JPH01220218A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain good output in the entire frequency range by incorporating cobalt-modified iron oxide having the specified values of average major axis lengths and ratios of Fe<3+> and Fe<2+> and forming magnetic layers in such a manner that a 1st magnetic layer and the 2nd magnetic layer having specific coercive forces, residual magnetic flux densities and layer thicknesses are superposed. CONSTITUTION:The 1st magnetic layer contains the cobalt-modified iron oxide having the average major axis length in a 0.27-0.35mum range and the ratio of the Fe<3+> and Fe<2+> in a 100:1-100:2.6 molar ratio range. The coercive force of said layer is in a 360-440Oe range, the residual magnetic flux density is >=1,800G and the layer thickness is in a 4.0-5.7mum range. The 2nd magnetic layer contains the cobalt-modified iron oxide having the average major axis length in a 0.17-0.27mum range and the ratio of the Fe<3+> and Fe<2+> in a 100:5.0-100:6.5 molar ratio range. The coercive force is in a 550-720Oe range, the residual magnetic flux density is >=1,700G and the layer thickness is in a 0.3-1.2mum range. The good output is thereby obtd. in the entire frequency band.

Description

[Detailed description of the invention] [Field of invention] The present invention relates to a magnetic recording medium.

More specifically, the present invention relates to a magnetic recording medium having at least two magnetic layers.

[Background of the Invention] In recent years, there has been a demand for audio cassette tapes that have low noise levels, well-balanced frequency characteristics, and good output characteristics in all frequency bands. Recently, music sources have become increasingly digital, such as compact discs, and are becoming ultra-high-fidelity and extremely low-noise, so the above-mentioned demands are becoming stronger for audio cassette tapes as well.

Furthermore, with regard to video tapes, there is a demand for tapes with higher video output and lower noise levels than ever before.

To achieve these performances, for example, when iron oxide-based magnetic powder is used as the magnetic powder used in magnetic recording media, the noise level is reduced by using magnetic powder with a short average major axis length. There are known ways to do this. However, although this method can reduce the noise level, the frequency characteristics are still not satisfactory.

A multilayer magnetic tape consisting of two magnetic layers has been proposed as an effective method for improving these frequency characteristics. range, the residual magnetic flux density is in the range of 1600 to 2000G, and the coercive force of the lower layer is 40
Range of 0 to 700 Oe, residual magnetic flux density of 1600 to 20
00G range, and CO compound-coated iron oxide or Co and Fe compound-coated iron oxide is used as the ferromagnetic powder, and the upper layer has a BET specific surface area of 2.
8 to 45rn'/g, and the lower layer contains B of the above iron oxide.
Those with an ET method specific surface area of 18 to 25 rrf/g are used. It is claimed that by adopting such a configuration, it is possible to obtain a magnetic recording medium that has excellent reproduction output and low noise in all frequency bands.

However, in the above magnetic recording medium.

The difference in coercive force between the upper and lower layers is too large, so the thickness of the upper layer is 1
When using a thick film of 4 m or more, especially for audio cassette tapes, the bias value increases, and the frequency response becomes sagging (lower output in the midrange) and the distortion rate increases.
Furthermore, the low frequency MOL also decreases.

Furthermore, coarse-grained ferromagnetic powder with a relatively small specific surface area is used in the lower layer of the magnetic recording medium. For this reason, the surface smoothness of the lower layer is not sufficient, and even if a magnetic layer with high coercive force is provided on this layer, the smoothness of the upper layer will also be poor, making it difficult to obtain excellent high-frequency output. Can not.

In order to obtain this smoothness, if the upper layer is made thick enough to modify the surface condition of the lower layer, it will cause a decrease in low-frequency output, a sagging phenomenon in the middle, an increase in distortion rate, and a decrease in erasing characteristics as described above. Although the upper layer has a high coercive force, since fine particles of ferromagnetic powder are used, if the layer thickness becomes too thick, the transfer characteristics will deteriorate.

Therefore, in the above magnetic recording medium, there is a large difference in coercive force between the upper and lower layers, the surface smoothness of the lower layer is not good, and the thickness of the upper layer is thick, so the frequency characteristics, strain rate, transfer characteristics, etc. It cannot be said that it has been sufficiently improved.

[Object of the Invention] An object of the present invention is to provide a magnetic recording medium such as a magnetic tape that has good output in all frequency bands, low bias noise, and high sensitivity.

In particular, it is an object to provide an audio cassette tape that has no middle sag (reduction in output in the middle range) in frequency characteristics, has a high maximum output level (MOL) in the ultra-treble range, and has a high MOL in the low and middle ranges.

Furthermore, the present invention provides an audio cassette tape with low bias noise and excellent transfer characteristics.

[Summary of the Invention] The present invention provides a magnetic recording medium in which a first magnetic layer and a second magnetic layer are provided in this order on the surface of a non-magnetic support, in which the first magnetic layer has a The magnetic powder has an average major axis length in the range of 0.27 to 0.35 μm, and a molar ratio of Fe'+ to Fe2◆ of 100:I.
N100: contains cobalt-modified iron oxide in the range of 2.6, and the coercive force (HC1) of the first magnetic layer is 360 to 4.
40 Oe, its residual magnetic flux density (Br1) is 1800 G or more, and its layer thickness is 4.0 to 5.7
gm, and the second magnetic layer has an average major axis length of 0.17 to 0.27.
the second magnetic layer comprises cobalt-modified iron oxide having a value in the range of μm and a molar ratio of Fe 3+ to Fe2◆ in the range of 100:5.0 to 100:6.5; The coercive force (He2) is in the range of 550 to 720 Oe, the residual magnetic flux density (Br1) is 1700 G or more, and the layer thickness is in the range of 0.3 to 1.2 μm. Located on magnetic recording media.

Preferred embodiments of the magnetic recording medium of the present invention are as follows.

■) Coercive force (He1) of the first magnetic layer is 330-430O
The magnetic recording medium described above is characterized in that the magnetic recording medium is in the range of e.

2) The coercive force (He2) of the second magnetic layer is 550 to 720
The magnetic recording medium described above is characterized in that the magnetic recording medium is in the range of Oe.

3) The magnetic recording medium described above, wherein the second magnetic layer has a layer thickness in a range of 0.5 to 1.0 μLm.

[Detailed Description of the Invention 1 The present invention is a magnetic recording medium basically comprising a first magnetic layer and a second magnetic layer provided in this order on the surface of a non-magnetic support.

The first magnetic layer has an average major axis length in the range of 0.27 to 0.35 pLm as a ferromagnetic powder contained in the magnetic layer, and the ratio of Fe3+ to Fe2° is 100 in molar ratio. :1
Contains cobalt-modified iron oxide in the range of ~100:2.6. The coercive force (HC1) of the first magnetic layer containing the cobalt-modified iron oxide is in the range of 360 to 440 Oe, and the residual magnetic flux density (Br1) is 18
00G or more, and the layer thickness is 4.0 to 5.0G. It is in the range of full m.

The second magnetic layer is made of ferromagnetic powder with an average major axis length of 0.
It has a value in the range of 17 to 0.2 full m, and the ratio of Fe 3+ to Fe 3+ is loO in molar ratio: 5.0 to 100.
: Contains cobalt-modified iron oxide in the range of 6.5. The coercive force (Hez) of the second magnetic layer containing cobalt-modified iron oxide is in the range of 550 to 720 Oe, the residual magnetic flux density (Br+) is 1700 G or more, and the layer thickness is 0. It is characterized by being in the range of .3 to 1.2 μm.

That is, the configuration of the magnetic recording medium of the present invention is as follows.

First, for the first magnetic layer which is the lower layer: [1] In order to obtain a smooth surface, a cobalt-modified iron oxide having a relatively small average major axis length (ie, a large BET specific surface area) is used.

[2] The above-mentioned cobalt-modified iron oxide having a large average major axis length is
Since the transcription characteristics tend to be inferior, Fe'ゝ and Fe3
The transfer characteristics are improved by reducing the ratio of Fe'' so that the molar ratio of

[3] By using the above cobalt-modified iron oxide having a large average major axis length, the magnetic susceptibility as a ferromagnetic powder is made as high as possible, and the degree of filling of the ferromagnetic powder in the resulting magnetic layer is increased.
It also improves magnetic flux density. However, the value of the average major axis length is suppressed to the extent that it does not adversely affect the transfer characteristics in [2].

It has the purpose and structure described above.

The second magnetic layer, which is the upper layer, has: [1] In order to have excellent frequency characteristics, the average major axis length of the cobalt-modified iron oxide is a small value (i.e., BET
Use one with a large normal surface.

[2] The layer thickness should be 1.2 gm or less in order to have excellent frequency characteristics and particularly high output in high frequencies.

[3] Magnetic layers using cobalt-modified iron oxide with a large average major axis length tend to have poor transfer characteristics, but because the upper layer accounts for a small proportion of the total magnetic layer thickness, the transfer characteristics of the lower layer are less effective. dominates the entire magnetic layer. Therefore, F e
The molar ratio of 3+ and Fe2' also improves the transfer characteristics by reducing the ratio of Fe2+, but such settings are not made, and Zoo: 5.0 ~ 5.0 is set to obtain a high magnetic susceptibility.
100:6.5 in order to improve electromagnetic conversion characteristics, that is, output and sensitivity in all frequency bands.

It has the purpose and structure described above.

In the magnetic recording medium of the present invention, which is composed of two magnetic layers (upper and lower), the lower layer has particularly good surface smoothness and good transfer characteristics, and the upper layer has good output in all frequency bands and has low sensitivity. It has high isoelectromagnetic conversion characteristics. By dividing the roles in this way, the above-mentioned various performances of the entire magnetic layer are obtained.

Specifically, the first magnetic layer (lower layer) has an average major axis length of 0.27 to 0.35μ as ferromagnetic powder contained in the magnetic layer.
m (BET ratio slope surface area is 25~31rr1''
/g) (7) with values in the range F e 3+ and F e
It contains cobalt-modified iron oxide whose molar ratio to 3+ is in the range of 100:1-Zoo:2.6. If the average major axis length is less than 0.27 μm, sufficient transfer characteristics cannot be obtained even if the Fe' ratio is reduced, and if it exceeds 0.35 gm, the surface smoothness deteriorates. . and Fe2
When the molar ratio of Fe is in the range of Zoo:1 to 100:2.6 and the molar ratio of Fe is less than 1, the σS of the ferromagnetic powder decreases and the maximum magnetic flux when it becomes a magnetic layer decreases. Density etc. deteriorate, and if it exceeds 2.6, the transfer characteristics will be poor.

Coercive force of the first magnetic layer containing this cobalt-modified iron oxide (
HC1) has a value in the range of 360 to 440 Oe, and its residual magnetic flux density (Br1) is 1800 G or more,
Furthermore, the layer thickness is in the range of 4.0 to 5.7 μm. When the coercive force (HC1) is less than 360 Oe, the high frequency output is low, and when it exceeds 440 Oe, the bias value increases and the low frequency output decreases. Preferably it is in the range of 370 to 430 Oe. Also, the residual magnetic flux density (Br+) is.

When it becomes less than 1800G, the low frequency output decreases. moreover,
If the layer thickness is less than 4.0 gm, it is difficult to secure low-frequency output.
If it exceeds 5.7 μm, the output will be saturated and become excessive.

Further, the second magnetic layer, which is the upper layer, has an average major axis length in the range of 0.17 to 0.27 μm as a ferromagnetic powder, and has F
The molar ratio of e 3+ and Fe 3+ is 100:5.
Contains cobalt-modified iron oxide in the range of 0 to 100:6.5. If the average long axis length is less than 0.1 full m, the particles are too fine, and the degree of filling of the ferromagnetic powder in the magnetic layer is reduced, and the orientation is also deteriorated, resulting in a reduction in Br, output, sensitivity, etc. do. If it exceeds 0.35 μm, bias noise will increase. Also, Fe3ゝ and Fe3
In the molar ratio of Zoo:5 to 100:6.5, when the molar ratio of Fe'' becomes less than 5, the σS of the ferromagnetic powder decreases, and the maximum magnetic flux density when it becomes a magnetic layer decreases. If it exceeds 6.5, the transfer characteristics will be poor.

Coercive force of the second magnetic layer containing this cobalt-modified iron oxide (
He2) has a value in the range of 550 to 720 Oe, the residual magnetic flux density (Br1) is 1700 G or more, and the layer thickness is in the range of 0.3 to 1.21 Lm. When the coercive force (Hc+) is less than 550 Oe, the high-frequency output is low;
When it exceeds 720 Oe, the low frequency output becomes low, the distortion rate increases, and the erasure rate also decreases. Preferably 600-70
It is in the range of 0 Oe. Moreover, when the residual magnetic flux density (Br1) becomes less than 1700G, the low frequency output decreases. Furthermore, the thickness of one layer is 0.31Lm! - At full capacity, the effect of using fine ferromagnetic powder in the upper layer becomes difficult to achieve, bias noise increases, and high-frequency output decreases. When it exceeds 1.2 μm, the distortion rate increases and the low-frequency output decreases.

Preferably, the layer thickness is 0.5 to 1. It is in the range of 0 μm.

The method for producing cobalt-modified iron oxide used in the present invention, in which the ratio of Fe 3+ to Fe 3+ is within the above range, is already known, and is carried out, for example, as follows.

First, γ-Fe, O, is dispersed in water, and C.

S04 and Fe50. Add NaOH to the colander to make it alkaline and heat. Then, the obtained surface treated γ-Fe 20. γ-F modified with Co and Fe by washing with water, heating and drying in nitrogen gas
Obtain e2O3. During the above steps, the ratio of Fe'+ to Fe2° is the same as Coco and Fe50. This can be done by changing the ratio and amount.

The magnetic recording medium of the present invention is manufactured, for example, as follows.

The non-magnetic support used in the present invention is polyethylene terephthalate (PET), t! From polyesters such as polyethylene naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, vinyl resins such as polyvinyl chloride and polyvinylidene chloride, synthetic resins such as polycarbonate, polyamide, polyamideimide, and polyimide. non-magnetic metal foil such as aluminum or copper; metal foil such as stainless steel foil; paper, ceramic sheet, etc.

The magnetic layer in the magnetic recording medium of the present invention is a layer in which ferromagnetic powder is dispersed in a binder.The ferromagnetic powder used in the present invention is cobalt-modified iron oxide, but if desired, other ferromagnetic There are no particular restrictions on the use of powders, examples include γ-Fe203. Fe, 04, CO gold-containing 7) Fe Co4, Cry□, Co-N1-P alloy, Fe-Co-Ni alloy, and other known ferromagnetic powders can be mentioned.

The binder solution for producing the magnetic paint used in the present invention is a binder solution containing a resin component, a solvent, and, if necessary, a lubricant and an abrasive.

The resin component is a conventionally known thermoplastic resin, thermosetting resin,
Alternatively, reactive resins or mixtures thereof are used. s1
Examples of one component include vinyl chloride copolymers (e.g., vinyl chloride/vinyl acetate copolymer, vinyl chloride/vinyl acetate/vinyl alcohol copolymer, vinyl chloride/vinyl acetate/acrylic acid copolymer, chloride Vinyl/vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, ethylene/vinyl acetate copolymer, -3O, Na or -3O
(vinyl chloride copolymers into which polar groups such as 2Na and epoxy groups have been introduced), cellulose derivatives such as nitrocellulose resins, acrylic resins, polyvinyl acetal resins, polyvinyl butyral resins, epoxy resins, phenoxy resins, polyurethane resins (e.g. , polyester polyurethane resin, polyurethane resin into which a polar group such as -So, Na or -5O2Na is introduced, and polycarbonate polyurethane resin).

Furthermore, when a curing agent is used, a polyisocyanate compound is usually used. The polyisocyanate compound is selected from those commonly used as curing agent components for polyurethane resins and the like.

Further, when performing curing treatment by electron beam irradiation, a compound having a reactive double bond (eg, urethane acrylate) can be used.

Examples of solvents used in the production of magnetic paints include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, ethyl isobutyl ketone, and cyclohexanone; esters such as methyl acetate, ethyl acetate, butyl acetate, and acetic acid glycol monoethyl ether; Glycol ethers such as ether, glycol dimethyl ether and dioxane; Aromatic hydrocarbons such as benzene, toluene and xylene; Ethylene cucolide, ethylene cucolide,
Carbon tetrachloride, chloroform, ethylene chlorohydrin,
Examples include chlorinated JI and hydrogen chloride such as dichlorobenzene, and these solvents can be used alone or in combination. Particularly preferred are polar solvents such as ketones or solvents containing polar solvents.

The magnetic powder is uniformly kneaded and dispersed with the binder solution. This kneading 1 dispersion can be carried out using a two-roll mill, a three-roll mill, an open knee goo, a pressurized knee goo, or a continuous kneader 1.
Generally, a method is used in which pre-dispersion is performed using a twin-screw continuous kneading mixer or the like, followed by post-dispersion using a sand grinder, ball mill, or the like.

Since it is necessary to obtain a high residual magnetic flux density for the magnetic paint used in the present invention, a twin-screw continuous cross-talk mixer is particularly preferred among the above devices as the device used for kneading the cobalt-modified iron oxide. By using the above, the ferromagnetic powder can be mixed (pre-dispersed) while applying a high shearing force.Furthermore, when diluting this mixed material, perform a fine dispersion process and then perform the above dispersion process. It is preferable that

Magnetic paint contains abrasives, lubricants,
Of course, any of various additives such as dispersants and antistatic agents may be added depending on the purpose.

Next, a method for manufacturing the magnetic recording medium of the present invention will be described. For coating, a magnetic paint prepared by the above-mentioned crosstalk dispersion method using the above-mentioned materials is applied onto a non-magnetic support by the following method. First, magnetic layer forming components such as the above-mentioned specific cobalt-modified iron oxide as a resin component and ferromagnetic powder for the first magnetic layer and a hardening agent blended as desired are mixed and dispersed together with a solvent as described above to form the first magnetic layer. Prepare a coating solution for A coating liquid for the second magnetic layer is prepared in the same manner for the second magnetic layer.

The method for producing a magnetic recording medium of the present invention involves applying a coating liquid for a first magnetic layer onto the surface of a non-magnetic support while it is running, and then applying a coating liquid continuously on top of the coating layer while the coating layer is in a wet state. The coating liquid for the second magnetic layer is applied so that the layer thickness of the second magnetic layer after drying is 0.3 to 1.
．． It is preferable to apply the coating so that it is within the range of 2 mm.The method of continuously coating the two layers is, for example, when a reverse roller is used as the coating machine, the reverse roller is applied so that the non-magnetic support under running is sandwiched between the two layers. The first magnetic layer may be installed in succession and applied, or the first magnetic layer may be placed at intervals within a range that allows the first magnetic layer to remain wet (that is, the applied layer still contains solvent and exhibits tackiness). Two units may be installed and applied.

In addition to reverse roller, methods for applying magnetic paint include air doctor coating, blade coating, rod coating, extrusion coating, air knife coating, squeeze coating, impregnation coating, transfer roll coating, gravure coating, kiss coating, cast coating, and spray coating. , spin codes and other methods are available.

The coating layer of the magnetic paint is applied so that the thickness of the magnetic layer of the obtained magnetic recording medium (the total layer thickness of the first magnetic layer and the second magnetic layer) is usually within the range of 0.5 to 10 gm. applied.

A backing layer may be provided on the side of the non-magnetic support used in the present invention that is not coated with magnetic paint. Normally, the back layer is the side of the non-magnetic support that is not coated with magnetic paint. This is a layer formed by applying a back layer-forming paint in which particulate components such as abrasives and antistatic agents and a binder are dispersed in an organic solvent.

Note that an adhesive layer may be attached to the surface of the nonmagnetic support on which the magnetic paint and the back layer forming paint are applied.

Usually, the coated layer of magnetic paint is dried after being subjected to a treatment for orienting the ferromagnetic powder contained in the coated layer of magnetic paint, that is, a magnetic field orientation treatment.

After being dried in this way, the coating layer is subjected to a surface smoothing treatment using, for example, a super calender roll, which removes the solvent during drying. Since the voids generated by the removal are eliminated and the filling rate of the ferromagnetic powder in the magnetic layer is improved, a magnetic recording medium with high electromagnetic conversion characteristics can be obtained.

The thus cured laminate is then cut into a desired shape.

The cutting can be carried out under normal conditions using a normal cutting machine such as a slitter.

Although the magnetic recording medium of the present invention has been described as having an upper and lower two-layer system, it may have three or more layers as a whole as long as it maintains the above specified properties and includes two magnetic layers.

[Effects of the Invention] The magnetic recording medium of the present invention includes cobalt-modified iron oxide having the average major axis length values and the ratio of Fe 3+ and Fe2° specified as described above, and the magnetic layer A first magnetic layer (lower layer) and a second magnetic layer (upper layer) having a specific coercive force (Hc2) and a specific residual magnetic flux density and a specific layer thickness are layered.

That is, in the magnetic recording medium of the present invention, which is composed of the above-mentioned two upper and lower magnetic layers, the lower layer has particularly good surface smoothness and good transfer characteristics, and the upper layer has good output and high sensitivity in all frequency bands. It has equal electromagnetic conversion characteristics. By dividing the roles in this manner, it is possible to obtain the above-mentioned extremely excellent performances of the entire magnetic layer, that is, good output in all frequency bands, low bias noise, high sensitivity, and excellent transfer characteristics.

Next, an example of the present invention and a comparative example will be described. In addition,
In the following examples, 1 part 1 means r parts by weight, unless otherwise specified.

[Example 1] The cobalt-modified iron oxide used as the ferromagnetic powder is shown in Table 1 for the first magnetic layer and in Table 2 for the second magnetic layer.

Table 1 Ferromagnetic Hc Average major axis length Fe2°/Fe'' Magnetic powder Tomei (Oe) (μm) (X100
) Paint name a 360 0.30 2.3
Ab 380 0.30 2.3
Bc 410 0.30 2.3
Cd 440 0.30
2.3 De 465 0.30
2.3 Ef 410 0.25 2.
2 Fg 410 0.27 2.3
Gh 410 0.34 2.3 Hi
410 0.37 2.4 I
'j 410 0.30 0.5 Jk
410 0.30 1.1 K+ 4
10 0.30 2.6 Lm 410
0, 30 2, 9 M Table 2 Ferromagnetic Hc Average major axis length F e ” / F e
” Magnetic powder Tomei (Oe) (μm)
(X100) Paint name n 560 0.23
6.0 No 600 0.23 6
．． 0 0p 680 0.23 B, I
Pq 740 0.23 6. IQ
r 770 0.23 6.2 Rs
680 0.15 5.9
SL 680 0.17 6. OTu
680 0.27 6.1 'Uv 680
0.30 6.2 Vw 680 0
．． 23 4.7 Wx 680 0.23
5.2 Xy 680 0.2
3 6.4 Yz 680
0.23 8.7 Magnetic material for second magnetic material was manufactured through the following four steps.

Cross-wire step: 100 parts of cobalt-modified iron oxide C for the first magnetic layer, 15 parts of hydroxyl group-containing vinyl chloride, vinyl acetate, copolymer (electrified vinyl 100OG: manufactured by Denki Kagaku Kogyo), 13 parts of methyl ethyl ketone, 13 parts of butyl acetate.
The mixture was kneaded while being continuously added to a twin-screw continuous mixer (KRC kneader T-4: manufactured by Kurimoto Iron Works) to obtain a mixed material.

Kneading and diluting step: 141 parts of the above mixed material, 3 parts of polyurethane resin (manufactured by Dainippon Ink and Chemicals: Crispon 7209, concentration 45%), 28 parts of methyl ethyl ketone, 28 parts of butyl acetate.
The mixture was continuously added to a twin-screw continuous mixer and then kneaded and diluted to obtain a diluted product.

Mixing and dispersion step and fine dispersion step; Above diluted product 200 parts Oleic acid 0.5 part Myristic acid 2.0 parts Dimethylpolysiloxane (degree of polymerization 60) 0.2 parts Methyl ethyl ketone
50 parts butyl acetate
57 parts were mixed and dispersed using Daysilver, then finely dispersed using a sand grinder, and the resulting dispersion was filtered through a filter having an average pore size of Igm to obtain the magnetic paint C for the first magnetic layer. Manufactured.

Magnetism/material for second magnetic layer It was manufactured using the following four steps.

Kneading step; Cobalt-modified iron oxide for second magnetic layer 100 parts Hydroxyl group-containing vinyl chloride Vinyl acetate 15 parts Copolymer (
Denka Vinyl 100OG: manufactured by Denki Kagaku Kogyo ■) Carbon black (average particle size: 30 mg) 1 part methyl ethyl ketone 13 parts cyclohexanone 13 parts The above composition was kneaded by continuously adding it to a twin-screw continuous mixer to form a mixer. I got it.

Kneading and diluting step: 142 parts of the above kneaded product, 3 parts of polyurethane resin (manufactured by Dainippon Ink & Chemicals: Crispon 7209, concentration 45%), 28 parts of methyl nityl ketone, 28 parts of butyl acetate.
The mixture was continuously added to a two-wheeled continuous kneading mixer and then kneaded and diluted to obtain a diluted product.

Mixing and dispersion step and fine dispersion step; Above diluted product 201 parts Oleic acid 0.5 parts Myristic acid 2.0 parts Dimethylpolysiloxane (degree of polymerization 60) 0.5 parts a-AfL20sC average particle size: 0.5 μm) l , 0 parts methyl ethyl ketone 50 parts butyl acetate
56 parts were mixed and dispersed using Daysilver, then finely dispersed using a sand grinder, and filtered through a filter having an average pore size of 1 μm to produce a magnetic paint N for a second magnetic layer.

The magnetic paint C for the first magnetic layer was coated on a polyethylene terephthalate support with a thickness of 6.3 μm so that the layer thickness after drying was 5 μm, and while the coated layer was in a sticky state, the second magnetic layer was coated. Apply magnetic paint N for the second magnetic layer so that the layer thickness after drying is 0.8 lm, perform magnetic field orientation treatment while it is in a sticky state, and dry to form a magnetic layer. did. After that, a super calender treatment was performed. and 3
．． An audio cassette tape was produced by slitting the tape to a width of 8 mm.

[Examples 2 to 7] Audio cassette tapes were produced in the same manner as in Example 1, except that the magnetic paint for the first magnetic layer and the magnetic paint for the second magnetic layer shown in Table 3 were used in Example 1. did.

[Comparative Examples 1 to 6] Audio cassette tapes were produced in the same manner as in Example 1, except that the magnetic paint for the first magnetic layer and the magnetic paint for the second magnetic layer shown in Table 3 were used in Example 1. did.

[Reference Example 1] In Example 1, C was used as the magnetic paint for the first magnetic layer and coated on the support so that the layer thickness after drying was 5.8 μm, and the magnetic paint for the second magnetic layer was An audio cassette tape was produced in the same manner as in Example 1, except that no coating was applied.

In the above Examples 1 to 7 and Comparative Examples 1 to 6, the influence of the composition of the second magnetic layer, which is the upper layer, was particularly observed. Performance was shown compared to the single-layer magnetic layer of Reference Example 1.

Table 3 shows the constitution of each of the above and the physical properties obtained from the evaluation method described below.

The measurement conditions for the above physical properties are as follows.

The measured magnetic field (Hm
) At 2 kOe, coercive force (He), residual magnetic flux density (
The value of Br) was measured.

Frequency characteristics: -20 dB human power level/l in no-pull position
/ Each frequency in (315Hz, 1kHz, Ei,
The output level was measured at 3kHz, 10kHz, 16kHz) and expressed as a relative value when the output level of the tape of Reference Example 1 was expressed as OdB.

1 piece ff1i millet top (MOL) Commercially available measuring tape deck (manufactured by Nakamichi ■, model 582)
MOL315 in normal position using
Each characteristic of MOLlo and MOL16 was measured.

(1) MOL315 The maximum output when the third-order high frequency of the 315 Hz signal becomes 3% was investigated. Note that the values shown are relative values when the MOL315 value of the tape of Reference Example 1 is set as OdB.

(2) We investigated the saturation output of a 10kHz signal on MOLlo. Note that the values shown are OdB of the SQL 10k value of the tape in Reference Example 1.
This is the relative value when .

(3) We investigated the saturation output of a 16kHz signal on MOL16. Note that the values shown are relative values when the value of 5QL16 of the tape of Reference Example 1 is set as OdB.

Bias Noise (BN) Bias noise is the output level after passing through the auditory correction circuit. The values listed are the BN of the tape in Reference Example 1.
It is a relative value when the value of is OdB.

Transfer characteristics After recording a 1 kHz signal within one rotation of the reel, no signal recording was performed for 10 rotations, and this was repeated three times. This tape was left at a temperature of 30±1° C. for 24 hours and then played back, and the playback output levels of the original signal and the transferred signal were measured, and the difference therebetween was determined.

兇” The ratio of third-order high-frequency distortion to the reproduction output level when a 315Hz OdB signal is input is shown.

In the following Examples 8 to 13 and Comparative Examples 7 to 12, the influence of the composition of the first magnetic layer, which is the lower layer, was particularly examined. Performance was shown compared to the single-layer magnetic layer of Reference Example 1.

[Examples 8 to 13] Audio cassette tapes were produced in the same manner as in Example 1, except that the magnetic paint for the first magnetic layer and the magnetic paint for the second magnetic layer shown in Table 4 were used in Example 1. did.

[Comparative Examples 7 to 12] Audio cassette tapes were produced in the same manner as in Example 1, except that the magnetic paint for the first magnetic layer and the magnetic paint for the second magnetic layer shown in Table 4 were used in Example 1. did.

Table 4 shows the configuration of each side and the physical properties obtained from the evaluation method described above.

In the following Examples 14 to 17 and Comparative Examples 13 to 17,
In particular, we looked at the layer thicknesses of the first magnetic layer and the second magnetic layer. Performance was shown compared to the single-layer magnetic layer of Reference Example 1.

[Examples 14 to 17] Audio cassette tapes were produced in the same manner as in Example 1, except that the magnetic paint for the first magnetic layer and the magnetic paint for the second magnetic layer shown in Table 5 were used in Example 1. did.

[Comparative Examples 13 to 17] Audio cassette tapes were produced in the same manner as in Example 1, except that the magnetic paint for the first magnetic layer and the magnetic paint for the second magnetic layer shown in Table 5 were used in Example 1. did.

Table 5 shows the configuration of each side and the physical properties obtained from the evaluation method described above.

As mentioned above, Table 3 shows the second magnetic layer (particularly the carbon content).
Types of O-modified iron oxide), Table 4 shows the first magnetic layer (same as above),
Table 5 shows the layer thickness and its effect on physical properties.

As is clear from these tables, a specific CO-modified iron oxide is used, and the upper and lower two layers each have specific magnetic properties,
The magnetic recording medium of the present invention having a specific layer thickness can ensure all the performances of frequency characteristics, maximum output, distortion rate, bias noise, and transfer characteristics in a well-balanced manner.

Patent applicant: Fuji Photo Film Co., Ltd. Agent: Yasushi Yanagawa, patent attorney: Procedural amendments filed on December 23, 1988

Claims (1)

[Claims] 1. A magnetic recording medium comprising a first magnetic layer and a second magnetic layer provided in this order on the surface of a non-magnetic support, wherein the first magnetic layer is composed of a ferromagnetic material contained in the magnetic layer. As a powder, the average long axis length is in the range of 0.27 to 0.35 μm, and the molar ratio of Fe^3^+ to Fe^2^+ is 10.
The first magnetic layer contains cobalt-modified iron oxide in the range of 0:1 to 100:2.6, and the coercive force (HC_1) of the first magnetic layer is 3.
The residual magnetic flux density (B
r_1) is 1800G or more, and the layer thickness is 4.
and the second magnetic layer has an average major axis length of 0.17 to 0.27 μm.
a cobalt-modified iron oxide having a value in the range of μm and a molar ratio of Fe^3^+ to Fe^2^+ in the range of 100:5.0 to 100:6.5, and The coercive force (Hc_2) of the second magnetic layer is in the range of 550 to 720 Oe, the residual magnetic flux density (Br_1) is 1700 G or more, and the layer thickness is in the range of 0.3 to 1.2 μm. A magnetic recording medium characterized by.