Magnetic recording medium and production thereof

A magnetic recording medium comprising a non-magnetic substrate and a magnetic layer formed thereon which contains chromium oxide ferromagnetic powder comprising chromium oxide of the formula: EQU CrO.sub.x wherein x is not smaller than 1.74 and not larger than 1.91, which has improved long term storage stability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetic recording medium and production 
thereof. More particularly, it relates to a magnetic recording medium 
comprising a magnetic layer containing chromium oxide, which has improved 
long term storage stability. 
2. Description of the Prior Art 
It is well known that chromium dioxide (CrO.sub.2) powder has a 
ferromagnetic property and is used as a magnetic powder of a magnetic 
recording medium. Chromium dioxide powder is commercially produced by 
oxidation of Cr.sub.2 O.sub.3 and CrO.sub.3 according to following 
reactions: 
##STR1## 
Coercive force and/or particle size of the chromium dioxide powder can be 
adjusted by the addition of Sb.sub.2 O.sub.3, Fe.sub.2 O.sub.3, TeO.sub.2, 
RhO.sub.2, IrO.sub.2 or the like to produce magnetic powder for magnetic 
recording. Recently, the amount of the chromium dioxide powder consumed as 
the magnetic powder has been slightly decreasing since a magnetic powder 
comprising iron oxide or iron oxide covered with cobalt has been 
developed. 
This is because chromium dioxide contains Cr(IV) which is chemically 
unstable so that it is inevitable that the saturation magnetization of the 
chromium dioxide inherently deteriorates in the air with the passage of 
time. 
However, the chromium dioxide powder has an excellent property such that 
when recorded information is written over, namely when the recorded 
information of the magnetic recording medium is erased for re-recording 
the information is satisfactorily erased. Since this good overwriting 
property of the chromium dioxide powder is suitable for a magnetic 
recording medium for use as the external memory of a computer, which 
should have good repeated recording properties, the chromium dioxide 
powder is being reevaluated. 
Under these circumstances, some attempts have been made to suppress the 
deterioration of the saturation magnetization with the passage of time due 
to chemical instability of chromium dioxide while maintaining the inherent 
advantages of chromium dioxide. 
The first attempt is to convert the surface layer of the chromium dioxide 
powder particle from dioxide to hydroxide of Cr(III) which is more stable 
than chromium dioxide and comprises treating the chromium dioxide powder 
with a reducing agent, such as sodium sulfite (cf. Italian Pat. No. 
27333A/79 and U.S. Pat. Nos. 3,512,930 and 3,529,930). 
The second attempt is to remove reactive materials such as adsorbed water 
present on the surface of the chromium dioxide powder particles and 
comprises heating the chromium dioxide powder at a temperature of 
330.degree. to 370.degree. C. in a nitrogen atmosphere (cf. G. Bassile, 
Material Review Bulletin, 17, (1982) 1197). 
However, the above attempts cannot prevent deterioration of the saturation 
magnetization of the chromium dioxide powder with the passage of time, and 
a saturation magnetic flux density of a magnetic recording medium 
comprising such modified or thermally treated chromium dioxide powder 
decreases by 10 to 30% from the original value after the recording medium 
is kept standing in the air at 60.degree. C. for 4 weeks. 
This may be partly because the particle size of the chromium dioxide powder 
is made smaller to satisfy the requirements for high density recording and 
in turn its specific surface area increases so that reactivity of the 
particle surfaces increases and thereby the saturation magnetization more 
easily deteriorates than with the conventional chromium dioxide powder 
having a larger particle size, and partly because mutual reactivity 
between a binder and the chromium dioxide powder is increased by 
functional groups of an additive which is added to increase dispersibility 
of the chromium dioxide powder in the binder. 
According to the study of the present inventors to find a good technique to 
prevent the deterioration of the saturation magnetization of the chromium 
dioxide powder with the passage of time, it was concluded that, by the 
above first attempt, although CrOOH is formed on the surface of the 
chromium dioxide powder particle, it may deteriorate the chemical 
stability of the chromium dioxide powder. Namely, according to the first 
attempt, even when CrOOH is formed on the surface of the chromium dioxide 
powder particle, it easily attracts water because of its hydrophilicity. 
This results in insufficient stability of the chromium dioxide powder. 
By the above second attempt, the chromium dioxide powder is heated at a 
temperature of from 330.degree. to 370.degree. C. in an atmosphere of 
inert gas, such as nitrogen, to remove crystallization water of chromium 
dioxide or adsorbed water on the particle surface in order to increase the 
stability of chromium dioxide. However, the result is not satisfactory. 
This is because not only CrOOH but also chromium dioxide has strong 
bonding force with a reactive material such as water. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide novel chromium oxide 
ferromagnetic powder. 
Another object of the present invention is to provide a magnetic recording 
medium having good overwriting properties and improved stability so that 
its magnetic properties are less deteriorated after long term storage. 
A further object of the present invention is to provide a method for 
preventing deterioration of magnetic powder comprising chromium oxide by 
simple thermal treatment. 
Yet, another object of the present invention is to provide a method for 
producing a magnetic recording medium having good overwriting properties 
and magnetic stability.

DETAILED DESCRIPTION OF THE INVENTION 
In view of the above described circumstances, in accordance the present 
inventors have with the present invention replaced a part of chromium 
dioxide with a more stable chromium (III) oxide, since the chromium 
dioxide powder inherently readily bonds with water and the like and its 
properties will deteriorate. To achieve this, reaction conditions have 
been carefully selected while keeping in mind that oxides of chromium 
other than chromium (IV) are easily formed, since chromium dioxide is of 
course being an oxide of chromium, and that the formed chromium oxides 
should be stably present and not disappear. 
As a result, it has been found that a desirable result can be obtained when 
the chromium dioxide powder is heated at a temperature not lower than 
410.degree. C. until saturation magnetization decreases to 46 to 87% of 
that of the untreated chromium dioxide (.sigma..sub.s :86.3 emu/g). 
Therefore, according to one aspect of the present invention, there is 
provided chromium oxide ferromagnetic powder comprising chromium oxide of 
the formula: 
EQU CrO.sub.x 
wherein x is not smaller than 1.74 and not larger than 1.91 
(1.74.ltoreq.x.ltoreq.1.91). 
According to another aspect of the present invention, there is provided a 
magnetic recording medium comprising a non-magnetic substrate and at least 
one magnetic layer formed thereon which contains the chromium oxide 
ferromagnetic powder of the present invention. 
FIG. 1 shows changes of saturation magnetization .sigma..sub.s (black 
circles) and coercive force Hc (white circles) when the chromium dioxide 
powder having saturation magnetization of 86.3 emu/g and coercive force of 
516 Oe is heated in the air for one hour at various temperatures. From 
these results of FIG. 1, it is seen that saturation magnetization and 
coercive force due to magnetism of the chromium dioxide powder are not 
deteriorated when the powder is heated at a temperature lower than 
400.degree. C. 
FIG. 2 shows changes of saturation magnetization (black circles) and 
coercive force (white circles) when magnetic powder having saturation 
magnetization of 77.4 emu/g and coercive force of 505 Hc, which is 
produced by hydroxidising the surfaces of the chromium dioxide powder 
particles and supposed to have CrOOH on the surfaces, is heated in the air 
for one hour at various temperatures. From the fact that saturation 
magnetization of the magnetic powder increases in a temperature range 
lower than 400.degree. C. and decreases in a temperature range higher than 
400.degree. C., it can be said that when the magnetic powder is heated at 
a temperature higher than 410.degree. C., CrOOH is reconverted to 
CrO.sub.2 and substantially no CrOOH is present on the surfaces of the 
chromium dioxide powder particles. 
According to the invention, therefore, the reason for heating the chromium 
dioxide powder at a temperature of not lower than 410.degree. C. is to 
remove CrOOH and to produce desired chromium oxide. 
The gradual conversion from CrO.sub.2 to a non-magnetic material comprising 
Cr.sub.2 O.sub.3 (Cr(III) oxide) by heating at a temperature not lower 
than 410.degree. C. can be traced by measuring decrease of saturation 
magnetization or weight change. While the theoretical weight decrease is 
9.5% when all the chromium dioxide powder is converted to Cr.sub.2 
O.sub.3, the weight loss found in the completely nonmagnetized powder is 
about 9% for the raw material chromium dioxide powder of the present 
invention. This means that the thermally treated magnetic powder according 
to the present invention comprises a major amount of chromium (III) oxide 
and a slight amount of chromium (IV) oxide. An average valency of chromium 
in the oxide is closer to trivalency than to tetravalency and x in 
CrO.sub.x falls in the range from 1.71 to 1.91. 
In this connection, U.S. Pat. No. 3,512,930 discloses an intermediate 
chromium oxide of Cr(IV) and Cr(III) of the formula: 
EQU Cr.sub.y O.sub.z 
wherein a ratio of z/y (=x) is not smaller than 2.0 and not larger than 
3.0. However, such intermediate chromium oxide does not have sufficient 
effect on improvement of stability of chromium dioxide. 
On the contrary, the magnetic recording medium utilizing the chromium oxide 
ferromagnetic powder treated according to the present invention shows 
excellent stability. That is, when the chromium oxide ferromagnetic powder 
according to the present invention is kept standing in the air at 
60.degree. C. for 4 weeks, a deterioration ratio of saturation 
magnetization does not exceed 6%. 
It is easily recognized by the conventional gravimetric method that the 
atomic composition of the extremely stabilized chromium oxide 
ferromagnetic powder according to the present invention is such that the 
atomic ratio of chromium and oxygen, namely x in the formula: CrO.sub.x is 
not smaller than 1.74 and not larger than 1.91, and that preferably the 
weight ratio of chromium(IV) oxide to a total weight of chromium (IV) 
oxide and other chromium oxides is at least 48% by weight. Further, by a 
sample vibration type magnetometer, the chromium oxide according to the 
present invention is analyzed to have saturation magnetization of 40 to 75 
emu/g. 
For effectively producing the chromium oxide according to the present 
invention, the chromium dioxide powder containing substantially no CrOOH 
is heated at a temperature not lower than 410.degree. C. till saturation 
magnetization decreases to 46 to 87% of original saturation magnetization 
of untreated chromium dioxide powder (.sigma.s=86.3 emu/g) to produce 
CrO.sub.x wherein x is as defined above. 
When the chromium dioxide powder contains CrOOH, it may be heated at a 
temperature of 250.degree. to 400.degree. C. in a nonreducing atmosphere 
to reconvert CrOOH to chromium dioxide and then heated at a higher 
temperature as described in the above, or the chromium dioxide powder 
containing CrOOH as such may be heated at a higher temperature as 
described in the above. 
The thermally treated ferromagnetic powder of the present invention is used 
as such or coated with a conventional organic material, and then dispersed 
in a synthetic resin base binder together with an organic solvent to 
prepare a magnetic paint, which is coated on a non-magnetic substrate made 
of, for example, a synthetic resin film or a non-magnetic metal plate and 
dried to produce a stabilized magnetic recording medium of the present 
invention. 
As described in the above, the chromium dioxide powder should be heated at 
a temperature not lower than 410.degree. C. In view of uniformity and 
efficiency of the thermal treatment, preferably the chromium dioxide 
powder is heated at a temperature of 410.degree. to 600.degree. C., more 
preferably 420.degree. to 500.degree. C. The heating atmosphere should not 
be such atmosphere which prohibits the formation of CrO.sub.x wherein x is 
as defined in the above. Preferably, the heating is carried out in the air 
or a non-reducing atmosphere. The heating time is usually 0.2 to 6 hours. 
The ferromagnetic chromium oxide powder according to the present invention 
has improved affinity with a highly reactive resin having various polar 
groups and/or functional groups. 
Preferred Embodiments 
The present invention will be explained further in detail by following 
examples in which parts are by weight unless otherwise indicated. 
EXAMPLE 1 
Each of following ferromagnetic chromium oxide powder (1,500 g) was heated 
in a muffle furnace of 50 cm in length, 30 cm in width and 30 cm in depth 
at a temperature of Table 1 and for a period of time of Table 1 in the air 
under 1 atm. 
Ferromagnetic Chromium Oxide Powder 
(a) Ferromagnetic chromium dioxide powder 
Major axis: 0.4 .mu.m 
Axis ratio: 10 
Specific surface area: 25 m.sup.2 /g 
Saturation magnetization: 86.3 emu/g 
Coercive force: 516 Oe 
Deterioration ratio: 29% 
(after kept standing at 60.degree. C. in the air for 4 weeks) 
(b) Ferromagnetic chromium oxide powder particle surfaces of which were 
converted to the hydroxide of chromium (III) according to U.S. Pat. No. 
3,512,930 
Major axis: 0.4 .mu.m 
Axis ratio: 10 
Specific surface area: 25 m.sup.2 /g 
Saturation magnetization: 77.4 emu/g 
Coercive force: 505 OE 
Deterioration ratio: 24% 
(after kept standing at 60.degree. C. in the air for 4 weeks) 
TABLE 1 
______________________________________ 
Sample Ferromagnetic Temp. Heating time 
No. powder (.degree.C.) 
(hrs) 
______________________________________ 
1 a 350 1 
2 a 390 1 
3 a 420 2.5 
4 a 420 4 
5 a 410 6 
6 a 500 0.5 
7 b 350 1 
8 b 390 1 
9 b 410 1 
10 b 420 1 
11 b Not heated 
12 b Not heated 
______________________________________ 
Properties of each sample ferromagnetic powder are shown in Table 2. 
TABLE 2 
______________________________________ 
Deterio- 
Sample 
x in .sigma..sub.s 
Hc ration Ratio of 
No. CrO.sub.x 
(emu/g) (Oe) ratio (%) 
Cr(IV)/non-Cr(IV) 
______________________________________ 
1 2.0 86 495 14.1 1/0 
2 2.0 86 470 11.0 1/0 
3 1.86 65 460 2.5 0.72/0.28 
4 1.80 53 446 2.0 0.60/0.40 
5 1.87 67 460 2.9 0.74/0.26 
6 1.91 76 440 3.0 0.82/0.18 
7 2.0 86 483 14.5 1/0 
8 2.0 86 450 12.0 1/0 
9 1.91 76 445 4.5 0.82/0.18 
10 1.89 71 437 3.0 0.77/0.23 
11 2.0 86 510 29.0 1/0 
12 -- 72 507 24.5 -- 
______________________________________ 
Each ferromagnetic chromium oxide powder having the above properties (100 
parts) was mixed with vinyl chloride/vinyl acetate/vinyl alcohol copolymer 
(VAGH manufactured by U.C.C.) (9 parts), polyurethane resin (Estan 5701 
manufactured by Goodrich) (8 parts), myristic acid (1 part), n-butyl 
stearate (0.5 part), carbon black (2 parts), cyclohexanone (144 parts) and 
toluene (144 parts) and dispersed in a ball mill for 75 hours. Thereafter, 
polyisocyanate compound (Colonate L manufactured by Nippon Urethane) (3 
parts) was added and thoroughly mixed to prepare a magnetic paint. Then, 
the magnetic paint was coated on a substrate made of a polyester film 
having a thickness of 20 .mu.m with a gravure coater to a thickness of 4 
.mu.m after drying and planished. The film with the magnetic coating was 
cut to produce a magnetic tape, which was assigned the same number as that 
of the sample of the ferromagnetic powder. 
TABLE 3 
______________________________________ 
Residual magnetic 
Squareness Coercive 
Deterio- 
Tape flux density Br 
ratio force ration 
No. (G) Br/Bs (Oe) ratio (%) 
______________________________________ 
1 1,760 0.80 500 12.0 
2 1,720 0.81 485 11.7 
3 1,510 0.82 465 5.5 
4 1,480 0.79 450 4.6 
5 1,605 0.80 460 4.0 
6 1,650 0.81 440 3.0 
7 1,710 0.80 470 10.9 
8 1,700 0.79 450 10.1 
9 1,635 0.80 440 4.9 
10 1,610 0.81 435 4.2 
11 1,770 0.81 505 16.3 
12 1,780 0.81 500 14.0 
______________________________________ 
Properties of the ferromagnetic powder and the magnetic tape shown in 
Tables 2 and 3 were measured as follows: 
Deterioration ratio 
Deterioration ratios of the ferromagnetic powder and the magnetic tape are 
calculated from the original saturation magnetization and residual 
magnetic flux density and those measured after the ferromagnetic powder 
and the magnetic tape are kept standing in the air at 60.degree. C. for 4 
weeks. 
Atomic ratio x (CrO.sub.x) 
The atomic ratio x is calculated from the weight of CrO.sub.x and 
saturation magnetization, and the weight of the sample after heating it 
till saturation magnetization becomes 0 (zero) with assuming that only 
CrO.sub.2 component has magnetism. 
Cr(IV)/non-Cr(IV) ratio 
This ratio is calculated from the weight change of the sample caused by the 
thermal treatment carried out in measuring the atomic ratio x. 
Saturation magnetization, Coercive force and Saturated magnetic flux 
density 
These properties are measured with a sample vibration type magnetometer 
(VSM-3 manufactured by Toei Kogyo) by applying a magnetic field of 5kOe. 
As understood from the above results, the ferromagnetic powder samples and 
the magnetic tapes of the present invention (Nos. 3, 4, 5, 6, 9 and 10) 
suffer from less deterioration than the conventional ones. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the present invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
to be included within the scope of the following claims.