Magnetic recording medium

A magnetic recording medium comprising a substrate and magnetic recording double-layers formed on the substrate, wherein the magnetic material of the magnetic recording under-layer is composed essentially of an alloy or metal magnetic powder having a specific surface area of at most 30 m.sup.2 /g as measured by BET method, and the magnetic material of the magnetic recording top-layer is composed essentially of an iron oxide or Co-adsorbed iron oxide powder having a specific surface area of at least 25 m.sup.2 /g as measured by BET method.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetic recording medium, and more 
particularly to a magnetic recording medium having a double layered 
structure. 
2. Description of the Prior Art 
In recent years, there have been commercial developments of tapes wherein 
an alloy powder or metal powder is used as magnetic powder for high 
performance audio recording tapes. However, such a fine metal powder is 
very active in contrast with an iron oxide powder. In an extreme case, 
when exposed in air, such a metal powder undergoes spontaneous ignition. 
Accordingly, it is an important subject to provide a some safety measure. 
In general, the finner the metal powder, the more active it becomes. It is 
well known that if the metal powder is made coarse, it will be stabilized. 
However, the noise (AC bias noise) characteristics will be deteriorated by 
an increase of the particle size. Under the circumstances, it is obliged 
to use fine particles with a specific surface area of at least 30 m.sup.2 
/g as measured by BET method. It would be possible to reduce the danger 
during the process for the production of the tape and also to minimize a 
change with time of the tape itself if the particle size could be enlarged 
to some extent so that a stabilized metal powder would be employed. In 
this respect, the present inventors have made a study to find out whether 
or not it is possible to use a magnetic metal having a more or less large 
particle size and yet to adequately reduce the noise. 
On the other hand, an iron oxide-type magnetic powder in the form of fine 
particles is widely used for magnetic tapes. The iron oxide-type magnetic 
powder is chemically stable and gives a low noise level. However, the 
transfer characteristics and the low frequency sensitivity tend to 
deteriorate as the fine particles become finner. Accordingly, it has been 
difficult to employ a super fine particulate powder. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a magnetic recording 
medium having a low noise level and a high sensitivity in low frequency 
region. 
As a result of an extensive research, the present inventors have found it 
possible to provide a magnetic recording medium having a highly sensitive 
electric characteristics without impairing the noise characteristic even 
when a alloy or metal powder having a relatively large particle size is 
employed. 
Namely, the present invention provides a magnetic recording medium 
comprising a substrate and magnetic recording double-layers formed on the 
substrate, wherein the magnetic material of the magnetic recording 
under-layer is composed essentially of an alloy or metal magnetic powder 
having a specific surface area of at most 30 m.sup.2 /g as measured by BET 
method, and the magnetic material of the magnetic recording top-layer is 
composed essentially of an iron oxide or Co-adsorbed iron oxide powder 
having a specific surface area of at least 25 m.sup.2 /g as measured by 
BET method. 
The fine iron-oxide type powder used for the top-layer is per se usually 
poor in the transfer characteristic and the low frequency sensitivity. 
Likewise, the alloy or metal powder used for the under-layer and having a 
relatively large particle size is per se likely to have an inferior noise 
characteristic. In the magnetic recording medium of the present invention 
having the above double layered structure, the properties of the top and 
under layers provide a synergistic effect whereby not only the drawbacks 
of the fine iron-oxide type powder are totally eliminated in spite of the 
top-layer made of such fine iron-oxide type powder, but also the noise 
characteristic is remarkably improved in spite of the under layer made of 
an alloy or metal powder having a relatively large particle size. Further, 
it has been found that there is no substantial property change with time, 
and the stability of the alloy or metal powder during the preparation of 
the magnetic recording medium can be secured. Thus, according to the 
present invention, drawbacks which used to be difficult to avoid when the 
top- or under-layer was used alone, do not appear, and the merits of the 
top- and under-layers cooperate to provide the synergistic effect. 
Now, the present invention will be described in detail with reference to 
the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
It has been found that in order to adequately attain the object of the 
present invention, it is desirable that the thickness of the top-layer is 
at most equal to the thickness of the under-layer. If the ratio in the 
thickness of the two layers (top-layer thickness/under-layer thickness) 
exceeds 1.0, problems such as an decrease in the sensitivity or an 
increase of the transfer are likely to result. On the other hand, if the 
ratio is less than 0.20, there will be a problem such as an increase of a 
noise. Therefore, in order to obtain a tape wherein an adequate low 
frequency sensitivity, noise and transfer are well balanced, it is 
preferred that the ratio of the top-layer thickness to the under-layer 
thickness is within the range of from 0.20 to 1.0. This tape (having a 
double layered structure) preferably has Br of at least 2000 gauss. 
Now, the present invention will be described with reference to Examples. 
However, it should be understood that the present invention is by no means 
restricted to these specific Examples. 
EXAMPLES 
(1) Preparation of coating materials for the under-layer and preparation 
tapes 
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Parts by weight 
______________________________________ 
Metal magnetic powder 100 
(with various surface areas) 
Vinyl chloride-vinyl alcohol copolymer 
15 
(VAGH manufactured by U.C.C.) 
Polyurethane resin 10 
(N-2304 manufactured by Nippon 
Polyurethane Co.) 
Oleic acid 2 
Alumina powder 2 
(average particle size: 0.5 .mu.m) 
MEK 85 
Toluene 85 
Cyclohexanone 85 
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The above composition for the coating material was mixed and dispersed in a 
ball mill for about 48 hours to obtain a magnetic coating material. To 
this coating material, 5 parts by weight of a polyisocyanate (COLONATE L 
manufactured by Nippon Polyurethane Co.) was added as a curing agent, and 
thoroughly mixed and stirred. Then, the mixture was coated on a 
polyurethane terephthalate film having a thickness of 8 .mu.m while 
applying orientation treatment, and then dried. Then, the coated film was 
subjected to surface treatment by super calendering, and further subjected 
to heat curing treatment. 
The average particle size of the metal magnetic powder in the above coating 
composition was varied, i.e., magnetic powders having specific surface 
areas of 20.5, 23.3, 24.1, 27.6 and 35.8 m.sup.2 /g, respectively, as 
measured by BET method, were used. The respective coating materials were 
designated as coating material M.sub.1 to M.sub.5, and the respective 
tapes (each having a single layered structure) were designated as tapes a 
to e. (See Table 1.) 
(2) Preparation of coating materials for the top-layer and preparation of 
tapes 
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Parts by weight 
______________________________________ 
Co--adsorbed iron oxide powder 
100 
(with various surface areas) 
Vinyl chloride-vinyl acetate- 
18 
vinyl alcohol copolymer (VAGH) 
Polyurethane resin (N-2304) 
7 
Lecithin 2 
.alpha.-SiC (average particle size 0.2 .mu.m) 
1 
Vinyl chloride stabilizer 
0.5 
Myristic acid 1 
MEK 85 
MIBK 85 
Toluene 85 
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The above composition was mixed and dispersed in a ball mill for about 48 
hours to obtain a magnetic coating material. This coating material was 
coated on a polyethylene terephthalate film having a thickness of 8 .mu.m 
while applying orientation treatment. After drying, the coated film was 
subjected to surface treatment by super calendering. 
The specific surface area of the Co-adsorbed iron oxide powder in the above 
coating composition was varied, i.e. magnetic powders having specific 
surface areas of 22.9, 26.1, 27.3, 32.0 and 39.1 m.sup.2 /g, respectively, 
were used as the Co-adsorbed iron oxide powder. The respective coating 
materials were designated as coating materials F.sub.1 to F.sub.4, and the 
respective tapes (each having a single layered structure) were designated 
as tapes f to j. (See Table 1.) 
(3) Preparation of double-layered tapes 
Coating materials M.sub.2 and M.sub.5 prepared in the above-mentioned 
manner were, respectively, coated on polyethylene terephthalante films 
having a thickness of 8 .mu.m so that the thickness after the calender 
treatment would be about 3 .mu.m, and then subjected to calender treatment 
and heat curing. Thereafter, coating materials F.sub.1, F.sub.3, F.sub.4 
and F.sub.5 are, respectively, coated thereon so that the thickness after 
the calender treatment would be about 2 .mu.m, and then subjected to 
calender treatment to obtain sample tapes k to n. (See Table 1.) 
These tapes were cut into audio tapes with a width of 3.81 mm. Then, 
various characteristics were measured. The results are shown in Table 1. 
Among them, tapes l and m represent the tapes of the present invention. 
In the Table, Hc is coercive force, Br is a residual magnetic flux density, 
S-315 is a reproduction output at 315 Hz, S-16-K is a reproduction output 
at 16 KHz (relatively to a standard tape) and P.T is a transfer 
characteristic. 
TABLE 1 
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Magnetic powders 
Characteristics of tapes 
Specific sur- 
Thickness of Bias 
Layer structure 
Coating 
face area 
layers Hc Br S-315 
S-16-K 
noise 
P.T. 
of tapes 
Tapes 
materials 
(m.sup.2 /g) 
(.mu.) (Oe) 
(G) 
(dB) 
(dB) 
(dB) 
(dB) 
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Single under- 
a M.sub.1 
20.5 4.2 667 
3300 
2.9 1.6 2.0 53.4 
layer b M.sub.2 
23.3 4.8 697 
3490 
3.1 3.2 1.3 52.3 
c M.sub.3 
24.1 4.1 773 
3040 
2.1 4.2 1.7 51.8 
d M.sub.4 
27.6 3.7 786 
3120 
2.1 5.3 1.7 52.3 
e M.sub.5 
35.8 4.2 827 
2750 
1.3 5.2 0.2 48.8 
Single top- 
f F.sub.1 
22.9 4.9 721 
1790 
0.4 3.3 -0.1 
52.1 
layer g F.sub.2 
26.1 5.0 692 
1760 
0.1 1.6 -0.8 
50.7 
h F.sub.3 
27.3 4.7 707 
1620 
-0.4 
2.5 -1.2 
49.3 
i F.sub.4 
32.0 3.9 696 
1700 
-0.7 
2.0 -2.2 
47.6 
j F.sub.5 
39.1 3.8 682 
1400 
-1.1 
1.9 -3.8 
44.9 
Double-layers 
k F.sub.1 /M.sub.2 
-- Top/Under 
707 
2810 
1.9 3.5 0.5 52.5 
2.1/2.9 
l F.sub.3 /M.sub.2 
-- 2.1/3.0 
703 
2710 
1.7 2.8 -1.1 
52.1 
m F.sub.5 /M.sub.2 
-- 2.0/3.2 
698 
2650 
1.5 2.0 -3.0 
50.2 
n F.sub.4 /M.sub.5 
-- 2.2/3.0 
774 
2350 
0.6 2.3 -1.8 
47.4 
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Based on the results of Table 1, the relationships between the transfer 
characteristic, the low frequency sensitivity (a relative value of the 
reproduction output at 315 Hz) and the AC bias noise are shown in FIGS. 1 
and 2. It is evident from FIG. 1 that the tapes having adequately low AC 
bias noise and adequately low transfer, are tapes l, m, f and g falling in 
the hatched portion. However, as is apparent from FIG. 2, tapes f and g 
have poor low frequency sensitivity, whereas tapes l and m have high 
sensitivity. As a whole, the magnetic tapes l and m exhibit outstanding 
characteristics where the noise, transfer and low frequency sensitivity 
are well balanced. 
The tapes showing high sensitivity in FIG. 2 all have Br of at least 2000 
G. The higher the value of Br, the higher the low frequency sensitivity. 
Accordingly, a single-layered tape using an alloy powder has higher 
sensitivity than the double-layered tapes. However, the alloy magnetic 
powder and the magnetic tape made thereof tend to be unstable and will 
have greater change with time as the specific surface area increases in 
contrast with the oxide magnetic powder, as shown in FIG. 3. Furthermore, 
in some cases, there is a danger of catching fire during the production. 
Therefore, it is desired to decrease the activity by selecting the 
specific surface area of at most 30 m.sup.2 /g. However, if such a low 
specific surface area is selected (see tapes a, b, c and d), the noise 
tends to increase. Whereas, as is evident from the tapes l and m, the 
noise can be substantially reduced without a substantial reduction of the 
low frequency sensitivity by using alloy powder having a low specific 
surface area of at most 30 m.sup.2 /g for the under-layer and an oxide 
magnetic powder having a specific surface area of at least 25 m.sup.2 /g 
for the top-layer. The oxide magnetic powder used for the top-layer has a 
poor low frequency sensitivity. Nevertheless, when used as the top-layer 
in combination with the under-layer to form the double-layered structure 
and the ratio of the top-layer thickness to the under-layer thickness is 
set to be not higher than 1.0, the desired characteristics of the 
under-layer can fully be produced without a substantial reduction of the 
low frequency sensitivity. Besides, since the specific surface area of the 
magnetic powder of the top layer is sufficiently large, the noise level 
can be adequately lowered, and no substantial adverse effect will be 
brought about from the under-layer. Furthermore, with respect to the 
transfer characteristic, the desirable influence of the small specific 
surface area of the under-layer overcomes the inferior transfer 
characteristic of the top-layer and substantially eliminates the adverse 
effect of the latter. 
On the other hand, while the top-layer is influential to the noise and the 
high frequency sensitivity (the short wave recording sensitivity), it does 
not require so high Br for a short wave recording, and accordingly it is 
possible to obtain an adequate high frequency sensitivity by using an 
oxide magnetic powder for the top-layer.