JPH01235024A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To prevent powder dislodgement, drop-out, sticking, clogging, etc., and to obtain excellent electromagnetic conversion characteristics and traveling durability by using specific polyurethane as a binder of the magnetic layer of the magnetic recording medium, specifying the content thereof and specifying the crystallite size of ferromagnetic metal powder to <=250Angstrom . CONSTITUTION:The crystallite size of the ferromagnetic metal powder is <=250Angstrom and the binder contains the polyurethane having at least one kind of the S-contg. group selected from the groups expressed by formula I. The content of the binder to be compounded is specified to 15-40wt.% of the weight of the ferromagnetic metal powder. The polyurethane contg. the S-contg. group at 0.01-5mm equiv. of the total quantity of the polyurethane is more preferred. The excellent electromagnetic conversion characteristics and traveling durability are thereby obtd.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium that has excellent electromagnetic conversion characteristics and running durability, and that does not corrode the head. .

[Background of the Invention] Conventionally, γ-Fe2O3 has been used as magnetic powder for magnetic recording media.
, Fe ffoa, and fine particle powders such as those obtained by adsorbing or doping cobalt ions are used. In recent years, with the demand for higher performance videotapes and audiotapes, high signal density and high playback output for short wavelength recording are required.In order to cope with these trends,
In particular, magnetic powders such as Fe, Fe-Co, or Fe
, Ferromagnetic metal fine powder mainly composed of C01Ni, etc. is now being used. However, since such ferromagnetic metal fine powder has a small crystallite size and a very large magnetic moment, the particles tend to aggregate with each other, resulting in ferromagnetism and metal fine powder. Uniform dispersion of powders into binders has become extremely difficult.

To address these problems, binders containing carboxyl groups, sulfonic acid groups, etc. in their molecular structures have been proposed with the aim of improving the affinity of binders for magnetic powder (Japanese Patent Application Laid-Open No. Publication No. 57-92422, etc.)
However, due to insufficient dispersibility of ferromagnetic metal fine powder,
The surface properties of the magnetic coating film were poor, the residual magnetic flux density and the squareness ratio were unsatisfactory, and furthermore, there were drawbacks such as easy powder falling off and poor durability.

To this end, binders with a higher content of carboxyl groups and sulfonic acid groups in their molecular structures have been proposed, but such binders are corrosive to metals and may cause head problems. This causes problems such as corrosion of the magnetic material and demagnetization due to corrosion of the ferromagnetic metal fine powder. In particular, magnetic recording media containing fine ferromagnetic metal powder as a magnetic material have a high coercive force and must be recorded and reproduced with high output and low noise. However, such heads are particularly susceptible to corrosion and do not allow the use of bonding agents such as those described above.

Furthermore, binders with non-corrosive polar functional groups such as hydroxyl groups and epoxy groups have been proposed (Japanese Unexamined Patent Publication No. 5
6-117329, Japanese Patent Publication No. 1984-1684, etc.). However, such binders have little effect on improving the dispersibility of the magnetic material when fine ferromagnetic metal powder, which tends to agglomerate, is used as the magnetic material. There are practical problems with the binder used in magnetic recording media.

Furthermore, in order to improve the dispersibility of the ferromagnetic metal fine powder and smoothen the surface of the coating film, it is possible to add surfactants, treat the ferromagnetic powder with silicone oil, etc., and improve the reactivity with the binder. Techniques have been proposed, such as adding a silane coupling agent with There is a need for binders that exhibit better dispersibility in powders.

[Object of the Invention] The object of the present invention is to provide a binder that can sufficiently disperse the ferromagnetic metal fine powder while using the ferromagnetic metal fine powder, and is not corrosive to the ferromagnetic metal fine powder or the head. By using a magnetic recording medium, powder falling, dropout, sticking, clogging, etc. are significantly improved, and the magnetic recording medium has excellent electromagnetic conversion characteristics, running durability, and excellent compatibility with the head. It is to provide.

[Structure of the Invention] The present invention provides a magnetic recording medium comprising a non-magnetic support and a magnetic layer formed by dispersing ferromagnetic metal powder on the support in a binder. The crystallite size of the powder is 250λ or less, and the crystallite size of the crystal S
0 The mixture contains a polyurethane having at least one S-containing group selected from the group consisting of -5H group, -C3H group, and -〇SH group, and the blending amount of the binder is the same as that of the ferromagnetic material. The magnetic recording medium is characterized in that the amount thereof is 15 to 40% by weight of the metal powder.

In a preferred embodiment of the present invention, the polyurethane has 0.0% of the S-containing groups. has an OI of ~5 meq/g and 10.000
~200,000, more preferably ~30,000
The above magnetic recording medium is characterized in that it is a polyurethane having a weight average molecular weight of 150,000.

Another preferred embodiment of the present invention resides in the above magnetic recording medium, wherein the magnetic layer further contains an abrasive having a Mohs hardness of 6 or more and a fatty acid having 12 to 22 carbon atoms. .

[Detailed Description of the Invention] The present invention uses ferromagnetic metal powder as the magnetic material. For example, the ferromagnetic metal powder has a metal content of 75% by weight or more, and a metal content of 80% by weight.
At least % by weight of at least one ferromagnetic metal or alloy (
Examples: Fe, Co, Ni, Fe-Co, Fe-Ni, Co
-Ni, Co-N1-Fe), and the metal content is 20f
Other components (e.g., AI, Si,
S, Sc, Ti, V, Cr, Mn, Cu, Zn.

Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba
, Ta, W, Re, Au, Hg, Pb, Bi, La, C
Mention may be made of alloys which may contain e, Pr, Nd, B, P). Further, water containing a small amount of the ferromagnetic metal,
It may also be a substance containing hydroxide or oxide.

The method of producing these ferromagnetic metal powders is well known;
The ferromagnetic metal powder in the present invention may be manufactured according to such a known method.

There is no particular restriction on the shape of the ferromagnetic metal powder in the present invention, and usually needle-like, granular, dice-like, rice-grain-like, and plate-like shapes can be used.

The crystallite size of the ferroelectric metal powder in the present invention is 25
It is preferably 0 or less, particularly 200^ or less. Here, the crystallite size means the crystallite size determined by X-ray diffraction.

The binder of the magnetic layer in the present invention includes -3H group, S
It is characterized by being a binder containing polyurethane having at least one S-containing group selected from the group consisting of O -C3H groups and -C5H groups.

The S-containing group may be directly bonded to the main chain of the polyurethane, or may be bonded to the side 8n.

The polyurethane preferably has the S-containing group in an amount of 0.01 to 5 milliequivalents/g based on the total amount of the polyurethane. The content of the above S-containing "^" is -
If the amount is less than or greater than the above range, the dispersibility of the ferromagnetic metal powder will deteriorate, and the electromagnetic conversion characteristics of the magnetic recording medium will deteriorate.

Furthermore, the above polyurethane has a price of 10,000 to 200,000
0, and particularly preferably one having a weight average molecular weight of 30,000 to 150,000. If the weighted average molecular weight i4 of the polyurethane is less than 10,000, the running durability of the tape tends to be poor.

The polyurethane can be produced from the polyol containing the S-containing group, a diisocyanate, and, if necessary, a chain extender, by a method known per se for producing polyurethane.

11 The polyol containing the S-containing group is a compound in which the S-containing group is bonded to the main chain or side of a polyol such as polyether diol, polyester diol, polycarbonate diol, or polycaprolactone diol. It is.

Typical examples of the polyether polyols include polyalkylene gelylcols such as polyethylene glycol and polypropylene glycol.

The above polyester polyol can be synthesized, for example, by polycondensation of a dihydric alcohol and a dibasic acid, ring-opening polymerization of lactones, such as caprolactone, and the like. Typical dihydric alcohols include ethylene glycol, propylene glycol, butanediol, 1.6
- Glycols such as hexanediol and cyclohexane dimetatool can be exemplified. Furthermore, typical dibasic acids include adipic acid, pimelic acid, azelaic acid, sebacic acid, phthalic acid, and terephthalic acid.

Further, the polycarbonate polyol can be prepared, for example, by the following formula [I] HO-R1-OH[I] [wherein R1 is, for example, =(CH2)n (n=3 to
14) Polycarbonate polyol with a molecule of 1i300 to 20,000 and a hydroxyl value of 20 to 300 synthesized by condensation or transesterification with an acid ester, dialkyl carbonate or diaryl carbonate, or a polycarbonate polyol with the general formula [111% Formula % [11] [wherein R2 is an alkylene group having 3 to 6 carbon atoms,
1,4-11.3- or 1,2-phenylene group or 1,4-11.3- or 1.2-cyclohexylene group, ], which is determined by condensation with a dihydric carboxylic acid having a molecular weight of 400-30,000, hydroxyl value 5-30
0 polycarbonate polyester polyol.

Other polyols, such as polyether polyols, polyester ether polyols, and polyesters, may be blended with the above polyol in an amount up to 90% by weight of the above polyol.

The polyisocyanate used to form the polyurethane by reacting with the polyol is not particularly limited, and commonly used polyisocyanates can be used.

For example, hexamethylene diisocyanate, toridine diisocyanate, isophorone diisocyanate, 1.3
-Xylylene diisocyanate, 1.4-xylylene diisocyanate, cyclohexane diisocyanate, toluidine diisocyanate, 2.4-1 lylene diisocyanate, 2.6-tolidine diisocyanate, 4,4
Examples include ゛-diphenylmethane diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, 1,5-naphthylene diisocyanate, 4,4-diphenylmethane diisocyanate, 3,3-dimethylphenylene diisocyanate, dicyclohexylmethane diisocyanate, and the like.

Examples of the chain extender include the aforementioned polyhydric alcohols, aliphatic polyamines, alicyclic polyamines, and aromatic polyamines.

In addition to the S-containing group, the polyurethane may further include, for example, -COOM, -5o, M, -opO,M, -0M
(Here, M represents a hydrogen atom, sodium, or potassium).

In the magnetic recording medium of the present invention, the content of the polyurethane in the binder is 10 to 50% by weight based on the total binder.
In particular, it is preferably 20 to 40% by weight.

As the binder for the magnetic recording medium of the present invention, other binders may be used in combination with the above polyurethane.

Other binders that can be used in combination are not particularly limited, and known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof, which are conventionally used as binders for magnetic recording media, can be used. can.

The thermoplastic resin has a softening temperature of 150°C or less,
Those with an average molecular weight of about 10,000 to 300,000, such as vinyl chloride vinyl acetate copolymer, vinyl chloride vinylidene chloride copolymer, vinyl chloride acrylonitrile copolymer, acrylic acid 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-silicon resin , nitrocellulose-polyamide resin, polyvinyl fluoride, vinylidene chloride acrylonitrile copolymer, butadiene acrylonitrile copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate) , nitrocellulose, etc.), styrene-butadiene copolymers, polyester resins, chlorovinyl ether acrylate copolymers, amino resins, and various synthetic rubber-based thermoplastic resins.

Among these thermoplastic resins, vinyl chloride copolymers are particularly preferred since they can improve the dispersibility of the ferromagnetic metal powder.

Specific examples of preferred vinyl chloride copolymers include vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-maleic acid copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, and vinyl chloride-acetic acid copolymer. Vinyl-maleic acid-vinyl alcohol copolymer, vinyl chloride-vinyl propionate-vinyl maleate copolymer, vinyl chloride-vinyl propionate-vinyl alcohol copolymer,
Examples include vinyl chloride-vinyl acetate-acrylic acid copolymers, vinyl chloride-vinyl acetate-acrylic acid-vinyl alcohol copolymers, and oxidized versions of these copolymers.

In particular, vinyl chloride copolymers containing polar groups such as carboxylic acid groups or their salts, sulfonic acid groups or their salts, phosphoric acid groups or their salts, amino groups, and hydroxyl groups are used to improve the dispersibility of magnetic materials. preferable.

Furthermore, polar groups (e.g. -COOM, -SO3M, -0
Polyurethanes having PO, M, -0M, etc., where M represents a hydrogen atom, sodium, or potassium, are other binders that may be preferably used in combination to improve dispersibility.

When the vinyl chloride copolymer is used as a binder in the present invention together with the polyurethane having an S-containing group, the weight ratio of vinyl chloride copolymer/total polyurethane is +00/20 to 100/200. It is preferable.

In addition, the above-mentioned thermosetting resin or reactive resin has a molecular weight of 200,000 or less in the state of a coating liquid, and its molecular weight becomes extremely large when heated after coating and drying, such as phenol resin. , phenoxy resin,
Epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, silicone resin, acrylic reaction resin, epoxy-polyamide resin, nitrocellulose melamine resin, mixture of high molecular weight polyester resin and isocyanate prepolymer, methacrylate Examples include mixtures of copolymers and diisocyanate prepolymers, mixtures of polyester polyols and polyisocyanates, urea formaldehyde resins, mixtures of low molecular weight glycol/high molecular weight diol/triphenylmethane triisocyanate, polyamine resins, and mixtures thereof.

The binder may further contain a compound having two or more incyanate groups (polyisocyanate). Examples of such polyisocyanates include tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate,
Isocyanates such as xylylene diisocyanate, naphthylene-1,5-diisocyanate, 0-toluidine diisocyanate, isophorone diisocyanate, triphenylmethane triisocyanate, reaction products of these isocyanates with polyalcohols, and condensation of these isocyanates. Examples include the produced polyisocyanate. The above polyisocyanates are, for example, Coronate from Nippon Polyurethane Industry Co., Ltd., Coronate HL, Coronate H1 Coronate EH, Coronate 2030, Coronate 2031, Coronate 2036, Coronate 3015, Coronate 3.
041, Coronate 2014, Millionate MR, Millionate MTL, Daltosec 1350, Daltosec 2170, Daltosec 2280, Takeda Pharmaceutical Co., Ltd.
From, Takenate D-102, Takenate D-11ON
, Takenate D-200, Takenate D-202, Sumidur-N75, from West German Bayer Co., Ltd., Desmodule L, Desmodule IL, Desmodule N, Desmodule HL, Dainippon From Ink Kagaku Kogyo Co., Ltd., Parnock
It is sold under trade names such as D850 and Parnock-D802. The amount of these polyisocyanates is preferably 5 to 40 parts by weight per 100 parts by weight of the total binder containing polyisocyanates.

The blending ratio of the ferromagnetic metal fine powder and the total binder in the magnetic layer of the magnetic recording medium of the present invention is 10
15 to 40 parts by weight of total binder per 0 parts by weight, especially 20 to 3 parts by weight
Preferably, it is 0 parts by weight. If the blending ratio of the binder is less than the above range, the running durability of the magnetic recording medium will deteriorate, and if it is more than the above range, the packing density of the ferromagnetic powder will become low, making it difficult to achieve the desired electromagnetic characteristics. or the coefficient of friction tends to increase.

There are no particular restrictions on the non-magnetic support in the present invention,
Those commonly used can be used. Examples of materials forming the nonmagnetic support include various synthetic resin films such as polyethylene terephthalate, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamideimide, and polyimide, and metal foils such as aluminum foil and stainless steel foil. be able to. There is also no particular limit to the thickness of the non-magnetic support, but it is generally 2.5 to 100 μm, preferably 3 to 80 μm.
It is m.

The magnetic recording medium of the present invention has the same structure as a conventionally known magnetic recording medium, except that the binder in the magnetic layer is specified as described above, so the technology can be used as appropriate. can.

For example, when manufacturing the magnetic layer of the magnetic recording medium of the present invention, ferromagnetic fine powder and a binder, an organic or inorganic filler,
Known additives (materials) such as carbon black, dispersants, antistatic agents, lubricants, abrasives, etc. are kneaded with a solvent to form a magnetic paint.

The above-mentioned filler is not particularly limited, and for example, the average particle size is in the range of 0.01 to 0.8 μm, preferably 0.06 to
Commonly used particulate fillers in the 0.4 μm range can be used. Examples of the above fillers include graphite, tungsten disulfide, boron nitride, calcium carbonate,
Particles such as aluminum oxide, iron oxide, titanium dioxide, magnesium oxide, curved lead oxide, calcium oxide, lithopone and talc can be mentioned, and these can be used alone or in mixtures.

As carbon black, furnace black, thermal black, color black, acetylene black, etc. can be used. The properties of carbon black include an average particle size of 5 to 1000ml ('f
ff, submicroscope), nitrogen adsorption method specific surface area is 1 to 800
m”/g, pH 4-11 (JIs 6221)
, dibutyl phthalate oil absorption is 10 to 800 mu/1
00g (JIS 6221) is preferred. Regarding the size of carbon black, 5 to 100 mμ of carbon black is used for the purpose of lowering the surface electrical resistance of the coating film, and 50 to 10 mμ of carbon black is used for the purpose of controlling the strength of the coating film.
00 mμ carbon black, and finer particle carbon black (100 mμ) for smoothing to reduce spacing loss for the purpose of controlling the surface roughness of the coating film.
Coarse particle carbon black (50 mμ or more) is used for the purpose of roughening the surface and lowering the coefficient of friction. Fine particle carbon black and coarse particle carbon black may be used together. Furthermore, carbon black whose surface is partially graphitized or grafted can also be used.

Examples of dispersants include fatty acids having 9 to 22 carbon atoms (e.g., caprylic acid, capric acid, lauric acid, myristic acid, valmitic acid, stearic acid, oleic acid, elaidic acid, linoleic acid, lylunic acid, stearolic acid) ), metal soaps consisting of the above fatty acids and alkali metals (e.g. lithium, sodium, potassium) or alkaline earth metals (e.g. magnesium, calcium, barium), esters of the above fatty acids and part of the hydrogen of their compounds; Compounds in which all fluorine atoms are substituted, amides of the above fatty acids,
Known dispersants such as aliphatic amines, higher alcohols, polyalkylene oxide alkyl phosphates, alkyl phosphates, alkyl borate 5-sarcosinates, alkyl ether esters, trialkyl polyolefin oxy quaternary ammonium salts, and lecithin are used. can be mentioned. When using a dispersant, it is usually 0.05 to 20 parts by weight per 100 parts by weight of the binder used.
Used in parts by weight range.

As dispersing agents, in particular saturated compounds having from 12 to 32 carbon atoms, such as lauric acid, myristic acid, valmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, lyric acid, stearolic acid, etc. Alternatively, it is preferable to use unsaturated fatty acids.

Examples of antistatic agents include conductive fine powders such as carbon black and carbon black graft polymers; natural surfactants such as saponins; nonionic surfactants such as alkylene oxides, glycerins, and glycidols; higher alkyl amines. cationic surfactants such as quaternary ammonium salts, salts of pyridine and other heterocyclic compounds, phosphoniums or sulfoniums;
Anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfuric ester groups, and phosphoric ester groups; Examples include amphoteric surfactants such as amino acids, aminosulfonic acids, and sulfuric or phosphoric esters of amino alcohols. be able to. When using the above-mentioned conductive fine powder as an antistatic agent, it is used in a range of 0.2 to 20 parts by weight per 100 parts by weight of ferromagnetic powder, and when a surfactant is used, it is used in an amount of 0.2 to 20 parts by weight per 100 parts by weight of ferromagnetic powder. It is used in a range of .1 to 10 parts by weight.

Examples of lubricants include the above-mentioned fatty acids, higher alcohols, fatty acids consisting of monobasic fatty acids with 12 to 20 carbon atoms, such as butyl stearate, and sorbitan oleate, and monohydric or polyhydric alcohols with 3 to 20 carbon atoms. esters,
In addition to mineral oils, animal and vegetable oils, olefin low polymers, α-olefin low polymers, known lubricants and plastic lubricants include silicone oil, graphite fine powder, molybdenum disulfide fine powder, Teflon fine powder, etc. be able to. The amount of lubricant added can be arbitrarily determined according to known techniques.

Examples of abrasive materials include α-alumina, fused alumina, silicon carbide, chromium oxide, cerium oxide, corundum, artificial diamond, α-iron oxide, garnet, emery (main components: corundum and magnetic iron XL), and garnet. , silica, silicon nitride, boron nitride, molybdenum carbide, boron carbide, tungsten carbide, titanium carbide, tripoly, diatomaceous earth, dolomite, etc. are representative examples from the viewpoint of durability of the magnetic layer of the magnetic recording medium. . In particular, it is preferable to use a combination of one to four types of abrasives having a Mohs hardness of 6 or more.

The average particle size of the abrasive is 0.005 to 5 microns,
In particular, it is preferably 0.05 to 2 microns. Preferably, the abrasive is used in an amount ranging from 0.01 to 20 parts by weight per 100 parts by weight of ferromagnetic powder.

There are no particular restrictions on the solvent used during kneading, and solvents commonly used for preparing magnetic paints can be used.

There is no particular restriction on the kneading method, and the order of addition of each component can be set as appropriate.

For the preparation of magnetic coatings, conventional kneading machines are used, such as two-roll mills, three-roll mills, ball mills, pebble mills, thoron mills, sand gliders - 1 Szegvari attritor, high-speed impeller dispersion machines, high-speed stone mills, high-speed impact mills, dispers, Kneader-1 high speed mixer,
Examples include homogenizers and ultrasonic dispersers.

Note that the above-mentioned additives such as dispersants, antistatic agents, and lubricants are not strictly limited to having only the above-mentioned effects; for example, if the dispersant is a lubricant, Alternatively, it may act as an antistatic agent. Therefore, it goes without saying that the effects of compounds etc. exemplified by the above classifications are not limited to those listed in the above classifications, and when using substances that have multiple effects, the amount of is preferably determined by considering the action and effect of the substance.

In addition, detergent dispersants, viscosity index improvers, pour point depressants,
It is also possible to add anti-foaming agents and the like.

The magnetic paint thus prepared is applied onto the aforementioned non-magnetic support. The coating can be applied directly onto the non-magnetic support, but it can also be applied onto the non-magnetic support via an adhesive layer or the like.

Examples of coating methods on non-magnetic supports include air doctor coating, blade coating, rod coating, extrusion coating, air knife coating, squeeze coating, impregnation coating,
reverse roll coat, transfer roll coat,
Methods such as gravure coating, kiss coating, cast coating, spray coating, and spin cording can be used, and methods other than these can also be used.

In this way, the magnetic paint can be manufactured so that the thickness of the magnetic layer after drying is generally in the range of about 0.5 to 10 μm, particularly in the range of about -95 to 7.0 μm.
It is applied in a range of μm.

The magnetic layer coated on the non-magnetic support is then treated to orient the ferromagnetic powder and then dried. Furthermore, after surface smoothing treatment is performed as necessary, the material is cut into a desired shape.

A known back layer may be provided on the surface of the nonmagnetic support on which the magnetic layer is not provided.

Next, Examples and Comparative Examples of the present invention will be shown.

In each category, "parts" indicate "parts by weight."

[Examples 1 to 6] A magnetic layer forming coating solution prepared by the following composition and method was applied to the surface of a 10 μm thick polyethylene terephthalate base (non-magnetic support), and subjected to magnetic field alignment treatment using a cobalt magnet. After the application, it was dried in an atmosphere at a temperature of 100° C. for 1 minute to form a magnetic layer with a thickness of 3.0 μm.

Thereafter, the magnetic layer was calendered and
It was cut into inch-sized tapes to create VHS video tapes.

Preparation of coating liquid Ferromagnetic Fe-Ni alloy powder 100 parts [
Ni content: approximately 5% by weight, crystallite size: average major axis length = 0.3 μm] Vinyl chloride copolymer... (type and amount shown in Table 1) Polyurethane [polar group shown in Table 1 A polyurethane synthesized from a polyol (the main skeleton of which is shown in Table 1), a diisocyanate, and a chain extender, in the amount shown in Table 1] Polyisocyanate (T amount shown in Table 1) (Coronate 3041, manufactured by Nippon Polyurethane) Carbon black 2 parts (average particle size: 40 mμ) α-An20.・・・・・・・・・1 part Stearic acid ・・・・・・・・・1 part Butyl stearate ・・・・・・・・・1 part Methyl ethyl ketone ・・・・・・300 parts The above composition was placed in a ball mill and processed for 50 hours to thoroughly mix and disperse the composition to prepare a coating solution for forming a magnetic layer.

[Comparative Examples 1 to 4] The composition of the magnetic layer forming coating solution in Examples 1 and 2 was as follows:
Example 1 except that the type or content of polar groups in polyurethane, or the amount of vinyl chloride copolymer or polyurethane used were changed as shown in Table 1.
A videotape was created in the same manner as in 2. and 2.

The characteristics of the videotapes obtained in each Example and Comparative Example prepared as described in Section 1 were evaluated by the following method, and the results are shown in Table 2.

[Gloss] Measured using a Suga gloss meter. The incident light angle was 45 degrees.

[Reproduction output] Image signal 50 T RE (The In5titut
e of Radi.

Engineers) video signals were recorded at a standard recording current. The average value of the envelope of this reproduced RF output was measured using an oscilloscope, and calculated using the following formula.

Reproduction output (aB) = 20 to glo v /
v.

V: Average value Vo: Reference value [Video color signal noise (C/N)] Image signal 30IR with 100% amplitude color subcarrier superimposed
The video signal of E was recorded using a standard recording current. This was reproduced and passed through a 1 kHz high-pass filter and a 50' OHZ low-pass filter, and the color signal noise AM component of the reproduced output signal was measured using a noise measuring device. Measure the reference sample in the same way, and calculate the difference in dB between the obtained reference value and the measured value for the sample.
It was displayed in

[Still Life] A video signal of 50 IRE was recorded, this was reproduced in still mode, the reproduced RF output level was recorded with a recorder, and the time until the signal level decreased to 1/2 was measured.

[Dropouts] The number of dropouts after 250 passes were repeated. To drop out, press t at the dropout counter.
The playback output level is 16dB for a period of 5xto-s seconds or more.
The number of items decreased by 7 minutes or more was displayed.

[Output Decrease] The regenerated output after repeatedly running 100 times was compared with that before running, and the amount of decrease was expressed as OdB before running.

From the results in Table 2, the video tapes obtained in each example were found to have lower gloss, playback output, C/N, still life, dropout, and lower output than the video tapes obtained in each comparative example. It is clear that the method is also extremely superior.

In particular, in the comparative example, the decrease in output was significant, and it is clear that corrosion of the head had occurred.

[Effects of the Invention] Since the magnetic recording medium of the present invention contains a specific polyurethane as a binder for the magnetic layer of the magnetic recording medium, even though ferromagnetic metal powder is used as the magnetic material,
This magnetic recording medium has extremely excellent electromagnetic conversion characteristics and running durability, and has the remarkable effect of not corroding the head.

Claims (1)

[Claims] 1. In a magnetic recording medium comprising a non-magnetic support and a magnetic layer formed by dispersing ferromagnetic metal powder on the support in a binder, the crystallite size of the ferromagnetic metal powder is 250.
Å or less, and the binder is at least one type of S selected from the group consisting of -SH group, ▲ group, ▲ group, ▲ group, etc.
1. A magnetic recording medium, comprising a polyurethane having a containing group, and the amount of the binder is 15 to 40% by weight of the ferromagnetic metal powder.