Note: Descriptions are shown in the official language in which they were submitted.
~s~
MAGNETIC RECOR~ CARRIER AND METHOD OF MAKING IT
The invention relates to a magnetic record carrier having a
substrate provided with a magnetic coating whose thickness
is in the ~m range. The coating consists of a binder,
magnetic particles and, optionally, abrasion-resistant
particles. The invention also relates to a method of making
such record carriers wherein iron oxide particles, the
binder and, optionally, abrasion-resistant particles are
applied onto the substrate and the binder is cured.
In the field of magnetic record carriers, particularly
magnetic disks and magnetic tapes, there is a tendency
toward incxeasingly higher storage densities. High storage
densities require a thin magnetic coating having a high
coercive field intensity. The read signal has to be high,
thus requiring -that the magnetic particles have a high
magnetization. ~Iigh magnetization is also required to
enable use of small magnetic particles which are needed to
obtain a signal with a low noise level.
Small particles of pure iron have the necessary high
magnetization, but such particles have so far been used only
in connection with magnetlc tapes having relatively thick
(approximately 10 ~m) magnetic layers, since the regular
distribution of iron particles within a layer is difficult
to achieve. Usually, magnetic coatings are applied by depositing
a dispersion of magnetic particles in a fluid matrix onto a
substrate. The dispersion of the particles within the fluid
matrix is determined by the attractive and repulsive forces
between the particles. The repulsive forces determine the
dispersion stability and are of an electrostatic nature. The
forces are counteracted by the attractive forces which are
van der Waal forces, and, in the case of magnetic particles,
magnetostatic attraction forces. In the case of iron particles,
these magnetostatic attraction forces are so high that
dispersion becomes very difficult. Therefore, the dispersion
stability could only be achieved in highly viscous systems
used for making relatively thick magnetic layers on magnetic
tapes. In the production of magnetic disks where the magnetic
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coatings are much thinner the mixtures of magnetic particles
and binder have a much lower viscosity. The dispersion of
iron particles in such mixtures is, there~ore, incomple~e,
or the iron particles agglomerate into clusters which not
only have an undesirably high diameter but in which the
magnetic moments are partly compensated. Consequently, the
magnetization is low and inhomogeneous over the layer. It is
pointed out that, owing to the thickness of the magnetic
coating in the conventional magnetic tapes, an inhomogeneity
caused by non-optimum dispersion of the magnetization of the
iron particles or iron particle aggregates can be tolerated.
This is another reason why iron particles have been widely
used in magnetic tapes but not in the magnetic disks which
require a much thinner magnetic coating.
The problem of poor dispersion characteristics is also
encountered in connection with Fe2O3 particles, although to
a lesser exten-t. U.S. Patent 4,280,918 describes a method for
improving the dispersion characteristics of Fe2O3 particles
by coating the particles with SiO2. U.S. Patent 4,333,961
also describes a method of applying Fe2O3 particles coated
with SiO2 and of uniform size onto a substrate in the form
of a mono-layer. As reported by T. Sueyoshi et al. in a
lecture entitled "Morphology of Fine Iron Particles" in
September 1983 a-t the M.R.M. Conference in Ferrara, Italy,
SiO2-coated Fe2O3 particles could be reduced to Fe in a
hydrogen atmosphere at temperatures between 250 and 350C.
However, it has been equally evident that to disperse the
thus produced SiO2-coated iron particles in a binder of low
viscosity, or to deposit these as a mono-layer on a substrate
is almost as difficult as the same process with uncoated
iron particles.
It is the object of the invention to provide a magnetic
record carrier with a thin magnetic coating having a high
uniform magnetization over the entire layer, and an uncom-
plicated process for reproducibly making such a magnetic
record carrier.
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~he magnetic record carrier according to the invention has
uniformly high magnetization. The magnetic coating has a
high packing density of the magnetic particles in view of
its low thickness and the necessary high storage density.
There is no partial aggregation of the iron particles, and
no partial compensation of the magnetic moments of the iron
particles.
In an advantageous embodiment of the magnetic record carrier
according to the invention the iron particles are dispersed
in a binder. The iron particles may be coated with an sio2
layer.
In yet another advantageous embodiment oE the magnetic
record carrier according to the invention a magnetic coating
is formed of a layer of SiO2-coated iron particles deposited
on the substrate, and a binder layer thereupon. The binder
layer can also contain abrasion-resistant particles. This
embodiment has the advantage of providing a substantially
thinner magnetic coating.
By first applying the magnetic particles in the form of
Fe2O3 onto the substrate, fixing them in their final location
and position, and only then reducing them to iron the method
disclosed by the invention elegantly avoids the dispersion
problem which had yet been unsolved in connection with iron
particles.
In an advantageous embodiment of the method, the iron oxide
particles and optionally the abrasion-resistant particles
are dispersed in a mixture of the binder with a solvent, the
dispersion is deposited on the substrate and cured, and
finally or overlappingly reduced in a hydrogen-containing
atmosphere. Prior to the dispersion, the iron oxide
particles can be coated with SiO2 or with a mixture of SiO2
and A12O3. Owing to the process selected the binder
viscosity is so high upon reduction that there can no longer be
any agglomeration and/or re-orientation of the iron
particles formed despite the strong forces which the iron
particles exert upon each other. Surprisingly, the contact
between the iron oxide particles and the hydrogen-containing
gas is
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apparently not substantially impeded by the binder and the
coating. The dispersion of the coated iron oxide particles
is even more complete than that of the uncoated iron oxide
particles. The additional process step of coating will be
used in situations where an extremely homogeneous
distribution of magnetic particles of the same diameter in
the magnetic coating is of decisive importance.
In another embodiment of the method, the iron oxide particles
enveloped with SiO2 or SiO2/A12O3 are applied, by means of
an active layer previously deposited on the substrate, onto
the substrate in a thin layer, subsequently reduced in a
hydrogen-containing atmosphere, and then coated with a
mixture comprising the binder, a solvent and, optionally,
abrasion-resistant particles. The mixture is then cured.
The coated iron oxide particles applied in a thin layer are
fixed to the substrate by means of electrostatic forces.
Surprisingly, the electrostatic Eorces are so strong that
even the high magnetostatic forces formed upon the reduction
of the iron oxide in the iron cannot tear the thus formed
coated iron oxide particles from their anchoring and thus
modify their position and their orientation relative to each
other.
In the following, the invention will be described with
reference to the drawings. The drawings show the following:
igs. 1 to 3 schematic cross-sectional views
of three embodiments of the magnetic
record carrier as disclosed by the
invention.
The magnetic record carriers described are magnetic disks
and magnetic tapes. Quite generally, these magnetic record
carriers have a structure composed of a substrate and a
magnetic coating thereupon. Substrates can be rigid or
flexible. Rigid substrates are usually employed for magnetic
disks, but there also exist magne-tic disks with flexible
substrates, such as the so-called floppy disks. Magnetic
tapes obviously all use flexible substrates. Conventional
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rigid substrates consis-t of aluminum, aluminum alloys such
as aluminum magnesium, or silicon. A11 these materials are
heat-resistant. Flexible substrate materials are of an
organic nature. They may comprise, for example, polyesters.
A frequently used example is marketed by Du Pont under the
trademark Mylar. If a magnetic record carrier with a flexible
substrate is to be made in accordance with the present
invention, the substrate has to be heat-resistant. Polyester
is not heat resistant. In such a case, polyimide would be a
suitable substrate material. The magnetic layer on the
substrate has a thickness between 0.1 and 0.81~m, preferably
O.3~m. With layer thicknesses within the above range the
necessary high storage densities can be realized. The
magnetic coating consists of a binder, the magnetic particles
and, optionally, abrasion-resistant particles. The term
binder refers to the matrix containing the magnetic particles
in the finished magnetic coating, or covering the magnetic
particles in the finished magnetic coating as well as the
material which is converted into this matrix in the course
of the production of the magnetic coating. A typical binder
is a material selected from the group of temperature~stable
polymers comprising epoxide phenol resins, polyimide,
melamine, polyamide and polypyrrolidine. The magnetic
particles consist of iron or of cobalt-doped iron. The
particles are acicular and have a length to thickness ratio
between 5:1 and 10:1. The length of the iron particles is
between 0.08 and 0.8J~m. The optionally existing
abrasion-resistant particles which are added to prevent
abrasion upon the sliding of a magnetic head on the magnetic
layer are preferably of spherical shape, and have a diameter
less than or equal to the thickness of the magnetic layer.
They preferably consist of a hard ceramic material such as
aluminum oxide, silicon carbide or silicon nitride.
With reference to Figs. 1 to 3, three different embodiments
of the magnetic record carrier will be described.
Fig. 1 depicts, in a schematic cross-section, a magnetic
record carrier such as a magnetic disk with a rigid substrate
which differs from the known magnetic disks only in that the
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magnetic particles consist of iron rather than Fe2O3, and
the iron particles are smaller than the iron oxide particles
in the known magnetic disks. In the embodiment of Fig. 1, a
magnetic coating is applied onto a substrate 1. The coating
consists of a binder 2, magnetic iron particles 3 dispersed
therein, and, optionally, abrasion-resistant particles lnot
shown), preferably of aluminum oxide. The thickness of the
magnetic coating and the size of the iron particles is in
the range of the above given values.
Fig. 2 is a sectional view of another embodiment of the
record carrier. The embodiment of Fig. 2 differs from that
of Fig, 1 in that the magnetic iron particles 3 are coated
with a layer consisting of silicon dioxide or a mixture of
silicon dioxide/aluminum oxide. The coating is between 2 and
50 nm thick, preferably between 8 and 20 nm. In comparison
with the embodiment of Fig. 1, the coating increases the
reliability of practically all iron particles existing
individually in a uniform distribution in the magnetic
layer. In -the embodiments of Figs. 1 and 2, the surface
density of the magnetic particles is approximately 10 to
101 particles/cm2 depending on the pigment volume concen-
tration and the particle size.
The embodiment of Fig. 3 clearly differs in its structure
from those depicted in Figs. 1 and 2. A substantially
blanket mono-layer consisting of iron particles 3 with a
coating ~ is deposited onto substrate 1. The thickness of
the mono-layer is between approximately 0.02 and approxi-
mately 0.OSJ~m. A layer of binder 2 is applied onto the
layer of magnetic particles. Binder 2 can also contain
abrasion-resistant particles (not shown). In order to
improve the adhesion of the iron particles to the substrate,
a thin active layer 5 is provided between substrate 1 and
the layer of iron oxide particles 3. Layer 5 can consist of
a mixture of an epoxide resin and a polyamide, or it can be
formed by a mono-layer of a cationic polyelectrolyte produced
out of an aqueous solution of polyethyleneimine or polyacrylamide.
Layers 5 and the layers consisting of iron particles 3 and
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;~5~ >~
binder 2 have a combined thickness between approximately
0.05 and 0.3 ~m. Coating 4 of iron par-ticles 3 can be
composed in the same manner and of precisely the same
thickness as in the case of the coated iron particles in the
embodiment of Fig. 2. In the embodiment of Fig. 3 the
surface density of the particles is again approximately 10
to 101 particles/cm2.
The method as disclosed by the invention uses many of the same
process steps - as are used in the production of known
magnetic disks. Thus, according to the invention, the
magnetic particles and the binder which optionally includes
abrasion-resistant particles are applied onto the substrate.
The binder used has a low-molecular state, and is mixed with
a solvent or a solvent mixture. The mixture has a low
viscosity so that it can be applied onto the substrate by
means of spinning. After the deposition of the binder-
containing mixture the solvent is evaporated and the binder
is cured or polymerized. In a las-t process step there
usually follows the polishing of the then present magnetic
coating. The method as disclosed by the invention essentially
differs from the known method of making magnetic disks in
that the particles applied onto the substrate in the form of
iron oxide are reduced to iron, directly after application
in those cases where the magnetic particles are deposited
individually, i.e. without the binder, and after the binder
curing where the magnetic particles are applied dispersed in
the binder.
In the following~ the production of the embodiments of the
magnetic record carrier as disclosed by the invention as
represented in Figs. 1 to 3 will be described, with the
record carrier produced being assumed to be a magnetic disk with
a rigid substrate in each instance. The manufacturing
processes described however can be applied without modification
to the production of magnetic disks with a flexible substrate.
In making magnetic tapes, only the process for depositing
the binder-containing mixture has to be modifiedO
GE9-85-002
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In the embodiment according to Fig. 1, iron oxide particles
are disperseA together with the binder, as e.g. an epoxide
resin preparation, usually abrasion-resistant particles,
e.g. of A12O3 and optionally admixtures like a cross-linking
agent in a solvent or solvent mixture. Such dispersions are
produced in agitators, ball or bead mills. ~he dispersion
preferably consists (in percent by weight) of approximately
60 ~ of a solvent mixture (mixture consisting of xylol,
ethylamylketone and isophorone), and approximately 40 ~ of a
solid substance mixture (approximately 50 % binder, approxi-
mately 49 % Fe2O3 and approximately 1 % A12O3). The finished
dispersion is used to coat the magnetic disk substrate
preferably consisting of an aluminum magnesium alloy, and
subsequently the desired layer thickness (preferably approx-
imately 0.3~Lm) is reached by means of spinning-off. During
the coating process, a permanent magnet is preferably
arranged under the substrate, and the dispersion is applied
in the location with the gap of the magnet beneath, i.e. in
the place with the highest field intensity. The magnetic
particles are aligned in rotation direction in parallel to
the substrate surface. There follows the curing of the
applied magnetic coating by heating for approximately
2 hours to a temperature in the order of 200C. Subsequently,
at a temperature between 250 and ~00C, preferably 325C,
the iron oxide particles are reduced to pure iron in a
hydrogen atmosphere. Reduction can also be effected in an
atmosphere containing nitrogen and hydrogen, but the reduction
times would then have to be extended, and the temperatures
used would have to be raised. The binder employed has to be
so resistant from a chemical and thermal point of view that
it does not decompose under reduction conditions. This
applies to the epoxide resin mentioned as an example. Other
binders satisfying this condition are polyimide, melamine,
polyamide and polypyrrolidine. The modifications of the
magnetic characteristics caused by the reduction are listed
in the first column of the table below. The last process
step is the polishing of the surface of the magnetic coating.
Finally~ the magnetic coating is equipped with a lubricant
possibly after a cleaning step.
GE9-85-002
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To make -the embodiment of Fig. 2, the iron oxicle particles
are first coated with an SiO2 or SiO2/Al2O3 layer. An
SiO2/Al2O3 coating is preferably made in that the iron oxide
particles are dispersed in water and subsequently processed,
with predetermined PH values in the basic range being
maintained and at increased temperatures first with a
silicate and subsequently with an aluminum salt solution.
SiO2/A12O3 layers can be produced on iron oxide particles as
follows:
45 g Fe2O3 particles of a uniform length, preferably between
O.1 and 1.0j~m, which are commercially available, are heated
with 300 ml distilled water, and stirred, at approximately
20,000 rpm to 90C. The P~l is adjusted by the drop-by-drop
admixture of an NaOH solution at 10Ø After the admixture
of 20 ml of a 40 ~ sodium si]icate solution the PH is
rapidly lowered to 9.0 by the drop-by-drop admixture of
hydrochloric acid. Stirring is continued for one hour, with
the temperature of 90C being maintained and deviation from
the PH of 9.0 + 0.1 being corrected by the admixture of NaOH
or HCl, respectively. After the expiration of the reaction
time, 20 ml of a 50 ~ aluminum sulfate solution are admixed
very rapidly, and the PH is immediately reduced to 8.0 by
means o-F hydrochloric acid admixture. After another 60 minutes
reaction time, with stirring being continued and a reaction
temperature between 93 and 95C being maintained, the
process is terminated. The -thus coated iron oxide particles
are filtered, thoroughly rinsed with distilled water, dried
at 150C, and finally pulverized.
For making an iron oxide particle coating consisting of
colloidal silicon dioxide an advantageous method consists in
that an aqueous suspension of the iron oxide particles and
an aqueous suspension of colloidal silicon dioxide particles
is made. In the first mentioned suspension, the PH is fixed
at a level where the iron oxide particles receive a positive
elec-trostatic charge. The suspension containing silicon
dioxide particles is set to the same PH' with the effect
that the silicon dioxide particles adopt a negative elec-
trostatic charge. If both suspensions are joined together,
GE9-85-002
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the iron oxide particles are covered with the silicon
dioxide particles with which they irreversibly bond. This
method is known from US-Patent 4 2~0 918. The individual
process steps are as follows:
y-Fe2O3 particles of the desired granular size are mixed
with a suitable acid, e.g. hydrochloric acid, and the
mixture is stirred for a time. The iron oxide particles have
a tendency to form aggregates. Stirring facilitates the
separation of the aggregates into individual particles as
the bridges between the particles are dissolved. After
stirring, the PH Of the mixture is adjusted to a suitable
value in order to generate a positive electrostatic charge
on the magnetic particles. Iron oxide particles have a
pronounced positive electrostatic charge in the PH range
between 3 and 6. For that reason the PH Of the mixture is
adjusted to a value within this range. Furthermore, a
mixture of colloidal silicon dioxide particles and an acid
is made. The PH is adjusted to approximately the same value
as in the mixture containing the magnetic particles. Colloidal
silicon dioxide particles have a considerable negative elec-
trostatic charge in the PH range between 3 and 6. The two
mixtures are cleaned and subsequently stirred, preferably
with an additional ultrasonic energization to support the
reaction. The colloidal silicon dioxide particles with their
negative electrostatic charge are attracted by the iron
oxide particles which have a positive charge. Preferably,
the mixture contains more colloidal silicon dioxide particles
than necessary in order to completely cover the iron oxide
particles. This surplus is advantageous because it supplies
a sufficient amount of silicon dioxide particles for rapidly
coating an iron oxide particle separated from other iron
oxide particles before it can again be attracted by other
iron oxide particles. After coating, the iron oxide particles
are irreversibly connected with the adsorbed mono-layer of
protective colloidal particles, and they are thus sufficiently
distanced from each other so that their mutual magnetic
attraction and thus their tendency to form aggregates has
been considerably reduced.
GE9-85-002
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Exemplified embodiment demonstrating the method wi-th yet
further detail: -
5 g y iron oxide powder was mixed with 50 ml of a 5 % HCl,
and exposed for 3 minutes to an ultrasonic treatment with a
power of 400 watts. Subsequently, 12 ml of concentrated H~l
were admixed, and the mixture was stirred for 40 minutes.
The iron oxide particles were then rinsed with water until a
PH of 3.5 had been reached. Furthermore, a suspension
containing 5 g colloidal silicon dioxide particles was mixed
with a cationic ion exchange resin, and stirred, until a PH
of 3.5 was reached. As an alternative it would have been
possible to implement the PH change by the admixture of
dilute sulfuric acid or hydrochloric acidO The ion exchange
resin was removed by means of filtration, and the suspension
of the colloidal silicon dioxide particles was admixed to
the iron oxide particle suspension. The mixture was then
exposed for 10 minutes to ultrasonic treatment with a power
of 400 watts. A silicon dioxide particle surplus and other
non-magnetic material was then removed by means of magnetic
sedimenta-tion. Subsequently, the P~l f the mixture was
increased to a value in the vicinity of 9.5, first by the
admixture of water and various decanting operations, and
then by the admixture of a suitable base, as e.g. sodium
hydroxide. Finally, the coated iron oxide particles were
filtered and dried.
The coated iron oxide particles are used for making a
magnetic disk as represented in Fig. 2. The process applied
is exactly the same as for making the magnetic disk of Fig.
1, with the only difference that for making the embodiment
of Fig. 1 the iron oxide particles are not coated. The
alterations of the magnetic properties thus caused in
reduction are listed in the second column of the table.
For realizing the embodiment depicted in Fig. 3 of the
magnetic record carrier as disclosed by the invention,
coated iron oxide particles that had been made as described
above are applied onto the substrate as a mono-layer. The
process of deposition is described in U.S. Patent ~,333,961.
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The substrate has to be prepared for receiving the coa-ted
iron oxide particles. This preparation is efEected by
applying an active layer onto the substrate. The process of
deposition is based on an electrostatic interaction between
ionic particles with a surface of an opposite ionic charge, in
an aqueous environment. This requires a charged surface
layer or active layer. There exist several ways of making
such an ac-tive layer.
In accordance with one of these ways the active layer is
generated by dissolving an epoxide resin or a polyamide in
cyclohexanone and spinning the solution onto the substrate.
An epoxide resin known as Epon 1004 and marketed under that
trademark by Shell Corporation, can be used. A polyamide
known as Versamid 100 marketed under that trademark by
Henkel Corporation may also be used. The deposited layer is
approximately 50 nm thick. The layer is tempered at
approximately 150C for about 30 minutes in order to partly
polymerize it, or to reduce the percentage of the
low-molecular fraction, respectively without all NH2- and
NH-groups being eliminated. The NH2- and NH-groups are the
source of the positive surface charge in an aqueous
environment.
Alternatively, the active layer can be produced by applying
a mono-layer of a cationic polyelectrolyte out of an aqueous
solution such as polyethyleneimine or polyacrylamide onto
the substrate. Subsequently, the surplus is rinsed off, and
the forming of the active layer is thus finished. The
active layer can also be made by applying methods like
ultraviolet grafting or plasma polymerization. To deposit
the magnetic particles on the disk substrate, after the
generation of the active layer, it is preferable to place
the substrate onto a slowly rotating coating unit which is
made to rotate with a speed between 0.1 and 5 rpm over the
gap of a permanent magnet. The iron oxide particles coa-ted
with silicon dioxide or SiO2/A12O3 are injected or pumped in
the form of an aqueous dispersion onto the substrate over
the area of the highest magnetic field intensity of the magnet.
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The magnetic particles are aligned in -the direction of
rotation in parallel to the substrate surface. The magnetic
particles are oriented while they still exist in their
dispersed form prior to the actual coating process. The
negatively charged particles bond with the positively
charged active layer on the substrate owing to electrostatic
interaction. As soon as -the particles are placed on the
surface they remain there. Owing to the electrostatic
repulsion between the magnetic particles, any magnetic
particle surplus is easily rinsed off. The coated substrate
is then dried by spinning, and heated to approximately 150C
for abou-t 15 rninutes. The process described can be modified
so that the substrate is first coated with a thin water
film, and the process is then continued as specified above.
The deposited layer has a high pigment volume concen-tration,
and good magnetic orientation. The magnetic particles do
not show any agglomeration tendency.
In the next process step, the coated iron oxide particle layer
is exposed to a hydrogen atmosphere at an increased temperature.
The layer is preferably exposed for one hour at a temperature
of 350C to a hydrogen stream of 30 cm3 per minute. After
reduction, the modifications to the magnetic characteristics
caused by the reduction were measured for seven samples. The
measured values are listed in the last column of the table.
A microscopic examination of the layer revealed that the
reduction had in no way altered the position of the particles
in the layer.
Onto the structure existing after reduction a dispersion is
applied which, apart from the absence of the magnetlc
particles, is llke the dispersion used for the production of
the magnetic record carriers shown in Eigs. 1 and 2. Subse-
quently, the layer containing the binder is cured, at a
temperature of approximately 250C for approximately 1 hour.
The conditions (viscosity of the dispersion, rotational
speed) are selected in such a way that the material deposited
on the substrate after curing is between approximately 0.05
and 0.3~m, preferably about 0.1~m thick. The structure now
GE9-85-002
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available is subsequently polished. Owing to the application
of the binder-containing layer the magnetic coating now has
excellent mechanical stability and abrasion-resistance. The
magnetic characteristics are not altered by such application.
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