JPH0242624A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve a traveling property and durability and to obtain electromagnetic conversion characteristics by providing a back coating layer contg. specific nonmagnetic powder and binder to the recording medium. CONSTITUTION:The nonmagnetic powder contains a carbon black having 20-40mum average primary particle size, carbon black having 50-100mum average primary grain size and zinc oxide powder having 0.3-1.5mum average grain size. The back coating layer contg. this nonmagnetic powder and binder is provided on the surface of a nonmagnetic base on the side opposite to the surface to be provided with a magnetic layer. The back coating layer sufficiently and uniformly dispersed with the nonmagnetic powder having the carbon black can, therefore, be provided. The traveling property and durability are improved in this way and the good electromagnetic conversion characteristics are obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to magnetic recording media. More specifically, the present invention relates to a magnetic recording medium that has excellent runnability and durability, and is capable of realizing good electromagnetic conversion characteristics.

[Conventional technology and invention try to solve it! l1g] In recent years, magnetic recording media have been developed, for example, with video angles of 7° and order 9.
It is widely used for various purposes such as Otabu and floppy disks.

It is desirable that these magnetic recording media have excellent running properties.

The excellent runnability of the magnetic recording medium means, for example, that it runs evenly, resulting in better electromagnetic conversion characteristics, and that the magnetic recording medium is not rubbed excessively while running, resulting in improved durability. can be improved.

The running performance of such a magnetic recording medium is determined not only by the surface condition of the magnetic layer provided on one side of the non-magnetic support but also by the Jl.
i The other side (back side) of the magnetic support on which the magnetic layer is not provided
The surface condition of the material also has a large influence.

Therefore, in order to improve running properties, it has been proposed to provide a back coat layer on the back surface of the nonmagnetic support.

The backcoat layer is usually formed from a non-magnetic powder and a binder.

The non-magnetic powder has the effect of making the surface of the back coat layer appropriately rough, reducing the contact area and lowering the coefficient of friction.

By providing such a back coat layer, the running properties of the magnetic recording medium can be improved.

However, providing a backcoat layer may cause new problems.

For example, when the non-magnetic powder aggregates and creates unevenness on the surface of the back coat layer, the unevenness is transferred to the magnetic layer, resulting in a decrease in electromagnetic conversion characteristics that reduces output or generates noise. (2) When used, the back coat layer becomes easily scraped, and the scraped parts adhere to the magnetic layer or magnetic head, resulting in a decrease in electromagnetic conversion characteristics.

Therefore, in the back coat layer, it is preferable that the non-magnetic powder is uniformly dispersed in the binder.

By the way, as a method of providing a back coat layer to improve the running properties of a magnetic recording medium, for example, Japanese Patent Laid-Open No. 57-1999
No. 130234, JP-A-58-161135.

JP-A-57-5:1B25 and JP-A-58-2415 show examples of using inorganic powder as a non-magnetic powder in the back coat layer, and many of these also include limitations on the particle size. It shows what was done.

In addition, for example, Japanese Patent Laid-Open No. 52-174 has proposed the use of carbon black in place of the above-mentioned inorganic powder. This is due to the use of carbon black as the non-magnetic powder, which improves runnability due to the roughening effect.

This may be because the purpose of the back coat layer is to provide an antistatic effect and a light shielding effect, or because the average particle size of the carbon black used is small (10 to 20 mu), the carbon black in the back coat paint is The dispersibility was extremely poor, and carbon black IB4J particles were present in the back coat layer formed.

There is a problem in that unevenness occurs on the surface of the back coat layer.

Furthermore, to solve the problems mentioned above.

Attempts have been made to use carbon black with a relatively small average primary particle size of 10 to 60 JLm in combination with carbon black with an average primary particle size of 100 #Lm or more (Japanese Patent Laid-Open No. 60-45938, Japanese Patent Laid-Open No. 45938). Showa 60-45939
No., JP-A-60-25023, JP-A-60-38725
No., JP-A-60-107729, JP-A-59-185
No. 027, JP-A-59-22:19:17, JP-A-5
7-111828, JP-A-50-147308, etc.)
.

However, simply adjusting the particle size of carbon black does not sufficiently disperse carbon black in the back coat layer, and it is difficult to achieve excellent runnability, durability, and good electromagnetic conversion characteristics. The problem is that it is difficult.

This invention has been made based on the above circumstances.

That is, an object of the present invention is to provide a back coat layer in which non-magnetic powder containing carbon black is sufficiently and uniformly dispersed, and to achieve excellent runnability and durability and good electromagnetic conversion characteristics. The objective is to provide a magnetic recording medium that can achieve

[Means and effects for solving the above problems] In order to solve the above problems, the inventors have worked hard to solve the above problems.

As a result of repeated studies, we found that a magnetic recording medium provided with a back coat layer containing a specific non-magnetic powder has a good dispersion state of the non-magnetic powder in the back coat layer, and has excellent runnability and durability. The present invention was achieved by discovering that it is possible to achieve good electromagnetic conversion characteristics.

That is, the invention for solving the above problem is as follows.

Carbon black with an average primary particle size of 20 to 40 m4, carbon black with an average primary particle size of 50 to 100 m, and carbon black with an average primary particle size of 0.5 to 1. The present invention is a magnetic recording medium characterized by being provided with a back coat layer containing a nonmagnetic powder including zinc oxide powder which is OJLm, and a binder.

The back coat layer, nonmagnetic support, and magnetic layer that constitute the magnetic recording medium of the present invention will be explained below.

(Back coat layer) +i? The back coat layer (i) is provided on the surface of the nonmagnetic support opposite to the surface on which the magnetic layer is provided.

The back coat layer contains non-magnetic powder and a binder.

An important point in this invention is that the non-magnetic powder is a carbon black having an average particle diameter of 20 to 40 mB [hereinafter sometimes referred to as carbon black A]. ]
and carbon black having an average primary particle size of 50 to 100 mIL [hereinafter sometimes referred to as carbon black B].

] and zinc oxide powder having an average particle size of Oo5 to 1.0 gm.

As a specific example of carbon black A, for example, manufactured by Columbia Carbon Co., Ltd. [trade name]; Raven 525
0,1255,1250,1200°1170.104
0.10:15. +030°+020,890,850
, 825 Manufactured by Cabot [Product name] Niblack Burls.

Regal 400, 600.50OR, SOD.

3:10,99゜Palcan XC-72,P manufactured by Mitsubishi Kasei [Product name]; CF9゜# 50,52,45,44,40,32,30,40
00MA-100,7,8,11.

Specific examples of carbon black B include, for example, "Raben+4." manufactured by Columbia Carbon Co., Ltd. [trade name];
16,22,410,420.430450° Manufactured by Cabot [Product name]; Sterling 83. Examples include V.

In the back coat layer, the carbon black A
The blending weight ratio of carbon black B and carbon black B (carbon black included/carbon black B) is preferably 50/
l to 2/3, more preferably 20/1 to 1/l
It is.

The zinc oxide powder may be manufactured by either a dry method or a wet method, and preferred is non-uniform zinc oxide manufactured by the French method.

The average particle size of the zinc oxide powder is 0.3 to 1.57 tm.
preferably 0.5-1. It is OJLITI.

The average particle size of the zinc oxide powder is expressed as a value determined by the air permeation method described below.

The air permeation method is based on the following general Kozeny-Karman ( In this method, the average particle diameter is determined using Equation (1) of Ozeny Fist Carman.

q: Powder filling permeation air amount (c, c) △P: Pressure difference between both sides of the packed bed (g/cm") A: Cross section of the packed bed 8 & (Cpeng 2) t; Air filling of Qc, c, Layer penetration time (sec)
W: Powder weight machine (g) In the above formula (1), ρ, η, L, A, and ε can be measured independently, so give Q and 1, and calculate △P for this. By measuring, Sw can be found.

Entering this value of Sv into the following relational expression (2), the average particle diameter 1
The layer is calculated.

Here: Sw: Specific surface area of powder (cs”/g) ε: Porosity of powder packed bed p; Powder density (g/cs+') η: Viscosity coefficient of air (g/c 5ee) L: Thickness of powder packed bed (C■) A measuring device such as 5S-100 (Shimadzu Corporation) is available.

In the back coat layer, the blending amount of the zinc oxide is gi weight ratio zinc oxide/carbon black A + carbon black B) with respect to the blending amount of carbon black including the carbon black A and the carbon black B.
is usually 1150~l/1, and furthermore 1/lO~
Preferably it is 2/3.

In the back coat layer, the blending amount of the non-magnetic powder is preferably 50 to 5 parts by weight based on 100 parts by weight of the binder.
00 parts by weight, more preferably 60 to 400 parts by weight.

As the non-magnetic powder, in addition to carbon black A, carbon black B, and the zinc oxide, organic fillers and inorganic fillers may be used in combination.

As the organic filler, acrylic styrene resin,
Examples include benzoguanamine resin powder, melamine resin powder, phthalocyanine pigment, polyester resin powder, polyamide resin powder, polyimide resin powder, and borin-ionized ethylene resin powder.

In particular, benzoguanamine-based and/or melamine-based resin powders are preferred for use in combination with carbon black.

Examples of fillers include silicon oxide, titanium oxide, aluminum oxide, calcium carbonate, zinc sulfate, tin oxide, chromium oxide, silicon carbide, calcium carbide, α-F.
e20. , talc, kaolin, boron nitride, zinc fluoride, and molybdenum dioxide.

As a binder in the back coat layer.

For example, thermoplastic resins, thermosetting resins, reactive resins, electron beam irradiation cured resins, or mixtures thereof, which have been conventionally used in magnetic recording media, can be used.

As the binder, polyurethane resin and fibers are used.
Preferably, a polyisocyanate is added to the resin.

The polyurethane resin is synthesized by a reaction between a polyol and a polyisocyanate, and depending on the selection of the polyol, the main chain of the polyurethane has ether bonds, -intra-film ester bonds, carbonate ester bonds,
Alternatively, it may contain a combination of two or more of these.

Furthermore, the polyurethane resin may have fluorine, silicon, or sulfone groups in its main chain or side chain in order to improve lubricity or dispersibility.

The molecular weight of the polyurethane resin is preferably from 500 to 200,000.

As the cellulose resin, cellulose ether, cellulose acetic acid ester, cellulose organic acid ester, etc. can be used.

The cellulose ether includes methylcellulose,
Examples include ethylcellulose, propylcellulose, isopropylcellulose, butylcellulose, methylethylcellulose, methylhydroxyethylcellulose, carboxymethylcellulose sodium salt, hydroxyethylcellulose, benzylcellulose, cyanoethylcellulose, vinylcellulose, nitrocarboxymethylcellulose, diethylaminoethylcellulose, aminoethylcellulose, and the like.

Examples of the cellulose inorganic ester include nitrocellulose, cellulose sulfate, and cellulose phosphate.

Cellulose organic acid esters include acetylcellulose, propionylcellulose, butyrylcellulose, methacryloylcellulose, chloroacetylcellulose,
Examples include β-oxybrobionyl cellulose, benzoyl cellulose, p-1-luenesulfonate cellulose, acetylpropionyl cellulose, acetyl butyryl cellulose, and the like. Among these, nitrocellulose is preferred.

By using a fiber-based resin, especially nitrocellulose, as the binder, the heat resistance, toughness,
It can improve block resistance, reduce the coefficient of friction, have an outstanding effect on preventing interlayer adhesion, and particularly improve running stability at high temperatures and high humidity.

In addition, in terms of production, in Hank coat paints containing nitrocellulose, the non-magnetic powder is quickly dispersed, the dispersion state is stable, and re-agglomeration of the non-magnetic powder is difficult to occur.

When using a polyurethane resin and a cellulose resin as the binder, the mixing weight ratio of the cellulose resin to the polyurethane resin is normal.

0.05 to 0.01, preferably 0.1 to 5.0
It is.

In this invention, polyisocyanate may be added to the binder as a curing agent.

As the void isocyanate, aromatic polyisocyanate or aliphatic polyisocyanate may be used.

Examples of aromatic polyisocyanates listed in Iia include tolylene diisocyanate (TD I ) and adducts of these incyanates and active hydrogen compounds, with an average molecular weight in the range of +(10 to 3.(100)). suitable.

Examples of the 1t1 aliphatic polyisocyanate include hexamethylene diisocyanate (HMD I) and adducts of these incyanates and active hydrogen compounds, and those having an average molecular weight in the range of 100 to 3,000 are preferred; Among the aliphatic polyisocyanates, non-alicyclic polyisocyanates and adducts of these compounds and active hydrogen compounds are preferred.

When adding the polyisocyanate, the amount of the polyisocyanate added is usually 1/20 to 7/10, preferably 1/10 to 7/10, based on the weight of the binder.
It is 2.

In addition to the nonmagnetic powder and the binder, a dispersant, an antistatic agent, and the like may be added to the back coat layer.

Examples of the dispersant include lecithin, phosphoric acid ester, amine compound, alkyl sulfate, JIr1 fatty acid amide, higher alcohol, polyethylene oxide, sulfosuccinic acid, sulfosuccinic acid ester, known surfactants, and salts thereof. I'll come. Furthermore, salts of polymers having negative functional groups (for example, -COON, -po□H) can also be used as dispersants.

These 311 types may be used alone, or 2M or more may be used in combination.

The antistatic agents include the carbon black, graphite, tin oxide-antimony oxide compounds,
Conductive powders such as titanium oxide-tin oxide-antimony oxide compounds; Natural surfactants such as saponin; Anno? Nonionic surfactants such as kylene oxide type and glycidol type; higher alkyl amines; cationic surfactants such as 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 acid ester groups, diamino acids, amino sulfonic acids, and bifacial active agents such as sulfuric acid or phosphoric acid esters of amino alcohols.

In addition, in this invention, it is preferable not to use a commonly used lubricant in the back coat layer.

- By using commonly used lubricants,
The coefficient of friction of the back coat layer increases, which may lead to a decrease in running durability such as abnormal running due to increased torque.

The thickness of the back coat layer is usually 0.1 to 5.0μ.
m, preferably 0.2 to 36 lLm.

The surface roughness of the backcoat layer is 0.005 mm in terms of runnability and electromagnetic conversion characteristics at the center if average roughness (Ra) of cot off 0.08 mm. [It is preferably 15 ILm or less.

Such a backcoat layer is provided on the nonmagnetic support described below. However, the surface of the nonmagnetic support on which the back coat layer is provided is the other surface of the nonmagnetic support from the surface on which the magnetic layer is provided.

(Nonmagnetic Support) Materials for forming the nonmagnetic support include, for example, polyethylene terephthalate and polyethylene-2,6-
Mention may be made of polyesters such as naphthalates, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and plastics such as polycarbonate. Further, metals such as Cu, An, and Zn, glass, and various ceramics such as so-called new ceramics (eg, boron nitride, silicon carbide': J), etc. can also be used.

There is no particular restriction on the form of the non-magnetic support, and it may be in any form such as tape, sheet, card, disk, etc.

The thickness of the non-magnetic support is, for example, usually 3 to 1100 pL, preferably 5 to 50 gm when it is in the form of a tape or sheet, and usually 30 to 1100 pL when it is in the form of a disk or card. It is.

(Magnetic Layer) The magnetic layer is provided on one surface of the nonmagnetic support.

The magnetic layer is a layer formed by dispersing the ferromagnetic powder in a binder.

Examples of the ferromagnetic powder include γ-Fe, O, G
o containing y Fe, 03. Iron oxide magnetic powder such as Fe*Oa, Co-containing y-Fe:IO, Fe-A1 alloy powder, Fe-AJI-P alloy powder, Fe-Ni-C
o alloy powder, Fe-Mn-Zn alloy powder, re-Ni
-Zn alloy powder, Fe-Go-Ni-Cr alloy powder.

Examples include Fc-Co-N1-P alloy powder, Go-Ni alloy powder, Go-P alloy powder, and ferromagnetic alloy powder containing ferromagnetic metals such as Fe, Ni, and Co as main components.

There is no particular restriction on the shape of the ferromagnetic powder, and for example, acicular, spherical, or ellipsoidal shapes can be used.

There is no particular restriction on the shape of the ferromagnetic powder, and for example, acicular, spherical, or ellipsoidal shapes can be used.

The binder includes, for example, a thermoplastic resin, a thermosetting resin 1-reactive resin, which has been conventionally used in magnetic recording media,
7! It is possible to use a t-irradiation curable resin or a mixture thereof, and the same resin as that used in the back coat layer can be used.

These may be used alone or in combination of two or more.

In the magnetic layer, in addition to the Moeki ferromagnetic powder and the binder, a lubricant, an antistatic agent, a hardening agent, etc. can be blended.

Furthermore, an abrasive can be added to the magnetic layer if necessary.

As the abrasive, for example, fused alumina, silicon carbide, chromium oxide, corundum, artificial plandum, artificial diamond, garnet, emery (main components: corundum and magnetite), etc. are used.

The average particle size of these abrasives is usually 0.05 to 5.0.
It is le m.

When blending the abrasive, the blending ratio of the abrasive is as follows:
Usually 0.5 to 100 parts by weight of the ferromagnetic powder
It is 20 parts by weight.

Note that in the back coat layer and the magnetic layer,
The lubricant, the antistatic agent, etc. do not have a sole function; for example, a - compound may act as a lubricant and an antistatic agent.

Therefore, the above-mentioned classifications in this invention indicate the main actions, and the actions of the classified compounds are not limited by the actions shown in the classification.

Next, a method for manufacturing the magnetic recording medium of the present invention will be explained.

(S! Method, etc.) The magnetic recording medium of the present invention is produced by, for example, preparing a magnetic coating material by cross-dispersing the magnetic layer forming components such as the ferromagnetic powder and binder in a solvent, and then using the obtained magnetic coating material. After applying and drying on one side of the non-magnetic support, and cross-dispersing back coat layer forming components such as the non-magnetic powder and the binder in a solvent to produce a back coat paint: It can be manufactured by applying the obtained coating material to the other surface of the non-magnetic support and drying it.

Examples of the solvent used for kneading and dispersing the magnetic layer forming components include acetone, methyl ethyl ketone (for use),
Ketones such as methyl isobutyl ketone (MlllK) and cyclohexanone; alcohols such as methanol, ethanol, propatool and butanol; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, propyl acetate and ethylene glycol monoacetate; Ethers such as diethylene glycol dimethyl ether, 2-ethoxyethanol, tetrahydrofuran, and dioxane; Aromatic hydrocarbons such as benzene, toluene, and xylene; methylene chlorite, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, dichlorobenzene, etc. halogenated hydrocarbons and the like can be used.

When kneading the composition of the magnetic coating components, the ferromagnetic powder and other magnetic coating components are charged into a kneader simultaneously or individually one after another. For example, the ferromagnetic powder is first added to a solution containing a dispersant, kneaded for a predetermined period of time, then the remaining components are added, and the mixing is continued to produce a magnetic paint.

In case of crosstalk, it is possible to use crosstalk machines in each village. Examples of this mixer include a Miki roll mill, a Miki roll mill, a ball mill, a pebble mill, a sight grinder, a Sqegvari attritor, a high-speed impeller dispersion machine, a high-speed stone mill, a high-speed impact mill, a disper kneader-1 high-speed mixer, a homogenizer, and an ultrasonic mixer. Examples include a disperser.

The coating solution of the magnetic layer forming component thus prepared is coated on one side of the non-magnetic support by a known method.

The back coat layer forming component can also be prepared by the same method as the magnetic layer forming component, and the coating liquid of the back coat layer forming component is applied to the other surface of the nonmagnetic support by a known method.

Application methods that can be utilized in this invention include, for example, gravure roll coating, wire bar coating, doctor blade coating, Sohar roll coating, dip coating, air knife coating, calendar coating, squeegee coating, kiss coating, and fan coating. Examples include tin coating.

After applying the magnetic layer forming R& acid component and the back coat layer forming component in this way, a magnetic field alignment treatment is performed as necessary in an undried state, and a surface smoothing treatment is usually performed using a super calender roll or the like. Let's do it.

The magnetic recording medium of the present invention can be obtained by performing the above-described surface smoothing treatment and then cutting the medium into a desired shape.

The magnetic recording medium of the present invention can be used, for example, by cutting it into a long shape, as a magnetic tape such as a video tape or audio tape, or by cutting it into a disk shape.
It can also be used as a floppy disk, etc. Furthermore, it can also be used in a cart-like, cylindrical, or other form like a normal magnetic recording medium.

[Example] Next, Examples and Comparative Examples of the present invention will be shown to further specifically explain the present invention.

In addition, in the examples and comparative examples described below, 1
``bu'' shall represent ``juwabu''.

(Example 1) After mixing and dispersing a magnetic layer composition having the composition shown below using a ball mill, a polyfunctional isocyanate was added as a curing agent to
A magnetic paint was prepared by adding 50% of the total amount and filtering through an Igm filter.

゛Go-containing -Fe, 0. ...... Polyurethane... Vinyl chloride-vinyl acetate copolymer Butyl stearate... Myristic acid... Stearic acid...・・・・・・ Alumina・・・・・・・・・・・・ Carbon black・・・・・・ Lecithin・・・・・・・・・ Cyclohexanone・・・・・・ Methyl ethyl ketone・・...Toluene number/Saiken...a- 100 parts, 8 parts, 12 parts, 0.8 parts, 0.5 parts, 0.5 parts, 5 parts, 0.5 parts, 4 parts, 40 parts, 60 parts of the obtained magnetic coating liquid was applied to one side of a 13 gm thick polyethylene terephthalate film using a reverse roll coater so that the dry thickness after calendaring was 4.5 μm, and heated. The solvent was removed and the surface was smoothed using a super calender.

Next, a back coat paint was prepared by dispersing the back coat layer composition having the composition shown in Table 1 using a ball mill for 24 hours.

The obtained back coat coating liquid was applied to the magnetic layer-formed side and the other side of the polyethylene terephthalate film using a reverse roll coater so that the dry thickness was 1101 L, and the solvent was removed under heating. Then, a back coat layer was formed.

Next, a sample tape was prepared by slitting it into 1/2 inch width using a cutting device.

(a) Pack the sample tape into a VIIS cassette and place it under NV-6200 (temperature 20°C, humidity 50%).
A deck (manufactured by Matsushita Electric) was used, and the vehicle was run repeatedly 200 times. Regarding the tape (2 (10 times bus tape)), the output fluctuation range, dynamic friction coefficient of the back coat layer, interlayer friction coefficient of the back coat layer, skew and jitter were measured, and tape damage and back coat layer abrasion were visually evaluated. .

Furthermore, the dynamic friction coefficient of the back coat layer and the interlayer friction coefficient of the back coat layer were measured for the sample tape (virgin tape) that had not been run yet.

In addition, the measurement of each characteristic was performed as follows.

Output fluctuation range: 20°C under the conditions of temperature 20℃ and humidity 60%
The width of the output fluctuation during reproduction of the 0-time bass tape was determined.

Motion friction coefficient: Under conditions of temperature 23'C1 humidity 60%.

Runability tester manufactured by Yokohama System Co., Ltd. (TDT-:100-
D), set the inlet tension to 20g, and set the diameter to 3.
．． A sample tape was wound 180' around a No. 81 stainless steel pin, ran for 3.3 cm for 1 second, and the exit tension after - minutes was measured and calculated from the following equation (3).

Interlayer friction coefficient: Under the conditions of temperature 23°C and humidity 60%,
Motion 1! Using a device similar to the one used to measure the friction coefficient, the inlet tension was set to 20 g, the magnetic layer was wound on the L side around a stainless steel drum with a diameter of 62 mm, and the sample tape was attached on top of it. The back coat layer surface was run at a speed of 0.3 cm/sec, and the exit tension after - minutes was measured and calculated from equation (c).

Skew measurement: The color bar signal was recorded on a sample tape and was run 200 times using a video deck (IIR-6500, manufactured by Victor Corporation) under conditions of a temperature of 40°C and humidity of 80%. When this occurred, the image distortion at the switching point was measured on the monitor screen, and this was expressed in seconds.

Jitter measurement: Sample tape after running 200 times on VTR
Measurement was performed using a Shi-7ter measuring meter (manufactured by 0 Kuro Denki Co., Ltd.).

(b) An interlayer adhesion test was conducted as follows.

Wrap a 1/2 inch wide tape with a pressure of 1 kg at a temperature of 6.
After being left for 20 hours under conditions of 0''C and 80% humidity, it was left to stand at room temperature for an additional 24 hours and rewound, and those with resistance when pulled apart were evaluated as present, and those without were evaluated as no.

(c) The surface roughness [Ra] of the back coat layer surface was measured as follows. Three-dimensional roughness measuring instrument 5E-3F
K (manufactured by Koita Research Institute) Cout-off 0.25, stylus force 30
(2) It was determined by measuring the length of the sample surface (bank coat layer surface) at 2.5 g.

(d) Chroma S/N was measured as follows.

+111-7100 (manufactured by Japan Victor ■) was used to record 4.5M11z at the maximum recording current, and the noise iJE during reproduction was measured.

The results are shown in Table 2.

(Examples 2 to 7 and Comparative Examples 1 to 4) Prototype tapes were prepared in the same manner as in Example 1 except that the back coat layer composition having the composition shown in Table 1 was used. The quality was measured using the following methods.

The results are shown in Table 2.

(This page, blank space below) (Evaluation) As is clear from Table 2, in the magnetic recording medium of the present invention, the non-magnetic powder is uniformly dispersed in the back coat layer, so the surface of the back coat layer is It has roughness, a low coefficient of friction, and very little scraping of the back coat layer or damage to the tape.

Furthermore, it has excellent skew and jitter characteristics due to stable running, small output fluctuation range, no deterioration of chroma S/N, sufficient durability, and good magnetic conversion characteristics. There is.

[Effects of the invention J According to this invention.

(1) By providing a back coat layer containing carbon black, it has antistatic effect and light shielding effect, and (2) Forms a back coat layer in which non-magnetic powder is sufficiently uniformly dispersed. Because you can.

(3) Due to the moderate roughening effect of the back coat layer,
It is possible to provide a magnetic recording medium that has advantages such as excellent running properties, excellent durability, and the ability to realize good electromagnetic conversion characteristics.

Claims (1)

[Claims] (1) Non-magnetic material comprising carbon black with an average primary particle size of 20 to 40 mμ, carbon black with an average primary particle size of 50 to 100 mμ, and zinc oxide powder with an average particle size of 0.3 to 1.5 μm. A magnetic recording medium comprising a back coat layer containing powder and a binder.