Magnetic recording medium comprising a ferromagnetic thin film and a protective layer of fluorine based resin

A magnetic recording medium is disclosed, comprising a non-magnetic support having provided thereon a magnetic layer comprising a ferromagnetic metal thin film, wherein a protective layer made mainly of a fluorine-based resin represented by formula (I): ##STR1## wherein m is an integer of 1 or more, and n is an integer of 10 or more, is provided on the ferromagnetic metal thin film.

FIELD OF THE INVENTION 
The present invention relates to a magnetic recording medium having a 
ferromagnetic metal thin film magnetic layer. More particularly, it is 
concerned with a metal thin film-type magnetic recording medium exhibiting 
excellent running properties and durability under wide humidity range 
conditions. 
BACKGROUND OF THE INVENTION 
Heretofore, a widely used magnetic recording medium type involved coated 
type media prepared by coating a non-magnetic support with a an organic 
polymer binder in which a ferromagnetic powder as magnetic material is 
dispersed, and then drying the magnetic coating composition. In recent 
years, with an increasing demand for high density recording, so-called 
metal thin film-type magnetic recording media, in which a ferromagnetic 
metal thin film as a magnetic layer is formed by a vapor deposition method 
such as vacuum deposition, sputtering or ion plating, or a plating method 
such as electric plating or electroless plating, have been receiving 
attention because their electromagnetic characteristics can be greatly 
increased. 
Theoretical and practical requirements for magnetic recording media for 
high density recording include high coercive force and low film thickness. 
Further, metal thin film-type magnetic recording media can be easily 
decreased in thickness by an order of magnitude as compared with coating 
type magnetic recording media and are high in saturated magnetic flux 
density. 
In particular, metal thin film formation by the vacuum deposition method 
provides great advantages because it does not need waste treatment as 
required in plating method, and its process is straightforward and a rate 
of deposition can be controllably increased. As a method of forming a 
magnetic layer having desirably high coercive force and squareness ratio 
by means of vacuum deposition, an oblique deposition method is known as 
described in, for example, U.S. Pat. Nos. 3,342,632 and 3,342,633. 
However, problems associated with magnetic recording media having a 
ferromagnetic metal thin film include weather resistance, running 
properties and durability. Also, problems stem from the fact that the 
magnetic recording medium is brought into high speed relative movement 
with a magnetic head in the course of recording, reproduction or erasing 
of magnetic signals. Under such operating conditions, running should be 
carried out both smoothly and in a stabilized manner, and at the same 
time, inadvertent contact with the head, abrasion or breakage should be 
avoided. Under such circumstances, and in order to increase running 
properties and durability, the provision of a lubricant layer or a 
protective layer on the ferromagnetic metal thin film has been studied. 
For example, one known protective layer is prepared by dissolving a 
thermoplastic resin, a thermo-setting resin, fatty acid, a metal salt of 
fatty acid, fatty acid ester, or alkyl phosphate perfluoropolyether-based 
compound, and then coating the resulting solution upon the metal thin 
film. 
In addition, it has recently been developed to increase durability by using 
protective coating compounds having a branched perfluoroalkenyl group 
(JP-A-61-107528 (the term "JP-A" as used herein means an "unexamined 
published Japanese patent application")). 
However, the metal thin film-type magnetic recording medium prepared 
according to the above conventional practice fails to exhibit sufficiently 
satisfactory running properties under wide humidity range conditions 
ranging from low humidity to high humidity. In addition, its durability is 
nonoptimal particularly in a low humidity region. 
Further, increasing the thickness of the protective layer or the lubricant 
layer is accompanied by reduction of electromagnetic characteristics due 
to loss of spacing between the head and the tape. 
Accordingly, the industry has awaited more improvements in metal thin 
film-type magnetic recording media which overcome the above-mentioned 
drawbacks of the conventional arrangements. 
SUMMARY OF THE INVENTION 
The objective of the present invention is to overcome the aforementioned 
problems encountered in the prior art. The main object of the present 
invention is to provide a metal thin film-type magnetic recording medium 
which exhibits excellent running properties and durability over a wide 
range of humidity conditions. 
The present invention relates to a magnetic recording medium comprising a 
non-magnetic support having provided thereon a magnetic layer comprising a 
ferromagnetic metal thin film, wherein a protective layer mainly 
comprising a fluorine-based resin represented by formula (I) as shown 
below is provided on the ferromagnetic metal thin film. 
##STR2## 
wherein m is an integer of 1 or more and preferably 20 to 500 and n is an 
integer of 10 or more and preferably 50 to 1,000. 
DETAILED DESCRIPTION OF THE INVENTION 
The fluorine-based resin of the present invention as represented by formula 
(I) comprises, in place of a repeating unit of CF.sub.2 CF.sub.2 of the 
conventional fluorine-based resin as described in U.S. Pat. No. 4,431,702, 
the depicted repeating unit segment represented by the formula: 
##STR3## 
as derived by introduction of a divalent polar group having the formula: 
##STR4## 
(hereinafter this repeating unit is referred to as a "polar unit") and a 
repeating unit represented by the formula: 
##STR5## 
(hereinafter this repeating unit is referred to as a "general unit"). In 
formula (I), the order of combination of n of the general units and m of 
the polar units is not critical, and the general and polar units may be 
either in a block form or in a random form. 
The salient feature of the present invention resides in hat the 
fluorine-based resin of the present invention, having the above specified 
structure of formula (I), is provided on the surface of the ferromagnetic 
metal thin film as a protective layer. 
Since the fluorine-based resin of the present invention contains the polar 
groups in the polar units, affinity of the polar groups to the 
ferromagnetic metal thin film and ultimately the affinity of the 
fluorine-based resin of the present invention to the ferromagnetic metal 
thin film are increased by the anchor effect of the polar groups to the 
ferromagnetic metal thin film. This increase of affinity produces the 
effect that the above protective layer is more difficult to peel apart 
from the ferromagnetic metal thin film as compared to conventional 
protective layers. 
Moreover, since the fluorine-based resin of the present invention contains 
the polar units, it can be easily dissolved in a solvent and, therefore, 
the present invention has an advantage in that the protective layer can be 
easily formed on the ferromagnetic metal thin film. By way of comparison, 
in the case of conventional fluorine-based resins such as 
polytetrafluoroethylene, since there are no suitable solvents capable of 
dissolving such resins, it is not easy to provide a protective layer on 
the surface of the ferromagnetic metal thin film from a practical 
standpoint. 
Moreover, the general units in the fluorine-based resin of the present 
invention are extremely hydrophobic especially compared with the 
conventional perfluoropolyether containing oxygen in the repeating unit, 
and the polar units are more hydrophilic than the general units. Also in 
the present invention, the general units contribute mainly to the 
excellent durability and weather resistance under high humidity conditions 
and the polar units can compensate for the durability under high humidity 
condition. 
Moreover, since the surface energy of the fluorine-based resin of the 
present invention is small, there is the additional advantage that a 
lubricant can be provided within the protective layer or in the surface 
portions thereof, and because of good affinity of the fluorine-based resin 
to the lubricant, the powerto retain the lubricant in the interior of the 
protective layer or in the surface thereof, i.e., lubricity, is maintained 
at a high level. 
Although the protective layer may be made of the fluorine-based resin of 
formula (I) alone,, it may be made of a mixture of the fluorine-based 
resin of formula (I) as a main component and other known resins as 
described in, for example U.S. Pat. Nos. 4,880,692, 4,798,775, 4,049,871, 
3,597,273, 3,634,137, 3,821,025, 4,068,040, 4,154,895, 4,429,017, 
4,409,299, and 4,333,988. Example of known resins to be used with the 
fluorine-based resin include polyvinylidene chloride, polyvinyl chloride, 
a vinyl chloride copolymer (e.g., vinyl chloride-vinyl acetate copolymer, 
vinyl chloride-vinyl acetate-vinyl alcohol copolymer, etc.), and 
polyurethane. 
The present inventors have discovered that by providing the fluorine-based 
resin of the present invention on the surface of a ferromagnetic metal 
thin film provided as a magnetic layer on the non-magnetic support that 
the abrasion resistance of the magnetic recording medium to members such 
as a magnetic head, a guide and a pole under severe conditions such as low 
humidity or high humidity is markedly increased and, at the same time, the 
coefficient of friction of the magnetic recording medium to running unit 
members is decreased. 
The above advantageous effects are even more enhanced by using in 
combination a fluoropolyether-based lubricant having a fluoropolyether 
structure with a resin of formula (I). This fluoropolyether-based 
lubricant may be used in two embodiments: one is such that the 
fluoropolyether-based lubricant is present in the interior of the 
protective layer in admixture with the fluorine-based resin of formula 
(I), and the other embodiment is a laminate structure wherein the 
fluoropolyether-based lubricant is present on the surface of the 
protective layer. It is to be noted, however, that the above mixed 
structure embodiment and laminate structure embodiment are not necessarily 
mutually exclusive, and they may be both used in combination in the same 
recording medium. 
The object of the present invention is attained by forming the protective 
layer comprised mainly of the fluorine-based resin represented by formula 
(I) on the surface of a ferromagnetic metal thin film provided on the 
non-magnetic support by techniques such as electric plating, sputtering, 
vacuum deposition and ion plating. 
As the fluorine-based resin of the present invention, Teflon AF sold by 
Dupont Co. and so on are commercially available. 
In formula (I), the ratio of n for the general unit to m for the polar unit 
is generally 5/95 to 80/20 and preferably 10/90 to 60/40. The weight 
average molecular weight of the formula (I) resin used is 1,500 to 15,000 
and preferably 2,000 to 12,000. 
Since the fluorine-based resin of the present invention contains the polar 
groups: 
##STR6## 
in the polar units, it has suitable polarity and thus is relatively easily 
soluble in solvents. Thus, the protective layer can be formed on the 
ferromagnetic metal thin film of the magnetic recording medium of the 
present invention by coating a solution of the fluorine-based resin of the 
present invention in a solvent. Therefore, the advantages associated with 
the present invention are in that the formation of the protective layer 
can be made relatively straight forward and that characteristics of the 
medium can be more improved by coating a mixed solution of the 
fluorine-based resin of formula (I) and other arbitrarily selected 
components capable of co-existence in the protective layer, and the 
thickness of the protective layer can be controlled relatively easily. 
As discussed above, for conventional fluorine-based resins, there are no 
solvents capable of dissolving them. Thus, there is no suitable method for 
applying the conventional fluorine-based resins to the magnetic recording 
medium. For this reason, a conventional practice involves forming a thin 
film of polytetrafluoroethylene on the magnetic layer by attaching the 
polytetrafluoroethylene powder by rubbing. In accordance with this method, 
however, it is not possible to form a uniform film, and the effects are 
not sufficiently satisfactory. 
In the production of the magnetic recording medium of the present 
invention, the protective coating made mainly of the fluorine-based resin 
of formula (I) of the present invention can be formed by the following 
methods: 
(1) a method in which the fluorine-based resin of the present invention is 
dissolved in a suitable organic solvent such as described hereinafter, 
coated or sprayed on a substrate and then dried, 
(2) a method in which the fluorine-based resin is melted and melt-coated on 
the magnetic layer, 
(3) a method in which the fluorine-based resin is attached onto the 
magnetic layer through rubbing, 
(4) a method in which the fluorine-based resin is dissolved in an organic 
solvent, and the magnetic recording medium is dipped in the resulting 
solution to make the fluorine-based resin as absorbed on the surface of 
the magnetic layer, and 
(5) a method in which a monomolecular film of the fluorine-based resin is 
formed on the surface of the magnetic layer by, for example, a 
Langmuir-Project method. 
Solvents which can be used to dissolve the fluorine-based resins of formula 
(I) of the present invention include perfluoroalkanes, 
perfluorocycloalkanes, perfluorocycloethers, low molecular weight 
perfluoropolyethers, and mixtures thereof. 
The thickness of the protective layer made mainly of the fluorine-based 
resin of formula (I) of the present invention is generally 10 to 200 .ANG. 
and preferably 30 to 100 .ANG., and the coated amount of the 
fluorine-based resin of formula (I) is generally 2 to 20 mg/m.sup.2 and 
preferably 5 to 10 mg/m.sup.2. The protective layer of the present 
invention generally contains 30 to 100 wt. % and preferably 60 to 100 wt. 
% of the fluorine-based resin based on the weight of the protective layer. 
If the amount of the fluorine-based resin present is too large, the anchor 
effect of the polar group in the polar unit does not work sufficiently, 
and the fluorine-based resin causes head contamination and head clogging. 
On the other hand, if the amount of the fluorine-based resin present is 
too small, the protective effect of the fluorine-based resin is weakened, 
and the object of the present invention cannot be attained. 
In the magnetic recording medium of the present invention, in order to 
increase the adhesion of the protective layer to the ferromagnetic metal 
thin film, the surface of the ferromagnetic metal thin film can be treated 
with surfactants such as fatty acid or various coupling agents prior to 
the provision of the protective layer. 
The protective layer may be either a single layer or be made of a plurality 
of layers. 
For formation of the protective layer, additionally, a vacuum deposition 
method, a method of attaching the powder by rubbing, and so forth can be 
employed. 
In the magnetic recording medium of the present invention, as described 
above, a general lubricant can be used in the protective layer in 
admixture with the fluorine-based resin of the present invention, or be 
laminated on the protective layer. In the former, the general lubricant 
can be contained in an amount of 2 to 50 wt. % and preferably 5 to 30 wt. 
% based on the weight of the protective layer. In the latter, the general 
lubricant can be coated in an amount of 2 to 200 mg/m.sup.2 and preferably 
5 to 30 mg/m.sup.2. 
General lubricants which can be used include fluoropolyethers (e.g., 
fluorinated polyethers and modified fluorinated polyethers), fatty acids, 
metal soaps, fatty acid amides, fatty acid esters, higher aliphatic 
alcohols, monoalkyl phospates, dialkyl phosphates, trialkyl phosphates, 
paraffins, silicone oil, animal and vegetable oils, mineral oils, higher 
aliphatic amines; inorganic fine powders such as graphite, silica, 
molybdenum disulfide and tungusten disulfide; resin fine powders such as 
polyethylene, polypropylene, polyvinyl chloride, an ethylene-vinyl 
chloride copolymer, and polytetrafluoroethylene; .alpha.-olefin polymers; 
and unsaturated aliphatic hydrocarbons which are liquid at ordinary 
temperature. 
If the fluoropolyether-based lubricants among the above lubricants are used 
in combination, the durability is increased cumulatively to some extent. 
The fluoropolyethers may be a homopolymer or copolymer and have a 
repeating unit of --(C.sub.a F.sub.b Cl.sub.c H.sub.d))-- wherein a is an 
integer of 1 to 5, b is an integer of 1 or more and c and d each is 0 or 
an integer of 1 or more, provided than b+c+d is 2a. 
Preferred examples of the fluoropolyethers are modified or unmodified 
perfluoropolyethers having a molecular weight of 1,500 to 15,000. 
More specifically, Crytox 143 series (polyfluoropropylene oxide) produced 
by Dupont Co., Fonbrine Y series (perfluoropropylene 
oxide-perfluoromethylene oxide copolymers) produced by Montefluos Co., 
Fonbrine Z series (perfluoroethylene oxide-perfluoromethylene oxide 
copolymers) produced by Montefluos Co., and the above fluoropolyethers 
compounds derived by introducing a functional group (e.g., --COOH, --OH, 
and --CONH.sub.2) into one or both ends of the above compounds, and 
compounds in which a plurality of perfluoropolyether chains are extended 
from the central atom group can be used. 
Of these compounds, the compounds in which a plurality of perfluoroether 
chains are extended from the central atom group, and which are low in the 
coefficient of friction and are excellent in durability. 
Specific examples of the compounds having such a fluoropolyether structure 
are shown below. 
##STR7## 
Materials which can be used for the ferromagnetic metal thin film include 
ferromagnetic metals such as iron, cobalt and nickel; and ferromagnetic 
alloys such as Fe--Co, Fe--Ni, Co--Ni, Fe--Rh, Co--P, Co--B, Co--Y, 
Co--La, Co--Ce, Co--Pt, Co--Sm, Co--Mn, Co--Cr, Fe--Co--Ni, Co--Ni--P, 
Co--Ni--B, Co--Ni--Ag, Co--Ni--Nd, Co--Ni--Ce, Co--Ni--Zn, Co--Ni--Cu, 
Co--Ni--W, and Co--Ni--Re. 
This material is provided on the non-magnetic support by a technique such 
as electric plating, electroless plating, gas phase plating, sputtering, 
vacuum deposition or ion plating to form the ferromagnetic metal thin 
film. The thickness of the film, which used as the magnetic recording 
medium, is 0.02 to 2 .mu.m and preferably 0.5 to 1.0 .mu.m. 
The above ferromagnetic metal thin film may also contain, in addition to 
the above-mentioned materials, any of O, N, Cr, Ga, As, Sr, Zr, Nb, Mo, 
Rh, Pd, Sn, Sb, Te, Pm, Re, Os, Ir, Au, Hg, Pb, and Bi. 
The surface profile of the magnetic layer is not particularly critical. 
However, in the case where the surface has projections 10 to 1,000 .ANG. 
in height, the resulting magnetic recording medium is excellent in running 
properties and durability. 
The thickness of the support is preferably 4 to 50 .mu.m. In order to 
increase the adhesion of the ferromagnetic metal thin film or to improve 
magnetic characteristics, the support may be provided with a subbing 
layer. As the subbing layer, for example, a filler-containing resin layer 
in which fine particles of SiO.sub.2 or CaCO.sub.3 are uniformly dispersed 
can be employed. 
Supports which can be used in the present invention include plastic bases 
such as polyethylene terephthalate, polyimide, polyamide, polyvinyl 
chloride, cellulose triacetate, polycarbonate, polyethylene naphthalate, 
and polyphenylene sulfide, and Al, Ti, and stainless steel. 
In order to increase running durability, it is effective to provide minute 
projections on the surface of the support prior to formation of the 
ferromagnetic metal thin film, because this results in formation of 
suitable replicated irregularities on the surface of the magnetic layer. 
The density of minute projections on the surface of the support is 
preferably 2.times.10.sup.6 to 2.times.10.sup.8 /mm.sup.2 and the height 
of the projection is preferably 1 to 50 nm. Such minute projections can be 
provided by conventional methods as described in, for example, U.S. Pat. 
Nos. 4,839,217, 4,952,449, 4,898,753, 4,138,386, 4,687,700, 3,419,460, 
4,761,327, 4,786,558, 4,615,939, and 4,619,869. 
The magnetic recording medium of the present invention may be in any 
desired form, such as tape, sheet, card or disk. Particularly preferred 
are tape- and disk-like forms. 
In the metal thin film-type magnetic recording medium of the present 
invention, the fluorine-based resin of formula (I) of the present 
invention, comprising the polar unit containing the polar groups and the 
general unit having hydrophobicity, is provided on the surface of the 
magnetic layer as the protective layer. Thus, the magnetic recording 
medium of the present invention is excellent in peeling resistance and 
particularly in lubricity under low and high humidity conditions and, 
therefore, exhibits good durability, running properties and abrasion 
resistance. In addition, the fluorine-based resin of formula (I) of the 
present invention is easily soluble in solvents unlike conventional 
fluorine-based resins and, therefore, produces the effect that the 
protective layer can be formed with great ease. 
The present invention is described in greater detail with reference to the 
following examples although it is not limited thereto.

EXAMPLE 1 
On a 13 .mu.m-thick polyethylene terephthalate film was provided a 
cobalt-nickel magnetic film (film thickness 150 nm) by oblique vacuum 
deposition to produce an original material for a magnetic recording 
medium. As a vapor source, an electron beam vapor source was used. A 
cobalt-nickel alloy (Co: 80 wt. %, Ni: 20 wt. %) was charged, and the 
oblique vacuum deposition was performed in a vacuum of 5.times.10.sup.-5 
Torr in such a manner that the incident angle was 50.degree.. 
Teflon AF1600 (produced by Dupont Co.), a fluorine-based resin of formula 
(I) of the present invention, was dissolved in a 1:1 (by weight) mixed 
solvent of a low molecular weight perfluoropropylene oxide polymer (m.w. 
300-400) and perfluorooctane, and the resulting solution was coated on the 
magnetic metal thin film of the above original material by means of a bar 
coater in such a manner that the amount of the solution coated was 10 
mg/m.sup.2 to thereby form a protective layer of the fluorine-based resin 
of the present invention. 
After drying of the protective layer, a solution of a lubricant of a 
perfluoropropylene oxide polymer sold under the trade name of Crytox 143AZ 
(produced by Dupont Co.) in Freon 113 was coated on the above protective 
layer by means of a bar coater in an amount of 10 mg/m.sup.2 to thereby 
form a lubricant layer. 
The thus manufactured material of the magnetic recording medium was then 
slit to 8 mm width to form a sample of 8 mm video magnetic tape. 
EXAMPLE 2 
A 8 mm video magnetic tape sample was produced in the same manner as in 
Example 1 except that the lubricant layer as an uppermost layer was not 
formed. 
EXAMPLE 3 
A 8 mm video magnetic tape sample was produced in the same manner as in 
Example 1 except that the lubricant layer as the uppermost layer was not 
formed, and that the protective layer containing the fluorine-based resin 
of formula (I) of the present invention was formed as described below. 
Teflon AF2400 (produced by Dupont Co.), a fluorine-based resin of formula 
(I) the present invention, and Crytox 143AY (produced by Dupont Co.), a 
lubricant of perfluoropolypropylene oxide, were mixed in a weight ratio of 
10:2, and the resulting mixture was dissolved in a 1:1 (by weight) mixed 
solvent of low molecular weight perfluoropolypropylene oxide and 
perfluorooctane. The solution thus prepared was coated in an amount of 10 
mg/m.sup.2 by means of a bar coater to form a protective layer of the 
fluorine-based resin of the present invention. 
Comparative Example 1 
A 8 mm video magnetic tape sample was produced in the same manner as in 
Example 1 except that the lubricant layer was formed by coating the 
ferromagnetic metal thin film with a solution of perfluoropolypropylene 
oxide polymer (Crytox 143AY) in Freon 113, in an amount of 10 mg/m.sup.2, 
and no other layers were formed. 
The 8 mm video magnetic tapes as obtained above were measured for 
coefficient of friction to a stainless steel bar under the following two 
conditions: 
Condition A: 25.degree. C., 70% RH, 
Condition B: 25.degree. C., 15% RH, and also for still durability on a 8 mm 
VTR under Conditions C and D as described hereinafter, by the following 
methods. 
Measurement of Coefficient of Friction 
The magnetic layer surface of the magnetic tape sample was brought into 
contact with a stainless steel pole at a tension (T.sub.1) and a winding 
angle of 180.degree., and a tension (T.sub.2) necessary for running the 
magnetic tape at a rate of 3.3 cm/sec was measured. The coefficient of 
friction, .mu., was calculated from the following equation: 
##EQU1## 
Measurement of Still Durability 
Still durability was evaluated by measuring a period until, when a pause 
button was pushed at the time of reproduction of image on a 8 mm video 
VTR, Model FUJIX-8 M-6 (produced by Fuji Photo Film Co., Ltd.) (provided 
the function to limit a still reproduction time was removed) under the 
following conditions: 
Condition C: 25.degree. C., 15% RH, 
Condition D: 25.degree. C., 5% RH, no image appeared. 
The results are shown in Table 1. 
TABLE 1 
______________________________________ 
Coefficient of Friction 
Still Durability 
Con- Con- Con- Con- 
dition A dition B dition C dition D 
______________________________________ 
Example 1 
0.21 0.20 more than 
42 min 
60 min 
Example 2 
0.22 0.20 more than 
more than 
60 min 60 min 
Example 3 
0.21 0.20 more than 
more than 
60 min 60 min 
Com. Ex. 1 
0.32 0.24 40 min 20 min 
______________________________________ 
From the results of Table 1, it can be seen hat in Examples 1 to 3 in which 
the fluorine-based resin of formula (I) of the present invention is 
provided as the protective layer, the coefficient of friction under high 
and low humidity conditions is low and the still durability is excellent 
as compared with Comparative Example 1 in which a conventional 
fluorine-based lubricant is merely provided on the magnetic layer. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.