JPH07220265A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium excellent in such characteristics as RF output, chroma output, running durability and still durability at high temp. and humidity or low humidity and suitable for use as a digital recording medium. CONSTITUTION:At least 1st, 2nd, 3rd and 4th layers are successively formed on a substrate. When the particle diameter (average particle diameter in the case of spherical particles or average major axis size in the case of acicular particles) of a filler contained by the largest weight among all fillers in the 1st layer, that in the 2nd layer, that in the 3rd layer and that in the 4th layer are represented by A (nm), B (nm), C (nm) and D (nm), respectively, the 4th layer is a magnetic layer and the relations of A> B and C>D are satisfied. When the crystallite size of a filter contained by the largest weight among all fillers in the 1st layer, that in the 2nd layer, that in the 3rd layer and that in the 4th layer are represented by (a) (nm), (b) (nm), (c) (nm) and (d) (nm), respectively, the 4th layer is a magnetic layer and the relations of a>b and c>d are satisfied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium,
More specifically, the present invention relates to a magnetic recording medium which is suitable as a recording medium for digital use and has excellent surface properties and electrical characteristics and running properties.

[0002]

2. Description of the Related Art In a conventional magnetic recording medium, a magnetic powder is made into a fine powder, or an upper layer is a magnetic layer,
Higher quality is achieved by forming a multi-layer structure in which the lower layer is a non-magnetic layer (see JP-A-63-187418).

However, in the former case, the magnetic powder is simply made finer, and in the latter case, JP-A-63-187418.
In the case of No. 6, since the shape of the non-magnetic powder is inappropriate, the dispersibility of the magnetic powder or the non-magnetic powder in the coating material for forming the magnetic layer or the non-magnetic layer is poor, and the calenderability in the calendar process deteriorates. , As a result,
The surface property of the magnetic recording medium does not reach a preferable state, and it is difficult to obtain a magnetic recording medium having excellent electrical characteristics and running properties required as a digital recording medium.

Generally, the surface property of the uppermost layer in a multi-layer magnetic recording medium is strongly influenced by the surface property of a layer which is in contact with the uppermost layer, and this tendency becomes more remarkable as the film thickness of the uppermost magnetic layer becomes thinner.

Therefore, in order to improve the surface property of the uppermost magnetic layer, it is necessary to improve the surface property of the layer underlying the uppermost layer. For that purpose, it is effective to make the powder used in the layer underlying the uppermost layer into fine particles and improve the dispersibility by using a binder containing a polar group (JP-A-4-57217). However, as the powder used for the lower layer is made finer, the physical properties such as Young's modulus deteriorate,
There was a problem that the running durability of the tape at low temperatures and the head touch characteristics (envelope characteristics) were deteriorated.
The problem is that as the tape becomes thinner for lengthening,
It has become more and more important.

[0006]

A magnetic recording medium having three or more constituent layers in which a magnetic layer and a nonmagnetic layer are combined is known from Japanese Patent Application Laid-Open No. 2-260231. However, it has not been possible to make the following characteristics compatible at a high level.

1) RF output, chroma output 2) Running durability under high temperature and high humidity (40 ° C, humidity 80%) and low humidity (20 ° C, humidity 20%) 3) Still durability (0 ° C, humidity) In view of the above problems, an object of the present invention is to provide a magnetic recording medium which improves the surface properties of the magnetic recording medium and is excellent in electrical characteristics and running properties and which is suitable as a digital recording medium. .

[0008]

According to the present invention, the above problems can be solved by the following constitution.

That is, at least the first, second, third, and fourth layers are formed on the support in this order from the side closer to the support, and the filler which is contained in the largest proportion by weight of all the fillers in each layer is formed. The particle size (average particle size in the case of spherical particles, average major axis length in the case of acicular particles) is A for the first, second, third, and fourth layers, respectively.
(Nm), B (nm), C (nm), and D (nm), the fourth layer is a magnetic layer and A> B, C> D. The first, second, third, and fourth layers are formed in this order from the side closer to the support, and the crystallite size of the filler contained in the largest proportion by weight of the total filler of each layer is set to the first, second, and third layers. A for the third and fourth layers
(Nm), b (nm), c (nm), d (nm), the fourth layer is a magnetic layer, and a> b, c> d is at least a magnetic recording medium on a support. The first, second, third, and fourth layers are formed in this order from the side closer to the support, and the Young's modulus of each layer is Y ₁ (kg / mm ² ), Y ₂ (kg / mm ² ), Y ₃ (k
g / mm ² ), Y ₄ (kg / mm ² ), the fourth layer is a magnetic layer, and Y ₁ > Y ₂ and Y ₄ > Y ₃ are satisfied. By using a magnetic recording medium in which the layers are non-magnetic layers and the third and fourth layers are magnetic layers, the above characteristics 1) to 3) can be improved.

In a preferred embodiment of the present invention, the particle diameters A, B, C and D of the filler in each layer are AB ≧ 20, C.
It is preferable that -D≥20, and AB≥50 and CD≥50.
Is more preferable. Further, it is usually 50 ≦ D ≦ 350, preferably 50 ≦ D ≦ 200, and more preferably 80 ≦ D ≦ 170. Also, usually 5 ≦ B ≦ 500,
5 ≦ B ≦ 400 is preferable, and 5 ≦ B ≦ 300 is more preferable.

The crystallite size of the filler in each layer is preferably ab ≧ 5, cd ≧ 5, and a−
More preferably, b ≧ 10 and cd−10. Also, usually 5 ≦ d ≦ 35, 5 ≦ b ≦ 100, and 5 ≦ d ≦ 20, 5 ≦
It is preferable that b ≦ 50, and it is more preferable that 5 ≦ d ≦ 15 and 5 ≦ b ≦ 35.

The Young's modulus of each layer is Y ₄ −Y ₃ ≧ 5
0, Y ₁ −Y ₂ ≧ 50 is preferable, Y ₄ −Y ₃ ≧ 100,
It is more preferable that Y ₁ -Y ₂ ≧ 100.

Further, it is preferable to use a so-called wetton wet multi-layer coating method in which the first to fourth layers are multi-layer coated while they are in a wet state.

In the present invention, in addition to the above-mentioned first to fourth layers, if necessary, an undercoat layer may be provided between the first layer and the support for the purpose of improving adhesion, or the fourth layer may be provided. It is also possible to provide an overcoat layer on this layer (the uppermost magnetic layer). It is preferable that the undercoat layer and the overcoat layer as well as the first to fourth layers are multilayer-coated by the wet on wet method.

The crystallite size (Crystal
(Size) is used in the same meaning as what is also called Crystallite Size. By measuring the integrated width of the (110) diffraction line of Fe using an X-ray diffractometer, it is measured by the Scherrer method based on Si powder. can get. The average major axis length of the powder is an average value of 500 major axis lengths of the powder measured by a transmission electron microscope photograph.

Further, magnetic powder, non-magnetic powder and the like having a predetermined crystallite size or average major axis length can be appropriately selected from commercially available products.

To control the crystallite size and average major axis length of magnetic powder, non-magnetic powder, etc.
Selection, reaction temperature during reaction such as oxidation and reduction, reaction time, pressure during reaction, reaction conditions such as pH and sintering inhibitor,
This can be performed by appropriately adjusting the types and amounts of various elements used as the shape control agent.

The magnetic powder used in the uppermost magnetic layer is preferably ferromagnetic metal powder or hexagonal ferrite powder, and particularly preferably ferromagnetic metal powder.

The thickness of the third layer (the layer which is in contact with the uppermost magnetic layer) is preferably 0.1 μm or more, more preferably 0.2 μm or more in order to keep the surface property of the layer good. preferable. However, if it becomes too thick, high-frequency characteristics will deteriorate due to film thickness loss, so 0.1-0.8 μm, more preferably 0.2-0.5
It is recommended to set to μm.

In order to achieve a Young's modulus higher than that of the second and third layers in the first and fourth layers, in addition to the control of the crystallite size of the powder and the average particle size, yielding is performed. Use measures such as increasing the hardness of the binder used, increasing the ratio of thermoplastic resin to polyurethane, increasing the amount of curing agent, and reducing the amount of additives, such as using polyurethane with dots. More preferably.

Next, the structure of the magnetic recording medium of the present invention will be sequentially described in detail.

(A) Non-magnetic support Examples of materials for forming the non-magnetic support include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, cellulose triacetate and cellulose diacetate. Cellulose derivatives such as, and plastics such as polyamide, aramid resin, and polycarbonate.

The form of the non-magnetic support is not particularly limited, and is mainly tape-like, film-like, sheet-like, card-like,
There are disc shape and drum shape.

The thickness of the non-magnetic support is not particularly limited, but in the case of a film or sheet, it is usually 2-100.
μm, preferably 3 to 50 μm, about 30 μm to 10 mm in the case of a disc or card, and appropriately selected depending on the recorder etc. in the case of a drum.

The non-magnetic support may have a single-layer structure or a multi-layer structure. Further, this non-magnetic support may be subjected to surface treatment such as corona discharge treatment.

A back coat layer is provided on the surface (back surface) of the non-magnetic support, on which the magnetic layer is not provided, for the purpose of improving the running property of the magnetic recording medium, preventing charging, and preventing transfer. It is preferable that an undercoat layer be provided between the magnetic layer and the non-magnetic support.

(B) Uppermost layer (fourth layer) which is a magnetic layer The fourth layer contains magnetic powder. Further, a binder and other components can be contained if necessary.

The thickness of the uppermost layer is usually 0.01 to 0.
It is 7 μm, preferably 0.02 to 0.6 μm, and particularly preferably 0.02 to 0.4 μm. When the thickness is less than 0.01 μm, output may not be obtained during reproduction because recording is not sufficient, while when it is greater than 0.7 μm,
A sufficient reproduction output may not be obtained due to the film thickness loss, which is not preferable.

(B-1) Magnetic Powder In the present invention, various kinds of known magnetic powders can be used for the uppermost layer which is a magnetic layer, but it is more preferable to contain a ferromagnetic metal powder.

The content of this ferromagnetic metal powder is usually 60 to 95 wt%, preferably 70 to 90 wt%, and particularly preferably 75 to 95 wt% with respect to the total solid content in the layer.
It is 85 wt%.

Examples of the magnetic powder used in the present invention include ferromagnetic iron oxide powder, ferromagnetic metal powder, and hexagonal plate powder.

Among these, ferromagnetic metal powder and hexagonal ferrite powder can be preferably used, and it is particularly preferable to contain ferromagnetic metal powder.

As the ferromagnetic iron oxide powder, γ-Fe is used.
FeOx (1.33 <x <with ₂ O ₃ , Fe ₃ O ₄ or intermediate iron oxides of these
Examples thereof include a compound represented by 1.5) and a compound added with Co (cobalt-modified) represented by Co-FeOx (1.33 <x <1.5).

Examples of the ferromagnetic metal powder include Fe, Co, Fe-Al system, Fe-Al-Ni system, Fe-Al-Zn system, Fe-Al-Co system, and F system.
e-Al-Ca system, Fe-Ni system, Fe-Ni-Al system, Fe-Ni-Co system, Fe-Ni-
Si-Al-Mn system, Fe-Ni-Si-Al-Zn system, Fe-Al-Si system, Fe-Ni-Zn
System, Fe-Ni-Mn system, Fe-Ni-Si system, Fe-Mn-Zn system, Fe-Co-Ni-P
Examples thereof include ferromagnetic metal powders such as those based on Ni-Co, Ni-Co, and Fe, Ni, Co, and the like. Among these, the Fe-based metal powder has excellent electrical characteristics.

On the other hand, in terms of corrosion resistance and dispersibility,
Fe-Al system, Fe-Al-Ca system, Fe-Al-Ni system, Fe-Al-Zn system, Fe-Al
Fe-Al type ferromagnetic metal powders such as -Co type, Fe-Ni-Si-Al-Co type and Fe-Co-Al-Ca type are preferable.

Particularly preferred ferromagnetic metal powder for the purpose of the present invention is a metal magnetic powder containing iron as a main component, and Al or Al and Ca, in which Al is in a weight ratio of Fe: Al = 100: 0.5.
~ 100: 20, Ca: Fe: Ca = 100: 0.1 to 1 by weight
It is desirable to contain it in the range of 00:10.

By setting the ratio of Fe: Al in such a range, the corrosion resistance is remarkably improved, and by setting the ratio of Fe: Ca in such a range, electromagnetic conversion characteristics are improved and dropout is reduced. Can be made.

Although the reason why the electromagnetic conversion characteristics are improved and the dropout is reduced is not clear, it is considered that the coercive force is improved and the agglomerates are reduced due to the improved dispersibility.

The ferromagnetic powder used in the present invention has a major axis length of 50 to 350 nm, preferably 50 to 200 nm.
nm, and more preferably 80 to 170 nm. When the major axis length of the ferromagnetic powder is within the above range, the surface properties of the magnetic recording medium can be improved and the electrical characteristics can be improved.

The ferromagnetic powder used in the present invention preferably has a coercive force (Hc) of usually 600 to 5,000 Oe. If this coercive force is less than 600 Oe,
The electromagnetic conversion characteristics may deteriorate and the coercive force is 5,00.
When it exceeds 0 Oe, recording may not be possible with an ordinary head, which is not preferable.

The ferromagnetic powder preferably has a saturation magnetization (σ _s ) which is a magnetic characteristic, usually 70 emu / g or more. If the saturation magnetization is less than 70 emu / g,
Electromagnetic conversion characteristics may deteriorate. In particular, when the ferromagnetic powder is a ferromagnetic metal powder, the saturation magnetization amount is preferably 120 emu / g or more.

Further, according to the present invention, the specific surface area by the BET method is 30 m ² / g or more in accordance with the increase in recording density.
In particular, a ferromagnetic metal powder of 45 m ² / g or more can be preferably used.

This specific surface area and its measuring method are described in detail in "Measurement of powder" (co-authored by JMDallavelle and Clyeorr Jr., translated by Muta et al., Published by Sangyo Tosho KK).
In addition, "Chemical Handbook" Application, P1170-1171 (Edited by The Chemical Society of Japan;
It is also described in Maruzen Co., Ltd., issued April 30, 1966.

The specific surface area is measured, for example, by measuring the powder at 105 ° C.
After degassing while heat-treating for 13 minutes before and after removing what is adsorbed to the powder, introduce this powder into the measuring device and set the initial pressure of nitrogen to 0.5 kg / m ² Measure at liquid nitrogen temperature (-105 ℃) for 10 minutes.

As the measuring device, for example, a counter soap (manufactured by Yuasa Ionics Co., Ltd.) is used.

Examples of the hexagonal plate-like powder include hexagonal ferrite. Such a hexagonal ferrite is composed of barium ferrite, strontium ferrite, etc., and even if part of the iron element is replaced with another element (for example, Ti, Co, Zn, In, Mn, Ge, Hb). Good. Regarding this ferrite magnetic material, IEEE
It is described in detail in trans on MAG-18 16 (1982).

Of these, barium ferrite can be preferably used in the present invention.

As an example of the barium ferrite (hereinafter referred to as Ba-ferrite) magnetic powder, an average particle size in which a part of Fe is replaced by at least Co and Zn (length of diagonal line of plate surface of hexagonal ferrite) Is 400 to 900Å, and the plate ratio (value obtained by dividing the length of the diagonal line of the hexagonal ferrite plate surface by the plate thickness) is 2.0 to 10.0, preferably 2.0 to 6.0, and the coercive force is Ba-ferrite having a (Hc) of 450 to 1500 Oe can be mentioned.

The above magnetic powder may be used alone or in combination of two or more kinds.

(B-2) Binder The binder contained in the fourth layer is typically a vinyl chloride resin such as polyurethane, polyester and vinyl chloride copolymer, and these resins are used. −
SO ₃ M, —OSO ₃ M, —COOM, —PO (OM ¹ ) ₂ and a repeating unit having at least one polar group selected from sulfobetaine groups are preferably contained.

However, in the above polar group, M represents a hydrogen atom, an alkali atom such as Na, K or Li, or an alkyl group.

The above polar group has the function of improving the dispersibility of the magnetic powder, and the content in each resin is 0.1 to 8.0 mol%, preferably 0.2 to 6.0 mol%. This content is 0.
If it is less than 1 mol%, the dispersibility of the magnetic powder will be reduced, and if the content exceeds 8.0 mol%, the magnetic coating will easily gel. The weight average molecular weight of each resin is 15,0
The range of 00 to 50,000 is preferred.

The content of the binder is 100
8 to 25 parts by weight, preferably 10 to 20 parts by weight
Parts by weight.

The binder is not limited to one kind, and two or more kinds can be used in combination. In this case, the ratio of the polyurethane and / or polyester to the vinyl chloride resin is 90:10 by weight. 10:90, preferably
It is in the range of 70:30 to 30:70.

It is preferable that an epoxy group is introduced into the vinyl chloride copolymer. This is because the thermal stability of the polymer is improved by doing so.

When an epoxy group is introduced, the content of the repeating unit having an epoxy group in the copolymer is 1 to
30 mol% is preferable, and 1 to 20 mol% is more preferable.

As the monomer for introducing the epoxy group, for example, glycidyl acrylate is preferable.

Regarding the technique for introducing a polar group into a vinyl chloride-based copolymer, JP-A-57-44227 and JP-A-58-108052.
Issue 59-8127, Issue 60-101161, Issue 60-235814, Issue 6
0-238306, 60-238371, 62-121923, 62-146
No. 432, No. 62-146433, etc., and these can be used in the present invention.

(B-3) Other Components In the present invention, in order to improve the quality of the uppermost magnetic layer, an abrasive, a lubricant, a durability improver, a dispersant,
Additives such as an antistatic agent and conductive fine powder can be contained as other components.

A known substance can be used as the polishing agent.

The average particle size of this abrasive is usually 0.
The thickness is 05 to 0.6 μm, preferably 0.05 to 0.5 μm, and particularly preferably 0.05 to 0.3 μm.

The content of the abrasive in the uppermost layer is usually 3 to 20 parts by weight, preferably 5 to 15 parts by weight, particularly preferably 5 to 10 parts by weight.

Fatty acids and / or fatty acid esters can be used as the lubricant. In this case, the amount of fatty acid added is preferably 0.2 to 10% by weight, more preferably 0.5 to 5% by weight, based on the magnetic powder. 0.2 added
If it is less than 10% by weight, the running property tends to decrease, and if it exceeds 10% by weight, fatty acids may exude to the surface of the magnetic layer,
The output is likely to decrease.

The amount of the fatty acid ester added is preferably 0.2 to 10% by weight with respect to the magnetic powder, and particularly preferably
It is 0.5 to 5% by weight. If the added amount is less than 0.2% by weight, the still durability is apt to deteriorate, and if it exceeds 10% by weight, fatty acid ester exudes to the surface of the magnetic layer,
The output is likely to decrease.

When a fatty acid and a fatty acid ester are used in combination to further enhance the lubricating effect, the weight ratio of the fatty acid and the fatty acid ester is preferably 10:90 to 90:10.

The fatty acid may be a monobasic acid or a dibasic acid, preferably having 6 to 30 carbon atoms, and 12 to 22 carbon atoms.
Is more preferable.

As the lubricant other than the above fatty acids and fatty acid esters, known substances can be used, and for example, silicone oil, fluorinated carbon, fatty acid amide, α-olefin oxide and the like can be used.

Examples of the curing agent include polyisocyanate. Examples of the polyisocyanate include aromatic polyisocyanates such as adducts of tolylene diisocyanate (TDI) and active hydrogen compounds, and hexamethylene diisocyanate ( There are aliphatic polyisocyanates such as adducts of HMDI) with active hydrogen compounds. The weight average molecular weight of the polyisocyanate is preferably in the range of 100 to 3,000.

As the dispersant, for example, those described in paragraph No. 0093 of JP-A-4-214218 can be used. These dispersants are usually used in the range of 0.5 to 5% by weight based on the magnetic powder.

As the antistatic agent, cationic surfactants such as quaternary amines; anionic surfactants containing acid groups such as sulfonic acid, sulfuric acid, phosphoric acid, phosphoric acid ester and carboxylic acid;
Examples thereof include amphoteric surfactants such as aminosulfonic acid; natural surfactants such as saponin. The above-mentioned antistatic agent is usually added in the range of 0.01 to 40% by weight with respect to the binder.

Further, in the present invention, conductive fine powder can be preferably used as the antistatic agent. Examples of the antistatic agent include carbon black, graphite, tin oxide, silver powder, silver oxide, silver nitrate, organic compounds of silver, metal particles such as copper powder, and metal oxides such as zinc oxide, barium sulfate and titanium oxide. Examples include pigments coated with a conductive material such as a tin oxide coating or an antimony solid solution tin oxide coating.

The average particle size of the conductive fine powder is
Usually, it is 5 to 700 nm, preferably 5 to 200 nm, and more preferably 5 to 50 nm.

The content of the conductive fine powder is 0.1 to 10 parts by weight, preferably 0.1 to 2 parts by weight, more preferably 0.1 to 1 part by weight, based on 100 parts by weight of the magnetic powder.

(C) Lower Layer (First Layer, Second Layer, Third Layer) The lower layer is composed of at least three layers and is formed with a plurality of layers between the non-magnetic support and the uppermost layer which is the magnetic layer. It

As the lower layer, for example, a magnetic layer (C-1) containing a magnetic powder, a nonmagnetic layer (C-2) containing a nonmagnetic powder, and a layer (C-3) containing a high magnetic permeability material. , Or a layer composed of a combination of these layers. In the present invention, the first layer and the second layer are preferably non-magnetic layers (C-2), and particularly preferably non-magnetic layers containing acicular non-magnetic powder. As the third layer, the magnetic layer (C-1) is preferable, and the magnetic layer containing iron oxide magnetic powder is particularly preferable.

The total thickness of the first layer and the second layer is usually 0.1 to 2.5 μm, preferably 0.2 to 2.0 μm,
It is particularly preferably 0.5 to 2.0 μm. The thickness is 2.5 μm
If it is larger than the above, the surface roughness of the upper layer surface after stacking becomes large, so-called multi-layer surface roughness may occur, and preferable electromagnetic conversion characteristics may not be obtained, while if it is smaller than 0.1 μm, at the time of calendaring. The effect of the lower layer in 2) is lost, and high smoothness cannot be obtained, which may deteriorate electromagnetic conversion characteristics, which is not preferable.

(C-1) Magnetic Layer The lower magnetic layer contains magnetic powder. Also, it contains a binder and other components as necessary.

(C-1-1) Magnetic Powder The magnetic powder contained in the lower magnetic layer is not particularly limited, and the compounds exemplified in (B-1) can be preferably used. These magnetic powders may be used alone or in combination of two or more.

Of these magnetic powders, iron oxide is preferable, and Co-containing iron oxide is particularly preferable. When the lower magnetic layer contains Co-containing iron oxide, the reproduction output becomes good in the region where the recording wavelength is large, particularly in the region of 1 μm or more.

The content of the magnetic powder is usually 70 to 90% by weight, preferably 75 to 85% by weight, based on the total solid content in the layer.

(C-1-2) Binder The binder contained in the lower magnetic layer is (B
-The compounds exemplified in 2) can be used,
The amount is usually 5 to 2 with respect to 100 parts by weight of the ferromagnetic powder.
It is 5 parts by weight, preferably 10 to 20 parts by weight.

(C-1-3) Other Components As other components contained in the lower magnetic layer,
The compounds exemplified in (B-3) can be used. The amount is not particularly limited and can be appropriately selected as long as the object of the present invention is not impaired.

(C-2) Nonmagnetic Layer The nonmagnetic layer contains nonmagnetic powder. Also, it contains a binder and other components as necessary.

(C-2-1) Non-Magnetic Powder In the present invention, various known non-magnetic powders can be appropriately selected and used.

Examples of the non-magnetic powder include carbon black, graphite, TiO ₂ , barium sulfate, ZnS and MgC.
O ₃ , CaCO ₃ , ZnO, CaO, tungsten disulfide, molybdenum disulfide, boron nitride, MgO, SnO 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 stone, silicon nitride, boron nitride, silicon carbide, molybdenum carbide, boron carbide , Tungsten carbide, titanium carbide, tribolite, diatomaceous earth, dolomite and the like.

Of these, preferred are carbon black, CaCO ₃ , TiO ₂ , barium sulfate, α-Al ₂ O ₃ and α-Fe ₂ O.
Inorganic powder such as ₃ , α-FeOOH, Cr ₂ O _3, etc.
-Fe ₂ O _3, α-FeOOH are preferable, especially preferred are α-
Fe ₂ O ₃ .

In the present invention, a non-magnetic powder having a needle-like powder shape can be preferably used. By using the acicular non-magnetic powder, the smoothness of the surface of the non-magnetic layer can be improved, and the smoothness of the surface of the uppermost layer composed of the magnetic layer laminated thereon can also be improved.

The major axis length of the non-magnetic powder used in the second layer is usually 5 to 500 nm or less, preferably 5 to 400 nm.
nm or less, and particularly preferably 5 to 300 nm or less.

The axial ratio of the non-magnetic powder is usually 2 to
20, preferably 5 to 15, particularly preferably 5
Is ~ 10. The axial ratio mentioned here means a ratio of the major axis length to the minor axis length (major axis length / minor axis length).

Further, in the present invention, it is preferable that the non-magnetic powder is surface-treated with a Si compound and / or an Al compound. When the non-magnetic powder subjected to such surface treatment is used, the surface condition of the uppermost layer which is the magnetic layer can be improved. The content of Si and / or Al is 0.1 to 10% by weight of Si based on the non-magnetic powder, and A
l is preferably 0.1 to 10% by weight, more preferably
Si is 0.1-5 wt%, Al is 0.1-5 wt%, especially Si
Is preferably 0.1 to 2% by weight and Al is 0.1 to 2% by weight.
Also, the weight ratio of Si and Al should be Al / Si ≧ 1. The surface treatment can be carried out by the method described in JP-A-2-83219. From the viewpoint of imparting conductivity, Sn
It is preferably surface-treated with O ₂ , an antimony solid solution SnO _2, or the like.

The content of the non-magnetic powder in the non-magnetic layer is usually 50 to 99% by weight, preferably 60 to 95% by weight, based on the total of all components constituting the non-magnetic layer.
And particularly preferably 70 to 95% by weight. When the content of the nonmagnetic powder is within the above range, the surface state of the uppermost layer which is the magnetic layer and the nonmagnetic layer can be improved.

(C-2-2) Binder The binder contained in the lower non-magnetic layer is as follows.
The compounds exemplified in (B-2) can be used, and the amount thereof is usually 5 to 150 parts by weight, preferably 10 to 120 parts by weight, relative to 100 parts by weight of the non-magnetic powder.

(C-2-3) Other Components As the other components contained in the lower non-magnetic layer, the compounds exemplified in (B-3) can be used. The amount is not particularly limited as long as it does not impair the object of the present invention, and can be appropriately selected.

(C-3) Layer Containing High Magnetic Permeability Material The layer containing the high magnetic permeability material contains a high magnetic permeability material. Also, it contains a binder and other components as necessary.

(C-3-1) High magnetic permeability material As a high magnetic permeability material, its coercive force Hc is 0 <Hc ≦ 1.0 × 1.
0 ⁴ (A / m), preferably 0 <Hc ≦ 5.0 × 10 ³ (A / m). When the coercive force is within the above range, the effect of stabilizing the magnetized region of the uppermost layer is exhibited as the high magnetic permeability material. When the coercive force exceeds the above range, desired properties may not be obtained due to the manifestation of properties as a magnetic material, which is not preferable.

In the present invention, it is preferable to appropriately select a material having a high coercive force as the high magnetic permeability material. Examples of such a high magnetic permeability material include a metal soft magnetic material and an oxide soft magnetic material.

As the metal soft magnetic material, Fe-Si alloy, Fe-Al alloy (Alperm, Alfenol, Alfer), permalloy (Ni-Fe binary alloy, and Mo, Cu, Cr, etc. were added thereto. Multi-component alloys), sendust (Fe-Si-Al {9.6 wt% Si, 5.4% Al, composition with the balance being Fe}), Fe-Co alloy and the like. Among them, sendust is preferable as a preferable metal soft magnetic material. In addition,
The metal soft magnetic material as the high magnetic permeability material is not limited to those exemplified above, and other metal soft magnetic materials can be used. The high-permeability material can be used alone or in combination of two or more.

As the oxide soft magnetic material, spinel type ferrites such as MnFe ₂ O ₄ , Fe ₃ O ₄ , CoFe ₂ O ₄ and NiFe _{2 are used.}
O ₄ , MgFe ₂ O ₄ , Li _0.5 Fe _2.5 O ₄ , Mn-Zn ferrite, N
i-Zn ferrite, Ni-Cu ferrite, Cu-Z
Examples include n-based ferrite, Mg-Zn-based ferrite, and Li-ZN-based ferrite. Among these, Mn-Zn ferrite and Ni-Zn ferrite are preferable. These soft oxide magnetic materials can be used alone or in combination of two or more.

This high-permeability material is made into a fine powder using a ball mill or other crushing device, and its particle size is 1 to 1,000 m.
It is preferably μ, particularly 1 to 500 mμ. In order to obtain such a fine powder, in the metal soft magnetic material,
It can be obtained by spraying a molten alloy in a vacuum atmosphere. Further, the soft oxide magnetic material can be made into a fine powder by a glass crystallization method, a coprecipitation firing method, a hydrothermal synthesis method, a flux method, an alkoxide method, a plasma jet method or the like.

In the layer containing the high magnetic permeability material, the content of the high magnetic permeability material is 10 to 99% by weight, preferably 50 to 95% by weight, and more preferably 60 to 90% by weight.
When the content of the high magnetic permeability material is within the above range, the effect of stabilizing the magnetization of the uppermost layer can be sufficiently obtained. Further, if the content of the high magnetic permeability material is less than 50% by weight, the effect as the high magnetic permeability layer may not be obtained, which is not preferable.

The layer containing the high magnetic permeability material may contain non-magnetic particles.

(C-3-2) Binder As the binder contained in the layer containing the high magnetic permeability material in the lower layer, the compounds exemplified in (B-2) can be mentioned, and the amount thereof is High permeability material 10
It is usually 5 to 30 parts by weight, preferably 1 to 0 parts by weight.
0 to 25 parts by weight.

(C-3-3) Other Components As the other components contained in the layer containing the high magnetic permeability material in the lower layer, the compounds exemplified in (B-3) can be mentioned. The amount is not particularly limited as long as it does not impair the object of the present invention, and can be appropriately selected.

-Manufacture of Magnetic Recording Medium- In the magnetic recording medium of the present invention, it is preferable that the upper layer is laminated by a so-called wet on wet method performed when the lower layer is in a wet state. This wet on wet method is disclosed in JP-A-2-251265.
No. 2-268862, the method used to manufacture the known multi-layered structure type magnetic recording medium can be appropriately adopted.

For example, generally, magnetic powder, binder,
A high-concentration paint is prepared by kneading a dispersant, a lubricant, an abrasive, an antistatic agent, etc. and a solvent, and then the high-concentration paint is diluted to prepare a coating paint, and then this paint is non-magnetically supported. Apply to the surface of the body.

Examples of the solvent include, for example, JP-A-4-1422
Those described in paragraph No. 0119 of No. 18 and the like can be used. In kneading and dispersing the magnetic layer forming components, various kneading and dispersing machines can be used.

Examples of this kneading and dispersing machine include those disclosed in Japanese Patent Laid-Open No.
Examples thereof include those described in paragraph No. 0112 of 4-214218. Among the above kneading dispersers, the kneading dispersers capable of providing a power consumption load of 0.05 to 0.5 KW (per 1 Kg of magnetic powder) are a pressure kneader, an open kneader, a continuous kneader, a two-roll mill and a three-roll mill. is there.

In this wet-on-wet multi-layer coating, since the uppermost magnetic layer is applied while the lower layer located below the uppermost layer is in a wet state, the surface of the lower layer (that is, the boundary with the uppermost layer). The surface becomes smooth and the surface property of the uppermost layer becomes good, and the adhesiveness between the upper and lower layers is also improved. As a result, especially for high density recording, high output and low noise are required, for example, the performance required as a magnetic tape is satisfied, and high durability performance is required. The peeling is eliminated, the film strength is improved, and the durability is sufficient. Further, the wet on wet multi-layer coating method can reduce dropout and improve reliability.

-Surface smoothing-In the present invention, next, surface smoothing may be performed by calendering.

Thereafter, if necessary, burnishing or blade treatment is performed for slitting.

In the surface smoothing treatment, the calender conditions are temperature, linear pressure, C / S (coating speed).
Etc. can be mentioned.

In the present invention, the above temperature is usually 50
~ 140 ° C, the above linear pressure is 50 ~ 400kg / cm, the above C / S is 20
It is preferable to hold at ~ 1,000 m / min. If these values are not satisfied, it may be difficult or impossible to keep the surface properties of the magnetic recording medium in good condition.

The thickness of the uppermost layer obtained as a result of the above treatment is 0.6 μm or less, preferably 0.02 to 0.6 μm.
When the thickness of the layer exceeds 0.6 μm, electrical characteristics are deteriorated, so that a magnetic recording medium suitable as a digital recording medium, which is the object of the present invention, cannot be obtained.

[0114]

Embodiments of the present invention will be described below.

The components, ratios and operation sequences shown below can be variously modified without departing from the scope of the present invention. In the following examples, all "parts" are "parts by weight".

Examples 1 to 9 and Comparative Examples 1 to 6 The following magnetic composition for uppermost layer (fourth layer) and lower layer (first layer)
Each of the components of the magnetic composition for layers 3 to 3) was kneaded and dispersed using a kneader and a sand mill to prepare a magnetic coating material for the uppermost layer (fourth layer) and a coating material for the lower layers (first to third layers). However, the average major axis length, crystallite size, film thickness, and polyurethane are shown in Table 1,
2 was changed appropriately.

{Magnetic coating for uppermost layer (fourth layer) -a} 100 parts of ferromagnetic metal powder (Sample No. 1) (average major axis length: 120 nm, Hc: 1700 Oe, crystallite size 12 nm, BET: 60m ² / g, Fe: Al: Ca = 100: 4: 1 (weight ratio)) Potassium sulfonate group-containing vinyl chloride resin 10 parts (Nippon Zeon Co., Ltd. MR-110) Sodium sulfonate group-containing polyurethane resin 10 parts (Toyobo Co., Ltd., UR-8300) α-alumina (0.15 μm) 8 parts Carbon black (40 nm) 0.5 part Stearic acid 1 part Butyl stearate 1 part Cyclohexanone 100 parts Methyl ethyl ketone 100 parts Toluene 100 parts {Lower layer Coating A for (first and second layers) A} α-Fe ₂ O ₃ 100 parts (major axis length: 100 nm, minor axis length: 20 nm, crystallite size 18 nm BET: 55 m ² / g, Si, Al compound (α -fe Si content 0.1% by weight relative to the ₂ O _3, Al content 0.9 wt%) with a surface treatment) sulfonated Vinyl chloride resin containing um group 12 parts (Zeon Corporation, MR-110) Sodium sulfonate group containing polyurethane resin 8 parts (Toyobo Co., Ltd., UR-8700) α-alumina (0.2 μm) 5 Part Carbon black (15 nm) 10 parts Stearic acid 1 part Butyl stearate 1 part Cyclohexanone 100 parts Methyl ethyl ketone 100 parts Toluene 100 parts Obtained top magnetic layer (4th layer) magnetic coating (a), 3rd layer magnetic coating 5 parts of a polyisocyanate compound (Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to each of B and the lower layer (first and second layers) coating material A.

As the lower layer (first layer) polyurethane, as shown in Table 1, a sodium sulfonate group-containing polyurethane resin (manufactured by Toyobo Co., Ltd., UR-8200) was used.
Used instead of -8700.

{Third layer paint B} The lower layer paint B is Co-γ-Fe ₂ O ₃ (Hc: 650 instead of α-Fe ₂ O ₃ in the lower layer paint A).
Oe, average major axis length 190 nm, crystallite size 35 nm, Si content in magnetic powder 0.8% by weight, Al content 0.1% by weight) were obtained in the same manner as the lower layer coating material A.

Examples 1 to 9 and Comparative Examples (1) to (6) As shown in Tables 1 and 2, the above-mentioned magnetic coating for the uppermost layer (fourth layer) and three lower layers (first to third). Layer) coating material was applied on a polyethylene-2,6-naphthalate film with a thickness of 6 μm, which was subjected to an undercoating treatment, by a wet on wet method,
Magnetic field orientation treatment is performed while the coating film is undried, followed by drying, and then surface smoothing treatment with a calendar,
A magnetic layer consisting of a lower layer and an uppermost layer having the thickness shown in Tables 1 and 2 was formed.

Further, a coating material having the following composition was applied to the surface (back surface) of the polyethylene-2,6-naphthalate film on the side opposite to the magnetic layer, the coating film was dried, and the above-mentioned calendar conditions were used. According to the above procedure, a back coat layer having a thickness of 0.8 μm was formed by calendering to obtain a wide original anti-magnetic tape.

Carbon black (Raven 1035) 40 parts Barium sulfate (average particle size 300 nm) 10 parts Nitrocellulose 25 parts Polyurethane resin 25 parts (N-2301 manufactured by Nippon Polyurethane Co., Ltd.) Polyisocyanate compound 10 parts (Nippon Polyurethane) (Coronate L, Co., Ltd.) Cyclohexanone 400 parts Methyl ethyl ketone 250 parts Toluene 250 parts {4th layer magnetic paint-b} In the uppermost layer magnetic paint a, instead of the ferromagnetic metal powder, Co-substituted barium ferrite (Hc:
1100 Oe, BET45m ² / g, average particle size 30nm, crystallite size 30
nm, σs: 64emu / g Same as a except that plate ratio 4) was used.

[0123]

[Table 1]

[0124]

[Table 2]

The original magnetic tape thus obtained was slit to prepare an 8 mm wide magnetic recording medium for video. The following evaluations were performed on this magnetic recording medium. The results are shown in Tables 3 and 4.

(Measurement method) <RF output, chroma output> Sony 8 mm video camera CCVD-900 was used to measure the RF output using the video signal and the chroma output using the chroma signal to obtain a reference tape (8 mm SG manufactured by Konica Corporation). ) Was evaluated as 0 dB.

<Durability at high temperature and high humidity> High temperature and high humidity (40
Recording the entire length of the tape using S-550 (manufactured by Sony Corporation) under the environment of 80 ° C and 80% humidity, and if the RF output reduction of 2 dB or more continues for 1 second or more during playback, the head is clogged. Counted the number of times.

<Running durability at low temperature> In a low temperature environment (20 ° C, 20% humidity), the full length recording of the tape was performed using S-550 (manufactured by Sony Corporation), and the RF output of 2 dB or more was reproduced. When the decrease continued for 1 second or more, the number of times was counted as head clogging.

<Still durability at low temperature> Sony 8mm
Using a video camera EV-S900, temperature 0 ℃, humidity 20%
Shows the time (in minutes) until the RF output decreases by 2 dB in the steal mode.

<Young's modulus> Tensilon HTM-100 manufactured by Orientec Co. was used and the tensile sample length was 200 mm and width was 12.65 mm.
Sample at a pulling speed of 100 mm / min and chuck pressure contact force of 50 kg,
The load fe that causes 1% elongation is calculated, and the Young's modulus E is calculated by the following formula.

E = [fe / (12.65 × d)] × 100 Young's modulus E _{T of} the entire tape (thickness d _T ) according to the above equation,
The Young's modulus E _M of the magnetic layer coating film (thickness d _M ) is calculated from the Young's modulus E _B of the support (thickness d _B including the back coat layer) by the following formula.

E _M = (E _T / d _T −E _B / d _B ) / d _M

[0133]

[Table 3]

[0134]

[Table 4]

As can be seen from the above results, the sample of the present invention has excellent magnetic recording characteristics and durability as compared with the comparative sample.

[0136]

The structure of the present invention improves the surface properties of a magnetic recording medium and also has excellent electrical characteristics and running properties.
A magnetic recording medium suitable as a digital recording medium is provided.

Claims (4)

[Claims] 1. At least a first layer, a second layer, a third layer, and a fourth layer are formed on a support in this order from the side closer to the support, and a filler that is contained in the largest proportion by weight of all fillers in each layer is formed. The particle size (average particle size in the case of spherical particles, average major axis length in the case of needle-shaped particles) is A (nm), B (nm) and C for the first, second, third and fourth layers, respectively. (Nm) and D (nm), the fourth layer is a magnetic layer, and the magnetic recording medium has A> B and C> D. 2. At least a first layer, a second layer, a third layer, and a fourth layer are formed on a support in this order from the side closer to the support, and a filler which is contained in the largest proportion by weight of all fillers in each layer is formed. When the crystallite size is a (nm), b (nm), c (nm), d (nm) for the first, second, third, and fourth layers, respectively, the fourth layer is a magnetic layer. Yes, a magnetic recording medium with a> b, c> d. 3. At least first, second, third, and fourth layers are formed on a support in this order from the side closer to the support, and the Young's modulus of each layer is Y ₁ (kg / mm ² ). , Y ₂ (kg / m
m ² ), Y ₃ (kg / mm ² ), Y ₄ (kg / mm ² ), the fourth layer is a magnetic layer and Y ₁ > Y ₂ and Y ₄ > Y ₃ Medium. 4. The magnetic recording medium according to claim 1, wherein the first and second layers are nonmagnetic layers, and the third and fourth layers are magnetic layers.