JPH09297916A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a C/N without decreasing the productivity by controlling an irregular coating state on the interface between a lower layer and an upper layer to a specified range. SOLUTION: This magnetic recording medium is a wet-on-wet coating type magnetic recording medium and is produced by applying a coating material for the lower layer containing a pigment powder, binder and org. solvent on a nonmagnetic supporting body, and then applying a coating material for the upper layer containing a ferromagnetic powder, binder and org. solvent while the lower layer is in a wet state to form the lower layer 2 and the upper layer 3. In the magnetic recording medium having a double-layer structure, the irregular coating state (change rate on the interface) of the interface 4 between the lower layer and upper layer is controlled to 1 to 6nm so as to decrease the noise level and to improve C/N. If the irregular coating state on the interface 4 can be controlled to 1 to 6nm, the medium can be obtd. without largely decreasing the coating rate or drying rate, and thereby, the obtd. wet-on-wet coating type magnetic recording medium has excellent reproducibility and productivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called multilayer coating type magnetic recording medium, and more particularly to improvement of interfacial property between a lower layer and an upper layer.

[0002]

2. Description of the Related Art As a recording medium used in audio equipment, video equipment, computer equipment, etc., a magnetic paint prepared by dispersing ferromagnetic powder, a binder and various additives together with an organic solvent is used as a non-magnetic support. There is known a so-called coating type magnetic recording medium in which a magnetic layer is formed by coating and drying the magnetic recording layer on the top.

Such a coating type magnetic recording medium has undergone various improvements in order to meet the demand for high density recording in the field of magnetic recording.

In order to realize high density recording in a coating type magnetic recording medium, fine metal particles are used as a ferromagnetic powder, and the medium surface is super smoothed to minimize spacing loss and at the same time. It is also important to reduce output loss due to self-demagnetization loss.

Among them, as a method of suppressing self-demagnetization loss, thinning the magnetic layer is effective. However, if the thickness of the magnetic layer is simply reduced to, for example, 1 μm or less, the surface shape of the non-magnetic support is likely to appear on the surface of the magnetic layer, which makes it difficult to smooth the surface of the magnetic layer. For this reason, in the case where the magnetic layer is thinned, a multi-layer coating type configuration in which a non-magnetic coating layer is interposed between the non-magnetic support and the magnetic layer is often adopted. By interposing the non-magnetic layer in this way, a thickness is obtained between the surface of the non-magnetic support and the surface of the magnetic layer, and the surface shape of the non-magnetic support is less likely to appear on the surface of the magnetic layer. Therefore, a thin magnetic layer is formed with a smooth surface shape.

As a method for producing such a multi-layer coating type magnetic recording medium, a non-magnetic coating material for the lower layer is applied and dried, and a magnetic coating material for the upper layer is applied on the dried lower layer coating film. The so-called wet-on-dry coating method, which uses a die head that has two slits for extruding the non-magnetic coating material for the lower layer and the magnetic coating material for the upper layer, is used to dry the non-magnetic support on the non-magnetic support. A simultaneous multi-layer coating method (wet-on-wet coating method) in which a paint and a magnetic paint are simultaneously applied has been proposed. Among them, the simultaneous multilayer coating method can form a coating film having a uniform thickness with few coating defects and coating streaks, and the formed lower layer and upper layer have high adhesiveness and excellent durability.

[0007]

By the way, in the coating type magnetic recording medium, it is known that the surface property of the magnetic layer greatly affects the noise level in the vicinity of the carrier frequency. Here, especially in the case of a multi-layer coating type magnetic recording medium, it is considered that the surface property of the upper magnetic layer and the interfacial property between the lower layer and the upper layer influence the noise of the reproduced signal. However, the relationship between the interface between the lower layer and the upper layer and the reproduced signal has hardly been studied so far, and the actual condition is that no proper evaluation criterion of the interface from the viewpoint of the reproduced signal has been found.

Therefore, the present invention has been proposed in view of such a conventional situation, and the interfacial property between the lower layer and the upper layer is
It is an object of the present invention to find an appropriate evaluation standard and to provide a magnetic recording medium from which a good reproduced signal with less noise can be obtained.

[0009]

In order to achieve the above-mentioned object, the magnetic recording medium of the present invention comprises a non-magnetic support, a lower layer coating comprising a pigment powder, a binder and an organic solvent, and In the magnetic recording medium in which one or more lower layers and the upper layer are formed, by applying the upper layer coating composed of the ferromagnetic powder, the binder and the organic solvent while the lower layer coating is in a wet state, the lower layer and the upper layer are formed. Unevenness of the interface of 1 to 6n
m.

When the coating unevenness at the interface between the lower layer and the upper layer is suppressed to 6 nm or less, the carrier frequency is 0.3 to 1 dB.
The noise level in a band separated by a certain degree is reduced, and a high C / N ratio can be obtained. The lower limit value of coating unevenness of 1 nm at the interface between the lower layer and the upper layer is set from the viewpoint of productivity, and by controlling the coating unevenness at this interface to 1 to 6 nm, C / An improvement in N ratio is achieved.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

FIG. 1 shows an example of a magnetic recording medium to which the present invention is applied.

In this magnetic recording medium, a lower layer paint comprising a pigment powder, a binder and an organic solvent is coated on a non-magnetic support 1, and then, while the lower layer paint is in a wet state, it is combined with a ferromagnetic powder. This is a multi-layer coating type magnetic recording medium in which a lower layer 2 and an upper layer 3 are formed by applying an upper layer coating material containing an agent and an organic solvent.

In the present invention, in such a multi-layer coating type magnetic recording medium, in order to reduce the noise level and improve the C / N ratio, coating unevenness at the interface 4 between the lower layer and the upper layer (variation amount of the interface). ) Is regulated to 1 to 6 nm.

That is, in the multi-layer coating type magnetic recording medium, the coating unevenness at the interface between the lower layer and the upper layer induces the output fluctuation of the reproduction waveform, and 0.3 to 1 MHz from the carrier frequency.
When the carrier signal has a frequency of 5 MHz, the noise appears as amplitude-modulating noise in a band of 4 to 4.7 MHz and a band of 5.3 to 6 MHz. That is, the greater the coating unevenness at the interface 4 between the lower layer and the upper layer, the higher the noise level in this band and the lower the C / N ratio. The upper limit of the coating unevenness at the interface 4 between the lower layer and the upper layer regulated by the present invention is such that C /
It is set from the viewpoint of the N ratio, and if the coating unevenness of the interface 4 is 6 nm or less, 0.3 from the carrier frequency.
The noise level in the band apart by about 1 dB is reduced, and C /
The N ratio is improved.

The coating unevenness at the interface 4 between the lower layer and the upper layer is controlled by the coating speed, the drying speed of the coating film, the physical properties of the coating material, and the lip shape of the die coater used for coating. For example, the slower the coating speed and the drying speed, the better the interface between the lower layer and the upper layer. However, considering productivity, it is not desirable to slow down the coating speed and the drying speed. The lower limit value of the coating unevenness on the interface 4, 1 nm, which is regulated by the present invention, is set from such a point. The coating unevenness of 1 to 6 nm can be realized without slowing down the coating speed and the drying speed, and a multilayer coating type magnetic recording medium excellent in both reproduction characteristics and productivity can be obtained.

In such a multi-layer coating type magnetic recording medium, as the material of the lower layer 2, the upper layer 3 and the non-magnetic support 1, any of those usually used in this type of magnetic recording medium is used. It is possible.

First, the upper layer 3 is formed by an upper magnetic coating material comprising a ferromagnetic powder, a binder and an organic solvent.

As the ferromagnetic powder, γ-Fe ₂ O ₃ ,
In addition to Fe ₃ O ₄ , Co-modified iron oxide, synthetic fine powder containing iron as a main component, modified barium ferrite, modified strontium ferrite and the like are used.

As the binder, modified or non-modified vinyl chloride resin, polyurethane resin or polyester resin may be used alone or in combination. Further, a fibrous resin, a phenoxy resin or a thermoplastic resin having a specific usage, a thermosetting resin, a reactive resin, or an electron irradiation curable resin may be used in combination. The group to be introduced for modification of the resin, -SO ₃ M, -OSO ₃ M and improving dispersibility of the magnetic powder for the purpose, -COO
M and —PO (OM ′) ₂ (where M represents an alkali metal atom such as Na and M ′ represents an alkali metal atom such as Na or an alkyl group) and the like are preferable.

The solvent for forming the coating material may be an organic solvent selected from ethers, esters, ketones, aromatic hydrocarbons, aliphatic hydrocarbons, chlorinated hydrocarbons and the like.

On the other hand, the lower layer 2 may be a non-magnetic layer or a magnetic layer.

The non-magnetic layer is formed by a non-magnetic paint composed of non-magnetic powder, a binder and an organic solvent.

As the non-magnetic powder, alumina, titanium oxide, silicon oxide, inorganic pigment powder such as α-iron oxide, carbon black and the like are used.

As the binder and the organic solvent, any of those exemplified in the upper layer 3 can be used.

Also, when the lower layer 2 is a magnetic layer, any of the materials exemplified for the upper layer 3 can be used.

In the magnetic paint and non-magnetic paint, if necessary, a dispersant such as lecithin, a lubricant such as stearic acid, an antistatic agent such as carbon black, an abrasive such as chromium oxide, and a rust preventive. May be added.

The lower layer coating material and the upper layer coating material made of such a material are formed by a so-called wet-on-wet coating method (wet multi-layer coating method) in which the upper layer coating film is applied over the lower layer coating film in a wet state. The lower layer 2 and the upper layer 3 are formed by coating.

FIG. 2 shows an example of a coating apparatus for coating the coating material by the wet-on-wet coating method.

This coating apparatus has two slit portions (slit portion 11 for lower layer coating material,
Die head 18 having slit 12) for upper paint
(4 lip type die head). That is, in this die head, the two slit portions 11, 1
A lower paint reservoir 13 and an upper paint reservoir 14 to which a lower paint and an upper paint are supplied, respectively, are formed on the back side of 2.
The lower layer paint and the upper layer paint supplied to the paint pools 13 and 14 are extruded through the slits 11 and 12 to the tip of the die head. On the other hand, the support 15 to which the paint is applied
Travels in the direction A in the figure from the slit 11 for the lower layer paint to the slit 12 for the upper layer paint along the tip surface of the die head.

The non-magnetic support 15 running in this way
First, when passing through the slit portion 11 for the lower layer paint, the lower layer paint extruded from the slit portion 11 is applied to the surface to form the lower layer coating film 16. Then, when passing through the slit portion 12 for the upper layer paint, the upper layer paint extruded from the slit portion 12 is applied on the lower layer coating film 16 in a wet state, and two layers of the coating films 16 and 17 are formed. .

The magnetic recording medium shown in FIG. 1 has a single lower layer, but the lower layer may have a multi-layered structure. In the case of a multi-layered structure, either a non-magnetic layer or a magnetic layer is used. It may have a structure in which a large number of these are laminated or a combination of a non-magnetic layer and a magnetic layer.

[0033]

EXAMPLES Examples of the present invention will be described below based on experimental results.

First, three types of magnetic tapes (tape a, tape b, tape c) were manufactured as follows.

An upper layer paint and a lower layer paint were prepared based on the following compositions. The coating was performed by adding a pigment, a binder, an additive and a solvent to a kneader, kneading them and dispersing them in a sand mill according to a conventional method. The amount of the non-volatile components during kneading is 85% by weight. The dispersion time was set to 5 hours for the upper layer paint and 3 hours for the lower layer paint.

<Composition of upper coating material> Fe-based metal ferromagnetic powder 100 parts by weight Polyvinyl chloride resin 14 parts by weight (Polymerization degree 150, 5 × 10 ⁻⁵ mol / g of sulfonic acid sodium salt as polar functional group) Polyester polyurethane resin 6 parts by weight (containing 1 × 10 ⁻⁴ mol / g of sulfonic acid sodium salt as a polar functional group) Additive: carbon 2 parts by weight Al ₂ O ₃ 5 parts by weight (primary particle size 0.09 μm, this Al ₂ O ₃ was dispersed in an organic solvent to form a slurry, which was mixed with another coating composition. The center particle size of this slurry is 0.17 μm) Methyl ethyl ketone 150 parts by weight Cyclohexanone 150 parts by weight <lower layer Composition of paint> α-iron oxide 100 parts by weight (Si3 atom% treated product: needle shape ratio 8, major axis length 0.18 μm) Polyvinyl chloride resin 14 parts by weight (Polymerization degree: 150, sulfonic acid sodium salt as polar functional group: 5 × 10 ⁻⁵ mol / g) Polyester polyurethane resin: 6 parts by weight (sulfonic acid sodium salt: 1 × 10 ⁻⁴ mol / g as polar functional group) ) Methyl ethyl ketone 105 parts by weight Cyclohexanone 105 parts by weight And, to the upper coating material, 1 part by weight of butyl stearate and 1 part by weight of stearic acid are added and stirred for 30 minutes, and further 4 parts by weight of polyisocyanate is added as a curing agent. It was Further, 1 part by weight of butyl stearate and 1 part by weight of stearic acid were added to the lower layer coating material and stirred for 30 minutes, and further 2 parts by weight of polyisocyanate was added.

The lower layer coating material and the upper layer coating material were simultaneously multilayer-coated on a polyethylene terephthalate base having a thickness of 4.5 μm by a 4-lip type die coater, and a magnetic field orientation treatment was performed on the wet upper layer by a solenoid coil. Then, the lower layer and the upper layer were formed by drying the coating film and performing calendering treatment and curing treatment. The film thickness of the formed coating film after drying is 0.15 μm in the upper layer and 2.0 μm in the lower layer.

Then, a back layer coating material was prepared based on the following composition.

<Composition of Back Layer Paint> 100 parts by weight of carbon black (trade name: Asahi # 50) 100 parts by weight of polyester polyurethane (product name: Nipolan N-2304) 500 parts by weight of methyl ethyl ketone Toluene 500 parts by weight A back layer was formed by coating and drying on the surface opposite to the side on which the lower layer and the upper layer of the magnetic support were formed. The thickness of the back layer after drying is 0.5.
μm.

Then, the tape raw material on which the respective layers were formed in this manner was cut into 8 mm widths to produce magnetic tapes (tape a, tape b, tape c).

In the tapes a, b, and c, the interface between the lower layer and the upper layer was changed by changing the coating conditions. Table 1 shows the coating unevenness at the interface between the lower layer and the upper layer of the tape a, the tape b, and the tape c.

The coating unevenness was measured by a thickness unevenness measuring device shown in FIG.

This thickness unevenness measuring apparatus includes a light emitting section 31 for emitting light, a diaphragm lens 32, a stage 33 on which the sample tape T is placed, a light receiving section 34 for receiving the laser beam from the light emitting section 31, The monitor display unit 35 displays the amount of light received by the light receiving unit 34.

The light emitting section 31 serves as a light source for emitting the laser beam P1. Here, the wavelength of this laser beam P1 was set to 780 nm.

The diaphragm lens 32 adjusts the spot radius of the laser beam P1 emitted from the light emitting section 31. In this device, the laser light P1 is emitted from the diaphragm lens 32.
Is narrowed down to 10 μm.

The stage 33 is for mounting the sample tape T, and is arranged in the vertical direction (y direction) and the horizontal direction (x direction).
It is possible to move within a certain range in both directions.
Therefore, by moving this stage, the thickness of the coating layer of the sample tape is subjected to line analysis or surface analysis. The stage 33 is connected to the monitor display unit 35, and the movement amount of the stage 33 is displayed on the monitor display unit 35.

Even if such a stage 33 is not used, for example, by transporting a long sample tape between the light emitting section 31 and the light receiving section 3, the coating thickness of the sample tape can be measured. You can

The light receiving section 34 receives the transmitted light P2 transmitted through the sample tape T. This light receiving section 34
Is connected to the monitor display unit 35, and this light receiving unit 3
The amount of light received at 4 is displayed on the monitor display unit 35.

The monitor display section 35 includes, for example, the light emitting section 31.
It has a built-in arithmetic processing function for calculating the thickness of the coating layer based on the intensity of the laser beam P1 emitted from the light source and the intensity of the transmitted light P2 received by the light receiving unit 34. That is, the thickness of the coating layer is the absorbance A shown by the transmittance = 1/10 ^Abs.
proportional to bs. In this monitor display unit, the transmittance is calculated from the intensity of the laser light P1 emitted from the light emitting unit 31 and the intensity of the transmitted light P2 received by the light receiving unit 34, and the absorbance Abs is calculated from this transmittance. . Then, the thickness of the coating layer is output based on the proportional relationship between the thickness of the coating layer and the absorbance Abs described above. Here, the thickness distribution of the upper layer of the coating layer reflects the height distribution of the interface between the lower layer and the upper layer. Therefore, here, the thickness unevenness of the upper layer is referred to as coating unevenness at the interface.

[0050]

[Table 1]

Next, the tape a, the tape b, and the tape c
For, the output fluctuation of the reproduced waveform and the power spectrum of the reproduced signal were examined.

The output fluctuation of the reproduced waveform was examined as follows. That is, the drum tester and Functi
A single sine wave signal having a long wavelength (recording frequency: 500 kHz) was recorded and reproduced by an on Generator. At this time, the relative speed between the tape and the head is 3.
33 m / sec. Then, the reproduced waveform was observed with an oscilloscope, the peak value of the amplitude was taken into a spectrum analyzer, and the fluctuation was observed.

Further, the power spectrum of the reproduced signal is calculated by the drum tester and the FunctionGenerator.
The carrier signal of 5 MHz was recorded and reproduced by, and the power spectrum of the reproduced signal was observed by a spectrum analyzer.

FIG. 4 shows the output fluctuation of the reproduced waveform, FIG. 5 shows the power spectrum of the reproduced signal, and FIG. 6 shows an enlarged part of this power spectrum.

First, referring to FIG. 4, the tapes b and c having relatively small uneven coating on the interface suppress the output fluctuation of the reproduced waveform, while the uneven coating on the interface is larger than 6 nm. In a, this output fluctuation is very large. The output fluctuation of the reproduced waveform is caused by the coating unevenness at the interface between the upper layer and the lower layer.

Next, referring to FIG. 5, in each of tape a, tape b, and tape c, the carrier signal frequency is 5 MH.
A power spectrum having a peak at z is obtained, and a detailed comparison of these results shows that the carrier signal frequency is 5 MHz.
band of z ± 0.3 to 1 MHz, that is, 4 to 4.7 MH
There is a difference in noise level between z and the band of 5.3 to 6 MHz.

Of these, the band of 4 to 4.7 MHz is enlarged and shown in FIG. 6, but the noise level in this band is suppressed low for tapes b and c in which the coating unevenness at the interface is relatively small. On the other hand, in the case of the tape a in which the coating unevenness on the interface is larger than 6 nm, the noise level is about 3 dB higher than that of the tapes b and c. For example 4.35M
Table 2 shows the C / N ratio with respect to 5 MHz when the output at Hz is taken as the noise level.

[0058]

[Table 2]

From the above results, the coating unevenness at the interface between the lower layer and the upper layer of the magnetic tape increases the amplitude fluctuation of the reproduced waveform,
It is shown that when the coating unevenness at the interface becomes larger than 6 nm, the noise level (amplitude modulation noise) in the band of 0.3 to 1 MHz from the carrier signal frequency remarkably increases. Therefore, in order to increase the C / N ratio, the coating unevenness at the interface between the lower layer and the upper layer needs to be 6 nm or less.

However, the coating unevenness at the interface between the lower layer and the upper layer is controlled by the coating speed, the drying speed, the physical properties of the coating material and the lip shape of the die coater used for coating. For example, the slower the coating speed and the drying speed, the better the interface between the lower layer and the upper layer. However, if the coating speed and the drying speed are too slow, the productivity will be low. In other words, the coating unevenness at the interface is suppressed and C
There is a contradictory relationship between increasing the / N ratio and increasing productivity. This is schematically shown in FIG. 7, and if the lower layer and the upper layer are formed under the condition that the coating unevenness at the interface is 1 nm or less, practical productivity cannot be obtained.

Therefore, considering both productivity and C / N ratio, the coating unevenness at the interface between the lower layer and the upper layer is 1 to 6 nm.
It is necessary to form under the condition that

[0062]

As is apparent from the above description, the present invention is obtained by evaluating the interface between the lower layer and the upper layer from the viewpoint of noise level, and the coating unevenness at the interface between the lower layer and the upper layer is obtained. Is regulated within the range of 1 to 6 nm. As a result, it is possible to obtain a magnetic recording medium having a low noise level, a high C / N ratio, and excellent productivity.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing one structural example of a magnetic recording medium of the present invention.

FIG. 2 is a schematic diagram showing a coating apparatus for performing the simultaneous multilayer coating method.

FIG. 3 is a schematic diagram showing a configuration of a thickness unevenness measuring device.

FIG. 4 is a characteristic diagram showing an output fluctuation of a reproduced waveform.

FIG. 5 is a characteristic diagram showing a power spectrum of a reproduced signal.

FIG. 6 is a characteristic diagram showing the power spectrum of FIG. 5 partially enlarged.

FIG. 7 is a schematic diagram showing the relationship between coating unevenness at the interface between the lower layer and the upper layer, and the C / N ratio and productivity.

[Explanation of symbols]

 1 non-magnetic support 2 lower layer 3 upper layer

Claims (3)

[Claims] 1. A non-magnetic support is coated with a lower layer coating comprising a pigment powder, a binder and an organic solvent, and while the lower layer coating is in a wet state, a ferromagnetic powder, a binder and an organic solvent are used. A magnetic recording medium in which one or more lower layers and an upper layer are formed by applying the above upper layer coating composition, wherein the coating unevenness at the interface between the lower layer and the upper layer is 1 to 6 nm. 2. The magnetic recording medium according to claim 1, wherein two layers, a lower layer and an upper layer, are formed on a non-magnetic support, and the pigment powder of the lower layer coating material is a non-magnetic pigment powder. 3. The magnetic recording medium according to claim 1, wherein the thickness of the upper layer is 0.2 μm or less.