JPH01185823A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a high-density magnetic recording medium having an excellent travelling property and durability by incorporating metal powder and >=2 kinds of carbon blacks having different average grain sizes into a back coat layer. CONSTITUTION:The metal powder and >=2 kinds of the carbon blacks having the different average grain sizes are incorporated into the back coat layer. Transparent plastic films made of polyester, polyimide, aramid, etc., are usable for a base. The metal powder and carbon blacks which constitute the back coat layer are prepd. together with a binder and a solvent into a coating compd. which is applied on the base. The large-grain size carbon black improves the traveling property and the metal powder and the small-grain size carbon black improve the smoothness and electrical conductivity and, therefore, the surface characteristic, electrical conductivity and traveling property are provided to the recording medium.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a high-density recording magnetic recording medium that has excellent light-shielding properties, running properties, and electrical conductivity.

(Prior Art) Conventionally, it has been widely practiced to provide a back coat layer on the back surface of a magnetic tape such as a video tape for the purpose of improving running performance, durability, and reinforcing the tape.

Furthermore, in recent years, these magnetic recording media are required to have higher density, and it has been found that ultrafine hexagonal powder such as Ba ferrite is suitable as a magnetic powder for this purpose.

This is because this fine powder is a hexagonal plate-shaped crystal and has an axis of easy magnetization in a direction perpendicular to the plate surface, thereby providing a high-density magnetic recording medium. Further, the requirements for the back coat layers used in these high-density magnetic recording media are becoming increasingly strict, and reductions in the coefficient of friction, improvements in conductivity, surface properties, etc. are required.

(Problems to be Solved by the Invention) However, it is extremely difficult for conventional backcoat layers to satisfy all of the requirements for backcoat layers for recently developed high-density magnetic recording media. for example,
A back coat layer made of only small particle size carbon coated together with a binder resin is convenient for ensuring surface properties and conductivity, but it is difficult to maintain Il during driving. It has a large Jtlj coefficient and poor durability.

In order to ensure runnability and durability, the average particle size of carbon black is often increased or large particle size carbon is mixed in, but in this case, the large particle size carbon The surface of the coating layer becomes rough, and this is transferred to the magnetic layer, causing problems such as dropouts and decreased output. Such problems are particularly noticeable in high-density magnetic recording media made of ultrafine hexagonal ferromagnetic powder such as Ba ferrite, which has recently been used, because it is necessary to maintain extremely high surface precision. Furthermore, the use of large particle size carbon causes deterioration of the electrical conductivity of the back coat.

Another problem with magnetic recording media using hexagonal ferromagnetic powders such as Ba ferrite is that these magnetic powders have high infrared transmittance, making it difficult to detect the magnetic layer. The point is that you have to rely on layers. For this reason, countermeasures such as increasing the thickness of the back coat layer have been taken, but new problems such as an increase in the total thickness of the tape have arisen.

The present invention aims to solve various problems in the back coat layer of the magnetic recording medium and thereby provide a high-density magnetic recording medium with excellent running properties and durability.

[Structure 1 of the Invention (Means for Solving the Problems) That is, the magnetic recording medium of the present invention is a magnetic recording medium in which a magnetic layer is formed on the surface of a support and a back coat layer is provided on the back surface. Metal powder in the coating layer,
It is characterized by containing two or more types of carbon black having different average particle sizes.

As the support used in the magnetic recording medium of the present invention, a transparent plastic film made of polyester, polyimide, aramid, etc. can be used.

In the present invention, the metal powder and carbon black constituting the back coat layer are made into a paint together with a binder resin and a solvent, and the paint is applied onto the support. Is there a dispersant in this paint, if desired? iI! organic fillers or metal oxides, such as abrasives or latex particles;
Inorganic fillers such as metal sulfides and metal fluorides may be added.

Examples of the binder resin include polyurethane resin, boester resin, polycarbonate resin, polyacrylic resin,
Polyamide resin, epoxy resin, phenol resin, polyether resin, phenoxy resin, melamine resin, vinyl butyral resin, furan resin, vinyl chloride resin, vinyl acetate resin, vinyl alcohol resin. Mixtures or copolymers of these four types are used. .

As the metal powder constituting the back coat layer, iron, chromium, cobalt, nickel, steel, tantalum, zirconium, aluminum, and alloy powders thereof can be used, and the particle size thereof is preferably 1000 nm to 5000 rv. Although these metal powders have a great effect of blocking infrared rays, they can increase the surface roughness of the back coat layer, increase the surface resistance of the coating film, and cause wear and tear on the tape guide parts, so please be careful when handling them. Requires consideration.

According to experiments conducted by the present inventors, these metal powders are used in combination with carbon black having an appropriate average particle size, or the back coat layer has a two-layer structure, and the 11th El contains metal powder. It has been confirmed that most of these problems can be solved by including carbon black of small particle size in the second layer.

Generally, many types of carbon black are commercially available having a predetermined particle size distribution and average particle diameter, but the carbon black constituting the back coat layer of the present invention has an average particle diameter in the range of 1100n to 4000n. Two or more types of carbon black having different average particle sizes are used.

The reason why two or more types of particles with different average particle sizes are used in the present invention is as follows.

That is, when a carbon black having a small average particle size is applied together with a binder resin, smoothness and conductivity are improved, but a coating film having a large coefficient of dynamic friction and poor running properties is obtained. Further, when crabbon black having a large average particle size is applied together with a binder resin, a coating film with excellent running properties is formed, but on the other hand, a coating film with a rough surface is obtained.

Therefore, in order to achieve both surface properties, conductivity, and runnability, it is necessary to use two or more types of carbon blacks having different average particle sizes.

Further, in addition to carbon black, metal powder is added to the back coat layer of the magnetic recording medium of the present invention in order to improve the shielding rate against infrared rays. Since the above-mentioned properties of carbon black are affected by the addition of metal powder, it is desirable to limit the particle size and amount of metal powder added within an appropriate range. The particle size of the metal powder is preferably in the range of 1100 nm to 5000 nm, and the amount added is preferably 50% or less based on the total amount with carbon black.

It is desirable that the back coat layer in the present invention has a two-layer structure.

With a back coat layer having a two-layer structure, it is extremely easy to adjust various properties. That is, in a back coat layer having a two-layer structure, a layer 11 containing carbon black and metal powder with a relatively large average particle size is first formed on a support, and then a layer 11 containing carbon black with a relatively large average particle size and a metal powder is formed on the first layer. By forming a second layer containing carbon black having an average particle size smaller than that of the carbon black in the first layer, it is possible to achieve appropriate smoothness of the surface of the first WI and to ensure conductivity. In some cases, large particle size carbon black or metal powder may be added to the second layer, but it is desirable that the amount added be smaller than that of the first layer.

In addition, in this specification, large particle size carbon black means
It refers to those with an average particle size of 500n1 to 4000n-, and small particle size carbon black refers to those with an average particle size of 1100n to 500n.
Say something n-. When the bank coat layer has a two-layer structure, the eleventh step is to use large particle carbon black, especially 2000 carbon black.
Carbon black having an average particle size of rv or more is applied alone or a mixture of large particle size carbon black, small particle size carbon black, and metal powder is applied together with a binder resin. The second layer is coated with small particle size carbon alone or a mixture of large particle size carbon and small particle size carbon black together with a binder resin. Since the 21st layer is provided for the purpose of improving the surface smoothness of the first layer, it is necessary that the average particle size of the entire carbon black contained therein is smaller than that of the first layer.

The coating thickness of the second layer is limited in order to take advantage of the runnability of the first layer, and is most preferably in the range of 500 nm to 5000 na+.

The above-mentioned first layer and second layer are first coated with the first M using a commonly used coating machine such as a reverse coater, gravure coater, flow coater, spray coater, etc., and then coated on the first WJ in the same manner. Apply the second layer using a coating machine.

The coating may be carried out immediately after coating the first layer or after the first layer coating has been cured to some extent. When applying the second layer, it is important to take care not to damage the underlying second layer, but this can be solved by adjusting the coating speed and the method of supplying the paint.

The durability of the coating film of the back coat layer can be improved by adding, for example, a polyisocyanate-based or amine-based curing agent to the components of the film and curing it in a suitable atmosphere.

Note that the present invention covers all commonly used magnetic layers.

That is, the magnetic powder used in the magnetic layer of the magnetic recording material of the present invention includes magnetite, γ-ferrite, C
o-modified γ-ferrite, Ca-deposited γ-ferrite,
Examples include, but are not limited to, metal powders mainly composed of Fe, hexagonal-based magnetic powders represented by C'02 and Ba-ferrite, and mixtures thereof.

Particularly preferred among these magnetic powders is the general formula % (%) (wherein A is any one of Ba, Sr, Ca, Pb, and Ca, and H is Zn, Co, Ti, and Ni).
1Hn, In, Cu, Ge, Nb, Sn, 2
At least one selected from the group of r, tlf and A°C
Species element, m is 0 or a number less than 1, n is 5.4 to 60
Each represents the number of . However, if H is divalent or 41j
In the case of elements with a valence of 5 or more, H is a combination of two or more elements with an average valence of 3. ) is a hexagonal plate-shaped hexagonal ferrite.

Among these hexagonal ferrites, those with a grain size (the length of the diagonal line of the plate surface) of 0.01 to 0.3 μm and an axial ratio (value obtained by dividing the length of the diagonal line by the plate thickness) of 2 to 10. is particularly preferred.

As the binder resin for the magnetic layer, the same binder resin used for the back coat layer can be used.

The mixing ratio of the magnetic powder and the binder is usually in the range of 10 to 35 parts by weight of the binder resin per 100 parts by weight of the magnetic powder.

In addition, in order to cure the coating film together with these binder resins, it is also possible to mix a curing agent such as a polyvalent isocyanate.

(Function) The backcoat layer in the magnetic recording medium of the present invention has improved infrared shielding rate, which was a problem with conventional backcoat layers, and provides a backcoat layer that is compatible with surface properties and runnability. For example, in the case of a two-layer structure, high conductivity of the second layer can be easily ensured, so that a magnetic recording medium with little charge can be obtained. In general, since the coating thickness can be adjusted, the stiffness of the magnetic recording medium can be adjusted using the back coat layer.

(Example) Examples of the present invention will be described below.

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

Example Fe-Ni alloy powder (average particle size: 1500rv) 2
0 parts large particle size carbon black 20I?
(Average particle size: 2000n++) Nitrocellulose 5 Sulfonated urethane resin (glass transition point 50°C) 20 Methyl ethyl ketone/cyclohexane (1 part 1 mixed solvent) 500 n After mixing the above materials, disperse with a sand grinder for 3 hours did. After filtration, the prepared coating material was mixed well with 6.5 parts of a polyisocyanate curing agent, and coated on a polyester film to a thickness of 0.5 .mu.m using a gravure coater to form a first layer. Next, a second layer of paint was produced using the following composition and the same process.

Small particle size carbon (average particle size: 200) 50 parts Nitrocellulose 20 Polyester resin 50 The obtained paint was filtered, 20 parts of a polyisocyanate curing agent was added, and coated on No. 1111 with a gravure coater. It was applied to a thickness of 2μ.

The film thus obtained had a surface roughness of 0.15 .mu.- and a surface resistance of 3.times.10 .OMEGA./SQ. These values are based on surface roughness of 040μm and surface resistance of 1×1 when only the first layer is used.
It was found that the surface properties were improved and the conductivity was significantly improved compared to 0Ω/SQ.

When this coating film was wrapped around a hard chrome-plated rod of 4 mm diameter and subjected to repeated friction, the coefficient of dynamic friction was as low as 0.25, and this value hardly changed even after 100 repeated m rubbings. On the other hand, the first layer is placed on the polyester film.
The coefficient of dynamic friction of the film in which only the above No. 211 was applied to a thickness of 1μ without applying any other layer was also as high as 0.35.
After repeating the friction 20 times, it increased to 0.6 or more.

Also, above 2 [! Wavelength 9 of coating film and second layer only coating film
When the transmittance at 00n- was measured, values of 3.5% and 0.5% were obtained, confirming that the metal powder has a remarkable light blocking effect.

Next, a paint with the composition shown below was applied to the opposite side of the back layer to form a magnetic layer, and then a similar test was conducted.
In this case as well, the same excellent characteristics as above were obtained.

Co-Ti substituted Ba ferrite powder 100
parts (average particle size o, oeμm, HC6000e, specific surface area 267Ii10) Polyurethane resin 4 IT[
CA-118 (Morton) Carbon (average particle size 30 μm) 3 Phosphoric ester surfactant 3 Stearic acid 0.5 Mixed solvent of methyl ethyl ketone and toluene (50150) 150
#Chromium oxide powder 3
Vinyl chloride-vinyl acetate copolymer 14 Mixed solvent (50150) 150 #[
Effects of the Invention As explained above, the magnetic recording medium of the present invention contains metal powder and two or more types of carbon black with different average particle sizes in the back coat layer, so that large particle size carbon The black improves running properties, and the metal powder and small particle size carbon black improve smoothness and conductivity, so it has excellent surface properties, conductivity, and running properties.

Claims (1)

[Claims] (1) In a magnetic recording medium in which a magnetic layer is formed on the surface of a support and a back coat layer is provided on the back surface, the back coat layer contains a metal powder and two or more types of carbon black having different average particle sizes. A magnetic recording medium characterized by containing.