JPS59213027A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium having a magnetic layer which is low enough in both surface electric resistance and optical transmissivity, also excellent in surface smoothness, and sufficient in durability, by providing a layer formed by combining carbon black whose specific surface area is specific. CONSTITUTION:A titled medium is provided with a layer containing carbon black CB1 whose specific surface area (BET value) B1 is 40m<2>/g<B1< 200m<2>/g, and carbon black (CB2) whose specific surface area (BET value) B2 is 200m<2>/g<= B2<500m<2>/g. By specifying CB1 within said range, a light shielding property of the layer can be made enough, and also, a dispersion property of CB1 into the layer can be improved. Also, by specifying CB2 as mentioned above, conductivity can be made enough. In this way, the surface electric resistance and optical transmissivity of a magnetic layer 3, etc. are lowered enough, also a surface smoothness of the layer (namely, a dispersion property of carbon black) is improved, and the durability of the layer can be improved by suppressing the quantity of carbon black to be added.

Description

DETAILED DESCRIPTION OF THE INVENTION 1. Industrial Application Field The present invention relates to magnetic recording media such as magnetic tapes and magnetic sheets.

2. Prior Art Conventionally, in magnetic recording media, when static electricity accumulates, discharge occurs between the media and the magnetic head, which tends to generate noise, and dust, etc., can be attracted and cause dropouts. . On the other hand, for video, a method is known in which the running of the tape is adjusted by detecting the difference in light transmittance between a tape portion having a magnetic layer and a leader tape portion.

For these reasons, the surface electrical resistance of the magnetic layer is generally set to 10
It is required that the optical transmittance of the tape portion where the magnetic layer is located be 0.05 cm or less when measured with 900 nm light. For this purpose, carbon black particles are usually added to the magnetic layer.

For example, according to Japanese Patent Publication No. 57-4968, conductive carbon black with a surface area of soo m'/r or more and a particle size of 30 mμ or less and light-shielding carbon black with a surface area of 5 oom2/r or less and a particle size of 40 mμ or more are magnetic. are added to each layer. However, in this magnetic tape, the conductive carbon black has a surface area of
If it is too large (2 or more), the oil absorption will be large and it will be dispersed in the magnetic layer, and if ptq <, the dispersion stability will be poor.
Otherwise, the surface properties of the tape may deteriorate.

Furthermore, Japanese Patent Application Laid-Open No. 57-200934 describes carbon black with a specific surface area (BET value) of 140 m"71 or more and a particle size of 20 mμ or less, and a carbon black with a specific surface area (BET value) of 40 m2
/f or less and carbon-free with a particle size of 50 mμ or more,
Magnetic layers to which higher fatty acids are added are shown.
However, in this magnetic tape, carbon black with a specific surface area of 40 m"/f or less and a particle size of 50 mμ or more has poor coloring power and electrical conductivity, so it is necessary to increase the amount of carbon black added.
This has the disadvantage that the mechanical properties of the magnetic layer deteriorate.

3. Purpose of the Invention The purpose of the present invention is to provide a magnetic recording medium having layers such as a magnetic layer which have sufficiently low surface electrical resistance and light transmittance, excellent surface smoothness, and sufficient durability. be.

4. Structure and effects of the invention, that is, the magnetic recording medium according to the invention has a specific surface area of CBET
Value) Bl is 40m”/l < Bl < 200m2
/ y carbon black (hereinafter sometimes referred to as CB work), and the specific surface area (BE value) B2 is 200
According to the present invention, CB is is added mainly for light shielding, but its specific surface area B is 40 m2/y (B.

It is extremely important to specify the specific surface area in the range <200 m"/f. The specific surface area in the range of 200 m"/f can provide sufficient light-shielding properties of the layer and at the same time improve the dispersibility of CB i in the layer. If the specific surface area of CB1 is outside this range and is less than 40 m''/f, the particle size will be too large and the light shielding properties will be poor, and the amount added will need to be increased more than necessary. If the trench is more than 200 m''/2, the particle size will be too small and the dispersibility in the layer will be poor.On the other hand, this CB.

In both cases, CB2 is added to provide sufficient conductivity, but its specific surface area is adjusted to zoom2/r≦B2 (500m2/
It is also very important to specify point 2.

That is, if the CBtO specific surface area B2 is less than 200 m''/, the particle size will be too large and the conductivity will be extremely insufficient even with the addition of carbon fiber, and the soom''/
This is because if it is more than f, the particle size will be too small and the dispersibility of CB 2 will deteriorate on the contrary.

Therefore, according to the present invention, CBi for light shielding and conductive C
Only by limiting each specific surface area of B2 to the above-mentioned specific ranges can the surface electrical resistance and optical transmittance of the magnetic layer etc. be sufficiently reduced, and the surface smoothness of the layer (i.e. carbon-free, lubricant-free, etc.) be sufficiently reduced. It is possible to improve the durability of the layer by improving the dispersibility) and by suppressing the amount of carbon black added.

This has been experimentally confirmed as shown in FIG. The specific surface area range of each of these carbon-free materials is fundamentally different from the range shown in the prior art.
The invention is based on new and original conditions discovered by the inventor. In addition, in order to more fully achieve the object of the present invention, each of the above specific surface areas should be 100≦B, (zoo, 200≦
It is desirable that B2≦300. Regarding the particle size of each carbon black, it is desirable that CBs is 20 μm or less and CB is 40 to 50 μm, which corresponds to the above specific surface area range according to the present invention.

In addition, in the present invention, the above-mentioned "specific surface area" refers to the surface area of υ per unit weight, and is a physical quantity that is completely different from the average particle diameter. For example, even if the average particle diameter is the same, the specific surface area There are those with a large specific surface area and those with a small specific surface area. To measure the specific surface area, first, carbon black powder is degassed while being heated at around 250°C for 30 to 60 minutes to remove what is adsorbed to the powder, and then introduced into a measuring device. , the initial pressure of nitrogen is 0.5ky
γ/m2, and the liquid nitrogen temperature (-195
(°C) adsorption measurement (a method for measuring the specific surface area generally referred to as the B, E, T method. For details, see J, Am
e.

Chem, Soc, 60309 (1938)). As the device for measuring the specific surface area (BET value), it is possible to use the "Powder Measuring Device (Kantanbu)" jointly manufactured by Yuasa Battery Co., Ltd. and Yuasa Ionics Co., Ltd. A general explanation of the specific surface area and its measurement method can be found in “Powder Measurement J (J, M,
Co-authored by DALLAVALLE and CLYDEORR Jr., translated by others; published by Sangyo Toshosha). (published on the 30th of May). (In addition, in the above-mentioned "Chemical Handbook",
Although the specific surface area is simply described as surface area (m2/1γ), it is the same as the specific surface area in this specification. ) 5 Examples Hereinafter, the present invention will be explained in more detail with reference to Examples.

First, to explain the effect of adding carbon black in more detail, carbon black is added to lower the surface electrical resistance and light transmittance as mentioned above, but when the amount added increases, the mechanical properties of the layer decrease. Since this causes significant deterioration, it is usually necessary to add carbon black to the binder in an amount of 5 to 35% by weight (preferably 10 to 25% by weight). In the present invention, due to the specific surface area within the above range, there is no need to increase the amount of carbon black added, and it can be set within the above range of 5 to 35% by weight, and the mechanical properties of the layer (for example, the magnetic layer Not only can powder fall-off be maintained well, but also the desired surface electrical resistance (10 9 Ω·m or less) and light transmittance (0.05 ohm or less) can be obtained.

In addition, carbon black with a small particle size (large surface area) is advantageous because it improves the surface resistivity and light transmittance of the coating film when it contains the same weight of carbon black, but on the contrary, it This makes dispersion difficult, resulting in surface roughness and pinhole formation due to poor dispersion, which increases the surface resistivity and light transmittance, and causes deterioration of electromagnetic conversion characteristics.

Acicular magnetic powder easily breaks during dispersion, and if it becomes overdispersed, it will break and the electromagnetic conversion characteristics will be significantly reduced. Therefore, when applying it to a magnetic layer, dispersion is usually stopped when the dispersion of the magnetic powder reaches its maximum. Stop. In this case, if a carbon plastic resin with poor dispersibility is used, it is often not dispersed sufficiently, resulting in separation of carbon black in the paint, roughening of the surface of the paint film, generation of pinholes, etc. However, this can be solved by the present invention. This can be effectively prevented by using carbon black.

In the present invention, conductive CBz is used in order to reduce the surface resistivity of the magnetic layer, etc. to a sufficient range, but it is preferable that such carbon 2 racks have particles connected to each other in the shape of a bunch of grapes. It is desirable to use a material that is solid, porous, and has a large specific surface area, ie, a material with a high stracture level. Examples of such carbon black include, for example, Conductex manufactured by Columbia Bobon.
tex) 975 (specific surface area 270 rl"/f, particle size 46 mμ), Conductex 950 (specific surface area 245
rl”/r, particle size 46 mμ), Palcan manufactured by Cabot (
Cabot Vulcan) XC-72 (specific surface area 2
57 m2/2, particle size 1 Bmμ), etc. can be used.
These carbon plastics have a large specific surface area, but when applied to a magnetic layer, they can be sufficiently dispersed before the magnetic powder is completely dispersed. Outside the scope of the present invention, if the specific surface area of CB2 is 500 m2/g or more, conductivity and light shielding properties are good in a completely dispersed forest state, but even when the dispersion of magnetic powder is completed, the dispersion of CB z is not completed, the surface of the paint film becomes rough,
This may cause pinholes to occur. If the specific surface area is less than 2 oom''/r, the effect of adding carbon black cannot be expected.

On the other hand, in order to reduce the light transmittance of the magnetic layer etc., it is possible to use the above-mentioned carbon blank CB2, but in the present invention, although the electrical conductivity is poor, the light shielding property is good (the original conductive carbon By adding a small amount of carbon CB, which has a smaller surface area than the conductive carbon and has excellent dispersibility, it is possible to obtain a synergistic effect that is more significant than when using conductive carbon alone. That is, by adding the light-shielding carbon C11 together with the conductive carbon CB2, the light transmittance can be sufficiently reduced, and the amount of conductive carbon added can be significantly reduced, so the overall amount of carbon black added can be reduced. The mechanical properties and surface smoothness of the layer are significantly improved. Such light-shielding carbon black CB1 has a small particle size, a relatively low structuring level, and a relatively low specific surface area, such as Raben (Raben manufactured by Columbia Carbon Co., Ltd.).
ven) 2000 (specific surface area 1som”/r, particle size 1
9mμ), 2100.1170.1000, 4100 manufactured by Mitsubishi Kasei Corporation, ≠75.4=44, ≠40, ≠3
5, ≠30, etc. can be used.

There is a certain preferable range for the mixing ratio (weight ratio) of each of the above carbon blacks, and CB2/CB1=90/4o
~so/'so is good, and 80/20 to 60/40 is even better. If this mixing ratio is greater than 0/10, the proportion of conductive carbon black CB will increase, resulting in insufficient light-shielding properties, and if it is smaller than 50150, there will be less conductive carbon black, CB, resulting in a surface ratio Resistance will increase.

FIG. 2 shows a magnetic recording medium according to the present invention, for example a magnetic tape, in which a subbing layer 2 (
(which may be omitted) and a magnetic layer 3 are laminated. Based on the present invention, the magnetic layer 3 has a specific surface area (BET value) B1 of 40 m2/f (B1 (2
00m2/f light-shielding carpon pla, CB s,
Specific surface ffl (BET value) B2 is 200m”/≦B
2 (500 m2/liter conductive carbon black CB 2
are contained in predetermined amounts, respectively.

Note that these double-bonded blacks may be contained not only in the magnetic layer 3 but also in the support 1 and the undercoat layer 2, or they may be contained in at least one of each layer 3.1.2. When the carbon black is incorporated into the support 1, the carbon black may be kneaded into the support.

In addition, examples of the ferromagnetic powder in the magnetic powder tf used in the present invention include γ-Fe20s powder, Fe3O4 powder, Co-containing 7 Fe2us powder, and CO-containing Fe s O.
4 powder, CrOx powder, Fe powder, Co powder and Fe −
Various ferromagnetic materials such as metal powders such as Co--Ni alloys and alloys thereof can be widely used.

The magnetic paint according to the present invention includes ferromagnetic powder, a binder,
A dispersant, each of the above-mentioned carbon blanks CBt, CB2, etc., is mixed and dispersed together with a coating solvent, and this magnetic coating is coated on a non-magnetic support [7] to form a magnetic layer, which is used as a magnetic recording medium according to the present invention. .

As the binder for the magnetic layer of the present invention, for example, conventionally known thermoplastic resins, thermosetting resins, reactive resins, electron beam curable resins, and mixtures thereof are used.

As a thermoplastic resin, 1. Softening temperature is below 150°C,
Those with an average molecular weight of io, ooo to 200.000 and a degree of polymerization of about 200 to 2,000, such as vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer Coalescence, acrylic ester-acrylonitrile copolymer, acrylic ester-vinylidene chloride copolymer, acrylic ester-styrene copolymer, methacrylic ester-acrylonitrile copolymer, methacrylic ester-vinylidene chloride copolymer, meccryl Acid ester-styrene copolymer, urethane elastomer, polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose diacetate, cellulose) triacetate, cellulose globionate, nitrocellulose, etc.) styrene-butadiene copolymer, polyester resin, chlorovinyl ether-acrylic acid di, 1.
f k copolymers, amine resins, various synthetic rubber-based thermoplastic resins, mixtures thereof, and the like are used.

The thermosetting resin or reactive resin has a molecular weight of 20° or less in the state of a coating solution, but after coating and drying, the molecular size becomes infinite due to reactions such as condensation and addition. Moreover, among these resins, those which soften but do not melt before the resin is thermally decomposed are preferable.
Specifically, examples include phenolic resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, silicone resins, acrylic reaction resins, mixtures of high molecular weight polyester resins and incyanate prepolymers, and methacrylate copolymers. and diisocyanate prepolymers, mixtures of polyester polyols and polyinsyanates, urea formaldehyde resins, mixtures of low molecular weight glycols/high molecular weight diols/triphenylmethane triisocyanates, polyamine resins, and mixtures thereof.

Examples of electron beam irradiation-curable resins include unsaturated prepolymers,
For example, maleic anhydride type, urethane acrylic type, polyester acrylic type, polyether acrylic type, polyurethane acrylic type, polyamide acrylic type, etc., or as a polyfunctional monomer, ether acrylic type, urethane acrylic type, phosphate ester acrylic type, aryl type , hydrocarbon type, etc.

These binders may be used alone or in combination, and other additives may be added as required.

When the present invention is applied to a magnetic layer, the mixing ratio of the ferromagnetic powder and the binder is 5 to 40,074 parts by weight, preferably 10 to 20 parts by weight of the binder to 100 parts by weight of the ferromagnetic powder.
It is preferable to use it in a range of 0 parts by weight. If the amount of binder is too large, the recording density of the magnetic recording medium will be lowered, and if it is too small, the strength of the magnetic layer will tend to decrease, and durability will be reduced, powder falling off, etc. will easily occur.

Furthermore, in order to improve the durability of the magnetic recording medium according to the present invention, various hardening agents can be contained in the magnetic layer etc., for example, isocyanate can be contained.

Aromatic isocyanates that can be used are, for example, tolylene diisocyanate (TDI), 4,4'-diphenylmethane diisocyanate (1!ID I), xylylene diisocyanate (MDI), metaxylylene diisocyanate (MXDI) and these. There is a fee for isocyanate and active hydrogen compound, etc. 0 tsubo + ward F
On the other hand, aliphatic isocyanates include hexamethylene diisocyanate (IIMDI), lysine isocyanate, and trimethylhexamethylene diisocyanate (IIMDI).
TMDI) and adducts of these incyanates and active hydrogen compounds.

Examples of the alicyclic incyanate include methylcyclohexane-2,4-diisocyanate [structural formula: 2GO xyl isocyanate] [structural formula: %formula% cyanate and its adduct of an active hydrogen compound.

In the magnetic recording medium of the present invention, a magnetic paint is prepared by mixing and dispersing the magnetic powder, the binder, and each additive in an organic solvent, and after adding the aromatic isocyanate and aliphatic incyanate, It is prepared by coating it on a support (for example, a polyester film) and drying it if necessary.

The amount of incyanate added is 1 to 10 to the binder.
0% by weight is preferred. If it is less than 1%, the hardening of the magnetic layer tends to be insufficient, and if it is more than 100%, the magnetic layer tends to become sticky even though it is hardened.

In order to obtain a more preferable magnetic layer, the amount of incyanate to be added is 3 to 30 volts to the binder.

Examples of the incyanate include an adduct of diisocyanate and a trivalent polyol, a pentamer of diisocyanate, and a decarboxylation compound of 3 moles of diisocyanate and water.

Examples of these include an adduct of 3 moles of tolylene diisocyanate and 10 moles of trimethylol, an adduct of 3 moles of metaxylylene diisocyanate and 1 mole of trimethylolpropane, a pentamer of tolylene diisocyanate,
There are pentamers consisting of 3 moles of tolylene diisocyanate and 2 moles of hexamethylene diisocyanate, decarboxylated products obtained by reacting 3 moles of hexamethylene diisocyanate and 1 mole of water, etc., and these are easily obtained industrially. .

The coating material used to form the magnetic layer and the like may contain additives such as dispersants, lubricants, abrasives, and other antistatic agents, if necessary.

Dispersants that may be used include lecithin, which has 8 to 18 carbon atoms, such as caprylic acid, capric acid, lauric acid, myristic acid, valmitic acid, stearic acid, oleic acid, elaidic acid, linoleic acid, linuric acid, etc. fatty acids (
R-COOH (R is a saturated or unsaturated alkyl group having 7 to 17 carbon atoms); alkali metals (Li, Na, K, etc.) or alkaline earth metals (Mg + Ca, Ba etc.).

In addition, higher alcohols having 12 or more carbon atoms and their (IbK sulfuric esters) can also be used. Commercially available general surfactants can also be used.
These dispersants may be used alone or in combination of two or more.

However, as lubricants, silicone oil, graphite, molybdenum disulfide, tungsten disulfide, monobasic fatty acids with 12 to 16 carbon atoms, and the total number of carbon atoms in the fatty acids 21 to 23 carbon atoms are used. Fatty acid esters (so-called waxes) consisting of monohydric alcohols can also be used. These lubricants are added in an amount of 0.2 to 20 parts by weight per 100 parts by weight of the binder.

Abrasive materials that may be used include commonly used materials such as fused alumina, silicon carbide, chromium oxide, corundum, artificial corundum, diamond, artificial diamond,
Garnet, emery (main ingredients: corundum and magnetite), etc. are used. These abrasives have an average particle size of 0.05
~5μ in size is used, particularly preferably 0
．． It is 1 to 2μ.

These abrasives contain 02 to 2 parts by weight per 100 parts by weight of the binder.
Other antistatic agents that may be added in an amount of 0 parts by weight include Gura powder; natural surfactants such as saponin; nonionic surfactants such as alkylene oxide, glycerin, and glycidol; Cationic surfactants such as higher alkylamines, quaternary ammonium salts, pyridine, other heterocycles, phosphoniums or sulfoniums; carboxylic acids,
Examples include anionic surfactants containing acidic groups such as sulfonic acid, phosphoric acid, sulfuric acid ester groups, and phosphoric ester groups; amphoteric surfactants such as amino acids, aminosulfonic acids, and sulfuric or phosphoric esters of amino alcohols.

Examples of solvents for magnetic paints or solvents used when applying magnetic paints 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, trityl acetate, butyl acetate, ethyl lactate, ethylene glycol monoacetate: ethylene glycol dimethyl ether, diethylene glycol monoethyl ether, dioxane,
Ethers such as tetrahydrofuran; aromatic hydrocarbons such as benzene, toluene and xylene; halogenated hydrocarbons such as methylene chloride, ethylene chloride, tetracarbon L, chloroform and dichlorobenzene, etc. can be used.

Support materials include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, plastics such as polycarbonate, and metals such as AtXzn. , glass, so-called knee ceramics (for example, boron nitride and silicon carbide), and ceramics such as porcelain and earthenware are used.

The thickness of these non-magnetic supports is approximately 3 to 100 μm, preferably 5 to 50 μm in the case of a film or sheet, and 30 μm to 10 m in the case of a disk or card.
If it is drum-shaped, it is cylindrical, and its shape is determined depending on the recorder used.

The above-mentioned support has a so-called baku coat (Ba
ckcoat).

Coating methods for forming a magnetic layer etc. by coating the magnetic paint etc. on the support include air doctor coating, blade coating, and coating method '7. --)--A7--), squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, claher coat, kiss coat, cast coat, -spray coat, etc. can be used, and other methods are also possible.

The magnetic layer coated on the support by such a method is optionally subjected to a treatment for orienting the ferromagnetic powder in the layer, and then the formed magnetic layer is dried.

Further, the magnetic recording body of the present invention is manufactured by subjecting it to surface smoothing processing or cutting it into a desired shape, if necessary.

In this case, the orientation magnetic field is approximately 500 to 35
The drying temperature is approximately 50~1000C.
The drying time is preferably about 3 to 10 minutes.

Hereinafter, the present invention will be explained with reference to specific examples.

The components, proportions, order of operations, etc. shown below may be changed in various ways without departing from the spirit of the invention.

In addition, in the examples below, all parts are 1 part by weight.
represents.

Example 1 First, the following composition was prepared.

CO-containing - Fe2Og 100 parts Polyurethane 14 parts (Nistan 5701 manufactured by Goodlinch) Nitrocellulose
5 parts (Seltsuba BT manufactured by Asahi Kasei Co., Ltd.
Hi) 1 part of vinyl chloride-vinyl acetate copolymer (
VAGH) Conductex 975 manufactured by Union Carbide (BET value 270m2/7, particle size 46
y11μ) 3.5 parts Raven 2000 (BET value 180m”/f, particle size 19
mμ) 15 parts lecithin 5 parts myristic acid 1 part valmitic acid butyl ester 1 part alumina 4 parts methyl ethyl ketone 50 parts cyclohexanone 100 parts
After stirring and mixing for 2 hours, 5 parts of a polyfunctional isocyanate (curing agent) was added, and the mixture was filtered through a filter with an average pore size of 1 μm.

The obtained magnetic paint was applied onto a 14 μm thick polyester film using a reverse roll coater, and after drying, the magnetic layer was surface-treated using a super calendar roll.
A wide magnetic film having a magnetic layer with a thickness of 5 μm was obtained. This film was cut into a width of 'A' to produce a magnetic tape for video.

Example 2 In the coating composition of Example 1, 3 parts of Conductex 975 and 2 parts of Raven 2000 were used as carbon black.
A magnetic tape was prepared in the same manner as in Example 1.

Comparative Example 1 In the paint composition of Example 1, Rapen 2000 was not added as carbon black, and Conductex 975 was added.
A magnetic tape was prepared in the same manner as in Example 1, except that 5 parts of chloride was added.

Comparative Example 2 In the paint composition of Example 1, Conductex 975
Ketzen Black EC (
A magnetic tape was prepared in the same manner as in Example 1 by adding 5 parts of BET value 900 m2/2 and particle size 30 m/'.

Comparative Example 3 In the coating composition of Example 1, Raven 2000 (7)
A magnetic tape was prepared in the same manner as in Example 1 except that 5 parts of LC Rabeysoo (BET value 3 sm''/Ps particle size 56 μm) was added instead.

Surface electrical resistance, light transmittance, surface gloss, and video characteristics were measured for each of the above magnetic tapes, and the results are shown in the table below. These measurement methods were as follows.

Surface electrical resistance: Measured according to the usual method.

Original transmittance: A part of the tape's magnetic layer is irradiated with white light and the ratio of transmitted light to incident light is measured.

Gloss: Measured at an angle of 60° using a variable angle photometer, and the standard plate is expressed as 100%.

Video characteristics: 4 MHz playback output was set as RF output, and video S/N and color S/N were measured using Shibaku Color Video Noise Meter 9250 (relative values are shown with the measured value of Example 1 set as 0).

(The following margins continue on the next page) From these results, the sample (Example 1.2) in which Conductex 975 and Raven 2000 were used together as carbon black according to the present invention showed good values for each property. When Conductex 975 was used alone (comparison row 1), the light transmittance deteriorated and the dispersibility was poor, resulting in poor gloss and video characteristics.
It can be seen that when Ketjenblack was used as the substitute for No. 5 (Comparative Example 2), the dispersibility was poor and all performances deteriorated.

However, when Raben 500 is used instead of Raben 2000 (Comparative Example 3), the light transmittance is too high, and in order to bring this to an acceptable value (0.04% or less), it is necessary to significantly increase the amount added. It is understood that this causes the mechanical properties to deteriorate.

Example 3 The following composition of the subbing layer phase was prepared.

Conductex! J75 14 parts Silane 20
00 6 parts polyurethane
20 parts VAGH 40 parts Methyl in butyl ketone 150 parts Toluene 150 parts This was coated on polyethylene terephthalate to form an undercoat layer.

In addition, in the magnetic coating composition in Example 1, the amounts of magnetic powder and carbon black were 103 parts of Co-containing Co-Fears, 2.1 parts of Conductex 975, and 2.1 parts of U-Pen 2000.
A magnetic layer was formed in the same manner except that the amount was changed to 0.9 parts.

The presence of the undercoat layer allows the amount of magnetic powder in the magnetic layer to be increased accordingly.

1, the obtained tape performance was as follows.

It was confirmed that in the magnetic recording medium of the present invention, there is a relationship shown in FIG. 2 between the specific surface area of carbon black added and the surface resistance.

[Brief explanation of the drawing]

The drawings show examples of the present invention, in which Fig. 1 is a diagram showing changes in characteristics depending on the specific surface area of carbon black, and Fig. 2 is an enlarged cross-sectional view of a portion of a magnetic tape. 1, -, , Nonmagnetic support 2 Undercoat layer 3 Magnetic layer. Agent: Patent Attorney Hiroshi Aisaka (and 1 other person) (Voluntary) Proceedings: January λψ, 1981, Indication of the case: 1982 Patent Application No. 86889 2, Name of the invention: Magnetic recording medium 3, Relationship with the case of the person making the amendment Patent applicant address: 1-26-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Name:
(127) Roku Konishi Photo Industry Co., Ltd. 4, Agent 6, Number of inventions increased by amendment 7, Subject of amendment (11, Memorandum, page 19, line 12, “(so-called low)”
First, delete J. (To 2, correct “binder” on page 19, line 13 of the same as “magnetic powder.”) (3) Correct “binder” on page 20, line 2 of the same as “magnetic powder.” 1 - He

Claims (1)

[Claims] 1. Specific surface area (BET value) B1 is 40 m”/f <
Carbon black with Bl <200m"/f and specific surface area (BET value) B2 of 2oom"/f≦&<5
1. A magnetic recording medium characterized by having a layer containing carbon black of 00 m''/t.