JPS5856229A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium having an excellent reproducing output at all bands with a low level of noise, by setting the specific surface area, the coercive force, the residual magnetic flux density, etc. within a specific range respectively for the magnetic powder of iron oxide of each of two magnetic layers. CONSTITUTION:The magnetic powder of iron oxide having 18-25m<2>/g specific surface area and obtained by a BET process is coated on a nonmagnetic substrate 1. Thus the 1st magnetic layer 4 is obtained. This layer 4 has 250- 400 oersted coercive force HC1 and 1,600-2,000G residual magnetic flux density Br. Then the magnetic powder of iron oxide having 28-45m<2>/g specific surface area and obtained by the BET process is coated on the layer 4 to obtain the 2nd magnetic layer 5 which has 250-500 oersted coercive force HC2. In this case, the HC2 is set larger than the HC1.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic recording medium, particularly a magnetic recording medium having two magnetic layers.

Conventional magnetic recording media, for example, magnetic tapes with broken layers of γ-Fe203 magnetic powder (specific surface area of magnetic powder 20 nX/g)
However, although the coercive force and residual magnetic flux density are large, and the reproduction output is large over the entire band, the bias noise is also large.

In view of the above points, the present invention provides a magnetic recording medium having excellent reproduction output and low noise (bias noise) in all bands.

Based on various experiments and researches, the present inventor has found that in magnetic recording media, when iron oxide magnetic powder with a large specific surface area is distributed on the surface of the magnetic layer, bias noise is reduced. We have found that distributing iron oxide magnetic powder with a small specific surface area in the lower layer) increases the reproduction output over the entire band.

Based on the results of this research, the present invention provides a magnetic layer in a magnetic recording medium with a two-layer structure made of iron oxide magnetic powder, and the specific surface area and coercive force H of each magnetic powder in both the upper and lower magnetic layers.
The C1 residual magnetic flux density Br, layer thickness, etc. are all selected to reduce bias noise while maintaining high reproduction output in the entire band.

The magnetic recording medium according to the present invention will be described in detail below with reference to an example in which γ-Fe203 magnetic powder is used as the iron butyride magnetic powder. The specific surface area of the magnetic powder is determined by the BET method.

Example 1 γ-B' with specific surface area 20i/g and coercive force 3200e
e203 magnetic powder・・・・・・・・・・・・・・・・・・
...400 parts by weight lecithin...
・・・・・・・・・・・・・・・・・・・・・・・・
...4 parts by weight methyl ethyl ketone...
・・・・・・・・・・・・・・・・・・700 parts by weight Cyclohexanone・・・・・・・・・・・・・・・・・・
...... 50 parts by weight of the above composition was added and after dispersion treatment in a ball mill for 24 hours, polyisocyanate (Desmodicol L-75 Bayer A, G!
A heavy plate was added and high-speed shear dispersion was performed for 2 hours to obtain a magnetic paint. This is called magnetic paint (A).

2 Specific surface area 20fn'/9, coercive force 3200e γ-
Fe203 magnetic powder・・・・・・・・・・・・・・・
...400 parts thermoplastic polyurethane resin...
......33.3 parts by weight Vinyl chloride-vinyl acetate copolymer...33.3 parts by weight Lecithin...
・・・・・・・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・Quadruple methyl ethyl ketone・・・・・・・・・・・・・・・・・・・・・
650 parts by weight cyclohexanone...
. . . 50 parts by weight of f parts of the above composition were added, and a magnetic paint was obtained by following the same steps as the magnetic paint (A) of the first example. This is referred to as magnetic paint (B).

3. Specific surface 1* 20tr? /11, coercive force 3200
e no r -Fe2O3 magnetic powder・・・・・・・・・・・・
400 parts by weight Thermoplastic polyurethane resin 25 parts by weight Vinyl chloride-vinyl acetate copolymer・・25 parts by weight lecithin・・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
...4 parts by weight methyl ethyl ketone...
・・・・・・・・・・・・・・・625 parts by weight Cyclohexanone・・・・・・・・・・・・・・・・・・・・・・
. . . 50 parts by weight of the above composition was added, and a magnetic paint was obtained by following the same process as the magnetic paint (A) of the first example. This is called magnetic paint (C).

4. The r-Fe2O3 magnetic powder of the magnetic paint (C) of the third example has a specific surface area of 20rrl/11 and a coercive force of 4200e.
A magnetic paint was obtained by the same process as the magnetic paint (A) of the first example except that the magnetic paint was replaced with γ-Fe203 magnetic powder of (Others being the same). Each of these is referred to as a magnetic paint (D).

5 The γ-Fe203 magnetic powder of the magnetic paint (C) of the third example has a specific surface area of 30 m''/, !i', and coercive force of 320.
Magnetic paints were obtained by the same process as the magnetic paint (A) of the first example except that r -Fe2O3 magnetic powders of 0e, 4200e and 5200e were used (other things being the same).

These are referred to as magnetic paints (E), CF) and (G), respectively.

6. The γ-Fe203 magnetic powder of the magnetic paint (A) of the first example has a specific surface area of 40 m'/l/s and a coercive force of 3200.
e.

Magnetic paints were obtained by the same process as the A-based paint (A) except that 7-Fe2O3 magnetic powders of 4200e and 5200e were used (other things being the same). Remove the magnetic paint (H), (
I) and (J).

However, the specific surface area and coercive force of the γ-Fe203 magnetic powder are controlled by appropriately selecting the size and/or shape of the magnetic powder particles.

The properties of each of the magnetic paints (A) to (J) described above are summarized in the table below. However, P/B indicates the ratio of binder B to magnetic powder P.

Therefore, using each of the above magnetic paints (A) to (J),
As shown in Figure 1, a magnetic paint is applied on a non-magnetic substrate (for example, a polyethylene terephthalate film with a thickness of 12μ) (1), subjected to magnetic field orientation treatment, dried, and then treated with a super calendar roll to form a single magnetic coating. A magnetic recording medium (3) comprising a layer (2) is produced.

In addition, using similar magnetic paints (A) to (J), the first magnetic layer (lower #) f4) was formed in the same process as above as shown in Figure 2.
A two-layer magnetic recording medium (6) according to the present invention, in which a second magnetic layer (upper layer) (5) is formed in the same process as above after the first magnetic layer (4) is sufficiently hardened. Create.

The magnetic properties (residual magnetic flux density Br, coercive force Hc), reproduction output (MOL), bias noise, etc. of the magnetic recording media (3) and (6) prepared as described above were measured.

The results are shown in FIGS. 3 and 4.

Figure 3 shows a single magnetic layer (
2) Characteristics of magnetic recording medium (3) (single layer examples (1) to 23
), FIG. 4 shows the characteristics of the two-layer magnetic recording medium (6) according to the present invention (Examples (1) to (2(I) of the present invention).

The magnetic recording medium (magnetic tape) used for the measurement was 1
This is an audio tape cut into 8 inch width. The measurement method for each characteristic is as follows.

1. The residual magnetic flux density Sr is the residual magnetic flux density when measured with an external magnetic field of 20000e. The unit is gauss (gauss)
s).

2 Coercive force HC is coercive force when measured with an external magnetic field of 20000e. The unit is Oersted (Oe).

3. For MOL (maximum output level: reproduction output), a reference frequency (315H2) and a high frequency (l QkHz) are used. This is the maximum saturation output level.

4. Bias noise is audible correction filter A (JIs)
This is the noise when using. The unit is decibel (dl)
).

5. Tape speed is 4.8 tm/Se'.

The MOL (315Hz, 10kHz) and bias noise are based on the single layer example (1) in Figure 3 (□ab
) is expressed as a relative value (db).

From FIG. 3 and FIG. 4, it is clear that the two-layer magnet according to the present invention has a specific surface area of the magnetic powder of the upper second magnetic layer (5) larger than that of the lower first magnetic layer (4). It can be seen that the recording medium (6) has a higher reproduction output over the entire low and high frequency bands than the conventional single-layer magnetic recording medium (3), and also significantly reduces bias noise.

The specific surface area of the iron oxide magnetic powder of the lower first magnetic layer (4) is preferably in the range of around 20 rrl/11 (18 to 25 ns"/g), and if it deviates from this range, the low-frequency reproduction output decreases.
The specific surface area of the iron oxide magnetic powder of the second magnetic layer (5) on the groove upper layer is preferably in the range of 28 to 45 rn'/g, and if it deviates from this range, the bias noise will not be reduced. On the other hand, the coercive force 1 (cl) and residual magnetic flux density Br1 of the magnetic layer (4) of feed 1 are 250 to 400 oersted and 1600 to 2000 Gauss, respectively.
Outside the 0 Gauss range, the low frequency output decreases. Second
It is preferable that the coercive force HC2 of the magnetic layer (5) is 250 to 500 Elsdet; if it is out of this range, the high frequency range will not be extended;
Furthermore, recording bias and erasure are subject to restrictions on the recording/reproducing apparatus side. Further, the relationship between the coercive force Hct of the first magnetic layer (4) and the coercive force lIc2 of the second magnetic layer (5) is preferably HCI<HO2.

The thickness of the first magnetic layer (4) is 2.0μ or more and 60μ
It is preferable that the thickness is less than 20μ, and if it is thinner than 20μ, the reproduction output in the low range will decrease, and if it is thicker than 60μ, only the reproduction output in the low frequency range will increase too much, and the balance between low and high frequencies will be poor.Also, the second magnetic layer (5) The thickness is preferably in the range of 0.2 to 20μ, and 0.2μ to 20μ.
If it is thinner than 2μ, the bias noise will not decrease, and it will be 2.0
As μ becomes thicker, the low-frequency reproduction output decreases.

As described above, in the present invention, based on the characteristic results shown in FIG. 4, the first magnetic layer (4) and the second magnetic layer (5) are sequentially formed on the non-magnetic substrate (1) as shown in FIG. The first magnetic layer (4) of the magnetic recording medium (6) comprising BE
The specific surface area by T method is 18-25 m'/! i'
It is formed by coating iron oxide magnetic powder, and its coercive force HCI
250 to 400 oersted, residual magnetic flux density Br is 1
600 to 2000 Gauss, and the second magnetic layer (5)
is formed by applying iron oxide magnetic powder with a specific surface area of 28 to 45 n7/I by the BET method, and its coercive force HC2 is 25
0 to 500 oersted.

According to the present invention, bias noise can be reduced while maintaining high reproduction output over the entire band, which is superior to conventional single-layer magnetic recording media (such as metal tape). A magnetic recording medium can be provided.

[Brief explanation of drawings]

FIG. 1 is an enlarged cross-sectional view of a conventional magnetic recording medium, FIG. 2 is an enlarged cross-sectional view of a magnetic recording medium according to the present invention, FIG. 3 is a table showing the characteristics of each side of a single-layer magnetic recording medium, and FIG. FIG. 4 is a table showing the characteristics of each embodiment of the two-layer magnetic recording medium according to the present invention. In the figure (11 is a non-magnetic substrate, (4) is a first magnetic layer, (5
) is the second magnetic layer. Figure 1 Figure 2

Claims (1)

[Claims] It consists of a non-magnetic substrate, a first magnetic layer on the substrate, and a second magnetic layer on the first magnetic layer, and the first magnetic layer is made of B.
It is formed by coating iron oxide magnetic powder with a specific surface area of 18 to 25 nl/g by the ET method, and its coercive force HC1 is 250 nl/g.
~400 oersted, residual magnetic flux density Br ~1600
2000 Gauss, and the second magnetic layer is formed by coating iron oxide magnetic powder with a specific surface area of 28 to 45 nj/ji by BET method, and its coercive force Hc2i is 250 to 500.
1. A magnetic recording medium characterized in that the coercive force HCI<coercive force HC2.