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The present invention relates to magnetic recording media which comprise a
non-magnetic base which is provided with a magnetic layer consisting of
finely divided anisotropic magnetic material dispersed in a binder,
wherein the binder is an OH-containing polyurethane crosslinked with a
polyisocyanate.
Magnetic recording media are used for the recording and playback of audio
and video signals and data. The constantly increasing demands made on
these recording media call for further improvements in their magnetic and
electroacoustic properties. For example, the trend toward higher recording
densities in the case of all the said fields of application makes the
production of thinner magnetic layers necessary. For this reason, the
packing density of the magnetic material in the magnetic layer and the
remanence in the recording direction must be considerably increased, and
the distribution of the magnetic material in the layer, and the surface
smoothness and homogeneity of the layer must be greatly improved, since
the faithful recording of signals makes great demands on the quality of a
magnetic layer. A magnetic layer must be capable of recording and
reproducing both high- and low-frequency signals without any variation in
their amplitude.
In order to achieve these properties, it is necessary not only that the
magnetic pigment should be distributed very uniformly in the organic
binder, but also that the magnetic layer should be magnetically very
sensitive in the recording direction. The anisotropy of the acicular
pigment particles is utilized to achieve a high orientation ratio. The
liquid dispersion comprising the magnetic pigment and the organic binder
solution is applied as a coating to an inert base and is then exposed to a
magnetic field, so that the magnetizable needles are oriented in the
recording direction. This procedure is followed directly by a drying
operation, in which the particles in the binder are locked in their
oriented positions. A measure of the degree of particle orientation
achieved, and hence of the sensitivity of the recording medium, is the
ratio of the residual induction B.sub.R to the saturation induction
B.sub.S of the dry magnetic layer, measured in the recording direction.
The distribution of the magnetic particles in the organic binder and their
orientation by the magnetic field are effected by the nature of the
polymer employed, the effect being particularly pronounced when a finely
divided pigment is used. There is a wide choice of organic binders, and
combinations thereof, for magnetic powders. Examples of conventional
binders include polyacrylates, nylons, polyesters, polyurethanes, phenoxy
resins, vinyl chloride/acrylonitrile copolymers and copolymers of vinyl
chloride, vinyl acetate and vinyl alcohol. The majority of the polymers
listed are relatively hard and brittle, whereas the usual mechanical
stressing of the magnetic layer requires an elastomeric, frequently
relatively soft, formulation. Therefore, polyurethane elastomers are
frequently combined with relatively brittle polymers, such as phenoxy
resins, vinyl chloride/vinyl acetate copolymers, polycarbonates, etc., or
plasticizers are introduced into the layer. Such polyurethanes are
prepared by reacting hydroxyl-containing polyethers or polyesters with
polyisocyanates. Usually, polyester-urethane elastomers as described in,
for example, German Published Application DAS No. 1,106,959, or
polyether-urethane elastomers as described in U.S. Pat. No. 2,899,411, are
employed. To improve the tape running properties, the above polymer
mixtures are frequently crosslinked with polyisocyanates. Hence, the hard
resins employed also frequently possess OH groups.
However, the disadvantages of using these binder systems are the high
solvent demand, the long dispersing time and the 2-stage dispersing
procedure required. Furthermore, the particular properties of the magnetic
materials are not satisfactorily displayed in these binder systems. This
is evident from the fact that the resulting recording media have a low
orientation ratio, low remanence and hence poor sensitivity at short and
long wavelengths, and an unsatisfactory maximum output level.
German Pat. No. 814,225 describes the use of bifunctional or higher
functional polyoxy compounds, preferably polyesters, which still possess
hydroxyl groups, in combination with polyisocyanates. The substances
mentioned in that publication, however, do not form films but are low
molecular weight products and hence tend to block before they have reacted
completely.
To overcome the disadvantages of German Pat. No. 814,225, German Published
Application DAS No. 1,130,612 proposes mixing 5-25% of a high-polymeric
physically drying surface coating binder with the polyester-polyisocyanate
binder. However, the process has the disadvantage that an additional
binder component ist required, with the result that the preparation
process is lengthened by a further step. Moreover, a 2-stage milling
operation is required in order to achieve optimum properties. German
Published Application DAS No. 1,283,282 is similar to German Published
Application DAS No. 1,130,612, except that in the former rubber is added
to the polyester-urethane binder.
In German Published Application DAS No. 1,571,128, the binder contains from
0 to 75 percent by weight of a polymeric matrix material, the remainder
being an elastomer. The polymeric matrix material used comprises one or
more copolymers selected from a group consisting of soluble
hydroxyl-containing resins having a molecular weight of not less than
2,000.
In choosing the conventional binder systems, it was the object in each case
to overcome or at least mitigate disadvantages and deficiencies, such as
long dispersing time, two-stage dispersing procedure, poor pigment
wetting, high solvent demand during the production of the dispersion, low
orientation ratio, poor sensitivity at short and long wavelengths, low
maximum output level at long and short wavelengths and inadequate
signal-to-print-through ratio of the recording layers. With regard to
obtaining optimum properties, the solutions proposed hitherto are either
inadequate or successful in only some cases.
It is an object of the present invention to provide magnetic recording
media which possess improved electroacoustic properties, in particular
with regard to sensitivity and maximum output level at long and short
wavelengths, as a result of the use of suitable binders which permit the
magnetic material to be dispersed in a smaller amount of solvent in a
short time by one-stage procedure.
We have found that this object is achieved, and that magnetic recording
media which comprise a magnetic layer which is applied to a non-magnetic
base and consists of a dispersion of an anisotropic magnetic material in
an organic binder consisting essentially of an OH-containing polyurethane
binder crosslinked with a polyisocyanate satisfy the requirements set, if
the polyurethane is a thermoplastic polyurea-urethane which has an OH
number of from 10 to 120 and is obtained from
IA. 1 mole of a polydiol having a molecular weight of from 400 to 4,000,
IB. from 0.2 to 10 moles of a diol of 2 to 18 carbon atoms,
IC. from 0.1 to 4 moles of a primary or secondary aminoalcohol of 2 to 20
carbon atoms, and
II. from 1.20 to 13 moles of a diisocyanate of 6 to 30 carbon atoms, the
proportion of NCO groups in the diisocyanate being from 65 to 95%, based
on Components IA to IC, of the equivalent amount of OH and NH groups, with
the proviso that the non-pigmented crosslinked film has a tensile strength
greater than 15 N/mm.sup.2, an elongation at break greater than 30%, a
modulus of elasticity greater than 150 N/mm.sup.2 and a pendulum hardness
of from 25 to 140 sec.
Equally suitable for the purpose of the invention are similar binders
obtained from
IA. 1 mole of a polydiol having a molecular weight of from 400 to 4,000,
IB. from 0.2 to 9 moles of a diol of 2 to 18 carbon atoms,
IC. from 0.1 to 4 moles of a primary or secondary aminoalcohol of 2 to 20
carbon atoms,
ID. from 0.01 to 1 mole of a triol of 3 to 18 carbon atoms, and
II. from 1.25 to 13 moles of a diisocyanate of 6 to 30 carbon atoms, the
proportion of NCO groups in the diisocyanate being from 65 to 95%, based
on Components IA to ID, of the equivalent amount of OH and NH groups.
The OH-containing polyurea-urethane binders which are crosslinked with the
polyisocyanates and can be used for the novel recording media possess, as
non-pigmented films, a tensile strength (according to DIN 53,455) greater
than 15, preferably greater than 25, N/mm.sup.2, an elongation at break
(according to DlN 53,455) greater than 30%, preferably greater than 50%, a
modulus of elasticity (according to DIN 53,457) greater than 150,
preferably greater than 200, N/mm.sup.2, and a pendulum hardness
(according to DIN 53,157) of from 25 to 140, preferably from 40 to 110
sec.
The OH-containing polyurea-urethane binders used according to the invention
are thermoplastic polyureaurethanes having an OH number of from 10 to 120,
preferably from 20 to 100, and a number average molecular weight of from
1,000 to 40,000. In the structure of these polymers, it has proved
advantageous if some of the OH terminal groups, preferably more than 70%,
in particular more than 90%, are present in the radicals:
##STR1##
where R is --(CH.sub.2)--.sub.n, R.sup.1 is H, --CH.sub.3 or
--(CH.sub.2).sub.n --CH.sub.3 and n is from 1 to 10.
Polymers having this structure are less thermoplastic than those without
these terminal groups. Moreover, such a structure permits an increase in
the content of terminal OH groups, with the result that when crosslinking
with the polyisocyanate is effected, the degree of crosslinking can be
varied within wide limits, according to the demands made on the magnetic
layer. The urea groups, which increase the dispersibility of conventional
magnetic materials, are also of advantage.
To prepare these polymers, a polydiol having a molecular weight of from 400
to 4,000, preferably from 700 to 2,500, is employed as Component IA,
suitable compounds being the conventional polyesterols, polyetherols,
polycarbonates and polycaprolactones.
Advantageously, the polyesterols are predominantly linear polymers which
have terminal OH groups, preferably 2 such groups, and an acid number of
less than 10, preferably less than 3. The polyesterols can be obtained in
a simple manner by esterifying an aliphatic dicarboxylic acid of 4 to 12,
preferably 4 to 6, carbon atoms with an aliphatic glycol, preferably one
of 2 to 12 carbon atoms, or by polymerizing lactones of 3 to 6 carbon
atoms. Examples of suitable aliphatic dicarboxylic acids are glutaric
acid, pimelic acid, suberic acid, sebacic acid, dodecanedioic acid and
preferably adipic acid and succinic acid. The dicarboxylic acids can be
used individually or as a mixture. In preparing the polyesterols, it may
be advantageous to replace the dicarboxylic acids with the corresponding
acid derivatives, such as a carboxylic acid ester where the alcohol
radical is of 1 to 4 carbon atoms, a carboxylic anhydride or a carboxylic
acid chloride. Examples of suitable glycols are diethylene glycol,
pentane-1,5-diol, decane-1,10-diol and 2,2,4-trimethylpentane-1,5-diol,
but ethane-1,2-diol, butane-1,4-diol, hexane-1,6-diol and
2,2-dimethylpropane-1,3-diol are preferably used. Depending on the desired
properties of the polyurethanes, the polyols can be used either alone or
mixed together in various proportions. Suitable lactones for the
preparation of the polyesterols are
.alpha.,.alpha.-dimethyl-.beta.-propiolactone, .gamma.-butyrolactone and
preferably .epsilon.-caprolactone.
The polyetherols are essentially linear substances which possess ether
bonds and terminal hydroxyl groups and have a molecular weight of about
600-4,000, preferably 1,000-2,000. Suitable polyetherols can be readily
prepared by polymerizing a cyclic ether, eg. tetrahydrofuran, or by
reacting one or more alkylene oxides, where alkylene is of 2 to 4 carbon
atoms, with an initiator whose molecule contains two bonded active
hydrogen atoms. Examples of alkylene oxides are ethylene oxide,
1,2-propylene oxide, epichlorohydrin, 1,2-butylene oxide and 2,3-butylene
oxide. The alkylene oxides can be used individually, in succession or as a
mixture. Examples of suitable initiators are water, glycols, eg. ethylene
glycol, propylene glycol, butane-1,4-diol and hexane-1,6-diol, amines, eg.
ethylenediamine, hexamethylenediamine and 4,4'-diaminodiphenylmethane, and
aminoalcohols, eg. ethanolamine. Like the polyesterols, the polyetherols,
too, can be used either alone or as a mixture.
Diols of 2 to 18, preferably 2 to 6, carbon atoms, eg. ethane-1,2-diol,
propane-1,3-diol, butane-1,4-diol, hexane-1,6-diol, pentane-1,5-diol,
decane-1,10-diol, 2-methylpropane-1,3-diol, 2,2-dimethylpropane-1,3-diol,
2,2-dimethylbutane-1,4-diol, 2-methyl-2-butylpropane-1,3-diol,
neopentylglycol hydroxypivalate, diethylene glycol, triethylene glycol and
methyldiethanolamine, are employed as Component IB.
In order to obtain novel recording media possessing special properties, it
is advantageous if, in the preparation of the OH-containing
polyurea-urethane, Component IB consists completely or partially of a
diamine of 2 to 15 carbon atoms, eg. ethylenediamine,
hexamethylene-1,6-diamine, 4,9-dioxadodecane-1,12-diamine or
4,4'-diaminodiphenylmethane, or an aminoalcohol, eg. monoethanolamine,
monoisopropanolamine or 4-methyl-4-aminopentan-2-ol.
Similarly, the diol used as Component IB may furthermore be completely or
partially replaced by water or by the primary or secondary aminoalcohols
listed below for Component IC.
These aminoalcohols (Component IC) of 2 to 20, preferably 3 to 6, carbon
atoms, include monoethanolamine, diethanolamine, monoisopropanolamine,
diisopropanolamine, methylisopropanolamine, ethylisopropanolamine,
methylethanolamine, 3-aminopropanol, 1-ethylaminobutan-2-ol,
4-methyl-4-aminopentan-2-ol and N-(2-hydroxyethyl)-aniline. Secondary
aminoalcohols are particularly suitable since they form an adduct at the
chain end and hence improve the solubility of the polymer.
Methylethanolamine, diethanolamine and diisopropanolamine have proved
particularly advantageous.
The triols used (Component ID) are compounds of 3 to 18, preferably 3 to 6,
carbon atoms, examples of appropriate compounds being glycerol,
trimethylolpropane and hexanetriol. Low molecular weight reaction
products, for example of glycerol or trimethylolpropane with ethylene
oxide and/or propylene oxide, are also suitable. The presence of triols
during the polyaddition leads to a branched end product; this branching is
advantageous for the mechanical properties of the polyurethane, provided
that localized crosslinking does not take place.
For the formation of the OH-containing polyureaurethanes, the components
listed under I are reacted with an aliphatic, cycloaliphatic or aromatic
diisocyanate of 6 to 30 carbon atoms (Component II). Particularly suitable
compounds for this purpose are toluylene 2,4-diisocyanate, toluylene
2,6-diisocyanate, m-phenylene diisocyanate, 4-chlorophenylene
1,3-diisocyanate, naphthylene 1,5-diisocyanate, hexamethylene
1,6-diisocyanate, cyclohexylene, 1,4-diisocyanate, tetrahydronaphthylene
1,5-diisocyanate, diphenylmethane diisocyanate, dicyclohexylmethane
diisocyanate and isophorone diisocyanate. The OH-containing polyurethanes
based on toluylene diisocyanate and isophorone diisocyanate are readily
soluble in tetrahydrofuran and dioxane. Mixtures of diisocyanates, eg. of
toluylene diisocyanate with diphenylmethane diisocyanate, are also
advantageous.
Components I and II are employed in the following ratio: from 1.20 to 13
moles of diisocyanate, from 0.2 to 10, preferably from 0.5 to 5, moles of
the straight-chain aliphatic diol of 2 to 18 carbon atoms and, if
appropriate, from 0.01 to 1, preferably from 0.15 to 0.5, mole of triol
can be employed per mole of polydiol. The amount of straight-chain diol
used depends partly on the molecular weight of the polydiol employed. The
isocyanate should be present in an amount which is 5-35% less than the
stoichiometric amount, based on the amounts of NH-containing or
hydroxyl-containing compounds, so that at the end of the reaction there is
virtually no free, unreacted isocyanate present, while free, unreacted
hydroxyl groups remain. However, for practical reasons it is often
advantageous, in a preliminary reaction of Components IA, IB, ID and II,
to use an excess of diisocyanate of from 5 to 40%, preferably from 10 to
30%, based on the amount required for complete conversion of the
reactants, so that the ratio of the number of hydroxyl groups employed to
the number of isocyanate groups in this reaction stage is from about
1:1.05 to 1:1.4, preferably from about 1:1.1 to 1:1.30. In the second
reaction stage, Component IC is then added in an amount such that the
number of NH equivalents corresponds to the NCO content, ie. from 0.1 to
4, preferably from 0.3 to 2.5, moles per mole of Component IA, or the
NCO-containing prepolymer is added to the aminoalcohol, so that the amino
groups react with the isocyanate. Even in the case of polyurethanes which
do not contain triols, variation of the aminoalcohols gives products
having an OH functionality of from 2 to 4, the OH groups being
predominantly at the chain ends. If the polyurethanes used contain triols,
the OH functionality is increased accordingly. This composition is of
advantage for film formation and for the final crosslinking of the
OH-containing polyurethane with the polyisocyanate. If, in this second
reaction stage, the NCO groups are slightly in excess of the NH or
NH.sub.2 groups, some of the aminoalcohol is incorporated into the
molecule and results in a branching point, depending on the aminoalcohol.
If an excess of NH groups is used, the aminoalcohol is not completely
incorporated into the polymer until the crosslinking reaction has taken
place. Hence, by varying the terminal groups, it is possible to match the
polymer to the particular requirements, eg. film-forming ability and
dispersibility.
The thermoplastic elastomeric OH-containing polyurea-urethanes having the
above composition are preferably prepared by a 2-stage process, in
solution, in the presence or absence of a catalyst and other assistants
and/or additives. It is also possible to prepare these products by the
solvent-free batch process. However, because of the possible presence of a
triol and the reaction of the amine with NCO groups, gel particles are
formed to at least some extent during polyaddition in the absence of a
solvent, and the reaction is therefore carried out in general in solution.
The risk of complete crosslinking occuring locally, as happens in the case
of polyaddition in the absence of a solvent, is usually avoided in the
polyaddition in solution.
Preferably used solvents for the preparation of the polyurethanes are
cyclic ethers, eg. tetrahydrofuran and dioxane, and cyclic ketones, eg.
cyclohexanone. Depending on the field of use, the polyurethanes can of
course also be dissolved in another strongly polar solvent, such as
dimethylformamide, N-methylpyrrolidone, dimethyl sulfoxide or ethylglycol
acetate. It is also possible to mix the above solvents with aromatics, eg.
toluene or xylene, or esters, eg. ethyl or butyl acetate.
Examples of suitable catalysts for the preparation of the polyurethane and
for the crosslinking reaction are tert.-amines, eg. triethylamine,
triethylenediamine, N-methylpyridine and N-methylmorpholine, metal salts,
eg. tin octoate, lead octoate and zinc stearate, and organometallic
compounds, eg. dibutyl-tin dilaurate. The amount of catalyst to be
employed depends on its activity. In general, it has proved advantageous
to use from 0.005 to 0.3, preferably from 0.01 to 0.1, parts by weight per
100 parts by weight of polyurethane.
In the 2-stage polyaddition process, the diisocyanate is placed first in
the reactor, and Components IA, IB and ID, with or without a catalyst,
assistants and additives, in a solvent, are then added at from 20.degree.
to 90.degree. C., preferably from 30.degree. to 70.degree. C., in the
course of from 0.5 to 5 hours. The components are then allowed to react
until the desired NCO content is reached, after which Component IC is
added in the 2nd stage, or Component IC is initially introduced and the
polymer is then added. In the 2-stage process, the first stage is carried
out using an NCO excess, based on Components IA, IB and ID.
The processing of the solution of the OH-containing polyurea-urethane
binder with magnetic materials and assistants into a magnetic dispersion,
and the application thereof to the base material to give the magnetic
recording medium may be carried out in a conventional manner.
The anisotropic magnetic materials which can be used are the conventional
ones, but the choice of pigment is a factor which substantially determines
the properties of the resulting magnetic layer. Examples of these
materials are gamma-iron(III) oxide, finely divided magnetite, non-doped
ferromagnetic chromium dioxide or cobalt-modified gamma-iron(III) oxide.
Acicular gamma-iron(III) oxide and ferromagnetic chromium dioxide are
preferred. The particle size is in general from 0.2 to 2 .mu.m, preferably
from 0.3 to 0.8 .mu.m.
As is conventionally the case, the magnetic layer may also contain small
amounts of additives, eg. dispersants and/or lubricants, and fillers,
which are admixed during dispersion of the magnetic pigment or during the
production of the magnetic layer. Examples of such additives are fatty
acids or isomerized fatty acids, eg. stearic acid, or their salts with
metals of main groups I to IV of the periodic table of elements,
amphoteric electrolytes, eg. lecithin, and fatty acid esters, waxes,
silicone oils, conductive carbon, etc. The additives are used in a
conventional amount, which is in general less than 10 percent by weight,
based on the magnetic layer.
The said OH-containing polyurea-urethane binders are generally used alone
for the production of the magnetic layers, and they permit very short
dispersing times. They can be crosslinked using a large number of organic
di-, tri- or polyisocyanates or isocyanate prepolymers having a molecular
weight of not more than 10,000, preferably from 500 to 3,000. Preferred
compounds are polyisocyanates which possess more than 2 NCO groups per
molecule. Polyisocyanates which are based on toluylene diisocyanate,
hexamethylene diisocyanate or isophorone diisocyanate and are obtained by
polyaddition to diols and triols or by biuret and isocyanurate formation
have proved particularly suitable. An adduct of toluylene diisocyanate
with trimethylolpropane and diethylene glycol is particularly
advantageous.
Depending on the properties which the recording material is required to
have, the amount of polyisocyanate component added can vary from as much
as 70%, preferably as much as 50%, less than the stoichiometric amount to
an excess of not more than 100%, preferably not more than 50%, the
percentages being based on the OH groups of the polyurethane binder to be
crosslinked.
However, when magnetic recording media according to the invention are to be
used for special purposes, it may be advantageous to add a second binder
component in an amount of from 10 to 50, preferably from 20 to 40, parts
by weight, based on the resulting total amount of binder. Particularly
suitable further binders are vinyl chloride polymers which are
substantially compatible with the polyurethane binder, phenoxy resins,
preferably those polycondensates obtained from epichlorohydrin and
bisphenol A, and polyvinylformal binders or high molecular weight
nonreactive polyurethane elastomers as described in, for example, German
Published Application DAS No. 1,106,959.
The novel recording material contains from 1 to 10, in particular from 3 to
6, parts by weight of magnetic material per part by weight of binder or
binder mixture. A particular advantage is the fact that the excellent
pigment-binding ability of the special polyurethanes permits a high
loading of magnetic material in the magnetic layer without the mechanical
properties being adversely affected or the service characteristics being
noticeably affected.
The non-magnetic and non-magnetizable bases used are conventional rigid or
flexible ones, in particular films obtained from linear polyesters, eg.
polyethylene terephthalate, which are in general from 4 to 200, in
particular from 10 to 36, .mu.m thick. More recently, the use of magnetic
layers on paper bases for electronic computing and accounting machines has
become important; the novel coating materials can be advantageously used
for this purpose, too.
The magnetic recording medium according to the invention can be produced in
a conventional manner. Advantageously, a magnetic dispersion is produced
in a dispersing apparatus, eg. a tubular ball mill or a stirred ball mill,
from the magnetic material and a solution of the binder or binders, with
the addition of dispersants and other additives, the polyisocyanate
crosslinking agent is mixed in, and the dispersion is filtered and then
applied to the non-magnetic base using a conventional coating apparatus,
eg. a knife coater. As a rule, the magnetic particles are oriented before
the fluid coating mixture has dried on the base; drying is advantageously
carried out at from 50.degree. to 90.degree. C. for from 2 to 5 minutes.
The magnetic layers can be subjected to a conventional surface treatment,
eg. calendering in which the coated base is passed between polished
rollers, with the application of pressure and optional heating at from
25.degree. to 100.degree. C., preferably from 60.degree. to 80.degree. C.
It has proved very advantageous to carry out calendering before
crosslinking is complete, since the OH polymers in the non-crosslinked
state are very thermoplastic but do not exhibit tackiness. The thickness
of the magnetic layer is in general from 2 to 20, preferably from 4 to 10,
.mu.m. Where magnetic tapes are to be produced, the coated webs are slit,
in the longitudinal direction, to the usual widths.
Compared with magnetic recording media obtained using, as binders, prior
art polyurethanes or polyurethane mixtures with suitable harder surface
coating resin components, the magnetic recording media according to the
invention possess improved electroacoustic properties, in particular a
higher maximum output level at short and long wavelengths as well as
higher sensitivity. Another big advantage is that it is possible with the
OH-containing polyurea-urethane to process conventional magnetic materials
into homogeneous, highly pigmented dispersions in conventional dispersing
apparatus, the operation being easy to carry out and, in particular,
requiring very little time and energy. The fact that up to 40% less
solvent is required in the dispersing operation should also be singled
out. A further advantage is that the crosslinking of the polymers suitable
for the novel magnetic recording media results in magnetic layers which
are stable even at elevated temperatures and high humidity levels.
In the Examples and Comparative Experiments which follow, parts and
percentages are by weight, unless stated otherwise. Parts by volume bear
the same relation to parts by weight as the liter to the kilogram.
EXAMPLE A
A solution of 150 g of a polyester having a molecular weight of 1,000 and
obtained from adipic acid and butane-1,4-diol, 31.8 g of diethylene
glycol, 4.47 g of trimethylolpropane and 200 g of tetrahydrofuran was
added dropwise to a solution containing 108.75 g of toluylene diisocyanate
(isomer ratio 8:2) and 114 g of tetrahydrofuran in the course of 2 hours
at 60.degree. C. 1 drop of dibutyl-tin dilaurate was added before the
beginning of the addition, and a further drop 1 hour after the addition
was complete. The solution was stirred at 60.degree. C. until the NCO
content was 1.72%, after which it was cooled to 45.degree. C. and 18.78 g
of methylethanolamine were added.
The resulting product had a solids content of 50%, an OH number of 45 and a
K value of from 22 to 24.
EXAMPLE 1
1,000 g of a Co-doped iron oxide pigment having a coercive force of 30
kA/m, 1,130 g of a solvent mixture of equal amounts of tetrahydrofuran and
dioxane, 35 g of a dispersant based on a mixture of a monophosphate with
the salt of a sulfosuccinic acid, 335 g of a 50% strength solution, in
tetrahydrofuran, of the hydroxyl-containing polyurethane solution
described in Example A, 1.0 g of a commercial silicone oil, 2.0 g of
hydroquinone, 2.0 g of n-butyl stearate and 10 g of isostearic acid were
dispersed for 70 hours in a steel ball mill having a capacity of 6 liters
and containing 8 kg of steel balls with a diameter of from 4 to 6 mm. The
resulting dispersion was then forced under pressure through a filter
having 5 .mu.m pores, 26.0 g of a 75% strength solution, in ethyl acetate,
of a triisocyanate obtained from 3 moles of toluylene diisocyanate and 1
mole of trimethylolpropane were added while stirring vigorously, and
immediately afterward the dispersion was applied to a 12 .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, dried at from 50.degree. to 90.degree. C., passed between
heated rollers (70.degree. C.) at a nip pressure of 200 kg/cm to
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