JP2632198B2 - Magnetic recording media - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a magnetic recording medium having a multilayer magnetic layer, and more particularly to a magnetic recording medium having improved electromagnetic conversion characteristics.

[Conventional technology]

Conventionally, a magnetic recording medium such as a magnetic tape generally disperses a ferromagnetic material in a binder dissolved by a solvent on the support while continuously transferring a belt-shaped nonmagnetic support in the longitudinal direction. A coating solution is applied, and then the coating solution is dried and solidified, and then the support is cut.

However, in the production of magnetic tapes, in order to increase the sensitivity and improve the S / N ratio, the ferromagnetic material is aligned in the transport direction of the non-magnetic support, and the squareness ratio (saturation flux density Bm
Therefore, it is necessary to increase the residual magnetic flux density (Br) by the following formula. Therefore, when a magnetic tape or the like is conventionally manufactured, the transfer direction of the non-magnetic support is determined by a permanent magnet or a solenoid while the coating liquid is not dried. In this method, the direction of the easy axis of magnetization of the ferromagnetic material is aligned (oriented) in the transfer direction by applying the above magnetic field.

In recent years, there has been a strong demand for increasing the storage capacity and recording / reproducing output of magnetic recording media, that is, magnetic disks and magnetic tapes.

Needless to say, in order to increase the storage capacity, it is necessary to increase the information recording density per unit area of the magnetic recording medium. On the other hand, to increase the information recording density, the writing magnetic flux generated from the magnetic head must be reduced. Therefore, the magnetic head must be downsized and the amount of generated magnetic flux must be reduced.

In order to invert the direction of magnetization with such a small amount of reduced magnetic flux, it is necessary to reduce the volume of the magnetic layer. Therefore, if the thickness of the magnetic layer is not reduced, complete magnetization reversal occurs. You cannot do that.

For these reasons, it is necessary to make the magnetic layer thinner in order to meet the above demand.

Further, in order to increase the recording / reproducing output, it is necessary to increase the residual magnetic flux of the magnetic layer. One method for this is to increase the thickness of the magnetic layer, but if the magnetic layer is made thicker, the high-frequency characteristics deteriorate. Therefore, in order to increase the residual magnetic flux and improve the high-frequency characteristics, it is necessary to increase the coercive force (Hc) of the magnetic material in addition to reducing the thickness of the magnetic layer. That is, it is conceivable to make the magnetic layer made of a high coercive force magnetic material thin. However, in this case, the high frequency characteristics are improved, but the low frequency characteristics are reduced. Also, if the magnetic layer made of a high coercive force magnetic material is made thicker, a magnetic field acts only on the surface at a high frequency, which makes it difficult to erase.

Therefore, as a magnetic recording medium having these characteristics, a thin magnetic layer made of a high coercivity magnetic material is formed on a relatively thick magnetic layer made of a magnetic material having a relatively low coercive force and a large residual magnetic flux. A magnetic recording medium is ideal. For this reason, conventionally, a production method in which these are divided into two layers and applied and subjected to an orientation treatment has been adopted.

However, when two layers are applied, the characteristics, that is, the squareness ratio, of the thin magnetic coating film having a dry thickness of the upper layer of 2 μm or less are significantly improved as compared with the case of applying the upper magnetic coating film having a thickness of more than 2 μm. Turned out not to be.

Therefore, the cause of this problem is clarified. In the case of a thin magnetic coating film having an upper layer thickness of 2 μm or less, the drying speed is increased because the thickness is reduced, and the viscosity of the coating film increases before a magnetic field is applied. However, it has been found that this is because the ferromagnetic material becomes difficult to move.

Furthermore, when a thin magnetic layer is applied on the dried magnetic layer,
In addition to the above phenomena, it was found that the solvent in the magnetic coating solution was absorbed by the lower magnetic layer, and the viscosity increased sharply, making it difficult for the ferromagnetic material to move.

Therefore, in order to solve the above-mentioned drawbacks, to increase the recording capacity and the recording / reproducing ability, and to provide a method of manufacturing a magnetic recording medium having good sensitivity and SN ratio, a magnetic recording medium is composed of at least two magnetic layers on a non-magnetic support. When a thin magnetic layer having a dry thickness of 2 μm or less as the uppermost layer is provided, at least two or more magnetic layers are simultaneously provided by simultaneous multi-layer coating, and a magnetic field is applied during undulation to achieve high-performance magnetic recording. It has been proposed that a medium can be manufactured (JP-A-62-212933).

In other words, a magnetic layer is formed by applying a magnetic coating solution on a non-magnetic support that runs continuously, providing a magnetic layer, applying a magnetic field to the magnetic layer while the magnetic layer is not dried, and then drying the magnetic layer. In a method of manufacturing a medium, at least two magnetic layers are simultaneously coated in a multi-layer manner, and a magnetic field is applied while both layers are not dried to perform orientation treatment.

However, although the magnetic recording medium thus obtained has the remarkable effect of improving the above characteristics, there is still room for improvement in output, S / N of color signals, and analog audio output.

In a magnetic recording medium having a multilayered magnetic layer,
It is known that lowering the coercive force of the lower layer and increasing the coercive force of the upper layer can improve the frequency characteristics and sensitivity in the low-frequency and high-frequency bands, and this type of magnetic recording medium is usually a polyester film or the like. First, a magnetic paint containing an iron oxide-based ferromagnetic material having a relatively low coercive force is applied and dried to form a first (lower) magnetic layer, and a comparison is made on the lower magnetic layer. It is made by applying and drying a magnetic paint containing a cobalt-containing iron oxide ferromagnetic material having a high magnetic coercive force.

However, if the Hc of the lower magnetic layer is lowered, it is possible to shift the bias peak, but the difference in Hc between the upper layer and the lower layer causes undulation in the frequency characteristics. Therefore, a specific wavelength is good, but if the wavelength deviates even a little, an optimum current cannot be obtained and the output becomes low. Hc of lower layer
Lowering the thickness makes it very difficult to design the thickness of the upper layer.
That is, it has been found that when a thin upper magnetic layer is provided on the unevenness of the thick lower magnetic layer, the thickness of the upper layer tends to fluctuate, and there is a difference in Hc. Therefore, a magnetic recording medium having excellent output and S / N over a wide band from a high band to a low band has been demanded.

[Means and actions for solving the problem]

The object of the present invention is to provide a magnetic recording medium in which a first magnetic layer and a second magnetic layer each containing a ferromagnetic material and a binder are provided in this order on a surface of a nonmagnetic support, wherein the ferromagnetic material has a needle-like iron oxide or alloy. Wherein the first magnetic layer and the second magnetic layer are oriented in the longitudinal direction, the squareness ratio of the first magnetic layer is 0.02 or more smaller than the squareness ratio of the second magnetic layer, and the second magnetic layer Can be solved by a magnetic recording medium characterized by having a thickness of 1.5 μm or less.

In JP-A-62-212933, Hc is greatly increased in the upper layer and the lower layer.
, And the frequency characteristics in a wide band are obtained. Regarding the squareness ratio, since the dispersion time is not particularly changed, the squareness ratio is the same for both the upper layer and the lower layer. In the present invention, the value of Hc is not so large, and the difference in the squareness ratio between the upper layer and the lower layer improves the output and sensitivity in a wide band. That is,
Reducing the squareness ratio is advantageous for the lower layer having a broad bias curve, because the lower layer may have a lower output in the high frequency range, and the optimum value of the recording current is smaller than the upper layer.
In addition, since Hc does not have much difference and has a difference in squareness ratio, even if there is a slight variation in the thickness of the thin upper layer of 1.5 μm or less, it does not immediately cause an output variation.
On the other hand, since the upper layer has a high Hc and a large squareness ratio, the upper layer is designed for short wavelength recording.

 Preferred embodiments of the present invention are as follows.

1) A first layer is coated on a support, and a second layer is coated thereon while in a wet state, and then simultaneously oriented, dried,
A magnetic recording medium obtained by surface treatment.

2) A magnetic recording medium, wherein the squareness ratio of the second layer is 0.86 or more.

3) A magnetic recording medium wherein the coercive force of the second layer is from 400 Oe to 2200 Oe and the coercive force of the first layer is from 1 cup to 0.1 cup of the second layer.

4) A magnetic recording medium, wherein the particle size of the ferromagnetic material in the first layer is larger than the particle size of the ferromagnetic material in the second layer.

5) A magnetic recording medium wherein the acicular ratio of the ferromagnetic material of the first layer is smaller than the acicular ratio of the ferromagnetic material of the second layer.

[Detailed Description of the Invention]

As the ferromagnetic material used in the present invention, known needle-shaped materials can be used, and in particular, iron oxide, cobalt-containing iron oxide,
An alloy containing iron as a main component is preferable because it has high electromagnetic conversion characteristics in all bands.

The difference between the squareness ratios of the first layer and the second layer is 0.02 or more, especially 0.04
The above is preferred. If the difference in the squareness ratio is less than 0.02, the electromagnetic conversion characteristics in a long wavelength range are not improved, which is not preferable. 0.4
The following is preferred.

The needle ratio of the ferromagnetic material used for the first layer is preferably from 2 to 10, particularly preferably from 3 to 8. The needle ratio of the ferromagnetic material used for the second layer is larger than that of the first layer, preferably 3 to 15, and more preferably 4 to 12. These needle ratios may be set when the ferromagnetic material is manufactured, or may be adjusted when the ferromagnetic material is dispersed.

The coercive force of the second layer is preferably from 350 to 2500 Oe,
To 2200 Oe are preferred. The specific surface area is 15m ² / g~80m ² / g, crystal size is preferably 150 to 500 angstroms. The lower ferromagnetic material preferably has a lower coercive force, a smaller specific surface area, a larger crystal size, and a smaller needle ratio than the upper ferromagnetic material.

If the coercive force is less than 350, the high-frequency characteristics of an audio normal position tape are inferior, which is not preferable. Also, 25
If it exceeds 00 Oe, recording and erasing with a normal magnetic head become difficult, which is not preferable. The coercive force of the lower layer is preferably equal to or less than that of the upper layer. For deep recording such as audio, about 1.0 to 0.5 times the upper layer, and for recording mainly on the surface layer such as video and digital, it is preferably 1.0 to 0.1 times.

Conventionally known binders can be used for the binder used in the present invention.

In the present invention, a nonmagnetic powder such as carbon black and an abrasive may be contained in addition to the ferromagnetic material.

In the present invention, the thickness of the upper layer is preferably 1.5 μm or less, more preferably 1 μm or less. The thickness of the upper layer is 1.
If it exceeds 5 μm, no improvement in electromagnetic conversion characteristics is observed, which is not preferable.

The thickness of the lower layer in the present invention is preferably 2.0 μm or more, and more preferably 2.5 μm or more. The thickness of the lower layer
If it is less than 2.0 μm, it is not preferable because the moldability in a calender is reduced or the electromagnetic conversion characteristics are not improved. The thickness of the lower layer is preferably 7.0 μm or less. If it exceeds 7.0 μ, no increase in reduction sensitivity is observed.

The reason why a magnetic recording medium having excellent electromagnetic conversion characteristics can be obtained by the present invention is not clear, but is considered as follows.

Since the squareness ratio of the lower layer is smaller than that of the upper layer, the coercive force of the lower layer alone becomes small and recording is easy. Since the orientation of the lower layer becomes more random and can be separated from the closest packing state of the acicular particles, voids due to entanglement between the particles increase. It is considered that since the space in the lower layer is increased, the moldability during the calendering process is improved, the surface of the magnetic layer is easily smoothed, and the electromagnetic conversion characteristics are excellent.

That is, the squareness ratio of the lower layer is smaller than that of the upper layer, so that the coercive force of the lower layer alone becomes small, and even if the upper layer is separated in thickness from the head, writing can be performed with a weak magnetic flux.

The coating method used in the present invention uses a conventionally known coating method.
Although it is possible to provide a layer by coating, it is difficult to smoothly coat the second layer to 1.5 μm or less for the purpose of the present invention.

Japanese Patent Application No. 62-1246 discloses a coating method free from such disadvantages.
Japanese Patent Application Laid-Open No. 63-146 discloses a simultaneous multi-layer coating method as shown in JP-A-63-146.
As shown in JP-A-209-63 and JP-A-63-146211, it is possible to increase the efficiency of improving the electromagnetic conversion characteristics by layering.

In the present invention, it is preferable to perform the above-mentioned simultaneous multilayer coating.

In the simultaneous multi-layer coating, the orientation is also performed at the same time. Therefore, in order to achieve the object of the present invention, it is necessary to use a coating liquid with low orientation for the lower layer. Such a coating solution is
It can be obtained by using a ferromagnetic material having a small acicular ratio or dispersing a magnetic paint so as to reduce the squareness ratio.

A ferromagnetic material having a small acicular ratio can be obtained by a method such as lowering the alkali concentration during the goethite reaction or adding a reaction inhibitor such as an organic acid. The squareness ratio can also be reduced by a method such as grinding a ferromagnetic material with a Simpson mill or the like, dispersing the magnetic paint with a strong force such as a ball mill, or dispersing for a long time.

When a ferromagnetic material having a relatively large acicular ratio is used, dispersion is performed until the acicular ratio is reduced by dispersion. Alternatively, the particles are dispersed by a dispersing machine in which an impact force is applied such that the needle ratio becomes small.

Alternatively, the dispersion is stopped when the ferromagnetic material is not completely dispersed to the primary particles.

It can be achieved by a method such as using a resin having a relatively low dispersing power as a binder for the lower layer.

〔Example〕

Hereinafter, the present invention will be described in more detail with reference to examples of the present invention. Parts represent parts by weight.

Example 1 First magnetic layer (lower layer) Cobalt-modified iron oxide 100 parts Hc680Oe, S _BET 30 m ² / g, crystallite size 250 mm, needle ratio 1: 8 Vinyl chloride-vinyl acetate-maleic anhydride copolymer 10 parts Composition ratio 86: 13: 1 Degree of polymerization 400 (MPR-TM manufactured by Nissin Chemical Co., Ltd.) 5 parts polyester polyurethane resin (Chrisbon 7209 manufactured by Dainippon Ink Co., Ltd.) 3 parts carbon black (Conductex SC made by Columbia Carbon) Butyl stearate (Nippon Oil & Fats Butyl Stearate) 1 part Stearic acid (Nippon Oil & Fats NAA 173K) 2 parts Butyl acetate 100 parts Cyclohexanone 50 parts Second magnetic layer (upper layer) Cobalt-modified iron oxide 100 parts Hc700Oe, S _BET 35m ² / g, crystallite size 200 mm, needle ratio 1:10 vinyl chloride-vinyl acetate-maleic anhydride copolymer 12 parts composition ratio 86: 13: 1 degree of polymerization 400 (MPR-TM manufactured by Nissin Chemical Co., Ltd.) polyester Polyurethane resin (Dainihon Crisbon 7209 manufactured by Ink Corporation 6 parts Carbon black 3 parts (Asahi Carbon # 35) α-alumina (AKP 30 manufactured by Sumitomo Chemical) 3 parts Butyl stearate 1 part Stearic acid 2 parts Butyl acetate 100 parts Methyl ethyl ketone 100 parts Above For each of the two paints, each component was kneaded and dispersed using a sand mill. In the dispersion for the first layer, the squareness ratio was changed by changing the dispersion time (2 hours to 20 hours). To the resulting dispersion was added 5 parts of a polyisocyanate (Coronate L-75 manufactured by Nippon Polyurethane Co.) for the first layer coating solution and 6 parts for the second layer coating solution.
Forty parts were added, and the mixture was filtered using a filter having an average pore diameter of 1 μm to prepare coating liquids for forming the first magnetic layer and the second magnetic layer, respectively.

The thickness of the obtained first magnetic layer coating solution after drying is 3.3
The second magnetic layer has a thickness of 0.7 μm
Is applied on a 15 μm-thick polyethylene phthalate support at the same time, while both layers are still in a wet state, oriented by a cobalt magnet and a solenoid, dried, super-calendered, and bulk thermo-treated. After performing the above, slitting was performed to a 1/2 inch width to produce a video tape. Table 1 shows the tapes thus obtained and their properties. Practical level is 0dB for each characteristic
(The standard is a commercial product of Fuji Film Co., Ltd.).

Table 1 Measurement squareness ratio first magnetic layer: value measured by removing only the upper layer from the predetermined coating thickness tape or multilayer coating tapes in a single layer _{^{(▲ φ L r ▼ / ▲}} φ L m ▼: ▲ ^L _r ▼ is the value before converting the thickness of the lower layer to Br, ▲ φ ^L _m ▼ is the value before converting the thickness of the lower layer to Bm.) Second magnetic layer: Saturation magnetic flux of the tape coated on the multilayer Calculated from the value obtained by subtracting the value of the first layer from the density and residual magnetic flux density [(▲ φ ^T _r
▼-▲ φ ^L _r ▼) / (▲ φ ^T _m ▼-▲ φ ^L _m ▼): ▲ φ ^T _r ▼ is the value before the total Br thickness conversion, and ▲ φ ^T _m ▼ is the total Bm The value before converting the thickness, ▲ φ ^L _m ▼ is the value before converting the thickness of the lower layer Bm, therefore, ▲ φ ^T _m ▼-▲ φ ^L _m ▼ is the value before converting the thickness of the upper layer Bm value. Measuring machine: Toei Kogyo Co., Ltd. VSM P-1, Hm = 5KOe Sweep temperature 3 min / cycle VS 4.2MHz output. SAG T-120 manufactured by Fuji Photo Film Co., Ltd.
Relative value when is set to 0 dB.

 Measurement deck NV-8200 manufactured by Matsushita Electric Industrial Co., Ltd.

CS 629KHz output.

Y S / N, S / N of luminance signal after luminosity correction.

C S / N, S / N of color signal after luminosity correction.

AS, 1KHz analog audio output.

Af, analog audio output difference between 10KHz and 1KHz.

As shown in Table 1 Nos. 1 to 5, VS, YS / N, and Af were obtained by providing a magnetic layer having a small squareness ratio in the first layer (lower layer).
CS, C without impairing the electromagnetic conversion characteristics in the short wavelength range such as
It can be seen that the electromagnetic conversion characteristics in the long wavelength region such as S / N and AS can be improved. Also, a multilayer coated magnetic recording medium according to the present invention.
It can be seen that Nos. 1 to 5 and 7 have better electromagnetic conversion characteristics in all bands than the conventional single-layer magnetic recording media Nos. 9 and 10.

Also, the electromagnetic conversion characteristics in the entire band are superior to those of No. 6 having the same squareness ratio and No. 8 of the second layer having a large thickness (No. 8).

As described above, according to the present invention, a magnetic recording medium having excellent electromagnetic conversion characteristics in all bands can be easily obtained.

Example 2 First magnetic layer (lower layer) Fe-Zn-Al alloy (90-5-5) 100 parts Hc1100 Oe, S _BET 40 m ² / g, crystallite size 200Å Vinyl chloride resin containing sodium sulfonate (Nihon Zeon ( MR-110) 10 parts Polyester polyurethane resin (UR-83 manufactured by Toyobo Co., Ltd.)
00) 5 parts Carbon black (Conductex SC made by Columbia Carbon) 3 parts Butyl stearate (butyl stearate made by NOF) 1 part Stearic acid (NAA-173K made by NOF) 2 parts Methyl ethyl ketone 150 parts Cyclohexanone 100 parts 2nd magnetic Layer (upper layer) Fe-Zn-Al alloy (90-5-5) 100 parts Hc 1500 Oe, S _BET 50 m ² / g, crystallite size 150Å Sodium sulfonate-containing PVC resin (Nippon Zeon Co., Ltd.)
10 parts Polyester polyurethane resin (UR-83 manufactured by Toyobo Co., Ltd.)
00) 5 parts carbon black 1 part (Asahi Carbon # 35) α-alumina (Sumitomo Chemical HIT50) 5 parts butyl stearate 1 part stearic acid 2 parts butyl acetate 150 parts methyl ethyl ketone 100 parts Using the above two paints, Dispersed in the same manner as in Example 1,
A coating solution was obtained.

The first magnetic layer coating solution obtained on a 10 μm-thick polyethylene terephthalate was subjected to simultaneous multi-layer coating so that the thickness after drying was 2.0 μm and the thickness of the second magnetic layer was 0.5 μm. While still wet, the powder was oriented by a cobalt magnet and solenoid, dried, subjected to a super calendar treatment, subjected to a bulk thermo treatment, and slit to a width of 8 mm to obtain a video tape. Table 2 shows the tapes thus obtained and their properties.

Measurement method in Table 2 Br / Bm: Same as in Table 1 VS: 5 MHz output, SAG P- manufactured by Fuji Photo Film Co., Ltd.
Relative value when 120 is set to OdB Measurement deck SONY S-800 CS: 0.75MHz output YS / N: S / N of luminance signal corrected for visual perception.

 C S / N: S / N of color signal after luminosity correction.

As shown in Table 2, it can be seen that the effects of the present invention can be similarly obtained even when the coating formulation of the magnetic layer changes.

Continuation of the front page (56) References JP-A-63-102037 (JP, A) JP-A-59-77628 (JP, A) JP-A-59-117732 (JP, A)

Claims (1)

(57) [Claims] 1. A magnetic recording medium comprising: a first magnetic layer and a second magnetic layer each containing a ferromagnetic material and a binder provided on a surface of a non-magnetic support in this order; Or the first magnetic layer and the second magnetic layer are oriented in the longitudinal direction, the squareness ratio of the first magnetic layer is smaller than the squareness ratio of the second magnetic layer by 0.02 or more, and the second magnetic layer is an alloy. A magnetic recording medium, wherein the thickness of the magnetic layer is 1.5 μm or less.