JPS61139922A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve largely the magnetic characteristic and electromagnetic conversion characteristic by depositing cobalt to the seed crystal of needle-like magnetic iron oxide having average <=5vol% of particle porosity and using such crystal as an essential component for the magnetic powder to be incorporated into a magnetic layer. CONSTITUTION:The magnetic particle to be incorporated into the magnetic layer of the magnetic recording medium consists essentially of the needle-like iron oxide having average <=5wt%, more preferably <=2wt% particle porosity which is used as the seed crystal and to which the cobalt is deposited. Since the seed crystal has the low porosity, the formation of the multiple magnetic domain structure of the particles is prevented and the surface of the magnetic particles after the deposition of the cobalt-contg. layer is made smooth. The dispersion of the magnetic powder in the magnetic paint is thus improved and the magnetic powder density of the magnetic layer is increased, by which the magnetic characteristic and electromagnetic conversion characteristic are largely improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to magnetic recording media such as various magnetic tapes and magnetic disks including audio tapes and video tapes.

[Prior Art] A typical magnetic recording medium is one in which a magnetic layer containing magnetic powder, which is a magnetic recording element, and a binder that binds the powder is provided on a non-magnetic support such as a polyester film. In order to improve the recording density of such magnetic recording media, T-Fe203, Fe3O4 and T-Fe20. Cobalt-coated acicular iron oxide, in which a cobalt-containing layer is provided on the surface of acicular magnetic iron oxide, such as an intermediate between Fe3O4 and Fe3O4, is widely used.

Incidentally, the acicular magnetic iron oxide which is the above-mentioned nucleus crystal is usually obtained by oxidizing ferrous hydroxide precipitated by a reaction between an aqueous ferrous salt solution and an aqueous alkaline solution. It is produced by using α-iron oxyhydroxide (α-FeOOH) as a raw material, dehydrating and reducing it by heating in a gas phase to obtain Fe3O4, and further heating and oxidizing it as necessary to obtain T-Fe203. (Reference unknown). Therefore, these acicular magnetic iron oxide particles generally have a high particle vacancy of 8 to 10% capacity on average, due to the volatilization of crystallized water during the heating and dehydration process (which creates pores and depressions due to their cleavage). Acicular magnetic iron oxide with such a high porosity has a multi-domain structure due to the Lorentz magnetic field generated by the pores, which reduces the magnetic force per particle. There is also a tendency to increase the porosity of the magnetic layer containing .

However, in the case of cobalt-coated acicular magnetic iron oxide as mentioned above, conventionally, even if the acicular magnetic oxide particles, which are the nucleus crystals, have some pores or recesses, these particles are larger than a certain level. Since it is covered by providing a cobalt-containing layer, there is no problem, and it has been thought that there is no particular room for improvement in the magnetic properties and electromagnetic conversion properties of a magnetic recording medium using this.

[Problems to be Solved by the Invention] However, despite the above-mentioned conventional wisdom, the present inventors have investigated and found that even in cobalt-coated acicular magnetic iron oxide, the properties of the acicular magnetic iron oxide of the nucleus crystals are It has been found that the magnetic characteristics and electromagnetic conversion characteristics of the magnetic recording medium are greatly affected by this, and that there is still room for improvement in this respect. Therefore, it is an object of the present invention to further improve the magnetic properties and electromagnetic conversion properties of a magnetic recording medium using cobalt-coated acicular magnetic iron oxide as the magnetic powder compared to conventional media.

[Means for Solving the Problems] In the course of intensive studies for the above purpose, the inventors added a specific crystallization control agent to an aqueous suspension of ferric hydroxide and heated it. Focusing on the fact that various methods have been proposed for obtaining acicular magnetic iron oxide with low porosity by reducing and optionally oxidizing α-Fe203 powder obtained by An attempt was made to use acicular iron oxide with low porosity as the nucleus of cobalt-coated acicular magnetic iron oxide. As a result, since the nucleus crystals have a smooth surface with low porosity, the surface of the particles after coating with cobalt also becomes smooth, so when this is used as magnetic powder, entanglement between magnetic particles in magnetic paint is prevented. The magnetic paint is coated on a non-magnetic support, and the dispersion is very good.
It has been found that a large amount of magnetic powder can be contained per unit volume in the magnetic layer formed by drying. By increasing the density of the magnetic powder in the magnetic layer, the magnetic recording medium becomes more magnetic than the conventional case where cobalt-coated acicular magnetic iron oxide with high porosity acicular magnetic iron oxide as the nucleus is used. It was found that the characteristics and electromagnetic conversion characteristics were greatly improved, and this invention was made.

That is, the present invention provides a cobalt-coated acicular iron oxide in which a cobalt-containing layer is deposited on a core crystal of acicular magnetic iron oxide having an average particle porosity of 5% by volume or less on a non-magnetic support. The present invention relates to a magnetic recording medium provided with a magnetic layer containing magnetic powder mainly composed of.

[Structure and operation of the invention]

In this invention, the cobalt-coated acicular magnetic iron oxide used as the main body of the magnetic powder is one in which acicular magnetic iron oxide with an average particle porosity of 5% by volume or less, preferably 2% by volume or less, is used as the nucleus crystal. It is. In other words, when iron a has a ζ porosity, the formation of a multi-domain structure in the particles is prevented, and
After the cobalt-containing layer has been applied, the surface of the magnetic powder particles becomes smooth, and the magnetic powder is well dispersed in the magnetic paint, making it possible to increase the density of the magnetic powder in the magnetic layer, thereby improving the magnetic properties of the magnetic recording medium. Conversion characteristics are improved.

Such low porosity core crystals are Fe50 and T-Fe20.
For example, an aqueous suspension of ferric hydroxide is made alkaline in the presence of a crystallization control agent selected from water-soluble compounds capable of coordinating iron. A method in which α-Fe is heated under the
20. , α-Fe203 powder with low particle porosity is obtained by adding seed crystals, etc., and this is heated and reduced to form Fe3
O4 or this Fe50. T-F
It can be obtained by using e203 or an intermediate oxide between it and Fe3O4.

By using such nuclear crystals, the magnetic powder coated with the cobalt-containing layer can be better dispersed in the magnetic paint, improving the sensitivity characteristics of the magnetic recording medium and also achieving good results in reducing noise. It will be done.

In addition, in the above manufacturing method, the porosity can be easily set by appropriately selecting conditions such as the type of crystallization control agent and the pH of the aqueous suspension of ferric hydroxide. This selection can be easily made by those skilled in the art.

Furthermore, regarding the method for producing acicular magnetic iron oxide used as the nucleus crystal in this invention, Japanese Patent Publication No. 55-4694 and Japanese Patent Publication No. 55-4694,
-22416 Publication, Special Publication No. 56-17290,
U.S. Patent No. 4,202,871 and JP-A-57-
It is described in detail in the 92527 publication. As crystallization control agents, various compounds such as those described in the above-mentioned literature, such as polycarboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, polyamines, organic phosphonic acids,
Thiocarboxylic acid, polyhydric alcohol, β-dicarbonyl compound, aromatic sulfonic acid 1. Salts and esters of these and other phosphates are used.

In addition, as for the particle size of the nucleus crystal, the average major axis diameter is 0.1 to 1.
．． 0 μ, average axial ratio (average major axis diameter/average minor axis diameter) is 5 to 2
Preferably, it is in the range of 0. If the average major axis diameter is too small, good results will not be obtained in sensitivity characteristics, and if it is too large, good results will not be obtained in noise reduction.

In this invention, cobalt-coated acicular magnetic iron oxide with a cobalt-containing layer provided on the surface of the core crystals is used as the main component of the magnetic powder, but the means for attaching the cobalt-containing layer is a conventional high-porosity needle. The same method as used for obtaining cobalt-coated acicular magnetic iron oxide having acicular magnetic iron oxide as the nucleus may be used. For example, by dispersing low-porosity nuclear crystals in an aqueous solution containing cobalt salts and ferrous salts, and adding an alkaline aqueous solution to this suspension and stirring under heating, cobalt can be deposited on the nuclear crystals. A layer containing iron oxide is applied.

The above cobalt salts include cobalt sulfate, cobalt chloride,
Cobalt nitrate, etc., ferrous salts, '-' are ferrous sulfate, ferrous chloride, etc., and alkaline aqueous solutions include NaO.
H or KOH aqueous solution, NH3 water, etc. are used.

The amount of cobalt salt to be used is such that the cobalt atoms in the finally obtained cobalt-containing iron oxide layer are 0.0% by weight relative to the core crystals.
It is preferably used in a range of 0.05 to 10%, more preferably in a range of 0.1 to 5%. If the amount of cobalt salt is too low, the characteristics of cobalt-coated acicular magnetic iron oxide will be weakened, and the difference in magnetic properties will be small compared to those without cobalt deposited.On the other hand, if the amount of cobalt salt is too high, the characteristics of cobalt-coated acicular magnetic iron oxide will be diminished, and on the other hand, if there is too much cobalt salt, cobalt will The uniformity of deposition of the containing layer is poor. The ratio of cobalt salt to ferrous salt used is preferably about 0 to 5 (ferrous salt/cobalt salt) in terms of molar ratio.

The reaction for depositing the above cobalt-containing layer is typically carried out at a temperature of 2
Initial pH 13 when adding 0 to 100°C1 alkaline aqueous solution
~14, reaction time is approximately 1 to 10 hours.

In addition, in this invention, cobalt-coated acicular magnetic iron oxide using the above-mentioned low-porosity nucleus crystals is mainly used as the magnetic powder, but the whole magnetic powder is generally used as long as the above-mentioned effects of the invention are not impaired. Within the range of 50% by weight or less, cobalt-coated acicular iron oxide with a conventional high porosity acicular magnetic iron oxide as a nucleus may be used in combination.

The magnetic recording medium of the present invention is produced by preparing a magnetic paint containing the above-mentioned magnetic powder, a binder component, and other additive components as necessary, and coating it on a known non-magnetic support such as a polyester film to a predetermined thickness (usually is coated to a thickness of 4 to 7 μm after drying, subjected to orientation treatment, dried to form a magnetic layer, and then subjected to known treatments such as calendering.

The above-mentioned magnetic layer has a coercive force in the range of 380 to 750 oersted (Oe), and its porosity is lower than conventional ones due to the properties of the magnetic powder itself as described above, and is generally 20% by volume. Below, in a particularly preferred embodiment, 5 to 10
% by volume, and the magnetic powder is packed with high density.

As the binder component, any of the binder components conventionally used in this type of magnetic recording medium, such as vinyl chloride-vinyl acetate copolymers, polyurethane resins, cellulose resins, polyvinyl butyral resins, and polyisocyanate compounds, can be used. is also available. Further, as additive components, dispersants, lubricants, abrasives, antistatic agents, reinforcing agents, etc. can be used as appropriate.

[Effects of the Invention] The magnetic recording medium according to the present invention is characterized in that the magnetic powder contained in the magnetic layer is mainly composed of cobalt-coated acicular magnetic iron oxide with low porosity acicular magnetic acidic iron oxide as the nucleus. Compared to conventional cobalt-coated acicular magnetic iron oxide magnetic powder using high-porosity core crystals, the smooth surface of the core crystals makes it easier to form particles after cobalt deposition. The surface becomes smooth, and the magnetic particles are less entangled with each other in the magnetic paint, resulting in very good dispersion. As a result, the density of the magnetic powder in the magnetic layer can be increased, which greatly improves the magnetic properties and electromagnetic conversion properties.

〔Example〕

Examples and comparative examples of the present invention will be shown below.

Hereinafter, parts refer to parts by weight.

Example 1 T-Fe203 powder with particle porosity O volume % [specific surface area 25
．． 5 m'/y, average major axis diameter 0.4/'', average axial ratio 10,
Coercive force (Hc) 3400e1 Saturation magnetization (σs) 7
2.5 emu/V) 300y, cobalt sulfate (Co
S04・7H20) 42.9y and ferrous sulfate (Fe 5
04 ・7 HzO) 127.4y dissolved in an aqueous solution 1.41! It was dispersed into Next, aqueous solution 11 in which 240.9 y of sodium hydroxide (NaOH) was dissolved was added to this suspension, and the mixture was reacted at 45°C for 6 hours with stirring. Wash the formed precipitate with water.

It was filtered and dried at 120°C in the atmosphere to obtain magnetic powder consisting of cobalt-coated acicular r-Fe203. This magnetic powder has a coercive force (Hc) of 6100e and a saturation magnetization amount (σ5)
It was 74.5 emu/y. Incidentally, an electron micrograph of this magnetic powder is shown in FIG.

Using the thus obtained magnetic powder, a mixture consisting of the following composition was mixed and dispersed in a ball mill for 70 hours to prepare a magnetic paint.

Magnetic powder (cobalt coated T-Fe203 powder...240
Palmitic acid...2.4 parts Methyl isobutyl ketone...225 parts Toluene...
225 parts of this magnetic paint was applied onto a polyester film having a thickness of 11 μm so that the thickness after drying would be 5 μm.

After drying and calendering, the tape was cut into A-inch width to produce a magnetic tape.

Example 2 T-Fe203 with particle porosity O volume % used in Example 1
Instead of powder, T-Fe, 03 powder with a particle porosity of 2% by volume [
Specific surface area 31rrl19', average major axis diameter Q, 3 pg
, average axial ratio 12, coercive force (Hc) 3500e, saturation magnetization (σ5) 71 emu/! i! ] Magnetic powder consisting of cobalt-coated acicular T-Fe203 was obtained in the same manner as in Example 1 except that 300 y was used. This magnetic powder has a coercive force (Hc) of 6300e and a saturation magnetization of 74 emp/
It was y.

A magnetic tape was produced in the same manner as in Example 1 using the thus obtained magnetic powder.

Comparative Example T-Fe20s with particle porosity of 0% by volume used in Example 1
Instead, T-Fe203 powder with a particle porosity of 8% by volume [specific surface area 26rr//y1 average major axis diameter 0.45μ, average axial ratio 12, coercive force (Hc) 3550e, saturation magnetization amount (σ
Magnetic powder consisting of cobalt-coated acicular T-Fe203 was obtained in the same manner as in Example 1 except that S) 72 emu/y) 300 y was used. This magnetic powder has a coercive force (Hc) of 6150
e, and the saturation magnetization amount was 74.5 emu/y. Incidentally, an electron micrograph of this magnetic powder is shown in FIG. A magnetic tape was produced in the same manner as in Example 1 using the thus obtained magnetic powder.

For each of the magnetic tapes of the above examples and comparative examples, the magnetic properties include coercive force (Hc), squareness ratio, and residual magnetic flux density QS.
r) as electromagnetic conversion characteristics at 315Hz and 12.5
When the KHz sensitivity and AC bias noise level were measured, the results shown in the table below were obtained. Note that all electromagnetic conversion characteristics are reference ratios.

As is clear from the above table, the magnetic tape (Examples 1 and 2) according to the present invention, which uses cobalt-coated acicular magnetic iron oxide using low-porosity nucleus crystals as magnetic powder, is different from the conventional high-porosity magnetic tape (Examples 1 and 2). It can be seen that the magnetic tape has superior magnetic properties and electromagnetic conversion properties as compared to a magnetic tape (comparative example) that uses cobalt-coated acicular magnetic iron oxide as magnetic powder using a cobalt-coated core crystal (comparative example). Furthermore, from a comparison between Figures 1 and 2, it is clear that when a high-porosity core crystal is used as in the past, the particles still have pores and irregularities after the cobalt-containing layer is applied. It is clear that the cobalt-coated acicular magnetic iron oxide using low-porosity nuclear crystals like the magnetic powder used in this invention has a smooth surface, and from this external shape it is clear that it can be used in magnetic paints. It can be seen that the dispersion of the particles is improved, and it is possible to achieve high-density packing in the magnetic layer.

[Brief explanation of drawings]

Figure 1 is an electron micrograph at a magnification of 30,000 times showing the particle structure of the cobalt-coated acicular magnetic iron oxide used in Example 1 according to the present invention, and Figure 2 is a cobalt-coated needle used in a comparative example. FIG. 2 is an electron micrograph at a magnification of 30,000 times showing the particle structure of magnetic iron oxide. Patent applicant: Hitachi Maxell, Ltd.jJj 1 1-1. ; 7 ′ ・Zo 1, -・

Claims (1)

[Claims] (1) Mainly composed of cobalt-coated acicular magnetic iron oxide formed by coating a cobalt-containing layer on core crystals of acicular magnetic iron oxide with an average particle porosity of 5% by volume or less on a non-magnetic support. A magnetic recording medium provided with a magnetic layer containing magnetic powder.