JPH09180187A - Production of magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a magnetic recording medium suppressing the occurrence of drop-out and excellent in running durability as well as electromagnetic transducing characteristics by making stress free from the max. value under the condition of a specified shear rate when an upper coating film is formed. SOLUTION: A lower coating film is formed on a nonmagnetic substrate by coating with a nonmagnetic coating material prepd. by dispersing nonmagnetic powder and a binder in a solvent. While the lower magnetic film is in a wet state, an upper coating film is formed on the lower film by coating with a magnetic coating material prepd. by dispersing magnetic powder and a binder in a solvent. With respect to the magnetic coating material, stress is made free from the max. value under the condition of a shear rate of 0.025 (1/s) in the relation between strain and stress measured with a strain control type viscometer. The objective magnetic recording medium suppressing the occurrence of dropout and excellent in running durability is produced in a high yield.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a magnetic recording medium, and more particularly to a method for manufacturing a magnetic recording medium by a wet-on-wet coating method.

[0002]

2. Description of the Related Art Conventionally, in video tapes, audio tapes, magnetic disks, etc., ferromagnetic powders such as ferromagnetic iron oxide, Co-modified iron oxide, CrO ₂ and ferromagnetic alloy powder are dispersed in a binder. A so-called coating type magnetic recording medium in which a magnetic layer is formed by coating the prepared magnetic coating material on a non-magnetic support is widely used.

In recent years, in the field of magnetic recording, recording density and wavelength shortening are progressing, and even in the above-mentioned coating type magnetic recording medium, such recording density and wavelength shortening can be achieved. There is a growing demand to have corresponding properties.

Here, in the coating type magnetic recording medium,
As a method for improving the electromagnetic conversion characteristics in the high density recording area, thinning of the magnetic layer can be mentioned. When the magnetic layer is made thin, self-demagnetization loss at the time of recording and thickness loss at the time of reproducing are reduced, and electromagnetic conversion characteristics are effectively improved.

However, in this case, if the thickness of the magnetic layer is reduced to, for example, 2 μm or less, the surface shape of the non-magnetic support is likely to be raised on the surface of the magnetic layer, and the surface of the magnetic layer becomes rough. If so, the electromagnetic conversion characteristics are deteriorated due to spacing loss, and dropouts frequently occur.

Therefore, in the coating type magnetic recording medium, a lower non-magnetic layer having a relatively large thickness is interposed between the magnetic layer and the non-magnetic support, whereby the surface shape of the non-magnetic support becomes the surface of the magnetic layer. A multi-layer coating type structure has been proposed which is difficult to appear in. In this multi-layer coating type magnetic recording medium, since the thin magnetic layer can be formed on the smooth surface, excellent electromagnetic conversion characteristics can be obtained in the short wavelength region.

By the way, such a multilayer coating type magnetic recording medium has heretofore been coated with a non-magnetic coating material on a non-magnetic support and dried, followed by calendering if necessary, and then the drying. It is manufactured by a so-called dry-on-wet coating method in which a magnetic coating material is applied onto the lower non-magnetic layer.

However, in this coating method, since the lower non-magnetic layer and the upper magnetic layer are formed on separate lines, the manufacturing process is complicated, and the thickness of the upper magnetic layer is thin due to the following reasons. There is a limit to stratification.

That is, in order to make the upper magnetic layer thin, a method of reducing the coating amount of the upper coating material itself or adding a large amount of a solvent to the coating material to dilute the concentration is adopted.

However, when the coating amount of the coating material is reduced, the coating film starts to be dried before the leveling is sufficiently performed, so that coating defects such as streaks and marking patterns (in the case of the grab a roll coating method) remain. , The yield is very bad.

On the other hand, when the concentration of the coating is diluted,
Many voids are formed in the coating film, the filling degree of the ferromagnetic powder becomes low, and the coating film strength becomes insufficient.

Therefore, as described in Japanese Patent Laid-Open No. 63-191315, the wet coating is applied to the lower coating film while the lower coating film is in a wet state. A method (wet-on-wet coating method) has been proposed.

In this wet-on-wet coating method, a lower layer coating material is applied to form a lower layer coating film, and then an upper layer coating material is applied onto the lower layer coating film. Wet coating method in which the coating material for the lower layer is sequentially coated, and an extrusion coater in which two slits for pushing the coating material for the lower layer and the coating material for the upper layer are formed in close proximity are used. A simultaneous multi-layer coating system is known in which paints are simultaneously coated.

In such a wet-on-wet coating method, even if the coating amount of the upper layer coating material is reduced, it is dried at the same time as the lower layer coating material, so that the coating film should be dried before being sufficiently leveled. There is no such thing as
The upper magnetic layer is formed with good surface properties. Also,
The adhesion between the lower and upper layers is good, and good running durability is obtained.

[0015]

However, with respect to the wet-on-wet coating method, various studies have been made when both the lower layer coating material and the upper layer coating material are magnetic coating materials. And there is a lack of knowledge about applying magnetic paint. When applying the non-magnetic paint and the magnetic paint under the condition that both the lower layer paint and the upper layer paint are magnetic paint, the lower layer upper layer interface is disturbed and pinholes are generated in the upper layer. There may be a case where cissing occurs in the magnetic layer. Further, surface roughness occurs in the upper magnetic layer depending on the physical properties of the coating material, and the surface roughness causes spacing loss and R
This may cause defective F envelope, deterioration of running property and durability, increase of dropout, and the like. Such surface roughness of the upper magnetic layer shows no sign when each coating material is applied as a single layer, and is a phenomenon that appears only when multi-layer coating is performed.

Therefore, the present invention has been proposed in view of such conventional circumstances, and the lower layer upper layer interface can be formed smoothly and the upper magnetic layer can be formed with good surface property,
An object of the present invention is to provide a method of manufacturing a magnetic recording medium that has excellent electromagnetic conversion characteristics, a good shape of an RF envelope, a dropout, and a magnetic recording medium that is excellent in running durability at a high yield. And

[0017]

Means for Solving the Problems As a result of intensive studies made by the present inventors in order to achieve the above-mentioned object, the disturbance of the interface between the lower layer and the upper layer, which occurs when the non-magnetic paint and the magnetic paint are applied in multiple layers, It has been found that the surface roughness of the upper magnetic layer is basically related to the non-Newtonian anomalous flow of the paint.

The method for producing a magnetic recording medium of the present invention has been completed based on such findings, and is prepared by dispersing a nonmagnetic powder and a binder together with a solvent on a nonmagnetic support. It is prepared by dispersing the magnetic powder and the binder together with a solvent onto the lower layer coating film while the lower layer coating film is in a wet state after applying the non-magnetic coating composition described above to form the lower layer coating film. When forming the upper coating film by applying the magnetic coating material, in the relationship between the strain and the stress measured by the strain control type viscometer of the magnetic coating material,
It is characterized in that the maximum value of stress does not appear under the condition of shear rate 0.025 (1 / s).

In the relation between strain and stress measured by a strain control type viscometer of the above magnetic paint, the shear rate is 0.0
A maximum value appears in the stress under the condition of 25 (1 / s). When the maximum value of this stress is σmax and the strain when the stress reaches the maximum value σmax is γmax, the following conditions are satisfied. To do.

Σmax <100 (Pa) γmax <0.1 By using the magnetic coating material satisfying the above conditions as the coating material for the upper layer, abnormal flow is suppressed, and the lower layer upper layer interface and the upper magnetic layer surface are kept in good condition. Will be formed.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described.

To prepare a multi-layer coating type magnetic recording medium having a lower non-magnetic layer and an upper magnetic layer by the wet-on-wet coating method, a non-magnetic powder and a binder together with a solvent are provided on a non-magnetic support. After applying the non-magnetic coating material prepared by dispersing to form the lower layer coating film, while the lower layer coating film is in a wet state, the magnetic powder and the binder are dispersed together with the solvent on the lower layer coating film. By applying the magnetic paint prepared by the above, the upper coating film is formed.

In such a wet-on-wet coating method, the interface between the lower layer and the upper layer is disturbed depending on the composition of the coating material.
Further, surface roughness occurs in the upper magnetic layer, which causes deterioration of the battery conversion characteristics of the medium, poor RF envelope, deterioration of running property and durability, increase of dropout, and the like. The inventors of the present invention examined such disturbances in the lower-layer upper-layer interface and surface roughness of the upper magnetic layer, and these are mainly caused by the non-Newtonian abnormal flow of the coating. It was speculated that

Therefore, in the present invention, in order to suppress such an abnormal flow of the coating material and make the lower layer upper layer interface and the upper magnetic layer surface in a good state, the upper layer magnetic coating material is measured by a strain control type viscometer. The relationship between the strain γ and the stress σ is regulated.

Before explaining the relationship between the strain γ and the stress σ, the yield value of the paint, which is considered to greatly affect the coating properties of the paint, will be described.

First, an ideal viscous liquid flow curve and a flow curve of a dispersion liquid in which a solid powder used for forming the upper magnetic layer is dispersed in a solvent are also shown in FIG.

An ideal viscous liquid exhibits a fluidity called Newtonian flow. In this Newtonian flow, the stress σ
When looking at the relationship between the shear rate dγ / dt, the flow curve becomes a straight line passing through the origin, as shown in FIG.

On the other hand, the dispersion in which the solid powder is dispersed in the solvent exhibits pseudoplastic non-Newtonian flow. In pseudoplastic non-Newtonian flow, stress σ and shear rate dγ / d
Looking at the relationship of t, as shown in FIG. 1, the flow curve does not pass through the origin, and the flow curve becomes linear in the range where the stress σ is large, but becomes curved in the range where the stress σ is small. In such pseudoplastic flow, the point at which the flow curve crosses the stress axis is the lowest shear force at which the liquid begins to flow, and this shear force is called the yield value. The yield value serves as an index of coating properties of the paint, and it is considered important to control the yield value in order to suppress abnormal flow of the paint.

However, the stress σ and the shear rate dγ / d
In practice, t is measured only at a certain large value. Therefore, when obtaining the intercept of the stress axis as the yield value, the flow curve must be extrapolated to dγ / dt = 0 to obtain the intercept of the stress axis.

Here, when the flow curve is linear, the flow curve can be extrapolated relatively easily and the yield value can be obtained. However, when the flow curve is composed of a straight line portion and a curved line portion as in the case of the dispersion liquid, the flow curve obtained by extrapolation contains individual errors or becomes more or less arbitrary. Therefore, it is difficult to accurately determine the yield value.

On the other hand, Casson plot is also proposed to show the relationship between the stress σ of the dispersion and the shear rate dγ / dt. According to this Casson plot, a linear flow curve is obtained in many dispersion liquids, and the yield value is obtained from this flow curve.

However, in the case of the magnetic coating generally used in the wet-on-wet coating system, the shear rate dγ /
Even if the measurement area of dt is limited to near zero, Casson
No linearity is observed in the plot, and the yield value cannot be accurately obtained even with this Casson plot.

According to "Rheology for Chemists" (written by Shigeharu Onoki), not all paints have a finite yield value. It is considered inappropriate to use the required yield value as an index of coating properties.

Therefore, in the present invention, attention is paid to the relationship between the strain γ and the stress σ measured by a strain control type viscometer under the condition that the shear rate is 0.025 (1 / s), and this is applied to the coating properties of the paint. It will be used as an index.

FIG. 2 shows an example of the relationship between the strain γ and the stress σ when the steady flow measurement is performed on the magnetic coating material under the condition of a minute shear rate (0.025 (1 / s)). The measuring device, measuring mode, and geometry are as follows.

Apparatus: Strain control type viscometer (Rheometrics, trade name RFS-II type) Measurement mode: Steady flow measurement (shear rate 0.025 (1 /
s)) Geometry: Titanium, 50 mm diameter cone plate (0.04 rad) In this magnetic paint, a region where the strain γ is small (a in the figure)
In the region), the stress σ increases in proportion to the strain γ, but when the strain γ exceeds a certain value, the stress σ suddenly decreases as the strain γ increases. When the strain γ further increases, the stress σ takes a constant value. That is, the stress σ and the shear rate dγ / dt are proportional (σ = η
(Dγ / dt)). Thus, in this paint, the stress σ changes as a curve having the maximum value σmax in the relationship between the strain γ and the stress σ.

Here, in the region a, the stress σ is the strain γ
The reason for this is that the dispersion liquid behaves as an elastic body, and σ = Gγ (G: elastic modulus,
(γ: strain, σ: stress). It is presumed that the formation of such an elastic region in a dispersion liquid such as a magnetic paint is closely related to the formation of a three-dimensional network structure due to the interaction between particles in the dispersion liquid. Boundary of elastic region, that is, maximum value of stress σma
It is considered that x has the same meaning as the yield value described above.

Therefore, the maximum value σmax of the stress serves as an index of the coating properties of the paint, and the coating properties of the paint can be controlled by this. That is, the maximum value σmax of this stress and the maximum value σ of this stress are regulated by the present invention.
By using a magnetic paint whose distortion γmax corresponding to max satisfies the following condition as the upper-layer paint, abnormal flow is suppressed, and the lower-layer upper-layer interface and the upper magnetic layer surface are formed in good condition. .

Σmax <100 (Pa) γmax <0.1 However, the maximum value σmax of the stress does not appear in all of the paint, and a clear maximum value of the stress σ may not appear in some paint compositions. is there. Thus distortion γ
Relationship between the stress and the stress σ, good coating properties can be obtained even when a magnetic paint that does not show the maximum value in the stress σ is used, and the lower upper layer interface and the upper magnetic layer surface are formed in a good state. .

The maximum value σmax of the stress in the magnetic paint and the strain γmax with respect to this maximum value σmax are determined by the kind and molecular weight of the binder used in the paint, the content of the solid powder component such as magnetic powder, the kind of solvent and the degree of dispersion. It is adjusted by controlling.

Here, specific materials for the upper magnetic layer include those shown below.

First, the upper magnetic layer is composed of a ferromagnetic powder and a binder. As the ferromagnetic powder, γ-Fe is used.
₂ O ₃ , Co-containing γ-Fe ₂ O ₃ , Co-coated γ-Fe ₂ O ₃ ,
CrO ₂ and ferrites represented by magnetite, that is, Fe ₃ O ₄ , Co-containing Fe ₃ O ₄ , and Co-deposited F
e ₃ O ₄ and the like.

Among these ferrites, those having a plate shape and having an easy axis of magnetization in the direction perpendicular to the plate surface are suitable as the ferromagnetic powder. Ferrite having such an easy axis of magnetization includes hexagonal ferrite. Hexagonal ferrite includes barium ferrite, strontium ferrite, and the like, and a part of iron element is another element (for example, Ti, Co, Zn, In, Mn, G).
e, Nb, etc.). Among hexagonal ferrites, barium ferrite is particularly preferable, and further, barium ferrite in which a part of Fe is substituted with at least Co and Zn and having an average particle size (length of diagonal line of plate surface of hexagonal ferrite) Is 300
900 angstrom, 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
It is preferably 10.0 and the coercive force is 450 to 1500 Oe.

Alternatively, a metallic magnetic powder may be used as the ferromagnetic powder. As the metal magnetic powder, F
e, Co, etc., as well as Fe-Al based, Fe-Al
-Ni-based, Fe-Al-Zn-based, Fe-Al-Co-based,
Fe-Al-Ca system, Fe-Ni system, Fe-Ni-Al
System, Fe-Ni-Co system, Fe-Ni-Si-Al-M
n-based, Fe-Ni-Si-Al-Zn-based, Fe-Al-
Si-based, Fe-Ni-Zn-based, Fe-Ni-Mn-based, F
e-Ni-Si system, Fe-Mn-Zn system, Fe-Co-
Examples of the alloy powder include Ni-P-based and Ni-Co-based alloy powders containing Fe, Ni, Co and the like as main components.

Of these, the Fe-based magnetic powder has excellent electrical characteristics. Further, in terms of corrosion resistance and dispersibility, Fe
-Al system, Fe-Al-Ca system, Fe-Al-Ni system,
Fe-Al-Zn system, Fe-Al-Co system, Fe-Ni
-Si-Al-Zn system, Fe-Ni-Si-Al-Mn
Fe-Al-based alloy powders such as iron-based alloys are preferred.

The shape of these metallic magnetic powders has an average major axis length of 0.5 μm or less, preferably 0.01 to 0.4 μm,
It is more preferably 0.01 to 0.3 μm and have an axial ratio (average major axis length / average minor axis length) of 12 or less, preferably 10 or less. For example, Fe—Al based ferromagnetic metal powder (Fe: Al weight ratio = 100: 5) having an average major axis length of 0.16 μm and a coercive force Hc of 1580 Oe.
A material having a saturation magnetization amount σs of 120 emu / g exhibits extremely excellent characteristics. The average major axis length and average minor axis length can be determined by observation with a transmission electron microscope.

As described above, as the ferromagnetic powder, oxide magnetic powder or metal magnetic powder can be used, but in any case, the saturation magnetization (σs) is preferably 70 emu / g or more. 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 the high density recording area, the specific surface area by the BET method is preferably 45 m ² / g or more.

Typical binders are polyurethane resins, polyester resins, vinyl chloride resins such as vinyl chloride copolymers, and the like.

These resins are --SO ₃ M, --OSO ₃ M,
-COOM, -PO (OM ') ₂ (where M represents a hydrogen atom or an alkali metal such as Na, K, Li, and M'
Preferably represents a hydrogen atom or an alkali atom such as Na, K or Li, an alkyl group) and a repeating unit having at least a polar group selected from sulfobetaine groups. These polar groups have the effect of improving the dispersibility of the ferromagnetic powder, and the content is 0.1 to 8.0.
It is good to be mol%, 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. On the other hand, if the content exceeds 8.0 mol%, the magnetic coating material tends to gel. The weight average molecular weight of the resin is preferably in the range of 15,000 to 50,000.

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 reacting a polyol with a polybasic acid. Incidentally, other polar group-introduced polyesters can also be synthesized by a known method.

Polyurethane is synthesized by the reaction of polyol and polyisocyanate. As this polyol, a polyester polyol obtained by reacting a polyol with a polybasic acid is generally used. In addition,
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 polyurethane and / or polyester and a vinyl chloride resin are mixed and used, the weight ratio is 90:
10 to 10:90, preferably 70:30 to 30:70
It is good to be in the range.

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

That is, as the resin used in combination, a vinyl chloride-vinyl acetate copolymer having a weight average molecular weight of 10,000 to 200,000, a vinyl chloride-vinylidene chloride copolymer, a vinyl chloride-acrylonitrile copolymer,
Butadiene-acrylonitrile copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (nitrocellulose etc.), styrene-butadiene copolymer, phenol resin, epoxy resin, urea resin, melamine resin,
Examples thereof include phenoxy resin, silicone resin, acrylic resin, urea formamide resin, and various synthetic rubber resins.

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

In order to improve the running durability of the medium, the upper magnetic layer is usually an abrasive, a lubricant, a durability improver, a dispersant, an antistatic agent used in this type of magnetic recording medium. You may add additives, such as an agent and electroconductive fine powder.

As the polishing agent, Japanese Patent Laid-Open No. 4-214218 is used.
Solid powders as described in the publication can be used. The average particle size of this abrasive is 0.05 μm to 0.6 μm, preferably 0.05 μm to 0.5 μm, and more preferably 0.05 μm to 0.3 μm. The amount of the abrasive added is 3 to 100 parts by weight of the magnetic powder.
The amount is 20 parts by weight, preferably 5 to 15 parts by weight, more preferably 5 to 10 parts by weight.

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 6 to 6 carbon atoms.
30 is preferable, and 12 to 22 is more preferable.

The amount of these fatty acids and fatty acid esters added is preferably 0.2 to 10% by weight, more preferably 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. When the amount exceeds 10% by weight, the fatty acid exudes on the surface of the magnetic layer,
The output tends to decrease. On the other hand, if the amount of the fatty acid ester added is less than 0.2% by weight, still durability is particularly insufficient. If it exceeds 10% by weight, the fatty acid ester is likely to seep out to the surface of the magnetic layer and the output tends to be reduced. When using a fatty acid and a fatty acid ester together,
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 acid and fatty acid ester. Examples of the lubricant used in combination include silicone oil, carbon fluoride, fatty acid amide, and α-olefin oxide.

Polyisocyanate or the like is used as the curing agent. Examples of polyisocyanates include aromatic polyisocyanates such as adducts of tolylene diisocyanate (TDI) and active hydrogen compounds, and aliphatic polyisocyanates such as adducts of hexamethylene diisocyanate (HMDI) and active hydrogen compounds. is there. The weight average molecular weight of these polyisocyanates is 100 to 3,000.
A range of 0 is desirable.

As a dispersant, Japanese Patent Laid-Open No. 4-214218 is used.
Compounds such as those described in the publication can be used. These dispersants are suitably used in the range of 0.5 to 5% by weight with respect to the magnetic powder.

As an antistatic agent, Japanese Patent Laid-Open No. 4-2142 is known.
Surfactants such as those described in JP 18 can be used. The addition amount of these antistatic agents is preferably in the range of 0.01 to 40% by weight with respect to the binder. In addition,
Conductive fine powder may be added as an antistatic agent. As the conductive fine powder, for example, carbon black, graphite, tin oxide, silver powder, silver oxide, silver nitrate, silver organic compounds, metal particles such as copper powder, zinc oxide, barium sulfate,
Examples include pigments such as metal oxides such as titanium oxide that have been coated with a conductive substance such as a tin oxide coating or an antimony solid solution tin oxide coating. The average particle size of these conductive fine powders is 5 to 700 nm, preferably 5 to 2 nm.
It is good that it is 00 nm. The amount of the conductive fine powder added is appropriately 1 to 20 parts by weight, preferably 2 to 7 parts by weight, based on 100 parts by weight of the magnetic powder.

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

First, as the non-magnetic powder, carbon black, graphite, TiO ₂ , barium sulfate, Zn
S, MgCO ₃ , CaCO ₃ , ZnO, CaO, tungsten disulfide, molybdenum disulfide, boron nitride, MgO, S
nO ₂ , SiO ₂ , Cr ₂ O ₃ , α-Al ₂ O ₃ , α-Fe ₂
O ₃ , α-FeOOH, SiC, cerium oxide, corundum, artificial diamond, α-iron oxide, garnet, garnet, silica, silicon nitride, silicon carbide, molybdenum carbide,
Examples thereof include boron carbide, tungsten carbide, titanium carbide, tribolite, diatomaceous earth and dolomite. this house,
In particular, carbon black, CaCO ₃ , TiO ₂ , barium sulfate, α-Al ₂ O ₃ , α-Fe ₂ O ₃ , α-FeOOH,
Inorganic powders such as Cr ₂ O ₃ are preferred. The non-magnetic powder may be surface-treated with a Si compound and / or an Al compound. Use of the surface-treated non-magnetic powder improves the surface properties of the upper magnetic layer. The surface treatment is preferably performed such that the Si and Al contents are 0.1 to 10% by weight with respect to the nonmagnetic powder.

The shape of the non-magnetic powder is preferably needle-like. By using the needle-shaped nonmagnetic powder, the smoothness of the surface of the nonmagnetic layer is improved, and as a result, the surface of the upper magnetic layer laminated thereon is also smooth. The major axis length, minor axis diameter and axial ratio (major axis diameter / minor axis diameter) of the non-magnetic powder are
The following range is preferable. That is, the major axis diameter of the non-magnetic powder is 0.50 μm or less, preferably 0.40 μm or less, and more preferably 0.30 μm or less. The minor axis diameter is 0.10 μm or less, preferably 0.08 μm or less, and more preferably 0.06 μm or less. The axial ratio (major axis diameter / minor axis diameter) is 2
-20, preferably 5-15, more preferably 5-1
0 is appropriate. Specific surface area is 10 to 250 m ² / g,
It is preferably 20 to 150 m ² / g, more preferably 30 to 100 m ² / g. When the non-magnetic powder having the above major axis diameter, minor axis diameter, axial ratio and specific surface area is used, the surface property of the lower non-magnetic layer becomes good,
The surface property of the upper magnetic layer laminated thereon is also in a good state.

The amount of the nonmagnetic powder mixed in the lower nonmagnetic layer is
50 based on the total amount of all the constituents of the non-magnetic layer.
It is suitable that the amount is ˜99 wt%, preferably 60 to 95 wt%, and more preferably 70 to 95 wt%. By setting the mixing amount of the non-magnetic powder within this range, the surface properties of the lower non-magnetic layer and thus of the upper magnetic layer become good.

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

Further, in the lower non-magnetic layer, various additives exemplified in the upper magnetic layer can be added. The addition amount is not particularly limited as long as the viscoelasticity of the lower layer coating material satisfies a predetermined condition.

Using the above materials, the lower non-magnetic layer,
To form the upper magnetic layer, first, the non-magnetic paint for the lower layer,
A magnetic coating material for the upper layer is prepared.

For the magnetic coating material for the upper layer, various additives such as the ferromagnetic powder, the binder and the dispersant, the lubricant, the polishing agent and the antistatic agent, which have been exemplified above, were kneaded together with the solvent to prepare a high-concentration magnetic coating material. Then, it is prepared by diluting and dispersing this high-concentration magnetic paint. In addition, the non-magnetic coating material for the lower layer is a high-concentration non-magnetic coating material prepared by kneading the above-mentioned various additives such as non-magnetic powder, binder and dispersant, lubricant, abrasive, antistatic agent, etc. with a solvent. After that, it is prepared by diluting and dispersing this high-concentration non-magnetic paint.

As the solvent for forming the paint, a solvent usually used in this kind of magnetic recording medium, for example, JP-A-4-
What is described in 214218 gazette is used. This solvent may be used alone or in combination of two or more kinds.

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

However, regarding the kind and molecular weight of the binder used in the magnetic coating material for the upper layer, the content of the solid powder component, the kind of the solvent, and the degree of dispersion, the maximum value σmax of the stress and the strain γmax with respect to this maximum value σmax are predetermined. Set to meet the conditions.

For the lower non-magnetic layer and the upper magnetic layer, the non-magnetic coating material for the lower layer and the magnetic coating material for the upper layer thus prepared are prepared by
It is formed by coating and drying on a non-magnetic support.

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 non-magnetic supports may have a single-layer structure or a multi-layer structure. Further, for example, surface treatment such as corona discharge treatment may be applied.

The thickness of the non-magnetic support is not particularly limited. For example, when the medium is a film or sheet,
It is suitable to be 2 to 100 μm, preferably 3 to 50 μm. In the case of a disc or card, 30
In the case of a drum shape, it is appropriately selected depending on the design of the recorder or the like.

To form the upper magnetic layer and the lower non-magnetic layer, for example, a coating film forming system as shown in FIG. 3 is used.

That is, in this coating film forming system, the non-magnetic support 11 on which the coating film is formed is conveyed from the supply roll 13 toward the winding roll 12, and along this conveying direction. Coating device 14, orientation magnet 1
5, the dryer 16, and the calendar device 17 are arranged in this order.

In such a coating film forming system, the coating material for the upper layer and the coating material for the lower layer are first multi-layered on the non-magnetic support 11 by the coating device 14.

This coating device 14 is, as shown in FIG.
Lower layer extrusion coater 1 for applying lower layer coating material
8 and the upper layer extrusion coater 19 for applying the upper layer coating so that the lower layer extrusion coater 18 is the introduction side of the non-magnetic support 11 and the upper layer extrusion coater 19 is the delivery side of the non-magnetic support 11. It is located and configured.

The lower layer extruding coater 18 and the upper layer extruding coater 19 are provided with slits 20 and 22 through which the coating material is extruded, and the coating material is supplied to the back side of the slit portions 20 and 22. Paint reservoirs 21 and 23 are provided. In such extrusion coaters 18 and 19, the coating material supplied to the coating material reservoirs 21 and 23 is extruded to the tip end portions of the coater through the slit portions 20 and 22.

On the other hand, the support 11 to which the coating material is applied has the lower layer extrusion coater 1 along the tip surfaces of the lower layer extrusion coater 18 and the upper layer extrusion coater 19.
8 is conveyed in the direction of the arrow Dd in the drawing toward the upper layer extrusion coater 19.

Non-magnetic support 1 conveyed in this way
First, when passing through the lower layer extrusion coater 18, first, the slit portion 20 of the lower layer extrusion coater 18 is provided.
The lower layer coating material extruded from is applied to the surface to form the lower layer coating film 24. And, the upper layer extrusion coater 1
When passing through 9, the upper layer coating material extruded from the slit portion 22 of the upper layer extrusion coater 19 is applied onto the lower layer coating film 24 in a wet state, and two layers of coating films 24 and 25 are sequentially formed. .

Incidentally, these extrusion coaters 18, 19
The paint may be supplied to the via an in-line mixer.

Lower layer coating film 24 formed as described above
The upper coating film 25 is sequentially conveyed to the orienting magnet 15, the dryer 16 and the calender device 17.

In the orientation magnet 15, the upper coating film is subjected to magnetic field orientation treatment. As the orientation magnet 15, a longitudinal orientation magnet or a vertical orientation magnet is appropriately selected according to the type of magnetic powder contained in the upper coating film. The magnetic field of these orientation magnets is preferably about 20 to 10,000 Gauss.

In the dryer 16, the lower layer coating film and the upper layer coating film are dried by hot air from the nozzles arranged above and below the dryer 16. At this time, the drying condition is that the temperature is about 30.
It is preferable to set the temperature to 120 ° C and the drying time to about 0.1 to 10 minutes.

Then, the lower layer coating film and the upper layer coating film that have passed through the dryer 16 are further guided to a calender device 17 and subjected to surface smoothing treatment. Under the conditions of the surface smoothing treatment by the calendar device 17, temperature, linear pressure, conveyance speed, etc. are important. That is, the temperature is 50 to 140 ° C., the linear pressure is 50 to 1000 kg / cm, and the conveying speed is 20 to 10
It is preferably 00 m / min. If these conditions are not satisfied, the surface properties of the upper layer may be impaired.

Although the lower non-magnetic layer and the upper magnetic layer are formed as described above, in this coating film forming process, the stress σ does not have a clear maximum value or has a maximum value σmax as the upper layer coating material. Even if it is, since the predetermined condition is satisfied, the abnormal flow of the paint can be suppressed. Therefore, the interface between the upper layer and the lower layer is formed smoothly, and the upper magnetic layer is formed with good surface property. Further, the adhesion between the lower and upper layers is also good.

In this coating system, the lower layer paint,
The upper layer paint is applied with a separate coater,
As shown in FIG. 5, an extrusion coater 26 in which the lower layer extrusion coater and the upper layer extrusion coater are integrated.
May be used.

In addition to the extrusion coater, a reverse roll, a gravure roll, an air doctor coater, a blade coater, an air knife coater, a squeeze coater,
An impregnation coater, a transfer roll coater, a kiss coater, a cast coater, a spray coater or the like may be used. At this time, the coating methods for the lower layer coating material and the upper layer coating material may be the same or different. Therefore, for example, a reverse roll and an extrusion coater can be combined, or a gravure roll and an extrusion coater can be combined to apply the upper layer coating material and the lower layer coating material.

Further, in the above constitution, the lower layer coating material and the upper layer coating material are sequentially applied, but an extrusion coater in which two slits are formed close to each other is used, and the lower layer coating material and the upper layer coating material are formed by this extrusion coater. You may make it apply | coat simultaneously the coating material.

That is, as shown in FIG. 6, the extrusion coater 27 used in the simultaneous multi-layer coating method has two slit portions (the lower layer slit portion 2) through which the paint is extruded at the tip portion.
8. The upper layer slit portion 30) is formed in close proximity, and the lower layer coating material and the lower layer coating material reservoir 29, to which the lower layer coating material and the upper layer coating material are respectively supplied, are provided on the back sides of the two slit portions 28, 30.
An upper layer paint reservoir 31 is provided. In the extrusion coater 27, the lower layer coating material and the upper layer coating material supplied to the coating material reservoirs 29 and 31 are extruded to the tip end portion of the extrusion coating device through the slits 28 and 30. On the other hand, the support 11 to which the coating material is applied is conveyed in the direction D in the figure from the lower layer slit portion 28 toward the upper layer slit portion 30 along the tip surface of the extrusion coater.

The non-magnetic support 1 thus conveyed.
1 includes a lower layer slit portion 28 and an upper layer slit portion 3
When passing 0, the lower layer coating extruded from the lower layer slit portion 28 is applied, and the upper layer coating extruded from the upper layer slit 30 is applied onto the lower layer coating material to form a two-layer coating film 24. , 25 are formed simultaneously.

The magnetic film on which the lower non-magnetic layer and the upper magnetic layer are thus formed is then subjected to burnishing treatment or blade treatment as required to obtain a desired medium shape. It becomes a magnetic recording medium.

As the medium shape, any tape shape, film shape, sheet shape, card shape, disk shape, drum shape, or any other shape commonly used in magnetic recording media can be employed.

In the magnetic recording medium thus manufactured, the interface between the lower layer and the upper layer is smooth, and the surface property of the upper layer is good, so that the electromagnetic recording characteristics are excellent and the shape of the RF envelope is also good. Dropout is suppressed. Further, since the lower layer and the upper layer have high adhesiveness, peeling of the film is unlikely to occur, high film strength is obtained, and excellent running durability is obtained.

In order to improve the electromagnetic conversion characteristics in the high density recording area, the film thickness of the upper magnetic layer is 0.3 μm or less, preferably 0.1 to 0.3 μm. When the thickness of the upper magnetic layer exceeds 0.3 μm, the electrical characteristics are deteriorated and it becomes insufficient as a medium applied to, for example, a digital recording system.

The above is the manufacturing process of the magnetic recording medium having the basic constitution. Further, a back coat layer is provided on the surface of the non-magnetic support on which the lower upper layer is not provided, or the lower layer and the non-magnetic support are further provided. You may make it provide an undercoat layer between and. These back coat layer and undercoat layer are formed according to a usual method.

[0102]

EXAMPLES Examples of the present invention will be described below based on experimental results.

First, the maximum value σmax of stress and the maximum value σmax of the upper layer magnetic coating material adopted in the present embodiment are shown.
Table 1 shows the strain γmax with respect to.

[0104]

[Table 1]

However, in the table, “-” in the columns of σmax and γmax represents the case where no clear maximum value was found in the stress σ. The binder component, vinyl chloride resin, is a potassium sulfonate group-containing vinyl chloride resin (manufactured by Nippon Zeon Co., Ltd., product name MR-110), and the polyurethane resin is a sodium sulfonate group-containing polyurethane resin (manufactured by Toyobo Co., Ltd.). The product name is UR-8700). The solvent is a mixed solvent in which methyl ethyl ketone, toluene and cyclohexanone are mixed in equal weight.

In the following experiments, a multilayer coating type magnetic recording medium was prepared using the upper layer coating material having such properties and its characteristics were evaluated.

Example 1 First, according to the following composition, the respective components of the magnetic coating material for the upper layer and the non-magnetic coating material for the lower layer were weighed and kneaded and dispersed using a kneader and a sand mill, respectively, to thereby form the magnetic coating material for the upper layer. , A non-magnetic coating material for the lower layer was prepared.

<Magnetic Coating for Upper Layer> 100 parts by weight of ferromagnetic iron powder (coercive force Hc: 1600 Oe, specific surface area by BET method: 55 m ² / g, major axis diameter: 0.25 μm, acicular ratio: 10, saturated) Magnetization amount s: 120 emu / g) Binder: Potassium sulfonate group-containing vinyl chloride resin 20 parts by weight (Nippon Zeon Co., Ltd. trade name MR-110) α-alumina 5 parts by weight Myristic acid 1 part by weight Butyl stearate 1 part by weight Parts Solvent 360 parts by weight (Methyl ethyl ketone: toluene: cyclohexanone (weight ratio) = 1: 1: 1 mixed solvent) <Non-magnetic coating material for lower layer> 100 parts by weight of α-Fe ₂ O ₃ (specific surface area by BET method: 52m ² / g, major axis length: 0.15 [mu] m, acicular ratio: 6) binder: 10 parts by weight vinyl potassium sulfonate group-containing chloride resin (Nippon Zeon Co. Trade name MR-110) Sodium sulfonate group-containing polyurethane resin 10 parts by weight (Toyobo Co., Ltd. trade name UR-8700) Butyl stearate 1 part by weight Mixed solvent 220 parts by weight (methyl ethyl ketone: toluene: cyclohexanone (weight ratio) = 1 A mixed solvent having a composition of 1: 1) 5 parts by weight of a polyisocyanate compound (trade name Coronate L manufactured by Nippon Polyurethane Industry Co., Ltd.) is added to each of the magnetic coating material for the upper layer and the non-magnetic coating material for the lower layer thus prepared. did. The relationship between the strain γ and the stress σ of the upper magnetic coating material was measured with a strain control viscometer under the condition of a shear rate of 0.025 (1 / s).
No clear local maximum was observed in.

Then, the magnetic coating material for the upper layer and the non-magnetic coating material for the lower layer were multi-layered on a polyethylene terephthalate support having a thickness of 7.5 μm by a wet-on-wet coating method, and the coating film was undried. A magnetic field orientation treatment was performed for a certain period of time, followed by drying and surface smoothing treatment with a calendar to form a lower non-magnetic layer and an upper magnetic layer. The thickness (dry film thickness) of the lower non-magnetic layer was 1.5 μm and that of the upper magnetic layer was 0.15 μm.

Further, on the surface of the above-mentioned polyethylene terephthalate support opposite to the side on which the lower non-magnetic layer and the upper magnetic layer were formed, a back coating composition having the following composition was applied, dried, and surface smoothed by calendering. By performing the treatment, a back coat layer having a thickness of 0.8 μm was formed, and a wide original anti-magnetic tape was obtained.

<Backcoat coating material> Carbon black 40 parts by weight (average particle size 26 nm) Barium sulfate 10 parts by weight (average particle size 300 nm) Nitrocellulose 25 parts by weight Polyurethane resin 25 parts by weight (Nippon Polyurethane Industry Co., Ltd., trade name N -2301) Polyisocyanate compound 10 parts by weight (Nippon Polyurethane Industry Co., Ltd., trade name Coronate L) Mixed solvent 900 parts by weight (Methyl ethyl ketone: toluene: cyclohexanone (weight ratio) = 1: 1: 1 mixed solvent) The obtained original anti-magnetic tape was cut into a tape width of 8 mm to produce a magnetic tape.

Examples 2 to 6 Magnetic tapes were produced in the same manner as in Example 1 except that the magnetic coating materials for the upper layer shown in Table 1 were used. In the magnetic paint, the composition other than the binder component and the solvent is the same as in Example 1.

Comparative Examples 1 to 10 Use of the magnetic coating materials for the upper layer shown in Table 1, that is, the maximum value σmax of stress and the maximum value σmax.
A magnetic tape was produced in the same manner as in Example 1 except that the strain γmax for was set outside the predetermined range. In the magnetic paint, the composition of components other than the binder component and the solvent component is the same as in Example 1.

The surface roughness, surface roughness Ra, electromagnetic conversion characteristics, RF envelope and number of dropouts of the produced magnetic tape were measured. The measuring method is as follows.

Surface roughness: The tape surface was observed using a differential interference microscope. At this time, when the surface was judged to be considerably smooth, it was recorded as ◯, when it was judged to be slightly rough, it was marked as Δ, and when it was judged to be considerably rough, it was recorded as x.

Surface roughness Ra (nm): JIS B 06
It is the center line average roughness Ra defined by 01. The surface roughness Ra was measured using a tally step roughness meter manufactured by Taylor Hopson. Stylus is 2.5
× 0.1 μm, needle pressure 2 mg, cut-off filter 0.33 Hz, measurement speed 2.5 μm / s, reference length 0.5 mm. In the roughness curve,
The unevenness of 0.01 μm or more was cut.

Electromagnetic conversion characteristics: chroma S / N, lumi S / N
Was evaluated by measuring. Note that the chroma S / N was measured by using a noise meter (manufactured by Shibasoku) to determine the difference between the S / N of the reference tape (manufactured by Sony Corporation) and the sample tape in the 100% white signal. The Lumi S / N was measured by using a noise meter (manufactured by Shibasoku) to find the difference in S / N between the reference tape (manufactured by Sony) and the sample tape in the chroma signal. These measurement data were recorded as total values when the value on the reference tape (manufactured by Sony Corporation) was 1 dB.

RF envelope: 8 sample tapes
It was run on a millideck (product name S-550 manufactured by Sony Corporation), the RF envelope at that time was displayed on an oscilloscope, and the ratio between the maximum value and the minimum value of the envelope was obtained and evaluated.

Number of Dropouts: The number of dropouts (the number of dropouts) of −12 dB / 5 μs detected per minute was measured by a measuring instrument manufactured by Shibasoku Co., Ltd. under the trade name of VHO1BZ.

Table 2 shows the results of these measurements.

[0121]

[Table 2]

As shown in Table 2, whether there is a clear maximum value σmax in the stress σ in the magnetic coating material for the upper layer,
Alternatively, the maximum value σmax of stress and this maximum value σmax
The magnetic tapes of Examples 1 to 6 in which the strain γmax with respect to the condition satisfies a predetermined condition have good surface properties, electromagnetic conversion characteristics and RF envelope are good, and dropout is sufficient. It is suppressed.

On the other hand, the maximum value σmax of the stress as the upper layer coating material and the strain γma with respect to this maximum value σmax.
In the magnetic tapes of Comparative Examples 1 to 6 in which x does not satisfy the predetermined condition, the surface of the upper magnetic layer is rough, so that the electromagnetic conversion characteristics are insufficient and dropouts frequently occur. To do. In addition, the shape of the RF envelope is also poor.

From this, it was found that controlling the relationship between the strain γ and the stress σ in the magnetic coating material for the upper layer is effective in producing a multi-layer coating type magnetic recording medium having good characteristics.

[0125]

As is clear from the above description, in the present invention, when a multilayer coating type magnetic recording medium is manufactured, a clear maximum value of the stress σ is not recognized as the magnetic coating for the upper layer, Alternatively, since the maximum value σmax of the stress and the strain γmax with respect to the maximum value σmax satisfy a predetermined condition, the interface between the lower layer and the upper layer is not disturbed, and the upper layer has a good surface property. Therefore, according to the present invention, excellent electromagnetic conversion characteristics and R
The shape of the F envelope is also good, dropout is suppressed, running property and durability are excellent, and a magnetic recording medium suitable for high-density recording can be manufactured with a high yield.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a flow curve in Newtonian flow and a flow curve in pseudoplastic flow together.

FIG. 2 is a characteristic diagram showing a relationship between strain γ and stress σ measured by a strain control type viscometer of magnetic paint.

FIG. 3 is a schematic diagram showing a coating film forming system for forming a lower non-magnetic layer and an upper magnetic layer by a wet-on-wet coating method.

FIG. 4 is a schematic view showing an example of a coating apparatus of the coating film forming system.

FIG. 5 is a schematic view showing another example of a coating device.

FIG. 6 is a schematic view showing still another example of the coating device.

Claims (2)

[Claims] 1. A lower-layer coating film is formed by applying a non-magnetic coating material prepared by dispersing a non-magnetic powder and a binder together with a solvent onto a non-magnetic support to form a lower-layer coating film. While forming the upper coating film by applying a magnetic coating material prepared by dispersing a magnetic powder and a binder together with a solvent on the lower coating film, a strain control type viscosity of the magnetic coating material is formed. In the relation between strain and stress measured by a meter, the shear rate is 0.025 (1 /
A method of manufacturing a magnetic recording medium, wherein a maximum value of stress does not appear under the condition of s). 2. A non-magnetic coating material prepared by dispersing a non-magnetic powder and a binder together with a solvent on a non-magnetic support to form a lower-layer coating film, and then the lower-layer coating film is in a wet state. While forming the upper coating film by applying a magnetic coating material prepared by dispersing a magnetic powder and a binder together with a solvent on the lower coating film, a strain control type viscosity of the magnetic coating material is formed. In the relation between strain and stress measured by a meter, the shear rate is 0.025 (1 /
A maximum value appears in the stress under the condition of s). When the maximum value of this stress is σmax and the strain when the stress reaches the maximum value is γmax, the following condition is satisfied. Production method. σmax <100 (Pa) γmax <0.1