Magnetic recording media

Magnetic recording media consist of a nonmagnetic substrate and one or more magnetizable layers which are firmly applied thereon and are based on a magnetic material which is finely dispersed in a polymer binder and further conventional additives and essentially consists of ferromagnetic chromium dioxide, and the said recording media are particularly stable to the chemical decomposition due to moisture and oxidizable compounds and hence to deterioration in the magnetic properties, owing to the addition of from 0.1 to 8% by weight of an organic compound from the group consisting of the antioxidants, based on the amount of chromium dioxide, to the magnetic layer.

The present invention relates to magnetic recording media consisting of a 
nonmagnetic substrate and one or more magnetizable layers which are firmly 
applied thereon and are based on a magnetic material which is finely 
dispersed in a polymer binder and further conventional additives and 
essentially consists of ferromagnetic chromium dioxide and is particularly 
stable to the chemical decomposition due to moisture and oxidizable 
compounds and hence to deterioration in the magnetic properties. 
Acicular, ferromagnetic chromium dioxide, its preparation and the use of 
this material for magnetic recording media have often been described. 
Magnetic recording media which contain chromium dioxide generally have 
superior magnetic properties compared with recording media based on other 
magnetic oxides. 
However, it is also known that the magnetic properties of recording media 
containing chromium dioxide deteriorate in the course of time. 
Ferromagnetic chromium dioxide in powder form is substantially stable in 
the absence of moisture and there is furthermore no detectable change in 
the magnetic properties over a long time. However, it has been observed 
that chromium dioxide can be attacked both by water and by other 
materials, for example the organic polymer binders used in the preparation 
of magnetic recording media, with decomposition to give nonmagnetic 
components. In the case of magnetic recording media, this means not only a 
loss of magnetic and hence electroacoustic properties but also an adverse 
effect on the mechanical properties. This deterioration is further 
accelerated at relatively high temperatures. There has therefore been no 
lack of attempts to overcome these disadvantages. For example, U.S. Pat. 
No. 3,512,930 describes the treatment of chromium dioxide powder with a 
reducing agent. In other processes, alumina coatings (U.S. Pat. No. 
3,687,726) or those consisting of sparingly soluble metal phosphates (U.S. 
Pat. No. 3,686,031) are produced. The application of metal compounds whose 
cations are capable of forming sparingly soluble chromates has also been 
disclosed (DE-B 21 52 331). JA-A-21200/76 proposes applying magnetic iron 
oxides to the surface in order to coat the chromium dioxide particles, 
while DE-A-27 49 757 describes the application of iron (III)-containing 
oxidic precipitates to the chromium dioxide. EP-B 0078042 describes a 
stabilization process in which metals, e.g. iron, zinc, cobalt or 
manganese, are incorporated into the surface of the chromium dioxide 
particles. Attempts have also been made to increase the stability merely 
by subjecting the chromium dioxide to a heat treatment in an inert gas 
atmosphere (EP-B 0029687) or in air (EP-B 0248402). 
However, all these processes have the disadvantage that the magnetic 
properties of the treated chromium dioxide materials are greatly reduced 
due to a non-magnetic surface layer which is achieved either by coating 
with foreign compounds or by means of a decomposition layer, and the 
recording media produced using the chromium dioxide materials obtained by 
these processes nevertheless do not have sufficient long-term stability to 
meet the present requirements in particular in the electronic computing 
sector. Furthermore, the attempts to stabilize magnetic recording media 
containing chromium dioxide by the addition of ionic compounds whose 
cation forms a sparingly soluble chromate directly to the dispersion, as 
proposed in, inter alia, DE-A 21 62 332, did not give sufficient 
stability, in particular the required long-term stability. 
It is an object of the present invention to provide magnetic recording 
media containing chromium dioxide which, without significant deterioration 
in the magnetic properties, have improved stability to the chemical 
decomposition due to moisture and oxidizable, generally organic compounds 
and thus ensure the required long-term stability of the magnetic 
recording. 
We have found that this object is achieved by a magnetic recording medium 
consisting of a nonmagnetic substrate and at least one magnetizable layer 
which is firmly applied thereon and is based on a magnetic material which 
is finely dispersed in a polymer binder and further conventional additives 
and essentially consists of ferromagnetic chromium dioxide, if the 
magnetizable layer additionally contains from 0.1 to 8% by weight, based 
on the amount of chromium dioxide, of an organic compound from the group 
consisting of the sterically hindered phenols, cresols, aromatic amines, 
benzotriazoles, triazine derivatives, esters of phosphorus-containing 
acids, phenolic phosphoric esters, their metal salts, benzophenones, 
substituted phenyl benzoates and alkyl esters of thio acids. 
Examples of the compounds added to the novel recording medium are: 
phenyl-.alpha.-naphthylamines, N-isopropyl-N'-phenyl-p-phenylenediamine, 
N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine, 
N,N'-di-(1,4-dimethylphenyl)-p-phenylenediamine, 
2,2,4-trimethyl-1,2-dihydro-6-ethoxyquinoline, 
phenylene-.alpha.-naphthylamine, condensate of an alcohol and 
.alpha.-naphthylamine, di-.beta.-naphthyl-p-phenylenediamine, 
disalicylideme-N-methyldiisopropylenediamine, benzofuran derivatives, 
2,2'-methylenebis-(4-methyl-6-tert-butylphenol), 
N,N'-di-sec-butyl-p-phenylenediamine, 
2,2-methylenebis-(4-methyl-6-cyclohexylphenol), 4,4'-dihydroxybiphenyl, 
2-mercaptobenzimidazole, zinc salt of 2-mercaptobenzimidazole, Al salt of 
N-nitrosocyclohexylhydroxylamine, K salt of 
N-nitrosocyclohexylhydroxylamine, 2,6-di-tert-butyl-p-cresol, 
hexane-1,6-diol bis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate, 
calcium 3,5-di-tert-butyl-4-hydroxybenzylmonoethylphosphonate, 
1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)-benzene, 
1,1,3-tris-(5-tert-butyl-4-hydroxy-2-methylphenyl)-butane, 
1,3,5-tris-(3,5-di-tert-butyl-3-methylphenol), 
4,4'-thiobis-(6-tert-butyl-3-methylphenol), 1,1'-thiobis-(2-naphthol), 
2,2'-methylenebis-(6-tert-butyl-4-methylphenol), tris-(nonylphenyl) 
phosphite, 2,2'-thiodiethyl 
bis-[3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate], octadecyl 
3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, 
1-phospha-2,6,7-trioxabicyclo[2.2.2]-oct-4-ylmethyl 
3,5-di-tert-butyl-4-hydroxyphenylpropionate/zinc stearate, dilauryl 
thiodipropionate, di-(alkoxyanilide) oxalate, N,N'-diacetyladipic acid 
dihydrazide, N,N'-dibenzaloxalic acid dihydrazide, 
2-(2-hydroxy-5-methylphenyl)-2H-benzotriazole, 
2-(2-hydroxy-3,5-di-tert-butylphenyl)-2H-benzotriazole, 
2-(2-hydroxy-3,5-di-tert-amylphenyl)-2H-benzotriazole, 
1,1'-thiobis-(2-naphthol), 
1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)-benzene, 
1,1,5-tris-(5-tert-butyl-4-hydroxy-2-methylphenyl)-butane, mixture of 
tris-(nonylphenyl) phosphite and 
3-methyl-4,6-bis-.alpha.-methylbenzylphenol, 
3-methyl-4,6-bis-.alpha.-methylbenzylphenol, 
2,2'-methylenebis-(6-tert-butyl-4-methylphenol), 
4,4'-butylidenebis-(6-tert-butyl-3 -methylphenol), 
2,6-di-tert-butyl-p-cresol, tris-(nonylphenyl) phosphite, 
4,4'-thiobis-(6-tert-butyl-3-methylphenol), pentaerythrityl 
tetrakis-[3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate], octadecyl 
3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate, 2-ethyl-n-hexyl 
S-(3,5-di-tert-butyl-4-hydroxybenzyl)-thioglycolate, 
bis-(2,2,6,6-tetramethyl-4-piperidyl)sebacate, molecular weight: 481 
N,N'-bis-[3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)-propionyl]-hydrazine, 
2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-tert-butyl-anilino)-1,3,5-triazi 
ne, triethylene glycol 
bis-3-(3-tert-butyl-4-hydroxy-5-methylphenyl)-propionate, 
N,N'-hexamethylenebis-(3,5-di-tert-butyl-4-hydroxyhydrocinnamide), diethyl 
3,5-di-tert-butyl-4-hydroxybenzylphosphonate, lauryl 
.beta.,.beta.'-thiodipropionate, myristyl .beta.,.beta.'-thiodipropionate 
or distearyl .beta.,.beta.'-thiodipropionate. 
These compounds are added in an amount of from 0.1 to 8, in particular from 
0.2 to 4, % by weight, based on the amount of magnetic material, during 
the preparation of the dispersion forming the magnetic layer. However, it 
is also possible to treat the magnetic material with the stated compounds 
before introduction into this dispersing process. Preferably, however, the 
addition is effected before or during dispersing. This allows additional 
use to be made of a dispersing action and a good, uniform distribution can 
be achieved. If the magnetic layer is produced using other conventional 
additives which, in addition to having other effects, such as improving 
the frictional properties and leveling, also promote dispersing, the 
advantageous properties are fully retained when the stabilizer is added. 
A suitable magnetic material essentially consisting of chromium dioxide is 
finely divided, acicular chromium dioxide having a mean particle length of 
from 0.1 to 2, in particular from 0.1 to 0.9 .mu.m, alone or as a mixture 
with not more than 40% by weight of ferrimagnetic iron oxides, especially 
acicular gamma-iron(III) oxide and cobalt-modified gamma-iron(III) oxide. 
When the magnetic iron oxides used were those of the bertholide type, it 
was found that the polyunsaturated compounds present in the magnetic layer 
of the novel recording media also effect stabilization with respect to the 
iron(II) content which can be changed by oxidation. 
Suitable binders for dispersing the finely divided magnetic material are 
the binders known for the preparation of magnetic layers, such as a 
copolyamide which is soluble in conventional solvents, a polyvinylformal, 
a polyurethane elastomer, mixtures of polyisocyanates and fairly high 
molecular weight polyhydroxy compounds or vinyl chloride polymers 
containing more than 60% of vinyl chloride molecular building blocks, for 
example a vinyl chloride copolymer with one or more comonomers, such as a 
vinyl ester of a monocarboxylic acid of 2 to 9 carbon atoms, or an ester 
of an aliphatic alcohol of 1 to 9 carbon atoms and of an ethylenically 
unsaturated carboxylic acid of 3 to 5 carbon atoms, such as the esters of 
acrylic acid, methacrylic acid or maleic acid, or a copolymer of vinyl 
chloride with one or more of these carboxylic acids themselves as 
comonomers or hydroxyl-containing vinyl chloride copolymers which can be 
prepared by partial hydrolysis of vinyl chloride/vinyl ester copolymers or 
direct copolymerization of vinyl chloride with hydroxyl-containing 
monomers, such as allyl alcohol or 4-hydroxybutyl or 2-hydroxyethyl 
(meth)acrylate. 
Preferably used polyurethane elastomer binders are commercial elastomeric 
polyester urethanes obtained from adipic acid, butane-1,4-diol and 
4,4'-diisocyanatodiphenylmethane, as described in, for example, German 
Published Application DAS 1,106,959 or German Published Application DAS 
2,753,694. The polyurethanes can be used as the sole binder but is 
preferably used as a mixture with other polymers, for example 
polyvinylformal, a phenoxy resin or a PVC copolymer. Preferably from 5 to 
40% of the second binder component are added. Any crosslinking of the 
magnetic recording medium which may be required depending on the binder 
system and the property profile of the tape is effected by reacting the 
polyurethanes or polyurethane binder mixtures with polyisocyanates. Many 
organic di-, tri- or polyisocyanates or isocyanates prepolymers having a 
molecular weight of up to 10,000, preferably from 500 to 3,000, can be 
used for the crosslinking. Polyisocyanates which carry more than 2 NCO 
groups per molecule are preferred. Polyisocyanates based on toluylene 
diisocyanate, hexamethylene diisocyanate or isophorone diisocyanate, which 
are formed by polyaddition with di- or triols by biuret and isocyanurate 
formation, have proven particularly suitable. An adduct of toluylene 
diisocyanate with trimethylolpropane and diethylene glycol is particularly 
advantageous. The amount of polyisocyanate may vary very considerably 
depending on the binder system. OH-containing polyureaurethane binders 
which are crosslinked with polyisocyanate and in which the OH-containing 
polyureaurethane is obtained by reacting a polydiol, a diol and a primary 
or secondary amino alcohol and, if required, a triol with a diisocyanate 
are also advantageous. 
Preferably used solvents are water, cyclic ethers, such as tetrahydrofuran 
and dioxane, and cyclic ketones, such as cyclohexanone. Depending on the 
application, the polyurethanes can also be dissolved in other strongly 
polar solvents, such as dimethylformamide, N-methylpyrrolidone, dimethyl 
sulfoxide or ethylglycol acetate. It is also possible to mix the stated 
solvents with aromatics, such as toluene, xylene and esters, such as ethyl 
or butyl acetate. 
Further known additives for improving the magnetic layer can be added to 
the dispersions. Examples of such additives are fatty acids, 
polycarboxylic acids, mono-, di or polysulfonic acids, phosphoric acids 
and mixtures thereof or esters of salts with metals of the first to fourth 
group of the Period Table, as well as waxes, lecithins, silicone oils, 
fluorocarbons, and in addition fillers, such as carbon black, graphite, 
powdered quartz and/or nonmagnetizable silicate-based powder. In general, 
such additives are present in a total amount of less than 10% by weight, 
based on the magnetic layer. 
The magnetic layers are produced in a known manner. For this purpose, the 
magnetic material is dispersed with the compound from the group consisting 
of the sterically hindered phenols having aromatic amino groups, the 
binder, the dispersant and sufficient solvent in a dispersing apparatus, 
for example a tubular ball mill or a stirred ball mill, with or without 
further additives. To obtain the advantageous binder/pigment ratio, these 
components can be added to the mixture either in solid form or in the form 
of 10-60% strength solutions or 30-60% strength dispersions. It has proven 
advantageous to continue dispersing until an extremely fine distribution 
of magnetic material is achieved, which may take from 1 to 5 days. A 
completely homogeneous magnetic dispersion is obtained by subsequent 
repeated filtration. 
The magnetic dispersion is then applied to the nonmagnetizable substrate by 
means of a conventional coating apparatus, for example a knife coater. 
Suitable nonmagnetic and nonmagnetizable substrates are the conventional 
substrates, in particular films of linear polyesters, such as polyethylene 
terephthalate, in general in thicknesses of from 4 to 200 .mu.m, in 
particular from 6 to 36 .mu.m. Before the still liquid coating mixture is 
dried on the substrate, which is advantageously effected at from 
50.degree. to 90.degree. C. in the course of from 10 to 200 seconds, the 
anisotropic magnetic particles are oriented along the intended recording 
direction by the action of a magnetic field. Magnetic layers can then be 
calendered and compacted on conventional apparatus by passing them between 
heated and polished rollers, if necessary at from 50.degree. to 
100.degree. C., preferably from 60.degree. to 80.degree. C. The thickness 
of the magnetic layer is in general from 1 to 20 .mu.m, preferably from 2 
to 10 .mu.m. 
The novel recording media have substantially improved stability to the 
chemical decomposition due to moisture and oxidizable compounds compared 
with recording media whose magnetic layer does not contain the organic 
compound from the group consisting of the sterically hindered phenols 
having aromatic amino groups. This means that the undesirable 
decomposition which has a very adverse effect on the magnetic properties, 
i.e. disproportionation of the chromium dioxide into chromate and 
chromium(III) ions is substantially suppressed. Another advantage is that 
the novel recording media have improved recording properties due to 
increased residual induction and improved orientation of the anisotropic 
magnetic materials.

The Examples which follow illustrate the invention and compare it with 
prior art experiments. In the Examples and Comparative Experiments, parts 
and percentages are by weight, unless stated otherwise. The magnetic 
properties were measured using a vibrating sample magnetometer in a 
magnetic field of 100 kA/m. The coercive force H.sub.c in [kA/m], the 
residual induction M.sub.r and the saturation magnetization M.sub.m in 
[mT] and the orientation ratio Rf, i.e. the ratio of the residual 
induction in the playing direction to that in the crosswise direction, 
were determined. In addition, the stability of the magnetic recording 
media was investigated by measuring chromate formation by the eluate test 
according to DIN 38414/S4 and determination of total chromium in the 
stated eluate. 
Base Polymer A 
In a heatable reaction vessel having a capacity of 150,000 parts by volume 
and equipped with a stirrer and a reflux condenser, 6,600 parts of a 
polyester of adipic acid and butanediol (molecular weight 1,100), 730 
parts of butanediol, 80 parts of trimethylolpropane and 3,862 parts of 
diphenylmethane 4,4'-diisocyanate were dissolved in 26,000 parts of 
tetrahydrofuran and the solution was heated to 55.degree. C. The 
components were reacted to a final viscosity of 25 Pa.s and the mixture 
was then diluted to a solids content of 12.5% with 52,900 parts of 
tetrahydrofuran. At the same time, the reaction was stopped by adding 50 
parts of diethanolamine. The K value of the resulting polymer was 63, 
measured as a 1% strength solution in dimethylformamide. 
EXAMPLE 1 
100,000 parts of steel balls, 16,000 parts of the 12.5% strength solution 
of the polyurethane elastomer stated in Example A, 10,000 parts of a 10% 
strength solution of a polyvinylformal consisting of 82% of vinylformal, 
12% of vinyl acetate and 6% of vinyl alcohol units, 135 parts of N-tallow 
fat-1,3-diaminodioleate, 270 parts of zinc stearate, 40 parts of 
polyisobutene (C.sub.24 -C.sub.28), 135 parts of 
2,6-di-tert-butyl-p-cresol and 13,500 parts of a ferromagnetic chromium 
dioxide (H.sub.c =40 kA/m) having a mean particle size of 0.5 .mu.m and a 
length/width ratio of 4:1 and 4,500 parts of tetrahydrofuran were 
introduced into a steel ball mill having a capacity of 100,000 parts by 
volume, and dispersing was carried out for about 190 hours. The dispersion 
was then filtered under pressure through a filter having a pore diameter 
of 5 .mu.m. A 20 .mu.m thick polyethylene terephthalate film was coated 
with the dispersion using a knife coater and, after passing through a 
magnetic field, the coating was dried at from 60.degree. to 100.degree. C. 
The magnetic layer was compacted and calendered by passing it between 
heated rollers (70.degree. C., nip pressure 200 kg/cm). The resulting 
thickness was 5 .mu.m. The coated film was then slit into 3.81 mm wide 
tapes. 
The results are shown in the Table. 
COMATIVE EXPERIMENT 1 
The procedure described in Example 1 was followed, except that 
2,6-di-tert-butyl-p-cresol was not added. 
EXAMPLE 2 
The procedure described in Example 1 was followed, except that 
N-isopropyl-N'-phenyl-p-phenylenediamine was used instead of 
2,6-di-tert-butyl-p-cresol. 
EXAMPLE 3 
The procedure described in Example 1 was followed, except that the Al salt 
of N-nitrosocyclohexylhydroxylamine was used instead of 
2,6-di-tert-butyl-p-cresol. 
EXAMPLE 4 
The procedure described in Example 1 was followed, except that 200 parts of 
disalicylidene-N-methyldiisopropylenediamine were used instead of 135 
parts of 2,6-di-tert-butyl-p-cresol. 
EXAMPLE 5 
100,000 parts of steel balls, 5,000 parts of the 12.5% strength solution of 
the polyurethane elastomer stated in Example A, 3,000 parts of a 10% 
strength solution of a polyvinylformal, consisting of 82% of vinylformal, 
12% of vinyl acetate and 6% of vinyl alcohol units, 135 parts of N-tallow 
fat-1,3-diaminodioleate, 270 parts of zinc stearate, 40 parts of 
polyisobutene (C.sub.24 -C.sub.28), 185 parts of 
2-(2-hydroxy-5-methylphenyl)-2H-benzotriazole and 13,500 parts of a 
ferromagnetic chromium dioxide pigment having a mean particle size of 0.5 
.mu.m and a length/width ratio of 4:1 and 4,500 parts of tetrahydrofuran 
were introduced into a steel ball mill having a capacity of 100,000 parts 
by volume, and dispersing was carried out for 70 hours. Thereafter, 11,000 
parts (12.5% strength) of polymer A and 7,000 parts of the stated 10% 
strength polyvinylformal solution were added and dispersing was continued 
for a further 20 hours. The dispersion was then removed from the mill and 
filtered under pressure through a filter having a pore diameter of 5 
.mu.m. After the filtration process, 17 g of a 50% strength solution of a 
triisocyanate, obtained from 3 moles of toluylenediisocyanate and 1 mole 
of trimethylolpropane, were added per kg of dispersion with vigorous 
stirring. The dispersion was then applied to an 8 .mu.m thick polyethylene 
terephthalate film by means of a conventional knife coater. The coated 
film was passed through a magnetic field to orient the magnetic particles 
and then dried at from 50.degree. to 90.degree. C. After drying, the 
magnetic layer was compacted and calendered, so that it was 5 .mu.m thick. 
The coated film was then slit into 3.8=1 mm wide tapes. 
The results are shown in the Table. 
EXAMPLE 6 
The procedure described in Example 5 was followed, except that 
2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-tert-butylanilino)-1,3,5-triazin 
e was added instead of 2-(2-hydroxy-5-methylphenyl)-2H-benzotriazole. 
TABLE 
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Magnetic 
properties Eluate value 
H.sub.c 
M.sub.m [mg CrO.sub.3 / 
Total chromium 
[kA/m] 
[mT] Rf 1 H.sub.2 O] 
[mg Cr/1 H.sub.2 O] 
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Example 1 
40.1 168 2.8 1.5 0.7 
Comparative 
40.5 172 2.8 26.4 14 
Experiment 1 
Example 2 
40.5 173 2.7 2.8 1.0 
Example 3 
40.1 170 2.9 2.1 1.0 
Example 4 
40.3 165 2.7 1.4 0.8 
Example 5 
40.4 168 3.1 1.9 1.0 
Example 6 
40.4 165 3.0 1.3 0.8 
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