JP2000285447A - Production of magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a magnetic recording medium suitable for a system employing a floating magnetic head. SOLUTION: The method for producing a magnetic recording medium comprises a step for coating a nonmagnetic support 1 with a magnetic paint principally comprising a magnetic powder and a binder and drying the magnetic paint to form a magnetic layer, a step for making smooth the surface of the magnetic layer by calendaring, a step for punching the nonmagnetic support 1 formed with the magnetic layer into a disc and making a magnetic disc, and a step for unwarping the magnetic disc by heat treating it at a temperature higher than the glass transition point of the binder and lower than the glass transition point of the nonmagnetic support 1, wherein the temperature rising rate is set at 1.0 deg.C/min or below in the step for unwarping by heat treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a disk-shaped multilayer coating type magnetic recording medium suitable for high-density recording.

[0002]

2. Description of the Related Art Magnetic recording media using ferromagnetic metal powders have been strongly demanded for high-density recording in recent years. In addition to various formats for audio and video, high-density floppy disks and backup data cartridges have been developed. Started to be used in computer peripherals, and became the mainstream of current magnetic recording media.
There are also some remarkable improvements in characteristics.

[0003] The development of such high-density recording technology is not only due to the progress of the medium, but it cannot be overlooked that it is due to the technological progress of the head-disk interface. Above all, in the case of high-density floppy disks, unlike the sliding heads used in conventional micro floppy disk drives, the use of floating heads, which float with a very small height on the disk, has been increasing. It is considered one of the key points of recording technology.

[0004]

However, in a system using a floating type head, when a disk is strongly sandwiched from both upper and lower sides, such as a sliding type head, deformation of the disk which is not so problematic, especially curling, is not considered. The so-called “warp” strongly affects the flying characteristics of the head, and as a result, affects the electromagnetic conversion characteristics and running durability.

[0005] The cause of such warpage is as follows.
It is roughly classified into those derived from the non-magnetic support and those derived from the steps after application of the magnetic paint. The former depends on the orientation direction of the polymer crystal at the time of producing the non-magnetic support (film). Considering that the crystal orientation contributes to maintaining the physical properties and strength of the film, Removal is difficult. On the other hand, also in the latter, although consideration is given to the process, such as devising the running path of the film in coating and calendering, the situation has not yet reached a satisfactory level.

Accordingly, the present invention has been proposed in view of such circumstances, and provides a method of manufacturing a magnetic recording medium capable of performing high-density, large-capacity recording while minimizing disk deformation. The purpose is to do so.

[0007]

A method of manufacturing a magnetic recording medium according to the present invention, which has achieved the above-mentioned objects, comprises applying a magnetic paint mainly composed of a magnetic powder and a binder on a non-magnetic support, drying the applied magnetic paint. After a coating step of forming a magnetic layer, a calendering step of smoothing the surface of the magnetic layer, and punching a non-magnetic support on which the magnetic layer is formed into a disk, and a cutting step of manufacturing a magnetic disk, For the magnetic disk, a heat treatment step for heat removal at a temperature higher than the glass transition point of the binder and lower than the glass transition point of the nonmagnetic support is performed. , Characterized in that the temperature rise rate is 1.0 ° C./min or less.

According to the method of manufacturing a magnetic recording medium according to the present invention, the processing temperature of the heat treatment step is higher than the glass transition point of the binder and the glass transition point of the nonmagnetic support. By making it lower, deformation of the magnetic recording medium can be suppressed without impairing the crosslinking of the coating film, and a magnetic recording medium suitable for high-density, large-capacity recording can be provided.

Further, according to the present method, in the above-described heat treatment step for removing a warp, the rate of rise is set to 1.0 ° C./min or less.
You. In this case, according to this method, deformation occurring on the main surface of the magnetic recording medium can also be suppressed.

Further, in the present method, after the completion of the heat treatment step for removing the warp, the cooling may be performed at a temperature decreasing rate of 1.0 ° C./min or less. In this case, according to this method, deformation occurring on the main surface of the magnetic recording medium can be further suppressed.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a method for manufacturing a magnetic recording medium according to the present invention will be described in detail with reference to the drawings.

In the method for manufacturing a magnetic recording medium according to the present invention, a magnetic disk as shown in FIG. 1 is manufactured. This magnetic disk is formed by applying upper layers 2 and 2 and lower layers 3 and 3 on both surfaces of a non-magnetic support 1 in a multilayer manner.

The upper layer 2 is a magnetic layer mainly composed of a ferromagnetic metal powder and a binder.

The ferromagnetic metal powder used for the upper layer 2 includes, for example, metals such as Fe, Co, and Ni;
Fe-Ni, Fe-Al, Fe-Ni-Al, Fe-A
I-P, Fe-Ni-Si-Al, Fe-Ni-Si-
Al-Mn, Fe-Mn-Zn, Fe-Ni-Zn, C
o-Ni, Co-P, Fe-Co-Ni, Fe-Co-
Ni-Cr, Fe-Co-Ni-P, Fe-Co-B,
Fe-Co-Cr-B, Mn-Bi, Mn-Al, Fe
Alloys such as -Co-V, iron nitride, iron carbide and the like.
Of course, to these ferromagnetic metal powders, Al, Si, P, which are added for the purpose of preventing sintering during reduction or maintaining the shape, etc.
An appropriate amount of a light metal element such as B may be contained. Most commonly, Fe alone or Fe-Co, Fe-Ni,
An alloy obtained by adding Al and / or Si to a Fe-Co-Ni alloy for the purpose of preventing sintering is used.

The specific surface area of these ferromagnetic metal powders is 2
It is 0-90 m < ² > / g, preferably 25-70 m < ² > / g. When the specific surface area is in the above range, high-density recording can be performed with the formation of fine particles of the ferromagnetic powder, and a magnetic recording medium having excellent noise characteristics can be obtained.

The ferromagnetic metal powder may be used alone or in combination of two or more. Further, hexagonal plate-like ferrites can also be used. M-type, W-type, Y-type, and Z-type barium ferrites, strontium ferrites, calcium ferrites, lead ferrites, and for controlling the coercive force of these, -Ti, Co-Ti-Zn, Co-Ti-
Nb, Co-Ti-Zn-Nb, Cu-Zr, Ni-T
What added i etc. can also be used.

On the other hand, the lower layer 3 is a layer mainly composed of a nonmagnetic powder and a binder, or a layer mainly composed of a magnetic powder having a small amount of magnetization and a small coercive force and a binder. By providing the lower layer 3 having surface smoothness and coating strength on the nonmagnetic support 1, the magnetic layer can be thinner than the upper layer 2 (magnetic layer) formed directly on the nonmagnetic support 1 as a single layer. Can be

Specific examples of the nonmagnetic powder used for the lower layer 3 include α-iron oxide, goethite, and acicular titanium oxide. These non-magnetic pigments include:
Surface treatment may be performed for the purpose of improving dispersibility, imparting conductivity, adjusting light transmittance, and the like.

These non-magnetic pigments have an average particle size (average long axis length) in order to ensure surface smoothness after coating.
It is necessary that the thickness be 0.3 μm or less, preferably 0.2 μm or less. Further, the needle ratio is 5 or more, preferably 10
In the above case, the effect of suppressing curling is exhibited. When the acicular ratio is 30 or more, the dispersibility is deteriorated, and the surface roughness is deteriorated.

As the magnetic powder used for the lower layer 3, any of the above-described magnetic powders used for the upper layer 2 can be used, but the influence on the upper layer 2 which is a magnetic layer is minimized. For this purpose, a material having a small amount of magnetization and a small coercive force is used. For example, a material having a relatively low output, a material having a relatively low output such as γ-iron oxide, cobalt-modified γ-iron oxide, magnetite, cobalt-modified magnetite, and chromium dioxide is used. In addition, these magnetic powders can be subjected to a surface treatment in consideration of dispersion stability with a non-magnetic pigment that is simultaneously dispersed.

These magnetic powders have an average particle size (average major axis length) in order to ensure surface smoothness after coating.
It is necessary that the thickness be 0.3 μm or less, preferably 0.2 μm or less. In order to suppress warpage, the needle ratio is preferably 5 or more, more preferably 10 or more. However,
When the needle ratio is 30 or more, the dispersibility is rather deteriorated,
It is not preferable because the surface roughness deteriorates.

Further, the above-mentioned non-magnetic pigments can be used in combination with the following pigments. For example, carbon black, especially carbon black having a structured structure, goethite, rutile titanium oxide, anatase titanium oxide, tin oxide, tungsten oxide, silicon oxide, zinc oxide, chromium oxide, cerium oxide, titanium carbide, The effect remains the same even when used in combination with BN, α-alumina, β-alumina, γ-alumina, calcium sulfate, barium sulfate, molybdenum disulfide, magnesium carbonate, calcium carbonate, barium carbonate, strontium carbonate, barium titanate, etc. .

As described above, when the above pigments are used in combination, it is necessary to pay attention to the size of the particles to be used together. In order to ensure the surface smoothness after coating, the average particle size (for needle-like particles, the average (Long axis length) should be 0.3 μm or less, preferably 0.2 μm or less. The non-magnetic pigment may be doped with an appropriate amount of impurities depending on the purpose. The specific surface area of the non-magnetic pigment is 5 to
It is 100 m ² / g, preferably 20 to 70 m ² / g. When the specific surface area is in the above range, the lower layer 3 does not hinder the smoothing of the non-magnetic pigment with fine particles.

The lower layer 3 needs to have a smooth surface capable of withstanding high-density recording and, at the same time, have a high electrical conductivity. When the electric conductivity is low, that is, when the surface electric resistance is high, not only the phenomenon that discharge noise is easily generated but also the phenomenon of dust collection caused by static electricity occurs, so that it becomes a cause of so-called dropout and reliability. This makes it impossible to provide a magnetic disk with excellent performance.

Therefore, carbon black or the like may be added to the lower layer 3 in addition to the nonmagnetic powder. For example, as carbon black effective for conductivity,
Carbon black having a structure structure is known. In particular, 100 m
It is preferable to use a carbon black having a structure structure and an oil absorption of 1/100 g or more. On the other hand, the addition of carbon black having a structure structure inevitably impairs the surface properties, so the amount of addition must be kept to the minimum necessary.

As the binder used for the above-mentioned upper layer 2 and lower layer 3, known thermoplastic resins, thermosetting resins, reactive resins and the like conventionally used as binders for magnetic recording media are used. It is possible and the number average molecular weight is 5,000 to
Those of 100,000 are preferably used. However, in general, when the particles are miniaturized, the voids between the particles are also miniaturized unless the geometrical arrangement changes. Therefore, it has a strong affinity for non-magnetic powder, magnetic powder, and ferromagnetic metal powder, and has excellent fluidity in order to wet the fine space between particles and to uniformly cover and disperse with the binder. It is necessary to use a binder.

As the thermoplastic resin, vinyl chloride, vinyl chloride copolymer, vinyl acetate, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylic acid ester Ruacrylonitrile copolymer, acrylate-vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylate-acrylonitrile copolymer, acrylate-vinylidene chloride copolymer, methacrylate-vinylidene chloride copolymer Coalesce, methacrylate-vinyl chloride copolymer, methacrylate-ethylene copolymer,
Polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose), Styrene butadiene copolymer, polyurethane resin, polyester resin, amino resin, synthetic rubber and the like can be mentioned.

Examples of the thermosetting resin or the reactive resin include a phenol resin, an epoxy resin, a polyurethane curing resin, a urea resin, a melamine resin, an alkyd resin, a silicone resin, a polyamine resin, and a urea formaldehyde resin.

All of the above binders include -SO ₃ M, -OSO ₃ M, and -C for the purpose of improving the dispersibility of the pigment.
Polar functional groups such as OOM and P = O (OM) ₂ may be introduced. Here, M in the formula is a hydrogen atom or an alkali metal such as lithium, potassium, and sodium.
Further, the polar functional groups include -NR ¹ R ² , -NR ¹
A side chain type having a terminal group of R ² R ³⁺ X ⁻ , NR ¹ R ²⁺
X ^- there is one of the main chain type. Where R ¹ , R ² , R ³
Is a hydrogen atom or a hydrocarbon group, and X ^- is a halogen element ion such as fluorine, chlorine, bromine, iodine, or an inorganic or organic ion. Also, -OH, -SH, -CN,
There are also polar functional groups such as epoxy groups. The amount of these polar functional groups is from 10 ^-1 to 10 ^-8 mol / g, preferably from 10 ^-1 to 10 ^-8 mol / g.
^-2 to 10 ^-6 mol / g. These binders can be used alone, or a plurality of binders can be blended and used.

Examples of the solvent used when preparing the upper layer 2 and the lower layer 3 described above include ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; alcohol solvents such as methanol, ethanol and propanol; Methyl acetate, ethyl acetate,
Butyl acetate, propyl acetate, ethyl lactate, ester solvents such as ethylene glycol acetate, diethylene glycol dimethyl ether, 2-ethoxyethanol, ether solvents such as tetrahydrofuran, dioxane, aromatic hydrocarbon solvents such as benzene, toluene, xylene,
Examples thereof include halogenated hydrocarbon solvents such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, and chlorobenzene. These solvents are appropriately mixed and used.

Further, the upper layer 2 and the lower layer 3 can contain a lubricant, non-magnetic reinforcing particles, and the like, if necessary.

Examples of the lubricant include graphite, molybdenum disulfide, tungsten disulfide, silicone oil, fatty acids having 10 to 22 carbon atoms, and fatty acids having 2 to 26 carbon atoms.
And terpene compounds, oligomers thereof, and fluorine-based lubricants. The above-mentioned lubricant is usually added to the upper and lower layers 2 and 3 because the absolute amount of the lubricant is insufficient if only the upper layer 2 is added.

The non-magnetic reinforcing particles include aluminum oxide (α, β, γ), chromium oxide, silicon carbide, diamond, garnet, emery, boron nitride, titanium carbide, silicon carbide, titanium carbide, titanium oxide (rutile). , Anatase) and the like. The amount of the particles is at most 20 parts by weight, preferably at most 10 parts by weight, more preferably at most 5 parts by weight, based on 100 parts by weight of the ferromagnetic metal powder. The Mohs hardness of the non-magnetic reinforcing particles is 4 or more, preferably 5 or more, and more preferably 6 or more. The specific gravity is 2 to 6 g / cm ³ , preferably ^{3 to} 6 g / cm ³ .
5 g / cm ³ , and the average primary particle size is 1.0 μm.
m, preferably 0.5 μm or less, more preferably 0.3 μm or less.

It is also possible to use a polyisocyanate for crosslinking and curing the above binder. Examples of the polyisocyanate include toluene diisocyanate and its adduct, and alkylene diisocyanate and its adduct. The amount of the polyisocyanate to be added to the binder is 5 to 80 parts by weight, preferably 10 to 50 parts by weight, based on 100 parts by weight of the binder. These polyisocyanates are used in both upper and lower layers 2, 3
It is also possible to use it only for the upper layer 2. When used for the upper and lower layers 2 and 3, the amount of each compound can be added to each layer in an equal amount, or can be changed at an arbitrary ratio.

Further, the non-magnetic support 1 used here
A known material conventionally used as a support for a magnetic recording medium can be used. For example, polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, celluloses such as cellulose triacetate and cellulose diacetate, vinyl resins, polyimides and polycarbonates. Examples include a molecular substrate or a substrate formed of metal, glass, ceramics, or the like.

Hereinafter, a method for manufacturing the magnetic disk will be described in detail.

(1) First, an upper layer paint and a lower layer paint are prepared.

The paint for the upper layer and the paint for the lower layer can each be prepared according to a conventional method. That is, in the upper layer paint, a ferromagnetic metal powder, a binder, a solvent, and various additives are mixed, and after kneading and dispersed, in the lower layer paint, a nonmagnetic powder, a binder, a solvent, and various additives are added. It can be obtained by mixing, kneading and dispersing.

The kneading apparatus used in the kneading step includes, for example, a continuous twin-screw kneader, a continuous twin-screw kneader capable of multi-stage dilution, a kneader, a pressure kneader, a roll kneader and the like.

The dispersing device used in the dispersing step includes, for example, a roll mill, a ball mill, a horizontal sand mill, a vertical sand mill, a spike mill, a pin mill,
Examples include a tower mill, DCP, agitator, homogenizer, and ultrasonic disperser.

(2) A lower layer paint and an upper layer paint are applied in one layer on one surface of the non-magnetic support, and are introduced into a dryer.
After winding the same, the lower layer paint and the upper layer paint are similarly applied on the other surface of the non-magnetic support in a multi-layered manner, and the resultant is introduced into a dryer. Then, the non-magnetic support having the magnetic layers formed on both surfaces is guided to a calender, and the surface of the magnetic layer is smoothed and then wound around a winding roll.

It is to be noted that, instead of the above-described production sequence, it is also possible to produce the product in the order of application on one side, drying and calendering, application on the other side, drying and calendering.

Examples of the coating apparatus for applying the above-mentioned upper layer coating material and lower layer coating material on the non-magnetic support in multiple layers include a gravure coater, a knife coater, a blade coater, a reverse roll coater, and a die coater. As a lip configuration of the die coater, there are a two-lip system, a three-lip system, a four-lip system, and the like.

As in the present invention, in order to apply two kinds of paints such as a paint for the upper layer and a paint for the lower layer in a multilayer manner, a method of applying and drying one layer at a time (so-called wet-on-dry coating method) is applied. Alternatively, a method in which a magnetic layer is applied on a non-magnetic layer in a wet state that has not been dried (so-called wet-on-wet coating method = simultaneous wet multilayer coating method) may be applied. However, it is preferable to perform simultaneous wet multilayer coating by a wet-on-wet multilayer coating method from the viewpoint of the uniformity of the coating film, the adhesiveness of the upper and lower interfaces, and the productivity.

When this wet-on-wet method is applied, for example, as shown in FIG. 2, two paint reservoirs 13 and 14 for supplying a lower paint 11 and an upper paint 12, respectively, Using a die coater 18 having two slits 16 and 17 for discharging the paints 11 and 12 toward the non-magnetic support 15, the two slits 1 are moved toward the non-magnetic support 15 traveling in the direction of arrow A.
The paint 11 for the lower layer and the paint 12 for the upper layer may be simultaneously discharged from 6, 17 to form a paint film of the paint 12 for the upper layer on the paint film of the paint 11 for the lower layer.

In the wet-on-wet method, the upper layer paint 12 is applied on the coating film of the lower layer paint 11 which is kept wet, so that the surface of the lower layer (that is, the boundary surface with the upper layer) is formed. And the surface properties of the upper layer are improved, and the adhesion between the upper and lower layers is also improved.
As a result, the performance required for a magnetic recording medium that requires high output and low noise, particularly for high-density recording, is satisfied, film peeling is eliminated, and film strength is improved.
In addition, dropout can be reduced, and reliability is improved.

On the other hand, in the case of the wet-on-dry method, it is necessary to select a paint having sufficient solvent resistance to the paint for the upper layer as the paint for the lower layer.

The boundary between the upper and lower layers formed by the wet-on-wet multi-layer coating method, except that a clear boundary substantially exists, except that the components of both layers are mixed with a certain thickness. Although there are cases where regions exist, any cases are included in the scope of the present invention.

(3) Next, the thus-prepared raw material is punched into a predetermined size.

(4) Next, the magnetic disk punched to a predetermined size is left in a constant-temperature facility at room temperature, and then the temperature is raised to a predetermined processing temperature, and a heat treatment for warp removal is performed.

Here, the treatment temperature of the heat treatment for removing the warp, that is, the temperature in the constant temperature equipment is higher than the glass transition point of the binder used for the upper layer and the lower layer, and is higher than the glass transition point of the nonmagnetic support. Low temperature.

When the processing temperature is lower than the glass transition point of the binder in the upper layer and the lower layer, although the warpage is small, the cross-linking is not sufficiently performed in the upper and lower layer coatings, and the coating strength is not sufficient. Poor sex. On the other hand, the higher the processing temperature, the greater the crosslinkability of the coating film. However, if the processing temperature is higher than the glass transition point of the nonmagnetic support, the warpage of the magnetic disk is rather increased, which is not preferable.

At the start of the heat treatment step, the rate of increase in the treatment temperature is increased at 1.0 ° C./min or less. If the rate of temperature rise is higher than 1.0 ° C./min, there is a possibility that the main surface of the magnetic disk will have irregularities and the amount of deformation will be large. As described above, if the main surface of the magnetic disk is deformed, the contact with the head becomes unstable. Therefore, by performing the temperature rise rate of the magnetic disk at 1.0 ° C./min or less after the heat treatment for removing the warp, it is possible to manufacture a magnetic disk in which the deformation occurring on the main surface is suppressed.

In particular, at this time, the rate of temperature rise is 1.0 ° C.
/ Min, and preferably from normal temperature to the heat treatment temperature. Thus, the rate of temperature rise is 1.0 ° C.
By raising the temperature from the room temperature to the heat treatment temperature while maintaining the temperature to not more than / min, the warpage can be surely removed.

(5) Next, after performing the warp removing heat treatment as described above, the magnetic disk is cooled at a predetermined temperature decreasing speed.

Here, the magnetic disk is cooled at a temperature lowering rate of 1.0 ° C./min or less. Set the temperature decrease speed to 1.
If the rate is higher than 0 ° C./min, there is a possibility that irregularities may occur on the main surface of the magnetic disk and the amount of deformation may increase. As described above, if the main surface of the magnetic disk is deformed, the contact with the head becomes unstable. Therefore, by performing the temperature reduction rate of the magnetic disk at 1.0 ° C./min or less after performing the warp removing heat treatment,
It is possible to manufacture a magnetic disk in which deformation occurring on the main surface is suppressed.

(6) Finally, the magnetic disk that has been subjected to the warp removing heat treatment is accommodated in a diskette.

Through the steps (1) to (6), it is possible to obtain a magnetic disk in which the amount of warpage is suppressed to 1 / 3.5 mm or less per inch of diameter. As described above, after punching to a predetermined size, the heat treatment is performed at a processing temperature lower than the glass transition point of the binder of the magnetic layer and higher than the glass transition point of the nonmagnetic support, thereby suppressing the warpage of the magnetic disk. be able to. As a result, even in a system using a flying head, excellent electromagnetic conversion characteristics and running durability can be obtained, and the magnetic recording medium is suitable for high-density, large-capacity recording.

The amount of warpage is obtained as follows. First, as shown in FIG.
The center hole 20a of the magnetic disk 20 is
The magnetic disk 20 is rotated in the circumferential direction by a rotating device 22 fixed by the support member 1 and having a support member 21 attached thereto. The microscope 2 is movable in the thickness direction of the magnetic disk 20 (the direction B in the figure) and can measure the moving distance.
3, the difference x between the highest part and the lowest part of the magnetic disk 20, that is, the amount of warpage can be obtained.

[0060]

EXAMPLES Hereinafter, the present invention will be described by way of specific examples with reference to experimental examples.

<Experiment 1> In Experimental Example 1, the magnetic disk shown in FIG. 1 was manufactured, and the influence of the difference in the processing temperature in the heat treatment step for warp removal was examined.

Here, the amount of warpage is obtained by the measuring device shown in FIG.

Sample 1 First, an upper layer (magnetic layer) 2 having the composition shown below was prepared.
The lower layer (nonmagnetic layer) 3 was made into a paint. According to a conventional method, a paint is prepared by mixing a pigment, a binder, an additive, and a solvent, kneading the mixture with a continuous kneader, and then dispersing the mixture with a sand mill. The paint for the upper layer was dispersed by a sand mill for 5 hours, and the paint for the lower layer was dispersed by a sand mill for 3 hours.

<Composition of Upper Layer Coating Material> Fe-based metal ferromagnetic powder 100 parts by weight (average major axis length = 0.2 μm, needle ratio = 9, saturation magnetization = 130 Am ² / kg, coercive force = 135 kA / m ) Polyvinyl chloride resin (degree of polymerization: 150, 14 parts by weight, potassium oxysulfonate salt containing 5 × 10 ⁻⁵ mol / g as a polar functional group) Polyester polyurethane resin, 6 parts by weight (sodium sulfonate salt, 1 × as a polar functional group) ^10-4 containing mol / g) additives (primary particle size 0.20μmAl ₂ O _3, added as a slurry) 5 parts by weight of stearic acid 3 parts by weight heptyl stearate 3 parts Methyl ethyl ketone 150 parts by weight cyclohexanone 150 parts by weight

<Composition of Lower Layer Paint> 100 parts by weight of α-iron oxide (average major axis length = 0.15 μm, needle ratio = 12, pH = 5.7) 11 parts by weight of carbon black Polyvinyl chloride resin (polymerization) Degree 150 17 parts by weight Potassium oxysulfonate salt is contained as a polar functional group at 5 × 10 ⁻⁵ mol / g) Stearic acid 1 part by weight Heptyl stearate 1 part by weight Methyl ethyl ketone 150 parts by weight Cyclohexanone 150 parts by weight

Next, 4 parts by weight of the polyisocyanate prepared as described above and 2 parts by weight of the polyisocyanate added to the lower layer paint were added to a 62 μm-thick polyethylene terephthalate using a 4-lip type die coater. Each coating material was simultaneously and multi-layer coated on a film (glass transition point = 69 ° C., surface roughness Ra = 8 nm) 1, non-oriented by a solenoid coil, and then dried. After forming the same coating film on the back surface, a calendering process is performed. Here, the coating thickness of each layer after drying was set to 0.25 μm for the upper layer 2 and 2.0 μm for the lower layer 3.

Next, as described above, the raw material having the upper and lower layers 2 and 3 formed on both surfaces of the film 1 was punched into a disk having a diameter of 3.5 inches.

Thereafter, the magnetic disk was set in a constant temperature room at room temperature, and the temperature was raised at a rate of 1.0 ° C./min to 45 ° C.
The treatment temperature was raised until the treatment temperature was 45 ° C., and the mixture was left for 48 hours. Thereafter, the magnetic disk was cooled to room temperature at a temperature decreasing rate of 1.0 ° C./min. The amount of warpage of Sample 1 thus obtained was 0.5 mm.

Samples 2 to 5 A heat treatment step was performed by changing the processing temperature to 55 ° C., 65 ° C., 75 ° C., and 85 ° C., respectively. Except for this, a magnetic disk was produced in the same manner as in Sample 1.

The amount of warpage of Sample 2 was 0.5 mm, the amount of warpage of Sample 3 was 0.6 mm, and the amount of warpage of Sample 4 was 1.
4 mm, and the amount of warpage of Sample 5 was 2.1 mm.

Characteristics Evaluation The electromagnetic conversion characteristics of the magnetic disks of Samples 1 to 5 were examined.

The electromagnetic conversion characteristics were determined by modifying a floppy disk drive (trade name: MPF-42B; manufactured by Sony Corporation), setting the disk rotation speed to 3600 rpm, and using a thin film head having a gap length of 0.2 μm as the head. The disk is run on the outermost track of the disk, and the ratio between the output of 35 MHz generated by the magnetic head and the noise from 0 to 17 MHz, that is, the so-called SN ratio is obtained. Table 1 shows the results.

[0073]

[Table 1]

As can be seen from the results shown in Table 1, the heat treatment was performed at a temperature higher than the glass transition point of the binder of 50 ° C. and lower than the glass transition point of the nonmagnetic support of 69 ° C. On the disc (Sample 2, Sample 3)
Good warpage is obtained with a small amount of warpage and a high SN ratio.

On the other hand, the magnetic disk (sample 1) heat-treated at a processing temperature lower than 50 ° C., which is the glass transition point of the binder, has a small amount of warpage, but the coating film is sufficiently crosslinked. The coating is destroyed by the head contact at the time of measurement, and the S / N ratio cannot be measured. Further, the magnetic disks (Samples 4 and 5) heat-treated at a temperature higher than 69 ° C., which is the glass transition point of the non-magnetic support, have a large amount of warpage, so that the head contact becomes unstable during actual running. , And the SN ratio deteriorates.

<Experiment 2> In Experiment 2, the effect on the surface of the magnetic disk due to the difference in the rate of temperature rise at the start of the heat treatment for warp removal is examined.

Samples 6 to 9 were heated at a rate of 0.5 ° C./min, 1.5 ° C./min, 2.0 ° C.
A magnetic disk was produced in the same manner as in Sample 3, except that the temperature was raised from room temperature to 65 ° C. at a rate of 5.0 ° C./min and 5.0 ° C./min.

With respect to the magnetic disks of the characteristic evaluation sample 3 and the samples 6 to 9, the deformation of the magnetic disk and the SN ratio were examined. As shown in FIG. 4, the amount of deformation of the magnetic disk was determined by measuring the difference y between the highest part and the lowest part of the unevenness generated on the main surface of the magnetic disk 20 under a microscope.

Table 2 shows the results.

[0080]

[Table 2]

As can be seen from the results in Table 2, in the magnetic disks (samples 3 and 6) in which the processing temperature was increased from room temperature to the processing temperature at a temperature rising rate of 1.0 ° C. or less, the amount of deformation should be suppressed to a low level. And a good SN ratio can be obtained.

On the other hand, the magnetic disk (Samples 7 to 9) in which the processing temperature was increased from room temperature to the processing temperature at a rate of increase of 1.0 ° C./min or more, had a large deformation amount, and no head contact was observed during actual machine running. As a result, the amount of decrease in the SN ratio becomes too large, which is not preferable.

<Experiment 3> In Experiment 3, the effect on the surface of the magnetic disk due to the difference in the temperature lowering speed of the magnetic disk after the completion of the heat treatment for warp removal is examined.

Samples 10 to 13 were cooled at a rate of 0.5 ° C./min, 1.5 ° C./min, 2.0 ° C.
/ Min, 5.0 ° C./min, except cooling to room temperature
A magnetic disk was manufactured in the same manner as in Sample 3.

With respect to the magnetic disks of the characteristic evaluation sample 3 and the samples 10 to 13, the deformation of the magnetic disk and the SN ratio were examined.
As shown in FIG. 4, the amount of deformation of the magnetic disk was determined by measuring the difference y between the highest part and the lowest part of the unevenness generated on the main surface of the magnetic disk 20 under a microscope, as shown in FIG. I asked by doing.

Table 3 shows the results.

[0087]

[Table 3]

As can be seen from the results in Table 3, in the magnetic disks (Samples 3 and 10) in which the processing temperature was decreased from room temperature to the processing temperature at a temperature lowering rate of 1.0 ° C. or less, the amount of deformation should be kept low. And a good SN ratio can be obtained.

On the other hand, the magnetic disk (Samples 11 to 13) in which the processing temperature is reduced from room temperature to the processing temperature at a rate of decrease of 1.0 ° C./min or more, has a large deformation amount, and the head contact does not occur during the actual running. Become stable, SN
The decrease in the ratio is too large, which is not preferable.

[0090]

As is apparent from the above description, according to the present invention, after the magnetic disk is stamped and formed, the temperature is higher than the glass transition point of the binder and higher than the glass transition point of the nonmagnetic support. Since the heat treatment is performed at a low temperature, deformation of the magnetic disk is suppressed without deteriorating the strength of the coating film, and high-density, large-capacity recording with a warpage of 1 / 3.5 mm or less per inch in diameter is achieved. It is possible to obtain a magnetic recording medium suitable for the above.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a configuration of a magnetic disk to which the present invention is applied.

FIG. 2 is a schematic diagram showing an outline of a coating apparatus to which a wet-on-wet system is applied.

FIG. 3 is a schematic diagram schematically showing an apparatus for measuring the amount of warpage of the magnetic disk.

FIG. 4 is a schematic cross-sectional view showing a state of irregularities generated on a main surface of the magnetic disk.

[Explanation of symbols]

 1 Non-magnetic support, 2 Upper layer, 3 Lower layer

Claims (5)

[Claims] A coating step of applying a magnetic paint mainly composed of a magnetic powder and a binder on a nonmagnetic support and drying to form a magnetic layer; a calendering step of smoothing the surface of the magnetic layer; A cutting step of punching the non-magnetic support having the magnetic layer formed thereon into a disk shape to produce a magnetic disk; and And a heat treatment step for performing a heat treatment at a temperature lower than the transition point.
A method for producing a magnetic recording medium, wherein the temperature is set to not more than ° C / min. 2. The magnetic recording medium according to claim 1, wherein in the warp removing heat treatment step, the temperature is raised from room temperature to a processing temperature while maintaining a temperature rising rate at 1.0 ° C./min or less. Production method. 3. The method for producing a magnetic recording medium according to claim 1, wherein in the coating step, a lower layer and an upper layer, which is the magnetic layer, are coated on the non-magnetic support by a simultaneous wet multilayer coating method. . 4. The method for manufacturing a magnetic recording medium according to claim 1, wherein after the completion of the heat treatment step for removing warpage, the temperature is lowered at a rate of 1.0 ° C./min or less. 5. The method according to claim 1, wherein in the applying step, the upper layer and the lower layer are formed on both main surfaces of the non-magnetic support.