Preparation of acicular, ferrimagnetic iron oxides

A process for the preparation of acicular ferrimagnetic iron oxides by reducing gamma-iron(III) oxide hydroxide to magnetite, at from 280.degree. to 620.degree. C., by means of an organic compound which is decomposable at this temperature in the presence of iron oxide, and, optionally, a stream of reducing gas, and, if desired, then oxidizing the magnetite with oxygen-containing gases, at from 150.degree. to 450.degree. C., to acicular ferrimagnetic iron oxide of the formula FeO.sub.x, where x is from above 1.33 to 1.50, wherein the organic compound used for the reduction is a non-polar glycerol ester of a fatty acid which is introduced into the aqueous reaction suspension during the preparation of the gamma-iron(III) oxide hydroxide.

The present invention relates to a process for the preparation of acicular 
ferrimagnetic iron oxides by reducing gamma-iron(III) oxide hydroxide to 
magnetite, at from 280.degree. to 620.degree. C., by means of an organic 
compound which is decomposable at this temperature in the presence of iron 
oxide, and, if desired, then oxidizing the magnetite with 
oxygen-containing gases, at from 150.degree. to 450.degree. C., to 
acicular ferrimagnetic iron oxide of the formula FeO.sub.x, where x is 
from above 1.33 to 1.50. 
Acicular ferrimagnetic iron oxides have long been employed extensively as 
magnetizable materials in the manufacture of magnetic recording media. 
Many processes for the preparation of the principally used compound 
gamma-iron(III) oxide have been disclosed. For example, as early a 
publication as British Pat. No. 675,260 describes a process for obtaining 
gamma-iron(III) oxide, wherein acicular .alpha.-iron oxide hydroxide 
(goethite) is dehydrated to .alpha.-iron(III) oxide, the latter is 
converted to magnetite in a reducing atmosphere at above 300.degree. C., 
and the magnetite is oxidized to acicular gamma-iron(III) oxide in air at 
below 450.degree. C. In the course of efforts to improve the crystalline, 
mechanical and magnetic properties of such materials, this process has 
undergone several modifications with respect to its individual stages and 
starting materials used. 
It has also been disclosed that ferrimagnetic iron oxides can be prepared 
by heating an iron oxide hydroxide with an organic substance. In this 
process, the organic substance is decomposed and the iron oxide hydroxide 
is reduced to magnetite, which can then either itself be used as the 
magnetic pigment or be converted into .alpha.-iron(III) oxide in the 
manner described. In as early a process as that disclosed in German Pat. 
No. 801,352, non-magnetic iron oxides are converted into magnetite by 
treatment with salts of short-chain carboxylic acids, followed by heating. 
According to U.S. Pat. No. 2,900,236, .alpha.-Fe.sub.2 O.sub.3 or 
.alpha.-FeOOH can be reduced using any organic substance which decomposes 
at below 540.degree. C. and produces little ash and tar. A number of 
subsequent publications described the reduction of .alpha.-iron oxides 
with organic substances, for example with higher hydrocarbons, alcohols, 
amines, fatty acids or their salts, oils, fats or waxes (German Laid-Open 
Application DOS No. 2,064,804, East German Pat. No. 91,017 and German 
Published Applications DAS No. 1,203,656 and DAS 1,771,327). The reduction 
of .gamma.-FeOOH with coconut oil fatty acid has also been described 
(German Published Application DAS No. 2,212,435). The essential feature of 
these processes is thorough mixing of the organic compound with the iron 
oxide hydroxide, or application of a very thin layer of the organic 
compound to the oxide. For this purpose, for example according to German 
Laid-Open Application DOS No. 2,064,804, the iron oxide used is 
impregnated with a liquid organic compound, and excess organic compound is 
removed. Solid organic substances are dissolved in a solvent, and the 
solution is brought into contact with the iron oxide. Coating methods have 
also been described in which an organic compound which is water-soluble or 
has been rendered water-soluble is added to an aqueous dispersion of the 
iron oxide hydroxide. Thus, according to German Published Application DAS 
No. 1,203,656, a water-insoluble soap is precipitated onto the iron oxide 
from a suspension of the latter in a solution of a water-soluble soap by 
the addition of an appropriate cation. As described in German Published 
Applications DAS No. 1,771,327 and DAS No. 2,212,435, an aqueous 
suspension of the iron oxide can be treated with a water-soluble soap 
based on a monocarboxylic acid, and the fatty acid can then be 
precipitated by acidification. In another possible method, a suitable acid 
is employed which is rendered water-soluble by the addition of morpholine. 
It is also necessary to ensure intimate mixing by using particularly 
effective mixers or by increasing the temperature. An .alpha.-iron oxide 
or .alpha.-iron oxide hydroxide in aqueous dispersion can also be coated 
with an organic compound having a betaine structure (German Published 
Application DAS No. 2,428,875), with an alkylphenol (German Published 
Application DAS No. 2,447,386) or with a water-soluble compound of the 
formula R.sup.1 R.sup.2 R.sup.3)P=0 (German Laid-Open Application DOS No. 
2,520,643). The addition of a water-soluble soap at as early a stage as 
the preparation of the iron oxide hydroxide has also been disclosed (East 
German Pat. No. 91,017). In an alkaline medium, it is also possible to use 
the long-chain fatty acid itself, this being converted to the soap by the 
alkali. However, as described in East German Pat. No. 74,306, these soaps 
also affect the geometry of the resulting iron oxide hydroxide, and are 
therefore employed, in addition to other surfactants, for example for 
controlling the length/width ratio. For this purpose, it is also possible 
to employ water-soluble macromolecular substances which act as protective 
colloids, eg. dextran, polyvinyl alcohol, etc. The length/width ratio of 
the resulting iron oxide hydroxide crystals decreases sharply with 
increasing concentration of the surfactants or of the protective colloids 
in the reaction solution. In accordance with German Laid-Open Application 
DOS. No. 2,461,937, carboxyl-containing polymers are added during the 
synthesis of .alpha.-FeOOH in order to produce magnetic pigments which 
give magnetic tapes having a particularly high signal-to-noise ratio. In 
order to be suitable for use in the process, these polymers must possess 
free carboxyl groups, and hence have an acid number of not less than 50. 
All of the compounds mentioned therefore possess both a hydrophilic and a 
hydrophobic component, and hence are at least somewhat water-soluble; they 
are polar and are surfactants or protective colloids. A disadvantage of 
such compounds is that they are often capable of existing only in certain 
pH ranges; for example, the soaps can exist only in the alkaline range. 
Furthermore, to completely dissolve these compounds in water, relatively 
high temperatures or assistants are often required. Many of these 
compounds contain heteroatoms, eg. S, P or N, which then remain, as 
inorganic radicals, on the pigment particles during further conversion of 
the iron oxide hydroxide and even after conversion to .gamma.-Fe.sub.2 
O.sub.3. In some cases, this may be desirable for shape retention, but in 
general these non-magnetic constituents have an adverse effect on the 
magnetic properties and make it more difficult to incorporate the magnetic 
material into the film-forming organic polymers. Another disadvantage of 
the processes described is that the said substances are applied to 
ready-prepared, or isolated and washed, iron oxide hydroxides. In spite of 
good dispersing, it is not possible to avoid a situation where 
agglomerates which are more or less randomly formed and hence are of 
various sizes are coated, ie. the coating on the individual particles 
forming the agglomerates is not coherent and results in magnetically 
inhomogeneous iron oxides after further treatment is carried out. As a 
result, additional process steps, such a resuspension, refiltration and 
rewashing, are required. 
It would therefore be desirable to add the coating substances to the 
reaction mixture before, during or after the preparation of the iron oxide 
hydroxide. However, the above disadvantages are particularly troublesome 
here, since the composition and the particle size and shape of the iron 
oxide hydroxide show a pronounced dependence on the process parameters, 
eg. temperature and pH, so that these cannot be varied at will. 
Frequently, organic substances are added during the reaction in order to 
influence the shape and size of the pigment particles. In these cases, the 
amounts employed are determined on the basis of the desired particle 
geometry, and cannot be fixed, for example, with a view to achieving 
optimum subsequent reduction. However, such a pronounced effect on the 
crystallization process itself may also be highly undesirable. In any case 
the tendency of the above compounds to foam is troublesome. Iron oxide 
hydroxides are prepared in a three-phase reaction (solid/liquid/gas), for 
which thorough mixing of the components, for example by vigorous stirring 
and by passing in a gas stream at high velocity, is absolutely necessary. 
Where the reaction mixture foams, such thorough mixing is not possible, 
and isolation of the precipitate, eg. by decanting, and washing are made 
more difficult. Moreover, undesirable flotation phenomena frequently occur 
during the reaction. Although the hydrophilic component of the substances 
used hitherto facilitates adduct formation with the iron oxide hydroxide, 
it also results in salts or ions from the reaction solution being readily 
included in the coating, with the result that they are then difficult to 
wash out again. 
It is an object of the present invention to provide a process which is free 
from the above disadvantages and gives acicular ferrimagnetic iron oxides 
which possess excellent magnetic properties, in particular a high coercive 
force coupled with a very narrow switching field distribution, and are 
hence useful for the production of magnetic recording media which have a 
low noise level. 
We have found that this object is achieved, and that, surprisingly, 
acicular ferrimagnetic iron oxides are obtained, in accordance with the 
invention, by reducing gamma-iron(III) oxide hydroxide to magnetite, at 
from 280.degree. to 620.degree. C., by means of an organic compound which 
is decomposable at this temperature in the presence of iron oxide, and, if 
desired, then oxidizing the magnetite with oxygen-containing gases, at 
from 150.degree. to 450.degree. C., to acicular ferrimagnetic iron oxide 
of the formula FeO.sub.x, where x is from above 1.33 to 1.50, if the 
organic compound used for the reduction is a non-polar glycerol ester of a 
fatty acid which is introduced into the aqueous reaction suspension during 
the preparation of the gamma-iron(III) oxide hydroxide. 
The acicular gamma-iron(III) oxide hydroxide which is suitable for the 
novel process, and is referred to as, inter alia, lepidocrocite, is known, 
as is its preparation. It has been disclosed that it can be prepared, for 
example, by precipitating iron(II) hydroxide from an iron(II) chloride 
solution with ammonia at pH 7 and at from 20.degree. to 50.degree. C., and 
then oxidizing the precipitated hydroxide with air while maintaining the 
pH (Schwertmann, Zeitschrift fur Anorg. Chemie 298 (1959), 337-348). 
German Pat. No. 1,223,352 discloses another process for the preparation of 
lepidocrocite, in which nucleation takes place as a result of 
precipitation from an iron(II) salt solution with an alkali metal base or 
an alkaline earth metal base and oxidation of the iron(II) hydroxide or 
carbonate with oxygen, air, an organic nitro compound or another oxidizing 
agent, and growth of the lepidocrocite seeds in the iron(II) salt solution 
is effected in the presence of metallic iron or with the simultaneous 
addition of equivalent amounts of iron(II) ions and a solution or 
suspension of an alkali or an alkaline earth, or with the simultaneous 
addition of equivalent amounts of iron(III) ions and a solution or 
suspension of an alkali metal base or an alkaline earth metal base, the 
oxidizing agent being used. The lepidocrocite can also be obtained in a 
similar manner if, in a first stage, a suspension of colloidal 
lepidocrocite seeds is produced by combining iron(II) chloride with an 
aqueous alkali, the concentration of the iron(II) chloride being about 
29.95-59.9 g per liter, and the resulting mixture is stirred vigorously 
while an oxygen-containing gas is fed in until the pH of the mixture is 
from 2.9 to 4.1, after which, in a second stage, the suspension is 
maintained while stirring vigorously at from 26.7.degree. to 60.degree. C. 
and at a pH of from 2.9 to 4.1, in the presence of an excess of iron(II) 
chloride, while an alkali and an oxygen-containing gas are fed in 
simultaneously and continuously until from 1.2 to 5 parts by weight, per 
part by weight of the seed, of the total product have been formed. 
In accordance with the invention, a non-polar glycerol ester of a fatty 
acid is then introduced into the aqueous reaction suspension during the 
preparation of the gamma-iron(III) oxide hydroxide. 
Appropriate glycerol esters of fatty acids of 8 to 23 carbon atoms have a 
solidification point below 20.degree. C. They occur, generally in the form 
of a mixture, in a number of natural oils, eg. peanut oil, soybean oil, 
castor oil and olive oil, and are neither water-soluble nor surface-active 
non polar. They are employed in an amount of from 0.5 to 10, preferably 
from 0.8 to 5.0, %, based on the iron oxide hydroxide. 
The said ester may be added to the reaction suspension at any desired point 
in time. However, it has proved particularly advantageous to introduce the 
glycerol ester of the fatty acid into the reaction suspension, at a pH of 
from 2 to 5, during the growth phase of the gamma-iron(III) oxide 
hydroxide. After the growth phase is complete, the resulting 
gamma-iron(III) oxide hydroxide is filtered off, washed with water to 
remove inorganic salts, and dried. The amount of organic substance which 
has been absorbed is then from 90 to 95% of the amount introduced. 
The gamma-iron(III) oxide hydroxide coated in this manner is then reduced 
to magnetite by heating at from 280.degree. to 620.degree. C., 
advantageously in continuous reduction stages with a mean residence time 
of from 30 to 150 minutes, under a stream of an inert gas, usually 
nitrogen. 
The magnetite obtained by the reduction is, if desired, then oxidized with 
oxygen-containing gases, advantageously under a stream of air, at from 
150.degree. to 450.degree. C., to acicular ferrimagnetic iron oxide of the 
formula FeO.sub.x, where x is from above 1.33 to 1.50. The oxidation is 
usually allowed to proceed until gamma-iron(III) oxide is obtained 
(x=1.5). 
In developing the novel process, it has been found that the coercive force 
and the specific surface area of the magnetic end product can be 
influenced by simultaneously metering in a reducing gas, eg. hydrogen, 
during the above reduction by means of a decomposable organic substances. 
By using appropriate mixtures of hydrogen and nitrogen during the 
reduction step, it is possible to vary the coercive force and the specific 
surface area within wide limits, as is shown by way of example in the 
Figure. 
When, as the decomposable organic compound which effects reduction to 
magnetite, a non-polar glycerol ester of a fatty acid is added directly to 
the reaction suspension in the preparation of the gamma-iron(III) oxide 
hydroxide in accordance with the novel process, coating of these particles 
takes place without the process conditions being restricted. The metering 
is simple and does not require any of the conventional process steps, eg. 
mixing, absorption from solution, and coating of the particles with an 
organic compound in the form of a vapor. Furthermore, there is no foaming, 
as is the case with the conventional additives, eg. sodium stearate, when 
air is passed through the reaction mixture; foaming gives rise to 
difficulties when the process is carried out. Although the glycerol esters 
of the fatty acids are non-polar, they are absorbed onto the particles. 
Since this takes place during formation of the particles in the 
suspension, the individual particles are coated. Agglomerates do not have 
first to be separated mechanically, resulting in possible damage to the 
acicular shape of the gamma-iron(III) oxide hydroxide particles. A 
particular advantage of a glycerol ester of a fatty acid is that it is 
non-polar and hence has no effect on the geometry of the particles during 
their growth. The uniform coating on all the particles results in the 
reduction proceeding uniformly. 
Compared with the gamma-iron(III) oxides obtainable by conventional 
conversion processes, the acicular, ferrimagnetic iron oxides prepared 
according to the invention, in particular the gamma-iron(III) oxide 
obtainable in this manner, are particularly homogeneous, exhibit good 
retention of the acicular shape and possess a particularly narrow 
switching field distribution. They are therefore outstandingly suitable 
for the production of magnetic recording media which possess a very good 
maximum output level at long wavelengths and an above-average maximum 
output level at short wavelengths coupled with a high signal-to-noise 
ratio and a good signal-to-print-through ratio. 
To produce a magnetic layer, the gamma-iron(III) oxide is dispersed in a 
polymeric binder. Suitable binders for this purpose are known compounds, 
such as homopolymers and copolymers of vinyl compounds, polyurethanes, 
polyesters and the like. The binder is used as a solution in a suitable 
organic solvent which may or may not contain further additives. The 
magnetic layer is applied to a rigid or flexible base, eg. a disk, a 
plastics film, or a card.

The Examples which follow illustrate the invention in comparison with 
comparative experiments based on the prior art. 
The specific surface area of the pigment was determined by the BET method, 
ie. nitrogen was adsorbed onto weighed evacuated gas-free pigment samples. 
The amount of nitrogen adsorbed is divided by the weight of the sample. 
The mean crystallite size in the individual particles was determined by 
X-ray diffraction. Using a Siemens X-ray goniometer with a counter, the 
profile of a reflection, eg. the (911) reflection, of a powder sample was 
measured. From this measurement and a knowledge of the apparatus 
constants, the mean crystallite size was deduced. For example, 
iron-filtered cobalt K.alpha. radiation is used as the radiation source. 
The properties of the magnetic powder were determined on an oxide sample 
having a tamped density d of 1.2 g/cm.sup.3, by means of a conventional 
vibrating sample magnetometer at 100 kA/cm field strength. The coercive 
force (Hc) is expressed in [kA/m], and the specific remanence (Mr/.rho.) 
and the specific saturation magnetization (Mm/.rho.) in [nTm.sup.3 /g]. 
The magnetic tape samples were also examined at a field strength of 100 
kA/m by means of a vibrating sample magnetometer. The coercive force Hc, 
the residual induction Mr in [mT] and the orientation ratio, ie. the ratio 
of the residual induction in the direction of particle orientation to that 
in the crosswise direction, are specified. The maximum output levels at 
long wavelengths and short wavelengths were measured according to DIN 
45,512, Part II, against IEC reference tape 1. The reference 
level-to-weighted noise ratio RGA is expressed relative to IEC reference 
tape 1, the RGA ratio of which is taken to be 0 dB. 
The switching field distribution becomes narrower with increasing 
orientation of the particles in the magnetic field, ie. with increasing 
squareness S=Mr/Mm, and the field required to switch 50% of the particles, 
ie. the remanence coercivity Hr, becomes smaller. 
S has to be measured using a saturation field which is not less than 10 
times the coercive force. In the Examples, the measurement was carried out 
at 8.times.10.sup.5 A/m. The lower the measured h.sub.25 value, the 
narrower the switching field distribution of the sample measured. 
EXAMPLE 1 
17.2 m.sup.3 of 8.5% strength iron(II) chloride solution were initially 
introduced into a stirred vessel having a capacity of 30 m.sup.3, and 3.20 
m.sup.3 of 15% strength sodium hydroxide solution were added, while 
stirring at 24.degree. C., to produce a precipitate from 55% of this 
solution. When the pH was monitored after precipitation, it was found to 
be 6.4-7.2. 1,650 m.sup.3 /hour of air were then injected into the stirred 
mixture at this temperature, and, after the passage of air had been 
continued for 3.5 hours, the pH of the suspension had decreased. At pH 3, 
the suspension was heated to 33.degree. C., and a further amount of 15% 
strength sodium hydroxide solution was then fed in. The latter operation 
was coupled with the pH measurement, the pH being set at 4.6.+-.0.2. After 
the temperature had been increased to 33.degree. C., 2,200 m.sup.3 /hour 
of air were passed in at pH 4.6.+-.0.2 for one hour, after which 25 kg of 
a glycerol ester mixture having an iodine number of 90, a solidification 
point of +6.degree. C. and a chain length of 14 to 18 carbon atoms were 
added to the suspension. After the passage of air had been continued for a 
further 3 hours, the oxidation rate was only 2% of that at the beginning 
of the growth phase. The reaction was then discontinued, and the 
suspension was washed chloride-free in a conventional filtration 
apparatus, using fully demineralized water, until the product contained 
0.1% of Cl, based on dry .gamma.-iron oxide hydroxide. 
The granulated filter residue was dried at 150.degree. C. in a conventional 
cabinet dryer. The specific surface area determined by the BET method was 
28.5 m.sup.2 /g; the pigment contained 1.9% of carbon, and the average 
length of the acicular particles was 0.5 .mu.m, the length/width ratio 
being 14:1. 
The product was then conveyed continuously, at a rate of 78 kg per hour, 
through a rotary tubular furnace subdivided into 4 heating zones. At the 
same time, a gas mixture, heated to 500.degree. C. and comprising 11 
m.sup.3 of hydrogen and 9 m.sup.3 of nitrogen, was passed in together with 
the solid. The internal temperatures of the 4 successive heating zones 
were 350.degree., 440.degree., 540.degree. and 550.degree. C. 
respectively. After a residence time of 40 minutes, the magnetite dropped, 
via a discharge gate flushed with nitrogen, into a second rotary tubular 
furnace, in which it was oxidized by means of a stream of air to 
.gamma.-iron(III) oxide, the internal temperature being 260.degree. C. and 
the mean residence time being 30 minutes. The results of the measurements 
on the product are shown in Table 1. 
EXAMPLE 2 
The procedure described in Example 1 was followed, except that, instead of 
11 m.sup.3 /hour of hydrogen and 9 m.sup.3 /hour of nitrogen, 20 m.sup.3 
/hour of hydrogen were fed into the reduction furnace. The results of the 
measurements are shown in Table 1. 
EXAMPLE 3 
The procedure described in Example 1 was followed, except that, instead of 
11 m.sup.3 /hour of hydrogen and 9 m.sup.3 /hour of nitrogen, 20 m.sup.3 
/nitrogen were fed into the reduction furnace. The results of the 
measurements are shown in Table 1. 
EXAMPLE 4 
The procedure described in Example 1 was followed, except that the ester 
added during the preparation of .gamma.-FeOOH was glycerol palmitate 
having an iodine number of 50, this ester being employed in the same 
amount. The results of the measurements are shown in Table 1. 
EXAMPLE 5 
The procedure described in Example 1 was followed, except that the amount 
of ester mixture added during the preparation of .gamma.-FeOOH was 32 kg. 
The results of the measurements are shown in Table 1. 
EXAMPLE 6 
The procedure described in Example 1 was followed, except that the internal 
temperatures of the successive zones of the reduction furnace were 
320.degree., 420.degree., 490.degree. and 530.degree. C. respectively. The 
results of the measurements are shown in Table 1. 
COMATIVE EXPERIMENT 1 
The procedure described in Example 1 was followed, except that the glycerol 
ester mixture was not added until the end of the growth phase. The mixture 
was stirred for one hour in the absence of air and was then filtered, the 
further procedure being as described in Example 1. The results of the 
measurements are shown in Table 1. 
COMATIVE EXPERIMENT 2 
The procedure described in Example 1 was followed, except that the organic 
compound added to the .gamma.-FeOOH suspension was tricresyl phosphate. 
The results of the measurements are shown in Table 1. 
COMATIVE EXPERIMENT 3 
The procedure described in Example 1 was followed, except that the organic 
compound added to the .gamma.-FeOOH suspension was soybean lecithin. The 
results of the measurements are shown in Table 1. 
COMATIVE EXPERIMENT 4 
The procedure described in Example 1 was followed, except that the organic 
compound added to the .gamma.-FeOOH suspension was stearic acid having a 
yield point of 55.degree. C. After filtration and drying, analysis showed 
that the residual amount of chloride ions was 0.6%, although the wash 
water was chloride-free before the end of the filtration. The results of 
the measurements are shown in Table 1. 
COMATIVE EXPERIMENT 5 
.gamma.-FeOOH was prepared as described in Example 1, except that an 
organic substance was not added. The lepidocrocite was filtered off, 
washed and dried, and the granulated pigment was converted into a 
suspension in a stirred kettle of 30 m.sup.3 capacity. Thereafter, 25 kg 
of the glycerol ester mixture described in Example 1 were added, and the 
suspension was heated at 35.degree. C. and stirred for a further 5 hours. 
The resulting suspension was then filtered, and the filter residue was 
dried at 150.degree. C. The other conditions were as described in Example 
1. The results of the measurements are shown in Table 1. 
COMATIVE EXPERIMENT 6 
.gamma.-FeOOH was prepared as described in Example 1, except that the ester 
mixture was not added. The product was fed into the reduction furnace at 
the same rate as in Example 1, the average residence time and the gas 
mixture employed likewise being as described in Example 1, and the ester 
mixture was injected continuously in an amount of 2.5%, based on the 
product fed in. The other conditions were as described in Example 1. The 
results of the measurements are shown in Table 1. 
COMATIVE EXPERIMENT 7 
.gamma.-FeOOH was prepared as described in Example 1, except that the ester 
mixture was not added. As the product was fed into the reduction furnace, 
stearic acid was metered continuously into the product feed tube upstream 
of the conveying screw, in an amount of 2.5%, based on the product fed in. 
The other conditions were as described in Example 1. The results of the 
measurements are shown in Table 1. 
COMATIVE EXPERIMENT 8 
.gamma.-FeOOH was prepared as described in Example 1, except that the ester 
mixture was not added. 78 kg/hour of the product was fed into the 
reduction furnace, together with a stream of 30 m.sup.3 /hour of nitrogen 
which had previously been passed through a vessel filled with stearic acid 
heated to 270.degree. C. The other conditions were as described in Example 
1. The results of the measurements are shown in Table 1. 
EXAMPLE 7 
A solution of 358 g of FeCl.sub.2.4H.sub.2 O in 2 liters of water was 
introduced into a 5 liter glass apparatus, and 1 liter of 1.8N NaOH was 
added dropwise to the vigorously stirred solution in the course of 10 
minutes in a stream of nitrogen. After a further 10 minutes, the gas 
stream was replaced by 200 liters/hour of air. As soon as the pH had 
dropped below 4, it was brought to 5-6 by dropwise addition of 1.8N NaOH 
and kept at this value, the passage of air being continued. The reaction 
was complete as soon as the pH remained constant without it being 
necessary to add sodium hydroxide solution. This was the case after 1,050 
ml of the latter had been added. 
TABLE 1 
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BET 
specific 
Crystallite Mr/ Mm/ % C in 
surface area 
size Hc [kA/m] 
[nTm.sup.3 /g] 
[nTm.sup.3 /g] 
FeOOH 
.gamma.-Fe.sub.2 O.sub.3 
__________________________________________________________________________ 
Example 
1 22.5 220 26.7 38 81 1.9 0.55 
2 20.1 260 28.2 39 82 1.9 0.50 
3 28.0 235 22.5 38 81 1.9 0.61 
4 21.9 240 27.0 38.5 82 2.4 0.68 
5 23.1 222 25.9 38 82 1.9 0.58 
6 23.8 210 24.8 39 83 1.9 0.60 
Comparative 
Experiment 
1 21.3 250 25.4 38 81 1.8 0.55 
2 18.9 290 27.2 38 80 1.5 0.35 
3 19.5 285 26.8 37.5 79 1.7 0.42 
4 17.8 270 27.8 38.5 81 1.9 0.51 
5 19.8 295 27.3 39 82.5 1.4 0.30 
6 20.3 300 26.9 38 80 -- 0.38 
7 20.5 280 26.8 38.5 83 -- 0.40 
8 18.2 340 29.2 40.5 84 -- 0.27 
__________________________________________________________________________ 
At this stage, 6.4 ml of an oil (cf. Table 2) were added, and the mixture 
was stirred for a further half an hour. The precipitate was then filtered 
off under suction, washed twice, and dried at 80.degree. C. in a vacuum 
shelf dryer. 
For conversion to .gamma.-Fe.sub.2 O.sub.3, the product was heated for 30 
minutes in a 500 ml spherical rotary quartz kiln in a stream of 5 
liters/hour of N.sub.2 at 520.degree. C., and was then oxidized for 30 
minutes at 350.degree. C. in a stream of 100 liters/hour of air. The 
results of the measurements are shown in Table 2. 
EXAMPLES A-O 
The magnetic materials of Examples 1 to 6 and of Comparative Experiments 1 
to 8 were processed in the following manner into magnetic dispersions 
which were used to manufacture magnetic tapes: 
9,000 parts (by weight here and hereinafter) of steel balls, 900 parts of 
the magnetic material, 22.5 parts of a long-chain amphoteric organophilic 
dispersant, 4.5 parts of a silicone oil, 4.5 parts of a mixture of 
isomeric carboxylic acids with a yield point of &lt;5.degree. C., 126 parts 
of a commercial isocyanate-free polyester-urethane obtained from adipic 
acid, butane-1,4-diol and 4,4'-dicyanatodiphenylmethane and having a K 
value of 61 (measured as a 1% strength solution in tetrahydrofuran) and 54 
parts of a PVC/ethyl maleate copolymer having a K value of 59 (likewise 
measured as 1% strength solution in tetrahydrofuran), and 2,200 parts of a 
mixture of equal parts of tetrahydrofuran and 1,4-dioxane were introduced 
into a steel ball mill which had a capacity of 6,000 parts by volume and 
was run at 72 r.p.m., the binders being introduced in the form of a 
solution in the solvent mixture mentioned. 
TABLE 2 
__________________________________________________________________________ 
Experiment 
Oil used 
S.sub.N.sbsb.2 [m.sup.2 /g] 
H.sub.c [kA/m] 
M.sub.m /.rho. [nTm.sup.3 /g] 
M.sub.r /.rho. [nTm.sup.3 /g] 
__________________________________________________________________________ 
a olive oil 
27.6 23.6 74 38 
b peanut oil 
26.7 23.5 78 40 
c soybean oil 
21.4 23.9 77 40 
__________________________________________________________________________ 
The entire mixture was milled for 80 hours at 35.degree. C. The magnetic 
dispersion obtained was filtered, after which it was cast, by means of a 
conventional coater, onto a 12 .mu.m thick polyethylene terephthalate 
film, the amount applied being such that, after drying in a tunnel dryer 
and then calendering on a multi-roll calender (to give an average 
peak-to-valley height of 0.10 .mu.m, measured using an apparatus 
manufactured by Perthen, Hanover, Germany), a 4.1 .mu.m thick magnetic 
coating was obtained. In the zone of the tunnel dryer where the coating 
was still wet, a magnet extending over the entire width of the coating 
oriented the magnetic particles in the desired direction. The coated webs 
thus obtained were slit into 3.81 mm wide magnetic tapes. 
The results of the measurements on the individual magnetic tapes are listed 
in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Maximum 
output Switching field 
Orienta- 
level at 
RGA distribution h.sub.25 
Hc Mr tion 1 kHz 
10 kHz 
ratio 
(field strength 
Example 
Material [kA/m] 
[mT] 
ratio 
[dB] 
[dB] 
[dB] 
= 800 kA/m) 
__________________________________________________________________________ 
A Example 1 26.3 170 
3.3 +1.0 
+1.5 
-0.5 
0.26 
B Example 2 27.2 168 
3.2 +0.8 
+1.9 
-0.8 
0.26 
C Example 3 22.5 162 
3.1 +1.5 
+0.2 
0 0.26 
D Example 4 26.2 172 
3.4 +1.1 
+1.6 
-0.4 
0.27 
E Example 5 25.2 175 
3.3 +1.0 
+1.2 
-0.2 
0.26 
F Example 6 24.1 177 
3.2 +1.4 
+0.6 
0 0.27 
G Comparative Experiment 1 
25.9 165 
2.9 +0.7 
+0.3 
-0.9 
0.29 
H Comparative Experiment 2 
26.0 155 
2.8 +0.8 
0 -1.0 
0.30 
J Comparative Experiment 3 
25.9 150 
2.7 +0.7 
+0.3 
-0.5 
0.31 
K Comparative Experiment 4 
26.7 145 
2.4 +0.3 
-0.5 
-1.5 
0.33 
L Comparative Experiment 5 
26.0 152 
2.6 +0.5 
+0.2 
-0.5 
0.31 
M Comparative Experiment 6 
25.8 155 
2.7 +0.5 
+0.5 
-0.8 
0.31 
N Comparative Experiment 7 
25.7 153 
2.7 +0.6 
+0.3 
-0.7 
0.32 
O Comparative Experiment 8 
28.0 148 
2.3 -0.5 
+1.8 
-1.8 
0.34 
__________________________________________________________________________