JP2005317095A - Manufacturing method of magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a magnetic recording medium which improves both of the polishing effect over a magnetic head and the strength enhancement effect on a magnetic layer, while highly maintaining electromagnetic conversion characteristics. <P>SOLUTION: A first coating material is produced by mixing magnetic powder, binder and alumina and separately a second coating material is produced by mixing using alumina, binder and solvent. Magnetic coating material is produced by adding the second coating material to the first coating material, and a magnetic recording medium 10 is produced by applying this magnetic coating material on a non-magnetism lower layer 2 and forming the magnetic layer 3. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a magnetic recording medium, and more particularly to a magnetic recording medium suitably used as, for example, a digital VTR tape or a data tape.

In recent years, thin-film multilayer magnetic recording media have been extensively developed for use in large-capacity floppy (registered trademark) disks and large-capacity data tapes.
With respect to such a thin film multi-layer type magnetic recording medium, conventionally, a technique for providing a nonmagnetic undercoat layer has been disclosed in order to control the influence of a nonmagnetic support when the magnetic layer is thinned (for example, , See Patent Document 1).
Further, by providing a large number of uniform protrusions on the surface of the non-magnetic support, a magnetic recording medium that achieves excellent electromagnetic conversion characteristics and has excellent running stability and running durability has been disclosed ( For example, see Patent Document 2.)

  However, in recent years, the technological innovation for high-density recording of magnetic recording media has been remarkable, and in order to achieve further high-density recording, each loss such as self-demagnetization loss, film thickness loss, separation loss, etc. is minimized. It is necessary to suppress. For this purpose, it is essential to reduce the thickness of the magnetic layer and achieve super smooth surface.

On the other hand, the thinning of the magnetic layer and the super smoothing of the surface as described above may impair the reliability of the medium (jitter at the tape, edge breakage, error rate at the disk) under high temperature and high humidity. .
Further, in order to achieve super-smoothing on the surface of the magnetic layer, if a super-smooth base without coarse protrusions is used as a non-magnetic support, inconvenience occurs when winding the raw material, resulting in a problem that process suitability is deteriorated.

  In order to solve such technical problems, the techniques disclosed in Patent Documents 1 and 2 cannot be adequately dealt with. In the future, the composition of the magnetic layer, the layer of the magnetic recording medium, There is a need to deal with various aspects such as the configuration and the surface characteristics of the nonmagnetic support.

By the way, the magnetic layer becomes easier to peel off as the film becomes thinner. When peeling occurs in the magnetic layer, it causes dropout, and it is necessary to prevent this.
As a means for preventing the peeling of the magnetic layer, a so-called wet-on-wet coating method is used in which a magnetic coating for forming a magnetic layer is applied while the nonmagnetic lower layer coated on the nonmagnetic support is wet. The method of forming a magnetic layer is mentioned.
In order to prevent the peeling of the magnetic layer, there is a method of adding an inorganic pigment or the like to the magnetic layer to increase the strength of the magnetic layer and improve the durability. If an inorganic pigment, which is a non-magnetic material, is included in the magnetic layer in order to improve the properties, the filling degree of the magnetic powder is inevitably lowered, leading to a decrease in output and a deterioration in electromagnetic conversion characteristics.

  That is, conventionally, there has been a technical problem that it is difficult to improve the durability of the magnetic layer while reducing the thickness of the magnetic layer to achieve high density recording.

In addition, since large-capacity data tapes used in computer devices and the like are repeatedly recorded and reproduced, if the polishing performance for the magnetic head is too great, the magnetic head will be damaged, so the polishing performance for the magnetic head must be reduced. There is.
When an inorganic pigment is contained in the magnetic layer as described above, the surface roughness of the magnetic layer is increased, and the magnetic head is scraped off by repeated recording and reproduction, resulting in damage to the magnetic head.

JP-A-57-198536 Japanese Patent Laid-Open No. 3-176807

  Therefore, in the present invention, in view of the conventional situation as described above, the film thickness is extremely thin, the surface characteristics are good, the running stability and running durability are excellent, and the electromagnetic conversion characteristics when using a short wavelength signal are good. The present invention also provides a method for producing a magnetic recording medium having extremely appropriate magnetic head polishing properties and excellent repetitive recording / reproducing characteristics over a long period of time.

  In the method of the present invention, a nonmagnetic lower layer in which at least nonmagnetic powder is dispersed in a binder on a nonmagnetic support and a magnetic layer in which at least magnetic powder is dispersed in the binder are sequentially stacked. A method for producing a magnetic recording medium comprising: a step of mixing a magnetic powder, a binder and alumina to produce a first paint; and a second paint obtained by mixing using alumina, a binder and a solvent. A step of adding a second paint to the first paint to produce a magnetic paint, and a step of applying a magnetic paint on the nonmagnetic lower layer to form a magnetic layer. And

According to the method of the present invention, it is possible to avoid a decrease in the packing density of the magnetic powder in the magnetic layer, which is a problem when the whole amount of alumina is contained as a powder in the magnetic layer of the coating type magnetic recording medium, and the output is It was possible to prevent the deterioration and the deterioration of the electromagnetic conversion characteristics.
On the other hand, a magnetic coating material is prepared by previously mixing the total amount of alumina with magnetic powder, a binder, and a solvent. Was prevented.
That is, according to the method of the present invention, it is possible to improve both the polishing effect on the magnetic head and the strength improvement effect of the magnetic layer while maintaining the electromagnetic conversion characteristics of the magnetic layer high, and the high magnetic characteristics and excellent running Both durability and characteristics were obtained.
Furthermore, by specifying the content of alumina for each process, it is possible to achieve proper magnetic head polishing (head wear) and to reliably produce a magnetic recording medium having high electromagnetic conversion characteristics. It was.

  The method for producing a magnetic recording medium of the present invention will be described in detail with reference to the drawings, but the present invention is not limited to the following examples, and conventionally known configurations are added, materials are changed, Various applications are possible without departing from the scope of the present invention.

As an example of the magnetic recording medium produced by the method of the present invention, a nonmagnetic lower layer 2 and a magnetic layer 3 are laminated on one main surface of a nonmagnetic support 1 as shown in the schematic configuration diagram of FIG. It has the composition which becomes.
A back coat layer 4 may be provided on the main surface (back surface) opposite to the magnetic layer forming surface of the nonmagnetic support 1 for the purpose of improving running performance, durability, antistatic, and transfer prevention. Good.
Further, an arbitrary writing layer or print recording layer may be provided, a forgery prevention layer may be provided, or a predetermined undercoat layer may be provided between the nonmagnetic lower layer 2 and the nonmagnetic support 1. Good. Further, an overcoat layer may be provided on the magnetic layer 3. Hereinafter, each of these layers will be described together with the method for producing a magnetic recording medium of the present invention.

First, the nonmagnetic support 1 will be described.
Examples of the material of the nonmagnetic support 1 include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and plastics such as polyamide and polycarbonate. Is mentioned.

The center line average roughness (Ra) of the nonmagnetic support 1 is preferably 12 to 28 nm, preferably 14 to 26 nm, and more preferably 16 to 24 nm.
The maximum roughness (Rt) of the nonmagnetic support 1 is preferably 95 to 220 nm, preferably 110 to 200 nm, and more preferably 120 to 180 nm.
Rt / Ra is preferably 10 or less, preferably 5 to 9, and more preferably 7 to 9.
On the surface of the nonmagnetic support 1, the number of protrusions having a protrusion height of 0.01 μm or more is preferably 200 or more per 1 mm of measurement length, but more preferably 250 to 1000.

In order to adjust the Ra, Rt, and the number of protrusions of the nonmagnetic support 1 to a suitable numerical range as described above, for example, on the back side of the nonmagnetic support 1 that is smooth and without waviness, Is contained in a proportion of 0.1 to 10% by weight, preferably 0.1 to 5% by weight, and the average film thickness is 0.01 to 1 μm, preferably 0. A protruding layer (not shown) having a thickness of 0.01 to 0.5 μm may be laminated.
The protrusion layer does not necessarily form a film as long as the particles are wrapped with a polymer. For example, the protrusion layer may have a mesh shape.
Further, a desired protrusion can be formed by mechanically forming unevenness on the back surface side of the nonmagnetic support 1 without stacking such a protrusion layer.

The nonmagnetic support 1 preferably has a total of Young's modulus in the longitudinal direction and Young's modulus in the width direction of 1300 kg / mm ² or more, more preferably 1400 kg / mm ² or more and 4000 kg / mm ² or less. .

The form of the nonmagnetic support 1 is not particularly limited, and may be any of tape, film, sheet, card, disk, drum, and the like.
The film thickness of the nonmagnetic support 1 is, for example, usually 3 to 100 μm, preferably 5 to 50 μm in the case of a film or sheet, and 30 μm to 10 mm in the case of a disk or card. The degree is preferable, and when it is a drum shape, it is appropriately selected according to the recorder or the like.

The nonmagnetic support 1 may have a single layer structure or a multilayer structure.
Further, the nonmagnetic support 1 may be subjected to a surface treatment such as a corona discharge treatment or may be provided with an arbitrary undercoat layer (not shown).

Next, the nonmagnetic lower layer 2 will be described.
The nonmagnetic lower layer 2 can be formed by applying a coating material prepared by dispersing nonmagnetic powder in a binder.

The nonmagnetic powder applied to the nonmagnetic lower layer 2 is preferably a material having a crystallite size of 10 to 100 nm and a Mohs hardness of 5 or more.
As the non-magnetic powder, a known non-magnetic powder used for a multilayer coating type magnetic recording medium can be appropriately used.
The crystallite size of the nonmagnetic powder is preferably 15 to 80 nm, more preferably 20 to 60 nm.
Specific materials for the nonmagnetic powder include, for example, titanium oxide, boron nitride acid, 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, garnet, garnet, silica, silicon nitride, silicon carbide and the like.
In particular, inorganic powders such as titanium oxide, barium sulfate, α-Al ₂ O ₃ , α-Fe ₂ O ₃ , α-FeOOH, and Cr ₂ O ₃ are preferable, and α-Fe ₂ O ₃ and α-FeOOH are particularly preferable. Α-Fe ₂ O ₃ is most preferable.
In addition, non-magnetic powder having a needle shape can also be used as appropriate.
When the needle-shaped nonmagnetic powder is used, the surface smoothness of the nonmagnetic lower layer 2 can be improved, and the surface smoothness of the uppermost layer composed of the magnetic layer 3 laminated thereon can be improved. Can do.

Here, the non-magnetic lower layer 2 means a layer that is completely non-magnetic (saturation magnetic flux density Bm is 0) and a substantially non-magnetic layer (a layer that is slightly magnetic, Is 0.01 to 100 gauss).
In particular, when needle-shaped α-Fe ₂ O ₃ is used as the filler in the layer, the Bm of the layer is usually about 0.01 to 100 gauss, but in this case as well, it is the nonmagnetic lower layer 2 And

The film thickness of the nonmagnetic lower layer 2 is preferably 0.2 to 2.5 μm, and more preferably 0.5 to 2.0 μm.
When the film thickness of the nonmagnetic lower layer 2 is 2.5 μm or less, a predetermined roughness can be formed on the surface of the magnetic layer in the multilayer coating, and preferable electromagnetic conversion characteristics can be obtained. On the other hand, when the film thickness of the nonmagnetic lower layer is 0.2 μm or more, high smoothness can be obtained during the calendar process, and the electromagnetic conversion characteristics can be improved.

  The shape and axial ratio of the nonmagnetic powder can be controlled by combining known methods such as selection of the starting material as a starting material, selection of oxidation-reduction conditions, selection of a sintering inhibitor, and the like.

In the nonmagnetic powder in the nonmagnetic lower layer 2, the major axis diameter of the needle-like powder or the number average particle diameter of the nonmagnetic needle-like nonmagnetic powder is preferably 20 nm or more and 250 nm or less, more preferably 220 nm or less, In particular, the thickness is desirably 200 nm or less.
The minor axis diameter of the acicular powder is preferably 10 nm or more and 100 nm or less, more preferably 80 nm or less, and particularly preferably 60 nm or less.

The axial ratio of the acicular nonmagnetic powder is preferably 2 to 20, more preferably 5 to 15, and particularly preferably 5 to 10.
The axial ratio here refers to the ratio of the major axis diameter to the minor axis diameter (major axis diameter / minor axis diameter).

The specific surface area of the nonmagnetic powder is preferably 10~250m ² / g, more 20~150m ² / g, it is particularly preferably 30 to 100 m ² / g.

  When a nonmagnetic powder having a major axis diameter, a minor axis diameter, an axial ratio, and a specific surface area in the above numerical range is used, the surface property of the nonmagnetic lower layer 2 can be improved, and the uppermost layer which is the magnetic layer 3 The surface property can also be made good, and the effect of improving the running property can be obtained.

The nonmagnetic powder is preferably surface-treated with a Si compound or an Al compound. When the nonmagnetic powder subjected to such surface treatment is used, the surface state of the upper magnetic layer 3 can be further improved.
In this case, the content of Si or Al is preferably 0.1 to 10% by weight of Si and 0.1 to 10% by weight of Al with respect to the nonmagnetic powder, more preferably Si. Is 0.1 to 5% by weight, Al is 0.1 to 5% by weight, Si is preferably 0.1 to 2% by weight, and Al is preferably 0.1 to 2% by weight.
The weight ratio of Si and Al is preferably Si <Al.
As for the surface treatment technology of Si and Al, the method described in JP-A-2-83219 can be applied.

The content of the nonmagnetic powder in the nonmagnetic lower layer 2 is 50 to 99% by weight, preferably 60 to 95% by weight, particularly preferably 70 to 95%, based on the total of all components constituting the nonmagnetic lower layer 2. It shall be weight%.
By selecting the content of the nonmagnetic powder within the above numerical range, the surface state of the magnetic layer 3 and the nonmagnetic lower layer 2 can be improved, and excellent electromagnetic conversion characteristics and running stability can be obtained.

Next, the magnetic layer 3 will be described.
The magnetic layer 3 is composed of magnetic powder, binder, alumina, conductive fine powder, lubricant, and other additives such as antistatic materials, dispersants, curing agents, and the like. It is formed by coating.
In the present invention, in particular, among the various components described above, at least magnetic powder, a binder, and alumina are mixed to produce a first paint, and separately from this, alumina, a binder, a solvent, Are mixed to produce a second paint, and the second paint is added to the first paint to produce a magnetic paint, and this magnetic paint is applied onto the nonmagnetic lower layer 2 to thereby form a magnetic layer. It has the feature in forming.

  Each content of alumina in the first paint and alumina in the second paint is 10 parts by weight or more and 20 parts by weight or less with respect to 100 parts by weight of the magnetic powder to be applied. Is preferred.

As the magnetic powder, so-called ferromagnetic metal powder is preferably applied.
The thickness of the magnetic layer 3 is preferably 0.05 to 0.3 μm, and more preferably 0.05 to 0.2 μm.

  Examples of the magnetic powder include Fe, Co, Fe—Al, Fe—Al—Ni, Fe—Al—Zn, Fe—Al—Co, Fe—Al—Ca, Fe—Ni, Fe -Ni-Al, Fe-Ni-Co, Fe-Ni-Si-Al-Mn, Fe-Ni-Si-Al-Zn, Fe-Al-Si, Fe-Ni-Zn, Fe A ferromagnetic metal powder mainly composed of Ni-Mn, Fe-Ni-Si, Fe-Mn-Zn, Fe-Co-Ni-P, Ni-Co, Fe, Ni, Co, etc. Can be mentioned. In particular, Fe-based metal powder has excellent electrical characteristics.

  From the viewpoint of corrosion resistance and dispersibility, Fe-Al, Fe-Al-Ca, Fe-Al-Ni, Fe-Al-Zn, Fe-Al-Co, Fe-Ni-Si-Al Fe-Al-based metal powders such as -Zn-based and Fe-Ni-Si-Al-Mn-based materials are suitable.

  In particular, the magnetic powder suitable for the present invention includes those containing iron as a main component, and Al, or Al and Ca, and for Al, Fe: Al = 100: 0.5 to 100 by weight ratio. : 20 and Ca are preferably contained in a weight ratio of Fe: Ca = 100: 0.1 to 100: 10.

By setting the ratio of Fe: Al within the above numerical range, an effect of improving the corrosion resistance can be obtained.
Further, by setting the Fe: Ca ratio within the above numerical range, electromagnetic conversion characteristics are improved and dropout is reduced.
The improvement in electromagnetic conversion characteristics and the effect of reducing dropout are thought to be due to an increase in coercivity due to an improvement in dispersibility, a decrease in aggregates, and the like.

The magnetic powder has an average major axis length of less than 0.15 μm, particularly 0.03 to 0.15 μm, more preferably 0.05 to 0.12 μm, and an X-ray particle size (crystallite size) of less than 20 nm, In particular, the thickness is preferably 5 to 17 nm.
The axial ratio (average major axis length / average minor axis length) is 12 or less, preferably 10 or less, and more preferably 4 to 9. These can be measured with a transmission electron microscope.

  By selecting the average major axis length, crystallite size, and axial ratio of the magnetic powder within the above numerical range, it is possible to improve the high frequency characteristics, particularly the output of the perpendicular recording component.

The average major axis length (in the case of acicular particles) and the number average particle diameter (in the case of spherical particles) of the magnetic powder and the nonmagnetic powder in the nonmagnetic lower layer 2 described above are determined by transmission electron micrographs. It is an average value obtained by measuring 500 major axis lengths or diameters (in the case of spherical particles) of powder or nonmagnetic powder.
The crystallite size was measured by the Scherrer method based on Si powder using an integral width of the (110) diffraction line of Fe with an X-ray diffractometer.

The method described in the X-ray diffraction manual (Rigaku Denki Co., Ltd.) is applied for the method of determination. It calculated | required by the correction | amendment (integral width) by Jones.
The axial ratio was obtained as a ratio of (average major axis length / average minor axis length) by measuring the average major axis length and the average minor axis length of 500 particles in an electron micrograph.

The magnetic powder preferably has a coercive force Hc in the range of usually 1000 to 5000 Oe (7.96 × 10 to 3.98 × 10 ² kA / m).
When the coercive force Hc is less than 1000 Oe (7.96 × 10 kA / m), the electromagnetic conversion characteristics may be deteriorated, and when the coercive force exceeds 5000 Oe (3.98 × 10 ² kA / m), This is not desirable because the magnetic head may become unrecordable.

The magnetic powder preferably has a saturation magnetization (σs), which is a magnetic property, of usually 120 emu / g or more, and particularly preferably 130 to 220 emu / g.
Furthermore, the specific surface area according to the BET method is preferably 30 m ² / g or more, particularly 45 m ² / g or more, depending on the recording density.

The specific surface area and the measuring method are described in detail in “Measurement of powder” (J. M. Dallavelle, Cleyorr Jr., co-authored by Hirota et al., Published by Sangyo Toshosha), and “Chemical Handbook” application Ed P1170-1171 (The Chemical Society of Japan: Maruzen Co., Ltd., issued April 30, 1966).
The specific surface area is measured by, for example, removing the powder adsorbed on the powder by heat treatment at around 105 ° C. for 13 minutes, and then introducing the powder into a measuring apparatus to reduce the initial pressure of nitrogen to 0. Set to 5 kg / m ² and measure with nitrogen at liquid nitrogen temperature (−105 ° C.) for 10 minutes.
As the measuring device, for example, a counter soap (manufactured by Yuasa Ionics Co., Ltd.) can be applied.

The magnetic powder preferably contains at least one of Fe, Al, and rare earth elements as its constituent elements.
That is, it is preferable to contain one or more rare earth elements selected from the group consisting of Sm, Nd, Y, Pr, and La.

In the magnetic powder, the abundance ratio of Fe, Al, and one or more rare earth elements selected from the group consisting of Sm, Nd, Y, Pr, and La in the overall composition is Al atoms with respect to 100 parts by weight of Fe atoms. 1 to 20 parts by weight, preferably 1 to 16 parts by weight of the rare earth element (preferably one or more selected from the group consisting of Sm, Nd, Y, Pr and La).
In addition, the abundance ratio of Fe, Al, and preferably a rare earth element (preferably one or more selected from the group consisting of Sm, Nd, Y, Pr, and La) on the surface is Al atoms with respect to 100 Fe atoms. The number is 70 to 300, and the number of rare earth elements is preferably 0.5 to 100.

  More preferably, the magnetic powder further contains Na and Ca as constituent elements, and the weight ratio of elements in the magnetic powder as a whole is 0.1 part by weight of Na atoms with respect to 100 parts by weight of Fe atoms. Less than, 0.1 to 2 parts by weight of Ca atoms, 2 to 10 parts by weight of Al atoms, 1 to 8 parts by weight of rare earth elements, and elements forming the surface of the magnetic powder. The average abundance ratio is 2-30 for the number of Fe atoms, 5-30 for the Ca atom, 70-200 for the Al atom, and 0 for the rare earth element. It is preferable that it is 5-30.

  Furthermore, the magnetic powder contains at least one of Co, Ni, and Si as its constituent elements, and the weight ratio of these elements in the entire magnetic powder is Co atoms with respect to 100 parts by weight of Fe atoms. Is 2 to 40 parts by weight, Ni atoms are 2 to 20 parts by weight, Si atoms are 0.3 to 5 parts by weight, Na atoms are less than 0.1 parts by weight, and Ca atoms are 0.8 parts by weight. 1 to 2 parts by weight, Al atoms are 1 to 20 parts by weight, rare earth elements are 1 to 16 parts by weight, and the average abundance of elements forming the surface of the magnetic powder is Fe atoms 100, Co atom number is less than 0.1, Ni atom number is less than 0.1, Si atom number is 20-130, Na atom number is 2-30, Ca atom number Is 5 to 30, Al atom number is 70 to 300, rare earth element The number of atoms is preferably 0.5 to 100.

The magnetic layer 3 preferably contains a conductive fine powder. For example, carbon black is a suitable example.
Carbon black having a number average particle diameter of 30 nm or more is preferable.
The conductive fine powder is a powder having an average particle size of 30 to 500 nm, preferably 45 to 300 nm, and preferably 0.1 to 2.0 wt with respect to 100 wt of the magnetic powder.

  In addition, when the nonmagnetic lower layer 2 contains conductive fine particles, an average particle diameter of 5 to 50 nm, preferably 10 to 40 nm, more preferably 10 to 30 nm is applied, and the filler 100 wt of the layer is used. On the other hand, it is preferably contained in an amount of 1.0 to 50 wt, preferably 3.0 to 30 wt, more preferably 5.0 to 20 wt.

Conductive fine powder includes carbon black, SnO ₂ , silver powder, silver oxide, silver nitrate, silver organic compounds, metal particles such as copper powder, and pigments such as zinc oxide, barium sulfate, titanium oxide, and other metal oxides. Are coated with a conductive material such as a tin oxide coating or an antimony solid solution tin oxide coating, and it is particularly preferable to use a pigment coated with a conductive material.

When carbon black is contained in the magnetic layer 3, if the average particle diameter is larger than 500 nm, the electromagnetic conversion characteristics deteriorate, so it is necessary to select the average particle diameter to be 500 nm or less.
Specific examples of the carbon black include Asahi # 60 (51 nm), Asahi # 55 (77 nm), Asahi Thermal (90 nm), Asahi # 50 (94 nm), Asahi # 35 (115 nm), Mitsubishi, manufactured by Asahi Carbon Black. Diablack G (84 nm) manufactured by Kasei Co., Ltd. Regal (RFAL) SRF-S (60 nm) manufactured by Cabot, Stirling NS (75 nm), HS100 (53 nm) manufactured by Denki Kagaku, MT manufactured by Seba Carbon -CI (270 nm) etc. are mentioned.

  When carbon black is contained in the nonmagnetic lower layer 2, for example, Seagull 600 (23 nm), Seast 6H (24 nm), Seast 6H (28 nm), Seast 116 (30 nm) manufactured by Tokai Electrode Co., Ltd., Asahi Carbon Black Asahi # 80 (23 nm), Colombian Carbon Corporation Conductex SC (17 nm), Conductex 975 (20 nm), Conductex 40-220 (20 nm), Mitsubishi Kasei Dia Black A (18 nm), Dia Black I ( 21 nm), Dia Black H (30 nm), Showa Denko Show Black O (30 nm), Cabot Monarch (MONARCH) 1300 (13 nm), Regal (REGAL) 400 (25 nm), VULCAN XC-72 ( 30nm), Vulcan (20nm), Balkan 9 (19nm), Black Pearls 2000 (15nm), and the like. These may be used alone or in combination of two or more.

As a method for adding carbon black, various conventionally known methods can be applied.
For example, carbon black fine particles and coarse particles may be simultaneously charged into a disperser and mixed, or only a part of them may be charged first, and the remaining amount may be charged when the dispersion has progressed to some extent.
When carbon black dispersion is particularly important, carbon black is kneaded with a magnetic powder or filler and binder by a three roll mill, a Banbury mixer, etc., and then dispersed by a disperser to obtain a paint.
In the nonmagnetic lower layer 2, when more importance is attached to the conductivity, if the carbon black is added as much as possible in the latter half of the dispersion process and the liquid preparation process, the structure structure of the carbon black is not easily cut. A so-called “carbon masterbatch” in which carbon black is previously kneaded with a binder may be used.

The particle size of carbon black can be directly measured visually with an electron microscope.
That is, it can be measured by cutting a magnetic recording medium, such as a tape, in the longitudinal direction to a thickness of about 700 mm and observing the obtained cross section with a transmission electron microscope (applied voltage 200 KV, magnification = 60000).
In this case, the diameter of each particle of carbon black is measured, and the number average particle diameter of N = 100 is defined as “number average particle diameter”.

Next, the binder (binder) in the nonmagnetic lower layer 2 and the magnetic layer 3 will be described.
As the binder, for example, polyurethane, polyester, vinyl chloride resin, phenoxy resin, cellulose resins and the like as a typical, these _{resins, -SO 3 M, -OSO 3 M} , - It preferably contains at least one polar group selected from COOM, —PO (OM ₁ ) ₂ and —OPO (OM ₁ ) ₂ .
In the above polar group, M is a hydrogen atom or an alkali metal such as Na, K, Li, and M ₁ represents a hydrogen atom, an alkali metal atom such as Na, K, Li, or an alkyl group.

The said polar group has the effect | action which improves the dispersibility of magnetic powder, and the content rate in each binder resin is 0.1-8.0 mol%, Preferably it is 0.5-6.0 mol%. When the content is less than 0.1 mol%, the dispersibility of the ferromagnetic powder is lowered, and when the content exceeds 8.0 mol%, the magnetic coating is easily gelled.
The weight average molecular weight of each resin is preferably in the range of 15,000 to 50,000.

The content of the binder in the magnetic layer is 10 to 40 parts by weight, preferably 15 to 30 parts by weight with respect to 100 parts by weight of the magnetic powder.
Only one type of binder may be applied, or two or more types may be used in combination.
For example, when a mixture of polyurethane and / or polyester and vinyl chloride resin is applied, the ratio of polyurethane and / or polyester to vinyl chloride resin is 90:10 to 10:90 by weight, Preferably it shall be the range of 70: 30-30: 70.

Specifically, when a polar group-containing vinyl chloride copolymer is used as a binder, 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 shown below It can synthesize | combine by addition reaction.
_{Cl-CH 2 CH 2 SO 3} M, Cl-CH 2 CH 2 OSO 3 M, Cl-CH 2 COOM, Cl-CH 2 -P (= O) (OM1) 2
From these compounds, Cl—CH ₂ CH ₂ SO ₃ Na is taken as an example to explain the above reaction.
_{-CH 2 C (OH) H- +} ClCH 2 CH 2 SO 3 Na → -CH 2 C (OCH 2 CH 2 SO 3 Na) H-.

  In addition, the polar group-containing vinyl chloride copolymer is charged with a predetermined amount of a reactive monomer having an unsaturated bond into which a repeating unit containing a polar group is introduced into a reaction vessel such as an autoclave, and a general polymerization initiator, for example, It can be obtained by carrying out a polymerization reaction using a radical polymerization initiator such as BPO (benzoyl peroxide) or AIBN (azobisisobutyronitrile), a redox polymerization initiator, a cationic polymerization initiator or the like.

  Specific examples of the reactive monomer for introducing the sulfonic acid or a salt thereof include unsaturated hydrocarbon sulfonic acids such as vinyl sulfonic acid, allyl sulfonic acid, methacryl sulfonic acid, p-styrene sulfonic acid, and salts thereof. Is mentioned.

  When introducing carboxylic acid or a salt thereof, for example, (meth) acrylic acid or maleic acid is used, and when introducing phosphoric acid or a salt thereof, for example, (meth) acrylic acid-2-phosphate is used. That's fine.

It is preferable that an epoxy group is introduced into the vinyl chloride copolymer. This improves the thermal stability of the polymer.
In the case of introducing an epoxy group, the content of the repeating unit having an epoxy group in the copolymer is preferably 1 to 30 mol%, more preferably 1 to 20 mol%. As a monomer for introducing an epoxy group, for example, crisidyl acrylate can be mentioned as a suitable example.

  Regarding the technology for introducing polar groups into the vinyl chloride copolymer, JP-A-57-44227, 58-108052, 59-8127, 60-101161, 60 No. 235814, No. 60-238306, No. 60-238371, No. 62-121923, No. 62-146432, No. 62-146433, and the like. It can be used as appropriate.

Next, the synthesis of polyester and polyurethane, which are suitable examples of the binder resin, will be described.
In general, polyesters are obtained by reacting polyols with polybasic acids. Using this known method, a polyester (polyol) having a polar group can be synthesized from a polyol and a polybasic acid partially having a polar group.

  Examples of the polybasic acid having a polar group include 5-sulfoisophthalic acid, 2-sulfoisophthalic acid, 4-sulfoisophthalic acid, 3-sulfophthalic acid, dialkyl 5-sulfoisophthalate, dialkyl 2-sulfoisophthalate, 4 -Dialkyl sulfoisophthalate, dialkyl 3-sulfoisophthalate, and sodium and potassium salts thereof.

  Examples of polyols include trimethylolpropane, hexanetriol, glycerin, trimethylolethane, neopentyl glycol, pentaerythritol, ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, 1,6- Hexanediol, diethylene glycol, cyclohexanedimethanol and the like can be mentioned. Polyesters with other polar groups introduced can also be synthesized by known methods.

Next, polyurethane will be described. This can be obtained by reaction of a polyol with a polyisocyanate.
As the polyol, a polyester polyol obtained by a reaction between a polyol and a polybasic acid is generally used. Therefore, if a polyester polyol having a polar group is used as a raw material, a polyurethane having a polar group can be synthesized. For example, it is preferable to apply an aromatic polyester polyurethane made using a polyester polyol having an aromatic ring and / or a polyester polyol containing a cyclic hydrocarbon residue.

Moreover, it is preferable to use polyisocyanate as a hardening | curing agent contained in a magnetic layer.
Examples of polyisocyanates include diphenylmethane-4,4′-diisocyanate (MDI), hexamethylene diisocyanate (HMDI), tolylene diisocyanate (TDI), 1,5-naphthalene diisocyanate (NDI), tolidine diisocyanate (TODI), lysine An isocyanate methyl ester (LDI) etc. are mentioned.

As another method for synthesizing a polyurethane having a polar group, an addition reaction between a polyurethane having a hydroxyl group and the following compound having a polar group and a chlorine atom is also effective.
_{Cl-CH 2 CH 2 SO 3} M, Cl-CH 2 CH 2 OSO 2 M, Cl-CH 2 COOM, Cl-CH 2 -P (= O) (OM 1) 2.
As techniques for introducing polar groups into polyurethane, JP-B-58-41565, JP-A-57-92422, JP-A-57-92423, JP-A-59-8127, JP-A-59-5423 are disclosed. 59-5424, 62-121923, etc., and any of these methods can be applied.

Moreover, as binder resin, the resin shown below can be used together by the usage-amount of 20 weight% or less in all the binders.
For example, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, butadiene-acrylonitrile copolymer, polyamide resin, polyvinyl butyral having a weight average molecular weight of 10,000-200000 , Cellulose derivatives (nitrocellulose, etc.), styrene-butadiene copolymers, phenol resins, epoxy resins, urea resins, melamine resins, phenoxy resins, silicone resins, acrylic resins, urea formamide resins, various synthetic rubber resins, etc. Can be mentioned.

Next, other components constituting the magnetic layer 3 and the nonmagnetic lower layer 2 will be described.
In order to improve the durability of the magnetic layer 3 and the nonmagnetic lower layer 2, it is desirable to contain polyisocyanate.
Examples of the polyisocyanate include aliphatic polyisocyanates such as adducts of aromatic diisocyanate and hexamethylene diisocyanate (HMDI) and active hydrogen compounds, such as adducts of tolylene diisocyanate (TDI) and active hydrogen compounds. Isocyanate.
The weight average molecular weight of the polyisocyanate is preferably in the range of 100 to 3000.

Further, a lubricant may be contained in the magnetic layer 3 or the nonmagnetic lower layer 2, and a lubricant layer may be formed on the magnetic layer 3.
As the lubricant, fatty acids and / or fatty acid esters can be used.
In this case, the amount of fatty acid added is preferably 0.2 to 10% by weight and more preferably 0.5 to 5% by weight with respect to the magnetic powder and nonmagnetic powder which are the main components in the layer.
If the added amount is less than 0.2% by weight, the runnability is likely to deteriorate, and if it exceeds 10% by weight, the fatty acid oozes out to the surface of the magnetic layer or the output is likely to decrease. .
Moreover, also in the case of fatty acid ester, the addition amount is preferably 0.2 to 10% by weight and more preferably 0.5 to 5% by weight with respect to the magnetic powder and nonmagnetic powder which are the main components in the layer.
If the addition amount is less than 0.2% by weight, the still durability is likely to deteriorate, and if it exceeds 10% by weight, the fatty acid ester is likely to ooze out on the surface of the magnetic layer or the output is likely to be reduced. It is.

When fatty acid and fatty acid ester are used in combination to further enhance the lubrication effect, the weight ratio of fatty acid and fatty acid ester is preferably 10:90 to 90:10.
The fatty acid may be a monobasic acid or a dibasic acid, and the carbon number is preferably 6 to 30, and more preferably 12 to 22.

  Specific examples of fatty acids include caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, isostearic acid, linolenic acid, oleic acid, elaidic acid, behenic acid, malonic acid, succinic acid, maleic acid Examples include acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, sebacic acid, 1,12-dodecanedicarboxylic acid, octanedicarboxylic acid and the like.

  The fatty acid preferably contains a melting point of 50 ° C. or higher. Specific examples of fatty acids having a melting point of 50 ° C. or higher include myristic acid, palmitic acid, pentadecylic acid, heptadecylic acid, stearic acid, behenic acid, arachidic acid, lignoceric acid, serotic acid, heptacosanoic acid, montanic acid, melicic acid, and laxelic acid Etc. Stearic acid is particularly preferable.

  Specific examples of fatty acid esters include oleyl oleate, isocetyl stearate, dioleyl malate, butyl stearate, butyl palmitate, butyl myristate, octyl myristate, octyl palmitate, pentyl stearate, pentyl palmitate, isobutyl Oleate, stearyl stearate, lauryl oleate, octyl oleate, isobutyl oleate, ethyl oleate, isotridecyl oleate, 2-ethylhexyl stearate, 2-ethylhexyl palmitate, isopropyl palmitate, isopropyl myristate, butyl Laurate, cetyl-2-ethyl hexalate, dioleyl adipate, diethyl adipate, diisobutyl adipate, diisodecyl adipate, oleyl stearate DOO, 2-ethylhexyl myristate, isopentyl palmitate, isopentyl stearate, diethylene glycol - mono - ether palmitate, diethylene - mono - ether palmitate and the like.

  Moreover, it is preferable to make the nonmagnetic lower layer 2 contain a fatty acid ester consisting of an unsaturated fatty acid and an unsaturated alcohol, or a glycerin ester. Preferable examples of the fatty acid ester include oleyl oleate, and a preferable example of the glycerin ester includes glycerin trioleate.

Examples of the unsaturated fatty acid component in the ester of the unsaturated fatty acid and the unsaturated alcohol include oleic acid, elaidic acid, linoleic acid, linolenic acid, and the like. Among them, oleic acid is most preferable. It is done.
Moreover, oleyl alcohol etc. are mentioned as an unsaturated alcohol component.
Specific examples of esters of these unsaturated fatty acid components and unsaturated alcohol components include oleyl oleate, oleyl elaidate, oleyl linoleate, oleyl linolenate, and the like.

Suitable examples of glycerin esters are listed below.
(1) Esters of glycerin and palmitic acid (16 carbon atoms) (however, the ester may be any of monoester, diester and triester (the same applies hereinafter)).
(2) Esters of glycerin and stearic acid (18 carbon atoms).
(3) Esters of glycerin and oleic acid (18 carbon atoms and one unsaturated carbon-carbon double bond).
(4) Esters of glycerin and linoleic acid (18 carbon atoms and two unsaturated carbon-carbon double bonds).
(5) Olive oil (natural product, a mixture of various fatty acid esters).
(6) Esters of glycerin and lauric acid (10 carbon atoms).
(7) Esters of glycerin and myristic acid (14 carbon atoms).
(8) Esters of glycerin and palmitic acid (16 carbon atoms).
(9) Esters of glycerin and isostearic acid (18 carbon atoms).
(10) Esters of glycerin and behenic acid (22 carbon atoms).
(11) 2-ethylhexanoic acid triglyceride.
(12) Behenic acid monoglyceride.
(13) Stearic acid monodiglyceride oleate.
(14) Diacetyl capric acid glyceride.
(15) Diacetyl palm fatty acid glyceride.
(16) Acetyl stearic acid glyceride.
(17) Diacetyl capric acid glyceride.
(18) Diacetyl palm fatty acid glyceride.
(19) Caprylic acid monodiglyceride.
(20) Acetyl stearic acid glyceride.
(21) Caprylic acid triglyceride.
(22) Fatty acid (C8, C10) triglyceride.
Among the glycerin esters shown above, they may be used alone or in combination of two or more. In addition to glycerin esters, other polyhydric alcohol esters such as sorbitan may be used in combination.

Further, in the nonmagnetic lower layer 2, as a lubricant, R ₁ C═OO (CHR ₂ CHR ₃ O) _n R ₄ (R ₁ is a linear or branched hydrocarbon group having 11 to 22 carbon atoms, R ₂ and R ₃ preferably contain H or CH ₃ , 1 ≦ n ≦ 10, and R ₄ contains a saturated or unsaturated hydrocarbon group having 1 to 22 carbon atoms.

Furthermore, R ₅ OC = OR ₆ (wherein R ₅ is a linear or branched hydrocarbon group having 1 to 18 carbon atoms, R ₆ is a linear or branched hydrocarbon group having 11 to 22 carbon atoms) as a fatty acid ester Is preferably contained.
Thus, by containing several different fatty acid esters and glycerin esters in the nonmagnetic lower layer 2, the magnetic layer 3 is appropriately replenished with a lubricant, and a stable lubricating action in a wide range of environments from high to low temperatures. Is exerted, and the durability is remarkably improved.
In addition to the fatty acid ester and glycerin ester, the nonmagnetic lower layer 2 preferably contains a plurality of fatty acids having different melting points from the viewpoint of improving durability.
The use of such a hybrid lubricant system that combines a number of different lubricants is important for the realization of high-density magnetic recording media with much higher density, higher durability, and improved error rate. Technology.

  Examples of the lubricant other than the fatty acid and fatty acid ester include silicone oil, graphite, carbon fluoride, molybdenum disulfide, tungsten disulfide, fatty acid amide, α-olefin oxide, and the like.

  Further, the magnetic layer 3 and the nonmagnetic lower layer 2 contain various additives such as a dispersant, a lubricant, an abrasive, an antistatic agent, and a filler as necessary.

Examples of the dispersant include those described in paragraph No. 0093 in JP-A-4-214218.
Usually, the dispersant is preferably contained in the range of 0.5 to 5% by weight with respect to the magnetic powder.

Specific examples of the abrasive include α-alumina and fused alumina.
The number average particle size of the abrasive is preferably 0.05 to 0.6 μm, more preferably 0.1 to 0.3 μm.

The nonmagnetic lower layer 2 and the magnetic layer 3 preferably contain α-alumina.
In particular, the nonmagnetic lower layer 2 preferably contains two or more kinds of nonmagnetic powders having a Mohs hardness of 5 or more.

Antistatic agents include conductive powders such as carbon black and graphite, cationic surfactants such as quaternary amines, and anionic surfactants containing acid groups such as sulfonic acid, sulfuric acid, phosphoric acid, phosphate ester, and carboxylic acid Agents, amphoteric surfactants such as aminosulfonic acid, and natural surfactants such as saponin.
The antistatic agent is preferably added in an amount of 0.01 to 40% by weight based on the binder.

(Magnetic recording medium manufacturing process)
Next, the manufacturing process of the magnetic recording medium 10 will be described below.
The magnetic recording medium 10 is preferably formed by a so-called wet-on-wet method in which the magnetic layer 3 is laminated when the nonmagnetic lower layer 2 is in a wet state.
As this wet-on-wet method, a method used for manufacturing a known multi-layer type magnetic recording medium can be appropriately employed.

In the formation of the magnetic layer 3, a magnetic powder, a binder, and alumina are mixed to produce a first paint, and an alumina and a binder are mixed using a solvent to produce a second paint. A magnetic paint is prepared by adding a second paint to one paint, and this magnetic paint is applied on the nonmagnetic lower layer 2 to form the magnetic layer 3.
Here, the respective contents of alumina in the first paint and alumina in the second paint are 10 parts by weight or more and 20 parts by weight or less with respect to 100 parts by weight of the magnetic powder. Is preferred.

  In addition, a dispersant, a lubricant, an abrasive, an antistatic agent, and the like are appropriately mixed and adjusted. In addition, as a solvent for adjustment, what is described in paragraph number 0119 in Unexamined-Japanese-Patent No. 4-214218 is applicable, for example.

In kneading and dispersing the paint, various kneading and dispersing machines can be used.
As this kneading and dispersing machine, for example, the one described in paragraph No. 0112 in JP-A-4-214218 can be applied.
For example, as a kneading and dispersing machine capable of providing a power consumption load of 0.05 to 0.5 KW (per 1 kg of magnetic powder), a pressure kneader, an open kneader, a continuous kneader, a two-roll mill, and a three-roll mill can be mentioned. .

The coating process will be specifically described.
First, each coating material for forming the magnetic layer 3 and the nonmagnetic lower layer 2 is applied to the film-like nonmagnetic support 1 fed from the supply roll by an extrusion extrusion coater in a wet-on-wet manner. Then, it is passed through a magnet for orientation or a magnet for oblique orientation, introduced into a dryer, and here, hot air is blown from nozzles arranged up and down to perform a drying treatment.
The dried non-magnetic support 1 with each coating layer is guided to a super calender apparatus comprising a combination of calendar rolls 38, and after being calendered here, it is wound on a winding roll.

  The magnetic film thus obtained can be cut into a tape having a desired width to produce, for example, a magnetic recording tape for an 8 mm video camera.

Each paint may be supplied to the extrusion coater through an in-line mixer.
As the wet-on-wet multilayer coating method, a combination of a reverse roll and an extrusion coater, a combination of a gravure roll and an extrusion coater, or the like can be applied.
Furthermore, 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, and the like can be combined.

In the multi-layer coating by the wet-on-wet method, the upper layer is applied while the lower layer is in a wet state, so that the surface of the lower layer (that is, the upper layer and the boundary surface) is smooth and the surface property of the upper layer is good. In addition, the contact between the upper and lower layers is improved.
As a result, the film satisfies the performance requirements of, for example, magnetic tape, which requires high output and low noise, especially for high-density recording. The film strength is improved and the durability is sufficient.
Further, the drop-out can be reduced by the wet-on-wet multilayer coating method, and the reliability can be improved.

  Solvents for coating preparation or dilution solvents at the time of application include, for example, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, alcohols such as methanol, ethanol, propanol, butanol, methyl acetate, ethyl acetate, acetic acid Esters such as butyl, ethyl lactate, ethylene glycol cenoacetate, glycol dimethyl ether, glycol monoethyl ether, ethers such as dioxane, tetrahydrofuran, aromatic hydrocarbons such as benzene, toluene, xylene, methylene chloride, ethylene chloride, tetrachloride Examples thereof include halogenated hydrocarbons such as carbon, chloroform and dichlorobenzene. These solvents may be used alone or in combination of two or more.

  The magnetic field in the orientation magnet or the vertical orientation magnet is about 20 to 10000 gauss, the drying temperature by the dryer is about 30 to 120 ° C., and the drying time is about 0.1 to 10 minutes. .

The magnetic recording medium is preferably subjected to surface smoothing by calendering.
Thereafter, slitting is performed by performing burnishing or blade processing as necessary. In the surface smoothing treatment, examples of calendar conditions include temperature, linear pressure, and C / S (coating speed). It is preferable to maintain the temperature at 50 to 140 ° C., the linear pressure at 50 to 400 kg / cm, and the C / S at 20 to 1000 m / min.

Hereinafter, the present invention will be described with specific examples.
However, the components, ratios, and operation order shown below can be variously changed without departing from the scope of the present invention.
In the following description, all “parts” are parts by weight.

  First, the magnetic layer forming paint and the nonmagnetic underlayer forming paint having the following composition formulas are kneaded and dispersed using a kneader and a sand mill, respectively, and the dispersion is prepared. It was prepared by adding 5 parts of Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd.

  By wet-on-wet method, on the polyethylene terephthalate film having a film thickness of 6 μm, the samples of Experimental Examples (1 to 15) were applied by the combinations shown in Table 1 below, and the magnetic field orientation was performed while the coating film was undried. After the treatment, followed by drying, the surface was smoothed with a calendar to produce a magnetic tape original. Then, it slit to 12.65 mm width, and it accommodated in the cassette, and obtained the magnetic tape used as a sample.

[Experimental Example 1]
(Coating composition for magnetic layer formation)
Fe-Al ferromagnetic metal powder: 100 parts by weight (Fe: Co: Al: Y = 100: 20: 8: 5 (weight ratio), average major axis length: 100 nm, axial ratio: 5, Hc: 2390 Oe, σs : 150 emu / g, crystallite size: 12 nm)
Sulphonic acid metal salt-containing vinyl chloride resin (manufactured by Nippon Zeon Co., Ltd., MR-104): 10 parts by weight Sulphonic acid metal salt-containing aromatic polyester polyurethane resin: 10 parts by weight (Toyobo Co., Ltd., UR-8200: Containing cyclohexane ring)
Alumina (α-Al ₂ O ₃ , number average particle size: 0.18 μm): 10 parts by weight Carbon black (number average particle size: 300 nm): 0.8 parts by weight Stearic acid: 1 part by weight Myristic acid: 1 part by weight Butyl stearate: 1 part by weight cyclohexanone: 100 parts by weight Methyl ethyl ketone: 100 parts by weight Toluene: 100 parts by weight In the sample of Experimental Example 1, the alumina in the magnetic coating was only powdered.

(Nonmagnetic underlayer coating composition)
α-Fe ₂ O ₃ : 100 parts by weight (average major axis length: 150 nm, average minor axis length: 15 nm, needle ratio: 10, crystallite size: 18 nm, Si in weight ratio to α-Fe ₂ O ₃ 0.2%, containing 1.0% by weight of Al with respect to α-Fe ₂ O ₃ )
Carbon black (number average particle diameter 20 nm): 16 parts by weight sulfonic acid metal salt-containing vinyl chloride resin: 6 parts by weight (manufactured by Nippon Zeon Co., Ltd., MR-110)
Sulfonate metal salt-containing aromatic polyester polyurethane resin: 3 parts by weight (manufactured by Toyobo Co., Ltd., UR-8300: containing cyclohexane ring)
Alumina (α-Al ₂ O ₃ , number average particle size: 0.2 μm): 6 parts by weight Stearic acid: 1 part by weight Myristic acid: 1 part by weight Butyl stearate: 1 part by weight Cyclohexanone: 100 parts by weight Methyl ethyl ketone: 100 parts by weight Part toluene: 100 parts by weight

[Experimental example 2]
The content of alumina in the magnetic layer forming coating was 20 parts by weight.
Alumina in the magnetic coating was only powdered.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 3]
The content of alumina in the magnetic layer forming coating was 30 parts by weight.
Alumina in the magnetic coating was only powdered.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 4]
When preparing the coating for forming the magnetic layer, first, 10 parts by weight of alumina is added to the magnetic powder or binder to prepare the first coating, and in a separate process, the binder, solvent, etc. are used. A second coating material to which 5 parts by weight of alumina was added was prepared, and a magnetic coating material was prepared by adding the second coating material to the first coating material.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 5]
When preparing the coating material for forming the magnetic layer, first, 20 parts by weight of alumina is added to the magnetic powder or the binder to prepare the first coating material. A second coating material to which 5 parts by weight of alumina was added was prepared, and a magnetic coating material was prepared by adding the second coating material to the first coating material.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 6]
When preparing the coating for forming the magnetic layer, first, 30 parts by weight of alumina is added to the magnetic powder or binder to prepare the first coating, and in a separate process, the binder, solvent, etc. are used. A second coating material to which 5 parts by weight of alumina was added was prepared, and a magnetic coating material was prepared by adding the second coating material to the first coating material.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 7]
When preparing the coating for forming the magnetic layer, first, 10 parts by weight of alumina is added to the magnetic powder or binder to prepare the first coating, and in a separate process, the binder, solvent, etc. are used. A second coating material to which 10 parts by weight of alumina was added was prepared, and a magnetic coating material was prepared by adding the second coating material to the first coating material.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 8]
When preparing the coating material for forming the magnetic layer, first, 20 parts by weight of alumina is added to the magnetic powder or the binder to prepare the first coating material. A second coating material to which 10 parts by weight of alumina was added was prepared, and a magnetic coating material was prepared by adding the second coating material to the first coating material.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 9]
When preparing the coating for forming the magnetic layer, first, 30 parts by weight of alumina is added to the magnetic powder or binder to prepare the first coating, and in a separate process, the binder, solvent, etc. are used. A second coating material to which 10 parts by weight of alumina was added was prepared, and a magnetic coating material was prepared by adding the second coating material to the first coating material.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 10]
When preparing the coating for forming the magnetic layer, first, 10 parts by weight of alumina is added to the magnetic powder or binder to prepare the first coating, and in a separate process, the binder, solvent, etc. are used. A second coating material to which 20 parts by weight of alumina was added was prepared, and a magnetic coating material was prepared by adding the second coating material to the first coating material.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 11]
When preparing the coating material for forming the magnetic layer, first, 20 parts by weight of alumina is added to the magnetic powder or the binder to prepare the first coating material. A second coating material to which 20 parts by weight of alumina was added was prepared, and a magnetic coating material was prepared by adding the second coating material to the first coating material.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 12]
When preparing the coating for forming the magnetic layer, first, 30 parts by weight of alumina is added to the magnetic powder or binder to prepare the first coating, and in a separate process, the binder, solvent, etc. are used. A second coating material to which 20 parts by weight of alumina was added was prepared, and a magnetic coating material was prepared by adding the second coating material to the first coating material.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 13]
When preparing the coating material for forming the magnetic layer, first, the first coating material is prepared without adding alumina to the magnetic powder, the binder, etc., and in a separate process, 10 parts by weight in the binder, the solvent, etc. A second paint to which alumina was added was prepared, and a second paint was added to the first paint to prepare a magnetic paint.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experiment 14]
When preparing the coating material for forming the magnetic layer, first, the first coating material is prepared without adding alumina to the magnetic powder, the binder, etc., and in a separate process, 20 parts by weight in the binder, the solvent, etc. A second paint to which alumina was added was prepared, and a second paint was added to the first paint to prepare a magnetic paint.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

[Experimental Example 15]
When preparing the coating material for forming the magnetic layer, first, the first coating material is prepared without adding alumina to the magnetic powder or the binder, and 30 parts by weight in the binder, the solvent, etc. in a separate process. A second paint to which alumina was added was prepared, and a second paint was added to the first paint to prepare a magnetic paint.
Other conditions were the same as in Experimental Example 1, and a sample magnetic tape was produced.

  Various sample magnetic tapes produced as described above were subjected to characteristic measurement and evaluation according to the following items.

[Playback output]
The output (dB) was measured using a recording / reproducing apparatus for magnetic tape, DVW500.
With respect to the value of the comparative (Ref) magnetic tape, the evaluation was ○ if it was −1.0 dB or more, Δ if −1.0 dB> x ≧ −1.5 dB, and × if it was> −1.5 dB. .

[Headwear]
With respect to the magnetic head, the magnetic tape was run in the full length playback mode with the magnetic tape recording / reproducing apparatus DVW500, and the change in the protruding amount of the magnetic head before and after running was measured.
In addition, about the evaluation of headwear, it followed the U-28 specification 9.12 of the LTO (Linear Tape Open) of TPC (Technology Providers Company) and the Abrasivity measurement method.
The Abrasivity measurement method is an index indicating the amount of wear of the magnetic head due to the magnetic tape contacting the magnetic head and running. Specifically, a square bar made of ferrite material or the like is set in a dedicated jig, and the head part of a tape drive or the like so that the corner of this square pole contacts the magnetic tape. And the magnetic tape is run for a predetermined time or a predetermined number of passes, and after running, the width of the worn abrasity bar is measured, and this is defined as the wear amount.
In this experiment, the amount of protrusion was measured using a protrusion measuring instrument (manufactured by TOHODENSHI KOGYO Co., Ltd.). The measurements were taken at the time of the first pass and 100 pass travel, and evaluated according to the following criteria.
○: Wear amount is 30 μm or more and 65 μm or less.
Δ: Wear amount is 25 μm or more and less than 30 μm.
X: Wear amount is less than 25 μm.

  Table 1 below shows the electromagnetic conversion characteristic evaluation based on the measurement result of the reproduction output, and the evaluation result of the headwear during the first pass and 100 pass travel.

  In Experimental Examples 1 and 2, the alumina in the magnetic layer forming coating was used only in the powder charging step, but in these cases, good results were obtained for the electromagnetic conversion characteristics. However, sufficient polishing power for the magnetic head was not exhibited, and sufficient evaluation of the headwear could not be obtained both in the initial stage and at the time of running many times.

In Experimental Example 3, a method was adopted in which the alumina in the coating material for forming the magnetic layer was contained only in the powder charging step. In this case, the content was increased as compared with Experimental Examples 1 and 2, so that the magnetic head As for the polishing power against the head wear, the evaluation of the head wear was good.
However, since the alumina was increased, the relative packing density of the magnetic powder was lowered, leading to deterioration of electromagnetic conversion characteristics.

In Experimental Examples 4 and 5, alumina in the magnetic layer forming coating material was contained in the powder charging step and mixing (separate mixing step) in a separately prepared coating material.
In these cases, good results were obtained for the electromagnetic conversion characteristics, but the amount of alumina in the separate mixing process was not sufficient, so that sufficient polishing power for the magnetic head was not exhibited, and both initial and multiple times of running However, sufficient evaluation of headwear was not obtained.

In Experimental Example 6, alumina in the magnetic layer forming coating material was contained in the powder charging step and mixing (separate mixing step) in a separately prepared coating material.
In this case, since the amount of alumina in the powder input was increased as compared with Experimental Examples 4 and 5, the polishing power for the magnetic head was improved, and the evaluation of the head wear was good.
However, since the alumina was increased, the relative packing density of the magnetic powder was lowered, leading to deterioration of electromagnetic conversion characteristics.

In Experimental Examples 7 and 8, alumina in the magnetic layer-forming coating material was contained in the powder charging step and mixing in a separately prepared coating material (separate mixing step).
In these cases, since the amount of alumina in both steps was suitable, excellent evaluation was obtained in both electromagnetic conversion characteristics and headwear.
Comparing the results of Experimental Examples 2 and 3 with Experimental Examples 7 and 8, even though the amount of alumina as the whole magnetic layer does not change, the charging method should be divided into the powder charging process and the separate mixing process. It was revealed that a magnetic recording medium having excellent characteristics can be obtained.
However, from the results of Experimental Examples 4 and 5, even if the alumina charging method is divided into a powder charging process and a separate mixing process, the alumina content in each process is less than 10 parts by weight, It was found that sufficient headwear improvement effect could not be obtained.

In Experimental Example 9, alumina in the magnetic layer forming coating material was included in the powder charging step and mixing (separate mixing step) in the coating material prepared separately.
In this case, although the evaluation of the headwear was good, since the amount of alumina in the powder charging process was large, the relative packing density of the magnetic powder was lowered, leading to deterioration of electromagnetic conversion characteristics.

In Experimental Examples 10 and 11, alumina in the magnetic layer forming coating material was included in the powder charging step and mixing (separate mixing step) in a separately prepared coating material.
In these cases, since the amount of alumina in both steps was suitable, excellent evaluation was obtained in both electromagnetic conversion characteristics and headwear.
When the results of Experimental Example 9 and Experimental Example 11 are compared, even if the amount of alumina as the entire magnetic layer is not changed, the amount of alumina in each process is suppressed to 20 parts by weight or less, and excellent electromagnetic conversion characteristics are obtained. It became clear that it was obtained.

In Experimental Example 12, alumina contained in the magnetic layer forming coating material was contained in the powder charging step and mixing (separate mixing step) in the coating material prepared separately.
In this case, although the evaluation of the headwear was good, since the amount of alumina in the powder charging process was large, the relative packing density of the magnetic powder was lowered, leading to deterioration of electromagnetic conversion characteristics.
Also from this result, it has been clarified that excellent electromagnetic conversion characteristics can be obtained when the amount of alumina in each step is suppressed to 20 parts by weight or less.

In Experimental Examples 13 and 14, the alumina in the magnetic layer was not added to the powder, but was contained only in mixing (separate mixing step) in a separately prepared paint.
In these cases, good evaluation was obtained for the electromagnetic conversion characteristics.
However, sufficient polishing power for the magnetic head was not exhibited, and sufficient evaluation of the headwear could not be obtained both in the initial stage and at the time of running many times.

In Experimental Example 15, the alumina in the magnetic layer was not contained in the powder, but was contained only in the mixing (separate mixing step) in a separately prepared paint.
In this case, the electromagnetic conversion characteristics are good. However, with regard to the headwear, a good evaluation was not obtained for the headwear during a large number of runnings.
Comparing Experimental Examples 7 and 10 with Experimental Example 15, even though the amount of alumina in the entire magnetic layer was not changed, the powder charging process and the paint prepared separately were used for the alumina charging method. It was clarified that excellent characteristics can be obtained by dividing into mixing (separate mixing step) and suppressing the amount of alumina in each step to 20 parts by weight or less.

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

Explanation of symbols

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

Claims (2)

On a non-magnetic support,
A nonmagnetic lower layer in which at least a nonmagnetic powder is dispersed in a binder;
A magnetic layer in which at least magnetic powder is dispersed in a binder, and a method for producing a magnetic recording medium in which multiple layers are sequentially formed,
Mixing the magnetic powder, the binder and alumina to produce a first paint;
Producing a second paint mixed with alumina, a binder, and a solvent;
Adding the second paint to the first paint to produce a magnetic paint;
Applying the magnetic coating material onto the non-magnetic lower layer to form the magnetic layer. Each content of alumina in the first paint and alumina in the second paint is 10 parts by weight or more and 20 parts by weight or less with respect to 100 parts by weight of the magnetic powder. The method of manufacturing a magnetic recording medium according to claim 1, wherein:

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