JPH02110817A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a medium with durability and ability of high-density record ing by specifying the specific surface area of metal magnetic powder and the ratio of iron atoms to aluminum atoms both contained in the powder, and incorporating polyurethane having negative functional groups which form intramolecular salt into a binder. CONSTITUTION:The metal magnetic powder has the specific surface area of >=45m<2>/g and the atomic ratio of iron to aluminum (iron atoms:aluminum atoms) of (100:1) - (100:20). The binder contains polyurethane having negative functional groups which form intramolecular salt. The metal magnetic powder having the above-mentioned specific surface area enables high-density recording and excellent S/N. By specifying the ratio of atoms of iron to aluminum in the metal magnetic powder, the amount of iron is enough so that the obtained metal magnetic powder essentially comprising iron has excellent electric characteristics and corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to magnetic recording media such as magnetic tapes, magnetic sheets, and magnetic disks.

(Background of the Invention) Recently, the density of magnetic recording media such as magnetic tapes has increased, and
With the shift to nitrogen, magnetic powders with smaller particle diameters are being used.

Generally, it is said that the S/N ratio of a magnetic recording medium is proportional to the square root of the number of particles of magnetic powder in the recording material involved in recording and reproduction. Therefore, when applying the same weight of magnetic powder,
The use of magnetic powder with a smaller particle size is more advantageous in improving the S/N. Further, if the magnetic powder is made into fine particles and its BET value is increased, the surface of the magnetic layer becomes smoother and there is less vibration loss, which is advantageous in obtaining high electromagnetic conversion characteristics. If metal magnetic powder is used, even higher density recording is possible and performance is further improved.

In other words, this metal magnetic powder has large saturation magnetization and coercive force,
It has excellent properties as a high-density recording material. However, on the other hand, when the BET value is high and the surface activity is large, the metal magnetic powder is subjected to air oxidation, and the magnetic properties gradually deteriorate as the oxidation progresses. Further, the metal magnetic powder has poor dispersibility in the binder, making it difficult to disperse, and in extreme cases, there are problems such as gelling of the binder resin in the paint.

In particular, as video tapes have become more portable in recent years, they have been exposed to a wide variety of environments, and their usage conditions have also varied. Therefore, video tapes are required to have high corrosion resistance and durability.

(Objective of the Invention) The object of the present invention is to enable high-density recording, have excellent electromagnetic conversion characteristics such as S/N ratio, and have good dispersibility of magnetic powder and corrosion resistance.
An object of the present invention is to provide a magnetic recording medium with excellent durability.

(Structure of the Invention and Its Effects) As a result of studies in accordance with the object of the present invention, it has been found that in a magnetic recording medium in which a magnetic layer contains a binder and metal magnetic powder dispersed in the binder, the above-mentioned The metal magnetic powder has a specific surface area of 45 m''/g or more, and the content ratio of iron atoms and aluminum atoms (iron atoms and aluminum atoms) contained in the metal magnetic powder is (100: l).
) to (100:20), and is achieved by a magnetic recording medium characterized in that the binder contains polyurethane in which a negative functional group forms an inner salt.

According to the present invention, the magnetic powder has a specific surface area of 45ffi2.
Since 7 g or more of metal magnetic powder is used, high-density recording is possible and a medium with excellent S/N ratio etc. can be provided.

In addition, by specifying the abundance ratio of iron atoms and aluminum atoms in the metal magnetic powder to (too: 1) to (100:20), the content of iron atoms is sufficient and the powder has excellent electrical properties. Not only can iron-based metal magnetic powder be provided, but the content of aluminum atoms is also sufficient, so that the corrosion resistance, which is a characteristic of aluminum atoms, can be fully exhibited. This range is further defined as (iron atom nialium atom) = (100: l
) to (100:8) is preferable.

However, when a large amount of iron atoms are present in the surface area of metal magnetic powder, the oxidation of individual particles is caused by the fact that it is a metal powder and has a specific surface area of 45 m"/g or more. Because it is easy to proceed and the particles are easy to aggregate,
There is a risk that the dispersibility will decrease and the durability of the medium will ultimately deteriorate. can be fully exerted and the progress of oxidation of magnetic powder can be controlled. Moreover, the dispersibility of particles is also improved.

Since the aluminum atoms are bonded to oxygen, the number of active sites present on the surface of each particle of the magnetic powder is reduced, and surface activity is suppressed. Therefore, the corrosion resistance and dispersibility of the metal magnetic powder are further improved, and the durability of the medium is improved.

The metal magnetic powder is preferably one obtained by a dry reduction method in which iron oxide is reduced with hydrogen or the like, and it is difficult to obtain the high electrical properties and high durability of the present invention with metal powder obtained by wet reduction with boron hydride or the like.

Examples of the metal magnetic powder used in the present invention include Fe-Aff-based, Fe-Al2-Zn-based, and Fe-^(2-Co-based) metal magnetic powders.

The iron-aluminum metal magnetic powder used in the magnetic recording medium of the present invention can be produced, for example, as follows. α-F
e00)1. Iron hydrates such as γ-FeOO) 1, or a
-Fe203, γ-Fezes, Fe50. Iron oxides such as iron oxides are reduced with Hl or the like at high temperatures.

For example, after reacting a ferrous compound (FeSO, FeCQ, s, etc.) with an alkaline component (e.g. NaOH, etc.),
7 or α-F
eOOH is α-Fe, 0. After converting into
I2 compounds (such as sodium aluminate) can be added.

The same applies to the case where a compound of an element other than Fs is added in addition to AI2.

Note that the above specific surface area is expressed by a BET value. To measure the specific surface area, for example, measure the powder at 30-60°C at around 250'C.
The powder is degassed while being heated for 1 minute to remove what it has adsorbed.Then, the powder is introduced into a measuring device, the initial pressure of nitrogen is set to 0.5Kg/m', and the liquid nitrogen temperature ( -1
Measure nitrogen adsorption at 95°C (for details, see J.
, Ame, Chem, Soc, 60309 (1938
). As the device for measuring the specific surface area (BET value), it is possible to use the "Powder Measuring Device (Cantersorb)" jointly manufactured by Yuki Battery Co., Ltd. and Yuki Ionics Co., Ltd.

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 (molecule J 1500 to 3000) such as polylactone polyol or polyether polyol with a polyfunctional aromatic or aliphatic isocyanate. As a result, polyester polyurethane, polyether polyurethane, and polycarbonate to polyurethane carbonated with phosgene or diphenylcarbonate are synthesized.

These polyurethanes are primarily produced by the reaction of polyisocyanates with polyol-/p and optionally other copolymers, and are produced by the reaction of urethane resins or urethane prepolymers containing free isocyanate groups and/or hydroxyl groups. It may be in the form of a urethane elastomer, or it may be in the form of a urethane elastomer that does not contain these reactive end groups.

Isocyanate components include various diisocyanate compounds, such as hexamethylene diisocyanate (IIM
DI), diphenylmethane diisocyanate (MD
I), hydrogenated MDI (II+2[)I), toluene diisocyanate (TDI), 1,5-naphthalene diisocyanate (NDI), tolidine diisocyanate 1- (TODI), lysine diisocyanate methyl ester (LDI) ), inphorone diisocyanate
(IPDI) etc. can be used. Also, if necessary, ■, 4
-butanediol, 1.6hexanediol, ■, 3-
A low-molecular polyfunctional alcohol such as butanediol is used to adjust the molecular weight, resin physical properties, etc.

The functional group forming the inner salt may be introduced into the iso7a-1 component, but it can also be introduced into the polyol component. may be introduced.

Polyether polyols in which negative functional groups form inner salts can be prepared by combining various dicarboxylic acid components, polyhydric alcohol components, and dicarboxylic acid components and/or negative functional groups in which negative functional groups form inner salts. It can be synthesized by polycondensing polyhydric alcohol components that form 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, diethylene glycol, polyhydric alcohols such as trimethylolpropane, hexanetriol, glycerin, trimethylolpropane, trimethylolethane, pentaerythritol, or these glycols and polyhydric Examples include two or more arbitrary types selected from alcohols.

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-5-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.

An example of the functional group forming the inner salt is the betaine group described below.

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

- A simple 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. Intramolecular amphoteric base of the present invention (betaine group, etc.)
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 introduction rate, it is easier to control φ because the polymer monomer has the functional group.

Examples of the betaine group include a sulfobetaine group, a phosphobetaine group, and a carboxine betaine 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.

qノ +: Contained in nirarethane.

x: -5ose, -o-so,.
-cooo-0-PO3He , -0PO,' , -
0PO, H, 0A: hydrogen or an alkyl group having 1 to 60 carbon atoms (eg, methyl group, ethyl group, etc.).

m: 1” IO integer.

(Example ψ B: -COO- or -CONH-R: an alkyl group, alkenyl group, or aryl group having 1 to 12 carbon atoms.

n, m: l” 10 integers.

Examples of betaine group-containing monomers that can be used include the compounds listed below, and the polyurethane resin used in the present invention uses these monomers. Although it is also available as a commercially available drug, it can be easily obtained by the following method.

1) Synthesis method using monochloroacetic acid R-N (CI, -COOH), +CQ-CH, -CO
OH et R-Alkyl groups such as methyl and ethyl 2) Synthesis method using monochlorosuccinic acid (!:H!-
A synthesis method using C00H propane sultone and a reaction for introducing betaine groups into a polymer as a polymer reaction will be described. In this method, a compound having a betaine group or the like is reacted with an OH group present at the end or side chain of polyurethane whose chain has been extended to a predetermined molecular weight by a polymerization reaction. 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. And polyurethane OH
By reacting the groups with unreacted NGO groups, a polyurethane into which betaine groups or the like have been 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.

＼so(The amount of betaine groups etc. introduced into the polyurethane resin of the present invention is preferably 0.01-1.0 mg+o(2/g, more preferably in the range of 0.1-0.5 mmoQ/g) .

If the amount of the polar group introduced is less than 0.01 mmol/g, a sufficient effect on the dispersibility of the ferromagnetic powder will not be observed. Further, the amount of the polar group introduced is 1. If it exceeds OmmoO/g, intermolecular or intramolecular aggregation tends to occur, which not only adversely affects dispersibility, but also causes selectivity to the solvent, which may make ordinary general-purpose solvents unusable.

Further, the number average molecular weight of the polyurethane resin according to the present invention is 5.
,000-100,000, more preferably 10.00
It is preferably in the range of 0 to 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 mol of N-methyljetanolamine saltone was reacted at a temperature of 120°C for 3 hours,
A sulfobetaine type polyfunctional monomer was obtained.

Next, 1.5 mol of adipic acid, 1.7 mol of l,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 with 5 ml of IIHg, and the reaction was carried out until the acid value reached 2 or less. The molecular weight of the obtained copolymerized polyester is M
It was w2500. 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. 20 parts of 1,4-butanediol was added and the reaction was further continued for 2 hours. 1 part was added and reacted for 1 hour. The molecular weight of the obtained polyurethane was Mv=3.
50,000, Mn=22,000.

The following betaine type polyurethane was synthesized following the above synthesis example.

(a) Average molecular weight for sulfobetaine type polyurethane: 22,000, Tg: -20°C1 polar group concentration: 0.04 mmol/g.

CH.

(-CO, -n-CH.+ Polyester /\Oe (c) Phosphobetaine type polyurethane Number average molecular weight; 30,000, Tg; 0°C, & Group t
lk degree: 0.07m+moff/g.

CH, CONHCHIC) If ÷ CHz'llll-CH*+ Polyester e (mouth) Carboxybetaine type polyurethane Number average molecular weight: 15,000, Tg; -10°C1 Polar group concentration: B. 1
mmol2/g.

8 or less The binder that can be used in combination with the betaine polyurethane resin in the present invention has an average molecular weight of about 10,000 to 2.
00,000, such as - urethane resin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, butadiene-acrylonitrile copolymer, boriamoto resin, polyvinyl Butyral, cellulose derivatives (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, etc.), styrene-butadiene copolymer, polyester resin, various synthetic rubbers, phenolic resin, epoxy resin, urea resin, melamine resin,
Phenoxy resin, silicone resin, acrylic reactive resin,
Examples include mixtures of high molecular weight polyester resins and incyanate prepolymers, mixtures of polyester polyols and polyisocyanates, urea formaldehyde resins, mixtures of low molecular weight glycols/high molecular weight diols/isocyanates, and mixtures thereof.

The above resins are -505M, -COOMl-PO(O
1d')* (where M is hydrogen or an alkali metal such as lithium, potassium, or sodium; M' is hydrogen, an alkali metal such as lithium, potassium, or sodium, or a hydrocarbon residue). Preferably, the resin is made of a resin. In other words, the polar groups of these resins improve their compatibility with the metal magnetic powder, thereby further improving the dispersibility of the magnetic powder and preventing agglomeration of the metal magnetic powder, further improving the stability of the coating liquid. This can also improve the durability of the medium.

The binder to be used, especially the vinyl chloride copolymer, is a vinyl chloride hetamine, a copolymerizable hetamine containing an alkali salt of sulfonic acid or phosphoric acid, and other copolymerizable hetamines as necessary. You can get it by doing this. Since this copolymer is based on vinyl synthesis, it is easy to synthesize, and various copolymer components can be selected, so that the properties of the copolymer can be optimally adjusted.

The metal of the above-mentioned sulfonic acid or phosphoric acid salt is an alkali metal (especially sodium, potassium, lithium),
Potassium is particularly preferred in terms of solubility, reactivity, yield, and the like.

The amount of the acidic group salt-containing monomer in the binder used in the present invention is preferably 0.1 to 30 mol%. If the amount of the salt-containing monomer is too large, the solubility in solvents is poor and gelation tends to occur. Moreover, if the amount of salt content is too small, desired characteristics cannot be obtained.

Preferably, the vinyl chloride copolymer further contains an epoxy group or a hydroxyl group. When using a mixture of binders, the blending ratio of the betaine type polyurethane resin and other resin is preferably 90 to 10 parts by weight, more preferably 80 to 20 parts by weight. If the above-mentioned blending ratio exceeds 90 parts by weight, the coating film becomes too brittle and the durability of the coating film is significantly deteriorated, and the adhesion to the support also deteriorates. Moreover, if the above-mentioned blending ratio is less than 10 parts by weight, the magnetic powder tends to fall off.

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, isocyanate can be contained.

Aromatic incyanates that can be used include, for example, tolylene diisocyanate (TDI) and adducts of these isocyanates and 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 isocyanates with active hydrogen compounds. Among these aliphatic incyanates and adducts of these isocyanates and active hydrogen compounds, those having a molecular weight in the range of 100 to 3,000 are preferred. Among the aliphatic isocyanates, non-alicyclic incyanates 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 in which magnetic powder, filler, etc. are dispersed and suspended in a binder, and a paint containing a dispersant, lubricant, etc. is used.

In addition, when carbon black is included in the magnetic layer, in combination with the action of α-alumina having the specific surface area described above, it can be used as a conductive carbon bra, which is further advantageous in terms of improving runnability and electromagnetic conversion characteristics. For example, Columbia Carbon's Conductex 9
75 (BET value (hereinafter abbreviated as BET) 250m”/g,
DBP oil absorption (hereinafter abbreviated as DBP) 170m12/g
r, particle size 24111μ), Conductex (Cond
uctex)-SC (BET220m2/g, DB
P115mQ/100g, particle size 20mμ), H3100 manufactured by Denki Kagaku Co., Ltd. (BET32m2/g, DBP18
0m12/100g, particle size 53mμ), #22B manufactured by Mitsubishi Kasei (BET55m2/g, DBP13)
1mQ/100g, particle size 40mμ), etc.

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.

The organic powder used in the present invention is preferably acrylic styrene resin, benzoguanamine resin powder, melamine resin powder, phthalocyanine pigment, polyolefin resin powder, polyester resin powder, polyamide resin powder, or polyimide resin. 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, a-FezO
x, 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 amino 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. The abrasives include commonly used materials such as alumina, silicon carbide, chromium oxide, corundum, artificial corundum, artificial diamond, pomegranate, and emery (
Main ingredients: corundum and magnetite) etc. 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.

Solvents to be added to the paint forming the constituent layers or diluting solvents during application of this paint include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; alcohols such as methanol, ethanol, globanol, butanol; Esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, and ethylene glycol monoacetate; Ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, and tetrahydro-7rane; Aromatic hydrocarbons such as benzene, toluene, and xylene ; methylene chloride,
Halogenated 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. Cu%AQ, metals such as Zn,
Glass, boron nitride, ceramics such as silicon carbide, etc. can also be used.

The thickness of these supports is approximately 3 to 100 μm in the case of a film or sheet, preferably 5 to 507JI11, and 30 μm to 1Os+ in the case of a disk or card.
If it is drum-shaped, it is used in a cylindrical shape, and its shape 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 coat etc. can be used, but these are not limited.

(Example) The present invention will be explained in detail below.

Note that the present invention is not limited to the illustrated embodiments.

In addition, in the following examples, all "parts" are parts by weight.

Preparation of Videotape First, a magnetic layer was formed on a polyethylene terephthalate base film having a thickness of 10 μm as a support in the following manner.

That is, after using a specified gold R11Ii powder and dispersing each component shown in Table 1, this magnetic paint was filtered with an lpH filter, 5 parts of 1 polyfunctional incyanetate was added, and 2 The magnetic layers were coated to a thickness of .5 μm and supercalendered to obtain magnetic layers having various compositions shown in Table 1.

However, for metal magnetic powder, the content ratio of iron atoms to aluminum atoms in the magnetic powder is shown.

Thereafter, a paint for the BC layer having the following composition was applied to the opposite side of the magnetic layer to a dry thickness of 0.4 microns.

Carbon black (average particle size 50μ1) 40 parts Barium sulfate 10 parts Nitrocellulose 25 parts N-2301 (made by Nippon Polyurethane) 25 parts Coronate L (// )
10 parts cyclohexanone
400 parts methyl ethyl ketone 25
0 parts toluene 25
0 parts A wide magnetic film having 80 magnetic layers of a predetermined thickness was thus obtained and wound up.

This film was cut to a width of ¥8m to make each videotape in Table 1 (corresponding to the numbers of each Example and Comparative Example). Table 1 shows the results of measuring the performance of the tapes obtained as described above. show.

The evaluation items are measured and displayed according to the following criteria.

Lumi S/N: Measured using a color video noise meter Shibasoku 925D/lJ. The bypass filter was set to 10 kHz, and the low pass filter was set to 4°2 MHz. VTR is 8mm
Use a video deck.

Still Image Lifespan: Shows the time in minutes for a still image to degrade by 2 dB. The larger the value, the higher the durability and wear resistance of the magnetic recording medium.

Remaining rate of saturation magnetization: The saturation magnetization of a sample tape measured after leaving it for one week in an atmosphere of 6000, 80% RH corresponds to what percentage of the saturation magnetization measured before leaving it.

Powder shedding: The sample tape was continuously run on a video deck at 40° C. 180% for 200 hours to measure powder shedding.

◎ Very good O Good △ Fairly good

◎ Very good ○ Good △ Fairly good In other words, it is extremely useful to use metal magnetic powder with a BET value of 45 m''/g or more, to specify the content ratio of iron atoms to aluminum atoms in the metal magnetic powder, and to use betaine type polyurethane as a binder. be.

Claims (1)

[Claims] In a magnetic recording medium in which a magnetic layer contains a binder and metal magnetic powder dispersed in the binder, the metal magnetic powder has a specific surface area of 45 m^2/g or more, and the metal magnetic powder has a The content ratio of iron atoms and aluminum atoms (iron atoms: aluminum atoms) is the atomic ratio (
100:1) to (100:20), and the binder contains polyurethane in which a negative functional group forms an inner salt.