Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
1999-11-03
2001-04-03
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S323000, C428S329000, C428S336000, C428S690000, C428S690000, C428S690000, C428S690000, C428S690000, C428S690000, C428S900000
Reexamination Certificate
active
06210775
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a magnetic recording medium, and more particularly to a thin high-density magnetic recording medium having a magnetic layer of not more than 1.0 &mgr;m in thickness which exhibits excellent electromagnetic characteristics, running properties, durability, and satisfactory productivity.
BACKGROUND OF THE INVENTION
Magnetic recording media comprising a non-magnetic support having thereon a magnetic layer comprising a binder having dispersed therein a magnetic powder, such as ferromagnetic iron oxide powder, Co-doped iron oxide powder, CrO
2
powder, and a ferromagnetic alloy powder have widespread applications as video tape, audio tape, and magnetic discs.
Short wave recording has recently been introduced to meet the demand for an increased recording density. For example, the recording wavelength for 8 mm-video tape has reached 0.54 m&mgr;. With this tendency, there has arisen a problem of so-called thickness loss on reproduction, that is, reproduction output is reduced as a function of increasing magnetic layer thickness.
In order to cope with this problem encountered in short wave recording, magnetic recording media using a thin film of a ferromagnetic metal have been put to practical use which have a very small thickness due to use of formation methods such as vacuum deposition techniques. Such metal-deposited recording media suffer little thickness loss and attain a very high reproduction output. However, production of such thin film magnetic metal-deposited recording media by vacuum evaporation of a metal on a non-magnetic support is less suited to mass production as compared with so-called coated type magnetic layers formed by the conventional coating techniques involving dispersions of ferromagnetic powders in a binder system. In addition, the metallic film is less reliable for long-term use because of susceptibility to air oxidation.
Therefore, it has been attempted to instead reduce the thickness of a magnetic layer formed by various manipulations of the conventional coating technique to thereby increase reproduction output. However, as the thickness of a magnetic layer is decreased to about 2 &mgr;m or less, the surface properties of a support are apt to strongly influence the surface properties of the magnetic layer resulting in deterioration of electromagnetic characteristics.
In order to reduce the thickness of a magnetic layer to minimize thickness loss of magnetic properties thereby achieving a high output while excluding the adverse influences of a support surface, it has been proposed to provide a thick non-magnetic layer between a non-magnetic support and a thin magnetic layer. For example, U.S. Pat. No. 2,819,186 discloses a magnetic recording medium comprising a support having thereon a hard and brittle magnetic layer having a magnetic substance content of 85% by weight or more and a thickness of not more than 0.25 mil as an upper layer and a soft and flexible non-magnetic lower layer having a higher thickness than the upper magnetic layer. JP-A-62-154225 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a magnetic recording medium having a magnetic layer thickness of 0.5 &mgr;m or less with a subbing layer provided between the magnetic layer and the support containing carbon black as a conductive fine powder and having a thickness greater than the magnetic layer so as to prevent surface resistance of the magnetic layer from increasing. JP-A-62-222427 discloses a magnetic recording medium comprising a support having thereon a subbing layer containing an abrasive having an average particle size of from 0.5 to 3 &mgr;m and a 1 &mgr;m or less thick magnetic layer containing a ferromagnetic powder, in this order, in which a part of the abrasive in the subbing layer projects through the magnetic layer so as to serve for cleaning of a magnetic head. Thus, it has been suggested to provide a non-magnetic lower layer immediately adjacent the support to reduce the thickness of a magnetic layer thereby to achieve high-density recording and, at the same time, to incorporate into the lower non-magnetic layer additives such as carbon black for static charge prevention or an abrasive for improvement in cleaning characteristics or durability.
However, conventional techniques for producing these magnetic recording media having a lower non-magnetic layer and an upper magnetic layer involve complicated processes. For example, such processes comprise first coating a non-magnetic layer on a non-magnetic support, then drying the non-magnetic layer, and then, if desired, followed by calendering, and thereafter coating a magnetic layer thereon. However, problems have been identified with these conventional techniques.
For instance, reduction in thickness of a magnetic layer is achieved either by reducing the application rate or by using an increased amount of a solvent in a magnetic binder coating composition to reduce the ultimate film concentration. Yet, when the former approach is taken, drying of the coating transpires too quickly before sufficient leveling can occur to leave surface defects such as streaks or traces of coating pattern, resulting in very poor yield. On the other hand, when the latter approach is taken, a thin coating composition provides a coating film with many voids, resulting in shortage of packing of a ferromagnetic powder or insufficient film strength.
In order to overcome these problems, it has been proposed to form a non-magnetic layer as a lower layer and a thin coat of a highly concentrated magnetic coating composition by a simultaneous coating system. For example, JP-A-63-191315 discloses a magnetic recording medium having a lower non-magnetic layer and an upper magnetic layer formed by simultaneous coating, in which the lower layer has a thickness of 0.5 &mgr;m or more and contains no polyisocyanate.
Extensive studies have hitherto been given to the above-described simultaneous coating system or successive wet coating system, called wet-on-wet coating, for formation of a plurality of magnetic layers. However, the same techniques cannot be applied to the lower non-magnetic layer to obtain satisfactory results. That is, where a lower non-magnetic layer and an upper magnetic layer are formed by wet-on-wet coating, disturbances occur in the interface between the upper and lower layers, causing pinholes or run-away of the magnetic coating composition.
Further, although a thick non-magnetic layer formed beneath a magnetic layer eliminates the influences of the surface roughness of the support, the problem of wearability against a recording head or durability is left unmitigated. The poor wearability or durability of conventional magnetic recording media having a non-magnetic lower layer appears attributable to curing of the lower layer comprising a thermosetting resin as a binder for reasons that the magnetic layer formed thereon is brought into contact with a head or other members without cushioning and that the magnetic recording media having such a cured lower layer lacks the desired flexibility.
This problem might be resolved by using a non-curing (thermoplastic) resin as a binder in the lower layer. However, when a magnetic layer is coated on a dry lower layer containing such a non-curing resin, as in the conventional technique, the lower layer is swollen with the organic solvent of the magnetic coating composition, giving undesired influences, such as turbulence of the magnetic coating composition, leading to impairment of the surface properties of the magnetic layer and deterioration of electromagnetic characteristics.
Furthermore, a magnetic coating composition must be diluted with a relatively large quantity of a solvent before it can be coated to a dry thickness of not more than 1.0 &mgr;m. Such a diluted coating composition is susceptible to agglomeration. Further, orientation of a ferromagnetic powder is apt to be disturbed during drying due to evaporation of the large quantity of organic solvent. When the medium has a non-continuous form, for e
Ejiri Kiyomi
Hayakawa Satoru
Inaba Hiroo
Saito Shinji
Fuji Photo Film Co. , Ltd.
Kiliman Leszek
Stroock & Stroock & Lavan LLP
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