JPH02260224A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain the magnetic recording medium for which hexagonal ferrite powder is used as a recording element and which has a high-density recording characteristic by specifying the thickness of a base film to <7mum and the electric resistance of a back coating layer to <=1X10<6>OMEGA/inch<2>. CONSTITUTION:A polyester film having <=7mum thickness is used as a nonmagnetic base and the back coating layer having 1X10<6>OMEGA/inch<2> electric resistance is formed on the other surface of the base. Namely, even if a conductive material, such as carbon black, is added into the magnetic layer, the amt. of the material to be added is decreased as far as possible to maintain the saturation magnetic flux density of the medium at a high value. On the other hand, the charge electrified on the magnetic layer side moves through the base film to the back coating layer side and the electric resistance is lowered down to the range where are no problems in practicability if the thickness of the base film and the electric resistance of the back coating layer are specified to specific values or below as the means of lowering the electric resistance in this state. The excellent electromagnetic conversion characteristics of the hexagonal ferrite powder are maintained in this way and the medium which is low in the electric resistance and does not generate electrification noises and traveling troubles is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magnetic recording medium that uses hexagonal ferrite powder as a recording element and has excellent high-density recording characteristics.

[Prior Art] In recent years, perpendicular magnetic recording as a magnetic recording method for magnetic tapes and the like has been attracting attention as a method that enables rapid high-density recording, and is being actively researched.

As a magnetic recording medium to which this perpendicular magnetic recording method is applied, one with a magnetic layer made of a thin metal film such as a Go-Cr alloy is seen as promising, but at present the corrosion resistance of the magnetic layer is not sufficiently ensured. Moreover, since the magnetic layer is a thin metal film, it is easily damaged by contact with the magnetic head and has poor durability.

On the other hand, in a magnetic recording medium with a coated magnetic layer that uses hexagonal ferrite powder, which is representative of Ba-ferrite, as a magnetic recording element, the ferrite powder is plate-shaped particles, and the ferrite powder is arranged in a direction perpendicular to the plate surface. Because it has an axis of easy magnetization, it is suitable for perpendicular magnetic recording, and because the magnetic powder is an oxide, it has excellent corrosion resistance.Moreover, the magnetic layer containing a binder has little wear even when it comes into contact with a magnetic head, and has good durability. and
It is considered to be the best overall perpendicular recording medium.

[Problems to be solved by the invention] However, hexagonal ferrite powder has extremely high electrical resistance, and therefore, media using hexagonal ferrite powder also have high electrical resistance, which causes problems such as charging noise and running trouble. Ta. In order to reduce the electrical resistance of this medium, a method is generally used to add a conductive substance such as carbon black to the medium together with hexagonal ferrite powder, but the saturation magnetization of hexagonal ferrite powder is different from that of gamma iron oxide or metal magnetism. Since it is smaller than other magnetic powders such as powder, there is a problem in that when a non-magnetic substance such as carbon black is added, the saturation magnetic flux density of the medium further decreases, resulting in a decrease in output.

The present invention was made to solve the above problems, and an object of the present invention is to provide a magnetic recording medium that can reduce the surface electrical resistance without reducing the saturation magnetic flux density of the magnetic layer.

[Means for solving the problem] The inventors have made this in order to solve the above problem,
The saturation magnetic flux density of the magnetic layer is maintained at a high value by not adding a conductive substance such as carbon black to the magnetic layer, or by minimizing the amount added even if it is added, while at the same time forming a magnetic layer. The surface electrical resistance of the magnetic layer is reduced by controlling the thickness of the support and the electrical resistance of the back coat layer formed on the back surface of the support to be below a certain value. In other words, in order to obtain excellent electromagnetic conversion, conductive substances such as carbon black should not be added to the magnetic layer, or even if they are added, the amount added should be as small as possible to increase the saturation magnetic flux density of the medium to a high value. I thought about keeping it.

On the other hand, in this state, although a high saturation magnetic flux density can be obtained, a problem arises in that the electrical resistance becomes significantly high. Therefore, as a means to reduce the electrical resistance, the thickness of the base film and the electrical resistance of the back coat layer are kept below a certain value.
It has been found that the electric charge on the magnetic layer side moves to the back coat layer side through the base film, and the electrical resistance is reduced to a level that poses no problem for practical use. That is, the thickness of the base film is set to 7 μm or less, and the electrical resistance of the back coat layer is set to I x 1×10 6 Ω/inch” or less.

It has been discovered that the charges on the magnetic layer side move through the base film to the back coat layer side and are discharged on the back coat layer side, resulting in a medium free of charging noise and running troubles. This structure of the medium maintains the excellent electromagnetic properties of hexagonal ferrite powder, resulting in low electrical resistance.
We succeeded in obtaining a medium that does not cause problems such as charging noise and running troubles in practical use.

[Structure and operation of the invention] The magnetic powder of the present invention may be an ordinary substituted hexagonal ferrite powder, for example, the general formula % (where A is at least one member selected from Ba, Sr, Pb, and Ca). element, M2+ is Cu + M n + C
M4+ is a divalent ion of at least one element selected from o + Zn t N 11 M g, and M4+ is Ti.

A tetravalent ion of at least one element selected from Zr and Sn, n is a real number of 3 to 7, X is 0 to 0.2, y is 0 to 0
．． 2.2 is 0.03 to 0.26). The average particle size of the particles is preferably about 0.02 to 0.5 μm; if it is too small, the saturation magnetization will be low, and if it is too large, the surface smoothness of the magnetic M will be reduced. Also, the coercive force is in the range of 200 to 20,000e, and the saturation magnetization is in the range of 55
It is desirable that it be at least ewru/g.

If the coercive force is too small, high recording density characteristics will be insufficient.
If it is too large, it will be difficult to record the signal.

Furthermore, if the saturation magnetization is too small, the output will decrease.

The method for obtaining the above-mentioned magnetic powder is not particularly limited, but for example, an aqueous solution of chloride or the like containing each element constituting the desired substituted ferrite is mixed with an alkaline aqueous solution to form a precipitate, and this precipitate is mixed. 15 in autoclave
After reacting for about 1 to 6 hours at a temperature of about 0 to 350 °C, the reaction product is washed, filtered, and dried to a temperature of about 400 to 1000 °C.
It can be obtained by heat treatment in air for several hours at a temperature of about °C.

The magnetic layer produced using the thus obtained magnetic powder and binder has extremely high electrical resistance. Therefore, in the present invention, the thickness of the base film on which the magnetic layer is formed and the back coat formed on the back surface of the base film are determined. By keeping the electrical resistance of the layer below a certain value, the surface electrical resistance of the magnetic layer is reduced. Therefore, it is usually not necessary to add a conductive substance such as carbon black to the magnetic layer, but a small amount may be added. However, if a large amount is added, the saturation magnetic flux density of the magnetic layer decreases, and there is no point in constructing the medium as in the present invention. In order to obtain a saturation magnetic flux density of about 1400 G or more necessary to obtain sufficient high-density recording characteristics, it is usually preferable to add the non-magnetic substance such as carbon black to about 4% by weight or less.

Next, regarding the base film, it is preferable to use a polyester film in terms of strength and surface properties.

The thickness needs to be 7 .mu.m or less in order to allow the charges accumulated on the magnetic layer side to escape through the base film to the back coat layer side. If it is thicker than this, almost no charge escapes, and the electrical resistance measured from the magnetic layer side becomes extremely high, causing problems such as charging noise and poor running. On the other hand, if it is too thin, the effect of reducing electrical resistance will improve, but the strength of the medium will decrease.

Therefore, the thickness of the polyester base film is usually 4 μm.
It is preferable to make it more than a certain degree.

Next, regarding the electrical resistance on the back coat layer side, I
If the electrical resistance is 1×10 6 Ω/inch or less, the charges accumulated on the magnetic layer side can be discharged. If the electrical resistance is higher than this, the charges can be discharged to some extent, but the effect is insufficient.

On the other hand, there is no particular limitation on the lower limit of the electrical resistance, and it is not too low to cause any problem.

To manufacture the magnetic layer of such a magnetic recording medium, a magnetic paint is prepared by adding and mixing the above-mentioned magnetic powder to an organic solvent along with a binder and various additives as necessary, according to a conventional method. , apply this paint onto the support using a roll coater.
The magnetic layer may be formed by coating with any coating means such as, and drying.

The binders mentioned above include vinyl chloride-vinyl acetate copolymers, polyvinyl butyral resins, cellulose resins, polyurethane resins, incyanate compounds, and radiation-curable resins, which have been conventionally used for magnetic recording media. A variety of types can be used. Examples of organic solvents include toluene, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, tetrahydrofuran,
Conventionally used materials such as ethyl acetate can be used. Further, as additives, lubricants, abrasives, light transmission preventive agents, etc. may be added as necessary.

In addition, for the back coat layer, a paint is prepared by adding and mixing carbon black to an organic solvent together with a binder and various additives blended as necessary, and this paint is applied to the support in the same way as when creating the magnetic layer. What is necessary is just to apply|coat to the back surface by arbitrary coating means, such as a roll coater, and to make it dry and form it. Binder for the above back coat layer, organic solvent,
As additives, those used when producing the magnetic layer can be used.

[Examples] The present invention will be specifically described below based on examples. In addition, in the following, parts mean parts by weight.

Production history Example: As a magnetic powder of the Juho group, coercive force 5400s, saturation magnetization 61.5e
Co--Ti substituted Ba-ferrite magnetic powder having an average particle size of 0.7 μm and mu/g was used. A magnetic paint having the following composition was prepared using this magnetic powder.

Magnetic powder 1000 parts Vinyl chloride-vinyl acetate-100 parts Vinyl alcohol copolymer (manufactured by UCC, VAG) Polyurethane resin 70 parts (manufactured by Dainippon Ink Chemical Co., Ltd., T5201) Trifunctional isocyanate compound 30 parts ( Manufactured by Nippon Polyurethane Co., Ltd.
coronate) myristic acid
20 parts stearin fin-butyl
20 parts toluene 8
00 parts Cylhexanone 800
Part Naori - A paint containing carbon black was prepared by further adding 10 to 40 parts of graphitized carbon black having an average particle size of 0.2 μm to the magnetic paint described above.

This magnetic paint was applied onto various polyester base films of different thicknesses so that the thickness of the magnetic layer after drying would be 2.5 μm, and calender treatment was performed to smooth the surface.

Back coat method A back coat paint was prepared with the following composition.

Granular α-Fe20. Powder 20 parts (average particle size: 0.2 μm) Vinyl chloride-vinyl acetate-100 parts Vinyl alcohol copolymer <manufactured by UCC, VAGH) Polyurethane resin 70 parts (manufactured by Dainippon Ink Chemical Co., Ltd., T520L) Trifunctional isocyanate 30 parts of compound (manufactured by Nippon Polyurethane Co., Ltd., coronated) myristic acid
2-part stearin Mn-butyl
2 parts toluene 60
0 parts cyclhexanone 600 parts Carbon black has an average particle size of 0.2μ
1 of graphitized carbon black was added in a range of 50 to 250 parts. The electrical resistance of the back coat layer was controlled by varying the amount of carbon black added within the above range.

Base Film Various polyester films with different thicknesses ranging from 5.8 μm to 11.7 μm were used.

Table 1 shows the coercive force, saturation magnetic flux density, electrical resistance measured from the magnetic layer side, and running of tapes prepared by changing the amount of carbon black added in the magnetic layer, the thickness of the base film, and the electrical resistance of the back coat layer. The results of examining performance and output are shown below.

The runnability was evaluated based on the runnability when the cut pieces were cut to a predetermined width, assembled into a cassette, and then run back and forth 1000 times at a speed of about im/sec. Further, the output was evaluated from the reproduction output when a 0.7 μm recording wavelength type signal was recorded using a ferrite head with a gap length of 0.2 μm. Note that the output is expressed as a relative output, with the value of the No. 10 tape as OdB.

[Effects of the invention] As is clear from Table 1, when the base film thickness is thicker than 7 μm, the electrical resistance of the back coat layer is reduced to 3×1.
Even if it is reduced to 0'Ω/sq, the electrical resistance does not decrease much, and the electrical resistance measured from the magnetic layer side does not reach the target txt.
o Not less than 11Ω/sq.

In addition, even with a tape using a base film with a thickness of 7 μm or less, in order to obtain the target electrical resistance of less than lXl011Ω/sq, the electrical resistance of the back coat layer must be
It can be seen that it is necessary to make it 10'Ω/sq or less.

Furthermore, adding carbon black to the magnetic layer reduces the electrical resistance to some extent, but the saturation magnetic flux density decreases significantly and the output decreases. It can be seen that in order to obtain a saturation magnetic flux density of 1400 G or more, the amount of carbon black added must be about 4 vt% or less. From the above results, it is clear that the saturation magnetic flux density of the magnetic layer using hexagonal ferrite powder is 14
00G and the electrical resistance measured from the magnetic layer side to prevent charging noise, running defects, etc.
It can be seen that in order to achieve Xl0"Ω/sq or less, the base film thickness must be 7 μm or less, and the back coat layer's electrical resistance must be lx10"Ω/sq or less.

Claims (1)

[Claims] (1) In a magnetic recording medium in which a magnetic layer using plate-shaped hexagonal ferrite powder as magnetic particles is formed on a non-magnetic support, a polyester film with a thickness of 7 μm or less is used as the non-magnetic support. , and by forming a back coat layer with an electrical resistance of 1 x 10^6 Ω/inch^2 or less on the other side of the support, the saturation magnetic flux density of the magnetic layer is 1400 Gauss or more, and the electric resistance measured from the magnetic layer side is A magnetic recording medium having a resistance of 1×10^1^1Ω/inch^2 or less.