JPH0762881B2 - Magnetic recording / reproducing system - Google Patents

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording / reproducing system in which a magnetic recording medium having a magnetic recording layer containing barium ferrite magnetic powder having specific characteristics and a ferrite head having a specific head gap are combined. Problems to be Solved by the Related Art and Invention Presently, magnetic recording media are widely used in the fields of audio, video, computers, magnetic disks, 8 m / m, etc. It is expected to be used in the field of high-density floppy disks, etc., and accordingly, the amount of information recorded on magnetic recording media is also increasing year by year. Therefore, there is an increasing demand for higher recording density for magnetic recording media. Is becoming more common. Conventionally, in magnetic recording media such as magnetic tapes, the magnetic characteristics have been improved by orienting the needle-shaped magnetic powder in the magnetic recording layer in the longitudinal direction, but the needle-shaped magnetic powder is oriented in the longitudinal direction. Although high output can be obtained in the low frequency band, there is a limit to high density recording. For this reason, recently, a magnetic recording medium has been proposed in which the magnetic powder is flat and the barium ferrite magnetic powder having an axis of easy magnetization in the vertical direction is used for the magnetic recording layer (Japanese Patent Laid-Open No. 57-195328). ). However, those using these barium ferrite magnetic powders have a short wavelength recording characteristic, but have a drawback that the erasing characteristic is inferior. Especially when used for a floppy disk, the overwrite (erasing rate) characteristic is poor. There is a problem in use. In addition, it is generally desirable that the relationship between the coercive force of the tape and the saturation magnetic flux density of the head material satisfies Bs> (5-6) Hc.
When applied to the [G] Mn-Zn ferrite single crystal, sufficient recording cannot be performed on a tape having a coercive force of about 800 [Oe] or more. Therefore, as a head material for a high coercive force tape, a metal-based material (sendust, amorphous) having a high saturation magnetic flux density has been reviewed from the Mn-Zn ferrite single crystal which has been the mainstream until now. However, the ferrite heads used so far have been used so far and have high reliability, so there is an advantage in that point compared to using alloys such as sendust and amorphous, and in combination with conventional reliable ferrite heads. An excellent magnetic recording medium containing barium ferrite magnetic powder is expected. Means for Solving the Problems The present inventors have previously predicted that the overwrite characteristics of the magnetic recording medium are essentially due to the magnetism of the magnetic powder itself, and based on the measured values in the powder state. Focusing on the fact that it can be predicted, we have pursued its relevance for barium ferrite magnetic powder in overwrite characteristics, and have proposed a magnetic recording medium with good overwrite characteristics using barium ferrite magnetic powder with a specific erase value or less. (Japanese Patent Application No. Sho 60-185676) (Japanese Patent Laid-Open No. Sho 62-46430), and as a result of extensive studies to solve the above-mentioned problems, barium ferrite having a specific coercive force (Hc) and a powder erasing value. It has been found that an excellent magnetic recording / reproducing system can be obtained by combining a magnetic recording medium using magnetic powder and a ferrite head having a specific head gap, and the present invention. It is those that have reached. That is, the present invention has a coercive force (Hc) of 750 or less, an erased value of −45 dB or less as measured by an AC powder erasing method, and a particle size of 0.15 μm.
The present invention relates to a magnetic recording / reproducing system characterized by combining a magnetic recording medium having a magnetic layer using barium ferrite magnetic powder and a ferrite head having a head gap of 0.4 to 0.55 μm. The measurement by the AC powder erasing method used in the present invention is performed by the previously proposed Japanese Patent Application No. 60-185675 (Japanese Patent Application Laid-Open No. 62-103567, Japanese Patent Publication No. 6-58344) and Japanese Patent Application No. 60-185676 (Japanese Patent Application No. 60-185676). Kaisho 62-4
No. 6430), the powder sample filled in a specified container is applied to the AC magnetic field application type degausser of this measurement, and the residual magnetization is measured. As shown in FIG. 1, in this AC magnetic field application type degaussing device, a magnetic powder sample container is attached to one end of a holding rod 3, and the other end of the holding rod rotates a rotation motor 5. It is fixed to the shaft. The magnetic powder container 2 at the tip of the holding rod 3 is configured so as to be positioned at the center of the two AC electromagnets 4 and 4'where the current is applied by the electromagnet current regulator 6. Then, the sample demagnetized by the AC magnetic field application type degaussing device 1 is next measured for its residual magnetization by a highly sensitive magnetization measuring device 7, and the erased value of the magnetic powder is obtained by the general formula (1) described in detail below. be able to. Any measuring device may be used as the magnetization measuring device, and a commercially available one is used. The mechanism of erasing by the head is considered as follows. As shown in FIG. 2, the value of remanent magnetization is Mr.
Is in the point of. Since the erasing head always generates an AC magnetic field having a constant amplitude, when attention is paid to an arbitrary minute portion (element) on the tape, as shown in FIG. 3, while the element passes through the center of the erasing head. The magnetic field direction is reversed several times, and the magnetic field is gradually attenuated. As a result, the magnetization of the element is
→ a → b → c → d → e → f route to converge on Me. The magnitude of Me depends on the maximum applied magnetic field Hm. The erase signal voltage is detected when the element with the magnetization of Me next passes through the read head. For the tape erase value ER, the original signal voltage corresponding to Mr is Er.
Letting Ee be the erase signal voltage at Hm (the head current Im is substituted because it cannot be measured), ER = 20 log (Ee / Er) (1). Where ER: erase value of magnetic recording medium Ee: erase signal voltage at specified applied magnetic field Hm (substitute with head current Im) Er: original signal voltage with respect to residual magnetization Mr in signal recording state Powder used in the present invention The principle of the erasure measurement method is an application of the same principle as the above-mentioned tape erasure, and by using this, the relationship with the overwrite characteristic was found. That is, in order to approximate the time change pattern of the magnetic field received by the powder sample to that of the tape element, the control method of the electromagnet current is set to the same as that shown in FIG. 3, and the sample is set in the plane of the magnetic field direction so as to correspond to the running of the tape. It rotates at high speed. The measurement procedure of the method for measuring the erased value of the magnetic powder will be described below with reference to FIG. (1) Fill the container 2 at the tip of the holding rod 3 with magnetic powder. (2) High-sensitivity measuring instrument 7 by applying a DC magnetic field of 10KOe to the sample
Measure the residual magnetization Mr. (3) The sample part of the holding rod 3 is at the center of the AC electromagnets 4 and 4'of the AC eraser 1 for the main measurement, and the other end is the rotation motor 5
To the rotation axis of. (4) A current is applied by the electromagnet current regulator 6. The applied current is increased from zero to I ₁ and again reduced to zero. At the same time the current is applied, the rotary motor also starts,
It rotates at a constant speed until the current becomes zero. (5) Remove the holding rod 3 and remanently magnetize with the magnetometer 7.
Measure M ₁ . (6) Applying a direct magnetic field of 10KOe to the sample again,
The state of (5) is repeated n times. (7) Erase current In at n times is In = I ₁ + n × 0.1 mA, m ≧ 1 (8) Erase value at n times with residual magnetization Mn ERn = 20 × log (Mn / Mr) ( 2) Calculate with. ERn: Erase value of magnetic powder Mn: Measured value of remanent magnetization with respect to In Mr: 10 Residual magnetization after applying Koe (9) As shown in Fig. 4, the horizontal axis is In and the vertical axis is ERn. ERn is obtained from the corresponding measured values of Mn by the above equation (2), plotted, and the relationship between In and ERn is subtracted by the least-squares linear regression equation to obtain the elimination value ER in IaA. As shown in FIG. 4, the erased value of the barium ferrite magnetic powder thus obtained is related to the overwrite characteristic of the magnetic recording medium. 6 is a graph showing the relationship between the overwrite property of the floppy disk of FIG. 5 and the erased value of barium ferrite magnetic powder. The erased value is the erased value of each magnetic powder when the erase current is 2.9A. The overwrite characteristic is a value measured on a medium by tape-forming each magnetic powder. For overwrite characteristics, write a 10KFRPI rectangular wave on the medium, and then overwrite 20KFRPI on it, and measure the output difference. The larger the output difference, the better the overwrite characteristics. At this time, a ferrite head, a gap of 0.4 μm, and a rotation speed of 300 rpm were used. From the figure, when the erased value is -45 (dB) or less, the overwrite characteristic is 26 dB, and when it is -50 (dB) or less, it is 30.
It becomes dB, and it is understood that the overwrite characteristic of the magnetic recording medium is improved and there is no problem in practical use. On the other hand, if the value is higher than -45 (dB), the recorded one will not be erased and will appear as an error, which is not preferable. Barium ferrite magnetic powder has a particle size distribution and coercive force (H
c) Since the distribution is bad, it is considered that the overwrite characteristic is deteriorated when it is used for a floppy disk, but if it is -45 (dB) or less, it can be used for a proppy disk. As described above, it was found that the poor overwrite property of barium ferrite is due to the bad erase value of the magnetic powder. Therefore, by adjusting the particle size distribution and Hc distribution of the magnetic powder, it is possible to solve the poor erase value of the magnetic powder. Therefore, we were able to improve the overwrite characteristics by suppressing the erase value of the magnetic powder to a certain fixed value of -45 dB or less. The barium ferrite magnetic powder used in the present invention also has a coercive force Hc of 750 Oe or less. When used as a medium, Hc
^{When ⊥} is 750 Oe or less, the overwrite property is improved. It was found that the overwrite property was not improved when Hc exceeded 800 Oe. When the barium ferrite magnetic powder has an Hc of 750 Oe or less and is out of the range of the erasing value -45 dB or less measured by the AC powder erasing method, the overwrite characteristic is not improved even when the head gap is 0.4 to 0.55 μm, which will be described later. The barium ferrite magnetic powder used in the present invention is a hexagonal plate-like powder and is represented by the chemical formula BaO.6Fe ₂ O ₃ , and in addition to this, some of Ba and Fe in this chemical formula are Ti, Cr, Co. , Zn, In, M
Those substituted with a metal such as n, Cu, Ge, Nb, Ca, Sr, Pb, Ni, Sn are also included. The barium ferrite magnetic powder has a diameter of 0.15 μm or less, preferably 0.1 μm or less, and the plate ratio is not limited, but a plate ratio of 6 or more, and more preferably 8 or more is easily oriented vertically. In this case, the upper limit of the plate ratio is not particularly limited,
Up to 40 can be manufactured. Here, the average particle size means an electron micrograph [scanning microscope (SEM) and transmission microscope (TE
M)], for example, about 50 cross sections of hexagonal barium ferrite particles are observed, and the measured values of the particle size are averaged. The average thickness is also the average of the measured values of electron micrographs. The plate ratio is a value of average particle diameter / average thickness. Alternatively, the average thickness can be measured by the half width of X-ray diffraction. Since barium ferrite has a hexagonal plate-like shape, it has a greater effect on the surface roughness than the acicular magnetic powder, and if it is larger than the above diameter, the surface roughness is greatly reduced, which is not preferable. When the particle size is in the above range, the vertical component is sufficiently utilized, the surface smoothness of the magnetic layer is improved, the noise is sufficiently low, and high density recording can be achieved. As a method for producing barium ferrite, there are 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 it goes without saying that any method can be used. The ferrite head of the present invention is, for example, an Mn-Zn single crystal (Mn
_{O, ZnO, Fe 2 O 3} ), a Ni-Zn single crystal _{(NiO, ZnO, Fe 2 O} 3) or the like. In addition, the present invention is also characterized by the size of the ferrite head gap.
Use the one of ~ 0.55μm. Combining a magnetic recording medium having a magnetic layer with barium ferrite magnetic powder having a coercive force Hc of 750 or less and a powder erasing value of -45 dB or less measured by an AC powder erasing method and a ferrite head having the head gap. Due to
The overwrite characteristic can be remarkably improved. The overwrite characteristic is preferably -26 dB or less. When the head gap is less than 0.4 μm, for example 0.3 μm, the overwrite characteristic is possible up to about −20 dB, but −
It is difficult to set it to 26 dB or less. Further, if the head gap exceeds 0.55 μm, the overwrite characteristic is sufficient, but it is not preferable because a sufficient recording density characteristic cannot be obtained. In the magnetic recording layer of the present invention, an organic binder, an inorganic pigment, a lubricant, a dispersant, an antistatic agent, etc., which are commonly used, can be used according to a conventional method. Conventionally, the organic binder used in the magnetic recording layer of the present invention is
Thermoplastic, thermosetting or reactive resins used for magnetic recording media or mixtures thereof are used, but curable resins, particularly radiation curable resins, are preferred from the viewpoint of the strength of the resulting coating film. The thermoplastic resin has a softening temperature of 150 ° C. or lower, an average molecular weight of 10,000 to 200,000, and a degree of polymerization of about 200 to 2,000, for example, vinyl chloride-vinyl acetate copolymer (including those containing carboxylic acid), Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (including carboxylic acid introduced), vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylic ester-acrylonitrile copolymer, acrylic ester -Vinylidene chloride copolymer, acrylic acid ester-styrene copolymer, methacrylic acid ester-acrylonitrile copolymer, methacrylic acid ester-vinylidene chloride copolymer, methacrylic acid ester-styrene copolymer, urethane elastomer, nylon-silicone Resin, nitrocellulose-polyamide resin , Polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer, butadiene-acrylonitrile copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, etc.), Styrene-butadiene copolymers, polyester resins, chlorovinyl ether-acrylic acid ester copolymers, amino resins, various synthetic rubber thermoplastic resins, and mixtures thereof are used. The thermosetting resin or reactive resin has a molecular weight of 200,000 or less in the state of a coating liquid, and by heating after coating and drying, the molecular weight becomes infinite due to reactions such as condensation and addition. Further, among these resins, those which do not soften or melt before the resin is thermally decomposed 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, nitrocellulose melamine resin, saturated polyester resin, mixture of high molecular weight polyester resin and isocyanate prepolymer, methacrylate copolymer A mixture of coalesce and diisocyanate prepolymer,
Mixture of polyester polyol and polyisocyanate, urea formaldehyde resin, low molecular weight glycol /
High molecular weight diol / triphenylmethane triisocyanate mixtures, polyamine resins, and mixtures thereof. Particularly, a mixture of an epoxy resin, a butyral resin, and a phenol resin, a mixture of an epoxy resin, polyvinyl methyl ether, and methylol phenol ether described in U.S. Pat. No. 3,058,844, and bisphenol A described in JP-A-49-131101. Mixtures of type epoxy resins with acrylic or methacrylic acid ester polymers are preferred. To cure such a thermosetting resin, generally, heating in a heating oven at 50 to 80 ° C. for 6 to 100 hours may be performed. It is preferable to use a binder obtained by curing a radiation curable compound. Specific examples of the radiation curable compound include acrylic double bonds having an unsaturated double bond showing radical polymerizability such as acrylic double bonds such as acrylic acid, methacrylic acid, or ester compounds thereof, and allyl close bonds such as diallyl phthalate. It is a resin in which a group of a double bond, an unsaturated bond such as a maleic acid or a maleic acid derivative, which is crosslinked by irradiation with radiation or polymerized and dried is contained or introduced in the molecule of the thermoplastic resin.
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, fumaric 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, VYBGX, VERR, VYES, VMCA, VAGH, VCARMAG52
0, VCARMAG528, and the like, and an acrylic double bond, a maleic acid double bond, and an allyl double bond are introduced into this by a method described below to perform radiation-sensitive modification. (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, 1,5 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, Byron 53S),
These are also subjected to radiation-sensitive modification. (3) Polyvinyl alcohol-based resin Polyvinyl alcohol, butyral resin, acetal resin, formal resin and copolymers of these components are used, and the hydroxyl groups contained in these resins are subjected to radiation-sensitive modification by the method described below. (4) Epoxy resin, phenoxy 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,
Butyl cellulose, acetyl cellulose and the like are preferable. Radiation-sensitive modification is carried out by the method described below by utilizing the hydroxyl groups in the resin. Other resins that can be used for radiation-sensitive modification include polyfunctional polyester resins, polyether ester resins, polyvinyl pyrrolidone resins and derivatives (PVP olefin copolymers), polyamide resins, polyimide resins,
Phenol resins, spiro acetal resins, acrylic resins containing at least one hydroxyl group-containing acrylic ester and methacrylic ester as a polymerization component are also effective. Examples of the elastomer or prepolymer will be given below. (1) Polyurethane Elastomer or Prepolymer The use of polyurethane is particularly effective in that it has good abrasion resistance and adhesion to a base film such as 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 consisting of polycondensation products of various polyesters such as linear saturated polyethers (polyethylene glycol, polypropylene glycol, polytetramethylene glycol), caprolactam, hydroxyl-containing acrylic acid ester, hydroxyl-containing methacrylic acid ester, etc. are effective. is there. It is very effective to modify these urethane elastomers by reacting the isocyanate group or hydroxyl group at the terminal with a monomer having an acrylic double bond or an allyl double bond, etc. to make it radiation sensitive. is there. In addition, those containing OH, COOH, etc. as polar groups at the ends are also included. Further, a mono- or diglyceride of a long-chain fatty acid having an unsaturated double bond and the like, and a monomer having an active double hydrogen reactive with an isocyanate group and having an unsaturated double bond having a radiation curability are 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 crosslinked 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 has 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 further 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 are styrene-butadiene rubber,
There are chlorinated rubbers, acrylic rubbers, isoprene rubbers and cyclized products thereof (CIR701 manufactured by Japan Synthetic Rubber), and elastomers such as epoxy modified rubber and internally plasticized saturated linear polyester (Toyobo Byron # 300) are also treated by the radiation sensitive modification treatment described below. Can be effectively used by applying. 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,
Trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, polyfunctional oligoester acrylate (Aronix M-7100, M-5400, 55
00, 5700 etc., Toagosei), acrylic modified urethane elastomer (Nipporan 4040), or those into which functional groups such as COOH are introduced, trimethylolpropane diacrylate (methacrylate), phenol ethylene oxide adduct Acrylate (methacrylate), a compound having an acryl group (methacryl group) or ε-caprolactone acryl group on the pentaerythritol condensed ring represented by the following general formula, 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 CH 3 ( special acrylate B) (3) [CH ₂ = CHCOOC ₃ H ₆ ) n-OCH ₂ ] _3- CCH ₂ CH ₃ (n≈3) (Special acrylate C) _{(8) CH 2 = CHCOO- (} CH 2 CH 2 O) 4 -COCH = CH 2 ( special acrylate H) Next, a radiation-sensitive binder synthesis example will be described. 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 Part, cyclohexanone
Charge 500 parts into a four-necked flask with 51 and dissolve under heating to 80 ℃.
After heating, add 61.4 parts of 2-hydroxyethylmethacrylate adduct * of tolylene diisocyanate, add 0.012 parts of tin octylate and 0.012 parts of hydroquinone at 80 ° C.
React in an N ₂ stream until the NCO reaction rate reaches 90%.
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 TD1348 part to 80 ° C in a four-necked flask of 11 in N ₂ stream, 260 parts of 2-ethylenemethacrylate, octyl 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 completion of the reaction, the product was taken out and cooled, and then white paste-like 2HEMA of TDI was obtained. B) Synthesis of acrylic modified butyral resin (radiation sensitive modified resin) Butyral resin Sekisui Chemical's BM-S 100 parts toluene 191.2
Part, and cyclohexanone 71.4 parts were charged into a 51-necked 4-necked flask, dissolved by heating, and heated to 80 ° C, and then the TDI 2HEMA adduct * was added.
In addition to 7.4 parts, 0.015 part of tin octylate and 0.015 part of hydroquinone are added and reacted at 80 ° C. in an N ₂ gas stream until the NCO reaction rate becomes 90% or more. 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 (Vylon RV-200 manufactured by Toyobo), 1
Dissolve 00 parts in 116 parts of toluene and 116 parts of methyl ethyl ketone by heating, raise the temperature to 80 ° C, add 3.55 parts of 2HEMA adduct * of TDI,
Add 0.007 parts of tin octylate and 0.007 parts of hydroquinone and react at 80 ° C in N ₂ stream until NCO reaction rate is 90% or more. d) ◎ Synthesis of acrylic modified acrylic resin (radiation-sensitive modified resin) Epoxy resin (Epicoat 1007 manufactured by Shell Chemical Co., Ltd.), 400 parts of which was heated and dissolved in 50 parts of toluene and 50 parts of methyl ethyl ketone, and then 0.006 parts of N, N-dimethylbenzylamine Then, 0.003 parts of hydroquinone was added to 80 ° C., 69 parts of acrylic acid was added dropwise, and the mixture was reacted at 80 ° C. until the acid value became 5 or less. ◎ Phenoxy resin Synthesis of acrylic modified product (Radiation sensitive modified resin) Phenoxy resin having OH group (PKHH: UCC molecular weight 3
600 parts, 1800 parts of methyl ethyl ketone and 1800 parts of methyl ethyl ketone are charged into a 31-necked 4-neck flask, heated and dissolved, and heated to 80 ° C, 6.0 parts of 2-hydroxyethyl methacrylate adduct of tolylene diisocyanate is added, and 0.012 parts of tin octylate and hydroquinone 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) 250 parts of diphenylmethane diisocyanate (MDI) type urethane prepolymer with terminal isocyanate (Nipporan 3119 manufactured by Nippon Polyurethane), 2HEMA32.5 parts, hydroquinone 0.07 parts, tin octylate 0.009 80 parts
After melting by heating to ℃, 43.5 parts of TDI is heated in the reactor at a temperature of 80 to 90 ℃
Add dropwise while cooling so that after the completion of the dropwise addition, react at 80 ° C 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 0.007 parts, tin octylate
Put 0.009 parts in a reaction can, heat and dissolve it to 80 ° C, add 43.5 parts of TDI while cooling so that the temperature in the reaction can is 80 to 90 ° C, and after completion of the addition, at 80 ° C a reaction rate of 95% or more Let it react until it becomes. g) Synthesis of acrylic modified polybutadiene elastomer (radiation-curable elastomer) Low molecular weight terminal hydroxyl group polybutadiene polyBD manufactured by Sinclair Petrochemical Co., Ltd. BD Rquitite Resin R-15, 250 parts, 2HE
MA32.5 parts, hydroquinone 0.007 parts, tin octylate 0.0
Put 09 parts in a reaction can, heat and dissolve at 80 ℃, and drop TDI 43.5 parts while cooling so that the temperature of the reaction can is 80 ~ 90 ℃, after the completion of dripping at a reaction rate of 95% or more at 80 ℃. Let it react until it becomes. It is known that a polymer is one that is disintegrated by irradiation with radiation and one that causes intermolecular crosslinking. Polyethylene, polypropylene, polystyrene, polyacrylic acid ester, polyacrylamide, polyvinyl chloride, polyester, polyvinylpyrrolidone rubber, polyvinyl alcohol, and polyacrolein are examples of those that cause cross-linking between molecules. With such a cross-linking polymer, a cross-linking reaction occurs even if the above-mentioned modification is not particularly performed. Therefore, in addition to the modified product, these resins can be used as they are for radiation cross-linking. Furthermore, according to this method, even a solventless resin that does not use a solvent can be cured in a short time,
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, cyclohexanone; alcohols such as methanol, ethanol, propanol, butanol; esters such as ethyl formate, methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, glycol acetate monoethyl ether, etc. Ethers such as isopropyl ether, ethyl ether, glycol dimethyl ether, glycol monoethyl ether and dioxane; aromatic hydrocarbons such as benzene, toluene, xylene and ethylbenzene; furans such as tetrahydrofuran and furfural;
Chlorinated hydrocarbons such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin and dichlorobenzene, and other dimethylformamides 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 OSi as inorganic filler
C, CaO, CaCO ₃ , zinc oxide, goethite, αFe ₂ O ₃ , talc, kaolin, CaSO ₄ , boron nitride, fluorinated graphite, molybdenum disulfide, ZnS and the like. 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 , Fe ₃ O ₄ , Fe
₂ O ₃ , ZrSiO ₄ , Sb ₂ O ₅ , SnO and the like are also 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 Product 100Å)
(Aerosil, Nippon Aerosil Co., Ltd.) and the like. Further, the ultrafine particle colloidal solution and the ultrafine aluminum oxide particles produced by the same vapor phase method as described above,
Also, titanium oxide and the aforementioned particulate pigments can be used. The amount of such inorganic pigments used is 1)
1 to 30 parts by weight for 100 parts by weight, and 1 for 2)
~ 30 parts by weight is appropriate, and if these are too much,
There is a drawback that the coating film becomes brittle and the dropouts increase. The diameter of the inorganic pigment is 0.1 μm for 1).
In the following, 0.05 μm or less is preferable, and with respect to 2), 0.7 μm or less, further 0.5 μm or less is preferable. The magnetic layer may contain a dispersant. As the dispersant, an organic titanium coupling agent, a silane coupling agent, or a surfactant is used, and as an antistatic agent, a natural surfactant such as saponin; a nonionic surfactant such as an alkylene oxide-based, glycerin-based, or glycidol-based surfactant;
Cationic surfactants such as higher alkyl amines, quaternary ammonium salts, pyridine and other heterocycles, phosphoniums or sulfoniums; including acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfuric acid ester group, phosphoric acid ester group Anionic surface active agents; amphoteric surface active agents such as amino 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, fluorinated 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 esters. As fatty acids, caprylic acid, capric acid, lauric acid,
Fatty acids with 12 or more carbon atoms (RCOO, such as myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, stearolic acid)
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 comprising a monobasic fatty acid having a number of 17 or more and a monohydric alcohol having a carbon number of 21 to 23 in total with the carbon number of the fatty acid is used, and an 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 acid 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, radiation-curable lubricants are also convenient to use. 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, etc., and when these lubricants are represented by the structural formula, CH ₂ ═CHCOOR, CH ₂ = CH-CH ₂ COOR, CH ₂ = CHCONHCH ₂ OCOR, RCOOCH ₂ —CH═CH _{2 and the} like, where R is a linear or branched saturated or unsaturated hydrocarbon group having a carbon number of 7 or more, preferably 12 or more and 23 or less. Can also be As the fluorine substitution product, CnF _{2n + 1} −, CnF _{2n + 1} (CH ₂ ) m − (where m = 1 to
5), CnF _{2n + 1} CH 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 magnetic recording layer containing no lubricant has a high coefficient of friction, which causes image fluctuations and jitters.Moreover, the high coefficient of friction particularly at high temperature causes the magnetic recording layer to be easily scraped, resulting in a rolling up. Is likely to occur. Further, in the disk medium, the running durability is inferior and the coating film is damaged. The dispersant and lubricant are preferably contained in 0.1 to 20% by weight with respect to the binder. If the amount of the binder is too large, blocking will occur, and if the amount of the binder is too small, adhesion will occur in the calender process, which is not preferable. The thickness of the magnetic recording layer of the present invention after coating and drying is 0.1 to 10
The range of μm is common. When the lubricant of the magnetic layer and the organic binder are radiation-curable, the active energy ray used for crosslinking is an electron beam using a radiation accelerator as a radiation source, and α60 using Co60 as a radiation source.
Rays, β-rays having Sr90 as a radiation source, X-rays having an X-ray generator as a radiation source, ultraviolet rays, and the like are used. Particularly, as a radiation source, a method of using radiation by a radiation heater is advantageous from the standpoints of controlling absorbed dose, introducing it into a manufacturing process line, and shielding ionizing radiation. Examples of the non-magnetic base material used in the present invention include polyesters such as polyethylene terephthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate, polyimide, polycarbonate, polysulfone, polyethylene naphthalate, aromatic aramid, aromatic polyester. Metal plates such as aluminum and aluminum, glass plates, etc. are 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, magnetic layers may be provided on both sides of the support. In particular, in the case of a floppy disk, it is preferable to provide a magnetic layer on both sides. The magnetic recording medium of the present invention may be provided with a back coat and a top coat, if necessary. Of these, the back coat preferably has a composition including a binder, a pigment, and a lubricant. As the binder, the radiation-curable resin used for the magnetic layer described above can be used. For example, (A) it has two or more unsaturated double bonds that are curable by radiation.
Plastic compound with a molecular weight of 5,000-100,000, (B)
Having one or more unsaturated double bonds that are curable by radiation, or not having radiation curability, molecular weight 3,000
To 100,000 rubbery compound, and (C) one or more unsaturated double bonds curable by radiation, molecular weight
200-3,000 compounds (A) 20-70% by weight, (B) 20
A combination used in a proportion of -80 wt% and (C) 10-40 wt% is preferred. Further, a thermoplastic resin and a thermosetting resin can be used, and those having an average molecular weight of 200,000 or less are preferable. Particularly preferred is a thermosetting resin (using a curing agent) composed of a combination of a fibrous resin (nitrified cotton etc.), a vinyl chloride-vinyl acetate-vinyl alcohol copolymer and urethane. As the pigment, the inorganic pigment used for the magnetic layer described above can be used. Regarding 1), binder 10
20 to 300 parts by weight for 0 parts by weight, 10 to 30 for 2)
It is preferable to include 0 part by weight. The lubricant used for the magnetic layer can be used as the lubricant. Among them, it is preferable to use fatty acid and / or fatty acid ester. 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 coating material, and the magnetic coating material is gravure coated on a substrate such as a polyester film. , Reverse roll coating, air knife coating, air doctor coating, blade coating, kiss coating, spray coating, etc., and apply orientation treatment so that the direction of easy magnetization of the magnetic powder is perpendicular to the magnetic layer. It may be carried out, dried, and preferably radiation-cured by a conventional method. Then, a back coat and a top coat may be provided if necessary. The orientation treatment follows a conventional method. Permanent magnets, DC magnets, and AC magnetic fields are typically used as orientation methods, and various combinations of these are used.
For example, various combinations such as a combination of vertical and horizontal, a combination of a permanent magnet or a DC magnetic field and an AC magnetic field, a mechanical orientation or a combination of the mechanical orientation and the above are used. Then, magnetic particles are oriented outside the magnetic field due to the demagnetizing field, and the particles are disturbed and dried in the magnetic field so that the orientation is not deteriorated. It needs to be dried so that the magnetic powder does not move. The magnetic field strength is preferably 1000 to 6000G. in this case,
In the present invention, since the plate ratio is easily oriented to 6 or more, 1000
Even about ~ 4000G will be sufficient for the purpose. In the recording medium of the present invention, a high-permeability metal thin film such as Permalloy or an undercoat layer having a coating film as shown below may be further provided between the support and the magnetic layer. You may use these together. The undercoat layer of the coating film preferably contains the above-mentioned radiation-curable compound, carbon black and / or a surfactant. The carbon black used 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 3 μm. Examples The present invention will be described in more detail with reference to Examples, but it goes without saying that the present invention is not limited thereto. The characteristics were evaluated as follows. (1) Linear recording density D ₅₀ (KFRPI) Rotation speed 300 rpm, head; ferrite head, output in low recording density area with gap 0.04 to 0.55 μm (E)
Is a linear recording density D ₅₀ (KFRPI) that is E / 2 in the high recording density area
Was measured. (2) Overwrite characteristics A rectangular wave of 10KFRPI is written on the medium with the above head gap, 20KFRPI is overwritten on it, and the output difference is measured. The larger the output difference, the better the overwrite characteristics. (3) Erase value Check the erase value of the magnetic powder when the erase current is 2.9A. (4) Plate-like ratio The cross section of 50 hexagonal barium ferrite particles was observed by electron micrograph [scanning microscope (SEM) and transmission microscope (TEM)], and the measured value of hexagonal grain size was measured. The average particle diameter / average thickness was calculated from the average particle diameter obtained by averaging and the average thickness obtained by averaging the measured values for the thickness, and defined as the plate ratio. Alternatively, these values can be measured by the full width at half maximum of 2θ by X-ray diffraction. (5) Vertical orientation degree The squareness ratio Br / Bm in the vertical direction of the magnetic tape was measured and the demagnetizing field was corrected. Example 1 An undercoat layer as shown below was formed on both the front surface and the back surface of a polyester (PET) film having a thickness of 75 μm. Undercoat layer Weight part Carbon black 20 mμm 50 (A) Acrylic modified vinyl chloride-vinyl acetate-vinyl alcohol copolymer Molecular weight 45,000 40 (B) Acrylic modified polyurethane elastomer Molecular weight 20,000 40 (C) Polyfunctional acrylate Molecular weight 1,000 20 Stearic acid 2 Butyl stearate 2 mixed solvent (MIBK / toluene = 1/1) 300 Disperse the above composition in a ball mill for 5 hours, apply it on the above polyester (PET) film to a dry thickness of 0.7 μm, and smooth the surface. After the treatment, an electron beam was applied to the undercoat layer in an N ₂ gas with an accelerating voltage of 150 KeV, an electrode current of 10 mA, an absorbed dose of 5 Mrad using an electron curtain accelerating device. 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, the hexagonal barium ferrite A shown in Table 1 (BaFe ₁₂ O ₁₈ in which Ba and Fe are partially substituted was synthesized by a hydrothermal synthesis method) was used to prepare a magnetic coating as follows. It was created. Magnetic layer 1 (Radiation curable magnetic layer) Weight part Barium ferrite Particle size variable 120 Carbon black 30mμ 10 α-Al ₂ O ₃ powder (0.5μ powder) 2 Solvent (MEK / toluene 50/50) 100 The above composition Mix for 3 hours in a ball mill to wet the hexagonal plate-shaped barium ferrite well. Next, vinyl chloride-vinyl acetate-vinyl alcohol copolymer (containing maleic acid) molecular weight 40,000 6 parts (converted to solid content) Acrylic double bond-introduced vinyl chloride / vinyl acetate copolymer (containing maleic acid) molecular weight 20,000 12 parts (solid Acrylic double bond-introduced polyether urethane elastomer Molecular weight 40,000 9 parts (solid content conversion) Pentaerythritol triacrylate 3 parts
Solvent (MEK / toluene 50/50) 200 parts Stearic acid 4 parts Butyl stearate 2 parts Mix well with a binder mixture and dissolve. This is put into a ball mill that has been previously treated with magnetic powder, and dispersed again for 42 hours. The magnetic coating material thus obtained was applied onto the undercoat layer and vertically aligned while being dried on a permanent magnet (3000 gauss), and then the solvent was continuously dried with an infrared lamp or hot air ( These may be used together at the same time), and after surface smoothing treatment, using an ESI electron curtain type electron beam accelerator, accelerating voltage 150K
The coating film was cured by irradiating it with an electron beam under an N ₂ atmosphere under the conditions of eV, electrode current 20 mA, and total irradiation amount 5 Mrad. The film thickness after curing was 2.0 μm for the magnetic layer. 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 characteristics of the sample thus prepared are shown in Table 1. Table 2 shows the results of overwriting characteristics when the head gap was changed from 0.3μ to 0.55μ. As shown in Tables 1 and 2, the head gap 0.4-
The 0.55 μm type has better overwrite characteristics than the 0.3 μm type. In this way, even with an overwrite of -20 dB with a gap of 0.3 μ, by setting a gap of 0.4 μ or more, by using magnetic powder with an erase value of −45 dB or less, the overwrite can be 26 dB or more. all right. Regarding the Hc of the magnetic powder, the sample m: Hc750 has a good overwrite property, and it is preferable to use the magnetic powder of Hc750 or less. The ones in Table 1 are plotted and shown in FIG. First, I decided that it can withstand practical use up to an overwrite of 20 dB, but since the standard of the overwrite characteristic of the current floppy disk is 26 dB, it is the fifth erase value corresponding to the area that meets this standard. From the figure, −45 dB or less is preferable. More preferably, the overwrite value is 28 dB and the erase value is -50 dB or less. Thus, it can be seen that the overwrite characteristic can be dealt with by the erase value of the magnetic powder. The gap length was measured at 0.45 μm below. Example 2 Hexagonal barium ferrite (BaFe ₁₂ O ₁₈ partially converted from Ba and Fe was synthesized by a hydrothermal synthesis method) Average particle size 0.08 μ
m, Hc = 720 120 parts by weight α-Al ₂ O ₃ (0.5 μ powder) 2 parts by weight Graphitized carbon # 4000B 20 mμ 12 parts by weight Dispersant (soybean oil crude lecithin) 3 parts by weight and solvent (MEK / cyclohexanone 70 / 30) A magnetic powder mixture was prepared in the same manner as in Example 1 using 100 parts by weight. Next, as a binder, vinyl chloride-vinyl acetate-vinyl alcohol copolymer (containing maleic acid) Molecular weight 20,000 6 parts by weight (as solid content) Acrylic modified phenoxy resin Molecular weight 35,000 6 parts by weight (as solid content) Acrylic modified poly Ether urethane elastomer molecular weight
20,000 18 parts by weight (solid equivalent) Solvent (MEK / Cyclohexanone 70/30) 200 parts by weight
Higher fatty acid-modified silicone oil 3 parts by weight
And 3 parts by weight of butyl myristate were mixed and dissolved. Then, a sample A was prepared by the same operation as in Example 1,
The characteristics were investigated. However, orientation treatment is AC magnetic field (3000G)
Was performed using. In Sample A, the radiation-curable binder was changed to the heat-curable binder and treated in the same manner. That is, vinyl chloride-vinyl acetate-vinyl alcohol copolymer (containing maleic acid) acrylic modified phenoxy resin, acrylic modified polyether urethane elastomer
30 parts by weight of 15 parts by weight of vinyl chloride-vinyl acetate-vinyl alcohol copolymer (VAGH manufactured by Union Carbide Co.) and urethane (Nipporan 3022 manufactured by Nippon Polyurethane Co.) 15
Sample B was prepared in the same manner as sample A, except that the parts were replaced by parts by weight. However, in this case, an isocyanate compound (Coronate L manufactured by Nippon Polyurethane Co.) was added to the magnetic paint after dispersion in an amount of 5
Parts by weight (calculated as solid content) were added. After smoothing the surface, at 80 ℃
It was heat cured for 48 hours. For sample A and B, use a roll of 8 inch diameter
The curl on the outside and the inside of the jumbo roll when it was wound 5000 m was compared. For curl measurement, a 5 inch punched floppy disk is placed on a plate plate and the curled height hcm is measured. The results are shown in Table 3. In the case of the thermosetting type, there was curling due to winding tightness inside the jumbo roll. In the case of radiation curing type,
There is no curl, which is preferable. Further, as shown in Table 4, the composition of Example 1 having the overwrite property is as shown in Table 4, and when compared with the current high density floppy as a comparative example, the particle size is 0.15 μm. The following are preferable in terms of linear recording density. Table 4 shows the data when the head gap is 0.5 μm. Those having a particle size of 0.15 μm or less are good, with a D ₅₀ of 40 KFRPI or more. Table 4 shows head gap of 0.55μ, 0.6μ
When set to, the sample in the table becomes 40,38KFRPI. Therefore, a gap of 0.55μ or less is preferable. With respect to the surface roughness, when it exceeds 0.08 μm, a characteristic deterioration occurs, which is not preferable for practical use. Effect of the Invention At the stage of investigating what is causing the bad overwrite property of barium ferrite, the magnetic powder was pursued. It was found that the bad overwrite property had a bad erasing value in the case of magnetic powder and that it exceeded Hc750. It was found that the overwrite property in the medium was improved by setting the powder erasure value of the magnetic powder to a certain constant level and setting Hc to a certain level or less.
Therefore, there was no problem in practical characteristics, and it could withstand use. The poor characteristics of barium ferrite itself could be solved.

[Brief description of drawings]

FIG. 1 is a block diagram of a magnetic powder demagnetizing device and a residual magnetization measuring device, FIGS. 2 and 3 are explanatory diagrams of the principle of the magnetic powder erasing measuring method, and FIG. 4 is an erasing value ERn with respect to the erasing current In of the magnetic powder.
FIG. 5 is a graph showing the relationship between the overwrite characteristic of a floppy disk and the erased value of barium ferrite magnetic powder.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-60-69822 (JP, A) Tadaaki Tsunoo "Tape recorder" (SHO-46-4-25) Nikkan Kogyo Shimbun P. 43

Claims (1)

[Claims] 1. A coercive force (Hc) of 750 Oe or less, an erased value of -45 dB or less measured by an AC powder erasing method, and a particle size of 0.15 μm.
A magnetic recording / reproducing system characterized by combining the following magnetic recording medium having a magnetic layer using barium ferrite magnetic powder and a ferrite head having a head gap of 0.4 to 0.55 μm.