Coating processes – Electrical product produced – Carbon coating
Reexamination Certificate
2003-06-24
2004-12-07
Pianalto, Bernard (Department: 1762)
Coating processes
Electrical product produced
Carbon coating
C427S131000, C427S132000, C428S900000
Reexamination Certificate
active
06827968
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a magnetic recording medium and, in particular, a process for producing a high density magnetic recording medium with excellent productivity.
2. Description of the Related Art
A recent trend in magnetic recording media has been for the recording density to increase and the recording wavelength to shorten. With regard to the magnetic recording media, there is a high demand for reducing the spacing loss by leveling the surface of a magnetic layer. As one method for leveling the surface of the magnetic layer, there is a method disclosed in JP-A-63-191315 (JP-A denotes a Japanese unexamined patent application publication) and JP-A-63-187418 in which a non-magnetic layer as a lower layer and a thin layer as an upper layer are applied by a simultaneous double layer coating method. As a method for further leveling the surface in this layer structure, there is a method in which an acicular powder is used as a lower layer non-magnetic powder, and a method in which titanium oxide, &agr;-iron oxide, etc. is used as a main powder, and fine particles of carbon black are used as a secondary powder, thus adjusting the liquid viscosity and thereby suppressing interfacial disturbance during double layering.
Furthermore, there is a method in which voids are maintained during formation of a coating by reducing the amount of binder in the lower non-magnetic layer, thus improving the calendering performance during a calendering treatment and thereby leveling the surface.
However, when non-magnetic liquids used in these methods are subjected to a high dispersion treatment by means of a sand mill, etc. using a high specific gravity dispersing medium (zirconia beads, etc.), the liquid viscosity tends to increase during the dispersion treatment or during storage after the dispersion treatment. This causes the problems of it becoming impossible for the liquid to be fed during the dispersion treatment and it being necessary to disassemble pipes for cleaning when halting the dispersion treatment, thus greatly decreasing the productivity due to the liquid loss and the increased operational load.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a high productivity process for producing a high density magnetic recording medium and, more particularly, to provide a process for producing a magnetic recording medium in which increased density and productivity can be achieved at the same time, and smoothness of the surface of a magnetic layer and suppression of the viscosity of a non-magnetic liquid can be achieved at the same time.
As a result of an intensive investigation by the present inventors into a process for producing a non-magnetic coating solution in order to achieve the above-mentioned object, a high productivity process for producing a magnetic recording medium has been accomplished by the production process described below, in which the surface of a magnetic layer of the magnetic recording medium is smooth, the medium exhibits good electromagnetic conversion characteristics, the low shear viscosity of the non-magnetic liquid is low, and the liquid loss is small.
That is, the present invention provides a process for producing a magnetic recording medium comprising a non-magnetic flexible support, a non-magnetic layer comprising a non-magnetic powder and a binder, and a magnetic layer comprising a ferromagnetic powder and a binder, the non-magnetic layer being provided on top of the support, the magnetic layer being provided on top of the non-magnetic layer, and the non-magnetic powder comprising two or more types of non-magnetic powder including carbon black and a non-magnetic powder other than carbon black, the process comprising a step of separately dispersing a non-magnetic liquid A and a non-magnetic liquid B, the non-magnetic liquid A comprising a binder and a non-magnetic powder other than carbon black and the non-magnetic liquid B comprising carbon black and a binder, and a subsequent step of stirring and mixing the non-magnetic liquid A and the non-magnetic liquid B to prepare a non-magnetic coating solution.
DETAILED DESCRIPTION OF THE INVENTION
The non-magnetic powder used in the production process of the present invention comprises two or more types of non-magnetic powder, including carbon black and a non-magnetic powder other than carbon black. The essential steps of the production process of the present invention include a step of separately dispersing a non-magnetic liquid A comprising a binder and a non-magnetic powder other than carbon black, and a non-magnetic liquid B comprising carbon black and a binder, and a subsequent step of stirring and mixing the non-magnetic liquid A and the non-magnetic liquid B to prepare a non-magnetic coating solution. Dispersing the non-magnetic liquid A and the non-magnetic liquid B separately can suppress the viscosity and the liquid loss of the non-magnetic liquids.
In the present invention, when the non-magnetic liquid A and the non-magnetic liquid B are stirred and mixed continuously without being kept stationary after they are separately dispersed, the two non-magnetic liquids can be uniformly mixed by stirring and mixing at a peripheral speed of 3 m/sec or higher.
On the other hand, when there is a period of 1 min or more between the time when dispersion of the non-magnetic liquid B is completed and the time when stirring and mixing of the non-magnetic liquid A and the non-magnetic liquid B is started, since the carbon black in the non-magnetic liquid B reaggregates to a large extent, it is difficult to break apart the aggregated carbon black by stirring and mixing at a peripheral speed of 18 m/sec or less, and the two non-magnetic liquids cannot be mixed uniformly. This causes the problems of D.O. and errors, due to spacing loss resulting from the surface roughness and the aggregates.
In order to break apart the aggregated carbon black, in the present invention it is preferable to mix and stir the non-magnetic liquids A and B at a peripheral speed of at least 25 m/s. Stirring and mixing at a peripheral speed of at least 25 m/s is effective when there is a period of 1 min or more between the time when dispersion of the non-magnetic liquid B is completed and the time when stirring and mixing of the non-magnetic liquid A and the non-magnetic liquid B is started.
In the present invention, it is preferable to use, as a high speed stirring machine, a thin-film spin system high speed mixer disclosed in JP-A-9-75698, JP-A-11-347388 (Japanese registered patent No. 3256801), and T. Asa, ‘21 Seiki no Kakuhangijutsu (Stirring technology in the 21st century)’ (Published by Kogyo Chosakai Publishing Co., Ltd on 20 Nov. 2000). It is generally known that in a conventional high speed stirring machine, cavitation occurs at a peripheral speed of 20 m/s or more and a shearing force cannot be applied to the liquid. Use of the thin-film spin system high speed mixer enables high speed stirring to be carried out at a peripheral speed of 25 m/s or more.
The thin-film spin system high speed mixer preferably used in the present invention is a stirring device having a container, a rotating shaft provided in the center of the container, and one or more stirrers mounted on the rotating shaft, the container having a circular cross section and being capable of containing a liquid that is to be treated by stirring. The stirrer has a radius that reaches the vicinity of the inner peripheral surface of the container, and the machine further includes a drive source for driving the stirrer at a high speed of at least 20 m/sec. By rotating the liquid being treated in response to the rotation of the stirrer, the liquid being treated is pressed against the inner surface of the container by centrifugal force and is rotated in a hollow thin film state. There can be one or plural stirrers according to the size of the container, but providing a plurality thereof enables the liquid being treated to be dispersed uniformly.
Specific examples of the th
Fuji Photo Film Co. , Ltd.
Pianalto Bernard
Sughrue & Mion, PLLC
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