Stock material or miscellaneous articles – Circular sheet or circular blank – Recording medium or carrier
Reexamination Certificate
1998-09-21
2001-05-22
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Circular sheet or circular blank
Recording medium or carrier
C428S216000, C428S328000, C428S690000, C428S690000, C428S690000, C428S900000, C427S548000
Reexamination Certificate
active
06235368
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium consisting of at least two layers which are cast on a nonmagnetic substrate, the lower layer containing a binder and at least one acicular pigment and the upper layer containing a binder and a ferromagnetic metal pigment and having a thickness of not more than 0.4 &mgr;m. The present invention also relates to a process for the production of such magnetic recording media which have no preferred magnetic direction.
BACKGROUND OF THE INVENTION
With modern magnetic recording media, there is a constant desire for higher storage capacities. faster access times and higher transmission rates of the stored information. For magnetic recording media, these requirements mean on the one hand a continuous increase in the relative speed between medium and magnetic head and on the other hand a continuous reduction in the thickness of the magnetic layer. For example, magnetic recording media having a high storage capacity now have magnetic layers which are less than about 1 &mgr;m thick and relative speeds between recording medium and head which are in the region of several meters per second.
Particularly in the case of data diskettes with storage capacities of 100 megabytes or more, the thickness of the recording layer is already substantially below 0.5 &mgr;m with relative speeds of more than 10 m/s. In this type of magnetic media, the magnetic head frequently remains on one and the same track during use, setting extreme requirements with respect to the abrasion resistance of the recording layer.
To meet this requirement, magnetic recording media in which a binder-free ferromagnetic metal layer was applied in a very small thickness by means of a vacuum technique have been developed in recent years. Although these metal evaporated recording media achieve a very high playback level, mass production of such media still presents considerable difficulties in comparison with magnetic recording media in which the magnetic pigments are dispersed in binders. Moreover, these ME tapes change under the influence of atmospheric oxygen. However, it has recently been possible to meet the requirement for small layer thickness also by means of a thin magnetic layer in which the finely divided magnetic particles are dispersed in a polymeric binder matrix and in which this layer is cast onto a nonmagnetic lower layer. Such recording media are described, for example, in U.S. Pat. No. 2 819 186, DE-A 43 02 516, EP 0 520 155, EP 0 566 100, EP 0 566 378, EP 0 682 802 and DE-A 44 43 896, 195 04 930, 195 11 872, 195 11 873, 195 11 875 and 195 11 876.
The abovementioned magnetic recording media describe media which have a two-layer structure and in which the upper, magnetic layer has a thickness of from 0.01 to slightly below 1 &mgr;m, preferably 0.1-0.4 &mgr;m. The thickness of the lower, nonmagnetic layer is 0.5-8 &mgr;m. The upper layer preferably contains finely divided magnetic metal or metal alloy particles while the lower, nonmagnetic layer contains finely divided nonmagnetic pigments which in some cases have an acicular structure, as described, for example, in the abovementioned publications 20 EP 0 566 378 and EP 0 682 802. From a comparison of the thicknesses of the part-layers, it is clear that the mechanical properties are essentially determined by the thicker lower layer.
In conventional recording media, the mechanical stability and the abrasion resistance of magnetic recording media can be increased by adding acicular pigments in contrast to pigments of isotropic shape, particularly when the pigments are arranged in a suitable manner. In the case of media which have a preferred direction, such as, for example, tape-like magnetic recording media, an arrangement in this direction is advisable, as described in the PCT application 96/30900 of the same Applicant. In the case of acicular magnetic pigments, this orientation can be further substantially improved by orienting the pigments in a strong magnetic field.
To ensure that the required high recording density is achieved, acicular pigment particles can be used only in the nonmagnetic lower layer for improving the mechanical properties. In this application, the orientation of the nonmagnetic acicular pigment particles is supported by the coating process itself since, owing to the shear gradient inevitably produced during application, some of the pigment needles are oriented in the coating direction.
However, in the case of disk-shaped recording media, i.e. floppy disks, an anisotropic orientation is harmful since the relative direction of movement of the magnetic head relative to the preferred direction of the pigment needles changes continuously during one revolution. Here on the contrary it is important as far as possible to ensure no anisotropic properties of the recording medium, i.e. neither mechanical nor magnetic.
It is an object of the present invention to provide a magnetic recording medium of the generic type stated at the outset which, in the ready-to-use state, has neither magnetic nor mechanical anisotropy and at the same time has improved abrasion resistance and good mechanical stability.
SUMMARY OF THE INVENTION
We have found that this object is achieved, according to the invention, by a magnetic recording medium which is composed of at least two layers, the lower layer containing at least one acicular magnetic pigment and, after coating and before drying, said magnetic recording medium being disoriented by magnetic treatment so that the magnetic pigments are present in random distribution in the layer.
DETAILED DESCRIPTION OF THE INVENTION
The invention, i.e. the composition of the magnetic recording medium and the processes for its production, is described in more detail below.
Known films of polyesters, such as polyethylene tere-phthalate or polyethylene naphthalate, and polyolefins, cellulose triacetate, polycarbonates, polyamides, poly-imides, polyamidoimides, polysulfones, aramids or aromatic polyamides are suitable as nonmagnetic flexible substrates.
According to the invention, the lower layer contains acicular magnetic pigments whose magnetic values are adjusted so that they do not interfere with the recording of the upper magnetic layer. Such pigments and their preparation are described in more detail in PCT application 96/30900 of the same Applicant for the case of a magnetic recording medium oriented in the longitudinal direction or in the casting direction. These are preferably pigments of the formula
Cr
a
O
x
nH
2
O
where
a is the average valency of the chromium with 3.1<a≦3.7
x is the oxygen equivalent resulting from the valency of the chromium
n is the water content with 0<n<(4−a)/2
having a ratio of the average length of the needles to the average diameter of the needles of from 3:1 to 12:1 and a length of from 50 to 150 nm. The specific surface area is from 50 to 100 m
2
/g and the coercive force H, is from 5 to 40, preferably from 5 to 25, kA/m. The preparation of these pigments is described in more detail in said PCT application so that there is no need to discuss them in more detail at this point. To ensure that the magnetic particles of the lower layer—referred to below as chromium oxide—move sufficiently in the magnetic disorientation zone, which will be described in more detail below, they must have an Hc of at least 25 kA/m and a specific powder magnetization of from 20 to 70 nTm
3
/g.
The further components of the lower layer, such as polymeric binders, further pigments, antistatic agents, carbon blacks, lubricants, crosslinking agents, solvents, wetting agents, and dispersing assistants, are described in more detail in the abovementioned documents. For example, polyurethanes and vinyl polymers which, in a preferred embodiment, have polar groups, for example sulfonate groups, can be used as polymeric binders. Furthermore, lubricants, such as fatty acids or fatty esters, crosslinking agents, such as polyisocyanate, and wetting agents and solvents, such as tetrahydrofuran, methyl ethyl ketone, cyclohexanone or dioxane
Jakusch Helmut
Stehle Peter
Traub Hansjorg
Wiegemann Hans-Bertram
EMTEC Magnetics GmbH
Keil & Weinkauf
Resan Stevan A.
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