Magnetic recording medium

A magnetic recording medium comprising a substrate, a magnetic layer provided thereon, a passivated film formed on the surface of said magnetic layer, and a lubricant-containing back-coat layer whose pH is adjusted to 6.3 or more on the back of said substrate.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
Field of the Invention 
This invention relates to a magnetic recording medium and, more 
particularly, to a magnetic recording medium of the ferromagnetic thin 
metal film type which is excellent in corrosion resistance and travelling 
properties, being kept from the corrosion caused by the back-coat layer, 
that is, a lubricant layer provided on the back of substrate. 
Related Art Statement 
The magnetic recording medium comprising a substrate and, formed thereon or 
on an interposed undercoat, a magnetic layer of thin ferromagnetic metal 
film containing cobalt as principal constituent is excellent in 
high-density recording characteristics but, on the other hand, has a 
problem of deterioration in corrosion resistance, beause the cobalt in the 
surface of magnetic layer is exposed to the atmosphere. In the prior art, 
in order to solve the above problem, the cobalt in the surface is 
subjected to passivation treatment by oxidation such as formation of a 
protective layer comprising cobalt oxide such as Co.sub.3 O.sub.4 by heat 
treatment of the surface of magnetic layer (U.S. Pat. No. 3,460,968), or 
by exposing the surface to an ozone atmosphere to form a complex oxide 
such as that of Co.sub.3 O.sub.4 and Co.sub.2 O.sub.3 [Japanese Patent 
Application "Kokai" (Laid-Open) No. 63,031/84] or by forming a protection 
layer containing cobalt oxides such as CoO and Co.sub.3 O.sub.4 by the 
glow discharge treatment of the surface [Japanese Patent Application 
"Kokai" (Laid-Open) No. 41,439/83]. 
In order to improve their travelling performance, conventional magnetic 
recording media having ferromagnetic thin metal films of the cobalt alloy 
type have been constructed in such a manner that the substrate, which is a 
component of the media, is provided on the back with a backcoat of 
lubricant layer. Such a magnetic recording medium, however, has a 
disadvantage in that when a magnetic recording medium in tape form is 
wound on a reel, that is, in the form of reeled tape, the thin 
ferromagnetic metal layer of the cobalt alloy type, which is a magnetic 
layer provided on the surface of the medium, is in close contact with a 
backcoat layer provided on the back of a substrate and is liable to be 
affected by the backcoat layer. If the backcoat becomes acidic due to the 
atmospheric moisture, the protective layer of passivated cobalt film is 
easily destructed or dissolved away, resulting in corrosion of the 
magnetic layer and, as a consequence, deterioration in characteristics and 
travelling performance of the magnetic recording medium. 
OBJECT AND SUMMARY OF THE INVENTION 
An object of this invention is to eliminate the above-noted disadvantage of 
the conventional magnetic recording medium in the prior art such that the 
backcoat layer, which is a lubricant layer provided on the back of a 
substrate of the recording medium, becomes acidic due to the effect of 
atmospheric moisture and causes corrosion of the magnetic layer in contact 
with the lubricant layer, the magnetic layer comprising cobalt or an alloy 
containing cobalt as principal constituent; and to provide a magnetic 
recording medium excellent in corrosion resistance and travelling 
performance. 
The present inventors found as a result of various experiments that when a 
passivated cobalt layer formed on the surface of a thin ferromagnetic 
metal layer, which is a magnetic layer comprising cobalt alone or cobalt 
as principal constituent, is exposed to an acidic solution or atmosphere 
of a pH value below 6.3, the passivated cobalt film becomes destroyed or 
dissolved away. It was further confirmed that when the backcoat layer, 
which is a lubricant layer in contact with the magnetic layer in the form 
of reeled tape, that is, a tape wound on a reel, has been adjusted to pH 
6.3 or above, the passivated cobalt layer on the surface of the magnetic 
layer in contact with the backcoat layer was not destroyed nor dissolved 
even if the atmospheric moisture had been condensed on the tape. There is 
thus obtained a corrosion-resistant magnetic recording medium of high 
reliability and high durability.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The adjustment of pH of the backcoat, i.e. lubricant layer, according to 
this invention is performed by adding a suitable amount of an alkaline 
compound to various lubricants which form the backcoat layer and adjusting 
pH to a prescribed value while measuring pH by means of a pH meter. A pH 
value of the backcoat layer below 6.3 is undesirable because of 
destruction of the passivated cobalt film under acid conditions below pH 
6.3. With the increase in alkalinity, the passivated cobalt film becomes 
more stable until the upper limit of pH 14 is reached. 
As lubricants which form the backcoat layer of the present magnetic 
recording media, there may be mentioned those which are customarily used, 
such as lubricants of the aliphatic, fluorine, silicone, and hydrocarbon 
types. The aliphatic lubricants include fatty acids, metal salts thereof, 
esters thereof, fatty acid amides, and aliphatic alcohols. Examples of 
desirable fatty acids are lauric acid, myristic acid, palmitic acid, oleic 
acid, stearic acid and behenic acid. Suitable metal salts of these fatty 
acids include salts with metals such as lithium, sodium, calcium, 
magnesium, aluminum, iron, cobalt, zinc, barium and lead. As examples of 
fatty acid esters, mention may be made of butyl stearate, octyl myristate, 
stearic acid monoglycerides, palmitic acid monoglyceride, oleic acid 
monoglyceride, and pentaerythritol tetrastearate. Examples of desirable 
fatty acid amides are caproic acid amide, capric acid amide, lauric acid 
amide, palmitic acid amide, behenic acid amide, oleic acid amide, linolic 
acid amide, and methylenebisstearic acid amide. Desirable fatty alcohols 
include stearyl alcohol and myristyl alcohol. There are also used 
chlorides such as trimethylstearylammonium chloride and stearoyl chloride 
and amines such as stearylamine, stearylamine acetate and stearylamine 
hydrochloride. 
Examples of desirable lubricants of the fluorine type are 
trichlorofluoroethylene, perfluoropolyethers, perfluoroalkyl polyethers, 
perfluoroalkyl carboxylic acids and the like. Examples of commercial 
products are Diflon #20 (Daikin Kogyo Co.), Krytox M and H, Vydex AR (Du 
Pont Co.), and Fomblin Z (Montedison Co.) 
The lubricants of the silicone type are silicone oils and modified silicone 
oils. The hydrocarbon lubricants include paraffins, squalane and waxes. 
The backcoat layer, that is, lubricant layer may contain additives such as 
rust preventives in addition to at least one of the above-listed 
lubricants constituting the lubricant layer. 
In adjusting the pH value of the backcoat layer by use of the above 
lubricants, if the pH of the lubricant being used is 6.3 or above, said 
lubricant can be used as such, whereas when the pH of the lubricant is 
below 6.3, the lubricant is used after having been adjusted to a 
prescribed pH value by the addition of an alkaline compound such as, for 
example, sodium hydroxide, potassium hydroxide or sodium 
hydrogencarbonate. The adjusted lubricant is dissolved in a suitable 
solvent such as, for example, toluene, methyl isobutyl ketone, methyl 
ethyl ketone, cyclohexanone, ethyl acetate, isopropyl alcohol, "Fleon", 
tetrahydrofuran, dimethylformamide, or dioxane and the resulting solution 
is applied by coating, spraying or dipping to form a backcoat layer 
adjusted to pH 6.3 or above. The alkaline compounds to be added to adjust 
the pH value of the backcoat layer include the above-noted sodium 
hydroxide and the like. In addition, there may be used any of the 
inorganic or organic compounds which do not destroy nor dissolve the 
passivated cobalt film formed on the magnetic layer and which can control 
the pH value within the range of from 6.3 to 14. 
EXAMPLES 
The invention is further illustrated in detail hereunder with reference to 
Examples and the accompanying Drawing. 
EXAMPLE 1 
A magnetic tape of the structure as shown in the FIGURE was prepared by the 
following procedure. 
A thin ferromagnetic metal film layer 2, 1500 .ANG. in film thickness, 
comprising Co-20 wt-% Ni alloy was evaporated at an oblique incident angle 
on a substrate of polyethylene terephthalate film 1, 9 .mu.m in thickness, 
in a vacuum of 6.times.10.sup.-5 Torr while introducing oxygen. The film 
on which the metal layer was formed was continuously wound up in roll 
form. The roll film was exposed to the atmosphere and again placed in a 
closed vessel. The vessel was then evacuated to 50 Torr and filled with 
oxygen to a pressure of 1.3 atmosphere. The sealed vessel was left 
standing for 150 hours to passivate the surface of thin ferromagnetic 
metal film layer 2, resulting in passivated cobalt film 3. A lubricant 
Fomblin Z, DIOL (Montedison Co.), which had been adjusted to pH 6.5 with 
sodium hydrogencarbonate, was applied to the backside of the substrate 1 
of polyethylene terephthalate film to form a backcoat layer 4. The 
resulting product was cut to a prescribed width to obtain a magnetic tape. 
EXAMPLE 2 
A magnetic tape was prepared in the same manner as in Example 1, except 
that Vydex 84 (Du Pont Co.) was used as the lubricant in place of Fomblin 
Z, DIOL and the backcoat layer was adjusted to pH 7.3. 
EXAMPLE 3 
A magnetic tape was prepared in the same manner as in Example 1, except 
that lauric acid was used as the lubricant in place of Fomblin Z, DIOL and 
the backcoat layer was adjusted to pH 7.5. 
COMATIVE EXAMPLE 1 
A magnetic tape was prepared in the same manner as in Example 1, except 
that myristic acid was used as the lubricant in place of Fomblin Z, DIOL 
and the backcoat layer was adjusted to pH 4.8. 
COMATIVE EXAMPLE 2 
A magnetic tape was prepared in the same manner as in Example 1, except 
that stearic acid was used as the lubricant in place of Fomblin Z, DIOL 
and the backcoat layer was adjusted to pH 6.1. 
The magnetic tapes prepared in Examples and Comparative Examples were each 
wound up in reel form to obtain reeled magnetic tapes, in which the 
backcoat layer and the surface of magnetic layer were kept in contact with 
each other. The tapes were left standing in an atmosphere of high 
temperature and high humidity at 60.degree. C. and 90% RH (relative 
humidity). The change with time in friction coefficient of the magnetic 
tape surface was measured. The corrosion resistance and the travelling 
performance were evaluated by the visual inspection of the surface 
condition of magnetic layer. The results were as shown in Table 1. The 
coefficient of friction was measured by a sliding type friction tester. 
The test results were expressed in terms of kinetic friction coefficient. 
TABLE 1 
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Friction coefficient and change in 
PH of surface condition of magnetic layer 
back- Number of days of standing in 
coat atmosphere of 60.degree. C. and 90% RH 
layer 0 10 20 30 
______________________________________ 
Example 1 
6.5 0.26 0.24 0.25 0.25 
Metallic luster 
Example 2 
7.3 0.27 0.26 0.26 0.26 
Metallic luster 
Example 3 
7.3 0.28 0.27 0.27 0.27 
Metallic luster 
Comparative 
4.8 0.35 &gt;1.0 -- -- 
Example 1 Metallic Corrosion 
-- -- 
luster all over 
the surface 
Comparative 
6.1 0.33 0.82 &gt;1.0 -- 
Example 2 Metallic Corrosion 
Corrosion 
luster specks all over 
the surface 
______________________________________ 
As is apparent from Table 1, the reeled magnetic tape having a backboard 
layer of PH 6.3 or above according to this invention showed even after a 
long storage in a high-temperature and high-humidity atmosphere an 
excellent corrosion resistance and a substantially unchanged friction 
coefficient, indicative of steady travelling performance, high durability, 
and high reliability of the magnetic recording medium of this invention. 
In the above Examples, sodium hydrogencarbonate was used as the alkaline 
compound for adjusting pH of the backcoat layer. It was confirmed that the 
same effect as that shown in the above Examples was exhibited when sodium 
hydroxide or potassium hydroxide was used or when other alkaline inorganic 
or organic compounds not destroying the passivated cobalt film were used. 
In the above Examples, commercial products such as Fomblin Z, DIOL 
(Montedison Co.) and Vydex 84 (Du Pont Co.) were used as the lubricant 
constituting the backcoat layer. It was also confirmed that the same 
effect as that shown in the above Examples was exhibited when aliphatic 
lubricants, fluorine-containing lubricants, silicone lubricants, or 
hydrocarbon lubricants were used. 
As the metallic materials for the thin ferromagnetic metal film used in 
magnetic recording media of this invention, there may be used cobalt alone 
or alloys comprising cobalt as principal constituent such as Co - Ni, Co - 
Cr, Co - P, Co - Pd, Co - Fe, Co - Ni - P, Co - Fe - Cr, Co - Ni - Cr, Co 
- Fe - Ni, Co - Ti, and Co - Cu. 
The materials for the substrate which can be used in the present magnetic 
recording medium are polyethylene terephthalate and other common plastic 
films such as synthetic resin films including polyamide, polyimide, and 
polyester films and composite plastic films. 
EFFECT OF THE INVENTION 
As described in detail above, even when stored in the form of reeled 
magnetic tape for a long period of time in an atmosphere of high 
temperature and high humidity, the magnetic recording medium of this 
invention provided with a backcoat layer having an adjusted pH value is 
kept from destruction of the corrosion resistance of the passivated cobalt 
film formed on the surface of a thin ferromagnetic metal film layer which 
is a magnetic layer comprising cobalt alone or cobalt as principal 
constituent. As a consequence, the excellent corrosion resistance and 
travelling performance are retained, indicating high durability and high 
reliability of the magnetic recording medium of this invention.