Coating processes – Magnetic base or coating – Magnetic coating
Reexamination Certificate
2001-05-04
2004-05-25
Pianalto, Bernard (Department: 1762)
Coating processes
Magnetic base or coating
Magnetic coating
C427S131000, C427S132000, C427S345000, C427S358000, C427S385500, C427S407100
Reexamination Certificate
active
06740353
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for producing a magnetic recording medium having a non-magnetic layer on a non-magnetic support and having a magnetic layer on the non-magnetic layer.
BACKGROUND ART
In recent years, the magnetic recording medium such as VTR tapes, audio tapes, computer tapes, etc. is required for high-density recording for increased performance, recording longer time, and reduced size and weight, and a magnetic recording system in which an MR head and a magnetic tape are combined is begun to be investigated.
As recording density is higher, a magnetic layer is thinning. For example, Japanese Examined Patent Publication No. 5-59490 (1993) discloses a simultaneous overlapping coating method of a non-magnetic lower coating layer and a magnetic layer onto a non-magnetic support with regard to a process for producing a magnetic recording medium having a thin magnetic layer with a thickness of 2 &mgr;m or less.
However, if a magnetic layer having a thickness of 0.08 &mgr;m or less is tried to be obtained by such a wet-on-wet coating, nonuniformity of a thickness of the magnetic layer has a significant impact on output variations.
For cases with a combination with the MR head reproducing system taken into account, for example, in Japanese Laid-open Patent Publication No. 10-312525 (1998), there is disclosed that the saturated magnetic flux density of the magnetic layer with the hexagonal ferrite powder is 300G to 1000G and the coercive force is 2000 Oe or more, or the saturated magnetic flux density of the magnetic layer with ferromagnetic metal powder is 800G 1500G and the coercive force is 2000 Oe or more. In Japanese Laid-open Patent Publication No. 10-302243 (1998), there is disclosed that the projection height on the magnetic layer surface and the reversal-volume of magnetization are prescribed and the coercive force of the magnetic layer is 2000 Oe or more.
However, in both Publications mentioned above, there is no detailed description in the relationship suited for an MR head between magnetic characteristics and a thickness of the magnetic layer, and there has not yet been obtained a magnetic layer having a sufficient sensitivity suited for the MR head. Also, according to Examples of the both Publications, though a magnetic layer having a thickness of 0.15 &mgr;m is prepared by the simultaneous overlapping coating method, it is difficult to thin a layer with a higher uniformity.
DISCLOSURE OF THE INVENTION
Objects of the Invention
Thus, an object of the present invention is to provide a process for producing a magnetic recording medium having a non-magnetic layer on a non-magnetic support and having a magnetic layer on the non-magnetic layer, by solving problems of the aforesaid prior art. Also, an object of the present invention is to provide a process for producing a magnetic recording medium having a magnetic layer that is a coating-type extremely thin layer and has a less thickness variation. Further, an object of the present invention is to provide a process for producing a magnetic recording medium suited for an MR (magneto-resistive) head reproducing system.
SUMMARY OF THE INVENTION
The present invention relates to a process for producing a magnetic recording medium having a non-magnetic layer on a non-magnetic support and having a magnetic layer on the non-magnetic layer.
The present invention is a process for producing a magnetic recording medium which comprises:
applying a non-magnetic layer coating material onto a non-magnetic support and drying the coating material to form a non-magnetic layer, and then
applying a magnetic layer coating material more excessively than an intended magnetic layer-wet thickness onto the non-magnetic layer by using a die nozzle coating followed by scraping excess amounts of the magnetic layer coating material to the intended magnetic layer-wet thickness by means of a bar to form a magnetic coating layer.
In the present producing process, it is preferable to apply 2 to 20 times as large amounts of the magnetic layer coating material as the intended magnetic layer-wet thickness onto the non-magnetic layer by using the die nozzle coating.
In the present producing process, a solid component concentration of the magnetic layer coating material is preferably 10% by weight or less.
In the present producing process, it is preferable to disperse the magnetic layer coating material again by means of an online dispersion apparatus immediately before applying the magnetic layer coating material onto the non-magnetic layer.
In the present producing process, after the non-magnetic layer coating material is applied onto the non-magnetic support and dried, it is preferable to cure the coating material.
In the present producing process, it is preferable that a radiation curing type binder resin is contained in the non-magnetic layer coating material and, after applying the non-magnetic layer coating material onto the non-magnetic support and drying the coating material, radiation curing of the coating material is carried out.
MODES FOR CARRYING OUT THE INVENTION
A magnetic recording medium produced in the present invention has a lower non-magnetic layer between a non-magnetic support and a magnetic layer as a coating layer constituent. On the underside of the non-magnetic support, a back-coat layer is formed as required. Alternatively, the magnetic layers may be formed on both sides of the non-magnetic support. Here, in the present invention, a lubricant coating layer and/or various coating layers for protecting the magnetic layer may be formed on an upper magnetic layer as required. Also, on the surface of the non-magnetic support, on which the magnetic layer is to be formed, an undercoat layer (adhesion facilitating layer) may be formed for improving adhesion between the coating layer and the non-magnetic support.
[Lower Non-magnetic Layer]
In the present invention, when a thickness of a magnetic layer is extremely thin such as, for example, 0.02 to 0.08 &mgr;m, the magnetic layer alone suffers a lack of a lubricant in the coating layer. Accordingly, role of a lower non-magnetic layer is to allow the non-magnetic layer contain a lubricant and supply the lubricant from the lower non-magnetic layer to the magnetic layer. Further, a surface roughness of a support film may be extensively mitigated by the intervention of this lower non-magnetic layer, and as a result, a surface smoothness of the magnetic layer is improved.
Therefore, it is desirable to allow the non-magnetic layer contain a carbon black. By allowing the non-magnetic layer to contain the carbon black, the lubricant can be held in the layer. It also has a role to lower a surface electric resistance of the magnetic layer.
For the carbon black contained in the non-magnetic layer, which is not particularly limited, a carbon black having an average particle size of about 10 to 80 nm is preferable. For such carbon black, the known one such as furnace carbon black, thermal carbon black, acetylene black, etc. may be used. Further, it may be used in a single or in a combination.
The BET specific surface area of such carbon black is 50 to 500 m
2
/g, preferably 60 to 250 m
2
/g. The carbon black that can be used in the present invention may be referred to “Carbon Black Almanac,” compiled by the Society of Carbon Black.
For the non-magnetic layer, in addition to carbon black, other non-magnetic powders may be used in combination. Examples of the other non-magnetic powders, which is not particularly limited, include &agr;-iron oxide, &agr;-alumina, Cr
2
O
3
, SiO
2
, ZnO, TiO
2
, ZrO
2
, SnO
2
and the like. Among these, when a needle-like &agr;-iron oxide having an average major axis length of 200 nm or less or a granular &agr;-iron oxide having an average particle size of 20 to 100 nm is used in combination with the carbon black, thixotropy properties of the coating material prepared by the only carbon black can be alleviated, and the coating layer can be hardened. In addition, for an abrasive, when &agr;-alumina or Cr
2
O
3
, having an average par
Pianalto Bernard
Rader & Fishman & Grauer, PLLC
TDK Corporation
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