JPH06162487A - Magnetic recording medium - Google Patents

Abstract

(57) [Abstract] [Purpose] A lower magnetic layer containing magnetic powder for long wavelength recording and a resin binder and an upper magnetic layer containing magnetic powder for short wavelength recording and a resin binder are formed on a non-magnetic substrate. In such a magnetic recording medium, the disturbance of the interface between the upper and lower magnetic layers is reduced to improve the surface property so that a high output can be obtained in a wide wavelength range. [Structure] Long-wavelength recording magnetic powder is Co-γ ferrite magnetic powder, and short-wavelength recording magnetic powder is Nb, Sn, Ni, and Zn.
Part of Fe is replaced by at least one element selected from among the following characteristics: SFD: 0.3 to 5, saturation magnetization σ _g : 50 to 70 emu / g,
A hexagonal ferrite powder having a coercive force Hc of 500 to 3,000 Oe: and a resin binder contained in the upper magnetic layer and the lower magnetic layer containing a sulfonic acid group or a salt thereof in the molecular chain and having the following characteristics. : Polyurethane resin having a molecular weight of 1,000 to 100,000 and Tg of -40 to + 40 ° C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high density magnetic recording medium such as a magnetic tape or a floppy disk used in the fields of audio, video and computer.

[0002]

2. Description of the Related Art Magnetic recording media are audio, video,
It has come to be widely used as a recording medium for recording / reproducing a large amount of information in a wide range of fields such as a computer, and accordingly, a demand for higher density recording is increasing.

Conventionally, in a coating type magnetic recording medium, a magnetic coating mainly composed of magnetic powder such as γ-ferrite or iron powder and a resin binder is coated on a non-magnetic substrate such as a polyester film and subjected to orientation treatment. After being subjected to a drying treatment, it is obtained by subjecting the formed coating film to a smoothing treatment with a calendar. A lubricant, an abrasive, a dispersant, an antistatic agent, a curing agent, or the like is further added to the magnetic paint, if necessary.

In order to significantly improve the magnetic recording density in such a magnetic recording medium, a perpendicular magnetic recording system using magnetization in a direction perpendicular to the coated surface of the support has been proposed.
It is getting attention. In this perpendicular magnetic recording system, for example, hexagonal ferrite powder, such as hexagonal ferrite powder, is used as an ultrafine particle hexagonal ferromagnetic powder having an easy axis of magnetization perpendicular to the plate surface. Magnetic recording is performed by orienting so as to be parallel to the surface and using residual magnetization in the direction perpendicular to the surface of the magnetic recording medium layer.

However, the magnetic recording medium obtained by such a perpendicular magnetization type recording system can obtain a high reproduction output in a short wavelength region of about 1 μm or less, while it is used for recording in a long wavelength region. Has a feature that it is difficult to obtain a high reproduction output. Therefore, there is a problem that it is difficult to obtain sufficient characteristics in the case of a medium such as a VTR tape which records and reproduces a signal in a long wavelength region such as an audio signal and a color signal.

As one means for compensating for such drawbacks, for example, a magnetic layer using a low coercive force Co-γ ferrite powder is provided as a lower magnetic layer, and a high coercive force Co-γ ferrite powder is provided above the magnetic layer. Alternatively, a magnetic layer using a magnetic powder having a high coercive force such as metal magnetic powder or hexagonal ferrite powder is provided as the upper magnetic layer, and the lower magnetic layer is used for long wavelength recording such as audio signals. A multi-layer coating type magnetic recording medium has also been proposed, in which it is attempted to perform recording separately for wavelength recording.

There are various methods for obtaining the above-mentioned magnetic recording medium having a multilayer structure. Among them, the simultaneous coating method (wet-on-wet method) is particularly noted because good surface smoothness can be obtained. Are gathering.
This simultaneous coating method is a method in which a lower layer coating film is coated using, for example, a slot die coater, and then the upper layer is coated before the organic solvent of the coating film evaporates and dries.

[0008]

However, even in the multilayer coating type magnetic recording medium having such a structure, satisfactory high frequency characteristics have not yet been obtained at the present time, and the upper magnetic layer is pressed. There were some problems such as easy demagnetization.

Further, in the coating type medium having such a multi-layer structure, in order to sufficiently exhibit the excellent characteristics of each layer, the thickness of the upper magnetic layer is usually set to, for example, 0.2 to 0.5.
It is desired to set in the range of submicron region of about μm. In order to form such an upper magnetic layer having a thickness in the submicron region, it is necessary to make the surface roughness of the upper magnetic layer as small as possible to enhance the smoothness of the surface. Further, the smoothness of the boundary surface between the upper magnetic layer and the lower magnetic layer also affects the smoothness of the surface of the upper magnetic layer, so that the disturbance of the boundary surface between these upper and lower magnetic layers is minimized,
It is also very important to improve the smoothness of the surface of the upper magnetic layer.

However, conventionally, when the types of magnetic powders used in the upper and lower magnetic layers are different, or the types of resin binders and other additives such as dispersants are different, a smooth magnetic layer surface is obtained. Was difficult to obtain. This is because when a magnetic coating material is applied onto a substrate to form a layer, the organic solvent of the coating film does not evaporate and dry, and the upper and lower magnetic layers are affected by a charge shock or an incompatibility of compatibility between the upper and lower layers. This is because the interface is likely to be disturbed. When the magnetic layer interface is disturbed in this way, the S / N of the obtained medium is lowered, and the medium becomes a noise-rich medium.

As the magnetic powder for the lower magnetic layer, for example,
A multilayer coating type magnetic recording medium that uses magnetic powder that is in-plane oriented such as Co-γ ferrite powder, and uses vertically oriented magnetic powder such as hexagonal magnetic powder as the magnetic powder for the upper magnetic layer. In addition, when the type of magnetic powder and the direction of magnetic field orientation are different between the lower layer and the upper layer, it was extremely difficult to create a medium by magnetic field orientation treatment after forming both upper and lower magnetic layers by the simultaneous coating method. .

For this reason, it has been attempted to form a lower magnetic layer on the substrate once and perform an alignment treatment, and then form a vertical alignment film on the upper layer, not by the simultaneous coating method. However, since the magnetic layer to be formed as the upper layer has a thin layer thickness in the submicron region, the calenderability is deteriorated, and a magnetic recording medium having a good surface property cannot be obtained. The magnetic recording medium manufactured in this way has a problem that the various properties such as S / N are deteriorated as a result of poor surface properties, and desired functions such as short wavelength characteristics cannot be sufficiently obtained.

The present invention has been made to address such a problem, and provides a high output in a wide wavelength range from a short wavelength range to a long wavelength range, and a boundary between an upper magnetic layer and a lower magnetic layer. It is an object of the present invention to provide a magnetic recording medium having less surface irregularity, excellent surface property, high S / N, and excellent running property and durability.

[0014]

The magnetic recording medium of the present invention comprises a non-magnetic substrate, a lower magnetic layer containing magnetic powder for long wavelength recording and a resin binder, and magnetic powder for short wavelength recording and a resin binder. In a magnetic recording medium formed by forming an upper magnetic layer containing, the long-wavelength recording magnetic powder is Co-γ ferrite magnetic powder, the short-wavelength recording magnetic powder is Nb, S
Part of Fe is replaced by at least one element selected from n, Ni, and Zn, and the following characteristics: SFD: 0.3-5 Saturation magnetization σ _g : 50-70 emu / g Coercive force Hc: 500- Hexagonal ferrite powder having 3,000 Oe: and the resin binder contained in the upper magnetic layer and the lower magnetic layer contains a sulfonic acid group or its salt in the molecular chain, and the following characteristics: Molecular weight: 1,000- It is characterized by comprising a polyurethane resin having 100,000 Tg: -40 to + 40 ° C.

In the present invention, the Co-γ ferrite magnetic powder used as the magnetic powder for long wavelength recording in the lower magnetic layer has a coercive force Hc of 300 to 1,000 Oe and a saturation magnetization of 70 to 85 emu /
Those having a grain size g and a grain length in the major axis direction of 0.2 to 1.0 μm are preferred.

On the other hand, the hexagonal ferrite powder used as the magnetic powder for short wavelength recording in the upper magnetic layer has uniaxial anisotropy in which the easy axis of magnetization is perpendicular to the grain plate surface, and the coercive force Hc: 500 to 3,000 Oe, more preferably 700 to 1,500
It is a magnetic powder of Oe. If the coercive force is less than 500 Oe, the recording signal on the recording medium does not remain sufficiently,
If it exceeds the above range, it becomes difficult to write a signal by a normal recording / reproducing head, which is not preferable. In addition, such a hexagonal ferrite has a plate ratio of 2.5 to 7,
The particle size is preferably 0.01 to 0.1 μm.

The hexagonal ferrite that can be used in the present invention is represented by M-type or W-type Ba-ferrite, Sr-ferrite, Ca-ferrite, Pb-ferrite, a solid solution thereof, or the following general formula. Ultrafine particles of ferrite such as ion-substituted products are exemplified.

General formula: AOn (Fe _{1-mM m} ) ₂ O ₃ (wherein A is any one element of Ba, Sr, Ca, Pb, M
Is Zn, Co, Ti, Ni, Mn, In, Cu, Ge, Nb, Sn, Zr, Hf, A
Of the elements consisting of l, etc., at least one element selected from Nb, Sn, Ni, and Zn is represented, and m is 0 to
0.2 and n represent the numbers 5.4 to 6.0, respectively. ) A polyurethane resin containing a sulfonic acid group or a salt thereof in the molecular chain according to the present invention has a molecular weight of 1,000 to 100,0.
00 can be used, but molecular weight of 3,000-
The range of 50,000 is more preferable. In the present invention, the polyurethane resin having the above characteristics is a magnetic powder 100
2 to 20 parts by weight, more preferably 4 parts by weight
Use it in the range of up to 10 parts by weight. If the amount of the polyurethane resin having the above characteristics exceeds 20 parts by weight, the ratio of the magnetic powder in the magnetic layer decreases, leading to a decrease in recording density, which is not preferable. If the amount is less than 2 parts by weight, it becomes impossible to form a medium having sufficient durability and coating strength. The polyurethane resin having the above characteristics is preferably used in at least 20% of all resin binders used in magnetic paint.

[0019]

The magnetic recording medium of the present invention maximizes the characteristics of each of the acicular magnetic powder forming the lower magnetic layer and the hexagonal magnetic powder forming the upper magnetic layer, and the functions of the upper and lower magnetic layers, That is, it is a multi-layer coating type magnetic recording medium in which the characteristics of the long wavelength region shared by the lower magnetic layer and the characteristics of the short wavelength region shared by the upper magnetic layer are ideally overlapped.

In the present invention, the magnetic conditions of the upper magnetic layer are given on the basis of the SFD (switching field distribution) of the magnetic powder for the upper magnetic layer, the saturation magnetization σ _g , and the coercive force Hc. It should be noted that SFD is regarded as a parameter showing the variation of coercive force Hc, and SFD
When is large, the noise component in magnetic recording is reduced, and when it is small, the packing factor of particles is increased. Therefore, by limiting the magnetic characteristics of the magnetic powder as described above and setting the magnetic conditions of the upper magnetic layer, it becomes possible to obtain a medium having a high output characteristic in the short wavelength region and a large S / N. .

The polyurethane resin having the above-mentioned characteristics according to the present invention is remarkably compatible with the hexagonal ferrite powder having the above-mentioned characteristics according to the present invention, which is used as a magnetic powder for short wavelength recording, as compared with other polyurethane resins. It is excellent. Therefore, in the present invention, by combining the magnetic powder and the resin binder, it becomes possible to form a magnetic layer having a high degree of dispersion of the magnetic powder and a sufficiently small film thickness.
As a result, it is possible to obtain a magnetic recording medium having excellent output characteristics in the short wavelength region without impairing the long wavelength characteristics.

Further, at this time, by using the same resin binder as the polyurethane resin for the lower magnetic layer, the dispersibility of the magnetic powder in the lower magnetic layer can be improved.
Not only that, but by using the resin binder common to the upper and lower layers, there is a possibility that interlayer disturbance at the upper and lower layer interfaces may occur even in the simultaneous coating method (wet-on-wet method) in which both layers are simultaneously formed in a non-dried state. Disappear. Therefore, a high-density magnetic recording medium having a high S / N with significantly less noise is formed.

[0023]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Example 1 First, the following coating composition was mixed as a <coating material for lower magnetic layer> and dispersed using a sand mill to obtain a magnetic coating material. The obtained coating material was further filtered through a filter having a pore size of 0.5 μm, and 4 parts by weight of trimethylolpropane adduct of tolylene diisocyanate was added as a curing agent per 100 parts by weight of magnetic powder and mixed. Paint>.

<Lower magnetic layer coating composition> Composition Co-γ ferrite powder 100 parts by weight (Hc: 700 Oe, specific surface area 40 m ² / g) Carbon black 2 parts by weight (average particle size: 40 nm, specific surface area 150 m ² / g) Sulfonic acid group-containing polyurethane resin 4 parts by weight (molecular weight 25,000 Tg = -20 ° C, sulfonic acid Na group content 0.
2 mmol / g) Vinyl chloride vinyl acetate copolymer resin 8 parts by weight (molecular weight 24,000-OH group is included) Phosphate ester 2 parts by weight Alumina 5 parts by weight (particle size 0.3 μm) Octyl myristate 2 parts by weight Methyl ethyl ketone / toluene 180 parts by weight Part (2/1 mixed solvent) Next, the following coating composition was mixed as a <coating material for upper magnetic layer> material and dispersed using a sand mill to obtain <coating material for upper magnetic layer>. The obtained coating material was further filtered through a pore size 0.3 μm filter, and then 4 parts by weight of trimethylolpropane adduct of tolylene diisocyanate was added as a curing agent per 100 parts by weight of magnetic powder and mixed, and the resulting mixture was mixed with <upper magnetic layer. Paint>.

<Coating for upper magnetic layer> 100 parts by weight of composition Co, Ti, Sn, Nb substituted Ba ferrite powder (average particle size 0.05 μm, specific surface area 42 m ² / g, SFD = 0.
55, the saturation magnetization _{σ g = 58.5 emu / g,} Hc = 1,380 Oe) 6 parts by weight of the sulfonic acid group-containing polyurethane resin (identical to those used in <lower magnetic layer coating>) sulfonic acid Na-containing vinyl chloride-based resin 6 parts by weight (molecular weight 23,000) 2 parts by weight phosphoric acid ester 2 parts by weight stearic acid 2 parts by weight butyl stearate 5 parts by weight alumina 5 parts by weight (particle size 7.3 μm) cyclohexanone / methyl ethyl ketone / toluene (1/1/1 mixed solvent) 180 parts by weight Part <Coating for lower magnetic layer> thus obtained and <
The above two types of magnetic coating materials for the upper magnetic layer> are separately supplied to two die coaters, and the lower layer and the upper layer are applied in this order on a polyester film having a thickness of 10 μm and passed through an in-plane orientation magnetic field of 4 kOe. After being soaked, it was dried to obtain a multilayer film in which the lower magnetic layer used Co-γ ferrite powder and the upper magnetic layer used Ba-ferrite powder. The thickness of the lower magnetic layer of the multilayer film is 2.5μ after drying.
m, and the film thickness after drying of the upper magnetic layer was 0.5 μm.

After forming this multi-layered magnetic layer, the surface was smoothed using a super calender and then aged at 50 ° C. for 2 days and allowed to cool. Then, this was slit into a width of 8 mm to prepare a tape sample.

Examples 2 to 5 Magnetic powder for short wavelength recording having the characteristics shown in Table 1
Four kinds of <coating material for upper magnetic layer> were made in the same manner as in <coating material for upper magnetic layer> of Example 1 except that Ba ferrite powder of four kinds was used.
It was created. The four types of Ba ferrite powders contain substitution elements Ti, Sn, Nb, and Zn in different combinations.

Then, each of the four types of <coating material for upper magnetic layer> thus obtained and <coating material for lower magnetic layer> prepared in Example 1 were combined, and the upper magnetic layer film was formed in the same manner as in Example 1. Four 0.5 mm thick multilayer 8 mm tape sample examples 2-5 were made.

Examples 6 to 9 As the magnetic powder for short wavelength recording, the same Co, Ti, Sn, Nb substituted Ba ferrite powder as the magnetic powder for short wavelength recording of Example 1 was used, and the SFD was within the range of the present invention. Four kinds of magnetic powder which are included in the above and different from those in Example 1 were used. And, other than that in the first embodiment
Four types of <upper magnetic layer coating materials> were prepared in the same manner as the upper magnetic layer coating material. Table 1 also shows various characteristics of these four magnetic powders.

Each of the four types of <coating material for upper magnetic layer> thus obtained was combined with <coating material for lower magnetic layer> prepared in Example 1, and four kinds of multi-layer 8 mm tapes were prepared in the same manner as in Example 1. Sample Examples 6-9 were made.

[0032]

[Table 1] Examples 10 to 14 As a resin binder for <coating material for upper magnetic layer>, Example 1
A polyurethane resin having the same sodium sulfonate group as the resin binder used in the <Coating for the upper magnetic layer>, which was different from Example 1 in the molecular weight and Tg of the present invention, was used. Then, in the same manner as in <Coating for upper magnetic layer> of Example 1 except for the above, five types of <coating for upper magnetic layer> were prepared. The number average molecular weight and Tg of these polyurethane resins having Na sulfonate groups are shown in Table 2.
It is shown in.

Then, each of the obtained 5 kinds of <coating material for upper magnetic layer> and <coating material for lower magnetic layer> prepared in Example 1 were combined, and 5 kinds of multilayered layers were prepared in the same manner as in Example 1. 8
mm tape sample Examples 10-14 were prepared.

Comparative Example 1 As the magnetic powder for short wavelength recording, the same Co, Ti, Sn, Nb substituted Ba ferrite powder as the magnetic powder for short wavelength recording of Example 1 was used, and the SFD was within the range of the present invention. No magnetic powder was used.
Then, <Coating for upper magnetic layer> was prepared in the same manner as in <Coating for upper magnetic layer> of Example 1 except the above. Various properties of this magnetic powder are shown in Table 1 together with various properties of the magnetic powder used in Examples 1 to 9.

The obtained <coating material for the upper magnetic layer>
And the <coating material for the lower magnetic layer> prepared in Example 1 were combined to prepare a multilayer 8 mm tape sample comparative example 1 in the same manner as in Example 1.

Comparative Examples 2 to 6 Example 1 was used as a resin binder for the <coating material for the lower magnetic layer>.
Instead of the sulfonic acid group-containing polyurethane resin in the formulation composition of the paint used in Example 1, a sulfonic acid group-free polyurethane resin (molecular weight 24,000 Tg = -5 ° C) was used.
Then, other than that, in the same manner as in Example 1, five types of multi-layer 8
mm tape sample comparative examples 2-6 were created. The number average molecular weight and Tg of these five polyurethane resins are shown in Table 2.
It is shown in.

Comparative Examples 7 to 9 Example 1 was used as a resin binder for the <coating material for the upper magnetic layer>.
Three polyurethane resins having the same sodium sulfonate group as the resin binder used in the <Coating for the upper magnetic layer> of which the molecular weight and Tg were not within the range of the present invention were used. Then, in the same manner as in <Coating for upper magnetic layer> of Example 1 except for the above, three types of <coating for upper magnetic layer> were prepared. The number average molecular weight and Tg of these three types of polyurethane resins having Na sulfonate groups are shown in Table 2.

Then, by combining each of the three kinds of <coating material for upper magnetic layer> thus obtained and <coating material for lower magnetic layer> prepared in Example 1, three kinds of coating materials were prepared in the same manner as in Example 1. Multilayer 8 mm tape sample comparative examples 7-9 were prepared.

Comparative Example 10 Comparative Example 5 was used as a resin binder for the <coating material for upper magnetic layer>.
Polyurethane resin (molecular weight 24,000 Tg = -5 ° C) containing no sulfone group, which was used as a resin binder for <coating material for lower magnetic layer> in ~ 9. Otherwise in the same manner as in Example 1, a multilayer 8 mm tape sample comparative example 10 was prepared.

[0040]

[Table 2] The characteristics of the 24 types of multilayer 8 mm tape samples of Examples 1 to 14 and Comparative Examples 1 to 10 prepared as described above were evaluated. Table 3 shows the evaluation results of Examples 1 to 9 and Comparative Example 1. The evaluation results of Examples 10 to 14 and Comparative Examples 2 to 10 are shown in Table 4. In measuring the output, a standard barium ferrite tape (single layer film) was set to 0 dB for comparison.

[0041]

[Table 3]

[Table 4] As is clear from Tables 3 and 4, according to the present invention,
The surface property of the tape medium sample is improved, and good tape characteristics are obtained. Further, when a polyurethane resin containing no sulfonic acid group or its salt is used in either the upper magnetic layer or the lower magnetic layer, the interface between the upper and lower layers tends to be disturbed, resulting in surface roughness of the multilayer tape. And the output and S / N also deteriorate. Therefore, it is understood that it is extremely important to make the resins of the upper and lower magnetic layers the same.

[0042]

As described above, according to the present invention, it is possible to select and combine magnetic powders and resin binders suitable for the upper magnetic layer and the lower magnetic layer of the magnetic recording medium having a multi-layer structure. High output is obtained in a wide range of wavelengths from the wavelength range to the long wavelength range, the interface between the upper magnetic layer and the lower magnetic layer is less disturbed, and the surface property is excellent, and the S / N is high, and the running property and durability are high. An excellent magnetic recording medium can be obtained.

[0043]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hajime Takeuchi Hajime, Komukai Toshiba-cho, Saiwai-ku, Kawasaki City, Kanagawa Prefecture

Claims (1)

[Claims] 1. A magnetic recording comprising a non-magnetic substrate, a lower magnetic layer containing long-wavelength recording magnetic powder and a resin binder, and an upper magnetic layer containing short-wavelength recording magnetic powder and a resin binder. In the medium, the long-wavelength recording magnetic powder is Co
-Γ ferrite magnetic powder, wherein the magnetic powder for short wavelength recording has a part of Fe replaced by at least one element selected from Nb, Sn, Ni, and Zn, and has the following characteristics: SFD: 0.3 5 Hexagonal ferrite powder having a saturation magnetization σ _g : 50 to 70 emu / g coercive force Hc: 500 to 3,000 Oe: and a resin binder contained in the upper magnetic layer and the lower magnetic layer is a molecule. A magnetic recording medium containing a sulfonic acid group or a salt thereof in the chain and comprising a polyurethane resin having the following characteristics: molecular weight: 1,000 to 100,000 Tg: -40 to + 40 ° C.