JP2006202415A - Manufacturing method of magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a magnetic recording medium having excellent surface smoothness and electromagnetic transducing characteristics, having enhanced productivity by curing a non-magnetic layer by irradiation with a smaller amount of electron beam. <P>SOLUTION: In the manufacturing method of the magnetic recording medium having at least a non-magnetic support, a lower non-magnetic layer on the non-magnetic support and an upper magnetic layer on the lower non-magnetic layer, a paint for the non-magnetic layer containing at least non-magnetic powder and an electron beam curing binder resin is applied on the non-magnetic support and dried to form an uncured lower non-magnetic layer, the uncured lower non-magnetic layer is irradiated with the electron beam having 1.0 to 4.0 Mrad irradiation amount to cure the lower non-magnetic layer and to obtain the cured lower non-magnetic layer having ≥80% gel fraction and a paint for the magnetic layer containing at least ferromagnetic powder and a binder resin is applied on the cured lower non-magnetic layer and dried to form the upper magnetic layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a magnetic recording medium, and more particularly to a method for manufacturing a magnetic recording medium excellent in surface smoothness and electromagnetic conversion characteristics.

  Conventionally, magnetic recording media have a magnetic layer on one surface of a nonmagnetic support, and a backcoat layer on the other surface of the nonmagnetic support to improve running durability.

  In recent years, high-density recording of magnetic recording media has been demanded in order to cope with an increase in the amount of recording data. For high density recording, the recording wavelength is shortened, and the magnetic layer is thinned. When the magnetic layer is thinned, the surface roughness of the support is reflected on the surface of the magnetic layer, the smoothness of the surface of the magnetic layer is impaired, and the electromagnetic conversion characteristics deteriorate. For this reason, for example, a nonmagnetic layer as an undercoat layer is provided on the surface of the support, and the magnetic layer is provided through the nonmagnetic layer.

  As the recording wavelength is shortened, the surface of the magnetic layer is required to be smoother from the viewpoint of spacing loss. For this reason, the surface of the nonmagnetic layer as the undercoat layer is also required to be smoother. The nonmagnetic layer is usually formed by applying and drying a coating for a nonmagnetic layer containing a nonmagnetic powder and a binder.

  For example, in Japanese Patent Application Laid-Open No. 2000-11353, for the production of a magnetic recording medium, a coating for a nonmagnetic layer containing an electron beam curable binder resin is applied on a nonmagnetic support and dried to form a lower nonmagnetic layer. It is disclosed that the upper magnetic layer is formed by performing electron beam irradiation, and then applying and drying a magnetic layer coating on the lower nonmagnetic layer. In column 9, lines 4 to 12 of the publication, “EB irradiation amount is preferably 1 to 10 Mrad, and more preferably 3 to 10 Mrad. If it is less than 3 Mrad, the stability of the coated surface of the upper magnetic layer is lacking, and irradiation exceeding 10 Mrad is performed. However, there is no difference in the physical properties of the medium, so it is not necessary.The higher the dose of the upper magnetic layer, the lower the dose of the upper magnetic layer. Therefore, it is preferable to irradiate EB separately before and after coating the upper magnetic layer for the best balance ”. That is, the EB irradiation amount is selected from the above range, but it is shown that the workability of the upper magnetic layer is in a trade-off relationship with the applicability of the upper magnetic layer and the physical properties of the medium.

  Japanese Patent Application Laid-Open No. 2004-63049 discloses a magnetic recording medium having a nonmagnetic layer containing a specific radiation curable polyurethane resin as a radiation curable binder resin. Paragraph [0029] of the publication discloses that an electron beam is preferable as the radiation, and that the electron beam irradiation amount is preferably 1 to 10 Mrad, and more preferably 3 to 7 Mrad.

JP 2000-11353 A JP 2004-63049 A

  An object of the present invention is to provide a method for producing a magnetic recording medium having improved productivity by curing a nonmagnetic layer with a smaller amount of electron beam irradiation and having excellent surface smoothness and electromagnetic conversion characteristics.

The present invention includes the following inventions.
(1) A method of manufacturing a magnetic recording medium having at least a nonmagnetic support, a lower nonmagnetic layer on the nonmagnetic support, and an upper magnetic layer on the lower nonmagnetic layer,
On the nonmagnetic support, a coating for a nonmagnetic layer containing at least a nonmagnetic powder and an electron beam curable binder resin is applied and dried to form an uncured lower nonmagnetic layer,
An uncured lower nonmagnetic layer is irradiated with an electron beam at an irradiation dose in the range of 1.0 to 4.0 Mrad, the lower nonmagnetic layer is cured, and a cured lower nonmagnetic layer having a gel fraction of 80% or more is obtained. Get,
A method of manufacturing a magnetic recording medium, comprising: applying a magnetic layer coating material containing at least a ferromagnetic powder and a binder resin on a cured lower nonmagnetic layer and drying to form an upper magnetic layer.

  (2) The method for producing a magnetic recording medium according to (1), wherein an electron beam sensitive modified vinyl chloride copolymer is used as at least part of the electron beam curable binder resin.

  (3) The method for producing a magnetic recording medium according to (2), wherein the electron beam-sensitive modified vinyl chloride copolymer has a (meth) acryloyl group in a side chain.

The gel fraction is the weight fraction of a resin that is insoluble in the solvent in the cured lower nonmagnetic layer, based on the total resin component weight. The measuring method is described in the examples.
The higher the gel fraction value, the more the curing or cross-linking reaction of the electron beam curable binder resin of the lower nonmagnetic layer, and the higher hardness and high surface smoothness of the lower nonmagnetic layer can be obtained.

  According to the present invention, a lower nonmagnetic layer is cured with a smaller amount of electron beam irradiation using a coating for a nonmagnetic layer containing an electron beam curable binder resin, and a lower surface layer having a high gel fraction and a smooth surface. A nonmagnetic layer is formed. Thereafter, since the upper magnetic layer is formed on the lower nonmagnetic layer having a high gel fraction and a smooth surface, a magnetic recording medium having excellent surface smoothness and electromagnetic conversion characteristics can be produced with high productivity.

  As an example of the magnetic recording medium produced in the present invention, a lower nonmagnetic layer having a thickness of 0.3 to 2.5 μm is provided on one surface of a nonmagnetic support, and a thickness of 0. An upper magnetic layer of 03 to 0.30 μm is provided, and a backcoat layer is provided on the other surface of the nonmagnetic support as necessary. In the present invention, a lubricant coating or various coatings for protecting the magnetic layer may be provided on the upper magnetic layer as necessary. In addition, an undercoat layer (an easy adhesion layer) may be provided on the one surface on which the magnetic layer of the nonmagnetic support is provided for the purpose of improving the adhesion between the coating film and the nonmagnetic support. .

[Lower nonmagnetic layer]
The lower nonmagnetic layer is formed using a coating for a nonmagnetic layer containing at least a nonmagnetic powder and an electron beam curable binder resin. Nonmagnetic powders include carbon black and nonmagnetic inorganic powders other than carbon black.

As carbon black contained in the nonmagnetic layer, furnace black for rubber, thermal black for rubber, black for color, acetylene black, and the like can be used. The specific surface area is preferably ⁵ to 600 m ² / g, the DBP oil absorption is 30 to 400 ml / 100 g, and the particle diameter is preferably 10 to 100 nm. Specifically, the carbon black that can be used can be referred to “Carbon Black Handbook”, edited by the Carbon Black Association.

  The blending amount of carbon black is 5 to 30% by weight, preferably 10 to 25% by weight in the lower nonmagnetic layer.

Various nonmagnetic inorganic powders other than carbon black can be used for the nonmagnetic layer. For example, acicular nonmagnetic iron oxide (α-Fe ₂ O ₃ ), CaCO ₃ , titanium oxide, barium sulfate, α-Al Inorganic powders such as ₂ O ₃ are mentioned.

  The blending ratio of carbon black and inorganic powder other than the carbon black is preferably 100/0 to 5/95 in terms of weight ratio (carbon black / inorganic powder). When the blending ratio of carbon black is less than 5 parts by weight, a problem occurs in surface electrical resistance.

  In the present invention, an electron beam curable resin is used as a binder for the lower nonmagnetic layer. As at least a part of the electron beam curable binder resin, it is preferable to use, for example, an electron beam sensitive modified vinyl chloride copolymer in an amount of 50% by weight or more.

  The vinyl chloride copolymer preferably has a vinyl chloride content of 50 to 95% by weight, particularly 55 to 90% by weight, and the average degree of polymerization is preferably about 100 to 500%. Particularly preferred is a copolymer of vinyl chloride and a monomer containing an epoxy (glycidyl) group. The vinyl chloride copolymer has been subjected to electron beam sensitive modification by introducing a (meth) acrylic double bond or the like by a known method.

  When the vinyl chloride copolymer is a copolymer of vinyl chloride and an epoxy group-containing monomer, for example, 2-isocyanate ethyl methacrylate (MOI: 2-methacryloyloxyethyl isocyanate) or 2-isocyanate ethyl acrylate (Meth) acryloyl group can be introduced into the epoxy group or hydroxyl group present in the side chain of the vinyl chloride copolymer through a urethane bond. Also, an isocyanate / acrylate-containing adduct (HPA-IPDI) having a structure in which hydroxypropyl acrylate (HPA) is added to one of the two isocyanate groups of isophorone diisocyanate (IPDI) via a hydroxy group is used. Thus, an acryloyl group can be introduced into the epoxy group or hydroxyl group present in the side chain of the vinyl chloride copolymer via a urethane bond.

  As described above, the vinyl chloride copolymer is subjected to electron beam sensitive modification with 2-isocyanatoethyl (meth) acrylate and electron beam sensitive modification with an isocyanate / acrylate-containing adduct (HPA-IPDI). A cured lower nonmagnetic layer having a gel fraction of 80% or more can be obtained with a low electron beam dose in the range of 0 to 4.0 Mrad. However, in order to obtain a cured lower nonmagnetic layer having a gel fraction of 80% or more at a lower electron beam dose, it is preferable to use electron beam sensitive modification with an isocyanate / acrylate-containing adduct (HPA-IPDI). . Details of the denaturation method are given in the examples.

  In addition to the electron beam curable vinyl chloride copolymer resin, if a diacrylate adduct (IPDI-2HPA) is present in the lower non-magnetic layer material as an electron beam curable polyfunctional monomer described later, the crosslinking density increases. For example, even with an electron beam irradiation dose as low as about 2.2 Mrad, a gel fraction of 84% can be achieved.

  As the electron beam curable resin, it is preferable to use the vinyl chloride copolymer in combination with an electron beam curable polyurethane resin.

  The polyurethane resin used in combination with the vinyl chloride resin is a general term for resins obtained by reaction of a hydroxyl group-containing resin such as polyester polyol and / or polyether polyol and a polyisocyanate-containing compound, and has a number average molecular weight of 5,000 to It is about 200,000 and has a Q value (weight average molecular weight / number average molecular weight) of about 1.5 to 4. The polyurethane resin has been subjected to electron beam sensitive modification by introducing a (meth) acrylic double bond by a known method.

  In addition to the vinyl chloride copolymer and the polyurethane resin, various known resins may be contained in the nonmagnetic layer in the range of 20% by weight or less of the total binder.

  In the present invention, in order to improve the crosslinking rate of the electron beam curable binder resin, an electron beam curable polyfunctional monomer, preferably a polyfunctional (meth) acrylate, may be used as a crosslinking agent.

  The polyfunctional (meth) acrylic monomer is not particularly limited. For example, ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, 1,6-hexane Examples include glycol di (meth) acrylate, pentaerythritol tetra (meth) acrylate, trimethylolpropane tri (meth) acrylate, and trimethylolpropane di (meth) acrylate.

The following diacrylate adducts are preferably used as the polyfunctional (meth) acrylic monomer.
A diacrylate adduct (IPDI-2HPA) having a structure in which hydroxypropyl acrylate (HPA) is added to each of two isocyanate groups of isophorone diisocyanate (IPDI) via a hydroxy group,
A diacrylate adduct (IPDI-2HEA) having a structure in which hydroxyethyl acrylate (HEA) is added to two isocyanate groups of isophorone diisocyanate (IPDI) via a hydroxy group,
Diacrylate adduct (TDI-2HPA) having a structure in which hydroxypropyl acrylate (HPA) is added to two isocyanate groups of tolylene 2,4-diisocyanate (TDI) via a hydroxy group.

  When used, the electron beam curable polyfunctional monomer is used in an amount of 1 to 50 parts by weight, preferably 5 to 40 parts by weight based on 100 parts by weight in total of the electron beam curable binder resin and the electron beam curable polyfunctional monomer. It is good to use part.

  The content of the binder resin used in the lower nonmagnetic layer is preferably 10 to 100 parts by weight, more preferably 100 parts by weight, in total, of 100 parts by weight of the nonmagnetic inorganic powder other than carbon black and carbon black in the lower nonmagnetic layer. Is 12-30 parts by weight. If the content of the binder is too small, the ratio of the binder resin in the lower nonmagnetic layer is lowered, and sufficient coating strength cannot be obtained. When the content of the binder is too large, the tape medium tends to be strongly curved in the tape width direction, and the contact with the head tends to be poor.

  The lower nonmagnetic layer preferably contains a lubricant as necessary. As a lubricant, regardless of whether it is saturated or unsaturated, fatty acids such as stearic acid and myristic acid, fatty acid esters such as butyl stearate and butyl palmitate, saccharides, and the like can be used alone or in combination of two or more. It is also possible to use a mixture of two or more fatty acids having different melting points, or a mixture of two or more fatty acid esters having different melting points. This is because it is necessary to continuously supply a lubricant corresponding to any temperature environment used for the magnetic recording medium to the surface of the medium.

  The lubricant content of the lower nonmagnetic layer may be appropriately adjusted according to the purpose, but is preferably 1 to 20% by weight based on the total weight of carbon black and the nonmagnetic inorganic powder other than carbon black.

  The coating material for forming the lower nonmagnetic layer is prepared by adding an organic solvent to each of the above components and mixing, stirring, kneading, dispersing, and the like by a known method. The organic solvent to be used is not particularly limited, and one or more of various solvents such as ketone solvents such as methyl ethyl ketone (MEK), methyl isobutyl ketone and cyclohexanone, and aromatic solvents such as toluene are appropriately selected. Use it. The addition amount of the organic solvent may be about 100 to 900 parts by weight with respect to 100 parts by weight of the total amount of polyfunctional monomers when used, such as carbon black, various inorganic powders other than carbon black, and binder resins. .

  The thickness of the lower nonmagnetic layer is usually 0.3 to 2.5 μm, preferably 0.3 to 2.3 μm. If the nonmagnetic layer is too thin, it is easily affected by the surface roughness of the nonmagnetic support. As a result, the surface smoothness of the nonmagnetic layer is deteriorated and the surface smoothness of the magnetic layer is also easily deteriorated. The conversion characteristics tend to be reduced. Further, since the light transmittance becomes high, it becomes a problem when the edge of the medium is detected by a change in the light transmittance. On the other hand, even if the nonmagnetic layer is thickened to some extent, the performance is not improved.

[Upper magnetic layer]
The upper magnetic layer contains at least a ferromagnetic powder and a binder resin.

In the present invention, it is preferable to use metallic magnetic powder or hexagonal plate-like fine powder as the ferromagnetic powder. The metal magnetic powder has a coercive force Hc of 118.5 to 237 kA / m (1500 to 3000 Oe), a saturation magnetization σs of 120 to 160 Am ² / kg (emu / g), and an average major axis length of 0.03 to 0.3. It is preferable that 1 μm, the average minor axis length is 10 to 20 nm, and the aspect ratio is 1.2 to 20. The Hc of the medium produced using the metal magnetic powder is preferably 118.5 to 237 kA / m (1500 to 3000 Oe). The hexagonal plate-like fine powder has a coercive force Hc of 79 to 237 kA / m (1000 to 3000 Oe), a saturation magnetization σs of 50 to 70 Am ² / kg (emu / g), an average plate particle size of 30 to 80 nm, a plate The ratio is preferably 3-7. The Hc of the medium produced using the hexagonal plate-like fine powder is preferably 94.8 to 238.7 kA / m (1200 to 3000 Oe).

  Here, the average major axis length of the ferromagnetic powder is obtained by separating and collecting the magnetic powder from the tape piece and measuring the major axis length of the powder from a photograph taken with a transmission electron microscope (TEM). it can. An example of the procedure is shown below. (1) Remove the back coat layer from the tape piece with a solvent. (2) The tape piece sample in which the lower nonmagnetic layer and the upper magnetic layer remain on the nonmagnetic support is immersed in a 5% NaOH aqueous solution, and heated and stirred. (3) Wash the coating film removed from the non-magnetic support and dry it. (4) The dried coating film is sonicated in methyl ethyl ketone (MEK), and the magnetic powder is adsorbed and collected using a magnetic stirrer. (5) Separate the magnetic powder from the residue and dry. (6) Collect the magnetic powder obtained in (4) and (5) on a dedicated mesh, prepare a sample for TEM, and take a photograph with TEM. (7) The major axis length of the magnetic powder in the photograph is measured and averaged (number of measurements: n = 100).

  The ferromagnetic powder may be contained in an amount of about 70 to 90% by weight based on the magnetic layer. If the content of the ferromagnetic powder is too large, the content of the binder is decreased, so that the surface smoothness due to calendering tends to deteriorate. On the other hand, if the content of the ferromagnetic powder is too small, a high reproduction output is obtained. I can't.

  The binder for the magnetic layer is not particularly limited, and a thermoplastic resin, a thermosetting or reactive resin, a radiation (electron beam or ultraviolet ray) curable resin, and the like are appropriately selected according to the characteristics and process conditions of the medium. Have been used.

  The content of the binder resin used in the magnetic layer is preferably 5 to 40 parts by weight, particularly preferably 10 to 30 parts by weight with respect to 100 parts by weight of the ferromagnetic powder. When the content of the binder is too small, the strength of the magnetic layer is lowered, and the running durability tends to be deteriorated. On the other hand, when the content of the binder is too large, the content of the ferromagnetic powder is lowered, so that the electromagnetic conversion characteristics tend to be lowered.

  Furthermore, in order to increase the mechanical strength of the magnetic layer and to prevent clogging of the magnetic head, the magnetic layer contains an abrasive having a Mohs hardness of 6 or higher, such as α-alumina (Mohs hardness 9). Such an abrasive is usually indefinite shape, prevents clogging of the magnetic head, and improves the strength of the coating film.

  The average particle diameter of the abrasive is, for example, 0.01 to 0.2 μm, and preferably 0.05 to 0.2 μm. If the average particle size is too large, the amount of protrusion from the surface of the magnetic layer becomes large, leading to a decrease in electromagnetic conversion characteristics, an increase in dropout, an increase in head wear, and the like. If the average particle size is too small, the amount of protrusion from the surface of the magnetic layer becomes small, and the effect of preventing head clogging becomes insufficient.

The average particle size is usually measured with a transmission electron microscope. The content of the abrasive may be 3 to 25 parts by weight, preferably 5 to 20 parts by weight with respect to 100 parts by weight of the ferromagnetic powder.
Further, in the magnetic layer, a dispersant such as a surfactant, a lubricant such as a higher fatty acid, a fatty acid ester, silicon oil, and other various additives may be added as necessary.

  The coating material for forming the magnetic layer is prepared by adding an organic solvent to each of the above components and mixing, stirring, kneading, dispersing and the like by a known method. The organic solvent to be used is not particularly limited, and those similar to those used for the lower nonmagnetic layer can be used.

  The thickness of the magnetic layer is 0.03 to 0.30 μm, more preferably 0.10 to 0.25 μm. If the magnetic layer is too thick, self-demagnetization loss and thickness loss increase.

  The centerline average roughness (Ra) of the magnetic layer surface is preferably 1.0 to 5.0 nm, more preferably 1.0 to 4.0 nm. If the Ra is less than 1.0 nm, the surface is too smooth, the running stability is deteriorated, and trouble during running tends to occur. On the other hand, when the thickness exceeds 5.0 nm, the surface of the magnetic layer becomes rough, and in a reproduction system using an MR type head, electromagnetic conversion characteristics such as reproduction output deteriorate.

[Back coat layer]
The backcoat layer is provided as necessary for improving running stability and antistatic of the magnetic layer, and the structure and composition are not particularly limited.For example, carbon black, nonmagnetic inorganic powder other than carbon black, And those containing a binder resin.

  The backcoat layer preferably contains 30 to 80% by weight of carbon black based on the backcoat layer.

In addition to the carbon black, various nonmagnetic inorganic powders can be used for the backcoat layer in order to control the mechanical strength. Examples of the inorganic powder include α-Fe ₂ O ₃ , CaCO ₃ , titanium oxide, Examples thereof include barium sulfate and α-Al ₂ O ₃ .

  The coating material for forming the backcoat layer is prepared by adding an organic solvent to each of the above components and mixing, stirring, kneading, dispersing and the like by a known method. The organic solvent to be used is not particularly limited, and those similar to those used for the lower nonmagnetic layer can be used.

  The thickness of the back coat layer (after calendering) is 1.0 μm or less, preferably 0.1 to 1.0 μm, more preferably 0.2 to 0.8 μm.

[Non-magnetic support]
The material used as the nonmagnetic support is not particularly limited, and may be selected from various flexible materials and various rigid materials according to the purpose, and may have a predetermined shape and size such as a medium according to various standards. Examples of the flexible material include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins such as polypropylene, and various resins such as polyamide, polyimide, and polycarbonate.

  The thickness of these nonmagnetic supports is preferably 3.0 to 15.0 μm. The form of the nonmagnetic support is not particularly limited, and may be any of a tape form, a sheet form, a card form, a disk form, and the like, and various materials are selected depending on the form and if necessary. Can be used.

[Manufacture of magnetic recording media]
In the present invention, each coating film (coating layer) is prepared by coating, drying, calendering, curing, etc., using each of the prepared non-magnetic layer forming coating material, magnetic layer forming coating material, and backcoat layer forming coating material. And forming a magnetic recording medium.

  In the present invention, the lower nonmagnetic layer and the upper magnetic layer are formed by a so-called wet-on-dry coating method. That is, first, a coating for a nonmagnetic layer is applied on one surface of a nonmagnetic support and dried, and a calender treatment is performed as necessary to obtain an uncured lower nonmagnetic layer. Thereafter, the uncured lower nonmagnetic layer is irradiated with an electron beam at an irradiation dose in the range of 1.0 to 4.0 Mrad, the lower nonmagnetic layer is cured, and the cured lower nonmagnetic layer has a gel fraction of 80% or more. Get a layer. Next, the magnetic layer coating material is applied, oriented and dried on the cured lower nonmagnetic layer to form an upper magnetic layer. The backcoat layer is provided as necessary, and the order of formation of the backcoat layer is arbitrary, that is, before the formation of the lower nonmagnetic layer, after the formation of the lower nonmagnetic layer and before the formation of the upper magnetic layer, the upper layer It may be any after the formation of the magnetic layer.

  The electron beam dose during curing of the lower nonmagnetic layer is selected from the range of 1.0 to 4.0 Mrad. When the irradiation amount is less than 1.0 Mrad, curing is insufficient and the gel fraction becomes less than 80%, which adversely affects the smoothness of the magnetic layer surface. On the other hand, when the irradiation amount exceeds 4.0 Mrad, there is no problem in terms of curing, but an irradiation time corresponding to the irradiation amount is required, and as a result, improvement in medium productivity cannot be expected. More preferably, the dose is in the range of 2.0 to 3.0 Mrad.

  In the present invention, since the electron beam curable resin as described above is preferably used for the lower nonmagnetic layer, the lower nonmagnetic layer is cured with a small electron beam dose in the range of 1.0 to 4.0 Mrad. Thus, the gel fraction of the lower nonmagnetic layer is 80% or more, preferably 85% or more. And since the coating material for magnetic layers is apply | coated on the hardened lower nonmagnetic layer, there is no disorder of the interface of a nonmagnetic layer and a magnetic layer. Therefore, a magnetic layer having excellent surface smoothness is obtained, and the electromagnetic conversion characteristics of the medium are also improved. It is preferable to calender the lower non-magnetic layer before applying the upper magnetic layer, because a magnetic layer having better surface smoothness can be obtained and the electromagnetic conversion characteristics can be further improved. For example, when the upper magnetic layer is applied, the center line average roughness (Ra) of the lower nonmagnetic layer surface is preferably 1.5 to 4.5 nm, more preferably 2.0 to 4.0 nm. When Ra exceeds 4.5 nm, the surface of the upper magnetic layer tends to be rough due to the influence of the roughness. On the other hand, there is not much need for Ra of less than 1.5 nm.

  As a coating method, various known coating means such as gravure coating, reverse roll coating, die nozzle coating, and bar coating can be used.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

[Synthesis example 1 of electron beam curable vinyl chloride resin used for non-magnetic paint]
Electron beam sensitive modification by IPDI-HPA adduct:
A 1 liter three-necked flask is charged with 424 parts by weight of isophorone diisocyanate (IPDI), 0.4 parts by weight of dibutyltin dilaurate, and 0.24 parts by weight of 2,6-tert-butyl-4-methylphenol (BHT). While controlling the reaction mixture at 60 ° C., 372 parts by weight of 2-hydroxypropyl acrylate (HPA) was added dropwise. After completion of the dropping, the reaction mixture was stirred at 60 ° C. for 2 hours, and then an IPDI-HPA adduct was obtained. In the obtained IPDI-HPA adduct body, the diacrylate adduct body (IPDI-2HPA) was also contained.

Next, 630 parts by weight of vinyl chloride resin MR110 manufactured by Nippon Zeon Co., Ltd. was dissolved in 2291 parts by weight of methyl ethyl ketone (MEK), and the water content of the solution was measured and found to be 0.03% by weight. Therefore, the water content of the solution was adjusted to 0.2% by weight and charged with 2.45 parts by weight of dibutyltin dilaurate and 0.09 parts by weight of 2,6-tert-butyl-4-methylphenol (BHT). After stirring at 70 ° C. for 3 hours, 352 parts by weight of the previously obtained IPDI-HPA adduct body was added. The reaction mixture was stirred at 70 ° C. for 15 hours, and after confirming the disappearance of the characteristic absorption (2270 cm ⁻¹ ) of the isocyanate group in the IR spectrum, the reaction mixture was taken out to obtain an electron beam curable vinyl chloride resin (R1). The obtained resin (R1) contained a diacrylate adduct (IPDI-2HPA).

[Synthesis example 2 of electron beam curable vinyl chloride resin used for nonmagnetic paint]
Electron beam sensitive modification with 2-isocyanatoethyl acrylate:
A 3-liter three-necked flask is charged with 500 parts by weight of vinyl chloride resin MR110 manufactured by Nippon Zeon Co., Ltd., 1250 parts by weight of methyl ethyl ketone (MEK), 0.5 parts by weight of dibutyltin dilaurate, and 0.3 parts by weight of hydroquinone, After stirring at 70 ° C. for 3 hours, 25 parts by weight of 2-isocyanatoethyl acrylate was added. The reaction mixture was stirred at 70 ° C. for 15 hours, and after confirming the disappearance of the characteristic absorption (2270 cm ⁻¹ ) of the isocyanate group in the IR spectrum, the reaction mixture was taken out to obtain an electron beam curable vinyl chloride resin (R2).

[Example 1]
(Preparation of non-magnetic paint)
Pigment Acicular α-FeOOH 80.0 parts by weight (average major axis length: 0.1 μm, crystallite diameter: 12 nm)
20.0 parts by weight of carbon black (trade name: # 950B, manufactured by Mitsubishi Chemical Corporation, average particle size: 17 nm, BET specific surface area: 250 m ² / g, DBP oil absorption: 70 ml / 100 g, pH: 8)
Electron beam curable vinyl chloride resin (R1) 12.0 parts by weight Electron beam curable polyester polyurethane resin 10.0 parts by weight (Toyobo Co., Ltd., trade name: TB-0216, (solid content) hydroxy-containing acrylic compound-phosphon Acid group-containing phosphorus compound-hydroxy-containing polyester polyol, average molecular weight: 13,000, P content: 0.2% (weight percentage), acrylic content: 8 mol / 1 mol) Dispersant Phosphoric acid surfactant 3 .2 parts by weight (trade name: RE610, manufactured by Toho Chemical Industry Co., Ltd.)
Abrasive material α-alumina 5.0 parts by weight (manufactured by Sumitomo Chemical Co., Ltd., trade name: HIT60A, average particle size: 0.18 μm)
NV (solid content concentration) = 33% (mass percentage)
Solvent ratio MEK / toluene / cyclohexanone = 2/2/1 (mass ratio)

The above materials were kneaded with a kneader and then dispersed in a horizontal pin mill in which 0.8 mm zirconia beads were filled at a filling rate of 80% (porosity: 50 vol%). Then further lubricant materials:

Lubricant Fatty acid 0.5 parts by weight (trade name: NAA180 manufactured by NOF Corporation)
Lubricant Fatty Acid Amide 0.5 parts by weight (trade name: Fatty Acid Amide S, manufactured by Kao Corporation)
Lubricant Fatty acid ester 1.0 part by weight (trade name: NIKKOLBS manufactured by Nikko Chemicals Co., Ltd.)

Add
NV (solid content concentration) = 25% (mass percentage)
Solvent ratio MEK / toluene / cyclohexane = 2/2/1 (mass percentage)
After being diluted so as to be dispersed, dispersion was performed. The obtained paint was further filtered through a filter having an absolute filtration accuracy of 3.0 μm to produce a non-magnetic paint.

(Preparation of magnetic paint)
Magnetic powder Fe-based acicular ferromagnetic powder 100.0 parts by weight (Fe / Co / Al / Y = 100/24/5/8 (atomic ratio), Hc: 188 kA / m, σs: 140 Am ² / kg, BET ratio (Surface area value: 50 m ² / g, average long axis length: 0.10 μm)
Binder resin Vinyl chloride copolymer 10.0 parts by weight (manufactured by Nippon Zeon Co., Ltd., trade name: MR110)
Binder resin Polyester polyurethane 6.0 parts by weight (trade name: UR8300, manufactured by Toyobo Co., Ltd.)
Dispersant Phosphoric surfactant 3.0 parts by weight (manufactured by Toho Chemical Co., Ltd., trade name: RE610)
Abrasive material α-alumina 10.0 parts by weight (manufactured by Sumitomo Chemical Co., Ltd., trade name: HIT60A, average particle size: 0.18 μm)
NV (solid content concentration) = 30% (mass percentage)
Solvent ratio MEK / toluene / cyclohexanone = 4/4/2 (mass ratio)

The above materials were kneaded with a kneader, and then dispersed as a pre-dispersion by a horizontal pin mill filled with 0.8 mm zirconia beads at a filling rate of 80% (porosity 50 vol%). Then, further
NV (solid content concentration) = 15% (mass percentage)
Solvent ratio MEK / toluene / cyclohexane = 22.5 / 22.5 / 55 (mass ratio)
After diluting so that it becomes, finish dispersion was performed.

  After adding 10 parts by weight of a curing agent (Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd.) to the obtained coating material, it was further filtered through a filter having an absolute filtration accuracy of 1.0 μm to prepare a magnetic coating material.

(Non-magnetic layer forming process)
On one side of a 6.2 μm thick PEN support, a nonmagnetic paint was applied by an extrusion coating method using a nozzle so that the thickness after calendering was 2.0 μm and dried. After that, with a calendar combining a plastic roll and a metal roll, processing was performed at a nip number of 4 times, a processing temperature of 100 ° C., a linear pressure of 3500 N / cm, a speed of 160 m / min, and further to 4.0 Mrad and an acceleration voltage of 200 kV. The lower nonmagnetic layer was formed by electron beam irradiation.

(Magnetic layer forming process)
On the nonmagnetic layer formed as described above, a magnetic coating material was applied with a nozzle so that the thickness after processing was 0.1 μm, oriented, and dried. Thereafter, a calender combining a plastic roll and a metal roll was used to process at a nip number of 4 times, a processing temperature of 100 ° C., a linear pressure of 3500 N / cm, and a speed of 160 m / min to form an upper magnetic layer.

  The magnetic recording tape raw material obtained as described above was thermally cured at 60 ° C. for 48 hours and slit (cut) to a width of 1/2 inch (= 12.650 mm), and the magnetic recording tape sample of Example 1 A data tape was prepared.

[Examples 2 to 4]
As shown in Table 1, the procedure of Example 1 was repeated except that the electron beam dose was changed to 3.4 Mrad (Example 2), 2.7 Mrad (Example 3), and 2.2 Mrad (Example 4). Thus, magnetic recording tape samples were prepared.

[Example 5]
As shown in Table 1, magnetic recording was performed in the same manner as in Example 1 except that 12.0 parts by weight of an electron beam curable vinyl chloride resin (R2) was used instead of the resin (R1) in the nonmagnetic paint. A tape sample was prepared.

[Comparative Examples 1-3]
As shown in Table 1, the electron beam irradiation dose was changed to 3.4 Mrad (Comparative Example 1), 2.7 Mrad (Comparative Example 2), and 2.2 Mrad (Comparative Example 3). Thus, magnetic recording tape samples were prepared.

[Evaluation of magnetic tape]
The following evaluation was performed for each magnetic recording tape sample.

(Observation of tape surface)
The magnetic layer surface of the tape was observed at a magnification of 100 times and a magnification of 200 times using a differential interference optical microscope. Table 1 shows the case where no dimple-like surface roughness was observed on the observation surface as ◯, and the case where dimple-like surface roughness was observed as x.
Dimple-like surface roughness is a phenomenon that occurs when a lower non-magnetic layer is insufficiently cured and a magnetic paint is applied thereon.

(Surface roughness (centerline surface roughness): Ra)
Using the “TALYSTEP system” (made by Taylor Hobson), Ra on the surface of the upper magnetic layer was measured based on JIS B0601-1982.
The measurement conditions were a filter of 0.18 to 9 Hz, a stylus 0.1 × 2.5 μm stylus, a stylus pressure of 2 mg, a measurement speed of 0.03 mm / sec, and a measurement length of 500 μm.
In addition, the measurement of Ra on the surface of the upper magnetic layer was performed after the final calendar process and the curing process.

(Gel fraction measurement method)
Using the release film as a support, a sample for gel fraction measurement was prepared as follows.
Each electron beam curable resin was adjusted to a solid content concentration of 20% by weight (solvent: MEK) to obtain a coating solution. The coating solution was applied onto the release film with an applicator and dried at 90 ° C. for 5 minutes to prepare a resin coating film having a thickness of 25 μm. This was used as a sample for gel fraction measurement.

The obtained uncured resin coating was irradiated with an electron beam at each dose shown in Table 1 to perform electron beam curing. Next, the cured resin coating film was peeled from the release film and cut into a size of about 1 cm × 4 cm. The weight of the cut cured resin coating film (measured as A (g)) was measured, and then the cured resin coating film was refluxed in MEK for 5 hours. After drying for a period of time, the weight (referred to as B (g)) was measured. From the obtained results, the gel fraction of the electron beam curable resin under each irradiation condition was determined by the following formula.
Gel fraction (%) = (B / A) × 100

The results are shown in Table 1.
From Table 1, in any of the tape samples of Examples 1 to 5, no dimple-like surface roughness was observed on the magnetic layer surface of the tape, and Ra on the upper magnetic layer surface was excellent.

  The tape samples of Comparative Examples 1 to 3 used the same electron beam curable vinyl chloride resin (R2) used in Example 5, but the gel fraction of the lower nonmagnetic layer had reached 80%. In addition, the lower nonmagnetic layer was not sufficiently cured, dimple-like surface roughness was observed, and Ra of the upper magnetic layer was increased.

Claims (3)

A method for producing a magnetic recording medium comprising at least a nonmagnetic support, a lower nonmagnetic layer on the nonmagnetic support, and an upper magnetic layer on the lower nonmagnetic layer,
On the nonmagnetic support, a coating for a nonmagnetic layer containing at least a nonmagnetic powder and an electron beam curable binder resin is applied and dried to form an uncured lower nonmagnetic layer,
An uncured lower nonmagnetic layer is irradiated with an electron beam at an irradiation dose in the range of 1.0 to 4.0 Mrad, the lower nonmagnetic layer is cured, and a cured lower nonmagnetic layer having a gel fraction of 80% or more is obtained. Get,
A method of manufacturing a magnetic recording medium, comprising: applying a magnetic layer coating material containing at least a ferromagnetic powder and a binder resin on a cured lower nonmagnetic layer and drying to form an upper magnetic layer.   The method for producing a magnetic recording medium according to claim 1, wherein an electron beam sensitive modified vinyl chloride copolymer is used as at least a part of the electron beam curable binder resin. The method for producing a magnetic recording medium according to claim 2, wherein the electron beam sensitive modified vinyl chloride copolymer has a (meth) acryloyl group in a side chain.