JP2835744B2 - Manufacturing method of magnetic recording medium - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a magnetic recording medium in which two or more magnetic layers are superposed.

[Background of the Invention]

Magnetic recording media used for various devices such as video tape recorders, etc. are manufactured by applying a magnetic paint on a non-magnetic support such as a polyester film, and then removing the magnetic particles in the magnetic coating to improve the electromagnetic conversion characteristics. An alignment process for aligning in a specific direction is performed. That is, for example, a magnetic coating material in which needle-like magnetic particles are dispersed together with a binder is applied to a non-magnetic support to a predetermined thickness, and the applied coating material has not yet dried and solidified, and the needle-like magnetic material in this coating film has not yet been solidified. When the particles can move,
The support coated with the magnetic paint is run in a magnetic field to orient the needle-like magnetic particles in the magnetic coating in the direction of the magnetic field, thereby increasing the squareness ratio in the orientation direction and increasing the magnetic recording medium. To improve sensitivity.

Conventionally, the orientation treatment employs a method of applying a DC magnetic field using a permanent magnet or a DC electromagnet. by the way,
In this method, even if the applied magnetic field is increased to increase the degree of orientation, the degree of orientation is not so much increased, but rather, if the magnetic field is increased to a certain degree or more, the surface smoothness of the magnetic coating film is deteriorated. It is said that there is.

Therefore, various methods for improving such an alignment treatment have been proposed. For example, Japanese Patent Publication No. Sho 49-30722 discloses an arrangement in which an auxiliary magnetic field device for superimposing an AC auxiliary magnetic field on a main alignment magnetic field is arranged near a main alignment device using a permanent magnet or a DC electromagnet. Further, as disclosed in JP-A-54-98205, there is disclosed a method of superimposing an AC magnetic field or mechanical vibration in a direction perpendicular to a main orientation direction by a DC magnetic field to facilitate alignment of particles. . Further, Japanese Patent Application Laid-Open
JP-A-88101 discloses the effect of applying an AC magnetic field in addition to a DC magnetic field in a magnetic disk orientation step to apply vibration to particles to facilitate orientation.

However, the conventional magnetic field orientations including any of these methods are all orientations by a DC magnetic field, and the AC magnetic field is given as an auxiliary weak magnetic field, and merely gives a vibration that facilitates the movement of particles at the time of orientation. .

And even if an alternating magnetic field is applied in this manner, basically, the alignment is performed by a DC magnetic field even if the intensity of the alignment magnetic field is in a small range equal to or less than the coercive force of the magnetic particles. Although a certain degree of orientation effect is produced, a sufficiently high squareness ratio cannot be obtained even if the orientation magnetic field is increased to be higher than the coercive force of the magnetic particles in order to obtain a high squareness ratio as described above. Rather, the surface properties of the coating film deteriorate as described above.

In particular, when magnetic particles having poor dispersibility or magnetic particles having large cohesiveness are used as the magnetic paint, and when the content ratio of the magnetic particles in the magnetic paint to the binder is large, good orientation is difficult to be achieved, and The mold ratio cannot be increased.

Regardless of whether or not an auxiliary AC magnetic field is applied as described above, when the orientation is substantially performed by a DC magnetic field, a sufficiently high squareness ratio cannot be obtained because of the orientation in the DC magnetic field orientation. It was pointed out that this was due to the fact that the magnetization reversal of the particles was hardly involved in the mechanism.

That is, conventionally, a magnetic recording medium coated with a magnetic paint is transferred to a magnetic field generating means that mainly applies a DC magnetic field to a state in which magnetic particles can move without drying the coating film. Generally, the magnetic field orientation is performed. In this case, the DC magnetic field applied to the magnetic particles of the magnetic field recording medium is set by the magnetic field generating means as soon as it enters the magnetic field generating means with a sharp rise. It does not rise to the required strength for the magnetic field, i.e. the orientation, but as it approaches the magnetic field generating means,
It shows a rise with a certain degree of inclination that is affected by the magnetic field by the magnetic field generating means. Therefore, in this case, even if the orientation magnetic field is selected to be larger than the coercive force of the magnetic particles, the magnetic particles always undergo a state of receiving a magnetic field equal to or less than the coercive force of the magnetic particles even for a short time. In this state, of course, no reversal of magnetization occurs, but the particles themselves begin to rotate. In this case, the magnetic particles having the direction of the spontaneous magnetization inclined in the direction of the orientation magnetic field can be oriented in the direction of the orientation magnetic field at a relatively small rotation angle φ of less than 90 °, but the direction of the spontaneous magnetization is When the direction is opposite to the direction of the aligning magnetic field, it is rotated by a large angle φ of 180 ° at the maximum to orient in the direction of the aligning magnetic field. Therefore, in this case, since a large movement is required until the alignment is completed, not only the alignment takes a long time but also the alignment treatment time, that is, even if the magnetic field is applied for a long time, the particle passing is not possible. The large rotations cause the two to be caught by each other and tend to stop in a state where they have not completed their orientation. This phenomenon becomes more remarkable especially when magnetic particles having poor dispersibility or magnetic particles having high cohesiveness are used as described above, or when the content ratio of the magnetic particles to the binder in the magnetic paint is increased.

Thus, when considering DC magnetic field orientation, the movement of individual particles during the orientation process is unreasonable, and sometimes it causes entanglement of the particles, which is the opposite effect of orientation, and the orientation magnetic field is used as a strong magnetic field. In addition, it has been suggested that the improvement of the orientation not only does not allow the substrate to be so much, but also causes the deterioration of the surface smoothness. In order to overcome this, for example, Japanese Patent Application Laid-Open No.
In Japanese Patent Application Laid-Open Publication No. H10-157, a method of manufacturing a magnetic recording medium is proposed in which an alternating magnetic field at least equal to the coercive force of magnetic particles is applied in parallel with the orientation direction to perform orientation treatment.
Japanese Patent Application Laid-Open No. 56-119938, characterized in that the magnetic recording is performed by a first step of applying a pulse magnetic field having a coercive force equal to or greater than the coercive force of the magnetic particles and a second step of applying a DC magnetic field in parallel with the direction of orientation. Media production methods have been proposed, and these techniques have been able to moderately increase the degree of orientation.

Incidentally, a technical idea of realizing the magnetization reversal by a DC magnetic field has begun to be considered.

Therefore, it is conceivable to apply such a technical idea of magnetization reversal not only to a single magnetic layer but also to a magnetic recording medium in which two or more magnetic layers are superposed.

That is, as shown in, for example, Japanese Patent Publication No. 53-21905 and many other documents, it is widely proposed to provide two or more magnetic layers on a non-magnetic support. In order to improve the degree of orientation of magnetic particles in a magnetic recording medium provided with two or more superposed magnetic layers, it is conceivable to apply a technical concept of magnetization reversal.

However, even when this technical idea is implemented, unlike a magnetic recording medium having only one magnetic layer on a non-magnetic support, magnetic recording is performed by superposing two or more magnetic layers on the non-magnetic support. The media did not give the desired results as expected.

[Disclosure of the Invention]

An object of the present invention is to provide a magnetic recording medium having two or more magnetic layers, in which substantially no disturbance occurs at the interface between the magnetic layers, the surface has high smoothness, and the orientation is improved. It is to provide a recording medium.

An object of the present invention is a method for manufacturing a magnetic recording medium in which two or more magnetic layers are superimposed, and after these magnetic layers are formed, these magnetic layers have a maximum coercive force. This is achieved by a method for manufacturing a magnetic recording medium, wherein the orientation treatment is performed with a magnetic field having a strength three times or more the coercive force of the magnetic particles.

In the present invention, the magnetic field having a strength three times or more is generated by the NN opposed magnet and / or the SS opposed magnet, and the ratio l / w of the effective length 1 and the effective width w of the magnet is used. Is 4/3
Desirably larger.

That is, in a magnetic recording medium having two or more magnetic layers, if the orientation treatment is performed at a magnetic field strength equal to or less than the coercive force of the magnetic particles having the smallest coercive force in these magnetic layers, sufficient orientation cannot be obtained, and The fact that the surface properties of the magnetic layer are not very satisfactory is the same as in the case of a magnetic recording medium having only one magnetic layer.

Further, when the orientation treatment of a magnetic recording medium having two or more magnetic layers at a magnetic field strength between the coercive force of the smallest magnetic particles and the coercive force of the largest magnetic particles, some of the Reversal of the magnetization direction of the magnetic particles occurs only in the magnetic layer,
The magnetic stability between the layer and the layer adjacent to the layer in which the direction of magnetization of the magnetic particles did not reverse did not occur, and the interface between the layers was disturbed, or even the surface of the magnetic layer was adversely affected. Appear, and irregularities occur on the surface, resulting in poor surface smoothness and poor electromagnetic conversion characteristics.

Incidentally, the magnetic field formed by the N-N opposed magnets or S-S counter magnets, as shown in FIG. 1, with a maximum in the vicinity of both ends of the magnet M ₁ and M _2, the distance from the magnet M _1, M ₂ Gradually weakens.

Now, suppose that the magnets M ₁ and M ₂ have a rectangular parallelepiped shape (length 1 and width w).
, The leakage magnetic field (the range is indicated by a) formed outside the width w of the magnets M ₁ and M ₂ is a≫w compared to the width w of the magnet, and the magnetic particles Is considered to have a greater influence on the orientation than the magnetic field within the magnet width w. Therefore, it is considered that increasing the leakage magnetic field a leads to an improvement in the degree of orientation. The leakage magnetic field a can be increased by increasing the length l of the magnet.

If the maximum strength B _max of the opposing magnetic field in FIG. 1 is equal to or less than the coercive force Hc of the magnetic particles of the magnetic recording medium, the magnetic particles oriented in the region of の will have a large rotation of up to 180 ° due to the magnetic field in the opposite direction. Then, it is reoriented in the direction of the magnetic field.
The orientation performed in the magnetic field in this case is totally useless.

However, when B _max ≧ Hc, the magnetic field B =
At Hc (indicated by Hc in FIG. 1), the direction of magnetization of the magnetic particles is reversed, and the subsequent magnetic field should act to further increase the orientation state of the magnetic particles oriented by the magnetic field. It is.

Therefore, if the orientation treatment is performed at a magnetic field strength equal to or higher than the coercive force of the magnetic particles having the largest coercive force, a magnetic recording medium having good orientation even with two or more magnetic layers and excellent recording and reproducing characteristics can be obtained. However, contrary to the expectation, when there are two or more magnetic layers, for example, an NN opposed magnet or an SS opposed magnet having a magnetic field strength about twice the coercive force of the magnetic particles having the largest coercive force. However, unlike the orientation treatment of a magnetic recording medium having only one magnetic layer, the recording / reproducing characteristics were not as expected.

However, if the orientation treatment of a magnetic recording medium having two or more layers is performed at a magnetic field strength that is three times or more the coercive force of the magnetic particles having the largest coercive force, for example, about three times the coercive force of the magnetic particles having the largest coercive force When the orientation processing is performed with the NN opposed magnet or the SS opposed magnet having the magnetic field strength, unlike the case where the orientation processing is performed using the NN opposed magnet or the SS opposed magnet having about twice the magnetic field strength, the magnetization reversal is all performed. The magnetization direction is reversed so that each magnetic particle is oriented in the direction of the magnetic field with the minimum rotation angle, and the magnetic particles are oriented in a short time, causing disturbance due to large rotation of the magnetic particles. Since the magnetic stability of the interface between the magnetic layers was maintained without performing the method, a magnetic layer having good surface smoothness, high orientation, and good electromagnetic conversion characteristics was obtained.

The maximum magnetic field strength B _{max of the} NN opposed magnetic field or the SS opposed magnetic field is three times or more the maximum coercive force Hc of the magnetic particles of the magnetic recording medium. The ratio l / w of the effective length 1 of the magnet in the direction perpendicular to the surface of the magnetic recording medium to the effective width w in the direction of travel of the magnetic recording medium is 4
It is desirable that the ratio is not less than / 3, but it is more preferable that
It is desirable that this is the case.

Incidentally, although the magnet M _1, M ₂ is no problem because the length l and width w of the magnets is the same as its geometric dimensions when a rectangular parallelepiped shape, when the magnets M _1, M ₂ is not a rectangular parallelepiped shape In consideration of this, a magnet having a rectangular parallelepiped shape corresponding to the magnetic field of the magnet not having the rectangular parallelepiped shape may be considered, and the calculation may be performed using the length l and the width w of the geometric dimensions of the magnet.

Also, the number of NN opposed magnets (or SS opposed magnets) is not limited to one, and two or more may be combined. In such a case, for example, NN opposed magnets, NN opposed magnets or S-
It is desirable that the same magnets such as the S counter magnet and the SS counter magnet are not arranged, but the NN counter magnets and the SS counter magnets are alternately arranged.

The present invention can be applied to a magnetic recording medium having two or more magnetic layers, and can be applied not only to two layers but also to three layers.

[Examples] Next, examples of the present invention and comparative examples are shown, and the present invention will be described more specifically.

(Example 1) After sufficiently kneading and dispersing each of the first magnetic layer composition and the second magnetic layer composition having the following compositions using a ball mill, a polyisocyanate compound was added to the first magnetic layer composition.
11 parts and 6 parts of a polyisocyanate compound were added to the second magnetic layer composition to prepare a magnetic paint.

First magnetic layer composition Co-γFe ₂ O ₃ (Hc = 600 Oe, average particle diameter 0.3 μm) 100 parts Vinyl chloride resin 10 parts Polyester polyurethane 5 parts α-Al ₂ O ₃ (average particle diameter 0.2 μm) 5 Part Carbon black 1 part Myristic acid 1 part Stearic acid 1 part Butyl stearate 1 part Methyl ethyl ketone 100 parts Cyclohexane 100 parts Toluene 100 parts Second magnetic layer composition Co-γFe ₂ O ₃ (Hc = 900 Oe, average particle diameter 0.2 μm 100 parts Vinyl chloride resin 10 parts Polyester polyurethane 5 parts α-Al ₂ O ₃ (average particle size 0.2 μm) 5 parts Carbon black 1 part Myristic acid 1 part Stearic acid 1 part Butyl stearate 1 part Methyl ethyl ketone 100 parts Cyclohexane 100 Part The first magnetic paint and the second magnetic paint obtained in this manner are applied to an apparatus as shown in FIG. 2 (in FIG. 2, 1 is a non-magnetic support, 2 is an unwinding roll, 3 Lower coating head, the upper coating head 4, 5 is the length l is the width w is 3cm in 6 cm N-N
(Or SS) counter magnet, 6 is a surface processed part, 7 is a back coat application head, 8 is a drying part, and 9 is a winding part) on a polyethylene terephthalate non-magnetic support having a thickness of 14.5 μm. First, after the first magnetic paint is applied, then the second magnetic paint is applied in a state where the first magnetic paint is not dried, and the magnetic field strength along the longitudinal direction of the non-magnetic support is 2700 Oe.
The orientation treatment was performed by the counter magnet 5 and the super calender treatment was performed to form a magnetic coating layer having a first magnetic layer as a lower layer and a second magnetic layer as an upper layer.

The dry thickness of the first magnetic layer was 2.0 μm, and the dry thickness of the second magnetic layer was 1.0 μm.

Next, a back coat paint composition having the following composition was dispersed in a ball mill for 5 hours to prepare a back coat paint, and this paint was dried to a thickness of 0.5 μm on the back surface of the support having the magnetic layer. And dried to form a back coat layer, thereby producing a wide magnetic tape for video.

Backcoat paint composition Carbon black 50 parts Nitrocellulose 20 parts Polyurethane resin 20 parts Methyl ethyl ketone 200 parts Toluene 200 parts (Examples 2 and 3) In Example 1, the magnetic field strength was set to 3600 Oe, 4500 Oe, and the others were exactly the same. went.

(Comparative Examples 1 to 6) In Example 1, the magnetic field intensity was 400 Oe, 800 Oe950 O
e, 1300 Oe, 1800 Oe, and 2300 Oe.

(Comparative Examples 7 to 13) For a magnetic recording medium provided with only the first magnetic layer in Example 1 to have a dry thickness of 3.0 μm (without the second magnetic layer), the magnetic field strength was 400 Oe, 800 Oe, 1300. Oe, 1800 O
The alignment treatment was performed at e, 2700 Oe, 3600 Oe, and 4500 Oe.

As for these video magnetic tapes, the squareness ratio of the component in the direction along the longitudinal direction of the non-magnetic support, the surface roughness of the magnetic layer surface, and Y-C / N and C-C / N were examined. Table 1 shows the results.

 The evaluation of each item is as follows.

Squareness ratio: The ratio (Br / Bm) between the residual magnetic flux density (Br) and the saturation magnetic flux density (Bm) was determined for the longitudinal component using a VSM under the conditions of the measurement magnetic field IOKOe.

Y-C / N, CC-N: Yokogawa Hewlett-Parkard 3
The measurement was performed using a Model 325 generator and a spectrum analyzer (Model TR4101) manufactured by Takeda Riken Co., Ltd. In addition, YC /
N records a 4.0 MHz signal, and C / C / N records a 0.6 MHz signal, and the reproduction output is measured.

Surface roughness (Ra): Measured with a cutoff value of 0.008 mm using a Surfcom 1500 surface roughness meter manufactured by Tokyo Seimitsu Co., Ltd.

As can be seen from Table 1, when the magnetic layer provided on the non-magnetic support is a single layer, if the orientation treatment is performed with a magnet having a magnetic field equal to or higher than the coercive force of the magnetic particles of the magnetic layer, the electromagnetic conversion characteristics can be improved. A magnetic recording medium excellent in quality can be obtained.

However, when the magnetic layer provided on the non-magnetic support is two or more layers, even if the orientation treatment is performed with a magnet having a magnetic field of at least the maximum coercive force among the magnetic particles of these magnetic layers, A magnetic recording medium having excellent electromagnetic conversion characteristics cannot be obtained.

That is, in the case of a magnetic recording medium in which two or more magnetic layers are superimposed, unless the magnetic field is subjected to an orientation treatment with a magnetic field having a magnetic field strength three times or more the coercive force of the magnetic particles in these magnetic layers, A magnetic recording medium with excellent conversion characteristics cannot be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining the magnetization reversal, and FIG. 2 is a schematic diagram of a magnetic recording medium manufacturing apparatus. M ₁ , M ₂ ... magnet, l ... length, w ... width, 1 ... non-magnetic support, 2 ... unwinding roll, 3 ... lower layer coating head, 4 ... upper layer coating head, 5 ... NN opposed magnets, 6... Surface treatment section, 7... Backcoat coating head, 8... Drying section, 9.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G11B 5/845

Claims (2)

(57) [Claims] 1. A method of manufacturing a magnetic recording medium comprising two or more magnetic layers superimposed on each other, wherein after forming these magnetic layers, the magnetic particles having the largest coercive force in these magnetic layers A magnetic recording medium having a magnetic field at least three times as strong as the coercive force. 2. A magnetic field having a strength three times or more is formed by an NN opposed magnet and / or an SS opposed magnet, and an effective length of the magnet in a direction perpendicular to a running direction of a magnetic recording medium. The ratio l / w of l to the effective width w in the running direction of the magnetic recording medium is 4/3.
The method for manufacturing a magnetic recording medium according to claim 1, wherein the method is larger.