GB2054411A - Magnetic recording medium - Google Patents

Abstract

The invention relates to a magnetic recording medium which provides on a base of non-magnetic material, in combination, an under layer of an acicular ferromagnetic metal powder prepared by dry reduction, and an upper layer of a chain ferromagnetic metal powder prepared by wet reduction, so as to make use of the advantageous properties of each. The thickness of the layers may be in the range of 0.5-4.0 mu m, and the upper-to-under layer thickness ratio is preferably in the range 1/3 - 4.

Description

SPECIFICATION Magnetic recording medium This invention relates to a magnetic recording medium, and more specifically to a magnetic recording medium suited for high density recording.

Ferromagnetic powders hitherto used in making magnetic recording media have been the powders of maghemite (Y-Fe203), maghemite either coated or doped with cobalt, magnetite (Fe3O4), cobalt-coated or doped magnetite, belthollide compounds of magnetite, chromium dioxide, etc. Their magnetic properties, such as coercive force (Hc) and maximum residual magnetic flux density (Br), are not sufficiently high to warrant the employment of those powders for the manufacture of magnetic recording media for the applications of tomorrow which will demand higher sensitivity for higher density recording.The resulting media are not so suitable for the magnetic recording of signals with short recording wavelengths of less than about one millimicron or for the recording over limited track widths (narrower than about 20 yam). The requirements becoming more and more exacting and severer for the magnetic recording media have stimulated efforts for the development of ferromagnetic powders which will possess the properties desirable for high density recording. One of promising material groups is that of ferromagnetic metal powders.

Ferromagnetic metal powders can be made in a variety of ways. For quantity production on the industrial basis, there are two typical processes: (1) Reduction of an acicular oxyhydroxide, with or without other metal content, or of an acicular iron oxide obtained from such an oxyhydroxide.

(2) Reduction of a solution containing a ferromagnetic metal salt by the addition of a reducing agent.

The ferromagnetic metal powders prepared by these processes will takc characteristics forms that depend on the kind of the starting material and the process used. The dry reduction (1), which involves reduction of an acicular oxyhydroxide or iron oxide in a reducing gas stream of hot hydrogen or the like, gives particles which are acicular, too, inheriting the shape from the mother salt. The particle size is such that the particles having a minor axis in the range of 200 - 1000 A and a major-to-minor-axis ratio of from 5 to 20 is suitable for use in fabricating the magnetic recording medium. As for the magnetic properties, the powder having a coercive force (Hc) in the range of 1000 - 2000 Oe and saturation magnetization (os) in the range of 100 - 180 emu/g appears to be obtained with ease.On the other hand, the ferromagnetic metal powders prepared by wet reduction, as by (2), tend to comprise chain particles, so called because spherical or granular particles grow as linked or connected in the form of a necklace. The magnetic properties of the powders most often obtained in this way are such that they have Hc in the range of 1000 - 2000 Oe and as in the range of 100 - 180 emu/g. The particle size desirable for use in the magnetic recording medium is 150 - 800 A in diameter and 500 - 10000 A in length.

When the acicular and chain particles are separately applied in mixture with a binder on a nonmagnetic base to form a magnetic recording medium, the resulting tapes will exhibit different properties, in packing density, residual magnetic flux density, tape surface quality, and head abrasion. The two have advantages and disadvantages as summarized below.

(A) When acicular particles alone are used to form the magnetic recording medium: Advantage 1) The packing density per unit volume is high, the residual magnetic flux density (Br) ranges from 3000 to 4000 G, and an output with long wavelengths (10 - 20 Fm or upwards) is easily obtained.

Disadvantages 1) Because of poor surface quality of the magnetized side, the tape as the end product has a high noise level, low SIN ratio, and limited short-wavelength output.

2) It causes much abrasion of the head.

(B) When chain particles alone are used to form the magnetic recording medium.

Advantages 1) Because of excellent surface quality of the magnetized side, the tape as the end produce has a low noise level, good SIN ratio, and high short-wavelength (1 - 2 Fm) output.

2) Abrasion of the head is less.

Disadvantage 1) The packaging density per unit volume is low, Br ranges from 1500 to 2500 G, and output of the long wavelengths (10 - 20 pm) is difficult to obtain.

When acicular and chain ferromagnetic powders are to be individually used in manufacturing magnetic recording media, the above-mentioned disadvantages hamper the perfection of practically useful products.

In this connection it is apparent from a comparison of the acicular and chain particles that their merits and demerits are opposite to each other in a mirror image fashion. Taking note of this, the present inventors arrived at the concept of ingeniously combining the two powders so that their strong points can be fully taken advantage of while the weak points of one can be offset by the advantages of the other and vice versa thereby to minimize the deleterious effects of their disadvantages. Thus, a magnetic recording medium of practically outstanding properties can be obtained by coating a base with acicular particles prepared by dry reduction, together with a binder, and then applying thereon chain particles prepared by wet reduction, together with a binder.

As briefly summarized above, the present invention provides a magnetic recording medium characterized by a double-layer coating structure formed on the base. Magnetic tapes with such a structure have already been developed. However, they consist, for example, of a CrO2 coat as the upper layer and a yFe2O3 coat as the under layer. The use of ferromagnetic metal particles is not contemplated nor any distinction is intentionally drawn between the acicular and chain forms, and naturally no particular consideration has been given to the surface quality, packing density, etc. of the magnetized side of the tape.The present invention is based on an inventive concept utterly different from the prior art in that, in the new field of ferromagnetic metal powders, a double-layer structure has been developed, taking note of the characteristics inherent to the particle shapes.

The present invention, as stated above, employs a magnetic metal powder composed mostly of chain particles in forming an upper layer on a base material and also employs a magnetic metal powder composed mostly of acicular particles in forming an under layer. The combined use of those powders gives a high density magnetic recording medium by far the superior in properties to the mdia using either powder alone.

The advantages of the invention are as follows: (1) Because the upper layer is composed of chain particles, the resulting tape has good surface quality, low noise level, and desirable SIN ratio.

(2) The upper layer of chain particles minimizes the head abrasion.

(3) Since the under layer largely contributes to the formation of long wavelengths, the properties attained will be better than when the chain particles alone are used.

(4) The double-layer structure of the two powders brings a tape improved in the surface quality over the tapes using either powder alone. Moreover, the noise level and SIN ratio are also ameiorated.

The metal particles to constitute the double-layer structure are mixed with a binder and are applied in the usual manner. The coating composition and the method of application are not dissimilar to those commonly employed. The thickness of the upper and under layers may be suitably chosen from the range of 0.5 - 4.0 Zm, and the upper-to-under-layer thickness ratio may be chosen, according to the magnetic recording application intended, from the range of 1/10 - 10, preferably from the range of 1/3 - 4.

The invention is illustrated by the following example.

Example Goethite was heat treated to form iron system acicular iron oxide. The resultant was reduced in a hot hydrogen stream, coated, and treated by immersion into a bath of sodium oleate when an acicular ferromagnetic metal powder was obtained. The particle dimensions were such that the average major-axis length was 0.20 Rm and the major-to-minor-axis ratio was 10. As regards the magnetic properties, Hc was 1300 Oe, as 160 emu/g and squareness ratio (SQ) 0.54.

In order to apply the powder on a base, a magnetic coating material of the following composition was prepared: Acicular particles............................... 300 parts by weight Polyester polyurethane 20 Vinyl chloride-vinyl acetate copolymer ........................................ 24 " Silicone oil......................................... 2 " Solvent 1000 The mixture was placed in a ball mill, kneaded for 10 hours, and after the addition of 1.5 parts of a triisocyanate compound ("kronet L" made by Nippon Polyurethane Ind. Co.), the whole mixture was kneaded for one more hour to obtain a magnetic coating material.

This coating material was applied on one side of a 15-um-thick polyethylene terephthalate film placed in a magnetic field, and the coat was dried with heat. Following a surface treatment, a coated film 3.1 um thick resulted.

Next, an aqueous solution of sulfates of iron cobalt and chromium and an aqueous solution of sodium borohydride were reacted in a DC magnetic field to precipitate a ferromagnetic metal powder. After washing with water, the powder was surface treated with sodium oleate to yield a chain ferromagnetic metal powder.

The particle size was 250 A in average diameter. Hcwas 1300 oe, SQwas 0.55, and as was 140 emu/g. Exactly in the same way as with the acicularferromagnetic metal powder described above, a magnetic coating material based on the chain particles was prepared and applied over the acicu lar-pa rticle coat of the tape to form thereon an upper layer varied in thickness. The upper coat was dried with heat. After super calendering, the coated tape was slitted into video tapes 1/2 inch in width. The properties of the magnetic tapes thus obtained are shown in a table appearing below. In the table, sample I represents a tape having only an acicular-particle layer and Sample V, a tape having only a chain-particle layer.The term "reproduction output" as used in the table is a value obtained by comparison of reproduction outputs at the frequency of five megahertz. By "head abrasion" is meant the amount of abrasion of the head (in ) after continuous passage of the tape past the head for 100 hours at a temperature between 200 and 250C and at a relative humidity between 60 and 80 %. "Video S/N" stands for value measured by SHIBASOKU's noise meter (model 925c) on the basis of the value of Sample V as O dB. "Surface quality" represents a relative value of the amount of reflected light at 250C on the basis of the value of Sample V as 100. The tape deck used for the measurement purpose was Matsushita Electric's VHS video tape-recorder Model NV-8800".

TABLE Under Upper Repro- Head Video Surface Sample layer layer duction abra (acicular (chain output sion SIN quality No. particle) particle) (/100 thickness thickness hrs) (um) (um) (dB) (dB) (%) 3.1 - -2.5 5-6 -2.0 60 II 3.1 0.8 +1.6 1-2 +1.0 130 Ill 3.1 1.5 +1.4 1-2 +1.0 120 IV 3.1 2.7 +0.7 1-2 +1.0 110 V - 3.3 0.0 1-2 0.0 100 As can be seen from the table, the tape of double-layer structure according to the invention is markedly improved over the single-layer tapes in the sensitivity at f = 5 MHz, in video S/N, and in surface quality with a considerably lower value of head abrasion than Sample I.

Claims (8)

1. A magnetic recording medium wherein a base of nonmagnetic material has thereon an under layer comprising an acicularferromagnetic metal powder prepared by dry reduction and an upper layer comprising a chain ferromagnetic metal powder prepared by wet reduction.

2. A magnetic recording medium according to claim 1, wherein said acicular ferromagnetic metal powder is obtained by reducing in a reducing gas stream an acicular oxyhydroxide, with or without other metal content, or acicular iron oxide prepared from such an oxyhydroxide.

3. A magnetic recording medium according to claim 2, wherein said ferromagnetic metal powder is made by reducing acicular iron oxide in a hot hydrogen stream.

4. A magnetic recording medium according to any of claims 1 to 3, wherein said chain ferromagnetic metal powder is obtained by reducing a solution containing a ferromagnetic metal salt by the addition of a reducing agent.

5. A magnetic recording medium according to any of claims 1 to 3, wherein said chain ferromagnetic metal powder is obtained by reacting a solution containing a ferromagnetic metal salt with an aqueous solution of sodium borohydride in a magnetic field.

6. A magnetic recording medium according to any of the preceding claims, wherein said acicular ferromagnetic metal powder has a minor-axis diameter in the range of 200 - 1000 A and a major-to-minor axis ratio in the range of 5 - 20, and said chain ferromagnetic metal powder has a particle diameter in the range of 150 - 800 and a length in the range of 500 - 10000 .

7. A magnetic recording media according to claim 1, substantially as hereinbefore described.

8. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.