JPH02105323A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve electromagnetic conversion characteristics and durability by specifying the transmittance of light of a specific wavelength of the recording medium having a required magnetic layer and a back coating layer to a specific value and incorporating specific polyurethane into a binder. CONSTITUTION:The magnetic recording medium having the magnetic layer formed by dispersing and incorporating magnetic material particles into a binder on one surface of a nonmagnetic base and the back coating layer formed by dispersing and incorporating a carbon black into the binder on the other surface is so formed that the light transmittance of 9,000Angstrom wavelength attains >=2% in the magnetic layer and 1 to 10% in the back coating layer and <=2% as the entire part of the recording medium. In addition, the polyurethane in which the negative functional group forms an intramolecular salt is incorporated into the binder. Then, the uniform dispersibility of the magnetic particles and the carbon black is enhanced and the good-quality magnetic tape which is improved in the electromagnetic conversion characteristics including S/N, obviates the generation of damages after repetitive traveling and is improved in the durability is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magnetic recording medium, and specifically relates to a magnetic recording medium using a resin film as a support.

[Background of the invention]

In recent years, as the density and S/N of magnetic materials, particularly video and computer recording media, have increased, magnetic particles with smaller particle diameters have come to be used.

Generally, S/ of magnetic recording media (hereinafter referred to as magnetic tape) is
The N ratio is said to be proportional to the square root of the number of magnetic particles in the recording material involved in recording and reproduction, so when applying the same weight of magnetic powder, the magnetic powder with a smaller particle size will The more it is used, the more advantageous it becomes to improving the S/N.

However, as magnetic particles become finer, their surface activity increases, making dispersion difficult.

Conventionally, various dispersants have been commonly used to improve dispersibility, but when an amount of dispersant is added to provide sufficient dispersibility, problems such as blooming and surface viscosity occur.

In order to avoid this difficulty, modified binders with polar groups such as sulfo groups and phospho groups introduced into binders for magnetic tape constituent layers containing magnetic particles and other fillers such as magnetic layers have been studied. I have cultivated myself (Japanese Patent Publication No. 56-7)
No. 4829, No. 57-44227, etc.), the film properties of the constituent layers are insufficient and do not give satisfactory results in terms of durability, durability, etc.

On the other hand, if the magnetic particles are made finer and the BET value is increased, the surface of the magnetic layer becomes smoother, which actually deteriorates the running durability of the tape. In other words, since the above magnetic tape violently slides against the magnetic head during recording and reproduction,
The smoothing of the surface increases the sliding contact area and increases the coefficient of friction, which actually deteriorates running performance. This also causes undesirable chain reactions such as wear of the magnetic layer during repeated use, falling off of the magnetic particles contained in the layer, and clogging of the magnetic lead due to the falling magnetic particles.

For video tapes that require even higher S/N, carbon black, carbon black, etc.
The trend is to remove fillers other than magnetic particles, such as abrasives, to increase the degree of filling of magnetic particles.

However, when carbon black is removed from the magnetic layer, the optical density of the magnetic tape decreases, making running control difficult and possibly causing stopping of running, and also causes a loss of electrical conductivity and electrification, which impairs running durability.

Furthermore, when a magnetic tape is run on a video deck, if the running properties and durability of the back side of the tape are not good, excessive tension will be applied to the running magnetic tape, causing the magnetic layer to rub against the magnetic head excessively. This not only causes damage to the magnetic layer and peeling off of the magnetic particles in the magnetic layer, but also changes in the strength of the tension at which the magnetic tape is wound, causing the winding pressure to fluctuate, causing the tape to become distorted. The edges become uneven,
When the tape is reused, the running of the tape becomes uneven.

When these things occur, the image or electromagnetic characteristics such as skew, jitter, and S/N deteriorate.

In order to improve these, it has been proposed to provide a back coat layer (hereinafter referred to as BC layer) on the back surface of the magnetic tape.

That is, inorganic powder is dispersed in a binder and the surface of the BC layer is roughened to reduce the contact area with guide balls etc., thereby reducing the coefficient of friction (for example, as disclosed in JP-A-57
-130234, 57-53825, 58-1
No. 61135, No. 5g-2415).

In addition, a method using carbon black instead of inorganic powder has been proposed (for example, Japanese Patent Publication No. 52-17401), aiming at the antistatic and light shielding effect based on the conductivity of carbon black, and the roughening effect due to its particles. However, the dispersibility of carbon black is extremely poor, and the agglomerated particles give a rough uneven surface to the /<・ink coating layer formed using this dispersion, and the agglomerated particles do not have a strong binding force with the binder, so they peel off. Not only is this easy, but when the tape is wound and comes into contact with each other, the large aggregated particles cause unevenness in the magnetic layer.

[Purpose of the invention]

In light of the above circumstances, the objects of the present invention are to provide (1) a magnetic tape with excellent uniform dispersion of magnetic particles and carbon blanks and good electromagnetic conversion characteristics, especially S/N characteristics; (2) a magnetic tape that can be repeatedly run; It is an object of the present invention to provide a highly durable magnetic tape that does not cause damage to the magnetic tape later.

[Structure of the invention]

The object of the present invention is to provide a magnetic recording medium having a magnetic layer containing magnetic particles dispersed in a binder on one side of a non-magnetic support, and a back coat layer containing carbon black dispersed in a binder on the other side. , the transmittance of light at a wavelength of 9000 is 2% or more in the magnetic layer and 1-10% in the back coat layer.
, and the amount of the magnetic recording medium as a whole is 2% or less, and is achieved by a magnetic recording medium containing polyurethane in which the binder has a negative functional group forming an inner salt.

Next, the present invention will be explained in detail.

The S/N of the magnetic tape is good if magnetic particles with a high BET value, for example 35 m''/g, are used in the magnetic layer, fillers such as carbon black are reduced, and the filling rate and light transmittance are high. When the transmittance of wavelength light falls into the region of 2% or more, it rapidly improves.On the other hand, when it is less than 2%, the S/N deteriorates due to excess filler such as carbon black.

In the BC layer, if there is too much carbon black with a light transmittance of less than 1%, the BC layer will become brittle and the BC layer will be worn away by running.On the other hand, if it exceeds 10%, the transmittance of the entire magnetic tape will decrease to 2% or less. This makes it difficult to control the vehicle, resulting in non-standard conditions.

Next, specific examples of carbon black used in the present invention include: manufactured by Columbia Carbon Co., Ltd.; Raven 5250.1255.1250.1200;
1170.1040.

1035, 1030.1020, 890, 850, 82
5. Raven MT-CI made by Cabot; Black Burls L, Regal 400, 600°50
OR, 500, 330, 99, Palcan xC-72. P
Manufactured by Mitsubishi Kasei; CF9# 50.52, 45, 44.40, 32, 30
．． Examples include 4000MA-100, 7, 8, 11, and the like.

Moreover, the BC layer according to the present invention contains BaSO4+ ZnO, Ti
Fillers such as E and Tic may be used in combination with carbon black.

Further, the total weight of the filler including carbon black and other fillers according to the present invention is 50 to 500 parts by weight, preferably 60 to 400 parts by weight, based on 100 parts by weight of the binder.

Polyurethane used as a binder for magnetic tape is
In addition to the good physical properties of polyurethane itself, the introduction of negative functional groups such as sulfone groups into the molecule improves the dispersion power for magnetic powder and fillers such as Carbon Co., Ltd., which are generally difficult to disperse. Can be done.

The polyurethane according to Shiraki's invention has a functional group introduced by forming an inner salt, and has a large dispersion power without impairing the physical properties of the binder or creating any other problems. The inner salt is preferably a betaine type, and more preferably a sulfobetaine type.

The "polyurethane in which the negative group forms an inner salt" according to the present invention can be used in the same way as in the usual polyurethane synthesis method, such as polycarbonate polyol, polyester polyol,
It is synthesized by reacting a high molecular weight polyol (molecular weight 8-) (molecular weight 500 to 3000) such as polylactone polyol or polyether polyol with a polyfunctional aromatic or aliphatic isocyanate. by this,
Polyester polyurethanes, polyether polyurethanes, and polycarbonate polyurethanes carbonated with phosgene or diphenyl carbonate are synthesized.

These polyurethanes are primarily produced by the reaction of polyisocyanates with polyols and optionally other copolymers, and may also be in the form of urethane resins or urethane prepolymers containing free isocyanate groups and/or hydroxyl groups; λ may also contain no reactive end groups (for example in the form of urethane elastomers).

As the isocyanate component, various diisocyanate compounds such as hexamethylene diisocyanate-I-(HM
DI), diphenylmethane diisocyanate (MDI)
), hydrogenated MDI (H+ffiMDr), toluene diisocyanate (TDI), 1.5-naphthalene diisocyanate (NDI), tolidine diisocyanate
(TODI), lysine diisocyanate methyl ester (LDI), isophorone diisocyanate (IPDI)
) etc. can be used. If necessary, a low-molecular polyfunctional alcohol such as (1), 4-butanediol, 1.6-hexanediol, (2), 3-butanediol is used to adjust the molecular weight and the physical properties of the resin.

The functional group forming the inner salt may be introduced into the isocyanate component, but it can also be introduced into the polyol component, and further into the above-mentioned low-molecular polyfunctional alcohol.

Polyester polyols in which the negative functional groups form an inner salt contain various dicarboxylic acid components, polyhydric alcohol components, and dicarboxylic acid components and/or negative functional groups in which the negative functional groups form an inner salt. It can be synthesized by polycondensing polyhydric alcohol components forming an inner salt.

Examples of the dicarboxylic acid component include terephthalic acid, isophthalic acid, sebacic acid, adipic acid, dimerizing luic acid, and maleic acid. As a polyhydric alcohol component,
Glycols such as ethylene glycol, propylene glycol, butylene glycol, and diethylene glycol, or polyhydric alcohols such as trimethylolpropane, hexanetriol, glycerin, trimethylolpropane, trimethylolethane, and pentaerythritol, or these glycols and polyhydric alcohols Two or more arbitrary types selected from these can be exemplified.

In order to produce a lactone polyester polyol in which the negative functional group forms an inner salt, the above-mentioned lactams such as S-caprolactam, α-methyl-1-caprolactam, S-methyl-8-caprolactam, and γ-butyrolactam are added. What is necessary is to introduce a functional group.

In order to produce a polyether polyol in which a negative functional group forms an inner salt, the above functional group may be introduced into ethylene oxide, propylene oxide, butylene oxide, or the like.

As the functional group forming the inner salt, a betaine group mentioned below can be exemplified.

A polyester polyol in which the negative functional group forms an inner salt will be further described.

A general method for synthesizing polyester is carried out by a condensation reaction between an acid component having an aliphatic or aromatic polyfunctional acid or a derivative thereof and an aliphatic or aromatic polyfunctional alcohol component. The intramolecular amphoteric base (such as a betaine group) according to the present invention may be contained in either the acid component or the alcohol component, or a method of introducing a betaine group or the like into the polymer as a polymer reaction may be used. However, in consideration of unreacted components and the introduction rate, it is easier to control if the polymer monomer has the functional group.

Examples of the betaine group include a sulfobetaine group, a phosphobetaine group, and a carboxybetaine group. Those having a sulfobetaine group or a phosphobetaine group are more preferred. The general formula of these betaine type functional groups is expressed as follows.

Nno ÷: State contained in nirarethane.

X: 50. . O5O3e, C00e0
-PQ3H0, -0PO8゜, -0P02H2θ-
11=A: Hydrogen or an alkyl group having 1 to 60 carbon atoms (eg, methyl group, ethyl group, etc.).

IIl: An integer from 1 to 10.

B: -Coo- or -CONH-R: carbon number 1-
12 alkyl, alkenyl or aryl groups.

n, m: l” 10 integers.

Examples of usable betaine group-containing monomers include the compounds listed below, but it goes without saying that the polyurethane resin used in the present invention is not limited to those using these monomers.

The monomer in which the negative functional group forms an inner salt is available as a commercially available drug, but it can be obtained by the following method: 1) Synthesis method using monochloroacetic acid R-N(CH2-COOH)2+C12-CH2- C.O.
OH2) Synthesis method CI using monochlorosuccinic acid (2-
Cool (3) The synthesis method using propane sultone and the reaction of introducing betaine groups etc. into the polymer as a polymer reaction will be described. This is O) present at the terminal or side chain of polyurethane whose chain has been extended to a predetermined molecular weight by a polymerization reaction.
One group is reacted with a compound having a betaine group or the like. In this case, first, a compound having a hydroxyl group and a betaine group is synthesized, and this is reacted with a polyfunctional incyanate such as a diisocyanate in equimolar amounts to form a reaction product between the NGO group on one side of the diisocyanate and the hydroxyl group in the above compound. obtain. Then, by reacting the OH groups of the polyurethane with the unreacted NCO groups, a polyurethane into which betaine groups and the like are introduced can be obtained.

Examples of the above-mentioned compounds having a hydroxyl group and a betaine group include, but are not limited to, the following compounds.

The amount of betaine groups etc. introduced into the polyurethane resin of the present invention is preferably in the range of 0.01 to 1.0 mmof2/g, more preferably in the range of 0.1 to 0.5 mmof2/g.

If the amount of the polar group introduced is less than 0.01 mmoff/g, a sufficient effect on the dispersibility of the ferromagnetic powder will not be observed. In addition, the amount of the above polar group introduced is 1.0 mmoff/g
If it exceeds this range, intermolecular or intramolecular aggregation tends to occur, which not only adversely affects dispersibility, but also causes selectivity to the solvent, which may make it impossible to use ordinary general-purpose solvents.

Further, the number average molecular weight of the polyurethane resin according to the present invention is 5,
000~100,000. More preferably 10,000
It is preferable that it is in the range of ~40.000. If the number average molecular weight is less than 5,000, the coating film-forming ability of the resin will be insufficient, and if the number average molecular weight exceeds 100,000, problems may occur in paint manufacturing processes such as mixing, transport, and coating. There is a possibility that this may occur.

(Synthesis Example) 1 mole of N-methyljetanolamine and 1 mole of propanesultone were reacted at a temperature of 120° C. for 3 hours to obtain a sulfobetaine type polyfunctional monomer.

Next, 1.5 mol of adipic acid, 1.7 mol of 1.4-butanediol, and 0.06 mol of the above sulfobetaine type acid-base polyfunctional monomer were charged, and the mixture was heated at 150 to 200°C for about 3 hours. The temperature was raised at 200°C for 4 hours, and the
Unreacted raw materials were removed at 5 mmHg, and the reaction was carried out until the acid value reached 2 or less. The molecular weight of the obtained copolymerized polyester is MW
It was 2500. 165 g of copolymerized polyester was dissolved in 300 parts of methyl ethyl ketone, 80 parts of diphenylmethane diisocyanate was added, and the mixture was reacted for 2 hours at 80°C. 4 parts of diol was added and reacted for 1 hour. The molecular weight of the obtained polyurethane is Mv=3,
50,000, Mn=22,000.

Polyurethane (a) according to the present invention was synthesized according to the above synthesis example, and (b) and (c) were synthesized in the same manner.

(a) Sulfobetaine type polyurethane number average molecular weight; 22,000, Tgi -20°C, polar group concentration; Q, 04mmoQ/gCI++ 0θ (b) Carboxybetaine type polyurethane number average molecular weight; 15,000, Tg; - 10°C1 polar group concentration; O,
O1mmoQ/g (c) Phosphobetaine deaf polyurethane number average molecular weight; 30,000, Tg; O°C polar group concentration;
0.07+mo(2/g0θ) In the present invention, in addition to the above-mentioned binder, conventionally used modified or unmodified vinyl chloride resins with polar groups, polyurethane resins, or polyester resins may be mixed as necessary. Furthermore, cellulose resins, phenoxy resins, thermoplastic resins with specific usage methods, thermosetting resins, reactive resins, electron beam curable resins, etc. may be used in combination.

The mixing ratio of the polyurethane forming the inner salt according to the present invention and the binder is 8/2 to 2/8, preferably 7
73-4/6.

The above-mentioned resins can be used as a constituent binder for the constituent layers of the present invention, such as the magnetic layer, back coat layer, protective layer or adhesive layer, by selecting the optimum type and amount, with their advantages and disadvantages being complementary.

In order to improve the durability of the constituent layers such as the magnetic layer of the magnetic tape of the present invention, various hardening agents can be contained, for example, incyanate can be contained.

Aromatic isocyanates that can be used include, for example, tolylene diisocyanate (TDI) and adducts of these isocyanates with active hydrogen compounds, and those having an average molecular weight of 100 to 3,000 are preferred.

Examples of aliphatic isocyanates include hexamethylene diisocyanate (HMDI) and adducts of these incyanates and active hydrogen compounds. Among these aliphatic incyanates and adducts of these incyanates and active hydrogen compounds, those having a molecular weight in the range of 100 to 3,000 are preferred. Among the aliphatic incyanates, non-alicyclic isocyanates and adducts of these compounds with active hydrogen compounds are preferred.

In the present invention, conventional techniques can be used to form the magnetic layer, back coat layer, or other constituent layers of the magnetic tape of the present invention.

The magnetic layer of the magnetic tape of the present invention is a dispersed magnetic layer using a paint containing magnetic powder, filler, etc. dispersed and suspended in a binder, and further containing a dispersant, lubricant, etc. as required.

Examples of magnetic materials include γ-Fe20s and coo
Fe2O3, Co deposited - Fe203, Fe50 and Co
Contains Fe3O4,C.

oxide magnetic materials, such as Fe, Nis Co, Fe
-Ni alloy, Fe-Co alloy, Fe-N1-P alloy, F
e-Ni-Co alloy, Fe-MnZn alloy, Fe-Ni
-Zn alloy, Fe-Co-Ni-Cr alloy, Fe-C.

N1-P alloy, Co-Ni alloy, Co-P alloy, Co-
Examples include various ferromagnetic materials such as Cr alloys and metal magnetic powders containing Fe, Ni, and Co as main components. Additives to these metal magnetic materials include Sl, Cu, Zn, and A.
fft Ps Elements such as Mn and Cr or compounds thereof may be included. Hexagonal ferrite such as barium ferrite and iron nitride are also used.

The coating material such as the magnetic coating material used to form the constituent layers such as the magnetic layer may optionally contain dispersants, lubricants,
Additives such as antistatic agents and fillers may also be included.

Dispersants used in the present invention include amine compounds, alkyl sulfates, fatty acid amides, higher alcohols,
Examples include polyethylene oxide, sulfosuccinic acid, sulfosuccinic acid ester, known surfactants, and salts thereof.

These dispersants may be used alone or in combination of two or more. These dispersants are added in an amount of 1 to 20 parts by weight per 100 parts by weight of the magnetic powder. These dispersants may be used to pre-treat the magnetic powder in advance.

In addition, as lubricants, silicone oil, graphite, carbon black graft polymer molybdenum disulfide, tungsten disulfide, lauric acid, myristic acid, monobasic fatty acids having 12 to 22 carbon atoms and the total number of carbon atoms of the fatty acids are used. Fatty acid esters made of monohydric alcohols having 13 to 40 carbon atoms can also be used. These lubricants are used in amounts of 0.2 to 20 parts by weight per 100 parts by weight of magnetic powder.
It is added in a range of parts by weight.

As the filler, organic powder or inorganic powder can be used individually or in combination.

As the organic powder used in the present invention, acrylic styrene resin, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment are preferable, but polyolefin resin powder, polyester resin powder, polyamide resin powder, and polyimide resin are preferred. Powder, polyfluoroethylene resin powder, etc. can also be used, and inorganic powders include silicon oxide, titanium oxide, aluminum oxide, calcium carbonate,
barium sulfate, zinc oxide, tin oxide, aluminum oxide,
Chromium oxide, silicon carbide, calcium carbide, α-Fe20
3. Talc, kaolin, calcium sulfate, boron nitride, zinc fluoride, and molybdenum dioxide.

Antistatic agents include carbon black, graphite, conductive powders such as tin oxide-antimony oxide compounds, titanium oxide-tin oxide-antimony oxide compounds; natural surfactants such as saponin; alkylene oxides, and glycerin. Cationic surfactants such as higher alkylamines, quaternary ammonium salts, pyridine, other heterocycles, phosphonium or sulfoniums: Carboxylic acid, sulfonic acid, phosphoric acid, sulfuric acid Examples include anionic surfactants containing acidic groups such as ester groups and phosphoric acid ester groups; amphoteric surfactants such as diamino acids, aminosulfonic acids, and sulfuric acid or phosphoric acid esters of amino alcohols.

In addition to the additives mentioned above, an abrasive may be added to the magnetic layer. As the abrasive, commonly used materials such as fused alumina, silicon carbide, chromium oxide, corundum, artificial corundum, artificial diamond, garnet, and emery (main components: corundum and magnetite) are used. These abrasives have an average particle size of 0.05 to 5 μm, particularly preferably 0.1 to 2 μm. These abrasives are added in an amount of 1 to 20 parts by weight per 100 parts by weight of the magnetic powder.

As the solvent blended in the paint forming the constituent layer or the diluting solvent during application of this paint, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; alcohols such as methanol, ethanol, propatool, butanol, etc. ; Esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, and ethylene glycol senoacetate; Ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, and tetrahydrofuran; Aromatic hydrocarbons such as benzene, toluene, and xylene; methylene chloride,
/X-logenated hydrocarbons such as ethylene chloride, carbon tetrachloride, chloroform, and dichlorobenzene can be used.

Examples of the support include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and plastics such as polyamide and polycarbonate. Metals such as Cu, A12, and Zn, glass, and ceramics such as boron nitride and silicon carbide can also be used.

The thickness of these supports is approximately 3 or 100 μm in the case of a film or sheet, preferably 5 to 50 μm, approximately 30 μm to 10 mm in the case of a disk or card, and cylindrical in the case of a drum. The type is determined depending on the recorder used.

Coating methods for forming the magnetic layer on the support include air doctor coating, blade coating, air knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, kiss coating, cast coating, Spray coats and the like can be used, but are not limited to these.

〔Example〕

The present invention will be specifically explained using examples.

Examples 1 to 3 and Comparative Examples (1) to (6) Magnetic paints of the following formulation I were sufficiently mixed and dispersed in a ball mill, 6 parts of polyfunctional incyanate was added as a hardening agent, and filtered with a 1 μm filter. It was coated on a polyethylene terephthalate base with a thickness of 13 μm using a reverse roll coater so that the dry film thickness was 4.5 μm. The film was subjected to supercalender treatment.

Next, the composition of formulation (1) (back coat (BC) paint) was dispersed in a ball mill for 24 hours to obtain a BC paint for each sample.

The paint is applied to the back side of the sample film using a reverse roll coater to a dry film thickness of 1.0 μm, and dried.
A BC layer was formed on each layer, and each part was slit to a width of 2 inches to prepare Example sample tapes 1 to 3 and Comparative example sample tapes (1) to 3.
I got (6).

Prescription ■ (Magnetic paint) (Parts by weight) C
o-containing - Fe203100 Sulfobetaine type polyurethane (synthesis example (a)) manufactured by Cucci Co., Ltd. b 0.8 0.5 0.5 parts by weight Nistan 5701 (gutstri chloride, vinyl acetate copolymer, stearate, butyl myristate, stearin) Acid alumina carbon black lecithin cyclohexanone methyl ethyl ketone toluene formulation II (BC paint) Sulfobetaine type polyurethane (synthesis example (a)) Nitrocellulose (Seltsuba; manufactured by Fukaisei) Polyisocyanate carbon black y Zinc oxide
2 toluene
750 Methyl ethyl ketone
1000 The formulation variation components of the sample and the characteristics of the magnetic tape are listed in Table 1.

In Examples 1 to 3 according to the present invention, there was very little abrasion of the BC layer and very little damage to the tape. Furthermore, since the running is stable, the skew and jitter characteristics are also excellent.

Furthermore, since it has excellent uniform dispersibility, it is possible to make the surface roughness extremely fine, the luminance S/N is high, and the durability in repeated running is sufficient.

Characteristic measurement method: 1) Lum1 S/N, RF output used Deck HR-57000 Noise Meter made by Victor Company of Japan Shibaku 925D/1 Our reference tape is OdB.

2) BC scraping Using HR-0180 (manufactured by Victor Japan), running was repeated 400 times for 5 minutes at the beginning of the tape, and the scraping of the BC layer after running was visually determined.

×　; Impractical O: No problem in practical use ◎; No damage occurs at all.

Claims (1)

[Claims] In a magnetic recording medium having a magnetic layer containing magnetic particles dispersed in a binder on one side of a non-magnetic support and a back coat layer containing carbon black dispersed in a binder on the other side, the transmission of light with a wavelength of 9000 Å is possible. The binder contains a polyurethane having a ratio of 2% or more in the magnetic layer, 1 to 10% in the back coat layer, and 2% or less in the entire magnetic recording medium, and in which the negative functional group forms an inner salt in the binder. Containing magnetic recording medium.