JP2002367159A - Method of manufacturing magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a high-density magnetic recording medium having a thin-layer magnetic layer with which a coating film having an excellent degree of orientation can be obtained in spite of the reduction in the film thickness of the magnetic layer and further the magnetic layer formed of the thin film can be made higher in packing and higher in durability and the uniform boundary between the magnetic layer and a nonmagnetic layer can be obtained. SOLUTION: The method of manufacturing the magnetic recording medium having the magnetic layer of <=0.3 μm in film thickness by coating the one surface of a nonmagnetic base with a nonmagnetic layer-forming coating material to form a nonmagnetic layer, then coating the surface to this nonmanagement layer with a magnetic layer-forming coating material, in which the layer-forming coating material described above contains at least ferromagnetic powder, a polishing material, a binder resin and an organic solvent and this organic solvent contains a solvent having a boiling point above 120 deg.C at a ratio higher than 50 wt.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method of manufacturing a magnetic recording medium, and more particularly to a method of manufacturing a high-density magnetic recording medium having a thin magnetic layer having a thickness of 0.3 μm or less.

[0002]

2. Description of the Related Art In response to recent demands for higher recording density of magnetic recording media, magnetic layers have been increasingly oriented to thinner, more filled, and more durable. Is becoming more advanced (high level).

In order to realize such a magnetic layer, proposals have conventionally been made such as forming the magnetic layer into a two-layer structure or providing an undercoat layer below the magnetic layer. Further, in these manufacturing methods, after once drying the layer located on the lower side, providing a magnetic layer located on the upper side, a so-called wet-on-dry coating method, or the layer located on the lower side and the upper layer Studies have been made on a so-called wet-on-wet coating method in which a magnetic layer and a magnetic layer are simultaneously provided in a wet state.

[0004] As a related art related to such a wet-on-wet coating method, Japanese Patent Publication No. 5-59490 discloses that a magnetic coating solution is applied on a continuously running non-magnetic support to form a magnetic layer. A method for producing a magnetic recording medium, comprising drying the magnetic layer while applying a magnetic field thereto while the magnetic layer is not dried, wherein the magnetic layer is provided simultaneously with the non-magnetic undercoat layer by multi-layer coating, and both layers are provided. A method for producing a magnetic recording medium, wherein a magnetic field is applied during the drying. "

[0005] Similarly, a prior art related to the wet-on-wet coating method, which relates to the improvement of the solvent composition of a coating material for forming a magnetic layer, is disclosed in Japanese Patent Publication No. 46455/1994.
In the publication, `` On a non-magnetic support, a lower layer formed by dispersing ferromagnetic powder in a binder solution is provided, and a magnetic coating solution obtained by dispersing ferromagnetic powder in a binder solution is coated thereon. In the method for producing a magnetic recording medium obtained by the above method, the solvent of the dispersion liquid of the lower layer contains 30% by weight or more of an organic solvent having an evaporation rate index of less than 50, and the solvent of the magnetic coating solution is an organic solvent having an evaporation rate index of less than 50. The solvent is 0 to 40% by weight and the content of the lower layer is equal to or less than that of the lower layer, and the magnetic coating solution is applied while the lower layer is not dried to form a multilayer coating layer. A method for producing a magnetic recording medium, characterized in that it is dried to a range of 0.3 to 3% by weight and then calendered.

On the other hand, Japanese Patent Application Laid-Open No. H11-213379 discloses a method of manufacturing a magnetic recording medium by a wet-on-dry coating method which has excellent electromagnetic conversion characteristics in a high frequency range and excellent productivity. In the example of this publication,
The composition of the solvent in the coating material for forming the magnetic layer is methyl ethyl ketone / toluene / cyclohexanone = 1/1/2.
Are used.

[0007]

However, the wet-on-wet method described in Japanese Patent Publication No. 5-59490 and Japanese Patent Publication No. 6-46455, in which the magnetic layer is coated while the non-magnetic undercoat layer is wet. Regarding the coating method, the interface between the magnetic layer and the non-magnetic layer becomes non-uniform regardless of the solvent composition of the magnetic layer forming coating material to be used, which causes output fluctuation and is preferable as a method for producing a high-density magnetic recording medium. It was not something.

Further, in the solvent composition in the coating material for forming the magnetic layer used in the examples in JP-A-11-213379, a stress of 10 to 20 mm occurs in the longitudinal direction of the magnetic recording medium when the magnetic layer is applied, and When used as a serpentine magnetic recording medium,
In some cases, this may cause a deterioration in the error rate and may cause a problem that the degree of orientation is not sufficiently increased.

Further, when the magnetic layer is coated on the non-magnetic layer, if the thickness of the magnetic layer is reduced to reduce the thickness, the drying speed of the coating of the magnetic layer is small because the thickness of the coating is small. Is faster, in other words, the solvent is more likely to evaporate, and the coating film dries quickly, so that even after magnetic field alignment treatment, the degree of orientation of the magnetic powder in the coating film does not sufficiently increase. There was a tendency. In addition, when the drying speed of the magnetic layer coating film is extremely high, solvent traces evaporated from the coating film remain as porous on the coating film surface, making it impossible to achieve high filling and high durability of the target magnetic layer. In some cases, such a problem occurred.

Accordingly, an object of the present invention is to provide a coating film having an excellent degree of orientation even if the thickness of the magnetic layer is reduced, and to further increase the filling and durability of the thinned magnetic layer. It is another object of the present invention to provide a method for manufacturing a high-density magnetic recording medium having a thin magnetic layer, which can obtain a uniform magnetic layer / non-magnetic layer interface.

[0011]

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, when manufacturing a magnetic recording medium having a magnetic layer / non-magnetic layer, the magnetic layer has a thickness of 0.3 μm.
m, the thinner the film, the faster the drying speed of the coating film, the poorer the magnetic field orientation of the magnetic powder in the coating film, and the worse the surface properties of the coating film (magnetic layer). Was found. From this finding, when providing a thin magnetic coating film having a magnetic layer thickness of 0.3 μm or less, it is possible that the magnetic coating film can be kept in an undried state until the magnetic field alignment treatment is completed. It turned out to be important for improvement. Therefore, as a result of further intensive studies focusing on the solvent composition of the magnetic layer forming paint, it has been found that the above object can be achieved by including a predetermined amount of a solvent having a boiling point or higher in the organic solvent of the magnetic layer forming paint. As a result, the present invention has been completed.

That is, according to the method of manufacturing a magnetic recording medium of the present invention, a coating material for forming a non-magnetic layer is formed by coating a coating material for forming a non-magnetic layer on one surface of a support for a non-magnetic layer. In the method of manufacturing a magnetic recording medium having a magnetic layer having a thickness of 0.3 μm or less, the coating for forming a magnetic layer,
It contains at least a ferromagnetic powder, an abrasive, a binder resin and an organic solvent, wherein the organic solvent contains more than 50% by weight of a solvent having a boiling point of 120 ° C. or more.

[0013] In the production method of the present invention, the nonmagnetic layer-forming paint is applied to one surface of the nonmagnetic support, dried, calendered, and crosslinked and cured to form a nonmagnetic layer. After the step, the wet-on-dry coating method of applying the above-mentioned coating material for forming a magnetic layer on the non-magnetic layer, performing an orientation treatment, drying and calendering can be suitably applied.

The solid content of the coating material for forming a magnetic layer is preferably 5 to 25% by weight, and the boiling point is 120 ° C.
The above solvent is particularly preferably cyclohexanone. Further, it is preferable that the non-magnetic layer forming paint contains at least carbon black and a binder resin, and the binder resin is a radiation-curable binder resin.

In the present invention, by revising the solvent composition of the coating material for forming the magnetic layer, it is possible to maintain the wet state of the magnetic coating film and apply the magnetic field while the magnetic coating film is not dried to perform the magnetic field orientation treatment. As a result, the degree of orientation of the magnetic powder can be greatly improved, and at the same time, the magnetic layer can be highly filled and durable.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a specific configuration of the present invention will be described in detail. First, a non-magnetic support that can be used in the method for producing a magnetic recording medium of the present invention, a coating material for forming a non-magnetic layer applied on one surface thereof, and a magnetic layer applied on the formed non-magnetic layer The forming paint will be specifically described.

[Non-magnetic support] The material used as the non-magnetic support is not particularly limited, and may be selected from various flexible materials and various rigid materials according to the purpose, and may be tape-shaped or sheet-shaped according to various standards. What is necessary is just to have a predetermined shape and dimensions such as
There is no particular limitation on the form. Examples of the material include, for example, polyesters such as polyethylene terephthalate and polyethylene naphthalate; polyolefins such as polypropylene; and various resins such as polyamide, polyimide and polycarbonate.

The thickness of these nonmagnetic supports is 3.0 to 1
It is preferably 5.0 μm, and various materials can be appropriately selected and used according to the form and as needed.

The surface roughness of the nonmagnetic support used in the present invention is determined by measuring the center line average surface roughness Ra (JIS-B-060).
In 1), the thickness is 10 nm or less, preferably 8 nm or less, more preferably 7 nm or less. If Ra exceeds 10 nm, the film projections will be pushed up on the surface of the magnetic layer, and the electromagnetic conversion characteristics will be deteriorated. The surface roughness of the non-magnetic support is freely controlled by the size and amount of the filler added to the non-magnetic support as needed. Examples of these fillers include oxides and carbonates such as Ca, Si, Ti, and Al, and organic resin fine powders such as acryl. Preferably, a combination of Al ₂ O ₃ and organic resin fine powder is used. It is.

[Non-magnetic layer forming coating] The non-magnetic layer forming coating is a coating containing at least carbon black, a binder resin and an organic solvent.

The carbon black which can be used in the coating for the non-magnetic layer includes furnace black for rubber, thermal black for rubber, black for color, acetylene black and the like. Such carbon black has a specific surface area of 5 to 5.
600 m ² / g, DBP oil absorption 30 to 400 ml / 10
The average particle diameter is preferably 0 g and 10 to 100 nm. Specifically, the carbon black to be used can be appropriately selected with reference to “Carbon Black Handbook” edited by Carbon Black Association.

Various inorganic powders other than carbon black can be used for the coating for forming the non-magnetic layer.
Needle-like non-magnetic iron oxide (α-Fe ₂ O ₃ ) or the like can be used. In addition, by using spherical ultrafine iron oxide particles, high dispersibility can be obtained, and the filling rate of particles in the nonmagnetic layer can be increased. As a result, the surface properties of the nonmagnetic layer itself are improved, and thus the surface properties of the magnetic layer are improved, and the electromagnetic conversion characteristics are improved. In addition, CaC
Various non-magnetic powders such as O ₃ , titanium oxide, barium sulfate, α-Al ₂ O _{3 and the} like can be used.

The mixing ratio of the carbon black and the inorganic powder is preferably from 100/0 to 10/90 by weight. If the compounding ratio of carbon black is less than 10, a problem occurs in the surface electric resistance.

As the binder resin, conventionally known thermoplastic resins, thermosetting resins, radiation-curable resins and mixtures thereof can be used.

However, with a conventionally used thermoplastic resin and thermosetting resin, in order to obtain sufficient coating film properties, the non-magnetic layer coated raw roll is placed in an oven for a long time (for example, 70 ° C.). , 2-48 hours) for curing. In this case, there is a problem that the nonmagnetic layer coating film is deformed due to the tightening of the winding and the smoothness of the surface of the nonmagnetic layer is deteriorated, in addition to the trouble in the manufacturing process.

In order to solve such a problem, it is most preferable to use a radiation-curable binder resin as the binder resin of the paint for forming the nonmagnetic layer. By adopting a wet-on-dry coating method using a radiation-curable binder resin, a non-magnetic layer-forming paint is applied, dried, calendered, and irradiated with radiation to form three-dimensional cross-links. In applying the coating material for forming a magnetic layer, it is possible to obtain a favorable result which cannot be obtained with a thermoplastic resin or a thermosetting resin. That is, according to this method, the nonmagnetic layer has already been three-dimensionally crosslinked at the time when the magnetic layer is provided, so that the magnetic layer forming paint can be immediately applied without swelling due to the organic solvent. Therefore, continuation and simplification of the process can be achieved.

The radiation-curable binder resin that can be used in the present invention includes one or more unsaturated double bonds in the molecular chain which generate radicals by radiation and cure by crosslinking or polymerization. Refers to resin.

The coating material for forming a nonmagnetic layer is prepared by adding an organic solvent to the above components. The organic solvent used is not particularly limited, and one or two of various solvents such as ketone solvents such as methyl ethyl ketone (MEK), methyl isobutyl ketone and cyclohexanone, and aromatic solvents such as toluene.
More than one kind can be appropriately selected and used. The amount of the organic solvent to be added may be about 100 to 900 parts by weight based on 100 parts by weight of the total of the solid content of the magnetic powder and various inorganic particles and the binder resin.

The concentration of the solid content in the coating material for forming a nonmagnetic layer is preferably 5 to 45% by weight, more preferably 10 to 40% by weight. If the concentration is too low or too high, it is difficult to obtain a uniform coating film, which is not preferable.

[Coating for forming magnetic layer] The coating for forming the magnetic layer is a coating comprising at least a ferromagnetic powder, a binder resin, an abrasive and an organic solvent.

In order to form a multilayer coating film by applying a coating material for forming a magnetic layer on a non-magnetic layer, as described above, after coating the coating material for forming a non-magnetic layer on a non-magnetic support, A wet-on-wet coating method in which the magnetic layer forming paint is applied in a wet state, and a wet-in method in which the non-magnetic layer forming paint is applied, dried, calendered and cured, and then the magnetic layer forming paint is applied. There are two methods of an on-dry coating method.
Comparing both methods, the flatness of the interface between the magnetic layer and the non-magnetic layer,
The wet-on-dry coating method is preferred from the viewpoint of uniformity and the presence or absence of output fluctuation. In this case, in order to avoid swelling of the non-magnetic layer when applying the coating material for forming the magnetic layer, it is necessary that the crosslinking reaction of the binder resin contained in the non-magnetic layer is completed in the non-magnetic layer.

In the present invention, the organic solvent used in the coating material for forming the magnetic layer must contain more than 50% by weight of a solvent having a boiling point of 120 ° C. or higher, preferably 55% by weight to 90% by weight, More preferably, the content is 60% by weight or more and 80% by weight or less. Boiling point 120 ° C
When the amount of the above solvent is 50% by weight or less, the drying speed of the coating film of the magnetic layer is increased, so that it is difficult to improve the degree of orientation of the magnetic layer. In addition, when the thickness of the magnetic layer is reduced to reduce the thickness, the desired filling and durability of the magnetic layer cannot be achieved.

Examples of the organic solvent having a boiling point of 120 ° C. or more include cyclohexanone, methyl n-butyl ketone,
Examples thereof include ethyl n-butyl ketone, diisobutyl ketone, isophorone, methyl cellosolve, and ethyl cellosolve, and among them, cyclohexanone is particularly preferable.

The solid content in the coating material for forming a magnetic layer is preferably 5 to 25% by weight, more preferably 10 to 20% by weight. When the concentration is less than 5% by weight, a uniform coating film cannot be obtained. On the other hand, when the concentration exceeds 25% by weight, the drying speed of the coating film for forming a magnetic layer increases, and Because the solvent migrates to the undercoat layer coating,
It is difficult to improve the degree of orientation of the magnetic layer, which is the object of the present invention.

As the ferromagnetic powder used for the coating film for forming a magnetic layer, a metal alloy fine powder is preferable. As such a metal alloy fine powder, the holding force Hc is 1500 to 3000 Oe,
The saturation magnetization σs is 120 to 160 emu / g, and the average major axis diameter is 0.15 μm or less, preferably 0.05 to 0.10.
μm, an average minor axis diameter of 10 to 20 nm, and an aspect ratio of 1.2 to 20 are desirable. Ni, Zn, Co, Al, Si, Y,
Other rare earths may be used.

Such a ferromagnetic powder may be contained in an amount of about 70 to 90 parts by weight in the composition of the magnetic layer. If the content of the ferromagnetic powder is too large, the content of the binder resin decreases, so that the surface smoothness due to the calendar processing tends to deteriorate. On the other hand, if the amount is too small, a high reproduction output cannot be obtained.

As the binder resin of the coating material for forming a magnetic layer, conventionally known thermoplastic resins, thermosetting resins, radiation-curable resins and mixtures thereof are used. Specific examples include polyurethane resins, vinyl chloride copolymers, vinyl chloride-acrylate copolymers, and vinyl chloride-
Vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer,
Acrylate-acrylonitrile copolymer, acrylate-vinylidene chloride copolymer, methacrylate-vinylidene chloride copolymer, methacrylate-ethylene copolymer, polyvinyl fluoride-
Vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyamide resin,
Polyvinyl butyral, cellulose derivatives (eg, cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, etc.), styrene-butadiene-based copolymer, polyester-chlorovinyl ether acrylate-based copolymer, amino Resins and synthetic rubber-based thermoplastic resins can be used. Among them, a combination of a polyurethane resin and a vinyl chloride resin is most preferable.

The content of the binder resin used in the coating material for forming the magnetic layer is preferably 5 to 40 parts by weight, particularly preferably 10 to 30 parts by weight, per 100 parts by weight of the ferromagnetic powder.
If the content of the binder resin is too small, the strength of the magnetic layer is reduced, so that the running durability tends to be deteriorated. On the other hand, if the content is too large, the content of the ferromagnetic powder decreases, and the electromagnetic conversion characteristics decrease.

The coating material for forming a magnetic layer contains α-alumina (Mohs hardness of 9) and chromium oxide (Mohs hardness of 9) to increase the mechanical strength of the magnetic layer and to prevent clogging of the magnetic head. , Silicon carbide (Mohs hardness of 9.5), silicon oxide (Mohs hardness of 7), aluminum nitride (Mohs hardness of 9), boron nitride (Mohs hardness of 9.5), etc., having a Mohs hardness of 6 or more, preferably a Mohs hardness of 9 or more. It is preferable to include at least one abrasive. These are usually
It has an irregular shape and has the effect of preventing clogging of the magnetic head and improving the strength of the coating film. The average particle size of the abrasive is preferably 0.05 to 0.2 μm, and is preferably smaller than the thickness of the magnetic layer. If the average particle size is too large, the amount of protrusion from the surface of the magnetic layer becomes large, resulting in a decrease in electromagnetic conversion characteristics, an increase in dropout, an increase in head wear, and the like. On the other hand, if the average particle size is too small, the amount of protrusion from the surface of the magnetic layer becomes small, and the effect of preventing head clogging becomes insufficient. The average particle size is usually measured by a transmission electron microscope. The content of the abrasive is 3 to 25 parts by weight, preferably 5 to 20 parts by weight based on 100 parts by weight of the ferromagnetic powder.

Further, a dispersing agent such as a surfactant, a lubricant such as a higher fatty acid, a fatty acid ester, or a silicone oil, or other various additives may be added to the coating material for forming a magnetic layer, if necessary. .

[Paint for forming back coat layer] The back coat layer is provided as necessary for improving running stability and preventing the magnetic layer from being charged. The coating for forming the back coat layer preferably contains 30 to 80% by weight of carbon black. If the content of carbon black is too small, the antistatic effect tends to decrease, and the running stability tends to decrease. Also, since the light transmittance is likely to be high, there is a problem in a method of detecting the end of the tape by a change in the light transmittance. On the other hand, if the content of carbon black is too large, the strength of the back coat layer decreases, and running durability tends to deteriorate. Any carbon black may be used as long as it is commonly used, and the average particle size is preferably about 5 to 500 nm. The average particle size is usually measured by a transmission electron microscope.

The backcoat layer forming paint may contain, in addition to the carbon black, non-magnetic inorganic powders such as various abrasives mentioned in the description of the magnetic layer in order to increase the mechanical strength. The content of the nonmagnetic inorganic powder is preferably 0.1 to 100 parts by weight of carbon black.
5 parts by weight, more preferably 0.5 to 2 parts by weight. The average particle size of the non-magnetic inorganic powder is preferably 0.1 to 0.5 μm. If the content of such a nonmagnetic inorganic powder is too small, the mechanical strength of the back coat layer tends to be insufficient, and if it is too large, the amount of abrasion of the tape sliding path guide and the like tends to increase.

In addition, if necessary, a dispersant such as a surfactant, a lubricant such as a higher fatty acid, a fatty acid ester, or a silicone oil, and other various additives may be added.

The binder resin, crosslinking agent, solvent and the like used for the backcoat layer forming paint may be the same as those used for the magnetic layer forming paint described above. The content of the binder resin is
For a total of 100 parts by weight of the solid content, preferably 15 to 2
00 parts by weight, more preferably 50 to 180 parts by weight. If the content of the binder resin is too large, the friction with the medium sliding contact path becomes too large, the running stability is reduced, and a running accident is likely to occur. In addition, problems such as blocking with the magnetic layer occur. On the other hand, when the content of the binder resin is too small, the strength of the back coat layer is reduced, and the running durability tends to be reduced.

The solid content concentration in the coating material for forming the back coat layer is preferably 5 to 25% by weight, more preferably 8 to 2% by weight.
0% by weight. If the concentration deviates from this range, a uniform coating film cannot be obtained, resulting in deterioration of the surface properties.

[Manufacturing Method] The magnetic recording medium of the present invention can be manufactured by applying the above-described coating material for forming a non-magnetic layer and the coating material for forming a magnetic layer on the non-magnetic support in this order.

In the wet-on-dry coating method which can be suitably applied in the present invention, the coating material for forming a non-magnetic layer is applied on one surface of the non-magnetic support according to a known method, and dried. The non-magnetic layer is formed by calendering and crosslinking and hardening.

When a radiation-curable binder resin is used in the coating material for forming the non-magnetic layer, the radiation used is preferably an electron beam. The irradiation amount is 1 to 10 Mrad, preferably 3 to 7 Mrad, and the irradiation energy (acceleration voltage) is preferably 100 kV or more. Further, it is desirable that the irradiation of the radiation be performed in-line after coating, drying and calendering and before winding, but may be performed after winding.

The center line average surface roughness Ra of the formed nonmagnetic layer is preferably 8.0 nm or less after the smoothing treatment.
6.0 nm or less is more preferable, and 5.0 nm or less is most preferable. If it exceeds 8.0 nm, the interface between the upper magnetic layer and the lower non-magnetic layer becomes non-uniform, which may cause output fluctuation.

The thickness of the formed non-magnetic layer is set to 0.1.
It is 1 to 2.5 μm, preferably 0.3 to 2.3 μm. When the non-magnetic layer is too thin, the surface properties of the non-magnetic support are easily affected, and as a result, the surface properties of the non-magnetic layer are deteriorated, and the surface properties of the magnetic layer are also likely to deteriorate. It tends to decrease. In addition, since the light transmittance increases, there is a problem in detecting the tape end based on a change in the light transmittance. Also, even if the nonmagnetic layer is thicker than a certain degree,
I can no longer expect the improvement of performance, rather, when providing a coating,
The thickness tends to be non-uniform, the application conditions are severe, and the surface smoothness tends to be poor.

Next, the above-mentioned coating material for forming a magnetic layer is applied to the formed non-magnetic layer according to a known method, subjected to an orientation treatment, dried, and preferably subjected to calendar processing.

The orientation treatment is performed to orient the magnetic powder in the magnetic layer.
It may be a longitudinal direction, a vertical direction, or an oblique direction with respect to the running direction of the medium. In order to direct the orientation in a predetermined direction, a permanent magnet such as a ferrite magnet or a rare earth magnet, an electromagnet, or a magnetic field generating means such as a solenoid is used. A plurality of these magnetic field generating means may be used in combination, and further, an appropriate drying step may be provided in advance before the orientation, or drying may be performed at the same time as the orientation so that the orientation after the drying is the highest. . After such a magnetic field orientation treatment is performed, each coating film is dried in a drying device (drying oven). Specifically, drying is usually performed by a known drying means such as hot air, far infrared rays, an electric heater, and a vacuum device provided inside a drying furnace. Drying temperature is usually 40-200
° C, preferably 60 to 180 ° C, more preferably 80 ° C.
The temperature may be appropriately selected within the range of from 150 ° C. to 150 ° C. depending on the heat resistance of the nonmagnetic support, the solvent composition, the solid content of the coating material, and the like. If the drying temperature is lower than 40 ° C., the drying efficiency is low, and the amount of the residual solvent tends to increase, which is not preferable. On the other hand, when the temperature exceeds 200 ° C., the solvent in the coating film evaporates too rapidly, which deteriorates the surface properties of the magnetic recording medium. The drying furnace may have a temperature gradient, and the atmosphere in the drying furnace may be general air or an inert gas.

The coating film subjected to the magnetic field orientation treatment and the drying treatment is preferably calendered. Also,
After the calendering treatment, it is preferable to perform a heat curing treatment at 40 ° C. to 80 ° C. and / or an electron beam irradiation treatment in order to accelerate the curing of the coating film.

The thickness of the formed magnetic layer is 0.30 μm
Hereinafter, it is preferably 0.05 to 0.30 μm, more preferably 0.10 to 0.25 μm. If the magnetic layer is too thick, self-demagnetization loss and thickness loss increase, and the required electromagnetic conversion characteristics cannot be obtained.

Further, the center line average roughness Ra of the surface of the magnetic layer
Is 1.0 to 8.0 nm, preferably 2.0 to 7.0 n
m. When Ra is less than 1.0 nm, the surface is too smooth, running stability is deteriorated, and trouble during running is likely to occur. On the other hand, if the thickness exceeds 8.0 nm, the surface of the magnetic layer becomes rough, and the electromagnetic conversion characteristics such as reproduction output deteriorate.

After the formation of the magnetic layer, a back coat layer is provided if necessary by a known method. The thickness (after calendering) of the formed back coat layer is as follows:
0 μm or less, preferably 0.1 to 1.0 μm, more preferably 0.2 to 0.8 μm. If the thickness of the back coat layer exceeds 1.0 μm, the friction between the back coat layer and the medium sliding path becomes too large, and running stability tends to decrease, which is not preferable.

After forming the back coat layer, the magnetic recording medium is processed into a desired tape shape or the like by a slitter, and if necessary, the magnetic layer surface and / or the back coat layer surface is subjected to secondary processing such as polishing and cleaning. Is prepared.

[0058]

EXAMPLES Hereinafter, the present invention will be described in more detail by showing specific examples of the present invention. A coating material for forming a magnetic layer, a coating material for forming a nonmagnetic layer, and a coating material for forming a backcoat layer were respectively prepared according to the following composition table.

[Coating for Magnetic Layer Formation] Binder Resin Solution Preparation Vinyl chloride resin (manufactured by Zeon Corporation: MR-110) 10 parts by weight Polyester polyurethane resin (manufactured by Toyobo Co., Ltd .: UR-8300) 7 parts by weight MEK 12 2.6 parts by weight Toluene 12.6 parts by weight Cyclohexanone 37.8 parts by weight The above composition was charged into a hypermixer, mixed and stirred to obtain a binder resin solution.

Kneading The following composition was put into a pressure kneader and kneaded for 2 hours. α-Fe magnetic powder 100 parts by weight (Hc = 1820 Oe, σs = 130 emu / g, BET = 57 m ² / g, average major axis length = 0.10 μm) α-Al ₂ O ₃ (Sumitomo Chemical: HIT- 82, average particle size = 0.13 μm) 12 parts by weight Binder resin solution 40 parts by weight

The following composition was added to the slurry after kneading to adjust the viscosity to an optimum for dispersion treatment. Binder resin solution 40 parts by weight MEK 9 parts by weight Toluene 9 parts by weight Cyclohexanone 27 parts by weight

Dispersion The above slurry was subjected to dispersion treatment in a sand mill.

Viscosity adjusting solution The following composition was charged into a hypermixer and mixed for 1 hour.
The mixture was stirred to obtain a viscosity adjusting liquid. 0.5 parts by weight of stearic acid 0.5 parts by weight of myristic acid 0.5 parts by weight of butyl stearate 126 parts by weight of MEK 126 parts by weight of toluene 378 parts by weight of cyclohexanone

After the viscosity-adjusting liquid was mixed and stirred with the viscosity-adjusted dispersion slurry, dispersion treatment was again performed with a sand mill to obtain a coating material.
The paint was circulated and filtered using a depth filter having an absolute filtration accuracy of 1.0 μm.

To 100 parts by weight of the paint after the final paint filtration, 0.8 part by weight of an isocyanate compound (Coronate L, manufactured by Nippon Polyurethane) was added, stirred and mixed, and circulated through a depth filter having an absolute filtration accuracy of 1.0 μm. Was performed to obtain a final paint for the magnetic layer.

[Coating for forming non-magnetic layer] Binder resin solution preparation Electron beam-curable vinyl chloride resin 10 parts by weight (vinyl chloride-epoxy containing monomer copolymer, average degree of polymerization = 310, epoxy content = 3% by weight %, S content = 0.6% by weight, acrylic content = 6 / molecule, Tg = 60 ° C.) Electron beam-curable polyester polyurethane resin 7 parts by weight (phosphorus compound-hydroxy-containing polyester polyurethane, number average molecular weight = 13,000, acrylic content = 6 / molecule, Tg = 10 ° C.) MEK 21 parts by weight Toluene 21 parts by weight Cyclohexanone 21 parts by weight

The above composition was charged into a hypermixer and stirred to obtain a binder resin solution.

Kneading The following composition was charged into a pressure kneader and kneaded for 2 hours. 10 parts by weight of needle-like α-Fe ₂ O ₃ (manufactured by Toda Kogyo Co., Ltd .: DPN-250BW, average major axis length = 0.15 μm, specific surface area = 53 m ² / g) 90 parts by weight of carbon black (manufactured by Columbian Carbon: Raven 760B, average particle size = 30 nm, specific surface area = 70 m ² / g, DPB oil absorption = 48 ml / 100 g) Binder resin solution 40 parts by weight

The following composition was added to the slurry after kneading to adjust the viscosity to an optimum for dispersion treatment. Binder resin solution 40 parts by weight MEK 15 parts by weight Toluene 15 parts by weight Cyclohexanone 15 parts by weight

Dispersion The above slurry was subjected to dispersion treatment in a sand mill.

Viscosity Adjusting Liquid The following composition was charged into a hypermixer and stirred to obtain a viscosity adjusting liquid. 0.5 parts by weight of stearic acid 0.5 parts by weight of myristic acid 0.5 parts by weight of butyl stearate 100 parts by weight of MEK 100 parts by weight of toluene 100 parts by weight of cyclohexanone

After adjusting the viscosity and dispersing the final paint into the slurry, the above viscosity adjusting solution was mixed and stirred, and then subjected to a dispersion treatment again with a sand mill to obtain a paint.
The paint was circulated and filtered using a depth filter having an absolute filtration accuracy of 1.0 μm to obtain a final paint for forming a nonmagnetic layer.

[Coating for Backcoat Layer Formation] Binder Resin Solution Preparation Vinyl Chloride-Vinyl Acetate-Vinyl Alcohol Copolymer 65 Parts by Weight (Monomer Weight Ratio = 92: 3: 5, Average Degree of Polymerization = 420) Polyester Polyurethane Resin (Toyobo: UR-8300) 35 parts by weight MEK 260 parts by weight Toluene 260 parts by weight Cyclohexanone 260 parts by weight

The above composition was charged into a hypermixer and stirred to obtain a binder resin solution.

Dispersion The following composition was charged into a ball mill and dispersed for 24 hours. 80 parts by weight of carbon black (Conductex SC, manufactured by Columbian Carbon Co., average particle size = 20 nm, BET = 220 m ² / g) 1 part by weight of carbon black (Sevaccarb MT, manufactured by Columbian Carbon Co., average particle size = 350 nm, BET) = 8 m ² / g) 1 part by weight α-Fe ₂ O ₃ (manufactured by Toda Kogyo KK: TF100, average particle size = 0.1 μm) 880 parts by weight binder resin solution

Viscosity Adjusting Liquid The following composition was charged into a hypermixer and stirred to obtain a viscosity adjusting liquid. 1 part by weight of stearic acid 1 part by weight of myristic acid 2 parts by weight of butyl stearate 210 parts by weight of MEK 210 parts by weight of toluene 210 parts by weight of cyclohexanone

After the above viscosity adjusting liquid was mixed and stirred with the viscosity-adjusted dispersion slurry, dispersion treatment was again performed for 3 hours by a ball mill. The paint was circulated and filtered using a depth filter having an absolute filtration accuracy of 1.0 μm.

To 100 parts by weight of the paint after the final paint filtration, 1 part by weight of an isocyanate compound (Coronate L, manufactured by Nippon Polyurethane Co.) was added.
The mixture was stirred, mixed, and circulated and filtered using a depth filter having an absolute filtration accuracy of 1.0 μm to obtain a back coat paint.

[Preparation of Magnetic Recording Medium] Example 1 (Wet-on-dry coating method (W / D)) For forming the above-mentioned non-magnetic layer on the surface of a non-magnetic support (a polyethylene terephthalate film having a thickness of 6.2 μm) The coating material was applied, dried, calendered, and cured by irradiating an electron beam in a nitrogen gas atmosphere. The above-mentioned coating material for forming a magnetic layer was applied on the obtained non-magnetic layer, subjected to an orientation treatment, dried, and then subjected to calendar processing. The thickness of the magnetic layer / nonmagnetic layer after the calendar processing was 0.2 / 1.8 μm. Further, the above-mentioned coating material for forming a back coat layer was applied to the back surface of the nonmagnetic support. After drying, calendering was performed. The thickness of the back coat layer after calendering is 0.5μ
m.

The obtained roll was allowed to stand at room temperature for 24 hours, then cured in a heating oven at 60 ° C. for 24 hours, cut into 8 mm width and 1/2 inch width, assembled into a cassette, and sampled with a magnetic tape. And

Examples 2 and 3 and Comparative Examples 1 and 2 (wet
On-dry coating method) Solvent composition MEK /
Magnetic tape samples were prepared in the same manner as in Example 1 except that toluene / cyclohexanone = 20/20/60 was changed to various solvent compositions shown in Table 1 below.
And Comparative Examples 1 and 2.

Example 4 (Wet-on-wet coating)
Method (W / W)) The above-mentioned coating material for forming a non-magnetic layer is applied to the surface of a non-magnetic support (a polyester terephthalate film having a thickness of 6.2 μm), and while the coating material for forming a non-magnetic layer is in a wet state,
The coating material for forming a magnetic layer was applied, thereafter, subjected to an orientation treatment, dried, calendered, irradiated with an electron beam in a nitrogen gas atmosphere, and cured. The thickness of the magnetic layer / nonmagnetic layer after the calendar processing was 0.2 / 1.8 μm. Further, a paint for a back coat layer was applied to the back surface of the nonmagnetic support. After drying, calendering was performed. The thickness of the back coat layer after calendering was 0.5 μm. After leaving the obtained roll at room temperature for 24 hours, it was cured in a heating oven at 60 ° C. for 24 hours, cut into 8 mm width and イ ン チ inch width, and incorporated into a cassette to obtain a magnetic tape sample.

The following tests were performed on the magnetic tape samples of Examples 1 to 3 and Comparative Examples 1 to 3. <Degree of Orientation> The magnetic properties of the magnetic tape sample were measured using a vibrating sample magnetometer (VSM-V type, manufactured by Tsukuei Kogyo Co., Ltd.). The maximum external magnetic field was 10 kOe. The degree of orientation is defined as (Br / Bm) MD / (Br / Bm) TD,
Br is the residual magnetic flux density, Bm is the saturation magnetic flux density, MD is the longitudinal direction of the tape, and TD is the tape width direction.

<Output Fluctuation> When a sine wave signal having a frequency of 750 kHz was recorded and reproduced with an optimum recording current, the RF output was converted to a spectrum analyzer (Advantest: TR417).
Measured in 1). The settings of the spectrum analyzer are center frequency 750kHz, frequency span 0H
z and the sweep time are set to optimal values, and the output fluctuation (Vp−
p) was confirmed. VTR used is Sony EV-S9
00 (Hi8 format VTR).

<Error Rate> A random signal is written over a 1/2 inch wide magnetic tape sample over the entire length of the tape, and an uncorrectable error occurs during a read check immediately after writing, and rewriting (rewriting) at another location
The number of times was measured. Measure the total write capacity and measure 1M
It was calculated as an error value per B. The drive used is Q
UAntum DLT-7000 (DLT5 mode)
And The results obtained are shown in Table 1 below.

[0086]

[Table 1] (*) In the solvent composition, the ratio of MEK / toluene / cyclohexanone was shown by weight%.

In Comparative Examples 1 and 2, when the content ratio of cyclohexanone in the solvent of the magnetic layer forming paint was 50% by weight or less, a streak was generated when the magnetic layer forming paint was applied, and a linear serpentine type magnetic recording medium was formed. To use as a problem in terms of error rate. Also, since the drying was completed before the magnetic powder was sufficiently oriented,
This caused a decrease in the degree of orientation.

In the wet-on-wet coating method in the fourth embodiment, although no stress occurs, the magnetic layer /
The interface of the non-magnetic layer became non-uniform, and output fluctuation occurred.

[0089]

As described above, according to the method for manufacturing a magnetic recording medium of the present invention, a coating film having an excellent degree of orientation can be obtained even if the thickness of the magnetic layer is reduced. In addition, it is possible to increase the filling and durability of the thinned magnetic layer and to obtain a uniform interface between the magnetic layer and the nonmagnetic layer.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Akihiko Seki 1-13-1 Nihonbashi, Chuo-ku, Tokyo Inside TDK Corporation (72) Inventor Katsuhiko Yamazaki 1-13-1 Nihonbashi, Chuo-ku, Tokyo TDK In-house F term (reference) 5D006 BA10 BA11 BA19 CA01 CA04 EA01 EA04 FA02 FA09 5D112 AA03 AA05 BB09 BD01 BD07 CC01 CC03 DD04 GB06

Claims (5)

[Claims] A coating material for forming a nonmagnetic layer is applied on one surface of a nonmagnetic support, and a coating material for forming a magnetic layer is applied on the nonmagnetic layer formed. In the method of manufacturing a magnetic recording medium having a magnetic layer, the coating material for forming a magnetic layer contains at least a ferromagnetic powder, an abrasive, a binder resin and an organic solvent, and the organic solvent is
A method for producing a magnetic recording medium, comprising more than 50% by weight of a solvent having a boiling point of 120 ° C. or higher. 2. A step of applying the coating for forming a nonmagnetic layer on one surface of the nonmagnetic support, drying, calendering, and crosslinking and curing to form a nonmagnetic layer,
2. The production method according to claim 1, further comprising a step of applying the coating material for forming a magnetic layer on the nonmagnetic layer, subjecting the coating material to an orientation treatment, drying and calendering. 3. The coating material for forming a magnetic layer having a solid content concentration of 5%.
The method according to claim 1 or 2, wherein the content is from 25 to 25% by weight. 4. The method according to claim 1, wherein the solvent having a boiling point of 120 ° C. or higher is cyclohexanone. 5. The non-magnetic layer forming coating material according to claim 1, wherein the coating material contains at least carbon black and a binder resin, and the binder resin is a radiation-curable binder resin. Manufacturing method.