JP2007265547A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium for high density recording which is nearly free from printing at the reverse side, has satisfactory head contact and has both high electromagnetic transducing characteristics and excellent running durability. <P>SOLUTION: The magnetic recording medium has a non-magnetic layer including a non-magnetic powder and a binder and a magnetic layer including ferromagnetic powder and a binder in this order on one surface of a non-magnetic supporting body and a back coat layer including carbon black and a binder on the other surface. The back coat layer has ultrafine indentation hardness lower than ultrafine indentation hardness of the magnetic layer in the range of 20 to 40 kg/mm<SP>2</SP>(196 to 392 MPa) at 80°C and 6 mgf load. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium suitable as an external recording medium for recording computer data.

  In recent years, the need for high-density recording has increased, and a magnetic recording medium having high electromagnetic conversion characteristics has been demanded. In addition, reliability is required when data is repeatedly used and stored. Therefore, in addition to excellent electromagnetic conversion characteristics, good running durability is also required for high-density recording magnetic recording media.

  Furthermore, in recent computer data storage tapes, it is necessary to reduce the tape thickness in order to increase the capacity. However, the thinned tape causes irregularities (jumping) of the tape winding surface, resulting in poor winding, and the tape edge of the protruding part is broken, or tape deformation called cinching or spokeing occurs. Such tape deformation also causes an increase in error rate.

  Further, with a thinned tape, the contact between the magnetic layer surface and the head becomes unstable, and the head contact becomes worse. Due to such deterioration per head, shallower dents in the magnetic layer also cause dropouts, resulting in increased error rates. This tendency is particularly remarkable in the state of high surface recording density (short recording wavelength, narrow track).

  In order to improve running durability, attempts are made to form protrusions on the backcoat layer surface by adding coarse carbon black having a particle size of 0.2 μm or more to the backcoat layer or providing protrusions on the base surface. Has been done. However, when the protrusions of the backcoat layer are formed by such a method, if the magnetic recording medium is stored in a roll state during the manufacturing process, or if the magnetic tape is stored in a state of being wound around a reel hub after commercialization, the protrusions of the backcoat layer are formed. A so-called “show-through” occurs, which is transferred onto the surface of the magnetic layer to form a minute dent. As a result, not only the electromagnetic conversion characteristics deteriorate, but also there is a disadvantage that the minute dropout increases and the error rate increases.

  In order to solve such a problem of “back-through”, attempts have been made to smooth the surface of the backcoat layer (see Patent Document 1). However, although the show-through is reduced by improving the smoothness of the backcoat layer, there is a problem that the winding shape becomes worse due to the smoothness of the backcoat, and errors increase after storage. In addition, attempts have been made to increase the hardness of the magnetic layer surface to suppress show-through, but the head contact deteriorates, leading to an increase in errors.

Patent Document 2 discloses a magnetic recording medium in which the amount of plastic deformation and indentation hardness of the magnetic layer are defined for the purpose of achieving both the running durability of the magnetic layer and the per head. However, in the case of a magnetic recording medium with a higher density, there has been a demand for better per-head securing. On the other hand, if the magnetic layer is made flexible for securing per head, there is also a problem that dropout due to the depression of the magnetic layer increases.
Japanese Patent Laid-Open No. 11-259851 JP 2002-237024 A

  Under such circumstances, the present invention has been made for the purpose of providing a magnetic recording medium for high-density recording having both high electromagnetic conversion characteristics with less show-through and good head contact and excellent running durability. It is.

As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge.
The show-through occurs when the back coat layer and the magnetic layer are in pressure contact when the magnetic recording medium is rolled up and stored or processed. In particular, during heat treatment (thermo treatment) after calendar treatment or during high temperature storage, the bulk core side is more tightly wound than the outside, so tension is increased and show-through tends to occur.
As described above, conventionally, it has been proposed to increase the hardness of the magnetic layer to prevent the show-through, but the hardness is a measured value at room temperature, which is the same as that of the medium at a high temperature as in the above-described thermo treatment. No state was taken into account. On the other hand, the present inventors have repeatedly studied and newly developed a means for measuring ultra-fine indentation hardness at a high temperature, thereby making it possible to evaluate the state of the medium at a high temperature. When evaluated by this new measurement method, if the backcoat layer has a larger degree of softening at high temperatures than the magnetic layer, there is less back surface exposure and good magnetic surface properties can be obtained. Was newly found. The present invention has been completed based on the above findings.

That is, the means for achieving the above object is as follows.
[1] A nonmagnetic layer containing a nonmagnetic powder and a binder and a magnetic layer containing a ferromagnetic powder and a binder in this order on one side of the nonmagnetic support, and carbon black on the other side And a magnetic recording medium having a backcoat layer containing a binder,
The back coat layer has an ultra indentation hardness in the range of 20 to 40 kg / mm ² (196 to 392 MPa) at a temperature of 80 ° C. and a load of 6 mgf, which is smaller than the ultra indentation hardness of the magnetic layer. A characteristic magnetic recording medium.
[2] The magnetic recording medium according to [1], wherein the back coating layer has an indentation hardness of 20 to 30 kg / mm ² (196 to 294 MPa) at a temperature of 80 ° C. and a load of 6 mgf.
[3] The ultra fine indentation hardness of the back coat layer at a temperature of 80 ° C. and a load of 6 mgf is 5 kg / mm ² (49 MPa) or more larger than the ultra fine indentation hardness of the magnetic layer at a temperature of 80 ° C. and a load of 6 mgf. The magnetic recording medium according to [1] or [2].
[4] The magnetic layer according to any one of [1] to [3], wherein the magnetic layer has an ultra fine indentation hardness in a range of 30 to 50 kg / mm ² (292 to 490 MPa) at a temperature of 80 ° C. and a load of 6 mgf. Magnetic recording medium.
[5] The magnetic layer according to any one of [1] to [4], wherein the magnetic layer has an ultra fine indentation hardness in a range of 40 to 50 kg / mm ² (392 to 490 MPa) at a temperature of 80 ° C. and a load of 6 mgf. Magnetic recording medium.
[6] The magnetic layer and the backcoat layer each have an ultra fine indentation hardness in a range of 50 to 80 kg / mm ² (490 to 784 MPa) at a temperature of 33 ° C. and a load of 6 mgf. A magnetic recording medium according to claim 1.
[7] The magnetic layer and the backcoat layer each have an ultra fine indentation hardness in a range of 55 to 70 kg / mm ² (539 to 686 MPa) at a temperature of 33 ° C. and a load of 6 mgf. A magnetic recording medium according to claim 1.
[8] In the magnetic layer, the number of protrusions having a height of 20 nm or more measured with an atomic force microscope is in the range of 10 to 100/900 μm ² , and the centerline average surface roughness Ra is in the range of ² to 5 nm. The magnetic recording medium according to any one of [1] to [7].
[9] The backcoat layer according to any one of [1] to [8], wherein the backcoat layer contains 2 to 10 parts by mass of carbon black having an average particle size of 230 to 300 nm with respect to 100 parts by mass of the binder. Magnetic recording media.

  According to the present invention, it is possible to reduce the back surface image due to tightening in the heat treatment process in the bulk, etc., so that the magnetic surface can be smoothed without reducing the coarse particle carbon necessary for stable running durability. Can be realized. Thereby, since dropout is reduced, a favorable C / N characteristic can be obtained. Thus, according to the present invention, a magnetic recording medium having both running durability and good electrical characteristics can be obtained.

Hereinafter, the present invention will be described in more detail.

The magnetic recording medium of the present invention has a nonmagnetic layer containing nonmagnetic powder and a binder and a magnetic layer containing ferromagnetic powder and a binder in this order on one surface of a nonmagnetic support, and the other side. A magnetic recording medium having a back coat layer containing carbon black and a binder on the surface, the back coat layer being in the range of 20 to 40 kg / mm ² (196 to 392 MPa) at a temperature of 80 ° C. and a load of 6 mgf. And the micro indentation hardness is smaller than the ultra indentation hardness of the magnetic layer.
As described above, in the magnetic recording medium of the present invention, the backcoat layer has appropriate flexibility at a high temperature and is softer than the magnetic layer. As a result, the granular component protruding from the surface of the backcoat layer sinks to the backcoat layer side when exposed to high temperature in a state where the medium is wound and tightened, such as during a thermo treatment, and thus, for example, ensuring running durability Therefore, even when an appropriate amount of coarse particle carbon black is added to the back coat layer, the show-through to the magnetic layer side can be remarkably reduced.

The ultra-fine indentation hardness (DH) has a triangular pyramid shape with a radius of curvature of 100 nm, a blade angle of 65 °, and a ridge angle of 115 ° under a measurement environment of 80 ° C. ± 2.5 ° C. The maximum load 6 mgf (P _max ) and the maximum displacement (H _max ) when the diamond indenter having the shape is unloaded when the load is continuously increased and the indentation load 6 mgf is reached on the surface of the layer to be measured. From this, the value is obtained by the following equation (1).
DH (kg / mm ² ) = 3.7926 × 10 ⁻² {P _max / (H _max ) ² } (1)

In the magnetic recording medium of the present invention, the backcoat layer has an ultrafineness of 20 to 40 kg / mm ² (196 to 392 MPa) at a temperature of 80 ° C. and a load of 6 mgf, which is smaller than the ultrafine indentation hardness of the magnetic layer. Has a small indentation hardness. When the ultra-fine indentation hardness is less than 20 kg / mm ² , it is difficult to ensure running durability because the backcoat layer is excessively flexible. On the other hand, when the ultra-fine indentation hardness exceeds 40 kg / mm ² , the back coat layer is hard, so when the medium is exposed to a high temperature by a thermo treatment or the like, the granular component protruding from the surface of the back coat layer is Without sinking to the side, show-through to the magnetic layer occurs and causes deterioration of electromagnetic conversion characteristics. The ultra micro indentation hardness is preferably in the range of 20 to 30 kg / mm ² (196 to 294 MPa).

  On the other hand, the ultra fine indentation hardness of the magnetic layer measured at a temperature of 80 ° C. and a load of 6 mgf is larger than the ultra fine indentation hardness of the back coat layer. Thus, if the balance between the hardness of the magnetic layer and the backcoat layer is adjusted so that the magnetic layer is harder than the backcoat layer at high temperatures, the medium is wound and the surface of the backcoat layer and the surface of the magnetic layer are wound. When exposed to a high temperature in a state of being in close contact, the particulate component protruding from the surface of the backcoat layer can be sunk into the backcoat layer side. In the present invention, the show-through to the magnetic layer can be remarkably reduced by providing the backcoat layer with cushioning properties.

From the above viewpoint, the difference in ultra-indentation hardness between the magnetic layer and the backcoat layer at a temperature of 80 ° C. and a load of 6 mgf is preferably 5 kg / mm ² (49 MPa) or more, and is 12 to 20 kg / mm ² ( 118 to 196 MPa) is more preferable. Further, nanoindentation hardness measured at a temperature 80 ° C. and a load 6mgf magnetic layers, for example 30~50kg / mm ² (292~490MPa), preferably in the range from 40~50kg / mm ² (392~490MPa) It is.

In order to secure electromagnetic conversion characteristics and running durability, it is preferable to control the hardness of the magnetic layer and the back coat layer at the running temperature. If the magnetic layer is too flexible at the running temperature, debris (powder falling) may occur due to high-speed sliding with the magnetic head, and if it is too hard, the head contact will deteriorate and cause a decrease in output. In addition, if the backcoat layer is too flexible at the running temperature, the running durability may be reduced, and if it is too hard, residues from the backcoat layer are likely to be generated and dropout may be increased. From the above viewpoints, it is preferable that the magnetic layer and the backcoat layer have an ultra fine indentation hardness in the range of 50 to 80 kg / mm ² (490 to 784 MPa) at a temperature of 33 ° C. and a load of 6 mgf. More preferably, it is the range of 55-70 kg / mm < ² > (539-686 MPa). The ultra micro indentation hardness is a value obtained by the method described above at a temperature of 33 ° C. ± 0.1 ° C.

  The ultra indentation hardness of the magnetic layer and the back coat layer at 33 ° C. and 80 ° C. is combined with (1) the type of binder resin used for the magnetic layer and the back coat layer, and (2) the back coat layer. (3) Calendar conditions, (4) Backcoat layer thickness. Details of each control method will be described below.

(1) Kind and combination of binder resin Different binder resins are used for the magnetic layer and the backcoat layer. Thereby, the flexibility of both layers can be changed. If a resin with a high glass transition temperature (Tg) is used as the binder resin, the layer becomes hard. Therefore, the binder resin used for the magnetic layer has a higher Tg than the binder resin used for the backcoat layer. By doing so, the back coat layer can be made more flexible than the magnetic layer. As the binder resin used for each layer, it is preferable to select a resin having a glass transition temperature of about 20 to 150 ° C., for example. If the glass transition temperature is less than 20 ° C, the surface of the magnetic layer or the backcoat layer tends to be tacky, and if it exceeds 150 ° C, the hardness may be too high and become brittle.

  Also, the flexibility of both layers can be changed by changing the combination of binder resins used in the magnetic layer and the backcoat layer. For example, polyurethane resin and vinyl chloride resin (polymer or copolymer) are used for the magnetic layer, and polyurethane resin and fiber resin (for example, cellulose acetate, cellulose diacetate, cellulose propionate) are used for the back coat layer. , Nitrocellulose) can make the backcoat layer more flexible than the magnetic layer. Further, the hardness of the magnetic layer can be increased by increasing the amount of the curing agent used in the magnetic layer.

For example, for the magnetic layer, the addition amount of the vinyl chloride resin in the magnetic layer is a, the glass temperature is Tg1, the addition amount of the polyurethane resin is b, the glass transition temperature is Tg2, the addition amount of the curing agent is c, When the glass transition temperature is Tg3, the glass transition temperature Tg _mag of the magnetic layer can be approximated to a value calculated as (Tg1 × a + Tg2 × b + Tg3 × c) / (a + b + c). In the present invention, the combination and addition amount of resins used in the magnetic layer are determined so that the glass transition temperature of the magnetic layer calculated as described above is, for example, 75 to 89 ° C., preferably 84 to 87 ° C. Can do.

  Similarly, for the backcoat layer, the addition amount of the nitrocellulose resin in the backcoat layer is d, the glass transition temperature is Tg4, the addition amount of the polyurethane resin is e, the glass transition temperature is Tg5, and the addition of the curing agent. When the amount is f and the glass transition temperature is Tg6, the glass transition temperature Tgback of the backcoat layer can be approximated to a value calculated as Tg = (Tg4 × d + Tg5 × e + Tg6 × f) / (d + e + f). In the present invention, the combination and addition amount of resins used in the backcoat layer are determined so that the glass transition temperature of the backcoat layer calculated as described above is, for example, 59 to 120 ° C., preferably 59 to 88 ° C. can do. Specifically, for example, when a nitrocellulose resin, a polyurethane resin, and a polyisocyanate are used as a binder in the backcoat layer, the nitrocellulose resin is 10 to 41% by mass (more preferably) in the total binder. Is 13 to 28% by mass), polyurethane resin is 43 to 109% by mass (more preferably 78 to 109% by mass), and polyisocyanate is 13 to 48% by mass (more preferably 13 to 30% by mass). It is preferable to use so that it may be contained. By using each component in the above proportion, a backcoat layer having a degree of softening at a high temperature much higher than that of the magnetic layer can be obtained even though the hardness is equivalent to that of the magnetic layer at room temperature. .

(2) Drying conditions of the backcoat layer In the drying process of the backcoat layer, the solvent is more likely to be volatilized as the drying temperature is increased and dried more rapidly, and the solvent is more likely to remain as the drying is performed at a relatively low temperature. As the residual solvent in the backcoat layer increases, the backcoat layer becomes softer. Therefore, it is preferable to perform a drying treatment so that an appropriate amount of solvent remains in the backcoat layer. The amount of residual solvent in the backcoat layer is preferably 0.2 to 1.5 mg / m ² , more preferably 0.8 to 1.2 mg / m ² . In general, in the drying step, the drying process is performed by stepwise increasing the ambient temperature at which the support after the backcoat layer is applied is conveyed. In the present invention, for example, a backcoat layer having a relatively large amount of residual solvent can be obtained by passing an atmosphere at a temperature of about 70 to 90 ° C. and then passing an atmosphere at a temperature of about 100 to 105 ° C.

(3) Calendar conditions The stronger the calendar conditions, the higher the hardness of the magnetic layer. As the calender roll, there is an elastic roll such as a heat-resistant plastic roll or a metal roll, and the layer becomes harder when the metal roll is used. Therefore, in order to obtain moderate flexibility, it is preferable to use an elastic roll or a combination of an elastic roll and a metal roll. Moreover, since a layer becomes so hard that calendar temperature, a linear pressure, and a conveyance speed are raised, it is preferable to set said each condition according to the calendar roll to be used.

(4) Thickness of the backcoat layer The thicker the backcoat layer, the greater the residual solvent amount and the more flexible and the cushioning properties. The thickness of the back coat layer is preferably 0.3 to 1.6 μm, more preferably 0.4 to 0.8 μm.

In the present invention, by appropriately combining the above control methods, the ultra-fine indentation hardness of the magnetic layer and the back coat layer at a temperature of 33 ° C. and 80 ° C. can be set within a desired range.
Next, details of the magnetic layer, nonmagnetic layer, nonmagnetic support, and backcoat layer in the magnetic recording medium of the present invention will be described.

[Nonmagnetic layer]
The nonmagnetic layer included in the magnetic recording medium of the present invention is a layer containing a nonmagnetic powder and a binder. This non-magnetic layer needs to be substantially non-magnetic so as not to affect the electromagnetic conversion characteristics of the magnetic layer on the non-magnetic layer, but is small enough not to affect the electromagnetic conversion characteristics of the magnetic layer. The magnetic powder may be contained.

  Examples of the nonmagnetic powder include nonmagnetic inorganic powder and carbon black. The non-magnetic inorganic powder is preferably relatively hard, and preferably has a Mohs hardness of 5 or more (more preferably 6 or more). Examples of nonmagnetic inorganic powders include α-alumina, β-alumina, γ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, silicon nitride, titanium carbide, titanium dioxide, silicon dioxide, Mention may be made of boron nitride, zinc oxide, calcium carbonate, calcium sulfate and barium sulfate. These can be used alone or in combination. Among these, titanium oxide, α-alumina, α-iron oxide, or chromium oxide is preferable. The average particle size of the nonmagnetic inorganic powder is preferably in the range of 0.01 to 1.0 μm (preferably 0.01 to 0.5 μm, particularly preferably 0.02 to 0.1 μm). In addition, 3 to 25% by mass (preferably 3 to 20% by mass) in the nonmagnetic powder is preferably one that can function as a so-called abrasive having a Mohs hardness of 5 or more (preferably 6 or more). .

In addition to non-magnetic inorganic powder, the non-magnetic layer is made of carbon for the purpose of imparting conductivity to the magnetic layer to prevent electrification and ensuring the smooth surface properties of the magnetic layer formed on the non-magnetic layer. Black can be added. The carbon black used for the nonmagnetic layer preferably has an average particle size of 35 nm or less (more preferably 10 to 35 nm). The specific surface area is preferably 5 to 500 m ² / g (more preferably 50 to 300 m ² / g). The DBP oil absorption is preferably in the range of 10 to 1000 mL / 100 g (more preferably 50 to 300 mL / 100 g). The pH is preferably 2 to 10, the water content is 0.1 to 10%, and the tap density is preferably 0.1 to 1 g / cc.

  As carbon black, those obtained by various production methods can be used. Examples of these include furnace black, thermal black, acetylene black, channel black and lamp black. Specific product examples of carbon black include BLACKPEARLS 2000, 1300, 1000, 900, 800, 700, VULCAN XC-72 (above, manufactured by Cabot Corporation), # 35, # 50, # 55, # 60 and # 80. (Above, manufactured by Asahi Carbon Co., Ltd.), # 3950B, # 3750B, # 3250B, # 2400B, # 2300B, # 1000, # 900, # 40, # 30, and # 10B (above, Mitsubishi Chemical Corporation )), CONDUCTEX SC, RAVEN, 150, 50, 40, 15 (above, Columbian Carbon Co., Ltd.), Ketjen Black EC, Ketjen Black ECDJ-500 and Ketjen Black ECDJ-600 (above, Lion Akzo ( Product)).

  The amount of carbon black added to the nonmagnetic layer is, for example, 3 to 25 parts by mass, preferably 4 to 20 parts by mass, and more preferably 5 to 15 parts by mass with respect to 100 parts by mass of the total nonmagnetic inorganic powder.

  Examples of the binder used for the nonmagnetic layer include thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof. Examples of thermoplastic resins include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal. And a polymer or copolymer containing vinyl ether as a structural unit. Examples of the copolymer include vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylate ester-acrylonitrile copolymer, acrylate ester-vinylidene chloride copolymer. Polymer, Acrylate ester-Styrene copolymer, Methacrylate ester-Acrylonitrile copolymer, Methacrylate ester-Vinylidene chloride copolymer, Methacrylate ester-Styrene copolymer, Vinylidene chloride-Acrylonitrile copolymer , Butadiene-acrylonitrile copolymer, styrene-butadiene copolymer, and chlorovinyl ether-acrylic ester copolymer.

  In addition to the above, polyamide resin, fiber-based resin (cellulose acetate butyrate, cellulose diacetate, cellulose propionate, nitrocellulose, etc.), polyvinyl fluoride, polyester resin, polyurethane resin, various rubber resins, etc. are also used. can do.

  Examples of thermosetting resins or reactive resins include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, Mention may be made of a mixture of polyester resin and polyisocyanate prepolymer, a mixture of polyester polyol and polyisocyanate, and a mixture of polyurethane and polyisocyanate.

  Examples of the polyisocyanate include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triphenylmethane triisocyanate, and the like. Isocyanates, products of these isocyanates with polyalcohols, and polyisocyanates formed by condensation of isocyanates.

  As the polyurethane resin, known resins having a structure such as polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, and polycaprolactone polyurethane can be used.

  In the present invention, the binder used for the nonmagnetic layer is vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-maleic anhydride copolymer. A combination of at least one resin selected from coalescence and nitrocellulose and a polyurethane resin, or a combination thereof with a polyisocyanate as a curing agent is preferable.

As the binder, if necessary to enhance the durability of the layer obtained more excellent _{dispersibility, -COOM, -SO 3 M, -OSO} 3 M, -P = O (OM) 2, -O _{-P = O (OM) 2 (} M represents a hydrogen atom or an alkali metal,.), - OH, -NR 2, -N + R 3 (. R is representative of a hydrocarbon group), epoxy group, -SH It is preferable to use one in which at least one polar group selected from —CN and the like is introduced by copolymerization or addition reaction. Such polar groups are preferably introduced into the binder in an amount of 10 ^-1 to 10 ^-8 mol / g (more preferably 10 ^-2 to 10 ^-6 mol / g).

  The binder content in the nonmagnetic layer can be, for example, 5 to 50 parts by mass (preferably 10 to 30 parts by mass) with respect to 100 parts by mass of the nonmagnetic powder. When a vinyl chloride resin, a polyurethane resin, and a polyisocyanate are used as a binder in the nonmagnetic layer, the vinyl chloride resin is 5 to 70% by mass and the polyurethane resin is 2 to 50% in the total binder. It is preferable to use so that the polyisocyanate is contained in an amount in the range of 2 to 50% by mass.

  The nonmagnetic layer may include a lubricant. The lubricant in the nonmagnetic layer oozes out to the surface of the magnetic layer, thereby reducing the friction between the magnetic surface and the magnetic head, the drive guide pole and the cylinder, and maintaining the smooth sliding state. can do. Examples of the lubricant include fatty acids and fatty acid esters. Examples of fatty acids include acetic acid, propionic acid, octanoic acid, 2-ethylhexanoic acid, lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, arachic acid, oleic acid, linoleic acid, linolenic acid, elaidic acid, And aliphatic carboxylic acids such as palmitoleic acid or mixtures thereof.

  Examples of fatty acid esters include butyl stearate, sec-butyl stearate, isopropyl stearate, butyl oleate, amyl stearate, 3-methylbutyl stearate, 2-ethylhexyl stearate, 2-hexyl decyl stearate, Acylation of butyl palmitate, 2-ethylhexyl myristate, butyl stearate and butyl palmitate, oleyl oleate, butoxyethyl stearate, 2-butoxy-1-propyl stearate, dipropylene glycol monobutyl ether with stearic acid Diethylene glycol dipalmitate, hexamethylene diol acylated with myristic acid to give a diol, and various ester compounds such as glycerin oleate. Rukoto can. These can be used alone or in combination. The addition amount of the lubricant in the nonmagnetic layer can be, for example, in the range of 0.2 to 20 parts by mass with respect to 100 parts by mass of the total nonmagnetic powder.

  Although the thickness of a nonmagnetic layer is not specifically limited, For example, it can be 0.5-2.5 micrometers. Preferably it is 1.0-2.0 micrometers, More preferably, it is 1.2-1.6 micrometers.

[Magnetic layer]
The magnetic layer is basically formed of a ferromagnetic powder and a binder. The magnetic layer usually contains a lubricant, conductive powder (for example, carbon black) and an abrasive. Examples of the ferromagnetic powder include γ-Fe ₂ O ₃ , Fe ₃ O ₄ , FeOx (x = 1.33 to 1.5), CrO ₂ , Co-containing γ-Fe ₂ O ₃ , Co-containing FeOx (x = 1.33 to 1.5), ferromagnetic alloy powder (ferromagnetic metal powder) containing Fe, Ni, or Co as a main component (75% by mass or more), and plate-shaped hexagonal ferrite powder. . In the present invention, it is preferable to use a ferromagnetic metal powder or a plate-shaped hexagonal ferrite powder as the ferromagnetic powder. Particularly preferred is a ferromagnetic metal powder.

The ferromagnetic metal powder has a specific surface area of preferably 30 to 70 m ² / g, and a crystallite size determined by X-ray diffraction method is preferably 50 to 300 Å. If the specific surface area is too small, it will not be possible to sufficiently cope with high density recording, and if it is too large, it will not be possible to disperse sufficiently, so that it will not be possible to form a smooth magnetic layer, and thus it will not be able to cope with high density recording.

The ferromagnetic metal powder may contain at least Fe, specifically, a single metal or an alloy mainly composed of Fe, Fe—Co, Fe—Ni, Fe—Zn—Ni, or Fe—Ni—Co. It is. Fe alone may be used. Regarding the magnetic properties of these ferromagnetic metal powders, in order to achieve a high recording density, the saturation magnetization (σs) is, for example, 110 A · m ² / kg or more, preferably 120 A · m ² / kg or more, 170 A · m ² / kg or less. The coercive force (Hc) is, for example, in the range of 1950 to 2650 oersted (Oe) (156 to 212 kA / m), preferably 2000 to 2500 Oe (160 to 200 kA / m). And the average major axis length of the powder calculated | required with a transmission electron microscope is 0.5 micrometer or less, for example, Preferably it is 0.01-0.3 micrometer, and an axial ratio (long-axis length / short-axis length, acicular ratio) is For example, 5 to 20, preferably 5 to 15. In order to further improve the characteristics, non-metals such as B, C, Al, Si, and P, or salts and oxides thereof may be added during the composition. Usually, an oxide layer is formed on the particle surface of the ferromagnetic metal powder for chemical stabilization.

The plate-shaped hexagonal ferrite powder used for the magnetic layer has a specific surface area of 25 to 65 m ² / g, a plate-like ratio (plate diameter / plate thickness) of 2 to 15, and an average plate diameter of 0.02. It is preferable that it is -1.0 micrometer. The plate-shaped hexagonal ferrite powder is difficult to record at high density if the particle size is too large or too small for the same reason as the ferromagnetic metal powder. The plate-shaped hexagonal ferrite is a ferromagnetic material having a flat shape and an easy axis of magnetization in a direction perpendicular to the flat plate surface. Specifically, barium ferrite, strontium ferrite, lead ferrite, calcium ferrite, and those Substituents such as cobalt can be listed. Of these, cobalt substitutes of barium ferrite and cobalt substitutes of strontium ferrite are particularly preferable. To the plate-shaped hexagonal ferrite, elements such as In, Zn, Ge, Nb, and V may be added as necessary to improve the characteristics. Regarding the magnetic properties of these plate-shaped hexagonal ferrite powders, in order to achieve a high recording density, the saturation magnetization (σs) is, for example, 50 A · m ² / kg or more, preferably 53 A · m ² / kg or more. is there. The coercive force is, for example, in the range of 700 to 2000 Oe (56 to 160 kA / m), and preferably in the range of 900 to 1600 Oe (72 to 128 kA / m).

The moisture content of the ferromagnetic powder is preferably 0.01-2% by mass. It is preferable to optimize the water content depending on the type of the binder. The pH of the ferromagnetic powder is preferably optimized depending on the combination with the binder used, and the pH is usually in the range of 4 to 12, preferably in the range of 5 to 10. The ferromagnetic powder preferably has at least a part of its surface coated with Al, Si, P, or an oxide thereof as required. The amount of the surface treatment used is usually 0.1 to 10% by mass with respect to the ferromagnetic powder. The coated ferromagnetic powder can suppress the adsorption of a lubricant such as fatty acid to 100 mg / m ² or less, so that the desired effect can be achieved even if the amount of lubricant added to the magnetic layer is reduced. The ferromagnetic powder may contain soluble inorganic ions such as Na, Ca, Fe, Ni, and Sr, but the content is preferably as small as possible. Usually, if it is 5000 ppm or less, it does not affect the characteristics. The ferromagnetic powder and the method for producing the same are described in, for example, Japanese Patent Application Laid-Open No. 7-22224.

  The ferromagnetic powder used in the present invention includes Al, Si, P, Ti, and rare earth elements (Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er. , Tm, Yb, Lu) and the like are preferably treated with a known substance as an anti-caking agent. In particular, it is preferably treated with at least Y (yttrium). The anti-caking agent is disclosed in, for example, JP-A Nos. 52-134858, 56-114833, 57-73105, JP-A-6-25702, and JP-A-6-36265. .

  A lubricant can be added to the magnetic layer, and the lubricant described as being usable for the nonmagnetic layer can be used. The normal addition amount of the lubricant is, for example, in the range of 0.2 to 20 parts by mass (preferably 0.25 to 10 parts by mass) with respect to 100 parts by mass of the ferromagnetic powder.

The magnetic layer has various purposes, such as reducing the surface electrical resistance (R _S ) of the magnetic layer, reducing the dynamic friction coefficient (μ _K value), improving the running durability, and ensuring the smooth surface properties of the magnetic layer. Carbon black can be added. As the magnetic layer, carbon black described as being usable for the nonmagnetic layer can be used. However, the carbon black used in the magnetic layer preferably has an average particle diameter in the range of 5 nm to 350 nm (more preferably 10 nm to 300 nm). Two or more carbon blacks having different average particle diameters can be used. The amount of carbon black added is usually in the range of 0.1 to 30 parts by mass (preferably 0.2 to 15 parts by mass) with respect to 100 parts by mass of the ferromagnetic powder.

An abrasive can also be added to the magnetic layer. Examples of the abrasive include fused alumina, silicon carbide, chromium oxide (Cr ₂ 0 ₃ ), corundum, artificial corundum, diamond, artificial diamond, garnet, emery (main components: corundum and magnetite). . These abrasives preferably have a Mohs hardness of 5 or more (preferably 6 or more) and an average particle size of 0.05 to 1 μm (more preferably 0.2 to 0.8 μm). . The addition amount of the abrasive is usually in the range of 3 to 25 parts by mass (preferably 3 to 20 parts by mass) with respect to 100 parts by mass of the ferromagnetic powder.

  As the binder used for the magnetic layer, those described as the binder for the nonmagnetic layer can be used. The binder in the magnetic layer is usually used in the range of 5 to 50 parts by mass (preferably 10 to 30 parts by mass) with respect to 100 parts by mass of the ferromagnetic powder. It is preferable to use a combination of a vinyl chloride resin, a polyurethane resin, and a polyisocyanate as a binder in the magnetic layer. In that case, the vinyl chloride resin is 5 to 70% by mass in the total binder, and the polyurethane resin. Is preferably used in an amount ranging from 2 to 50% by weight and polyisocyanate in an amount ranging from 2 to 50% by weight. In the present invention, as described above, in order to balance the hardness with the back coat layer, it is preferable to adjust the glass transition temperature of the binder resin, the combination of the resins, the amount of the curing agent used, and the like.

In the present invention, as described above, the show-through to the magnetic layer can be reduced by balancing the hardness of the magnetic layer and the backcoat layer at high temperatures where the show-through is likely to occur. A magnetic layer having high smoothness and reduced dents that cause dropout can be obtained. On the surface of the magnetic layer, the number of protrusions having a height of 20 nm or more measured with an atomic force microscope is in the range of 10 to 100/900 μm ² (preferably 10 to 50/900 μm ² ). The center line average surface roughness Ra of the magnetic layer is in the range of 2 to 5 nm (preferably 2 to 3 nm). In particular, in the present invention, by making the backcoat layer flexible at a high temperature as described above, it is possible to remarkably reduce the dents on the surface of the magnetic layer due to the back surface reflection, particularly inside the bulk roll. Dropouts originating from the dents on the layer surface can be reduced, and the electrical characteristics of the medium can be significantly improved. If the number of protrusions with a height of 20 nm or more measured with an atomic force microscope is within the above range, the distance between the head and the magnetic recording medium (spacing loss) is small, and good electrical characteristics can be obtained, and high temperature Even in a harsh environment of high humidity (for example, 40 ° C.-80% RH), the head and the magnetic recording medium do not stick to each other, and good running performance can be obtained.

  The thickness of the magnetic layer is, for example, 0.05 to 0.2 μm, preferably 0.08 to 0.12 μm. In the present invention, even if the magnetic layer is a thin layer, as described above, by balancing the hardness of the magnetic layer and the backcoat layer particularly at high temperatures, the show-through is reduced and excellent electromagnetic characteristics are achieved. can do.

[Back coat layer]
The back coat layer is a layer containing at least carbon black and a binder, and is preferably a layer in which carbon black and an inorganic powder having a Mohs hardness of 5 to 9 are dispersed in the binder. In addition, the backcoat layer of such a structure is described, for example in Unexamined-Japanese-Patent No. 9-115134, The backcoat layer in this invention can also be comprised similarly to these structures.

  Two types of carbon black having different average particle sizes are preferably used in combination. Specifically, it is preferable to use in combination fine particle carbon black having an average particle size of 10 to 20 nm and coarse particle carbon black having an average particle size of 230 to 300 nm. In general, by adding fine carbon black as described above, the surface electrical resistance of the backcoat layer can be set low, and the light transmittance can also be set low. Depending on the magnetic recording apparatus, there are many which use the light transmittance of the tape and are used for the operation signal. In such a case, the addition of fine carbon black is particularly effective. In addition, particulate carbon black is generally excellent in retention of liquid lubricants, and contributes to reduction of the friction coefficient when used in combination with lubricants. On the other hand, the coarse particulate carbon black having a particle size of 230 to 300 nm has a function as a solid lubricant, and also forms minute protrusions on the surface of the backcoat layer to reduce the contact area, thereby reducing the friction coefficient. Contributes to the reduction of

  Specific products of the particulate carbon black include the following. The parenthesized values are average particle diameters. RAVEN2000B (18 nm), RAVEN1500B (17 nm) (above, Colombian Carbon Co., Ltd.), BP800 (17 nm) (Cabot Corp.), PRINTEX90 (14 nm), PRINTEX95 (15 nm), PRINTEX85 (16 nm), PRINTEX75 (17 nm) (and above) , Manufactured by Degussa), # 3950 (16 nm) (manufactured by Mitsubishi Chemical Corporation). Examples of specific products of coarse particle carbon black include thermal black (270 nm) (manufactured by Kerncarb) and RAVEN MTP (275 nm) (manufactured by Colombian Carbon).

  When two types having different average particle sizes are used for the back coat layer, the content ratio (mass ratio) of fine particle carbon black of 10 to 20 nm and coarse particle carbon black of 230 to 300 nm is the former: the latter = 98. : It is preferable to exist in the range of 2-75: 25, More preferably, it is the range of 95: 5-85: 15. The content (total content) of carbon black in the backcoat layer is usually in the range of 30 to 80 parts by mass, preferably in the range of 45 to 65 parts by mass with respect to 100 parts by mass of the binder. . Moreover, in order to ensure running durability, the coarse particle carbon black is preferably used in an amount of 2 to 10 parts by mass (more preferably 4 to 8 parts by mass) with respect to 100 parts by mass of the binder resin. . As described above, in the present invention, the balance between the hardness of the back coat layer and the magnetic layer at high temperatures is achieved, so that even when a predetermined amount of coarse carbon black is used to ensure running durability, There is an advantage that the show-through to the layer can be reduced.

  In the backcoat layer, an inorganic powder having a Mohs hardness of 5 to 9 can be added for the purpose of imparting high running durability to the tape and strengthening the backcoat layer. Further, by using an inorganic powder having a Mohs hardness of 5 to 9, an appropriate polishing force is generated in the backcoat layer, and adhesion of the dropped carbon black to the tape guide pole or the like can be reduced. The average particle size of the inorganic powder having a Mohs hardness of 5 to 9 is preferably in the range of 80 to 250 nm, more preferably in the range of 100 to 210 nm.

Examples of the inorganic powder having a Mohs hardness of 5 to 9 include α-iron oxide, α-alumina, and chromium oxide (Cr ₂ O ₃ ). These powders may be used alone or in combination. Among these, α-iron oxide or α-alumina is preferable. The content of the inorganic powder having a Mohs hardness of 5 to 9 in the back coat layer is usually 3 to 30 parts by mass, preferably 3 to 20 parts by mass with respect to 100 parts by mass of the carbon black.

  The back coat layer can contain a lubricant. The lubricant can be appropriately selected from the lubricants listed as the lubricant that can be used for the nonmagnetic layer described above. In the backcoat layer, the lubricant is usually added in the range of 1 to 5 parts by mass with respect to 100 parts by mass of the binder.

  As the binder for the backcoat layer, those described as the binder for the nonmagnetic layer can be used. Preferably, the structure is a combination of nitrocellulose resin, polyurethane resin, polyester resin and polyisocyanate. When a nitrocellulose resin, a polyurethane resin, and a polyisocyanate are used in combination as a binder in the backcoat layer, as described above, the nitrocellulose resin is 10 to 41% by mass (more preferably, in the total binder). 13 to 28% by mass), polyurethane resin is 43 to 109% by mass (more preferably 78 to 109% by mass), and polyisocyanate is 13 to 48% by mass (more preferably 13 to 30% by mass). It is preferable to use it so that it is included. The binder for the backcoat layer is usually used in the range of 5 to 250 parts by mass (preferably 10 to 200 parts by mass) with respect to 100 parts by mass of the carbon black of the backcoat layer. In the present invention, as described above, in order to balance the hardness with the magnetic layer, it is preferable to adjust the glass transition temperature of the binder resin, the combination of the resins, the amount of the curing agent used, and the like.

[Additive]
A dispersing agent can be added to the coating liquid for forming each layer of the magnetic tape in order to favorably disperse the magnetic powder or nonmagnetic powder in the binder. If necessary, a plasticizer, conductive particles (antistatic agent) other than carbon black, an antifungal agent, and the like can be added to each layer. Examples of the dispersant include 12 to 18 carbon atoms such as caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid and stearic acid. A fatty acid (RCOOH; R is an alkyl group having 11 to 17 carbon atoms, or an alkenyl group), a metal soap composed of an alkali metal or an alkaline earth metal of the fatty acid, a compound containing fluorine of the fatty acid ester, Fatty acid amide, polyalkylene oxide alkyl phosphate ester, lecithin, trialkyl polyolefinoxy quaternary ammonium salt (alkyl is 1 to 5 carbon atoms, olefin is ethylene, propylene, etc.), sulfate, copper phthalocyanine, etc. Can be used. These may be used alone or in combination. In particular, it is preferable to use a combination of copper oleate, copper phthalocyanine, and barium sulfate in the backcoat layer. In any layer, the dispersant is preferably added in the range of 0.5 to 20 parts by mass with respect to 100 parts by mass of the binder.

[Non-magnetic support]
Nonmagnetic supports that can be used in the present invention include biaxially stretched polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyamide, polyimide, polyamideimide, aromatic polyamide, polybenzoxazole, and the like. be able to. These supports may have been previously subjected to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like. The support that can be used in the present invention has a centerline average surface roughness of 0.1 to 20 nm, preferably 1 to 10 nm at a cutoff value of 0.25 mm, and has excellent surface smoothness. It is preferable to have. These supports preferably have not only a small center line average surface roughness but also no coarse protrusions of 1 μm or more. The thickness of the support is, for example, 4 to 15 μm, preferably 4 to 9 μm. When it is thin, the unevenness of the back layer is easily captured by handling tension, and this can be effectively suppressed by using a high-Tg polyurethane resin for the magnetic layer. When the thickness is 7 μm or less, it is preferable to use an aromatic polyamide such as PEN or aramid.

[Production method]
The magnetic recording medium of the present invention is preferably a magnetic tape. The magnetic tape can be obtained by slitting the original magnetic tape in the longitudinal direction. The thickness of the entire tape is, for example, 3 to 20 μm, and it is preferably 4 to 10 μm, and more preferably 4 to 8 μm, for increasing the capacity. Below, the manufacturing method of a magnetic tape is demonstrated.

  In the production of the magnetic tape, a step of sequentially forming a nonmagnetic layer and a magnetic layer on one side of a wide elongated support and a back layer on the other side in order according to a usual method was obtained. A step of cutting the original magnetic tape into a predetermined width is included. In addition, each process normally performed, such as a drying process, an orientation process, a calendar process, or a winding process, is suitably performed during each said process or before and after each process.

  As the calendering conditions, the temperature of the calender roll is in the range of 60 to 120 ° C, preferably in the range of 70 to 115 ° C, particularly preferably in the range of 80 to 100 ° C, and the pressure is 100 to 400 kg / cm (98 to 392 kN). / M), preferably 200 to 350 kg / cm (196 to 343 kN / m), particularly preferably 250 to 350 kg / cm (245 to 343 kN / m). As described above, it is preferable to use an elastic roll or a combination of an elastic roll and a metal roll as the calendar roll. As the elastic roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyamideimide or the like can be used.

  In the present invention, it is preferable to provide a magnetic layer on the nonmagnetic layer while it is in a wet state. That is, the magnetic layer is applied by a so-called wet-on-wet method in which the magnetic layer coating liquid is applied onto the magnetic layer while the formed nonmagnetic layer is in a wet state after the non-magnetic layer coating liquid is applied. It is preferably formed using a method.

Examples of the application method by the wet-on-wet method include the following methods.
(1) Using a gravure coating, roll coating, blade coating, or extrusion coating apparatus, a nonmagnetic layer is first formed on a support, and while the nonmagnetic layer is in a wet state, the support pressurization type ext A method of forming a magnetic layer with a roux coating device (see JP-A-60-238179, JP-B-1-46186, JP-A-2-265672).
(2) A method in which a magnetic layer and a nonmagnetic layer are formed almost simultaneously on a support using a coating apparatus comprising a single coating head provided with two slits for coating solution (Japanese Patent Laid-Open No. 63-88080, (See Kaihei 2-17921 and 2-265672).
(3) A method in which a magnetic layer and a nonmagnetic layer are formed on a support almost simultaneously using an extrusion coating apparatus with a backup roller (see JP-A-2-174965).
In the present invention, the nonmagnetic layer and the magnetic layer are preferably formed using a simultaneous multilayer coating method.

  As a method of cutting the obtained magnetic tape raw material into a predetermined width, it is preferable to use, for example, a method described in JP-A-2001-273629.

  Next, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the embodiment shown in the examples. Hereinafter, the display of “parts” represents “parts by mass”.

[Examples 1-5, Comparative Examples 1-3]
Preparation of coating solution for nonmagnetic layer formation Nonmagnetic powder α-Fe ₂ O ₃ hematite 80 parts Average major axis length: 0.15 μm
Specific surface area by BET: 52 m ² / g
pH: 8
Tap density: 0.8g / ml
DBP oil absorption: 27-38ml / 100g
Surface treatment layer: Al ₂ O ₃ , SiO ₂
Carbon black 20 parts Average primary particle size: 16 nm
DBP oil absorption: 80ml / 100g
PH: 8.0
Specific surface area by BET method: 250 m ² / g
Volatile content: 1.5%
12 parts of vinyl chloride copolymer MR-104 manufactured by Nippon Zeon
Polyester polyurethane resin 5 parts Neopentyl glycol / caprolactone polyol / MDI = 0.9 / 2.6 / 1, —SO ₃ Na group content: 1 × 10 ⁻⁴ eq / g
α-Al ₂ O ₃ (average particle size: 0.1 μm) 1 part butyl stearate 1 part stearic acid 1 part methyl ethyl ketone 100 parts cyclohesanone 50 parts toluene 50 parts

Preparation of Coating Solution for Magnetic Layer Formation Table 1 shows the amounts of vinyl chloride copolymer, polyester polyurethane resin, and curing agent described below.
Ferromagnetic metal powder 100 parts Composition: Fe / Co = 100/30 (atomic ratio)
Hc: 191 kA (2400 Oe)
Specific surface area by BET method: 48 m ² / g
Crystallite size: 130Å
Surface treatment layer: Al ₂ O ₃ , Y ₂ O ₃
Particle size (major axis length): 0.06 μm
Needle ratio: 6
σs: 120 A · m ² / kg (120 emu / g)
Vinyl chloride copolymer part a MR-110 made by Nippon Zeon
Polyester polyurethane resin part b Byron UR8300 manufactured by Toyobo
Curing agent part c Coronate 3040 made by Nippon Polyurethane
α-Al ₂ O ₃ (average particle size: 0.23 μm) 5 parts Carbon black (average particle size 0.08 μm) 0.5 parts
Butyl stearate 1 part Stearic acid 5 parts Methyl ethyl ketone 90 parts Cyclohesanone 30 parts Toluene 60 parts

Preparation of Backcoat Layer-Forming Coating Solution Table 1 shows the amounts of the following nitrocellulose resin, polyurethane resin, and curing agent added.
100 parts of fine carbon black powder (manufactured by Cabot, BP-800, average particle size: 17 nm)
Coarse carbon black powder 10 parts (Carncarb, thermal black, average particle size: 270 nm)
α-iron oxide 15 parts (TF100 manufactured by Toda Kogyo Co., Ltd., average particle size 110 nm, Mohs hardness: 5.5)
Nitrocellulose resin d part polyurethane resin e part Nippon Polyurethane N2301
Curing agent part f
Coronate 3040 manufactured by Nippon Polyurethane
Dispersant: Copper oleate 5 parts
Copper phthalocyanine 5 parts
Barium sulfate 5 parts Methyl ethyl ketone 2200 parts Butyl acetate 300 parts Toluene 600 parts

About said nonmagnetic layer coating liquid, after kneading | mixing each component with an open kneader, it was disperse | distributed using the sand mill. 5 parts of polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) is added to the obtained dispersion, and 40 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone is added, followed by filtration using a filter having a pore size of 1 μm to apply a nonmagnetic layer. A liquid was prepared.
For the magnetic layer coating solution, components other than the curing agent were kneaded with an open kneader and then dispersed using a sand mill. A curing agent was added to the obtained dispersion, and 40 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone was added, followed by filtration using a filter having a pore size of 1 μm to prepare a magnetic layer coating solution.
Regarding the backcoat layer coating solution, components other than the curing agent were kneaded with an open kneader and then dispersed using a sand mill. A curing agent was added to the obtained dispersion, and 2200 parts of methyl ethyl ketone, 300 parts of butyl acetate and 600 parts of toluene were further added, and the mixture was further kneaded with a continuous kneader and then dispersed using a sand mill. The obtained dispersion was filtered using a filter having a pore size of 1 μm as described above.

  The obtained nonmagnetic layer coating solution and magnetic layer coating solution are further dried so that the nonmagnetic layer has a thickness of 1.5 μm after drying and the magnetic layer has a thickness of 0.15 μm after drying. A simultaneous multilayer coating was performed on a 7 μm thick support (biaxially stretched polyethylene terephthalate) so that the total thickness of the tape was 8.7 μm, and both layers were still wet. Oriented and dried by a cobalt magnet having a magnetic force of 3T (3000G) and a solenoid having a magnetic force of 0.15T (1500G).

  After drying the magnetic layer and the nonmagnetic layer, a calendar process was performed under the conditions shown in Table 2 using a seven-stage calendar apparatus composed of calendar rolls shown in Table 2.

  After the calendar treatment, the back coat layer was applied to a thickness shown in Table 1 after drying. Further, in order to adjust the hardness of the backcoat layer, the temperature of the dry zone (first dry zone + second dry zone) after application of the backcoat layer is adjusted as shown in Table 1. Thus, the residual solvent amount was adjusted.

  After the formation of the back coat layer, the magnetic layer, the non-magnetic layer and the back coat layer were strengthened, and were subjected to a storage treatment for 36 hours in an environment at 80 ° C. in order to prevent thermal deformation of the flexible support. The bulk length was set to 5000 m for evaluation of back coat surface exposure.

After the storage process, the sheet was cut into ½ width using a cutting apparatus shown in FIG. 1 of JP-A-2001-273629. The slit conditions were unified to the following conditions and cut.
Slit speed: 300m / min Meshing depth: 0.5mm
Peripheral speed ratio between upper blade and lower blade: 1.05

The following evaluation was performed about the obtained magnetic tape.
(1) Ultra micro indentation hardness (measuring device)
An ultra-fine indentation hardness tester (model ENT-1100a) manufactured by Elionix Co., Ltd. was used as a measuring device. The main equipment use is as follows.
・ Load generation method: electromagnetic force type ・ Indenter: Triangular pyramid indenter, blade angle 65 °, ridge angle 115 °, made of diamond ・ Load range: 2 mgf to 100 gf (20 μN to 1 N)
・ Load resolution: 0.2μN
・ Displacement measurement method: Capacitance detection of indenter movement ・ Displacement range: ~ 20μm
・ Displacement reading resolution: 0.3 nm
(Measurement condition)
Each magnetic tape is cut into 5mm x 5mm, fixed to a dedicated sample stage (13mm x 19mm) made of CF5 (Novinite, casting) with instant adhesive (trade name: Aron Alpha), dried, and put into measurement environment The measurement was performed after leaving for about 30 minutes.
Test load: 6 mgf (58.8 μN)
-Number of divisions: 100
・ Step interval: 20msec
・ Measurement environment: 33 ℃ ± 0.1 ℃ and 80 ℃ ± 2.5 ℃
N number of measurements: Nine measurements were made on the surface of the magnetic layer or the back coat layer, and the above equation (5) was used using the five intermediate data except for data with noise, data with a large maximum displacement, and data with a small maximum displacement. The ultra-fine indentation hardness (DH) was determined by 1).

(2) Measurement of protrusions The number of protrusions having a height of 20 nm or more from the reference surface on the surface of the magnetic layer was determined by measuring a measurement area of 30 × 30 μm with AFM (model SPA500) manufactured by SII Corporation. The reference plane is a plane in which the upper volume and the lower volume are equal, and is determined by the AFM used for measurement.

(3) Measurement of Centerline Average Surface Roughness Ra Measurement area 40 × 40 μm was measured with AFM (model SPA500) manufactured by SII Corporation.

(4) C / N evaluation Recording head (MIG, gap: 0.15 μm, track width: 18 μm, magnetic field strength (Bs): 1.8 T) and reproducing MR head (shield type: gap between shields: 0.2 μm, track width 4 μm) was attached to a drum tester and measured. A single-frequency signal with a recording wavelength of 0.2 μm (50 MHz) is recorded at a head-medium relative speed of 10 m / sec. C / N (dB). During reproduction, a bias current was applied to the MR head so as to maximize the reproduction output.

(5) About DO (dropout) evaluation A signal is recorded on a magnetic tape at a recording track width of 15 μm, a recording wavelength of 0.36 μm, a tape feed speed of 2.5 m / sec, and a lead track width of 7.5 μm using an MR head. The signal was reproduced at a tape feed speed of 2.5 m / sec. At this time, the number of dropouts per 1 MB of recording when the output decreased by 50% or more for a time of 0.08 μsec or more was measured.

(6) Head dirt evaluation The head after running for C / N evaluation was observed with a digital microscope manufactured by Keyence Corporation to evaluate the dirt state.
○: Little head dirt ×: Little head dirt

(7) Alfe Silver wear <Change rate of wear width>
The wear width of the AlFeSil prism when the magnetic recording medium is run under the following running conditions is a, and after the running, the same running part of the magnetic recording medium is the same as the above for the same AlFeSil prism under the same conditions as above. The increment b of the wear width of the AlFeSil prism when contacted under the same running conditions was determined, and the change rate b / a of the wear width was calculated.
(Running conditions)
The magnetic layer surface is brought into contact with one ridge side of the AlFeSil prism at a wrap angle of 12 degrees so as to be orthogonal to the longitudinal direction of the AlFeSil prism (the prism defined in ECMA-228 / AnnexH / H2), and magnetic recording is performed in this state. The medium is run at 580 × 100 Pass at a speed of 3 m / sec under a tension of 100 gf.
The above results are shown in Table 3.

Evaluation Results As shown in Table 3, in the magnetic tapes of Examples 1 to 5, the magnetic layer and the backcoat layer have appropriate hardness at room temperature, and the backcoat layer is flexible at high temperatures. Such magnetic tapes of Examples 1 to 5 had less dropout and better C / N than the magnetic tapes of Comparative Examples 1 to 3. In addition, the magnetic tapes of Examples 1 to 5 had less Alfesilver wear and good per head.

  According to the present invention, it is possible to provide a magnetic recording medium for high-density recording that has both high electromagnetic conversion characteristics and excellent running durability.

Claims (9)

A nonmagnetic layer containing a nonmagnetic powder and a binder and a magnetic layer containing a ferromagnetic powder and a binder on one side of the nonmagnetic support in this order, and carbon black and a binder on the other side A magnetic recording medium having a backcoat layer comprising
The back coat layer has an ultra indentation hardness in the range of 20 to 40 kg / mm ² (196 to 392 MPa) at a temperature of 80 ° C. and a load of 6 mgf, which is smaller than the ultra indentation hardness of the magnetic layer. A characteristic magnetic recording medium. ^2. The magnetic recording medium according to claim 1, wherein the back coating layer has an ultra-fine indentation hardness of 20 to 30 kg / mm ² (196 to 294 MPa) at a temperature of 80 ° C. and a load of 6 mgf. The ultra fine indentation hardness at a temperature of 80 ° C and a load of 6 mgf of the backcoat layer is 5 kgmm ² (49 MPa) or more larger than the ultra fine indentation hardness of the magnetic layer at a temperature of 80 ° C and a load of 6 mgf. 2. The magnetic recording medium according to 2. 4. The magnetic recording medium according to claim 1, wherein the magnetic layer has an ultra fine indentation hardness in a range of 30 to 50 kg / mm ² (292 to 490 MPa) at a temperature of 80 ° C. and a load of 6 mgf. . 5. The magnetic recording medium according to claim 1, wherein the magnetic layer has an ultra fine indentation hardness in a range of 40 to 50 kg / mm ² (392 to 490 MPa) at a temperature of 80 ° C. and a load of 6 mgf. . 6. The magnetic layer and the backcoat layer according to claim 1, wherein the magnetic layer and the backcoat layer have an ultra fine indentation hardness in a range of 50 to 80 kg / mm ² (490 to 784 MPa) at a temperature of 33 ° C. and a load of 6 mgf. Magnetic recording media. 7. The magnetic layer and the back coat layer according to claim 1, wherein the magnetic layer and the back coat layer have an ultra fine indentation hardness in a range of 55 to 70 kg / mm ² (539 to 686 MPa) at a temperature of 33 ° C. and a load of 6 mgf. Magnetic recording media. The magnetic layer has a number of protrusions having a height of 20 nm or more measured with an atomic force microscope in a range of 10 to 100/900 μm ² and a center line average surface roughness Ra in a range of ² to 5 nm. Item 8. The magnetic recording medium according to any one of Items 1 to 7. The magnetic recording medium according to claim 1, wherein the back coat layer contains 2 to 10 parts by mass of carbon black having an average particle size of 230 to 300 nm with respect to 100 parts by mass of the binder.