JP2000163738A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the durability of a double-layer coating type magnetic recording medium and to inhibit the progress of rusting. SOLUTION: Each face of a nonmagnetic substrate 1 is doubly coated with a nonmagnetic layer 2 as a lower layer and a magnetic layer 3 as an upper layer and a hard carbon protective coat 4 is formed on the magnetic layer 3. The thickness of the protective coat 4 is 1-20 nm. Since the strength of the protective coat 4 is very high, durability is significantly improved. Since the protective coat 4 has also a corrosion preventing effect, the deterioration of characteristics, e.g. due to the corrosion of a ferromagnetic powder is inhibited.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-layer coating type magnetic recording medium, and more particularly to improvement in running durability and rust characteristics of a magnetic recording medium for data storage.

[0002]

2. Description of the Related Art Conventionally, as a magnetic recording medium such as a magnetic tape or a magnetic disk, a magnetic coating prepared by dispersing a ferromagnetic powder in a binder is applied to a non-magnetic support to form a magnetic layer. The formed, so-called coating type magnetic recording medium is widely used.

In recent years, in the field of magnetic recording, higher recording densities and shorter wavelengths have been progressing, and the above-mentioned coating type magnetic recording media have been required to have characteristics that can cope with them. .

[0004] As a method of improving the electromagnetic conversion characteristics in a high-density recording area in a coating type magnetic recording medium, first, a thin magnetic layer can be mentioned. Thereby, the self-demagnetization loss at the time of recording and the thickness loss at the time of reproduction are reduced, and the electromagnetic conversion characteristics are effectively improved.

[0005] However, as a problem accompanying the thinning of the magnetic layer, there is a problem of spacing loss due to surface irregularities. This is a problem caused by the fact that when the magnetic layer is thinned, the surface state depends on the surface properties of the non-magnetic support. If the surface of the non-magnetic support is rough, For example, the surface of the magnetic layer becomes rough, which results in deterioration of electromagnetic conversion characteristics and induces dropout.

[0006] Further, there is an unavoidable problem that the durability is reduced due to the thinning. In the case of a data cartridge that employs a helical scan method that requires an improvement in data transfer rate with rapid increase in recording density, the surface of the magnetic layer and the magnetic head slide at high speed. Similarly, in the case of a magnetic disk such as a large-capacity floppy (registered trademark) disk, which has been attracting attention in recent years, the disk is exposed to severe conditions against the sliding of the head because high-speed rotation is inevitable.

As described above, as the magnetic layer becomes thinner, it is necessary to control the surface properties and to ensure durability against a high-speed sliding system.

In view of the above, for a coating type magnetic recording medium, a multilayer coating type structure in which a relatively thick lower non-magnetic layer is interposed between a magnetic layer and a non-magnetic support has been proposed. In this multilayer coating type configuration, the surface shape of the magnetic layer does not follow the surface shape of the nonmagnetic support due to the interposition of the lower nonmagnetic layer, and the surface of the magnetic layer can be freely designed. In addition, by giving various functions to the lower non-magnetic layer, it is possible to improve durability against a high-speed sliding system.

[0009]

However, in order to improve the quality of the above-mentioned multilayer coating type magnetic recording medium, the reliability under a wide range of environment such as high temperature, high humidity and low temperature is secured. Must. Under such a severe environment, durability and storage characteristics are important issues.

[0010] Here, as a method for addressing the problem of durability, a. Method of improving physical properties of coating film by changing type and amount of binder b. Method of controlling the surface properties by adjusting the size and amount of additive particles such as abrasives c. Increasing the amount of lubricant added and a method of forming a lubricant layer on a coating film can be mentioned.

However, the above-described method has the following disadvantages.

For example, in the method described in a above,
When the paint characteristics are significantly changed, the coating properties may be deteriorated.

In the method described in (b), the durability tends to be improved by increasing the amount of the additive.
Electromagnetic conversion characteristics deteriorate. Since the surface property is closely related to the electromagnetic conversion characteristics, the amount of the additive is restricted to some extent.

In the method described in (c) above, if the amount of the lubricant present on the surface is large, the durability tends to be improved. However, the spacing decreases due to the thickness of the lubricant layer, the output decreases due to clogging of the head, and the medium production. Various adverse effects such as interlayer adhesion of the raw roll at the time are involved.

Therefore, in the above-mentioned multilayer coating type magnetic recording medium, it is necessary to improve the strength of the coating film itself.

As for the preservation characteristics, a method of providing an additive having a rust-preventive effect on the surface of the magnetic layer or providing a synergistic effect of lubricity and rust-preventive property by providing a lubricant layer is provided. But it is hardly enough.

Accordingly, the present invention has been proposed in view of such conventional circumstances, and a multilayer-coated magnetic recording medium in which a magnetic layer and a non-magnetic layer are multilayer-coated on a non-magnetic support. An object of the present invention is to significantly improve the strength of a coating film and improve its durability, and also to suppress the progress of rust even in a high temperature and high humidity environment as a synergistic effect.

[0018]

Means for Solving the Problems In order to achieve the above object, as a result of intensive studies, a hard carbon film (a so-called diamond carbon film: DLC film) has been formed on a multilayer coating type magnetic recording medium. It was found that the strength was significantly improved and the progress of corrosion was suppressed.

The present invention has been completed on the basis of this finding, and comprises a lower magnetic layer and a lower nonmagnetic layer coated on a nonmagnetic support, and a hard carbon protective film is formed on the upper magnetic layer. Are formed.

Since the strength of the hard carbon protective film is very high, by providing it on the upper magnetic layer, the durability is greatly improved.

The hard carbon protective film also has a rust-preventing effect, and suppresses deterioration of characteristics due to, for example, corrosion of the ferromagnetic powder.

[0022]

Embodiments of the present invention will be described below.

The multilayer coating type magnetic recording medium can be obtained by applying a magnetic coating material prepared by dispersing a magnetic powder and a binder together with a solvent on a non-magnetic support in a layered manner. Alternatively, after forming a lower layer coating by applying a lower layer nonmagnetic paint prepared by dispersing a nonmagnetic powder and a binder together with a solvent, for the upper layer prepared in the same manner as described above on the lower layer coating. It is obtained by forming a top coat by applying a magnetic paint. In the present invention, the latter form is adopted.

Specifically, in the case of a magnetic disk, as shown in FIG. 1, for example, a lower non-magnetic layer 2, an upper magnetic layer 3, and a hard carbon (DLC) protective film 4 are formed on both sides of a non-magnetic support 1.
Are sequentially formed.

Although the thickness of each layer is arbitrary, for example, the dry thickness of the upper magnetic layer 3 is preferably 3 μm or less,
The dry thickness of the lower non-magnetic layer 2 is preferably 10 μm or less.

Specific examples of the material of the upper magnetic layer in the present invention include the following.

First, the upper magnetic layer is made of a ferromagnetic powder, a binder and the like. Any known ferromagnetic powder can be used, and an oxide magnetic powder may be used. Magnetic powder may be used.

As the oxide magnetic powder, for example, γ-F
e ₂ O ₃ , Co-containing γ-Fe ₂ O ₃ , Fe ₃ O ₄ , Co-containing γ
—Fe ₃ O ₄ , Co-coated —Fe ₃ O ₄ , CrO _{2 and the} like. Examples of the metal magnetic powder include Fe, Co, N
i, Fe-Co, Fe-Ni, Fe-Co-Ni, Co
-Ni, Fe-Co-B, Fe-Co-Cr-B, Mn
-Bi, Mn-Al, Fe-Co-V, etc. Further, for the purpose of improving these various characteristics, Al, S
Metals such as i, Ti, Cr, Mn, Cu, and Zn may be added. Of these, Fe-based magnetic powders have excellent electrical characteristics. In terms of corrosion resistance and dispersibility, Fe-Al, Fe-Al-Ca, Fe-A
l-Zn system, Fe-Al-Co system, Fe-Ni-Si-
F such as Al-Zn system, Fe-Ni-Si-Al-Mn system
An e-Al alloy powder is preferred. The shape of these ferromagnetic powders has an average major axis length of 0.5 μm or less, preferably 0.01 to 0.4 μm, more preferably 0.01 to 0.4 μm.
It is 0.3 μm and the axial ratio (average major axis length / average minor axis length) is 12 or less, preferably 10 or less.

In any case, the saturation magnetization (σs) is 7
It is preferably at least 0 emu / g. If the saturation magnetization is less than 70 emu / g, sufficient electromagnetic conversion characteristics may not be obtained.

Further, from the viewpoint of enabling recording and reproduction in a high-density recording area, the specific surface area by the BET method is 45 m ² / g.
It is preferable that it is above. Also, hexagonal ferrites such as barium ferrite, iron nitride, and the like can be used.

The barium ferrite is a barium ferrite in which a part of Fe is substituted by at least Co and Zn, and has an average grain size (diagonal length of the plate surface of hexagonal ferrite) of 300 to 900 angstroms.
The plate ratio (value obtained by dividing the length of the diagonal line of the plate surface of hexagonal ferrite by the plate thickness) is 2.0 to 10.0, and the coercive force is 450 to
1500 Oe is preferred.

Typical examples of the binder include a polyurethane resin, a polyester resin, and a vinyl chloride resin such as a vinyl chloride copolymer.

These resins include -SO ₃ M, -OSO ₃ M,
—COOM, —PO (OM ′) ₂ (where M represents a hydrogen atom or an alkali metal, M ′ represents a hydrogen atom or an alkali atom or an alkali group) and at least one polar group selected from sulfobetaine groups. It is preferable to contain a repeating unit having the same. These polar groups have the effect of improving the dispersibility of the ferromagnetic powder, and have a content of 0.1.
1-8. Preferably, it is O mol%, more preferably 0.2 to 6.0 mol%. When the content of the polar group is less than 0.1 mol%, the dispersibility of the magnetic powder is reduced. Conversely, the content is 8.
If it exceeds 0 mol%, the magnetic paint tends to gel. The weight average molecular weight of the resin is 15,000 to 50.
It is preferably in the range of 000.

The vinyl chloride copolymer containing a polar group is obtained by an addition reaction between a copolymer having a hydroxyl group such as a vinyl chloride-vinyl alcohol copolymer and a compound having a polar group and a chlorine atom. Can be synthesized.

The polyester is synthesized by a reaction between a polyol and a polybasic acid. It should be noted that a polyester into which another polar group has been introduced can also be synthesized by a known method.

Polyurethane is synthesized by a reaction between a polyol and a polyisocyanate. As this polyol, a polyester polyol obtained by reacting a polyol with a polybasic acid is generally used. When a polyester polyol having a polar group is used as a raw material, a polyurethane having a polar group can be synthesized.

These resins may be used alone or in combination of two or more. For example,
When a mixture of polyurethane and / or polyester and a vinyl chloride resin is used, the weight ratio is 90: 1.
The range is from 0 to 10:90, preferably from 70:30 to 30:70.

Further, the following resins may be used together in an amount of 50% by weight or less of the total binder.

The resins used in combination are vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, butadiene-acrylonitrile copolymer having a weight average molecular weight of 10,000 to 200,000. , Polyamide resin, polyvinyl butyral, cellulose derivative, styrene-butadiene copolymer, phenol resin, epoxy resin, urea resin,
Examples include melamine resin, phenoxy resin, silicone resin, acrylic resin, urea formamide resin, various synthetic rubber resins, and the like.

As the binder, the above resins are used, and the mixing amount of these binders in the upper magnetic layer is preferably 8 to 25 parts by weight with respect to 100 parts by weight of the ferromagnetic powder. It is preferably from 10 to 20 parts by weight.

For the purpose of controlling the surface properties in order to improve the running durability and the like of the medium, the upper magnetic layer usually contains abrasives, dispersants, antistatic agents and the like used in this type of magnetic recording medium. May be added. However, the addition of a lubricant should be avoided, and the hard carbon protective film (D
A lubricant layer is formed on the LC protective film. This is because, when a lubricant is added, the adhesiveness to the DLC film is reduced due to the effect of the lubricant oozing on the surface of the magnetic layer.

The abrasive has an average particle size of 0.05 to
The thickness is 0.6 μm, preferably 0.05 to 0.3 μm.

The addition amount is suitably 3 to 20 parts by weight, preferably 5 to 10 parts by weight, per 100 parts by weight of the ferromagnetic powder.

As the lubricant, fatty acids and fatty acid esters are used alone or in combination. The fatty acid may be a monobasic acid or a dibasic acid, and has a carbon number of 6 to
30 is preferable, and it is more preferable that it is 12-22.

The addition amount of these fatty acids and fatty acid esters is preferably from 0.2 to 10% by weight, more preferably from 0.5 to 5% by weight, based on the magnetic powder.

When the amount of the fatty acid is less than 0.2% by weight, the running property of the medium is not sufficiently improved.
If it exceeds 0% by weight, the fatty acid easily oozes out on the surface of the magnetic layer and the output is liable to decrease.

On the other hand, if the amount of the fatty acid ester is less than 0.2% by weight, the still durability is particularly insufficient. On the other hand, when the content exceeds 100% by weight, the fatty acid ester easily oozes out on the surface of the magnetic layer and the output is liable to decrease. In addition,
The ratio of fatty acid to fatty acid ester is 10:90 by weight.
~ 90: 10 is preferred.

A known lubricant may be used in combination with the above fatty acids and fatty acid esters. Examples of the lubricant used in combination include silicone oil, carbon fluoride, fatty acid amide, olefin oxide and the like.

As the curing agent, polyisocyanate or the like is used. Examples of the polyisocyanate include an aromatic polyisocyanate such as an adduct of tolylene diisocyanate (TDl) and an active hydrogen compound, and an aliphatic polyisocyanate such as an adduct of hexamethylene diisocyanate (HMDl) and an active hydrogen compound. Can be The weight average molecular weight of these polyisocyanates is 100 to 30.
Desirably, it is in the range of 00.

As the dispersant, compounds described in JP-A-4-214218 can be used. These dispersants are suitably used in the range of 0.5 to 5% by weight based on the magnetic powder.

As the antistatic agent, JP-A-4-2142
Surfactants as described in JP-A-18 can be used. The addition amount of these antistatic agents is preferably in the range of 0.01 to 40% by weight based on the binder.

Any of conventionally known materials can be used as these additives, and there is no particular limitation.

On the other hand, the lower non-magnetic layer is composed of a non-magnetic powder, a binder and the like.

First, as the non-magnetic powder, for example, carbon black, graphite, TiO ₂ , barium sulfate, ZnS, MgCO ₃ , CaCO ₃ , ZnO, CaO,
Tungsten disulfide, molybdenum disulfide, MgO, Sn
O ₂ , SiO ₂ , Cr ₂ O ₃ , α-Al ₂ O ₃ , α-Fe
Examples include ₂ O ₃ , SiC, cerium oxide, silicon nitride, silicon carbide, molybdenum carbide, and titanium carbide. Of these, inorganic powders such as carbon black, TiO ₂ , barium sulfate, α-Al ₂ O ₃ , α-Fe ₂ O ₃ and Cr ₂ O ₃ are particularly preferred.

These nonmagnetic powders include Si compounds and Al
It may be surface-treated with a compound. In the surface treatment, the content of Si and Al is 0.1 to
It is preferable to carry out so as to be 10% by weight.

The shape of the non-magnetic powder may be acicular or spherical, and more preferably acicular. By using the needle-shaped nonmagnetic powder, the smoothness of the surface of the lower nonmagnetic layer is improved, and as a result, the surface of the upper magnetic layer laminated thereon is also smooth. However, the major axis diameter, minor axis diameter and axial ratio (major axis diameter / minor axis diameter) of the nonmagnetic powder are preferably in the following ranges.

That is, the major axis diameter of the nonmagnetic powder is 0.5 μm
Or less, preferably 0.30 μm or less.

The minor axis diameter is preferably 0.1 μm or less, more preferably 0.06 μm or less. Shaft ratio (Long shaft diameter /
The short axis diameter is suitably 2 to 20, preferably 5 to 10. The specific surface area is 10 to 250 m ² / g, preferably 3
It is good to be 0-100 m < ² > / g. When the nonmagnetic powder having the major axis diameter, the minor axis diameter, the axial ratio, and the specific surface area as described above is used, the surface of the upper magnetic layer laminated thereon is also in a good state.

The mixing amount of the nonmagnetic powder into the lower nonmagnetic layer is as follows:
The total amount of all components constituting the nonmagnetic layer is 50
It is appropriate that the content be from 99 to 99% by weight, preferably from 70 to 95% by weight. By setting the mixing amount of the nonmagnetic powder in this range, the surface properties of the lower nonmagnetic layer and the upper magnetic layer are improved.

As the binder, any of the resins exemplified for the upper magnetic layer can be used. The mixing amount of the binder is 5 to 150 parts by weight, preferably 10 to 120 parts by weight, based on 100 parts by weight of the nonmagnetic powder.

Further, in order to improve the surface properties, additives such as an abrasive, a dispersant, and an antistatic agent may be added to the lower nonmagnetic layer in the same manner as the upper magnetic layer. However, for the same reason as described above, the addition of a lubricant to the paint should be avoided.

Using the above materials, a lower non-magnetic layer,
To form the upper magnetic layer, first, a lower non-magnetic paint and an upper magnetic paint are prepared.

The paint for the upper layer is the ferromagnetic powder exemplified above,
After preparing a high concentration magnetic paint by kneading various additives such as a binder and a dispersant, an abrasive, an antistatic agent with a solvent,
It is prepared by diluting and dispersing this high-concentration magnetic paint.

The non-magnetic paint for the lower layer is prepared by kneading various additives such as the binder and dispersant, lubricant, abrasive and antistatic agent exemplified above together with a solvent to prepare a high-concentration magnetic paint. It is prepared by diluting and dispersing this high-concentration magnetic paint.

Examples of the organic solvent used in the coating include those commonly used in magnetic recording media of this type, for example, ketones such as acetone, methyl ethyl ketone, diisobutyl ketone and cyclohexanone, and benzene, toluene, xylene and chlorobenzene. Any of aromatic hydrocarbons, hexane and the like can be used. These organic solvents may be used alone or in combination of two or more.

As the kneading and dispersing machine, any of those described in, for example, JP-A-4-214218 can be used. In particular, 0.05 to 0.5 kW (magnetic powder 1
A pressure kneader, an open kneader, a continuous kneader, a two-roll mill, and a three-roll mill are suitable because they can provide a power consumption load of (per kg).

The upper magnetic layer and the lower non-magnetic layer are formed by applying the upper magnetic paint and the lower non-magnetic paint thus prepared to a non-magnetic support and drying.

Here, as the non-magnetic support, for example,
Polyethylene terephthalate, polyethylene-2,6-
Examples include polyesters such as naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and plastics such as polyamide, aramid resin and polycarbonate. These nonmagnetic supports may have a single-layer structure or a multilayer structure. Further, for example, a surface treatment such as a corona discharge treatment may be performed.

The thickness of the nonmagnetic support is preferably about 30 μm to 10 mm in the case of a disk or a card.

As a method of applying the magnetic paint for the upper layer and the non-magnetic paint for the lower layer to the non-magnetic support, a gravure coating method,
A blade coating method, a paint extruding method, etc. may be mentioned, and any conventionally known coating method may be used. Further, any of the following multilayer coating methods may be used as the multilayer coating method.

After coating and drying the non-magnetic paint on the non-magnetic support, a calendering treatment is applied as necessary, and the magnetic paint is applied on the dried lower non-magnetic layer. On-wet coating method-A so-called wet-on-wet method in which an upper coating film is formed on a lower coating film while the lower coating film is in a wet state as described in JP-A-63-191315. Coating method As a method of forming a multilayer coating film, for example, FIGS. 2 and 3
And a coating film forming system as shown in FIG.

That is, in this coating film forming system, the non-magnetic support 13 on which the coating film is formed is conveyed from the supply roll 11 to the take-up roll 12, and along the conveyance direction. The coating device 14, the magnet 15 for orientation, the dryer 16, and the calendar device 17 are arranged in this order.

In this coating film forming system, the coating material 14 first applies the upper magnetic coating material 20 and the lower nonmagnetic coating material 21 on the nonmagnetic support in a multilayer manner. An extruder coater 18 for the lower layer paint for applying the lower layer paint and an extruder coater 19 for the upper layer paint for applying the upper layer paint are arranged in the application device.

Each of the extrusion coaters has a slit 22 formed at the tip thereof for extruding the paint, and a paint reservoir 23 for supplying the paint is provided on the back side. The paint supplied to the paint pool 23 is supplied to the slit 22
Through the coater tip.

On the other hand, the support 13 to which the coating material is applied is transported along the tip of the extrusion coater from the lower layer extrusion coater 18 to the upper layer extrusion coater 19 in the direction of the arrow in the figure.

The non-magnetic support conveyed in this manner is coated with a lower layer coating material extruded from the slit portion of the lower layer extrusion coater 18 to form a lower layer coating film.
The upper layer coating material extruded from the slit portion of the upper layer extrusion coater 19 is applied onto the wet lower layer coating film, and two layers of coating films are sequentially formed.

The lower coating film and the upper coating film formed as described above are sequentially conveyed to the magnet 15 for orientation, the dryer 16 and the calender 17.

In this coating system, the lower layer paint and the upper layer paint are applied by separate coaters. However, as shown in FIG. 4, a lower layer extrusion coater and an upper layer extrusion coater are integrated. Extrusion coater 24
May be used. Further, in addition to the extrusion coater, a gravure roll, a blade coater, an air doctor coater, an impregnated coater, or the like may be used.

Further, in the above configuration, the lower layer non-magnetic paint and the upper layer magnetic paint are sequentially applied. However, an extrusion coater having two slits formed in close proximity is used. The paint and the upper layer paint may be applied simultaneously.

That is, as shown in FIG. 5, the extrusion coater 25 used in the simultaneous multi-layer coating method has two slit portions 22 (a lower layer slit portion and an upper layer slit portion) at the tip end of which are extruded in close proximity. A lower layer paint reservoir 26 and an upper layer paint reservoir 27 to which each paint is supplied are provided on the back surface of the two slit portions. In this extrusion coater, the lower layer paint and the upper layer paint supplied to the paint pool are extruded to the tip of the coater through a slit. On the other hand, the support 13 to which the coating material is applied is transported along the tip of the extrusion coater from the lower layer slit to the upper layer slit in the direction of the arrow in the figure.

The non-magnetic support conveyed in this way is coated with the lower layer paint extruded from the lower layer slit portion when passing through the lower layer slit portion and the upper layer slit portion. The upper layer paint extruded from the upper layer slit is applied on the paint, and two layers of coating films are simultaneously formed.

In the magnetic recording medium such as the magnetic tape according to the present invention, the surface on which the magnetic layer and the non-magnetic layer are formed on the non-magnetic support is used in order to improve the running property and reduce the electric resistance. A back coat layer may be provided on the surface opposite to the above. As the material used for the back coat layer, conventionally known materials can be used, and any non-magnetic layer in which a non-magnetic powder having an antistatic effect or a friction reducing effect such as carbon black is dispersed in a binder can be used. Good.

Then, a calender treatment is performed to further flatten the coating surface properties. A heat-resistant resin roll such as polyamide or epoxy is used as the calender roll. The resin roll and the steel roll are alternately installed, and the application roll is sandwiched between the rolls, and the roll is run while applying pressure and temperature. At this time, instead of using the resin roll, the processing may be performed by replacing the resin roll with a steel roll.

As for the calendering conditions, the temperature was 5
The pressure is preferably 0 ° C. or more, the pressure is 150 kg / cm or more in linear pressure, and the speed is 20 m / min or more.

A hard carbon (DLC) protective film is formed on the magnetic layer of the raw roll obtained as described above.

Here, the DLC protective film is, for example, a carbon film whose peak is observed at a position corresponding to diamond in Raman spectroscopy, and is a hard film having excellent physical characteristics. This DLC protective film has already been adopted so far to improve the durability and rust prevention of the metal magnetic thin film type magnetic recording medium, and can be formed by the same method as the forming method.

Examples of the method include a sputtering method, a chemical vapor deposition method (CVD method) using a hydrocarbon-based gas, a vacuum deposition method, an ion plating method, and the like. Note that the sputtering method includes a magnetron sputtering method and a facing target method, and the CVD method includes a plasma CVD method, an ECR plasma CVD method, an arc jet plasma CVD method, and the like.

Although the thickness of the DLC protective film is optional,
The thickness is preferably 1 to 20 nm. If the thickness of the DLC protective film is less than 1 nm, the effect cannot be sufficiently obtained. Conversely, if it exceeds 20 nm, the electromagnetic conversion characteristics will be degraded due to spacing.

A lubricant layer may be provided on the DLC protective film for the purpose of improving running durability. As a lubricant,
Fatty acids and fatty acid esters are used alone or in combination. The fatty acid may be a monobasic acid or a dibasic acid, and preferably has 6 to 30 carbon atoms, and more preferably 12 to 22 carbon atoms.

The amount of these fatty acids and fatty acid esters applied (top coat) is preferably 5 to 100 mg / m ² . If the application amount is less than 5 mg / m ² ,
If the running property and the still durability of the medium are not sufficiently improved, and if it exceeds 100 mg / m ² , the fatty acid or fatty acid ester is likely to ooze on the surface of the magnetic layer or to lower the output.

The ratio between the fatty acid and the fatty acid ester is preferably from 10:90 to 90:10 by weight. In addition, a known lubricant may be used in combination with the above fatty acid and fatty acid ester. Examples of the lubricant used in combination include silicone oil, carbon fluoride, fatty acid amide, olefin oxide and the like.

As a method for forming these lubricant layers, a lubricant solution dissolved in a solvent may be applied or sprayed. Further, it may be impregnated in a lubricant solution.

[0093]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments to which the present invention is applied will be described.

In each of the examples, the respective components of the magnetic paint for the upper layer and the non-magnetic paint for the lower layer were weighed and kneaded and dispersed using a kneader and a sand mill in accordance with the following composition to obtain the magnetic paint for the upper layer. A non-magnetic paint for the lower layer was prepared, and a multilayer coated magnetic disk was prepared and tested.

Example 1 <Composition of magnetic coating for upper layer> P / B ratio = 6 100 parts by weight of ferromagnetic iron powder (specific surface area by BET method: 50 m ² / g, coercive force: 16000 e) Sodium sulfonate group-containing polyurethane Resin 4 parts by weight (Toyobo Co., Ltd., trade name MG0144) Potassium sulfonate group-containing vinyl chloride resin 16 parts by weight (Nippon Zeon Co., Ltd., trade name MR110) Carbon black (Asahi Carbon Co., Ltd., trade name # 50) 2 parts by weight alpha-alumina (trade name HlT50) 2 parts Methyl ethyl ketone 200 parts toluene 150 parts by weight cyclohexanone 200 parts by weight <composition of lower Layer nonmagnetic coating> α-Fe ₂ 0 ₃ 100 parts by weight (specific surface area by the BET method: 52m ² / g ) Sodium sulfonate group-containing polyurethane resin 4 parts by weight (trade name MG01, manufactured by Toyobo Co., Ltd.) 44) Potassium sulfonate group-containing vinyl chloride resin 16 parts by weight (manufactured by Zeon Corporation, trade name MR110) Methyl ethyl ketone 100 parts by weight Toluene 50 parts by weight 100 parts by weight cyclohexanone Further, the dispersion liquid of the upper magnetic paint and the lower nonmagnetic paint 5 parts by weight of a polyisocyanate compound (trade name, Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) was added.

The magnetic paint for the upper layer and the non-magnetic paint for the lower layer thus prepared were coated on a polyethylene terephthalate having a thickness of 60 μm using a coating apparatus shown in FIG. A lower non-magnetic layer having a thickness of 0.5 μm was simultaneously coated. At this time, the upper magnetic coating film was randomly subjected to a magnetic field orientation treatment while the coating film was in an undried state. Thereafter, drying was performed to form a multilayer coating film. Then, a multilayer coating film was similarly formed on the surface opposite to the side on which the multilayer coating film was obtained. Thereafter, a wide original web magnetic sheet was obtained by performing a surface smoothing process using a calendar.

Next, a DLC protective film was formed on the surface of the magnetic sheet obtained as described above. In this example, a DC magnetron sputtering apparatus as shown in FIG. 6 was used.

Here, an outline of the apparatus will be described.

The DC magnetron sputtering device is
Vacuum chamber 31 provided with gas inlet 32 and exhaust port 33
The cathode electrode 34 and the anode electrode 35 are arranged to face each other, and the target 3
6, and a magnet 37 for forming a magnetic field is arranged on the anode electrode 35 side.

Then, the coating roll 30 produced as described above is sent out in the direction of the arrow, passes between the electrodes 34 and 35, and is formed under the following conditions so that the DLC protective film has a thickness of 4 nm. Then, it was wound up on a winding roll 38.

Next, a DLC protective film was formed on the surface opposite to the surface on which the DLC protective film was formed in the same manner.

<Film formation conditions> Apparatus: DC magnetron sputtering apparatus Target: carbon Sputter gas: Ar Vacuum degree at the time of film formation: 8 × 10 ^-1 Pa Film thickness: 4 nm The magnetic sheet thus obtained has a diameter of 3 After punching into a 0.5 inch disk, the sample was left in a 60 ° C. oven for 20 hours (curing treatment) to obtain a sample sheet. The sample sheet was subjected to an electromagnetic conversion characteristic, a coating film strength, and a high-temperature and high-humidity storage test.

<Evaluation Method> Each evaluation method of the above-described sample will be described below.

The electromagnetic conversion characteristics were measured using a drive obtained by modifying a 2 MB FDD (manufactured by Sony Corporation, MPF-73W) to 600 rpm, using a recording wavelength of 1.5 μm and a recording frequency of 500 k.
The output on the outer peripheral side in Hz was measured. Here, the measurement result is an average value of n = 5, and a comparison was made using a sample of Comparative Example 2 described below as a reference medium.

The film strength was measured using a thin film scratch tester (RHE)
SCA, CSR-02) was used.

This measuring apparatus performs an indentation test in which the indenter needle is slightly vibrated horizontally and the indenter needle is pushed into a tilted specimen at a constant speed, and an indentation test is performed to determine the bonding strength between the coating and the base material. This is to evaluate the abrasion resistance characteristics of the film. The film strength was measured according to the following measurement conditions.

Measurement conditions Indenter needle tip diameter 15 μm Spring constant 22.5 g / mm Load rate 12 mN / m Specimen inclination angle 3 degrees Pushing speed 10 μm / s Measurement environment 25 ° C./40% rh The completely destroyed load (complete damage point) was read.

The high-temperature and high-humidity storage test was performed at 60 ° C./relative humidity of 9
After being stored in a 0% constant temperature bath for 10 days, the magnetic properties were measured, and the change in the magnetic properties before and after storage was examined.

Example 2 A sample was prepared and evaluated in the same manner as in Example 1 except that the thickness of the DLC protective film was changed to 8 nm.

Example 3 A sample was prepared and evaluated in the same manner as in Example 1 except that the thickness of the DLC protective film was changed to 16 nm.

Example 4 A sample was prepared and evaluated in the same manner as in Example 1 except that the thickness of the DLC protective film was changed to 30 nm.

Comparative Example 1 In the same manner as in Example 1, an upper magnetic paint and a lower non-magnetic paint were prepared, applied, and calendered to obtain a wide original sheet.

The magnetic sheet thus obtained was punched into a disk having a diameter of 3.5 inches, and then a sample was prepared as described below. The same evaluation as in Example 1 was performed.

Comparative Example 2 A sample was prepared in the same manner as in Comparative Example 1 except that 10 parts by weight of butyl stearate was added to both the upper layer paint and the lower layer paint to prepare each paint. The same evaluation as in Example 1 was performed.

The results are shown in Table 1.

[0116]

[Table 1]

From Table 1, it can be seen that the samples of Examples 1, 2, 3 and 4 having the DLC protective film had a larger film thickness than those of Comparative Example 1 having no DLC protective film and Comparative Example 2 having a lubricant added. It can be seen that the strength is large. Also, DL
As the thickness of the C protective film increases, the film strength increases.

However, the fourth embodiment is inferior in output value. This is considered to be a spacing loss due to the film being too thick (a loss when the distance between the head and the recording layer is large).

FIG. 7 shows the relationship between the thickness of the DLC protective film and the output ratio.
The upper limit of the thickness of the DLC protective film is 20 nm. If the output ratio is less than 95%, short wavelength output cannot be ensured, and it is difficult to deal with high-density floppy disks and the like.

The sample having the DLC protective film is
Deterioration of magnetic characteristics is small. This is probably because the DLC protective film suppressed the contact between the magnetic layer and air, thereby suppressing the oxidation reaction of the magnetic particles.

[0121]

As is apparent from the above description, in the present invention, the hard carbon protective film (DLC protective film) is provided on the upper magnetic layer of the multilayer coating type magnetic recording medium. The film strength is improved, and the durability can be greatly improved.

The hard carbon protective film also exerts a rust-preventing effect, and can suppress the progress of rust even under, for example, a high temperature and high humidity environment.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a main part showing an example of a magnetic disk to which the present invention is applied.

FIG. 2 is a schematic diagram showing an example of a wet-on-wet coating film forming system.

FIG. 3 is a schematic view showing an example of an extrusion coater.

FIG. 4 is a schematic view showing another example of an extrusion coater.

FIG. 5 is a schematic view showing still another example of an extrusion coater.

FIG. 6 is a schematic diagram showing an example of a DC magnetron sputtering device for forming a DLC protective film.

FIG. 7 is a characteristic diagram showing a relationship between a thickness of a DLC protective film and an output ratio.

[Explanation of symbols]

1 Non-magnetic support, 2 Lower non-magnetic layer, 3 Upper magnetic layer, 4 Hard carbon protective film

Claims (2)

[Claims] 1. A magnetic recording medium comprising a non-magnetic support, a lower non-magnetic layer and an upper magnetic layer being coated in a multi-layered form, and a hard carbon protective film formed on the upper magnetic layer. 2. The hard carbon film has a thickness of 1 to 20 n.
2. The magnetic recording medium according to claim 1, wherein m is m.