetic recording mediums have become smoother, and the coefficient of
friction has become extremely large. Thus, durability concerning the
sliding contact with equipment has become a serious problem. Further,
magnetic recording mediums are used under various environmental
conditions, and information recorded thereon are usually stored for a long
period of time ranging from several years to several tens of years.
Accordingly, magnetic recording mediums are demanded to have running
durability and shelf stability under a wide variety of environmental
conditions. Under these circumstances, conventional lubricants have become
unable to produce satisfactory effect.
Further, small-sized video tape recorders such as cam coders and 8-mm VTR
are often used outdoor. Therefore, magnetic recording mediums are demanded
to endure use in a wide range of environmental conditions.
To improve magnetic properties required for high-density magnetic
recording, e.g., high-definition recording, digital recording, etc., metal
thin film type magnetic recording mediums that use a ferromagnetic metal
thin film as a magnetic layer and that are useful for high-density
magnetic recording have been expected to replace the conventional magnetic
recording mediums. However, in a metal thin film type magnetic recording
medium, the magnetic layer is protected by only an extremely thin metallic
oxide layer. Therefore, it has been demanded to provide a lubricant layer
material which is capable of not only ensuring running properties and
durability but also markedly improving shelf stability of metal thin film
type magnetic recording mediums. Further, metal thin film type magnetic
recording mediums are being improved so as enable achievement of
higher-density recording by smoothing the magnetic layer surface and also
changing the composition of the magnetic layer from CoNi--O to Co--O or
Co--Fe that contains Co--O, etc. so as to attain high coercive force (Bm).
With the conventional metal thin film type magnetic recording mediums that
have a high cobalt content, however, it has heretofore been difficult
obtain stable running properties, durability and corrosion resistance.
Studies have also been conducted of a method for improving durability by
forming on the magnetic layer a protective layer of, for example, silica,
alumina, titania, graphite, amorphous carbon, cobalt oxide, titanium
nitride, chromium carbide, etc. With this method, however, the thickness
of the protective layer must be increased in order to obtain satisfactory
protective action, resulting in spacing loss. Accordingly, the output
reduces, and the achievement of high recording density is hindered,
unfavorably.
Under the above-described circumstances, hydrocarbon- or
fluorine-containing lubricants which have various structures, and which
are excellent in lubricating performance have been examined as being
lubricants for metal thin film type magnetic recording mediums, and it is
known that an organic fluorine compound having a hydrophilic functional
group in a molecule thereof is capable of improving running durability to
a considerable extent.
However, even if a lubricant having excellent characteristics is used, a
smooth metal thin film type magnetic recording medium having improved
electromagnetic transducing characteristics suffers from the problem that
the lubricant attached to the surface of the magnetic layer is gradually
lost because of repeated sliding movement, resulting in deterioration of
the characteristics.
One approach that has heretofore been made to improve the characteristics
is to introduce various polar groups into the lubricant. For example,
Japanese Patent Application Laid-Open (KOKAI) No. 59-119537 (U.S. Pat. No.
No. 4,537,832) and Japanese Patent Application Post-Examination
Publication No. 04-50644 disclose a technique in which a carboxyl group,
an ester group, and a phosphoric ester are introduced. The disclosed
technique involves, however, the problem that repeated running durability
at low temperature is inferior. There have also been made many studies to
improve the characteristics by using two or more different kinds of
lubricant in combination.
Japanese Patent Application Laid-Open (KOKAI) No. 62-141625 discloses the
use of a carboxylic acid and a fluorine-containing ester in combination.
With this method, the repeated running durability at low temperature can
be markedly improved in comparison to a case where each lubricant is used
alone. However, the magnetic recording medium using a carboxylic acid and
a fluorine-containing ester in combination has the problem that it is
inferior in corrosion resistance, particularly shelf stability under
high-temperature and high-humidity conditions.
Japanese Patent Application Laid-Open (KOKAI) Nos. 62-236120 (U.S. Pat. No.
4,735,848) and 62-103824 disclose the use of a phosphoric triester or a
phosphoric diester and a fluorine-containing ester lubricant. However,
since triesters and diesters exhibit weak adsorption to a magnetic film,
the wear resistance cannot be improved. Therefore, satisfactory
characteristics cannot necessarily be obtained.
Japanese Patent Application Laid-Open (KOKAI) No. 04-205712 proposes a
method in which a lubricant layer is formed from two different kinds of
lubricant, and in which the amounts of lubricant at the magnetic layer
side and at the back coat layer side are specified. With this method,
however, no satisfactory lubricating properties can be obtained.
Japanese Patent Application Post-Examination Publication No. 57-29767 and
Japanese Patent Application Laid-Open (KOKAI) Nos. 58-188326, 60-63711,
60-63712, 62-209718 and 01-211215 disclose a method in which a lubricant
is applied to a back coat layer provided on the side of a magnetic
recording medium which is reverse to the side thereof on which a magnetic
layer is formed, thereby enabling the lubricant to be appropriately
supplied from the back coat layer to the magnetic layer surface where the
lubricant is gradually lost by sliding movement. Such a coating method
enables an improvement in repeated running durability. However, when a
lubricant is coated on only the back coat layer, durability is difficult
to ensure; when a lubricant is coated on both the back coat layer and the
magnetic layer, a surplus of lubricant is inevitably present on the
magnetic layer. Consequently, the coefficient of static friction rises,
giving rise to problems such as undesired sticking of the magnetic
recording medium.
In terms of corrosion resistance, which is a problem in practical use of
magnetic recording mediums that use a ferromagnetic metal thin film as a
magnetic layer, the required corrosion resistance cannot sufficiently be
ensured by only a fluorine-containing lubricant having a polar group in a
molecule thereof. Accordingly, it has been proposed to use such a
lubricant in combination with a rust preventive. However, it has
heretofore been difficult to ensure the required corrosion resistance by
the combined use of a lubricant and a rust preventive that are selected
from those which are generally known.
An object of the present invention is to provide a magnetic recording
medium which has the coefficient of friction stabilized at a low level,
and which is excellent in repeated running durability and corrosion
resistance and capable of high-density magnetic recording in particular.
SUMMARY OF THE INVENTION
The present invention provides a magnetic recording medium having a
magnetic layer of a ferromagnetic metal thin film formed over a
non-magnetic base, wherein the improvement has a lubricant layer
containing a phosphoric monoester compound which is represented by the
following formula (1), and a fluorine group-containing carboxylic acid
ester compound having an alkylene oxide group, which is represented by the
following formula (2) or (3), on the magnetic layer or on an inorganic
protective layer formed over the magnetic layer:
R.sup.1 --OPO(OH).sub.2 Formula ( 1)
where R.sup.1 is a hydrocarbon group having from 8 to 26 carbon atoms.
Rf.sup.1 --R.sup.2 --COO--(R.sup.3 O).sub.n --R.sup.4 FORMULA ( 2)
Rf.sup.2 --R.sup.2 --COO--(R.sup.3 O).sub.n --R.sup.4 --Rf.sup.3Formula ( 3
)
where Rf.sup.1, Rf.sup.2, Rf.sup.3 : (CF.sub.3).sub.a CF.sub.3 -a
(CF.sub.2).sub.b -- a: 1 to 3 a+b: 1 to 11 n=l to 8
R.sup.2 is an alkylene or alkenyl group having from 2 to 14 carbon atoms;
R.sup.3 is an alkylene group having from 1 to 4 carbon atoms; and
R.sup.4 is an alkylene group having from 2 to 18 carbon atoms.
In the above-described magnetic recording medium, an inorganic protective
layer may be provided between the ferromagnetic metal thin film and the
lubricant layer.
In the above-described magnetic recording medium, a back coat layer may be
provided on the side of the non-magnetic base which is reverse to the side
on which the ferromagnetic metal thin film is provided. The back coat
layer contains the above-described alkylene oxide-containing fluorinated
carboxylic acid ester compound.
In the above-described magnetic recording medium, the ferromagnetic metal
thin film is formed from a ferromagnetic metal containing not less than 90
at. % cobalt.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present inventors have found that a magnetic layer having a highly
smooth surface and high electromagnetic transducing characteristics can be
obtained by providing a lubricant layer which contains a phosphoric
monoester compound and a fluorine-containing carboxylic acid ester on the
magnetic layer or on an inorganic protective layer formed over the
magnetic layer, and further forming a layer in which the
fluorine-containing carboxylic acid ester is present on the phosphoric
monoester compound, thereby smoothing the surface and further raising the
coercive force (Bm) of the magnetic layer material, and that the magnetic
layer thus formed exhibits high running durability and shelf stability.
Our research conducted thereafter has revealed that, among lubricants
used, a fluorine-containing carboxylic acid ester that is liquid at
ordinary room temperature is particularly effective in improving still
durability and other durability under various environmental conditions,
and that such a lubricant exhibits excellent characteristics without
precipitating during storage even if the amount of lubricant added to the
back coat layer is somewhat increased. Known fluorine-containing
carboxylic acid esters which are liquid at ordinary room temperature are
those which contain an unsaturated bond in a molecule thereof or have a
branched structure, thereby reducing the interaction between molecules.
These lubricants exhibit excellent characteristics as described above,
but, on the other hand, they have the disadvantage that it is difficult to
orient them densely on the magnetic layer or the protective layer after
coating because of the weak interaction between molecules. Consequently,
the .mu. value becomes relatively high. Carboxylic acid esters having an
unsaturated bond have a problem in terms of long-term shelf stability
because the unsaturated bond is readily oxidized in the air.
Under these circumstances, we conducted studies in order to obtain a
lubricant free from the above-described disadvantages, and as a result,
have found that a lubricant having an alkylene oxide bond in a molecule of
a fluorine-containing carboxylic acid ester is liquid at ordinary room
temperature, and it exhibits favorably low .mu. value and superior shelf
stability while maintaining excellent characteristics of the liquid
lubricant. The present invention has been accomplished on the basis of
this finding.
The present invention is a magnetic recording medium which is provided with
a lubricant layer that includes a lubricant consisting essentially of a
phosphoric monoester compound, and a lubricant consisting essentially of a
fluorine-containing carboxylic acid ester having an alkylene oxide group.
The phosphoric monoester compound is strongly adsorbed on the magnetic
layer and also capable of improving corrosion resistance to a considerable
extent. Examples of such phosphoric monoester compounds include monolauryl
phosphate, monohexadecyl phosphate, etc.
The hydrophobic chain of the phosphoric monoester compound is preferably a
straight-chain saturated hydrocarbon group, but it may be an unsaturated
hydrocarbon group, or a hydrocarbon group having a branched structure in
which a side chain has been introduced. Regarding the length of the
saturated hydrocarbon group, the number of carbon atoms in the main chain
is preferably in the range of from 8 to 26. If the number of carbon atoms
is smaller than 8, no favorable protective effect can be obtained,
whereas, if it is larger than 26, crystallizability increases, causing the
coatability to reduce. As a result, the lubricating effect reduces.
In the formula (2) or (3) representing a fluorine compound which is used to
produce the magnetic recording medium of the present invention, it is even
more preferable that a in Rf.sup.1, Rf.sup.2 and Rf.sup.3 should be from 1
to 2, a+b should be from 1 to 10, and n should be from 1 to 3. Further,
R.sup.2, R.sup.3 R.sup.4, Rf.sup.1, Rf.sup.2 and Rf.sup.3 may be branched,
and it is even more preferable that R.sup.2 should be an alkylene group
having from 2 to 10 carbon atoms, R.sup.3 should be an alkylene or alkenyl
group having from 1 to 4 carbon atoms, and R.sup.4 should be an alkylene
group having from 3 to 14 carbon atoms.
If R.sup.2 is not smaller than 11, the compound cannot readily be adsorbed
on the magnetic layer, resulting in a rise in the coefficient of friction.
If R.sup.2 or R.sup.3 exceeds the upper limit of the above range,
durability is deteriorated; particularly, the reduction of output
increases, unfavorably, If R.sup.2 or R.sup.3 is not larger than the lower
limit of the above range, the solubility of the compound in a solvent
becomes insufficient, or the volatility of the compound becomes
undesirably high. Consequently, the compound cannot exhibit satisfactory
characteristics.
The alkylene oxide group in the fluorine-containing carboxylic acid ester
compound of the present invention is capable of lowering the melting point
of the ester. Thus, it is possible to obtain uniformity of the coating
film and attain favorable running durability under various service
environmental conditions. Further, the fluorine-containing carboxylic acid
ester compound having an alkylene oxide group is considerably superior in
stability to fluorine-containing carboxylic acid ester compounds having an
unsaturated bond which have heretofore been known, and hence considered to
be capable of maintaining the original characteristics even if it is
stored for a long time.
Furthermore, the fluorine-containing carboxylic acid ester compound of the
present invention has an alkylene oxide group of specific length which is
introduced into the center of a molecule thereof so as to lie adjacent to
the ester group, thereby enhancing the adsorption of the molecule to the
surface of the magnetic recording medium. Further, since the alkylene
oxide group is not so corrosive as an oxo acid in the same way as the
ester group, the compound exhibits excellent corrosion resistance.
Accordingly, even if the compound is coated in excess to such an extent
that there are molecules which are not adsorbed on the surface of the
magnetic recording medium, the molecules are less likely to corrode the
surface of the magnetic layer or the magnetic head. Further, an alkyl
group is bonded adjacently to the alkylene oxide group, thereby weakening
corrosiveness due to the hydrophilic nature of the alkylene oxide group,
and thus improving the corrosion resistance.
In addition, since both the ester group and the alkylene oxide group
display small interaction between molecules, it is easy for the molecules
to behave independently. Accordingly, the liquid lubricating properties
can be enhanced, and thus the coefficient of friction of the magnetic
recording medium can be effectively reduced.
The following are specific examples of fluorine compounds usable for the
magnetic recording medium of the present invention:
C.sub.8 F.sub.17 COO(CH.sub.2 CH.sub.2 O).sub.2 C.sub.12 H.sub.25
C.sub.6 F.sub.13 C.sub.8 H.sub.16 COO(CH.sub.2 CH.sub.2 O).sub.6 C.sub.2
H.sub.4 C.sub.4 F.sub.9
C.sub.8 F.sub.17 C.sub.14 H.sub.28 COO(C.sub.6 H.sub.12 O)C.sub.8 H.sub.17
(CF.sub.3).sub.2 CFC.sub.10 H.sub.20 COO(C.sub.6 H.sub.12 O).sub.4 C.sub.14
H.sub.29
C.sub.8 F.sub.17 C.sub.2 H.sub.4 COO(C.sub.8 H.sub.16 O).sub.2 C.sub.5
H.sub.10 CF(CF.sub.3).sub.2
These alkylene oxide-containing fluoroalkyl carboxylic acid esters can be
obtained by synthesizing a corresponding carboxylic acid chloride and an
alkoxy alcohol containing or not containing fluorine in the presence of a
base such as triethylamine, and purifying the resulting ester by vacuum
distillation or recrystallization.
In the magnetic recording medium of the present invention, the lubricant
layer can be formed on the magnetic recording medium surface by either an
intermix application method in which a phosphoric monoester compound and a
fluorine-containing carboxylic acid ester compound having an alkylene
oxide group, which are used as lubricants, are dissolved in an organic
solvent, and the resulting solution is coated on the surface of the
magnetic recording medium, or a successive application method in which
after the phosphoric monoester compound has been coated, the
fluorine-containing carboxylic acid ester compound having an alkylene
oxide group is coated. However, since the phosphoric monoester compound
has excellent adsorptivity, it is preferable to form, first, a lubricant
layer of a phosphoric monoester compound and, thereafter, form a lubricant
layer of a fluorine-containing carboxylic acid ester compound having an
alkylene oxide group.
It is particularly preferable to employ a method in which, after a solution
of a phosphoric monoester compound in an organic solvent has been coated
on the surface of the magnetic recording medium and dried, a back coat
layer containing a lubricant of a fluorine-containing carboxylic acid
ester compound having an alkylene oxide group is formed, and then the
magnetic recording medium is wound up, thereby transferring the lubricant
of the fluorine-containing carboxylic acid ester compound contained in the
back coat layer to the surface of the magnetic recording medium. To allow
the back coat layer to contain a fluorine-containing carboxylic acid ester
compound having an alkylene oxide group, it is possible to employ a method
in which a fluorine-containing carboxylic acid ester compound having an
alkylene oxide group is coated on the back coat layer prepared in advance,
or a method in which, when the back coat layer is to be produced, the back
coat layer coating solution is mixed with a fluorine-containing carboxylic
acid ester compound having an alkylene oxide group. When a lubricant layer
is formed in this way, it is possible to produce an adsorption film having
a phosphoric monoester compound densely oriented therein in comparison to
a case where a lubricant which consists essentially of a phosphoric
monoester compound and a fluorine-containing carboxylic acid ester
compound having an alkylene oxide group is coated directly on the surface
of the magnetic layer. With the above-described method, it is possible to
form a film structure in which the fluorine-containing carboxylic acid
ester compound having an alkylene oxide group is mainly present on the
adsorption film of the phosphoric monoester compound or between the
adsorption films. Such a film structure enables favorable lubricating
properties to be attained. Further, since a fluorine-containing carboxylic
acid ester compound having an alkylene oxide group has slightly higher
adsorptivity than that of a fluorine-containing carboxylic acid ester
compound having a similar structure but containing no alkylene oxide
group, the compound is readily adsorbed on the surface of the magnetic
recording medium, and surplus lubricant can hardly exist on the magnetic
recording medium surface. Therefore, the fluorine-containing carboxylic
acid ester compound of the present invention shows a favorably low .mu.
value and is unlikely to cause stick-slip or other similar problem in
comparison to other fluorine-containing carboxylic acid ester compounds
which are liquid at ordinary room temperature.
With the lubricant film having the above-described arrangement, when the
magnetic recording medium is run in a VTR, the fluorine-containing
carboxylic acid ester compound gradually reduces in amount by sliding
contact with the magnetic head and the guide poles, but the
fluorine-containing carboxylic acid ester compound contained in the back
coat layer is transferred to the surface of the magnetic recording medium
when wound up. Therefore, there is substantially no change in the amount
of fluorine-containing carboxylic acid ester compound which is present on
the surface of the magnetic recording medium. Thus, favorable running
durability can be obtained.
With the conventional method, when a fluorine-containing lubricant is
coated directly on the surface of the magnetic recording medium by using
an organic solvent, even if the lubricant is soluble in the organic
solvent, fine coating unevenness and drying unevenness are likely to occur
during coating and drying process. With the method in which a
fluorine-containing carboxylic acid ester compound is added to the surface
of the back coat layer, such coating unevenness is unlikely to occur, and
the number of kinds of usable lubricant and organic solvent advantageously
increases.
Various generally known methods can be used to apply a phosphoric monoester
compound and a fluorine-containing carboxylic acid ester compound having
an alkylene oxide group to the magnetic recording medium. For example,
when the compounds of the present invention are to be coated on the
magnetic recording medium surface in the form of a solution, a wire bar
coating method, a gravure coating method, etc. may be used. Since the
compounds of the present invention are soluble in ordinary organic
solvents, e.g. methyl ethyl ketone, ethanol, etc., the magnetic recording
medium can be produced without using a fluorine-containing organic
solvent, which is costly and highly destructive of the ozone layer.
The coating weight of the phosphoric monoester compound on the magnetic
layer is preferably in the range of from 1 to 20 mg/m.sup.2, particularly
preferably from 1 to 10 mg/m.sup.2. If the coating weight is excessively
low, wear resistance becomes insufficient, whereas, if the coating weight
is excessively high, problems arise in terms of running properties and
durability.
The phosphoric monoester compound is preferably coated on the magnetic
layer, but it may be contained in the back coat layer. However, if the
proportion in which the phosphoric monoester compound is present in the
back coat layer is excessively high, it becomes difficult for the
fluorine-containing carboxylic acid ester compound to transfer to the
surface of the magnetic recording medium, causing the running durability
to be deteriorated.
The amount of the fluorine-containing carboxylic acid ester compound having
an alkylene oxide group present on the surface of the magnetic recording
medium is preferably in the range of from 1 to 20 mg/m.sup.2, particularly
preferably from 3 to 10 mg/m.sup.2. If the amount of the
fluorine-containing carboxylic acid ester compound present on the magnetic
recording medium surface is excessively small, the repeated running
durability is deteriorated, whereas, if it is excessively large, the
coefficient of friction rises. In a case where the fluorine-containing
carboxylic acid ester compound having an alkylene oxide group is mixed in
the back coat layer or coated on the surface thereof, the amount of the
compound present in or on the back coat layer is preferably in the range
of from 5 to 40 mg/m.sup.2. If the amount of the fluorine-containing
carboxylic acid ester compound present in or on the back coat layer is
excessively small, the supply of the fluorine-containing carboxylic acid
ester compound to the magnetic layer surface becomes insufficient, causing
the repeated running durability to be deteriorated. If the amount of the
fluorine-containing carboxylic acid ester compound present in or on the
back coat layer is excessively large, both the magnetic layer surface and
the back coat layer surface undesirably increase in the coefficient of
friction.
It is also possible to use another lubricant in combination with the
lubricants of the present invention. A lubricant jointly used may be
coated directly on the magnetic layer. However, if it is applied to the
back coat layer in the same way as the fluorine-containing carboxylic acid
ester compound having an alkylene oxide group, the repeated running
durability and other properties are even more effectively improved. It is
also possible to use a plurality of lubricants in combination with the
lubricants of the present invention.
For example, fluorine-containing carboxylic acid esters represented by
compounds having the following structures are effective in improving the
repeated running durability:
C.sub.8 F.sub.17 COOC.sub.18 H.sub.37
C.sub.8 F.sub.17 (CH.sub.2).sub.2 COOC.sub.8 F.sub.17
C.sub.17 F.sub.35 COO(CH.sub.2).sub.2 C.sub.8 F.sub.17
C.sub.17 F.sub.33 COO(CH.sub.2).sub.2 C.sub.8 F.sub.17
C.sub.8 F.sub.17 (CH.sub.2).sub.10 COO(CH.sub.2).sub.2 C.sub.8 F.sub.17
If a generally known perfluoro polyether is jointly used, still durability
and other properties are even more effectively improved. Examples of
perfluoro polyethers usable in the present invention include
perfluoromethylene oxide, perfluoroethylene oxide polymer,
perfluoro-n-propylene oxide polymer (CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n,
perfluoroisopropylene oxide polymer (CF(CF.sub.3)CF.sub.2 O).sub.n, etc.
Perfluoro polyethers used in the present invention may contain a polar
group, e.g. an alcohol, a methyl ester group, etc. Specific examples are
KRYTOX143AZ and 157SL, manufactured by Du Pont Co., Ltd., FOMBLINZ-DOL and
Z-DEAL, manufactured by Montefluos, and so forth. When such a perfluoro
polyether is to be coated, it is necessary to use a fluorine-containing
organic solvent.
When a perfluoro polyether is to be used, it may be mixed in a back coat
layer coating solution in the same way as a fatty ester compound.
Alternatively, the perfluoro polyether may be mixed in a fatty ester
compound when the compound is to be coated on the back coat layer produced
in advance.
If a fluoroalkane in which a part of a hydrocarbon with no functional group
introduced therein is replaced by fluorine is jointly used, it is possible
to produce a lubricant film by using an ordinary organic solvent, and it
is also possible to improve the still durability of the magnetic recording
medium.
Although the magnetic recording medium of the present invention has
excellent anticorrosion properties, the corrosion resistance can be
further enhanced by using a heterocyclic rust preventive in combination
with the lubricant. Examples of rust preventives usable in the present
invention include nitrogen-containing heterocyclic compounds, e.g.
benzotriazole, benzimidazole, purine, pyrimidine, etc., derivatives
obtained by introducing an alkyl side chain or the like into the nuclei of
these nitrogen-containing heterocyclic compounds, nitrogen- or
sulfur-containing heterocyclic compounds, e.g. benzothiazole,
2-mercaptobenzothiazole, a tetrazinedene cyclic compound, a thiouracil
cyclic compound, etc., and derivatives of these compounds.
Examples of tetrazinedene cyclic compounds usable for this purpose are
those which are represented by the following formula:
##STR1##
where R is a hydrocarbon group selected from the group consisting of an
alkyl group, an alkoxy group, and an alkylamide group.
Tetrazinedene cyclic compounds having from 3 to 26 carbon atoms are
particularly preferable. In the case of an alkoxy group, R in ROCOCH.sub.2
-- may be C.sub.3 H.sub.7 --, C.sub.6 H.sub.13 --, or phenyl. In the case
of an alkyl group, R may be C.sub.6 H.sub.13 --, C.sub.9 H.sub.19 --, or
C.sub.17 H.sub.35 --. In the case of an alkylamide group, R in
RNHCOCH.sub.2 -- may be phenyl, or C.sub.3 H.sub.7 --.
Examples of thiouracil cyclic compounds are those which are represented by
the following formula:
##STR2##
where R is a hydrocarbon group having at least 3 carbon atoms, which may
contain a substituent other than carbon and hydrogen.
When a rust preventive is to be used, it may be applied to either the
magnetic layer or the back coat layer. In view of the compatibility with
running durability and corrosion resistance required to withstand repeated
sliding movement, however, addition of a rust preventive to the back coat
layer is particularly effective.
The coating weight of the rust preventive is preferably in the range of
from 0.1 to 5.0 mg/m.sup.2, more preferably from 0.3 to 3 mg/m.sup.2.
In the present invention, it is possible to use any magnetic layer which is
formed from a known metal or alloy consisting essentially of cobalt by
oblique-incidence vacuum deposition in an atmosphere containing a trace of
oxygen. To improve electromagnetic transducing characteristics, it is
particularly preferable to use Co--O, Co--Fe that contains Co--O, etc., in
which not less than 90% of the metal atoms constituting the magnetic layer
is cobalt. It is further preferable that not less than 95%, more
preferably not less than 97%, of the metal atoms should be cobalt.
The thickness of the magnetic layer is preferably in the range of from 100
nm to 3,000 nm, more preferably from 120 nm to 2,000 nm.
Ferromagnetic metal thin films in which almost all the metal atoms
constituting the magnetic layer is cobalt have heretofore been considered
to be excellent in magnetic properties but inferior in weathering
resistance and unfavorable in terms of running properties and durability
in practical use. However, the use of a lubricant layer as in the present
invention makes it possible to produce a magnetic recording medium which
is excellent in corrosion resistance, running properties and durability,
and which is satisfactorily fit for practical use even if the
ferromagnetic metal thin film has a chemical composition in which not less
than 90% of the metal atoms is cobalt, as described above.
As a non-magnetic base for the magnetic recording medium of the present
invention, a synthetic resin film having a thickness of 3 .mu.m to 10
.mu.m is preferably used. Examples include polyethylene terephthalate,
polyethylene naphthalate, polyimide, polyamide, and polyamide-imide. With
a view to improving running properties, it is preferable to apply an
inorganic filler having a particle diameter of from 50 .mu.m to 200 .mu.m
to the surface of the non-magnetic base. The non-magnetic base may also be
arranged so that such a filler is contained inside the base, and
irregularities are formed on the surface of the base.
To improve electromagnetic transducing characteristics, the non-magnetic
base may be formed in a multilayer structure or may have a non-magnetic
primary coat layer and an intermediate layer.
If the running durability is insufficient in the present invention, it is
preferable to form a known inorganic protective layer, e.g., a carbon
film, an oxide film, a nitride film, etc., on the magnetic layer and to
form a lubricant layer over the inorganic protective layer. By doing so,
the characteristics of the magnetic recording medium can be markedly
improved. Such an inorganic protective layer is preferably formed from a
carbon film having a structure selected from among amorphous, graphite and
diamond structures, and a mixture of these structures. A rigid carbon
film, which is generally known as "diamond-like carbon", is particularly
preferable. Diamond-like carbon can be produced by a plasma CVD method
using hydrocarbon of low molecular weight, e.g., methane, as a raw
material, or a sputtering method using a graphite target. In comparison to
graphite-like carbon film, diamond-like carbon film exhibits low
lubricating properties, but it has high hardness and hence displays
exceedingly superior wear resistance when an appropriate lubricant layer
is formed thereon.
Since the lubricant layer in the present invention has excellent
adsorptivity, it produces satisfactory effect when coated on the
above-described inorganic protective layer. However, if the adsorption of
the phosphoric monoester compound to the carbon film is insufficient, the
surface of the carbon protective film should be treated by argon plasma or
oxygen plasma, or exposure to a high-humidity atmosphere, or wet oxidizing
treatment. By such surface treatment, the adsorptivity can be improved.
As has been described above, the magnetic recording medium of the present
invention has a lubricant layer formed from a phosphoric monoester
compound and a fluorine-containing carboxylic acid ester compound, as a
fluorine-containing lubricant, which is formed by introducing an alkylene
oxide group into an ester, and which is liquid at ordinary room
temperature. Thus, the lubricant layer exhibits excellent characteristics
of liquid lubricant, together with a favorably low .mu. value and
excellent shelf stability. Further, since alkylene oxide is hydrophilic,
the lubricant is adsorptive to the magnetic layer. Accordingly, the film
will not be broken even during low-speed running, and the coefficient of
friction is reduced. Thus, the lubricant exhibits excellent corrosion
resistance. Further, since the alkylene oxide group exhibits no acidic
properties, it produces favorable effects without causing head corrosion
or other similar problem.
The present invention will be explained below more specifically by way of
Examples of the present invention and Comparative Examples.
EXAMPLE
On a polyethylene terephthalate film of 10 .mu.m in thickness having a
spherical silica filler of 18 nm in particle diameter, metal cobalt was
obliquely deposited at an incidence angle of 45.degree. in an
oxygen-containing atmosphere, thereby obtaining a ferromagnetic metal thin
film consisting of a stack of two magnetic layers which were made of
columnar crystals of ferromagnetic metal such that each layer had a
thickness of 70 nm, and the directions of the columnar crystals in the two
layers were the same. After the magnetic layer had been uncurled by heat
treatment, a methyl ethyl ketone solution of each of compounds A shown in
Tables 1 and 2 was coated on the magnetic layer so that the coating weight
of the compound A was 18 .mu.mol/m.sup.2, and then dried. Subsequently, a
resin composition consisting essentially of carbon black and a resin
binder was coated on the side of the base which was reverse to the side on
which the magnetic layer had been formed, and dried to produce a back coat
layer. Next, an isopropanol solution of each of compounds B shown in
Tables 1 and 2 was coated on the back coat layer so that the coating
weight of the compound B was 12 .mu.mol/m.sup.2, and then dried.
Thereafter, each magnetic recording medium was slit into a tape of 8 mm in
width, thus preparing magnetic recording tapes of sample Nos. 1 to 20.
Next, a magnetic layer was produced in the same way as the above-described
samples. Thereafter, a diamond-like carbon film of 10 nm in thickness was
formed on the magnetic layer, and then the compounds A and B of sample No.
21 in Table 2 were coated thereon. In addition, a sample having no
lubricant applied to either the diamond-like carbon film or the back coat
layer was produced as sample No. 22.
Each sample was evaluated by the following methods. The results of the
evaluation are shown in Tables 1 and 2. In the tables, "(Comp)" attached
to sample Nos. denotes a comparative example.
[Evaluation Method]
1 Measurement of coefficient of friction (.mu. value)
Each sample of magnetic tape was brought into contact with a stainless
steel pole (material: SUS420J) at a winding angle of 180.degree. under a
tension (T.sub.1) of 20 g at 23.degree. C. and 70% RH. A tension (T.sub.2)
required to run the sample at a speed of 3.3 cm/sec. under the
above-described condition was measured, and the friction coefficient .mu.
of the magnetic tape was determined from the following formula on the
basis of the measured value:
.mu.=1/.pi..multidot.1n(T.sub.2 /T.sub.1)
2 Measurement of still durability
Still A: A color bar image was recorded on each sample tape at a running
tension of 20 g by using an 8-mm VTR (FUJIX-M6, a product of Fuji Photo
Film Co., Ltd.) under the environmental conditions of 23.degree. C. and
10% RH. Then, the recorded image was reproduced in the still playback mode
with the still playback limiting mechanism left inoperative, and a time
taken until the playback output reached -6 dB with respect to the initial
output was measured to evaluate the still durability.
Still B: A color bar image was recorded on each sample tape by using an
8-mm VTR (FUJIX-V88, a product of Fuji Photo Film Co., Ltd.) at
-10.degree. C. Then, the recorded image was reproduced in the still
playback mode with the still playback limiting mechanism left inoperative,
and a time taken until the playback output reached -6 dB with respect to
the initial output was measured to evaluate the still durability.
Still C: Still durability of each sample tape was measured under the
conditions for still A after storage for 1 week under the environmental
conditions of 60.degree. C. and 50% RH.
3 Evaluation of repeated running durability
Each sample magnetic tape of 60 minutes in length was continuously
subjected to 100 running passes for playback by using an 8-mm VTR (SONY
CV10, a product of Sony Corporation) under the environmental conditions of
23.degree. C., 10% and 70% RH. After 100 running passes, the output was
measured, and the relative value with respect to the initial output was
defined as a reduction in the playback output. After the running, the head
was observed with a microscope to evaluate head contamination as follows:
.circleincircle. . . . no contamination was observed on either of the
sliding and non-sliding contact surfaces of the head;
.largecircle. . . . slight contamination was observed on the non-sliding
contact surface of the head, although the sliding contact surface was not
contaminated;
.DELTA. . . . considerable contamination was observed on the non-sliding
contact surface, although the sliding contact surface was not
contaminated;
.DELTA.X . . . contamination was observed on the non-sliding contact
surface, and slight contamination was also observed on the sliding contact
surface; and
X . . . contamination was observed on both the sliding and non-sliding
contact surfaces.
4 Resistance to corrosion from sulfurous acid gas
Each sample magnetic tape was stored for 72 hours under the environmental
conditions of 27.degree. C., 80% RH and sulfurous acid gas content of 1
ppm. After the storage, the surface of the tape was visually observed to
evaluate the resistance to corrosion from sulfurous acid gas as follows:
.largecircle. . . . there was no substantial change in the luster of the
magnetic layer surface during the storage.
.DELTA. . . . corrosion was observed, although metallic luster was left on
the whole surface of the tape; and
X . . . part or the whole of the magnetic layer was corroded away.
As has been described above, the present invention provides a magnetic
recording medium having a magnetic layer of a ferromagnetic metal film
formed over a non-magnetic base, in which a lubricant layer is formed over
the magnetic layer from a phosphoric monoester compound and a
fluorine-containing carboxylic acid ester compound, as a
fluorine-containing lubricant, which is formed by introducing an alkylene
oxide group into an ester, and which is liquid at ordinary room
temperature. Thus, the lubricant layer exhibits excellent characteristics
of liquid lubricant, t | | |
