JPH0760506B2 - Magnetic recording medium - Google Patents

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Technical Field The present invention particularly relates to a coating type magnetic recording medium having a magnetic layer containing hexagonal plate-like magnetic powder. More specifically, it is intended to improve the durability running property, the overwrite property and the output when the hexagonal plate-like magnetic powder is used for a disk medium. 2. Description of the Related Art In recent years, with respect to magnetic recording, there has been a strong demand for improvement in recording density in order to achieve a large recording capacity and a small recording size. Conventionally, as a coating type magnetic recording medium having excellent merits in productivity, etc., it has a magnetic layer containing acicular magnetic powder such as γ-Fe ₂ O ₃ and Co deposited γ-Fe ₂ O _3. Things have been widely used. However, in a magnetic recording medium containing such magnetic powder, the recording density is naturally limited. Therefore, as a measure for enabling higher recording density, a method using residual magnetization in the direction perpendicular to the surface of the magnetic recording medium has been proposed. As a medium used in such a vertical system, for example,
There is a magnetic recording medium containing a hexagonal plate-like magnetic powder such as Ba ferrite or Sr ferrite in the magnetic layer. By using such a medium, higher recording density can be realized, but when a conventional ring type magnetic head is used as a disk medium, sufficient durability running property, overwrite characteristic and output cannot be obtained. However, there is a strong need for improvement in this respect. II OBJECT OF THE INVENTION It is an object of the present invention to provide a magnetic recording medium, particularly a disk medium, which has excellent durability and overwrite characteristics, high recording density and output, and good magnetic characteristics and durability. is there. III DISCLOSURE OF THE INVENTION Such an object is achieved by the present invention described below. That is, the present invention has an average particle size of 0.02 to 0.2 on the support.
μm, having a magnetic layer having a film thickness of 0.4 to 1.8 μm containing a plate-like magnetic powder having an average plate-like ratio of 2 or more and less than 6 and a binder, and having a squareness value in the vertical direction of the medium as y, vertical The magnetic recording medium is characterized in that y ≦ −0.001x + 1.5, 450 ≦ x <750, and 0.75 <y, where x (Oe) is the value of the coercive force in the direction. IV Specific Structure of the Invention Hereinafter, the specific structure of the present invention will be described in detail. A magnetic recording medium of the present invention (hereinafter simply referred to as a medium) has a magnetic layer containing the following hexagonal plate-like magnetic powder and a binder. The thickness of such magnetic layer is 0.4 to 1.8 μm, especially
It is 4 to 1.6 μm, and more preferably 0.4 to 1.4 μm. When this value exceeds 1.8 μm, the overwrite characteristic is deteriorated, and at the same time, when a so-called ring-type head is used, overwriting is performed while maintaining high recording density. The lower limit of the film thickness is 0.4 μm, and if it is less than 0.4 μm, it becomes difficult to apply the magnetic paint, and it becomes difficult to obtain a recording medium having a smooth magnetic layer surface. Therefore, the envelope characteristic is deteriorated. As the characteristics of the medium of the present invention using such magnetic powder,
When the value of vertical squareness ratio Br⊥ / Bm⊥ in the direction perpendicular to the film surface is y and the value of coercive force Hc⊥ in the vertical direction is x (Oe), the characteristic values of these media are given by the following formulas. Within range. That is, it has characteristics within the range represented by y ≦ −0.001 + 1.5 450 ≦ x <750 0.75 <y. When the value of the perpendicular squareness ratio Br⊥ / Bm⊥ and the value of the coercive force Hc⊥ are out of the above ranges, the durability running property of the medium is not improved.
In addition, the overwrite characteristic and the recording density may decrease. Further, in the range represented by the above relational expression, y ≦ −0.001 + 1.5 500 ≦ x ≦ 700 0.75 <y is more preferable. The upper limit of the value y of the vertical squareness ratio Br⊥ / Bm⊥ is 1 or less, and particularly y is preferably 0.76 to 0.99. Within such a range of the present invention, the overwrite characteristics are satisfied, the recording density characteristics are excellent, and the vertical squareness ratio is increased, so that the effect of the plate-like particles is further increased and the running durability is improved. The property becomes good. The vertical squareness ratio and the coercive force Hc⊥ of the medium are measured as follows. That is, the magnetization curve is measured in the direction perpendicular to the medium surface, and this is corrected for demagnetizing field. Then, the residual magnetization Br⊥, the saturation magnetization Bm⊥, and the coercive force Hc⊥ are measured from this.
The vertical squareness ratio is the ratio of Br⊥ / Bm⊥. Even in the case of a so-called double-sided recording medium, the above measuring method may be used. The hexagonal plate-like magnetic powder used in the present invention is preferably a hexagonal ferrite powder such as barium ferrite or strontium ferrite. The average particle size of such magnetic powder is from 0.02 to
0.2 μm, especially 0.02 to 0.15 μm, more preferably 0.02 to
It is 0.10 μm. The average thickness of the magnetic powder is 0.10 μm or less, especially 0.003 μm.
˜0.075 μm, more preferably 0.003 to 0.05 μm. Here, the average particle size refers to, for example, about 50 cross sections of hexagonal barium ferrite particles observed by an electron microscope photograph [scanning microscope (SEM) and transmission microscope (TEM)] It is an average of the measured values. The average thickness is also the average of the measured values of electron micrographs.
The average plate-like ratio is a value of average particle diameter / average thickness. Alternatively, these values can be measured by the full width at half maximum of 2θ by X-ray diffraction. If the average particle size exceeds 0.20 μm, the surface roughness decreases and the linear recording density characteristic deteriorates. Further, the average plate-like ratio, which is a value obtained by dividing the average particle diameter by the average thickness, is 2 or more and less than 6, and particularly preferably 2 to 4. If the average plate shape ratio is 6 or more, the overwrite characteristic is deteriorated. If it is less than 2, it is difficult to increase the recording density. As the barium ferrite used, hexagonal barium ferrite such as BaFe ₁₂ O ₁₉ or Ba of barium ferrite,
Part of Fe is Ca, Sr, Pb, Co, Ni, Ti, Cr, Zn, In, Mn,
Examples include those substituted with Cu, Ge, Nb, Zr, Sn and other metals. You may use these together. Further, it may be hexagonal strontium ferrite SrFe ₁₂ O ₁₉ or one obtained by substituting it according to the above. Examples of the method for producing barium ferrite or the like include a ceramic method, a coprecipitation-firing method, a hydrothermal synthesis method, a flux method, a glass crystallization method, an alkoxide method, a plasma jet method, and the like, and any method is used in the present invention. Good. For details of these methods, refer to “Ceramics 18 (1983) No. 10” by Yoshiyasu Koike and Osamu Kubo. The coercive force of such magnetic powder is less than 800 Oe, especially 450-75
It is 0 Oe, more preferably 500 to 750 Oe. The binder used when the magnetic powder is a magnetic coating is a radiation-curable resin, a thermoplastic resin, a thermosetting or reactive resin, or a mixture thereof is used. It is preferable to use a radiation curable resin. The thermoplastic resin has a softening temperature of 150 ° C. or lower, an average molecular weight of 10,000 to 200,000 and a polymerization degree of 200 to 2,000. Such a thermosetting resin or a reactive resin also has such a degree of polymerization, and by heating after coating and drying,
The molecular weight becomes infinite by reactions such as condensation and addition. Among these resins, those that do not soften or melt before the resin thermally decomposes are preferable. Specifically, for example, phenol resin, epoxy resin, polyurethane curable resin, urea resin, butyral resin, formal resin, melamine resin, alkyd resin, silicone resin, acrylic reaction resin, polyamide resin, epoxy-polyamide resin, saturated polyester resin. , Polycondensation resins such as urea-formaldehyde resin or mixtures of high molecular weight polyester resins and isocyanate prepolymers, mixtures of methacrylate copolymers and diisocyanate prepolymers, mixtures of polyester polyols and polyisocyanates, low molecular weight glycols / high molecular weight Mixtures of the above-mentioned polycondensation resin and a crosslinking agent such as an isocyanate compound, such as a mixture of diol / triphenylmethane triisocyanate, vinyl chloride-vinyl acetate (including carboxylic acid-containing) , Vinyl chloride - vinyl alcohol -
Mixtures of vinyl copolymer resins such as vinyl acetate (including carboxylic acid-containing), vinyl chloride-vinylidene chloride, vinyl chloride-acrylonitrile, vinyl butyral, vinyl formal, etc. and a cross-linking agent, nitrocellulose, cellulose acetobutyrate, etc. A mixture of a base resin and a crosslinking agent,
A mixture of a synthetic rubber system such as butadiene-acrylonitrile and a cross-linking agent, and further a mixture thereof is preferable. And, in particular, a mixture of an epoxy resin, a butyral resin, and a phenol resin, a mixture of an epoxy resin described in US Pat. No. 3,058,844, a polyvinyl methyl ether, and a methylol phenol ether, and JP-A-49-131101.
A mixture of the bisphenol A type epoxy resin described in the above item and an acrylic acid ester or methacrylic acid ester polymer is preferable. In order to cure such a thermosetting resin, generally, it may be heated at 50 to 80 ° C. for 6 to 100 hours in a heating oven. As the binder, a binder obtained by curing a radiation curable compound, that is, a binder using a radiation curable resin is particularly preferable. Specific examples of the radiation-curable compound include acrylic double bonds such as acrylic acid, methacrylic acid, or ester compounds thereof having an unsaturated double bond having radical polymerizability, and an allyl double bond such as diallyl phthalate. It is a resin containing or introducing into a molecule of a thermoplastic resin a group such as a heavy bond, an unsaturated bond such as maleic acid or a maleic acid derivative, which is crosslinked or polymerized by irradiation with radiation. In addition, any compound having an unsaturated double bond that undergoes cross-linking polymerization upon irradiation with radiation can be used. The following unsaturated polyester resin is a resin containing a group that is crosslinked or polymerized and dried by irradiation with radiation in the molecule of the thermoplastic resin. A polyester compound having a radiation-curable unsaturated double bond in its molecular chain, for example, a saturated polyester resin composed of the ester bond of polybasic acid and polyhydric alcohol of (2) below and part of the polybasic acid is changed to maleic acid. An unsaturated polyester resin containing a radiation-curable unsaturated double bond may be mentioned. The radiation-curable unsaturated polyester resin is prepared by adding maleic acid, fumaric acid, etc. to at least one polybasic acid component and at least one polyhydric alcohol component, in the usual manner, that is, in the presence of a catalyst, at 180 to 200 ° C under a nitrogen atmosphere. After dehydration or dealcoholization reaction, heat up to 240-280 ℃, 0.5-1mmHg
It can be obtained by condensation reaction under reduced pressure. The content of maleic acid, furamic acid or the like is 1 to 40 mol%, preferably 10 to 30 mol% in the acid component due to crosslinking during production, radiation curability and the like. Examples of the thermoplastic resin that can be modified into a radiation curable resin include the following. (1) Vinyl chloride copolymer Vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl alcohol copolymer, vinyl chloride-vinyl alcohol-vinyl propionate copolymer, vinyl chloride-vinyl acetate-malein Acid copolymer, vinyl chloride-vinyl acetate-vinyl alcohol-maleic acid copolymer, vinyl chloride-vinyl acetate-terminal OH side chain alkyl group copolymer, for example VRO manufactured by UCC
H, VYNC, VYEGX, VERR, VYES, VMCA, VAGH, UCARMAG52
0, UCARMAG528 and the like, and an acrylic double bond, a maleic acid double bond, and an allyl double bond are introduced into this to perform radiation-sensitive modification. These may contain a carboxylic acid. (2) Saturated polyester resin Saturated polybasic acid such as phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid, sebacic acid and ethylene glycol, diethylene glycol, glycerin, trimethylolpropane, 1,2 propylene glycol, 1 , 3
Obtained by ester linkage with polyhydric alcohols such as butanediol, dipropylene glycol, 1,4 butanediol, 1,6 hexanediol, pentaerythritol, sorbitol, glycerin, neopentyl glycol, 1,4 cyclohexanedimethanol Examples include saturated polyester resins or resins obtained by modifying these polyester resins with SO ₃ Na or the like (for example, Vylon 53S), and these also undergo radiation sensitive modification. (3) Polyvinyl alcohol resin Polyvinyl alcohol, butyral resin, acetal resin, formal resin and copolymers of these components,
Radiation-sensitive modification is performed on the hydroxyl groups contained in these resins. (4) Epoxy resin, phonoxy resin Epoxy resin obtained by reaction of bisphenol A with epichlorohydrin, methyl epichlorohydrin, for example, Shell Chemical (Epicoat 152, 154, 828, 1001, 1004, 100
7), made by Dow Chemical (DEN431, DER732, DER511, DER33
1), made by Dainippon Ink (Epiclon 400, 800), and phenoxy resin (PKHA, PKHC, PKHH) made by UCC, which is a high-polymerization resin of the above epoxy, a copolymer of brominated bisphenol A and epichlorohydrin, large Products manufactured by Nippon Ink Chemical Co., Ltd. (Epiclon 145, 152, 153, 1120) are available, and those containing a carboxylic acid group are also included. Radiation-sensitive modification is carried out using the epoxy groups contained in these resins. (5) Fibrin derivatives Various types are used, but particularly effective ones are nitrification cotton, cellulose acetobutyrate, ethyl cellulose,
Radiation-sensitive modification is carried out by utilizing the hydroxyl group in the resin, which is preferably butyl cellulose, acetyl cellulose, or the like. Other resins that can be used for radiation-sensitive modification include polyfunctional polyester resins, polyetherester resins, polyvinylpyrrolidone resins and derivatives (PVP olefin copolymers), polyamide resins, polyimide resins, phenolic resins, spiroacetal resins, An acrylic resin containing at least one of a hydroxyl group-containing acrylic ester and a methacrylic ester as a polymerization component is also effective. Examples of the elastomer or prepolymer will be given below. (1) Polyurethane Elastomer or Prepolymer Use of Polyurethane Abrasion Resistance and Base Film,
For example, it is particularly effective in that it has good adhesion to a PET film. As an example of the urethane compound, as the isocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1,3-xylene diisocyanate, 1,4
-Xylene diisocyanate, 1,5-naphthalene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 3,3'-dimethyl-4,4'-
Diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 3,3'-dimethylbiphenylene diisocyanate, 4,4'-biphenylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, Desmodur L, Desmodur N, etc. Polyvalent isocyanate and linear saturated polyester (ethylene glycol, diethylene glycol, glycerin, trimethylolpropane, 1,4-butanediol,
Polyhydric alcohols such as 1,6-hexanediol, pentaerythritol, sorbitol, neopentyl glycol, 1,4-cyclohexanedimethanol and phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid, sebacic acid Such as polycondensation with saturated polybasic acid),
Polyurethane elastomers and prepolymers composed of polycondensation products of various polyesters such as linear saturated polyether (polyethylene glycol, polypropylene glycol, polytetramethylene glycol), caprolactam, hydroxyl-containing acrylic acid ester, hydroxyl-containing methacrylic acid ester, etc. are effective. is there. By reacting a terminal isocyanate group or hydroxyl group of these urethane elastomers with a monomer having an acrylic double bond or an allyl double bond,
Radiation-sensitive modification is very effective. It also includes those containing OH, COOH or the like as a polar group at the terminal. Further, a monomer having an unsaturated double bond having an active hydrogen that reacts with an isocyanate group and having a radiation-curable unsaturated double bond, such as a mono- or diglyceride of a long-chain fatty acid having an unsaturated double bond, is also included. (2) Acrylonitrile-Butadiene Copolymer Elastomer Acrylonitrile butadiene copolymer prepolymer with a terminal hydroxyl group commercially available as Sinclair Petrochemical PolyBD Retied Resin or an elastomer such as Hiker 1432J manufactured by Nippon Zeon Co., Ltd. The heavy bond is suitable as an elastomer component which causes radicals to be cross-linked and polymerized by radiation. (3) Polybutadiene Elastomer A prepolymer having a low molecular weight terminal hydroxyl group such as poly BD retied resin R-15 manufactured by Sinclair Petrochemical Co. is particularly preferable in view of compatibility with a thermoplastic resin. R-15
Since the molecular end of the prepolymer is a hydroxyl group, it is possible to increase the radiation sensitivity by adding an acrylic unsaturated double bond to the end of the molecule, which is more advantageous as a binder. Also cyclized polybutadiene, CBR-M90 made by Japan Synthetic Rubber
1 also has excellent properties when combined with a thermoplastic resin. Other suitable thermoplastic elastomers and prepolymers thereof include styrene-butadiene rubber, chlorinated rubber, acrylic rubber, isoprene rubber and cyclized products thereof (CIR701 manufactured by Japan Synthetic Rubber), epoxy-modified rubber, internal Elastomers such as plasticized saturated linear polyester (Toyobo Byron # 300) can also be effectively used by subjecting them to radiation-sensitive modification treatment. As the compound having a radiation-curable unsaturated double bond used in the present invention as an oligomer or a monomer, styrene, ethyl acrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol dimethacrylate, 1,6- Hexane glycol diacrylate, 1,6-hexane glycol dimethacrylate,
N-vinylpyrrolidone, pentaerythritol tetraacrylate (methacrylate), pentaerythritol triacrylate (methacrylate), trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, polyfunctional oligoester acrylate (Aronix M-7100, M-5400, 5500, 5700, Toa Gosei), urethane elastomer (Nippon Run 4040)
Modified acrylics, or those into which functional groups such as COOH have been introduced, trimethylolpropane diacrylate (methacrylate) phenol ethlenenoxide adduct acrylate (methacrylate), pentaerythritol condensation represented by the following general formula A compound having an acryl group (methacryl group) or ε-caprolactone-acryl group on the ring, In the formula, a compound of m = 1, a = 2, b = 4 (hereinafter referred to as a special pentaerythritol condensate A), a compound of m = 1, a = 3, b = 3 (hereinafter referred to as a special pentaerythritol condensate B) , M = 1, a = 6, b = 0 compound (hereinafter referred to as special pentaerythritol condensate C), m = 2, a = 6, b = 0 compound (hereinafter referred to as special pentaerythritol condensate D) , And special acrylates represented by the following general formula. _{1) (CH 2 = CHCOOCH 2} ) 3 -CCH 2 OH ( special acrylate _{A) 2) (CH 2 =} CHCOOCH 2) 3 -CCH 2 OH 3 ( special acrylate B) 3) [CH ₂ = CHCO (OC ₃ H ₆ ) _n- OCH ₂ ] _3- CCH ₂ CH ₃ (special acrylate C) _{8) CH 2 = CHCOO- (CH} 2 CH 2 O) 4 -COCH = CH 2 ( special acrylate H) _{12) AM-N n -M-} A A: acrylate, M: 2 dihydric alcohol N: 2 dibasic acid (special acrylate L) will now be described radiation-sensitive binder Synthesis Example. a) Synthesis of acrylic modified vinyl chloride vinyl acetate copolymer resin (radiation sensitive modified resin) 750 parts of partially saponified vinyl chloride-vinyl acetate copolymer having OH groups (average degree of polymerization n = 500) and toluene 1250 Parts, 500 parts of cyclohexanone are charged into a four-necked flask of 51, heated and dissolved.
After heating at 0 ℃, add 61.4 parts of 2-hydroxyethylmethacrylate adduct * of tolylene diisocyanate, add 0.012 parts of tin octylate and 0.012 parts of hydroquinone, and the NCO reaction rate becomes 90% in N ₂ gas stream at 80 ℃. Let it react. After completion of the reaction, the mixture is cooled and 1250 parts of methyl ethyl ketone is added for dilution.

[* Preparation of 2-hydroxyethylmethacrylate (2HEMa) adduct of tolylene diisocyanate (TDI) After heating 348 parts of TDI in a four-necked flask of 11 in N ₂ stream at 80 ° C, 260 parts of 2-ethylenemethacrylate, octylic acid 0.07 parts of tin and 0.05 parts of hydroquinone are used in the reactor at a temperature of 80
The reaction is completed by stirring at 80 ° C for 3 hours after completion of the dropwise addition while controlling the cooling so that the temperature becomes ~ 85 ° C. After the reaction was completed, it was taken out, cooled, and then white paste TDI
I got 2 HEMA. B) Synthesized to acrylic modified Bratil resin (radiation sensitive modified resin) Butyral resin Sekisui Chemical's BM-S 100 parts toluene 191.2
, 5 parts of cyclohexanone and 71.4 minutes of cyclohexanone were charged into a four-necked flask of 51, heated and dissolved, and after heating at 80 ° C, 7.4 parts of 2HEMA adduct * of TDI was added, and 0.015 parts of tin octylate and 0.015 parts of hydroquinone were added, and at 80 ° C. NCO reaction rate in N ₂ gas flow is 90
React until it becomes over%. After the reaction is complete, it is cooled and diluted with methyl ethyl ketone. c) Saturated polyester resin Synthesis of acrylic modified product (Radiation sensitive modified resin) Saturated polyester resin (Toyobo Byron RV-200), 100
Parts were dissolved in 116 parts of toluene and 116 parts of methyl ethyl ketone by heating and heated to 80 ° C, 3.55 parts of 2HEMA adduct * of TDI was added, and 0.007 parts of tin octylate and 0.0 of hydroquinone were added.
Add 07 parts and react at 80 ° C in N ₂ stream until the NCO reaction rate becomes 90% or more. d) ◎ Synthesis of modified acrylic resin of epoxy resin (radiation sensitive modified resin) 400 parts of epoxy resin (Epicoat 1007 manufactured by Shell Chemical Co., Ltd.) was dissolved in 50 parts of toluene and 50 parts of methyl ethyl ketone by heating,
N, N-dimethylbenzylamine 0.006 parts, hydroquinone
Add 0.003 parts to 80 ° C and add 69 parts acrylic acid dropwise.
The reaction is carried out at 0 ° C until the acid value becomes 5 or less. ◎ Synthesis of modified phenoxy resin acrylics (radiation sensitive modified resin) Phenoxy resin with OH group (PKHH: UCC molecular weight 30
000) 600 parts, methyl ethyl ketone 1800 parts were charged into a 31 four-necked flask, heated and dissolved, and heated to 80 ° C, 6.0 parts of tolylene diisocyanate 2-hydroxyethyl methacrylate adduct was further added, and tin octylate 0.012 parts and hydroquinone were added. Add 0.012 parts and react at 80 ° C in N ₂ stream until the NCO reaction rate becomes 90%. The phenoxy modified product has a molecular weight of 35,000 and one double bond per molecule. e) Synthesis of urethane elastomer acrylic modified product (radiation-curable elastomer) Diphenylmethane diisocyanate (MDI) -based urethane prepolymer with terminal isocyanate (Nipporan 3119 made by Nippon Polyurethane) 250 parts, 2HEMA 32.5 parts, hydroquinone 0.07 parts, tin octylate 0.009 80 parts
After melting by heating at ℃, 43.5 parts of TDI is heated to 80 ~ 90
Add dropwise while cooling to reach 80 ° C
React until the reaction rate reaches 95% or more. f) Synthesis of polyether-based urethane-terminated elastomer-modified acrylic (radiation-curable elastomer) Polyether PTG-500 manufactured by Nippon Polyurethane Company, 250 parts, 2
HEMA32.5 parts, hydroquinone 0.07 parts, tin octylate 0.
After putting 009 parts in a reaction vessel and heating to 80 ° C. to dissolve, 43.5 parts of TDI was added dropwise while cooling so that the temperature in the reaction vessel was 80 to 90 ° C., and after completion of the dropping, a reaction rate of 95 at 80 ° C. React until it becomes over%. g) Synthesis of acrylic modified polybutadiene elastomer (radiation curable elastomer) Sinclair Petrochemical's low molecular weight terminal hydroxyl group polybutadiene poly BD LIQUIT resin R-15 250 parts, 2HEMA
32.5 parts, hydroquinone 0.07 parts, tin octylate 0.009
Part into a reaction vessel, heat solubility to 80 ° C, drop TDI 43.5 parts while cooling so that the temperature inside the reaction vessel is 80 to 90 ° C, and after completion of dropping, a reaction rate of 95% or more at 80 ° C. Let it react until it becomes. It is known that a polymer is one which is disintegrated by irradiation with radiation and one which causes intermolecular crosslinking. Those that cause cross-linking between molecules include polyethylene, polypropylene, polystyrene, polyacrylic acid ester,
Polyacrylamide, polyvinyl chloride, polyester,
There are polyvinylpyrrolidone rubber, polyvinyl alcohol, and polyacrolene. With such a cross-linked polymer, a cross-linking reaction occurs even if the above-mentioned modification is not particularly performed, and therefore, in addition to the modified product, these resins can be used as they are. To cure such a radiation curable resin, various known methods may be used. When ultraviolet rays are used for curing, a photopolymerization sensitizer is added to the above-mentioned radiation-curable compound. The photopolymerization sensitizer may be a conventionally known one, for example, benzoin methyl ether, benzoin ethyl ether, α-methylbenzoin, benzoin compounds such as α-chlordeoxybenzoin, benzophenone, acetophenone, bisdialkylaminobenzophenone and the like. Examples thereof include ketones, quinones such as acetoraquinone and phenanthraquinone, and sulfides such as benzyl disulfide and tetramethylthiuram monosulfide. The photopolymerization sensitizer is 0.1 to 10% by weight based on the resin solid content.
The range of is desirable. For ultraviolet irradiation, for example, an ultraviolet light bulb such as a xenon discharge tube or a hydrogen discharge tube may be used. On the other hand, when an electron beam is used, the radiation characteristics are
It is convenient to use a radiation accelerator with an accelerating voltage of 100 to 750 KV, preferably 150 to 300 KV, and to irradiate it with an absorbed dose of 0.5 to 20 megarads. In particular, as the irradiation source, the method of using an electron beam by a radiation heater and the method of using the above-mentioned ultraviolet rays are advantageous from the viewpoints of controlling absorbed dose, introducing into a manufacturing process line, shielding ionizing radiation, etc. . Furthermore, according to this method, even a solventless resin that does not use a solvent can be cured in a short time, and thus such a resin can be used. By using such a radiation-curable resin, a so-called jumbo roll having a large diameter is free from winding tightness, and there is no difference in electromagnetic conversion characteristics inside and outside the jumbo roll, and the characteristics are improved. Also, because it can be done online, productivity is improved. Magnetic powder / binder is 1 / 1-9 / 1 by weight ratio, especially 2 / 1-8
It is preferably / 1. When the ratio is less than 1/1, the saturation magnetic flux density becomes low, and when it exceeds 9/1, the surface roughness becomes poor due to poor dispersion and the coating film becomes brittle, which is not preferable. In the present invention, a non-reactive solvent is used if necessary. The solvent is not particularly limited, but is appropriately selected in consideration of the solubility and compatibility of the binder. For example, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, esters such as ethyl formate, ethyl acetate and butyl acetate, alcohols such as methanol, ethanol, isopropanol and butanol, and aromatic carbonization such as toluene, xylene and ethylbenzene. Hydrogen, ethers such as isopropyl ether, ethyl ether and dioxane, and furans such as tetrahydrofuran and furfural are used as a single solvent or a mixed solvent thereof. These solvents are used in a proportion of 10 to 10000 wt%, especially 100 to 5000 wt% with respect to the binder. The magnetic layer may contain an inorganic pigment. As the inorganic pigment, 1) conductive carbon black, graphite, graphitized carbon black,
2) SiO ₂ , TiO ₂ , Al ₂ O ₃ , Cr ₂ O ₃ as an inorganic filler,
SiC, CaO, CaCO ₃ , zinc oxide, goethite, γ-Fe ₂ O ₃ ,
Talc, kaolin, CaSO ₄ , boron nitride, graphite fluoride, molybdenum disulfide, ZnS, etc. In addition to these, the following fine particle pigments (aerosil type, colloidal type): SiO ₂ , Al ₂ O ₃ , TiO ₂ , ZrO ₂ , Cr ₂ O ₃ , Y ₂ O ₃ , CeO ₂ , F
_{e 3 O 4, Fe 2 O} 3, ZrSiO 4, Sb 2 O 5, SnO 2 and the like may also be used. These fine particle pigments are, for example, in the case of SiO ₂ , ultra fine particle colloidal solution of silicic acid anhydride (Snowtex, water system, methanol silica sol, etc., Nissan Chemical), ultra fine particle anhydrous silica produced by burning purified silicon tetrachloride (standard Article 10
0Å) (Aerosil, Nippon Aerosil Co., Ltd.) and the like. Further, the ultrafine particle colloidal solution described above and aluminum oxide in the form of ultrafine particles produced by a vapor phase method similar to the above, titanium oxide and the above-mentioned fine particle pigment may be used. It is appropriate to use 1 to 30 parts by weight with respect to 100 parts by weight of the binder for 1) and 1 to 30 parts by weight for 2) with respect to 1). It has the disadvantage of becoming brittle and having more dropouts. Regarding the diameter of the inorganic pigment, it was 0.1 μm for 1).
In the following, 0.05 μm or less is more preferable, and in 2), 0.7 μm or less, further preferably 0.05 μm or less. The magnetic layer may contain a dispersant. Organic titanium coupling agents, silane coupling agents and surfactants as dispersants, natural surface active agents such as saponins as antistatic agents; nonionic surface active agents such as alkylene oxides, glycerin, glycidols; higher alkyl amines. Surfactants such as salts, quaternary ammonium salts, pyridine and other heterocycles, phosphonium or sulfonium compounds; anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfuric acid ester group and phosphoric acid ester group Agents: Amphoteric activators such as amine acids, aminosulfonic acids, sulfuric acid or phosphoric acid esters of amino alcohols, etc. are used. The magnetic layer may contain a lubricant. As the lubricant, silicone oil, fluorine oil, fatty acid, fatty acid ester, paraffin, liquid paraffin, surfactant and the like can be used as the lubricant conventionally used in this type of magnetic recording medium. Preference is given to using fatty acid esters. Examples of the fatty acid include caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, stearic acid and the like having 12 or more carbon atoms (RCOO
H and R are alkyl groups having 11 or more carbon atoms), and fatty acid esters include fatty acid esters consisting of monobasic fatty acids having 12 to 16 carbon atoms and monohydric alcohols having 3 to 12 carbon atoms, carbon A fatty acid ester or the like consisting of a monobasic fatty acid having a number of 17 or more and the number of carbon atoms of the fatty acid, and a monohydric alcohol having a carbon number of 21 to 23 is used, and the alkali metal or alkali of the fatty acid is used. Metal soaps made of earth metals, lecithin, etc. are used. As the silicone, those which are modified with fatty acids and those which are partially modified with fluorine are used. As the alcohol, a higher alcohol is used, and as the fluorine, one obtained by electrolytic substitution, telomerization, oligomerization or the like is used. Among the lubricants, it is convenient to use radiation-curable lubricants. Since these suppress backside transfer to the back side, there are advantages such as prevention of dropout, reduction of output difference due to inner and outer diameters when wound in a roll, and online manufacturing. To have. As the radiation-curable lubricant, a compound having a molecular chain exhibiting lubricity and an acrylic double bond in the molecule, for example, acrylic acid ester, methacrylic acid ester, vinyl acetate ester, acrylic acid amide compound, vinyl alcohol ester , Methyl vinyl alcohol ester, allyl alcohol ester, glyceride, and the like. When these lubricants are represented by structural formulas, CH ₂ = CHCOOR, CH ₂ = CH-CH ₂ COOR, CH ₂ = CHCONHCH ₂ COOR, In _{RCOOCH = CH 2, RCOOCH 2 -CH} = CH 2 or the like, wherein R is a linear or branched, saturated or unsaturated hydrocarbon group, the number of carbon atoms is 7 or more, preferably 12 to 23, These can also be fluorine-substituted products. As the fluorine-substituted compound, CnF _{2n + 1} −, CnF _{2n + 1} (CH ₂ ) _m − (where m = 1 to 5), CnFnCH ₂ CH ₂ NHCH ₂ CH ₂ −, Etc. Specific preferred examples of these radiation-curable lubricants include:
Examples thereof include stearic acid methacrylate (acrylate), stearyl alcohol methacrylate (acrylate), glycerin methacrylate (acrylate), glycol methacrylate (acrylate), and silicone methacrylate (acrylate). The dispersant and the lubricant may be contained in the binder in an amount of 0.1 to 20 parts by weight. As the support, polyesters such as polyethylene terephthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate, polyimide, polycarbonate, polysulfone, polyethylene naphthalide, aromatic aramid, aromatic polyester,
Further, a metal plate such as aluminum or a glass plate is used, but not limited to these. Among these, it is particularly preferable to use polyester, polyamide, polyimide or the like. In the magnetic recording medium of the present invention, a magnetic layer is usually provided on both sides of the support. Examples of such a medium include a floppy disk and a hard disk. Further, an undercoat, a backcoat and a topcoat may be provided if necessary. When the back coat is provided, it is preferable that the back coat is composed of a binder, a pigment and a lubricant. The magnetic recording medium of the present invention may be produced by a conventional method. Magnetic powder is mixed and dispersed with a binder, an organic solvent and the like to prepare a magnetic paint, and the magnetic paint is gravure coated on a substrate such as a polyester film. , Reverse roll coat, air knife coat, air doctor coat, blade coat, kiss coat, spray coat, etc. are applied using a method, and if necessary, an alignment treatment by a magnetic field in the horizontal direction or a vertical direction is performed, and then dried. Radiation curing is preferably performed according to a conventional method. Then, a back coat and a top coat may be provided if necessary. The orientation treatment may be performed by various methods according to a conventional method in order to obtain the above-described predetermined perpendicular squareness ratio and coercive force. For example, permanent magnets, DC magnets, and AC magnetic fields are typically used as orientation methods, and various combinations thereof, such as vertical and horizontal combinations, horizontal orientation, permanent magnets, or combinations of DC magnetic fields and AC magnetic fields, mechanical Various ones such as a mechanical orientation, a mechanical orientation and the above combination are used. Of these, particularly in the present invention, it is preferable to perform mechanical alignment or vertical alignment. The magnetic field strength used for orientation is preferably 1000 to 6000G. In the recording medium of the present invention, a high-permeability metal thin film such as Permalloy or an undercoat layer of various coating films may be further provided between the support and the magnetic layer. You may use these together. The undercoat layer of the coating film contains the above-mentioned thermosetting resin or radiation-curable compound, a conductive pigment, an inorganic filler, a lubricant, a dispersant for a surfactant, and the like, if necessary. Carbon black is preferable as the conductive pigment. The carbon black may be one produced by any method such as furnace, channel, acetylene, thermal, lamp, etc., but acetylene black, furnace black, channel black, roller and disk black, and German naphthalene black are preferable. The particle size of carbon black may be any, but preferred is 10 as measured by electron microscopy.
˜100 mμm, particularly preferably 10 to 80 mμm. Further, with respect to the particle size, when the particle size exceeds 100 mμm, the surface roughness of the undercoat layer surface is deteriorated, which causes deterioration of the characteristics after coating the magnetic layer. On the other hand, if it is less than 10 mμm, the dispersion is not successful and the surface roughness of the undercoat is deteriorated. A special type of carbon black is graphitized carbon black, and graphitized carbon black can also be used in the present invention. By providing such an undercoat layer, it is possible to prevent sticking of the medium to the head, sticking of guide rollers, calendar rollers and the like during the manufacturing process such as the coating process and the occurrence of discharge noise and the like. The thickness of the undercoat layer is preferably about 10Å to 5 μm. As a magnetic head to be used, for example, in a coating type magnetic recording medium, a ring type head, which has a proven track record in stability, can be used, so that it is more advantageous than a thin film type magnetic recording medium. Especially, the coercive force Hc ⊥ in the vertical direction of the medium
Is low, it is possible to use a ferrite head with a proven track record up to now, which is even more advantageous. When a ring-type head is used, the head gap affects the overwrite characteristics of the medium and the characteristics such as the linear recording density D ₅₀ . Therefore, the gap of such head is 0.35-0.5μ.
m, and particularly preferably 0.4 to 0.5 μm. If the gap becomes large, the linear recording density D ₅₀ becomes poor, and if the gap becomes small, the overwrite characteristic becomes poor. V. Specific Effect of the Invention According to the present invention, it is possible to obtain a magnetic recording medium which has excellent durability and running properties, good overwrite characteristics, high recording density, and good magnetic characteristics. Such magnetic recording media include various floppy disks,
Used for audio, video, video floppy, image file, computer disk, magnetic disk, magnetic card, etc. It is particularly suitable for floppy disks, computer disks, and magnetic disks. VI Specific Examples of the Invention Hereinafter, the present invention will be described in more detail by showing specific examples of the invention. Example 1 An undercoat layer as shown below was formed on both the front and back surfaces of a 75 μm thick polyester (PET) film. Undercoat layer Weight part CaCO ₃ 50 mμm 10 Carbon 20 mμm (graphitized carbon) 30 Vinyl chloride-vinyl alcohol copolymer (VAGH manufactured by Union Carbide) 50 Polyurethane elastomer (BF, Goodrich Esten 5703) 50 Stearic acid 2 Butyl stearate 2 Mixed solvent (MIBK / toluene = 1/1) 300 After the above mixture was dispersed in a ball mill for 5 hours, 20 parts by weight of an isocyanate compound (Coronate L manufactured by Nippon Polyurethane Company) was further added, and this mixture was added to the above polyester (PET ) A film was applied on the film to a dry thickness of 3 μm, surface smoothing was performed, and then heat curing was performed at 80 ° C. for 48 hours. A magnetic layer made of a magnetic coating as shown below was further formed on both surfaces of such an undercoat layer to prepare various samples. That is, hexagonal barium ferrite having a coercive force as shown below (Ba, Fe in BaFe ₁₂ O ₁₉ is partially C
The magnetic powders of which o and Ti were substituted were synthesized by the hydrothermal method). The average particle size of these magnetic powders was 0.07 μm, and the average plate ratio was 4. The substituted metal was analyzed by fluorescent X-ray and converted with Fe as 100%. Using these magnetic powders, a magnetic paint was prepared as follows. Barium ferrite 120 parts by weight α-Al ₂ O ₃ (0.5 μm powder) 2 parts by weight Carbon black (20 mμm) 10 parts by weight Solvent (MEK / toluene: 50/50) 100 parts by weight The above composition is mixed in a ball mill 3 The barium ferrite was well mixed by mixing for a period of time. Next, the binder shown below was put into a ball mill containing the magnetic powder mixture, and mixed and dispersed again for 42 hours. Binder composition Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (UCARMAG528 manufactured by Union Carbide Co., Ltd.) 15 parts by weight (solid type conversion), urethane (Nipporan 3022 manufactured by Nippon Polyurethane Co., Ltd.) 15 parts by weight (solid type conversion), solvent (MEK / cyclohexanone; 70/30) 200 parts by weight, 3 parts by weight of higher fatty acid-modified silicone oil, and 3 parts by weight of butyl myristate were mixed and dissolved. Further, after dispersion, 5 parts by weight (as solid content) of an isocyanate compound (Coronate L manufactured by Nippon Polyurethane Company) was added to the magnetic coating material. The magnetic coating material thus obtained was applied onto the undercoat layer by gravure coating, and various alignment treatments shown below were performed, and then the solvent was dried by a far infrared lamp or hot air. (Alignment treatment) Treatment 11: An alignment treatment was performed while drying in a vertical magnetic field (3000 G). Treatment 12: In addition to treatment 11, the pressure applied to the calendar processing was increased. Process 13: An N-S facing main magnet (3000G) having a yoke on the entrance side of the main magnet was used to perform an alignment process while drying in a vertical magnetic field. Treatment 14: In Treatment 13, the coating speed was increased and the magnetic field was further increased. Then, after performing a surface smoothing treatment, heat curing was performed at 80 ° C. for 48 hours to cure the coating film. The thickness of the coating film (magnetic layer) after curing was 1.5 μm. The film thickness was measured with an electronic micrometer. These coating films were formed on both sides of the film to give a double-sided coat. The coercive force in the perpendicular direction of these various medium samples was as shown in Table 1 below. For each of the above samples, the vertical squareness ratio, the durability running property, the overwrite property and D ₅₀ shown below were determined. (1) Vertical Squareness Ratio The squareness ratio Br⊥ / Bm⊥ in the vertical direction of the magnetic recording medium sample was measured and the demagnetizing field was corrected. When magnetic layers were formed on both sides of the support, the measurement was performed for each magnetic layer. (2) Durability Running property Measured at room temperature with the current floppy disk drive. (3) Overwrite characteristics A rectangular wave of 10KFRPI is written on a magnetic recording medium sample by a ring type head (gap 0.4μm), and 20KFR is written on it.
PI was overwritten and the output difference was measured. The larger the output difference, the better the overwrite characteristics. (4) Linear recording density D ₅₀ (KFRPI) Rotation speed 300 rpm, head; ferrite head, linear recording in which the output (E) in the low recording density area becomes 0.4 in the high recording density area with a gap of 0.4 μm. The density D ₅₀ (KFRPI) was measured. The results are shown in Table 1. Example 2 An undercoat layer as shown below was formed on both the front and back surfaces of the polyester (PET) film used in Example 1. Undercoat layer Weight part Carbon black 20 mμm 50 (A) Acrylic modified salt Bee vinegar Bi-vinyl alcohol Copolymer Molecular weight 45,000 45 (B) Acrylic modified polyurethane elastomer Molecular weight 5,000 45 (C) Pentaerythritol triacrylate 10 Stearic acid 2 Stearic acid Butyl 2 mixed solvent (MIBK / toluene = 1/1) 300 Disperse the above mixture in a ball mill for 5 hours, apply it on the above polyester (PET) film to a dry thickness of 0.7 μm, and perform surface smoothing treatment. The electron beam was applied to the undercoat layer in an N ₂ gas at an accelerating voltage of 150 KeV, an electrode current of 10 mA, an absorbed dose of 5 Mrad using an electron curtain type electron beam accelerator. A magnetic layer made of a magnetic coating as shown below was further formed on both surfaces of such an undercoat layer to prepare various samples. That is, first, Ba, F of hexagonal barium ferrite BaFe ₁₂ O ₁₉ having an average grain size of 0.05 μm and an average plate ratio of 3 is used.
A magnetic paint was prepared in the following manner by using a material in which a part of e was replaced by Co7% and Ti2%. Barium ferrite (Hc = 550 Oe) 120 parts by weight α-Al ₂ O ₃ (0.5 μm powder) 2 parts by weight Carbon black (20 mμm) 10 parts by weight Solvent (MEK / toluene; 50/50) 100 parts by weight The above composition Was mixed in a ball mill for 3 hours to thoroughly wet the barium ferrite. Next, as a binder, vinyl chloride-vinyl acetate-vinyl alcohol copolymer (maleic acid 1% content; MW40,000) 8 parts by weight (solid component conversion), acrylic double bond-introduced vinyl chloride-vinyl acetate copolymer (Containing 1% maleic acid; MW20,000) 10 parts by weight (as solid content), acrylic double bond-introduced polyether urethane elastomer (MW40,000) 9 parts by weight (as solid content), pentaerythritol triacrylate 3 200 parts by weight of solvent (MEK / toluene; 50/50) 4 parts by weight of stearic acid and 2 parts by weight of butyl stearate were mixed and dissolved. This was placed in a ball mill containing the magnetic powder mixture, and mixed and dispersed again for 42 hours. The magnetic paint thus obtained is applied by gravure coating on the above undercoat layer, and then the solvent is dried with a far infrared lamp or hot air while performing magnetic field orientation while drying in a vertical magnetic field (3000 G). Let After the surface smoothing treatment, using an ESI Electro-curtain type electron beam accelerator, accelerating voltage 150 KeV, electrode current 20 mA, irradiating electron beam under N ₂ atmosphere under the condition of total irradiation amount 5 Mrad, The coating was cured. The coercive force Hc⊥ in the vertical direction as a medium sample was 600 Oe. The thickness of the coating film (magnetic layer) after curing was various as shown in Table 2. The film thickness was measured with an electronic micrometer. These coating films were formed on both sides of the film to give a double-sided coat. The samples thus produced are referred to as Samples 201 to 208 (Table 2). The vertical squareness ratio, the overwrite characteristics, and D ₅₀ of these samples were examined. [Example 3] The PET film and the undercoat layer were the same as in Example 1, and the magnetic layer was changed as follows. That is, the average particle size is 0.06 μm, the average plate ratio is 5,
Other mixture composition, medium preparation method, etc. are described below by using magnetic powder (coercive force 600 Oe) in which part of Ba and Fe of BaFe ₁₂ O ₁₉ is replaced by Co (9%) and Ti (3%). The same procedure as in Example 1 was carried out except that the binder composition was changed as shown, and the orientation treatments were variously changed to the treatments 11, 12, and 13 described above. The thickness of the magnetic layer after the layer was dried was set to 1.5 μm. Binder composition Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (1% maleic acid content; M20,000) 10 parts by weight (solid component conversion), acrylic modified phenoxy (MW35,000) 6 parts by weight (solid component conversion) ), Acrylic modified polyether urethane elastomer (MW2
0,000) 24 parts by weight (in terms of solid content), solvent (MEK / cyclohexanone; 70/30) 200 parts by weight, higher fatty acid-modified silicone oil 3 parts by weight, butyl myristate 3 parts by weight Various types as shown in Table 3 Vertical squareness ratio, durability running property, overwrite property and D ₅₀ were measured for the media samples. The coercive force Hc⊥ in the vertical direction as a medium sample is 65
It was 0 Oe. The results are shown in Table 3. From the above results, the effect of the present invention is clear. It was confirmed that in Examples 1 to 3, curling was more likely to occur when the binder composition was a thermosetting resin than in a radiation curable resin due to winding tightness during thermosetting. Therefore, from the practical viewpoint, it is preferable that the binder composition is a radiation curable resin.

Claims (5)

[Claims] 1. A magnetic layer having a film thickness of 0.4 to 1.8 μm, containing a plate-like magnetic powder having an average particle size of 0.02 to 0.2 μm and an average plate ratio of 2 or more and less than 6 and a binder on a support. If the squareness value of the medium in the vertical direction is y and the coercive force value in the vertical direction is x (Oe), then y ≦ −0.001x + 1.5, 450 ≦ x <750, and 0.75 <y A magnetic recording medium characterized by the following. 2. The magnetic recording medium according to claim 1, wherein the plate-shaped magnetic powder is a hexagonal plate-shaped magnetic powder. 3. The magnetic recording medium according to claim 1, wherein the medium is a disk medium. 4. The magnetic recording medium according to any one of claims 1 to 3, wherein x is 500 to 700 Oe. 5. The magnetic recording medium according to any one of claims 1 to 4, wherein the binder contained in the magnetic layer is a radiation curable binder.