JP2001134920A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium having excellent electromagnetic conversion characteristics optimum for high density recording and durability. SOLUTION: Relating to the magnetic recording medium having a non- magnetic substrate 1 and a magnetic layer 3 formed by applying a magnetic paint consisting of at least magnetic powder and a bonding agent which are dispersed in a solvent, the bonding agent contains a polyvinyl acetal resin having >=80 deg.C glass transition temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium such as an audio tape, a video tape and a magnetic disk, and more particularly to a magnetic recording medium having a magnetic layer coated with a magnetic paint.

[0002]

2. Description of the Related Art In recent years, in the field of magnetic recording media, high-density recording has been promoted by using short-wavelength signals.
In order to realize high-density recording in a coating type magnetic recording medium provided with a magnetic layer formed by applying a magnetic paint, it is required to improve the electromagnetic conversion characteristics for short wavelength signals.

As a technique for improving the electromagnetic conversion characteristics in a high-density recording area in a coating type magnetic recording medium, there is a method of reducing the thickness of a magnetic layer. By making the magnetic layer thinner,
It is possible to reduce the self-demagnetization loss at the time of recording and the thickness loss at the time of reproduction that occur when a short wavelength signal is used. As a result, the magnetic recording medium has effectively improved electromagnetic conversion characteristics.

However, when a short wavelength signal is used for high density recording, the output of the magnetic recording medium tends to decrease. Therefore, what is being studied to improve the output characteristics with respect to the short-wavelength signal is to increase the average number of particles of the magnetic powder per unit volume of the magnetic layer. For example, the average major axis length is 0.13
By using a magnetic powder of fine particles having a size of not more than μm, the average number of particles of the magnetic powder per unit volume of the magnetic layer can be increased. Thereby, recording demagnetization can be reduced, and as a result, C / N can be improved.

In a coating type magnetic recording medium, a hexagonal ferrite powder exhibiting crystal orientation, for example, barium ferrite (BaF) is used as a magnetic powder suitable for high-density recording.
e ₁₂ O ₁₉ ) and the like are being used. Hexagonal ferrite powder has attracted attention as a magnetic powder that can be used for both the perpendicular recording method and the in-plane recording method for realizing high-density recording because the orientation of the C axis, which is the axis of easy oxidation, is easy. ing.

[0006]

However, in the coating type magnetic recording medium, if the thickness of the magnetic layer is reduced to, for example, 0.5 μm or less, the contact pressure generated between the magnetic head and the magnetic recording medium is reduced. The magnetic layer may not be able to withstand. In this case, the magnetic layer is scraped and powder fall occurs. As a result, in a magnetic recording medium having a thin magnetic layer, there is a possibility that problems such as dropouts occur frequently during recording and reproduction.

When the magnetic layer is thinned, the magnetic layer may be plastically deformed. The plastic deformation of the magnetic layer causes deterioration of the flatness of the surface of the magnetic layer. That is, if plastic deformation occurs in the magnetic layer, the surface of the magnetic layer undulates. If the magnetic layer surface has undulation and is not of a uniform thickness, the above-mentioned problems such as dropout become more prominent. Therefore, it is necessary to increase the strength of the magnetic layer in order to suppress plastic deformation of the magnetic layer.

In order to increase the strength of the magnetic layer, it is conceivable to use a resin having a glass transition temperature of about 80 ° C. or higher as a binder for the magnetic paint. For example, nitrocellulose is one of the resins having a glass transition temperature of 80 ° C. or higher.

However, the conventional glass transition temperature is 80 ° C.
Since the magnetic paint containing the above resin as a binder has poor dispersibility, it is not possible to sufficiently disperse the fine magnetic powder having an average major axis length of 0.13 μm or less in the magnetic paint.

[0010] In particular, since the above-mentioned barium ferrite is required to be a powder having a small average particle size and a large plate ratio, it is difficult to sufficiently disperse it in a magnetic paint. Here, the average particle size refers to the average major axis of the C-axis plane of the crystal in the hexagonal ferrite powder. The plate ratio is a ratio between the average major axis and the thickness in the C-axis direction.

For this reason, in a coating type magnetic recording medium, when the magnetic layer contains a resin having a glass transition temperature of about 80 ° C. or higher as a binder, high-density recording is realized by using a short wavelength signal. The required electromagnetic conversion characteristics could not be obtained.

The present invention has been proposed based on such a conventional situation. By forming a magnetic layer having high strength and highly dispersed fine magnetic powder,
An object of the present invention is to provide a magnetic recording medium having excellent durability and excellent electromagnetic conversion characteristics that are optimal for high-density recording.

[0013]

A magnetic recording medium according to the present invention, which has achieved the above-mentioned objects, comprises a non-magnetic support, and a magnetic paint comprising at least a magnetic powder and a binder dispersed in a solvent. In the magnetic recording medium having the magnetic layer to be formed, the binder contains a polyvinyl acetal resin having a glass transition temperature of 80 ° C. or higher.

In the magnetic recording medium according to the present invention configured as described above, the magnetic layer contains a polyvinyl acetal resin having a glass transition temperature of 80 ° C. or more as a binder.
As a result, the magnetic layer has a high strength and a highly dispersed magnetic powder of fine particles.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the magnetic recording medium according to the present invention will be described below in detail with reference to the drawings.

As shown in FIG. 1, a magnetic recording medium to which the present invention is applied includes a non-magnetic support 1, a non-magnetic layer 2 formed on the non-magnetic support 1, and a non-magnetic support 2 on the non-magnetic support 2. And the magnetic layer 3 formed on the substrate.

As the nonmagnetic support 1, any of a polyethylene terephthalate film, a polyethylene naphthalate film, an aramid film and the like can be used.
Further, the shape of the nonmagnetic support 1 is not particularly limited, and may be any shape such as a tape shape, a disk shape, and a card shape.

The non-magnetic layer 2 is formed by applying a non-magnetic paint comprising at least a non-magnetic powder and a binder. As the non-magnetic powder, a conventionally known non-magnetic powder, for example, α-Fe ₂ O ₃ can be used. The binder contained in the nonmagnetic layer 2 includes:
A conventionally known binder, for example, a vinyl chloride resin can be used. The magnetic layer 3 is formed by applying a magnetic paint in which at least a magnetic powder and a binder are dispersed in a solvent.

As the magnetic powder, any conventionally known magnetic powder can be used, and both oxide magnetic powder and metal magnetic powder can be used.

As the oxide magnetic powder, for example, γ-F
e ₂ O ₃ , Co-containing γ-Fe ₂ O ₃ , Fe ₃ O ₄ , Co-containing γ
—Fe ₃ O ₄ , Co-coated —Fe ₃ O ₄ , CrO _2, etc. Examples of the metal magnetic powder include Fe, Co, and N.
i, Fe-Co, Fe-Ni, Fe-Co-Ni, Co
-Ni, Fe-Co-B, Fe-Co-Cr-B, Mn
-Bi, Mn-Al, Fe-Co-V and the like.
Further, for the purpose of improving various characteristics of the metal magnetic powder, a metal magnetic powder to which a metal component such as Al, Si, Ti, Cr, Mn, Cu, and Zn is added can be used.

The average major axis length of the magnetic powder is preferably 0.13 μm or less. Further, it is more preferably 0.10 μm or less, particularly preferably 0.08 μm or less. If the average major axis length of the magnetic powder is longer than 0.13 μm, it is difficult to increase the output for a short wavelength signal. Therefore, the average major axis length without magnetic powder is 0.13.
By setting the thickness to μm or less, it is possible to improve the recording characteristics for a short wavelength signal.

The magnetic powder has an average particle size of 0.1.
It is preferable to use a hexagonal ferrite powder having a particle size of 1 μm or less and a plate ratio of 2 or more. Further, the average particle size is more preferably 0.05 μm or less, and particularly preferably 0.03 μm or less. As the hexagonal ferrite, it is possible to appropriately select and use conventionally known ones having various metal ions, and specific examples include barium ferrite powder.

The hexagonal ferrite powder having an average particle size of 0.1 μm or less and a plate ratio of 2 or more has excellent magnetic properties. This realizes the excellent electromagnetic conversion characteristics.

As the binder contained in the magnetic layer 3, a polyvinyl acetal resin having a glass transition temperature of 80 ° C. or higher is used. Here, the glass transition temperature refers to JI of thermal analysis.
It is specified in the S standard (JISK7121) or the ISO standard for thermal analysis (ISO11357-1 / -2). Further, a polyvinyl acetal resin having a glass transition temperature of 100 ° C. or more is more effective, and a polyvinyl acetal resin having a glass transition temperature of 110 ° C. or more is most effective. The magnetic layer 3 has high strength by containing a polyvinyl acetal resin having a glass transition temperature of 80 ° C. or higher as a binder.

As the polyvinyl acetal resin, a polyvinyl acetal resin represented by the structural formula (1) is used.
The polyvinyl acetal resin is a general term for a resin obtained by reacting polyvinyl alcohol obtained by saponifying polyvinyl acetate with an aldehyde, and polyvinyl alcohol is intramolecularly cyclized to form an acetal group. As the aldehyde, various aldehydes such as formaldehyde, acetaldehyde and butyraldehyde can be used.

When a polar group-containing polyvinyl acetate obtained by copolymerizing a required amount of a vinyl monomer having a sulfonate group is used as a raw material, a polyvinyl acetal resin containing a polar group is synthesized. By controlling the copolymerization amount of the vinyl monomer having a sulfonate group, the number of polar group units is adjusted.

Further, by controlling the degree of the saponification reaction of polyvinyl acetate, the number of units of vinyl acetate is adjusted. Further, as a result of the acetalization reaction of polyvinyl alcohol obtained by saponifying polyvinyl acetate, the number of acetal groups and hydroxyl groups is adjusted by controlling the degree of acetalization (the ratio of acetalized hydroxyl groups). .

The polyvinyl acetal resin represented by the structural formula (1) has a degree of polymerization of 100 to 1,000,
The molar composition of the acetal unit represented by k in the formula, the vinyl acetate unit represented by l in the formula, the vinyl alcohol unit represented by m in the formula, and the polar group unit represented by p in the formula is k + 1 + m + p = 100, 55 ≦ k ≦ 9
Those having the ratio of 0, 1 ≦ l ≦ 20, 5 ≦ m ≦ 30, and 0 ≦ p ≦ 3 are preferable.

When k is less than 55, the glass transition temperature is 8
It is difficult to synthesize a resin having a temperature of 0 ° C. or higher, and the hardness of the resin becomes insufficient. Therefore, the durability of the magnetic layer 3 may not be sufficiently secured. On the other hand, if k exceeds 90, synthesis is difficult.

L has an effect on the compatibility and durability with other resins used in the magnetic layer 3, and in order to maintain their properties well, the range of 1 ≦ l ≦ 20 is satisfied. Is preferred.

M is preferably in the range of 5 ≦ m ≦ 30 in view of compatibility with other resins used in the magnetic layer 3 and reactivity with isocyanate as a curing agent.

P greatly affects the dispersibility of magnetic powder and the like. In particular, since finely divided magnetic powder has poor dispersibility, by using a resin having a polar group as a binder, it is possible to improve the dispersibility of the magnetic powder in the magnetic paint, and as a result, the magnetic paint Dispersibility can be improved. If p exceeds 3, the solvent solubility of the polyvinyl acetal resin becomes poor, and there is a possibility that the polyvinyl acetal resin lacks practicality.

The polar group X is -OSO ₃ Na, -SO ₃ N
_{a, -SO 3 K, -SO 3} H, -COOH, -OPO (O
_{H) 2, -PO (OH)} 2, -NR 2, -NR 3 Cl, -N
R ₃ Br, —NR ₃ I and the like.

The alkyl group R is represented by C _n H _{2n + 1} (n = 0,
1, 2, 3, 4, 5, 6). When n = 0, R is hydrogen H.

The glass transition temperature of the polyvinyl alcohol resin can be adjusted by changing the alkyl group R, controlling the degree of acetalization (the number of acetal units), and converting some of the hydroxyl groups of the vinyl alcohol unit with an aromatic compound such as phenyl isocyanate. It can be adjusted by a method such as denaturation.

Specifically, the smaller the n of the alkyl group R, the higher the glass transition temperature. Further, the higher the degree of acetalization, that is, the greater the number of acetal units, the higher the glass transition temperature. Further, the more the hydroxyl group of the vinyl alcohol unit is modified with an aromatic compound having a functional group capable of reacting with the hydroxyl group, the higher the glass transition temperature becomes.

As a polyvinyl acetal resin in which a part of the hydroxyl group of the vinyl alcohol unit is modified with an aromatic compound, specifically, a part of the hydroxyl group of the vinyl alcohol unit of the polyvinyl acetal resin represented by the structural formula (1) is For example, it is preferable to use a polyvinyl acetal resin modified with phenyl isocyanate shown in Chemical formula 2 and having a structure shown in Chemical formula 3 as a binder contained in the magnetic layer. Since such a polyvinyl acetal resin has a higher glass transition temperature, the magnetic layer 3 has a higher strength.

[0038]

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[0039]

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It is also possible to modify a part of the hydroxyl groups of the vinyl alcohol unit with an aromatic compound shown in Chemical formula 4.

[0041]

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The amount of the polyvinyl acetal resin mixed with the magnetic paint is preferably 1 to 30 parts by weight, more preferably 2 to 25 parts by weight, and more preferably 5 to 20 parts by weight based on 100 parts by weight of the magnetic powder. Is most preferred.

As the binder to be contained in the magnetic paint, this polyvinyl acetal resin may be used alone or in combination of two or more. Further, the polyvinyl acetal resin and other known binders may be used in combination.

Examples of the binder that can be used in combination include a polyurethane resin and a vinyl chloride copolymer. Specifically, the weight average molecular weight is 5000 to 2000
A vinyl chloride-vinyl acetate copolymer, a vinyl chloride-vinylidene chloride copolymer, a vinyl chloride-acrylonitrile copolymer, a butadiene-acrylonitrile copolymer, a polyamide resin, a polyester resin, a cellulose resin (such as nitrocellulose), Examples include a styrene-butadiene copolymer, a phenol resin, an epoxy resin, a urea resin, a melamine resin, a phenoxy resin, a silicone resin, an acrylic resin, a urea formamide resin, and various synthetic rubber resins.

The magnetic paint may contain a dispersant. As the dispersant, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid and fatty acids having 12 to 18 carbon atoms such as oleic acid, salts of these alkali metals or salts of alkaline earth metals or Examples thereof include amides and azo compounds having a carboxyl group and a sulfonic acid group.
These dispersants are 0.5 to 5% by weight based on the magnetic powder.
It is preferable to use within the range.

The magnetic paint can contain an abrasive. Examples of the abrasive include α-alumina, fused alumina, chromium oxide, titanium oxide, α-iron oxide, silicon oxide, silicon nitride, tungsten carbide, molybdenum carbide, boron carbide, corundum, zinc oxide, cerium oxide, magnesium oxide, and the like. Can be mentioned. The average particle size of these abrasives is preferably in the range of 0.05 μm to 0.8 μm, and more preferably in the range of 0.05 μm to 0.3 μm. The amount of the abrasive is preferably in the range of 3 to 20 parts by weight, more preferably 5 to 10 parts by weight, based on 100 parts by weight of the magnetic powder. By adding the abrasive to the magnetic paint within the above range, the magnetic recording medium has good surface properties and abrasiveness.

The magnetic paint may contain a curing agent for the purpose of crosslinking and curing the binder. As the curing agent, a polyisocyanate or the like is used. Examples of the polyisocyanate include an aromatic polyisocyanate such as an adduct of tolylene diisocyanate and an active hydrogen compound, and an aliphatic polyisocyanate such as an adduct of hexamethylene diisocyanate and an active hydrogen compound. The weight average molecular weight of these polyisocyanates is 100 to 3
000 is preferred. Further, a curing agent for epoxy resin such as triazinedithiol or ethylenediamine may be used for the epoxy group-containing resin.

The magnetic paint can contain a lubricant. As the lubricant, a fatty acid or a fatty acid ester is used alone or in combination. The fatty acid may be a monobasic acid or a dibasic acid, and has a carbon number of 6 to
It is preferably 30. In addition, a known lubricant may be used together with the fatty acid or the fatty acid ester. Examples of the lubricant which can be used in combination include silicone oil, carbon fluoride, fatty acid amide, olefin oxide and the like.

Examples of the solvent used for forming the magnetic paint and the non-magnetic paint include known solvents which have been conventionally used, for example, methyl ethyl ketone, toluene, cyclohexanone and the like.

When preparing the magnetic paint and the non-magnetic paint, a kneader used in the kneading step, a diluting disperser used in the diluting and dispersing step, and the like are used. The kneading step is a step of dispersing a pigment containing a pigment (a non-magnetic powder or a magnetic powder) having a relatively high solid content and a resin at a high shear. The dilution and dispersion step is a step of dispersing a pigment containing a pigment having a relatively low solid content, a resin, and the like by the impact force of beads or the like.
Conventionally known machines can be used for the kneader and the dilution / dispersion machine. Specific examples of the kneader include a continuous twin-screw kneader (extruder), a co-kneader, and a pressure kneader. In addition, examples of the diluting and dispersing machine include a vertical sand mill, a horizontal sand mill, a spike mill, a pearl mill, a double cylinder pearl mill, and the like.

When forming the non-magnetic layer 2 and the magnetic layer 3, a non-magnetic paint and a magnetic paint are applied on the non-magnetic support 1 in this order. At this time, a so-called wet-on-dry method in which a non-magnetic paint and a magnetic paint are applied and dried one layer at a time, and a so-called wet-on-wet method in which a magnetic paint is applied over a non-magnetic paint in a wet state. There is. Of these, the wet-on-wet method is preferably used. At this time, a die coater is mainly used as a coating device.

The die coater has a die head having two slits so that the non-magnetic paint and the magnetic paint are extruded, respectively. Non-magnetic support 1 to which paint is applied
Along the tip surface of the die head where the paint is extruded,
It travels from the slit for non-magnetic paint to the slit for magnetic paint. The non-magnetic support 1 running in this way is coated with the non-magnetic paint extruded from the slit when passing through the slit for the non-magnetic paint.
Then, when passing through the slit for the magnetic paint, the magnetic paint extruded from the slit is applied onto the wet non-magnetic paint.

When a magnetic layer is formed by applying a magnetic paint on a non-magnetic support, gravure coating,
Conventionally known methods for applying a magnetic paint, such as an extrusion coat, an air doctor coat, and a reverse roll coat, can be used.

It is preferable that the thickness of the magnetic layer 3 is 0.5 μm or less. By setting the thickness of the magnetic layer 3 to 0.5 μm or less, it becomes preferable as a recording medium using a short-wavelength signal. If the thickness of the magnetic layer 3 is greater than 0.5 μm, desired electromagnetic conversion characteristics may not be obtained.

According to the above-mentioned method, at least the magnetic layer 3 is formed on the non-magnetic support 1, and the wet coating film is dried, and then, if necessary, subjected to a surface smoothing treatment such as a calender treatment. A long sheet is obtained.

Then, the long sheet is slit or punched into a desired shape, for example, a tape medium is slit into a desired width, and a disk medium is punched into a disk, thereby obtaining a magnetic recording medium. Is produced.

In the magnetic recording medium configured as described above, the magnetic layer 3 has a glass transition temperature of 80 ° C. as a binder.
As described above, it contains the polyvinyl acetal resin represented by the structural formula (1). Therefore, as a magnetic recording medium,
The durability of the magnetic layer 3 is ensured.

When the thickness of the magnetic layer 3 is 0.5 μm or less, the self-demagnetization loss at the time of recording and the thickness loss at the time of reproduction that occur when a short-wavelength signal is used are reduced. And the electromagnetic conversion characteristics are effectively improved.

Further, when the average major axis length of the magnetic powder contained in the magnetic layer 3 is 0.13 μm or less, the magnetic recording medium has excellent electromagnetic conversion characteristics and can perform high-density recording. .

The magnetic powder has an average particle size of 0.1.
By using a hexagonal ferrite powder having a thickness of 2 μm or less and a plate ratio of 2 or more, the magnetic recording medium is optimal for high-density recording and has excellent electromagnetic conversion characteristics.

The magnetic layer 3 contains a polyvinyl acetal resin in which a part of the hydroxyl groups of the polyvinyl acetal resin represented by the structural formula (1) is modified with an aromatic compound such as phenyl isocyanate as a binder. As a magnetic recording medium, the durability of the magnetic layer 3 is further improved.

The magnetic recording medium according to the present invention has a non-magnetic support on a surface opposite to the surface on which the magnetic layer is provided.
A back coat layer may be provided for the purpose of improving the runnability of the magnetic recording medium, and preventing charge and transfer. Further, the magnetic recording medium according to the present invention may be provided with an undercoat layer between the nonmagnetic layer and the nonmagnetic support for the purpose of enhancing the adhesiveness of the nonmagnetic layer. . As the material of the backcoat layer and the undercoat layer, any of those commonly used in magnetic recording media can be used.

[0063]

EXAMPLES Hereinafter, specific examples and comparative examples of the present invention will be described. In this example and comparative examples, disk-shaped magnetic disks were manufactured as described below, and characteristics of these magnetic disks were evaluated.

Example 1 According to the following composition, each component of a magnetic paint and a non-magnetic paint was weighed and kneaded and dispersed using a continuous twin-screw kneader and a sand mill to prepare a magnetic paint and a non-magnetic paint. did.

<Magnetic paint composition> Ferromagnetic powder: 100 parts by weight of metal needle-shaped magnetic powder (coercive force Hc: 2200 [Oe], specific surface covering by BET method: 50 m ² / g, average long axis length: 0 13 μm, needle ratio: 3, saturation magnetization σs: 145 emu / g) Binder-1: 6 parts by weight polyvinyl chloride acetal resin containing sodium sulfonate group (0.8 mol% of metal sulfonate group as a polar group was used. The contained polyvinyl acetate having a degree of polymerization of 300 was saponified to produce polyvinyl alcohol, and then acetalized with acetaldehyde and butyraldehyde, with a degree of acetalization of 75 mol%, a hydroxyl group of 18 mol%, and a glass. (Transition temperature is 80 ° C.) Binder-2: 6 parts by weight of vinyl chloride resin containing potassium sulfonate base 5 parts by weight of α-alumina 1 part by weight Butyl stearate 1 part by weight Solvent 350 parts by weight of myristic acid (methyl ethyl ketone: toluene: cyclohexanone (weight ratio) = 1: 1: a mixed solvent of 1 and a composition)

<Nonmagnetic coating composition> 100 parts by weight of α-Fe ₂ O ₃ (specific surface area by BET method: 52 m ² / g, major axis diameter: 0.15 μm, needle ratio: 6.5) Binder -3: 20 parts by weight of a vinyl chloride resin containing potassium sulfonate base ・ 1 part by weight of myristic acid ・ 1 part by weight of butyl stearate ・ 350 parts by weight of solvent (methyl ethyl ketone: toluene: cyclohexanone (weight ratio) = 1: 1: 1) After adding 5 parts by weight of a polyisocyanate compound as a curing agent to each of the magnetic paint and the non-magnetic paint adjusted as described above, the magnetic paint and the non-magnetic paint are wet-on.・ Thickness 5 by wet coating method
A multi-layer coating was performed on the front and back main surfaces of a non-magnetic support made of a polyethylene terephthalate having a thickness of μm, and then a calender treatment was performed. Then, a long sheet having a 0.5 μm-thick magnetic layer was formed.

Next, the long sheet thus obtained was punched into a 3.5-inch diameter disk-shaped disk having a 1-inch through-hole to produce a magnetic disk.

Example 2 Example 1 was repeated except that the thickness of the magnetic layer was changed to 0.1 μm.
A magnetic disk was produced in the same manner as described above.

Example 3 A magnetic disk was prepared in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 0.1 μm, and that the binder-1 was changed to a polyvinyl acetal resin having a glass transition temperature of 110 ° C. Produced. The degree of acetalization of the polyvinyl acetal resin was 78 mol%.

Example 4 Binder-1 was changed to a polyvinyl acetal resin containing no metal sulfonate group, and the magnetic powder had an average major axis length of 0.1.
A magnetic disk was manufactured in the same manner as in Example 1 except that the diameter was changed to 15 μm.

Example 5 A magnetic disk was manufactured in the same manner as in Example 1 except that the binder-1 was changed to a polyvinyl acetal resin containing no metal sulfonate group.

Example 6 A magnetic disk was produced in the same manner as in Example 1, except that the average length of the magnetic powder was changed to 0.08 μm.

Example 7 A magnetic disk was produced in the same manner as in Example 1 except that the binder-1 was changed to a polyvinyl acetal resin containing 3.0 mol% of a metal sulfonate group as a polar group.

Example 8 Binder-1 was reacted with phenylisocyanate on a polyvinyl acetal resin in which a part of hydroxyl groups was modified with an aromatic compound, specifically, a polyvinyl acetal resin containing sodium sulfonate group. A magnetic disk was prepared in the same manner as in Example 1, except that the polyvinyl acetal resin was changed to a mol% and a phenyl-containing component of 10 mol%.

Comparative Example 1 A magnetic disk was prepared in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 0.7 μm, and that the binder-1 was changed to a polyvinyl acetal resin having a glass transition temperature of 70 ° C. Produced. The degree of acetalization of the polyvinyl acetal resin was 72 mol%.

Comparative Example 2 A magnetic disk was produced in the same manner as in Example 1 except that the binder-1 was changed to a glass transition temperature of 70 ° C. The degree of acetalization of the polyvinyl acetal resin was 72 mol%.

Comparative Example 3 A magnetic disk was produced in the same manner as in Example 1, except that the binder-1 was changed to a nitrocellulose resin having a glass transition temperature of 120 ° C.

With respect to Examples 1 to 7 and Comparative Examples 1 to 3 produced as described above, the RF output and the number of dropouts were measured, and the electromagnetic conversion characteristics and durability were evaluated. The method for measuring these characteristic values and the method for evaluating the effects are shown below.

<Electromagnetic conversion characteristics (RF output)> Magnetic disk recording / reproducing device (trade name: MPF-42B, manufactured by Sony Corporation)
The disk was rotated at 3600 rpm, and a magnetic head was run on the outermost tracks of the magnetic disks of Examples and Comparative Examples using a thin-film head having a gap length of 0.2 μm. 35MHz generated in
The electromagnetic conversion characteristics were evaluated by measuring the output component of.
The measurement data was a relative value when the measurement value obtained in Example 1 was set to 0 dB. As evaluation judgment, a very good level is ◎, a good level is ○, a level without problems is △,
Poor was indicated by x.

<Durability (Number of Dropouts)> The number of times of drop of output (-12 dB / 5 μs) detected per minute (number of dropouts) was measured. As evaluation judgments, a very good level was indicated by ◎, a good level was indicated by ○, a problem-free level was indicated by △, and a defect was indicated by ×.

Table 1 shows the thickness of the magnetic layers, the glass transition temperature of the polyvinyl acetal resin, the amount of the polar group of the sulfonate group, and the average major axis length of the magnetic powder in the examples and comparative examples.

[0082]

[Table 1]

As is apparent from Table 1, Examples 1 to 8 in which the magnetic layer contains a polyvinyl acetal resin having a glass transition temperature of 80 ° C. or higher as a binder show excellent RF output and the number of dropouts is small. It was found that the glass transition temperature of the polyvinyl acetal resin was lower than 80 ° C., as shown in Comparative Example 2. The desired durability cannot be obtained because the strength of the polyvinyl acetal resin is weak and the hardness of the magnetic layer is insufficient.

As shown in Comparative Example 3, when a nitrocellulose resin was used instead of a polyvinyl acetal resin as a binder having a glass transition temperature of 80 ° C. or higher,
The dispersibility of the magnetic powder of the fine particles contained in the magnetic paint cannot be improved, and the electromagnetic conversion characteristics cannot be improved.

Further, as shown in Comparative Example 1, the glass transition temperature was lower than 80 ° C. and the thickness of the magnetic layer was 0.5 μm.
If it is larger than m, desired electromagnetic conversion characteristics cannot be obtained due to self-demagnetization loss during recording and thickness loss during reproduction.

Further, from Examples 1 and 2, Example 2 in which the thickness of the magnetic layer was 0.1 μm was
It was found that the electromagnetic conversion characteristics were better as compared with Example 1 which was μm. Therefore, it was found that the electromagnetic conversion characteristics of the magnetic recording medium were further improved by reducing the thickness of the magnetic layer to 0.5 μm or less.

Further, from Examples 2 and 3, Example 3 having a glass transition temperature of 110 ° C. has a glass transition temperature of 8
As compared with Example 2 at 0 ° C., it was found that the durability was excellent. Therefore, the higher the glass transition temperature,
It was confirmed that the magnetic recording medium had excellent durability. This means that as the glass transition temperature increases,
This is because the strength of the polyvinyl acetal resin also increases.

Further, from Examples 1 and 6, Example 6 in which the average major axis length of the magnetic powder is 0.08 μm is compared with Example 1 in which the average major axis length of the magnetic powder is 0.13 μm. It was found that the electromagnetic conversion characteristics were good.
Therefore, it was found that as the average major axis length of the magnetic powder was as small as 0.13 μm or less, the magnetic recording medium had improved electromagnetic conversion characteristics.

Further, from Examples 1 and 8, it was found that a polyvinyl acetal resin in which a part of hydroxyl groups was modified with an aromatic compound having a functional group capable of reacting with hydroxyl groups was used as a binder contained in the magnetic layer. Example 8 is a resin having a higher glass transition temperature.
It turned out that it becomes what was more excellent in durability. Therefore, as a binder to be contained in the magnetic layer, it is more preferable to use a polyvinyl acetal resin modified with an aromatic compound having a functional group capable of reacting a part of the hydroxyl group with the hydroxyl group, as a magnetic recording medium having the magnetic layer Was found to have better durability.

Next, magnetic disks using hexagonal ferrite powder, specifically, barium ferrite powder as magnetic powder were produced as examples and comparative examples, and various characteristics of these magnetic disks were evaluated. Was.

Example 9 According to the following composition, each component of a magnetic paint and a non-magnetic paint was weighed and kneaded and dispersed using a continuous twin-screw kneader and a sand mill to obtain a magnetic paint and a non-magnetic paint. It was adjusted.

<Magnetic paint composition> Ferromagnetic iron fine powder: 100 parts by weight of barium ferrite powder (coercive force Hc: 2500 [Oe], average particle size: 0.1 μm, plate ratio: 2, saturation magnetization σs:・ Binder-1: 6 parts by weight Polyvinyl chloride acetal resin containing sodium sulfonate base (Saponification of polyvinyl acetate having a polymerization degree of 300 and containing 0.8 mol% of a metal sulfonate group as a polar group) Then, acetalization was carried out with acetaldehyde and butyraldehyde, with a degree of acetalization of 75 mol%, a hydroxyl group of 18 mol%, and a glass transition temperature of 80 ° C.) 2: Potassium sulfonate base-containing vinyl chloride resin 6 parts by weight ・ α-alumina 5 parts by weight ・ Myristic acid 1 part by weight ・ Butylster Allate 1 part by weight ・ Solvent 350 parts by weight (Mixed solvent having a composition of methyl ethyl ketone: toluene: cyclohexanone (weight ratio) = 1: 1: 1)

<Nonmagnetic paint composition> 100 parts by weight of α-Fe ₂ O ₃ (specific surface area by BET method: 52 m ² / g, major axis diameter: 0.15 μm, needle ratio: 6.5) Binder -3: 20 parts by weight of a vinyl chloride resin containing potassium sulfonate base ・ 1 part by weight of myristic acid ・ 1 part by weight of butyl stearate ・ 350 parts by weight of solvent (methyl ethyl ketone: toluene: cyclohexanone (weight ratio) = 1: 1: 1) After adding 5 parts by weight of a polyisocyanate compound as a curing agent to each of the magnetic paint and the non-magnetic paint adjusted as described above, the magnetic paint and the non-magnetic paint are wet-on.・ Thickness 5 by wet coating method
A multi-layer coating was performed on the front and back main surfaces of a non-magnetic support made of a polyethylene terephthalate having a thickness of μm, and then a calender treatment was performed. Then, a long sheet having a 0.5 μm-thick magnetic layer was formed.

Next, the long sheet thus obtained was punched into a 3.5-inch diameter disk-shaped disk having a 1-inch through-hole to produce a magnetic disk.

Example 10 Example 9 was repeated except that the thickness of the magnetic layer was changed to 0.1 μm.
A magnetic disk was produced in the same manner as described above.

Example 11 A magnetic disk was prepared in the same manner as in Example 9 except that the thickness of the magnetic layer was changed to 0.1 μm, and that the binder-1 was changed to a polyvinyl acetal resin having a glass transition temperature of 110 ° C. Produced.

Example 12 Binder-1 was changed to a polyvinyl acetal resin containing no metal sulfonate group, and the magnetic powder had an average particle size of 0.1.
A magnetic disk was manufactured in the same manner as in Example 9 except that the magnetic disk was changed to 11 μm.

Example 13 A magnetic disk was prepared in the same manner as in Example 9 except that the binder-1 was changed to a polyvinyl acetal resin containing no metal sulfonate group.

Example 14 A magnetic disk was produced in the same manner as in Example 9, except that the barium ferrite powder was changed to one having an average particle size of 0.03 μm.

Example 15 A magnetic disk was produced in the same manner as in Example 9 except that the binder-1 was changed to a polyvinyl acetal resin containing 3.0 mol% of a metal sulfonate group as a polar group.

Comparative Example 4 A magnetic disk was prepared in the same manner as in Example 9 except that the thickness of the magnetic layer was changed to 0.7 μm and the binder-1 was changed to a polyvinyl acetal resin having a glass transition temperature of 70 ° C. Produced.

Comparative Example 5 A magnetic disk was produced in the same manner as in Example 9 except that the binder-1 was changed to a polyvinyl acetal resin having a glass transition temperature of 70 ° C.

Comparative Example 6 A magnetic disk was produced in the same manner as in Example 9 except that the binder-1 was changed to a nitrocellulose resin having a glass transition temperature of 120 ° C.

With respect to Examples 9 to 15 and Comparative Examples 4 to 6 manufactured as described above, the electromagnetic conversion characteristics and the number of dropouts were measured. The method for measuring these characteristic values and the method for evaluating the effect are the same as those described above.

As a result of the above property evaluation, Table 2 shows the thickness of the magnetic layer, the glass transition temperature of the polyvinyl acetal resin, the amount of the polar group of the sulfonate group, and the average particle size of the barium ferrite powder in the examples and comparative examples.

[0106]

[Table 2]

As is clear from Table 2, Examples 9 to 15 in which the magnetic layer contains a polyvinyl acetal resin having a glass transition temperature of 80 ° C. or more as a binder are similar to Examples 1 to 8 described above. It was found that the magnetic layer had excellent electromagnetic conversion characteristics and the durability of the magnetic layer was ensured.

In Examples 9 to 15 in which barium ferrite powder was used as the magnetic powder, the electromagnetic conversion characteristics optimum for high-density recording were the same as in Examples 1 to 8 in which metal needle-like magnetic powder was used as the magnetic powder. It was found to be excellent.

On the other hand, as shown in Comparative Example 5, when the thickness of the magnetic layer is 0.5 μm or less, but the glass transition temperature of the polyvinyl acetal resin is lower than 80 ° C., As in Example 2, the desired durability cannot be obtained.

Further, as shown in Comparative Example 6, when a nitrocellulose resin was used instead of the polyvinyl acetal resin as the binder having a glass transition temperature of 80 ° C. or more, the electromagnetic conversion was performed in the same manner as in Comparative Example 3 described above. The characteristics cannot be enhanced.

Further, as shown in Comparative Example 4, the glass transition temperature was lower than 80 ° C. and the thickness of the magnetic layer was 0.5 μm
If it is larger than the above, desired electromagnetic conversion characteristics cannot be obtained as in Comparative Example 1 described above.

[0112]

As is clear from the above description, in the magnetic recording medium according to the present invention, the magnetic layer contains a polyvinyl acetal resin having a glass transition temperature of 80 ° C. or higher as a binder. As a result, the magnetic layer has a high strength and a highly dispersed magnetic powder of fine particles. Therefore,
A magnetic recording medium having this magnetic layer has excellent durability and excellent electromagnetic conversion characteristics optimal for high-density recording.

[Brief description of the drawings]

FIG. 1 is a sectional view of a magnetic recording medium to which the present invention is applied.

[Explanation of symbols]

 1 Non-magnetic support, 2 Non-magnetic layer, 3 Magnetic layer

Claims (6)

[Claims] 1. A magnetic recording medium having a nonmagnetic support and a magnetic layer formed by applying a magnetic paint obtained by dispersing at least a magnetic powder and a binder in a solvent, wherein the binder is glass. A magnetic recording medium comprising a polyvinyl acetal resin having a transition temperature of 80 ° C. or higher. 2. The magnetic recording medium according to claim 1, wherein said magnetic layer has a thickness of 0.5 μm or less. 3. The polyvinyl acetal resin according to claim 1, 2. The magnetic recording medium according to claim 1, wherein 4. The magnetic recording medium according to claim 3, wherein the polyvinyl acetal resin has a part of hydroxyl groups modified with an aromatic compound having a functional group capable of reacting with the hydroxyl groups. 5. An average major axis length of said magnetic powder is 0.13 μm.
2. The magnetic recording medium according to claim 1, wherein m is equal to or less than m. 6. The magnetic powder has an average particle size of 0.1 μm.
2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is a hexagonal ferrite powder having a platelet ratio of 2 or more.