Production of a magnetic recording medium

Details Production of a magnetic recording medium Production of a magnetic recording medium

1995-02-01

1997-02-11

Pianalto, Bernard

Coating apparatus

Immersion or work-confined pool type

Running length work, longitudinally traveling

427131, 4274242, 427548, 427549, 427550, 427599, B05D 312

Patent

active

056016476

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for applying one or more magnetizable layers one on top of the other to a flexible nonmagnetic substrate by means of an extrusion coater having at least two coater slots.
There are several methods for the application of such magnetizable layers. A dispersion containing the magnetic pigments can be applied to the film by immersion methods, knife coaters, reverse roll coaters or extrusion coaters. The extrusion coater has the advantage of a closed recycle-free system which meets the relevant cleanliness requirements for the production of a modern high-performance magnetic tape.
Extrusion coaters for the simultaneous application of two layers one on top of the other have been described in EP 0 392 810, DE 40 11 279 and U.S. Pat. No. 5,072,688. Here, two different dispersions flow one on top of the other inside the coating block and emerge from the extruder orifice as a double layer without mixing, directly onto the substrate moving freely past the extruder orifice. The layer fills the void between substrate and coater orifice. The constancy of the wet layer thickness over the web width and length is controlled by the uniformity of the metered amount and the running of the substrate. Owing to the system-related fluctuations in these properties, deviations of up to several .mu.m in the wet layer thickness may occur, which is far too high in the case of wet layer thicknesses of about 10 .mu.m, as required for modern magnetic tapes.
Furthermore, DE 19 07 212 describes an extrusion coating process which avoids the effect of the fluctuating film contact pressure by passing the substrate about 1 mm away from the extruder orifice, over the pole of a magnet. The magnetic dispersion emerging from the extruder orifice is drawn to the substrate by the action of the magnetic edge located directly behind the film web. In this case, the uniformity of thickness over the width still depends on the constancy of the metered amount, which constancy is associated with fluctuations. When precision metering pumps are used, the problem is reduced to the constancy of the amount delivered over the web width. In the apparatus described, moreover, only a single layer can be cast.
In addition, in the production of magnetic recording media, the object is to extend the coating of the magnetic dispersions uniformly over the width of the substrate, in order to avoid thickening of the layer at the edges of the films, due to the cohesive forces of the dispersions. This may be effected, for example, by providing stripping brushes at the coating edges; according to DE 29 36 035, another solution comprises arranging magnetic circles, which are adjusted to an angle of from 0.5.degree. to 10.degree. between coated edge and centre line of the slot, at the coated edges immediately after application of the magnetic dispersion to the substrate. These apparatuses require a significant technical complexity and must furthermore be continuously serviced in order to maintain and to monitor the constancy of the edge coat.
FR 2 321 742 describes a process for the production of multilayer magnetic recording media, slot dies being mounted, between coating means which are a distance apart, for extracting the solvent contained in the magnetic dispersion or for blowing in air to avoid undercoating with solvent.
It is an object of the present invention to provide a process which is capable, particularly at high coating speeds, of applying one or more layers simultaneously, at least one of which contains a layer of magnetic pigments and of ensuring the constancy of the wet layer thickness over the total coating width of .ltoreq.0.5 .mu.m, relatively regardless of effects of the metered amount and the rheology.
It is a further object of the present invention to provide a process of the abovementioned generic type, in which no layer thickening or at most minimum layer thickening occurs at the cast edges, without additional measures in connection with the coated web.
We have found that these objects are achieved, according to

REFERENCES:
patent: 3681138 (1972-08-01), Ankenbrand et al.
patent: 5072688 (1991-12-01), Chino et al.
patent: 5186754 (1993-02-01), Umemura et al.

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