JP2002100022A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure superior running durability while retaining good electromagnetic transducing characteristics. SOLUTION: In the magnetic recording medium with a nonmagnetic substrate 1, a middle layer 2 formed on the substrate 1 and containing a magnetic powder and/or a nonmagnetic powder and a binder and a magnetic layer 3 formed on the middle layer 2 and containing a ferromagnetic powder and a binder, the magnetic layer 3 further contains a pigment having a Mohs hardness of 3 to <6, the average particle diameter of the pigment is larger than the thickness of the magnetic layer 3, and when the glass transition temperature of the binder in the magnetic layer 3 and that of the binder in the middle layer 2 are represented by Tg (U) and Tg (L), respectively, the inequality Tg (U)>Tg (L) is satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium, and more particularly to an improvement in electromagnetic conversion characteristics and running durability.

[0002]

2. Description of the Related Art For example, as a magnetic recording medium used in a video tape recorder or the like, a non-magnetic support such as a polyester film and a ferromagnetic powder, a binder, an organic solvent, various additives are provided on the non-magnetic support. A so-called coating type magnetic recording medium including a magnetic layer formed by applying a magnetic paint prepared by mixing and dispersing an agent or the like is known. There is also known a so-called metal magnetic thin film type magnetic recording medium in which a magnetic layer is formed by applying a ferromagnetic material on a nonmagnetic support by a vacuum thin film forming technique. Among these, coating-type magnetic recording media have been dominant in the field of magnetic recording because of their excellent productivity, durability and corrosion resistance.

In a coating type magnetic recording medium, studies have been made to reduce the thickness of the magnetic layer to reduce self-demagnetization loss during recording and thickness loss during reproduction. However, in the case of a single-layer coating type magnetic recording medium in which a magnetic layer is formed on a nonmagnetic support, when the magnetic layer is thinned, the surface shape of the nonmagnetic support is significantly reflected on the surface of the magnetic layer. In addition, the surface smoothness of the magnetic layer deteriorates.

Therefore, a multilayer coating type magnetic recording medium in which an intermediate layer is interposed between a nonmagnetic support and a magnetic layer has been proposed. In this multilayer coating type magnetic recording medium, since the magnetic layer and the non-magnetic support are not in direct contact, the surface of the magnetic layer is not affected by the surface shape of the non-magnetic support. Therefore, it is possible to form a thin magnetic layer having good surface smoothness. The intermediate layer may be a magnetic layer having magnetism or a non-magnetic layer that is non-magnetic.

[0005]

Incidentally, the strength of the magnetic layer tends to decrease as the magnetic layer becomes thinner.
For this reason, in a magnetic recording medium having a thinned magnetic layer, securing running durability is a major issue. As a general method of improving running durability, there is a method of using a material having a high glass transition temperature as a binder contained in a magnetic layer to increase the strength of the magnetic layer.

However, even if the magnetic layer containing such a material as a binder is subjected to a surface treatment such as a calendar treatment, it has been difficult to smooth the surface of the magnetic layer.
For this reason, as described above, it is difficult to improve the surface smoothness of the multilayer coating type magnetic recording medium in which the glass transition temperature of the binder contained in the magnetic layer and the intermediate layer is specified. As a result, there is a problem that a spacing loss occurs and electromagnetic conversion characteristics are deteriorated.

In other words, in the magnetic recording medium of the multilayer coating type, it was not possible to realize good electromagnetic conversion characteristics while ensuring running durability.

The present invention has been proposed in view of such conventional circumstances, and has as its object to provide a magnetic recording medium having excellent running durability while maintaining good electromagnetic conversion characteristics. I do.

[0009]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have made various studies and found that a specific pigment is contained in the magnetic layer and that the binder contained in the magnetic layer is not contained. It has been found that by defining the glass transition temperature and the glass transition temperature of the binder contained in the intermediate layer, a magnetic recording medium having excellent running durability can be obtained while maintaining good electromagnetic conversion characteristics. Was.

The present invention has been completed on the basis of such findings, and comprises a nonmagnetic support, a magnetic powder and / or a nonmagnetic powder and a binder, and is formed on the nonmagnetic support. Recording medium comprising a magnetic layer containing a ferromagnetic powder and a binder, and a magnetic layer formed on the intermediate layer, wherein the magnetic layer contains a pigment having a Mohs hardness of 3 or more and less than 6. The average particle size of the pigment is larger than the thickness of the magnetic layer, and the glass transition temperature of the binder contained in the magnetic layer is set to T.
g (U), and when the glass transition temperature of the binder contained in the intermediate layer is Tg (L), Tg (U)> Tg (L)
It is characterized by being.

The magnetic layer has a Mohs hardness of 3 or more and less than 6, and contains a pigment having an average particle size larger than the thickness of the magnetic layer. In this magnetic recording medium, Tg (U)> Tg
Since the relationship (L) is satisfied, even if a pigment having an average particle diameter larger than the thickness of the magnetic layer is added to the magnetic layer, the exposure of the pigment exposed on the surface of the magnetic layer by a surface treatment such as a calendar treatment is performed. It is possible to optimize the amount.

Therefore, in the magnetic layer and the intermediate layer,
For example, the coating film properties such as abrasion resistance and still durability are greatly improved, and the surface smoothness of the magnetic layer is improved, so that the spacing loss is reduced.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a magnetic recording medium according to the present invention will be described in detail.

As shown in FIG. 1, a magnetic recording medium to which the present invention is applied has a nonmagnetic support 1 and a magnetic powder and / or a nonmagnetic powder formed on one main surface of the nonmagnetic support 1. And an intermediate layer 2 containing a binder and a magnetic layer 3 formed on the intermediate layer 2 and containing a ferromagnetic powder and a binder.

The magnetic layer 3 is formed by applying a magnetic paint prepared by mixing a magnetic layer composition such as a ferromagnetic powder and a binder onto the intermediate layer 2.

The thickness of the magnetic layer 3 is preferably 0.03 μm or more and 0.3 μm or less. By setting the thickness of the magnetic layer 3 in the above range, the magnetic recording medium has excellent electromagnetic conversion characteristics. If the thickness of the magnetic layer 3 exceeds 0.3 μm, there is a possibility that the output will decrease due to recording demagnetization. Further, it is difficult to reduce the thickness of the magnetic layer 3 to less than 0.03 μm in manufacturing.

The magnetic layer 3 contains a pigment having an average particle size larger than the thickness of the magnetic layer and a Mohs hardness of 3 or more and less than 6 (hereinafter, simply referred to as pigment). Since this pigment has a Mohs' hardness of 3 or more and less than 6, it does not polish the magnetic head like an abrasive, but since the average particle size is larger than the thickness of the magnetic layer 3, the pigment effect is caused by the so-called pigment effect. 3 is a strong coating film.

When a pigment having a Mohs hardness of less than 3 is used, the pigment itself is destroyed by sliding with a magnetic head. On the other hand, if a pigment having a Mohs hardness of more than 6 is used, the pigment itself will polish the magnetic head.

Examples of pigments having a Mohs hardness of 3 or more and less than 6 include carbon black, graphite, calcium carbonate, antimony, fluorite, barite, silver, dolomite, copper, magnesium oxide, platinum, palladium, opal, and the like. Iron, asbestos, manganese, apatite, nickel and the like can be used.
Among these pigments, it is particularly preferable to use carbon black. As the carbon black, a known carbon black commonly used in this type of magnetic recording medium can be used.

If the average particle size of the pigment is smaller than the thickness of the magnetic layer 3, the durability of the magnetic layer 3 becomes insufficient. Therefore, the average particle size of the pigment is preferably as large as possible. However, when the thickness is larger than the sum of the thickness of the magnetic layer 3 and the thickness of the intermediate layer 2, the pigment is largely exposed from the surface of the magnetic layer 3 even if a part of the pigment is embedded in the intermediate layer 2. For this reason, the spacing between the magnetic head and the magnetic recording medium increases, and the electromagnetic conversion characteristics deteriorate. Therefore, the average particle size of the pigment depends on the thickness of the magnetic layer 3 and the thickness of the intermediate layer 2.
Is required to be smaller than the total thickness.

By the way, in a general magnetic recording medium, when the magnetic layer contains an abrasive or non-magnetic reinforcing particles having an average particle diameter larger than the thickness of the magnetic layer, the non-magnetic reinforcing particles and the like are removed from the surface of the magnetic layer. Due to the large exposure, there has been a problem that the spacing between the magnetic head and the magnetic recording medium increases, and the electromagnetic conversion characteristics deteriorate.

On the other hand, a magnetic recording medium to which the present invention is applied has a magnetic layer 3 containing the above pigment, and has a glass transition temperature T B of a binder contained in the magnetic layer 3.
g (U) and the glass transition temperature of the binder contained in the intermediate layer 2 as Tg (L), Tg (U)> Tg
(L).

As described above, when the relationship of Tg (U)> Tg (L) is satisfied, even if the pigment contained in the magnetic layer 3 has an average particle size larger than the thickness of the magnetic layer 3, for example, By performing surface treatment such as calendar treatment,
Part of the pigment is embedded in the intermediate layer 2. The amount of the pigment exposed on the surface of the magnetic layer 3 can be reduced to a small extent because a part of the pigment is embedded in the intermediate layer 2. In other words, by performing a calendar process on the magnetic recording medium,
The amount of the pigment exposed on the surface of the magnetic layer 3 can be set in an optimum range. Therefore, the surface smoothness of the magnetic layer 3 is improved, and the spacing loss is reduced.

The amount of the pigment to be added to the magnetic layer 3 is as follows:
The content is preferably 0.1% by weight to 10% by weight based on the ferromagnetic powder. When the amount of the pigment is 0.1% by weight to 10% by weight with respect to the ferromagnetic powder, the magnetic recording medium surely has excellent running durability and electromagnetic conversion characteristics.

If the amount of the pigment is less than 0.1% by weight, sufficient running durability may not be obtained. On the other hand, if the amount of the pigment exceeds 10% by weight, the electromagnetic conversion characteristics may be deteriorated.

Examples of the binder contained in the magnetic layer 3 include copolymers such as vinyl chloride, vinyl acetate, vinyl alcohol, vinylidene chloride, acrylate, methacrylate, styrene, butadiene, and acrylonitrile;
Alternatively, in addition to a copolymer obtained by combining two or more of these, a polyurethane resin, a polyester resin, an epoxy resin, or the like having a higher glass transition temperature than the binder contained in the intermediate layer 2, that is, Tg (U) > Tg (L)
Those satisfying the following relationship can be used. Also, Tg
If the relationship (U)> Tg (L) is satisfied, two or more of these binders can be used in combination.

Among these, it is particularly preferable to use a polyurethane resin, and the content of the polyurethane resin is preferably 30% by weight or more of the binder contained in the magnetic layer 3.

In the binder contained in the magnetic layer 3, T
g (U) is preferably 100 ° C. or more and 130 ° C. or less. If Tg (U) is less than 100 ° C., sufficient durability may not be obtained. On the other hand, Tg (U) is 13
When the temperature exceeds 0 ° C., the crosslinking reaction hardly occurs, and there is a possibility that the coating film becomes brittle.

The glass transition temperature of the binder is adjusted by selecting a chain extender, for example, when a polyurethane resin is used as the binder.

The ferromagnetic powder is used as a ferromagnetic powder in a coating type magnetic recording medium, such as an iron oxide ferromagnetic powder, a chromium oxide ferromagnetic powder, a metal ferromagnetic powder, and a hexagonal ferrite powder. Any of the available materials can be used.

The magnetic layer 3 preferably contains an abrasive. It is preferable to include a powder of α-iron oxide, alumina, aluminum oxide, chromium oxide, titanium oxide, silica, or the like as an abrasive.

The magnetic layer 3 preferably contains non-magnetic reinforcing particles. Non-magnetic reinforcing particles include aluminum oxide (α, β, γ), chromium oxide, silicon oxide,
Diamond, garnet, emery, boron nitride, titanium carbide, silicon carbide, titanium carbide, titanium oxide such as rutile and tanatase, and the like can be used.

Further, the magnetic layer 3 can contain various additives such as a dispersant, a lubricant, an antistatic agent, a rust preventive, and a curing agent which are generally used in a coating type magnetic recording medium. is there.

The intermediate layer 2 is prepared by applying an intermediate paint prepared by mixing an intermediate layer composition such as a magnetic powder and / or a nonmagnetic powder and a binder onto one main surface of the nonmagnetic support 1. It is formed. The intermediate layer 2 may be non-magnetic or magnetic.

Magnetic Intermediate Layer 2 Containing Magnetic Powder
In this case, a ferromagnetic powder of the system exemplified as the material to be contained in the magnetic layer 3 can be used. When the non-magnetic intermediate layer 2 contains a non-magnetic powder and is used as a non-magnetic intermediate layer 2, hematite (α) which is usually used in a coating type magnetic recording medium is used.
-Fe2O3), non-magnetic powders such as titanium oxide can be used.

The binder contained in the intermediate layer 2 is the same as the binder contained in the magnetic layer 3 described above, but has a lower glass transition temperature than the binder contained in the magnetic layer 3; That is, any material that satisfies the relationship of Tg (L) <Tg (U) can be used.

In the binder contained in the intermediate layer 2, T
g (U) is preferably 20 ° C. or more and 80 ° C. or less. When Tg (L) is less than 20 ° C., the intermediate layer 2 is excessively crushed by a surface treatment such as a calendar treatment, and the surface of the magnetic layer 3 is excessively smoothed. On the other hand, when Tg (L) exceeds 80 ° C., the pigment may not be sufficiently embedded in the intermediate layer 2 even if a surface treatment such as a calendar treatment is performed.

Incidentally, the intermediate layer 2 can contain various additives such as a dispersant, a lubricant, an antistatic agent, a rust preventive and a curing agent which are usually used in a coating type magnetic recording medium. is there.

As the non-magnetic support 1, a polymer substrate formed using a polymer material such as polyethylenes, polyesters, polyolefins, celluloses, vinyl resins, polyimides and polycarbonates;
A metal substrate made of aluminum alloy, titanium alloy or the like, a ceramic substrate made of aluminum glass or the like, a glass substrate, or the like can be used.

In this magnetic recording medium, if necessary, a top coat layer made of a lubricant, a rust preventive, or the like may be provided on the magnetic layer 3 or the intermediate layer 2 and the nonmagnetic support 1 may be provided. To
An undercoat layer can be provided for the purpose of controlling the surface properties of the non-magnetic support 1 and the like. In addition, on the other main surface of the non-magnetic support 1 opposite to the main surface on which the magnetic layer 3 and the like are provided, a back (not shown) is provided for the purpose of improving runnability, preventing charge, preventing transfer, and the like. It is possible to provide a coat layer. As the material of the top coat layer and the back coat layer, any of the materials usually used in this type of magnetic recording medium can be used.

In manufacturing this magnetic recording medium, an intermediate paint prepared by dispersing the above-mentioned intermediate layer composition in a solvent is applied on one main surface of the non-magnetic support 1 to form an intermediate layer 2 To form Next, a magnetic paint prepared by dispersing the above-described magnetic layer composition in a solvent is applied on the intermediate layer 2 to form the magnetic layer 3. After the intermediate layer 2 and the magnetic layer 3 are dried, a calendering process for smoothing the surface of the magnetic layer 3 is performed.

In preparing the magnetic coating material and the intermediate coating material, examples of the solvent for mixing the above various compositions to form a coating material include ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone.
Ester solvents such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, glycol monoethyl ether,
Glycol ether solvents such as glycol dimethyl ether and glycol monoethyl ether, dioxane, aromatic hydrocarbon solvents such as benzene, toluene and xylene, aliphatic hydrocarbons such as hexane and heptane, methylene chloride, ethylene chloride, carbon tetrachloride, Chlorinated hydrocarbon solvents such as chloroform, ethylene chlorohydrin, and dichlorobenzene can be used. One of these solvents may be used alone, or two or more thereof may be used in combination.

As an application method for applying the intermediate paint and the magnetic paint prepared as described above, any of the conventionally known application methods can be applied. However, a die coater having a plurality of slit portions through which the paint is extruded is formed. It is preferable to apply a so-called wet-on-wet coating method in which a magnetic coating material is overlaid and applied on the intermediate layer 2 in a wet state. By applying the wet-on-wet coating method, the intermediate layer 2 and the magnetic layer 3 having good surface smoothness can be formed.

The magnetic recording medium constituted as described above has a magnetic layer 3 containing a pigment having a Mohs hardness of 3 or more and less than 6 and having an average particle diameter larger than the thickness of the magnetic layer 3. The glass transition temperature of the binder contained in
g (U) and the glass transition temperature of the binder contained in the intermediate layer 2 as Tg (L), Tg (U)> Tg
(L), the coating properties such as abrasion resistance and still durability of the magnetic layer 3 and the intermediate layer 2 are greatly improved, and the surface smoothness of the magnetic layer 3 is improved, and spacing loss is reduced. Reduce. Therefore, this magnetic recording medium has excellent running durability while maintaining good electromagnetic conversion characteristics.

[0045]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a magnetic recording medium according to the present invention will be described.
This will be described in detail based on specific experimental results. Here, a plurality of magnetic tapes each including a nonmagnetic support, a nonmagnetic layer that is nonmagnetic as an intermediate layer, and a magnetic layer were produced.

<Experiment 1> Sample 1-1 First, each component of the magnetic paint was weighed according to the following composition.

<Magnetic coating composition> Metal magnetic powder 100 parts by weight Binder: vinyl chloride copolymer 0 parts by weight Polyurethane resin (Tg = 130 ° C.) 16 parts by weight Carbon black (average particle diameter: 0.25 μm) 1 part by weight, 1 part by weight of myristic acid, 1 part by weight of stearic acid ester, 4 parts by weight of isocyanate-based curing agent. The holding power of the ferromagnetic powder was 2,400 Oe, and the specific surface area was 47 m ² / g. Mohs hardness of 3 or more, 6
Carbon black was used as the less than pigment.

The above components and a mixed solvent of methyl ethyl ketone, toluene and cyclohexanone were mixed to a solid content concentration of 28% by weight, and kneaded and dispersed using a sand mill to prepare a magnetic paint.

Next, each component of the non-magnetic paint was weighed according to the following composition.

<Non-magnetic paint composition> 100 parts by weight of α-Fe ₂ O ₃ needle-shaped powder 13 parts by weight of vinyl chloride copolymer 1 part by weight of myristic acid 1 part by weight of stearic acid ester The above components and methyl ethyl ketone , A mixed solvent of toluene and cyclohexanone to obtain a solid content of 39% by weight, and further kneaded and dispersed using a sand mill to prepare a non-magnetic paint.

Then, the magnetic paint and the non-magnetic paint prepared as described above are mixed with a non-magnetic support having a thickness of 8.5.
Two layers were simultaneously coated on a μm polyethylene terephthalate film to form a nonmagnetic layer having a thickness of 1.8 μm and a magnetic layer having a thickness of 0.2 μm. Then, after performing an orientation treatment on the wet magnetic layer, the magnetic layer and the nonmagnetic layer were dried, and further subjected to a calendaring treatment and a curing treatment.

Next, a back coat layer was formed on the other main surface of the non-magnetic support opposite to the one main surface on which the magnetic layer was formed. As described above, the raw magnetic recording medium on which the non-magnetic layer, the magnetic layer, and the back coat layer were formed was slit to produce a magnetic tape having a width of 8 mm. And
This magnetic tape was assembled in a cassette to produce a cassette tape. In this magnetic tape, Tg
(U) was 130 ° C. and Tg (L) was 70 ° C. The measurement of the glass transition temperature was performed as described below.

<Measurement Method of Glass Transition Temperature> First, a binder for measuring the glass transition temperature was applied on a polyethylene terephthalate film to form a coating film having a thickness of 30 μm. Next, the polyethylene terephthalate film on which the coating film was formed was cut, and the width was 10 mm and the length was 25 mm.
Sample. Then, as a measuring device, Bipron DDV-II-EA manufactured by Toyo Baldwin Co., Ltd. was used.
While heating this sample, the frequency 1 was applied to the edge in the width direction of the sample.
The temperature at which the sine wave stretching strain of 0 Hz was applied and the maximum tan δ of the phase difference δ between both vectors of the sine wave stress appearing at the other end was obtained as the glass transition temperature. The heating rate for heating the sample was 5 ° C./min, and the heating temperature range was −100.
C. to 150C.

Samples 1-2 to 1-4 A cassette tape was prepared in the same manner as in Sample 1-1, except that carbon black having an average particle size shown in Table 1 shown below was used.

Sample 1-5 to Sample 1-8 Tg (U) is 70 ° C. and Tg (L) is 70 ° C.
A cassette tape was produced in the same manner as in Sample 1-1, except that carbon black having an average particle size described in Table 1 shown below was used.

Samples 1-9 to 1-12 Except that carbon black having a Tg (U) of 70 ° C. and a Tg (L) of 100 ° C. and having an average particle size shown in Table 1 below is used. Produced a cassette tape in the same manner as in Sample 1-1.

The binder contained in the magnetic layer was a vinyl chloride copolymer having a glass transition temperature of 70 ° C., a polyurethane resin having a glass transition temperature of 0 ° C., and a binder having a glass transition temperature of 130 ° C. Tg (U) is adjusted by appropriately changing the mixing ratio with a certain polyurethane resin. Also, the binder contained in the non-magnetic layer has its Tg (L) adjusted in the same manner as in the above-mentioned method of adjusting the glass transition temperature of the binder contained in the magnetic layer.

The sample 1 prepared as described above
Various measurements were performed on the cassette tapes Nos. 1 to 1-12, and the following characteristics were evaluated from the obtained measurement results.

[Electromagnetic Conversion Characteristics] A digital video tape recorder (product name: DVW-
A500, manufactured by Sony Corporation.
m, that is, a single frequency signal having a frequency of 7 MHz was simultaneously recorded and reproduced, and the RF conversion was measured to evaluate the electromagnetic conversion characteristics. The RF output was expressed as a relative value of the reproduced output of the magnetic tape of Sample 1-8 as 0 dB.

[Running Durability] As a measuring machine, an 8 mm video deck (trade name: EV-S55, manufactured by Sony Corporation) was remodeled, the algorithm for releasing the still function was removed, and still was continued unless a stop command was issued. In the environment of a temperature of −5 ° C., a cassette tape on which a signal is recorded is still-run, and 3d from the initial output.
Time until B decreases (hereinafter referred to as still time)
Was measured to evaluate the running durability.

The above measurement results are shown in Table 1 together with the average particle size, Tg (U) and Tg (L) of carbon black.

[0062]

[Table 1]

From Table 1, it can be seen that the magnetic layer containing carbon black having an average particle size larger than the thickness of the magnetic layer is provided, and
Samples 1-3 and 1-4 satisfying g (U)> Tg (L) have good RF output, long still time, excellent running durability, and good electromagnetic conversion characteristics. I understood.

On the other hand, the average particle size of the carbon black is equal to or less than the thickness of the magnetic layer, and Tg (U) ≦ Tg
It was found that Sample 1-5, Sample 1-6, Sample 1-9, and Sample 1-10, which are (L), could not obtain good results for each characteristic.

Samples 1-7, 1-8, 1-11 and 1-12 in which the average particle size of the carbon black is larger than the thickness of the magnetic layer but Tg (U) ≦ Tg (L) are satisfied. It was found that the desired RF output could not be obtained.

Further, Samples 1-1 and 1-2 in which Tg (U)> Tg (L) but the average particle size of the carbon black is equal to or less than the thickness of the magnetic layer are Samples 1-3 and Samples It was found that the still time was shorter than that of 1-4.

Accordingly, the magnetic tape is provided with a magnetic layer containing a pigment having a Mohs hardness of 3 or more and less than 6 and having an average particle size larger than the thickness of the magnetic layer, and Tg (U)> Tg
It was found that when the relationship of g (L) was satisfied, running durability was excellent and good electromagnetic conversion characteristics were obtained.

<Experiment 2> In Experiment 2, the average particle size of the carbon black was fixed, and the difference in the characteristics of the magnetic recording medium due to the difference in Tg (U) and Tg (L) was evaluated.

Sample 2-1 to Sample 2-15 : A vinyl chloride copolymer having a glass transition temperature of 70 ° C., and a polyurethane resin having a glass transition temperature of 0 ° C.
The above-mentioned method for adjusting the glass transition temperature of a binder contained in a magnetic layer by using a binder whose Tg (U) is adjusted by appropriately changing the compounding ratio with a polyurethane resin having a glass transition temperature of 130 ° C. Sample 1-except that a binder whose Tg (L) was adjusted in the same manner as described above was used for the nonmagnetic layer.
In the same manner as in Example 1, a cassette tape was produced.

Sample 2 prepared as described above
Various measurements were performed on the cassette tapes of Sample Nos. 1 to 2-15 in the same manner as the above-described measurements, and the characteristics were evaluated. Table 2 shows the measurement results together with Tg (U) and Tg (L).

[0071]

[Table 2]

From Table 2, it was found that the RF output of Sample 2-6 in which Tg (U) ≦ Tg (L) was low.

Here, sample 2-3 and sample 2-
In comparison with Sample No. 4, it was found that Sample 2-3 in which Tg (U) was 100 ° C or higher had a longer still time than Sample 2-4 in which Tg (U) was lower than 100 ° C. Further, according to Sample 1-3 in Table 1, Tg (U) was 130 ° C.
However, it was found that the still time was long.

Further, Sample 2-7 and Sample 2-8
Sample 2 having a Tg (L) of 80 ° C.
-8 was found to have a higher RF output than Sample 2-7 in which Tg (L) exceeded 80 ° C.

Further, comparing Sample 2-14 with Sample 2-15, Sample 2-14 having a Tg (L) of 20 ° C. is better than Sample 2-15 having a Tg (L) of less than 20 ° C. It was also found that the still time was long.

Therefore, the magnetic tape has a Tg (U) of 10
It was found that when the temperature was 0 ° C. or more and 130 ° C. or less and the Tg (L) was 20 ° C. or more and 80 ° C. or less, it had better running durability and better electromagnetic conversion characteristics.

<Experiment 3> In Experiment 3, the difference in the characteristics of the magnetic recording tape due to the difference in the average particle diameter of carbon black and the difference in the content of carbon black was evaluated.

Sample 3-1 to Sample 3-20 Sample 2 except that the content of carbon black and the average particle size of carbon black were as shown in Table 3 below.
Tg (U) is 100 ° C. and Tg
A cassette tape containing a magnetic tape having (L) 70 ° C., a magnetic layer having a thickness of 0.2 μm, and a nonmagnetic layer having a thickness of 1.8 μm was produced.

Sample 3 manufactured as described above
Various measurements were performed on the cassette tapes of Sample Nos. 1 to 3-20 in the same manner as described above, and the characteristics were evaluated. The above measurement results, the content of carbon black,
The results are shown in Table 3 together with the average particle size of carbon black.

[0080]

[Table 3]

The amount of carbon black added is 0.1% by weight.
Samples 3-3 and 3-4 which are less than 60 minutes have a still time of 60 minutes or less and do not have practical running durability. On the other hand, it was found that Samples 3-7 and 3-8 in which the addition amount of carbon black was 0.1% by weight had practically sufficient running durability.

Samples 3-19 and 3-20 in which the addition amount of carbon black exceeds 10% by weight have sufficient running durability, but have low RF output and have desired electromagnetic conversion characteristics. I haven't. On the other hand, Sample 3-1 in which the amount of carbon black added was 10% by weight.
5 and sample 3-16 were found to have practically sufficient electromagnetic conversion characteristics.

Further, samples 3-1, 3-2, and 3-3 where Tg (U)> Tg (L) but the average particle size of carbon black is equal to or less than the thickness of the magnetic layer.
5, sample 3-6, sample 3-9, sample 3-1
0, sample 3-13, sample 3-14, sample 3
It was found that Sample No. -17 and Sample 3-18 were short in still time and inferior in running durability.

Therefore, the magnetic tape has a Mohs' hardness of 3 or more and less than 6, and an average particle diameter of the pigment is larger than the thickness of the magnetic layer, for example, carbon black is added in an amount of 0.1 to 0.1.
It has been found that when the content is 1% by weight or more and 10% by weight or less, excellent running durability and electromagnetic conversion characteristics are surely obtained.

<Experiment 4> In Experiment 4, the difference in the characteristics of the magnetic recording medium due to the difference in the thickness of the magnetic layer was evaluated.

Sample 4-1 to Sample 4-6 except that the thickness of the magnetic layer was as shown in Table 4
In the same manner as in -1, a cassette tape was produced.

Sample 4-7 A sample was formed without forming a nonmagnetic layer. A magnetic paint having the same composition as in Example 1 was directly applied on a nonmagnetic support to produce a magnetic tape having a magnetic layer having a thickness of 3 μm, that is, a so-called single-layer coating type magnetic tape. Then, this magnetic tape was assembled in a cassette to produce a cassette tape.

Sample 4 manufactured as described above
RF output measurements were performed on the cassette tapes 1 to 4-7. Table 4 shows the above measurement results. In addition,
The RF output was shown as a relative value when the RF output of the magnetic tape of Sample 4-7 was set to 0.

[0089]

[Table 4]

From Table 4, it was found that the smaller the thickness of the magnetic layer, the higher the RF output. However, it was found that when the thickness of the magnetic layer was less than 0.03 μm, application was difficult. The thickness of the magnetic layer is 0.3 μm
It was found that Sample 4-5 had better RF output than Sample 4-6 in which the thickness of the magnetic layer exceeded 0.3 μm. Therefore, it was found that when the thickness of the magnetic layer was 0.03 μm or more and 0.3 μm or less, more excellent electromagnetic conversion characteristics were obtained.

[0091]

According to the magnetic recording medium of the present invention, the magnetic layer contains a pigment having a Mohs hardness of 3 or more and less than 6, and the average particle size of the pigment is larger than the thickness of the magnetic layer. This results in a strong coating film. Further, in this magnetic recording medium, when the glass transition temperature of the binder contained in the magnetic layer is Tg (U) and the glass transition temperature of the binder contained in the intermediate layer is Tg (L), Tg ( U)> Tg
(L), so that even if a pigment having an average particle size larger than the thickness of the magnetic layer is added to the magnetic layer, the amount of the pigment exposed on the surface of the magnetic layer by a surface treatment such as a calendar treatment is optimized. It is possible.

Therefore, in this magnetic recording medium, the physical properties of the coating film such as abrasion resistance and still durability in the magnetic layer and the intermediate layer are remarkably improved, the surface smoothness of the magnetic layer is improved, and the spacing loss is improved. , And has excellent running durability while maintaining good electromagnetic conversion characteristics.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration example of a magnetic recording medium.

[Explanation of symbols]

 1 Non-magnetic support, 2 Intermediate layer, 3 Magnetic layer

Claims (6)

[Claims] 1. A non-magnetic support, comprising a magnetic powder and / or a non-magnetic powder and a binder, an intermediate layer formed on the non-magnetic support, a ferromagnetic powder and a binder, A magnetic recording medium comprising a magnetic layer formed on the intermediate layer, wherein the magnetic layer contains a pigment having a Mohs hardness of 3 or more and less than 6, and the average particle size of the pigment is larger than the thickness of the magnetic layer. Tg is the glass transition temperature of the binder contained in the magnetic layer.
(U), wherein Tg (U)> Tg (L), where Tg (L) is the glass transition temperature of the binder contained in the intermediate layer. 2. The magnetic recording medium according to claim 1, wherein the pigment is carbon black. 3. The magnetic recording medium according to claim 1, wherein the thickness of the magnetic layer is not less than 0.03 μm and not more than 0.3 μm. 4. The above Tg (U) is 100 ° C. or higher and 130
2. The magnetic recording medium according to claim 1, wherein the temperature is lower than or equal to ° C. 5. The magnetic recording medium according to claim 1, wherein said Tg (L) is 20 ° C. or more and 80 ° C. or less. 6. The magnetic recording medium according to claim 1, wherein the magnetic layer contains a polyurethane resin as a binder.