JPS59144042A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To ensure high sensitivity with every frequency band by providing the 1st magnetic layer containing metallic magnetic powder having a specific surface area set within a specific range and the 2nd magnetic layer containing metallic magnetic powder having a specific surface area and setting the coercive force set within a specific range for both layers with a small difference of coercive force between both layers. CONSTITUTION:The 1st magnetic layer containing the metallic magnetic powder having a 20-30m<2>/g specific surface area (BET method) with 1,000-1,300 Oe coercive force is formed on a substrate. Then the 2nd magnetic layer containing the metallic magnetic powder having >=32m<2>/g specific surface area (BET method) with the coercive force equal to the 1st magnetic layer is formed on the 1st layer. The thickness ratio is set in a (1:1)-(1:10) range between the 2nd and 1st magnetic layers, and at the same time the difference of coercive force is set at <=20 Oe between both layers. Thus it is possible to obtain a magnetic recording medium which is free from any slump with the frequency characteristics and has high sensitivity with every frequency band with a low level of noises.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic recording medium having a two-layer magnetic layer structure, and its object is to reduce sag in frequency characteristics (
The object of the present invention is to provide b) a magnetic recording medium that has good frequency characteristics and sensitivity in all frequency bands and has low noise.

It has long been known that in magnetic recording media with a two-layer magnetic layer structure, sensitivity and frequency characteristics in low-frequency and high-frequency bands can be improved by lowering the coercive force of the lower layer and increasing the coercive force of the upper layer. ing. It is also known that the sensitivity and frequency characteristics can be further improved by using metal magnetic powder with a large amount of saturation magnetization as the magnetic powder in these magnetic layers, and such magnetic recording media usually have a substrate such as polyester film. First, a magnetic paint containing metal magnetic powder with a relatively low coercive force is applied on top and dried to form a lower magnetic layer, and then a metal magnetic powder with a relatively high coercive force is applied on top of this lower magnetic layer. Apply magnetic paint,
It is made by drying to form an upper magnetic layer.

However, in this type of conventional magnetic recording medium, the sensitivity and frequency characteristics in low and high frequency bands are improved by making the coercive force of the upper magnetic layer higher than that of the lower magnetic layer, and the sensitivity and frequency characteristics in the low frequency and high frequency bands are improved. The larger the difference in coercive force between the ceramic layers, the better the sensitivity and frequency characteristics obtained. On the other hand, as the difference in coercive force between the ceramic layers increases, a sagging phenomenon occurs in the frequency characteristics, and the frequency characteristics in the midrange gradually deteriorate. There are some difficulties.

On the other hand, in order to improve the electromagnetic conversion characteristics, metal magnetic powder with a large amount of saturation magnetization is also used by incorporating it into a single magnetic layer. Because of the high magnetic flux density, excellent electromagnetic conversion characteristics such as high sensitivity and output can be obtained, and the mid-range frequency characteristics do not deteriorate as in the case of magnetic recording media with a two-layer magnetic layer structure. However, when this type of metal magnetic powder is used, noise cannot be sufficiently reduced. For this reason, noise has been reduced by using fine metal magnetic powder with a large specific surface area, but in this case, although the noise can be sufficiently reduced, the particles are so fine that the orientation is poor. However, the problem is that the low-frequency output level decreases.

The inventors conducted various studies in view of the current situation, and found that a metal magnetic powder having a specific surface area of 20 to 32 rrr/g by the BET method was used as the magnetic powder for the lower magnetic layer, and a magnetic powder for the upper magnetic layer was used. By using metal magnetic powder with a specific surface area larger than 32 nf/g by the BET method, the coercive force of both the upper and lower magnetic layers is set to 1000 to 1300 oersteds, and the difference in coercive force between the ceramic layers is made to be 20 oersteds or less. As a result, it was discovered that good frequency characteristics and sensitivity could be obtained in the entire frequency band, and noise could be sufficiently reduced, without the occurrence of sagging in the frequency characteristics and the deterioration of the frequency characteristics in the middle range.This led to the creation of this invention. reached.

In this invention, the metal magnetic powder used in the upper magnetic layer is a fine powder with a specific surface area of more than 32nf/g by the BET method in order to sufficiently reduce noise and obtain sufficient output in the high frequency band. It is preferable to use In addition, the metal magnetic powder used in the lower magnetic layer is used in order to obtain sufficient output in the low frequency band.
It is preferable to use a material with a specific surface area of 20 to 32 nf/g by the method.If the specific surface area is larger than 32 ffr/g and it is fine, the orientation will be poor and the output in the low frequency band will be poor.
5 ～～～ The force cannot be made sufficiently good.
- The metal magnetic powders used in both the upper and lower magnetic layers include, for example, metal powders such as iron, cobalt, and nickel, or alloy powders thereof, and iron containing AI% Cr.
, Mn5S t, Zn, and other alloy powders are preferably used.

The coercive force of both the upper and lower magnetic layers is determined in order to improve the sensitivity and frequency characteristics in low frequency and high frequency bands, and to prevent the frequency characteristics from deteriorating in the middle range due to the sagging phenomenon in the frequency characteristics. Tomo 1000-1300
It is preferable that the difference in coercive force between the upper and lower magnetic layers be within the range of 20 Oersteds, and if the difference in coercive forces is greater than 20 Oersteds, a sagging phenomenon occurs in the frequency characteristics. Note that if the difference in coercive force between the upper and lower magnetic layers is 20 oersted or less,
The coercive force of both the upper and lower magnetic layers may be higher or lower.

In addition, the thickness of both the upper and lower magnetic layers is preferably within the range of 1:1 to 1:10 in terms of the ratio of the upper magnetic layer thickness to the lower magnetic layer thickness. If the upper magnetic layer is too thick, the characteristics in the low frequency band cannot be sufficiently improved.
If it is too thin, noise cannot be sufficiently reduced.

The formation of both the upper and lower magnetic layers can be carried out according to a conventional method. For example, first, BE
A magnetic paint containing a metal magnetic powder having a specific surface area of 20 to 32 at/g by the T method, a binder resin, a sintering agent and other additives is applied by a conventional method and dried to form a lower magnetic layer. Next, on this lower magnetic layer, a magnetic paint containing a metal magnetic powder having a specific surface area of more than 32n/g by the BET method, a binder resin, an organic solvent, and other additives is applied by a conventional means and dried. The upper magnetic layer may be formed using the above steps.

The binder resins used in both the upper and lower magnetic layers include conventionally widely used binder resins such as vinyl chloride-vinyl acetate copolymers, polyvinyl butyral resins, cellulose resins, polyurethane resins, and isocyanate compounds. used.

Further, as the organic solvent, methyl isobutyl ketone, methyl ethyl ketone, cyclohexanone, toluene, ethyl acetate, tetrahydrofuran, dimethylformamide, etc. are used alone or in combination of two or more.

In addition, various additives commonly used in magnetic paints,
For example, dispersants, moisturizing agents, abrasives, antistatic agents, and the like may be optionally added.

Next, embodiments of the invention will be described.

Example 1 <Magnetic paint for lower layer made by m> α-Fe magnetic powder (coercive force 1220 100 parts by weight Oersted, saturation magnetization amount 139 emu 7g% B ET method specific surface area 24.8
rrr/g) VAGH (manufactured by U.C.C., USA, vinyl salt 10-vinyl acetate-vinyl alcohol copolymer) Pandex T-5201 (manufactured by Dainippon Ink Co., Ltd., urethane elastomer) Coronet ) L (Japan Polyurethane 3 Trifunctional low molecular weight isocyanate compound manufactured by Kogyo Co., Ltd.) Myristic acid 2 Toluene
92 Methyl isobutyl ketone 92 This composition was mixed and dispersed in a ball mill for 72 hours to prepare a magnetic paint for the lower layer.

<Preparation of magnetic paint for upper layer> α-Fe magnetic powder (coercive force 1230 100 parts by weight Oersted, saturation magnetization 138 emu 7g % B ET method specific surface area 33.6
rrr/g) VAGH14 Takelac E-551T (Take 1) 6 Yakuhin Kogyo Co., Ltd., urethane prepolymer) Coronate L 2 Myristic acid 2 Silicone oil
0.3〃Carbon Black 2
"α F8203 powder 4 Toluene 92 Methyl isobutyl ketone 92 This composition was mixed and dispersed in a ball mill for 72 hours to prepare a magnetic paint for the upper layer.

<Manufacture of double-layer magnetic tape> After applying the above magnetic paint for the lower layer on a polyester base film with a thickness of about 12μ, drying, and performing surface treatment,
Further curing was performed at 60° C. for 24 hours to form a lower magnetic layer with a thickness of 2.5 μm.

Next, the above-mentioned magnetic paint for the upper layer is further applied on this magnetic layer, dried, and subjected to surface treatment to form an upper magnetic layer with a thickness of 1.5μ, and then cut to a predetermined width to form a double-layer magnetic tape. I made it.

Comparative Example 1 In the composition of the upper layer magnetic paint in Example 1, the coercive force was 1230 oersted, and the saturation magnetization was 138 emu/g.
, instead of α-0 Fe magnetic powder with a specific surface area of 33.6♂/g by the BET method, a coercive force of 1310 oersted,
A double-layer magnetic tape was prepared in the same manner as in Example 1, except that the same amount of α-Fe magnetic powder having a saturation magnetization of 138 gmu 7 g and a specific surface area of 33.9 rrr/g by the BET method was used.

Comparative Example 2 A magnetic tape was produced in the same manner as in Example 1 except that the formation of the upper magnetic layer was omitted and the thickness of the lower magnetic layer was changed from 2.5 μm to 4.0 μm.

Comparative Example 3 A magnetic tape was produced in the same manner as in Example 1 except that the formation of the lower magnetic layer was omitted and the thickness of the upper magnetic layer was changed from 1.5 μm to 4.0 μm.

Regarding the magnetic tapes obtained in Examples and Comparative Examples, the coercive force of both upper and lower magnetic layers, residual magnetic flux density, and rectangular shape (Br/8m
), sensitivity at 315■2, frequency characteristics at 12.5 KIIz, maximum output level at 315 Hz and 2 KHz, saturation output level at 10 KHz, and AC noise were measured.

The table below shows the results.

3 As is clear from the above table, the double layer magnetic tape obtained in Comparative Example 1 had a low maximum output level at 2 KHz and sagging in the frequency characteristics, whereas the double layer magnetic tape obtained in Example 1 Magnetic tape does not exhibit this kind of sag;
In addition, the double N magnetic tape obtained in Example 1 has better frequency characteristics and sensitivity in all frequency bands and lower AC noise than the magnetic tapes obtained in each comparative example. It can be seen that the magnetic recording medium obtained has no sag in the frequency characteristics, has good frequency characteristics and sensitivity in the entire frequency band, and has sufficiently reduced noise.

Patent applicant: Hitachi Maxell, Ltd.

Claims (1)

[Claims] 1. On the substrate, the specific surface area by BET method is 20 to 32r.
Coercive force containing metal magnetic powder of rF/g is 1000 to 1
forming a first magnetic layer of 300 oersted, and further forming thereon a second magnetic layer having a coercive force of 1000 to 1300 oersted containing metal magnetic powder with a specific surface area larger than 32 m/g by BET method, A magnetic recording medium characterized in that the difference in coercive force between the upper and lower magnetic layers is 20 oersted or less.