Process for preparing acicular ferromagnetic material consisting essentially of iron-containing chromium dioxide

An acicular, ferromagnetic material which has a high coercive force and essentially consists of iron-containing chromium dioxide is prepared by thermal decomposition of hydrated chromium(III) chromate under superatmospheric pressure.

The present invention relates to an acicular ferromagnetic material 
essentially consisting of iron-containing chromium dioxide, which material 
is distinguished in particular by a high coercive force, and a process for 
its preparation. 
Acicular chromium dioxide, its preparation and the use of this material for 
magnetic recording media have been described in a number of publications. 
Magnetic recording media which contain chromium dioxide generally have 
superior magnetic properties compared with recording media based on other 
magnetic oxides, the said properties being attributable to the high values 
for coercive force, specific remanence and saturation magnetization and in 
particular to the uniform shape and the small dimensions of the acicular 
chromium dioxide particles. 
In addition to the synthesis of the chromium dioxide by a 
synproportionation reaction under hydrothermal conditions from 
chromium(III) and chromium(VI) oxides (cf. inter alia EP-A 27 640), the 
preparation of acicular chromium dioxide by thermal decomposition of 
hydrated chromium(III) chromate is also known. For example, DE-A 22 10 059 
describes a process in which Cr.sub.2 (CrO.sub.4).sub.3.nH.sub.2 O, where 
n is from 1 to 8, is decomposed at from 250 to 500.degree. C. and under 
from 30 to 1,000 bar to give chromium dioxide. The product formed in this 
process has a coercive force which is too low for the recording media now 
customary and which decreases even further at higher values of n. Where 
the values of n are greater than 8, the remanent magnetization and the 
saturation magnetization then also decrease owing to the CrOOH then 
simultaneously formed. The magnetic properties can be improved by a 
process according to DE-A 23 32 854, in which a substance which undergoes 
exothermic decomposition under the given reaction conditions is mixed with 
the chromium(III) chromate. Modification of the chromium dioxide with 
lanthanum, yttrium, barium and strontium is also supposed to improve the 
magnetic characteristics, according to DE-A 29 19 572. Furthermore, 
modified chromium dioxide can also be prepared from chromium(III) 
chromates having a higher water content (degree of hydration n from 8 to 
12) (DE-B 25 20 030 and DE-B 26 48 305). 
The common feature of all these processes for the preparation of chromium 
dioxide from chromium(III) chromates is that, in order to avoid a decrease 
in the saturation magnetization due to the formation of CrOOH, only 
chromium(III) chromates having a relatively low water content, ie. a 
degree of hydration n of not more than 12, can be used. The products 
having a low water content are substantially in powder form and therefore 
require more expensive safety measures, for example encapsulation of the 
apparatuses or the installation of extraction apparatuses. In order to 
overcome these problems, the use of a chromium(III) chromate having a 
degree of hydration greater than 13 has therefore been proposed. This 
process also has the advantage that the magnetic properties of the 
resulting chromium dioxide can be improved, contrary to earlier 
assumptions. 
The improvements form part of the efforts aimed at improving the magnetic 
properties of the chromium dioxide, in particular the coercive force, so 
that this material can also be used for the preparation of magnetic 
recording media required for the newly developed recording methods 
employing high recording density, especially in the video sector. Although 
it has been disclosed (FR-B-25 02 384) that a chromium dioxide having a 
very high coercive force can be prepared by doping with iridium, this 
process cannot be implemented for economic reasons. The same also applies 
to the coating of the chromium dioxide with cobalt compounds, which 
permits a substantial increase in the coercive force (Tatsuru Namikawa et 
al., Nippon Kugaku Kaishi 1980 (2), 194-198). 
It is an object of the present invention to provide an acicular, 
ferromagnetic chromium dioxide material which, without the addition of 
iridium and without coating with cobalt, has a very high coercive force in 
conjunction with high values for the saturation magnetization. 
We have found that this object is achieved by an acicular, ferromagnetic 
material which essentially consists of iron-containing chromium dioxide 
and which contains less than 2 ppm of iridium and less than 50 ppm of 
cobalt, based on the amount of chromium dioxide, and has a coercive force 
greater than 61 kA/m and a magnetization of more than 70 nTm.sup.3 /g, 
measured in a magnetic field of 380 kA/m. 
The present invention furthermore relates to a process for the preparation 
of this material by decomposing a water-containing chromium(III) chromate 
of the formula Cr.sub.2 (CrO.sub.4).sub.3.nH.sub.2 O having a degree of 
hydration n of not less than 13 at from 200 to 500.degree. C. and under 
from 50 to 700 bar, wherein the water-containing chromium(III) chromate is 
heated to 200-500.degree. C. in a high pressure reactor and cooled 
immediately after this temperature has been reached and the high pressure 
reactor is vented. 
The novel process can be carried out in a particularly advantageous manner 
if the chromium(III) chromate having a degree of hydration n of not less 
than 13, preferably from 14 to 20, is obtained by adding water to 
chromium(VI) oxide (CrO.sub.3) in an amount such that, after partial 
reduction of the CrO.sub.3 with an organic material, the required water 
content of the chromium(III) chromate results. The amount of water formed 
during the reduction must be taken into account here. 
Since, in the preparation of chromium(III) chromates having a high water 
content, the amount of water added to the CrO.sub.3 is generally not 
sufficient to effect complete dissolution, it is advantageous if the 
CrO.sub.3 /H.sub.2 O starting suspension is subjected, together with the 
modifiers, to thorough dispersing, for example with a dissolver stirrer. 
The reduction of the CrO.sub.3 is highly exothermic. To prevent the 
reaction suspension from being heated to an excessive extent, it is 
advisable to cool the reaction vessel during the reduction. To moderate 
the generally vigorous reaction between organic material and CrO.sub.3, it 
has proven advantageous to dilute the reducing agent with water. However, 
in order to avoid changing the total water content of the chromium(III) 
chromate in this procedure, the amount of water used for partial 
dissolution of the CrO.sub.3 must be reduced by an amount corresponding to 
the amount of water used for dilution of the reducing agent. Of the n 
moles of H.sub.2 O of the Cr.sub.2 (CrO.sub.4).sub.3.nH.sub.2 O, it has 
proven most advantageous to use four moles for diluting the reducing agent 
and the remaining (n-4) moles for the preparation of the CrO.sub.3 
suspension. 
Suitable reducing agents for the chromium(III) chromate preparation are all 
conventional reducing agents, but preferably readily oxidizable organic 
compounds, such as alcohols and aldehydes, in particular less volatile 
polyhydric alcohols, such as glycol or glycerol. The Cr.sup.6+ /Cr.sup.3+ 
molar ratio in the chromium(III) chromate should as far as possible be 
1.5, but an excess or a deficiency of Cr.sup.6+ may also be employed. 
In the novel process, modified chromium dioxide materials are prepared, and 
it is appropriate to use known modifiers, such as antimony, tellurium, 
iron and/or their compounds and combinations of these substances. Usually, 
combinations of tellurium oxide and/or antimony oxide, if necessary in the 
form of the potassium antimonyl tartrate (EP-A 198 110), and iron oxide 
are particularly suitable. The modifiers are used in amounts of from 0.05 
to 10% by weight, calculated as oxide and based on the resulting chromium 
dioxide. The modifiers should as far as possible be introduced into the 
CrO.sub.3 suspension before the addition of the reducing agent. 
The novel process is carried out, for example, by adding less water to the 
CrO.sub.3 than would be required to obtain the predetermined degree of 
hydration of not less than 13 in the chromium(III) chromate. After the 
addition of the modifiers to the viscous mass, the mixture is thoroughly 
dispersed using suitable apparatuses. Thereafter, the resulting suspension 
is reduced in a stirred vessel provided with a dropping funnel and reflux 
condenser and coolable from outside, with thorough mixing, by slow 
addition of an organic reducing agent, for example glycerol, to which the 
remaining amount of water required for obtaining the final water content 
of the chromium(III) chromate has been added. The resulting pasty 
chromium(III) chromate is transferred to a stainless steel reaction vessel 
and then converted to chromium dioxide in a high pressure reactor under 
from 50 to 700 bar and at from 200 to 500.degree. C. In the novel process, 
it is essential that, immediately after reaching the reaction temperature, 
the resulting material is cooled again, it being essential for the 
temperature to be reduced rapidly below, in particular, 220-250.degree. 
C. This ensures homogeneous incorporation of the iron ions in the crystal 
lattice of the chromium dioxide. Simultaneously with or after cooling, the 
high pressure reactor is vented. The chromium dioxide is removed 
mechanically from the reaction vessel and is milled. 
For further improvement of the synthesized chromium dioxides, thermal 
aftertreatment under oxidizing conditions at from 150 to 400.degree. C. 
has been disclosed (DE-B 19 05 584). If necessary, the chromium dioxide 
obtained in the manner described can be further stabilized in a known 
manner, for example by the surface action of reducing agents (DE-B 1 925 
541) or of alkalis (DE-A 3 600 624), by heating under an inert gas (EP-A 
29 687) or by treatment with water-insoluble organic compounds and a 
surfactant (DE-A-36 10 411). 
The novel material essentially consisting of chromium dioxide has a 
coercive force of more than 61, preferably from 61 to 78, kA/m, the 
magnetization being more than 70 nTm.sup.3 /g, measured in a magnetic 
field of 380 kA/m. Surprisingly, these high values for the coercive force 
are not achieved either by doping the chromium dioxide with iridium or by 
coating with cobalt. 
Thus, this chromium dioxide material can particularly advantageously be 
used in magnetic recording media which are intended to be used for 
recording high information densities. The novel chromium dioxide is 
processed to magnetic recording media by known methods. For the production 
of the magnetic layer, from 2 to 10 parts by weight of chromium dioxide 
together with the binder and the suitable dispersants, lubricants and 
further conventional additives in a total amount of not more than 10% by 
weight, based on the chromium dioxide, are processed to give a dispersion. 
The dispersion thus obtained is filtered and is applied with a 
conventional coating apparatus, for example a knife coater, in one or more 
thin layers on the nonmagnetic substrate or in a thin layer on the 
magnetic recording medium already provided with another magnetic layer. 
Before the liquid coating mixture is dried at from 50 to 90.degree. C., 
magnetic orientation of the chromium dioxide particles may be carried out. 
For special surface treatment of the magnetic layer, the coated film webs 
are passed between heated, polished rollers under pressure. Thereafter, 
the thickness of the magnetic layers are usually from 1.5 to 12 .mu.m. 
The binders used for the magnetic layers are the known polymer binders, 
such as acrylate copolymers, polyvinyl acetates, eg. polyvinylformal or 
polyvinylbutyral, relatively high molecular weight epoxy resins, 
polyurethanes and mixtures of these and similar binders. Elastomeric 
linear polyesterurethanes which are soluble in a volatile organic solvent 
and are virtually free of isocyanate groups have proven advantageous; 
these polykurethanes can be prepared by reacting a polyester of an 
aliphatic dicarboxylic acid of 4 to 6 carbon atoms, such as adipic acid, 
and one or more aliphatic diols of 3 to 10 carbon atoms, such as 1,2- and 
1,3-propylene glycol, 1,4-butanediol, diethylene glycol, neopentylglycol 
or 1,8-octanediol, with a diisocyanate of 6 to 24, in particular 8 to 20, 
carbon atoms, such as toluylene diisocyanate or 
4,4'-diisocyanatodiphenylmethane, preferably in the presence of a fairly 
small amount of a glycol of 4 to 10 carbon atoms, such as 1,4-butanediol, 
which produces chain extensions. Polyesterurethanes obtained from adipic 
acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane are preferred. 
Preferred polyesterurethanes have a Shore A hardness of from 70 to 100, a 
tensile strength of from 40 to 42 N/mm.sup.2 (according to DIN 53,455) and 
an elongation at break (according to DIN 53,455) of about 440-560%. The K 
value according to H. Fikentscher (Cellulose-Chemie 13 (1932), page 58 et 
seq.) is from 40 to 60 (1% strength in dimethylformamide) for the 
particularly suitable polymer binders. 
The Examples which follow illustrate the invention and compare it with the 
prior art. For the resulting chromium dioxide, the specific surface area 
SSA in [m.sup.2 /g] according to DIN 66,132 was determined using a 
Strohlein areameter from Strohlein, Dusseldorf, by the one-point 
difference method due to Haul and Dumbgen, and the magnetic properties 
were determined using a vibrating sample magnetometer in a magnetic field 
of 380 kA/m, these properties being the coercive force H.sub.c in [kA/m] 
and the specific remanence Mr/.rho. and the saturation magnetization 
M.sub.s /.rho. in [nTm.sup.3 /g]. The mean tap density was .rho.1.3 
g/cm.sup.3. The amount of cobalt and the amount of iridium were determined 
by atomic absorption spectrometry. The X-ray patterns were recorded using 
an automatic Siemens D 500 with copper K.alpha. radiation. To determine 
the lattice constants, the CrO.sub.2 samples were mixed with MICA (MBS 
standard SRM No. 675) as an internal standard and the measurements were 
carried out at 20.degree. C. The position of the CrO.sub.2 (110) line was 
related to the theoretical value of the MICA line (theta 26.774.degree. 
C.) and the lattice constant a in [.ANG.] was calculated from the measured 
difference. The stated length of the CrO.sub.2 particles is an arithmetic 
mean determined by statistical evaluation and measurement of about 200 
particles in electron micrographs.

EXAMPLE 1 
447 g of water were added to 1.5 kg of CrO.sub.3 in a stirred glass vessel 
initially open at the top, and dispersing was carried out for 10 minutes 
using a dissolver stirrer at 1430 rpm. The amount of water initially taken 
was insufficient for complete dissolution of the CrO.sub.3. Thereafter, 
3.3 g of TeO.sub.2 (=0.26% by weight, based on the CrO.sub.2 being formed) 
and 45.3 g of Fe.sub.2 O.sub.3 (=3.6% by weight) were added to the 
suspension, and the reaction vessel was sealed with a lid provided with a 
reflux condenser and a dropping funnel. The reaction vessel was 
furthermore equipped with a stirrer driven by a powerful motor. 334 g of 
water and 118.4 g of glycerol was slowly passed into the reaction 
suspension from the dropping funnel in the course of 120 minutes while 
refluxing, cooling the reactor wall with ice and stirring the CrO.sub.3 
suspension vigorously. A Te/Fe-modified chromium(III) chromate having an 
H.sub.2 O content n of 16 resulted. 
The Cr.sub.2 (CrO.sub.4).sub.3.16 H.sub.2 O obtained in the above-mentioned 
manner was placed in a cylindrical stainless steel vessel and heated to 
350.degree. C. under 350 bar in a high pressure reactor and cooled again 
immediately after the temperature had been reached. The reactor was then 
vented. After cooling to room temperature, the CrO.sub.2 formed was 
removed mechanically from the reaction vessel. 
The reaction product was heated for 80 minutes at 180.degree. C. It had a 
coercive force of 71.4 kA/m, a specific magnetization M.sub.s /.rho. of 92 
nTm.sup.3 /g and a remanence of 43 nTm.sup.3 /g. The iridium content was 
less than 2 ppm and the cobalt content was less than 30 ppm. The SSA was 
20 m.sup.2 /g, the lattice constant a was 4.430.+-.0.0015 .ANG. and the 
mean particle length was 300 nm. 
EXAMPLE 2 
447 g of water were added to 1,500 g of CrO.sub.3 in a stirred glass vessel 
initially open at the top, and dispersing was carried out for 10 minutes 
using a dissolver stirrer. The amount of water initially taken was 
insufficient for complete dissolution of the CrO.sub.3. Thereafter, 45.3 g 
of Fe.sub.2 O.sub.3 (3.6% by weight), 1.89 g of TeO.sub.2 (0.15% by 
weight) and 5.77 g of potassium antimonyl tartrate (0.2% by weight) were 
added and the mixture was dispersed again for 10 minutes using the 
dissolver stirrer. The reaction vessel was then sealed with a lid equipped 
with a reflux condenser and a dropping funnel. 
117.3 g of glycerol in 924 g of water were added dropwise in only 48 
minutes while refluxing, cooling the reactor wall with ice and stirring 
the CrO.sub.3 suspension. A Te/Sb/Fe-modified chromium(III) chromate 
having an H.sub.2 O content n of 16 resulted. 
The product obtained was introduced in 170 g portions into cylindrical 
stainless steel vessels and converted in a high pressure reactor under the 
following conditions: 
EXAMPLE 2a 
Heating to 320.degree. C. (maximum pressure 176 bar) followed by immediate 
cooling to room temperature and subsequent venting of the reactor. 
EXAMPLE 2b 
Heating to 340.degree. C. (maximum pressure 224 bar) followed by immediate 
cooling to room temperature and subsequent venting of the reactor. 
The product was removed from the reaction vessel, washed with water until 
the conductivity of the washwater had reached &lt;100 .mu.S/cm and dried. The 
products had the following magnetic properties: 
______________________________________ 
H.sub.c M.sub.s /.rho. 
M.sub.s /.rho. 
SSA 
[kA/m] [nTm.sup.3 /g] 
[nTm.sup.3 /g] 
[m.sup.2 /g] 
______________________________________ 
Example 2a 78.0 87.5 42.8 19 
Example 2b 66.7 93.4 46.1 22 
______________________________________ 
The lattice constants a of the pigment from Example 2a was 4.432.+-.0.0015 
.ANG., and that of the pigment from Example 2b was 4.426.+-.0.0015 .ANG.. 
The mean particle length of the pigment from Example 2a was 350 nm and 
that of the pigment from Example 2b was 300 nm. 
EXAMPLE 3 
The procedure described in Example 2 was followed, except that the dopants, 
45.3 g of .gamma.-Fe.sub.2 O.sub.3 (3.6% by weight) and 3.28 g of 
TeO.sub.2 (0.26% by weight), were dispersed together with the initially 
taken mixture of 1.5 kg of CrO.sub.3 and 447.5 g of H.sub.2 O for 10 
minutes using the dissolver stirrer. 
The resulting Te/Fe-modified chromium(III) chromate having 16 molecules of 
water of crystallization was placed in a cylindrical stainless steel 
vessel and heated to 318.degree. C. under 240 bar in a high pressure 
reactor and cooled again immediately after the temperature had been 
reached. Thereafter, the pressure was let down and the resulting CrO.sub.2 
was removed from the reactor. 
The reaction product was heated for 80 minutes at 180.degree. C. in the 
air, then washed with water until the washwater had a conductivity of &lt;100 
.mu.S/m and finally surface-treated according to DE-B 19 25 541 with a 
sodium sulfite solution. 
The dried end product had the following properties: 
______________________________________ 
H.sub.c M.sub.s /.rho. 
M.sub.r /.rho. 
SSA 
[kA/m] [nTm.sup.3 /g] 
[nTm.sup.3 /g] 
[m.sup.2 /g] 
______________________________________ 
73.2 90.1 41.2 36 
______________________________________ 
The lattice constant a was 4.428.+-.0.0015 .ANG. and the average needle 
length was 300 nm (FIG. 1). 
EXAMPLE 4 
The procedure described in Example 2 was followed, except that 45.3 g of 
.gamma.-Fe.sub.2 O.sub.3 (3.6% by weight) and 4.42 g of TeO.sub.2 (0.35% 
by weight) were used as dopants. 
The resulting Te/Fe-modified chromium(III) chromate having 16 molecules of 
water of crystallization was placed in a cylindrical stainless steel 
vessel and heated to 340.degree. C. under 216 bar in a high pressure 
reactor, been reached. After venting of the reactors, the resulting 
CrO.sub.2 was removed and cooled again immediately after the temperature 
had from the reactor and washed with water until the washwater had a 
conductivity of &lt;100 .mu.S/m. 
The end product had the following properties: 
______________________________________ 
H.sub.c 
Mm/.rho. Mr/.rho. SSA a 
[kA/m] [nTm.sup.3 /g] 
[nTm.sup.3 /g] 
[m.sup.2 /g] 
[.ANG.] 
______________________________________ 
61.4 95.3 45.5 31.4 4.428 .+-. 0.0015 
______________________________________ 
COMATIVE EXPERIMENT 1 
The procedure described in Example 1 was followed, except that, after the 
reaction temperature of 350.degree. C. had been reached, the reaction 
mixture was kept at this temperature for a further 4 hours. 
After the working up procedure as stated in Example 1, a coercive force of 
59.8 kA/m, a specific magnetization of 92 nTm.sup.3 /g and a remanence of 
42.1 nTm.sup.3 /g resulted. The iridium content was less than 2 ppm and 
the cobalt content less than 30 ppm. 
The specific surface area was determined as 19 m.sup.2 /g, the lattice 
constant was 4.428.+-.0.0015 .ANG. and the mean particle size was 300 nm. 
COMATIVE EXPERIMENT 2 
The procedure described in Example 2 was followed, except that dopants were 
not added. The products obtained after preparation of the suspension was 
subjected to a high pressure treatment and subjected to the conditions 
below. 
COMATIVE EXPERIMENT 2a) 
Heating to 350.degree. C. (maximum pressure 320 bar) followed by immediate 
cooling to room temperature and subsequent venting of the reactor. 
COMATIVE EXPERIMENT 2b) 
Heating to 330.degree. C. (maximum pressure 232 bar) followed by immediate 
cooling to room temperature and subsequent letting down of the reactor. 
The resulting powder was removed mechanically from the reaction vessels. 
The following powder properties were determined: 
______________________________________ 
H.sub.c Mm/.rho. Mr/.rho. 
SSA 
.rho. [kA/m] [nTm.sup.3 /g] 
[nTm.sup.3 /g] 
[m.sup.2 /g] 
______________________________________ 
2a 0 2.9 0.1 2 
2b 0 2.1 0.1 1 
______________________________________ 
The resulting powder was identified as .alpha.-CrOOH by X-ray diffraction. 
COMATIVE EXPERIMENT 3 
380 g of H.sub.2 O were initially taken in a reaction vessel having a 
volume of 2 1. First 973 g of CrO.sub.3 were added while stirring, 
followed by 48.5 g of .gamma.-Fe.sub.2 O.sub.3 (3.6% by weight, based on 
chromium dioxide) and 5.56 g of potassium antimonyl tartrate (0.18% by 
weight) based on chromium dioxide. 480 g of chromium(III) oxide were now 
introduced with constant stirring, and stirring was continued for a 
further 30 minutes. The reaction suspension was then treated in an 
autoclave under the conditions below. 
COMATIVE EXPERIMENT 3a) 
Heating to 330.degree. C., keeping at this temperature for 4 hours (maximum 
pressure 380 bar) followed by immediate cooling to room temperature, and 
subsequent venting of the reactor. 
COMATIVE EXPERIMENT 3b) 
Heating to 350.degree. C., keeping at this temperature for 4 hours (maximum 
pressure 310 bar), followed by immediate cooling and subsequent venting of 
the reactor. 
COMATIVE EXPERIMENT 3c) 
Heating to 320.degree. C., keeping at this temperature for 4 hours (maximum 
pressure 288 bar), followed by immediate cooling and subsequent venting of 
the reactor. 
The products were heated for 80 minutes at 180.degree. C. in the air. The 
following results were obtained: 
______________________________________ 
H.sub.c M.sub.s /.rho. 
M.sub.r /.rho. 
SSA 
.rho. [kA/m] [nTm.sup.3 /g] 
[nTm.sup.3 /g] 
[m.sup.2 /g] 
______________________________________ 
3 a) 58.4 98.2 47.3 32 
3 b) 58.5 98.7 48.2 33 
3 c) 58.3 99.2 49.4 31 
______________________________________ 
The lattice parameters a of the products were 4.427.+-.0.0015 .ANG. in all 
three experiments and the mean needle lengths were 280-300 nm. 
COMATIVE EXPERIMENT 4 
121.7 g of CrO.sub.3, 47.5 g of H.sub.2 O and 60 g of Cr.sub.2 O.sub.3 were 
dispersed with a dissolver stirrer, with formation of a viscous dark 
paste. The resulting mass was placed in a cylindrical stainless steel 
vessel and heated to 350.degree. C. in a high pressure reactor (maximum 
pressure 304 bar) and then kept at this temperature for 4 hours. The 
mixture was then cooled and the reactor vented at room temperature. 
The resulting CrO.sub.2 was removed mechanically from the reaction vessel. 
The following powder properties were determined: 
______________________________________ 
H.sub.c M.sub.s /.rho. 
M.sub.r /.rho. 
SSA 
[kA/m] [nTm.sup.3 /g] 
[nTm.sup.3 /g] 
[m.sup.2 /g] 
______________________________________ 
31.2 101.9 43.9 13.0 
______________________________________ 
The mean needle length is 250 nm and the lattice constant a is measured as 
4.424.+-.0.0015 .ANG.. 
EXAMPLE B1 
In a mill having a capacity of 500 parts by volume and containing 100 parts 
by volume of steel balls of 1.5 mm diameter, 40 parts of the chromium 
dioxide obtained in Example 1 and aftertreated with a sodium sulfite 
solution were mixed with 175 parts of a 13% strength solution of a 
thermoplastic polyesterurethane of adipic acid, 1,4-butanediol, 
4,4'-diisocyanatodiphenylmethane in a mixture of equal amounts of 
tetrahydrofuran and dioxane, 150 parts of a 13% strength solution of a 
commercial polyvinylformal in a mixture of equal amounts of 
tetrahydrofuran and dioxane, 24 parts of a solvent mixture of equal 
amounts of tetrahydrofuran and dioxane and 1 part of zinc stearate, and 
the mixture was dispersed for 4 hours. Thereafter, the same amounts of the 
two binder solutions, 13.5 parts of the stated solvent mixture and 0.1 
part of a commercial silicone oil were added and dispersing was continued 
for a further 30 minutes. The dispersion was then filtered and was applied 
in a conventional coating apparatus, by means of a knife coater, to a 
polyethylene terephthalate film in a thickness such that, after drying and 
calendering, the thickness of the resulting dry layer was 5.5 .mu.m. 
Directly after the liquid dispersion has been poured on, the acicular 
chromium dioxide particles were oriented by means of a magnetic field 
along the recording direction. The magnetic properties measured on the 
tape samples, ie. the coercive force H.sub.c in [kA/m], the remanent 
magnetization M.sub.r in [mT] and the relative remanence Mr/Ms, the 
orientation ratio Rf, the ratio of the residual induction in the playing 
direction to that in the crosswise direction and the switching field 
distribution SFD according to Williams and Comstock (AIP Conf. Proc. 5 
(1971), 738) are shown in the Table. 
EXAMPLES B 2-B5 
The procedure described in Example B 1 was followed, except that the 
chromium dioxide materials shown in the Table were used. 
TABLE 
______________________________________ 
CrO.sub.2 H.sub.c Mr 
from [kA/m] [mT] Mr/Ms RF SFD 
______________________________________ 
B 1 Example 1 69.3 146 0.89 3.4 0.22 
B 2 Example 2a 77.5 147 0.85 2.5 0.21 
B 3 Example 4 63.9 146 0.89 4.0 0.25 
B 4 Comp. 58.2 150 0.88 3.1 0.23 
Exp. 1 
B 5 Comp. 58.8 147 0.87 3.3 0.20 
Exp. 3a) 
______________________________________