JP4415187B2 - Magnetic recording medium - Google Patents

Description

  The present invention relates to a multilayer coating type magnetic recording medium, and more particularly to a high density type magnetic recording medium having an extremely thin magnetic recording layer.

In recent years, the amount of information has increased due to the use of digital recording and the like, and in the future, the direction of further higher density and shorter wavelength recording will be headed.
Accordingly, in high-density magnetic recording media used in various magnetic recording devices, the magnetic layer is thinned and the surface of the magnetic layer is smoothed in order to improve short wavelength output and S / N characteristics. Has been planned.

On the other hand, magnetic recording systems using magnetoresistive heads (MR heads, GMR heads, etc.) have been devised and put into practical use in response to higher recording densities of magnetic recording media. As a result, there is a problem that electrostatic breakdown of the magnetic head occurs.
In order to solve such a problem, a technique has been proposed in which carbon black is contained in the lower nonmagnetic layer while the surface of the magnetic layer is smoothed to reduce the surface electrical resistance of the magnetic layer (for example, Patent Documents). 1 and 2).

On the other hand, as described above, the magnetic layer has been made thinner in order to cope with high-density recording and short-wavelength recording. However, if this layer is made extremely thin, for example, 100 nm or less, a phenomenon in which electromagnetic conversion characteristics deteriorate. Was confirmed.
As a result, when the surface of the magnetic layer was observed in an enlarged manner, a fine pinhole phenomenon occurred, and it was found that this caused recording loss and caused deterioration in electromagnetic conversion characteristics.

  Regarding such problems, conventionally, technical proposals such as specifying the area ratio of the nonmagnetic portion on the surface of the recording layer by examining the paint properties have been made, but in the future, the magnetic layer will be made thinner. It is considered that the effect of sufficiently suppressing the pinhole phenomenon on the surface of the magnetic layer cannot be obtained.

JP 2000-163739 A JP 2002-260214 A JP 2001-8220 A JP 2001-250219 A

  In view of the above-described conventional problems, the present invention relates to a high-density magnetic recording medium having an extremely thin magnetic layer, effectively reducing the surface pinhole phenomenon and realizing excellent electromagnetic conversion characteristics. It was decided to provide a possible magnetic recording medium.

The magnetic recording medium of the present invention comprises, on at least one main surface of a nonmagnetic support made of polyethylene terephthalate , a lower nonmagnetic layer containing at least inorganic particles made of iron oxide and / or titanium oxide and a binder, and at least A magnetic layer containing a magnetic powder and a binder and having a film thickness of 50 nm or less is formed by a wet-on-wet method , the magnetic layer has a film thickness of d (nm), and the lower non-magnetic layer When the major axis of the inorganic particles contained in is defined as a (nm), the following relationship is assumed.
a ≦ 3.0 × d−60 (1)

  According to the present invention, a high-density recording type magnetic recording medium capable of effectively reducing the pinhole phenomenon on the surface of the magnetic layer and realizing excellent electromagnetic conversion characteristics is obtained.

  The magnetic recording medium of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following examples, and conventionally known configurations are added, materials are changed, and the gist of the present invention is described. Various applications are possible within the range that does not change.

A magnetic recording medium according to an example of the present invention is formed by laminating a lower nonmagnetic layer 2 and a magnetic layer 3 on one main surface of a nonmagnetic support 1 as shown in the schematic configuration diagram of FIG. The back coat layer 4 is formed on the main surface.
Hereinafter, each of these layers will be described.

As the nonmagnetic support 1, any material used in conventionally known magnetic recording media can be applied.
For example, polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins such as polyethylene and polypropylene, cellulose derivatives such as cellulose triacetate, cellulose diacetate and cellulose acetate butyrate, vinyl resins such as polyvinyl chloride and polyvinylidene chloride, Examples thereof include plastics such as polycarbonate, polyimide, polyamide and polyamideimide, metals such as paper, aluminum and copper, light alloys such as aluminum alloys and titanium alloys, ceramics and single crystal silicon.
The form of the non-magnetic support 1 is not particularly limited, and may be appropriately selected depending on the final shape of the magnetic recording medium, such as a film, tape, sheet, disk, card, drum, etc. Select.

Next, the lower nonmagnetic layer 2 will be described.
The lower nonmagnetic layer 2 has a configuration containing at least inorganic particles, a binder, and various additives. The lower nonmagnetic layer 2 can be formed by applying a paint in which these materials are mixed using a predetermined solvent.

  As the inorganic particles contained in the lower nonmagnetic layer 2, any of various inorganic fine particle powders conventionally applied to the lower nonrecording layer of the magnetic recording medium can be used. For example, alumina, iron oxide, silicon carbide, Chromium oxide, cerium oxide, goethite, silicon nitride, titanium carbide, titanium oxide, silicon dioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, disulfide Examples include molybdenum. These may be used alone or in combination. The shape of the inorganic particles may be any of a needle shape, a spherical shape, a plate shape, and a dice shape.

  Further, it is preferable to add a conductive agent to the lower nonmagnetic layer 2 in order to suppress electrostatic breakdown of the magnetic head. Any known conductive agent can be used, and examples thereof include carbon black and conductive titanium oxide.

As the binder contained in the lower nonmagnetic layer 2, any binder resin that has been conventionally applied to the production of a coating type magnetic recording medium can be used.
For example, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-maleic acid copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer Polymer, acrylic ester-acrylonitrile copolymer, acrylic ester-vinylidene chloride copolymer, methacrylic acid-vinylidene chloride copolymer, methacrylic ester-styrene copolymer, thermoplastic polyurethane resin, phenoxy resin, polyvinyl fluoride , Vinylidene chloride-acrylonitrile copolymer, butadiene-acrylonitrile copolymer, acrylonitrile-butadiene-methacrylic acid copolymer, polyvinyl butyral, cellulose derivative, styrene-butadiene copolymer, polyester resin, phenol Fat, epoxy resins, thermosetting polyurethane resins, urea resins, melamine resins, alkyd resins, urea - formaldehyde resin, or mixtures thereof.
In particular, polyurethane resin, polyester resin, acrylonitrile-butadiene copolymer or the like is applied to impart flexibility, and cellulose derivative, phenol resin, epoxy resin, or the like can be applied to impart rigidity. preferable.
Moreover, durability of the lower nonmagnetic layer 2 can be improved by using, for example, an isocyanate compound as a crosslinking agent.

As the solvent for adjusting the coating material for forming the lower nonmagnetic layer 2, any solvent that has hitherto been applied to the production of a coating type magnetic recording medium can be used.
For example, ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, ester solvents such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate and glycol acetate monoethyl ester, glycol ethers such as glycol monoethyl ether and dioxane Examples thereof include organic solvents, aromatic hydrocarbon solvents such as benzene, toluene and xylene, and organic chlorine compound solvents such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin and dichlorobenzene.

Next, the magnetic layer 3 will be described.
The magnetic layer 3 has a configuration containing at least a ferromagnetic powder, a binder resin, and various additives, and can be formed by applying a paint in which these are mixed using a predetermined solvent.
As the ferromagnetic powder, any known material that has been applied for coating-type magnetic recording media can be applied. Examples thereof include ferromagnetic iron oxide, ferromagnetic chromium dioxide, ferromagnetic alloy, and iron nitride. It is done.
Any binder can be used as the binder for forming the magnetic layer 3 as long as it is a binder applied to a coating type magnetic recording medium, and any of the binders for forming the lower non-magnetic layer described above can be applied.
Moreover, as a solvent for adjusting the coating material for forming the magnetic layer, any conventionally known solvent can be applied, and any of the above-mentioned solvents for forming the lower non-magnetic layer can be applied.

The film thickness of the magnetic layer shall be 100 nm or less. If it exceeds 100 nm, PW50 (pulse width at 50% of the peak of the isolated reproduction wave) becomes large, and the high-density recording characteristics deteriorate.
When reproducing a magnetic recording medium in a system using an MR head, if the magnetic layer is relatively thick and Br is 0.25 T or more, the specifications of the MR head (the saturation magnetic flux density of the MR element, the film thickness and the SAL) The saturation phenomenon of the MR head occurs depending on the saturation magnetic flux density and film thickness of the film, and the C / N characteristics are degraded.

In the magnetic recording medium of the present invention, when the thickness of the magnetic layer 3 is d (nm) and the major axis length of the inorganic particles contained in the lower nonmagnetic layer 2 is a (nm), It specifies to the thing which has the relationship of Formula (1). The following formula was derived by plotting the relationship between the thickness of the magnetic layer when the electromagnetic conversion characteristics were practically good and the long axis length of the inorganic particles in the lower nonmagnetic layer and calculating by the least square method. It is.
a ≦ 3.0 × d−60 (1)
Thereby, it was confirmed that the pinhole phenomenon on the surface of the extremely thin magnetic layer 3 can be effectively reduced, and excellent electromagnetic conversion characteristics can be obtained.

  The back coat layer 4 is formed as necessary, and is formed of various additives such as a binder, inorganic particles, a lubricant, and an antistatic agent.

  In the present invention, in place of the backcoat layer 4, the lower nonmagnetic layer 2 and the magnetic layer 3 described above are laminated on the other main surface side to have recording layers on both main surfaces. A large-capacity magnetic recording medium can also be obtained.

Next, a method for producing a magnetic recording medium will be described.
First, a predetermined nonmagnetic support 1 is prepared. Next, the coating material for the lower nonmagnetic layer 2 and the coating material for the magnetic layer 3 are prepared.
These coating materials are prepared by kneading and dispersing the above-described materials together with a predetermined solvent. Any known kneading method can be applied to the kneading dispersion method, and the method is not particularly limited. Examples thereof include a method using a continuous biaxial kneader (extruder), a kneader, a pressure kneader and the like.
The lower nonmagnetic layer 2 and the magnetic layer 3 are formed by laminating the respective paints using a conventionally known coating method such as gravure coating, extrusion coating, air doctor coating, reverse roll coating, and the like.
Further, the magnetic layer 3 may be formed on the lower nonmagnetic layer 2 by a so-called wet-on-dry method in which each paint is sequentially applied and dried. Alternatively, the magnetic layer 3 may be formed by a so-called wet-on-wet method in which multiple layers are applied.

  In addition, the material of the nonmagnetic support 1, the inorganic particles contained in the lower nonmagnetic layer 2, the binder, and other additives such as a dispersant, an abrasive, an antistatic agent, an antirust agent, etc. As the magnetic powder, binder, other additives, and solvent for adjusting the coating material, any conventionally known one can be applied and is not limited at all.

[Examples 1 to 8], [Comparative Examples 1 to 4], [Reference Tape]
In the following, various samples were prepared for tape-like magnetic recording media. The present invention is not limited to the following examples. As shown in FIG. 1, a sample magnetic tape is formed by laminating a lower nonmagnetic layer 2 and a magnetic layer 3 on one main surface of a nonmagnetic support 1, and a backcoat layer on the other main surface. It is assumed that 4 is formed.

First, a magnetic layer forming paint (magnetic paint) having the following composition was prepared.
[Magnetic paint composition]
Ferromagnetic powder: 100 parts by weight (iron-cobalt alloy metal magnetic powder (average major axis length 0.06 μm))
First binder: 9 parts by weight (vinyl chloride copolymer (average polymerization degree 300))
Second binder: 9 parts by weight (polyester polyurethane resin (weight average molecular weight 41200, Tg 40 ° C.)
Lubricant: Stearic acid: 1 part by weight Butyl stearate: 2 parts by weight Solvent: Methyl ethyl ketone: 20 parts by weight Toluene: 20 parts by weight Cyclohexanone: 10 parts by weight

According to the above composition, the mixture was kneaded with a kneader, further diluted with methyl ethyl ketone, toluene, and cyclohexanone, and then dispersed in a sand mill to obtain a dispersion for forming a magnetic layer.
Thereafter, 4 parts by weight of polyisocyanate (Japanese polyurethane curing agent “Coronate L”) was added and stirred to prepare a magnetic coating.

Next, a coating material for forming the lower nonmagnetic layer was prepared.
First, four types of first inorganic particles (acicular iron oxide and spherical titanium oxide having different long superaxial lengths) shown in Table 1 below were prepared.

  Next, an arbitrary particle was selected from the first inorganic particles shown in Table 1 above, and a dispersion for forming the lower nonmagnetic layer was prepared according to the composition shown below.

[Dispersion composition for forming the lower non-magnetic layer]
First inorganic particles (selected from inorganic particles A to D in Table 1): 100 parts by weight Second inorganic particles: carbon black: 24 parts by weight
(Particle size 20 nm, DBP oil absorption 120 ml / 100 g)
First binder: Vinyl chloride copolymer (average polymerization degree 300): 9 parts by weight Second binder: Polyester polyurethane resin: 9 parts by weight
(Weight average molecular weight 41200, Tg 40 ° C.)
Lubricant: butyl stearate: 2 parts by weight Stearic acid: 1 part by weight Organic solvent: methyl ethyl ketone: 20 parts by weight
Toluene: 20 parts by weight
Cyclohexanone: 10 parts by weight

  The above materials were kneaded and further diluted with a solvent, and then dispersed in a sand mill to obtain a dispersion for forming the lower nonmagnetic layer. Thereafter, 3 parts by weight of polyisocyanate (Japanese polyurethane curing agent “Coronate L”) was added to prepare a coating material for forming the lower non-magnetic layer.

As the nonmagnetic support 1, a 5.0 μm-thick polyethylene terephthalate film is prepared. First, the lower nonmagnetic layer-forming coating material prepared as described above is applied to a thickness of 1.0 μm, Subsequently, a coating for forming the magnetic layer was applied.
Then, the magnetic field orientation process was performed, and it dried and wound up. Further, a calendar process was performed and a curing process was performed.

  Next, a backcoat layer-forming dispersion was prepared according to the following composition, and 10 parts by weight of a polyisocyanate (a Japanese polyurethane curing agent “Coronate L”) was added to prepare a backcoat layer coating material. And this coating material was apply | coated to the surface on the opposite side to a magnetic layer formation surface, and the backcoat layer 4 with a film thickness of 0.6 micrometer was formed.

[Backcoat layer dispersion composition]
Inorganic powder (carbon black): 100 parts by weight (particle size 40 nm, DBP oil absorption 112.0 ml / 100 g)
Binder: Polyester polyurethane resin: 13 parts by weight (weight average molecular weight 71200)
Binder: Phenoxy resin (average polymerization degree 100): 43 parts by weight Binder: Nitrocellulose resin (average polymerization degree 90): 10 parts by weight Solvent: Methyl ethyl ketone: 500 parts by weight Toluene: 500 parts by weight

A wide magnetic tape was produced as described above, slit into a 1/2 inch width, and further incorporated into a 1/2 inch one reel cassette for 500 m to obtain a sample magnetic tape.
About the sample magnetic tape produced as mentioned above, the surface observation by the electron microscope (SEM) of the magnetic layer formation surface and the measurement of the electromagnetic conversion characteristic were performed.
An observation method and a measurement method are shown below.

[Surface observation]
The surface of the magnetic layer was observed with an electron microscope (SEM) at a magnification of 50,000 times, the area of pinholes in a specific area (2.1 μm × 1.7 μm = 3.57 μm ² ) was calculated, and the ratio was expressed in%.

[Electromagnetic conversion characteristics]
Using a fixed electric machine equipped with a recording magnetic head (MIG, gap length 0.15 μm), a signal with a wavelength of 0.3 μm is recorded, and then a reproducing magnetic head (MR head, gap length 0.2 μm) Was used to reproduce the signal.
The C / N characteristics when the noise level was set at ± 2 MHz from the output of each single frequency and the reproduced signal were measured.

  In Table 2 below, the major axis length of the inorganic particles contained in the lower nonmagnetic layer 2 of the sample magnetic tapes of Examples 1 to 8 and Comparative Examples 1 to 4, the film thickness of the magnetic layer 3, and the above (1) The numerical value on the right side of the equation, the pinhole ratio, and the electromagnetic conversion characteristics are shown.

In Table 2, the thickness of the magnetic layer 3 is set to 100 nm, and the reference tape having the structure in which the inorganic particles A are contained in the lower nonmagnetic layer 2 is used as a reference. The results of evaluating the samples are shown.
When the film thickness of the magnetic layer 3 is d (nm) and the major axis length of the inorganic particles contained in the lower nonmagnetic layer 2 is a (nm), a relationship of a ≦ 3.0 × d−60 is established. In each of Examples 1 to 8, the pinhole area was less than 20%, and electromagnetic conversion characteristics superior to the reference tape were obtained.
On the other hand, in Comparative Examples 1 to 4 in which the relationship of the above formula is not established, the pinhole area exceeded 20%, and the electromagnetic conversion characteristics deteriorated as compared with the reference tape.

  As described above, according to the present invention, the magnetic layer thickness d (nm) and the long axis length a (nm) of the inorganic particles contained in the lower nonmagnetic layer satisfy a predetermined relationship. As a result of the identification, even in an extremely thin magnetic recording medium having a magnetic layer thickness of 100 nm or less, the pinhole phenomenon was suppressed and excellent electromagnetic conversion characteristics were obtained.

1 is a schematic sectional view of a magnetic recording medium of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Nonmagnetic support body 2 ... Underlayer nonmagnetic layer, 3 ... Magnetic layer, 4 ... Backcoat layer, 10 ... Magnetic recording medium

Claims (1)

Containing at least one main surface of a nonmagnetic support composed of polyethylene terephthalate, a lower nonmagnetic layer containing at least inorganic particles composed of iron oxide and / or titanium oxide and a binder, and at least a magnetic powder and a binder to a film thickness 50 nm or less of the magnetic layer is is overlaid formed by wet-on-wet method,
The thickness of the magnetic layer as d (nm), when the long axis of the inorganic particles contained in the lower non-magnetic layer was changed to a (nm), magnetic that have a relationship of the following formula (1) Care recording medium.
a ≦ 3.0 × d−60 (1)

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